TECHNICAL PAPER #6
TREATMENT AND DISPOSAL
Stephen A. Hubbs
R. Bruce Robinson
C. D. Spangler
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209
Tel: 703/276-1800 * Fax:
Understanding Sewage Treatment and Disposal
Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
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
They are not intended to provide construction or
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
almost entirely by VITA Volunteer technical experts on a
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
and Margaret Crouch as project manager.
Hank Stonerook, author of this paper, is a principal with
Resources Management-Midwest, Inc.
He has published
several articles dealing with wastewater management and
and has served as a technical consultant on international
projects during his affiliation with the U.S. Peace Corps.
Reviewers Stephen A. Hubbs, R. Bruce Robinson, Ira Somerset,
C.D. Spangler are also specialists in the area.
Hubbs is a research
engineer with the Louisville Water Company, Louisville,
is an assistant professor at the University of
Tennessee where he teaches courses in wastewater management
is a sanitary engineer by education and a
regional shellfish specialist with the U.S. Food and Drug
of the Department of Health and Human Services, where
he studies and evaluates the effects of sewage on
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
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
SEWAGE TREATMENT AND DISPOSAL
Volunteer Henry Stonerook
The treatment and disposal of domestic wastes--sewage--is
more and more important as ever-increasing rural populations
and urbanization threaten existing potable water supplies in
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
aquatic life. Proper
collection, treatment, and disposal of
sewage is necessary to promote healthful conditions and
the quality of the world's water resources.
Domestic wastes can conveniently be separated into body
(feces and urine) and gray water, which is all the other
wastes of the household, including both laundry and kitchen
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
types of sewage treatment systems in use throughout the
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
Appropriate sewage treatment technology,
including on-site, composting, land application, and
systems, are discussed as possible alternatives for
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
Table 1 presents the major pollutants
contained in wastewater, typical measurement parameters, and
potential environmental impact of the pollutants.
Most of these pollutants are present to one degree or
any type of sewage discharge.
Residential sewage is composed of
several components, including discharges from toilets,
bathing facilities, and laundry facilities.
Table 2 provides a
Principal Pollutants in Wastewater
Biochemical oxygen demand
(BOD); chemical oxygen
Total suspended solids
Fecal coliform bacteria
Determine amount of
([NH.sub.3] - N test)
Determine amount of
([PO.sub.4] - P test)
Depends on toxin
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
ammonia nitrogen, and orthophosphate anticipated from an
household consisting of 3.2 persons using conventional
plumbing fixtures and detergents.
Similar discharges from
developing countries might be expected to be more
the amount of water used per household is lower, but the
of waste is about the same.
A major environmental problem caused by too much sewage
into a lake or other confined body of water is
Eutrophication is a natural aging process that is greatly
accelerated by the discharge of ammonia and phosphates.
nutrients promote the excessive growth of algae, which
depletes the dissolved oxygen content of the water
Residential Wastewater Discharge Composition
(Milligrams per Liter)
(*) Gallons per
capita per day.
(**) The total
flow of wastewater is 41.4 gallons per capita per
John B. Winneberger, Manual
of Grey Water Treatment
(Ann Arbor, Michigan: Ann Arbor
reduces the variety of aquatic life and the quality of the
itself and imparts unpleasant tastes and odors.
discharge of untreated or partially treated sewage will
such pollution of the water.
In areas with a significant housing density, sanitary sewers
built to remove the wastewater to a treatment facility or
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
a health hazard. For
example, with combined sewers comes the
danger that, during a rainstorm, raw sewage could enter a
conduit and pollute water used for drinking or bathing.
addition, the cost of treating the combined storm and
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
four most common materials used in the construction of
These materials are chosen because of their resistance to
and their strength and flow properties.
corrosion, which occurs when wastewater is confined or slow
moving, can affect concrete and asbestos-cement sewers as
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.
replacement costs must be considered as well as construction
Sewage collection systems are designed according to a basic
plus an allowance in infiltration through sewer joints.
wastewater discharge ranges from 40 to 50 gallons per capita
day in homes having flush toilets, sinks, showers, and
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
from 70 to 100 gallons per day.
Where flows of this magnitude
occur, minimum sewer size is generally eight inches in
Sewer sizes vary according to the flow being conveyed and
function of slope, velocity, and internal roughness of the
(holes equipped with covers) are built to
gain access to the sewers from ground level for cleaning and
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
tanks, where many solids can settle out and not cause
in the pipe. These
tanks constitute pretreatment facilities.
Solids collected in the tanks must be removed periodically,
usually at 1- to 2-year intervals, by pumping them into tank
trucks and treating the material in special treatment
Pressure sewers, combined with grinder pumps following
a wet well or effluent pumps following settling in septic
have also been used to transport sewage to the treatment
These systems are relatively inexpensive to construct, but
maintenance and power costs associated with their operation
Furthermore, skilled maintenance is required.
Various combinations of short collector systems and
pretreatment or treatment facilities serve as alternative
in cases where housing densities cannot justify expensive
gravity collection systems.
TECHNOLOGIES FOR WASTEWATER TREATMENT
PHYSICAL, BIOLOGICAL, AND CHEMICAL TREATMENT TECHNOLOGIES
Wastewater is treated using one or a combination of
including physical, biological, and chemical systems.
units typical of each of these systems are given in Table 3.
3. Process Units Typical of Various
- trickling filter pH
- rotating contactors
- sludge digestion
- sludge digestion
Physical systems include processes that pump, that remove
by screening or settling, or that equalize flow
Bar screens, both mechanical and hand cleaned, are used to
large objects and serve to protect downstream mechanical
Grit (inorganic, fine solids such as sand, coffee grounds,
egg shells, etc., which are relatively heavy) is removed by
Removing grit also helps to protect pumps
and equipment from abrasion and prevent the settling of
materials in other treatment units.
A simple grit tank consists
of a channel through which wastewater flows at a constant
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
to the sludge treatment facility.
Overflow from the tanks exists
through a system of weirs for further treatment or
surface skimmer is often employed to remove floating solids
equalization facilities are tanks that serve to
regulate and dampen the variable peaks of flow that occur
normal day's time or as a result of severe inflow caused by
Biological systems employ both aerobic and anaerobic systems
stabilize wastewater and sludge.
The most common of these, the
activated sludge system, involves adding air to the
to promote the growth of aerobic microorganisms that feed on
digest the organic material.
Detention times of two to six hours
are necessary to stabilize the waste, which requires large
capable of holding two to six hours of the average daily
Air is blown into these tanks to promote the growth of
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
of microorganisms, is returned to the aeration tanks to
This is a highly skilled operation and is
very expensive to build and operate.
Another type of aerobic treatment system is the trickling
Incoming wastewater, which is first settled, is distributed
uniform rate over a medium of rock or plastic upon which
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
high level of operator skill.
Anaerobic systems are commonly used to digest the settled
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
capabilities for cold climates.
In many cases, a floating cover
allows for the production of methane gas and the mixing of
digesters, if well insulated or operated in
warm climates, need little or no input of energy to
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.
is a function of sludge feed rate, moisture content, the
of volatile contents of the sludge, and the amount of toxic
Large quantities of moisture and toxics will
retard sludge digestion and minimize methane gas production.
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
Chemicals are used extensively in wastewater treatment
for disinfection (chlorine) and sludge thickening
They are also used extensively in industrial wastewater
to adjust the pH and to remove heavy metals.
Chemical costs and
handling properties, however, make them rather poor choices
sewage treatment systems for developing countries.
sewage treatment plant employing screening, grit removal,
settling, trickling filter, final setting, disinfection, and
anaerobic sludge digestion is presented in Figure 1.
The technologies described in Section IV are designed to
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,
other, simpler methods are likely to be more suitable for
For ordinary households or family groups, body wastes are
disposed of in a sanitary latrine.
Health authorities in most
countries have developed plans for such installations.
important considerations are that the pit should be designed
it will not pollute ground water or permit access by insects
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
fresh fecal material.
A good arrangement is to have two pits.
When the first is full, the slab and building are moved to
second pit and the first is covered with earth and allowed
compost. When the
second pit is full, the first pit is cleaned
out and the slab and building moved back over it and the
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
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
premises. It should
not stand to form puddles, which may result
in mosquito propagation.
Sewer systems are expensive--usually two or three times the
of a water supply system.
Sewers also require a good flow of
water or material will settle in the sewers, resulting in
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,
expensive treatment is necessary.
If water is in ample supply water-flush toilets can be used
institutions such as hospitals, schools, and government
In such installations, a plumbing system in the building
can collect the toilet wastes and gray water together and
them to a sewer that takes them a short distance from the
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
and evaporation and the overflow can be used for irrigation.
In tropical climates the pond can be loaded at a rate of
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,
about 60 feet wide and 140 feet long (approximately 20
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
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
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
water will get to the design depth.
Once the pond is operating,
maintenance is simple and requires only part-time ordinary
to check on the inlet flow and the retaining dikes, to cut
grass and weeds on the dikes, and to remove any aquatic
in the shallow areas along the dike so as to discourage
Various other alternative technologies for wastewater
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.
4. Some Popular Low-Cost Technologies
for Wastewater Treatment
Use(*) Technology Design
W Land area; soil type;
crops grown; climate
potential to pollute
water and crops;
may attract flies
and parasitic worms;
may cause odors
S Detention time; air
requirement; moisture turning
additive to mix
sludge with compost
W Soil type; topography;
Large areas required
groundwater; depth to
S Detention time;
Can produce a fuel;
moisture content soil
W Land area; climate;
topography; crops potential
W/S Detention time;
overflow rate and
attract flies; may
(*) W =
wastewater; S = sludge.
Land treatment relies on bacteria and organisms present in
as well as the soil's physical characteristics to stabilize
The sewage is stored in lagoons prior to
being spread over fields through channels or piping
the sewage has been thoroughly, treated the crops grown in
fields can be used for animal feed.
However, for sewage that has
not been treated adequately, the land application site
be set aside and no crops on it should be consumed by
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
to solve an environmental problem and create a useful
This product, a soil conditioner, contains some nutrient
the form of nitrogen and phosphorous.
Composting is a natural
process that occurs when aerobic microorganisms live in an
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
content to approximately 60 percent.
In many cases, a bulking
agent such as wood chips or leaves is added to help achieve
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
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
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
device (e.g., septic or interceptor tank).
They are a
means to dispose of wastewater without having to discharge
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
health problems. The
length of a leaching field depends on the
amount of sewage to be treated (i.e., number of persons
the type of sewage, and the types of soils present.
Where an excess of evaporation occurs, evapotranspiration
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
in the ground to be further broken down by the
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.
Biogas generators are process units that make use of
digestion as a means to stabilize waste and produce
systems are designed to digest animal and human solid
they can be used as a treatment mechanism for sludge.
waste decomposes with the aid of anaerobic microorganisms to
produce methane gas, which can be recovered and used as a
As with composting, an optimum carbon to nitrogen ratio
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
Adding the correct amount of waste
material to the unit as well as mixing the material
and removing the digested product from the unit are
Biogas generators can be designed for
small-scale use in one or several homes in many countries;
they only partially solve the sewage problem.
Because they cannot
handle wash water or other types of wastewater, an
means of sewage treatment for these wastes must be provided.
Aquaculture systems have become popular as a relatively
means to provide advanced treatment where it is
specially selected aquatic vegetation, large amounts of
biodegradable material, suspended solids (SS), and other
can be removed from wastewaters.
Water is allowed to
flow through channels at a slow rate where aquatic plants
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
Pretreatment of the waste such as in a series of
lagoons must be provided to remove the solids and partially
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
cost, produces a good effluent, and is mechanically simple.
Imhoff tank, shown in Figure 3, is a large, deep tank
an upper compartment for settling and a lower compartment
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.
tropical climates where the temperature does not vary
the foaming and odor problem will be reduced.
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
spread over land directly or applied to drying beds for
disposal. Since the
discharge from these tanks is not of
high quality, it may require further treatment in lagoons or
Many books have been written on sewage treatment and
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
Sludge Management." Civil
May 1982, p. 62.
Fey, Robert T.
"Cost-Minded Community Chooses Small Diameter
System." Water and Sewage Works,
June 1978, p. 58.
Golveke, Clarence G.
Biological Reclamation of Solid Wastes.
Pennsylvania: Rodale Press, 1977.
Metcalf and Eddy.
Wastewater Engineering. New York,
Norris, D.P., and Troyan, J.J.
"Cost-Effectiveness of On-Site and
Sewerage Alternatives." Civil
Otis, R.J., and Stewart, D.E.
"Alternative Wastewater Facilities
Unsewered Communities in Rural America."
Management Project. Madison,
Rich, Linvil G.
Low-Maintenance, Mechanically Simple
New York, New York.:
Winneberger, John H.
Manual of Grey Water Treatment Practice.
Michigan: Ann Arbor Science, 1974.
CONTACT FOR ADDITIONAL INFORMATION
American Society of Agricultural Engineers
2950 Niles Road
Michigan 49085 USA
American Society of Civil Engineers
345 East 47th
New York, New
York 10017 USA
EPA Small Wastewater Flows Clearinghouse
Virginia 26526 USA
Environmental Research Information Center
Research and Development
Environmental Protection Agency
Inter-American Association of Sanitary
Maryland 20833 USA
National Sanitation Foundation
Michigan 48106 USA
Pan American Health Organization
525 23rd Street,
University of Wisconsin - Extension
Engineering and Applied Science
432 North Lake
Wisconsin 53706 USA
Water Pollution Control Federation
Pennsylvania Avenue, N.W.
10. World Bank
1818 H Street,
11. World Health
20 Avenue Appia
1211 Geneva 27
TERMS USED IN SEWAGE TREATMENT AND DISPOSAL
Activated Sludge System:
A biological treatment system employing
aerobic growth, and recycled sludge.
oxygen. Refers to the addition of
oxygen to the
stabilization process of wastewater and sludge.
Ammonia: A nitrogen
compound that, in combination with phosphates
or by itself,
promotes algal growth. In large
this compound is
toxic to aquatic life.
oxygen. The treatment or stabilization
sludge in the absence of oxygen.
method of sewage treatment employing aquatic
plants to absorb
Biochemical Oxygen Demand (BOD):
A measure of the organic materials
wastewater and the amount of oxygen they
consume over a
length of time, usually five days, at 20 [degrees] C.
Facilities that promote the growth of
to reduce the strength of organic material in
The addition of chemicals to wastewater or
neutralize harmful compounds or enhance thickening
Sewers that carry wastewater from homes and
well as runoff from rain.
aerobic treatment method generally used for
animal or human wastes that are essentially
Detention Time: The
time a unit of sewage is retained in a treatment
means, usually chemical, to treat wastewater to
Reduction of the variability of flows by holding
the sewage in a
tank so that the flow to the treatment plant
over the day.
excessive growth of algae in a body of water.
A treatment means using plants to take up
release it to the atmosphere. Some is
physical treatment process used to remove solids by
wastewater through a graded medium.
Sewers that are installed at a downward slope to
wastewater without the use of pumps.
Grit: Larger solids of
primarily inorganic nature in wastewater,
egg shells, coffee grounds, which settle
out quickly when
the velocity is decreased in the grit
entering sanitary sewers from springs or
entering sanitary sewers through leaky pipe joints
ponds that hold wastewater and use aerobic and/
methods to stabilize wastes. They are
to store water
for long periods of time.
Leach: To remove
soluble constituents from (a substance) by the
action of a
Methane: The major
gas generated from the anaerobic decomposition
of sludges or
The amount of water contained in a known volume
of solids (e.g.,
On-Site Disposal: A
means of sewage treatment designed for one or
a small group of
households without connections to a central
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.
hydrogen. The symbol that denotes a
hydrogen ion concentration. On a scale
zero to 14,
seven represents neutrality. Numbers
indicate acidity; greater than seven indicate
compounds that are known to promote
of algae if present in high concentrations.
Physical units such as pumps, filters,
tanks, that serve to move, screen, or contain
overall term used to characterize unwanted material,
substances in the environment.
Pipes of small diameter used for conveying
it is pumped; these pipes are usually preceded
stage of treatment, usually screening, to
solids or grit.
Reuse: A term
employed when talking about using treated wastewater
as a water
Sewers designed to carry only wastewaters from
businesses, and industries.
Sludge: The material
that settles out from wastewater.
Soil additive that acts as a bulking agent and
Suspended Solids (SS):
A measure of the amount of solids present
the solids are removed by drying at a low
Toxic Material: A
material, usually man-made, that at certain
can kill aquatic life or be a hazard to human
Physical, biological, or chemical systems or
used to reduce the strength of pollutants.
Trickling Filter: A
biological treatment system using aerobic
stabilize wastewater by trickling through a medium
combination of human waste and used water
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.