r/EPA
United States
Environmental Protection
Agency
Office of Wastewater Enforcement
and Compliance, Office of Water
Washington. DC 20460
November 1991
Guidance for NPDES
Compliance Inspectors
Evaluating Sludge
Treatment Processes
"?•'.?'.• Printpd on Recycled Paper
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GUIDANCE FOR NPDES COMPLIANCE INSPECTORS:
INSPECTION OF SLUDGE TREATMENT UNIT PROCESSES
November 1991
Submitted to:
U.S. Environmental Protection Agency
Office of Wastewater Enforcement and Compliance
401 M Street, SW
Washington, DC 20460
Submitted by:
Science Applications International Corporation
7600-A Leesburg Pike
Falls Church, VA 22043
EPA Contract No. 68-C8-0066, WA Nos. C-2-6 (E) and C-3-6 (E)
SAIC Project Nos. 01-0834-03-0606-001 and 01-0834-03-2156-001
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ACKNOWLEDGEMENT
This document was prepared under the technical direction of Lee Okster, Enforcement Division,
Office of Water Enforcement and Permits, U.S. Environmental Protection Agency. Assistance was
provided to EPA by Science Applications International Corporation of McLean, Virginia, under EPA
Contract No. 68-C8-0066. Susan Moore managed the SAIC participation in this effort under Work
Assignment Nos. C-l-11, C-l-41, C-2-6 and C-3-6. The principal authors are: Yvonne Ciccone, Steve
Dowhan, Keith Eckert, Jack Faulk, William Hahn, Brian Hillis, Mark Klingenstein, Susan Moore, Ruth
Much, Christopher Vilord, and Mary Waldron.
The objective of the authors was to compile information into one reference for inspectors charged
with the responsibility of evaluating sludge treatment processes. The authors drew information from
many references, as indicated in the bibliography. In particular, the authors would like to acknowledge
five EPA documents from which a great deal of information was excerpted in whole or in part:
• NPDES Compliance Inspection Manual
• Process Design Manual for Sludge Treatment and Disposal
• Operations Manual, Sludge Handling and Conditioning
• Field Manual for Performance Evaluation and Troubleshooting at Municipal Wastewater
Treatment Facilities
• Inspectors Guide for Evaluation of Municipal Wastewater Treatment Plant
Special thanks are also extended to the review team for their time and effort to ensure that this
document was as useful as possible. This review team included Robert Bastian, Anne Lassiter, Lee
Okster, and Mark Charles of EPA Office of Wastewater Enforcement and Compliance; Dianne Stewart,
Jessica Kaplan, and Robert Linett of SAIC; Bob Brobst, Sludge Coordinator for EPA Region 8; John
O'Grady, Sludge Coordinator for Region 5; and Robert Heiniger, Solid Waste Enforcement Division,
Maryland Department of the Environment.
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TABLE OF CONTENTS
INSPECTION OF SLUDGE TREATMENT UNIT PROCESSES
1. INTRODUCTION 1-1
2. GRAVITY THICKENING 2-1
3. DISSOLVED AIR FLOTATION THICKENING 3-1
4. CENTRIFUGATION 4-1
5. AEROBIC DIGESTION 5-1
6. ANAEROBIC DIGESTION 6-1
7. HEAT TREATMENT 7-1
8. WET AIR OXIDATION 8-1
9. INCINERATION 9-1
10. COMPOSTING 10-1
11. CHEMICAL STABILIZATION AND CONDITIONING 11-1
12. VACUUM FILTER 12-1
13. FILTER PRESS 13-1
14. BELT FILTER PRESS 14-1
15. SLUDGE DRYING BEDS 15-1
16. SLUDGE DRYING LAGOONS 16-1
17. HEAT DRYING 17-1
18. DISINFECTION 18-1
11
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TABLE OF CONTENTS (Continued)
Page
APPENDIX A: CHECKLISTS
Gravity Thickening A-3
Dissolved Air Flotation Thickening A-7
Centrifugation A-13
Aerobic Digester A-17
Anaerobic Digester A-23
Heat TreatmentAVet Air Oxidation A-29
Incineration A-35
Composting A-41
Chemical Stabilization/Conditioning A-49
Vacuum Filter A-55
Filter Press A-61
Belt Filter Press A-65
Sludge Drying Beds A-69
Sludge Drying Lagoons A-73
Heat Drying A-77
Beta or Gamma Irradiation A-83
APPENDIX B: BIBLIOGRAPHY
111
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LIST OF TABLES
1 Gravity Thickener Typical Loadings and Performance 2-6
2 Flotation Thickener Operation and Performance 3-6
3 Operating and Design Conditions for Aerobic Sludge Digestion 5-5
4 Supernatant Characteristics from Anaerobic Digesters 6-10
5 Operating and Design Conditions for Anaerobic Sludge Digestion 6-11
6 Typical Dosage Ranges for Chemical Conditioning 11-5
7 Typical Dewatering Performance Data for Rotary Vacuum Filters-
Cloth Media 13-5
8 Typical Dewatering Performance Data for Rotary Vacuum Filters-
Coil Media 13-6
9 Typical Results of Pressure Filtration 13-4
10 Typical Data for Various Types of Sludges Dewatered on a Belt Press 14-7
11 Typical Performance Data for Drying Beds 15-7
12 Suggested Minimum and Optional Monitoring for Heat Drying Processes 17-8
13 Troubleshooting Guide for Heat Drying Operations 17-13
14 Operating Parameters for Achieving Pathogen Reduction 18-4
15 Processes Determined to Be Equivalent to PSRP or PFRP 18-5
IV
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LIST OF FIGURES
Page
1 Sludge Management Alternatives 1-4
2 Gravity Thickener 2-4
3 Dissolved Air Flotation Thickener 3-2
4 Continuous Countercurrent Solid Bowl Conveyor Discharge Centrifuge 4-2
5 General Schematic of Imperforate Basket Centrifuge 4-3
6 Schematic of a Disc Nozzle Centrifuge 4-5
7 Summary of the Anaerobic Digestion Process 6-2
8 Configuration of Anaerobic Digesters 6-4
9 Fixed and Floating Digester Covers 6-6
10 General Thermal Sludge Conditioning Flow Scheme for a
Non-Oxidative System 7-2
11 Volatile Solids and COD Content of Sludge Treated by Wet Air Oxidation 8-2
12 Flow Chart for High Pressure/High Temperature Wet Air Oxidation 8-4
13 Cross-Section of a Multiple-Hearth Furnace 9-3
14 Cross-Section of a Fluidized Bed Furnace 9-5
15 Process Zones in a Multiple-Hearth Furnace 9-9
16 Cutaway View of a Drum or Scraper-Type Rotary Vacuum Filter 12-2
17 Operating Zones of a Rotary Vacuum Filter 12-3
18 Rotary Vacuum Filter System 12-7
19 Side View of a Filter Press 13-2
20 Typical Sand and Gravel Drying Bed Construction 15-2
21 Typical Paved Drying Bed Construction 15-4
22 Cross-Section of a Wedge-Wire Drying Bed 15-4
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LIST OF FIGURES (Continued)
23 Flash Dryer System
Page
17-3
24 Rotary Kiln Dryer .......................................... 17~5
25 Schematic for a Rotary Dryer ...................................
26 Equipment Layout for Electron Irradiation Facility ...................... ^-7
27 Schematic of Gamma Irradiation Facility ............................ 18-10
VI
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INSPECTION OF SLUDGE TREATMENT UNIT PROCESSES
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1. INTRODUCTION
This is a companion document to the guidance on evaluating compliance with sludge requirements
during NPDES inspections. This document was compiled to serve as a reference providing detailed
information about sludge treatment processes to NPDES inspectors. While it is not a design manual or
an operation and maintenance manual, it does include a description of each process configuration and its
major components. It summarizes process control considerations. This manual also contains a checklists
to facilitate an evaluation of the performance of each unit process. This document covers sludge
treatment processes commonly used throughout the United States. Because additional technologies are
being developed or introduced from other countries, this manual will be updated periodically.
In inspecting sludge processing treatment trains, it is important that the inspector be cognizant of
certain fundamentals:
• Sludge processing is an integral part of any biological wastewater treatment system. Sludge is
the primary by-product of successful treatment, and efficient removal of sludge from biological
treatment systems is essential to the successful operation of these systems.
- Successful sludge processing, like successful wastewater treatment, requires the proper
integration of a number of unit processes in order to effect desired change hi sludge volumes
and characteristics. Because of this, the inspector must go beyond the evaluation of
individual unit processes and use these evaluations of the individual unit processes as the
basis for an overall evaluation of the solids handling train.
• Sludge processing arguably poses the greatest challenges hi wastewater treatment from the
standpoints of design, operation and maintenance. As a result, shortcomings hi sludge
processing systems are very common. These shortcomings not only prevent compliance with
40 CFR Part 257, but they also frequently contribute to the treatment plant's noncompliance with
its NPDES permit limitations.
In order to adequately assess both individual unit processes and the sludge processing train as a
whole, it is necessary for the inspector to first fully understand all of the functions performed by sludge
processing. The inspector must then combine this understanding with knowledge of the mechanics of
inspecting and evaluating each of the individual unit processes. To assist the reader hi achieving these
goals the remainder of this manual is broken into separate chapters, each addressing a different sludge
treatment process.
1-1
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The remainder of this chapter discusses solids processing as a whole and describes the basic
functions carried out by each of the general categories of unit processes. The following chapters provide
an overview of the various technologies used to accomplish each basic function. Each chapter is
organized to first describe the technology/unit process, and then, provide guidance to the inspector on
how to go about evaluating the design, operation, and maintenance of each technology/unit process.
1.1 BACKGROUND ON SLUDGE TREATMENT
As noted previously, sludge generation is a byproduct of primary, secondary, and advanced
wastewater treatment processes. In evaluating a particular wastewater treatment plant's sludge processing
train, it is important to ascertain the characteristics of the sludge(s) being processed. In general, three
types of raw sludge are likely to be generated by POTWs.
• Primary Sludge—This sludge consists of material removed from the raw wastewater by
sedimentation. As such, raw primary sludge typically displays the following characteristics:
Total solids (percent of wastewater): 2 to 8
- Volatile solids (percent of TS): 60 to 80
Grease content (percent of TS): 6 to 30
Raw primary sludge is typically grey in color and has an offensive odor. It typically contains
heavy solids, fecal matter, food particles, and vegetative matter.
• Secondary Sludge—This sludge consists primarily of excess microorganisms from the biological
populations responsible for effecting secondary treatment. In the case of activated sludge
systems, waste activated sludge (WAS) is removed from the system by the operators in order
to maintain a relatively constant population size. In fixed film systems, such as trickling filters
and rotating biological contractors, the sludge produced is the result of the continual sloughing
of organisms from the biological "film." Typical characteristics of raw waste activated sludges
are as follows:
Total solids (percent of wastewater): _<.! percent
- Volatile solids (percent of TS): 75 to 80 percent
Biological sludges are brown in color, exhibit a visible floe structure, and have an inoffensive
odor when fresh. Settling and compaction characteristics of waste activated sludges are poor
compared to primary sludge. This is due primarily to the presence of "bound water" within the
bacterial cells present in biological sludges.
• Other—Other sludges which may be encountered by the inspector may include:
Chemically precipitated sludges—These sludges may result from chemically assisted primary
or secondary sedimentation, or from advanced wastewater treatment processes. The
1-2
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characteristics of these sludges are dependent on both the unit process into which the
chemical(s) are introduced and the chemical(s) used.
- Combined sludges—most commonly combined sludge consists of primary and thickened
secondary sludge.
The primary goals of sludge processing are to:
1. Facilitate the removal from the treatment system the amounts of sludge necessary to allow
proper, efficient treatment of wastewater.
2. Allow disposal or reuse of the sludge in an efficient, environmentally sound manner that protects
both human health and impacts on the natural environment.
In order to achieve these goals, sludge processing trains typically incorporate unit processes which fall
into several basic functional categories; these categories are described briefly below.
• Thickening—Carried out to increase the concentration of solids hi the sludge. This is done
primarily to improve the efficiency with which further processing (e.g., stabilization,
dewatering) is carried out.
• Stabilization—Carried out to minimize the tendency of the sludge to putrefy following disposal
hi the environment, to reduce the level of pathogens hi the raw sludge, and (hi the case of
anaerobic digestion) to produce methane gas that can be used as a fuel source.
• Conditioning—Generally practiced to improve the dewatering characteristics of the sludge.
• Dewatering—Involves the removal of a significant fraction of the liquid hi the sludge. This is
done to minimize the volume of sludge to be disposed, reduce the tendency for the sludge to
attract disease vectors following disposal, and to meet landfill minimum moisture content
limitations.
• Disinfection—Carried out to reduce the pathogen content of the sludge. Many stabilization
processes also provide some degree of disinfection.
Sludge stabilization and disinfection can be completely separate processes, but frequently these
functions are performed hi a single process.
Figure 1 provides an overview of the various unit processes which can be used to carry out the
functions described above.
1-3
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THICKENING STABILIZATION CONDITIONING DEWATERING DISPOSAL
DISINFECTION
Primary
Sludge
Primary and
Secondary
Sludge
Chemical
Sludge
Secondary
Sludge
Heat
Treatment
Stabilization
Marketing and
Distribution
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1.2 UNIT PROCESS EVALUATION
This remainder of this document provides information and guidance intended to assist the inspector
in evaluating specific sludge unit processes. Processes covered are grouped according to the basic
functions described in the previous section; these processes and the corresponding page designation are
as follows:
Function Page
• Thickening
Gravity Thickening 2-1
Dissolved Air Flotation Thickening 3-1
Centrifugation 4-1
• Stabilization
Aerobic Digestion 5-1
Anaerobic Digestion 6-1
Heat Treatment 7-1
Wet Air Oxidation 8-1
Incineration 9-1
Composting 10-1
Chemical Stabilization
and Conditioning 11-1
• Conditioning
Chemical Stabilization
and Conditioning 11-1
Heat Treatment 7-1
• Dewatering
Vacuum Filter 12-1
Filter Press 13-1
Belt Press 14-1
Centrifuge 4-1
Sludge Drying Beds 15-1
Sludge Drying Lagoons 16-1
Heat Diyers 17-1
• Disinfection 18-1
Each unit process subsection provides information on the following:
• Process Configuration and Components
• Process Control Considerations
• Process Performance Evaluation
1-5
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2. GRAVITY THICKENING
Gravity thickening is the most commonly used sludge concentration process in the United States.
Gravity thickness are similar to sedimentation basins used in primary and secondary treatment and serve
to gently agitate the sludge to aid in its concentration by promoting the release of trapped waster and
gases. Thickening hi a concentration technique hi which relatively thin sludges such as waste activated
sludges are increased hi solids content hi order to reduce the total sludge volume, which, in turn, allows
a reduction hi the size of subsequent treatment units. A tremendous reduction in volume can be achieved
through a modest increase hi solids content. To aid in this process, chemicals may be used to enhance
the gravity thickening of waste activated sludges.
During the gravity thickening process, solids settle by gravity to the bottom of the thickener forming
a sludge blanket with a partially clarified liquid or supernatant above. The supernatant flows over the
effluent weirs and is returned to the treatment plant headworks. Thickening takes place as the sludge
particles compact at the bottom of the tank. As the drive unit turns the sludge collection mechanism the
blanket is gently stirred, which helps compact the sludge solids and release water from the mass. Sludge
solids are scraped toward a center well and withdrawn.
The efficiency of a gravity thickener is influenced by the following sludge characteristics:
• Type of sludge
• Age of sludge
• Sludge temperature
• Solids concentration.
Both the type and age of sludge to be thickened can have pronounced effects on the overall
performance of gravity thickeners. Fresh primary sludge usually can be concentrated to the highest
degree. If the primary sludge is septic or allowed to go anaerobic, hydrogen sulfide (t^S), methane
(CHJ, and carbon dioxide (COj) gases may be produced (gasification). If gas is produced, it will attach
to sludge particles and carry these solids to the surface. The net effects of gas production due to
anaerobic conditions will be reduced thickener efficiency and solids concentration, and increased sludge
volumes.
2-1
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Biological secondary sludges are not as well suited for gravity thickening as primary sludge.
Secondary sludges contain large quantities of microorganisms. These biological solids are composed of
approximately 85 to 90 percent water by weight within the cell mass. The "bound" water contained
within the cell wall makes the sludge less dense than primary sludge solids. Therefore, gravity thickening
of only biological secondary sludge is rarely, if ever, practiced. More commonly, biological secondary
sludge is combined with primary sludge prior to gravity thickening.
The fact that biological solids contain large volumes of cell water and are often smaller or finer in
size than primary sludge solids makes them harder to separate by gravity concentration. The age of the
biological secondary sludge also plays an important role in the efficiency of the gravity thickening
process. Generally, as the age of the biological secondary sludge increases, nitrate levels in the sludge
will also increase. If this sludge is retained under anoxic conditions (as is likely in gravity thickeners),
denitrification will occur and result in rising solids and excessive solid concentrations in the thickener
overflow. If the "old" sludge is kept in an aerobic condition, it will thicken more readily than a younger
sludge. One method of ensuring that a gravity thickener remains aerobic is through the addition of
"dilution" water. This is a clear, relatively high dissolved oxygen (DO) water (typically plant effluent)
that acts to provide needed oxygen to the thickener.
One other problem associated with activated sludge is "sludge bulking." A predominance of
filamentous organisms in the sludge results in a bulking sludge which settles and compacts poorly. This
in turn results in lower solids concentrations and the increased likelihood of solids loss from the gravity
thickener. Low pH, low DO, and/or low nutrients may cause growth of filamentous organisms in the
aeration tanks.
Another sludge characteristic which affects the degree of thickening is the temperature of the sludge.
As the temperature of the sludge (primary and biological secondary) increases, the rate of biological
activity is increased and the sludge tends to gasify and rise at a faster rate. During warm weather
operation the settled sludge has to be removed at a faster rate from the thickener than during cold weather
operation. When the sludge temperature is lower during the winter, biological activity and gas production
proceed at a lower rate.
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2.1 PROCESS CONFIGURATION AND COMPONENTS
Gravity thickeners are typically circular and resemble circular clarifiers. The main components of
gravity thickeners, as shown in Figure 2 are:
• Inlet and distribution assembly
• Sludge rake to move the sludge to a sludge hopper
• Vertical steel members or "pickets" mounted on the sludge rake
• Effluent or overflow weir
• Scum removal equipment.
The inlet or distribution assembly usually consists of a circular steel skirt or baffle which originates
above the water surface and extends downward approximately 2 to 3 ft. below the water surface. The
sludge to be thickened enters the assembly, and flows downward under the steel skirt and through the
tank where the solids settle to the bottom. The inlet assembly provides for an even distribution of sludge
throughout the tank and reduces the possibility of short-circuiting to the effluent end of the thickener.
The sludge rake provides for movement of the settled (thickening) sludge. As the rake slowly
rotates, the settled solids are moved to the center of the tank where they are deposited in a sludge hopper.
The tank bottom is usually sloped towards the center to facilitate the movement of sludge to the collection
point. Typically, sludge pumps used to remove the thickened sludge from the collection point include
centrifugal recessed-impeller type pumps or positive displacement type pumps.
The vertical steel members (pickets) that are usually mounted on the sludge rake assembly provide
for gentle stirring or flocculation of the settled sludge as the rake rotates. This gentle stirring action
serves two purposes. Trapped gases in the sludge are released to prevent rising of the solids. Also,
stirring prevents the accumulation of a large volume of solids floating on the thickener surface that must
be removed as scum, and which will create nuisance and odor problems.
The effluent or thickener overflow flows over a continuous weir located on the periphery of the
thickener. This weir usually includes an effluent baffle to retain floating debris and a scum scraper and
collection system to remove these floatables.
2-3
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i
I
*<
o
Raised position
of truss arm
Effluent weir
Scraper blades
Hopper plow
Effluent
Elevation
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22 PROCESS CONTROL CONSIDERATIONS
Typically, the flow through the thickener is continuous and should be controlled to be as constant
as possible. Monitoring of the influent, effluent, concentrated sludge streams, and sludge blanket depth
should be done at least once per shift, and should include collection of samples for later laboratory
analysis. Table 1 lists typical gravity thickener loadings and performance parameters.
Under normal operating conditions, water at the surface should be relatively clear and free from
solids and gas bubbles. The sludge blanket depth is usually kept around 5 to 8 ft. The speed of the
sludge collectors should be fast enough to allow the settled solids to move towards the sludge collection
pump. The bottom sludge collectors should not be operated at speeds that will disrupt the settled solids
and cause them to float to the surface. Sludge withdrawal rates should be sufficient to maintain a constant
blanket level. Solids content in sludge withdrawn can frequently be increased by using intermittent
withdrawal. This helps prevent "ratholing" or "piping".
Process controls which can affect the performance of a gravity thickener include:
• Solids and hydraulic loadings
• Solids and hydraulic detention times
• Sludge blanket depths.
2.2.1 Solids and Hydraulic Loadings
The hydraulic loading or overflow rate is the total number of gallons applied per square foot of
thickener surface area per day (gpd/ft2). The solids loading is the total number of pounds of solids
applied per square foot of thickener surface area per day (Ibs/day/ft2). To achieve the optimum solids
loading rate with the solids concentrations typically fed to gravity thickeners, a low hydraulic loading
would be necessary. However, this low hydraulic loading causes excessive detention tunes, which results
hi septic conditions. Dilution water, provides a means to increase the hydraulic loading to the optimum
hydraulic loading rate without increasing solids loading rates. As mentioned above, dilution water is also
used to add oxygen to the sludge that is fed to gravity thickeners.
The solids and hydraulic loadings are affected by the efficiency of the clarifiers and the sludge
wasting rates. Increased sludge wasting rates will increase the hydraulic load on the thickener. Increased
efficiency of primary and secondary clarifiers will increase the solids load on the thickener.
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TABLE 1. GRAVITY THICKENER TYPICAL LOADINGS AND PERFORMANCE
Sludge type
Raw primary
Raw primary + FeCl3
Raw primary + low lime
Raw primary + high lime
Raw primary + WAS*
Raw primary + (WAS + FeCl3)
(Raw primary + FeCl3) + WAS
Digested primary
Digested primary + WAS
Digested primary + (WAS + FeCl3)
WAS
Trickling filter
Primary + trickling filter
Influent Solids
Concentration,
(percent)
2.0-5.0+
2.0+
5.0
7.5
2.0
1.5
1.8
8.0
4.0
4.0
1.0
1.0
2.0
Typical Solids
Loading Rate,
nb/ffVdav)
20 to 30
6
20
25
6 to 10
6
6
25
15
15
5 to 6
8 to 10
12 to 20
Thickened Sludge
Concentration,
(percent)
8.0 to 10
4.0
7.0
12.0
4.0 to 9.0
3.0
3.6
12.0
8.0
6.0
2.0 to 3.0
7.0 to 10.0
5.0 to 9.0
*WAS = Waste activated sludge
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2.2.2 Solids and Hydraulic Detention Times
The solids detention time is based on the amount of solids applied, the depth and concentration of
the sludge blanket, and the quantity of solids removed from the bottom of the thickener. The hydraulic
detention time is dependent upon the hydraulic loading. An excessive hydraulic detention time can allow
septic conditions to develop and produce odors. Short detention times can cause a washout of solids and
adversely affect the operation of the wastewater treatment processes. The solids detention time can be
controlled by controlling the depth of the sludge blanket.
2.23 Sludge Blanket Depths
The sludge blanket depth influences the solids detention time and degree of thickening. If the
blanket is maintained at too high a level and the solids detention time is excessive, gasification may
develop with subsequent rising sludge and deterioration of effluent quality. A certain amount of blanket
depth is desirable to obtain the maximum possible sludge concentration. The objective is to maintain as
much depth as possible to get the highest concentration of solids without the process going septic.
Operations should maintain 4-6 foot free settling zone above the sludge blanket.
23 PROCESS PERFORMANCE EVALUATION
When evaluating the performance of a gravity thickener the inspector should compare the actual
operating conditions to recommended conditions. An inspection checklist is included in Appendix A.
The inspection checklist is designed to assist the inspector in gathering the information and making the
calculations required to make the comparison.
The specific areas of concern regarding gravity thickeners are:
• Surface and overflow quality
• Sludge blanket depth and thickened sludge concentrations.
The overflow should be relatively clear and the liquid surface should be free of gas bubbles. The
effluent weirs should be level and free of debris. The sludge blanket depth should be deep enough to
obtain a good sludge concentration but not so deep that it produces gasification.
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3. DISSOLVED AIR FLOTATION THICKENING
Sludge thickening by dissolved air flotation (DAF) is a process well-suited for biological wastewater
treatment sludges. In this process, solids separation/concentration is achieved by making the solids float
and form a concentrated layer of sludge. DAF is especially effective on activated sludge, which is
difficult to thicken by gravity because of its low specific gravity.
Typically, a pressurized recycle stream is saturated with air. This supersaturated stream is mixed
with the sludge to be thickened at atmospheric pressure. The thousands of minute air bubbles released
at atmospheric pressure from the recycle stream adhere to, or are trapped by, the biological floes. The
resulting matrix of air and solids has a bulk specific gravity of less than 1.0. This low specific gravity
results in the floe rising to the surface of the thickener, where it can be removed from the liquid phase.
A sludge layer 8 to 24 hi. thick forms on the surface of the tank and can be removed by a skimming
mechanism for further processing. Flotation aids such as polymers can be used to increase performance.
3.1 PROCESS CONFIGURATION AND COMPONENTS
Figure 3 shows a typical air flotation system. Part of the effluent from the flotation unit is pumped
to a retention tank at 60 to 70 psig. Air is fed into the pump discharge line at a controlled rate and mixed
by the reaeration pump. The flow through the recycle system is controlled by a valve. Effluent recycle
ratios can range from 30 to 150 percent of the influent flow. The recycle flow and sludge feed are mixed
hi a chamber at the entrance to the unit. If flotation aids are used, they usually are fed into this mixing
chamber. The sludge particles are floated to the surface and the clarified effluent or subnatant flows over
a weir. The thickened sludge is removed by a skimmer. Bottom sludge collectors are used to remove
any settled sludge or grit.
The dissolved air system employs either a compressed air supply or an aspirator-type air injection
assembly to obtain a pressurized air-water solution. The key components of DAF thickener units are:
• Air compressor
• Pressurized retention tank
• Recycle pump
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Sludge Removal Mechanism •
Recycle Row
Bottom Sludge Collector
Unit Sludge Feed
Ullll C^IMUCIIl
Aux. Recycle Connect.
(Primary Tank 1
or Plant Effluent) \
(>R
Flotation Unit
^
4—
j
ecirculation Pump
V V_
ftmmtmm
I
*~~f
Thickened Sludge
Discharge
. Unit Sludge Feed
Recycle Flow
Reaeration Pump
Retention Tank
(Air Dissolution)
FIGURE 3. DISSOLVED AIR FLOTATION THICKENER
3-2
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• Pressure release valve
• Inlet or distribution assembly
• Sludge scrapers
• Effluent baffle.
The sludge to be thickened may be introduced at the bottom of the unit, through a distribution box,
and blended with a pressurized recycle stream. Alternatively, the entire influent stream is injected with
air and pressurized. Compressed air is either introduced into the retention tank directly or at some point
upstream of the retention tank. The pressurized and air-saturated liquid then flows to the distribution or
inlet assembly and is released at atmospheric pressure through a pressure-release valve. The decrease
in pressure causes the air to come out of solution hi the form of thousands of minute air bubbles. These
bubbles make contact with the sludge solids hi the distribution box and attach to the solids, causing the
solids to rise to the surface. An effluent baffle is provided to keep floated solids from going into the
effluent. This baffle extends approximately 2 to 3 in. above the water surface and 12 to 18 in. below
the surface. Clarified effluent flows under the baffle and leaves the unit through an effluent weir.
3.2 PROCESS CONTROL CONSIDERATIONS
Typically, the flow through the thickener is continuous and should be set to be as constant as
possible. Monitoring of the influent, effluent, and float sludge streams should be done at least once per
shift, and composite samples should be taken for later laboratory analysis. In addition, ajar "float" test
should be performed frequently to provide a visual indicator of the influent stream's tendency to form
a float layer, and of any inclination to leave fine solids in the subnatant. This test is usually performed
by drawing a sample of the pressurized influent flow into a glass cylinder, and observing the formation
of the float layer and clear subnatant.
Under normal operating conditions, the effluent stream should be relatively free of solids (less than
100 mg/1 suspended solids) and should resemble secondary effluent. The float solids will have a
consistency resembling that of cottage cheese. The depth of the float solids should extend approximately
6 to 18 hi. below the surface. The surface farthest from the float solids collection should be scraped
clean of floating solids with each pass of the sludge collection scrapers. If the sludge blanket is allowed
to build up (become too thick) and drop too far below the surface, thickened (floated) solids will be
carried under the effluent baffle and contaminate the subnatant.
3-3
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Proper control of a DAF thickener is based on:
• Retention tank pressure
• Recycle ratio
• Feed solids concentration
• Detention period
• Air-to-solids ratio
• Type and quality of sludge
• Solids and hydraulic loading rates
• Use of chemical aids.
Air pressure used in flotation is important because it determines the size of the air bubbles formed
and can affect the solids concentration and the subnatant (separated water) quality. Increases in pressure
or air flow typically produce greater float solids concentrations and a lower effluent suspended solids
concentration. There is an upper limit, however, because too much air breaks up the floe structure.
Recycle ratio and feed solids concentration are related. Additional recycle of clarified effluent does
two things:
• It allows more air to be dissolved because there is more liquid.
• It dilutes the feed sludge.
Dilution reduces the effect of particle interference on the rate of separation. Concentration of sludge
increases as the sludge blanket detention time increases.
The air-to-solids ratio is also important because it affects the sludge rise rate. The air-to-solids ratio
is the ratio of air feed to dry sludge solids feed, by weight. The air-to-solids ratio needed depends mostly
on sludge characteristics such as the sludge volume index (SVI). The most common air-to solids ratio
used for an activated sludge thickener is 0.02.
The quality of the activated sludge, as well as me solids concentration, will affect the performance
of the thickener. If the SVI of the activated sludge exceeds 200, the concentration of the float solids will
generally decrease.
-------
If either the solids or hydraulic loading becomes excessive, effluent quality declines and thickened
sludge concentrations are reduced. Typical maximum hydraulic loading or overflow rate is 0.80 gpm/ft2
at solids concentrations of 5,000 mg/1.
Chemical flotation aids (polymers) improve thickening and solids capture. The dosage must be
determined for each specific sludge, but dosages of 5 to IS Ibs/ton of sludge solids are common.
33 PROCESS PERFORMANCE EVALUATION
When evaluating the performance of a DAF thickener compare the actual operating conditions to
recommended operating conditions. Typical operating conditions for DAF thickeners are presented in
Table 2. An inspection checklist is included hi Appendix A. The inspection checklist is designed to
assist the inspector in gathering the information and making the calculations required to make the
comparison.
The inspector should visually inspect the effluent quality and float sludge characteristics. The
effluent from the DAF units should be relatively clear. Well-operated units should produce effluents
equivalent hi appearance to secondary clarifier effluent. If an unusually high amount of suspended solids
are exiting the unit in the effluent, the following parameters should be evaluated:
• Hydraulic and solids loading rates—Excessive loadings will cause solids to washout.
• Air to solids ratio—A low ratio will indicate an inadequate air application rate, and a high ratio
will indicate an excessive air application rate. An excessive rate will sheer the sludge floe and
prevent flotation.
3-5
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TABLE 2. FLOTATION THICKENER OPERATION AND PERFORMANCE
Operating Parameter
Solids loading, Ibs dry solids/hr/ft2
of surface
With chemicals
Without chemicals
Influent solids concentration, mg/1
Air-to-solids ratio
Blanket thickness, in.
Retention tank pressure, psig
Recycle ratio, % of influent flow
Expected Performance
Float solids concentration, %
Solids removal, %
With flotation aid
Without flotation aid
Ranee
2to5
Ito2
5,000 to 10,000 min.
0.02 to 0.04
8 to 24
60 to 70
30 to 150
Typical
2
1
5,000 min.
0.03
3 to 7
95
50 to 80
3-6
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4. CENTRIFUGATION
Centrifugation concentrates sludge solids by increasing the gravitational force. As sludge is rotated
in the centrifuge, solids, which are heavier than water, are sedimented. Centrifuges may be used as
thickening devices for activated sludge or as dewatering devices for digested or conditioned sludges.
Waste activated and digested sludges, which have a specific gravity similar to water due to their high
content of biological cell-bound water, are not amenable to some types of thickening/dewatering
processes. Centrifugation achieves particle separation by enhancing the difference in specific gravity
between the sludge solids and water.
4.1 PROCESS CONFIGURATION AND COMPONENTS
Three types of centrifuges (solid bowl, basket, disc) are typically used to thicken or dewater
wastewater sludges. The following section discusses the basic operation of each type of centrifuge.
The continuous-feed, solid-bowl, decanter centrifuge (or solid-bowl centrifuge) is the most widely
used type of centrifuge for dewatering sewage sludge (Figure 4), because it operates in a continuous flow-
through mode and because it has a low cost/capacity ratio. The solid bowl centrifuge consists of a
rotating bowl having a cylindrical-conical shape and a screw conveyor. Sludge enters the rotating bowl
through a stationary feed pipe that extends into the hollow shaft of the rotating screw conveyor, and is
distributed through ports into the rotating bowl. The gravitational force causes the solids to settle out on
the inner surface of the rotating bowl. The screw conveyor moves the sludge solids across the bowl, up
the inclined beach (conical section of bowl) and to the outlet ports. The lighter liquid, or centrate, pools
above the sludge layer and flows towards the centrate outlet ports located at the large end of the machine.
The pool depth is maintained by baffles located before the centrate outlet ports.
The basket centrifuge is also referred to as the imperforate-bowl, knife-discharge type, and is a batch
dewatering unit that rotates around the vertical axis (Figure 5). Sludge is fed into the unit at the bottom
center of the bowl through a stationary feed pipe. Sludge solids form a cake on the inside of the rotating
bowl while centrate flows over the top lip of the bowl. The duration of the feed time is
U.S. EPA Headquarters Library
Mail code 3201
4-1 1200 Pennsylvania Avenue NW
Washington DC 20460
-------
COVER
DIFFERENTIAL SPEED
GEAR BOX
MAIN DRIVE SHEAVE
CENTRATE
DISCHARGE
FEED PIPES
(SLUDGE AND
CHEMICAL)
BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
I
FIGURE 4. CONTINUOUS COUNTERCURRENT SOLID BOWL CONVEYER DISCHARGE CENTRIFUGE
-------
FEED
POLYMER
SKIMMINGS
KNIFE
CAKE
CAKE
FIGURE 5. GENERAL SCHEMATIC OF IMPERFORATE BASKET CENTRIFUGE
4-3
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controlled by either a preset timer or a centrate monitor. The centrate monitor shuts the feed pump off
when a certain level of suspended solids appears in the centrate. Deterioration in the centrate indicates
that the centrifuge bowl is filled with solids, and separation can no longer take place.
Once the feed is stopped the bowl begins to decelerate. When the bowl has decelerated to a certain
point a plow is activated and scrapes the solids from the bowl wall. The solids drop out the bottom into
a hopper, the plow retracts, and the bowl accelerates, starting a new cycle.
Figure 6 features a cut-away view of a disc nozzle centrifuge. The feed normally enters through the
top (bottom feed is also possible) and passes down through a feed well in the center of the rotor. An
impeller within the rotor accelerates and distributes the feed slurry, filling the rotor ulterior. The heavier
solids settle outward toward the circumference of the rotor under increasingly greater centrifugal force.
The liquid and the lighter solids flow inward through the cone-shaped disc stack. These lighter particles
are settled out on the discs, and migrate out to the nozzle region. The clarified liquid passes on through
the disc stack into the overflow chamber and is then discharged through the effluent line.
The centrifugal action causes the solids to concentrate as they settle outward. At the outer run of
the rotor bowl, the high energy imparted to the fluid forces the concentrated material through the rotor
nozzles. One part of this concentrated sludge is drawn off as the thickened product and another is
recycled back to the base of the rotor and pumped back into the concentrating chamber; there, it is
subjected to additional centrifugal force and is further concentrated before it is once again discharged
through the nozzles. This recirculation is advantageous because it increases the overall underflow by
flushing action; allows the use of larger nozzles, thus decreasing the potential for nozzle plugging; and
helps achieve a stable separation equilibrium that lends itself to precise adjustment and control.
Nozzle wear can be a major problem with this process. Grit in the sludge greatly accelerates nozzle
deterioration. Grit removal prior to centrifuging is very important.
4-4
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FEED
FEED
EFFLUENT
DISCHARGE
EFFLUENT
DISCHARGE
CONCENTRATING
CHAMBER
SLUDGE
DISCHARGE
RECYCLE FLOW
ROTOR
BOWL
ROTOR
NOZZLES
SLUDGE
DISCHARGE
FIGURE 6. SCHEMATIC OF A DISC NOZZLE CENTRIFUGE
4-5
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4.2 PROCESS CONTROL CONSIDERATIONS
There are several variables that determine the performance of solid bowl centrifuges. Bowl speed
is one of the most important, since centrifugal force speeds up the separation process. At any given pool
depth, an increase in bowl speed provides more gravitational force, providing greater clarification and
faster sedimentation.
The sludge feed rate is also very important. Lower sludge feed rates result in increased solids
separation. If the sludge feed is increased, the residence period decreases, and the solids recovery will
decrease. The sludge feed rate can be controlled to optimize the performance of a centrifuge.
Differential speed between bowl and conveyor (sludge feed) is provided by the backdrive assembly.
This speed between bowl and conveyor is important as it determines the Solids Retention Time (SRT).
A longer residence tune allows the solids the opportunity to become more concentrated.
The use of polymers has allowed more materials to be dewatered by centrifuges. The degree of
solids recovery (percent solids of sludge cake) can vary over wide ranges, depending on the sludge and
the amount of polymer used.
4.3 PROCESS PERFORMANCE EVALUATION
An inspection checklist is included in Appendix A. The inspection checklist is designed to assist the
inspector in gathering the information and in evaluating the performance of the centrifuge.
The centrifuge process is a mechanized process and as such more attention must be paid to the
maintenance of the units. The equipment should be greased and oiled as specified by the equipment
manufacturer. Any major overhaul of the equipment should be conducted by qualified personnel.
Routine inspection and repair of the centrifuge should include an evaluation of:
• Shear pins
• Main bearings
• Seals
• Conveyor bushings
• Thrust bearing seal
• Feed and discharge ports.
4-6
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During the operation of a centrifuge the operators should routinely check the oil reservoir (if present)
level, and cooling water and oil temperatures; and check for vibration, noises, and leaks.
The centrifuge sludge cake and centrate should be checked daily. The centrate should be relatively
clear and free of solids. The sludge cake should be in the range of 10 to 30 percent solids. The systems
solids recovery should be in the range of 60 to 95 percent.
When evaluating a centrifuge operation, the inspector should evaluate the general operating condition
of the unit. He/she should pay special attention to any leaks, worn parts, vibrations, or noises—which
are all signs of inadequate maintenance. The maintenance records should be reviewed to ensure that worn
parts are being inspected and replaced appropriately.
4-7
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5. AEROBIC DIGESTION
The function of aerobic digestion is to stabilize waste primary sludge, waste biological sludge, or
a combination of these by long-term aeration. This process results in reduction of volatile solids and
pathogens. A stable sludge with a low oxygen demand, good settling characteristics, and no offensive
odor is produced.
During aerobic digestion, organic substrate is oxidized to carbon dioxide, water, and ammonia. As
the digestion proceeds, ammonia is oxidized to nitrate nitrogen. The oxidation of ammonia can create
a pH drop if the alkalinity is insufficient to buffer the solution. The following reaction may be used to
describe the overall aerobic digestion process for the oxidation of both carbonaceous and nitrogenous
substances:
C5H7N02 + 7O2 > 5C02 + 3H2O + NO3~ + H+
Recently, the addition of pure oxygen to the aeration system has been used hi aerobic digester
design. In conventional digesters, concentrations of influent sludge volatile suspended solids (VSS) must
be no more than 3 percent for retention times of 15 to 20 days. Above this percentage, oxygen from
atmospheric air cannot be dissolved into the digesting sludge fast enough to keep the biological reaction
going. However, pure oxygen can dissolve in sludge nearly five tunes as fast as can oxygen from the
air. As a result, pure oxygen aeration allows a more concentrated sludge feed. By feeding a more
concentrated sludge, either the SRT can be longer or the total pounds of sludge digested per day can be
increased. Pure oxygen digesters usually are closed so that oxygen is not lost to the atmosphere.
5.1 PROCESS CONFIGURATION AND COMPONENTS
Aerobic digesters are typically a single-stage process that can be operated in a batch, semibatch, or
continuous mode. Batch and semibatch operations are by far the most common. In the batch mode the
digester tank is filled with raw sludge and aerated for 2 to 3 weeks. The aeration is then stopped and
the stabilized solids are allowed to settle. The clarified liquid is decanted and the settled solids are
removed.
5-1
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The semibatch mode of operation is similar to the batch mode, with the following exceptions:
• Raw sludge is added every couple of days.
• Hie clarified liquid is periodically decanted.
• Solids are generally held in the digester for long periods of tune before they are removed for
further treatment or disposal.
In the continuous mode of operation, solids are pumped directly from the clarifiers into the aerobic
digester. The digestion basin operates at a fixed level, with the overflow going to a solids-liquid
separator. Thickened and stabilized solids are either recycled back to the digestion tank or removed for
further processing.
Aerobic digesters are designed to provide an oxygen rich, well-mixed environment for the digestion
of organic matter and reduction of pathogenic organisms. Aeration and mixing are usually accomplished
simultaneously by either diffused air aerators or mechanical mixers. Diffused air aerators should be
capable of supplying 20 to 35 ft? per minute (cfm) of air per 1,000 ft3 of digester volume. Mechanical
aerators should supply 0.5 to 4.0 horsepower per 1,000 ft3 of digester volume. Typically, the tanks are
uncovered and installed above ground. However, tanks may be covered and the walls insulated to
minimize heat loss and to prevent freezing.
5.2 PROCESS CONTROL CONSIDERATIONS
40 CFR Part 257 specifies minimum residence times and temperatures that the sludge must remain
in the digester. These time and temperature requirements are set to ensure that the proper amount of
pathogen and volatile solids reduction occurs for the facility's ultimate disposal option. If the sludge is
to be land applied and the requirements for a PSRP are to be met (see 40 CFR Part 257), the residence
time requirements range from 60 days at 15°C to 40 days at 20°C. If properly operated, an aerobic
digester is capable of achieving a volatile solids reduction of 40 to 50 percent and a pathogen reduction
of 90 percent.
5-2
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Proper control of an aerobic digester is based on:
• Solids retention time (SRT)
• Temperature
• Volatile solids loading
• Air supply requirements.
5.2.1 Solids Retention Time
In general, as the SRT is increased, the efficiency of the aerobic digestion process is also increased.
The solids retention time is limited by the digester volume and sludge loading rates. In order to
maximize the SRT, the sludge should be thickened as much as possible prior to digestion.
5.2.2 Temperature
The efficiency and rate of aerobic digestion is directly related to the temperature. As the
temperature of the system decreases, the rate of biological activity also decreases. A decrease in
biological activity will result in a decreased rate of destruction of the biomass, and the potential for
unstabilized sludge to exit the digester. Desirable aerobic digestion temperatures are approximately 65°
to 80°F. In colder climates, provisions may have to be made to heat the digester (or conserve self-
generated heat by using a cover and/or a more efficient aerator) to maintain temperatures in the desirable
range. Actual temperatures hi aerobic digesters depend on the temperature and volume of sludge fed to
the digester and the temperature of the air coming from the blowers to the digester.
5.2.3 Volatile Solids Loading
Volatile solids loading is an estimate of the quantity of organic matter applied to the digester. The
optimum volatile solids loading for aerobic digestion depends on the treatment plant and is generally
determined by pilot and/or full-scale experimentation. In general, volatile suspended solids loadings for
effective aerobic stabilization vary from 0.07 Ib VSS/day/fi3 to 0.20 Ib VSS/day/ft3, depending on the
temperature and type of sludge. Operation outside of the recommended range can result in decreased
treatment efficiency.
5-3
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5.2.4 Air Supply Requirement
The air requirements of an aerobic digester are governed by a need to keep the digester solids in
suspension (well mixed) and to maintain a dissolved oxygen (DO) concentration of 1 to 2 mg/1. The
quantity of air required will vary depending on the sludge type, temperature, and concentration; and on
the activity of biomass within the digester. Obviously, as the concentration and/or activity increases,
more air is required to satisfy the oxygen requirements of the biomass and to keep solids in suspension.
The residual DO is a measure of the quantity of oxygen supplied beyond that used by the biomass. The
residual DO within the digester should always be greater than 1.0 mg/1. If the digester DO falls below
1.0 mg/1, aerobic processes will be negatively impacted.
5.3 PROCESS PERFORMANCE EVALUATION
When evaluating the performance of an aerobic digester the inspector should compare the actual
operating conditions to recommended operating conditions. Typical operating conditions for aerobic
digesters are presented in Table 3. An inspection checklist, included in Appendix A, is designed to assist
the inspector hi gathering the information and making the calculations required to make the comparison.
53.1 Design Evaluation
In evaluating the design adequacy of an aerobic digestion system, the inspector should consider the
following:
• Air supply system—The existing system should be capable of providing sufficient oxygen and
mixing.
• Digester volume—The digester should be large enough to provide a sufficient solids retention
time, provide for storage of sludge, and handle all solids generated by the wastewater treatment
systems.
5-4
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TABLE 3. OPERATING AND DESIGN CONDITIONS FOR AEROBIC SLUDGE DIGESTION
Solids retention time (days)
Activated sludge only IS to 20 days
Activated sludge + primary 20 to 25 days
Volatile suspended solids loading 0.024 to 0.14
Ob VSS/fWday)
Diffused air requirements (cfm/1,000 ft3)
Activated sludge only 20 to 35
Primary and activated sludge > 60
Mechanical mixer requirements 1.0 to 1.25
(hp/1,000 ft3)
Minimum dissolved oxygen (mg/1) 1.0 to 2.0
Temperature (liquid) (°F) >59
Reactor pH (s.u.) > 6.5
Volatile suspended solids reduction (%) 35 to 50
5-5
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5.3.2 Operation and Maintenance Evaluation
When evaluating the aerobic digester operation, the following parameters should be considered:
• Dissolved oxygen levels—To ensure adequate solids reduction, the digester dissolved oxygen
level of the sludge in the digester should be maintained at between 1.0 and 2.0 mg/1. If the DO
falls below 1.0 mg/1, filamentous organisms will begin to form and inhibit digestion. If the DO
is maintained above 2.0 mg/1, energy is being wasted.
• Digester temperature—Temperature has a significant influence on biological activity. The
minimum temperature of the digester should be 59 °F.
• Digester pH—The pH should be above 6.5. The extended digestion tunes of aerobic digestion
are conducive to nitrification. Nitrification will lower the pH, which in turn could inhibit the
carbonaceous digestion process.
• Feed sludge—The undigested sludge should be monitored for total solids (TS), total volatile
solids (TVS), pH, and flow rate. These parameters are useful in determining digester loading
rates and volatile solids reduction levels.
• Digested sludge—The digested sludge should be monitored for TS, TVS, pH, and flow rate to
determine the volatile solids reduction levels and solids retention times.
• Supernatant—The supernatant should be monitored for flow rate, biochemical oxygen demand
(BOD), total suspended solids (TSS), and pH to measure digestion efficiency and to determine
the loading on the wet-end treatment processes.
5-6
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6. ANAEROBIC DIGESTION
Anaerobic digestion is the biological degradation of complex organic substances in the absence of
free oxygen. During these reactions, energy is released and much of the volatile organic matter is
converted to methane, carbon dioxide, and water. Since little carbon and energy remain available to
sustain further biological activity, the remaining solids are rendered stable.
Anaerobic digestion involves several successive fermentations carried out by a mixed culture of
microorganisms. This web of interactions compromises two general degradation phases: acid formation
and methane production. Figure 7 shows, in simplified form, the reactions involved in anaerobic
digestion.
In the first phase of digestion, microorganisms including facultative bacteria convert complex organic
substrates to short-chain organic acids. These volatile organic acids tend to reduce the pH, although
alkaline buffering materials are also produced. Organic matter is converted into a form suitable for
breakdown by the second group of bacteria. In the second phase, strictly anaerobic bacteria (called
methanogens), convert the volatile acids to methane (CH4), carbon dioxide (COa), and other trace gases.
When an anaerobic digester is working properly, the two phases of degradation are hi dynamic
equilibrium; that is, the volatile organic acids are converted to methane at the same rate that they are
formed from the more complex organic molecules. As a result, volatile acid levels are low hi a working
digester. However, methane formers are inherently slow-growing, with doubling times measured in days.
In addition, methanogenic bacteria can be adversely affected by even small fluctuations hi pH, substrate
concentrations, and temperature. In contrast, the acid formers can function over a wide range of
environmental conditions and have doubling times normally measured hi hours. As a result, when an
anaerobic digester is stressed by shock loads, temperature fluctuations, or an inhibitory material, methane
bacterial activity begins to lag behind that of the acid formers. When this happens, organic acids cannot
be converted to methane as rapidly as they form. Once the balance is upset, intermediate organic acids
accumulate and the pH drops resulting in further inhibition of the methanogens and process use.
6-1
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Acid Methane
Formation Production
t t
1 1
Raw
Sludge
Complex
Substrate
^
Micro-
organisms
w
Principally
Acid Formers
r T
Stable and Intermediate
Degradation Products
Organic Acids, CO2,
H2O, and Cells
r
Micro-
organisms
Methane r
Bacteria
CH4+C02+ °.the/ e+nd
4 2 Products
H20, H2S
f\, II ^^^^^ | t\L — L |_
Degradation Products
FIGURE 7. SUMMARY OF THE ANAEROBIC DIGESTION PROCESS
6-2
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Anaerobic digesters may be "low-rate" or "high-rate." The primary difference between the two is
the rate at which the degradation of organic matter occurs. The low-rate system is not mixed and the
degradation process is generally slower than the high-rate. The high-rate system is well-mixed and
receives a higher solids loading rate. This forces the degradation of organic matter to occur much faster.
Consequently, the high-rate digester is more susceptible to an upset that the low-rate and requires more
intensive process controls.
The process has been successful when fed primary sludge, combinations of the primary sludge and
secondary sludge, and (to a lesser extent) thickened secondary sludge. Anaerobic digestion converts
about SO percent of the organic solids to liquid and gas, greatly reducing the amount of sludge to be
disposed. About two-thirds of the gas produced hi the process is methane, with a heat value of 600
BTU/standard cubic foot (scf). About 15 scf of gas is formed per pound of volatile solids (VS)
destroyed. Anaerobic digester gas has been used extensively in wastewater treatment plants for many
years to heat digesters and buildings, and as fuel for engines that drive pumps, air blowers, and electrical
generators. In a few areas, it has been used as a Grade 6 healing oil to heat municipal office buildings.
6.1 PROCESS CONFIGURATION AND COMPONENTS
The configuration of an anaerobic digester is typically either a single-stage process or a two-stage
process (Figure 8). In the low rate, single-stage process, three separate layers form as decomposition
occurs. A scum layer is formed at the top of the digester, and below it are supernatant and sludge layers.
The sludge zone has an actively decomposing upper layer and a relatively stabilized bottom layer. The
stabilized sludge settles at the base of the digester and the supernatant is usually returned to the plant
influent. In a single-stage, high rate process the digester is heated and mixed, and supernatant is not
withdrawn. In the two-stage process sludge stabilizes in the first stage, while the second stage provides
settling and thickening. The digester is heated to between 85° and 95 °F, and usually provides 10 to 20
days detention of sludge. More recently, packed bed anaerobic digesters have been used. These
innovative configurations may offer advantages in certain circumstances over the more traditional designs.
These configurations are not widely demonstrated to date hi municipal plants.
6-3
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Single-stage Anaerobic Digester
Gas Removal
Sludge inlets
Scum Layer
Supernatant Layer
Active
Digestion
Zone
Supernatant
Outlets
Digested Sludge
Sludge
Outlets
Two Stage Anaerobic Digester
Sludge
Inlet
Sludge
Heater
First Stage
Completely Mixed
Mixed
Active
Digestion
Zone
biuage
Outlet
w
\
i
i
i
»
1
1
Sludge
Inlet
^
i
i
Scum Layer
Supernatant Layer
Digested Sludge
Second Stage
Unmixed
Supernatant
Outlets
Sludge
Outlets
FIGURE 8. CONFIGURATION OF ANAEROBIC DIGESTERS
6-4
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Anaerobic digesters are designed to provide an oxygen free, warm, and well-mixed environment for
the digestion of organic matter and reduction of pathogenic organisms. This environment is effected
through three major systems hi a digester:
• Covers
• Heaters
• Mixers.
6.1.1 Covers
Digesters have either fixed or floating covers, as shown hi Figure 9. Fixed covers are made of
concrete or steel and may be flat, conical, or dome shaped. It is difficult to make concrete covers gas-
tight because concrete tends to develop cracks. Sludge must be removed from fixed-cover units without
letting air into the system, which could form an explosive mixture. For this reason, fixed-cover digesters
have water-level controls to make the overflow equal to inflow. Floating covers may be either the type
that rest directly on the liquid and have limited gas storage, or the gas-holder type that rests on a cushion
of gas and is provided with side skirts. Floating covers are the safest digesters to operate since there
is little chance of creating an explosive mixture under the cover. The gas-holder type is used to store
gas as it is produced. The pressure developed inside the tank causes the cover to lift as much as 6 ft
or more above the minimum height.
6.1.2 Heaters
Digesters can be heated by:
Hot-water coils within the digester—Hot water coils inside the digester have been used widely
hi the past. The main disadvantage of using coils is that they corrode and cake .with sludge,
which results in reduced heat transfer efficiency.
Recirculating sludge through an external heat exchanger—The external heat exchanger with
recirculation of the sludge is the most often used method of heating. This method provides good
scum control with no pipes inside the digester.
Direct contact of hot gas with sludge —Direct flame heating has been used where gas is mixed
into the sludge in small heating tanks.
• Steam injection—Steam injection has been used hi only a few cases.
6-5
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FIXED COVER
Fixed Cover
Pressure Vacuum Relief
Supernatant Overflow
Floating Cover
Pressure Vacuum Relief
Boating Cover
Supernatant Overflow
Gas Holder Cover
Pressure Vacuum Relief
Gas Holder Cover
Supernatant Overflow
FIGURE 9. FIXED AND FLOATING DIGESTER COVERS
6-6
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6.13 Mixers
Mixing can be provided by:
• Recirculating sludge through an exterior heat exchanger
• Mechanically mixing or pumping the sludge within the digester
• Releasing compressed digester gas through diffusers near the bottom of the digester, through
several pipes discharging above the top of the cone.
6.2 PROCESS CONTROL CONSIDERATIONS
The anaerobic process is mostly controlled by the methane-forming bacteria. These bacteria grow
slowly and have generation times which range from just less than 2 days to about 22 days. Methane
formers are very sensitive to pH, sludge composition, and temperature. If the pH drops below 6.5,
methane does not form and the organics in the sludge do not decrease. The methane bacteria are very
active in the mesophilic range (between 80° and 110°F), and hi the thermophilic range (between 113°
and 149°F). Most of the anaerobic digesters hi the United States operate within the mesophilic
temperature range.
Proper control of anaerobic sludge digestion is based on:
• Food supply
• Detention time and temperature
• Mixing
• pH and alkalinity
• Gas production.
6.2.1 Food Supply
Microorganisms are most effective when food (raw sludge feed) is provided hi small amounts at
frequent intervals or on a continuous basis. If too much sludge is added rapidly to the primary digesters,
the first (acid-forming) step may produce acid faster than the organisms needed for the second (gas-
forming) step can break them down. This results hi incomplete digestion, and causes bad odors.
6-7
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The sludge fed to the digester should be as thick as possible without clogging pumps and piping.
Thin sludge takes up too much digester space and adds excess water which must be heated.
6.2.2 Time and Temperature
Less detention time usually is needed for complete digestion as temperature increases. Most
digesters are designed to operate in the 90 °F to 95 °F temperature range. If the temperature falls much
below this range, more time is needed for digestion. Complete digestion usually occurs in about 15 days
hi a well mixed, properly heated digester. A temperature change of 2° or 3°F can be enough to disturb
the balance between the acid and methane formers.
40 CFR Part 257 specifies minimum residence times and temperatures that sludge must remain hi
the digester if the sludge product is to meet PSRP requirements. These time and temperature
requirements are set to ensure that the proper amount of pathogen and volatile solids reduction occurs
to support the facility's ultimate disposal option. To meet these minimum requirements, sludge must be
digested in the absence of air at residence times ranging from 60 days at 20°C to 15 days at 35°C to
55°C. There must also be a 38% volatile solids reduction. The regulations do not stipulate any PFRP
requirements for anaerobically digested sludge.
Raw sludge feed should be well-mixed with the contents of the primary digester. This helps to
ensure that the organisms have adequate contact with their food supply, and that the contents of the
digester are uniformly heated. The mixing system operation should be closely monitored.
6.2.4 pH and Alkalinity
Anaerobic digestion is relatively effective within the pH range of 6.5 to 7.5; however, the optimum
range is 6.8 to 7.2. Outside these ranges, digestion efficiency drops rapidly. Bicarbonate alkalinity
should be kept at a minimum level of 1,000 mg/1 as calcium carbonate (CaCO3) for good pH control.
To determine the bicarbonate alkalinity, both the volatile acid concentration and the total alkalinity must
be measured. The bicarbonate alkalinity is then calculated as shown:
Bicarbonate Alkalinity = (Total Alkalinity/0.8 Volatile Acids)
6-8
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The 0.8 factor in the above equation is needed to convert the volatile acid units from mg/1 as acetic
acid to mg/1 as CaC03, the equivalent alkalinity unit. The volatile acid to total alkalinity ratio should be
kept below 0.5 for good digester operation.
If the digester volatile acid concentration increases, pH will decrease unless bicarbonate alkalinity
is added. Two of the most popular forms of bicarbonate alkalinity are lime and sodium bicarbonate.
Lime additions beyond a bicarbonate alkalinity of 500 to 1,000 mg/1 will react with carbon dioxide, form
a precipitate, and have little effect on digester pH. Sodium bicarbonate does not react with carbon
dioxide, and although it is more expensive than lime, smaller amounts are needed because it does not
precipitate out of solution.
Chemicals can be added to the digestion system at several points. It is best to feed the chemicals
with metering pumps for good control. Chemicals can be added directly to the digester to make big
changes in bicarbonate alkalinity. The EPA publication, Operations Manual—Anaerobic Digestion (EPA
430/9-76-001) contains detailed guidance on chemical addition.
6.2.5 Gas Production
Gas production is one of the most important measurable digestion parameters. Overall digester
performance is reflected by the total volume, rate, and composition of gas produced. Generally, the gas
production should be between 13-18 ft3 of digester gas/lb volatile solids destroyed. Differences in
average gas production at a plant usually mean a change in the degree of digestion or a change in the
character of the sludge being fed. Gas from a properly operating digester contains about 65 to 69 percent
methane and 30 to 35 percent carbon dioxide. If more than 35 percent of the gas is carbon dioxide,
there is probably something wrong with the digestion process.
6.2.6 Supernatant Return
As shown in Figure 8, supernatant (the liquid above the sludge zone) is displaced as sludge is added
to the digester. Table 4 presents typical digester supernatant quality data. Inadequate digestion can result
in poor quality supernatant, which can lower overall plant performance when the supernatant is recycled.
Usually, supernatant is returned to the head of the plant; however, this recycle stream may greatly
increase the BOD, COD, TSS, and ammonia nitrogen loading on the plant. The supernatant should be
pretreated or returned to plant units where it will have the least effect. Additionally, it is best to return
the supernatant when the raw wastewater flow is at its daily low.
6-9
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TABLE 4. SUPERNATANT CHARACTERISTICS FROM ANAEROBIC DIGESTERS
Suspended solids
BOD5
COD
Ammonia as NH3
Total phosphorus as P
Trickling Filters*
(mg/1)
500 to 5,000
500 to 5,000
2,000 to 10,000
400 to 600
100 to 300
Activated
Sludge Plants*
(mg/1)
5,000 to 15,000
1,000 to 10,000
3,000 to 30,000
500 to 1,000
300 to 1,000
"Includes primary sludge.
63 PROCESS PERFORMANCE EVALUATION
In evaluating anaerobic digestion systems, the inspector is cautioned to bear in mind the following:
• In many plants, the anaerobic digestion system is the most complex unit process, from both a
technology/hardware stand point and from an operations stand point.
• Anaerobic digestion is a form of biological treatment, and as such requires a coherent process
control strategy. This strategy should incorporate target values, regular monitoring, trend
tracking, etc.
When evaluating the performance of an anaerobic digester the inspector should compare the actual
operating conditions to recommended operating conditions. Typical operating conditions for both low-
rate and high-rate anaerobic digesters are presented in Table 5. An inspection checklist is
included in Appendix A. The inspection checklist is designed to assist the inspector in gathering the
information and making the calculations required to make the comparison.
6-10
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TABLE 5. OPERATING AND DESIGN CONDITIONS
FOR ANAEROBIC SLUDGE DIGESTION
Temperature
Mesophylic
Thermophylic
pH
Optimum
General Range
Gas Production
Per pound VS added
Per pound VS destroyed
Gas Composition
Methane
Carbon dioxide
Hydrogen sulfide
Volatile Acids Concentration
General Range
Alkalinity Concentration
Normal Operation
Volatile Solids Loading
Low-rate
High-rate
Digester Capacity Based on Design
Population Equivalent (PE)
Low-rate
High-rate
Solids Retention Time (SRT), days
Low-rate
High-rate
Digested Solids Concentration (%)
Low-rate
High-rate
85°F to 95°F
113°Ftol49°F
6.8 to 7.2
6.5 to 7.5
6to8fP
16 to 18 ft3
65 to 69%
31 to 35%
trace
200 to 800 mg/1
2000 to 3500 mg/1
0.02 to 0.05 Ib VS/fWday
O.OStoO.lSlbVS/fWday
4 to 6 ff/PE
0.7 to 1.5 fWPE
30 to 60
10 to 20
4to6
4 to 6
6-11
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6.3.1 Design Evaluation
In evaluating the design adequacy of an anaerobic digestion system, the inspector should consider
the systems configuration and capacity. Keeping hi mind whether the system is high or low-rate, the
inspector should evaluate the following design elements:
• Volatile solids—Loading rate and nominal solids retention time should be calculated (per
checklist).
• Cover design—Check to see that adequate guides are provided for floating covers, to prevent
tipping.
• Gas storage—Check to see that adequate storage capacity (or a proper flare) is provided.
• Mixing
If gas mixing is used, check to see if adequate injection points are provided to mix the entire
volume of the digester. Also check the condition of the injection system.
- If recirculation is provided for mixing, check for piping configuration which might promote
short-circuiting within the digester, and recirculation pump capacities and condition.
• Heating—Look for the use of in-tank coils; these are typically found hi older systems and are
usually caked with solids, lowering their efficiency. In systems using external heat exchangers,
note the date of last cleaning. If pre- and post-heat exchanger temperatures are monitored, look
for a downward trend hi the post-heat exchanger temperature value (indicative of failing heat
exchangers).
• Insulation—In colder climates, check to see if tankage is bermed and covers are appropriately
insulated.
6.3.2 Operation and Maintenance Evaluation
Due to the complexity of the anaerobic digestion process, efficient operation requires that a
comprehensive process control program be in place. The operator must monitor various parameters to
maintain conditions conducive to the microorganisms involved hi the digestion process. As previously
noted, the digestion process relies on two major groups of microorganisms having significantly different
growth rates and nutritional requirements. This complexity of requirements increases the difficulty of
operation and makes the system more prone to upsets. An upset digester may lead to a deterioration in
the plant effluent quality as a result of increased loadings on the wet-end treatment processes; these
increased loadings result from poor quality supernatant and/or decreased solids processing capacities.
6-12
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The inspector must therefore not only evaluate the operation of the digester, but must also consider what
effect, if any, the digester operation is having on the wet-end operations and effluent quality.
When evaluating the digester operation, the following parameters should be considered:
• Digester temperature—Temperature has a significant influence on biological activity. The
optimum digestion temperature ranges are given in Table 5 for mesophylic and thermophylic
digestion; use these ranges.
• Digester pH—The pH should be in the range of 6.8 to 7.2 in order for complete digestion to
occur. Since pH changes very slowly, it must be monitored in conjunction with other
parameters for effective process control.
• Alkalinity—An effective digestion process exists at a total alkalinity of 2000 to 3500 mg/L.
• Volatile acids—Volatile acids are intermediate products of digestion which should be monitored
hi conjunction with pH and alkalinity. A typical volatile acids concentration ranges from 200
to 800 mg/1. A volatile acids/alkalinity ratio in excess of 0.8 is indicative of process failure.
Generally, at ratios greater than 0.8, methane production is inhibited.
• Digester gas—Effective digestion should produce from 12 to 18 ft3 of gas per pound of volatile
solids destroyed. The composition of the gas is another indicator of digester efficiency.
Generally, most smaller POTWs do not monitor the composition of the digester gas, so this
information may not be readily available.
• Feed sludge—The undigested sludge should be monitored for Total Solids (TS), Total Volatile
Solids (TVS), pH, and flow rate. These parameters are useful in determining digester loading
rates and volatile solids reduction levels.
• Digested sludge—The digested sludge should be monitored for TS, TVS, pH, and flow rate to
determine the volatile solids reduction levels and solids retention times.
• Supernatant—The supernatant should be monitored for flow rate, BOD, TSS, and pH to measure
digestion efficiency and to determine the loading on the wet-end treatment processes.
6-13
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7. HEAT TREATMENT
The heat treatment process involves heating wastewater sludge to temperatures of 350° to 400 °F
(177° to 240°C) in a reaction vessel under pressures of 250 to 400 psig (1,723 to 2,758 kN/nf) for
periods of 15 to 40 minutes. This significantly changes the sludge dewatering characteristics, primarily
by breaking down the structure of microbial cells in waste activated sludges and releasing the water bound
in the cell. The process effectively sterilizes the sludge by destroying pathogenic organisms. The heat-
treated sludge retains the characteristics of a very dense, well-digested domestic sludge. The heat-treated
sludge has excellent dewatering characteristics and does not normally require chemical conditioning to
dewater well on mechanical equipment, yielding cake solid concentrations of 40 to 50 percent. The
process is suitable for many types of sludges that cannot be stabilized biologically because of the presence
of toxic materials and is relatively insensitive to changes in sludge composition. The heat treatment
process generates odorous off-gases that must be collected and treated before release, and a liquor, or
decant, with high concentrations of organics (creating a high chemical oxygen demand COD), ammonia
nitrogen, and color.
Many heat treatment systems in use today are of the oxidation type. In this system high pressure
air is introduced into the sludge feed upstream of the heat exchanger. Some oxygen is consumed in the
process which generates heat, improves heat transfer rates, and reduces supplemental heat requirements.
Another modification of this process involves using higher temperatures and pressures in the reaction
vessel, and adding air under high pressures. These processes, known as low-pressure and high-pressure
wet air oxidation, are described in the next chapter. The wet air oxidation process produces results
nearly identical to the non-oxidation heat treatment process.
7.1 PROCESS CONFIGURATION AND COMPONENTS
Heat treatment systems are technically complex, even though the process itself is relatively straight
forward. A typical non-oxidation heat treatment system is shown in Figure 10.
Thickened sludge is first passed through a grinder or grinder pump to reduce the maximum particle
size. The resulting slurry is then pressurized and sent through one or more heat exchangers. From the
heat exchangers, the preheated slurry enters the reaction vessel, where the conditioning reactions occur.
Sufficient pressure is maintained to prevent boiling of the sludge. The treated slurry is subsequently
cooled in the heat exchanger; gases are pulled off in a vapor-liquid separator and reduced to atmospheric
7-1
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RAW SLUDGE
SLUDGE-WATER -
SLUDGE HEAT
EXCHANGER
POSITIVE
DISPLACEMENT
PUMP
DECANT
LIQUOR
CAKE
FIGURE 10. GENERAL THERMAL SLUDGE CONDITIONING FLOW SCHEME
FOR A NON-OXTOATIVE SYSTEM
7-2
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pressure through a pressure control valve. In many systems, the gases are processed to eliminate odors.
Gas cleanup methods include wet scrubbing, activated carbon absorption, after burning with fossil fuel,
and catalytic oxidation. With the last two methods, energy recovery is possible through the use of heat
recovery boilers and gas-liquid heat exchangers. Slurry from the gas-liquid separator passes through a
liquid-level control valve and is dewatered for final disposal. The liquid phase is recycled to the
treatment plant or given separate treatment for reduction of the residual soluble organics.
Heat treatment system failures are associated with the high pressures involved, heat exchanger
scaling and corrosion, and required supernatant liquid treatment.
The basic components of a heat treatment system are as follows:
• Grinders—These units are used to reduce sludge particle size. This reduction prevents problems
with the handling of the sludge throughout the rest of the system and, more importantly,
increases the ratio of surface area to mass for the sludge particles.
• Low-and-high pressure feed pumps—Together, these pumps achieve the pressures necessary to
support wet air (flameless) oxidation. Low-pressure pumps are typically centrifugal types, while
high-pressure pumps are typically positive displacement pumps.
• Heat exchangers—A series of heat exchangers are used to recover heat from the reactor effluent
stream; this heat is used to preheat the incoming sludge. Heat exchangers may be sludge-to-
sludge, or sludge-to-water-to-sludge types.
• Reactor—The reactor serves to provide the necessary residence time at the design temperature
and pressure. Reactors are cylindrical hi shape, and are constructed of either stainless steel or,
in some cases, titanium.
• Boiler—The boiler provides the steam used as a source of supplemental heat.
• Separator—The separator is the pressurized vessel in which the treated sludge stream is split into
two phases: the off-gases and the treated slurry.
7.2 PROCESS CONTROL CONSIDERATIONS
Proper control of a heat treatment system consists largely of maintaining temperature and pressure
conditions at the system's design flow rate. The primary control parameters for heat treatment of sludge
are:
7-3
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• Temperature/pressure—These two intimately related parameters determine the degree of
stabilization/conditioning mat takes place. Temperature is typically held at 350° to 400°F, and
pressure at 250 to 400 psig. Under these conditions, only a nominal degree of organics
oxidation (typically less than 5 percent) will occur.
• Feed sludge percent solids—Typically 3 to 6 percent. Sludge with too low a percentage of solids
will require excessive supplemental heat, while sludge much over 6 percent solids is difficult to
pump at high pressure. Six percent solids is probably close to optimal for most systems.
• Volatile solids reduction (VSS)—This is the primary performance evaluation criteria. Reduction
should be commensurate with the temperature and pressure of the system. (See Section 8.2.
VSS reduction for heat treatment is typically less at given temperatures and pressures, than for
wet air oxidation systems.)
• Decant—In a properly functioning system, the treated sludge should exhibit good settling
characteristics, and decant solids should be relatively low (< 1,000 mg/1). The decant produced
by heat treatment typically exhibits very high levels of organics, ammonia, and color, as
follows:
Substances in Concentration Range,
Decant (Liquor) mg/1 (except as shown)
COD 2,500 to 22,000
BOD 1,600 to 12,000
NH3-N 30 to 700
Phosphorus 70 to 100
Color 2,000 to 8,000 units
The recycle liquor can be very difficult to treat, offensive-smelling, and can upset wastewater
treatment processes. The high concentration of organics and ammonia indicates the potential
impact that recycling the liquor can have on the wastewater treatment processes. It is important
to recognize the significance of the recycle load in the management of the overall plant
operation. Pretreatment of these recycles and/or their consideration in the design as secondary
treatment are necessary.
7.3 PROCESS PERFORMANCE EVALUATION
In evaluating a heat treatment system, the inspector should bear in mind the following:
• In most plants in which it is installed, heat treatment will be the most complex, maintenance-
intensive unit process.
• The strength of the decant liquor sidestream causes its handling to be a problem in most plants.
The inspector should include an evaluation of the impact of this sidestream on plant operations.
7-4
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When evaluating one of these systems, the inspector should obtain the manufacturer's O&M manual
for the system being evaluated. An inspection checklist is provided in Appendix A. The checklist is
structured to aid the inspector hi gathering the information needed to properly evaluate a heat treatment
system.
7.3.1 Design Evaluation
The inspector should consider the design capacity of each heat treatment system or "train" at the
POTW. The inspector should evaluate design capacity hi terms of gpm of sludge produced, detention
time hi the reactor, and the intended temperature and pressure of the operation.
In evaluating the systems configuration, the inspector should consider:
• The level of redundancy provided, both hi terms of number of "trams" and hi individual
components. The level of "availability" expected by the manufacturer (75 percent is typically
specified and is optimistic) can affect the required redundancy. If individual components are
"available" and can be shipped quickly when needed then the number of units needed on-site as
"back-up" can be reduced.
• The "serviceability" of both the individual units specified, and of the overall system layout.
• Provisions for acid washing and manual scale removal, acid washing is a necessity for most
plants.
• Material of construction of the pumps, piping, heat exchangers and reactors.
• Decant handling—Adequate treatment should be provided to handle the decant, or the plant's
wastewater treatment units (specifically biological treatment) should have been sized to
accommodate the load imposed by this sidestream.
• Gas handling—There should be provisions to handle all off-gases. In addition, the decant tank,
ash storage tanks, and dewatering facilities may require gas handling to control odor problems.
The basic control parameters, discussed earlier, are:
• Temperature
• Pressure
• Sludge feed rate
• Percent solids of sludge feed.
7-5
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In general, all of these parameters should be closely monitored, so as to detect deviations from
design operating conditions. Given the complexity of these systems, and the differences that exist
between the various systems in use, it is important that the manufacturer's recommended operating
procedures form the basis for the plant operating strategy.
As most "operations" problems with heat treatment systems are in reality maintenance problems, it
is important mat a comprehensive preventative and reactive maintenance program be in place. Such a
program should include:
• Routine inspections of all components
• Scheduled cleaning/descaling of the piping, heat exchangers, reactors, and decanting system
• Procedures for evaluating operations data on a daily basis to detect impending problems
• Annual indepth inspection, to include pressure testing, and checks for pipe erosion and
component fatigue.
7-6
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8. WET AIR OXIDATION
Sludge, like many other complex organics, may be oxidized at high temperatures, in the presence
of water, in what can be described as flameless combustion. This process, known as "wet air oxidation,"
is related to heat treatment (see last section) but differs in that it 1) includes air injection, 2) typically
occurs at higher temperatures and pressures, and 3) as a result, includes a significant level of oxidation
of organic matter. Wet air oxidation parallels incineration in terms of the ash-like, largely inorganic
residue produced.
In wet air oxidation, sludge and air are introduced into a reaction vessel at high temperatures (400°
to 700°F) and pressures (500 to 1500 psi) and held under those conditions while oxidation proceeds.
Because the oxidation of sludge is exothermic, under the proper conditions (high enough sludge
concentration and internal heat system recovery) this process can proceed with no external heat supplied
to the reactor.
The end products of this process are:
• (Largely) inorganic "ash"
• Off-gases
• Liquid phase.
Off-gases from this process include oxygen, nitrogen, carbon dioxide, water vapor, oxides of
nitrogen, and sulfur. The liquid phase, which typically separates rapidly from the ash (generally in
proportion to the degree of oxidation), is rich hi a variety of complex, soluble organics.
Figure 11 is a composite representation of the results of wet oxidation for a typical sewage sludge.
The figure shows volatile solids content or COD content hi the solid phase, and the total sludge as a
function of total oxidation in both phases. The vertical distance between the two curves is the solids or
COD content in the liquid phase. Up to approximately 50 percent total oxidation, reduction in the
volatile solids or COD in the liquid phase is minimal; above 50 percent, the volatile solids and COD of
both phases are reduced to low values. At 80 percent total oxidation, about 5 percent of the original total
volatile solids hi the sludge is in the solid phase, and 15 percent is in the liquid phase.
8-1
-------
1-
01
I
H
01
D
_J
u.
Z
o.
O
1
Q
O
d
CO
g
_j
01
H
0
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60
70
80 90 100
OXIDATION- %
FIGURE 11. VOLATILE SOLIDS AND COD CONTENT OF SLUDGE TREATED BY WET AIR
OXIDATION
8-2
-------
The degree to which organic materials are oxidized is a function of temperature, reaction time, and
the quantity of *ir (or oxygen) supplied. The process may be applied to dilute suspensions of sludge;
however, a solids content between 4 and 6 percent minimizes reactor volume and allows a thermally self-
sustaining reaction. Solids concentrations greater than approximately 10 percent create problems with
mixing and with consequent mass transfer of the oxygen. There are insufficient data to indicate any
advantage hi using pure oxygen rather than air as the oxidant source, but studies are being conducted to
evaluate the impact of oxygen enrichment and supercritical conditions on system performance and cost.
8.1 PROCESS CONFIGURATION AND COMPONENTS
Wet air oxidation, while generally simple hi concept, requires a rather complex assemblage of
equipment hi order to carry out the process on a flow-through basis. The wet air oxidation process is
shown schematically in Figure 12.
Sludge first passes through a grinder to reduce maximum feed solids size to less than 1/4 hi. (0.64
cm). The resulting slurry is then pressurized. Oxygen is supplied in the form of high-pressure air; the
amount of air required for complete oxidation of typical, domestic sludge solids is about 7.5 Ibs per
10,000 Btu. The pressure required for successful wet air oxidation is that necessary to prevent phase
charge (vaporization) at the design operating temperature.
The sludge-air mixture is then passed through one or more heat exchangers, where it is heated to
close to the desired reaction temperature by the reactor effluent stream and introduced into the reactor
for oxidation. Temperatures of between 400° and 700°F and pressures of between 500 and 1,800 psig
are used, with detention tunes of 40 to 60 minutes. The oxidized slurry is cooled hi the heat exchanger,
gases are removed hi a vapor-liquid separator, and the gases are reduced to atmospheric pressure through
a pressure control valve. The gases are typically processed to eliminate odors, since wet ah- oxidation
is known for its pungent, "musky" odor. Gas cleanup methods include wet scrubbing, activated carbon
absorption, after burning with fossil fuel, and catalytic oxidation. With the last two methods, energy
recovery is possible through the use of heat recovery boilers and gas-liquid heat exchangers.
8-3
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SLUDGE
HEAT
GROUND
SLUDGE
STORAGE
TANK
t Mmi^^,
T_SLUDGE
SLUDGE
FEED PUMP
AIR
HIGH
PRESSURE
SLUDGE PUMP
(POSITIVE
DISPLACEMENT)
d
NGER
s. f
§
CO
a
ui
N
a
X
o
, STEAM
t
O
CN
z
(N
8
A
^^
___ SLUDGE & 1
REACTOR
STEAM
INJECTION
AIR COMPRESSOR
STERILE
NON-PUTRESCIBLE
SOLIDS
ALTERNATE METHODS
OF DEWATERING
FILTER PRESS
VACUUM FILTER
CENTRIFUGE
DRAINAGE BEDS
LAGOONS
LEVEL
CONTROL
VALVE
AIR
TO
PRESSURE ATMOSPHERE
CONTROL
VALVE
GAS
CLEAN-UP
UNIT (1)
SEPARATOR
BOILER
(START-UP
STEAM)
OXIDIZED
SLUDGE
SLURRY
(1) WET SCRUBBING, CARBON
ABSORPTION. OR AFTERBURNING
SUPERNATANT
FIGURE 12. FLOW CHART FOR HIGH PRESSURE/HIGH TEMPERATURE
WET AIR OXIDATION
8-4
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Slurry from the gas-liquid separator passes through a liquid-level control valve and is dewatered for
final disposal. At high degrees of oxidation, the residual solids resemble ash from thermal incineration.
These residual solids are easily dewatered to a high solids content by conventional means, such as vacuum
or pressure filtration. The liquid phase is recycled to the treatment plant or given separate treatment for
reduction of the residual soluble organics.
High pressure/high temperature wet air oxidation processes generate excessive heat when they
operate with a high heating value sludge and an adequate solids content (approximately 6 percent). Still,
a source of high pressure steam (separate boiler or an existing plant system) must be provided for startup.
Because of the relatively high maintenance requirements (and subsequent low availability) of
individual pieces of equipment hi wet air systems, redundancy of virtually all of the individual system
components described below is common.
The basic components of a wet air oxidation system are as follows:
• Grinders—These units are used to reduce sludge particle size. This reduction of particle size
prevents problems with the handling of the sludge throughout the rest of the system and, more
importantly, increases ratio of surface area to mass for the sludge particles.
• Low- and high-pressure feed pumps—Together, these pumps achieve the pressures necessary to
support wet air (flameless) oxidation. Low-pressure pumps are typically centrifugal types, while
high-pressure pumps are typically type positive displacement pumps.
• Heat exchangers—A series of heat exchangers are used to recover heat from the reactors effluent
stream; this heat is used to preheat the incoming sludge/air mixture. Preheating is necessary if
oxidation is to occur without the use of supplemental heat. In general, bundled, jacketed tube-
type exchangers are used. Due to the corrosive/abrasive nature of sludge under high pressure,
these units are typically constructed of stainless steel or, in some instances, titanium.
• Reactor—The reactor serves to provide the necessary residence time at the design temperature
and pressure. Reactors are cylindrical in shape and constructed of either stainless steel or, in
some cases, titanium.
• Startup boiler—The boiler provides the steam used as a source of supplemental heat during
system startup.
• Separator—The separator is the pressurized vessel in which the oxidized sludge stream is split
into two phases: the off-gases and the oxidized slurry. Automatic valves control the flow of
gases and slurry from the separator.
8-5
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• Decant tank—From the separator, the oxidized sludge slurry may be directed to what is, in
effect, a gravity separator, or clarifier. The settled "ash" is pumped to a dewatering unit while
the overflow, or decant, is typically either treated separately or returned to the plant headworks.
8.2 PROCESS CONTROL CONSIDERATIONS
Proper control of wet air oxidation consists primarily of maintaining, at the design flow rate, proper
temperature and pressure conditions. This requires the operation and maintenance of a complex,
maintenance-intensive system.
• Temperature/pressure—These are the primary control parameters which determine the degree
of oxidation which will take place. As noted previously, the minimum pressure required at a
given temperature is that needed to prevent vaporization of the water hi the sludge. The
following summarizes the relationship between temperature/pressure and the degree of oxidation
achieved by a given system:
Oxidation category COD Reduction. % Temp.T °F Pressure, psi
Low 5 350 to 400 300 to 500
Intermediate 40 450 750
High 92 to 98 675 1,650
• Detention time—Must be adequate to permit oxidation to proceed to the desired degree, but
should not be excessive (typically 40 to 60 minutes).
• Feed sludge percent solids—Typically 3 to 6 percent. Sludge that has too low a percentage of
solids will require a supplemental heat source, while sludge that is much over 6 percent solids
is difficult to pump at high pressure. Six percent solids is probably close to optimal for most
systems.
• Volatile solids reduction—This is the primary performance evaluation criteria. It is likely to
vary with site specific design criteria of the unit. Evaluate VSS reduction by comparing current
operating data with 1) design criteria and 2) historical data on VSS reduction rates. Reduction
should be commensurate with the temperature and pressure of the system.
8-6
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• Dgejnt—In a properly functioning system, the oxidized sludge should exhibit good settling
characteristics, and decant solids should be relatively low (< 1,000 mg/1). The decant produced
by wet air oxidation typically exhibits very high levels of organics, ammonia, and color, as
follows:
Substances in Concentration Range,
Decant (liquor') mg/1 (except as shown')
COD 100 to 17,000
BOD 3,000 to 15,000
NH3-N 400 to 1,700
Phosphorus 20 to 150
Color 1,000 to 6,000 units
The recycle liquor can be very difficult to treat, offensive-smelling, and can upset wastewater
treatment processes. The high concentrations of organics and ammonia illustrate the potential impact
that recycling of the liquor can have on the wastewater treatment processes. It is important to
recognize the significance of the recycle load in the management of the overall plant operation.
83 PROCESS PERFORMANCE EVALUATION
In evaluating a wet air oxidation system, the inspector should bear hi mind the following:
• In virtually any plant hi which it is installed, wet air oxidation will be by far the most complex,
maintenance-intensive unit process.
• The strength of the decant liquor sidestream causes its handling to be a problem in many plants.
The inspector should include an evaluation of the impact of this sidestream on plant operations
hi his/her inspection.
An inspection checklist is provided hi Appendix A. This checklist is structured to aid the inspector
in gathering the information needed to properly evaluate a wet air oxidation system. When evaluating
one of these systems, the inspector should first obtain the manufacturer's O&M manual for the system
being evaluated.
8-7
-------
8.3.1 Design Evaluation
The inspector should consider the following when evaluating the capacity of a wet air oxidation
system:
• The design capacity of each "train" in gpm, and the reactor detention time
• The intended temperature and pressure of operation
• The nominal (expected) heat value of the sludge to be processed.
In evaluating the systems configuration, consider:
• The level of redundancy provided, both in terms of number of "trains" and in individual units.
The level of "availability" expected by the manufacturer (75 percent is typically specified and
is optimistic) can affect the required redundancy. If individual units are "available" and can be
shipped quickly when needed then the number of units needed on-site as "back-up" can be
reduced.
• The "serviceability" of both the individual units specified, and of the overall system layout.
• Provisions for acid washing and manual scale removal
• Material of construction of the pumps, piping, heat exchangers and reactors.
• Decant handling—Adequate treatment should be provided to handle the decant, or the plant's
wastewater treatment units (specifically biological treatment) should have been sized to
accommodate the load imposed by this sidestream.
• Gas handling—There should be provisions to handle all off-gases. In addition, the decant tank,
ash storage tanks, and dewatering facilities may require gas handling to control odor problems.
83.2 Operations and Maintenance Evaluation
The basic control parameters, discussed above, were:
• Temperature
• Pressure
• Feed rate
• Feed sludge percent of solids
• Volatile solids reduction.
8-8
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In general, all of these parameters should be closely monitored, so as to detect deviations from
design operating conditions. Given the complexity of wet air oxidation systems, and the differences that
exist between the various systems hi use, it is important that the manufacturer's recommended operating
procedures form the basis for the operations strategy.
As most "operations" problems with wet air systems are hi reality maintenance problems, it is
important that a comprehensive preventive, and reactive maintenance program be in place. Such a
program should include:
• Routine inspections of all components
• Scheduled cleaning/descaling of the piping, heat exchangers, reactors, and decanting system
• Procedures for evaluating operations data on a daily basis to detect impending problems
• Annual indepth inspection, to include pressure testing, and checks for pipe erosion and
component fatigue.
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9. INCINERATION
The incineration of sewage sludge reduces the total volume and mass of residuals generated by
wastewater treatment and destroys all organic matter in the sludge. The resultant end-product is a sterile,
odor-free ash containing inert particles and heavy metals.
Typically, sludge may be incinerated in either multiple-hearth or fluidized bed incinerators.
However, other furnace types such as electric-infrared and rotary kiln have been used. Complete
combustion of sludge results in the conversion of the combustible constituents into carbon dioxide, water,
and sulfur dioxide. To ensure that complete combustion is achieved, excess air 20 to 150 percent more
than theoretically required for combustion is supplied to the incinerator.
Municipal sewage sludge has a heat value ranging from less than 7,000 to 10,000 Btu/lb of dry
solids. Primary sludges have higher heat values than secondary sludges. Higher heat values may be
observed if there is a significant grease concentration present. Conversely, lower heat values may exist
if the sludge was conditioned with inorganic chemicals. Although sludge contains a significant amount
of combustible material, it cannot be burned autogenously (without supplemental fuel) unless it is
dewatered to at least 25 percent solids. Since most of the supplemental fuel used in sludge incineration
is needed to evaporate water from the sludge, incineration is typically preceded by a dewatering process
to conserve fuel consumption.
To control air pollutants, all sludge incinerators are equipped with scrubbers. Most systems
currently use wet scrubbers for paniculate removal, the most effective being the Venturi impingement
scrubber. The sludge incinerator scrubbers in present use will generally not remove the products of
incomplete combustion, i.e., hydrocarbons, oxides of nitrogen and sulfur, carbon monoxide, and smoke.
Therefore, proper operation that ensures complete combustion is essential for air pollution control.
The final step in sludge incineration involves ash disposal. The ultimate ash disposal method will
be dependent on the heavy-metal concentration of the ash and its classification as a hazardous or
nonhazardous waste. The ash is typically stored on an interim basis at the POTW site. The ash may be
stored as a slurry in lagoons or stockpiled on the POTW grounds with ultimate disposal typically in
landfills. Dry ash typically is very fine and is low in density. For this reason it is frequently transferred
as a slurry and stored hi lagoons on site, and wet down for truck transport.
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9.1 PROCESS CONFIGURATION AND COMPONENTS
The two most common types of sludge incinerators employed are multiple-hearth or fluidized bed
incinerators. The configuration and components of these two types of incinerators are presented in the
following sections.
9.1.1 Multiple-Hearth Incinerator
A multiple-hearth incinerator is the type of incinerator that has been used most often for municipal
wastewater sludge incineration. As Figure 13 shows, a multiple-hearth incinerator has a cylindrical steel
shell around several solid refractory hearths and a central rotating shaft with rabble arms. The dewatered
sludge enters at the top through a flapgate and drops down through the incinerator, from hearth to hearth,
by the action of the rabble arms. The hearths are made of high-heat, heavy-duty fire brick. The drop
holes are located hi each hearth hi such a manner that results hi the sludge being alternately fed towards
the periphery or central shaft as the solids fall from hearth to hearth. The capacity of these incinerators
depends on the total area of the enclosed hearths. They are designed with diameters ranging from 54 hi.
to 21 ft. 6 hi., and with 4 to 11 hearths. Capacities of multiple-hearth incinerators range from 200 to
8,000 Ib/hr of dry sludge, with typical operating temperatures of 1,400° to 1,7GO°F.
Each hearth usually has two doors, fitted to cast iron frames and designed to close reasonably tight.
Each door has an observation port that can be opened. Since the furnace may operate at temperatures
of up to 2,000°F, the shaft and rabble arms are cooled by ah- supplied in controlled amounts from a
blower located at the bottom of the shaft. The shaft is motor-driven and speed can be adjusted from
approximately 0.5 to 1.5 rpm. Two or more rabble arms are connected to the shaft at each hearth. Each
rabble arm has a central tube for conducting ah- from the central shaft to the end of the rabble arm. The
air may be discharged to atmosphere or returned to the bottom hearth as preheated air for combustion
purposes.
The rabble arms provide mixing action as well as rotary and downward movement of the sludge.
Combustion ah- flow is countercurrent to that of the sludge. Some hearths have oil or gas burners to
provide startup or supplemental heat. Sludge is constantly turned and broken into smaller particles by
the rotating rabble arms. This process exposes the sludge surface to the hot furnace gases so that rapid
and complete drying, as well as burning, of sludge occurs.
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COOLING AIR
DISCHARGE
SLUDGE CAKE,
SCREENINGS,
AND GRIT-
SCUM
AUXILIARY
AIR PORTS
RABBLE ARM
2 OR 4 PER
HEARTH
GAS FLOW
CLINKER
BREAKER
BURNERS
SUPPLEMENTAL
FUEL
COMBUSTION AIR
SHAFT COOLING
AIR RETURN
SOLIDS FLOW
DROP HOLES
ASH
DISCHARGE
FIGURE 13. CROSS-SECTION OF A MULTIPLE-HEARTH FURNACE
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The multiple-hearth system usually has an instrumentation system that sends operating data to a
control panel. Temperatures can be recorded for each hearth, for exhaust gas, and for scrubber inlet gas.
The temperature on each hearth can be controlled to within ± 40°F. Breakdowns such as
burner shutdown, furnace over temperature, draft loss, and feed belt shutdown can be monitored. If there
is a power or fuel failure, the furnace should be shut down automatically, and the shaft cooling air fan
should be run on standby power to prevent shaft deformation.
9.1.2 Fluidized Bed Incinerator
The fluidized bed incinerator is a cylindrical refractory lined vessel with a grid near the bottom to
support a sandbed. The grid is comprised of a series of tuyeres (air diffusers) through which combustion
air is supplied. When the incinerator is shut down, the tuyeres prevent the sand from dropping down into
the windbox. A typical fluid bed reactor used for combustion of wastewater sludges is shown in Figure
14. Dewatered sludge enters above the bottom grid, and air flows upward at a pressure of 3.5 to 5.0 psig
to fluidize the mixture of hot sand and sludge. Auxiliary fuel can be supplied to the sand bed to maintain
temperatures necessary for complete combustion to occur. The reactor is a single chamber unit where
moisture evaporation and combustion occur at 1,400° to 1,500°F. Because of the large heat reservoir
hi the bed, and a rapid distribution of fuel and sludge throughout the bed, there is good contact between
fuel and oxygen. The sand bed keeps the organic particles until they are reduced to mineral ash. The
motion of the bed grinds up the ash material that is constantly stripped from the bed by the upflowing
gases. The heat reservoir provided by the sandbed also allows faster startup when the unit is shut down
for short periods, e.g., overnight. As an example, a unit can be operated 4 to 8 hours a day with little
reheating after restarting because the sandbed is such a large heat reservoir.
Exhaust gases are usually scrubbed with treatment plant effluent. Particles are separated from the
liquid in a hydrocyclone, with the liquid stream returned to the head of the plant. An oxygen analyzer
in the stack controls the air flow into the reactor. The auxiliary fuel feed rate may be controlled by a
temperature recorder. To lower fuel costs, an air preheater can be used with a fluidized bed.
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SAND
FEED
THERMOCOUPLE
SLUDGE
INLET
FLUIDIZING
AIR INLET
REFRACTER
ARCH
EXHAUST AND ASH
PRESSURE TAP
.SIGHT
Y GLASS
BURNER
TUYERES
PRESSURE TAP
STARTUP
-i PREHEAT
hBURNER
S FOR HOT
WINDBOX
FIGURE 14. CROSS-SECTION OF A FLUTOIZED BED FURNACE
9-5
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An instrumentation system is an inherent component of a fluidized bed incinerator. Thermocouples
and pressure taps are installed at several locations to monitor the temperature or pressure at various points
in the system.
92 PROCESS CONTROL CONSIDERATIONS
Economical and complete combustion in both incinerator types is dependent on an effective process
control program. Frequent monitoring and comprehensive record-keeping are essential to an efficient
incineration process. The parameters of primary concern are:
• Sludge cake total solids and volatile solids content
• Sludge feed rate
• Combustion air feed rate
• Drying, combustion, and freeboard zone temperatures
• Exhaust gas oxygen and carbon monoxide levels.
9.2.1 Sludge Cake Solids Content
To reduce auxiliary fuel requirements and to increase the incinerator throughput, the sludge must
be dewatered prior to incineration. Increasing solids concentrations will generally reduce the fuel
requirement, as mere will be less water to evaporate before combustion. If sufficient volatiles are
present, autogenous combustion may occur at total solids concentrations greater than 35 percent. Since
the sludge solids concentration does not remain consistent, samples of sludge cake should be taken and
analyzed at least once per shift.
9.2.2 Sludge Feed Rate
The feed rate should be monitored to determine the sludge loading rate to the incinerator. Sludge
loading rate is the weight of wet sludge fed to the reactor per square foot of reactor bed per hour
(Ibs/hr/ft2). Loading rates out of the normal range for that type of incinerator may indicate operating
problems, poor management, or other items that may lead to noncompliance. If the incinerator is
overloaded, then incomplete combustion will occur. Incomplete combustion will result in increased
pollutant emissions and in odor levels in the exhaust stack.
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The inspector should also check records of sludge loading rates to determine if the sludge feed rate
is relatively constant. If there are rapid fluctuations in the sludge feed rate, the operator must vary the
air feed and combustion temperature in order to ensure complete combustion. This makes the operation
unsteady and causes early failure of many components due to excess wear and thermal stress. A constant
sludge feed rate is desirable to avoid having to make these operating adjustments.
923 Mr Feed Rate
The supplied air rate will vary depending on sludge constituents and moisture content. Excess air
is the amount of air supplied beyond the theoretical air requirements for complete combustion. Providing
sufficient excess air ensures complete combustion of organics and minimizes the emission of hazardous
air pollutants. Since the feed rate and composition of the sludge vary, incinerator operators may have
to vary air feed rates in order to maintain adequate excess air. If air rates are inadequate, complete
combustion will not occur. Excessive air feed may result in higher emissions of nitrogen oxides, low
combustion temperatures, or increased auxiliary fuel consumption depending on the amount of excess air.
Generally, excess air supplies should be as follows:
• Multiple-hearth furnaces—100 percent or more excess air
• Fluidized bed furnaces—20 to 45 percent excess air
• Electric-infrared furnaces—30 to 70 percent excess air.
The air feed rate should be monitored to ensure adequate oxygen is supplied for complete
combustion.
9.2.4 Temperature Monitoring
A combustion temperature of 1,200°F to 1,600°F is normal and considered necessary for oxidation
of high molecular weight organics. However, operation temperatures above 1,800°F may result in
increased emissions of metals, ash melting (slagging), and damage to the refractory material. Rapid
changes in temperature indicate that there are operational problems that should be resolved. Temperature
changes can be caused by many events, such as increased moisture in the sludge, changes in excess air,
flame outs, and changes in the fuel values going to the incinerator. Several temperature locations are
normally required to be monitored. The inspector should check that all thermocouples or other devices
are hi working order and are calibrated as required.
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In a multiple-hearth furnace it is important to monitor the temperature at the individual hearths. As
can be seen from Figure 15, several distinct processes are occurring at different hearth levels and
temperatures. It is therefore imperative that the temperature in each zone is continuously monitored
during sludge incineration. Additional temperature monitoring may be conducted on the preheated
combustion air supply and stack exhaust gas.
In a fluidized bed incinerator, the temperature should be continuously monitored in the windbox,
bed, freeboard, and exhaust sections. For complete combustion to occur, the bed temperature should be
maintained at approximately 1,400°F. Excessively high bed temperatures may be due to excessive
auxiliary fuel feed rates or high grease concentrations. The freeboard temperature is normally kept in
the range of 1,500° to 1,600°F.
9.2.5 Exhaust Gas Monitoring
Generally, the oxygen content in the exhaust gas is maintained at between 3 and 6 percent. Low
oxygen concentrations and increased carbon monoxide levels may be indicative of excess sludge loading
rates, inadequate air supply rates, or excess auxiliary fuel rates. Periodic measurements of the following
parameters should also be conducted: particulates, hydrocarbons, heavy metals, and oxides of sulfur and
nitrogen.
The concentrations of oxygen and carbon monoxide in the exhaust or off-gas are also indicative of
combustion efficiency. Combustion efficiency can be calculated as follows:
Combustion Efficiency = Cone CO2 in exhaust gases
Cone C02 + CO in exhaust gases
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NORMAL
SLUDGE/ASH
TEMPERATURES
NORMAL
AIR
TEMPERATURES
DRYING ZONE
V\\\\
600° to
\\\\\
w 1400° to
1700°F
COMBUSTION ZONE
1400° to
1700°F
\X\\\
\\\\
1400° to
1800°F
\\\
FIXED CARBON
BURNING ZONE
\\\\\
1400° ta
1800°F
ASH COOLING
ZONE
SLUDGE
FLOW
AIR
FLOW
FIGURE 15. PROCESS ZONES IN A MULTIPLE-HEARTH FURNACE
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93 PROCESS PERFORMANCE EVALUATION
Evaluation of the incineration process relies heavily on inspection of operating parameters. For all
records of measurements taken automatically, the inspector should attempt to verify that data recorded
are the same as those being measured. The inspector should record the time and reading of instruments
during the facility inspection and compare them with the charts or printouts generated. It is very common
for charts to be recording incorrect data. In particular, the inspector should verify the following
parameters:
• Sludge feed rate—The inspector should look at incinerator records to determine the range of
sludge feed rates. These rates should be compared to the values listed in the O&M manual as
typical loading rates for that type of incinerator to see if the incinerator is overloaded.
• Percent solids of sludge feed—The inspector should look at operating records to determine the
average solids content of sludge feed and the average rate of auxiliary fuel consumption as rapid
or large changes in conditions can make incineration difficult and result in violations.
• Excess air—The inspector should evaluate both the amount of excess air and the variability of
its supply.
• Combustion temperature—The inspector should inspect plant records to determine if the
incinerator is operating at appropriate combustion temperatures, and should record temperatures
at the time of the inspection directly from the instrument.
• Exhaust gas—The inspector should review records of oxygen and carbon monoxide content of
the exhaust or off-gas to aid in the evaluation of combustion efficiency.
When evaluating the performance of a sludge incinerator, the inspector should obtain as much design
and operating information as possible from the operation and maintenance manual. All sewage sludge
incinerators constructed or modified after June 11,1973 are required (under the New Source Performance
Standards at 40 CFR Part 60) to have performance tests run as they come on line. These air pollution
tests are frequently used to define the optimal range for the above described operating parameters. Also,
the air pollution permit conditions should be reviewed prior to the inspection, as these will generally
define operating conditions. Finally, the operation of the sludge dewatering should be evaluated
concurrently with the incinerator inspection. The efficiency of the sludge dewatering system will
significantly impact the operation of the incinerator. An inspection checklist for incinerators is included
in Appendix A. This checklist is designed to assist the inspector in gathering the information necessary
to evaluate incinerator operations.
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10. COMPOSTING
Composting is the aerobic decomposition of the sludge organic constituents at elevated temperatures.
Recently, some facilities have initiated anaerobic composting schemes for sewage sludge. Composted
sludge from municipal wastewater treatment plants can contain significant levels of nutrients and may be
suitable for use in soil enhancement for plant growth. Composting is performed to create a stable,
humus-like material, and occurs hi two phases: stabilization and maturation.
In the stabilization (active) phase, biological activity causes the temperature to rise to a thermophilic
level, followed by a gradual decrease to ambient levels. The population of microorganisms increases
rapidly as the temperature rises and easily oxidized organic compounds are metabolized. Excess released
energy results in a rapid rise in temperature to the range of 40° to 60°C. At these temperatures,
pathogenic organisms will be greatly reduced or completely destroyed. The ultimate rise hi temperature
is influenced by the availability of oxygen and the air flow through the compost piles.
As the energy source is depleted, biological activity slows and the temperature slowly returns to
ambient levels. At this stage, maturation of any undegraded organic matter occurs. The composting
process is complete when there is a marked drop hi temperature and no subsequent significant increase
in temperature occurs when the mature compost is aerated.
10.1 PROCESS CONFIGURATION AND COMPONENTS
Composting can be conducted in either unconfined or confined composting systems. Unconfined
composting is conducted in either windrow piles or forced-air static piles. The basic steps to be followed
in those two processes are similar, but the processing technology for the composting stage differs
appreciably. In the windrow method, oxygen is drawn into the pile by natural convection and turning,
whereas hi the static pile method, aeration is induced by forced-air circulation.
Confined (in-vessel) composting is conducted in an enclosed container or basin. The systems are
designed to minimize odor and process time by controlling environmental conditions such as air flow,
temperature, and oxygen concentration.
10-1
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The components of an unconfined composting operation generally include equipment for moving and
mixing the compost mixture. A typical compost operation would include a dump truck, front end loader,
drum screen, and a windrow turner or aeration blower assembly. The composting process can require
the use of a large amount of land area.
10.2 PROCESS CONTROL CONSIDERATIONS
The efficiency of a composting operation is dependent upon the sludge characteristics, initial
moisture content, uniformity of the mixture, frequency of aeration or windrow turning, and climatic
conditions. The characteristics of the sludge being composted will affect the amount and type of bulking
material that is required. The bulking material provides porosity, which facilitates moisture control for
the sludge. Typically, recycled compost, wood chips, and sawdust are used as bulking agents.
Dewatered municipal sludges are usually too wet to satisfy optimum composting conditions. The
optimum moisture content is in the range of 50 to 60 percent water. If the mixture is over 60 percent
water, the proper structural integrity will not be obtained. If the moisture content is less than 40 percent,
moisture may limit the rate of decomposition. The moisture content can be reduced by blending the
sludge with a dry bulking material or recycled product, and dewatering the sludge to as great an extent
as economically possible.
The dewatered sludge and bulking agent should be uniformly mixed. Uniform mixing will create
the proper texture, ensure that the moisture is consistent throughout the pile, and allow air to flow
through the pile and provide oxygen.
Sufficient air should be supplied to the pile, either by forced aeration or windrow turning, to
maintain oxygen levels of between 5 and 15 percent. Insufficient oxygen levels will create anaerobic
conditions, which will stop the composting and create odors. Excessive aeration can have the effect of
cooling the pile, which in turn will slow the composting process. Oxygen and temperature readings
should be monitored either continuously or several times per day. Readings should be taken at several
locations within the pile.
10-2
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To comply with the process to significantly reduce pathogens (PSRP) requirements of 40 CFR
Part 257, the compost pile must be maintained at a minimum operating temperature of 40°C for at least
5 days. During this time, the temperature must be allowed to rise above 55 °C for at least 4 hours to
ensure pathogen destruction. Typically, this is done near the end of the active composting phase to
prevent inactivation of the microbes responsible for metabolizing the organic fraction of the sludge.
To comply with the processes to further reduce pathogens (PFRP) requirements of 40 CFR Part 257,
the hi vessel and static aerated compost piles must be maintained at a minimum operating temperature
of 55°C for at least 3 days. The windrow pile must be maintained at a minimum operating temperature
of 55 °C for 15 days. Additionally, there must be at least 3 turnings of the pile during this tune period.
Climatic conditions play an important role in windrow composting. During wet weather conditions
or hi cold climates the compost tune can significantly increase.
10.3 PROCESS EVALUATION
When evaluating the performance of a composting operation, the inspector will rely mostly on
sensory observations. An inspection checklist is provided hi Appendix A. This checklist is designed to
assist the inspector in gathering the information required to evaluate the performance of the compost
operation. Some of the key areas to be evaluated are briefly discussed below.
The inspector should visually inspect the piles to ensure they are mixed thoroughly. The mixed
material should be relatively homogenous, without large clumps of sludge. The mixture should not look
too moist or wet. If it does, the inspector should measure the moisture content.
The inspector should measure the temperature and oxygen levels throughout the compost piles. The
operator's records should be inspected to ensure that the pile is meeting the tune and temperature
requirements in 40 CFR Part 257.
The inspector should inspect the compost site for signs of runoff. The runoff should be collected
and treated. The runoff is usually returned to the treatment plant headworks.
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Significant odors will generally be associated with the compost operations especially from active
piles. Therefore, some sites may have odor control systems in operation. Odor from "cured" compost
piles should be minimal if the organic fraction has been sufficiently reduced.
The finished compost should have a moisture content in the range of 40 to SO percent and a volatile
solids content of 40 percent or less.
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11. CHEMICAL STABILIZATION AND CONDITIONING
Chemical conditioning is a process in which chemicals are added to sludge to improve its dewatering
characteristics. Inorganic salts (commonly lime or ferric chloride) or organic compounds (long-chain
polymers) may be used alone or hi combinations to accomplish this conditioning. Conditioning does not
itself reduce the quantity or water content of the sludge, but rather improves the efficiency of subsequent
solids handling steps. The purpose of chemical stabilization is to make the sludge less odorous and
putrescible and to reduce the pathogenic organisms, as well as improve the potential for dewatering the
sludge.
Inorganic chemicals used for sludge conditioning commonly include ferric chloride, used alone or
in conjunction with lime or alum, or lime used alone. Organic conditioning agents, usually referred to
as polyelectrolytes or polymers, include a variety of long-chain organic molecules which may be either
anionically or cationically charged or be electrically neutral.
Physically, wastewater sludges consist of a mixture of solid phases suspended in an aqueous solution
of dissolved substances. The surfaces of the solid phase tend to acquire charge by preferential absorption
of ions from the solution or by ionization of component functional groups. The solid phases of domestic
wastewater sludges characteristically possess a negative charge. This electrostatic charge hinders the
separation of the solid phase from the water in two ways: First, water molecules, being polar, are
strongly attached to the surface of the solids. Second, the like charges on individual solid particles cause
them to repel. Sludge conditioning addresses both of these problems by introducing molecules of opposite
charge. These molecules attract the oppositely charged sludge particles, neutralize their surface charge,
and agglomerate the smaller particles into larger, denser particles with low electrical charge.
Use of lime as a conditioning agent has the additional advantage that it will not only condition the
sludge, but will biologically stabilize it as well. The addition of lime creates a highly alkaline
environment that inactivates or destroys pathogens. Lime stabilization is practiced to meet the
requirements of 40 CFR Part 257 (processes to significantly reduce pathogens), which states that
sufficient lime must be added to produce a pH of 12 after 2 hours of contact. Because the addition of
lime does not reduce the volatile organics, if the pH drops below 11, renewed bacteria and pathogen
growth can reoccur. Therefore, the pH must be maintained at above 12 for 2 hours to ensure pathogen
11-1
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destruction and to provide enough residual alkalinity so that the pH does not drop below 11 for several
days.
11.1 PROCESS CONFIGURATION AND COMPONENTS
The configuration for chemical sludge conditioning usually consists of one or more bulk storage
vessels for the conditioning of chemicals, chemical feeders to accurately add the chemical agents to the
sludge, and a conditioning tank in which the conditioning chemicals and sludge are mixed. The
conditioning tank is usually located immediately adjacent to the solids processing unit that will receive
the conditioned sludge. The lime stabilization process uses the same type of equipment as conditioning,
although the process can take place before or after dewatering.
Lime can be purchased in dry form. Depending on the size of the system, the lime may be
purchased in bags, trucks, or rail cars. Large volumes are handled with screw conveyors, bucket
elevators, or pneumatic transfer lines. The most common forms purchased are hydrated lime (Ca.(OH)^)
and pebble quicklime (CaO). Because quicklime is hygroscopic, water tight and airtight storage must be
provided. Attention should be given to dust control with manual handling or pneumatic transfer systems.
Pebble quicklime requires "slaking" (conversion of CaO to Ca(OH)2 by mixing with water) prior to
use. Commercial designs vary in regard to the combinations of water to lime, to slaking temperature and
slaking tune, in obtaining the "milk of lime" suspensions. Slaked lime requires only enough water to
form the desired "milk of lime" slurry concentration (usually 3 percent).
Lime is not corrosive to steel and therefore steel, iron, rubber plastic, polyvinyl chloride (PVC), and
concrete may be used for transfer and storage of lime or "milk or lime" solutions. Lime solutions rapidly
cloud glass piping, therefore the use of glass rotometers on lime solution lines is not recommended.
Two types of feeders, volumetric and gravimetric, are used to meter dry lime. Volumetric feeders
deliver fixed volumes of material; gravimetric feeders deliver constant weights of material. Variations
in the bulk density of dry lime usually make gravimetric feeders more accurate. Lime solutions can be
fed with positive displacement metering pumps.
11-2
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Ferric chloride is usually purchased in liquid solution with a ferric chloride content of 35 to 45
percent. Shipping concentrates vary from summer to whiter due to the relatively high crystallization
temperature of solutions at these concentrations. Liquid ferric chloride is shipped in 5- and 13-gal
carboys, tank trucks, and rail cars.
Ferric chloride is highly corrosive to iron and steel. Storage tanks and transfer lines should be
constructed of fiberglass reinforced plastic (FRP), PVC, or rubber- or plastic-lined steel. Because of the
tendency of ferric chloride solution to stain or deposit, glass tube rotometers are not appropriate. Feeding
of ferric chloride solutions is usually accomplished with rubber diaphragm positive displacement pumps
or rotodip pumps.
Alum (aluminum sulfate) is usually purchased in liquid form. The typical solution concentration is
49 percent as A,(SO4)2 • 14H2O. Liquid alum is corrosive to mild steel, but not 316 stainless
steel. Storage tanks and transfer lines should be constructed of 316 stainless steel, FRP, PVC, plastics,
or lead. Feeding of alum solutions may be accomplished with positive displacement pumps, rotodip
feeders, or centrifical pumps equipped with rotometers. Pumps should be constructed of 316 stainless
steel or plastics.
Polymers (polyelectrolytes) may be purchased hi dry or liquid form. Small systems most commonly
purchase liquid polymer solution in 33- or 35- gal drums. Liquid polymer solutions are generally stored
in 316 stainless steel, FRP- or plastic-lined tanks. These high-concentration solutions may be diluted in
a day tank prior to feeding. Liquid polymers or dry polymer solutions are fed using metering pumps or
rotodip feeders.
Larger systems purchase dry polymer, which is batch-mixed prior to use. Batch-mixing of dry
polymers requires continuous mixing until all polymer is dissolved to prevent the formation of semisolid
lumps commonly called "fish eyes." After the polymer is completely dissolved, it must be aged for 8
to 24 hours prior to use. Because of the tendency of dry polymer to absorb moisture, it is usually
purchased hi bags or cardboard drums rather than in bulk. Dry polymer should be stored in a cool, dry
place. Extended storage should be avoided.
The chemical conditioning tank is usually located immediately adjacent to the subsequent solids
processing unit. The tank's volume should be appropriate for the solids dewatering unit's capacity. If
11-3
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chemical stabilization is not followed by dewatering, the chemical stabilization tank is sized based on the
daily sludge volume treated. Construction materials for the tank and mixer should be appropriate for the
sludge, and conditioning chemical, characteristics. For most municipal wastewater sludges, steel or
concrete tanks may be used.
For chemical conditioning, sufficient mixing to completely disperse the conditioning chemicals should
be provided. Mixing is accomplished by either diffused air or mechanical agitators. Excessive mixing
which will break up the resulting floe through shear action, should be avoided. Storage of the floe after
it has formed should be minimal to prevent it from settling in the tank, or require excessive mixing to
keep in suspension. In contrast, the mixing requirements for lime stabilization may require more
agitation to promote rapid and even mixing throughout the tank and keep solids hi suspension.
11.2 PROCESS CONTROL CONSIDERATIONS
The primary means of process control for chemical conditioning is to vary the dosage of the
conditioning chemicals. A rapid laboratory method for optimizing chemical dosage is the Standard Jar
Test. This test uses a six-place paddle stirrer to mix equal samples of the sludge to be conditioned. Six
different doses of a conditioning chemical can then be added and mixed. The mixer is then turned off
and the sludge is observed. If enough of the conditioned chemical has been added, an immediate
agglomeration of suspended solids will be noted, clear interstitial water will be present, and the suspended
solids will settle in less than 5 minutes.
Although the Standard Jar Test is useful for determining chemical dosages, the true performance
evaluation criteria for chemical conditioning is the product produced by the dewatering step. Excessive
moisture content, filter media, blinding, poor cake release, or high filtrate or centrate solids, are all
indications of poor chemical conditioning performance. Operating records for the
conditioning/dewatering operation should correlate chemical dosages with dewatering system performance
to determine long-term optimum conditioning requirements.
Feed rates for chemical conditioning of sludges are extremely variable depending on the process
used, the nature of the sludge, and the type of chemical. Table 6 shows typical ranges of dosages.
-------
TABLE <5. TYPICAL DOSAGE RANGES FOR CHEMICAL CONDITIONING
FeCl3, Ib Lime/lb CaO Polymer lb/
Dry Ton Solids Dry Ton Solids Dry Ton Solids
Raw primary + waste
activated sludge 40-50 110-300 15-20
Digested primary + waste
activated sludge 80 -100 160 - 370 30-40
Elutriated primary + waste
activated sludge* 40 -125 — 20-30
* Elutriated sludge results from a process whereby the sludge is washed with fresh water or plant
effluent to reduce the demand for conditioning chemicals and to improve settling of filtering
characteristics (sludge handling and conditioning).
The lime stabilization process is mainly controlled by the pH of the sludge-lime mixture, lime
dosage, and mixing time. Lime should be added continuously until the desired pH level is reached. This
can be done manually or by an automatic pH control. If the control is manual, the operator must monitor
the pH several times a shift. The lime needed to reach the desired pH level is affected by the type,
chemical makeup, and percent solids of the sludge. Therefore, the exact dosage can only be determined
by actual experimentation at the plant.
The sludge pH must be maintained at 12 for 2 hours for adequate stabilization (PSRP). The mixing
time can be adjusted to provide a detention time hi the tank of at least 30 minutes. The lime-treated
sludge can also be transferred to a contactor vessel in which mixing is continued and additional lime is
added, if necessary, to maintain the desired pH. Mixing tune is usually a function of lime slurry feed
rate and is not limited by the mixing capacity of the system. Therefore, mixing is best reduced by
increasing the capacity of the lime slurry tank. In lime stabilization, the lime is frequently added after
dewatering. This is a satisfactory means for achieving lime stabilization provided the lime and the sludge
are thoroughly mixed.
11-5
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Process control measurements should include:
• Continuous monitoring of the flow of the feed sludge to the conditioning or stabilization unit
• Continuous or periodic monitoring of the chemical dosage
• Daily sampling of feed sludge for total solids, suspended solids, and alkalinity
• Daily sampling of conditioned sludge for suspended solids
• Continuous monitoring of pH in mixing tank
• Daily (or continuous) sampling of pH of stabilized sludge.
11.3 PROCESS PERFORMANCE EVALUATION
In evaluating chemical conditioning systems, the inspector is cautioned to bear hi mind that the
chemical conditioning operation and subsequent dewatering operation are interrelated. Poor performance
of the dewatering operation may reflect improper operation of the conditioning step.
When evaluating the performance of a chemical stabilization process, the inspector should check the
pH of the lime-treated sludge as it exits the mixing tank and 2-hour-old lime-treated sludge. An
inspection checklist is included in Appendix A. The checklist is structured to aid the inspector hi
gathering the information needed to properly evaluate a chemical conditioning or chemical stabilization
system.
113.1 Design Evaluation
In evaluating the design adequacy of chemical conditioning and stabilization systems, the inspector
should consider the following:
• Chemical storage facilities—Storage facilities should be designed to maintain a 15- to 30-day
inventory of chemicals based on average use. Smaller inventories could result in the facility
running out if there is some disruption in the supply. Larger inventories may increase the
likelihood that dry chemicals will absorb moisture and form lumps, or that solids or liquids will,
through age, loose reactivity. Storage facilities should be watertight or airtight if appropriate.
Dust control should be provided if needed.
• Operating strategy—There should be a well-defined operating strategy based on either bench
scale testing or dewatering facility performance, to ensure that chemical dosage rates are
adjusted in response to changing sludge characteristics.
11-6
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• Safely.—Because of the corrosive nature of dry conditioning chemicals, attention should be given
to dust control measures, particularly when pneumatic transfer systems or manual handling are
used. Storage, handling, and feeder equipment should be kept clean, with leaks and spills
cleaned up immediately. Concentrated polymer solutions/and dried alum solutions are extremely
slippery and if spilled will present a significant hazard to individuals working in the area.
• Calibration—Gravimetric feeders should be regularly calibrated and the calibration records
maintained at the site. Similar calibration records should be kept for pH meters, and other
control instrumentation.
• Redundancy—There should be sufficient redundancy in the system so that the failure of any one
component (e.g., a metering pump) does not put the entire system out of service. Where
possible, if more than one component is used all components should be the same make and
model. This will ininimize the spare parts inventory required and simplify maintenance
activities.
• Mixing—Mixing of the chemical conditioning/stabilization tank is necessary to ensure intimate
contact between the conditioning/stabilization chemicals and the solids. Under-mixing will result
in poor solids capture in the dewatering step. Over-mixing is equally a problem, as it will break
up the floe which has formed and redistribute fines into suspension. The mixer should be an
appropriate size, and excessive mixing tune should be avoided. Floe break-up or excessive
mixing is not of concern for the chemical stabilization process.
• Conditioning tank detention time—The size of the conditioning tank should be appropriate for
the dewatering step that follows it. For batch-type dewatering equipment (e.g., plate and frame
pressure filters) only the solids required for one batch should be conditioned at one time. For
continuous dewatering operations, detention time in the conditioning tank should be no longer
than necessary to adequately mix the chemicals and allow the floe to form. Lime stabilization
requires a minimum detention time of 30 minutes.
• Visual observations—A sample taken from the conditioning tank should show rapid
agglomeration of solids, crystal clear interstitial water, and rapid settling tendencies. The
supernate should not be cloudy or contain fines. A sample of the stabilized sludge should show
apHof 12.
11-7
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12. VACUUM FILTER
Vacuum filters have been widely used for dewatering both raw and digested wastewater sludges.
The earlier predominant methods were the drum or scraper-type rotary vacuum filters. The belt filter
with natural or synthetic fiber cloth, woven stainless steel mesh, or coil springs media now predominate.
In vacuum filtration, a vacuum applied downstream of the media forces the liquid phase through the
porous media, leaving behind the solids to form a cake. A horizontal cylindrical drum, covered with a
porous medium, is partially submerged in a vat of liquid sludge. As it slowly rotates, vacuum applied
immediately under the filter medium draws solids to form a cake on the surface of the filter medium.
Suction continues to dewater the solids adhering to the belt as it rotates out of the liquid. Then the
vacuum is stopped while the cake is removed, and the medium is washed by water sprays before
reentering the vat.
Figure 16 shows the cutaway view of a vacuum filter. The drum surface is partitioned into several
sections around its circumference. Each section is sealed from its adjacent section and the ends of the
drum. A separate drain line connects each section to a rotary valve at the axis of the drum. Bridge
blocks in the valve divide the sections into the three zones which correspond to the parts of the filtering
cycle: the cake forming zone, the cake drying zone, and the cake discharging zone. A vacuum is applied
to the cake forming zone and the cake drying zone of the valve. As each of the drain lines pass through
the different zones in the valve the vacuum is applied to each of the drum sections.
Figure 17 illustrates the three operating zones encountered during a complete revolution of the drum.
About 10 to 40 percent of the drum surface is submerged in a vat containing a previously conditioned
sludge slurry. This portion of the drum is the cake forming zone. Vacuum applied to the submerged
drum section causes filtrate to pass through the media and sludge particles to be retained on the media.
As the drum rotates, each section is successively carried through the cake forming zone to the cake drying
zone. This zone is also under vacuum and begins at the point where a drum section emerges from the
sludge vat. The cake drying zone represents 40 to 60 percent of the drum surface and terminates at the
point where vacuum is shut off to each successive section. At this point, the sludge cake and drum
section enter the cake discharge zone, where sludge cake is removed from the media.
12-1
-------
CLOTH CAULKING
STRIPS
AUTOMATIC VALVE
AIR AND
FILTRATE
LINE
DRUM
FILTRATE PIPING
CAKE SCRAPER
SLURRY AGITATOR
VAT
AIR BLOW-BACK LINE
SLURRY FEED
FIGURE 16. CUTAWAY VIEW OF A DRUM OR SCRAPER-TYPE
ROTARY VACUUM FILTER
12-2
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PICK-UP OR FORM
ZONE
FIGURE 17. OPERATING ZONES OF A ROTARY VACUUM FILTER
12-3
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12.1 PROCESS CONFIGURATION AND COMPONENTS
There are two variations of the vacuum filter: the drum filter and the belt filter. The drum filter
operates continuously with vacuum applied in the cake forming zone and cake drying zone. In the cake
discharge zone a positive air pressure is maintained in the segment just ahead of the sludge scraper blade
to aid in removal of the dried cake. A fine spray may be used to clean the filter medium with a catching
trough beneath to dispose of the washings. This type of filter has been largely replaced by the belt filter.
Belt rotary vacuum filters differ from the drum or scraper-type units in that the belt medium leaves
Irum during cake discharge and washing. The belt medium may be of cloth or stainless steel mesh
-»il snrinjrs
the drum
or coil springs
• Cloth- or stainless steel mesh-medium filters—A traveling woven cloth (synthetic or natural-
fiber) or metal mesh belt serves as the filter medium. At the end of the drying zone the belt
leaves the drum, passing over a small-diameter discharge roll that facilitates cake discharge.
There may also be a small-diameter curved bar between the point where the belt leaves the drum
and the discharge roll. This bar aids in maintaining belt dimensional stability and adequate cake
discharge. A scraper blade may also be present to obtain cake release from cloth media. The
belt can be washed on both sides, if desired, before positioning back on the drum.
• Coil-medium filters—Two layers of stainless steel springs wrapped around the drum act to
support the initial solids deposit in the cake forming zone. The solids, hi turn, serve as the filter
medium. When the two layers of springs leave the drum they are separated from each other.
The sludge cake is lifted off the lower layer of coil springs, and discharged off the upper layer
with the aid of a positioned tine bar. The two coil spring layers are then washed separately by
spray nozzles and returned to the drum just before the drum reenters the sludge vat.
Tables 7 and 8 contain typical performance data for cloth and coil media as affected by type of
sludge, chemical dosage, and feed solids concentrations.
The principal components of a vacuum filter system are illustrated in Figure 18. Sludge is drawn
directly from clarifiers, holding tanks, or thickening tanks and discharged to a conditioning tank, where
it is mixed (pump and agitator) with chemical coagulants (using pumps if chemical feed is automatic).
The sludge is pumped through a feed chute to a vat under the filter. The vat is equipped with an agitator
that uniformly distributes solids across the face of the filter. The filter drum drain lines are
12-4
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TABLE 7. TYPICAL DEWATERING PERFORMANCE DATA FOR
ROTARY VACUUM FILTERS-CLOTH MEDIA
Sludge Type Feed Solids Cone.
percent
RawP
WAS
P + WAS
P + TF
Anaerobically digested
P
P + TF
P + WAS
P3
t/i Elutriated anaerobicallv digested
P
P + WAS
Thermally conditioned
P + WAS
4.5
2.5
3
4
4
3
5
5
4.5
6
-9.0
-4.5
-7
-8
-8
-7
-10
-10
- 8
-15
Chemical Dosage1
FeCl3
20
60
25
20
30
40
40
25
30
-40
-100
-40
-40
-50
-60
-60
-40
-60
O3
kg/Mg dry solids
CaO
80-
120-
90-
90-
100-
150-
125-
0-
0-
0
100
360
120
120
130
200
175
50
75
Yield2
kg dry solids/m2/hr
17
5
12
15
15
17
20
15
20
-40
-15
-30
-35
-35
-40
-40
-35
-40
CakeSolids
percent
27-35
13-
18-
23-
25-
18-
20-
•20
•25
30
32
25
27
27-35
18-
35-
25
45
XA11 values shown are for pure FeCl3 and CaO. Dosage must be adjusted for anything else.
2Filter yield depends to some extent on feed solids concentration. Increasing the solids concentration normally gives a higher yield.
3Some heat-treated sludge requires some conditioning to maintain recovery at a high level.
1 Ib/ton = 0.5 kg/Mg
1 Ib/ft2/hr = 4.9 kg/m2/hr
Key: P
WAS
TF
Raw primary
Waste activated sludge
Trickling filter
-------
TABLE 8. TYPICAL DEWATERING PERFORMANCE DATA FOR
ROTARY VACUUM FILTERS-COIL MEDIA
Sludge Type Feed Solids Cone.
percent
i_>
to
drs
RawP
TF
P -f WAS
Anaerobically digested
P + TF
P + WAS
Elutriated anaerobically digested
P
8-
4-
3-
5-
4-
8-
10
6
5
8
6
10
Chemical Dosage1
FeCl3
20
20
10
25
25
10
-40
-30
-30
-40
-40
-25
kg/Mg dry solids
CaO
80
50
90
120
100
15
-120
-70
-110
-160
-150
-60
Yield2
kg dry solids/m2/hr
30-
30-
12-
20-
17-
20-
40
40
20
30
22
40
CakeSolidjf
percent
28-
20-
18-
27-
20-
28-
32
32
25
33
25
32
^11 values shown are for pure FeCl3 and CaO. Dosage must be adjusted for anything else.
2Filter yield depends to some extent on feed solids concentration. Increasing the solids concentration normally gives a higher yield.
1 Ib/ton = 0.5 kg/Mg
1 Ib/ft2/hr = 4.9 kg/mz/hr
Key: P Raw primary
WAS Waste activated sludge
TF Trickling filter
-------
SLUDGE INLET
SILENCER
CONVEYOR t~l
WATER
FILTRATE
PUMP
VAT
\
VACUUM
PUMP
FIGURE 18. ROTARY VACUUM FILTER SYSTEM
12-7
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connected to a combination vacuum receiver and filtrate pump prior to the vacuum pump. The principal
purpose of the receiver is air-liquid separation of the filtrate. Air taken from the top of the receiver is
discharged to the atmosphere through a wet-type vacuum pump, while water from the bottom is removed
by a filtrate pump. Other components of the unit include water wash sprays.
12 2 PROCESS CONTROL CONSIDERATIONS
Optimum performance is one that balances maximum sludge cake output (yield), desired cake
dryness and filtrate clarity (solids removal efficiency). Sludge cake dryness relates to the amount of
solids in the filtrate: the drier the sludge cake, the higher the content of solids in Ihe filtrate. The cake
should not be dried more than is necessary for final disposal. At the same tune, the filtrate solids should
be kept to a practical minimum, as these solids impose a load on the plant treatment units receiving this
filtrate. Principal design components that affect dewatering efficiency include:
• Type of sludge and conditioning—Primary sludge is easier to dewater than secondary biological
sludges. Proper conditioning causes sludge to release its water and lowers the vacuum
requirements.
• Solids concentration—The higher the suspended solids concentration of the feed sludge, the
greater will be the production rate of the filter and the cake suspended solids concentration. The
design range for the feed suspended solids concentration is between 3 and 10 percent. Below
3 percent it becomes difficult to produce sludge filter cakes thick enough or dry enough for
adequate discharge from the filter media. If the concentration is greater than 10 percent, the
sludge becomes difficult to pump, mix with chemicals, and distribute after conditioning to the
filter. The sludge treatment processes preceding the dewatering process affect the feed
suspended solids concentration to the filter and need to be designed and operated to achieve the
optimum for the filter.
• Type of filter media—Considerations during the design selection process include desired
liquid/solid separation, filtrate of acceptable clarity, easy release of filter cake, mechanical
strength for long life expectancy, chemical resistance to the materials being handled, minimal
resistance to filtrate flow, and minimal blinding or clogging. Monofilament fabrics seem the
most resistant to blinding and have a long life; cloth filters produce cleaner filtrate. Coil springs
perform poorly with sludges that contain particles that are extremely fine and resistant to
flocculation.
12-8
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Changes in the sludge characteristics may require minor adjustments to operating conditions to
achieve optimum performance. The following operational factors impact filter performance:
• Vacuum level—Vacuum is controlled by:
- Amount of conditioning—Proper conditioning causes sludge to release its water and lowers
the vacuum requirements.
Drum speed—Vacuum rises with the drum speed.
- Sludge level in filter vat—Vacuum drops as the vat level lowers.
• Degree of drum submergence—A full vat provides maximum cake forming time and minimum
cake drying time, resulting hi a thicker but wetter cake.
• Cycle time—The drum speed is controlled by a variable speed drive. The slower the drum
speed, the thicker and drier the cake and the lower the vacuum needed. As the drum speeds up,
it has less tune to remove the water, and the vacuum increases to compensate for less time.
12.3 PROCESS PERFORMANCE EVALUATION
123.1 Design Evaluation
The principal design variables that impact the vacuum filter operation are conditioning chemicals
(type and dosage), filter media used, feed solids concentration, and solids loading rate. Changes in
upstream processes—including sludge production (volume or type), sludge mixture, conditioning
procedures, and sludge holding times (before conditioning and dewatering)—can affect the efficiency of
the filter performance. The inspector should evaluate whether the filter, when operating within design
parameters, is able to achieve the desired cake characteristics and filtrate quality while keeping up with
the sludge production.
123.2 Operation and Maintenance Evaluation
The inspector should evaluate whether the process control measurements being performed provide
operators with the necessary information to determine filter performance and necessary changes.
Adequate process control measures include testing the sludge feed and sludge cake for total solids once
per day, and testing the filtrate for suspended solids once per day. Though not pertinent to filter process
control, measuring the filtrate flow continuously and testing the filtrate for BOD on a regular basis will
provide data for calculating the filtrate loading on the wastewater treatment unit processes.
12-9
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The inspector should check to see that routine operating procedures are performed. The Filter's
O&M manual will provide the specific procedures to follow but the inspector can inquire about general
procedures, such as whether the system is inspected at least twice a shift. When in operation, the vacuum
valve, sludge influent valve, filtrate flow valve, and chemical conditioning valves should all be open, and
the vat drain valve should be closed. The sludge pump, conditioning pump, conditioning tank agitator
drive, and filter vat agitator should all be operating (operation of me filter vat agitator is optional
depending on need). Drum and belt drives, and water sprays, should be operating.
The inspector should also check to see how often maintenance procedures are performed. The
system should be shutdown periodically, and dram valves opened, lines flushed, the filter medium and
tanks thoroughly washed and inspected, and drum chain lubricators checked. The inspector should also
check the spare parts inventory to see if it includes the following:
• Drive mechanism parts such as sprockets, chains, gears, motors, bearings
• Vacuum mechanism parts such as hoses, fittings, pumps, gauges.
To evaluate filter performance, the inspector should visually check the following aspects of filter
operation:
• Cake characteristics—Check to see that cake release is easy and that there is no excessive cake
cracking before release. Thin cake with poor dewatering is indicative of poor performance;
probable causes to check include filter media blinding, improper chemical coagulant dosage,
inadequate vacuum or leaks hi vacuum system or seals, excessive drum speed, or drum
submergence too low.
• Blinding or clogging of filter media—Check the wash water pressure or quantity used. Blinding
or clogging may also result from excessive amount of lime being added to the sludge for
conditioning.
• High solids in filtrate—Probable causes would be an improper chemical coagulant dosage, low
solids concentration hi the feed sludge, filter media blinding, or excessive mesh size in filter
media.
12-10
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• Vibrating receiver—Probable causes would be:
- A clogged filtrate pump; check filtrate pump output to see if it's clogged.
- Air leak in the suction line.
A duty drum face.
- Missing seal strips.
• High vat level—Check to see if:
- Feed rate is too fast
- Drum speed is too slow
- Drain lines are clogged
- Vacuum pump is operating
- Filtrate pump is off or clogged.
• Low vat level—Check to see if the feed rate is too fast or the vat drain valve is open (should be
closed).
Some odors are generated by the vacuum filtration operation, but proper conditioning and ventilation
should minimize the problem. For safety reasons, the work areas should be free of grease, sludge, oil,
or other debris.
12-11
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13. FILTER PRESS
The filter press is a batch device used to dewater sludges. There are several types of presses
available, but the most common consists of vertical plates that are held in a frame and pressed together
between a fixed and moving end. A cloth is mounted on the face of each individual plate. Despite its
name, the filter press does not squeeze or press sludge. Instead, when the press is closed; the sludge is
pumped into the press at pressures up to 225 psi and passed through feed holes in the trays along the
length of the press. The water passes through the cloth, while the solids are retained and form a cake
on the surface of the cloth. Filter presses usually require a precoat material (typically incinerator ash or
diatomaceous earth) to aid in solids retention on the cloth and release of the cake. Sludge feeding is
stopped when the cavities or chambers between trays are filled. Drainage ports are provided at the
bottom of each press chamber. The filtrate is collected hi these, taken to the end of the press, and
discharged to a common drain.
13.1 PROCESS CONFIGURATION AND COMPONENTS
A typical vertical plate filter press is shown in Figure 19. The press consists of feed pumps gears,
drives, chains, sprockets, bearing brackets, electrical contacts, suction lines and sumps, cloths, and rubber
surfaces. All of these components require routine inspection and maintenance.
13.2 PROCESS CONTROL CONSIDERATIONS
If the filter press is operated as recommended, with sufficient washing and air drying tune between
cycles, the cake should have the highest possible solids content. The cake also should discharge from
the press with a minimum of debris left behind. Discharge of a wet cake can lead to dirty cloths on the
lower stile faces, making it difficult to obtain a good seal on this gasket area when closing the press.
It is usually possible to develop an excellent relationship between filtrate flow rate (which decreases
as the cycle progresses) and cake moisture for a given sludge. That is, for any given filtrate flow rate
a corresponding filter cake concentration can be expected.
13-1
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FIGURE 19. SIDE VIEW OF A FILTER PRESS
13-2
-------
Whether or not to precoat is an operational question. The precoat is the placement of an initial
coating on the filter cloth prior to application of the sludge. The precoat acts as an additional filtration
membrane and also aids in a clean removal of sludge from the cloth. If the investment in a precoat
system has been made, its use should reduce manpower requirements for media cleaning and may provide
better performance.
If the press is operated as recommended, but performance is unsatisfactory, a different type of cloth
may give better results. The addition of precoat may also aid hi performance.
13 J PROCESS PERFORMANCE EVALUATION
An inspection checklist is included in Appendix A. The inspection checklist is designed to assist the
inspector in garnering the information needed to properly evaluate the performance of the pressure filter.
When evaluating the performance of a pressure filter, the inspector should pay special attention to the
overall condition and maintenance of the press. Mechanized parts should be inspected for wear,
corrosion, and proper adjustment. Prior to filling, the operator should ensure that the plate filters are
clean and have no holes. Small pinholes in the filter can significantly reduce the performance of the
system.
The filter cake solids and cycle length can be compared against the performance data presented in
Table 9. If the filter is not producing adequately high cake solids, the inspector should evaluate the
following:
• Whether maximum feed pump pressures are being achieved.
• Whether the filter cloths are in good condition. The inspector should particularly note any tears
or holes.
• Whether a precoat is used.
• Whether all filtrate drainage and sludge feed passages are free and unobstructed.
• Whether the manufacturer's procedures (especially end-of-cycle procedures) are being followed.
13-3
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TABLE 9. TYPICAL RESULTS OF PRESSURE FILTRATION
Sludge Type
Primary
Primary + FeCl3
Primary + 2 Stage
Primary + WAS
Primary + (WAS
FeCl3)
(Primary + FeCl3)
+ WAS
WAS
WAS + FeCl3
Digested Primary
Digested Primary
+ WAS
Digest Primary +
(WAS + FeCl3)
Tertiary Alum
Tertiary Low Lime
Conditioning
5% FeCl3, 10% lime
100% ash
10% Lime
None
5% FeCl3, 10% lime
150% ash
5% FeCl3, 10% lime
10% lime
7.5% FeCl3
250% Ash
5% FeCl3, 10% Lime
6% FeCl3, 30% Lime
5% FeCl3, 10% Lime
100% Ash
5% FeCl3, 10% Lime
10% Lime
None
Feed
Solids. %
5
4*
7.5
8*
8*
3.5
'
5*
5*
8
6-8*
6-8*
4*
8*
Typical
Cycle
Length. Hr
2
1.5
4
1.5
2.5
2.0
3
4
2.5
2.0
3.5
2
2
3
6
1.5
% Solids
Filter Cake
Solids. %
45
50
40
50
45
50
45
40
45
50
45
40
45
40
35
55
* Thickening used to achieve this solids concentration
13-4
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14. BELT FILTER PRESS
Belt filters are designed to press sludge between two tensioned, moving belts that are porous. The
belts are passed through various diameter rollers that squeeze out the water and thus generate a dried
sludge that can easily be removed from the belts. Design is usually based on the sludge generation rate
of the treatment plant rather than on the wastewater flow to the plant. Belt filter presses are advantageous
in that, with minimal energy requirements, they are capable of producing a very dry cake. Conversely,
this type of dewatering is very sensitive to sludge characteristics, is hydraulically limited, and has a short
filter media life as compared with other dewatering devices that use filter media.
Although belt filters are available in numerous designs and may be quite complex, all operate under
the same simple three-step process: chemical conditioning, gravity drainage, and compression.
Chemical conditioning is vital to the efficient operation of a belt filter press. Properly conditioned
sludge results hi flocculation of the small particles into larger stronger particles that bridge the openings
in the filter belt and thus remain on the belt. Key to the selection of a chemical conditioner is the ability
of the floe to withstand the pressures generated during the dewatering process without passing through
the filter or squeezing out from between the belts. Polymers have been shown to be the most successful
conditioning agent. Typically, cationic polymers are used, although a two-polymer system that uses a
cationic polymer followed by either an anionic or nonionic polymer may ease cake removal from the belt.
Attempts have been made to use other types of chemical conditioners, such as lime, without success. If
lime is to be used to stabilize the sludge for landfilling, the lime should be added to the sludge after
dewatering by the belt filter process.
Gravity drainage occurs as the conditioned sludge is discharged onto the moving belt. Free water
(interstitial water in the sludge slurry) readily separates from the slurry and is recycled back through the
treatment process. The efficiency of this drainage depends on the type of sludge, quality of the sludge,
conditioning, belt screen mesh, and design of the drainage zone. Typically, gravity drainage occurs on
a flat or slightly inclined belt for a period of 1 to 2 minutes. A 5 to 10 percent increase in solids
concentration should be expected in the gravity drainage zone; that is, 1 to 5 percent feed produces 6
to 15 percent solids prior to compression. The uniform distribution of sludge across the belt is vital for
maximum speed through the pressing operation, and to prevent blinding of the belt mesh, uneven belt
tension, and distortion.
14-1
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The final stage of the belt filtration process consists of compressing the sludge. This step is initiated
as soon as the sludge is subjected to an increase in pressure. This pressure can come from the
compression of the sludge between belts or from the application of a vacuum on the lower belt. The area
where the belt filter begins to compress the sludge (known as the low-pressure zone or wedge zone) is
key to preparing a firm, even sludge cake that can withstand the shear forces to which it is subjected by
the rollers. As the sludge cake progresses through a series of rollers in the filter, high pressures are
exerted by the upper and lower belts. This increased pressure causes flexing of the sludge cake, which
results in the release of water and the further compaction of the sludge cake. Some belt presses have an
independent high-pressure zone that uses additional belts or hydraulic cylinders to increase pressure and
produce a drier cake.
Filter cake is removed from the belts using a scraper mechanism, which drops the sludge into a
hopper or conveyor belt for transfer to the sludge management area. After the cake is removed, a spray
of water is applied to the underside of the belt to rid the belt of any remaining solids. This spray rinse
water is mixed with the filtrate and recycled back through the treatment plant, either to primary or
secondary treatment.
Odors are often a problem with belt filter presses. These odors can be controlled by allowing
adequate ventilation, by using fresh sludge or by using oxidizing chemicals. Potassium permanganate
or hydrogen peroxide can be used to oxidize the odor-causing chemicals (predominantly hydrogen sulfide)
into odorless compounds. In addition, using potassium permanganate can also improve the dewaterability
of the sludge, reduce the amount of polymer required, and eliminate sulfide from the filtrate recycle.
14.1 PROCESS CONFIGURATION AND COMPONENTS
Typically, belt filter presses operate in a semicontinuous mode, based on the volume of sludge to
be dried. Best results are achieved when the belt press is operated under the same conditions at all times,
the lone variable being whether or not sludge is being applied to the belt. This ensures that once a steady
state operation is achieved, variables such as application rate and roller speed will not change. Of course,
as with any system, minor adjustments will occasionally be needed to fine-tune the unit to the changing
conditions of the sludge.
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Belt presses vary in size, roller configuration, filter porosity, and composition of belt material. In
addition, some manufacturers offer vacuum-assisted belt presses that may increase the solids content of
the sludge.
Belt filters are designed to provide a high solids content sludge at minimal cost. This is
accomplished through six major systems in the press:
• Chemical conditioning
• Dewatering belts
• Rollers
• Belt tracking and tensioning system
• Controls and drives
• Belt washing.
14.1.1 Chemical Conditioning
Chemical conditioning typically takes place in a small tank (70 to 100 gallons) that is positioned
approximately 2 to 3 ft before the belt filter, in a rotating drum attached to the top of the press, or in an
in-line baffled injector. It is also recommended that a second small tank be installed further up the line
(25 ft) hi situations where a longer contact time may be needed to properly condition the sludge. These
polymer conditioning units are typically supplied by the manufacturer along with the belt press. The feed
point for any odor-suppressing oxidizer (e.g., potassium permanganate) should be upstream of the
polymer feed point by a distance which will allow for approximately 1 minute of contact time prior to
polymer addition.
14.1.2 Dewatering Belts
The dewatering belts, typically made of monofilament polyester fibers, come in various weave
combinations, permeabilities, and particle retention capabilities—all of which influence performance of
the press. The determination of the correct belt usually requires testing with actual sludge to determine
me most appropriate belt construction parameters. For plants already in operation, this is a simple
procedure. For newly designed plants, however, this evaluation must be based on information obtained
from similar plants mat are processing a similar type of sludge.
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There are two basic types of belts, split and continuous. Split belts are joined together with a device
called a clipper seam. Split belts are the most common type of belt and can be used on all types of belt
presses. The continuous, or seamless belt, can only be used on certain presses and are more difficult
to install, but may have longer life spans.
14.13 Rollers
The rollers are the main mechanical component of the belt press. The rollers set the pressure and
force that dewater the sludge; therefore, the proper design and control of this equipment is necessary for
a dry sludge cake. The number, size, and shaft diameter of the rollers are the key design parameters for
a belt press. At a given belt tension, as roller diameter decreases, pressure on the cake increases.
14.1.4 Belt Tracking and Tensioning System
The belt tracking and tensioning system is the key control, once the number and size of the rollers
have been determined and installed. Tensioning allows the press operator adjustments to match the sludge
composition, while the tracking system ensures that the belt is operating at its maximum design
efficiency. Poor tracking causes excessive wear on the belts as well as not providing the driest possible
sludge.
14.1.5 Control and Drives
Process controls typically include automatic startup and shutdown, tracking and tensioning of belts,
pressure gauges, operating time meters, and sludge and polymer pump controls. It is important that the
startup and shutdown procedures are automated in the correct sequence to ensure additional manpower
is not needed for this procedure. For example, starting the sludge pumps before the conveyor belt would
cause a pile of sludge to build up that may interfere with the operation of the system. The polymer and
sludge feed pumps must also shut off automatically if any operations downstream of these pumps should
fail.
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14.1.6 Belt Washing
The belt washing system is designed to apply a steady stream of water onto the backside of the belts
after the cake has been removed. This allows the belt to track smoothly back around and apply a uniform
layer of sludge. Occasionally, facilities will use recirculated water from the press as wash water,
although operational problems are more likely to occur because of the high solids content recirculated.
Washwater typically is applied at a rate of more than half the slurry application rate. Therefore,
secondary effluent, rather than a potable water, is usually used as washwater to save costs.
14.2 PROCESS CONTROL CONSIDERATIONS
Once familiar with the equipment, the press operator should be able to evaluate the operation by
visual inspection. Control of the sludge solids content leaving the belt press is affected primarily by five
parameters:
• Chemical conditioning
• Percent solids of incoming sludge
• Loading rate of sludge
• Operating speed of the belt
• Compression of the rollers.
Chemical Conditioning
The appropriate polymer for chemical conditioning is usually determined by jar testing. The
optimum dosage is the amount at which above that little or no increase in floe size or supernatant clarity
is noted. Because sludge characteristics can change, as can chemical costs, many facilities have a dual
polymer feed system that can feed either liquid or dry polymer. Underconditioned sludge will not dram
well hi the gravity drainage section, resulting hi either an exceptionally wet sludge or uncontrolled
discharge of slurry in that section. Overconditioned sludges drain so rapidly that the sludge does not have
time to distribute uniformly over the belt. Overconditioned and underconditioned sludges both can cause
blinding of the filter media. Inclusion of a sludge blending tank prior to the press can help to minimize
this problem and ensure that the polymer is uniformly distributed through the slurry.
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14-2-2 Percent Solids of Incoming Sludge
Generally, a thicker incoming sludge will produce a drier cake. Therefore, it is preferable to apply
as thick a sludge as possible to the belt filter. This is quite often achieved through the use of a sludge
thickening process prior to the application of the sludge to the belt press.
14.2.3 Loading Rate of Sludge
The application rate of sludge to the press has a significant effect on performance of die unit. Each
unit will have a design operating range; the press should be operated within this range. A typical belt
press will have a hydraulic loading rate of about 40 gpm/m (12 gpm/ft) of belt width. If the loading rate
is too high or too low, the unit will not operate efficiently. Too high a rate of application can generate
a poorly dewatered sludge, as can application of too low a rate of sludge. The ideal application rate is
the maximum rate at which there is no noticeable drop in performance.
14.2.4 Operating Speed of the Belt
As the sludge application rate increases, the belt speed of the press should be increased. The actual
speed of the unit depends on the desired characteristics of the sludge cake. Obviously, the slower the
operating speed, the better the dewatering of the sludge. The optimum speed for any user's particular
case is best determined through trial and error. Once the optimum speed is determined, little or no
adjustment should be needed.
14.2.5 Compression of the Rollers
As with the speed of the belt, the best compression of the rollers should be determined through trial
and error. Once set, the compression should not require adjustment.
14 J PROCESS PERFORMANCE EVALUATION
When evaluating the performance of a belt press, the inspector should compare the actual operating
conditions to the recommended operating conditions. Operating conditions for various types of sludges
dewatered on a belt filter press are presented hi Table 10. The inspection checklist in Appendix A is
designed to assist the inspector in gathering the information and making the calculations required to make
the comparison between actual operating conditions and design conditions.
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TABLE 10. TYPICAL DATA FOR VARIOUS TYPES OF SLUDGES DEWATERED ON A BELT PRESS
Sludge Type
Feed Solids
percent
Raw
P
WAS
P + WAS
P + TF
Anaerobically Digested
P
WAS
P + WAS
Aerobically Digested
P + WAS
p + TF
Oxygen Activated
WAS
Thermally Conditioned
P + WAS
3- 10
0.5-4
3-6
3-6
3-10
3-4
3-9
1 -3
4-8
1 -3
4-8
Solids
Loading Rate
kg/hr/m belt
width
360 - 680
45 - 230
180 - 590
180 - 590
360 - 590
40 - 135
180 - 680
90 - 230
135 - 230
90 - 180
290 - 910
Polymer Dose
g/kg
Cake Solids
percent
1 -5
1 - 10
1 - 10
2-8
1 -5
2- 10
2-8
2-8
2-8
4- 10
0
28-35
20-35
20-35
20-40
25-36
12-22
18-35
12-30
12-30
15-23
25 - 40+
Key: P Raw primary
WAS Waste activated sludge
TF Trickling filter
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143.1 Design Evaluation
In evaluating the design adequacy of a belt press, the inspector should consider the following:
• Press capacity—The system should be designed to handle sludge at the rate generated by the
treatment plant. A backup system should be available for down times to prevent the
accumulation or disposal of sludge.
• Belt tracking—The belt system should include an automatic adjusting device to periodically
correct roller adjustment. This reduces labor requirements of manually adjusting the belts and
is also more efficient at ensuring proper alignment.
• Spray nozzles—If the plant effluent or recycled filtrate is used as washwater, a high efficiency
filtration system should be included prior to the spray nozzles to prevent clogging. Some spray
nozzles will contain stainless steel brushes in the spray header to automatically clean the nozzles
without removing them from the press. Also, the spray nozzles should be designed such that
the stream of water reaches the entire surface of the belts.
• Process controls—Control equipment should be located away from the belt press, preferably in
a control room, to protect these controls from the moist and corrosive operating conditions.
• System integration—The system should be designed and installed by one supplier. Typically,
performance is more efficient when equipment comes from one supplier rather than from
several.
14.3.2 Operation and Maintenance Evaluation
In evaluating the belt press operation, the inspector should consider the following parameters:
Process controls—The following measurements should be conducted by the plant operators to
ensure optimum operations:
- Feed Slurry and Dewatered Sludge—The feed sludge and the dewatered sludge cake should
be monitored for total solids and flow.
Filtrate and Wash Water—The filtrate and wash water should be monitored for biochemical
oxygen demand, suspended solids, total solids, and flow.
Filter Cake—The sludge cake should have a uniform thickness across the entire width of the belt
without squeezing out the sides during operation. The scrapers also should remove the majority
of the cake, with the wash water removing the residual sludge remaining on both sides of the
belts.
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Preventive maintenance—Belt presses require a good preventive maintenance program because
of their susceptibility to equipment malfunctions (many moving parts) and the corrosive nature
of the waste. Preventive maintenance should include periodic inspections of:
- V-Belts, drives, and gear reducers
- Filter belts and tracking mechanism
- Rollers, bearings, and bores
- Bearing brackets
- Baffles
- Electrical contacts in starters and relays
- Suction lines and pumps
- Chemical mixing tanks and pumps.
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15. SLUDGE DRYING BEDS
Drying beds are a widely used method of dewatering municipal sludge in the United States. They
are generally used for dewatering well-digested sludge. Attempts to air dry raw sludge usually result in
odor problems.
Digested and/or conditioned sludge is discharged onto a drying bed and allowed to dewater and dry
under natural conditions. After the sludge is applied to the porous drainage media, dissolved gases are
released and rise to the surface, floating the solids and leaving a layer of liquor at the bottom. The liquor
drains through the porous media (usually sand) and is collected hi the underdrain system and usually
returned to the plant for further treatment. Drying beds drain very slowly at first, but after approximately
three days, the rate of drying increases. As the sludge dries, cracks develop in the surface, allowing
evaporation to occur from the lower layers and accelerating the drying process. After maximum drainage
is reached, the dewatering rate gradually slows down and evaporation continues until the moisture content
is low enough to permit sludge removal. Dry sludge may be removed periodically from the beds, by
special conveyors or with other loading equipment, for ultimate disposal.
Chemical addition (such as polymers) has been used to enhance drying bed performance. In northern
climates, a freeze/thaw/dram cycle has been used to allow use of outdoor beds in regions that experience
sub-freezing weather. Reed beds (hi which plant growth in the beds is encouraged) have also been used.
15.1 PROCESS CONFIGURATION AND COMPONENTS
Drying beds for sludge dewatering are operated hi parallel. Most facilities provide more than one
drying bed to ensure that there will be enough available drying space to handle the digested sludge
generated by the treatment process.
Drying beds generally consist of 1- to 3- ft high retaining wall enclosing a porous drainage media.
The drainage media may be made up of various sandwiched layers of sand and gravel, combinations of
sand and gravel and cement strips, slotted metal media, or a permanent porous media.
Of these, the sand and gravel beds, shown in Figure 20, are most common. Generally, sand and
gravel beds are comprised of 4 to 9 in. of sand (0.3 to 1.2 mm diameter) over an 8- to 18-in. layer of
gravel (gravel size is usually 1/8 to 1 in. diameter). The water drains to an underdrain system which
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Gate
Sludge
FIGURE 20. TYPICAL SAND AND GRAVEL DRYING BED CONSTRUCTION
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consists of perforated pipe at least 4 in. in diameter. The underdrain pipes are usually spaced between
8- and 20-ft apart, depending on the size of the beds. The pipes must have a slope of at least 1 percent
to allow the drainage water to flow back to the treatment plant.
Another type of drying bed in use is a paved drying bed (Figure 21). Typically, these beds have either
a concrete or asphalt paved surfaced sloping at least 1.5 percent towards a drainage media consisting of
sand over gravel. The stabilized sludge is put on the paved portion and the water drains down the slope
to the drainage media. The water men collects in an underdrain pipe that runs the length of the drainage
media.
Another less common type of drying bed is the wedge-wire (or wedgewater) drying bed (Figure 22).
The bed consists of a shallow rectangular watertight basin fitted with a false floor of stainless steel or
preformed polyurethane panels. These panels have wedge-shaped slotted openings of 0.01 in. (0.25 mm).
The false floor is made watertight with caulking where the panels abut the walls. An outlet valve to
control the rate of drainage is located underneath the false floor. Water or plant effluent enters the bed
from beneath the panels (or wedge-wire septum) until a depth of approximately 1 in. (2.5 cm) over the
wedge-wire septum is attained. This water serves as a cushion that permits the sludge as it is slowly
introduced to float without causing upward or downward pressure across the wedge-wire surface. The
water further prevents compression or other disturbance of the colloidal particles. After the bed is filled
with sludge, the initially separate water layer and the drainage water are allowed to percolate away at a
controlled rate, through the outlet valve. After the free water has been drained, the sludge further
concentrates by drainage and evaporation until there is a requirement for sludge removal.
The final type of drying bed available is a vacuum-assisted drying bed. These beds are relatively
uncommon. They consist of a reinforced concrete bottom ground slab, a layer of stabilized aggregate
several inches thick, and a rigid multimedia top. This space between the concrete bottom slab and the
rigid multimedia top is also the vacuum chamber and is connected to a vacuum pump. Sludge is applied
to the surface of the multimedia top until it is entirely covered. The vacuum system is then started to
remove the water from the sludge.
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GATE
V: SLAB
FIGURE 21. TYPICAL PAVED DRYING BED CONSTRUCTION
CONTROLLED DIFFERENTIAL HEAD IN VENT
BY RESTRICTING RATE OF DRAINAGE
VENT
1
PARTITION TO FORM VENT
WEDGEWIRE SEPTUM
/ OUTLET VALVE TO CONTROL TO CONTROL
f RATE OF DRAINAGE
FIGURE 22. CROSS-SECTION OF A WEDGE-WIRE DRYING BED
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Drying beds are sometimes enclosed in a green-house type glass structure or have roofs covering
them to increase drying efficiency in wet or colder climates. In addition, enclosing the drying beds helps
to control odor and insects, and improves the overall appearance of the plant. It is important that totally
enclosed drying beds are well-ventilated to allow moisture to escape. Enclosed beds generally need only
67 to 75 percent of the area required for an open bed.
15 2 PROCESS CONTROL CONSIDERATIONS
Treatment plant operations have less control over the performance of drying beds than they do over
mechanical dewatering systems. Performance of drying beds is affected by such factors as weather,
sludge characteristics, the design of the drying bed, chemical conditioning, and the depth of sludge. To
qualify as Processes to Significantly Reduce Pathogens (PSRP), a drying bed must meet the operating
parameters in 40 CFR Part 257. Not more than 9 in. of sludge can be applied to the drying bed and what
is applied must be left to dry for 3 months. During 2 of the 3 months the average daily temperature must
be above 0°C (32 °F). Air-dried sludge does not meet Processes to Further Reduce Pathogens (PFRP)
requirements unless used in conjunction with another treatment process that qualifies as PFRP.
Through experience, each operator will determine the optimum depth of sludge that can be applied
to the drying beds. The typical depth of application is 8 to 12 in. Factors that should be considered
when applying sludge to the bed are the type of sludge and the moisture content. Generally, sludge with
a high grit content will dewater rapidly, while sludge containing grease drain slower. The age of the
sludge is important as well. Aged sludge dries slower than new sludge. Primary sludge dries faster than
secondary sludge and digested sludge dries faster than raw sludge. It is important that wastewater sludge
be well digested for good drying. In well-digested sludge, gases tend to float the sludge solids and leave
a clear liquid layer, which drains through the sand. Other factors affecting the depth at which sludge is
applied include the area of sand bed available and the need to draw sludge from the digesters. Fresh
sludge should never be applied on top of dried sludge to a bed. The exception would be if reed beds are
used.
A thinner layer of sludge will dry more rapidly, permitting quick removal and reuse of the bed. An
8 in. layer should dry in about 3 weeks in the open during reasonably dry weather. A 10 in. layer of
the same sludge will take 4 weeks, so that the 25 percent additional sludge actually takes 30 percent more
time to dry. In some cases it may be desirable to apply sludge in a layer thinner than 8 niches. The best
operation can only be determined by trial and error, and may also vary seasonally.
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Chemicals can be used on sludges that are hard to dewater or for overloaded beds. The chemicals
most commonly added to sludges to aid in dewatering are alum, ferric chloride, and organic
polyelectrolytes. Chemical conditioning of sludge was discussed earlier under Chemical Stabilization and
Conditioning.
The best time to remove dried sludge from drying beds depends on a number of factors, such as
subsequent treatment by grinding or shredding, the availability of drying bed area for application of
current sludge production, labor availability, and, of course, the desired moisture content of the dried
sludge. Sludge can be removed by shovel or forks at a moisture content of 40 percent; however, if it
is allowed to dry to 60 percent moisture, it will weigh only half as much and is still easy to handle. If
the sludge gets too dry (80 to 90 percent solids), it will be dusty and will be difficult to remove because
it will crumble as it is removed. The useful capacity of the drying beds can be maximized by always
removing the sludge as soon as it has reached the desired dry ness.
15.3 PROCESS PERFORMANCE EVALUATION
153.1 Design Evaluation
An inspection checklist for sludge drying beds is provided in Appendix A. The checklist is designed
to assist the inspector in gathering the information required to adequately evaluate drying bed operations.
While drying beds are rather simple in nature, the inspector should be aware that there are certain factors
that affect the design adequacy of the beds for a particular plant. The most important consideration when
evaluating the adequacy of a drying bed is the solids loading on a dryweight basis, applied yearly, per
square foot of drying bed area. Drying beds are normally sized based upon required square feet of bed
area per capita served by the treatment plant. The area required depends on climate and sludge
conditioning prior to drying. By using a covered bed, the drying efficiency is increased.
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Covered beds can handle a higher solids loading rate than uncovered beds. Table 11 shows typical
performance in terms of solids loading rate and moisture content of dried sludge for covered and
uncovered beds.
TABLE 11. TYPICAL PERFORMANCE DATA FOR DRYING BEDS
Open Beds Covered Beds
Solids loading rate
Ib/yr/ft2 up to 25 up to 40
Moisture content of dried
sludge, percent 50 to 60 50 to 60
Another design consideration an inspector will want to evaluate is the method of sludge cake removal
from the drying beds. Most plants remove the sludge cake manually, which requires that the sludge be
dried between 30 and 40 percent solids. Mechanized systems only require the cake to have a 20 to 30
percent solids content, thus reducing the amount of drying tune. A reduction in drying time allows more
sludge to be handled.
15.3.2 Operations and Maintenance Evaluation
Sludge drying beds are relatively simple to operate and maintain, but certain steps must be taken to
ensure good performance and aesthetics. After sludge is applied to the beds, lines should be drained and
flushed with water to prevent plugging and high pressures caused by gases resulting from the
decomposing sludge.
After the sludge cake is removed from a sand media filter, the bed should be levelled and raked to
ensure that it can drain sludge properly. The depth of the sand should be checked regularly. More sand
should be added when the depth is below 4 niches.
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Odors, flies and vegetation growth on the beds are other problems (excepting plant growth on reed
beds) that may occur in the drying beds. These should all controlled. Odors are typically treated with
chemicals. These are either sprayed into the air to mask the odor, or are added to the sludge to prevent
the odor. Flies are controlled by the destruction of breeding, or by traps and poisons. They are most
effectively controlled in the larvae stage by sprinkling calcium borate or borax in the sludge, especially
in the cracks of the drying cake. Vegetation is easily controlled either by physically removing the plant,
or, in bad cases, by using herbicides.
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16. SLUDGE DRYING LAGOONS
Sludge lagoons are similar to sand beds in that sludge is periodically drawn from a digester, placed
in a lagoon, and removed after a period of drying. Unlike sand drying beds, drying lagoons do not have
an underdrain system for drainage water removal. Lagoons operate by periodically decanting the
supernatant back to the treatment plant and by evaporation. Sludge lagoons are periodically dredged to
remove sludge for ultimate disposal.
16.1 PROCESS CONFIGURATION AND COMPONENTS
Treatment plants using lagoons for sludge dewatering should have more than one lagoon on site.
The units are operated hi parallel; allowing the plant operator to apply sludge to one lagoon while
leaving another lagoon to dry. Sludge lagoons are very basic treatment units. Some lagoons have plastic
or rubber bottom linings, while many others have a natural earth base. Supernatant and rainwater drain-
off points are normally provided on most lagoons. The drain-off liquid is usually returned to the plant
for further treatment. Unlike drying beds, lagoons are always open and not covered to protect from the
weather. Covering lagoons is impractical due to their larger size.
16 2 PROCESS CONTROL CONSIDERATIONS
Very little process control can be performed on drying lagoons once the sludge has been applied.
The plant operator does have control over the type of sludge being applied to the lagoon. Untreated or
lime-treated sludges, and sludges with a strong supernatant, are generally not suited for dewatering in a
lagoon. These types of sludges cause odor problems hi the treatment plant.
Lagoon performance is dependent upon climatic conditions. Geographic areas that have high annual
precipitation and/or low temperatures are not suited for sludge dewatering lagoons. Lagoons are best
utilized in regions that are hot and arid. Operators should ensure that sludge is evenly distributed across
the basin during application. In most regions with drying lagoons, the depth of the applied sludge after
excess supernatant has been drawn off should not exceed 15 in. to prevent excess drying tune. In arid
regions, the sludge can be applied to a greater depth due to the higher evaporation rate. Sludge takes a
long tune to dewater in a lagoon. Generally, if sludge is applied to a depth of 15 hi. or less, it will
usually dry between 40 and 60 percent solids in 3 to 5 months, depending on the weather. When sludge
is to be used for soil conditioning, it can be stored for further drying. One operational approach for
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lagoons is a 3-year cycle in which the lagoon is loaded for 1 year, dries for 18 months, is cleaned, and
is then allowed to rest for 6 months.
16.3 PROCESS PERFORMANCE EVALUATION
163.1 Design Evaluation
The factor determining the design of the sludge lagoons is the solids loading rate. A solids loading
rate typically used in the design of lagoons is 2.2 to 3.4 Ib/yr/ft3 of lagoon capacity. Other designs are
1 fWcapita for primary digested sludges in a dry climate, and 3 to 4 ftVcapita for activated sludge plants
where the annual rainfall is greater than 36 inches. A 2 ft-high dike with a sludge depth of 15 in. (after
decanting) is often used. Sludge removal is normally done using a front end loader. The moisture
content of sludge cake in most areas, except for the more arid, is too high to permit removal by manual
means.
163.2 Operation and Maintenance Evaluation
Overall, operation and maintenance of sludge lagoons requires little effort on the part of the plant
operator. There are, however, some things that should be done to create a good maintenance program.
The lagoon dikes and liner should be regularly inspected, and any damage should be repaired to prevent
sludge leaking. Before applying sludge to the lagoon, the bottom of the basin should be leveled and any
vegetation growing there removed. Sludge application lines and valves should be regularly checked. In
the winter, the sludge lines should be drained to prevent freezing. Excess rain or snow that has
accumulated on the lagoon should be decanted to increase evaporation efficiency. In addition, weeds,
odors, and insects should be kept to a minimum.
Records must be kept on the sludge loading, percent solids hi sludge and decant, quantity and depth
in the lagoon, date sludge is applied, drying time and rainfall. This will provide the operator with the
information necessary to determine the optimal time of sludge removal from the lagoon by comparing
sludge moisture content with time for drying under particular climatic conditions.
An inspection checklist for sludge lagoons is provided in Appendix A. The checklist is designed
to assist the inspector in gathering the information required to adequately evaluate drying lagoon
operations.
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17. HEAT DRYING
Heat drying is used to reduce moisture content and pathogens in stabilized or conditioned sludge.
Sludge that has been heat-dried may then be used as fertilizer, soil amendment, or, since the water
content and volume is considerably reduced, the sludge may be used as cover material hi a landfill.
Sludge is usually prepared for heat drying by mechanical dewatering to reduce its moisture content. The
resulting sludge cake is then heat dried to reduce moisture from an initial level of roughly 80 percent to
a level of 5 to 10 percent hi the finished product. Heat drying is distinct from incineration hi that the
solids are held to temperatures too low (140° to 200°F) to result in destruction of organic matter. Heat
drying is usually accomplished by direct heat transfer involving interaction of hot gases with sludge
particles or through indirect heat transfer where a heated surface transfers heat to the sludge cake. Water
vapor is removed by a flow of moist gas, most often air.
Types of air flows in sludge dryers can be cocurrent (moving with the sludge flow), countercurrent
(moving against the sludge flow) or crosscurrent (moving across the sludge flow). In most direct drying
operations (flash, spray, and some rotary dryers), odor distillation is minimized, and energy efficiency
is best accomplished by using a cocurrent air flow.
The three stages of heat drying are initial drying, steady-state drying, and final drying. Initial drying
occurs during a short period as the sludge temperature and drying rate are raised to the level of
steady-state drying; little drying occurs during this first phase. During steady-state drying, the longest
of the phases, the temperature at the interface between the wet sludge and the gas is kept at the wet bulb
temperature of the gas. In this phase, where drying occurs most rapidly, moisture evaporated from the
surface of the material is replaced by moisture from the interior of the sludge. Final drying is the phase
during which the surface of the sludge is only partially saturated and, although the temperature at the
sludge-gas interface is higher, drying rates are significantly lower than during the steady-state phase.
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Regulations addressing Processes to Further Reduce Pathogens in sludge (40 CFR Part 257) define
heat drying as a process in which,
Dewatered sludge cake is dried by direct or indirect contact with hot gases, and
moisture content is reduced to 10 percent or lower. Sludge particles reach temperatures
well in excess of 80°C or the wet bulb temperature of the gas stream in contact with
the sludge at the point where it leaves the dryer is in excess of 80°C.
A correctly operated heat drying process that maintains temperatures at these levels should ensure the
reduction of pathogens (such as bacteria, viruses, or helminth ova) below detectable levels (EPA, 1989).
Heat drying produces a dried sludge material, a moist exhaust gas, and sometimes a liquid
sidestream. Dusty, odorous or contaminated materials are not easily accepted for use as fertilizer or soil
conditioner. Some level of finishing of the dried sludge (screening, pelletizing or granulating) may be
required. Exhaust gases may need afterburning to reduce odors and particulates. Other techniques used
hi treating exhaust gases include cyclonic dust separators, wet scrubbers, electrostatic precipitators, and
baghouses. Liquid sidestreams are sometimes produced by these ah- pollution control devices. These
sidestreams are often returned to the headworks of the POTW, but may sometimes require separate
treatment.
Flash dryers and rotary dryers are the more common types of heat drying techniques employed in
the United States. Other methods include spray dryers, and a patented multiple-effect evaporation method
known as the Carver-Greenfield process. Each of these techniques is discussed in the following sections.
17.1 PROCESS CONFIGURATION AND COMPONENTS
17.1.1 Flash Dryer
Flash drying rapidly removes moisture through spraying or injecting solids into a stream of heated
gas. A typical flash drying process, marketed by CE-Raymond, is shown in Figure 23. The first step
in flash drying involves mechanical mixing of some portion of the waste stream of previously dried sludge
with wet sludge cake to improve handling characteristics. The resulting sludge mix and hot gases
17-2
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CYCLONE
EXHAUST
GAS
VAPOR FAN
AUTOMATIC
DAMPERS
INDUCED
DRAFT FAN
EXPANSION
JOINT
EXPANSION
JOINT
EXPANSION
JOINT
DOUBLE
FLAP VALVE
MANUAL
DRY
DIVIDER
COMBUSTION
AIR PREHEATER
DRY PRODUCT
CONVEYOR
WET SLUDGE
CONVEYOR
DEODORIZING
PREHEATER
DISCHARGE SPOUT
AUTOMATIC
DAMPERS
COMBUSTION AIR FAN
REMOTE
MANUAL
DAMPERS
CAGE MILL
HOT GAS DUCT
FIGURE 23. FLASH DRYER SYSTEM (COURTESY CE-RAYMOND)
17-3
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from the incinerator are mechanically agitated in a cage mill. Air velocities in the cage mill are typically
65 to 100 ft/sec, with inlet gas temperatures reaching 1,300°F. This stage accomplishes the drying in
a matter of seconds. A cyclonic dust separator is used to separate the solids from the gas stream, which
are then passed on to the deodorizing preheater and incinerator. Exhaust gases then pass through a
combustion preheater, together with inlet air. A portion of this mixture is exhausted to the atmosphere
after scrubbing.
17.1.2 Rotary Dryer
The components common to rotary dryers are shown in Figure 24. Rotary dryers function in a
manner similar to horizontal cylindrical kilns. Sludge is prepared for processing hi rotary dryers, as for
flash dryers, by mixing with previously dried sludge. The resulting sludge cake/dried sludge mixture is
added to one end of the dryer. The rotation of the cylinder (5 to 8 rpm) as well as different internal
arrangements of vanes, paddles or other devices agitate and break up the material, facilitating moisture
transfer. Often, rotary dryers are slightly tilted to facilitate transport of the sludge through the device.
Other designs use a central shaft with agitators to transport and agitate the sludge mixture. Heating of
the sludge in direct rotary dryers occurs as hot gases (1,200°F) are passed through the rotating cylinder
at speeds ranging from 4 to 12 ft/sec. Indirect rotary dryers use a jacket carrying hot gases to heat the
steel cylinder's surfaces. Sometimes the central shaft is similarly heated. Indirect-direct rotary dryers
direct the hot gases used to heat the surfaces through the drying sludge material before venting.
Residence time for sludge passing through the dryer ranges from 20 to 60 minutes. As with flash drying,
the resulting gases are passed through a cyclonic dust separator to separate out coarser particles of dried
sludge. Options for the exhaust gases from the cyclone are shown hi Figure 25.
17.1.3 Spray Dryer
Spray-drying operations, like flash drying, result in nearly instantaneous drying of the sludge
particles. Three steps are involved: liquid atomizing, gas/droplet mixing, and drying of the liquid
droplets. Centrifugal dishes or bowls are most commonly used as atomizers, although some installations
use high-pressure nozzles. Atomizing breaks the liquid sludge into fine droplets, exposing greater surface
to the hot gases used to dry the sludge. The atomizing device directs the droplets into a vertical tower
where they pass downward through a rising gas stream introduced at roughly 1,300°F (705°C).
17-4
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PRODUCT
FIGURE 24. ROTARY KILN DRYER
17-5
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AIR
CHEMICAL
SCRUBBER
_fc. DIRECT DISCHARGE
TO ATMOSPHERE
ATMOSPHERE
•FUEL
• ATMOSPHERE
BURNER
1500°F
SCRUBBER
•ATMOSPHERE
FEED SLUDGE
ALTERNATIVES AVAILABLE FOR EXHAUST GAS DEODORIZATION
AND PARTICULATE REMOVAL
FIGURE 25. SCHEMATIC FOR A ROTARY DRYER
17-6
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The droplets lose their moisture, fall through the gas stream and are collected at the bottom of the dryer.
The gas stream is passed through a cyclonic dust separator before being exhausted through air pollution
control devices.
17.1.4 Carver-Greenfield
The patented Carver-Greenfield Process, marketed by Foster Wheeler Energy Corporation and
Dehydro-Tech Corporation, uses multiple-effect evaporation to remove moisture from a mixture of sludge
and light oil. Major steps hi this process involve oil/sludge mixing, multiple-effect evaporation, oil-solid
separation, and condensate-oil separation. Oil helps the sludge/oil slurry maintain flowing characteristics
and minimizes the formation of scale and corrosion of heat exchanging surfaces. After it is mixed, the
slurry is passed through a grinder to minimize clogging of the evaporator tubes. Water is removed from
the slurry by falling-film evaporation as the material flows down evaporator tubes hi a thin film. Steam
generated as the slurry passes through the evaporator tubes is used to heat subsequent tubes, enhancing
the efficiency of heat transfer. Oil remaining in the sludge is centrifuged out of the resulting product.
Miscibility characteristics of the light oil used facilitate separation of the oil from water hi the condensate.
Condensate water from the evaporation and separation processes will contain ammonia and dissolved
organic materials, requiring additional treatment in most cases. Gases exhausted from this process should
be sent to a boiler or incinerator for odor removal through thermal destruction.
17.2 PROCESS CONTROL CONSIDERATIONS
Larger facilities may operate heat drying equipment on a continuous basis, while smaller facilities
may only operate dryers on a shift or intermittent basis. The inspector should review the POTW's
normal procedures in this regard. Startup and shutdown procedures should be clearly spelled out to avoid
process inefficiencies.
Physical control considerations in heat drying involve maintaining as constant a process rate as
possible. Table 12 provides suggestions for monitoring operational parameters. A diagram provided with
the table illustrates the sample locations discussed in the table. Careful monitoring of the process using
observation as well as analytical testing will result hi more efficient and predictable processing of sludge.
If temperature ranges necessary for proper operation of the drying processes are not maintained,
17-7
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TABLE 12. SUGGESTED MINIMUM AND OPTIONAL MONITORING
FOR HEAT DRYING PROCESSES
Stack Gas
Fuel/Air
Mixture
Vapor
Pneumatic
Conveyance
Line
Dewatered Slud
Dried Sludge Return
Cyclone
Dried
Sludge
Suggested Minimum 1
rH
(8
e
*H
U
a
o
Percent Solids
Temperature
Sludge Feed
Rate
Oxygen
Particulates
SP2-, NOX,
CO, C02
Fuel
Consumption
Air Flow
Ash Content
Nutrient
Content
Density
Toxicity
Sample
Frequency
I/day
Continuous
Continuous
Continuous
As required
by APCD*
As required
by APCD*
Continuous
Continuous
1 /month
1 /month
1 /month
1 /month
Sample
Location
Dewatered Sludge
Dried Sludge
Furnace, Stack
gas, dewatered
and dried sludge
Dewatered Sludge
Stack Gas
Stack Gas
Stack Gas
Furnace Input
Furnace Input
Dried Sludge
Dried Sludge
Dried Sludge
Dried Sludge
Sample
Method
Grab
Record
Continuously
Record
Continuously
Record
Continuously
Record or
Grab
Record or
Grab
Record
Continuously
Record
Continuously
Grab
Grab
Grab
Grab
Reason
for Sample
Process
Control
Process
Control
Process
Control
Furnace
Control
Air Pollution
Control
Air Pollution
Control
Furnace
Control
Furnace
Control
Determine
characteristics
prior to
use or
disposal.
•Air Pollution Control District
17-8
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alarms or other devices should signal operators of unsuitable conditions to enable rapid correction of
problems.
Process control also requires frequent inspection and monitoring of the heat drying components.
Procedures should call for inspection of heat drying equipment on a regular basis. An odor-free product
with proper percent moisture indicates a properly functioning system. Procedures should call for
checking and noting various parameters such as operating temperatures and pressures; and sludge, feed,
fuel, and ah* flow rates.
Maintenance of the proper moisture content in sludge cake processed by rotary or flash dryers is
especially critical to proper functioning of these systems. Too much moisture in the sludge feed can
create serious energy inefficiencies. The proper mixture of incoming sludge and previously dried sludge
fed to rotary or flash dryers is also a critical variable. Conveyance equipment used hi rotary dryers and
flash dryers may be prone to clogging if the mixture of wet sludge and previously dried sludge is allowed
to reach too high a moisture content, but an adjustment of the respective flows of material to obtain a
drier mix can resolve this problem. The proper mixture should be determined by trial and error at a level
that permits easy handling without caking.
Efficient operation of heat drying also requires that the quantity of hot gases used hi the process be
optimized. Dusting problems may limit air flow rates, especially in rotary dryers. The quantity of hot
gas should be just sufficient to dry the sludge. The optimum gas flow, in turn, depends on the sludge
mixture produced and should be determined through operational results. Dusting problems may limit air
flow rates, especially hi rotary dryers. Operating temperatures should be maintained at levels
recommended by the manufacturer of the process. Too low a temperature will not result hi sufficient
drying; too high a temperature can result in high energy costs.
173 PROCESS PERFORMANCE EVALUATION
The inspector should review the facility from the standpoint of its design as well as its operation and
maintenance procedures. An inspection checklist is provided in Appendix A. This checklist is designed
to aid the inspector hi gathering information needed to properly evaluate a heat drying system.
17-9
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17 J.I Design Evaluation
The inspector should investigate the presence of common design shortcomings. The first of these
is the development of large clumps or "clinkers" in the dried sludge. Grinding equipment may be used
to process the sludge as part of finishing the dried material for distribution, or preprocessing might be
considered.
Another common design problem involves excessive wear and corrosion to mixers, air locks,
dampers, cage mills, and other metal equipment. POTWs with only coarse screening for grit removal
may have greater problems with abrasion. Also, high pressure nozzles used to atomize sludge during
spray drying are susceptible to abrasion and clogging. Centrifugal dishes or bowls are more common
for this reason.
Another significant design problem can result from inadequate or improperly designed air pollution
controls resulting in unacceptable particulates or odors. The inspector should review the design
specifications for air pollution equipment installed to address these pollutants. Most commonly, odors
are removed by afterburning and particulates by scrubbing, precipitation or baghouses.
Capacities of equipment and storage areas are an important design issue. Heat drying equipment
is usually available in various design modules ranging from handling capacities of 40 to 2,400 tons/h of
wet sludge cake. The inspector should review the capacities of the processes and associated storage
capacities by first reviewing the number and capacities of dryers being used. If the drying operation is
continuous, sufficient excess drying capacity should exist to allow dryers to be taken out of service for
maintenance activities while maintaining treatment for all sludge produced. A minimum of 3 days of
peak production is suggested (EPA, 1979). In cases where drying operations are not continuous, storage
facilities should be adequate for peak sludge cake production during off-shift periods as well as for
scheduled maintenance activities. Similarly, adequate storage facilities for dried product must be available
if distribution is undertaken. If sales or distribution of dried sludge are seasonal, this capacity may need
to be quite large. In processes calling for incineration or subsequent processing of the dried sludge,
storage requirements will depend on the capacities of these processes. For this reason, and to eliminate
space requirements, stockpiling should be minimized by seeking a regular market or outlet for the
material.
17-10
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173.2 Operations and Maintenance Evaluation
System performance, measured as the percent moisture in the finished product, can range from 2
to 10 percent. The inspector should review the manufacturers' operations manuals to determine the target
moisture percentage for which the system was designed.
As previously mentioned, conveyances, drying shells and other sludge handling equipment can be
easily eroded through the abrasive action of dried sludge. Ferric chloride, often used as a dewatering
aid, can result in corrosive conditions which accentuate this problem. Procedures at the facility should
call for frequent, regular inspections of parts and equipment. Plant components used in heat drying,
which should be regularly inspected as part of standard plant procedures, include the following:
• Drives and gear reducers
• Sludge belt conveyors
• Pneumatic conveyers and pumps
• Bearings and bearing brackets on all equipment
• Cage mills and mixers
• Electrical contacts in all equipment, especially relays and starters
• Burners
• Furnaces and ancillary equipment.
Periodic pro-active replacement of heavily used components may be necessary. Proper coatings
should be used to minimize wear and corrosion. The WPCF Manual of Practice Number 17: Paints and
Protective Coatings for Wastewater Treatment Facilities provides useful information on this topic.
Heat exchangers and other components exposed to high temperatures and/or scaling should be
regularly inspected on a schedule recommended by the manufacturer. In cases where certain equipment
demonstrates a predictable service life, regular replacement should be scheduled under plant maintenance
procedures.
The inspector should check to ensure that pathogen reduction is occurring in accordance with 40
CFR Part 257. Check to be certain that temperatures cited earlier for pathogen reduction are maintained.
17-11
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The inspector should determine whether adequate storage exists for sludge feed and dried product.
Dried product should not be exposed to rewetting. This can allow regrowth of organisms and subsequent
decomposition with associated odors. Dried product should be stored in a manner that will minimize the
potential for rewetting or fires.
As reviewed above, most heat drying processes generate an exhaust with odors and particulates. In
evaluating air pollution control equipment, the inspector should review the exhaust gas treatment devices
and associated manufacturers' operational specifications. The inspector should also review limits and
monitoring requirements found in plant air permits, as well as records of monitoring results kept at the
plant. Paniculate removal efficiencies as high as 96 to 97 percent may be required. Table 12 outlines
suggested monitoring of air pollutants and other operational parameters.
Table 13 presents a troubleshooting guide for heat drying operations. Problems addressed include
improper drying, decreased flow in pneumatic lines, decreased flow in fans and ducts, excessive
particulate emissions, and excessive odors. Probable causes, recommended monitoring to confirm
problems, and suggested solutions are provided.
i,
Safety should always be of concern at POTWs. In particular, the complex equipment and high
temperatures used in heat drying can create numerous opportunities for employee injury. Heavy dust
and/or grease can cause fire hazards due to the combination of combustible particles, rapid air velocities,
and high temperatures. Inspection procedures should include careful review of potential fire hazards from
grease or dust accumulation and provide for prompt housekeeping to minimize hazards. Fire-fighting
procedures should have been taught to personnel and appropriate equipment should be available.
Safety procedures should also specify monitoring techniques to minimize the potential for hazards
during grab sampling from hot equipment. Warning signs should be placed at locations where workers
are likely to contact hot surfaces. Equipment that could create hazards when malfunctioning (such high-
speed fans) should be equipped with warning sensors and recording devices to help signal and predict
breakdowns.
17-12
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TABLE 13. TROUBLESHOOTING GUIDE FOR HEAT DRYING OPERATIONS
INDICATORS/OBSERVATIONS
1. Sludge not properly dried.
2. Decreased sludge flow in
pneumatic lines.
3. Decreased flow in fans and
ductwork.
4. Excessive participates in stack
gas.
5. Excessive odors in stack gas.
PROBABLE CAUSE
la. Furnace temperature too low.
Ib. Ratio of wet to dried sludge
too high.
Ic. Quantity of hot combustion
gases sent to dryer too low.
Id. Moisture content of feed
sludge too high.
2a. Caking or blockage of line
with wet mixture of sludge.
3a. Grease accumulation.
4a. Faulty or poorly operating
pollution control equipment.
Sa. Temperature of afterburner too
low.
CHECK OR MONITOR
la. Furnace temperature.
Ib. Moisture content of wet/dry
sludge mixture.
Ic. Hot gas flow.
Id. Percent solids of feed sludge.
2a. Moisture content of wet/dry
sludge mixture.
3a. Visually inspect ducting, fans.
4a. Pollution control equipment.
5a. Afterburner temperature.
SOLUTIONS
la. Increase temperature as
required. II
Ib. Change ratio to provide drier II
mixture.
Ic. Increase flow of combustion
gases.
Id. Check operation of II
dewatering equipment II
preceding heat drying II
equipment. Increase percent [I
solids output. |
2a. Change ratio to provide drier II
mixture. ||
3a. Steam clean equipment as
required. ||
4a. Correct operation of pollution II
control equipment - see 1
manufacturer's manual. ||
5a. Operate afterburner between
1 ,200-1 ,400°F (650-700°C). |
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18. DISINFECTION
Wastewater sludge disinfection, the destruction or inactivation of pathogenic organisms in the sludge,
is carried out principally to minimize public health concerns. Destruction is the physical disruption or
disintegration of a pathogenic organism, while inactivation, as used here, is the removal of a pathogen's
ability to infect. Another concern is to minimize the exposure of humans and domestic animals to
pathogens in the sludge. At the present time hi the United States, the use of procedures to reduce the
number of pathogenic organisms is a requirement before the distribution or sale of sludge or sludge-
containing products for use as a soil amendment, or before land application. Since the final use or
disposal of sludge may differ greatly with respect to public health concerns, and since a great number of
treatment options effecting various degrees of pathogen reduction are available, the system chosen for
reduction of pathogens should be tailored to the demands of the particular situation. Apart from the
sludge stabilization methods discussed elsewhere hi this manual, another method for achieving additional
disinfection involves irradiation of the finished sludge.
To make inspection easier using this manual, this section on disinfection is organized somewhat
differently man the preceding ones dealing with other solids handling processes. After addressing the
four categories of pathogens and pathogen reduction by the previously described sludge treatment
processes, this discussion is subdivided into two major sections that each deal with one of the two
methods of irradiation, beta and gamma. The appropriate configuration and component, control
considerations and process performance evaluations are all discussed under each irradiation method.
18.1 PATHOGENS
A pathogen, or pathogenic agent, is any biological organism that can cause disease in the host
organism. Those organisms or agents fall into four broad categories: viruses, bacteria, parasites, and
fungi. Viruses, bacteria, and parasites are the primary pathogens that are present at some levels in sludge
as a result of human activity. Fungi are secondary pathogens and are only numerous in sludge when
given the opportunity to grow during some stage of the treatment or storage process.
18.1.1 Viruses
Viruses are obligate parasites and can only reproduce by dominating the internal processes of host
cells and using the hosts' resources to replicate themselves. Therefore, viral levels will not increase in
sludge. Different viruses show varying resistance to environmental factors such as heat and moisture.
18-1
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Typical total virus concentrations in untreated wastewaters are 1,000 to 10,000 plaque-forming units
(PFU) per 100 ml; treated effluent concentrations are 10 to 300 PFU per 100 ml. Wastewater treatment,
particularly chemical coagulation or biological processes followed by sedimentation, concentrates viruses
in sludge. Raw primary and waste activated sludges typically contain 10,000 to 100,000 PFU per 100
ml.
18.1.2 Bacteria
Bacteria are single-celled organisms that range in size from slightly less than one micron (jit) in
diameter to 5/i wide by 15/i long. Among the primary pathogens, only bacteria are able to reproduce
outside the host organism. They can grow and reproduce under a variety of environmental conditions.
High heat is more effective for inactivating bacteria, although some species form heat-resistant spores.
Pathogenic bacterial species generally grow best at a pH between 6.5 and 7.5. The ability of bacteria
to reproduce outside a host is an important factor. Although sludge may be disinfected, it can be
reinoculated and recontaminated.
18.1.3 Parasites
Parasites include protozoa, nematodes, and helminths. Pathogenic protozoa are single-celled animals
that range in size from 8/1 to 25/t. Protozoa are transmitted by cysts, the nonactive and environmentally
insensitive form of the organism. Their life cycles require that a cyst be ingested by a human or another
host. The cyst is transformed into an active organism in the intestines, where it matures and reproduces,
releasing cysts in the feces. Due to their need for a host organism, parasites, like viruses, do not increase
in numbers in sludge.
Nematodes include roundworms and hookworms. These organisms may reach sizes up to 14 hi. in
the human intestine, and may invade other tissues. This situation is especially common when man ingests
the ova of a roundworm common to another species, such as the dog. The nematode does not stay in
the intestine, but migrates to other body tissue, such as the eye, and encysts. The cyst, similar to that
formed by protozoa, causes inflammation and fibrosis hi the host tissue. Pathogenic nematodes cannot
spread directly from human to human. The ova discharged in feces must first embryonate at ambient
temperature, usually hi the soil, for at least 2 weeks.
18-2
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Helminths include flatworms, such as tapeworms, that may be more than 12 in. (30 cm) long. The
most common types in the United States are associated with beef, pork, and rats. Transmission occurs
when man ingests raw or inadequately cooked meat, or the eggs of the tapeworm. In the less serious
form, the tapeworm develops in the intestine, maturing and releasing eggs. In the more serious form,
it localizes in the ear, eye, heart or central nervous system.
Parasite cysts are insensitive to many sludge treatment processes although, as sludge ages, viable
cysts decrease. Heat is effective against cysts; radiation may also be effective.
18.1.4 Fungi
Fungi are single-celled nonphotosynthesizing organisms that reproduce by developing spores, which
form new colonies when released. Fungi are secondary pathogens in wastewater sludge, and large
numbers have been found growing in compost. The pathogenic fungi are most dangerous when the
spores are inhaled by people whose systems are already stressed by a disease such as diabetes or by
immunosuppressive drugs. Fungi spores, especially those ofAspergillusfumigatus, are ubiquitous in the
environment and have been found in pasture lands, hay stacks, manure piles, and the basements of most
homes.
18.2 PATHOGEN REDUCTION DURING SLUDGE TREATMENT PROCESSES
Sludge stabilization processes are ideally intended to reduce putrescibility, decrease mass, and
improve treatment characteristics such as dewaterability. Many stabilization processes also accomplish
substantial reductions in pathogen concentrations. In addition, some dewatering processes reduce
pathogen levels. These processes have been discussed previously. Federal regulations (40 CFR Part 257)
specify holding times and temperatures that are considered adequate to achieve pathogen reduction by
each sludge stabilization process. These operational requirements are summarized in Table 14.
Additional processes, evaluated and approved by the Pathogen Equivalency Committee, are listed in Table
15.
18-3
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TABLE 14. OPERATING PARAMETERS FOR ACHIEVING PATHOGEN REDUCTION
Sludge
Treatment Process
Aerobic Digestion
Anaerobic Digestion
Processes to Significantly
Reduce Pathogens (PSRP)*
60 days at 15°C
40 days at 20°C
Volatile solids reduction (VSR) of at
least 38%
60 days at 20 °C
15 days at 35 - 55°C
VSR of at least 38%
Processes to Further Reduce
Pathogens fPFRPl**
10 days at 55 - 60°C with VSR at
least 38%
N/A
Heat Treatment
Wet Air Oxidation
Incineration
Composting
N/A
N/A
N/A
5 days at 40 °C and temperature must
exceed 55 °C for 4 hours during this
period
180°C for 30 minutes
Must reduce pathogens to level
equivalent to other PFRPs.
Must reduce pathogens to level
equivalent to other PFRPs.
Within-vessel 3 days at 55 °C
Static aerated pile 3 days at 55°C,
Windrow 15 days at 55 °C with a
minimum of 5 turnings of pile
Chemical
Stabilization
Product pH of 12 after 2 hours of contact N/A
Air Drying Beds
At least 3 months with sludge piled to a
maximum depth of 23 cm/9 in. Two
months of this period temperatures must
average above 0°C on a daily basis
N/A
Heat Drying
Electron and
Gamma Ray
Irradiation
Pasteurization
N/A
N/A
N/A
Sludge temperature > 80°C
Moisture content reduced to
Dosage at least 1.0 at megarad at
room temperature (20°C) (used in
conjunction with PSRP which
reduce volatile solids)
70°C for 30 minutes (used in
conjunction with PSRP which
reduce volatile solids)
*PSRPs reduce, but do not eliminate pathogens. PSRPs typically achieve a 90% reduction in virus and bacteria.
**PFRPs reduce pathogens to below detectible levels.
18-4
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TABLE 15. PROCESSES DETERMINED TO BE EQUIVALENT TO PSRP OR PFRP
Town of Telluride, Colorado
Comprehensive Materials
Management, inc., Houston,
Texas
N-Viro Energy Systems Ltd.,
Toledo, Ohio
Public Works Department,
Everett, Washington
Haikey Creek Wastewater
Treatment Plant, Tulsa,
Oklahoma
Ned K. Burleson & Associates,
Inc., Fort Worth, Texas
Scarborough Sanitary District,
Scarborough, Maine
Mount Holly Sewage Authority,
Mount Holly, New Jersey
N-Viro Energy Systems Ltd.,
Toledo, Ohio
Miami-Dade Water and Sewer
Authority, Miami, Florida
Process Description
Combination oxidation ditch, aerated storage, and drying process. Sludge is treated in an oxidation
ditch for at least 26 days and then stored in an aerated holding tank for up to a week. Following
dewatering to 18% solids, the sludge is dried on a paved surface to a depth of 2 feet. The sludge is
turned over during drying. After drying to 30% solids, the sludge is stockpiled prior to land
application. Together, the drying and stockpiling steps take approximately 1 year. To ensure that
PSRP requirements are met, the stockpiling period must include one full summer season.
Use of cement kiln dust (instead of lime) to treat sludge by raising sludge pH to at least 12 after 2
hours of contact. Dewatered sludge is mixed with cement kiln dust in an enclosed system and then
hauled off for land application.
Use of cement kiln dust and lime kiln dust (instead of lime) to treat sludge by raising the pH.
Sufficient lime or kiln dust is added to sludge to produce a pH of 12 for at least 12 hours of contact.
Anaerobic digestion of lagooned sludge. Suspended solids had accumulated in a 30-acre aerated
lagoon that had been used to aerate wastewater. The lengthy detention time in the lagoon (up to 15
years) resulted in a level of treatment exceeding (hat provided by conventional anaerobic digestion.
The percentage of fresh or relatively unstabilized sludge was very small compared to the rest of the
accumulation (probably much less than 1 % of the whole).
Oxidation ditch treatment plus storage. Sludge is processed in aeration basins followed by storage in
aerated sludge holding tanks. The total sludge aeration time is greater than the aerobic digestion
operating conditions specified in the Federal regulations of 40 days at 20°C (68°F) to 60 days at 15 °C
(59°F). The oxidation ditch sludge is then stored in batches for at least 45 days in an unaerated
condition or 30 days under aerated conditions.
Aerobic digestion for 20 days at 30°C (86°F) or 15 days at 35 °C (95 °F).
Static pile aerated 'composting" operation that uses fly ash from a paper company as a bulking agent.
The process creates pile temperatures of 60° to 70°C (140° to 158°F) within 24 hours and maintains
these temperatures for up to 14 days. The material is stockpiled after 7 to 14 days of "composting"
and then marketed.
Zunpro 50-gpm low-pressure wet air oxidation process. The process involves heating raw primary
sludge to 177° to 204°C (350° to 400°F) in a reaction vessel under pressures of 250 to 400 psig for
15 to 30 minutes. Small volumes of air are introduced into the process to oxidize the organic solids.
Advanced alkaline stabilization with subsequent accelerated drying.
• Alternative 1: Fine alkaline materials (cement kiln dust, lime kiln dust, quicklime fines, pulverized
lime, or hydrated lime) are uniformly mixed by mechanical or aeration mixing into liquid or
dewatered sludge to raise the pH to greater than 12 for 7 days. If the resulting sludge is liquid, it
is dewatered. The stabilized sludge cake is then air dried (while pH remains above 12 for at least
7 days) for at least 30 days and until the cake is at least 65 % solids. A solids concentration of at
least 60% is achieved before the pH drops below 12. The mean temperature of the air surrounding
the pile is above 5"C (41 °F) for the first 7 days.
• Alternative 2: Fine alkaline materials (cement kiln dust, lime kiln dust, quicklime fines, pulverized
lime, or hydrated lime) are uniformly mixed by mechanical or aeration mixing into liquid or
dewatered sludge to raise the pH to greater than 12 for at least 72 hours. If the resulting sludge is
liquid, it is dewatered. The sludge cake is then heated, while the pH exceeds 12, using exothermic
reactions or other thermal processes to achieve temperatures of at least 52°C (126 °F) throughout
the sludge for at least 12 hours. The stabilized sludge is then air dried (while pH remains above
12 for at least 3 days) to at least 50% solids.
Anaerobic digestion followed by solar drying. Sludge is processed by anaerobic digestion in two well-
mixed digesters operating in series in a temperature range of 35° to 37eC (95° to 99°F). Total
residence time is 30 days. The sludge is then centrifuged to produce a cake of between 15 to 25%
solids. The sludge cake is dried for 30 days on a paved bed at a depth of no more than 46 cm (18
inches). Within 8 days of the start of drying, the sludge is turned over at least once every other day
until the sludge reaches a solids content of greater than 70%. The PFRP approval was conditional on
the microbiological quality of the product.
PSRP
National
PSRP
PSRP
PSRP
PSRP
PFRP
PFRP
National
PFRP
Conditional
PFRP
18-5
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18 J PATHOGENIC DESTRUCTION USING BETA IRRADIATION
Beta rays (high-energy electrons) are projected through wastewater sludge, by an appropriate
generator, to destroy or inactivate pathogens. The electrons produce both biological and chemical effects
as they scatter off material in the sludge. Direct ionization by the electrons may damage molecules of
the pathogens, particularly the DNA in bacteria cell nuclei, and the DNA or RNA of the viruses. The
electrons also induce secondary ionizations in sludge as they penetrate. Secondary ionization directly
inactivates pathogens, and produces oxidizing and reducing compounds that in turn attack pathogens.
The pathogen-reducing power of the electron beam (e-beam) depends on the number and the energy
of electrons impacting the sludge. E-beam dose rates are measured in rads; one rad is equal to the
absorption of 4.3 x 10"* Btu per pound of material. Since the radiation distributes energy throughout the
volume of material regardless of the material penetrated, the degree of disinfection with an irradiation
system is essentially independent of the sludge solids concentration, within the maximum effective
penetration depth of the radiation. The penetrating power of electrons is limited, with a maximum range
of 0.2 in. (0.5 cm) in water or sludge slurries, when the electrons have been accelerated by a potential
of 1 million volts (MeV).
For e-beam disinfection to be effective, some minimum dosage must be achieved for all sludge being
treated. This effect is attained by dosing above the average dosage desired for disinfection. One method
used to ensure adequate disinfection is to limit the thickness of the sludge layer radiated so that ionization
intensity of electrons exiting the treated sludge is about 50 percent of the maximum initial intensity. If
electron irradiation is combined with some other stabilization process and is operated at a dosage of 1.0
megarad at room temperature, the pathogen destruction will meet the PFRP requirements.
18.3.1 Process Configuration and Components
The major system components of an electron irradiation unit (shown in the schematic in Figure 26)
include: sludge screener, sludge grinder, sludge feed pump, sludge spreader, electron beam power
supply, electron accelerator, electron beam scanner, and sludge removal pump. A concrete vault houses
the electron beam, providing shielding for the workers from stray irradiation, especially X-rays. X-rays
are produced by the interaction of the electrons with the nucleus of atoms in the mechanical equipment
and in the sludge. The pumps must be progressive cavity or similar types to ensure smooth
18-6
-------
HIGH VOLTAGE CABLE
ELECTRON
BEAM
POWER
SUPPLY
ELECTRON
ACCELERATOR
CONCRETE
SHIELDING
ELECTRON
BEAM
SCANNER
SLUDGE SLUDGE
SCREEN GRINDER
SLUDGE
FEED
PUMP
T
SLUDGE
SPREADER
SLUDGE
REMOVAL
PUMP
FIGURE 26. EQUIPMENT LAYOUT FOR ELECTRON IRRADIATION FACILITY
18-7
-------
sludge feed. Screening and grinding of sludge prior to irradiation is necessary to ensure that a uniform
layer of sludge is passed under the e-beam.
18.3.2 Process Control Considerations
The electrons are first accelerated. They leave the accelerator in a continuous beam that is scanned
back and forth at 400 times per second across the sludge. The sludge is scanned as it falls free in a thin
film from the end of die inclined ramp. The dosage is varied by adjusting the height of the underflow
weir and, hence, the sludge flow rate.
Instrumentation needs for an e-beam facility should include flow measurement of and temperature
probes in the sludge streams entering and leaving the irradiator. Alarms as well as monitoring should
be used to indicate variation in sludge flow and high or low radiation doses.
18.33 Process Performance Evaluation
The inspector should review pathogen reduction records to evaluate unit performance and to
evaluate the process control measurements. These measures include testing the sludge before and after
radiation to calculate pathogen reduction, recording the temperature of the sludge stream before and after
leaving the irradiation unit, and recording the sludge flow and radiation dosage.
The inspector should inquire about the routine operating procedures. The unit O&M manual should
be consulted to determine the specific procedures that should be followed. Valves and pumps should be
operational and subject to periodic maintenance. The O&M manual should also be consulted to determine
the periodic maintenance procedures and frequency.
Safety measures such as warning signs for radioactive material or X-rays, audible alarms for
radiation and critical equipment, and periodic testing of emergency safety procedures and equipment
should be evaluated.
18-8
-------
18.4 PATHOGEN DESTRUCTION USING GAMMA IRRADIATION
Gamma irradiation produces effects similar to those from an electron beam. However, gamma rays
differ from electrons in two major ways. First, gamma rays are very penetrating; a layer of water 25 in.
(64 cm) thick is required to stop 90 percent of the rays from a cobalt-60 (CO-60) source; in comparison,
a 1-MeV electron can only penetrate about 0.4 in. (1 cm) of water. Second, gamma rays result from
decay of a radioactive isotope. Decay from a source is continuous and uncontrolled; it cannot be turned
off and on. The energy level (or levels) of the typical gamma ray from a given radioactive isotope are
also relatively constant. Once an isotope is chosen for use as a source, the applied energy can only be
varied with exposure tune.
If gamma irradiation is combined with some other stabilization process and is operated at 1.0
megarad at room temperature, the pathogenic destruction will meet the PFRP requirements.
18.4.1 Process Configuration and Components
Two general types of gamma systems have been proposed for wastewater sludge disinfection. The
first is a batch-type system for liquid sludge, where the sludge is circulated in a closed vessel surrounding
the gamma ray source (depicted in Figure 27). Dosage is regulated by detention and source strength.
The second system is for dried or composted sludge. A special hopper conveyor is used to carry the
material for irradiation to the gamma ray source. Conveyor speed is used to control the dosage.
Instrumentation should include radiation detectors, and flow metering for the wet sludge system.
When either facility is operating, arrangements must be made for periodic radiation safety inspection.
The disinfection effectiveness should also be tested by periodic sampling of the sludge before and after
disinfection.
18.4.2 Process Performance Evaluation
As with the beta irradiation, process performance evaluation of a gamma irradiation unit involves
review of process control measurements and pathogen reduction records to evaluate unit performance.
These measures include testing the sludge before and after radiation to determine pathogen reduction,
recording the sludge flow and monitoring source strength. The inspector should inquire about
18-9
-------
SLUDGE
INLET
VENT
GROUND
LEVEL
CONCRETE
SHIELDING
SLUDGE
COBALT
RODS
SLUDGE
OUTLET
FIGURE 27. SCHEMATIC OF GAMMA IRRADIATION FACILITY
18-10
-------
routine operating procedures, periodic maintenance, and radiation detection alarms and inspections and
other emergency and routine safety procedures.
No units have been installed in the U.S.A. although Sandra Laboratories in Albuquerque, NM
successfully operated a pilot facility designed to treat dried sludge conveyed through the unit hi bulk or
hi bags.
18-11
-------
APPENDIX A
INSPECTION CHECKLISTS
FOR SLUDGE TREATMENT PROCESSES
-------
This appendix contains checklists that correspond to each of the unit processes described in
Appendix A. The checklists were developed to assist the inspector in conducting evaluations of the
processes and in documenting the inspection findings. The checklists are included as a separate appendix
to facilitate quick access. The page number of each checklist hi listed below.
Page Number
Checklist In Appendix
Gravity Thickening A-3
Dissolved Air Flotation Thickening A-7
Centrifugation A-13
Aerobic Digester A-17
Anaerobic Digester A-23
Heat Treatment/Wet Air Oxidation A-29
Incineration A-35
Composting A-41
Chemical Stabilization/Conditioning A-49
Vacuum Filter A-55
Filter Press A-61
Belt Filter Press A-65
Sludge Drying Beds A-69
Sludge Drying Lagoons A-73
Heat Drying A-77
Beta or Gamma Irradiation A-83
A-l
-------
PERFORMANCE
GRAVITY T
Facility Name:
Contact Name:
Inspector Name:
^~^—^^—i^——^—^i^
L DESIGN INFORMATION
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
It
1.
2.
3.
4.
5.
6.
7.
8.
9.
Number of units
Type of sludge thickened:
Ratio of combined sludges (secondary: primary)
What is the thickener diameter?
What is the thickener depth?
What is the thickener volume?
What is the thickener design overflow rate?
What is the thickener design solids loading rate?
What is the supernatant return location?
Is the thickener covered?
If so, is it properly ventilated?
Are off-gases treated?
PROCESS INFORMATION
What is the sludge application rate?
What is the frequency of sludge application?
What is the thickened sludge pumping rate? ^
What is the frequency of thickened sludge application
What is the influent sludge concentration?
What is the thickened snlids concentration?
What is the supernatant TSS concentration?
What is the supernatant BOD concentration?
What is the sludge blanket depth?
^^^^^^^^^^^^^^^^^^^== ^=a; __
I EVALUATION
HICKENER
==^====================^===
NPDES Permit:
Telephone:
Date:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^BBI^^^^^^^^^^M^BBHHHM^^^^^MMHHMHBHi
In operation
ft
ft
gal
gal/dav/ft2
Ibs/dav ft2
D Yes D No
D Yes D No
D Yes D No
gal/dav or Ibs/dav
min/hr
gal/dav
? min/hr
% solids
% solids
mg/1
mg/1
ft
A-3
-------
n. PROCESS INFORMATION (Cwitinued)
10.
11.
12.
13.
14.
15.
in.
i.
2.
3.
4.
5.
6.
7.
8.
Are floating solids or gas bubbles present?
Are there odors in the vicinity of the thickener?
Is chemical conditioning used?
(If yes, refer to chemical conditioning/stabilization section)
Dosage based on: D Jar tests D Operating experience D Other
Describe the operating strategy for the thickener:
Are adequate operating records maintained?
MAINTENANCE INFORMATION
Is there an adequate preventative maintenance program?
Is there adequate equipment redundancy?
Is the spare parts inventory adequate?
Housekeeping adequate?
Are effluent weirs level and clean?
Visual evidence of short circuiting in the thickener?
Components out of service
Out of service davs in (year)
Out of service davs in (Veart
Out of service days in (year)
What is the current mechanical condition of the unit?
D Yes D No
D Yes D No
D Yes D No
D Yes D No
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
D Yes** D No
D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
A-4
-------
« SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/slip/fall:
•"V. .OTHER OBSERVATIONS
A-5
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-6
-------
PERFORMANCE EVALUATION
DISSOLVED AIR FLOTATION (DAF) THICKENER
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
I. DESIGN INFORMATION
, 1. Number of units:
In operation:
. hrs/day
2. Period of operation:
3. Type of sludge fed: D Primary Sludge
If combined sludge, what is the ratio by volume?
4. Thickener shape:
5. Thickener size:
days/week
D Secondary Sludge D Combined
6. Thickener volume:
cuft
7. Design influent flow:
8. Subnatant return location:
sqft
.sqft
_gal
gal/day
1. Describe operational strategy:
2. Sludge feed rate:
gal/day
3. Daily operating time:
4. Raw sludge solids concentration:
5. Thickened sludge solids concentration:
6. Subnatant suspended solids content: _
7. Floating sludge depth: _ __
8. Effluent recycle ratio: _ .
9. Air flow rate: _ .
10. Retention tank pressure:
. Ibs/day
hr
in
cu ft/min
Monitoring
Frequency
A-7
-------
II. PROCESS INFORMATION (Continued)
11. Is sludge being effectively removed by the skimmer?
12. Is the skimmer operated continuously?
13. Duration of typical skimmer on/off cycle:
14. Avg. operating speed of skimmer:
15. Are the effluent weirs clean and level?
16. Is effluent clear and relatively free of solids?
17. Is polymer used? (If so, what type?)
Refer to Chemical Conditioning/Stabilization Section.
18. Hydraulic loading rate
(Raw sludge plus recycle flows divided by surface area):
19. Solids loading rate
(Solids application rate divided by surface area):
20. Air to solids ratio
(Air flow rate divided by solids application rate):
21. Percent solids removal efficiency:
22. Location of supernatant return in plant:
supernatant return rate:
Supernatant solids concentration:
23. Are adequate operating records maintained?
mg/1.
D Yes D No**
D Yes D No
ft/min
D Yes D No**
D Yes D No**
gal/min/ft2
Ibs/hr/ft2
gal/day
D Yes D No**
HI. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program? D Yes D No**
2. Is there adequate equipment redundancy? D Yes D No**
3. Is the spare parts inventory adequate? D Yes D No**
4. Housekeeping adequate? D Yes D No**
5. Are air diffusers and tanks inspected at least once per year? D Yes D No**
6. Are mixing, pumping, and blower equipment inspected annually for worn blades
and impellers? D Yes D No**
A-8
-------
MAINTENANCE INFORMATION (Continued)
7. Are air filters serviced at regular intervals?
8. Components out of service
Out of service
Out of service
Out of service
D Yes D No**
days in
. days in.
. days in
.(year)
.(year)
-(year)
9. What is the currently mechanical condition of the unit?
** Please elaborate in V. OTHER OBSERVATIONS
D Good D Poor**
, SABETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fall:
V. OTHER OBSERVATIONS
-------
VI. * PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-10
-------
COMPARISON OF ACTUAL DISSOLVED AIR FLOTATION
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
PARAMETER
Hydraulic Loading gpm/sq ft
Solids Loading Ib/hr/sq ft
Raw Sludge Concentration mg/1
Subnatent TSS Concentration mg/1
Solids Removal Percent w/Flotation Aid
w/o Floating Aid
Air to Solids Ratio
Depth of Floating Solids (inches)
Floating Solids Concentration (Percent)
ACTUAL
DESIGN
TmCAL
0.5-2.0
0.5-2.0
5,000
<100
95
50-80
0.03
8-24
3-7
A-ll
-------
Facility Name:
• ' M
Contact Name:
••—•i • •- •^••••i—^
Inspector Name:
•••••••*••••
j L DESIGN INFORMATION
1. What type of centrifuge is present?
D Solid Bowl D Basket
2. Manufacturer's name:
j 3. Is the centrifuge used for thickening or
4. Number of units _^___
5. Type of sludge processed
6. Design Criteria:
Sludge feed solids concentration range (min - max)
Sludge feed rate. gal/min
Solids capture
^--—..-—.^___^__
PERFORMANCE EVALUATION
CENTRIFUGATION
^—
NPDES Permit:
— _
Telephone:
——«^—••«•
Date:
Disc Nozzle
. dewatering purposes?
Number in operation
Expected solids concentration of centrifuged sludge
Motor operating current
Bearing operating temperature
————«.
. PROCESS INFQRMATIONF
—"i '" H ii
1. Describe the operating strategy:
2- What is the operating period?
3- What is the sludge feed rate?
4- What is the total solids concentration of the feed sludge?
What is the total solids concentration of the centrifuged sludge?
What is the total solids concentration of the centrate?
hr/day
days/week
Ibs dry solids/hr
-------
n.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
ra.
i.
2.
3.
PROCESS INFORMATION (Continued)
What is the solids capture?
Location of centrate return in plant:
Is the feed sludge chemically conditioned prior to centrifuging? D Yes
(If yes, refer to chemical conditioning/stabilization checklist for additional evaluation parameters.)
What is the operating temperature of the bearings?
What is the motor operating current under load?
What is the monitoring frequency of the following:
Sludge feed rates
Feed sludge solids
Centrifuged sludge solids
Centrate solids
Motor operating current
Bearing temperatures
Is the centrifuge flushed during the shutdown phase? D Yes
If yes. for what period of time?
Are there excessive blockages in the sludge feed pipe due to rags? D Yes
Is there excessive wear of internal components due to high concentrations of grit? D Yes
Does the sludge feed rate to the centrifuge result in excessive motor current
or frequent torque overloads? D Yes
Does the centrifuge show signs of excessive vibrations? D Yes
Are the operating records adequate? CD Yes
Are there documented standard operating procedures for startup and shutdown of
the centrifuges? (If yes, attach copy to report.) D Yes
MAINTENANCE INFORMATION
Is there an adequate preventative maintenance program? D Yes
Is there adequate equipment redundancy? HD Yes
Is the spare parts inventory adequate? D Yes
%
D No
op
amperes
D No
D No
D No
D No
D No
D No
D No
,
n NO**
D No**
D No**
A-14
-------
MAINTENANCEINFORMATION (Continued)
4. Is the housekeeping adequate? D Yes D No**
5. What is the frequency of major overhauls?
5. Are air diffusers and tanks inspected at least once per year? D Yes D No**
6. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
8. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
» SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fall:
v, OTHER OBSERVATIONS
A-15
-------
VI. PROCESS'
(Sketch or replace with plant schematic)
A-16
-------
g"™ "- ' — — ^— -^^^— .•.•. i •'"-"—• ii .— —^-^— . — _
PERFORMANC1
AEROBIC ]
— i^ -^-— .— _ — ^_^.^ _^_ ^__ _ ___^_^_^^^
Facility Name:
Contact Name:
Inspector Name:
^^^__^^^___^^^^^_l^_^__^_______l^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
L DESIGN INFORMATION
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
ir,
i.
2.
3.
g^ .
£ EVALUATION
DIGESTER
=====
NPDES Permit:
Telephone:
Date:
•— — •— ^— — — — — — «^— •
Type of Digester (check one):
D Primary D High Rate D Secondary D Low Rate
Number of units: Tn operation-
Type of sludge digested:
D Primary Sludge D Secondary Sludge
If combined sludge, what is the ratio by volume?
D Combined Sludge
Mode of operation: D Batch D Semi-Batch D Continuous
Digester dimensions (L x W x D) ft
Total volume of digester:
cuft gal
Design sludge application rate: gal/dav Ibs/dav
Design volatile solids loading:
Hydraulic retention time:
What type of aeration equipment is used:
Ibs/cu ft/dav
davs
If diffused air is used do air diffusers require frequent cleaning? D Yes D No
Aeration source: D Air D Pure Oxygen
Air suDolv cauacitv: CFM/1 ,000 cu ft horsepower Off/1 .000 cu ft)
Are the digesters open or covered:
l*ROeESS INFORMATION
Describe operational strategy:
Sludge application rate: gal/day
Frequency of application:
Ibs/dav
hr
A-17
-------
H. PROCESS INFORMATION (Continued)
4. Raw sludge solids concentration:
5. Raw sludge volatile solids content: _
6. Digested sludge solids concentration:
7. Digested sludge volatile solids content:
8. Digested sludge removal rate: gal/day
9. Reactor solids concentration:
10. Reactor volatile solids content:
11. Reactor temperature (average):
12. Reactor dissolved oxygen:
13. Reactor pH:
14. Sludge recycle rate to the digester:
IS. Are there foaming problems?
16. Are there odor problems?
17. Location of supernatant return in plant:
Supernatant return rate:
Supernatant solids concentration:
18. Are adequate operating records maintained?
. Ibs/day
. mg/1
ft
Monitoring
Frequency
D Yes D No
D Yes** D No
. gal/day
mg/l_
Yes D No**
. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
3. Is the spare parts inventory adequate?
4. Housekeeping adequate?
5. Are air diffusers and tanks inspected at least once per year?
6. Are mixing, pumping, and blower equipment inspected annually for worn blades
and impellers?
7. Are air filters serviced at regular intervals?
D Yes
D Yes
D Yes
E! Yes
D Yes
Q No**
D No**
D No**
D No**
D No**
D Yes D No**
Cl Yes D No**
A-18
-------
Ig; HfoflNTENANCE INFORMATION (Continued)
g. Components out of service
Out of service days in (year)
Out of service days in (year)
^____^_^______ Out of service days in (year)
9. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
. SAHETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fall:
* OTHER OBSERVATIONS
A-19
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-20
-------
COMPARISON OF ACTUAL AEROBIC DIGESTER
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
PARAMETER
Solids Retention Time (days)
Temperature (Fahrenheit)
Volatile Solids Reduction %
Volatile Solids Loading
(lb VS./cu ft/dy)
Air Requirements
Diffuser System (cfin/1,000 cu ft)
Activated Sludge
Primary & Activated Sludge
Air Requirements
Mechanical System
(hp/l,000cuft)
Dissolved Oxygen Minimum (mg/1)
Reactor pH
ACTUAL
DESIGN
TYPICAL
10-20
>59
0.024-0.14
20-35
>60
1.0-1.25
1.0-2.0
>6.5
40CFR257
From 60 days at 59°F to 40
days at 68 °F
38
A-21
-------
PERFORMANCE EVALUATION
ANAEROBIC DIGESTER
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
L DESIGN INFORMATION
1. Type of digester (check one):
D Primary D High Rate
2. Number of units:
D Secondary
3. Type of sludge digested:
D Primary Sludge D Secondary Sludge
If combined sludge, what is the ratio by volume?
4. Type of cover: D Fixed ID Floating
5. Sludge application rate: gal/day
6. Digester diameter:
7. Digester depth:
D Low Rate
In operation:
Combined Sludge
8. Total volume of digester:
cuft
9. Design volatile solids loading:
10. Hydraulic retention time:
11. Digester heating mechanism:
12. Digester mixing mechanism:
. Ibs/day
ft
ft
gal
Ibs/cu ft/day
days
H. PROCESS INFORMATION
1. Describe operational strategy:
2. Sludge application rate:
gal/day
Ibs/day
A-23
-------
n.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
PROCESS ': --'.\
Raw sludge solids concentration: %
Raw sludge volatile solids content: %
Digested sludge solids concentration: %
Digested sludge volatile solids content: %
Digested sludge removal rate: gal/day Ibs/day
Digester volatile acids: mg/1
Digester pH:
Digester temperature: °F
Digester alkalinity: mg/1
Volatile acids/alkalinity ratio:
Depth of scum blanket: ft
Depth of grit layer: ft
Gas production: fWdav
Gas composition: a. Methane %
b. Carbon dioxide %
c. Hydrogen sulfide %
Active digester volume (total digester volume LESS scum and grit layer):
Monitoring
Frequency
fiVdav
Volatile solids loading (volatile solids application rate PER active digester
volume): Ibs/ftVdav
Volatile solids reduction: (sludge application rate times percent volatile solids
reduction): Ibs/dav
Gas production rate per Ib. Volatile solids reduced (gas produced divided by volatile solids
reduction): f
Solids retention time (digester solids mass divided by solids discharge rate)
tVlb VS Destroyed
day
Location of supernatant return in plant:
Supernatant return rate: gal/day
Supernatant solids concentration: mg/1
Are adequate operating records maintained? D Yes D No
A-24
-------
MABSTTENANCE INFORMATION
1. Is there an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
3. Is the spare parts inventory adequate?
4. Housekeeping adequate?
5. Are regular inspections made of:
a. Gas safety devices?
b. Gas piping system, compressors and scrubbers?
c. Water seals?
d. Manometers?
e. Digester structure and heat transfer system?
f . Scum blanket build-up?
g. Pumping system?
6. Components out of service
Out of service days in
Out of service days in
Out of service days in
7. What is the current mechanical condition of the unit?
** Please elaborate in V- OTHER OBSERVATIONS
.(year)
_(year)
-(year)
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D No**
D No**
n NO**
D No**
D No**
D No**
D No**
D No**
D No
D No
D No
D Good D Poor**
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
A-25
-------
3. Ventilation:
4. Chemical:
5. Trip/fall:
6. Confined space:
A-26
-------
PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
-------
COMPARISON OF ACTUAL ANAEROBIC DIGESTER
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
PARAMETER
Solids Retention Time
Temperature (Fahrenheit)
Volatile Solids Reduction %
.^^.^•^^•^^^•^^—^^.^^—^—••^^^•••^^^^^^^^—M^^— •••««"—•— '
PH
Gas Production
Per Pound VS. Added
(cu ft/lb VS. Added)
Per Pound VS. destroyed
(cu ft/lb VS. destroyed)
Gas Composition (%)
Methane
Carbon Dioxide
Hydrogen Sulfide
Volatile Acids Cone, (mg/1)
Alkalinity Cone, (mg/1)
Volatile Solids Loading
Low-Rate (Ib VS./cu ft/day)
High-Rate (Ib VS.cu ft/day)
Solids Retention Time (days)
Low-Rate
High-Rate
ACTUAL
DESIGN
.•UUai^WMHriMM^Hfe^^Mriiin
TYPICAL
98
6.8 to 7.2
6-8
16-18
65-69
31-35
Trace
200-800
2,000-3,500
0.02-0.05
0.05-0.15
30-60
10-20
40 O» 257
From 60 days at 68 °F to 15
days at 95-131 °F
35
A-28
-------
PERFORMANCE EVALUATION
HEAT TREATMENT/WET AIR OXIDATION
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
I. DESIGN INFORMATION
1. Manufacturer:
2. Number of units(trains):
D Wet Air D Heat Treatment
Number in service:
3. Type of sludge treated: D Primary
If mixed, what is the ratio by volume?
D Secondary
D Chemical
D Mixed
4. Design sludge flow (influent) per unit gpm.
5. Design reactor temperature °F
6. Design reactor pressure psig.
7. Design air feed scfm
8. Design influent solids concentration percent. Design solids loading_
_lbs/hr per train.
9. Number of heat exchangers per train
10. Material of construction:
a. Heat exchangers:
b. Reactors:
11. Describe decant treatment/handling:
12. Describe off-gas handling:
13. Describe decant and dewatering air treatment:
-------
n. PROCESS INFORMATION
1. Describe the process control strategy:
2. Describe process control monitoring, including points monitored, parameters and frequency:
3. Influent sludge flow/unit.
Reactor pressure(s):
4. Influent sludge solids
. gpm
.psig
5. Influent particle size (Max)
6. Influent chloride cone.
mg/1
7. Volume of sludge treated daily
8. Hours of operation per day
.gal
9. Percent volatile solids in treated sludge
10. Percent solids:
a. Reactor effluent
b. Decanted sludge
c. Dewatered sludge.
d. Decant
11. Recycle liquor flow
12. Is odor a problem? _
13. Are alarms provided for:
a. Equipment failures
b. High/low pressure
c. High/low temperature
Reactor temperature(s)
Air feed rate
Influent volatile solids
Influent corrosivity
Dry solids treated daily
Hours of operation per week
How often?
D Yes
D Yes
D Yes
. scfin
n
Ibs
.gpd
n NO
D No
D No
A-30
-------
III. PROCESS INFORMATION (Continued) I
|-
II 14. Describe operational problems:
I
1
15. Are operating records adequate?
D Yes D No
[ffl, MABST^fANCl INBORMATJON |
II 1. Is there an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
1
II 3. Is the spare parts inventory adequate?
1
4. Is general housekeeping adequate?
1 5. Has the availability of any train been less than 75%? Describe: _
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
I
"
I 6 Frequency of acid washing:
7. Frequency of general inspections: __
Frequency of solvent washing:
8. Frequency of scale inspections:
a. Heat exchangers
b. Reactors ___
c. Piping .
d. Decant tank
9. Mass of scale removed manually per year
10. Frequency of pressure system check
Ibs
11. Has fitting, piping and elbow erosion been a problem?
If yes,:
a. Is plant grit removal adequate? _ .
b. Is plant screening adequate? . .—
D Yes
D No
c. Is plant sludge degritting provided?
-------
. MAINTENANCE INFORMATION (Controlled)
12. Components out of service:
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
13. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
IV. SAFETY CONSIDERATIONS jgM^^^
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fall:
6. Confined space:
A-32
-------
(Sketch or replace with plant schematic)
A-33
-------
COMPARISON OF ACTUAL HEAT TREATMENT/WET AIR OXIDATION
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
PARAMETER
Sludge Flow per Unit gpm
Reactor Temperature (Far.) Heat Ttmt.
Wet Air
^•^^^^MM^^^B^^^^Mm^B^^^^^^^^B^^HH^^— H^^HV^^^W^WVHB^^^^—
Detention Time (Min.) Heat Ttmt.
Wet Air
Reactor Pressure (psig) Heat Ttmt.
Wet Air
Air Feed lbs/10,000 BTU
Influent Solids Percent
Solids Loading Ibs/hr train
Decanted Sludge TSS Concentration
mg/1
Dewatered Sludge TSS Concentration
mg/1
ACTUAL
^•^^•••MM-^MBMH^^
DESIGN
mimmiiaiiiimiiii*iiimiiii^^^*iai**iiiiim
TYPICAL
350-400
400-700
^•••^^^^^••••••••^^•••••••"••i"
15-40
40-60
250-400
500-1,500
7.5
3.6
< 1,000
30-50
40 OH 257
Sludge must be maintained
at 180 degrees C. for 30
minutes.
l—"*-B*^—^^^^^^^^^^^^^™B*--'^^^^^^—-^^^^^^^^™"Ma"™^^^B
A-34
-------
1— — ======S^S^=========— ;
PERFORMANCE EVALUATION
INCINERATION
^==8===================^^
Facility Name: NPDES Permit:
Contact Name: Telephone:
Inspector Name: Date:
^^^^^^^H^mm^H^^HHHj^^HKH^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^BBBBBMHBBBBBHHHBB^BBHBM
I, DESIGN INFORMATION
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
n.
A.
B.
What type of incinerator is used?
D Multiple hearth (MH) D Fluidized bed (FB)
Other (specify)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^HMM^H^^H^^MHBHBBBBJjjj^P^^^MMIHI
Number of units Niimh«r ii ni^n^n
Incinerator dimensions:
Height ft Diameter ft
For MH: Number of hearths
Air blower capacity
Theoretical air requirements
Design sludge loading rate (Ibs/ftVhr or Ibs/hr): avg
Type of auxiliary fuel used:
Type of air pollution control device:
Height of incinerator stack:
PROCESS INFORMATION
Describe the operating strategy:
Sludge Feed
1. Thickened Sludge:
Total Solids
Volatile Solids
Area ft2
scfin
scfin
max
Monitoring
Frequency
%
%
A-35
-------
n.
B.
C.
PROCESS INFORMATION (Continued)
2. Dewatered Sludge Cake:
Total solids
Volatile solids
3. Thickened sludge feed rate to dewatering
How often is it monitored?
4. Average sludge loading rate to the incinei
Incinerator
1 . What is the operating period?
%
%
system:
•ator:
hr/day
2. Does the facility monitor and record the following:
D Pressure drop across the air pollution
D Oxygen concentration of exhaust gas
control equipment
D Operating temperature of every hearth in multiple hearth furnaces or the bed and
freeboard temperature of fluidized bed incinerators
D Fuel feed to the incinerator
Monitoring
Frequency
gal/lir
Ibs/ftVhr or Ibs/hr
days/week
Monitoring
Frequency
3. What are the incinerator operating temperatures?
a. Multiple Hearth
Upper hearths (drying)
Middle hearths (combustion)
Lower hearths (cooling)
Stack exhaust
b. Fluidized Bed
Preheated air
Bed
Freeboard
Stack exhaust
c. Is the combustion temperature relatively stable or does it fluctuate significantly?
A-36
-------
Jn. PROCESS INFORMATION (Continued)
C. 4.
5.
6.
7.
8.
9.
10.
D. Air
1.
2.
3.
4.
5.
6.
'-
For FB incinerators: What are the pressure readings for the following:
Windbox . ,
Bed
Freeboard
If available, provide the following exhaust gas information:
Concentration of CCy
Concentration of CO:
Concentration of Cs:
Concentration of N?:
What is the average combustion efficiency?
What is the actual air feed rate?
What is the percentage of excess air to the incinerator?
What is the auxiliary fuel feed rate? gal/hr |
How often and in what quantities is scum incinerated?
%
%
%
%
%
scfin
%
jal/ton solids incinerated
Pollution Control
Is the facility subject to NSPS requirements? D Yes D No
Is the facility subject to mercury monitoring requirements? (emits more than
160 grams mercury per day) D Yes D No
Is the facility subject to beryllium monitoring? D Yes D No
Is the facility subject to PCB monitoring? D Yes D No
Is the facility in compliance with its air emission standards? D Yes D No
If no, list parameters not in compliance:
Parameter Emissk
in Bate Bate
A-37
-------
H. PROCESS INFORMATION (Continued)
D. 7. What is the pressure drop across the air pollution control device? inches of water
8. What is the average rate of particulate emissions? Ibs/tons solids incinerated
9. Obtain copy of last emissions performance test results and attach to this report.
E. Ash Management
1. How much ask is produced? _
2. Is the ash handled D wet or D dry?
3. How is the ash stored prior to disposal?
4. How is the ash disposed?
5. Is this disposal method in accordance with 40 CFR 257 or 261-268? D Yes D No
MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program? D Yes d No**
2. Is there adequate equipment redundancy? D Yes D No**
3. Is the spare parts inventory adequate? CD Yes CD No**
4. Is housekeeping adequate? CD Yes CD No**
5. How often and in what quantities is sand replaced in the FB incinerator?
6. What is the frequency of calibration of all temperature and pressure sensors?
7. How often are the freeboard water spray nozzles replaced in the FB incinerator?
8. How often is the incinerator shutdown and the interior inspected?
9. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
10. What is the current mechanical condition of the unit? CD Good CD Poor**
** Please elaborate in V. OTHER OBSERVATIONS
A-38
-------
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fell:
A-39
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-40
-------
PERFORMANCE EVALUATION
COMPOSTING
Facility Name:
B^,,,,,!,^^^^!!!^^^!!!*™™™!---^^ II •
1 Contact Name:
f
| Inspector Name:
• = : ~
NPDES Pennit:
Telephone: |
Date: |
FL DESIGN INFORMATION I
1. Composting method (check one): D In-Vessel
2. Type of sludge composted:
D Primary Sludge D Septage [
If combined sludge, what is the ratio by volume?
3. Composting capacity: dry tons solids/day at
4. Pile or in-vessel dimensions:
5. Bulking agent used:
[U Windrow
Secondary Sludge
% total solids
8. Is finished compost screened:
If yes, what type of screen is used:
9. Expected finished compost characteristics
D Aerated Static Pile
Combined Sludge
Hit mix ratio:
Active phase davs
Curing phase days
D Yes D No
Production rate:
ydVdry ton sludge moisture content:
Volatile solids content:
10. Number of air blowers:
11. Total blower capacity: .
scfm
12. Method of mixing windrow or in-vessel contents:
13. Ancillary equipment (loaders, dump trucks, etc.):
Type
Quantity
-------
I. ]
14.
n.
i.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
DESIGN INFORMATION (Continued)
Type of odor control system:
Describe operational strategy:
Dewatered sludge cake characteristics:
Total solids content: %
Volatile solids content: %
Moisture content: %
Sludge processing rate: d
Sludge to bulking agent mix ratio:
Is the pile uniformly mixed: D Yes
Actual composting period Active phase days Curing p
Describe the monitoring locations for temperature:
Average active phase temperature:
Is the temperature uniform throughout the compost mixture: D Yes
Is the mixture maintained at a minimum of 40°C for five days and at a temperature
exceeding 55°C for four hours to meet the PSRP requirements of 40 CFR Part 257? D Yes
For Static Aerated Pile and In-vessel operations is the mixture maintained at a min-
imum of 55°C for three days to meet the PFRP requirements of 40 CFR Part 257? D Yes
For Windrow operations is the mixture maintained at a minimum of 55°C for fifteen
days and the pile turned at least five times to meet the PFRP requirements of 40
CFR Part 257? Q Yes
Describe the monitoring locations for oxygen:
i:S::i:i:£::::i:^:-:-^
Monitoring
Frequency
ry tons solids/day
i D No**
base days
HAHHA^^HaH^BA^^K^fl^^^^B^H^^HHVAHA^B
°C
i D No
; D No**
$ D No**
5 D NO**
A-42
-------
0, PROCESS INFORMATION (Continued)
14. Average active phase oxygen level:
IS. Is the oxygen content uniformly distributed throughout the compost mixture: D Yes D No
16. How often are the windrow piles turned and what is the determining factor for turning the piles?
17. Static Aerated Pile or In-vessel Methods
a. Are the blower run times adjusted during the active phase?
b. If die blower run times are adjusted, what is the controlling factor?
c. Aeration type: Forced-pressure
d. Is the odor control system in use?
e. Has the system experienced freezing of blower or air lines during
cold weather periods?
18. Are there odor problems:
If yes, provide source(s):
D Yes
n NO
19. Is there an insect or rodent problem at the site?
20. Is runoff from the site collected?
If yes, How is it treated?
21. Finished compost characteristics
Production rate:
Moisture content:
Volatile solids content:
22. How is finished product stored on site?
Vacuum-induced
D Yes D No D NA
D Yes
D Yes
D No
D No
D Yes
D Yes
D No
D No
ydVdry ton sludge
23. How much finished product is currently on-site?
24. Describe how die finished product is distributed:
.yd3
A-43
-------
II. PROCESS' INFORMATION (ContM^i^^giii^fjii • ,., .. :
25. What is the monitoring frequency of:
a. Feed sludge quality and quantity:
b. Finished product quality:
c. Temperature and oxygen levels in compost mixture:
d. Blower run times:
26. Is there a written standard operating procedure (SOP)? D Yes
If yes, attach a copy to this report.
27. Are adequate operating records maintained? D Yes
m. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program? D Yes
2. Is there adequate equipment redundancy? D Yes
3. Is the spare parts inventory adequate? D Yes
4. Is housekeeping adequate? D Yes
5. What is the frequency of calibration of the temperature and oxygen probes?
6. Static air piles: How often are the air distribution lines cleaned to ensure uniform air flow?
7. Components out of service
Out of service davs in (veart
Out of service davs in (year)
Out of service days in (year)
8. What is the current mechanical condition of the unit? D Good
** Please elaborate in V. OTHER OBSERVATIONS
IV. SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
•' ' • ' • '•• '..-.,
- • • • - • ' .-.''•' • •'.:
D No
D No**
D No**
D No**
D No**
D No**
D Poor**
A-44
-------
, SAFETY CONSIDERATIONS
3. Ventilation:
4. Chemical:
5. Trip/fall:
6. Airborne Pathogens (Aspergillus):
A-45
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-46
-------
COMPARISON OF ACTUAL COMPOSTING
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
PARAMETER
ACTUAL
DESIGN
TYPICAL
Moisture Content of Influent Sludge (%)
50-60
Temperature of Compost Pile (°F)
120-150
Oxygen Level within Compost Pile (%)
5-15
NOTE: As per 40 CFR 257:
PSRP requires that the sludge must be maintained at
104°F for five days, must exceed 131°F for at least
four hours.
PFRP requires that for Within Vessel and Forced Air
Static piles, the temperature must be at least 131 °F
for 3 days. Windrow piles must maintain a
temperature of 131 °F or more for at least 15 days
and during this period the windrow must be turned a
minimum of 5 times.
A-47
-------
PERFORMANCE EVALUATION
CHEMICAL STABILIZATION/CONDITIONING
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
DESIGN INFORMATION
1. What type of sludge is being conditioned (primary, waste activated, combined)
2. What is the design rate of the conditioning system
3. Chemical(s) used for conditioning:
4.
5.
6.
7.
Chemical storage inventory
Is chemical feed system D manual D automatic?
If dry feeders used, are they D volumetric or D gravimetric?
Are feeders automatically paced?
Paced to: D Row D pH D Sludge concentration
gal/day?
purchased (dry) (as liquid)
. purchased (dry) (as liquid)
, purchased (dry) (as liquid)
days?
D Yes
G No
. PROCESS INFORMATION
1. Describe operational strategy:
2. Conditioning chemical concentration as fed to sludge?
3. Current chemical dosage rates?
(Ibs/dry ton solids)
(Ibs/dry ton solids)
(Ibs/dry ton solids)
A-49
-------
H. PROCESS INFORMATION (Continued)
4. Dosages based on: D Jar tests D Operating experience D Other
5. Conditioning tank mixing? D insufficient D adequate D excessive
6. Visual observations of conditioning tank sample:
7. Describe the operating strategy:
8. Are adequate operating records maintained? D Yes D No
IIL MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program? CD Yes D No**
2. Is there adequate equipment redundancy? CD Yes D No**
3. Is the spare parts inventory adequate? D Yes D No**
4. Housekeeping adequate? D Yes D No**
5. Are adequate calibrations done and records maintained? (pH meters,
flow meters, scales, etc.) D Yes D No**
6. Visual evidence of excessive dust in the area? D Yes** D No
7. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
8. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
Hazards noted (describe):
1. Moving equipment:
A-50
-------
V SAEETY CONSIDERATIONS
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/slip/fall:
6. Confined space:
V. OTHER OBSERVATIONS
A-51
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-52
-------
COMPARISON OF ACTUAL CHEMICAL STABILIZATION/CONDITIONING
CONDITIONS TO DESIGN AND TYPICAL CONDITIONS
Illiilliiill
TYPICAL
CHEMICAL FEED RATE Ib/dry ton solids
A. Ferric chloride
a. Ray Primary Sludge + WAS
b. Digested Primary Sludge + WAS
c. Elutriated Primary + WAS
40-80
80-100
40-125
8. Lime
a. Raw Primary Sludge + WAS
b. Digested Primary Sludge + WAS
c. Elutriated Primary + WAS
110-300
160-370
C. Polymer
a. Raw Primary Sludge + WAS
b. Digested Primary Sludge + WAS
c. Elutriated Primary + WAS
15-20
30-40
20-30
NOTE: As per 40 CFR 257:
Chemical Stabilization processes must produce a pH
of 12 after 2 hours of chemical contact.
A-53
-------
PERFORMANCE EVALUATION
VACUUM FILTER
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
MMUMMMMMI
I, DESIGN INFORMATION
Date:
1. Manufacturer:
2. Number of units:
Number in service:
3. Media: D Cloth D Coil springs
4. Design loading lbs(dry)/sq ft/hr/unit Design vacuum:
5. Effective area/unit sq ft Gross design load unit
6. Type of sludge treated: D Primary D Secondary D Chemical D Mixed
inches Hg
lbs(dry)/hr
If mixed, what is ratio by volume?
7. Design influent solids concentration
8. Design drum speed(range) rpm. Percent submergence (range)
9. Location of filtrate return in plant:
10. Sludge pumping: D automatic D manual
11. Chemical conditioning used:
12. Chemical feed: D automatic D manual
13. Adequate alarms provided?
D Yes D No
II. PROCESS INFORMATION
1. Describe operational strategy:
2. Describe process control monitoring, including points monitored, parameters and frequency:
A-55
-------
. PROCESS INFORMATION (Cohtihited)
3. Influent sludge flow/unit.
4. Solids loading
gpm
5. Percent solids in cake:
6. Vacuum:
. Yield
lbs(dry)/sq ft/hr per unit
7. Efficiency of solids capture.
8. Hours of operation per day
_. Hours of operation per week
9. Frequency of pump operation
10. Percent solids:
a. Cake
b. Filtrate(TSS)
11. Filtrate flow
12. Does cake separate freely?
13. Does the media blind?
14. Describe operational problems:
IS. Are operating records adequate?
Influent sludge solids %
lbs(dry)/sq ft/hr per unit.
Yes
. inches Hg
min/hr
D Yes D No
D Yes D No
D No
Iff. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
3. Is the spare parts inventory adequate?
4. Is general housekeeping adequate?
5. Has the availability of any unit been less than 75%? Describe:
Yes
Yes
Yes
Yes
D No**
D No**
D No**
D No**
6. Frequency of general inspections:
a. Vacuum system:
b. Chemical feed system:
A-56
-------
ItfAINTENANCE INFORMATION (Continued)
c. Media:
d. Pumps:
e. Cake conveyor:
7. Has fitting, piping and elbow erosion been a problem? D Yes D No
If yes,:
a. Is plant grit removal adequate?
b. Is plant screening adequate?
c. Is plant sludge degritting provided?
8. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
9. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
IV, SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fell:
A-57
-------
V. OTHER OBSERVATIONS
A-58
-------
PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-59
-------
I PERFORMANCE EVALUATION
1 FILTER PRESS
| Facility Name:
1 Contact Name:
1 Inspector Name:
[t DESIGN INFORMATION
1. Number of units
| 2. What is the filter process volume?
1 3. Is sludge pumping D Manual D Automatic?
1 4. Design sludge feed rate:
H 5. Design feed solids:
NPDES Permit:
Telephone:
Date:
In ooeraiion
ft3
eal/dav
%
FiL PROCESS INFORMATION
H 1. Describe operational strategy:
1
H 2. What is the average volume of influent sludge flow?
3. What is the influent sludge percent solids?
H 4. Operating period: days/week
5. What is the concentration of solids in the sludge cake
H 6. What is the TSS concentration of the filtrate?
D 7. Are chemical conditioners used?
1 If Yes what type of chemicals are used?
gal/day
%
hrs/day
? %
mg/1
D Yes D No
1 (refer to Section on Stabilization/Chemical Conditioning)
1 8. Is chemical feed D manual D automatic?
I 9. Are the filter plates clean and free of pinholes? D Yes D No
10. Does the monitoring program meet the operations and maintenance manual
recommendations? D Yes D No
I 11. Are operating records adequate? D Yes D No
111 Are the filter plates precoated? D Yes D No
1 Tf ves with what' —
A-61
-------
. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program? D Yes D No**
2. Is there adequate equipment redundancy? D Yes D No
3. Is the spare parts inventory adequate? D Yes D No
4. Housekeeping adequate? D Yes D No**
5. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
6. How often is the media cleaned?
7. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
IV. SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/slip/fall:
6. Confined space:
A-62
-------
Y, OTHER OBSERVATIONS
A-63
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-64
-------
Facility Name:
Contact Name:
Inspector Name:
NPDES Permit:
Telephone:
Date:
PERFORMANCE EVALUATION
BELT FILTER PRESS
L DESIGN INFORMATION
I. Number of units
In operation
2. Mode of operation: D Batch D Semi-Batch D Continuous
3. Operating period: hours/day
days/week
4. What type of sludge is being processed D Primary sludge D Secondary sludge D Combined sludge
If combined sludge, what is the ratio by volume?
5. What is the design sludge feed rate gal/day Ibs/hr
6. What is the design feed solids concentrations?
7. What is the belt width?
solids
(ft) (meters)
8. Polymer feed system? D Liquid
9. Where can polymer be added?
Dry D Both
10. Wash water makeup: D Plant effluent
Recycled D Potable water
. PROCESS INFORMATION
1. What is the current sludge application rate?
2. Influent sludge solids content? % solids (average)
3. Belt speed?
. gal/day or.
Ibs/hr
4. Polymer dosage? gpm
5. Dosage based on: D Jar tests D Operating experience
6. Dewatered sludge solids contents? % solids (average)
7. Filtrate flow rate? .
% solids (maximum)
ft/second
Ibs/ton
D Other
8. Filtrate TSS content?
9. Where is filtrate returned to plants?
10. Solids recovery? .
% solids (maximum)
gpm
mg/1
H. Are there odor problems?
D Yes** D No
A-65
-------
H. PROCESS INFORMATION (Continued)
12. Is an odor control system installed?
13. Odor control chemicals used?
D Yes D No
14. Describe the operating strategy:
15. Are adequate operating records maintained?
D Yes D No
ffl. MAINTENANCE INFORMATION
1. Is there an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
3. Is the spare parts inventory adequate?
4. Housekeeping adequate?
5. Does belt show evidence of wear?
6. Belt tension properly adjusted?
7. Components out of service
Out of service davs in ( veart
Out of service days in (yeart
Out of service days in (year)
8. What is the current mechanical condition of the unit?
** Please elaborate in V. OTHER OBSERVATIONS
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
D Yes** D No
D Yes D No**
D Good D Poor**
IV. SAFETY CONSIDERATIONS
Hazards noted (describe):
1 . Moving equipment:
2. Electrical:
A-66
-------
?; SAFETY CONSIDERATIONS (Continued)
3. Ventilation:
4. Chemical:
5. Trip/slip/feU:
6. Confined space:
A-67
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-68
-------
I PERFORMANCE EVALUATION
SLUDGE DRYING BEDS
1 Facility Name:
1 Contact Name:
1 Inspector Name:
1 1. DESIGN INFORMATION
NPDES Permit:
Telephone:
Date:
I 1. Type of drying bed D Sand D Wedge-wire D Paved D Vacuum
1 2. Ate the beds D Enclosed D Covered D Open?
3. Type of sludge applied: D Primary D Secondary D Combined
If combined, what is the ratio by volume?
1 4. Sludge application rate:
1 5. Number of separate drying bed compartments:
6. Total surface area:
7. Population served by the treatment plant:
8. Drying area provided:
D 9. Do the beds have an underdrain system?
gal/dav
ft2
ft2
D Yes D No
1 M. PROCESS INFORMATION
1. Describe operational strategv:
2. Is the sludge digested before application?
3. Average sludge application rate:
4. Solids loading rate:
5. Typical drying time:
D Yes D No
gal/dav
gal/dav
davs
%
7. Sludge removed D Manually D Mechanically?
0 ?• Tvoical averace sand deoth in sand drying beds: ln
A-69
-------
II. PROCESS INFORMATION (Contimiisd)
9. Where does the drainage return to?
10. What is the average TSS of the drainage water returning to the plant?
11. Are there splash plates or diffusion devices in place when sludge is being applied
to the beds? D Yes
12. What is the average depth that sludge is applied to the drying beds?
13. Are odors a problem?
14. Are flies a problem?
IS. Is record keeping adequate?
No
D Yes D No
D Yes D No
D Yes D No
HI. MAINTENANCE INFORMATION
1. Do plant personnel rake and level the sand beds after sludge is removed?
2. Are sand beds maintained to a depth of at least 4 in. of sand?
3. Is there excessive vegetation growing on the drying beds?
4. Is there any leakage of sludge from one compartment to another?
5. Are sludge lines flushed out to prevent freezing in cold weather?
6. Is there an adequate preventative maintenance program?
7. Is there adequate equipment redundancy?
8. Is the spare parts inventory adequate?
9. Housekeeping adequate?
10. Are adequate calibrations done and records maintained? (pH meters, flow
meters, scales, etc.)
11. Components out of service
Out of service
Out of service
Out of service
days in
days in
days in
.(year)
.(year)
.(year)
12. What is the current mechanical condition of the unit?
** Please elaborate in V. OTHER OBSERVATIONS
U Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D Yes
D No
D No
D No
D No**
D No
D No
D No**
D No**
D No**
D No**
D Good D Poor**
A-70
-------
SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3; Ventilation:
4. Chemical:
5. Trip/fell:
6. Confined space:
, OTHER OBSERVATIONS
A-71
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-72
-------
PERFORMANCE EVALUATION
SLUDGE DRYING LAGOONS
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
f» DESIGN INFORMATION
1. Number of lagoons:
2. Total volume of lagoons:
3. Total lagoon surface area:
4. Sludge application rate: _
gal
ft2
gal/day
5. Type of sludge applied to the lagoon: D Primary
If combined, what is the ratio by volume?
6. Total population served by treatment plant:
D Secondary D Combined
itt PKOCSSS INFORMATION
1. Describe operational strategy:
2.
3.
4.
5.
6.
7.
Is the sludge digested before application?
Average sludge application rate:
Solids loading rate:
Typical drying time:
D Yes D No
gal/day
gal/day
days
Solids concentration in dried sludge:
Are the lagoons rested after sludge removal?
If yes, for how long?
D Yes D No
Is there a provision to draw off supernatant or precipitation?
Where does the supernatant go?
Are there any odor problems?
Yes I-" No
D Yes D No
A-73
-------
H. PROCESS INFORMATION (Continued)
10. Are there any insect problems? d Yes D No
11. What is the average depth that sludge is applied to in the lagoon? in
12. Are records adequate? d Yes D No
HI. MAINTENANCE INFORMATION
1. Is there excessive weed growth on the lagoons? D Yes D No
2. Are all of the partitions in all of the dikes in good repair to prevent sludge
leaking from the lagoon? D Yes D No
3. Are the sludge feed lines flushed to prevent freezing in cold weather? D Yes D No
4. Is there an adequate preventive maintenance program? D Yes D No**
5. Is there adequate equipment redundancy? D Yes D No**
6. Is the spare parts inventory adequate? D Yes D No**
7. Housekeeping adequate? D Yes D No**
8. Are adequate calibrations done and records maintained? (pH meters, flow meters,
scales, etc.) D Yes D No
9. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
10. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
IV. SAFETY CONSIDERATIONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
A-74
-------
4. Chemical:
5. Trip/fell:
6. Confined space:
I V. OTHER OBSERVATIONS
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-76
-------
PERFORMANCE EVALUATION
HEATING DRYING
Facility Name:
NPDES Permit:
Contact Name:
Telephone:
Inspector Name:
Date:
I, DESIGN INFORMATION
1. Manufacturer:
2. Number of units:
Number in service:
3. Typeunit(s): D Rotary D Flash D Spray D Carver-Greenfield D Other
4. Design loading lbs(dry & wet)/hr/unit
5. Type of sludge treated: D Primary D Secondary D Chemical D Mixed
If mixed, what is the ratio by volume? ______
6. Design wet cake moisture content % Design dry cake moisture content.
7. Dewatering technology in use: „
8. Sludge finishing: D Classified D Pelletized D Granulated
9. Air pollution control used: __
10. Parameters limited(air) and limits:
11. Does air pollution equipment generate liquid sidestreams?
Describe:
How handled?:
Where directed to?:
12. Type of air flow: D Concurrent D Countercurrent
Design air velocities: •
Crosscurrent
Design gas temperature:
13. Is wet cake mixed with dry cake?
If so, design mix ratio: .
14. Is wet cake ground?:
fpm
D Yes D No
-------
I. DESIGN INFORMATION (C^tiriulfrliM •
15.
16.
n.
i.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Capacity of:
a. Wet cake storage:
b. Dry cake storage:
Is dry storage provided?
Adequate alarms provided?
PROCESS INFORMATION
Describe operational strategy:
Describe process control monitoring, including points monitored, parameters
Wet cake moisture %. Dry cake moisture %. Moisture o
practiced): %.
Solids loading
Yield
Hours of operation per day . Hours of operation per week
Outlet eas temperature: °F. In-vessel gas temperature:
Air flow rate
Problems with wet and/or dry cake handling; describe:
Problems with dry cake; dust, odors,contamination, etc.; describe:
Problems with "clinkers" in dry sludge?
tons/cu yds
tons/cu yds
D Yes D No
D Yes D No
and frequency:
f mixed sludge (if mixing is
IbsCdry and wetVhr per unit
lbs(drv & "wef'Vhr per unit
°F.
scfm
D Yes D No
A-78
-------
iT ROCESS INFORMATION (Continued)
11. Compliance with air permits; describe:
12. Describe operational problems:
5. Are operating records adequate?
D Yes D No
IIL MAINTENANCE INFORMATION
1. Is mere an adequate preventative maintenance program?
2. Is there adequate equipment redundancy?
3. Is the spare parts inventory adequate?
4. Is general housekeeping adequate?
5. Has the availability of any unit been less than 75%? Describe:
G Yes
D Yes
D Yes
D Yes
D No
D No
D No
D No
Frequency of general inspections:
a. Conveyor/pneumatic systems:
b. Mixing system:
c. Air heating systems:
d. Air pollution control systems: __ ___
Has abrasion/corrosion-related wear of equipment been a problem; if so, describe:
8. In spray systems; has
wear or clogging posed problems; if so describe:
-------
. MAINTENANCE INFORMATION
9. Components out of service
Out of service days in (year)
Out of service days in (year)
Out of service days in (year)
10. What is the current mechanical condition of the unit? D Good D Poor**
** Please elaborate in V. OTHER OBSERVATIONS
IV. SAFETY CONSJDERAHONS
Hazards noted (describe):
1. Moving equipment:
2. Electrical:
3. Ventilation:
4. Chemical:
5. Trip/fall:
6. Confined space:
A-80
-------
PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
-------
- •^================
PERFORMANCI
BETA OR GAMM
^.^•^ , ••^•g,,, ..-. aa!B,,,,^^ _ _ •_,..,,,,,=__^
Facility Name:
Contact Name:
Inspector Name:
•^^••••••^•••••••I^^^^^^^^^^^^^^^^^^^^^M
^^^^^^^^ ^^^^•"^^^^^^^^^^^^^^^^^^^^^^^•^^^^••^•^M
L DESIGN INFORMATION
1.
2.
3.
4,
5.
6.
7.
8.
0.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Number of units
Type of sludge: D Primary D Secondary C
If combined, what is ratio bv voluem?
Mode of operation: D Batch D Continuous
Dosage
========
I EVALUATION
[A IRRADIATION
^^^SEi^BS^SiE^^^^^SSS^ESSSSSSSSSE^^^Si^^^SESSSS^SSESSSSSSSSES^^SISSSSSESS^^^^^^S
NPDES Permit:
Telephone:
Date:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^HHHHHHHHHHI^HIH^^^^^^^IIBHHHi^^BBl^ll^^^^^^^^^^^^H^H^HHHHHJjjJI
In operation
D Combined
Steel-lined concrete vault for radiation source (gamma)? D Yes D No
Steel-lined source handling pool (gamma)? D Yes D No
Radiation alarm (gamma)? CD Yes D No
Fire suppression system (gamma radiation unit for compost)? D Yes D No
PROCESS INFORMATION
Describe operational strategy:
Kludge flow rate (for beta ray radiation unit):
Influent sludge temperature (for beta ray radiation um
Effluent sludge temperature (for beta ray radiation un
Dosage:
Conveyor speed (gamma radiation unit for compost):
Detention time (batch-type pamma radiation unit:
Influent pathogen concentration:
T?twiiK»npv a«H Hiiratinn of onftratinti!
sal/hr
tt: °C
tt: °C
ft/sec
min
mg/1
mg/1
days/""^if /shift
A-83
-------
H. PROCESS INFORMATION (Continued)
11.
12.
13.
14.
15.
16.
17.
in.
i.
2.
3.
4.
5.
6.
7.
IV.
How frequent is operation inspected?
Sludge pumping: D Manual D Automatic
How often do the sludge pumps run?
If multiple units are used, is the flow distributed evenly?
Is the process control monitoring program adequate for the O&M manual
recommendations?
Are operating records adequate?
Visual observations of the process:
MAINTENANCE INFORMATION
Is there an adequate preventative maintenance program?
Is there adequate equipment redundancy?
Is the spare parts inventory adequate?
Housekeeping adequate?
Are adequate calibrations done and records maintained? (pH meters, flow meters,
scales, etc.)
Components out of service
Out of service days in (year)
Out of service days in ( veart
Out of service days in (veart
What is the current mechanical condition of the unit?
SAFETY CONSIDERATIONS
hrs/day
min/hr
D Yes D No
D Yes D No
D Yes D No
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
D Yes D No**
D Good D Poor**
Hazards noted (describe):
1 . Moving equipment:
2.
Electrical:
A-84
-------
. SAFETY CONSIDERATIONS (Continued)
3. Ventilation:
4. Chemical:
5. Trip/fell:
6. Confined space:
. O1HER OBSERVATIONS
-------
VL PROCESS SCHEMATIC
(Sketch or replace with plant schematic)
A-86
-------
APPENDIX B
BIBLIOGRAPHY
-------
BIBLIOGRAPHY
Clark, J.W., Wiessman, W., Hammer, M., Water Supply Pollution Control. (Harper and Row
Publishers, 1977).
Gulp, G.L., and Folks Heim, N. Field Manual for Performance Evaluation and Troubleshooting at
Municipal Wastewater Treatment facilities. U.S. Environmental Protection Agency, 430/9-78-001, Jan.
1978.
Advanced Waste Treatment - Field Study Training Program. U.S. Environmental Protection Agency
1987.
Operations Manual. Sludge Handling and Conditioning. Office of Water Program Operation, U.S.
Environmental Protection Agency, 430/9-78-002, Feb. 1978.
Process Design Manual for Sludge Treatment and Disposal. Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, 625/1-79-011 Sept. 1979.
Process Design Manual for Suspended Solids Removal. U.S. Environmental Protection Agency, 625/1-
75-0032, Jan. 1975.
Hinrichs, D.J., Inspectors Guide for Evaluation of Municipal Wastewater Treatment Plants. U.S.
Environmental Protection Agency, 430/9-79-010 April 1979.
Steel, E.W., McGhee, T.J., Water Supply and Sewerage. (McGraw-Hill Book Company, 1979).
Guidance for Writing Case-by-Case Permit Requirements for Municipal Sewage Sludge. Office of Water
Enforcement and Permits, EPA, May 1990.
Summary of Environmental Profiles and Hazard Indices for Constituents of Municipal Sludge. Office
of Water Regulations and Standards, EPA, July 1985.
Use and Disposal of Municipal Wastewater Sludge. Intra-Agency Sludge Task Force, EPA 625/10-
84-003, September 1984.
Overview of Sewage Sludge and Effluent Management. Office of Technology Assessment, U.S.
Congress, C/R-36b/#10, March 1986.
Evaluation of Sludge Management Systems. Office of Water Program Operations, EPA 430/9-80-001,
MCD-61, February 1980.
Municipal Sludge Management: EPA Construction Grants Program. Office of Water Program
Operations, EPA 430/9-76/009, April 1976.
Municipal Sludge Management: Environmental Factors. Office of Water Program Operations, EPA
430/9-77/004, October 1977.
B-l
-------
Metcalf and Eddy Inc., Wastewater Engineering: Treatment Disposal/Reuse. (McGraw-Hill Book
Company, 1979).
H. SAMPLING SLUDGE QUALITY
POTW Sludge Sampling and Analysis Guidance Document. Office of Water Enforcement and Permits,
EPA, August 1989.
Sampling Procedures and Protocols for the National Sewage Sludge Survey. Office of Water Regulations
and Standards, EPA, August 1988.
Analytical Methods for the National Sewage Sludge Survey. Office of Water Regulations and Standards,
EPA, August 1988.
m. PATHOGENS
Control of Pathogens in Municipal Wastewater Sludge. Center for Environmental Research Information,
EPA 625/10-89/006; September 1989.
Pathogen Risk Assessment Feasibility Study. Office of Research and Development, EPA 670/2-73/098,
December 1973.
IV. LAND APPLICATION
Land Application of Municipal Sludge. Municipal Environmental Research Laboratory, EPA 625/1-
83/016, October 1983.
Application of Sewage Sludge to Cropland. Office of Water Program Operations, EPA 430/9-76/013,
November 1976.
Applications of Sludge on Agricultural Land. Municipal Construction Division, Office of Research and
Development, EPA 600/2-78/131b, June 1978.
Land Treatment of Municipal Wastewater. EPA Center for Environmental Research Information, EPA
625/1-81-013, October 1981.
Sewage Disposal on Agricultural Soils: Chemical and Microbiological Implications. Office of Research
and Development, EPA 600/2-78/13 Ib, June 1978.
Loeht, R.C., Pollution Control for Agriculture. (Academic Press Inc., 1984).
V. LANDFILLING
Municipal Sludge Landfills. Environmental Research Information Center, Office of Solid Waste, EPA
625/1-78/010, SW-705, October 1978.
B-2
-------
VI. DISTRIBUTION AND MARKETING
Composting of Municipal Wastewater Sludges. EPA Center for Environmental Research Information,
EPA 625/4-85-014, August 1985.
Composting Processes to Stabilize and Disinfect Municipal Sewage Sludge. Office of Water Program
Operations, EPA 430/9-81-011, MCD-79, June 1981.
VH. INCINERATION
Municipal Wastewater Sludge Combustion Technology. EPA Center for Environmental Research
Information, EPA 625/4-85-015, September 1985.
. MISCELLANEOUS
Dewatering Municipal Wastewater Sludges. Office of Research and Development, EPA 625/1-87/014,
September 1987.
Odors Emitted from Raw and Digested Sewage Sludge. Office of Research and Development, EPA
670/2-73/098, December 1973.
Process Design Manual for Dewatering Municipal Wastewater Sludges. Office of Research and
Development, EPA 625/1-82-014, October 1982.
Radioactivity of Municipal Sludge. Office of Water Regulations and Standards, EPA, April 1986.
B-3
*O.S. Government Printing Office : 1992 312-014/40070
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