Working for Clean Water
An Information Program for Advisory Groups
Municipal
Wastewater Processes
Details
Will flow reduction measures
remove the need for new wastewater treatment facilities?
Are all conventional, alternative,
and innovative options considered?
What are the economic, environmental, and
social tradeoffs for each alternative?
Does the preferred alternative fit in with
the lifestyle of the community?
Does the community have the resources necessary
to construct and operate the facilities?
Citizen Handbook
-------
This program was prepared by
The Pennsylvania State University
Institute of State & Regional
Affairs
Middletown, PA 17057
Dr. Charles A. Cole
Project Director
Dr. E. Drannon Buskirk, Jr.
Project Co-Director
Prof. Lama Cfar. Stoltzftis
Editor
This unit was prepared by
Charles A. Cole and John B,
Nesbitt
Advisory Team for the Project
David Elkinton. State of West
Virginia
Steve Frisfaman, private citizen
Michele Frome, private citizen
John Hammond, private citizen
Joan Jurancich, State of California
Richard Hetherington, EPA
Region 10
Rosemary Henderson, EPA
Region 6
George Hoeasel, EPA Region 3
George Neiss, EPA Region 5
Ray Pfortner, EPA Region 2
Paul Pinault, EPA Region 1
Earlene Wilson, EPA Region 7
Dan Burrows, EPA Headquarters
Ben Gryetko, EPA Headquarters
Robert Hardaker, EPA
Headquarters
Charles Kaoffinan, EPA
Headquarters
Steve Maier, EPA Headquarters
EPA Project Officer
Barry H. Jordan
Office of Water Programs
Operations
Acknowledgements
Typists:
Ann Kirsch, Jan Russ, Tess
Startoni
Student Assistants:
Fran Costanzi, Kathy DeBatt,
Michael Lapano, Mike Moulds,
Terry Switzer
Illustrator.
Charles Speers
Graphics support was provided by
the Office of Public Awareness,
U.S. Environmental Protection
Agency.
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Municipal Wastewater Processes: Details
Communities may save 40 percent of their
local wastewater treatment costs by using
alternative or innovative technologies. This
is an attractive incentive, but many of
these communities will still choose
conventional modes of treatment. Why? It
is because most are dependable, and they
produce wastewater that is
environmentally acceptable.
Whatever method of treatment is chosen,
the task is the same: the separation of
pollutants (mainly solids, but also
dissolved materials) from water. This
separation is accomplished by biological,
chemical, and physical methods. Most
approaches are patterned after Mother
Nature's methods of water purification, but
are accelerated and are concentrated to
keep up with our huge volumes of
wastewater.
Although the principles of the treatment
processes are simple, the technologies can
be complicated. Understanding these
technologies is made more difficult by the
technical language in which the processes
are sometimes discussed. When an
advisory group discusses wastewater
treatment options, it must be familiar with
the requirements and limitations of these
processes. An understanding of treatment
processes can begin by following the path
of a drop of wastewater as it travels
through treatment facilities.
A Drop of Wastewater
Upon entering a treatment plant, a
wastewater drop (and billions like it)
usually flows through a series of
preliminary processes — screening, grit
removal, and/or shredding. These processes
either remove the coarse materials from
the wastewater, or make them smaller for
further treatment. The drop then
undergoes a stage of primary treatment.
During this treatment phase, solids that
float or settle are separated from the
wastewater.
Some pollutants that remain are removed
by secondary treatment processes. These
methods usually involve biological
treatment. Organisms, mainly bacteria,
through their metabolic functions convert
the pollutants into forms which are easier
to remove from wastewater. Secondary
treatment is now required as a minimum
for all wastewaters.
The drop may undergo advanced waste
treatment for the removal of substances not
ordinarily taken out at other stages of
treatment. Dissolved nutrients and some
Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Disinfection
(if required)
Effluent
Solids
disposal
Categories of wastewater
treatment processes
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organic materials are removed with
advanced treatment. These advanced
processes may follow previous stages, or
they may be used instead of them. As
compared to other options, advanced waste
treatment is costly.
Waste materials that are removed by the
treatment processes go to facilities for
handling solids. These materials, called
sludge, are ultimately disposed of by land
application, incineration, or other means.
Before the treated drop is discharged into a
lake or stream, it may be disinfected to
reduce the risk of disease. The drop then
returns to the natural water cycle. It may
collect impurities and immediately undergo
treatment, or it may not appear in
wastewater again for centuries.
The number of treatment processes and the
degree of treatment usually depend upon
the uses of the receiving waters. Treated
wastewaters discharged into a small
stream used for a domestic water supply
will require a considerably higher level of
treatment than wastewater discharged into
water used solely for transportation.
Effluent criteria are thus established for
each wastewater treatment facility. Two
principal criteria for assessing the
efficiencies of many wastewater treatment
processes are the removal of suspended
solids and BOD. Many solids serve as food
for organisms present in the sewage. As
organisms such as bacteria feed on organic
matter (carbon-containing substances),
oxygen is consumed in direct proportion to
the amount of nutrients present. These
organisms cause a biochemical oxygen
demand (BOD). The measurement of BOD
thus represents the amount of organic
matter present in water.
Effluent requirements are only one factor
to be considered in selecting wastewater
treatment alternatives. Others include:
• Wastewater characteristics
• Environmental effects
• Resource requirements
(energy and chemicals)
• Monetary costs
• Sludge handling and disposal
• Process reliability and flexibility.
Primary Treatment
Primary wastewater treatment removes
those pollutants which will either
accumulate on a screen or settle. The
screen removes large floating objects such
as rags and sticks that may clog pumps
and small pipes. The debris removed from
the screen is usually buried in a landfill.
Some plants use a device known as a
comminutor, which combines the functions
of a screen and a grinder. This device
shreds the solid material in the
wastewater. The pulverized matter
remains in the wastewater to be removed
later in a settling tank.
After the wastewater has been screened or
comminuted, it passes into a grit chamber
where cinders and small stones are
allowed to settle to the bottom. A grit
chamber is highly important for cities with
combined sewer systems. It removes the
grit or gravel that washes off streets or
land during a storm and ends up at
treatment plants. This material is usually
Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Solids
disposal
Disinfection
(if required)
Effluent
-------
Primary wastewater treatment
taken from the tank, washed, and buried
in landfills near the treatment plant.
After the screening and grit removal, the
wastewater still contains suspended solids.
Some can be removed from the sewage in a
sedimentation tank or primary clarifier.
Wastewater flows through the tank very
slowly. During a two-hour period, the
suspended solids gradually sink to the
bottom. This mass of settled solids is called
raw primary sludge. It is removed from the
primary clarifier tank by mechanical
scrapers and pumps, and is transferred to
sludge processing operations. Floating
materials, such as grease and oil, rise to
the surface of the sedimentation tank
where they are collected by a
surface-skimming system. They are
removed from the tank for further
processing, usually to a sludge digester.
In primary treatment only the heavier
particles are removed. The very fine
suspended solids and dissolved substances
are taken out in subsequent treatment
operations.
Secondary Treatment
The major purpose of secondary treatment
is to remove the BOD-causing substances
that escape primary treatment, and to
remove more of the suspended solids. In
most cases the secondary processes
function by biological means. They are
designed to provide the proper
surroundings for the breakdown of organic
materials by microorganisms. A variety of
approaches are used to establish a growth
Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Disinfection
(if required)
Effluent
Solids
disposal
-------
environment. These secondary processes
usually supply oxygen, and/or provide
surfaces on which the microbes can grow.
Some possibilities include:
• Trickling filter
• Activated sludge tank
• Oxidation pond and lagoon
• Rotating biological contactor
• Activated biofilter
• Aquaculture
• Land treatment.
Land treatment of effluent has gained
considerable popularity in recent years.
Wastewater with at least primary
treatment is applied to vegetated soils.
Biological, chemical, and physical actions
remove contaminants from the water. Land
treatment is discussed in more detail in
the section on advanced wastewater
treatment.
Trickling Filter
A trickling filter consists of a bed of coarse
materials, such as rocks, slats, or plastics,
over which wastewater is applied by
rotating pipes or fixed nozzles.
As the wastewater trickles through the
bed to underdrains, microbial growth
occurs on the surface of the materials.
Microorganisms consume most of the
organic matter in the sewage. However,
the microorganisms sloughed off the filter
surfaces result in suspended solids in the
wastewater. Thus, the flow from the
Trickling filter
Outlets
Distributor arm
Drains
trickling filter is passed through a
sedimentation basin which collects these
solids by allowing them to settle. This
sedimentation basin is referred to as a
secondary clarifier or a final clarifier, to
differentiate it from the sedimentation
basin used for settling at the primary
treatment phase. Solids from this clarifier
are further treated in the sludge handling
operation.
Rock trickling filters have performed well
for decades. In recent years other materials
have found increased use, such as plastic
rings, corrugated plastic sheets, and
redwood slats. These materials offer a
larger surface area for the growth of
microbes, and more open space for air flow
than rock. They also weigh less so it is
possible to construct a taller filter bed that
uses less land area than a rock filter.
A typical overall efficiency of a municipal
wastewater trickling filter treatment plant
is about 85 percent removal of BOD and
suspended solids, which corresponds to
about 30 milligrams per liter of each in the
final effluent. Trickling filters have long
been a popular treatment process.
Trickling Filter
Advantages
• Simple process and equipment
• Responsive to variable pollutant
loads
• Minimal operator skills
• Minimal plant maintenance
• Low energy requirements relative to
activated sludge
Disadvantages
• Vulnerable to cold weather
• Reduced treatment efficiency in
winter
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Activated Sludge
The activated sludge process is a biological
wastewater treatment technique in which
a mixture of wastewater and biological
solids (microorganisms and wastes) is
agitated and aerated. The biological solids
are subsequently separated from the
treated wastewater. A portion of these
solids is returned to the aeration process as
it is needed. As the microorganisms grow
and are mixed with the air, the individual
organisms clump together to form an
active mass of microbes called activated
sludge.
In the conventional activated sludge
process, the wastewater flows continuously
into an aeration tank where air mixes the
activated sludge with the wastewater, and
supplies the oxygen needed for the
microbial growth. The mixture from the
aeration tank flows to a secondary clarifier
where the activated sludge is settled. Most
of the settled biological sludge is returned
to the aeration tank to continue rapid
breakdown of the organic materials.
Because more activated sludge is produced
than can be used in the process, some of
Activated Sludge
Advantages
• Treats various wastewater
compositions
• Meets various effluent standards
• Compared to trickling filter:
higher quality of effluent, slightly
lower capital costs, and smaller
land area requirement
Disadvantages
• Need for careful operational controls
• High energy requirements
the returned sludge is separated for final
treatment and disposal. In conventional
systems the wastewater is typically
aerated for 6-8 hours in long, rectangular
aeration basins. Air is introduced either by
injecting it near the bottom of the aeration
tank, or by mechanical mixers located at
the surface.
Many variations of this conventional
system have improved the process
performance. These variations depend on
adjustments in treatment time, method of
aeration, or in use of pure oxygen rather
than air. Approaches known as contact
stabilization, extended aeration, and ditch
oxidation are all variations of the basic
process.
Activated sludge
Aeration tank
Settling tank
Influent
fr Waste
sludge
5
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Oxidation pond
Oxidation Pond and Lagoon
Large, relatively shallow basins used for
treating wastewater through the
interaction of sunlight, wind, algae, and
oxygen are called oxidation ponds, lagoons,
or stabilization ponds. They are one of the
most common treatment systems. They
account for about one-third of all secondary
treatment plants in the United States.
About 90 percent of the ponds are used in
towns with less than 10,000 people (1
million gallons per day treatment
capacity). Primary processes are sometimes
used for the pretreatment of wastes, but
this added cost is usually not justified.
The most critical factor in this process
involves the supply of oxygen. If oxygen is
insufficient, acceptable treatment will not
occur. To eliminate the dependence of
algal-produced oxygen and to reduce the
area required by the ponds, aeration
equipment is sometimes used to supply
oxygen. Such a system is called an aerated
lagoon. Air can be supplied by a
compressor that injects air into the pond
through tubing on the pond bottom, or by
mechanical aerators installed at the
surface of the pond. Aerated ponds are
typically about one-fifth the size of a
conventional oxidation pond. Aerated
lagoons are usually followed by a second
settling pond. A pond can often accomodate
15 to 60 days of wastewater flows. In
conventional ponds, sludge is removed by
dredging.
Oxygen Carbon dioxide. Ammonia. Phosphate
Bacteria
Anaerobic
Oxidation ponds usually meet secondary
treatment requirements for the removal of
BOD. However, they occasionally fail to
meet secondary requirements for
suspended solids removal because of the
algae in the pond effluent. Effluent
suspended solids requirements for ponds
have been relaxed in most states because
of this algae concentration.
Oxidation Pond
Advantages
• Ease in construction, operation, and
maintenance
• Low construction costs
• Minimal equipment maintenance
• Effective removal of disease-causing
organisms
Disadvantages
• Large space requirement for
conventional pond
• Weed problems and dike failures
• Difficulty in meeting effluent
requirements due to algae
• Complex operations and high costs if
algae removal is required.
Rotating Biological Contactor
This process, also sometimes called the
biodisc or rotating biological surface,
consists of a series of closely-spaced plastic
discs mounted on a horizontal shaft. They
are rotated while about one-half of their
surface area is immersed in wastewater.
Oxygen is absorbed onto a film of
wastewater on the discs. These devices
provide a surface for the growth of
microorganisms. As the microbes become
dislodged, they are kept in suspension by
the moving discs. As the treated
wastewater flows from the reservoir below
the discs, it carries the suspended growths
to a settling basin for removal.
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Rotating Biological Contactor
Advantages
• No recycling of sludge
• Minimal maintenance on
mechanical equipment
• Higher pollutant removal than
• ger pout
trickling filter
• As compared to activated sludge:
less susceptible to upset and washout,
and fewer process decisions by
operator
Disadvantages
• Needs to be protected from weather
by covers
• Reduced efficiency in cold climates
• No long-term operating experience
inlLS.
Activated Biofilter
This process combines features of both the
trickling filter and activated sludge
systems. The process recirculates both the
effluent and the settled sludge from the
secondary clarifier thus creating a mixed
liquid. The trickling filter media used in
this system is made up of redwood slats.
Oxygen is supplied by the splashing of the
wastewater between layers of the redwood
slats, and by the movement of the
wastewater across the layer of microbes
attached to the slats. Supplemental
aeration is sometimes provided in an
aeration tank between the filter and
clarifier.
Activated Biofilter
Advantages
• Stable operations and minimal
process upsets
• Improvement of activated sludge
efficiency
• As compared to a trickling filter:
needs less area, and is more
vulnerable to cold temperatures
Disadvantage
• Requirement for supplemental
aeration
Aquaculture
Aquaculture is the growing of plants or
animals in water. Aquacultural systems for
wastewater treatment include both natural
and artificial wetlands and other systems
that usually involve the production of
algae and other plants. The natural
wetlands suitable for treatment may
closely resemble a bog. Water hyacinth, a
large fast-growing plant, is found
throughout the South, and is being used
for waterwater treatment. The growing
plants have a high capacity for using both
nutrients and organic matter in the
wastewater.
Aquaculture
Advantages
• Low energy requirements
• Low capital and operating costs
• Useful for polishing effluents
• Possible plant by-products
Disadvantages
• Climate-limited to southern U.S.
• Requires large land area
• Toxic materials may affect plants
Secondary Treatment
Considerations
Most secondary wastewater treatment
processes are well developed, but choosing
technologies for a facility cannot be done in
a supermarket fashion. Many different
factors must be considered, including
process benefits and drawbacks. For
example, a trickling filter can save energy,
but it may cost more for construction.
Capital, energy, chemicals, and land costs
can be traded off, depending on particular
processes.
-------
In evaluating treatment alternatives,
various considerations can be taken into
account. They include: capital cost,
operation and management costs, energy
requirements, land requirements,
treatment reliability, climate impact,
operator simplicity, response to shock
loads, effects of toxic materials, and sludge
production.
In planning a wastewater treatment
facility several parties work together,
including the grantee, the consultant,
and the advisory group. Difficult
technical decisions have to be made.
A potential for conflict exists. As
community representatives, the
advisory group must see that
community concerns enter the
discussion. Although the advisors
usually have no water quality training,
they must communicate with the
consultants on technical matters.
Questions must be asked without
unrealistic second-guessing. In
selecting treatment processes the
following questions need answers:
• What is the source of wastewater, and
can the quantity of water be reduced?
• Can the community afford to pay for
and operate particular processes?
• What are the reasons for using a
particular pollutant removal scheme —
climate, experience of the consultants,
process reliability, monetary costs,
suitability to the problems of the area,
or what?
• Does the plan permit future
modifications and additions to the
system?
• Are innovative or alternative
solutions as well as multiple uses
considered?
• Do the choices fit in with the values
of the community?
• HOWT will the treatment alternatives
affect the environment?
Evaluation of Secondary Treatment Alternatives
System*
Conventional activated
sludge
Pure oxygen activated
sludge
Rock trickling filter
Plastic trickling filter
Activated biofilter
Rotating biological
contactor
Oxidation pond with
filtration
Aerated lagoon with
filtration
Land treatment
inreiemja Hating
Treatment
Reliability
M
H
H
H
M
M
H
H
H
H
Land
Requirement
M
L
M
L
M
L
H
H
H
L
Capital
Cost
M
H
M
M
M
H
L
L
M
L
Energy
Requirement
M
M
L
L
M
L
L
L
M
L
Operating
Coxt
H
H
M
M
M
M
L
M
M
L
Climate
Impact
L
L
M
M
M
M
H
H
H
L
Sludge
Production
H
H
M
M
M
M
L
L
L
L
Relatin-Ratings: High =# Medium =M LL-U- = /-
"80-90 percent removal of BOD
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Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Disinfection
(if required)
Effluent
Solids
disposal
Advanced Processes
Conventional secondary processes do not
remove all pollutants. Some that remain
may be of major concern. Processes are
available to remove these additional
pollutants. Besides solving tough pollution
problems, these processes improve the
effluent quality to the point where it is
adequate for many reuse purposes. They
may convert what was originally
wastewater into a valuable resource too
good to throw away, such as the reuse of
effluent by industries.
In the past the advanced processes were
often called "tertiary wastewater
treatment" or just advanced wastewater
treatment. They can be subdivided into
"advanced secondary wastewater
treatment" and "advanced wastewater
treatment" categories. However, the
following sections describe available
advanced processes without dividing them
into their two separate classifications.
Phosphorus Removal
Phosphorus is one of the components of
wastewater that can seriously disrupt the
ecological balance of our waters. To meet
water quality standards, many cities are
required to reduce phosphorus to low
concentrations in wastewater discharges.
Phosphorus is not removed to any
appreciable extent in conventional primary
or secondary treatment. However, it can be
removed by relatively minor modifications
to existing municipal wastewater
treatment facilities. Phosphorus removal
processes involve:
• Chemical precipitation
• Biological removal
• Land treatment.
In the chemical precipitation processes,
chemicals called coagulants — substances
such as aluminum sulfate (alum), lime, or
ferric chloride — are added to the
wastewater. These substances cause the
solids in the wastewater to coagulate and
clump together so as to settle faster. If the
proper amount of coagulant is added, it
also converts the phosphorus in the
wastewater into an insoluble form that can
be removed by settling. Approximately 90
percent of the phosphorus and suspended
solids, and an additional amount of the
BOD normally present in a secondary
effluent can be removed through
precipitation.
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Chemical coagulation
Coagulant
Influent [i| £-
Settling tank
Rapid mix
Slow mix (Flocculation)
Chemical sludge
The necessary amount of coagulant varies
among localities, depending on the
characteristics of the wastewater being
treated. Large amounts of chemicals are
usually required for the maximum removal
of phosphorus, while a much smaller
quantity may be adequate for just
suspended solids removal.
Chemical Precipitation
Advantages
• Removal of BOD, phosphorus, and
suspended solids
• Simple process controls
• Improved reliability of secondary
treatment
• Significant separation of metals,
bacteria, and viruses
Disadvantages
• High COSt
• Large quantities of chemical sludge
for disposal
• Some chemicals (alum, ferric
chloride) are not reusable
• Increase in wastewater dissolved
solids
• Untested full-scale operations
(biological removal)
In biological removal, a modified activated
sludge process is operated so that the
microbes take up the required amount of
phosphorus. The phosphorus is then
separated from the activated sludge in a
stripping process. These actions remove
phosphorus from the wastewater, and
either significantly reduce or eliminate the
chemicals required for precipitation. This
removal of BOD and suspended solids is
equivalent to, or better than, the results of
the conventional activated sludge process.
Biological removal may be the most
economical process for phosphorus removal
other than land treatment. However,
cost-effectiveness analysis will make these
determinations on a case-by-case basis.
The land treatment process is another
option for phosphorus removal. Land
treatment involves putting wastewater
onto land rather than discharging it into
lakes and streams. Phosphorus and other
nutrients are separated from the water by
growing plants or soil processes as the
water passes through.
Land Treatment
Advantages
• Recycling of nutrients such as
phosphorus, nitrogen, and organic
matter
• Increased crop production
• Recreation and open space potential
• Retention of water in watersheds
• No chemical sludge
Disadvantages
• Scarcity of suitable sites
• Relatively large land requirements
• Seasonal operation may be
necessary in colder climates
10
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Filtration
The process of passing wastewater through
a granular bed such as fine sand and/or
coal to remove suspended matter is called
filtration. Modern wastewater filters are
often made up of a mixture of two or three
different materials (coal, sand, and garnet
are commonly used) of varying sizes and
densities. These materials form a
multimedia filter which is coarse at the
upper surface and becomes uniformly finer
with depth. Efficient filtration of a
chemically-treated effluent can reduce
suspended solids to almost zero and
phosphorus to 0.1 mg/L or less.
Wastewater is passed downward through
the filter until the filter becomes clogged
with material removed from the
wastewater. The filter is then cleaned by
reversing the flow (called backwashing).
The backwash water is then returned to
the head of the treatment facility.
Microscreening is another type of filtration.
Microscreens are cylindrical drums
covered by a metallic filter fabric. They
rotate slowly in a tank with two
compartments, so that water enters a drum
from one end and flows out through the
filtering fabric. The waste solids are
retained on the surface of the rotating
screen. These solids are flushed from the
screen and collected in a hopper or trough
inside the drum for return to the secondary
treatment plant. Microscreens can usually
reduce the suspended solids concentration
in activated sludge effluent from
20-25 mg/L to 6-10 mg/L.
Influent
Filter media
Effluent
Filtration
Advantages
• Control of suspended solids in
secondary effluent
• Additional removal of phosphorus
and suspended solids in
coagulation-sedimentation processes
• Increased treatment reliability
• Easily automated and time-tested
• Minimal operator attention
• Minimal space requirements
Disadvantage
• Processing of backwash wastes
Carbon Adsorption
Even after secondary treatment,
coagulation-sedimentation, and filtration,
some organic materials that are resistant
to biological breakdown will remain in the
effluent. One removal method for this
material involves activated carbon.
Activated carbon is a finely-ground carbon
with a very large surface area. Organic
contaminants are removed by adsorption,
which is the attraction and accumulation
of one substance (waste) on the surface of
another (carbon). After the adsorption
capacity has been reached, the carbon can
be restored by heating it in a furnace at a
temperature sufficiently high to drive off
the adsorbed materials.
Activated carbon is utilized in two forms,
powdered and granular. The powdered
carbon is mixed with the wastewater for
several minutes to allow adsorption to
occur. It then is removed by settling —
usually with the assistance of a coagulant.
The carbon adsorption is achieved by
passing the wastewater through long
columns or beds of the carbon.
Multimedia, filter
11
-------
Treatment preceding carbon adsorption can
be varied, depending on the desired final
effluent quality. Carbon adsorption often
follows processes such as secondary
treatment, coagulation-sedimentation, and
filtration. By combining these processes, a
colorless, odorless, sparkling-clear effluent
can be produced. It can be free of bacteria
and viruses, and can contain a BOD of less
than 1 mg/L and an organic concentration
of less than 10 mg/L. This water quality is
suitable for many purposes.
Another approach called independent
physical-chemical treatment also uses
carbon. In this method biological secondary
processes are eliminated altogether.
Carbon is the sole means of removing
organic substances. The raw wastewater is
usually coagulated and settled (and
sometimes filtered) before it is passed
through the carbon system. This approach
provides a degree of treatment better than
biological secondary treatment followed by
carbon adsorption. The approach is useful
in meeting temporary treatment
requirements, or in cases where space is
very limited. The process is usually more
costly than the biological secondary
processes.
Nitrogen Control
Nitrogen plays a fundamental role in the
aquatic environment. However, if excessive
amounts of nitrogen are discharged into
waterways, serious pollution problems can
result. During conventional biological
wastewater treatment, almost all the
nitrogen in the wastewater is converted
into ammonia and/or nitrates. Although
ammonia in wastewater has low toxicity
for humans, it can: consume dissolved
oxygen in the receiving water; damage
aquatic life; corrode copper fittings;
increase the chlorine requirements for
disinfection. On the other hand, nitrates at
high concentrations may be toxic to
infants.
Ammonia nitrogen can be reduced in
concentration or removed from wastewater
by several processes. These processes are:
• Biological nitrification and
denitrification
• Land treatment
• Physical-chemical methods such as
ammonia stripping and selective ion
exchange.
Carbon Adsorption
Advantages
• Removal of organic materials
passing through biological secondary
treatment processes
• Accommodates wide variations in
flows, wastewater quality, and
concentration of toxic materials
space requirement
• Needs minimal space
•gee
'Relatively expensive
i High energy requirement for carbon
• Equipment for carbon regeneration
and reuse is ill-suited for small plants
and requires very careful operator
control
Biological Nitrification and Denitrification
In this process nitrogen-containing matter
such as protein is broken down in two
biological steps. First, the nitrogenous
matter is converted into nitrates
(nitrification) by providing oxygen in the
proper amount. The nitrification step is
usually accomplished by using activated
sludge, a trickling filter, or a rotating
biological contactor. It may follow or be
combined with secondary treatment for the
removal of BOD. This action may
accompany the biological conversion of the
nitrates into nitrogen gas (denitrification).
In many cases, carrying out only the
nitrification step may be adequate to meet
effluent requirements.
12
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New biological processes that accomplish
nitrification, denitrification, and the
biological removal of phosphorus have been
recently developed and patented. However,
these processes have not yet been used
extensively on a plant-size scale.
The efficiency of biological nitrification is
usually 80 to 90 percent conversion of
ammonia to nitrate. The combined
nitrification-denitrification process can
remove up to 80 percent of the total
nitrogen.
Land Treatment
Land treatment of wastewaters can provide
moisture and nutrients necessary for crop
growth. Wastewater usually contains
substantial amounts of nitrogen and
phosphorus that are useful for crop
production. The natural processes remove
the nutrients by plant growth, and the water
is returned to the hydrologic cycle. The
wastewater is treated on the soil by slow
rate irrigation, overland flow, or
infiltration-percolation.
secondary effluent can be converted to
ammonia gas by raising the pH to high
(alkaline) values. The gaseous ammonia
can then be released by passing the high
pH effluent through a stripping tower. The
use of lime permits the simultaneous
coagulation of suspended solids and the
removal of phosphorus, while at the same
time adjusting the pH for the stripping
process.
The concentration of ammonia emitted
from the tower is very low — well below
odor levels, and does not cause air
pollution problems. However, lime scaling
and energy requirements make the process
unattractive. This type of system was
abandoned at the Lake Tahoe, California,
advanced wastewater treatment facility.
Biological Nitrification
Advantages
• Design and operations similar to
secondary treatment processes
• Low sludge volumes
• Minimal air or water quality side
effects
Disadvantages
• Large space requirements relative
to secondary treatment
• Vulnerable to upsets by toxic
substances, equipment failures, or
operator error
• High energy usage
Physical-Chemical Methods
A process that removes gaseous ammonia
from water by agitating the water-gas
mixture in the presence of air is called
ammonia stripping. Ammonia nitrogen in
Ammonia Stripping
Advantages
• Simple technology
• Minimal space requirements
Disadvantages^
• Decreased efficiency in cold
temperatures
• Inoperable in freezing conditions
• Lime scaling in tower
• High electrical energy use
In selective ion exchange, ammonium ions
in solution are exchanged for sodium or
calcium ions. The process operation
resembles a water softener, except that the
material being removed is ammonium
nitrogen rather than water hardness. The
bed must be regenerated periodically so
that its capacity to remove ammonia is
restored. The process is very efficient. It
can remove 95-97 percent of the ammonia
nitrogen.
13
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Selective Ion Exchange
Advantages
• High removal efficiency
• Immune to temperature variations
• Useful fertilizer product
• Controllable process
• Minimal space requirements
Disadvantages
• Complex equipment and operations
• High capital costs
• Disposal of waste product
Concerns About Advanced
Treatment
Much thought needs to be given to the
planning of advanced wastewater
treatment (AWT) systems.
The advisory group can contribute by
asking the following questions at the
start of the discussions:
• Have community-wide options such
as wastewater flow reduction and
changed water uses been explored that
will diminish the need for AWT?
• Is AWT really needed to meet surface
water quality standards?
• Has land treatment been considered
as an alternative to AWT?
• Can the community afford the
on-going chemical and energy expense
of AWT?
• Are there sufficient disposal sites in
the area for AWT sludge?
• Will the treatment facilities have
competent personnel for dealing with
complex AWT processes?
• Will the community's welfare be
endangered if an AWT process fails?
What recourse will the community
have?
Evaluation of Advanced Treatment Alternatives
System
Phosphorus Removal
Chemical precipitation
Biological removal
Land treatment
Nitrogen Control
Nitrification
Ammonia stripping
Ion exchange
Land treatment
Organic Matter Removal
Carbon adsorption
i Preferred Rating:
Treatment
Reliability
H
L
H
M
M
H
H
H
H
Land
Requirement
L
L
H
L
L
L
H
L
L
Capital
Cost
L
M
M
M
H
M
M
H
L
Energy
Requirement
M
M
M
M
H
L
M
H
L
Operating
Cost
H
M
L
M
H
H
L
H
L
Climate
Impact
L
L
H
L
H
L
H
L
L
Relative Rating?: H = H Medium = M Lou- = L
14
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Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Solids
disposal
Disinfection
(if required)
Effluent
Disinfection
The last step in a treatment plant is
sometimes the addition of a disinfectant to
the treated wastewater to kill pathogenic
(disease-causing) bacteria and viruses. This
process differs from sterilization, which is
the killing of all living organisms. The
addition of chlorine gas or some other
chemical form of chlorine is the process
most commonly used for wastewater
disinfection in the United States. The
wastewater then flows into a basin, where
it is held for about 30 minutes to allow the
chlorine to react with the pathogens. Some
concern about the formation of chlorination
by-products as potential carcinogens exists,
but the use of chlorine has proven to be a
very effective means of disinfecting both
wastewaters and water supplies.
Many European countries use ozone rather
than chlorine for disinfection. Ozone is an
energetic form of oxygen that readily
reacts with many substances. In the
United States, ozone generators are used
to purify air, among other uses.
Sludge Handling
In purifying wastewaters another problem
is created — sludge handling. The sludge
is made of materials separated from the
raw wastewater. It consists primarily of
organic substances and solids such as the
precipitates produced in some advanced
treatment. Whatever the wastewater
process, there is always something that
must be burned, buried, treated for reuse,
or disposed of iri some way.
Except when land treatment is used,
higher degrees of wastewater treatment
usually result in larger amounts of sludge
that must be handled. The satisfactory
treatment and disposal of sludge can be the
single most complex and costly operation in
a conventional wastewater treatment
system. Without sludge treatment, even the
best wastewater treatment process is
incomplete.
The basic operations of sludge treatment
are:
• Conditioning
• Thickening
• Stabilization
• Dewatering
• Disposal.
15
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Influent
Primary
treatment
Secondary
treatment
Advanced
waste
treatment
Disinfection
(if required)
Effluent
Solids
disposal
Although various combinations of
equipment and processes are used in
treating sludges, the basic alternatives are
fairly limited. The ultimate depository of
the sludge materials could be either land,
air, or water. Current policies discourage
disposal practices such as ocean dumping.
Air quality considerations require air
pollution equipment as part of the sludge
incineration process so that sludge cannot
be discharged into the air. Thus, the sludge
in some form eventually will be returned to
the land.
Sludge Conditioning
Several methods of conditioning sludge to
ease the separation of the liquid and solids
are available. The principal ways involve
chemicals, or heat and pressure.
Chemical coagulants such as ferric
chloride, lime, or organic polymers are
commonly used. Ash from incinerated
sludge has also found use as a conditioning
agent. These substances are mixed with
the sludge just ahead of the thickening or
dewatering processes. Chemical sludge
conditioning is used at hundreds of
municipal treatment plants.
Another conditioning approach is to heat
the sludge at high temperatures and
pressures. Under these conditions, much
like those of a pressure cooker, water
bound in the solids is released.
Another method involves the application of
heavy doses of chlorine to the sludge. This
is a relatively new approach. Because of
the acidic effects of the chlorine, it also
stabilizes the organic sludges.
Sludge Thickening
After the sludge has been conditioned, it is
often thickened before further processing.
Thickening is usually accomplished in one
of two ways:
• Solids are floated to the top of the liquid
• Solids are allowed to settle to the bottom.
The goal is to remove as much water as
possible before the final dewatering or
disposal of the sludge.
Thickened
sludge
Sludge
Water
Flotation thickening
16
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Flotation Thickening
Advantage
• Separation of light particles of
activated sludge from wastewater
Disadvantages
• Compressed air requirements
• Control difficulties
In flotation thickening, air under pressure
is injected into the sludge to float solids to
the top of liquid. The process typically
increases the solids content five times.
Gravity thickening, essentially a
sedimentation process similar to those
which occur in all settling tanks, allows
solids to settle to the bottom. Gravity
thickening also can increase primary
sludge solids by five times. The current
trend is towards using gravity thickening
for primary sludges, and flotation
thickening for activated sludge. The
thickened products are then blended for
further processing.
Gravity Thickening
Advantage
• Simple operation
Disadvantages
• Occasional odor problems
• Difficulty in separation of light
particles
Sludge
Water
Thickened
sludge
Small, rotating drum screens have been
introduced recently for thickening sludge.
These screens are similar to a large
kitchen strainer. Polymer-conditioned
sludge is fed to the inside of the drum.
Water passes through the screen and is
returned to the wastewater treatment
process. The thickened sludge falls from
the open end of the strainer.
Sludge Screening
Advantage
• Small space requirements
Disadvantage
• Requires careful operational
controls
Sludge Stabilization
Sludge stabilization biologically breaks
down the organic solids so that they are
more stable (less odorous and less
putrescible), are easier to dewater, and
have less mass. If the sludge is to be
dewatered and burned, stabilization
normally is not used. Many municipal
plants do not use incineration. Instead they
rely on sludge digestion to stabilize the
organic sludges before final disposal. Two
basic processes are in use: anaerobic
digestion, and aerobic digestion.
Anaerobic digestion involves the
breakdown of solids in an environment
that is devoid of oxygen. Most modern
anaerobic digesters use tanks in a
two-stage process. In the first stage
biological digestion occurs. The second
stage is used for storing and concentrating
the digested sludge. The second operation
may be an open tank, an unheated tank, or
a sludge lagoon. As the organic solids are
broken down by anaerobic bacteria, liquids
and gases are formed. A relatively clear
liquid, called supernatant, can be
withdrawn and recycled to the treatment
system. Methane and carbon dioxide also
are formed. The digester gas containing
methane is a usable fuel. It is used
principally for heating the first digestion
Gravity thickening
17
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tank. It also can be used for boiler and
internal combustion engines that are, in
turn, used for pumping sewage, operating
blowers, and generating electricity. An
efficiently operating anaerobic digester
converts about 50 percent of the organic
solids to liquid and gaseous forms.
Anaerobic Digestion
Advantages
• Production of a useful by-product,
methane
• Reduces the final volume of sludge
for disposal
Disadvantages
* Sensitive to variations in amounts
of sludge and toxic materials
* Increased safety requirements
• Results in a supernatant with high
concentration of pollutants that must
be treated
Aerobic digestion is accomplished by
injecting air into the organic sludges. Its
most extensive use has been in relatively
small activated sludge plants. However, it
is receiving increased attention for larger
facilities. For example, the Metropolitan
Denver Sewage Disposal District uses
aerobic digestion for sewage flows over
100 mgd. Solids reduction efficiency is
similar to the anaerobic processes.
Aerobic Digestion
Advantages
* Stable operation, not sensitive to
upset
• Results in relatively clean
supernatant
Disadvantages
• Requires considerable amount of
electricity
• Difficulty in thickening solids by
gravity settling
* Generates no useful product such as
methane
Sludge Dewatering
Water may be extracted from sludge by
various approaches:
• Sandbeds
• Vacuum filters
• Centrifuges
• Pressure filters
The most widely used method of sludge
dewatering involves drying the sludge on
sandbeds. These beds are especially
popular in small plants because of their
simple operation and maintenance. They
usually consist of a layer of sand placed
over gravel. Sludge is drawn from the
digester, placed on the sandbed, and
allowed to stand until it is dried by
drainage and evaporation. In good weather,
the solids can be concentrated several-fold
within six weeks. At that time, the sludge
will resemble moist soil. Sandbeds are
sometimes enclosed by glass in
greenhouse-type structures to protect the
sludge from rain, and thus shorten the
drying period. This arrangement is also a
form of solar heater.
As the number of secondary treatment
facilities grow, the use of more compact
and more efficient mechanical-dewatering
systems is increasing. These systems
include vacuum filters, centriftiges, and
pressure filters.
Sandbed Dewatering
Advantages
• Simple operations
• Low energy usage
Disadvantages
• High space requirement
• Vulnerable to weather
18
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A vacuum filter consists of a cylindrical
drum covered with a filter medium or
fabric, which rotates partially submerged
in a vat of conditioned sludge. A vacuum is
applied inside the drum to extract water,
leaving behind the solids, called filter cake,
on the filter medium. Vacuum filtration of
sludge results in a sludge cake dry enough
for disposal in a landfill, or by application
to the land as a relatively dry soil
conditioner.
Vacuum filtration is the most popular
mechanical sludge-dewatering method for
municipalities, with over 1,500
installations. While this method requires
more skilled operation than a drying bed,
it has the advantages of occupying much
less space and being more controllable.
Vacuum Filtration
Advantages
• Not vulnerable to weather
• Small space requirements
Disadvantages "
• Skilled operations requirements
• High electrical energy consumption
Centrifuges are also a popular means of
dewatering municipal sludges. A centrifuge
uses centrifugal force to separate sludge
solids from the liquid. Polymers used for
sludge conditioning are also injected into
the centrifuge. The solids are spun to the
outside of a bowl from which they are
scraped. The liquid is returned to the head
of the facility for further treatment.
Centrifiigation
Advantages
• Minimal space needs
• Large separational forces for small
particles
• Not vulnerable to weather effects
• Relatively odor-free operation
Disad v ant age
• Extensive maintenance
requirements
Pressure filtration is also an effective
means of sludge dewatering that is finding
increased use in the United States. Sludge
is dewatered by pumping it at high
pressure through a filter or a belt running
between rollers. A very dry sludge cake
results. Although popular in Europe for
years, pressure filtration only recently has
undergone extensive use in the United
States. Interest has been spurred by recent
improvements in equipment. However, the
capital costs are high.
Ultimate Disposal
Several options exist for the final disposal
of sludge. Sometimes it can be used as a
soil conditioner or low-grade fertilizer. It
also may be burned or disposed of through
wet air oxidation.
Fertilizer and Soil Conditioner
Municipal sludge contains essential plant
nutrients and useful trace elements. It
thus has potential as a fertilizer or soil
conditioner. Before serving these uses, the
sludge is usually stabilized by digestion or
some other process to control
microorganisms and odors. After
stabilization, the sludge can be used as a
fertilizer or soil conditioner in several
alternative forms:
• Liquid sludge directly from the
stabilization process
• Dewatered sludge
• Dewatered and dried sludge
* Composted sludge.
19
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Many municipalities apply liquid sludge to
croplands. This sludge is not used for root
crops or crops consumed raw by people
because of health considerations. It is
frequently used for pastureland or corn,
wheat, and forage crops. Small towns often
haul the sludge in trucks that also spread
the sludge on the land. Large cities usually
find pumping the sludge through pipelines
to the disposal sites to be the cheapest
method of sludge transportation.
To reduce the volume of material handled,
dewatering is sometimes used before
applying the sludge to the land. In small
treatment plants, sludge removed from
drying beds is often stockpiled for use by
the community or by local citizens. Larger
cities may use mechanical dewatering
systems, with the sludge cake hauled to
the disposal sites where it is plowed into
the ground.
The drying of dewatered sludge by heat
further reduces the volume. Several major
U.S. cities, including Houston and
Milwaukee, dry their sludge for use as a
soil conditioner.
Incineration
Advantages
• Almost complete destruction of
sludge
• Small space requirement
Disadvantages
• High capital cost
• High fuel cost
• Extensive maintenance
requirements
• Air pollution potential
The wet air oxidation process is based on
the principle that a substance capable of
burning can be broken down in the
presence of very hot water under pressure.
The oxidized solids and liquid can be
separated by settling, vacuum filtration, or
centrifugation.
Sludge Reduction
If sludge use as a soil conditioner is
impractical, or if a land site is not suitable
for the disposal of dewatered sludge,
communities may turn to the alternative of
sludge reduction. Sludge reduction involves
decreasing the mass of solids through
methods such as incineration and wet air
oxidation.
Incineration completely evaporates the
moisture in the sludge, and burns the
organic solids to an ash. To minimize the
amount of fuel used, the sludge must be
dried as completely as possible before
incineration. Incinerators have the
advantage of small space requirements, but
suffer from long start-up periods, complex
operations, and high energy costs.
Wet Air Oxidation
Advantage
• Very small space requirement
Disadvantages
• High capital cost
• Highly-skilled operators needed to
handle maintenance and safety
problems
• Produces highly polluted liquid that
must be recycled or treated
20
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Advanced Treatment Sludges
The chemical coagulation-sedimentation
process in advanced waste treatment
produces large volumes of chemical
sludges. No other advanced process creates
a significant sludge problem. If lime is
used as a coagulant, the sludge can be
dewatered by the usual separation
techniques (vacuum filters, centrifuges,
and filter presses). The sludge can then be
passed through an incinerator in a process
called recalcining. This process drives off
water and carbon dioxide, leaving behind a
reusable form of lime. This method reduces
both the amount of new lime that must be
purchased, as well as the volume of
residues for final disposal. The lime sludge
is dewatered and buried in cases where
recalcining is too expensive.
If salts of iron or aluminum, such as alum
or ferric chloride, are used as the coagulant,
these chemicals at this time cannot be
recovered and reused for phosphorus
removal. These sludges, then, are
dewatered, with the same alternatives for
disposal as the organic sludges from
secondary treatment.
Planning for Sludge
Disposal
In facility planning, sludge disposal is
often ignored during the initial
evaluation of wastewater treatment
alternatives — a disastrous mistake.
The monetary cost of sludge disposal
about equals the cost of treating the
wastewater alone. Relevant questions
for advisory groups include:
• What effluent and/or sludge quality is
needed for the long-term use of
disposal techniques?
• What are the sludge disposal options
and their related costs?
• How will the disposal techniques
affect the environment?
• Does the choice match the
preferences of the community?
Land application is a good alternative
for sludge disposal. However, potential
hazards exist when joint industrial-
municipal treatment is used. Industrial
wastes may contain heavy metals or
other toxic substances that limit the
disposal of sludge on land. Properly
controlled sludge may be applied to
land without problems developing. The
advisory group may help locate
available land disposal sites, and lead
public discussion of the best method of
sludge disposal for the community.
Selection of Processes
The array of treatment processes is
extensive. A major portion of facility
planning involves choosing one of them.
Over a hundred different techniques,
options, and processes exist for wastewater
treatment. In determining the best solution
to a wastewater problem, these
alternatives should be evaluated carefully
in light of specific local conditions. Among
the factors that should be considered are:
• Wastewater amount and characteristics
(domestic, commercial, and industrial uses)
• Effluent requirements
• Environmental effects
• Public acceptance
• Resource consumption
• Sludge handling
• Process complexity, reliability, and
flexibility
• Implementation capability
• Monetary costs.
The bottom line for most people is how
much a system costs. Both nonmonetary
and monetary costs are involved.
Environmental, social, and indirect effects
such as land development are the principal
nonmonetary considerations. Monetary
costs consist mainly of capital, operations,
replacement, and management
expenditures. The costs should be
presented in a form that has meaning for
the taxpayer, such as dollars per household
per year. These costs, especially for
operations, are increasing rapidly due to
escalating energy costs.
21
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Selected Resources
More Environmental Pollution Control Alternatives: Municipal Wastewater. Publication
Number EPA-625/5-76-012. Washington, DC: U.S. Environmental Protection Agency,
May 19?6 ?9 pp Qrder Number 5012.
This document is an excellent non-technical discussion of available municipal
wastewater treatment processes. It describes the processes, gives costs and
energy requirements, and discusses their efficiency, advantages, and
disadvantages. It is available from CERI, Technology Transfer, U.S.
Environmental Protection Agency, Cincinnati, OH 45268. Give the order
number and publication title when ordering.
Innovative and Alternative Technology Assessment Manual. MCD-53. Washington, DC:
U.S. Environmental Protection Agency, September, 1978. 388 pp.
This document contains fact sheets for 117 different wastewater treatment
process variations. Each fact sheet describes a process and its modifications. It
discusses technology status, applications, limitations, equipment manufacturers
(list only), environmental impacts, and references. Process diagrams and costs
are also given. It is available from the General Services Administration,
Centralized Mailing List Services, Building 41, Denver Federal Center, Denver,
CO 80225. Give the document number MCD-53 and the title when ordering.
Proceedings from National Conferences on Shopping for Sewage Treatment: How To Get
the Best Bargain for Your Community or Home. Draft. Washington, DC: U.S.
Environmental Protection Agency, April and June 1978. 120 pp.
This document is a collection of small papers presented at two conferences in
Denver, CO, and Washington, DC. The papers mainly pertain to wastewater
treatment technologies, and citizen involvement in the facilities planning
process. Brief comments concerning other topics are also included. It is available
from the Office of Water Program Operations, U.S. Environmental Protection
Agency, Washington, DC 20460.
VanNote, Robert H. et al. A Guide to the Selection of Cost-Effect ive Wastewater Treatment
Systems. Publication Number EPA-430/9-75-002. Washington, DC: Office of Water
Programs Operations, U.S. Environmental Protection Agency, July 1975. 229 pp. Order
Number PB-244-417/2BE.
This document presents information which can be used to determine the
alternative municipal wastewater treatment schemes that will meet specific
effluent guidelines. Procedures and information which can be used in
determining the cost of each alternative are also given. It costs $28 a copy; a 15
percent discount is given for orders of 20 or more copies. It can be ordered from
the National Technical Information Service, 5285 Port Royal Road, Springfield,
VA 22161.
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Glossary
Activated Sludge — sludge that has been
aerated and subjected to bacterial action; used
to remove organic matter in raw sewage during
secondary waste treatment.
Adsorption — attraction and accumulation of
one substance on the surface of another.
Advanced Treatment — processes beyond the
secondary or biological stage; accomplishes
removal of most suspended solids and nutrients
such as phosphorus and nitrogen.
Aeration — circulation of oxygen through a
substance; aids in purification.
Aerobic Digestion — breakdown of organic
matter by bacteria in the presence of oxygen.
Alkaline — wastewater with a pH above 7.0;
contains relatively few hydrogen ions as
compared to an acid.
Ammonia Stripping — process in which
gaseous ammonia is removed from water by
agitating a water-gas mixture in the presence
of air.
Anaerobic Digestion — breakdown of organic
matter by bacteria in the absence of oxygen.
Aquaculture — growth of plants or animals in
effluent as a part of a treatment scheme.
Biochemical Oxygen Demand (BOD) —
amount of dissolved oxygen in water required
in the biological process of breaking down
organic matter.
Biodisc — a large rotating plastic disc which
provides a surface area for the attachment and
growth of microorganisms.
Biological Contactor — a series of
closely-spaced biodiscs that provide a large
surface area for the biological removal of
organic pollutants from wastewater.
Carcinogen —- cancer-causing agent.
Centrifugation — the separation of sludge
particles from the liquid by a rapidly rotating
drum.
Chemical Precipitation — treatment
technique that utilizes chemicals, known as
coagulants, to cause solids in the wastewater to
clump together and settle.
Clarifier — sedimentation tank used for the
removal of settleable solids.
Coagulation — a clumping of particles in
wastewater to settle out impurities; often
induced by chemicals such as lime or alum.
Combined Sewer — drainage system that
carries both sewage and stormwater runoff.
Comminutor — a machine that grinds up
large objects in the raw wastewater entering a
sewage treatment plant.
Conditioning — treatment of sludge with
chemicals or heat so that the water may be
readily separated.
Cost-Effectiveness Analysis — determination
of whether a project or technique is worth
funding; both monetary and non-monetary
factors are involved.
Criteria — a rule or basis for criticism or
judgment.
Denitrification — anaerobic biological
conversion of nitrates into nitrogen gas.
Dewatering — the separation of water from
sludge by vacuum, pressure, or drying
processes.
Disinfectant — a chemical such as chlorine
that is added to the wastewater to kill bacteria.
Dissolved Solids — total of disintegrated
organic and inorganic material contained in
water.
Ecology — study of relationships between
organisms and their surroundings.
Effluent — treated or untreated wastewater
discharged into the environment.
Filtration — process of passing wastewater
through a granular bed or fine screen for
removing suspended matter that cannot be
removed by sedimentation.
Grit Chamber — a tank where sand, cinders,
and small stones are removed from wastewater
by settling.
Hydrologic Cycle — the flow of water through
the air, land, and liquid environments.
Infiltration — the action of water moving
through small openings in the earth as it seeps
downward.
Irrigation — application of water to vegetation
to improve its production.
Lagoon — a pond containing wastewater in
which organic wastes are removed under
aerobic or anaerobic conditions.
Land Treatment — process of applying
wastewater to the land for removal of
pollutants; sludge (the solids removed from
wastewater) also may be disposed on land, but
it is not called land treatment.
Nitrification — conversion of
nitrogen-containing substances such as proteins
into nitrates by bacteria.
Nitrogenous — containing the element
nitrogen.
Organic Matter — carbon-containing
substance.
Overland Flow — land application technique
in which wastewater is applied to gently
sloping ground planted with vegetation.
Oxidation Pond — a natural or man-made
pond where wastewater is processed through
the interaction of sunlight, wind, aquatic
organisms, and oxygen.
pH — hydrogen ion concentration in a solution.
Percolation — flow of water down through
pores or spaces in rock or soil.
Polymer — chemical compound consisting of
repeating structural units.
Primary Treatment — first stage of
wastewater treatment; removal of floating
debris and solids by screening and
sedimentation.
Secondary Treatment — treatment of
wastewater to remove all floating and
settleable solids; biochemical oxygen
demanding substances (BOD) and suspended
solids are reduced to a concentration of no more
than 30 mg/L in the effluent.
Sedimentation — settling out of solids in
wastewater or stormwater by gravity.
Sludge — accumulated solids and water
extracted from wastewater.
Stabilization — digestion of the organic solids
in sludge so that they may be handled without
causing a nuisance or health hazard.
Supernatant — the relatively clear liquid that
forms on the top of the digested sludge in the
second tank of a two-stage anaerobic digestion
process.
Suspended Solids (SS) — small particles of
solid pollutants in sewage that cause cloudiness
and require treatment for removal.
Thickening — separation of as much water as
possible from sludge by gravity or flotation
techniques.
Trickling Filter — a secondary treatment
process where wastewater seeps through a film
of microorganisms growing on stones or a
synthetic medium. As the wastewater trickles
through the media, the microorganisms
metabolize most of the organic pollutants.
Vacuum Filter — a cylindrical drum filter
which uses a vacuum to separate the solids
from the water.
Wet Air Oxidation — process of breaking
down solids in wastewater under conditions of
high temperature and pressure.
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Working for Clean Water is a
program designed to help advisory
groups improve decision making in
water quality planning. It aims at
helping people focus on essential
issues and questions by providing
trained instructors and materials
suitable for persons with
non-technical backgrounds. These
materials include a citizen
handbook on important principles
and considerations about topics in
water quality planning, an
audiovisual presentation, and an
instructor guide for elaborating
points, providing additional
information, and engaging in
problem-solving exercises.
This program consists of 18
informational units on various
aspects of water quality planning:
• Role of Advisory Groups
• Public Participation
• Nonpoint Source Pollution:
Agriculture, Forestry, and Mining
• Urban Stormwater Runoff
• Groundwater Contamination
• Facility Planning in the
Construction Grants Program
• Municipal Wastewater Processes:
Overview
• Municipal Wastewater Processes:
Details
• Small Systems
• Innovative and Alternative
Technologies
• Industrial Pretreatment
• Land Treatment
• Water Conservation and Reuse
• Multiple Use
• Environmental Assessment
• Cost-Effectiveness Analysis
• Wastewater Facilities Operation
and Management
• Financial Management
The units are not designed to
make technical experts out of
citizens and local officials. Each
unit contains essential facts, key
questions, advice on how to deal
with the issues, and
clearly-written technical
backgrounds. In short, each unit
provides the information that
citizen advisors need to better
fulfill their role.
This program is available through
public participation coordinators at
the regional offices of the United
States Environmental Protection
Agency. D
This information program was
financed with federal funds from
the U.S. Environmental Protection
Agency under Cooperative
Agreement No. CT900980 01. The
information program has been
reviewed by the Environmental
Protection Agency and approved
for publication. Approval does not
signify that the contents
necessarily reflect the views and
policies of the Environmental
Protection Agency, nor does the
mention of trade names or
commercial products constitute
endorsement of recommendation
for use. Q
This project is dedicated to the
memory of Susan A. Cole.
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