EPA-430/1-76-018
ORIENTATION TO WASTEWATER
TREATMENT OPERATION
TRAINING MANUAL
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
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EPA-430/1-76/018
January 1975
ORIENTATION TO WASTEWATER TREATMENT OPERATION
This course is designed for water pollution agency
personnel who are relatively unfamiliar with waste-
water treatment methods and treatment plant
operation, and whose job functionH require a general
knowledge in these areas. An overview of the
principal waste treatment processes currently in use
la provided. General design and operational requirements
for treatment facilities are considered.
On completion of the course the student will have a
working knowledge of wastewater treatment processes,
design parameters, and operation and maintenance
requirements (or effective treatment. He will
understand the function of individual treatment units.
and will be able to evaluate the capability of a treatment
facility to produce a desired degree of treatment.
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Water Program Operations
National Training and Operational Technology Center
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DISCLAIMER
Reference to commercial products, trade names, or
manufacturers is for purposes of example and illustration.
Such references do not constitute endorsement by the
Office of Water Program Operations, U. S. Environmental
Protection Agency.
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CONTENTS
Title or Description Chapter Number
Introduction 1
Why Treat Wastes 2
Wastewater Facilities 3
Pretreatment: Racks, Screens, Comminucors and 4
Grit Removal
Sedimentation and Flotation 5
Trickling Filters 6
Activated Sludge 7
Sludge Digestion and Handling 8
Waste Treatment Ponds 9
Disinfection and Chlorination 10
Flow Measurements 11
Plant Safety and Good Housekeeping 12
Sampling 14
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INTRODUCTION
The material herein consists of extracts from a manual used in a home study
program for wastewater treatment plant operators, developed by Sacramento
State College, in cooperation with the California Water Pollution Control
Association, under a Technical Training Grant from the Water Quality Office,
U. S. EPA. Only a small portion of the content of the manual is included.
Much material dealing with details of plant operation and maintenance,
laboratory analyses, Mathematics, Analysis and Presentation of Data, and Report
Writing, has not been included, since it is not considered pertinent to the
objectives of this course.
If you are interested in obtaining the complete home study manual, or in
participating in the home study program write to:
Professor Kenneth Kerri
Department of Civil Engineering
Sacramento State College
6000 Jay Street
Sacramento, California 95819
Title of the manual is "Operation of Wastewater Treatment Plants: A Field
Study Training Program"
Charge for the manual alone is $25.00. Charge for taking the course is $30.00.
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CHAPTER 2. WHY TREAT WASTES?
2.0 PREVENTION OF POLLUTION
The operator's main job is to protect the many users of receiving
waters. He must do the best he can to remove any substances
which will unreasonably affect these users.
Many people think any discharge of waste to a body of water is
pollution. However, with our present system of using water to
carry away the waste products of home and industry, it would be
impossible and perhaps unwise to prohibit the discharge of all
wastewater to oceans, streams, and groundwater basins. It is
possible under present day technology to treat wastes in such
a manner that existing or potential receiving water uses are not
unreasonably affected. Definitions of pollution include any
interference with beneficial reuse of water or failure to meet
water quality requirements. Any questions or comments regarding
this definition must be settled by the appropriate enforcement
agency.
2.1 WHAT IS PURE WATER?
Water is a combination of two
parts hydrogen and one part
oxygen, or H20. This is true,
however, only for "pure" water
such as might be manufactured
in a laboratory. Water as we
know it is not "pure" hydrogen
and oxygen. Even the distilled
water we purchase in the store
has measurable quantities of
various substances in addition
to hydrogen and oxygen. Rain
water, even before it reaches
the earth, contains many sub-
stances. These substances,
since they are not found in
"pure" water, may be consider-
ed "impurities". When rain
falls through the atmosphere,
it gains nitrogen and other
gases. As soon as the rain
flows overland it begins to
Fig. 2.1 Water + Impurities
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dissolve from the earth and rocks such substances as calcium,
magnesium, sodium, chlorides, sulfates, iron, nitrogen,
phosphorus, and many other materials. Organic matter (matter
derived from plants and animals) is also dissolved by water
from contact with decaying leaves, twigs, grass, or small
insects and animals. Thus it should be realized that a fresh
flowing mountain stream may pick up many natural "impurities",
some possibly in harmful amounts, before it ever reaches
civilization or is affected by the waste discharges of man.
Many of these substances, however, are needed in small amounts
to support life and be useful to man. Concentrations of im-
purities must be controlled or regulated to prevent harmful
levels in receiving waters.
QUESTIONS
2.1A What are some of the dissolved substances
in water?
2.IB How does water pick up dissolved substances?
2.2 TYPES OF WASTE DISCHARGES
The waste discharge that first comes to mind in any discussion
of stream pollution is the discharge of domestic wastewater.
Wastewater contains a large amount of organic waste.1 Industry
also contributes substantial amounts of organic waste. Some
of these organic industrial wastes come from vegetable and
fruit packing; dairy processing; meat packing; tanning; and
processing of poultry, oil, paper and fiber (wood), and many
more.
1 Organic waste (or-GAN-nick). Waste material which comes from
animal or vegetable origin. Organic waste generally will be
consumed by bacteria and other small organisms. Inorganic
wastes are chemical substances of mineral origin and may con-
tain carbon and oxygen, whereas organic wastes contain mainly
carbon and hydrogen along with other elements.
2-2
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Another classification of wastes is inorganic wastes.2 Domestic
wastewater contains inorganic material as well as organic, and many
industries discharge inorganic wastes which add to the mineral
content of receiving waters. For instance, a discharge of salt
brine (sodium chloride) for water softening will increase the
amount of sodium and chloride in the receiving waters. Some
industrial wastes may introduce inorganic substances such as
chromium or copper, which are very toxic to aquatic life. Other
industries (such as gravel washing plants) discharge appreciable
amounts of soil, sand or grit, which also may be classified as
inorganic waste.
There are two other major types of wastes that do not fit either
the organic or inorganic classification. These are heated
(thermal) wastes and radioactive wastes. Waters with temper-
atures exceeding the requirements of the enforcing agency may
come from cooling processes used by industry and from thermal
power stations generating electricity. Radioactive wastes
are usually controlled at their source, but could come from
hospitals, research laboratories, and nuclear power plants.
QUESTIONS
2.2A Several of the following contain significant
quantities of organic material. Which are
they?
a. Domestic Wastewater
b. Cooling Water from Thermal Power Stations
c. Paper Mill Wastes
d. Metal Plating Wastes
e. Tanning Wastes
2.2B List four types of pollution.
2.3 EFFECTS OF WASTE DISCHARGES
Certain substances not removed by wastewater treatment processes
can cause problems in receiving waters. This section reviews
some of these substances and discusses why they should be treated.
2 Inorganic waste (IN-or-GAN-nick). Waste material such as sand,
salt, iron, calcium, and other mineral materials which are not
converted in large quantities by organism action. Inorganic
wastes are chemical substances of mineral origin and may contain
carbon and oxygen, whereas organic wastes are chemical substances
of animal or vegetable origin and contain mainly carbon and
hydrogen along with other elements.
2-3
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2.30 Sludge and Scum
If certain wastes (including domestic wastewater) do not
receive adequate treatment, large amounts of solids may
accumulate on the banks of the receiving waters, or they
may settle to the bottom to form sludge deposits or float
to the surface and form rafts of scum. Sludge deposits
and scum are not only unsightly; but if they contain
organic material, they may also cause oxygen depletion and
be a source of odors. Primary treatment3 units in the waste-
water treatment plant are designed and operated to remove
the sludge and scum before they reach the receiving waters.
2.31 Oxygen Depletion
Most living creatures need oxygen to survive, including fish
and other aquatic life. Although most streams and other
surface waters contain less than 0.001% dissolved oxygen
(10 milligrams of oxygen per liter of water, or 10 mg/1),4
most fish can thrive if there are at least 5 mg/1 and other
conditions are favorable. When oxidizable wastes are dis-
charged to a stream, bacteria begin to feed on the waste
and decompose or break down the complex substances in the
waste into simple chemical compounds. These bacteria also
use dissolved oxygen (similar to human respiration or
breathing) from the water and are called aerobic bacteria.5
As more organic waste is added, the bacteria reproduce
3 Primary treatment. A wastewater treatment process consist-
ing of a rectangular or circular tank which allows those
substances in wastewater that readily settle or float to
be separated from the water being treated.
4 Milligrams per liter, mg/1 (MILL-i-GRAMS per LEET-er). A
measure of the concentration, by weight of a substance per
unit volume. For practical purposes, one mg/1 is equal to
one part per million parts (ppm). Thus, a liter of water
with a specific gravity of 1.0 weighs one million milligrams;
and if it contains 10 milligrams of dissolved oxygen, the
concentration is 10 milligrams per million milligrams, or
10 milligrams per liter (10 mg/1), or 10 parts of oxygen per
million parts of water, or 10 parts per million (10 ppm).
5 Aerobic bacteria (AIP.-0-bick back-TEAR-e-ah) . Bacteria
which will live and reproduce only in an environment con-
taining oxygen which is available for their respiration,
such as atmospheric oxygen or oxygen dissolved in water.
Oxygen combined chemically, such as in water molecules,
II20, cannot be used for respiration by aerobic bacteria.
2-4
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rapidly; and as their population increases, so does their use
of oxygen. Where waste flows are high the population of
bacteria may grow large enough to use the entire supply of
oxygen from the stream faster than it can be replenished by
natural diffusion from the atmosphere. When this happens
fish and most other living things in the stream which require
dissolved oxygen die.
Fig. 2.2 Oxygen depletion
Therefore, one of the principal objectives of wastewater treat-
ment is to prevent as much of this "oxygen-demanding" organic
material as possible from entering the receiving water. The
treatment plant actually removes the organic material the same
way a stream does, but it accomplishes the task much more
efficiently by removing the wastes from the wastewater.
Secondary treatment6 units are designed and operated to use
natural organisms such as bacteria in the plant to stabilize7
and remove organic material.
Another effect of oxygen depletion, in addition to the killing
of fish and other aquatic life, is the problem of odors. When
6 Secondary treatment. A wastewater treatment process used to
convert dissolved or suspended materials into a form more
readily separated from the water being treated.
7 Stabilize. To convert to a form that resists change.
Organic material is stabilized by bacteria which convert
the material to gases and other relatively inert substances.
Stabilized organic material generally will not give off
obnoxious odors.
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all the dissolved oxygen has been removed, anaerobic bacteria8
begin to use the oxygen which is combined clleinica1 ly w ith
other elements in the form of chemical compounds, such as
sulfate (sulfur and oxygen), which are also dissolved in the
water. When anaerobic bacteria reirove the oxygen froir. sulfur
compounds, hydrogen sulfide (H2S) is released which has a
"rotten egg" odor. This gas is not only very odorous, but it
also erodes concrete and can discolor and remove paint from
homes and structures. Hydrogen sulfide also may form explosive
mixtures with air and is capable of paralyzing your respiratory
center. Other products of anaerobic decomposition (putrefaction:
PU-tree-fack-SHUH) also can be objectionable.
2.32 Other Effects
Some wastes adversely aifect the clarity and color of the
receiving waters, making them unsightly and unpopular for
recreation.
Many industrial wastes are highly acid or alkaline, and
either condition can interfere with aquatic life, domestic
use, and other uses. An accepted measurement of a waste's
acidity or alkalinity is its pH.9 Before wastes are dis-
charged they should have a pHTimilar to that of the receiving
water.
Waste discharges may contain toxic substances, such as heavy
metals or cyanide, which may affect the use of the receiving
water for domestic purposes or for aquatic life.
8 Anaerobic bacteria (AN-air-0-bick back-TEAR-e-ah). Bacteria
tliat live and reproduce in an environment containing no
"iree" or dissolved oxygen, Anaerobic bacteria obtain
their oxygen supply by breaking down chemical compounds
which contain oxygen, such as sulfate (SCiJ and nitrate
(NC'3) .
9 pH. Technically, this is the logarithm o± the reciprocal
of the hydrogen-ion concentration, which will be explained
in Chapter 14, Laboratory Procedures and Chemistry. For
now, it is sufficient to understand that pH may range
from 0 to 14, where 0 is most acid and 14 is most alkaline,
and 7 is neutral. Most natural waters have a pH between
6.5 and 8.5.
2-6
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Taste- and odor-producing substances may reach levels in the
receiving water which are readily detectable in drinking water
or in the flesh of fish.
Treated wastewaters contain nutrients10 capable of encourag-
ing excess algae and plant growth in receiving waters. These
growths hamper domestic, industrial, and recreational uses.
Conventional wastewater treatment plants do not remove a
major portion of the nitrogen and phosphorus nutrients.
QUESTIONS
2.3A What causes oxygen depletion when organic
wastes are discharged to the water?
2.3B What kind of bacteria cause hydrogen sulfide
gas to be released?
2.33 Human Health
Up to now we have discussed the physical or chemical effects
that a waste discharge may have on the uses of water. More
important, however, may be the effect on human health through
the spread of disease-producing bacteria and viruses. Initial
efforts to control human wastes evolved from the need to prevent
the spread of diseases. Although untreated wastewater contains
many billions of bacteria per gallon, most of these are not
harmful to humans, and some are even helpful in wastewater
treatment processes. However, humans who have a disease which
is caused by bacteria or viruses may discharge some of these
harmful organisms in their body wastes. Many serious outbreaks
of communicable diseases have been traced to direct contamination
of drinking water or food supplies by the body wastes from a
human disease carrier.
10 Nutrients. Substances which are required to support living
plants and organisms. Major nutrients are carbon, hydrogen,
oxygen, sulfur, nitrogen and phosphorus. Nitrogen and
phosphorus are difficult to remove from wastewater by con-
ventional treatment processes because they are water soluble
and tend to recycle.
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Some known examples of diseases whi-ch may be spread through
wastewater discharges are:
Fig. 2.3 Diseases
Fortunately these organisms that grow in the intestinal tract
of diseased humans are not likely to find the environment in
the wastewater treatment plant or receiving waters favorable
for their growth and reproduction. Although many of these
pathogenic organisms11 are removed by natural die-off during
the' normal" treatment processes, sufficient numbers can remain
to cause a threat to any downstream use involving human contact
or consumption. If these uses exist downstream, the
treatment plant must also include a disinfection12 process.
The disinfection process historically employed is the addition
of chlorine. Proper chlorination of a we 11-treated waste will
usually result in essentially a complete kill of these patho-
genic organisms. The operator must realize, however, that
11 Pathogenic organisms (path-o-JEN-nick OR-gan-iz-ums).
Bacteria or viruses which can cause disease. There are
many types of bacteria which do not cause disease and
which are not called pathogenic.
12 Disinfection (DIS-in-feck-shun). The process by which
pathogenic organisms are killed. There are several ways
to disinfect, but chlorination is the most frequently
used method in water and wastewater treatment.
2-8
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breakdown or malfunction of equipment could result in the
discharge at any time of an effluent which contains patho-
genic organisms.
QUESTIONS
2.3C Where do the disease-causing organisms
in wastewater come from?
2.3D What is the term which means "disease-
causing"?
2.3E What is the most frequent means of dis-
infecting treated wastewater?
2.4 SOLIDS IN WASTEWATER
One of the primary functions of a treatment plant is the
remc\al of solids from wasiewater.
2.40 Types of Solids
In Section 2.2 you read about the different types oi7 pollution:
organic, inorganic, thermal, and radioactive. For a normal
municipal wastewater which contains domestic wastewater as well
as some industrial and commercial wastes, the concern of the
treatment plant designer and operator usually is to remove the
organic and inorganic suspended solids, to remove the dissolved
organic solids (the treatment plant does little to remove
dissolved inorganic solids), and to kill the pathogen!c organ is ms
by disinfection. Thermal and radioactive wastes require special
treatment.
Since the main purpose of the treatment plant is removal of
solids from the wastewater, a detailed discussion of the types
of solids is in order. Figure 2.4 will help you understand the
different terms.
2.41 Total Solids
For discussion purposes assume that you obtain a one-liter
sample of raw wastewater entering the treatment plant. Heat
this sample enough to evaporate all the water and weigh all
the solid material left (residue); it weighs 1000 milligrams.
Thus, the total solids concentration in the sample is lOOn
milligrams per liter (mg/1). This weight includes both
dissolved and suspended solids.
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2.42 Dissolved Solids
How much is dissolved and how much is suspended? To determine
this you could take an identical sample and filter it through a
very fine-mesh filter such as a membrane filter or fiberglass.
The suspended solids will be caught on the filter, and the
dissolved solids will pass through with the water. You can
now evaporate the water and weigh the residue to determine the
weight of dissolved solids. In Fig. 2.4 the amount is shown
as 800 mg/TiThe remaining 200 mg/1 is suspended solids,
Dissolved solids are also called fi1terable residue.
2.43 Suspended Solids
Suspended solids are composed of two parts: settleable and
nonsettleable. The difference between settleable and nonset-
tleable solids depends on the size, shape, and weight per unit
volume of the solid particles; larger-sized particles tend to
settle more rapidly than smaller particles. It is important
to know the amount of settleable solids in the raw wastewater
for design of settling basins [primary units), sludge pumps,
and sludge handling facilities. Also, measuring the amount
of settleable solids entering and leaving the settling
basin allows you to calculate the efficiency of the basin
for removing the settleable solids. A device called an
Imhoff Cone*3 is used to measure settleable solids in
milliliters per liter, ml/1. (The example in Fig. 2.4 shows
a settleable solids concentration of 130 mg/1. The settled
solids in the Imhoff Cone had to be dried and weighed by proper
procedures to determine their weight.
It is possible to calculate the weight of nonsettleable sondb
by subtracting the weight of dissolved and settleable solids
from the weight of total solids. In Fig. 2.4 the nonsettleable
solids concentration is shown as 70 mg/1. Suspended solids are
also called nonfilterable residue.
13 Imhoff Cone. A clear cone-shaped container marked
with graduations used to measure the volumetric
concentration of settleable solids in wastewater.
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99.9%
I
\00%
1OOO
mg/1
800
mg/1
me/I
sg/i^r
\*>0
mg/1
I
•^mi/iwuuuar
40 WPS
j
T
OR
1.IT02 6V
3
Fig. 2.4 Composition of solids in raw wastewater
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2.44 Organic and Inorganic Solids
For total solids or for any separate type of solids, such as
dissolved, settleable, or nonsettleable, the relative amounts
of organic and inorganic matter can be determined. This
information is important for estimating solids handling
capacities and for designing treatment processes for removing
the organic portion in waste. The organic portion can be
very harmful to receiving waters.
2.45 Floatable Solids
There is no standard method for the measurement and evaluation
of floatable solids. Since treatment units are designed to
remove these solids, it is important for you to be aware of
floatable solids in raw wastewater and treated effluent.
Floatable solids are undesirable in the plant effluent from an
aesthetic viewpoint because the sight of floatables in receiv-
ing waters indicates the presence of inadequately treated
wastewater.
2.5 ADDITIONAL READING
For a detailed discussion of the physical and chemical compo-
sition of wastewater you may wish to refer to:
1. MOP 11, pp 4-7
2. New York Manual, pp 1-10
3. Texas Manual, pp 1-18
QUESTIONS
2.4A An Imhoff Cone is used to measure
solids.
2.4B Why is it necessary to measure settleable solids?
2.4C Total solids are made up of and
solids, both of which contain
organic and inorganic matter.
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CHAPTER 3. WASTEWATER FACILITIES
3.0 COLLECTION, TREATMENT, DISPOSAL
Facilities for handling wastewater are usually considered to have
three major components or parts: collection, treatment, and
disposal. For a municipality, these components make up the
"sewerage" system or wastewater facilities; but for an individual
industry which handles its own wastewater, the same three components
are necessary. This training course is directed primarily to plant
operators for municipalities, so the discussion in this and later
chapters will be in terms of municipal wastewater facilities.
3.1 COLLECTION OF WASTEWATER
Collection and transportation of wastewater to the treatment plant
is accomplished through a complex network of pipes and pumps of
many sizes.
Major water using industries which contribute waste to the collec-
tion system may affect the efficiency of a wastewater treatment
plant, especially if there are periods during the day or during
the year when these industrial waste flows are a major load on
the plant. For instance, canneries are highly seasonal in their
operations; therefore, it is possible to predict the time of year
to expect large flows from them. A knowledge of the location of
commercial and industrial dischargers in the collection system
may enable an operator to locate the source of a problem in the
plant influent, such as oil from a refinery or a gas station.
The length of time required for wastes to reach your plant can
also affect treatment plant efficiency. Hydrogen sulfide gas
(rotten egg gas) may be released by anaerobic bacteria feeding
on the wastes if the flow time is quite long and the weather is
hot; this can cause odor problems, damage concrete in your plant,
and make the wastes more difficult to treat. (Solids won't settle
easily, for instance.) Wastes from isolated subdivisions located
far away from the main collection network often have this "aging"
problem.
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3.10 Sanitary, Storm, and Combined Sewers
For most sewerage systems the sewer coming into the treatment
plant carries wastes from households and commercial establish-
ments in the city or district, and possibly some industrial
waste. This type of sewer is called a sanitary sewer.* All
storm runoff from streets, land, and roofs of buildings is
collected separately in a storm sewer,2 which normally dis-
charges to a water course without treatment. In some areas
only one network of sewers has been laid out beneath the
city to pick up both sanitary wastes and storm water in a
combined sewer.3 Treatment plants that are designed to handle
the sanitary portion of the wastes sometimes must be bypassed
during storms due to inadequate capacity, allowing untreated
wastes to be discharged into receiving waters. Separation of
combined sewers into sanitary and storm sewers is very costly
and difficult to accomplish,
Even in areas where the sanitary and storm sewers are separate,
infiltration'* of groundwater or storm water into sanitary
sewers through breaks or open joints can cause high flow
problems at the treatment plant. Replacement or sealing of
leaky sections of sewer pipe is called for in these cases.
The treatment plant operator is generally the first to know
about infiltration problems because of the unusually high
flows he observes at the plant during periods of storm water
runoff.
1 Sanitary Sewer (SAN-eh-tar-ee SUE-er). A sewer intended
to carry wastewater from homes, businesses, and industries.
2 Storm Sewer. A separate sewer that carries runoff from
storms, surface drainage, and street wash, but that ex-
cludes domestic and industrial wastes.
3 Combined sewer. A sewer designed to carry both sanitary
wastewaters and storm or surface water runoff.
4 Infiltration (IN-fill-TRAY-shun). Groundwater that seeps
into pipes through cracks, joints, or breaks.
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Sanitary sewers are normally placed at a slope sufficient to
produce a velocity of approximately two feet per second. This
velocity will usually prevent the deposition of solids that may
clog the pipe or cause odors. Manholes are placed every 300 to
500 feet to allow for
inspection (Fig. 3.1) and
cleaning of the sewer.
When low areas of land must
be sewered or where pipe
depth under the ground
surface becomes excessive,
pump stations (Fig. 3.2)
are normally installed.
These pump stations lift
the wastewater to a higher
point from which it may
again flow by gravity,
or the wastewater may be
pumped under pressure
directly to the treatment
plant. A large pump station
located just ahead of the
treatment plant can create
problems by periodically
sending large volumes of
flow to the plant one minute,
and virtually nothing the
next minute.
Fig. 3.1
Manholes allow
inspection of the
collection system
QUESTIONS
3.1A Why should the operator be familiar with the
wcstewater collection and transportation network?
3.IB List three types of sewers.
3.1C What problem may occur when it takes a long time
for wastewater to flow through the collection
sewers to the treatment plant?
3.ID Why are combined sewers a problem?
3-3
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3.2 TREATMENT PLANTS
Upon reaching a wastewater treatment plant, the wastewater flows
through a series of treatment processes (Fig. 3.3) which remove
the wastes from the water and reduce its threat to the public
health before it is discharged from the plant. The number of
treatment processes and the degree of treatment usually depend
on the uses of the receiving waters. Treated wastewaters dis-
charged into a small stream used for a domestic water supply
and swimming will require considerably more treatment than waste-
water discharged into water used solely for navigation.
To provide you with a general picture of treatment plants, the
remainder of this chapter will follow the paths a drop of waste-
water might travel as it passes through a plant. You will be
introduced to the names of the treatment processes, the kinds
of wastes the processes treat or remove, and the location of the
processes in the flow path. Not all treatment plants are alike;
however, there are certain typical flow patterns that are
similar from one plant to another.
When wastewater enters a treatment plant, it usually flows through
a series of pretreatment processes—screening, shredding, and
grit removal. These processes remove the coarse material from
the wastewater. Flow-measuring devices are usually installed
after pretreatment processes to record the flow rates and
volumes of wastewater treated by the plant.
N'ext the wastewater will generally receive primary treatment.
During primary treatment some of the solid matter carried by
the wastewater will settle out or float to the water surface
where it can be separated from the wastewater being treated.
Secondary treatment processes usually follow primary treatment
and commonly consist of biological processes. This means that
organisms living in the controlled environment of the process
are used to partially stabilized (oxidize) organic matter not
removed by previous treatment processes and to convert it into
a form which is easier to remove from the wastewater.
Waste material removed by the treatment processes goes to
solids handling facilities and then to ultimate disposal.
Waste treatment ponds may be used after pretreatment, primary
treatment, or secondary treatment. Ponds are frequently con-
structed in rural areas where there is sufficient available
land.
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WAT-EC.
Fig. 3.2 Collection sewer profile
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PJ2/MAQY
ANT?
REMOV& ZOC&, JZOOTS
ANP
SAMP AND 4PAV&L
H&LP4
OIL
Fig. 3.3 Flow diagram of wastewater treatment plant processes
3-6
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••• f
BAR SCREEN
Hand raked
Plolform
Str.Tn
MAN
ftiVATlON
(Courtesy Water Pollution Control Federation)
Fig. 3.4 Bar screens
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Advanced methods of waste treatment are being developed for
general cleanup of wastewater or removal of substances not
removed by conventional treatment processes. They may follow
the treatment processes previously described, or they may be
used instead of them. Before treated wastewater is discharg-
ed to the receiving waters, it should be disinfected to
prevent the spread of disease.
In the following sections these treatment processes will be
briefly discussed to provide an overall concept of a treatment
plant. Details will be presented in later chapters to provide
complete information on each of these processes.
3.3 PRETREATMENT
3.30 General
Pretreatment processes commonly consist of screening, shredding,5
and grit removal to separate coarse material from the wastewater
being treated.
3.31 Screening
Wastewater flowing into the
treatment plant will occasionally
contain pieces of wood, roots,
rags, and other debris. To pro-
tect equipment and reduce any
interference with in-plant flow,
debris and trash are usually
removed by a bar screen (Fig.
3.4). Most screens in treatment
plants consist of parallel bars
placed at an angle in a channel
in such a manner that the waste-
water flows through the bars.
Trash collects on the bars and
is periodically raked off by
hand or by mechanical means.
In most plants these screenings
are disposed of by burying or
burning. In some cases they are
automatically ground up and re-
turned to the wastewater flow for
removal by a later process.
Fig, 3.5 Screened
§ ground
5 Shredding. A mechanical treatment process which cuts large
pieces of wastes into smaller pieces so they won't plug pipes
or damage equipment (comminution).
3-8
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3.32 Shredding
Devices are also available which cut up or shred material while
it remains in the wastewater stream. The most common of these
are the barminutor (Fig. 3.6) and the comminutor (Fig. 3.7).
One of these devices usually follows a bar screen.
Fig. 3.6 Barminutor Fig. 3.7 Comminutor
(Courtesy Chicago Pump)
3-9
-------
3.33 Grit Chambers
Most sewer pipes are laid at a slope steep enough to maintain a
wastewater flow of two feet per second (fps). If the velocity
is reduced slightly below
that, say to 1.5 fps,
some of the larger,
heavier particles will
settle out. If the
velocity is reduced to
about 1 fps, heavy
inorganic material such
as sand, eggshells, and
cinders will settle; but
the lighter organic
material will .remain in
suspension. The settled
inorganic material is
referred to as grit.6
Grit should be removed
early in the treatment
process because it is
abrasive and will rapidly
wear out pumps and other
equipment. Since it is
mostly inorganic, it can-
not be broken down by any
biological treatment pro-
Fig. 3.8 Removal of eggshells
cess and thus should be
removed as soon as possible,
Grit is usually removed in a long, narrow trough called a Grit
Chamber (Fig. 3.9). The chamber is designed to provide a flow-
through velocity of 1 fps. The settled grit may be removed
either by hand or mechanically. Since there is normally some
organic solid material deposited along with the grit, it is
usually buried to avoid nuisance conditions. Some plants are
equipped with "grit washers" that clean some of the organic
material out of the grit so that organic solids can remain in
the main waste flow to be treated.
Many treatment plants have aerated grit chambers in which com-
pressed air is added through diffusers to provide better separation
of grit and other solids. Aeration in this manner also "freshens"
a "stale" or septic wastewater, helping to prevent odors and
assist the biological treatment process.
6 Grit. The heavy mineral material present in wastewater,
such as sand, gravel, cinders, and eggshells.
3-10
-------
jye?.^7 V :
\ff^s^-^^t--'
Srfe.- '**"-.*
-* — ^ • ^
Fig. 3.9 Grit chamber
WPCF MOP No. 11,
Operation of Wastewater Treatment Plants
QUESTIONS
3.3A Why is grit removed early in the treatment
process?
3.3B What is usually done with grit which has been
removed from the wastewater?
3-11
-------
3.4 FLOW MEASURING DEVICES
Although flow measuring devices are not for treating wastes,
it is necessary to know the quantity of wastewater flow so
adjustments can be made on pumping rates, chlorination rates,
aeration rates, and other processes in the plant. Flow rates
must be known, also, for calculation of loadings on treatment
processes and treatment efficiency. Most operators prefer to
have a measuring device at the headworks of their treatment
plant.
The most common measuring device is a Parshall Flume (Fig. 3.10).
Basically it is a narrow place in an open channel which allows
the quantity of flow to be determined by measuring the depth of
flow. It is a widely used method for measuring wastewater because
its smooth constriction does not offer any protruding sharp
edges or areas where wastewater particles may catch or collect
behind the metering device.
Another measuring device used in open channels is a weir7 (Fig.
3.10). A weir is a wall placed across the channel over which
the waste may fall. It is usually made of thin metal and may
have either a rectangular or V-notch opening. Flow over the
weir is determined by the depth of waste going through the
opening. A disadvantage of a weir is the relatively dead
water space that occurs just upstream of the weir. If the
weir is used at the head end of the plant, organic solids
may settle out in this area. When this occurs odors and
unsightliness can result. Also, as the solids accumulate the
flow reading may become incorrect.
A good measuring device for flows of treated or untreated waste-
water is a Venturi meter (Fig. 3.10). It is a special section
of contracting pipe, and it measures flow in much the same way
as a Parshall Flume. It does not offer any sharp obstructions
for particles to catch on. Magnetic flow meters (Fig. 3.10)
also are being used successfully to measure wastewater flows.
7 Weir (weer). A vertical obstruction such as a wall or plate,
placed in an open channel and calibrated in order that a
depth of flow over the weir can easily be converted to a
flow rate in MGD (million gallons per day).
3-12
-------
VENTURI METER
C1«««i»j »ot»n
End Photo of Parshall Flume
(Drauinga courtesy of
Pollution Control Federation)
Fig. 3.10 Flow meters
-------
3.5 PRIMARY TREATMENT
We have previously discussed the reduction in velocity of the
incoming waste to approximately one foot per second in order
to settle out heavy inorganic material or grit. The next step
in the treatment process is normally called sedimentation or
primary treatment. In this process the waste is' directed" into
and through a large tank or basin. Flow velocity in these
tanks is reduced to about 0.03 foot per second, allowing the
settleable solids to fall to the bottom of the tank, thus
making the wastewater much clearer. It has therefore become
common practice to call these sedimentation tanks "clarifiers".
The first clarifier that the wastewater flows into is called a
primary clarifier. We will discuss later the need for another
clarifier after the biological treatment process. This second
clarifier is called a secondary clarifier.
Clarifiers normally are either rectangular (Fig. 3.11) or
circular fFig. 3.12). Primary clarifiers are usually designed
to provide 1.5 to 2 hours detention time.8 Secondary clarifiers
usually provide slightly more time.
Generally the longer the detention time provided, the more
removal of solids that takes place. In a tank with two hours
detention time, approximately 60 percent of the suspended solids
in the raw wastewater will either settle to the bottom or float
to the surface and be removed. Removal of these solids will
usually reduce the Biochemical Oxygen Demand (BOD)9 of the
waste approximately 30 percent. The exact removal depends on
the amount of BOD contained in the settled material.
All primary clarifiers, no matter what their shape, must have
a means for collecting the settled solids (called sludge10) and
8 Detention Time. The time required to fill a tank at a
given flow or the theoretical time required for a given
flow quantity of wastewater to flow through the tank.
9 Biochemical Oxygen Demand or BOD (BUY-o-KEM-ik-cull
OX-zi-gen de-MAND). The BOD indicated the rate of
oxygen utilized by wastewater under controlled con-
ditions of temperature and time.
10 Sludge (sluj). The settleable solids separated from
liquids during processing or deposits on bottoms of
streams or other bodies of water.
3-U
-------
SLUDGE COLLECTOR
DRIVE UNIT
EFFLUENT IEIRS
SLUDGE COLLECTOR CHAIN
AND FLIGHTS
CROSS COLLECTOR
CHAIN AND FLIGHTS
Fig. 3.11 Rectangular clarifier
(Courtesy Jeffrey)
3-15
-------
EFFIUENT IE III
DRIVE UNIT
sr.U« DISCHtRCE Hfii.
INfLDENT
SUMP
Fig. 3.12 Circular clarifier
3-16
-------
the floating solids (called scum). In rectangular tanks,
sludge and scum collectors are usually wooden beams ("flights")
attached to endless chains. The collector flights travel on
the surface, in the direction of the flow, conveying grease
and floatable solids down to the scum trough to be skimmed
off to the solids (sludge) handling facilities. The flights
then drop below the surface and return to the influent end
along the bottom, moving the settled raw sludge to the sludge
hopper. The sludge is periodically pumped from the hopper to
the sludge handling facilities.
In circular tanks, scrapers or "plows", attached to a rotating
arm, rotate slowly around the bottom of the tank. The plows
push the settled sludge toward the center and into the sludge
hopper. Scum is collected by a rotating blade at the surface.
As in the case of the rectangular tank, both scum and sludge
are usually pumped to the solids or sludge handling facilities.
The clear surface water of the primary tank flows out of the
tank by passing over a weir. The weir must be long enough to
allow the treated water to leave at a low velocity; if it leaves
at a high velocity, particles settling to the bottom or those
already on the bottom may be picked up and carried out of the
tank.
QUESTIONS
3.5A What is the purpose of "flights" or "plows" in
a clarifier?
3.SB What happens to the sludge and scum collected
in a primary clarifier?
3-17
-------
3.6 SECONDARY TREATMENT
3.60 General
In many treatment plants the wastewater flows out of the
primary clarifier into another unit where it receives secondary
or biological treatment. This means that the wastewater is
exposed to living organisms (such as bacteria) which eat the
dissolved and nonsettleable organic material remaining in the
waste. The two processes used almost universally for biological
treatment are the trickling filter and activated sludge. These
are both aerobic biological treatment processes, which means
the organisms require dissolved oxygen (Fig. 3.13) in order to
live, eat, and reproduce.
Fig. 3.13 Organisms require dissolved oxygen
3.61 Trickling FiIter
The trickling filter is one of the oldest and most dependable
of the biological treatment processes. Most of these plants
are removing 65 to 85% of the BOD and suspended solids present
in the influent.
3-18
-------
The trickling filter is a bed of \h to 5-inch rock, slag
blocks, or specially manufactured "media"11 over which settled
wastewater from the primary clarifier is distributed (Fig.
3.14). The settled wastewater is usually applied by an
overhead rotating distributor and trickles over and around
the media as it flows downward to the effluent collection
channel. Since the media and the voids in between them are
large (usually 2.5- to 4-inch diameter), and since the applied
wastewater no longer has any large particles (they settled
out in the clarifier), the trickling filter does not remove
solids by a filtering action. It would be more correct to
call the filter a biological contact bed or biological reactor
since this is the function it performs. The filter bed offers
a place for aerobic bacteria and other organisms to attach
themselves and multiply as they feed on the passing waste-
water. This process of feeding on, or decomposing, waste is
exactly the same as the process occurring in the stream when
waste is discharged to it. In the trickling filter, however,
the organisms use the oxygen which enters the waste from the
surrounding air, rather than using up the stream's supply of
dissolved oxygen. Thus the voids between the media must be
large so sufficient oxygen can be supplied by circulating air.
The wastewater being distributed on the filter usually has
passed through a primary clarifier, but it still contains
approximately 70 percent of its original organic matter,
which represents food for organisms. For this reason a
tremendous population of organisms develops on the media.
This population continues to grow as more waste is applied.
Eventually the layer of organisms on the media gets so thick
that some of it breaks off (sloughs off) and is carried into
the filter effluent channel. This material is normally called
humus. Since it is principally organic matter, its presence in
a stream would be undesirable. It is usually removed by
settling in a s e condary c1ar ifier. Humus sludge from the
secondary clarifier is usually returned to the primary clari-
fier to be resettled and pumped to the sludge handling facili-
ties along with the "raw" sludge which settles out as previously
described.
11 Media. The material in a trickling filter over which
settled wastewater is sprinkled and then flows over and
around during treatment. Slime organisms grow on the
surface of the media and treat the wastewater.
3-19
-------
TRICKLING FILTER
I. OliTMWTM fUW>tT IMUMNI
r.msrimwTo* AMU
avtNf
(Courtesy Water Pollution Control Federation)
Fig. 3.14 Trickling filter
3-20
-------
3.62 Activated Sludge
Another biological treatment unit that is used in secondary
treatment, following the primary clarifier, is the aeration
tank. When aeration tanks are used with the sedimentation
process, the resulting plant is called an activated sludge
plant. The activated sludge process is widely used by large
cities and communities where land is expensive and where
large volumes must be highly treated, economically, without
creating a nuisance to neighbors. The activated sludge plant
is probably the most popular biological treatment process
being built today for larger installations or small package
plants. These plants are capable of BOD and suspended solids
reduction of up to 90 or 99%. The activated sludge process
is a biological process, and it serves the same function as
a trickling filter. Effluent from a primary clarifier is
piped to a large aeration tank (Fig. 3.15). Air is supplied
to the tank by either introducing compressed air into the
bottom of the tank and letting it bubble through the waste-
water and up to the top, or by churning the surface mechanically
to introduce atmospheric oxygen.
Aerobic bacteria and other organisms thrive as they travel
through the aeration tank. With sufficient food and oxygen
they multiply rapidly, as in a trickling filter. By the time
the waste reaches the end of the tank (usually 4 to 8 hours),
most of the organic matter in the waste has been used by the
bacteria for producing new cells. The effluent from the
tank, usually called "mixed liquor", consists of a suspension
containing a large population of organisms and a liquid with
very little BOD. The activated sludge forms a lacey network
that captures pollutants.
The organisms are removed in the same manner as they were
in the trickling filter plant. The mixed liquor is piped
to a secondary clarifier, and the organisms settle to the
bottom of the tank while the clear effluent flows over the
top of the effluent weirs. This effluent is usually clearer
than a trickling filter effluent because the suspended
material in the mixed liquor settled to the bottom of the
clarifier more readily than the material in a trickling
filter effluent. The settled organisms are known as
activated sludge. They are extremely valuable to the treatment
3-21
-------
TYPICAL ACTIVATED SLUDGE TANK
•in
Ink* ch«
-------
process. If they are removed quickly from the secondary clari-
fier, they will be in good condition and hungry' For more food
(organic wastes)(fig.
3.16). They are there-
fore pumped back (re-
circulated) to the
influent end of the
aeration tank where
they are mixed with
the incoming waste-
water. Here they
begin all over again
to feed on the organic
material in the waste,
decomposing it and
creating new organisms.
Left uncontrolled, the
number of organisms
would eventually be
too great, and therefore
some must periodically
be removed. This is
accomplished by pumping
a small amount of the
activated sludge to the
primary clarifier. The
organisms settle in the
clarifier along with
the raw sludge and are
removed to the sludge
handling facilities.
There are many variations of the conventional activated sludge
process, but they all involve the same basic principle. These
variations will be discussed in Chapter 7, Activated Sludge.
Fig. 3.16
Hungry organisms
ready for
more food
3.63 Secondary Clarifiers
As previously mentioned, trickling filters and activated sludge
tanks produce effluents that contain large populations of micro-
organisms and associated materials (humus). These microorganisms
must be removed from the flow before it can be discharged to the
receiving waters. This task is usually accomplished by a
secondary clarifier. In this tank the trickling filter humus or
activated sludge separates from the liquid and settles to the
3-23
-------
bottom of the tank. It is removed to the primary clarifier to
be resettled with the primary sludge" or returned to the begin-
ning of the secondary process to continue treating the waste-
water. The clear effluent flows over a weir at the top of the
tank.
QUESTIONS
3.6A Would it be a good idea to use trickling filter
media of various sizes so it could pack together
better?
3.6B Why is a secondary clarifier needed after a
trickling filter or aeration tank?
3.6C Activated sludge can be pumped from the secondary
clarifier to .
3-24
-------
3.7 SOLIDS HANDLING AND DISPOSAL
3.70 General
Solids removed from wastewater treatment processes are commonly
broken down by a biological treatment process called sludge
digestion. After digestion and dewatering the remaining
material may be used for fertilizer or soil conditioner.
Some solids, such as scum from a clarifier, may be disposed of
by burning or burial.
3.71 Digestion and Dewatering
Settled sludge from the primary clarifier and occasionally
settled sludge from the secondary clarifier are periodically
pumped to a digestion tank. The tank is usually completely
sealed to exclude any air from getting in (Fig. 3.17). This
type of digester is called an anaerobic digester because of
the anaerobic bacteria that abound in' the tank. Anaerobic
bacteria thrive in an environment devoid of dissolved oxygen
by using the oxygen which is chemically combined with their
food supply.
Two major types of bacteria are present in the digester. The
first group starts eating on the organic portion of the sludge
to form organic acids and carbon dioxide gas. These bacteria
are called "acid formers". The second group breaks down the
organic acids to simpler compounds and forms methane and
carbon dioxide gas. These bacteria are called "gas formers".
The gas is usually used to heat the digester or to run engines
in the plant. The production of gas indicates that organic
material is being eaten by the bacteria. A sludge is usually
considered properly digested when 50 percent of the organic
matter has been destroyed and converted to gas. This normally
takes approximately 30 days if the temperature is kept at
about 95°F.
Most digestion tanks are mixed to continuously bring the food
to the organisms,to provide a uniform temperature, and to avoid
the formation of thick scum blankets. When a digester is not
being mixed the solids settle to the bottom, leaving an amber-
colored liquid above the sludge known as supernatant. The
3-25
-------
* ..
II
Jthri
SLUDGE DIGESTER
Fixed cover
Vacuum fcjposs tat
/wMchKkwtM
usteif 8
$M**OW!lf
(Courtesy Water Pollution Control Federation)
Fig. 3.17 Sludge digester
3-26
-------
supernatant is displaced from the tank each time a fresh
charge of raw sludge is pumped from the primary clarifier.
The displaced supernatant usually is returned from the
digester back to the plant headworks and mixed with in-
coming raw wastes. Supernatant return should be slow to
prevent overloading or shock loading of the plant.
Above the supernatant level a scum blanket will usually
develop. Scum blankets consist of grease, soap, rubber
goods, hair, petroleum products, plastics, and filter tips
from cigarettes. These scum blankets may contain most of
the added food or sludge. Digestion organisms are usually
below the supernatant and little digestion will occur if
the organisms and food don't get together. Control of
scum blankets consists of mixing the digester contents
and burning or burying skimmings instead of pumping them
to the digester.
Above the scum blanket or normal water level is the gas
collection area. Digester gas is normally about 70% methane
and 30% carbon dioxide. When mixed with air, digester gas
is extremely explosive (Fig. 3.18).
"<
' -•' 3 ; > "•
: 5 .&»*.-, ** ••ssj.'rf ^
*%-\N,% 'MMJ^ M V*/
-------
Fig. 3.19 Sludge drying bed
(Courtesy Water Pollution Control Federation)
VACUUM FILTER
Fig. 3.20 Vacuum filter
(Courtesy Water Pollution Control Federation)
3-28
-------
Some of today's activated sludge treatment plants are equipped
with aerobic digesters. An" aerobic digester is ususally an
open tank with compressed air being blown through the sludge.
Destruction of organic matter is accomplished by bacteria
which require dissolved oxygen to survive. One advantage
of this process is that there is no explosive gas being pro-
duced. On the other hand, this is also a disadvantage since
the anaerobic digester gas is used as a fuel for boilers and
engines around the plant. Aerobic sludge from an aerobic
digester doesn't thicken as readily as sludge from an an-
aerobic digester. Aerobic sludge filters about as well as an
equivalent concentration of anaerobic sludge.
3.72 Incineration
Burning of wet sludge by wet oxidation or of dewatered sludge
are possible methods of ultimate disposal; however, the process
must not create an air pollution problem. To prevent skimmings
from clarifiers causing operational problems, incineration or
burial are used.
QUESTIONS
3.7A What two basic types of bacteria are present
in an anaerobic digester?
3.7B Why are digesters mixed?
3.7C List some of the ways to dispose of digested
sludge.
3-29
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3.8 WASTE TREATMENT PONDS
A. special method of. biological treatment deserving attention
is wastewater treatment ponds (Hig. 3.21). They do not
resemble the concrete and steel structures or the mechanical
devices that have been previously discussed. But these simple
depressions in the ground are capable of producing an effluent
comparable to some of the most modern plants with respect to
BOD and bacteria reduction.
In some treatment plants, wastewater being treated may flow
through a coarse screen and flow meter before it flows through
a series of ponds. In other plants the ponds may be located
after primary treatment, while in some plants they are placed
after trickling filters. The type of treatment processes and
the location of ponds are determined by the design engineer
on the basis of economics and the degree of treatment required
to meet the water quality standards of the receiving waters.
When wastewater is discharged to a pond, the settleable solids
fall to the bottom just as they do in a primary clarifier.
The solids begin to decompose and soon use up all the dissolved
oxygen in the nearby water. A population of anaerobic bacteria
then continues the decomposition, much the same as in an
anaerobic digester. As the organic matter is destroyed, methane
and carbon dioxide are released. When the carbon dioxide rises
to the surface some of it is used by algae, which convert it to
oxygen by the process of photosynthesis.12 This is the same
process used by living plants. Aerobic bacteria, algae, and
other microorganisms feed on the dissolved solids in the upper
layer of the pond much the same way they do in a trickling
filter or aeration tank. Algae produce oxygen for the other
organisms to use.
Some shallow ponds (3 to 6 feet deep) have dissolved oxygen
throughout their entire depth. These ponds are called aerobic
ponds. They usually have a mechanical apparatus adding oxygen
plus their oxygen supply from algae.
12 Photosynthesis (foto-SIN-tha-sis). A process in which
chlorophyll (green plant tissue) converts carbon dioxide
and inorganic substances to oxygen and additional plant
material utilizing sunlight for energy. Land plants grow
by the same process.
3-30
-------
STABILIZATION POND
shoytd e* imiy
K> ipr«v
-------
Deep (8 to 12 feet), heavily loaded ponds may be devoid of
oxygen throughout their depth. These ponds are called anaerobic
ponds. At times, these ponds can be quite odorous, and they
are used in sparsely populated areas only.
Ponds that contain an aerobic top layer and an anaerobic
bottom layer are called facultative ponds. These are the
ponds normally seen in most areas. If they are properly
designed and operated,they are virtually odor free and produce
a well-oxidized (low BOD) effluent.
Occasionally ponds are used after a primary treatment unit.
In this case, they are usually called oxidation ponds. When
they are used to treat raw wastewater, they are called raw
wastewater lagoons or waste stabilization ponds.
The effluent from ponds is usually moderately low in bacteria.
This is especially true when the effluent runs from one pond
to another or more (series flow). The long detention time,
usually a month or more, is required in order for harmful
bacteria and undesirable solids to be removed from the pond
effluent. If the receiving waters are used for water supply
or body contact sports, chlorination of the effluent may still
be required.
QUESTION
3.8A How are facultative ponds similar to:
1. a clarifier?
2. a digester?
3. an aeration tank?
3-32
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3.9 ADVANCED METHODS OF TREATING/WASTEWATER
The treatment processes described so far in this chapter are
considered conventional treatment processes. As our population
grows and industry expands, more effective treatment processes
will be required. Advanced methods of waste treatment may
follow conventional processes, or they may be used instead of
these processes. Sometimes advanced methods of waste treat-
ment are called tertiary (TER-she-AIR-ee) treatment because
they frequently follow secondary treatment. Advanced methods
of waste treatment include coagulation-sedimentation (used in
water treatment plants), adsorption, and electrodialysis.
Other new treatment processes that may be used in the future
include reverse osmosis, chemical oxidation, and the use of
polymers.
Advanced methods of treatment are used to reduce the nutrient
content fnitrates and phosphates) of wastewater to prevent
blooms of algae in lakes, reservoirs, or streams. Carbon
filters are used to reduce the last traces of organic materials.
In some parts of the arid west advanced methods are used to
enable the use of the plant effluent for recreational reser-
voirs.
QUESTION
3.9A If wastewater from a secondary treatment
plant were coagulated with alum or lime
and settled in a clarifier, would this be
considered a method of advanced waste
treatment?
3-33
-------
3.10 DISINFECTION
Although the settling process and biological processes remove
a great number of organisms from the wastewater flow, there
remain many thousands of bacteria in every milliliter of
wastewater leaving the secondary clarifier. If there are
human wastes in the water, it is possible that some of the
bacteria are pathogenic, or harmful to man. Therefore,
if the treated wastewater is discharged to a receiving water
that is used for a drinking water supply or swimming or wading,
the water pollution control agency or health department will
usually require disinfection of the effluent prior to discharge.
Disinfection is usually defined as the killing of pathogenic
organisms. The killing of all organisms is called sterilization.
Sterilization is not accomplished in treatment plants as the
final effluent after disinfection always contains some living
organisms due to the inefficiency of the killing process.
Disinfection can be accomplished by almost any process that
will create a harsh environment for the organisms. Strong
light, heat, oxidizing chemicals, acids, alkalies, poisons,
and many other substances will disinfect. Most disinfection
in wastewater treatment plants is accomplished by chlorine,
which is a strong oxidizing chemical.
Chlorine gas is used in most treatment plants although some
of the smaller plants use a liquid chlorine solution as their
source. The dangers in using chlorine gas, however, have
prompted some of the larger plants to switch to hypochlorite
solution (bleach) even though it is more expensive.
Chlorine gas is withdrawn from pressurized cylinders containing
liquid chlorine and mixed with water or treated wastewater to
make up a strong chlorine solution. Liquid hypochlorite solution
can be used directly. The strong chlorine solution is then mixed
with the effluent from the secondary clarifier. The effluent
is then directed to a chlorine contact basin. The basin can be
any size or shape, but better results are obtained if the tank
is long and narrow. This shape prevents rapid movement or short
circuiting through the tank. Square or rectangular tanks can
be baffled to achieve this effect (Fig. 3.22). Tanks are usually
designed to provide approximately 20 to 30 minutes theoretical
contact time, although the trend is to longer times. If the
plant's outfall line is of sufficient length, it may function as
an excellent contact chamber since short circuiting will not occur.
3-34
-------
CHLORINE CONTACT BASIN
(Courtesy Water Pollution Control Federation)
Fig. 3.22 Chlorine contact basin
3-35
-------
3.11 ADDITIONAL READING
Some books you can read to obtain further information on
the treatment plant and the various processes involved are:
a. MOP 11
b. New York Manual
c. Texas Manual
d. Sewage Treatment Practices, by Bloodgood
e. Babbitt, Harold E., and E. Robert Bauman, Sewerage and
Sewage Treatment, John Wiley and Sons, New York, Eighth
Edition, 1958. $10.75.
f. Summary Report, Advanced Waste Treatment, July 1964-
July 1967, U. S. Department of Interior, FWPCA, WP-20-
AWTR-19. Available from the Publications Office, Ohio
Basin Region, Environmental Protection Agency, Water
Quality Office, Cincinnati, Ohio 45226.
g. Santee Recreation Proceedings, Santee, California, U.S.
Department of Interior, FWPCA, WP-20-7 (1967).Available
from Publications Office source given in (f) above.
hf A Primer on Waste Water Treatment, prepared by the Office
of Public Information, Federal Water Quality Administration,
OVA-12, October 1969. Available from Superintendent of
Documents, U.S. Government Printing Office, Washington,
D.C. 20402. Price $0.55.
3-36
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4.1 INTRODUCTION TO PRETREATMENT
In various ways, a little or a lot of almost everything finds
its way into sewers and ends up at the wastewater treatment
plant. Cans, bottles, pieces of scrap metal, sticks, rocks,
bricks, plastic toys, plastic lids, caps from toothpaste tubes,
towels and other rags, sand--all are found in the plant
influent.1
These materials are troublesome in various ways. Pieces of
metal, rocks, and similar items will cause pipes to plug, may
damage or plug pumps, or jam sludge collector mechanisms in
settling tanks (c1arifiers).2 Sand, eggshells, and similar
materials (grit) can plug pipes, cause excessive wear in pumps,
and use up valuable space in the sludge digesters.3
If a buried or otherwise inaccessible pipe is plugged, or a
sludge collector mechanism jams, or a critical pump is put out
of commission, serious consequences can result. Reduced plant
efficiency allows a heavy pollutional load on the receiving
waters, causing health hazards to downstream water users, sludge
deposits in stream or lake (with resultant odors and unsightliness),
and sometimes causing the death of fish and other aquatic life.
Also, a good deal of hard (sometimes rather unpleasant) work is
involved, and usually there are heavy (and unbudgeted) expenses.
With these things in nind, it is evident that an important part
of a wastewater treatment plant is the equipment used to remove
the rocks and other materials as early as possible. These items
of equipment are screens, racks, comminutors, and grit removal
devices and are called pretreatment facilities. See Fig. 4.1 for
location of these processes in a typical plant.
1 Influent (IN-flu-ent). Wastewater or other liquid—raw or
partly treated—flowing into a reservoir, basin, treatment
process, or treatment plant.
2 Clarifier (KLAIR-i-fire) (settling tank, sedimentation basin)
A tank or basin in which wastewater is held for a period of
time so that the heavier solids settle to the bottom and the
lighter material will float to the water surface.
3 Digester (die-JEST-er). A tank in which sludge is placed to
allow sludge digestion to occur. Digestion may occur under
anaerobic (more common) or aerobic conditions.
4-1
-------
AWT?
Fig. 4.1 Flow diagram of typical plant
4-2
-------
4.2 SCREENS AND RACKS
Parallel bars may be placed at an angle in a channel in such
a manner that the wastewater will flow through the bars, but
the large solids will be caught on the bars. These bars are
commonly called racks when the spacing between them is 3" to
4" or more. When the spacing is about 1" to 2", they are
usually called bar screens.
4.20 Manually Cleaned Bar Screens
Manually cleaned bar screens (Fig. 4.2) require frequent
attention. As debris collects on the screen, it blocks the
channel, causing the wastewater to back up into the sewer.
This, in turn, causes organ i c mate ri a Is ** to settle out;
the dissolved oxygen5 is depleted; and septic6 conditions
develop, producing hydrogen sulfide which causes a rotten egg
odor and is corrosive to concrete, metal, and paint. If
cleaning of the screens is infrequent, the sudden rush (when
they do get cleaned) of septic wastewater creates a sudden
"shock" load on the plant, sometimes resulting in a poor
quality plant effluent.7
^ Organic Material. Material which comes from animal or
vegetable sources. Organic material generally can be
consumed by bacteria and other small organisms. Inorganic
materials are chemical substances of mineral origin and
may contain carbon and oxygen, whereas organic materials
contain mainly carbon and hydrogen along with other elements.
5 Dissolved Oxygen. Atmospheric oxygen dissolved in water
or wastewater, usually abbreviated DO.
6 Septic (SEP-tick). Wastewater devoid of dissolved oxygen.
If severe, the wastewater turns black, giving off foul
odors and creating a large oxygen demand.
7 Effluent (EF-lu-ent). Wastewater or other liquid—raw,
partially or completely treated--flowing from a basin,
treatment process, or treatment plant.
4-3
-------
TROUGH
Fig. 4.2 Manually cleaned bar screen
ELEVATOR MECHANISM
Fig. 4.3 Mechanically cleaned bar screen
4-4
-------
Cleaning of bar screens is accomplished with a rake with tines
(prongs) which will fit between the bars. Extreme caution
should be taken when raking the screen--footing may be poor
due to the water and grease underfoo'ty lack of enoujgh room to stand,
location of the receptacle for the debris'," e't'c. You should look
this area over carefully to spot hazards and take corrective action.
Good housekeeping, a guard rai1, a hanger or other storage for
the rake, jgboo footing, etc', will' greatly reduce the possibility
of injury.
4.21 Mechanically Cleaned Screens
Mechanically cleaned screens (Fig. 4.3) overcome the problem of
wastewater backing up and greatly reduce the time required to
take care of this part of your plant. There are various types
of mechanisms in use, the more common being traveling rakes
which bring the debris up out of the channel and into hoppers
or other receptacles. You should keep these units well lubricated
and adjusted. Follow the manufacturer's recommendations carefully.
A few minutes spent in proper maintenance* procedures can save hours
or days of trouble and help to keep the plant operating efficiently.
Occasionally some debris will be present which the equipment
cannot remove. Periodic checks should be made so that these
materials can be removed by hand. To determine if some material
is stuck in the screen, divert the flow through another channel
or "feel" across the screen with a rake or similar device.
4-5
-------
Always shut the unit off first. Never reach into the operating range
of machinery while it is running. Slow-moving equipment is especially
hazardous. Because it moves slowly, it does not appear dangerous.
However, most geard-down machinery is so powerful that it can crush
almost any obstruction. A HUMAN HAND. FOR INSTANCE, OFFERS LITTLE
RESISTANCE TO THIS TYPE OF EQUIPMENT.
Various other mechanical methods are in use, involving actual coarse
screens or perforated sheet metal. These units are automatically
cleaned with scrapers, rotating brushes, water sprays, or air jets.
The screens may be in the form of belts, discs, or drums set in a
channel so that the wastewater flows through the submerged portion,
with the collected debris being removed as it passes the brushes or
sprays.
4.3 DISPOSAL OF SCREENINGS
The material removed from the screens is very offensive and hazardous.
It produces obnoxious odors and draws rats and. flies. Burial,
incineration, and shredding or grinding are three common methods
of disposal. If the screenings are buried, at least six inches
of earth cover must be provided immediately. The final earth
cover must be deep enough to prevent flies from reaching the
screenings through cracks caused by settling. At small plants with
manually cleaned bar screens, an enterprising operator can make
a "press" from a piece of steel pipe or casing, using a heavy screw,
rack and pinion, or even an automobile jack to provide pressure, to
dewater the screenings before disposal. The practice of using
grinders (shredders, disintegraters, etc.) to cut up screenings and
return them to the effluent can impose a great load on following
treatment processes.
4-6
-------
4.4 COMMINUTION (com-min-00-shun)
Comminutors are devices which act as a cutter and a screen.
Their purpose is to shred (comminute) the solids and leave
them in the wastewater. This overcomes problems of screen-
ings disposal. As with screens, they are mounted in a
channel, and the wastewater flows through them. The rags,
etc., are shredded by cutters (teeth) until they can pass
through the openings. Pieces of wood and plastic are rejected
and must be removed by hand. Most of these units have a
shallow pit in front of them to catch rocks and scrap metal.
The flow to the comminutor should be shut off periodically
and the debris removed from the trap. The frequency of
checking the trap can be determined from experience. However,
it is not wise to allow more than a few days between checks.
A comminutor consists of a rotating drum with slots for the
wastewater to pass through (Fig. 4.4). Cutting teeth are
mounted in rows on the drum. The teeth pass through cutter
bars or "combs" with very small clearances so that a shearing
action is obtained. The wastewater passes into the vertically
mounted drum through the slots in the drum and flows out the
bottom. A rubber seal, held in place by a bolted-down ring,
prevents leakage under the drum. This seal should be checked
whenever the rock and scrap metal trap is checked.
Some comminutors also have a mercury seal (Fig. 4.5) to keep
water out of the bearings. This is because these units are
designed so that, at their rated capacity, the top of the
drum will be under several inches of water. This head loss8
will be specified in the manufacturer's instructions. The
mercury seal should be checked annually or after a particularly
heavy flow. Drain the mercury; weigh it (the amount of mercury
will be specified by weight); and if it is dirty, strain it
through some heavy material (such as denim or chamois) before
putting it back in the comminutor. (You will probably have to
8 Head Loss. "Head" is a common term used in discussing
pumps. It is a way of expressing pressure in terms of
the height of vertical column of water. In this case,
the head loss is the height to which the water must build
up in front of the drum until there is sufficient pressure
to force that particular amount of water through the slots
(Fig. 4.4).
4-7
-------
MOTOR
ROTATING CUTTING SCREEN
i
oo
FLOW
\
HEAD LOSS
Fig. 4.4 Comminutor
-------
MOTOR
AND
REDUCTION GEARS
SUPPORT STRUCTURE
AND BEARINGS
MERCURY Fll IR PLUG
MERCURY SEAL
MERCURY DRAIN PLUG
CUTTER AND SHAFT MOVE
TOGETHER AS UNIT
-STATIONARY CUTTER BAR
Fig. 4.5 Mercury seal in comminutor
4-9
-------
squeeze the mercury through the cloth or, if laboratory equipment
is available, use a suction flask.) Add more mercury if needed.
CAUTION
CAUTION
CAUTION
Mercury is poisonous. Breathing the fumes can be fatal or cause
loss of hair and teeth. Wash up thoroughly after handling it.
Remove gold rings, etc., from your hands first, as they may end up
coated with mercury. If your ring is thus coated, it will have to
be heated to bum off the mercury. If you must handle or work with
mercury, be sure to work over a large tray in order to catch any
spills. Plenty of fresh air ventilation is an absolute must;.
There are many variations of the comminutor. One of the more common
ones has the trade name of "barminutor" (Fig. 4.6). This unit con-
sists of a bar screen made of U-shaped bars and a rotating drum with
teeth and "shear bars". The rotating drum travels up and down the
bar screen. Careful attention must be given to maintaining the oil
level in these machines; otherwise, water may get into the bearings.
Consult the manufacturer's instructions for detailed procedures.
4-10
-------
Fig. 4.6 Barminutor
4-11
-------
4.5 GRIT REMOVAL
Grit (sand, eggshells, cinders, etc.) is the heavier mineral
matter in wastewater which will not decompose or "break down".
It causes excessive wear in pumps. A mixture of grit, tar,
grease and other cementing materials can form a solid mass in
pipes and digesters that will not move by ordinary means.
Consequently, grit should be removed as soon as possible after
reaching the plant.
4.6 GRIT CHAMBERS [Fig. 4.7)
The simplest means of removing grit from the wastewater flow
is to pass it through channels or tanks which allow the velocity
of flow to be reduced to a range of 0.7 to 1.4 ft/sec. The
objective is to allow the grit to settle to the bottom, while
keeping the lighter organic solids moving along to the next
treatment unit. Experience has shown that a flow-through
velocity of one foot per second (ft/sec) is best.
Velocity is controlled by several means. With multiple-
channel installations, the operator may vary the number of
channels (chambers) in service at any one time to maintain a
flow velocity of approximately one ft/sec in the grit chambers.
Other methods involve the use of proportional weirs (Fig. 4.8)
at the outlet for automatic regulation.
WATER
/SURFACE
Fig. 4.8 Proportional weir
4-12
-------
GRIT SETTLING AREA
CENTER WALL
SLIDE GATES
WEIRS (WHEN USED)
GRIT SETTLING AREA
STOP GATES
Insert when
cleaning to
prevent backflow
Fig. 4.7 Grit Chamber
-------
The proportional weir in Fig. 4.8 will tend to decrease the
velocity in the grit chambers when the flows increase because
the exit area will decrease, thus increasing the depth of water
flow in the channel. If the operator wishes to increase the
velocity in a grit chamber,'he could use a proportional weir
and turn it over so the exit area increased as the flow in-
creased. This would tend to keep the depth of water flow in
the channel low and cause higher velocities. A barrier with
a variable height at the outlet of the grit chamber can be
used instead of a proportional weir to regulate velocities.
Flow velocities also may be regulated by the shape of the grit
chamber instead of placing devices at the outlet. Some grit
chambers have cross-sectional shapes similar to a proportional
weir. The operator may regulate the velocities in a grit
chamber by using boards to change cross-sectional shape, but
he should seriously consider any maintenance or operational
problems that might develop when trying to keep the grit chamber
clean,
A simple -method of estimating the velocity is to place a stick
in the channel and time its travel for a measured distance.
Calculate as follows:
... . - , Distance traveled, ft
Velocity, ft/sec = Time, sec
Example:
A stick travels 25 feet in 20 seconds.
Solution: 1.25
ir i -4. r*. / Distance, ft
Velocity, ft/sec = Time) sec
25 ft
20 sec
1.25 ft/sec
0
The actual velocity probably will be slightly higher than your
estimate, but it is a very quick way to check the grit chamber
velocity.
4-14
-------
Removal of grit ranges from use of a scoop shovel to various
types of collectors and conveyors. For hand-cleaned chambers,
the frequency of cleaning is determined by experience, If
the channel can be removed from service during the cleaning
operation, the job is made easier, and no grit is washed into
the plant,
Since there is always a small amount of organic matter in the
grit chamber, disposal of grit should be treated the same as
screenings. Burial is the most satisfactory disposal method.
Failure to quickly cover grit results in odors and attracts
flies and rats.
Cleaning grit chambers manually can be quite hazardous. Take
precautions against
slipping and back
strain. Beware of
dangerous gases when
workin g in covered
grit chambers."
There are many types
of mechanical grit
collector mechanisms.
Common ones are chain-
driven scrapers
(called "flights")
(Fig. 4.10) that are
moved slowly along
the bottom and up
an incline out of
the water to a
hopper, or along
the bottom to an
underwater trough
where a screw con-
veyor lifts the grit to a storage hopper or truck. Some
designs use conveyor belts with buckets attached.
An aerated grit chamber is actually a tank with a sloping
bottom and a hopper or trough in the lower end (Fig. 4.11).
Air is injected along the wall of the tank above the trough.
The rolling action of the water in the tank moves the grit
along the bottom to the grit hopper. Grit is removed from
the hopper by a conveyor system.
Aerated grit chambers are most frequently found at activated
sludge plants where there is a readily available air supply,
and the pre-aeration helps to "freshen" the wastewater. The
older wastewater becomes the more difficult it is to treat. A
freshening process tends to make later processes more effective.
4-15
-------
Fig. 4,10 Chain-driven scrapers (flights)
(Courtesy Jeffrey Mfg. Co.)
A grit chamber with a slower flow velocity than recommended may
allow appreciable organic matter to settle out with the grit.
This mixture of grit and organic matter is called detritus.1(
In some plants grit chambers are called detritus tanks. Organic
matter may be separated from the grit by blowing air through or
washing the detritus to resuspend the organic matter. Centri-
fuges also are used to separate grit from sludge or organic matter
from grit.
10 Detritus (de-TRI-tus).
by wastewater.
The heavy, coarse material carried
4-16
-------
WATER SURFACE
o o o
a
O
o o
I
M
~J
AIR
0 °
0 ° o
o o
o
o
OOP
V
GRIT HOPPER AND COLLECTOR MECHANISM
ts: y--y--r
r /•.. /..-;/
Fig. 4.11 Aerated grit chamber
-------
4.7 QUANTITIES OF GRIT
Plants having well-constructed separate wastewater collection
systems can usually expect to average 1 to 4 cu ft of grit
per million gallons. These quantities have been rising in
recent years due to household garbage grinders. They can
also be expected to increase during storm periods.
Plants receiving waste from combined collection systems can
expect to average 4 to 15 cu ft of grit per million gallons
with peaks during storm periods many times higher. Grit
collected during storm periods has been reported at over
500 cu ft per million gallons, probably the result of flow
from broken sewers or open channels.
Records of grit quantities should be kept in the same manner
as for screenings.
QUESTION
4.7A Your plant has an average flow of 2.0 MGD.
An average of 4 cu ft of grit is removed
each day. How many cu ft of grit per MG
of flow are removed?
4.8 GRIT WASHING
In some cases it is necessary or desirable to use grit as
fill material. Since a small amount of organic material
settles out with the sand, etc., it becomes necessary to
"wash" the grit. There are a number of devices built for
this purpose. Most use water to wash the grit as it is
being removed from the grit chamber (Fig. 4.12). In aerated
grit chambers, the grit is ordinarily free enough of organics
that it may be considered "washed". Chapter 5 of the Water
Pollution Control Federation's Manual of Practice No. 11
has additional information and should be read carefully by
the operator.
4-18
-------
I
I-J
vo
AUX. WASH WATER
(USUALLY WASTEWATER)
GRIT FtbU
SCREW-TYPE
IMPELLER
WATER AND ORGAN ICS OUTLET
TO HOPPER, TRUCK, ETC.
Fig. 4.12 Grit washer
-------
QUESTION
4.8A Why is it sometimes necessary or desirable
to "wash" grit?
4.9 PREAERATION
Preaeration is a wastewater treatment process used to freshen
wastewater, remove gases, add oxygen, promote flotation of
grease, and aid coagularion. The freshening of wastewater
improves the effectiveness of following treatment processes.
The process is usually located before primary sedimentation
(Fig. 4.1). Other processes used to accomplish freshening
include ozonation and prechlorination.
Preaeration consists of aerating wastewater in a channel or
separate tank for 10 to 30 minutes. Aeration may be accomplished
by either mechanical surface aeration units or diffused air
system.11 Air application rates with a diffused air system
normally range from 0.5 to 1.0 cu ft of air per gallon of waste-
water treated.
4.10 ADDITIONAL READING
a. MOP 11, pages 17-24
b. New York Manual, pages 27-29
c. Texas Manual, pages 160-173
d. Sewage Treatment Practices, by Bloodgood,
pages 19-22 and 26-34
11 See Chapter 7, Activated Sludge, for a discussion of
aeration facilities.
4-20
-------
CHAPTER 5. SEDIEENTATION AND FLOTATION
(Lesson 1 of 3 Lessons)
5.0 INTRODUCTION
Raw or untreated wastewater contains some materials which will
settle to the bottom or float to the water surface readily when
the wastewater velocity is allowed to become very slow. Sewers
are designed to allow the raw wastewater to flow rapidly to
prevent this from happening. Grit chambers (see Chapter 4) are
designed to allow the wastewater to flow at a slightly slower
rate than in the sewers so that heavy, inorganic grit will
settle to the bottom where it can be removed. Settling tanks
decrease the wastewater velocity far below the velocity in a
collection sewer.
In most municipal wastewater treatment plants, the treatment unit
which immediately follows the grit chamber (see Figs. 5.1 and
5.2 for typical plant layout) is the sedimentation and flotation
unit. This unit is sometimes called a settling tank, sedimentation
tank, or clarifier. The most common name is primary clarifier,
since it helps to clarify or clear up the wastewater.
A typical plant (Figs. 5.1 and 5.2) may have clarifiers located
at two different points. The one which immediately follows the
bar screen or comminutor or grit chamber (some plants don't have
all of these) is called the primary clarifier, merely because it
is the first clarifier in the plant. The other, which follows
the biological treatment unit (if there is one), is called the
secondary c1arifier. The two types of clarifiers operate almost
exactly the same way. The reason for having two types is that
the biological treatment unit converts more solids to the
setteable form, and they have to be removed from the treated
wastewater.
The main difference between the two types of clarifiers is in the
sludge density handled. Primary sludges are usually denser than
secondary sludges. Effluent from a secondary clarifier is normally
clearer than primary effluent.
5-1
-------
FUNCTION
-i BEMOVS
T2CMON/AU
P8/MM2V
41?
BIOLOGICAL
Fig. 5.1 Flow diagram of typical plant
5-2
-------
SCREENING
GRIT
REMOVAL
> •
FLOW
METER
PRE-AERATION
(
V.
PRIMARY
CLARIFICATION
(N0.1)
PRIMARY
< I CLARIFICATION
(NO.2)
SUPERNATANT & SECONDARY SLUDGE RETURN
BIOLOGICAL
TREATMENT
SOLIDS
CENTERING
CHLORINE
CONTACT
TO
RECEIVING
WATERS
Fig. 5.2 Plan diagram of a typical primary wastewater treatment plant
-------
Solids which settle to the bottom of a clarifier are scraped
to one end (in rectangular clarifiers) or to the middle
(circular clarifiers) into a sump. From the sump the solids
are pumped to the sludge handling or sludge disposal system.
Systems vary from plant to plant and include sludge digestion,
vacuum filtration, incineration, land disposal, lagoons and
burial. Figures 5.3 and 5.4 show detailed sketches of rec-
tangular and circular clarifiers.
Disposal of skimmed solids varies from plant to plant. They
may be buried with material cleaned off the bar screen, in-
cinerated, pumped to the digester, or they may be even sold
for their grease and oil content. Pumping skimmed solids to
a digester is not considered good practice because skimmings
can cause operational problems in digesters.
This chapter contains information on start-up, daily operation,
and maintenance procedures; sampling and laboratory analyses;
some problems to look out for; safety; and basic principles of
sedimentation and flotation. You may wish to refer to the two
chapters containing details of laboratory analyses and mathe-
matics for further information.
5-4
-------
on
t
t/i
SCUM BAFFLE
EFFLUENT WEIRS
SLUDGE COLLECTOR
-DRIVE UNIT
SCUM SKIMMER AND TROUGH
EFFLUENT TROUGH
TARGET BAFFLE
SLUDGE COLLECTOR CHAIN
AND FLIGHTS
CROSS COLLECTOR
CHAIN AND FLIGHTS
SLUDGE
WITHDRAWAL
Fig. 5.3 Rectangular sedimentation basin
-------
EFFLUENT WEIR
DRIVE UNIT
VERTICAL
DRIVE CAGEH
COUNTER
BALANCE
WEIGHTS
SLUDGE
WITHDRAWAL
PIPE
INFLUENT
BLADES AND SCRAPER
SQUEEGEES
AND/OR SUCTION
MECHANISM
SUMP
Fig. 5.4 Circular clarifier
-------
5.2 SAMPLING AND LABORATORY ANALYSIS
5.20 General
Proper analysis of representative samples is the only con-
clusive method of measuring the efficiency of clarifiers.
Tests may be conducted in the plant at the site where the
sample is collected or in the laboratory. The particular
tests depend upon whether the effluent from the clarifier
goes to another treatment process or is discharged to
receiving waters.
The frequency of testing and the expected ranges will vary from
plant to plant. Strength of the wastewater, freshness, charac-
teristics of the water supply, weather, and industrial wastes
will all serve to affect the "common" range of the various test
results.
Tests
Dissolved
Oxygen (DC)
Settleable
Solids
PH
Frequency
Daily
Daily
Daily
Location
Effluent
Influent
Effluent
Influent
Effluent
Common Range
0
5
0.5
6.5
6.5
- 2 mg/1
- 15 ml
- 4 ml
- 8.0*
- 8.0*
3.
4. Temperature
5. BOD
6. Suspended
Solids
7. Chlorine
Residual
(if needed)
8. Coliform
Group
Bacteria
(if needed)
Daily Influent 50 - 85°*
Weekly Influent 150 - 400 mg/1
(Minimum) Effluent 60 - 160 mg/1
Weekly Influent 150 - 400 mg/1
(Minimum) Effluent 60 - 150 mg/1
Daily Plant
Effluent 0.5 - 3.0 mg/1
Weekly Effluent 500,000 - 100,000,000
per 100 ml
*Depends on region, water supply and discharges to the collection
system
5-7
-------
5.21 Sampling
Samples of the influent to the clarifier and the effluent from
it will give you information on the clarifier efficiency for
removal of solids,
bacteria, and BOD. As
with all sampling, the
purpose is to collect
samples which represent
the true nature of the
wastewater or stream
being sampled. The
amount of solids, BOD,
bacteria, and the
clarity and pH will
probably vary through-
out the day, week, and
year. You must determine
these variations in order
to understand how well
your clarifier is doing
its job.
5.22 Calculation of Clarifier Efficiency
To calculate the efficiency of any wastewater treatment process,
you need to collect a sample of the influent and the effluent
of the process, preferably composite samples for a 24-hour period.
The particular water quality indicators (BOD, suspended solids)
you are interested in are measured and the efficiency is calcu-
lated. You can calculate the efficiency of a clarifier in
removing several different items, such as efficiency in removing
BOD or efficiency in removing suspended solids. Calculations of
treatment efficiency are for process control purposes. Your main
concern must be the quality of the plant effluent, regardless of
percent of wastes removed.
5-8
-------
Example;
The influent BOD to a primary clarifier is 200 mg/1, and the
effluent BOD is 140 mg/1. What is the efficiency of the primary
clarifier in removing BOD?
Formula:
Efficiency, % = C,1.". - Outi) IOQ%
In
(200 mg/1 - 140 mg/1)
200 mg/1
= (60 mg/1) 1QO%
200 mg/1
= (.30) 100%
= 30% BOD Removal
5.23 Typical Clarifier Efficiencies
Following is a list of some typical percentages for primary
clarifier efficiencies:
Expected
Removal
Efficiency
Settleable solids 90% to 95%
Suspended solids 40% to 60%
Total solids 10% to 15%
Biochemical oxygen demand 25% to 35%
Bacteria 25% to 75%
pH will generally not be affected significantly by a clarifier.
You can expect wastewater to have a pH of about 6.5 to 8.0,
depending on the region, water supply and wastes discharged
into the collection system.
5-9
-------
Clarifier efficiencies are affected by many factors, including:
1. Types of solids in the wastewater, especially if there
is a significant amount of industrial wastes.
2. Age of wastewater when it reaches the plant. Older
wastewater becomes stale or septic, and solids do
not settle properly because gas bubbles form under
them.
3. Rate of wastewater flow as compared to design flow.
4. Mechanical conditions and cleanliness of clarifier.
5-10
-------
5.3 SLUDGE AND SCUM PUMPING
The particles which settle to the floor of the clarifier are
called sludge. The accumulated sludge should be removed fre-
quently, and this is accomplished by mechanical cleaning devices
and pumps in most tanks. (See Fig. 5.3 and 5.4) Mechanically
cleaned tanks need not be shut down for cleaning. Septic
conditions3 may develop rapidly in primary clarifiers if sludge
is not removed at regular intervals. The proper interval is
dependent on many conditions and may vary from thirty minutes
to eight hours, and as much as twenty-four hours in a few
instances. Experience will dictate the proper frequency of
removal. Sludge septicity can be recognized when sludge gas-
ificationlf causes large clumps of sludge to float on the water
surface. Septic sludge is generally very odorous and acid
(has a low pH).
Excess water should be eliminated from the sludge if possible
because of its effects on the volume of sludge pumped and on
digester operation. A good thick primary sludge will contain
from 4.0 to 8.0 percent dry solids as indicated by the Total
or Suspended Solids Test in the laboratory. Conditions which
may affect sludge concentration are the specific gravity, size
and shape of the particles, and temperature, and turbulence
in the tank.
3Septic conditions (SEP-tick). A condition produced by anaerobic
organisms. If severe, the wastewater turns black, giving off
foul odors and creating a heavy oxygen demand.
^Sludge gasification. A process in which soluble and suspended
organic matter are converted into gas. Sludge gasification
will form bubbles of gas in the sludge and cause large clumps
of sludge to rise and float on the water surface.
5-11
-------
Withdrawal (pumping) rates should be slow in order to prevent
pulling too much water with the sludge. While the sludge is
being pumped, take samples frequently and examine them visually
for excess water. If the samples show a "thin" sludge, it is
time to stop pumping. Practice learning to recognize the
differences between thin or concentrated sludges. There are
several methods for determining "thick" or "thin" sludge with-
out a laboratory analysis:
1. Sound of the sludge pump.
The sludge pump will
usually have a different
sound when the sludge is
thick than when it is
thin.
2. Pressure gauge readings.
Pressure will be higher
on the discharge side of
the pump when sludge is
thick.
3. Sludge density gauge
readings.
4. Visual observation of a
small quantity (gallon
or less).
5. Watch sludge being pumped
through a site glass in
the sludge line.
When you learn to use the indicators listed above, you should compare
them frequently with lab tests. The laboratory Total Solids Test is
the only accurate method for determining exact density. However,
this analytical procedure is too slow for controlling a routine
pumping operation. Muiy operators use the centrifuge test to obtain
quick results.
floating material (scum) may leave the clarifier at the effluent
unless a method has been provided for holding it back. A baffle
is generally provided in the tank at some location to collect scum.
Primary clarifiers often have a scum collection area where the scum
is skimmed off by some mechanical method, usually a skimming arm or
a paddle wheel. If mechanical methods are not provided, use hand
tools such as skimming dipper attached to a broom handle.
Frequently check the scum trough to be sure it is working properly.
Clean the box with a brush and hot witer. Scum may be' disposed of
by burning or burial.
5-12
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CHAPTER 5. SEDIMENTATION AND FLOTATION
(Lesson 2 of 3 Lessons)
5.6 PRINCIPLES OF OPERATION
5.60 General
Sedimentation and flotation units are designed to remove physically
those solids which will settle easily to the bottom or float easily
to the top. Sedimentation is usually the principal basis of design
in such units and will be discussed in more detail in this section.
Flotation of fats, oils, hair, and other light material also is
very important to protect the esthetics of receiving waters.
The sedimentation and flotation units commonly found are:
1. Primary clarifiers
2. Secondary clarifiers
3. Flotation units
' 4. Imhoff tanks
This section will describe each unit individually as it relates to
another process or as a process by itself.
5.61 Primary Clarifiers
The most important function of the primary clarifier is to remove
as much settleable and floatable material as possible. Organic
settleable solid removal is very important because it causes a
high demand for oxygen (BOD) in receiving water or subsequent bio-
logical treatment units in the treatment plant.
Many factors influence the design of clarifiers. Settling charac-
teristics o.f suspended particles in water are probably the most
important considerations. The design engineer must consider the
speed at which particles will settle in order to determine the
correct dimensions for the tank. Rapid movement of water
(velocity) will hold most particles in suspension and carry them
along until the velocity of water is slowed sufficiently for
particle settling. The rate of downward travel (settling) of a
particle is dependent on the weight of the particle in relation
5-13
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to the weight of an equal volume of water (specific gravity),5
the particle size and shape, and th'e temperature of the liquid.
Organic settleable solids are seldom more than 1 to 5 percent heavier
than water; and, therefore, their settling rates are slow.
If the horizontal velocity of water is slowed to a rate of 1.0 to 2.0
feet of travel per minute (grit chamber velocities were around 1 ft/sec)
most particles with a specific gravity of 1.05 (5% more than water) will
settle to the bottom of the container. Specific gravity of water is
1.000 at 4.0 degrees Celsius (formally Centigrade) or 39°F; it weighs
8.34 Ibs per gallon. Wastewater solids with a specific gravity of 1.05
will weigh 8.76 Ibs per gallon (1.05 times 8.34 Ibs equals 8.76 Ibs per
gallon). The relationship of the particle settling rate to liquid
velocity may be explained very simply by use of a sketch (Fig. 5.5).
o
2
4
6
in
N
X
^ v^/7
Nx Jjcv
XNx$
VERTICAL
T SETTLING RATE=
1 FT/6 MIN OR
10 FT/60 MIN
ill
LENGTH - 200 FT
HORIZONTAL FLOW OF WATER-
200 FT/100 MIN
y/C/* DIRECTION OF
x
-------
bottom of the tank at a rate of 1.0 foot in 6 minutes, it will rest on
the floor of the tank in 60 minutes if the tank is 10 feet deep. If
the particle settles at the rate of 10 feet in 60 minutes, it should
settle in the first 60 percent portion of the tank because the liquid
surrounding it requires 100 minutes to flow through the tank.
There are many factors which will influence settling characteristics
in a particular clarifier. A few of the more common ones are as
follows:
Temperature. Water expands as temperature increases (above 4°C) and
contracts as temperature decreases (above 4°C). Below 4°C the oppo-
site is true. In general, as water temperature increases, settling
rate of particles increases; and, as temperature decreases, so does
the settling rate. Molecules6 of water react to temperature changes.
They are closer together when liquid temperature is lower; thus,
density7 increases and water becomes heavier per given volume because
there is more of it in the same space. As water becomes more dense,
the density difference between water and solid particles becomes less;
and therefore the particles settle slower. This is illustrated in
Fig. 5.6.
HATER MOLECULES ARE EXPANDED.
THIS ALLOWS FOR EASY SETTLING.
HATER MOLECULES ARE CLOSE.
PARTICLE SETTLING DIFFICULT.
•ARM WATER
100°C (LESS DENSE)
(7.989 LBS/GAL)
COLD WATER
4°C (MORE DENSE)
(8.335 LBS/GAL)
Fig. 5.6 Influence of temperature on settling
6 Molecules (MOLL-ee-kules). The smallest portion of an element
or compound retaining or exhibiting all the properties of the
substance.
7 Density (DEN-sit-tee). The weight per unit volume of any sub-
stance. The density of water (at 4°C) is 1.0 gram per cubic
centimeter (gms/cc) or about 62.4 Ibs per cubic foot. If one
cubic centimeter of a substance (such as iron) weighs more than
1.0 gram (higher density), it will sink or settle out when put
in water. If it weighs less (lower density, such as oil), it
will rise to the top and float. Sludge density is normally
expressed in gms/cc.
5-15
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Short Circuits. As wastewater enters the settling tank, it
should be evenly dispersed across the entire cross section of
the tank and should flow at the same velocity in all areas
toward the discharge end. When the velocity is greater in
some sections than in others, serious "short circuiting" may
occur. The high velocity area may decrease the detention
time in that area, and particles may be held in suspension
and pass through the discharge end of the tank because they
do not have time to settle out. On the other hand, if velocity
is too low, undesirable septic conditions may occur. Short
circuiting may easily begin at the inlet end of the sedimen-
tation tank (Fig 5.7). This is usually prevented by the use
of weir plates, baffles, port openings, and by proper design
of the inlet channel. Short circuiting also may be caused by
turbulence and stratification of density layers due to temperature
or salinity.
5-16
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GH VELOCITY AREA
POOR SETTLING,
XX X
* x x
" LOW VELOCITY AREA
SEPTIC CONDITIONS AND ODORS
Top View Looking Down
X X. X
* X y.
Side View - Warm Influent
Side View - Cold Influent
Fig. 5.7 Short circuiting
5-17
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Detention Time.8 Wastewater should remain in the clarifier
long enough to allow sufficient settling time for solid par-
ticles. If the tank is too small for the quantity of flow
and the settling rate of the particles, too many particles
will be carried out the effluent of the clarifier. The relation-
ship of "detention time" to "settling rate" of the particles
is important. Most engineers design for about 2.0 to 3.0 hours of
detention time. This is, of course, flexible and dependent on
many circumstances.
Detention time can be calculated by use to two known factors:
1. Flow in gallons per day (gpd)
2. Tank dimensions
Example:
The flow is 3.0 million gallons per day (MGD), or 3,000,000 gal/day.
Tank dimensions are 60 feet long by 30 feet wide by 10 feet deep.
What is the detention time?
8 Detention Time. The time required to fill a tank at a given
flow or the theoretical time required for a given flow of
wastewater to pass through a tank.
5-18
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Formulas .*
Detention _ TankVolume, cu ft x 7.5 gal/cu ft x 24 hr/day
Time, hrs " Flow, gal/day
Tank Volume, cu ft = Length, ft x Width, ft x Depth, ft
Calculations:
Tank Volume, cu ft = Length, ft x Width, ft x Depth, ft
= 60 ft x 30 ft x 10 ft
= 18,000 cu ft
Detention _ Tank Volume, cu ft x 7.5 gal/cu f t x 24 hr/day
Time, hrs " Flow, gal/day
= 18.000 cu ft x 7.5 gal/cu ft x 24 hr/day
' 3~,000,000 gal/day
= 5,240,000 gal-hr/day 24 18,000
3,000,000 gal/day x7.5 180
120 1,440,000
= 1.08 hours 168_ 1 800 0
180.0 3,240,000
Evaluation. If detention time is only 1.08 hours and if labora-
tory tests indicate poor removal of solids, then additional tank
capacity should be placed into operation (if available) in order
to obtain additional detention time. You must realize that flows
fluctuate considerably during the day and night and any calculated
detention time is for a specific flow.
Discussion. The formula given in this section allows you to
calculate the theoretical detention time. Actual detention time
is less than the detention time calculated using the formula and
can be measured by the use of dyes, tracers, or floats.
5-19
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Weir Overflow Rate. Wastewater leaves the clarifier by flowing over
weirs and into effluent troughs (launders)9 or some type of weir
arrangement. The number of lineal10 feet of weir in relation to the
flow is important to prevent short circuits or high velocity near
the weir or launder which might pull settling solids into the
effluent. The weir overflow rate is the number of gallons of waste-
water that flow over one lineal foot of weir per day. Most designers
recommend about 10,000 to 20,000 gallons per day per lineal foot of
weir. Higher weir overflow rates have been used for materials with
a high settling rate or for intermediate treatment. Secondary clari-
fiers and high effluent quality requirements generally need lower
weir overflow rates than primary clarifiers. The calculation for
weir overflow rate requires two known factors:
1. Flow in gpd
2. Lineal feet of weir
Example:
The flow is 5.0 MGD in a circular tank with a 90-foot weir
diameter.11 What is the weir overflow rate?
10
Launders (LAWN-ders). Sedimentation tank effluent troughs.
When the flow leaves a sedimentation unit, it usually flows
into a trough after it leaves the tank. The top edge of
the trough over which wastewater flows as it enters the
trough is considered a weir.
Lineal (LIN-e-al). The length in one direction of a line.
For example, a board 12 feet long has 12 lineal feet in its
length.
11
Weir Diameter (weer), Circular
clarifiers have a circular weir
within the outside edge of the
clarifier. All the water leaving
the clarifier flows over this
weir. To find the length of this
weir, the weir diameter must be
known. The diameter is the
length of a line from one edge
of a weir to the opposite edge
and passing through the center
of the circle formed by the weir.
DIAMETER
CIRCULAR
WEIR
DIAMETER
SECTION
5-20
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Formu1as:
Flow Rate, gpd
Weir Overflow, gpd/ft
Length of Weir, ft
Length of Circular Weir = 3.14 x Weir Diameter, ft
Calculations:
Length of Cir-
cular Weir, ft = 3.14 x (Weir Diameter, ft)
= 3.14 x 90 ft
= 283 Lineal Feet of Weir 282.60
Weir Over- _ Flow Rate, gpd 17.668
flow, gpd/ft = Length of Weir, ft 283/5,000,000
2 83
5.000.000 gal/day 2 170
= 283""!?"1 981
= 17,668 gpd/ft
2 220
2 264
5-21
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Surface Settling Rate or Surface Loading Rate. This term is
expressed in terms of gpd/sq ft of tank surface area. Some
designers and operators have indicated that the surface
loading rate has a direct relationship to the settleable solids
removal efficiency in the settling tank. The suggested
loading rate varies from 300 to 1200 gpd/sq ft, depending on
the nature of the solids and the treatment requirements. Low
loading rates are frequently used in small plants in cold
climates. In warm regions, low rates may cause excessive
detention which could lead to septicity. The calculation for
surface loading rate requires two known factors:
1. Flow in gpd
2. Square feet of liquid surface area
Example:
The flow in a secondary plant is 5.0 MGD in a tank 90 feet long
and 35 feet wide. What is the surface loading rate?
Formula;
,* f j- n ^ j / r* Flow Rate, gpd
Surface Loading Rate, gpd/sq ft = Area> gq ft
Calculations:
Surface Area, sq ft = Length, ft x Width, ft
= 90 ft x 35 ft
= 3150 sq ft
Surface Loading _ Flow Rate,
Rate, gpd/sq ft " Area, sq ft
5,000,000 gpd
3150 sq ft
= 1587 gpd/sq ft
5-22
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Detention Time, Weir Overflow Rate, and Surface Loading Rate are
three mathematical methods of"checking the performance of exist-
ing facilities against the design values. However, laboratory
analysis of samples is the only reliable method of measuring
clarifier efficiency. If laboratory results indicate a poorly
operating clarifier, the mathematical methods may help you to
identify the problem.
QUESTIONS
5.6A What is "short circuiting" in a clarifier?
5.6B Why is "short circuiting" undesirable?
5.6C How can "short circuiting" be corrected?
5.6D A circular clarifier has a diameter of 80 feet and
an average depth of 10 feet. The flow of waste-
water is 4.0 MGD. Calculate the following:
1. Detention Time, in hours
2. Weir Overflow Rate, in gpd/ft
3. Surface Loading Rate, in gpd/sq ft'
5-23
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CHAPTER 5. SEDIMENTATION AND FLOTATION
(Lesson 3 of 3 Lessons)
5.62 Secondary Clarifiers or Final Settling Tanks
Secondary clarifiers usually follow a biological process in the
flow pattern of a treatment plant. (See Figs. 5.1 and 5.2.)
The most common biological processes are the Activated Sludge
Process12 and the Trickling Filter.l3
In some plants a chemical process may be used instead of a bio-
logical process, but the latter is far more common for municipal
treatment plants.
The final settling tank is sometimes referred to as a "humus tank"
when used after a trickling filter to settle out sloughings11*
from the filter media. Filter sloughings are a product of bio-
logical action in the filter; the material is generally quite
high in BOD and will degrade the effluent quality unless it is
removed. The specific description of trickling filters is covered
in Chapter 6.
12 Activated Sludge Process (ACK-ta-VATE-ed sluj). A biological
wastewater treatment process in which a mixture of wastewater
and activated sludge is aerated and agitated. The activated
sludge is subsequently separated from the treated wastewater
(mixed liquor) by sedimentation, and wasted or returned to the
process as needed.
13 Trickling Filter. A treatment process in which the wastewater
trickles over media that provide the opportunity for the form-
ation of slimes which clarify and oxidize the wastewater.
14 Sloughings (SLUFF-ings). Trickling filter slimes that have
been washed off the filter media. They are generally quite
high in BOD and will degrade effluent quality unless removed.
5-24
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Secondary clarifier detention times are about the same as for
primary clarifiers, but the surface loading and weir overflow
rates are generally lower due to the less dense characteristics
of secondary sludges. The following are ranges of loading
rates for secondary clarifiers used after biological filters:
Detention Time - 1.0 to 2.0 hours
Surface Loading Rate - 300 to 1200 gpd/sq ft
Weir Overflow Rate - 5,000 to 15,000 gpd/lineal ft
The amount of solids settling out in a secondary clarifier
following a trickling filter will be very irregular due to a
number of varying conditions in the biological treatment process.
In general, you can expect to pump about 30% to 40% as much
sludge from the secondary clarifier as from the primary; thus,
total sludge pumping will increase by that amount. These figures
indicate how the trickling filter "creates" settleable solids
which were not present in the raw wastewater in settleable form.
The sludge in the secondary settling tank will usually have a
completely different appearance and characteristics than the
sludge collected in a primary settling tank. It will usually
be much darker in color, but should not be grey or black. A
grey sludge usually indicates insufficient biological stabili-
zation (treatment). Sludge will turn black if it is
allowed to stay in the secondary clarifier too long. If this
happens, then the return sludge or waste sludge pumping rate
should be increased or the time of pumping lengthened or made
more frequent. Secondary sludges generally require continuous
or frequent pumping at a rate sufficient to maintain a
reasonably concentrated sludge and a low sludge blanket in the
clarifier.
The particle sizes may be very irregular with generally good
(rapid) settling characteristics. The sludge may appear to
be a fluffy humus type of material and will usually have little
or no odor if sludge removal occurs at regular intervals. The
sludge collected in the final settling tanks is sometimes dis-
posed of by transferring to a primary settling tank to be mixed
with primary sludge, and it is sometimes transferred directly
to the digestion system, depending on the particular plant design
and the characteristics of the sludge.
Final settling tanks which follow the activated sludge process
are designed similarly to those used for the trickling filter,
except that they are more conservative in design because the
sludge tends to be less dense. Their purpose is identical,
except that the particles to be settled are received from the
aeration tank rather than the trickling filter. Most final
sedimentation tanks used with the activated sludge process are
5-25
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mechanically cleaned due to the importance of rapidly returning
sludge to the aeration tank. (This is explained in Chapter 7,
Activated Sludge.) The sludge volume in the secondary tank will
be greater from the activated sludge process than from the
trickling filter process.
The standard laboratory tests used to measure solids removal in
primary settling tanks are used also for secondary settling
tanks.
QUESTIONS
5.6E Why are secondary clarifiers needed in secondary
treatment plants?
5.6F What usually is done with the sludge that settles
out in secondary clarifiers?
5-26
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5.7 FLOTATION PROCESSES
Wastewater always contains some solids in suspended form that
neither settle nor float to the surface and therefore remain
in the liquid as it passes through the clarifier. Dissolved
solids will, of course, travel through the clarifiers because
they are unaffected by these units. There are two other types
of solids in wastewater known as "Colloids" and "Emulsions"
that are very difficult to remove.
A "colloid" is a particle held in suspension due to its very
small size and its electrical charge. It is usually less that
200 millimicrons15 in size, and generally will not settle
readily.If organic, it exerts a high oxygen demand, so its re-
moval is desirable.
An "emulsion" is a liquid mixture of two or more liquid sub-
stances not normally dissolved in one another, but one liquid
held in suspension in the other. It usually contains suspended
globules of one or more of the substances. The globules
usually consist of grease, oil, fat, or resinous substances.
This material also exerts a high oxygen demand.
One method for removing emulsions and colloids is by a "flotation
process", pumping air into the mixture to cause the suspended
material to float to the surface where it can be skimmed off.
15 Millimicron (MILL-e-MY-cron). One thousandth of a micron
or a millionth of a millimeter.
5-27
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The particles can be flocculated16 with air or chemical coagulants17
and forced or carried to the liquid Surface by minute air bubbles.
Figure 5.8 shows the chain of events in the flotation process.
' ,HYU •'••';! ':. i.v'V'i
•W;-;!;:^':-'^-:.;
$?tM'M&
..-...-. ,.••$"•'(
SMALL PARTICLES
KILL NOT SETTLE.
SMALL PARTICLES
IN FLOCCULATED
FORM.
FLOCCULATED PAR
TICLES ATTACHED
TO AIR BUBBLES.
BUBBLES CARRY
PARTICLES TO
SURFACE.
ACCUMULATED
SCUM OR FOAM
ON SURFACE.
MOST AIR BUB-
BLES ARE
RELEASED.
Fig. 5,8 Flotation process
Most of the air bubbles are released at the liquid surface. Particles
are removed in the form of scum or foam by skimming.
There are two common flotation processes in practice today:
1. Vacuum Flotation. The wastewater is aerated for a short
time in a tank where it becomes saturated with dissolved
air. The air supply is then cut off and large air bubbles
pass to the surface and into the atmosphere. The waste-
water then flows to a vacuum chamber which pulls out dis-
solved air in the form of tiny air bubbles which float
the solids to the top.
2. Pressure Flotation. Air is forced into the wastewater
in a pressure chamber where the air becomes dissolved in
the liquid. The pressure is then released from the
wastewater, and the wastewater is returned to atmospheric
pressure where the dissolved air is released from solution
in the form of tiny air bubbles. These air bubbles rise
to the surface and, as they rise, they carry solids to
the surface.
16 Flocculated (FLOCK-you-lay-ted). An action resulting in the
gathering of fine particles to form larger particles.
17 Coagulants (ko-AGG-you-lents). Chemicals added to destabilize,
aggregate, and bind together colloids and emulsions to improve
settleability, filterability, or drainability.
5-28
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Any flotation process is based upon release of gas bubbles in
the liquid suspension (Fig. 5.8) under conditions in which the
bubbles and solids will associate with each other to form a
combination with a lower specific gravity than the surrounding
liquid. They must stay together long enough for the combin-
ation to rise to the surface and be removed by skimming.
QUESTIONS
5.7A Why is the "flotation process" used in some waste-
water treatment plants?
5.7B Would you place the flotation process before or
after primary sedimentation?
5.7C Give a very brief description of:
1. Colloid
2. Emulsion
5.7D Give a brief description of the Vacuum Flotation
process.
5-29
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5.8 IMHOFF TANKS
Imhoff tanks are rarely constructed today. Your plant may
consist of only an Imhoff tank if it serves a very small com-
munity or if it was constructed many years ago. It is quite
possible that you may never have operating responsibility for
one of these units. They will be discussed for general know-
ledge and for the few operators who will have operating
responsibility for therc.
The Imhoff tank combines sedimentation and sludge digestion
in the same unit. There is a top compartment where sedimen-
tation occurs and a bottom compartment for digestion of settled
particles (sludge). The two compartments are separated by a
floor and a slot designed to allow settling particles to pass
through to the digestion compartment (Fig. 5.9).
Wastewater flows slowly through the upper tank as in any other
standard rectangular sedimentation unit. The settling solids
pass through the slot to the bottom sludge digestion tank.
Anaerobic digestion of solids is the same as in a separate
digester. Gas bubbles are formed in the digestion area by
bacteria. As the gas bubbles rise to the surface they carry
solid particles with them. The slot is designed to prevent
solids from passing back into the upper sedimentation area as
a result of gasification where they would pass out of the unit
with the effluent.
The same calculations previously used for clarifiers can be used
to determine loading rates for the settling area of the Imhoff
tank. (Chapter 8, Sludge Digestion, will explain the anaerobic
process in the sludge digestion area of this unit.) Some typical
values for design and operation of Imhoff tanks are:
Settling Area
Wastewater Detention Time - 1.0 to 4.0 hours
Surface Settling Rate - 600 to 1200 gpd/sq ft
Weir Overflow Rate - 10,000 to 20,000 gpd/ft
Suspended Solids Removal - 45% to 65%
BOD Removal - 25% to 35%
Digestion Area
Digestion Capacity - 1.0 to 3.0 cu ft/person
Sludge Storage Time - 3 to 12 months
5-30
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GAS VENTS
\
SETTLING COMPARTMENT
SLUDGE DIGESTION
COMPARTMENT
SLUDGE WITHDRAWAL LINE
Fig. 5.9 Imhoff tank
5-31
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Here are a few operational suggestions:
1. In general, there is no mechanical sludge scraping device
for removing settled solids from the floor of the settling
area. Solids may accumulate before passing through the slot
to the digestion area. It may be necessary to push the accu-
mulation through the slot with a squeegee or similar device
attached to a long pole. Dragging a chain on the floor and
allowing it to pass through the slot is another method for
removing the sludge accumulation.
2. Scum from the sedimentation area is usually collected by hand
tools in a separate container for disposal. It may also be
transferred to the gas venting area where it will work down
into the digestion compartment. Scum in the gas vents should
be kept soft and broken up by soaking it periodically with
water or by punching holes in it and mixing it with the liquid
portion of the digestion compartment. The addition of 10 pounds
of hydrated lime per 1000 connected population per day may be
helpful for controlling odors from the gas vent area and also
for adjusting the chemical balance of the scum for easier
digestion.
3. Some Imhoff tanks have the piping and valving to reverse the
direction of flow from one end toward the other end. If
possible, the flow should be reversed periodically for the
purpose of maintaining an even sludge depth in the digestion
compartment. The sludge level in the digestion area must be
lower than the slot in the floor of the settling area to
prevent plugging of the slot. A line of gas bubbles directly
over the slot indicates the sludge level in the digestion
chamber is too high.
4. The explanation of sludge digestion in Chapter 8 will
supply information that can be applied to the digestion
area in the Imhoff tank. Neither sludge mixing nor heating
devices are used in an Imhoff tank. Sludge loading rates,
withdrawal rates, laboratory tests, and visual appearance
of sludges are very similar to what they are in an unheated
digester. If visual appearance is the only method you have
of judging the sludge, it is safe to assume that if sludge
in the digestion area is relatively odorless or has a musty
smell and is black or very dark in color, the process is
working satisfactorily.
5-32
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The laboratory testing program for an Imhoff tank should be
complete enough to identify operational problems and to supply
necessary information to regulatory agencies. The following
minimum program is suggested, assuming adequate laboratory
facilities, personnel, and size of the system.
TYPICAL
SUGGESTED ANALYSIS USUAL RANGE REMOVAL %
Settling Area
Settleable Solids 3.0 - 10.0 ml/1 75 - 90
Suspended Solids 200 - 400 mg/1 45-65
pH 6.7 - 7.3
Alkalinity 100 - 300 mg/1
BOD 200 - 500 mg/1 25 - 35
Digestion Area
PH 6.7 - 7.3
Alkalinity 1000 - 3000 mg/1
Vol. Acids 100 - 500 mg/1
Efficiency of operation can be determined by measuring the settle-
able solids, suspended solids, or BOD of the influent and effluent.
5-33
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QUESTIONS
5.8A What are the two components of an Imhoff tank?
5.8B Describe the sludge from an Imhoff tank which
is operating properly.
5.8C How could you maintain a fairly level sludge
blanket in the digester portion of an Imhoff tank?
5.8D How can you force settled material into the
digestion compartment?
5.9 SEPTIC TANKS
Septic tanks are used mostly for treating the wastewater from
individual homes or from small populations (such as camps)
where sewers have not been provided. They operate very much
like an Imhoff tank except there is not a separate digestion
compartment. Detention time is usually long (12 to 24 hours)
and most settleable solids will remain in the tank. They
must be pumped out and disposed of periodically to prevent
the tank from filling up. Part of the solids in the septic
tank are liquified and discharged with the wastewater into
the soil mantle. Conditions are not favorable for rapid
gasification and most waste stabilization occurs in the soil.
Septic tank effluent is usually disposed of in underground per-
forated pipes called "leach lines", and sampling of effluent may
be impossible. The ability of the soil mantle to leach the
septic tank effluent is the critical factor in subsurface waste
disposal systems.
For additional information on septic tanks, refer to the Manual of
Septic Tank Practice, U.S. Public Health Service, Washington, D.C.
5-34
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CHAPTER 6. TRICKLING FILTERS
(Lesson 1 of 3 Lessons)
6.0 INTRODUCTION
6.00 General Description
In the initial chapters of this course, you have learned about
physical methods of wastewater treatment. In general, these
techniques (processes) consist of the screening of large particles,
settling of heavy material, and floating of light material by pre-
liminary and primary treatment units (screen, grit chamber, clarifier).
Although primary treatment is very efficient for removing settleable
solids, it is not capable of removing other, lighter suspended solids
or dissolved solids which may exert a strong oxygen demand on the
receiving waters.
In order to remove the very small suspended solids (colloids) and
dissolved solids, most waste treatment plants now being built include
"secondary treatment".1 This additional process increases overall
pTaht removal of suspended solids and BOD to 90% or more. The two
most common secondary treatment processes are trickling filters and
activated sludge. This chapter will deal with trickling filters.2
1 Secondary treatment. A wastewater treatment process used to
convert dissolved or suspended materials into a form more
readily separated from the water being treated.
2 Trickling filters are sometimes called biofilters, accelo
filters, or aero-filters, depending on the recirculation pattern.
6-1
-------
Figures 6.1 and 6.2 show where a trickling filter is usually-
located in a plant.
More trickling filters, have been built in this country than
any other type of secondary treatment device. Most trickling
filters are large in diameter, shallow, cylindrical structures
filled with stone and having an overhead distributor. (See
Fig. 6.3.) Many variations of this design have been built.
Square or rectangular filters have been constructed with fixed
sprinklers for wastewater distribution.
6.01 Principles of Treatment Process
Trickling filters, or biological oxidation beds, consist of
three basic parts:
1. The media (and retaining structure)
2. The underdrain system
3. The distribution system
The media provide a large surface area upon which a biological
slime growth develops. This slime growth, sometimes called a
zoogleal film,3 contains the living organisms that break down
the organic material. The media may be.rock, slag, coal, bricks.
redwood blocks, molded plastic or any other sound,
durable material. The media should be of such sizes and stacked
in such a fashion to provide voids for air to ventilate the filter
and keep conditions aerobic. For rock, the size will usually be
from about two inches to four inches. Although actual size is
not too critical, it is important that the media be uniform in
size to permit adequate ventilation. The depth ranges from about
three to eight feet.
The underdrain system has a sloping bottom, leading to a center
channel, which collects the filter effluent. It also supports
the media and permits air flow. Common methods are the use of
spaced redwood stringers, or any of a number of prefabricated
blocks of concrete, /vitrified clay, or other material.
3 Zoogleal Film (ZOE-glee-al). A complex population of organisms
that form a slime growth on the trickling filter media and
break down the organic matter in the wastewater. These sliwes
consist of living organisms, silt, and other debris. Slime
growth is a more common definition.
6-2
-------
PDNCflONJ
R5MOVS KOCH4,
FEE AEEATlON
AMP
Fig. 6.1 Flow diagram of treatment plant
6-3
-------
RECIRCUUTION LIKE
PRIMARY
CLARIFICATION
PRIMARY
TRICKLING
FILTER
SECONDARY
TRICKLING
FILTER
PRETREATMENT
PRIMARY
CLARIFICATION
SECONDARY
CLARIFIER
, . HUMUS SLUDGE
SUPERNATANT
ANAEROBIC
DIGESTER
(PRIMARY)
ANAEROBIC
DIGESTER
(SECONDARY)
CHLORINE CONTACT
TO
RECEIVING
WATERS
Fig. 6.2 Plan to typical trickling filter plant
-------
DISTRIBUTOR ARM
ROTATION
STAY ROD TURN
BUCKLE
OUTLET ORIFICE-
TRAILING EDGE &'.
SPLASH PLATES .
SUPPORT GRILL &
UNDERDRAINAGE SYSTEM
SLOPED FLOOR
UNDERDRAIN CHANNEL
OUTLET BOX
OUTLET PIPE
\
Fig. 6.3 Trickling filter
6-5
-------
The distribution system, in the vast majority of cases, is a
rotary-type distributor which consists of two or more horizontal
pipes supported a few inches above the filter media by a central
column. The wastewater is fed from the column through the hori-
zontal pipes and is distributed over the media through orifices
located along one side of each of these pipes (or arms).
Rotation of the arms is due either to the "jet-like" or rotating
water sprinkler reaction from wastewater flowing out the orifices
or by some mechanical means. The distributors are equipped with
a mercury or mechanical type seal at the center column to pre-
vent leakage and protect the bearings, guy rods for seasonal
adjustment of the pipes (arms) to maintain them in a horizontal
position, and quick-opening gates at the end of each arm to
permit easy flushing.
Today the fixed nozzle distribution system is not as common as
the rotary type. Each fixed nozzle consists of a circular orifice
with an inverted cone-shaped deflector mounted above the center
which breaks the flow into a spray. Some types have a steel ball
in the inverted cone. (See Fig. 6.5.) The fixed nozzle system
requires an elaborate piping system to insure relatively even
distribution of the wastewater. Flow is usually intermittent and
is controlled by automatic siphons which regulate the flow from
dosing tanks. (See Fig. 6.5.) The nozzles extend six to twelve
inches above the media and are shaped so that an overlapping spray
pattern exists at the start of dosing when the head in the dosing
tank is the greatest. The pattern is carefully worked out to pro-
vide a relatively even distribution of the wastewater.
6.02 Principles of Operation
The maintenance of a good growth of organisms on the filter media
is crucial to successful operation.
6-6
-------
DISCHARGE LEVEL
STEEL BALL
AUTOMATIC SIPHON,
OR DOSING CHAMBER
AIR VENT
FIXED-SPRAY NOZZLES
Fig. 6.5 Siphon and nozzle details for fixed spray filters
-------
The term "filter" is rather misleading, indicating that solids
are separated from liquid by a strailiing action, but this is not
the case. Passage of wastewater through the filter causes the
development of a gelatinous coating of bacteria, protozoa, and
other organisms on the media. This growth of organisms absorbs
and utilizes much of the suspended colloidal and dissolved organic
matter from the wastewater as it passes over the growth in a
rather thin film. Part of this material is utilized as food for
production of new cells, while another portion is oxidized to
carbon dioxide and water. Partially decomposed organic matter
together with excess and dead film is continuously or periodically
washed (sloughed) off and passes from the filter with the effluent.
For the oxidation (decomposition) processes to be carried out,
the biological film requires a continuous supply of dissolved
oxygen, which may be absorbed from the air circulating through
the filter voids (spaces between the rocks or other media).
Adequate ventilation of the filter must be provided; therefore
the voids in the filter media must be kept open. Clogged voids
can create operational problems, including ponding and reduction
in overall filter efficiency.
A method of increasing the efficiency of trickling filters is to
add recirculation. Recirculation is a process in which filter
effluent is recycled and brought into contact with the biological
film more than once. Recycling of filter effluent increases the
contact time with the biological film and helps to seed the lower
portions of the filter with active organisms. Due to the increased
flow rate per unit of area, higher velocities occur which tend to
cause more continuous and uniform sloughing of excess growths,
thus preventing ponding and restriction of ventilation. This
increased hydraulic loading also decreases the opportunity for
snail and filter fly breeding. It has been observed that the
thickness of the biological growth is directly related to the
organic strength of the wastewater (the higher BOD, the thicker
the layers of organisms). By the use of recirculation, the strength
of wastewater applied to the filter can be diluted, thus preventing
excessive build-up.
Recirculation may be constant or intermittent and at a steady or
fluctuating rate. Recycling may be practiced only during periods
of low flow to keep rotary distributors in motion, to prevent
drying of the filter growths, or to prevent freezing. Recirculation
in proportion to flow may be utilized to reduce the strength of the
wastewater applied to the filter while steady recirculation of a
constant amount keeps the distributors in operation and also tends
to even out the highs and lows of organic loading, but involves
higher pumping costs.
6-8
-------
It is generally agreed that any organic waste which can be
successfully treated by other aerobic biological processes
can be treated on trickling filters. This includes, in
addition to domestic wastewater, such wastewaters as might
come from food processing, textile and fermentation industries,
and certain pharmaceutical processes. Industrial wastewaters
which cannot be treated are those which contain excessive con-
centrations of toxic materials, such as pesticide residues,
heavy metals, and highly acidic or alkaline wastes.
For maximum efficiency, the slime growths on the filter media
should be kept fairly aerobic. This can be accomplished by
proper design of the wastewater collection system and proper
operation of primary clarifiers, or by pretreatment of the
wastewater by aeration or addition of recycled filter effluent.
The air supply to the slimes may be improved by increased air
or wastewater recirculation. The thin slime growth may be
aerobic on the surface, but anaerobic next to the media. A
trickling filter media of rock or slag can accumulate slimes
only on the outside surface, but manufactured media provides
considerably more surface area per unit of dead space.
The temperature of the
wastewater and of the
climate also affects
filter operation, with
temperature of the waste-
water being the more
important. Of course,
temperature of the
wastewater will vary
with the weather.
Within limits, activity
of the organisms increases
as the temperature rises.
Therefore, higher loadings
and greater efficiency
are possible in warmer
climates if aerobic conditions
can be reasonably maintained
in the filter.
6-9
-------
6.12 Daily Operation
Once growth on the media has been established and the plant is in
"normal operation", very little routine operational control is
required. Careful daily observation is important. Items to be
checked daily are:
1. Any indication of ponding
2. Filter flies
3. Odors
4. Plugged orifices
5. Roughness or vibration of the distributor arms
6. Leakage past the mercury seal
Occasionally the underdrains should be checked for accumulation of
debris in order to prevent stoppages.
Refer to the appropriate paragraphs in the following section on
operational problems for procedures to correct these conditions.
Operation of clarifiers is interconnected with trickling filter
operation. If the recirculation pattern permits, it is a good idea
to return filter effluent to the primary clarifier. This is a very
effective odor control measure. In some plants, increasing the
recirculation rate will increase the hydraulic loading on the
clarifier. Be sure the hydraulic loading remains within the
engineering design limits. If the hydraulic loading is too low,
septic" conditions may develop in the clarifier, while excessively
high loadings may wash solids out of the clarifier.
Recirculation during low inflow periods of the day and night may
help to keep the slime growths wet, minimize fly development and
wash off excessive slime growths. It may be necessary to reduce
or stop recirculation during high flow periods to avoid clarifier
problems from hydraulic overloading. Recirculation of final
clarifier effluent dilutes influent wastewater and recirculated
sludge improves slime development on the media.
6-10
-------
You should, by evaluating your own operating records, adjust the
process to obtain the best possible results for the least cost.
Use the lowest recirculation rates that will yield good results
(but not cause ponding or other problems) to conserve, power.
Power costs are a large item in a plant budget. Also, reduced
hydraulic loadings mean better settling in the clarifiers, re-
sulting in less chlorine usage in plants which disinfect the
final effluent, since organic matter exerts a high chlorine demand.
If filter effluent, rather than secondary clarifier effluent, is
recirculated, the hydraulic loading on the secondary clarifier is
not affected.
6-11
-------
6.2 SAMPLING AND ANALYSIS
6.20 General
The trickling filter is a biological treatment unit and therefore
loadings and efficiencies of the unit are normally determined on
the basis of influent characteristics (inflow and biochemical oxygen
demand (BOD) test) and required quality of effluent or receiving
waters (dissolved oxygen and solids).
The frequency
ot each test and expected ranges will vary from plant to plant,
Strength of the wastewater, freshness, characteristics of the
water supply, weather, and industrial wastes will all serve to
affect the "common" range of the various test results.
6.21 Typical Trickling Filter Plant Lab Results
1.
2.
3.
4.
5.
6.
7.
8.
Test
Dissolved
Oxygen
Settleable
Solids
PH
Temperature
BOD
Suspended
Solids
Chlorine
Residual
Co li form
Bacteria
Frequency
Daily
Daily
Daily
Daily
Weekly
(Minimum)
Weekly
(Minimum)
Daily
Weekly
(Minimum)
Location
Prim. Effl.
Influent
Influent
Final Effl.
Influent
Influent
Prim. Effl.
Final Effl.
Influent
Prim. Effl.
Final Effl.
Final Effl.
Final Effl.,
Chlorinated
Common Range
1.0
5
6.8
7.0
-
150
60
15
150
60
15
0.5
50
- 2.0 mg/1
- 15 ml/1
- 8.0
- 8.5
—
- 400 mg/1
- 160 mg/1
- 40 mg/1
- 400 mg/1
- 150 mg/1
- 40 mg/1
- 2.0 mg/1
- 700/100 :
9. Clarity
Daily
Final Effl.
1 - 3 ft
6-12
-------
NOTES; Results of tests listed on the previous page as "Primary
' " Effluent" may vary at'different plants due to the many
variations in recirculation patterns and activities of
the waste dischargers into the collection system.
Settleable solids tests of the effluent may be required
by some regulatory agencies. If your plant is operating
efficiently, the settleable solids will be so low as to
oe unreadable. In this case, record as "Trace".
Dissolved Oxygen and Settleable Solids or Clarity Tests
on trickling filter effluent are sometimes useful in evalu-
ating problems when they occur. 'The operator should know
what range is "common" for his plant.
An easy test that should be made periodically by the operator is to check
the distribution of wastewater over the filter. Pans of the same
size are placed level with the rock surface at several points along
the radius of a circular filter. The distributor arm should then
be run long enough to almost fill the pans. The arm is then stepped
and the amount or depth of water in each pan is measured. The amount
in each pan should not differ from the average by more than 5%. If
the distribution is not uniform, the orifices must be adjusted.
6-13
-------
CHAPTER 6. TRICKLING FILTERS
(Lesson 3 of 3 Lessons)
6.6 CLASSIFICATION OF FILTERS
6.60 General
Depending upon the hydraulic and organic loadings applied, filters
are classified as standard-rate, high-rate, or roughing filters.
Further designations, such as single-stage, two-stage, series or
parallel, and others are used to indicate the flow pattern of the
plant. The hydraulic loading applied to a filter is the total volume
of liquid, includin g reci rculat ion, expressed as gallons per day
per square foot of filter surface area (gpd/sq ft). The organic
loading is expressed
as the pounds of BOD
applied per day per
1000 cubic feet of
filter media
(Ibs BOD/day/1000 cu ft).
Where recirculation is
used, an additional
organic loading will
be placed on the
filter; however, this
added loading is omitted
in most calculations
because it was included
in the influent load.
6.61 Standard-Rate Filters
The standard-rate filter is operated with hydraulic loading range
of 25 to 100 gals/day/sq ft, and an organic BOD loading of 5 to 25
Ibs/day/1000 cu ft. The filter media is usually 6 to 8 feet in depth,
with application to the filter by a rotating distributor, although
many are equipped to provide some recirculation during low flow periods.
The filter growth is often heavy and in addition to the bacteria and
protozoa10 many types of worms, snails, and insect larvae can be found.
10 Protozoa (pro-toe-ZOE-ah). A group of microscopic animals,
principally of one cell, that sometimes cluster into colonies.
6-14
-------
The growth usually sloughs off at intervals, noticeably in
spring and fall. The effluent from a standard-rate filter
treating municipal wastewater is usually quite stable with
BODs as low as 20 to 25 mg/1.
6.62 High-Rate Filters
High-rate filters were the result of trying to reduce costs
associated with standard-rate filters or attempting to treat
increased wasteloads with the same facility. Studies indi-
cated that essentially the same BOD reductions could be obtained
at the higher design loadings.
High-rate filters are normally 3 to 5 feet deep with recommended
loadings being 100 to 1000 gal/day/sq ft and 25 to 300 Ibs BOD/
day/1000 cu ft. These filters are designed to receive waste-
water continually, and practically all high-rate installations
utilize recirculation.
Due to the heavy flow of wastewater over the media, more uniform
sloughing of the filter growths occurs,.. This sloughed material
is somewhat lighter than from a standard-:xA%? unit and therefore
more difficult to settle. Effluent with BODs as low as 20 to 50
mg/1 is sometimes produced by plants treating municipal waste-
water.
6.63 Roughing Filter
A roughing filter is actually a high-rate filter receiving a very
high organic loading. Any filter receiving an organic loading of
over 300 Ibs of BOD/day/1000 cu ft of media is considered to be
in this class. This type of filter is used primarily to reduce
the organic load on subsequent oxidation processes such as a
second-stage filter or activated sludge process. Many times
they are used in plants which receive strong organic industrial
wastes. They are also used where an intermediate (50-70% BOD
removal) degree of treatment is satisfactory.
Operation of the filter is basically the same as for the high-rate
filters with recirculation. Qvera.ll BOD reductions are much lower,
but reductions per unit volume of filter media are greater.
6-15
-------
6.64 Filter Staging
Fig. 6.7 shows various filter and clarifier layouts. The decision
as to the number of filters (or stages) required is one of design
rather than operation. In general, however, at smaller plants
where the flow is fairly low, the strength of the raw wastewater
is average, and effluent quality requirements are not too strict,
a single-stage plant (one filter) is often sufficient and most
economical. In slightly overloaded plants the addition of some
recirculation capability can sometimes improve the effluent quality
enough to meet receiving water standards without the necessity of
adding more stages.
In two-stage filter plants, two filters are operated in a series.
Sometimes a secondary clarifier is installed between the two
filters. Recirculation is almost universally practiced at two-
stage plants with many different arrangements being possible.
Choice of recirculation scheme used is based on consideration of
which arrangement produces the best effluent under the particular
conditions of wastewater strength and other characteristics.
(See Fig. 6.7.)
QUESTIONS
5.6A What are the three general classifications of trick-
ling filters?
6.6B What are the principal differences between standard-
rate,and high-rate filters?
6-16
-------
RECIRCULATION LINE
INFLUENT
EFFLUENT
PRIMARY CLARIFIER JRICKUNG FILTER SECONDARY CLARIFIER
Typical Single-stage Recirculation Patterns
Typical Two-Stage Recirculation Patterns
Fig. 6.7 Trickling filter recirculation patterns
6-17
-------
6.7 LOADING PARAMETERS
6.70 Typical Loading Rates
STAN DARD-RATE PILTE R:
Media - 6 to 8 ft depth, growth sloughs
periodically
Hydraulic Loading - 25 to 100 gal/day/sq ft
Organic (BOD) Loading - 5 to 25 Ibs BOD/1000 cu ft
HIGH-RATE FILTER:
Media - 3 to 5 ft depth, growth sloughs
continually
Hydraulic Loading - 100 to 1000 gal/day/sq ft
Organic (BCD) Loading - 25 to 300 Ibs BOD/10DO cu ft
6.71 Computing Hydraulic Loading
In computing hydraulic loadings, several bits of information must
be gathered. To figure the hydraulic loading, we must know:
1. The gallons per day applied to the filter, and
2. The surface area of the filter.
NOTE; Hydraulic loadings are expressed as:
gal/sq ft/day,31 or
gal/day/sq ft = gpd/sq ft.11
Both expressions mean the same. The hydraulic rate indi-
cates the number of gallons of wastewater per day applied
to each square foot of surface area or the gallons of water
applied to each square foot each day.
11 Loadings as well as test results should always be presented
using the same units. Theoretically a rate should have the
time unit ^ast (gal/sq ft/day); however, because flows are
calculated as gal/day, it is easier to understand if loadings
are reported as gal/day/sq ft. The Water Pollution Control
Federation's MOP No. 6, Units of Expression for Wastes and
Wacte Treatment, uses both terms.
6-18
-------
Suppose we have a high-rate filter that is fed by a pump rated at
2100 gpm, and the filter diameter is 100 feet.
Hydraulic Loading,
gpd/sq ft
Flow Rate, gpd
Surface Area, sq ft
For our problem, we must obtain the flow rate in gpd and surface
area12 in square feet or ft2.
(a) Flow Rate,
gal 60 min 24 lirs
— a •- - • X - - L — ^^- X - - x — " —
min hr day
= 3,024,000 gal/day
(b) Surface Area,
sq ft
0.785 x (Diameter, ft)2
0.785 x 100 ft x 100 ft
7850 sq ft
(c)
Hydraulic
Loading,
gpd/sq ft
Flow Rate, gpd
Surface Area, sq ft
5,024,000 gpd
7850 sq ft
= 385 gpd/sq ft
385
7850 / 3,024,000.
2 355 0
669 00
628 00
41 000
39 250
1 750
12 Area of a
Circle, sq ft
= 0•785 x Diameter) ft x Diameter, ft, or
= 0.785 D2
6-19
-------
It is important to note in computing hydraulic loadings that when
filter effluent is recirculated to the filter influent, recirculated
flow must be added to the primary clarifier effluent flow in order
to calculate the total hydraulic loading. When filter effluent is
recirculated to the primary clarifier influent, recirculated flow
must be added to the clarifier influent flow.
6.72 Computing Organic (BOD) Loading
Using the same filter as in the above example of hydraulic loading,
assume that the laboratory test results show that the wastewater
being applied to the filter has a BOD of 100 mg/1. We need to know
the pounds of BOD applied per day and the volume of the media in
cu ft.
NOTE; Organic (BOD) loadings are expressed as:
Ibs BOD/1000 cu ft/day, or
Ibs BOD/day/lOQO cu ft.
Both expressions mean the same. The organic loading indi-
cates the pounds of BOD applied per day to the volume of
filter media for treatment.
Organic (BOD) Loading, _ BOD Applied, Ibs/day
Ibs BOD/day/1000 cu ft " Volume of Media in 1000 cu ft
To solve this problem we must first calculate the BOD applied in
Ibs/day and volume of media in cu ft.
cu ft
Volume of Media, . . . _ , .
fi. = (Surface Area, sq ft) (Depth, ft)
= (7850 sq ft) (3 ft)
= 23,550 cu ft
Volume of Media, .
in 1000 cu ft '
r* •«. ^
cu ft units)
= 23.5 thousand cubic feet
6-20
-------
(BOD*
.
M mg day gal
= 2522 Ibs BOD/day
25.22016
Organic BOD Loading, _ ^ BOD Applied, Ibs BOD/ day ^
Ibs BOD/day/ 1000 cu ft ~ Volume of Media (in 1000 cu ft)
107.
2522 Ibs BOD/day /-- •; -------
= 23.5 (1000 cu ft) 23.5/2522.0
= 107 Ibs BOD/day/ 1000 cu ft 172 0
164 5
In computing BOD loadings, it is standard practice to ignore the BOD
of the recirculated effluent, where recirculation is used. To attempt
to perform this calculation (using the recirculated load) is compli-
cated and makes it difficult to compare your loadings and resulting
effluent quality with other plants.
13 The units of this formula can be proved by remembering that
one liter equals one million milligrams.
mg _ mg _ _mj
L " 1,000,000 mg " M mg
Therefore,
if. BCD X MGD X 8.34 ^ = S|£ x Sjlti. x £. . Ib BODAUr.
6-21
-------
CHAPTER 7. ACTIVATED SLUDGE
(Lesson 1 of 8 Lessons)
7.0 INTRODUCTION
7.00 General
When wastewater enters an activated sludge plant, the pretreatment
processes (Chapter 4) remove the coarse or heavy solids (grit) and
other debris, such as roots, rags, and boards. Primary clarifiers
(Chapter 5) remove much of the floatable and settleable material.
Normally settled wastewater is treated by the activated sludge
process, but in some plants the raw wastewater flows from the pre-
treatment processes directly to the activated sludge process.
7.01 Definitions
ACTIVATED SLUDGE (Fig. 7.1). Activated sludge consists of sludge
particles produced in raw or settled wastewater (primary effluent)
by the growth of organisms in aeration tanks in the presence of
dissolved oxygen. The term "activated" comes from the fact that
the particles are teaming with bacteria, fungi, and protozoa.
ACTIVATED SLUDGE PROCESS (Fig. 7.1). The term activated sludge
process refers to a method or process of wastewater treatment.
In this treatment process there is maintained a biological culture
consisting of a large number of organisms. All of them require
food (wastewater or substrate) and oxygen to make the process work.
The bacterial population is maintained at some mass (solids concen-
tration) 1 to balance the food available from the wastewater for the
microorganisms2 (food/microorganism ratio) with the oxygen input
capability of the plant equipment.
1 Solids Concentration. The solids in the aeration tank
carry bacteria that feed on wastewater.
2 Microorganisms, Very small organisms that can be seen
only through a microscope. Some microorganisms use
the wastes in wastewater for food and thus remove or
alter much of the undesirable matter.
7-1
-------
AERATION TANK
SECONDARY CLARIFIER
EFFLUENT
Fig. 7.1 Activated sludge and activated sludge process
7-2
-------
7.02 Process Description
Secondary treatment in the form of the activated sludge process
(Figs. 7.2 and 7.3) is aimed at oxidation and removal of soluble
or finely divided suspended materials that were not removed by
previous treatment. This is accomplished in an aeration tank
by aerobic organisms within a few hours when the water is being
treated while it flows through the tank. Soluble or finely
divided suspended solids are intended to be stabilized3 in the
aeration tank by partial oxidation to form carbon dioxide, water,
sulfates, and nitrates. Remaining solids are intended to be con-
verted to a form where they can be settled and removed as sludge
during clarification.
After the aeration period the wastewater is routed to a secondary
settling tank for a liquid-organism (water-solids) separation.
Settled organisms are quickly returned back to the aeration tank.
The resultant clarifier effluent is usually chlorinated and dis-
charged from the plant.
Conversion of dissolved and suspended material to settleable solids
is the main objective of high-rate activated sludge processes,
while low-rate processes stress oxidation. The oxidation may be
by chemical or biological processes. In the activated sludge process,
the biochemical oxidation carried out by living organisms is stressed.
The same organisms also are effective in conversion of substances to
settleable solids if the plant is operated properly.
3 Stabilized Waste. A waste that has been treated or decomposed
to the extent that, if discharged or released, its rate and
state of decomposition would be such that the waste would not
cause a nuisance or odors.
7-3
-------
FUNCTION
REMOVAL
P&IMM2Y
AMP
BOC&.
ANP HELP* KEMDVE: O/L
K/LL
Fig. 7.2 Flow diagram of a typical plant
7-4
-------
PRETREATMENT
EXCESS
ACTIVATED
SLUDGE
AERATION
TANK
RETURN
ACTIVATED
SLUDGE
SECONDARY
CLARIFIER
SUPERNATANT
ANAEROBIC
DIGESTER
(PRIMARY)
SOLIDS
DEWATERING
DIGESTER
(SECONDARY
CHLORINE
CONTACT
TO
RECEIVING
'WATERS
CHLORINATION
Fig. 7.3 Plan layout oi" a typical activated sludge plant
-------
7.1 REQUIREMENTS FOR CONTROL
Control of the activated sludge process is based on evaluation
of and action upon several interrelated factors to favor effective
treatment of the influent wastewater. These factors include:
1. Effluent quality requirements.
2. Wastewater flow, concentration, and characteristics
of the wastewater received.
3. Amount of activated sludge (containing the working
organisms) to be maintained in the process relative
to inflow,
4. Amount of oxygen required to stabilize wastewater
oxygen demands and to maintain a satisfactory level
of dissolved oxygen to meet organism requirements,
5. Equal division of plant flow and waste load between
duplicate treatment units (two or more clarifiers or
aeration tanks).
6. Transfer of the pollutional material (food) from the
wastewater to the floe mass (solids or workers) and
separation of the solids from the treated wastewater,
7. Effective control and disposal of inplant residues
(solids, scums, and supernatants) to accomplish
ultimate disposal in a nonpollutional manner.
8. Provisions for maintaining a suitable environment for
the work force of living organisms treating the wastes.
Keep them healthy and happy.
Effluent quality requirements may be stated by your regularoty
agency in terms of percentage removal of wastes. Current regu-
lations frequently specify allowable quantities of wastes that
may be discharged. These quantities are based upon flow and
concentrations of significant items such as solids, oxygen demand,
coliform bacteria, nitrogen, and oil as specified by your regulatory
agencies.
The effluent quality requirements largely determine the mode of
activated sludge operation and the degree of control required.
For example, if an effluent containing 50 mg/1 of suspended solids
and BOD (refers to five-day BOD) is satisfactory, a high-rate
activated sludge process is likely to be applicable. If the limit
7-6
-------
is 10 mg/1, the high-rate process would not be suitable.
If a high degree of treatment is required, very close process
control and additional treatment after the activated sludge process
may be needed.
Flow concentrations and characteristics of the influent are subject
to limited control by the operator. Municipal ordinances may
prohibit discharge to the collection system of materials signifi-
cantly damaging to treatment structures or safety. Control over
wastes dumped into the collection system requires inspection to
insure compliance. It may be necessary to require alternate means
of disposal, pretreatment, or controlled discharge of significantly
damaging items to permit dilution to an acceptable level by the
time the waste arrives at the treatment plant,
The material entering the aeration tanks is mixed with the acti-
vated sludge to form a mixture of sludge, carrier water, and
influent solids. These solids come from roofs or streets in
combined sewer systems and also from the discharges from homes,
factories, and businesses. Included in the return sludge solids
are many different types of helpful living organisms that were
grown during previous contact with wastewater. These organisms
are the workers in the treatment process. They use the incoming
wastes for food and as a source of energy for their life processes
and for the reproduction of more organisms. These organisms will
use more food contained in the wastewater in treating the wastes.
The activated sludge also forms a lacy mass that entraps many
materials not used as food.
Some organisms (workers)
will require a long time
to use the available food
in the wastewater at a
given waste concentration.
Many organisms will compete
with each other in the use
of available food (waste)
to shorten the time factor
and increase the portion of
waste stabilized. The ratio
of food to organisms is a
primary control in the
activated sludge process.
Organisms tend to increase
with waste (food) load and
time spent in the aeration
tank. Under favorable con-
ditions the operator will
remove (sludge wasting) the
excess organisms to maintain
7-7
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the required number of workers for effective waste treatment,
Therefore, removal of organisms from the treatment process
(sludge wasting) is a very important control technique.
Oxygen, usually supplied from air, is necessary to sustain the
living organisms and for oxidation of wastes to obtain energy
for growth. Insufficient oxygen will inactivate aerobic organ-
isms, make facultative4 organisms work less efficiently, and
favor production of foul-smelling intermediate products of
decomposition and incomplete reactions.
An increase in organisms in an aeration tank will require greater
amounts of oxygen. More food in the influent encourages more
organism activity and more oxidation; consequently, more oxygen
is required in the aeration tank. An excess of oxygen is required
for complete waste stabilization. Therefore, the dissolved oxygen
(DO) content in the aeration tank is an essential control test.
Some minimum level of oxygen must be maintained to favor the desired
type or organism activity to achieve the necessary treatment effi-
ciency.
Flows must be distributed evenly among two or more similar treat-
ment units. If your plant is equipped with a splitter box or a
series of boxes, it will be necessary to periodically check and
estimate whether the flow is being split as intended.
Activated sludge solids concentrations in the aerator and the
secondary clarifier should be determined by the operator for
process control purposes. Solids are in a deteriorating condition
as long as they remain in the secondary clarifier. Depth of sludge
blanket in the secondary clarifier and concentrations of solids in
the aerator are very important for successful wastewater treatment.
Centrifuge tests will give a quick estimate of solids concentrations
and locations in the units. Precise solids tests should be made
periodically for comparison with centrifuge solids tests. Before
any changes are made in the mode of operation, precise solids
measurements should be obtained. Settleability tests show the
degree and volume of solids settling that may be obtained in a
secondary clarifier; however, visual plant checks show what is
actually happening.
Facultative (FACK-ul-tay-tive). Facultative bacteria can use either
molecular (dissolved) oxygen or oxygen obtained from food materials.
7-8
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Primary clarifiers remove easily settleable or floatable material.
Activated sludge tends to convert soluble solids to suspended
cell mass material and to gather and agglomerate5 particles too fine
to settle rapidly into readily separated material. If the soluble
solids transfer fails, then the process fails to provide a satis-
factory effluent.
There must be organisms, oxidizing conditions, and suitable time
to cause the conversion of soluble solids and to agglomerate the
fine particles to form a floe mass.
This floe mass consists of millions of organisms (1012 to 1018/100 ml
in a good activated sludge), including bacteria, fungi, yeast,
protozoa, and worms. When a floe mass is returned to the aerator
from the final clarifier, the organisms grow as a result of taking
food from the inflowing wastewater. The surface of the floe mass
is irregular and promotes the transfer of wastewater pollutants
into the solids by means of mechanical entrapment, absorption,
adsorption, or adhesion. Many substances not used as food also
are transferred to the floe mass, thus improving the quality of
the plant effluent.
Material taken into the floe mass is partially oxidized to form
cell mass and oxidation products. Ash or inorganic material (silt
and sand) taken in by the floe mass increases the density of the
mass. Mixing in the aerator promotes collisions and thus produces
larger floe masses. The net effect after the aeration period is
to form a floe mass which will separate from the wastewater and
settle to the bottom of the secondary clarifier. This sludge contains
most of the residual contaminants and organisms.
Growth of organisms and accumulated residues produce solids for
disposal (waste activated sludge). Certain materials are converted
and removed from the wastewater to the atmosphere in the form of
stripped gases (carbon dioxide or other volatile gases), and also
as water and as solids (sludge). To produce a good effluent the
operator must strive to minimize the return of these solids (other
than as return sludge) to the process. They must be removed from
the wastewater being treated and disposed of in the plant by a
manner which prevents any material from returning to the plant flow.
For example, maintain as high a concentration of solids in the return
sludge as possible to reduce the amount of water needed to return
these solids back to the aerator. Don't pump waste activated sludge
5 Agglomerate. To cause the growing or coming together of dispersed
suspended matter into larger floes or particles which settle rapidly.
7-9
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directly to an anaerobic digester because it will return to the
aeration tank as supernatant with an added load on the organisms.
The organisms have already attempted to treat the solids once and
won't be too effective next time. If screenings are removed at
the headworks, don't grind them up and return them to the plant
flow. Once material is removed from the wastewater, keep it out,
except as necessary to maintain the process.
To maintain the working organisms in the activated sludge, you
must provide a suitable environment. Intolerable concentrations
of acids, bases, and other toxic substances are undesirable and
may kill the working organisms. Unduly fluctuating loads may cause
overfeeding, starvation, and other factors that are all capable
of upsetting the activated sludge process. Insufficient oxygen
can cause an unfavorable environment which results in decreased
organism activity.
An outstanding example of a toxic substance added by operators is
the uninhibited use of chlorine for odor control (prechlorination).
Chlorination is for disinfection. Chlorine is a toxicant and
should not be allowed to enter the activated sludge process because
it is not selective with respect to type of organisms damaged.
It may kill the organisms that you should be retaining as workers.
Chlorine is effective in disinfecting the plant effluent after
treatment by the activated sludge process.
The successful operation of an activated sludge plant requires the
operator to be aware of the many factors influencing the process
and to check them repeatedly. The actual control of the process
as outlined in this section is relatively simple. Control consists
of maintaining the proper solids (floe mass) concentration in the
aerator for the waste (food) inflow by adjusting the waste sludge
pumping rate and regulating the oxygen supply to maintain a satis-
factory level of dissolved oxygen in the process.
QUESTIONS
7.1A Why is air added to the aeration tank in the
activated sludge process?
7.IB What happens to the air requirement in the aeration tank
when the strength (BOD) of the incoming water increases?
7.1C What factors could cause an unsuitable environment for
the activated sludge process in an aeration tank?
END OF LESSON 1 OF 8 LESSONS
on
ACTIVATED SLUDGE
7-10
-------
CHAPTER 7. ACTIVATED SLUDGE
(Lesson 2 of 8 Lessons)
7.2 BASIC VARIABLES AND RECORD KEEPING
7.20 General
Wastewater flows and constituents fluctuate daily. The activated
sludge plant operator attempts to maintain the process at some
balanced state that will be capable of handling the minor variations
in flows or wastewater characteristics and produce the desired
quality of effluent. To accomplish this goal he must establish his
process on known data and knowledge obtained at other plants and
relate them to his plant. After his plant becomes operational, he
then must relate his control procedures to his own experience. The
variations that affect his operation are derived from two sources:
(1) the dischargers to the collection system and (2) inplant
operational variables.
7.21 Variables in Collection System
7.210 Combined Sewer Systems
During storms the treatment plant will receive an increase in flow
which may cause the following problems:
1. Reduced wastewater time in treatment units (hydraulic
overload).
2. Increased amounts of grit and silt which lower the
volatile (food) content of the solids.
3. Increased organic load during initial washout of
accumulated sewer deposits.
4. Rapid changes in wastewater temperature and solids content.
7.211 Waste Dischargers to the System
Various industries and businesses can cause considerable fluctuation
in flows and waste characteristics entering a plant. You should become
7-11
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acquainted with the managers
of plants whose discharges
could upset your treatment
processes, Convince these
men in a friendly manner how
vital it is to your plant
processes and the receiving
waters for you to be notified
of any potentially harmful
discharges. Try to obtain
their cooperation and request
them to notify you whenever
an accidental spill, a process
change, or a cleaning operation
occurs which could cause un-
desirable waste discharges.
This requires diplomacy to
obtain cooperation from dis-
chargers to regulate their own
discharges and to reduce the
number of midnight dumps.
7.212 Maintenance of the Collection System
Advance notice of collection system maintenance crew activities
can be very helpful. If a lift station has been out of service
for a period of time, large volumes of septic wastewater could cause
a shock load on your treatment processes. Similar problems could
be created when a blockage in a line is cleared or a new line is
connected to the system. Analysis of inflow quantities and charac-
teristics when these flows reach a treatment plant can indicate
whether or not they will cause a serious problem.
7.22 Operational Variables
Continual review of laboratory test results is essential in determining
whether a treatment plant is discharging effluent of the required
quality in terms of such water quality indicators as COD, suspended
solids, and nitrogen. If the desired quality of the plant effluent
is not achieved, the operator must determine what factor or factors
have changed to upset plant performance and thus reduce efficiency.
Important factors that could have changed include:
1. Higher COD or BOD load applied to the aerator
(influent load).
2. More difficult to treat wastes have adversely
changed influent characteristics.
7-12
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3. Unsuitable mixed liquor suspended solids concentra-
tion in the aerator.
4. Lower or higher rate of wasting activated sludge.
5. Unsuitable rate of returning sludge to the aerator
could adversely influence mixed liquor suspended solids.
6. Higher solids concentrations in digester supernatant6
returned to the plant flow, or return too rapid.
7. Dropping of oxygen concentration in the aerator below
desirable levels.
Examination of plant records should reveal the items which have
changed that could have upset the treatment process.
QUESTIONS
7.2A What two major variables affect the way an activated
sludge plant is operated?
7.2B What variables in the collection system can affect the
operation of an activated sludge plant?
7.2C What problems can be caused in an activated sludge plant
when excessive storm water flows through the process?
6 Supernatant (sue-per-NAY-tent). Liquid removed from settled
sludge. Commonly refers to the liquid between the sludge on
the bottom and the scum on the surface of an anaerobic digester.
This liquid is usually returned to the influent wet well or the
primary clarifier.
7-13
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7.24 Typical Lab Results for an Activated Sludge Plant
Typical results of lab tests for an activated sludge plant are
provided to assist in the evaluation of lab results and plant
performance. Remember that every plant is different and is
influenced by different conditions.
Test
COD
BOD
SUSPENDED
SOLIDS
DISSOLVED
OXYGEN
CHLORINE
RESIDUAL
(30 min.)
Location
Influent
Primary Effl.
Final Effl.
(Conv. Act. SI.)
Influent
Primary Effl.
Final Effl.
(Conv. Act. SI.)
Influent
Primary Effl.
Mixed Liquor
Return Sludge
Final Effl.
(Conv. Act. SI.)
Mixed Liquor
Final Effl. (Outfall)
Final Effl.
Common Range
300
200
30
150
100
10
150
60
1000
2000
10
2
2
0.5
- 700
- 400
- 70
- 400
- 280
- 20
- 400
- 160
- 4500
- 10,000
- 20
- 4
- 6
- 2.0
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1*
COLIFORM GROUP
BACTERIA, MPN
CLARITY
(Secchi Disc)
PH
Final Effl.
(Chlorinated)
Final Effl.
Influent
Effluent
23 - 700/100 ml
3-8
6.8 - 8.0
7.0 - 8.5
ft
Regulatory agencies normally specify a chlorine residual
remaining after a certain time period.
7-14
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7.25 Design Variables
Several different types of activated sludge plants have been
built using various flow arrangements, tank configurations, or
oxygen application equipment. However, all of these variations
are essentially modifications of the basic concept of conven-
tional activated sludge.
7.250 Aeration Methods
Two methods are commonly used to supply oxygen from the air to
the bacteria—mechanical aeration and di ffus ed aeration. Both
methods are mechanical processes with the difference being
whether the mechanisms are at or in the aerator or at a remote
location.
Mechanical aeration devices agitate the water surface in the
aerator to cause spray and waves by paddle wheels
mixers, rotating brushes, or some other method of splashing
water into the air or air into the water where the oxygen can
be absorbed.
Mechanical aerators in the tank tend to be lower in installation
and maintenance costs. Usually they are more versatile in terms
of mixing, production of surface area of bubbles, and oxygen
transfer per unit of applied power.
Diffused air systems use a device called a diffuser
which is used to break up the air stream from the blower system
into fine bubbles in the mixed liquor. The smaller the bubble,
the greater the oxygen transfer due to the greater surface area
of rising air bubbles surrounded by water. Unfortunately, fine
bubbles will tend to regroup into larger bubbles while rising
unless broken up by suitable mixing energy and turbulence.
7-15
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7.251 Variation of Activated Sludge Process
The activated sludge plant may be operated in any one of three
operational zones on the basis of "sludge age"10 which is an
expression of pounds of organic loading added per day per pound
of organisms maintained in the particular process. Sludge age
is a control guide that is widely used and is an indicator of
the length of time a pound of solids is maintained under aeration
in the system. If the amount of solids under aeration remains
fairly constant, then an increase in the influent solids load
will decrease the sludge age. Use of this measure of sludge
age is recommended for the new activated sludge plant operator
because of the ease in understanding this approach. The experienced
operator may not accept this method of control because it ignores
the soluble COD that is related to the solids production but not
measured by suspended solids tests on the influent.
The following values are typical sludge ages for different types
of municipal activated sludge plants with negligible industrial
wastes. Actual loadings must be related to the type of waste and
local situation.
1§ High-Rate. A high-rate activated sludge plant operates at
the highest loading of food to microorganisms; the sludge
age ranges from 0.5 to 2.0 days. Due to this higher loading
the system produces a lower quality of effluent than the
other types of activated sludge plants. This system requires
greater operational surveillance and control and is more
easily upset.
2. Conventional. Conventional activated sludge plants are the
most common type in use today. The loading of food to micro-
organisms is approximately 50% lower than in a high-rate plant,
and the sludge age ranges from 3.5 to 7.0 days. This method
of operation produces a high quality of effluent and is capable
of absorbing some shock loads without effluent quality being
adversely affected.
10 Sludge Age, days =
(Suspended Sol, in Mixed Liq.t mg/1) (Aerator Vol., MG) (8.34 Ibs/gal)
(Suspended Sol. in Primary Effl., mg/1) (Flow, MGD) (8.34 Ibs/gal)
Suspended Solids Under Aeration, Ibs
Suspended Solids Added, Ibs/day
7-16
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3. Extended Aeration. Extended aeration is commonly employed in
smaller package-type plants or so-called complete oxidation
systems. These are the most stable of the three processes
due to the light loading of food to microorganisms, and the
sludge age is commonly greater than ten days. Effluent
suspended solids commonly are higher ttian found under con-
ventional loadings.
For a summary of the loadings for different types of activated
sludge processes, see Table 7-1.
There are other variations of activated sludge processes such as
contact stabilization, step-feed, Kraus and complete mix which are
discussed in Section 7.9.
QUESTIONS
7.2G List two methods of supplying oxygen from air to bacteria
in the activated sludge process.
7.2H Write the formula for calculating sludge age.
END OF LESSON 2 OF 8 LESSONS
on
ACTIVATED SLUDGE
7-17
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TABLE 7-1
AERATION TANK CAPACITIES AND PERMISSIBLE LOADINGS*
PROCESS
Modified or
"HIGH-RATE"
Conventional
Extended
Aeration
PLANT DESIGN
FLOW, MGD
All
To 0.5
0.5 to 1.5
1.5 up
All
AERATION RETENTION
PERIOD, HOURS
BASED ON DESIGN FLOW
2.5 up
7.5
6.0 to 7.5
6.0
24
PLANT DESIGN
LOAD
Ib BOD/day
2000 up
To 1000
1000 to 3000
3000 up
All
AERATOR LOADING
Ib BOD per day/lb MLSS
1/1 (or less)
1/2 to
1/4
As high as 1/10
to
As low as 1/20
SLUDGE
AGE,
DAYS
0.5 - 2.0
3.5 - 7.0
10
or
Longer
I
H-"
00
* Recommended Standards for Sewage Works (10 State Standards), Great Lakes-Upper Mississippi
River Board of State Sanitary Engineers, 1968 Edition, published by Health Education Service,
P. 0. Box 7283, Albany, New York 12224. Price, $1.00.
-------
CHAPTER 7. ACTIVATED SLUDGE
(Lesson 8 of 8 Lessons)
7.8 AERATOR LOADING PARAMETERS
7.80 General
Sludge age has been suggested as the method for controlling the
solids in the activated sludge process. Other operational con-
trols used successfully by operators include the waste load
(food)/sludge volatile solids (organisms) ratio and the mean
cell residence time (MCRT). Mathematically, one can show that
aerator loadings based on sludge age, food/organism ratio, and
MCRT are theoretically similar. In each case the operator
selects a number or value for the parameter to start with on
the basis of experience and data available from other plants.
He then adjusts this value until he finds an operating range
which produces the best quality effluent for his plant.
In each case, the critical factor is the food/organism relation-
ship which cannot be precisely estimated for any specific plant.
The operator attempts to maintain in the aerator tank sufficient
solids (organisms) to use up the incoming waste (food). He
doesn't want too many organisms nor too few organisms in the
aeration tank in relation to the incoming food. Operation of
the activated sludge process requires removing the organisms
(settled activated sludge) from the secondary clarifier as
quickly as possible. The organisms are either returned to the
aerator to use the incoming food, or they are wasted. Therefore,
a critical decision is to determine the amount of solids to be
wasted. This procedure has been discussed and an example pro-
vided in Section 7.52 for the sludge age aerator loading parameter.
Select a method to operate your plant and stick with it. Don't
continually try to switch from one method to another.
7.81 Food/Organism Ratio
The food-to-organism loading ratio is based upon the food provided
each day to the microorganism mass in the aerator. Food (waste)
provided is preferably measured by the COD of the influent to the
aerator. COD is recommended because test results are available
7-19
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within four hours and process changes can be made before the
process becomes upset. Many operators load aerators on the
basis of the BOD test, but results five days later are too
late for operational control. The ratio of food load provided
each day to the volatile solids in the aerator is the recip-
rocal of the sludge age (see Table 7-1, Section 7.25). Typical
loading parameters have been established for the three opera-
tional zones of activated sludge and are summarized as follows:
1. Hi_gh-Rate
COD: 1 Ib COD per day/1 Ib of MLVSS18 under aeration.
BOD: >0.5* Ib BOD per day/1 Ib of MLVSS under aeration.
2. Conventional
COD: 0.5 to 1.0 Ib COD per day/1 Ib of MLVSS under aeration.
BOD: 0.25 to 0.5 Ib BOD per day/1 Ib of MLVSS under aeration.
3. Extended Aeration
COD: <0.2* Ib COD per day/1 Ib MLVSS under aeration.
BOD: 0.05 to 0.10 Ib BOD per day/1 Ib MLVSS under aeration.
* > means greater than. Greater than 0.5 Ib BOD.
< means less than, Less than 0.2 Ib COD.
7.82 Calculation of Food/Organism Aerator Loading
Determine the amount of mixed liquor volatile suspended solids to
be maintained in the aerator of the conventional plant studied in
this chapter. Assume a food/organism ratio of 0.5 Ib COD per day/
1 Ib of mixed liquor volatile suspended solids under aeration.
Frequently this loading is expressed as 50 Ibs COD per day/100 Ibs
of MLVSS.
18 MLVSS means Mixed jLiquor Volatile Suspended £olids.
7-20
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Information needed:
1. Average COD of primary effluent, ISO mg/1
2. Average daily flow, 4,0 MGD
3, Average volatile content of mixed liquor
suspended solids, 80%
Find pounds of COD provided aerator per day.
Aerator
Loading, = Prim. Effl. COD, mg/1 x Daily Flow, MGD x 8.34 Ibs/gal
Ibs COD/day
= 150 mg/1 x 4.0 MGD x .8.34 Ibs/gal
= 5004 or 5000 Ibs COD/day
Find desired pounds of Mixed Liquor Volatile Suspended Solids under
aeration, based upon 0.5 Ib COD 'per day/1 Ib of MLVSS. ""
MLVSS, _ Primary Effluent COD, Ibs/day
Ibs = Loading Factor in Ibs COD/day/1 Ib MLVSS
ML
5000 Ibs COD/da
= 0.5 Ib COD/day/Ib MTVSS
5000
= 0.5/lbs MLVSS
= 10,000 Ibs MLVSS under aeration
The MLVSS is a measure of the organisms in the aerator available to
work on the incoming waste (food). When operating your plant on the
basis of MLVSS, you should determine any flucuations that may occur
during the week and make appropriate adjustment.
7-21
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If the COD load applied to the aerator increases or drops to a
significantly different level for two consecutive days, a new
mixed liquor solids value should be calculated and activated
sludge wasting adjusted to achieve the new value of solids desired
under aeration. Calculation of waste sludge rates is outlined in
Sections 7.52 and 7.53.
7.83 Mean Cell Residence Time (MCRT)
Another approach for solids control used by operators is the Mean
Cell Residence Time (MCRT) or Solids Retention Time (SRT). This
is a refinement of the sludge age. Both terms are almost the
same. The equation for MCRT is:
MCRT, _ Pounds of Suspended Solids in Total Secondary System
days = Lbs of Susp Sol Wasted/day + Lbs of Susp Sol Lost in EffI/day
The most desirable MCRT for a given plant is determined experimentally
just as with the use of sludge age or the mixed liquor volatile sus-
pended solids concentration. The desired MCRT for conventional plant
operation should fall between 5 and 15 days. (Don't confuse this time
with the recommended range for Sludge Age of 3.5 to 10 days.)
A way of determining MCRT for the example plant in this chapter
would be as follows:
Required Data:
1. Aerator = 1,000,000 gals
2. Final clarifier volume = 500,000 gals
Total secondary system volume = 1.5 MG
3. Wastewater flow to aerator = 4.0 MGD
4. Waste sludge flow for past 24 hrs = 0.075 MGD
5. Mixed liquor suspended solids
concentration = 2400 mg/1
6. Waste sludge (or return sludge)
suspended solids concentration = 6200 mg/1
7. Final effluent suspended solids
concentration = 12 mg/1
7-22
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7.84 Calculation of Mean Cell Residence Time
MCRT,
days
MCRT
Suspended Solids in Total Secondary System, Ibs
Susp Sol Wasted, IbsV'day + Susp Sol Lost in Effl, Ibs/day
Susp Sol in Mixed Liq, mg/1 x (Aerator, MG + Final Clari-
fier Vol. MG] x 8.54 Ibs/gal .
(Susp Sol in Waste, mg/1 x Waste Rate, MGD x 8.34 Ibs/gal)
+(Susp Sol in Effl, mg/1 x Plant Flow, MGD x 8.34 Ibs/gal)
2400 mg/1 x (1.0 MG + 0.5 MG) x 8.54 Ibs/gal
(6200 mg/1 x 0.075 MGD xS.34 Ibs/gal)
+ (12 mg/1 x 4.0.MGD x 8.34 Ibs/gal)
2400 mg/1 x 1.5 MG x 8.34 Ibs/gal
(6200 mg/1 x 0.075 MGD X 8.34 Ibs/gal)
+ (12 mg/1 x 4.0 MGD x 8.34 Ibs/gal)
30,024 Ibs
3878 Ibs/day + 400 Ibs/day
30,024 Ibs
4278 Ibs/day
= 7.0 days
If you are operating the plant on the basis of MCRT and the plant
operates satisfactorily at the MCRT of 8, 9, 10, 11,^or even 15
days, the main method of control is to adjust the waste sludge
rate to maintain the MCRT at the desired number of days.
7-23
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Rearranging the equation on the previous page, calculation of the
sludge waste rate from the system merely means plugging in the
chosen MCRT (use 7 days) and solids figures.
Example;
Waste
ci j Susp Sol in System, Ibs c c , . crr. ., ,,
Sludge, = V . ' 7 —l . Susp Sol in Effl, Ibs/day
Ibs/day MLK1' days
2400 mg/1 x 1.5 MG x 8.54 Ibs/gal 12 mg/1 x 4.0 MGD
7 days " x 8.34 Ibs/gal
30,024 Ibs Ann ., .,
= 7 days - 4°° lbS/day
= 4289 Ibs/day - 400 Ibs/day
= 3889 Ibs/day
The waste sludge pumping rate of 3878 Ibs/day appears to be correct
to maintain a Mean Cell Residence Time of 7 days.
QUESTIONS
7.8A Why is it sometimes necessary to waste some activated sludge?
7.8B If you calculate that your plant has 12,000 pounds of mixed
liquor volatile suspended solids under aeration and you need
9,000 pounds under aeration, how many pounds should be wasted?
7.8C -What should be the waste sludge pumping rate (GPM) if a plant
should be wasting 3000 pounds per day and the concentration
of return sludge is 6000 mg/1?
7.8D Estimate the waste sludge rate (Ibs/day) from an activated
sludge plant operating at an MCRT of 10 days. The system
contains 40,000 pounds of suspended solids and the effluent
has a suspended solids concentration of 10 mg/1 at a flow
of 5 MGD.
7-24
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7.9 MODIFICATIONS OF THE ACTIVATED SLUDGE PROCESS
7.90 Reasons for Other Modes of Operation
Modification of the conventional activated sludge process has been
developed to improve operational results under certain circumstances,
Some of these conditions may be:
1. Current or actual loadings are in excess of
design loading for conventional operation.
2. Wastewater constituents require added nutrients
to properly treat influent waste load;
3. Flow or strength of waste varies seasonally.
7.91 Contact Stabilization (Fig. 7.9)
Operation of an activated sludge plant on the basis of contact
stabilization requires two aeration tanks. One tank is for
separate reaeration of the return sludge for a period of at
least four hours before it is permitted to flow into the other
aeration tank to be mixed with the primary effluent requiring
treatment. Loading factors are the same as for conventional
activated sludge, but at times the solids in the aeration tank
may be almost twice as high as normal ranges in conventional
plants.
If the solids content in aeration tank "A" (mixed liquor aerator,
Fig; 7.9) and aeration tank "B" (return sludge aeration only) are
combined, the loading ratio .of food/organisms is the same as
conventional operation, but if you only look at aeration tank "A"
where the load is applied, we approach double the load ratio
established for conventional activated sludge.
Contact stabilization attempts to have organisms assimilate and
store large portions of the influent waste load in a short time
(as short as 30 minutes). The activated sludge is separated from
the treated wastewater in the secondary clarifier and returned to
the reaeration tank "B". No new food is added to the reaeration
tank and the organisms must use the waste material they collected
and stored in the first aeration tank. When the stored food is
used up, the organisms begin searching for more food and are
ready to be returned to tank "A".
Process controls for a contact stabilization plant are the same
as those described for a conventional plant in this chapter. When
a plant has exceeded design flows, or is subject to periodic high
7-25
-------
INFLUEN
I
N>
MIXED LIQUOR
AERATOR 'A'
REAERATED
RETURN
SLUDGE
RETURN SLUDGE
REAERATION
AERATOR 'B1
Fig. 7.9 Plan layout of contact stabilization plant
-------
flows or shock waste loads, then contact stabilization is capable
of treating the plant influent because a ready reserve of organisms
is available in the reaeration tank "B".
7.92 Kraus Process (Fig. 7.10)
The Kraus process is a modification of conventional activated
sludge, and the process is patented by its developer. The process
is widely used when the wastewater contains a much greater ratio
of carbonaceous to nitrogenous material than found in normal
domestic wastewater.
This imbalance commonly occurs when wastes from canneries or
dairies are treated. When the organisms use all of a limiting
constituent, they refuse to remove the remaining portions of
the other constituents. Normally this nutrient deficiency is
nitrogenous material which is readily available in anaerobic
digester supernatant and sludges. Feeding anaerobic digester
supernatant or digester sludge to the aeration system will
usually supply the proper nutrients to maintain the balance.
The method of application is very important.
In the Kraus process, the return sludge is sent to the reaeration
aerator ("B") to be mixed with the digested sludge from a completely
mixed digester. In the reaeration tank ("B"), the digested sludge
and the return sludge are mixed, reaerated, and then sent to the
mixed liquor aerator ("A"). The amount of digested sludge introduced
to the system is determined by laboratory evaluation and by carbo-
naceous material removal through the system.
The same controls apply as described for controlling a conventional
activated sludge plant. The main objective is to properly balance
nutrients; however, one added advantage (similar to contact stabili-
zation) is the ability to maintain a large mass of organisms under
aeration in a relatively smaller system.
7.93 St ep-Feed Ae ra t i on (Fig. 7.11)
Step-feed aeration actually is a step-feed process based on con-
ventional activated sludge loading parameters. The difference
between step-feed and conventional operation is that in conventional
activated sludge the primary effluent and return sludge are intro-
duced at one point only, the entrance to the aeration tanks. In
step-feed aeration the return sludge is introduced separately and
in many cases allowed a short reaeration period by itself at the
entrance to the tank. The primary effluent is admitted to the aeration
tanks at several different locations. These locations distribute
7-27
-------
i
N3
CO
INFLUENT
ANAEROBIC
DIGESTER
AERATION
TANK 'A'
AERATION
TANK 'B'
DIGESTER SLUDGE
SECONDARY
CLARIFIER
EFFLUENT
RETURN
SLUDGE
Fig. 7.10 Kraus process
-------
MODES OF FLOW
100% Primary Effluent
Conventional Flow Activated Sludge Process
MODE 1
Return Sludge
Flow
Aerator
Effluent
MODE 2
Return Sludge
25%
25%
Flow
Step-Feed
25%
\ \ \
25% Primary Effluent
\
Aerator
Effluent
MODE 3
Return Sludge
Step-Feed
33-1/3% 33-1/3% 33-1/3%
\
\
Flow
Aerator
Effluent
Step-Feed
MODE 4
Return Sludge
0%
\
-V
0% 50%
\ \
1
1
Flow |
50%
\
Aerator
Effluent
or Contact Stabilization
100% Influent
Several possible modes ,of feeding primary effluent to the aeration
tanks. Some tanks may have more or fewer points of discharge into
the tank.
Fig. 7.11 Modes of step aeration
7-29
-------
the waste load over the aeration tank and reduces oxygen sags in
an aerator. If you introduce the influent near the outlet end
of the aeration tank, the process will become similar to contact
stabilization.
Step-feed aeration distributes the oxygen demand from the wastewater
over the entire aerator instead of concentrating it at the inlet end.
Some of its advantages over conventional operation include less
aeration volume to treat the same volumes of wastewater, better con-
trol in handling shock loads, and better control of the solids entering
the secondary clarifiers. When a conventional plant is operating
above design loads and the secondary clarifiers cannot handle
the solids load, switching to step-feed aeration or contact
stabilization allows the operator to maintain the desired amount.
of solids under aeration. Successful operation requires good waste
storage transfer into the solids in the short time interval before
the waste reaches the effluent end of the aeration tank.
This mode of operation is controlled by the same procedures
used for the conventional process except that the mixed liquor
suspended solids determinations must be made at each point of
wastewater addition to measure the waste content and dilution
factor provided by the primary effluent to determine the total
pounds of solids in the aeration tank.
7.94 Complete Mix (Fig. 7.12)
The complete mix mode of operation is a design modification of
tank mixing techniques to insure equal distribution of applied
waste load, dissolved oxygen, and return sludge throughout the
aeration tank. The theory of this modification is that all
parts of the aeration tank should be similar in terms of amounts
of food, organisms, and air. This is accomplished by providing
diffuser location and application points of influent and return
sludge to the aerator at several locations. Providing a similar
condition throughout the entire aeration tank allows a food/organism
ratio of 1/1 and still produces effluent qualities comparable to
conventional operation. Generally, smaller aeration tanks are
more completely mixed than larger ones. Usually aeration is more
efficient in a complete mix facility such as illustrated in
Fig. 7.12 because of the locations of the air headers.
7-30
-------
4
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p
s 1
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1
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_ EFFLUENT
FROM AERATOR
PLAN VIEW
AIR HEADERS - NOTE VARIOUS POSITIONS
(HERRING BONE PATTERN)
RAW WASTE AND RETURN
SLUDGE INLET POINTS
Fig. 7.12 Air header locations in complete mix
7-31
-------
7.95 Modified Aeration (Fig. 7.13).
PRIMARY EFFLUENT
OR RAW WASTEWATER
RETURN
SLUDGE
10%
AERATION TANK
2 HOURS DETENTION
EFFLUENT
EXCESS SLUDGE TO PRIMARY INFLUENT
IF PRIMARY CLARIFIER AVAILABLE
SECONDARY
CLARIFIER
EXCESS SLUDGE
TO THICKENER
Fig. 7.13 Modified aeration
Modified aeration is also known as high-rate activated sludge.
Frequently it is used as intermediate treatment where the dis-
charge requirements demand higher treatment than primary, but
not as high as conventional activated sludge, in terms of BOD
and suspended solids removals.
Either raw wastewater or primary effluent is applied to an aeration
tank with a detention time of two hours and a mixed liquor suspended
solids concentration of less than 1000 mg/1. Air requirements are
lower because of fewer organisms (solids) under aeration. Effluent
quality ranging from primary treatment to conventional activated
sludge treatment can be achieved by the operator by controlling
the air supply, aeration period, and the pounds of solids under
aeration.
7-32
-------
7.10 ACKNOWLEDGEMENT
Mr. F. J. Ludzack, Chemist, National Training Center, Federal
Water Quality Administration, provided many helpful comments to
the development of this chapter. • His contributions are grate-
fully appreciated.
7.11 ADDITIONAL READING
a. MOP 11, pages 108-122.
b. New York Manual, pages 58-69.
c. Texas Manual, pages 236-282.
d. Sewage Treatment Practices, pages 55-62.
e. Jenkins, D., and Garrison, W.E., "Control of Activated Sludge
by Mean Cell Residence Time," JWPCF, Vol. 40, No. 11, p. 1905
(November 1968).
f. McKinney, R.E., and O'Brien, W.J., "Activated Sludge—Basic
Design Concepts," JWPCF, Vol. 40, No. 11, p. 1831 (November
1968).
g. Stewart, M.J., "Activated Sludge Process Variables--The Complete
Spectrum," Water and Sewage Works Magazine, Reference Volume,
p. R-241 (November 30, 1964).
h. Aeration Practice, MOP No. 5, Water Pollution Control Federation,
3900 Wisconsin Avenue, Washington, D.C. 20016. $3.00 to
members, $6.00 to others.
or
Journal Water Pollution Control Federation, Vol. 41, Nos. 11
and 12, and Vol. 42, No. 1.
END OF LESSON 8 OF 8 LESSONS
on
ACTIVATED SLUDGE
7-33
-------
CHAPTER 8. SLUDGE DIGESTION AND HANDLING
(Lesson 1 of 5 Lessons)
8.D INTRODUCTION
Settled solids removed from the bottom and floating scums removed
from the top of clarifiers and sedimentation tanks are a. watery,
odorous mixture called raw sludge and scum. Frequently this raw
sludge is pumped to a sludge digester for treatment before disposal.
In the anaerobic sludge digester, the most common kind, bacteria
decompose the organic solids in the absence of dissolved oxygen.
Figure 8.1 shows the location of an anaerobic sludge digester in
a typical plant. Figures 5.2, 6.2, and 7.2 also show plan views
of the location of sludge digestion and handling facilities in
relation to other treatment processes.
8.00 Purpose of Sludge Digestion
Anaerobic digestion1 reduces wastewater solids from a sticky, smelly
mixture to a mixture that is relatively odor free, readily dewaterable,2
and capable of being disposed of without causing a nuisance.
In the process organic solids are liquefied, the solids volume is
reduced, and valuable methane gas is produced in the digester by the
action of two different groups of bacteria living together in the same
environment. One' group consists of saprophytic organisms,3 commonly
referred to as "acid formers". The second group, which utilized the
1 Anaerobic Digestion (AN-air-0-bick). Wastewater solids and water
(about 5% solids, 95% water) are placed in a large tank where
bacteria decompose the solids in the absence of dissolved oxygen.
At least two general groups of bacteria act in balance:
saprophytic bacteria (see Footnote 3) and methane fermenters
break down the acids to methane, carbon dioxide, and water.
2 Dewaterable. A material is considered dewaterable if water will
readily drain from it. Generally raw sludge dewatering is more
difficult than water removal from digested sludge.
3 Saprophytic Organisms (SAP-pro-FIT-tik). Organisms living on dead
or decaying organic matter. They help natural decomposition of the
organic solids in wastewater.
8-1
-------
FUNCTION!
ANP
BIOLOGICAL CHEMICALS *.
MIL
Fig. 8.1 Flow diagram of typical plant
8-2
-------
acid w.t.cluced by the saprophytes, are the "methane fermenters".
The insthr^e fezwenters are not as abundant in raw wastewater as
are fhe acid formers. The methane fermenters desire a pH range
of 6.5 to 8,0 and will reproduce only in that range.
The object of good digester operation is to maintain suitable
cendi'cicr.s ir< the digester for a growing (reproducing) population
of both acid formers and methane fermenters. You must do this by
the contiol of food supply (organic solids), vo1 ati 1e ac id/aIXa1inity
relationship, mixing, and temperature. Generally, you have done your
job properly if the digester reduces the volatile (organic) solids
content by between 40 and 60% of what they were in the raw sludge.
To obtain the desired degree of organic solids reduction may require
from 5 to 120 days of digestion time. The time required depends on
how good a job you are required to do on digesting the sludge, and on
the adequacy of mixing, the organic loading rate, and the temperature
at which the bacterial culture is maintained.
8.01 How Sludge Digestion Works (by William Garber)
The equations shown in Fig. 8.2 illustrate one way of outlining what
happens in a digester. These equations indicate two general types
of reactions:
1. Acid forming reactions which proceed at a rate
dependent upon temperature, pM, and food conditions.
2. Methane fermentation reactions which proceed at a
rate dependent upon temperature, pH, and food conditions.
You must try to operate an anaerobic sludge digester so that the rate
of acid formation and methane fermentation are approximately equal;
otherwise the reaction will get out of balance. The most common condition
of unbalance that occurs is that the methane fermenters, which are
sensitive anaerobes, fail to keep pace and the digester becomes acid
because the rate at which acids are converted is too low.
The literature has been full of terms such as "Standard-Rate" and
"High-Rate" digestion. These terms refer to digester loading and
not to the rates of bacterial action. In "High-Rate" systems,
mixing is used to obtain the best possible distribution of the
substrate (food) and seed (organism) so that more bacterial reaction
can occur.
8-3
-------
RAW
COMPLEX
""
FATS
PRlNClRALl-V
AC IP FORMES
SAPBOPMYTlC
FACULTATIVE
00
ORGANIC AC1P-S
PRIMCIPALLV
PGOPUCP5
C02,4420.
OC6ANIC ACIP4,
co 2.
ME THANE CA20QN
ANP
Pg-6
PCOPUCT6
Fig. 8.2 Reactions in a digester
-------
Mixing is the most important factor in the so-called "High-Rate"
processes, and it is considered to accomplish the following:
1. Utilize as much of the total content of a digester
as possible.
2. Quickly distribute the raw sludge food throughout the
volume of sludge in the tank.
3. Put the microorganisms in contact with the food.
4. Dilute the inhibitory by-products of microbiological
reactions throughout the sludge mass.
5. Achieve good pH control by distributing buffering alka-
linity throughout the digestion tank.
6. Obtain the best possible distribution of heat through
the tank.
7. Minimize the separation of grit and inert solids to the
bottom or floating scum material to the top.
A digester may be operated in one of three temperature zones or
ranges, each of which has its own particular type of bacteria. The
lowest range (in an unheated digester) utilizes psychroph i1ic (cold
temperature loving) bacteria. ** Temperature of the sludge inside
tends to adjust to the outside temperature. However, below 50°F
little or no bacterial activity occurs and the necessary reduction
in sludge volatiles (organic matter) will not occur. When the
temperature increases above 50°F, bacterial activity increases to
a measurable rate and digestion starts again. The bacteria appear
to be able to survive temperatures well below freezing with little
or no harm. The psychrophilic upper range is around 68°F. Digestion
in this range requires from 50 to 180 days, depending upon the degree
of treatment (solids reduction) required. Few digesters are designed
today to operate in this range, but there are many still in use,
including most Imhoff tanks and similar unheated digesters with no
mixing devices. Generally these digesters are not very effective in
digesting sludge.
** Psychrophilic Bacteria (organisms) (sy-kro-FILL-lick). A group
of bacteria that thrive in temperatures below 68°F.
8-5
-------
The middle range of organisms are called the mesophilic (medium
temperature loving) bacteria5; they-thrive between about 68°F
and 113°F. This is the most common operational range, with
temperatures usually being maintained at about 95 °F to 98°F.
Digestion at that temperature may take from 5 to 50 days or more
(normally around 25 to 30 days), depending upon the required
degree of volatile solids reduction and adequacy of mixing. The
so-called "High-Rate" processes are usually operated within the
mesophilic temperature range. These are nothing more than pro-
cedures to obtain good mixing so that the organisms and the food
can be brought together to allow the digestion processes to pro-
ceed as rapidly as possible. With the most favorable conditions
the time may be no more than five days for an intermediate.level
of digestion.
The third range of organisms are called thermophilic (hot tempera-
ture loving) bacteria,6 and they thrive above 113'°F". The time
required for digestion in this range falls between 5 and 12 days,
depending upon operational conditions and degree of volatile
solids reduction required. However, the problems of maintaining
temperature, sensitivity of the organisms to temperature change,
and some reported problems of poor solids-liquid separation are
reasons why only a few plants have actually been operated in the
thermophilic range.
You cannot merely raise the temperature of the digesters and have
a successful operation in another range. The bacteria must have
time to adjust to the new temperature zone and to develop a
balanced culture before continuing to work. An excellent rule
for digestion is never change the temperature more than one degree
a day to allow the bacterial culture to become acclimated (adjust
to the temperature changes).
Secondary digestion tanks are sometimes used to allow liquids
(supernatant)7 to separate from the solids, to provide a small
amount of additional digestion, and to act as a "seed" source (the
settled, digested sludge). However, digestion tanks generally have
too small a "surface area to depth" ratio to be good sedimentation
tanks. .Separation of solids from liquids is more efficient in
5 Mesophilic Bacteria (mess-0-FILL-lick). A group of bacteria
that thrive in a temperature range between 68°F and 113nF.
6 Thermophilic Bacteria (thermo-FILL-lick). A group of bacteria
that thrive in temperatures above 113°F.
7 Supernatant (sue-per-NAY-tent). In a sludge digestion tank,
the supernatant is the liquor between the surface scum and
the settled sludge on the bottom of the tank.
8-6
-------
lagoons or in tanks designed for separation. If a significant
amount of digestion occurs in the secondary tank, the result
may be poor separation of solids. Secondary digesters should be
used for solids concentration and for a reservoir of alkalinity
and seed sludge which may be returned to the primary digester
when needed.
You have certain other items you can use for control in addition
to mixing and temperature selection. These include:
1. Varying the sludge concentration or water added to
the system.
2. Varying the rate and frequency of feeding, with continuous
feed the most desirable.
3. Closely controlling grit and skimming in order that
capacity of the tank is affected as little as possible
by these materials.
4. Cleaning regularly to maintain capacity.
5. A good maintenance program to maintain the maximum
degree of flexibility.
6. Maintaining records and laboratory control in order
that process condition is known at all times.
Although digestion is a complex process and only a portion of its
theory is understood, enough is known to allow you to exercise
good operational control. For sludge digestion as for any of the
wastewater processes, remember that for the most successful opera-
tion you need to do the following:
1. Understand the theory of the process so you know what
you are basically trying to do.
2. Know your facilities thoroughly so that you can attain
maximum flexibility of operation.
3. Keep careful records and use laboratory analyses to
follo« the process continually.
4. Maintain your facilities in the best possible condition
at all times.
8-7
-------
8.1 COMPONENTS IN THE ANAEROBIC SLUDGE DIGESTION PROCESS
To understand and operate an anaerobic sludge digester, the operator
must be familiar with the location and function of the various com-
ponents of the digestion facility.
8.10 Pipelines and Valves
Raw sludge pipelines are usually constructed of cast iron or steel
to withstand pumping pressures. In recent years glass-lined or
epoxy-lined sludge lines have been used to alleviate the problem
of grease deposits. These deposits cut capacity and may cause
stoppages. Some plants use "go-devil" type cleaners and/or hot
chemical solutions such as T.S.P. instead.
The valves used in sludge and scum lines are mostly of the plug
type. They give positive control where a gate or butterfly valve
may become blocked by rags or other material which will not allow
the valve to seat. In some cases a gate or butterfly valve is indi-
cated because a quick closing plug valve could result in water hammer
and damage the pipeline.
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8-8
-------
QUESTIONS
8.10A Why are plug type valves used in sludge lines?
8.10B Why should a positive displacement pump never
be started against a closed valve?
8.IOC Why should a sludge line never be closed at both
ends?
8.11 The Digester
Digestion tanks may be cylindrical or cubical in shape. Most tanks
constructed today are cylindrical. The floor of the tank is sloped
so that sand, grit, and heavy sludge will tend to be removed from
the tank. Most digesters constructed today have either fixed or
floating covers.
A. Fixed Cover Tanks
A fixed cover tank has a stationary roof, generally slab, conical,
or cone-shaped, and constructed of concrete or steel. Both types
of covers are normally designed to maintain no more than an eight-inch
water column of gas pressure on the tank roof (Fig. 8.3), but some are
designed for pressures of 25 inches or more. The domed cover is
designed to hold a larger volume of gas. Any type of mixing device
may be used with a fixed cover tank, and the tank must be equipped
with pressure and vacuum relief valves.
A fixed cover digester can have an explosive mixture in the tank when
sludge is withdrawn if proper precautions are not taken to prevent air
from being drawn into the tank. Each time a new charge of raw sludge
is added, an equal amount of supernatant is displaced because the tank
is maintained at a fixed level.
B. Floating Cover
A floating cover moves up and down with the tank level and gas pressure.
Normally the vertical travel of the cover is about eight feet, with
stops (corbels) or landing edges for down (lowering) control and
8-9
-------
^
WATER SEAL
-NORMAL WATER LEVEL
INLET BOX
"fl "^SUPERNATANT BOX
AND TUBES
SLUDGE DRAW OFF LINE
NOTE: USUALLY WATER SEALS ARE LOCATED
IN THE VERTICAL SIDEWALLS OF
FLOATING COVER DIGESTERS, RATHER
THAN AS SHOWN ON ROOF.
oo VACUUM RELIEF
0
WATER SEAL
iMPHF'j OF ._ — r
WATER PRESSURE ^^^T
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^** — DIGESTER GAS PRESSURE ^^
FLAME ARRESTOR
•SEDIMENT TRAP
GAS PIPING
ATMOSPHERIC
PRESSURE
CONDENSATE DRAIN
DIGESTER ROOF
I ig. 8.3 IVater seal on digester
-------
maximum water level for upward travel. Maxirruni v/ater level is
controlled by an overflow pipe that must be kept clear to prevent
damage to the floating cover by overfilling. Gas pressure is
dependent upon the weight of the cover. The advantages of a
floating cover include less danger of explosive mixtures forming
in the digester, better control of supernatant withdrawal, and
better control of scum blankets. Disadvantages include higher
construction and maintenance costs.
C. Digester Depth
A typical operation depth for digesters is around 20 feet (side
wall water level depth). The bottom slopes downward to the center
of the tank. A gas space of two to three feet is usually provided
above normal liquid sludge level, but some floating covers allow
more room for gas storage.
D. Raw Sludge Inlet
Typically the raw sludge feed is piped to the top of the primary
digester and admitted on the side opposite the supernatant over-
flow pipe (Fig. 8.4) to the secondary digester. Typically this
line also carries any recirculated digester sludge in the system
so that the raw sludge is immediately seeded with bacteria as it
enters the tank.
E. Supernatant Tubes (Fig. 8.4)
On a fixed cover digester there may be three to five supernatant
tubes set at different levels for supernatant removal. Normally
only one tube is used at a time. The tube used is selected to
return the supernatant liquor with the lowest quantity of solids
back to the primary clarifier, or to sludge drying beds, provided
space is available.
A single adjustable tube is also used at some plants. On the
floating cover digester there is usually only one supernatant
tube. This may be adjusted to pull supernatant liquor from various
levels of the tank by raising or lowering the tube. In smaller
plants the supernatant withdrawal may be done only once or twice
a day, because the floating cover allows the tank to handle volume
changes. An adjustable tube usually allows a supernatant with the
least solids content to be selected. The digester should be visually
checked a minimum of once per day for liquor levels to prevent over-
filling and structural damage to the. tank.
8-11
-------
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SUPERNATANT TUBES MAY BE VARIOUS LENGTHS OTHER THAN SHOWN
9 FT. LONG
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SECONDARY DIGESTER
-------
F. Sludge Draw-off Lines
The sludge draw-off lines are typically placed on blocks along
the sloping floor of the digester. Sludge is withdrawn from the
center of the tank. Very seldom are they placed under the floor
of the digester because they would not be accessible in case of
blockages. These lines are normally six inches in diameter and
equipped with plug valves. The lines are used to transfer the
digester sludge periodically to a sludge disposal system of
either drying beds or some type of dewatering system. These
lines also transfer seed sludge from the secondary digester to
the primary digester and recirculate bottom sludge to seed and
break up a scum blanket.
QUESTIONS
8.11A Why should you .maintain no more than an eight-inch
water column of gas pressure on the roof of a fixed
cover digester?
8.11B Why must a fixed cover digester be equipped with
pressure and vacuum relief valves?
8.11C What are the advantages of a floating cover in
comparison with a fixed cover digester?
8.11D Why is it desirable to mix recirculated digester
sludge with raw sludge?
END OF LESSON 1 OF 5 LESSONS
on
SLUDGE DIGESTION AND HANDLING
8-13
-------
CHAPTER 8. SLUDGE DIGESTION AND HANDLING
(Lesson 2 of 5 Lessons)
8.12 Gas System (Fig. 8,5)8
The anaerobic digestion process produces 7 to 12 cubic feet of gas
for every pound of volatile matter destroyed, depending upon the
characteristics of the sludge. The gas consists mainly of methane
(CHiJ and carbon dioxide (C02). The methane content of the gas in
a properly functioning digester will vary from 65 to 70%, with
carbon dioxide running around 30 to 35% by volume. One or two
percent of the digester gas is composed of various other gases.
Digester gas (due to the methane) possesses a heat value of approxi-
mately 500 to 600 BTU9 per cubic foot, whereas natural gas with a
higher methane content may range from 900 to 1200 BTU per cubic foot.
Digester gas is utilized in plants in various ways: for heating the
digesters, for heating the plant buildings, for running engines, for
air blowers for the activated sludge process, or for electrical power
for the plant.
WARNING
CAKi
VA/AY4>. WH£N AAIX£P WITH
It CAN ^O12M gXPLO4lV€ MIXTURES , AMP IT
AL40 CAM CAU^B ASPHYXIATION OR OXV6EN
MUST MOT
8 Many figures in this section were made available courtesy of
VAREC, Inc., 301 East Alondra Blvd., Gardena, California 90247.
Mention of commercial products or manufacturers is for illustra-
tive purposes and does not imply endorsement by Sacramento State
College, EPA/WQO, or any other state or federal agency.
9 BTU: British Thermal Unit. The amount of heat required to raise
the temperature of one pound of water one degree Fahrenheit.
8-14
-------
The gas system removes the gas from the digester to a point of use,
or to be burned in the waste gas burner as excess. The following
items are components of the gas system.
A. Gas Dome
This is a point in the digester roof where the gas from the tank is
removed. On fixed cover tanks there may also be a water seal
(Fig. 8.3) incorporated to protect the tank structurally from excess
positive pressure,10 or vacuum created by withdrawal of sludge or
gas too rapidly.
If gas pressure is allowed to build up to 11 inches of water column
pressure, it will escape around the water seal to the atmosphere
without lifting the roof. If sludge is drawn or gas used too
rapidly, the vacuum could exceed eight inches and break the water
seal, thus allowing air to enter the tank. Without the water seal,
the vacuum could become great enough to collapse the tank. Air in
the tank creates an explosive condition. In addition, sulphuric
acid corrosion is often found where air is consistently in contact
with the gas. The pipeline between the gas storage tank and the'
digester will protect the digester from water seal leaks, if the
line is clear. When liquids are pumped into the digester, gas can
go out the line to the storage tank and when liquids are pumped out
of the digester, gas can return through the line.
QUESTIONS
8.12A What are the two main gaseous components of
digester gas after gas production has become
well established?
8.12B What are some uses of digester gas?
8.12C Why must the digester gas be controlled with
extreme caution?
10 Positive Pressure. A positive pressure is a pressure greater
than atmospheric. It is measured as pounds per square inch
(psi) or as inches of water column. A negative pressure
(vacuum) is less than atmospheric and is sometimes measured
in inches of mercury.
8-15
-------
TYPICAL FLOW AND INSTALLATION DIAGRAM
MULTIPLE DIGESTER GAS SYSTEM
DIGESTER
(Fixed or floating roof)
DIGESTER
(Fixed or floating roof)
FULL SIZE I7"X22" PRINTS
OF THIS SCHEMATIC
AVAILABLE ON REQUEST
This schematic is for general guidance purposes
only and is not intended to represent a specific design.
Fig. 8.5 Digester gas system
Courtesy of VAffiC
-------
B. Pressure Relief and Vacuum Relief Valves
(Fig. 8,6, VAREC Fig. No. 5800-81)
The pressure relief valve and the vacuum relief valve both are
attached to a common pipe, but each works independently. The
pressure relief valve is equipped with a seat and weighted with
lead washer weights. Each weight is stamped with its equivalent
water column height11 such as 1" H20 or 3" H20. There should be
sufficient weights, combined with the weight of the pallet, to
equal the designed holding pressure of the tank. The gas pressure
is normally established between six inches and eight inches of
water. If the gas pressure in the tank exceeds the pop-off setting,
then the valve will open and vent to the atmosphere for a couple
of minutes, through the pressure relief valve. This should occur
before the water seal blows out. The water seal can be broken
when a tank is overpumped or gas removal is too slow.
The vacuum relief valve operates similarly to the pressure relief
valve except that it relieves negative pressures to prevent the
tank from collapsing. Operating of either one of these valves is
undesirable, because this allows the mixing of digester gas with
air and can create an explosion outside the tank if the pressure
relief valve opens and inside the tank if the vacuum relief opens.
WARNING
AHP UPP&ie L. IM
These two valves should be checked at least every six months for
proper operation.
11 Water Column Height. When pressure builds up in a digester,
the gas pressure would force water up a tube of water connected
to the outside of the digester. The higher the water column
height, the greater the gas pressure.
8-17
-------
PRESSURE RELIEF AND
VACUUM BREAKER VALVE
WITH FLAME ARRESTER
for Use on Digesters and Gas Holders
The "Varec" Figure No. 5800-81 unit consists
of a Figure No. 2000-81 Pressure Relief and
Vacuum Breaker Valve and a Figure No. 50-91
Flame Arrester. Maximum protection against ex-
cessive pressure and vacuum is afforded and acci-
dental ignition of sludge gas within the digester
and gas holder from external sources is eliminated.
Valve is light weight and corrosion resistant
construction. Interior parts are readily accessible
for inspection and maintenance purposes. Pallets
are dead weight loaded and include replaceable
synthetic rubber sludge gas resistant seat inserts
to insure gas tight seating and long life service
with minimum maintenance. Seat rings, pallets
and guide posts are anodized for extra corrosion
protection and are removable.
Flame arrester consists of a flame arresting bank
assembly enclosed within a gas-tight housing. The
bank consists of a multiple number of individual
corrugated stamped sheets and is readily remov-
able from the housing for inspection and cleaning
purposes, The arrester is listed by Underwriters
Laboratories and is approved by Associated Fac-
tory Mutual Laboratories.
FIGURE NO. 5800-81
SETTINGS
Valves are furnished with variable pressure
settings from 2" to 10" of water in increments of
1" of water. Vacuum setting is 2" of water unless
otherwise specified.
STANDARD MATERIALS OF CONSTRUCTION
Valve is substantially aluminum (impervious
to the attack of sludge gas) throughout except
for synthetic rubber pallet seat inserts and steel
studs, nuts and screws.
Flame arrester bank is all aluminum and the
housing consists of cast aluminum ends and cast
iron side and cover plates. Gaskets are graphited
asbestos.
3-13
-------
C. Flame Arresters (Fig. 8.8, VAREC Fig. No. 450)
A typical flame arrester is a rectangular box holding approximately
50 to 100 corrugated aluminum plates with punched holes. If a flame
should develop in the gas line, it would be cooled below the ignition
point as it attempted to pass through the baffles, but gas could flow
through with little loss in pressure.
To prevent explosions, flame arresters should be installed:
1. Between vacuum and pressure relief valves and the
digester dome.
2. After sediment trap on gas line from digester.
3. At waste gas burner.
4. Before every boiler, furnace, or flame.
Flame arresters should be serviced every three months by
valving the gas off, pulling one end plate, and sliding
the baffle cartridge out of the housing. A build-up of
scale, salts from condensate, and residue build-up on the
plates restricts gas flow.
The cartridge in the flame arrester is designed to slide open so
the baffles may be separated and washed without complete dismantling.
When the unit is reassembled it should be tested for leaks by swabbing
a soapsuds solution over potential leaky areas and inspecting for
bubbles.
D. Thermal Valves
Another protective device installed near a flame source and near
the gas dome is the thermal valve. This valve is round, with a
weighted seat attached to a stem. The stem sets on a fusible disk
holding the seat up. If enough heat is generated by a flame, the
fusible element melts and drops the stem and valve seat to cut off
gas flow. Most valves are equipped with a wing nut on top of the
valve body. If the wing nut is removed, it uncovers a glass tube
which shows visually if the stem is up. If the stem cannot be seen,
then the valve is closed, and no gas can flow. If this occurs, the
valve is removed and heated in boiling water to remove the melted
8-19
-------
fusible slug. A new slug is installed (slightly larger than an
aspirin tablet), the stem replaced'on top of it, and the valve is
ready for service. These valves should be dismantled at least
once a year in order to be positive that the stem is free to fall
and not gummed up with residue or scale from the gas.
Figure 8.9 (VAREC Fig. No. 440) shows a flame arrester connected
to a pressure relief valve.
QUESTIONS
8.12G How would you service a flame arrester?
8.12H Why should you check the thermal valves at least
once a year?
8-20
-------
FLAME TRAP ASSEMBLY
FIGURE NO. 450
Assembly consists of "Varec" Flame Trap Fig. No.
53-81 and Thermal Operated Shutoff Valve Fig. No. 430.
It is usually installed in all gas lines to gas utilization
equipment, as close as possible to the points of com-
bustion, and in lines leading from each digester and
gasholder. May be installed in either horizontal or
vertical pipelines.
It is designed to arrest and stop flame propagation
— and to stop explosion waves, thus insuring protection
of major equipment.
FEATURES
Simple and positive flame trap. The fusible element
melts at 260' and stops gas flow within 15 seconds.
Compression type fusible element prevents shutoff valve
closing unless contacted by flame. Three extra fusible
elements shipped with each unit.
Since this unit is manufactured of aluminum, it resists
the attack of any of the corrosive elements common
to sludge gas.
Indicator rod shows when valve is in normal open
position.
The "Varec" Flame Trap Fig. No. 53-81 of this unit
is listed by the Underwriters' Laboratories and approved
by Associated Factory Mutual Laboratories.
Net free area through flame arresting bank is approxi-
mately four times corresponding pipe size. Each passage-
way has a net free area of approximately 0.042 sq. inches.
By actual test these units have more flow capacity with
less pressure drop than any known contemporary device.
Flow capacity curves are shown on the following page
to assist in selecting the correct size of equipment.
Flame Trap element is easy to inspect and clean. It
has good vertical and horizontal drainage. Drip Trap
connection is provided in case Unit is installed at low
point in line.
MATERIALS OF CONSTRUCTION
Flame Trap Housing — aluminum and cast iron
Flame Trap Element — aluminum
Thermal Valve Body & Cover — aluminum
Guide Stem — stainless steel
Sight Glass — pyrex
Cover and cap gaskets — graphited asbestos
Sight glass gasket — synthetic rubber
Spring — stainless steel
8-21
-------
PRESSURE RELIEF AND FLAME TRAP ASSEMBLY
FIGURE NO. 440
Assembly consists of "Varec" Figure No. 386 Back Pressure
Regulator, a "Varec" Figure No. 53-81 Flame Trap and a
Thermal Shutoff Control unit.
It is usually installed in the waste gas line, just upstream
of the waste gas burner.
It is designed to maintain a predetermined back pressure
throughout the gas system so that only surplus gas is wasted,
and to stop flame and explosion waves.
FEATURES
Simple, foolproof, sensitive in operation and a positive
flame trap. The fusible element melts at 260"F. and stops
gas flow within 15 seconds. Compression type fusible element
prevents shutoff valve closing unless contacted by flame.
Three extra fusible elements supplied with each unit.
Since the main bodies of the unit are constructed of
aluminum and the stems, needle valve, and other important
moving parts are of 18.8 stainless steel, this unit resists the
attack of any of the corrosive elements common to sludge
gas.
The "Varec" Flame Trap Fig. No. 53-81 of this unit is
listed by the Underwriters' Laboratories and approved by
Associated Factory Mutual Laboratories. The Flame Trap
clement is easy to inspect and clean. Drip Trap connection
is provided in case unit is installed at a low point in line.
Net free area through flame arresting bank is approxi-
mately four times corresponding pipe size. Each passageway
has a net free area of approximately 0.042 sq. inches. By
actual test these units have more flow capacity with less pres-
sure drop than any known contemporary device.
Flow capacity curves are shown on the following page to
assist in selecting the correct size of equipment.
The Back Pressure Regulator unit is equipped with setting
indicator so operator can easily adjust setting to requirements.
RANGE OF OPERATION
Range of operation is 2 to 12 inches water. Special springs
available for higher operating pressures. Equipment supplied
by factory set at 6 inches of water if not specified otherwise.
Operator can adjust to his requirements.
MATERIALS OF CONSTRUCTION
Regulator Body — cast aluminum
Diaphragm Case — cast aluminum
Bonnet — cast aluminum
Spring - Cadmium-plated steel
Diaphragm — corded synthethic rubber
Cap — brass
Thermal Shutoff Valve — aluminum, brass & stainless
steel
Flame Trap Housing — heavy cast aluminum ends and
cast iron side and cover plates
Flame Trap Element — aluminum
8-22
-------
E. Sediment Traps
A sediment trap is a tank 12 to 15 inches in diameter and two
to three feet in length. It is usually located on top of the
digester near the gas dome. The inlet gas line is near the top
of the tank and on the side. The outlet line comes directly from
the top of the sediment tank. The sediment trap is also equipped
with a perforated inner baffle, and a condensate drain near the
bottom. The gas enters the side at the top of the tank, passes
down and through the baffle, then up and out the top. Moisture
is collected from the gas in the trap, and any large pieces of
scale are trapped before entering the gas system. The trap should
be drained of condensate frequently but may have to be drained twice
a day during cold weather, because greater amounts of water will be
condensed.
F. Drip Traps--Condensate Traps
(Fig. 8.10, VAREC Fig. Nos. 245 and 246)
Digester gas is quite wet and in traveling from the heated tank to
a cooler temperature the water condenses. The water must be trapped
at low points in the system and removed, or it will impede gas flow
and cause damage to equipment, such as compressors, and interfere
with gas utilization. Traps are usually constructed to have a storage
space of one to two quarts of water. All drip traps on gas lines
should be located in the open air and be of the manual operation type.
Traps should be drained at least twice a day and possibly more often
in cold weather. Automatic drip traps are not recommended because
many automatic traps are equipped with a float and needle valve
orifice and corrosion, sediment, or scale in the gas system can keep
the needle from seating. The resulting leaks may create gas concen-
trations with a potential hazard to life and equipment.
G. Gas Meters
Gas meters may be of various types, such as bellows, diaphragm,
shunt flow, propeller, and orifice plate or differential pressure.
They are described in detail in the metering section of Chapter 11,
Maintenance.
H. Manometers
Manometers are installed at several locations to indicate gas pressure
within the system in inches of a water column.
8-23
-------
I. Pressure Regulators (Fig. 8.11, VAREC Fig. No. 387)
Pressure regulators are typically installed next to and before the
waste gas burner. Such regulators are usually of the diaphragm
type and control the gas pressure on the whole digester gas system.
They are normally set at eight inches of water column by adjusting
the spring tension on the diaphragm. Whenever an adjustment of a
pressure setting is made, check the gas system pressure with a
manometer for the proper range. If the gas pressure in the system
is below eight inches of water column, no gas flows to the waste
burner. When the gas pressure reaches eight inches of water column,
the regulator opens slightly, allowing gas to flow to the burner.
If the pressure continues to increase, the regulator opens further
to compensate. The only maintenance this unit requires is on the
thermal valve on the discharge side which protects the system from
back flashes. This unit is spring loaded and controlled by a fusible
element that vents one side of the diaphragm, thus stopping the
gas flow when heated. Maintenance includes checking for proper
operation of the regulator and of the fusible element. Gas
regulators are also placed at various points in the system to
regulate the gas pressure to boilers, heaters, and engines. Diaphragm
conditions in the regulators should be checked at periodic intervals.
J. Waste Gas Burner (Fig. 8.12)
Waste gas burners are used to burn the excess gas from the digestion
system. The waste gas burner is equipped with a continuous burning
pilot flame, so that any excess gas will pass through the gas regu-
lator and be burned. The pilot flame should be checked daily to be
sure that it has not been blown out by wind. If the pilot is out,
gas will be vented to the atmosphere creating an odorous and
potentially explosive condition.
QUESTIONS
8.121 How frequently should you drain a sediment trap?
8.12J Why must drip or condensate traps be installed in
gas lines?
8.12K What is a deficiency in automatic drip and conden-
sate traps?
8.12L How would you adjust the gas pressure of the digester
gas system?
8.12M Why should the pilot flame in the waste gas burner
be checked daily?
8-24
-------
DRIP TRAPS
Automatic
Varec Drip Traps are for collection and safe
removal of condensate from gas lines and equip-
ment. Drip traps should be installed at all low points
in gas pipe systems where condensation will collect.
The Varec Figure No. 245 Automatic Drip
Trap employs a float operated needle valve which
automatically drains off collected condensate. This
feature is particularly desirable where a closed dis-
charge to drain is permissible and where condensate
occurs too frequently for manual operation.
Standard construction is alum, body and cover,
stainless steel ball float and needle valve assembly
and graphited asbestos gasket. Available with l/2",
", 1" NPT connections.
FIG. NO. 245 AUTOMATIC
Rotating Disc Type
The Varec Figure No. 246 Drip Trap is manu-
ally operated. Handle rotates disc from open inlet
position to drain position. Ports and disc are so
arranged that gas cannot escape regardless of disc
position. Both ports and shaft are positively sealed
by synthetic rubber "O" rings. Vent hole is provided
to allow inflow of air to bowl while draining.
Standard construction is cast aluminum bowl
and handle. Aluminum cover plate and disc are
anodized. Other working parts are stainless steel.
Heavy duty construction throughout. Available in
2!/2 quart capacity with 1" NPT connections.
FIG. NO. 246
ROTATING DISC TYPE
8-25
-------
BACK PRESSURE
REGULATOR
SINGLE PORT
The Figure No. J86 Regulator Valve is designed to
control upstream pressure in sludge gas lines. Positive
shut-off as well as accurate control is provided. Pointer
type indicator, in weather-proof bonnet, facilitates set-
ting adjustment. No weights or dismantling necessary
to make adjustment.
Valve is the single port type operated by a spring loaded
diaphragm.
Setting range is 2" W.C. to 12* W.C. as standard. Higher
settings available (20" W.C. maximum) on special order.
MATERIALS OF CONSTRUCTION:
Heavy cast aluminum valve body, diaphragm housing
and pallet, stainless steel operating shaft, heavy corded
synthetic rubber diaphragm and cadmium plated steel
spring.
FIGURE NO. 386
PRESSURE (REDUCING)
REGULATOR
SINGLE PORT
The Figure No. 387 Regulator Valve is designed to
control downstream pressure in sludge gas lines. Posi-
tive shut-off as well as accurate control is provided.
Pointer type indicator, in weather-proof bonnet, facili-
tates setting adjustment. No weights or dismantling
necessary to make adjustment.
Valve is single port type operated by a spring loaded
diaphragm.
Setting range is 2" W.C. to 12" W.C. as standard. Higher
settings available (20" W.C. maximum) on special order.
MATERIALS OF CONSTRUCTION:
Heavy cast aluminum valve body, diaphragm housing
and pallet, stainless steel operating shaft, heavy corded
synthetic rubber diaphragm and cadmium plated steel
spring.
FIGURE NO. 387
8-26
-------
VAREC
WASTE GAS BURNER
r— FIG. 239
\ BUILDING-
\T/
VAREC
FLAME CHECK
FIG. 52
GAS PIPING SCHEMATIC
ENCLOSED INSTALLATION
VAREC FIG.70-81 EXPLOSION RELIEF VALVES
VAREC
REMOTE COVER
POSITION INDICATOR
FIG. 102
VAREC
3 UNIT MANOMETER
FIG. 217
VAREC
PRESSURE RELIEF 8
FLAME TRAP ASSEMBLY
-FIG.440
VAREC
FLAME CHECK
FIG52-
GAS SUPPLY
TO LABORATORY
VAREC
PRESSURE REDUCING
REGULATOR-FIG. 387
-BUILDING
U METER
3y others)
METER
(By others)
VAREC
CHECK VALVE
FIG. 211-92
GAS SUPPLY TO
SERVICE EQUIPMENT
TO WASTE GAS BURNER
PI LOT SUPPLY TO
WASTE GAS BURNER—7
NOTE-INSTALL DRIP TRAPS AT ALL LOW POINTS
VAREC
DRIP TRAP
FIG.245 OR 246
VAREC
DRIP TRAP
FIG. 245 OR 246
VAREC
FLAME TRAP
ASSEMBLY-FIG.450
GAS SUPPLY FROM
DIGESTER
VAREC
SEDIMENT a DRIP TRAP
ASSEMBLY- FIG. 233, 218. 246
FULL SIZE I7"X22" PRINTS
OF THIS SCHEMATIC
AVAILABLE ON REQUEST
This schematic is (or general guidance purposes
only and is not intended to represent a specific design.
Fig. 8.12 Waste gas burner
Courtesy of VAEEC
-------
8.13 Sampling Well (Thief Hole)
(Fig. 8.13, VAREC Fig. Nos. 42-81 and 48-81)
The sampling well consists of a 3- or 4-inch pipe (with a hinged
seal cap) that goes into the digestion tank, through the gas zone,
and is always submerged a foot or so into the digester sludge.
This permits the sampling of the digester sludge without loss of
digester gas pressure, or the creation of dangerous conditions
caused by the mixing of air and digester gas. However, caution
must be used not to breathe gas which will always be present in
the sample well and will be released when first opened. A sampling
well is sometimes referred to as a "thief hole".
8.14 Digester Heating
Digesters can be heated in several ways. Newer facilities typically
provide digesters that are heated by recirculating the digester sludge
through an external hot water heat exchanger. Digester gas is used
to fire the boiler, which is best maintained between 140 and 180°F
for proper operation. The hot water is then pumped from the boiler
to the heat exchanger where it passes through one jacket system,
while the recirculating sludge passes through an adjacent jacket,
picking up heat from the hot water. In some units the boiler and
exchanger are combined and the sludge also is passed through the unit.
Circulation of 130°F water through pipes or heating coils attached
to the inside wall of the digester is another method of heating
digesters, although not too common in newer plants. This approach
creates problems of cooking sludge on the pipes and insulating them,
thus reducing the amount of heat transferred. Some facilities use
submerged combustion of the gas with heat exchange between the hot
gaseous products evolved and the liquid sludge.
Other plants inject steam directly into the digesters for heating.
The steam is produced in separate boilers or is recovered in connection
with vapor phase cooling of engines. Careful treatment of the
evaporated water to prevent scaling of the system is necessary so the
practice is generally confined to plants with good laboratory control.
QUESTIONS
8.13A Why should a digester have a special sampling well?
8.14A What causes a reduction in the amount of heat trans-
ferred from coils within the digester?
8-28
-------
SAMPLING HATCH or
HANDHOLE COVER
Non-sparking Gas-tight
VAREC Sampling Hatches or Hand-
hole Covers are for use on digester covers
or roofs. Insurance requirements are com-
plied with in that this equipment is
non-sparking, self-closing and gas-tight.
Construction is non-corrosive in sludge
gas service.
Above photo shows simplicity
of operation
Figure No. 42-81 incorporates a standard
125 lb. A.S.A. flanged base for mounting.
It is of extra heavy construction, basically
of aluminum throughout. Specialty features
are included such as a safety foot pedal for
quick opening, a hand wheel which may be
padlocked closed, and a synthetic rubber
insert in cover to insure a gas-tight seal.
Figure No. 42-81
Flanged Base
Figure No. 48-81 is substantially same as
Figure No. 42-81 in that it includes all the
specialty features and is of same materials
of construction. However, the base is for
Standard Pipe Thread mounting.
8-29
Figure No. 48-81
Screwed Base
-------
8.15 Digester Mixing
Mixing is very important in a digester. The ability of the mixing
equipment to keen the tank completely mixed speeds digestion greatly.
Several important objectives
are accomplished in a well-
mixed digester.
a. Inoculation12 of the raw
sludge immediately with
microorganisms.
b. Prevention of a scum
blanket from forming.
c. Maintenance of homogeneous
contents within the tank,
including even distribution
of food, organisms, alka-
linity, heat, and waste
bacterial products.
d. Utilization of as much of
the total contents of the
digester as possible and
minimization of the build-
up of grit and inert solids
on the bottom.
A. Gas Mixing
This type of mixing is the most generally used in recent years, and
various approaches have been patented by manufacturers. Gas is pulled
from the tank, compressed, and discharged through gas outlets or orifices
within the digester, or at some point several feet below the sludge
surface. The gas rising to the surface through the digesting sludge
carries sludge with it, creating a gas lift with a rolling action of
the tank contents. The gas mixer may be operated on either a start
and stop or a continuous basis, depending upon tank conditions. The
components required for gas mixing include inlet and discharge gas
lines, a positive displacement compressor, and a stainless steel gas
line header in the digester. The gas header is equipped with a cross
arm to hold a specified number of gas outlets, and may be mounted in
a draft tube. The gas compressor is sized for the digester and may
range from 30 to 200 cfm of gas.
12 Inoculation (in-NOCK-you-LAY-shun).
culture into a system.
Introduction of a seed
8-30
-------
Work with "natural gas evolution" mixing at the Los Angeles County
Sanitation District's plants has indicated that loadings of over
0.4 pounds of volatile solids per cubic foot per day were possible,
but that if the loading dropped below 0.3 pounds immediate strati-
fication occurred. In terms of gas recirculation, adequate mixing
has been calculated from this study to be of the order of 500 cfm
(cubic feet per minute) per 100,000 cubic feet of tank capacity
if released at about a 15-foot depth. If released at a 30-foot
depth, about 250 cfm per 100,000 cubic feet of tank capacity should
be satisfactory. If hydraulic processes are used, either by re-
circulation or by draft tubes and propellers, then something like
30 HP per 100,000 cubic feet of tank capacity is required.
Maintenance requires that the condensate be drained from the lines
at least twice a day, that the diffusers be cleaned to prevent high
discharge pressures, and that the compressor unit be properly lubri-
cated and cooled.
QUESTIONS
8.ISA Why should a digester be kept completely mixed?
8.15B What maintenance is necessary for the proper
operation of digester mixing by the use of gas?
8-31
-------
B. Mechanical Mixing
Propeller mixers are found mainly on fixed cover digesters. Normally
two or three of these units are supported from the roof of the tank
with the props submerged 10 to 12 feet in the sludge. An electric
motor drives the propeller stirring the sludge.
Draft tube propeller mixers are either single or multiple unit
installations. The tubes are of steel and range from 18 to 24
inches in diameter. The top of the draft tube has a rolled lip
and is located approximately 18 inches below the normal water level
of the tank. The bottom of the draft tube may be straight or
equipped with a 90° elbow. The 90° elbow type is placed so that
the discharge is along the outside wall of the tank to create a
vortex (whirlpool) action.
The electric motor driven propeller is located about two feet
below the top of the draft tube. This type unit usually has
reversible motors so the prop may rotate in either direction.
In one direction the contents are pulled from the top of the
digester and forced down the draft tube to be discharged at the
bottom. By operating the motor in the opposite direction, the
digested sludge is pulled from the bottom of the tank and dis-
charged over the top of the draft tube to the surface.
If two units are in the same tank, an effective operation for
breaking up a scum blanket is operating one unit in one direction
and the other unit in the opposite direction, thereby creating a
push-pull effect. The draft tube units are subject to shaft bearing
failure due to the abrasiveness of sludge, and due to corrosion by
hydrogen sulfide (H2S) in the digester gas. Maintenance consists
of lubrication and, if belt-driven, adjustment of belt tension.
A limitation of draft tube type mixers is digester water level. If
the water level is maintained at a constant elevation, a scum blanket
forms on the surface. The scum blanket may be a thick layer and the
draft will only pull liquid sludge from under the blanket, not dis-
turbing it. Lowering the level of the digester to just three or four
inches over the top of the draft tube forces the scum to move over
and down the draft tube. This applies mainly to single direction
mixers.
Pumps are sometimes used to mix digesters. This method is common
in smaller tanks. When external heat exchangers are employed, a
larger centrifugal pump is used to recirculate the sludge and dis-
charge it back into the digester through one or two directional
nozzles at the rate of 200 to 1000 gpm.
8-32
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The tank may or may not be equipped with a draft tube such that the
pump suction may be from the top- or valved from the bottom of the
digester. Control of scum blankets with this method of mixing is
dependent upon how the operator maintains the sludge level and
where the pump is pulling from and discharging to the digester.
Maintenance of the pump requires normal lubrication and a good pump
shaft sealing water system. The digested sludge is abrasive and
pump packing, shafts, wearing
rings, and impellers are rapidly
worn. Another problem associated
with pump mixing is the clogging
°f tne PumP impeller with rags,
rubber goods, and plastic material.
A pump may run for days not pump-
ing due to cl°88ing because the
* *, if^^A iTr liJr operator was not checking the
equipment for proper operation.
Pressure gauges .should be
installed on the pump suction
and discharge pipes. When a
gauge reading different than
normal occurs, the operator has
an indication that some condition
has changed that requires checking.
QUESTIONS
8.15C How would you break up a scum blanket in a digester
with two or more draft tube propeller mixers?
8.15D Why should pressure gauges be installed on mixing
pump suction and discharge lines?
END OF LESSON 2 OF 5 LESSONS
on
SLUDGE DIGESTION AND HANDLING
8-33
-------
CHAPTER 8. SLUDGE DIGESTION AND HANDLING
(Lesson 3 of 5 Lessons)
8.2 OPERATION OF DIGESTERS
A digester can be compared with your own body.
Both require food; but if fed too much, both
become upset. Excess acid will upset both.
Both like to be warm, with a body temperature
of 98.6°F near optimum. Both have digestive
processes that are similar. Both discharge
liquid and solid waste. Both utilize food
for cell reproduction and energy. If some-
thing causes upsets in a digester, just think
how you would react if it happened to you
and recall what would be the proper remedy.
The remedies for curing upset digesters will
be discussed throughout this chapter.
8.20 Raw Sludge and Scum
Raw sludge is normally composed of solids
settled and removed from the primary and
secondary clarifiers. Raw sludge contains
carbohydrates, proteins, and fats, plus
organic and inorganic chemicals that are
added by domestic and industrial uses of water.
Solids are composed of organic (volatile) and inorganic material with the
volatile content running from about 60% to 80% of the total, by weight.
Some plants do not have grit removal equipment; so the bulk of the
inert (inorganic) material such as sand, eggshells, and other debris
will end up on the bottom of the digester occupying active digestion
space. The rate of debris accumulation is predictable so that the
amount is a function of the period of time between digester cleanings.
Where cleaning has been neglected, a substantial portion of the active
volume of the digester becomes filled with inert debris. Scum-forming
products, such as kitchen grease, soaps, oils, cellulose, plastics,
and other floatable debris, are generally all organic in nature but
may create problems if the scum blanket in the digester is not con-
trolled. Control is by providing adequate mixing and heat.
Several products end up in the digester that are not desirable because
the bacteria cannot effectively utilize or digest them, and they cannot
be readily removed by the normal process. These products include:
8-34
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1. petroleum products and mineral oils
2. rubber goods
3. plastics (back sheets to diapers)
4. filter tips from cigarettes
5. hair
6. grit (sand and other inorganics)
Consequently, these items tend to accumulate in the digester and,
without adequate mixing, may form a hard, floating mat and a sub-
stantial bottom deposit. On the other hand, a well-mixed tank may
also present operational problems. For example, the material
shredded by a comminuter or barminuter may become balled together
by the mixing action and plug the digester supernatant lines.
Scum from the primary clarifiers is comprised mainly of grease and
other floatable material. It may be collected and held in a scum
box and then pumped to the digester once a day, or it may be added
continuously or at a frequency necessary to maintain the proper
removal of scum from the raw wastewater flow. Many operators prefer
not to pump scum to the digesters, but to dispose of it by burning or
burial. Scum may also refer to the floating and gas buoyed material
found on the surface of poorly mixed digesters. This material may
contain much cellulose, rubber particles, mineral oil, plastic,
and other debris. It may become 5 to 15 feet thick in a digester,
but should not occur in a properly operating digester. A thick
scum layer will reduce the active digestion capacity of a digester.
8.21 Starting a Digester
When wastewater solids are first added to a new digester, naturally
occurring bacteria attack the most easily digestible food available,
such as sugar, starches, and soluble nitrogen. The anaerobic acid
producers change these foods into organic acids, alcohols, and
carbon dioxide, along with some hydrogen sulfide. The pH of the
sludge drops from 7.0 to about 6.0 or lower. An "acid regression
stage"13 then starts and lasts as long as six to eight weeks. During
this time ammonia and bicarbonate compounds are formed, and the pH
gradually increases to around 6.8 again, establishing an environment
for the methane fermentation or alkaline fermentation phase. Organic
acids are available to feed the methane fermenters. Large quantities
of methane gas are produced as well as carbon dioxide, and the pH
increases to 7.0 to 7.2. Once alkaline fermentation is well estab-
lished, strive to keep the digesting sludge in the 7.0 to 7.2 pH range.
13 Acid Regression Stage. A time period when the production of
volatile acids is reduced. During this stage of digestion
ammonia compounds form and cause the pH to increase.
8-35
-------
If too much raw sludge is added to the digester, the acid
fermenters will predominate, driving the pH down and creating an
undesirable condition for the methane fermenters. The digester
will go sour or acid again. When a digester recovers from a sour
or acid condition, the breakdown of the volatile acids and forma-
tion of methane and carbon dioxide is usually very rapid. The
digester may then foam or froth, forcing sludge solids through
water seals and gas lines and causing a fairly serious operational
problem. A sour digester usually requires 30 to 60 days to recover.
As noted at the beginning of this section, the first group of
organisms must do its part before food is available to the next
group. Once the balance is upset, so is the food cycle to the
next group. When the tank reaches the methane fermentation phase,
there is sufficient alkaline material to buffer the acid stage and
maintain the process. Operational actions such as poor mixing,
addition of excess food, excess water supplied to dilute the alka-
line buffer, over-drawing digested sludge, or improper temperature
changes can cause souring again.
The simplest way to start a digester is with seed sludge (actively
digesting material) from another digester. The amount of seed to
use is dependent upon factors such as mixing processes, digester
sizes, and sludge characteristics, but amounts between 10 and 50%
of the digester capacity have been used.
8-36
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8.22 Feeding
Food for the bacteria in the digester is the sludge from the primary
and secondary clarifiers. Make every effort to pump as thick a
sludge to the digester as possible. This may be accomplished by
holding a blanket of sludge as long as possible in the primary clari-
fier, long enough to allow sludge concentration, but not long enough
for sludge to start rising. In some plants concentration is accom-
plished in separate sludge thickening or flotation tanks.
Better operational performance occurs when the digester is fed several
times a day, rather than once a day because you are avoiding temporary
overloads on the digester and you are using your available space more
effectively. If the plant is producing only 500 gallons of 6% sludge
a day, one feeding may be allowable; however, for volumes much greater
than 500 gallons a day, several pumpings a day should be used. This
not only helps the digestion process, but maintains better conditions
in the clarifiers, permits thicker sludge pumping, and prevents coning1**
in the primary clarifier hopper. On fixed cover digesters frequent
feeding spreads the return of digester supernatant over the entire day
instead of a return in one slug with possible upset of the secondary
treatment system. Sludge is usually concentrated by holding a thick
blanket on the bottom of the clarifier; but if sludge sets for a pro-
longed period, lowest layers may stick to the bottom and will no longer
flow with the liquid. When pumping is attempted, liquid flows but
solids remain in the hopper in a cone around the outlet.
It is never desirable to pump thin sludge or water to a digester.
A sludge is considered thin if it contains less than 4% solids
(too much water). Reasons for not pumping a thin sludge include:
1. Excess water requires more heat than may be available.
2. Excess water reduces holding time of the sludge in the digester.
3. Excess water forces seed and alkalinity from the digester,
jeopardizing the system due to insufficient buffer15 for
the acids in the raw sludge.
11+ Coning (CONE-ing) . A condition that may be established in a sludge
hopper during sludge withdrawal when part of the sludge moves toward
the outlet while the remainder tends to stay in place. Development
of a cone or channel of moving liquid surrounded by relatively
stationary sludge.
15 Buffer. A measure of the ability or capacity of a solution or liquid
to neutralize acids or bases. This is a measure of the capacity of
water or wastewater for offering a resistance to changes in the pH.
Buffer capacity is measured by titration with standard alkali and
acid until the pH reaches some reference or end point (a pH of 4.5
or 8.5). The higher the volume (ml) of known reagent requirements,
the higher the buffer capacity.
8-37
-------
Sludge concentrations above about 12% solids will usually not
digest well in conventional digestion tanks since adequate mixing
cannot be obtained. This, in turn, leads to improper distribution
of food, seed, heat, and metabolic products so that the souring
and a stuck16 digester results. However, most plants have diffi-
culty in obtaining a raw sludge of 8% solids. Where a trickling
filter or activated sludge process is used as the secondary system,
sludges may have a solids range from 1 to 3%. A good activated
sludge is likely to be oxidized to the point of negligible action
in an anaerobic digester.
Feeding a digester must be regulated on the basis of laboratory
test results in order to insure that the volatile acid/alkalinity
reltaionship does not start to increase and become too high. See
Section 8.4B.
QUESTIONS
8.22A How would you attempt to pump as thick a
sludge as possible to a digester?
8.22B Why should sludge be pumped occasionally throughout
the day rather than as one slug each day?
8.22C Why should the pumping of thin sludge be avoided?
16 Stuck. A "stuck" digester does not decompose the organic
matter properly. Some operators refer '.o it as constipated.
It is characterized by low gas production, high volatile acid/
alkalinity relationship, and poor liquid-solids separation.
A digester in a stuck condition is sometimes called a "sour"
digester.
8-38
-------
8.23 Neutralizing a Sour Digester
The recovery of a sour digester can be accelerated by neutralizing
the acids with a caustic material such as soda ash, lime, or
ammonia, or by transferring alkalinity in the form of digested
sludge from the secondary digester. Such neutralization increases
the pH to a level suitable for growth of the methane fermenters
and provides buffering material which will help maintain the required
volatile acids/alkalinity relationship and pH. When ammonia is added
to a digester, an added load is eventually placed on the receiving
waters. The application of lime will increase the solids handling
problems. Soda ash is more expensive than lime, but doesn't add as
much to the solids deposits. Transferring secondary digester sludge
has the advantage of not adding anything extra to the system that
was not there at an earlier time and, if used properly, will reduce
both the effluent load and the solids handling problem.
If digestion capacity and available recovery time are great enough,
it is probably preferable to simply reduce loading while heating
and mixing so that natural recovery occurs. However, there are
often conditions in which such neutralization is necessary.
When neutralizing a digester, the prescribed dose must be carefully
calculated. Too little will be ineffective, and too much is both
toxic and wasteful. In considering dosage with lime, the small
plant without laboratory facilities could use as a rough guide a
dosage of about one pound of lime added for every 1000 gallons of
sludge to be treated. Thus, a 188,000-gallon digester full of
sludge would receive 188 pounds of lime. A more accurate method is
to add sufficient lime to neutralize 100% of the volatile acids in
the digester liquor.
8-39
-------
8.24 Enzymes
In recent years several products containing "commercial" enzymes
or other hiocatalysts (BUY-o-CAT-a-lists) have been marketed for
starting digesters, controlling scum, or simply to maintain oper-
ation. Such biocatalysts or enzymes have never been shown to be
effective in controlled tests and could, in fact, cause as much
harm as good. A biological system such as found in the digesters
develops a balanced enzyme and biocatalyst system for the conditions
under which it is operating. The quantities of natural enzymes
developed within the digesting sludge are many, many times greater
than any amount you could either add or afford to purchase.
8.25 Foaming
Large amounts of foam may be generated during start-up by the almost
explosive generation of gas during the time of acid recovery. Foam-
ing is the result of active gas production while solids separation
has not progressed far enough (insufficient digestion). It is
encouraged during start-up by overfeeding. Foaming can be prevented
by adequate mixing of the digester contents before foaming starts.
Bacteria can go to work very quickly when they have the proper
environment. Almost overnight they can generate enough gas to
create a terrific mess of black foam and sludge. The foam not only
plugs gas piping systems, but can exert excess pressures on digester
covers, cause odor problems, and ruin paint jobs on tanks and build-
ings .
7 Enzymes (HN-zimes). F:nzymes are substances produced by
"living organisms that speed up chemical changes.
8-40
-------
To clean up the mess, first drop the level of the digester a
couple of feet by withdrawing some supernatant. Next, cut off
the gas system and flush it with water. Then hose the outside
of the digester off as soon as possible or the paint will be
stained a permanent grey. Drain and refill the water seal to
remove the water fouled by the foaming. Use a strainer type
skimming device to remove any rubber goods and plastic materials
that have entered the water seal.
To control the foaming the best method is to stir the tank gently
to release as much of the trapped gas from the -Team as possible.
Some operators even stop mechanical mixing equipment and stir
with long, woeden poles. Try not to add too much water from the
cleaning hoses as this reduces the temperature and dilutes the
tank, which could create conditions for more foaming later. Do
not feed the tank heavily, preferable not at all, until the foaming
has subsided.
Foaming may occur when a thick sludge blanket is broken up, tempera-
ture changes radically, or the sludge feeding to the digester is
increased. Avoid any conditions that give the acid formers the
opportunity to produce more food than the methane fermenters can
handle, because when the methane fermenters are ready, they may
work too fast.
If there had been adequate mixing, foaming problems would not have
developed. Start mixing from top to bottom of the tank before foam-
ing starts, not afterwards.
8.26 Gas Production
When a digester is first started, extremely odorous gases are produced,
including a number of nitrogen and sulphur compounds such as skatole,
indole, mercaptans, and hydrogen sulfide. Many of these are also
produced during normal digestion phases, but they are generally so
diluted by carbon dioxide and methane that they are hardly notice-
able. Their presence can be determined by testing if so desired.
During the first phases of digester start-up, most of the gas is
carbon dioxide (C02) and hydrogen sulfide (H2S). This combination
will not burn and therefore is usually vented to the atmosphere.
When methane fermentation starts and the methane content reaches
around 60%, the gas will be capable of burning. Methane
production eventually should predominate, generating a gas
8-41
-------
with 65% to 70% methane and 30% to 35% C02 by volume. Digester
gas will burn when it contains 56% 'methane, but is not usable
as a fuel until the methane content approaches 62%. When the
gas produced is burnable, it may be used to heat the digester
as well as for powering engines and for providing building
heating.
QUESTIONS
8.24A What is the function of enzymes in digestion?
8.25A How would you attempt to control a foaming digester?
8.25B What preventive measures would you take to prevent
foaming from recurring?
8.26A Why is the gas initially produced in a digester not
burnable?
8-42
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8.27 Supernatant and Solids
Plants constructed today are typically equipped with two separate
digestion tanks or one tank with two divided sections. One tank
is called the primary digester and is used for heating, mixing,
and breakdown of raw sludge. The second tank, or secondary
digester, is used as a holding tank for separation of the solids
from the liquor. To accomplish such separation, the secondary
tank must be quiescent (qui-ES-sent) (without mixing)^
Most of the sludge stabilization work is accomplished in the
primary digester, and 90% of the gas production occurs there.
It is desirable to very thoroughly mix the primary tank, but it
is undesirable to return the digested mixture to the plant as a
supernatant. Therefore, when raw sludge is pumped to the primary
digester, an equal volume is transferred to the secondary digester,
and settled supernatant from the secondary digester is returned
to the plant.
In the primary digester the binding property of the sludge is broken,
allowing the water to be released. In the secondary digester the
digested sludge is allowed to settle and compact, with some digestion
continuing. When the solids settle they leave a light amber colored
liquor zone between the top of the settled sludge and the surface
of the digester. By adjusting or selecting the supernatant tube,
the liquor with the least solids is returned to the plant.
The settled solids in the secondary digester are allowed to compact
so that a minimal amount of water will be handled in the sludge
dewatering system. These solids are excellent seed or buffer
sludge in case the primary digester becomes upset. A reserve of
30 to 100 thousand gallons should always be held in the secondary
digester. This represents a natural enzyme reserve and may save
the system during a shock load. Primary and secondary
sludge digesters should be operated as a complement to each other.
If you need more seed or buffering capacity in the primary digester,
it should be taken from the secondary digester.
The secondary tanks should be mixed frequently, preferably after
sludge has been withdrawn and supernatant will not be returned to
the plant. Usually secondary digesters are provided with mixers or
recirculating pumps, preferably arranged for vertical mixing. This
periodic mixing prevents coning of solids on the bottom of the tank
and the formation of a scum blanket on the top. Mixing also helps
the release of slowly produced gas that may float solids or scum.
If your plant has only one digester, stop mixing for one day before
withdrawing digested sludge to drying beds.
8-43
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8.28 Rate of Sludge Withdrawal
The withdrawal rate of sludge from either digester should be no
faster than a rate at which the gas production from the system is
able to maintain a positive pressure in the digester (at least
two inches of water column). If the draw-off rate is too fast,
the gas pressure drops due to volume expansion.
WARNING: If continued, a negative pressure develops on
the system (vacuum). This may create an explosive hazard
by drawing air into the digester. If the primary digester
has a floating cover, the sludge may be drawn down to
where the cover rests on the corbels without danger of
losing gas pressure.
Some operators prefer to pump raw sludge or wastewater to a digester
during digested sludge drawoff to maintain a positive pressure. If
gas storage lines permit it, return gas to the digester to maintain
pressure in the digester.
QUESTIONS
8.27A What is the purpose of the secondary digester?
8.27B When raw sludge is pumped to the primary digester,
what happens in the secondary digester?
8.27C How is the level of supernatant withdrawal selected?
8.28A How would you determine the rate of sludge withdrawal?
END OF LESSON 3 OF 5 LESSONS
on
SLUDGE DIGESTION AND HANDLING
8-44
-------
CHAPTER 8. SLUDGE DIGESTION AND HANDLING
(Lesson 4 of 5 Lessons)
8.3 DIGESTER SLUDGE HANDLING
After sludge has passed through a digestion system, it must be
dewatered and disposed of.
Small treatment plants are usually provided with sludge drying beds,
while larger plants utilize mechanical dewatering and drying systems,
Discharge by pipeline or barge to the ocean is sometimes used.
8.30 Sludge Drying Beds (See Fig. 8.14)
The drying process is accomplished through evaporation and percolation
of the water from the sludge after it is spread on a drying bed. The
drying bed is constructed with an underdrain system covered with
coarse crushed rock. Over the rock is a layer of gravel, and then a
layer of pea gravel covered with six to eight inches of sand,
Before sludge is applied, loosen the compacted sand layer by using a
sludge fork with tines eight to twelve inches long. Stick the tines
of the fork into the sand bed and rock it back and forth several times.
This is to loosen the sand only, and care should be taken that the
gravel and sand layer are not mixed. After the whole surface of the
bed is loosened, rake it with a garden rake to break up the sand clods.
Then level the bed by raking or dragging a 4" x 6" or 2" by 12" board
on ropes to smooth the surface.
Sludge is then drawn to the bed from the bottom of the secondary di-
gester. Draw the sludge slowly so as not to create a negative pressure
in the digester and to prevent coning of sludge in the bottom of the
digester. A thick sludge of 8% solids travels slowly, and if the draw-
off rate is too fast, the sludge around the pipe flows out and the
thicker sludge on the bottom moves too slow to fill the void. Conse-
quently, the thinner sludge above the draw-off pipe moves in; and when
it does, the supernatant level is reached, thus allowing almost nothing
but water to go to the drying bed. The thin sludge and supernatant
flowing down to the draw-off pipe washes a hole (shaped like a cone) in
the bottom sludge. When this occurs it sometimes may be remedied by
"bumping". This is accomplished by quickly closing and opening the
draw-off valve on a gravity flow system, which creates a minor shock wave
and sometimes washes the heavier sludge into the cone. If the digested
sludge is pumped to the drying bed, quickly start and stop the pump
using the power switch to create the "bumping" action.
8-45
-------
CLEAN OUT AT GRADE-
•M
TM;
SLUDGE LINE
BED 1
BED UNDERDRAIN
BED 2
REMOVABLE REDWOOD
PLANKS FOR BED FILLING
I M
SLUDGE INLET
TO BEDS
WALKWAY ON TOP OF
BED WALL
BED DRAIN LINE RETURNING TO PLANT HEADWORKS
PLAN SECTION
CLEAN OUT AT
AT GRADE
• t
SAND BED SURFACE
yo o oo o
SLUDGE INLET
o ooo oooo oo
\
CROSS SECTION
DRAIN LINES
SAND
PEA GRAVEL
CRUSHED ROCK
DRAIN LINE
Fig. 8.14 Sludge drying bed
8-46
-------
To draw sludge slowly is time consuming and requires frequent
checks to be sure it does not thicken and stop flowing completely
or cone and run too fast.
The sludge being drawn to the bed is sampled at the beginning of
the fill, when the bed is half full, and just before the bed is
filled to the desired level. The samples may be mixed together
or analyzed separately for total and volatile solids.
The depth to which the sludge is applied is normally around 12
inches, but sometimes it is as deep as 18 inches in arid regions.
If it is deeper, the time required for drying is too long. A
bed filled with 20 inches of sludge would require approximately
the same time to dry as a bed loaded with 14 inches, dried and
removed and filled with another load 14 inches deep.
WARNING
NO SMOKIN6 OR
Auowep IN t-ne vtciNirrv wuece
15
4HOWM
£ MUMBLE OF
AMP fcl)£3l£^ OM lite -SURf ACe
AMP Plt2£^ CAU-^eP BV AM
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After a bed of sludge is drawn, the sludge draw-off line should
be flushed and cleared with water so the solids won't cement in
the line and one end of the line left open for gas to escape, if
it forms. Be sure to drain the line if freezing is a problem.
In warm weather, a good sand bed will have the sludge dry enough
for removal within four weeks. The water separates from the
sludge and drains down through the sand. Evaporation also dries
the sludge and will cause it to crack.
8-47
-------
When the sludge has formed cracks clear to the sand, it may then
be removed by hand with forks. The one major drawback of sand
beds is that heavy equipment, such as a skip loader, cannot be
used because the weight could damage the underdrain system. Also,
the scraping action could mix the sand with the gravel or remove
some of the sand with the dried sludge which will have to be
replaced.
Some operators lay 2" x 12" boards across the sand for wheel-
barrows or light trucks and fork the sludge cake into them to
haul to a disposal site. The dried sludge cake is normally
three to six inches thick and is not heavy unless a large amount
of grit was present in the sludge. The operator calculates the
amount of cake in cubic feet by the depth of the dry sludge cake
and surface area of the bed. The total dry pounds is arrived at from
the total solids in the sludge samples when the sludge was drawn.
Dried sludge makes an excellent soil conditioner and a low-grade
fertilizer. However, in many states air dried digested sludge may
only be used on lawns, shrub beds, and orchards and cannot be used
on root crop vegetables unless heat dried (at 1450°F), or unless
it has been in the ground that the crop is to be planted in for
over one year. It is always best to check with the state or local
health department before dried sludge is used on a food crop.
If a bed of "green" sludge (partially digested) is accidentally
drawn, it will require special attention. The water will not drain
rapidly, odors will be produced, and the water held provides an
excellent breeding ground for nuisance insects. Flies, rat-tail
maggots, psychoda flies, and mosquitoes will breed profusely in this
environment. An application of dry lime spread over the bed by
shovel, and a spraying of a pesticide, is beneficial. The sludge
from such a bed should never be used for fertilizer.
Dry sludge cake will burn at a slow smoldering pace, producing quite
an offensive odor; therefore, don't allow it to catch fire.
WARNING
A SU)P<3£ P0WD&I20C PDST IS
ANP KicKec? INTO THE- AIC IT wi LL
SIM1LAC TO A
MILU.
MIES 4TT?AYS A££ &&T ttf£V
A
8-48
-------
8.31 Blacktop Drying Beds (See Fig, 8.15)
This type of bed has become prevalent in the past few years and
has several advantages if designed properly. It is made of black-
top or asphalt with both sides sloping gradually to the center to
a one-foot wide drain channel. The drain channel runs the full
length of the bed with a three- or four-inch drain line on the
bottom. The drain line is covered with rock, gravel, and sand
as in a sand bed. The drain line usually has a cleanout at the
upper end, and a control valve on the discharge end.
When the bed is to be used, the cleanout on the drain line is
removed and the line is flushed with clear water and the clean-
out cover replaced. It is recommended that the drain line valve
be closed and the drain line and drain channel be filled with
water to the top of the sand, so that the sand is not sealed
with sludge. Sludge is then admitted to the bed. Some plants
have operated successfully without pre-filling the collection
system with water.
The depth the sludge is applied to the bed is between 18 and 24
inches.
The sludge is sampled in the same way as when using a sand bed,
except one additional sample is taken in a glass jar or beaker
and set aside. By watching the jar of sludge, you can observe
at some time during the first 24 to 36 hours that the sludge
will rise to the top, leaving liquor on the bottom. This is
primarily caused by the gas in the sludge. (Later, the sludge
will again settle to the bottom and the liquor will be on the
surface.) The drain valve on the drying bed should be opened
when the sludge separates and rises to the top of the jar. The
liquor collected in the sludge bed drains is normally returned
to the primary clarifiers.
After the sludge has started to crack and has a crust, drying
time may be reduced by driving a vehicle through the bed to mix
the sludge. When the cake is dry a skip loader is used to clean
the bed.
Blacktop beds may be able to handle two to three times as much
sludge as sand beds in a given period of time.
8-49
-------
BED DRAIN LINE RETURN TO PLANT HEADVORKS r j"
wm
\
i
|
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M
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ENTRANCE RAMP WITH REDWOOD STOP LOG
PLAN VIEW
PLANT MIX BLACKTOP^
SLUDGE LINE
CROSS SECTION ONE BED
Fig. 8.15 Blacktop drying bed
8-50
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8.32 Sludge Lagoons
Sludge lagoons are deep ponds that hold digested sludge and, in
some instances, supernatant. Digested sludge is drawn to the
lagoon periodically and may require a year or two to fill. When
the lagoon is full, sludge is discharged into another lagoon
while the first one dries. This drying period can require a
year or two before the sludge is removed. Some large cities have
used lagoons for many years, avoiding the use of covered secondary
digestion tanks.
QUESTIONS
8.31A How would you attempt to reduce drying time in
blacktop beds?
8.32A How does a sludge lagoon operate?
8-51
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8.33 Withdrawal to Land
Wet sludge can be spread on land to reclaim the land or on farm
land and ploughed in as a soil conditioner and fertilizer. Used
with lagoons this gives a flexible system., This is an excellent
method of sludge disposal wherever applicable, because it returns
the nutrients to the land and completes the cycle as intended by
nature.
Transporting sludge to the disposal site is accomplished by tank
truck or pipeline. The application of wet sludge to the land
depends upon the topography and the crop to be raised on that
land. When applied to grass or low ground cover crops, appli-
cation may be by spraying from the back of the tank truck while
driving over the land, by the use of irrigation piping, or by
shallow flooding.
The best method, but most costly, is leveling the land, constructing
ridges and furrows, and then pumping the sludge down the furrows
similar to irrigation practices used in arid regions. This method
is not only capable of reclaiming land unsuitable for growing
plants and trees, but may yield crops equal to or greater than
those raised with commercial fertilizers.
Some precautions that must be practiced with this method of sludge
disposal include:
1. Never apply partially digested ("green") sludge or scum.
2. Residential areas must not be located near land disposal
sites.
3. Land disposal sites must not be located on a flood plain
where the sludge may be washed into the receiving waters
during flooding.
4. Domestic water wells must not be located on the land
receiving the sludge.
5. Root crop vegetables must not be grown on the land.
6. Cooperation with the landowner as to application time,
drying, and covering must be guaranteed.
7. Access to the land during wet weather must be provided.
8-52
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8.34 Mechanical Dewatering
In plants where large volumes of sludge are handled and drying
beds are not feasible, mechanical dewatering may be used.
Mechanical dewatering falls into two methods: vacuum filters
and centrifuges. Each is capable of reducing the moisture
content of sludge to 60% to 80%, leaving a wet, pasty cake
containing 20% to 40% solids. This cake may then be disposed
of as land fill, barged to sea, dried in furnaces for fertilizer,
or incinerated to ash in furnaces or wet oxidation units.
A. Vacuum Filters [Fig. 8.17J.
For digested sludge to be dewatered by this method usually requires
a conditioning of the sludge by the addition of chemicals.
Elutriation (e-LOO-tree-a-shun) is the washing of the digested
sludge in plant effluent in a suitable ratio of sludge to effluent.
Elutriation may be accomplished in from one to three separate tanks,
similar to small rectangular clarifiers. The sludge is pumped to
the elutriation tank and mixed with plant effluent. Next this
mixture is admitted to the other tanks to establish a counter-
current wash. The sludge is then allowed to settle and is collected
by flights and pumped to the next elutriation tank. After one to
three washings it is then pumped to the conditioning tanks. The
main purpose of the elutriation tanks is to remove the fine sludge
particles which require large amounts of chemicals for coagulation.
It also removes amino acids and salts which may have a small coagu-
lant demand. After elutriation the sludge will react with the
chemicals better and produce better cake. The elutriate (effluent
from elutriation tanks) is returned to the primary clarifiers and
may result in a very heavy recirculating load since it is chiefly
fine solids. Many treatment plants have discontinued the practice
of elutriation. Although the process saves approximately $1 per ton
of dry solids handled on chemical costs, the costs are excessive for
treating the elutriate (wash water) in the biological treatment
processes.
Sludge conditioning is accomplished by the addition of various
coagulants or flocculating agents such as ferric chloride, alum,
lime, and polymers. In the conditioning tank the amount of chemical
solution added is normally established by laboratory testing of sludge
grab samples by adding various chemical concentrations to the grab
samples to obtain a practical filtration rate by vacuum. This test
establishes the operating rate for the chemical feed pumps or
rotameters from the chemical head tanks, which is normally less
than 10% of the dry sludge solids rate to the conditioning tank.
(Both rates could be in pounds per 24 hours.) In this tank the
8-53
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chemical is mixed into the sludge by. gentle agitation for several
minutes. The conditioned sludge then flows to the filter bath
where it is continuously and gently agitated. After operation
has started, chemical feed is regulated according to cake appear-
ance and behavior.
Filter drums are 10 to 18 feet in diameter, and 12 to 20 feet in
length, They may use cloth blankets of dacron, nylon, or wool,
or use steel coil springs in a double layer, to form the outer
drum covering and filter media. The drum inside is a maze of
pipe work running from a metal screen and wood surface skin, and
connecting to a rotating valve port at each end of the drum.
Cloth blankets are stretched and caulked to the surface of the
filter drums with short sections of 1/4" cotton rope at every
screen section. The sides of the blanket are also stretched and
stapled to the end of the drums. The nap18 of the blanket should
be out. After the blanket is stretched completely around the drum,
it is then wrapped with two strands of 1/8" stainless steel wire,
approximately 2" apart for the full length of the drum.
The installation of a blanket may require several days, and it
lasts from 200 to 20,000 hours. The life of the blanket depends
greatly on the blanket material, conditioning chemical, backwash
frequency, and acid bath frequency. An improper adjustment of the
scraper blade, or accidental tear in the blanket, will usually
require its replacement.
Both cloth blankets and coil spring filters require a high pressure
wash after 12 to 24 hours of operation, and in some instances, an
acid bath after 1000 to 5000 operating hours.
The filter drum is equipped with a variable speed drive to turn the
drum from 1/8 to 1 rpm. Normally, the lower rpm range is used to
give the filter time to pick up sufficient sludge as it passes through
the conditioned sludge tub under the filter. Normally less than 1/5
of the filter surface is submerged in the tub and pulling sludge to
the blanket or springs by vacuum to form the cake mat. As that area
passes through the conditioned sludge, the vacuum holds a layer 1/8
to 1/2 inch thick of sludge to the media, and continues to pull the
water from the sludge to approximately 210 degrees from the bottom
point of the filter after it leaves the vat. This is the drying
cycle. At this point the vacuum is released and a light air pressure
(3.0 psi) is applied to the inside of the blanket, lifting the sludge
18
Nap. The soft fuzzy surface of the fabric.
8-54
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O9
EXTERNAL PIPING
COIL SPRINGS
WASH HEADERS
FILTER
VALVE
DRUM
GE LEVEL
NLETi DRAIN
Fig. 8.17 CoiIfliter elevation
Courtesy of KomKne-Sanderson Engineering Corporation
-------
so that it falls from the blanket into a hopper or conveyor belt.
The drum then rotates past a scraper blade to remove sludge that
did not fall. The applied air is then phased out as that
section starts into the filter tub, and vacuum is applied in
order to pick up another coating of sludge.
The thickness of the sludge cake and moisture content depend
upon the sludge, chemical feed rate, drum rotation speed, mixing
time, and condition of the blanket or coil springs. A filter may
blank out (lose sludge cake) for any of the above reasons or due
to the loss of vacuum or filtrate pumps. Filtrate is the liquor
separated from the sludge by the filter; it is returned to the
primary clarifiers.
QUESTIONS
8.33A What are some of the advantages of applying sludge
to land?
8.34A How is sludge disposed of in many large plants or
areas where drying beds are not feasible?
8.34B How would you prepare digested sludge for drying by
vacuum filtration?
8.34C How would you determine the chemical feed rate to
condition sludge?
8.34D What factors influence the life of a filter blanket?
8-56
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B. Centrifuge
Centrifuges are gaining in popularity for dewatering raw or primary
sludges for furnaces or incineration units. Their use on digested
sludge is becoming more widespread and is expected to replace vacuum
filters as the prime digested sludge dewatering device. Most
digested sludges are conditioned with polymers before being fed to
a centrifuge.
Centrifuges are various sized cylinders that rotate at high speeds.
The sludge is pumped to the center of the bowl where centrifugal
force established by the rotating unit separates the lighter liquid
from the denser solids. The centrate19 is returned to the primary
clarifiers, and the sludge cake is removed to a hopper or to a con-
veyor for disposal.
The feed rate, pool depth, centrifuge rpm, and other factors
determine the condition of the discharge cake or slurry and the
quality of centrate. The centrate usually contains a high amount
of suspended solids that become difficult to handle in the primary
clarifiers and digesters. A large amount of grit in the sludge greatly
increases the wear rate on the centrifuge. Similar to the wash water
from the elutriation process, centrate from vacuum filters also exerts
a difficult load on biological treatment processes.
QUESTIONS
8.34E Centrifuges are commonly used to dewater
what types of sludges?
8.34F How would you regulate the condition of
the sludge cake from a centrifuge?
END OF LESSON 4 OF 5 LESSONS
on
SLUDGE DIGESTION AND HANDLING
19 Centrate. The liquor leaving the centrifuge after
most of the solids have been removed.
8-57
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CHAPTER 8. SLUDGE DIGESTION AND HANDLING
(Lesson 5 of 5 Lessons)
8.4 DIGESTER CONTROLS AND TEST INTERPRETATION
A. Temperature
A thermometer is usually installed in the recirculated sludge line
from the digester to the heat exchanger. This thermometer will
accurately measure the temperature of the digester contents when
circulation is from bottom to top. The temperature from the di-
gester is recorded and should be maintained between about 95 and
98°F for mesophilic digestion. Never change the temperature more
than 1°F per day. Accurate temperature readings also may be taken
from the flowing supernatant tube or from the heat exchanger sludge
inlet line. The same temperature should be maintained at all levels
of the tank.
B. Volatile Acid/Alkalinity Relationship
The volatile acid/alkalinity relationship is the key to successful
digester operation. As long as the volatile acids remain low and
the alkalinity stays high, anaerobic sludge digestion will occur in
a digester. Each treatment plant will have its own characteristic
ratio for proper sludge digestion (generally less than 0.1). When
the ratio starts to increase, corrective action must be taken imme-
diately. This is the first warning that trouble is starting in a
digester. If corrective action is not taken immediately or is not
effective, eventually the C02 content of the digester gas will in-
crease, the pH of the sludge in the digester will drop, and the
digester will become sour.
A good procedure is to measure the volatile acid/alkalinity relation-
ship at least twice a week, plot the volatile acid/alkalinity
relationship against time, and watch for any adverse trends to develop.
Whenever something unusual happens, such as an increased solids load
from increased waste discharges or a storm, the volatile acids/
alkalinity relationship should be watched closely.
8-58
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The volatile acid/alkalinity relationship is an indication of
the buffer capacity of the digeste'r contents. A high buffer
capacity is desirable and is achieved by a low ratio which exists
when volatile acids are low and the alkalinity is high (120 mg/1
volatile acids/2400 mg/1 alkalinity). Excessive feeding of raw
sludge to the digester, removal of digested sludge, or a shock
load such as produced by a storm flushing out the collection
system may unbalance the volatile acid/alkalinity relationship.
A definite problem is developing when the volatile acid/alkalinity
relationship starts increasing. Once the relationship reaches the
vicinity of 0.5/1.0 (1000 mg/1 volatile acids/2000 mg/1 alkalinity),
serious decreases in the alkalinity usually occur. At a relation-
ship of 0.5/1.0 the concentration of C02 in digester gas will start
to increase. When the relationship reaches 0.8 or higher, the pH
of the digester contents will begin to drop. When the relation-
ship first starts to increase, ample warning is given for corrective
action to be taken before problems develop and digester control is
lost.
Response to an Increase in Volatile Acid/Alkalinity Ratio:
When the ratio starts to increase, extend mixing time of digester
contents, control heat more evenly, and decrease sludge withdrawal
rates. Mixing should be vertical mixing from the bottom of the tank
to the top of any scum blanket. If possible, some of the concen-
trated sludge in the secondary digester should be pumped back to
help correct the ratio. In addition, the primary digester should
not be operated as a continuous overflow unit when raw sludge is
added, but it should be drawn down to provide room for some sludge
from the secondary digester too. During heavy rains when extra
solids are flushed into the plant, it may be necessary to add some
digested sludge to the primary digester. Use the volatile acid/
alkalinity ratio as a guide to determine the amount of digested
sludge that should be returned to the primary digester for control
purposes.
C. Digester Gas (C02 and Gas Production)
This is a useful test to record. The change of C02 in the gas is
an indicator of the condition of the digester. Good digester gas
will have a C02 content of 30 to 35%. The volatile acid/alkalinity
relationship will start to increase before the carbon dioxide (C02)
content begins to climb. If the C02 content exceeds 42%, the
digester is considered in poor condition and the gas is close to the
burnable limit (44 to 45% C02).
8-59
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Gas production in a properly operating digester should be con-
stant if feed is reasonably constant. If the volume produced
gradually starts falling, trouble of some sort is indicated.
D. £H
pH is normally run on raw sludge, recirculated sludge, and super-
natant. This information is strictly for the record and not for
plant control. The raw sludge, if stale, will be acid and run in
the range of 5.5 to 6.8. Digester liquors should stay around 7.0
or higher. pH is usually the last indicator to change and gives
little warning of approaching trouble. It is therefore the least
desirable control method.
QUESTIONS
8.4A Where would you obtain the temperature of a digester?
8.4B Why is the volatile acid/alkalinity relationship very
useful in digester control?
8.4C What should be done when the volatile acid/alkalinity
relationship starts to increase?
8.4D Why is pH a poor indicator of approaching trouble in
a digester?
8-60
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E. Solids Test
Samples are collected of the raw sludge, recirculated sludge, and
supernatant. Each sample is tested for total solids and volatile
solids.
The information from these tests is used to determine the pounds
of solids handled through the system, the digester loading rates,
and the percent of reduction of the organic matter destroyed by
the digester, All of these tests are necessary for the maintenance
of close digester operation.
F. Volume of Sludge
Volumes of sludge are needed to determine the pounds of solids
handled through the system. In smaller plants which use a positive
displacement pump, the volume of raw sludge is determined by the
volume the pump displaces during each revolution. For instance, a
10-inch piston pump with a 3-inch stroke will discharge one gallon
per revolution. These pumps are equipped with a counter on the end
of the shaft and are seldom operated faster than 50 gpm.
8-61
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J. Digester Supernatant
The total solids test is run on the digester supernatant to deter-
mine the solids load returned to the plant. The total solids in
the digester supernatant should be kept below 1/2 of 1% (0.005 or
5000 mg/1) . High solids content in the supernatant usually indicates
that too much seed or digested sludge is being withdrawn from the
digester. This kind of withdrawal could increase the volatile acid/
alkalinity relationship which is also undesirable.
Another simple method for checking supernatant is to draw a sample
into a 1000 ml graduate and let it stand for four or five hours.
The sludge on the bottom of the graduate should be below 50 ml, with
an amber colored liquor above it. If supernatant solids are allowed
to build too high, an excessive solids and BOD load is placed on the
secondary system and primary clarifier. Sludge withdrawn from the
secondary digester or supernatant removal tubes should be changed to
a different level in the digester where the liquor contains the least
amount of solids when the supernatant load becomes too heavy on the
plant.
Plants should be designed to allow all sludge solids and liquids to
go to a lagoon or some such system for final or ultimate disposal,
rather than returning them to the plant.
K. Computing Digester Loadings
Digester loadings are reported as pounds of volatile matter per cubic
foot or 1000 cubic feet of digester volume per day. The loading rate
should be around 0.15 to 0.35 pounds of volatile solids per cubic foot
in a heated and mixed digester. For an unmixed or cold digester, the
loading rate should not exceed 0.05 pounds of volatile matter per
cubic foot, assuming that each cubic font contains approximately 0.5
pounds of preiigested solids.
8-62
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8.6 AEROBIC SLUDGE DIGESTION
8.60 Introduction
Aerobic digestion of solids occurs, whether intentional or not, in
any of the conventional secondary treatment processes. In the ex-
tended aeration process, the aerobic digestion process is continued
almost to the maximum obtainable limit of volatile matter reduction.
A separate aerobic digester is intended mainly to insure that re-
sidual solids from aerobic biological treatment processes are digested
to the extent that they will not cause objectionable odors during
disposal. The aerobic digester is a separate operation following
other processes to extend
decomposition of solids and
regrowth of organisms to a
point where available energy
in active cells and storage
of waste materials are suf-
ficiently low to permit the
material to be considered
stable enough for discharge
to some ultimate disposal
operation. Neither aerobic
nor anaerobic sludge digestion
completes the oxidation of
volatile materials in the
digester.
Important comparisons between
aerobic and anaerobic sludge
digestion are summarized in
the following sections.
Anaerobic Sludge Digestion
1. Does not use aeration as part of the process.
2. Works best on fresh wastes that have not been treated
by prior stabilization processes.
3. Uses putrefaction as a basic part of the process.
8-63
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4. Tends to concentrate sludge and improves drainability.
5. Produces methane gas that provides energy for other
operations.
6. Generates major digestion products consisting of solids,
carbon dioxide, water, methane, and ammonia.
7. Produces liquids that may be difficult to treat when
returned to the plant.
8. Generates sludges that need additional stabilization
before ultimate disposal.
Aerobic Sludge Digestion
1. Has lower, equipment costs, hut operating costs are higher,
2. Tends to produce less noxious odors.
3. Produces liquids that usually are easier to treat
when returned to the plant.
4. Generates major digestion products consisting of residual
solids, carbon dioxide, water, sulfates, and nitrates.
Most of these products are close to the final stabilization
stage.
5. May achieve nitrogen removal by stopping aeration long
. enough to allow the conversion of nitrates to nitrogen
gas. Aeration must be restarted before sulfates are
converted to sulfides (H2S).
6. Tends to work better on partially stabilized solids from
secondary processes that are difficult to treat by the
anaerobic digestion process.
7. Produces a sludge that has a higher water content. Aerobic
sludges are difficult to concentrate higher than 4 percent
solids.
8. Uses oxygenation and mixing provided by aeration process
equipment.
9. Has less hazardous cleaning and repairing tasks.
10. Works by aerobic decay which produces less odors when
operated properly.
8-64
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8.61 Process Description
Aerobic digestion tanks may be either round or rectangular, eighteen
to twenty feet deep, with or without covers, depending on geographical
location and climatic conditions. The tanks use aeration equipment
(mechanical or diffused air) to maintain aerobic conditions. Each
tank has a sludge feed line at mid-depth of the tank, a sludge draw-
off line at the bottom of the tank, and a flexible, multilevel super-
natant draw-off line to remove liquor from the upper half of the tank.
Covers are used in colder climates to help maintain the temperature
of the waste being treated. Covers should not be used if they
reduce evaporative cooling too much and the liquid contents become
too warm. When the liquid becomes too warm offensive odors may
develop and the process effluent will have a very poor quality.
Aerobic digestion requires the waste solids to be held at least
twenty days in the digester. Detention time depends on the origin
of the sludge being treated. Twenty days will provide sufficient
digestion time for sludges from an extended aeration process where
the sludges are already well digested. Sludges from a contact
stabilization process require more than twenty days. When temper-
atures are very low the sludge may have to be held until the
weather warms in the spring.
8.62 Operation
Aerobic digesters are operated under the principle of extended
aeration from the activated sludge process, relying on the mode or
region called endogenous22 respiration. Aerobic digestion consists
of continuously aerating the sludge without the addition of new food,
other than the sludge itself, so the sludge is always in the endogenous
region. Aeration continues until the volatile suspended solids are
reduced to a level where the sludge is reasonably stable, does not
create a nuisance or odors, and will readily dewater.
22 Endogenous (en-DODGE-en-us): A diminished level of respiration
in which materials previously stored by the cell are oxidized.
8-65
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8.64 Operational Problems
A. Scum
The aerobic digesters will have to be skimmed periodically to remove
floating grease and other material that will not digest. This
material should be disposed of by incineration or burial with the
scum collected from the primary clarifier.
B. Odors
Odors should not be a problem in aerobic digestion unless insufficient
oxygen is supplied or a shock load reaches the aerobic digestion tanks.
If an odor problem does occur, a very effective cure is to recycle
sludge from the bottom of the second or third tank back to the first
tank. This is also good practice in activated sludge plants that have
bulking.problems because sludge from the last aerobic digester responds
very quickly when returned to an aerator.
C. Floating Sludge
Floating sludge may become quite thick in the second and third tanks
when aeration is stopped during removal of the supernatant. To avoid
clogging, the supernatant draw-off line should be installed so the
withdrawal point is from two to six feet below the water surface. The
floating sludge is a problem only during supernatant removal. Scum
and solids must be removed from the supernatant to prevent interference
with other treatment processes and degradation of the plant effluent.
8.65 Maintenance Problems
Usually this process requires very little maintenance. Routinely hose
the side walls of open tanks for appearance and fly control.
8-66
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A. Diffuser Maintenance
If diffused air is used for aeration, only open orifice or nozzle
type diffusers should be installed because of the daily stopping
of air flow during supernatant removal.
B. Aeration Equipment
Aeration equipment should be operated continuously except when
settling is needed for supernatant removal. Both settling and
supernatant removal should be accomplished in 0.5 to 1.5 hours.
QUESTIONS
8.6A Why do some plants have aerobic digesters?
8.6B What are some of the advantages of aerobic digestion
in comparison with anaerobic digestion?
8.6C What dissolved oxygen levels should be maintained
in aerobic digesters?
8-67
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8.7 ADDITIONAL READING
a. MOP 11, pages 39-88.
b. New York Manual, pages 85-116.
c. Texas Manual, pages 303-396 and 413-444.
d. Sewage Treatment Practices, pages 63-79.
e. Anaerobic Sludge Digestion, WPCF Manual of Practice No. 16,
Water Pollution Control Federation, 3900 Wisconsin Avenue,
Washington, D.C. 20016. Price $1.50 to members; $3.00 to
others. Indicate your member association when ordering.
f. Dague, Richard R., Digester Control, J. Water Pollution
Control Federation, Vol. 40, No. 12, p 2021 (December 1968).
g. Sludge Dewatering, WPCF Manual of Practice No. 20, Water
Pollution Control Federation, 3900 Wisconsin Avenue,
Washington, D.C. 20016. Price $3.00 to members; $5.00
to others. Indicate your member association when ordering.
END OF LESSON 5 OF 5 LESSONS
on
SLUDGE DIGESTION AND HANDLING
8-68
-------
CHAPTER 9. WASTE TREATMENT PONDS
USED FOR TREATMENT OF WASTEWATER AND OTHER WASTES
(Lesson 1 of 3 Lessons)
9.1 INTRODUCTION
Shallow ponds (three to five feet deep) are often used to treat
wastewater and other wastes instead of, or in addition to, conven-
tional waste treatment processes. (See Figs. 9.1 and 9.2 for
typical plant layouts.) Wastes which are discharged into ponds
are treated or stabilized1 by several natural processes acting
at the same time. Heavy solids settle to the bottom where they
are decomposed by bacteria. Lighter suspended material is broken
down by bacteria in suspension.
Dissolved nutrient materials, such as nitrogen and phosphorous,
are utilized by green algae which are actually microscopic plants
floating and living in the water. The algae utilize carbon dioxide
(C02) and bicarbonates to build body protoplasm. In so growing
they need nitrogen and phosphorous in their metabolism much as
land plants do. Like land plants, they release oxygen and some
carbon dioxide as waste products.
In recent years, ponds have become more popular as treatment
facilities. Extensive studies of their performance have led to a
better understanding of the natural processes by which ponds treat
wastes. Information is also available which can help operators to
regulate the pond processes for efficient waste treatment.
9.2 HISTORY OF PONDS IN WASTE TREATMENT
The first wastewater collection systems in the ancient Orient and
in ancient Europe were discharged into adjacent bodies of water.
These systems accomplished their intended purpose until overloading,
as in modern systems, made them objectionable.
1 Stabilized Wastes. A waste that has been treated or decomposed
to the extent that if discharged or released, its rate and state
of decomposition would be such that the waste would not cause a
nuisance or odors.
9-1
-------
In ancient times, ponds and lakes were purposefully fertilized
with organic wastes to encourage the 'growth of algae which, in
turn, greatly increased the production of fish due to the food
supply provided by the algae, '["his practice still persists and
is a recognized art in Germany.
Hvidently, the first ponds constructed in the United States were
built for the purpose of excluding waste waters from intrusion into
places where they would be objectionable, (\ice constructed, these
ponds performed a treatment process that finally became recognized
as such. The tendency over the years has been to equate pond treat-
ment efficiency with the non-emission cf odors. Actticlly, the
opposite is true a? the greatest organic load destroyed per unit
of area ("high treatment efficiency") may be accompanied by objection-
able odors.
Armed with the current scientific knowledge of pondinp and utilizing
the experience of both successes and failures, engineers have designed
and constructed a great number of ponds performing a variety of
functions since 195.S. Ponds that have been designed with adequate
engineering, backed by the research of a qualified biological con-
sultant, and operated in a purposeful manner have produced successful
results.
Ponding of wastewater as a complete process offers the following
advantages for smaller installations, provided land is not costly
and the location is isolated from residential, commercial, and
recreational areas:
> 1. Poes not require expensive equipment.
2. Does not require highly trained operating personnel.
3. Is economical to construct.
0-2
-------
INF.
FLOW
METER
L.
BAR SCREEN
w
V
: 'PWDS ' J
w
F
DISINFECTION
EFF.
^-
Measure,
record
flow
Remove
coarse
mater ial
Biologica
Process
Chlorine Contact
(Kill pathogenic
organi sms)
o
I
/
— *" ^^ /
1
INFLUENT ^ ^ J \ ,. ^
1 V UK ini v 5H=
V-,i..i—.u;"" • :':::" -'-- "-'iiiiinrX
CHLORINATION
\z::," '^-^J -:•' ":'"„— ll/ T
N^x/'-v- \r/ EFFLUENT
Fig. 9.1 Typical plant; ponds only
-------
4. Provides treatment that is equal to or superior to
some conventional processes.
5. Makes a satisfactory short-term method of treating
wastewater on a temporary basis until a permanent
plant can be constructed.
6. Is adaptable to fluctuating loads.
7. Is probably the most trouble-free of any treatment
process when utilized correctly, provided a consistently
high quality effluent is not required.
QUESTIONS
9.2A If a pond is giving off objectionable odors, are the
wastes being effectively treated? Explain your answer.
9.2B Discuss the advantages of ponds.
9-4
-------
INF.
FLOW
METER
k
PRELIM.
TREATMENT
t
r
PRIMARY
TREATMENT
>.
1
SECONDARY
TREATMENT
^.
f
JW$
DISIN-
FECTION
Measure,
record
flow
•o
i
On
Screening,
grit
removal
(Remove
coarse
material)
Sedimen-
tation
(Remove
settleable
and floating
materials)
Biological
Process
(Remove sus-
pended and
dissolved
sol ids)
v
DIGESTION
AND
SLUDGE
HANDLING
(Solids
- Disposal)
INFLUENT
CHLORINATION
TO DIGESTION
Fig. 9.2 Typical plant; ponds after secondary treatment
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9.3 POND CLASSIFICATIONS AND USES
Ponding of raw wastewater, as a complete treatment process, is
used to treat the wastes of single families as well as large
cities up to the size of the city of Melbourne, Australia, which
handles 78 million gallons of wastewater per day. Ponds designed
to receive wastes with no prior treatment are often referred to
as raw wastewater (sewage) lagoons or stabilization ponds. This
requires sizable areas of land.
Ponds are quite commonly used in series (one pond following another)
after a primary wastewater treatment plant to provide additional
clarification, BOD removal, and disinfection. These ponds are
sometimes called oxidation ponds.
Ponds are sometimes used in series after a trickling filter plant,
thus giving a form of "tertiary"2 treatment. These are sometimes
called polishing ponds.
Ponds placed in series with each other can provide a high quality
effluent which is acceptable for discharge into most watercourses,
if stringent disinfection standards are not required.
It is possible to have a great many different variations in ponds
due to depth, operating conditions, loading, etc., and a bold
line of distinction is often impossible. Current literature
generally uses three broad pond classifications: aerobic,
anaerobic, and facultative.
Aerobic ponds are characterized by having dissolved oxygen distri-
buted throughout their contents practically all of the time. They
usually require an additional source of oxygen other than the rather
minimal amount that can be diffused from the atmosphere at the water
surface. The additional source of oxygen may be supplied by algae,
by mechanical agitation of the surface, or by bubbling air through
the pond.
2 Tertiary (TER-she-AIR-ee). Tertiary refers to the third treat-
ment process or the process following a secondary treatment
process, such as a trickling filter. Some refer to tertiary
treatment as advanced waste treatment, meaning processes that
remove wastes not normally removed by conventional (secondary)
treatment processes.
9-6
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Anaerobic ponds, as the name implies, are usually without any dis-
solved oxygen throughout their "entire depth. Treatment depends on
fermentation of the sludge at the pond bottom. This process, under
certain conditions, can be quite odorous, but it is highly efficient
in destroying organic wastes. Anaerobic ponds are mainly used for
industrial processing wastes, although some domestic waste ponds
find their way into this category when they become badly overloaded.
Facultative (FACK-ul-tay-tive) ponds are the most common type in
current use. The upper portion (supernatant) of these ponds is
aerobic, while the bottom layer is anaerobic. Algae supply most
of the oxygen to the supernatant. Facultative ponds are most common
because it is almost impossible to maintain completely aerobic or
anaerobic conditions all the time at all depths of the pond.
Pond uses may be classified according to detention time. A pond
with a detention time of less than three days will perform in ways
similar to a sedimentation or settling tank. Some algal growth
will occur in the pond, but it will not have a major effect on the
treatment of the wastewater.
Prolific algal growth will be observed in ponds with detention
periods from three to around 20 days, but large amounts of algae
will be found in the pond effluent. In some effluents, the
stored organic material may be greater than the amount in the
influent. Detention times in this range merely allow the organic
material to change form and delay problems until the algae settle
out in the receiving waters'. Effluent BODs may show considerable
reductions from influent BOD concentrations, but this is because
BOD is a rate estimate (oxygen used during a 5-day period). The
rate of oxygen used is temporarily slowed down, but will increase
when anaerobic decomposition of settled dead algal cells starts.
Longer detention periods in ponds provide time for algal sedimen-
tation, hopefully in ponds with anaerobic conditions on the bottom
and aerobic conditions on the surface. Combined aerobic-anaerobic
treatment provided by long detention periods produces definite
stabilization of the influent.
QUESTIONS
9.3A What is the difference between raw wastewater (sewage)
lagoons, oxidation ponds, and polishing ponds?
9.3B What is the difference between the terms aerobic,
anaerobic, and facultative?
9.3C Describe three possible uses of ponds.
9-7
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9.4 EXPLANATION OF TREATMENT PROCESS
Waste disposal ponds are classified according to their dissolved
oxygen content. Oxygen in an aerobic pond is distributed through-
out the entire depth practically all the time. An anaerobic pond
is predominantly devoid of oxygen most of the time because oxygen
requirements are much greater than the oxygen supply. In a
facultative pond, the upper portion is aerobic most of the time,
whereas the bottom layer is predominantly anaerobic.
In aerobic ponds, organic matter contained in the wastewater^is
first converted to carbon dioxide and ammonia, and finally, in the
presence of sunlight, to algae. Algae are simple one-cell micro-
scopic plants which are essential to the successful operation of
both 'aerobic and facultative ponds.
By utilizing sunlight through photosynthesis,3 the one-celled
plant uses the oxygen in the water molecule to produce free oxygen,
making it available to the aerobic bacteria that inhabit the pond.
Each pound of algae in a healthy pond is capable of producing 1.6
pounds of oxygen on a normal summer day. Algae subsist on carbon
dioxide and other nutrients in the wastewater. Algae occur in a
pond without seeding and multiplying greatly under favorable conditions.
In anaerobic ponds, the organic matter is first converted by a group
of organisms called the "acid producers" to carbon dioxide, nitrogen,
and organic acids. In an established pond, at the same time, a
group called the "methane fermenters" breaks down the acids and
other products of the first group to form methane gas and alkalinity.
Water is another end product of organic reduction.
In a successful facultative pond, the processes characteristic
of aerobic ponds occur in the surface layers, while those similar
to anaerobic ponds occur in its bottom layers.
During certain periods sludge decomposition in the anaerobic zone
is interrupted and it begins to accumulate. If sludge accumulation
3 Photosynthesis. A process in which organisms with the aid of
chlorophyll (green plant enzyme) convert carbon dioxide and
inorganic substances to oxygen and additional plant material,
utilizing sunlight for energy. Land plants grow by the same
process.
9-8
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occurs and decomposition does not set in, it is probably due to
lack of suitable bacteriological'population, low pH,14 presence
of inhibiting substances, or a low temperature. Under these
circumstances the acid production will continue at a slower rate,
but the rate of gas (methane) production slows down considerably.
Sludge storage in ponds is continuous with small amounts stored
during warm weather and larger amounts when it is cold. During
low temperatures the bacteriological population cannot multiply
fast enough to handle the waste. When warm weather comes, the
"acid producers" start in decomposing the accumulated sludge deposits
built up during the winter. If the organic acid production is too
great, a lowered pH will occur with the possibilities of an upset
pond and resulting hydrogen sulfide odors.
Hydrogen sulfide is ordinarily not a problem in properly designed
and operated ponds because it dissociates (divides) into hydrogen
and hydrosulfide ions at high pH and may form insoluble metallic
sulfides or sulfates. It is because of this high degree of dis-
sociation and the formation of insoluble metallic sulfides that
ponds having a pH above 8.5 do not emit odors, even when hydrogen
sulfide is present in relatively large amounts.
All of the organic matter that finds its way to the bottom of a
stabilization pond through the various processes of sludge
decomposition is subject to me th an e f ermen t at i on, provided that
proper conditions exist or become established.
In order for methane fermentation to exist, an abundance of organic
matter must be deposited and continually converted to organic acids.
An abundant population of methane bacteria must be present. They
require a pH level within the sludge of from 6.5 to 7.5, alkalinity
of several hundred mg/1 to buffer (neutralize) the organic acids
(volatile acid/alkalinity relationship), and suitable temperatures.
pH. pH is an expression of the intensity of the alkaline or
acid strength of water. Mathematically, pH is the logarithm
(base 10) of the reciprocal of the hydrogen ion concentration.
pH =
The pH may range from 0 to 14, where 0 is the most acid, 14
the most alkaline, and 7 is neutral. Most natural waters
usually have a pH between 6.5 and 8.5.
9-9
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Once methane fermentation is established, it accounts for a
considerable amount of the organic load removal.
QUESTIONS
9.4A How is oxygen produced by algae?
9.4B Where does the algae found in a pond come from?
9.4C What happens to unstable organic matter in a pond?
9-10
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9.5 POND PERFORMANCE
The treatment efficiencies that can be expected by ponds vary
more than most other treatment devices. Some of the many
variables are:
1. Physical Factors
a. type of soil
b. surface area
c. depth
d. wind action
e. sunlight
f. temperature
g. short circuiting
h. inflow variations
2. Chemical Factors
a. organic material
b. pH
c. solids
d. concentration and nature of waste
3. Biological Factors
a. type of bacteria
b. type and quantity of algae
c. activity of organisms
d. nutrient deficiencies
e. toxic concentrations
The performance expected from a pond depends upon its design.
The design, of course, is determined by the waste discharge
requirements or the water quality standards to be met in the
receiving waters. Overall treatment efficiency may be about
the same as primary treatment (only settling of solids), or
it may be equivalent to the best secondary biological treat-
ment plants. Some ponds, usually those located in hot, arid
areas, have been designed to take advantage of percolation
and high evaporation rates so that there is no discharge.
Depending on design, ponds can be expected to provide BOD re-
movals of from 50 to 90%. Facultative ponds, under normal
design loads with 50 to 60 days detention time, will usually
remove approximately 90 to 95% of the coliform bacteria and
70 to 80% of the BOD load approximately 80% of the time.
9-11
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Physical sedimentation by itself has been found to remove
approximately 90% of the suspended solids in three days, and
about 80% of the dissolved organic solids in ten days. How-
ever, in a pond with a healthy algae and bacteria population,
a phenomenon known as bioflocculation5 can occur which will
remove approximately 85% of both suspended and dissolved solids
within hours. Bioflocculation is accelerated by increased tem-
perature, wave action, and high dissolved oxygen content.
Pond detention times are sometimes specified by regulatory
agencies to assure adequate treatment and removal of bacteria.
Many agencies specify effluent or receiving water quality
standards in terms of median and maximum MPN values that should
not be exceeded. In critical water use areas chlorination or
other means of disinfection can be used to further reduce the
coliform level.
A pond is generally regarded as not fulfilling its function
when it creates a visual or odor nuisance, or leaves a high BOD,
solids, grease, or coliform group bacteria concentration in the
discharge.
QUESTIONS
9.5A What is bioflocculation?
9.5B What biological factors influence the treatment
efficiency of a pond?
9.5C What factors indicate that a pond is not fulfilling
its function (operating properly)?
END OF LESSON 1 OF 3 LESSONS
on
WASTE TREATMENT PONDS
5 Bioflocculation. A condition whereby organic materials tend
to be transferred from the dispersed form in wastewater to
settleable material by mechanical entrainment and assimilation.
9-12
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CHAPTER 9. WASTE TREATMENT PONDS
(Lesson 2 of 3 Lessons)
9.6 STARTING THE POND
One of the most critical periods of a pond's life is the time that
it is first placed in operation. If at all possible, at least one
foot of water should be in the pond before wastes are introduced.
The water should be turned into the pond in advance to prevent
odors developing from waste solids exposed to the atmosphere. Thus
a source of water should be available when starting a pond.
It is a good practice to
start ponds during the
warmer part of the year
because a shallow starting
depth allows the contents
of the pond to cool too
rapidly if nights are cold.
Generally speaking, the
warmer the pond contents,
the more efficient the
treatment processes.
Algal blooms will normally
appear from seven to twelve
days after wastes are intro-
duced into a pond, but it
generally takes at least 60
days to establish a thriving
biological community. A
definite green color is
evidence that a flourishing
algae population has been established. After this length of time
has elapsed, bacterial decomposition of bottom solids will usually
become established. This is generally evidenced by bubbles coming
to the surface near the pond inlet where most of the sludge deposits
occur. Although the bottom is anaerobic, travel of the gas through
the aerobic surface layers generally prevents odor release.
9-13
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Wastes should be discharged to the ppnd intermittently during
the first few weeks with constant monitoring of the pH. The
pH in the pond should be kept above 7.5 if possible. Initially
the pH of the bottom sludge will be below 7 due to the digestion
of the sludge by acid-producing bacteria. If the pH starts to
drop, discharge to the pond should be diverted to another pond
or diluted with make-up water if another pond is not available
until the pH recovers. A high pH is essential to encourage a
balanced anaerobic fermentation (bacterial decomposition) of
bottom sludge. It also is indicative of 'high algal activity
since removal of the carbonates from the water in algal metabolism
tends to keep the pH high. A continuing low pH indicates acid
production which will cause odors.
QUESTIONS
9.6A Why should at least one foot of fresh water cover
the pond bottom before wastes are introduced?
9.6B Why should ponds be started during the warmer part
of the year if at all possible?
9.6C What does a definite green color in a pond indicate?
9.6D When bubbles are observed coming to the pond surface
near the inlet, what is happening in the pond?
9-14
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9.7 DAILY OPERATION AND MAINTENANCE
Because ponds are deceptively simple, they are probably neglected
more than any other type of wasYewater treatment process.Many
of the complaints that arise from ponds are the result of neglect
or poor housekeeping. Following are listed the day-to-day
operational and maintenance duties that will help to insure peak
treatment efficiency and to present your plant to its neighbors
as a well-run waste treatment facility.
9.70 Scum Control
Scum accumulation is a common characteristic of ponds and is
usually the greatest in the spring of the year when the water
warms and vigorous biological activity resumes. Ordinarily,
wind action will dissipate scum accumulations and cause them
to settle; however, in the absence of wind or in sheltered areas,
other means must be used. If scum is not broken up, it will dry
on top and become crusted. It is not only more difficult to break
up then, but a species of blue-green algae is apt to become
established on the scum which can give rise to disagreeable odors.
If scum is allowed to accumulate, it can reach proportions where
it cuts off a significant amount of sunlight from the pond.
Rafts of scum cause a very unsightly appearance in ponds and
can quite likely become a source of botulism that will have a
devastating effect on waterfowl and shore birds which may be
attracted to the facility.
Many methods of breaking up scum have been used, including agitation
with garden rakes from the shore, jets of water from pumps or tank
trucks, and the use of outboard motors on boats in large ponds.
Scum is broken up most easily if it is attended to promptly.
9.71 Odor Control
It is probably inevitable that, at some time, odors will come from
a wastewater treatment plant no matter what kind of process is used.
Most odors are caused by overloading (see Section 9.117 to determine
pond loading) or poor housekeeping practices and can be remedied
by taking corrective measures. However, there are times, such as
when unexpected shutdowns occur, that plant processes may be upset
and cause odors. For these unexpected occurrences it is strongly
advised that a careful plan for emergency odor control be available.
Odors usually occur during the spring warmup in colder climates
because biological activity is reduced during cold weather.
9-15
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For ponds, recirculation from aerobic units, the use of floating
aerators, and heavy chlorination should be considered as means
to reduce odors. Recirculation from an aerobic pond to the inlet
of an anaerobic pond (1 part recycle flow to 6 parts influent
flow) will reduce or eliminate odors. Usually floating aeration
and chlorination equipment are too expensive to have setting idle
waiting for an odor problem to develop. Odor masking chemicals
also have been promoted for this purpose and have some uses for
concentrated specific odor sources. However, in almost all cases,
process procedures of the type mentioned previously are preferable.
In any event, waiting until the emergency arises before planning
for odor control is poor procedure. Often several days are needed
to receive delivery of materials or chemicals if they are required.
Try to have possible alternate methods of control ready to go if
they are needed.
In some areas, sodium nitrate has been added to ponds as a source
of oxygen to prevent odors. To be effective, sodium nitrate must
be dispersed throughout the water in the pond. Once mixed in the
pond it acts very quickly because many common organisms (faculta-
tive groups) may use the oxygen in nitrates instead of dissolved
oxygen. Liquid sodium hydrochloride or chlorine solution is a
faster acting solution, but not necessarily the best chemical be-
cause it will interfere with biological stabilization of the wastes.
9.72 Weed Control
Weed control is an essential part of good housekeeping and is not
a formidable task with modern herbicides and soil sterilants.
Weeds around the edge are most objectionable because they allow a
sheltered area for mosquito breeding and scum accumulation. In
most average ponds there has been little need for mosquito control
when edges are kept free of weed growth. Aquatic weeds, such as
tules, will grow in depths shallower than three feet, so an operating
pond level of at least this depth is necessary. Tules may emerge
singly or well scattered but should be removed promptly by hand as
they will quickly multiply from the root system. Weeds also can
hinder pond circulation.
9.73 Insect Control
Mosquitoes will breed in sheltered areas of standing water where
there is vegetation or scum to which the egg rafts of the female
mosquito can become attached. These egg rafts are fragile and
will not withstand the action of distrubed water surfaces such as
caused by wind action or normal currents. Keeping the water edge
clear of vegetation and keeping any scum broken up will normally
give adequate control. Shallow, isolated pools left by a receding
pond level should be drained or sprayed with a larvacide.
9-16
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Any of several minute shrimp-like animals may infest the pond
from time to time during the warmer months of the year (March-
November) . These predators live on algae and at times will
appear in such numbers as to almost clear the pond of algae.
During the more severe infestations there will be a sharp drop
in the dissolved oxygen of the pond, accompanied by a lowered
pH. This is a temporary condition because the predators will
outrun the algae supply, and there will be a mass die-off of
insects which will be followed by a rapid greening up of the
pond again.
Ordinarily there should be no great concern about these infesta-
tions because they soon balance themselves; however, in the case
of a heavily loaded pond, a sustained low dissolved oxygen con-
tent may give rise to noxious odors. In that event any of several
commercial sprays can be used with excellent control. Dibrom-8
has been used with good results.
Chironomid midges are often produced in wastewater ponds in sufficient
numbers to be serious nuisances to nearby residential areas, farm
workers, recreation sites, and industrial plants. When emerging in
large numbers they may also create traffic hazards. At present
the only satisfactory control is through the use of insecticides
such as parathion, Abate, Sursban, and Fenthion. Control measures
are time consuming and may be difficult, particularly if there is
a discharge to a receiving stream. If possible, lower the level in
the ponds enough to contain a day's inflow before applying an
insecticide. Holding the insecticide for at least one day will
kill more insects and reduce the effect of the insecticide on re-
ceiving waters. For better results, insecticides should be applied
on a calm day and any recirculation pumps should be stopped.
9.74 Levee Maintenance
Levee slope erosion caused by wave action is probably the most
serious maintenance problem. If allowed to continue, it will
result in a narrowing of the levee crown which will make accessi-
bility with maintenance equipment most difficult.
If the levee slope is composed of easily erodable material, the
only long-range solution is the use of bank protection such as
stone riprap or broken concrete rubble.
Levee tops should be crowned so that rain water will drain over the
side in a sheet flow rather than flowing a considerable distance
along the levee crown and gathering enough flow to cause erosion
when it finally spills over the side and down the slope.
If the levees are to be used as roadways during wet weather, they
should be paved or well graveled.
9-17
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9.75 Headworks and Screening
It is important to clean the bar screen as frequently as possible.
The screen should be visited at least once or twice a day with more
frequent visits during storm periods. Screenings should be disposed
of daily in a sanitary manner, such as by burial, to avoid odors
and fly breeding.
Many pond installations have grit chambers at the headworks to
protect raw wastewater lift pumps or prevent plugging of the
influent lines. There are many types of grit removal equipment.
Grit removed by the various types of mechanical equipment or by
manual means will usually contain small amounts of organic matter
and should therefore be disposed of in a sanitary manner. Disposal
by burial is the most common method.
9.76 Some Operating Hints
Anaerobic ponds should be covered and isolated for odor control
and followed by aerobic ponds. Floating polystyrene planks can
be used to cover anaerobic ponds and can be painted for protection
from the sun. These will help to confine odors and heat and tend
to make the anaerobic ponds more efficient.
Placing ponds in series tends to cause the first pond to become
overloaded and may never allow it to recover; the overload may
be carried to the next pond in series. Feeding ponds in parallel
allows you to distribute the incoming load evenly between units.
Whether ponds are operated in series or in parallel6 should depend
on the loading situation.
When operating ponds in series, the accumulation of solids in the
first pond may become a serious problem after a long period of use.
Periodically the flow should be routed around the first pond. This
pond should then be drained and the solids removed and buried.
PONDS IN SERIES
PARALLEL
9-18
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3. It can be helpful to provide for a large amount of recircu-
lation, say 25 to 100%. This- allows the algae and other
aerobic organisms to become thoroughly mixed with incoming
raw wastewater. At the same time, good oxygen transfer can
be attained by passing the incoming water over a deck or
other type of aerator. This procedure can cause heat loss,
however.
4. Heavy chlorination at the recirculation point can assist in
odor control, but will probably interefere with treatment.
5. As with any treatment process, it is necessary to measure the
important parameters (DO fluctuations during a 24-hour period
and solids) at frequent, regular intervals and plot them so
that you have some idea of the direction the process is taking
in time to take corrective action when necessary.
6. When solids start floating to the surface of a pond during the
spring or fall overturn, the pond should be taken out of service
and cleaned. Measurement of the sludge depth on the bottom of
a pond also will indicate when a pond should be cleaned.
7. Before applying insecticides or herbicides, be sure to check
with appropriate authorities regarding the long term effects
of the pesticide you plan to use. Do not apply pesticides
that may be toxic to organisms in the receiving waters.
QUESTIONS
9.7A Why should scum not be allowed to accumulate on the
surface of a pond?
9.7B How can scum accumulations be broken up?
9.7C What are the causes of odors from a pond?
9.7D What precaution would you take to be prepared for an
odor problem which might develop?
9.7E Why are weeds objectionable in and around ponds?
9.7F How can weeds be controlled and removed in and around
ponds?
9.7G Why should insects be controlled?
9.7H Why should a pond be lowered before an insecticide is
applied?
9.71 Why are the contents of ponds recirculated?
9-19
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9.8 SURFACE AERATORS
Surface aerators have been used in two types of applications:
1. To provide additional air for ponds during the night
or during cold weather, or for overloaded ponds.
2. To provide a mechanical aeration device for ponds operated
as an aerated lagoon. Aerated lagoons operate similar to
an activated sludge aeration tank without returning any
settled activated sludge.
In both cases the aerators are operated by time clocks with estab-
lished on-off cycles. Laboratory tests on the dissolved oxygen in
a pond indicate the time period for on and off cycles to maintain
aerobic conditions in the surface layers of the pond. Adjustments
in the on-off cycles are necessary when changes occur in the
quantity and quality of the influent and seasonal weather conditions.
Some experienced operators have correlated their lab test results to
pond appearance and regulate the on-off cycles using the following
rule: If the pond has foam on the surface, reduce the operating
time of the aerator; and if there is no evidence of foam on the pond
surface, increase the operating time of the aerator.
Maintenance of surface aerators should be conducted in accordance
with manufacturer's recommendations.
9.9 SAMPLING AND ANALYSIS
9.90 General
Probably the most important sampling that can be accomplished easily
by any operator is routine pH and dissolved oxygen analysis. It is
very desirable to make pH, temperature, and dissolved oxygen tests
several times a week, and occasionally during the night, throughout
operation of the pond. These values should be recorded because
they will serve as a valuable record of performance. The time of day
should be varied occasionally for the tests so that the operator be-
comes familiar with the pond's characteristics at various times of
the day. Usually the pH and dissolved oxygen will be lowest just at
sunrise. Both will get progressively higher as the day goes on,
reaching their highest point in late afternoon.
9-20
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It is especially important to remember to avoid getting any
atmospheric oxygen into the sample taken to measure dissolved
oxygen. This is most necessary when' samples are taken in the
early morning or if the dissolved oxygen in the pond is low from
overloading. If possible, measure the dissolved oxygen with an
electric probe, being careful not to allow the membrane on the
end of the probe to be exposed to the atmosphere.
Ponds often have clearly developed individuality, each being a
biological community that is unique unto itself. Identical adjacent
ponds receiving the same influent in the same amount often have a
different pH and a different dissolved oxygen content at any given
time. One pond may generate considerable scum while its neighbor
is devoid of scum. For this reason, each pond should be given
routine testing as regards to pH and dissolved oxygen. Such
testing may indicate an unequal loading because of the internal
clogging of influent or distribution lines that might not be
apparent from visual inspection. Tests also may indicate differences
or problems that are being created by a build-up of solids or solids
recycle.
As an operator becomes familiar with operating a pond, he can soon
learn to correlate the results of some of the chemical tests with
visual observations. A deep green sparkling color generally indi-
cates a high pH and a satisfactory dissolved oxygen content. A dull
green color or lack of color generally indicates a declining pH and
a lowered dissolved oxygen content, A grey color indicates the pond
is being overloaded.
9.91 Frequency and Location of Lab Samples
The frequency of testing and expected ranges of test results vary
considerably from pond to pond, but you should establish those
ranges within which your pond functions properly. Test results will
also vary during the hours of the day. Table 9-1 summarizes the
typical tests, locations, and frequency of sampling.
Tests of pH, DO, and temperature are important indicators of the
condition of the pond, whereas BOD, coliform, and solids tests
measure the efficiency of the pond in treating wastes. BOD is
also used to calculate the loading on the pond.
In order to estimate the organic loading on the pond, the operator
must have some knowledge of the biochemical oxygen demand (BOD) of
the waste and the approximate average daily flow. Influent BOD and
solids will vary with time of day, day of week, and season, but a
pond is a good equalizer if not overloaded. Recirculation will help
an overloaded pond.
9-21
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TABLE 9-1
FREQUENCY AND LOCATION OF LAB SAMPLES
Test
Frequency
Location
Common Range
pH*
Dissolved
Oxygen (DO)*
Temperature
BOD
Coliform Group
Bacteria
Chlorine Residual
Suspended Solids
)
Dissolved Solids
Daily
Daily
Daily
Weekly
Weekly
Daily
Weekly
Weekly
Pond
Pond
Effluent
Pond
Influent
Effluent
Effluent
Effluent
Influent
Effluent
Influent
Effluent
7.5+
0.5-2.0 mg/1
*pH values above 9.0 and DO levels over 15 mg/1 are not uncommon.
BODs should be measured on a weekly basis. Samples should be taken
during the day at low flow, medium flow, and high flow. The average
of these three tests will give a reasonable indication of the organic
load of the wastewater being treated. If it is suspected that the
BOD varies sharply during the day or from day to day, or if unusual
circumstances exist, the sampling frequency should be increased to
obtain a clear definition of the variations. If the pond DO level
is supersaturated the sample must be aerated to
remove the excess oxygen before the BOD test is performed.
9-22
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9.92 Expected Treatment Efficiencies7
Table 9-2 is provided as a guide to indicate probable removal
efficiencies of typical ponds.
TABLE 9-2
EXPECTED RANGES OF REMOVAL BY PONDS
Item
Detention Time
Expected Removal
BOD
BOD (facultative pond)
Coliform Bacteria
(facultative pond)
Suspended Solids
Dissolved Solids
50 to 60 days
i
50 to 60 days !
After 3 days
After 10 days
50 to 90%
70 to 80%9
90 to 95%
90%
80%
7 Waste Removal, % = (In - 0"*? x 100%
In
8 Facultative Pond (FACK-ul-tay-tive). The most common type
of pond in current use. The upper portion (supernatant) is
aerobic while the bottom layer is anaerobic. Algae supply
most of the oxygen to the supernatant.
9 Expected removal approximately 80% of the time with poorer
removals during the remainder of the time.
9-23
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CHAPTER 9. WASTE TREATMENT PONDS
(Lesson 3 of 3 Lessons)
9.11 DESIGN CRITERIA
A review of some common design criteria will give an insight to
the theory and operation of a pond.
9.110 Location
The general considerations for the location of other types of
wastewater treatment plants also apply to the location of ponds.
Isolation should be as great as can be economically provided.
Attention to the direction of prevailing winds with due regard
for present and projected downwind residential, commercial, and
recreational development is of utmost importance.
9.111 Chemistry of Waste
Before the design of any pond is undertaken, it should be determined
whether there are any possible toxic effects (interfere with algal or
bacterial growth) from the waste. Some natural water supplies may
have a high sulfur content or other chemicals that limit the possibility
of desired sludge decomposition.
Certain wastes, such as dairy products and wine products, are
difficult to treat because of their low pH. Any processing waste
should be carefully investigated before one can be certain that it
can be successfully treated by ponding. Some process wastes contain
powerful fungicides and disinfectants that may have a great inhibitive
effect on the biological activity in a pond.
9.112 Headworks and Screening
A headworks with a bar screen is desirable to remove rags, bones, and
other large objects that might lodge in pipes or control structures.
A trash shredder is a luxury that may not be warranted. Any
material that gets past an adequate bar screen will in all
probability not harm the influent pump. Any fecal matter will
be pulverized in going through the pump.
9-24
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9.113 Flow Measuring Devices
It is highly desirable that an influent measuring device be installed
to give a direct reading on the daily volume of wastes that are
introduced into the ponds. This information, along with a BOD
measurement of the influent, is required to estimate the organic
loading on the pond. Comparison of influent and effluent flow
rates is necessary for estimating percolation and evaporation
losses.
A measuring device provides basic data for prediction of future
plant expansion needs or for detecting unauthorized or abnormal
flows. Reliable, well-kept records on flow volume help justify
budgets and greatly assist an engineer's design of a plant expansion
or new installation.
9.114 Inlet and Outlet Structures
Inlet structures should be simple and foolproof and should be
standard manufactured articles so that replacement parts are
readily available. Telescoping friction fit tubes (see Fig. 9.4)
for regulating spill or discharge height should be avoided because
a biological growth may become attached and prevent the tubes from
telescoping if they are not cleaned regularly.
A submerged inlet will minimize the occurrence of floating material
and will help conserve the heat of the pond by introducing the
warmer wastewater into the depths of the pond. Warm wastewater
introduced at the bottom of a cold water mass will channel to the
surface and spread unless it is promptly and vigorously mixed with
cold water. Warm wastewater spilled onto the surface of the pond
will spread out in a thin layer on the surface and not contribute
to the warmth of the lower regions of the pond where heat is needed
for bacterial decomposition. Inlet and outlet structures should be
so located in relation to each other to minimize possible short
circuiting.
Valves that have stems extending into the stream flow should be
avoided. Stringy material and rags will collect and form an
obstruction and may render the valve inoperative.
Free overfalls (Fig, 9.5) at the outlet should be avoided to minimize
release of odors, foaming, and gas entrapment which may hamper pipe
flows. Free overfalls should be converted to submerged outfalls if
they are causing nuisances and other problems.
9-25
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THREADED STEM
WHEEL HANDLE
VALVE BOX
V-NGTCH
FRICTION FIT
BETWEEN PIPES
Fig. 9.4 Telescoping friction fit tubes
for regulating discharge
9-26
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POND
FOAM
STREAM
FREE 0VERFALL--UNDESIRABLE
SUBMERGED OUTLET--NO FOAMING PROBLEMS
Fig. 9.5 Free overflow and submerged outlet
9-27
-------
If a pond has a surface outlet, floating material can be kept
out of the effluent by building a simple baffle around the out-
let. The baffle can be constructed of wood or other suitable
material. It should be securely supported or anchored.
9.115 Levee Slopes
The selection of the steepness of the levee slope must depend on
several variables. A steep slope erodes quicker from wave wash
unless the levee material is of a rocky nature or else protected
by riprap. However, a steep slope minimizes waterline weed growth.
It is more difficult to operate equipment and to perform routine
maintenance on steep slopes. A gentle slope will erode the least
from wave wash, is easier to operate equipment on, and is easier
to perform routine maintenance on. However, waterline weed growth
will have a much greater opportunity to flourish.
9.116 Pond Depths
The operational depth of ponds deserves considerable attention.
Depending upon conditions, ponds of less than three feet of depth
may be completely aerobic if there are no solids on the bottom
[unlikely) because of the depth of sunlight penetration. This
means that the treatment of wastes is accomplished essentially by
converting the wastes to algae cell material. Ponds of this
shallow depth are apt to be irregular in performance because
algae blooms will increase to such proportions that a mass die-
off will occur with the result of all algae precipitating to the
bottom and thereby adding to the organic load. Such conditions
could lead to the creation of an anaerobic pond. The bottoms of
shallow ponds will become anaerobic when solids collect on the
bottom and after sunset.
Discharges from shallow, aerobic ponds contain large amounts of
algae. To operate efficiently these ponds should have some means
of removing the algae grown in the pond before the effluent is
discharged to the receiving waters. If the algae are not removed
from the effluent, the organic matter in the wastewater is not re-
moved or treated and the problem is merely transferred to some
downstream pool.
An observed phenomenon of lightly loaded, shallow secondary ponds
and tertiary ponds is that they are apt to become infested with
filamentous algae and mosses that not only limit the penetration
of sunlight into the pond but hamper circulation of the pond's
contents and clog up inlet and outlet structures. When the loading
is increased, this condition improves.
9-28
-------
Pond depths of four feet or more allow a greater conservation
of heat from the incoming wastes to foster biological activity
as the ratio between pond volume and pond area is more favorable.
In facultative ponds, depths over four feet provide a physical
storage for dissolved oxygen accumulated during the day to carry
over through the night when no oxygen is released by the algae,
unless floating algae and poor circulation keep all the oxygen
near the surface. This physical storage of DO is very important
during the colder months when nights are long.
A pond operating depth of at least three feet is recommended to
prevent tule and cattail growth. Ponds less than three feet deep
should be lined to prevent troublesome weed growth. Weeds that
emerge along the shore line can be effectively controlled by
spraying with any of several products available.
QUESTIONS
9.11A Why are some wastes not easily treated by ponds?
9.11B What is the minimum recommended pond operating depth?
9.11C Why should the inlet to a pond be submerged?
9.11D Why should the outlet be submerged?
9. HE How could problems created by a surface outlet be
reduced or corrected?
9.11F Why should free overfalls be avoided?
9.11G Why are shallow ponds apt to be irregular in performance?
9.11H Why should the influent to a pond be metered?
9-29
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9.117 Pond Loading
The waste loading on a pond is generally spoken of in relation
to its area, and may be stated in several different ways:
1. Ibs of BOD per day per acre = Ibs BOD/day/acre
CThis is called organic loading.)
2. inches (or feet) of depth added per day
CThis is called hydraulic loading or overflow rate.)
or 3. persons (or population served) per acre
(This is called population loading.)
Detention time is related directly to pond hydraulic loading,
which is actually the rate of inflow of wastewater. It may be
expressed as million gallons per day (MGD), or as the number of
acre-inches per day or acre-feet per day (one acre-foot covers
one acre to a depth of one foot or twelve inches and is equal to
43,560 cu ft). We must know the pond volume in order to determine
detention time; this is most easily computed on an acre-foot basis.
A. Detention Time
Pond Volume (ac-ft)
Detention (in days)
Influent Rate (ac-ft/day)
This equation does not take into consideration water which may be
lost through evaporation or percolation. Detention time may vary
from 30 to 120 days, depending on the treatment requirements to
be met.
B. Population Loading
Loading calculated on a population-served basis is expressed simply
as:
,. _ , Population Served, persons
No. of Persons per Acre = —*•— r-=—-p-*-
Area of Pond, ac
The population loading may vary from 50 to 500 persons per acre,
depending on many local factors.
9-30
-------
C. Hydraulic Loading
The hydraulic loading or overflow rate is expressed as:
T , , Inflow (ac-in per day)
Inches per day = —•;• • • • * "•
^ Pond Area, ac
The hydraulic loading may vary from half an inch to several inches
per day, depending on the organic load of the influent.
NOTE: If the wastewater inflow rate is known in million
gallons per day (MGD), it can be converted to an
equivalent number of acre-inches per day as follows:
Inflow, acre-inches per day = (Inflow, MGD) x 36.8 10
If the pond detention time is known, the hydraulic loading can
also be calculated, as follows:
_ . , Depth of Pond, in
Inches per day = -—*—— ——*—•:
v Detention Time, days
D. Organic Loading
The organic loading is expressed as:
- CHOP, mg/1) (Flow, MGD) (8. 54 Ibs/gal) H
day per acre) " Pond area • ac
Typical organic loadings may range from 10 to 50 Ibs BOD per day
per acre.
10 i MGD = 1*000*000 Sal x 1 cu ft x L>£ x A2J2. = 36.8 *
day 7.48 gal 43,560 sq ft 1 ft day
11 Recall Ibs/day = (Cone. mg/M mg)(M gal/day)(8.34 Ibs/gal)
9-31
-------
9.12 ACKNOWLEDGMENT
Liberal use has been made of the many papers presented by Professor
W. J. Oswald of the University of California at Berkeley on the sub-
ject of the treatment of wastes by ponding.
9.13 ADDITIONAL READING
a. New York Manual, page 71
b. Texas Manual, pages 283-302
c. Raw Sewage Lagoons in California, by California State Depart-
ment of Public Health, Bureau of Sanitary Engineering, Berkeley,
California, May 1969.
d. Waste Stabilization Lagoons - Design, Construction, and Operation
Practices Among Missouri Basin States, Missouri Basin Engineering
Health Council, 1960. Reproduced by U.S. Public Health Service,
Region VI, Kansas City, Missouri.
END OF LESSON 3 OF 3 LESSONS
on
WASTE TREATMENT PONDS
9-32
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CHAPTER 10. DISINFECTION AND CHLORINATION
(Lesson 1 of 4 Lessons)
10.0 PRINCIPLES OF WASTEWATER DISINFECTION WITH CHLORINE
10.00 Introduction
Wastewater contains organisms from both the healthy and sick
people discharging their wastes into the collection system.
Disease-producing organisms are potentially present in all
wastewaters, and these organisms must be removed or killed before
treated wastewater can be discharged to the receiving waters.
The purpose of disinfection is to destroy pathogenic organisms1
and thus prevent the spread of water-borne diseases.
The conventional waste treatment processes described in previous
chapters remove pathogens from wastewater in varying degrees.
The destruction and removal of pathogens is brought about in
several ways:
1. Physical removal through sedimentation and filtration
2. Natural die-away of organisms in an unfavorable environ-
ment during storage
3. Destruction by chemicals introduced for treatment purposes
1 Pathogenic (path-o-JEN-nick) Organisms. Bacteria or viruses
which can cause disease (typhoid, cholera, dysentery). There
are many types of bacteria which do not cause disease and which
are not called pathogenic. Many beneficial bacteria are found
in wastewater treatment processes actively cleaning up organic
wastes.
10-1
-------
Although the number of microorganisms in polluted waters is
reduced by treatment processes and natural purification, the
term disinfection is used in practice to describe treatment
processes that have as their major'objective the killing of
pathogenic organisms (Fig. 10.1). Because chlorine and some
of its compounds disinfect so well, and because they are
available at reasonable cost, they have been used almost to
the exclusion of other disinfecting agents. This chapter on
disinfection will be concerned primarily with the principles
and practice of chlorine disinfection.
10.01 Disinfection
The main use of chlorine in domestic waste treatment is dis-
infection. Strictly defined, disinfection is the destruction
of all pathogenic organisms, while sterilization is the total
destruction or removal of all microorganisms. When wastewater
effluents are discharged to receiving waters which may be used
as a source of public water supply, shellfish growing areas,
or for recreational purposes, treatment for the destruction of
pathogenic organisms is required to minimize the health hazards
of pollution of these receiving waters. Such treatment is known
as disinfection.
Chlorination for disinfection purposes requires killing essentially
all of the pathogens in the domestic waste effluent. Many other
sensitive organisms in contact with chlorine are destroyed too.
No attempt is made to sterilize wastewater, which is both un-
necessary and impractical. In some instances sterilization would
be detrimental where other treatment, dependent upon the activity
of the saprophytes,2 follows chlorination. Chlorine is a non-
selective killer. It affects organisms on the basis of sensitivity,
growth rate, concentration and exposure time.
2 Saprophytes (SAP-pro-fights). Organisms living on dead or
decaying organic matter; they help natural decomposition of
the organic solids in wastewater.
10-2
-------
PDNCflOM
A&EATIOM
PX2JMAX2V
WA6TEWATEK.
AVP HELP*'PBMOVg OS2-
&IOL06KTAL
Fig. 10.1 Typical flow diagram of wastewater treatment plant
10-3
-------
To accomplish disinfection, sufficient chlorine must be added
to satisfy the chlorine demand3 and leave a residual^ chlorine^
that will destroy bacteria.The residual must be maintained
for a sufficient "contact time" to insure killing the pathogens.
For most wastewater, extending chlorine contact time can be
more effective than increasing dosages.
Special laboratory equipment is necessary to measure the effec-
tiveness of chlorination for reducing the number of bacteria.
The tests require several days to complete. Thus bacterial
examinations are not generally practical for the day-to-day
control of the application of chlorine. For many years dis-
infection requirements often specified an orthotolidine5
chlorine residual of 0.5 milligrams per liter after a chlorine
contact time of thirty minutes. Compliance with this require-
ment generally resulted in MPNs6 of about 3000 coliform7
organisms per 100 ml (California, 1966), However, this result-
ing MPN may vary considerably (several orders of magnitude) from
plant to plant. Considering dilution with water having a low coliform
content, this standard appeared suitable when public contact with
the waters was limited. Today people are living more intimately
with wastewater than ever before. Wastewater effluents are
3 Chlorine Demand. Chlorine demand is the difference between the
amount of chlorine added to wastewater and the amount of residual
chlorine remaining after a given contact time. Chlorine demand
may change with dosage, time, temperature, nature and amount of
impurities in water. Chlorine Demand = Chlorine Applied -
Chlorine Residual.
4 Residual Chlorine. Residual Chlorine is the amount of chlorine
remaining after a given contact time and under specified conditions.
5 Orthotolidine (or-tho-TOL-i-dine). Orthotolidine is a colorimetric
indicator of chlorine residual in which a yellow-colored compound
is produced,
6 MPN. MPN is the Most Probable Number of coliform group organisms
per unit volume expressed as density of organisms per 100 ml.
7 Coliform (COAL-i-form). The coliform group of organisms is a
bacterial indicator of contamination. This group has as one
of its primary habitats the intestinal tract of human beings.
Coliforms also may be found in the intestinal tract of warm-
blooded animals, and in plants, soil, air, and the aquatic en-
vironment .
10-4
-------
quality, it has
to disinfection
used for irrigating
lawns, parks, ceme-
teries, freeway
planting, golf
courses, college
campuses, athletic
fields, and other
public areas.
Recreational lakes
used for boating,
swimming, water
skiing, fishing, and
other water sports
are frequently made
up partially and,
in a few cases,
solely of treated
effluents. As public
contact has increased
and diluting waters
have decreased or
become of poor
become obvious that more consideration must be given
practices.
QUESTIONS
10.OA What is the purpose of disinfection? Why is this important?
10.OB How are pathogenic bacteria destroyed or removed from water?
10.OC Why is chlorination used for disinfection?
10.OD Why are wastes not sterilized?
10-5
-------
10.02 Reaction of Chlorine in Wastewater
In order to determine where in the treatment process and how much
chlorine should be applied to accomplish the purpose desired, it
is necessary to know the action of chlorine when added to waste-
water.
Chlorine is an extremely active chemical that will react with many
compounds to produce may different products. If a small amount of
chlorine is added to wastewater, it will react rapidly with such
substances as hydrogen sulfide, thiosulfates (industiral wastes),
and ferrous iron. Under these conditions, chlorine is converted to
chloride and little or no disinfection will result. If enough
chlorine is added to react with all of these substances, called re-
ducing compounds, a little more chlorine added will react with
ammonia or other nitrogenous compounds present and form chloramines,
which have disinfecting action. Again, if enough chlorine is added
to react with all the reducing compounds and all the nitrogenous
matter, this chlorine will react with organic matter to produce
chlororganic compounds8 or other combined forms of chlorine, which
have slight disinfecting action. Finally, if enough chlorine is
added to react with all of the above compounds, any additional
chlorine will form free available chlorine (HOC1) which has the
highest disinfecting action. (Fig. 10.2, Page 10-9)
The exact mechanism of this disinfection action is not fully known.
In some theories, chlorine is considered to exert a direct action
against the bacterial cell, thus destroying it. A more recent
theory is that the toxic character of chlorine inactivates the
enzymes9 upon which the living microorganisms are dependent for
utilizing their food supply. As a result, the organisms die of
starvation. From the point of view of wastewater treatment, the
mechanism of the action of chlorine is much less important than
its effects as a disinfecting agent.
8 Chlororganic (chlor-or-GAN-nick). Chlororganic compounds are
organic compounds combined with chlorine. These compounds
generally originate from or are associated with living or dead
organic materials
9 Enzymes (EN-zimes). Enzymes are substances produced by living
organisms that speed up chemical changes.
10-6
-------
The quantity of reducing substances, both organic and inorganic,
in wastewater varies, so the amount of chlorine that has to be
added to wastewater for different purposes will vary. The
chlorine used by these organic and inorganic reducing substances
is defined as the chlorine demand. It is equal to the amount
added minus that remaining as combined chlorine after a period
of time, which is generally thirty minutes. Thus,
Chlorine Demand = Chlorine Dose - Chlorine Residual
Although significant kill of sensitive organisms occurs while
the chlorine demand is being satisfied, disinfection is caused
primarily by that amount remaining after the chlorine demand
has been satisfied. This quantity of chlorine in excess of the
chlorine demand is defined as residual chlorine and is expressed
as milligrams per liter (mg/1).
It should be noted that in wastewater treatment chlorination
is not normally to the "break point" (Fig. 10.2) so that a
free residual would exist. The "break point" for good secondary
effluent would be a chlorine dosage of approximately 150 mg/1.
Thus we are talking primarily about a combined residual. However,
with some of the more advanced treatment processes in which a
high degree of nitrification occurs, treatment to free chlorine
residuals beyond the break point is possible at a chlorine dose
of less than 25 mg/1.
Both chlorine addition and contact time are essential for organism
kill.Experimental determination of the best combination of
combined residual and contact time is necessary to insure both
proper chlorination and minimum use of chlorine. Changes in pH
affect the disinfection ability of chlorine and the operator must
reexamine the best combination of chlorine addition and contact
time when the pH fluctuates.
It must be emphasized that wastewaters are not and need not be
carried to a free residual for effective bactericidal action
at the present time in most locations. With increasingly stringent
receiving water standards requiring higher quality effluents in
the future, the need for disinfection to the free chlorine residual
is a distinct possibility. Complete disinfection ("kill" of patho-
genic bacteria and viruses) is assured mainly by chlorination to a
free available chlorine residual.
Calculation of the chlorine dosage and chlorine demand is illustrated
in the following problem.
10-7
-------
EXAMPLE:
A chlorinator is set to feed 50 pounds of chlorine per 24
hours; the wastewater flow is at a rate of 0.85 MGD, and the
chlorine as measured by the OT (orthotolidine) tests after
thirty minutes of contact is 0.5 mg/1. Find the chlorine
dosage and chlorine demand in mg/1.
Chlorine Feed
or Dose, mg/1
Chlorine
Demand, mg/1
59.
0.85 / 50.00
42 5
7 50
7 65
50 Ibs chlorine/day
0.85 MG/day
59 Ibs chlorine per MG
59 Ibs chlorine/MG
8.34 Ibs/gal
7.1 Ibs chlorine/million pounds water
7.1 ppm (p_arts £er million parts)
7.1 mg/1
chlorine Dose mg/1 - Chlorine Residual, mg/1
a
7.1 mg/1 - 0.5 mg/1
6.6 mg/1
QUESTIONS
10. DE How does chlorine react with wastewater?
10. OF How much chlorine must be added to waste-
water to produce disinfecting action?
10. OG How is the chlorine demand determined?
10. HH How is the chlorine dosage determined?
10.01 Calculate the chlorine demand of treated
domestic wastewater if:
Flow Rate = 1.2 MGD
Chlorinator = 70 Ibs of chlorine per 24 hours
Residual = 0.4 mg/1 after thirty minutes
10-8
-------
;'
o
i
CHLORINE
RESIDUAL,
mg/l
INITIAL
CHLORINE
DEMAND
COMBINED
RESIDUAL
CHLORINE
OXIDATION OF
COMBINED RESI-
DUAL MATERIALS
(CHLORAMINES)
BREAK POINT
FOR SECONDARY
IKASTE1NATER,
APPROXIMATELY
25 TO 150 MG/L
FREE CHLORINE
RESIDUAL ON A
1 TO 1 BASIS
CHLORINE DOSAGE, mg/l
Fig. 10.2 Break-point chlorination curve
-------
10.03 Rules of Disinfection
The State of California presently (1969) specifies the coliform
MPN in the effluent as a primary standard for effectiveness of
disinfection. It has been established that the bacteria causing
enteric10 diseases are less resistant to the chlorine than the
non-pathogenic intestinal bacteria, designated as the coliform
group. For this reason the destruction of the coliform group
of bacteria generally provides an effective criterion of waste-
water disinfection. However, certain viruses, spores, and
pathogenic bacteria inside solids may be more resistant than
coliform group bacteria to chlorine. When a chlorine residual
criterion is also set, it is considered to be a secondary
standard and is valid only if, and as long as, bacterial kill
meets the MPN standard. One sample is not as meaningful as a
series of samples indicating trends.
Studies have shown great variation in MPNs in chlorinated waste-
water samples even under apparently similar conditions. These
variations occur for numerous reasons, some of which are as
follows:
1. MPN does not directly measure the true number of coli-
form bacteria present, but rather is an expected or
probable number based on analysis of samples from a
large population (all of the wastewater flowing by
the sampling point).
2. Small samples from large amounts of a source are not
representative unless the source is uniform, and cer-
tainly wastewater is far from uniform.
3. Many variables affect the number of coliform bacteria
present in a chlorinated waste: numbers and charac-
teristics of bacteria prior to chlorination, concen-
tration and nature of the specific agent accomplishing
the disinfection, accessibility of the disinfectant
to the microorganisms, and various environmental factors,
4. The test is not always performed under ideal conditions.
For example, culture media dilutions or other factors
may be unfavorable for valid coliform counts.
10 Enteric: Intestinal
10-10
-------
Several different methods including MPNs, membrane filters, and
fecal coliforms may be specified for defining adequate disinfection.
The format used is geared to fit the specific discharge and the
downstream uses of the receiving waters. Check with your state
regulatory agency for the requirements applicable to your plant.
Because of limited laboratory facilities available at most waste-
water treatment plants, the following statement has been included
in disinfection requirements issued in California:
"Methods other than bacterial testing for the demonstra-
tion of effectiveness of disinfection will be accepted
after the discharger has provided sufficient labora-
tory data showing that statistically sound correlation11
exists at all times between bacterial results and
measurements produced by the alternate proposed method."
Many of the smaller dischargers in California have asked the
State for assistance in correlating chlorine residual and coli-
form MPN. The State has conducted studies at several plants.
The studies have not been of a research nature, but were con-
ducted for the purpose of determining whether disinfection as
being practiced at the specific plants was adequate to protect
the public health. Following are some of the findings from
these studies:
1. It is difficult to maintain a consistently high
degree of disinfection at most wastewater treat-
ment plants. Chlorination is apparently more
effective in a well-clarified effluent than one
in which significant suspended solids are present.
A lump of solids may consume available chlorine
before the chlorine penetrates the particle.
Organisms imbedded within the particle are thus
protected from the chlorine and are not disin-
fected.
2. Thorough mixing of chlorine solution with the waste-
water is essential to achieve maximum efficiency of
coliform kill for a given chlorine dosage.
3. Higher chlorine residuals (after a given contact time)
are required for primary treated wastes than for
secondary treated wastes to effect a comparable coli-
form quality.
4. Two-stage chlorination (pre- and post-chlorination)
provides more consistent production of low coliform
density than postchlorination alone.
11 Correlation: Relationship
10-11
-------
5. Generally speaking, a correlation exists between chlorine
residual and coliform density. (Coliform densities de-
crease with increased chlorine residuals.) The individu-
alities of wastewater treatment plants and their effluent
conditions, as well as sampling and analysis techniques,
make it difficult to apply a correlation determined from
one plant to other plants.
6. Chlorine residuals, and corresponding feed rates, required
to afford a desired disinfection level vary from day to
day and from morning to afternoon at most treatment plants.
7. Increases in chlorine residuals above a certain point do
not appear to reduce coliform densities significantly.
8. Increases in detention time above a certain point do not
appear to reduce coliform densities significantly.
9. The actual contact time in most chlorine contact chambers
is considerably less than the theoretical contact time.
10. Samples of wastewater chlorinated in a laboratory do not
give results comparable to those obtained in chlorine
contact chambers.
11. The better the treatment the more effective the disinfection
at a given chlorine dosage.
10.04 Chlorine Requirement
The object of disinfection is the destruction of pathogenic bacteria,
and the ultimate measure of the effectiveness is the bacteriological
result. The measurement of residual chlorine does supply a tool for
practical control. If the residual chlorine value commonly effec-
tive in most wastewater treat-
ment plants does not yield
satisfactory bacteriological
kills in a particular plant,
the residual chlorine that
does must be determined and
used as a control in that
plant. In other words, the
0.5 mg/1 residual chlorine,
while generally effective, is
not a rigid standard but a
guide that may be changed to
meet local requirements.
One special case would be the use of chlorine in the effluent from
a plant serving a tuberculosis hospital. Studies have shown that a
residual of at least 2.0 mg/1 should be maintained in the effluent
from this type of institution, and that detention time should be
10-12
-------
at least two hours at the average rate of flow instead of the
thirty minutes which is normally used for basis of design.
Two-stage chlorination may be particularly effective in this
case.
It will generally be found that in a domestic waste the
following dosages of chlorine are a reasonable guideline to
produce chlorine residual adequate for disinfection. Indivi-
dual plants may require higher or lower dosages, depending upon
type and amount of suspended and dissolved organic compounds
in the chlorinated sample.
TYPE OF TREATMENT DOSAGE
(Based on Average Flow)
Primary plant effluent 20 - 25 mg/1
Trickling filter plant effluent 15 mg/1
Activated sludge plant effluent 8 mg/1
Sand filter effluent 6 mg/1
QUESTIONS
10.OJ Which is more resistant to chlorination, bacteria
causing enteric diseases or non-pathogenic intestinal
bacteria, designated as the coliform group?
10.OK Why does one find great variation in MPNs in chlori-
nated wastewater samples even under apparently similar
conditions?
10.OL What are some of the findings of studies attempting to
correlate chlorine residual and coliform MPN?
10.OM How is the effectiveness of the chlorine residual for
a particular plant determined?
END OF LESSON 1 OF 4 LESSONS
on
DISINFECTION AND CHLORINATTON
10-13
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CHAPTER 10. DISINFECTION AND CHLORINATION
(Lesson 2 of 4 Lessons)
10.1 POINTS OF CHLORINE APPLICATION
10.10 Collection System Chlorination
One of the primary benefits of up-sewer chlorination is to prevent
the deterioration of structures. Other benefits include odor and
septicity control, and possibly BOD reduction to decrease the load
imposed on the wastewater treatment processes. In some instances,
the maximum benefit may result from a single application of chlorine
at a point on the main intercepting sewer before the junction of
all feeder sewer lines. In others, several applications at more
than one point on the main intercepting sewer or at the upper ends
of the feeder lines may prove most effective. Chlorination should
be considered as a temporary or emergency measure in most cases,
with emphasis being placed on proper design. Aeration also is
effective in controlling septic conditions in collection systems.
Although many problems result from improper design or design for
future capacity requirements, the need for hydrogen sulfide pro-
tection exists under the best of conditions.
10.11 Prech1orin at i on
Prechlorination is defined as the addition of chlorine to wastewater
at the entrance to the treatment plant, ahead of settling units and
prior to the addition of other chemicals.
In addition to its application for aiding disinfection and odor
control at this point, prechlorination is applied to reduce plant
BOD load, as an aid to settling, to control foaming in Imhoff
units, and to help remove oil. Current trends are away from pre-
chlorination to up-sewer aeration for control of odors.
10.12 Plant Chlorination
Chlorine is added to wastewater during treatment by other processes,
and the specific point of application is related to the results
desired. The purpose of plant chlorination may be for control and
prevention of odors, corrosion, sludge bulking, digester foaming,
filter ponding, filter flies, and as an aid in sludge thickening.
Here again, chlorination should be an emergency measure.
10-14
-------
in.13 Post ch1orin ation
Postchlorination is defined as the addition of chlorine to municipal
or industrial wastewater following other treatment processes. This
point of application should be before a chlorine contact unit12 and
after the final settling unit in the treatment plant. This is the
most effective place for chlorine application after treatment and
on a well clarified effluent. Postchlorination is employed primarily
for disinfection. As a result of chlorination for disinfection, some
reduction in BOD may be observed; however, chlorination is rarely
practiced solely for the purpose of BOD reduction.
QUESTIONS
10.1A What is the purpose of up-sewer chlorination?
10.IB Where should chlorine be applied in sewers?
10.1C What are the reasons for prechlorination?
10.ID Why might chlorine be added to wastewater during
treatment by other processes?
10.IE What is the objective of postchlorination?
12 Chlorine Contact Unit. A baffled basin that provides
sufficient detention time for disinfection to occur.
10-15
-------
10.2 CHLORINATION PROCESS CONTROL
10.20 Chlorinator Control
The control of chlorine flow to points of application is accom-
plished by six basic methods and a seventh method which combines
two of the basic six.
10.200 Manual Control
Feed rate adjustment and starting and stopping of equipment is
done by hand.
10,201 Start-Stop Control
Feed rate adjustment by hand, starting and stopping (by inter-
rupting injector water supply) controlled by starting of waste-
water pump, flow switch, level switch, etc.
10.202 Step-Rate Control
Chlorinator feed rate is varied according to the number of waste-
water pumps in service. As each pump starts, a pre-set quantity
of chlorine is added to the flow of chlorine existing at starting
time. This system can be applied conveniently with installations
employing up to eight pumps.
10.203 Timed Program Control
Chlorine feed rate is varied on a timed step-rate basis regulated
to correspond to the times of flow changes or by using a time-
pattern transmitter which employs a revolving cam cut to match a
flow pattern.
10.204 Flow Proportional Control
Chlorinator feed rate is controlled by a system which converts
wastewater flow information into a Chlorinator control value.
This can be accomplished by a variety of flow metering equipment,
including all process control instrumentation presently available
and nearly all metering equipment now in use on wastewater systems,
10-16
-------
10.205 Chlorine Residual Control
Chlorine feed rate is controlled to a desired chlorine residual
Cusually combined chlorine) level. After mixing and reaction
time (about five minutes maximum), a wastewater sample is
titrated by an amperometric13 analyzer-recorder (or indicator).
As the residual chlorine level varies above or below the
desired (setpoint) level, the chlorinator is caused to change
its feed rate to bring the chlorine residual back to the
desired level.
10.206 Compound Loop Control
Any "automatic" control system (step-rate, timed program, flow
proportional, or residual) can be employed in two ways:
(1) by positioning the feed rate valve, or (2) by varying the
vacuum differential across the feed rate valve. Compound loop
control employs both controls simultaneously. For instance,
a flow proportional (or step-rate, or timed program) control
system may position the feed rate valve, and a residual control
system may vary the vacuum differential across the feed rate
valve. Thus, changes in flow cause changes in feed rate valve
position, but changes in chlorine demand may occur without any
flow change. When this happens the residual analyzer detects
a change in chlorine residual and by varying the vacuum differ-
ential across the feed rate valve causes the chlorinator to
change rates to meet the desired chlorine residual level.
Various combinations of compound loop control can be employed.
Generally speaking, the part of the system requiring the fastest
response should be applied to valve positioning (since it
responds faster). If flow changes are rapid, flow control
should be by valve position. If flow and demand change rates
are nearly the same, the magnitude of change may dictate
the selection of control.
13 Amperometric (am-PURR-o-MET-rick). A method of measurement
that records electric current flowing or generated, rather
than recording voltage. Amperometric titration is an elec-
trometric means of measuring concentrations of substances
in water.
10-17
-------
The selection of control methods should be based on treatment
costs and treatment results (require-d or desired). A waste
discharger must normally meet a disinfection standard. A small
treatment plant might do this with a compound loop control
system costing several thousand dollars, but may save less than
one hundred dollars a year in chlorine consumed. In this case
the expense would not be justified. A manual system might be
employed which would meet the maximum requirements and over-
chlorinate at a minimum requirement periods. It is not unheard
of for a plant to have maximum chlorine residual requirements
because of irrigation and/or marine life tolerances. In these
cases the uncontrolled or promiscuous application of chlorine
cannot be considered, no matter how large the added cost.
A chlorine residual level may be required at some point down-
stream from the best residual control sample point. In this
case a residual analyzer should be used to monitor and record
residuals at this point. It may also be employed to change
the control set point of the controlling residual analyzer.
Ultimate control of dosage for disinfection rests on the results
desired, that is, the bacterial level or concentration acceptable
or permissible at the point of discharge. Determination of
chlorine requirements according to the current edition of
Standard Methods for the Examination of Water and Wastewater is
the best method of control. You must remember that the chlorine
requirement or chlorine dose will vary with wastewater flow, time
of contact, temperature, pH, and major waste constituents such as
hydrogen sulfide, ammonia, and amount of dead and living organic
matter.
QUESTIONS
10.2A How can chlorine gas feed be controlled?
10.2B Control of chlorine dosage depends on the bacterial
desired.
10.2C Define amperometric.
10-18
-------
10.23 Chlorine Solution Discharge Lines, Diffusors, and Mixing
10.230 Solution Discharge Lines
Solution discharge lines are made from a variety of materials
depending upon the requirements of service. Two primary requisites
are that it must be resistant to the corrosive effects of chlorine
solution and of adequate size to carry the required flows. Addi-
tional considerations are pressure conditions, flexibility (if
required), resistance to external corrosion and stresses when
underground or passing through structures, ease and tightness of
connections, and the adaptability to field fabrication or alteration.
Development of plastics in the past several years has contributed
greatly to chemical solution transmission. Polyvinyl chloride (PVC)
pipe and black polyethylene flexible tubing have all but eliminated
the use of rubber hose. Both are generally less expensive and both
outlast rubber in normal service. The use of hose is almost ex-
clusively limited to applications where flexibility is required or
where extremely high back pressures exist.
PVC and polyethylene can be field fabricated and altered. PVC
should be Schedule 80 to limit its tendency to cold flow and
partially collapse under vacuum conditions, or for higher pressure
ratings if required. Schedule 80 PVC may be threaded and assembled
with ordinary pipe tools or may be installed using solvent welded
fittings.
Rubber lined steel pipe has been used for many years where resistance
to external stresses is required. It cannot be field fabricated or
altered and is thus somewhat restricted in application. PVC lining
of steel pipe has not yet become economically competitive, but
other plastics have been developed which can readily compete with
rubber lining and are adaptable to field fabrication and alteration.
Never use neoprene hose to carry chlorine solutions because it will
become hard and brittle in a short time.
10.231 Chlorine Solution Diffusors
These diffusors are normally constructed of the same materials used
for solution lines. Their design is an extremely important part of
a chlorination program. This importance is almost completely related
to the mixing of the chlorine solution with the wastewater being
treated; however, strength, flexibility, etc., also must be given
10-19
-------
consideration. In most circular, filled conduits flowing at
0.25 ft/sec (or greater) a solution injected at the center of
the pipeline will mix with the entire flow in ten pipe diameters.
Mixing in open channels can be accomplished by the use of a
hydraulic jump (Fig. 10.4) or by sizing diffusor orifices so
that a high velocity (about 16 ft/sec) is attained at the
diffusor discharge. This accomplishes two things: (1) intro-
ducing a pressure drop to get equal discharge from each orifice,
and (2) imparting sufficient energy to the surrounding waste-
water to complete the mixing. Generally speaking, a diffusor
should be supplied for each two to three feet of channel depth.
HIGH FLOW X> HYDRAULIC LOW FLOW
VELOCITY X^ JUMP VELOCITY
Fig. 10.4 Hydraulic jump
10.232 Mixing
Mixing is extremely important ahead of a chlorine contact tank
or a residual sampling point. Since a contact tank is usually
designed for low velocity, little mixing occurs after waste-
water enters it. It is therefore necessary to achieve mixing
before the contact tank is entered. The same is true for a
chlorine residual sampling point; otherwise erratic results will
be obtained by the residual analyzing system.
QUESTIONS
10.2H Why does little mixing of the chlorine solution
with wastewater occur in chlorine contact basins?
10.21 Chlorine solution discharge lines may be made of
or
10-20
-------
CHAPTER 10. DISINFECTION AND CHLQRINATION
(Lesson 3 of 4 Lessons)
10.3 SAFETY AND FIRST AID
All persons handling chlorine should be thoroughly aware of its
hazardous properties. Personnel should know the location and use
of the various pieces of protective equipment and be instructed
in safety procedures. For additional information on this topic, see
the Water Pollution Control Federation's Manual of Practice No. 1,
Safety -in Wastewater Works, and the Chlorine Institute's Chlorine
Manual, 4th edition.19
10.30 Chlorine Hazards
Chlorine is a gas, heavier than air, extremely toxic and corrosive
in moist atmospheres. Dry chlorine gas can be safely handled in
steel containers and piping, but with moisture must be handled in
corrosion-resisting materials such as silver, glass, teflon, and
certain other plastics. Chlorine gas at container pressure should
never be piped in silver, glass, teflon, or any other plastic material.
The gas is very irritating to the mucous membranes of the nose, to the
throat, and to the lungs; a very small percentage in the air causes
severe coughing. Heavy exposure can be fatal. (See Table 10-1.)
WARNING
A
CHLORINE- 6A4,
AN17 C\\&CK Ffcl? TM£ ^M£U- OP
GO INTO A IZOOM COKTAINIMCET
IN TH£ Al£ Wlf-HOUT A
CLOTHING ANU?4-\eup
f=|20M
FIC.E
19 Write to: Water Pollution Control Federation, 3900 Wisconsin
Avenue, Washington, D.C. 20016; price to WPCF members, $0.75;
others, $1.50. The Chlorine Institute, Inc., 342 Madison Avenue,
New York, New York 10017; price $0.75.
10-21
-------
TABLE 10-1
PHYSIOLOGICAL RESPONSE TO CONCENTRATIONS OF CHLORINE GAS20
Effect
Parts of Chlorine Gas
Per Million Parts of Air
By Volume (ppm)
Slight symptoms after several
hours' exposure
Detectable odor
60-minute inhalation without
serious effects
Noxiousness
Throat irritation
Coughing
Effects dangerous to one-half
to one hour
Death after a few deep breaths
1
3
4
5
15
30
40
1000
20 Adapted from data in U.S. Bureau of Mines Technical Paper
248 (1955).
10-22
-------
10.31 Why Chlorine Must Be Handled With Care
You must always remember that chlorine is a hazardous chemical
and must be handled with respect. Concentrations of chlorine
gas in excess of 1000 ppm may be fatal after a few breaths.
Because the characteristic sharp odor of chlorine is notice-
able even when the amount in the air is small, it is usually
possible to get out of the gas area before serious harm is
suffered. This feature makes chlorine less hazardous than
gases such as carbon monoxide, which is odorless, and hydrogen
sulfide, which impairs your sense of smell in a short time.
Inhaling chlorine causes general restlessness, panic, severe
irritation of the throat, sneezing, and production of much
saliva. These symptoms are followed by coughing, retching
and vomiting, and difficulty in breathing. Chlorine is par-
ticularly irritating to persons suffering from asthma and
certain types of chronic bronchitis. Liquid chlorine causes
severe irritation and blistering on contact with the skin.
10.32 Protect Yourself From Chlorine
Every person working with chlorine should know the proper ways
to handle it, should be trained in the use of self-contained
breathing apparatus, and should know what to do in case of
emergencies.
WARNING
AN? IN&PF^CfW^ IN
NOT
OE
Here are some items you should always remember in order to
protect yourself and others from possible injury:
10-23
-------
a. In an emergency, only authorized persons with adequate
safety equipment should be in the danger area. Have
your fire department examine your chlorine handling
facilities and safety equipment so they will be aware
of what you have and the possible dangers. They are
well trained in the use of breathing apparatus and
may be able to help you in an emergency, especially if
they are familiar with chlorine hazards.
b. In any chlorine atmosphere, short shallow breathing is
safer than deep breathing. Recovery from exposure depends
on the amount of chlorine inhaled, so it is important to
keep that amount as small as possible.
c. Clothing contaminated with liquid or gaseous chlorine
continues to give off chlorine gas and irritate the body
even after leaving a contaminated area. Therefore, con-
taminated clothing should be removed immediately and the
exposed parts of the body washed with a large amount of
cool water.
The use of a breathing apparatus is advisable during these
operations. All caution should be taken to prevent any
liquid from coming in contact with clothing not designed
for protection, because the liquid can penetrate the cloth
and cause skin problems.
Learn the correct way of using the breathing apparatus,
practice using it regularly, and take safety drills
seriously. What you learn may save your life. The fire
department is well trained in the use of breathing apparatus
and can be very helpful in training.
10-24
-------
e. If you have found a chlorine leak and left the area before
the leak was stopped, you should use an apparatus with a
separate air supply when you return and repair the leak.
Never rely on a cannister type mask for protection in
repairing chlorine leaks. Cannister masks are not recom-
mended because they do not supply oxygen. They only remove
chlorine, if they are effective. Some agencies allow the
use of cannister type masks; however, most operators who
have had experience repairing chlorine leaks do not use
cannister masks because of their short shelf life (ap-
proximately three to four months) and inability to provide
adequate protection against high concentrations of chlorine.
Extensive ventilation is recommended.
f. Cooperate in taking care of all safety equipment, handling
it carefully, and returning it to its proper storage place
after use. Defective equipment, or equipment which you
can't find when you need it, will not protect you.
g. Always be sure that you know the location of first aid
cabinets, breathing apparatus, showers, and other safety
equipment. Review emergency instructions regularly to
be sure you know them.
h. Notify your police department that you need help if it
becomes necessary to stop traffic on roads and to evacuate
persons in the vicinity of a chlorine leak.
10.33 First Aid Measures
a. Be sure you know the location of breathing apparatus, first
aid kits, and other safety equipment at all times.
b. Remove clothing contaminated with liquid chlorine at once.
Carry patient away from gas area—if possible to a room
with a temperature of 70°F. Keep patient warm, with
blankets if necessary. Keep him quiet.
c. Place patient on his back with his head higher than the
rest of his body.
d. Call a doctor and fire department immediately. Immediately
begin appropriate treatment.
e. Eyes. If even small quantities of chlorine have entered
the eyes, hold the eyelids apart and flush copiously with
lukewarm running water. Continue flushing for about fifteen
minutes. Do not attempt any medication except under specific
instructions from a physician.
10-25
-------
£. Skin. Get patient under a shower immediately, clothes
and all. Remove clothing while'the shower is running.
Wash the skin with large quantities of soap and water.
Do not attempt to neutralize chlorine with chemicals.
Do not apply salves or ointments except as directed by
a physician.
g. Inhalation. If the patient is breathing, place him in a
comfortable position; keep him warm and at rest until a
physician arrives.
If breathing seems to have stopped, begin artificial
respiration immediately. Mouth-to-mouth resuscitation
or any of the approved methods may be used. Oxygen
should be administered if equipment and trained personnel
are available.
Automatic artificial respiration is considered preferable
to manual, but only when administered by an experienced
operator.
Rest is recommended after severe chlorine exposure.
h. Throat Ir ritati on. Drinking milk will relieve the dis-
comforts of throat irritation from chlorine exposure.
Chewing gum or drinking spirits of peppermint also will
help reduce throat irritation. Follow emergency rules
given by your physician. In the absence of such rules,
the first aid steps above are suggested.
Taken in part from Chlorine Safe Handling Pamphlet, published by
The Chemical Division of PPG Industries, Inc.
QUESTIONS
10.3A What are the hazards of chlorine gas?
10.3B What type of breathing apparatus is recommended when
repairing a chlorine leak?
10.3C What first aid measures should be taken if a person
comes in contact with chlorine?
10-26
-------
10.4 CHLORINE HANDLING
10.40 Chlorine Containers
10.400 Cylinders
Cylinders containing 100 to 150 pounds of chlorine are convenient
for the average small consumer. These cylinders are usually of
seamless steel construction (Fig. 10.5).
A fusible plug is placed in the valve, below the valve seat.
This plug is a safety device. The fusible metal
softens or melts at 158° to 165°F, to prevent building up of
excessive pressures and the possibility of rupture due to a fire
or high surrounding temperatures.
Cylinders will not explode and can be handled safely.
The following are procedures for handling chlorine cylinders.
1. Move cylinders with a properly balanced hand truck with
clamp supports that fasten at least two-thirds of the
way up the cylinder.
2. 100- and 150-pound cylinders can be rolled in a vertical
position. Lifting of these cylinders -should be avoided
except with approved equipment. Never lift with chains,
rope slings, or magnetic hoists.
3. Protective cap should always be replaced when moving a
cylinder.
4. Cylinders should be kept away from direct heat (steam
pipes, radiators, etc.).
5. Cylinders should be stored in an upright position.
10-27
-------
Chlorine Cylinder
Protection
Hood
Valve
Neck Ring
Cylinder
Body
If— Foot Ring
I
Net
Cylinder
Contents
100 Lbs.
150 Lbs.
Approx.
Tare,
Lbs.'
73
92
Dimensions,
Inches
A
8V.
10V.
B
54 Vj
54%
"Stamped inn1 we:gh on Vinder shoulder
does hui include /alve piotecti ,n hood.
Fig. 10.5 Chlorine cylinder
(Courtesy of PPG Industries, Inc., Chemical Division)
10-28
-------
10.401 Ton Tanks
Ton tanks are of welded construction and have a loaded weight
of as much as 3700 pounds. They are about 80 inches in length
and 30 inches in outside diameter. The ends of the tanks are
crimped inward to provide a substantial grip for lifting clamps
(Fig. 10.7).
The following are some characteristics of and procedures for
handling ton tanks.
Most ton tanks have eight openings for fusible plugs and valves
[Fig. 10.7]. Generally, two operating valves are
located on one end near the center and six or eight fusible
metal safety plugs, three or four on each end. These are
designed to melt within the same temperature range as the
safety plug in the cylinder valve.
WARNING
IT \^V£Xy IMPORTANT T-MAT
^HOULP NOT ££• TAAAP£-R£rP W\T-H
ANW
IM
TAMK VAMU- SB
Ton tanks are shipped by rail in multi-unit tank cars. Single
units may be transported by truck or semi-trailer.
Ton tanks should be handled with a suitable lift clamp in con-
junction with a hoist or crane of at least two-ton capacity
(Fig. 10.7).
Ton tanks should be stored and used on their sides, above the
floor or ground, on steel or concrete supports. They should
not be stacked more than one high.
10-29
-------
Net Weight of Chlorine .. .2000 Ibs. 2-Ton Minimum 15/16
Tare Wt. of Tank (average) 1550 Ibs. Capacity Hoist
Gross Weight Full (average)
3550 Ibs.
Spacer —
each end
Chlorine Gas Eduction Pipe
Valve
Protectio
Hood
Chlorine
Liquid
Fusible Plugs,
(at least 3 each end)
Fig. 10.7 Ton tank lifting beam
(Courtesy of PPG Industriest Inc., Chemical Division)
10-30
-------
Ton tanks should be placed on trunnions which are equipped
with rollers so that the withdrawal valves may be positioned,
one above the other. The upper valve will discharge chlorine
gas, and the lower valve will discharge liquid chlorine (see
Fig. 10.7). Trunnion rollers should not exceed 3-1/2 inches
in diameter so that the containers will not rotate too easily
and be turned out of position. Roller shafts should be equipped
with a zerk type lubrication fitting and slotted for even
lubrication. Roller bearings are not advised because of the
ease with which they rotate. Locking devices are not required
when these rules are observed.
10.402 Chlorine Tank Cars
Chlorine tank cars are of 16-, 30-, 55-, 85-, or 90-ton capacity.
All have four-inch cork board insulation protected by a steel
jacket. The dome of the standard car contains four angle valves
plus a safety valve. The two angle valves located on the axis
line of the tank are equipped for discharging liquid chlorine.
The two angle valves at right angles tc the axis of the tank
deliver liquid chlorine.
The following are some procedures for unloading chlorine tank cars.
Unloading of tank cars should be performed by trained personnel
in accordance with Interstate Commerce Commission (ICC) regulations.
In most situations chlorine is withdrawn from tank cars as a
liquid and then passed through chlorine evaporators. Sometimes
dry air is passed into the tank car through one of the gas valves
to assist in liquid withdrawal. This practice is referred to as
"air padding".
QUESTIONS
10.4A How may chlorine be delivered to a plant?
10.4B What is the purpose of the fusible plug?
10-31
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CHAPTER 10. DISINFECTION AND CHLORINATION
(Lesson 4 of 4 Lessons)
10.5 CHLORINATION EQUIPMENT AND MAINTENANCE (by J. L. Beals)
10.50 Chlorinators
Chlorine may be delivered from a feeder by one of two methods:
1. Solution feed, commonly practiced, in which the chlorine
gas is controlled, metered, introduced into a stream of
injector water, and then conducted as a solution to the
point of application.
2. Direct feed, sometimes called dry feed, in which the gas
is introduced directly through a suitable diffuser at
the point of application. This method is used only when
a source of injector water at adequate pressure, or power
for an injector pump, is not available. Operating diffi-
culties experienced in metering dry chlorine gas directly
to the point of application make this type of equipment
a "last resort".
Following are the common types of feeders used in wastewater treat-
ment plants.
10.500 Vacuum-Solution Feed Chlorinators
This type of equipment (Fig. 10.10) comprises in excess of 90% of
all gas chlorination equipment in service today in water and waste-
water treatment operations. The primary advantage of vacuum
operation is safety. If a failure or breakage occurs in the
vacuum system, the chlorinator either stops the flow of chlorine
into the equipment or allows air to enter the vacuum system rather
than allowing chlorine to escape into the surrounding atmosphere.
In case the chlorine inlet shut-off fails, a vent valve discharges
the incoming gas to the outside of the chlorinator building.
The operating vacuum is provided by a hydraulic injector. The
injector operating water absorbs the chlorine gas, and the resultant
chlorine solution is conveyed to a chlorine diffusor through corro-
sion resistant conduit.
10-32
-------
A vacuum chlorinator also includes a vacuum regulating valve
to dampen fluctuations and give Smooth operation. A vacuum
relief prevents excessive vacuum within the equipment.
A typical vacuum control chlorinator is shown in Fig. 10.10.
Chlorine gas flows from a chlorine container to the gas inlet
(located above the circled Y in the middle right of the figure).
After entering the chlorinator the gas passes through a spring
loaded pressure regulating valve which maintains the proper
operating pressure. A rotameter is used to indicate the rate
of gas flow. The rate is controlled by a V-notch variable
orifice. The gas then moves to the injector where it is dis-
solved in water and leaves the chlorinator as a chlorine solution
(HOC1) ready for application.
10.501 Partial Vacuum, Pressure Type, and Pulsating Type
Chlorinators
Aside from the pressure type which has been described previously,
these types of equipment are limited in application and few remain
in service. Pulsating and partial vacuum chlorinators are primarily
designed for extremely low feed rates. Vacuum-solution feed equip-
ment can feed less than 0.25 Ibs/day. The reduced cost of hypo-
chlorination has almost eliminated their use.
10.51 Hypoch1orin at ors
Hypochlorinators are devices that are used to feed chlorine in the
form of calcium, sodium, or lithium hypochlorite. Hypochlorites
are available as liquids or various forms of solids (powder, pellets),
and in a variety of containers or in bulk.
QUESTION
10.5A How is chlorine delivered (fed) to the
point of application?
10-33
-------
VENT
-VACUUM REGULATING VALVE
-8" TO BB' WATER VACUUM SIGNAL
VACUUM TRANSMITTER
16''TO96" WATER \MCUUM
V-NOTCH
VARIABLE ORIFICE
PRESSURE GAUGE
2O PSIG MINIMUM
ROTAMETER
''TO 2''WATER
PRESSURE DROP
YTOZ
PRESSURE-VACUUM
RELIEF VALVE
OPENS AT
I" TO 2* WATER PRESSURE
35" TO 4O" WATER W3JUM
22" TO 24"
MHTER \MCUUM
(UNBIASED)
MANUAL FEED
RATE ADJUSTER
GAS PRESSURE
REGULATING VALVE
REMOTE FROM
CHLORINATOR
INJECTOR VACUUM
GAUGE
lO'Hg MINIMUM
CONTROL VACUUM
CHECK VALVE
INJECTOR WATER
SUPPLY
o
O
c <
O c
=» 3
2. °
2- 2.
Fig. 10.10 Vacuum solution feed chlorinator
(Courtesy Wallace & Tiernan)
O
O
O
o
I
o
o
o
§
o-n
-------
10.6 OTHER USES OF CHLORINE
10.60 Odor Control
Chlorination of wastewater for odor control is used to inhibit
the growth of odor-producing bacteria and to destroy hydrogen
sulfide (H2S), the most common odor nuisance, which has the
smell of rotten eggs. Hydrogen sulfide, in addition to creating
an odor nuisance, can be an explosion hazard when mixed with air
in certain concentrations. Breathing H2S can impair your ability
to smell, and too much will paralyze your respiratory center,
causing death in severe cases. It also can cause corrosion of
metals and concrete, being particularly damaging to electrical
equipment even in low concentrations.
The presence of hydrogen sulfide may be detected in significant
quantities in any collection and treatment system where sufficient
time is allowed for its development. It may be expected to be
present most often in new systems where flows are extremely low
in comparison with design capacity, and particularly in lift
stations where pump operating cycles may be at a low frequency.
Collection systems which serve large areas often allow time for
H2S development even when operating at design capacity.
The purpose of this section is not to discuss the reasons for
odor production, but rather their elimination or control by .
chlorination; however, the correction of an odor problem will
usually require a decision being made between system modification
and treatment. Sometimes both may be required. Choices of this
type often hinge on the costs involved, and it will frequently
be found that modifications to major system components are far
more costly than treatment. When this is the case, chlorination
is usually the most economical solution. Other solutions include
the use of air or ozone.
Sulfides develop whenever given time to do so. The rate of
sulfide production increases with temperature (about 7% on the
average with each 1°C increase in wastewater temperature).
The odors which are controllable with chlorine are specifically
hydrogen sulfide which can be inactivated by chlorination at
levels well below the chlorine demand point. This is commonly
referred to as "sub-residual chlorination". The reason that
this is true is based on the fact that the C12 + H2S reaction
precedes most other chlorine-consuming reactions. Since it is
known that bacterial kills occur at sub-residual levels, it is
logical that odor-producing bacteria can be reduced in numbers
10-35
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without satisfying the chlorine demand. This can be accomplished
without significantly interfering with organisms beneficial to
the treatment processes.
The quantities of chlorine required to accomplish control of
odors vary widely from plant to plant and at any given plant
fluctuate over a broad range. Hydrogen sulfide is generally
found in higher concentrations when flows are low. For this
reason it is usually not economical to chlorinate for odor
control in direct proportion to flow. Tests should be run over
periods which include all the various conditions which could
possibly affect odor production in order that a basis for
treatment may be established.
When the requirements are known, the primary concern is to
apply chlorine at the proper location. The best locations are
generally up-sewer ahead of the plant influent structures,
and up-sewer ahead of lift stations. This is done to allow
mixing and reaction time before the waste reaches a point of
agitation.
Sometimes force mains empty into the gravity sections of a
collection system several hours after pumping. If odor problems
result, a treatment point should be placed upstream at a point
where the sewer is still under pressure and flowing full; thus
treatment can be completed before odors are released to the
atmosphere.
Hydrogen sulfide should not be considered merely an odor nuisance.
It must always be kept in mind that it can create an explosion
hazard, it can paralyze your respiratory center, and it should
always be considered a source of corrosion. For these reasons,
odor masking agents should not be used except possibly as addi-
tional treatment for odors not eliminated by chlorination.
Excessive use of masking agents could prevent detection of a
serious problem condition.
10.61 Protection of Structures
The destruction of hydrogen sulfide in wastewater also reduces
the production of suliuric acid that is highly corrosive to
sewer systems and structures. This is particularly significant
where temperatures are high and time of travel in the sewer
system is unusually long. The treatment is similar to that for
odor control: chlorination sufficient to prevent hydrogen sulfide
10-36
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formation or to destroy hydrogen sulfide that has been produced
(about 2 mg/1 chlorine per mg/1 'of hydrogen sulfide"). Sulfide
problems also may be corrected by oxygenation in sewers. The
choice between oxygenation and chlorination will usually depend
on the costs involved.
10.62 Aid to Treatment
Among its many uses, chlorine improves treatment efficiency in
the following ways.
10.620 Sedimentation
Prechlorination at the influent of a settling tank improves
clarification by improving settling rate, reducing septicity23
of raw wastewater, and increasing grease removal. Maximum
grease removal is achieved when chlorination is combined with
aeration ("aero-chlorination"). It is an expensive procedure,
and some studies have indicated that benefits are minimal.
Generally grease removal in this manner is considered a bene-
ficial side effect or "bonus" reaction to chlorine which is
essentially applied for other reasons. Excess chlorination
ahead of secondary processes can inhibit the bacterial action
critical to the process and decrease sedimentation efficiency.
10.621 Trickling Filters
Continuous chlorination at the filter influent controls slime
growths and destroys filter fly larvae (Psychoda). Generally
the chlorine is applied to produce a residual of 0.5 mg/1
(continuous) at the orifices or nozzles. Caution should be used
because some filter growth may be severely damaged by excessive
chlorination. Suspended solids will increase in a trickling
filter effluent after chlorination for filter fly control. Also,
it will be difficult to evaluate filter performance on the basis
of BOD removals because chlorine can interfere with the BOD test.
As a general statement, it would be well to look closely at
23 Septicity (sep-TIS-it-tee) is the condition in which organic
matter decomposes to form foul-smelling products associated
with the absence of free oxygen.
10-37
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loadings, operation, and general adequacy of the process when
filter fly chlorination is continuously necessary, because
continuous chlorination may be an expensive alternative for
adequate design and operation.
10.622 Activated Sludge
Chlorination of return sludge reduces bulking of activated sludge
that is caused by overloading. The point of application should
be where the return sludge will be in contact with the chlorine
solution for about one minute before the sludge is mixed with
the incoming settled wastewater. Chlorine is also commonly
used to control filamentous organisms. Again, chlorine used in
this manner is an expensive alternative for adequate design and
operation. The main effort should be directed toward process
improvement, considering chlorination mainly as an emergency
solution. Never forget that chlorine is toxic to organisms that
are needed to treat the incoming wastes.
10.623 Reduction of BOD
Chlorination of raw wastewater to produce residual of 0.5 mg/1
after 15 minutes of contact may cause a reduction of 15 to 30%
in the BOD of the wastewater (Baity, 1929). Generally a reduction
of at least 2 mg/1 of BOD is obtained for each mg/1 of chlorine
absorbed up to the point at which the residual is produced.
Snow (1952) has shown that the BOD reduction also depends on the
condition of the wastewater. He reported a 10% reduction in
fresh wastewater and a 25 to 40% reduction in stale wastewater.
Both real and apparent effects of chlorination are evident in the
wastewater and in the test bottle.
QUESTIONS
10.6A How can odors be controlled? Why?
10.6B How can sulfuric acid damage to structures be mini-
mized or eliminated? Why?
10-38
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10.7 ACKNOWLEDGMENTS
Portions of the information contained in this chapter were taken
in part from Chapter 17, Disinfection and Chlorination, Water
Pollution Control Federation Manual of Practice No. 11; and
from Chapter 7, Chlorination of Sewage, Manual of Instruction
for Sewage Treatment Plant Operators (New York Manual). Both
publications are excellent references for additional study.
Mr. J. L. Beals provided many helpful comments.
10.8 REFERENCES
Baity, H.G., "Reduction of BOD in Sewage by Chlorination",
Sewage Works J., 1, 279 (1929).
California State Department of Public Health, "Laws and Regu-
lations Relating to Ocean Water-Contact Sports Areas" (1958).
California State Department of Public Health, "Statewide Stan-
dards for the Safe Direct Use of Reclaimed Waste Water for
Irrigation and Recreational Impoundments" (1968).
California State Department of Public Health, "Laws and Regu-
lations Relating to Swimming Pools" (1966).
California State Department of Public Health, "Some Experience
with Disinfection of Waste Water in California", G.E. Browning
and F.R. McLaren (1966).
Chlorine Institute, "Chlorine Manual" (1969).
Dow Chemical, "Dow Chlorine Handbook" (1966).
New York State Department of Health, "Manual of Instruction for
Sewage Plant Operators" (1966),
PPG Industries, Inc., Chemical Division, "Chlorine Safe Handling
Pamphlet".
Snow, W.B., "Biochemical Oxygen Demand of Chlorinated Sewage",
Sewage and Industrial Wastes, 24, 689 (1952).
U.S. Bureau of Mines, Technical Paper 248 (1955).
U.S. Public Health Service, "Drinking Water Standards" (1962).
10-39
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Water Pollution Control Federation, Manual of Practice No. 1,
"Safety in Waste Water Works".
Water Pollution Control Federation, Manual of Practice No. 4,
"Chlorination of Sewage and Industrial Wastes" (under revision).
Water Pollution Control Federation, Manual of Practice No. 11,
"Operation of Waste Water Treatment Plants" (1968).
10.9 ADDITIONAL READING
a. MOP 11, pages 127-135.
b. Mew York Manual, pages 73-83.
c. Texas Manual, pages 397-412,
d. "Chlorine—Safe Handling", PPG Industries, Inc., Chemical
Division, One Gateway Center, Pittsburgh, Pennsylvania 15222.
e, Chlorination Guide, Water and Sewage Works Magazine, Scranton
Publishing Company, 355 East Wacker Drive, Chicago, Illinois 60601.
Price, $1.25.
f. Chlorine Manual (4th edition), The Chlorine Institute, Inc.,
342 Madison Avenue, New York, New York 10017. Price $0.75.
g. Safety in Waste Water Works, MOP No. 1, Water Pollution Control
Federation, 3900 Wisconsin Avenue, Washington, D.C. 20016.
Price to WPCF members, $0.75; others, $1.50.
Films on chlorine safety also are available from the Chlorine Institute
and PPG Industries, Inc.
END OF LESSON 4 OF 4 LESSONS
on
DISINFECTION AND CHLORINATION
10-40
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CHAPTER 11. MAINTENANCE
(Lesson 6 of 6 Lessons)
11.2 FLOW MEASUREMENTS—METERS AND MAINTENANCE
11.20 Flow Measurements, Use and Maintenance
Flow measurement is the determination of the quantity of a mass in
movement within a known length of time (Fig. 11.19). Usually the
mass which may be solid, liquid, or gas is contained within physical
boundaries such as tanks, pipelines, and open channels or flumes.
The limits of such physical or mechanical boundaries provide a
measurable dimensional area that the mass is passing through. The
speed at which the mass passes through these boundaries is related
to dimensional distance and units of time; it is referred to as
velocity. Therefore, we have the basic flow formula:
Quantity = Area x Velocity
Q = AV
or
Q, cu ft/sec = (Area, sq ft)(V, ft/sec)
The performance of a treatment facility cannot be evaluated or
compared with other plants without flow measurement. Individual
treatment units or processes in a treatment plant must be observed
in terms of flow to determine their efficiency and loadings. Flow
measurement is important to plant operation as well as to records of
operation. It is essential that the devices used for such measurement
be understood, be used properly, and most important, be maintained so
that information obtained is accurate and dependable.
MASS
AREA
Fig. 11.19 Flow mass
11-1
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11,21 Manufacturers' and Operators* Responsibilities
Equipment and instrument manufacturers should be required to
furnish instruction manuals and parts lists. In the parts list
it should be required that the manufacturer designate recommended
spare parts, and such parts should be obtained and be available
for use.
Instrumentation and flow measurement devices should be considered
as fragile mechanisms. Rough handling will damage the units in
as serious a manner as does neglect. Treat the devices with care,
keep them clean, and they will perform their designated functions
with accuracy and dependability.
11.22 Various Devices for Flow Measurement
The selection of a type of flow metering device, and its location,
is made by the designer in the case of new plant construction. It
is also possible that a metering device will have to be added to
an existing facility. In both cases the various types available,
their limitations, and criteria for installation should be known.
Often the criteria for installation must be understood for the
proper use and maintenance of a fluid flow meter. Metering devices
commonly used in treatment facilities include:
2H22.
Constant
Differential
Head Area
Velocity
Meter
Common Name
Rotameter
Weirs
Rectangular
Cipoletti
V-Notch
Proportional
Flumes
Parshall
Palmer-Bowlus
Nozzles
Propeller
Application
Liquids and Gases
a. Chlorination
Liquids—partially filled
channels, basins, or clari-
fiers
a. Influent
b. Basin control
c. Effluent
d. Distribution
Liquids—partially filled
pipes and channels
a. Influent
b. Basin control
c. Effluent
d. Distribution
Liquids—channel flow ,
clean water piped flow
11-2
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Common Name
Application
Velocity
Meter
Magnetic
Differential
Head
Shuntflo
Venturi Tube
Flow Nozzle
Orifice
Displacement
Piston
Diaphragm
Liquids and sludge in
closed pipe
a. Influent
b. Basin control
c. Sludge recirculation
d. Distribution
Gases—closed pipe
a. Digester gas
Gases and liquids
in closed pipes
a. Influent
b. Basin control
c. Effluent
d. Digester gas
e. Distribution
Gases and liquids in
closed pipes
a. Plant water
b. Digester gas
A description of how each device works is in reality a definition
of the meter type.
Constant Differential—A mechanical device called the "float" is
placed in a tapered tube in the flow line. The difference in pressures
above and below the float causes the float to move with flow variations.
Instantaneous rate of flow is read out directly on a calibrated scale
attached to the tube.
Head Area--A mechanical constriction or barrier is placed in the open
flow line causing an upstream rise in liquid level. The rise or "head"
(H) is a function of velocity of flow and when referenced to empirical
flow formula provides an indication of the flow rate. When first
starting to pump sludge in a long line, the pressure may increase con-
siderably before the sludge starts flowing.
Velocity Meters—The velocity of the liquid flowing past the measure-
ment point through a given area gives a direct relation to flow rate.
The propeller type is turned by fluid flow past propeller vanes which
move gear trains. These gear trains are used to indicate the fluid
velocity or flow rate. The velocity of liquid flow past the probes of
a magnetic meter is related to electrical formula and read out as the
flow rate through secondary instrumentation. (See Section 11.24.) Pitot
tubes are used to measure the velocity head (H) in flowing water to
give the flow velocity (V = /TgTT). (fig. 11.20)
11-3
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T H where V=~V2gH
FLOW
Fig. 11.20 Pitot tube
Differential Producers--A mechanical constriction (Fig. 11.21)
in pipe diameter (reduction in pipe diameter) is placed in the
flow line shaped to cause the velocity of flow to increase
through the restriction. When the velocity increases, a pressure
drop is created at the restriction. The difference between line
pressure at the meter inlet and reduced pressure at the throat
section is used to determine the flow rate which is indicated by
a secondary instrument.
DIFFERENTIAL
PRESSURE
FLOW
CONSTRICTION
Fig. 11.21 Differential producer
Displacement Units—Liquids or gas enters, fills a tank or
chamber of known dimensions, activates a mechanical counter,
and empties the tank in readiness for another filling. Mech-
anical gearing activated by chamber fill and evacuation actuates
a counter which is referenced to time and thus flow rate is
determined.
11-4
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11.23 Meter Location
The selection of a particular type of meter or measuring device
and its location in a particular flow line or treatment facility
is usually a decision made by the plant designer. Ideally the
flow should be in a straight section before the meter. In open
channels the flow should not be changing directions, nor should
waves be present in the metering section above the measuring
device. Valves, elbows, and other items that chould disrupt the
flow ahead of a meter can upset the accuracy and reliability of
a flow meter. Most flow meters are calibrated (checked for
accuracy) in the factory, but they also should be checked in
their actual field installation. When a properly installed and
field calibrated meter starts to give strange results, check for
obstructions in the flow channel and the flow metering device.
QUESTIONS
11.2A What is flow measurement?
11.2B Write the fundamental flow formula.
11.2C Why should flow be measured?
11.2D List several types of flow measuring devices.
11.2E If a flow meter does not read properly, what items
should be checked as potential causes of error?
11-5
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11.24 Conversion and Readout Instruments and Controls
Conversion and readout instrumentation is used to convert the initial
measurement (for example, depth of water) to a more commonly used
number or value (depth of water in a Parshall flume to flow of water
in MGD). The type of device depends upon what the' sensor (device)
measures and what kind of results are desired. Often the conversion
device only will transmit the signal (depth of water) to another
meter which will interpret the signal and convert it to a usable
number (flow in MGD). Instruments used with flow measurement equip-
ment are classified as transmitters, receivers, recorders, controllers,
and summators or totalizers. All of the different devices available
are too numerous to list. Most devices used today will fall into the
classifications outlined in the following paragraphs.
11.240 Mechanical Meters
Mechanical meters are those devices which measure the variable
flow indicator and convert this value into a usable number. Con-
version of the flow variable to a scale or meter giving the usable
number may be by gear trains, hydraulic connections, magnetic
sensing, electrical connections, and many other devices.
11.241 Transmitters
Transmitters send the flow variable, as measured by the measuring
device, to another device for conversion to a usable number.
Variables are transmitted mechanically, electrically, and pneu-
matically.
11.242 Receivers
Receivers pick-up the transmitted signal and convert it to a usable
number. Receivers may present the measurement as an instantaneous
flow rate, record the flow on a chart against time, and total or sum
the flow during a time period. Receivers may have one, two, or all
three of these features.
11.243 Controllers
Controllers are similar to receivers except they are capable of
comparing received signals with other values and sending corrective
or adjusting signals when necessary. The compared value may be
manually set or it may be based on another received signal. The
11-6
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correction or adjustment may be proportional to the size of the
deviation of the compared values, may be a gradual adjustment,
or may provide a predetermined correction based on the size of
the deviation and your objectives.
Selection and adjustment of controllers should be done by a
specialist in the field or the manufacturer's representative.
Maintenance must be done according to manufacturer's instructions.
11.25 Sensor Maintenance
Each individual sensing meter will have its own maintenance require-
ments. In any instrument, the sensor is the most common source of
problems. Fortunately, the electronics or drive are easy to check.
The important and common maintenance requirements are tabulated
below in relation to meter types. Not all the maintenance problems
can be listed. It is a proven fact that if preventive maintenance
is regularly applied the uncommon problem is a rare occurrence.
The most important single item to be considered in maintenance is
good housekeeping. This must take many forms since it is applied
to various devices. Good housekeeping, the act of providing pre-
ventive maintenance for each of the various sensors, includes being
sure that foreign bodies are not interfering with the measuring
device. Check for and remove deposits which will accumulate from
normal use. Repair the sensor or measuring device whenever it is
damaged.
Common preventive maintenance suggestions:
Motor Type Suggested Maintenance
Constant Differential Disassemble and clean tube and float
Rotameters when deposits are observed.
Head Area
Weirs: Flow formula is based on square
Rectangular clean edges to the meter shape with
Cipoletti free fall over the weir. Clean and
V-Notch brush off deposits as accumulated.
Proportional Keep clear of foreign bodies and
interference.
11-7
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Motor Type
Head Area
Flumes:
Parshall
Palmer-Bowlus
Nozzles
Velocity Meter
Propeller
Shuntflo
Magnetic
Differential
Producers
Displacement
Suggested Maintenance
Normally used with float wells, keep
sensor line between well and flume
clean; clean off deposits.
Should not be used on anything but
clear water. Grease and check yearly.
Keep dampening chamber fluid level to
line; periodically drain to remove
collected sediment.
Manufacturers are providing various
cleaning mechanisms to clean the
internal parts regularly. If you as
an operator manually operate, be sure
to perform maintenance on schedule;
if automatically, check action fre-
quently. Provide for periodic meter
removal from line and physically
clean meter.
Venturi, nozzle, and orifice hydraulic
connections should be back-flushed
regularly. Installation should be
arranged for internal surface cleaning
on a reasonable schedule.
Periodically drain and flush. Keep
greased as necessary; check frequently
on operation.
External connections between the sensing and conversion and readout
devices should be checked to ensure such connections are clean in
appearance and connections are firm. Be sure no foreign obstruction
will interfere or promote wear. On mechanical connections, grease
as directed; on hydraulic or pneumatic connections, disconnect and
ensure free flow in the internal passage.
11-8
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11.26 Conversion and Readout Instrument Maintenance
Both the mechanically actuated unit and the transmitters will have
direct sensor connections. Cleaning and checking on a regular
schedule is essential to avoid problems with the usual accumulation
of foreign material. Maintenance for the internal parts to either
device is minimized when the sensor connections are clean and
operable. Normal wear will occur and is increased when sediments
and deposits are not removed regularly as directed. Lubricate
mechanical components as directed by the equipment manufacturers'
instrument manuals. Do not over-lubricate, because it causes
other difficulties equally
as troublesome as under-
lubrication.
Receiver maintenance is
limited to periodic check-
ing of mechanical parts,
proper lubrication, and
good housekeeping within
the unit. Moisture
should be eliminated by
heat if required. Pneumatic
instruments should be watched
carefully to ensure that
foreign particles which
might be introduced by the
air supply do not cause
clogging in the actuating
elements. Pneumatic systems
are usually protected by air
filters or traps at the
supply source and individual units at the instrument. Filters should
be cleaned and blown down on a regular schedule to ensure their efficient
operation in cleaning the air supply. In the case of clogging of
small orifices and devices of the pneumatic system, do not attempt to
pjressurize the system at higher than normal operating pressure for
cleaning. Such action will 'damage internal parts. Follow procedures
as outlined by the manufacturer and as shown in the instruction manuals.
Most reputable manufacturers are equipped to provide repair service
in the case of worn parts, or mechanical failure. It is recommended
that major service be left to trained employees of the manufacturer.
It is preferred that manufacturers have field service available for
repair on the plant premises; however, if such service is not avail-
able, the device should be returned to the factory.
11-9
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Many manufacturers have a maintenance contract service available
wherein a trained service employee periodically, on a prescribed
schedule, checks the instrument in all ways including accuracy
and wear factors. Such periodic checking allows for replacement
of parts prior to a complete breakdown. Parts which would normally
wear over a time period are replaced by this serviceman who will
anticipate such need from an experience factor.
Do not attempt instrument service, parts replacement, or
repair work unless you have read the instruction manual
thoroughly and you understand what you are doing. Follow
the procedures as set forth in the instruction manual care-
fully.
All instruments are connected to a power supply of some source.
That power supply is potentially dangerous unless handled properly.
Be sure all electrical power is shut off and secured so that
others cannot unintentionally switch the source on. On electrical
and electronic devices the electrical power used and/or generated
within the device is exceptionally dangerous, both to the man
and to the other component equipment. Do not attempt service
unless you are qualified to do so.
Recording charts often seem to accumulate at a rapid rate, and a
decision must be made whether to store or destroy old records.
Inconvenient as it may be, records should be retained. They are
the backbone of reference information needed for future planning
and plant expansion when necessary. Above all, if properly used,
they are an index for efficiency checks unparalleled in value.
Storage space may be minimized by preparing summary records, micro-
film photocopy, or selective sampling and storage of the usual and
unusual.
QUESTIONS: .
11.2F What is the purpose of transmitting instruments?
11.2G What is the most important item in maintaining flow
meters?
11.2H What should you do with old recording chart records?
11-10
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CHAPTER 12. PLANT SAFETY AND GOOD HOUSEKEEPING
(Lesson 1 of 3 Lessons)
12.0 INTRODUCTION—WHY SAFETY?
A cat may have nine lives, but you have only one! Protect it!
Others may try, but only your efforts in thinking and acting
safely can ensure you the opportunity of continuing to live your
single life!
You are working at an occupation that has an accident frequency rate
second only to that of the mining industry! Not a very desirable
record.
Your employer has the responsibility of providing you with a safe
place to work. But you, the operator who has overall responsibility
for your treatment plant, must accept the task of seeing to it that
your plant is maintained in such a manner as to continually provide
a safe place to work. This can only be done by constantly thinking
s afety.
You have the responsibility of protecting yourself and other plant
personnel or visitors by establishing safety procedures for your
plant and then by seeing that they are followed. Train yourself to
analyze jobs, work areas, and procedures from a safety standpoint.
Learn to recognize potentially hazardous actions or conditions. When
you do recognize a hazard, take immediate steps to eliminate it by
corrective action. If correction
is not possible, guard against
the hazard by proper use of warn-
ing signs and devices and by the
establishing and maintaining of
safety procedures. As an indi-
vidual, you can be held liable
for injuries or property damage
as a result of an accident caused
by your negligence.
REMPMBER; "ACCIDENTS DON'T JUST
HAPPEN--THEY ARE CAUSED"!! Ho>v
true it is! Behind every accident
there is a chain of events which
lea.d to an unsafe act, unsafe
condition, or a combination of both,
THINK SAFETY!
SAftfV
12-1
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Accidents may be prevented by using good common sense, applying
a few basic rules, and particularly by acquiring a good knowledge
of the hazards peculiar to your job as a plant operator.
The Bell system has one of the best safety records of any
industry. A variation of their successful policy statement
is:
"There is no job so important
nor emergency so great
that we cannot take time
to do our work safely."
Although this chapter is intended primarily for the wastewater
treatment plant operator, the operators of many small plants
have the responsibility of sewer maintenance also. Therefore
the safety aspects of both sewer maintenance and plant operation
will be discussed.
12.1 KINDS OF HAZARDS
You are equally exposed to accidents whether working on the
collection system or working in a treatment plant. As a worker,
you may be exposed to:
1. Physical injuries
2. Infections and infectious diseases
3. Oxygen deficiency
4. Toxic or suffocating gases or vapors
5. Radiological hazards
6. Explosive gas mixtures
7. Fire
8. Electrical shock
9. Noise
12.10 Physical Injuries
The most common of physical injuries are cuts, bruises, scrapes,
and broken bones. Injuries can be caused by moving machinery.
Falls from or into tanks, deep wells, catwalks, or conveyors can
be disabling. Most of these can be avoided by the proper use of
ladders, hand tools, and safety equipment, and by following estab-
lished safety procedures.
12-2
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12.11 Infections and Infectious Diseases1
Although treatment plants and plant personnel are certainly not
expected to be "pristine pure", personal cleanliness is a great
deterrent to infections and infectious diseases. Immunization
shots for protection against typhoid and tetanus are essential.
Make it a habit to thoroughly wash your hands before eating or
smoking, or going to the lavatory. If you have any cuts or
other broken skin areas on your hands, wear proper protective
gloves when in contact with wastewater or sludge in any form.
Bandages covering wounds should be changed frequently.
Do not wear your work clothes home, because diseases may be
transmitted to your family. Provisions should be made in your
plant for a locker room where each employee has a locker. Work
clothes should be placed
or hung in lockers and not
thrown on the floor. Your
work clothes should be
cleaned at least weekly or
more often if necessary.
If your employer does not
supply you with uniforms and
laundry service and you must
take your work clothes home,
launder them separately from
your regular family wash.
All of these precautions will
reduce the possibility of you
and your family becoming ill
because of your contact with
wastewater.
1 You must attempt to avoid skin infections and infectious
diseases such as typhoid fever, dysentery, hepatitis, and
tetanus.
12-3
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12.12 Oxygen Deficiency
Oxygen deficiency may exist in any enclosed, and particularly
below grade (ground level) , unventilated structure where a gas
heavier than air, such as carbon monoxide, has displaced the air.
AN
UKiV£MT
A MANHOLE
STRUCT LJK& WITHOUT
POR OXV^rEN ITBPlO^MCV AND
Ventilation may be provided by fans or blowers. Equipment is
available to measure oxygen deficiency and must be used whenever
you enter a potentially hazardous area. Try your local fire
department for sources of this type of equipment in your area.
12.13 Toxic or Suffocating Gases or Vapors
Toxic or suffocating gases may come from industrial waste dis-
charges or from the decomposition of domestic wastewater. You
must become familiar with the waste discharges into your system.
On pages 174 and 175 of The New York Manual, Table 10, Common
Dangerous Gases Encountered in Sewers and at Sewage Treatment
Plants, contains information on the simplest and cheapest safe
method of testing for gases.
12.14 Radiological Hazards
The newest of hazards to plant operators is a result of the in-
creasing use of radioactive isotopes in hospitals, research labs,
and various industries. Check your sewer service area for the
possible use of these materials. If you are receiving a discharge
that may contain a radioactive substance, contact the contributor
of the discharge. He will usually cooperate with monitoring this
type of waste.
12-4
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12.15 Explosive Gas Mixtures
Explosive gas mixtures may develop in confined areas in treatment
plants from mixtures of air and methane, natural gas, manufactured
fuel gas, or gasoline vapors. Explosive ranges can be detected by
using a combustible gas indicator. Avoid explosions by keeping
open flames away from areas potentially capable of developing ex-
plosive mixtures by providing adequate ventilation with fans or
blowers.
12.16 Fire
Burns from fires can cause very serious injury. Avoid the accumulation
of flammable material and store any material of this type in approved
containers at proper locations. Know the location of fire fighting
equipment and the proper use of the equipment.
12.17 Electrical Shock
Electrical shock frequently causes serious injury. Do not attempt
to repair electrical equipment unless you know what you are doing.
12.18 Noise
Loud noises from gas engines and gas or electric blowers can cause
permanent ear damage. Operators and maintenance men must wear the
proper ear protecting devices whenever working in noisy areas for
any length of time.
QUESTIONS
12.1A How can you prevent the spread of infectious
diseases from your job to you and your family?
12.IB What should you do before entering an unventilated,
enclosed structure?
12.1C What are potential sources of toxic or suffocating
gases or vapors?
12-5
-------
CHAPTER 12. PLANT SAFETY AND GOOD HOUSEKEEPING
(Lesson 2 of 3 Lessons)
12.21 Treatment Plants and Pumping Stations
Because hazards found in pumping stations are identical to those'
found in treatment plants, the items discussed hereafter may be
applied to both situations.
12.210 Headworks
Structures and equipment in this category may consist of bar screens,
racks, comminuting or grinding equipment, pump rooms, wet pits, and
chlorination facilities.
1. Bar Screens or Racks. These may be either manually or auto-
matically cleaned.When manually cleaning screens or racks, be
certain that you have a clean, firm surface to stand upon. Re-
move all slimes, rags, greases, or other material that may cause
you to slip. GOOD HOUSEKEEPING IN THESE AREAS IS MANDATORY.
When raking screens, leave plenty of room for the length of your
rake handle so as not to be thrown off balance by striking a
wall, railing, or light fixture. Wear gloves to avoid slivers
from the rake handle or scraping your knuckles on concrete.
Injury may allow an infection to enter your body.
Place all material in a container that may be easily removed
from the structure. Do not allow material to build up on the
working surface.
If your rack area is provided with railings, check to see that
they are properly anchored before you lean against them. If
removable safety chains are provided, never use these to lean
against or as a means of providing extra leverage for removing
large amounts of material.
A hanging or mounting bracket of some type should be used to
hold the rake when not in use. Do not leave it lying on the
deck.
If mechanically raked screens or racks are installed, never
work on the electrical or mechanical part of this equipment
12-6
-------
without first turning the unit off by means of a push-
button lockout for momentary stoppages, and by turning
off, locking out, and tagging the main circuit breaker
if it is necessary to remove or make a major adjustment
or repair to the unit.
TO
AMP
NOf
MAM
%N
The time and date the unit was turned off should be noted
on the tag, as well as the reason it was turned off. The
tag should be signed by the man who turned the unit off,
No one should then turn on the main breaker and start the
unit until the tag has been removed by the person who placed
it there, or until he has specific instructions from the
person who tagged the breaker. Your local safety equipment
supplier can obtain these tags for you.
2. Comminuting or Grinding Equipment. This equipment may consist
of barminutors, comminutors, grinders, or disintegrators.
NEVER work on the mechanical or electrical parts of the unit
without first locking out the unit at either a push-button
lockout or the main circuit breaker of the control panel.
Be certain the breaker is properly tagged as explained in the
previous section.
Good housekeeping is essential in the area of comminuting
equipment. Keep all walking areas clean and free of slimes,
oils, greases, or other materials. Hose down all spills
immediately. Provide a proper place for equipment and tools
used in this area.
See that proper guards are installed and kept in place around
cables, cutters, hoists, revolving gears, and high-speed
equipment such as grinders. If it is necessary to remove the
guards prior to making adjustments on equipment, be certain
that they are reinstalled before restarting the unit.
12-7
-------
DO NOT
START
THIS EQUIPMENT
BEING
\
REPAIRED
STATE COMPENSATION
INSURANCE FUND
OF
CALIFORNIA
Fig. 12.2 Typical warning tag
(Source: State Compensation Fund of California)
12-8
-------
MAN
WORKING
ON LINE
DO NOT CLOSE THIS
SWITCH WHILE THIS
TAG IS DISPLAYED
SIGNATURE:
This is the ONLY person authorized to remove this tag.
INDUSTRIAL INDEMNITY/INDUSTRIAL UNDERWRITERS/
INSURANCE COMPANIES
Z& 4E210— R66
Fig. 12.3--Typical Warning Tag (Con't).
Source: Industrial Indemnity/Industrial Underwriters/Insurance Cos,
12-9
-------
3. Pump Rooms, The same basic precautions apply here as they
do to any type of enclosed room or pit where wastewater or
gases may enter and accumulate.
Always provide adequate ventilation to remove gases and supply
oxygen. If the room is below ground level and provided with
only forced air ventilation, be certain the fan is on before
entering the area. Wear a harness with a safety line (as
for manhole work) when entering pits, wet wells, tanks, and
below-ground pump rooms.
The tops of all stairwells or ladders should be protected by
a removable safety chain. Keep this chain in place when the
stairwell or ladder is not being used.
Never remove guards from pumps, motors, or other equipment
without first locking out or turning off equipment at main
breaker and properly tagging. Always replace all guards
before starting units.
Guards should be installed around all rotating shaft couplings,
belt drives, or other moving parts normally accessible.
Maintain good housekeeping in pump room. Remove all oil and
grease, and clean up spills immediately.
If you have a multi-level pump building, never remove and
leave off equipment removal hatches unless you are actually
removing or replacing equipment. Be sure to provide barricades
or ropes around the opening to prevent falls. Be extremely
cautious when working around openings that have raised edges.
These are hazardous because you can stumble over them easily.
Never start a positive displacement pump against a closed valve.
On piston pumps, the yoke over the ball check could break and
endanger personnel in the vicinity.
All emergency lights used in these areas should be explosion
proof. Be sure to keep light shields in place and replace
immediately when broken. Permanent lights should be of an
approved explosion-proof type. Until the area has been checked
for an explosive atmosphere, NO OPEN FLAMES (such as a welding
torch) OR SMOKING SHOULD BE ALLOWED.
12-10
-------
CAUTION
A
our of- TH
^. HP VOU PO NOT
015 A^& MOT FAMILIAR W iTH TW^ 6QU \PAAfisn;
4. Wet Pits—Sumps. Covered wet pits or sumps are potential
death traps. Never enter one by yourself. Use a safety
harness and have sufficient personnel available to lift
you out. Always use forced air to ventilate the area, and
check for explosive gases and oxygen deficiency before
entering. Also, be particularly alert for hydrogen sulfide
gas. Use your nose initially, but do not continue to depend
upon it as you will become insensitive to the odor. A
small, reasonably priced hydrogen sulfide detection unit
may be purchased. Check with your local safety equipment
supplier.
After you have determined the atmosphere is safe, use extreme
care in climbing up and down access ladders to pit areas. The
application of a nonslip type coating on ladder rungs is
helpful. If available, a truck hoist is safer than a ladder
for entering pit areas.
Watch your footing on the floor of pits and sumps. They are
very slippery.
Never attempt to carry tools or equipment up or down ladders
into pits or sumps. Always use bucket and handline or sling
for this purpose.
Only explosion-proof lights and equipment should be used in
these areas.
A good safety practice is to turn off all chlorination, whether
located upstream or directly in sump, and allow ample time
before entering the area. This, with forced ventilation, will
give time for the area to be cleared of chlorine fumes.
12-11
-------
12.211 Grit Chambers
Grit chambers may be of various designs, sizes, and shapes;
but they all have one thing in common: they get dirty. Good
housekeeping is neededl Keep walking surfaces free of grit,
grease, oil, slimes, or other material that will make a slippery
surface.
Before working on mechanical or electrical equipment, be certain
that it is turned off and properly tagged (Figs. 12.2 and 12.3).
Install and maintain guards on gears, sprockets, chains, or other
moving parts that are normally accessible.
If it becomes necessary to enter the chamber, pit, or tank for
cleaning or other work, do so with extreme caution. If this is
a covered area, provide and maintain adequate ventilation to
remove gases from the area and to supply oxygen to the workers.
Use only explosion-proof lights. Always check for explosive
gases and oxygen deficiency before entering.
Be sure of your footing when working in these structures.
Rubber boots with a nonskid cleat type sole should be worn.
Step slowly and cautiously as there is usually an accumulation
of slippery material or slimes on the bottom. Use hand holds
and railings; if none are available, install them now.
\
Use ladders, whether vertical or ships ladders, cautiously.
If possible, apply nonslip material or coatings to ladder rungs,
Keep handrails free of grease and other slippery substances.
If it is necessary to take tools or equipment into the bottom
area, lower these in a bucket or sling by handline. Never
attempt to carry items up or down a ladder.
Chlorination safety is discussed in Chapter 10.
12-12
-------
12.212 Clarifiers or Sedimentation Basins
The greatest hazard involved in working on or in a clarifier
is the danger of slipping. If possible, maintain a good non-
skid surface on all stairs, ladders, and catwalks. This may
be done by using nonskid strips or coating. Be extremely
cautious during freezing weather. A small amount of ice can be
very dangerous.
Your housekeeping program should include the brushing or clean-
ing of effluent weirs and launders (effluent troughs). When
it is necessary to actually climb down into the launder, always
wear a harness with a safety line and have someone with you. A
fall may result in a very serious injury.
Be cautious when working on the bottom of a clarifier. When
hosing down, always hose a clean path to walk upon. Avoid
walking on the remaining sludge whenever possible.
Always turn off and lock out or turn off and tag clarifier
breaker before working on drive unit. If necessary, adjustments
may be made on flights or scrapers while the unit is in operation;
but keep in mind that, although these are moving quite slowly,
there is tremendous power behind their movement. Stay clear of
any situation where your body or the tools you are using may get
caught under one of the flights or scrapers.
Guards should be installed over or around all gears, chains,
sprockets, belts, or other moving parts. Keep these in place
whenever the unit is in operation.
Railing should be installed along the tank side of all normal
walkways. If the unit is elevated above ground, railings should
be installed along the outside of all walkways, also. Check with
your State Safety Office for requirements on railing installation.
12.213 Digesters and Digestion Equipment4
Digesters and their related equipment include many hazardous areas
and potential dangers.
Also see "Safe Work Procedure No. 2, Entering and Working in
Digesters", Jour. Water Poll. Control Fed., Vol. 42, No. 3,
Part 1, p 466 (March 1970).
12-13
-------
No smoking and no open flames should be allowed in the vicinity
of digesters, in digestion control buildings, or in any other
areas or structures used in the sludge digestion system. This
includes pipe galleries, compressor or heat exchanger rooms,
and others. All these areas should be posted with signs in a
conspicuous place which forbid smoking and open flames. Methane
gas produced by anaerobic conditions is explosive when mixed with
the proper proportion of air.
All enclosed rooms or galleries in this system should be well
ventilated with forced air ventilation. Before entering any
enclosed area or pit which is not ventilated, a check should be
made for explosive gases and hydrogen sulfide. Do not depend
upon your nose for hydrogen sulfide (H2S) detection in these
areas. A small amount of H2S in the air will make your sense
of smell immune to the odor in a short period of time. Use an
H2S detector.
When you are working in these areas, forced air ventilation with
a portable blower should be provided. Again, do not go into an
area by yourself where H2S is present. Have someone watch you.
Never enter a partially empty or completely empty digester with-
out first thoroughly ventilating the structure and then checking
for an explosive atmosphere and the presence of hydrogen sulfide
gas. Explosion-proof lights and nonsparking tools5 and shoes
should always be used when working around, on top of, or in a
digester unless it has been completely cleaned and emptied,
continuously ventilated by a blower, and constant checks are
made of the atmosphere in the tank.
WITHIN AM
Be certain that guardrails are installed along the edges of the
digester roof or cover in areas where it is necessary to work
close to the edge. A fall from the top of a digester could be
fatal.
5 Nonsparking tools are especially manufactured for use in areas
where potentially explosive mixtures of gases may be present.
12-14
-------
Explosion blew off top of digester
and landed on top of pickup truck
Fig. 12.4 Blown-up digester
12-15
-------
When working on equipment such as draft tube mixers, compressors,
diffusers, etc., be certain that the unit which operates or
supplies gas to these types of equipment is properly locked out
and appropriately tagged (Figs. 12.2 and 12,3).
If you have a heated digester, read and heed the manufacturer's
instructions before working on the boiler or heat exchanger.
Know that the gas valve is turned off before attempting to
light the pilot. Be certain that the fire box has been venti-
lated according to the manufacturer's instructions before
lighting the pilot,
CAUTION
C5A4
OUT i N A MOP£r£AT-e w i N p.
ATTEMPT TO EetiGHTT+AE: UNIT,
THAT T-H^ MAIH VALVe
ANC7 TH£" 4TACK. ALU2W&P TO
A p£W M/NUT£4,. MAMV
T-H^lie HAltZ ANP
RZOMA
f-ROM
When it becomes necessary to clean tubes or coils in a heat
exchanger, turn the unit supplying hot water off far enough in
advance to allow the heat exchanger to cool. Never dpen the
unit without doubly checking the water and sludge temperatures.
Be certain that they have cooled down to body temperature or
lower.
Before working on any sludge pump, whether it is centrifugal
or positive displacement, be certain; that the unit is turned off
and properly tagged (Figs. 12.2 and 12.3).
Positive displacement pumps should be equipped with an air
chamber and a pressure switch to shut the unit off at a pre-
set pressure. Never start a positive displacement pump against
a closed discharge valve because pressure could build up and
burst a line or damage the pump. If you have closed this valve
in order to inspect or clean the pump, double check to be sure
that it is open before starting the unit.
12-16
-------
Sludge pump rooms should be well ventilated to remove any
gases that might accumulate from leakage, spillage, or from
a normal pump cleaning,, If you spill digesting sludge, clean
it up immediately to prevent the possible accumulation of gases.
Provide thorough^ regularly scheduled inspection and maintenance
of your gas collection system, Inspect drip traps regularly.
The so-called "automatic" drip trap is known to jam open fre-
quently, allowing gas to escape.
Good maintenance of flame arrestors will ensure that they will
be able to perform their job of preventing a backflash of the
flame.
QUESTIONS
12.2J How can the danger of slipping be reduced on
slippery surfaces?
12.2K Why should no smoking or open flames be allowed
in the vicinity of digesters?
12.21 What safety precautions would you take before
entering a recently emptied digester?
12,2M What would you do before relighting a waste gas
burner?
12.2N Why should you never start a positive displacement
pump against a closed discharge valve?
12-17
-------
12.214 Trickling Filters
When it becomes necessary to inspect or service a rotating
distributor, stop the flow of wastewater to the unit and allow
it to come to rest.
NeVBC, -4TAMP 0& WALk: OH THE
AA&PlA WHIU£ TU£ CCO>cTiM6
1^7 IN MOTION.
Provide an approved ladder or stairway for access to the media
surface. Be positive this is free from obstructions such as
hose bibs, valve stems, etc.
Extreme caution should be used when walking on the filter media.
The biological slimes make the media very slippery. Move
cautiously and be certain of your footing.
ALLOW AMVOKJ^ TO Rl (7£- A
Although a rotating distributor moves fairly slowly, the force
behind it is powerful. An operator who has fallen off and been
dragged by a distributor is fortunate if he can walk away under
his own power.
WARNING
TOXIC.
Always wear rubber gloves when handling mercury. When cleaning
mercury, follow the manufacturer's recommendations. Do so only
in the open in a well-ventilated room. Be sure to have a tray
under the working area during mercury clean-up. It is extremely
difficult to recover mercury from the floor. Dry mercury vaporizes
slowly, and mercury vapors also are toxic.
12-18
-------
Refrain from smoking and eating when handling mercury. Always
wash your hands thoroughly when finished.
When inspecting underdrains, check to determine that the
channels or conduits are adequately ventilated. Gases are not
normally a problem here, but may be if there is a build-up of
solids which have become septic.
If it becomes necessary to jack up a distributor mechanism
for inspection or repair, always provide a firm base off the
media or drainage system for the jack plate. A firm base may
be provided by wooden planks which will spread the weight over
a large area. However, sometimes the only way to obtain firm
support is to remove the media and use the drainage system as
a firm base. Remember you are lifting a heavy weight. Do not
attempt inspection or repair work until the distributor has been
adequately and properly blocked in its raised position.
12.215 Aerators
Guardrails should be installed on the tank side of usual work
areas or walkways. If the tank is elevated above ground, guard-
rails should also be installed on the ground side of the tank.
An operator should never go into unguarded areas by himself.
When working on Y-walls, or other unguarded areas where work
is done infrequently, at least a two-man team should do the work.
Approved life preservers with permanently attached hancllines
should be accessible at strategic locations around the aerator.
You should wear a safety harness with a life line when servicing
aerator spray nozzles and other items around an aerator.
An experiment in England found that if an operator fell into a
diffused aeration tank, he should be able to survive because air
will collect in the clothing and tend to help keep him afloat.6
Drownings apparently occur when a person is overcome by the
initial shock or there is nothing to grab hold of to keep afloat
or to pull oneself out of the aerator.
When removing or installing diffusers, be aware of the limitations
of your working area. Inspect and properly position hoists and
other equipment used in servicing swing diffusers.
6 Kershaw, M.A., "Buoyancy of Aeration Tank Liquid", Jour.
Water Poll. Control Fed., Vol. 33, No. 11, p 1151 (Nov. 1961).
12-19
-------
When it is necessary to work in an empty aerator, lower your-
self into the aerator with a truck hoist if one is available.
Ladders are awkward and dangerous; but if portable ladders must
be used, properly position them so that they will not slip or
twist. A good practice is to tie the top of the ladder so that
it cannot slip. Be extremely careful when using fixed ladders
as they become very slippery. The floor of the aerator also
is likely to be extremely slippery.
If your plant is in an area subject to freezing weather, be
aware of possible ice conditions around these units and use
caution accordingly.
12.216 Ponds
Ponds of any kind present basically the same hazards. Therefore,
the following safety measures will apply to ponds in general.
If it is necessary to drive a vehicle on top of the pond levees,
maintain the roadway in good driving condition by surfacing it
with gravel of asphalt. Do not allow chuck holes or the formation
of ruts. Be extremely cautious in wet weather. The material
used in the construction of most levees becomes very slippery
when wet. Slippery conditions should be corrected using crushed
rock or other suitable material.
Never go out on the pond for sampling or other purposes when by
yourself. Someone should be standing by on the bank in case you
get into trouble. Always wear an approved life jacket when
working from a boat or raft on the surface of the pond. And,
as in any boating activity, do not stand up in the boat while
performing work.
I
12-20
-------
12.217 Chlorine
.
4-WXIsJPL.B WITH CAUTION /
The most common causes of accidents involving chlorine gas are
leaking pipe connections and over-chlorinating.
Chlorine bottles or cylinders should be stored in a cool, dry
place, away from direct sunlight or from heating units. Some
heat is needed to cause desired evaporation and to control
moisture condensation on tanks. Chlorine bottles or cylinders
should never be dropped or allowed to strike each other with
any force. Cylinders should be stored in an upright position
and secured with a chain, wire rope, or clamp. They should
be moved only by hand truck and should be well secured during
moving. One-ton tanks should be blocked so that they cannot
roll. They should be lifted only by an approved lifting bar
with hooks over the ends of the containers. Never lift a
bottle or cylinder with an improvised sling.
Connections to cylinders and tanks should be made only with
approved clamp adaptors or unions. Always inspect all surfaces
and threads of the connector before making connection. If you
are in doubt as to their conditions, do not use the connector.
Always use a new approved type gasket when making a connection.
The reuse of gaskets very often will result in a leak. Check
for leaks as soon as the connection is completed. Never wait
until you smell chlorine. If you discover even the slightest
leak, correct it immediately, as leaks tend to get worse rather
than better. Like accidents, chlorine leaks generally are caused
by faulty procedure or carelessness.
Obtain from your chlorine supplier and post in a conspicuous
place (outside the chlorination room) the name and telephone
number of the nearest emergency service in case of severe leak.
Cylinder storage and chlorinator rooms should be provided with
means of ventilating the room. As chlorine is approximately two
and a half times heavier than air, vents or an exhaust fan should
be provided at floor level. Ideal installations have a blower
mounted on the roof to blow air into the room and are vented at
the floor level to allow escaped chlorine to be blown out of the
building.
12-21
-------
Always enter enclosed cylinder storage or chlorinator rooms
with caution. If you smell chlorine when opening the door to
the area, immediately close the door, turn on ventilation,
and seek assistance.
Never attempt to enter an atmosphere of chlorine when by
yourself or without an approved air supply and protective
clothing. Aid can usually be obtained from your local fire
department, which will normally have available a self-
contained breathing apparatus which will allow a person to
enter safely into an atmosphere of chlorine.
An excellent booklet may be obtained from PPG Industries, Inc.,
Chlorine—Safe Handling.7 Safety information on chlorine
handling is also contained in Chapter 10, Disinfection and
Chlorination. Your local chlorine supplier will probably pro-
vide you with all the information you need to handle and use
chlorine safely. It is your responsibility to obtain, read,
and understand safety information and to practice safety.
12,218 Applying Protective Coatings
CAUTION! When applying protective coatings in a clarifier
or any other tank or pit, whether enclosed or open topped, use
protective equipment to prevent skin burns from vapors from
asphaltic or bitumastic coatings. This may involve the use of
protective clothing as well as protective creams to be applied
to exposed skin areas. An air supply must be used when paint-
ing inside closed vessels or in an open deep tank. Many paint
fumes are heavier than air; therefore, ventilation must be from
the bottom upward.
Check with your paint supplier for any hazards involved in
using his products.
7 PPG Industries, Inc., Chemical Division, One Gateway Center,
Pittsburgh, Pennsylvania 15222.
12-22
-------
CHAHI-.i- .17. PLANT SAF-im AN)GOODH)USEKElgP.ING
12,3 SAFETY IN THE LABORATORY0
In addition to all safety practices and procedures mentioned in
the previous sections of this chapter, the collecting of samples
and th? performance of laboratory tests require that you be aware
of the ?p°'l':ln hazards involve'? in this typ*? of "X'rk.
Laboratories us« many hazardouc chemicals. The?" v.hQmif °1? sh
be kept in I'mjt.ed amounts and u/wl with respect;, You.v i'hv
supplier p-',y be able to ?"pply you with a safety -^annnj ,
12.30 Collecting Samples
•=:--?• .?-^ l_i <-'JHT=£35ica«»OTqr=A»3-J-- n»~>
Whenever possibleD rubber gloves should be worn when your hands
may come in direct con*:act with wastewat.er or sludge. When you
have finished sampling, always wash the gloves thoroughly before
removing tl^m. After removing the gloves, v^sh your hands
thoroughly, using a disinfectant type soap.
CQU&CT ANY ^AM'PLB^ WITH
HANP^ \f:
4KIN
-J
Do not climb over or go beyond guardrails or chains when collecting
samples, U?« sapipJe po]ns, ropes, etc.,, ?.r necessary to collect
sampler.
8 Also see "CRC Handbook of Laboratory Safety", by Norman V.
Steere, Chemical Rubber Publishing Company, 18901 Cranwood
Parkway, Cl^velandj, Ohio 44128. Price $24.50.
12-23
-------
12.31 Equipment Set-Up and Performance of Tests
Following are some basic procedures to follow when working in
the laboratory:
1. Use proper safety goggles or face shield in all tests where
there is danger to the eyes.
IOOV- /NTO T"H£: OP£"N ErMP OP-
A dDWTAWER PUWNkEr A REACTION OR
Use care in making rubber-to-glass connections. Lengths of
glass tubing should be supported while they are being inserted
into rubber. The ends of the glass should be flame polished9
to smooth them out, and a lubricant such as water should be
used. Never use grease or oil. Gloves or some other form of
protection for the hands should be used when making such
connections. The tubing should be held as close to the end
being inserted as possible to prevent bending or breaking.
Never try to force rubber tubing or stoppers from glassware.
Cut the rubber as necessary to remove it.
Always check labels on bottles to make sure that the proper
chemical is selected. Never permit unlabeled or undated con-
tainers to accumulate around or in the laboratory. Keep
storage areas organized to facilitate chemical selection for
use. Clean out old or excess chemicals. Separate flammable,
explosive, or special hazard items for storage in an approved
manner. See Section 12.9, Additional Reading, Reference 10.
LASE-UEP, IN PI CATl M6 COMTEK^
AMP PAT£- gOTTl_-£ \A//v5- OPE-h4B^
?. ALL
AMP AN\TlPOT£:.
9 Flame Polished, Sharp or broken edges of glass (such as the end
of a glass tube) are flame polished by placing the edge in a flame
and rotating it. By allowing the edge to melt slightly, it will
become smooth.
12-24
-------
4. Never handle chemicals with the bare hand. Use a spoon or
spatula for this purpose.
5. Be sure that your laboratory is adequately ventilated.
A
\\OOV
HAVING-
Even mild concentrations of fumes or gases can be dangerous.
6. Never use laboratory glassware for a coffee cup or food dishc
This is particularly dangerous when dealing with wastewaters.
7. When handling hot equipment of any kind, always use tongs, as-
bestos gloves, or other suitable tools. Burns can be painful
and can cause more problems (encourage spills, fire, and shock).
8. When working in the lab, avoid smoking and eating except at
prescribed coffee breaks or at the lunch period.
4AA0^1 N 6 OR EATIN Gr.
9. Do not pipette chemicals or wastewater samples by mouth. Always
use a suction bulb on an automatic burette.
10. Handle all chemicals
and reagents with
care. Read and be-
come familiar with
all precautions or
warnings on labels.
Know and have avail-
able the antidote
for all poisonous
chemicals in your
lab.
11. A short section of
rubber tube on each
water outlet is an
excellent water
flusher to wash
away harmful
12-25
-------
chemicals from the eyes and skin. It is easy to reach and
can quickly be directed on the exposed area. Eyes and
skin can be saved if dangerous materials are washed away
quickly.
12. Dispose of all broken or cracked glassware immediately.
Chipped glassware may still be used if it is possible to
fire polish the chip in order to eliminate the sharp edges.
This may be done by slowly heating the chipped area until
it reaches a temperature at which the glass will begin to
melt. At this point remove from flame and allow to cool.
AW
Always use a suitable glove or tool.
13.
14. Wear a protective smock or apron when working in the lab.
This may save you the cost of replacing your work clothes
or uniform. Protective eye shields should be worn too.
QUESTIONS : :
12.3A What safety precautions would you take when collecting
laboratory samples from a plant influent?
12.3B Why should you always wash your hands before eating?
12,3C Why should chemicals and reagents be handled with care?
12-26
-------
12.5 WATER SUPPLIES
Inspect your plant to see if there are any cross-connections
between your potable (drinking) water and items such as water
seals on pumps, feed water to boilers, hose bibs below grade
where they may be subject to flooding with wastewater or sludges,
or any other location where wastewater could contaminate a
domestic water supply.
If any of these or other existing or potential cross-connections
are found, be certain that your drinking water supply source is
properly protected by the installation of an approved back-flow
prevention device.
It is a good practice to have your drinking water tested at
least monthly for coliform group organisms. Sometimes the best
of back-flow prevention devices do fail.
You may find in your plant that it will be more economical to
use bottled drinking water. If so, be sure to tack up con-
spicuous signs that your water is not drinkable. This also
applies to all hose bibs in the plant from which you may obtain
water other than a potable source. This is a must in order to
inform visitors or absent-minded or thirsty employees that the
water from each marked location is not for drinking purposes.
QUESTION
12.5A Why do some wastewater treatment plants use bottled
water for drinking purposes ?
12-27
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12.6 SAFETY EQUIPMENT AND INFORMATION
Post conspicuously on your bulletin board the location and types
of safety equipment available at your plant (such as first aid
kit, breathing apparatus, explosiometers, etc.). You, as the
plant operator, should be thoroughly familiar with the operation
and maintenance of each piece of equipment. You should review
these at fixed intervals to be certain that you can safely use
the piece of equipment as well as to be sure that it is in
operating condition.
Contacts should be made with your local fire and police depart-
ments to acquaint them with hazards at your plant as well as to
inform them of the safety equipment that is necessary to cope
with problems that may arise. Quite often it is possible to
arrange a joint training session with these people in the use
of safety equipment and the handling of emergencies. They also
should know access routes to and around the treatment plant.
If you have any specific problems of a safety nature, do not
hesitate to contact officials in your state safety agency. They
can be of great assistance to you. And do not forget your equip-
ment manufacturers; their familiarity with your equipment will
be of great value to you.
Also posted in conspicuous places in your plant should be such
information as the phone numbers of your fire and police depart-
ments, ambulance service, chlorine supplier or repairman, and
the nearest doctor who has agreed to be available on call.
Having these immediately available at telephone sites may save
your or a fellow worker's life. Check and make sure these
numbers are listed at your plant. If they are not listed,
ADD THEM NOW.
QUESTION
12.6A What emergency phone numbers should be listed in a
conspicuous place in your plant?
12-28
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14o3 SAMPLING, by Joe Nagano„ from California Water Pollution
Control Association Operators Laboratory Manual
14,30 Importance
Before any laboratory tests are performed, it is highly important
to obtain a proper, representative sample. Without a representative
sample, a test should not even be attempted because the test result
will be incorrect and meaningless. A laboratory test without a good
sample will most likely lead to erroneous conclusions and confusion,
The largest errors produced in laboratory tests are usually caused
by improper sampling, poor preservation/ or lack of enough mixing""
during compositing0 and testing.
14,31 Accuracy of Laboratory Equipment
Laboratory equipment, in itself, is generally quite accurate.
Analytical balances weigh to 0.1 milligram. Graduated cylinders,,
pipettest and burettes usually measure to 1% accuracy, so that the
errors introduced by these items should total less than 5%, and
under the worst possible conditions only 10%. Under ideal conditions
let us assume that a test of raw wastewater for suspended solids
should run about 300 mg/1. Because of the previously mentioned
equipment or apparatus variables, the value may actually range
from 270 to 330 mg/1. Results in this range are reasonable for
operation. Other less obvious factors are usually present which
make it quite possible to obtain results which are 25, 50, or even
100% in error, unless certain precautions are taken. Some examples
will illustrate how these errors are produced.
The City of Los Angeles Terminal Island Treatment Plant is a
primary treatment facility with a flow of 8 million gallons per
day. It has an aerated grit chamber, two circular 85-foot clari-
fiers of 750,000 gallon capacity, and two digesters 100 and 75 feet
in diameter.
5 Composite (Proportional) Samples (com-POZ-it). Samples collected
at regular intervals in proportion to the existing flow and then
combined to form a sample representative of the entire period of
flow over a given period of time.
14-1
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Monthly summary calculations based upon the suspended solids test
showed that about 8,000 pounds of suspended solids were being
captured per day during sedimentation assuming 200 mg/1 for the
influent and 100 mg/1 for the effluent. However, it also appeared
that 12,000 pounds per day of raw sludge solids were being pumped
out of the clarifier and to the digester. Obviously, if sampling
and analyses had been perfect, these weights would have balanced.
The capture should equal the removal of solids. . A study was made
to determine why the variance in these values was so great. It
would seem logical to expect that the problem could be due to
(1) incorrect testing procedures, (2) poor sampling, (3) incorrect
metering of the wastewater or sludge flow, or (4) any combination
of the three or all of them.
In the first case, the equipment was in excellent condition.
The operator was a conscientious and able employee who was
found to have carried out the laboratory procedures carefully
and who had previously run successful tests on comparative
samples. It was concluded that the equipment and test proce-
dures were completely satisfactory.
14.32 Selection of a Good Sampling Point to Obtain
a Representative Sample
A survey was then made to determine if sampling stations were in
need of relocation. By using Imhoff cones and running settleable
solids tests along the influent channel and the aerated grit
chamber, one could quickly recognize that the best mixed and
most representative samples were to be taken from the aerated
grit chamber rather than the influent channel.
The settleable solids ran 13 ml/1 in the aerated grit chamber
against 10 ml/1 in the channel. By the simple process of
determining the best sampling station, the suspended solids
value in the influent was corrected from 200 mg/1 to the more
representative 300 mg/1. Calculations, using the correct
figures, changed the solids capture from 8,000 pounds to 12,000
pounds per day and a balance was obtained.
This study clearly illustrates the importance of selecting a
good sampling point in securing a truly representative sample.
It emphasizes the point that even though a test is accurately
performed, the result may be entirely erroneous and. meaning Less
insofar as use for process control is concerned, unless a good
representative sample is taken. Furthermore, a good sample is
highly dependent upon the sampling station. Whenever possible,
14-2
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select a place where mixing is thorough and the wastewater quality
is uniform. As the solids concentration increases, above about
200 mg/1, mixing becomes even more significant because the waste-
water solids will tend to separate rapidly with the heavier solids
settling toward the bottom, the lighter solids in the middle, and
the floatables rising toward the surface. If, as is usual, a
one-gallon portion is taken as representative of a million-gallon
flow, the job of sample location and sampling must be taken
seriously.
14.33 Time of Sampling
Let us consider next the time and frequency of sampling. In
carrying out a testing program, particularly where personnel
and time are limited due to the press of operational responsi-
bilities, testing may necessarily be restricted to about one
test day per week. If the operator should decide to start his
tests early in the week, by taking samples early on Monday
morning he may wind up with some very odd results.
One such incident will be cited. During a test for ABS (alkyl
benzene sulfonate), samples were taken early on Monday morning
and rushed into the laboratory for testing. Due to the detention
time in the sewers, these wastewater samples actually represented
Sunday flow on the graveyard shift, the weakest wastewater obtain-
able. The ABS content was only 1 mg/1, whereas it would normally
run 8 to 10 mg/1. So the time and day of sampling is quite important,
and the samples should be taken to represent typical weekdays or
even varied from day to day within the week for a good cross-section
of the characteristics of the wastewater.
14.34 Compositing and Preservation of Samples
Since the wastewater quality changes from moment to moment and
hour to hour, the best results would be obtained by using some
sort of continuous sampler-analyzer. However, since operators
are usually the sampler-analyzer, continuous analysis would
leave little time for anything but sampling and testing. Except for
tests which cannot wait due to rapid chemical or biological change
of the sample, such as tests for dissolved oxygen and sulfides, a
fair compromise may be reached by taking samples throughout the
day at hourly or two-hour intervals.
When the samples are taken, they should be immediately refrigerated
to preserve them from continued bacterial decomposition. When all
of the samples have been collected for a 24-hour period, the samples
from a specific location should be combined or composited together
according to flow to form a single 24-hour composite sample.
14-3
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To prepare a composite sample, (1) the rate of wastewater flow
must be metered and (2) each grab sample must then be taken
and measured out in direct proportion to the volume of flow
at that time. For example, Table I illustrates the hourly flow
and sample volume to be measured out for a 12-hour proportional
composite sample.
TABLE I
DATA COLLECTED TO PREPARE PROPORTIONAL COMPOSITE SAMPLE
Flow
Time MGD Factor Sample Vol
Flow
Time MGD Factor Sample Vol
6 AM 0.2
7 AM 0.4
8 AM 0.6
9 AM 1.0
10 AM 1.2
11 AM 1.4
100
100
100
100
100
100
A sample composited in
20
40
60
100
120
140
this
12 N 1.5 100
1 PM 1.2 100
2 PM 1.0 100
3 PM 1.0 100
4 PM 1.0 100
5 PM 0.9 100
manner would total 1140 ml.
150
120
100
100
100
90
1140
Large wastewater solids should be excluded from a sample, particu-
larly those greater than one-quarter inch in diameter.
A very important point should be emphasized. During compositing
and at the exact moment of testing, the samples'must be Vigorously
remixed so^ that they will be of the same composition ancT as well
mixed as when they were originally sampled. Sometimes such remixing
may become lax, so that all the solids are not uniformly suspended.
Lack of mixing can cause low results in samples of solids that
settle out rapidly, such as those in activated sludge or raw waste-
water. Samples must therefore be mixed thoroughly and poured
quickly before any settling occurs. 'If this is not done, errors
of 25 to 50% may easily occur. For example, on the same mixed
liquor sample, one person may find 3,000 mg/1 suspended solids
while another person may determine that there are only 2,000 mg/1
due to poor mixing. When such a composite sample is tested, a
reasonably accurate measurement of the quality of the day's flow
can be made.
If a 24-hour sampling program is not possible, perhaps due to
insufficient personnel or the absence of a night shift, single
representative samples should be taken at a time when typical
characteristic qualities are present in the wastewater. The
samples should be taken in accordance with the detention time
14-4
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required for treatment. For example, this period may exist
between 10 AM and 5 PM for the sampling of raw influent. If
a sample is taken at 12 Noon, other samples should be taken
in accordance with the detention periods of the serial processes
of treatment in order to follow this slug of wastewater or plug
flow. In primary settling, if the detention time in the pri-
maries is two hours, the primary effluent should be sampled at
2 PM. If the detention time in the succeeding secondary treat-
ment process required three hours, this sample should be taken
at 5 PM.
14.35 Sludge Sampling
In sampling raw sludge and feeding a digester, a few important
points should be kept in mind as shown in the following illus-
trative table.
For raw sludge from a primary clarifier at Los Angeles' Terminal
Island Plant, the sludge solids varied considerably with pumping
time as shown by samples withdrawn every one-half minute.
TABLE II
DECREASE IN PERCENT TOTAL SOLIDS DURING PUMPING
Pumping Time
In Minutes
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Total Solids
Percent
7.0
7.1
7.4
7.3
6.7
5.3
4.0
2.3
2.0
1.5
Cumulative
Solids
Average
7.0
7.1
7.2
7.2
7.1
6.8
6.4
5.9
5.5
5.1
14-5
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Table II shows that the solids were heavy during the first
2.5 minutes, and thereafter rapidly became thinner and
watery. Since sludge solids should be fed to a digester
with solids as heavy as possible and a minimum of water,
the pumping should probably have been stopped at about
3 minutes. After 3 minutes, the water content did become
greater that desirable.
In sampling this sludge, the sample should be taken as a
composite by mixing small equal portions taken every 0.5
minutes during pumping. If only a single portion of sludge
is taken for the sample, there is a chance that the sludge
sample may be too thick or too thin, depending upon the
moment the sample is taken, A composite sample will pre-
vent this possibility.
It should also be emphasized again that as a sludge sample
stands, the solids and liquid separate due to gasification
and flotation or settling of the solids, and that it is
absolutely necessary to thoroughly remix the sample back
into its original form as a mixture before pouring it for
a test.
When individual samples are taken at regular intervals
in this manner, they should be carefully preserved to
prevent sample deterioration by bacterial action. Re-
frigeration is an excellent method of preservation and
is generally preferable to chemicals since chemicals may
interfere with tests such as BOD and COD.
14.36 Sampling Devices
Automatic sampling devices are wonderful timesavers and should be
employed where possible. However, like anything automatic,
problems of which the operator should be aware do arise in their
use. Sample lines to auto-samplers may build up growths which
may periodically slough off and contaminate the sample with a
high solids content. Very regular cleanout of the intake line
is required. Another problem occurred at Los Angeles' Hyperion
Plant when the reservoir for the automatic sampler was attacked
by sulfides. Metal sulfides flaked off and entered the sample
container producing misleading high solids results. The
reservoir was cleaned and coated with coal-tar epoxy and little
further difficulty has been experienced.
14-6
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Manual sampling equipment includes dippers, weighted bottles,
hand-operated pumps, and cross-section samplers. Dippers con-
sist of wide-mouth corrosion resistant containers (such as
cans or jars] on long handles that collect a sample for testing.
A weighted bottle is a collection container which is lowered
to a desired depth. At this location a cord or wire removes
the bottle stopper so the bottle can be filled. Sampling pumps
allow the inlet to the suction hose to be lowered to the sampling
depth. Cross-sectional samplers are used to sample where the
wastewater and sludge may be in layers, such as in a digester or
clarifier. The sampler consists of a tube, open at both ends,
that is lowered at the sampling location. When the tube is at
the proper depth, the ends of the tube are closed and a sample
is obtained from different layers.
Many operators build their own sampler (Fig. 14.1) using the
material described below:
1. Samp 1 ing Bucket. A coffee can attached to an e'ight-foot
length of 1/2-inch electrical conduit or a wooden broom
handle with a 1/4-inch diameter spring in a four-inch loop.
2. Sampling Bottle. Plastic bottle with rubber stopper equipped
with two 3/8-inch glass tubes, one ending near bottom of
bottle to allow sample to enter and the other ending at the
bottom of the stopper to allow the air in the bottle to
escape while the sample is filling the bottle.
For sample containers, wide-mouth plastic bottles are recommended.
Plastic bottles, though somewhat expensive initially, not only
greatly reduce the problem of breakage and metal contamination,
but are much safer to use. The wide-mouth bottles ease the
washing problem. For regular samples, sets of plastic bottles
bearing identification labels should be used.
14.37 Summary
1. Representative samples must be taken before any tests are
made.
2. Select a good sampling location.
3. Collect samples and preserve them by refrigeration.
4. If possible, prepare 24-hour composite samples. Mix samples
thoroughly before compositing and at the time of the test.
14-7
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1/2" Conduit
Length to Suit
V
1/4" Spring to Retain Sample Bottle
Coffee Can
Glass Tube Vent
I
Quart
Plastic
Bottle
J
n Rubber Stopper \
^
1
-*-
^— - --_
\\
11
Glass Tube - Cut to
fit 1/2" clearance from
bottom of bottle
Fig. 14.1 Sampling bottle
14-8
PURCHASE ORDER H1387
PRINT ORDER. 0842
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