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                        TECHNICAL PAPER #6
                       UNDERSTANDING SEWAGE
                      TREATMENT AND DISPOSAL
                          Hank Stonerook
                       Technical Reviewers
                        Stephen A. Hubbs
                        R. Bruce Robinson
                           Ira Somerset
                          C. D. Spangler
                  1600 Wilson Boulevard, Suite 500
                    Arlington, Virginia 22209 USA
               Tel:  703/276-1800 * Fax:   703/243-1865
             Understanding Sewage Treatment and Disposal
                         ISBN:  0-86619-206-9
            [C] 1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Leslie Gottschalk
as primary editor, Julie Berman handling typesetting and layout,
and Margaret Crouch as project manager.
Hank Stonerook, author of this paper, is a principal with Environmental
Resources Management-Midwest, Inc.   He has published
several articles dealing with wastewater management and disposal,
and has served as a technical consultant on international development
projects during his affiliation with the U.S. Peace Corps.
Reviewers Stephen A. Hubbs, R. Bruce Robinson, Ira Somerset, and
C.D. Spangler are also specialists in the area.   Hubbs is a research
engineer with the Louisville Water Company, Louisville,
Kentucky.  Robinson is an assistant professor at the University of
Tennessee where he teaches courses in wastewater management and
treatment.  Somerset is a sanitary engineer by education and a
regional shellfish specialist with the U.S. Food and Drug Administration
of the Department of Health and Human Services, where
he studies and evaluates the effects of sewage on shellfish-growing
areas.  Spangler, a sanitary engineer, has been involved
in water and wastewater for a number of years.   He has worked for
the U.S. Public Health Service, the World Bank, and as a private
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
             by VITA Volunteer Henry Stonerook
The treatment and disposal of domestic wastes--sewage--is becoming
more and more important as ever-increasing rural populations
and urbanization threaten existing potable water supplies in many
areas of the world.  Health problems and diseases are often
related to inadequate sewage treatment.   Pollution of rivers and
lakes results in fish kills and destruction of other forms of
aquatic life.  Proper collection, treatment, and disposal of
sewage is necessary to promote healthful conditions and maintain
the quality of the world's water resources.
Domestic wastes can conveniently be separated into body wastes
(feces and urine) and gray water, which is all the other liquid
wastes of the household, including both laundry and kitchen waste
water.  Body wastes are the most hazardous due to the possibility
of contact with intestinal disease organisms.   Gray water ordinarily
has few disease organisms unless the laundry has contained
garments soiled by fecal discharges.
This paper is not meant to be an in-depth study of many kinds and
types of sewage treatment systems in use throughout the world.
Rather, it serves only as an introduction.   Included is a discussion
of sewage and its characteristics; the collection of sewage;
and a brief discussion of physical, biological, and chemical
treatment systems.  Appropriate sewage treatment technology,
including on-site, composting, land application, and aquaculture
systems, are discussed as possible alternatives for developing
nations.  A glossary of terms used in this paper and common to
discussions of sewage treatment systems is also included.
The physical and chemical characteristics of wastewater vary
according to both time of day and type of wastewater discharged
(residential/industrial).  Table 1 presents the major pollutants
contained in wastewater, typical measurement parameters, and the
potential environmental impact of the pollutants.
Most of these pollutants are present to one degree or another in
any type of sewage discharge.   Residential sewage is composed of
several components, including discharges from toilets, sinks,
bathing facilities, and laundry facilities.   Table 2 provides a
       Table 1.   Principal Pollutants in Wastewater
Type of                    Measurement               Environmental
Pollutant                   Parameter                  Impact
Biodegradable        Biochemical oxygen demand      Reduce oxygen
organics             (BOD); chemical oxygen         content of
                     demand (COD)                   receiving water
Suspended            Total suspended solids         Turbidity;
material             (TSS)                          sediment
Pathogenic           Fecal coliform bacteria        Health hazard
Ammonia              Determine amount of            Reduces oxygen
                     ammonia in                     content; toxic
                     wastewater                     to aquatic life;
                     ([NH.sub.3] - N test)          promotes algal
Phosphate            Determine amount of            Promotes algal
                     phosphate in                   growth
                     ([PO.sub.4] - P test)
Toxic                Depends on toxin               Hazardous to
materials            present                        aquatic life and
                                                    plants; may be
                                                    toxic to humans
range of flows and pollution loads in terms of the amount of
biological oxygen demand (BOD), chemical oxygen demand (COD),
ammonia nitrogen, and orthophosphate anticipated from an average
household consisting of 3.2 persons using conventional "western-style"
plumbing fixtures and detergents.   Similar discharges from
developing countries might be expected to be more concentrated as
the amount of water used per household is lower, but the amount
of waste is about the same.
A major environmental problem caused by too much sewage discharged
into a lake or other confined body of water is eutrophication.
Eutrophication is a natural aging process that is greatly
accelerated by the discharge of ammonia and phosphates.   These
nutrients promote the excessive growth of algae, which further
depletes the dissolved oxygen content of the water body.  This
  Table 2.   Residential Wastewater Discharge Composition
                   Flow of                  (Milligrams per Liter)
Type of          Wastewater                        Ammonia    Ortho-
Facility           (gpcd)(*)     BOD        COD     Nitrogen  phosphate
Kitchen sink         3.6         676       1,380        5.4     12.7
Bathtub              8.5         192         282        1.3      1.0
Bathroom sink        2.1         236         383        1.2     48.8
Laundry machine      7.4         282         725       11.3    171.0
Toilet              19.8         313         896       37.1     77.4
Average             (**)         310         755       20.5     71.4
    (*) Gallons per capita per day.
   (**) The total flow of wastewater is 41.4 gallons per capita per
   Source:   John B. Winneberger, Manual   of Grey Water Treatment
            Practice (Ann Arbor, Michigan:  Ann Arbor Science,
reduces the variety of aquatic life and the quality of the water
itself and imparts unpleasant tastes and odors.   Limiting the
discharge of untreated or partially treated sewage will prevent
such pollution of the water.
In areas with a significant housing density, sanitary sewers are
built to remove the wastewater to a treatment facility or disposal
area.  Although combined sewers (sewers that collect both
wastewater and storm water) cost much less to build than do
sewers that separate wastewater from storm water, they can become
a health hazard.  For example, with combined sewers comes the
danger that, during a rainstorm, raw sewage could enter a bypass
conduit and pollute water used for drinking or bathing.   In
addition, the cost of treating the combined storm and sanitary
wastes is high.  Most new sewer construction makes use of separate
sanitary sewers for these reasons.
Clay tile, concrete, asbestos-cement, and PVC plastic are the
four most common materials used in the construction of sewers.
These materials are chosen because of their resistance to corrosion
and their strength and flow properties.   However, sulfide
corrosion, which occurs when wastewater is confined or slow
moving, can affect concrete and asbestos-cement sewers as can
some industrial (toxic) materials.   Sulfide corrosion is accelerated
by high temperatures.  Clay pipe or PVC plastic may be a more
advisable choice of material under those conditions.   However,
replacement costs must be considered as well as construction
Sewage collection systems are designed according to a basic flow
plus an allowance in infiltration through sewer joints.   Actual
wastewater discharge ranges from 40 to 50 gallons per capita per
day in homes having flush toilets, sinks, showers, and laundry
facilities.  Allowing for infiltration through sewer joints and
inflow from miscellaneous clear water direct connections (e.g.,
catch basins, drains), per capita flow can be expected to range
from 70 to 100 gallons per day.   Where flows of this magnitude
occur, minimum sewer size is generally eight inches in diameter.
Sewer sizes vary according to the flow being conveyed and are a
function of slope, velocity, and internal roughness of the
conduit.  Manholes (holes equipped with covers) are built to
gain access to the sewers from ground level for cleaning and
inspection.  The manholes are placed at 300- to 500-foot intervals
and at those points where changes in direction and slope
Smaller sewers (i.e., those with diameters less than eight
inches) have been used in conjunction with septic or interceptor
tanks, where many solids can settle out and not cause obstruction
in the pipe.  These tanks constitute pretreatment facilities.
Solids collected in the tanks must be removed periodically, i.e.,
usually at 1- to 2-year intervals, by pumping them into tank
trucks and treating the material in special treatment facilities.
Pressure sewers, combined with grinder pumps following storage in
a wet well or effluent pumps following settling in septic tanks,
have also been used to transport sewage to the treatment plant.
These systems are relatively inexpensive to construct, but the
maintenance and power costs associated with their operation can
be high.  Furthermore, skilled maintenance is required.
Various combinations of short collector systems and dispersed
pretreatment or treatment facilities serve as alternative designs
in cases where housing densities cannot justify expensive
gravity collection systems.
Wastewater is treated using one or a combination of processes
including physical, biological, and chemical systems.   Process
units typical of each of these systems are given in Table 3.
               Table 3.  Process Units Typical of Various
                         Wastewater Technologies
Physical System         Biological System             Chemical System
Pumping                 Aerobic systems              Precipitation
Screening                - lagoons                   Coagulation
Flow equalization        - trickling filter          pH adjustment
Settling                 - rotating contactors       Disinfection
Grit removal             - sludge digestion
Filtration              Anaerobic systems
                         - sludge digestion
                         - lagoons
                        Land treatment
                        Subsurface disposal
Physical Technologies
Physical systems include processes that pump, that remove solids
by screening or settling, or that equalize flow fluctuations.
Bar screens, both mechanical and hand cleaned, are used to remove
large objects and serve to protect downstream mechanical equipment.
Grit (inorganic, fine solids such as sand, coffee grounds,
egg shells, etc., which are relatively heavy) is removed by
controlled settling.  Removing grit also helps to protect pumps
and equipment from abrasion and prevent the settling of these
materials in other treatment units.   A simple grit tank consists
of a channel through which wastewater flows at a constant velocity
independent of the volume of discharge.   Settling tanks, which
are rectangular or circular in shape, are designed to remove
solids and are sized according to the velocity of the flow
through the tank.  Solids settle out and fall to the bottom of
the tank.  These tanks employ a subsurface scraping mechanism to
direct the settled solids or sludge to a pump well for discharge
to the sludge treatment facility.   Overflow from the tanks exists
through a system of weirs for further treatment or discharge.  A
surface skimmer is often employed to remove floating solids and
scum.  Flow equalization facilities are tanks that serve to
regulate and dampen the variable peaks of flow that occur over a
normal day's time or as a result of severe inflow caused by
Biological Technologies
Biological systems employ both aerobic and anaerobic systems to
stabilize wastewater and sludge.   The most common of these, the
activated sludge system, involves adding air to the wastewater
to promote the growth of aerobic microorganisms that feed on and
digest the organic material.   Detention times of two to six hours
are necessary to stabilize the waste, which requires large tanks
capable of holding two to six hours of the average daily flow.
Air is blown into these tanks to promote the growth of aerobic
organisms.  Large amounts of power are required to mix and aerate
the tanks.  Settling tanks follow the activated sludge system,
and some of the settled sludge, containing a high concentration
of microorganisms, is returned to the aeration tanks to promote
microorganism growth.  This is a highly skilled operation and is
very expensive to build and operate.
Another type of aerobic treatment system is the trickling filter.
Incoming wastewater, which is first settled, is distributed at a
uniform rate over a medium of rock or plastic upon which aerobic
organisms attached themselves and grow.   These organisms attack
the sewage, reducing it in strength.   The dead organisms and
other solids are removed in settling tanks.   Flow is also recycled
with this system.  Although not as complex as activated sludge,
this is a delicate, complex treatment method that also requires a
high level of operator skill.
Anaerobic systems are commonly used to digest the settled solids;
they are less commonly used as wastewater treatment systems.
Anaerobic digesters are enclosed tanks of 20 feet or more in
depth, sometimes insulated and equipped with external heating
capabilities for cold climates.   In many cases, a floating cover
allows for the production of methane gas and the mixing of the
sludge.  Anaerobic digesters, if well insulated or operated in
warm climates, need little or no input of energy to function.
They usually decompose wastes at temperatures of 35 to 40 [degrees] C.  Gas
produced from the decomposition can be captured and used to
provide fuel to operate natural gas pumps.   Digester performance
is a function of sludge feed rate, moisture content, the amount
of volatile contents of the sludge, and the amount of toxic
materials present.  Large quantities of moisture and toxics will
retard sludge digestion and minimize methane gas production.
Chemical Technologies
Chemical treatment systems are designed to remove pollutants
through the addition of certain chemicals.   Capital costs for
these systems are usually low, but operating costs can be significant.
Chemicals are used extensively in wastewater treatment
for disinfection (chlorine) and sludge thickening (dewatering).
They are also used extensively in industrial wastewater treatment
to adjust the pH and to remove heavy metals.   Chemical costs and
handling properties, however, make them rather poor choices for
sewage treatment systems for developing countries.   A typical
sewage treatment plant employing screening, grit removal, primary
settling, trickling filter, final setting, disinfection, and
anaerobic sludge digestion is presented in Figure 1.

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The technologies described in Section IV are designed to treat
wastewater and sludge effectively.   They are generally very expensive,
however, and require extensive operation and maintenance.
As such, they may be applicable for larger population
areas, which can afford their construction and maintenance, but
other, simpler methods are likely to be more suitable for smallscale
For ordinary households or family groups, body wastes are best
disposed of in a sanitary latrine.   Health authorities in most
countries have developed plans for such installations.   The most
important considerations are that the pit should be designed so
it will not pollute ground water or permit access by insects or
rodents.  The pit will become full over several years depending on
its size and the number of users.   When full it can be cleaned
out; this is a disagreeable job and may result in exposure to
fresh fecal material.  A good arrangement is to have two pits.
When the first is full, the slab and building are moved to the
second pit and the first is covered with earth and allowed to
compost.  When the second pit is full, the first pit is cleaned
out and the slab and building moved back over it and the second
pit is covered and allowed to compost.   If a water-seal latrine is
used, the slab and building can be permanent.   The sewage is
carried behind the latrine where it can be distributed to one of
two pits for alternate use and composting.
Gray water is usually used for irrigation of garden plots,
shrubs, or trees and scattered to help settle the dust around the
premises.  It should not stand to form puddles, which may result
in mosquito propagation.
Sewer systems are expensive--usually two or three times the cost
of a water supply system.  Sewers also require a good flow of
water or material will settle in the sewers, resulting in blockages.
In the United States between 40 and 50 percent of domestic
water goes to flush toilets.   This is a great waste of water and
can create a problem when discharged into streams and lakes, so
expensive treatment is necessary.
If water is in ample supply water-flush toilets can be used in
institutions such as hospitals, schools, and government buildings.
In such installations, a plumbing system in the building
can collect the toilet wastes and gray water together and deliver
them to a sewer that takes them a short distance from the building
to a small stabilization pond.   Such a pond is less expensive
to build than a septic tank and tile field.   It will also have
fewer operating problems, because it will lose water from seepage
and evaporation and the overflow can be used for irrigation.
In tropical climates the pond can be loaded at a rate of 2,500
people per acre (or 6,000 per hectare).   For a population of 500
people the pond would only be one-fifth of an acre in area, or
about 60 feet wide and 140 feet long (approximately 20 meters
wide by 50 meters long).  The length should be about two to three
times the width.  The pond should be at least three feet deep (1
m) and should be deeper at the inlet end to allow for sludge
accumulation.  The inlet pipe should extend about one-quarter of
the way into the pond.  Over time, the pond will develop a rich
green algal culture that, with the bacteria, will break down the
organic materials in the sewage.   Many ponds have fish, frogs, and
ducks as residents.  A properly designed pond will have little or
no odor and what odors that might occasionally occur usually
cannot be detected beyond 300 feet (100 m).   A newly constructed
pond may take some months before the bottom will be sealed and
water will get to the design depth.   Once the pond is operating,
maintenance is simple and requires only part-time ordinary labor
to check on the inlet flow and the retaining dikes, to cut the
grass and weeds on the dikes, and to remove any aquatic vegetation
in the shallow areas along the dike so as to discourage
mosquito propagation.
Various other alternative technologies for wastewater treatment
have developed over the years.   Table 4 lists the technologies,
their intended use (wastewater or sludge), and their design
parameters, and provides comments to each technology.
            Table 4.  Some Popular Low-Cost Technologies
                      for Wastewater Treatment
Technology   Use(*)     Technology Design        Comments
Land treat-     W      Land area; soil type;   Reuse potential
ment                   crops grown; climate   wastewater; pre-
                                              treatment required;
                                              potential to pollute
                                              water and crops;
                                              may attract flies
                                              and parasitic worms;
                                              may cause odors
Composting      S      Detention time; air     Soil conditioner;
                       requirement; moisture  turning required;
                       content                need additive to mix
                                              sludge with compost
Leaching        W      Soil type; topography; Large areas required
field                  groundwater; depth to
                       bedrock; area
Anaerobic       S      Detention time;         Can produce a fuel;
digesters              moisture content       soil conditioner;
                                              cannot treat
Aquaculture     W      Land area; climate;     Pretreatment required;
                       topography; crops      potential
                                              adverse health effects
Imhoff tank    W/S     Detention time;         Treats wastewater
                       overflow rate          and sludge; low-cost
                                              energy; requires
                                              maintenance; may
                                              attract flies; may
                                              cause odors
         (*) W = wastewater; S = sludge.
Land treatment relies on bacteria and organisms present in soil
as well as the soil's physical characteristics to stabilize
pretreated sewage.  The sewage is stored in lagoons prior to
being spread over fields through channels or piping systems.  If
the sewage has been thoroughly, treated the crops grown in these
fields can be used for animal feed.   However, for sewage that has
not been treated adequately, the land application site should
be set aside and no crops on it should be consumed by animals or
humans.  Care should be taken in selecting sites so that pollution
of ground water or surface water cannot occur due to
percolation or runoff from the land treatment site.
Composting of sludge and/or human and animal waste offers a means
to solve an environmental problem and create a useful product.
This product, a soil conditioner, contains some nutrient value in
the form of nitrogen and phosphorous.   Composting is a natural
process that occurs when aerobic microorganisms live in an optimum
environment that is a function of the carbon to nitrogen (C
to N) ratio of the mixture.  Care must be taken to keep this
ratio at approximately 25 to 30 parts of carbon to 1 part of
nitrogen, to maintain an adequate supply, and limit the moisture
content to approximately 60 percent.   In many cases, a bulking
agent such as wood chips or leaves is added to help achieve these
conditions.  Temperatures in a properly composted mixture exceed
40 [degrees] C for several days.   The compost process requires approximately
10 to 14 days, and should be followed by several weeks of
curing.  Oftentimes, the composted product is screened to recover
the bulking agent before it is used.   The screened material, if
it has aged long enough, can be bagged and stored or sold in bulk
for use as a soil conditioner.   Adding compost to farmland can
reduce the amount of fertilizer required for crops.
Leaching fields are generally used in conjunction with a pre-treatment
device (e.g., septic or interceptor tank).   They are a
means to dispose of wastewater without having to discharge it to
a watercourse.  Proper soil types are necessary for the construction
of leaching fields.  A tight, nonporous clay soil is generally
unsuitable since the leached sewage cannot pass through it.
The sewage then comes to the surface, causing odors and potential
health problems.  The length of a leaching field depends on the
amount of sewage to be treated (i.e., number of persons connected),
the type of sewage, and the types of soils present.
Where an excess of evaporation occurs, evapotranspiration systems
are effective.  These systems employ a raised distribution field
with the crops or trees grown on top.   The vegetation draws up the
moisture and transpires it, leaving the residual solids trapped
in the ground to be further broken down by the microorganisms
present there.  These systems are generally limited to small
clusters of homes, but several can be scattered throughout a
community.  A sketch of a typical leaching and evapotranspiration
system is given in Figure 2.

14p13.gif (600x600)

Biogas generators are process units that make use of anaerobic
digestion as a means to stabilize waste and produce fuel.  These
systems are designed to digest animal and human solid wastes; or
they can be used as a treatment mechanism for sludge.   The solid
waste decomposes with the aid of anaerobic microorganisms to
produce methane gas, which can be recovered and used as a fuel.
As with composting, an optimum carbon to nitrogen ratio (i.e., 25
to 30 parts of carbon to 1 part of nitrogen) is required for
proper operation.  A detention time of at least 30 days is required
for stabilization.  Adding the correct amount of waste
material to the unit as well as mixing the material thoroughly
and removing the digested product from the unit are important
operational parameters.  Biogas generators can be designed for
small-scale use in one or several homes in many countries; but
they only partially solve the sewage problem.   Because they cannot
handle wash water or other types of wastewater, an additional
means of sewage treatment for these wastes must be provided.
Aquaculture systems have become popular as a relatively low-cost
means to provide advanced treatment where it is required.  Utilizing
specially selected aquatic vegetation, large amounts of
biodegradable material, suspended solids (SS), and other nutrients
can be removed from wastewaters.   Water is allowed to
flow through channels at a slow rate where aquatic plants are
grown.  These plants are harvested periodically and can then be
composted further or digested anaerobically.   The complete aquaculture
system is labor intensive, but requires minimum energy
and equipment.  Pretreatment of the waste such as in a series of
lagoons must be provided to remove the solids and partially treat
the sewage prior to its disposal.   The resulting system requires
large land areas on which to operate.
lmhoff tanks offer a treatment means that is relatively low in
cost, produces a good effluent, and is mechanically simple.   An
Imhoff tank, shown in Figure 3, is a large, deep tank employing

14p15.gif (600x600)

an upper compartment for settling and a lower compartment for
anaerobic digestion.  Gases escape through vents along the sides
of the tank.  Proper tank design can limit operating problems
such as foaming, scum formation, and malodorous sludge.   In
tropical climates where the temperature does not vary greatly,
the foaming and odor problem will be reduced.   Proper operation,
including daily cleaning of the side vents, will promote optimum
operation of the system.  Sludge withdrawal should occur only two
or three times a year, and the resulting digested product can be
spread over land directly or applied to drying beds for subsequent
disposal.  Since the discharge from these tanks is not of
high quality, it may require further treatment in lagoons or
leaching fields.
Many books have been written on sewage treatment and disposal,
and no one source is authoritative.   Several reference sources
are listed below.  Most of these are written for use in developed
Bastian, Robert K.  "Natural Treatment Systems in Wastewater
     Treatment and Sludge Management."  Civil Engineering-ASCE,
     May 1982, p. 62.
Fey, Robert T.  "Cost-Minded Community Chooses Small Diameter
     Gravity System."  Water and Sewage Works, June 1978, p. 58.
Golveke, Clarence G.  Biological Reclamation of Solid Wastes.
     Emmaus, Pennsylvania:  Rodale Press, 1977.
Metcalf and Eddy.  Wastewater Engineering.  New York, New York:
     McGraw-Hill Book Company, 1977.
Norris, D.P., and Troyan, J.J.   "Cost-Effectiveness of On-Site and
     Community Sewerage Alternatives."  Civil Engineering-ASCE,
     December 1977, p. 84.
Otis, R.J., and Stewart, D.E.   "Alternative Wastewater Facilities
     for Small Unsewered Communities in Rural America."  Small
     Scale Waste Management Project.  Madison, Wisconsin:  University
     of Wisconsin, July 1976.
Rich,  Linvil G.   Low-Maintenance, Mechanically Simple Wastewater
      Treatment Systems.   New York, New York.:   McGraw-Hill Book
     Company, 1980.
Winneberger, John H.  Manual of Grey Water Treatment Practice.   Ann
     Arbor, Michigan:  Ann Arbor Science, 1974.
 1.   American Society of Agricultural Engineers
     2950 Niles Road
     St. Joseph, Michigan 49085 USA
 2.   American Society of Civil Engineers
     345 East 47th Street
     New York, New York 10017 USA
 3.   EPA Small Wastewater Flows Clearinghouse
     Centennial House
     Morgantown, West Virginia 26526 USA
 4.   Environmental Research Information Center
     Office of Research and Development
     U.S. Environmental Protection Agency
     Cincinnati, Ohio 45268 USA
 5.   Inter-American Association of Sanitary Engineering
     AIDIS-USA Section
     18729 Considine Drive
     Brookeville, Maryland 20833 USA
 6.   National Sanitation Foundation
     Technical Services Division
     3475 Plymouth Street
     Ann Arbor, Michigan 48106 USA
 7.   Pan American Health Organization
     525 23rd Street, N.W.
     Washington, D.C. 20037 USA
 8.   University of Wisconsin - Extension
     College of Engineering and Applied Science
     432 North Lake Street
     Madison, Wisconsin 53706 USA
 9.   Water Pollution Control Federation
     2626 Pennsylvania Avenue, N.W.
     Washington, D.C. 20037 USA
10.  World Bank
     1818 H Street, NW
     Washington, D.C. 20433 USA
11.  World Health Organization
     20 Avenue Appia
     1211 Geneva 27
Activated Sludge System:  A biological treatment system employing
     forced aeration, aerobic growth, and recycled sludge.
Aerobic:  With oxygen.  Refers to the addition of oxygen to the
     treatment or stabilization process of wastewater and sludge.
Ammonia:  A nitrogen compound that, in combination with phosphates
     or by itself, promotes algal growth.  In large concentrations
     this compound is toxic to aquatic life.
Anaerobic:  Without oxygen.  The treatment or stabilization of
     wastewater or sludge in the absence of oxygen.
Aquaculture:  A method of sewage treatment employing aquatic
     plants to absorb pollutants.
Biochemical Oxygen Demand (BOD):   A measure of the organic materials
     present in wastewater and the amount of oxygen they
     consume over a length of time, usually five days, at 20 [degrees] C.
Biological Treatment:  Facilities that promote the growth of
     microorganisms to reduce the strength of organic material in
Chemical Treatment:  The addition of chemicals to wastewater or
     sludge to neutralize harmful compounds or enhance thickening
     or settling capabilities.
Combined Sewers:  Sewers that carry wastewater from homes and
     businesses as well as runoff from rain.
Composting:  An aerobic treatment method generally used for
     sludges or animal or human wastes that are essentially
Detention Time:  The time a unit of sewage is retained in a treatment
Disinfection:  A means, usually chemical, to treat wastewater to
     kill pathogens.
Equalization:  Reduction of the variability of flows by holding
     the sewage in a tank so that the flow to the treatment plant
     is equalized over the day.
Eutrophication:  The excessive growth of algae in a body of water.
Evapotranspiration:  A treatment means using plants to take up
     moisture and release it to the atmosphere.  Some is removed
     directly through evaporation.
Filtration:  A physical treatment process used to remove solids by
     forcing wastewater through a graded medium.
Gravity Sewers:  Sewers that are installed at a downward slope to
     convey wastewater without the use of pumps.
Grit:  Larger solids of primarily inorganic nature in wastewater,
     including sand, egg shells, coffee grounds, which settle
     out quickly when the velocity is decreased in the grit
Infiltration:  Water entering sanitary sewers from springs or
     storm sewers.
Inflow:  Water entering sanitary sewers through leaky pipe joints
     or manholes.
Lagoons:  Shallow ponds that hold wastewater and use aerobic and/
     or anaerobic methods to stabilize wastes.  They are designed
     to store water for long periods of time.
Leach:  To remove soluble constituents from (a substance) by the
     action of a percolating liquid.
Methane:  The major gas generated from the anaerobic decomposition
     of sludges or solid wastes.
Moisture Content:  The amount of water contained in a known volume
     of solids (e.g., sludge).
On-Site Disposal:  A means of sewage treatment designed for one or
     a small group of households without connections to a central
Organics:  Carbon substances that break down in the presence of
Oxygen Content:  The amount of dissolved oxygen in wastewater.
Pathogens:  A name given to a group of organisms known to cause
     diseases or to upset human body functions.
pH:  Potential hydrogen.  The symbol that denotes a measurement of
     the effective hydrogen ion concentration.  On a scale of
     zero to 14, seven represents neutrality.  Numbers less
     than seven indicate acidity; greater than seven indicate
Phosphates:  Phosphorous compounds that are known to promote
     excessive growth of algae if present in high concentrations.
Physical Treatment:  Physical units such as pumps, filters,
     screens, or tanks, that serve to move, screen, or contain
Pollutant:  An overall term used to characterize unwanted material,
     chemicals, or substances in the environment.
Pressure Sewers:  Pipes of small diameter used for conveying
     wastewater after it is pumped; these pipes are usually preceded
     by some pretreatment device.
Pretreatment:  First stage of treatment, usually screening, to
     remove large solids or grit.
Reuse:  A term employed when talking about using treated wastewater
     as a water source.
Sanitary Sewers:  Sewers designed to carry only wastewaters from
     homes, businesses, and industries.
Sludge:  The material that settles out from wastewater.
Soil Conditioner:  Soil additive that acts as a bulking agent and
     holds moisture.
Suspended Solids (SS):  A measure of the amount of solids present
     in wastewater; the solids are removed by drying at a low
     temperature (105 [degrees] F).
Toxic Material:  A material, usually man-made, that at certain
     concentrations can kill aquatic life or be a hazard to human
Treatment Systems:  Physical, biological, or chemical systems or
     combinations used to reduce the strength of pollutants.
Trickling Filter:  A biological treatment system using aerobic
     means to stabilize wastewater by trickling through a medium
     of rocks.
Wastewater/Sewage:  A combination of human waste and used water
     from households, businesses, and industrial processes.
Weir:  An obstruction placed across a stream to divert the water
     to make it flow through a desired channel, which may be a
     notch or opening in the weir itself.