Water and Air Effluents Treatment Handbook


Water and Air Effluents Treatment Handbook

Author: NPCS Board of Consultants & Engineers
Format: Paperback
ISBN: 9788178331195
Code: NI217
Pages: 576
Price: Rs. 1,275.00   US$ 125.00

Published: 2009
Publisher: Asia Pacific Business Press Inc.
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Water treatment describes those processes used to make water more acceptable for a desired end use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end use. Water quality analytical techniques are considered in the context of EEC directives on the quality of the aquatic control of all effluents is entering it. The principal methods of water analysis are reviewed and it indicated in view of destructive and hazardous role of pollution, it become necessary that the very nature of atmosphere, the various air effluent are present there to save the environment from the harmful effect. Effluent can be treated in different ways, it is classified as; preliminary treatment, primary treatment, secondary treatment and complete final treatment. Waste water obtained from industries is generally much more polluted than the domestic or even commercial waste water. Industrial wastewater cannot be always treated easily by the normal methods of treating domestic waste waters. Depending on the quantum, concentration, toxicity and presence of non biodegradable organics in an industrial wastewater, its treatment may consist of any one or more processes such as equalization, neutralization, physical treatment, chemical treatment and biological treatment. The atmosphere contains hundreds of air pollutants from natural or from anthropogenic sources. All such pollutants are called primary pollutants for example; sulphur oxides, carbon monoxide, nitrogen oxides, lead etc. Secondary pollutants are the chemical substances, which are produced from the chemical reactions of primary pollutants or due to their oxidation etc. A high growth in vehicle population brings in its wake urban air pollution problems unless timely appropriate steps to control vehicle emissions are under taken.
Some of the fundamentals of the book are quality and characteristics of effluents, collection of sewage samples for physical and, chemical testing, disposing of effluents, disposal of wastewaters in lakes and management of lake waters, disposal of sewage effluents on land for irrigation, classification of treatment processes, treatment of industrial effluents, methods of treating industrial wastewaters, strategies for management of industrial wastes, combined industrial municipal wastes, a process for upgrading paper mill effluent by water hyacinth, ventilation for controlling indoor air pollution, the environment and its pollution, disposal of environmentally hazardous radioactive effluents and biomedical wastes, air pollution, its control and monitoring, fuels from waste etc.
This book is an effort to put together the various options available to meet the water and air effluent available for the environmental protection. The book presents a concise but through an overview of state of technology for water and air effluent treatment. The water and air effluent treatments are organized into chapters by broad problem area, treatment of industrial effluent, industrial waste management, etc. This will be helpful to technocrats, consultants, educators, architects, industry executive, students and others concerned with saving environment problem.

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Contents

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1. QUALITY AND CHARACTERISTICS OF EFFLUENTS
IMPORTANCE OF STUDY
Decay or Decomposition of Sewage
CHARACTERISTICS OF EFFLUENTS
Physical Characteristics of Sewage and Their Testing
Chemical Characteristics of Sewage and Their Testing
Total Solids, Suspended Solids and Settleable Solids
Population Equivalent
Relative Stability
Collection of Sewage Samples for Physical and,
Chemical Testing
Bacteriological Characteristics and Testing
2 DISPOSING OF EFFLUENTS
DISPOSAL BY DILUTION
Conditions Favouring Disposal By Dilution
Standards of Dilution for Discharge of
Waste waters into Rivers
Dilution in Rivers and Self Purification of Natural Streams
Disposal of Wastewaters in Lakes and
Management of lake Waters
Disposal of Wastewater in Sea Water
DISPOSAL ON LAND
Disposal of sewage effluents on land for for Irrigation
Quality Standards For Wastewater Effluents to be Discharged on Land For Irrigation
'Effluent Irrigation' and 'Sewage Farming'
-Difference Thereof
Methods of Applying Sewage Effluents to Farms
Sewage Sickness
Crops Grown in Sewage Farms and Their Hygienic Aspect
Dilution Method Vs. Land Disposal Method for
Disposal of Sewage
3. TREATMENT OF EFFLUENTS
Classification of Treatment Processes
SCREENING
Types of Screens, Their Designs and Cleaning
Comminutors
Disposal of Screenings
GRIT REMOVAL BASINS
Grit Chambers
Detritus Tanks
Design of ParaboIic Grit Chamber provided-with-a-Par shall Flume
TANKS FOR REMOVING OILS AND GREASE
Skimming Tanks
Vacuators
Disposal of Skimmings
Necessity and Use of Skimming Tanks in India
SEDIMENTATION
General Introduction
Principle of Sedimentation
Theory of Sedimentation
Sedimentation Tanks
Disadvantages or Demerits of Coagulation in Sewage Treatment
SECONDARY TREATMENT THROUGH BIOLOGICAL
FILTRATION OF SEWAGE
(Aerobic Attached Culture)
Introduction to Sewage Filtration
Contact Beds for Biological Filtration of Sewage
Intermittent Sand Filters for Biological Filtration of Effluents
Trickling Filters for Biological Filteration of Sewage
Construction and Operation of Trickling Filters
Recirculation of Treated Sewage and its Use in High Rate Trickling Filters
Other Miscellaneous Types of Filter
SECONDARY SEDIMENTATION
Secondary Settling Tanks or Humus Tanks
DIGESTION AND DISPOSAL OF PRIMARY AND
SECONDARY SLUDGE
Sludge and Its Moisture Content
Sludge Digestion Process
Stages in the Sludge Digestion Process
Factors Affecting Sludge Digestion and Their Control
Sludge Digestion Tank or Digestors
(Aerobic Suspended Culture)
Disposal of Digested Sludge
Use of Lagoons for Disposal of Raw Sludge
SECONDARY TREATMENT THROUGH ACTIVATED
SLUDGE PROCESS
(Aerobic Suspended Culture)
Definition of Activated Sludge Process
Various Operations and Units of an Activated Sludge Plant
Bulking and Foaming Sludge in an Activated Sludge Treatment Plant
DESIGN CONSIDERATIONS INVOLVED IN AN
ACTIVATED SLUDGE PLANT
Aeration Tank Loadings
Sludge Volume Index (S.V.I.)
Sludge Recycle and Rate of Return Sludge
Wasting of Excess Sludge (Qw)
Modifications of the Basic Activated Sludge Process
Size and Volume of the Aeration Tank
Qxygen Requirement of the Aeration Tanks
Advantages and disadvantages of an Activated Sludge Plant
Activated Sludge Process Vs Trickling Filter Process and the Choice of One
SECONDARY TREATMENT THROUGH ROTATING
BIOLOGICAL CONTRACTORS
(Aerobic Attached Culture)
Rotating Biological Contractors (RBCS)
AEROBIC STABILISATION UNITS
(Aerobic Suspended Culture)
Oxidation Ponds and Stabilisation Ponds
Oxidation Ditches (Pasveer Type) or Extended Aeration Lagoons
Mechanically Aerated Lagoons
ANAEROBIC STABILISATION UNITS
Anaerobic and Facultative Stabilisation Ponds
Septic Tanks
Advantages
Disadvantages
Imhoff Tanks
Clarigesters
High Rate Anaerobic Systems
CHLORINATION OF SEWAGE
Disinfection of Sewage by Using Chlorine
Design of Inlet Chamber
Design of Screen Chamber
Design of Grit Chamber
Design of Aeration Tanks
Design of Secondary Clarifier
Return Sludge Pump House:
Design of Sludge Drying Beds
4. TREATMENT OF INDUSTRIAL EFFLUENTS
Introduction
Methods of Treating Industrial Wastewaters
Thermal Pollution
5. INDUSTRIAL WASTE WATER EFFLUENTS
Introduction
TERMINOLOGY
TREATMENT LEVELS
Primary Treatment
Secondary Treatment
LAGOONS AND SEPTIC TANKS
TYPES OF INDUSTRIAL WASTES
Strategies for Management of Industrial Wastes
QUANTITY OF INDUSTRIAL WASTES
METALS AND METAL PRODUCTS
Geographical Aspects of the Industry
Technological Control Methods for Steel Mill Wastes
CHEMICAL AND ALLIED PRODUCTS
PAPER AND ALLIED PRODUCTS
PETROLEUM & COAL PRODUCTS
Petroleum
Coal
FOOD & KINDRED PRODUCTS
Cannery Wastes
Frozen Foods
Dairy Products
Meat
Bakery Products
Poultry
MACHINERY AND TRANSPORTATION EQUIPMENT
STONE, CLAY & GLASS PRODUCTS
RUBBER AND PLASTICS
LUMBER AND WOOD PRODUCTS
TEXTILE MILL PRODUCTS
Cotton
Synthetics
COMBINED INDUSTRIAL-MUNICIPAL WASTES
COST ASPECTS OF POLLUTION CONTROL IN THE
PULP AND PAPER INDUSTRY
6. RECLAMATION OF TEXTILES EFFLUENTS
Experimental Procedure
Results and Discussion
Conclusion
7. A PROCESS FOR UPGRADING PAPER MILL
EFFLUENT BY WATER HYACINTH
Experimental Procedure and Results
Phase I
Phase II
Phase III
Discussion
Conclusion
Salient Features
8. DISPOSAL OF SOLID EFFLUENTS AND REUSE
Definition, Classification, Quantity and Composition of Refuse
Collection, Removal and Carriage of Refuse
Disposal of Refuse
Advantages
Disadvantages
Merits
Demerits and Limitations
9. VENTILATION FOR CONTROLLING INDOOR
AIR POLLUTION
Sources, Effects and Status of Indoor Air Pollution
Definition of Ventilation
Effects of Occupancy of a Space
Purpose of Ventilation
Extent of Ventilation Required and Ventilation Standards
Systems of Ventilation
Advantages
Disadvantages
10. THE ENVIRONMENT AND ITS POLLUTION
Biosphere and Environment
Physical and Biological Environment
Ecosystem and Ecological Balance of Nature
Impact of Man on Biosphere
Pollution and Conservation of Environment
Status of Administrative Control on Environment in India
Status of Water Pollution--Monitoring and Control in India
Status of Air Pollution--Monitoring and Control in India.
11. DISPOSAL OF ENVIRONMENTALLY HAZARDOUS RADIOACTIVE EFFLUENTS AND BIOMEDICAL WASTES
RADIOACTIVE WASTES
Radioactive Elements and Radioactive Radiations
Impacts of Radioactivity on Life and Environment
Disposal of Radioactive Wastes
BIO-MEDICAL WASTES
Biomedical Wastes and Their Hazards on Health and Environment
Legal Laws on Management of Medical Wastes in India
COLLECTION AND TREATMENT OFBIOMEDICAL WASTES
Colour Coding of Biomedical Wastes and Their Collection in Different Coloured Bins or Bags
Collection of Sharp Wastes
Labeling for Identification of Bio-medical Wastes
Storage of Bio-medical Waste
Transportation of Bio-medical Wastes to the Treatment and Disposal Site
Treatment and Disposal of Biomedical Wastes
12. AIR POLLUTION, ITS CONTROL AND
MONITORING
Air Pollutants, Their Effects, and Sources of Origin
Dispersion of Air Pollutants into the Atmosphere
AIR POLLUTION CONTROL
The Natural Self-Cleansing Properties of the Environment
Dilution Method for Controlling Air Pollution from Stationary Sources (Factories)
Controlling Air Pollution from Stationary Sources by Installing Engineering Devices
Controlling Air Pollution from Automobiles
13. VEHICULAR AIR POLLUTION AND MEASURES
FOR ITS CONTROL
Types of Vehicle Emissions
Emission Characteristics of Indian Vehicles
Vehicle Emission Control Technology and Fuel Efficiency
Inspection and Maintenance
Conclusions
14. FUELS FROM WASTE
Characteristics of Wastes for Fuels
Wastes as Fuel
Wood-Waste Combustion Systems
Municipal-Waste Combustion
Other Wastes as Fuel
Conversion Systems Applied to Wastes
Conclusions

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QUALITY AND CHARACTERISTICS OF EFFLUENTS 

Importance of Study

When untreated sewage is discharged into some river stream floating solids present in the discharged sewage may be washed up on to the shore, near the point of disposal, where they decompose and create foul smells and bad odours. The large amount of organic matter present in the discharged sewage will also consume the dissolved oxygen from the river stream in getting oxidized, and may thus seriously decrease the dissolved oxygen of the river stream, causing fish kills and other undesirable effects. In addition to these effects, the discharged sewage will contaminate the river water with pathogenic bacteria. Hence, even though municipal sewage is 99.9 per cent water, it requires treatment, if nuisance is to be avoided.

The extent and type of treatment required, however, depends upon the character and quality of both sewage and source of disposal. For example, a small community at the seaside might discharge its unaltered sewage directly into the ocean without any ill effects, but if this city were located inland on a small stream, a high degree of treatment might be needed.

In the olden times, the waste waters from a community were not so much contaminated as they are today. The urbanization, industrial growth, and the improved standards of living, have increased the strength and quantity of municipal sewage in recent years to a point where dilution alone can no longer be relied upon to prevent the undesirable effects of pollution. In many cases, more advanced treatment of wastewater is essential to prevent undue pollution. This is much more so, when the disposed sewage is likely to contain industrial wastewaters.

Hence, it is absolutely necessary to study the characteristics and behaviour of sewage, to ensure its safe disposal. This study will help us in determining the degree and type of treatment required to be given to a given sewage, and thus avoid the pollution of the source of its disposal.

Before we discuss the various physical, chemical and biological (bateriological) characteristics of sewage, let us first discuss as to how the sewage gets decayed, and what happens to it with the passage of time.

Decay or Decomposition of Sewage

Most of the organic matter present in sewage is unstable and decomposes readily through chemical as well as biological actions. Till the advent of biological methods, chemical treatment of sewage was, therefore, being adopted to remove the organic matter from the sewage. However, after the advent of biological degradation methods, this type of treatment has become our foremost choice.

These bacteria will then utilize the free oxygen as electron acceptor, thereby oxidizing the organic matter to stable and unobjectionable end products. The stable end products like nitrates, carbon dioxide, sulphates are formed, respectively for the three forms of matter, i.e. nitrogenous carbonaceous and sulphurous matter. Water, heat, and additional bacteria will also be produced in this biological oxidation, which can be represented by the following equations

It may also be noted that during the decomposition of nitrogenous organic matter, the ammonia formed in the initial stages, may linger on till the end, depending upon the available oxygen, retention time, temperature, biological activity, etc., because the facultative bacteria are incapable to break ammonia to nitrates.

The intermediate products formed in the aerobic oxidation of the three types of organic matter are shown in their respective cycles, in Fig. l (a), (b) and (c). These cycles are known as nitrogen cycle, carbon cycle, and sulphur cycle, for nitrogenous, carbonaceous, and sulphurous organic matter, respectively.

Anaerobic Decomposition. If free dissolved oxygen is not available to the sewage, then anaerobic decomposition, called putrefaction, will occur. Anaerobic bacteria as well as facultative bacteria operating anaerobically, will then flourish and convert the complex organic matter into simpler organic compounds of nitrogen, carbon, and sulphur. These anaerobic bacteria, infact, survive by extracting and consuming the bounded molecular oxygen present in compounds like nitrates (NO3) and sulphates (SO4). Gases like ammonia, nitrogen, hydrogen sulphide, methane, etc. are also evolved in this decomposition, producing obnoxious odours.

Steps in the Nitrogen cycle. Nitrogenous organic matter get oxidized to ammonia, then to nitrites, and finally to nitrates, which when consumed by plants, through photosynthesis, form plant proteins (plant life). The plant proteins, when consumed by animals, form animal proteins. The wastes produced by animals and their dead bodies, will again form nitrogenous organic matter, thus completing the nitrogen cycle.

There may be some short circuits of the cycle, as shown by dotted lines say for example, the dead plants may also, on death, lead to formation of organic matter directly, without changing into animal protein. Similarly, nitrates on denitrification may get converted into free nitrogen (and sometimes to ammonia), which may be converted into plant proteins, as it may be used by certain bacteria residing in the plant roots. This is called nitrogen fixation.

Steps in the Sulphur Cycle. This cycle is similar to nitrogen cycle. The sulphurous organic matter, on oxidation, produces H2S gas, which on further oxidation, changes to sulphur, and then finally to sulphates (SO4~). Sulphates, when consumed by plants through photosythesis, change into plant proteins which when eaten by animals, change into animal proteins. The wastes produced by animals and their dead bodies will again form sulphurous organic matter, thus completing the sulphur cycle.

 

There may be short circuits in the cycle, as shown by the dotted lines organic sulphurous matter may be directly formed by the death of the plants, without the formation of animal proteins.

Similarly, sulphates in the absence of oxygen will be converted into H2S, by the process of reduction.

Steps in the Carbon Cycle. The carbonaceous organic matter, on oxidation, releases carbon dioxide, which is its final end product. This CO2, when used by plants through photosynthesis, gets converted into plant carbohydrates, fats and proteins (sugars) which when eaten by animals, change into animal fats and proteins. The wastes produced by animals and their dead bodies will again form carbonaceous organic matter, thus completing the carbon cycle.

There may be short circuits in the cycle, as shown by the dotted lines. Organic carbonaceous matter may be directly formed by the death of plants.

Similarly, the plant life gives off CO2 at night, and the animal life gives off CO2 during respiration. Both these respiration processes are shown by the dotted lines in the above figure. various stages, at which these gases are evolved are shown in Fig. 2. which represents nitrogen, carbon and sulphur cycles together, for the above anaerobic decomposition. The final equations, representing this decomposition, are given below

The organic acids including alcohols produced in Eq. 4, are further converted into methane gas (CH4), carbon dioxide gas (CO2), etc., if methane forming bacteria are also especially present in the sewage. This conversion is represented by the equation.

An understanding of these cycles will help us in determining the stage of decomposition of sewage by testing for the products of decay. For example, a well oxidised sewage will contain nitrates and sulphates, but very little ammonia and hydrogen sulphide. On the other hand, lesser oxidised sewage will contain nitrites and sulphur instead of nitrates and sulphates.

Basis of Biological Treatment. Numerous bacteria are found in waste waters of a community. So much so, that about 5 to 50 billion bacteria are generally present per litre of sewage. Out of these bacteria, only a small number are harmful to man, and are called pathogens. But a large number of bacteria present in sewage, called non pathogens are, however, important aids, in the process of decomposition, and are thus useful to us in sewage treatment. The fundamental basis of our treatment given to sewage, therefore, is to provide an environment favourable to the action of the aerobic and anerobic bacteria, that stabilise the organic matter present in sewage either through aerobic or anaerobic decomposition.

Treatment units which work on oxidation alone (i.e. aerobic decomposition) are aeration tanks, contact beds, intermittent sand filters, trickling filters, and oxidation ponds.

Treatment units which work on putrefaction alone (i.e., anaerobic decomposition) are septic tanks, Imhoff tanks, sludge digestion tanks, and anaerobic lagoons.

CHARACTERISTICS OF EFFLUENTS

The quality of effluents can be checked and analyzed by studying and testing its physical, chemical and bacteriological (biological) characteristics, as explained below

Physical Characteristics of Sewage and Their Testing

Physical examination of sewage is carried out in order to determine its physical characteristics. This includes tests for determining (i) turbidity (ii) colour (iii) odour and (iv) temperature. These tests are summarized below

Turbidity. Sewage is normally turbid, resembling dirty dish water or wastewater from baths having other floating matter like fecel matter, pieces of paper, cigarette ends, match sticks, greases, vegetable debris, fruit skins, soaps, etc. The turbidity increases as sewage becomes stronger. The degree of turbidity can be measured and tested by turbidity rods or by turbidimeters, as is done for testing raw water supplied.

Colour. The colour of sewage can normally be detected by the naked eye, and it indicates the freshness of sewage.

If its colour is yellowish, grey, or light brown, it indicates fresh sewage. However, if the colour is black or dark brown, it indicates stale and septic sewage. Other colours may also be formed due to the presence of some specific industrial wastes.

Odour. Fresh sewage is practically odourless. But, however, in 3 to 4 hours, it becomes stale with all oxygen present in sewage being practically exhausted. It then starts omitting offensive odours, especially that of hydrogen sulphide gas, which is formed due to decomposition of sewage.

Temperature. The temperature has an effect on the biological activity of bacteria present in sewage, and it also affects the solubility of gases in sewage. In addition, temperature also affects the viscosity of sewage, which, in turn, affects the sedimentation process in its treatment.

The normal temperature of sewage is generally slightly higher than the temperature of water, because of additional heat added during the utilization of water. The average temperature of sewage in India is 20°C, which is near about the ideal temperature for the biological activities. However, when the temperature is more, the dissolved oxygen content (D.O.) of sewage gets reduced.

Chemical Characteristics of Sewage and Their Testing

Tests conducted for determining the chemical characteristics of sewage help in indicating the stage of sewage decomposition, its strength, and extent and type of treatment required for making it safe to the point of disposal. Chemical analysis is, therefore, carried out on sewage in order to determine its chemical characteristics. It includes tests for determining

Total solids, suspended solids, and settleable solids

pH value

Chloride content

Nitrogen content

Presence of fats, greases, and  oils.

Sulphides, sulphates and H2S gas.

Dissolved oxygen

Chemical oxygen demand (C.O.D.)

Bio chemical oxygen demand (BOD).

These tests are briefly discussed below

Total Solids, Suspended Solids and Settleable Solids

Sewage normally contains very small amount of solids in relation to the huge quantity of water (99.9%). It only contains about 0.05 to 0.1 per cent (i.e. 500 to 1000 mg/1) of total solids. Solids present in sewage may be in any of the four forms suspended solids, dissolved solids, colloidal solids, and settleable solids.

Suspended solids are those solids which remain floating in sewage. Dissolved solids are those which remain dissolved in sewage just as salt in water. Colloidal solids are finely divided solids remaining either in solution or in suspension. Settleable solids are that portion of solid matter which settles out, if sewage is allowed to remain undisturbed for a period of 2 hours. The proportion of these different types of solids is generally found to be as given below

It has been estimated that about 1000 kg of sewage contains about 0.45 kg of total solids, out of which 0.225 kg is in solution, 0.112 kg is in suspension, and 0.112 kg is settleable.

 

DISPOSING OF EFFLUENTS

 

The various treatments that may be given to raw sewage before disposing it of, it shall be worthwhile to first discuss the various methods and sources of disposal of sewage. The study of the sources of disposal is important, because the amount of treatment required to be given to sewage depends very much upon the source of disposal, its quality and capacity to tolerate the impurities present in the sewage effluents, without itself getting potentially polluted or becoming less useful.

There are two general methods of disposing of the sewage effluents

      Dilution i.e. disposal in water and

      Effluent Irrigation or Broad Irrigation or Sewage Farming, i.e. disposal on land.

Disposal by dilution is more common of these two methods, which are described below

DISPOSAL BY DILUTION

Disposal by dilution is the process whereby the treated sewage or the effluent from the sewage treatment plant is discharged into a river stream, or a large body of water, such as a lake or sea. The discharged sewage, in due course of time, is purified by what is known as self purification process of natural waters. The degree and amount of treatment given to raw sewage before disposing it of into the river stream in question, will definitely depend not only upon the quality of raw sewage but also upon the self purification capacity of the river stream and the intended use of its water.

Standards of Dilution for Discharge of Waste waters into Rivers

The ratio of the quantity of the diluting water to that of the sewage is known as the dilution factor and depending upon this factor, the Royal Commission Report on Sewage Disposal has laid down the following standards and degrees of treatment required to be given to a particular sewage.

The above standards have been operative in England since 1912, and had also been followed in India without much variance.

However, with the increasing pollution of surface streams due to indiscriminate discharge of domestic and industrial, waste waters  without bothering to look into the available dilution ratios, it has become imperative to limit the concentrations of various pollutants being discharged in to the surface water sources along with the sewage and industrial effluents. The tolerance limits for such constituent pollutants have therefore been prescribed by various countries, including India. These limits are based upon the desirability of giving full fledged treatment to sewage and industrial liquid wastes, up to minimum level of secondary treatment.

The Bureau of Indian Standards (BIS), previously known as Indian Standard Institution (ISI), has therefore laid down its guiding standards for sewage effluents, vide IS 4764 1973, and for industrial effluents vide IS 2490 1974, as shown in table 2. These tolerance limits are supposed to be the national guidelines for guiding the various State Pollution Control Boards for prescribing their legally enforceable standards, depending upon the water quality and dilution available in their respective surface water sources, and the type of effluents produced by the different industries.

When the industrial wastewaters are disposed of in to public sewers, then also, their quality has to be checked, by following the standards prescribed by IS 3306 1974.

Dilution in Rivers and Self Purification of Natural Streams

When sewage is discharged into a natural body of water, the receiving water gets polluted due to waste products, present in sewage effluents. But the conditions do not remain so for ever, because the natural fores of purification, such as dilution, sedimentation, oxidation reduction in sun light, etc., go on acting upon the pollution elements, and bring back the water into its original condition. This automatic purification of polluted water, in due course, is called the self purification phenomenon. However, if the self purification is not achieved successfully either due to too much of pollution discharged into it or due to other causes, the river water itself will get polluted, which, in turn, may also pollute the sea where the river outfalls.

The various factors on which these natural forces of purification depend are (a) temperature (b) turbulence (c) hydrography such as the velocity and surface expanse of the river stream (d) available dissolved oxygen, and the amount and type of organic matter present, (e) rate of reaeration, etc.

Besides affecting the dilution and sedimentation rates, the temperature also affects the rate of biological and chemical activities, which are enhanced at higher temperatures and depressed at lower temperatures. The dissolved oxygen content of water, which is very essential for maintaining aquatic life and aerobic conditions (so as to avoid the anaerobic decomposition and subsequent nuisance) is also influenced by temperature. At higher temperatures, the capacity to maintain the D.O. concentration is low while the rate of biological and chemical activities is high, causing thereby rapid depletion of D.O. This is likely to lead to anaerobic conditions, when the pollution due to putrescible organic matter is heavy.

The turbulence in the body of water helps in breaking the surface of the stream or lake, and helps in rapid re aeration from the atmosphere. Thus, it helps in maintaining aerobic conditions in the river stream, and in keeping it clear. Too much of turbulence, however, is not desirable, because it scours the bottom sediment, increases the turbidity, and retards algae growth, which is useful in reaeration process. Wind and undercurrents in lakes and oceans cause turbulences which affect their self purification.

The Hydrography affects the velocity and surface expanse of the river stream. High velocities cause turbulence and rapid reaeration, while large surface expanse (for the same cubic contents) will also have the same effects.

The larger the amount of dissolved oxygen presents in water, the better and earlier the self purification will occur.

The amount and the type of organic matter and biological growth present in water will also affect the rate of self purification. Algae which absorbs carbon dioxide and gives out oxygen, is thus, very helpful in the self purification process.

The rate of reaeration i.e. the rate at which the D.O. deficiency is replenished, will considerably govern the self purification process. The greater is this rate, the quicker will be the self purification, and there will be no chances of development of anaerobic conditions.

Zones of Pollution in a River Stream. A polluted stream undergoing self purification can be divided into the following four zones

Zone of degradation

Zone of active decomposition

Zone of recovery and

Zone of cleaner water

These zones are discussed below

Zone of degradation or Zone of pollution. This zone is found for a certain length just below the point where sewage is discharged into the river stream. This zone is characterized by water becoming dark and turbid with formation of sludge deposits at the bottom. D.O. is reduced to about 40% of the saturation value. There is an increase in carbon dioxide content reoxygenation (i.e. re aeration) occurs but is slower than de oxygenation.

      These conditions are unfavourable to the development of aquatic life and as such, algae dies out, but fish life may be present feeding on fresh organic matter. Moreover, certain typical bottom worms such as Limondrilus and Tubifex appear with sewage fungi, such as sphaerotilusnatans.

Zone of active decomposition. This zone is marked by heavy pollution. It is characterized by water becoming greyish and darker than in the previous zone. D.O. concentration falls down to zero and anaerobic conditions may set in with the evolution of gases like methane, carbon dioxide, hydrogen sulphide, etc., bubbling to the surface, with masses of sludge forming an ugly scum layer at the surface. As the organic decomposition slackens due to stabilization of organic matter, the re aeration sets in and D.O. again rises to the original level (i.e. about 40%).

In this zone, bacteria flora will flourish. At the upper end, anaerobic bacteria will replace aerobic bacteria, while at its lower end, the position will be reversed. Protozoa and fungi will first disappear and then reappear. Fish life will be absent. Algae and Tubifex will also mostly be absent. Maggots and Psychoda (sewage fly) larvae will, however, be present in all but the most septic sewage.

Zone of recovery. In this zone, the river stream tries to recover from its degraded condition to its former appearance. The water becomes fearer, and so the algae reappears while fungi decreases. B.O.D. falls down and D.O. content rises above 40% of the saturation value Protozoa, Rotifers, Crustaceans and large plants like Sponges, Bryozons, etc. also reappear. Bottom organisms will include Tubifex, Mussels, Snails, etc. The organic material will be mineralised to form nitrates, sulphates, phosphates, carbonates, etc.

Zone of Cleaner Water. In this zone, the river attains its original conditions with D.O. rising up to the saturation value. Water becomes attractive in appearance and Game fish (which requires atleast 4 to 5 mg /1 of D.O.) and usual aquatic life prevails. Same pathogenic organisms may still, however, survive and remain present, which confirms the fact that when once a river water has been polluted, it will not be safe to drink it, unless it is properly treated.

Indices of Self Purification. The stage of self purification process can be determined by the physical, chemical and biological analysis of the water. Colour and turbidity are the physical indices, while D.O., B.O.D. and suspended solids are the chemical indices which can mark the stages of purification. Moreover, the biological growth present in water can also indicate the stage of purification process, as different types of micro and macro organisms will exist in polluted water under different conditions, as discussed in the previous sub article.

The different zones of pollution (i.e. various stages in the self purification process) and the physical, chemical and biological indices, characteristics of each zone, are shown in Fig. 1.

The Oxygen Deficit of a Polluted River Stream. The oxygen deficit D at any time in a polluted river stream is the difference between the actual D.O. content of water at that time and the saturation D.O. content at the water temperature i.e.

            In order to maintain clean conditions in a river stream, the oxygen deficit must be nil, and this can be found out by knowing the rates of de oxygenation and re oxygenation.

De oxygenation Curve. In a polluted stream, the D.O. content goes on reducing due to decomposition of volatile organic matter. The rate of de oxygenation depends upon the amount of the organic matter remaining to be oxidised at the given time (i.e. Lt as well as on the temperature of reaction (i.e. T). Hence, at a given temperature, the curve showing depletion of D.O. with time, i.e. deoxygenation curve (Refer curve 1 of Fig. 2) is similar to the first stage B.O.D. curve. It can also be expressed mathmatically.

Re oxygenation Curve. In order to counter balance the consumption of D.O. due to de oxygenation, atmosphere, supplies oxygen to the water, and the process is called re oxygenation. The rate at which the oxygen is supplied by the atmosphere to the polluted water depends upon

The depth of the receiving water (rate is more in a shallow depth)

The condition of the body of water (rate is more in a running stream than in a quiescent pond)

The saturation deficit or the oxygen deficit (i.e. the deficit of D.O. below the saturation value) and

The temperature of water.

Depending upon these factors, the rate of re oxygenation can also be expressed mathematically and plotted in the form of a curve called re oxygenation curve (Refer curve II Fig. 2).

Oxygen Deficit Curve. In a running polluted stream exposed to the atmosphere, the de oxygenation as well as the re oxygenation go hand in hand. If de oxygenation is more rapid than the re oxygenation, an oxygen deficit results.

Note. If the D.O. content becomes zero, aerobic conditions will no longer be maintained and putrefaction will set in.

The amount of resultant oxygen deficit can be obtained by algebraically adding the de oxygenation and re oxygenation curves (see curve III Fig. 2). The resultant curve so obtained is called the oxygen sag curve or the oxygen deficit curve. From this curve, the oxygen deficit and oxygen balance (i.e., 100 D) per cent in a stream after a certain lapse of time, can be found out.

It can also be seen that when the de oxygenation rate exceeds the re oxygenation rate, the oxygen sag curve shows increasing deficit of oxygen but when both the rates become equal, the critical point is reached, and then finally when the rate of de oxygenation falls below that of re oxygenation, the oxygen deficit goes on decreasing till becoming zero.

This is the important first stage equation in which L is the B.O.D. of the mixture of sewage and stream, and F (KD and KR also) corresponds to the temperature of the mixture of sewage and stream at the outfall.

The above equations are of practical value in predicting the oxygen content at any point along a stream, and thus help us in estimating the degree of waste treatment required, or of the amount of dilution necessary, in order to maintain a certain D.O. in the stream.

Disposal of Wastewaters in Lakes and Management of lake Waters

Lake Pollutants. Disposal of wastewaters in confined lakes or reservoirs is much more harmful than its disposal in flowing streams and rivers. Water quality management in lakes in entirely different from that in rivers.  It is infact the phosphorous (a nutrient largely contained in industrial as well as domestic wastewaters), which seriously affects the water quality of lakes and is hence considered as the prime lake pollutant. Oxygen demanding wastes may be the other important lake pollutants. The toxic chemicals from industrial wastewaters may also sometimes very adversely effect some special classes of the lakes. However, phosphorous (a nutrient) constitutes the most important lake pollutant, and needs special study in water quality management of lakes.

A study of the lake systems is essential to understand the role of phosphorous in lake pollution. The study of lakes is called limnology.

Stratification in Lakes. The water of a lake gets stratified during summers and winters, as discussed below

During summer season, the surface water of a lake gets heated up by sunlight and warm air. This worm water being lighter, remains in upper layers near the surface, until mixed downward by turbulence from winds, waves, boats and other forces. Since such turbulence extends only to a limited depth from below the water surface, the top layers of water in the lake become well mixed and aerobic. This warmer, well mixed and aerobic depth of water is called epilimnion zone. The lower depth, which remains cooler, poorly mixed and anaerobic, is called the hypolimnion zone. There may also exist an intermediate zone or a dividing line, called thermocline, as shown in Fig. 3 (a).

The change from epilimnion to hypolimnion can be experienced while swimming in a lake. When you swim in top layers horizontally, you will feel the water warmer and if you dive deeper, you will find the water cooler. The change line will represent monocline. The depth of epilimnion zone depends upon the size of the lake for the same temperature changes. It may be as little as I m in small lakes and may be as large as 20 m or more in large lakes. This depth also depends upon the storm activity in the spring when stratification is developing. A major storm at the right time will mix the warmer water to a substantial depth and thus create a deeper epilimnion zone than its normal depth. Once formed, lake stratification is very stable, and can only be broken by exceedingly violent storms. As a matter of fact, as summer progresses, this stability increases the epilimnion continues to warm, while the hypolimnion remains at a fairly constant temperature.

With the onset of winter season, the epilimnion cools, until it is more dense than the hypolimnion. The surface water then sinks, causing  overturning. The water of the hypolimnion rises to the surface, where it cools and again sinks. The lake, thus becomes completely mixed, making it quite aerobic. In regions of freezing temperatures, when the temperature drops below 4°C, the above process of overturning (or turn over) stops, because water is most dense at this temperature. Further cooling or freezing of the water surface results in winter stratification, as shown in Fig. 3 (b).

With the passing of winters and commencement of spring season, the surface water again warms up and overturns, and lake becomes completely mixed. The lakes in regions of temperate climate will, therefore, have at least one, if not two, cycles of stratification and turn over every year.

Biological Zones in Lakes. Lakes have been found to exhibit distinct zones of biological activity, largely determined by the availability of light and oxygen. The most important biological zones are

      Euphotic zone

      Littoral zone and

      Benthic zone.

These zones are shown in Fig. 4, and briefly discussed below

Euphotic zone. The upper layer of lake water through which sunlight can penetrate, is called the euphotic zone. All plant growth occurs in this zone. In deep water, algae grow as the most important plants, while rooted plants grow in shallow water near the shore.

The depth of the euphotic zone is reduced by the turbidity, which blocks sunlight penetration. In most lakes, the turbidity is due to algal growth although colour and suspended clays may substantially reduce sunlight penetration in some lakes. It is important to note that the bottom of euphotic zone only rarely coincides with the thermocline.

The depth of the euphotic zone can be approximated and measured by a simple device, called the secchi disk, shown in Fig. 8.5. This disk is lowered into the lake water until the observer can no longer distinguish between the boundaries between its white and black quadrants. This depth, called the secchi disk depth, does not correspond exactly to the depth of euphotic zone, but provides a good approximation of its extent. This secchi disk depth also provides a measure of the aesthetic quality of water. The greater is the secchi disk depth, the clearer is the water.                                           

Littoral zone. The shallow water near the shore, in which rooted plants grow, is called the littoral zone. The extent of the littoral zone depends on the slope of the lake bottom, and the depth of the euphotic zone. The littoral zone cannot extend deeper than the euphotic zone, as shown in Fig. 8.3.

Benthic zone. The bottom sediments in a lake comprises what is called the benthic zone. As the organisms living in the overlying water die, they settle down to the bottom, where they are decomposed by the organisms living in the benthic zone. Bacteria are always present in this zone. The presence of higher life forms, such as worms, insects and crustaceans however, depends upon the availability of oxygen at the lake bottom.

Productivity of a Lake. The productivity of a lake is defined as a measure of its ability to support a food chain. Since the algae form the base of this food chain, which is required by the other forms of living organisms to thrive, its presence measures the lake productivity. Although, more productive lakes will have a higher fish population, yet since such a lake will have to support heavier algal growth, its water quality will be reduced, because of the undesirable changes that occur as algal growth increases. Moreover, due to reduced water quality, the most desirable fish which flourish in better quality waters, will be lost.

A lakes productivity level may, therefore, be determined by measuring the amount of algal growth that can be supported by the available nutrients. This productivity level of a lake is thus, reflective of the water quality of the lake. As the productivity of a lake increases, its water quality reduces. Because of the important role productivity plays in determining water quality, it forms a basis for classifying lakes.

Depending upon the increasing level of its productivity, the lakes may be classified as

 Oligotrophic lakes

Mesotrophic lakes

Eutrophic lakes and

Senescent lakes.

There four types are discussed below

Oligotrophic lakes. Oligotrophic lakes have a low level of productivity due to a severely limited supply of nutrients to support algal growth. The water of such a lake is therefore clear enough as to make its bottom visible upto considerable depth. In such a case, the euphotic zone often extends into the hypolimnion, which is aerobic. Oligotrophic lakes, therefore, support cold water game fish. An important example of such a lake is offered by the Tahoe Lake on California Nevada border in USA.

Mesotrophic lakes. The lakes having medium productivity levels, with medium growth of algae and turbidity, are usually classified as mesotrophic lakes. In such a lake, although substantial depletion of oxygen may occur in the hypolimnion, yet it remains aerobic.

Eutrophic lakes. Eutrophic lakes do have a high level of productivity, because of an abundant supply of algal nutrients. The flourishing growth of algae makes the lake water to be highly turbid, so that the euphotic zone may extend only partially into the epilimnion.

As the algae die, they settle to the lake bottom, where they are decomposed by benthic organisms. In a eutrophic lake, this decomposition is sufficient to deplete the hypolimnion of oxygen during summer stratification.

Because the hypolimnion is anaerobic during summer, such a lake will only support warm water fish. As a matter of fact, all types of cold water fish are driven out of the lake, before the hypolimnion becomes anaerobic, because they generally require dissolved oxygen levels of at least 5 mg/L. Highly eutrophic lakes may also have large mats of floating algae that typically impart unpleasant tastes and odours to the water.

Senescent lakes. These are very old shallow lakes, having thick organic sediment deposits at their bottoms. Rooted water plants abundantly grow in such shallow ponds, which ultimately become marshes.

Eutrophication of Lakes. Eutrophication is a natural process under which lakes get infested with algae and silt up gradually to become shallower and more productive through the entry and cycling of nutrients like carbon, nitrogen and phosphorous. The initially clear water oligotrophic lakes, therefore, gradually turn into mesotrophic, eutrophic, and senescent stages, due to continuous entry of silt and nutrients.

This natural process of eutrophication infact can always get its carbon and nitrogen requirements from the atmospheric gases like CO2 and (NO2) while the requirement of phosphorous is met by its presence in natural run off due to disintegration of rocks, which produce phosphorous. The increased phosphorous in lake water, entering either through the agricultural use of its drainage area or through the entry of domestic and industrial waste waters, will cause accelerated eutrophication of lakes, and is called cultural eutrophication. The natural process of eutriphication thus, gets intensified by the entry of wastewater discharges into the lakes, causing permanent damage to its water quality and siltation. Eutrophic lakes are, however, not necessarily polluted, but pollution contributes to eutrophication.

The water quality management for a lake, therefore, aims at reducing its eutrophication to atleast at  its  natural  level, by controlling and reducing the input of phosphorous in the lake water. Once the input of phosphorous, (which directly controls the production of chlorophyll and hence the algal development) is reduced, the phosphorous concentration will gradually fall down, as the existing phosphorous will get buried into the sediment or is flushed out of the lake along with the excess river flow feeding the lake.

Strategies suggested for reducing eutrophication by removal of phosphorous by its precipitation by addition of lime to the lake water or by dredging out the phosphorous rich sediment from the lake bottom, will not succeed until the entry of phosphorous is not curtailed. It, therefore, becomes imperative to stop the entry of sewage and industrial wastewaters, which largely contain phosphorous, into the lakes. Even the treated sewage will not be low in phosphorous, since phosphorous can be removed only by costly advanced methods of waste treatment. The lake waters should, therefore, not be used even for discharge of treated sewage. The phosphorous content of the domestic sewage can, however, be reduced by banning the use of phosphorous rich polyphosphates in detergents, which presently contribute heavy input of phosphorous in domestic sewage, as large as twice that contributed by human excreta. Several advanced countries have, therefore, banned the use of phosphates in detergents and soaps. India has yet to follow suit.

The continuous entry of seeping septic tank effluents from cottages and houses built adjoining the lakes, through the sub soil towards the lake, will also cause phosphorous pollution in the lake, after the soil gets too saturated to absorb any further phosphorous, finally passing it on to the lake. The time it takes for phosphorous to break through to the lake depends on the type of soil, the distance to the lake, the amount of wastewater generated, and the concentration of phosphorous in the seeping wastewater. Entry of phosphorous through such sources should also be controlled to reduce cultural eutrophication of lakes, by collecting the septic tank effluents in sewers, to be carried up to the treatment plants, before disposing it off, safely.

The use of fertilizers in fields in the drainage area should also be controlled to reduce the entry of phosphorous through the surface run off flowing over such fields and finally entering the lakes. Treatment of such catchments to reduce soil erosion will also help in reducing phosphorous entry into the surface runoff.

TREATMENT OF EFFLUENTS

Classification of Treatment Processes

Effluents before being disposed of either in river streams or on land, has generally to be treated, so as to make it safe. The degree of treatment required, however, depends upon the characteristics of the source of disposal, as discussed in the previous chapter.

Effluents can be treated in different ways. Treatment processes are often classified as

Preliminary treatment

Primary treatment

Secondary (or Biological) treatment and

Complete final treatment, as discussed below

Preliminary Treatment. Preliminary treatment consists solely in separating the floating materials (like dead animals, tree branches, papers, pieces of rags, wood, etc.), and also the heavy settleable inorganic solids. It also helps in removing the oils and greases, etc. from the sewage. This treatment reduces the BOD of the wastewater, by about 15 to 30%. The processes used are Screening for removing floating papers, rags, clothes, etc. Grit chambers or Detritus tanks for removing grit and sand and Skimming tanks for removing oils and greases.

Primary Treatment. Primary treatment consists in removing large suspended organic solids. This is usually accomplished by sedimentation in Settling basins.

The liquid effluent from primary treatment, often contains a large amount of suspended organic material, and has a high BOD (about 60% of original).

Sometimes, the preliminary as well as primary treatments are classified together, under primary treatment.

The organic solids, which are separated out in the sedimentation tanks (in primary treatment), are often stabilised by anaerobic decomposition in a digestion tank or are incinerated. The residue is used for land fills or soil conditioners.

Secondary Treatment. Secondary treatment involves further treatment of the effluent, coming from the primary sedimentation tank. This is generally accomplished through biological decomposition of organic matter, which can be carried out either under aerobic or anaerobic conditions. In these biological units, bacteria will decompose the fine organic matter, to produce clearer effluent.

The treatment reactors, in which the organic matter is decomposed (oxidised) by aerobic bacteria are known as aerobic biological units and may consist of (i) Filters (intermittent sand filters, as well as trickling filters) (ii) Aeration tanks, with the feed of recycled activated sludge (i.e. the sludge, which is settled in secondary sedimentation tank, receiving effluents from the aeration tank) (iii) Oxidation ponds and Aerated lagoons. Since all these aerobic units, generally make use of primary settled sewage, they are easily classified as secondary units.

The treatment reactors, in which the organic matter is destroyed and stabilized by anaerobic bacteria, are known as anaerobic biological units and may consist of Anaerobic lagoons, Septic tanks, Imhoff tanks, etc. Out of these units, only anaerobic lagoons make use of primary settled sewage, and hence, only they can be classified under secondary biological units. Septic tanks and Imhoff tanks, using raw sewage, are, therefore, not classified as secondary units.

The effluent from the secondary biological treatment will usually contain a little BOD (5 to 10% of the original), and may even contain several milligrams per litre of DO.

The organic solids/sludge separated out in the primary as well as in the secondary settling tanks, will be disposed of by stabilizing them under anaerobic process in a Sludge digestion tank.

The Final or Advanced Treatment. This treatment is sometimes called tertiary treatment, and consists in removing the organic load left after the secondary treatment, and particularly to kill the pathogenic bacteria. This treatment, which is normally carried out by chlorination, is generally not carried out for disposal of sewage in water, but is carried out, while using the river stream for collecting water for re use or for water supplies. It may, however, Sometimes be adopted, when the outfall of sewage is very near to the water intake of some nearby town.

The sewage treatment is, therefore, usually confined up to secon­dary treatment only. Well in fact, various physical, chemical and biological processes are available for treatment, depending upon the particular requirements. The choice of the treatment methods depends on several factors, including the disposal facilities available. Actually, the distinction between primary, secondary and tertiary treatment is rather arbitrary, since many modern treatment methods incorporate physical, chemical and biological processes in the same operation.

The various treatment units, which may be used for treating sewage, and the extent of BOD, solids and bacteria removed by them, have been summarized in Table 1.

The individual operations of these units may be combined in different ways, depending upon the topography and other local needs, so as to create different types of treatment plants.

Fig. 1 shows diagrammatic sketches of some standard types of sewage treatment plants, which give full treatment, and may be adopted under different conditions. The first plant [Fig. 1 (a)] consists of Imholf tanks and low rate trickling filters, and may be adopted for very small towns, although the use of Imhoff tanks has become old and obsolete these days. The second plant [Fig. 1 (b)] consists of sedimentation tanks and high rate trickling filters, and is suitable for cities of small and medium sizes. The third type of treatment plant [Fig. 1 (c)] consists of a sedimentation tank and an activated sludge treatment plant, and is suitable for larger cities, where continuous attendance and supervision is possible.

These three different types of treatment plants contain different combinations of treatment units. These combinations can also be changed, and some other combination made, depending upon the local needs. The topography and geology will influence the choice of the units to be adopted. For example, the trickling filter plant consumes high head and requires steep ground slope to avoid pumping Imholf tanks are vary deep and can not be constructed economically under adverse underground conditions. Similarly, the size of the plant will also influence the type of units to be housed in it. For example, smaller sized plants cannot house such units which require continuous skilled attendance and supervision, and as such, they should house such units, which do not employ complicated mechanical equipment.

All these treatment units will now be discussed in details.

SCREENING

Screening is the very first operation carried out at a sewage treatment plant, and consists of passing the sewage through different types of screens, so as to trap and remove the floating matter, such as pieces of cloth, paper, wood, cork, hair, fibre, kitchen refuse, fecal solids, etc. present in sewage. These floating materials, if not removed, will choke the pipes, or adversely affect the working of the sewage pumps. Thus, the main idea of providing screens is to protect the pumps and other equipments from the possible damages due to the floating matter of the sewage.

Screens should preferably be placed before the grit chambers (described in the next article). However, if the quality of grit is not of much importance, as in the case of land fillings, etc., screens may even be placed after the grit chambers. They may sometimes be accommodated in the body of the grit chambers themselves.

Types of Screens, Their Designs and Cleaning

Depending upon the size of the openings, screens may be classified as coarse Screens, medium screens, and fine screens.

      Coarse screens are also known as Racks, and the spacing between the Bars (i.e. opening size) is about 50 mm or more. These screens do help in removing large floating objects from sewage. They will collect about 6 litres of solids per million litre of sewage. The material separated by coarse screens, usually consists of rags, wood, paper, etc., which will not putrefy, and may be disposed of by incineration, burial, or dumping.

      In medium screens, the spacing between bars is about 6 to 40 mm. These screens will ordinarily collect 30 to 90 litres of material per million litre of sewage. The screenings usually contain some quantity of organic material, which may putrefy and become offensive, and must, therefore, be disposed of by incineration, or burial (not by dumping).

Rectangular shaped coarse and medium screens are now a days widely used at sewage treatment plants. They are made of steel bars, fixed parallel to one another at desired spacing on a rectangular steel frame, and are called bar screens. The screens are set in a masonry or R.C.C. chamber, called the screen chamber.

Now a days, these screens are generally kept inclined at about 30 to 60° to the direction of flow, so as to increase the opening area, and to reduce the flow velocity and thus making the screening more effective. While designing the screens, clear openings should have sufficient total area, so that the velocity through them is not more than 0.8 to 1 m/sec. This limit, placed on velocity, limits the head loss through the screens, and, thus, reduces the opportunity for screenings to be pushed through the screens.

The material collected on bar screens can be removed either manually or mechanically. Manual cleaning is practised at small plants with hand operated rakes. The inclined screens help in their cleaning by the upward stroke of the rake. Large plants, however, use mechanically operated rakes, which move over the screens, either continuously or intermittently.

The cleaning of screens by rakes will be hindered by cross bars, if at all provided. They are, therefore, generally avoided.

Screens are sometimes classified as fixed or movable, depending upon whether the screens are stationary or capable of motion.

Fixed screens are permanently set in position. A most commonly used bar type screen is shown in Fig. 2.

Movable screens are stationary during their operating periods. But they can be lifted up bodily and removed from their positions for the purpose of cleaning. A common movable bar medium screen is a 3 sided cage with a bottom of perforated plates. It is mainly used in deep pits ahead of pumps.

      Fine Screens have perforations of 1.5 mm to 3 mm in size. The installation of these screens proves very effective, and they remove as much as 20% of the suspended solids from sewage. These screens, however, get clogged very often, and need frequent cleaning. They are therefore, used only for treating the industrial wastewaters, or for treating those municipal wastewater, which are associated with heavy amounts of industrial wastewaters. These screens will considerably reduce the load on further treatment units.

Brass or Bronze plates or wire mesh is generally used for constructing fine screens. The metal used should be resistant to rust and corrosion.

The fine screens may be disc or drum type, and are operated continuously by electric motors. Fig. 3 shows a typical disc type of fine screen, which is cleaned by a cone brush.

Comminutors

Comminutors or Shredders are the patented devices, which break the larger sewage solids to about 6 mm in size, when the sewage is screened through them. Such a device consists of a revolving slotted drum, through which the sewage is screened (Fig. 4). Cutters mounted on the drum, shear the collected screenings against a comb, until they are small enough to pass through 5 mm to 10 mm wide slots of the drum. These are usually arranged in pairs to facilitate repairs and maintenance. Comminutors are of recent origin, and eliminate the problem of disposal of screenings, by reducing the solids to a size which can be processed elsewhere in the plant. They should always be preceded by grit chambers to prevent their excessive wear.

Such devises are used only in developed countries like USA, and generally not adopted in our country.

Disposal of Screenings

The material separated by screens is called the screenings. It contains 85 to 90% of moisture and other floating matter. It may also contain some organic load which may putrefy, causing bad smells and nuisance. To avoid such possibilities, the screenings are disposed of either by burning, or by burial, or by dumping. The dumping is avoided when screenings are from medium and fine screens, and are likely to contain organic load, as pointed out earlier. The screenings may also sometimes be broken up by a comminutor and then passed on to the grit chamber.

Burning of the screenings is done in the incinerators, similar to those used for burning garbage. The process of burning is called Incineration. The screenings are first dried with suns heat by spreading on ground or by compressing through hydraulic or other presses, so as to reduce the moisture content to about 60%. The incineration is carried out at temperatures of about 760 to 815°C. This will avoid bad smells.

The screenings may also be disposed of by burial. The process is technically called Composting. In this process, the screenings are buried in 1 to 1.5 m deep trenches, and then covered with 0.3 to 0.45 m of porous earth. In due course of time, oxidation reduction of screenings will take place, and the contents can be used as manure.

Another method of disposing of the screenings is by dumping them in low lying areas (away from the residential areas) or in large bodies of water, such as sea. Dumping in sea will be suitable only where strong forward currents do exist to take the dumped material away from the shore line. The dumping on land for raising low lying areas is also adopted only when screenings are from the course screens, and not from the medium or fine screens, and as such not containing much organic load.

Digestion of screenings along with the sewage sludge in a sludge digestion tank has also been tried, but not found successful.

GRIT REMOVAL BASINS

Grit Chambers

Grit chambers, also called grit channels, or grit basins, are intended to remove the inorganic particles (specific gravity about 2.65), such as sand, gravel, grit, egg shells, bones, etc. of size 2 mm or larger to prevent damage to the pumps, and to prevent their accumulation in sludge digestors. Girt chambers are, infact nothing but like sedimentation tanks, designed to separate the intended heavier inorganic materials by the process of sedimentation due to gravitational forces, and to pass forward the lighter organic materials. They may be placed either before or after the screens. Many engineers, howevers, prefer to place them before the screens, as to avoid silting of the screen chambers.

A grit chamber is an enlarged channel or a long basin, in which the cross section is increased, so as to reduce the flow velocity of sewage to such an extent that the heavy inorganic materials do settle down by gravity, and the lighter organic materials remain in suspension, and, thus, go out along with the effluent of the grit basin. The important point in the design of the grit basins is that the flow velocity should neither be too low as to cause the settling of lighter organic matter, nor should it be so high as not to cause the settlement of the entire silt and grit present in sewage. The flow velocity should also be enough to scour out the settled organic matter, and reintroduce it into the flow stream. Such a critical scourting velocity is, infact, given by the modified Shields formula, which states that Critical scour velocity.

For grit particles of 0.2 mm (d), the above formula gives critical velocity values of 0.11 to 0.25 m/sec. This fixes the limits for optimum flow velocity for design of grit basins. In practice a flow velocity of about 0.25 to 0.3 m/sec is adopted for the design of grit basins.

In order to prevent large increase in flow velocity at peak hours, due to increased discharge, and thus, to avoid the scouring of the settled grit particles from the bottom, it is preferable to design the grit chambers for D.W.F. (Dry weather flow), and to provide additional units for taking increased discharge at peak hours. If, however, a single unit is to be designed, or there are large variations in discharge, then the grit chamber is designed for generating optimum velocity at peak discharge and a velocity control section, such as a proportional flow weir or a parshall flume (venturiflume), is provided at the lower (effluent) end of the grit channel, which helps in varying the flow area of the section in direct proportion to the flow, and thus, helps to maintain a constant velocity in the channel (within the permissible limits of 5 to 10% over the designed value), even at varying discharges,.

It has also been proved that when a proportioning flow weir is used as a velocity control device, then a rectangular cross section is required for the grit channel but however, when a pars hall flume is used as a velocity control device, then a parabolic cross section is required for the grit channel, inorder to keep the flow velocity constant, as shown in Fig. 5 (a) and (b).

The depth and detention time provided for a grit basin are inter dependent, and are based on the considerations of settling, Velocity of inorganic particles through water. A detention time of about 40 to 60 seconds (1 minute) is generally sufficient for a water depth of about 1 to 1.8 m. After fixing the depth and the detention time we can easily design the tank dimensions, as its length will then be equal to velocity x detention time.

As stated earlier generally two to three separate chambers in parallel (as shown in Fig. 6.) should be provided one to pass the low flow, and the other to pass (along with the first of course) the high flow. This will also help in manual cleaning of the chambers, as one unit can work while the other is shut down for cleaning.

     The grit chambers can be cleaned periodically at about 3 weeks interval, either manually, mechanically or hydraulically Hand cleaning is done only in case of smaller plants (of capacity less than about 4.5 million litres per day), while mechanical or hydraulic cleaning is adopted for larger plants. In manual cleaning, grit is removed by shovels, etc., by hand in mechanical cleaning, grit is removed with the help of machines and in hydraulic cleaning, grit is removed by the force of water jet directed from a central point and removed through the pipes in the side walls or bottom of the chamber.

The removed grit may contain some organic matter, and can be washed prior to its disposal, if necessary, by using certain patented machines, and the wash water returned to the plant influent. Washed grit may still contain about 1 to 5% of putrescible organic matter.

The silt and grit, etc. removed by the grit chambers can be easily disposed of either by burial or burning (incineration) or for raising low lying areas by dumping. It cannot be used for preparation of concrete, as it contains sufficient organic matter

Detritus Tanks

Detritus tanks are nothing but grit chambers designed to flow with a smaller flow velocity (of about 0.09 m/sec) and longer detention periods (about 3 to 4 minutes) so as to separate out not only the larger grit, etc., but also to separate out the very fine sand particles, etc. Due to this, a large amount of organic matter will also settle out along with the inorganic grit, sand, etc. This organic material is then separated from the grit by control of currents in the tank through baffles, or by controlled aeration of the flow through the tank. The rising air bubbles will then separate the lighter organic matter from the descending grit. The grit is removed continuously by means of scraper mechanism. All other details of detritus tanks remain the same as those of a grit chamber.

Design of ParaboIic Grit Chamber provided with a Par shall Flume

Parshall Flume. A parshall flume, also called a venturi flume, is a horizontally constricted vertical throat in an open channel, as shown in Fig. 8. Such a venturi flume, as we know, can be used as a discharge measuring device, and also as a velocity control device. This device is made use of for its latter purpose in a grit channel.

The venturi flume, as a velocity control device, is preferable to the proportional flow weir, etc., as it involves negligible head loss, and can also work under submerged conditions for certain limits. These limits of submergence are 50% in case of 0.15 m throat width, and 70% for wider throat widths upto 1 m. Another advantage of a venturiflume is that one control section can be installed for 2 to 3 grit chambers. Moreover, the venturi flume is a self cleaning device, and there is no problem of clogging.

TANKS FOR REMOVING OILS AND GREASE

Skimming Tanks

Skimming tanks are sometimes employed for removing oils and grease from the sewage, and placed before the sedimentation tanks. They are, therefore, used where sewage contains too much of grease or oils, which include fats, waxes, soaps, fatty acids, etc. These materials may enter into the sewage from the kitchens of restaurants and, houses, from motor garages, oil refineries, soap and candle factories, etc. They are, thus, normally present in large amounts in the industrial wastewaters.

If such greasy and oily matter is not removed from the sewage before it enters further treatment units, it may form unsightly and odourous scums on the surface of the settling tanks, or interfere with the activated sludge treatment process, and inhibit biological growth on the trickling filters

These oil and greasy materials may be removed in a skimming tank, in which air is blown by an aerating device through the bottom. The rising air tends to coagulate and congeal (solidify) the grease, and cause it to rise to the surface (being pushed in separate compartments), from where it is removed.

The typical details of a skimming tank are shown in Fig. 10. It consists of a long trough shaped structure divided into two or three lateral compartments by means of vertical baffle walls (having slots in them) for a short distance below the sewage surface, as shown. The baffle walls help in pushing the rising coagulated greasy material into the side compartments (called stilling compartments). The rise of oils and grease is brought about by blowing compressed air into the sewage from diffusers placed at the bottom of the tank. The collected greasy materials are removed (i.e. skimmed off) either by hand or by some mechanical equipment. It may then be disposed of either by burning or burial.

TREATMENT OF INDUSTRIAL EFFLUENTS

Introduction

Wastewaters obtained from industries are generally much more polluted than the domestic or even commercial Wastewaters. Still, however, several industrialists try to discharge their effluents into our natural river streams, through unauthorized direct discharges. Such a tendency on the part of the industries may pollute the entire river water to a grave extent, thereby making its purification almost an impossible task. Sometimes, the industries discharge their polluted wastewates into municipal sewers, thereby making the task of treating that municipal sewage, a very difficult and a costly exercise.

The industries are, therefore, generally prevented by legal laws, from discharging their untreated effluents. It therefore, becomes necessary for the industries to treat their Wastewaters in their individual treatment plants, before discharging their effluents either on land or lakes or rivers, or in municipal sewers, as the case may be.

Methods of Treating Industrial Wastewaters

Industrial wastewaters, as pointed out above, usually contain several chemical pollutants and toxic substances in too large proportions. The characteristics of the produced wastewater will usually vary from industry to industry, and also vary from process to process even in the same industry. Such industrial wastewaters cannot always be treated easily by the normal methods of treating domestic wastewaters, and certain specially designed methods or sequence of methods may be necessary.

In order to achieve this aim, it is generally always necessary and advantageous to isolate and remove the troubling pollutants from the wastewaters, before subjecting them to usual treatment processes. The sequence of treatment processes adopted should also be such as to help generate useful by products. This will help economize the pollution control measures, and win encourage the industries to develop such treatment plants.

Depending upon the quantum, concentration, toxicity and presence of non biodegradable organics in an industrial wastewater, its treatment may consist of any one or more of the following processes

Equalization

Neutralisation

Physical treatment

Chemical treatment and

Biological treatment.

These processes are briefly discussed below

Equalisation. Equalisation consists of holding the waste water for some pre determined time in a continuously mixed basin, to produce a uniform wastewater. Such an arrangement will, of course be necessary when the wastewater produced by the industry varies in characteristics and quantity over the entire day.

Neutralisation. Neutralisation means neutralising the excessive acidity or alkalinity of the particular wastewater, by adding alkali or acid, respectively, to the wastewater. This may be achieved either in the equalisation tank, where possible, or a separate neutralisation tank may be used.

Physical Treatment. Physical treatment consists of separating the suspended inorganic matter by physical processes, like sedimentation   and floatation.

Sedimentation. Sedimentation as we know, is employed to  separate the heavier settleable solids, and hence sedimentation tank may be provided only when the wastewater contains a high percentage of such heavy inorganic solids.

Floatation. Floatation consists of creation of fine air bubbles in the waste tank, by introduction of air into the tank from the bottom. The rising air bubbles, attach themselves to the fine suspended particles, increasing their buoyancy, and finally lifting them to liquid surface for consequent removal by skimming.

Chemical Treatment. Chemical treatment is often necessary before the biological treatment, though sometimes, it may not be required at all. Sometimes, it may however, serve as the final stage of treatment.

Since chemical treatment is a costly and expensive exercise, care should be taken to see, if it could be avoided altogether, to achieve our required goals.

The chemical treatment is used to recover the dissolved organic matter from the wastewater, and may consists of one or more of the following processes

Reverse osmosis or hyper filtration

Electrodialysis

Chemical oxidation

Chemical coagulation or chemical precipitation

Adsorption

Deionisation

Thermal reduction and

Air stripping.

All these processes are briefly pointed out below

Reverse Osmosis. In reverse osmosis treatment process, the wastewater containing dissolved salts are filtered through semipermeable membranes at a pressure higher than the osmotic pressure, as discussed in article 9.29.6(3) Vol. 1. Such a treatment requires pre treatments like (a) activated carbon adsorption or (b) chemical precipitation followed by some kind of filtration.

Electrodialysis. In electrodialysis treatment process, dissolved salts from wastewater are separated by passing an electriccurrent through the wastewater tank, installed with ion exchange membranes, as discussed in article 9.29.6(2)  Vol. I. This treatment process also requires some pretreatment, as is required in reverse osmosis process.

Chemical Oxidation. In chemical oxidation, chemicals like chlorine and ozone are used to reduce substances like ammonia and cyanid etc. from the wastewater, in addition     to reducing BOD load on biological treatments.

Chemical Coagulation. Chemical coagulation, as we know, is adopted in treating raw water supplies, and helps in sedimentation of unsettleable micro and colloidal impurities, which get absorbed in the gelatinous flocs, formed by the chemical reactions between coagulants, or between the coagulant and the alkalies present in raw water.

Biological Treatment. Biological treatment of industrial wastewaters is necessary, when they contain large quantities of biodegradable substances. Such biological treatment may be used either with or without acclimatisation.

Laboratory tests on wastewater for determining its  ratio, will help in determining the type of treatment required. Say for example, if this ratio is more than 0.6, the wastewaters are biologically treatable without acclimatisation and if the ratio is less than 0.6 and upto 0.3, then acclimatisation is needed for biological treatment and if the ratio is less than 0.3, biological treatment may not be necessary.

Acclimatisation consists of the gradual exposure of the waste water in increasing concentration to the seed or initial microbiological population under a controlled condition.

The criteria for selecting a particular conventional biological treatment process, may differ for different types of industrial was   tewaters. The system parameters for a given type of industrial wastewater may be determined by laboratory experiments.

In the absence of any actual test data, the performance data of a similar type of industrial wastewater may be used for design.

It has also been observed that most of the industrial wastewaters do not contain sufficient nutrients for micro biological growth and hence nutrients like urea (containing nitrogen), superphosphates (containing phosphorous) etc. may have to be added to the reactors. For balanced growth of micro organisms in a biological treatment reactor, the ratios of BOD Nitrogen Phosphorous should be 100 5 1 for aerobic systems, and 100 2.5 0.5 for anaerobic systems.

In certain cases, special types of micro organisms are found to cause better biological oxidation, since commonly available micro­biological population may fail to achieve oxidation.

Acute Toxic wastewaters must be handled with special care and specific treatments. Say for example, toxic wastes, like cyanides, formaldehydes, phenols, etc. may need acclimatised growth of special type of bacteria. Toxic metals, like copper, zinc, chromium etc. may need pre separation chemically, as otherwise, they may interfere with the biological oxidation by tying up the enzymes, essentially required for microbial growth.

Segregation of strong wastewaters from weak wastewaters may also sometimes help in reducing problems in industrial wastewater treatment plants.

In conclusion, it can be stated that the selection of a sequence of particular treatment processes depends on the characteristics of the wastewaters, and also upon the permissible requirements of the effluents.

The treatment processes needed for different industries will, there­fore, generally vary from one industry to another.

The attempts made for treating industrial wastewaters, simply by biological methods, though cheaper, have generally failed in the absence of making efforts for pre recovery of certain pollutants or chemicals from the wastewater by using chemical methods. However, use of simple biological methods have fairly succeeded in industries, like fruit processing, dairies, slaughterhouses, and textiles.

Physical and chemical treatments prior to biological treatment, for separation of troubling pollutants, like chromium, arsenic, cyanide, mercury, several nitrogenous substances, lignin, etc., is very necessary and important for industries like fertilizers, dyes and pigments, pesticides, electroplating, paper and pulp, etc. Suitable pre treatments to the wastewaters of such industries, before subjecting them to the biological treatment is thus, the prime requirement for designing and planning the treatment plant for a particular industry.

Possible large scale re use of the treated water in the industry is another important factor, which must be considered, while deciding the sequence of treatment processes for a particular industry. Such a possible reuse, if can be made possible, will help in large scale economy in the industry.

The pollution characteristics of certain typical Indian industries along with suggested treatments with flowcharts are broadly reflected in Tables 11.1 and 11.2 so as to serve as a first hand guide to the design engineers. For detailed planning and design of the treatment works, however, the design engineer will have to be well conversant with the detailed operations and stages involved in the given industry, study the characteristics of the wastewaters generated at different places in the industry, and to finally work out the most satisfactory and economical sequence of treatment processes for that industry, using his intuition, intelligence, experience, and of course, ingenuity.

INDUSTRIAL WASTE WATER EFFLUENTS

Introduction

Industries use huge quantities of the nations waters and are the major factor in the continuing rise in water pollution. They utilize over    15 trillion gallons of water but, prior to discharge, treat less than 5 trillion gallons. In terms of a single pollution parameter, BOD, the waste generated by industries is equivalent to that generated by a      population of over 360 million people. Even more undesirable than the BOD loads of industrial effluents are the enormous quantities of mineral and chemical wastes from factories which steadily become more complex and varied. They include metals such as iron, chromium, nickel, and copper salts such as compounds of sodium, calcium, and magnesium acids such as sulfuric and hydrochloric petroleum wastes and brines phenols cyanides ammonia toluene blast furnace wastes greases many varieties of suspended and dissolved solids and numerous other waste compounds. These wastes degrade the quality of receiving waters by imparting tastes odors and color and through excess mineralization, salinity, hardness, and corrosion. Some are toxic to plant and animal life.

The variety and complexity of inorganic and organic components contained in industrial effluents present a serious liquid waste water treatment control problem in that the pollution and toxicity effects of       these constituents are of greater significance than those found in domestic   waste waters.

Conventional waste water treatment technology which is often barely adequate for existing waste types offers even less promise of providing the type and degree of treatment which will be required in the near future. Therefore, industrial pollution control technology must be developed to achieve effective and economical control of pollution from such varied industries as those producing metals and metal products, chemicals and allied products, paper and allied products, petroleum and coal products, food and kindred products, textiles and leather goods.

TERMINOLOGY

Several of the more common terms encountered in sewage treatment technology are given below to assist the reader in developing his vocabulary in this field and in understanding what follows.

Activated sludge process removes organic matter from sewage by saturating it with air and biologically active sludge.

Adsorption is an advanced way of treating wastes in which carbon removes organic matter not responsive to clarification or biological treatment.

Aeration tank serves as a chamber for injecting air into water.

Algae are plants which grow in sunlit waters. They are a food for fish and small aquatic animals and, like all plants, put oxygen into the water.

Bacteria are the smallest living organisms which literally eat the organic parts of sewage.

BOD, or biochemical oxygen demand, is the amount of oxygen necessary in the water for bacteria which consume the organic sewage.      It is used as a measure in telling how well a sewage treatment plant is working.

Chlorinator is a device for adding chlorine gas to sewage to kill infectious germs.

Coagulation is the clumping together of solids to make them settle out of the sewage faster. Coagulation of solids is brought about with the use of certain chemicals such as lime, alum, or polyelectrolytes.

Combined sewer carries both sewage and storm water runoff.

Comminutor is a device for the catching and shredding of heavy solid matter in the primary stage of waste treatment.

Diffused air is a technique by which air under pressure is forced into sewage in an aeration tank. The air is pumped down into the sewage through a pipe and escapes through holes in the side of the pipe.

Digestion of sludge takes place in heated tanks where the material can decompose naturally and the odors can be controlled.

Effluent is the liquid that comes out of a treatment plant after completion of the treatment process.

Electrodialysis is a process by which electricity attracts or draws the mineral salts from sewage.

Floc is a clump of solids formed in sewage when certain chemicals     are added.

Flocculation is the process by which certain chemicals form clumps of solids in sewage.

Incineration consists of burning the sludge to remove the water and reduce the remaining residues to a safe, nonburnable ash. The ash can then be disposed of safely on land, in some waters, or into caves or other underground locations.

Interceptor sewers in a combined system control the flow of the sewage to the treatment plant. In a storm, they allow some of the sewage to flow directly into a receiving stream. This protects the treatment plant from being overloaded in case of a sudden surge of water into the sewers. Interceptors are also used in separate sanitation systems to collect the flows from main and trunk sewers and carry them to the points of treatment.

Ion is an electrically charged atom or group of atoms which can be drawn from waste water during the electrodialysis process.

Lateral sewers are the pipes that run under the streets of a city and      into which the sewers from homes or businesses empty.

Lagoons are scientifically constructed ponds in which sunlight. algae, and oxygen interact to restore water to a quality equal to effluent from a secondary treatment plant.

Mechanical aeration begins by forcing the sewage up through a pipe in a tank. Then it is sprayed over the surface of tank, causing the waste stream to absorb oxygen from the atmosphere.

Microbes are minute living things, either plant or animal. In sewage, microbes may be germs that cause disease.

Mixed liquor is the name given the effluent that comes from the aeration tank after the sewage has been mixed with activated sludge and air.

Organic matter is the waste from homes or industry which is of plant or animal origin.

Oxidation is the consuming or breaking down of organic wastes or chemicals in sewage by bacteria and chemical oxidants.

Oxidation pond is a manmade lake or body of water in which wastes are consumed by bacteria. It is used most frequently with other waste treatment processes. An oxidation pond is basically the same as a sewage lagoon.

Primary treatment removes the material that floats or will settle in sewage. It is accomplished by using screens to catch the floating objects and tanks for the heavy matter to settle in.

Pollution results when something animal, vegetable, or mineral reaches water, making it more difficult or dangerous to use for drinking, recreation, agriculture, industry, or wildlife.

Polyelectrolytes are synthetic chemicals used to speed the removal of solids from sewage. The chemicals cause the solids to coagulate or clump together more rapidly than chemicals like alum or lime.

Receiving waters are rivers, lakes, oceans, or other water courses that receive treated or untreated waste waters.

Salts are the minerals that water picks up as it passes through the air, over and under the ground, and through household and industrial uses.

Sand filter removes the organic wastes from sewage. The waste water is trickled over the bed of sand. Air and bacteria decompose the wastes filtering through the sand. The clean water flows out through drains in the bottom of the bed. The sludge accumulating at the surface must be removed from the bed periodically.

Sanitary sewers, in a separate system, are pipes in a city that carry only domestic waste water. The storm water runoff is taken care of by a separate system of pipes.

Secondary treatment is the second step in most waste treatment systems in which bacteria consume the organic parts of the wastes. It is accomplished by bringing the sewage and bacteria together in trickling filters or in the activated sludge process.

Sedimentation tanks help remove solids from sewage. The waste water is pumped to the tanks where the solids settle to the bottom or float on top as scum. The scum is skimmed off the top, and solids on the bottom are pumped out to sludge digestion tanks.

Septic tanks are used to treat domestic wastes. The underground tanks receive the waste water directly from the home. The bacteria in the sewage decompose the organic waste and the sludge settles on the bottom of the tank. The effluent flows out of the tank into the ground through drains. The sludge is pumped out of the tanks, usually by commercial firms, at regular intervals.

Sewers are a system of pipes that collect and deliver waste water to treatment plants or receiving streams.

Sludge is the solid matter that settles to the bottom of sedimentation tanks and must be disposed of by digestion or other methods to complete waste treatment.

Storm sewers are a separate system of pipes that carry only runoffs from buildings and land during a storm.

Suspended solids are the wastes that will not sink or settle in sewage.

Trickling filter is a bed of rocks or stones. The sewage is trickled over the bed so the bacteria can break down the organic wastes. The bacteria collect on the stones through repeated use of the filter.

Waste treatment plant is a series of tanks, screens, filters, and other processes by which pollutants are removed from water.

TREATMENT LEVELS

There are at present two basic methods of treating wastes. They are called primary and secondary. In primary treatment, solids are allowed to settle and are removed from the water. Secondary treatment, a further step in purifying waste water, uses biological processes.

Primary Treatment

As sewage enters a plant for primary treatment it flows through a screen which removes large floating objects such as rags and sticks that may clog pumps and pipes. The screens vary from coarse to fine from those consisting of parallel steel or iron bars with openings of about half an inch or more to screens with much smaller openings.

Screens are generally placed in a chamber or channel in a position inclined with respect to the flow of the sewage to make cleaning easier. The debris caught on the upstream surface of the screen can be raked off manually or mechanically. Some plants use a device known as a comminutor which combines the functions of a screen and a grinder. These devices catch and then cut or shred the heavy solid material. In this method the pulverized material remains in the sewage flow to be removed later in a settling tank.

After the sewage has been screened, it passes into what is called a grit chamber where sand, grit, cinders, and small stones are allowed to settle to the bottom. A grit chamber is especially important for cities with combined sewer systems because it will remove the grit or gravel that washes off streets or land during a storm and ends up at the treatment plants. The unwanted grit or gravel from this process is usually disposed of by filling land near a treatment plant.

In some plants, another screen is placed after the grit chamber to remove any further material than might damage equipment or interfere with later processes.

With the screening completed and the grit removed, the sewage still contains suspended solids. These are minute particles of matter that can be removed from the sewage by treatment in a sedimentation tank. When the speed of the flow of sewage through one of these tanks is reduced, the suspended solids will gradually sink to the bottom. This mass of solids is called raw sludge.

Various methods have been devised for removing sludge from the tanks. In older plants it was removed by hand. After a tank had been in service for several days or weeks, the sewage flow was diverted to another tank. The sludge in the bottom of the out of service tank was pushed or flushed with water to a nearby pit and then removed for further treatment or disposal.

Almost all plants built within the past 30 years have included mechanical means for removing the sludge from sedimentation tanks. In some plants it is removed continuously and in others at intervals. To complete the primary treatment the sludge free effluent is chlorinated to kill harmful bacteria and then discharged into a stream or river. The chlorination also helps to reduce odors.

Although 30 per cent of the municipalities in the United States give only primary treatment to their sewage, this process by itself is considered entirely inadequate for most needs. Municipalities and industry, faced with increased amounts of wastes and wastes that are more difficult to remove from water, have turned to secondary and even advanced waste treatment.

Secondary Treatment

Secondary treatment removes up to 90 per cent of the organic matter in sewage by making use of the bacteria it contains. The two principal processes for secondary treatment are trickling filters and the activated sludge process. The effluent from the sedimentation tank in the primary stage of treatment flows or is pumped to a facility using one or the other of these processes.

Trickling Filter. A trickling filter is simply a bed of stones from three to ten feet deep through which the sewage passes. Bacteria gather and multiply on these stones until they can consume most of the organic matter in the sewage. The cleaner water trickles out through pipes in the bottom of the filter for further treatment. The sewage is applied to the bed of stones in two principal ways. One method consists of distributing the effluent intermittently through a network of pipes laid on or beneath the surface of the stones. Attached to these pipes are smaller, vertical pipes which spray the sewage over the stones. Another much used method consists of a vertical pipe in the center of the filter connected to rotating horizontal pipes which spray the sewage continuously upon the stones.

Activated Sludge Process. The trend today is toward the use of the activated sludge process instead of trickling filters. The former process speeds up the work of the bacteria by bringing air and sludge heavily laden with bacteria into close contact with the sewage.

In the activated sludge process the sewage from the settling tank in primary treatment is pumped to an aeration tank where it is mixed with air and sludge loaded with bacteria and allowed to remain for several hours. During this time, the bacteria break down the organic matter. From the aeration tank the sewage, now called mixed liquor, flows to another sedimentation tank to remove the solids. Chlorination of the effluent completes the basic secondary treatment. The sludge, now activated with additional millions of bacteria and other tiny organisms, can be used again by returning it to an aeration tank for mixing with new sewage and ample amounts of air.

The activated sludge process, like most other techniques, has advantages and limitations. The size of the units needed is small so that they require comparatively little land space. Also, the process is free of flies and odors. But it is more costly to operate than the trickling filter, and it sometimes loses its effectiveness when faced with difficult industrial wastes.

An adequate supply of oxygen is necessary for the activated sludge process to be effective. Air is mixed with sewage and biologically active sludge in the aeration tanks by three different methods. The first, mechanical aeration is accomplished by drawing the sewage from the bottom of the tank and spraying it over the surface, thus causing the sewage to absorb large amounts of oxygen from the atmosphere. In the second method, large amounts of air under pressure are piped down into the sewage and forced out through openings in the pipe. The third method is a combination of mechanical aeration and the forced air method.

The final phase of the secondary treatment consists of the addition of chlorine to the effluent coming from the trickling filter or the activated sludge process. Chlorine is usually purchased in liquid form and injected into the effluent as a gas 15 to 30 minutes before it is discharged into a watercourse. If done properly, chlorination will kill more than 99 per cent of the harmful bacteria in an effluent.

Tertiary Treatment. Tertiary treatment is used when the waste stream must meet strict requirements governing recreational bodies of water, or must approach drinking water standards. This may require one or several of the following processes slow filtration rapid filtration with activated carbon adsorption by activated carbon application of ozone high rate chlorination or use of other oxidizing chemical or lagooning.

At each plant the question may arise as to what degree of treatment is actually required. Water quality criteria imposed by different waste stream discharges may vary widely. Even within the same state, or for a particular river basin, different limits for each of the contaminants may be set for the section of the river under consideration.

LAGOONS AND SEPTIC TANKS

There are many well populated areas in the United States that are not served by any sewer systems or waste treatment plants. Lagoons and septic tanks are the usual alternatives in such situations.

A septic tank is simply a tank buried in the ground to treat the sewage from an individual home. Waste water for the home flows into the tank where bacteria in the sewage break down the organic matter and the cleaner water flows out of the tank into the ground through subsurface drains. Periodically the sludge or solid matter in the bottom of the tank must be removed and disposed of.

In a rural setting, with the right kind of soil and the proper location, the septic tank is a safe and effective means of disposing of strictly domestic wastes. Septic tanks should always be located so that none of the effluent can seep into wells used for drinking.

Lagoons or, as they are sometimes called, stabilization or oxidation ponds, also have several advantages when used correctly. They can give sewage primary and secondary treatment or they can be used to supplement other processes. A lagoon is a scientifically constructed pond, usually three to five feet deep, in which sunlight, algae, and oxygen interact to restore water to a quality equal to or better than effluent from secondary treatment. Changes in the weather affect how well a lagoon will break down the sewage.

When used with other waste treatment processes lagoons can be very effective. A good example is the Santee, California, water recla­mation project. After conventional primary and secondary treatment by activated sludge, the towns waste water is kept in a lagoon for 30 days. Then the effluent, after chlorination, is pumped to land located immediately above a series of lakes and allowed to trickle down through sandy soil into the lakes. The resulting water is of such good quality that the residents of the area can swim, boat, and fish in the lake water.

TYPES OF INDUSTRIAL WASTES

Industrial wastes exceed if one includes the steam electric generating industry the combined total of all other liquid wastes of human activities in terms of volume, probably averaging over 200 billion gallons per day and they contain thousands of potentially polluting elements and compounds, often in high concentrations.

Because of the large gaps in the information base, it is not possible to hypothesize with any certainty that industrial wastes are more damaging on a national basis than the effects of unconstrained runoff and in the Western States, damages from intense irrigation and water management practices may or may not exceed damages caused by industrial wastes. But there need be no hesitation in making the judgement that for the nation as a whole, routine discharges of industrial wastes exceed in polluting impact, sewered domestic wastes, urban runoff, mining, transportation, or accidental spills. Their sheer volume and the directly toxic influences of many kinds of industrial wastes (acids, heavy metals, some persistent organic compounds) are sufficient to justify the judgement, even though no comprehensive industrial waste inventory has been compiled.

The great variety of industrial pollutants argues against an attempt to catalog them, but for the purposes of general description we may recognize at least five distinct categories of wastes from industrial sources

Oxygen demanding materials wasted to water by industrial processes have been calculated to amount to 29.7 billion pounds of five day chemical oxygen demand (BOD5) per year having an aggregate discharged strength, after treatment, of 11 billion pounds of BOD5 per year. The estimate, then, places the oxygen demand of industrial wastes before treatment at 3.5 times that of sewered domestic wastes, and the after treatment strength at 5.5 times that of the after treatment strength of domestic wastes.

Settleable and suspended solids wasted to water by industrial processes are calculated to amount to about 24 billion pounds per year before treatment, over 7 billion pounds after treatment, with the relationship between industrial and domestic wastes quite similar to that for BOD.

Many materials which impart acidity or alkalinity or which contain radioactivity are added to water in undeterminable amounts by manufacturing activities. Often these are di­rectly toxic. Such materials cause a permanent change in water quality that is not reduced by natural assimilation. Dilution and neutralization the latter usually adding to the concentration of total dissolved solids are the only remedies in such cases.

Heat in the amount of about 9,152,000 million Btus is currently discharged annually into water by industrial processes, most of it by the electrical generating industry. Although some control is exercised through recycling procedures, particularly in arid areas, the major part of the nations industrial cooling water is discharged directly to streams. Waste heat is a pollutant in that it reduces the utility of water for additional cooling and may radically alter aquatic ecology. It also contributes to the polluting effect of water borne materials by accelerating chemical reaction rates and by reducing the solubility of gases including oxygen.

      Toxic compounds occur in water as a result of industrial processes, either through direct discharge by establishments producing such compounds (e.g., factories engaged in the       production of pesticides or pharmaceuticals), by the synergistic   interaction of materials in water, or through the food chain. Again, no quantitative assessment is available, and for the second category of toxins, even determining probability of occurrence cannot be attempted.

Within these general categories of pollutants are included most of the possible sources of recognized water quality problems. Only bac­terial and viral presences fall outside of the group of polluting effects to which industry contributes materially. (Although these are not entirely out of the range of parameters to be included in the polluting activities of industry meat packing plants, and other food processors in lesser measure, contribute to the presence and viability of water borne bac­teria.) Manufacturing must stand near the top, and it very probably leads, in any list of potential sources of water pollution.

And for any possible strategy of water pollution control, industrial wastes are of critical importance. Over the last ten years, the amount and composition of industrial output has been such that for every incremental pound of BOD that has been generated directly by population increase, twenty more have been generated by increased industrial output. Increased per capita production is the essence of improvement in the standard of living, and the production of wastes is an inescapable concomitant of the production of goods. So that as population and per capita production continues to advance, we can anticipate a continuing and unavoidable advance in the volume of wastes to be managed.

Fortunately, industry has added rapidly to its inventory of waste treatment facilities over the last decade and a half, and it appears that provision for waste treatment is routinely designed into new plants and plant additions. Estimated daily discharge of BOD from all sources in 1968 was little greater than in 1957, in spite of significant increase in population, the incidence of sewering, and industrial production. While additional treatment of municipal wastes had an unquestionable influence on the economys ability to contain the level of waste discharges, the preponderance of industrial wastes and their more rapid rate of increase would have made containment impossible if incremental industrial­ waste treatment effectiveness had not occurred at more advanced rates than incremental industrial production.

Strategies for Management of Industrial Wastes

Unlike the public sector, where only variations on a single theme of waste treatment are possible, industrial pollutants can be managed through at least four distinct strategies. (1) The obvious procedure is the installation of industrial waste treatment plants. Treatment effectiveness through factory operated plants is estimated to have been increasing at a 6.9 per cent annual rate (in terms of BOD reduction) since the early nineteen sixties. (2) A second and increasingly prevalent procedure is to discharge industrial wastes to public systems for treatment. The aggregate amount of BOD from industrial sources discharged to public sewers is estimated to have increased at an 8.4 per cent annual rate through the late nineteen sixties and industry is estimated to account for half of the current BOD loading to metropolitan area waste treatment plants. (3) Process modification and changed product formulations are probably the most effective as well as the most efficient means of reducing wastes. The outstanding example is the shift of the pulp and paper industry from the sulfite pulping process to the sulfate process, calculated to have been responsible for a greater reduction in the aggregate level of BOD than the combination of all of the waste treatment plants in the U.S. More recent example include the rising prevalence of cooling water recycle, development of biodegradable detergents, and substitution of reclaimable hydrochloric acid for sulphuric acid in metal pickling liquors. (4) In the absence of alternative control procedures, industry has on occasion abandoned a product line or production procedure. At the present time, phosphorus based detergents, DOT, mercury battery production of chlorine and alkali, and the Solvay process for production of soda ash all seem likely candidates for abandonment.

QUANTITY OF INDUSTRIAL WASTES

Table. 1 shows reported quantities of industrial waste waters discharged in 1967. Waste load estimates, based upon an estimate of the average quantity of pollutant per product unit, indicate that the chemical, paper, and food and kindred industrial groups generated about 90 per cent of the BOD5 in industrial wastewater before treatment.

Similar statistics on net waste load discharges are not completely available. However, indications are that the extent of industrial waste water treatment is not greater than that currently practiced for municipal waste waters.

Industrial wastes differ markedly in chemical composition, physical characteristics, strength, and toxicity from wastes found in normal domestic sewage. Every conceivable toxicant and pollutant of organic and inorganic nature can be found in industrial waste waters. Thus, the BOD5 or solids content often are not adequate indicators of the quality of industrial effluents. For example, industrial wastes frequently con­tain persistent organics which resist the secondary treatment procedures applied normally to domestic sewage. In addition, some industrial effluents require that specific organic compounds be stabilized or that trace elements be removed as part of the treatment process.

It is therefore necessary to characterize each industrial waste water to permit comparative pollutional assessments to be made for individual industries as well as industry groups. Characterization will permit classifying the components of industrial waste waters into as few as four basic classes of pollutants to more readily collate pollution statistics and to evaluate economics of methods of treatment as well as to project least cost methods. Proposed generalized basic classification parameters are BOD, COD, SS, and TDS into which all known pollutants can be classed. Also required is the establishment of a relative pollution comparative index for all significant pollutants. This index, in combination with the known characteristics and volume of a waste water, will determine the relative gross pollution severity of all industrial wastes and establish a basis for comparing the severity of pollution from different industries.

Table 3 presents permissible criteria for surface water for public supplies as obtained from the Report of the Committee on Water Quality Criteria, April 1, 1968. The addition of an assumed BOD5 value of 5 mg/l to these criteria permits comparisons of the listed pollutants to be made against a unit of BOD. Under these circumstances it is relatively apparent that pollutants such as endrin and phenol (on a mg/l concentration equivalent basis) are 5000 times more critical as pollutants than BOD. Further work in this area will permit establishment of more accurate priorities in terms of our nations most critical needs.

All major industrial groups and industries suspected of making significant contributions to water pollution. These have been selected on the basis of a process water intake of at least 1 billion gallons per year and with regard to the potential for pollution from the process use of the water.

The industries number approximately 150 and potentially represent equally numerous waste waters of significantly different characteristics for which treatment technology must either be developed or upgraded. The inter changeability of treatment technology between similar types of waste waters is anticipated.

A wide spectrum of technology is available for controlling and treating industrial water pollution. Some of the more important unit operations and unit processes are given in Table 5.

In spite of the complexity and magnitude of industrial pollution, initial estimates of the costs for clean waters from industrial sources have been made. Table 2 gives the types of treatments and costs for major industries, while Table 3 presents the estimated industrial capital requirements to abate pollution by 1973 to the extent of providing 85 per cent treatment effectiveness. It is seen that the capital requirements for this task are substantially less than the estimated capital require­ments for municipal treatment or collection facilities for separating combined sewers, while the gross pollution load contributed is substantially greater than either. This indicates that the average cost of industrial waste treatment is substantially less than for municipal waste treatment when based on treatment cost per Ib BOD. If these estimates are reasonably accurate it would appear that for the most part industrial pollution control to an equivalency with secondary treatment is within reach at a reasonable cost.

Alternatives in waste water treatment are However by product recovery and utilization techniques can reduce the cost of treatment and frequently prove to be less expensive than other methods of disposal.

Recycled water ultimately may be the most valuable product due to supply shortages, increasing water supply costs, increasing water treatment costs, and mounting municipal sewerage charges. The recovery of product fines, useable water, and thermal energy are key methods of reducing over all waste treatment costs and must always be considered.

Frequently, waste streams can be eliminated or significantly reduced by process modifications. One notable example is the application of save rinse and spray rinse tanks in plating lines. This measure brings about a substantial reduction in waste volume as well as a net reduction in metal dragout.

RECLAMATION OF TEXTILE EFFLUENTS

Phenols due to their wide spread usage are often found in the effluents from certain industrial plants. The examples of their usage include the intermediates in the production of plastics, drugs dyes etc. Effluents of coking plants, brown coal distillery plants, pulps and paper industry, textile processing plants are the major source of phenols in the environment. Thus a wide variety of phenols get introduced into aquatic environment. Chlorinated phenols are one of the environmentally harmful compounds, having been proved to be toxic to many water organisms even at 0.1 ppm level. Furthermore, 0.01 ppm level of chlorophenols suffices to impart an extremely disagreeable taste and odour to water. Thus it becomes necessary to process such waters in order to eliminate phenols. The removal of phenols and chlorinated phenols by activated charcoals has been considered the most efficient process. In view of the high cost and tedious procedure for the regeneration of activated carbon there is a continuing search for low cost efficient adsorbents. Recently, we have reported some guar derivatives with potential utility in removing colour from the textile effluents. As a part of our research programme for the reclamation of textile effluents we investigated the sorption and desorption characteristics of phenol and some chlorophenols on the guar derivatives. These phenols are added to the thickenings used in the textile printing as preservative.

Experimental Procedure

Adsorbents. The adsorbents used in this study are quaternary aminised trimethyl and tripropyl aminohydroxypropyl guars referred onwards as TMAHP, TEAHP and TPAHP guars. These were synthesised as described earlier.

Phenols. Phenol, 2 chlorophenol, 2, 4 dichloro phenol, 2,4.6  trichlorophenol and pentachlorophenol were of LR grade and were   purified by distillation or by repeated crystallisation, there purity was chec ked by TLC.

Textile effluents. Replicate samples of textiles effluents were collected from two open drains of Jodhpur industrial area at a specific point marked for this purpose. The effluents were collected in PVC bottles by standard procedure. The effluents were highly coloured, alkaline and foul smelling. The detailed analysis of it has been, already, reported. All other chemicals were of Analar grade and were used as such. All the solutions were prepared in double distilled water obtained from an all glass still.

Analytical technique. The adsorption isotherms were obtained by agitating 0.05 to 0.5 g of the adsorbent with 100 ml aqueous solution of phenols in skrew cap jars. The agitation was continued for 60 min at ambient temperature (25 + 0.2°C). The initial pH of the solution was adjusted by adding requisite amount of dilute acid or alkali solution. The equilibrated solutions were centrifuged for 10 min at 10,000 RPM in a T 24 model (GDR) centrifuge and analysed for the residual phenol concentration by 4 aminoantipyrene method as described in ASTM.

The waste water samples were allowed to stand for 4 hr the     supernatant liquid was decanted, adjusted for pH and 100 ml of it was    agitated for 1 hr with the different amount of guar derivatives. The  waste waters were than centrifuged and analysed for residual total phenol concentrations by 4 amino antipyrene method using Shimadzu model 240 spectrophotometer.

Results and Discussion

The synthesised guar derivatives show favourable adsorption for phenol and chlorophenols from their aqueous solutions. The sorption and desorption of phenols in our case may be better explained using Freundlich adsorption equation.

      Effect of pH. The introduction of trialkylamino substituent on to guar imparts it a strongly basic an ion exchange character that permit the adsorption of phenol ion/molecule by the ion exchange process over a wide range of pH 3 10. The effect of pH on the adsorption of phenol on guar derivatives was investigated for 2, 4 dichlorophenol   and pentachlorophenol on TMAHP guar in the pH range 3.5 10.0.  The values of K and 1/n at different pH were calculated from the linear   plots between log x/m and logC, these are given in the Table 2. From these values it is apparent that the adsorption is favoured in the pH range of 4 7 (Fig. 2). This is probably due to the neutralisation of the anionic sites and negative potential of the surface of guar derivatives at lower pH. The reduced adsorption at higher pH may be attributed to            the abundance of hydroxyl ion and consequent ionic repulsion between the surface partial negative charge and the phenolate ions.

Effect of the initial phenol concentration. The effect of the initial phenol concentration on the sorption and desorption behaviour of the guar derivatives was investigated for 2, 4 dichlorophenol and pentachlo rophenol at 50 and 100 mg/1 initial concentrations.

The values of K and 1/n were calculated from the linear plots of log x/m and log C (Fig. 3). The adsorption capacity, K, was found higher for the initial lower concentration of phenol concerned. This appears due to the larger adsorbent surface available to the phenolate ions at lower concentrations.

Studies with textile effluents the adsorption of total phenol in the textile effluents on TMAHP guar was studied at pH 3,4 and 8.5. The percentage decrease in the total phenol concentration in the effluent was plotted against the amount of TMAHP guar added. The per cent decrease in the phenol concentration was found proportional to the amount of the guar derivative up to a certain amount and then approaches to a limiting value (Fig. 4). The removal was maximum at pH 4. This is in accordance to our previous results obtained for the adsorption of anionic dyes and phenols on TMAPH guar. Approximately 70 90% of the total phenol was removed in the first half an hour and no significant removal was observed subsequently.

The removal of the individual phenol by the guar derivative was evaluated by treating 1litre textile effluent with TMAHP guar at pH 6.0 and estimated the residual phenol concentration by high performance liquid chromatography (HPLC). The textile effluent was subjected to the extraction with methylene chloride before and after treatment with guar derivative, separately, the extract was then concentrated and re dissolved in methanol. The isocratic elutions were made with mobile phase being methanol water mixture (5545, v/v) at a flow rate of 1 ml per minute. The detection limit of UV detector was kept 2, as calculated from the noise to signal ratio. Fig. 5 shows the chromatographic separation of the phenolic compounds in the methylene chloride extract. These results show that all the chlorophenols are adsorbed equally well and the derivative show no preferential adsorption to a particular phenol. The per cent decrease in the concentration of individual phenol was evaluated from the calibration curves, obtained by chromatographing standard sample of phenol and chlorophenols under identical experimental conditions. These values have been given in Table 3.

 

A PROCESS FOR UPGRADING PAPER MILL EFFLUENT BY WATER HYACINTH

 

A novel and effective low cost treatment for pulp and paper mill effluent is discussed in this paper. The effluent is treated in stabilization lagoons with water hyacinth, Eichhornia crassipes (Mart) Solms, found abundantly in tropical and humid countries of the world. Since large volumes of effluent are generated in a pulp and paper mill, the need for an effective, economic and simple process for its treatment is of primary consideration to the paper industry.

Some of the conventional methods of treatment, i.e., activated sludge, Trickling filteration and stabilization lagoons, etc., are well known. Due to heavy capital investment and high operational costs, these methods are generally uneconomical to install and operate. Even the low cost waste treatment process based on lagoon stabilization requires extensive land areas since the detention time necessary to achieve the desired pollution reduction is very high, ranging from 30 to 40 days, has been observed by several investigators that water hyacinth possesses the ability to substantially reduce the concentrations of organic matter, minerals and heavy metals present in the waste waters.

Water hyacinth, which has rapidly spread throughout the warm regions of the world, is considered as one of the most noxious weeds creating a problem in its eradication. However, this weed possesses good potential for reduction of pollution load of sewage and industrial effluents, and also as a source of energy food and livestock food, fertilizers, and other products. Wolverton and McDonald have rightly mentioned that the neglected water hyacinth has now begun to gain its respectability by offering a relatively simple and economically attractive solution to some of the most pressing problems.

Water hyacinth, atropical plant of exquisite beauty with broad glossy green leaves and light lavender flowers splotched with canary yellow, is found in ditches, ponds and streams. The plant is highly prolific and reproduces mainly by vegetative offsprings. Its growth rate is so high that it can double its number every 8 10 days in warm, nutrient enriched waters forming a huge floating mat.

Details of life cycle of water hyacinth have been studied. Its ecological adoptibility throughout the warm regions of the world has been reviewed by Holm Chatterjee studied the details of chemical composition of water hyacinths whereas Wolverton and McDonald reported chemical composition of water hyacinths grown in sewage lagoons.

For reduction of pollutants from the industrial and domestic waste water by water hyacinth culture, Sinha have carried out laboratory investigations by growing in oxidation ponds with digested sugar wastes and effluents from septic tanks. MC Donald studied the effect of this plant on nutrient and pollutant removal from sewage and industrial effluent lagoons Neuse performed an experiment at the Willamson Creek Plant in Austin, Texas, USA involving hydraulic testing of water hyacinth culture unit specially designed to remove algae from stabilization pond effluent. Widyanto have studied the effects of agriculture, domestic and industrial pollutants on water hyacinths.

The National Space Technology Laboratory (NSTL), Mississippi, has also carried out extensive studies on the utilization of water hyacinths for treatment of various sewage and industrial effluents.

Experimental Procedure and Results

The above mentioned reports indicate that there is considerable scope for utilizing water hyacinths for upgrading industrial effluents but no work has so far reported methods for treating the pulp and paper mill effluent with water hyacinth. The present work on upgrading the paper mill effluent was initiated with the following objectives

      To study the techno economic feasibility of water hyacinth utilization in the upgrading of combined paper mill effluent.

      To identify, evaluate and optimize the influencing parameters responsible for the treatment of paper mill effluent with water hyacinth.

For measurement of pollution load, COD estimations have been carried out throughout the studies. BOD measurements have not been carried out in the belief that if the process could be effective for removal of constituents contributing to COD, it would be easier control BOD values to the desired levels. The combined effluent from an integrated pulp and paper mill has been used for various studies reported here.

Phase I

Survivability of the plant. Effluents with varying pH were collected. For obtaining the high and low range of pH, dilute NaOH and HC1 were added in calculated doses. In each of the pots, plants were grown and their survivability was observed. Intermittently, the pH of the effluent was also noted (Fig. 1).

Studies on survivability at different pH values, 2 to 12, showed that water hyacinth could survive and grow well within a wide pH range of 4 10 beyond which it become dry. Furthermore, the dried plants did not rejuvenate even after placing them in a suitable nutrient medium. It is interesting to note that in the case of plants which survived in the pH range of 4 10, the final pH of the effluent was always around 7 (neutral) within a week, irrespective of the initial pH of the effluent.

In general, the growth of water hyacinth was rapid in the combined pulp and paper mill effluent. It was also observed that addition of small quantities of sewage waters accelerated the plant growth.

Studies on detection period vs reduction in pollution load. Effluent was kept for stabilization with a predetermined quantity of water hyacinth. Samples at odays as well as at different time intervals during treatment were drawn and analysed for pH (Table 1) and COD (Fig. 2).

It is seen that the treated effluent (760 ppm COD) showed around 70 per cent of COD reduction within 9 days as compared to only 27 per cent without hyacinth treatment.

Optimization of the surface area coverage by water hyacinthThe surface area of the experimental pots was divided into four equal sections. After filling the pots with effluent, approximately 25%, 50%, 75% and 100% of the area was covered with water hyacinth in one, two, three and four sections respectively. Samples were analysed for pH and COD (Table 2).

An optimum coverage of 75 per cent of the area available by water hyacinth can be considered as adequate since beyond this no appreciable reduction in COD is observed. Also, full coverage of the surface area hinders the growth of the plant and aeration of the surface.

These parameters were followed in the subsequent phases of the work.

Phase II

Detection period. The effluent after 4 hours of settling was fed with water hyacinth covering 75% of surface area. Samples were analysed for pH and COD initially as well as after different days (Table 3).

In bigger capacity experimental tanks (200 litres) COD reduction was low as compared to earlier experiments in the laboratory using smaller tanks of 30 litres because of the high volume of effluent to be treated in proportion to the plant content. In a typical experiment, it is seen that the reduction of COD achieved is almost two fold when 1.5 kg wet biomass of hyacinth is added to 200 litres of effluent.

Recycling of the plant. Recycling possibility of the same stock of hyacinth was studied by repeatedly using the same set of hyacinth for seven consecutive treatments by changing the effluent, in the container. The samples were analyzed for pH and COD in the beginning and after 3 days for each set.

For recycling it has been seen that the same stock of hyacinths can be used without any decrease in efficiency for more than seven cycles keeping the detention period at 3 days in each set.

Ratio of effluent to plant. To  study  the effect of volume (height) of the effluent on the efficiency of treatment, studies were carried out in experiment tanks which were filled with the effluent to different volumes, i.e., 100%, 75% and 50% to which a fixed quantity of hyacinth was added. Samples were analyzed for pH and COD after different detention periods (Table 4).

The extent of COD reduction obtained with different volumes of effluent showed that the overall reduction has an increase relation with the height of the effluent. The lesser the height per plant, the higher the pollution reduction, i.e., the reduction in COD is directly proportioned to the total volume of effluent treated.

Treatment of effluent with high pollution load. To study if longer detection would be helpful for treatment of effluents with high pollution loads, experiments were carried out for longer durations (approximately for a month). Samples were analyzed for pH and COD for different days of detention (Fig. 3).

A maximum of 60% reduction in COD could be obtained within a detention period of one month in case of a typical effluent having about 1500 ppm COD.

Treatment of effluent from paper machines. Unsettled effluent from the paper machine was also collected as it contains very high suspended matter and high inorganic content, and is, therefore, quite different in characteristics and its composition from the combined paper mill effluent and allowed to settle for 4 hours before treating with water hyacinth. Reduction in pH and COD was noted (Fig. 4).

Around 90% of COD reduction was achieved within a short detention time of 6 days. Beyond 6 days the reduction in COD was insignificant.

Based on the encouraging results obtained in Phase I and Phase II it was decided to carry out large scale trials on experimental lagoons (Phase III).

Phase III

A pilot lagoon was constructed (Fig. 5) and the effluent was filled to a height of 1 m, allowed to settle for 4 hr and then 75% of surface area of the lagoon was covered with water hyacinth. After settling, the initial values of pH and COD of the effluent were measured and subsequent measurements were carried out for different periods for progressive treatment with water hyacinths. Samples were collected from the top and bottom surfaces for different days of detention (Fig. 6).

To simulate the working of a pilot lagoon with a detention period of 15 days in actual operation in the mills l/15th of the treated effluent was removed from bottom and the same quantity of fresh effluent was added from the top to make up the volume. The incoming effluent was analyzed for its COD after settling. This process was continued for 7 days and the samples collected from the bottom of the ponds were analyzed for pH and COD (Table 5).

The results in the pilot lagoon were quite encouraging with a healthy growth of the hyacinth. The per cent reduction was quite comparable with the earlier experimental results. Approximately 70 80% COD reduction could be achieved in the lagoon within 15 days for the combined effluent with around 600 ppm of COD.

Our results on percentage of pollution reduction are quite encouraging. Around 70 80% of COD reduction has been achieved within 15 days detention in a lagoon treatment with hyacinth in a batch process or continuous process. Similar trends in the reduction of pollution load were also in different effluents. Around 80% of suspended solids were removed from the treated effluent in comparison to the 15% in the untreated one within the first few days.

In conventional biological treatment processes, the biological degradation and subsequent assimilation of impurities are sometimes carried out by phytoplanktonic algae. But because of the easy decomposition of algal cells, algae laden effluent is considered as the most undesirable features of stabilization ponds. Water hyacinth is found to check the growth of algae and has been considered as one of the tools for removing algae from the lagoons. Although, water hyacinth does not produce oxygen through its roots, it purifies the waste by means of a complicated mechanism, regarding which Sinha considered that cellular enzyme system accomplishes oxidation without oxygen by removal of hydrogen. They also found that roots of hyacinths contain some dehydrogenation enzymes which possibly absorb hydrogen and also act as election transferring agents to accomplish the higher rate of oxidation. Along with the biodegradation of the organic matter, water hyacinth can also remove toxic metals. In the static laboratory experiments as reported by NSTL scientist Eichhornia rapidly absorbs gold, silver, cobalt strontium, cadmium, nickel, lead and mercury very efficiently. Water hyacinth can also absorb or metabolize phenols and other organic toxic compounds from effluents and drinking water supplies. Hence, removal of toxic material by absorption leads to reduction in the toxicity of the system. Further, treatment with water hyacinth also reduces the overall turbidity of the water.

Whatever may be the probable cause of biodegradation and removal of toxic materials water hyacinth gives a new dimension in the field of upgrading the industrial wastes with special reference to the waste from Pulp and Paper Industry.

DISPOSAL OF SOLID EFFLUENTS AND REUSE

Definition, Classification, Quantity and Composition of Refuse

All solid and semi solid wastes of a community, except human excreta and sullage is classified under the general term refuse, Refuse, thus, represents the dry wastes or solid wastes of the society, and includes garbage, ashes, rubbish, dust, etc. as defined below

Garbage. It includes all sorts of putrescible organic wastes, obtained from kitchens, hotels, restaurants, etc. All waste food articles, vegetable peelings, fruit peelings, etc., are thus, included in this term. These wastes are organic in nature, and thus, likely to decompose quickly, producing foul odours and health hazards. They may also result in breeding of flies, mosquitoes, insects, etc. Hence, garbage must be disposed of, properly and quickly. When it is scientifically processed and composted, then it is possible to obtain valuable products, like grease, hog wood, fertiliser, etc. from garbage. The density of garbage usually varies between 450 to 900 kg/m3.

Ashes. Ashes as all of us know denote the incombustible waste products from hearths and furnaces, and houses or industries. The density of ashes generally varies between 700 to 850 kg/m3. Its quantity is getting reduced in modern days due to the increasing use of cooking gas and kerosene oil, and lesser use of cooking coal, in houses. In industrial towns, however, this quantity may be quite appreciable.

Rubbish. Rubbish includes all non putrescible wastes except ashes. It, thus, includes all combustible and non combustible wastes, such as rags paper pieces, broken pieces of glass and furniture, card boards, broken crockery, etc. Rubbish is lighter, and normally has a density varying between 50 to 400 kg/m3. It may create greater nuisance during the autumn and summer, as it may be scattered by high winds.

The usual density of refuse (mixture of all types of dry wastes) generally varies between 300 to 600 kg/m3.

Besides the above technical classification based on the type of wastes, the refuse may also be classified, depending on its source, as (i) house refuse (ii) street refuse and (iii) trade refuse. All these terms are quite apparent, and known to all of us, and hence need no explanation.

The quantity of solid wastes (refuse) produced by a society depends upon the living standards of its residents. The industrialization of modern society has resulted in a vast increase in the amount of refuse generated per person.

In an average modern city, each citizen produces about 0.3 to 0.8 kg of solid domestic waste per day. The quantity generated is generally found to be on a lower side, when the degree of commercialization and industrialization is on a lower level. Say for example, a city like Delhi, produces about 4000 tonnes/day of solid wastes whereas. New York produces as much as 25,000 tonnes/day of solid wastes.

The quantity of refuse produced in a city not only depends upon the type of the city and on the living standards of the residents, but also depends upon the seasons. Say for example, in India, average summer refuse is about 25% higher than the yearly average, due to larger carriage and consumption of fruits, like mangoes, melons, etc.

The average composition of refuse (by weight) is estimated to be as shown in Table 1.

The quantity of rubbish in Indian refuse, as compared to that in U.S.A. is very small as in India, large quantities of papers, cardboards, plastics, synthetic polymers, rags, etc. are picked up and removed enroute, by the rag pickers (men, women and children) before the refuse reaches the disposal site. This, infact, reduces the calorific value of Indian refuse. On the other hand, the quantity of garbage in U.SA is very small, because of the use of garbage grinders, and use of tinned and readymade packed foodstuffs.

Collection, Removal and Carriage of Refuse

In India, the refuse is generally collected in individual houses in small containers, and from there, it is collected by sweepers in small hand driven lorries/carts, and then dumped into the masonry chambers constructed along roadsides, by municipalities. The refuse is finally carted away by municipal trucks, for further disposal during some day time. The methods adopted here are highly unsatisfactory, and need tremendous improvements and changes.

Infact, the house sweepers and street scavangers, do not bother much for carrying refuse properly, and they go on scattering it here and there, while carrying it up to the municipal chambers. Many of them do not bring hand lorries even. Even at the entrance point of the municipal refuse chamber, they just throw the refuse, scattered alround in and out of its gate. The street animals would further scatter it, leading to all round scattering of the refuse, almost everywhere, resulting in highly insanitary conditions and health hazards.

Similarly, the municipal trucks, do not generally clear and clean the refuse chambers properly, and the residual refuse remains dumped for a long time, resulting in its composition and evolution of obnoxious gases, and consequently causing health hazards. Even municipal trucks, while carting refuse, are generally not closed bodied, and hence go on throwing refuse on way, and also giving pungent smell. This is happening so, even in Delhi, the capital of India.

The process of refuse collection and its carriage, therefore, needs vast changes in this country. The refuse, should, therefore, infact be collected by municipal trucks directly from the houses. Roadside refuse collection masonry chambers, need complete elimination. Municipal trucks should be completely closed, and should visit homes and houses, twice a day, once in the morning and once in the evening, to collect household refuse and street sweepings.

The transporting trucks (vehicles) should also be of high quality, of special design and be properly maintained. They should be strong, durable, and water tight, and be made of stainless steel with smooth interior, having round corners and edges, for facility of cleaning. They should have a low loading line, say upto about 1.5 m, so that minimum of time and effort is required in filling them. They should have a cover, which should be made as a part of the body with hatches which can be opened during collection. Mechanical devices should be installed in these vehicles, for lifting the body to the sides or back, or for pushing the refuse out, so that they can be quickly and easily emptied.

The practice of open burning of tree leaves and grass clippings, as prevailing in our country, also needs to be stopped. Such leaves and grasses, instead of being burnt, should be mulched and used for compost.

Disposal of Refuse by Sanitary Land Filling. In this method of refuse disposal, refuse is carried and dumped into the low lying area under an engineered operation, designed and operated according to the acceptable standards, as not to cause any nuisance or hazards to public health or safety.

The refuse is dumped and compacted in layers of 0.3   0.6 m or so, and after the days work when depth of filling becomes about 1.5 m, it is covered by good earth of about 15 cm to 30 cm thickness, so that the refuse is not directly exposed. This filling is done by dividing the entire site into smaller portions, as explained in article 12.3.1.1. The compaction is done by movement of bull dozers, trucks, etc. before starting filling the second layer of refuse.

Filling of low lying areas should generally be done by leaving a minimum distance of 6 m from the surrounding area. Insecticides like DDT, creosote, cresol, etc. should also be sprayed on the layers to prevent breeding of mosquitoes and flies. A final cover of about 0.6 metre of earth is laid and compacted at the top of the filled up land to prevent rodents from burrowing into the refuse.

With the passage of time, the filled up refuse will get stabilized due to the decomposition of organic matter and subsequent conversion into stable compounds. The land filling operation is essentially a biological method of waste treatment, since the waste is stabilized by aerobic as well as anaerobic bacterial processes.

Initially, the bacterial decomposition occurs under the aerobic conditions, because a certain amount of air is trapped within the landfill. However, the oxygen in the trapped air is soon exhausted within a few days and the long term decomposition occurs under anaerobic conditions.

The entire period of refuse stabilisation can infact, be divided into five distinct phases (i) During the first phase of operation, aerobic bacteria and fungi, which are dominant, deplete the available oxygen to effect oxidation of organic matter. As a result of aerobic respiration, the temperature in the fill increases, (ii) In the second phase, anaerobic and facultative bacteria develop to decompose the organic matter and H2 and CO2 gases are thus evolved through acidogenic activity. (iii) In the third phase, methanogenic bacteria develop to cause evolution of methane gas. (iv) In the fourth phase of decomposition, the methanogenic activity gets stabilized. (v) In the fifth stage, the methanogenic activity subsides, representing depletion of the organic matter and ultimately, the system returns to aerobic conditions within the land fill.

For better biological degradation, the moisture content of the dumped material should be high, say not less than 60% or so, which is sometimes maintained by the aerobic decomposition brought out by fungi, or sometimes by sub soil water.

The refuse, in managed landfills, may usually get stabilized, generally within a period of 2 to 4 months and settle down by 20 40% of its original height. The filled up land can infact, be used for developing some green land, parks, or other recreational spots. Unequal settlement and odour trouble, may however, be there, and hence, normally, for the first 1 2 years, the land is grassed or planted, fenced, and left out as reserved green land. This can, on a later date, be preferably used for developing some regular play grounds, or picnic spots. Such sites may also sometimes be used for constructing houses though they are not generally preferred, because such constructions may prove to be costly due to their deeper foundations for avoiding unequal settlements. Such houses, may further pose problems like those of bad odours and cracks in walls and plasters, on a later date.

This method of refuse disposal is very suitable to the heavier type of Indian refuse, and also to the rural communities, hostels, camps, etc. Hence, it is widely adopted in our country. So much so, that about 90%    of Indian refuse is disposed of in this manner.

The area method is used when it is not possible to further excavate at the chosen land fill site, especially when the groundwater is high. In this method, the entire land fill site is first of all divided into a number of sub areas by constructing embankments and roads. The sub areas are called sub division cells or sometimes simply as cells. However, to differentiate them from smaller heaps of waste fill, which are filled daily at the site and are called cells, it is preferable to call these sub division areas as sub division cells.

The development of roads on the embankments will help in moving the trucks bringing the solid waste from the city over them, as to enable their being emptied in any of the sub division cells. These sub division cells may be numbered, and may be taken up for filling with waste, one after the other.

The work of filling in a chosen sub division cell is started by bringing and filling the refuse (solid waste) along one of the cross embankment or bundi, such as the one shown in Fig. 12.1, and marked as AB. The waste is simply brought and dumped on the ground, spread in layers of about 0.5 m thickness, and compacted. Another layer of 0.5 m thickness is then placed on top of the previous layer, and also compacted. Layering and compacting are repeated until a height of about 1.5 m is reached. At this point and at the end of a working day, a cover of earth of about 0.15 to 0.3 m thickness is compacted on the top and side slopes of the compacted heap, which is called a cell. This cover is called the daily cover.

Fig. 1 shows the prospective view of such a land filling site, in which you can locate the cross earthen levee (AB). To the right hand side of this levee is a completed unit of compacted solid waste or fill, shown shaded, which is bound by the daily earth cover on side as well as on top. This small fill unit is called a cell. Another cell to the right of the previous cell is then filled on the next day and completed in the same manner. The daily cell filling, then proceeds to the right, until the entire bottom most horizontal span (length) gets filled. In the figure, a total of six horizontal cells are filled in the span. Cells are then further constructed on top of the previous cells to complete another horizontal span of cells in the second lift. A partially filled second lift is shown in Fig. 1 itself. The earth cover existing between the bottom cells (1st lift) and the cells just above them (2nd lift) is known as the intermediate cover. The vertical height of the horizontal cells of a given lift including the top cover, is known as the lift.

VENTILATION FOR CONTROLLING INDOOR AIR POLLUTION

Environment, as we all understand, is nothing but our surroundings, which can be badly affected by smokes, smells, dusts, gases, oxygen deficits, noises, and vibrations, etc. When such substances or actions hazardously affect our environment, we call them as pollutants, and the process whereby the surroundings get adversely affected, is known as environmental pollution.

A public health engineer, who is responsible for removing all kinds of wastes of a society, is evidently responsible for cleaning the environment from such pollutants, and, thus, to ensure a healthy and wholesome surroundings, to ensure health and happiness for the people.

Environmental sanitation, which evidently means cleaning of the environment, therefore, becomes the major task of a public health engineer, and this task primarily, includes collection and disposal of refuse and sewage from houses, buildings, and other public areas, the subject which has already been dealt in the earlier chapters of this book. Provision of sufficient and wholesome air to the buildings and residents for controlling indoor air pollution, is also included as a work of environmental sanitation, and hence, usually, included in the subject of Public Health Engineering more so, because it is the wholesome air, on which depends the health of the public.

However, before we discuss design aspects governing ventilation of buildings, we shall first describe the harmful effects caused by indoor air pollution and its present status.

Sources, Effects and Status of Indoor Air Pollution

In a developing country like India, the most important source of indoor air pollution is combustion of domestic fuel (such as cow dung, wood, and crop residues) used for cooking, on which 80% (1991 census) of our population relies. It has further been estimated by Indian Council of Medical Research (ICMR) New Delhi that globally 30 lakh people die every year due to air pollution, out of which 18 lakh people die due to indoor air pollution in developing countries. In India alone, 5.89 lakh people die annually due to indoor air pollution (4.96 lakh in rural areas and 0.93 lakh in urban areas).

The indoor air pollution has infact, been found to be much worse than the outdoor air pollution, since a pollutant released indoor is thousand times more likely to reach the lungs than a pollutant released outdoors. It is generally the women and young children and infants who face the maximum adverse effects of indoor air pollution. In poor households, there is no separate kitchen, and people usually stay in the same room where they cook, or burn fuel to heat during winters. Moreover, women who work on cooking, and their young children particularly the infants, always necessarily stay at the place of cooking or burning of traditional fuel. This exposes them to continuous indoor air pollution over long hours. This proves worst for the children (up to the age of about 5 years), whose lungs are in the developing stages.

The Environment Health Scientists of California have, infact, strongly linked the indoor air pollution, with acute respiratory infections and chronic obstructive lung disease in children below five years of age. Cross country analysis by Anita Zaidi of Agha Khan Hospital in Islamabad (Pakistan) also shows a strong link between traditional biomass fuel use and infant mortality. The experts have concluded that traditional fuel emits large quantities of dangerous pollutants and are often burnt in poorly ventilated conditions. Hence they are responsible for substantial ill health in the country.

Promoting the use of cleaner fuels, improved stoves and better ventilation of homes and kitchens should therefore be given top priority to reduce indoor air pollution. Moreover, children and pregnant women, who are most susceptible to ill effects of indoor air pollutions, need to be protected on top priority from indoor air pollution.

Purpose of Ventilation

Ventilation, as stated earlier, is meant for supply of fresh air, and to replace the old hot used up (exhausted) air. The ventilation ensures the removal of bad effects of occupancy of an enclosed space

By providing necessary oxygen to remove oxygen deficit caused by respiration

By removing and diluting CO2 in the air

By lowering down the temperature by removing hot used up air and replacing it by colder fresh air

By reducing humidity and

By reducing body odours.

Extent of Ventilation Required and Ventilation Standards

In olden days, it was thought that the poisonous CO2 released by inhabitants is mainly responsible for causing pollution in houses and other public buildings. It was also thought that CO2 content increasing beyond 0.06% in the room, would cause very harmful effects. Accordingly, the ventilation standards were framed on limiting the CO2 of the used up air to 0.06%, as against the normal content of 0.04% of fresh air.

This purification standard of 0.06% of CO2 means that the air gets contaminated when its CO2 content increases from 0.04% to 0.06%. In other words, addition of 0.02% of CO2 in the air by respiration will contaminate the air, which further means that the addition of 0.02 cum of CO2 will contaminate the air of a room of 100 cum capacity.

Moreover, since an adult person releases 17 litres (0.017 cum) of CO2 per hour, we can, conclude that 0.02 cum of CO2 will be released by an adult in minutes. Hence, an adult person, if kept in a closed room of 100 cum volume, will contaminate its air in 70.6 minutes.

If we consider a room of an average size 10 ft. × 10 ft. × 10 ft. i.e. 1000 ft3 = 28 cum volume, we find that such a room of 28 cum capacity will be contaminated by a single adult in

In other words, a room of 28 cum volume will be contaminated by the presence of a single individual in 20 minutes, and hence air of this room will need to be changed at every 20 minutes i.e. 3 changes per hour will be required, with total air requirement of 3 x 28 = 84 cum per hour.

This will, however, be true only if there is no automatic ventilation through the loose door and window fittings, and also the ventilation that takes place when the doors of a room are occasionally opened. Due to such automatic ventilations, the requirement of air will get reduced. On the other hand, the requirement of air will increase if the number of inhabitants is more than 1, or if the room volume occupied per person is less than 28 cum. Moreover, this requirement of air is based upon the assumption that 0.06% CO, content will start causing discomfort to the inhabitants, needing air replacement whereas, infact, in these days, it has been established that CO2 contents upto 1% or so, can be easily withstood. Due to these reasons, the air requirement was considered much more in olden times than in modern days. The air requirement of as high a value as 50 cum per hour per person was not considered infrequent in olden days. This requirement has, nowadays, not only been toned down to about 15 30 cum/hr, but rather the entire concept of ventilation has undergone a change.

Now a days, it has been established that maximum air change is required not for keeping CO2 under control, but is to ensure proper heat dissipation and cooling of the human body, as explained below

The metabolism in all living beings produces heat, which is partly (20%) consumed in their different physical and chemical activities. The remaining 80% heat has to be dissipated through conduction, convection, and evaporation, so as to maintain the thermal equilibrium of the body.

The blood in the body carries the heat to fine capillaries near the skin, and from there, it dissipates into the atmosphere by conduction. Due to this, the temperature of the surrounding air rises, which sets up the convection currents. Fresh air comes nearby, and carries away heat. If the rate of its passage is slow, the body feels discomfort whereas if this passage is accelerated through fans, etc. more relief is secured.

During summer season, the temperature difference between the body (at 37°C) and the surrounding air, becomes very low, and the heat gradient becomes very flat, and hence the rate of conduction falls down. Mechanical ventilation then becomes most essential.

When the surrounding temperature increases even beyond the body temperature, as happens in a tropical country like India, where 40 to 45 °C temperature is quite common during summers, both conduction and convection stops functioning. The evaporation of the body sweat can only cause cooling. The rate of this evaporation also reduces when humidity is high. Hence, on a hot and humid day, body feels more discomfort. The comfort can then be increased by Increasing the air movement by using fans, and also by increasing the surface area of evaporation by removing clothings. Lungs also help in removal of heat through convection and evaporation. In hot weather, deeper and frequent breathing expels more heat.

It thus becomes evident that ventilation is mainly required to control the body heat, and not to overcome CO2 alone. The rate of ventilation required for body cooling exceeds the rate required for removing other bad effects of occupancy, such as decrease of O2, increase of odours etc.

Hence, the air changes are required and provided these days on the basis of body cooling alone, and not on the consideration of CO2.

On this consideration and in actual practice, the fresh air is supplied at the rate of 15 to 30 cum per hour per person, depending upon the type of building. When the number of occupants cannot be easily determined, the rate of air supply may be based upon the number of air changes to be provided.

Table 1 gives some common accepted standards in this regard.

On minimum side, a window area of 0.052 m2 per person should generally be provided, so as to ensure admission of atleast 28 cum (i.e. 1000 cft) of air per hour with a velocity not greater than 9 m/min.

Another recommendation is to provide about th of the floor area in the living rooms for windows. Every room should preferably be provided with atleast 2 windows, and at least one of them should face open space or a varandah. Kitchens must be provided with more window area.

Provision of deflectors (also called fan lights) of 30 cm height at the bottom or top of a window, opening inward, permits the ventilation of the room, even when windows are closed, as shown in Fig. 1. (a) and (b).

In case of slopy roofs, ridge ventilators may be provided, as shown in Fig. 2. Such ventilators are useful in taking out used vitiated air from large halls.

In hot summer months, during day time, hot outside air may be warmer than the inside room air, and the ventilators may then reverse their functions. In other words, the bottom openings may start letting out the room air, whereas the top openings may start admitting outside air to fill up the partial vacuum created thereby. The inside of room will, therefore, become worse, unless the admitted air is cooled down by some method. Khas curtains may, therefore, be hanged at the roof level ventilators to cool the air by evaporation. But during night time, when outside temperature falls, all windows and ventilators may have to be kept open for allowing them to function in a normal manner.

 

THE ENVIRONMENT AND ITS POLLUTION

Biosphere and Environment

From our knowledge of Geology, we know that the solid earth and its interior is known as lithosphere, and the gaseous layers surrounding the earth upto a distance of about 700 km, composes the atmosphere. The atmosphere is further sub divided into (i) troposphere (ii) stratosphere and (iii) ionosphere, depending upon the distance of the gaseous layers from the surface of the earth.

The entire collection of water over the earth as well as inside the earth is called the hydrosphere.

A relatively narrow belt of lithosphere and atmosphere, a little below and above the surface of the land, and in water and air, which largely contains living organisms (such as plants and animals, including human life), is called the biosphere. Biosphere is, therefore, that particular zone on earth, where the lithosphere, the hydrosphere, and the atmosphere come into contact with one another. It is, in fact, that portion on the earth, where alone, life is in existence.

There is a continuous exchange of matter between these three elements (i.e. lithosphere, hydrosphere, and atmosphere). Say for example, plants draw their food from the nutrients and moisture found in the soil layers of the lithosphere. Similarly, plants use carbon dioxide from the atmosphere, and sunlight for their growth. Dead plants and animals are decomposed by bacteria and become soil nutrients. Some of this may be dissolved by the running water and added to the hydrosphere. The evaporation of water from the hydrosphere, and subsequent condensation of water vapour in the atmosphere and consequent rainfall on the earth, provide water supply for the organisms in the biosphere.

Physical and Biological Environment

The four major elements, i.e. (i) land, (ii) water, (iii) air, and (iv) living organisms (plants and animals), together constitute what is known as environment or ecosystem.

Environment can be further sub divided into

Physical environment and

Organic or Biological environment

Land, water, and air together, infact, forms one group of environ­ment, called physical environment whereas, the living organisms, form another group of environment, called biological environment.

While the physical environment (land, water and air) is essential for existence of life in various forms the biological environment provides the necessary food, so very essential for the sustenance of man on the earth. Man, as a matter of fact, cannot survive on the earth without plant and animal life.

Ecosystem and Ecological Balance of Nature

The various living organisms constituting the biological environ­ment are, infact, dependent upon each other for their survival, and each, inturn, depends on the physical environment of the area in which it lives. Ecosystem, as stated earlier, is the physical environment together with the organisms, which live therein. Ecology is the science which deals with the inter relationships between the various organisms and their relationship with the physical environment.

Plants growing on land and in water provide food for herbivorous animals (generally smaller and weaker, plant eating organisms including elephant) and these herbivorous animals, inturn, become the food for the carnivorous animals (flesh eating animals, like lions). The relative number of these different types of organisms in the biosphere is, however, such that there is no scarcity of food for any organism. The smaller organisms are much more in number and their growth and reproduction is also much faster than those of larger organisms, which are fewer in number and reproduce slowly. This ensures the availability of sufficient food for larger animals.

In the natural environment, in the biosphere, there, infact, exists a perjeet balance or equilibrium between the various organisms, and this is known as ecological balance. In this equilibrium state, the relative numbers of different organisms in a particular environment remain constant. This ecological balance may, however, get disturbed, when changes take place in the natural environment, which may consequently change the relative numbers of the different organisms in the biosphere. If the numerical ratio balance between the different organisms is disturbed, then naturally, there becomes a dearth of food for certain particular organisms, which may ultimately lead to large scale mortality of those particular organisms. A new equilibrium is finally re established under the changed conditions. In this period of readjustment, evidently, certain old species get extinct, and new species may be born. This process of evolution of new species and extinction of old species is a continuous process.

The present existing pattern of organisms in the biosphere has been reached as a result of gradual evolution and extinction over several million years of earths history. During the changing physical environment, various organisms adapted themselves, and survived but still however, various particular species which could not tolerate the changing environment, died out, and became extinct. New species were also born in the new changing environment. Man, infact, came into existence, as a result of large scale environmental changes that took place about one to two million years ago.

Impact of Man on Biosphere

After the arrival and reproduction of man on the earth, a large scale impact has been caused on the biosphere, due to his unchecked actions. Say for example, large scale deforestation of forests for residential and agricultural land uses has changed the habitat of organisms living in the forests. His hunting of animals, has led to the extinction of certain animal species. He has also developed new types of domesticated animals as well as plants to serve his own needs. His using pesticides and insecticides in the agriculture farms have also affected the relative proportion of various organisms in the biosphere. Such continuing imbalances in the biosphere, if not checked, will certainly prove disastrous to the very existence of man himself, because it is the biosphere, on which depends, the life as well as the progress of human civilization.

Pollution and Conservation of Environment

The development of civilization and rapid industrialization by man has caused a great damage to the ecosystem. Things have worsened because no attention or a very little attention has been paid towards protecting the environment, while executing industries and other developmental projects.

Associated with any development, there is bound to be some amount of environmental degradation. An ecological survey and effective measures for protecting the environment, are therefore, essentially required, before any developmental project is undertaken. But unfortunately, while developing any industry or commercial or even urban properties, we have not bothered to look at the environmental degradation, likely to be caused by those establishments, either through our ignorance or through our sheer greed, for not spending any money on things which do not immediately affect us, individually. There are infact, people among us who believe that there is more money in destroying the environment (such as those who felt trees, and kill wild animals, unauthorized), rather than in conserving it. Tomorrow is not their immediate concern.

Several examples of mans interference with the natural equi­librium may be mentioned. The excessive use of coal, petroleum, and natural gas for industries, automobiles, and power generation, has created enormous problems of pollution of environment.

Pollution of air is strikingly marked in the industrial and congested cities of U.S., Europe, Japan, and even in India.

The smokes from factories, coke ovens and furnaces, steam engines, etc. exhaust fumes from automobiles, power plants, etc., injurious chemical fumes from oil refineries, zinc refineries, chemical industries, metallurgical plants, iron and steel plants, incineration plants, etc. evolution of radioactive gases and suspended radiactive dusts from atomic explosions and accidental discharges from nuclear reactors, etc. have polluted the air to such an extent that special steps are now required at several places for reducing such air pollutions.

The increase in CO2 content of the environment has been respon­sible for gradual heating up of our globe, by a process called green­house effect. The CO2 layer, infact, acts like a glass cover used for a green house, which allows the outside heat to enter the green house, but does not allow the inside heat to go out. Similar to a glass house, CO2 layer is transparent to short wave radiation from the sun, but absorbs the longer wave radiation from the earth. The net result is , gradual heating up of the earth. It is feared that by the year 2100, CO2 in our atmosphere will be doubled, leading to 40C rise in the world temperature. Such continuous warming of earth may cause the glaciers to recede and ice to melt at poles. This may cause a rise in sea level by about 0.65 m. which ultimately may submerge most of our islands and coastal cities.

The salty sea water spreading to land may also lead to infertility of soil and spoil the underground water.

Pollution of water is another aspect of environmental pollution. The waters of rivers, lakes, and oceans are, now a days being polluted on a large scale, by the outflow of effluents from factories and industries. Water pollution also results from the disposal of solid urban wastes, such as plastics, rubbers, paper, untreated wastewaters and sewage, etc. Water pollution also interferes with the growth of organisms living in the water bodies, thus retarding the natural purification process caused by such organisms.

All such environmental pollutions, lead to the spread of diseases through polluted air and water. The use of insecticides and pesticides has led to the concentration of DDT and other harmful chemicals in plants and animal food products, which when consumed by man, cause diseases.

In order to check such drastic ill effects of environmental pollu­tion on the health of the people as well as the plants and the animals, it is extremely necessary that we wake up from our long slumber, and initiate some drastic remedial measures to protect our natural wholesome environment. Legislative laws and their effective implementation is most important to stop the greedy and/or ignorant people from spoiling our environment.

Status of Administrative Control on Environment in India

In our country, no attention was paid for controlling the environ­mental effects of developmental projects, almost till the year 1968 or so. It was in the 4th five year plan period (1968 1973), when, for the first time, environmental aspects were introduced for harmonious development.

In the year 1970, the Government of India, appointed a committee under the chairmanship of Pandit Pitamber Pant (a member of the planning commission), which prepared a country report for presenting in the U.N. Conference on Human Environment, in 1972. Soon thereafter, a National Committee on Environmental Protection and Coordination was set up in the Deptt. of Science and Technology, for advising the G.o.I. on environmental matters.

The 5th five year plan (1973 1977) continued to stress upon the environmental considerations. During this plan period, a central law was enacted, under the name of Water (Prevention and Control of Pollution) Act, 1974. This law was meant for checking and preventing water pollution, which had become quite prominent by that time, at several places in the country.

As a result of this legislative enactment of 1974, a central board, called Central Board for the Prevention and Control of Water Pollu­tion, was constituted for monitoring and detecting pollutions of water bodies, and for initiating remedial measures, including prosecutions in courts, so as to prevent the influential industrialists and municipalities from throwing their industrial wastes and sewages into our precious water bodies.

This act of 1974 also stipulated that the various States of India will separately constitute such pollution control boards in their respective States, and the central board will serve as a watchdog body to advice and watch the performance of State boards, besides exercising its separate authority in respect of union territories, which are directly governed by the centre.

Another important act, called the Water (Prevention and Control of Pollution) Cess Act, 1977, was also passed by the parliament, which has proved quite effective in reducing the quantities of industrial wastes, as the act promotes recycling and reuse of the wastes.

Protection of environment was further stressed in the 6th five year plan (1980 1985), which contained a separate chapter on Environment and Development. During this plan period, a separate Department of Environment was set up on November 1, 1981, at the level of central cabinet. Air pollution was also recognised, and a central legislation, called Air (Prevention and Control of Pollution) Act, 1981, was enacted. The water pollution control boards were given the additional charge of looking after air pollution control also.

A third act, called The Environment (Protection) Act, 1986 has also been promulgated by the Parliament after the occurrence of the Bhopal gas tragedy. This act extends to whole of India, and central Government has been empowered for taking any measures, which in its opinion are necessary for improving and protecting the environment. Under section 9 of this act, it has also been made obligatory on the part of industries to prevent and mitigate environmental pollution, which may be caused due to any accident or unforseen act at their industry. Under section 10 and 11 of this act, Government officers, empowered by the Central Government, may now enter the premises of the industries and collect samples, and/or carry out any reasonable action deemed fit for environmental protection.

Severe fines and penalties, including imprisonments upto 5 years have been prescribed for failures and continued failures on the part of industries failing to comply with the act, under section 5 of the act.

Moreover, in order to check the emitted smokes from badly maintained automobiles, containing too much of lead, carbon monoxide and paniculate matter, a fourth legislation, called Motor Vehicles Act, 1988 has been passed by the parliament. The implementation of exhaust standards framed under Central Motor Vehicles Rules 1989, was tol come into force w.e.f. 1 7 1989. However, due to non availability of monitoring equipments, like smoke meters and gas analysers, for checking the quality of exhaust emissions, and also due to political and other influences, this act largely remains on the statue books of the country.

Even after five years of the enactment of this legislation, nothing serious is being done to check and prevent the polluting vehicles, barring a few pollution checks carried out on private cars, in Delhi. The authorities are thus, simply ralaxing, blaming interference from the politicians and the courts.

In order to prevent loopholes in the effective implementation of these environmental laws, the Government is now thinking to constitute special environmental courts, for speedy trial of the offenders of anti pollution laws and also to carry out annual environmental audits, to easily detect disobedience of such laws by individual industries. The earlier it is done the better.

 

 

DISPOSAL OF ENVIRONMENTALLY HAZARDOUS RADIOACTIVE EFFLUENTS AND BIOMEDICAL WASTES

RADIOACTIVE WASTES

Radioactive Elements and Radioactive Radiations

Radioactive elements like thorium, strontium, iodine, plutonium, phosphorous, carbon, manganese, radium, cobalt, zinc, etc. existing as radioactive isotopes of different designation numbers, when present in the natural environment, disintegrate, releasing ionised radiations or rays. These emitted radiations are very powerful and contain enormous energy. These rays are even capable of penetrating through thick steel sheets.

The radiactive disintegration may be defined as the spontaneous breakup of the nucleus of an atom of a radioactive element or its isotope. The breakup of nucleus itself is important here, as compared to that of the entire atom or the molecule in other types of hazardous wastes. In dimensions of radioactivity, the nucleus of a radioactive atom itself is too large, making it unstable. This unstable nucleus breaks up, and the atom changes to another one, which is lower in mass. The radiations or the radioactivity generated due to such breakup of the nucleus of the radioactive atom, which is found to be extremely hazardous to life, consists of (i) alpha particles (ii) beta particles and (iii) gamma radiations, as discussed below

Alpha Radiations. The release of alpha radiations from the nucleus of a radioactive element is equivalent to the release of positively charged helium nucleus, consisting of two neutrons and two protons. This release causes the parent atom to loose four atomic mass, since an atomic mass (also called atomic mass number) is the sum of the number of protons and the number of neutrons in the nucleus. The loss caused in atomic number in such a break up will be equal to 2, because atomic number is equal to the number of protons in the nucleus.

Disintegration of Uranium 238 to release alpha particles, given by eq. (1) can be quoted as an example of this type of breakup of radiactive element, which is explained below

When U 238 releases the alpha particles, the atomic number decreases by 2, and atomic mass (or atomic mass number) decreases by 4. The atomic number of Uranium is 92. From the periodic table (given at the end of the book as Appendix Table A 8), the atom with an atomic number of 90 is Thorium (Th). Hence, the equation is written as

            Alpha particles released in the above equation, are slow moving ionising particles, possessing weak penetration power, and can be stopped even by 8 cm of air or by thin sheets. They are deflected by electric and magnetic fields, but are strongly ionising with a doubly ionised helium nucleus, as brought out above.

Beta Radiations. Beta radiations are the release of electrons from a radioactive nucleus, caused by the breakup of a neutron into a proton and an electron. Due to this break up of one neutron, the nucleus will have one increased proton, and hence one increased positive charge or one increase in its atomic number. Since the mass of the released electron is very small, the reaction will not change the atomic mass, which equals to the number of protons and number of neutrons (here the total number of neutrons and protons remains unchanged, since one neutron is changed into one proton with the release of one electron). However, due to change in charge (increased charge), the parent atom changes to another atom of higher atomic number. Being an electron, beta radiation has a negative charge.

Beta particles are hence defined as high velocity electrons, which can penetrate thin aluminium sheets, and are strongly deflected by electric and magnetic fields.

Gamma Radiations. Gamma radiation or gamma ray, as it is also called, has no charge or mass, being simply an electromagnetic radiation that travels at the speed of light. Being an electromagnetic radiation, gamma radiation may be thought of as a wave or a photon having very short wave length in the range of 103 to 10 7 mm.

When an electron in an atom moves from a higher energy level (excited state) to the lower stable level, energy is radiated. An analogous phenomenon also occurs in a nuclear reaction a nucleus in an excited state releases the gamma rays when it transforms into a more stable lower form. Gamma radiation may accompany either alpha or beta radiation. X rays are also a sort of gamma rays.

Gamma rays are hence defined as very high frequency photons, which can penetrate several cm thick lead, and are not deflected by magnetic fields.

Nuclear Fission. When an orbital electron goes from a higher unstable energy level to a lower stable energy level, energy gets liberated. This is what happens in a nuclear reaction, where a nucleus, Fissions to transform to a lower stable form, releasing energy, which is tapped.

Since a fission is a breakup of the nucleus, at least two fission fragments shall form in the process. In 1 out of 10,000 fissions, three fission fragments are formed instead of two.

Consider a fission reaction using Uranium 235 (U 235). In the process, the Uranium atom captures a neutron, making the nucleus unstable, causing break up. Since the atomic mass of a neutron is 1, the sum of the atomic masses of the product species must be 235 + 1 = 236, as illustrated below

Mo and La are the two fission fragments. The total of atomic masses on reactants and products of reaction, both totalling 236.

Since they are too small to emit alpha particles, fission fragments are beta or gamma emitters only. Reaction (17.3) yields 204 MeV of energy.

Where three fission fragments are formed (1 in 10,000 cases), the third fragment is tritium (H 3). Since it is a chemical form of hydrogen, it exchanges freely with the non radioactive form of hydrogen in the cooling water used in nuclear reactors. Containment of this fragment is, thus, difficult. Tritium (H3) has a long life of 12.3 years.

Table 1. lists the fission fragments that can be produced in the fission reactor. Some of the products are very harmful to life. Say for example, Cs 137 concentrates in muscles, Sr 90 concentrates in bones, and 1 131 concentrates in thyroid gland. The safe disposal of radioactive wastes is therefore of paramount importance and is hence discussed below in brief.

Impacts of Radioactivity on Life and Environment

When human body is subjected to the ionised radiations, large scale hazards, like cancers, shortening of life span, deformations, and genetic changes may occur, depending upon the quantum of radiation/ exposure although, however, smaller and calculated doses of such radiations are used on human body for medical diagonosis through X rays, etc. for detecting the mal functioning of different organs of the body.

The quantum of radiation, which determines the time of exposure, is generally measured in several units.

The most commonly used unit of measuring radiation dose is rad. 1 millirad is equal to 103 × rad, and one kilo rad is 1000 rad. 100 rad is represented by another latest unit, called gray (Gy). Numerically 1 rad also equals the old unit of 1 rem (roentogen). Roentogen is represented by R or r. All these units depend directly or indirectly on the capability of the radioactive materials to cause ionisation in their environment. All the basic units used in radiation are given in Appendix Table A 7 at the end of the book.

Inorder to ensure that such radiations do not cause harmful effects on the body, it is necessary that the quantum of these radiations, to which the body is subjected to, be restricted to safe standards. International Commission on Radiological Protection, by their research and past experience on the use of X rays and radium, has stipulated the limiting radiation exposures per year per normal person.

These standard values of annual radiation exposure are given below in table. 2.

Larger concentrations of radio activity not only proves dangerous to humans, but also to plants, animals, and birds. It therefore, becomes imperative for man to watch and control the presence of such dangerous radiations in the environment.

Fortunately, however, radio activity emissions in natural environment have not been detected anywhere, going above or even nearer to the permissible limits, except at places where atomic weapons are exploded, or where some accidental discharges eminate from nuclear reactors/ stations.

The Chernobyl atomic accidental discharge in Russia, which occurred on April 26, 1986, is well known to all of us. This accidental nuclear discharge had not only killed around 30 people, but had scattered radioactive fallout over a vast area, the large scale genetic effects of which, are yet to come up. Its radioactive fallouts have been detected far and wide, even over a vast area of Europe. Inspite of several measures taken to avoid the long range genetic effects of radioactivity in these areas, and more than 50 lakh people being constantly undergoing regular medical checkups, the crisis of confidence is, growing up, as birth defects in farm animals and increased ill health among children and villagers, are being reported.

In a British Medical Research Council (BMRC) study, it has been concluded that two atomic isotopes, named Strontium 90 and Cesium 137 are generally produced from fallouts from nuclear weapon testings and nuclear power stations. Strontium 90, like calcium, gets incorporated into the bones, and as such, the milk produced by animals into whose bodies strontium 90 has been concentrated, may become radioactively contaminated.

Iodine 131 is another highly dangerous isotope. This element can enter the food chain at any level, and become concentrated in human body, consuming such foods. Once within the body, it can damage white blood cells, bone marrow, spleen lymph nodes, and can cause lung tumors, skin cancers, sterilities, defective eye sights, etc.

These three isotopes have, thus, been singled out for contaminat­ing the pasture milk and air, and thereby posing radioactive threats to life.

In the above British M.R.C. study, a dose called Emergency Reference Level (E.R.L.) has also been defined, as the concentration of radioactivity, which different parts of the body can tolerate, varying from 10 to 60 rads. This reference level is a guide to initiate counter measures like evacuation, etc., as and when the radioactivity in a particular area is detected to be more than the reference level. Other counter measures, like giving anti atomic drugs to remove the bad effects of radioactivity may also sometimes be adopted say for example, contamination of Iodine 131 may be removed by treating the affected people with ordinary iodine tablets.

Fortunately, radioactivity released in the natural environment, under normal conditions, known as the back ground radiation, is found to be hardly 100 200 milli rads, a value much lower than the tolerance limits. However, in case of accidental nuclear discharges, anti radioactivity measures may have to be initiated, as pointed out above.

Disposal of Nuclear Wastes in India. The position of disposal of nuclear wastes in India is, though not very acute at present, but is liable to become so in future, as our nuclear programme gets momentum. We have three sources of nuclear wastes i.e. (i) the nuclear mines (ii) the nuclear power plants and (iii) the nuclear research reactors.

Our nuclear mines for extracting uranium ore are located at Jaduguda village in Bihar state. Once the uranium ore is extracted at these mines it is sent to a facility centre in Hyderabad for processing to secure uranium. Earlier, the waste from the processing plant was being dumped in a nearby pond, but since it

 

AIR POLLUTION, ITS CONTROL AND MONITORING

 

Air Pollutants, Their Effects, and Sources of Origin

Air, as we all know, is most essential for life. It has been established that man can hardly survive for 5 minutes without air although, however, he can survive for 5 days without water, and for 5 weeks without food.

This life supporting important natural element sometimes be­comes our bitter enemy, as and when it gets polluted, since it causes a number of diseases in our body. The harmful effects caused to human body by the polluted air depends upon the type and concentration of the pollutants present in the air.

The polluted air, not only is harmful to man, but is also harmful to all types of life, including plants, animals, and birds. It is also harmful to non living materials, like metals, marbles and other stones, woods, paints, papers, etc. which get spoiled by the contact with polluted air, either due to the mere physical corrosive action of polluted air, or/and due to the chemical attack of the pollutants on such materials.

The atmosphere, as a matter of fact, contains hundreds of air pollutants from natural or from anthropogenic sources. All such pollutants are called primary pollutants.

Certain less important primary pollutants are H2S, H2F and other fluorides, methyl and ethyl mercaptans, etc., which are usually rarely found in our general atmosphere, although if present, may prove quite harmful.

These primary pollutants often react with one another or with water vapour, aided and abetted by the sunlight, to form entirely a new set of pollutants, called secondary pollutants. These secondary pollutants are the chemical substances, which are produced from the chemical reactions of natural or anthropogenic pollutants or due to their oxidation, etc., caused by the energy of the sun. These new pollutants are often more harmful than the original basic chemicals that produce them.

H2SO4 is formed by the simple chemical reaction between SO2 and H2O vapour, and is a much more toxic pollutant than SO2, having far reaching effects on environment, since it causes acid rains.

Other secondary pollutants like ozone, formaldehyde, PNA, etc. are formed by photochemical reactions, caused by sun light between two primary pollutants. Say for example, O3 is formed due to photochemical reaction between hydrocarbons (HC) and nitrogen oxide (NO). Similarly, aldehydes may be formed by photochemical oxidation of hydrocarbons in the atmosphere.

All these major air pollutants are now discussed below in details, mentioning their hazardous effects on human body, and alongwith their sources of origin.

Sulphur Dioxide. Sulphur dioxide is an irritant gas, and when inhaled, affects our mucous membranes. It increases the breathing rate and causes oxygen deficits in the body, leading to bronchial spasms in some of the affected persons. Patients of asthma are very badly affected by this pollutant.

Some quantity of atmospheric sulphur dioxide (SO2) may oxidise to form sulphur trioxide (SO3), which when inhaled, may dissolve in the body fluids to form sulphuric acid (H2SO4), which is a very strong corrosive acid. SO3, thus, causes high and worse irritation even at lower concentrations, leading to severe bronchospasm.

SO2 is also responsible for causing acidity in fogs, smokes and in rains, and hence is the major source of corrosion of buildings and metal objects.

SO2, mainly originates in the atmospheric air from the refineries and chemical plants, smelting operations, and burning of fuels. Thermal power plants may emit SO2 quantities, as high as 1/10th of the coal burnt by them. Open burning of garbage as well as municipal incineration plants may also emit sulphur dioxide in the air.

The specified standard for SO2 under U.S. Ambient Air quality standards is 80 mg/m3 which approximates to 0.03 ppm, at 20°C.

Carbon Monoxide. Carbon monoxide possesses about 200 times affinity for blood hemoglobin (Hb,) than oxygen. Eventually, when inhaled, CO replaces O2 from the hemoglobin, and form what is known as carboxy hemoglobin (CO.Hb,).This carboxy hemoglobin is of no use for respiratory purposes, and hence when about half of the hemoglobin of the blood is used up in forming carboxy hemoglobin, death becomes a certainty.

Persons dying of carbon monoxide inhalations exhibit characteristic bright pink colour of the flesh due to the presence of pink coloured carboxy hemoglobin in their bloods.

Carbon monoxide also affects the central nervous system, and is even responsible for heart attacks, and high mortality rates.

Carbon monoxide chiefly originates from automobile exhausts, and is caused by incomplete combustion of organic matter.

In cities, it is found in as high concentrations as about 60 mg/m3 (54 ppm) with still higher concentrations in tunnels, garrages, and near the road intersections and running automobiles.

The specified standard for CO under US Ambient Air Standards is 10 mg/m3 (9 ppm).

Oxides of Nitrogen. Out of seven known varieties of oxides of nitrogen, only nitric oxide (NO) and nitrogen dioxide (NO2) are found to be somewhat injurious to human health. NO2 is considered to be more injurious than NO.

Eye and nasal irritations are the common problems caused by about 28 mg/m3 (15 ppm) of NO2 and respiratory discomfort may occur even with brief exposure, when its concentration rises to about 47 mg/m5 (25 ppm).

Many deaths are reported to have occurred in a fire in a clinic in Clevland in U.S.A. in May 1929, due to evolution of NO2 from the burning of X ray films.

The NO2 mainly originates into the atmosphere from automobile exhausts, incineration plant, furnace smokes, etc., as it is caused by the combustion of organic matter.

The specified standard for NO2, under U.S. Ambient Air Quality Standards, is 100 ug/m3 (0.05 ppm).

Hydrogen Sulphide. Hydrogen sulphide, as is well known to all of us, is a foul smelling gas with a typical odour of rotten eggs. Exposure to this gas for short periods may lead to loss of smell sense. This gas may also cause headaches, conjunctivitis, sleeplessness, and pain in the eyes. Its higher concentrations may block oxygen transfer, and damage the nerve tissues.

H2S gas, however, is generally not found in any trouble some concentrations in our general atmosphere, mainly because, it is not emitted in automobile exhausts, since it gets burnt to SO2. H2S is, therefore, not included in the Ambient air quality Standards.

Still, however, this gas is produced in industries, like oil refining, rubber, tanneries, plants manufacturing sulphur dyes, and artificial silk by the viscose process, etc., and may, therefore, pose a threat to the people working in those particular plants, and in the nearby areas, due to chances of its accidental leakages.

Methyl and Ethyl Mercaptans. These are other compounds of sulphur, which may be of interest to us in air pollution, because of their strong odours. These compounds, however, are generally neither found in our general environment, nor are they harmful to us. As a matter of fact, they are added to domestic and industrial gas supplies, so as to detect gas leakages due to their pungent smells.

Hydrogen Fluoride and other Fluorides. Fluorides, present in air may range from those which are extremely irritant and corrosive like H2F, to relatively non reactive compounds. Their smaller concentrations may produce fluorosis in cattle and plants. They are, however, less harmful to human beings.

They are emitted into the atmosphere by aluminium plants, steel plants, phosphate fertiliser plants, etc. They are also produced by burning of coal. H2F is also used in refining and in some chemical industries, and hence its localised threats near such plants, always persist.

Fortunately, H2F concentrations in city air are generally found to be much lower, and do not pose any problems. Its max. concentration found in most cities is around 0.025 ppm. is, therefore, not included in the ambient air quality standards.

Lead (Pb). Lead is mainly injected into the atmosphere through the exhausts of automobiles, particularly, by automobiles running on petrol. Normally, inorganic lead has been detected in urban areas in concentrations of about 0 2 ug/m3, with higher values in areas of heavy traffic. The concentrations of lead in inhaled air, may cause irritation of mucous membranes of nose, throat and lungs. Lead poisoning may also cause damage to gastrointestinal tracts, liver and kidney. It may also cause abnormalities in pregnancy and fertility. Lead poisoning is also found to be responsible for retarding mental growth in children.

The specified standard for lead, under U.S. Ambient Air Quality Standards, is 1.5 ug/m3.

Hydrocarbons (HC). Hydrocarbons, as we all know, are compounds containing only hydrogen and carbon. The hydrocarbons are generally divided into two categories, i.e.

      Aliphatic group of hydrocarbons and

      Aromatic group of hydrocarbons.

The aliphatic hydrocarbons include alkanes (methanes), alkenes (olefins), and alkynes. Out of these three varieties, the alkenes (olefins) have been found to be unsaturated and highly reactive in atmosphere through photochemical reactions. Alkanes (methanes) are simply inert hydrocarbons, and do not react photo chemically. The third variety, i.e. alkynes, though quite reactive, is generally not found present in the atmosphere, and hence is of no importance to us in the air pollution studies.

Due to this photochemical reactionary property of the alkenes (olefin) type of hydrocarbons, it has become very important to study and monitor the presence of such hydrocarbons in the air.

The presence of hydrocarbons in the city air, is therefore attract­ing a lot of attention, because the olefin type of hydrocarbons react with other pollutant gases, forming new pollutants, which are even more harmful than the individual original pollutants.

Hydrocarbons are chiefly released into the atmosphere by auto­mobile exhausts. Paraffins and olefins have been found to be chief hydrocarbons found present in large quantities in the air of Los Angeles city of U.S.A., where heavy number of automobiles ply every day. Hydrocarbons are also released into the atmosphere by smokes of incinerators, through fumes of oil refineries, and also by evaporation of gasoline at service stations.

Benzene and its related aromatic hydrocarbons are also extensively used for solvent extraction purposes, and many of these hydrocarbons are found to cause body cancers.

Aldehydes and ketones may also be considered under hydrocar bons, because they may also be formed by the oxidation of hydrocar bons in the atmosphere, although they may primarily be released by automobiles and incinerators along with hydrocarbons. Substances like formaldehyde cause irritation of eyes, skins and lungs, and hence, may be quite injurious to health.

The presence of hydrocarbons in the environment may prove to be quite hazardous, and hence needs to be properly monitored.

The specified standard for non methane types of hydrocarbons under U.S. Ambient Air Quality Standards is 160 ug/m3 (0.24 ppm).

Carcinogenic Substances. The poly nuclear group of aromatic hydrocarbons is considered to be quite important to air pollution studies, because many of these compounds have been shown to be carcinogenic (likely to produce cancers). Increase in lung cancers in cities have been blamed on these hydrocarbons, caused by automobile exhaust emissions. Out of many varieties of such hydrocarbons, Benzo (a) pyrene has been found to be the most carcinogenic hydrocarbon, followed by Benzo (e) acephenanthrylene, and Benzo (j) flouroranthene. Various other varieties, with less carcinogenic properties are also known.

The Ambient Air Quality Standards adopted by Environment Protection Agency (EPA) of USA, does not specify any particular standard for these hydrocarbons. However, attempts are being made to reduce exposer to these benzene compounds in industries from the present level of 10 ppm (30 mg/m3) to 1 ppm (3 mg/m3).

Insecticides and Pesticides. Insecticides, like DDT are not only harmful for insects, for killing of which they are generally used, but are also harmful to man. The indoor spraying of DDT causes its concentrations in domestic pets as well as in humans. So much so, that DDT has even been found in the milk of mothers. The presence and absorption of DDT may injuriously affect our central nervous system, and may attack other organs of the body. For these reasons, the use of DDT has, infact, been banned in U.S.A.

Similarly, Pesticides are also quite harmful to human health. Their growing use in agricultural farming is not without threat, because, if such substances are absorbed by the expectant mothers, they may cause premature labour, and even abortions.

The recent Bhopal Gas Leakage Tragedy from the factory of Union Carbide of India, manufacturing pesticides is a pointer to the threats of spontaneous serious air pollutions that may result from such industries, which utilise or produce poisonous gases in their manufacturing processes. Insecticides and pesticides do come under such industries and hence, liable to pose such serious threats to life, even in future.

Allergic Agents. Various microscopic substances, contained in air, may cause allergic reactions in sensitive human bodies. Such substances are called aero allergens.

The reactions to such substances occur in our body mainly in the skin and the respiratory tract. Sneezing is one symptom of allergy, followed by skin troubles and/or bronchitis, asthma, etc.

Such organic allergens have been largely found to originate from living things like plants, animals, etc., although however, finely powdered industrial materials may also sometimes cause allergic reactions in sensitive persons.

Pollen grains and fungal spores from local plants are one of the worst allergens. About 20 micro metre in diameter, ragweed pollen is usually deposited within 90 m of the parent plant, and hayfever and asthma sufferers coming within that range are liable to suffer severe allergic reactions and asthma attacks.

The high degree of allergy caused by pollens and spores, originat­ing from the local plants, makes our beautiful city of Bangalore, a dreaded place for asthmatics and allergic persons. Instances have even been cited, where not only the asthmatics have had asthma attacks on them in Bangalore, but even normal persons who had never exhibited asthma tendencies earlier, got attacks of asthma in this city of fine climate. The Asthma Research Society has infact, identified 75 types of air borne pollens and 120 types of spores here among which the pollen of parthenium plant was found to be the highest (41%), followed by grass pollen (28.8%), and pollens of cassia species (11.8%).

Besides the pollens and spores, other important allergens are animal hairs, furs, feathers, dusts, spices, cotton flakes, flour, tobacco etc. besides certain chemical compounds. Say for example, SO2 and compounds of cabalt and beryilium are found to cause allergic reactions in certain sensitive persons. Similarly, commercial fur dye paraphenylenediamine is an allergic agent, capable of causing dermatitis in addition to bronchial asthma. Factories processing the castor beans for oil extraction, release a powdery material in to the air, which is a strong allergen.

Radioactive Isotopes. The evil effects of radioactive substances have been thoroughly discussed under article 16.3. But still, however, we would like to repeat here that the three isotopes, namely strontium  90, Cesium 137, and Iodine 131 have been singled out as the main products of atomic explosions and accidental discharges from atomic and nuclear reactors although, however, other isotopes may also be present.

The serious health hazards caused by such radioactive emissions are anemia, cancers, shortening of life spans, and above all, the genetic effects, like sterility, embroyo defects, congenital malforma­tions, etc. Radioactivity is notorious for its delayed and long term evil effects on human health.

Ozone. The presence of ozone gas in the air may cause irritation in the respiratory tract, reaching much deeper into the lungs than the oxides of sulphur.

The origin of this gas into the atmosphere is probably caused indirectly, by the process of photochemical air pollution. In this process, two pollutants unite together in the presence of sun light, producing a third pollutant.

Since ozone has been generally found to occur in the highly motorised areas, particularly during day time, it is believed that it is produced by the photochemical reaction of hydrocarbons and nitrogen oxide.

Possibility of formation of such photochemical smogs is quite high in places where number of plying automobiles is too high, and where inversion smog conditions prevail in the atmosphere. Los Angeles city in USA is facing such problem of photochemical smog, which also causes intense eye irritation, unusual odour, and reduction in the atmospheric visibility.

Delhi city is also becoming prone to such problems, and Delhis atmosphere needs to be keenly and immediately watched by the authorities, before it becomes too late and irreversible.

VEHICULAR AIR POLLUTION AND MEASURES FOR ITS CONTROL

The population of vehicles in India particularly that of mopeds, scooters and motorcycles, has been growing at a fast rate, in the recent past. At present, India is the second largest producer of two wheelers, after Japan, and the production of these vehicles will almost double to 2 million vehicles/year in the next five years. During the same period, the production of cars and diesel vehicles is estimated to increase by 60 70% fo 135,000 and 190,000 vehicles/year respectively.

According to an estimate the projected population of different types of vehicles is shown in Fig. l. One sees a totally different picture of vehicle mix in India compared to US and European countries. The two wheelers far outnumber the cars and diesel vehicles unlike developed countries. The Directorate General of Technical Department2 projects a still higher growth in the population of two wheelers. The pattern of fuel consumption is also very different in India as compared to US and Europe. First scooters, motorcycles, etc. consume more than 50 per cent of total gasoline at present and their share is expected to grow still further. Secondly, the consumption of diesel fuels is about seven times higher than that of gasoline.

A high growth in vehicle population brings in its wake urban air pollution problems unless timely and appropriate steps to control vehicle emissions are undertaken. Although the present population of vehicles in Indian cities is much smaller than in US and Europe, the existing vehicle types and engine designs, coupled with their older age and congested and slow moving traffic, perhaps has aggravated the situation such that the signs of vehicular air pollution are already apparent in the large cities in India.

The emission control measures help to a significant extent in the development of more efficient engines and also in their efficient operation. In this paper, emission characteristics of the present Indian vehicles are given. The type of emission control technology required and its effects on vehicle fuel economy are also discussed.

Types of Vehicle Emissions

It is well known that the gasoline vehicles contribute mostly to carbon monoxide, un burned hydrocarbons (fuel), nitrogen oxides and lead participates to the atmosphere. The diesel vehicles are the primary culprits in emitting black smoke, in addition to the gaseous emissions mentioned above. The gaseous emissions pose health hazards by themselves as well as generate hazardous secondary pollutants, e.g. ozone, peroxyacyl and aryl nitrates through reactions in the atmosphere. On the other hand, the diesel smoke is considered largely a nuisance and causes poor visibility for the traffic following the offending diesel vehicle. The diesel exhaust is also malodorous due to presence of traces of aldehydes and other oxygenated hydrocarbons. Depending upon the sulphur content of diesel fuels, the diesel vehicles also emit varying amount of sulphur dioxide.1 into the atmosphere.

Emission Characteristics of Indian Vehicles

The initial emission control measures have generally been directed towards carbon monoxide from gasoline vehicles and black smoke from diesel vehicles. This has been followed by the control of unburned hydrocarbons, nitrogen oxides and lead removal from gasoline. In view of the above, a vehicle emission survey comprising of idle carbon monoxide emissions from gasoline vehicles and free acceleration smoke from diesel vehicle was conducted in Delhi for the Department of Environment, Government of India3. The main findings of the survey are given below.

Idle CO emissions from gasoline vehicles   The carbon monoxide emissions from about 600 inservice vehicles were measured and the results are shown in Fig. 2. The passenger cars, generally, gave higher idle CO emissions than the two and three wheelers.

The two and three wheelers in India are mostly powered by crankcase scavenged two sroke engines. An inherent design feature of these engines is short circuiting of fresh air fuel mixture during cylinder scavenging process. The lower volume concentration of CO in the exhaust gases of two and three wheelers compared to cars is largely due to dilution of combustion products by the short circuited fresh fuel air mixture. However, the fresh mixture short circuiting results in very high concentration of un burned hydrocarbons of up to 45,000 ppmc in the exhaust gases of 2 stroke engines.

The proposed Indian Standards for CO emissions from gasoline vehicles and the percentage of vehicles surveyed which meet these standards are shown in Table 1.

Free acceleration diesel smoke The smoke emissions from buses, trucks and minibuses are shown in Fig. 3. Again more than 600 vehicles of all categories were tested. The minibuses and trucks were observed to be excessively high emitters of black smoke. The proposed Indian Standards specify a limit of 65HSU (Hartridge Smoke Unit) under free acceleration. The srrioke limits ranging from 40 to 60 HSU have been in force in several European countries. Table 2 gives the percentage of the vehicles surveyed which meet the proposed IS Smoke limit. Excessively high smoke from the trucks and minibuses resulted largely from over fuelling and poor mechanical condition of the engines of these vehicles.

Mass emission data   The idle CO and free acceleration smoke serve as good indicators of the emission potential of the gasoline and diesel vehicles respectively as far as these two pollutants are concerned. However, the mass of different pollutants emitted by the vehicle is the most accurate measure of its pollution characteristics. Using US EPA (Environmental Protection Agency) and ECE (Economic Commission for Europe) driving cycles, which simulate the typical urban driving schedules in these countries, the mass emission data from scooters, motorcycles, mopeds, cars and diesel engines were measured3. Table 3 shows the average mass emission characteristics of Indian vehicles.

The control technology required depends on the type and design of engine/vehicle, the pollutant to be controlled and the extent of reduction required. Moderate to large reductions in the vehicle emissions have been possible by those technologies which also result in the improvement of vehicle fuel economy. Stringent controls and more so relating to the nitrogen oxides, require the technology which may result in the fuel economy loss. In the following sections, the emission control technology which may be required initially in India and their effect on the vehicle fuel economy are discussed.

Four stroke gasoline engine vehicles passenger cars   The first exhaust emission standards in US and uptill now in Europe have been met primarily through the following techniques.

Leaner mixture operation, improved design and calibration of carburettors,  idle mixture adjustments and faster idle speed, improved spark timing control and use of electronic ignition systems and reduction in quench zones in combustion chamber, etc.

In Europe, 50 to 60 per cent reduction in CO and HC and about 20 per cent reduction in NOX have resulted through the above improvements. The design modifications in intake manifolding to improve mixture preparation and reduce maldistribution amongst different engine cylinders and use of high turbulence combustion chamber have made possible to run the engines much leaner than before.

The advantages of lean mixture operation become obvious from Fig. 4 which shows the effect of air fuel ratio on the emissions. Lean mixture operation results in Substantial reductions in CO and HC emissions. NOx emissions however, peak at about 5 to 10 per cent leaner than the stoichiometric mixture. The typical variation of fuel consumption with air fuel ratio is also shown in Fig. 4. The range of air fuel ratios used on cars before 1970, and around 1980 in Europe are also marked. With the leaner operation of the cars, fuel economy benefits and the accompanying reductions in CO and HC are evident. With improvements in the combustion chamber design the mixture can be further leaned to obtain reduction in all the three pollutants as well as the fuel consumption.

Extra lean operation of the engine results in power loss but reduces also the octane number requirement. Thus, compression ratio of the lean burn engines can be increased such that the same power output is obtained. Up to 10 per cent reduction in fuel consumption and large reductions in the emissions are possible through these changes.

Improved carburettor design, production and calibration have given significant reductions in emissions. Carburettor metering tolerance was cut from 12 % to 6 % richer than the lean limit. Several manufacturers in US and Europe maintained only a 3 per cent tolerance band on carburettor metering characteristics as shown in Fig. 5.

Very wide variations in the carburettor metering characteristics of even up to 10 to 15% were, common in the pre emission control era. Reducing this tolerance through better quality control will give low emissions with improvements in the vehicle fuel economy.

Emission control technologies, e.g. exhaust gas recirculation (EGR), exhaust gas treatment by catalytic or thermal reactors, are required to obtain emission reductions of more than 75%. However, these do not provide the fuel economy benefits. Use of EGR, in fact, deteriorates the engine efficiency.

Two stroke gasoline engine powered two and three wheelers The principal advantages of two stroke engines are low cost, high specific power output, low weight, simplicity of design and ease of maintenance. However, depending upon the engine operating conditions, 15 to 40 per cent of the air fuel mixture supplied to the two stroke engine is short circuited to the exhaust without taking part in combustion. Table 3 also showed that the hydrocarbon emissions from the two stroke engine powered vehicles under urban driving conditions amounted to 22 30 per cent of the fuel supplied. Presence of a high amount of residual gases in the combustion chamber results in very low NOx emissions. Thus, the main pollutants from 2 stroke engines are carbon monoxide and hydrocarbons.


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