PROCESS DESIGN MANUAL FOR
UPGRADING EXISTING WASTEWATER TREATMENT PLANTS
For
ENVIRONMENTAL PROTECTION AGENCY
Technology Transfer
By
ROY F. WESTON, INC.
Environmental Scientists and Engineers
West Chester, Pennsylvania
Program No. 17090 GNQ
Contract No. 14-12-933
October, 1971
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The mention of trade names or commercial products in this manual is for illustration
purposes, and does not constitute endorsement or recommendation for use by the
Environmental Protection Agency.
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ABSTRACT
The main purposes of this manual are to examine situations that necessitate upgrading
of existing municipal wastewater treatment plants and to discuss and evaluate the corrective
actions that are required to upgrade these existing plants. Upgrading to overcome organic
and hydraulic overloadings and/or to meet more stringent treatment requirements is
considered. The information presented in this manual is specifically adapted to plants having
capacities of less than 5 mgd. This particular capacity was selected because most of the
existing municipal wastewater treatment plants in the United States have capacities of
less than 5 mgd.
The manual emphasizes that operational improvement and modifications to existing unit
operations be considered as the logical initial approach to upgrading existing treatment
plants, before major expansion of existing facilities is implemented.
Because of the numerous alternatives available for upgrading an existing treatment plant,
it is necessary to understand thoroughly the fundamentals of the various unit operations
commonly used in municipal wastewater treatment plants. Therefore, this manual examines
in depth the capabilities, limitations, and interrelationships of the various unit processes.
The manual also examines hypothetical situations requiring upgrading of unit operations
and describes "order of magnitude" costs associated with the upgrading of various unit
operations.
One chapter of the manual presents case histories of upgrading of existing wastewater
treatment plants to illustrate the approaches actually used in these circumstances. The
operation and maintenance requirements of the upgraded treatment plants are also briefly
examined in the manual.
This manual was submitted in fulfillment of Project Number 17090 GNQ,
Contract 14-12-933, under the sponsorship of the Office of Water Programs, Environmental
Protection Agency.
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CONTENTS
Chapter Page
ABSTRACT iii
CONTENTS v
FOREWORD ix
1 INTRODUCTION 1-1
2 INVESTIGATIVE APPROACH 2-1
2.1 Examination of Need for Upgrading 2-1
2.2 Study of Plant Performance History 2-3
2.3 Identification of Problem Areas 2-4
2.4 Consideration of Applicable Upgrading Techniques 2-5
2.5 References 2-5
3 FLOW EQUALIZATION 3-1
3.1 General 3-1
3.2 Determination of Equalization Requirements 3-1
3.3 Process Designs and Cost Estimates 3-4
4 TECHNIQUES FOR UPGRADING TRICKLING FILTER PLANTS 4-1
4.1 General 4-1
4.2 Trickling Filter Processes 4-1
4.3 Trickling Filter Performance Factors 4-3
4.4 Trickling Filter Design Formulas 4-11
4.5 Applicability of Various Trickling Filter Design Formulas 4-19
4.6 Laboratory and Pilot-Scale Treatability Studies 4-19
4.7 Trickling Filter Upgrading Techniques and Design Basis 4-21
4.8 References 4-37
5 TECHNIQUES FOR UPGRADING ACTIVATED SLUDGE PLANTS 5-1
5.1 General 5-1
5.2 Activated Sludge Processes 5-1
5.3 Activated Sludge Design Considerations 5-15
5.4 Pilot Studies 5-26
5.5 Activated Sludge Upgrading Techniques and Design Basis 5-31
5.6 References 5-48
6 CLARIFICATION AND CHEMICAL TREATMENT 6-1
6.1 General , 6-1
6.2 Primary Clarification 6-1
6.3 Secondary Clarification 6-1
6.4 Chemical Treatment , 6-2
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CONTENTS
(continued)
Chapter Page
6.5 Other Approaches to Improvement of Clarification 6-7
6.6 Chemical Feeders 6-12
6.7 Process Designs and Cost Estimates 6-17
6.8 References 6-18
7 EFFLUENT POLISHING TECHNIQUES 7-1
7.1 General 7-1
7.2 Polishing Lagoons 7-1
7.3 Micro straining 7-7
7.4 Multi-Media, Coarse-Media, and Moving-Bed Filters 7-11
7.5 Activated Carbon Adsorption 7-15
7.6 Process Designs and Cost Estimates 7-20
7.7 References 7-24
8 PRE-AERATION AND POST-AERATION PRACTICES 8-1
8.1 Pre-Aeration 8-1
8.2 Post-Aeration 8-3
8.3 References 8-9
9 DISINFECTION AND ODOR CONTROL 9-1
9.1 General 9-1
9.2 Disinfection 9-1
9.3 Odor Control 9-2
9.4 Other Uses of Chlorine 9-6
9.5 References 9-7
10 SLUDGE THICKENING 10-1
10.1 Air Flotation 10-1
10.2 Gravity Thickening 10-5
10.3 References 10-14
11 SLUDGE DIGESTION 11-1
11.1 Anaerobic Digestion 11-1
11.2 Aerobic Digestion 11-19
11.3 References 11-31
12 SLUDGE DEWATERING 12-1
12.1 Vacuum Dewatering 12-1
12.2 Drying Beds 12-10
12.3 Centrifugation 12-13
12.4 References 12-24
VI
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CONTENTS
(continued)
Chapter Page
13 CASE HISTORIES OF TREATMENT PLANT UPGRADING 13-1
13.1 General 13-1
13.2 Case History No. 1 13-1
13.3 Case History No. 2 13-6
13.4 Case History No. 3 13-11
13.5 Case History No. 4 13-15
13.6 Case History No. 5 13-18
13.7 Case History No. 6 13-21
13.8 References 13-23
14 OPERATION AND MAINTENANCE REQUIREMENTS FOR
UPGRADED PLANTS 14-1
14.1 General 14-1
14.2 Responsibility of the Design Engineer 14-1
14.3 Instrumentation and Automatic Operation 14-2
14.4 Operation and Maintenance Requirements 14-6
14.5 References 14-8
ACKNOWLEDGEMENTS
WRSIC
VII
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FOREWORD
The formation of the Environmental Protection Agency marks a new era of environmental
awareness in America. The Agency's goals are national in scope and encompass broad
responsibility in the area of air and water pollution, solid wastes, pesticides, and radiation.
A vital part of EPA's national water pollution control effort is the constant development
and dissemination of new technology for wastewater treatment.
It is now clear that only the most effective design and operation of wastewater treatment
facilities, using the latest available techniques, will be adequate to meet the future water
quality objectives and to ensure continued protection of the Nation's waters. It is essential
that this new technology be incorporated into the contemporary design of waste treatment
facilities to achieve maximum benefit of our pollution control expenditures.
The purpose of this manual is to provide the engineering community and related industry
a new source of information to be used in the planning, design, and operation of present
and future municipal wastewater treatment facilities. It is recognized that there are a
number of design manuals, manuals of standard practice, and design guidelines currently
available in the field that adequately describe and interpret current engineering practices
as related to traditional plant design. It is the intent of this manual to supplement this
existing body of knowledge by describing new treatment methods, and by discussing the
application of new techniques for more effectively removing a broad spectrum of
contaminants from wastewater.
Much of the information presented is based on the evaluation and operation of pilot,
demonstration, and full-scale plants. The design criteria thus generated represent typical
values. These values should be used as a guide and should be tempered with sound
engineering judgment based on a complete analysis of the specific application.
This manual is one of the four now available through the sponsorship of the Environmental
Protection Agency to describe recent technological advances and new information in the
following subject areas:
Granular Carbon Adsorption
Phosphorus Removal
Upgrading Existing Plants
Suspended Solids Removal
These manuals are the first edition copies and will be updated as warranted by the advancing
state of the art to include new data as it becomes available, and to refine design criteria
as additional full-scale operational information is generated.
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CHAPTER 1
INTRODUCTION
The ability of wastewater treatment plants to perform at required levels of efficiency
becomes more critical as water pollution abatement programs achieve their objectives.
Deviations from design performance, which were formerly of lesser consequence, now
become paramount because of their impact on the receiving waters. Improved process
monitoring and plant operation will obviously reduce the incidence of inadequate
performance, but many cases are the result of more basic deficiencies in the treatment
system. Such deficiencies can arise from: 1) inadequate initial design; or 2) increased or
changed load applied to the system. Another consideration in achieving the required levels
of efficiency is the upgraded treatment required to maintain the desired water quality
in the receiving waters.
Regardless of the cause, the result is that an inadequately treated effluent is discharged.
The historical solution to such a problem has been plant expansion along the same lines
as the original facility, or addition of conventional unit processes to add secondary or,
in a relatively few cases, tertiary treatment to the system. Depending on its application,
a generalized approach such as this does not necessarily make optimum use of the previously
existing facilities nor of the expanded facilities. The situation is further complicated where
regional treatment systems are proposed for the future and existing facilities are inadequate
for the interim period. In such cases, a solution must make optimum use of available
technology, with minimum capital expenditure.
Upgrading of wastewater treatment plants may be required to handle higher hydraulic
and organic loadings to meet existing effluent quality and/or to meet higher treatment
requirements. Any of these situations requires optimization of existing facilities before
consideration of additional treatment facilities. It is necessary that a distinction be made
between upgrading to accommodate higher hydraulic and organic loads, and upgrading
to meet stricter treatment requirements. Existing facilities can be made to handle higher
hydraulic and organic loads at slightly reduced treatment efficiency by process
modifications, whereas meeting higher treatment requirements usually requires significant
expansion and/or modification of existing facilities.
Rapid urbanization, development of industries, and stricter treatment requirements often
necessitate unanticipated upgrading of treatment plants or premature implementation of
upgrading programs. Many existing treatment plants are not capable of meeting the more
stringent performance levels required by today's water quality standards. In addition, there
are needs for interim improvements. These considerations, plus economic pressures to
optimize pollution abatement expenditures, make it mandatory that a logical and
technically sound approach to upgrading existing treatment facilities be established. This
is especially true because of the numerous alternatives available for consideration prior
to the selection of a method for upgrading an existing facility. It is for this reason that
a major plant expansion, i.e., complete duplication of existing unit treatment processes,
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for the purposes of this manual will be considered the least attractive upgrading procedure
available, since this approach does not consider optimization of existing facilities.
Therefore, the purpose of this manual will be to present necessary information for
considering various courses of action with regard to an impending or existing plant overload
situation, or with regard to increasing the efficiency to meet stricter water quality standards.
The diversity of causes that necessitate upgrading of existing plants precludes the use of
this manual as a conventional design manual. Therefore, it is aimed at establishing a
framework of possible alternative methods of upgrading overloaded treatment plants having
capacities up to 5 mgd. This maximum capacity was selected because over 94 percent
of the existing treatment plants in the United States in 1968 had capacities of less than
5 mgd (1). Also, past experience has indicated that plants smaller than 5 mgd often have
a higher proportion of operational and upgrading problems than do plants of larger size.
To facilitate the information presented in the subsequent sections, only plants treating
"typical" domestic wastewaters will be considered.
Particular upgrading procedures are stressed as interim methods which may be implemented
with a minimum amount of effort and capital expenditure prior to a more elaborate
upgrading or even a major plant expansion. Cost information has been compiled and
estimates prepared for the upgrading of individual unit processes. When available, cost
information has also been included for the reported case histories on plant upgrading.
Due to the varying complexity of existing plants, the real benefit of the subsequent cost
information will be as a tool for developing comparative capital costs for various upgrading
techniques. Particular unit process cost information must be used cautiously, since the
complexity of the individual situation will dictate the costs required for upgrading.
The aspects of nutrient removal, although extremely important and oftentimes responsible
for upgrading action at many treatment plants, will not be discussed since a separate manual
will be published by EPA on this topic.
References
1. Statistical Summary 1968 Inventory Municipal Waste Facilities in the United States.
Federal Water Quality Administration: Government Printing Office, 1971.
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CHAPTER 2
INVESTIGATIVE APPROACH
2.1 Examination of Need for Upgrading
The need for upgrading an existing wastewater treatment plant may arise for one or more
of the following reasons:
1. Lack of proper plant operation and control.
2. Inadequate plant design.
3. Changes in wastewater flow or characteristics.
4. Changes in treatment requirements.
2.1.1 Lack of Proper Plant Operation and Control
One of the primary considerations in evaluating an overloaded plant is in the area of
improper plant operation and control. An incorrectly operated or maintained plant will
never be able to perform according to design. Therefore, no physical upgrading should
be considered before the engineer is assured that the plant is being operated to yield
its maximum efficiency.
There seem to be two main reasons for the large number of smaller-capacity plants
throughout the country which are poorly operated or maintained:
1. The smaller community or sanitary district will not or cannot provide funds
for the employment of qualified operators.
2. The lack of appropriation of operating funds limits the extent of any scheduled
maintenance program.
In addition, many wastewater treatment plants do not have a laboratory equipped to
analyze the wastewater samples from the various units to assess their performance. Improper
operation, coupled with inadequate laboratory control, increases the probability of
inadequate treatment. For this reason, the smaller community or sanitary district should
make sure its plants are staffed with an adequate number of competent operators and
laboratory personnel. Further, sufficient funds should be made available to insure a proper
maintenance program.
2.1.2 Inadequate Plant Design
In the past, the problems associated with inadequately designed wastewater treatment plants
have been a major concern of individual state agencies. For this reason, most states have
adopted conservative design guidelines and review procedures which must be followed unless
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the engineer has operating data which will substantiate a less conservative viewpoint. The
implementation of these procedures by regulatory agencies has substantially reduced the
problem of inadequate plant design as applied to municipal wastewater.
The major exception to this is in the area of joint municipal and industrial wastewater
treatment. Design of wastewater treatment facilities based strictly on state standards may
not be applicable when a significant amount of industrial wastewater is discharged to a
municipal plant. When this situation occurs, sufficient wastewater sampling and treatability
studies should be performed to establish parameters necessary for the design of the
treatment plant.
In the past, one of the areas in the design of treatment plants not given much consideration
was flexibility. The following design considerations can greatly increase the flexibility
allowed to the treatment plant operator:
1. Splitter boxes before and after individual unit processes, for greater ease in
operation and maintenance.
2. Piping associated with the aeration basin designed with enough flexibility to
facilitate implementation of various activated sludge modifications.
3. Sufficient blower capacity to meet fluctuating organic loads to the aeration basin.
4. Sufficient recycle capacity for trickling filters to meet fluctuating loads.
5. Chlorination capacity with an incremental factor to be utilized for operational
control, e.g., odor control.
2.1.3 Changes in Wastewater Flow or Characteristics
Two major problems facing engineers in the design of wastewater treatment plants are:
1) forecasting changes in population and wastewater flow; and 2) the operational problems
caused by changes in the characteristics of municipal wastewaters due to the rapid
industrialization of an area. Forecasting changes in population and wastewater flows in
connection with upgrading of a treatment plant may be quite burdensome, but generally
will not be subject to as much uncertainty as in similar forecasting for a relatively
undeveloped area. In many cases, the maximum anticipated growth is defined by saturation
of the tributary area. Potential extension of this area must also be considered, and is
often limited by topographical constraints and political boundaries.
In-line measurement and analysis of existing wastewater flows, analysis of local area growth
patterns, examination of local influences such as land use planning studies, zoning
regulations, wastewater discharge ordinances, and full use of State, County, and local
planning agencies can all be extremely useful in judging the future expected flows for
a given upgrading situation.
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Rapid industrialization in the plant's service area can cause major operational problems
in existing plants, which may require upgrading of various unit processes to handle increased
hydraulic and organic loads. The alert community before issuing a building permit should
make the industry aware that pretreatment may be required for wastewater containing
toxic materials, or for those having an unusually high percentage of organic or inorganic
material compared to typical domestic wastewaters. Equalization of industrial wastewater
discharges may be helpful in minimizing diurnal flow variations to treatment plants and
in distributing shock loads of high-strength wastewaters.
Population equivalent is a reasonable method of equating the organic content or flow
contribution of industries to the ordinary per capita contribution present in domestic
wastewaters. In many cases, even a relatively small industry may contribute a significantly
higher loading than, i the existing population. Population equivalents for many industrial
wastewaters should .be based on COD analysis rather than on BOD 5, since extremely strong
or toxic wastes may: show an artificially low BOD^ value.
2.1.4 Changes in Treatment Requirements
Increased pressure on the part of the Federal and State governments and a more ecologically
minded public are requiring local communities and sanitary districts to enforce existing
water quality standards. In addition, many regulatory agencies are stipulating increased
organic and solids removal, minimum dissolved oxygen concentration in the plant effluent,
a minimum consistent BOD and SS removal during low-flow periods, and removal of
substantial portions of the nitrogenous oxygen demand. For example, the Potomac
Enforcement Conference has recently required 85 percent removal of nitrogenous oxygen
demand from wastewaters treated in the Washington, D.C. metropolitan area (1). Therefore,
to meet existing and future requirements, upgrading of existing treatment facilities will
often be required.
2.2 Study of Plant Performance History
It is the responsibility of the engineer engaged in upgrading an existing wastewater
treatment plant to examine the plant's performance history thoroughly, as the first step
in an engineering study. Treatment plant operating records serve two basic functions:
1) providing the operating information necessary for process monitoring and control; and
2) providing the historical record of plant performance. Data collected from a typical
plant should include: 1) daily total flow; 2) maximum and minimum flow rates within
a day; 3) volume of air added; 4) a series of concentration parameters such as BOD,
suspended solids, COD, phosphates, nitrogen, and temperature; 5) consumption of chlorine,
coagulants, and neutralizing chemicals; and 6) sludge production.
The engineer should examine all treatment plant records to become familiar with the type
of sampling and flow measurement techniques employed by the plant personnel and verify
their accuracy. One difficult area in assessing plant performance data is the variation in
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reliability of influent flow measurement data. Often, plant flow is obtained from some
type of flow recording instrument, and reliable flow information is possible only when
the treatment plant operator makes it a point to calibrate the flow measuring and recording
instruments periodically. A representative portion of the operating data should be evaluated
by the engineer. If operational data are not available, then it is the responsibility of the
engineer to collect sufficient data to proceed with his upgrading evaluation.
2.3 Identification of Problem Areas
After evaluating the plant's operating records, it is necessary to determine the factors
which influence the plant's current performance. There are four problem areas whose effect
should be considered in any upgrading situation:
1. Hydraulic and organic overloading.
2. Inadequate organic removal.
3. Inadequate solids removal.
4. Inadequate sludge handling.
The performance of different unit processes within a treatment plant is affected to varying
degrees by an increase in hydraulic and organic loading. The relationship between the
increases in flow and in organics is also an important consideration. For example, a
significant increase in flow without a corresponding increase in organics will generally not
be as detrimental as when the increase in flow is also accompanied by a correspondingly
large increase in organics.
Nevertheless, the efficiency of most unit operations is affected by hydraulic and organic
overloading. The increased flow will increase overflow rates and will decrease available
detention time in primary and secondary clarifiers. An increased overflow rate in the
primary clarifier will in turn decrease suspended solids and BOD removal at this unit.
As a result, solids and BOD loads to secondary treatment processes are increased. Also,
hydraulic overloading reduces the compaction of solids that normally takes place in
clarifiers and increases the volume of sludge to be handled.
Organic overloading significantly increases the:
1. Organic load per unit of aeration volume.
2. Lbs. BOD applied per pound of MLVSS under aeration.
3. Demand for more oxygen.
These effects significantly decrease the efficiency of activated sludge treatment. A similar
effect can be seen in trickling filter plants. In activated sludge plants, an increase in organic
loading can reduce the operational stability of the process by causing sludge bulking. Solids
removal efficiency in secondary clarifiers is thereby reduced, significantly minimizing the
amount of solids that can be carried in the aeration basin. The carryover of biological
solids from the final clarifier increases the effluent BOD.
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Many treatment plants have anaerobic digestion facilities designed on the basis of a certain
volume per capita. Increases in the number of people to be served, together with the
increased volume of sludge resulting from poor clarifier performance caused by hydraulic
overloading, makes the existing volume in many anaerobic digesters inadequate. This
situation creates an operational problem leading to inadequate sludge digestion. The poorly
digested sludge further complicates dewatering on vacuum filters or sand beds, because
it does not dewater rapidly.
The above discussion clearly indicates that problem areas are interrelated and should be
assessed concurrently to determine the logical combination of applicable upgrading
procedures. Since optimization of existing facilities is necessary in any upgrading situation,
an understanding of various modifications available for the different unit processes is
essential. This is the reason for defining the capabilities and limitations of various unit
processes in subsequent chapters of this manual.
2.4 Consideration of Applicable Upgrading Techniques
Technology in the field of wastewater treatment in the past decade has provided many
innovative upgrading procedures to meet deficiencies in existing processes. Various research
projects sponsored by EPA have resulted in a better understanding of various unit processes.
In addition, new types of equipment for wastewater treatment have enlarged the range
of alternatives available for consideration in upgrading treatment plants.
It has long been recognized that the performance of a wastewater treatment plant is affected
by variations in the influent flow. Equalization of extreme flows can dampen the
fluctuations in loading to a plant.
Various processes and process equipment are being marketed and successfully used to
increase removal in primary and secondary clarifiers. They include the use of chemical
coagulation, peripheral-feed clarifiers, and inclined-tube settlers. These procedures, in many
cases, have the effect of maintaining good solids removal while maximizing the hydraulic
throughput in the existing facilities. Chemical addition in primary and secondary clarifiers
can increase solids capture and BOD removal. In addition, several types of screening devices
are available as possible substitutes for primary clarification.
Several modifications of the conventional activated sludge process, including step aeration,
contact stabilization, and complete mixing, have been adequately studied and have been
used to upgrade various treatment plants. A most significant development in the activated
sludge treatment process came with the full-scale demonstration of the feasibility and
effectiveness of using oxygen aeration as a substitute for air aeration. Plants using oxygen
aeration are now being designed, with the single largest plant, at Detroit, Michigan,
designed for a flow of 300 mgd.
Another method of upgrading an overloaded secondary plant is to provide additional
treatment ahead of the existing biological treatment facilities. The use of plastic media
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trickling filters should be considered when roughing treatment is indicated. Plastic media
filters have been successfully used as roughing filters in industrial wastewater treatment,
and it is very likely that they will be used in the future for upgrading of municipal treatment
plants.
The true effect of a nitrified effluent on dissolved oxygen in receiving waters has just
recently been recognized and substantiated (2). For this reason, some regulatory agencies
are requiring nitrification of treatment plant effluents during summer periods, and in some
cases are contemplating a year-round nitrification requirement. Nitrification during summer
months may be accomplished through modifications to the existing treatment units, such
as addition of chemicals to the primary clarifier to decrease the organic loading to existing
aeration units. However, dependable year-round nitrification will require a two-stage
biological treatment system.
On many occasions, treatment plants which are functioning satisfactorily (design flow not
exceeded) are required to improve solids or BOD removal because of more stringent water
quality standards. This additional treatment can often be achieved by polishing the
treatment plant effluent. Several methods are currently available and have been used
successfully, including aerobic and facultative lagoons, microstraining, multi-media
filtration, and activated carbon treatment.
Although considerable effort has been made in the study of organic removal processes,
the area of sludge handling and dewatering has not received corresponding attention.
Inadequate digestion and sludge handling facilities often adversely affect the overall
treatment plant operation. The return of supernatant or filtrate from thickening or
dewatering units to the head of the plant can impose high oxygen demands on the system
and add substantial amounts of fine solids which are difficult to remove in the secondary
clarifier. The high concentration of nutrients and organics in such streams and the periodic
nature of the return flow often necessitate separate treatment, especially when nutrient
removal is a consideration.
Various sludge-handling developments which have been successful are: 1) high-rate
anaerobic digestion; 2) aerobic digestion; 3) thickening of sludge prior to digestion to
increase the capacity of existing digesters; 4) the use of chemicals to improve thickening
and dewatering of sludges; and 5) the use of heat treatment processes for the disposal
of sludges. The use of aerobic digestion is likely to alleviate many of the operational
problems associated with the anaerobic treatment of sludges.
Having briefly presented a general overview of the technology available for upgrading,
the engineer should keep in mind various considerations which will affect the overall
economics of upgrading:
1. The physical condition of existing plant equipment and structures as it relates
to the use of existing facilities in an upgrading situation.
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2. The length of time before a major expansion will be required, based on
population and wastewater flow projections.
3. The time required for implementation of various upgrading techniques.
4. Compatability of upgrading procedures with future planned expansions. For
example, if the engineer determines that contact stabilization will not work well
on a particular wastewater, then step aeration may not be the most economical
interim step. Perhaps completely mixed activated sludge or oxygen aeration
would be more logical. The reasoning behind this type of decision will be
explained further in subsequent chapters.
5. Financial resources available to the community.
6. Costs of the various upgrading techniques that can be used to achieve essentially
the same result. The operation and maintenance costs, as well as the capital
costs, may be substantially different. Therefore, economic comparison of
available alternatives is necessary.
2.5 References
1. Nitrogen Removal from Wastewaters. Federal Water Quality Administration,
Publication ORD-17010, October, 1970.
2. Courchaine, Robert, Significance of Nitrification in Stream Analysis - Effects on the
Oxygen Balance. Journal Water Pollution Control Federation, 40, No. 5, pp. 835-847
(1968).
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CHAPTER 3
FLOW EQUALIZATION
3.1 General
The cyclic nature of wastewater flows, in terms of volume and strength, is well established.
While the concept of flow equalization has been employed in the field of water supply
and in the treatment of some industrial wastes, it has not been widely accepted in the
municipal pollution control field. Anticipated problems with solids settling, odor, and
septicity can be cited as the major factors limiting its use.
Recently, interest in flow equalization for municipal treatment has increased due to the
advent of stricter water quality standards, the elimination of plant bypassing, and the
increased removal efficiencies that are possible when biological or chemical treatment
processes are operated at or near steady-state conditions.
There are two major objectives in the design of flow equalization basins. The first of
these is simply to dampen the diurnal flow variations that normally exist in typical
municipal wastewater collection systems, and thus achieve a constant or nearly constant
flow rate through the downstream treatment processes. In this type of system, little
consideration is given to controlling the concentration changes that take place during
storage. The major design factors are supplying enough air to keep the basin aerobic and
providing adequate mixing to prevent solids deposition. Consideration should be given to
locating equalization basins both at the treatment plant site and at strategic upstream
locations in the tributary collection system. The upstream locations may offer the added
advantage of relieving trunk sewer overload during peak flow periods.
The second objective of flow equalization is to provide the capacity to distribute shock
loads of toxic or treatment-inhibiting substances over a reasonable period of time to prevent
system failure and to minimize the periodic discharge of harmful contaminants to the
receiving stream or surface impoundment. The measurement or estimation of
time-dependent concentration profiles and flow-through curves is normally used to analyze
the flow characteristics of these systems for determining the effects of tank geometry,
effluent weir placement, and mixing regime on changes in contaminant concentrations
through the basin.
In all cases, the added costs of flow equalization must be measured against the reduction
in downstream process costs and the increased efficiencies that can be achieved by operating
these processes under relatively constant loading conditions.
/
3.2 Determination of Equalization Requirements
To determine the appropriate equalization basin volume, it is necessary to plot an inflow
mass diagram of the hourly fluctuations for a typical daily wet-weather wastewater flow.
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Figure 3-1 shows the hourly fluctuations for a typical plant. Superimposed on Figure
3-1 is the inflow mass diagram for the hourly flows, the ordinate of which is obtained
by accumulating the hourly flows and converting them into equivalent volumes of
wastewater.
In Figure 3-1, the slope of line A represents the average rate at which the wastewater
is pumped from the equalization basin to the downstream treatment units, which for the
particular wet-weather flow in Figure 3-1 is 10,000 gallons/hour. This slope is determined
by drawing a straight line through the origin and point C, which is the end of the inflow
mass diagram. The maximum required capacity of the equalization basin is determined
by drawing lines B and D parallel to line A and tangent to the inflow mass diagram
at its maximum and minimum points, E and F. The vertical distance between lines B
and D represents the minimum required equalization volume of 30,000 gallons, which
is approximately 12.5 percent of the average daily wet-weather flow in this example.
In addition to the volume required to equalize typical wet-weather peaks, the basin must
be sized to accommodate any anticipated concentrated plant wastewater streams. Anaerobic
digester supernatant and sludge dewatering filtrate are periodically discharged to the front
end of the treatment plant and usually have higher organic and nutrient concentrations
than typical municipal wastewater. Due to their periodic discharge, these flows create shock
load conditions which reduce plant efficiency. COD and ammonia concentrations of 10,000
to 20,000 mg/1 and 1,000 mg/1, respectively, are common and consequently create a high
oxygen demand. Equalization of these loadings is extremely beneficial to overall plant
performance.
The following table is an estimate of the equalization basin's volume requirements for
the example shown in Figure 3-1:
Table 3-1
Equalization Requirements
Source Range of Equalization Needs
percent of flow
Wet Weather Flow Equalization 12.5
Digester Supernatant 0.3 to 1.4
Sludge Dewatering Filtrate 0.5 to 1.5
TOTAL 13.3 to 15.4
The maximum volume requirements to equalize wet-weather flows will depend on the
magnitude of the infiltration and extraneous surface water that enters the wastewater
collection system. In some cases, it may not be economically feasible to equalize extreme
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FIGURE 3-1
EQUALIZATION REQUIREMENTS FOR A TYPICAL WET WEATHER FLOW
OJ
-! 150
£ 100
EQUALIZATION BASIN VOLUM
30 xlO3 GALS
• AVERAGE FLOW 10.000 GAL./HR
3AM
12
TIME OF DAY
-------
peaks of wet-weather flow. Regulatory requirements involving prohibition of treatment
plant bypassing, however, will favor the construction of some type of equalization facility.
Examination of the plant's past flow records will facilitate the selection of a particular
wet-weather flow for design purposes.
For successful operation of equalization basins, mixing and aeration of the wastewater
are required. Mixing is necessary to prevent deposition of solids in the basin, and aeration
is required to prevent septicity.
A typical equalization flow schematic is shown in Figure 3-2. The flow enters the
equalization basin by gravity, and the basin contents are pumped to the primary treatment
units using continuous-flow, variable-speed pumps. The maximum pumping capacity of
the equalization pump station should be sufficient to handle the maximum flow expected,
even though the equalization basin may be designed to equalize a somewhat smaller flow.
3.3 Process Designs and Cost Estimates
Capital cost estimates, prepared for equalization facilities for plants having capacities of
1, 3, and 5 mgd, are shown in Table 3-2. These costs do not include land costs,
contingencies, engineering design, and bonding.
Table 3-2
Capital Costs for Equalization Facilities
(ENR Index 1,500)
Capital Costs for
Plant Size Equalization Facilities
mgd In Thousand Dollars
1 $210
3 450
5 600
The above costs are based on the typical flow diagram shown in Figure 3-2. The volume
of the equalization basin was based on 15 percent of the treatment plant daily capacity.
The basin dimensions should be selected to avoid interference between aerators and to
minimize fluctuations in basin water level. Aerator manufacturers recommend a minimum
basin size of 15 to 50 feet square and a minimum depth of 5 to 8 feet, depending on
the particular aerator horsepower used. To avoid large volumes of dead storage in the
equalization basin as a result of aerator operating requirements, the use of a compartmented
basin is suggested. For example, two compartments of a four-compartment basin could
be used to equalize diurnal flow variations, while all four compartments could be used
to equalize wet-weather peaks. When floating surface .aerators are used, care must be
exercised to maintain a minimum water level to protect the aerator. This may be
3-4
-------
FIGURE 3-2
SCHEMATIC FLOW DIAGRAM OF EQUALIZATION FACILITIES
RAW
WASTEWATER
DIGESTER SUPERNATANT
AND SLUDGE DEWATERING
FILTRATE
EQUALIZATION BASI
WITH MECHANICAL
FLOATING AERATORS
N
VARIABLE
STATION
SPEED PUMPING
PRIMARY
TREATMENT
SECONDARY
TREATMENT
FINAL
EFFLUENT
3-5
-------
accomplished by compartmentalization as previously mentioned, or by low-level controls
on the pump and aerator. Mixing requirements for wastewaters having a suspended solids
concentration of approximately 200 mg/1 range from 0.02 tp 0.04 hp/1,000 gallons of
maximum storage volume.
In addition to the mixing requirement, aeration to prevent septicity must also be
considered. Oxygen should be supplied to the equalized flow at a sufficient rate
(approximately 15 mg C^/l/hr) to prevent septic odor problems. Mechanical aerators are
one method of furnishing both the mixing and aeration requirements. The oxygen transfer
capabilities of mechanical aerators operating under standard conditions vary from 3 to
4 Ibs O2/hp-hr.
The costs in Table 3-2 were developed for a reinforced concrete basin equipped with
floating aerators. The floating aerator is anchored to the periphery of the basin and is
permitted to fluctuate with the water level. The pumps are variable-speed centrifugal pumps.
If ground conditions are satisfactory, the use of an earthen lagoon will reduce the cost
of the equalization basin significantly.
3-6
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CHAPTER 4
TECHNIQUES FOR UPGRADING TRICKLING FILTER PLANTS
4.1 General
In 1968 there were more than 3,700 trickling filter plants in the United States serving
over 28 million people. In contrast, there were approximately 2,100 activated sludge plants
serving 41 million people (1). In the past, the trickling filter plant has been considered
the ideal plant for populations of 2,500 to 10,000.
Several reasons have justified this popularity. One is its economy, not only in first cost,
but also in operation; another is its relative simplicity of operation, which does not require
as highly skilled operators as activated sludge plants require.
Although the effluent from a trickling filter plant is generally of lesser quality than that
from an activated sludge plant, trickling filter performance has been considered adequate
in many rural areas where stream assimilative capacity is relatively large in relation to
population. However, increased urbanization and more stringent water quality standards
will require that many existing trickling filter plants be upgraded to improve the quality
of treatment provided.
Upgrading of a trickling filter may be required due to hydraulic or organic overloading,
higher effluent quality requirements, or both. In general, decreasing hydraulic or organic
overloading in existing facilities will not produce a significant increase in BOD removal
above the original design value. Therefore, additional treatment facilities will be needed
if increased BOD removal is required. It is emphasized that the upgrading of an existing
plant should utilize the existing units as much as possible.
4.2 Trickling Filter Processes
Trickling filtration consists of uniform distribution of wastewater over the trickling filter
media by a flow distributor. A large portion of the wastewater applied to the filter rapidly
passes through it, and the remainder slowly trickles over the surface of the slime. BOD
removal occurs by biosorption and coagulation from the rapidly moving portion of the
flow and by progressive removal of soluble constituents from the more slowly moving
portion of the flow.
The quantity of biological slime produced is controlled by the available food, and the
growth will increase as the organic load increases until a maximum effective thickness
is reached. This maximum growth is controlled by physical factors including hydraulic
dosage rate, type of media, type of organic matter, amount of essential nutrients present,
and the nature of the particular biological growth.
4-1
-------
In the past, trickling filters have been classified as either low (standard), intermediate,
high, or super-rate filters based on hydraulic and organic loading rates.
4.2.1 Low-Rate Trickling Filters
Low-rate trickling filters are designed to handle organic loadings of 10 to 20 Ibs. of
BOD/1,000 cu.ft./day, and hydraulic loadings of 2 to 4 million gallons/acre/day (mgad).
In general, low-rate filters do not use recirculation to maintain a constant hydraulic loading,
but use either suction-level controlled pumps or a dosing siphon. Dosing tanks are small,
usually with only 2-minute detention time based on twice the average design flow so
that intermittent dosing is minimized. Even so, at small plants the low night-time flows
may result in intermittent dosing. If the interval between dosings is long (e.g., greater
than one or two hours), the efficiency of the process will be affected since the character
of the biological slime will be altered due to lack of moisture.
Under normal operations, the low-rate filter and secondary clarifier may average
85 percent BOD removal. By the addition of recirculation during periods of low flows
(so that the filter is always wet), it is possible to increase filter efficiency to 90 percent
and even higher in some instances (2).
In most low-rate filters, only the top 2 to 4 feet of the filter media have appreciable
biological slime. As a result, the lower portions of the filter may be populated by
autotrophic nitrifying bacteria which oxidize ammonia nitrogen to nitrite and nitrate forms.
If the nitrifying population is sufficiently well established and if climatic conditions are
favorable, a well operated low-rate filter, in addition to providing good BOD removal,
can produce a highly nitrified effluent. The positive effect that a nitrified effluent has
in reducing the total oxygen demand in receiving waters is being increasingly utilized in
the formulation of water quality standards.
4.2.2 Intermediate-Rate Trickling Filters
Intermediate-rate trickling filters are generally designed to treat hydraulic loadings of
4 to 10 mgad and corresponding organic loadings ranging from 15 to 30 Ibs. BOD/1,000
cu.ft./day, including recirculation. In the past, there have been some cases where the organic
loading in the intermediate range stimulated considerable biological filter growth and the
rate of hydraulic loading was not sufficient to eliminate clogging of the trickling filter
media (2). This clogging situation can be remedied somewhat by utilizing relatively large
stone, 3 to 4 inches in diameter. However, it should also be noted that many plants
operate in this intermediate range with no reported operational problems (2) (3). In
practice, some engineers will design a high-rate filter to operate in the intermediate range
during the early period of its operating life, when average flows are substantially below
the average design flows.
4-2
-------
4.2.3 High-Rate Trickling Filters
High-rate trickling filters have hydraulic loadings of 10 to 30 mgad and organic loadings
up to 90 Ibs. BOD/1,000 cu.ft./day, including recirculation. In all high-rate filters, some
form of recirculation is used in order to maintain a relatively constant hydraulic loading.
The correspondingly higher loadings result in an overall BOD removal efficiency that is
somewhat lower than that obtainable from a low-rate trickling filter. The higher organic
loadings in high-rate filters preclude the development of nitrifying bacteria in the lower
section of the filter. Hence, these plants will seldom exhibit any incipient nitrification.
4.2.4 Super-Rate Trickling Filters
Super-rate trickling filters have evolved as a result of the development of various types
of synthetic trickling filter media. Past experience has indicated that hydraulic loadings
of 150 mgad and higher, including recirculation, may be accommodated in super-rate
trickling filters. A discussion of synthetic media characteristics is presented in
Section 4.3.2.
4.3 Trickling Filter Performance Factors
There are numerous factors that affect the performance of trickling filters. Some of these
are:
1. Wastewater Characteristics.
2. Trickling Filter Media.
3. Trickling Filter Depth.
4. Recirculation.
5. Hydraulic and Organic Loading.
6. Ventilation.
7. Temperature of Applied Wastewater.
4.3.1 Wastewater Characteristics
Domestic wastewaters vary in composition and strength, depending on the relative amounts
of industrial wastewater and infiltration present. The rate of BOD removal from a domestic
wastewater in a trickling filter generally exceeds the BOD removal rate from an industrial
wastewater which has a high percentage of dissolved BOD. This is due to the high percentage
of colloids in domestic wastewater, and to the apparent increased ability of the filter
to remove this colloidal material. A reasonable explanation for this is that some of these
materials are removed by biological flocculation and not by oxidation and synthesis of
new cells.
The strength of wastewaters can vary substantially over a daily period. One method of
dampening these fluctuations is to recirculate filter effluent through the primary clarifier.
4-3
-------
4.3.2 Trickling Filter Media
The introduction of synthetic media for trickling filters has extended the range of hydraulic
and organic loading well beyond the range of stone media. Table 4-1 presents a comparison
of physical properties of various types of trickling filter media. Two properties which
are of interest are specific surface area and percent void space. Greater surface areas permit
a larger mass of biological slimes per unit volume, while increased void space allows for
higher hydraulic loadings and enhanced oxygen transfer. The ability of synthetic media
to handle higher hydraulic and organic loadings is directly attributed to the higher specific
surface area and void space of these media compared to stone media and blast furnace
slag, as shown in Table 4-1.
Table 4-1
Comparative Physical Properties of Trickling Filter Media
Units Specific
Nominal per Unit Surface
Packing Size cu.ft. Weight Area Void Space
inches Ibs./cu.ft. sq.ft./cu.ft. percent
Plastic Media 20 x 48 2-3 2-6 25-30 94-97
Del-pak Redwood Media 47'/2 x 47'/2 x 35% - 10.3 14 —
Granite 1-3 - 90 19 46
Granite 4 - 13 60
Blast Furnace Slag 2-3 51 68 20 49
4.3.3 Trickling Filter Depth
Most low-rate trickling filters are designed with depths ranging from 5 to 7 feet, while
high-rate filters are designed with depths of 3 to 6 feet. The relatively deep low-rate filters
improve the nitrification potential.
The treatment efficiency of a synthetic media trickling filter is much more responsive
to variations in depth than a stone media trickling filter. For this reason, the depth selection
for a super-rate trickling filter is a major design parameter, as will be illustrated later
in this chapter.
4.3.4 Recirculation
The practice of effluent recirculation can be used to improve the efficiency and operation
of stone media trickling filters. For example, it can minimize the pperational problems
associated with intermittent dosing of low-rate trickling filters. Recirculation ratios of
0.5 to 4.0 have been used in high-rate filters; Caller and Gotaas (4) have demonstrated
that a recirculation ratio of greater than 4 does not materially increase the efficiency
of filters and is also uneconomical.
-------
There are many possible flow configurations which may be used with a single or two-stage
high-rate trickling filter plant. Some of the more common flow diagrams which have been
presented in the Water Pollution Control Federation's Sewage Treatment Plant Design
Manual (MOP No. 8) have been included in Figure 4-1.
Decisions regarding the use of any one of the flow configurations shown in Figure 4-1
are based on an examination of the relative economics and, in some cases, the preferences
of the design engineer.
Recirculation as applied to the plastic media involves a •< slightly different concept than
has been previously applied to stone filters. Various types of super-rate filter media have
different minimum wetting rates, i.e., a rate of flow per unit area which will induce a
biological slime throughout the depth of the media. This minimum wetting rate typically
ranges from 0.5 to 1.0 gpm/sq.ft., depending on the geometric configuration of the media.
Therefore, recirculation in plastic media filters is practiced to maintain the desired wetting
rate for a particular medium. Generally, increasing the hydraulic loading substantially above
the minimum wetting rate decreases the BOD removal through the filter (5).
4.3.5 Hydraulic and Organic Loading
Two major parameters which affect the performance of a trickling filter are its hydraulic
and organic loading rates. An attempt was made to correlate the efficiency of the secondary
portion of various trickling filter plants to their corresponding hydraulic and organic loading
rates. The results are shown in Figures 4-2 and 4-3 for stone media trickling filter plants
having various recycle rates. It is clear from Figures 4-2 and 4-3 that hydraulic loading
will more closely predict the performance of a stone media trickling filter than organic
loading. A similar effect is obtained for plastic media trickling filters as shown in
Figures 4-4 and 4-5.
4.3.6 Ventilation
Proper ventilation of trickling filters is essential to the maintenance of aerobic conditions
throughout the filter media. The Ten-States Standards recommend that all drains, channels,
and pipes be sized such that not more than 50 percent of their cross-sectional area will
be submerged at the design hydraulic loading (15). If the trickling filter is constructed
on or near grade, provision for ventilation will be less critical than if the topography
necessitates construction well below grade. In these latter instances, forced ventilation or
ventilation shafts may be a consideration. However, many design engineers are of the
opinion that forced ventilation is generally not justified (3).
4-5
-------
FIGURE 4-1
COMMON FLOW DIAGRAMS FOR SINGLE AND
TWO-STAGE HIGH-RATE TRICKLING FILTERS
SINGLE-STAGE
R
R S, S
TWO-STAGE
LEGEND
S SLUDGE RETURN
R RECIRCULATED FLOW
CD PRIMARY CLARIFIER
O TRICKLING FILTER
E33 INTERMEDIATE CLARIFIER
^ FINAL CLARIFIER
NOTE: "REPRINTED WITH PERMISSION FROM "SEWAGE TREATMENT PLANT DESIGN'
MANUAL OF PRACTICE NO. 6. WATER POLL. CONTROL FEDERATION
WASHINGTON. D.C : MANUAL OF ENS PRACTICE NO 36 AMER SOC
CIVIL ENGR., NEW YORK. N.Y. (1959). "
4-6
-------
£ 40
FIGURE 4-2
EFFECT OF HYDRAULIC LOADING ON
STONE MEDIA TRICKLING FILTER PERFORMANCE
LEGEHD*
McCABE & ECKENFELDER(B)
BURGESS ET. AL. (7)
CALLER & GOTAASC4)
BENZIE ET. AL.(B)
NATIONAL RESEARCH COUNCIL(9)
BETHLEHEM, PA. (10)
"NUMBERS IN PARENTHESES INDICATE REFERENCES
20 —
0.2
0.3
0.4
0.5
0.6
HYDRAULIC LOADING,GPM/FT2
(INCLUDING RECYCLE)
4-7
-------
FIGURE 4-3
EFFECT OF ORGANIC LOADING ON
STONE MEDIA TRICKLING FILTER PERFORMANCE
100
• . o
A
*o>
A° A
I A
•
A
-^ • A
• ^L
0 A
^i
i
A
*•
»
0 *
k
1
LEG
A
A
END*
A
A
A
__•
•m
• • *
*
* McCABE & ECKENFELDER ( 6 )
• BURGESS ET. AL. ( 7 )
A CALLER & GOTAAS ( 4 )
• BENZIE ET. AL. ( 8 )
0 NATIONAL RESEARCH COUNCIL ( 9 )
• BETHLEHEM, PA. (10)
"NUMBERS IN PARENTHESES INDICATE REFERENCES
1 1
80
60
40
20
20
40 60 80 100
ORGANIC LOADING LBS BOD/DAY/IOOO FT3
(INCLUDING RECYCLE)
120
140
4-8
-------
FIGURE 4-4
EFFECT OF HYDRAULIC LOADING ON PERFORMANCE OF
PLASTIC MEDIA TRICKLING FILTERS
LEGEND*
• DOW CHEMICAL CO. PILOT PLANT #1 (II)
A GERMAIN (12)
• MOORE (13)
O DOW CHEMICAL CO. PILOT PLANT #2 (11)
• DOW CHEMICAL CO. PILOT PLANT #3 (11)
A SEDALIA, MO. (14)
NUMBERS IN PARENTHESES INDICATE REFERENCES
I 2 3
HYDRAULIC LOADING, GPM/FT2 (INCLUDING RECYCLE)
-------
FIGURE 4-5
EFFECT OF ORGANIC LOADING ON PERFORMANCE OF
PLASTIC MEDIA TRICKLING FILTERS
100
80
60
40
20
LEGEND*
• DOW CHEMICAL CO. PILOT PLANT#1 ,(II )
A GERMAIN (12)
• MOORE (13)
O DOW CHEMICAL CO. PILOT PLANT #2 (11)
• DOW CHEMICAL CO. PILOT PLANT #3 (11)
* SEDALIA, MO. (14)
*NUMBERS IN PARENTHESES INDICATE REFERENCES
100 200 300
ORGANIC LOADING LBS BOD/1000 FT3/ DAY
(INCLUDING RECYCLE)
400
4-10
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4.3.7 Temperature of Applied Wastewater
The efficiency of trickling filters is affected by temperature changes. The effect of
temperature on filter performance is expressed by the following relationship (16) (17):
ET = E200T~20
where:
8 = Constant varying from 1.035 to 1.041
Ej = Filter efficiency at temperature, T
£20 = Filter efficiency at 20°C
T = Wastewater temperature, °C
Filter performance was observed to vary 21 percent between summer and winter months
in several high-rate filters in Michigan (8). The effect of temperature was especially
pronounced in high-rate filters due to the cooling effect of recirculation. It has been
reported that covering of filters in cold climates does not substantially increase the
performance because the filter covering does not increase the temperature of the applied
wastewater (18).
4.4 Trickling Filter Design Formulas
Several attempts have been made to delineate the fundamentals of the trickling filter process
based on actual operating data from trickling filter plants correlating several variables that
affect trickling filter operation. Analyses of operating data were made to establish equations
or curves that best fitted the available data. The results of these data analyses led to
the development of the following various trickling filter formulations:
1. National Research Council Formula.
2. Ten-States Standards.
3. Velz.
4. Rankin.
5. Caller and Gotaas.
6. Schulze.
7. Eckenfelder.
Although the trickling filter formulas represent attempts to include many of the variables
that can affect trickling filter operations, the use of any one of these formulas does not
universally reflect the actual performance of filters.
4-11
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4.4.1 National Research Council Formula (NRC) (9)
The NRC formulation was the result of an extensive analysis of operational records from
stone-media trickling filter plants serving military installations. Based on data analysis, the
NRC recommended the following formulas for predicting the performance of stone-media
trickling filters:
First or Single Stage:
c. - 100
• W\ '/2
1 +0.0085 777:
Second Stage:
100
E? =
j , 0.0085
where:
EI = Percent BOD removal efficiency through the first-stage filter and
clarifier
W = BOD loading (Ibs./day) to the first or single-stage filter, not including
recycle
V = Volume of the particular filter stage in acre-ft.
F = Recirculation factor for a particular stage, (1 + R)/(l + 0.1
R = Recirculation ratio = recirculated flow/plant influent flow
E2 = Percent BOD removal efficiency through the second-stage filter and
clarifier
W^ = BOD loading (Ibs./day) to the second-stage filter, not including recycle
Some of the limitations of the NRC formulas are:
1. Military wastewater is characteristically more concentrated than average domestic
wastewaters.
2. The effect of temperature on trickling filter performance is not considered.
3. NRC formulas indicate that organic loading has a greater influence on filter
efficiency than hydraulic loading. This is probably because of the concentrated
nature of the wastewaters.
4. Applicability is limited to concentrated domestic wastewaters because no factor
is included to account for differing treatability rates.
5. The formula for second-stage filters is based on the existence of intermediate
settling tanks following the first-stage filters.
4-12
-------
A comparative plot of trickling filter operational data with the predicted value using the
first or single-stage NRC formula is shown in Figure 4-6. It is clear from Figure 4-6 that
the use of the NRC formula may result in substantial deviation from the actual performance
of a trickling filter.
4.4.2 Ten-States Standard Design Guidelines
The data analysis of plants located in the colder northern regions of the United States
by the Great Lakes-Upper Mississippi River Board of Sanitary Engineers led to the
development of design guidelines for trickling filters. In the 1968 edition of the Ten-States
Standards, the Board has presented a loading curve for single-stage stone media filters
which is reproduced in Figure 4-7 (15). In developing Figure 4-7, loading due to
recirculation has not been considered.
The limitations of this design curve are:
1. The formulation is based on data obtained from colder regions.
2. Hydraulic loadings are considered to have no influence on the efficiency of the
filter.
3. Applicability is limited to domestic wastewaters within a specific concentration
range.
4.4.3 Velz Formula (21)
In 1948, Velz proposed the first major formulation delineating a fundamental law as
contrasted to previous attempts based on data analysis. The Velz formula relates the BOD
remaining at depth D as follows:
where:
L = Total removable BOD, mg/1
LD = Removable BOD at depth D, mg/1
D = Filter depth, ft.
K = Constant
Removable BOD in the Velz formula is defined as the maximum fraction of applied BOD
removed at a specific hydraulic loading range.
4-13
-------
FIGUftE 4-6
COMPARISON OF TRICKLING FILTER OPERATING DATA WITH NRC FORMULA
100
BO
60
40
20
LEGEND*
A
O
BETHLEHEM, PA.(lfl)
BURGESS ET. AL.(7)
DEEDS & DATA (19)
HOMACK (20)
McCABE & ECKENFELDER (6)
GALLED & GOTAAS (4)
NATIONAL RESEARCH COUNCIL (9)
*NUMBERS JN PARENTHESES INDICATE REFERENCES
2000
4000
LBS BOO/DAY
EQUIVALENT ACRE-FT
6000
8000
-------
35
30
25
20
CO
OQ
15
10
FIGURE 4-7
TEN STATE STANDARD DESIGN GUIDELINE (15)
0 10 20 30 40 50
BOD APPLIED - IBS. PER 1000 CU.FT.
'INCLUDE BOD REMOVAL IN THE TRICKLING FILTER AMD SECONDARY CLARIFIER.
4-15
-------
4.4.4 Rankin Formula
In 1955, Rankin developed empirical formulas based on the Ten-States Standards, including
the following equation for a single-stage plant (6):
where:
Le = BOD of settled filter effluent, mg/1
La = BOD of primary effluent, mg/1
R = Recirculation ratio
For two-stage, high-rate filters, the Ten-States Standards recommends that "the BOD load
applied to the second-stage filter, recirculation included, shall not exceed two times the
BOD expected in the settled effluent. When the effluent of the first-stage filter is applied
directly to the second stage without intermediate settling, the assumed BOD removal by
the first stage shall not exceed 50 percent.(6)" Based on the previous statements, Rankin
developed the following equations:
= 0.5 La
and
where:
La = BOD of primary effluent, mg/1
Lej = BOD of the unsettled effluent of first-stage filter, mg/1
Le2 = BOD of the settled effluent of second-stage filter, mg/1
R2 = Recirculation ratio of second-stage filter
4.4.5 Caller and Gotaas Formula
In 1964, the last major effort to forecast the performance of stone filters was attempted
by Caller and Gotaas (4) using multiple regression analysis of data from existing plants.
4-16
-------
Based on regression analysis, the following equation was developed:
Le- (i + r)0.78(1+D)0.67a0.25
where:
K =
i0.28T0.15
Le = Unsettled filter effluent BOD, mg/1
Lj = Filter influent BOD, mg/1
D = Filter depth, feet
i = Influent flow, mgd
r = Recirculation flow, mgd
a = Filter radius, feet
T = Wastewater temperature, °C
The Caller and Gotaas formula recognized the effects of recirculation, hydraulic loading,
filter depth, and wastewater temperature as being important in understanding the
performance of a trickling filter. They further indicated that recirculation improves the
performance of a filter, but established a 4:1 ratio as a practical upper limit for
recirculation.
4.4.6 Schulze Formula
In 1960, Schulze (22) postulated that the time of liquid contact with the biological mass
is directly proportional to the filter depth and inversely proportional to the hydraulic
loading rate; this is expressed as follows:
Qn
where:
t = Liquid contact time, minutes
C = Constant
D = Filter depth, feet
Q = Hydraulic Loading rate, gpm/sq.ft.
n = Exponent characteristic of the filter media
4-17
-------
Combining the time of contact with the first-order equation for BOD removal, in an
adaptation of the Velz theory, Schulze derived the following formula:
k = g-KD/Q11
where:
Le = BOD of unsettled filter effluent, mg/1
Lj = BOD of filter effluent
K = Treatability constant
n = Exponent characteristic of the filter media
D = Filter depth, feet
In 1965, Germain applied the Schulze formulation to a plastic media (Dowpac) filter as
follows (12):
h = e-KD/Q11
where:
Lo = BOD of primary effluent (not including recirculation), mg/1
Le = BOD remaining, mg/1
D = Depth of filter, feet
Q = Hydraulic load, gpm/sq.ft. (not including recirculation)
K = Treatability constant
n = Exponent characteristic of filter media
Germain found that K and n for Dowpac media treating domestic primary effluent were
0.088 and 0.5, respectively.
4.4.7 Eckenfelder Formula
In 1963, Eckenfelder modified the equations of Schulze to include the effect of changes
in filter depth on the BOD removal per unit of depth. Eckenfelder proposed the following
equations (23) (24):
_ J T e
L0 " i , CPU-"*) and L°= i+R where:
ffl"
LJ = Influent BOD (not including recirculation), mg/1
L0 = Influent BOD (including recirculation), mg/1
Le = BOD of unsettled filter effluent, mg/1
R = Recirculation ratio
With A in acres, D in feet and Q in mgd:
C = 2.5; l-m = 0.67; n = 0.5
4-18
-------
4.5 Applicability of Various Trickling Filter Design Formulas
The design engineer has available several formulas for trickling filter designs, and the
decision to use one in preference to another is often difficult. The availability of several
formulas often raises doubts concerning their validity in the mind of the design engineer.
An attempt has been made by Hanumanulu (25) to compare the actual performance of
a 12-ft. deep stone media trickling filter with that predicted using NRC, Ten-States
Standards, Velz, Eckenfelder, and Caller and Gotaas formulas. The filter was operated
at a constant flow without recycle as well as with a 1:1 recirculation ratio. It was found
that Velz, Ten-States Standards, and the NRC formulas predict filter efficiencies that are
closer to observed values when operated without recycle, while the Eckenfelder and Caller
and Gotaas formulas predict efficiencies closer to observed values for filters operated with
recirculation.
Ordon (26) calculated the volume of filter media required to achieve specified BOD
removals using the NRC, Eckenfelder, and Caller and Gotaas1 formulas. The wastewater
flow, BOD, and temperature were assumed as 1 mgd, 100 mg/1, and 20°C, respectively.
The volume of filter calculated by the different formulas is shown in Table 4-2.
Inspection of Table 4-2 indicates characteristic trends which the designer should be aware
of before using any of these formulas. In Table 4-2, when recirculation was zero, the
filter volumes calculated from the NRC and Eckenfelder formulas were essentially the
same, while the Caller and Gotaas formula gave volumes which were significantly different.
However, when recirculation was considered, the NRC design volumes were generally quite
conservative, while the volumes calculated by the Eckenfelder and Caller and Gotaas
formulas were more nearly the same. In general, the NRC formulas would seem to apply
when recirculation is not considered, when seasonal temperature differentials are minor,
and when the wastewater load is highly variable and of high strength.
4.6 Laboratory and Pilot-Scale Treatability Studies
Trickling filters traditionally have been designed using one of the several formulas cited
previously. The use of treatability studies for design of trickling filters has been hampered
by the lack of suitable laboratory-scale testing methods, and has generally been restricted
to the plastic-media filters, with pilot units being supplied by the manufacturers of plastic
media on a rental basis. The pilot units available from the plastic media manufacturers
require considerable manpower and funds to obtain the meaningful data needed for design
purposes. Treatability studies for evaluation of stone-media filter design parameters are
usually not performed.
However, it is interesting to note that advances are being made in the development of
a practical laboratory-scale piloting facility for both stone and plastic media. Based on
the concept of contact time as introduced by Schulze, the trickling filter process may
be modeled by using an inclined plane to support biological growth (27).
4-19
-------
Table 4-2
Trickling Filter Volumes for
Various Organic Removals as Calculated
by Different Design Formulas
(All Volumes in Thousands of Cubic Feet)
£
0
Recirculation
Ratio
0
1
2
3
4
5
6
50%
NRC'
2.7
1.7
1.4
1.2
1.0
0.9
_
ROD Rerr
ECK2
3.8
0.96
0.42
0.24
0.15
0.12
0.08
i oval
G&G3
0.2
0.12
0.12
0.12
0.12
0.12
0.12
60%
NRC
6
3.6
2.8
2.6
2.4
2.2
_
BOD Rem
ECK
8.5
2.2
0.95
0.55
0.35
0.24
0.17
OVflt
G&G
1.2
0.28
0.26
0.26
0.26
0.26
0.26
__2Q%_RQD Removal
NRC
15
8.8
6.8
6.1
5.8
5.7
_
ECK
20
5
2.3
1.3
0.8
0.6
0.4
G&G
10
1.8
1.2
0.9
0.9
0.9
0.9
_Z5_&BOD Removal
NRC
23
15
11
9.9
9.3
8.8
_
ECK
32
8
3.5
2
1.5
0.92
0.67
G&G
42
5
2.4
1.8
1.6
1.5
1.5
80% BOD Removal
NRC
40
25
20
18
17
16
_
ECK
58
15
7
4
2.5
1.8
1.4
.G&G
300
23
7.3
4.3
3.6
3.0
-
90% BOD Removal
NRC
210
130
105
90
85
80
-
.ECK.
290
75
35
120
14
9
6
G&G
-
400
170
80
45
-
'NRC - National Research Council
2ECK - Eckenfelder
3G&G - Caller and Gotaas
Design Conditions
Filter Influent Flow = 1 mgd
Filter Influent BOD = 100 mg/1
Wastewater Temperature = 20°C
-------
Wastewater is introduced at a variable rate to the top of a slimed inclined plane. The
plane's inclination may be varied to change the contact time. As previously discussed,
Schulze's formula relates the contact time to the depth and hydraulic loading, as well
as to the physical characteristics of the filter media. BOD removal is then assumed to
vary with the following first-order removal equation:
where:
Le = BOD of unsettled filter effluent, mg/1
LJ = BOD of filter influent, mg/1
K = Treatability constant
t = Contact time, minutes
The inclined plane method furnishes data on BOD removal, contact time, hydraulic loading,
and recirculation ratios.
Since the basic purpose of either a laboratory or pilot-plant evaluation is to study variables
that affect filter performance, any treatability studies should be of sufficient duration,
and should consider the following variables as they affect the filter performance:
1. Applied BOD loading.
2. Hydraulic loading.
3. Recirculation.
4. Wastewater temperature.
The data thus obtained from treatability studies can be evaluated using the various trickling
filter formulas previously discussed.
4.7 Trickling Filter Upgrading Techniques and Design Basis
Upgrading of trickling filter plants may be required because the plants are hydraulically
and/or organically overloaded, because of the need for increased treatment efficiency, or
both. Upgrading to relieve overloaded conditions and upgrading to improve removal
efficiency to meet higher water quality standards will be covered in the following sections.
4.7.1 Upgrading to Relieve Organic and Hydraulic Overloading
Trickling filter plants may be upgraded to relieve hydraulic and/or organic overloading
by any one of the following three general procedures:
1 . Upgrading an existing single-stage filter to adequately handle an increased load,
either organic or hydraulic.
4-21
-------
2. Upgrading a single-stage trickling filter to a two-stage biological system.
3. Upgrading an existing two-stage trickling filter to a multiple-stage biological
system.
There are several factors that should be considered prior to upgrading a trickling filter
plant. Since upgrading varies from plant to plant, only general observations can be made.
Items to be considered are:
1. Check the hydraulic capacity of the trickling filter distributor arm to determine
the recommended operating range.
2. Investigate the ventilation in all pipes, channels, and drains. As previously
discussed, not more than 50 percent of any conduit's cross-section should be
submerged under average hydraulic design loading.
3. Decide whether to use direct recirculation after the filter or recirculation of
the clarified effluent. One study has indicated that direct recirculation of filter
effluent is as effective as recycling clarified effluent (28).
4. Evaluate the capability of the secondary clarifier to determine if additional
capacity is required and if the sludge-collection mechanism is performing
correctly.
5. Check and evaluate the capacity of the sludge-handling facilities. Upgrading
secondary treatment facilities usually results in an increased sludge production.
The following examples are illustrative in nature and are not based on actual performance
data unless specified in the text.
4.7.1.1 Upgrading a Single-Stage Trickling Filter-Conversion From Low-Rate
to High-Rate (Example A)
Upgrading of a hydraulically overloaded low-rate trickling filter can be accomplished by
converting it to a high-rate trickling filter through recirculation. This upgrading procedure
has been used successfully in the following areas: Edgerton, Wisconsin; Flandreau, South
Dakota; Pueblo, Colorado; and Coeur d'Alene, Idaho (29).
Example A will help to illustrate the design considerations involved in upgrading a low-rate
filter to a high-rate filter. A flow diagram for the overloaded plant appears in Figure 4-8.
Table 4-3 contains a comparison of the original design values of the low rate filter before
it was overloaded, as well as the operating data from the overloaded plant before upgrading.
The flow increased from 185,000 gpd to 370,000 gpd, while the BOD and suspended
solids in the effluent increased from 30 mg/1 and 23 mg/1, respectively, to 44 mg/1 and
36 mg/1.
4-22
-------
FIGURE 4-8
UPGRADING A LOW-RATE TRICKLING FILTER
TO A HIGH-RATE TRICKLING FILTER
EXAMPLE A
PRIMARY EFFLUENT
185,000 6PD
TRICKLING
FILTER
•O
vJ
SECONDARY
CLARIFIER
SLUDGE
EFFLUENT
TREATMENT SYSTEM BEFORE UPGRADING
LOW-RATE TRICKLING FILTER
EXISTING
TRICKLING
FILTER
NEW RECIRCULATION PUMPING STATION
EXISTING SECONDARY
CLARIFIER
PRIMARY EFFLUENT -*-j
370,000 GPD
EFFLUENT
RECIRCULATION 185,000 GPD
ADDITIONAL
REQUIRED
CAPACITY
SLUDGE
TREATMENT SYSTEM AFTER UPGRADING
HIGH-RATE TRICKLING FILTER
4-23
-------
Table 4-3
Operational and Design Data for Example A
Description
Flow - gpd
Influent BOD, mg/1
Influent SS, mg/1
Primary Clarifier
Overflow Rate, gpd/sq.ft.
Percent BOD Removal
Percent SS Removal
Trickling Filter
Depth, feet
Hydraulic Loading, mgad 1
Organic Loading, Ibs. BOD/1,000 cu.ft./day1
Recirculation Ratio
Secondary Clarifier
Overflow Rate, gpd/sq.ft.
Secondary Treatment
Percent BOD Removal
Percent SS Removal
Overall Plant Performance
Percent BOD Removal
Percent SS Removal
Effluent BOD, mg/1
Effluent SS, mg/1
Original
Design
Before
Overloading
185,000
230
210
700
32
55
6
4.6
23
0
800
81
75
87
89
30
23
Overloaded
Operating
Condition
370,000
210
200
1,400
23
48
6
9.2
48
0
1,600
73
65
79
82
44
36
Upgraded
Design
Calculations
370,000
210
200
700
32
55
6
13.8
47
0.5
800
79
75
86
89
30
22
Includes recirculation.
4-24
-------
To upgrade the plant to its previous performance, it was decided to renovate the plant
so that it could treat the flow of 370,000 gpd as a high-rate filter. The initial step was
to evaluate the quantity of recycled flow to be returned ahead of the filter. This can
be done by using one of the trickling filter design formulas presented in the previous
section. The results of the upgrading calculations are presented in Table 4-3, and the
upgraded flow diagram is presented in Figure 4-8.
To implement this upgrading, several factors were investigated. The hydraulic capacity
of ,the existing distributor arm was found to be deficient and therefore replaced. The
hydraulic head available to the filter was found to be limiting; therefore, the new distributor
arm was motorized. The existing filter media and underdrains were found to be in good
condition. The hydraulic capacity of the drains was evaluated and found to be sufficient.
A recirculation pumping station was constructed with variable-speed pumping capacity
regulated with flow-proportioning pump controls. The final clarification capacity was
increased to accommodate the larger flows. In addition, the primary clarification capacity
would also have to be increased, but this cost is not considered in this unit operations
section.
The capital costs associated with this upgrading were estimated at $96,000 ($519 per
1,000 gpd of incremental upgraded capacity) and were allocated as follows:
Trickling Filter Modification $51,000
Recirculation Facilities 15,000
Secondary Clarifier Expansion 30,000
TOTAL $96,000l
4.7.1.2 Upgrading a Single-Stage Trickling Filter - Conversion from High-Rate
to Completely-Mixed Activated Sludge (Example B)
In 1965, the Ontario Water Resources Commission set 15 mg/1 of suspended solids and
BOD as the objectives for secondary treatment plant effluents. Such an effluent quality
could not be achieved with an existing high-rate trickling filter plant at Gravenhurst, Ontario
(30).
To upgrade the high-rate filter, the plant was converted to completely-mixed activated
sludge. The filter was converted to an aeration tank 40 feet in diameter by removing
the media and raising the concrete sidewalls by seven feet to a total height of 12 feet.
A 10-hp mechanical aerator was installed. The duo-clarifier (combination primary and
* These costs are based on ENR index of 1500 and contain no contingency for engineering
design, bonding, and construction supervision.
4-25
-------
secondary clarifier) was converted to a 40-foot diameter secondary clarifier, and a new
35-foot diameter primary clarifier was constructed. A 100-percent sludge recycle capacity
was provided.
The previously described upgrading technique resulted in the following measured
improvements:
Before After
Parameter Upgrading Upgrading
Dry weather design flow, gpd
Influent organic load, Ibs. BOD/day
Effluent BOD, mg/1
300,000
360
>20
375,000
540
15-20
The capital costs for this upgrading were estimated at $70,000 and were allocated as
follows:
Tank modification
Secondary clarifier modification
TOTAL
$55,000
15.000
$70,000
1
These costs do not include upgrading of any other unit treatment processes, e.g., primary
clarification.
4.7.1.3 Upgrading a Single-Stage Trickling Filter to a Two-Stage Biological
System - Conversion From a Single-Stage to a Two-Stage Filtration
System (Example C)
Organically overloaded low, intermediate, and high-rate trickling filters may be upgraded
by converting them to two-stage filtration systems, utilizing rock media for both stages.
Example C depicts such an upgrading, and it will illustrate the major considerations to
be evaluated.
A flow diagram for Example C before upgrading is shown in Figure 4-9. A summary
of the operating data is presented in Table 4-4. Treating a flow of 6 mgd, the
intermediate-rate filter produced a final effluent with BOD and SS concentrations of
99 mg/1 and 85 mg/1, respectively. To improve the organically overloaded conditions, it
was decided to design a high-rate filter and intermediate clarifier to operate ahead of
the existing, intermediate-rate filter. The appropriate recirculation ratio and filter volume
were calculated using one of the design formulas previously discussed. The results of these
* These costs are based on ENR index of 1500 and contain no contingency for engineering
design, bonding, and construction supervision.
4-26
-------
FIGURE 4-9
UPGRADING A SINGLE-STAGE TRICKLING FILTER
TO A TWO-STAGE FILTRATION SYSTEM
EXAMPLE C
INTERMEDIATE-RATE
TRICKLING FILTER
PRIMARY
EFFLUENT
6.0 M6D
SLUDGE
SECONDARY
CLARIFIER
FINAL
EFFLUENT
TREATMENT SYSTEM BEFORE UPGRADING
SINGLE-STAGE INTERMEDIATE-RATE TRICKLING FILTER
RECIRCULATION 7.5 MGD
NEW RECIRCULATION PUMPING STATION
SLUDGE
PRIMARY
EFFLUENT
6.0 MGD
FINAL
1
EFFLUENT
1 ST. STAGE-NEW NEW
HIGH-RATE FILTER INTERMEDIATE
CLARIFIER
2ND STAGE-EXISTING EXISTING
INTERMEDIATE-RATE SECONDARY
FILTER CLARIFIER
TREATMENT SYSTEM AFTER UPGRADING
TWO-STAGE TRICKLING FILTRATION SYSTEM
(BOTH STAGES UTILIZE ROCK MEDIA)
4-27
-------
Table 4-4
Operational and Design Data for Example C
Overloaded Upgraded
Operating Design
Description Condition Calculations
Flow - mgd 6 6
Influent BOD, mg/1 355 355
Influent SS, mg/1 340 340
Primary Clarifier
Overflow Rate, gpd/sq.ft. 750 750
Percent BOD Removal 38 38
Percent SS Removal 60 60
Trickling Filter - 1st Stage
Depth, feet - 4
Hydraulic Loading, mgadl - 19
Organic Loading, Ibs. BOD/1,000 cu.ft./day1 - 110
Recirculation Ratio - 1.25
Intermediate Clarifier
Overflow Rate, gpd/sq.ft.l - 1,000
Percent BOD Removal - 1 st Stage - 81.7
Percent SS Removal - 1st Stage - 70.0
Trickling Filter - 2nd Stage
Depth, feet 7 7
Hydraulic Loading, mgad 1 8.3 8.3
Organic Loading, Ibs. BOD /1,000 cu.ft./dayJ 50 10
Recirculation Ratio 0 0
Final Clarifier
Overflow Rate, gpd/sq.ft. 800 800
Percent BOD Removal - 2nd Stage 55 50
Percent SS Removal - 2nd Stage 38 63
Overall Plant Performance
Percent BOD Removal 72 94
Percent SS Removal 75 96
Effluent BOD, mg/1 99 20
Effluent SS, mg/1 85 15
1 Includes recirculation.
4-28
-------
calculations are presented in Table 4-4, and the upgraded flow diagram is shown in
Figure 4-9. The upgraded effluent is expected to contain 20 mg/1 of BOD and 15 mg/1
of SS.
This type of upgrading, in which a complete set of units is added, is far less complicated
than a renovation of existing tankage. The most important consideration in this type of
upgrading is that sufficient hydraulic head be available to operate the individual unit
processes properly. In Example C, the major capital costs include providing new high-rate
filters, intermediate clarifiers, a recirculation pumping station regulated with
flow-proportioning pump controls, and the appropriate piping. The capital costs associated
with this upgrading were estimated to be $1,500,000 and were allocated as follows:
Trickling Filter Additions $ 1,000,000
Recirculation Facilities 100,000
Intermediate Clarification 400,000
TOTAL $1,500,000l
4.7.1.4 Upgrading a Single-Stage Trickling Filter to a Two-Stage Biological
System - Conversion of a Single-Stage Filter to a Filtration/Activated
Sludge System (Example D)
If the hydraulic and organic loads to a high-rate filter are such that it would not produce
a high degree of BOD removal, it is possible to upgrade the facility by the addition of
an activated sludge unit immediately downstream from the existing filter. In this situation,
the existing overloaded trickling filter acts as a roughing filter, and the subsequent activated
sludge unit provides the treatment capacity needed to obtain the desired BOD removal.
Example D illustrates the major considerations in this type of upgrading. The flow diagram
of the overloaded plant appears in Figure 4-10, and operating data for the overloaded
period are presented in Table 4-5. The existing plant was upgraded by the addition of
a completely-mixed activated sludge system The calculations for the upgrading are
summarized in Table 4-5, and the upgraded flow diagram is shown in Figure 4-10.
Implementation of this upgrading technique would reduce the effluent BOD an estimated
150 mg/1, from 220 mg/1 to 70 mg/1. Details concerning the design of an activated sludge
system treating an effluent from a single-stage biological treatment process are presented
in a later section.
Construction costs for Example D include the costs for a completely-mixed aeration basin,
floating mechanical aerators, and an activated sludge recirculation pumping station. In
1 These costs are based on an ENR index of 1500 and contain no allowance for engineering
design, bonding, and construction supervision.
4-29
-------
FIGURE 4-10
UPGRADING A HIGH-RATE TRICKLING FILTER
TO A TWO-STAGE FILTRATION/ACTIVATED SLUDGE SYSTEM
EXAMPLE D
PRIMARY
EFFLUENT
2.0 MGD
RECIRCULATION 6.0 MGD
TRICKLING FILTER
RECIRCULATION
PUMPING STATION
SECONDARY
CLARIFIER
FINAL
EFFLUENT
TREATMENT SYSTEM BEFORE UPGRADING
HIGH-RATE TRICKLING FILTER
PRIMARY
EFFLUENT
2.0 MGD
RECIRCULATION 6.0 MGD
EXISTING
TRICKLING
FILTER
•EXISTING RECIRCULATION
PUMPING STATION
r-NEW
\ COMPLETELY MIXED/
AERATION TANK
NEW 100% SLUDGE
RECYCLE FACILITIES
-EXISTING
SECONDARY
CLARIFIER
FINAL
EFFLUENT
TREATMENT SYSTEM AFTER UPGRADING
FILTRATION/ACTIVATED SLUDGE SYSTEM
4-30
-------
Table 4-5
Operational and Design Data for Example D
Overloaded Upgraded
Operating Design
Description Conditions Calculations
Flow, mgd 2 2
Influent BOD, mg/1 550 550
Influent SS, mg/1 400 400
Primary Clarifier
Overflow Rate, gpd/sq.ft. 700 700
Percent BOD Removal 20 20
Percent SS Removal 40 40
Trickling Filter
Depth, feet 6 6
Hydraulic Loading, mgadl 50 50
Organic Loading, Ibs. BOD/1,000 cu.ft. /day 260 260
Recirculation Ratio 3.0 3.0
Percent BOD Removal as a Roughing Unit - 50
Completely-Mixed Aeration Tank
Detention Time Based on Average Flow, hours 1 - 3.0
Sludge Recycle Capacity, percent of design flow - 100
Volumetric Loading, Ibs. BOD/day/1,000 cu.ft.
of Aeration Tank Volume - 50
Secondary Clarifier
Overflow Rate, gpd/sq.ft. 700 700
Percent BOD Removal2 50 84
Percent SS Removal2 40 67
Overall Plant Performance
Percent BOD Removal 60 87
Percent SS Removal 50 80
Effluent BOD, mg/1 220 70
Effluent SS, mg/1 144 80
1 Includes recirculation
2In secondary units including the roughing filter.
4-31
-------
addition, the secondary clarifier was modified to use a suction-type sludge removal
mechanism. The cost for this upgrading has been estimated at $320,000, and is broken
down as follows:
Aeration Tank $190,000
Sludge Recirculation 70,000
Clarifier Modifications 60,000
TOTAL $3 20,000l
4.7.1.5 Upgrading a Single-Stage Trickling Filter to a Two-Stage Biological
System - Addition of a Super-Rate Roughing Filter to a Single-Stage
Trickling Filter (Example E)
An organically overloaded high-rate trickling filter may be upgraded by placing a synthetic
media super-rate filter immediately upstream to act as a roughing unit. Example E is
presented to illustrate the engineering considerations which must be evaluated.
Figure 4-11 contains flow diagrams of the secondary treatment system before and after
upgrading. Table 4-6 contains operational data from the overloaded plant. The roughing
filter was sized using the Schulze formula presented previously. Design data for the roughing
filter is also summarized in Table 4-6. By removing 30 percent of the applied BOD in
the roughing filter (not including recirculation), it was possible to reduce the recirculation'
ratio from 3 to 2 on the existing high-rate filter.
The construction costs include the roughing filter, a recirculation pumping station with
flow-proportioning controls, and all appropriate piping. This upgrading was estimated to
cost $215,000 and is allocated as follows:
Roughing Filter $190,000
Recirculation Facilities 25,000
TOTAL $215,000*
4.7.1.6 Upgrading an Existing Two-Stage Trickling Filter to a Multiple-Stage
Biological System
Fewer options are available for upgrading a hydraulically or organically overloaded
two-stage filter than for upgrading a single-stage filter. In general, there are three options
available to the engineer faced with upgrading an overloaded two-stage filter:
These costs are based on an ENR index of 1500 and contain no allowance for engineering
design, bonding, and construction supervision.
4-32
-------
FIGURE 4-11
UPGRADING A HIGH-RATE TRICKLING FILTER USING
A SUPER-RATE TRICKLING FILTER AS A ROUGHING UNIT
EXAMPLE E
PRIMARY
EFFLUENT.
2.0 MGD
RECIRCULATION 6.0 MGD
TRICKLING
FILTER
SECONDARY
CLARIFIER
RECIRCULATION
PUMPING STATION
FINAL
EFFLUENT
TREATMENT SYSTEM BEFORE UPGRADING
HIGH-RATE TRICKLING FILTER
PRIMARY
EFFLUENT
2.0 MGD
NEW
SYNTHETIC MEDIA
ROUGHING FILTER
NEW RECIRCULATION
PUMPING STATION
1.5 MGD
EXISTING
TRICKLING
FILTER
EXISTING
SECONDARY
CLARIFIER
EXISTING
RECIRCULATION
PUMPING
STATION
FINAL
EFFLUENT
TREATMENT SYSTEM AFTER UPGRADING
ROUGHING FILTER PRECEEDING EXISTING HIGH-RATE FILTER
4-33
-------
Table 4-6
Operational and Design Data for Example E
Description
Flow, mgd
Influent BOD, mg/1
Influent SS, mg/1
Primary Clarifier
Overflow Rate, gpd/sq.ft.
Percent BOD Removal
Percent SS Removal
Roughing Filter
Depth, feet
Hydraulic Loading, mgad 1
Organic Loading, Ibs. BOD/1,000 cu.ft./day1
Recirculation Ratio
Percent BOD Removal
Trickling Filter
Depth, feet
Hydraulic Loading, mgad 1
Organic Loading, Ibs. BOD/1,000 cu.ft./day1
Recirculation Ratio
Secondary Clarifier
Overflow Rate, gpd/sq.ft.1
Percent BOD Removal2
Percent SS Removal2
Overall Plant Performance
Percent BOD Removal
Percent SS Removal
Effluent BOD, mg/1
Effluent SS, mg/1
Overloaded
Operating
Conditions
2
550
400
700
20
40
6
50
260
3.0
700
50
40
60
50
220
144
Upgraded
Design
Calculations
2
550
400
700
20
40
11
62.6
430
0.75
30
6
38
147
2.0
520
84
67
87
80
70
80
1 Includes recirculation
2In secondary units including roughing unit
4-34
-------
1. Construction of a roughing filter preceding the existing system.
2. Construction of an activated sludge system following the existing system.
3. Construction of a separate parallel biological treatment system.
A detailed discussion will not be presented here, since most of the engineering
considerations pertaining to these three options have been examined in previous sections.
4.7.2 Upgrading to Increase Organic Removal Efficiency
Upgrading techniques previously discussed relate to the ability of existing facilities to handle
increased hydraulic or organic loads by providing modifications to meet existing effluent
standards. However, there may be a need to meet higher effluent standards even though
the existing facilities are not hydraulically or organically overloaded. Table 4-7 contains
suggested alternatives for improving effluent quality under these conditions. The main
purpose of the table is to present various alternatives and to suggest a range of anticipated
improvement in performance for each alternative.
It should be emphasized that, in cases where unit processes are added to existing facilities,
the improvement in overall organic removal will be a direct function of the BOD removal
achieved in the "add-on" unit process, e.g., a polishing lagoon. However, where unit
processes precede existing units, e.g., the use of a roughing filter, the overall BOD removal
may not be increased in direct proportion to the amount achieved in the "add-on" process.
A detailed discussion on polishing lagoons, microstrainers, filters, activated carbon, and
clarifier modifications appears in subsequent chapters. The applicability of alternatives to
individual cases should be evaluated in detail prior to the implementation of a particular
upgrading procedure.
4-35
-------
\
Table 4-7
Upgrading Techniques for Improvement of Trickling Filter Plant Efficiency
Incremental BOD Removal
Addition Preceding Modification to Addition Following Across the Added or
Existing Unit Existing Unit Existing Unit Modified Process
percent
1. Low-Rate Trickling Filter
Add Recirculation
during low-flow
periods 0-10
^ 2. High-Rate Trickling Filter
OJ
O^ Increase Recirculation 0-10
3. Two-Stage Trickling Filter
_1
Roughing Trickling Filter
(Rock or Synthetic Media) 20-40
Chemical Addition
To Primary Clarifier 30-50
2nd Stage Activated Sludge- 30-70
Polishing Lagoon 30-60
Multi-media Filters 50-80
Microstraining 30-80
Activated Carbon 60-80
'Generally not amenable to modifications for increasing treatment efficiency.
2 A consideration if year-round nitrification is required.
-------
4.8 References
1. Statistical Summary 1968 Inventory Municipal Waste Facilities in the United States.
Federal Water Quality Administration: Government Printing Office, 1971.
2. McKinney, R., Microbiology for Sanitary Engineers. New York: McGraw Hill Book
Company, Inc., 1962.
3. Sewage Treatment Plant Design. Water Pollution Control Federation Manual of
Practice No. 8, Washington, D.C., 1959.
4. Caller, W.S., and Gotaas, H.B., Analysis of Biological Filter Variables. Journal of
the Sanitary Engineering Division, ASCE, 90, No. 6, pp. 59-79 (1964).
5. Reynolds, L.B., and Chipperfield, P.N.J., Principles Governing the Selection of Plastic
Media for High-Rate Biological Filtration. Presented at the International Congress on
Industrial Waste Water, Stockholm, Sweden, 1970.
6. McCabe, J., and Eckenfelder, W., Biological Treatment of Sewage and Industrial
Wastes. New York: Reinhold Publishing Company, 1956.
7. Burgess, F.J., et al, Evaluation Criteria for Deep Trickling Filters. Journal Water
Pollution Control Federation, 33, No. 8, pp. 787-816 (1961).
8. Benzie, W., Effects of Climatic and Loading Factors on Trickling Filter Performance.
Journal Water Pollution Control Federation, 35, No. 4, pp. 445-455 (1963).
9. Sewage Treatment at Military Installations. National Research Council, Sewage Works
Journal, 18, No. 5, pp. 787-1,028 (1946).
10. Bethlehem, Pa.: Private communication with William Grim Plant Operator, November,
1970.
11. Waste Water Treatment. Midland, Michigan: The Dow Chemical Company, 1965.
12. Germain, J., Economic Treatment of Domestic Waste by Plastic - Medium Trickling
Filters. Presented at the 38th Annual Conference of the Water Pollution Control
Federation, Atlantic City, N.J., October, 1965.
13. Moore, R., Pilot Plant Testing for Municipal Sewage Treatment. Journal of Sanitary
Engineering Division, ASCE, 96, No. 2, pp. 573-591 (1970).
14. Sedalia, Mo.: Private communication with R.W. Cunningham Director of Public
Works, December 9, 1970.
4-37
-------
15. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of State Sanitary Engineers, 1968.
16. Rowland, W.E., Flow Over Porous Media as in a Trickling Filter. Proceedings-12th
Purdue Industrial Waste Conference, pp. 435-465 (1957).
17. Eckenfelder, W.W., Industrial Water Pollution Control. New York: McGraw-Hill Book
Company, 1966.
18. Sheahan, J.P., Use of Styrofoam for Trickling Filter Covers. Proceedings-20th Purdue
Industrial Waste Conference, pp. 572-582 (1965).
19. Deeds and Data. Journal Water Pollution Control Federation, 31, No. 3, pp. 315-320
(1959).
20. Homack, P., Discussion of Article by R. Rankin. Transactions of the American Society
of Civil Engineers, 120, pp. 836-841 (1955),
21. Velz, C.J., A Basic Law for the Performance of Biological Beds. Sewage Works Journal,
20, No. 3, pp. 245-261 (1960).
22. Schulze, K.L., Load and Efficiency of Trickling Filters. Journal of Water Pollution
Control Federation, 32, No. 3, pp. 245-261 (1960).
23. Eckenfelder, W.W., Trickling Filter Design and Performance. Transactions of the
American Society of Civil Engineers, 128, Part III, pp. 371-398 (1963).
24. Eckenfelder, W.W., and Barnhart, W., Performance of a High-Rate Trickling Filter
Using Selected Media. Journal Water Pollution Control Federation, 35, No. 12,
pp. 1,535-1,551 (1963).
25. Hanumanulu, V., Effect of Recirculation on Deep Trickling Filter Performance.
Journal of Water Pollution Control Federation, 41, No. 10, pp. 1,803-1,806 (1969).
26. Ordon, C., Discussion of Article by Baker and Graves (Feb. 1968). Journal of the
Sanitary Engineering Division, ASCE, 94, No. 3, pp. 579-583 (1968).
27. Maier, W., et al, Simulation of the Trickling Filter Process. Journal of the Sanitary
Engineering Division, ASCE, 93, No. 4, pp. 91-112 (1967).
28. Culp, G., Direct Recirculation of High-Rate Trickling Filter Effluent. Journal of Water
Pollution Control Federation, 35, No. 6, pp. 742-747 (1963).
4-38
-------
29. Environmental Protection Agency: Private Communication with D. Lussier,
Construction Grants Division, December 22, 1970.
30. Economical Sewage Treatment Plant Conversion at Gravenhurst. Water and Pollution
Control, 106, No. 1, pp. 26-27 (1968).
4-39
-------
CHAPTER 5
TECHNIQUES FOR UPGRADING ACTIVATED SLUDGE PLANTS
5.1 General
The conventional activated sludge process as originally developed has undergone significant
changes, primarily due to a better understanding of the theory involved and to the
experience accumulated over the years in successful operation of the process. Today, it
remains the most versatile and efficient of the available biological treatment processes.
Historically, the activated sludge process has been used in larger cities, where the ratio
of river assimilative capacity to waste load is small. In the past decade, there has been
a trend toward its use by smaller communities to meet the more stringent requirements
stipulated by regulatory agencies.
Existing overloaded conventional activated sludge plants pose a problem to the maintenance
of established water quality standards. Various modifications of the conventional process
developed over the years permit reduced detention time in the aeration tanks; the
applicability of these process modifications in the efficient upgrading of existing plants
will be examined and discussed.
5.2 Activated Sludge Processes
Basically, the activated sludge process uses microorganisms in suspension to oxidize soluble
and colloidal organics to CO2 and H2O in the presence of molecular oxygen. During the
oxidation process, a portion of the organic material is synthesized into new cells. A part
of the synthesized cells then undergoes auto-oxidation in the aeration tanks, the remainder
forming excess sludge. Oxygen is required in the process to support the oxidation and
synthesis reactions. In order to operate the process on a continuous basis, the solids
generated must be separated in a clarifier for recycle to the aeration tank, with the excess
sludge from the clarifiers being withdrawn for further handling and disposal.
5.2.1 Conventional Activated Sludge
A schematic of the conventional activated sludge process is shown in Figure 5-1. The
wastewater is commonly aerated for a period of 6 to 8 hours (based on the average design
flow) in the presence of a portion of the secondary sludge (1). The rate of sludge return
expressed as a percentage of the average wastewater design flow is normally about
25 percent, with minimum and maximum rates of 15 and 75 percent. The plug flow
mixing configuration is used to condition the biological organisms for improved
clarification. This is accomplished in rectangular tanks, designed so that the total tank
length is generally 5 to 50 times the width. Operational data from various conventional
activated sludge plants are summarized in Table 5-1.
5-1
-------
Table 5-1
Operational Data from Various Conventional Activated Sludge Plants
Plant
Location
Michigan
Illinois
Ohio
Indiana
Maryland
Michigan
Wisconsin
Indiana
Indiana
Maryland
Maryland
Influent
Flow
mgd
5.0
288.0
86.9
14.9
3.9
8.0
7.6
3.9
5.5
8.0
7.7
Sludge
Recycle
percent
32.0
47.9
25.0
30.0
32.0
15.5
51.6
30.8
28.5
26.0
25.0
Influent
mg/1
182
129
91
161
254
118
157
134
113
155
148
BOD
Effluent
mg/1
19
11
12
14
32
6
36
14
6
10
15
Aeration
Tank
MLSS
mg/1
1,844
1,930
2,180
2,420
1,808
2,801
1,094
2,625
1,680
2,040
2,240
Organic
Loading
Ibs. BOD/dav
Ib. MLSS
0.34
0.17
0.12
0.16
0.39
0.15
0.38
0.21
0.19
0.23
0.20
Volumetric
Loading
Ibs. BOD/dav
1 ,000 cu.ft.
39
21
17
24
44
26
26
35
20
29
25
Aeration
Detention
Time1
hours
6.99
8.7
7.73
10.0
8.8
6.7
9.1
5.7
8.2
7.7
8.2
Air Supplied
per Ib. of
BOD Removed
cu.ft./lb.
770
876
1.600
733
500
690
690
886
435
1.260
1.900
Secondary
BOD Removal
Efficiency
percent
89.6
91.5
86.9
91.0
86.8
94.9
77.1
89.7
94.7
93.5
89.9
Referc
2
2
2
2
2
2
2
2
3
3
Excluding sludge recycle.
-------
FIGURE 5-1
CONVENTIONAL ACTIVATED SLUDGE FLOW DIAGRAM
RAW „
WASTEWATER '*
1
PRIMARY
SEDIMENTATION
1 SLUDGE TO
DIGESTER
i
UJ
CO
ea
=3
_l
CO
oe
=3
H-
UJ
ac
6$
m
C*l
DERATION ~N
' )
QTANK "
6-8 HOURS -^
f DETENTION
V*
i
RETURN AND EXCESS SLUDGE
FINAL
CLARIFIER
ALTERNATE
» FINAL
EFFLUENT
EXCESS SLUDGE TO
"DIGESTER OR
EXCESS SLUDGE
THICKENER
The following factors have been cited (1) as limitations in the design and use of the
conventional activated sludge process:
1. BOD loadings are limited to about 35 Ibs./1,000 cu.ft./day.
2. A high initial oxygen demand is experienced in the head end of the aeration
tank.
3. The final clarifier is subjected to high solids loadings.
i
i
4. It is necessary to increase sludge recirculation proportionately with increasing
BOD loadings.
5. Detention times are in the range of 6 to 8 hours.
6. There is a lack of operational stability with variations in hydraulic and organic
loadings.
Some of these limitations have stimulated the development and use of various activated
sludge modifications, such as step aeration, contact stabilization, completely-mixed,
two-stage activated sludge, and the use of oxygen aeration instead of air as a source of
dissolved oxygen. These modifications are discussed in subsequent sections.
5-3
-------
5.2.2 Step Aeration
The step aeration modification is illustrated in Figure 5-2. Unlike the conventional flow
pattern, the influent wastewater is introduced at various points along the length of the
aeration tank. The Ten-States Standards recommends a sludge return rate (based on the
average wastewater flow) of 50 percent, with minimum and maximum rates of
20 and 75 percent. In actual cases, this rate has been found to be as high as 100 percent.
Splitting up the influent flow to the aeration tank reduces the initial oxygen demand
usually experienced in the conventional process, and distributes the organic loading more
uniformly over the length of the aeration tank. This appears to afford a more efficient
utilization of the biological population of the tank, which explains the fact that organic
loadings up to 50 Ibs. BOD/1,000 cu.ft./day have been treated. Operational data from
various step aeration processes are summarized in Table 5-2.
FIGURE 5-2
STEP AERATION FLOW DIAGRAM
AERATION TANK
3-4 HOURS DETENTION TIME
WASTEWATER
SLUDGE TO
DIGESTER
RETURN SLUDGE
FINAL
EFFLUENT
^EXCESS SLUDGE
There is some question regarding the air requirements for this modification. Generally,
the step aeration process will not utilize any more air than a conventional system treating
comparable flows. In some cases, such as in New York City (4) where the sludge recycle
is approximately 25 percent, the air required is about half that normally used for a
conventional process.
5-4
-------
Table 5-2
Operational Data from Various Step Aeration Plants
Plant
Location
New York
New York
New York
New York
New York
New York
Maryland
Washington, D.C.
Washington, D.C.
Indiana
Indiana
Indiana
Indiana
Influent
Flow
mgd
110
20.7
92
50
95
31
16.9
0.125
0.125
12.8
12.4
19.3
34.3
Sludge
Recycle
percent
24
49
35
28
28
28
24
65
50
92
77.3
49.7
31.7
Influent
mg/l
74
137
100
120
115
100
140
82
110
124
134
139
131
BOD
Effluent
mg/l
12
3
8
' 6
16
12
11
18
32
15
14
17
18
Aeration
Tank
MLSS
mg/l
1,170
3,520
1,110
3,300
3,300
4,400
2,120
6,400
2,050
2,900
2,600
2,750
3,360
Organic
Loading
Ibs. BOD/day
Ib. MLSS
0.49
0.10
0.42
0.31
0.28
0.13
0.54
0.08
0.37
0.19
0.17
0.22
0.22
Volumetric
Loading
Ibs. BOD/day
1 ,000 cu.ft.
36
23
30
71
58
37
58
32
48
33
29
41
45
Air
Supplied
cu.ft. /gal.
_
-
-
0.43
0.54
0.59
-
1.5-2.0
1.5-2.0
-
-
-
_
Air Supplied
per Ib. of
BOD Removed
cu.ft./lb.
910
910
933
-
-
-
-
-
-
1.240
1.090
1.080
911
Aeration
Detention
Time1
hours
3.1
8.4
4.9
2.5
2.9
4.2
3.8
3.8
5.0
5.3
7.0
5.0
4.3
Secondary
BOD Removal
Efficiency
percent
83.8
94.2
92.2
94.0
86.0
90.0
92.3
84.0
71.0
88.8
.88.8
87.8
86.3
Referei
4
4
4
4
4
4
3
5
5
4
4
4
4
Excluding sludge recycle.
-------
This decrease in air requirements is attributed to its more effective utilization. In step
aeration systems which utilize higher sludge recycle, the air requirements approach those
of the conventional system. A significant design consideration in these latter step aeration
systems is that since the detention times are lower than for the conventional system,
the air supply system and diffusion equipment must be modified to supply approximately
the conventional volume of air to a tank approximately one-half the conventional size.
In the conventional process, the mixed liquor concentration is intended to be relatively
constant throughout the aeration tank, while in the step aeration process the concentration
decreases as the return sludge becomes further diluted with the influent flow This principle
is illustrated in Figure 5-3 (6). A lower solids loading may, in some cases, improve clarifier
performance.
5.2.3 Contact Stabilization
The principles involved in the contact stabilization modification were initially demonstrated
in the upgrading of an existing hydraulically overloaded conventional plant in
Austin, Texas. The design capacity was upgraded from 6 mgd to 15 mgd using a contact
stabilization flow pattern (7).
Laboratory studies and field work have demonstrated that wastewater BOD in the colloidal
or insoluble state is rapidly removed from wastewater in a relatively short contact time
by the combined physical processes of biological flocculation, adsorption, and
enzyme-complexing. This offers the possibility of substantial reduction in plant volume
for wastewaters largely in these forms. In the contact stabilization process, after the
biological sludge is separated from the wastewater in the clarifier, the concentrated sludge
is further aerated in another aeration tank (called the stabilization tank). Here, the
flocculated and adsorbed BOD is stabilized (Figure 5-4). In addition to a shorter total
contact time, the contact stabilization modification has the advantage of being able to
handle greater shock and toxic loadings because of the biological buffering capacity of
the stabilization tank, and the fact that at any given time the majority of the activated
sludge is isolated from the main stream of the plant flow.
FIGURE 5-4
CONTACT STABILIZATION FLOW DIAGRAM
RAH ,
WASTEWATER
PRIMARY
SEDIMENTATION
1
SLUDGE TO
DIGESTER
1 —
CONTACT TANK
DETENTION
STABILIZATION TANK
2-6 HOURS
DETENTION
FINAL
CLARIFIER
RETURN ,
SLUDGE ,
EXCESS
SLUDGE
FINAL
EFFLUENT
Operational data from four contact stabilization processes are summarized in Table 5-3.
5-6
-------
FIGURE 5-3
COMPARISON OF SOLIDS LOADING ON THE FINAL CLARIFIER
FOR CONVENTIONAL AND STEP AERATION SYSTEMS
Mode 1 - Conventional
25% Return
Sludge
SS= 1 0,000 mg/L
100% Primary Effluent
A
2,000
B
2,000
C
2,000
D
2,000
Aerator
Effluent
SS=, 2,000 mg/L
Average Aerator (MLSS) Concentration
2,000 mg/L
Mode 2 - Step Aeration
25%
Primary
Effluent
1
25% Return
Sludge
SS = 6,240 mg/L
1 i
A
3,120
'
B
2,080
i
C
1,560
' D
1,248
Aerator
Effluent
SS= 1,248 mg/L
Average Aerator (MLSS) Concentration
2,002 mg/L
Example assumes negligible suspended solids in the primary effluent and final effluent.
5-7
-------
00
Table 5-3
Operational Data from Various Contact Stabilization Plants
Plant
Location
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Texas
New Jersey
Maryland
Influent
Flow
mgd
8.3
7.2
7.8
8.0
7.4
7.4
7.1
7.0
5.1
2.5
8.0
Sludge
Recycle
percent
37.4
41.3
32.7
33.3
34.6
35.1
50.2
51.4
71.1
70.0
50.0
Influent
mg/l
200
228
195
295
262
208
333
320
216
312
200
BOD
Effluent
mg/l
24
10
14
28
12
10
25
24
16
30
8
Contact
Tank
MLSS
mg/l
2,530
3,930
2,620
2,440
3,210
3,020
2,760
2,290
2,540
4,000
1,620
Contact
Tank
Aeration
Time'
minutes
28
33
30
29
32
32
33
34
46
100
78
Stabilization
Tank
MLSS
mg/l
7,700
1,000
6,850
7,500
7,320
8,580
6,300
7,550
6,700
6,700
4,200
Stabilization
Tank
Aeration
Time2
minutes
148
172
156
142
168
168
170
175
240
144
78
Organic
Loading
Ibs. BOD/dav
Ib. MLSS
0.25
0.17
0.25
0.35
0.23
0.43
0.49
0.35
0.17
0.32
0.38
Volumetric
Loading
Ibs. BOD/day
1 ,000 cu.ft.
108
99
97
148
94
156
196
148
69
104
-
Secondary
BOD Removal
Efficiency
percent
88.0
95.6
92.8
90.5
95.4
95.2
92.5
92.5
92.6
90.4
96.0
Reference
7
7
7
7
7
7
7
7
7
8
3
Based on influent flow excluding sludge recycle.
on sludge recycle flow.
-------
The time required for stabilization is a function of contact time, temperature, and strength
of the waste. An increase in contact time normally reduces the stabilization time
requirements (9). An important design consideration is the need for adequate stabilization
to ensure satisfactory secondary clarifier performance. The relative detention times and
air requirements of the contact and stabilization tanks have been reported by Lesperance
(9) and are summarized in Table 5-4.
Table 5-4
Comparison of Contact Stabilization Detention Times
and Air Requirements
Detention Time Contact Tank Stabilization Tank
hours based on forward based on sludge
plus recycle flow recycle
Wastewater containing mostly insoluble 0.5 2.0
BOD (domestic wastewater), minimum
Wastewater containing mostly soluble 2 3
BOD, minimum
Most commonly used 0.5 to 1.0 2 to 6
Air Requirements Contact Tank Stabilization Tank
percent of total percent percent
Wastewater containing mostly insoluble BOD 40 60
Wastewater containing mostly soluble BOD 60 40
Most of the benefits of contact stabilization are achieved if the organic load is present
mainly in a colloidal state. Generally, the greater the fraction of soluble BOD, the greater
the required contact time. As a result, the required aeration volume of this process
approaches that of the conventional process as the relative amount of soluble BOD in
the wastewater increases.
Ten-States Standards, however, specify significantly higher contact and stabilization times
than those previously cited, especially for the smaller sized plants as indicated in Table 5-5
(10).
5-9
-------
Table 5-5
Suggested Design Guidelines
Plant Design
Flow
mgd
to 0.5
0.5 to 1.5
1.6 and up
Contact Time1
hours
3.0
3.0 to 2.0
2.0 to 1.5
Stabilization Time^
hours
6.0
6.0 to 4.0
4.0 to 3.0
1 Based on average design forward flow
2fiased on average design recycle flow
These values were no doubt selected to compensate for the extreme flow variations that
occur at small treatment plants. However, when Ten-States Standards design criteria are
applied to smaller plants, they may result in poor quality effluents (11). McKinney (12)
has indicated that, in typically designed contact stabilization plants, all of the stabilization
of the organic matter in the raw wastewater occurs in the contact zone; therefore, only
endogeneous respiration occurs in the stabilization tank. This situation results in partial
stabilization of the sludge in the contact tank, which causes poor settling characteristics
in the secondary clarifier.
5.2.4 Completely-Mixed Activated Sludge
In the past at many small activated sludge package plants, oxygen was supplied by
mechanical aerators, which provided nearly completely-mixed conditions. However, the
specific advantages of the completely-mixed system are just recently being realized (13)
(14).
One of the main advantages of the completely-mixed process is related to the introduction
of influent waste and the recycled sludge uniformly throughout the aeration tank, as
indicated in Figure 5-5. This allows for uniform oxygen demand throughout the aeration
tank. This flow pattern also adds some operational stability when treating slug loads of
industrial wastes. Operational data from four plants utilizing complete-mix are presented
in Table 5-6.
FIGURE 5-5
COMPLETELY-MIXED FLOW DIAGRAM
RAW
Wl'iTFWITFB OR ...
PRIMARY EFFLUENT
/ AERATION \
V 1-3 H
i
OURS /
RETURN SLUDGE
FINAL
CLARIFIER
.FINAL
EFFLUENT
• - IKISS SLUDGE
5-10
-------
Table 5-6
Operational Data from Various Completely-Mixed Activated Sludge Plants
Plant
Location
Nebraska
Nebraska
Nebraska
Nebraska
Nebraska
Texas
Texas
Texas
Texas
Illinois
Illinois
Illinois
Illinois
Influent
Flow
mgd
3.4
4.1
5.0
0.38
0.43
0.29
0.29
0.30
0.37
1.6
1.94
1.91
1.55
Sludge
Recycle
percent
50
100
200
26
40
82
100
145
100
21
21
25
25
Influent
mg/1
250
270
280
225
227
115
141
123
180
102
80
80
108
BOD
Effluent
mg/1
15
13.5
6
25
32
9
25
19
17
8
13
19
18
Aeration
Tank
MLSS
mg/1
4,500
4,500
4,500
4,230
5,460
3,820
5,000
5,540
5,620
6,500
6,000
6,500
6,300
Organic
Loading
Ibs. BOD/day
Ib. MLSS
0.27
0.32
0.38
0.48
0.42
0.21
0.20
0.16
0.29
0.17
0.195
0.18
0.20
Volumetric
Loading
Ibs. BOD/day
l.OOOcu.ft.
80
97
116
126
142
50
62
54
103
74
73
72
79
Aeration
Detention
Time1
hours
5.0
4.4
3.8
2.6
2.5
3.7
3.7
2.2
3.0
2.20
1.75
1.80
2.2
Air Supplied
per Ib. of
BOD Removed
cu.ft./Ib.
500
500
560
-
-
'
-
-
-
1,670
1,900
1,380
1,290
Secondary
BOD Removal
Efficiency
percent
94
95
98
89
86
92.5
82.0
83.0
91.0
92.5
84.0
76.0
83.0
Reference
13
13
13
15
15
16
16
16
16
17
17
17
17
Excluding sludge recycle
-------
5.2.5 Two-Stage Activated Sludge
Two-stage activated sludge is essentially two separate activated sludge processes operating
in series, as shown in Figure 5-6. One of the chief advantages of this flow scheme is
in the area of nitrification. The two separate sludge systems permit the development of
two specialized microbial populations. In the first stage, the bulk of the carbonaceous
material is removed by a wide variety of heterotrophic organisms commonly found in
activated sludge. The reduction of BOD in the first stage permits an accumulation of
the slower growing nitrifying oragnisms in the second stage which oxidize the ammonia
nitrogen to the nitrate form.
FIGURE 5-6
TWO-STAGE ACTIVATED SLUDGE FLOW DIAGRAM
RAW
VASTEVATE1
OR PRIMARY
EFFLUENT
FIRST
STAGE
CLARIFIES
RETURN
SLUDGE
•— 1
SECOND STAGE
AERATION TANK
^
- «/
(^
^* "N
C
v^
IE TURN SLUDGE
4
— -v
fc
STAGE
CURIFIER
i
FINAL
EFFLUENT
EXCESS SLUDGE
EXCESS SLUDGE
Although attention has recently been given to this modification, it is generally not
considered economical for upgrading unless nitrification is a major consideration. This point
is emphasized by examining the operational data presented in Table 5-7. The incremental
BOD removal obtained in the second stage generally is not competitive with alternative
carbonaceous removal options unless nitrification is also required. The advantage of
satisfying the oxygen demand of the ammonia nitrogen normally discharged should not
be underestimated since this can amount to as high as 70 percent of the total oxygen
demand of the plant secondary effluent (20). Note the high air requirement in Ibs. air
supplied/lb. BOD removed in the second stage (Table 5-7). A portion of this air is used
for ammonia oxidation.
5.2.6 Oxygen Aeration
The Linde Division of the Union Carbide Corporation recently introduced an activated
sludge system utilizing oxygen instead of air and termed it the UNOX process.
Subsequently, several other companies have introduced oxygen aeration contacting systems.
5-12
-------
Table 5-7
Operational Data of Two-Stage Activated Sludge Plants
Plant
Location
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Pennsylvania
Influent
Flow
mgd
2.27
2.24
1.47
1.28
2.10
0.21
0.21
0.21
0.21
0.21
0.21
Sludge
Recycle
percent
27
19
48
46
27
56
56
56
56
56
56
Influent
mg/1
204
220
271
249
223
138
266
104
133
110
134
BOD
Effluent
mg/1
35.5
22.7
32.4
32.8
40.5
12.0
29.0
19.0
25.0
13.0
10.0
Aeration
Tank
MLSS
mg/1
3,760
2,020
2,350
1.980
1.920
3,150
3,150
2,650
2,650
2,650
2,650
Organic
Loading
Ibs. BOD/day
Ib. MLSS
0.86
1.56
1.10
1.02
1.56
1.0
1.9
0.92
1.20
0.95
1.10
Volumetric
Loading
Ibs. BOD/day
l.OOOcu.ft.
182
197
160
128
188
205
375
154
196
158
186
1st Stage
BOD Removal
Efficiency
percent
82.2
89.2
87.3
83.4
81.8
95.8
89.0
81.8
81.4
88.5
92.7
Air Supplied
per Ib. of
BOD Removed
cu.ft.Ab.
1,180
1,180
1,180
1,180
1,180
820
820
820
820
820
820
Aeration
Detention
Time1
hrs.
1.6
1.7
2.5
2.9
1.8
0.7
0.7
0.7
0.7
0.7
0.7
' Excluding sludge recycle.
-------
Table 5-7
(continued)
2nd Stage Performance
Sludge
Recycle
percent
24
11
23
29
14
19
19
19
19
19
19
BOD
Influent
mg/1
36
23
32
33
41
12
29
19
25
13
10
Effluent
mg/1
12
11
19
19
17
7
8
4
21
g
7
Aeration
Tank
MLSS
mg/1
1,520
1,600
g20
goo
935
1,500
1,510
1,350
1,350
1,350
1,350
Organic
Loading
Ibs. BOD/day
Ib. MLSS
0.34
0.21
0.36
0.34
0.59
0.21
0.47
0.35
0.49
0.24
0.20
Volumetric
Loading
Ibs. BOD/day
l,000cu.ft.
33.0
21.0
19.0
17.0
35.0
20.5
44.0
29.0
41.5
20.5
17.0
2nd Stage
BOD Removal
Efficiency
percent
67.0
53.7
40.4
42.5
59.0
41.6
72.2
79.0
16.0
38.5
30.0
Air Supplied
per Ib. of
BOD Removed2
cu.ft./lb.
4,600
4,600
4,600
4,600
4,600
4,600
4,100
4,100
4,100
4,100
4,100
Aeration
Detention
Time3
hours
1.6
1.7
2.5
2.9
l.g
-
0.7
0.7
0.7
0.7
0.7
Overall
Secondary
BOD Removal
percent
94.0
95.0
93.0
93.0
93.0
95.0
96.0
96.0
81.0
93.0
95.0
Reference
18
18
18
18
18
19
19
19
19
19
19
^Including that needed for ammonia oxidation.
•^Excluding sludge recycle.
-------
A schematic diagram of the UNOX system is shown in Figure 5-7 (21). In the oxygen
aeration process, the aeration tank is staged by using baffles, and is completely covered
to provide a gas-tight enclosure. The influent wastewater, recycled sludge, and oxygen
gas are introduced into the first stage, and then flow to subsequent stages. The oxygen
is produced at the plant site by either a cryogenic unit or, in the case of smaller plants,
a molecular sieve device. A liquid oxygen storage unit is generally recommended to
eliminate the duplication of units usually specified by most State Health Departments.
Table 5-8 summarizes operational data from two plants utilizing oxygen aeration. The
following is a list of possible advantages of the oxygen aeration process (22):
1. Reduced capital cost.
2. Reduced operating cost.
3. Reduced sludge production.
4. More reliable process control.
5. More effective odor control.
6. Reduced land area.
7. High D.O. in the final effluent.
Of course, the potential economic advantages are a function of local factors and should
be confirmed in comparison with other upgrading alternatives.
5.3 Activated Sludge Design Considerations
Initially, plant operators, through a trial and error procedure, developed the most efficient
operating criteria for conventional activated sludge plants as well as for the modifications
of the process. Out of this evolution, basic design criteria were developed. These criteria
are still in use today and, in many cases, are rigidly adhered to by regulatory agencies.
A limitation of these design criteria is that volumetric loading (Ibs. BOD/1,000 cu.ft./day)
has been considered preferable, for design purposes, to organic loading considerations
(Ibs. BOD/day/lb. MLVSS). Many of the modifications have shown that organic loading
is an important consideration; in fact, a higher volumetric loading has been achieved for
modifications of the conventional process at the same organic loading used in the
conventional process.
Basic parameters of interest in the design of an activated sludge process are:
1. BOD removal for specific operating conditions.
2. Oxygen (air) requirements for synthesis of organisms and for endogenous
reactions.
3. Sludge production.
4. Oxygen transfer rates in wastewater.
5. Nutrient requirements.
6. Separation and return of activated sludge.
5-15
-------
FIGURE 5-7
SCHEMATIC DIAGRAM Of MULTI-STAGE
OXYGEN AERATION SYSTEM (21)
ON
AERATION
TANK COVER
OXYGEN
FEED GAS
WASTE
LIQUOR
FEED
RECYCLE
SLUDGE
GAS RECIRCULATION
COMPRESSORS
EXHAUST
"GAS
MIXED LIQUOR
EFFLUENT TO
CLARIFIER
PROPELLER
SPARGER
-------
Table 5-8
Operational Data from Two Oxygen-Aeration Activated Sludge Plants
Plant
Location
New York
New York
New York
New York
New York
New York
New York
Washington, D.C.
Washington, D.C.
Washington, D.C.
Influent
Flow
mgd
1.33
1.29
1.38
1.19
1.36
1.41
1.64
0.07
0.10
0.10
Sludge
Recycle
percent
53
54
56
45
42
38
32
50
31.5
38
Influent
mg/1
237
221
249
283
270
304
269
115
102
116
BOD
Effluent
mg/1
22
18
19
19
9
11
15
19
12
14
Aeration
Tank
MLSS
mg/1
5,890
6,810
6,840
5,890
7,400
5,700
5,560
4,140
6,000
8,120
Organic
Loading
Ibs. BOD/day
Ib. MLSS
0.35
0.27
0.38
0.37
0.31
0.47
0.50
0.31
0.25
0.22
Volumetric
Loading
Ibs. BOD/dav
l.OOOcu.ft.
126
115
140
132
142
166
170
80
90
108
Aeration
Detention
Time'
hours
2.9
3.0
2.8
3.3
2.9
2.8
2.4
2.2
1.7
1.7
Secondary
BOD Removal
Efficiency
percent
91.0
92.0
93.0
91.5
97.0
97.0
94.5
84.0
88.2
88.0
Reference
21
-) 1
21
21
21
21
21
5
5 '
5
Excluding sludge recycle.
-------
5.3.1 BOD Removal Rates for Specific Operating Conditions
Eckenfelder (23) has indicated that a linear arithmetic relationship exists between BOD
removal rate (mg BOD/hr./gm VSS) and effluent BOD (mg/1), as shown in Figure 5-8
for typical data from various completely-mixed activated sludge plants. The variations in
the BOD removal relationship in Figure 5-8 are influenced by the presence of various
proportions of domestic and industrial wastes.
Using Figure 5-8, the detention time required to achieve a specific effluent BOD can be
obtained as follows:
La-Le
Sar'
where:
t = Detention time, hours
La = Influent BOD to aeration tank, mg/1
Le = Clarifier effluent BOD, mg/1
Sa = Mixed liquor volatile suspended solids (MLVSS), mg/1
r' = BOD removal rate, mg BOD/hr./gm VSS
Weston (24) developed a log-log relationship between a BOD removal rate constant (r)
and a loading ratio (LO/SO), defined by the equation:
r —
where:
r = BOD removal rate constant, min"'
Lo = BOD of wastewater after mixture of raw wastewater or primary effluent with
sludge recycle, mg/1
Le = Clarifier effluent BOD, mg/1
te = Aeration tank detention time, minutes (including recycle)
Lj = Raw wastewater or primary effluent BOD, mg/1
R = Sludge recycle as percent of influent flow
S0 = MLVSS, mg/1
5-18
-------
FIGURE 5-8
BOD REMOVAL CHARACTERISTICS
FOR VARIOUS COMPLETELY MIXED ACTIVATED SLUDGE PLANTS (23)
Q
O
CD
1
Q
O
CO
100
80
60
40
20
READILY REMOVABLE
ORGANICS
RESISTANT
ORGANICS
20 40 60 80
EFFLUENT BOD (mg/l)
100
5-19
-------
The aeration tank detention time is related to process efficiency (E) and BOD removal
rate (r) by the following:
— _
100-E x r
where:
LO
Operational data from over 20 plants with various activated sludge modifications were
analyzed using the Weston procedure to determine BOD removal rate constants. Operational
data for this analysis were taken from references (2) through (8), (11), (13) through (19),
(21), and (25) through (27). The results are summarized in Figure 5-9. It should be pointed
out that the BOD removal rate curves represent only average kinetics with no temperature
correction applied, and the various loading ratios were determined using MLSS, not MLVSS.
For these reasons, these curves are not recommended for design purposes, but are included
merely to illustrate the relative kinetic rates of the modifications. The presence of
significant quantities of industrial wastes, which may have different removal rate
characteristics, would modify or displace the curves shown.
Note that each modification in Figure 5-9 except step aeration results in an increased
r and, therefore, a subsequent decrease in detention time (assuming the efficiency remains
constant). The basic reason for the non-consistency in the step aeration data is that the
S0 value (MLSS) is the average value in the aeration tank. In the step aeration process,
the concentration decreases markedly as it proceeds through the aeration tank, but the
average concentration is quite similar to the conventional process. Therefore, for the design
of step aeration systems, volumetric loading can probably be used, as will be discussed
subsequently.
The BOD removal rate curve for the partially treated wastewater in Figure 5-9 represents
data obtained from the second stage of three different two-stage biological treatment plants
(18) (19) (27). Two of these plants use activated sludge as the first stage, while the third
plant uses trickling filtration. The BOD removal rates for the partially treated wastewaters
are markedly lower than those of conventional activated sludge because the organics
remaining in the first-stage effluent are more resistant to biological degradation than those
entering a conventional plant.
Figure 5-10 was prepared using the same operational data as used for Figure 5-9 to show
a correlation between volumetric and organic loading rates. The results shown in
Figure 5-10 indicate that, for the same organic loading, the volumetric loading increases
for various modifications of the conventional process, thereby reducing the required
aeration volume.
5-20
-------
FIGURE 5-9
RELATIONSHIP BETWEEN BOD REMOVAL RATE CONSTANTS
AND LOADING RATIOS
FOR VARIOUS ACTIVATED SLUDGE MODIFICATIONS
LEGEND
• CONVENTIONAL PROCESS
• STEP AERATION PROCESS
O COMPLETELY MIXED PROCESS
X OXYGEN AERATION PROCESS
A CONTACT STABILIZATION PROCESS
(OVERALL T)
A CONTACT STABILIZATION PROCESS
(CONTACT D
• PARTIALLY TREATED WASTEWATER
LOAD RAT 10-2-
Sn
5-21
-------
FIGURE 5-10
RELATIONSHIP BETWEEN VOLUMETRIC AND ORGANIC LOADINGS
FOR VARIOUS ACTIVATED SLUDGE MODIFICATIONS
LEGEND
• CONVENTIONAL PROCESS
* STEP AERATION PROCESS
O COMPLETELY MIXED PROCESS
• OXYGEN AERATION PROCESS
A CONTACT STABILIZATION PROCESS
240
200
° 160
CO
I—
Ll_
O
120
a 80
40
OPERATED AS A PLUG FLOW CONVENTIONAL
ORGANIC LOADING, LBS. BOD/DAY/LB SUSPENDED SOLIDS
UNDER AERATION
5-22
-------
Various relationships between BOD removal and organic loading were developed using the
same operational data that were used to prepare Figures 5-9 and 5-10, and these
relationships are presented in Figure 5-11. Contact stabilization, completely mixed, and
the oxygen aeration modifications generally show a slightly higher percent BOD removal
at the same loading than does the conventional activated sludge process.
5.3.2 Air Requirements for Synthesis of Organisms and for Endogenous Reactions
Table 5-9 contains ranges for the cubic feet of air required per pound of BOD removed
for the various activated sludge modifications previously discussed.
Table 5-9
Air Requirements for
Various Activated Sludge Modifications
Process Standard cu.ft. Air/lb. BOD Removed
Conventional 1,000 to 700
Step Aeration 700 to 500
Contact Stabilization 750
Completely-Mixed 600
Source: Eckenfelder (23)
5.3.3 Sludge Production
Normally, the activated sludge processes generate excess sludge in relation to the organic
loading maintained in the system (9). For common ranges of organic loadings, namely
0.3 to 0.6 Ib. BOD/lb.MLVSS/day, it has been observed that the amount of excess sludge
produced in the conventional and various modified processes generally varies between
0.5 and 0.7 Ib. VSS/lb. BOD removed (6) (13) (28). In contrast to these observations,
preliminary results obtained using oxygen aeration indicate that, for organic loadings of
0.4 to 0.8 Ib. BOD/lb. MLVSS/day, the excess sludge production was 0.3 to 0.45 Ib.
VSS/lb. BOD removed (28).
5.3.4 Oxygen Transfer Rates in Wastewater
Oxygen transfer rates in wastewater are affected by various physical and chemical variables,
e.g., temperature, degree of turbulent mixing, liquid depth in the aeration tank, oxygen
composition of aerating gas, type of aeration device, and chemical characteristics of the
wastewater. The major area often overlooked in the past by design engineers has been
the effect of an industrial waste on the overall oxygen transfer rate of a system. Where
5-23
-------
FIGURE 5-11
RELATIONSHIP BETWEEN BOD REMOVAL AND ORGANIC LOADING
FOR VARIOUS ACTIVATED SLUDGE MODIFICATIONS
95
£ 90
CO
CO
85 .
CONTACT
STABILIZATION
COMPLETELY
MIXED
CONVENTIONAL
AND STEP
AERATION
CONVENTIONAL PROCESS
STEP AERATION
O COMPLETELY MIXED PROCESS
• OXYGEN AERATION PROCESS
A CONTACT STABILIZATION PROCESS
0.0
0.
0.2
0.3
0.4
0.5
ORGANIC LOADING, LBS. BOD/DAY/LB SUSPENDED SOLIDS
UNDER AERATION
5-24
-------
the industrial waste makes up a large proportion of the total flow it is desirable to verify
oxygen transfer rates in the laboratory for proper sizing of aeration units. Oxygen transfer
capability of several aeration devices are indicated in Table 5-10.
Table 5-10
Comparison of the Aeration Costs of Various Systems
Type of Power^ Relative Relative
Aeration System Ib. 02/hp/hr.1 kwh/lb. O2 Cost/Day Power Cost Capital Cost
Diffused-Air, Fine-Bubble 2.1 0.35 1.26 1.75 3.2
Diffused-Air, Coarse-Bubble 1.4 0.55 1.98 2.75 2.5
Mechanical Aeration, Vertical Shaft 3.7 0.20 0.72 1.0 1.0
Agitator Sparged System 2.1 0.35 1.26 1.75 2.25
'Oxygen transfer capabilities shown are for standard conditions in tap water, i.e. 20°C, 760 mm barometric pressure, and
initial dissolved oxygen equal to 0 mg/1.
^Dollars/lb. C>2/day based on 1.5 cents/kwh power cost.
Source: Mechanical Aeration Seminar (31)
5.3.5 Nutrient Requirements
It is necessary that sufficient nitrogen and phosphorus be present in a wastewater such
that neither nutrient becomes the limiting factor in microbial growth reactions encountered
in the activated sludge process. Normally, supplemental nutrients are not required for
municipal wastewater treatment plants because adequate quantities are available in domestic
wastewaters to make organic carbon the limiting macronutrient. For optimum operation
of the activated sludge process, the minimum ratio of raw wastewater BOD:N:P is 60:3:1
(29).
5.3.6 Separation and Return of Activated Sludge
Basically, the ability of activated sludge to be separated in a final clarifier does not change
appreciably for the various modifications. Clarifier requirements are, therefore, essentially
the same, regardless of the modification implemented, provided that operating conditions
remain the same. It is for this reason that the Ten-States Standards recommends an average
surface overflow rate of 800 gpd/sq.ft. for all of the activated sludge modifications
previously discussed, except contact stabilization, when the design capacity is over 1.5 mgd.
For the contact stabilization process, the Ten-States Standards recommends an average
surface overflow rate of 700 gpd/sq.ft., because with this modification primary
sedimentation is often omitted.
5-25
-------
In the past, the importance of the final clarifier as an integral unit of the activated sludge
process has not been fully recognized. However, recent work by Dick (30) indicates that
the final clarifier design is an important aspect in the design of an activated sludge process
and that improper clarifier design is often the cause of inefficient BOD and suspended
solids removal. He recommends that both aspects of clarification and thickening be
considered in the design of final clarifiers.
The recent appreciation of the importance of the final clarifier on activated sludge process
efficiency has raised doubt as to the advisability of designing clarifier overflow rates solely
on the basis of average or nominal design flow. Many engineers now prefer to size clarifiers
on the basis of maximum daily flow. This technique provides greater protection against
system solids washout, at the expense of a somewhat larger clarifier. Depending on
wastewater characteristics, geometric configuration, and pretreatment considerations, a
range of 1,000 to 1,600 gpd/sq.ft. is suggested as a guideline for the maximum allowable
surface overflow rate in an activated sludge final clarifier.
Control of sludge recycle is probably the most important operational tool the plant operator
has at his disposal to intellegently manage the sludge inventory. Therefore, it is extremely
important to provide sufficient sludge recycle capacity to give the operator the required
operating flexibility to handle the highly variable and fluctuating waste loads characteristic
of many plants.
Two techniques which have been used to control sludge recycle are:
1. Automatically varying the recycle flow to maintain a set relationship to influent
flow.
2. Controlling the recycle pumps by a sludge blanket sensor to maintain a
predetermined blanket height in the final clarifier.
A firm sludge recycle capacity of at least 50 percent is recommended for the conventional
and step aeration processes; at least 100 percent is recommended for the contact
stabilization and completely-mixed modifications. Firm capacity is defined as the available
pumping capacity with the largest pump out of service.
5.4 Pilot Studies
The use of pilot facilities for investigating the upgrading of existing activated sludge plants
is strongly indicated in many cases, to ensure that optimum design parameters are selected.
There are two general types of piloting facilities available: batch or continuous-flow
systems. Continuous-flow systems may range in size from bench-scale to 5 or 10-gpm
units. The basic objective of either a batch or a continuous-flow study is to generate
parameters necessary for design. Some of the parameters of basic interest are BOD removal
rates, oxygen requirements, and sludge production.
5-26
-------
5.4.1 Batch Studies
Batch laboratory-scale units are subject to all of the inherent difficulties of biological
oxidation systems, with the added magnified complexities of large surface-to-volume ratios,
small quantities of sludge mass in the reactor, and the undesirable factors associated with
slug feeding of wastewater. In spite of these inherent difficulties, batch studies have
attractive features in that they afford an economic and efficient controlled method of
developing fundamental information concerning the applicability of various activated sludge
modifications. However, use of the continuous-flow system is preferable to obtain design
parameters since it approximates the operation of an actual plant, permitting evaluation
of the effects of variations in treatability characteristics, as well as of variations in
wastewater loading or strength.
5.4.2 Continuous-Flow Studies
Continuous-flow units, as opposed to batch units, must be used on wastewaters which
exhibit biostatic or exert toxic effects. However, most of the municipal waters do not
exhibit this property unless there is a significant discharge of untreated industrial wastes.
A schematic diagram of a continuous-flow aeration unit is shown in Figure 5-12. Basically,
the system consists of a wastewater feed tank provided with a mixer to blend the
wastewater, prior to feeding to the aeration unit. The use of a mixer in addition to blending
prevents solids deposition in the feed tank. The wastewater from the feed tank and the
recycle sludge from the secondary clarifier are pumped to the aeration tank using peristalic
type pumps. The aeration in laboratory units is normally supplied through porous diffusers,
and the air supply is controlled by the use of rotameters. The wastewater, after treatment,
flows by gravity to a clarifier where the mixed liquor solids are separated. In the
laboratory-scale clarifier, care must be taken to prevent solids deposition on the side walls
of the clarifier. To accomplish this, the clarifier should have a scraper mechanism which
aids in both the settling and removal of the mixed liquor solids. If 24-hr, composite
sampling of feed wastewater and clarifier effluent are required, provision should be made
to pump these streams into refrigerated sample bottles.
The selected size of the aeration system depends on the wastewater strength and the desired
detention time. Slow pumping rates are difficult; therefore, for longer detention times,
larger aeration volumes are desirable. Also, higher organic loadings due to high strength
wastes require a larger aeration volume.
Two approaches may be applied for the acclimation and growth of a culture of
microorganisms for use in a continuous-flow system An available activated sludge culture
may be utilized as the source of microorganisms, with the normal feed to that system
being gradually replaced by the wastewater under investigation until satisfactory
performance on that wastewater is obtained. Alternatively, culture development can begin
with a small quantity of seed organisms and a wastewater feed diluted below the toxicity
5-27
-------
FIGURE 5-12
SCHEMATIC OF A CONTINUOUS-FLOW AERATION UNIT
oo
HASTEIATER
FEED TANK
MIXER
.Q.
d°
AIR
SUPPLY
VROT01ETER
FEED
PUMP
(PERISTALIC
TYPE)
MIXEO LIQUOR
AERATION
TANK
SLUDGE RECYCLE
POROUS
DIFFUSER
SETTLED EFFLUENT
SECONDARY
CLARIFIER
SLUDGE PUMP
(PERISTALIC TYPE)
WASTE SLUDGE
-------
threshold (if toxicity exists). As the biological mass develops, the toxicity threshold is
redetermined and wastewater concentration is increased accordingly until the culture is
capable of handling wastewater at 100 percent concentration. The latter technique is
preferred because it provides the best opportunity to observe the growth characteristics
of the biological culture as well as potential problems with acute or chronic toxicity.
When the culture is capable of functioning on the undiluted wastewater, data are collected
on the performance of the system, beginning with a low-feed rate and increasing the feed
rate until performance near that for the anticipated design is achieved. For various organic
loadings (Ibs. BOD/day/lb. MLVSS), the performance and characteristics of the system
should be evaluated in terms of:
1. BOD removal.
2. COD removal.
3. Oxygen consumption.
4. Concentration of biological solids.
5. Characteristics of biological culture (microscopic appearance and settling rates).
6. Physical nature of the effluent (suspended solids, odor, color, etc.).
5.4.2.1 BOD Removal Rate Determinations
The data collected from continuous-flow units can be analyzed using either the Eckenfelder
or Weston procedures in order to define the appropriate BOD removal rate constant for
design conditions as previously discussed (23) (24).
5.4.2.2 Oxygen Uptake Requirements
The oxygen consumption data obtained in the continuous-flow pilot unit are evaluated
to obtain energy and endogenous oxygen requirements as shown in Figure 5-13, which
is a schematic of an oxygen uptake curve for a typical continuous-flow activated sludge
unit (29). The slope of the line (m) represents the oxygen required for cell synthesis,
while the ordinate intercept (b) represents the oxygen required for endogenous respiration.
The net oxygen consumption is expressed by the following formula:
= m s. BOD removed + fc
day
d\
/
where:
O2 = Ibs. oxygen uptake/day
m = energy oxygen, Ibs. oxygen uptake/lb. BOD removed
b = endogenous oxygen, Ibs. oxygen uptake/day /lb. VSS under aeration
5-29
-------
FIGURE 5-13
DETERMINATION OF OXYGEN UPTAKE REQUIREMENTS (29)
CO CO
Q CO
POUNDS BOD REHO/EO DAY POUND VSS UNDER AERATION
HI = ENERGY 02 (LBS. 02/LB BOD REMOVED)
b = ENDOGENOUS 02 (LBS. 02/OAY/LB VSS UNDER AERATION)
5-30
-------
5.4.2.3 Sludge Production
Sludge production in an activated sludge system is expressed as the net effect of two
processes as follows:
1. A production of new organisms resulting from the synthesis of a portion of
the organic material removed.
2. A reduction of the weight of organisms under aeration by the process of
self-oxidation or endogenous respiration.
Figure 5-14 is a schematic representation of sludge production from a continuous-flow
pilot plant (29). The slope of the line (m) represents sludge synthesis, while the ordinate
intercept (b) represents the endogenous destruction of solids. The net sludge production
is expressed by the following equation:
VSS produced/day = m1 (Ibs. BOD removed/day) - b1 (Ibs. VSS under aeration)
where:
m * = sludge synthesis (Ibs. VSS produced/lb. BOD removed)
endogenous destruction
Ib. VSS under aeration)
b - endogenous destruction of sludge (Ibs. VSS destroyed/day/
Sludge production, like any other biological process, is temperature dependent. Therefore,
sludge production data obtained from a pilot study must be adjusted for the temperature
ranges which are to be experienced by the full-scale plant.
5.5 Activated Sludge Upgrading Techniques and Design Basis
Upgrading of conventional activated sludge plants may be required because the plants are
hydraulically and organically overloaded, because of the need for increased treatment
efficiency, or both. Upgrading to relieve overloaded conditions and upgrading to improve
removal efficiency to meet higher water quality standards are covered in the following
two sections.
5.5.1 Upgrading to Relieve Organic and Hydraulic Overloading
The following activated sludge modifications are examined as they apply to the upgrading
of an existing overloaded conventional activated sludge plant:
1. Step aeration and contact stabilization - these processes are combined because
of their similarities.
2. Completely-mixed.
3. Oxygen aeration.
4. Use of activated sludge to treat partially treated wastewater.
5-31
-------
FIGURE 5-14
DETERMINATION OF SLUDGE PRODUCTION CHARACTERISTICS (29)
POUNDS BOD REMOVED DAY POUND VSS UNDER AERATION
m'- SLUDGE SYNTHESIS (IBS. VSS/LB BOD REMOVED)
b'- ENDOGENOUS DESTRUCTION OF SLUDGE (LBS. VSS/DAY/POUND
VSS UNDER AERATION)
5-32
-------
Before examining each individual upgrading procedure, several general statements can be
made. Operational data, BOD removal rate constants, and volumetric loadings previously
discussed indicate that all of the activated sludge modifications are applicable for upgrading
an overloaded conventional activated sludge plant. These modifications will require
consideration of renovating the air system to supply more air per unit of aeration tank
volume. This is because the volumetric loadings for various modifications are substantially
higher than for the conventional process as shown in Figure 5-10, even though the air
requirements per pound of BOD removed decrease slightly as indicated in Table 5-9.
Therefore, to upgrade using one of the previously discussed modifications, it will generally
be necessary to install an air system which will be capable of supplying ah- at a higher
rate than was previously required by the conventional process. This may be done either
by enlarging the existing air supply facilities, or by adding surface or mechanical aerators
where applicable.
Table 5-10 contains a comparison of capabilities and costs of various aeration systems
commonly employed in the activated sludge process (31). The mechanical aerator and
agitator sparger systems are illustrated in Figure 8-2 in the Post-Aeration Section. The
data presented in Table 5-10 indicate the increased oxygen transfer capability and the
lower capital and operating costs for the mechanical aerator. Even though mechanical
aerators afford a high transfer efficiency, their use in an existing basin may pose problems
because the geometric configurations required for their most efficient use may be quite
different than the existing basin configuration. Most existing conventional plants use either
a fine or coarse bubble diffused air system. The fine bubble system is more efficient
and cheaper to operate, but on the other hand represents a greater capital investment
and a costlier maintenance problem than a coarse bubble system.
The use of mechanical aerators and agitator sparger systems has gained popularity in the
recent decade. The aerator has a high oxygen transfer efficiency, but this advantage is
partially lost in upgrading a plug flow type aeration tank due to adverse geometric
configuration requiring multiple units. This was found to be true for an upgrading
investigation performed for the City of Baltimore, Maryland (32). In an economic
comparison, it was found that the annual costs for a diffused versus a mechanical aeration
system were approximately equal because of the existing configuration of the plug flow
basins. However, it was recommended that mechanical aeration definitely be considered
for future aeration tank expansion.
The agitator sparger system has an operational advantage over the diffused air unit (coarse
or fine bubble) in that during low flows the air may be reduced but the mixing will
be maintained due to the action of the turbine agitator.
5-33
-------
5.5.1.1 Step Aeration and Contact Stabilization (Examples A and B)
Step aeration has been used successfully as an upgrading technique in New York City;
Indianapolis, Indiana; and numerous other locations. Contact stabilization has been used
in Austin, Texas; York, Pennsylvania; and Bergen County, New Jersey.
The step aeration and contact stabilization processes have been grouped together because
both modifications can be incorporated into the upgraded design at a minimum capital
investment. Added flexibility in the use of the two modifications is accomplished by sizing
the influent step aeration piping so that the entire flow may be introduced in the last
bay of the aeration tank, thus permitting operation as a contact stabilization process.
This type of upgrading, if applicable, generally requires a minimal capital investment.
Again, it must be stressed that if the soluble BOD in the wastewater is expected to increase
significantly over the design period of the plant due to an increase of industrial wastewater
discharged to the municipal plant, then contact stabilization may not be as efficient as
other alternatives. Hence, upgrading under this condition should preferentially consider
use of the completely mixed or oxygen aeration modifications. Examples A, B, and C
are desk-top analyses to illustrate the design considerations involved in upgrading a
conventional activated sludge plant to step aeration, contact stabilization, and
completely-mixed flow patterns, respectively.
A schematic flow diagram for upgrading a conventional plant to step aeration is presented
as Example A in Figure 5-15. A comparison of the original design values for the
conventional plant before it was overloaded as well as the data from the overloaded plant
before upgrading are shown in Table 5-11. The upgrading of the plant was required due
to an increase of flow from 5 to 8.4 mgd, which increased the effluent BOD from 20 mg/1
to 35 mg/1.
To upgrade the plant to its previous performance, it was decided to employ step aeration.
The design performance of the upgraded plant is also shown in Table 5-11. To implement
this upgrading, it was necessary to modify the influent piping, renovate the air system
in order to deliver 700 cu.ft. of air/lb. BOD removed, and to expand the average sludge
recycle capacity to 2.1 mgd, or 25 percent of the upgraded flow In addition, the
secondary clarifier capacity was increased to maintain an average overflow rate of
800 gpd/sq.ft. Due to the increased hydraulic load, the primary clarification capacity and
other ancillary operations, such as excess sludge handling and disposal facilities, would
also require evaluation for upgrading to match the increased capacity and performance
of the activated sludge process. The cost for additional primary clarifier or sludge handling
capacity will not be considered in this unit operations section.
5-34
-------
FIGURE 5-15
UPGRADING A CONVENTIONAL ACTIVATED SLUDGE PROCESS
TO STEP AERATION
EXAMPLE A
PRIMARY
EFFLUENT
5.0 MGD
AERATION TANK
SECONDARY
CLARIFIER
CZ^rr
25% SLUDGE RECYCLE!
I
FINAL
EFFLUENT
EXCESS SLUDGE
TREATMENT SYSTEM BEFORE UPGRADING
CONVENTIONAL ACTIVATED SLUDGE (DIFFUSED AIR SYSTEM)
AERATION TANK
PRIMARY
8.4 MGD
* *
r
%
V
f
*
%
Tt T L
ADDITIONAL
REQUIRED
CAPACITY
IVI, SLUDGE RECYCLE T
^ FINAL
~~~^ EFFLUENT
| SECONDARY
i CLARIFIER
EXCESS SLUDGE
TREATMENT SYSTEM AFTER UPGRADING
STEP AERATION PROCESS
5-35
-------
Table 5-11
Upgrading Conventional
Activated Sludge to Step Aeration - Example A
Description
Flow, mgd
Influent BOD, mg/1
Primary Treatment
Percent BOD Removal
Aeration Tank
MLSS, mg/1
Sludge Recycle, percent
Air Requirement, cu.ft. air/lb. BOD removed
Volumetric Loading, Ibs. BOD/day/1,000 cu.ft.
Organic Loading, Ibs. BOD/day/lb. MLSS
Detention Time in Aerator, minutes^
Secondary Clarifier
Overflow Rate, gpd/sq.ft.
Secondary Treatment
Percent BOD Removal
Effluent BOD, mg/1
Original
Design
Before
Overloading
5.0
200
30
800
86.0
20
Overloaded
Design
Condition
8.4
200
30
1
1,280
75.0
35
Upgraded
Design
Condition
8.4
200
30
2,000
25
800
35
0.34
300
-
15
—
62
0.88
180
2,000
25
700
62
0.54
180
800
86.0
20
1 Requires modification of primary clarifier to handle increased hydraulic load to achieve 30 percent
BOD removal.
Excluding sludge recycle.
5-36
-------
The capital costs for upgrading these secondary units were estimated at $410,000
($120 per 1,000 gpd of incremental upgraded capacity) and were allocated as follows:
Aeration Tank Modification $ 160,000
Secondary Clarifier Expansion 250,000
TOTAL $410,000l
Example B illustrates upgrading a conventional activated sludge plant equipped with
mechanical aerators to contact stabilization. A schematic flow diagram of the plant before
and after upgrading is shown in Figure 5-16. Table 5-12 contains design data from the
plant while overloaded and after it was upgraded. The plant was upgraded from 1.2 to
3.0 mgd using the contact stabilization modifications. The effluent BOD was upgraded
from 40 mg/1 to 20 mg/1.
Capital costs for this modification include revamping the influent piping, expanding the
sludge recycle to 75 percent of the upgraded flow, and installing new mechanical aerators
capable of delivering 3.5 Ibs 02 per hp/hour under standard conditions. In this example,
the primary clarifier was incorporated into the secondary clarification facilities, thereby
providing an average overflow rate of 780 gpd/sq.ft. To implement these modifications,
the capital costs were estimated at $370,000 ($206 per 1,000 gpd of incremental upgraded
capacity) and were allocated as follows:
Aeration Tank Modifications $340,000
Conversion of Present Primary to
Secondary Clarifier 30,000
TOTAL $370,0001
5.5.1.2 Completely-Mixed Activated Sludge (Example C)
Past experience with completely-mixed domestic activated sludge on a large scale has been
quite successful, although somewhat limited. Completely-mixed plants have been installed
at Grand Island, Nebraska; Freeport, Illinois; South Tahoe, California; and
Albany, Oregon. McKinney (13) and Smith (14) have reported the usefulness of this
process for upgrading an overloaded activated sludge plant. When the completely-mixed
process is to be considered as an upgrading technique for a conventional plant, the
geometric configuration of the aeration basin poses a major problem. The plug flow
hydraulic pattern must be altered to a completely-mixed pattern.
These costs are based on ENR Index of 1500 and contain no contingency for engineering
design, bonding, and construction supervision.
5-37
-------
FIGURE 5-16
UPGRADING A CONVENTIONAL ACTIVATED SLUDGE PROCESS
TO CONTACT STABILIZATION
EXAMPLE B
AERATION TANK
PRIMARY
/
^
SECONDARY
CLARIFIER
T I
1.2 MGD I .
SLUDGE RECYCLE
FINAL
EFFLUENT
TREATMENT SYSTEM BEFORE UPGRADING
CONVENTIONAL ACTIVATED SLUDGE (MECHANICAL AIR SYSTEM)
RAW WASTEWATER
3.0 MGD
STABILIZATION
TANKS
I
i .
75% SLUDGE RECYCLE
ADDITIONAL,REQUIRED
CAPACITY PROVIDED BY
PRIMARY CLARIFIERS
TREATMENT SYSTEM AFTER UPGRADING
CONTACT STABILIZATION PROCESS
FINAL
EFFLUENT
5-38
-------
Table 5-12
Upgrading Conventional Activated
Sludge to Contact Stabilization - Example B
Description
Flow, mgd
Influent BOD, mg/1
Primary Clarifier
Overflow Rate, gpd/sq.ft.
BOD Removal, percent
Aeration Tank
Volumetric Loading, Ibs. BOD/day/1,000 cu.ft.
Sludge Recycle, percent
Detention Time, hours
Contact Basin
Stabilization Basin
Secondary Clarifier
Overflow Rate, gpd/sq.ft.
BOD Removal in Secondary Units
SS Removal in Secondary Units
Effluent BOD, mg/1
Effluent SS, mg/1
Overloaded
Design
Condition
3.0
200
1,200
20
44
15 ,
4.4-
960
75
75
40
30
Upgraded
Design
Condition
3.0
200
60'
75
4.:
780C
90
90
20
18
* Primary clarifier converted to secondary Clarifier.
^Total organic loading increases due to elimination of primary treatment.
Based on influent flow plus 15 percent sludge recycle to the total basin.
^Based on influent flow plus 75 percent sludge recycle to the contact basin.
^Based on 75 percent sludge recycle to the stabilization basin.
"Reduction in OFR is achieved by converting the primary clarifier to a secondary basin.
5-39
-------
Example C is presented to illustrate some of the various engineering considerations which
must be evaluated before implementing this type of upgrading procedure. The original
flow diagram of Example C is similar to Example A shown in Figure 5-15. Original design,
overloaded, and upgraded design data for Example C are presented in Table 5-13.- The
plant was upgraded from 5.0 mgd to 10.0 mgd using a completely mixed flow pattern
as shown in Figure 5-17. The influent wastewater and recycled sludge piping were modified
so that the flow would be uniformly distributed throughout the aeration tank. Four new
longitudinal effluent weirs were installed in the aeration tanks to induce a traverse flow
pattern as indicated in Figure 5-17. A new agitator sparger air system was installed capable
of supplying 600 cu.ft. of air/lb. of BOD removed. In addition, the average sludge recycle
capacity was increased to 60 percent of the upgraded flow and the clarification capacity
was increased to maintain an average overflow rate of 800 gpd/sq.ft. As in Example A,
an upgrading of the total plant would require consideration of expanded primary
clarification and sludge handling facilities.
The capital costs for the upgrading were estimated at $700,000 ($140 per 1,000 gpd of
incremental upgraded capacity) and were allocated as follows:
Aeration Tank Modifications $280,000
Secondary Clarifier Expansion 420,000
TOTAL $700,000!
5.5.1.3 Oxygen Aeration (Example D)
There are special areas for consideration in upgrading an existing conventional activated
sludge plant by use of: oxygen aeration. In addition to those of concern when converting
to other activated sludge modifications, the following are listed:
1. The foundation or pile capacity must be checked against the increased loading
of the oxygen aeration dissolution system.
2. The structural integrity of the aeration tank walls must be checked due to the
increased loading, if pre-cast concrete tank covers are used.
3. Baffling may be required to sectionalize the aeration tank in order to be
compatible with various oxygen aeration systems.
4. Existing tank aeration piping may have to be removed back to the tank header.
* These costs are based on ENR Index oft 15 00 and contain no contingency for engineering
design, bonding, and construction supervisions
5-40
-------
FIGURE 5-17
UPGRADING CONVENTIONAL ACTIVATED SLUDGE
TO A COMPLETELY-MIXED SYSTEM
EXAMPLE C
TREATMENT SYSTEM AFTER UPGRADING TO
COMPLETELY-MIXED PROCESS
NEW
EFFLUENT
«EIR — .
PRIMARY
t
-
10.0 MGD
*
|_
«*.
mm
t
-»
t
«__
_^
t
fc
<
60% SLUDGE RECYCLE
*-^
ADDITIONAL REQUIRED
CAPACITY
FINAL
EFFLUENT
4 SECONDARY CLARIFIER
EXCESS SLUDGE
TYPICAL CROSS -SECTION OF UPGRADED
AERATION TANK (26)
F
'1
M
JT— DRIVE UNIT
n
s~
XpEFFLUENT
JlEI.
SPARGER
RING
I
o
o
^
FLAT
BLADE
TURBINE
r
~~\ B3 —*£-( \
n a c=^4~V
J
}>
r*
—INFLUENT
. PI PI HP,
5-41
-------
Table 5-13
Upgrading Conventional Activated
Sludge to a Completely-Mixed System - Example C
Description
Flow, mgd
Influent BOD, mg/1
Primary Treatment
Percent BOD Removal
Aeration Tank
MLSS, mg/1
Sludge Recycle, percent
Air Requirements, cu.ft. air/lb. BOD removed
Volumetric Loading, Ibs. BOD/day/1,000 cu.ft.
Organic Loading, Ibs. BOD/day/lb. MLSS
Detention Time In Aerator, minutes^
Secondary Clarifier
Overflow Rate, gpd/sq.ft.
Secondary Treatment
Percent BOD Removal
Effluent BOD, mg/1
Original
Design
Before
Overloading
5.0
200
30
800
86
20
Overloaded
Design
Condition
10.0
305
30
1
1,600
62
80
Upgraded
Design
Condition
10.0
305
30
2,000
25
820
35
0.34
300
2,000
25
—
107
1.04
150
3,000
60
600
107
0.69
150
800
91
20
1
Requires modification of primary clarifier to handle increased hydraulic load to achieve 30 percent
BOD removal.
r\
^Excluding sludge recycle.
5-42
-------
5. Provide protection against the potential explosion hazard of pure oxygen or
oxygen-enriched air.
6. Provide protection against potential accelerated corrosion due to pure oxygen
or oxygen-enriched air.
Some of the aspects of using oxygen aeration which may make it economically attractive
include:
1. The oxygen generation equipment may be placed outside and does not require
a protective enclosure.
2. Expensive renovation of the blower building is eliminated.
3. Capital and operational costs may be reduced compared to diffused air systems.
However, the cost differential between the systems decreases with decreased plant
size (21).
4. Reduced sludge production.
Use of oxygen aeration for upgrading municipal treatment plants handling extremely large
flows is in the design stage in Detroit, Michigan, and is under consideration in
New York City. In New York City, a 20-mgd section of an existing modified air aeration
plant will be converted to oxygen aeration to upgrade treatment efficiency. Oxygen aeration
at Detroit will be utilized to expand a 300-mgd section of the existing primary treatment
plant to secondary treatment.
Example D presents the modification of a diffused air, conventional activated sludge plant
to oxygen aeration. Original design data, overloaded, and upgraded design data for this
example are presented in Table 5-14. The conversion to oxygen aeration permits the
capacity of the plant to be increased from 2 to 6 mgd.
The capital costs include covering the existing aeration basin, oxygen generation and
dissolution equipment, increasing the sludge recycle capacity to 50 percent of the upgraded
flow, and maintaining a secondary clarifier overflow rate of approximately 800 gpd/sq.ft.
-------
Table 5-14
Upgrading Conventional Activated
Sludge to an Oxygen Aeration System - Example D
Description
Flow, mgd
Influent BOD, mg/1
Primary Treatment
Percent BOD Removal
Aeration Tank
MLSS, mg/1
Sludge Recycle, percent
Air Requirements, cu.ft. air/lb. BOD removed
Oxygen Requirements, Ibs. O2/lb. BOD removed
Volumetric Loading, Ibs. BOD/day/1,000 cu.ft.
Organic Loading, Ibs. BOD/day/lb. MLSS
Detention Time in Aerator, minutes^
Secondary Clarifier
Overflow Rate, gpd/sq.ft.
Secondary Treatment
Percent BOD Removal
Effluent BOD, mg/1
Original
Design
Before
Overloading
2
200
30
800
86
20
Overloaded
Design
Condition
6
200
30
1
Upgraded
Design
Condition
6
200
30
2,000
25
800
—
35
0.34
300
2,000
25
_
_
105
1.02
100
4,000
50
_
1.2
105
0.51
100
64
50
800
86
20
1 Requires modification of the primary clarifier to handle increased hydraulic load to achieve 30 percent
BOD removal.
'Excluding sludge recycle.
5-44
-------
The capital costs were estimated at approximately $700,000 ($175 per 1,000 gpd of
incremental upgraded capacity) and were allocated as follows:
Aeration Tank Modifications $ 130,000
Oxygen Generation and Dissolution
Equipment1 400,000
Secondary Clarifier Expansion 170,000
TOTAL $700,0002
5.5.1.4 Use of Activated Sludge Process for Treatment of Partially-Treated
Effluent
This modification is by far the simplest of all upgrading procedures to implement since
the activated sludge process will be built as an addition to an existing facility. The
partially-treated effluent may result from a roughing filter or even an organically overloaded
activated sludge process. The second-stage activated sludge process can be built using any
modification as previously discussed. An economic comparison should be made before
making a decision on the activated sludge modification to be used. Table 5-15 summarizes
design information recommended for two-stage activated sludge when nitrification is not
considered essential (33). If nitrification is required, aeration to provide at least 5 Ibs.
oxygen/lb. of ammonia nitrogen should be provided in addition to the air requirements
for carbonaceous BOD removal.
Table 5-15
Two-Stage Activated Sludge Design Guidelines
Design
Description Parameter
Aeration Tank
Minimum first-stage detention time, hrs.' I .y
Minimum second-stage detention time. hrs.' 1.5
Air supply for first-stage, cu.ft. air/lb. BOD applied to plant influent 1,000
Air supply for second-stage, cu.ft. air/lb. BOD applied to plant influent 2,000
Settling Tank
Minimum first-stage detention time, hrs.- 2.4
Minimum second-stage detention time, hrs.2 30
Maximum average first-stage overflow rate, gpd/sq.ft. 1,200
Maximum average second-stage overflow rate, gpd/sq.ft. 800
'Based on design flow not including recirculated sludge.
2Based on average daily design flow.
Source: Pennsylvania Department of Health (33)
Capital Costs were taken from reference (21).
These costs are based on ENR index of 1500 and contain no contingency for engineering
design, bonding, and construction supervision.
-------
If year-round nitrification is a design criteria, then the following information in Table 5-16
may be useful in the preliminary sizing of the second-stage process units (34). It should
be stressed that the detention time required to achieve nitrification is strongly dependent
upon the temperature of wastewater and the concentration of mixed liquor solids
maintained in the system.
Table 5-16
Design Guidelines for Second-Stage Units
to Include Consideration of Nitrification
Design
Description Parameter
Aeration Tank
Optimum pH range 8.2 to 8.6
Maximum Influent BOD, mg/1 40 to 50
Tank Configuration plug flow
MLVSS, mg/1 1,000 to 2,500
D.O. at average loading, mg/1 3.0
Minimum D.O. at peak loads, mg/1 1.0
Sludge recirculation, percent 50 to 100
Detention time based on average flow, hr. 2 to 6
Oxygen requirements (stoichiometric), Ibs. 02/lb. NH3-N 4.6
Settling tank
Average allowable overflow rate, gpd/sq.ft. 800
5.5.2 Upgrading to Increase Organic Removal Efficiency
Upgrading techniques previously discussed relate to the ability of existing facilities to handle
increased hydraulic or organic loads by providing modifications to meet existing effluent
standards. However, there may be a need to meet higher effluent standards even though
the existing facilities are not hydraulically or organically overloaded. Table 5-17 contains
suggested alternatives for improving effluent quality under these conditions. The main
purpose of the table is to present various alternatives and to suggest a range of anticipated
improvement in performance for each alternative.
It should be emphasized that, in cases where unit processes are added on to existing
facilities, the improvement in overall organic removal will be a direct function of the
BOD removal achieved in the "add-on" process. However, where unit processes precede
existing units, e.g. the use of a roughing filter, the overall BOD removal may not be
increased in direct proportion to the amount achieved by the "add-on" process.
-------
Table 5-17
Upgrading Techniques for Improvement of Activated Sludge Treatment Plant Efficiency
Addition Preceding
Existing Unit
Roughing Trickling Filter
(Rock or Synthetic Media)
Chemical Addition
To Primary Clarifier
Existing Process
Activated Sludge
Addition Following
Existing Unit
2nd Stage Activated Sludge
Polishing Lagoon
Multi-media Filters
Microstraining
Activated Carbon
1
Incremental BOD
Removal Across
the Added Process
percent
20-40
30-50
30-70
30-60
50-80
30-80
60-80
1
A consideration if year-round nitrification is required.
-------
A detailed discussion on polishing lagoons, microstrainers, filters, activated carbon, and
clarifler modifications appears in subsequent chapters. The applicability of these alternatives
to individual cases should be evaluated in detail prior to the implementation of a particular
upgrading procedure.
5.6 References
1. Sawyer, C., Activated Sludge Modifications. Journal Water Pollution Control
Federation, 32, No. 3, pp. 232-244 (1960).
2. Haseltine, T.R., A Rational Approach to the Design of Activated Sludge Plants.
Included in Biological Treatment of Sewage and Industrial Wastes, ed. by McCabe,
J., and Eckenfelder, W.W., New York: Reinhold Publishing Company, 1956.
3. Phosphate Study at the Baltimore Back River Wastewater Treatment Plant.
Environmental Protection Agency, Program Number 17010 DFV, September, 1970.
4. Torpey, W., and Chasick, A.H., Principles of Activated Sludge Operation. Included
in Biological Treatment of Sewage and Industrial Wastes, ed. by McCabe, J., and
Eckenfelder, W.W., New York: Reinhold Publishing Company, 1956.
5. Private communications with F. Bishop, Chief, Blue Plains - Washington, D.C. Pilot
Plant, Environmental Protection Agency, Washington, D.C., January 10-11, 1971.
6. Torpey, W., Practical Results of Step Aeration. Sewage Works Journal, 20, No. 5,
pp. 781-788 (1948).
7. Ulbrich, A., and Smith, M., Operation Experience with Activated Sludge - Biosorption
at Austin, Texas. Sewage and Industrial Wastes, 29, No. 4, pp. 400-413 (1957).
8. Grich, E., Operating Experience with Activated Sludge Reaction. Journal Water
Pollution Control Federation, 33, No. 8, pp. 856-863 (1961).
9. Lesperance, T.W., A Generalized Approach to Activated Sludge. Reprinted from Water
and Wastes Engineering by Reuben H. Donnelly Corporation, New York City,
New York.
10. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of State Sanitary Engineers, 1968.
11. Dague, R., et al, Contact Stabilization: Theory, Practice, Operational Problems and
Plant Modifications. Presented at the 43rd Annual Conference - WPCF,
Boston, Mass. (October, 1970).
5-48
-------
12. McKinney, R., Research and Current Developments in the Activated Sludge Process.
Journal Water Pollution Control Federation, 37, No. 12, pp. 1696-1704 (1965).
13. McKinney, R., et al, Evaluation of a Complete Mixing Activated Sludge Plant. Journal
Water Pollution Control Federation, 42, No. 5, pp. 737-752 (1970).
14. Smith, H., Homogeneous Activated Sludge - Three Parts. Water and Wastes
Engineering, 4, No. 7,8,10, pp. 46-50, 56-63, 50-53 (1967).
15. Hammer, M., and Tilsworth, T., Field Evaluation of a High Rate Activated Sludge
System. Water and Sewage Works, 115, No. 6, pp. 261-266 (1968).
16. Private communication with M.E. Holding, Water Reclamation Research Center,
Dallas, Texas, January, 1971.
17. Private communication with C.L., Swanson, Sanitary Engineer, EPA, Cincinnati, Ohio
November 6, 1970.
18. Private communication with Department of Civil Engineering, Pennsylvania State
University, University Park, Pennsylvania, January, 1968.
19. Simpson, R.W., Activated Sludge Modification. Water and Sewage Works, 106, No. 10,
pp. 421-426 (1959).
20. Earth, E.F., et al, Chemical - Biological Control of Nitrogen and Phosphorus in
Wastewater Effluent. Journal Water Pollution Control Federation, 40, No. 12,
pp. 2,040 - 2,054 (1968).
21. Albertsson, J., et al, Investigation of the Use of High Purity Oxygen Aeration in
the Conventional Activated Sludge Process. Federal Water Quality Administration,
Program Number 17050 DNW, May, 1970.
22. McWhirter, J.R., Use of High Purity Oxygen Aeration in the Conventional Activated
Sludge Process. Presented at the 63rd Annual Meeting of the American Institute of
Chemical Engineers, Chicago, Illinois, December 3, 1970.
23. Eckenfelder, W.W., Theory of Design. Included in The Activated Sludge Process in
Sewage Treatment Theory and Application, Presented at a Seminar at the University
of Michigan, February, 1966.
24. Weston, R.F., Fundamentals of Aerobic Biological Treatment of Wastewater. Public
Works, 94, No. 11, pp. 74-83 (1963).
25. Boon, A.G., The Role of Contact Stabilization in the Treatment of Industrial Waste
and Sewage. Journal of Effluent and Water Treatment, 9, No. 6, pp. 319-326 (1969).
5-49
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26. Jackson, R., et al, Short-Term Aeration Solves Activated Sludge Expansion Problems
at Sioux Falls. Journal Water Pollution Control Federation, 37, No. 2, pp. 255-261
(1965).
27. Private communication with Leonard Waller, Plant Superintendent, South River Water
Pollution Control Plant, Atlanta, Georgia, January 27, 1971.
28. Union Carbide Unox System Wastewater Treatment. Union Carbide Corporation,
Linde Division, 1970.
29. Eckenfelder, W.W., Industrial Water Pollution Control. New York: McGraw-Hill Book
Company, 1966.
30. Dick, R., Role of Activated Sludge Final Settling Tanks. Journal of Sanitary
Engineering Division, ASCE, 96, No. 2, pp. 423-436 (1970).
31. Mechanical Aeration Seminar, Presented by Eimco Corporation in New York, N.Y.,
1969.
32. Letter Report to the City of Baltimore, Maryland, Roy F. Weston, Inc. July 14, 1970.
33. Sewerage Manual. Sanitary Water Board - Pennsylvania Department of Health
Publication No. 1, Harrisburg, Pennsylvania, 1969.
34. Sawyer, C.N., Design of Nitrification and Denitrification Facilities. Presented at a
Symposium on Design of Wastewater Treatment Facilities, Presented by Environmental
Protection Agency, Cleveland, Ohio, April 22-23, 1971.
5-50
-------
CHAPTER 6
CLARIFICATION AND CHEMICAL TREATMENT
6.1 General
Improved solids separation in primary and secondary clarifiers, either by operational
changes or by addition of chemicals, is usually accompanied by concurrent reductions
in BOD values in the overflow. Therefore, improvements in the clarification process can
be advantageously used to meet specific treatment requirements, particularly where the
treatment plants are experiencing hydraulic and organic overloads. The solids separation
process can be improved by adding additional clarification area, by chemical treatment
of wastewaters, or by use of more efficient settling devices.
6.2 Primary Clarification
Increased solids separation in primary clarifiers has the following advantages in addition
to increasing their hydraulic capacity:
1. An increase in quantity of primary sludge produced (which can be more readily
thickened and dewatered than secondary sludge).
2. A decrease in quantity of secondary sludge produced.
3. A decrease in organic loading to secondary treatment process units.
The primary clarifier performance significantly influences the extent of secondary treatment
required and, in most cases, affects the overall effluent quality of existing treatment plants.
Also, since clarification is the most economical way to remove suspended and colloidal
pollutants, every effort should be made to improve the primary clarification process before
additional facilities are considered.
6.3 Secondary Clarification
The performance of conventional secondary wastewater treatment systems is determined
by comparing the quality of the overflow from secondary clarifiers to that of the incoming
wastewater. The biological treatment unit converts a portion of the soluble and insoluble
organic pollutants to suspended organic solids (biological). Unless these organic solids are
effectively removed in the secondary clarifiers, the treatment process cannot be considered
a successful operation. Fortunately, the biological solids flocculate and separate readily
by gravity, provided that the rise velocity in the clarifier is maintained below the settling
velocity of the floe particles. The following conditions, either alone or in combination,
will disrupt the secondary clarifier performance:
6-1
-------
1. Hydraulic overloading, which causes the rise velocity of the wastewater in the
secondary clarifier to exceed the settling velocity of the solids.
2. Organic overloading of the biological treatment units, which results in an
increased solids load to secondary clarifiers.
In addition to the above conditions, improper inlet and outlet design often cause short
circuiting of wastewater, thereby reducing the overall BOD removal efficiency. Improper
sludge withdrawal techniques can also cause solids carryover to the clarifier effluent.
6.4 Chemical Treatment
Chemical addition to primary and secondary clarifiers in this manual is concerned only
with increased solids and BOD removal. Chemical treatment for phosphorus removal is
covered in detail in the process design manual for phosphorus removal.
6.4.1 Chemical Addition to Improve Clarification
At first, chemicals were used to improve the efficiency of primary clarification systems.
Later, when these systems were followed by secondary treatment processes, the practice
of adding chemicals to upgrade primary treatment because unnecessary. However, the
technique of adding chemicals to the primary clarifier is still an effective upgrading
procedure for a secondary plant when the following conditions exist (1):
1. Wastewater flow is intermittent or varies greatly.
2. Space available for additional clarification facilities is limited.
3. Industrial wastes that would interfere with biological treatment are present.
4. Plant is hydraulically and/or organically overloaded.
5. Improvements in existing treatment performance are required as an interim
measure before the addition of new facilities.
The chemicals commonly used in wastewater treatment are the salts of iron and aluminum
lime, and synthetic organic poly electrolytes. The iron (ferrous and ferric) and aluminum
salts (sodium aluminate or alum) react with the alkalinity and soluble orthophosphate
in wastewater to form precipitates of the respective metallic hydroxides or phosphates.
In addition, they destabilize the colloidal particles that would otherwise remain in
suspension. These precipitates, along with the destabilized colloids, flocculate and settle
readily in a clarifier.
Sodium aluminate is a basic salt and can be advantageously used for wastewaters containing
low to moderate amounts of alkalinity. Alum, being an acid salt, is best suited for
wastewater high in alkalinity. While both alum and sodium aluminate exhibit great
capability for total phosphorus removal, the use of alum introduces six times as much
dissolved solids to the wastewater as does sodium aluminate (2). Normally, lime is used
6-2
-------
to precipitate hydrous oxides of iron and aluminum when the alkalinity of wastewaters
is low. The reaction of iron and aluminum salts is pH-dependent and has to be evaluated
for each case to determine the most effective pH range and the optimum chemical dosage.
The addition of lime alone is also effective in coagulating wastewater. The positive calcium
ions help to destabilize colloidal particles while precipitating soluble orthophosphates as
hydroxyapatite. Since lime treatment takes place at high pH (9.0-11.5), the effluent from
this process will normally require pH adjustment before biological treatment. In some
cases, natural recarbonation from biological oxidation is adequate to maintain the pH within
acceptable limits.
6.4.2 Use of Chemicals in Primary Clarifiers
The effect of polyelectrolyte addition (used either alone or in combination with inorganic
coagulants) on primary clarifier performance is shown in Table 6-1. For comparative
purposes, the performance of the clarifiers before and after the addition of chemicals
is shown. As indicated, the average values for suspended solids and BOD removals were
37.7 percent and 31 percent, respectively, before chemical treatment. As a result of
chemical addition, suspended solids and BOD removal efficiencies increased to 64.7 percent
and 46.7 percent, respectively. It is also evident from Table 6-1 that the effect of
polyelectrolyte addition was pronounced where the existing clarifier performance was poor,
as indicated by initial low suspended solids removal. The above data illustrate that the
proper selection and application of polyelectrolytes and chemicals to raw wastewater can
significantly improve primary clarifier performance.
When considering the addition of chemicals to primary clarifiers, it is important to examine
the effect of increased primary clarifier efficiency on subsequent treatment units. The
increased removal of suspended solids and BOD from raw wastewater can affect the
downstream biological process in several ways. If the BOD load to the aerator falls below
0.25-0.35 Ib. BOD/lb. MLVSS/day for extended periods of time, nitrification conditions
can develop in the aerator. This can reduce the total oxygen demand of the effluent,
but will impose an added oxygen demand on the aeration facility because the oxidation
of one pound of ammonia nitrogen requires about 4.5 pounds of oxygen.
A decrease in loading to the aerator will normally require more careful management of
sludge to insure stable operation of the aeration basin. However, the quantity of excess
activated sludge generated under these reduced loading conditions will be substantially
less than that generated under normal loading conditions, and this may be considered
an added advantage of adding chemicals to the primary clarifier.
Little information has been generated regarding the periodic addition of chemicals to the
primary clarifier for controlling peak organic or hydraulic loads. This approach, while not
always applicable, can frequently be used to maintain system stability during temporary
overload.
6-3
-------
Table 6-1
Type and Amount
of
Chemical Added
Purifloc-A21 (0.95 mg/l)
DOW-SA1193 (0.2 mg/l)
Purifloc- A21 (1 mg/l)
Purifloc-A21 (0.75 mg/l)
Purifloc-A21 (0.89 mg/l)
DOW-SA1193 (0.25 mg/l)
Purifloc-A21 (1 mg/l)
FeCI2 + NaOH + Purifloc - A23 (0.3 mg/l)
FeCl2 + NaOH + Purifloc - A23 (0.3 mg/l)
Purifloc-A2! (1 mg/l)
Purifloc - A23 (0.25 mg/l)
FeCl3 + Purifloc - A23
FeCI3 + Purifloc - A23
Purifloc -A21 (0.74 mg/l)
Purifloc-A21M (1.14 mg/l)
MEAN
'WAS - Waste activated sludge
PS - Primary sludge
Effect of Chemical Treatment on Primary Clarifier Performance
Performance Preceding Chemical Treatment
SS Removed
mg/l
13
13
157
26
113
120
107
230
104
52
93
93
percent
12
12
43
18
43
47
47
62
49.7
31
33
33
50
43
BOD Removed
mg/l percent
28
28
82
50
135
111
83
47
53
53
26
26
23
22
37
31
43.8
31
34
34
36
Weight Weight
Ratio of Performance After Chemical Treatment Ratio of
WAS/PS1 SS Removed
mg/l
0.61 75
0.61 72
281
69
159
0.8 151
169
379
173
0.79
1 .44 80
196
213
percent
65
55
76
52
60
61
62
79
76.8
51
74
68
63
63
BOD Removed WAS/PS!
mg/l
46
36
127
87
154
74
105
58
102
97
percent
48 0.31
37 0.41
33
.
37
0.46
46
39
57.8
0.28
46.4 0.67
61
53
45
Reference
3
3
3
3
3
3
3
4,5
4,5
6
7
8
8
9
9
93.5
37.7
68.5
31
168
64.7
87
46.7
-------
Schmidt and McKinney (10) studied phosphorus removal by lime addition to the primary
clarifier of a treatment system which also included secondary treatment. In this study,
the system was operated at a pH value of 9.5, which during biological treatment was
reduced to a value between 7 and 8. Therefore, no neutralization was required. The lime
precipitation step reduced the BOD by 60 percent, suspended solids by 90 percent, and
total phosphorus by 80 percent. However, Schmidt and McKinney indicated that the
lime-primary sludge was gelatinous in nature and required polyelectrolyte treatment prior
to dewatering by vacuum filtration. They further indicated that the mass of primary sludge
is about twice that obtained by conventional settling, although the total mass of primary
and secondary sludge produced is increased by less than 50 percent (10). Lime addition
to primary clarifiers for phosphorus removal has been used in many locations. In all cases,
significant improvements in both suspended solids and BOD removal were noted. Table 6-2
presents the results of some of these studies.
Table 6-2
Lime Addition to Primary Clarifiers
Location
Duluth,
Minnesota
Rochester,
New York
Lebanon,
Ohio
Lime Added
mg/1 CaO
75
125
140
145
Percent
Removal Before
Lime Addition
Percent
Removal After
Lime Addition
Remarks
BOD
50
55
SS BOD
70
70
60
75
50
-SS_
75 —
90 —
80-90 Jar tests
— 66
74
Pilot plant
Reference
12
12
11
As mentioned above, the addition of lime to the primary clarifier can be expected to
increase the primary sludge mass to about twice that obtained by conventional primary
settling, depending on the alkalinity of the incoming wastewater. Therefore, a complete
evaluation of the sludge handling facilities must be made when considering this technique.
For instance, some states have cautioned against this practice when the primary sludge
is to be anaerobically digested.
Freese, et al, (9) studied the application of polyelectrolytes for raw wastewater flocculation
in the District of Columbia's Water Pollution Control Plant. The plant also recirculated
thickener overflow and digested sludge elutriate to the primary clarifier. Even though the
primary clarifier performance improved with the addition of chemicals, the solids input
from the sludge elutriation process remained the same. As a result, the full benefit of
polyelectrolyte addition was not realized. This indicated that separate treatment of elutriate
is required, since polyelectrolyte addition apparently does not enhance the capture of
the fine solids which are normally discharged in the elutriate. These fine solids often
accumulate in the solids-handling system
Mogelnicki (13) reported on the effect of polyelectrolyte addition in primary clarifiers
on the overall BOD removal. The data reported by Mogelnicki, covering both activated
6-5
-------
Percent
Removal Before
Polyelectrolyte
Addition
BOP SS
26 —
23 43
Percent
Removal After
Polyelectrolyte
Addition
BOD SS^
48 —
33 76
Percent
Removal Before
Polyelectrolyte
Addition
BOD SS
83 —
79 72
Percent
Removal After
Polyelectrolyte
Addition
BOD SS_
90 —
85 84
sludge and trickling filter processes, are shown in Table 6-3. These data indicated that
polyelectrolyte addition to primary clariflers increases the overall BOD removal by
approximately 7 percent.
Table 6-3
Effect of Polyelectrolyte Addition in Primary Clarifier
on Overall BOD Removal
Primary Clarifier Total Plant
Percen
Removal B
Type of
Wastewater
Activated Sludge'
Trickling Filter1
'With 1 mg/lPuriflocA-21
Source: Mogelnicki (13)
6.4.3 Use of Chemicals in Secondary Clarifiers
There is little published information available on the use of chemicals to improve secondary
clarifier performance. This is probably due to the fact that aerobic biological sludges
flocculate and settle readily if normal growth conditions are maintained. However, upsets
in secondary clarifier performance can occur as a result of increased hydraulic and/or
solids loading or development of a filamentous or bulking sludge. When one or more of
these conditions exist, the use of inorganic chemicals and/or polyelectrolytes has been
successful in some instances in obtaining a satisfactory effluent.
Singer, et al, (14) studied the effect of adding cationic and anionic polyelectrolytes to
improve settling characteristics of bulking activated sludge in the laboratory. Their studies
indicated that cationic polyelectrolytes at a concentration of 2-3 mg/1 were effective in
coagulating a bulking activated sludge but that the anionic polyelectrolyte tested had no
effect on improving settling. Goodman and Mikkelson (15), on the basis of full-scale studies,
concluded that application of cationic polyelectrolytes to primary clarifier effluent at the
rate of 0.1 Ib./ton of secondary dry solids increased overall BOD removal efficiency to
95 percent and decreased the loss of solids in the secondary effluent of the activated
sludge plant.
Based on studies conducted at the Hanover treatment plant by the Metropolitan Sanitary
District of Chicago, Zenz and Pivnicka (16) have shown that the addition of alum to
aeration tanks (primarily intended for soluble phosphorus removal) improved flocculation
of activated sludge. However, their results indicated that increasing amounts of alum floe
escaped through the final clarifiers as the dosage of alum increased from an A1:P weight
ratio of 1.54 to 1.85. The addition of alum to the aeration tank favored the development
of lower organisms, while the higher forms such as protozoa and metazoa were absent
6-6
-------
when using alum. Zenz and Pivnicka also indicated that the precipitated phosphate was
not released during anaerobic digestion and was permanently removed from the treatment
system.
Laughlin (17) has reported adding 460 gallons/day of alum (17 percent alum solution)
to the secondary clarifier of a trickling filter plant treating 1.5 mgd of wastewater. The
preliminary results indicated a reduction in effluent BOD and SS concentrations from
20 mg/1 and 15 mg/1, respectively, to 10 mg/1 and 10 mg/1. The phosphorus concentration
in the effluent was reduced from 8 mg/1 to 1 mg/1. In addition, Laughlin (17) has reported
problems of reduced alkalinity in sludge undergoing digestion when alum was used as
coagulant in primary treatment at the Richardson, Texas plant. The addition of alum
to raw wastewater was discontinued after 9 days total operation to prevent pH depression
in the digester.
From the above studies, it can be concluded that alum, iron, or polyelectrolyte addition,
either in the primary or secondary treatment process, can be used advantageously to
improve the overall performance of the treatment system including phosphorus removal.
Lime addition may not be feasible for upgrading activated sludge secondary clarifiers
because of the potential adverse effect of recirculated lime sludge on mixed liquor microbial
characteristics. Lime addition to either trickling filter or activated sludge secondary clarifiers
will require pH adjustment of the effluent before discharge to the receiving waters. Lime
addition to primary clarifiers may be used, if consideration is given to controlling the
pH within acceptable limits for the subsequent processes, and to changes in sludge
characteristics and handling requirements.
6.5 Other Approaches to Improvement of Clarification
6.5.1 Design and Operational Factors
In many cases, poor clarifier performance is the result of poor operation or inadequate
design even when the hydraulic load has not exceeded the design values. It is essential
to correct these deficiencies through modifications before any consideration is given to
other upgrading techniques, such as chemical addition. Inadequate design factors which
affect clarifier performance include the following:
1. Poor inlet and outlet design.
2. Poor sludge withdrawal system.
3. Absence of scum removal devices.
Poor inlet or outlet design can cause excessive turbulence or short circuiting in the clarifiers,
resulting in the escape of solids in the effluent. Fall (18) has described a system in the
Greater Peoria Sanitary District Sewage Treatment Plant (employing the Kraus
Modification) where changes in inlet and outlet design improved clarifier performance.
He reported that conversion of center-feed square primary and secondary clarifiers to
peripheral-feed systems permitted overflow rates as high as 4,100 gpd/sq.ft. without any
6-7
-------
apparent loss in solids removal efficiency, and further that combinations of primary and
secondary sludge were concentrated to as much as 6 percent in the clarifiers. Typical
cross sections of a circular clarifier with center and peripheral feeds are illustrated in
Figure 6-1.
Poor sludge withdrawal systems in secondary clarifiers often cause sludge accumulation,
thus creating oxygen-deficient conditions. As a result, the sludge will sometimes gasify
due to denitrification, and will rise to the surface and overflow the weirs. This condition
can be partially corrected by installing automatic sludge-withdrawal devices, sludge blanket
finders, and sludge density meters to facilitate proper sludge management practices.
Instrumentation commonly used for this purpose is discussed in detail in Chapter 14.
West (19) has described several case histories where improved operational conditions
increased the efficiency of clarifiers. Table 6-4 shows the methods and performance
obtained from the above studies. Installation of a suction-type sludge-withdrawal device,
in lieu of a scraper mechanism, and of a scum removal device is strongly recommended
for improving secondary clarifier performance. The scraper mechanism and hydraulic
suction device for sludge removal are illustrated in Figure 6-1.
6.5.2 Use of Tube Settlers
According to the classical theory of discrete particle settling, the efficiency of suspended
particle removal in a sedimentation basin is solely a function of overflow rate and is
independent of depth and detention time. If the above theory is applicable to raw
wastewater or activated sludge floe settling, then the clarifier performance could be
improved by introducing a number of trays or tubes in the existing clarifiers. However,
the introduction of trays has been found to be unsuitable on a practical basis because
of the sludge collection and removal devices required. A new device called a tube settler,
which employs the above approach of trays or tubes in the existing clarifiers, has recently
become available. Tube settlers (of various lengths) are usually installed in modules at
an angle to the horizontal, and are made of a light-weight, durable material. A typical
inclined-tube settler module is shown in Figure 6-2 (20). Figure 6-3 shows a typical
clarifier cross-section with the tube settler modules installed (20). Even though tube settlers
can be used at any angle of inclination, Hernandez and Wright (21) have recommended
an angle of 60°, for self-cleaning purposes.
Tube settlers have been used in primary and secondary clarifiers to improve performance
as well as to increase throughput in existing clarifiers. Conley and Slechta (22) and Gulp,
et al (20) have described the performance of several plant-scale installations of tube settlers
in primary and secondary clarifiers. The results of their studies indicate that the overflow
rates in primary clarifiers can be increased to 5,000 gpd/sq.ft. while producing the same
quality effluent as the control unit without the settlers. Tube settlers enhance the ability
to capture settleable solids at high overflow rates because the depth of settling has been
reduced to a few inches in the tube. It should be realized that tube settlers do not improve
6-8
-------
FIGURE 6-1
TYPICAL CLARIFIER FEED AND SLUDGE REMOVAL MECHANISMS
\
0
SLUDGE
371
INFLUENT
CIRCULAR CENTER FEED CLARIFIER WITH
A SCRAPER SLUDGE REMOVAL SYSTEM
^*
EFFLUENT
INFLUENT
EFFLUENT
SLUDGE
CIRCULAR PERIPHERAL FEED CLARIFIER WITH*
HYDRAULIC SUCTION SLUDGE REMOVAL SYSTEM
6-9
-------
Table 6-4
Effect of Clarifier Operational Improvements on Overall Effluent Quality
Plant Location & Capacity
Sioux Falls, South Dakota
3.5 mgd
Operational Improvement
Sludge Blanket Finder for Secondary
Clarifier
Turbidimeter for Effluent Quality
Increased Sludge Recirculation Capacity
Improvement in Overall Effluent Quality
Effluent BOD reduced from 20 to 10 mg/1
Effluent SS reduced from 35 to 13 mg/1
Metropolitan St. Louis
Sewer District, Missouri
21 mgd
Sludge Blanket Finder for Secondary
Clarifier
Turbidimeter for Effluent Quality
Increased Air Supply
Reduced Sludge Recirculation
Effluent BOD reduced from 40 to 9 mg/1
Effluent SS reduced from 92 to 16 mg/1
BOD removal efficiency increased from
73 to 94 percent
SS removal efficiency increased from
46 to 92 percent
Source: West (19)
-------
FIGURE 6-2
INCLINED TUBE SETTLER MODULE (20)
FIGURE 6-3
INSTALLATION OF TUBE SETTLERS AT THE
WICKAM, PENNSYLVANIA SEWAGE TREATMENT PLANT (20)
TUBE EFFLUENT LAUNDER
THIS PORTION OF
CLARIFIER REMOVED
FROM SERVICE
MIXED LIQUOR
INLET
AIR LIFT
SLUDGE RETURN
6-11
-------
the efficiency of primary clarifiers that are already achieving very high (40-60 percent)
removals of suspended solids. Moreover, tube settlers will neither remove colloidal solids
that remain in suspension nor induce additional coagulation to effect added particle
removal.
Tube settlers have been used to improve secondary clarifier performance where the clarifiers
were subjected to overflow rates of 900 to 2,800 gpd/sq.ft. (20) (22). Prior to the
installation of tube settlers, the effluent solids concentration varied between
8 and 1,480 mg/1. This range was reduced to 4-156 mg/1 after the installation.
Fouling due to attachment and growth of biological slime on the sides of the tubes is
sometimes a problem. Some form of cleaning device (water jet or air) is required so that
the solids build-up can be removed occasionally. Conley (22) has recommended the use
of 1.0 gpm/sq.ft. as a maximum overflow rate and 35 Ibs./sq.ft./day as a maximum solids
loading for the design of secondary clarifiers with tube settlers. Since the flocculating
and settling characteristics of sludge vary from plant to plant, each case should be evaluated
separately for suitable design criteria. Small pilot units are available from the manufacturer
for this purpose.
The performance of clarifiers provided with tube settlers at various installations is
summarized in Table 6-5. Little information is available at the present time to establish
cost information on tube settlers. However, an estimating cost figure of
12 to 20 dollars/sq.ft. for tube settlers with an installation cost of 5 to 15 dollars/sq.ft.
has been recommended by the manufacturer (22).
6.6 Chemical Feeders
Table 6-6 contains a summary of properties and characteristics of chemicals commonly
used in wastewater treatment. Most chemicals used in wastewater treatment are added
to the unit treatment process in solution. Dry chemicals may be fed to dissolving tanks
by either volumetric of gravimetric feeders. Gravimetric feeders are more accurate and
dependable, but cost more than volumetric feeders.
One type of volumetric feeder uses a continuous belt from under the hopper to the
dissolving tank. A mechanical gate mechanism regulates the depth of material on the belt,
and the rate of feed is governed by the speed of the belt and/or the height of the gate
opening. The hopper normally is equipped with a vibratory mechanism to reduce arching.
This type of feeder is not usually suited for easily fluidized materials. Another type employs
a screw or helix. Rate of feed is governed by the speed of screw or helix rotation. Some
screw-type designs are self-cleaning, while others are subject to clogging.
Most of the other types of volumetric feeders fall into the positive-displacement category,
involving some form of moving cavity of a specific or variable size. In operation, the
chemical falls by gravity into the cavity and is more or less fully enclosed and separated
6-12
-------
Table 6-5
Performance of Clarifiers Using Tube Settlers
- Surface Overflow Rate.
^Percent removal rather than concentration.
Operational Data Using
Plant Location
Philomath,
Oregon
Philomath,
Oregon
Hopewell Township,
Pennsylvania
Miami,
Florida
Type
Trickling
Filter
Trickling
Filter
Activated
Sludge
Activated
Sludge
Size
mgd
0.15
0.15
0.13
1.0
Tube
Location
Secondary
Clarifier
Primary
Clarifier
Secondary
Clarifier
Secondary
Clarifier
Existing Facility
SOR1
gpm/sq.ft.
0.6
0.84
0.34
1.3
Eff. SS
mg/1
60-70
40-45 2
60-70
500
Tube Settlers
SOR1
gpm/sq.ft.
3.3-4.6
2.1-3.3
2-3
1.7
Eff. SS
mg/1
60-70
34-412
27
33
Source: Conley and Slechta (22)
-------
Table 6-6
Properties and Characteristerics of Selective Chemicals
Used in Wastewater Treatment
Chemical Name
and
Formula
Aluminum sulfate
A12(S04)3- 14H20
Ferric Chloride
FeCl3 Solution
FeCl3 • 6H2O
FeCl3 - Anhydrous
Ferric Sulfate
Fe2 (S04)3 • 9H20
Ferrous Sulfate
FeSO4 ' 7H2O
Sodium Aluminate1
Na2O • A1203
Calcium Hydroxide
Ca(OH)2
Calcium Oxide
CaO
Polyelectrolyte^
Shipping
Container
100-200 Ibs.
Bags or Bulk
Barrels, Bulk
Barrels
Drums
Bags, Drums
Bags, Barrels,
Bulk
Bags, Drums
50 Ibs. Bags,
Barrels, Bulk
SO Ibs. Bags,
Barrels, Bulk
SO Ibs. bags
Weight
Ibs./cu.ft.
60-67
121bs./gal.
60-65
65-70
70-72
63-66
50-60
25-70
55-70
75
Solubility
in Water
gm./lOOcc.
78.8 (30°C)
35-45% Solution
91.1 (20°C)
74.4 (0°C)
Very Soluble
60.2 (30°C)
—
23.4 (40°C)
Forms
Ca(OH)2
—
Storage
Container
Materials
Iron, Steel
Glass, Rubber,
Concrete
Glass, Rubber,
Concrete
Glass, Rubber,
Concrete
Plastic, Rubber,
Stainless Steel
Asphalt, Concrete
Iron, Plastic,
Rubber
Asphalt, Concrete,
Rubber
Asphalt, Concrete,
Rubber
—
Handling
Characteristics
Dusty
Acid, Corrosive
Acid, Corrosive
Acid, Corrosive
—
—
Need Hopper
Agitation
Dusty
Dusty
—
Feed
Regulation
Solution
—
Solution
Solution
Solution
Solution
Solution
Slurry
Slurry
Solution
Strength of
Solution (%)
and
Characteristics
Acid and Corrosive
<45%
Acid and Corrosive
Acid and Corrosive
<25%
Acid and Corrosive
Acid and Corrosive
—
Alkali
<25%
Alkali
0.25-6.0
Suitable
Handling
Material
for Solution
Lead, Rubber,
Plastics
Plastic, Glass
Rubber
—
Plastic, Glass,
Rubber
Plastic, Glass
Iron, Plastic,
Rubber
Plastic, Rubber
Plastic, Rubber
—
'Also available in liquid form
^There are over 200 polyelectrolytes marketed for wastewater treatment. Information on particular poly electrolyte is available from
the manufacturer
Sources: Water Pollution Control Federation (1)
Fair and Geyer (23)
-------
from the hopper's feed. The size of the cavity and the rate at which the cavity moves
and is discharged govern the amount of material fed. The positive control of the chemical
may place a low limit on rates of feed. One unique design is the progressive-cavity metering
pump, a non-reciprocating type. Positive-displacement feeders often utilize air injection
to enhance flowability of the material.
The basic drawback of volumetric feeder design, i.e. its inability to compensate for changes
in materials density, is overcome by modifying the volumetric design to include a
gravimetric or loss-in-weight controller. This modification allows for weighing of the
material as it is fed. The beam balance type measures the actual mass of material, and
is considerably more accurate over a period of time than the less common spring-loaded
gravimetric designs.
Gravimetric feeders are used where feed accuracy of about 99 percent is required for
economy, as in large-scale operations, and for materials which are used in small, precise
quantities. It should be noted, however, that even gravimetric feeders cannot compensate
for weight added to the chemical by excess moisture. Many volumetric feeders may be
converted to a loss-in-weight basis by placing the entire feeder on a platform scale tared
to neutralize the weight of the feeder.
Good housekeeping and need for accurate feed rates dictate that the gravimetric feeder
be shut down and thoroughly cleaned on a regular basis. Although many of these feeders
have automatic or semi-automatic devices which compensate to some degree for
accumulated solids on the weighing mechanism, accuracy is affected, particularly on humid
days, when hygroscopic materials are fed. In some cases, built-up chemicals can actually
jam the equipment.
No discussion of feeders is complete without at least a passing reference to dissolvers,
because any metered material must be accurately mixed with water to provide a chemical
solution of desired strength. Most feeders, regardless of type, discharge their material to
a small dissolving tank, which generally is equipped with a nozzle system and/or mechani'cal
agitator depending on the solubility of the chemical being fed.
One particular area that requires careful consideration is the dispersion of dry
polyelectrolytes to make feeding solutions. Long-chain polymers are very difficult to
dissolve, and special equipment is often necessary. Figure 6-4 depicts three typical
techniques for dissolving polymers (24). The simplest method is the manual-vortex
technique, in which dry polymer is manually dropped into the vortex produced by the
mixer. The manual-aspiration technique utilizes the aspiration principle to wet the polymer;
water at a pressure of at least 20 psi is forced into the mixing funnel producing a downward
stream which traps the dry polymer.
The third method illustrated in Figure 6-4 is the automatic-wetting spray technique. This
system tends to replace the manual measuring of chemicals and free the plant operator
6-15
-------
MECHANICAL MIXER
FIGURE 6-4
TYPICAL HANDLING AND APPLICATION TECHNIQUES FOR POLYELECTROLYTES (24)
DRV CHEMICAL
MANUALLY FED
WATER SUPPLY
— WETTING (
DISSOLVING TANK
WATER SUPPLY
SHUT-OFF VALVE
WATER METER (OPTIONAL
•POLYMER DISPENSER ( MIXING FUNNEL
' f*\—MECHANICAL MIXER
1
AL )•>
-==-^k=
&
DRAIN VALVE -/
-DISSOLVING TANK
/-HOLDING TANK
&-. / (TWICE SIZE OF DISSOLVING TANK)
^
JJQ METERING PUMP
». MANUAL-VORTEX TECHNIQUE
B. MANUAL-ASPIRATOR TECHNIQUE
STORAGE HOPPER
DRY POLYMER
FEEDER
FEEDER BASE
FLOW CONTROL
VALVE -\
SOLENOID VALVE
WETTING CHAMBER
WITH SPECIAL
WETTING SPRAY
AND MIXER
MIXING TANK
WITH MIXER (
LEVEL SWITCH
WATER SUPPLY
METERING PUMP
C. AUTOMATIC-WETTING SPRAY TECHNIQUE
-------
for other duties. Polymer is discharged evenly onto a water spray in a small wetting
chamber, where it is trapped and dropped into the vortex. The entire operation may be
controlled automatically by level switches and solenoid valves. Once the polymer is in
solution, no agitation is required. However, some manufacturers recommended diluting
the solution further before applying it to the unit treatment process.
Positive displacement or plunger-type pumps generally are used to meter polymer solutions,
but diaphragm pumps can also be used and/or adapted to handle them For the more
viscous solutions (> 1,000 cp), the speed of a mechanical diaphragm pump should be
limited to 105 strokes/minute (24). In addition, ball-type suction and discharge valves
are recommended. The design of polymer-dispersing systems should recognize the
temperature dependence of viscosity and the consequent effects on measuring and pumping.
The capacity of a dissolver is based on detention time, which is directly related to the
solubility or wettability of the chemical. Therefore, the dissolver must be large enough
to provide the necessary detention for both the chemical and the water at the maximum
feed rate. At lower feed rates, the strength of solution or suspension leaving the dissolver
will be less, but the detention time will be approximately the same unless the water supply
to the dissolver is reduced. When the water supply to any dissolver is controlled for the
purpose of forming a constant-strength solution, then mixing within the dissolver must
be accomplished by mechanical means, because sufficient power will not be available from
the mixing jets at low rates of flow.
Specific factors influencing chemical feed rates per volume of water, detention times, and
selection of materials of construction are available in the literature (25). Alum, lime, and
ferrous sulfate have been found to require about 5 minutes detention time at about
0.5 Ib./gallon. Ferric sulfate requires longer detention times (20 to 30 minutes) than the
other granular chemicals. Further practical experience with a number of these chemicals
is available in Gulp and Gulp (26). Hot-water dissolvers decrease the required tank volume.
The foregoing descriptions give some indication of the wide variety of materials involved.
Because of this variety, a modern facility may contain any number and variety of feeders,
with combined or multiple materials capability. Ancillary equipment to the feeder also
varies according to the material to be handled. Liquid feeders involve a limited number
of design principles, principally to account for density and viscosity ranges. Solids feeders,
relatively speaking, vary considerably due to the wide ranges of physical and chemical
characteristics, feed rates, and the degree of precision and repeatability required.
6.7 Process Designs and Cost Estimates
Process units were designed and capital costs were developed for three examples of chemical
treatment in primary clarifiers:
EXAMPLE 1: Alum and polyelectrolyte addition at concentrations of 20 mg/1 (as
and 0.5 mg/1, respectively.
6-17
-------
EXAMPLE 2: Ferric chloride and polyelectrolyte addition at concentrations of
20 mg/1 (as Fe^+) and 0.5 mg/1, respectively.
EXAMPLE 3: Lime precipitation using 150 mg/1 of quick lime (CaO).
Capital costs were developed for a capacity of 1 mgd for the first two cases and 10 mgd
for the lime precipitation method. The chemical treatment systems using alum and ferric
chloride consist of bulk chemical storage facilities, transfer pumps, rapid mixing tanks,
and flocculating basins. When lime is used, the treatment system includes a storage bin,
lime feeder, and mixing tank. The costs are presented in Table 6-7.
"""•• Table 6-7
Capital Costs for Chemical-Addition Facilities
(ENR Index 1500)
Example Capital Cost^
1 - Alum & Polyelectrolyte $ 74,000
2 - Ferric Chloride & Polyelectrolyte 63,000
3-Lime 150,000
* These costs contain no contingency for engineering design,
bonding, and construction supervision.
6.8 References
1. Sewage Treatment Plant Design. Water Pollution Control Federation Manual of
Practice No. 8, Washington, D.C., 1959.
2. Brenner, R.C., Phosphorus Removal by Mineral Addition. Nutrient Removal and
Advanced Water Treatment Symposium, Presented by Federal Water Pollution Control
Administration, Cincinnati, Ohio, April 29-30, 1969.
3. Anon, Effects of Raw Sewage Flocculation in Secondary Waste Treatment Plants.
Midland, Michigan: The Dow Chemical Co.
4. Wukasch, R.F., The Dow Process for Phosphorus Removal. Paper presented at the
Phosphorus Removal Symposium, Presented by Federal Water Pollution Control
Administration, Chicago, 111., June, 1968.
5. Wukasch, R.F., New Phosphate Removal Process. Water and Wastes Engineering, 5,
No. 9, pp. 58-60 (1968).
6-18
-------
6. Voshel, D., and Sak, J.G., Effect of Primary Effluent Suspended Solids and BOD
on Activated Sludge Production, Journal Water Pollution Control Federation, 40,
No. 5, Part 2, pp. R203-R212 (1968).
7. Wirts, J.J., The Use of Organic Poly electrolyte for Operational Improvement of Waste
Treatment Processes. Federal Water Pollution Control Administration,
Grant No. WPRD 102-01-68, May, 1969.
8. Applications of Chemical Precipitation Phosphorus Removal at the Cleveland Westerly
Wastewater Treatment Plant. Prepared for the City of Cleveland, Ohio, by the Dow
Chemical Co., Midland, Mich. (April, 1970).
9. Freese, P.V., Hicks, E., Bishop, D.F., and Griggs, S.H.,/?aw Wastewater Flocculations
with Polymers at the District of Columbia Water Pollution Control Plant. Federal
Water Quality Administration, Contract No. WPRD 53-01-67.
10. Schmidt, L.A., and McKinney, R.E., Phosphate Removal by a Lime-Biological
Treatment Scheme. Journal Water Pollution Control Federation, 41, No. 7,
pp. 1,259-1,279 (1969).
11. Villiers, Ronald V., Municipal Waste-water Treatment by Single Stage Lime
Clarification and Activated Carbon. Internal EPA paper, Robert A. Taft Water
Research Center, Cincinnati, Ohio.
12. Process Design Manual for Phosphorus Removal. Black and Veatch Consulting
Engineers, Environmental Protection Agency, Contract No. 14-12-936,
Washington, D.C. (1971).
13. Mogelnicki, S., Experiences in Polymer Applications to Several Solids - Liquids
Separation Process. Proceedings - Tenth Sanitary Engineering Conference - Waste
Disposal from Water and Wastewater Treatment Processes, University of Illinois,
February 6 - 7, 1968.
14. Singer, P.C., Pipes, W.O., and Hermann, E.R., Flocculation of Bulked Activated Sludge
with Poly electrolytes. Journal Water Pollution Control Federation, 40, No. 2, Part 2,
pp. 21-129 (1968).
15. Goodman, B.C., and Mikkelson, K.A., Advanced Wastewater Treatment. Chemical
Engineering Desk Book Issue, 77, pp. 75-85, April 27, 1970.
16. Zenz, D.R., and Pivnicka, J.R., Effective Phosphorus Removal by the Addition of
Alum to the Activated Sludge Process. Proceedings - 24th Industrial Waste
Conference, Purdue University, pp. 273-301 (1969).
6-19
-------
17. Laughlin, James, Modifications of a Trickling Filter Plant to Allow Chemical
Precipitation. Advanced Waste Treatment and Water Reuse Symposium, Presented by
Environmental Protection Agency, Dallas, Texas, January 12-14, 1971.
18. Fall, E.B., Jr., Redesigning Existing Treatment to Increase Hydraulic and Organic
Loading. Presented at the 43rd Annual Conference - WPCF, Boston, Mass.
(Oct., 1970).
19. West, A.F., Case Histories of Plant Improvement by Operations Control, Nutrient
Removal and Advanced Waste Treatment. Federal Water Pollution Control
Administration, Ohio Basin Region, Cincinnati, Ohio (1969).
20. Culp, G.L., Hsiung, K.Y., and Conley, W.R., Tube Clarification Process, Operating
Experience. Journal Sanitary Engineering Division, ASCE, 95, No. 5, pp. 829-847
(1969).
21. Hernandez, J., and Wright, J., Tube Settler Design. Presented at the 25th Industrial
Waste Conference, Purdue University (May, 1970).
22. Conley, W.R., and Slechta, A.F., Recent Experiences in Plant Scale Application of
the Settling Tube Concept. Presented at the 43rd Annual Conference - WPCF, Boston,
Mass. (Oct., 1970).
23. Fair, G., and Geyer, J., Water Supply and Wastewater Disposal. New York: John
Wiley and Sons, Inc., 1966.
24. Russo, F., and Carr, R., Polyelectrolyte Coagulant Aids and Flocculents: Dry and
Liquid, Handling and Application. Water and Sewage Works, 117, No. 11, pp.
R-72/R-76 (1970).
25. Water Treatment Plant Design. New York: American Water Works Association, Inc.,
1969.
26. Culp, R.L., and Culp, G.L., Advanced Wastewater Treatment. New York: Van
Nostrand-Reinhold Company, 1971.
6-20
-------
CHAPTER 7
EFFLUENT POLISHING TECHNIQUES
7.1 General
The use of effluent polishing for secondary effluent is a relatively new idea which is
receiving increasing attention as a practical and economical method of upgrading to obtain
increased organic and suspended solids removal from existing treatment facilities. It appears
to be particularly applicable in those cases (and there are many) where it is necessary
to increase efficiency by an overall amount of 10 to 20 percent in order to meet stricter
water quality standards.
Four unit processes are considered in this manual for effluent polishing: 1) polishing
lagoons; 2) microstraining; 3) filtration, including mixed, multi-media, and moving-bed
filters; and 4) activated carbon adsorption. The reader is referred to the process design
manual for suspended solids removal for an in-depth discussion of microstraining and
filtration.
7.2 Polishing Lagoons
Polishing lagoons offer an opportunity for increased organic and solids removal at a
minimum cost. There are two types of polishing lagoons which can be used, aerobic and
facultative.
7.2.1 Aerobic Lagoons
Aerobic lagoons are generally subdivided into two groups:
1. Shallow lagoons, with depths in the range of 2.5 to 4.0 feet.
2. Deep lagoons, with aeration devices included to insure maintenance of aerobic
conditions.
The shallow aerobic lagoon is one in which the algae-bacterial inter-relationship is optimized
by providing as much light penetration as possible, and by maximizing photo-synthetic
efficiency and bacterial oxidation of organic wastes. Operational data from a shallow
aerobic lagoon are presented in Table 7-1 (1). The data indicate consistent BOD removals
throughout the year, but marked increase in the concentration of suspended solids in
the effluent during the summer periods, when algae activity is at its peak. The decreased
solids removal without concurrent reduction of BOD removal during the summer months
is caused by algae carryover in the effluent. This indicates that algae present in the effluent
do not exert a significant amount of BOD demand during the five-day incubation used
in the standard BOD test. The substantial increase in effluent suspended solids during
the summer period, however, is a major disadvantage of the shallow lagoon as a dependable
year-round polishing technique.
7-1
-------
Table 7-1
Operational Data from a Shallow Aerobic Polishing Lagoon
Plant Time
Location Period
Indian Creek, 1963
Kansas June 12-13
July 10-11
Aug. 27-28
-j
to Dec. 1-2
1964
Jan. 28-29
March 6-7
April 10-11
Flow
mgd
2.3
1.9
2.1
1.5
1.3
1.7
2.8
BOD
Ibs./day
In
377
179
194
380
416
383
839
Out
188
144
81
191
171
213
256
BOD
Removal
percent
50.2
19.5
58.3
49.8
59.0
44.4
69.4
Surface
Organic
Loading
Ibs. BOD/acre/day
60.9
29.0
31.6
61.3
67.1
61.8
135
SS
Ibs./day
In
563
240
199
534
691
284
489
Out
990
488
480
210
367
128
117
SS
Removal
percent
- 77
-103
-140
61
45
23
77
Pond size - 6.2 acres
Depth - 2.5 ft.
Odorous in Spring
Source: Loehr and Stephenson (1).
-------
An alternative to the shallow lagoon is the deep, aerated, lagoon. These deeper lagoons
can operate at greater surface organic loadings than shallow lagoons and yet maintain
higher organic removals. Since oxygen is supplied to the basin by mechanical devices rather
than furnished by the algae-bacterial biosymbiotic relationship, the algae production in
the aerated lagoon is minimal compared to the shallow lagoon. Operational data for two
aerated effluent polishing lagoons (8 to 10 feet deep) are presented in Table 7-2 (2).
Table 7-2
Removal Efficiencies for
Deep Aerated Effluent Polishing Lagoons
Surface BOD SS
Plant Location Organic Loading Removal Removal
Ibs. BOD/acre/day percent percent
Washington Borough, N.J.1 230 63 78
East Windsor Township, N.J.2 134 75 75
1 Low-rate trickling filter plant
^Contact stabilization plant
Source: Hinde Engineering (2)
The lagoon at the Washington Borough Plant has average influent BOD and suspended
solids concentrations of 43 mg/1 and 70 mg/1, respectively. The average effluent BOD
and suspended solids concentrations are 16 mg/1 and 15 mg/1, respectively. The East
Windsor Plant's polishing lagoon receives organic and solids concentrations as high as 80
mg/1, while the effluent concentrations are generally about 15 mg/1.
The deep aerated lagoon utilizes various types of aeration devices to supply the necessary
oxygen to stabilize the organic matter. The aeration devices also must provide sufficient
mixing to disperse oxygen uniformly and to prevent solids deposition. Because mechanical
aeration devices supply considerably more oxygen per unit horsepower than air diffusion
devices, a generalized approach for sizing mechanical aerators (floating or fixed) is discussed
below.
Eckenfelder (3) has indicated that;the power levels per 1,000 gallons of aeration tank
capacity required to maintain solids under suspension and to disperse oxygen uniformly
throughout the basin are 0.02 to<:0.03!hp'/KQOO gallons and 0.006 - 0.01 hp/1,000 gallons,
respectively.
Edde (4) studied the degree of mixing:provided by mechanical:aerators used in treating
wastewater from pulp mills.IHis study indicates that a velocity greater than 0.4 ft./sec.
7-3
-------
should be maintained in the basin to prevent solids deposition, and that mixing energy
input varies with the size of the aeration unit. The following values were given as sufficient
mixing energy to disperse oxygen uniformly throughout the basin (4):
Table 7-3
Mechanical Mixing Energy Required for
Oxygen Dispersion
Size of Aerators Mixing Energy
hp hp/1,000 gal.
100 0.014
50 0.018
20 0.021
The above discussions indicate that mechanical aerators can be designed to provide either
complete mixing of solids including oxygen dispersion, or just to provide uniformly
dispersed oxygen. In the latter case, solids deposition will occur in the basin.
Based on the reported organic surface loadings, the approximate land requirements for
treating secondary effluent are as follows:
Table 7-4
Lagoon Land Requirements
Type Land Requirements
acres/mgd
Shallow aerobic lagoons 4.0
Deep aerated lagoons 1.0
7.2.2 Facultative Lagoons
Facultative lagoons are characterized by two distinct zones - aerobic and anaerobic.
Hydraulic and organic loadings are such that the dissolved oxygen in the lower section
of the lagoon is depleted but an aerobic layer is maintained near the surface. A cross-section
of a typical facultative lagoon is shown in Figure 7-1.
At Peoria, Illinois, Fall (5) investigated the efficiency of a 10-foot deep polishing lagoon
operated for 9-month periods each as a facultative lagoon and as an aerated lagoon. The
results of his work are summarized in Table 7-5. It is interesting to note that both the
BOD and the suspended solids concentrations in the effluent did not change appreciably
during the period when the lagoon was operated aerobically as compared to the facultative
operation. Fall also has stated that during the two winters of operation there was no
7-4
-------
FIGURE 7-1
TYPICAL CROSS SECTION OF A FACULTATIVE LAGOON
2-3 FT. AEROBIC ZONE
7-5
-------
Table 7-5
Comparison of Operational Data from Facultative and Aerated Polishing Lagoons
Description
Type of Secondary Plant
Flow, mgd
Lagoon Size, acres
Average Pond Depth, feet
Influent BOD, mg/1
Effluent BOD, mg/1
Percent BOD Removal
Influent SS, mg/1
Effluent SS, mg/1
Percent SS Removal
Detention Time, days
Organic Surface Loading, Ibs. BOD/acre/day
Air Applied, cu.ft. air/lb. BOD applied
Odor
Minimum Temperature of Lagoon During Study,°F
Sources: Peoria - Fall (5)
Decatur - Reynolds (6)
Springfield - Hickman (7)
Peoria,
Illinois
Aerated
Activated
Sludge
0.66
0.45
10
58
34
41
55
17
67
1.8
710
223
None
48
Facultative
Activated
Sludge
0.65
0.45
10
62
30
52
55
18
67
1.82
747
0
None
48
Decatur,
Illinois
Facultative
Trickling
Filter
6.8
8.4
5.5
30
18
40
61
31
49
2.3
218
0
None
52
Springfield,
Missouri
Facultative
Activated
Sludge
18.7
10
12
83
30
64
69
26
62
1.61
1,292
0
-------
ice on the pond. The lowest temperature of the pond effluent was 48°F, and this was
recorded after 5 days during which temperatures were below 0°F. Facultative operation
of the lagoon produced small amounts of algae in the pond during the summer period,
but no odor problems, were noted during the operation of this lagoon.
Operational data from the facultative effluent polishing lagoon in Decatur, 111., also shown
in Table 7-5, indicate that BOD and suspended solids removals averaged 40 percent and
49 percent, respectively, while operating under an organic surface loading of 218 Ibs
BOD/acre/day (6). As seen in Table 7-5, the facultative lagoon at Springfield, Mo., receives
much higher surface organic loadings (approximately 1,290 Ibs. BOD/acre/day) and still
performs creditably, with average BOD and suspended solids removals of 64 and 62 percent,
respectively (7).
A major disadvantage of using a facultative lagoon is the fact that the effluent will have
a minimal dissolved oxygen content. Springfield, Mo., solved this problem by using cascade
aeration (See Chapter 8, Section 8.2 on Post-Aeration). The effluent from the polishing
lagoon flows over a series of 5 weirs with a total drop of 75 inches. The average dissolved
oxygen in the effluent (September, 1970 through March, 1971) was 7.0 mg/1 with a
minimum and maximum, respectively, of 4.0 and 9.9 mg/1 (7).
7.3 Microstraining
Microstraining has application in effluent polishing chiefly as a method of removing
additional suspended solids (and their associated BOD) from wastewater treatment plant
effluents. A typical unit is shown in Figure 7-2. (8). The microstrainer consists of a rotating
drum with a peripheral screen. Influent wastewater enters the drum internally and passes
radially outward through the screen, with deposition of solids on the inner surface of
the drum screen. At the top of the drum pressure jets remove the deposited solids. This
backwash water is then collected and returned to the head of the plant.
The screens employed in microstrainers have extremely small openings and are made from
a variety of metals and plastics. Individual manufacturers have specific designs and sizes
for the particular needs of any potential installation. One manufacturer offers the following
grades of microfabric (9):
Table 7-6
Microstrainer Fabric Sizes
No. of
Openings per
Opening sq.in.
microns
23 165,000
35 80,000
60 60,000
7-7
-------
FIGURE 7-2
TYPICAL MICROSTRAINER UNIT (8)
DRIVE UNIT-
SCREENING
FABRIC
WASH WATER
JETS
INFLUENT CHAMBER
EFFLUENT WEIR
EFFLUENT CHAMBER
7-8
-------
The weave and shape of individual fabric wires are such that they allow the water from
the backwashing jets to penetrate the screen and remove the solids mat which forms on
the inside of the screen during its passage through the feed stream. Bodien and Stenburg
(8) have noted that only about one-half of the applied washwater actually penetrates the
screen; the rest flows down the outer perimeter into the effluent chamber.
Previously-strained effluent can be used as washwater.
Although the microstrainers have small openings, the openings themselves cannot account
for the removal efficiency of the unit. Actually, the mat of previously trapped solids
provides the fine filtration which characterizes the unit performance. This being the case,
Lynam et al (10) showed quantitatively that the slower the rate of drum rotation, the
better the product water. Another factor which becomes important in light of this mat
phenomenon is the nature of the solids applied to the microstraining process. For example
Lynam et al were unsuccessful in filtering the resulting chemical floe when secondary
effluent was coagulated ahead of the microstrainer unit.
As a section of the screen passes through its cycle, it becomes clogged rapidly as the
solids mat forms. The continuous cleansing afforded by the backwashing jets at the apex
of its travel must be augmented in some way to prevent the buildup of screen-clogging
slimes over a period of time. Ultraviolet light placed in close proximity to the screen
has been somewhat successful in slowing the development of these slimes. In general,
however, units must be taken out of service on a regular basis (once/week, for example)
to have the metal screens cleaned with a chlorine solution. In some instances, a similar
cleaning with an acid solution may be required on occasion to deal with iron or manganese
buildup on the metal screen. In cases where oil and grease problems occur, a hot water
and/or steam treatment can be used to remove these materials from the screen.
One of the advantages of using a microstrainer is its low head requirement. It is, therefore,
advantageous to transfer secondary effluent, without pumping, to a tertiary microstraining
installation in order to minimize the shear forced imparted to the fragile biological floe.
Head loss through the microstraining unit, including inlet and outlet structures is about
12 to 18 inches (9). Across the screen, a 6-inch limit is usually imposed at peak flows.
Head losses in excess of this value are prevented by bypass weirs. Head loss buildup is
reduced by increasing the rate of drum rotation and by increasing the pressure and flow
of the backwashing jets. These adjustments can be made manually or automatically.
Other operating parameters include the hydraulic and solids loading on the unit. Lynam
et al (10) found that the solids loading was the limiting factor in microstraining of activated
sludge secondary effluent. Maximum capacity was found to be 0.88 Ibs./day/sq.ft. at a
hydraulic loading of 6.6 gpm/sq.ft. Excessively high solids during upset periods can reduce
the capacity of the unit drastically from design levels. Chlorination immediately ahead
of microscreening units should be avoided to protect the screens.
Operational data from various installations are presented in Table 7-7. The microstrainers
using 23-micron fabric exhibited average solids removals ranging from 57 to 89 percent,
7-9
-------
Table 7-7
Microstrainer Operational Data
Location
Brampton, Ontario
Lebanon, Ohio
Chicago, Illinois
Luton, England
Bracknell, England
Plant
Size
mgd
0.1
Pilot
Pilot
3.0
3.6
7.2
Feed
Type
A.S.I
Effluent
A.S.
Effluent
A.S.
Effluent
A.S.
Effluent
Effluent
from A.S.
and T.F.2
T.F.
Effluent
Fabric
Opening
microns
23
23
35
23
35
35
SS
Removal
percent
57
89
73
71
55
66
Effluent
SS
mg/1
—
1.9
7.3
=3.0
7.3
5.7
BOD
Removal
percent
54
81
61
74
30
32
Effluent
BOD Backwash
mg/1 % of flow
— —
— 5.3
~ 5.0
«3.0 3.0
— 3.0
8.4
.S. - Activated Sludge
.F. - Trickling Filter
Reference
10
11
9,11
-------
while the 35-micron fabric exhibited removals of 55 to 73-percent. In practice, the coarser
35-micron fabric is generally usedi for the removal of coarse solids. Maintenance of
microstrainers can be quite costly, since most units will require cleaning at least once
a week as previously mentioned. :.iu:i
Typical design parameters for microstrainers are presented in Table 7-8. These parameters
must be evaluated to determine the proper microstrainer design for existing conditions.
Table 7-8
Typical Microstrainer Design Parameters
Parameter Value
Drum Speed, rpm 0.7 - 4.3
Filter Fabric, microns 23 and 35
Average Hydraulic Loading *, gal/sq.ft./hr.
23-micron fabric 600
35-micron fabric 800
Backwash Pressure, psi 20 - 80
Amount of Backwash Water, percent of average flow 3-6
Maximum Hydraulic Loss through Screen, inches 6
1 Based on submerged screen area.
Sources: Diaper (9) and Lynam (10).
7.4 Multi-media, Coarse-media, and Moving-bed Filters
Historically, sand filtration has not been an efficient method of polishing secondary
treatment plant effluent because of low application rates, high head losses, and the need
for frequent backwashing. This is largely because the normal backwashing of a sand filter
results in a size-graded filter with the finest grains in the upper layers. The resulting
stratification removes the bulk of the suspended matter in the upper levels, with a
consequent inefficient use of the remaining depth of the filter.
However, developments in mixed, multi-media, and deep-bed coarse-media filters have
necessitated a re-evaluation of the role of filtration in effluent polishing. In general, these
modifications permit deeper penetration of the media by the suspended and colloidal
contaminants; thus, there is a more effective utilization of the filter depth as compared
to conventional sand filters. The increased utilization of filter depth is somewhat offset
by the fact that increased backwashing rates and larger quantities of washwater are required
to backwash the media properly.
Deep-bed coarse-media sand filters are a modification of the typical rapid sand filter. The
deep-bed filter has a minimum of 4 feet of media as compared to the 2.5 feet or less
7-11
-------
of the usual rapid sand filter. The media in a deep-bed filter will generally range between
1 and 3 mm. in diameter, while the media in the rapid sand filter typically are less than
1 mm. in diameter.
In addition to coarse, mixed, and multi-media filters, a new filtering technique known
as a moving-bed filter (MBF) has been developed by Johns-Manville Corporation (12).
A schematic of the MBF is shown in Figure 7-3. The unit is basically a sand filter, but
as the filter surface becomes clogged, the filtering medium is moved forward by means
of a hydraulically-actuated mechanical diaphragm The clogged filter surface is removed
mechanically or by gravity, to the extent that a fresh and clean filtering surface is exposed
to the incoming chemically treated liquid. The unit is thus a form of countercurrent
extraction device which has the capability of functioning on a continuous basis and does
not have to be taken off stream for cleaning or backwashing.
The sand and accumulated sludge fall into a hopper and are washed and separated. The
sand is then returned to the base of the filter unit.
Table 7-9 contains operational data from some of the various filtration processes previously
discussed. Based on the available data, the mixed and coarse-media filters have significantly
higher application rates while still maintaining a high degree of solids removal. However,
it must be pointed out that the data for the MBF were developed for a phosphate removal
study and, therefore, if BOD and suspended solids removals were the only considerations,
the alum dosages could probably be reduced.
The use of pilot studies in the design of filtration units is recommended because of the
numerous variables which govern the efficiency of the filtration processes. Some of the
variables affecting filtration are: media depth, grain size, grain material, rate of filtration,
in-flow solids concentration, characteristics of the suspension, water temperature, head
loss, and backwash requirements.
7-12
-------
FIGURE 7-3
SCHEMATIC OF THE MOVING-BED FILTER (MBF) (12|
INFLUENT
WASTE WASH WATER
DISCHARGE
7-13
-------
Filter Type
Deep-Bed Coarse-Media
Gravity downflow
Gravity downflow
Pressure upflow
Pressure up How
Neptune Microfloc
Mixed-Media
Moving-Bed
Table 7-9
Effluent Polishing Results - Filtration
Feed
Type
T.F.1
Effluent
T.F.1
Effluent
T.F.1
Effluent
A.S.2
Effluent
E.A.3
Effluent
T.F4
Effluent
T.F.5
Effluent
Media
Size
nun.
1 .0-2.0
0.9-1.7
0.9-1.7
0.9-1.7
0.25-2.0
0.6-0.8
0.6-0.8
Filter
Depth
ft.
—
2-3
5
5
2.5
4.2
4.2
Hydraulic
Loading
gpm/sq.ft.
6
3
3
3
5
i
i
SS
Removal
percent
70
67
85.5
77
74
47
67
Effluent BOD
SS Removal
mg/l percent
5-7 55
58
5.0 74
— —
— 88
71
— 80
Effluent
BOD Reference
mg/l
1 3
1 4
2.5 14
1 4
1 C
1 -)
1 _
1 2
'T.F.-Trickling Filter
-A.S. - Activated Sludge
- E.A. - Extended Aeration
100 mg/l alum and 0.2-0.75 mg/l anionic polymer
200 mg/l alum and 0.2-0.75 mg/l anionic polymer
-------
7.5 Activated Carbon Adsorption
The limitations of conventional biological treatment processes in regard to reliable
achievement of a high degree of organic removal (particularly of certain compounds which
are refractory to biodegradation), along with increasingly strict water quality standards,
emphasize the need for a supplementary organic removal process. Thus, activated carbon
is presently being used to provide tertiary treatment of biologically treated effluents.
Moreover, experience gained from the operation of activated carbon plants for tertiary
treatment of wastewater suggests that activated carbon need not be restricted to a polishing
role, but can be used as an alternative to biological treatment. Replacement of conventional
biological treatment by activated carbon (i.e. secondary treatment application) is
emphasized in the EPA process design manual for carbon adsorption. The discussion in
the following pages is concerned exclusively with the tertiary application of carbon.
Activated carbon for wastewater treatment can be used either in the powdered or in the
granular forms. The impracticality of economical regeneration has restricted the use of
powdered carbon in wastewater treatment, although this problem is being resolved.
Consequently, since the use of powdered activated carbon in wastewater treatment is not
widespread, the discussion in the following section is limited to granular activated carbon.
7.5.1 Process Principles and Design Factors
The adsorption of organic materials from wastewater onto the activated carbon involves
complex physical and chemical interactions. Biological degradation of adsorbed materials
also occurs, and this can significantly enhance the overall treatment performance (16)
(17).
The ability of activated carbon to adsorb large quantities of dissolved materials from
wastewater is due to its highly porous structure and to the resulting large surface area,
which provides many sites for adsorption of dissolved materials.
Important factors in the design of activated carbon treatment facilities include:
pretreatment requirements; particle size; hydraulic loading and contact time; regeneration
losses; flow configuration; and required effluent quality.
7.5.1.1 Pretreatment Requirements
Treatment of wastewater by activated carbon requires that the influent total suspended
solids concentration be less than about 50 mg/1. This is essential in order to use the activated
carbon bed as an adsorption medium and to minimize its filtration function. If the solids
loading is much higher than 50 mg/1, a filter may be needed in advance of downflow
carbon beds, or upflow carbon beds may be required for feasible operation.
7-15
-------
7.5.1.2 Particle Size
Theoretically, carbon particle size primarily affects the rate of adsorption and not the
capacity of the carbon. Adsorption rates are greater for smaller particle sizes than for
larger particle sizes. However, adsorbents close to saturation will be less affected by particle
size than adsorbents in their virgin state (16).
Data from Lake Tahoe indicate that there will be a reduction in adsorption capacity of
about 20 to 35 percent in going from 12 x 40 mesh carbon to 8 x 30 mesh carbon
at a relatively short contact time (16). This apparent difference in adsorption capacity
attributable to particle size is minimized at longer contact times (18). Since finer particle
sizes are susceptible to greater head losses, 12 x 40 mesh carbon is probably not suitable
for use in downflow columns (18).
7.5.1.3 Hydraulic Loading Rate and Contact Time
Contact time, hydraulic loadings, and bed depth are interrelated physical parameters. Of
the three, contact time is clearly the most important. Since the activated carbon treatment
of wastewater requires that a definite contact time be established to complete the
adsorption process, any increase in applied hydraulic load necessitates a deeper carbon
column to maintain the same contact time.
Data obtained at the Pomona, California Pilot Plant indicate that Total Organic Carbon
(TOC) removal does not vary significantly after fifteen minutes contact time for hydraulic
loading rates of 4, 7, and 10 gal./min./sq.ft. (16). It was further noted that for equivalent
contact times, the percent TOC removal was similar for hydraulic loading rates of 4, 7
and 10 gal./min./sq.ft. These results indicate that contact time is more important than
applied hydraulic loadings, and is in fact the most important design factor in carbon
adsorption systems.
Typical hydraulic loading rates and contact times used in various locations are shown
in the following table:
Table 7-10
Typical Hydraulic Loading Rates and
Contact Times
Plant Location
Pomona
Lake Tahoe
Nassau County
Hydraulic
Loading Rate
gpm/sq.ft.
7
8
7.5
Contact Time1
minutes
36
18
24
Type of
Treatment
Tertiary
Tertiary
Tertiary
1 Empty Bed
Source: Zanitch and Morand (19)
7-16
-------
It should be noted that both gravity and pumped flow systems are available; gravity flow
systems are not likely to be practical at hydraulic loading rates above about 4 gpm/sq.ft.
7.5.1.4 Effect of Regeneration
Activated carbon requires regeneration when its adsorption capacity is exhausted.
Considerable effort has been expended to determine the effect of regeneration on
adsorption capacity of the carbon. However, since few research groups have regeneration
facilities, only limited data are available. Results obtained at Pomona (16) indicate that
the adsorptive capacity decreases by approximately 35 percent after 7 regeneration cycles,
as indicated in Figure 7-4. It was also determined that regeneration does not affect the
degree of organic removal in subsequent exhaustion cycles. This loss of capacity is not
necessarily a critical factor, since it is necessary to make up physical losses of carbon
after each regeneration cycle. These losses are caused by several factors: carbon is burned
and lost through the stack as combustion products; or is abraded into dust in the course
of handling. Further "losses" are due to the buildup of inorganic ash in the carbon particles
during repeated use and regeneration.
7.5.1.5 Flow Configuration
Depending on the dissolved organic and suspended solids loading, any of several optional
flow configurations can be adopted:
1. Downflow Beds in Series - the lead contactor is removed, regenerated, and
replaced in line at the downstream end, the other contractors being moved up
in sequence.
2. Downflow Beds in Parallel - parallel beds are arranged in a.staggered exhaustion
pattern so that when one is exhausted and removed from service, the product
of the others can be blended with that portion of flow normally treated by
the exhausted contactor to maintain the required product quality for the entire
plant.
3. Upflow Beds (expanded or partially expanded) - no head loss is built up, and
no backwashing is necessary; post-filtration is required; the same series and
parallel considerations apply as for downflow operation.
4. Upflow (moving bed) - exhausted lower strata of the bed are continuously
removed and replaced (at the top of the bed) by virgin carbon.
Downflow beds always require backwashing unless a pre-filtration step is added. Upflow
beds do not require backwashing since no head losses build up; however, post-filtration
is necessary.
7-17
-------
FIGURE 7-4
EFFECT OF REACTIVATION ON ADSORPTION CAPACITY (17)
^ 30 -
3 4 5
NUMBER OF REGENERATIONS
7-18
-------
7.5.1.6 Effluent Quality
In addition to the above design considerations, the question of effluent quality standards
should not be neglected. It should be clear at this point that the carbon adsorption process
can be readily controlled and designed to achieve virtually any desired organic removal
efficiency. It is probably unique among tertiary processes in this respect.
7.5.2 Laboratory and/or Pilot Plant Investigations
Activated carbon removes dissolved materials from wastewaters by a combination of three
mechanisms: adsorption, filtration, and biological degradation. Therefore, in order to judge
the effectiveness of activated carbon for wastewater treatment, both laboratory and pilot
testing are required.
The adsorption mechanism can be evaluated in the laboratory by running adsorption
isotherms. Actual plant conditions should be simulated with regard to temperature, pH,
and pre-treatment. A detailed isotherm procedure is given in many books, as well as in
the above mentioned carbon design manual. Only a brief description will be given here.
Adsorption isotherms are normally conducted by contacting a sample of wastewater with
varying amounts of pulverized carbon for a standard interval of time. The wastewater
sample is analyzed for TOC, COD, or BOD (as deemed necessary), both before and after
contacting with the pulverized activated carbon. The treated water should be coarse-filtered
prior to analysis to eliminate carbon fines. The isotherm is a plot of the amount of solute
adsorbed per unit weight of carbon as a function of residual concentration of solute.
The isotherm is empirically represented by the following expression (14):
x/m = KC */n
where:
x = weight of solute adsorbed
m = weight of carbon
C = equilibrium concentration of solute in solution after adsorption
K and n are constants
The isotherms are normally plotted on a log-log scale. The extrapolation of the isotherm
line to the initial concentration (abscissa) gives the theoretical adsorption capacity of that
carbon when it is in equilibrium with the influent concentration.
The advantage of isotherms are; 1) they are relatively simple tests to perform; 2) they
indicate whether the desired degree of treatment can be readily achieved; and 3) they
give the approximate adsorptive capacity of the carbon in a column application. However,
isotherm results should not be used to extrapolate carbon capacities and dosages to full-scale
plant size.
7-19
-------
Typical isotherms obtained for the same carbon with different secondary wastewater
effluents were reported by Masse (20) (see Figure 7-5). The results shown in Figure 7-5
indicate that the adsorptive capacity of carbon with respect to COD varies from 0.37
to 0.42 Ib. of COD per Ib. of carbon. This is equivalent to 0.22 to 0.25 Ib. of carbon/1,000
gallons of throughput. The carbon requirements (per unit volume of wastewater treated)
obtained from isotherms are conservative (i.e. high), because removal by adsorption alone
is estimated.
Since isotherms cannot measure the quantity of organics removed by filtration and
biological action, pilot column testing must be conducted to evaluate the effect of these
factors. Column testing helps to determine: 1) the required contact time; 2) the adsorptive
capacity of the carbon; 3) the pressure drop across the beds and backwash requirements
for downflow operation; and 4) the shape of the column exhaustion wave front.
The column used in pilot testing should have a diameter of at least 4 inches. The depth
of column depends on the range of contact times being considered, as does the hydraulic
loading. Normally, 2 to 4 columns are used in series, since this arrangement permits
evaluation of the effect of different contact times on effluent quality. When loading
granular carbon into the test column (a "wet" packing procedure is recommended), care
must be exercised to avoid entrapping air within the carbon column. Air entrapment causes
channeling and reduces the contact area, which in turn yields false test results. The process
design manual for carbon adsorption describes the conduct of pilot operations in more
detail.
7.5.3 Uses of Activated Carbon for Upgrading Treatment Plants
Activated carbon has been used successfully to provide tertiary treatment at three locations:
Lake Tahoe, Pomona, and Nassau County. Experience at these places clearly demonstrates
the ability of activated carbon to produce effluents with very low levels of organics. At
Lake Tahoe, the secondary effluent is treated with lime followed by clarification and
mixed-media filtration prior to treatment with the activated carbon; at Pomona, secondary
effluent is treated directly in activated carbon columns; in Nassau County, secondary
effluent is alum-clarified prior to treatment in activated carbon columns. Some operating
results as well as design parameters for these carbon column installations are shown in
Table 7-11.
7.6 Process Designs and Cost Estimates
A cost comparison has been prepared for various effluent polishing processes, and the
results are presented in Table 7-12. The cost information on the MBF was obtained from
a phosphate removal study (12) and, therefore, is higher than would be expected for
an application not including phosphorus removal. The data on microstraining and sand
filters were taken from Smith and McMichael (22) and updated to an ENR index of 1500.
Process designs and cost estimates for tertiary treatment by granular activated carbon have
been examined in an earlier EPA publication, prepared by the M.A. Kellog Co. and
Swindell-Dressier Co., Divisions of Pullman, Inc., (16).
7-20
-------
FIGURE 7-5
COD ISOTHERMS USING VIRGIN CARBON
AND DIFFERENT SECONDARY WASTEWATER EFFLUENTS (20)
1.0
to
0.01
=0-37
\ 100.0
(C) RESIDUAL COO CONG. (MG/L)
-------
Table 7-11
Summary of Operating Results Using Activated Carbon for Tertiary Treatment
to
N)
Operating Data
Capacity
Source of Waste
Secondary Treatment
Pre-treatment
Carbon Type
Column Configuration
Column Dimensions
Nominal Contact Time
Loading Rate
Carbon Column Performance
COD, mg/1
. BOD, mg/1
Color, Pt-Co Units
Carbon Dosage
Pomona
200 gpm
Domestic
Standard Activated Sludge
Chlorination
16 x 40 mesh
4-Stage Downflow
6' dia. x 9'deep
36 minutes
7 gpm/sq.ft.
Influent Effluent
47
10
30 3
350 Ibs/million gallon
Lake Tahoe
1,800 gpm
Domestic
Standard Activated Sludge
Coagulation and Filtration
8 x 30 mesh
2-Upflow in Parallel
12'dia. x 14'deep
13 minutes
8 gpm/sq.ft.
Influent Effluent
20-30 2-10
5-20 2-5
20-50 5
250 Ibs/million gallon
Nassau County
400 gpm
Domestic
High-rate Activated Sludge
Coagulation and Filtration
8 x 30 mesh
4-Stage Downflow
8' dia. x 6' deep
24 minutes
7.5 gpm/sq.ft.
Influent Effluent
5
500 Ibs/million gallon
Source: Zanitch and Morand (19)
-------
Table 7-12
to
OJ
Economic Comparison of Various Effluent Polishing Processes
(ENR Index 1500)
Capital Costs
Thousands of Dollars
Yearly Costs
1
cents/1,000 gallons
Design
Flow
mgd
1.0
Deep2
Aerated
Lagoon
Micro-3
straining
39
Sand3
Filters
120
MBF4
302
Multi-Media3
Filters
80
Deep0
Aerated
Lagoon
—
o
Micro-
straining
4.5
•3
Sand3
Filters
7.6
MBF4
14
4.0
10.0
270
135
290
2.0
3.9
5.7
4.6
3.4
^Yearly costs include: amortization and interest at 4.5 percent for 25 years, operating and maintenance costs.
^Capital cost data obtained from Pierce (21).
3Cost data obtained from Smith and McMichael (22).
^MBF cost data obtained from Bell, et al (12) include operation and maintenance costs.
^Capital cost data obtained from Neptune Microfloc (23).
^Costs estimated by using information furnished in Pierce (21).
-------
7.7 References
1. Loehr, R., and Stephenson, R.. An Oxidation Pond as a Tertiary Treatment Device.
Journal of the Sanitary Engineering Division, ASCE, 91, No. 3, pp. 3M4 (1965).
2. Private Communication with James Neighbor, Vice-President, Hinde Engineering
Company, Highland Park, Illinois, October 28, 1970.
3. Eckenfelder, W.W., Engineering Aspects of Surface Aerator Design. Presented at the
22nd Industrial Waste Conference, Purdue University (May, 1967).
4. Edde, G., Field Research Studies of Hydraulic Mixing Patterns in Mechanically Aerated
Stabilization Basins. Presented at the International Congress in Industrial Wastewater,
Stockholm, Sweden (November, 1970).
5. Fall, E., Retention Pond Improves Activated Sludge Effluent Quality. Journal Water
Pollution Control Federation, 37, No. 9, pp. 1,194 - 1,202 (1965).
6. Reynolds, Jeremiah, Decatur Tertiary Treatment Plan Proves its Worth. Water and
Sewage Works, 115, No. 12, pp. 584 - 553 (1968).
7. Hickman, Paul, Polishing and Secondary Effluents and Treatment Bypasses. Presented
at the 26th Industrial Waste Conference, Purdue University (May 4, 1971).
8. Bodien, D.G., and Stenburg, R.L.,Microstraining Effectively Polishes Activated Sludge
Plant Effluent. Water and Wastes Engineering, 3, No. 9, pp. 74 - 77 (1966).
9. Diaper, E.W.J., Tertiary Treatment by Micros training. Water and Sewage Works, 115,
No. 6, pp. 202 - 207 (1969).
10. Lynam, B., et al, Tertiary Treatment at Metro Chicago by Means of Rapid Sand
Filtration and Microstrainers. Journal Water Pollution Control Federation, 41, No. 9,
pp. 247 - 279 (1969).
11. Truesdale, G., and Birkbeck, A., Tertiary Treatment Process for Sewage Works
Effluents. Journal of the Institute of Water Pollution Control, April, 1967.
12. Bell, G.R., et al, Phosphorus Removal Using Chemical Coagulation and a Continuous
Countercurrent Filtration Process. Federal Water Quality Administration,
Program No. 17919 EDO, June, 1970.
13. Private Communication with Peter Kaye, Municipal Sales Manager, Dravo Corporation,
Pittsburgh, Pennsylvania, June 2, 1971.
7-24
-------
14. Convery, J.J., Solids Removal Processes. Nutrient Removal and Advanced Waste
Treatment Symposium, Presented by Federal Water Pollution Control Administration,
Cincinnati, Ohio, April 29-30, 1969.
15. Gulp, G.L., and Hanse, S., Extended Aeration Effluent Polishing by Mixed-Media
Filtration. Water and Sewage Works, 114, No. 2, pp. 46-51 (1967).
16. Appraisal of Granular Carbon Contacting, Report Nos. TWRC 11 and 12, Federal
Water Pollution Control Administration, Ohio Basin Region, Cincinnati, Ohio.
17. Parkhurst, J.D., Dryden, F.D., McDermott, G.N. and English, J., Pomona Activated
Carbon Pilot Plant, Journal Water Pollution Control Federation. 39, No. 10, Part 2,
pp. R70-R81 (1967).
18. Gulp, R.L. and Gulp, G.L., Advanced Wastewater Treatment. New York: Van
Nostrand-Reinhold Company, 1971.
19. Zanitch, R.H., and Morand, J.H., Tertiary Treatment of a Combined Wastewater with
Granular Activated Carbon. Presented at 3rd Mid Atlantic Industrial Waste
Conference, University of Maryland, (Nov., 1969).
20. Masse, A.N., Organic Residue Removal. Nutrient Removal and Advanced Waste
Treatment Symposium, Presented by Federal Water Pollution Control Administration,
Cincinnati, Ohio, April 29-30, 1969.
21. Pierce, J., Aerated Lagoons Treat Secondary Effluent. Water and Sewage Works, 117,
No. 5 (1970).
22. Smith, R., and McMichael, W., Cost and Performance Estimates for Tertiary
Wastewater Treating Processes. Federal Water Pollution Control Administration,
Cincinnati, Ohio, June, 1969.
23. Private Communication with John Atherholt, A. B. and G. Associates, Narberth,
Pennsylvania, Manufacturer's Representative for Neptune Microfloc, Inc., June, 1971.
7-25
-------
CHAPTER 8
PRE-AERATION AND POST-AERATION PRACTICES
8.1 Pre-Aeration
Pre-aeration of wastewater has been practiced for over 50 years throughout the United
States, mainly for the purpose of odor control and/or the prevention of septicity. Initially,
short aeration periods ranging up to 15 minutes were used. As aeration periods were
lenghtened, the additional benefits of grease separation and flocculation of solids became
evident (1).
Efforts have been made by Roe (1), and by Seidel and Baumann (2) to study the effects
of pre-aeration on primary clarifier performance. In 1951, Roe tabulated operational data
from 38 plants using pre-aeration. These results are presented in Figure 8-1 (1). The data
in Figure 8-1 illustrate the effect of pre-aeration time on subsequent suspended solids
(SS) removal in the primary clarifier. The quantities of air used varied between 0.06 and
0.15 cu. ft ./gallon. Roe further found that in order to maintain proper agitation, plant
operators varied the air supply between 1.0 and 4.0 cu.ft./lineal foot of tank, depending
on the physical tank layout and type of aeration equipment used. BOD removal could
not be correlated with the observed solids removal.
In 1961, Seidel and Baumann conducted a comparative study at the Ames, Iowa, secondary
treatment plant to determine the effect of pre-aeration on primary clarifier performance.
They determined that with 45 minutes detention and an aeration rate of 0.1 cu.ft.
air/gallon, BOD and SS removals were increased by 7 to 8 percent in the primary tank.
The 7 to 8 percent increased removals in the primary tank may or may not be realized
on overall plant performance, e.g., part of the incremental solids removed as a result of
pre-aeration might have been removed in the secondary clarifier without using pre-aeration.
Seidel and Baumann also evaluated pre-aeration economics based on conventional design
standards and found that the cost of pre-aeration increases the total annual operating
cost by 2 to 3 percent. However, they felt a slight improvement in clarifier efficiency
would narrow or eliminate the cost differential.
The Ten-States Standards recommend detention times of 30 minutes for effective solids
flocculation and at least 45 minutes for appreciable BOD reduction with pre-aeration (3).
In addition, it is stated that the use of polyelectrolytes may substantially reduce these
detention times.
Although the use of aerated grit chambers is becoming increasingly popular as a
pretreatment unit in wastewater treatment plants, their use should not be expected to
substantially increase the BOD or SS removal in the primary clarifier.
8-1
-------
FIGURE 8-1
EFFECT OF PREAERATION TIME ON SOLIDS REMOVAL (1)
60 MIN
50 MIN
40 MIN
30 MIN
20 MIN
15 MIN
DATA BASED ON 2 HOUR
DETENTION TIME IN
SEDIMENTATION BASIN
100 200 300 400
SUSPENDED SOLIDS IN RAW SEWAGE - MG/L
500
8-2
-------
8.1.1 Applicability to Plant Upgrading
Due to the limited amount of SS and BOD removal achieved as a result of pre-aeration,
its use for the upgrading of solids and organic removal is limited. However, the use of
pre-aeration for preventing septicity of raw wastewater and grease removal should be
considered in the overall evaluation of pre-aeration as a unit process.
8.1.2 Process Design and Cost Estimates
Capital cost estimates for pre-aeration facilities for three plants with capacities of 1, 3,
and 5 mgd are shown in Table 8-1:
Table 8-1
Capital Costs for Pre-Aeration Facilities1
(ENR 1500)
Capital Costs for
Plant Size Pre-Aeration Facilities
m§d In Thousand Dollars
1 $ 75
3 148
5 215
Does not include land costs, contingencies, engineering design or
bonding.
The pre-aeration facilities were based on a detention time of 45 minutes and an air supply
of 0.1 cu.ft./gallon. The basin was of reinforced concrete construction and the air
requirement was supplied by floating mechanical aerators.
8.2 Post-Aeration
Many states are considering or have already enacted legislation requiring the maintenance
of minimum dissolved oxygen concentrations in wastewater treatment plant effluents. This
is required since most water quality standards specify a minimum dissolved oxygen (D.O.)
concentration of 4.0 mg/1, while most secondary plants usually discharge effluents ranging
between 0.5 and 2.0 mg/1.
There are at least four methods available for the post-aeration of a wastewater treatment
plant's effluent. These are shown in Figure 8-2. Most of these devices were initially
developed for water treatment and are now being used in the wastewater treatment field.
8.2.1 Diffused Aeration
Diffusion aerators are usually placed in concrete tanks which are commonly 9 to 15 feet
deep and 10 to 30 feet wide. Ratios of width to depth should not exceed two, if effective
8-3
-------
FIGURE 8-2
VARIOUS POST AERATION DEVICES
OXYGEN SOURCE OR
AIR COMPRESSOR
1
o
K. DIFFUSED AERATION
of?
B-2 PUMP TYPE AERATOR
HEAD LOSS
C. CASCADE AERATOR
uu uu
B-l TURBINE TYPE AERATOR
TURBINE
LTI
AIR LINE -
SPARGER
B-3 AGITATOR SPARGED SYSTEM
VENTURI ASPIRATOR
0. U-TUBE AERATOR
8-4
-------
mixing is to be obtained. Tank length is governed by the desired detention period, which
usually varies from 10 to 30 minutes. The maximum air required is typically
0.15 cu.ft./gallon.
The use of oxygen aeration in the activated sludge process may eliminate the need for
post-aeration. Oxygen-aerated mixed liquor discharged to the secondary clarifier usually
has a D.O. of at least 6.0 mg/1 (4).
8.2.2 Mechanical Aeration
Mechanical aerators are generally grouped in two broad categories: turbine types and pump
types, as shown in Figure 8-2. In all types, oxygen transfer occurs through a vortexing
action and/or from the interfacial exposure of large volumes of liquid sprayed over the
surface.
To avoid interference between units, aerator manufacturers recommend a minimum basin
size of 15 to 50 feet square and a minimum depth of 5 to 8 feet, depending on the
horsepower of the aerator.
One aerator design equation proposed by Kormanik for a post-aeration basin is (5):
p =
= 0.347 Q (CC0 + Rrt)
N
Ir
'-
-
SW
0
T-20
where:
P = Horsepower required
Q = Wastewater effluent flow, mgd
T = Design temperature of effluent, °C
No = O2 transfer efficiency under standard conditions in tap water,
Ib. O2/hp-hr.
Fg = Correction factor related to a change in basin geometry
C = Required final D.O. level after post-aeration, mg/1
Co = D.O. concentration of the incoming wastewater effluent, mg/1
Csw = O2 saturation concentration of effluent, mg/1, where Csw = Cs x
C20 = D.O. saturation of tap water at 20°C, mg/1
Cs = D.O. saturation of tap water at temperature T, mg/1
Rr = O2 utilization rate, mg/l/min.
a = O2 transfer coefficient of the effluent (alpha factor)
t = Detention time in minutes
T? = Aerator efficiency correction
0 = Oxygen saturation coefficient in wastewater (beta factor)
6 = Temperature coefficient varying between 1 .02 and 1 .024
8-5
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8.2.3 Cascade Aeration
Cascade aeration takes advantage of the effluent discharge to create a series of steps or
weirs over which the flow moves. The objective is the maximization of turbulence to
increase oxygen transfer. Head requirements vary from 3 to 10 ft., depending upon the
initial D.O. and the desired increase. If the necessary head is not available, effluent pumping
is required.
In England, the Water Research Laboratory has performed investigations to qualify as
much as possible the layout of cascade aeration schemes. Barrett and others proposed
the following formulae (6):
r = Cs- Ca/Cs- Cb
r = 1 + 0.11 ab (1 + 0.046T) h
where:
r r= The deficit ratio
Cs = Oxygen saturation value corresponding to temperature T, mg/1
Ca = Oxygen concentration above the weir, mg/1
Cb = Oxygen concentration below the weir, mg/1
a = Water quality parameter equal to 0.8 for a waste water treatment
plant effluent
b = Weir geometry parameter equal to unity for a free weir and 1.3 for
the step weirs used in their experimental work
T = Water temperature in °C
h = Height in feet through which water falls
For example, to raise the D.O. concentration of a wastewater treatment plant effluent
from 0.5 mg/1 to 4.5 mg/1 at 20°C, the overall height requirement for a series of step
weirs would be approximately 4.0 feet. However, it should be pointed out that the values
of the parameters a and b are somewhat arbitrary and need further refinement to
substantiate preliminary results.
8.2.4 U-Tube Aeration
The U-tube aerator consists of two basic components: a conduit to provide a vertical
U-shaped flow path and a device for entraining air into the stream flow in the down
leg of the conduit as indicated in Figure 8-2. The entrainment device is one of two types:
1) aspirator; or 2) compressor and diffuser. In either case, the entrained air is carried
along the down leg of the tube because the water velocity exceeds the buoyant rising
velocity of the air bubbles.
Various design considerations include air-to-water ratio, tube cross-sectional area, and depth.
The maximum air-to-water ratio practicable is a function of the velocity through the system.
At velocities of approximately 4 fps, 20 percent air-to-water injection is about the limit
8-6
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for satisfactory operation (7). The hydraulic head requirements for plants of 5 mgd or
less should be less than 5 feet. If sufficient head is not available, the flow may be pumped
through the U-tube.
Speece and Orosco (7) have suggested that one economic method of construction for
deep U-tubes, greater than 20 feet in depth, would be a circular hole bored into the
soil. The hole would be cased and a smaller pipe then suspended a few feet from the
bottom of the hole as shown in Figure 8-2. The diameter of the smaller pipe is selected
so that its cross-sectional area is approximately equal to the cross-sectional area of the
annular space between the two pipes. Thus, the velocity of the water will be approximately
equal in both legs of the U-tube.
Presently there are no known U-tube installations in wastewater treatment plants for
post-aeration. However, their applicability is presently being investigated by EPA as a
possible pre-aeration device in sewer lines. The possibilities of using a U-tube as a
post-aeration device seem good at this time. The additional benefits of no moving parts
and little or no associated labor and maintenance make the U-tube device extremely
attractive.
Of all the types of post-aeration methods, it is likely that mechanical aeration and U-tube
aeration will find extensive application in the future.
8.2.5 Process Designs and Cost Estimates
Mitchell and Lev (8) have prepared a cost comparison between mechanical, diffused, and
U-tube aeration for post-aeration of a 10-mgd treatment plant effluent. This cost
comparison is presented in Figure 8-3. The costs are expressed as amortized capital cost
per pound of oxygen dissolved. The amortization was based on 4.5 percent for 25 years.
As shown in Figure 8-3, the gravity U-tube aeration device is the most economical for
the particular conditions investigated. However, it must be cautioned that these amortized
capital costs represent only 20 to 50 percent of the total yearly cost of operating these
devices. Maintenance, operation, and power charges may be substantial. These costs were
not included since there is a scarcity of reliable operating costs for aeration systems.
However, when U-tube aeration can be operated under conditions where head is available,
it is likely to be the cheapest of all devices currently on the market. This is due to the
fact that U-tube aeration devices have low maintenance and power requirements.
8-7
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FIGURE 8-3
CAPITAL COST COMPARISONS FOR POST AERATION
OF SECONDARY TREATED EFFLUENT(S)
ENR=1500
OPERATING CONDITIONS
PRESSURES.5 PSIA
TEMPERATURE-VS^F
SAT. D.0.=8.33 MG/L
INITIAL D.O.-l.O MG/L
FLOW 10 MGD
U-TUBE
AERATION
PUMPED)
MECHANICAL
AERATION
DIFFUSED
AERATION
U-TUBE
AERATION
GRAVITY
0246
FINAL DISSOLVED OXYGEN CONCENTRATION, MG/L
8-8
-------
8.3 References
1. Roe, F., Pre-aeration and Air Flocculation. Journal Water Pollution Control
Federation, 23, No. 2, pp. 127-140 (1951).
2. Seidel, H., and Baumann, E., Effect of Pre-aeration on the Primary Treatment of
Sewage. Journal Water Pollution Control Federation, 33, No. 4, pp. 339-355 (1961).
3. Recommended Standards for Sewage Works. Great Lakes - Upper Mississippi River
Board of State Sanitary Engineers, 1968.
4. Albertsson, J., et al, Investigation of the Use of High Purity Oxygen Aeration in
the Conventional Activated Sludge Process. Federal Water Quality Administration,
Program Number 17050 DNW, May, 1970.
5. Kormanik, R., Simplified Mathematical Procedure for Designing Post Aeration
Systems. Journal Water Pollution Control Federation, 41, No. 11, pp. 1956-1958
(1969).
6. Barrett, M.J., et al, Aeration Studies of Four Weir Systems. Water and Water
Engineering, 64, No. 9, pp. 407-413 (1960).
7. Speece, R., and Orosco, R., Design of U-tube Aeration Systems. Journal of the
Sanitary Engineering Division, ASCE, 96, No. 3, pp. 715-726 (1970).
8. Mitchell, R.C., and Lev, A.D., The U-tube for Water Aeration. Federal Water Pollution
Control Administration, Contract No. 14-12-434, March, 1970.
8-9
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CHAPTER 9
DISINFECTION AND ODOR CONTROL
9.1 General
Disinfection and odor control are two areas which are receiving increased attention from
regulatory agencies through the establishment and enforcement of rigid bacteriological
effluent standards and air pollution standards.
9.2 Disinfection
In 1968, 41 percent of all municipal wastewater plants in the United States were using
chlorination for disinfection purposes (1). Other disinfectants, ozone, for example, are
currently being studied by EPA to evaluate their potential use in disinfecting wastewater
treatment plant effluents.
The following table was taken from the Water Pollution Control Federation's Sewage
Treatment Plant Design Manual and contains ranges of chlorine dosages recommended for
disinfection (2):
Table 9-1
Chlorine Dosage Ranges
Waste^ Chlorine Dosage
mgTT
Raw Sewage 6 to 12
Raw Sewage (septic) 12 to 25
Settled Sewage 5 to 10
Settled Sewage (septic) 12 to 40
Chemical Precipitation Effluent 3 to 10
Trickling-Filter Effluent 3 to 10
Activated Sludge Effluent 2 to 8
Sand Filter Effluent 1 to 5
To be effective for disinfection purposes, a chlorine residual of 0.2 to 1.0 mg/1 is
recommended, with a contact time of not less than 15 minutes at peak flow rates (2).
It is possible to maximize the efficiency of an existing chlorine contact tank by improving
the flow pattern through the tank. This could be accomplished by using baffles and by
locating the chlorine addition point where complete mixing with the wastewater is assured.
Beyond these modifications, it may not be possible to substantially increase the
performance of chlorine contact tanks without increasing tank capacity to provide adequate
detention time.
9-1
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At the present time, ozone is being used for disinfection purposes in the water treatment
field. Preliminary information indicates that ozone disinfection of potable water requires
only 5 minutes of contact time to accomplish the same degree of disinfection as 15-minute
contact with chlorine (3). In the future, it may be that ozone could be used effectively
for wastewater disinfection purposes.
9.3 Odor Control
Wastewater treatment plants serving large municipalities are generally characterized by
extensive collection systems with correspondingly high detention times. For example, the
Washington, D.C. Pollution Control Plant serves areas as far as 25 to 30 miles away. This
type of situation often leads to odor problems during summer periods. Odor problems
are characteristically most critical during the plant's low flow periods (approximately
9 p.m. to 4 a.m.), due to increases in the sewer detention time.
9.3.1 Odor Generation and Characteristics
Odors from wastewater treatment plants can usually be attributed to three sources: septic
raw wastewater, overloaded secondary treatment facilities, and sludge treatment practices.
Septicity in wastewaters is caused by the depletion of dissolved oxygen due to long
residence in sewers and the subsequent increase in anaerobic activity. As wastewater
becomes anaerobic, facultative and anaerobic bacteria flourish. These bacteria utilize
nitrates and sulfates present in wastewater as their oxygen source. The reduction of sulfate
ions produces the highly odorous gas, hydrogen sulfide. Other odorous gases which may
be present are indole, skatole, mercaptans, disulfides, volatile fatty acids, and ammonia.
Increased summer temperature and extended sewer detention times can result in the rapid
build-up of hydrogen sulfide and carbon dioxide according to the following reactions (4):
anaerobic
S04= + organic bac*gria > S= + H2O + CO2
At a pH level below 8, the equilibrium shifts toward the formation of non-ionized H2S
and is about 80 percent complete at pH 7. At pH 8 and above, most of the reduced
sulfur exists in solution as HS" and S= ions (4). H2S is noticeable even in the cold when
present in water to the extent of 0.5 mg/1. When present to the extent of 1.0 mg/1,
it becomes very offensive (5).
Overloaded secondary treatment facilities are also a potential source of odor. If the air
supply to an activated sludge aeration tank is inadequate, odorous conditions usually
develop. It is also possible that a properly sized air supply system can strip odorous gases
from septic wastewater.
9-2
-------
Odors associated with sludge treatment occur in thickening, digestion, and sludge
dewatering facilities. Thickeners may receive both septic primary and secondary sludges.
Gases from well-operated digesters may contain small quantities of H2S, which are usually
destroyed by normal flaring of digester gas. The predominant odor in digested sludge is
ammonia, although traces of volatile organic acids may be present.
9.3.2 Odor Measurement
Odor data are generally qualitative rather than quantitative in nature. The two available
quantitative methods are the H2S determination and the Threshold Odor Number (TON).
The latter method is only semi-quantitative in that determination of the TON is dependent
on the olefactory senses of the individual performing the analysis. This can be
de-personalized somewhat by using a panel to determine the TON value for a sample.
9.3.3 Odor Control Methods Available
The various methods available for control of odors emanating from a wastewater treatment
plant are:
1. Changes in the operational procedures and new techniques.
2. Chemical treatment or pre-treatment, which might include chlorine, ozone, lime,
or powdered carbon.
3. Collection and treatment of noxious gases.
9.3.3.1 Changes in Operational Procedures and New Techniques
Odors associated with septic wastewater are generally not amenable to solution through
operational changes within the treatment plant itself. The applicability of in-sewer aeration
methods for reduction of odors and hydrogen sulfide corrosion is currently being
investigated by EPA. Among the procedures being evaluated are U-tube installations and
pure oxygen injection into force mains.
Many sludge odors in a plant are a direct result of an improperly operated or overloaded
anaerobic sludge digester. Improved temperature control and better mixing of digester
contents may alleviate the odor problem
9.3.3.2 Chemical Treatment
Chlorination is probably the most widely used of the chemical treatment processes available
to control odors because it is effective and most treatment operators have had experience
in handling chlorine. The presence of chlorination facilities at the plant for disinfection
is a further reason for its utilization.
9-3
-------
Chlorination is used for two purposes: to retard biological action which produces odors,
and to react chemically with odorous sulfur compounds, oxidizing them to innocuous
sulfur forms, usually free colloidal sulfur.
Table 9-2 contains a summary of odor reduction data for a chlorinated raw domestic
waste water. A pre-chlorination dosage of 10 mg/1 at maximum flow was recommended
for odor control. This dosage is substantiated by the data in Table 9-1 (6). In addition,
another incremental 5 mg/1 of chlorine capacity was recommended as an adequate margin
of safety for peaks in sulfide levels or chlorine demand.
Table 9-2
Effect of Chlorine on Odor Reduction For A
Raw Domestic Wastewater (6)
Detention Time in Minutes
Chlorine*
Dosage
mg/1
0
5
10
25
50
*pH = 7
Temperature = 85°F
Ozone has been added to wastewaters for odor control with some favorable results. Because
of the extremely high reactivity of ozone, a much higher ozone demand is generally
exhibited by a wastewater than would be exhibited for chlorine. However, the use of
oxygen aeration in secondary treatment may have an added benefit, since the exhaust
gas could provide the ozone generator with an economic source of oxygen. Due to the
high cost of ozone generation, the use of ozone for odor control may be limited (6).
Lime and powdered carbon have also been used in various applications for odor control.
The addition of lime to septic wastewater raises the pH. Since the solubility of F^S
increases with increasing pH, less t^S evolves, thereby decreasing the odor level. Powdered
activated carbon adsorbs odor-causing materials and, thereby, decreases the odor level.
The results of a laboratory odor study are presented in Table 9-3 (7). Concentrations
of less than 10 mg/1 of powdered activated carbon were successful in providing significant
odor reduction.
1
Sulfide
mg/1
1.7
0.5
0.2
0.2
0.2
120
Sulfide
mg/1
1.7
0.7
0.2
0.2
0.2
9-4
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Table 9-3
Effect of Powdered Activated Carbon on Odor Reduction
Raw Domestic Activated Carbon Cone, of
Plant Waste water 1 Treated Effluent Activated Carbon
TON TON mg/1
Charlottesville, Va. 300 100 3.8
Hershey Estates, Pa. 140 100 5.0
Butler, Pa. 280 200 10.0
Sample temperature = 60°C
9.3.3.3 Collection and Treatment of Noxious Gases
The covering of odorous unit process facilities to localize odors is a method which can
be used to prevent odors from reaching the atmosphere. The major expenses of this method
are the covering of the units and collection and treatment of the evacuated gases. In
cold climates, covering units can lead to conditions of high humidity and indoor fog if
proper ventilation is not provided. Many municipal plants, e.g., Cedar Rapids, Iowa (8),
and Elmira, New York (8), are using low-cost, formed-in-place styrofoam domes on odorous
treatment units.
The treatment methods usually considered for evacuated gases include simple or catalytic
combustion, ozonation, and chemical oxidation.
Combustion methods require heating the gases to approximately 800°F to 900°F for
catalytic combustion and approximately 1,300°F to 1,400°F for simple combustion.
Operating costs for these methods are primarily determined by the amount of air to be
heated. Ozonation, while somewhat affected by the volume of gas collected, is primarily
affected by the quantity of odorous materials to be controlled.
9.3.4 Effects on Subsequent Units
A consideration in using chlorine for odor control is that the chlorine dosage should not
produce a high residual chlorine level which may in turn be detrimental to secondary
biological units. When using lime for odor control, consideration must also be given to
increased sludge production.
9.3.5 Process Designs and Cost Estimates
A cost estimate has been prepared for two odor-control systems (chlorination and powdered
activated carbon) for 1, 3, and 5 mgd treatment plants. The capital costs are presented
in Table 9-4. The chemicals are added to raw wastewater before the downstream treatment
units.
9-5
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Table 9-4
Capital Costs for Odor Control Systems
ENR Index 1500
Plant Capital Costs for Odor Control Systems
Size Chlorination Powdered Activated Carbon
mgd 10mg/l 10mg/l
1 $41,0002 $33,000
3 43.0002 46,000
5 45,0002 64,000
*These costs do not include a contingency for engineering design, bonding, and
construction supervision.
^Smallest size commercially available chlorinator.
The chlorination system included a gas chlorinator capable of delivering 10 mg/1 of chlorine
during peak flow rates. A building, scale, and other necessary appurtenances were included.
The powdered activated carbon system included a 15-day storage hopper and a volumetric
feeder capable of delivering 10 mg/1 at peak flow rates. In addition, a 1-day capacity
slurry tank, pump, building, and associated piping were included.
9.4 Other Uses of Chlorine
In the operation of wastewater treatment plants, chlorine has been found useful as an
upgrading technique. Some of the various applications of chlorine are as follows (2) (9)
(10):
1. Destruction or control of undesirable growths of algae and slime-forming bacteria
in pipelines and conduits.
2. Control of filter flies, clogging, and ponding in trickling filters. Chlorine applied
for approximately 8 hours to produce a residual of 1 to 2 mg/1 in the
distributor arm will generally unclog the filter. Residuals of 20 to 50 mg/1 will
eliminate ponding by causing the filter to unload all of its biological slime.
3. Improvement in wastewater coagulation.
4. Improvement in the separation of grease from wastewaters.
5. Reduction of the immediate oxygen requirements of return activated sludge and
digester supernatant return.
6. Chlorination of return activated sludge may be effective in the control of sludge
bulking. A chlorine dose of 1 to 10 mg/1 based on the volume of return sludge
9-6
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has been used, provided the chlorine application point is located to allow for
2 to 3 minutes of mixing before being discharged to the aeration basin.
9.5 References
1. Statistical Summary 1968 Inventory Municipal Waste Facilities, in the United States.
Federal Water Quality Administration: Government Printing Office, 1971.
2. Sewage Treatment Plant Design. Water Pollution Control Federation Manual of
Practice No. 8, Washington, D.C., 1959.
3. O2 and 0$ - Rx for Pollution. Chemical Engineering, 77, No. 2, pp. 46-48 (1970).
4. Sawyer, C., Chemistry for Sanitary Engineers. New York: McGraw-Hill Book
Company, 1960.
5. Nordell, E., Water Treatment for Industrial and Other Uses. New York: Reinhold
Publishing Corporation, 1961.
6. Roy F. Weston, Inc., Engineer's Preliminary Report Odor Control Studies Washington,
D.C. Water Pollution Control Plant. December, 1967.
7. Aqua Nuchar for Odor Control in Waste Treatment. Covington, Virginia: Westvaco
Corporation.
8. Dow Domes-Environmental Enclosures. Midland, Michigan: Dow Chemical Company,
1968.
9. Fair, G., and Geyer, J., Water Supply and Waste-Water Disposal. New York: John
Wiley and Sons, Inc., 1954.
10. Operation of Wastewater Treatment Plants. Water Pollution Control Federation Manual
of Practice No. 1, Washington, D.C., 1970.
9-7
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CHAPTER 10
SLUDGE THICKENING
10.1 Air Flotation
The use of air flotation for upgrading is limited primarily to thickening of sludges prior
to dewatering. Used in this way, the efficiency and/or capacity of the subsequent
dewatering units can be increased and the volume of supernatant from the following
digestion units can be decreased. Existing air flotation thickening units can be upgraded
by the optimization of process variables, and by the utilization of polyelectrolytes.
Air flotation thickening is best applied to thickening waste activated sludge. With this
process, it is possible to thicken the sludge to 6 percent, while the maximum concentration
attainable by gravity thickening without chemical addition is 2-3 percent (1). The air
flotation process can also be applied to mixtures of primary and waste activated sludge.
The greater the ratio of primary sludge to waste activated sludge, the higher the permissible
solids loading to the flotation unit. Due to the high operating costs, it is generally
recommended that air flotation be considered only for thickening waste activated sludge
(2).
10.1.1 Process and Design Considerations
The most commonly used type of air flotation unit is the dissolved air pressure flotation
unit. A schematic flow diagram for a typical unit is illustrated in Figure 10-1. In this
unit, the recycled flow is pressurized from 40 to 70 psig and then saturated with air
in the pressure tank. The pressurized effluent is then mixed with the influent sludge and
subsequently released into the flotation tank. The excess dissolved air then separates from
solution, which is now under atmospheric pressure, and the minute (average diameter 80
microns) rising gas bubbles attach themselves to particles which form the sludge blanket
(3). The thickened blanket is skimmed off and pumped to the downstream sludge handling
facilities while the subnatant is returned to the plant.
The following table is a summary of typical parameters used in the design of air flotation
thickening units:
Table 10-1
Air Flotation Parameters
Parameter Typical Value
Air pressure, psig 40-70
Effluent Recycle Ratio, percent of influent flow 30-150
Detention time, hours 3
Air to solids ratio, Ibs air/lb solids 0.02
Solids loading, Ibs/sq.ft./day 10-50
Polymer Addition, Ibs/ton dry solids 10
10-1
-------
FIGURE 10-1
SCHEMATIC OF AN AIR FLOTATION UNIT
SKIMMER MECHANISM
PRESSURE TANK
EFFLUENT
RECYCLED
RISE ZONE
\
i
FLOUTED SLUDGE
EFFLUENT
AIR
RECYCLED
EFFLUENT
INFLUENT SLUDGE
-------
In addition to the above parameters, the feed solids concentrations and the type and
quality of the sludge affect the performance of the air flotation unit. A detailed discussion
of all the previous parameters can be found in the following references (2) (3) (4) (5).
Bench-scale flotation units have been utilized for air flotation designs, but poor correlations
have generally been obtained with full-scale performance (3) (5). Therefore, pilot units
usually are recommended to determine optimum recycle rates, chemical requirements, and
general applicability of air flotation to sludge thickening.
Typical operating data for various air flotation units is presented in Table 10-2. Combined
primary and activated sludge produces a more concentrated float sludge than waste
activated sludge alone. Polymer and/or chemical addition allows greater solids loading and
improves solids recovery without substantially increasing the float solids concentration.
10.1.2 Use of Air Flotation for Upgrading Existing Sludge Handling Facilities
As an upgrading technique, air flotation is best applied to thickening waste activated sludge.
The process produces a sludge concentration of 4 to 6 percent, thus decreasing the volume
of sludge to be handled in subsequent solids handling units. The capacity and efficiency
of an air flotation process can normally be improved with polymer addition of less than
10 pounds/ton of dry solids.
10.1.3 Process Designs and Cost Estimates
The following example illustrates the use of prethickening by air flotation prior to anaerobic
digestion.
Existing anaerobic digesters were experiencing unstable operation due to the increased
volume of sludge produced by an increase in plant flow from 10 to 16 mgd. Operational
data from the overloaded and the, upgraded plant are presented in Table 10-3. Prior to
upgrading, the waste activated sludge was recycled to the primary clarifier. The volume
of the combined sludge was 100,000 gpd at 3 percent solids. The increased sludge volume
due to the plant overloading decreased the detention time in the digesters from 17 to
11.25 days. To improve the operation of the existing digesters, it was necessary to reduce
the sludge volume to increase the digester detention time. To reduce sludge volume,
thickening of the waste activated sludge by air flotation was considered.
10-3
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Table 10-2
Air Flotation Performance
Description
Activated Sludge
Activated Sludge1
Activated Sludge
Activated Sludge
Activated Sludge
Activated Sludge
Activated Sludge
Combined (Primary and
Activated) Sludge
Combined (Primary and
Activated) Sludge
Combined (Primary and
Activated) Sludge
Solids
Loading
Ibs/sq.ft./day
12 to 18
24 to 48
13.9
7.1
19.8
26.2
28.8
24 to 30
21
46.6
40.7
Influent
Suspended
Solids
percent
0.5 to 1.5
0.5 to 1.5
0.81
0.77
0.45
0.80
0.46
1.5 to 3.0
0.64
2.30
1.77
Float
Solids
percent
4.0 to 6.0
4.0 to 5.0
4.9
3.7
4.6
6.5
4.0
6.0 to 8.0
8.6
7.1
5.3
Solids
Recovery
percent
85 to 95
95 to 99
85
99
83
93
88
85 to 95
91
94
88
Reference
6
7
8
8
8
8
8
6
8
8
8
3 to 6 Ibs polyelectrolyte/ton.
-------
Table 10-3
Example of
Upgrading Sludge Handling Facilities
Using Air Flotation
Description Overloaded Plant Upgraded Plant
Total Primary Solids Produced 15,300 Ibs 15,300 Ibs
Total Primary Solids — 36,800 gpd (5%)
Total Waste Activated Sludge Produced 9,800 Ibs 9,800
Total Waste Activated Sludge Volume — 29,400 gpd (4%)
Total Combined Solids Produced 25,100 Ibs 25,100 Ibs
Total Combined Solids Volume 100,000 gpd (3%) 66,200 gpd (4.5%)
Digester Hydraulic Detention Time 11.25 days 17 days
The flow diagram for the upgraded plant is shown in Figure 10-2. As a result of separate
thickening of the waste activated sludge, it is expected that the primary sludge can be
concentrated to 5 percent in the primary clarifiers. The total volume of sludge discharged
to the anaerobic digestion facility due to the separate thickening of the waste activated
sludge and the improved solids concentrations in the primary clarifier is expected to be
66,220 gpd, compared to 100,000 gpd prior to upgrading. The air flotation system was
designed using an air pressure of 50 psig, an effluent recycle of 100 percent, and a solids
loading of 25 Ibs/day/sq.ft. It is anticipated that the polymer dosage requirements will
be 5 Ibs/ton of dry solids.
The capital costs for air flotation thickening in this example are estimated at $156,000
(ENR 1500). These costs include two air flotation units, a polymer addition system, and
appropriate connecting piping. They do not include an allowance for engineering design,
bonding, and construction supervision.
10.2 Gravity Thickening
Gravity thickening is the most common process in use today for the concentration of
sludge prior to digestion and/or dewatering. Thickeners can contribute to the upgrading
of sludge handling facilities as follows:
1. Increase the capacity of overloaded digesters or subsequent sludge handling units.
2. Reduce the size and increase the efficiency of sludge digestion or dewatering
units.
3. Improve primary clarifier performance by providing continuous withdrawal of
sludge, thereby insuring maximum removal of solids.
10-5
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FIGURE 10-2
UPGRADING SLUDGE HANDLING FACILITIES USING AIR FLOTATION
RAW
WASTEWATER
PRIMARY
CLARIFICATION
PRIMARY
SLUDGE
FINAL
CLARIFICATION
RETURN SLUDGE
AUXILIARY RECYCLE
J>IR
FLOTATION EFFLUENT
AIR FLOTATION
UNIT
FINAL
EFFLUENT
WASTE ACTIVATED
m—'SLUDGE
POLYMER
DIGESTED SLUDGE
TO DEWATERING
10-6
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The process is simple and is the least expensive of the available thickening processes. The
reduction in size and improvement in efficiency of subsequent sludge handling processes
often can offset the cost of gravity thickening. The process also allows equalization and
blending of sludges, thereby improving the uniformity of feed solids to the following
processes. Existing gravity thickeners can be upgraded by providing continuous feed and
drawoff, by diluting the feed solids, and by chemical addition.
10.2.1 Process Considerations
Gravity thickening is characterized by zone settling. The four basic settling zones in a
thickener are:
1. The clarification zone at the top containing the relatively clear supernatant.
2. The hindered settling zone where the suspension moves downward at a constant
rate and a layer of settled solids begins building from the bottom of the zone.
3. The transition zone characterized by a decreasing solids settling rate.
4. The compression zone where consolidation of sludge results solely from liquid
being forced upward around the solids.
To date, many attempts have been made to simulate zone settling in a batch settling
test to generate design information which would be applicable to a continuous unit. Various
theories have been developed for analyzing batch settling data and they have been reviewed
and discussed in the literature (9) (10) (11) (12). Most of the theories assume that the
settling velocity of sludge at a given concentration in a small batch cylinder is similar
to the velocity in prototype thickening units. However, it has also been recognized that
other parameters are involved, such as cylinder depth, cylinder diameter, mixing conditions,
and sludge characteristics. All have a definite influence on thickening characteristics. The
cumulative effect of these parameters is such that when batch settling test data are used
for unit sizing, the result is an oversized unit. For this reason, batch settling test results
must be scaled down (10). Edde and Eckenfelder (11) have developed a mathematical
model from batch and full-scale thickening data. The model facilitates determination of
solids loading at a given sludge blanket depth, initial feed solids, underflow concentration,
and hydraulic loading. This technique is particularly useful for determining gravity thickener
design parameters when upgrading existing wastewater treatment plants.
The performance of a gravity thickener depends a great deal upon the type of sludge
to be thickened. Generally, poor performance is achieved when thickening activated sludge
alone. An underflow concentration of 2.0 to 3.0 percent is usually the maximum attainable.
Typical performance data for gravity thickeners are presented in Table 10-4.
10-7
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Table 10-4
Gravity Thickening Process Performance
Type of Sludge
Primary Sludge
o
oo
Activated Sludge
Primary and Secondary Sludge
Mass Loading
Ibs/sq.ft./day
24.2
31.2
24.9
17.2
13.2
16.3
21.0
20.0
2.0-3.5 *
19
3.1
7.8
17.9
37.5
28.5o
20.02
4.6
Influent Solids
Concentration
percent
0.4
0.6
0.4
0.3
0.2
0.2
1.06
0.87
0.20
0.20
0.2
0.6
1.2
0.7
0.5
1.1
1.1
Effluent Solids
Concentration
percent
4.5
4.6
4.9
4.1
3.8
4.2
3.0
2.8
2.3-2.8
0.74
4.5
6.3
8.1
4.0
4.5
4.4
7.8
Reference
13
13
13
13
13
13
1
1
14
14
13
13
13
13
13
1
14
1
Polyelectrolyte addition.
Ratio 1:1 (primary to secondary sludge), by volume.
-------
10.2.2 Design Considerations
Both solids loading and hydraulic surface loadings must be considered when designing
gravity thickeners. Experience indicates that solids or mass loading generally governs the
design (15). The following mass loadings (3) (16) have been used for thickener design
for different types of sludge:
Table 10-5
Mass Loadings for Designing Thickeners
Type of Sludge Mass Loading
Ibs/sq.ft./day
Primary Sludge 22
Primary and Trickling Filter Sludge 15
Primary and Waste Activated Sludge 6-10
Waste Activated Sludge 4-8
The dry solids ratio of waste activated to primary sludge governs the acceptable solids
loading to be used in thickener design. As this ratio increases, the acceptable solids loading
decreases. In other words, the addition of waste activated sludge reduces the acceptable
solids loading to the thickeners.
Most thickeners are operated at a hydraulic loading of 600 to 800 gpd/sq.ft. of surface
area (16). Thickeners with hydraulic loadings less than 400 gpd/sq.ft. have been found
to produce odors (16). To achieve hydraulic loadings in the acceptable range, secondary
effluent is normally blended with the combined waste sludge before feeding the resulting
uniform diluted sludge to the thickeners.
As mentioned previously, solids loading is generally the controlling parameter and dictates
the required surface area of the thickener. For example, if a solids loading of 10
Ibs/sq.ft./day is used for a combined primary and activated sludge and typical performance
efficiencies are desired, the calculated hydraulic loading will be in the order of 100
gpd/sq.ft. Effluent dilution will be required to achieve the recommended 600 to 800
gpd/sq.ft.
Most continuous thickeners today are circular and designed with a side water depth of
approximately 10 feet. While sludge blanket depth is an important parameter, it has been
reported that underflow solids concentrations are independent of sludge blanket depths
greater than 3 feet (3).
10-9
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In the design of gravity thickeners, it is important that operational flexibility be provided.
Such flexibility includes the ability to regulate the quantity of dilution water, adequate
sludge pumping capacity so that solids concentrations are not limited, continuous feed
and underflow pumping, protection devices against torque overload, and a sludge blanket
detection device.
10.2.3 Upgrading Existing Facilities with Gravity Thickening
Gravity thickening is generally used prior to digestion processes. It can also be used as
a combined thickening and equalization process prior to sludge dewatering. Another
application is in areas where sludge hauling is utilized and there is a need to reduce the
volume of sludge to be hauled. In all cases, gravity thickening will yield higher underflow
solids concentrations than obtainable with primary sedimentation, and the efficiency of
subsequent digestion and dewatering facilities will improve. Hence, gravity thickening
should always be considered in upgrading existing solids handling and dewatering facilities.
10.2.4 Upgrading Existing Gravity Thickeners
Improved thickening can be obtained by diluting the sludges to be thickened. It has been
reported that a feed solids concentration of 0.5 to 1.0 percent is optimum and that dilution
reduces the interference between the settling particles (3).
Torpey (17) used dilution for thickening combined primary and secondary sludges in the
development of the Densludge System. A feed sludge concentration of less than 1 percent
produced underflow concentrations of 11.2 and 6 percent for combined primary and
modified waste activated sludge, and combined primary and conventional waste activated
sludge, respectively. To obtain these dilute feed sludge concentrations, dilution water was
pumped from the primary or secondary clarifier and blended with the combined sludges
prior to thickening.
Thickening systems at New York City's Tallmans Island and Bowery Bay pollution control
facilities utilize the processes developed by Torpey and presently obtain 4-6 percent
underflow solids concentration with a yearly average of 4.5 percent (18). Both plants
are operating using a combination of the step and activated aeration processes. A similar
design with digested sludge recirculation at Bergen County, New Jersey, produces an
underflow concentration of 5.2 to 7.5 percent with a yearly average of 6.3 percent (19).
In both cases, the lower underflow concentrations occur during the summer months.
The improved thickening due to dilution can also be attributed to the fines that are washed
from the sludge and returned to the plant through the thickener overflow. Experiences
at Bergen County, New Jersey, show that, even with digested sludge recirculated to the
thickener, the overflow from the thickener does not appreciably affect the over-all BOD
removal efficiency of the treatment plant (20).
10-10
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Similar experiences were reported at the Bowery Bay Pollution Control Plant. However,
the air requirements were increased from 0.28 to 0.31 cu.ft./gal. (21) by the return of
thickener supernatant to the aeration tanks.
Experiences with the use of polyelectrolytes for upgrading gravity thickening have been
reported at Amarillo, Texas (14). The original plant was designed to handle 7.5 mgd,
but was receiving a flow of up to 10.5 mgd. A 55-foot diameter gravity thickener was
being used to thicken combined primary and waste activated sludge. However, bulking
occurred due to the overloaded conditions and the high ratio of primary sludge to waste
activated sludge. In-plant recycling of solids resulted.
To minimize the problem, only activated sludge was thickened in the gravity thickener.
Polyelectrolyte addition was utilized in the thickener to improve sludge blanket control
and to obtain maximum underflow solids concentration. Anionic polyelectrolytes were
ineffective, but a cationic polyelectrolyte permitted a solids loading of 4.5 to
7.0 Ibs/day/sq.ft. while maintaining an underflow solids concentration of 2.6 percent, at
a cost of $1.10 to $3.64/ton of dry solids. Polyelectrolyte was used here to successfully
control the sludge blanket height. This practice was continued until the plant was upgraded
to 12 mgd (14).
For the expansion to 12 mgd at Amarillo, Texas, an existing 70-foot diameter final clarifier
was modified for thickening the waste activated sludge. The overflow from this thickener
is mixed with the primary sludge to dilute the feed sludge to the existing primary sludge
thickener (14).
Operation of the waste activated sludge thickener showed that, at a solids loading of 2 to
3.5 Ibs/sq.ft./day, only a 2.4 percent underflow solids concentration could be obtained.
Polymer addition was tried once again to increase the underflow solids concentration during
a 142-day program, but proved unsuccessful.
In Chicago, the addition of polymer at dosages of less than 10 Ibs/ton dry solids increased
the solids loading by 2 to 4 times, but there was no benefit in solids thickening (1).
From these two examples, it appears that polymer addition improves solids capture and
reduces solids overflow, but has little or no effect on improving solids underflow
concentration. The use of pickets has been tried in order to improve thickening and was
shown to be successful at Chicago (1), but was unsuccessful at Amarillo, Texas (14).
10.2.5 Process Designs and Cost Estimates
The following example illustrates the use of gravity thickening before anaerobic digestion.
Existing anaerobic digesters were experiencing unstable operation due to the increased
amount of sludge generated by an increase in plant flow from 10 to 16 mgd. Operational
10-11
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data from the overloaded plant without gravity thickening and the upgraded plant with
thickening are presented in Table 10-6. The volume of the combined primary and
secondary sludge was 100,000 gpd at 3 percent solids prior to upgrading. The increased
sludge volume due to the plant overloading decreased the detention time in the digesters
from 17 to 11.25 days.
Table 10-6
Example of
Upgrading Sludge Handling Facilities
Using Gravity Thickening
Description Overloaded Plant Upgraded Plant
Loadings to the Anaerobic Digester
Total Primary & Secondary Solids 25,100 Ibs 25,100 Ibs
Total Volume Primary & Secondary Sludge 100,000 gpd (3%) 66,200 gpd (4.5%)
Digester Hydraulic Detention Time 11.25 days 17 days
To improve the operation of the existing digesters, it was decided to reduce the sludge
volume by gravity thickening. The flow diagram of the upgraded plant is shown in
Figure 10-3. The total volume of sludge discharged to the anaerobic digestion facility
as a result of gravity thickening was reduced from 100,000 gpd at 3 percent solids to
66,200 gpd at 4.5 percent solids. The gravity thickener was designed using a mass loading
of 10 Ibs/sq.ft./day. A hydraulic overflow rate of 600 gpd/sq.ft. was achieved by recycling
final effluent to the mixing chamber ahead of the thickener as shown in Figure 10-3.
The capital costs for gravity thickening in this example are estimated at $258,000 (ENR
1500). These costs include one gravity thickener, a mixing chamber, effluent recycle
capacity, and an allowance for appropriate connecting piping. They do not include the
allowance for engineering design, bonding, and construction supervision.
10-12
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FIGURE 10-3
UPGRADING SLUDGE HANDLING FACILITIES USING A GRAVITY THICKENER
RAW
THICKENER
OVERFLOW
FINAL
SEDIMENTA-
TION
WASTE ACTIVATED SLUDGE
EFFLUENT RECYCLE WATER
TO DIGESTER OR
»• SLUDGE DEWATERING
FACILITIES
-------
10.3 References
1. Ettelt, G. A., and Kennedy, T., Research and Operational Experience In Sludge
Dewatering at Chicago. Journal Water Pollution Control Federation, 38, No. 2, pp.
248-257 (1966).
2. Jones, Warren H., Sizing and Application of Dissolved Air Flotation Thickeners. Water
and Sewage Works, 115, No. 11, pp. R177-178 (1968).
3. Burd, R. S., A Study of Sludge Handling and Disposal. Federal Water Pollution Control
Administration, Publication WP-20-4, May, 1968.
4. Mulbarger, M. C., and Huffman, D., Mixed Liquor Solids Separation by Flotation.
Journal of the Sanitary Engineering Division, ASCE, 96, No. 4, pp. 861-871 (1970).
5. Ettelt, G. A., Activated Sludge Thickening by Dissolved Air Flotation.
Proceedings-19th Industrial Waste Conference, Purdue University, pp. 210-244
(1964).
6. Katz, W. J., and Geinopolos, A., Sludge Thickening by Dissolved-Air Flotation. Journal
Water Pollution Control Federation, 39, No. 6, pp. 946-958 (1967).
7. Koogler, J. B., Operational Report of the Biddeford, Maine Sludge Disposal System.
Peapack, New Jersey: Komline-Sanderson Engineering Company, 1966.
8. Katz, W. J., and Geinopolos, A., Concentration of Sewage Treatment Plant Sludges
by Thickening. Proceedings - Tenth Sanitary Engineering Conference - Waste
Disposal from Water and Wastewater Treatment Processes, University of Illinois,
February 6-7, 1968.
9. Dick, Richard, and Ewing, Benjamin, Evaluation of Activated Sludge Thickening
Theories. Journal of the Sanitary Engineering Division, ASCE, 93, No, 4, pp. 9-29
(1967).
10. Vesilind, Arne, Design of Prototype Thickeners from Batch Settling Curves. Water
and Sewage Works, 115, No. 7, pp. 302-307 (1968).
11. Edde, Howard, and Eckenfelder, W., Theoretical Concept of Gravity Sludge
Thickening; Scale-Up Laboratory Units to Prototype Design. Journal Water Pollution
Control Federation, 40, No. 8, pp. 1486-1498 (1968).
12. Dick, R., Thickening. Included in Water Quality Improvement by Physical and
Chemical Processes, ed. by Gloyna, E., and Eckenfelder, W. W., Austin, Texas:
University of Texas Press, 1970.
10-14
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13. Ford, D., General Sludge Characteristics. Included in Water Quality Improvement by
Physical and Chemical Processes, ed. by Gloyna, E., and Eckenfelder, W. W., Austin,
Texas: University of Texas Press, 1970.
14. Jordon, V. J., and Scherer, C. H., Gravity Thickening Techniques at a Water
Reclamation Plant. Journal Water Pollution Control Federation, 42, No. 2, pp.
180-189 (1970).
15. Schroepfer, G. J., and Ziemke, N. R., Factors Affecting Thickening in Liquid Solids
Separation. National Institute of Health, Sanitary Engineering Report No. 156S,
March, 1964.
16. Sparr, A., and Grippi, V., Gravity Thickeners for Activated Sludge. Journal Water
Pollution Control Federation, 41, No. 11, pp. 1886-1904 (1969).
17. Torpey, W.N., Concentration of Combined Primary and Activated Sludges in Separate
Thickening Tanks. Journal of the Sanitary Engineering Division, ASCE, 80, No. 1,
pp. 1-17 (1954).
18. Private Communication with J. Donnellon Department of Public Works,
New York City, December 10, 1970.
19. Zablatzky, H. R., and Baer, G. T., High Rate Digester Loadings. Journal Water
Pollution Control Federation, 43, No. 2, pp. 268-277 (1971).
20. Private Communication with H. R. Zablatzky, Superintendent, Bergen County Sewer
Authority, Little Ferry, New Jersey, December 15, 1970.
21. Torpey, W. N., and Milbinger, N. R., Reduction of Digester Sludge Volume by
Controlled Recirculation. Journal Water Pollution Control Federation, 39, No. 9, pp.
1464-1474 (1967).
10-15
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CHAPTER 11
SLUDGE DIGESTION
11.1 Anaerobic Digestion
Anaerobic digestion is one of the most frequently employed processes for sludge treatment.
The process converts the biodegradable portion of the sludge solids to inoffensive gases.
In contrast to raw sludge, which is difficult to dewater, offensive to the senses, and laden
with pathogenic organisms, the residue after digestion (digested sludge) is relatively easy
to dewater, non-offensive, and contains few pathogens. Thus, anaerobic digestion achieves
ultimate disposal by gasifying a portion of the sludge and by preparing the remainder
for ultimate disposal by other methods. In addition, the end products have recycle
potential. The major gaseous end product is methane, which is often used as a source
of fuel in wastewater treatment plants. The digested sludge is an excellent soil conditioner
and has found some utility for this purpose.
11.1.1 Biochemical Theory
Operation, control, and design of this process require an understanding of the fundamental
biochemistry and bacteriology involved. Consequently, a brief review of these will be given
here.
Anaerobic digestion of sludge is a complex biochemical process employing several groups
of anaerobic and facultative organisms. In general, the process can be considered to consist
of two steps. In the first step, facultative organisms called "acid formers" degrade the
complex organics of wastewater sludge to volatile organic acids. Acetic acid is the primary
acid formed, with propionic and butyric acids of secondary importance. In the second
step, these volatile acids are fermented to methane and carbon dioxide by a group of
strict anaerobes called "methane bacteria."
The more important of these two phases is the methane fermentation phase because:
1. The only mechanism of COD or BOD removal is the production of methane.
Acid production only solubilizes the complex organics; it does not accomplish
stabilization.
2, This step has been found to be the rate-limiting step in the reaction sequence.
The primary reason why the methane fermentation step is rate limiting is that the
reproduction rate for these organisms is quite low relative to that of other groups of
bacteria. For example, the doubling time of the acid formers is several hours while that
of the methane formers is, under ideal conditions, four days. Thus, even if a temporary
difficulty in the system arose, it would be much harder for the methane organisms to
11-1
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adjust than for the acid formers. In addition, it has been found that the environmental
conditions required to maintain optimum performance by the methane organisms are much
more restrictive than for the acid forming organisms. Consequently, most of the effort
in design and operation should be expended to make sure that the methane fermentation
step is carried out as efficiently as possible.
11.1.2 Environmental Conditions for Optimum Performance
Knowledge of the range of environmental conditions which favor optimum performance
of this process is not as extensive as desired. For this reason, the rate of failure with
this process is higher than for other waste treatment processes. A summary of the state
of knowledge is given below.
11.1.2.1 pH
Tight pH control is required for this process because methane bacteria are extremely
sensitive to slight changes in pH. The usual pH range required is 6.6 to 7.4. In general,
it is wise to maintain the pH as close to 7.0 as possible.
In an anaerobic digester, pH is maintained by a bicarbonate buffer system due to the
great quantity of carbon dioxide produced during methane fermentation. The pH is a
function of the bicarbonate alkalinity of the digesting liquor and the fraction of CC>2
in the digester gas. Figure 11-1 prepared by McCarty (1) illustrates this relationship.
Because of the significance of pH control in digester operation, it is most important that
the dynamic nature of buffer destruction and formation in the digester be understood.
This process is reviewed in the following equations for simple carbohydrates such as glucose.
The equations mentioned are equally applicable to digestion of sludge.
> 3 CH3COOH
3 CH3COOH + 3 NH4HCO3 - > 3 CH3COONH4 + 3 H2O + 3 CO2
3 CH3COONH4+ 3H20 ^ethane bacteria > 3 ^ +
The first equation represents the breakdown of glucose to acetic acid by acid forming
bacteria. The acid is neutralized, as shown in the second equation, by the biocarbonate
buffer. If sufficient buffer is not present, the pH would drop, and the conversion of acetate
to methane, as shown in the third equation, would be inhibited. During the third reaction,
the buffer consumed in the second reaction is reformed.
In the digestion process, a dynamic equilibrium between buffer formation and destruction
is maintained when the process is proceeding satisfactorily. However, when an upset occurs,
it is usually the methane bacteria rather than the acid formers which are adversely affected.
Therefore, net buffer consumption takes place, and the process is in danger of pH failure.
When this occurs, an external source of alkalinity must be added to maintain the pH
in the proper range.
11-2
-------
FIGURE 11-1
RELATIONSHIP IETWEEN pH AND BICARBONATE CONCENTRATION (1)
50
40
30
- 20
OtJ
2
10
250
500 1000 2500 5000 10.000
HCOJ CONCENTRATION - MG/L AS CaC03
25.000
-------
Figure 11-1 indicates that the bicarbonate alkalinity should be maintained at a minimum
level of 1,000 mg/1 as CaCO3 to ensure adequate pH control. The conventional titration
procedure for digester alkalinity determination does not discriminate between the various
forms of alkalinity, which is unfortunate because only bicarbonate alkalinity buffers in
the correct pH range for good digestion. The other major form of alkalinity measured
by this test is that produced by the volatile acids. To determine the bicarbonate alkalinity,
both the volatile acid concentration and the total alkalinity must be measured. Then,
Bicarbonate Alkalinity = (Total Alkalinity - 0.8 Volatile Acids)
The 0.8 factor in the above equation is required to convert the volatile acid units from
mg/I as acetic acid to mg/1 as CaCO3, the equivalent alkalinity unit.
It should be noted that in the second and third equations, ammonium bicarbonate was
used as the form of the alkalinity. This represents the situation in wastewater sludge
digestion where large concentrations of ammonium result from the destruction of protein.
In fact, the maximum value of the total alkalinity is set by the concentration of the
ammonium ion. The carbon dioxide generated in the methane fermentation will not form
negatively-charged bicarbonate (the buffer) unless an equivalent quantity of cation is
present. This is provided by the destruction of natural protein with the release of
positively-charged ammonium. If a cation is not present to force formation of bicarbonate
buffer, self-regulation of pH in the digestion process is not possible. In this case, alkaline
material must be added continuously to control the pH. For example, the anaerobic
degradation of glucose, illustrated in the first three equations, would require the addition
of an external source of alkalinity. It is not necessary that ammonium bicarbonate be
utilized for this purpose; in fact, in terms of cost and avoidance of potential toxicity,
another bicarbonate salt might be favored.
In general, this difficulty will not be experienced in wastewater sludge digestion unless
either a high carbohydrate fraction from ,an industrial waste is present in the sludge or
a very thin sludge is being treated.
11.1.2.2 Temperature
The temperature response of methane bacteria is the same as other bacterial groups.
Although thermophilic methane bacteria exist, it is generally not economically feasible
to heat sludge to this temperature range. Thus, digestion of wastewater sludge is conducted
in the mesophilic range. The optimum temperature in this range is 35°C (95°F). More
important than maintenance of a particular temperature is maintenance of the chosen
temperature at a constant level. A temperature change of 2 or 3 degrees F is sufficient
to disturb the dynamic balance between the acid formers and the methane formers. This
will lead to an upset because the acid formers will respond much more rapidly to changes
in temperature than will the methane bacteria.
11-4
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11.1.2.3 Nutrients
The major stumbling block in the application of anaerobic treatment to industrial
wastewaters is the lack of knowledge of the nutritional requirements of the methane
bacteria. Speece and McCarty (2) have done the most definitive work on the macro-nutrient
and micro-nutrient requirements of these organisms. As these authors indicate, domestic
wastewater appears to contain all of the nutrients required by the methane organisms.
Thus, difficulty can be expected in digestion only when a considerable fraction of the
sludge is of industrial origin.
11.1.2.4 Toxic Materials
A review of this subject has been provided by Kugelman and Chin (3). These authors
indicate that toxicity in general is due to an excess quantity of any material, even for
a substance normally considered a nutrient. It was also indicated that a quantitative
definition of the concentration at which a substance starts to exert a toxic effect is difficult
to define because this could be modified by antagonism, synergism, and acclimation. In
addition, the degree of stress on the process as defined by the organic loading and biological
solids retention time can significantly affect "toxicity."
Substances which may be present in municipal sludge in concentration ranges which can
produce toxicity include heavy metals, sulfides, surface-active agents, light metals, and
certain organics. All of these can gain entrance to wastewater sludge from industrial sources.
In addition, light-metal cations will enter sludge if an alkaline material is added to control
the pH. Several papers (3) (4) (5) review the best engineering data available on toxicity.
Reference should be made to these papers for complete information. General information
on some substances is given in Table 11-1.
Table 11-1
Concentrations Which Will
Casue a Toxic Situation
in Wastewater Sludge Digestion
Substance Concentration
mg/1
Sulfides 200
Heavy Metals1 >1
Sodium 5,000- 8,000
Potassium 4,000-10,000
Calcium 2,000- 6,000
Magnesium 1,200- 3,500
Ammonium 1,700- 4,000
Free Ammonia 150
1 Soluble
11-5
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It must be emphasized that the values in this table are only guides. If toxicity is suspected,
a thorough analysis of all the chemical constituents of the sludge must be made before
definite conclusions can be drawn. Potential solutions to toxicity problems, other than
elimination from the wastewater, should be evaluated in small-scale digesters of the type
used in laboratory investigations.
11.1.3 Process Kinetics
Lawrence and McCarty (6) have reviewed the kinetics of anaerobic digestion. They indicated
that the overall process kinetics are controlled by the methane bacteria. In addition, they
found that the removal efficiency could be characterized by the equation:
Ks(l+Kd-SRT)
So~SRT'Km-(l+Kd-SRT)
C> - .--_-!—L
where:
Ks, Km, and Kd = Kinetic constants
S0 = Influent substrate concentration
E = Substrate removal efficiency expressed in decimal form
SRT = Biological Solids Retention Time
The engineer and/or plant operator can control only the SRT. Thus, this is the fundamental
design and control parameter which must be used. SRT is the biological solids retention
time and is analogous to the sludge age parameter used in activated sludge system design.
For a digestion system without sludge recycle, the SRT is numerically equal to the HRT
(hydraulic retention time). This analysis indicates a fallacy which is prevalent at present
in digestion criteria. Digesters are designed at present on one of three criteria: volume
per individual served, weight of volatile solids per unit volume of digester per unit time,
and HRT. Of these, the only valid criterion is HRT.
Values for the kinetic constants discussed above were determined experimentally by
Lawrence and McCarty (7). These values indicate that at 35°C the absolute minimum
SRT for anaerobic digestion is 3 to 4 days. This value agrees with the minimum HRT
determined by Torpey (8) in field studies. For design purposes, a longer HRT should
be utilized to provide a safety factor against upsets and to allow for fluctuations in sludge
volume. In addition, it has been shown that in some situations the rate-limiting step is
solubilization of grease and/or protein, which requires HRT values longer than four days.
Suggested retention times for high-rate digesters are shown and discussed in the following
section.
11-6
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11.1.4 Present Digestion Systems
Prior to a discussion of procedures for upgrading the performance of digestion systems,
a description of existing digestion systems will be presented. Figure 11-2 illustrates the
two types of digestion systems in use at present.
In the conventional system, also known as a low-rate system, the tank is not mixed and,
in some cases, is not heated. Sludge is added at the top and withdrawn at the bottom.
Stratification develops in the system due to a lack of mixing. In general, this can be
classed as a plug-flow system. This system is rather inefficient by the normal design criteria
utilized, as illustrated in Table 11-2 (9). Because of the lack of mixing and consequent
stratification, much of the digester volume is wasted, and many operational problems result.
In this type of digester, acidification takes place in the top and middle layers, while methane
fermentation is confined to the lower layers. This leads to areas of low and high pH
in the system, which restrict optimum biological activity. Grease breakdown is poor because
the grease tends to float to the top of the digester while the methane bacteria are confined
to the lower levels. Methane bacteria are removed with the digested sludge and are not
recycled to the top, where they are required. During progression from top to bottom
of the digestion tank, the sludge is compressed and gradually dewatered. The water
separated from the sludge forms the supernatant layer. The supernatant is high in nitrogen,
phosphorus, BOD, COD, and suspended solids. It places an additional organic load on
the biological treatment section of the plant and recycles excess nitrogen and phosphorus
through the plant. Chemicals added for pH control are not dispersed throughout the tank,
and their effectiveness is limited.
Table 11-2
Typical Design Criteria for Low-Rate and High-Rate Digesters
Parameter Low-Rate High-Rate
Solids Retention Time (SRT), days 30 to 60 10 to 20
Solids Loading, Ib. VSS/cu.ft./day 0.04 to 0.1 0.15 to 0.40
Volume Criteria, cu.ft./capita
Primary Sludge 2 to 3 1-1/3 to 2
Primary Sludge + Trickling Filter Sludge 4 to 5 2-2/3 to 3-1/3
Primary Sludge + Waste Activated Sludge 4 to 6 2-2/3 to 4
Combined Primary + Waste Biological
Sludge Feed Concentration, percent solids
(dry basis) 2 to 4 4 to 6
Digester Underflow Concentration,
percent solids (dry basis) 4 to 6 4 to 6
Source: Burd (9)
11-7
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INLET
FIGURE 11-2
DIGESTION SYSTEMS
GAS WITHDRAWAL
GAS
/ / /. SCUM LAYER. / / /
SUPERNATANT
ACTIVE LAYER
v\ \\* ^^^
\\\ STABILIZED
^\A \ SOLIDS
I
SOLIDS
REMOVAL
CONVENTIONAL
— -^OUTLET
GAS WITHDRAWAL
INLET *•—
GAS
OUTLET
HIGH RATE
11-8
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The high-rate system differs from the low-rate system in that the contents are well mixed,
either continuously or intermittently, and the digester is heated. This procedure avoids
all of the difficulties inherent in low-rate systems. Consequently, this system operates well
at lower HRT values and higher organic loading rates. (See Table 11-2).
Various mixing systems have been successfully utilized in digesters. These include:
1. A central draft tube through which sludge is circulated by a turbine mixer set
in the table.
2. Propellers (mounted from the roof) which stir the sludge.
3. Gas circulation through diffusers in the base of the digester.
4. Gas injection into the top layer of the sludge.
Specific design of these systems can be obtained from the various manufacturers.
Sludge heating is accomplished either by circulating hot water through coils in the inner
wall of the digestion tank or by circulating sludge through an external heat exchanger.
The latter method is preferred since it was found the coils inside the digestion tank are
easily caked with partially dried sludge. The sludge circulation rate to the external heat
exchanger is set to achieve one complete turnover of the tank contents in 24 to 48 hours.
Design of a high-rate digestion system must include a heat balance to determine fuel
requirements. The WPCF Manual of Practice No. 8(10) presents in detail the procedure
for making such a heat balance. This discussion includes valuable data on the fuel value
of sludge gas and the insulation characteristics of typical digester construction material.
One difficulty with high-rate digestion is that the sludge leaving the digester is thinner
than the incoming sludge (due to solids destruction). To concentrate the sludge, secondary
digesters have been added to many high-rate digestion systems. In effect, these are settling
tanks since they are neither heated nor mixed. They also serve as a source of seed sludge
in case of digester upset. The secondary digester capacity is usually 2 to 4 times that
of the primary digester.
11.1.5 Upgrading Existing Anaerobic Digestion Facilities
Improperly functioning digestion systems can be upgraded by applying procedures which
will make the systems more closely approach the theoretical optimum performance. The
conditions which will produce optimum performance of this process have been given in
the preceding sections of this chapter. Specific upgrading techniques will now be discussed.
11.1.5.1 Process Monitoring and Biochemical Control
The first step in any upgrading technique is constant monitoring of the process for
biochemical upset. This can be accomplished with the aid of the volatile acid and alkalinity
tests and by a digester gas analysis. Any sudden rise of volatile acids indicates that the
11-9
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system is out of biochemical balance. A rise in the CO2 fraction in the gas or a decrease
in methane production per pound of volatile solids added will also indicate upset. However,
the volatile acid test is more sensitive. When an upset occurs, an alkaline material must
be added to maintain the bicarbonate alkalinity above 1,000 mg/1 as CaCO3- An easily
soluble bicarbonate salt, such as NaHCO3, is best for this purpose. Care must be exercised
not to exceed the level at which the cation of the alkaline material will cause toxicity.
If this is a potential problem, a mixture of alkaline salts should be used. Kugelman and
McCarty (4) have described methods of preventing cation toxicity by adding appropriate
quantities of cation antagonists.
Control of pH during an upset is only a stop-gap measure. The cause of the upset must
be located and eliminated. Sometimes this is easy. For example, heavy-metal toxicity can
be completely eliminated by precipitation of the metal in the digester as the sulfide (11).
In other cases, only exclusion of the toxin from the system will suffice.
11.1.5.2 Upgrading Techniques
The major upgrading technique for low-rate digesters is conversion to high-rate digestion.
To maintain high-rate digestion, the following conditions are necessary:
1. Solids thickening to maintain volatile solids loading in the range of
0.15 to 0.4 Ib. VSS/cu.ft./day.
2. Complete mixing of digester contents.
3. Solids feed and withdrawal at a uniform rate.
4. Temperature control system capable of maintaining a uniform temperature range
of 30 to 35°C.
5. A solids retention time of 10 to 20 days.
The principal techniques used for upgrading high-rate digesters are to increase feed solids
concentration, provide a secondary digester for liquid-solids separation, and increase SRT
by recycling digested sludge to the primary digester.
The relationship between thickening of solids, detention time, and solids loading has been
illustrated by Sawyer (12) and is shown in Figure 11-3. This relationship points out the
importance of thickening the solids prior to digestion.
In most cases, thickening is best accomplished in a separate thickening unit. (Thickening
techniques are discussed in Chapter 10 of this manual.) If air flotation is chosen,
consideration should be given to minimizing the bound air in the float sludge before
pumping it to the digester.
Thickening can also be accomplished in a clarifier and controlled by a sludge density
meter as reported by Garrison, (13) and Sironen and Lee (14), or by using automatic
11-10
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FIGURE 11-3
RELATIONSHIPS BETWEEN SLUDGE SOLIDS,
DIGESTER LOADINGS, AND DETENTION TIME (12)*
400
350
300
- 250
200
150
100
50
% SOLIDS,
FEED SLUDGE
10
15
20
25
30
DETENTION TIME, DAYS
*OPTIMUM TEMPERATURE RANGE 85-95°F
11-11
-------
sludge blanket control devices. Both of these techniques require some visual operator
control, as described in the WPCF Manual of Practice No. 16 (15), and will most likely
reduce the efficiency of the clarification system.
Pre-thickening is generally required when solids loadings greater than 0.2 Ib.VSS/cu.ft./day
are used. At these levels, liquid-solids separation in the digestion system becomes more
difficult (as previously described), especially if secondary digester capacity is limiting.
Hence, if supernatant is drawn from this process, it could be detrimental to the efficiency
of the secondary biological system unless properly distributed.
There are restrictions on the degree to which raw sludge can be thickened. These include
the difficulty of pumping thick sludge and the maintenance of adequate mixing in the
digester. Generally, sludge can be thickened to about 8 percent solids without the need
to install special pumping or additional mixing equipment. If it is desired to thicken beyond
this point, adequate studies of the sludge flow characteristics must be made to evaluate
pumping and mixing requirements.
Potential toxic effects may also limit the degree of sludge thickening. Thickening will
result in high ammonium and bicarbonate concentrations in the digester. The higher
alkalinity will tend to raise the pH and convert the ammonium ion to free ammonia
according to the following equation, raising the possibility of free ammonia toxicity:
If the pH is maintained below 7.2, free ammonia toxicity can generally be avoided. Thus,
when digesting thickened sludge, process control is extremely important. If the ammonium
ion concentration and pH are high, it may be necessary to add acid to keep the pH
below 7.2. The only acid which should be used for this purpose is hydrochloric acid.
Sulfuric acid addition would yield sulfate, which would eventually be reduced to corrosive
H2S under anaerobic conditions. Nitric acid addition would release nitrate, which under
certain conditions is toxic to methane bacteria.
The addition of mixing by itself can have a significant beneficial effect on digester
performance. At the City of Pontiac, Michigan (16), modifications were made to an existing
digester by adding a gas recirculation unit to improve mixing. Mixing inhibited scum
formation, improved heat transfer, and provided a more stable digestion process. Many
other communities have had similar success (17).
Experiences at Chicago (18) have shown that digesters can be upgraded to operate at
volatile solids loadings of 0.2 Ib./cu.ft./day and at solids retention times of 10 days.
Complete mixing is necessary to achieve these operating results and enables a wide variation
in loadings.
11-12
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Bergen County, New Jersey, is the best reported example of upgrading low-rate digesters
to high-rate digesters (19). In 1951, a 20-mgd activated sludge plant was constructed with
four 1.3-million gallon digesters. In 1961, the capacity of the plant was increased to
50 mgd, and the existing digesters were modified to high-rate. In 1969, additional studies
were conducted on a full-scale basis, and it was found that two of the original digesters
could handle the entire loading from the existing 50-mgd activated sludge plant. On this
basis, it has been projected that the original four digesters will be able to handle the
increased solids loading when the plant is expanded to 75 mgd. It is significant to note
that even at these higher loadings, the anaerobic process was very stable and the efficiency
of the process remained the same.
Successful upgrading of existing digestion facilities at Bergen County was accomplished
by the following methods:
1. Completely mixing the digester contents.
2. Pre-thickening the primary and secondary sludge to an average concentration
of 6.3 percent (range 5.2 to 7.5 percent).
3. Increasing the applied solids loading from 0.22 Ib./cu.ft./day to
0.5 Ib./cu.ft./day as a result of thickening.
4. Decreasing the liquid detention time from 22 to 10 days.
The addition of a second stage to a high-rate digester enables digestion in the primary
stage and solids-liquid phase separation to occur in the secondary stage. In some areas,
this phase separation may be important. It appears that, as the VSS loading in the primary
digester increases, the detention time in the secondary sludge digester should be increased.
The loading to two-stage high-rate digestion systems may be increased by recirculating
the digested sludge from the secondary stage back to the primary stage, since this effectively
increases the SRT at the same digester HRT. As with activated sludge systems, the limiting
factor is the solids-liquid phase separation. Perhaps a degasification process can be included
between the primary and secondary tanks to aid in the separation, as is done with the
anaerobic contact process (20). When satisfactory phase separation is obtained in the
secondary digester, adequate SRT's can be maintained while decreasing hydraulic retention
times to less than 10 days (21). A more stable digestion process then results, due to
higher populations of methane bacteria in the primary stage and to the lessening of toxicity
effects at longer SRT's.
Digesters can also be upgraded by recirculating a portion of the digested sludge back to
the thickening units and mixing it with the incoming combined sludge and effluent recycle
water. This procedure has been reported by Torpey and Melbinger (22) with modified
aeration plants in New York City, and was originally adopted to improve the pumping
characteristics of highly concentrated digester feed sludge of 10 to 14 percent solids. It
appears that the thickening process was improved because the digested sludge was
incorporated into the pore spaces of the mixed primary and waste activated sludge, thereby
11-13
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eliminating the typical gel structure produced by grease in the raw sludge. It was also
found that recycling digested sludge improved digester performance due to the seeding
of the combined sludge prior to digestion and because of the greater SRT thereby afforded.
Volatile solids reduction was also increased. A digested sludge recycle of 50 percent
appeared to be optimum for the New York City plants, and a net volume reduction of
digested sludge from 197 cu.ft./million gallons to 112 cu.ft./million gallons was achieved.
Fines are washed out in the thickener by the high volume of effluent recycle water used
and are returned to the aeration basins. This thickener supernatant return increased the
aeration requirements of the New York City plants slightly, but had little apparent effect
on BOD removal (average of 68 percent removal with recirculation and an average
63 percent without recirculation). However, when efficiencies greater than 90 percent
BOD removal are required, thickener supernatant return will definitely be a major concern.
It must be pointed out that conversion to high-rate digestion is not a cure-all, especially
if digested sludge is to be dewatered prior to final disposal. Methane production and volatile
solids reductions are approximately the same at high-rate as at standard-rate, but indications
are that dewatering of high-rate sludge is difficult (9). To obviate this difficulty, Sawyer
(12) has suggested that secondary digesters in high-rate digestion systems be two to four
times the capacity of high-rate primary digesters, to provide adequate time for solids
conditioning. This technique has been reported at Grand Rapids, Michigan (23), where
the volatile solids loading to the primary digester is in excess of 0.25 Ib./cu.ft./day. The
ratio between secondary and primary digester capacity is 3.5:1. Secondary digester
underflow solids exceed 10 percent and supernatant solids average less than 2 percent
of the raw solids load. This indicates that the economics of decreasing the detention time
in the primary digesters should be weighed against providing the additional capacity in
the secondary digesters when solids dewatering is required. Such an evaluation would not
be required if the digested sludge is to be disposed of on land, because the degree of
sludge conditioning is not the same as that required for sludge dewatering.
11.1.6 Anaerobic Supernatant Treatment
The return of supernatant liquor from digesters or thickeners to the treatment facilities
is an important consideration. Such supernatants contain a significant quantity of volatile
solids, organic matter, and high concentrations of nutrients, particularly nitrogen and
phosphorus, as indicated in Table 11-3. The supernatant return problem can be reduced
significantly by treating the supernatant with the lime precipitation process followed by
ammonia stripping, as shown schematically in Figure 11-4. Operational data for this process
are shown in Table 11-3 for a lime concentration of 6,000 mg/1 (24). The data indicate
substantial reductions in nitrogen, phosphorus, organics, and solids.
11-14
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FIGURE 11-4
LIME PRECIPITATION PROCESS FOR ANAEROBIC
DIGESTER SUPERNATANT (24)
SLURRY OF
SLAKED LIME
NH3 GAS RELEASED
TO ATMOSPHERE
1
DIGESTER
SUPERNATANT ^
AIR STRIPPING
TO REMOVE CO 2
AND RAISE pH
TO
RE
AN
SUPERNATANT
DH 8.1-8.3 ^
E p
CHEMICAL
PRECIPITATION
AND SETTLING
CALCINING
) REUSE
EFFLUENT
oH 10.8-11.4..
LIME
SLUDGE
COUNTER-CURRENT
AIR STRIPPING
A
COMPRE
AIR
i
SSED
IHLHIC.U
SUPERNATANT ^
(WITH REDUCED
QUANTITIES OF
NUTRIENTS)
-------
Table 11-3
Operational Data for the Lime Precipitation Process for
Anaerobic Digester Supernatant
Concentration, mg/1
Parameter Influent Effluent
PH 7.1 10.7
Total Solids 4,985 2,753
Total Volatile Solids 3,330 1,821
Suspended Solids 2,905 1,190
Volatile Suspended Solids 2,530 930
COD 5,407 2,919
Total Carbon 3,075 1,214
Total Organic Carbon 1,624 914
Ortho - PO4 (as P) 91 5.9
Total Phosphate (as P) 141 37
NH3-Nitrogen (as N) 818 7261
1572
Organic Nitrogen (as N) 282 176
'Effluent not air stripped after the lime treatment.
^Effluent air stripped after the lime treatment.
Source: Bennett (24)
11.1.7 Process Designs and Cost Estimates
Two examples of upgrading existing anaerobic digesters are presented in this section.
11.1.7.1 Example A
In this example, upgrading of two-stage low-rate digestion facilities was required due to
the increase in plant flow from 1 mgd to 3 mgd. A gravity thickener was added prior
to digestion. Primary digester performance was improved by adding gas mixing and installing
external heat exchangers to control the temperature more accurately. The comparison
between existing and upgraded design conditions is presented in Table 11-4. The flow
diagram of the upgraded plant is shown in Figure 11-5.
A mixing chamber ensures proper blending of sludges and effluent water prior to thickening.
With thickening, the solids concentration and the volatile solids loading to the primary
digester were increased from 2 to 5 percent and from 0.036 to 0.108 Ib./cu.ft./day,
respectively.
11-16
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Table 11-4
Upgrading an Existing Low-Rate Digestion System Using
Pre-thickening of the Combined Sludge and Improvements to the
Primary Digester - Example A
Parameter Original Design
Plant Flow, mgd
Combined Sludge Characteristics
Volume, gpd
Solids Contribution, Ibs/day
%vss
Gravity Thickener
Number
Solids Loading, Ibs./sq.ft./day
Effluent Water Required to Dilute Sludge, gpd
Hydraulic Loading, gpd/sq.ft.
Thickened Sludge Volume, gpd
Digester
Number - Primary Digesters
Number - Secondary Digesters
Primary Digester Characteristics
Secondary Digester Characteristics
Digester Volume Allocation, cu.ft./capita/day
Digester Volume (Total), cu.ft.
Hydraulic Retention Time, days (Total)
days (Primary)
VSS Loading, Ib./cu.ft./day (Total)
Ib./cu.ft./day (Primary)
9,100(2%)
1,530
70
1
1
Limited heating and mixing
No heating or mixing
6 (Low-rate)
60,000
49.4
24.7
0.018
0.036
Upgraded Design
3
27,300 (2%)
4,590
70
1
10
248,100
600
10,920(5%)
1
1
New gas mixing and improved heating
No heating or mixing
2 (High-rate)
60,000
41.0
20.5
0.054
0.108
-------
FIGURE 11 -5, EXAMPLE A
UPGRADING AN EXISTING LOW RATE DIGESTION SYSTEM
USING PRE-THICKENING OF THE COMBINED SLUDGE
AND IMPROVEMENTS TO THE PRIMARY DIGESTER
SECONDARY
CL»RIFIC»TION
NEI UNITS
11-18
-------
Digested sludge recirculation was utilized, with provisions for recirculating 50 percent of
the volume of sludge to be digested back to the thickener. Torpey found that this technique
improved VSS reduction (22). The capital costs of this upgrading were estimated at
$118,000! (ENR 1500) and were allocated as follows:
Thickener $ 64,000
Digester Renovation 54,000
TOTAL $118,000
The cost of this upgrading is estimated at $39,500/1,000 Ibs./day of increased solids
loading.
11.1.7.2 Example B
This example illustrates upgrading of existing digesters to increase capacity by converting
both low-rate primary and secondary digesters to high-rate digesters. This is illustrated
in Figure 11-6. It is noted that the existing four digesters have all been upgraded, but
that the fourth digester is normally used as a storage tank and a backup digester only
during periods of operational problems. All upgraded digesters are provided with complete
mixing, uniform solids feeding and withdrawal, and uniform temperature control. The
design parameters for the existing and upgraded digesters are shown in Table 11-5.
Generally, some form of pre-thickening is required to obtain 4 percent solids in the feed
sludge. However, experience at Grand Rapids, Michigan (25) has shown that concentrations
as high as 7 percent can be obtained with very close operational control of the primary
clarifiers. Therefore, pre-thickening of sludge is assumed not to be necessary in this example.
With this conversion, it must be recognized that there may be problems with the dewatering
characteristics of the sludge. This modification will be useful where ultimate disposal land
sites can accept 5 to 7 percent digested sludge. The cost associated with this modification
is $92,000, or $3,000/1,000 Ibs./day of increased solids loading. The greatly reduced cost
of upgrading in Example B compared to Example A is due to availability of mixing and
heating equipment in the existing digesters.
11.2 Aerobic Digestion
Aerobic digestion is an effective means for upgrading existing overloaded sludge digestion
facilities, particularly as applied to the digestion of waste activated sludges. It offers a
low capital cost means of achieving stabilization and volume reduction of wastewater
sludges. Most package type activated sludge plants include aerobic digestion. The process
These costs do not include a contingency for engineering design, bonding, and
construction supervision.
11-19
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FIGURE 11-6, EXAMPLE B
UPGRADING EXISTING TWO-STAGE DIGESTERS
TO PRIMARY HIGH-RATE DIGESTERS
RECYCLE OR RETURN SUDCE
RAI ,
RASTEWATER
1
Minor
CLARIFICATION
-L
1 TRICKLING
FILTER
T
SECONDARY
CLARIFICATION
FINAL
EFFLUENT
NOTE:
I. SLUDK STORAIi UNIT TO BE USED AS IAHUP PHIMRV OIKSTER
11-20
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Table 11-5
Parameter
Plant Flow, mgd
Digester
Number - Primary Digesters
Number - Secondary Digesters
Primary Digester Characteristics
Secondary Digester Characteristics
Digester Volume Allocation, cu.ft./capita/day
Digester Volume (Total), cu.ft.
Sludge Volume to Digesters, gpd
Solids Loading to Digesters, Ibs./day
%vss
Hydraulic Retention Time, days (Total)
days (Primary)
VSS Loading, Ib./cu.ft./day (Total)
Ib./cu.ft./day (Primary)
Upgrading Existing Two-Stage Digesters
to High-Rate Digesters - Example B
Original Design
12.5
2
2
Mixing and heating provided (2 units)
No mixing or heating
6
300,000
61,000(3%)
15,200
70
36.8
18.4
0.035
0.070
Upgraded Design
37.5
1 (Storage Unit)
Mixing and heating provided (3 units)
Storage unit and standby
primary digester
2
225,000 (Active)
75,000 (Storage)
137,000(4%)
45,600
70
12.3 (3 units)
0.142 (3 units)
-------
can also be applied to the digestion of primary sludges or to combinations of primary
and secondary sludges. It has been indicated that aerobic digestion is competitive with
anaerobic digestion for activated sludge plants up to a size of at least 8 mgd (26).
The typical concentrations of various constituents present in aerobic and anaerobic
supernatant liquors, shown in Table 1 1-6 (24) (27), indicate that the effect of supernatant
return on biological units would definitely be less pronounced when aerobic digestion
is used.
Table 11-6
Comparison of Aerobic and Anaerobic
Supernatant Liquors
Aerobic Anaerobic
Parameters Supernatant Supernatant
pH 5.6 7.1
BOD, mg/1 16 , -
COD,mg/l - 5,407
Volatile Solids, mg/1 39.5 3,330
Ammonia Nitrogen, mg/1 1.75 818
Fewer operational problems are associated with aerobic digestion than with anaerobic
digestion. Hence, less laboratory control and daily maintenance are required. Also, the
dangers of gas explosions are eliminated because the only gaseous by-products of aerobic
stabilization are carbon dioxide and water vapor.
11.2.1 Process Considerations
Aerobic digestion is accomplished by aerating waste sludge until it is stable and relatively
nuisance free. In the aerobic digestion of waste sludges, two different forms of oxidation
take place, as shown in the following two reactions. First, a portion of the organic substrate
in the wastewater sludge is oxidized and the remainder is converted to cell mass. Second,
the cell mass produced or present is oxidized until only a relatively inert fraction remains.
organic matter + 02 - — - > cellular matter + CC>2 +
cellular matter + CH — — - — - - > digested sludge + COo + HoO
* destruction ^ z
Once the organic substrate is removed from the system, the biological cells must begin
to use their own stored cellular material and dead cells as food. This self-oxidation of
cell material (endogenous respiration) reduces the amount of cellular material remaining.
Approximately 1 5 days of detention time are required to stabilize waste biological sludges
and to reduce the volatile suspended solids (VSS) by 40 to 60 percent (28). The oxygen
11-22
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requirements for the aerobic digestion process, exclusive of nitrification requirements, are
in the range of 3 to 4 mg/l/hr./1,000 mg/1 MLSS under aeration for the endogenous
respiration phase. This is less than the requirements for the oxidation of raw organic matter.
Both pH and alkalinity are reduced in a properly functioning aerobic digester when
nitrification takes place. Nitrification lowers pH according to the following reaction:
NH4+ + 1.5 O2
bacteria
NO2
2H
H2O
The second step of nitrification is as follows:
bacteria
N02 - + 0.5 02
N03
Theoretically, 7.1 Ibs. of CaCC>3 alkalinity are destroyed per Ib. of ammonia nitrogen
oxidized, since the two protons released neutralize one mole of CaCO3 according to the
following reaction:
2H+ + CaCO3
H2CC>3 + Ca
The aerobic digester operational data shown in Table 11-7 indicate the relation between
ammonia, nitrite, and nitrate nitrogen as a function of detention time (29). Table 11-7
also indicates that, in the normal temperature ranges of 15° to 35°C, an increase in
temperature increases the rate of VSS reduction.
Table 11-7
Aerobic Digestion Operational Data
Detention
Time
days
5
10
30
60
5
10
15
30
60
5
10
Temperature
°C
15
15
15
15
20
20
20
20
20
35
35
VSS
Reduction
percent
pH Alkalinity
mg/1
21
32
40.5
46
24
41
43
44
46
26
45
7.6
7.6
6.6
4.6
7.6
7.6
7.8
5.4
5.1
7.9
8.0
510
380
81
23
590
390
560
31
35
630
540
NH3-N
mg/1
54
3.2
4.0
38
54
4.9
7.0
28
7.0
14
10.0
NO2-N
mg/1
Trace
1.28
0.36
0.23
Trace
0.59
2.27
0.19
0.51
0,18
0.08
NO3-N
mg/1
None
64
170
835
None
60
29
275
700
None
None
Source: Jaworski (29)
11-23
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There are two methods of operating aerobic digesters. One is on a continuous basis, and
the other is on a modified or intermittent batch basis. Ritter (30) has reported that it
is good practice to stop wasting sludge into the digester for a period of up to 5 days
prior to making a sludge withdrawal when using an intermittent batch operation.
Continuous digester operation requires provision for continuously decanting the
supernatant, but offers the advantages of equalizing air requirements and providing a slow
continuous supernatant return.
Published data on phosphate removal in aerobic digesters is extremely limited. Recent
testing in Pennsylvania indicated that orthophosphate removal was generally negligible (30).
The aerobic digester was operated on an intermittent basis with the supernatant periodically
decanted. The operating data revealed that, on days when the digester supernatant was
decanted, the effluent concentrations of orthophosphate exceeded the influent
concentration by as much as 200 percent (43 mg/1 compared to 14 mg/1). This was
explained by the fact that when aeration is in progress, phosphate is adsorbed by sludge
cells. Conversely, when aeration is terminated, the phosphate is released by sludge cells.
If low effluent phosphate concentrations are required from aerobic digesters, then
automatic supernatant decanting should be used without stopping the aeration, as in the
continuous operation described above.
It has been shown that the detention time in an aerobic digester treating waste sludge
from a contact stabilization operation should be 15 days, to obtain satisfactory thickening
and dewatering (26). However, it has also been reported that satisfactory digestion of
waste trickling filter sludge does not occur until after 30 days of digestion (31). These
reports illustrate the need for running a small (55-gallon) aerobic digestion pilot plant
on an existing wastewater sludge to help evaluate aerobic digestion characteristics prior
to installing a plant-scale unit.
11.2.2 Design Basis
In designing an aerobic digestion system, care must be taken to see that the characteristics
of the sludge to be digested are fully identified. As previously mentioned, this can best
be done with pilot plant studies. Through such studies, the stabilization time, oxygen
requirements, and volatile suspended solids reduction can be determined. A procedure for
analyzing aerobic digestion kinetics has been reported by Reynolds (32).
Table 11-8 contains a summary of parameters used in the design of aerobic digestion
units for municipal wastewater sludges. As previously discussed, when phosphorus removal
is a consideration, continuous operation may be necessary. If phosphorus removal is not
a design criterion, then the digester may be operated intermittently. When considering
aeration requirements, recognition must be given to that needed for oxidation of the
ammonia present.
11-24
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Table 11-8
Aerobic Digestion Design Parameters
Parameter
Detention Time, days
Air Requirements
diffuser system, cfm/1,000 cu.ft.
cfm/l,000cu.ft.
mechanical system, gp/1,000 cu.ft.
Value
15-20
20-25
20-35!
>902
1.0-1.25
mg O2/l/hr./l,000 mg/1 MLSS
Minimum Dissolved Oxygen, mg/1
Temperature, °C
3.0
1.0
Volatile Solids Reduction, percent
Tank Design
40-50
Power Costs
$/yr./lb. BOD removed
$/yr./capita
2.18
0.37
Remarks
Waste Activated Sludge Alone
Primary + Waste Activated Sludge
Enough to keep the solids in suspension
and maintain a D.O. between 1-2 mg/1.
This level is governed by mixing
requirements. Most mechanical aerators
in aerobic digesters require bottom
mixers for solids concentration greater
than 8,000 mg/1, especially if deep
tanks (>12 feet) are used.
Reference
If sludge temperatures are lower than
15°C, additional detention time should
be provided so that stabilization will
occur at the lower biological reaction
rates.
Aerobic digestion tanks are open and
generally require no special heat transfer
equipment or insulation. For small
treatment systems (0.1 mgd), the tank
design should be flexible enough so that
the digester tank can also act as a sludge
thickening unit. If thickening is to be
utilized in the aeration tank, sock-type
diffusers should be used to minimize
clogging.
These cost data are based upon three
operational plants in Pennsylvania.
9,31
9
26
30
30
1 Waste activated sludge alone.
^Primary and waste activated sludge.
11-25
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11.2.3 Use of Aerobic Digestion for Upgrading Sludge Handling Facilities
11.2.3.1 Use of Existing Facilities
As an upgrading technique, aerobic digestion can be carried out in existing unused tankage,
such as old Imhoff tanks or old clarifiers. If unusually shaped basins are used, attention
should be placed on ensuring that complete mixing will be achieved and that dead spots
will be prevented. Potential dead spots can be filled and covered with concrete. Air-diffusion
systems are more easily adapted to unusual basin shapes than are surface aerators.
11.2.3.2 Supplemental Aerobic Digestion
Aerobic digestion can be used in conjunction with existing anaerobic digesters. In Monroe,
Wisconsin, and Corpus Christi, Texas, it has been found best to digest the primary sludge
anaerobically and the waste biological sludge aerobically (33). The advantage of this
segregation is that the primary sludge is not diluted by the waste biological sludge and
that the anaerobically digested primary sludge subsequently filters better on a vacuum
filter when it does not contain the waste biological sludge. Both digestion systems produce
a stabilized nuisance-free sludge.
11.2.3.3 Conversion of Anaerobic Digesters to Aerobic Digestion
If existing anaerobic digesters are overloaded and for some reason cannot be upgraded
as described in Section 11.1, they can be converted to aerobic digesters. Aerobic digesters
yield similar volatile suspended solids (VSS) reductions and are relatively odor free. Hence,
this conversion may be applicable to small overloaded plants in residential areas. Aerobic
digestion will usually require an increase in the blower capacity of the air supply system,
which would increase the yearly operating cost. However, these increased costs could be
offset by savings in maintenance requirements. An alternative method of supplying the
additional air is by using mechanical surface aeration where tank geometry permits.
If an existing anaerobic digester is converted to operate as an aerobic digester, the cover
should be removed. Experience at one midwestern city indicated that the combination
of an air diffusion system and a covered digester increased the air temperature inside
the digester to 135°F and the sludge temperature to 97°F (34). At these high temperatures,
objectionable odors were produced, and the addition of air caused the release of these
odors to the surrounding residential area. These high temperatures, in turn, reduced the
ability of the air supply to maintain the desired dissolved oxygen levels. An increase in
air supply to the system was then required, which resulted in a subsequently higher liquid
temperature.
11-26
-------
11.2.4 Process Designs and Cost Estimates
Two examples are presented to illustrate upgrading of sludge handling facilities using aerobic
digestion.
11.2.4.1 Example A
As a result of upgrading a trickling filter plant from 0.66 mgd to 0.88 mgd capacity,
an existing two-stage anaerobic digestion system experienced operational problems. Original
design data for the plant are listed in Table 11-9. Excess primary clarifier capacity was
available due to an original conservative clarifier design. The excess capacity could be
advantageously used as an aerobic digestion basin. The aerobic digestion of a portion of
the combined sludge permitted a decreased loading to the anaerobic digestion system,
resulting in a more stable operation. Modifications used for this upgrading are shown in
Figure 11-7, and the upgraded design data are presented in Table 11-9.
Table 11-9
Upgrading Sludge Handling Facilities at a
Trickling Filter Plant Using Aerobic Digestion - Example A
Description Original Design Upgraded Plant
Plant Design Capacity 0.66 0.88
Primary Clarifier (3 units)
Overflow Rate, gpd/sq.ft. 273J 6002
Anaerobic Digester
Detention Time, days 16.5 21.8
Volatile Solids Loading,
Ib. VSS/cu.ft./day 0.1063 0.0803
Sludge Solids, Ibs./day 1,125 850
Sludge Volume, gpd 3,370 (4%) 2,545 (4%)
Aerobic Digester (1 unit)
Detention Time, days - 30
Volatile Solids Loading,
Ib. VSS/cu.ft./day - 0.0583
Air Requirements, cfm - 500^
Sludge Solids, Ibs./day - 650
Sludge Volume, gpd - 1,950 (4%)
'Based on 3 units including recycle.
2fiased on 2 units including recycle.
3Based on 70% VSS in sludge added to digester.
464cfm/l,000cu.ft.
11-27
-------
FIGURE 11-7
UPGRADING BY USE OF SUPPLEMENTAL AEROBIC DIGESTION
EXAMPLE A
TRICKLING FILTER
RECYCLE SLUDGE
f
RAW „ ,.I ,
HtSTEWATER WET
* WELL j
T t
l —
l
1, 2
SLUDGE
WELL
—
l
I
ANAEROBIC
DIGESTER
*
DDIM1DV CFTTI 1 II P TANK
PRIMARY SETTLING TANK
AEROBIC DIGESTER
(FORMERLY PRIMARY SETTLING TANK)
I
AIR
TO
TRICKLING
FILTER
j '
TO
SLUDGE
* DE»ATERING
FACILITY
LEGEND
»ASTEIATER
SLUDGE
SUPERNATANT
11-28
-------
The capital costs for upgrading the digestion facilities were estimated at $32,000^
(ENR 1,500).
11.2.4.2 Example B
Due to continued operational problems with an existing anaerobic digestion facility, a
community has decided to convert from anaerobic to aerobic digestion. The waste activated
sludge from a 4-mgd activated sludge plant is settled along with raw wastewater in the
primary clarifiers and pumped directly to the digestion system at an average solids
concentration of 3 percent.
In the upgraded system, the two aerobic digesters are to be operated continuously at
a total detention time of 37.4 days. This detention time is in excess of the 20 to 25 days
required and will allow the aerobic digesters to handle increased solids loadings in the
future. Due to the continuous operation of the digester, there will be no supernatant
disposal problem associated with the digester operation. A summary of the upgraded design
data is presented in Table 11-10.
Table 11-10
Aerobic Digester Upgraded Design Parameters - Example B
Description Value
Plant Design Flow, mgd 4
Number of Digesters 2
Digester Volume (each), cu.ft. 75,000
Total Solids Added to Digesters, Ibs./day 7,500
Volatile Solids Added to Digesters
(70 percent), Ibs./day , 5,250
Sludge Volume (3 percent), gpd 30,000
Digester VSS Loading (each), Ib. VSS/cu.ft./day 0.035
Hydraulic Detention Time (total), days 37.4
Aeration Requirements (each), cfm 4,500
1 (60 cfm/1,000 cu.ft.)
Both tanks are provided with diffused air equipment to supply the required amount of
air. For this conversion, the digester covers were removed to ensure that the heat produced
by the air diffusion system was dissipated. A simplified flow diagram is shown in
Figure 11-8.
These costs do not include a contingency for engineering design, bonding, and construction
supervision.
11-29
-------
FIGURE 11-8
CONVERSION OF ANAEROBIC TO AEROBIC DIGESTION
EXAMPLE B
DIGESTER I
(EXISTING STRUCTURE)
AIR HEADER
1 T
DIGESTER 2
(EXISTING STRUCTURE)
BLOWERS
COMBINED PRIMARY AND
WASTE BIOLOGICAL SLUDGE*"
REMOVE
COVER
DIGESTED SLUDGE
TO DENATERING
FACILITIES
EXISTING SLUDGE
PUMPS
-------
The estimated total capital costs for the upgrading are $175,000* (ENR 1500), broken
down as follows:
Air System $135,000
Renovation to Existing Tank 40,000
TOTAL $175,000
11.3 References
1. McCarty, P.L., Anaerobic Waste Treatment Fundamentals. Public Works 95, No. 9,
pp. 107-112 (1964).
2. Speece, R.L., and McCarty, P.L., Nutrient Requirements and Biological Solids
Accumulation in Anaerobic Digestion. Proceedings of the International Conference
on Water Pollution Research, Pergamon Press, 1962.
3. Kugelman, I.J., and Chin, K.K., Toxicity Synergism and Antagonism in Anaerobic
Waste Treatment Processes. Presented before Division or Air, Water and Waste
Chemistry, American Chemical Society, Houston, Texas, February, 1970.
4. Kugelman, I.J., and McCarty, P.L., Cation Toxicity and Stimulation in Anaerobic
Waste Treatment. Journal Water Pollution Control Federation, 37, No. 1, pp. 97-115
(1965).
5. Lawrence, A.W., Kugelman, I.J., and McCarty, P.L., Ion Effects in Anaerobic
Digestion. Technical Report No. 33, Department of Civil Engineering, Stanford
University, March, 1964.
6. Lawrence, A.W., and McCarty, P.L., Unified Basis for Biological Treatment Design
and Operation. Journal of the Sanitary Engineering Division, ASCE, 96, No. 3,
pp. 757-778 (1970).
7. Lawrence, A.W., and McCarty, P.L., Kinetics of Methane Fermentation in Anaerobic
Treatment. Journal Water Pollution Control Federation, 41, No. 2, Part 2,
pp. R1-R17 (1969).
8. Torpey, W.N., Loading to Failure of a Pilot High Rate Digester. Sewage and Industrial
Wastes, 27, No. 2, pp. 121-133 (1955).
These costs do not include a contingency for engineering design, bonding, or
construction supervision.
11-31
-------
9. Burd, R.S., A Study of Sludge Handling and Disposal. Federal Water Pollution Control
Administration, Publication WP-20-4, May, 1968.
10. Sewage Treatment Plant Design. Water Pollution Control Federation Manual of
Practice No. 8, Washington, D.C., 1959.
11. Lawrence, A.W., and McCarty, P.L., The Role of Sulfide in Preventing Heavy Metal
Toxicity in Anaerobic Treatment. Journal Water Pollution Control Federation, 37,
No. 3, pp. 392^09 (1965).
12. Sawyer, C., Anaerobic Units. Proceedings of a Symposium on Advances in Sewage
Treatment Design, Metropolitan Section-Sanitary Engineering Division, ASCE, New
York, 1961.
13. Garrison, W.E., et al, Gas Recirculation - Natural, Artificial. Water and Wastes
Engineering, 1, No. 5, pp. 8-9 (1964).
14. Sironen, E.R., and Lee, D., Sludge Density Control with Sonar. Journal Water
Pollution Control Federation, 42, No. 2, pp. 298-301 (1970).
15. Anaerobic Sludge Digestion. Water Pollution Control Federation Manual of
Practice No. 16, Washington, D.C., 1968.
16. Meyers, H.V., Improved Digester Performance through Mixing. Journal Water Pollution
Control Federation, 33, No. 11, pp. 1,185-1,187 (1961).
17. Langford, L.L., P.F.T. - Pearth Multipoint Gas Recirculation. Water and Sewage
Works, 108, No. 10, pp. 382-383 (1962).
18. Lynam, Bart, et al, Start-Up and Operation of Two High-Rate Digestion Systems.
Journal Water Pollution Control Federation, 39, No. 4, pp. 518-535 (1967).
19. Zablatzky, H., and Peterson, S., Anaerobic Digestion Failures. Journal Water Pollution
Control Federation, 40, No. 4, pp. 581-585 (1968).
20. Schroepfer, G.J., et al, The Anaerobic Contact Process as Applied to Packing House
Wastes. Sewage and Industrial Wastes, 27, No. 4, pp. 460-486 (1955).
21. Dague, R., Application of Digestion Theory to Digester Control. Journal Water
Pollution Control Federation, 40, No. 12, pp. 2,021-2,031 (1968).
22. Torpey, W., and Melbinger, N., Reduction of Digested Sludge Volume by Controlled
Recirculation. Journal Water Pollution Control Federation, 39, No. 9,
pp. 1,464-1,474 (1967).
11-32
-------
23. Voshel, D., Gas Recirculation and CRP Operation. Wastes Engineering, 34, No. 9,
pp. 452455 (1963).
24. Bennett, G., Development of a Pilot Plant to Demonstrate Removal of Carbonaceous,
Nitrogenous and Phosphorus Materials from Anaerobic Digester Supernatant and
Related Process Streams. Federal Water Quality Administration, Program
Number 17010 FKA, May, 1970.
25. Voshel, D., Sludge Handling at Grand Rapids, Michigan Wastewater Treatment Plant.
Journal Water Pollution Control Federation, 38, No. 9, pp. 1,506-1,517 (1966).
26. Smith, A.R., Aerobic Digestion Gains Favor. Water and Wastes Engineering, 8, No. 2,
pp. 24-25 (1971).
27. Walker, J.D., Aerobic Digestion of Waste Activated Sludge. Presented at the Ohio
Water Pollution Control Conference, Cleveland, Ohio, June 15, 1967.
28. Barnhart, E., Application of Aerobic Digestion to Industrial Waste Treatment.
Proceedings-16th Industrial Waste Conference, Purdue University, pp. 612-618 (1961).
29. Jaworski, N., et al, Aerobic Sludge Digestion. Presented at the Conference on
Biological Waste Treatment, Manhattan College, N.Y., April 20-22, 1960.
30. Ritter, L., Design and Operating Experiences Using Diffused Aeration for Sludge
Digestion. Journal Water Pollution Control Federation, 42, No. 10, pp. 1,782-1,791
(1970).
31. Pentz, H., Experimental Aerobic Digester Treating Sludge from Standard Rate
Trickling Filter Plant. Presented at the Pennsylvania State Water Pollution Control
Federation Conference, August, 1969.
32. Reynolds, T., Aerobic Digestion of Waste Activated Sludge. Water and Sewage Works,
114, No. 22, pp. 37-42 (1967).
33. Dreier, D.E., Aerobic Digestion of Solids. Proceedings-18th Industrial Waste
Conference, Purdue University, pp. 123-139 (1963).
34. Private communication with C. L. Swanson, EPA, Cincinnati, Ohio, November 12,
1970.
11-33
-------
CHAPTER 12
SLUDGE DEWATERING
12.1 Vacuum Filtration
In 1967, there were slightly more than 1,500 vacuum filter installations in the United
States in wastewater treatment service (1). The majority of these installations are in the
larger municipal plants, where scarcity of available land often places sludge drying beds
in an unfavorable economical position. The high degree of operator skill required for
efficient vacuum filter operation is more likely to be available at larger plants than at
smaller plants. Another factor which discourages extensive use of vacuum filters for sludge
dewatering at small plants is the requirement for sludge conditioning prior to filtration.
Small plants often do not have the necessary storage and handling facilities to purchase
conditioning chemicals in economical bulk quantities.
12.1.1 Process Considerations
Process operating considerations for vacuum filtration include:
1. Control of feed solids concentration.
2. Chemical requirements for sludge conditioning.
3. Sludge mixing and flocculation.
4. Drum speed and drum submergence.
5. Filter fabric characteristics.
Each of these parameters affects the filter yield, economy of operation, and filter cake
characteristics. The feed solids concentration can be controlled by pre-thickening the sludge.
In general, the higher the feed solids concentration, the higher the filtration rate and
the filter yield. The relationship is not linear, however, as was noted at one installation
where doubling the feed solids concentration more than doubled the filtration rate (2).
Few, if any, raw or digested wastewater sludges can be successfully dewatered without
some form of chemical conditioning using ferric chloride, lime, and/or polyelectrolytes.
Proper sludge conditioning requires a determination of optimum chemical dosages.
Experience and careful laboratory monitoring of the sludge characteristics are key factors
in maintaining the proper chemical proportions and concentrations.
Optimum sludge mixing and flocculation under varying conditions require that sludge
conditioning tanks be provided with variable-speed mixer drives, removable weirs to vary
the sludge detention time, and multiple points of chemical application as indicated in
Figure 12-1. The sludge slurry must be agitated sufficiently to maintain uniformity;
however, excessive agitation should be avoided so that the conditioned slurry particles
are not sheared and broken up.
12-1
-------
FIGURE 12-1
TYPICAL VACUUM FILTER FLOW DIAGRAM
N)
tb
COAGULANT
POLYMER
FLOW CONTROL
SLUDGE
c
c
)
J
p
r i
/
14
G3
9. 1
V
/-
y
t
a
V
S
SLUDGE CONDITIONING TANKS
FILTRATE RETURN
TO PLANT
AIR TO
ATMOSPHERE
SILENCER
WATER TO PLANT
VACUUM
PUMP
WASHINGS
RETURN
TO PLANT
TO FILTER
CLOTH WASH
FROM WATER SOURCE
-------
Drum speed and drum submergence are important factors in the operation of vacuum
filters since they affect filter yield and filter cake moisture. Increasing the drum
submergence generally results in increased filter yield, but produces a filter cake with
higher moisture content. Decreasing the drum speed, i.e., increasing the cycle time, has
the opposite effect of decreasing the filter yield, but produces a cake with lower moisture
content.
Much information is available from the various manufacturers of vacuum filters and filter
media on the selection of a proper medium. The range of filter fabrics available for
metal-belt or coil-type filters is limited when compared with the great number of cloths
available for use with conventional drum or belt-type filters. Laboratory experimentation,
using the Filter Leaf Test, provides information on filter farbic blinding, cake discharge
characteristics, and filtrate quality, which is of use in selecting the best fabric for a given
sludge.
12.1.2 Evaluation of the Vacuum Filtration Process
Experience has shown that there are considerable variations in filtration rate, not only
between different sludge types, but also between the same types of sludges at different
locations. The discrepancies in filter test results at different plants are usually related to
variations in feed solids concentrations, particle size distributions, industrial waste
components in the raw wastewater, etc. Obviously, then, it is of major concern to have
a laboratory technique which can accurately predict the performance of a full-scale vacuum
filter prior to its installation.
The Buchner Funnel Test and the Filter Leaf Test are commonly used in laboratory testing
programs for estimating the filterability of sludges. When the amount of representative
sludge is limited (less than 10 liters), it is advisable first to perform the Buchner Funnel
Test to determine optimal chemical dosage and sludge filtration characteristics. The Filter
Leaf Test can then be run at the optimum condition to determine filter yield. If a large
amount of sludge is available, the Buchner Funnel Test can be eliminated and the Filter
Leaf Test run instead.
The main purpose of the Buchner Funnel Test is to evaluate the optimum chemical
requirements based on a comparison of the specific resistance of chemically treated sludge
with that of untreated sludge. An approximate filter yield can also be calculated from
the Buchner Funnel Test. Basically, the Buchner Funnel Test consists of filtering 100 ml
of sludge, either raw or conditioned, through filter paper under a vacuum of
20 to 25 inches of mercury. The volume of filtrate (V, in ml) with time is noted and
plotted against elapsed time/volume (t/V in sec./ml) to obtain the slope of the resulting
line. Using the above information, the specific resistance of the sludge is calculated from
the following equation:
12-3
-------
r= 6.91 x 106
where:
r = Specific resistance, sec2/gm
b = Slope of plot (V vs. t/V), sec/ml2
A = Area of filter, sq.cm.
P = Filtration vacuum, psig
H = Absolute viscosity of filtrate, centipoise
c = Initial suspended solids concentration, mg/ml
The dimensions of the variables in the above equation are in units typically measured
in the laboratory. The conversion constant, 6.91 x 10 , reduces the variables to units
which are dimensionally consistent. The specific resistance as calculated by the above
equation would be expressed as sec^/gm.
The approximate filter yield can be estimated from the specific resistance data using the
following relationship (3):
L= 0.0357
/mPCi(1
\ 0R£
\Ci-Cf /
where:
L = Filter yield, Ibs./sq.ft./hr.
Cj and Cf = Initial and final moisture content of the sludge, percent
m = Percentage of time for which vacuum acts during cycle
B = Time for one drum revolution, minutes (usually between 1.5 and 5 minutes)
ju = Absolute viscosity of filtrate, centipoise
R = r x 10-"7', gm/sec2
r = Specific resistance, sec2/gm
P = Filtration vacuum, psig
The Filter Leaf Test techniques are simple, and the test can,be.:run with minimum effort.
With careful laboratory techniques, results will be closely indicative of full-scale vacuum
filter operation and need only be corrected by a design factor used to compensate for
partial fabric blinding over a long period of operation. A scale-up design factor of 0.9
is typically used. The advantage of the Filter Leaf Test'is that the filter yield is measured
and not merely calculated using an empirical equation.
The Filter Leaf Test is usually performed on a;0.'l sq.ft. filter leaf. Different filter fabrics
should be evaluated at a specified vacuum pressure. Varying doses and types of chemicals
should also be tested to establish chemical' conditioning requirements.
The main objective of the Filter Leaf Test is to evaluate the effect of different fabrics,
fabric forms, and drying times on filter yield!.The;basiccsteps in performing a Filter Leaf
Test consist of the following:
12-4^
-------
1. Submerge the filter leaf in the sludge slurry and apply vacuum for a fixed form
time.
2. Remove the filter leaf from the sludge slurry to allow the cake to dry for a
fixed drying time.
3. Remove the cake from the filter leaf and measure the weight and moisture
content of the cake.
4. Measure the filtrate suspended solids to determine the filtrate characteristics.
The filter yield is then calculated using the following equation:
T _ dry sludge weight (gm) x number of filtration cycles/hr.
453.6 x area of test filter leaf (sq.ft.)
where: L = Filter yield in Ibs./sq.ft./hr.
12.1.3 Upgrading Existing Vacuum Filters
The need to upgrade an existing vacuum filter is usually due to an increase in the solids
loading. Under these circumstances, existing vacuum filters are required to dewater at an
increased filtration rate; otherwise, additional filtration capacity must be provided. The
filtration rate of an existing vacuum filter can sometimes be increased by careful attention
to operating conditions and judicious use of polyelectrolytes, where applicable, to improve
sludge conditioning.
Most older filter installations were designed to use inorganic chemicals, such as ferric
chloride and lime, as sludge conditioners. Within the last five years, organic polyelectrolytes
have begun to replace inorganic chemicals as sludge conditioners, and these polymers offer
an attractive advantage in more economical storage, handling, and feeding equipment.
Polymers are also less corrosive and are frequently less expensive than inorganic chemicals
within normal dosage ranges.
The yield obtained when using polymers in municipal sludge conditioning is generally higher
than when using inorganic chemical conditioners. This statement is supported by studies
conducted by the Dow Chemical Company (4) in which polyelectrolytes and inorganic
conditioning chemicals were compared on the same sludges. The results shown in
Table 12-1 indicate that polymer conditioning not only increases filter yield, but
significantly reduces chemical requirements for conditioning sludge.
The use of polymers in improving the operation of vacuum filters has been practiced
at treatment plants in Bay City, Michigan (5), and Kansas City, Missouri (6), among others.
The Bay City Wastewater Treatment Plant provides primary treatment for 7 mgd and
produces approximately 450 tons of dry solids/year. Until 1961, raw primary sludge was
12-5
-------
Table 12-1
Vacuum Filtration Results Comparing Inorganic Chemicals
with Purifloc C-31 on Municipal Sludge
to
ON
Location
1. Municipal STP
Municipal STP
2. Municipal STP
Municipal STP
3. Municipal STP
Municipal STP
4. Municipal STP
Municipal STP
5. Atlanta, Clayton
Atlanta, Clayton
6. Municipal STP
Municipal STP
7. Municipal STP
Municipal STP
8. Municipal STP
Municipal STP
9. Municipal STP
Municipal STP
10. Atlanta • South River
1 1 . Municipal STP
Municipal STP
12. Municipal STP
Municipal STP
Type of Sludee
Raw primary
Raw primary
Raw primary
Raw primary
Raw primary
Raw primary
Raw primary
Raw primary
Digested primary
Digested primary
Digested primary
Digested primary
Elutriated/digested/primary
Elutriated/digested/primary
Elutriated/digested/primary
Elutriated/digested/primary
Elutriated/digested/primary
Elutriated/digested/primary
Elutriated/digested
primary and secondary
Digested primary and secondary
Digested primary and secondary
Elutriated/digested/primary
and secondary
Elutriated/digested/primary
Tvpe of Filter
K-S
K-S
K-S
K-S
Eimco drum
Eimco drum
Eimco drum
Eimco drum
D-O drum
D-Odrum
drum
drum
D-O drum
D-O drum
D-O drum
D-O drum
Eimco drum
Eimco drum
D-O drum
Eimco
Eimco
DO drum
D-O drum
Filler Media
coil
coil
coil
coil
open synthetic
open synthetic
open synthetic
open synthetic
long napped dacron
long napped dacron
44 x 44 saran
44 x 44 saran
napped polyester
napped polyester
napped polyester
napped polyester
napped polyester
napped polyester
long napped dacron
synthetic
synthetic
napped dacron
napped dacron
Chemical
FeCl3
Lime
C-31
Fe2(S04)3
Lime
C-31
FeCl3
Lime
C-31
FeCI3
Lime
C-31
Fe7(SO4)3
C-3~l
FeCI3
Lime
C-31
FeCI3
C-31
FeCl3
C-31
Fe2(S04)3
C-31
Lime
Fe2(SO4)3
C-31
FeCI3
Lime
C-31
Fe2(S04)3
Lime
C-31
Dosage
Ibs./ton dry solids
162.4
166.8
14.0
60
106
8.4
80.0
280.0
18.0
78.0
390.0
20.0
(S6.06/T)
(S6.S8/T)
66.0
206.0
17.0
56.8
10.2
100.0
8.0
108.0
9.0
(S8.90/T)
(S8.79/T)
6IO.O
-------
conditioned with ferric chloride (FeClg) and either kiln-dried pebble lime or spent carbide
(calcium hydroxide formed as the result of chemical action in making acetylene). Bay
City's conditioned sludge is dewatered on vacuum filters having an effective area of
150 sq.ft. The vacuum filter cake is incinerated. In 1961, polyelectrolytes were tried as
sludge conditioners in an attempt to improve filter yield. Results of filter operation for
1959-1964 using FeCl^, lime, and polyelectrolytes (5) are presented in Table 12-2.
These results clearly indicate that use of polyelectrolytes increased the filter yield and
significantly reduced vacuum filtration operation time.
The cost of chemicals for sludge conditioning was found to be $9.93/ton of dry solids
using FeCl^ and kiln dried lime, $6.85/ton of dry solids when using FeCl^ and carbide
lime, and $7.00/ton of dry solids when using polyelectrolytes.
The following advantages were realized at Bay City when polyelectrolytes were used for
sludge conditioning:
1. Equipment and floor space savings.
2. Improved housekeeping.
3. Improved safety.
4. Reduced quantities of ash, with a large reduction in ash handling and storage.
5. Reduction in operating time, with resulting savings in operating and maintenance
costs.
In Kansas City, 115 mgd of wastewater are treated in two plants, and the resulting sludge
is pumped to a central location for sludge dewatering (6). Polyelectrolytes were selected
for sludge conditioning in the dewatering operation. Dewatered sludge is incinerated. The
specifications for sludge dewatering included: a filter capacity of 2,300 Ibs./hr. for each
of eight filters, a filtration rate of 6.2 Ibs./sq.ft./hr. at minimum solids concentration,
and a maximum moisture content of 75 percent in the cake. The cost of polymer
conditioning was initially estimated at $4.30/ton of dry solids. During the first six months
of operation, however, the actual cost of polymer was $10.61/ton dry solids. This cost
was reduced by $2.20/ton of dry solids during the subsequent four-year period of operation.
Despite the higher than estimated cost for sludge conditioning, the experiences of the
Kansas City Treatment Plant with polyelectrolytes have been satisfactory, since both the
filter yield and cake moisture content specifications are routinely met.
One area where polyelectrolytes have not been as effective as inorganic chemical
conditioning is in the vacuum filtration of waste activated sludge directly from the
underflow of a secondary clarifier (without thickening). Preliminary information from an
oxygen aeration study conducted at Batavia, New York, indicates that the optimum
conditioning was ferric chloride at a dosage of 200 Ibs./ton of dry solids (7). The waste
activated sludge concentration from the oxygen aeration system varied between
12-7
-------
Table 12-2
A Comparison Between Lime/Ferric Chloride and Polyelectrolytes
for Conditioning Raw Primary Sludge
to
00
Year
1959
1960
1961-62
1 962-63
1963-64
Dry
Solids
tons
461
580
424
415
437
Solids
in
Feed
Sludge
percent
11.2
11.2
10.9
10.7
10.2
Amount of Substance Added Solids
Filter
Yield Lime FeCh
Ibs./sq.ft./hr.
3.1 162.000 31.000
3.1 225.000 44.000
5.3 — —
5.5 — - —
6.3
pounds in
C-31 C-32 C-149 A-21 Cake
percent
,in i
___ « -.._ HU. I
._ __ __ _.. 39.0
— 5.5(>2 — " — 35.9
10.300 — — -- 34.5
Total of ull three
polymers
7.671 34.6
Solids
Recovery
percent
64.1
62.1
75.6
73.7
75.9
Operation
Time
hours
2,125
2,420
1,11')
1,114
1,301
Source:
Sherbick (5).
-------
2 and 3 percent. Filter yield increased from 1 to 5 Ibs./sq.ft./hr. as the cycle time was
decreased from 6 to 2 minutes/revolution. The moisture content varied between
75 and 85 percent, with .the higher moisture content generally corresponding with lower
cycle times. Likewise, in the City of Milwaukee, unthickened (diffused air) waste activated
sludge is vacuum filtered using ferric chloride (8). The filter yield ranges between
1 and 3 Ibs./sq.ft./hr. for an average cycle time of 3.5 to 4.0 minutes/revolution (cake
moisture content 80 to 85 percent).
12.1.4 Process Designs and Cost Estimates
The following example will serve to illustrate design and cost considerations when upgrading
vacuum filter installations by converting from inorganic chemical to polyelectrolyte sludge
conditioning.
An existing vacuum filter installation annually conditions and filters 300 tons (dry basis)
of mixed digested primary and secondary sludge with a filter yield of 5 Ibs./sq.ft./hr.
In the past, sludge conditioning has been accomplished using 65 Ibs. of ferric chloride
and 200 Ibs. of lime per ton of dry solids. To reduce the costs involved in bulk chemical
handling and to increase filter yield, the applicability of a polyelectrolyte system for sludge
conditioning is investigated.
The optimum polyelectrolyte dosage is found to be 20 Ibs./ton of dry solids, added to
the digested sludge in a 1-percent solution. The polyelectrolyte addition results in a
subsequent filter yield of 8 Ibs./sq.ft./hr. This upgrading procedure enables the vacuum
filter to decrease its operating time by over 60 percent, thereby decreasing operational
and maintenance costs.
The capital cost for the polyelectrolyte application system is estimated at $6,000
(ENR Index 1500). This cost includes all required tankage, pumps, and mixers. In
actuality, this cost is probably high since some of the equipment could probably be salvaged
from the existing inorganic chemical addition system.
12-9
-------
12.2 Drying Beds
The dewa taring of digested sludge on drying beds has long been practiced in the United
States. Historically, sludge drying beds have been used for communities of many sizes,
as illustrated in the following table (9):
Population Size
Group
Less than 1,000
1,000 to 5,000
5,000 to 10,000
10,000 to 25,000
25,000 to 50,000
50,000 to 100,000
More than 100,000
TOTAL
Table 12-3
Distribution of Sludge-Drying Beds
by Population Size Groups
Total Number
of Plants
3,780
4,990
1,437
1,164
473
269
452
12,565
Number with
Sludge-
Drying Beds
1,237
2,759
857
659
217
121
196
6,046
Percentage
with
Drying Beds
33
55
59
57
46
45
43_
48
The popularity of drying beds for dewatering sludge is due to their operational flexibility,
simplicity, and low maintenance costs. Disadvantages include their large land requirement
and inability to dewater effectively during inclement weather.
12.2.1 Process Considerations
One of the difficulties in developing a rational design for sludge beds is the multitide
of variables which affect the drying rate of sludges when applied to sand beds. In practice,
it is difficult to isolate these variables and evaluate them quantitatively. Some of the more
important variables are (10):
1. Climate and atmospheric conditions.
a. Temperature
b. Humidity
c. Rainfall
d. Wind velocity
e. Barometric Pressure
f. Solar Radiation
2. Depth of sludge application.
3. Presence or absence of coagulants.
4. Sludge moisture content.
12-10
-------
5. Source and type of sludge.
6. Extent of sludge digestion.
7. Sludge age.
8. Sludge composition.
9. Sludge concentration when applied.
10. Sludge bed construction.
Notwithstanding the magnitude of the problems involved, some generalizations concerning
the applicability of these factors can be made. When possible, decisions regarding the
specific effect of any or all of these factors should be based on bench-scale testing.
Quon and Johnson (11) have indicated that well-digested sludge should be applied to drying
beds in depths of 6 to 9 inches, with 8 inches appearing to give optimum drying rates.
Sludge should be properly digested before being applied to the drying beds. Raw or poorly
digested sludge dewaters slowly on the drying beds and produces strong odors. Sludge
that has been overly digested exhibits high density which also impairs drainage. Aerobically
digested sludge usually has good dewatering characteristics and, when applied to sand drying
beds, drains well (2).
It has been widely accepted that under normal conditions, practically all of the drainage
of digested sludge occurs during the first three days following the filling of the drying
bed (1). After this initial period, it was felt that evaporation was largely responsible for
additional dewatering of the sludge. Recent studies indicate this is not the case. In an
extended study, it was found that the initial rate of drainage was small, but that it increased
with time (11). After approximately three days, the drainage rate increased and the sludge
surface dropped substantially. This phenomenon is explained by considering that air trapped
in the voids of the sand bed is not free to move and thus impedes the initial flow of
water through the filter. Eventually, this air is liberated, allowing a greater flow to pass
through the sand bed. After a period of maximum drainage, the drainage rate gradually
decreases due to the build-up of solids on the sand surface, which offers resistance to
further filtration. Once this point is reached, evaporation from the free water surface
accounts for further dewatering. Experiments in some installations have shown that
tile-drained sludge beds dry 25 percent faster than beds with an impervious bottom (11).
In certain areas with adverse climatic conditions, the use of glass-covered beds, while
expensive, has been found to increase the total output of dewatered sludge by 100 percent
(12). In many cases, this increase makes glass-covered beds considerably cheaper in the
long run. Adequate ventilation must be provided in constructing covered beds so that
maximum evaporation rates may be maintained.
12-11
-------
12.2.2 Design Basis
Present-day design practices are still based largely on comparisons with existing plants
in the area, or upon empirical recommendations of various regulatory agencies. The
following sludge drying bed area requirements are specified in the Ten-States Standards
for domestic wastewater treatment plants located in northern United States (13).
Table 12-4
Sludge-Drying Bed Area Requirements
Type of Sludge
Primary digested
Primary and humus digested
Primary and activated digested
, Primary and chemically precipitated digested
Open Beds
1.0 to 1.5
1.25 to 1.75
1.75 to 2.5
2.0 to 2.5
Area of Drying Beds
sq.ft./capita
Covered Beds
0.75 to 1.0
1.0 to 1.25
1.25 to 1.5
1.25 to 1.5
In the southern United States, reduced areas are often practical because of more favorable
climatic conditions.
12.2.3 Upgrading Existing Facilities
It is possible to upgrade an overloaded sludge drying bed by the following methods:
1. Improving the performance of upstream facilities, e.g., thickeners, digesters.
2. Adding chemicals to increase sludge dewatering.
3. Covering open beds wherever climatic conditions adversely affect performance.
Chemicals such as alum, ferric chloride, and, more recently, polyelectrolytes have been
used as flocculants to improve dewatering capacity of sludge drying beds. The use of
these chemicals increases the permissible annual sludge loadings to the drying beds by
increasing the number of sludge draws per year.
In general, the chemicals allow greater amounts of water to drain from the sludge, thereby
decreasing the amount of water to be removed through the slower evaporation process.
Bed loadings for chemically treated and untreated sludge should be evaluated by laboratory
and field testing to determine the effectiveness of chemical addition on sludge dewatering.
Buchner Funnel Tests can predict dewatering rates on drying beds in the same manner
as that predicted for vacuum filter performance (14). Care must be taken to avoid adding
excess amounts of chemicals, which might bind sand particles and lower dewatering rates.
12-12
-------
The available coagulants are not equally effective for sludge dewatering. Alum has been
used successfully at a dosage of 1 Ib. of alum per 100 gallons of digested sludge (1).
On the other hand, polyelectrolyte has been used at dosages as low as
0.05 Ib. per 100 gallons of digested sludge (10).
In many parts of the country, the common practice is to cover sludge beds to protect
them from rainfall and severe winter conditions. Recent work in northern Texas indicated
that, during the dry season, covers retarded the drying rate rather than accelerating it
(10). These data point to the fact that drying of sludge under covered conditions is not
necessarily advantageous when weather conditions are more favorable for natural drying.
12.2.4 Process Designs and Cost Estimates
An open drying bed at an existing activated sludge plant was originally designed based
on a population equivalent of 20,000 and a land requirement of 2 sq.ft./capita, and was
loaded at a rate of 10 Ibs. of dry solids/yr./sq.ft. As a result of upgrading secondary
treatment units, it was necessary to increase the loading to 15 Ibs./yr./sq.ft. to
accommodate increased sludge quantities. Two alternatives were available for upgrading
the existing drying beds. It was possible either to cover the beds and reduce the area
requirements to approximately 1.35 sq.ft./capita, or to add 1 Ib. of alum per 100 gallons
of digested sludge to decrease the drying time by approximately 50 percent.
Covering the drying beds was estimated to cost $200,000, or $2,000/ton dry solids/yr.
of increased cake yield (ENR Index 1500). The alum slurry feed system and flocculation
tank was estimated at $28,000, or $280/ton dry solids/yr. of increased cake yield.
Based on comparison of these capital cost estimates for upgrading sludge drying beds,
it would appear that chemical addition would be the most economic alternative provided
that the climatic conditions would not adversely affect its operation. In order to make
a definite conclusion, it would be necessary to compare yearly operating costs which would
include chemical costs.
12.3 Centrifugation
Centrifuges have been used for many years by various industries for clarifying liquids,
concentrating solids, separating immiscible liquids, and purifying oils. However, their use
in the wastewater field for sludge dewatering is not as widespread as is the use of vacuum
filters. Recent improvements in centrifuge design, efforts by the centrifuge industry to
enter the wastewater treatment field, and broader dissemination of centrifuge performance
data have encouraged increased use of centrifuges for thickening and dewatering of primary,
secondary, and combined wastewater sludges. Centrifuges have good potential for upgrading
overloaded solids handling facilities due to their flexibility in operation and lesser space
requirements compared to vacuum filters.
12-13
-------
Solid-Bowl
Continuous
6 to 60
1 to 200
*
1 to 1 5 tons/hr.
1,000 to 6,000
3,200 max.
5 to 250
Basket
Batch
12 to 60
100 max.
0.1 to 30
1 ,000 Ibs.-max.
2;500 max.
2,000 max.
100 max.
Disc
Continuous
8 to 30
10 to 300
0.1 to 10
10to3,000gal./hr.
4,500 to 10,000
12,000 max.
10 to 125
12.3.1 Types of Centrifuges
There are three general classifications of centrifuges that can be applied to sludge thickening
and dewatering: solid-bowl, disc, and basket centrifuges. These are illustrated in Figure 12-2
(15). The capabilities of these units in processing wastewater sludges are summarized in
Table 12-5 (16).
Table 12-5
Summary of Centrifuge Characteristics
Centrifuge Description
Description
Bowl diameter, in.
Flow rate, gpm
Solids in feed, percent
Solids Discharged
Speed, rpm
Centrifugal Force, G
Motor horsepower
*Any liquid or slurry which can be pumped.
Source: Townsend (16).
The most popular type of centrifuge today is the solid-bowl because of its dependable
performance and low maintenance requirements. The solid-bowl machine has a spinning
cylinder which causes particles to settle out along its inner wall; Solid-bowl centrifuges
are especially suited to dewatering primary wastewater sludge and mixtures of primary
and waste biological sludge. They are also able to dewater waste biological sludge alone,
but some form of polymer addition is required in order to operate at an economical
feed rate and to obtain solids concentrations above 5 or 6 percent. For most sludges,
to achieve solids recovery in the range of 80 to 95 percent with a solid-bowl centrifuge
requires the addition of polymers to the sludge.
The basket centrifuge is a tubular type centrifuge with a solid bowl and, therefore, is
similar to the solid-bowl centrifuge in that the solids settle out along the inner wall due
to centrifugal force. The solids are removed on an automated batch basis. Because of
the large bowl diameter, the basket centrifuge is operated at slower speeds. The centrifuge
can be operated on automatic cycle for programmed filling and skimming. Abrasion occurs
only with the skimming tool. Hence, for the most part, this is a low-speed, low-maintenance
unit.
The application of basket centrifuges to the wastewater treatment field is relatively new.
Field tests of this unit have been successful in thickening waste activated sludge and indicate
12-14
-------
FIGURE 12-2
VARIOUS CLASSIFICATIONS OF CENTRIFUGES (15)
GEAR BOX
r
DRIVE SHEAVE
FEED
LIQUID SOLID
DISCHARGE DISCHARGE
SOLID BOWL CENTRIFUGE
CAKE DISCHARGE *•
FEED
'CLARIFIED EFFLUENT
SOLID BOWL BASKET CENTRIFUGE
FEED
EFFLUENT DISCHARGE
SLUDGE DISCHARGE
DISC TYPE CENTRIFUGE
12-15
-------
that this unit may be increasingly utilized in small plants to improve sludge dewatering
operations. Concentrations of 9 to 10 percent solids can be produced, without the use
of polymers, with solids recoveries of 80 to 90 percent.
The disc centrifuge utilizes a vertical disc stack, with subsequent sludge discharge through
nozzles located around the periphery of the disc stack. The use of disc centrifuges for
thickening in Sioux Falls, S.D. (17) and Chicago, 111. (18) resulted in plugging of the
disc stack and nozzles. It has been found that these problems can be minimized by using
screens in the feed line to the centrifuge. Recently, it has been reported that disc centrifuges
can increase the solids concentrations of waste activated sludge from 0.5-1.0 percent to
a concentration of 5.0-6.0 percent without the use of polymers (17).
12.3.2 Process Considerations
Process variables for centrifugation are feed rate, sludge solids characteristics, feed
consistency, temperature, and chemical additives. Machine variables are bowl design, bowl
speed, pool volume, and conveyor speed (2). Major factors of importance in the product
sludge are cake dryness and solids recovery. To increase cake dryness, the following
guidelines are important (2):
1. Increase feed rate.
2. Decrease feed solids concentration.
3. Increase temperature.
4. Do not use flocculants.
5. Increase bowl speed.
6. Decrease pool volume.
7. Decrease conveyor speed.
Guidelines for increasing solids recovery are as follows (2):
1. Decrease feed rate.
2. Increase feed solids concentration.
3. Increase temperature.
4. Use flocculants.
5. Increase bowl speed.
6. Increase pool volume.
7. Decrease conveyor speed.
The above—mentioned guidelines indicate that most of the variables which improve cake
dryness tend to decrease the solids recovery. This is an important feature of centrifuge
operation. Therefore, operation of a centrifuge should be optimized to obtain the desired
balance between cake dryness and solids recovery.
12-16
-------
The following advantages are associated with the use of a centrifuge:
1. Capital cost is low in comparison with other mechanical equipment.
2. Operating costs are moderate, provided flocculants are not required.
3. The unit is totally enclosed so that odors are minimized.
4. The unit is simple and will fit in a small space.
5. Chemical conditioning of the sludge is often not required.
6. The unit is flexible in that it can handle a wide variety of solids
and can function as a thickening as well as a dewatering device.
7. Little supervision is required.
8. The centrifuge can dewater some industrial sludges that cannot be
handled by vacuum filters.
Disadvantages of its use are:
1. Without the use of chemicals, solids capture is often poor.
2. Chemical costs can be substantial.
3. Trash must often be removed from the centrifuge feed by screening.
4. Percent cake solids are often lower than those resulting from vacuum
filtration.
5. Maintenance costs are high.
6. Fine solids which escape the centrifuge (in the centrate) may resist
settling when recycled to the head of the treatment plant and gradually build
up in concentration and eventually raise effluent solids levels.
i
12.3.3 Design Considerations |
Centrifuges are usually selected on the basis of tests with smaller, geometrically-similar
machines. The use of a continuous centrifuge for testing purposes is not realistic if sludge
supply is limited, as when evaluating a small pilot plant. In the past, no laboratory
procedure has been available to predict prototype performance. This is in direct contrast
to the laboratory procedures, namely the Buchner Funnel Test and the Filter Leaf Test,
which have been developed to assess the dewatering characteristics of the vacuum filter.
Recently, Vesilind (19) developed a laboratory procedure which makes it possible to predict
prototype centrifuge performance on the basis of percent solids recovery. By trial and
error, it was found that the following model relates laboratory to prototype data:
0.1
Estimated Percent Recovery = 100
where:
Cf = Feed solids concentrations, mg/1
S = Centrate solids concentration, mg/1, measured after spinning in a laboratory
centrifuge at a desired centrifugal force and appropriate time.
P = Percent of sludge not penetrated as determined from a sludge penetrometer.
12-17
-------
No attempt was made to determine percent cake solids in this study. However, it is known
that percent solids recovery and cake solids have an inverse relationship, i.e., the higher
the solids recovery, the lower the percent cake solids. The numerical difference between
predicted and actual solids recovery from Vesilind's formula can be expected to be within
plus or minus 10 percent.
12.3A Centrifuge Performance in Sludge Thickening and Dewatering
Table 12-6 contains operating data supplied by various centrifuge manufacturers as well
as that reported in the literature for various combinations of municipal wastewater sludges.
These data indicate that the solid-bowl centrifuge is the most adaptable to the various
combinations of wastewater sludges.
Raw primary and digested primary sludges dewater easily. With polymer addition, a
centrifuge can produce 25 to 40 percent cake solids with better than 90 percent recovery.
When trickling filter sludge is added to either of these sludges, the percent cake solids
drops to 20 to 25 percent, and the polymer dosage to obtain 90 percent recovery
increases. Factors responsible for this loss in efficiency include lower feed solids and the
less favorable dewatering characteristics of biological sludge compared to primary sludge.
Waste activated sludge, by itself without conditioning, is difficult to thicken or dewater.
This is readily evident in Table 12-6 and is not peculiar to the centrifuge process. A
disc centrifuge can thicken waste activated sludge to 5 to 7 percent recovery without
polymers. A basket centrifuge can also thicken waste activated sludge, but large amounts
of polymer are required to make the sludge scrollable. To obtain 8 to 10 percent cake
solids, polymers at a cost of $15 to $20/ton of dry solids are required to obtain greater
than 90 percent recovery.
The use of centrifuges for sludge dewatering has been considered recently by several
municipalities. At one large southeastern city, a centrifuge test program was conducted
to determine the applicability of centrifuge dewatering of raw primary, digested primary,
co-settled ,raw primary and waste activated, mixed digested, and primary digested plus
thickened waste activated sludges (20). With a solid-bowl centrifuge, 55 to 85 percent
recovery was obtained without the use of polyelectrolytes depending upon the feed rate.
Recovery levels of 85 percent or better were achieved with 0.5 to 5 pounds of strong
cationic polyelectrolyte per ton of dry solids for combined raw primary and waste activated
sludges, primary digested plus waste activated sludges, and mixed digested sludges.
Thickening of waste activated sludge using a disc machine was found to be feasible and
produced 5 to 7 percent solids without polyelectrolytes. Problems were encountered in
trying to use a solid-bowl centrifuge on the combination of thickened waste activated
and primary digested sludges. Higher levels of polyelectrolyte were required as the
proportion of activated sludge to primary sludge increased. When operating with a mixture
of 2/3 primary digested sludge and 1/3 thickened waste activated sludge on a dry solids
basis, 50 to 60 percent recovery was achieved without polyelectrolytes. To increase
recovery to 80 to 90 percent, polyelectrolyte dosages of 10 to 20 pounds/ton of dry
solids were required.
12-18
-------
Table 12-6
Centrifuge Performance Data
Type of Sludge
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Raw
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Primary - Digested
Raw Primary + T.F.
Raw Primary + T.F.
Raw Primary + T.F.
Raw Primary + T.F.
Raw Primary + T.F.
Raw Primary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Digested Pr mary + T.F.
Raw Primary + Waste
Activated
Raw Primary + Waste
Activated
Waste Activated
(after roughing filter)
Waste Activated
(after roughing filter)
Percent
Centrifuge Type Capacity Feed Solids
gpm
Solid Bowl
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
23-26 9-12
23-36 9-12
27.5 8.8
27.5 8.4
27.5 9.2
6
6
9.9-22 9.6
5.5
5.6
Solid Bowl 40-80
Solid Bowl 40-80
Disc 50-80 0.7
Disc 60-270 0.7
Percent
Cake Solids
30-40
30-40
25-35
28-40
30-40
28-50
28-50
32-37
25-28
26-37
30-40
30-40
24-30
25-35
20-30
22-28
22-28
26-30
22-30
24-30
25-35
23
20-26
21-25
20-25
25+
20.2
20-28
22-36
25
18-25
18-25
18-20
18-20
5-7
6.1
Percent
Recovery
70-80
95
90+
70-90
50-90
65-80
80-98
84-93
88-95
85-96
44
84
97
70-90
95
90+
70-85
85-90
75-85
85
81-90
87-95
90+
65-75
96-100
82-96
83-90
85-94
40
90
90+
60-75
85
60-85
95+
40-60
85
93-87
97-80
Polymer
Requirement
$/ton Ibs./ton
None
5-10
2-4
None
1.5-2.5
None
1 .0-2.5
6.4
3-5
2-7
None
3.94
5.72
None
5-10
3-6
None
3-6
0
0.5-1
8
3-7
3-6
-
4-9
5-8
4-10
6-9
None
6.74
4-8
-
2.5-3.5
None
8-16
None
4-5
None
None
Referen
20
20
20
20
20
20
20
21
21
21
23
23
23
22
TT
">2
22
20
20
20
21
•>i
?!
21
21
21
21
23
23
22
22
20
21
21
20
20
20
20
12-19
-------
Type of Sludge
Waste Activated
Waste Activated
Waste Activated
Waste Activated
Conventional
Thickened
Contact Stabilization
or Extended Aeration
Thickened Waste
Activated Sludge
(by Disc Centrifuge)
Raw Primary + Waste
Activated
Digested Primary
Digested Primary +
Waste Activated
Digested Primary +
Thickened Waste Activated
Aerobic Digested
(contact stabilization)
Heat Treatment Sludge
Zimpro
Porteous
Chemical Sludges
Lime-Phosphate
Lime-Treatment in
Primaries
Tertiary Phosphate
Removal (Chemical
+ Waste Activated)
Centrifuge Type
Disc
Disc
Basket
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bow
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Solid Bowl
Table 1 2-6
(continued)
Percent Percent
Capacity Feed Solids Cake Solids
gpm
150 0.75-1.0 5-5.5
400 4.0
33-70 0.7 9-10
10-12 1.5 9-13
8-10
5-15
5-12
5-12
40-60 5-6 15-17
30-80 5-6 15-17
15-23
19-25
18-24
18-24
70-140 4 >17
4 >17
50-120 22-30
30-150 25-32
2.8 15-19
15-20
15-20
18-24
18-24
40-140 6 20-33
30-80 6 20-22
10-14
10-14
30-45
30-50
35^0
40-45
16-20
Percent
Recover
90+
80
70-90
90
90-100
90-100
90+
70-85
50-65
85
90+
50-75
50-80
95
20-50
85
80-95
50-75
85-90
50-60
85-100
50-70
95
40-80
85
90+
50-60
85-90
85-90
75-85
96-98
85-90
Polymer
Requirement
$/ton
None
None
None
15-20
5-10
2-5
None
5-10
None
None
6-20
None
12
None
10-35
None
10-20
None
13-17
5-10
None
None
None
None
6-7
11-17
None
3-8
2.4-10
1.0
3.4
22
20
24
22
20
20
25
25
20
20
20
20
25
25
20
20
22
22
22
22
22
20
22
12-20
-------
At a southwestern community, field tests were conducted to determine the applicability
of centrifugation for dewatering combined primary and secondary digested sludges (20).
The results indicated that a solid-bowl centrifuge could be used to replace concrete drying
beds. With the use of a strong cationic polyelectrolyte at a concentration of 3 to 4 Ibs./ton
of dry solids, the sludge could be dewatered to 17 to 18 percent with a solids recovery
of 85 percent. Solids recovery was increased to 98 to 99 percent recovery when the
polyelectrolyte dosage was raised to 5 to 6 Ibs./ton of dry solids.
In El Paso, Texas, a land problem necessitated replacement of the sludge drying beds (26).
Centrifuges were used to dewater the digested sludge at a 6 to 7 percent feed solids
concentration. The centrifuges produced 20 to 22 percent cake solids with 85 to
90 percent recovery, at a polyelectrolyte dosage of 2 to 3 Ibs./ton of dry solids.
The use of a cationic polyelectrolyte was evaluated for a combination of 80 percent raw
Imhoff and 20 percent digested sludges (20). The cationic polyelectrolyte was effective
in improving sludge dewatering at economical dosage levels (2 to 3 Ibs./ton of dry solids).
With an average feed solids concentration of 8 percent, a cake solids of 35 percent at
95 percent solids recovery was obtained.
Centrifuges can also be used to replace vacuum filters for sludge dewatering. Large plants
are particularly interested in evaluating the centrifuge as an alternative to other mechanical
sludge dewatering devices. Savings attributed to decreased operational and maintenance
costs have been noted. Pre-treatment devices to further decrease maintenance costs are
also evaluated. A cyclone can be used ahead of centrifuges to remove a large fraction
of the sand and other abrasives. In addition, screening of the centrifuge feed material
and/or use of solids grinding is recommended to eliminate the possibility of conveyor
feed zone pluggage.
A disc centrifuge has been field tested for thickening waste activated sludge at an eastern
Pennsylvania community with good success (27). Using a 30-in. centrifuge with a 150-hp
motor and 300-gpm feed rate, the disc centrifuge produced a 5 percent underflow with
90 percent solids recovery when the Sludge Volume Index was generally "less than 100.
Since the plant did not have primary treatment, it was necessary to install a screening
device ahead of the centrifuge. The screening effectiveness was demonstrated in that the
nozzles of the centrifuge did not plug. The combination of effective screening and a
patented recirculating system (providing a larger nozzle size) was instrumental to the good
performance.
12.3.5 Use of Centrifuge for Upgrading Sludge Handling Facilities
The performance of centrifuges in various applications clearly indicates that centrifugation
should be considered when the upgrading of solids handling facilities is required. Centrifuges
are a flexible upgrading device because of their applicability in both the thickening and
the dewatering of various mixtures of sludges.
12-21
-------
When used as a thickening device, a centrifuge can upgrade an overloaded anaerobic digester
by reducing the volume of feed sludge, thereby increasing digester detention time. In
addition, centrifuges can also be used to supplement existing overloaded gravity thickeners.
When used as a dewatering device, they can supplement existing overloaded vacuum filters.
12.3.6 Process Designs and Cost Estimates
Two examples are given to illustrate the upgrading of thickening and dewatering facilities
through the use of centrifuges.
12.3.6.1 EXAMPLE A
Anaerobic digestion facilities at an existing activated sludge plant are overloaded; as a
result, the detention time in the digesters has been reduced to 11.25 days, thereby causing
unstable operation of the digesters.
To upgrade the digestion facilities, it is decided to thicken the waste activated sludge-
by using a disc centrifuge, as shown in Figure 12-3. It is assumed that polyelectrolyte
addition is not necessary. The volume of the waste activated sludge, 117,000 gpd at
1 percent solids (9,800 Ibs. dry solids/day) before centrifugation, is reduced to 29,250 gpd
at 4 percent solids by the centrifugation step. Combination of the thickened waste
activated sludge with 36,800 gpd of 5 percent primary sludge results in an overall increase
in digester detention time to approximately 17 days.
The capital cost for this upgrading procedure is estimated at $215,000 (ENR 1500)
($17,200/ton of total plant dry solids/day, or $43,900/ton of waste activated dry
solids/day). This cost includes one standby disc centrifuge, in-line screens, sludge pumps,
and appurtenances, but does not include an allowance for engineering design, bonding,
and construction supervision.
12.3.6.2 EXAMPLE B
Vacuum filter facilities at an existing activated sludge plant are overloaded due to a recent
increase in plant capacity from 10 to 20 mgd. Space limitation at the plant prohibits
the installation of another filter.
It is decided to add a centrifugation facility to supplement existing vacuum filter facilities.
Fifty percent of the mixed digested sludge, 12,000 Ibs./day of dry solids (20 gpm by
volume), is to be treated in two alternately used solid-bowl centrifuges. At an estimated
5 percent feed solids and with a polymer dosage of 3 to 6 Ibs./ton of dry solids, it is
expected that the solid-bowl centrifuge will produce 20 to 25 percent cake solids at
85 percent solids recovery. Facilities for sludge dewatering include 2 solid-bowl centrifuges,
each capable of handling 15 to 20 gpm, sludge feed pumps, polymer addition facilities,
and other necessary appurtenances.
12-22
-------
FIGURE 12-3
EXAMPLE A
UPGRADING DIGESTION BY THICKENING WITH DISC CENTRIFUGE
RAW WASTEHATER
SLUDGE DEWATERING
12-23
-------
The capital cost for this upgrading procedure is estimated at $218,000 (ENR 1500)
($18,175/ton of total plant dry solids/day, or $36,350/ton of dry solids/day actually
processed by the centrifuges). This cost does not include an allowance for engineering
design, bonding, and construction supervision.
12.4 References
1. Sludge Dewatering. Water Pollution Control Federation Manual of Practice No. 20,
Washington, D.C., 1969.
2. Burd, R.S., A Study of Sludge Handling and Disposal. Federal Water Pollution Control
Administration, Publication WP-20-4, May, 1968.
3. Eckenfelder, W.W., and O'Connor, D.J., Biological Waste Treatment. New York:
Pergamon Press, 1961.
4. Sludge Conditioning with Purifloc. Dow Chemical Company, 1966.
5. Sherbick, J.M., Synthetic Organic Flocculants Used for Sludge Conditioning. Journal
Water Pollution Control Federation, 37, No. 8, pp. 1,180-1,183 (1965).
6. Hopkins, G., and Jackson, R., Polymers in the Filtration of Raw Sludge. Journal
Water Pollution Control Federation, 43, No. 4, pp. 689-698 (1971).
7. McDowell, M.A., et al, Continued Evaluation of Oxygen Use in the Conventional
Activated Sludge Process. Preliminary Results of EPA Contract No. 14-12-867,
Batavia, N.Y., 1971.
8. Personal Correspondence Between Dr. Joseph Farrell (EPA-WQO) and
Mr. R.S. Powell (City of Milwaukee), dated January 12, 1971.
9. Statistical Summary 1968 Inventory Municipal Waste Facilities in the United States.
Federal Water Quality Administration: Government Printing Office, 1971.
10. Jennett, J.C., and Santry, I., Jr., Characteristics of Sludge Drying. Journal of the
Sanitary Engineering Division, ASCE, 95, No. 5, pp. 849-863 (1969).
11. Quon, J., and Johnson, G., Drainage Characteristics of Digested Sludge. Journal of
the Sanitary Engineering Division, ASCE, 92, No. 2, pp. 67-82 (1966).
12. Nebiker, J., Drying of Wastewater Sludge in the Open Air. Journal of Water Pollution
Control Federation, 39, No. 4, pp. 608-626 (1967).
13. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of State Sanitary Engineers, 1968.
12-24
-------
14. Nebiker, John H., et al, An Investigation of Sludge Dewatering Rates. Journal Water
Pollution Control Federation, 41, No. 8, Part 2, pp. R255-R266 (1969).
15. Lawson, George, R., Equipment and Chemicals - An Approach to Water Pollution.
Investment Dealer's Digest, August 5, 1969.
16. Townsend, Joseph, What the Wastewater Plant Engineer Should Know about
Centrifuges. Water and Wastes Engineering, 6, No. 11, pp. 41-44 (1969).
17. Bradney, L., and Bragstad, R.E., Concentration of Activated Sludge by Centrifuge.
Sewage and Industrial Wastes, 27, No. 4, pp. 404-411 (1955).
18. Ettlet, G.A., and Kennedy, J., Research and Operational Experience in Sludge
Dewatering at Chicago. Journal Water Pollution Control Federation, 38, No. 2,
pp. 248-257 (1966).
19. Vesilind, A., Estimation of Sludge Centrifuge Performance. Journal of the Sanitary
Engineering Division, ASCE, 96, No. 3, pp. 805-818 (1970).
20. Private Communication with George Patenaude, Philadelphia District Representative,
Sharpies-Stokes Division, Pennwalt Corporation, Wynnewood, Pennsylvania, October
27, 1970.
21. Albertson, O., and Guidi, E., Centrifugation of Waste Sludges. Journal Water Pollution
Control Federation, 41, No. 4, pp. 607-628 (1969).
22. Private Communication with Gene Guidi, Sales Manager, Environmental Control
Equipment, Bird Machine Company, Walpole, Massachusetts, February 22, 1971.
23. Hercofloc Flocculant Polymers For Use in Sludge Conditioning. Hercules Incorporated,
Environmental Services Division, Wilmington, Delaware, Bulletin ESD-102A, 1969.
24. Eckenfelder, W.W., Industrial Water Pollution Control. New York: McGraw-Hill Book
Company, 1966.
25. Albertson, O., and Guidi, E., Advances in the Centrifugal Dewatering of Sludges.
Water and Sewage Works, 114, No. 11, pp. 133-142 (1967).
26. El Paso Loses Drying Beds in Boundary Action. Water and Sewage Works, 117, No. 2,
pp. 26 - 27 (1970).
27. Private Communication with Laurence Sheker, Resident Manager, Environmental
Equipment and Systems Division, Dorr-Oliver Incorporated, Camp Hill, Pennsylvania,
April 22, 1970.
12-25
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CHAPTER 13
CASE HISTORIES OF TREATMENT PLANT UPGRADING
13.1 General
The capabilities and limitations of various unit processes have been discussed in preceding
chapters. However, in any wastewater treatment plant, the operation of an individual
process affects the operation of other processes in the treatment system. Therefore in
upgrading situations, emphasis should be placed on the effects of upgrading on the
treatment plant as a whole rather than on a particular unit operation.
Various case histories are discussed to illustrate procedures which have been used to upgrade
wastewater treatment facilities. Cost estimates for these cases are furnished when such
information is available.
13.2 Case History No. 1
Upgrading Using Chemical Addition to Primary Clarifiers and Conversion of Conventional
Activated Sludge to Contact Stabilization (1)
Plant 1 before upgrading was a parallel activated sludge and trickling filter plant. The
original nominal design capacity of the plant was 3.6 mgd, but the average influent flow
had increased to 6.0 mgd. The plant was removing significantly less than 90 percent of
the BOD during the summer and as little as 60 percent during the winter.
Upgrading of this plant was an interim measure, since the plant was scheduled to be replaced
in 5 years with a regional system. Figure 13-1 is a flow diagram of the upgraded treatment
system. Table 13-1 summarizes plant operating conditions prior to upgrading and plant
design conditions after upgrading. The conventional activated sludge system originally
designed to treat a flow of 1.2 mgd, was upgraded to a contact stabilization system. This
was accomplished by increasing the capacity of the mechanical aerators from a total of
40 hp up to 110 hp (60 hp in the contact basin and 50 hp in the stabilization basin)
and by modifying the basin's piping. Since experience has shown that contact stabilization
works well without primary clarification, the existing primary clarifier was converted to
a secondary clarifier to decrease the activated sludge secondary clarifier overflow rate.
The contact stabilization process was designed to handle a flow of 3.0 mgd.
The trickling filter portion of the plant was originally designed to operate at 2.4 mgd
as a two-stage system. However, during periods of hydraulic overloading, the trickling filters
were operated in parallel to increase the hydraulic capacity.
Plant operating data indicated that the secondary portion of the parallel-operated trickling
filter system was removing 70-80 percent of the primary effluent BOD. Therefore, it was
13-1
-------
FIGURE 13-1
CASE HISTORY NO. 1
FLOW DIAGRAM OF PLANT INCLUDING UPGRADED UNIT PROCESSES
U)
to
m-
7
• ECNtNICU MR SCREEK
HMO GRIT CHII8ER
POUELECTROLITE
T1NK
0.91 POUEUCnOUTE
IET IELL >KD
PUIPING STITION
STIBILIZITION HHIlONl CODTICT IEIAIIOK
i
PR!«
EKISTUt
SECOHDISt CURIFIERl
CONVERTED
PRIKIIY CLIRIFIER
»£» CHLORINE
CONTICT CHIME*
c
»•
—
Fll«l _,.
EFFLIEIT
LtllFIED EFFLUEIT
PROPOSED I.HE IDOIIIOK
SISIEU
-------
Table 13-1
Case History No. 1 - Plant Operating and Design Conditions
Operating Conditions
Operation Before Upgrading
Pre-treatment Facilities (Bar Screen, Aerated Grit Chamber)
Average Flow, mgd 6-0
Activated Sludge Plant
Primary Clarifier Overflow Rate, gpd/sq.ft.1 1,200
Flow to Aeration Basin, mgd . 3.0
Volumetric Loading in Aeration Tank, Ibs. BOD/day/1,000 cu.ft.
Average Sludge Recycle, percent 30
Secondary Clarifier Overflow Rate, gpd/sq.ft.1 960
BOD Removal in Secondary Treatment, percent 75
Suspended Solids Removal in Secondary Treatment, percent 75
Trickling Filter Plant
Primary Clarifier Overflow Rate, gpd/sq.ft.1 820
BOD Removal in Primary Treatment, percent 20
Flow to Trickling Filter, mgd 3.0
Hydraulic Loading, mgd/acre-ft.^ -
Organic Loading, Ibs. BOD/day/1,000 cu.ft.3
Recirculation Ratio 1.0
Secondary Clarifier Overflow Rate, gpd/sq.ft.4 1,000
BOD Removal in Secondary Treatment, percent 70 to 80
Sludge Handling Facilities
Vacuum Filter Operation 16hrs./day
5 days/week
Design Conditions
After Upgrading
6.0
3.0
602
100
740
90
90
820
50 to 60
3.0
6.25
72
19
1.0
1,000
70 to 80
20 hrs./day
6 days/week
Remarks
Primary clarit'ier converted
to secondary clarificr.
O2 supply, not organic
loading, was limiting prior
to upgrading.
Reduction in overflow rate
is achieved by converting the
primary clarifier to a secondary
unit.
Chemical coagulation
For each stage
First Stage
Second Stage
For each stage
Increased operation to
accommodate increased
sludge quantities
'Clarifier overflow rates based on average flow
^Organic loading increases due to increase in flow and elimination of primary treatment.
3Does not include recirculation.
4Based on average flow plus recycle.
13-3
-------
decided to confine the upgrading to improvement of the removal efficiency of the primary
clarifier. The selected approach was chemical addition, namely lime and anionic
polyelectrolyte. Laboratory testing indicated that as much as 60 percent BOD removal
and 75 percent suspended solids removal could be attained in the primary clarifier with
the addition of 1.0 mg/1 polyelectrolyte and 200 mg/1 of lime. With this modification,
the trickling filter system could again be operated as a 2-stage system at a flow of 3.0 mgd.
A summary of the measured performance of the existing system and the anticipated
performance after upgrading is given below in Table 13-2.
Table 13-2
Summary of Treatment Performance
for Case History No. 1
Design Flow, mgd
Average Flow, mgd
BOD Removal, percent
Effluent BOD, mg/1
SS Removal, percent
Effluent SS, mg/1
Measured Performance
Before Upgrading
3.6
6.0
80
40
85
30
Anticipated Performance
After Upgrading
6.0
6.0
90
20
90
20
The cost of upgrading the plant was estimated at $510,000, and the increased annual
operating costs were estimated at $74,800. These costs are broken down in Table 13-3.
13-4
-------
Table 13-3
Summary of Upgrading Costs
for Case History No. 1
(ENR Index 1500)
ESTIMATED CONSTRUCTION COST
Chemical Addition/Flocculation $ 45,000
Conversion to Contact Stabilization 340,000
Mechanical Refurbishing 25,000
Electrical Refurbishing 50,000
Construction Contingency 50,000
Total Estimated Construction Cost1 $510,000
Unit Capital Cost of Upgrading $212/1,000 gpd
of upgraded flow
ESTIMATED ANNUAL OPERATING COST INCREASE
Operation and Maintenance $ 5,700
Electrical 9,100
Chemical 52,000
Sludge Disposal 8,000
Total Annual Operating Cost Increase $ 74,800
Unit Annual Operating Cost Increase 3.4 cents/1,000 gallons
of treated flow
'Costs do not include engineering, legal, and administrative fees.
13-5
-------
13.3 Case History No. 2
Upgrading an Existing High-Rate Trickling Filter Plant Using a Series Activated Sludge
Process (2)
Plant 2 before upgrading was a high-rate trickling filter plant whose original design capacity
of 12.5 mgd was being approached. The initial treatment requirements were 85 percent
BOD and SS removals, and removals of approximately 80 percent were actually being
obtained. The treatment requirements were then changed to include 90-95 percent removal
of the total oxygen demand (the sum of the ultimate carbonaceous oxygen demand and
the nitrogenous oxygen demand). This additional requirement indicated that most of the
ammonia nitrogen had to be removed or converted to the nitrate form.
Figure 13-2 is a flow diagram of the upgraded treatment system. Table 13-4 summarizes
plant operating conditions prior to upgrading and plant design conditions after upgrading.
An in-plant survey indicated that the existing equipment was in good condition and could
be incorporated into the upgrading scheme. After an evaluation of alternative upgrading
possibilities, the existing plant was upgraded from 12.5 mgd to 22.5 mgd using a series
treatment of activated sludge and trickling filtration. The activated sludge system was
designed with a detention time of 3 hours, with the activated sludge effluent being
subsequently treated in the trickling filter to take advantage of the incipient nitrifying
ability of the trickling filter.
Another major consideration in the upgrading was the disposal of waste activated sludge.
To reduce the volume of primary and waste activated sludge, a gravity thickener was
incorporated into the design. Although the existing two-stage anaerobic digesters had
sufficient volume to handle the thickened sludge from the upgraded plant, operational
problems were being experienced in the existing plant due to incomplete mixing and
non-uniform temperature control in the primary digesters. The original design included
mechanical mixing using draft tubes and internal heating coils. To improve digester
performance, gas recirculation was used for better mixing, and an external heat exchanger
was incorporated to facilitate maintenance of a uniform temperature throughout the
primary digesters. When the upgraded plant comes on line, it is expected to perform as
indicated in Table 13-5.
13-6
-------
FIGURE 13-2
CASE HISTORY NO. 2
FLOW DIAGRAM OF PLANT INCLUDING UPGRADED UNIT PROCESSES
EXPANDED
PRETREITHENT
FICUITIEI
P
p
s
3
fr
ROPOSEO
UIP
TITIOI
•
i
— »
PROPOSED IEBHIOII B1SIKS
±T T
_ _ 1 -
r
"" PROPOSED SLUDCE
RECYCLE PUIIPS
J^,
p
\
^
ROPOSEO SECO(0»«
If— 1
' ^
1 ^-1*"
'EXISTING
SPUITE
BOX
FINK
EFFLUENT,,
Ll
EXISTING II1IC«LI«C FILTERS
-------
Table 13-4
Case History No. 2 - Plant Operating and Design Conditions
Operating Conditions Design Conditions
Operation Before Upgrading After Upgrading
Pre-treatment Facilities (Bar Screen,
Grit Chamber, Raw-Sewage Pump
Station) Avg. Capacity, mgd 12.5 22.5
Primary Clarifiers
Overflow Rate, gpd/sq.ft. 7661 7852
BOD Removal, percent 56 30
SS Removal, percent 74 60
Aeration Basins
Detention Time, hours - 3.0
Volumetric Loading, Ibs. BOD/day/
l,000cu.ft. - 30
Organic Loading, Ibs. BOD/day/lb.
MLVSS - 0.25
Sludge Recycle, percent of average flow - 60
Mechanical Aerator Capacity, hp _ 750
Intermediate Clarifiers
Overflow Rate, gpd/sq.ft. 785
Trickling Filter
Hydraulic Loading, mgad 30 21.6
Organic Loading, Ibs. BOD/day/.
l,000cu.ft. 66 21
Recirculation Ratio 1.5-* 0
Final Clarifiers
Overflow Rate, gpd/sq.ft. 7664 785
Sludge Thickener
Design Underflow Concentration - 4.0
Design Solids Loading, Ibs. SS/day/sq.ft. - 10.0
Detention Time, hours - 10.0
Anaerobic Digesters
Loading, Ibs. VSS/day/cu.ft. 0.05 0.12
Detention Time, days 34 14
1 Based on average design flow plus 75% secondary sludge recirculation.
^Excluding sludge recycle.
^The recirculation ratio includes 75% secondary sludge recirculation to the primary clarifier
influent and 75% secondary clarifier effluent recirculation to the trickling filter influent.
^Based on average design flow plus 75% secondary clarifier effluent recirculation.
13-8
-------
Table 13-5
Summary of Treatment Performance
for Case History No. 2
Before Upgrading After Upgrading
00 ^
Average Design Flow, mgd 12.5 •
BOD Removal, percent 81 ..
Effluent BOD, mg/1 41 ^
SS Removal, percent 81
Effluent SS, mg/1 40 ^
Effluent NH3-N mg/1 27 J-
Effluent NOs-N1, mg/1 <1
1 Approximate
The construction costs for treatment facilities were estimated at $4,130,000, and the
increased annual operating costs due to upgrading were estimated at $224,000. These costs
—* « * 4 ,-t S
are broken down in Table 13-6.
13-9
-------
Table 13-6
Summary of Upgrading Costs
for Case History No. 2
(ENR Index 1500)
ESTIMATED CONSTRUCTION COST
Expansion of Pre-treatment Facilities $ 165,000
Expansion of Primary Treatment 190,000
Addition of Activated Sludge Treatment and
Intermediate Clarification 2,030,000
Expansion of Sludge Handling 54,000
Expansion of Control Building, Chlorination Facilities, etc. 260,000
Piping, Electrical, Instrumentation, etc. 890,000
Construction Contingency 541,000
Total Estimated Construction Cost* $4,130,000
Unit Capital Cost of Upgrading $413/1,000 gpd
of upgraded flow
ESTIMATED ANNUAL OPERATING COST INCREASE
Labor $ 30,000
Power 86,000
Maintenance and Supplies 108,000
Total Annual Operating Cost Increase $224,000
Unit Annual Operating Cost Increase 2.7 cents/1,000 gallons
of treated flow
* Costs do not include engineering, legal, and administrative fees.
13-10
-------
13.4 Case History No. 3
Use of Roughing Filter to Upgrade an Existing Low-Rate Trickling Filter Plant (3) (4)
(5)
Case History No. 3 involves the upgrading of an existing low rate trickling filter plant
in Huber Heights, Ohio. The original plant was designed in August, 1956 for a flow of
0.7 mgd, with 85 percent BOD and suspended solids removals. The community developed
so rapidly that by 1970, the average flow had increased to 2.3 mgd. The flow diagrams
for the original and upgraded plant are shown in Figure 13-3.
Operational and performance data for the overloaded plant for 1962, when the plant was
receiving 1.15 mgd, are compared with corresponding data after the plant was upgraded
to 2.3 mgd in Table 13-7.
The comminuter and primary clarifiers were replaced with three Hydrasieve units of 1-mgd
capacity each. These units are stationary screens capable of removing 20 to 35 percent
of the BOD and suspended solids. A Hydrasieve unit is illustrated in Figure 13-4, along
with a schematic flow diagram through the unit. These screens generally require no power
and little maintenance.
The plastic media roughing filter used in the upgrading has an application rate of
approximately 2.5 gpm/sq.ft. Present BOD removal is about 25 to 35 percent through
the roughing unit. Because of the increased hydraulic loading, it was necessary to expand
the secondary clarification and chlorine contact tank capacities. The abandoned primary
clarifiers were converted into sludge thickeners. This step, in addition to conversion of
the anaerobic digester to a high-rate unit using gas recirculation for mixing, enabled the
sludge handling system to process the increased quantity of sludge produced.
This case history points out the fact that an existing plant may be gradually upgraded
to handle a three-fold increase in flow with the use of innovative techniques and newly
applied process equipment. The capital costs for upgrading the capacity of the plant were
estimated at approximately $300,000 (ENR 1500), or $187 per 1,000 gallons/day of
upgraded flow capacity.
13-11
-------
FIGURE 13-3
CASE HISTORY NO.3
COMPARISON OF ORIGINAL AND UPGRADED FLOW DIAGRAMS
LEGEND
ORIGINAL PLANT
UPGRADED PLANT
K)
WASTEWATER
SLUDGE
ANAEROBIC
DIGESTION
Q-
DRYING
BEDS
I
I
_*_
ULTIMATE DISPOSAL
SECONDARY
CLARIFIERS
REMOVALS,
L_JCOMMINUTION
. r--r
SLUDGE THICKENERS
ANAEROBIC
DIGESTION
DRYING
BEDS
ULTIMATE DISPOSAL
CHLORINE CONTACT
TANK
HYDRASIEVE
PLASTIC MEDIA
ROUGHING FILTER
SECONDARY
CLARIFIERS
CHLORINE CONTACT
TANK
-------
Table 13-7
Case History No. 3 - Plant Operating and Performance Data
Operation Before Upgrading After Upgrading
(1962)
Average Flow Rate, mgd 1.15 2.3
Primary Clarifier
Overflow Rate1, gpd/sq.ft. 1,170 _ 5
Hydrasieve Slot Size, inches - 0.06
BOD Removal, percent 352 253
SS Removal, percent - 6 223
Plastic Media Roughing Filter
Hydraulic Loading4, gpm/sq.ft. - 2.5
Organic Loading4, Ibs. BOD/day/1,000 cu.ft. - 520
Recirculation Ratio - «2.0
BOD Removal, percent - 30
Trickling Filter (Stone Media)
Hydraulic Loading, mgad 6.0 12.0
Organic Loading, Ibs. BOD/day/1,000 cu.ft. 56.2 87.0
Secondary Clarifiers
Overflow Rate, gpd/sq.ft. 1,170 750
Overall Plant Performance
BOD Removal, percent 83 85
Effluent BOD, mg/1 41 37
SS Removal, percent - 6 84
Effluent SS, mg/1 - 6 40
1 Based on average flow rate.
^Based on primary clarifier performance.
3Based on hydrasieve performance only.
^Including recirculation.
5 Primary clarifiers converted to gravity thickeners.
^Operating data not available.
13-13
-------
FIGURE 13-4
HYDRASIEVE SCREENING UNIT*
13-14
'COURTESY OF THE BAUER BROS. co. - SPRINGFIELD, OHIO
-------
13.5 Case History No. 4
Upgrading an Existing High-Rate Trickling Filter by Conversion to a Super-Rate Filter
System (6)
The North Treatment Plant at Sedalia, Missouri, is a high-rate trickling filter plant, designed
for 1.25 mgd, and was removing 85 percent of the BOD in 1963. However, the Water
Pollution Board set a final effluent BOD of 20 mg/1, which Sedalia could not meet with
the existing facilities. The 1963 plant flow diagram is illustrated in Figure 13-5.
The plant was upgraded to treat an average design flow of 2.5 mgd. The existing
stone-media filter was renovated to operate with plastic media. In addition, a second
plastic-media filter was constructed. The two plastic-media filters are operated in parallel
with a total recirculation ratio of 1.55. One additional primary clarifier and one additional
secondary clarifier were installed.
To remove additional BOD and suspended solids, a shallow aerobic polishing lagoon was
constructed after the secondary clarifiers, and a vacuum filter was added to reduce the
volume of digested sludge. A flow diagram of the upgraded plant is also shown in
Figure 13-5. Table 13-8 contains a summary of operating data for the 1963 overloaded
period; in addition, the upgraded design criteria are listed, along with actual operational
data for the post-upgrading period. It should be noted that the effluent BOD was improved
from 115 mg/1 to 11 mg/1 after upgrading, which is below the 20 mg/1 requirement. It
should also be pointed out that Missouri has no suspended solids removal requirements
for plants with a flow of less than 10 mgd.
The capital costs of upgrading the plant were estimated at $2,600,000 (ENR Index 1,500),
or $2,080 per 1,000 gpd of incremental upgraded capacity.
13-15
-------
FIGURE 13-5
CASE HISTORY NO.4
COMPARISON OF ORIGINAL AND UPGRADED FLOW DIAGRAMS
LEGEND
- WASTEWATER
-- SLUDGE
OVERLOADED PLANT
UPGRADED PLANT
ON
GRIT REMOVAL
COMMINUTION
ANAEROBIC
DIGESTION
DRYING
BED
ULTIMATE
DISPOSAL
NOTE: CONVERTED TO USE PLASTIC MEDIA.
PRIMARY
CLARIFIER
ANAEROBIC
DIGESTION
f.
I
_*.
f"-
4*
^EXISTING
TRICKLING
FILTER
DRYING
BEDS
VACUUM
FILTER
l__
SECONDARY
CLARIFIER
ULTIMATE
DISPOSAL
GRIT REMOVAL
COMMUNITION
PRIMARY
CLARIFIERS
NEW
PLASTIC
MEDIA
FILTER
SECONDARY
CLARIFIERS
POLISHING
LAGOON
-------
Table 13-8
Case History No. 4 - Plant Operating and Design Data
1963 1969
Operating Upgraded Operating
Operation Data Design Data
Average Daily Flow, gpm 1.25 2.5 1.80
Raw Wastewater BOD, mg/1 768 576 450
Primary Clarifiers
Overflow Rate^gpd/sq.ft. 1,000 1,000 720
BOD Removal, percent 40 40 60
Trickling Filters
Hydraulic Loading^, mgad 24.5 31.8 25
Organic Loading^, Ibs. BOD/day/
l,000cu.ft. 226 72.6 20
Recirculation Ratio 1.0 1.55 1.55
Final Clarifiers
Overflow Rate1, gpd/sq.ft. 755 755 545
Secondary BOD Removal, percent 75 93.2 86.8
Polishing Lagoon (Shallow Aerobic)
Maximum BOD Loading, Ibs. BOD/
acre/day - 68 30
BOD Removal, percent - 12 54
Vacuum Filtration Rate,
Ibs./sq.ft./hour - 5.0 -3
Overall Plant Performance
BOD Removal, percent 85 96.5 97.7
Effluent BOD, mg/1 115 20 11
1 Based on average daily flow.
^Including Recirculation.
3 Lack of sufficient operating data.
13-17
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13.6 Case History No. 5
Upgrading a Contact Stabilization Package Plant to a "Modified" Completely-Mixed Flow
Pattern (7)
Coralville, Iowa, had a contact stabilization package plant which was providing detention
times of 2.6 and 6.5 hours, respectively, in the contact and stabilization zones, based
on a forward flow of 867,000 gallons per day, not including sludge recycle. A typical
plan view of the contact stabilization package plant is shown in Figure 13-6.
As previously discussed in Chapter 5, a contact-zone detention time of this magnitude
may result in improper plant operation because the sludge becomes partially stabilized
in this zone and acquires poorer settling characteristics. After investigation, this was found
to be the case in Coralville. Operational data from the plant before upgrading are
summarized in Table 13-9. The effluent BOD and suspended solids were 26 and 24 mg/1,
respectively.
To improve the plant's performance, it was decided to modify the flow pattern as indicated
in Figure 13-6. The influent piping was modified so that the raw wastewater was evenly
distributed into what originally was the stabilization zone. No wastewater was introduced
into the former contact zone. Mixed liquor in the upgraded system proceeded from the
former stabilization zone through the former contact zone to the secondary clarifier. The
return sludge was introduced into the former stabilization zone at one point only.
Therefore, the upgrading resulted in a "modified" completely-mixed flow pattern, with
an overall detention time of 9.1 hours for an average flow of 867,000 gpd.
Performance data for the upgraded plant are included in Table 13-9. The effluent BOD
and suspended solids concentrations were lowered to 13 and 6 mg/1, respectively, by the
upgrading procedure. The costs associated with this modification are primarily due to piping
changes. No cost breakdown was available for the modification.
13-18
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FIGURE 13-6
CASE HISTORY NO. 5
UPGRADING A CONTACT STABILIZATION PACKAGE PLANT
TO A COMPLETELY-MIXED FLOW PATTERN (5)
STABILIZATION ZONE
CONTACT ZONE
SLUDGE RECYCLE
EXCESS SLUDGE
WASTING LINE
BEFORE UPGRADING
CONTACT STABILIZATION FLOW PATTERN
AEROBIC DIGESTER
SETTLING ZONE
COMPLETELY-MIXED
AERATION TANK
SLUDGE RECYCLE
EXCESS SLUDGE
WASTING LINE
AFTER UPGRADING
MODIFIED COMPLETELY-MIXED FLOW PATTERN
INFLUENT
EFFLUENT
AEROBIC DIGESTER
SETTLING ZONE
13-19
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Table 13-9
Case History No. 5 - Plant Operating Data
Operation
Average Flow, mgd
Influent BOD, mg/1
Influent SS, mg/1
Aeration Tank
Sludge Recycle, percent
Contact Unit Volumetric Load,
Ibs. BOD/day/1,000 cu.ft.
Contact Unit Organic Load,
Ibs. BOD/day/lb. MLVSS
Contact Unit MLSS, mg/1
Secondary Clarifier
Overflow Rate^, gpd/sq.ft.
Overall Plant Performance
BOD Removal, percent
Effluent BOD, mg/1
SS Removal, percent
Effluent SS, mg/1
Contact Modified
Stabilization 1 Completely-Mixed 2
0.867
135
150
60
78
0.4
3,500
750
81
26
84
24
0.867
135
150
60
224
750
90
13
96
6
1 Before Upgrading
2 After Upgrading
3 Based on average flow
4fiased on total aeration volume.
13-20
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13.7 Case History No. 6
\
Upgrading Using Polyelectrolyte Addition Before the Primary Clarifier (8)
The Easterly Wastewater Treatment Works in Cleveland, Ohio, is a conventional activated
.sludge plant whose dry-weather design flow is 123 mgd. In 1968, the plant was
hydraulically overloaded. Operational data from the period are presented in Table 13-10,
identified as the 1968 Control Period. It is interesting to note that the overflow rate
of the primary clarifiers was 2,030 gpd/sq.ft., which is substantially above the accepted
values. As a result of the hydraulic overloading, the overall BOD and suspended solids
removals were only 79 and 85 percent, respectively.
To improve the overall plant performance, a polyelectrolyte addition program was initiated.
An anionic polyelectrolyte, Purifloc A-23, was added at an average dosage of 0.21 mg/1.
Since proper polyelectrolyte dispersal and uniform mixing into the entire waste flow
constitute an extremely important aspect of the flocculation process, it was decided to
add the polymer at the plant's two Venturi meters. These meters are located immediately
after the grit chamber and in front of the pre-aeration basin. Dye studies indicated that
there was a 7.5-minute travel time between the Venturi meters and the primary clarifiers.
Six of the 7,5 minutes would be spent in the pre-aeration basin. The gentle agitation
in the pre-aeration basin provided adequate flocculation of wastewater solids.
A summary of the effectiveness of the polymer addition on plant performance as compared
to performance during the previous control period is also presented in Table 13-10. The
improvement in primary suspended solids removal increased the volume of primary sludge
from 5.0 to 6.8 million gallons per month. Overall plant performance was improved, as
noted by the reduced effluent BOD and suspended solids concentrations. In addition to
the increased treatment efficiency, the polyelectrolyte addition was responsible for the
following benefits to the subsequent activated sludge process:
1. A 20 percent volume decrease in the amount of waste activated sludge produced.
2. A 22 percent reduction in air supply requirements, resulting in a power cost
savings of over $3,300 per month.
3. An increase in aeration tank D.O. concentration from an average of 3.2 mg/1
to 3.8 mg/1.
An economic comparison was made between polyelectrolyte addition and providing
additional tankage to equal the performance of the flocculation system The amortized
cost for the additional tankage was about $314,000 per year, while the chemical cost
was $150,000 per year, thereby indicating a considerable yearly savings in favor of the
polyelectrolyte alternative.
13-21
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Table 13-10
Case History No. 6 - Plant Operating Data
Operation
Influent BOD, mg/1
Influent SS, mg/1
Primary Clarifier
Overflow Rate, gpd/sq.ft.
BOD Removal, percent
SS Removal, percent
Sludge Solids Concentration, percent
Sludge Volume Pumped, million gallons/month
Aeration Tank
MLSS, mg/1
Organic Loading, Ibs. BOD/day/lb. MLSS
Dissolved Oxygen, mg/1
Waste Activated Sludge Concentration, percent
Waste Activated Sludge Pumped, million gallons per
month
Overall Plant Performance
BOD Removal, percent
Effluent BOD, mg/1
SS Removal, percent
Effluent SS, mg/1
1968
Control
Period
104
169
2,030
31
31
4.1
5.0
1,670
0.48
3.2
2.4
12.3
79.1
21.8
85.3
24.8
Polymer
Demonstration
Period
67
157
2,170
46
51
4.3
6.8
1,602
0.29
3.8
2.0
9.8
83.4
11.1
89.2
17.0
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13.8 References
1. Roy F. Weston, Inc., Confidential Concept Engineering Report, April 6, 1970.
2. Roy F. Weston, Inc., Confidential Concept Engineering Report, April 2, 1970.
3. Wittenmyer, J.D., and Sak, J.G., Plastic Media Roughing Filter Provides Most
Economical Plant Expansion. Presented at the Ohio Water Pollution Control
Association, June 15, 1967.
4. Wittenmyer, J.D., A Look at the Future Now. Presented at the Ohio Water Pollution
Control Conference, June 20 1969.
S. Private communication with J.D. Wittenmyer, Vice-President, Ohio Suburban Water
Company, Dayton, Ohio, January 22, 1971.
6. Bums & McDonnell Engineering Company, Report on Sewage Treatment Plant and
Sanitary Sewer Improvements for Sedalia, Missouri, 1963.
7. Dague, R.R., et al, Contact Stabilization: Theory, Practice, Operational Problems and
Plant Modifications. Presented at the 43rd Annual Conference - WPCF, Boston, Mass.
(October 4-9, 1970).
8. Wirts, J.J., The Use of Organic Poly electrolyte for Operational Improvement of Waste
Treatment Processes. Federal Water Pollution Control Administration, Grant
No. WPRD 102-01-68, May, 1969.
13-23
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CHAPTER 14
OPERATION AND MAINTENANCE REQUIREMENTS FOR UPGRADED PLANTS
14.1 General
The discussions in the preceding chapters of this manual demonstrate the fact that
upgrading existing wastewater treatment facilities involves the consideration and possible
inclusion of a multiplicity of processes and process equipment. Operation and maintenance
requirements for a plant incorporating such a diversity of processes are frequently complex.
Since this manual is directed principally to the process design of upgraded facilities, it
is not appropriate to the scope or purpose of this document to present a detailed operation
and maintenance manual. Rather, some guidelines and pertinent discussion will be presented
to assist in the development of an operation and maintenance program for an upgraded
plant.
14.2 Responsibility of the Design Engineer
One of the first and more obvious considerations to the engineer faced with upgrading
existing wastewater treatment facilities is the need to consider current plant operation
and maintenance, either as causes of problems requiring upgrading or as means of
accomplishing the required improvement in plant performance. As has been discussed in
the preceding chapters dealing with individual unit processes and operations, an important
first step in upgrading plant performance is to review operational and maintenance practices,
and the adequacy and competency of plant operating staff and supervision. It is frequently
possible to effect a significant improvement in plant performance, without the need for
capital expenditures, by modification of plant practices, by addition of operating and/or
maintenance staff, by improved or supplemented supervision, or by training of existing
staff.
In September, 1970, the Federal Water Quality Administration (now EPA-OWP) issued
"Federal Guidelines - Design, Operation and Maintenance of Wastewater Treatment
Facilities" (1). The latter section of this document includes "Guidelines for Operation
and Maintenance". Projects for which Federal grant assistance is requested are expected
to comply with these guidelines as well as with technical bulletins which the Environmental
Protection Agency will be issuing from time to time. The aforementioned guidelines provide
information on the basic minimum requirements for federally assisted projects on such
matters as personnel, records, reports, laboratory control, and process control, and present
a suggested guide for an operation and maintenance manual.
The design engineer has the responsibility to be fully knowledgeable of the operational
factors of the plant for which he is responsible and to relate that knowledge to the
alternative means of upgrading the plant's performance. For instance, in both the selection
and evaluation of alternative upgrading measures, he must consider total annual costs,
14-1
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including operation and maintenance, as well as capital construction costs. Some guidelines
to assist in such comparisons have been included in the previous discussions on unit
processes.
The design engineer should make it a point to obtain all pertinent plant operating records
and to solicit the observations, comments, and suggestions of the operating staff so that
optimum use may be made of the experience and knowledge of the particular plant and
system. He should also prepare or assist in the preparation of operation and maintenance
manuals for the upgraded plant, and in the on-site training of operating, maintenance
and supervisory personnel to ensure that the function, capabilities, and limitations of the
upgraded facilities are adequately communicated to those responsible for their operation.
He should be retained to remain involved during at least the initial operational period
to utilize most effectively his expertise and knowledge of the process.
The design engineer also has the responsibility to assess the adequacy of available
information on existing plant performance, to define the additional data required, and
to develop and direct or execute a program to obtain such information. A program to
obtain adequate design information can range anywhere from simple qualitative
observations through quantitative samplings and analysis programs to laboratory or
pilot-scale treatability studies. This charge presents a very real challenge to his professional
judgement, requiring the balancing of the constraints of economics, timing, and staff
availability against the need for a fully reliable and effective treatment facility. The
information presented in this manual provides direction and guidance to assist the design
engineer in the exercise of this professional judgement.
14.3 Instrumentation and Automatic Operation
Historically, wastewater treatment plants have not incorporated sophisticated
instrumentation or automated operation. However, with the advent of improved
instrumentation and the need in many cases to consistently maintain plant effluent quality,
the use of such systems may be indicated.
14.3.1 Instrumentation
In general, instrumentation leads to improved performance, increased efficiency, and
reduced operating costs. It also offers permanent records for system evaluation, and
generates data for required State regulatory reports. There are three basic types of
parameters which may be monitored in wastewater treatment: mechanical, physical, and
chemical. Table 14-1 contains a summary of instrumentation available for use in wastewater
treatment plants (2). Some common on-line instruments which have application for various
unit processes in a conventional wastewater treatment processes follow.
14-2
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Table 14-1
Wastewater Treatment Instrumentation
Primary Measuring Method
Pressure
Electric and electronic
Photoelectric
Differential pressure
Radiation absorption
Electrochemical reaction
Metering Function
Pressure
Level
Mechanical position or motion Level
Position
Temperature
Flow
Level
Turbidity
Total carbon analyzer
Flow
Ratio control
Specific gravity
Density
Moisture content
Level
D.O.
pH
ORP
C\2 concentration
Conductivity
Some Applications
Pump discharge pressure
Force main pressure
Wet-well level
Chemical storage level
High-water alarm
Gas storage pressure
Digester cover level
Valve position indication
High-water alarm
Incinerator temperature
Wet-well overflow
Diversion overflow
Digester temperature
Magnetic flow metering
Final effluent turbidity
BOD
Wastewater flow
Air flow
Cl2 flow
Treatment chemical flow
Flow proportioning
Sludge density control
Wet-well level
Sludge-well level
Filter cake moisture content
Aeration tank D.O.
Digester pH
Final effluent pH
Cl2 residual
Effluent D.O.
Source: Salvatorelli, JWPCF (2)
14-3
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14.3.1.1 Bar Screen Control
Instrumentation for bar screen control should include an inlet level indicator which actuates
the cleaning mechanism when the water reaches a predetermined level in the influent
chamber. In addition, a high water surface level alarm and a high torque alarm in the
bar screen cleaning mechanism should be considered.
14.3.1.2 Grit Chamber Control
Grit chamber controls should include a high torque alarm on motor controls and perhaps
a timer on grit removal mechanisms. Both the bar screen and grit chamber high torque
alarms may be connected to a high torque motor shut down.
14.3.1.3 Raw Wastewater Flow Measurements
Some means of measuring wastewater flow include flumes, venturi meters, orifice plates,
and magnetic flow meters. Flow measurement is required to provide a record of influent
flow to evaluate plant and unit process efficiencies and to dispense chemicals.
14.3.1.4 Primary Clarification Control
One method of measuring primary sludge is with a magnetic flow meter. Self heating
meters are available and quite useful where grease is a problem. Sludge depth can be
indicated using either infra-red adsorption, photo-electric opacity, ultra-sonic amplitude
detection, or radiation amplitude detection. Sludge pumping can then be initiated
automatically and controlled by a predetermined timing operation or by using a sludge
density meter in the clarifier underflow line.
14.3.1.5 Secondary Clarification Control
The motor controls of the secondary clarifier, like the primary clarifier, should be
connected to a high torque alarm and motor shut down switch. Control of secondary
sludge drawoff must be considered in conjunction with subsequent sludge dewatering
equipment. The control of secondary sludge is much more sensitive than control of primary
sludge.
Sludge blanket depth can be determined by the same means as used to determine primary
sludge depth. Present measuring and sensing equipment is not as sensitive as required for
most secondary sludges. A measurement of sludge blanket depth with a predetermined
timed sludge drawoff may prove to be the optimum instrumentation. Control of recycled
sludge is probably best accomplished by utilizing a magnetic flow meter as the flow element.
A conventional flow element can be used, but would be recommended only if supplemented
with a continuous purge system, inspection access, and a good maintenance program. The
control loop should include the flow element and controller with proportional and reset
controls. Waste sludge should likewise be metered and recorded.
14-4
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14.3.1.6 Dissolved Oxygen Control (Activated Sludge)
Reasonably reliable and accurate dissolved oxygen monitoring equipment has been
developed and field demonstrated in recent years. Automatic control of dissolved oxygen
(D.O.) in activated sludge plants is generally more useful and justifiable on the basis of
economics of operation rather than on reliability or quality of performance. That is,
automatic D.O. control can be used to more effectively match power input to oxygen
demand than to upgrade biological removal kinetics. Placement of the D.O. probe to obtain
a true average reading is a difficult problem. Assistance from the probe manufacturer should
be requested if necessary.
14.3.1.7 Trickling Filtration Systems
Trickling filtration generally requires less sophisticated instrumentation than does activated
sludge. The instrumentation for trickling filters generally includes control of effluent and
sludge recycle depending on the individual flow diagram. Control versatility must be
provided if a two-stage plant is to be operated in series and parallel.
14.3.1.8 Chlorination Systems
The chlorination system is generally controlled with a final residual analyzer. The system
should include recording as well as control equipment.
""" 14.3.1.9 Sludge Handling Systems
Instrumentation packages for sludge handling systems, including air flotation, anaerobic
digesters, vacuum filters, and centrifuges, are often supplied by the individual manufacturer.
Coordination is required to couple control to input and output systems, and to relay
required information to a central control panel, if used. Detailed discussion of the specific
instrumentation involved is omitted here because of the many approaches which may be
applicable.
14.3.1.10 Level Control
Level control may be critical in the operation of various unit processes. High and low
level alarm may be coupled with pump control as required. Pneumatic bubbler sensing
systems offer the simplest approach to level control. A water system that creates an air
pressure suitable for a level sensor is now available and is quite useful in systems where
there is no instrument air available.
14.3.1.11 Foam Detection
Foam levels can be detected by use of photoelectric cells which use a modulated output.
The output from the foam detection device can then actuate foam control equipment.
14-5
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14.3.2 Automated Instrumentation
Presently there are automated wet chemical procedures for COD, ammonia, phosphates,
and other specific materials. In addition, combustion procedures for total carbon (TC),
total organic carbon (TOC), and total oxygen demand (TOD) have been developed and
applied successfully. These automated analytical systems provide levels of sensitivity which
are not normally obtained in manual operation of the same procedures, and in some cases
can be used for process control.
14.3.3 Centralized Control Panel
The use of a centralized control panel should be considered in any fully instrumented
system. Instant data availability at a central control point permits immediate reaction to
alarm conditions and, with proper manual standby controls, gives the experienced operator
complete control over most situations.
14.3.4 Summary
The decision to utilize sophisticated instrumentation and/or automatic controls depends,
among other factors, on plant size and the importance of consistently maintaining effluent
quality.
There is a scarcity of information regarding the current investments made in controls for
wastewater treatment plants. Salvatorelli (2) reported that approximately 4.5 percent of
the total capital cost of a recently constructed municipal wastewater facility was invested
to completely monitor and control the facility. A somewhat analogous comparison is the
12 percent invested by chemical and petroleum industries in 1968 for manufacturing
process controls (3). Andrews (3) reported that, based strictly on an economic analysis,
a 9 percent investment in instrumentation and controls for a 100 mgd activated sludge
plant could be justified if the plant performance could be improved from 87 to 92 percent
BOD removal. Smaller plants (less than 5 mgd) may not be able to justify expenditures
for such instrumentation and controls even if plant performance could be improved as
above. The possibility should be analyzed carefully and weighed against other alternatives,
e.g., better use of the money through additional treatment capability.
14.4 Operation and Maintenance Requirements
Operation and maintenance requirements (and their associated costs) are generally defined
by three factors: chemicals, required power, and labor.
14.4.1 Chemicals
Chemical quantities and their associated annual costs can be readily estimated from required
dosage rates (average for annual costs, maximum for storage and feed rate requirements),
wastewater flow rates (average and maximum considerations as with dosage), and unit
14-6
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costs appropriate to the particular chemicals used. The designer should bear in mind that
the form in which the chemical is received, stored, or fed can have a significant impact
on the amount of operating labor required in its handling and feeding, as well as on
the initial construction costs of the handling and feeding facilities. This is an individual
consideration requiring evaluation of relative costs as delivered for the various forms, the
availability of existing storage, handling, and feeding facilities, and staffing policies.
14.4.2 Power
Incremental power requirements and costs are likewise relatively simple to estimate, being
the function of additional connected horsepower, its operating frequency and duration,
and unit power costs. Local utility policy with respect to demand rates, of course, must
be included in the economic comparison. In addition, recent recommendations by EPA
state that the treatment facility should be capable of satisfactory operation during
emergencies and power failures (1). To achieve this degree of reliability, duplicate sources
of power for essential plant elements must be provided.
14.4.3 Labor
EPA will be issuing recommended wastewater treatment plant staffing guides in the near
future. These guides will offer the engineer a means of more accurately estimating
manpower requirements by establishing a step-by-step methodology for itemizing those
tasks which must be performed at treatment plants, and for applying related manpower
needs to these tasks. One step further is the need for employing personnel who are qualified
to adequately and efficiently operate the wastewater treatment plant. Most states currently
have mandatory operator certification programs, while the majority of the others have
voluntary operator certification programs.
14.4.3.1 Operating Manpower Requirements
The first step in the development of estimates of additional operating manpower
requirements for upgraded facilities is a thorough analysis of existing operations to establish
the adequacy of present staff and operational procedures. This must be done whether
the upgrading procedure selected involves the modification of operating procedures or
requires the addition or modification of unit processes. The analysis should account for
specific local conditions, problems, and objectives.
Following this analysis of existing wastewater treatment facilities, the engineer must make
a judgement as to the efficiency of operations and the qualifications of the staff in relation
to performance of upgraded facilities. Factors effecting such a judgement include:
1. The physical arrangement and geographic dispersion of the system.
2. The relative complexity of the present versus the upgraded facilities.
14-7
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3. The level of training and experience of the present staff in relation to the level
required by upgraded processes.
4. The relative degree of laboratory and analytical control required for the upgraded
versus the existing facilities.
The operational analysis and estimate of manpower requirements should be done on an
individual basis for each plant being considered for upgrading. Manpower requirements
should be estimated and recommended as dictated by the specific local conditions.
14.4.3.2 Maintenance Manpower Requirements
Similar to the development of estimates for additional operating manpower requirements
for upgrading facilities, the first step in the estimation of additional maintenance manpower
requirements is a thorough analysis of existing maintenance staff and procedures. The
development and implementation of an appropriate preventive maintenance program for
wastewater treatment facilities alone can represent a significant upgrading procedure to
the extent that it provides for reliable, consistent plant operation and performance. Again,
the design engineer responsible for facility upgrading must exercise his judgement as to
the sufficiency of maintenance programs and the adequacy of the qualifications and
performance of the maintenance staff. This information must in turn be related to the
requirements of the upgraded processes and facilities. The factors influencing the engineer's
judgement on these aspects of facility performance are similar to those discussed previously
on the topic of operation requirements.
In the estimation of total maintenance costs for upgraded facilities, the engineer must
include maintenance materials and supplies in addition to maintenance labor. Again, he
is usually dealing with incremental costs and must assess the adequacy of present lubrication
practices, spare parts stocking and replacement practices, and similar maintenance
procedures involving expendable materials and supplies.
14.5 References
1. Federal Guidelines - Design Operation and Maintenance of Waste Water Treatment
Facilities. Federal Water Quality Administration, September, 1970.
2. Salvatorelli, J.J., Value of Instrumentation in Wastewater Treatment. Journal Water
Pollution Control Federation, 40, No. 1, pp. 101-111 (1968).
3. Andrews, John F., Control of Wastewater Treatment Plants - the Engineer as an
Operator. Water and Sewage Works, 118, No. 1, pp. 26-32 (1971).
14-8
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ACKNOWLEDGEMENTS
This manual was prepared by Roy F. Weston, Inc. under the sponsorship of the
Environmental Protection Agency. The technical guidance and assistance of the
Environmental Protection Agency staff during the preparation of the manual are gratefully
acknowledged.
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• I Accession Number
w
n I Subject Field & Group
050
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Environmental Protection Agency
f. \Tltlo
Process Design Manual for Upgrading Existing Wastewater Treatment Plants
JQlAotbotfsJ
P. Krlshnan
C.H. Hangan
16 1 p<0Jae">M
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