ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MUNICIPAL WASTEWATER potx

Stated briefly, microorganisms establish themselves on trickling filter slime or in activated sludge liquor and feed on waste material in the sewage.. Secondary treatment is an essential

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:

c) BOD (5 days 20°C)

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

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 dilureduc-tion 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 reacincuba-tion 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

y k

t⫽ ⬘ ⫺(L y)

Integration and taking of proper limits gives

y⫽L ⫺e⫺k t⬘ ⫽L ⫺ ⫺k t⬘

Phelps’ relation for decolorization of methylene blue with time

y L

t

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

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 treattreat-ment A few regulatory authorities have endeavored to classify treatment

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 convenaddi-tional 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 ⫽ ⬘ ⫺ ⬘k y1 k D2

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

t

1

0

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 requiretreat-ments 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

k

c

k t c

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)

Dc

reoxygenation

D.O D

critical deficit deoxygenation

time (1)+(2)

D.O s at.’

DO

D=O

FIGURE 3

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 Rectanrectan-gular 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

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

CI2

FIGURE 4

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

(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

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

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

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:

5) Activated carbon adsorption

6) Chemical clarification and precipitation

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

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 treatequip-ment 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

REFERENCES

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1914

2 Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of

Envi-ronmental Science and Engineering , 4 th Ed., Vol 1, Gordon and Breach,

Inc., New York 1998

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Book Co., New York, N Y., 1962

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Chem-ical Engineering, Montreal, 1981

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25 Streeter, H.W and E.B Phelps A study of the pollution and natural

purification of the Ohio river Public Health Bulletin 146, US Public

Health Service, Washington, D.C., 1925

26 UK Patent No 2,128,980

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28 US Patent No 4,500,428

JAMES R PFAFFLIN

Gillette, N.J

CAMERON MACINNIS

University of Windsor