ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MUNICIPAL WASTEWATER potx

727
MUNICIPAL WASTEWATER
Sewage is the spent water supply of a community. Because of
infiltration of groundwater into loose sewer pipe joints, the
quantity of groundwater is frequently greater than the quan-
tity of water initially consumed. Sewage is about 99.95%
water and 0.05% waste material.
A weaker (more dilute) sewage results from greater per
capital water consumption. Industrial wastes will contribute
to sewage strength. Sewage flow varies with time of day
and will be higher during daylight hours. Larger cities will
exhibit less variation with time than smaller cities. Many
small communities will have a flow in the late night hours
that is due almost entirely to infiltration.
Per capita production will vary from less than 100 gal-
lons per day for a strictly residential community to about
300 gallons per day for a highly industrialized area. The
concept of “population equivalent” is frequently applied
in evaluating industrial waste contributions to sewage flow
and planning for hydraulic, solids, and biochemical oxygen
demand loadings.
Wastewater treatment facilities have high initial capital
costs due to the need for large tanks, equipment and land
area. The designed life of a treatment plant is normally
equal to the life of the bonded indebtedness of the project.
It is expected that capacity will be just reached at the end
of this period. In a community with combined sanitary and
storm sewers it is often found necessary to bypass waste-
water flows during periods of heavy rain or, in low lying
areas, during high tides. Excess water may be held in deten-
tion basins until normal flow resumes and then treated in

the plant. Bypassed flow can be screened and chlorinated
before discharge. This subject is receiving increasingly
close attention.
Strength of sewage is usually expressed in terms of the
following parameters: total solids (sometimes called volatile
suspended solids, VSS), filterable solids, and biochemical
oxygen demand (BOD). Many methods are available for
determination of the foregoing. However, in order that oper-
ating and research data reported by workers in widely sepa-
rated areas be comparable, it is necessary to have analytical
methods which are simple, give consistent results and are
widely accepted. This need is filled by Standard Methods
for the Examination of Water and Wastewater. Contained in
each edition are methods for analysis of water and waste-
water which have been accepted by committees of experts
in various fields. Standard Methods is revised about every
five years.
Analysis of a typical American sewage is given below:
Total solids 600 mg/l
a) suspended solids 200 mg/l
1) settleable solids 120 mg/l
2) colloidal solids 80 mg/l
organic 60 mg/l
mineral 20 mg/l
b) Filterable solids 400 mg/l
1) colloidal solids 40 mg/l
organic 30 mg/l
mineral 10 mg/l
2) dissolved solids 360 mg/l
organic 100 mg/l

mineral 260 mg/l
c) BOD (5 days 20°C)
1) suspended 42 gms/cap. × day
settleable 19 gms/cap. × day
non-settleable 23 gms/cap. × day
2) dissolved 12 gms/cap. × day
Total 54 gms/cap. × day
Assuming an average daily flow of 100 gallons per capita,
54 gms/cap. × day = 0.12# BOD/cap. × day. This is a strictly
domestic sewage. Per capita BOD values can vary from about
0.10#/day to above 0.25#/day. A commonly accepted value
used for estimation is 0.17#/cap. × day.
In the above analysis, the determination of solids, min-
eral and organic, gives an indication of the loadings to be
placed on the plant. Volatile solids give the organic loading
and mineral solids are those which must be accommodated
by sedimentation equipment.
BOD is a measure of the loadings placed on the oxygen
resources of the receiving waters. Several methods have been
used over the years for determination of the amount of oxygen
necessary for stabilization of the waste. Permanganate and
other oxidizing agents were formerly used. One method still
finding application, but only as a general indication, is the
putrescibility, or methylene blue, test. Methylene blue, a dye,
decolorizes in the absence of dissolved oxygen. This test is
only quasiquantitative, but finds use in day to day operation
control of wastewater treatment plants. The method used
today is an incubation and dilution method. Dissolved oxygen
is determined at the beginning and end of a 5 day period by
the Alsteberg azide modification of the Winkler method.

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Water can hold about 10 mg/l dissolved oxygen. Since BOD
values can vary from less than 1 mg/l for a clean stream to
many thousands of milligrams per liter for some industrial
wastes, it is necessary to dilute stronger wastes.
The dissolved oxygen concentration of a seeded nutrient
dilution water is determined. A measured quantity of waste
is then added to a known volume of dilution water. Total
sample volume is usually about three hundred milliliters.
The mixture is then incubated at 20°C for five days. At the
end of this period the dissolved oxygen concentration of the
mixture is again measured. BOD is then equal to the reduc-
tion in dissolved oxygen divided by the dilution factor.
Figure 1 shows the exertion of BOD over an extended
period (more than 50 days). The trajectory displayed is one
which might be followed when waste is added to a rela-
tively clean stream and the watercourse acts as an incuba-
tion bottle. Such a complex reaction is of little utility for day
to day control. In order to make the concept of BOD more
useful, a simplified model is developed. The path followed
in the idealized model is displayed in Figure 2.

The lag phase is eliminated by use of seeded dilution water.
In this phase, microorganisms which utilize the waste material
for food are becoming acclimatized and assuming dominance
in the system. In the carbonaceous phase the dominant organ-
isms are feeding upon the more easily degraded compounds.
The nitrogenous phase, in which the character of the food and

the microbial population change, does not usually begin for at
least ten days. This portion is of interest in research on kinet-
ics but can be ignored in the following. It is hypothesized that
a simple monomolecular rate constant can describe the car-
bonaceous phase and the reaction is asymptotic to a limiting
value L at infinite time. L is known as the ultimate BOD and
is a mathematical artifact. The rate of reaction is assumed pro-
portional to the BOD still remaining,



d
d
1
y
k
t
Lyϭ
Ј
Ϫ().
Integration and taking of proper limits gives



yL e L
kt kt
ϭϪ ϭϪ
ϪϪ
ЈЈ
()( ).1110

11
Phelps’ relation for decolorization of methylene blue with
time



y
L
t
ϭϪ1 0.794
is equivalent to the model equation when k
1
equals
0.10/day.
In practice, the 5 day 20°C BOD is used to describe
the strength of a waste. Simple enumeration of the BOD
value tells nothing about the path by which the number was
reached since L and k
1
are not specified. Sewers and treat-
ment plants are not isothermal entities but it is necessary that
a common basis be established. One of the reasons advanced
for the choice of the five day period is the fact that almost all
rivers in England are within five days flow of the sea. Waste
once discharged to the infinite ocean is no longer of interest.
Times have changed and there is great concerns for effects of
waste on the “infinite ocean.”
In the literature reference to BOD means 5 day 20°C BOD
(BOD
5

) unless specifically stated otherwise. Recently, some
popular writers have erroneously used the term “ biological
oxygen demand.” There is no such parameter.
The reaction rate constant k
1
actually describes a series of
complex microbiological reactions. In the initial stages of
biological stabilization of sewage, carbonaceous material is
oxidized to carbon dioxide and nitrogenous material is oxi-
dized first to nitrite and then nitrate. One of the measures of
degree of stabilization of organic matter in an effluent is the
nitrate concentration. However, recent theories concerning
eutrophication have raised serious questions concerning the
desirability of high effluent nitrate concentrations.
time
lag phase
carbonaceous
phase
B.O.D
mg/l
nitrogenous
phase
FIGURE 1
B.O.D.
mg/l
L
y
L – y
y
time

FIGURE 2
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MUNICIPAL WASTEWATER 729
Sewage contains the waste of a city and its inhabitants.
It is possible to find almost any microorganism in sewage.
In fact, in the sewer system can be found quite unexpected
creatures. About 1960, sewer workers in New York City
found a number of live and exceedingly unfriendly alligators
in that city’s sewers. These animals, gifts to city residents,
had been disposed of through household toilets. The warm,
wet environment of the sewers, rich in food, was excellent
for rapid growth.
Traditionally, public health practice applied to water san-
itation has placed major emphasis on prevention of typhoid
fever, the causative organism of which is S. typhosa. This
disease is spread by the pathway of anus to mouth. S. typhosa
is a fairly delicate organism and is usually not found in high
concentrations. It is felt that if pollution arising from human
intestinal discharges is removed to a high degree, then S.
typhosa will also be removed. As BOD is used as an indi-
cator of pollutional loading on oxygen resources, so is the
indicator organism E. coli used as an indication of pollu-
tional loading due to human intestinal waste. E. coli is a
normal inhabitant of the human intestinal tract. It is excreted
in huge numbers and the presence of this organism in receiv-
ing waters may be evaluated quantitatively. The lactose
broth-brilliant green bile test is commonly used to determine
the presence of E. coli. A common soil organism, A. aero-
genes, gives a false positive test and further confirmatory

tests are necessary. Serial dilutions of the water being tested
are prepared and, on the assumption that one organism is
responsible for a positive test, the Most Probable Number
(MPN) of organisms is determined. The MPN is based on
statistical reasoning. Work by Kupchick, using the enzyme
urease to determine the presence of uric acid, has shown a
high degree of correlation between the concentration of this
acid in wastewater and the MPN.
Most pathogenic organisms are not hardy and do not com-
pete well for food. The use of E. coli as an indicator organism
is in the way of a margin of safety. This is consistent with
Phelps’ concept of multiple barriers.
Microorganisms form the basis of secondary, or biologi-
cal, wastewater treatment. Stated briefly, microorganisms
establish themselves on trickling filter slime or in activated
sludge liquor and feed on waste material in the sewage.
Large particles are removed in the primary settling por-
tion of treatment. The larger particles, grease, etc. are in some
ways not as objectionable as the colloidal or truly dissolved
materials. The larger particles are, of course, objectionable
from an aesthetic point of view, but the smaller particles
place more of an immediate load upon the receiving waters.
In almost all areas of environmental engineering surface
area is one of the controlling parameters. Microorganisms,
in carrying out their vital processes, utilize the waste as food
and the smaller particles offer greater surface area per unit
volume. Microbial activity is correspondingly greater and
the oxygen required is also greater.
Microorganisms require a readily available source
of oxygen. Aerobic conditions are said to exist when the

oxygen is in the form of dissolved molecular oxygen or
nitrates. At the point of exhaustion of nitrates and nitrites
the system is in the anaerobic state and the oxygen sources
are then sulfates, phosphates, borates, etc. Reduction of
sulfates to give mercaptans (HS
−
) carriers with it charac-
teristic vile odors. Secondary treatment is an essentially
aerobic process while conventional sludge digestion is an
anaerobic process.
In the trickling filter an activated sludge processes
microorganisms extract their food from the flowing waste-
water. By means of extracellular enzymes large molecules
are broken down so that they may pass through the microbial
cell wall by diffusion. The food is further broken down for
cell synthesis and energy by means of intracellular enzymes.
End products are largely carbon dioxide and water. The
waste material, now part of the cell mass, is removed in the
final stage of treatment.
The primary reason for treating any waste is the need for
avoiding nuisance and dangerous conditions in the environ-
ment. It is necessary to remove some of the waste so that the
remaining can be discharged to the surroundings. This is, in
effect, disposal by dilution. Discharge standards are usually
based on concentration and total quantity.
Sewage purification works were formerly constructed
for reasons based primarily on public health. More modern
thinking has expanded the original rationale to include pro-
tection of oxygen resources of the receiving waters. If the
second criterion is satisfied, the first will almost always be

also satisfied. Demand for clean waters is increasing even
more rapidly than population. Water will be used many
times in its passage to the sea. Lakes are essentially a closed
system. Leisure time is increasing and the question of water
pollution has reached the point where aesthetics is now a
significant parameter in planning. It is no longer possible to
treat a watercourse as a separate entity. The approach now
finding wider and wider application is that of basin manage-
ment. This is the systems approach. An excellent example
of this is the Ohio River Sanitation Commission. Here it has
been possible to rise above local jealousies and self inter-
est. The results are most gratifying and should serve as an
example to other areas. It is unfortunate, but true, that one
heavy pollution source can undo the efforts of many groups
with social responsibility and foresight.
Because objectives in waste management can change
drastically at political boundaries, it has occasionally been
necessary for intervention by representatives of larger politi-
cal entities when pollution problems effect, for example,
several states. Federal agencies in the Unites States and
other countries have come to play an increasingly larger role
in waste management, particularly when problems do not
respect political boundaries. The discussion presented here
involves primarily technology of wastewater treatment and
the underlying philosophy will not be extensively covered.
It has been mentioned that larger particles in wastewater
can be removed by physical means. Removal of colloidal
and dissolved components requires other methods and this
must be accomplished economically. Treatment is classified
as primary if it is without biological basis. Secondary treat-

ment is generally accepted as biological treatment. A few
regulatory authorities have endeavored to classify treatment
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730 MUNICIPAL WASTEWATER
degree on the basis of degree of removal. This relegates some
biological treatment to primary status. Many experts do not
agree with this change in definition. A widely accepted defi-
nition of tertiary treatment is the use of any process, in addi-
tion to conventional secondary treatment, for the purpose of
further removals. From the foregoing it should be noted that
chemical treatment, popular in the past, is a primary treat-
ment process.
Water carriage of waste was practised in the Minoan
civilization of Crete. It may be said that sanitation practices
reflect the level of a civilization. Certainly, sanitary practices
of the Middle Ages were of a rather low order. Methodical
control of waterborne diseases was not attained until the
19th century. It is of historical interest to note that knowl-
edge of the use of creosote for odor control at the Carlisle,
England sewage purification works gave inspiration to
Joseph Lister for the birth of asepsis in surgery.
The process by which wastewater is purified can best be
understood by following the waste from its source through
the collection system and treatment plant. Organic material
discharged to a watercourse will eventually undergo stabi-
lization. This is accomplished by natural processes (unit
operations) and a wastewater treatment plant basically sets
up, under controlled conditions, the processes which act in
the river. Indeed, it has often been said that a sewage treat-

ment plant is a river in miniature. In the river heavy particles
settle out and lighter particles float to the surface. Biological
decomposition takes place. Oxygen present is used by organ-
isms that accomplish decomposition. Some of the settled
material will be resuspended, increasing the organic load-
ing. While oxygen is being withdrawn by BOD this resource
is being replenished at a rate proportional to the deficit. The
oxygen deficit is the difference between the amount that can
be held at saturation (about 10 mg/l) and the amount actually
present.
Stated mathematically, the oxygen concentration in a
river as a function of time is



d
d
D
t
ky kDϭ
Ј
Ϫ
Ј
12
.
This expression considers only the effects of BOD and atmo-
spheric reaeration. A more complete equation can be writ-
ten but effects of oxygen production by algae and oxygen
reduction by benthal (bottom) deposits are not of major
significance here. Solving the above gives

D
kL
kk
D
t
kt kt kt
ϭϪϩϫ
ϪϪ Ϫ
Ј
1
1
0
10
11 2
−
{}
10 10 .
This expression is commonly known as the oxygen sag
equation and is displayed in Figure 3. It is of interest here
because it illustrates the rationale underlying waste treat-
ment requirements. Waste is treated so that undesirable
conditions do not develop in the receiving waters. In effect,
a limit has been placed on the allowable oxygen deficit.
Regulatory authorities usually require that a minimum dis-
solved oxygen level be maintained. Normally, this will be
stated as a percentage of dissolved oxygen saturation. This
is the largest permissible critical deficit. Game fish may

require a minimum of 5 mg/l D.O. while scavengers can
survive in a much lower quality water.
The critical deficit is given by



D
kL
k
c
kt
c
ϭ
ϫ
Ϫ
1
2
1
10
.
Once the maximum deficit is specified, the BOD loading
on the stream can be immediately estimated. The allowable
BOD loading will be that impressed on the watercourse by
the wastewater treatment plant effluent. It is of interest to
observe the effect of the condition of the river at the point
of discharge. A river in poor condition will have a large ini-
tial deficit, D
0
. This can raise the treatment requirements. It
can be seen that it is necessary to integrate the efforts on a

basin wide basis. Parameters other than just dissolved oxygen
must be controlled by the treatment processes. This is accom-
plished in some, or all, of the following steps.
Decomposition of the waste begins in the collection system.
Ease, or difficulty, of treatment depends to a large degree on
the condition of the sewage when it reaches the plant. Some
substances are not permitted in the sewage system. Gasoline
and other flammable substances, oil, hexavalent chromium are
examples of prohibited substances. These can damage either
the collection system or the treatment plant and processes. The
legal vehicle by which such materials are excluded is known
as a sewer ordinance.
It is most economical to collect sewage by gravity flow.
If topography does not permit, pumping will be necessary
in order to cross high points and to avoid excessively low
flow velocities and deposition of waste material in the pipes.
(1)
(2)
D
c
reoxygenation
D.O
D
critical deficit
deoxygenation
time
(1)+(2)
D.O.
s at.’
D

O
D=O
FIGURE 3
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MUNICIPAL WASTEWATER 731
A combined system will, at times, exceed the hydraulic
capacity of the treatment plant and flow must be either bypasses
or held in detention tanks until heavy flow has subsided.
Multiple treatment units are provided and treatment is
not interrupted during periods of maintenance or repairs.
Protection must be provided for pumps against large objects,
such as floating pieces of wood. Coarse racks, with clear
openings of more than 2 inches, may be placed at the entrance
to the plant. Racks to be placed in advance of grit chambers
and settling tanks will have clear openings of 1 to 2 inches. In
smaller plants racks are cleaned by hand while larger plants
have mechanically cleaned racks. Disposal is by burial,
incineration or digestion with sewage sludge. Mechanically
cleaned racks have smaller clear openings because head
losses are lower with continuous cleaning. Racks with clear
openings of 1 to 2 inches can be expected to give from 20
to 100 ft
3
of solids per 1000 people annually. Comminutors
macerate floating material into sizes sufficiently small to be
easily handled by centrifugal pumps. Racks and screens with
very small openings have been almost completely replaced
by comminutors.
Rate of flow into the plant will vary over a wide range

during any 24 hour period with smaller plants exhibiting
greater variation. Flow is measured by Parshall flume, Venturi
meter or Sutro (keyhole) weir. The Parshall flume, sometimes
called the open channel venturi, is most commonly used. The
device operates on the principle of critical flow and mea-
surement of water depth upstream of the flume throat. The
governing equation is of the form Q = cWH
3/2
, where Q is
the discharge per unit time, W is the throat width, H is the
water depth, and c is a constant. While c changes with throat
width, it is closely constant for a constant throat width. Flume
liners of reinforced fiberglass are all but replacing steel and
concrete liners. Ease of fabrication, close tolerance, and
corrosion resistance are advantages cited. The true Venturi
operates on closed pipe (pressure) flow and is usually found
in larger plants. The Sutro, or keyhole, weir is shaped as its
name suggests. Its principal advantage is maintenance of a
constant upstream velocity over a wide flow range, but it
does have a high energy loss and metering is lost when the
opening is submerged.
It is necessary to remove grit in order to protect pumps
against excessive wear and to maintain capacity of sludge
digesters. It has been found that digesters in plants serving
low lying sandy areas can, if grit removal is not efficient,
lose up to a third of capacity in but a few years. Grit cham-
bers must operate in a fairly narrow velocity range of from
0.75 to 1.25 ft/sec. Above this range deposited material is
scoured back into suspension and below the lower value
organic material settles out. The resulting material, called

detritus, is unsuitable for landfill uses due to its highly
putrescible nature. Grit chambers are usually designed to
remove particles with a specific gravity of 2.65 and a mean
diameter of 0.02 cm. Because flow variation with depth fol-
lows a parabolic function, Q = cWH · H
1/2
, the grit chamber
is often given an approximately parabolic shape and better
velocity control is attained. The amount of grit collected per
million gallons flow is found to vary from 1 to 12 cubic feet.
Grit is removed manually in small plants and continuously
by mechanical means in larger plants.
Settling tanks are provided for removal of larger, heavier
organic particles, oil, and grease. Oil, grease, and other
materials lighter than water are skimmed continuously from
the surface and led to digestion. Both circular and rectan-
gular surface configurations are used. Rectangular tanks
of the flow through variety have length to width ratios of
4/1 to 6/1. Circular tank size is usually limited by structural
requirements of trusses carrying skimming devices. Tank
depths vary from 7 to 15 feet. Bottoms are sloped about 1%
in rectangular tanks and about 8% in circular tanks to facili-
tate sludge removal.
Design is on the basis of hydraulic loading. A com-
monly used figure is 1000 gal/day × ft
2
surface area. It can
be expected that a BOD removal of 30% will be achieved in
a well operated primary sedimentation unit.
If treatment includes only screening, sedimentation, and

chlorination of effluent, the treatment is classed as primary.
Primary treatment, while inadequate for most areas, is
better than no treatment. The adequacy of secondary treat-
ment is now being seriously questioned. Nonetheless, it
fits the economics of the situation. Only in the 1960’s did
wastewater treatment become of interest to any but a small
number of people. Sanitary engineers were wont to say
“It may be sewage to you but it is bread and butter to me.”
Theirs was not a profession to which much glamour was
attached. Financing bodies were reluctant to invest adequate
sums in waste treatment facilities. Hopefully, this has now
changed.
There are two main processes utilized for biological
(secondary) treatment. These are (1) the trickling filter and
(2) activated sludge.
The trickling filter is not a true filter. It can best be
described as a pile of stones, or other coarse material, over
which sewage flows. This is the most widely used biological
treatment process. Present day biological treatment technol-
ogy is a logical development from sewage farms (irrigation
areas) to intermittent sand filters to contact (fill and draw)
beds to trickling filters and activated sludge units. Numerous
modifications of the basic processes have evolved but the
underlying principles remain unchanged. In biological
treatment a suitable environment is provided so that micro-
organisms may thrive under controlled conditions. The suit-
able environment is one rich in food and maintained in the
aerobic state. The zoogleal mass remains fixed on the filter
media in the trickling filter while the sewage flows past. In
the activated sludge process the sewage and organisms flow

together. In both cases the microorganisms come from the
sewage itself. Traditional secondary treatment plants operate
in the declining growth phase.
Irrigation by sewage provides water return and some
waste stabilization but this means of sewage disposal is in
conflict with sound public health practice and ought not be
used where there is a possibility that sewage can pass with
little change into the groundwater table. Irrigation is best
applied in arid regions. When it is utilized for food growing
areas, care must be taken so that edible plants and fruit are
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not contaminated. Odors are a problem and removals decline
markedly in cold weather.
Intermittent sand filters are much like the slow sand fil-
ters used for potable water production. The sewage is applied
to a sandy area and allowed to percolate downward. Raw
sewage may be applied at rates as high as 80,000 gal/acre ×
day and secondary effluents at rates as high as 800,000 gal/
acre × day. Application of the secondary effluent would be
considered tertiary treatment. Biological films that form on
the sand grains undergo continuous stabilization. It is neces-
sary to rest the bed between dosings so that objectional con-
ditions do not develop. Surface accumulations of solids must
be periodically removed. This method is not recommended
for areas underlain by fissured limestone.
Fill and draw beds operate as the name indicates. A tank,
packed with coarse material, is filled with sewage and allowed
to stand full. It is then drained and allowed to rest. Air is

drawn into the bed during filling and emptying. Loadings are
about 200,000 gal/acre × ft × day. There is little application
of this method today in treatment of municipal wastewater
but it does find use in industrial waste treatment. Fill and
draw beds are a batch operation and the trickling filter is a
continuous operation.
Sewage is distributed over a trickling filter by slowly
revolving arms equipped with nozzles and deflectors. Some
earlier plants had fixed nozzles but this is no longer done.
Revolving arms are driven by hydraulic head. Sewage dis-
charged is allowed to flow slowly downward through the
bed. Air is down into the bed by temperature differential,
thus maintaining a supply of oxygen for the process. Filter
media is usually stone. Sizes are in the range of 1 to 4 inches.
Packing of this size permits air to be drawn into the bed and
the bed is not clogged by biological slime. There appears
to be a trend toward more use of plastic filter media. Filter
depths range from 3 to 14 feet. A common depth is 6 feet.
After passage through the filter the sewage is collected
in tile underdrains. These underdrains serve two purposes:
(1) collection of filter effluent and (2) circulation of air
into the filter. The underdrains discharge to a main collec-
tion channel which, in turn, discharges to the final settling
(humus) tank. The importance of the function of the final
settling tank can be seen by an examination of what occurs
in the filter itself.
A new filter is “broken in” by applying sewage as in
normal operation. After a time the microbial (zoogleal) mass
establishes itself on the filter media and carries on the work
of waste stabilization. Waste material in the flowing sewage

(food) is first absorbed into the zoogleal mass and then
assimilated by the microorganisms. Much of the organic
waste material has, at this point, been utilized for cell syn-
thesis and energy. There must be continuous removal of
filter slime or the process becomes sluggish due to a lower
feeding rate of old organisms. Since waste material is now
a part of the filter slime there must be a means provided for
removal of sloughed off organisms or the waste material,
now in different form, would still appear in the plant efflu-
ent and little constructive would have been accomplished.
The required removal is carried out in the secondary settling
tank. A rate of application lower than that of the primary
tank is necessary here because of the different character
of the material to be removed. Rates in this portion of the
system are in the range of 600 gal/ft
2
× day.
A portion of the effluent is recirculated, as shown in Figure
4. This is done in order to (1) smooth out flow, (2) keep the
food concentration more constant, (3) lower the film thickness
and, thus, control the psychoda fly, and (4) reseed the applied
sewage with acclimatized organisms. The psychoda, or filter,
fly is a very small insect which breeds in trickling filter slime.
It does not bite but can be extremely bothersome because it
does get into the nose and mouth. The range of flight is short
but the creature can be carried great distances by the wind.
Control of the fly in its developmental phase can be achieved
by flooding the filter periodically or through chlorination of
influent.
Trickling filters can be classified on the basis of (1) hydrau-

lic loading per unit area and (2) applied pounds of BOD per
1000 ft
3
of filter volume.
The low rate trickling filter, with hydraulic loadings
of 2 to 4 million gallons per acre per day (mgad) and 10
to 20 pounds BOD per 1000 ft
3
, is usually found in use in
smaller plants. With proper operation, BOD removals of
80 to 85% can be routinely expected. Raising the applied
sewage to 10 mgad produced greater BOD removals per
unit filter volume but the effluent organic concentration was
found to be high. Influent organic concentration was reduced
by greater effluent recirculation and lower effluent organic
concentration was realized. Units that operate in the 10 to 40
mgad range are called high rate trickling filters. BOD load-
ings are up to 90 pounds per 1000 ft
3
, but removals to be
expected are in the range of 65 to 75%.
In the 4 to 10 mgad range operational difficulties were
frequently encountered and this range was avoided for many
years. It appears that, in this range, the hydraulic application
was inadequate to keep the filter slime from attaining exces-
sive thickness. Many plants had operated well in this range,
but other plants had many problems. The solution seems to
have been reached with use of relatively large, 2 to 4 inches,
filter stones.
Experimental plants using plastics media have recently

achieved very high removal efficiencies (97%) at hydraulic
loading rates of 100 mgad. Much of the microbial mass is
in the recirculated effluent and these plants are, in effect,
modifications of the activated sludge process. Organic
influent
Headworks
Primary
Sed.
Biological
Treatment
Secondary
Sed.
out
recirculation
excess to
digester
to digester
Primary Treatment
CI
2
FIGURE 4
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© 2006 by Taylor & Francis Group, LLC
MUNICIPAL WASTEWATER 733
loadings are in the region of 100 pounds BOD per 1000 ft
3

filter volume.
Filter packing by plastic media is finding wider appli-
cation. Design criteria, however, call for quite deep filters

and this appears to be uneconomical in terms of power
requirements.
Modifications of the trickling filter process over the
years have dealt with improvements in media, air circula-
tion, and loadings. One of great interest is that proposed by
Ingram. Sewage is introduced at various levels in a very deep
filter in an attempt to distribute the load more uniformly over
the whole filter depth. Hydraulic loadings up to 500 mgad
have been successfully achieved.
A new development in trickling filter technology is the
Rotating Biological Contractor (RBC). A rotating drum is
partially immersed in wastewater. A zoogleal mass devel-
ops on the drum surface, functioning in the same manner as
trickling filter slime. Such installations may be completely
enclosed in plant buildings, thus avoiding any effects of
extremes in outside temperature. An excellent example of
the application of this process for upgrading the municipal
treatment works at North Bergen, New Jersey.
Activated sludge serves the same function as trickling
filter slime. The major difference lies in the filter slime being
fixed to the filter media while the activated sludge is carried
along with the flowing wastewater. Development of the acti-
vated sludge process began with attempts to purify sewage by
blowing air into it. It was observed that after prolonged aera-
tion, flocs composed of voraciously feeding organisms devel-
oped. This floc settled after aeration was stopped. Addition of
fresh sewage to tanks containing the settled sludge produced
high purification in a practical time. The name activated
sludge was assigned this means of waste treatment. At first,
this was operated as a fill and draw system. Research showed

that continuous operation could be practiced and this is the
present means or operation. The process involves:
1) return of activated sludge to the aeration tank influ-
ent and discharge of excess sludge to digestion
2) aeration of the sludge-sewage mixture to maintain
purification and
3) settling of the aeration tank effluent to remove
floc before final discharge.
Step (3) is necessary for the same reason as the comparable
portion of the trickling filter process-removal of waste mate-
rial transferred to the microbial cell mass. Floc is formed
in the tank through aerobic growth of unicellular and fila-
mentous bacteria. Protozoa and other organisms will also
be found in the floc. This is a strictly aerobic process and
air requirements are high. Two aeration systems are used,
(1) diffused air units and (2) mechanical aeration. Air dif-
fusers are more commonly used in North America but
mechanical aeration systems may be found in plants of less
than 1 million gallons per day (mgd) capacity. Both methods
of aeration perform three functions, (1) transfer of oxygen
to the mixture and maintenance of aerobic conditions,
(2) intimate mixing of floc and sewage, and (3) keeping the
floc in suspension.
Aeration tanks are normally rectangular in cross section,
10 to 15 feet deep and 30 feet wide. Length to width should
be greater than 5 to 1 in order to avoid short circuiting.
Detention periods are from 4 to 6 hours. Air is introduced
from diffusers in such a way as to set up a spiral flow pat-
tern, thus aiding in mixing of floc and sewage and helping
to prevent dead spaces in the tank. It was found that oxygen

requirements decreased as the waste proceeded through the
tank. The number of diffusers was, therefore, increased at
the beginning of the unit and decreased at the effluent end.
This is now the common practice and is known as tapered
aeration. Mechanical aeration has the same function as air
diffusers but is accomplished by rotating paddles or brushes.
Peripheral velocity is about 2 ft/sec.
Floc returned to the aeration tank has the purpose as
trickling filter slime but floc concentration can be varied as
operation needs dictate. Returned sludge varies from about
10 to 30%. Mixed liquor suspended solids (MLSS) will
vary from 600 mg/l to 4000 mg/l, on a dry weight basis.
An important parameter in routine process control is the
ratio of the volume of MLSS to the dry weight of MLSS.
This is known as the sludge volume index (SVI) and is in
the range of 50 to 100 in well operating plants. When the
value approaches 200 operating difficulties can be expected.
Factors which promote or inhibit microbiological growth are
important and these include pH, temperature, and oxidation-
reduction potential (ORP). Hydrogen ion potential, pH, will
have a great effect on the dominant species of organisms.
Bacteria predominate above pH of 6.5 while fungi assume
greater importance below this value. There must be adequate
buffering capacity if metabolic products are acidic.
Modifications of the basic activated sludge process have
come about for solution to specific operating problems.
The municipal treatment plant at Peoria, Illinois received
a waste high in carbohydrates. The resulting nitrogen defi-
ciency caused a light and poorly settling activated sludge floc
with attendant poor waste stabilization. Kraus, for whom the

modification is named, aerated digester supernatant. This,
added to the influent, gave a nitrifying activated sludge. The
result was a readily settleable sludge with improved organic
removals.
New York City has plants scattered throughout the five
boroughs, treating more than 1 billion gallons of sewage per
day. A major modification resulting from experimentation
with plant operation has come from this city. In conventional
plants sewage was added at one end of the aeration tank and
allowed to flow through. This gave a high initial microbial
food supply and correspondingly high oxygen requirement.
The New York City modification involves introduction of
sewage at intervals along the tank. This smooths out the food
supply and lowers the oxygen requirements. The sewage is
added at discrete steps along the unit and the name applied
is step aeration.
A low mixed liquor suspended solids concentration of
200 to 500 mg/l is maintained in the high rate process. This
gives a high food to microbial mass ratio. This keeps the
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734 MUNICIPAL WASTEWATER
floc in the active growth phase, but excess food will be dis-
charged in the effluent. BOD removals are only 50 to 60%,
but in some areas this is acceptable. New York City applied
this method successfully because of a weak sewage and
low temperatures. Philadelphia and Los Angeles met with
indifferent success because of stronger sewage or higher
temperatures.
A process originated simultaneously and independently

by Smith and Eckenfelder made use of a phenomenon
observed by many researchers but dismissed as experimen-
tal error. When activated sludge and raw sewage are mixed
together in an aeration vessel there is a noted reduction in
BOD, followed by a rise and then another reduction. The
first decrease had been ignored by most research workers.
Smith and Eckenfelder found that this was due to adsorp-
tion of waste material onto the activated sludge floc. This
came from material desorbed from colloidal particles. Plants
at Austin, Texas, and Bergen County, New Jersey were con-
verted from overloaded to underloaded by changing to the
biosorption process.
A recent activated sludge process modification is the
Deep Shaft Process. As its name suggests, two concen-
tric deep shafts (120–150 m) are sunk into the ground.
Wastewater is injected into one of the concentric shafts and
the effluent is withdrawn from the other. A constant ambient
temperature is maintained due to the surrounding geological
formations. Compressed air is injected at the bottom, giving
high dissolved oxygen concentrations and provided intimate
mixing. Waste sludge is removed in clarifiers as in conven-
tional activated sludge plants.
Putrescible material collected from the primary settling
tanks and excess sludge from humus tanks must be disposed
of cheaply and efficiently. This material is highly unstable and
a potential nuisance source. Because it is putrescible it can
be stabilized by biological means, serving as food and energy
sources for microorganisms naturally found in the sludge.
Raw sludge is about 95% water, but the water is not easily
removed. As the sludge is broken down the water content is

lessened, and the volume is markedly reduced. A rough rule
is that sludge volume is reduced by half when water is low-
ered from 95 to 90%, and by two thirds when reduced from
95 to 85%. Fresh sludge has a gray color and can be easily
pumped. Its odor is most disagreeable, being due principally
to mercaptans. Digested sludge is black in color, granular
and has a slight tarry odor.
Sludge digestion is carried out in order to reduce the
volume of sludge to be handled, and reduce the number of
pathogens. Sludge is usually withdrawn at regular intervals
from primary and secondary tanks and led by gravity to a
sludge well. It is then pumped to the digester.
Mixing is very important for efficient sludge digestion.
Temperature is equally important.
Since destruction of sludge is carried on by microor-
ganisms, kinetics of their life processes will be temperature
dependent. It has been found that sludge temperature of about
95°F will give acceptably short detention times. Even shorter
detention times for the same quality of digested sludge can
be achieved with temperatures of about 125–130°F, but this
temperature range is not widely used for reasons of econom-
ics. Above 95°F an increase in detention time is noted, up to
110°F, and then again a decrease. The reason for this is the
changing character of the predominant organisms.
Heating of sludge for efficient digestion is carried out in
one of two ways. The older installations have hot water coils
in the periphery of the tank, and heat is transmitted to the
digesting sludge. Mixing was felt to be adequately effected
by turbulence due to gas generation. Mechanical mixers
have been used. It was found, however, that mixing was not

sufficient. In addition, heating of entire tank contents was
not achieved due to “baking” of sludge in the vicinity of the
heating coils. A second method of sludge heating and mixing
was developed, involving the use of external heat exchang-
ers. Sludge is pumped from the digestion tank through a
heat exchanger and returned to the tank. Two objectives are
accomplished (1) efficient mixing of sludge, thereby reduc-
ing the amount of inadequately digested sludge, (2) more
uniform temperature throughout the tank, thus reducing
digestion time. The use of external heat exchangers has
almost completely supplanted heating coils and internal
mixers in new plant design.
Sludge gas generated during digestion is approximately
72% methane and 28% carbon dioxide. Hydrogen and H
2
S
are present in trace amounts. Gas thus generated has a calo-
rific content of about 600 BTU/ft
3
. About 10 ft
3
of gas are
produced per cubic foot of raw sludge digested. Generally,
the amount of sludge gas produced is sufficient to provide
heat used in maintaining digesting sludge at the required
temperature, heating plant buildings, provide hot water and
incineration of digested sludge, when practiced, and fuel and
generators.
Volatile acids, reported as acetic acid, are perhaps the
most important parameters in control of sludge digestion.

Volatile acids below 1000 mg/l occur in a healthy digestion
process. Volatile acids of 6000 mg/l indicate a malfunction-
ing process. pH values of 6.8 to 7.2 are optimum. Values less
than 6.8 usually are due to excessive volatile acid produc-
tion. In the past liming of malfunctioning tank contents was
practiced in an effort to adjust pH to about 7.0. However,
the change in volatile acids production was due to changing
dominant process microorganisms. The lowered pH and high
volatile acids concentrations were a sign of a sick process,
rather than the cause.
Digested sludge is reasonably inert but it must be fur-
ther dewatered and the question of final disposal of raw and
digested sludge is one of the most pressing with which envi-
ronmental engineers must deal today.
Sludge can be dewatered on open or covered drying beds.
Open beds are exposed to the air and drying is accomplished
by drainage and evaporation. Covered beds resemble a
greenhouse. Temperatures are rather high and this aids evap-
oration. In both cases sludge is allowed to flow over sand
beds and let stand for a suitable period. The dried sludge is
then scraped from the beds.
Sludge can be dewatered by vacuum filtration. Filter drums
rotate slowly, picking up wet sludge at the bottom. A slight
vacuum is applied and the water drawn off is returned to the
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MUNICIPAL WASTEWATER 735
plant stream. At the end of the cycle the dewatered sludge is
removed by a scraper.
Dried sludge can be incinerated, taken to a landfill dis-

posal site, composed or subjected to superoxidation. Some
coastal cities barged sludge to sea. This was to have ceased
in 1981 but was permitted to continue for ten years. It is not
allowed after 1991. Incineration of sludge has the potential
for air pollution problems and often there is local opposition
to installation of an incinerator. Incinerators are expensive to
build and operate. Land disposal is expensive and disposal
sites frequently are considerable distances from the gener-
ating wastewater treatment plants. Groundwater pollution
can occur. Landfills which can accept digested sludge are
in short supply. Transportation costs are a quite significant
part of total disposal costs. Composting has been suggested
as a possible ultimate solution. The requirement of relatively
large land areas and odor production are problems.
A promising approach involves superoxidation. Here the
sludge is treated with a strong oxidizing agent. Volume is
reduced greatly and the end product is stable and inoffen-
sive. Transportation costs are thus reduced.
It is common practice to chlorinate effluent for bacte-
rial control. Regulations vary from state to state, but most
regulations require chlorination to a specified residual.
Requirements usually vary from season to season, the most
stringent rules governing the swimming season.
A phenomena not yet fully understood is that of after-
growth, wherein bacterial count is fairly low immediately
after effluent discharge but then suddenly rises to a high
figure.
In some plants chlorination of the influent is practiced
for the purpose of odor control.
Chlorination of storm water overflow is commonly prac-

ticed. In some cases storm water overflow is subjected to
simple sedimentation and/or screening, storage and chlori-
nation, then discharged after cessation of the storm.
One commonly used definition of tertiary treatment is any
treatment in addition to secondary (biological) treatment.
Tertiary treatment is practiced when an effluent of much
higher quality is required than is attainable with conventional
biological treatment. They type process used will depend on
the final effluent quality necessary and the economics of the
total process.
Commonly used tertiary treatment processes are listed
below:
1) Sand filtration
2) Microstrainers
3) Oxidation ponds
4) Foam separation
5) Activated carbon adsorption
6) Chemical clarification and precipitation
7) Ion exchange
Disposal of human and kitchen wastes in areas not served
by sewers and wastewater treatment plants presents unique
problems.
Disposal must be in the immediate vicinity of the source
of the wastes. In adequate controls are not exercised a closed
system may results. An example is Suffolk County (Long
Island), New York. Septic tanks are widely used, and there
is strong local opposition to the considerable expense of
installing sewers. Effluent from the septic tanks found its
way into the ground water which is the supply for much of
the county. Eventually, the problem was graphically pointed

up by the appearance of foaming detergents in water issuing
from the tap.
In more primitive societies waste disposal is a matter of
convenience. A “cat hole” or communal straddle trench is
utilized and covered when capacity is reached. This is the
same as the practice with privies and cesspools.
Privies, as the name implies, are simply open pits with
a structure to provide privacy. Human excreta is deposited
into the pit and is slowly stabilized. Stabilization is slow,
due principally to the presence of urine. Pits may be open
earth or concrete vault. Drawbacks for both types are odors
and fly problems. For the unlined pit there is the additional
problem of ground water pollution. Older privy construction
allowed access to flies around the edge of the pit. For proper
protection against flies there must be a tight seal around the
edge of the pit and adequate screening of openings in the
privy structure itself.
In the 1930s a large number of the older privies were
replaced by concrete vault types. Today, such methods of
waste disposal are found only in the smaller rural communi-
ties where there is no municipal collection system.
Cesspools are simply pits into which waste is allowed to
flow. The term leaching pit is sometimes used. Water seeps
into the ground, leaving solid matter in the pit. Construction
is of two types. A pit may be unlined, or it may be lined with
sewer pipe laid on end. Almost nowhere in the United States
are cesspools permitted by health authorities.
Septic tanks are widely used in smaller towns and out-
lying suburbs of larger cities. They are a combination sedi-
mentation tank and anaerobic digester. Sanitary and kitchen

wastes flow into the tank and grease and light material rise to
the top. Heavier particles settle to the bottom where anaerobic
stabilization occurs. Deflector plates are provided at inlet and
outlet in order to minimize short circuiting. Effluent flows to
a tile field where disposal is into the earth. The tile field is
composed of perforated field tile fed by a manifold. The tile is
underlain with granular material, usually gravel. Care must be
taken that the earth does not become clogged by material car-
ried over from the septic tank. Septic tanks are being replaced
as more and more areas are served by municipal systems.
Health authorities do not look with favor on septic tanks.
Capacity will be a function basically of the number of per-
sons or units served. Some experts feel that, in no case, should
capacity be less than 1500 gallons. Lesser volumes are permit-
ted in many codes and the thought that 1500 gallons ought to
be the minimum permitted arouses home builders and land
developers. Periodically it is necessary to employ a scavenger
service for emptying the tank of accumulated solids. Solids
thus collected may be discharged to a convenient treatment
system or directly to a wastewater treatment plant.
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The Imhoff Tank has not been discussed earlier because
it is similar in many respects to the septic tank. Use is gener-
ally confined to small communities and isolated installations.
Operation is a combination of sedimentation and anaerobic
digestion. This tank was invented by Karl Imhoff, who first
used them in the Essen District in Germany in 1907. The
tank is composed of two chambers, one above the other.

Surface configuration may be circular, square or rectangular.
Depth is 25′ to 35′.
Sewage flows through the upper chamber, at a low veloc-
ity (about 1 fps). Solids settle out and slide through a slot
into the bottom chamber. Detention period is about 2 hours.
Solids accumulating in the bottom, or digestion, chamber
have an initial water content of 85 to 95%. After proper diges-
tion of about 60 days the water content is reduced to about
50% and the volume is greatly reduced. Gases produced
during digestion are vented to the atmosphere by gas vents
located at the tank sides. Solids buoyed up by gas are pre-
vented from escaping to the upper tank by deflector plates.
Attempts were made to hasten digestion by heating the
lower compartment but were of limited success due to over-
turning of the tank contents.
Some rectangular tanks are arranged so that the direction
of flow can be reversed, with outlets becoming inlets and
vice versa.
In recent years manufacturers of waste treatment equip-
ment have endeavoured to supply complete treatment plants
for small communities or developments and isolated instal-
lations. Basically, these plants, called package plants, supply
primary treatment and sometimes some biological treatment
on a small scale without requiring extensive operating super-
vision. It is felt that such treatment is to be preferred to septic
tanks or only primary treatment (Imhoff Tanks, for example),
but such installations are not the ultimate solution.
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2. Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of Envi-
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Inc., New York. 1998.
3. Bewtra, J.K. and H.I. Ali, Physical and Chemical Treatment of Wastewa-
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Ed.,
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5. Camp, T.R., Trans. ASCE , 3, p. 895, 1942.
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JAMES R. PFAFFLIN
Gillette, N.J.
CAMERON MACINNIS
University of Windsor.
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© 2006 by Taylor & Francis Group, LLC
MUNICIPAL WATER REUSE: see WATER REUSE
MUNICIPAL WATER SUPPLY: see WATER TREATMENT