ENVIRONMENTAL POLLUTION
CONTROL ALTERNATIVES:
MUNICIPAL WASTEWATER
U.S. EPA
TECHNOLOGY TRANSFER
EPA-G25/5-76-012
U S Environmental Protection Agency
Region 5, Library (PL-12J)
' 77 Weft Jackson Boulevard, 12th Floor
Chicago IL 60604-3590
-------
This publication was prepared for the U.S. En-
vironmental Protection Agency, Technology
Transfer, by Gordon Gulp of Culp/Wesner/Culp,
El Dorado Hills, California. EPA coordination
and review were carried out by Robert E. Crowe
and Robert S. Madancy, Technology Transfer,
Cincinnati, Ohio.
NOTICE: The mention of trade names or commercial prod-
ucts in this publication is for illustration purposes, and does
not constitute endorsement or recommendation for use by
the U.S. Environmental Protection Agency.
-------
contents
WHAT'S IN WASTEWATER AND
WHY IT'S OF CONCERN 1
WHAT GENERAL TREATMENT
APPROACHES ARE AVAILABLE? 4
PRIMARY TREATMENT 6
SECONDARY TREATMENT 8
Trickling Filters 9
Activated Sludge 13
Oxidation Ponds 20
Other Secondary Processes 21
DISINFECTION 25
ADVANCED WASTEWATER
TREATMENT 27
Phosphorus Removal 28
Filtration 30
Carbon Adsorption 33
Nitrogen Control 38
Land Treatment 46
FLOW EQUALIZATION 52
SLUDGE TREATMENT
AND DISPOSAL 54
Sludge Conditioning 55
Sludge Thickening 58
Sludge Stabilization 60
Sludge Dewatering 62
Use of Sludge
as a Soil Conditioner 67
Sludge Reduction 70
AWT Process Sludges 71
EVALUATING ALTERNATIVES 72
-------
-------
Basically, wastewater is the flow of used
water from a community. The name is apt, for
wastewater is actually 99.94 percent water by
weight. The rest, 0.06 percent, is material dis-
solved or suspended in the water. The sus-
pended matter is often referred to as "sus-
pended solids" to differentiate it from pollutants
in solution.
While "sewage" usually connotes human
wastes, the term also includes everything else
that makes its way from the home to sewers,
coming from various drains, bathtubs, sinks,
and washing machines. A generally accepted
estimate is that each individual, on a national
average, contributes approximately 100 gallons
of water per day to a city's sewage flow.
Wastewater also comes from three other
sources: commercial, industrial, and storm and
ground water. Commercial wastewaters from
office buildings and small businesses include
both human wastes and water from cleaning or
other minor processes. Industrial wastewaters,
on the other hand, may consist of large volumes
of water used in processing industrial products.
The three basic types of sewage systems
that convey wastewater or stormwater are:
Sanitary sewer system—A system that car-
ries liquid and water-carried wastes from
residences, commercial buildings, indus-
trial plants, and institutions, together with
minor quantities of ground, storm, and
surface wastes that are not admitted in-
tentionally.
Storm sewer system—A system that carries
stormwater and surface water, street
wash and other wash waters, or drainage,
but excludes domestic wastewater and
industrial wastes.
Combined sewer system—A system in-
tended to receive both wastewater and
storm or surface water.
^Seepage is an undesired source of wastewa-
ter flow encountered in separate sewer sys-
tems that are in poor repair. Seepage occurs
when ground water enters sewer pipes through
cracks or loose joints. This problem usually oc-
curs only in older systems, and the improved
engineering, materials, and installation meth-
ods used now can keep unwanted ground water
out of separate sewage systems almost entire-
ly.
The wastewater components of major con-
cern are those which will deplete the oxygen
resources of the stream or lake to which they
are discharged, those which may stimulate un-
desirable growths of plants or organisms (such
as algae) in the receiving water, or those which
will have undesirable esthetic effects or ad-
verse health effects on downstream water
uses. The pollutants of concern are made up of
both organic and inorganic materials.
The organics in wastewater are derived from
both the animal and plant kingdoms and the
activities of man, who may synthesize organic
compounds. Organic compounds are normally
composed of a combination of carbon, hydro-
gen, oxygen, and, in some cases, nitrogen.
Other important elements, such as sulfur,
phosphorus, and iron, may also be present. The
principal groups of organic substances found in
wastewater are proteins (40-60 percent), car-
bohydrates (25-50 percent), and fats and oils
(10 percent). The use of water in a municipality
may add inorganic compounds, such as sul-
fates, chlorides, phosphorus, and heavy met-
als, which are also of concern from a pollution
control standpoint. Some of the organics and
inorganics are present in the wastewater as
suspended matter (i.e., suspended solids)
while the rest are in solution. Most of the sus-
pended solids can be simply removed by allow-
ing the wastewater to stand quietly to permit the
solids to settle. The soluble organic and inor-
ganic pollutants are more difficult to remove.
Of the organics found in wastewater, a sub-
stantial portion consists of biodegradable ma-
terials—those which serve as food sources for
bacteria and other micro-organisms. These
biodegradable substances include such com-
pounds as sugars, alcohols, and many other
compounds that may find their way into sewers.
The biological breakdown of these materials
consumes oxygen. The amount of oxygen re-
quired to stabilize the biodegradable organics is
measured by the biochemical oxygen demand
-------
(BOD) test. The higher the BOD, the more oxy-
gen will be demanded from the water to break
down the organics. This parameter is the most
widely used measure of organic pollution ap-
plied to wastewaters. It is used in sjzing treat-
ment facilities and in 'predicting the effects of
treated wastewater discharges on receiving
waters. If the oxygen demand of the treated
wastewater exceeds the oxygen resources of
the receiving water, then the oxygen will be
completely depleted and the stream or lake will
become septic near the wastewater discharge
point.
Because fish and many beneficial aquatic
plants require oxygen to survive, the removal of
BOD becomes a major goal of all wastewater
treatment plants. Many years ago our popula-
tion and industry was so sparse and scattered
that we could rely on Nature's treatment in
streams and lakes to remove BOD without over-
taxing the oxygen resources of our waters.
When our population and industrial activities
increased and the construction of treatment
facilities failed to keep pace, many of our
streams and lakes suffered a noticeable loss of
fish life. For example, before recent cleanup
measures were completed, the runs of salmon
up Oregon's Willamette River came to a virtual
halt as a result of lack of oxygen in the Portland
harbor. Wastewater treatment plants have now
been placed in operation in the Willamette Ba-
sin, and these plants are so effective in remov-
ing BOD that the salmon runs have been
restored—an excellent example of the positive
results that can be achieved by the use of avail-
able treatment processes.
Some of the organics in wastewater are not
biologically degradable and, thus, are not part
of the BOD. Some of these nondegradable or-
gaoies, such as pesticides, can have adverse
long-term effects and can contribute to taste,
odor, and color problems in downstream water
supplies. The cjTeinic^[oxygen_d^maj[idJC^pD)
test is used tofrieasureThe quantities of these
materials present. The COD value also reflects
biologically degradable materials; therefore,
the COD is higher than the BOD because more
compounds can be oxidized chemically than
biologically. Some of the COD-causing mate-
rials are organics that are very resistant to
breakdown in the environment; they are of par-
ticular concern where water is used for a munic-
ipal water supply downstream.
Fortunately, there are treatment techniques
available for removing wastewater COD as well
as BOD. These techniques are discussed later
in this publication.
Wastewater contains bacteria and viruses
that can transmit diseases. This consideration
can be especially critical if the receiving water is
used for recreation near the point of wastewater
discharge. As early as 1854, it was established
that cholera was transmitted by sewage-con-
taminated drinking water. A hepatitis epidemic
in Delhi, India, in 1955, was also traced to con-
tamination of a water supply by sewage. An
amebic dysentery outbreak in Chicago in 1933
from sewage-contaminated water caused 23
deaths. Thus, another important wastewater
treatment concern is often the removal of as
many pathogenic bacteria and viruses as pos-
sible before discharge of the wastewater. Be-
cause bacteria and viruses are of minute size,
they can be enmeshed in suspended solids in
the wastewater. The suspended solids can act
as a shield to protect bacteria and viruses from
contact with added disinfecting agents, ham-
pering the disinfection process. Thus, removal
of suspended solids is important to insure good
disinfection as well as to provide removal of
-------
some of the insoluble organic and inorganic
pollutants.
Wastewater also contains two elements—
phosphorus and nitrogen—that can stimulate
undesirable growths of algae in lakes and
streams. These algal growths can cause thick,
green, scumlike mats that interfere with boating
and recreation. They also may cause unpleas-
ant tastes and odors in water supplies and
operating problems in downstream water
treatment plants, and may exert a significant
oxygen demand after the algae die. Where re-
ceiving waters are particularly sensitive to
stimulation of algae, removal of phosphorus
and nitrogen is of concern.
Heavy metaL pollutants recently have re-
ceived a great deal of emphasis as a result of
the concern over mercury discharges. Many
heavy metals (such as mercury, silver, chromi-
um, lead, zinc, and cadmium) may find their
way into municipal wastewaters from commer-
cial or industrial sources—or even from a hob-
byist's darkroom! The toxic effects of those
metals can interfere with biological waste
treatment processes. If these metals enter the
receiving water in sufficiently high concentra-
tions, they can cause fish kills and create a
problem in downstream water supplies. In
smaller quantities they may not cause im-
mediate fish kills, but can enter the aquatic food
chain where they can accumulate and cause
long-term problems.
During use of water in a municipality, the min-
eral quality of the water is altered. Inorganic
salts containing calcium, magnesium, sodium,
potassium, chlorides, sulfates, and phosphates
are among the pollutants added. These pollut-
ants are normally referred to as total dissolved
solidsjQpS). Normal water treatment practices
at downstream locations do not remove these
solids. As a result, the dissolved-solids content
increases as a supply source such as a major
river passes through several users in series.
Excessive dissolved-solids concentrations can
result in unpalatable taste and some physiolog-
ical problems. A high dissolved-solids concen-
tration can also adversely affect irrigation use,
industrial use, or stock and wildlife watering.
Calcium and magnesium contribute to down-
stream water hardness. Control of the TDS can
be of concern in arid areas where little dilution is
available and where reuse of wastewater may
be desired (as in southern California).
As noted earlier, municipal wastewater is
usually 99.94 percent water; thus the concen-
trations of the pollutants discussed are very
dilute. These concentrations are usually ex-
pressed as milligrams of pollutant per liter of
water (mg/l). One mg/l of a pollutant is equiva-
lent to 1 part of the pollutant (by weight) in 1
million parts of water—or, as expressed in
another often-used term, 1 part per million
(ppm). One mg/l or 1 ppm, to put the terms in
perspective, is equivalent to 1 minute of time in
1.9 years or 1 inch in 16 miles. These statistics
emphasize that wastewater treatment proc-
esses designed to remove a few milligrams per
liter of a pollutant are similar to sifting a hay-
stack to remove the needle. However, the bal-
ance in Nature for survival or death of fish de-
pends on the presence or absence of only 2-3
mg/l of oxygen in the stream or lake, and unde-
sirable growths of algae can be stimulated by a
few tenths of a milligram of phosphorus and
nitrogen per liter. Typical concentrations of pol-
lutants in raw, untreated, municipal wastewa-
ters are as follows: BOD = 150-250 mg/l, COD
= 300-400 mg/l, suspended solids = 150-250
mg/l, phosphorus = 5-10 mg/l, nitrogen = 15-25
mg/l, and TDS = 400-500 mg/l.
-------
;•'•• .' ••: .--- ->-'"H; V-',.;
'/•'*'"*' >^ '*:> *'" ; *"f»-rj* >ff ?'; .',-,';' ~"
The alternatives for municipal wastewater
treatment fall into three major categories:
• Primary treatment
• Secondary treatment
• Advanced wastewater treatment
The major goal of primary treatjnBDt is to
remove from wastewater those pollutants
which will either settle (such as the heavier
suspended solids) or float (such as grease).
Primary treatment will typically remove about
60 percent of the raw sewage suspended solids
and 35 percent of the BOD. Soluble pollutants
are not removed. At one time, this was the
degree of treatment used by many cities. Now
Federal law requires that municipalities provide
the higher degree of treatment provided by
secondary treatment. Although primary treat-
ment alone is no longer acceptable, it is still
frequently used as the first treatment step in a
secondary treatment system. Thus, past in-
vestments in primary treatment facilities pro-
vide useful treatment functions when treatment
is upgraded to the secondary level.
The major goal of secondary treatment is to
/"remove the soluble BOD that escapes the
f primary process and to provide added removal
V of suspended solids. These removals are typi-
cally achieved by using biological processes,
providing the same biological reactions that
would occur in the receiving water if it had
adequate capacity to assimilate the wastewater
discharges. The secondary treatment process-
es are designed to speed up these natural
processes so that the breakdown of the de-
gradable organic pollutants can be achieved in
relatively short time periods in treatment units
that relieve our streams and lakes of the purifi-
cation burden. Although secondary treatment
may remove more than 85 percent of the BOD
and suspended solids, it does not remove sig-
nificant amounts of nitrogen, phosphorus,
COD, or heavy metals, nor does it completely
remove pathogenic bacteria and viruses.
These latter pollutants may require further re-
moval where receiving waters are especially
sensitive.
-------
In cases where secondary levels of treatment
are not adequate, new treatment processes are
applied to the secondary effluent to provide ad-
vanced wasfewafer treatment, or further re-
moval of the pollutants. Some of these proc-
esses may involve chemical treatment and fil-
tration of the wastewater—much like adding a
typical water treatment plant to the tail end of a
secondary plant—or they may involve applying
the secondary effluent to the land in carefully
designed irrigation systems where the pollu-
tants are removed by a soil-crop system. Some
of these new processes can remove as much
as 99 percent of the BOD and phosphorus, all
suspended solids and bacteria, and 95 percent
of the nitrogen, and can produce a sparkling
clean, colorless, odorless effluent indistin-
guishable in appearance from a high-quality
drinking water. Although these processes and
land treatment systems are often applied to
secondary effluent for advanced treatment,
they have also been used in place of conven-
tional secondary treatment processes.
Most of the impurities removed from the
wastewater do not simply vanish, although
some organics are broken down into harmless
carbon dioxide and water. Instead, most im-
purities are removed from the wastewater as
solids, leaving a residue called "sludge." Be-
cause most of the impurities removed from the
wastewater are present in the sludge, sludge
handling and disposal must be carefully carried
out to achieve satisfactory pollution control. Un-
treated sludge still consists largely of water—as
much as 98-99 percent. Many treatment plants
use a digestion process followed by a drying
process for sludge treatment. Sludge digestion
takes place in heated tanks where the material
can decompose naturally and the odors can be
controlled. Because digested sludge contains
about 95 percent water, the next step in treat-
ment must be the removal of as much of the
water as possible. Many small plants dry their
sludge on open drying beds made up of sand
and gravel. The sludge is spread on the bed and
allowed to dry. After a week or two of drying, the
residue is removed and used as a soil con-
ditioner or landfill. In most areas, the available
land around treatment plant sites is at a pre-
mium; as a result, other methods of sludge
treatment are finding increased use. In some
cases, the sludge is dewatered by mechanical
devices and then burned in incinerators. These
incinerators are carefully designed and
equipped with air pollution control equipment so
that the sludge-handling process does not add
to the pollution of the atmosphere. In other
cases, the sludge may be used as a soil con-
ditioner. The city of Milwaukee, Wisconsin, has
dewatered and dried its sewage sludges for
years. The dried material is bagged and sold
under the name "Milorganite" as a soil con-
ditioner. Sludge is also used in a semiliquid form
by cities such as Chicago, Illinois, for reclaiming
large land areas. Chicago's project will eventu-
ally restore 10,000 acres of unproductive
strip-mined land for use as a productive agricul-
tural area.
The rest of this publication will describe the
alternatives for wastewater treatment and
sludge handling in detail.
-------
primary
treatment
/Primary treatment removes from the waste-
f water those pollutants which will either settle
\put or float. As wastewater enters a plant for
primary treatment, it flows through a screen.
The screen removes large floating objects,
such as rags and sticks, that may clog pumps
and small pipes. The screens typically are
made of parallel steel or iron bars with openings
of about half an inch.
Screens are generally placed in a chamber or
channel in an inclined position to the flow of the
sewage to making cleaning easier. The debris
caught on the upstream surface of the screen
can be raked off manually or mechanically. The
debris removed from the screen is usually
buried in a landfill.
Some plants use a device known as a com-
minutor, which combines the functions of a
screen and a grinder. This device catches and
then cuts or shreds the heavy solid material.
The pulverized matter remains in the wastewa-
ter flow to be removed later in a settling tank.
After the wastewater has been screened, it
passes into a grit chamber, where sand, grit,
cinders, and small stones are allowed to settle
to the bottom. A grit chamber is highly important
for cities with combined sewer systems, be-
cause it will remove the grit or gravel that
washes off streets or land during a storm and
ends up at treatment plants.
The grit or gravel removed by the grit cham-
ber is usually taken from the tank, washed so
that it is clean, and disposed of by landfilling
near the treatment plant.
With the screening completed and the grit
removed, the wastewater still contains sus-
pended solids, some of which can be removed
from the sewage by treatment in a sedimenta-
tion tank. These tanks may be round or rectan-
gular, are usually 10-12 feet deep, and hold the
-------
wastewater for periods of 2-3 hours. Wastewa-
ter flows very slowly through them, so that the
suspended solids gradually sink to the bottom.
This mass of settled solids is called raw primary
sludge. The sludge is removed from the sedi-
mentation tank by mechanical scrapers and
pumps. Floating materials, such as grease and
oil, rise to the surface of the sedimentation tank,
where they are collected by a surface-skimming
system and removed from the tank for further
processing, usually in a sludge digester.
-------
secondary
treatment
/ The major purpose of secondary treatment is
[ to remove the soluble BOD that escapes pri-
1 mary treatment and to provide further removal
\pf suspended solids. A minimum of secondary
treatment is now required for municipalities. In
/'most cases, secondary processes are biologi-
Lcal in nature, designed to provide the proper
environment for the biological breakdown of
soluble organic materials. A great variety of
biological micro-organisms come into play—
bacteria, protozoa, rotifers, fungi, algae, and so
forth. All biological processes depend on bring-
ing these organisms into contact with the im-
purities in the wastewater so that they can use
these impurities as food. The organisms con-
vert the biodegradable organics into carbon
dioxide, water, and—just as when a person
consumes food—more cell material. This bio-
logical breakdown of organic material requires
oxygen. The basic ingredients needed for sec-
ondary biologic treatment are the availability of
many micro-organisms, good contact between
these organisms and the organic material, the
availability of oxygen, and the maintenance of
other favorable environmental conditions (for
example, favorable temperature and sufficient
time for the organisms to work). A variety of
approaches have been used in the past to meet
these basic needs. The most common ap-
proaches are called
8
-------
• Trickling filters
• Activated sludge
• Oxidation ponds (or lagoons)
In addition, some relatively new approaches to
secondary treatment, which do not fall in any of
the above categories, will be discussed. As
noted earlier, secondary levels of treatment can
also be achieved by nonbiological, physical-
chemical processes or by land treatment sys-
tems, which are discussed in later sections.
trickling
filters
A trickling filter consists of a bed of coarse
' material, such as stones, slats, or plastic mate-
rials, over which wastewater is applied in
drops, films, or spray from moving distributors
or fixed nozzles, and through which it trickles to
underdrains.
As the wastewater trickles through the bed,
microbial growth occurs on the surface of the
stone or packing in a "fixed film." The wastewa-
ter passes over the stationary microbial popula-
tion to provide the needed contact between the
micro-organisms and the organics. Trickling fil-
ters have long been a popular biologic treat-
ment process. The most widely used design for
many years was simply a bed of stones from 3
to 10 feet deep through which the wastewater
passed. The wastewater is typically distributed
over the surface of the rocks by a rotating arm.
-------
ti STi\'T\ /'A / \ r A / v / \ / ** /
^O ,
¥ ••
ocug.'c
S%!k;4; UnderdVain system S^^rls
::;£S;?>jfe-5 .w.f*.i(Jvs A*.l&:.v*-»ift--i> £fe>#S
Bacteria gather and multiply on these stones
until they consume most of the organic matter in
the sewage. The cleaner water trickles out
through pipes in the bottom of the filter. Rock
filter diameters of up to 200 feet are used. Trick-
ling filters are not primarily a filtering or straining
process as the name implies (the rocks in a rock
filter are 1-4 inches in diameter, too large to
strain out solids), but are a means of providing
large amounts of surface area where the mi-
cro-organisms cling and grow in a slime on the
rocks as they feed on the organic matter. Ex-
cess growths of micro-organisms wash from
the rock media and would cause undesirably
high levels of suspended solids in the plant
effluent if not removed. Thus, the flow from the
filter is passed through a sedimentation basin to
allow these solids to settle out. This sedimenta-
tion basin is referred to as a "secondary
clarifier" or a "final clarifier" to differentiate it
from the sedimentation basin used for primary
settling. To prevent the biological slimes from
drying out and dying during nighttime periods
when wastewater flows are too low to keep the
filter wet continuously, filter effluent is often re-
cycled to the trickling filter. Recirculation re-
duces odor potential and improves filter effi-
ciency as it provides another opportunity for the
microbes to attack any organics that escaped
the first pass through the filter. Another ap-
proach to improving trickling-filter performance
or handling strong wastewaters is the use of two
filters in series, referred to as a "two-stage"
trickling-filter system.
TYPICAL ONE-and
TWO-STAGE
TRICKLING-FILTER
SYSTEMS
influent >
Recycle
Primary
clarifier
Filter
Clarifier
Effluent
Recycle
Recycle
Influent-
Primary
ciarifier
\
First-stage
filter
\
r
Second-stage
filter
Clarifier
• Effluent
10
-------
Although rock trickling filters have performed
well for years, certain limitations have become
apparent. Under high organic loadings, the
slime growths can be so prolific as to plug the
void spaces between the rocks, causing flood-
ing and failure of the system. Also, the volume
of void spaces is limited in a rock filter, which
restricts the circulation of air in the filter and the
amount of oxygen available for the microbes.
This limitation, in turn, restricts the amount of
wastewater that can be processed. To over-
come these limitations, other materials for filling
the trickling filter have recently become popular.
These materials include modules of corrugated
plastic sheets, redwood slats, and plastic rings.
These media offer larger surface areas for
slime growths (typically 27 square feet of sur-
face area per cubic foot as compared to 12-18
square feet per cubic foot for 3-inch rocks) and
greatly increase void ratios for increased air
flow. The materials are also much lighter than
rock (by a factor of about 30), so that the trick-
ling filters can be much taller without facing
structural problems. While rock in filters is usu-
ally not more than 10 feet deep, synthetic media
depths are often 20 feet or more, reducing the
overall space requirements for the trickling-f ilter
portion of the treatment plant.
Corrugated pfasttc media
Wastewater
Siotogteal growfii
M Treated wastewater
-------
A typical overall efficiency of a trickling-filter
treatment plant is about 85-percent removal of
BOD and suspended solids for municipal
wastewaters, which corresponds to about 30
mg/l of suspended solids and BOD in the final
effluent. As do all the available processes, the
trickling-filter process has advantages and dis-
advantages.
Advantages. The basic simplicity of the
process is a major advantage. The incoming
load of pollutants can vary over a wide range
during the day without causing operating prob-
lems, minimizing the need for operator skills.
The mechanical equipment is simple, making
plant maintenance an easy task. Energy re-
quirements for the process are low.
Disadvantages. The process efficiency is af-
fected markedly by air temperature because of
the large, fixed surface area of the microbes
exposed to the air within the filter. Treatment
efficiency falls off in the winter and improves in
the summer. The actual contact time between
the organics and the microbes is limited and is
shorter than that achieved in the activated-
sludge process. As a result, some soluble BOD
that would be removed by the activated-sludge
process escapes a trickling-filter plant. Thus,
the overall efficiency is less than that of a well-
operated activated-sludge process. Coupled
with increasing rigid treatment requirements,
this disadvantage has favored, and has led to a
trend in new plant construction toward, the
activated-sludge process.
Energy Requirements. The process has a
low power consumption—about 150 kWh per
million gallons treated for the wastewater
treatment portion of the plant. The power con-
sumption is much less than that required for the
activated-sludge process. Trickling filters con-
sume no resources other than power.
Space Requirements. The precise space re-
quirements for a plant will depend on the design
criteria selected by the consultant as best
suited for the particular wastewater, the extent
of other facilities (such as laboratories, ware-
housing, shops, etc.), the method of sludge
handling used, and the layout best suited for the
specific site. Typically, a complete rock trickling
plant will occupy about 1 acre per million gal-
lons per day (mgd) of capacity. Taller filters
packed with synthetic media can reduce the
total space requirements by a factor of about 2.
Costs. The costs of wastewater treatment are
typically expressed in terms of costs per volume
of wastewater treated, often as cents per 1,000
gallons. Costs are composed of both the con-
struction (capital) costs and the daily costs to
operate and maintain the facility. Capital costs
are expressed as the annual costs, including
interest, to amortize the total investment in the
treatment facility. The costs are typically amor-
tized over a 20-year period. By adding together
the annual capital costs and the operation and
maintenance costs, a total annual cost is ob-
tained. The cost per 1,000 gallons is then de-
termined by dividing the total annual cost by the
total wastewater volume treated during the
year. Because the costs per 1,000 gallons will
vary with the portion of the available capacity
actually used, comparisons are usually made
based on the costs experienced when the facil-
ity operates at its full design capacity. Costs
also vary with plant size; economies of scale
are realized in larger plants. Cost estimates in
this publication will be based on the 1-10-mgd
capacity range, which encompasses most mu-
nicipal plants (10,000-100,000 population
served). Costs per 1,000 gallons will be higher
in smaller plants outside this range and lower in
larger plants. Based on early 1975 price levels,
the costs of trickling-filter treatment (not includ-
ing sludge-handling costs) range from 40-50
cents per 1,000 gallons at 1 mgd to 15-20 cents
per 1,000 gallons at 10 mgd (based on amortiza-
tion of capital costs over 20 years at 7 percent
interest). At a sewage flow of about 350 gallons
per day from an average residence, these
treatment costs are equivalent to $4.20-$5.25
per month per home at'1 mgd and $1.60-$2.10
per month at 10 mgd.
12
-------
activated
sludge
The activated-sludge process is a biological
wastewater treatment technique in which a
mixture of wastewater and biological sludge
(micro-organisms) is agitated ar^d aerated.
The biological solids are subsequently sepa-
rated from the treated wastewater and re-
turned to the aeration process as needed.
The activated-sludge process derives its
name from the biological mass formed when air
is continuously injected into the wastewater.
Under such conditions, micro-organisms are
mixed thoroughly with the organics under con-
ditions that stimulate their growth through use
of the organics as food. As the micro-organisms
grow and are mixed by the agitation of the air,
Sludge return
Waste sludge
CONVENTIONAL ACTIVATED SLUDGE
-------
the individual organisms clump together (floc-
culate) to form an active mass of microbes
called "activated sludoe.." In practice, the
wastewater flows continuously into an aeration
tank where air is injected to mix the activated
sludge with the wastewater and to supply the
oxygen needed for the microbes to break down
the organics. The mixture of activated sludge
and wastewater in the aeration tank is called
"mixed liquor." The mixed liquor flows from the
aeration tank to a secondary clarifier where the
activated sludge is settled out. Most of the set-
tled sludge is returned to the aeration tank to
maintain a high population of microbes to per-
mit rapid breakdown of the organics. Because
more activated sludge is produced than can be
used in the process, some of the return sludge
is diverted or "wasted" to the sludge-handling
system for treatment and disposal. In conven-
tional activated-sludge systems, the wastewa-
ter is typically aerated for 6-8 hours in long,
14
rectangular aeration basins with about 1 cubic
foot of air injected uniformly along the length of
the aeration basin for each gallon of wastewater
treated. Air is introduced either by injecting it
into diffusers near the bottom of the aeration
tank or by mechanical mixers located at the
surface of the aeration tank. The volume of
sludge returned to the aeration basin is typically
20-30 percent of the wastewater flow. There are
many variations of this conventional system
that have evolved over the years and that have
improved the process performance, as de-
scribed in the following paragraphs.
Early in the use of the conventional process,
it was found that the demand for oxygen in the
aeration tank was much greater at the inlet end
of the aeration basin, where the stronger incom-
ing wastewater entered, than at the outlet end,
where most of the oxygen-demanding mate-
rials had been stabilized. This discovery led to
the tapered aeration process, where a greater
-------
-------
Waste sludge
STB> AERATION
portion of the air was injected at the inlet end
than at the outlet end of the aeration basin. The
quantity of air used was the same, but its dis-
tribution was tapered along the aeration tank.
Another variation evolved in which the waste-
water flow was introduced at several points
rather than all at once. Although it is actually a
step feeding of wastewater, the process is
known as step aeration. Multiple feed points
spread the oxygen demand over more of the
aeration basin, which results in more efficient
use of the oxygen. Existing conventional plants
are often modified to the step aeration process
to increase their capacity. To extend even
further the benefits achieved with step aeration,
the cefljptetekju/x activated-sludge concept
may be used. In this system, the influent
wastewater is dispersed as uniformly as possi-
ble along the entire length of the aeration basin,
so that the oxygen demand is uniform from one
end to the other.
Another variation of activated sludge is the
contact stabilization process. In this approach,
the incoming wastewater is mixed briefly (20-30
minutes) with the activated sludge—just long
enough for the microbes to absorb the organic
pollutants from solution but not long enough for
them to actually break down the organics. The
activated sludge is then settled out and re-
turned to another aerated basin (stabilization
tank), in which it is aerated for 2-3 hours to
permit the microbes to break down the ab-
sorbed organics. Because the settled volume of
the activated sludge being aerated is much
smaller than the total wastewater flow, the total
size of the plant is reduced.
-------
Many small activated-sludge plants, often
sold as prefabricated steel package plants, use
the extended aeration form of activated sludge.
The p~roce§5 flow diagram is essentially the
same as in the complete mix system, except
that these small plants typically have no primary
treatment and aerate the raw wastewater for a
24-hour period rather than the 6-8 hours used in
conventional plants. The long aeration period
allows the activated sludge formed to be par-
tially digested within the aeration tank so that it
can be dewatered and disposed of without the
need for large sludge digestion capacity.
-------
tion has resulted in rapid acceptance by
consulting engineers, municipalities, and indus-
tries. The first full-scale application of this proc-
ess to the treatment of municipal wastewater
occurred in 1969 under a demonstration con-
tract from the U.S. Environmental Protection
Agency (EPA). In this demonstration project, a
total of 1.25 mgd of sewage was treated. Now,
there are many full-scale municipal wastewater
treatment plants that will use oxygen aeration in
various stages of design and construction. The
total amount of sewage to be treated by these
plants will soon be measured in billion gallons
per day. To provide efficient use of the oxygen,
the aeration tanks are often covered and the
oxygen is recirculated through several stages.
When the tanks are covered, high-purity oxy-
gen (over 90 percent) enters the first stage of
the system and flows through the oxygenation
basin concurrently with the wastewater under
treatment. Pressure under the tank covers is
essentially atmospheric and sufficient to main-
tain control and prevent backmixing from stage
to stage. This system allows for efficient oxygen
use at low power requirements. Mixing within
each stage can be accomplished either with
surface aerators or with a submerged rotating-
sparge system. As an alternative to the use of
covered basins, specially designed oxygen dif-
fusers can be used in open basins.
Sludge-concentrating hopper
Influent
Effluent
OXIDATION DITCH
A variation of the conventional process,
called the oxidation ditch, was developed in the
Netherlands~and has round use in the United
States. A surface-type aerator is used that pro-
vides aeration and circulates the wastewater
through the ditch.
Since 1970, there has been a great deal of
interest in systems using pure oxygen as a
substitute for air. The potential of oxygejiaera-:
18
Aeration
Control tank cover
SCHEMATIC DIAGRAM OF MULTISTAGE
OXYGEN AERATION SYSTEM
Aoitator
Recycle
sludge
-------
The number of stages and the type of mixing
device selected are variables that depend on
waste characteristics, plant size, land availabil-
ity, treatment requirements, and other similar
considerations. Pure oxygen allows the use of
much smaller aeration tanks (1.5-2 hours' aera-
tion rather than 6-8 hours), typically produces a
better settling activated sludge than conven-
tional air systems, and produces a sludge that is
easier to dewater. The oxygen used in the proc-
ess is typically generated onsite. The potential
advantages of the process have led many
localities to adopt this approach, including De-
troit, Michigan (900 mgd); New Orleans,
Louisiana (122 mgd); Middlesex County, New
Jersey (120 mgd); Louisville, Kentucky (105
mgd); Denver, Colorado (72 mgd); Montgomery
County, Maryland (60 mgd); Miami, Florida (55
mgd); Euclid, Ohio (22 mgd); New York City (20
mgd); Salem, Oregon (16 mgd); Deer Park,
Texas (6 mgd); and Tahoe Truckee Sanitation
Agency, California (5 mgd).
By now it is apparent that there are many
variations of the activated-sludge process,
each of which has advantages and disadvan-
tages relative to the others. The general proc-
ess of activated-sludge treatment, however,
does have some identifiable advantages and
disadvantages.
Advantages. The process is versatile be-
cause the design can be tailored to handle a
wide variety of raw wastewater compositions
and to meet a variety of effluent standards. The
process is capable of producing a higher quality
effluent than the trickling-filter process. A prop-
erly designed and operated activated-sludge
plant removes essentially all soluble BOD. The
secondary effluent BOD is made up primarily of
the oxygen demand exerted by the suspended
solids in the effluent. Typical effluent quality is
20-25 mg/l BOD and 20-25 mg/l suspended
solids, although, with careful operation, the
process has produced less than 10 mg/l BOD
and suspended solids at some plants. The
process is usually lower in capital costs than a
trickling-filter plant and requires less area.
Disadvantages. The process requires care-
ful operational control—more than that required
by a trickling filter. Energy requirements are
also higher.
Energy Requirements. The power require-
ments are relatively high—typically 625 kWh
per million gallons treated.
Space Requirements. Typically, a conven-
tional activated-sludge plant occupies about
0.5 acre per mgd of capacity. The pure oxygen
system significantly reduces space require-
ments.
Cosfs. Based on 1975 prices, the overall
activated-sludge process costs (exclusive of
sludge disposal costs) range from 45-55 cents
per 1,000 gallons at 1 mgd to 20-25 cents for
1,000 gallons at 10 mgd. These costs are
equivalent to $4.75-$5.80 per month per home
at 1 mgd and $2.10-$2.60 per month per home
at 10 mgd.
19
-------
oxidation
ponds
Oxidation ponds (also called "lagoons" or
"stabilization ponds") are large shallow ponds
designed to treat wastewater through the in-
teraction of sunlight, wind, algae, and oxygen.
They are one of the most commonly employed
secondary systems and account for about one-
third of all secondary plants in the United
States. About 90 percent of the ponds are used
in towns with less than 10,000 people (1-mgd
capacity). Primary treatment is sometimes
used as pretreatment, but the added cost is
usually not justified. Typically, raw wastewater
enters the pond at a single point in the middle of
the pond or at one edge. Ponds are usually 2-4
feet deep—at least deep enough to prevent
weed growths but not deep enough to prevent
mixing by wind currents. Shallow ponds are
usually aerobic—that is, oxygen is present-
through nearly all of their depth, with the only
portion devoid of oxygen (anaerobic) being the
sludge layer on the bottom of the pond. Some
ponds have been designed (and have worked
well) with depths of 10-20 feet, where the
anaerobic bottom zone becomes a greater por-
tion of the overall system. The pond may have
sufficient volume to accommodate from 15 to 60
days of wastewater flow, and it may be a fill-
and-draw or continuous flow-through opera-
tion. Algae grow by taking energy from the sun-
light and consuming the carbon dioxide and
inorganic compounds released by the action of
the bacteria in the pond. The algae, in turn,
release oxygen needed by the bacteria to sup-
plement the oxygen introduced into the pond by
wind action. The most critical factor is to insure
that enough oxygen will be present in the pond
to maintain aerobic conditions; if oxygen is in-
sufficient, odor problems will occur. The sludge
deposits from the pond eventually must be re-
moved by dredging.
Ponds are sometimes designed with several
cells in parallel to distribute the wastewater bet-
ter and avoid localized zones of high oxygen
demand caused by uneven deposits of sludges.
Several smaller parallel cells also reduce the
problems that can be encountered with wave
action in large ponds. Ponds are sometimes
placed in series for strong wastes or to permit
use of the last pond in a series as a polishing
step to provide higher removals of suspended
solids. Pond effluent is sometimes recirculated
to improve mixing in the pond.
To eliminate the dependence on algal-
produced oxygen and to reduce the area re-
quired by the ponds, aeration equipment is
sometimes installed in the pond to supply oxy-
gen. Such a system is called an aeratedja-
goon. Air can be supplied by a compressor that
"injects air into the pond through tubing installed
on the pond bottom or by mechanical aerators
installed at the surface of the pond. Aerated
ponds are typically about one-fifth the size of a
conventional oxidation pond and are actually a
form of the activated-sludge process. Aerated
lagoons are usually followed by a quiescent,
second-stage pond to remove the suspended
solids from the aerated-lagoon effluent.
Oxidation ponds usually meet secondary
treatment requirements for removal of BOD, but
frequently fail to meet secondary requirements
for suspended solids removal because of the
presence of algae in the pond effluent. Much
work is currently underway on various methods
of removing these algae; the most promising
alternatives to date are filtration through sand
beds at low rates, filtration through a bed of
rocks that may be a part of the dike system, and
a combination of chemical treatment of the
pond effluent and settling. These polishing
techniques may produce a degree of treatment
that exceeds the requirements for secondary
removals of both BOD and suspended solids.
Some municipalities have already decided to
use polishing systems on their existing lagoons
to meet new treatment standards rather than
abandon the ponds in favor of an all-new treat-
ment system.
AoVanfages. Oxidation ponds are easy to
construct, operate, and maintain. They are low
in construction costs and there is no mechani-
cal equipment to maintain. Because of their
long detention time, they are effective in remov-
ing disease-causing organisms.
Disadvantages. The relatively large space
requirements for conventional ponds are a dis-
advantage in many areas and for large cities.
Because the ponds are simple to operate, some
towns have virtually ignored them after installa-
tion, resulting in weed growths on the dikes and
even dike failures, in some cases, caused by
animals burrowing into the dikes. The frequent
need for removal of algae from the effluent to
meet secondary treatment requirements fully is
a disadvantage. Many systems for removal of
these algae introduce more complex operating
and maintenance requirements and higher
costs.
Energy Requirements. Oxidation ponds do
not consume power unless artificially aerated.
Completely mixed lagoons can use more
energy than the activated-sludge process.
Space Requirements. The actual require-
ments depend on the climate, but typically
20
-------
v^£-
&j&t?*1t*'^,':'\'s.vl
W%y?- «•;<,*> >**>, .&*&
* i ^f"' _n f _ «t*^ A
«T«A»f * CARBON W0XHJE * AMMONIA
range from 35 acres per mgd of capacity for
nonaerated ponds in warm climates to 85 acres
per mgd in cold climates using conventional
4-foot-deep lagoons.
Cosfs. The construction costs for a pond
range from about $2,000 per acre for ponds
greater than 25 acres to $6,000-$8,000 per
acre for ponds of 4 acres or less (excluding land
costs). Operation and maintenance costs are
usually about 20-25 percent of those for
trickling-filter or activated-sludge plants. Total
treatment costs for a 1-mgd plant will typically
be less than 20 cents per 1,000 gallons ($2.10
per month per home)—much less than
activated-sludge or trickling-filter treatment at
the same capacity. To this cost, however, the
cost of removing algae from the pond effluent
must often be added to meet secondary stand-
ards fully. These added costs may be as high
as 10 cents per 1,000 gallons (5-7 cents has
been estimated for sand filters without the use
of chemicals).
other secondary
processes
There are two recently developed processes
that do not fit precisely into the activated-sludge
or trickling-filter categories, but do capitalize on
some of the best features of both. These proc-
esses are
• Rotating biological contactors
• Activated biofilter
Rotating Biological Contactors. This process
ing biological surfaces) consists of a series of
closely spaced discs (10-12 feet in diameter)
21
------- OCR error (C:\Conversion\JobRoot\0000030W\tiff\20008S4V.tif): Saving image to "C:\Conversion\JobRoot\0000030W\tiff\20008S4V.T$F.T$F" failed.
-------
mounted on a horizontal shaft and rotated while
about one-hatf of their surface area is immersed
in wastewater. The process has been used in
Europe for several years. The discs are typi-
cally constructed of lightweight plastic. When
the process is placed in operation, the microbes
in the wastewater begin to adhere to the rotat-
ing surfaces and grow there until the entire sur-
face area of the discs is covered with a 1/16-
1/8-inch layer of biological slimes. As the discs
rotate, they carry a film of wastewater into the
air, where it trickles down the surface of the
discs, absorbing oxygen. As the discs complete
their rotation, this film mixes with the reservoir
of wastewater, adding to the oxygen in the res-
ervoir and mixing the treated and partially
treated wastewater. As the attached microbes
pass through the reservoir, they absorb other
organics for breakdown. The excess growth of
microbes is sheared from the discs as they
move through the reservoir. These dislodged
organisms are kept in suspension by the mov-
ing discs. Thus, the discs serve several pur-
poses. They provide media for the buildup of
attached microbial growth, bring the growth into
contact with the wastewater, and aerate the
wastewater and suspended microbial growth in
the wastewater reservoir. The speed of rotation
is adjustable. The attached growths are similar
in concept to a trickling filter, except that the
microbes are passed through the wastewater
rather than the wastewater being passed over
the microbes. Some of the advantages of both
the trickling-filter and activated-sludge proc-
esses are realized. As the treated wastewater
flows from the reservoir below the discs, it car-
ries the suspended growths out to a
downstream settling basin for removal. The
process can achieve secondary effluent quality
or better. By placing several sets of discs in
f^'MfitrP^>:-"'lft^ *?>'•£'-' -
*
series, it is possible to achieve even higher
degrees of treatment—including biological
conversion of ammonia to nitrates if desired.
The process is being used or planned for use at
some 50 U.S. installations, including those at
Battleground, Washington; Boynton Beach,
Florida; Cadillac, Michigan; Hopkinton, Iowa;
Omaha, Nebraska; Selden, Long Island, New
York; Edgewater, New Jersey; and Whitewater,
Wisconsin.
Advantages. There are no sludge or effluent
recycle streams. The mechanical equipment is
low speed, easing maintenance. Higher de-
grees of treatment are obtained than in a trick-
ling filter. The bulk (95 percent) of the microbes
is attached to the discs, making them less sus-
ceptible to washout and upset than in an
activated-sludge plant. The process requires
fewer process decisions by the operator than
does activated sludge. Because of the low hy-
draulic headless through the process, rotating
biological contactors frequently can be added
to an existing plant to improve performance
without the need to add pumping facilities.
Disadvantages. The disc process must be
covered for protection against freezing, precipi-
tation, wind, and vandalism. Efficiency is ad-
versely affected by cold temperatures unless
the treatment building is heated. There is not
yet any long-term operating experience with the
process in the United States.
Energy Requirements. The power require-
ments are about 400 kWh/mg.
Space Requirements. The overall plant
space requirements are about 0.5 acre per mgd
of capacity.
Coste. Savings in power costs are such that
the overall treatment costs are projected to be
somewhat lower than activated-sludge costs.
Lack of full-scale U.S. experience makes cost
generalization difficult.
^Activated Biofilters. This process combines
an attache?growth system with recirculation of
activated sludge over and through the media. In
23
-------
addition to recirculating effluent as typically
done in a trickling filter, the process also recircu-
lates settled sludge from the secondary clarifier.
The trickling-filter media used in this system is
made up of redwood slats. Through sludge re-
circulation, it is possible to build up a level of
suspended microbes comparable to that in an
activated-sludge system in addition to the popu-
lation of microbes attached to the redwood
media. Oxygen is supplied by the splashing of
the wastewater between layers of the redwood
slats and by the movement of the wastewater in
a film across the microbial layer attached to the
slats. The typical depth of the redwood media is
14 feet.
An aeration tank is often installed between
the filter and the secondary clarifier to provide
high degrees of treatment. With about 1 hour of
supplemental aeration, the process will pro-
duce an effluent with less than 20 mg/l BOD and
suspended solids. When supplemental aera-
tion is used, the redwood filter size can be re-
duced somewhat, and overall costs may actu-
ally be reduced because of lower waste sludge
quantities. The process is in use at Madera,
California (10 mgd); Idaho Falls, Idaho (17
mgd); Freemont (10.5 mgd) and Burwell (0.5
mgd), Nebraska; Henderson (15 mgd) and
Owensboro (8 mgd), Kentucky; Longmont,
Colorado (2.5 mgd); Mt. Vernon, Washington (4
mgd); Kalispell (3 mgd) and Helena (6.2 mgd),
Montana; and Forest Grove, Oregon (21 mgd).
Advantages. The combination of fixed mi-
crobial growth and high concentration of sus-
pended growths provides stable operation and
minimizes system upsets. The process can be
added ahead of existing activated-sludge ba-
sins to increase plant capacity or efficiency. The
process requires less area than a trickling-filter
plant and is less sensitive to cold temperature
effects.
Disadvantages. The supplemental aeration
process discussed earlier is often needed to
meet secondary treatment standards. Although
finding increased use, the process is relatively
new and there is no long-term experience to
draw from.
Energy Requirements. Power requirements,
with supplemental aeration, are 10-15 percent
less than for activated sludge.
Space Requirements. The overall space re-
quirements are comparable to an activated-
sludge plant—about 0.5 acre per mgd.
24
-------
disinfection
Disinfection is the killing of pathogenic (dis-
ease-causing) bacteria and viruses found in
wastewaters. This process differs from sterili-
zation, which is the killing of all living organisms.
The last treatment step in a secondary_pjant is
the addition of a disinfectant to thetreated
wastewater. The addition of chlorine^ aas or
some other form of chlorine, whichTs called
chlorination, is the process most commonly
used tor Wastewater disinfection in the United
States. The chlorine is injected into the waste-
water by automated feeding systems. The
wastewater then flows into a basin, where it is
held for about 30 minutes to allow the chlorine
to react with the pathogens. Chlorine is used
primarily in two forms: as a gas, or as a solid or
liquid chlorine-containing hypochlorite com-
pound. Gaseous chlorine is generally consid-
ered the least costly form of chlorine that can be
used in large facilities, but it can cause safety
hazards if not handled properly. Hypochlorite
forms have been used primarily in small sys-
tems (less than 5,000 persons), or in large sys-
tems, where safety concerns related
to handling chlorine gas outweigh economic
concerns. Although there is concern about the
formation of some byproducts resulting from
chlorination, the use of chlorine has proven to
be a very effective means of disinfecting waste-
waters and water supplies. To insure a con-
stant supply of chlorine and to avoid problems
of transporting chlorine through surrounding
residential areas, some municipalities have
elected to build facilities at their wastewater
treatment plants to generate their own chlorine
or hypochlorite from salt (sodium chloride).
-------
An alternative to chlorine is ozone, which is
widely used in Europe for disinfection of water
supplies. Ozone is produced at its point of use
by passing dry air between two high-potential
electrodes to convert oxygen into ozone. Re-
cent improvements in the technology of ozone
production have bettered the reliability and
economy of its generation. The advantages of
using ozone are that is has high germicidal
effectiveness, which is the greatest of all known
substances, and that the only residual material
left in the wastewater is more dissolved oxygen.
The electrical generation of ozone is an ener-
gy-intensive operation. Because ozone must
be produced electrically as it is needed and
cannot be stored, it is difficult to adjust treat-
ment to variations in ozone demand. The ozo-
nation process is included in the design of a
4-mgd wastewater treatment plant in Mahoning
County, Ohio, and several smaller plants using
ozone are under design or construction. How-
ever, there is not yet any significant full-scale
wastewater experience with the process in the
United States. The cost of ozone is typically
higher than the cost of the chlorine required to
accomplish the same degree of disinfection.
However, the cost of the disinfection process is
typically 1 cent per 1,000 gallons or less—so
insignificant a portion of overall treatment costs
that the minor difference in the cost of ozonation
and chlorination is not an overriding factor in
selection between these disinfection alterna-
tives.
-------
advanced
wastewater
treatment
Although secondary treatment processes,
when coupled with disinfection, may remove
over 85 percent of the BOD and suspended
solids and nearly all pathogens, only minor re-
movals of some pollutants—such as nitrogen,
phosphorus, soluble COD, and heavy met-
als—are achieved. In some circumstances, the
pollutants contained in a secondary effluent are
of major concern. In these cases, processes
capable of removing pollutants not adequately
removed by secondary treatment are used in
what is called "tertiary wastewater treatment"
(these processes have often Been called ad-
vanced wastewater treatment, or AWT for
short). The following sections describe avail-
able AWT processes. In addition to solving
tough pollution problems, these processes im-
prove the effluent quality to the point that it is
adequate for many reuse purposes and may
convert what was originally a wastewater into a
valuable resource too good to throw away.
27
-------
phosphorus
removal
Phosphorus has been identified as one of the
key factors in the disruption of the ecological
balance of our waters. To meet water quality
standards, many cities will be required to re-
duce phosphorus to low concentrations in
wastewater discharges. Excess phosphorus
enters our lakes and streams and stimulates
the growth of algae and other aquatic life forms
and causes them to grow in great profusion.
This overabundance of algae in our lakes and
streams causes objectionable odors and even-
tually results in depletion of the water—thus
killing off, or limiting, fish population. In conven-
tional wastewater treatment facilities, phos-
phorus is not removed to any appreciable
extent. Available processes now allow for effec-
tive removal of phosphorus by relatively minor
modifications to existing municipal wastewater
treatment facilities.
Coagulant Polymer (optional)
.1. D .1
Rapid
mix
In these processes, chemicals called "coagu-
lants"—such as aluminum sulfate (alum), lime,
or ferric chloride—are added. These coagu-
lants cause the solids in the wastewater to
coagulate and clump together so as to settle out
faster. The clumping together of solids is accel-
erated by slowly stirring (flocculating) the
wastewater after the coagulants have been add-
ed. After flocculation, the wastewater enters a
settling basin where the solids are settled out.
Thecoagulation-flocculation process increases
the rate at which the suspended solids settle. If
the proper amount of coagulant is added, the
^BBBBWP^R^rl
..• ' ff^ipj jfWf-iv-i. . -,••'' J
To disposal
coagulant reacts with the phosphorus in the
wastewater to convert it to an insoluble form
that can also be removed by settling, removing
90 percent of the phosphorus and suspended
solids normally present in a secondary effluent.
The land treatment process discussed later is
another available means of phosphorus re-
moval.
The coagulant does not necessarily have to
be added in a process downstream of the sec-
ondary process. Some plants add the coagu-
lant to the raw wastewater as it enters the plant
and remove the resulting solids in the primary
clarifier; others add the coagulant to the aera-
tion tank of an activated-sludge plant, where it is
mixed by the aeration process and the resulting
floe is removed in the secondary clarifier; and
others add the coagulant downstream.
In still another variation, the activated-sludge
process is operated so as to take up as much
phosphorus as possible in the activated-sludge
particles. The phosphorus is then stripped in a
digestion process from the activated sludge
after it has settled and while it is being returned
to the aeration tank, and the coagulant is
applied to the highly concentrated phosphorus
-------
stream from the stripping operation. Adding
coagulant to the raw wastewater has the advan-
tage over tertiary addition of also removing
some of the BOD from the secondary process,
reducing the size of the secondary biological
treatment units needed. Adding coagulant
either to the raw wastewater or to the aeration
basin allows removal of the chemical floe with-
out the need for a separate tertiary settling ba-
sin. However, adding coagulant downstream of
the secondary process with provision of a ter-
tiary settling basin offers greater removal of
suspended solids and improves overall system
reliability by providing a means for removal of
any solids that escape the secondary clarifier.
If lime is used as the coagulant, it causes an
increase in the pH of the wastewater. "pH" is a
measure of the acidity or alkalinity of the
wastewater. A pH value of 7 is used to describe
a perfectly neutral (neither alkaline nor acid)
wastewater. The higher the pH, the more al-
kaline is the wastewater (pH = 14 is the maxi-
mum end of the scale). The lower the pH, the
more acid is the wastewater (pH = 0 is the lower
end of the scale). A bonus resulting from the
use of high-pH lime coagulation is the removal
of certain heavy metals that may be present at
times in wastewater as a result of certain types
of industrial wastewater discharges. Concen-
trations of antimony, chromium, cadmium, cop-
per, iron, lead, manganese, nickel, silver, and
zinc will be reduced more than 90 percent if
present. High pH is also effective in killing a
substantial number of viruses and bacteria.
The amount of coagulants required varies
from locale to locale, depending on the charac-
teristics of the wastewater being treated. Quan-
tities may range from 375 to 3,000 pounds per
million gallons. Usually, the higher end of the
range is required for maximum removal of
phosphorus, while the lower end may be
adequate for just suspended solids removals.
Tests must be conducted to determine the
coagulant best suited for a given wastewater.
Consideration must be given to the local costs
of the alternative coagulants, sludge disposal,
and the local availability of the chemicals.
In addition to the foregoing coagulants,
synthetic organic chemicals called "polymers"
are sometimes used in very small amounts
(less than 10 pounds per million gallons) to
increase further the settling rate of the solids.
When used for this purpose, they are called
"settling aids."
There are many plants now using coagula-
tion for phosphorus removal, suspended solids
removal, or both. Among these are Escanaba,
Bay City, and Wyoming, Michigan; Contra
Costa Sanitary District, South Lake Tahoe, and
Orange County, California; Rochester, New
York; Alexandria, Virginia; Rocky River, Cleve-
land, and Sanduskv, Ohio: Palmetto and Tam-
pa, Florida; Boulder, Colorado; Richardson,
Texas; Piscataway, Maryland; and Michigan
City, Indiana.
Advantages. Coagulation-sedimentation is
a well-proven process that provides reliable
removal of BOD and suspended solids. Proc-
ess control is simple. When used downstream
of secondary treatment, it improves the overall
system reliability by providing a means to re-
move the excessive quantities of solids that
may escape occasionally from the biological
process. Coagulation-sedimentation also may
provide substantial removals of heavy metals,
bacteria, and viruses.
Disadvantages. Larger quantities of chemi-
cal sludge are usually generated. Although lime
sludges may be recovered and reused, alum or
ferric sludges cannot be. Also, the addition of
chemicals may result in an addition of dissolved
solids to the wastewater.
29
-------
filtration
Filtration is the process of passing wastewa-
ter through a filtering medium, such as fine
sand or coal, to remove suspended or colloidal
matte/: The goal of filtration in tertiary treatment
is the removal of suspended solids from a sec-
ondary effluent or the effluent from the coagula-
tion-sedimentation process. For example, the
effluent from the tertiary coagulation and
sedimentation typically will contain 3-5 mg/l
suspended solids and 0.5-1 mg/l phosphorus.
Efficient filtration of chemical effluent can re-
duce suspended solids to zero and phosphorus
to 0.1 mg/l or less. Filtration of secondary
effluents without chemical coagulation (plain
filtration) is also used. Typically, plain filtration
will reduce activated-sludge effluent
suspended solids from 20-25 mg/l to 5-10 mg/l.
Plain filtration is not effective on trickling-filter
effluents, because the trickling-filter process is
not as efficient in flocculating the microbes so
that they are in a form readily removed by filtra-
tion.
30
-------
Filtration of wastewater is typically achieved
by passing the wastewater through a granular
bed 30-36 inches deep, composed of relatively
small particles (less than 1.5 millimeter in size).
Some filters use deeper beds and coarser
materials to achieve similar results. Modern
wastewater filters are usually made up of a
mixture of two to three different materials or
media (coal, sand, and garnet are commonly
used) of varying sizes and specific gravities.
These materials form a filter (called a mul-
timedia filter), which is coarse at the upper sur-
face and becomes uniformly finer with depth.
Proper selection of filter media is extremely im-
portant in wastewater filtration, because the
wastewater solids content is variable and may
reach high levels if the processes upstream of
the filtration process are not operated properly.
Conventional filters such as those used widely
in water treatment, made up of only one grada-
tion of sand, may also be used in wastewater
treatment. This type of filter normally requires
more frequent backwashing or cleaning than
the multimedia filters.
Wastewater is passed downward through the
filter during its normal cycle of operation. Even-
tually, the filter becomes plugged with material
removed from the wastewater, and is then
cleaned by reversing the flow (called
"backwashing"). The upward backwash rate is
high enough that the media particles are sus-
pended and the wastewater solids are washed
from the bed. These backwash wastewaters
(usually less than 5 percent of the wastewater
flow treated) must be recycled to the wastewa-
ter treatment plant for processing. Filtration
may be accomplished in open concrete struc-
tures by gravity flow, or in steel pressure ves-
sels. The operation and control of the process
may be readily automated.
Operating
••*(*' "
Rate of flow and loss
Filter bed wash-
water troughs
Concrete filter
Pressure lines to
hydraulic valves from
Cast iron
manifold
PRESSURE FILTER - FILTER CYCLE SCHEMATIC
PRESSURE FILTER- BACKWASH CYCLE SCHEMATIC
-------
-I
TYPICAL MICROSCREEN UNIT
Effluent chamber
The use of multimedia filters in tertiary
wastewater treatment applications is well es-
tablished and successful. Illustrative wastewa-
ter installations include Louisville, Kentucky;
Lemont and Hatfield Township, Pennsylvania;
Aurora and Colorado Springs, Colorado; Ben-
seville and Barrington, Illinois; Bedford Heights
and Cleveland, Ohio; South Lake Tahoe,
Orange County, Vallejo, and Ventura, Califor-
nia; Beaverton, Oregon; Piscataway, Maryland;
Dallas, Texas; and Pontiac, Michigan.
Microscreening is another system used for
filtration. Microscreens are mechanical filters
that consist of a horizontally mounted drum, the
cylindrical surface of which is made up of a
special metallic filter fabric, and that rotates
slowly in a tank with two compartments, so that
32
-------
water enters the drum from one end and flows
out through the filtering fabric. The drum is usu-
ally submerged to approximately two-thirds of
its depth. The solids are retained on the inside
of the rotating screen, which has very fine open-
ings of 23-60 microns, and are washed from the
fabric through a row of jets fitted on top of the
machine. The wastewater containing the solids
flushed from the screen is collected in a hopper
or trough inside the drum for return to the sec-
ondary plant. Microscreens used in plain filtra-
tion applications can reduce activated-sludge-
effluent suspended solids from 20-25 mg/l to
6-10 mg/l. Microscreens have an advantage
over granular filters in that they operate con-
tinuously without the need for a separate
backwashing cycle. They have the disadvan-
tage of being more sensitive to variations in the
incoming suspended solid concentrations, and
they are not used for removal of chemical floe.
The largest U.S. installation of microscreening
in a wastewater system is at the Chicago Sani-
tary District's Northside plant, with a design
capacity of 15 mgd. Microscreening systems
have been used at Akron, Ohio (3 mgd);
Franklin Township (4 mgd), Hempfield Town-
ship (6 mgd) and Lionville, Pennsylvania (0.75
mgd); and Jackson Township, New Jersey (0.1
mgd).
Advantages. Effluent filtration provides a
means of controlling the suspended solids con-
tent of a secondary effluent and providing
added removals of phosphorus and suspended
solids from the coagulation-sedimentation
process. This positive control improves the
overall reliability of treatment as well as provid-
ing a higher degree of treatment. It is a well-
proven process, is readily automated, requires
little operator attention, and requires little
space.
Disadvantages. The process generates a
backwash waste stream, which, although small
in volume, must be recycled to the wastewater
plant for processing.
Energy Requirements. The power consump-
tion for filtration and backwashing is typically
about 95-100 kilowatt-hours (kWh) per million
gallons.
Space Requirements. The process and re-
lated auxiliary systems require 300-500 square
feet per mgd of capacity.
Cosfs. The costs may range from 15 cents
per 1,000 gallons ($1.60 per month per home) at
1 mgd to 6.5 cents per 1,000 gallons (70 cents
per month per home) at 10 mgd.
carbon
adsorption
Even after secondary treatment, coagulation,
sedimentation, and filtration, the soluble or-
ganic materials that are resistant to biological
breakdown will persist in the effluent. The per-
sistent materials are often referred to as "refrac-
tory organics," and are responsible for the color
found in secondary effluent. Secondary effluent
COD values are often 30-60 mg/l. The most
practical available method for removing these
materials is the use of activated carbon. Acti-
vated carbon removes organic contaminants
from water by adsorption, which is the attraction
and accumulation of one substance on the sur-
face of another. The amount of carbon surface
area available is the most important factor, be-
cause adsorption is a surface phenomenon.
The activation of carbon in its manufacture pro-
duces many pores within the particles, and it is
the vast areas of the walls within these pores
that account for most of the total surface area of
the carbon and that makes it so effective in
removing organics. After the capacity of the
carbon for adsorption has been exhausted, it
can be restored by heating the carbon in a
furnace at a temperature sufficiently high to
drive off the adsorbed organics. Keeping oxy-
gen at very low levels in the furnace prevents
the carbon from burning. The organics are
passed through an afterburner to prevent air
pollution. In small plants where the cost of an
onsite regeneration furnace cannot be justified,
it may be attractive to ship the spent carbon to a
central regeneration facility for processing.
Activated carbon used for wastewater treat-
ment may be either in a granularform (about 0.8
millimeter in diameter, the size of a fairly coarse
sand) or in a powdered form. The carbon in
powdered form is mixed with the wastewater for
several minutes to allow adsorption to occur
and then removed by settling—usually with the
assistance of a coagulant. The powdered form
is more difficult to handle (dust problems) and
more difficult to regenerate than the granular
form. Regeneration is essential to favorable
economics in wastewater treatment because of
the large quantities of carbon needed. For
these reasons, powdered carbon has not had
as widespread use in wastewater treatment as
has granular carbon. However, the powdered
form requires much less capital investment
than the granular form. There is continuing,
promising work on developing improved
methods for regenerating powdered carbon
that may permit realization of its potential bene-
fits.
33
-------
-------
i
Granular carbon adsorption is achieved by
passing the wastewater through beds of the
carbon that may resemble a gravity filter or that
may be housed in deep (20-25-foot) columns.
These carbon beds usually provide 20-40 min-
utes contact between the carbon and the
wastewater.
The degree of treatment provided before
carbon adsorption can be varied, depending on
the desired final effluent quality. Where very
high degrees of treatment are required, sec-
ondary treatment, coagulation-sedimentation,
and filtration usually precede carbon treatment.
Some organic materials (sugars, for example)
are very difficult to remove by adsorption but
are readily removed by activated sludge. Thus,
using biological treatment before carbon ad-
sorption insures the maximum removal of or-
ganics. The use of coagulation-sedimentation
and filtration as further pretreatment removes
small suspended particles that could plug the
small pores in the carbon particles, reducing
carbon efficiency. By combining these process-
es, a colorless, odorless, sparkling clear
effluent, free of bacteria and viruses, with a
BOD of less than 1 mg/l and a COD of less than
10 mg/l, can be produced. To put this COD in
perspective, many water supplies and several
treated drinking waters in the United States
have a COD of more than 10 mg/l. The water
quality is so good that it is suitable for many
reuse purposes. A utility district at South Lake
Tahoe, California, uses the above process
sequence and has used its effluent to create a
recreational lake that supports an excellent
trout fishery and that has been approved for
swimming by health authorities. Another plant
at Orange County, California, uses the carbon-
treated effluent to recharge its ground water
35
-------
supply. A plant at Windhoek, South Africa, recy-
cles its carbon-treated wastewater directly to
the drinking water system.
Plants in design for or already using activated
carbon for treatment of secondary effluent in-
clude Arlington (30 mgd), Occoquan Sewage
Authority (11 mgd), and Fairfax County (36
mgd), Virginia; Colorado Springs, Colorado (3
mgd); Dallas, Texas (100 mgd); Los Angeles (5
mgd), Orange County (15 mgd), and South Lake
Tahoe (7.5 mgd), California; Montgomery
County (60 mgd) and Piscataway (5 mgd),
Maryland; and St. Charles, Missouri (5.5 mgd).
Raw sewage
Grit removal
and screening
, *Hr?w ' "••*" j -^ .Coagulant *
* v,>^ '-^r^f ' Jfe^!.^^ AM/-> X'j^ '
.*^,*,
•v *-ki '-TSJia.,.
^-'f 1 W
! ••«-.
Another approach to using the ability of car-
bon to remove organics is called "independent
physical-chemical treatment" (IPC). In this ap-
proach, biological secondary processes are
eliminated altogether, and the carbon is the sole
means of soluble organics removal. In such a
system, the raw wastewater is usually coagu-
lated and settled (and sometimes filtered) be-
fore it is passed through the carbon system.
Such a system provides a degree of treatment
better than biological secondary but not as
good as that by biological secondary followed
by carbon adsorption. The IPC approach re-
duces the space requirements of a conven-
tional biological plant by a factor of about 4, and
the system is not affected by any toxic materials
that could upset a biological process (and, in
fact, removes most toxins). The approach is
useful in meeting treatment requirements that
are intermediate between secondary and the
most rigid AWT standards, or in cases where
space is very limited or troublesome industrial
toxins are present. The level of treatment is
higher than biological secondary, as are the
overall costs in most cases.
-------
There are several IPC plants in design or
operation, including Cortland (10 mgd), LeRoy
(1 mgd), and Niagara Falls (48 mgd), New York;
Cleveland Westerly and Rocky River (10 mgd),
Ohio; Fitchburg, Massachusetts (15 mgd); Gar-
land, Texas (30 mgd); Owosso, Michigan (6
mgd); Rosemount, Minnesota (0.6 mgd); and
Vallejo, California (13 mgd).
Advantages. Carbon adsorption removes
organic materials that cannot be removed by
biological secondary processes. The operation
can tolerate wide variations in flow or wastewa-
ter quality and requires little operator attention.
The process requires little space.
Disadvantages. The economics of the proc-
ess are improved markedly by use of carbon
regeneration and recycling, but regeneration
equipment is not readily adaptable to very small
plants (less than 3 mgd). The regeneration
process requires careful operator control.
Energy Requirements. Carbon adsorption
and regeneration typically consume about 500
kWh per million gallons.
Space Requirements. The carbon process
typically requires 300-500 square feet per mgd
of capacity.
Costs. At 10 mgd, the costs of carbon adsorp-
tion and regeneration are about 11 cents per
1,000 gallons ($1.15 per month per home), while
at 1 mgd, if carbon regeneration is not prac-
ticed, they may be as high as 35 cents per 1,000
gallons ($3.70 per month per home).
-------
nitrogen
control
Nitrogen in its many forms has long played a
fundamental role in the aquatic environment. It
is now apparent that ecological imbalances in
the natural environment have been caused, in
part, by the excessive discharges of nitroge-
neous materials to natural waterways. In cer-
tain forms, nitrogen is one of the major nutrients
supporting blooms of green and blue-green
algae in surface waters. Nitrogen not only has
nutrient value, but, in its various forms, can
represent as much as 70 percent of the total
oxygen demand of conventionally treated
municipal wastewater.
During conventional biological wastewater
treatment, almost all the nitrogen contained in
the wastewater is converted into ammonia
nitrogen. Although ammonia has very little tox-
icity to humans, treated wastewater effluent
containing ammonia has several undesirable
features.
• Ammonia consumes dissolved oxygen in
the receiving water.
• Ammonia can be toxic to fish life.
• Ammonia is corrosive to copper fittings.
• Ammonia increases the amount of chlorine
required for disinfection.
Ammonia nitrogen can be reduced in con-
centration or removed from wastewater by sev-
eral processes. These processes can be di-
vided into two broad categories: biological
Suspended growth
system
•S." Organic
Primary
methods and physical-chemical methods. The
physical-chemical category can be further di-
vided into the following processes.
• Ammonia stripping
• Selective ion exchange
• Breakpoint chlorination
Biological Nitrification-Denitrification. This
process is the biological conversion of nitroge-
nous matter into nitrates (nitrification), followed
compound
Fixed film system
denitrification
by the anaerobic biological conversion of the
nitrates to nitrogen gas (denitrification). The
process is based on the principle that the nitro-
gen compounds found in raw sewage may be
converted to the nitrate form in a properly de-
signed secondary biological process (the nitrifi-
cation process). These nitrates may then be
removed by further treatment in the absence of
oxygen. Under these anaerobic conditions, the
nitrogen is released as nitrogen gas (the denitri-
38
-------
fication process). Because nearly 80 percent of
the atmosphere consists of nitrogen, there is no
air pollution associated with the release of ni-
trogen from the wastewater to the atmosphere.
In some cases, carrying out only the nitrifica-
tion portion of the process may be adequate.
Nitrification is accomplished by providing oxy-
gen in the amount required in the biochemical
reaction to convert ammonia nitrogen to nitrate
nitrogen, or roughly 4.5 pounds of oxygen per
pound of ammonia nitrogen in the wastewater.
There are several alternative approaches to
biological nitrogen removal. The most reliable
performance has been found to occur when the
first step of treatment is an activated-sludge
step, with the resulting activated sludge settled
and recycled to this step of the process. This
step oxidizes most of the raw wastewater BOD.
The nitrification step can then be accomplished
in a suspended growth system similar to the
activated-sludge process, in a fixed-film system
consisting of a trickling-filter-like column of
stones or synthetic media, or with rotating
biological contactors. The organisms that carry
out the nitrifying step are very slow growing,
and, if they are lost from the suspended growth
system because of poor settling characteristics
or for other reasons, process performance may
suffer for many weeks until an adequate popu-
lation of nitrifiers can be established again.
Thus, the fixed-film system for nitrification of-
fers an advantage in that it provides greater
assurance of retention of the nitrifying orga-
nisms.
When the effluent from a wastewater treat-
ment plant is discharged to a receiving water
with a significant flow, such as a river, nitrate
nitrogen may not affect it adversely. In fact, a
nitrified effluent free of substantial quantities of
ammonia can offer several advantages:
• Nitrate nitrogen provides oxygen to sludge
beds and prevents the formation of septic
odors.
39
-------
• Nitrified effluents are more efficiently disin-
fected by chlorine treatment.
• A nitrified effluent reduces the oxygen de-
mand on the receiving waters.
The deciding factor ih determining whether
the discharge of a nitrified effluent to a free-
flowing receiving water is acceptable is the level
of nitrate nitrogen it contains. If the level is too
high, then further action is necessary to control
the nitrogen content of the effluent. This is also
the case when treated wastewater is dis-
charged to relatively still bodies of water, such
as lakes, reservoirs, and estuaries. In these
cases, even a highly nitrified effluent can have
harmful effects, such as fostering algal blooms.
If a nitrified effluent is determined unaccept-
able, then nitrogen removal by downstream use
of the denitrification process is required.
The denitrification step can be accomplished
either in an anaerobic activated-sludge system
(suspended growth system) or in a columnar
system (fixed-film system). The high degree of
biological treatment upstream of the denitrifica-
tion process leaves little oxygen-demanding
material in the wastewater by the time it
reaches denitrification. The desired nitrate re-
duction will occur only as a result of oxygen
demand being exerted in the absence of oxy-
gen in the wastewater. If denitrification is to be
practical, an oxygen-demand source must be
added to reduce the nitrates quickly. The most
common method of supplying the needed oxy-
gen demand is to add methanol in the denitrifi-
cation process.
The efficiency of biological nitrification-
denitrification is usually 80-90 percent nitrogen
removal. The process is in use or planned for
use at Central Contra Costa Sanitary District,
MODIFICATIONS OF
THE NITRIFICATION PROCESS
Open tank denitrification
(activated-sludge-type culture)
Oxygen-demanding
substance
I. Open tank
nitrification
High-rate
organic synthesis
Submerged
filter
Submerged
fitter
(fine media)
Sand fMter
•i**
£,;.
?#
»»v
f>!
5W
V*
v*;'
:?i%
1
1
„**
*«.
0***
y&
S
:"
i
1 1 1 1
-t
•
*' i
JtlwiaJ
III. Submerged filter
(coarse media)
40
-------
California (30 mgd); El Lago, Texas (0.5 mgd);
Tampa (50 mgd) and Orlando (12 mgd), Florida;
Hobbs, New Mexico (5 mgd); Salt Creek (50
mgd) and Waukegan (30 mgd), Illinois; and
Madison, Ohio (6 mgd). Nitrification (without
denitrification) is planned or in use at Washing-
ton, D.C. (309 mgd); Madison, Wisconsin (30
mgd); Flint (20 mgd), Jackson (17 mgd), and
Benton Harbor (13 mgd), Michigan; and
Waukegan (20 mgd), Highland Park (18 mgd),
and Gurnee (17 mgd), Illinois.
Advantages. The biological processes in-
volved are similar to those used in the past for
secondary treatment, both in design and opera-
tion. The process generates no significant
added sludge for disposal, nor does it have any
objectionable side effects on air or water quality.
Disadvantages. The process requires more
space than other methods of nitrogen removal.
The process can be upset by toxic materials.
The loss of microbes from any of the three
biological processes used in series resulting
from toxins, equipment failure, or operator error
can disrupt performance for many days.
Energy Requirements. Nitrification con-
sumes substantial added power, equivalent in
additional consumption to that of a conventional
activated-sludge plant (about 625 kWh per mil-
lion gallons). The denitrification process, using
a mixed-tank system, adds about another 300
kWh per million gallons.
Space Requirements. The space require-
ments depend on the configuration of nitrifica-
tion and denitrification units selected, but will
typically be 0.3-0.6 acre per mgd of capacity.
Costs. The costs may typically range from 30
cents per 1,000 gallons at 1 mgd ($3.15 per
month per home) to 16 cents per 1,000 gallons
at 10 mgd ($1.70 per month per home).
-------
Ammonia Stripping. This process removes
gaseous ammonia from water by agitating the
water-gas mixture in the presence of air. In
practice, the process is based on the principle
that nitrogen in the form of ammonium ions in
secondary effluent can be converted to am-
monia gas by raising the pH to high values. The
gaseous ammonia can then be released by
passing the high-pH effluent through a stripping
tower where the agitation of the water in the
presence of a large air flow through the tower
releases the ammonia. The use of lime in coag-
ulation-sedimentation permits simultaneous
coagulation for suspended solids and phos-
phorus removal and the necessary upward ad-
justment of pH for the stripping process.
The three basic steps in ammonia stripping
are (1) raising the pH of the water to form am-
monia gas, generally with the lime used for
phosphorus removal, (2) cascading the water
down through a stripping tower to release the
ammonia gas, and (3) circulating large quan-
tities of air through the tower to carry the am-
monia gas out of the system. The towers used
for ammonia stripping closely resemble con-
ventional cooling towers. The concentration of
ammonia in the offgas from the tower is very
low—well below odor levels—and does not
cause air pollution problems.
The major process limitation is the effect of
temperature on efficiency. As the air tempera-
ture drops, efficiency also drops. For example,
stripping removes about 95 percent of the am-
monia in warm weather (70° F air temperature)
but only about 75 percent of the ammonia when
the temperature falls to 40° F. The process be-
comes inoperable as a result of freezing prob-
lems within the stripping tower when the air
temperature falls very far below freezing. De-
velopment work is underway on a system that
42
-------
will recirculate the offgases from the tower to
minimize the temperature effects.
Ammonia stripping is in use at South Lake
Tahoe (7.5 mgd) and Orange County (15 mgd),
California; and Bucks County, Pennsylvania
(7.0 mgd). The Orange County plant will use
waste heat from an adjacent seawater desalt-
ing plant to control the temperature of the strip-
ping air.
Advantages. The process offers the lowest
cost method of nitrogen removal now available.
It is also the simplest to operate, and its simplic-
ity insures reliability. It requires little space.
Disadvantages. Cold weather adversely af-
fects performance, and prolonged periods of
freezing weather render the process inopera-
ble. Deposits resulting from the upstream lime
treatment can occur within the tower, and provi-
sions for controlling or removing the deposits
must be made.
Energy Requirements. Power requirements
are about 1,000 kWh per million gallons.
Space Requirements. Total space require-
ments are usually less than 700 square feet per
mgd of capacity.
Cosfs. Costs range from 9 cents per 1,000
gallons at 1 mgd (95 cents per month per home)
to 6 cents per 1,000 gallons at 10 mgd (65 cents
per month per home).
Selective Ion Exchange. By this process,
ammonium ions in solution are exchanged for
sodium or calcium ions displaced from an in-
soluble exchange material. The process opera-
tion resembles that of a water softener, except
that the material being removed is ammoni-
um-nitrogen rather than water hardness. Both
are ion-exchange processes, where the water
is passed through a bed of ion-exchange mate-
rial that has the ability to remove certain con-
stituents in exchange for a constituent of the
exchange material. The selective ion-exchange
process derives its name from the use of an
-------
VY -15*3% - *
r? ^viYs-StfaA* e*,-*-* •»*
|lteM|iii^Mi^w^lM * J'fct ^ ? „< ^fs
/ ^;',-%
»r< «<>"iy.-' -*<-* <-tc
^-C X: <'.;/•"" ". •*;..[ ' *
ion-exchange material that selectively removes
ammonium. The ion-exchange material is a
naturally occurring zeolite called "clinoptilolite."
Ammonium is removed by passing the waste-
water through a bed of clinoptilolite until the
capacity of the clinoptilolite has been used to
the point that ammonia begins to leak through
the bed. At this point, the clinoptilolite must be
regenerated so that its capacity to remove am-
monia is restored.
The clinoptilolite is then regenerated by pass-
ing concentrated salt solutions through the ex-
change bed. The ammonium-laden regenerant
volume is about 5-6 percent of the throughput
volume treated before regeneration. If the am-
monium is removed from the regenerant, the
regenerant can be reused. There are no regen-
erant brines to dispose of, avoiding a major
problem of conventional, nonselective ex-
change resins. Several techniques are availa-
ble for removal of ammonium from the regener-
ant, some of which release the ammonium as
nitrogen gas. Others recover the nitrogen in
reusable forms such as ammonium sulfate or
aqueous ammonia.
The process is very efficient and can remove
95-97 percent of the ammonium nitrogen. The
process is in use at or in design at Rosemount,
Minnesota (0.6 mgd); the Upper Occoquan
Sewage Authority, Virginia (10.9 mgd); and the
Tahoe Truckee Sanitation Agency, California (5
mgd).
Advantages. Efficiency for nitrogen removal
is very high, is readily controllable, and is not
sensitive to temperature variations. The proc-
ess lends itself well to the eventual recovery of
the nitrogen in a form that can be used as a
fertilizer. Space requirements are low.
44
-------
Disadvantages. Equipment and operation
are relatively complex and the capital costs are
high.
Energy Requirements. Power consumption
depends primarily on how the regenerant re-
covery process is handled, but will be about 100
kWh per million gallons in most cases.
Space Requirements. The space required
for the ion-exchange beds and related regener-
ant recovery system is usually less than 1,000
square feet per mgd.
Cosfs. The costs may range from about 20
cents per 1,000 gallons at 1 mgd ($2.10 per
month per home) to 12 cents per 1,000 gallons
at 10 mgd ($1.25 per month per home).
Breakpoint Chlorination. In this process,
chlorine is added to wastewater in such
amounts that the chlorine demand is satisfied
so that further addition of chlorine results in a
directly proportional chlorine residual. It is
used for nitrogen removal because chlorine,
when added to wastewater containing am-
monium nitrogen, reacts to form compounds
that, if enough chlorine is added, eventually are
converted to nitrogen gas. To achieve the con-
version, about 10 mg/l of chlorine must be
added per mg/l of ammonia nitrogen in the
wastewater. A typical secondary effluent am-
monia concentration of 20 mg/l requires the use
of about 1,700 pounds of chlorine per million
gallons treated—about 40 or 50 times more
than normally used in a wastewater plant for
disinfection only.
The facilities required for the process are
simple. Wastewater (after secondary or tertiary
treatment) flows into a mixing chamber where
the chlorine is added and thorough mixing is
provided. Because the large amount of chlorine
used has an acidic effect on the wastewater,
alkaline chemicals (such as lime) may be added
to the same chamber to offset this effect. The
nitrogen gas formed by the reactions is re-
leased to the atmosphere. The process can
achieve 99+ percent removal of the ammonium
nitrogen. The chemical additions are monitored
and controlled by a computer system, providing
automated operation. The amounts of chlorine
used provide very effective disinfection as well
as nitrogen removal. Because the process is
just as effective in removing 1 mg/l as 20 mg/l of
ammonium, it is used frequently as a polishing
step downstream of other nitrogen removal
processes. The low capital cost of the break-
point process makes it attractive for this pur-
pose. The process is used or planned for use at
Montgomery County, Maryland (60 mgd); Cort-
land, New York (10 mgd); Owosso, Michigan (6
mgd); Arlington County, Virginia (30 mgd); and
Orange County (15 mgd) and South Lake
Tahoe (7.5 mgd), California. The large
Montgomery County facility will generate its
own hypochlorite for the process at the waste-
water treatment plant.
Advantages. The principal advantages of
breakpoint chlorination are its high efficiency,
small space requirements, low capital costs,
assurance of disinfection, and the conversion of
ammonium to elemental nitrogen that presents
no disposal problem.
Disadvantages. The chlorine added results
in an increase in the chloride content of the
wastewater. If the effluent is not discharged to a
coastal estuary or mixed with large quantities of
freshwater, this increase may be significant if
there are downstream water supplies. The
process requires large quantities of chlorine.
Space Requirements. Total space require-
ments for the mixing chamber and related
chemical feed and storage are typically less
than 500 square feet per mgd of capacity.
Cosfs. The costs primarily depend on the
price of chlorine and the quantity of ammonium
to be removed, with little economy of scale in
the 1-10-mgd capacity range. Costs may range
from 11 to 15 cents per 1,000 gallons ($1.15-
$1.60 per month per home) for typical wastewa-
ters with 20-25 mg/l of ammonium.
Influent
Nitrogen
Reaction
basin
• Chlorine
pH control
chemicals
Effluent
45
-------
land
treatment
An alternative to the previously discussed
processes for producing an extremely high-
quality effluent is offered by an approach called
"land treatment." Land treatment is the applica-
tion of effluents, usually following secondary
treatment on the land by one of the several
available conventional irrigation methods. This
approach uses wastewater, and often the nu-
trients it contains, as a resource rather than
considering it as a disposal problem. Treatment
is provided by natural processes as the effluent
moves through the natural filter provided by the
soil, plants, and related ecosystem. Part of the
wastewater is lost by evapotranspiration, while
the remainder returns to the hydrologic cycle
through overland flow or the ground water sys-
tem. Most of the ground water eventually re-
turns, directly or indirectly, to the surface water
system.
Land treatment of wastewaters can provide
moisture and nutrients necessary for crop
growth. In semiarid areas, insufficient moisture
for peak crop growth and limited water supplies
make water especially valuable. The primary
nutrients (nitrogen, phosphorus, and potas-
sium) are reduced only slightly in conventional
secondary treatment processes, so that most of
these elements are still present in secondary
effluent. Soil nutrients are consumed each year
by crop removal and lost by soil erosion. Fer-
tilizer supply is highly dependent on energy in-
put, and recently has increased significantly in
price. Recycling wastes to the land so that the
nutrient cycle can be completed and soil fertility
maintained is an alternative that should be
given serious consideration.
Land application is the oldest method used
for treatment and disposal of wastes, with use
by cities recorded for more than 400 years.
Several major cities, including Berlin, Mel-
bourne, and Paris, have used "sewage farms"
for at least 60 years for waste treatment and
disposal. About 600 communities in the United
States reuse municipal wastewater treatment
plant effluent in surface irrigation systems,
mostly in arid or semiarid areas.
Evaporation -
transpiration
//// Rainfall
///// ///
City
Irrigated
forest
Treatment
and storage
lagoons
Subsurface
tile drainage
Impermeable
layer
Ground water
-------
Evaporation
Land treatment systems use one of the three
basic approaches:
• Irrigation
• Overland flow
• Infiltration-percolation
In the irrigation mode, the wastewater is
applied to the land by sprinkling or by surface
spreading. Sprinkling systems may be either
fixed or moving. Fixed sprinkling systems, often
called solid set systems, may be either on the
Spray or surface
application
Crop
(a) IRRIGATION
Slope variable
Deep
percolation
-------
ground surface or buried. Both types usually
consist of impact sprinklers on risers that are
spaced along lateral pipelines, which are in turn
connected to main pipelines. These systems
are adaptable to a wide variety of terrains and
may be used for irrigation of either cultivated
land or woodlands. There are a number of dif-
ferent moving sprinkling systems, but the
center pivot system is generally the most widely
used for wastewater irrigation.
The two main types of surface application
systems are ridge-and-furrow and flooding
techniques. Ridge-and-furrow irrigation is ac-
complished by gravity flow of effluent through
furrows from which it seeps into the ground.
The irrigation techniques all apply the
wastewater to the land so that some pollutants
are taken up in the growing plants, some are
transformed in the soil to harmless agents, and
some are held in the soil. Some of the purified
wastewater percolates through the soil to be-
come ground water, some is taken up by plants,
some runs off, and some evaporates. Typical
removals of pollutants from secondary effluent
by irrigation are BOD, 98 percent; COD, 80
percent; suspended solids, 98 percent; nitro-
gen, 85 percent; phosphorus, 95 percent; met-
als, 95 percent; and micro-organism, 98 per-
cent.
An example of an irrigation system is in Lub-
bock, Texas, where 15 mgd of secondary
effluent is applied to 2,300 acres of a farmer's
cropland. Crops consist of small grains—such
as wheat, barley, oats, and rye—cotton, and
many varieties of grain sorghums. Crop yields
exceed those achieved with conventional irriga-
tion.
A major spray irrigation project has been in-
stalled at Muskegon, Michigan. This 43.4-mgd
project irrigates 6,300 acres of a 10,000-acre
48
-------
site with secondary effluent. The wastewater
from several municipalities and industries in
Muskegon County is collected and treated in
aerated lagoons. The lagoon effluent is then
sprayed onto farmland for irrigation. Corn is the
primary crop grown at Muskegon. Crop yields
were as high as 92 bushels per acre in 1974 and
produced an income of $368,000. The system
was constructed for a cost of $42.0 million.
Treated wastewater collected from underdrains
beneath the irrigated area is of extremely high
quality, as reflected by analyses that show a
BOD of 2 mg/l, total organic carbon of 5 mg/l,
phosphate of 0.05 mg/l, and nitrogen of 3.2
mg/l.
In an overland flow system, the wastewater is
sprayed over the upper edges of sloping ter-
races and flows slowly down the hill and
through the grass and vegetative litter. Al-
though the soil is not the primary filter in this
mode, treatment efficiencies are high in a well-
run system. Typical removals are BOD, 92 per-
cent; suspended solids, 92 percent; nitrogen,
70-90 percent; phosphorus, 40-80 percent; and
metals, 50 percent. Soils best suited for this
approach are clays and clay loams with even,
moderate slopes (2-6 percent). Grass is usually
planted to provide a habitat for biota and to
prevent erosion. As the effluent flows down the
slope, a portion infiltrates into the soil, a small
amount evaporates, and the remainder flows to
collection channels. As the effluent flows
through the grass, the suspended solids are
filtered out and the organic matter is oxidized by
the bacteria living in the vegetative litter. Over-
land flow treatment has been used in the United
States primarily for treating high-strength
wastewater, such as that from canneries. In
Australia, overland flow or grass filtration has
been used for municipal waste treatment for
many years. However, there are no existing
-------
U.S. applications of this technique for municipal
wastes.
\r\infiltration-percolation systems, the prima-
ry goal is to recharge ground water by percolat-
ing as much wastewater as possible into the
ground by placing the wastewater (after second-
ary treatment) into spreading basins. The dis-
tinction between treatment and disposal for this
process is quite fine. Wastewater applied to the
land for the purpose of disposal is also undergo-
ing treatment by infiltration and percolation.
Typical removals of pollutants from secondary
effluent are BOD, 85-99 percent; suspended
solids, 98 percent; nitrogen, 0-50 percent;
phosphorus, 60-95 percent; and metals, 50-95
percent. Infiltration-percolation is primarily a
ground water recharge system, and does not
attempt to recycle the nutrients through crops.
Phoenix, Arizona, is now installing a 15-mgd
infiltration system to recharge ground water
used for unrestricted irrigation.
When properly designed and operated, a
land treatment system of the irrigation type can
produce an effluent quality comparable to that
produced by other AWT processes for phos-
phorus, suspended solids, BOD, heavy metal,
virus, and bacterial removal. Comparable nitro-
gen removals can also be produced, but the
nitrogen removal achieved in a land treatment
system depends directly on the specific design
and operating procedures used. For example,
while phosphorus is readily removed by chemi-
cal reactions with the soil, the chief mechanism
for nitrogen removal is uptake by crops. High
degrees of nitrogen removal require that
wastewater be applied to the land only during
the season of active crop growth. In many parts
of the country, this requirement and the need to
avoid applying wastewater to frozen land fre-
quently dictate that storage lagoons with capac-
ity to store 3-5 months of wastewater be con-
50
-------
structed to store wintertime wastewater flows.
Land treatment also efficiently removes heavy
metals. These metals may accumulate and
persist in the soil, however, and their long-term
effects must be evaluated carefully for the
specific wastewater and soil conditions in-
volved.
Examples of municipalities currently using
land treatment include Muskegon County,
Michigan (43.4 mgd); Tallahassee, Florida
(2.5 mgd); Oceanside (1.5 mgd), Pleasanton
(1.3 mgd), Golden Gate Park, San Francisco (1
mgd), Santee (1 mgd), and Bakersfield (12.3
mgd), California; St. Charles, Maryland (0.5
mgd); Colorado Springs, Colorado (5.5 mgd);
and Ephrata, Washington (0.44 mgd).
Advantages. Land treatment provides a very
advanced degree of treatment without generat-
ing any chemical sludges. It recycles the water
and the nutrients contained in the wastewater
for productive uses, and may even enable rec-
lamation of unproductive land while reducing
the use of other water resources. High degrees
of treatment are achieved without consumption
of resources such as chemicals and activated
carbon. Large open space areas are preserved
with potential for multiple recreational use dur-
ing the nonirrigation season. Operating costs
are less than for other tertiary processes, and
there is the potential for economic return from
sale of crops.
Disadvantages. The large land areas re-
quired may be a disadvantage, especially in
urbanized areas. Although there are many
existing systems, operation frequently has
been based on what was observed to work
without nuisance. The monitoring of effluent
quality and the determination of system design
limitations have often been inadequate. As a
result, there is little U.S. experience available
that is of direct assistance in designing a land
treatment system that will provide high levels of
treatment. Crop selection is restricted by health
and other factors dictated by wastewater treat-
ment rather than agricultural considerations.
Energy Requirements. Power requirements
for land treatment are extremely variable and
difficult to generalize. The type of irrigation sys-
tem used has a major effect; spray irrigation
consumes more power than overland flow. A
major variable is the amount of power required
to transport wastewater from its source to a
suitable land treatment site. Typical power con-
sumption by the irrigation system may range
from 1,000 to 2,500 kWh per million gallons.
Space Requirements. The space require-
ments are a function of the level of treatment
required and the soil type. They range from 100
to 600 acres per mgd of capacity.
Costs. Costs are also highly variable, de-
pending on space requirements for a specific
project, local land costs, specific irrigation sys-
tem used, etc. For l-mgd capacity, costs may
range from 20 cents per 1,000 gallons ($2.10
per month per home) to $1.09 per 1,000 gallons
($11.45 per month per home), while at 10 mgd,
they may range from 14 cents per 1,000 gallons
($1.50 per month per home) to $1 per 1,000
gallons ($10.50 per month per home). The cost
of land actually used as part of a treatment
system is eligible for Federal pollution control
grants. Costs to transport the wastes to a suit-
able land treatment site vary widely and are not
included in the foregoing estimates.
-------
flow
equalization
Flow equalization is not a treatment process
per se, but a technique that can be used to
improve the effectiveness of both secondary
and tertiary processes. Wastewater does not
flow into a municipal wastewater treatment
plant at a constant rate. The flow rate varies
from hour to hour, reflecting the living habits of
the area served. In most towns, the pattern of
daily activities begins with rising between 6 and
7 a.m., going to work between 8 and 9 a.m.,
lunch between 12 and 1 p.m., returning home
between 4 and 5 p.m., dinner at 6 or 7 p.m., and
bed by 11 p.m. This routine sets the pattern of
sewage flow and strength. Above-average
sewage flows and strength occur in midmorn-
ing. The constantly changing amount and
strength of wastewater to be treated makes
efficient process operation difficult. Also, many
treatment units must be designed for the
maximum flow conditions encountered, which
actually results in their being oversized for av-
erage conditions. The purpose of flow equaliza-
tion is to dampen these variations so that the
wastewater can be treated at a nearly constant
flow rate. Flow equalization, at low cost, can
52
VI
__ L_.
6
— — . a.m.
1 |
N 6
M
Time of day
VARIATIONS IN SEWAGE FLOW
DURING A TYPICAL DAY
-------
significantly improve the performance of an
existing plant and increase its useful capacity.
In new plants, flow equalization can reduce the
size and cost of the treatment units.
Flow equalization is usually achieved by con-
structing large basins that collect and store the
wastewater flow and from which the wastewa-
ter is pumped to the treatment plant at a con-
stant rate. These basins are normally located
near the head end of the treatment works, pref-
erably downstream of pretreatment facilities
such as bar screens, comminutors, and grit
chambers. Adequate aeration and mixing must
be provided to prevent odors and solids deposi-
tion.
Flow equalization will normally improve the
suspended solids removal in a primary clarifier,
stabilize the operation of the biological second-
ary processes, and improve secondary clarifier
performance. In AWT processes, flow equaliza-
tion eases control of chemical addition and
substantially reduces cost of facilities such as
filters and carbon columns by permitting them
to be sized for average flows rather than peak
flows.
The needed basins may be constructed of
earth, concrete, or steel, or may even some-
times be converted treatment units such as
former sludge lagoons, aeration basins, or
clarifiers. The cost of low equalization will vary
considerably from one application to another,
depending on the basin size, construction
selected, mixing and aeration requirements,
availability of land, location of facility, and pump-
ing requirements. Costs may range from 3-7
cents per 1,000 gallons at 1 mgd to 1-3 cents per
1,000 gallons at 10 mgd. These costs may be
more than offset by savings in downstream
treatment processes.
53
-------
BASIC SLUDGE-HANDLING ALTERNATIVES
sludge treatment
and disposal
In the process of purifying the wastewater,
another problem is created—sludge handling.
The higher the degree of wastewater treatment,
the larger the residue of sludge that must be
handled. Satisfactory treatment and disposal of
the sludge can be the single most complex and
costly operation in a municipal wastewater
treatment system. The sludge is made of mate-
rials settled from the raw wastewater—such as
rags, sticks, and organic solids—and of solids
generated in the wastewater treatment proc-
esses—such as the excess activated sludge
created by aeration or the chemical sludges
produced in some AWT processes. Whatever
the wastewater process, there is always some-
thing that must be burned, buried, treated for
reuse, or disposed of in some way.
The quantities of sludge involved are sig-
nificant. For primary treatment, they may be
2 500-3 500 gallons per million gallons of
wastewater treated. When treatment is up-
qraded to activated sludge, the quantities in-
crease by 15,000-20,000 gallons per million gal-
lons Use of chemicals for phosphorus removal
can add another 10,000 gallons. The sludges
withdrawn from the treatment processes are
still largely water, as much as 97 percent!
Sludge treatment processes, then, are con-
cerned with separating the large amounts of
water from the solid residues. The separated
water is returned to the wastewater plant for
processing.
54
Sludge
Thicken
Dewater
Burn
(oxidize)
Ash to
landfill
Digest
(stabilize)
Dewater
Landfill
Use as fertilizer
Dry
-------
The basic functions of sludge treatment are
• Conditioning—treatment of the sludge with
chemicals or heat so that the water may be
readily separated
• Thickening—separation of as much water
as possible by gravity or flotation process
• Dewatering—further separation of water
by subjecting the sludge to vacuum pres-
sure, or drying processes
• Stabilization—stabilization of the organic
solids so that they may be handled or used
as soil conditioners without causing a nui-
sance or health hazard through processes
referred to as "digestion"
• Reduction—reduction of the solids to a
stable form by wet oxidation processes or
incineration
Although a large number of alternative com-
binations of equipment and processes are used
for treating sludges, the basic alternatives are
fairly limited. The ultimate depository of the
materials contained in the sludge must either be
land, air, or water. Current policies discourage
practices such as ocean dumping of sludge. Air
pollution considerations necessitate air pollu-
tion facilities as part of the sludge incineration
process. Thus, the sludge in some form will
eventually be returned to the land. The follow-
ing paragraphs discuss the processes em-
ployed in the basic alternative routes by which
this may occur.
sludge
conditioning
Several methods of conditioning sludge to
facilitate the separation of the liquid and solids
are available. One of the most commonly used
is the addition of coagulants—ferric chloride,
lime, or organic polymers. Ash from incinerated
sludge has also found use as a conditioning
agent. Just as when coagulants are added to
the wastewater, chemical coagulants act to
clump the solids together so that they are more
easily separated from the water. In recent
years, organic polymers have become increas-
ingly popular for sludge conditioning. Polymers
are easy to handle, require little storage space,
and are very effective. The conditioning chemi-
cals are injected into the sludge just ahead of
thickening or dewatering processes and are
mixed with the sludge. Chemical sludge condi-
tioning is used at hundreds of municipal plants.
Another conditioning approach is to heat the
sludge at high temperatures (350-450° F) and
pressures (150-300 pounds per square per
square inch, or psi). Under these conditions—
much like those of a pressure cooker—water
bound up in the solids is released, improving
the dewatering characteristics of the sludge.
Commercial systems first grind the sludge and
then inject it into a reactor where high tempera-
ture and pressure are applied. The sludge flows
from the reactor to a settling tank, where the
solids are concentrated before being sent on to
the dewatering step. Units of this type have
been used at several plants, including Colorado
Springs, Colorado; Levittown and Lancaster,
Pennsylvania; Kalamazoo, Midland, and Grand
Haven, Michigan; Terre Haute, Indiana; Roths-
child, Wisconsin; Louisville, Kentucky; and Fort
Lauderdale, Florida. Several other new installa-
tions are now underway. Heat treatment has the
advantage of producing a sludge that dewaters
better than chemically conditioned sludge. The
process has the disadvantages of relatively
complex operation and maintenance and the
creation of highly polluted cooking liquors that,
when recycled to the treatment plant, impose a
significant added treatment burden.
Another approach to conditioning is the ap-
plication of heavy doses of chlorine to the
sludge under low pressure (30-40 psi). This
relatively new approach, because of the acidic
effects of the chlorine, also provides stabiliza-
tion of organic sludges.
The costs of sludge processes typically are
expressed in terms of dollars per ton of solids
processed. The solids are expressed in terms
of the dry weight of the solids present in the
sludge. The quantities of sludge to be proc-
essed vary greatly from one locale to another,
but a typical primary-plus-activated-sludge
plant will produce 1-1.5 tons of dry solids per
million gallons treated. Chemical conditioning
costs may range from $3 to $30 per ton—the
higher the proportion of activated sludge, the
more difficult and expensive the conditioning
process. Heat treatment costs typically are $20
to $40 per ton.
55
-------
HIGH PRESSURE
PUMP
GROUND SLUDGE
STORAGE
:|WATER
AIR COMPPRESSOR
BOILER
56
-------
-------
sludge
thickening
After the sludge has been conditioned, it is
often thickened before further processing.
Thickening is usually accomplished in one of
two ways: the solids are floated to the top of the
liquid (flotation thickening) or are allowed to
settle to the bottom (gravity thickening). The
goal is to remove as much water as possible
before final dewatering or disposal of the
sludge. The processes involved offer a low-cost
means of reducing sludge volumes by a factor
of 2 or more. The costs of thickening are usually
more than offset by the resulting savings in the
size and cost of downstream sludge-
processing equipment.
The flotation thickening process injects air
into the sludge under pressure (40-80 psi).
Under this pressure, a large amount of air can
be dissolved. The sludge then flows into an
open tank where, at atmospheric pressure,
much of the air comes out of solution as minute
air bubbles that attach themseJves to sludge
solids particles and float them to the surface.
Flotation is especially effective on activated
sludge, which is difficult to thicken by gravity.
The sludge forms a layer at the top of the tank;
this layer is removed by a skimming mechanism
for further processing. The process typically in-
creases the solids content of activated sludge
from 0.5-1 percent to 3-6 percent, greatly eas-
ing further dewatering.
Gravity thickening has been used widely on
primary sludges for many years. It is simple and
inexpensive. It is essentially a sedimentation
process similar to that which occurs in all set-
tling tanks. Sludge flows into a tank that is very
Unit
effluent
Sludge removal
mechanism
--«*
-; Sludge blanket *p
n^
(feed *•:* _, .
v Sludge
_ discharge
Recycle flow
Recycle
flow
Bottom sludge collector
Unit
sludge feed
58
FLOTATION THICKENER
-------
similar in appearance to the circular clarifiers
used in primary and secondary sedimentation;
the solids are allowed to settle to the bottom
where a heavy-duty mechanism scrapes them
to a hopper from which they are withdrawn for
further processing. The type of sludge being
thickened has a major effect on performance.
The best results are obtained with purely prima-
ry sludges. As the proportion of activated
sludge increases, the thickness of the settled
sludge solids decreases. Purely primary
sludges can be thickened from 1 -3 percent to 10
percent solids.
Costs of thickening are about $4-$10 per ton
(for the 1-10-mgd plant range) for gravity thick-
ening and $4-$26 per ton for flotation thicken-
ing. The current trend is toward using gravity
thickening for primary sludges and flotation
thickening for activated sludges, and then
blending the thickened sludges for further proc-
essing.
-------
Fixed cover
Digester gas outlet
sludge
stabilization
The principal purposes of sludge stabilization
are to break down the organic solids biochemi-
cally so that they are more stable (less odorous
and less putrescible) and more dewaterable,
and to reduce the mass of sludge. If the sludge
is to be dewatered and burned, stabilization is
not normally used. Many municipal plants do
not use incineration, however, and rely on
sludge digestion to stabilize their organic
sludges. There are two basic digestion process-
es in use. One is carried out in closed tanks
devoid of oxygen and is called "anaerobic di-
gestion." The other approach injects air into the
sludge to accomplish "aerobic digestion."
Most modern anaerobic digesters use a
two-stage process. The sludge is normally
heated by means of coils located within the
tanks or an external heat exchanger.
In the two-stage process, the first tank is used
for the biological digestion. It is heated and
equipped with mixing facilities. The second tank
is used for storage and concentration of di-
gested sludge and formation of a relatively clear
liquid (called "supernatant") that can be with-
drawn from the top of the tank and recycled to
the treatment plant. The second tank may be an
open tank, an unheated tank, or a sludge la-
goon. Tanks are usually circular, are seldom
CO
Floating cover
(often uncovered)
Supernatant layer
Digested
sludge
c
3
in
41
2. 3
CO O
First stage
(completely mixed)
Second stage
(stratified)
SCHEMATIC OF TWO-STAGE DIGESTION PROCESS
less than 20 feet or more than 115 feet in diame-
ter, and may be as deep as 45 feet or more. As
the organic solids are broken down by
anaerobic bacteria, methane gas and carbon
dioxide gas are formed. Methane gas is com-
bustible and must not be allowed to mix with air
or an explosive mixture may result. The diges-
ter gas containing methane is a usable fuel, a
fact that has been receiving increased atten-
tion. Digester gas may be used as fuel for boiler
and internal combustion engines that are, in
turn, used for pumping sewage, operating
blowers, and generating electricity. An effi-
ciently operating anaerobic digester converts
about 50 percent of the organic solids to liquid
and gaseous forms. The methane liberated has
the potential to generate about 30 kWh of elec-
tricity for every 100 people served. As com-
pared to aerobic digestion, anaerobic digestion
has the advantages of producing a useful by-
product (methane) rather than consuming pow-
er. It has the disadvantages that it is sensitive to
variations in sludge feed and can become eas-
ily upset if not carefully operated. It also pro-
duces a supernatant (which must be recycled to
the treatment plant) containing a high concen-
tration of soluble pollutants that are an added
load on the secondary process. Costs for
anaerobic digestion are typically $30-$40 per
ton of dry solids.
60
-------
Aerobic digestion is accomplished by aerat-
ing the organic sludges in an open tank re-
sembling an activated-sludge aeration tank. (In
fact, activated-sludge aeration tanks have been
converted to aerobic digesters.) Its most exten-
sive use has been in relatively small activated-
sludge plants. It is receiving increased attention
for larger plants, however, and has been used
at the Metropolitan Denver Sewage Disposal
District for sewage flows over 100 mgd. The
process can achieve about the same 50-
percent solids reduction achieved in the anaer-
obic process, while offering advantages of
being more stable in operation and recycling
fewer pollutants to the wastewater plant than
anaerobic digesters. It has the disadvantages
of higher power costs and does not produce an
energy source such as methane. Total costs are
typically $30-$50 per ton.
f^BJJrt^j^e,
^-
--»N .
-^
S*
VT*
, /,*"Vf
»f
-ȣ%?=-
l&&^ ^.^:^.--V
'^•¥^XN vp^::-^^- V
>:^ v\ \:C- '::/ l
* S
-<-X
:x-v
U
awK%w
to plant ,
1^
Settled sludge returned to aerodigester
AEROBIC DIGESTION SCHEMATIC
-------
sludge
dewatering
The most widely used method for sludge de-
watering in the past has been drying the sludge
on sandbeds. These beds are especially popu-
lar in small plants because of their simplicity of
operation and maintenance. They are usually
constructed of a layer of 4-9 inches of sand
placed over 8-18 inches of gravel. Sludge is
drawn from the digester, placed on the sand-
bed, and allowed to stand until dried by a com-
bination of drainage and evaporation. Drainage
is collected in pipes beneath the gravel and
returned to the wastewater plant for treatment.
In good weather, the solids content can be in-
creased to 45 percent (resembling moist dirt)
within 6 weeks and can reach as high as 85-90
percent. Sandbeds have sometimes been en-
closed by glass, greenhouse-type structures to
protect the sludge from rain and reduce the
drying period. In small plants, the dried sludge
is usually removed from the drying beds by
hand, while larger plants often use mechanical
equipment. Although sandbeds are simple to
operate, the space requirements can be a dis-
advantage when secondary sludge is involved.
Unless the beds are covered, the performance
can be markedly affected by weather. With in-
creased use of secondary treatment, the use of
more compact and more controllable
mechanical-dewatering systems is increasing.
Such systems include vacuum filters, cen-
trifuges, and pressure filters.
A vacuum filter basically consists of a cylin-
drical drum covered with a filtering material or
fabric, which rotates partially submerged in a
vat of conditioned sludge. A vacuum is applied
T8ACKW6 ROLL
E06E SENSING DEVICE
BOWED ROU
DOCTOR BLADE
WASH SPRAYS
ADJUSTABLE ROLl
FILTER VAIVE
FILTER DRUM
62
-------
inside the drum to extract water, leaving the
solids or "filter cake" on the filter medium. As
the drum completes its rotational cycle, a blade
scrapes the filter cake from the filter and the
cycle begins again. In some systems, the filter
fabric passes off the drum over small rollers to
dislodge the cake. There is a wide variety of
filter fabrics, ranging from Dacron to stainless-
steel coils, each with its own advantages. The
vacuum filter can be applied to digested sludge
to produce a sludge cake dry enough (30-40
percent solids) to handle and dispose of by
burial in a landfill or by application to the land as
a relatively dry fertilizer. If the sludge is to be
incinerated, it is not necessary to stabilize the
sludge by digestion. In this case, the vacuum
filter is applied to the raw sludge to dewater it.
The sludge cake is then fed to the furnace to be
incinerated. The costs of vacuum filtration may
range from $20 to $60 per ton of dry solids; the
greater the proportion of activated sludge, the
greater the costs of dewatering and the wetter
the sludge cake. Vacuum filtration has been the
most popular mechanical sludge-dewatering
method in the municipal field, with over 1,500
installations. While this method has the disad-
vantage of requiring more skilled operation than
a drying bed, it has the advantages of occupy-
ing much less space and being more control-
lable in performance than a drying bed.
Centrifuges are also a popular means of de-
watering municipal sludges. A centrifuge uses
centrifugal force to speed up the separation of
sludge particles from the liquid. In a typical unit,
sludge is pumped into a horizontal, cylindrical,
"bowl," rotating at 1,600-2,000 rpm. Polymers
used for sludge conditioning also are injected
into the centrifuge. The solids are spun to the
outside of the bowl where they are scraped out
by a screw conveyor. The liquid, or "centrate," is
returned to the wastewater treatment plant for
-------
I A
-------
treatment. The centrifuging process is usually
comparable to vacuum filtration in costs and
performance. It has the advantages of being
entirely enclosed, which may reduce odors, re-
quiring a small amount of space, being able to
handle some sludges that might otherwise plug
vacuum filter media, and exerting large separa-
tional forces on the sludge. It has the disadvan-
tage of being complex to maintain because of
the high speed of the equipment. If grit and sand
are not carefully removed, abrasion problems
will occur in the centrifuge.
Pressure filtration is also an effective means
of sludge dewatering that is finding increased
use in the United States. Sludge is dewatered
by pumping it at high pressure (up to 225 psi)
through a filter medium that is attached to a
series of plates. These plates are held together
in a frame between one fixed end and one mov-
ing end. Sludge is pumped into the chambers
between plates, so that the water passes
through the filter medium and the solids are
retained. Eventually, the pressure filter fills with
sludge solids. Pumping of sludge is then dis-
continued, and the moving end of the press is
pulled back so that the individual plates can be
moved to dislodge the filter cake. After the cake
is removed, the plates are pushed back to-
gether by the moving end and the cycle begins
again. Pressure filtration offers the advantages
of providing the dryest cake achievable by
mechanical dewatering methods, producing a
very clear filtrate for return to the treatment
plant, and frequently reducing chemical condi-
tioning costs. It has the disadvantages of being
a batch-type operation requiring operator atten-
tion at the end of each cycle and of requiring
periodic washing of the filter medium. The costs
for the dewatering step alone are often com-
parable to vacuum filtration and centrifugation,
but the dryer cake produced (often 50 percent
solids) can provide savings in total sludge-
handling costs. Although popular in Europe for
years, pressure filtration only recently has
found extensive use in the municipal field in the
United States. Interest has been spurred by
recent improvements in equipment. Major sys-
tems are in operation at Cedar Rapids, Iowa,
and Kenosha, Wisconsin, with many more in
the design or construction stage.
65
-------
-------
use of sludge
as a soil conditioner
Municipal sludge contains essential plant nu-
trients and useful trace elements, and has po-
tential as a fertilizer or soil conditioner. Before
such use, the sludge is nearly always stabilized
by digestion or some other process to control
pathogenic bacteria and viruses and to mini-
mize the potential for odors. There are then
several alternative forms of the sludge that can
be used as fertilizer or soil conditioner: liquid
sludge directly from the stabilization process,
dewatered sludge, or dewatered and dried
sludge.
Several cities apply liquid sludge to crop-
lands. This practice has the advantage of
eliminating dewatering costs, but the disadvan-
tage of increasing the volume of sludge that
must be handled and applied to the land. Such
sludge is not used for root crops or crops con-
sumed by people in the raw because of health
considerations. It is frequently used for pasture-
land or corn, wheat, or forage crops. Smaller
towns often haul the sludge in trucks that also
spread the sludge on the land. Large cities usu-
ally find pumping the sludge through pipelines
to the disposal site to be the cheapest method
of sludge transportation. The largest operation
using liquid sludge in the United States is that of
the Metropolitan Sanitary District of Chicago.
Digested sludge is hauled to strip-mined land
200 miles from Chicago and applied by spray-
ing to restore the land to productive use. Even-
tually, 10,000 acres will be fertilized with the
sludge, which will be transported by pipeline (it
is being barged to the site until the pipeline is
built). Crops grown include corn, soybeans, and
On left, crop without sludge; on right, crop with sludge applied
-------
winter wheat. Once the pipeline is complete,
sludge disposal costs, including digestion, are
expected to be about $35 per ton of dry solids.
To reduce the volume of material handled,
dewatering is sometimes used before applying
the sludge to the land. In small plants, sludge
removed from drying beds is often stockpiled
for use by the city or by local citizens. Larger
cities may use mechanical dewatering sys-
tems, with the sludge cake hauled to the dis-
posal site where it is plowed into the ground.
Large drying lagoons at the disposal site are
planned by the Metropolitan Denver Sewage
Disposal District to accomplish dewatering.
Heat drying of dewatered sludge reduces the
volume even further and provides a safe prod-
uct from a health standpoint. Several major U.S.
cities, including Houston and Milwaukee, dry
their sludge for use as a soil conditioner. Hous-
ton's dried sludge is sold to a contractor in
Florida, who has been using the product in cit-
rus groves for over 10 years. The sludge is
transported by rail or barge. The Milwaukee
Sewerage Commission markets its heat-dried
activated sludge under the trade name "Milor-
ganite," and this is a widely used soil con-
ditioner. It is sold, in 50-pound bags, to large
distributors, who in turn market the material
through jobbers in all 50 States and some
foreign countries. An average analysis of Milor-
ganite showed 6 percent nitrogen, 4 percent
phosphate, 0.4 percent potash, 5 percent mois-
ture, and numerous beneficial trace elements.
Although they have recovered some of their
sludge-processing costs, neither Milwaukee
nor Houston has made a profit from sludge
processing and sales. A changing supply in
inorganic fertilizers may make this approach
attractive to other cities.
Crops being grown with sludge from Chicago
68
-------
o
CO
-------
sludge
reduction
If sludge use as a soil conditioner is not prac-
tical, or if a site is not available for landfill using
dewatered sludge, cities may turn to the alter-
native of sludge reduction. Incineration com-
pletely evaporates the moisture in the sludge
and combusts the organic solids to a sterile ash.
To minimize the amount of fuel used, the sludge
must be dewatered as completely as possible
before incineration. If the sludge is dry enough,
no fuel may be needed except to start up the
furnace. The exhaust gases from an incinerator
must be treated carefully to avoid air pollution.
EPA has developed standards that insure that
air quality will not be impaired by municipal
sludge incinerators. The two most widely used
sludge incineration systems in the United
States are the multiple-hearth furnace and the
fluidized-bed incinerator.
The multiple-hearth furnace is the most
widely used wastewater sludge incinerator in
the United States today. It is simple and dura-
ble, and has the flexibility of burning a wide
variety of materials. There are over 120 of these
units installed for wastewater-sludge combus-
tion. A typical multiple-hearth furnace consists
of a circular steel shell surrounding a number of
hearths. Dewatered sludge enters at the top
and proceeds downward through the furnace
from hearth to hearth, moved by the rotary ac-
tion of rabble arms driven by a central shaft.
Gas or oil burners furnish heat for startup of the
furnace and supplemental heat, if needed, to
keep the temperature in the lower part of the
furnace at 1,500° F or higher. The flue gases are
70
-------
passed through a scrubbing device to control
air pollution.
The first Jluidized-bed multiple sludge in-
cinerator was installed in 1962, and there are
now several units operating in the United
States. The fluidized-bed incinerator is a verti-
cal steel cylinder filled with a bed of hot sand.
Combustion air flows up through the bed of
sand at a rate high enough to fluidize the sand.
Dewatered sludge is injected into the fluidized
sand where it is burned at 1,400-1,500° F. The
sludge ash is carried out the top with the
exhaust gases, and is removed in the air pollu-
tion control process.
Costs of sludge incineration are typically
$40-$60 per ton, depending on plant size, fuel
costs, and sludge composition. Costs for
multiple-hearth and fluidized-bed systems are
comparable, and a careful evaluation of local
conditions is needed to determine if one system
will have an economic advantage over the
other. Each has its own operational advan-
tages. The multiple-hearth furnace has the ad-
vantages of simpler maintenance and opera-
tion, but the disadvantages of requiring longer
time periods for startup to avoid sudden
changes in temperature that would damage the
insulating bricks in the furnace. Also, the teeth
on the internal rabble arms in the multiple
hearth sometimes pose a maintenance prob-
lem. The fluidized-bed system has the advan-
tage of more efficient fuel use. Because the
sandbed retains heat even after operation has
stopped, the incinerator is better suited for in-
termittent operation (one shift per day, for
example). However, operation and mainte-
nance are more complex. Some problems of
scale accumulation on the sand have been re-
ported. Both systems have demonstrated their
ability to incinerate municipal sludges reliably
without creating air pollution problems.
As an alternative to burning, organic sludge
can be oxidized by a process called "wet air
oxidation." This process is based on the princi-
ple that any substance capable of burning can
be oxidized in the presence of liquid water at
temperatures between 250° F and 700° F. The
process can operate on difficult-to-dewater
sludges where the solids are but a small per-
centage of the water streams, eliminating the
need for dewatering. The sludge is passed
through a grinder, and then into a reactor where
high temperature (500° F or more) and pressure
(1,000-1,700 psi) are applied. At this tempera-
ture, the high pressure is needed to keep the
water from turning into steam. Air is injected
also to speed oxidation. The oxidized solids and
liquid can be separated by settling or by vac-
uum filtration or centrifuging. Wet air oxidation
has the advantage of eliminating the dewater-
ing step and minimizing air pollution potential,
because oxidation takes place in water without
producing exhaust gases containing flyash or
dust. It has the disadvantage of creating a liquid
very high in BOD, phosphorus, and nitrogen,
which must be recycled through the wastewater
treatment process, imposing a significant
added treatment burden on the secondary
process. Maintenance problems may be com-
plex and the high-pressure/high-temperature
system introduces some safety considerations.
AWT process
sludges
As noted earlier, the coagulation-
sedimentation process produces large volumes
of chemical sludges. No other AWT process
creates a significant sludge problem. Although
spent activated carbon might be considered a
waste solid, it is usually regenerated and
reused and is a relatively dry solid that is easily
handled. If lime is the coagulant used in
coagulation-sedimentation, the sludge can be
dewatered by the same techniques discussed
earlier (vacuum filters, centrifuges, filter press-
es). It can then be passed through either a
multiple-hearth or a fluidized-bed furnace in a
process called "recalcining." This process
drives off water and carbon dioxide, leaving a
reusable form of lime behind. Recalcining re-
duces the volume of new lime that must be
purchased, as well as the volume of sludge
residual for disposal. The lime sludge has also
been dewatered and buried in cases where re-
calcining economics were not favorable. The
costs of lime recalcining are typically $30-$40
per ton of lime recovered. While these costs
are, in most cases, about the same as buying
new lime, an overall savings may result in that
sludge disposal costs are reduced substantially
by recycling most of the chemical sludge rather
than disposing of it. Overall costs of dewatering
chemical sludges and their ultimate disposal
can add 5-15 cents per 1,000 gallons to the cost
of wastewater treatment. If salts of iron or
aluminum, such as alum or ferric chloride, are
used as the coagulant, the chemicals cannot be
recovered and reused for phosphorus removal.
These sludges, then, are dewatered, with the
same alternatives for disposal as the organic
sludges from secondary treatment.
71
-------
evaluating
alternatives
It should be apparent that there are many
alternatives available for wastewater treatment
and for handling the sludges produced. There is
no panacea; each wastewater treatment prob-
lem must be evaluated carefully in light of
specific local conditions to determine the best
solution. Among the factors that must be con-
sidered are
• Nature of the raw wasfes—Current or fu-
ture industrial wastes could have a sig-
nificant effect on the capacity and perform-
ance of the treatment facility. Industrial pre-
treatment requirements may minimize
these effects, but the process should be
flexible enough to accommodate variations
in pretreatment efficiency.
• Effluent requirements—The required ef-
fluent quality has an obvious, major impact
on process selection.
• Process reliability—It is important that the
processes selected provide the maximum
degree of reliability.
• Sludge production—The ability to handle
sludges produced by a candidate process
in an economical and environmentally
satisfactory manner is also a critical factor
in process evaluation.
• Air pollution—A careful evaluation of the
ultimate fate of pollutants removed from
the wastewater must be made to insure
that water pollution control has not been
achieved at the expense of air pollution.
-------
• Resource consumption—Wastewater
treatment cannot be achieved without the
expenditure of resources such as power
and chemicals. It is obviously desirable to
minimize the consumption of these re-
sources, and the relative consumption by
alternative processes is a factor for con-
sideration.
• Space requirements—The relative space
requirements of alternative processes are
a factor in process selection.
• Safety considerations—Any potential
hazards within the plant boundaries, or
those which could affect the surrounding
area as a result of plant malfunction or >
transport of materials to or from the plant,
must be considered.
• Cosfs—It is obviously important to select a
process that can achieve the project goals
in the most cost-effective manner within
the constraints imposed by the foregoing
considerations.
73
-------
-------
slliltlll^^
':tfe)|ii^^
^•^^•^•^^^^•^'V-^-'^" -"ci*J!*i&^
ORGANIC SLUDGE!
HANDLING
_r
Land application (fertilizer)
f- Digestion — j _ Bury
i~ Dewatef — 1 Land application (fertilizer)
LDry - fertilizer
Dewater
burn
AVTT CHiM(CAL
SLUDGES
Wet air oxidation
— Recovery and reuse
_ Disposal
The best alternative system for pollutant re-
moval must be selected based on a case-by-
case study of efficiency and actual costs. Illus-
trative cost ranges have been presented
throughout this report. To illustrate the costs
associated with each increment of quality im-
provement, the next few pages present an
example of how processes may be added to a
conventional secondary plant to achieve re-
moval of pollutants such as phosphorus, nitro-
gen, and COD and further removal of BOD and
suspended solids, and the resulting costs. Of
course, there are many possible combinations
and process sequences that could be used. For
example, nitrogen removal could occur im-
mediately after the secondary process rather
than at the point shown. The overall costs would
not be affected, however. The example shows
how available processes can be added step by
step to an existing system in modular incre-
ments as needed if treatment standards con-
tinue to become more rigid. Current EPA re-
search efforts are aimed at finding more
economical processes to achieve high levels of
treatment. However, proven treatment technol-
ogy is available today to eliminate municipal
wastewaters as a significant source of pollution
and to convert them to a valuable resource if
water reuse is needed in an area.
tE AjLfERNAflVESRetATIVtTO REQUW1M4TS FOR KJtLWTAOT REMOVAL
75
-------
05
£ §
CENTS PER 1,000 GALLONS
S> £ ?F
lilifHI 11
ilffP'lfl
Ifssliil 18
DOLLARS PER MONTH PER HOME
-------
CENTS PER 1,000 GALLONS
• »
1
DOLLARS PER MONTH PER HOME
-------
. 2.65
CARBON ADSORPTION
-------
Many photographs in this publication were fur-
nished by:
Air Products
Autotrol Corporation
Beloit-Passavant Corporation
Calgon
Dorr-Oliver, Inc.
Dravo Corporation
Ecodyne
Envirotech Corporation
FMC Corporation
Ingersoll-Rand
Komline-Sanderson
Lakeside Equipment Co.
Lockwood Corporation
McDowell Co.
Metropolitan Denver Sewage Disposal District
Metropolitan Sanitary District of the County of
Milwaukee
Metropolitan Sanitary District of Greater
Chicago
Neptune Microfloc
Peabody Wells
Zimpro
Zurn Industries, Inc.
79
------- |