Industrialization is the process of social and economic change that transforms a human group from a pre-industrial society into an industrial one. It is a part of a wider modernization process, where social change and economic development are closely related with technological innovation, particularly with the development of large scale energy and metallurgy production. Industrial pollution hurts the environment in a range of ways, and it has a negative impact on human lives and health. Pollutants can kill animals and plants, imbalance ecosystems, degrade air quality radically, damage buildings, and generally degrade quality of life. India is a home to many industries. The sectors include Iron and Steel, Pulp and Paper, Food Processing, Chemicals, Aluminium Industry, Cement, Pharmaceuticals, Machine tools, Surface finishing Industries etc. However, the industrial growth happening at a breakneck speed has resulted in a significant contribution to the toxicity in the environment. Therefore industrial activities should comply with regulatory norms for prevention and control of pollution. There have been many guidelines for the industries and the pollution caused by them. The setup and implementation of these guidelines is a joint responsibility of the central and state governments along with the Central Pollution Control Board to curb such emissions. At present, the control of pollution from industrial installations remains a key issue in India. As urbanisation expands and cities grow the need to deal with the environmental impact becomes even more important to ensure sustainable development. This also entails handling increasing volumes of waste water. Efficient wastewater management exploiting the capacity optimally requires a thorough understanding of the pollutions sources origin and substance. Hence pollution sources must be mapped and identified.
This book is designed to assist in the identification and implementation of a cost effective program for industrial pollution monitoring, control, and abatement within the context of institutional and financial constraints present in India. The book is a complete guide on industrial pollution control in important industries like Iron and Steel, Pulp and Paper, Food processing, Chemicals, Aluminium industry, Cement, Pharmaceuticals, Paint industry and many more. This book will be very resourceful to all its readers, students, entrepreneurs, technical institution, scientist, etc.
THE NATURE OF AIR POLLUTION
we can discuss the nature of the equipment used to
control air pollution, we must first define what constitutes air
order to understand the nature of the specific problem which control
must be designed to meet.
our purposes, air pollution consists of participates
or impurities that are suspended in or conveyed by a moving stream of
air. This pollutant material may exist in liquid form (commonly
gaseous fume, or as solid particulate matter, including dust. It will
result in at least one of four detrimental effects: loss of valuable
at atmospheric nuisance or safety hazard, damage to the quality of the
manufacture product, or mostly plant and equipment maintenance.
are formed and classified in either of two
size categories: submicron or micron. Equipment design and collection
efficiently are directly related to particle size. Particles 1 micron
in size (1 micron
equals 1/25,400 in.)
are generally easily collectable; smaller (submicron) sizes are more
One exception is the gas molecule. Although extremely small (0.0 micron in size), it
is usually easily
removed by packed tower type methods
discussed at length later in this chapter.
first consideration in the proper selection of a
collector is the nature of the pollutant materials to be collected.
Origin of Air Pollutants
the chemical and physical characteristics of all
particles must be known in order to choose the proper collector(s). The
of the particle may furnish a clue as to whether the pollutant
fume, dust, mist, or just an undesirable gas.
formed by mechanical disintegration may
generally be classed in the micron size group.
These may be formed by pulverizing, crushing, and grinding. Generally,
particles are larger than 1 or 2 microns. But there are always
the rules. For example, a particle of dust which is subject to
“self-disruption,” as in a roller grinding mill, may produce particles
distribution into the submicron size range.
created by coarse disintegration such
as sawing, jaw crushing, or tumbling (as in
slow-moving rotary equipment) may be above the 15-micron range.
created by a physical change of state (such
as sublimation or chemical reformation) or
from an intensive heating and melting operation are generally
operations, in both the steel and nonferrous
industries, usually produce exhaust or combustion gases containing
metallurgical submicron fumes. The generation of finer fumes is
related to the heat intensity.
current areas which are known producers of extremely
submicron metallurgical fumes are:
Oxygen injection—to raise melting temperature at
Electric furnace—to reach temperatures above 2000°F.
having a composite combination of particles
may be found in air which has been swept through the inside of a plant
various processes, or which has been trapped in the hoolding around
secondary consideration in selecting a collector is
the feasibility of available collectors to suit the specific needs of
Factors such as particulate or gas recovery, the area which is to be
by the collector, power usage, disposal problems, maintenance,
parts, and situations governed by the overall process operation weigh
on the best choice for the type of collector. Often a combination of
collectors will offer many advantages.
INFORMATION REQUIRED PRIOR TO EQUIPMENT
each air pollution problem is unique, exact
preliminary knowledge of pollutants is required in order to design
equipment. Particle size, gas temperature, and corrosive constituents
factors influencing equipment selection and design. To acquire this
samples of dust and mist must first be collected.
acid mist (SO3)
illustrates how the chemical properties of a
pollutant can cause complications. Appearing as a visible fume with
odor, SO3 is a submicron aerosol mist
sulfuric acid upon combining with moisture. When this occurs on moist
tissues, a tingling feeling or burning sensation results. To be
scrubbed, SO3 must first be hydrated to
convert it into the
removable form of sulfuric acid. Wet conditioning prior to scrubber
performs the hydration. This requires a dual operation at the
plus energy of impaction.
plants have generally collected sulfuric
acid with an electrostatic precipitator. More recently, plants
sulfuric acid have utilized venturi scrubbers (operating at 30 in. W.G.
pressure drop) to make the characteristic whitish cloud of SO3 mist essentially disappear.
problem is involved with
the phosphorus pentoxide
methods are not sufficient for this submicron aerosol mist, venturi
operating at 30 in. W.G. pressure drop have proved efficient.
a rule, the submicron mist must be reduced to
approximately 1 or 1½ mg/scf to approach invisibility.
must take many factors into account when
selecting the correct method of treating a specific air pollution
type of control equipment embodies particular design
peculiarities which influence installation, maintenance, and efficiency.
manufacturer must also weigh the interrelated process
and environmental factors. Frequently, the preferred solution to one
side problems may indicate a choice other than that first apparent.
collection equipment eventually selected often represents the most
compromise. Alternately, a combination of equipment may be the answer.
Factors Affecting Equipment Specification
of the gas collector to the gas source can determine the
treatment required. As a general rule, a reasonably close location
many side problems. Among problems alleviated are conveyance of dust
which could settle in the ducts, and cool spots which cause
ultimate corrosion. Close placement also minimizes pressure drop losses
occur when gas is drawn through a long distance.
is also influenced by the nature of the dust loading.
If the dust is micron size or larger, in an essentially dry and ambient
condition, the collector may be installed as close to the source as
However, if the gas loading is sub-micron in size, factors such as
growth should be considered. Collection efficiency is somewhat enhanced
submicron particles have an opportunity to agglomerate with one another
traveling a long distance to a collector.
feasibility of a particular type of equipment
may hinge on space factors. If the source is on the roof, for example,
considerations of roof loading will affect collector design. These can
size, as in extremely large bag collectors or precipitators. In the
case of a
wet scrubber, on the other hand, weight may be the determining factor,
recycle tanks are requisite. Foundation factors of support materials
are therefore involved whether on the roof, near the ground, indoors,
consisting of heavy particles, tar vapors,
or organic solvents may create side problems varying from fire hazards
accumulation on the
walls. In this case, short duct systems are preferred. If impractical,
consideration should be given to spraying the ducts, necessitating
ducts (possibly 5°) to direct drainage toward the scrubber.
handling extremely hot gas, the collector must be
placed at some distance from the source to allow cooling in transit.
especially true in the case of bag collectors, where pretreatment of
by cooling, improves collection efficiency and protects the filter
These fabrics affect the overall operation and cost of the equipment,
require specific temperature ranges for maximum performance and
temperature may contribute to duct warpage.
cooling often causes a “wet-dry line” condition,
in which solids accumulate at the cooled duct walls.
temperatures for electrostatic precipitators
range from 350 to 500°F. The presence of such acid compounds as SO2 and SO3 requires
serious consideration. Mixtures of sulfur trioxide have a high dew
can create a corrosive atmosphere.
SO3 dew point should
be considered at all
extremely hot gases are best handled by wet
collectors, which effectively utilize several cooling techniques.
cooling to saturation is one means of reducing the temperature. For
scrubbing results, gas temperatures above 450°F should be saturated
entry into any wet scrubber.
oldest known method for removing dust from an
airstream is the cloth filter. It is versatile and highly efficient for
collecting solid particulate matter in a wide range of sizes.
a general rule, efficiency increases in direct
proportion to the amount of cloth area in the filter. Maximized cloth
delivers lower pressure drop requirements, greater reserve capacity for
or expansion, and longer media (fabric) life. Thus, efficient design of
dust collectors involves a compromise between the ideal of large bag
and their considerable cost. To maximize filter surface per unit
designers frequently determine size by selecting the highest filter
(velocity through the media expressed in feet per minute) consistent
The actual fabric of the filter is chosen on the basis
of its ability to withstand temperatures and stresses inherent in the
as well as its compatibility with the pollutants to be collected. The
are fibrous material, either natural or man made, in woven or felt
form. In the
case of woven fabric, the material serves as a base for the
accumulation of a
porous layer known as a filter cake. This cake heightens the Filter’s
efficiency by screening out submicron particles. With felt materials,
is accomplished as the gas travels through the maze of fibers in the
affecting fabric filter performance
These include the fineness and size distribution
of particulate matter, particle shape, agglomeration tendencies, static
or tendency, physical properties such as adhesion or sublimation, and
properties such as crystallization and polymerization reactivity.
factors include gas constituents, loading, media limitations,
humidity, desired differential pressure, turbulence, and dust origin.
temperature, for example, requires more cloth,
probably because of an attendant gas viscosity increase. This is
counterreacted by reduced density. Dust load is also a major
requiring more cloth area at higher loadings (more frequent cleaning or
precleaning). Certain continuous automatic collectors can handle more
without lowering cfm per square foot when loading surpasses a certain
normally in excess of 100 grains/cu ft. This is probably due to total
saturation of the air with dust, thus giving bag surfaces a
rate of accumulation per unit time.
application factors fall
into three basic categories. The first of
these is nuisance venting, which includes relief of transfer points,
and packing stations. The second, product collection may involve air
conveying-venting mills, flash driers, and classifiers. The third,
filtering, ranges from spray driers to kilns and reactors.
pressure is lost in a sudden contraction than
in a sudden enlargement, as seen in the formula
which Kc is
a function of the ratio of the cross-sectional area of the smaller to
the larger pipe. The following table gives the value of Kc for various area ratios:
Values of Kc for Sudden Contraction*
0.362 0.338 0.308 0.267 0.221 0.164 0.105 0.053 0.015
O’Brien and Hickox.
or orifice type scrubbers are employed in
installations requiring high-energy collection of submicron particles.
venturi’s basic construction and principles of
operation are noncomplex in nature. Since there are no internals,
accessible from the unit’s exterior.
Figure 10: Various
resistances to flow within the
collector cause a loss of pressure.
venturi is the most accurate of fluid meters, since
it contains no moving parts to impede the airflow. The unique shape of
venturi offers 98 percent velocity head (power consumption) recovery,
allowing efficient introduction of fluid to meet the gas crossflow in
throat region. Atomization and impaction occur as the injected liquid
shattered by the unscrubbed gas into minute droplets which collide with
carry away minute particles.
Because of the equipment’s simplicity, designers
can choose from a full range of construction materials to handle
corrosion, abrasion, or both, depending on the nature of the emission.
materials frequently specified are stainless steel or Hastelloy. Lead
may be employed for protection against concentrations of sulfuric acid.
(ceramic) linings are used for protection against high temperatures
excessively abrasive particles. Rubber linings provide protection
fluorine and phosphoric acid fume emissions. To minimize deterioration
eliminate maintenance, current designs frequently use plastics such as
glass and PVC (for nitric acid).
Atomization of the scrubbing liquid takes place
in the throat of the venturi. Here, the liquid is introduced at
pressure and is shattered into minute droplets by the onrushing gas
coarse particles, such as
found in the lime kiln
gases and flue gases from the powdered coal furnaces, efficient
be attained with lower velocities and water rates than those needed for
collection of submicron particles.
a general rule, a higher liquid rate is usually more
advantageous to use than a higher gas velocity. Figuring 3 gal/1,000
take an extreme example, the combination of high velocity and low
would probably create an unwetted void through the middle of the
venturi. If we
were to look down the venturi, we would see an area in the middle where
action does not take place. Hence, it is always preferable to lean
higher liquid rate to ascertain a proper liquid-to-gas impaction
level. At the
other extreme, a problem can occur where a low gas velocity exists with
high a liquid rate (12 gal/1,000 cfm, for example). Under these
liquid shattering may be reduced to such a point that the scrubber
operate like an “ejector,” which means that very poor collection
are achieved. In summary, as a general rule, the ideal operating range
venturi type scrubber varies from 5 to 8 gal of liquid/1,000 cu ft, so
maximum and minimum scrubber contact velocities of 140 fps at 300 fps
Even though venturi scrubbers occupy less ground space
than most bag collectors and precipitators, they can also clean gas
within a minimal equipment area. The venturi’s lack of internals,
eliminates the need for work stopping checks for plugging and
effective firestop in
applications where extremely fine, dry, or
combustible materials are involved, the venturi is capable of handling
some noxious gases, sticky dust, and moisture, with no secondary dust
operation costs may be low if
the collected sludge can be disposed of without
clarification. However, sludge that requires clarification and
raise the capital cost of the venturi plant to that of the
The basic disadvantage of the venturi scrubber is
the rise in operating cost—usually 50 to 60 percent above that of
cleaning—primarily because of the high power cost for fans and water
As mentioned above, further processing of the wet sludge poses a costly
includes fan cleaning, which must take place
at frequent intervals to prevent mud from collecting on the blades,
fan to go “out of balance.” In systems handling saturated gases,
design must make full allowance for corrosive gas handling to avoid
terms of psychological community relations, the
venturi (as well as any wet scrubber) is under a disadvantage in that
emits a steam plume, especially in cold weather. Although recent
cooling towers have made it possible practically to eliminate the
plume, it may
give the impression that cleaning is less satisfactory than in
Other Types of Wet Scrubbers
the most elemental of wet scrubbers. These are empty
towers utilizing liquid introduced via a bank of spray nozzles at the
Gases passing countercurrent to the falling drops are scrubbed clean of
particulars matter of larger than micron size. These towers may also be
coolers or as primary cleaners. Though these units were acceptable in
years, recent developments have surpassed their performance.
spray scrubbers combine
the spray technique with the mechanical
principle of centrifugal force. The cylindrical tower contains a
manifold, from which droplets are sprayed into the airstream, which
through a tangential duct. Centrifugal force created by the gas rotates
droplets at high velocities, enabling them to collide with and carry
particulate matter to the scrubber walls.
designed for specific installations, cyclonic
spray scrubbers can accommodate gas entrance velocities up to 200 fps,
low-energy pressure drops ranging from 2 to 6 in. W.G. They have been
for cleaning micron-sized particles created from mechanical
packed tower type of gas scrubber is a prime method
of scrubbing a true gas. In use since the 1800s, packed towers are used
removal of gaseous fumes, noxious gases, and entrained droplets in gas
installations for various chemical processes.
the packed tower is a vertical vessel
in which various fill material is wetted. Surface area provided by the
packings offers a basis for inducing interaction between the liquid and
phases. The air or gas enters the bottom of the tower and receives a
preliminary washing as the scrubbing liquid drains in an opposing flow
packed, irrigated bed. This liquid, which is pumped into the top of the
flows down over the packing bed. En route, it covers the surface areas
packing with a liquid film that accomplishes the major work of
Finally, the airstream passes through a mist eliminator section (a dry
pad) before it is permitted to exhaust.
in design Spray
towers with baffles or slotted plates are
also used to produce a circuitous path for the contaminated airstream.
the case of packing, these cause the contaminant to contact the water
High-pressure nozzles maximize air and water contact.
of the complexities involved in packed towers,
air pollution control specialists normally design equipment for each
industrial situation, extrapolating data from laboratory and/or pilot
studies to accommodate the variables inherent in the particular gas
The following guidelines will help to determine
the best uses of the packed tower.
obtain maximum throughput in a vessel so that there will be sufficient
contact between the gas to be collected and the liquid media, the gas
velocity, should range between 3 and 6 fps.
of Packing. Packing
should be specified to provide maximum
surface area along with minimum void area.
surface is a prerequisite for covering the
packing with a liquid film sufficient to contact and react with the gas
Obviously, the packing is not wettable during the absorption cycle,
interface reaction is reduced. In this case, more packing is required.
Similarly, if a tower has a very large packing bed with minimal surface
exposure, the liquid film created may be minimal. This contact will be
to the gas and droplets which exist between the packing, thereby
interaction of the gas and the liquid film on the packing surface.
the smaller the packing size, the more contact
offered, thereby requiring more energy (pressure drop) to pass the gas
the tower. At the same time, the smaller packing requires less height
in the tower.
larger quantities of packing with large void
space have proved to be equal in contact efficiency to smaller packing
greater surface area. Experience has shown that a nominal packing size
from 1 to 2 in. is optimum for the capacity of gas handled as well as
factors of grillage availability, water distribution, and pressure
tower’s packed “bed” may contain any of a
variety of materials in an assortment of shapes, each specifically
its compatibility with the constituent in the particular gas stream.
Conventional packings include Berl saddles or Raschig rings, which are
constructed in chemical stoneware. Porcelain and carbon are also
used. Specialized packings marketed today include those made of
and propylene. These packings are commercially known as Maspac, Pall
Tellerettes. Different packing materials will have
different pressure drop which in turn result in various horsepower
depends on such factors as the absorption coefficients,
the partial pressure of the gaseous contaminant, and the
of the tower. Usually effective irrigated packing heights range between
4 and 8
ft with a 9 in. high mist eliminator bed located above the spray region.
drop is normally described in terms of inches
water gage (W.G.). Increased water gage refers to the power consumed in
to move the gases through the equipment. This power is generally
the air moving device, which may be the blower or the fan. As a rule of
a 64 percent
consumes 1 hp/1,000 cfm of gas when developing approximately 4.3 in.
Approximately 408 in. W.G. is equivalent to 14.7 psi, or 27.4 in. W.G.
equivalent to 1 psi. As a general guide a low pressure drop through a
bed is associated with a high percentage of free or void space.
2 offers variables of the many packings that may be
used in the general equation for pressure drops.
Packed towers are greatly influenced by the
solubility of the gas to be collected.
defined as having “gas film controlling” whereas
difficult gases are defined as having “liquid film controlling.” Simple
which are readily water soluble (gas film controlling) are as follows:
HCl, SO3 in strong H2SO4,
SO2 by alkali
and NH3 solution, H2S
in caustic solution, also the evaporation and
condensation of liquids. Those gases which are not readily soluble
film controlling) are CO2,
H2 by water, the absorption of
CO2 in alkali solution, and the
chlorine in water.
or readily insoluble, gases require
special considerations which are beyond
the scope of this discussion. Furthermore, their products are not
associated with the air pollution nuisance purpose.
general design of packed towers is dependent upon
whether the gas contains any solid particles. Where the solid particles
concentrations higher than 5 mg/cu ft, a packed tower is not
water soluble gas
In a very simplified manner, we have listed those
gases which are readily removed using water on a once-through basis.
also times when the concentrations in water allow recirculation with
optimum removal. Such a concentration is largely dependent upon the gas
leaving the tower as well as the concentration of the contaminant in
stream. At high temperatures, gases have a tendency to revaporize from
surface. Accordingly, the recirculation cycle becomes limited as the
pressure above the liquid surface increases. Those gases which are
water contact are as follows: ammonia (NH3),
hydrogen fluoride (HF), hydrochloric acid
(HCl). Gaseous tetrafluoride (S1F4)
is also effectively removed, provided the
packing does not receive gas concentrations above 5 mg/scf as F. Upon
wetting S1F4 converts partially to the
which will foul and plug
the packing. Where the gases need additional treatment to convert them
soluble form, solubility is influenced by a neutralizing additive.
need such additional neutralizing treatment are sulfur dioxide (SO2),
hydrogen sulfide (H2S),
and nitric-nitrous oxides
packed tower type scrubber further lends itself to
the handling of gases which may have entrained droplets as from plating
various metallurgical operations. These droplets may consist of
as sulfuric, phosphoric, and nitric acids. Caution must be exercised
(particularly when dealing with sulfuric and phosphoric acid) to ensure
the droplets are true, and not a decomposition product existing as an
mist. These “aerosol mists,” as they are termed, occur as a white cloud
cannot be effectively removed by a packed tower scrubber.
of a packed tower may be further clarified in
that the tower uses a support grid to hold the packing or fill. The
grid has an
open pattern to permit an upflow of gas through it with minor
resistance to gas
of liquid distribution
The “weir box” is a unique liquid
distribution vehicle with V notches which permit uniform liquid
It is subject to reasonable leveling within the tower. The primary
of the weir box device is that the operator cannot tell whether a
uniform distribution exists and is effectively compensated for with
in gas flow. V notches in the weir box having spacing beyond 3 in. do
full coverage at the upper edge of the packing, resulting in loss of
packing height. V notches spaced at 3 in. may lose up to 2 ft of
height. Also, weir boxes are very costly and add excessive weight to
place of weir boxes, the spray pipe assembly
method has recently been demonstrated to be superior as it allows
adequate distribution throughout, and fuller use of packing height,
distribution exists at the upper edge of the packing. Further, removal
spray headers from the exterior is allowed. Disadvantage of the spray
occurs when recirculated water contains particles that may plug the
nozzles. But careful choice of nozzles will help prevent such a
as FRP or polypropylene for the shell housing—has
increased in recent years. The packing made of polypropylene and the
headers of PVC and spray jets of Teflon has proved to be ideal
Plastic is also advantageous as it is lightweight, noncorrosive to any
fluids mentioned above, easily repaired and assembled, and requires no
painting. Of course, there are some temperature limitations where PVC
employed, but this situation is primarily restricted to the pipe
does not prove to be a problem in most instances.
Although small quantities of fine micron sized
particles can be scrubbed by the packed tower, it is not generally
as a primary apparatus for the collection of micron particles, because
particles can close the space between the tower’s packing and cause it
become clogged, rendering the equipment inoperative. A good rule of
thumb is to
allow a maximum of 0.2 grain/scf of dust to enter a packed tower.
serious problem of blocked packing may result.
steam plume is a very important psychological
consideration which may affect the overall equipment selection and
wet type scrubber or wet type electrostatic
precipitator which is doing an excellent job will show a steam
upon becoming saturated with moisture. Although the steam will have no
oil the surrounding
area, its appearance may cause concern with the novice that the air
equipment is inadequate or malfunctioning.
plumes are the result of rapid cooling of gases or air carrying
moisture to below the saturation temperature. Obviously, gases from
scrubbers will quickly show a steam plume at the stack discharge.
colder winter months, the gases are subjected to a colder atmosphere.
thus air-condensed sooner and have shorter plume trails compared with
equivalent gases cooled by the summer atmosphere.
a guide, saturated gases which discharge from the
stack below 105°F will have a negligible appearance and will not create
questionable steam plume. At 105°F the volume of moisture content is
7 percent, whereas at higher saturation temperatures, the percentage
moisture by volume is as follows: 130°F, 15.0; 160°F, 32.5;180°F, 51.0.
In addition to appearance, steam plumes have other side
effects which include:
SO2 (or other
corrosive gases) may become aggravated through steam plume
absorbed by the newly formed droplets into sulfurous acid and then
on homes and industrial sites.
In some cases odoriferous constituents may be
entrapped by falling droplets to increase odor at ground elevation.
Methods for steam plume minimization
Cooling of Hot Gases. A
quick inspection of the psychrometric reveals
that the saturation, easily reached by all wet type scrubbers, is
upon the initial moisture content (Ib moisture/lb dry air) and the
gas temperature. Reduction of either or both of these conditions
the saturation temperature.
the example in the chart, point A is at an initial
temperature of 500°F and moisture content of 0.11 Ib moisture/Ib dry
therefore, the saturation temperature will be 150°F at point B. Point C
designated a reduced initial temperature at 140°F and the same moisture
content, 0.11 Ib moisture/Ib dry air. Now the saturation temperature
17°F reduction at the final saturation represents a
reduction from 25.2 per cent water vapor to 15.3 percent water vapor
(approximately a 40 percent water vapor content reduction).
Adiabatic saturation lines and percentage
saturation curves at temperatures ranging from 0 to 500ºF at a pressure
29.921 in. Hg.
is effected within a continuous S shaped duct
with sufficient surface exposure for radiation and cooling by
surrounding the ducts. Often the ducts may be arranged essentially
with U bends returning upward and downward. The bottoms of these U
be furnished with cleanout doors and hoppers to permit intermittent
or Direct Cooling of Saturated Gases. This
technique cools the already 100 percent
saturated gases with cool water. Sufficient coolness is required to
or condense water vapor down to the desirable lower saturation.
we see, in Fig. 18, point B at 150°F
saturated has a heat content of 267.0 Btu/lb dry air cooling to 105°F
saturated; the heat content is 73.6 Btu/lb air. Heat to be removed
— 73.6, or 193.4 Btu/lb dry air.
using a standard spray tower where available, cooling
water at 70 to 85°F may be introduced at 25 psig. Droplets in the range
microns in diameter will fall counterflow to the gas passage and carry
latent heat of the water vapor and sensible heat of the dry gas. As a
rule, the cool water leaving a properly designed tower will approach
or 15° F of the entrance gas temperature. Therefore, pounds of 80°F
needed equals 193.4/(150) — (80+ 15) or 3.51 Ib water/lb dry gas or
of a tower filled with drip-point grid tiles offers a
method to obtain benefit of maximum heat transfer with an approach of
approximately 5°F or less ( between the gas and liquid discharging).
by Mixing with Atmospheric Air. In
some special cases sensible cooling may be
obtained by the addition of atmospheric air having a low dew point
However, this method becomes impractical where already large saturated
of gas having high saturation temperatures are involved.
to Reduce Disposal
are generated by virtually every industrial
enterprise. These wastes can be solids or liquids. For example, the
pharmaceutical industry generates waste solvents when purifying ethical
and discarding manufactured products or raw materials that do not meet
specifications. Paper mills generate waste from wood pulping, trimmings
the paper-making machines, and from roll ends. Dry cleaners generate
oil, dirty filters used to recycle solvents, and spent solvents. The
variety of waste products is virtually endless.
are three main methods of waste disposal:
land disposal, which has been and continues to be predominant;
disposal with eventual drainage out to sea; and dispersal to the
Treatment before disposal is desirable for reducing the toxicity,
volume of the waste.
are available for recycling parts of
these wastes and developing new, marketable products. To be
viable, these products must meet minimum standards. For instance, to
saleable, recycled lubricating oil must meet standards for lubricating
and temperature stability set by the Society of Automotive Engineers.
solvents must meet commercial criteria for industrial solvents, such
cleaning effectiveness and corrosion inhibition. Recycled paper must be
suitable for cardboard manufacture, writing paper, or cellulosic
If these commercial constraints are not met, the recycled product
be discarded as waste.
a waste reduction project, the first goal is to
eliminate the waste as early in the production process as possible. It
however, rarely feasible to optimize a production process to the
no waste is produced. “Zero discharge” is a laudable goal, but it is
practical. However, even when the volume of waste from a production
minimized, waste reduction opportunities do not end. The waste can
processed to recover useful materials or treated to reduce its volume,
toxicity, or mobility, thus reducing its impact on human health and the
This chapter discusses and presents examples of a few of the treatment
technologies for reducing the environmental and health effects of
that are disposed of as waste.
USES OF TREATMENT
can be used to reduce the volume of
waste requiring disposal, to render a hazardous waste nonhazardous, or
recover a useful product or some other resource. The following examples
illustrate potential uses of treatment for waste reduction.
reduction can be used to reduce treatment
costs or to reduce the handling and disposal costs for residues
treatment. Volume reduction can be accomplished by using a variety of
• reuse of treated wastewater or other wastes
• treatment modifications to reduce the generation of
• segregated treatment to reduce hazardous waste mixtures
addition, incineration can be used to reduce
waste volume or to render a hazardous waste nonhazardous.
most extreme example of wastewater reduction
is a zero-discharge wastewater management system. In such a system,
is treated to sufficient quality that it can be reused in place of raw
within the facility that generated the wastewater. Typically,
cannot be reused in the original process is evaporated or used for some
beneficial use, such as spray irrigation.
discharge has been practiced for some time in
locations where water is scarce. Zero discharge may involve the use of
technologies for removing suspended solids, such as clarification and
filtration, and technologies for removing dissolved solids, such as
osmosis and evaporation. The actual technologies required depend on the
of the wastewater as well as the water-quality requirements of the
reuses the treated water. Complete demineralization is relatively
however, in some cases, wastewater discharges can be reduced
using less-expensive selective removal technologies.
Study 1 : Zero Discharge at an
Integrated Manufacturing Facility
and Objectives. A
corporation located a new manufacturing facility
in a rural area. This facility was to be highly integrated, with many
components manufactured onsite. Minimizing the impact of the facility
local environment was an important consideration; in addition, local
supplies and wastewater disposal capacity were limited.
of these factors, the company decided to
investigate the feasibility of in-plant recycling of wastewater. The
goal was zero discharge of wastewater from the site.
Management Computer Model.
To assist in evaluating water and wastewater
management alternatives, a computer model was developed of the
facility’s water use and wastewater production. The water management
included water usage and quality requirements for more than 50
processes in 9 separate manufacturing areas, taking into account the
constituents and pollutants added to the water. In addition to
processes, 20 water treatment processes were included in the model.
to Zero Discharge.
Approaches to achieving zero discharge include
at-source treatment and
treatment and recycle, or some combination of these in an integrated
management system. At-source treatment and recycle was appealing
because it would
treat waste streams in small, possibly modular, systems, and would be
for only those parameters not meeting water reuse requirements. The
disadvantage was that the large size of this facility would necessitate
multiple treatment and recycle systems. An end-of-pipe treatment system
have to be extensive and contain many unit processes in order to
the various flows at the facility. An integrated approach combined the
features of at-source and end-of-pipe treatment.
Water Management System.
The water quality model was used to analyze
numerous configurations of an end-of-plant wastewater treatment
end-of-pipe treatment system was recommended to receive all process
treat the wastewater, and recycle the treated water back to the
units. This was supplemented with some at-source treatment to reduce
of unit processes required in the end-of-source treatment to reduce the
of unit processes required in the end-of-pipe facility. An estimate of
capital costs for this approach is presented in Table 1.
Case Study 2: Wastewater Reuse at a
Coal-Fired Power Plant
An electric utility located in the eastern.
United States operates a large coal-fired power station. Recently,
regulators required the station to meet new stringent discharge limits
metals. Meeting the limits would require very expensive treatment of
discharge from the station.
The client contracted CH2M HILL to develop a
wastewater management plan for the station. CH2M HILL conducted
sampling and flow monitoring to identify flow and pollutant sources.
information was used to develop a mass-balance diagram of the power
Significant sources of flow include cooling-tower blowdown, ash-hopper
overflow, coal pile runoff, and miscellaneous waste streams.
concentrations of metals are present in enough waste streams so that
end-of-pipe treatment will be required.
allowance (30% of subtotal)
TOTAL ESTIMATED CONSTRUCTION COSTS
(15% of construction costs)
(10% of construction costs)
TOTAL CONCEPTUAL LEVEL CAPITAL COSTS
Treatment Process. The
power station currently generates an average
of 7 million gallons per day (mgd) of wastewater. The required size of
end-of-pipe treatment faculty for this volume of effluent is
mgd. Many of the waste streams are contaminated with suspended solids.
of solids from these waste streams can produce an effluent having a
quality rivaling that of the raw river water used at the facility.
streams represent more than 50% of the wastewater currently discharged
facility. The settled wastewater can be reused as cooling-tower makeup
ash-hoppers makeup water, and for other power plant processes. The
client and CH2M
HILL reached a consensus
that average wastewater discharge flows could be reduced to
2: presents order-of-magnitude capital costs for
the end-of-pipe treatment facility with and without wastewater reuse.
2: Comparison of Capital Costs for Treatment at a Coal-Fired Power
Capital Cost (s)
can save the
client $4.6 million in treatment costs. The estimated capital cost of
piping modifications required to reuse wastewater is approximately $1
Reducing Generation of Solid Residues
waste treatment practices can reduce the amount
of solid residues requiring disposal. This can be achieved by modifying
treatment chemistry to produce less sludge, or by removing water from
sludge to produce a drier cake.
Modification of Treatment Chemistry to
conventional treatment of a mixed-metal waste
containing hexavalent chromium, the pH of the waste stream is reduced
3 with a mineral acid (usually sulfuric). A reducing agent (sulfur
sodium metabisulfite) is then added to convert hexavalent chromium to
reduced trivalent state. After the reaction is complete, lime is added
the pH of the combined wastewater to approximately 9.5 in order to
heavy metals as hydroxides. A disadvantage of this treatment scheme,
is that it produces large quantities of sludge, in excess of the metals
targeted for removal. The sludge is produced because calcium carbonate
calcium sulfate are precipitated out of the wastewater, particularly
a hard alkaline wastewater.
research sponsored by the Air Force, it was found that
chrome reduction could be accomplished at neutral to slightly alkaline
a combination of ferrous sulfate and sodium sulfide as reducing agents
sodium hydroxide for precipitating heavy metals. The study found that
resulting iron hydroxide precipitate was effective in removing other
metals, such as cadmium and nickel, at a neutral to slightly alkaline
advantages of these modifications include eliminating an acidic chrome
step, eliminating the addition of acid and reducing the need for lime,
precipitating metals without also producing calcium sulfate or
process has been adopted at the Tinker AFB
industrial wastewater treatment plant to remove chrome, copper, nickel,
cadmium, and other metals from a 1-mgd combined industrial waste
Operating at a pH of 7.5 to 8.5, the treatment system achieves the same
reductions as the old process operating at a pH of 2.5 to 3, while
sludge volumes by two-thirds. The Air Force estimated that this process
modification would save $1,000 per day in chemical usage and sludge
a similar treatment process, ferrous iron is generated
electrochemically using sacrificial iron electrodes. Equipment has been
installed at a Navy ordnance plant in Pomona, California, to remove
traces of chromium and nickel. The pH of the reactor is held to a range
6 and 9 using caustic soda (sodium hydroxide). The precipitated metals
settled out in a clarifier and dewatered in a filter press. The process
reported to produce 75% less sludge when compared with acidic chrome
and lime precipitation.
industrial wastewater treatment facilities
typically use hydroxide precipitation for removing toxic metals. The
hydroxide solids are usually removed by clarification. Metal hydroxide
withdrawn from a clarifier typically have solids concentrations
1% to 2%. A pound of copper precipitated with
hydroxide produces 1.54 lb
of solid copper hydroxide. In a 1% sludge, this pound of copper
produces 154 lb
of sludge. Disposal of such a large volume would be expensive, even if
permitted (liquid waste disposal in hazardous waste landfills is
Therefore, most waste treatment plants dewater their sludge before
of a thickener can be useful before the sludge is
dewatered. Adding a thickener after or as part of the clarifier
additional sludge storage volume in addition to increasing the sludge
as much as 5 to 6%. (In our example, this equates to reducing the 154
copper hydroxide sludge to 30 lb.) Increasing solids by thickening
subsequent sludge dewatering in two ways. First it reduces the time
for dewatering, and second, it usually results in a drier sludge cake
dewatering has traditionally been accomplished in
open sand drying beds, especially in areas of the country with warm
low rainfall, and cheap land. However, because sludge from industrial
wastewater treatment plants is frequently hazardous, sludge drying beds
all of the design (and, potentially, the regulatory) requirements of a
hazardous waste landfill, with collection and treatment of leachate required.
dewatering is most frequently used because of regulatory and cost
types of mechanical dewatering devices are
typically used for treating sludge from industrial wastewater treatment
vacuum filters, belt presses, and plate-and-frame filter presses.
filters are used infrequently because they produce
the least-dry cake of the three mechanical methods. Vacuum filters
employ a precoating process, using diatomaceous earth, to improve
This is a disadvantage because some of the precoat is scraped off
process, adding to the volume of solids to be disposed of. In addition,
filters generally require higher energy use than the other mechanical
dewatering processes. In some applications (for example, in dewatering
hydroxide sludge), vacuum filters are preferred, because they are
gelatinous sludges and can produce a dry cake without blinding the
cloth, as occurs with the other filters.
costs for the smallest belt press are higher than
for the smallest plate-and-frame press. However, belt presses can be
for large waste treatment plants. Operating costs are lower because
operate continuously, as opposed to plate-and frame presses, which
a batch mode. Belt presses generally produce a cake having a range of
30% solids content.
filter presses are generally used for
facilities that produce a small volume of waste, because they are
operate and are available in a broad range of sizes. Sludge from the
or clarifier is pumped, typically using an air diaphragm pump, to the
of a filter press. The solids are retained by the synthetic cloth
and the liquid flows through the media and is returned to the
treatment plant for retreatment. After the pressure required to pump
the press reaches a maximum value, the hydraulic press that holds the
together is released, and a filter cake is discharged. Plate-and-frame
presses generally produce the driest sludge cakes (30% to 40% solids),
an advantage when disposal is based on weight or volume.
press operation generally requires little operator
attention except at the beginning and end of a cycle. Automatic plate
greatly reduce the manual labor required to remove the filter cake.
dewatered sludge will literally fall out of the press when it opens.
major maintenance cost of a mechanical sludge filter
is replacing filter cloths, which can be frequent when handling
having an extreme pH. However, metal hydroxide sludges generally have
moderately alkaline pH and are nonabrasive.
mechanical dewatering can increase the solids
content of a sludge from 30% to 40%. Using the 1 lb of copper as an
unthickened 1% sludge weighing 154 lb would be reduced to 5 lb of
(30% solids). This filter cake still contains 3.5 lb of water In
though this sludge appears to be dry on the surface, free water
escape during shipment. Landfill operators may then either reject the
(because of a ban on landfill disposal of free liquids) or require that
kiln dust be added to react with or soak up the excess moisture.
drying of the filter cake to 80% to 90% solids
content is feasible: The process reduces the weight of our hypothetical
from 5 to 2 lb (80% solids). eliminating the need for adding kiln dust
prevent the generation of free water during shipment.
SEGREGATED TREATMENT TO REDUCE HAZARDOUS
the past, operating a centralized treatment plant for
industrial waste has been more cost-effective than providing individual
treatment systems for each production unit in a large manufacturing
However, hazardous waste regulations have classified the residues from
treatment of wastes from certain industrial operations (i.e.,
or chromate-conversion coating of aluminum) as listed hazardous wastes,
regardless of their actual composition or toxicity. This is
complicated by the
RCRA mixture rule, which states that mixing any quantity of a
with a nonhazardous waste renders the entire mixture hazardous.
combination of these two regulations makes it
imperative that a manufacturer investigate whether combined treatment
waste streams makes sense. It is often economical to provide treatment
hazardous waste separately from all other wastes, especially when the
waste is only a small portion of total waste flow in the facility or
only a small portion of the total sludge when treated.
is an example in which an aerospace
manufacturing facility plans to provide separate treatment (and
recycle) of the
wastewater from chromate conversion coatings operations. Segregating
wastes will result in the industrial waste-water treatment plant
becoming classified as nonhazardous. Reducing the amount of waste to be
disposed of justifies the construction and operation of two separate
Case Study 3: Separate
Treatment of Waste From
Chromate Conversion Coating
and Objectives. CH2M
HILL performed a waste reduction study for a
major aerospace manufacturing facility in the western United States.
identified rinsewater from chromate conversion coating (Iriditing) of
as the main target for waste reduction efforts because it is
the sludge from the entire industrial wastewater treatment plant being
classified as hazardous. This rinsewater (approximately 15,000 gal/day)
mixed with other industrial wastes (212,000 gal/day) for treatment at a
industrial wastewater treatment plant.
U.S. EPA lists “wastewater treatment sludges from the
chemical conversion coating of aluminum” as hazardous under the
F019, specifically because they typically contain hexavalent chromium
Appendix VII). In addition to being used in the chromate conversion
process, hexavalent chromium compounds are used at the facility to
surfaces (remove oxide surface coating) when, preparing parts for
conversion coating. Table 3 shows the amounts of chromium discharged to
waste treatment plant from the three locations that employ chromate
coating at the facility. These data were estimated from composition of
process baths involved and from approximations by company personnel of
production loads on these processes and typical drag-out rates.
tons of dewatered sludge was produced at the
industrial wastewater treatment facility. The waste did not contain
concentrations of heavy metals, but because it was a listed waste, it
disposed of as hazardous at a cost of $210,000. Thus, less than 1 lb of
chromium discharged to the treatment plant resulted in more than 1,000
sludge per day being classified as hazardous. The cost of disposal was
significant incentive for eliminating chromium-containing wastewater
industrial treatment plant.
3: Chromium Discharges from Conversion Coating Facilities
the F019 classification of the industrial
wastewater treatment sludge could be achieved only by attaining zero
of rinse waters and baths from the chromate conversion coating and the
processes. To achieve this objective, a closed-loop rinsewater system
have to be installed, with all residues being hauled off the site for
as hazardous waste.
of Alternatives. Four
unit processes were considered necessary for
a closed-loop system:
water recycle is necessary because rinse water is
usually maintained at a low concentration of contamination to effect
rinsing. The existing operation produced approximately 15,000 gal of
rinsewater waste per day (125,000 lb) in the process of removing the 1
chromium per day. Alternatives considered for rinsewater cleanup were:
ion exchange (IX)
reverse osmosis (RO)
ion transfer membranes
chromium from the rinse water would eliminate
the major toxic constituent from this waste stream and simplify
any blow down stream. Alternatives considered for process chemical
reduction was considered necessary for reducing
the size of the rinsewater recycle system and the quantity of blowdown
to prevent buildup of salts in the system. Alternatives for volume
purification is necessary if process chemicals are
to be recovered from the rinsewater and returned to the bath, inasmuch
systems will return contaminants and useful chemicals. Otherwise,
are concentrated in the bath until the bath is no longer functional,
necessitating disposal of a large quantity of hazardous material. It
little sense to recover 1 lb of chromium per day if this recovery
the disposal of a 20,000-gal tank of Iridite solution containing more
lb of chromium. Also, because the concentration of impurities in the
solutions is much higher than that in the rinsewater, bath purification
easier than rinsewater cleanup. Furthermore, cations are removed in the
and are not carried over to the rinsewater; therefore, the volume of
regenerant is reduced significantly. Alternatives for purification of
chrome-containing process solutions were:
alternative technologies were evaluated for their
potential for waste reduction, effects on production, and relative
Existing users of the technologies were contacted to discuss their
experiences. Sites were visited to collect information on the
was recommended for rinsewater cleanup. IX is
an established technology and can also segregate chromic and nitric
cations and concentrate them for reuse. Deox solutions contain high
concentrations of nitric acid. Reduced rinse flows are accomplished by
providing some counter flow rinsing, so that four individual recycle
are used for the seven tanks. An electrodialytic bath maintenance unit
recommended for the principal deox bath. Finally, evaporation was
for concentrating IX regenerant solutions, either to enhance recovery
or to reduce
cost of disposal.
rinsewater is passed through a cartridge
to remove particles that could plug the IX resin. A cartridge filter
recommended because suspended solids loading is low in these acidic
rinsewaters. Also, a cartridge filter does not require backwashing, so
wastewater is generated. Cation exchange is used for removing metallic
impurities such as trivalent chromium, aluminum, sodium, and zinc. Then
anionic exchange resin is used to remove hexavalent chromium and
Finally, the rinsewater is treated with activated carbon to remove
organics that could foul IX resins if allowed to build-up in the
rinsewater system. The carbon is placed after, rather than in front of,
resin, because carbon removes hexavalent chromium, reducing the
cation exchange resin is regenerated with sulfuric
acid, removing the metals and returning the resin to its acid form. The
of this acidic regenerant solution is reduced by evaporation and
off the site.
and deoxidizer bath
anion exchange resin is regenerated with caustic soda
(sodium hydroxide). The resulting regenerant solution is a caustic
mixture of sodium
chromate, sodium nitrate, and sodium hydroxide. This regenerant is then
concentrated by evaporation.
of the low production of contaminants at Remote
Sites A and B, it is not cost-effective to provide these locations with
completely independent demineralizer units. It is more cost-effective
provide these locations with portable IX units, which are returned
to the Chem Mill facility for regeneration.
of Segregated Treatment.
Projected capital and operating costs for the IX
systems (a central system with regeneration facilities and two remote
are provided in Tables 4 and 5. The cost of installing this system is
approximately $294,000 and would result in a savings of $148,300 per
a resulting payback period of less than 2 years for this investment.
a bath maintenance system on Chem Mill Tank 3
(deox tank) would cost approximately $16,000. This would decrease
to the demineralizer system by approximately 90% from this source,
overall cation loading to the demineralizer system by approximately
annual savings from installing a bath maintenance system for this tank
provided in Table 6. The table shows an annual savings of $4,400, for a
projected payback period of approximately 3.6 years.
additional production benefit not included in this
analysis is improved consistency
increased operating life of the process solution, which reduces the
disposal. The nonquantified production benefits (and low capital cost)
sufficient to convince management to adopt this improvement, despite
relatively long period projected for payback.
RECOVERY OF A USEFUL PRODUCT
treatment technologies concentrate waste materials
into smaller volumes or convert wastes into less mobile forms (such as
hydroxide precipitation from wastewater). Useful products can be
waste streams. In some cases, that product is a raw material that can
as feed stock. In other cases, the recovered product is energy, such as
wastes are used to fuel a power-generation facility.
Case Study 5: Recovery of Copper at a
and Objectives. An
aerospace manufacturer operates a circuit-board
fabrication facility in New England. CH2M HILL was contracted to design
treatment plant for this facility. The principal objective was to
volume of sludge generated by the treatment plant.
from the treatment plant discharges to a
publicly owned treatment works. The effluent is required to comply with
and local pretreatment discharge limits. Wastes treated in the
consist mainly of continuous overflow rinses from scrubbing, cleaning,
electroless plating of copper, electroplating of copper, photoresist
processes, and etching. These wastes contain a mixture of regulated
(principally copper and lead) and complexing agents from electroless
plating and etching processes.
current maximum flow is approximately 90 gal/min. The
treatment facility is designed for 50 gal/min with a maximum hydraulic
of 90 gal/min. The presence of complexing agents renders conventional
precipitation treatment ineffective. One effective technique for
type of waste has been the addition of large doses of ferrous iron,
displaces copper and lead from their complexes, allowing their
metal hydroxides. This process, however, produces voluminous sludge
the precipitation of large quantities of ferrous hydroxide.
minimize hazardous waste, CH2M HILL recommended
treating the combined rinsewater waste by chelating IX, and recovering
copper from the spent regenerant solution using electrowinning.
are pumped directly to a pH adjustment tank.
Concentrated acidic and caustic wastes are pumped to holding tanks to
metered into the mixing tank to prevent slug loading of acids, alkalis,
metals to the treatment system. In the pH adjustment tank, the waste is
adjusted to a pH between 4 to 5 using sulfuric acid or caustic soda
hydroxide), because this pH range is optimal for selective IX removal
waste is then pumped through duplex cartridge
filters. It is estimated that weekly replacement of filters will be
The effluent from the filter passes through two activated carbon
series. Activated carbon adsorption removes organic compounds that
the IX resins, reducing the resins capacity and resulting in more
replacement. Two carbon adsorption columns are provided in series,
allowing complete use of the carbon in the first vessel, with the
providing polishing. Following complete breakthrough of organic
the first carbon column, that carbon should be replaced and the roles
two vessels reversed. It is estimated that annual replacement of carbon
required. The carbon vessels will be backwashable, with the backwash
be returned to the initial pH adjustment tank.
wastewater then flows through dual IX columns,
operated in series, using a chelating cationic IX resin. Following
the lead IX column resin is regenerated with sulfuric acid, rinsed
water, and returned to service as the lag, or polishing, unit. The
are returned to the rapid-mix tank for treatment. The waste regenerant
to a holding tank for copper recovery by electro-winning.
effluent from the IX units discharges to the final
rapid-mix tank for pH adjustment with sulfuric acid or caustic soda
discharge to the city sewer.
spent IX regenerant, electroless copper plating bath
growth, and sodium persulfate etching solutions are stored in separate
tanks. These solutions are pumped to a low-surface-area (LSA)
electrowinning unit for recovery of copper by electroplating onto flat
stainless steel plates. The electrowinning unit is batch operated.
batch is treated, the effluent is pumped to the acid or alkaline
and bled into the influent pH adjustment tank for additional treatment.
effluent is expected to contain 1 or 2 grams of copper per liter.
high-surface-area, high-mass-transfer (HMT) electrowinning
system is being considered for use instead of an LSA system. The
an HMT system is its reported ability to reduce effluent copper
to a few milligrams per liter. This equipment would reduce the load of
on the IX system resulting in less frequent regeneration. Disadvantages
HMT are increased power consumption and lower rates of metal recovery
(resulting in longer electrowinning times).
alternative to HMT electrowinning is an
atmospheric evaporator, which would increase the concentration of metal
IX regenerant prior to LSA electro-winning. Benefits include volume
improved electrowinning efficiency from increased metal concentration,
resulting in reduced frequency of IX regeneration, and potential reuse
regeneration acid following dilution to the original acid
reduction in volume benefits operation of the system in that fewer
required to be electrowinned. The required electrowinning cathode area
time for electrowinning are unchanged because they are dependent on
of metal to be removed rather than hydraulic volume throughput.
of evaporation include the requirement for heating an acidic solution,
problems associated with handling a heated concentrated acid, and
power and maintenance requirements.
systems were considered for installation at this
facility: LSA electrowinning with an atmospheric evaporator and HMT
electrowinning. Order-of-magnitude cost estimates for installing the
systems are provided in Table 13. Operating costs are listed in Table
Treatment for Resource or Energy Recovery.
processes can be used
for recovering minerals from waste streams and for recovering energy in
form of steam or electricity.
organic compounds, when burned, will generate
HCl gas and a small amount of chlorine gas. The HCl can be absorbed in
multistage absorption tower to manufacture hydrochloric acid at
varying from 6 to 24% HCl. These acid streams have been used for
in steel manufacture.
containing more than about 20% sulfur can be
burned. The sulfur dioxide can then be catalytically oxidized to sulfur
trioxide and absorbed to manufacture sulfuric acid. An alternative
be used to reduce the sulfur dioxide to make elemental sulfur.
previous examples are of well-established industrial
processes that have been modified to recover commercial products from
The use of these methods has been limited, thus far, to situations
waste streams are generated in a chemical production plant, and the
been added to the production process for waste treatment or air
work has been done on the feasibility of
treating ash produced by solids incinerators to recover iron from the
These methods have not yet proved fruitful because of the low price of
However, reclaiming noble metals from the manufacture of printed
and electronic parts is being actively considered as an alternative to
disposal. As with all recycling/reclamation projects, the
depends on the value of the reclaimed product and the cost of