a
primer
on
waste water
treatment
-------
INTRODUCTION
WILLIAM D. RUCKELSHAUS
Administrator
Environmental Protection Agency
DAVID D. DOMINICK
Acting Commissioner
Water Quality Office
CONTENTS
Collecting and treating wastes 1
Primary treatment 3
Secondary treatment 4
Lagoons and septic tanks 1
The need for further treatment of wastes 9
The types of pollutants 10
Advanced methods of treating wastes 14
Coagulation-sedimentation 15
Adsorption 15
Electrodialysis 17
The blending of treated water 17
New challenges for waste treatment 19
Chemical oxidation 20
Polymers and pollution 20
The problem of waste disposal 21
Common sewage treatment terminology 24
CWA-12
Revised March, 1971
For sale by the Superintendent of Documents,
U.S. Government Printing Office
Washington, D.C. 20402 - Price 55 cents
Thousands of waste treatment plants will be constructed
or expanded across the Nation during the years ahead
to control or prevent water pollution.
This increased construction activity is the result of the
passage of the Water Quality Act of 1965 which called for
the establishment of water quality standards for all the
interstate streams, coastal waters, and lakes, and the Clean
Water Restoration Act of 1966 which increased Federal
financial aid to cities to help build these needed plants.
Current and escalating demands to upgrade our environ-
ment have encouraged this trend.
Communities across the land will be planning, financing,
and building the facilities to meet the water quality stand-
ards. Some cities will be constructing plants where none
existed before. Others will be expanding inadequate
facilities while some communities will be adding more ad-
vanced methods to handle new types of wastes.
It won't happen overnight. From drawing board to opera-
tion takes time. In some cases, projects will be built in
stages. Consequently, more and more people will be
watching this developing progress toward cleaner water.
They will need to know more about waste treatment.
In this primer, the methods used now and processes being
developed for the future to treat wastes are explained.
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primer
on
waste water
treatment
AND TREATING WASTES
The most common form of pollution
control in the United States consists
of a system of sewers and waste treatment
plants. The sewers collect the waste water
from homes, businesses, and many indus-
tries and deliver it to the plants for treat-
ment to make it fit for discharge into
streams or for reuse.
There are two kinds of sewer systems-
combined and separate. Combined sewers
carry away both water polluted by human
use and water polluted as it drains off
homes, streets, or land during a storm.
In a separated system, one system of
sewers, usually called sanitary, carries
only sewage. Another system of storm
sewers takes care of the large volumes of
water from rain or melting snow.
Each home has a sewer or pipe which
connects to the common or lateral sewer
beneath a nearby street. Lateral sewers
connect with larger sewers called trunk or
main sewers. In a combined sewer system,
these trunk or main sewers discharge into
a larger sewer called an interceptor. The
interceptor is designed to carry several
times the dry-weather flow of the system
feeding into it.
During dry weather when the sewers are
handling only the normal amount of
waste water, all of it is carried to the
waste treatment plant. During a storm
when the amount of water in the sewer
system is much greater, part of the water,
including varying amounts of raw sewage,
is allowed to bypass directly into the re-
ceiving streams. The rest of the wastes
are sent to the treatment plant. If part of
the increased load of water were not di-
verted, the waste treatment plant would
be overloaded and the purifying processes
would not function properly. (A research,
development and demonstration program
is being conducted to solve this urban run-
off pollution problem. The aim is to de-
velop technology that will control and/or
treat combined sewer overflows, storm
water discharges and general washoff of
rainwater polluted by dirt or other con-
taminants on the land.)
Interceptor sewers are also used in sani-
tary sewer systems as collectors of flow
from main sewers and trunks, but do not
normally include provisions for bypassing.
Untreated sewage pours into stream from combined storm-sanitary sewer.
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Y\ \\
STORM WATER DISCHARGES
A waste treatment plant's basic function
is to speed up the natural processes by
which water purifies itself. In many cases,
nature's treatment process in streams and
lakes was adequate before our population
and industry -grew to their present size.
When the sewage of previous years was
dumped into waterways, the natural proc-
ess of purification began. First, the sheer
volume of clean water in the stream di-
luted the small amount of wastes. Bacteria
and other small organisms in the water
consumed the sewage or other organic
matter, turning it into new bacterial cells,
carbon dioxide, and other products.
But the bacteria normally present in
water must have oxygen to do their part
in breaking down the sewage. Water ac-
quires this all-important oxygen by ab-
sorbing it from the air and from plants
that grow in the water itself. These plants
use sunlight to turn the carbon dioxide
present in water into oxygen.
The life and death of any body of water
depend mainly upon its ability to main-
tain a certain amount of dissolved oxygen.
This dissolved oxygen — or DO — is what
fish breathe. Without it they suffocate.
If only a small amount of sewage is
dumped into a stream, fish are not affected
and the bacteria can do their work and
the stream can quickly restore its oxygen
loss from the atmosphere and from plants.
Trouble begins when the sewage load is
excessive. The sewage will decay and the
water will begin to give off odors. If
carried to the extreme, the water
lose all of its oxygen, resulting in the
death of fish and beneficial plant life.
Since dissolved oxygen is the key element
in the life of water, the demands on it are
used as a measure in telling how well a
sewage treatment plant is working. This
measuring device is called biochemical
oxygen demand, or BOD. If the effluent
or the end-product from a treatment plant
has a high content of organic pollutants,
the effluent will have a high BOD. In
other words, it will demand more oxygen
from the water to break down the sewage
and consequently will leave the water with
less oxygen (and also dirtier).
With the growth of the Nation, the prob-
lems of pollution have become more com-
plex. The increased amounts of wastes
and the larger demands for water have
reduced the capacity of running water to
purify itself. Consequently, cities and in-
dustry have had to begin thinking about
removing as much as possible of the
oxygen-demanding pollutants from their
sewage.
Adequate treatment of wastes along with
providing a sufficient supply of clean
water has become a major concern.
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STREET DRAIN
MUNICIPAL WASTES'
^OVERFLOW
INDUSTRIAL WASTES
w
$x
WASTEWATER TREATMENT PLAN
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BINED
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PRIMARY TREATMENT
At present, there are two basic ways of
treating wastes. They are called primary
and secondary. In primary treatment,
solids are allowed to settle and are re-
moved from the water. Secondary treat-
ment, a further step in purifying waste
water, uses biological processes.
As sewage enters a plant for primary
treatment, it flows through a screen. The
screen removes large floating objects such
as rags and sticks that may clog pumps
and small pipes. The screens vary from
coarse to fine—from those with 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 an inclined position to the
flow of the sewage to make cleaning
! -
FFLUENT
•n the up-
stream surface of the scree can be raked
off manually or mechanically.
Some plants use a device known as a com-
minutor which combines the functions of
a screen and a grinder. These devices
catch and then cut or shred the heavy
solid material. In the process, the pulver-
ized matter 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 highly 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 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 that might damage equip-
ment or interfere with later processes.
With the screening completed and the grit
removed, the sewage still -contains dis-
solved organic and inorganic matter along
with suspended solids. The latter consist
of 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, sludge removal was done
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 pit
near the tank, and then removed, usually
by pumping, for further treatment or dis-
posal.
Almost all plants built within the past 30
years have had a mechanical means for
removing the sludge from sedimentation
tanks. Some plants remove it continu-
ously while others remove it at intervals.
To complete the primary treatment, the
effluent from the sedimentation tank is
chlorinated before being discharged into
a stream or river. Chlorine gas is fed into
the water to kill and reduce the number
of disease-causing bacteria. Chlorination
also helps to reduce objectionable odors.
Although 30 percent 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.
Today's cities and industry, faced with in-
creased amounts of wastes and wastes that
are more difficult to remove from water,
have turned to secondary and even ad-
vanced waste treatment.
SCREEN
SECONDARY TREATMENT
Secondary treatment removes up to 90
percent of the organic matter in sewage by
making use of the bacteria in it. The two
principal types of secondary treatment are
trickling filters and the activated-sludge
process.
The trickling filter process or the acti-
vated sludge process is used mostly today.
After the effluent leaves the sedimenta-
tion tank in the primary stage of treat-
ment, it flows or is pumped to a facility
using one or the other of these processes.
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 con-
sists of distributing the effluent inter-
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mittently through a network of pipes laid
on or beneath the surface of the stones.
Attached to these pipes are smaller, verti-
cal 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.
The trend today is toward the use of the
activated sludge process instead of trick-
ling filters. This process speeds up the
work of the bacteria by bringing air and
sludge heavily laden with bacteria into
close contact with the sewage.
After the sewage leaves the settling tank in
primary treatment, it is pumped to an
aeration tank where it is mixed with air
and sludge loaded with bacteria and al-
Itmed 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 addi-
tional 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 lim-
itations. The size of the units necessary
for this treatment is small, thereby re-
quiring less land space and the process
is free of flies and odors. But it is more
costly to operate than the trickling filter,
and the activated sludge process some-
times loses its effectiveness when faced
with difficult industrial wastes.
An adequate supply of oxygen is neces-
sary for the activated sludge process to
be effective. Air is mixed with sewage
•
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Slowly revolving pipes in trickling filter
spray waste water over beds of stones
where bacteria consume organic matter.
and biologically active sludge in the aera-
tion tanks by three different methods.
The first, mechanical aeration, is accom-
plished 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 open-
ings 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, as
the most common method of disinfection,
to the effluent coming from the trickling
filter or the activated sludge process.
Chlorine is usually purchased in liquid
form, converted to a gas, and injected into
the effluent 15 to 30 minutes before the
treated water is discharged into a water-
course. If done properly, chlorination will
kill more than 99 percent of the harmful
bacteria in an effluent.
Aeration unit in sewage treatment plant mixes
oxygen from the air with waste water to
help bacteria break down organic compounds.
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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 may
act as less than satisfactory alternatives
at such locations.
A septic tank is simply a tank buried
in the ground to treat the sewage from
an individual home. Waste water from
the home flows into the tank where bac-
teria in the sewage may break down the
organic matter and the cleaner water flows
out of the tank into the ground through
sub-surface 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 may be a reasonable and temporary
means of disposing of strictly domestic
wastes. Septic tanks should always be
located so that none of the effluent can
seep into sources 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 sec-
ondary treatment or they can be used to
supplement other processes.
A lagoon is a scientifically constructed
pond, usually three to five feet deep, in
DIAGRAM OF A SEPTIC TANK
which sunlight, algae, and oxygen interact
to restore water to a quality that is often
equal to or better than effluent from sec-
ondary treatment. Changes in the weather
may change the effectiveness of lagoons.
When used with other waste treatment
processes, lagoons can be very effective.
A good example of this is the Santee,
California, water reclamation project.
After conventional primary and secondary
treatment by activated sludge, the town's
waste water is kept in a lagoon for 30
days. Then the effluent, after chlorina-
tion, is pumped to land immediately above
a series of lakes and allowed to trickle
down through sandy soil into the lakes.
The resulting water is of such good qual-
ity, the residents of the area can swim,
boat, and fish in the lake water.
Sunlight, algae, oxygen work together to purify waste water in a lagoon or oxidation pond.
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Industrial concentrations can cause
gross pollution without adequate treat-
ment. Note sewers dumping wastes.
Activated carbon is tested in tanks as an
improved method of removing organic
matter by adsorption. Organic com-
pounds cling to carbon and settle out.
8
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FURTHER TREATMENT OF WASTES
In the past, pollution control was con-
cerned primarily with problems caused
by domestic and the simpler wastes of
industry. Control was aimed principally
towards protecting downstream public
water supplies and stopping or preventing
nuisance conditions.
Pollution problems were principally local
in extent and their control a local matter.
This is no longer true. National growth
and change have altered this picture.
Progress in abating pollution has been
outdistanced by population growth, the
speed of industrial progress and techno-
logical developments, changing land prac-
tices, and many other factors.
The increased production of goods has
greatly increased the amounts of com-
mon industrial wastes. New processes in
manufacturing are producing new, com-
plex wastes that sometimes defy present
pollution control technology. The in-
creased application of commercial fertil-
izers and the development and widespread
use of a vast array of new pesticides are
resulting in a host of new pollution prob-
lems from water draining off land.
The growth of the nuclear energy field
and the use of radioactive materials
foreshadow still another complicating and
potentially serious water pollution situa-
tion.
Long stretches of both interstate and
intrastate streams are subjected to pollu-
tion which ruins or reduces the use of
the water for many purposes. Conven-
tional biological waste treatment proces-
ses are hard-pressed to hold the pollution
line, and for a growing number of our
larger cities, these processes are no
longer adequate.
Our growing population not only is pack-
ing our central cities but spreading out
farther and farther into suburbia and ex-
urbia. Across the country, new satellite
communities are being born almost daily.
The construction or extension of sewer
lines has not matched either the growth
rate or its movements. Sea water intrusion
is a growing problem in coastal areas. It
is usually caused by the excessive pump-
ing of fresh water from the ground which
lowers the water level, allowing salt water
to flow into the ground water area.
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THE TYPES OF POLLUTANTS
Present-day problems that must be met
by sewage treatment plants can be
summed up in the eight types of pollutants
affecting our waters.
The eight general categories are: common
sewage and other oxygen-demanding
wastes; disease-causing agents; plant nu-
trients; synthetic organic chemicals; inor-
ganic chemicals and other mineral sub-
stances; sediment; radioactive substances;
and heat.
Oxygen-demanding wastes—These are
the traditional organic wastes and am-
monia contributed by domestic sewage
and industrial wastes of plant and animal
origin. Besides human sewage, such
wastes result from food processing, paper
mill production, tanning, and other man-
ufacturing processes. These wastes are
usually destroyed by bacteria if there is
sufficient oxygen present in the water.
Since fish and other aquatic life depend
on oxygen for life, the oxygen-demanding
wastes must be controlled, or the fish die.
Disease-causing agents—This category in-
cludes infectious organisms which are
carried into surface and ground water
by sewage from cities and institutions,
and by certain kinds of industrial wastes,
such as tanning and meat packing plants.
Man or animals come in contact with
these microbes either by drinking the
water or through swimming, fishing, or
other activities. Although modern disin-
fection techniques have greatly reduced
the danger of this type of pollutant, the
problem must be watched constantly.
Plant nutrients—These are the substances
in the food chain of aquatic life, such as
algae and water weeds, which support
and stimulate their growth. Carbon, nitro-
gen and phosphorus are the three chief
nutrients present in natural water.
Large amounts of these nutrients are
10
Common sewage from homes, businesses
depletes oxygen supply in the water.
Blood and grease turn water brown
at drain from meat packing plant.
Floating algae create unsightly conditions.
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Chemicals in water from factory
turn stream into bubbling
mess and are difficult to remove.
produced by sewage, certain industrial
wastes, and drainage from fertilized
lands. Biological waste treatment
processes do not remove the phosphorus
and nitrogen to any substantial extent—
in fact, they convert the organic forms of
these substances into mineral form, mak-
ing them more usable by plant life. The
problem starts when an excess of these
nutrients over-stimulates the growth of
water plants which cause unsightly con-
ditions, interfere with treatment processes,
and cause unpleasant and disagreeable
tastes and odors in the water.
Synthetic organic chemicals—Included in
this category are detergents and other
household aids, all the new synthetic
organic pesticides, synthetic industrial
chemicals, and the wastes from their man-
ufacture. Many of these substances are
toxic to fish and aquatic life and possibly
harmful to humans. They cause taste
Acid forms in water draining from abandoned mine./
and odor problems, and resist conven-
tional waste treatment. Some are known
to be highly poisonous at very low con-
centrations. What the long-term effects
of small doses of toxic substances may
be is not yet known.
Inorganic chemicals and mineral sub-
stances—A vast array of metal salts, acids,
solid matter, and many other chemical
compounds are included in this group.
They reach our waters from mining and
manufacturing processes, oil field opera-
tions, agricultural practices, and natural
sources. Water used in irrigation picks up
large amounts of minerals as it filters
down through the soil on its way to the
nearest stream. Acids of a wide variety
are discharged as wastes by industry, but
the largest single source of acid in our
water comes from mining operations and
mines that have been abandoned.
r /
-------
n
Dirt from soil erosion pollutes water, reduces stream bed.
Many of these types of chemicals are
being created each year. They interfere
with natural stream purification; destroy
fish and other aquatic life; cause excessive
Thousands of fish are killed
each year by wastes in water
that reduce oxygen supplies.
hardness of water supplies; corrode ex-
pensive water treatment equipment; in-
crease commercial and recreational boat
maintenance costs; and boost the cost of
waste treatment.
Sediments—These are the particles of
soils, sands, and minerals washed from the
land and paved areas of communities
into the water. Construction projects are
often large sediment producers. While
not as insidious as some other types of
pollution, sediments are a major problem
because of the sheer magnitude of the
amount reaching our waterways. Sedi-
ments fill stream channels and harbors,
requiring expensive dredging, and they
fill reservoirs, reducing their capacities
and useful life. They erode power tur-
bines and pumping equipment, and reduce
fish and shellfish populations by blanket-
ing fish nests and food supplies.
12
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Steaming hot water pollutes river.
More importantly, sediments reduce the
amount of sunlight penetrating the water.
The sunlight is required by green aquatic
plants which produce the oxygen neces-
sary to normal stream balance. Sediments
greatly increase the treatment costs for
municipal and industrial water supply
and for sewage treatment where combined
sewers are in use.
Radioactive substances—Radioactive pol-
lution results from the mining and proces-
sing of radioactive ores; from the use of
refined radioactive materials in power re-
actors and for industrial, medical, and re-
search purposes; and from fallout follow-
ing nuclear weapons testing. Increased
use of these substances poses a potential
public health problem. Since radiation ac-
cumulates in humans, control of this type
of pollution must take into consideration
total exposure in the human environment
—water, air, food, occupation, and medi-
cal treatment.
Heat—Heat reduces the capacity of water
to absorb oxygen. Tremendous volumes
of water are used by power plants and
industry for cooling. Most of the water,
with the added heat, is returned to
streams, raising their temperatures. With
less oxygen, the water is not as efficient in
assimilating oxygen-consuming wastes and
in supporting fish and aquatic life. Un-
checked waste heat discharges can seri-
ously alter the ecology of a lake, a stream,
or even part of the sea.
Water in lakes or stored in impoundments
can be greatly affected by heat. Summer
temperatures heat up the surfaces, caus-
ing the water to form into layers, with the
cooler water forming the deeper layers.
Decomposing vegetative matter from nat-
ural and man-made pollutants deplete the
oxygen from these cooler lower layers
with harmful effects on the aquatic life.
When the oxygen-deficient water is dis-
charged from the lower gates of a dam,
it may have serious effects on downstream
fish life and reduce the ability of the
stream to assimilate downstream pollu-
tion.
To complicate matters, most of our wastes
are a mixture of the eight types of pollu-
tion, making the problems of treatment
and control that much more difficult.
Municipal wastes usually contain oxygen-
consuming pollutants, synthetic organic
chemicals such as detergents, sediments,
and other types of pollutants. The same
is true of many industrial wastes which
may contain, in addition, substantial
amounts of heat from cooling processes.
Water that drains off the land usually
contains great amounts of organic matter
in addition to sediment. Also, land drain-
age may contain radioactive substances
and pollutants washed from the sky, vege-
tation, buildings, and streets during rain-
fall.
13
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.(•>
ADVANCED METHODS
OF TREATING WASTES
These new problems of a modern so-
ciety have placed additional burdens
upon our waste treatment systems. To-
day's pollutants are more difficult to re-
move from the water. And increased
demands upon our water supply aggra-
vate the problem. During the dry season,
the flow of rivers decreases to such an
extent that they have difficulty in assimi-
lating the effluent from waste treatment
plants.
In the future, these problems will be met
through better and more complete meth-
ods of removing pollutants from water
and better means for preventing some
wastes from even reaching our streams in
the first place.
The best immediate answer to these prob-
lems is the widespread application of ex-
isting waste treatment methods. Many
cities that have only primary treatment
need secondary treatment. Many other
cities need enlarged or modernized pri-
mary and secondary systems.
But this is only a temporary solution.
The discharge of oxygen-consuming
wastes will increase despite the universal
application of the most efficient waste
treatment processes now available. And
these are the simplest wastes to dispose of.
Conventional treatment processes are al-
ready losing the battle against the
modern-day, tougher wastes.
The increasing need to reuse water now
calls for better and better waste treatment.
Every use of water—whether in home, in
the factory, or on the farm—results in
some change in its quality.
To return water of more usable quality
to receiving lakes and streams, new meth-
ods for removing pollutants are being de-
veloped. The advanced waste treatment
techniques under investigation range from
extensions of biological treatment capa-
ble of removing nitrogen and phosphorus
nutrients to physical-chemical separation
techniques such as adsorption, distillation,
and reverse osmosis.
These new processes can achieve any de-
gree of pollution control desired and, as
waste effluents are purified to higher and
higher degrees by such treatment, the
point is reached where effluents become
"too good to throw away."
Such water can be deliberately and di-
rectly reused for agricultural, industrial,
recreational, or even drinking water sup-
plies. This complete water renovation
will mean complete pollution control and
at the same time more water for the
Nation.
14
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COAGULATION—SEDIMENTATION
The application of advanced techniques
for waste treatment, at least in the next
several years, will most likely take up
where primary and secondary treatment
leave off. Ultimately, entirely new sys-
tems will no doubt replace the modern
facilities of today.
The process known as coagulation-sedi-
mentation may be used to increase the
removal of solids from effluent after pri-
mary and secondary treatment. Besides
removing essentially all of the settleable
solids, this method can, with proper con-
trol and sufficient addition of chemicals,
reduce the concentration of phosphate by
over 95 percent.
In this process, alum, lime, or iron salts
are added to effluent as it comes from the
secondary treatment. The flow then passes
through flocculation tanks where the
chemicals cause the smaller particles to
floe or bunch together into large masses.
The larger masses of particles or lumps
will settle faster when the effluent reaches
the next step—the sedimentation tank.
Although used for years in the treatment
of industrial wastes and in water treat-
ment, coagulation-sedimentation is classi-
fied as an advanced process because it is
not usually applied to the treatment of
municipal wastes. In many cases, the
process is a necessary pre-treatment for
some of the other advanced techniques.
ADSORPTION
Technology has also been developed to
effect the removal of refractory organic
materials. These materials are the stub-
born organic matter which persists in
water and resists normal biological treat-
ment.
15
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The effects of the organics are not com-
pletely understood, but taste and odor
problems in water, tainting of fish flesh,
foaming of water, and fish kills have been
attributed to such materials.
Adsorption consists of passing the effluent
through a bed of activated carbon gran-
ules which will remove more than 98 per-
cent of the organics. To cut down the cost
of the procedure, the carbon granules can
be cleaned by heat and used again.
An alternative system utilizing powdered
carbon is under study. Rather than pass
the effluent through a bed of granules,
the powdered carbon is put directly into
the stream. The organics stick to the car-
bon and then the carbon is removed from
the effluent by using coagulating chem-
icals and allowing the coagulated carbon
particles to settle in a tank.
The use of this finely ground carbon will
improve the rate at which the refractory
organics are removed. The potential wide-
spread use of powdered carbon adsorp-
tion depends largely on the effectiveness
of regenerating the carbon for use again.
Except for the salts added during the use
of water, municipal waste water that has
gone through the previous advanced proc-
esses will be restored to a chemical quality
almost the same as before it was used.
When talking of salts in water, salt is not
limited to the common kind that is used
in the home for seasoning food. In waste
treatment language, salts mean the many
minerals dissolved by water as it passes
through the air as rainfall, as it trickles
through the soil and over rocks, and as it
is used in the home and factory.
16
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EUECTRODIALYSIS
Electrodialysis is a rather complicated
process by which electricity and mem-
branes are used to remove salts from an
effluent. A membrane is usually made of
chemically treated plastic. The salts are
forced out of the water by the action of
an electric field. When a mineral salt is
placed in water it has a tendency to break
down into ions. An ion is an atom or a
small group of atoms having an electrical
charge.
As an example, the two parts of common
table salt are sodium and chlorine. When
these two elements separate as salt dis-
solves in water, the sodium and chlorine
particles are called ions. Sodium ions
have a positive charge while chlorine ions
have a negative charge.
When the effluent passes through the elec-
trodialysis cell, the positive sodium ions
are attracted through a membrane to a
pole or electrode that is negatively
charged. The negatively charged chlorine
ions are pulled out of the water through
another membrane toward an electrode
with a positive charge.
With the salts removed by the action of
the two electrodes, the clean water flows
out of the electrodialysis cell for reuse or
discharge into a river or stream.
As a city uses its water, the amount of
salts in the water increases by 300-400
milligrams per liter. Fortunately, electro-
dialysis can remove this buildup of salts.
In other words, this process returns the
salt content of the water back to where it
was or even better than when the city
first received the water.
THE BLENDING OF
TREATED WATER
Properly designed and applied, the meth-
ods that have been explained will be able
to supply any quality of water for any
reuse.
But none of these processes will stand
alone. They must be used in a series or a
parallel plan. In a series, all the sewage
passes through all the processes, one after
another, each process making a particular
contribution toward improving the water.
For example, the conventional primary
treatment removes the material that will
readily settle or float; the secondary bio-
logical step takes care of the decompos-
able impurities; coagulation-sedimenta-
tion, the third step, eliminates the
suspended solids; carbon adsorption re-
moves the remaining dissolved organic
matter; electrodialysis returns the level of
the salts to what it was before the water
was used; and, finally, chlorination pro-
vides the health safety barrier against
disease carriers.
Basically the same result can be achieved
by separating the effluent into two
streams. In this instance, all of the waste
receives the primary and secondary treat-
ment and then passes through the coagu-
lation-sedimentation and adsorption proc-
esses which remove the organic matter.
Half of the sewage is then treated by
evaporation and adsorption to remove all
impurities including the minerals. This
effluent, when blended with the other half,
can provide water with the desired level
17
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18
FILTER AND ADSORPTION
BW1
of minerals. After chlorination, the water
can be reused.
Almost any degree of water quality can
be achieved by varying the flow of the
two streams. This technique reduces the
treatment cost, since only a fraction of the
flow requires treatment with the more
expensive unit processes, such as distilla-
tion.
Distillation or evaporation basically con-
sists of bringing the effluent to the boiling
point. The steam or vapor produced is
piped to another chamber where it is
cooled, changing it back to a liquid. Most
of the unwanted polluting impurities re-
main in the original chamber. However,
some volatile substances may distill along
with the water and carry along foreign
materials that contribute objectionable
taste.
As most people have discovered, distilled
water has a flat, disagreeable taste caused
by the absence of minerals and air. But
by blending this pure water with water
that still contains some minerals, a clean,
better tasting water results. And just as
importantly, the more expensive distilla-
tion process is used on only part of the
effluent, and the rest of the waste water is
treated by the less costly procedures.
••--.•*•?/<
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NEW CHALLENGES
FOR WASTE TREATMENT
So far, the most readily available proc-
esses that will solve most current
pollution problems have been covered.
But the future holds many new challenges.
Scientists are still looking for the ultimate
system that will do the complete job of
cleaning up water, simply and at a reason-
able cost.
One such possible process under study is
reverse osmosis. When liquids with differ-
2nt concentrations of mineral salts are
separated by a membrane, molecules of
pure water tend to pass by osmosis from
the more concentrated to the less concen-
trated side until both liquids have the
same mineral content.
Scientists are now exploring ways to take
advantage of the natural phenomena of
osmosis, but in reverse. When pressure is
exerted on the side with the most minerals,
this natural force reverses itself, causing
the molecules of pure water to flow out of
the compartment containing a high salt
concentration.
This means that perfectly pure water is
being taken out of the waste, rather than
taking pollutants out of water as is the
traditional way. And this process takes
clean water away from everything — bac-
teria, detergents, nitrates.
Tests have shown that the theory works
well, resulting in water good enough to
drink. Efforts are now under way to de-
velop large membranes with long life.
Also, the process and equipment need to
be tested on a large scale.
-><*&?
«*-^.
w**r
Many other techniques to improve waste
19
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treatment are under development in
laboratories and in the field.
For example, special microscopic orga-
nisms are being tested for removing ni-
trates from waste water by reducing the
nitrates to elemental nitrogen.
CHEMICAL OXIDATION
Municipal waste waters contain many
organic materials only partially removed
by the conventional treatment methods.
Oxidants such as ozone and chlorine
have been used for many years to
improve the taste and odor qualities or
to disinfect municipal drinking water.
They improve the quality of water by
destroying or altering the structure of the
chemicals in the water.
However, the concentration of the organic
materials in drinking water supplies is
much less than it is in the waste-bearing
waters reaching treatment plants. Until
recently, the cost of the oxidants has pre-
vented the use of this process in the
treating of wastes. Now, improvements in
the production and application of ozone
and.pure oxygen may reduce costs suffi-
ciently to make their use practicable.
When operated in conjunction with other
processes, oxidation could become an ef-
fective weapon in eliminating wastes re-
sistant to other processes.
During the past 10 to 15 years, the
chemical industry has been working on
synthetic organic chemicals, known as
polyelectrolytes or polymers, to further
improve the separation step.
Formerly, polymers have proved effective
when used at a later stage of treatment—
the sludge disposal step. Sludge must be
dewatered so that it can be more easily
disposed of. By introducing polymers into
the sludge, the physical and chemical
bonds between the solids are tightened.
When this happens, the water can be ex-
tracted more rapidly.
Wider use of polymers is now being in-
vestigated. By putting polymers into
streams or rivers, it may be possible to
capture silt at specified locations so that
it can be removed in quantity.
If polymers are put into raw sewage,
waste treatment plants may be able to
combine a chemical process with the
standard primary and secondary stages.
And this method of removing solids can
be applied immediately without lengthy
and expensive addition of buildings or
new facilities.
The chemicals also hold promise as a
means of speeding the flow of waste
waters through sewer systems, thus, in
effect, increasing the capacity of existing
systems.
POLYMERS AND POLLUTION
In discussing the coagulation-sedimenta-
tion process, mention was made of the
use of alum or lime to force suspended
solids into larger masses. The clumping
together helps speed up one of the key
steps in waste treatment—the separation
of solids and liquids.
20
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No matter how good the treatment of
wastes, there is always something
left over. It may be the rags and sticks
that were caught on the screens at the very
beginning of the primary treatment. It
could be brine or it could be sludge—that
part of the sewage that settles to the
bottom in sedimentation tanks. Whatever
it is, there is always something that must
be burned, buried, or disposed of in some
manner.
It is a twofold problem. The sludge or
other matter must be disposed of to com-
plete a city's or industry's waste treat-
ment. And it must be disposed of in a
manner not to add to or upset the rest of
the environment.
If it is burned, it must be done in a way
not to add to the pollution of the at-
mosphere. This would only create an
additional burden for our already over-
burdened air to cope with. And air pollu-
tants by the action of rain and wind have
a habit of returning to the water, compli-
cating the waste treatment problem rather
than helping it.
There are many methods and processes
for dealing with the disposal problem,
which is sometimes referred to as the
problem of ultimate disposal. The most
common method for disposing of sludge
and other waste concentrates consists of
digestion followed by filtration and in-
cineration.
The digestion of sludge takes place in
heated tanks where the material can de-
compose naturally and the odors can be
controlled. As digested sludge consists of
90 to 95 percent water, the next step in
disposal must be the removal of as much
of the water as possible.
Water can be removed from sludge by use
of a rotating filter drum and suction. As
Sludge, solid matter left
after treatment, is spread
over the land to dry.
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the drum rotates in the sludge, the water
is pulled through the filter and the resi-
dues are peeled off for disposal. For more
effective dewatering, the sludge can be
first treated with a coagulant chemical
such as lime or ferric chloride to produce
larger solids before the sludge reaches the
filter.
Drying beds which are usually made of
layers of sand and gravel can be used to
remove water from sludge. The sludge is
spread over the bed and allowed to dry.
After a week or two of drying, the residue
will be reduced in volume and, conse-
quently, will be easier to dispose.
Incineration consists of burning the dried
sludge to reduce the residues to a safe,
non-burnable ash. The ash can be dis-
posed of by filling unused land or by
dumping it well out into the ocean. Since
most of the pollutants have been removed
by the burning, the ash should cause very
little change in the quality of the ocean
" waters.
A very promising new method of sludge
disposal gets rid of the unwanted sludge
and helps restore a ravaged countryside.
In many areas of the country, tops of hills
and mountains were sliced away to get at
the coal beneath. This strip mining left
ugly gashes and scars in otherwise beauti-
ful valleys of many States. It would take
nature many years to restore the denuded
areas.
With the new disposal idea, digested
sludge in semi-liquid form is piped to the
spoiled areas. The slurry, containing
many nutrients from the wastes, is spread
over the land to give nature a hand in re-
turning grass, trees, and flowers to the
barren hilltops.
Restoration of the countryside will also
help in the control of acids that drain from
mines into streams and rivers, endanger-
ing the fish and other aquatic life and
adding to the difficulty in reusing the
" Countryside restored after strip mining by using sludge as fertilizer for plants.
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water. Acids are formed when pyrite con-
taining iron and sulfur is exposed to the
air.
Sludge or other waste concentrates are
not always costly burdens. By drying and
other processes, some cities have pro-
duced fertilizers that are sold to help pay
for part of the cost of treating wastes. If
not sold to the public, some municipalities
use the soil enrichers on parks, road park-
ways, and other public areas.
Some industries have found they can re-
claim certain chemicals during waste
treatment and reuse them in manufactur-
ing or refining processes. Other firms have
developed saleable by-products from resi-
dues in waste treatment.
More studies are going on to find greater
use for sludge to help solve the disposal
problem and to help offset the cost of
waste treatment.
Sludge from paper mill removed from waste stream
and dried to prevent pollution of nearby river.
Revolving drum filters water from sludge
so residues can be disposed of more easily.
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Common sewage treatment terminology
Activated Sludge process removes
organic matter from sewage by sat-
urating it with air and adding bio-
logically active sludge.
Adsorption is an advanced way of
treating wastes in which activated car-
bon removes organic matter from
waste water.
Aeration Tank serves as a chamber
for injecting air into water.
Algae are plants which grow in sun-
lit waters. They are a food for fish
and small aquatic animals and, like
all plants, put oxygen in the water.
Bacteria are small living organisms
which often consume the organic con-
stituents of sewage.
BOD, or biochemical oxygen demand,
is the dissolved oxygen required by
organisms for the aerobic decomposi-
tion of organic matter present in
water. It is used as a measure in de-
termining the efficiency of a sewage
treatment plant.
Chlorinator is a device for adding
chlorine gas to sewage to kill infec-
tious 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 and iron salts.
Combined Sewer carries both sew-
age and storm water run-off.
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
out through holes in the side of the
pipe.
Digestion of sludge takes place in
tanks when the materials decompose,
resulting in partial gasification, lique-
faction, and mineralization of pol-
lutants.
Distillation in waste treatment con-
sists of heating the effluent and
then removing the vapor or steam.
When the steam is returned to a
liquid it is almost pure water. The
pollutants remain in the concen-
trated residue.
Effluent is the liquid that comes out
of a treatment plant after comple-
tion of the treatment process.
Elecrrodialysis is a process which
utilizes direct current and an arrange-
ment of permeable-active membranes
to achieve separation of the soluble
minerals from the water.
Floe is a clump of solids formed in
sewage by biological or chemical ac-
tion.
Flocculation is the process by which
clumps of solids in sewage are made
to increase in size by chemical, physi-
cal, or biological action.
Fungi are small, non-chlorophyll-bear-
ing plants which may play a useful
role in trickling filter treatment op-
erations.
Incineration consists of burning the
sludge to remove the water and re-
duce the remaining residues to a
safe, non-burnable ash. The ash
pan 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 re-
ceiving stream. This protects the
treatment plant from being over-
loaded in case of a sudden surge
of water into the sewers. Inter-
ceptors 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 empty the sewers from
homes or businesses.
Lagoons are ponds, usually man-made
to rigid specifications, in which sun-
light, algae, and oxygen interact to
restore water to a reasonable state of
purity.
Mechanical Aeration uses mechanical
energy to inject air into water, causing
the waste stream to abosorb oxygen
from the atmosphere.
Microbes are minute plant or ani-
mal life. Some microbes which may
cause disease exist in sewage.
Mixed Liquor is a mixture of acti-
vated sludge and waters containing
organic matter undergoing activated
sludge treatment in the aeration tank.
Organic Matter is the carbonaceous
waste contained in plant or animal
matter and originating from domestic
or industrial sources.
Oxidation is the addition of oxygen
which breaks down organic wastes
or chemicals in sewage by bacterial
and chemical means.
Oxidation Pond is a man-made lake
or body of water in which wastes
are consumed by bacteria. It is
used most frequently with other
waste treatment processes. An oxi-
dation 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 us-
ing screens to catch the floating
objects and tanks for the heavy
matter to settle in.
Pollution results when animal, vegeta-
ble, mineral or heat wastes or dis-
charges reach water, making it less
desirable for domestic, recreation, in-
dustry, or wildlife uses.
Polyelectrolytes are synthetic chemi-
cals used to speed the removal of
solids from sewage. The chemicals
cause the solids to flocculate 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.
24
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Salts are the minerals that water
picks up as it passes through the
air, over and under the ground, and
through household and industrial
Sand Filters remove some suspended
solids from sewage. Air and bacteria
decompose additional wastes filtering
through the sand. Cleaner water drains
from the bed. The sludge accumu-
lating at the surface must be re-
moved from the bed periodically.
Sanitary Sewers, in a separate sys-
tem, 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 sys-
tems in which bacteria consume the
organic parts of the wastes. It is
accomplished by bringing the sew-
age and bacteria together in trick-
ling 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
the top as scum. The scum is skimmed
off the top, and solids on the bottom
are pumped to incineration, digestion,
filtration or other means of final dis-
posal.
Septic Tanks are used for domestic
wastes when a sewer line is not avail-
able to carry them to a treatment
plant. The wastes are piped to un-
derground tanks directly from the
home or homes. The bacteria in the
wastes 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 set-
tles to the bottom, floats, or becomes
suspended in the sedimentation tanks
and must be disposed of by filtration
and incineration or by transport to
appropriate disposal sites.
Storm Sewers are a separate system
of pipes that carry only runoffs
from buildings and land during a
storm.
Sterilization is the destruction of all
living organisms. In contrast, disin-
fection is the destruction of most of
the living organisms.
Suspended Solids are the small par-
ticles of solid pollutants which are
present in sewage and which resist
separation from the water by con-
ventional means.
Trickling Filter is a support media for
bacterial growth, usually 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.
Virus is the smallest form of micro-
organism capable of causing disease.
Waste treatment plant being expanded to meet increasing demands
J
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the fight
4U.S. GOVERNMENT PRINTING OFFICE: 1971 O- 419 -407
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