MINIM
EPA 430/9-78-002
OPERATIONS MANUAL
Sludge Handling
and Conditioning
February 1978
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Water Program Operations
Municipal Operations Branch
Washington, D.C. 20460
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OPERATIONS MANUAL
SLUDGE HANDLING AND CONDITIONING
by
William F. Ettlich
Daniel J. Hinrichs
Thomas S. Lineck
Culp/Wesner/Culp-Clean Water Consultants
Box 40
El Dorado Hills, CA 95630
Contract No. 68-01-4424
EPA
Project Officer
Lehn Potter
Task Officer
Marie Perez
February, 1978
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
WASHINGTON, D.C. 20460
Forsale by the Superintendent of Documents, U.S. Government
Printing Office, Washington, D.C. 20402
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DISCLAIMER
This report has been reviewed by the U.S. Environmental Protection
Agency, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S. Environ-
mental Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
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ABSTRACT
This work was initiated with the overall objective of providing an
operations manual and guidance to assist in the proper operation and
maintenance of various sludge processing, conditioning and disposal
systems at wastewater treatment plants. Emphasis is placed on the
establishment of good operational procedures, testing, and effective
measures and procedures for detection and correction of operational
problems. The style and format of the manual is tailored to the needs
of the user.
The processing and disposal systems presented in the manual include
those designed to treat various types of sludge generated from primary,
secondary and chemical wastewater treatment processes. All of the
principal sludge unit processes and unit operations are included such as
sludge thickening, stabilization and conditioning, chemical and heat
dewatering, heat drying, and ultimate disposal systems.
Step-by-step operation and maintenance procedures are presented for
the various systems. Each section contains explicit instructions on
process control procedures for detecting and correcting operational
problems. Plant type and size are considered where applicable to operational
procedures. Guidance and procedures for tailoring and modifying the
technical information provided in the manual to individual and special
cases is included in useful troubleshooting guides, solutions to design
shortcomings, and descriptions of design variations of particular processes.
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CONTENTS
1. Introduction 1
2. Description of Manual 2
3. General Maintenance 6
4. General Process Startup Procedures 8
5. Operation and Maintenance Manuals 10
I Chemical Conditioning
II Gravity Thickening
III Flotation Thickening
IV Aerobic Digestion
V Thermal Treatment
VI Lime Treatment
VII Chlorine Treatment
VIII Centrifugation
IX Vacuum Filtration
X Pressure Filtration
XI Belt Filtration
XII Drying Beds
XIII Lagoons
XIV Heat Drying
XV Multiple Hearth Incineration
XVI Fluidized Bed Incineration
XVII Composting
XVIII Land Application
XIX Landfill
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SECTION 1
INTRODUCTION
This manual is an operation and maintenance guide or reference document
for use by operating personnel and other individuals interested in improving
the performance of municipal wastewater treatment plants. The primary pur-
pose of the manual is to provide operational and maintenance guidance for
various sludge unit processes. A secondary purpose of the manual is its
use as a supplemental text for various training courses. Consulting
engineers, designers, plant operators, educators, and students will also
find the manual a useful source document on operation and maintenance of
sludge processes.
The manual was developed as described in Section 2 based on state-of-
the-art investigation of published literature, manufacturer's operation and
maintenance publications, full scale plant experiences, and experience of
individuals.
This work was followed up by visits to actual operating facilities
of each type to document actual field experiences and verify procedures
outlined in the manual.
The manual was written to be readily usable by plant operating per-
sonnel. The manual will also be helpful to those who design facilities
and prepare plant operation and maintenance manuals.
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SECTION 2
DESCRIPTION OF MANUAL
This manual covers each of the sludge processes as a separate unit
process. Each of these process sections is complete within itself and
contains the following sub-sections as applicable to the specific unit
process.
Process Description
Typical Design Criteria and Performance
Staffing Requirements
Monitoring
Normal Operating Procedures
Control Considerations
Emergency Operating Procedures
Common Design Shortcomings
Troubleshooting Guide
Maintenance Considerations
Safety Considerations
Reference Material
PROCESS DESCRIPTION
This is a brief description of the unit process, the features, design
differences between variations of the process, mechanical and process
operation, and a description of the sidestreams. This paragraph serves as
an introduction to the manual for that unit process.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
This paragraph is designed to give a very brief indication of design
parameters important to the unit process and expected results. In most
cases, typical ranges are given for various types of sludges. Design
factors and performance vary widely, however, the information in this man-
ual is intended to illustrate typical current practice. This information
was developed primarily from full scale plant operating experiences.
STAFFING REQUIREMENTS
This paragraph contains recommended labor requirements for operation
and maintenance of the unit process. The data in this paragraph does not
represent an average of current full scale plant experience, but represents
the labor required to actually perform the operation, maintenance, and
monitoring at a satisfactory level, generally as outlined in the manual.
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It is difficult to accurately estimate unit process labor requirements.
The information in this manual will provide general guidance, but should
not be accepted as absolute. This information was developed from full
scale plant experience where satisfactory operation and maintenance is
performed, from estimates based on experience, and from other references.
Unless noted otherwise, labor requirements were developed from "Estimating
Costs and Manpower Requirements for Conventional Wastewater Treatment
Facilities", EPA Contract No. 14-12-462, October, 1971, as slightly
modified to reflect additional and current experience.
It is inferred and intended that labor requirements be additive, i.e.,
the total staffing for a complete sludge handling system would be the sum
of all unit processes as shown in this manual. This is a simplified approach
for estimating purposes only, because in many cases there may be some
efficiencies in monitoring, maintenance, and operational functions when a
number of unit processes are operated as a system. Therefore, care and
judgement must be used in applying the staffing information and, again,
it is intended only for general guidance.
MONITORING
The monitoring recommendations were developed from "Estimating Labora-
tory Needs from Municipal Wastewater Treatment Facilities", EPA-430/9-74-
002, June, 1973, and full scale plant experiences. The monitoring require-
ments and practices will vary widely depending on the exact process, the
size of the plant, and the needs of the local facility. The intent of this
paragraph is to show requirements for an average facility.
NORMAL OPERATING PROCEDURES
These step-by-step procedures for various operating modes were
developed from manufacturer's manuals and from actual full scale plant
manuals and operations. The procedures have been simplified to cover the
general case and may change slightly for a particular manufacturer's
equipment.
CONTROL CONSIDERATIONS
The intent of this paragraph is to describe the pertinent physical
and process control considerations and the practical application of these
considerations to the unit process. Guidance is provided to aid in tailor-
ing the technical process considerations to individual and special plant
cases. This paragraph includes the use of sensory observations. The
information in this paragraph was developed from technical reference
materials and from actual plant operating experiences.
EMERGENCY OPERATING PROCEDURES
This paragraph contains recommended actions in case of electrical
power failure or loss of other treatment units and the effect on the unit
process. Additional suggestions on provisions for maximizing plant
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reliability during emergencies resulting from failure of plant components
may be found in "Design Criteria for Mechanical, Electric, and Fluid System
and Component Reliability", EPA-430-99-001. Guidance on emergency
planning may also be found in "Emergency Planning for Municipal Wastewater
Treatment Facilities", EPA-430/9-74-013, February, 1974,
COMMON DESIGN SHORTCOMINGS
This information is based on actual full scale plant experiences. It
is intended to outline common plant design shortcomings and easily im-
plemented facility modifications to overcome the deficiency or compensate
for the deficiency. This information may not apply to all plant situations
and should be used only with competent engineering advice.
TROUBLESHOOTING GUIDE
This is a condensed action-reaction type presentation which covers a
number of common unit process problems, symptoms, and corrections.
MAINTENANCE CONSIDERATIONS
A sound maintenance management system can be a major, positive factor
in the successful long-term performance of a municipal wastewater system.
The agency responsible for the wastewater system must give full support to
the maintenance program if it is to be successful. The records from a
good maintenance system are also very useful in preparing realistic budgets
and in planning an adequate inventory of replacement parts. Where inade-
quate maintenance programs are a source of problems, the following references
will be helpful.
Manufacturer's operation and maintenance manuals.
"Maintenance Management Systems for Municipal Wastewater Facilities",
EPA 430/9-74-004, October, 1973.
"Considerations for Preparation of Operation and Maintenance Manuals",
EPA 430/9-74-001 .
"Operation of Wastewater Treatment Plants", Manual of Practice No. 11,
Water Pollution Control Federation (1976) (Chapter 30).
This manual outlines general maintenance considerations, but a detailed
program must be developed for each specific plant. Generally, the manu-
facturer's maintenance manual should be used as a basis for developing this
program.
SAFETY CONSIDERATIONS
Although safety related problems may not contribute to process malfunc-
tions, the operator should be alert for obviously hazardous conditions for
both his protection and the protection of the operating staff. Typical
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safety considerations are outlined in this manual for each unit process.
The following references will be useful.
"Safety in Wastewater Works", Manual of Practice No. 1, Water
Pollution Control Federation (1969).
"Operation of Wastewater Treatment Plants", Manual of Practice No.
11, Chapter 31, Water Pollution Control Federation (1976).
REFERENCE MATERIAL
Pertinent reference materials, formulas, and definitions are provided
as applicable to the specific unit process.
NOTE: Section 405 of the Federal Water Pollution Control Act provides for the
regulations concerning disposal of sewage sludge. These regulations will be
available after 1979.
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SECTION 3
GENERAL MAINTENANCE
There are a number of general maintenance considerations applicable to
a number of unit processes. Those considerations are listed in this section
and will not be repeated in each of the specific unit process sections.
Maintenance can be provided in-house or by outside contract services.
Certain special services must be performed by outside services in many
cases, but in-house maintenance for other requirements has generally been
more satisfactory than use of contract services. It is recommended that
in-house maintenance capability be developed for at least routine mainte-
nance services.
1. A good preventative maintenance program is essential to continuity
of service required for reliable plant operation. Preventative
maintenance should include regular inspection, painting, lubrica-
tion, and minor and major overhaul on a scheduled basis. A good
recordkeeping system is essential to a successful program.
2. Operating personnel should always be alert for unusual noises
and other sensory indications of impending problems. Such in-
dications should be checked out immediately upon detection to help
prevent more serious equipment failures.
3. Lubrication is essential to proper equipment operation. Manu-
facturer's recommendations should be followed as related to in-
tervals, methods, and types of lubricants. Overlubrication of
ball and roller bearings should also be avoided. Oil leaks
should be corrected promptly for safety reasons and because they
indicate a developing maintenance problem.
4. Painting of plant components at reasonable time intervals will
make cleaning an easier chore, and will help to prevent rust and
deterioration of tankage and equipment. Use of proper surface
preparation and application of proper coatings will provide
long term advantages even though the initial cost and time re-
quired may be greater. If surfaces are recoated at proper in-
tervals it may be that only the top coat will have to be applied
rather than complete replacement of base coats. Paint manufac-
turers provide excellent guidelines on the use of their products
for various services.
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5. Concrete surfaces and expansion joints should be inspected for
deterioration on an annual basis. Patchwork, recalking, and
sealing should be done promptly when the need for such repair
becomes apparent.
6. Drive systems should be inspected regularly for worn belts,
chains, pulleys, sprockets, and flexible couplings. Proper
lubrication should be provided. Drive systems should be ad-
justed at intervals to provide proper chain or belt tension.
7. Proper housekeeping is important to morale, safety, and mainte-
nance. Areas should be kept clean and spills should be cleaned
up promptly. Conditions that cause continuing clean-up problems
should be corrected so that the problems are eliminated or mini-
mized. Walkways should be kept clear of water, oil, grease,
leaves, snow, ice, and other similar conditions.
8. Valves and gates should be operated at regular intervals
(typically every month) to assure free operation and to check
on adequacy of lubrication.
9. Sludge lines should receive particular attention. These lines
should be inspected periodically for buildups and should be
flushed and cleaned as necessary to remove sludge and grit
accumulations. The required intervals must be determined based
on plant experience.
10. All maintenance work should consider the potential presence of
sewage gases. Maintenance personnel should be familiar with
the types of gases, potential locations where the gases can be
expected, monitoring procedures, proper ventilation procedures,
and proper work procedures.
11. Tankage should be drained at least once a year so submerged
equipment can be inspected and repaired. At the same time
protective coatings can be repaired.
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SECTION 4
GENERAL PROCESS STARTUP PROCEDURES
This section contains general process startup considerations which
are applicable to most unit processes. These considerations will not be
repeated in each manual. Careful and systematic process startup pro-
cedures will help to prevent damage to equipment and minimize safety
hazards to operating personnel.
1. Clean all debris from tankage, pipelines, and from the vicinity
of equipment. Assure that all packing material and shipping
tiedowns are removed per manufacturer' s instructions.
2. Check all protective coatings for damage and repair as
necessary.
3. Provide initial lubrication. Be sure all oil reservoirs are
properly filled. Remove any temporary protective coatings which
were applied for shipping protection.
4. Operate all valves, shafts, and other mechanical components
prior to filling with process liquid where possible. Adjust
drives, belt tension, alignment, and other items at this time.
5. Adjust weirs and troughs to approximate position.
6. Check electrical components for operational status. Check out
control circuits as possible on a "dry-run" basis prior to
operation of drive.
7. Check all motors for correct voltage connections at the terminal
box.
8. Check all motors for proper direction of rotation.
9. Pressurize piping with water to check for leaks, where possible.
10. Check out and calibrate instrumentation, controls, and safety
devices.
11. Check that the necessary chemicals are on hand for initial
operation.
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12. When tankage is filled, the weirs can be adjusted to final level.
13. Start up all support facilities such as service water, air
supply, hot water, and similar facilities.
14. Make sure safety equipment is available.
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SECTION 5
OPERATION AND MAINTENANCE MANUALS
10
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CHEMICAL CONDITIONING
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CONTENTS
Process Description .......................... 1-1
Typical Design Criteria and Performance ................ I-4
Staffing Requirements ......................... 1-5
Normal Operating Procedures ...................... -^"6
Startup .............................. z~6
Routine Operations ........................ ^~8
Shutdown ............................. 1-8
Control Considerations ........................ I-9
Physical Control ......................... 1-9
Emergency Operating Procedures .................... 1-9
Loss of Power .......................... 1-9
Common Design Shortcomings ...................... 1-9
Troubleshooting Guide ......................... 1-10
Safety Considerations ......................... 1-12
Reference Material .......................... 1-14
References ........................... 1-14
Glossary of Terms and Sample Calculations ............ 1-14
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PROCESS DESCRIPTION
chemicals
polymers
The most frequently encountered conditioning practice
is the use of ferric chloride either alone or in combination
with lime, although the use of polymers is rapidly gaining
widespread acceptance. Although ferric chloride and lime are
normally used in combination, it is not unusual for them to
be applied individually. Lime alone is a fairly popular
conditioner for raw primary sludge and ferric chloride alone
has been used for conditioning activated sludges. Lime treat-
ment to a pH of 10.4 or above has the added advantage of pro-
viding a significant degree (over 99 percent) of disinfection
of the sludge according to "Water Supply and Treatment", Bul-
letin 211, published by the National Lime Association.
Organic polymer coagulants, and coagulant aids have been
developed in the past 20 years and are rapidly gaining accep-
tance for sludge conditioning. These polymers are of three
basic types:
1. Anionic (negative charge) - serve as coagulants aids to
inorganic Al+++ and Fe+++ coagulants by increasing the
rate of flocculation, size, and toughness of particles.
2. Cationic (positive charge) - serve as primary coagulants
alone or in combination with inorganic coagulants such as
aluminum sulfate.
equipment
3. Nonionic (equal amounts of positively and negatively
charged groups in monomers) - serve as coagulant aids in
a manner similar to that of both anionic and cationic
polymers.
The popularity of polymers is primarily due to their ease
in handling, small storage space requirements, and their
effectiveness. All of the inorganic coagulants are difficult
to handle and their corrosive nature can cause maintenance
problems in the storing, handling, and feeding systems in
addition to the safety hazards inherent in their handling.
Many plants in the U.S. have abandoned the use of inorganic
coagulants in favor of polymers.
The facilities for chemical conditioning are relatively
simple and consist of equipment to store the chemical(s),
feed the chemical(s) at controlled dosages, place the
1-1
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chemical(s) in solution or slurry, and feed the solution to
the process as shown.in Figure 1-1.
SUPPLIER
TRANSPORT
STORAGE j *
CALIBRATED
FEEDER
CHEMICAL
MIXING
WITH
WATER
- \
SOLUTION
FED TO
PROCESS
Figure 1-1. Chemical conditioning system schematic.
The equipment used for storing and handling these
chemicals varies with the type of chemical used, liquid or
dry form of the chemical, quantity of chemical used, and plant
size. Storage requirements vary, but typically may be 15 to
30 days of use or 150 percent of the bulk transport capacity,
whichever is greater.
Dry ferric chloride is obtained in 18 and 40 gallon steel
drums. Storage requirements consist of a dry storeroom. Once
the drums are opened the contents should be used or mixed with
water and stored in solution. Liquid ferric chloride is
shipped in tank trucks or by rail tanker. Storage tanks must
ferric be lined with corrosion resistant material such as rubber,
chloride lead, stainless steel, Duriron or plastic. Fiberglass rein-
storage forced plastic (FRP) storage tanks have become more popular in
recent years. The handling equipment construction materials,
especially mixing tanks must also be heat resistant due to the
amount of heat given off when ferric chloride is mixed with
water. In areas of cold weather the storage and feed equip-
ment should be heated or the solution diluted to prevent
crystallization.
Lime is purchased in the dry form. There are several
compounds available, but pebble quicklime (CaO) and hydrated
lime Ca(OH)2 are the most commonly used for sludge condition-
ing. Hydrated lime is usually used for small facilities and
quicklime for large facilities.
Storage and handling facilities are the same for either
compound. Small plants using bagged lime require a water-
proof storage building. Large plants using bulk lime require
water tight and air tight storage units. Unlike ferric
chloride, lime is not corrosive to steel, so conventional
steel or concrete bins or silos can be used as shown in
Figure 1-2 (see following page).
Handling may be accomplished manually for bagged lime
lime or with bucket elevators and/or screw conveyors for bulk
handling systems. Pneumatic trucks and rail cars have recently been
lime
storage
1-2
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developed which blow the lime into the storage units, thus
eliminating the need for mechanical conveyors for unloading.
90° BEND
4' Min. Rod.
4" PIPE
60° CONE
COUPLING
DUST COLLECTOR
SAFETY VALVE
HIGH LEVEL
INDICATOR
LOW LEVEL
INDICATOR
VIBRATOR
(OR AIR PADS)
polymer
forms
Figure 1-2. Typical bulk storage tank.
Polymers for sludge conditioning are available in liquid,
powder, or pellet form. The polymers are purchased in con-
centrated form and mixed or diluted with water before use. It
has been found that polymer concentrations as low as 0.01 per-
cent perform efficiently in sludge conditioning. Presumably,
at such dilute concentrations the polymer molecules can per-
form at maximum efficiency.
Although the liquid form is the easiest to mix with
water, it is also expensive because of high transportation
costs. The most expensive form is pellets, that are also
easy to mix. Powders can be difficult to mix, but they are
the least expensive. Powdered polymers can cause housekeep-
ing problems by making floors extremely slippery and being
difficult to clean up.
1-3
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polymer
storage
feeders
Powdered polymer is available in 25 pound multi-wall
paper bags. Storage areas should be dry. Liquid polymer is
available in 5 gallon pails, 55 gallon steel drums, and tank
trucks. Liquid polymer should be stored in heated buildings
or tanks. Freezing does not harm the product, but low tem-
peratures create handling difficulties due to greatly increased
viscosity. Concentrated bulk storage should be in lined
tanks. Generally, bulk storage is not feasible except for
plants over 100 mgd because of the small feed dosages.
The feed systems consist of transfer from storage, mixing
a given solution strength, and metering the solution to pro-
cess or metering the chemical from storage to a slurry or solu-
tion and immediate transfer of the liquid to the process.
There are many equipment variations available. The chemicals
can be metered dry using dry feeders, in liquid form by
metering pumps, or known concentration solutions can be pre-
pared and then metered with metering pumps.
In large plants dry chemicals are transferred to hoppers
by screw conveyors. Dry feeders then add the chemical to
mixing or slurry tanks. Bulk quicklime is normally fed to a
slaking device where the oxides are converted to hydroxides,
producing a paste or slurry, which is then further diluted
before being piped or pumped to the process.
There are two types of dry feeders - volumetric and
gravimetric. As the names imply, the volumetric feeder meters
a repeatable volume of chemical and the gravimetric feeder
meters a repeatable weight of material.
There are six commonly used types of volumetric feeders.
They are roll feeder, screw conveyor, belt feeder, rotary
paddle feeder, oscillatory hopper feeder, and vibratory
feeder.
Gravimetric feeders can be classified into three groups:
pivoted belt, rigid belt and loss-in-weight hopper. These
feeders automatically compensate for difference in form, size,
or density to feed a repeatable weight of chemical. The feed
rate is adjusted by activally weighing the material being fed.
TYPICAL DESIGN CRITERIA & PERFORMANCE
Feed rates for chemical conditioning of sludges are
extremely variable depending on process used, nature of the
sludge, and type of chemical. Typical range of dosages are
as follows:
1-4
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FeCl3, Lime,
lb/ Ib CaO/ Polymer,
dry ton dry ton Ib/dry
solids solids 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 either fresh water or plant effluent to reduce
the demand for conditioning chemicals and to improve settling
of filtering characteristics.
STAFFING REQUIREMENTS
Labor requirements include unloading, storing, and feed-
ing chemicals and are shown in Table 1-1. Labor requirements
were developed from "Costs of Chemical Clarification of
Wastewater", EPA Contract No. 68-03-2186, final draft,
December, 1977.
TABLE 1-1. CHEMICAL CONDITIONING LABOR REQUIREMENTS
Chemical
Ferric chloride
Lime (slaked)
Lime (unslaked)
Polymer (dry)
Polymer (liquid)
Capacity,
Ib/hr
10
50
100
500
100
500
1,000
100
500
1,000
.5
1.0
5.0
10.0
.5
1.0
5.0
10.0
Operation &
maintenance
labor,
hr/yr
150
210
300
800
1,800
1,850
2,100
2,400
2,400
2,900
500
580
750
850
390
400
420
440
1-5
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NORMAL OPERATING PROCEDURES
Caution;
Check all equipment and work areas for spilled chemi-
cals. Chemicals, especially polymers, that have been
spilled on the floor can cause slippery areas if moisture
is also present. Some chemicals such as sodium hydroxide
are a safety hazard if spilled.
Startup
Lime Feeding System (including slaker)
1. Turn on the main water supply valve, thus allowing water
to fill the slaking chamber.
2. Start up the vapor and dust collection system.
3. When slaking chamber is approximately 1/4 full of water
close by-pass valve.
4. Turn on the water supply to the slaker spray nozzles.
These should be directed along the center of the
separator weirs. Spray should be centered along the
edge of the weir.
5. Start the slaker grit conveyor.
6. Adjust the water valve to pass the recommended water
rate. Optimum water quantity varies with the grade
of lime and the size of the grit particles that are
to be removed but typical ratios are 3 to 5 pounds of
water per pound of quicklime. Adjust flow by charac-
teristics of grit being removed. If lime is being
washed out with the grit, flow is too high and should
be reduced. Slurry temperature in the slaker should be
150°F to 170°F.
7. Turn on the paddle shaft drive.
8. Start chute vibrator or tapper.
9. Set feeder to desired feed rate and start.
10. Open the chute valve to admit lime to the feeder. The
feeder should start feeding lime to the slaking chamber
where it should be slaked into paste form, discharged
over the separation weirs, diluted to a slurry in the
separator chamber, and delivered through the slaker
discharge.
1-6
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11. After operation has stabilized, adjust the water valve
to obtain the desired slaker performance.
12. After slaker has been in operation for 15 or 20 minutes,
examine the grit being discharged from the grit remover.
If necessary, re-adjust the water flow to obtain optimum
grit removal. Decreasing the flow will result in the
removal of finer grit particles and will increase the
total amount of grit removed. Increasing the flow will
result in the removal of coarser grit and will decrease
the total amount of grit particles. It is possible to
remove an insert located where the slaker connects to
the grit conveyor. For a given flow, removal of the
insert will result in the removal of more fine particles
and increase the total amount of grit removed.
Dry Polymer Feed System
1. Weigh the amount of dry polymer to be mixed.
2. Determine amount of mixing water to be used to produce
the desired mixture concentration.
3. Turn on mixing water supply valve.
4. Pour measured amount of polymer through eductor funnel
while mixing water is flowing.
5. Turn on mixer when water level is up to the propeller.
6. Shut off mixing water when correct volume of mixing
water has been added.
7. Allow mixer to run 30 to 60 minutes, then shut off.
8. Transfer solution to feed tank if two tank system is
used.
9. Set feed pump using charts to determine setting for
desired dosage.
10. Check to see that feed line valves are open.
11. Start pump.
Liquid Chemical Feed Systems (all chemicals)
1. Dilute liquid chemical as needed if dilution system is
provided.
1-7
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2. Transfer dilute solution to feed tank.
3. Set feed pump using charts to determine setting for
desired dosage.
4. Check to see that feed line valves are open.
5. Start flow of dilution water (on discharge of pump) if
used.
6. Start feed pump.
Routine Operations
Shutdown
1. Inspect the system twice a shift and make necessary
dosage changes.
2. Make sure feeders are continuing to pump by observing
draw down in solution tanks occasionally.
3. If feeders are not automatically proportioned to flow
rate, the feed rate must be changed each time the flow
rate is changed.
Lime Feed System
1. Stop the feeder.
2. Close the valve in the chute which supplies lime to the
feeder.
3. Stop the slaker paddle shaft drive.
4. Stop the slaker grit conveyor drive.
5. Turn off water supply to slaker spray nozzles.
6. Shutdown the vapor and dust collection system.
7. Turn off water supply to slaker.
Other Chemical Feed Systems
The other systems are shutdown by turning off the feeder
and dilution water if used.
1-8
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CONTROL CONSIDERATIONS
Physical Control
If the feeders are automatically paced to flow, feed rate
adjustments are required only to compensate for varying dosage
requirements. If the feeder is not paced to flow the feed
rate must be adjusted each time the plant flow rate is changed.
The lime slaker requires occasional adjustment for variations
in lime quality.
EMERGENCY OPERATING PROCEDURES
Loss of Power
The chemical feed system will normally shutdown during
power outages becasue the drives are electrical. It may be
desirable to close the water supply valves after the feed
lines are flushed unless the water supply also shuts down
during the outage.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Dry feed chemicals
deposit in feeder.
2. Liquid chemicals
crystalize or
become too
viscous in
storage.
Solution
1. Provide mechanical mixers
for dissolving solids and
maintaining them in suspension
prior to delivery to feeder.
2a. Improve insulation.
2b. Order lower concentration
from supplier.
2c. Dilute slightly when chemical
is delivered to the storage
tank.
3. Feed system
capacity
inadequate.
2d. Heat the storage area.
3a. Add equipment.
3b. Increase chemical solution
strength (CAUTION: polymer
concentrations greater than
0.5 to 1 percent may be
difficult to prepare or too
viscous to feed.)
1-9
-------
TROUBLESHOOTING GUIDE
• i
INDICATORS/OBSERVATIONS
1. Cake or filtrate
solids decreases.
2. Air slaking occurring
during storage of
quicklime.
3. Feed pump suction or
discharge line
clogged.
4. Grit conveyor or
slaker inoperable.
5. Paddle drive on
slaker is overloaded.
6. Lime deposits in lime
slurry feed lines.
1 i
PROBABLE CAUSE
»•- ••— •
la. Improper chemical
dosage (assuming no
mechanical problem
in dewatering device)
Ib. Mechanical failure
in feed system.
2a. Adsorption of mois-
ture from atmosphere
when humidity is
high.
3a. Chemical deposits.
4a. Foreign material in
the conveyor .
5a. Lime paste too thick.
5b. Grit or foreign
matter interfering
with paddle action.
6a. Velocity too low.
CHECK OR MONITOR
la. Test for proper
dosage (Buchner
Funnel test, filter
leaf test, or jar
test) .
Ib. Visual inspection.
2a. Humidity, storage
facility not air-
tight.
3a. Visual inspection.
3b. Inspect check valves.
4a. Broken shear pin.
5a. Visual inspection.
5b. Visual inspection.
6a. Check velocity in
pipelines.
CHEMICAL CONDITIONING
SOLUTIONS
la. Correct dosage according to
test results.
Ib. Repair failure.
2a. Make storage facilities air-
tight, and do not convey
pneumatically .
3a. Provide sufficient dilution
water .
4a. Replace shear pin and remove
foreign material from grit
conveyor .
5a. Adjust compression on the
spring between gear reducer
and water control valve to
alter the consistency of the
paste.
5b. Remove grit or foreign mater-
ials or use a better grade of
lime.
6a. Maintain high slurry velocity
by use of a return line to the
slurry holding tank. Better
yet, the slurry should be
transported in troughs with
-------
TROUBLESHOOTING GUIDE
CHEMICAL CONDITIONING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
6b. Inadequate mixing.
6b. Inspect mixing in
slurry tank.
6b. Provide adequate mixing in
slurry storage tank.
7. Incomplete slaking
of quicklime.
7a. Too much water is
being added.
7a. Hydrate particles
coarse due to rapid
formation of a
coating.
7a. Reduce quantity of water added
to quicklime (ratio to weight
of water to lime should be
3:1 to 5:1 depending on lime
and slaker).
8. "Burning" during
quicklime slaking.
8a. Insufficient water
being added, resul-
ting in excessive
reaction tempera-
tures.
8a. Some particles left
unhydrated after
slaking.
8a. Add sufficient water for
slaking (See Solution 7).
9. Chemical feed line
ruptures.
Positive displace-
ment pump has been
started against a
closed valve or
plugged line.
Valves and line.
Open valves in feed line before
pump is started. Be sure that
line is open. A good procedure
is to start flow of dilution
water first.
10. Chemical concentra-
tion changes with-
out varying setting.
10. New load of chemical
with different mois-
ture content, den-
sity, or chemical
content.
LO. Analyze moisture
content, density,
chemical concentra-
tion.
10. Recalibrate and adjust feeder
accordingly.
-------
SAFETY CONSIDERATIONS
Safety
dust
ferric
chloride
first
aid
polymer
Dust from dry chemicals can be irritating to the res-
piration system if inhaled. In plant areas where chemical
dust may be present such as bag handling areas, unloading
areas, or around open feeders, workers should wear a light-
weight filter mask and tight fitting safety glasses with side
shields.
Great care should be taken to AVOID THE CONTACT OF ANHY-
DROUS FERRIC CHLORIDE WITH ANY PART OF THE BODY, and especi-
ally with the eyes. The moisture present in the eyes or on
the skin can cause sufficient heat to burn the skin. Ferric
chloride solutions should be handled with the same care as
acid solutions, since they can cause burns similar to those
caused by acids: They are also injurious to clothing and
cause difficult-to-remove stains. Personnel handling anhy-
drous ferric chloride or ferric chloride solutions should
wear overalls, rubber apron, rubber gloves and chemical gog-
gles. Floors, walls and equipment which are subject to
splashing should be protected with corrosion-resistant paint
or rubber mats.
If anhydrous ferric chloride comes in contact with the
skin or clothing, DO NOT WASH IMMEDIATELY WITH WATER. Severe
burns can result from the high heat produced when anhydrous
ferric chloride is dissolved. Wipe off the excess ferric
chloride first with a cloth, and then flood rapidly with
large amounts of water.
If liquid ferric chloride comes in contact with the skin
or clothing, wash it off immediately and thoroughly with water.
Dry polymer powder can be extremely irritating to eyes.
Eye protection should be worn when handling powder. If pow-
der gets in the eyes, flush with water. The major hazard
with polymer handling is powder spilled on the floor which
becomes wet thus causing extremely slippery surfaces. This
powder remains slippery until washed down with large volumes
of water.
lime
The problem of protection from quicklime burns is more
serious, particularly in hot weather when workers are per-
spiring freely. Besides using eye protection and respirators
workers exposed to quicklime dust should also wear proper
clothing, including long-sleeved shirt with sleeves and collar
buttoned, trousers with legs down over tops of shoes or boots,
head protection, and gloves. Clothing should not bind too
tightly around neck, wrists, or ankles. It is also advisable
to apply a protective cream to exposed parts of body,
1-12
-------
particularly neck, face, and wrists.
Freshly slaked lime in stiff putty or milk form can
produce burns when hot. After slurry is cool, contact with
skin is virtually harmless, the principal effect being removal
of natural skin oils. Therefore, workers who frequently
handle lime slurry should oil their skin, where exposed,
daily, using something similar to a petroleum jelly. This
will help prevent chapping and thus reduce danger from burns
or infection.
Workers inspecting or cleaning slakers should wear
safety goggles.
After handling lime, operators should shower. If cloth-
ing has been subject to lime dust, or splattered with lime
slurry, remove and launder. If possible, wear clean clothes
every shift.
If lime gets in eyes, flush with large amounts of cold
water immediately, followed by concentrated boric acid solu-
tion. Don't rub eyes if irritated by lime dust; doing this
will only add to the discomfort. If the symptoms persist, a
doctor should be consulted.
Lime burns should be treated similarly to caustic burns.
Wash thoroughly with soap and warm water, then with vinegar
to remove all lime. Apply burn ointment like boric acid salve
and cover with sterile bandage. Keep bandaged during healing
to prevent infection.
An efficient dust collecting and removal system is rec-
ommended at areas where lime is handled. An industrial vacu-
um can be used for cleaning up lime dust around and on equip-
ment. The cleaner should be emptied after each use.
Quicklime bags should be stored in a clean, dry place to
avoid moisture pickup. Otherwise the intense heat generated
from accidental contact with water may be enough to start a
fire in nearby flammable materials.
An important slaker safety measure is the installation
of a thermostatic valve to prevent overheating and possible
explosion. This could occur if the water supply fails and
the lime feed continues, allowing the lime to overheat and
produce excessive steam. The safety valve delivers a supply
of cold water as soon as maximum safe slaker temperature is
exceeded. An added safety feature is a high temperature
alarm device.
Another important safety precaution is to avoid using
the same conveyor or bin for alternately handling quicklime
1-13
-------
and one of the coagulants containing water of crystallization
such as aluminum or ferric sulfate. The water of crystalli-
zation may be absorbed by the quicklime and could generate
enough heat to cause a fire if the lime is in contact with
bags or other combustibles. Explosions have also been re-
ported to result from lime-alum mixtures in enclosed bins,
where the intense reaction heat (1,100°F) liberated sufficient
hydrogen from the water to create an explosive atmosphere.
Therefore, if the facilities are to be used alternately, they
should be cleaned thoroughly after every use. This hazard
would not apply to hydrated lime.
REFERENCE MATERIAL
References
1. National Lime Association, Lime Handling Application and
Storage, Washington, D.C. 20016.
2. BIF, "Engineering Data", West Warwick, R.I. 02893.
3. Penwalt Corporation, "Ferric Chloride", 3 Parkway,
Philadelphia, PA. 19102.
Glossary of Terms and Sample Calculations
1. Elutriated sludge results from a process whereby the
sludge is washed with either fresh water or plant
effluent to reduce the demand for conditioning chemicals
and to improve settling or filtering characteristics.
2. Anhydrous denotes materials without water; specifically,
water of crystallization.
3. A hypothetical graphical method of determining proper
polymer feed pump settings are shown in Figure 1-3 (see
following page). Graph A is used to determine total
polymer requirements in pounds per day at various dosage
rates. Graph B is then used to determine the required
feed rate polymer solution.
Finally, Graph C is used to determine correct polymer
pump speed in rpm. Graphs A and B apply to the general
case and Graph C applies to an assumed feed pump. A
graph similar to Graph C can be prepared for the actual
feed pump in a specific plant. For a specific pump the
feed rate may be a function of rpm or stroke setting as
applicable to the pump. As an example assume the
following conditions:
Sludge wasting rate = 7,000 Ib dry solids per day
1-14
-------
2000
4000 6000 8000
WASTE ACTIVATED SLUDGE, Ib/day
10POO
12,000
100
20
60 80
POLYMER FEED, Ib/day
100
120
Figure 1-3. Graphs A and B.
1-15
-------
1500
Q.
Q.
1000
500
20
40 60 80
POLYMER FEED, gal/hr
100
120
Figure 1-3. Graph C.
1-16
-------
Required polymer feed = 6 Ib per ton dry solids
(obtain from past experience, manufacturers recom-
mendations, and lab tests).
Polymer solution mixture = 1 percent.
Pump stroke = 1 1/2 inches.
Using the 6 Ib per ton curve on Graph A read
required polymer on vertical axis = 21 Ib per day.
Using the 1 percent curve on Graph B read required
polymer feed rate on the vertical axis = 10 gph.
Using the 1 1/2 inch stroke curve on Graph C read
required pump speed on the vertical axis = 300 rpm.
The following table is based on a polymei tiix tank 4'-0"
diameter x 4'-3" depth with a volume equiv^ ^nt of 7.85
gallons per inch of depth. Approximately 6 inches free-
board should be reserved for mixing purposes to eliminate
splashing. The following table illustrates a form to
use in preparing a mixing guide for preparation of stock
polymer solutions.
Vol. of
solution
to be
prepared
50 gals
100
150
200
250
300
350
Total available
tank depth req'd
(includes 6-inch
freeboard
13 inches
19
25
32
38
44
51
Pounds polymer to be
added to tank to make up
solution strength as noted
0.5%
2.1
4.2
6.3
8.4
10.5
12.6
14.7
0.75%
3.15
6.30
9.45
12.60
15.75
18.90
22.05
1.0%
4.2
8.4
12.6
16.8
21.0
25.2
29.4
1.5%
6.3
12.6
18.9
25.2
31.5
37.8
44.1
2.0%
8.4
16.8
25.2
33.6
42.0
50.4
58.8
1-17
-------
II
GRAVITY THICKENING
-------
CONTENTS
Process Description II-l
Typical Design Criteria and Performance II-3
Staffing Requirements II-3
Monitoring II-4
Normal Operating Procedures II-5
Startup II-5-
Routine Operations II-5
Shutdown II-5
Control Considerations II-5
Physical Control II-5
Process Control II-6
Emergency Operating Procedures II-9
Loss of Power II-9
Loss of Other Treatment Units II-9
Common Design Shortcomings II-9
Troubleshooting Guide 11-11
Maintenance Considerations 11-13
Safety Considerations 11-13
Reference Material 11-13
References 11-13
Glossary of Terms and Sample Calculations 11-13
-------
PROCESS DESCRIPTION
process
Gravity thickening is the most common sludge concentra-
tion process in use at wastewater treatment plants in the
United States. It is simple and inexpensive, but it may
not produce as highly concentrated sludges as other thickening
processes. Gravity thickening is a sedimentation process
which is similar to that which takes place in all settling
tanks; in fact, gravity thickening units physically and
operationally look like sedimentation tanks. The objective
of sludge thickening is to produce as thick a sludge as
possible at minimum cost. Chemicals may be used to aid
the process as described under CONTROL CONSIDERATIONS.
operation
design
differences
supernatant
return to
process
Solids settle by gravity to the bottom of the basin
forming a sludge blanket with a clearer liquid (supernatant)
above. The supernatant is removed from the basin over weirs
located near the top of the tank usually around the outer
circumference. Thickening takes place as the sludge par-
ticles move to the bottom of the tank and the water moves
toward the top. As the drive unit turns the 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, normally by
pumping.
Differences between various designs of gravity thicken-
ing units are mainly physical construction differences such
as the way the sludge raking arms are supported and driven,
arrangement of the arms, and whether skimming is provided.
These physical differences do not have an effect on operation
and maintenance except for the operator to note the mainte-
nance requirements of the equipment at his plant as shown in
the manufacturer' s instructions.
A typical gravity thickener is illustrated in Figure
II-l (see following page).
Thickener supernatant is usually returned to either the
primary or the secondary treatment process and normally
causes no problem to process operation. The respective
treatment process must be sized to treat the supernatant
flow and organic loading in addition to normal plant flow.
II-l
-------
MANUAL ARM LIFT
VERTICAL
ADJUSTABLE^"
DIFFUSER
-WEIR
TROUGH
INLET
PORT-
—-INFLUENT WELL
ARM 8 CONCENTRATOR ASSEMBLY
THICKENED SLUDGE
Figure II-l. Typical gravity thickener.
II-2
-------
TYPICAL DESIGN CRITERIA & PERFORMANCE
TABLE II-l.
Gravity thickeners are designed based on overflow rate
(hydraulic loading) and solids loadings. The principles
that apply are the same as those used in designing sedimenta-
tion tanks. Typically, a proposed design is checked for both
overflow rate and solids loading and the final selection is
based on a thickener design that will meet both of the design
considerations. Current practice in the United States calls
for design overflow rates of 400 to 800 gpd per square foot.
The design solids loadings will vary with the type of sludge
and typical loadings are shown in Table II-l along with
expected thickener performance. This table was developed
from information in "Process Design Manual for Sludge Treat-
ment and Disposal", EPA 625/1-74-006, October, 1974. Gravity
thickening should remove 90 percent of the solids in the feed
to the thickener as an average.
GRAVITY THICKENER TYPICAL LOADINGS AND PERFORMANCE
Sludge type
Influent
solids
concentration,
percent
Typical
solids
loading rate,
Ib/sq ft/day
Thickened
sludge
concentration,
percent
Raw primary
Raw primary + Fed 3
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
5.0
2.0
5.0
7.5
2.0
1.5
1.8
8.0
4.0
4.0
1.0
1.0
20-30
6
20
25
6-10
6
6
25
15
15
5-6
8-10
8.0-10
4.0
7.0
12.0
4.0
3.0
3.6
12.0
8.0
6.0
2-3
7-9
*WAS Waste activated sludge
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
gravity thickeners are shown in Table II-2 (see following
page). The requirements are based on the surface area of
the thickener and include thickening and removal of sludge
from the thickener, but do not include any allowances for
chemical addition.
II-3
-------
TABLE II-2.
GRAVITY THICKENER LABOR REQUIREMENTS
Thickener surface
acre, sq ft
Labor, hr/yr
Operation
Maintenance
Total
Less than 500
1,000
2,000
5,000
310
350
420
680
180
200
240
370
490
550
660
1,050
MONITORING
^SUPERNATANT
RECYCLE TO
PLANT INFLUENT
SLUDGE
INFLUENT
FROM
PREVIOUS
s'| SLUDGE
TREATMENT
PROCESS
THICKENED
SLUDGE TO
NEXT SLUDGE
TREATMENT
PROCESS
A. TEST FREQUENCY
R RECORD CONTINUOUSLY
D DAY
W WEEK
B. LOCATION OF SAMPLE
SI SLUDGE INFLUENT
TS THICKENED SLUDGE
SU SUPERNATANT
C, METHOD OF SAMPLE
G
R
GRAB SAMPLE
RECORD CONTINUOUSLY
D. REASON FOR TEST
P PROCESS CONTROL
C .*• COST CONTROL
E. FOOTNOTES:
1. FOR CONTROL OF PROCESS
RECEIVING THIS FLOW.
II-4
-------
NORMAL OPERATING PROCEDURES
Startup
1. Open thickener influent valve or gate and begin filling
thickener.
2. When rakes are covered start the thickener drive.
3. Start up chemical feed, where used.
4. Set up the thickened sludge pumping and controls and
place into operation.
5. Check operation of skimmer mechanisms, adjust so that
scum is drawn into skimmer, and turn on water sprays
if needed. (If thickener is equipped with skimmer)
Routine Operations
1. Inspect system twice a shift.
2. Carry out maintenance as required including cleanup
and washdown.
3. Take samples as outlined in MONITORING Section.
Shutdown
1. Shut down chemical feed systems.
2. Close thickener influent valve or gate.
3. Shut down the thickener drive, if desired.
4. Drain the thickener, if desired, or shut down the
thickened sludge pumping. Sludge should be pumped
from the thickener if it is to be shut down for more
than a day or two.
5. Turn off water sprays if thickener is equipped with
sprays.
CONTROL CONSIDERATIONS
Physical Control
Typically the flow through the thickener is continuous
and should be set for as constant a rate as possible.
II-5
-------
torque
sludge
pumping
The drive mechanism normally turns continuously and
contains a torque monitor which will shut down the drive
and sound an alarm if the drive mechanism is overloaded.
A review of Table II-l will show that for many sludges
the thickened sludge is only 2 or 3 times the concentration
of the influent sludge. In order to maintain the thickener
solids balance, the thickened sludge flow rate for these
cases must be 30 to 50 percent of the influent flow. In
most cases it will be advantageous to draw off thickened
sludge continuously at a flow rate approximately equal to:
Thickened sludge _ /Influent \ I Inf
\flow, gpm/ \Thi
flow rate, gpm
Influent solids,
Thickened solids,
It is important to maintain an adequate thickened sludge
flow rate or sludge will accumulate very rapidly in the
thickener.
Process Control
inspection
Efficient and consistent operation of sludge thickening
process and equipment depends on frequent monitoring, both
sensory and analytical.
The thickener should be inspected once a shift. The
supernatant should be relatively clear and the process
should be free of odors. The supernatant should be clear
enough to see down into the thickener at least one to two
feet. After gaining some operating experience it should be
possible to roughly judge the operation of the thickener by
visual appearance of the supernatant and surface of the
thickener.
odors
sampling
Odors from the thickener indicate that sludge is not
being withdrawn rapidly enough and is becoming septic in
the thickener. When the sludge becomes septic, gas is formed
and this gas mixes with the sludge to cause the sludge to
float to the surface. Sludge should be withdrawn on a more
frequent schedule to cure this problem. If the problem is
not cured, the supernatant will contain high BOD and SS and
may upset the rest of the treatment process when returned to
the main plant.
Sampling should be performed as outlined under MONITORING.
These samples may be obtained through valves provided in the
respective thickener piping. If sampling points are not pro-
vided, they should be installed to facilitate operation and
control of the process. Samples of the supernatant can be
obtained at the overflow weir.
II-6
-------
analysis
visual
analysis
sludge
blanket
solids
Samples should be analyzed according to procedures
specified in STANDARD METHODS and, in addition, should be
visually analyzed.
The influent sludge sample, when left undisturbed for
about 30 minutes in a beaker, should separate into a well
defined layer of sludge in the bottom and a relatively clear
liquid (supernatant) above. If the separation does not take
place in the beaker, problems can be expected in thickener
operation. Either the plant treatment process is not
operating properly or the sludge is not being properly pre-
treated prior to thickening (if chemicals are being added).
Very little sludge should settle from the sample of super-
natant in 30 minutes. If sludge does settle in the super-
natant sample, it indicates that either the thickener is
being overloaded (too dilute of an influent flow) or the
sludge level is too high and is carrying over the weirs.
The following major variables affect the operation of
gravity thickening and are discussed in this section.
1. Sludge blanket
2. Solids concentration
3. Liquid temperature and seasonal variations
4. Loading rates
5. Chemicals
6. Waste activated sludge
Thickening experience indicates the need to maintain a
sludge blanket in a thickener to assist in compaction. This
compaction results from layers of solids exerting a squeez-
ing or compressing force on those below. The sludge blanket
level is generally controlled by the sludge withdrawal rate.
Sludge should be removed from the thickener in small amounts
several times daily rather than in large amounts less often
to prevent the development of septic conditions within the
thickener. The sludge level should be kept well below the
top of the thickener. The level of the sludge can be
checked using a portable sludge level detector or by probing
for the top of the sludge blanket using a pole with a one
foot square plate fastened perpendicularly to the bottom of
the pole.
In general, a thicker sludge will be obtained with a
decrease in the sludge volatile solids content. The effect
of the initial solids concentration varies, but in general
it has been found that optimum results are achieved when the
influent solids concentration is between 0.5 and 1.0 percent.
Within this range, sludge compaction and supernatant clarity
are optimized.
II-7
-------
temperature
loading
rates
The liquid temperature has an effect on operation of the
thickening process. Generally, during warm weather periods
the blanket should be maintained at lower levels because of
accelerated biological degradation and the possibility of
septic conditions developing. Odor is' a good indicator of
this condition and the solution is to pump the thickened
sludge at a higher rate. Conversely, deeper sludge blankets
can be maintained when liquid temperatures are lower. The
liquid temperature may also affect the thickener performance
and, in some cases, the concentration of thickened sludge
in summer may be as low as 60 percent of that obtained in
winter operations.
Overflow and solids loading rates are also important.
The plant operating manual may call for specific loading
rates, however, if performance is not satisfactory, the
operation should be changed. For example, if it is found
that sludge is not thick enough, the operator should try
running at a lower overflow rate by decreasing the influent
sludge flow. The operator should also try to identify any
hydraulic disturbances within the thickener and these should
be corrected by proper modifications such as baffling. If
the sludge flow to the thickener is far below the design rate,
pumping of secondary effluent to the thickener to bring
hydraulic flows up closer to design overflow rates may help
the operation and also minimize odor generation. Typical
solids loading rates for various types of sludges are shown
in II-l.
polymers
chlorine
waste
activated
sludge
Polymers may be used to improve the performance of
gravity thickeners. Because of the large number of polymers
commercially available and the diversity of sludge types, it
is necessary to do a number of jar tests to determine an
optimum polymer and polymer dosage for a particular sludge.
Typical polymer doses are one to five milligrams per liter
(mg/1).
Some plants have provisions for feeding chlorine to the
thickener influent sludge. Chlorination of this sludge is
effective in the oxidation of hydrogen sulfide and in the
control of odor causing bacteria. A chlorine residual of
1 mg/1 in the thickener supernatant should be adequate for
odor control. Chlorine addition to thickeners increases
the cost of operation and cannot be justified unless odor
problems exist.
It is very difficult to gravity thicken waste activated
sludge because of the light nature of the waste activated
sludge solids. The addition of digested primary sludge to
waste activated sludge has been found to help in the thicken-
ing of these solids. The amount of digested sludge to be
-------
applied to the thickener and the resulting performance will
vary for each plant but, typically, should be in the range
of 30 to 70 percent of the influent sludge flow. The optimum
digested sludge flow can be determined by trying several
ratios within the range of 30 to 70 percent.
EMERGENCY OPERATING PROCEDURES
Loss of Power
Short power interruptions should not greatly affect
sludge thickening although electrical equipment will not
operate, the thickening process will not deteriorate if
power is regained within 30 minutes to an hour. If power
is unavailable for longer periods, septic conditions may
develop. The effect of potential septic conditions can be
partially or totally overcome by aerating or mixing the con-
tents of the thickener if practical and/or adding chlorine
to the thickener contents.
Loss of Other Treatment Units
The loss of other treatment units should not greatly
affect the operation of the thickener. The loss of the
digesters or other processes to which thickened sludge is
pumped may create a ,solids storage problem. It is not
desirable to store sludge in the thickener, but in an
emergency it can be used for sludge storage. In case of
a prolonged problem it may be necessary to haul sludge to
another treatment facility or disposal area.
COMMON DESIGN SHORTCOMINGS
Shortcoming Solution
1. Scum carries over 1. Install an adequate scum
effluent weirs. baffle just inside the
effluent weir. This baffle
should be slightly submerged
and extend approximately 6
inches above the water
surface.
2. Sludge contains 2. Install grit chamber at plant
excessive grit. headworks (major construction),
or eliminate sources of grit
entering sewer system. (Make
sure existing grit removal
facilities are being properly
operated.)
II-9
-------
Shortcoming
3. Short circuiting
flow through tank
causing poor solids
removal.
4. Corrosion of steel
components.
Solution
3. Modify hydraulic design and
install appropriate baffles
to disperse flow and reduce
inlet velocities.
4a. Coat surfaces with proper
paint. Industrial paint
suppliers and appliers can
be located in the yellow
pages of large city telephone
directories. These suppliers
can furnish complete recom-
mendations on proper coating
systems for various applica-
tions. See also WPCF MOP
No. 17.
4b. Install cathodic protection
system in tank. Suppliers
of this type of equipment
will provide design assis-
tance and can be found in
the yellow pages under
"Corrosion Control".
Generally, cathodic protection
should be installed only if
the surface is protected by
a coating which is at least
80 percent effective.
11-10
-------
TROUBLESHOOTING GUIDE
GRAVITY THICKENING
INDICATORS/OBSERVATIONS
1. Septic odor, rising
sludge.
2. Thickened sludge not
thick enough.
3. Torque overload of
sludge collecting
mechanism.
PROBABLE CAUSE
la. Thickened sludge
pumping rate is too
low.
Ib. Thickener overflow
rate is too low.
2a. Overflow rate is too
high.
2b. Thickened sludge
pumping rate is too
high.
2c. Short circuiting of
flow through tank.
3a. Heavy accumulation
of sludge.
3b. Foreign object
jammed in mechanism.
3c. Improper alignment
of mechanism.
CHECK OR MONITOR
la. Check thickened
sludge pumping system
for proper operation.
Ib. Check thickener col-
lection mechanism for
proper operation.
Ic. Check overflow rate.
2a. Check overflow rate.
2c. Use dye or other
tracer to check for
circuiting.
3a. Probe along front of
collector arms.
SOLUTIONS
la. Increase pumping rate of
thickened sludge.
Ib. Increase influent flow to
thickener - a portion of the
secondary effluent may be
pumped to thickener if
necessary to bring overflow
rate to 400 to 600 gpd/sq ft.
Ic. Chlorinate influent sludge.
2a. Decrease influent sludge flow
rate.
2b. Decrease pumping rate of
thickened sludge.
2c. Check effluent weirs: repair
or relevel. Check influent
baffles: repair or relocate.
3a. Agitate sludge blanket in
front of collector arms with
water jets. Increase sludge
removal rate .
3b. If problem persists drain
thickener and check mechanism
for free operation.
3c. Attempt to remove foreign
object with grappling device.
-------
TROUBLESHOOTING GUIDE
GRAVITY THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Surging flow.
4a. Poor influent pump
programming.
4a. Pump cycling.
4a. Modify pump cycling. Reduce
flow and increase time.
5. Excessive biological
growths on surfaces
and weirs (slimes,
etc.)
5a. Inadequate cleaning
program.
5a. Frequent and thorough cleaning
of surfaces. Apply chlorination.
6. Oil leak.
6a. Oil seal failure.
6a. Oil seal.
6a. Replace seal.
7. Noisy or hot bearing
or universal joint.
7a. Excessive wear.
7b. Improper alignment.
7c. Lack of lubrication.
7a. Alignment.
7b. Lubrication.
7a. Replace, lubricate, or align
joint or bearing as required.
8. Pump overload.
8a. Improper adjustment
of packing.
8b. Clogged pump.
8a. Check packing.
8b. Check for trash in
pump.
8a. Adjust packing.
8b. Clean pump.
9. Fine sludge particles
in effluent.
9a. Waste activated
sludge.
9a. Portion of WAS in
thickener influent.
9a. Better conditioning of the
WAS portion of the sludge.
9b. Thicken WAS in a flotation
thickener.
-------
MAINTENANCE CONSIDERATIONS
A good preventive maintenance program will reduce break-
downs which could be not only costly, but also very unpleasant
for operating personnel. Plant components including the
following should be inspected semiannually for wear, corrosion,
and proper adjustment:
1. Drives and gear reducers
2. Drive chains and sprockets
3. Shaft bearings and bores
4. Bearing brackets
5. Baffles and weirs
6. Electrical contacts in starters and relays
7. Suction lines and sumps
8. Skimming units
SAFETY CONSIDERATIONS
The gravity sludge thickening equipment presents no
special hazards, however, general safety considerations
should apply. At least two persons should be present when
working in areas not protected by handrails. Walkways and
work areas should be kept free of grease, oil, leaves and
snow. Protective guards and covers must be in a place un-
less mechanical/electrical equipment is locked out of
operation.
REFERENCE MATERIAL
References
1.
2.
3.
Standard Methods for the Examination of Water and
Wastewater.
American Public Health Association
1015 Eighteenth Street, N.W.
Washington, D.C. 20036
WPCF Manual of Practice No. 17
(WPCF MOP No. 17), Paints and Protective Coatings for
Wastewater Treatment Facilities.
WPCF Manual of Practice No. 11, Chapter 8, Operation
of Wastewater Treatment Plants, Sludge Conditioning.
Glossary of Terms and Sample Calculations
1. Overflow rate^ is the flow rate over the effluent weir
divided by the liquid surface area or the influent flow
rate minus the average sludge draw-off flow rate divided
11-13
-------
by the liquid surface area. This parameter is normally
expressed as gallons per day per square foot of surface
area. The following table shows thickener supernatant
flow rates which result in 600 gpd/sq ft overflow rates
for various size units and a sample calculation.
Supernatant
Thickener flow rate
diameter, for 600 gpd/sq ft
ft overflow rate
10 33
20 130
30 294
40 523
50 818
60 1,178
The following is a sample overflow rate calculation for a
30 foot diameter thickener operating at a flow of 300 gpm.
(flow, gpm) 60 min 24 hrs
Overflow rate = hr day
TTd2
4
(300) (60) (24) 4
(3.14) (30)*
= 611 gpd/sq ft
2. Removal is the removal of the solids through the thicken-
ing process expressed as a percentage of influent solids.
This is very difficult to calculate accurately over a
period of time unless rather extensive sampling and test-
ing is performed. The removal can be calculated at any
moment based on grab sampling as follows:
(average sludge flow)(sludge solids)
Removal = . _.,r ' \ ,. ^ '— x 100
(average influent flow)(influent solids)
(average supernatant flow)(supern. solids)
(average influent flow)(influent solids)
x 100
All corresponding parameters such as flows or solids
must be expressed in the same units.
As an example assume the following data all taken at
the same time.
Influent flow = 100 gpm Solids =1.0%
Sludge flow = 18.3 gpm Solids =5.0%
Supernatant flow = 81.7 gpm Solids = 0.1%
11-14
-ds)|
!LJ
-------
= 91.5%
= 100-
or
(100-18.3)(0.:
x 100
(100) (1)
_ —'
= 91.8%
3. Sludge concentration is the weight of solids per unit
weight of sludge. It can be calculated in percent as
follows:
weight of dry sludge solids
Concentration = . , . ^ —r—5 x 100
weight of wet sludge
4. Solids loading is the feed solids applied per day
divided by the liquid surface area of the thickener.
It is generally expressed as weight (pounds) of dry
solids per day per square foot.
5. Supernatant is the clarified liquid which forms above
the sludge layer during the settling process. The
supernatant is the effluent flow from the gravity
thickening process.
6. Weir loading is the supernatant flow rate divided by
the linear footage of effluent weir. It is normally
expressed as gallons per day per foot of weir.
11-15
-------
Ill
FLOTATION THICKENING
-------
CONTENTS
Process Description III-l
Typical Design Criteria and Performance III-3
Staffing Requirements III-3
Monitoring III-5
Normal Operating Procedures III-6
Startup III-6
Routine Operations III-6
Shutdown III-7
Control Considerations III-8
Physical Control III-8
Process Control III-8
Emergency Operating Procedures 111-10
Loss of Power 111-10
Loss of Other Treatment Units 111-10
Common Design Shortcomings 111-10
Troubleshooting Guide 111-12
Maintenance Considerations 111-14
Safety Considerations 111-14
Reference Material 111-14
References 111-14
Glossary of Terms and Sample Calculations 111-15
-------
PROCESS DESCRIPTION
process
operation
thickening
Sludge thickening by flotation is a process especially
effective for light sludges. This process causes the sludge
to float and the sludge is then skimmed from the surface of
the thickener. Flotation is especially effective on activated
sludge, which is difficult to thicken by gravity because of
its low specific gravity. Air is injected into the incoming
sludge under pressure. The sludge then flows into an open
tank, either rectangular or circular, where at atmospheric
pressure, much of the air comes out of solution as minute air
bubbles. These bubbles attach themselves to sludge particles
and float these particles to the surface. A sludge layer
forms on the top surface of the tank contents and this layer
is removed by a skimming mechanism for further processing.
A typical air flotation system is shown in Figure
III-l (see following page). A portion of the flotation
thickener effluent, or similar plant process stream, is pump-
ed to a retention tank at 60 to 70 psig. Air is fed into the
recirculation pump discharge line or the retention tank at a
controlled rate and mixed by the flow from the reaeration
pump discharging into the retention tank through eductors.
The flow through the recycle system is metered and controlled
by a valve located immediately before the mixing of the re-
cycle stream with the sludge feed. Effluent recycle ratios
range from 30 to 150 percent of the thickener influent flow.
The recycle flow and sludge feed are mixed in a chamber at
the flotation unit inlet. Flotation aids such as polymers,
if used, are normally fed into this mixing chamber. The
sludge particles float to the surface. The clarified effluent
is discharged under a baffle and over an adjustable weir which
controls the depth of penetration of the surface sludge skim-
ming mechanism. Bottom sludge collectors are used for removal
of any settled sludge or grit that may accumulate.
Sludge thickening occurs in the floating sludge blanket,
which is normally 8 inches to 24 inches thick. The buoyant
sludge and air bubble mixture forces the surface of the
blanket above the water level, allowing water to drain from
the sludge particles. Detention time in the flotation zone
is not critical, provided the particle rise rate is sufficient
and that horizontal flow velocity in the unit does not produce
scouring and rewetting of the sludge blanket.
III-l
-------
SLUDGE REMOVAL MECHANISM
UNIT
EFFLUENT
•'•'••.'.':';'•'.';.•'•• ^.•'l.Vi^> .•'.>/'/.-"FLOW. ZONE .-.
>:3;?^.: .^••••'•V-;S ^^;?Sifcy^
___ i^—^—rS^-v-Rj£
RECYCLE
FLOW
BOTTOM SLUDGE COLLECTOR
SLUDGE
DISCHARGE
X
RECYCLE
FLOW
UNIT
SLUDGE FEED
UNIT EFFLUENT
AUX. RECYCLE CONNECTION
(PRIMARY TANK OR
PLANT EFFLUENT)
AIR FEED
FLOTATION UNIT
RECIRCULATION PUMP
REAERATION PUMP
THICKENED SLUDGE
*— DISCHARGE
UNIT FEED
SLUDGE
RECYCLE
FLOW
- RETENTION TANK
(AIR DISSOLUTION)
Figure III-l. Dissolved air flotation system.
III-2
-------
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Flotation thickeners are typically designed based on
solids loading, overflow rate, and influent solids concentra-
tion. Typical operation and performance parameters are shown
in Table III-l and were developed from manufacturer's opera-.
tion manuals.
TABLE III-l. FLOTATION THICKENER OPERATION AND PERFORMANCE
Operation Parameter
Range
Typical
2
1
5,000 min
0.03
Solids loading, Ib dry
solids/hr/sq ft of
surface
With chemicals 2 to 5
Without chemicals 1 to 2
Influent solids concentration,
mg/1 5,000 min
Air to solids ratio 0.02-0.04
Blanket thickness, in 8-24
Retention tank pressure, psi 60-70
Recycle ratio, % of influent flow 30-150
Expected Performance
Float solids concentration, %
Solids removal, %
With flotation aid
Without flotation aid
3-7
95
50-80
Operating parameters for some actual plants are shown
in Table III-2 (see following page), which was summarized in
"Process Design Manual for Sludge Treatment and Disposal",
(EPA 625/1-74/006).
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
flotation systems are shown in Table III-3 (see following
page). The hours shown include thickening and removal of
sludge to the next unit process.
III-3
-------
TABLE III-2. DISSOLVED AIR FLOTATION - ACTUAL OPERATING CONDITIONS AND PERFORMANCE
H
H
Location
Bernardsville, N.J.
Bernardsville, N.J.
Abington, Pa.
Hatboro, Pa.
Morristown, N.J.
Oinaha, Nebr.
Omaha, Nebr.
Belleville, 111.
Indianapolis, Ind.
Warren, Mich.
Frankenmuth, Mich.
Oakmont.Pa.
Columbus, Ohio
Levittown.Pa.
Nassau Co .,N.Y.
Bay Park S.T.P.
Nassau Co. ,N.Y.
Bay Park S.T.P.
Nashville, Term.
Feed
M.L.a
R.S.*
R.S.
R.S.
R.S.
R.S.
M.L.
R.S.
R.S.
R.S.
M.L.
M;L.
R.S.
R.S.
R.S.
R.S.
R.S.
R.S.
Influent
ssmg/1
3,600
17,000
5,000
7,300
6,800
19,660
7310
18,372
2,960
6,000
9,000
6,250
6300
5,700
8,100
7,600
15,400
Subnatant
ssmg/1
200
196
188
300
200
118
50
233
144
350
80
80
40
31
36
460
44
% Removal
ss
94.5
98.8
96.2
96.0
97.0
99.8
99.4
98.7
95.0
95.0
99.1
98.7
99.5
99.4
99.6
94.0
99.6
Float
% Solids
3.8
4.3
2.8
6.0
4.0
3.5
5.9
8.8
6.8
5.7
5.0
7.8
6-9
6-8
8.0
5.0
5.5
4.4
3.3
12.4
Loading
Ib/hr/ft2
2.16
4.25
3.0
2.95
1.70
7.66
3.1
3.83
2.1
5.2
6.5
3.0
3.3
2.9
4.9
1.3
5.1
Flow
gpm/ft2
1.2
0.5
1.2
0.8
0.5
0.8
0.8
0.4
1.47
1.75
1.3
1.0
1.0
1.0
1.2
0.33
0.66
Remarks
Standard0
Standard
Flotation Aid''
After 1 2 ho urs holding
Flotation Aid
Standard
Flotation Aid
After 24 hours holding
Flotation Aid
Flotation Aid
Flotation Aid
After 12 hours holding
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Standard
Flotation Aid
"M.L. - Mixed liquor from aeration tanks.
*R.S. - Return sludge.
cStandard - Indicates no flotation aid and no holding before sampling.
^Flotation Aid - Indicates use of coagulant-flotation aid.
-------
TABLE III-3. FLOTATION THICKENER LABOR REQUIREMENTS
Thickener surface
Labor, hr/yr
area, sq ft
11
21
53
105
210
520
Operation
215
320
550
840
1,300
2,200
Maintenance
260
350
540
750
1,050
1,600
Total
475
670
1,090
1,590
2,350
3,800
MONITORING
^PORTION OF
PLANT
EFFLUENT
POLYMER
FEED
AIR
DISSOLUTION
TANK
AIR FEED
THICKENED
SLUDGE
SLUDGE
INFLUENT
J-
SU
-v
SUPERNATArtfT
RECYCLE TO
PLANT INFLUENT
TOTAL
§ SOLIDS
2
5 BOD
Q
K SUSPENDED
J2 SOLIDS
0
(9
w FLOW
ISI
to
t- _
Z Q
< 0
a. _
ALL
ALL
ALL
ALL
>
U
LU
^3
i n
00 LU
LU CC
1- LJ-
1/D
2/W
1/D
R
LL
O
z
5 LU
\— ]
<^ Q_
81
TS
SI
SU
SU
SU
u_
O
^ LU
O j
I OL
UJ |
5 co
G
G
G
R
K
Z ^
O H
00
< cr
LU O
cn u.
P
C
pd)
p
P(1.
A. TEST FREQUENCY
R - RECORD CONTINUOUSLY
D - DAY
W - WEEK
B. LOCATION OF SAMPLE
SI SLUDGE INFLUENT
TS THICKENED SLUDGE
SU SUBNATANT
C. METHOD OF SAMPLE
= GRAB SAMPLE
- RECORD CONTINUOUSLY
D. REASON FOR TEST
P PROCESS CONTROL
C COST CONTROL
E. FOOTNOTES:
1
FOR CONTROL OF PROCESS
RECEIVING THIS FLOW.
III-5
-------
If polymer is used to aid in the flotation process,
the optimal chemical dosage for the feed sludge should be
determined at the start of each shift using jar test proce-
dures. See the section on "Control Considerations" for more
discussion.
NORMAL OPERATING PROCEDURES
Startup
1. Close drain valves as required.
2. Open appropriate valves on the recycle water system.
a. If the unit has been drained, open the necessary
valves to the auxiliary water supply-.
b. If the unit has not been drained do not open
the auxiliary water supply valves.
3. Start the recirculation pump. If the unit has been
drained wait until it is full and the auxiliary water
supply valve has been closed before proceeding to
Step 4.
4. Start the air feed and adjust to the required flow.
5. Start the chemical feed system.
6. Allow unit to run 10 to 15 minutes before starting
influent sludge feed. This will charge the unit with
chemical and aerated water.
Routine Operations
A check on the following unit operations at least twice
per shift is recommended.
1. Visual check for proper chemical conditioning and mechan-
ical operation. For example: large floe carrying over
into recycle water indicates a problem with the reaera-
tion system. A very turbid effluent with no floe devel-
opment indicates a chemical deficiency or overloading of
the unit.
2. Flow
3. Skimmer speed setting
4. Recycle rate
5. Chemical supply
III-6
-------
6. Obtain and analyze samples as required
7. Chemical v-notch weir setting
8. Retention tank air cushion
A mechanical check should be made on the following units
at two hour intervals.
1. Pumps: chemical feed, recycle, reaeration, and sludge
sumps
2. Air manometer operation
3. Retention tank pressure
4. Sludge pit mixers
Shutdown
1. Shut off influent feed
2. Shut off chemical supply
3. If possible, allow unit to operate for 30 minutes before
shutting down the sludge removal system (skimmer
flights). This serves to clear the unit of suspensions
and the sludge removal system clears the water surface
of sludge. The unit can then be shut down with the
flotation retention tank filled with practically clean
water and the flotation unit primed for start-up.
4. Shut off air supply
5. Turn off reaeration pump
6. Turn off recirculation pump
7- Turn off sludge mechanism drive motor(s)
8. Shut off chain oilers
9. If no other units are operating to the same pit, shut
off sludge pit mixer and pump .
10. If the unit is to be shut down for an extended period
or for internal maintenance, it must be drained -
a. Open drain valves on air flotation unit and
retention tank -
III-7
-------
10. b. Flush the unit, flights, beaching plate,
baffles with the high pressure hose.
CONTROL CONSIDERATIONS
Physical Control
Typically the flow through the thickener is continuous
and should be set for as constant a rate as possible.
The drive mechanism normally turns continuously and
contains a torque monitor or shear pins which will shut down
° US the drive (and sound an alarm) if the drive mechanism is
overloaded.
Flow meters are normally provided for the flow through
the thickener, the recycle flow, and the air flow.
... A control is normally provided on the retention tank
retention
to automatically control the liquid level by blowing off
excess accumulations of air within the tank.
. The discharge pressure of the air compressor is normally
regulated by a pressure reducing valve and is typically set at
75 psi.
Process Control
Air pressure in flotation is important because it deter-
mines air saturation or size of the air bubbles formed. It
influences the degree of solids concentration and the sub-
a^r natant (separated water) quality. In general, either in-
creased pressure or air flow produces greater float (solids)
concentrations and a lower effluent suspended solids concen-
tration. There is an upper limit, however, as too much air
will tend to break up floe particles.
The air-to-solids ratio is important because it affects
the sludge rise rate. The air-to-solids ratio needed for a
air-to- particular application is a function primarily of the sludge's
solids characteristics such as SVI. The most common ratio used for
design of a waste activated sludge thickener is 0.03.
Additional recycle of clarified effluent does two things:
1. It allows a larger quantity of air to be dissolved
because there is more liquid.
recycle
2. It dilutes the feed sludge.
Dilution reduces the effect of particle interference on*
III-8
-------
chemicals
the rate of separation. Concentration of sludge increases
and the effluent suspended solids decrease as the sludge
blanket detention period increases.
Use of chemical flotation aids (polymers) provides
improved thickening and solids capture. The quantities
required must be determined for each specific sludge, but are
usually in the range of 5 to 15 pound chemical per ton of dry
solids.
chemical
setting
sampling
The approximate chemical dosage and feed pump setting
may be determined using the following procedure. Draw a
1,000 ml sample of representative influent sludge and with
a pipette mix in chemical taken directly from the chemical
mix tank. Note the ml of chemical required to produce a
pronounced firm, well defined flox. Calculate the ratio of
sludge volume to chemical volume, ml/ml. Using the daily
sludge flow determine the daily chemical addition as follows:
chemical flow, gpd = (sludge flow, gpd)
(j
ml chemical
ml sludge
Check the chemical feed pump literature and set the ad-
justment so the pump feeds 1.5 to 2.0 times the calculated re-
quirement of chemical solution. This feed rate can be reduced
to optimum as the plant operates. Overfeed of chemical pro-
duces very little additional benefit, but increases operating
costs.
Sampling should be performed as outlined under
MONITORING. These samples may be obtained through valves pro-
vided in the respective thickener piping. If sampling points
are not provided, they should be installed to facilitate op-
eration and control of the process. Samples of the super-
natant can be obtained at the overflow weir.
analysis
visual test
Samples should be analyzed according to procedures
specified in standard Methods and, in addition, should be
visually analyzed.
Operating experience will allow most operators to judge
the performance of the flotation thickener. A sludge rise
test, performed as follows, is useful to visually check
operating results. On most units, a sampling valve is pro-
vided on the inlet mixing chamber. When the unit is in
operation, a quart jar sample is withdrawn and the time for
the sludge to rise so that a clear separation between sludge
and liquid can be seen. Normal rise times are 10 to 25 sec-
onds, and experience will indicate an average time for each
particular plant. The relative depth of the blanket, sub-
natant clarity and general appearance of flocculated sludge
particles are also good visual indicators.
III-9
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
The air flotation unit should be shut down unless
emergency electrical generation is available. After power is
restored a normal start up should be performed and the unit
placed back in operation.
Loss of Other Treatment Units
Loss of chemical feed to the flotation unit will
generally affect performance. If this occurs, operating
parameters such as recycle ratio may require readjustment to
obtain the best possible performance. Best performance with-
out chemical feed will generally be very inferior to perform-
ance with chemical feed.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1.
2.
3.
4.
5.
Excessive wear in
sludge mechanism
chains and gears.
Poor results in
mixing chemicals
(polymers).
Early failure of
pressure gauges
and controls.
Sludge feed pumps
run on-off cycle
causing pulsating
feed to DAF unit.
Only primary
e.ffluent avail-
able for auxiliary
recycle.
Solution
1. Install automatic oilers.
2a. Install automatic batching
system or an aspirator wetting
system to assure initial wetting
of polymer (powders).
2b. Prepare a less concentrated
mixture of 0.25 to 0.5 percent
by weight of polymer to water.
3. Install such equipment on
panels isolated from equipment
vibration.
4. Install a flow indicator and
and flow control system to
provide consistent, controllable
inflow rate.
5. Install line so that secondary
effluent can be used for recycle
during periods when primary
effluent has more than 200 mg/1
solids or contains unusual
amounts of stringy materials.
111-10
-------
Shortcoming Solution
6. Wide variations 6. Install a mixing-storage
in feed solids tank to minimize fluctuations.
concentration occur
due to direct feed
of DAT from final
clarifier.
III-ll
-------
TROUBLESHOOTING GUIDE
FLOTATION THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Floated sludge too
thin.
la. Flight speed too
high.
Ib. Unit overloaded.
Ic. Polymer dosages too
low.
Id. Excessive air/solids
ratio.
le. Low dissolved air.
Ib. Proper operation
and calibration of
polymer pumps.
Ic. Proper operation
and calibration of
polymer pumps.
Id. Float appearance
(very frothy).
la. Adjust flight speed as
required.
Ib. Turn off sludge feed and allow
unit to clear or purge the
unit with auxiliary recycle.
Ic. Adjust as required.
Id. Reduce air flow to pressuri-
zation system.
le. See Item 2.
2. Low dissolved air.
2a. Reaeration pump off,
clogged, or mal-
functioning.
2b. Eductor clogged.
2c. Air supply mal-
function.
2a. Pump condition.
2c. Compressor, lines,
2a. Clean as required.
2b. Clean eductor.
2c. Repair as required.
3. Effluent solids too
high.
3a. Unit overloaded.
3b. Polymer dosages too
low.
3c. Skimmer off or too
slow.
3d. Low air/solids
ratio.
3a. See Item Ib.
3b. See Item Ic.
3c. Skimmer operation.
3d. Poor float forma-
tion with solids
settling.
3c. Adjust speed.
3d. Increase air flow.
-------
TROUBLESHOOTING GUIDE
FLOTATION THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3e. Improper recycle
flow.
3e. Recycle pump flow
rate.
3e. Adjust flow.
4. Skimmer blade leak-
ing on beaching
plate.
4a. Skimmer wiper not
adjusted properly.
4b. Hold-down tracks too
high.
4a. Adjust
5. Skimmer blade bind-
ing on beaching
plate.
5. Skimmer wiper not
adjusted properly.
5. Adjust
6. High water level in
retention tank.
6a. Air supply pres-
sure low.
6b. Level control system
bleeding continu-
ously.
6c. Insufficient air
injection.
6a. Compressor and air-
lines.
6b. Level control system.
6c. Compressor and air
lines.
6a. Repair
6b. Repair bleed system.
6c. Increase air flow.
7. Low water level in
retention tank.
7a. Recirculation pump
not operating or
clogged.
7b. Level control system
not bleeding air
properly.
7a. Pump operation.
7b. Level control.
7a. Inspect and clean.
7b. Repair
8. Low recirculation
pump capacity.
8. High retention tank
pressure.
8. Recirculation flow
rate.
8. Increase recirculation flow.
-------
MAINTENANCE CONSIDERATIONS
A good preventive maintenance program will reduce break-
downs which could be not only costly, but also very unpleasant
for operating personnel. The following are the major elements
which should be inspected semiannually for wear, corrosion,
and proper adjustment:
1. Drives and gear reducers
2. Chains and sprockets
3. Guide rails
mechanical 4. Shaft bearings and bores
5. Bearing brackets
6. Baffle boards
7. Flights and skimming units
8. Suction lines and sumps
SAFETY CONSIDERATIONS
The dissolved air flotation equipment presents no
special hazards, however, general safety considerations should
apply. At least two persons should be present when working
in areas not protected by handrails. Walkways and work areas
should be kept free of grease, oil, leaves and snow. Pro-
tective guards must be in place unless mechanical/electrical
equipment is locked out of operation.
The retention tank is a hydropneumatic tank and should
not be pressurized beyond the working pressure rating. The
tank should have a functional relief valve and should be
inspected on a regular basis for excessive corrosion.
REFERENCE MATERIALS
References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association,
1015 Eighteenth Street, N.W., Washington, D.C. 20036.
2. WPCF Manual of Practice No. 17 (WPCF MOP NO. 17), Paints
and Protective Coatings for Wastewater Treatment
Facilities.
3. WPCF Manual of Practice No. 11, Chapter 8, Operation of
Wastewater Treatment Plants, Sludge Conditioning.
4. Process Design Manual for Sludge Treatment and Disposal,
Chapter 4. EPA 625/1-74-006 U.S. EPA Technology
Transfer.
111-14
-------
5. Estimating Laboratory Needs for Municipal Wastewater
Treatment Facilities. EPA 430/9-74-002 U.S. EPA
Office of Water Program Operations, Washington, D.C.
20460.
Glossary of Terms and Sample Calculations
1. Overflow rate is the flow rate through the thickener
divided by the liquid surface area normally expressed
in gpd per sq ft.
2. Solids loading is the dry weight of sludge solids
per unit time per square foot of thickener surface
area. This is normally expressed as Ib dry sludge
solids per hr per sq ft of surface.
3. Air to solids ratio is the ratio of air feed to dry
sludge solids feed by weight. The weight of air is
0.08 times the flow rate in standard cu ft per min.
(0.08) (Air flow, cfm)
Ratio =
Influent dm solids, Ib
111-15
-------
IV
AEROBIC DIGESTION
-------
CONTENTS
Process Description IV-1
Typical Design Criteria and Performance ... IV-3
Staffing Requirements • IV-3
Monitoring IV^-5
Normal Operating Procedures ....... IV-6
Startup IV-6
Routine Operations .... IV-6
Shutdown IV-7
Control Considerations IV-7
Physical Control IV-7
Process Control IV-7
Emergency Operating Procedures . . IV-9
Loss of Power IV-9
Loss of Other Treatment Units IV-9
Common Design Shortcomings . . IV-9
Troubleshooting Guide . IV-11
Maintenance Considerations IV-13
Safety Considerations IV-13
Reference Material .............. IV-13
References . IV-13
Glossary of Terms and Sample Calculations .... IV-13
-------
PROCESS DESCRIPTION
process
operation
design
differences
high
purity
oxygen
Aerobic digestion is the separate aeration of waste
primary sludge, waste biological sludge, or a combination of
waste primary and biological sludges in an open or closed
tank(s) in the presence of dissolved oxygen. The purpose is
to further treat these sludges so that they will not cause
odors or other nuisances in final disposal. Aerobic digestion
also reduces the volume of sludge solids. Figure IV-1 (see
following page) is a schematic diagram of an aerobic digestion
system. Aerobic digestion is used commonly for package plants
and many times is a part of the package plant tankage.
Aerobic digestion is equally useful for larger plants espe-
cially for waste biological sludges.
Aerobic digestion is a completely mixed activated sludge
system with either batch or continuous flow input. The
contents of the digester are aerated for a period of 12 to 22
days depending on the type of sludge. As aeration takes place
the organisms consume the food. The food supply decreases
and the organisms begin to digest their own cell tissues for
energy. The organisms convert this cell tissue to carbon
dioxide, water and ammonia. The ammonia is subsequently
converted to nitrate as the digestion proceeds. The solids
are then separated from the liquid for disposal desired.
Solids, after adequate aerobic digestion, usually dewater
easily and do not cause odor problems.
There are some design variations among aerobic digestion
systems. Current practice varies according to whether or not
the separate sedimentation tank shown in Figure IV-1 is used.
Some designs use a batch-type system, where the sludge is
aerated and mixed for a number of days, settled without
mixing, and sludge and supernatant removed all in the same
tank. Aerobic digesters often use rectangular aeration tanks
and mechanical or diffused aeration systems.
Recently high purity oxygen (95 to 97 percent) aeration
has been used for aerobic digesters. In some cases oxygen
from atmospheric air (21 percent) cannot be dissolved into
the digesting sludge fast enough to meet the requirements of
the biological reaction. High purity oxygen can dissolve in
sludge nearly five times as fast as oxygen from the air and
permits a more concentrated sludge feed to the digester. High
purity oxygen digesters are covered or baffled to prevent a
high loss of oxygen to the atmosphere.
IV-1
-------
SLUDGE
H
(O
>' V
.\
:O
/.
:/
•& C,
\.
SETTLED SLUDGE RETURNED TO AERODIGESTER
J
u
SUPERNATANT
DIGESTED
SLUDGE
Figure IV-1. Schematic of aerobic digestion system.
-------
Supernatant from the digestion process is returned to
supernatant either the primary or the secondary treatment process and
return normally causes no problem to process operation. The respec-
to tive treatment process must be capable of handling the
process additional hydraulic flow resulting from the return of
supernatant.
TYPICAL DESIGN CRITERIA & PERFORMANCE
Typical design criteria are shown in "Process Design
Manual for Sludge Treatment and Disposal",(EPA 625/1-74-016)
and in Table IV-1 (see following page).
Current practice is to provide about 15 days of deten-
tion time for the digestion of waste biological sludge and
about 20 days when primary sludge is included. Loadings
vary from 0.1 to 0.2 pounds of volatile suspended solids (VSS)
per cubic foot per day. A 40 to 50 percent reduction in
volatile suspended solids content is normally obtained. The
supernatant may contain as little as 10 to 30 mg/1 BOD, 10
mg/1 ammonia nitrogen, and from 50 to 100 mg/1 nitrate
nitrogen. When nitrification occurs, both pH and alkalinity
are reduced.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
aerobic digesters are shown in Table IV-2. The requirements
are based on plant design flow and include removal of sludge
to the next unit process.
TABLE IV-2. AEROBIC DIGESTION LABOR REQUIREMENTS
Plant design flow, Labor, hr/yr
mgd
0.5
1
2
5
10
25
Operation
100
160
260
500
800
1,500
Maintenance
20
30
50
100
160
300
Total
120
190
310
600
960
1,800
IV-3
-------
TABLE IV-1
AEROBIC DIGESTION DESIGN PARAMETERS
Parameter
Value
Remarks
Solids retention
time, days
Solids retention
time, days
Volume allowance,
cu ft/capita
VSS loading,
pcf/day
Air requirements
Diffuser system,
cfm/1,000 cu ft
Diffuser system,
cfm/1,000 cu ft
Mechanical system,
hp/1,000 cu ft
10-15°
15-20b
3-4
0.024-0.14
20-35a
>60b
1.0-1.25
1.0-2.0
Minimum DO, mg/1
Temperature, C
VSS reduction, percent 35-50
Tank design
Power requirement,
BHP/10,000
Population Equivalent
Depending on temperature, type of sludge,
etc.
Depending on temperature, type of sludge,
etc.
Enough to keep the solids in suspension
and maintain a DO between 1-2 mg/1.
This level is governed by mixing require-
ments. Most mechanical aerators in aero-
bic digesters require bottom mixers for
solids concentration greater than 8,000
mg/1, especially if deep tanks (>12 feet)
are used.
If sludge temperatures are lower than 15 C,
additional detention time should be pro-
vided so that digestion will occur at the
lower biological reaction rates.
Aerobic digestion tanks are open and gen-
erally require no special heat transfer
equipment or insulation. For small treat-
ment systems (0.1 mgd), the tank design
should be flexible enough so that the
digester tank can also act as a sludge
thickening unit. If thickening is to be
utilized in the aeration tank, sock type
diffusers should be used to minimize
clogging.
8-10
Excess activated sludge alone.
Primary and excess activated sludge, or primary sludge alone.
IV-4
-------
MONITORING
TEMPERATURE
pH
TOTAL SOLIDS
TOTAL
VOLATILE
SOLIDS
DO
SETTLEABLE
SOLIDS
pH
SUSPENDED
SOLIDS
BOD
ALKALINITY
TEST
FREQUENCY
l/D
1/0
2/W
2/W
3/W
3/W
ID
(1)
ID
2/W
LOCATION OF
SAMPLE
p
p
I
DS
1
DS
P
p
S
S
S
P
o.
0
G
G
G
G
G
G
G
G
G
G
REASON
FOR TEST
H
H
H
H
P
H
H
H
p(2)
H
-~TCS
SUPERNATANT
"INFLUENT
SLUDGE
A. TEST FREQUENCY
DIGESTED SLUDGE
D
W
DAY
WEEK
B. LOCATION OF SAMPLE
I INFLUENT
DS • DIGESTED SLUDGE
S - SUPERNATANT
P • PROCESS
C. METHOD OF SAMPLE
G • GRAB SAMPLE
D. REASON FOR TEST
H > HISTORICAL KNOWLEDGE
P PROCESS CONTROL
FOOTNOTES:
1 WHEN DRAW OFF SUPERNATANT.
2. FOR CONTROL OF PROCESS RECEIVING
THIS FLOW.
IV-5
-------
NORMAL OPERATING PROCEDURES
Startup
1. Open digester influent valve or gate and begin filling
digester.
2. When diffusers are covered start the air blowers. If
mechanical aeration is used, start when the appropriate
liquid level is reached.
Routine Operations
Aerobic digestion is, for the most part, a self-regulat-
ing process. The exception is when the process is overloaded
or the equipment is inoperative.
1. Inspect system twice per shift.
2. Take samples as outlined in MONITORING Section.
3. Aerobic digesters may be operated in continuous or batch
flow modes.
a. Continuous flow, like the operation of a convention-
al activated sludge aeration tank and as shown in
Figure IV-1. A portion of the digested solids are
recycled and a portion are removed.
b. Batch flow, where the digester is operated accord-
ing to the procedure in the following paragraph.
4. The normal operating procedures for a batch flow
digester are as follows:
a. Fill digester and aerate for the time outlined
under CONTROL CONSIDERATIONS.
b. Turn off aeration equipment and allow the solids
to settle. This solid-liquid separation should be
limited to three or four hours to avoid clogging
of the air diffuser equipment.
c. Remove as much supernatant as possible. Sample as
outlined in MONITORING Section.
d. Remove the thickened, digested sludge. Sample as
outlined in MONITORING Section.
e. Add new sludge to the digester.
f. Turn on aeration equipment.
IV-6
-------
Shutdown
1. Shut off aeration.
2. Decant as much supernatant as possible.
3. Draw off the thickened sludge.
4. Wash down tank and aerators.
5. Drain the digester of its final contents.
CONTROL CONSIDERATIONS
Physical Control
dissolved
oxygen
batch
feed
continuous
feed
In most plants the aerobic digester is operated as a
self-regulating process with very little process control
required.
If the digester is equipped with a dissolved oxygen (DO)
meter, the aerators should be adjusted so that the DO level is
maintained between 1 and 2 mg/1 for efficient operation. It
has also been found that the digested sludge dewaters best
if the DO level is maintained within this range.
For batch-feed digesters, sludge should be added,
relatively uniform amounts daily, if possible. The volume
and concentration of sludge added each day should be as
uniform as possible. Sludge settling and drawoff may be
performed once a week, while sludge addition or feed is
practiced daily. In this way the sludge volume in the
digester will increase each day until the next decanting and
drawoff period.
The rate of sludge return from the settling basin to
the aeration basin of continuous feed digesters must be
adjusted in the same manner as for activated sludge treatment.
This return flow should be between 20 and 50 percent of the
sludge flow to the aeration basin.
Process Control
mixing
inspection
Mixing is very important in the operation of aerobic di-
gesters. The solids must be well mixed to provide contact be-
tween the organisms and the food supply. Mixing is usually ac-
complished by the aeration system, however, mechanical mixers
or mechanical aerators operating at lower power requirements
may be used to help in mixing. In many cases, floating aera-
tors are used because the operational water level in the
digester varies from time to time.
The digester should be inspected once per shift for
proper operation of aeration equipment and pumps. The con-
tents of the tank should be well mixed and relatively free of
odors.
IV-7
-------
odors
sampling
analysis
temperature
The aerobic digester should not produce detectable odors.
Odors will be produced if the sludge becomes septic. This
indicates poor aeration and/or poor mixing.
Sampling should be performed as outlined under
MONITORING. These samples may be obtained through valves
provided in the digester piping. If sampling points are not
provided, it may be necessary to obtain samples directly from
the digester contents.
Samples should be analyzed according to procedures
specified in Standard Methods.
The two major variables that effect the rate of aerobic
digestion are temperature, and solids retention time.
The rate of a biological reaction will increase as the
temperature increases. A rule of thumb is that the reaction
rate doubles for each 10 C rise in temperature. Although
this beneficial temperature effect has been observed in many
bench studies, actual aerobic digestion plant experience has
not supported fully this rule of thumb. Because of long
detention times and tank sizes, aerobic digestion is satis-
factory at most ambient temperatures. However, the energy
released by the process can cause temperatures to rise if the
aerobic digester is covered.
The solids retention time (SRT) is defined as the average
length of time that the solids are retained in the process.
For continuous feed systems this is:
total mass of solids in digester
mass of solids wasted/day
For a batch feed digester this is:
average mass of the solids in digester during batch
(mass of solids wasted from batch)(number of days in batch)
Some recommended SRT values are given below for operation
at 20°C.
Sludge type SRT, days
Activated only 12-16
Activated with no
primary settling 16-18
Primary plus activated or
trickling filter sludge 18-22
IV-8
-------
Carbon dioxide and the nitrate ion, two products of
aerobic digestion, tend to lower the pH of the digester. The
pH decrease depends on the stability of the bacteria and the
buffering capacity of the water which vary for each treatment
plant situation. In some cases, the decline in pH may be
enough that readjustment of pH is necessary. If readjustment
is necessary, an alkaline liquid or slurry such as sodium
hydroxide, sodium bicarbonate, or lime can be added to the
digester as required. Removing the CC>2 from the gas above a
closed top digester may also help to reduce the drop in pH.
EMERGENCY OPERATING PROCEDURES
Loss of Power
Short power interruptions should not greatly affect the
aerobic digestion process. Although electrical equipment
will not operate, the digestion process will be satisfactory
if power is regained within about 30 minutes to several hours.
If power is unavailable for longer periods, septic conditions
may develop. Septic odors can be overcome by adding chlorine,
however, this will affect the digestion process.
Loss of Other Treatment Units
Most sludges are thickened prior to aerobic digestion.
The loss of the thickener means that a more dilute sludge
(more water) will be sent to the digester. This may be
partially overcome by decanting supernatant from the
digester more frequently.
The loss of other processes to which the digested solids
are pumped may create a solids storage problem. The sludge
may be left in the digester for a few days longer than the
required retention time, however, in case of a prolonged
problem it may be necessary to haul sludge to another treat-
ment facility or disposal area.
COMMON DESIGN SHORTCOMINGS
Shortcoming Solution
1. No provisions 1. Install system to feed sodium
for pH adjustment bicarbonate to digester influent
and low pH occurs in or alkaline materials such as
aerobic digester. sodium hydroxide or lime to
digester.
2. Air diffusers plug 2. Replace diffusers with a type
frequently. with larger openings. This may
require additional blower
capacity due to lower oxygen
IV-9
-------
Shortcoming Solution
2. continued 2. transfer efficiency.
Install mechanical aeration
equipment.
3. Solids depositing 3. Increase mixing in digester
and accumulating by increasing aeration or
in digester due mixing.
to marginal mixing
capabilities.
IV-10
-------
TROUBLESHOOTING GUIDE
AEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Excessive foaming.
la. Organic overload.
la. Organic load.
Ib. Excessive aeration.
Ib. Dissolved oxygen.
la. (1) Reduce feed rate
(2) Increase solids in
digester by decanting
and recycling solids.
Ib. Reduce aeration rate.
2. Low dissolved
oxygen.
2a. Diffusers clogging.
2b. Liquid level not
proper for mechan-
ical aeration.
2c. Blower malfunction.
2d. Organic overload.
2a. Decant digester,
withdraw sludge and
inspect diffusers.
2b. Check equipment
specifications.
2c. Air delivery rate,
pipeline pressure,
valving.
2d. (See la)
2a. Clean diffusers or replace with
coarse bubble diffusers or
sock-type devices.
2b. Establish proper liquid level.
2c. Repair pipe leaks, set valves
in proper position, repair
blower.
3. Sludge has objec-
tionable odor.
3a. Inadequate SRT.
3b. Inadequate aeration.
3a. SRT.
3b. DO should exceed
1 mg/1.
3a. (See la).
3b. Increase aeration or reduce
feed rate.
Ice formation
damages mechanical
aerators.
4a. Extended freezing
weather.
4a. Check digester
surface for ice
block information.
4. Break and remove ice before
it causes damage.
pH in digester has
dropped to undesir-
able level (below
6.0-6.5).
5a. Nitrification is
occurring and waste-
water alkalinity is
low.
5a. pH of supernatant.
5a. Add sodium bicarbonate to
feed sludge or lime or sodium
hydroxide to digester.
-------
TROUBLESHOOTING GUIDE
AEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5b. In covered digester
CO2 is accumulating
in air space and is
dissolving into
sludge.
5b. Vent and scrub the digester
gas.
-------
MAINTENANCE CONSIDERATIONS
The maintenance program for the aerobic digester is
very similar to the program for the activated sludge process.
Mechanical equipment requiring regular attention includes the
aeration system, the mixing and the pumping equipment.
Air diffusers and tanks should be scheduled for inspec-
tion at least once a year. It is common for certain types of
aera ion fine bubble diffusers to clog over a period of time.
equipmen Scheduled draining of the tank(s) for inspection and service
should be done in the summer if possible.
Mixing, pumping, and blower equipment should be inspected
annually for worn blades and impellers. Seals, packing, and
ec , bearings should be inspected and serviced as recommended in
" the manufacturer's service manual. Air filters should be
serviced at regular intervals.
SAFETY CONSIDERATIONS
The aerobic digestion equipment presents no special
hazards, however, general safety considerations should apply.
At least two persons should be present when working in areas
not protected by handrails. Walkways and work areas should
be kept free of grease, oil, leaves and snow. Protective
guards and covers must be in place unless mechanical/
electrical equipment is locked out of operation.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater. Americal Public Health Association,
1015 Eighteenth Street, N.W., Washington, D.C, 20036.
2. WPCF Manual of Practice No. 17. (WPCF MOP No. 17),
Paints and Protective Coatings for Wastewater Treatment
Facilities.
3. WPCF Manual of Practice No. 11, Chapter 19. Operation
of Wastewater Treatment Plants, Aerobic Digestion.
Glossary of Terms and Sample Calculations
1. Sludge Concentration is the weight of solids per
unit weight of sludge. It can be calculated in percent
as follows:
„ . . weight of dry sludge solids n ...
Concentration = 2-^-; -^ r-2^ x 100
weight of wet sludge
IV-13
-------
2. Solids Retention Time (SRT) is the average time that
the solids remain in the process. For continuous feed
systems :
Total mass of solids in digester
. _ i --— — . . _ — . .-*
mass of solids wasted/day
For batch feed systems:
SRT =
_ average mass of the solids in digester during batch
(mass of solids wasted from batch) (number of days in batch)
As an example assume the following data:
Continuous feed :
Tank volume = 65,000 gal Solids =2.5%
Wasting rate = 2000 gal/day Solids = 5.0%
65,000 x 0.025
SRT = 2,000 x 0.05 = 16'3 days
Batch feed with sludge settling and drawof f once per
week:
Sludge volume in digester at beginning
of week: 40,000 gal
Sludge volume in digester at end
of week: 65,000 gal
Solids =2.5%
Total of supernatant and settled
sludge drawof f: 25,000 gal
Number of days in batch = 7
Average volume of sludge in digester =
40,000 + 65,000
- - - - = 52,500
2
52,500 x 0.025
25,000 x 0.025
7 = 15
3- Supernatant is the clarified liquid which forms above the
sludge layer during the settling process. The super-
natant is decanted from the aerobic digester and
returned to the plant.
IV-14
-------
THERMAL TREATMENT
-------
CONTENTS
Process Description V-l
Typical Design Criteria and Performance ........ V-3
Staffing Requirements V-4
Monitoring V-5
Normal Operating Procedures . ..... V-6
Cold Startup V-6
Hot Startup V-6
Routine Operations « V-7
Cold Shutdown V-8
Hot Shutdown ........... V-9
Control Considerations V-9
Physical Control . V-9
Process Control V-9
Emergency Operating Procedures ..... V-ll
Loss of Power ........... ........ V-ll
Loss of Other Treatment Units ......... ... V-ll
Common Design Shortcomings ........ ......... V-12
Troubleshooting Guide .......... .... V-13
Maintenance Considerations ...... ... V-19
Safety Considerations .................... V-20
Reference Material . V-21
References .......... V-21
Glossary of Terms and Sample Calculations . ...... V-21
-------
PROCESS DESCRIPTION
types
There are two basic processes for thermal treatment of
sludges. One, wet air oxidation, is the flameless oxidation
of sludges at temperatures of 450 to 550 F and pressures of
about 1200 psig. The other type, heat treatment, is similar,
but carried out at temperatures of 350 to 400 F and pressures
of 150 to 300 psig. Wet air oxidation reduces the sludge to
an ash and heat treatment improves the dewaterability of the
sludge. The lower temperature and pressure heat treatment is
more widely used than the oxidation process. The two pro-
cesses are similar and this manual covers both.
When the organic sludge is heated, heat causes water to
escape from the sludge. Thermal treatment systems release
water that is bound within the cell structure of the sludge
and thereby improves the dewatering and thickening character-
istics of the sludge. The oxidation process further reduces
the sludge to ash by wet incineration (oxidation). A typical
heat treatment process is shown in Figure V-l (see following
page). Sludge is ground to a controlled particle size and
heat pumped to a pressure of about 300 psi. Compressed air is
treatment added to the sludge (wet air oxidation only), the mixture is
process brought to a temperature of about 350 F by heat exchange with
treated sludge and direct steam injection, and then is pro-
cessed (cooked) in the reactor at the desired temperature and
pressure. The hot treated sludge is cooled by heat exchange
with the incoming sludge. The treated sludge is settled from
the supernatant before the dewatering step. Gases released
at the separation step are passed through a catalytic after-
burner at 650 to 705 F or deodorized by other means. In some
cases these gases have been returned through the diffused
air system in the aeration basins for deodorization.
wet
air
oxidation
process
The same basis process is used for wet air oxidation of
sludge by operating at higher temperatures (450 to 640 F) and
higher pressures (1200 to 1600 psig). The wet air oxidation
(WAO) process is based on the fact that any substance
capable of burning can be oxidized in the presence of water
at temperatures between 250 F and 700 F. Wet air oxidation
does not require preliminary dewatering or drying as required
by conventional air combustion processes. However, the
oxidized ash must be separated from the water by vacuum
filtration, centrifugation, or some other solids separation
technique.
V-l
-------
i
to
1
Sludge-E^-QXH^
GRINDER
AIR COMPRESSOR
TO INCINERATOR
GROUND
SLUDGE
HOLDING
TANK
HEAT
EXCHANGER
St
PUMP
1
POSITIVE
DISPLACEMENT
SLUDGE PUMP
OXIDIZED
SLUDGE
TANK
REACTOR
Exhaust Gas
PRESSURE
CONTROL
VALVE
VAPOR
COMBUSTION
UNIT
FILTER
PUMP
Treated
Boi
Water
ated f\
•ler -*J >
'ter I I
BOILER
Figure V-l. Thermal treatment system schematic.
-------
advantages
disadvantages
An advantage of thermal treatment is that a more readily
dewaterable sludge is produced than with chemical condition-
ing. Dewatered sludge solids of 30 to 40 percent (as opposed
to 15 to 20 percent with chemical conditioning) have been
achieved with heat treated sludge at relatively high loading
rates on the dewatering equipment (2 to 3 times the rates with
chemical conditioning). The process also provides effective
disinfection of the sludge.
Unfortunately, the heat treatment process ruptures the
cell walls of biological organisms, releasing not only the
water but some bound organic material; returns to solution
some organic material previously converted to particulate
form; and creates other fine particulate matter. The break-
down of the biological cells as a result of heat treatment
converts these previously particulate cells back to water and
fine solids. This aids the dewatering process, but creates a
separate problem of treating this highly polluted liquid from
the cells. Treatment of this water or liquor requires care-
ful consideration in design of the plant because the organic
content of the liquor can be extremely high.
TYPICAL DESIGN CRITERIA & PERFORMANCE
Thermal treatment units are sized based on the antici-
pated sludge flow rate (gpm). The flow rate determines the
detention time in the heat exchanger(s) which is typically
30 to 60 minutes.
The terms used to categorize the degree of wet oxidation
- low oxidation, intermediate oxidation, and high oxidation -
refer to the degree of reduction in the chemical oxidation
demand (COD) of the sludge. Higher temperatures are required
to effect higher degrees of oxidation, and the higher temp-
eratures, in turn, require the use of correspondingly higher
pressures in order to prevent flashing to steam or burning.
The operating temperature and pressure ranges for the
three oxidation categories are given below:
Oxidation
category
Low
Intermediate
High
COD reduction,
percent
5
40
92-98
Temp.,
°F
350-400
450
675
Pressure,
psi
300-500
750
1,650
With high oxidation the amount of sludge ash is about
the same as with air incineration.
V-3
-------
STAFFING REQUIREMENTS
Manpower estimates for thermal treatment are shown in
Table V-l, and are broken down into operation and maintenance
requirements. Operation includes time spent reading and
logging process data, controlling and adjusting the various
systems and components, and laboratory work. Maintenance
includes cleaning and repairing process components, general
upkeep of the process area, checking and repairing of controls
and instrumentation, and performing preventative maintenance.
In some plants these operation and maintenance functions may
vary or may overlap.
Labor requirements for major overhaul work such as
reactor cleaning; pipe and tube replacement; pump, compressor
and boiler working parts replacement and other similar items
are not included. For this type of work, except in large
plants, the skills of contracted specialists would normally
be utilized.
TABLE V-l. THERMAL TREATMENT LABOR REQUIREMENTS
Thermal treatment Labor, hr/yr
capacity, GPM Operation Maintenance Total
5
10
20
50
100
200
400
4,600
5,100
6,000
8,200
11,000
16,000
22,000
1,200
1,300
1,400
1,900
2,300
3,200
4,300
5,800
6,400
7,400
10,100
13,300
19,200
26,300
V-4
-------
MONITORING
TOTAL SOLIDS
s
3 TEMPERATURE
2
z
| pH
£ SUSPENDED
[2 SOLIDS
D BOD
FLOW
N
co
1—
2 Q
< O
1 «^
Q. —
ALL
ALL
ALL
ALL
ALL
ALL
O
2;
UJ
D
1-0
co UJ
uj or
h- u.
WD
Mn
1/D
1/D
2/W
R
u.
O
-z.
— UJ
-------
NORMAL OPERATING PROCEDURES
General
Cold Startup
Hot Startup
The procedures for starting heat treatment equipment
vary somewhat depending on the equipment manufacturer and the
other treatment at the plant. One characteristic common to
all heat treatment units is that the operating procedures are
sequential and must be systematically followed with no step
omitted. It is recommended that the operator read and under-
stand the various instruction manuals supplied with the equip-
ment at his plant before attempting any operational procedure.
With this in mind, the following generalized instructions are
only a guideline. A manufacturer's representative should
always be present during the initial process startup.
This procedure is used when the system is cold, drained
and depressurized.
1. Review cold startup valve positioning checklist.
2. Review cold startup instrumentation setpoint checklist.
3. Start high pressure pump (at specified flow rate) and
operate on water.
4. Start process air compressor.
5. Pressurize the system.
6. Start the boiler (steam supply).
7. Start grinder.
8. Switch pumping from water to sludge.
9. Increase sludge flow to full rate.
10. Prepare scrubber and fume incinerator.
11. Start the rake mechanism in the oxidized sludge
storage tank.
12. Start the treated sludge dewatering system.
This procedure is used when the reactor is filled with
hot sludge and pressurized (commonly called a "bottled"
V-6
-------
reactor), and the balance of the system is depressurized and
cool.
1. Review hot startup valve positioning checklist.
2. Review hot startup instrumentation setpoint checklist.
3. Prepare scrubber and fume incinerator.
4. Start the rake mechanism in the oxidized sludge storage
tank.
5. Start the high pressure pump (at specified flow rate)
and operate on water.
6. Start process air compressor.
7. Pressurize the system.
8. Start the boiler.
9. Unbottle the reactor.
10. Start grinder.
11. Switch from water to sludge.
12. Increase sludge flow to full rate.
13. Start the treated sludge dewatering system.
Routine Operations
Thermal treatment systems should always have an operator
in attendance when they are running. The lead operator should
be machinery oriented and able to do routine preventive
maintenance.
Each hour, the operator should:
1. Record all instrument readings on log sheet. Compare
with previous readings and investigate any unexplained
changes. Temperature or pressure deviations above or
below setpoint may be the first indication of trouble.
2. Adjust pumping system as necessary to maintain desired
sludge flow rate.
3. Adjust oxidation system as necessary to maintain
desired temperatures.
4. Examine each operating piece of equipment. Check
V-7
-------
lubrication, cooling water, operating temperatures,
leakage, sound, and vibration. Any unit which appears
to be operating abnormally should be closely watched and
the cause of the abnormal operation determined and
corrected without delay. The operator should not hesi-
tate to shut down the system if operating irregularities
persist without obvious cause, or become progressively
worse.
5. Take samples as required.
Cold Shutdown
This procedure is used to remove the system from
service where all components are to be depressurized and cool-
ed including the reactor.
1. Switch from sludge to water.
2. Close steam block valve to reactor.
3. Shut down the boiler.
4. Clean pressure control valves.
5. Reduce system pressure.
6. Bottle the reactor.
7. Depressurize the heat exchanger(s).
8. Blow down the reactor.
9. Shut down the high pressure pump.
10. Pressurize the reactor.
11. Shut down the air compressor.
12. Blow down the reactor (second blowdown).
13. Shut down the treated sludge dewatering equipment.
14. Shut down the rake mechanism in the oxidized sludge
storage tank.
15. Shut down the scrubber-fume incinerator.
16. Shut down the instrument air compressor and air dryer.
V-8
-------
Hot Shutdown
This procedure is used to remove the system from service,
but maintains the reactor in a bottled condition (filled and
pressurized) which simplifies the startup procedure.
1. Switch from sludge to water.
2. Reduce reactor pressure.
3. Bottle the reactor.
4. Shut down the boiler.
5. Raise system pressure.
6. Clean the pressure control valves.
7. Backwash downcomber line.
8. Shut down the process air compressor.
9. Shut down the high pressure pump.
10. Depressurize the system.
11. Shut down the treated sludge dewatering equipment.
12. Shut down the rake mechanism in the oxidized sludge
storage tank.
13. Shut down the scrubber-fume incinerator.
14. Shut down the instrument air compressor and dryer.
CONTROL CONSIDERATIONS
Physical Control
Four important physical variables control the performance
of wet oxidation units: temperature, air supply, pressure,
and feed solids concentration. Controls are normally pro-
vided for controlling reactor temperature, pressure, and
the air supply.
Process Control
The extent and rate of sludge solids oxidation are
determined by the reactor pressure and temperature. Much
higher degrees of oxidation and shorter reaction times are
possible at higher pressures and temperatures. The reactor
temperature and pressure affect the quality of the recycle
V-9
-------
temperature
air
flow
water (liquor) and the dewaterability of the oxidized sludge.
Reaction temperature should be kept as low as possible,
consistent with adequate conditioning of the sludge. Higher
temperatures cause more complete breakdown of the sludge
particles, releasing more cell water and thus releasing more
BOD into solution. Higher temperatures do provide a treated
sludge which dewaters readily, but at great sacrifice because
of the poorer quality of the recycle liquor.
As with conventional incinerators, an external supply
of oxygen (air) is required to attain nearly complete oxida-
tion. The air requirement for the wet oxidation process is
determined by the heat value of the sludge being oxidized,
and by the degree of oxidation desired. Thermal efficiency
and fuel requirements are functions of air input, so it is
important that the air flow not be higher than needed.
Because the input air becomes saturated with steam from con-
tact with the liquid in the reactor, it is also important to
control the air flow to prevent excessive loss of water from
the reactor.
holding
time
Increasing the holding time in the thermal reactor will
increase the breakdown of the sludge cells and degrade the
fibrous material. The effect is that the quality of the
recycle water will be poorer and the treated sludge will not
dewater as well. For example, in low oxidation at 350 to
400 F, the color of the recycle liquor increases from 2,150
units for a reaction time of 3 minutes, to 3,800 units at 15
minutes, to 5,500 units at 30 minutes.
The recycle liquor can be very difficult to treat,
offensive smelling, and can upset plant treatment processes;
therefore it must be considered carefully in operating thermal
treatment processes. Typical recycle liquor characteristics
are as follows.
recycle
liquor
Substances in
strong liquor
TSS
COD
BOD
NH3-N
Phosphorus
Color
Concentration range,
mg/1 (except as shown)
100 - 20,000
100 - 17,000
3,000 - 15,000
400 - 1,700
20 - 150
1,000 - 6,000 units
These high concentrations illustrate the potential
impact that recycle of the liquor can have on the wastewater
treatment processes. It is important that the operator rec-
ognize the significance of the recycle load in the management
of the overall plant operation. *
V-10
-------
An equal degree of filterability and settleability can,
within limits, be accomplished by various combinations of
time and temperature. For instance high temperature and short
time reaction time as compared to lower temperature and longer
and reaction time. Longer reaction time at low temperature treat-
temperature ment is usually the most economical. Overcooking (various
combinations of high temperatures and long reaction times)
actually breaks down the fibrous material itself (as compared
to simply releasing the cell water) and produces a more
difficult to dewater treated sludge.
The pH at which sludges are heat treated has an effect
on the dewaterability of the treated sludge. Treatment at
pH lower pH produces a more dewaterable treated sludge, but
corrosion problems are increased.
Cooling of heat treated sludges prior to atmospheric
exposure can reduce, but will not eliminate odor problems.
other Increasing the solids content of the sludge feed to the heat
consider- treatment process decreases operating costs, but increases
ations the content of dissolved COD, nitrogen, and phosphorus in the
recycle liquor and may reduce the dewaterability of the
treated sludge.
EMERGENCY OPERATING PROCEDURES
Loss of Power
In the event of a prolonged failure or one of undeter-
minable duration, the following procedure should be used.
Isolate (bottle) the reactor. Immediately thereafter,
reduce the pressure slowly with the pressure control valve
to transfer as much of the contents of the heat exchangers
as possible to the decant tank. This is to prevent blockage
from solids that might bake on the sides of the heat exchang-
er tubes or settle into the "U" bends.
The reason for doing this very soon after the power
failure is that the pressure in the instrument air receiver
will dissipate slowly and will be adequate for performing
this shutdown procedure for only a limited time until the air
pressure is lost.
Loss of Other Treatment Units
The loss of other treatment units should not greatly
affect the operation of the thermal treatment unit. Perform-
ance, however, may be affected if the incoming solids con-
centration changes.
V-ll
-------
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Effects of recycled
liquors on waste-
water treatment
process were not
adequately con-
sidered and plant
is upset.
2. Lack of or inad-
equate equipment
installed for
deodorization of
off-gases from
decant tanks,
thickeners, or
dewatering system.
3. Backup support
systems (boiler,
feed pumps,
grinders, air
compressors, etc)
not provided.
4. High temperatures
and presence of
calcium, sulfates,
or chlorides in the
sludge creates ex-
cessive scaling &
corrosion in heat
exchangers & reac-
tion vessels, and
piping.
Solution
la. Store liquors and recycle
during low flow night
time conditions.
Ib. Install separate treatment
system for liquors before
they are recycled (review
with consultant).
2a. Temporary solutions may
include addition of hydrogen
peroxide to open tanks or use
of masking chemicals.
2b. Install adequate deodorization
equipment (review with
consultant).
2c. Collect these off gases and
pipe back to the diffused air
system in the aeration basins.
3. Install backup components.
4. Use 316 stainless steel
or Titanium for materials
of construction.
V-12
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
1 . Odors
2. Raw sludge grinder
requires very fre-
quent maintenance.
3. Scaling of heat
exchangers.
4. Heat treatment
system down time
is substantial.
PROBABLE CAUSE
la. Odors being re-
leased in decant
tanks , thickeners ,
vacuum pump exhaust
or in dewatering.
lb. Odors being re-
leased when
recycle liquors
enter wastewater
treatment tanks.
2. Excessive grit in
raw sludge .
3a. Calcium sulfate
deposits.
3b. Operating tempera-
tures too high -
causing baking of
solids.
4. Inadequate operation
& maintenance
skills.
CHECK OR MONITOR
2. Operation of raw
sludge degritting
system and raw sew-
age grit removal .
3a. Efficiency of heat
transfer - difficult
to maintain reactor
temperatures .
SOLUTIONS
la. (1) Cover units, collect air
and deodorize it before
release by use of inciner-
ation, adsorption, or
scrubbing .
(.2) Cover open tank surface
with small floating plas-
tic balls to reduce
evaporation and odor loss .
lb. Pre-aerate liquors in covered
tank and deodorize off- gases.
2 . Maintain and properly operate
the raw sludge and raw sewage
degritting systems.
3a. Provide acid wash, in accordance
with manufacturer's instruc-
tions.
3b. Operate reactor at temperatures
below 390°F for heat condition-
ing of sludge.
3c. Use hydraulically driven
cleaning bullet to clean
inner tubes.
4. Contract for maintenance of
system & institute training
program for operators.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. Grinder has shut
down.
5a. Loss of seal water.
5b. Grinder has jammed.
5a. Seal water supply -
are valves open.
5b. Is grinder motor
reversing automati-
cally when over-
loaded.
5a. Establish flow of seal water.
5b. Remove obstruction.
6. Feed pumps are
overheating.
6a. Inadequate lubrica-
tion.
6b. Cooling water supply
inadequate.
6a. Oil levels.
6b. Cooling water.
6a. Lubricate pumps.
6b. Establish adequate flow of
cooling water.
7. Steam use is high.
7. Sludge concentra-
tion to heat treat-
ment unit is low.
7. Sludge concentra-
tion.
7. Operate thickener to maintain
6 percent solids if possible;
3 percent minimum.
8. Solids dewater
poorly.
8a. Anaerobic digestion
prior to heat
treatment.
8b. Temperatures not
maintained high
enough.
8b. Reactor tempera-
tures.
8a. Discontinue anaerobic digestion
of sludge to be heat treated.
8b. Temperature should be at least
35CPF.
9. High system
pressure.
9a. Blockage in
reactor.
9a. (1)
(2)
If relief
valves are
blowing, shut
down unit.
If relief
valves are not
blowing,
blockage was
temporary.
9a. (1) Remove blockage.
(2) Check pressures and tem-
peratures to note any
discrepancies from normal
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
9b. Pressure controller
set too high.
9c. Block valve closed.
9b. Pressure controller
setting.
9c. Block valve.
9b. Reduce set point on pressure
controller.
9c. Check system for proper
valving.
10. Feed pumps not
pumping adequate
flow.
lOa. Improper control
setting.
lOb. Leakage or plugging
in product check
valves.
lOc. Air trapped in
pump cylinders.
lOa. Control setting.
lOb. Inspect check valves
lOa. Adjust control setting.
lOb. Repair or replace check valves.
lOc. Bleed off air.
11. System pressure
is dropping.
lla. Pressure controller
set too low.
lib. Pressure control
valve trim is
eroded.
lla. Setting on pressure
controller.
lib. Inspect valve.
lla. Set pressure controller at
proper valve,
lib. Replace valve.
12. Oxidation tempera-
ture is rising.
12a. Inlet temperature
too high.
12b. Sludge feed rate
is too slow.
12c. Improper control
setting.
12d. Pump stopped or
slowed.
12a. Should not exceed
310°F for sludge
conditioning.
12b. Operation of sludge
feed pumps and feed
rate.
12c. Temperature control.
12d. Pump operation.
12a. Reduce temperature by diluting
incoming sludge with water.
12b. Increase sludge feed rate.
12c, Appropriately adjust control
setting.
12d. Start pump and/or increase
rate.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
12e. Volatile matter
such as gas or oil
being pumped
through the system.
12f. Pneumatic steam
valve not function-
ing properly.
12 f. Valve operation.
12e. Switch from sludge to water
and stop the process air
compressor.
12f. Repair malfunctioning valve.
13. Oxidation temper-
ature is falling.
CTi
13a. Heat exchanger
fouled.
13b. Reactor inlet temp-
erature is too low,
because of low
density sludge.
13c. High flow rate
being pumped
through system.
13d. Improper tempera-
ture control
setting.
13e. Pneumatic steam
valve not function-
ing properly.
13f. No signal air to
the temperature
control valve.
13g. Boiler not func-
tioning properly.
(see item 3)
13b. Should be at least
280°F.
13c. System flow rate.
13d. Temperature control
setting.
13e. Steam valve.
13g. Boiler operation.
13b. Reduce dilution of incoming
sludge.
13c. Reduce flow rate at high
pressure pump(s).
13d. Appropriately adjust.
13e. Repair malfunctioning valve.
13f. Check instrument air supply.
13g. Consult boiler manufacturer's
instruction manual for correc-
tive action.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
14. Scoring of air
compressor cylin-
der walls and
pistons.
14a. Carbon or other
foreign material
in compression
cylinder.
14a. Visual inspection.
14a, Maintain compressor system
to avoid material from
entering system.
15. Filter cake dif-
ficult to feed
into incinerator.
15a. Filter cake too
dry.
15a. Reduce temperature (and
pressure) of the treatment
system.
16.
Low system
pressure.
16a. High pressure pump
and/or process air
compressor and/or
boiler stopped.
16b. Intake filter
clogged.
16c. Pressure controller
set too low.
16d. Any of the blowdown
valves may be
partially opened.
16e. Leaking interstage
trap.
16f. Slipping drive
belts.
16b. Inspect filter for
clogging.
16c. Pressure controller
setting.
16d. Valves.
16f,
Drive belt
slippage.
16b. Clean or replace filter.
16c. Increase set point on pressure
controller.
16d. Check compressor valving.
16e. Check trap for proper opera-
tion.
16f. Adjust belt tension.
17. High temperature.
17a. Inadequate water
flow.
17b. Leaking cylinder
valves.
17a. Water flow.
17b. Cylinder valves.
17a. Adjust water flow.
17b. Repair and/or clean or replace.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
17c. Intercooler and/or
jackets plugged.
17d. No flow from the
force feed
lubricators.
17c. Visual inspection.
17d. (1) Low oil level.
(2) Malfunctioning
lubricator.
(3) Loose or worn
belt.
17c. Clean intercooler and/or
replace.
17d. (1) Add oil.
(2) Repair lubricator.
(3) Tighten loose belt, or
replace if worn.
18. Air compressor
safety valve
receiving.
M
00
18a. Pressure controller
set too high.
18b. No signal air
pressure to PCVs.
18c. One or more block
valves in the
system are closed.
18d. Plugged pressure
control valve (PCV)
18a. All system pressures
appear high.
18c. Valves closed.
18d. Visual inspection.
18a. Reduce set point on controller.
18b. Check instrument air supply.
18c. Check system for proper
valving.
18d. Switch to standby PCV and
clean plugged valve.
-------
MAINTENANCE CONSIDERATIONS
component
maintenance
system
maintenance
Thermal treatment unit maintenance must consider both
components and system. Component considerations consist of
routine inspection, preventative maintenance, and lubrication
schedules. The manual supplied with the equipment should be
consulted for this information. It is suggested that records
be kept to determine whether the maintenance program is being
followed. These records should include the inspection dates
and service performed.
System maintenance consists of a set of routine cleaning
procedures for removing scale buildup from system components
and piping. This usually involves periodic washdown with
5 percent solution of nitric acid in water. System components
requiring this type of cleaning usually include the heat
exchanger, the reactor, and the oxidized sludge decant tank.
This maintenance must be performed on a regular schedule to
maintain satisfactory operation. Instructions for this type
of cleaning are specific to the thermal treatment system in
question and depend on valving, piping and type of system
components.
An example of a cleaning sequence for a heat exchanger
is shown below:
1. Review heat exchanger cleaning valve positioning
checklist.
2. Start the solvent pump.
3. Flush the heat exchangers with water.
4. Fill the solvent tank.
5. Start the boiler.
6. Heat the water in the solvent tank.
7. Shut down the boiler.
8. Add nitric acid to the solvent tank.
9. Circulate the acid solution.
10. Dispose of the acid, generally, it must be neutralized.
11. Stop the solvent pump.
12. Start the oxidation unit as per hot startup instructions.
V-19
-------
The need for heat exchanger cleaning is indicated by
heat an increasing temperature differential between the reactor
exchanger inlet and outlet, and an increasing pressure drop through
cleaning the system.
The need for cleaning of piping is usually determined
by opening the pipe at regular intervals and performing visual
pipe inspections. One manufacturer recommends a solvent wash when
cleaning the scale buildup exceeds 1/8 inch. Pipeline cleaning pigs
may also be effective for some cleaning applications.
In addition to the above routine cleaning, a thorough
check for scale buildup inside the reactor should be
accomplished annually. If a scale problem is evident and
reactor the acid cleaning procedures are ineffective, it may be
cleaning necessary to remove the scale mechanically. This may be
accomplished by an air-driven turbine cutting tool or a high
pressure water blast. Local companies are usually available
with the equipment necessary for this type of cleaning.
An annual pressure check, usually a hydrostatic test,
pressure following the manufacturer's instructions, should be
test accomplished to insure the integrity of the pressure piping
and fittings.
SAFETY CONSIDERATIONS
Safety is an important consideration when operating
thermal treatment processes. Observation of temperatures
and pressures and visual checks of operating machinery are
the most important aspects of safe operation.
1. If at any time during operation the system temperatures
are abnormally high, stop the air compressor and switch
from sludge to water. Abnormally high temperatures are
shown in the manufacturer's manuals.
2. Any inspection or cleaning should be done with that
section of the system completely depressurized. Liquids
under pressure can cause serious harm to personnel if
suddenly discharged.
3. Proper protective equipment should be worn during
inspection and cleaning per manufacturer's recommenda-
tions.
4. Observe proper handling procedures when using acid
solutions for cleaning. Recommended safety procedures
should be obtained from the supplier and implemented
prior to handling of any acid in the plant.
V-20
-------
5.
6.
7.
8.
9.
10.
REFERENCE MATERIAL
References
Vessels should be well ventilated and completely isolated
before entering. Never enter a vessel without a lift
line held by someone outside the vessel and a reliable
source of air inside the tank.
Carbon coatings on high pressure air compressor discharge
valves indicate too much oil is being used to lubricate
the cylinder. Failure to correct this could result in
fires at the discharge of these cylinders.
Motor circuit disconnects should be locked open before
working on any machine.
Belt driven equipment should not be operated without
safety guards.
Follow State and Federal safety codes for this type of
equipment.
Wear proper masks when working around the supernatant
and liquor because of the gases and odors.
Standard Methods for the Examination of Water and Waste-
water. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
WPCF Manual of Practice No. 17 (WPCF MOP No. 17), Paints
and Protective Coatings for Wastewater Treatment
Facilities.
Glossary of Terms and Sample Calculations
1. Recycle liquor or "cooking liquor" is the liquid removed
from the sludge by decanting or thickening. Generally,-
the liquor is odorous and difficult to treat.
2. Off gases are the gases released from various open tanks
in the thermal treatment process. These gases are
odorous and must be collected and treated prior to
discharge to the atmosphere.
3. COD (chemical oxygen demand) is an important, rapidly
measured parameter for determining the oxygen equivalent
of that portion of the organic and inorganic matter in a
sample that is susceptible to oxidation by a strong chem-
ical oxidant. Thus, COD is the oxygen consuming organic
and inorganic matter present in wastewater.
V-21
-------
BOD or biochemical oxygen demand, is the amount of oxygen
required for the biological oxidation of degradable
organic content in a liquid, during a specified time, and
at a specified temperature. Results of the standard test
assessing wastewater strength usually are expressed in
mg/1 as 5-day 20°C BOD.
V-22
-------
VI
LIME TREATMENT
-------
CONTENTS
Process Description VI-1
Typical Design Criteria and Performance VI-1
Staffing Requirements VI-2
Monitoring VI-8
Normal Operating Procedures VI-9
Startup VI-9
Routine Operations VI-9
Shutdown VI-9
Control Considerations VI-9
Physical Control VI-9
Process Control VI-10
Emergency Operating Procedures VI-11
Loss of Power VI-11
Loss of Other Treatment Units VI-11
Common Design Shortcomings VI-11
Troubleshooting Guide VI-12
Maintenance Considerations VI-14
Safety Considerations VI-14
Reference Material VI-14
References VI-14
Glossary of Terms and Sample Calculations VI-14
-------
PROCESS DESCRIPTION
process
operation
supernatant
return to
process
The lime stabilization process can be used to treat
raw primary, waste activated, septage and anaerobically
digested sludges. The process involves mixing a large
enough quantity of lime with the sludge to increase the pH
of the mixture to 12 or more. This normally reduces bacterial
hazards and odor to a negligible value, improves vacuum filter
performance and provides satisfactory means of stabilizing
the sludge prior to ultimate disposal.
Lime slurry is normally added to the sludge in a mixing
tank. Mixing is accomplished by either diffused air or
mechanical agitators. Enough lime is added to increase the
sludge pH to the desired level. After this initial mixing
the lime treated sludge is transferred to a contactor vessel.
Mixing is continued in the contactor and additional lime is
added, if necessary, to maintain the desired pH. The sludge
remains in the contactor for a specified time period,
typically 30 minutes. The treated sludge is thickened and
stored or disposed of immediately.
Differences between various designs may affect operation
at individual plants. For example, the mixing, 30 minute
contact time and thickening required for this process may
all occur in one tank. In general, the operation and main-
tenance suggestions in this section apply to all systems,
however, the operator should note the particular requirements
of the equipment at his plant.
A typical process flowsheet for lime stabilization is
shown in Figure VI-1 (see following page).
Sludge supernatant is usually returned to either the
primary or the secondary treatment process and normally
causes no problem to process operation. The respective
treatment process must be able to handle the increase in
flow resulting from the return of supernatant.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Lime facilities for sludge stabilization are sized on
the basis of the daily sludge volume treated. The amount of
lime used varies but can be estimated using Table VI-1 (see
following page). Lime handling facilities are then based on
the quantity of lime needed for treatment.
VI-1
-------
SLUDGE/
LIME
MIXING
VESSEL
pH MONITOR
AND RECORDER
STABILIZED SLUDGE
TO THICKENER.
STORAGE, OR
IMMEDIATE DISPOSAL
sludge
disposal
Figure VI-1. Lime stabilization process flowsheet.
Mixing requirements for sludge slurries are also an
important design consideration. The level of agitation should
be high enough to keep solids suspended and spread the lime
slurry evenly and rapidly throughout the mixing tank.
Typical mixer requirements are shown in Table VT-2 (see
following pages).
Expected performance results are shown in Tables VI-3,
VI-4, and VI-5 (see following pages) for chemical properties,
bacterial composition and solids concentration of lime
stabilized sludge.
It may be difficult to find suitable disposal sites for
lime treated sludge. This should be considered carefully for
each site.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance includ-
ing unloading of lime and operation and maintenance of the
lime slaker, are shown in Table VI-6 (see following pages).
The requirements are based on pounds per hour of continuous
lime feed. These data were developed from "Costs of Chemical
Clarification of Wastewater", EPA Contract No. 68-03-2186,
final draft, December 1977.
VI-2
-------
TABLE VI-1. LIME REQUIRED FOR STABILIZATION TO pH 12 FOR 30 MINUTES
Sludge type
Primary sludge
Waste activated
sludge
Septage
Anaerobic
Sludge
solids, %
3-6
1-1.5
1-4.5
6-7
Average Ib
Ca (OH) 2/ton
dry solids
240
600
400
380
Range Ib
Ca (OH) 2/ton
dry solids
120- 340
420- 860
180-1,020
280- 500
Total
volume
treated
136,500
42,000
27,500
23,500
Average
total
solids
mg/1
43,276
13,143
27,494
55,345
Average
initial
pH
6.7
7.1
7.3
7.2
Average
final
PH
12.7
12.6
12.7
12.4
Includes some portion of waste activated sludge.
Data in this Table developed from:
1. "Stabilization and Disinfection of Wastewater Treatment Plant Sludges", 'USEPA Technology
Transfer, 1974.
2. "Lime Stabilized Sludge: Its Stability and Effect on Agricultural Land", EPA-670/2-75-012,
April, 1975.
-------
TABLE VI-2. TYPICAL MIXING REQUIREMENTS FOR SLUDGE SLURRIES
Tank
size,
gallons
5,000
15,000
30,000
75,000
100,000
Tank
diameter, Prime mover, hp
feet Shaft speed, rpm
9.6 7.5/125
5/ 84
3/ 56
13.9 20/100
15/ 68
10/ 45
7.5/ 37
17.5 40/ 84
30/ 68
25/ 56
20/ 37
23.75 100/100
75/ 68
60/ 56
50/ 45
26.1 125/ 84
100/ 68
75/ 45
Mixer
diameter,
inches
32
38
43
45
53
63
67
57
61
66
81
62
74
79
87
72
78
94
Data in this Table developed from "Stabilization and Disinfection of
Wastewater Treatment Plant Sludges", USEPA Technology Transfer, 1974.
VI-4
-------
TABLE VI-3. CHEMICAL PROPERTIES OF RAW AND LIME STABILIZED SLUDGES
•
Total Soluble Total
Alkalinity, COD, COD, phosphate,
Sludge type mg/1 mg/1 mg/1 mg/1
Raw primary 1,958 54,146 3,046 350
Lime stab, primary 4,313 41,180 3,556 283
Waste activated 1,265 12,810 1,043 218
Lime stab, waste
activated 5,000 14,697 1,618 263
H
1
ui Septage 2,245 24,940 1,223 172
Lime stab, septage 4,305 17,487 1,537 134
Anaerobic digested 3,406 66,372 1,011 580
Lime stab, anaer.
digest 11,400 58,692 1,809 381
Total
Soluble kjeldahl Ammonia
phosphate, nitrogen, nitrogen, Total
mg/1 mg/1 mg/1 solids, %
69 1,656 223 4.5
36 1,374 145 4.9
85 711 38 1.3
25 1,034 53 1.7
25 820 92 2.6
2 597 84 2.7
15 2,731 709 6.9
3 1,980 494 5.8
References same as Table VI-1.
-------
TABLE VI-4. COMPARISON OF BACTERIA IN ANAEROBIC DIGESTED VERSUS LIME STABILIZED SLUDGES
Anaerobically digested
Lime stabilized*
Primary
Waste activated
Septage
Fecal
coliform
/I 00 ml
1,450 x 103
4 x 103
16 x 103
265
Fecal
streptococci
/I 00 ml
270 x 103
23 x 103
61 x 103
665
Total
coliform
/100 ml
3
27,800 x 10
27.6 x 103
212 x 103
2,100
Salmonella
/100 ml
6
3**
3
3
Ps.
aeruginosa
/100 ml
42
3
13
3
* To pH equal to or greater than 12.0
** Detection limit = 3
References same as Table VI-1.
-------
TABLE VI-5. VOLATILE SOLIDS CONCENTRATION OF SLUDGES
Sludge Lime stabilized sludge
volatile solids volatile solids
solids concentration, concentration,
Sludge type mg/1 mg/1 ^__
Primary 73.2 54.4
Waste activated 80.6 54.2
Septage 69.5 50.6
Anaerobically digested 49.6 37.5
References same as Table VI-2.
TABLE VI-6. LIME TREATMENT LABOR REQUIREMENTS
Operation and maintenance
Lime feed, Ib/hr labor, hr/yr
10 966
100 1,183
1,000 1,975
10,000 6,570
VI-7
-------
MONITORING
lime addition
influent >
sludge
t
o
mixing
o
tank
stabilized
sludge to
thickener
Thickener
• supernatant
J
thickened, stabilized
sludge
Total Solids
Suspended Solids
pH
Alkalinity
Flow
Plant Size
(mgd)
All
All
All
All
All
Sample
Frequency
I/day
I/day
Continuous
I/day
Continuous
Sample
Location
Influent
Sludge
Influent
and
Stabilized
Sludge
Mixing
Tank
Influent
Sludge
Influent
Sludge
S amp 1 e
Method
Grab
Grab
Record
Continuously
Grab
Record
Continuously1
Reason for
Test
Process
Control
Process
Control
Process
Control
Process
Control
Process
Control
VI-8
-------
NORMAL OPERATING PROCEDURES
Startup
1. Open influent valve or gate and begin filling sludge
mixer tank.
2. Start up lime feed.
3. Start mixers when sludge level is high enough.
4. Set up pH control monitor.
5. Set up the stabilized sludge thickener controls, if
applicable, and place into operation.
6. Check operation of lime slaking mechanism.
Routine Operations
1. Inspect system twice per shift.
2. Carry out maintenance as required including clean up,
washdown, and lime handling.
Shutdown
3. Take samples as outlined in MONITORING section.
1. Shutdown lime feed system.
2. Close sludge mixing tank influent valve or gate.
3. Drain the system if desired or shutdown sludge pumping.
4. Turn off mixing mechanism when level is below agitators.
CONTROL CONSIDERATIONS
Physical Control
types
Although lime is available in a number of forms, the most
commonly used for sludge stabilization are quicklime and
hydrated lime. Quicklime (unslaked limed) is almost entirely
calcium oxide, (CaO). Quicklime does not react uniformly
when applied directly to sludge, but first must be converted
to the hydrated form, Ca(OH)2- Hydrated or slaked lime is a
powder obtained by adding sufficient water to quicklime to
satisfy its affinity for water.
VI-9
-------
conveying
feeding
mixing
Lime may be conveyed either mechanically by screw
conveyors or bucket conveyors, or pneumatically.
Lime is normally fed as a slurry because of its low
solubility in water. Other advantages of applying lime as a
slurry are that it is transported more readily as a slurry;
better dispersion of the lime in the sludge is accomplished;
preparation of the lime slurry with agitation reduces
the tendency for lime to settle in the treatment vessels.
Mixing is very important in the operation of a lime
stabilization process. The sludge and lime must be well
mixed to insure a uniform mixture. Excess lime is usually
required to compensate for poor mixing.
Process Control
inspection
sampling
The lime stabilization equipment should be checked
several times per shift to assure proper operation of sludge
mixing equipment, lime feeding equipment, pH control and
pumps.
Sampling should be performed as outlined under
MONITORING. These samples may be obtained through valves
provided in the system piping. If sampling points are not
provided, it may be necessary to obtain samples directly from
the tank contents.
analysis
PH
Samples should be analyzed according to procedures
specified in Standard Methods.
The lime stabilization process is mainly controlled by
the pH of the sludge-lime mixture. Lime should be added
continuously until the desired pH level is reached and there-
after, as required to maintain the desired pH. This can be
done monthly 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 affect-
ed by the type of sludge, its chemical makeup and percent
lime dose solids. Therefore, the exact dosage can only be determined
by actual experimentation at the plant.
Mixing time is usually a function of lime slurry feed
mixing rate and is not limited by the mixing capacity of the system.
time Therefore, mixing time is best reduced by increasing the
capacity of the lime slurry tank.
VI-10
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
Short power interruptions should not greatly affect lime
stabilization of sludge. Although electrical equipment will
not operate, the stabilization process will not deteriorate
if power is regained within about 30 minutes to an hour. If
power is unavailable for a longer period of time the pH of the
sludge-lime mixture may begin to fall. Septic conditions may
develop if only a small amount of lime was added before the
power failure occurred. The effect of potential septic con-
ditions can be partially or totally overcome by aerating or
mixing the contents of the system tanks and/or adding lime
or chlorine.
Loss of Other Treatment Units
The loss of other treatment units should not greatly
affect the operation of the lime stabilization process. If
the loss of any process following lime stabilization creates
a solids handling problem it may be necessary to haul sludge
to another treatment facility or disposal area.
COMMON DESIGN SHORTCOMINGS
Shortcoming
Solution
1.
Mechanical equip-
ment fouled with
rags and debris.
2.
3.
4.
Inadequate process
monitoring equip-
ment.
Lime solids settle
out prior to feed
point.
Strong odors
produced during
sludge stabiliza-
tion especially
with diffused air
mixing.
Remove rags and screened
debris from wastewater stream
prior to sludge treatment and
dispose of separately. Do not
run debris through a comminutor
and return debris to the treat-
ment process.
Run frequent manual tests or
install continuous pH monitor-
ing equipment.
Provide mechanical mixers for
dissolving solids and main-
taining them in suspension
prior to delivery to feed point.
Provide adequate ventilation
to dissipate odors created
during mixing. These odorous
gases may include ammonia which
is stripped from the sludge.
VI-11
-------
TROUBLESHOOTING GUIDE
LIME TREATMENT
INDICATORS/OBSERVATIONS
1. Air slaking
occurring during
storage of quicklime.
2 . Feed pump discharge
line clogged.
3. Grit conveyor or
slaker inoperable .
4. Paddle drive on
slaker is overloaded.
5. Lime deposits in
lime slurry feeder.
6. "Downing" or incom-
plete slaking of
quicklime .
PROBABLE CAUSE
la. Adsorption of
moisture from atmo-
sphere when humidity
is high.
2a. Chemical deposits.
3a. Foreign material in
the conveyor.
4a. Lime paste too thick.
4b. Grit or foreign
matter interfering
with paddle action.
5a. Velocity too low.
6a. Too much water is
being added.
CHECK OR MONITOR
la. Moisture in storage
facility from leaks
or humid atmosphere .
2a. Visual inspection.
3a. Broken shear pin.
4a. Visual inspection.
4b. Visual inspection.
6a. Hydrate particles
coarse due to rapid
formation of a
coating.
SOLUTIONS
la. Make storage facilities air-
tight, and do not convey
pneumatically.
2a. Provide sufficient dilution
water .
3a. Replace shear pin and remove
foreign material from grit
conveyor.
4a. Adjust compression on the
spring between gear Reducer
and water control valve to
alter the consistency of the
paste.
4b. Remove grit or foreign
materials , try to obtain lime
with a lower grit content, or
install grit removal facilities
in slaker or slurry line.
5a. Maintain continuously high
velocity by use of a return
line to the slurry holding
tank.
6a. Reduce quantity of water added
to quicklime (detention
slakers-waters to lime ratio
= 3^:1
Paste slaker ratio = 2:1).
H
-------
TROUBLESHOOTING GUIDE
LIME TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. "Burning" during
quicklime slaking.
7a. Insufficient water
being added, result-
ting in excessive
reaction temperature.
7a. Some particles left
unhydrated after
slaking.
7a. Add sufficient water for slak-
ing (See Solution 7).
8. Sludge retains defin-
ite offensive odor
after addition of
lime.
8a. Lime dose too low.
8a. Check pH in sludge-
lime mixing tank to
assure desired level
is reached.
8a. Increase lime dose, check pH
monitor for possible mal-
function.
H
I
-------
MAINTENANCE CONSIDERATIONS
mechanical
A good preventive maintenance program will reduce break-
downs which could be not only costly, but also very unpleasant
for operating personnel. Plant components including the
following should be inspected semiannually for wear,
corrosion, and proper adjustment:
1. Drives and gear reducers
2. Drive chains and sprockets
3. Shaft bearings and bores
4. Bearing brackets
5. Baffles and weirs
6. Electrical contacts in starters and relays
7- Suction lines and sumps
SAFETY CONSIDERATIONS
1.
2.
REFERENCE MATERIAL
The equipment for lime stabilization of sludge presents
no special hazards, however, general safety considera-
tions should apply. At least two persons should be
present when working in areas not protected by handrails.
Walkways and work areas should be kept free of grease,
oil, leaves and snow. Protective guards and covers must
be in place unless mechanical/electrical equipment is
locked out of operation. Wet lime sludge may increase
the possibility or severity of electrical shock hazards.
Safety practices for handling lime are contained in
"Safety Practice for Water Utilities", AWWA Manual M3.
References
1.
2.
3.
Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
WPCF Manual of Practice No. 17 (WPCF No. 17) Paints and
Protective Coatings for Wastewater Treatment Facilities.
Safety Practice for Water Utilities, No. M3.
American Water Works Association, 2 Park Avenue,
New York, N.Y. 10016.
Glossary of Terms and Sample Calculations
Lime dose is amount of lime required to satisfy the
chemical demand present in the sludge and raise the pH to
the desired level.
VI-14
-------
VII
CHLORINE TREATMENT
-------
CONTENTS
Process Description VII-1
Typical Design Criteria and Performance VII-4
Staffing Requirements VII-6
Monitoring VII-7
Sensory Observations VII-8
Normal Operating Procedures VII-8
Pre-Startup VII-8
Startup VII-8
Routine Operations VII-9
Shutdown VII-9
Control Considerations VII-10
Physical Control VII-10
Process Control VII-10
Emergency Operating Procedures VII-11
Loss of Power VII-11
Loss of Other Treatment Units VII-11
Common Design Shortcomings VII-12
Troubleshooting Guide VII-13
Maintenance Considerations VII-17
Safety Considerations VII-17
Reference Material VII-17
References VII-17
Glossary of Terms and Sample Calculations VII-18
-------
PROCESS DESCRIPTION
Stabilization by chlorine addition has been developed
and is marketed under the registered trade name "Purifax".
The chemical conditioning of sludge with chlorine varies
greatly from the more traditional methods of biological
digestion or heat conditioning. First, the reaction is al-
process most instantaneous. Second, there is very little volatile
solids reduction in the sludge. There is some breakdown of
organic material and formation of carbon dioxide and nitrogen;
however, most of the conditioning is by the substitution or
addition of chlorine to the organic compound to form new
compounds that are biologically inert.
The chemical form in which chlorine is present in water
is directly related to pH. The first reaction of chlorine is
with ammonia (combined available chlorine), however, this is
a small portion of the chlorine added for this process. Most
of the chlorine (free available chlorine) ends up as either
hydrochloric acid, HC1, or hypochlorous acid, HOC1. The HOC1
subsequently breaks down into nascent oxygen, O, and HC1.
Below pH 5, molecular chlorine, C12, appears in solution and
increases in concentration with decreasing pH. The equations
for the reaction of free available chlorine in water can be
summarized as follows:
Cl + 2H 0 >" HOC1 + HC1 + HO
HOC1-
+ Cl ' below pH5
Hypochlorous acid, HOCl, its subsequent by-product
nascent oxygen, 0, and molecular chlorine are all strong
oxidants. The hydrochloric acid is not an oxidant or a dis-
infectant, but does lower the pH of the solution.
Generally, the entire process consists of a macerator,
flow meter, recirculation pump, two reaction tanks, a chlorine
eductor, chlorinator, evaporator, a pressure control pump,
and 2 holding tanks. Variations are possible with the selec-
tion of the individual units depending on the nature of the
sludge. Conventional grit removal equipment used for the
plant influent will suffice for grit reduction of sludge
processed through the oxidation unit. If grit removal equip-
VII-1
-------
ment has not been provided for the plant then it should be
added to this system. The type of macerator selected depends
on the type of sludge being stabilized. The resulting maxi-
mum particle size should not exceed 1/4 inch. To provide
optimum utilization of chlorine the system should be preceded
by a sludge holding tank which includes some means of mixing
or agitation. This is especially necessary for treating pri-
mary sludge. The primary sludge solids concentration is typ-
ically higher at the beginning of a pumping cycle and lower
near the end of the cycle. Without provision of the holding
tank the chlorine requirement would be variable and over- or
under- chlorination a possibility. With the holding tank in
use the chlorine requirement can be set at a constant rate.
Use of thickeners ahead of the conditioning unit is optional.
The sludge processing rate will be reduced for thicker
sludges; specifically for primary or primary plus trickling
filter greater than 4 percent, primary plus waste activated
sludge greater than 2.6 percent, or waste activated sludge
greater than 1 percent. The lower processing rates offset the
reduction in volume obtained by thickening so there is no ad-
vantage in thickening to concentrations greater than those
given above for the different types of sludge.
A schematic of the Purifax process is shown on Figure
VII-1 (see following page). The sludge is first pumped
through a macerator to reduce the particle size for optimum
chlorine exposure. It is then mixed with conditioned sludge
ahead of the recirculation pump at a ratio of 3.8 gallons of
recirculated sludge for each gallon of raw sludge. The
operation combined flow is then pumped through the first reaction tank
where it is thoroughly mixed. A portion of the sludge then
flows to the second reaction tank while the remainder is
recirculated. Recirculation of a portion of the sludge aids
in mixing and provides better utilization of the chlorine.
The recirculation rate is normally held constant. A pressure
control pump at the discharge end of the second reaction tank
maintains a pressure of 30 to 40 psi on the entire system.
Chlorine is added to the recirculated sludge line ahead
of the recirculation pump. The passage of the conditioned
sludge through the eductor creates a vacuum which causes
chlorine gas to move from the chlorine supply into the sludge
line. The recirculation of the conditioned sludge through
the eductor satisfies the dilution requirements of the
chlorine gas without introducing additional water into the
system. The recirculation pump acts as a mixer for the raw
and conditioned sludges. Almost all of the reaction between
the sludge and chlorine takes place in the first reaction
tank. This tank provides 3 minutes of detention time at
design flow. The second reaction tank provides an additional
1.5 minutes of detention time. Operating the system under"
VII-2
-------
H
U)
II
SLUDGE
STORAGE
MACERATOR
SUPERNATANT
RETURN
TO PLANT
HOLDING TANK
SLUDGE
SLUDGE FEED FLOW METER
PUMP
ro
CHLORINIZER VAPORIZER
(IF REQUIRED)
PURIFAX UNIT
Figure VII-1. Schematic (courtesy of BIF)
-------
supernatant
return
to
process
pressure forces the chlorine to penetrate into the sludge
particles to insure a complete reaction.
Chlorine is supplied to the unit from a separate
chlorinator located in the same room as the chlorinators for
disinfecting the plant effluent. Because of the large volumes
of chlorine required for the Purifax unit, an evaporator is
used ahead of the chlorinator.
The sidestreams that require further treatment result
from the thickening and/or dewatering processes that follow
the oxidation unit. The characteristics of the supernatant
vary with the type of sludge being treated and the method
of thickening or dewatering. The oxidized sludge should be
contained in a holding tank or reservoir for at least 48
hours. This will allow time for the chlorine residual to
approach zero and the pH to raise from 3.5 or 4.0 to 5.0 or
6.0. The BODg and suspended solids concentrations obviously
are quite variable but each should be less than 350 mg/1. The
supernatant or filtrate sidestreams are routed to the plant
headworks for treatment with the incoming sewage.
If the oxidized sludge is dewatered without provisions
for the holding tank then sodium hydroxide or lime must be
added to raise the pH. The quantities of filtrate or super-
natant to be treated vary with the type of process(es) used.
In general, the quantity of supernatant or filtrate to be
treated is minor in terms of the total treatment plant
capacity.
TYPICAL DESIGN CRITERIA & PERFORMANCE
The loading rates shown in Table VII-1 (see following
page) apply to standard Purifax units.
VII-4
-------
TABLE VII-1. LOADING RATES
Type of sludge
Solids
concentration,
gpm/hp
Primary
Primary & trickling filter
Primary & waste act. sludge
Primary & waste act. sludge
Primary & waste act. sludge
Waste activated sludge
Waste activated sludge
Waste activated sludge
Waste activated sludge
Anaerobic digester supernatant
Anaerobic digester supernatant
Anaerobic digester supernatant
Anaerobic digester supernatant
Septic tank sludge
Septic tank sludge
Septic tank sludge
Trickling filter humus
Trickling filter humus
Trickling filter humus
<4
<4
<2.6
4.0
5.0
1.0
2.0
3.0
4.0
0.2
0.3
0.4
0.5
2.0
3.0
4.0
2.0
3.0
4.0
2 -3.5
2 -3.5
2 -3.5
1.5-2.5
1.1-2.1
2.9-5.1
2.2-3.9
1.5-2.6
0.8-1.3
2.9-5.1
2.5-4.5
2.1-3.8
1.8-3.2
2.9-5.1
2.5-4.5
2.2-3.9
2.9-5.1
2.5-4.6
2.2-4.0
chlorine
dosages
stabilized
sludge
character-
istics
Chlorine dosages range from 600 to 4800 mg/1 depending
on the type of sludge and solids concentration. Generally,
the units should be operated with the lowest concentration
shown for each sludge type shown in Table VII-1. At these
concentrations the chlorine dosage varies from 600 to 2000
mg/1 or .005 to .017 pounds per gallon. The actual dosage
used must be adjusted for each individual plant.
The stabilized sludge will have a pH of 2.5 to 4.5 and
chlorine residual of 200 to 400 mg/1. The stabilized sludge
will have chlorine smell and light brown color. Total solids,
suspended solids, and volatile solids concentrations will be
about the same as the raw sludge. When stored for 48 hours
the chlorine residual will have fallen to 0 and the pH will
have increased to 4.5 to 6.0. The organics will normally not
decompose even after several days of storage.
Table VII-2 Csee following page) shows the expected char-
acteristics for sidestreams from typical thickening and de-
watering operations as applied to the conditioned sludge.
VII-5
-------
TABLE VII-2. SIDESTREAM CHARACTERISTICS
Supernatant from Conditioned Sludge Holding
BOD5
Suspended solids
PH
Chlorine residual
Filtrate from Vacuum Filter
BOD5
Suspended solids
pH (with 20-30 Ib/ton NaOH)
Chlorine residual
Centrate from Solid Bowl Centrifuge
BOD 5
Suspended solids
pH (with 20-30 Ib/ton NaOH)
Chlorine residual
50-150 mg/1
50-200 mg/1
4.5-6.0
0
100-350
50-150
4.5-5.5
200-400 (unless stored)
200-400
300-500
4.5-5.5
200-400 (unless stored)
STAFFING REQUIREMENTS
The staff requirements shown below apply to the Purifax
process, macerator, pumps, and chlorination system. Dewater-
ing or thickening operation and maintenance are not included.
Package Chlorine Treatment Unit Labor Requirements
Operation
Maintenance
2 hr/shift/unit
3 hr/shift/unit
The chemical oxidation process is automated. The main
effort is visually checking the process and operating the
ancillary equipment. Most of the systems now in operation
are package type units.
VII-6
-------
MONITORING
SLUDGE
FEED""
L
PURIFAX UNIT
HOLDING
TANK
OS
SUPERNATANT
SLUDGE
Q
lil
I-
co
tu
CD
CD
CO
pH
SUSPENDED
V SOLIDS
VOLATILE
SOLIDS
CHLORINE
DEMAND
CHLORINE
RESIDUAL
ORP
LU
N
CO
h-
-7 f~^
< CD
_l ^
Q. C-
ALL
ALL
ALL
ALL
ALL
ALL
ALL
>
O
LU
^
H O
co LU
LU CL
1- U-
1/D
1/W
1/D
1/W
1/D
1/D
1/W
Z ~J
g Q.
I — ^
^ ^
0 w
O LL
-1 0
SF, O,
S, CS
CE
SF, O,
S, CS
SF, O,
S, CS
SF
O, S.
CS
O
LL
0
Q LU
O _|
(— ^
LU <
G
G
G
G
G
G
G
1—
co
O i
CO
< DC
LU O
DC LL
P
P
P
P
P
P
P
A. TEST FREQUENCY
D= DAY
W = WEEK
B. LOCATION OF SAMPLE
SF = SLUDGE FEED
O = OXIDIZED SLUDGE
S =SUPERNATANT
CS = CONDITIONED
C. METHOD OF SAMPLE
G = GRAB SAMPLE
D. REASON FOR TEST
P = PROCESS CONTROL
E. FOOTNOTES
VII-7
-------
Sensory Observations
The oxidized sludge should have a faint chlorine odor
after processing, with no chlorine odor after 2 days storage.
The material should be light gray in color. If these
characteristics change, the process control parameters should
be checked. Each individual plant will result in processed
sludge that has slightly varing sensory characteristics.
After the plant has operated for several weeks then the
sensory observations can be more critically reviewed. Major
differences in chlorine odor may result with too little or
too much chlorine. Darkening of the sludge 'color may result
if the process is not properly operating. If either of the
above occur, the chlorine residual and system pressure should
be checked immediately. If these parameters are within
acceptable ranges, then check for changes in the incoming
sludge.
NORMAL OPERATING PROCEDURES
Pre-Startup
chlorine
time
delays
Startup
Check operation of the chlorine pressure-reducing valve
by turning ON the power switch on the sludge oxidation unit
control panel. Turn the chlorine valve switch to the OPEN
position, observe operation of valve actuator.
Turn switch to the CLOSED position.
Adjust time delay relays according to the manufacturer's
instruction manual.
1. Adjust the chlorinator feed rate to a low setting.
2. Close the chlorine pressure-reducing valve bypass valve.
3. Turn on the chlorine supply to the chlorinator.
4. Turn the feed pump and macerator motor starter selector
switches to AUTO.
5. Turn both selector switches in the PURIFAX motor starter
panel to AUTO.
6. Turn the power switch and alarm switch ON and the
chlorine valve switch to AUTO.
VII-8
-------
7. Depress the START pushbutton. When the motors start,
adjust the speed of the pressure control pump to produce
30 psi at the process pressure gauge. If this is not
done before the timing relay times out, the system will
automatically shut down.
8. The vacuum gauge should read approximately 20 inches of
mercury. The vacuum gauge on the chlorinator should
read the same. The pump suction gauge should read
approximately 5 psi. The pump discharge gauge should
read approximately 30 psi.
9. Check the oxidation unit for obvious sludge leaks.
10. Belt adjustment - Adjust take-up on the recirculation
pump belts until only a slight bow appears in the
slack side.
11. Recheck the tension of new belts several times within
the first 50 hours of operation and adjust if necessary.
12. Thereafter check the tension periodically.
13. Install belt guard.
14. When sludge is introduced into the system, it may be
necessary to readjust the speed of the pressure control
pump.
15. Adjust the chlorinator feed rate to the calculated rate.
16. Check for water at each pump seal by disconnecting the
seal water tubing.
Routine Operations
Shutdown
The system operation is automatic after startup. Should
a problem develop causing deviation from established operating
limits, the system will automatically shutdown. The system
cannot be restarted until the problem causing the shutdown
has been corrected. The system should be checked twice a
shift.
Normal shutdown is a sequential operation initiated by
depressing the STOP pushbutton. The sequence of operation
is as follows:
VII-9
-------
1. Depressing the STOP pushbutton causes immediate
interruption of the circuit to the chlorine pressure-
reducing valve causing it to close and shut off the
chlorine gas supply.
2. The chlorine pressure switch senses the loss of chlorine
gas pressure and its contacts open. After a sufficient
time has elapsed to evacuate chlorine gas from the
piping, an OFF delay relay, 3TR, is de-energized,
closing the vacuum line valve.
3. When the vacuum valve closes, its auxiliary contacts
open causing interruption of the circuits to the recir-
culation pump and pressure control pump motor starters.
Auxiliary contacts in the starters open, interrupting
the circuit to the seal flushing water solenoid valve
and the remotely mounted control relay. The control
relay is de-energized stopping the feed pump and
macerator.
CONTROL CONSIDERATIONS
Physical Control
The control system is automatic, with little operator
attention required.
Process Control
There are three variables that affect operation. They
are throughput rate, chlorine feed rate, and system pressure.
The throughput rate has been designed for an expected solids
throughput concentration. The process chlorine feed is set based upon
the expected rate of solids fed to the oxidation unit. If
the actual solids concentration increases the throughput rate
should be lowered.
The chlorine feed rate is also adjusted based on through-
chlorine put rate s°lids concentration, and/or monitoring results.
!f tne chlorine residual increases or decreases beyond the
limits of the recommended range, check the chlorine demand
and reset the chlorine feed.
If the above parameters are correct and the oxidized
system sludge characteristics are not within recommended limits,
pressure check the unit pressure. This should be between 30 and 40
psi.
VII-10
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
The oxidation unit will not operate without power. Raw
sludge must be hauled to a landfill site or temporarily stored
if facilities are available. If stored, the sludge must be
processed when power is restored.
Loss of Other Treatment Units
Other treatment units that affect the oxidation unit
include raw sludge thickening and oxidized sludge dewatering.
If the raw sludge thickener is out of service the throughput
rate will not be affected unless the maximum capacity is
exceeded. If this occurs, the unit hours of operation will
have to be extended. The chlorine feed rate should be ad-
justed. The amount of adjustment is determined by the results
of a chlorine demand test.
If the oxidized sludge dewatering unit is out of
service, then the disposal transport system and disposal site
must be expanded to handle the increased volume.
Should an emergency occur requiring immediate shutdown
and over-ride of normal sequential shutdown, an EMERGENCY
STOP pushbutton is provided for this purpose. This device
interrupts power to all components in the oxidation unit
control circuit, shuts down all motors and closes the vacuum
line valve and the chlorine pressure-reducing valve. The
EMERGENCY STOP pushbutton is also used as a reset device to
restore the system to normal operating status after an alarm
situation has occurred.
The control system is designed to sense certain compo-
nent and system failures. Pressure switches are located to
sense over-pressure, excessive suction, low chlorine pressure
and low eductor vacuum. A flow switch senses low flow.
Motor starters sense motor overloading. Evaporator low
temperature switch senses low water temperature.
Whenever deviation from established operating limits is
sensed, lights indicating the cause of the problem will be ac-
tivated, an audible alarm will call attention to the problem,
and the system will be automatically shutdown until the prob-
lem is corrected. The audible alarm may be switched off. The
indicating lamps remain lighted until the problem is corrected
and the system reset.
A lock-out relay is included in the circuit that
allows indicating alarm lamps to light, the audible alarm
to sound, and prevents restarting without resetting the
VII-11
-------
system when shutdown occurs in an alarm situation. It also
prevents alarm devices from functioning during normal
shutdown.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1
Unit improperly
sized.
Solution
1. Change operating time.
Inadequate hold-
ing tank capacity.
2a. Add another holding tank.
2b. Use chemicals for pH
adjustment and chlorine
removal.
Inadequate storage
for new sludge
during power outage
or shutdown.
3a. Store sludge in clarifiers
(temporary).
3b. Haul sludge to landfill.
VII-12
-------
TROUBLESHOOTING GUIDE
CHLORINE TREATMENT
INDICATORS/OBSERVATIONS
1. Oxidation unit shuts
down (low chlorine
supply pressure) .
2. Oxidation unit shut-
down.
PROBABLE CAUSE
la. No chlorine supply
pressure.
Ib. Failure of electric
chlorine pressure-
reducing and shut-
off valve to open.
Ic. Failure of chlorine
pressure switch to
close.
Id. Electrical failure
in control panel.
2a. Failure of feed pump
motor to operate .
CHECK OR MONITOR
la. (1) Check chlorine
tanks .
(2) Check all
manual valves
in supply
piping.
(3) Check evapo-
rator .
Ib. Check chlorine valve
switch on control
panel (should be in
AUTO position) .
Ic. (1) Check position
of relay.
(2) Check pressure
setting and
switch contacts .
Id. See wiring diagram.
2a. (1) Check selector
switch on motor
starter.
(2) Check motor
overload
heaters .
(3) Check control
relay.
SOLUTIONS
la. (1) If empty, replenish supply.
(2) Should be fully open.
(3) See manufacturer's
instructions.
Ib. (1) Turn chlorine valve switch
on control panel to OPEN
position. If valve opens,
and if chlorine pressure is
indicated at chlorinator ,
the problem is in the
control panel.
(2) If valve remains closed,
the problem is in the
valve or valve operator.
Ic. (1) Should be de-energized.
(2) Adjust as needed.
Id. Correct where necessary.
2a. (1) Should be in AUTO
position.
(2) Correct if overloaded.
(3) Should be energized, if
not, problem is in control
panel or control relay. If
H
I
H1
U)
-------
TROUBLESHOOTING GUIDE
CHLORINE TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
2. Oxidation unit shut-
down. (Cont'd)
2b.
Failure of feed
pump to pump.
2c.
Failure of flow
meter receiver
contacts to close.
H
H
I
2d.
Process pressure
switch contacts
opened.
2e.
Pump suction
pressure switch
contacts opened.
2b. (1) Check lines for
obstructions.
(2) Check valves.
(3) Pull pump.
2c. (1) Check setting
of auxiliary
contacts.
(2) Check valve in
discharge
piping.
2d. (1) Check valve in
discharge
piping.
(2) Monitor pressure
control pump
speed.
2e. Obstruction in
inlet piping.
2a. (3) relay is energized,
relay has failed.
2b. (1) Clean lines.
(2) Open if necessary.
(3) Repair per manufacturer's
instructions.
2c. (1) Should be set for approx-
imately 50% of the minimum
system throughput rate.
(2) Should be fully open.
2d. (1) Should be fully open.
(2) Reduce speed.
2e. Remove obstruction.
3. Oxidation unit shuts
down (low vacuum).
3a. Break or leak in
chlorine vacuum
line piping.
3b. Plugged eductor
body.
3a. Locate leak.
3b. Disassemble
and inspect.
3a. Repair.
3b. Remove two pipe plugs, vacuum
gauge, switch assembly and
vacuum line valve. Clean all
openings.
-------
TROUBLESHOOTING GUIDE
INDICATORS/OBSERVATIONS
4. Macerator stopped.
PROBABLE CAUSE
3c. Recirculation pump
fails to deliver
required dynamic
head of 30 psi.
(discharge pressure
minus suction
pressure) .
3d. Failure of pressure
control pump to
maintain pressure in
the system.
3e. Failure of pressure
control pump motor
or recirculation
pump motor to
operate .
3f. Failure of vacuum
switch contacts to
close.
3g. Electrical failure
in control panel.
4. Jammed with debris.
CHECK OR MONITOR
3c. (1) Check drive
belt tension.
(2) Check for worn
impeller.
(3) Check for air
in piping.
3d. (1) Check pump
speed.
(2) Check for
worn impeller .
3e. (1) Check selector
switches on
the starter
panel.
(2) Check motor
starter over-
loads.
3f. (1) Check switch
assembly and
vacuum gauge .
(2) Check vacuum
setting and
switch contact.
(.3) Check for loss
of oil.
3g. Check wiring diagram.
4. Inspect.
SOLUTIONS
3c. (1) Adjust.
(2) Replace.
(3) Bleed off air at vent
plugs.
3d. (1) Increase speed.
(2) Replace.
3e. (1) Set on AUTO position.
(2) Reset.
3f. CD Replace.
(2) Clean and reset.
(3) Fix leak and/or refill.
3g. Repair.
4. Turn off power, remove
obstruction .
-------
TROUBLESHOOTING GUIDE
CHLORINE TREATMENT
INDICATORS/OBSERVATIONS
5. Indication of
vacuum at oxidation
unit but none at
chlorinator.
6. Depressing STOP
button on control
panel, unit continues
to run longer than
normal shutdown time.
*
PROBABLE CAUSE
5a. Diaphragm check
valve plugs.
5b. Failure of vacuum
line valve ball to
open.
6a. Failure of electric
chlorine pressure-
reducing and shut-
off valve to close.
6b. Failure of chlorine
pressure switch
contacts to open.
6c. Failure of vacuum
line valve limit
switch contacts
to open.
CHECK OR MONITOR
5a. Inspect.
5b. Inspect.
6a. Check chlorine
pressure at
chlorinator.
6b. Check position of
relay.
6c. (1) Inspect.
(2) Check valve
operator .
SOLUTIONS
5a. Disassemble and clean.
5b. (1) Replace with spare valve.
(2) Disassemble and replace
broken ball or shaft.
6a. (1) If there is pressure,
turn chlorine valve
switch on control panel
to the closed position.
(2) If there is still pressure,
the valve is stuck open -
replace.
(3) If there is no pressure,
the problem is in the
control panel (correct
wiring or fuse) .
6b. If energized, the pressure
switch is stuck open.
6c. (1) Replace with spare valve.
(2) If in the OPEN position,
problem is in motor,
gearing, limit switches,
or cams .
(3) If closed, problem is in
the limit switch or cam.
H
H
I
M
cn
-------
MAINTENANCE CONSIDERATIONS
Inspect motors at regular intervals; keep motors clean
and ventilation openings clear.
diaphragm Check valve may become clogged causing sludge to back-
check up into chlorinator. Periodically disassemble and clean.
valve Replace diaphragm and spring if they are deteriorated.
Valve ball and seats may become scored causing sludge
to back-up into diaphragm check valve. Periodically dis-
vacuum assemble by unscrewing union nuts, with valve in CLOSED
line position, remove carrier and ball by pressing on flat spot
valve on ball. Replace ball and seats if scored.
When reassembling valve, use caution. Only hand tighten
union nuts.
macerator Check the macerator twice daily for debris.
SAFETY CONSIDERATIONS
The major concerns are contact with the low pH and the
high chlorine concentration of the oxidized sludge. Human
contact with the oxidized sludge should be avoided. If this
occurs, shower immediately.
The macerator can be dangerous if maintenance is
attempted while the unit is turned on. Be sure the power
is off before doing maintenance work.
Safety practices for handling chlorine are contained in
"Safety Practice for Water Utilities", AWWA Manual M3.
Generation of chlorinated hydrocarbons may be a problem,
but the magnitude of any such problem cannot be determined at
this time.
REFERENCE MATERIAL
References
1. Safety Practice for Water Utilities, No. M3. American
Waterworks Association, 2 Park Avenue, New York, N.Y.
10016.
2. Standard Methods for the Examination of Water and Waste-
water. American Public Health Association, 1015
Eighteenth St., N.W., Washington, D.C. 20036
VII-17
-------
Glossary of Terms and Sample Calculations
1. Chlorine dosage is the amount of chlorine required to
oxidize the sludge (chlorine demand) plus the desired
residual. The dosage is computed as mg/1 concentration
and the chlorine feed system set at the equivalent
Ib/day feed rate.
Given a desired chlorine residual of 300 mg/1 and a
chlorine demand of 800 mg/1, the chlorine dosage and
resulting feed rate (for 12,000 gal/day throughput)
are computed as follows.
Chlorine dosage = Chlorine demand + desired residual
=300 mg/1 + 800 mg/1
= 1100 mg/1
Feed rate, Ib/day = 1100 mg/1 x 8.34 x .012 mgd
= 110 Ib/day
2. Throughput rate is the gallons of sludge fed to the
unit per unit time (gpm or gpd).
3. Oxidized sludge is the chemical oxidation unit effluent.
4. Conditioned sludge is the oxidized sludge that has also
been conditioned by a holding tank or chemical treatment
to raise the pH and reduce the chlorine residual.
5. Nascent oxygen is uncombined oxygen in molecular form
(O).
6. Oxidant is an agent which oxidizes a substance by remov-
ing one or more electrons from an atom, ion, or
molecule.
VII-18
-------
VIII
CENTRIFUGATION
-------
CONTENTS
Process Description VIII-1
Typical Design Criteria and Performance VIII-4
Staffing Requirements .... VIII-6
Monitoring VIII-7
Normal Operating Procedures VIII-8
Startup VIII-8
Routine Operations VIII-8
Shutdown VIII-9
Control Considerations VIII-9
Physical Control VIII-9
Process Control VIII-10
Emergency Operating Procedures VIII-11
Loss of Power VIII-11
Loss of Other Treatment Units VIII-11
Common Design Shortcomings VIII-11
Troubleshooting Guide VIII-12
Maintenance Considerations VIII-15
Safety Considerations VIII-15
Reference Material VIII-16
References VIII-16
Glossary of Terms and Sample Calculations VIII-16
-------
PROCESS DESCRIPTION
The centrifuge is essentially a sedimentation device in
which the solids-liquid separation is enhanced by rotating the
liquid at high speeds so as to subject the sludge to increased
gravitation forces.
Centrifuges have been used for both sludge thickening
and dewatering especially for waste activated sludge and
digested sludges. The disc type and the solid bowl centri-
applications fuges are well suited to thickening operations. Centrifuges
can be used to classify sludges according to relative specific
gravity. For instance, phosphorus rich sludge can be removed
from lime sludge to enable efficient recovery and reuse of
the lime.
Three types of centrifuges have been used for sludge
dewatering.
1. Solid bowl centrifuge - This is the most widely used
type for dewatering of sewage sludge. This centrifuge
assembly (Figure VIII-1, see following page) consists
of a rotating bowl and conveyor. The rotating bowl, or
shell, is supported between two sets of bearings and
includes a conical section at one end to form a de-
watering beach or drainage deck. Sludge enters the
rotating bowl through a stationary feed pipe extending
into the hallow shaft of the rotating screw conveyor
and is distributed through ports into a pool within the
rotating bowl.
The helical rotating conveyor moves the sludge solids
across the bowl, up the beaching incline to outlet ports
and then to a sludge cake discharge hopper.
As the liquid sludge flows through the bowl toward the
overflow devices, progressively finer solids are settled
centrifugally to the rotating bowl wall. The water or
centrate drains from the solids and back into the pool.
Centrate is discharged from the bowl through ports in
the end which maintain the pool in the bowl at the
desired depth.
Most solid bowl machines employ the countercurrent flow
of liquid and solids described above and illustrated
in Figure VIII-1. They are appropriately referred to as
VIII-1
-------
COVER
H
H
I
DIFFERENTIAL SPEED
GEAR BOX
MAIN DRIVE SHEAVE
/ROTATING
CONVEYOR
u
CENTRATE
DISCHARGE
FEED PIPES
(SLUDGE AND
CHEMICAL)
BEARING
BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
Figure VIII-1.
Continuous counter-current solid bowl conveyor discharge
centrifuge.
-------
"countercurrent" centrifuges. Recently a "cocurrent"
centrifuge design was introduced in which the solids
and liquid flow in the same direction. General con-
struction is similar to the countercurrent design
except there are no centrate ports in the bow] head.
Instead, the centrate is withdrawn by a skimming device
near the junction of the bowl and the beach.
2. Basket centrifuge - This centrifuge is also referred to
as the imperforate bowl, knife discharge type and is a
batch dewatering unit that rotates around the vertical
axis. The sludge is charged into the basket and forms
an annular ring as the unit rotates. The liquid
(centrate) is displaced over a baffle or weir at the
top of the unit. When the solids concentration reaches
the desired limit the centrifuge is stopped. A knife or
skimmer displaces the cake from the vertical wall and
out the bottom openings. A schematic is shown in
Figure VIII-2.
CHARGING
DISCHARGING
Figure VIII-2. Basket centrifuge in charge and
discharge cycle.
VIII-3
-------
3. Disc centrifuge - The disc centrifuge is continuous flow
variation of the basket centrifuge as shown in Figure
VIII-3. This centrifuge is prone to plugging and in some
cases the sludge may have to be screened prior to
centri fugation.
sidestream
Figure VIII-3. Disc-type centrifuge.
The centrate is usually returned to the plant influent
or some other appropriate point in the main treatment process.
Return of centrate to flotation thickeners has proven satis-
factory.
TYPICAL DESIGN CRITERIA & PERFORMANCE
loading
rates
A number of variables, including sludge feed rate, solids
characteristics, temperature, and conditioning processes in-
fluence the sizing of centrifugation equipment for a particu-
lar application. Of these, sludge feed rate is the parameter
most commonly used for sizing centrifuges. Single centrifuge
capacities range from 4 gpm to about 250 gpm. Typical feed
rates for several sizes of solid bowl centrifuges are shown in
Table VIII-1 for typical municipal waste sludge.
TABLE VIII-1. SOLID BOWL CENTRIFUGE TYPICAL FEED RATES
Machine size, in
Feed rate
Ib dry solids/hr
18
24
36
300 to 800
700 to 2,000
1,500 to 3,500
Expected centrifuge performance is shown in Table VIII-2
(see following page) for a number of conditions. These data
were developed from "Process Design Manual for Sludge Treat-
ment and Disposal", EPA 625/1-74-006 and actual plant data.
VIII-4
-------
TABLE VIII-2. EXPECTED CENTRIFUGE PERFORMANCE
Solid
Bowl, Dewatering Performance
Sludge Cake Characteristics
Waste water sludge type
Raw or digested primary
Raw or digested primary
trickling filter humus
Raw or digested primary
activated sludge
Activated sludge
Oxygen activated sludge
High- lime sludges
Lime classification
Heat treated sludge
Heat treated sludge
Solids
Solids, % recovery, %
25-35 90-95
28-35 70-90
, plus 20-30 80-95
25-35 60-75
, plus 15-30 80-95
15-25 50-65
8-9 80-85
8-10 80-85
50-55 90
40 75
30-50 85-90
30-50 92-99
Polymer
addition
2-4 Ibs/ton
no
5-15 Ibs/ton
no
5-20 Ibs/ton
no
5-10 Ibs/ton
3-5 Ibs/ton
no
no
no
2-5 Ibs/ton
Typical Thickening Performance
(Based on
Type of Centrifuge
sludge type
WAS Disc
EAS (after
roughing
filter) Disc
EAS Basket
EAS Solid-bowl
limited plant operating experience)
Underflow Solids
solids, Feed solids/ recovery.
% % %
4-5.5 0.75-1.0 80-90
5-7 0.7 80-97
9-10 0.7 90-70
5-13 0.4-1.5 70-90
85
90
95
Polymer
requirement ,
Ib/ton
None
None
None
None
<5
5-10
10-15
WAS = waste activated sludge
EAS = extended aeration waste sludge
VIII-5
-------
The use of polymers has increased the range of materials
that can be dewatered satisfactorily by centrifuges. The
degree of solids recovery can be regulated over rather wide
polymers ranges depending on the amount of polymer used. A wetter
sludge cake is usually produced when polymers are used
because of the increased capture of fines.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
centrifuges are shown in Table VIII-3. The requirements are
based on the annual solids feed rate. Centrifuge maintenance
requires highly trained personnel and varies considerably
depending on whether major maintenance is performed on site
or if machines are rebuilt by service shops.
TABLE VIII-3. CENTRIFUGE LABOR REQUIREMENTS
Annual dry
solids applied, Labor, hr/yr
dry tons/yr Operation Maintenance Total
100 700 200 900
500 1,000 300 1,300
1,000 1,500 400 1,900
5,000 4,000 1,000 5,000
10,000 7,000 1,800 8,800
50,000 30,000 8,000 38,000
VIII-6
-------
MONITORING
XCENTRATE RECYCLE
TO PLANT INFLUENT
NOTE:
SOLID BOWL TYPE SHOWN.
FLOW PATTERN IS SIMILAR
FOR OTHER MODELS.
TOTAL SOLIDS
5
§ BOD
Z
I
SUSPENDED
g SOLIDS
53 SETTLEABLE
O SOLIDS
FLOW
ISI
to
h- —
Z Q
Q. _
ALL
ALL
ALL
ALL
ALL
0
UJ
D
i O
if) UJ
UJ OC
H u-
1/D
1/W
I/O
1/H
R
u.
O
-,
O
t— _j
< Q-
81
-i to
S
C
CE
CE
CE
CE
u_
O
°LJU
0^
I 0.
UJ <
2 to
G
G
G
G
R
Z ^
O H
to
< OC
uj o
P
p(2l
P
P
P111
A. TEST FREQUENCY
H HOUR
D - DAY
W • WEEK
R - RECORD CONTINUOUSLY
B. LOCATION OF SAMPLE
S
C
CE
SLUDGE FEED
CAKE
CENTRATE
C. METHOD OF SAMPLE
G • GRAB SAMPLE
R • RECORD CONTINUOUSLY
D. REASON FOR TEST
P PROCESS CONTROL
E. FOOTNOTES:
I. DAILY OPERATION ASSUMED.
2. FOR CONTROL OF PROCESS
RECEIVING THIS FLOW.
VIII-7
-------
NORMAL OPERATING PROCEDURES
Startup
The manufacturer's handbooks should be consulted for
specific instructions covering all equipment variations,
however, the following general considerations should apply
to most units. Centrifuge construction and features vary
widely.
1. Confirm operation of sludge pumps, dewatered sludge
conveyor and centrate return pumps.
2. For oil lubricated centrifuges:
a. Check oil level. Open supply valve for oil-
cooling water, if applicable.
b. Start the oil pump, check oil temperature, and
adjust cooling water to desired flow, if
applicable.
3. For grease lubricated bearings make sure they are
properly greased.
4. Start the centrifuge drive motor. If any severe or
unusual vibration occurs shut the system down immed-
iately, however, never stop the lubricating system
before the centrifuge stops.
5. Open the sludge feed valve and check the ammeter
occasionally to make certain the centrifuge is not
being overloaded.
Routine Operations
Once in operation the centrifuge should run with very
little attention. A list of suggested routine operational
procedures follows and should be checked regularly as
applicable to the particular machine.
1. Check level in oil reservoir.
2. Check flow of oil to bearings.
3. Check flow of cooling water and oil temperature to
assure it is operating in the proper range.
4. Check machine vibration. When vibration becomes
noticeable, machine should be overhauled. Continued
VIII-8
-------
Shutdown
operation when vibration is excessive may damage
bearings and other machine parts.
5. Check ammeter reading to assure that centrifuge
loading is normal. An above average reading indicates
overloading.
6. Check bearings for unusual noises.
7. Check bearing temperatures by hand feel. Grease
lubricated bearings which run hot are probably
overly lubricated.
8. Check system for leaks.
In the event a torque overload occurs the system should
automatically shutdown. After the machine stops turn the
torque arm to determine if the machine is blocked by solids.
If the machine turns without obstruction reset the control
and allow a sufficient cooling period before restarting. If
the machine can not be cleared so it rotates freely, the
centrifuge may require disassembly. In this case consult
manufacturer's instructions.
1. Stop feed to centrifuge and thoroughly flush with hot
water or solvent. This is extremely important.
2. Turn the centrifuge off.
3. Continue flushing until the centrifuge stops.
4. Turn off the lubricating system including the cooling
water, but not until the centrifuge comes to a complete
stop.
5. If the flushing procedure does not remove all of the
deposits, the machine may have to be disassembled for
cleaning. In this case consult the manufacturer's
instructions.
6. The machine should not be restarted unless it has
been properly flushed and it can be turned by hand.
CONTROL CONSIDERATIONS
Physical Control
For proper operation and safety, centrifuge systems
require a certain amount of instrumentation such as motor
VIII-9
-------
shutdown, torque monitor, vibration monitor, running lights,
control switch, oil pressure indicator and low pressure
alarm, oil temperature indicator and ammeter.
Process Control
sludge feed
chemicals
There are several variables that can be controlled by
the operator to affect optimum centrifuge performance.
In general, the sludge variables that improve gravity
sedimentation also improve centrifugation. If the sludge
feed is increased, in continuous flow systems, the resident
period decreases and the solids recovery will decrease.
Also, within limits, if the slurry temperature is increased,
the solids recovery and the cake solids will improve. This
is rarely practiced since it is costly and generally not a
good economic alternative.
The use of chemicals, usually polymers, may improve
solids recovery, but a wetter cake is generally produced
because additional fines are captured.
For the solid bowl centrifuge, changes in the bowl
speed, pool depth, or conveyor speed will affect performance.
This is shown in Table VIII-4.
TABLE VIII-4. INFLUENCE OF MACHINE VARIABLES ON OPERATION
OF SOLID BOWL CENTRIFUGE
Process change
Cake moisture
Solids recovery
Increase bowl speed Decrease
Increase pool depth Increase
Increase conveyor speed Increase
Polymer feed Increase
Increase
Increase
Decrease
Increase
pool depth
The bowl is normally equipped with adjustable dams or
weirs for changing pool depth. Consult manufacturer's
instructions for changing the pool depth as this generally
requires partial disassembly.
The optimum settings for these variables depend on the
quality of the cake and centrate desired and on the feed
solids characteristics. Therefore, as with vacuum filters,
it is best to try various settings and establish a
centrifuge performance curve.
VIII-10
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
During a power outage the centrifuge should be cleaned
as well as possible in accordance with normal shutdown
procedures and all switches and valves shut off. This will
enable normal startup procedures to be followed when power
is restored. If the centrifuge shuts down because of tripped
relays, blown fuses, or the action of other safety devices
the cause must be determined and proper adjustments made
before restarting the equipment.
Loss of Other Treatment Units
The loss of treatment units that precede the centrifuge
may affect centrifuge performance and require control adjust-
ments. For example, the loss of the sludge thickener will
result in a wetter sludge feed to the centrifuge. A loss of
a process following the centrifuge should not have any affect
on operation unless an alternative method of sludge disposal
requires control adjustments to produce a cake with a
different solids concentration.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Excessive
corrosion of
parts; particularly
the rotating
conveyor.
2. Flushing supply
not strained and
plugs nozzles.
3. No means for re-
moval of bowl
assembly.
4. Rigid piping con-
nections to
centrifuge and
excessive vibra-
tion of piping.
5. Lack of adequate
degritting causes
excessive wear.
Solution
1. Replace components
affected with more suitable
materials.
2. Install strainer.
Install overhead hoist
or use portable lifting
frame.
Install flexible con-
nections.
5. Install degritting
system.
VIII-11
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Centrate clarity
inadequate.
la. Feed rate too high.
Ib. Low pool depth.
Ic. Conveyor screws
worn.
Id. Speed too high.
le. Feed solids too
high.
If. Chemical condition-
ing improper.
la. Flow records.
Ib. Setting of weirs.
Ic. Vibration; excessive
solids buildup in
machine.
Id. Pulley setting.
le. Spin test on feed
sludge - should
be <40% by volume.
If. Chemical feed rate.
la. Reduce flow.
Ib. Increase pool depth to improve
clarity.
Ic. Repair or replace conveyor.
Id. Change pulley setting for lower
speed.
le. Dilute the feed sludge.
If. Change chemical dosage.
2. Cake too wet.
2a. Feed rate too high.
2b. High pool depth.
2c. Speed too low.
2d. Excessive chemical
feed.
2a. Flow records.
2b. Setting of weirs.
2c. Pulley setting.
2d. Chemical feed rate.
2a. Reduce flow.
2b. Decrease pool depth to improve
dryness.
2c. Change pulley setting for
higher speed.
2d. Decrease chemical dosage.
3. Centrifuge torque
control trips.
3a. Feed rate too high.
3b. Feed solids too
high.
3c. Foreign material
in machine.
3a. Flow records.
3b. Spin test on feed
sludge - should be
<40% by volume.
3c. Inspect interior.
3a. Reduce flow.
3b. Dilute feed sludge.
3c. Remove conveyor and remove
foreign material.
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATION
INDICATORS/OBSERVATIONS
4. Excessive vibration.
PROBABLE CAUSE
3d. Gear unit is
misaligned.
3e. Faulty bearing,
gear, or spline.
4a. Improper lubrica-
tion.
4b. Improper adjustment
of vibration.
4c. Discharge funnels
may be contacting
centrifuge .
4d. Portion of conveyor
screw may be plugged
with solids causing
unbalance .
4e. Gear box improperly
aligned.
4f . Pillow block bear-
ing damage .
4g. Rotating parts out
of balance.
4h. Parts not tightly
assembled.
4i. Uneven wear of
conveyor.
CHECK OR MONITOR
3d. Vibration.
3e. Inspect.
4a. Check lubrication
system.
4b. Vibration isolators.
4c. Position of funnels.
4d. Interior of machine.
4e. Gear box alignment.
4f. Inspect bearings.
4i. Inspect conveyor.
SOLUTIONS
3d. Correct the alignment.
3e. Replace faulty parts.
4a. Provide correct lubrication.
4b. Adjust isolators.
4c. Reposition slip joints at
funnels.
4d. Flush out centrifuge.
4e. Align gear box.
4f. Replace bearings.
4g. Balance rotating parts.
4h. Tighten parts.
4i. Resurface, rebalance.
I
h-1
CO
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
Sudden increase in
power consumption.
5a. Contact between
bowl and accumu-
lated solids in
centrifuge case.
5b. Effluent pipe
plugged.
5a. Solids plows; look
for polished area
on outer bowl.
5b. Check for free
discharge of solids.
5a. Apply hard surfacing to areas
with water.
5b. Clear effluent pipe.
Gradual increase
in power consumption.
6a. Conveyor screw
wear.
6a. Conveyor condition.
6a. Resurface screw.
7. Spasmodic, surging
solids discharge.
7a. Pool depth too low.
7b. Conveyor screw
rough.
7c. Feed pipe (if
adjustable) too
near bowl beach.
7d. Machine vibration
excessive (See item
4).
7a. Weir position.
7b. Improper hard sur-
facing or corrosion.
7a. Increase pool depth.
7b. Rebuild conveyor screw.
7c. Move feed pipe to effluent end.
8. Centrifuge shuts
down (or will not
start).
8a. Tripped circuit
breaker or fuses.
8b. Overload relay
tripped.
8c. Torque control
tripped.
8d. Vibration switch
tripped.
8a. Electrical.
8b. Overload relay.
8c. (See item 3)
8d. (See item 4)
8a. Correct problem and restart.
8b. Flush machine, reset overload
relay.
-------
MAINTENANCE CONSIDERATION
For routine equipment maintenance considerations such as
oiling or greasing frequency or filter replacement consult
manufacturer's instructions.
A hoist in good working order should be available for
disassembly of the centrifuge.
A major consideration is whether or not the machine
overhaul is done locally or by returning the machine (or
parts) to a qualified repair shop. If qualified personnel
are available, the machine can be overhauled locally with
specialized work such as balancing performed by qualified
local shops. Otherwise, the machine or the worn parts should
be repaired by a qualified repair station or at the manu-
facturer 's shop.
Plant personnel should have spare parts and tools
for the following maintenance items if they are to do the
work:
1. Replace shear pins
2. Replace main bearings
mechanical 3. Replace seals
4. Replace conveyor bushings
5. Replace thrust bearing seal
6. Replace or repair feed and discharge ports
SAFETY CONSIDERATIONS
1. Make sure that protective guards which may have been
removed to service belts, gears, and other exposed
moving parts have been replaced.
2. Don't wear loose clothing when servicing or operating
the centrifuge.
3. Observe all electrical safety criteria.
4. Do not operate the machine under conditions that may
produce excessive vibration (bowl not properly flushed),
or if the vibration shutdown is inoperative.
VIII-15
-------
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
2. Process Design Manual for Sludge Treatment and Disposal,
EPA 625/1-74-006. U.S. Environmental Protection Agency,
Technology Transfer, Cincinnati, Ohio 45268.
3. WPCF Manual of Practice No. 11, Chapter 9, Operation of
Wastewater Treatment Plants, Sludge Dewatering.
4. Process Design Manual for Sludge Treatment and Disposal,
Chapter 4, EPA 625/1-74-006. U.S. EPA Technology
Transfer, Cincinnati, Ohio 45268.
5. Estimating Laboratory Needs for Municipal Wastewater
Treatment Facilities, EPA 430/9-74-002, June, 1973,
U.S. Environmental Protection Agency, Office of Water
Program Operations, Washington, D.C. 20460.
Glossary of Terms and Sample Calculations
Solids recovery is the ratio of cake solids to feed
solids for equal sampling times. It can be calculated
with suspended solids and flow data or with only
suspended solids data. The centrate solids must be
corrected if chemicals are fed to the centrifuge.
Recovery ~
fwet cake flow, lb_J(cake solids, %) (100)
\ _ hrj _
f«ret feed flow, lb\ (feed solids,
V
Recovery =
(cake solids, %) (feed solids, % - centrate solids, %) (100)
(feed solids, %) (cake solids, % - centrate solids, %)
2. Centrate solids must be corrected if chemicals are added
to centrifuge as follows. The centrate solids must be
corrected because the centrate is diluted by the extra
water from the chemical and chemical dilution water
feeds. The measured centrate solids, therefore, are
less than the actual solids would be without the added
water from the chemical feed.
VIII-16
-------
correction factor =
(feed rate,gpm)+(chemical flow,gpm)+(dilution vater,gpm)
feed rate, gpm
corrected centrate solids =
(measured centrate solids) (correction factor)
3. Solids feed rate is the dry solids feed to the
centrifuge.
(feed flow, gpm)
|"8.33 lb\ /feed solids, %\ /60 min')
\gal J\ 100 / \ hr /
4. Cake solids discharge rate is the dry solids cake
discharge from the centrifuge.
dry cake solids discharge rate =
(dry solids feed rate) (solids recovery)
VIII-17
-------
IX
VACUUM FILTRATION
-------
CONTENTS
Process Description IX-1
Typical Design Criteria and Performance IX-3
Staffing Requirements IX-3
Monitoring IX-5
Normal Operating Procedures IX-6
Startup IX-6
Routine Operations IX-6
Shutdown IX-6
Control Considerations IX-6
Physical Control IX-6
Process Control IX-7
Emergency Operating Procedure IX-9
Loss of Power IX-9
Loss of Other Treatment Units IX-9
Common Design Shortcomings IX-10
Troubleshooting Guide IX-11
Maintenance Considerations IX-14
Safety Considerations IX-14
Reference Material IX-14
References IX-14
Glossary of Terms and Sample Calculations IX-14
-------
PROCESS DESCRIPTION
process
equipment
variations
A vacuum filter basically consists of a cylindrical drum
which rotates partially submerged in a vat of sludge. The
filter drum is divided into compartments by partitions or
seal strips. A vacuum is applied between the drum deck and
filter medium causing filtrate to be extracted and filter
cake to be retained on the medium during the pickup and cake
drying cycle. The filter medium may be a cloth made of natu-
ral or synthetic fibers, stainless steel wire mesh or coil
springs. In the drum filter shown in Figure IX-1 (see
following page), the cake of dewatered sludge is removed by
a fixed scraper blade, however there are alternative designs
which use other methods for sludge removal.
The following major equipment variations are common:
1. Scraper - discharge mechanism: The filter drum operates
continuously with a vacuum pickup forming and filtering
zone, a vacuum cake drying zone, and a pressure blow
back or 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.
2. String discharge filters: Closely spaced strings around
the filter drum, the medium, and a set of discharge
and return rolls carry the sludge cake and then free it
from the medium and discharge it to a hopper. The
strings pass through a set of aligning combs before
returning to the drum.
3. Belt-medium filters: A traveling woven cloth or metal
belt serves as the filter medium and transports the
sludge cake to the discharge roll in a manner similar to
that of the string discharge filters. The belt can be
washed on both sides, if desired, before positioning
back on the drum.
4. Coil-medium filters: Two layers of stainless steel
springs wrapped around the drum act as the filter medium.
When the two layers of springs leave the drum they
separate in such a manner that the sludge cake is lifted
off the lower layer of coil springs and discharged off
IX-1
-------
CLOTH CAULKING
STRIPS
DRUM
x
AUTOMATIC VALVE
FILTRATE PIPING
CAKE SCRAPER
AIR AND FILTRATE
ySLURRY AGITATOR
VAT
AIR BLOW-BACK LINE
r SLURRY FEED
Figure IX-1. Cutaway view of a rotary drum vacuum filter.
-------
sidestreams
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.
The only sidestream is the filtrate which is the liquid
removed from the sludge during dewatering. Filtrate is re-
turned to a main plant treatment process. When filtrate
quality is poor, it is possible to build up a large proportion
of fine solids in the plant and reduce plant treatment
efficiency. In an activated sludge process, the filtrate may
be returned to a flotation or thickener process.
TYPICAL DESIGN CRITERIA & PERFORMANCE
Performance of the vacuum filtration process can vary
widely depending on the sludge type, sludge characteristics,
conditioning, type of vacuum filter, and loading rates.
Typical applications are shown in Table IX-1 (see following
page). These data were summarized from a number of sources
including full scale plant operations.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
vacuum filters are shown in Table IX-2. The requirements are
based on the surface area of the filter.
TABLE IX-2. VACUUM FILTRATION LABOR REQUIREMENTS
Vacuum filter
area, sq ft
50
100
200
400
800
1,600
3,200
5,000
Operation
1,020
1,750
2,800
4,400
7,300
11,900
18,600
25,400
Labor , hr/yr
Maintenance
180
300
500
800
1,300
2,100
3,400
4,600
Total
1,200
2,050
3,300
5,200
8,600
14,000
22,000
30,000
IX-3
-------
TABLE IX-1. VACUUM FILTRATION TYPICAL LOADINGS AND PERFORMANCE
Sludge type
Design assumptions
Typical Performance
Feed loading cake
solids, rates, solids,
% psf/hr %
Primary
Primary + FeCl3
Primary +
Low Lime
Primary +
High Lime
Primary + WAS
Primary +
(WAS + FeCl3)
(Primary +
+ WAS
Thickened to 10% solids
polymer conditioned
85 mg/1 FeCl3 dose
Lime conditioning
Thickening to 2.5% solids
300 mg/1 lime dose
Polymer conditioned
Thickened to 15% solids
600 mg/1 lime dose
Polymer conditioned
Thickened to 15% solids
Thickened to 8% solids
Polymer conditioned
Thickened to 8% solids
& lime conditioned
Thickened primary sludge
to 2.5%
Flotation thickened WAS
to 5%
Dewater blended sludges
10 8-10 25-38
2.5 1.0-2.0 15-20
15 6 32-35
15 10 28-32
4-5 16-25
20
3.5 1.5 15-20
Waste Activated
Sludge (WAS)
WAS + FeCl3
Digested primary
Digested primary
+ WAS
Digested primary
+ (WAS + FeCl3)
Tertiary alum
Thickened to 5% solids 5
Polymer conditioned
Thickened to 5% solids 5
Lime + FeCl3 conditioned
Thickened to 8-10% solids 8-10
Polymer conditioned
Thickened to 6-8% solids 6-8
Polymer conditioned
Thickened to 6-8% solids 6-8
FeCl3 + lime conditioned
Diatomaceous earth precoat 0.6-0.8
2.5-3.5
1.5-2.0
7-8
3.5-6
2.5-3
0.4
15
15
25-38
14-22
16-18
15-20
IX-4
-------
MONITORING
5 TOTAL SOLIDS
D
5
z
5 BOD
SJ SUSPENDED
(0 SOLIDS
LU
3 FLOW
UJ
N
CO
1- _
2 Q
< O
_l ">
Q. —
ALL
ALL
ALL
ALL
O
UJ
D
i O
00 LU
uj cc
(- U-
1/D
2/W
1/D
R
a.
O
Z
K ^J
< Q-
0 5
0<
-1 00
S
C
F
F
F
u.
O
DLU
I Q.
H- 5
5 oo
G
G
G
R
,
Z ^
O 1^
oo '
< cc
UJ o
cc u.
p
pID
p
pill
FILTRATE
/RECYCLE TO
PLANT
INFLUENT
A. TEST FREQUENCY
R RECunD CONTINUOUSLY
D DAY
W WEEK
B. LOCATION OF SAMPLE
S SLUDGE FEED
C SLUDGE CAKE
F FILTRATE
C. METHOD OF SAMPLE
GRAB SAMPLE
RECORD CONTINUOUSLY
D. REASON FOR TEST
P PROCESS CONTROL
E. FOOTNOTES:
1 FOR CONTROL OF PROCESS
RECEIVING THIS FLOW.
IX-5
-------
NORMAL OPERATING PROCEDURES
Startup
1. Open vacuum and sludge influent valve filtrate flow
valve, and chemical conditioning valves.
2. Start up sludge pump, chemical conditioning pump,
conditioning tank agitator drive, and filter vat
agitator.
3. When sludge nearly fills the filter vat start vacuum
pump and filtrate pump.
4. Start drum drive and check drum chain lubricators.
5. Start all other equipment such as wash sprays and
conveyor belt drives.
Routine Operations
1. Inspect system twice a shift.
2. Carry out maintenance as required including cleanup
and washdown.
3. Take samples as outlined in MONITORING section.
4. Make adjustments in conditioning and drain speed.
Shutdown
1. Shutdown chemical conditioning and sludge feed.
2. Stop filter vat agitator just before vat water level
drops below drum.
3. Stop vacuum and filtrate systems.
4. Open drain valves, flush lines, and hose down filter
medium and tanks.
5. Stop drum drive and water sprays.
CONTROL CONSIDERATIONS
Physical Control
Filter physical control is accomplished by drum speed,
general vacuum, and sludge feed rate. The drum speed is controlled
by a variable speed drive.
IX-6
-------
vacuum
control
The vacuum is controlled by:
1. Amount of Conditioning - proper conditioning causes the
sludge to release its water allowing the cake to open up
and lowering the vacuum requirements.
2. Drum Speed - The slowest drum speed produces the thick-
est, driest, cake and the lowest vacuum. As the drum
speeds up it has less time to remove the water and the
vacuum rises with the drum speed.
3. Sludge Level in the Filter Vat - A full vat provides
maximum contact time and minimum drying time, resulting
in a thicker cake and the highest vacuum. As the vat
level is lowered the vacuum drops.
4. Mechanical Devices - Some systems may be equipped with a
spring loaded vacuum release valve which can be set to
open at any desired vacuum level.
Process Control
yield
efficiency
Control of the vacuum filter systems should be based on
performance. The performance of vacuum filters may be
measured by various criteria such as the yield, the efficiency
of solids removal and the cake characteristics. Each of these
criteria is of importance, but one or the other may be partic-
ularly significant in a given plant. Yield is the most
common measure of filter performance. The yield is the filter
output and is expressed in terms of pounds of dry total solids
in the cake discharged from the filter, per square foot of
effective filter area per hour.
The second measure of filter performance is the
efficiency of solids removal. Basically, the vacuum filter
is a device used for separating solid matter from liquid,
and the actual efficiency of the process is the percentage
of feed solids recovered in the filter cake. Solids removals
on vacuum filters range from about 85 percent for coarse mesh
media to 99 percent with close weave, long nap media. The
recycled filtrate solids impose a load on the plant treatment
units, and should normally be kept to a practical minimum.
However, it may be necessary to reduce the filter efficiency
in order to deliver more filter output and thus keep up with
sludge production.
The filter cake quality is another measure of filter
performance, depending upon cake moisture and heat value.
Cake solids content varies from 20 to 40 percent by weight,
depending upon the type of sludge handled and the filter
cycle time and submergence. Delivery of a very dry cake does
IX-7
-------
cake
chemical
conditioning
tank
agitation
heat
treated
sludges
optimum
operation
cake
drying
not necessarily indicate good filter performance. Cake
moisture should be adjusted to the method of final disposal;
it is inefficient to dry the cake more than is required.
When the dewatered sludge is incinerated, a raw sludge cake
having a fairly high moisture content can be burned without
auxiliary fuel because of the higher volatile content, while
a digested sludge cake will have to be drier to burn without
make-up heat.
If chemical conditioning is used, the operator should
determine the best conditioning chemical for the feed sludge.
If the character of the feed sludge is subject to change, an
evaluation of conditioning agents should be made after each
change. Once an effective conditioner has been selected, the
next task is to determine the best chemical dosage rate. One
or more of the variables should be held constant and the
others varied systematically to develop a series of condition-
ing performance curves. The best chemical conditioning con-
sidering cost and required performance can then be determined.
Since all sludges vary, determine the best procedure
for operation of the filter vat agitator by experience. Some
sludges may require continuous use of the agitator, while
others may be best with no vat agitation (in this case the
sludge must be without agitation from start-up).
The effect of heat treatment on various municipal sludges
is to make all types of sludges readily dewaterable by vacuum
filtration with minimum chemical conditioning. Raw primary
sludges have been dewatered at rates as high as 40 psf/hr
and waste activated sludges at 7 psf/hr. Mixtures of raw
primary and secondary sludges subjected to heat treatment
should produce yields well over 10 psf/hr.
The filter can be operated for maximum sludge cake
output, for lowest chemical cost, for the driest cake or any
combination of these. All that is necessary is to strike
a balance between all the controls for the desired output.
Once a balance is achieved, it should be easy to maintain
by making small adjustments. Large changes in any one of the
operating parameters will effect all the others which means
striking a new balance.
The sludge cake should not crack until just before it
drops off the fabric. This will keep the vacuum pulling air
through the sludge, drying it until the last possible moment
rather than just pulling air through the cracks.
In general, the filter produces more cake as it runs
faster, however, since it is hard to judge production between
a thin, fast moving cake, and a thick, dry cake, the pro-
IX-8
-------
production
inspection
odors
sampling
analysis
duction should be based on the sludge pump speed. The higher
sludge pumping rate corresponds to a greater production. The
optimum filter drum speed is the fastest speed that will
produce a clean discharge of the cake. An exception to this
may occur when dewatered sludge is to be incinerated and a
very dry cake is desired. In this case, moisture content
and incinerator capacity govern the drum speed.
The quality of the cake should be observed along with the
breakup of the cake as it falls from the filter fabric. After
gaining some operating experience it should be possible to
roughly judge the operation of the filter by visual
appearance.
Some odors are generated by the vacuum filtration
process, but proper preconditioning, chemical conditioning,
and ventilation should minimize the problem.
Sampling should be performed as outlined under
MONITORING. These samples may be obtained through valves
provided in the respective thickener piping. If sampling
points are not provided, they should be installed to facili-
tate operation and control of the process. Samples of the
supernatant can be obtained at the overflow weir.
Samples should be analyzed according to procedures
specified in Standard Methods.
EMERGENCY OPERATING PROCEDURE
Loss of Power
Short power interruptions should not greatly affect the
vacuum filtration system. Although electrical equipment
will not operate, the process will not deteriorate if power
is regained within about 30 minutes to an hour. During a
prolonged outage, septic conditions may develop and it may
be advisable to shutdown, drain, and washdown the equipment.
Loss of Other Treatment Units
Loss of treatment units preceding or following the
vacuum filter should not affect filter operation significant-
ly except that the performance or yield may change.
IX-9
-------
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Improper filter
media specified
with result that
(a) filter blinds
or (b) has inade-
quate solids
capture.
2. Improper chemical
conditioning sys-
tem specified.
No provisions for
cleaning of filtrate
lines.
Cake does not
release well
from belt-type
filter.
Solution
1. Run filter leaf
tests with different
media. Replace media
with best one.
Run filter leaf tests
to determine proper
conditioning chemical
and dosage.
Install unions or tees
in filtrate line to
permit ready cleaning.
Add blade to supplement
discharge roll.
5. Filtrate pumps are
easily air bound.
5. Install an equalizing
line from high point of
receiver to the top of
the pump casing.
IX-10
-------
TROUBLESHOOTING GUIDE
VACUUM FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. High solids in
filtrate.
la. Improper coagulant
dosage.
Ib. Filter media
blinding.
la. Coagulant dosage.
Ib. Coagulant feeder
calibration.
Ic. Visually inspect
media.
la. Change coagulant dosage.
Ib. Recalibrate coagulant feeder.
Ic. Synthetic cloth -
detergent and steam wash
steel coils - acid clean cloth-
water wash or exchange for new.
2. Thin cake with poor
dewatering.
2a. Filter media blind-
ing.
2b. Improper chemical
dosage.
2c. Inadequate vacuum.
2d. Drum speed too high.
2e. Drum submergence
too low.
2a. Inspect media.
2b. See la.
2c. Amount of vacuum,
leaks in vacuum sys-
tem, leaks in seals.
2d. Drum speed.
2e. Drum submergence.
2a. See Ib.
2b. See la.
2c. Repair vacuum system
(See 3 also).
2d. Reduce drum speed.
2e. Increase drum submergence.
3. Vacuum pump stops.
3a. Lack of power.
3b. Lack of seal water.
3c. Broken V-belt.
3a. Heater tripped.
3b. Source of seal water.
3c. V-belt.
3a. Reset pump switch.
3b. Start seal water flow.
3c. Replace V-belt.
4. Drum stops rotating.
4a. Lack of power.
4a. Heater tripped.
4a. Reset drum rotation switch.
5. Receiver is vibrat-
ing.
5a. Filtrate pump is
clogged.
5a. Filtrate pump output.
5a. Turn pump off and clean.
-------
TROUBLESHOOTING GUIDE
VACUUM FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5b. Loose bolts and
gasket around
inspection plate.
5c. Worn ball check in
filtrate pump.
5d. Air leaks in suction
line.
5e. Dirty drum face.
5f. Seal strips missing.
5b. Inspection plate.
5c. Ball check.
5d. Suction line.
5e.
5f.
Drum face.
Drum.
5b. Tighten bolts and align gasket.
5c. Replace ball check.
5d. Seal leaks.
5e. Clean face with pressure hose.
5f. Replace seal strips.
H
X
6. High vat level.
6a. Improper chemical
conditioning.
6b. Feed rate too high.
6c. Drum speed too slow.
6d. Filtrate pump off or
clogged.
6e. Drain line plugged.
6f. Vacuum pump stopped.
6g. Seal strips missing.
6a. Coagulant dosage.
6b. Feed rate and solids
yield.
6c. Drum speed.
6d. Filtrate pump.
6e. Drain line.
6f. See item 3.
6g. Drum
6a. Change coagulant dosage.
6b. Reduce feed rate.
6c. Increase drum speed.
6d. Turn on (or clean) pump.
6e. Clean drain line.
6f. See item 3.
6g. Replace seal strips.
7. Low vat level.
7a. Feed rate too low.
7b. Vat drain valve
open.
7a. Feed rate.
7b. Vat drum valve.
7a. Increase feed rate.
7b. Close vat drain valve.
-------
TROUBLESHOOTING GUIDE
VACUUM FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8. High amperage draw
by vacuum pump.
8a. Filtrate pump
clogged.
8b. Improper chemical
conditioning.
8c. High vat level.
8d. Cooling water flow
to vacuum pump to
high.
8a. Filtrate pump output.
8b. Coagulant dosage.
8c. See item 6.
8d. Cooling water flow.
8a. Turn pump off and clean.
8b. Change coagulant dosage.
8c. See item 6.
8d. Decrease cooling water flow.
H
x
i
9. Scale buildup on
vacuum pump seals.
9a. Hard, unstable
water.
9a. Vacuum pump seals.
9a. Add a scale inhibitor.
-------
MAINTENANCE CONSIDERATIONS
A good preventive maintenance program will reduce
breakdowns which could be not only costly, but also very
unpleasant for operating personnel. Plant components includ-
ing the following should be inspected semiannually for wear,
corrosion, and proper adjustment:
1. Drives and gear reducers
2. Drive chains and sprockets
3. Shaft bearings and bores
mechanical 4. Bearing brackets
5. Baffles and weirs
6. Electrical contacts in starters and relays
7. Suction lines and sumps
8. Vacuum pump or system
SAFETY CONSIDERATIONS
At least two persons should be present when working
in areas not protected by handrails. Walkways and work
areas should be kept free of grease, sludge, oil, leaves
and snow. Protective guards and covers must be installed
unless mechanical/electrical equipment is locked out of
operation. Avoid acid cleaning of filtrate lines because
of the potential for explosions.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
2. WPCF Manual of Practice No. 17 (WPCF MOP No. 17),
Paints and Protective Coatings for Wastewater Treatment
Facilities.
3. WPCF Manual of Practice No. 20, Chapter 4, Sludge
Dewatering.
Glossary of Terms and Sample Calculations
Solids content, also called percent total solids, is
the weight of total solids in sludge per unit total
weight of sludge, expressed in percent. Water content
plus solids content equals 100 percent. This includes
all chemicals and other solids added to the sludge.
IX-14
-------
2. Sludge concentration is the weight of solids per unit
weight of sludge. It can be calculated in percent as
follows:
weight of dry sludge solids
Concentration = r—:— ~— x 100
weight of wet sludge
3. Loading rate is the loading of the dry weight basis
sludge solids divided by the area of the vacuum filter
drum. The dry weight of the solids must include chemi-
cals that are added.
4. Filtrate is the effluent or liquid portion of a sludge
removed by or discharged from a filter.
IX-15
-------
PRESSURE FILTRATION
-------
CONTENTS
Process Description X-l
Typical Design Criteria and Performance X-3
Staffing Requirements X-3
Monitoring X-5
Normal Operating Procedures X-6
Startup X-6
Routine Operations X-6
Cake Discharge X-7
Shutdown X-7
Control Considerations X-8
Physical Control X-8
Process Control X-8
Emergency Operating Procedures X-9
Loss of Power X-9
Loss of Other Treatment Units X-9
Common Design Shortcomings X-9
Troubleshooting Guide X-10
Maintenance Considerations X-12
Safety Considerations X-12
Reference Material X-12
References X-12
Glossary of Terms and Sample Calculations X-12
-------
PROCESS DESCRIPTION
process
operation
There are several types of presses available but the
most common consists of vertical plates which are held in a
frame and which are pressed together between a fixed and
moving end as shown in Figure X-l (see following page). A
cloth is mounted on the face of each individual plate.
Despite its name, the filter press does not close to squeeze
or press sludge. Instead, the press is closed and then
sludge is pumped into the press at pressures up to 225 psi
and passes through feed holes in the trays along the length
of the press. Filter presses usually require a precoat
material (incinerator ash or diatomaceous earth are typically
used) to aid in solids retention on the cloth and release
of the cake.
The water passes through the cloth, while the solids
are retained and form a cake on the surface of the cloth.
Sludge feeding is stopped when the cavities or chambers
between the trays are filled. Drainage ports are provided
at the bottom of each press chamber. The filtrate is
collected in these, taken to the end of the press, and
discharged to a common drain.
The dewatering step is completed when the filtrate flow
is near zero. At this point the pump feeding sludge to the
press is stopped and any back pressure in the piping is
released through a bypass valve. The electrical closing
gear is then operated to open the press. The individual
plates are next moved in turn over the gap between the plates
and the moving end. This allows the filter cakes to fall
out. The plate moving step can be either manual or automatic.
When all the plates have been moved and the cakes released,
the complete rack of plates is then pushed back by the moving
end and closed by the electrical closing gear. The valve to
the press is then opened, the sludge feed pump started, and
the next dewatering cycle commences. Filter presses are
normally installed well above floor level, so that the cakes
can drop onto conveyors or trailers positioned underneath
the press.
Filtrate quality should be very good (less than 100 mg/1
suspended solids) if the system is properly operated.
During the early part of the cycle, the drainage from a large
press can be in the order of 2,000 to 3,000 gallons per hour.
This rate falls rapidly to about 500 gallons per hour as the
X-l
-------
FIXED END
TRAVELING END
ELECTRIC
CLOSING GEAR
OOCOO
OPERATING HANDLE
Q.
design
differences
Figure X-l. Side view of a filter press.
cake forms and at the end of the cycle the rate is virtually
zero. Filtrate is normally returned to the plant treatment
process.
One modification to the filter press is a vertical
press with horizontal pressing modules. An endless filter
cloth passes through the stacked modules and then through a
washing chamber. Each module forms a cavity and is designed
with a sealing mechanism at the end opposite the point of
feed. The sealing mechanism is lowered onto the cloth during
the charging and pressing cycles and retracts during the
discharging cycle. Once the pumps have filled the modules
with sludge, an impervious diaphragm at the top of each module
is pneumatically activated to squeeze the water from the
sludge. After the dewatering step, the filter cloth advances
to remove the cake from the modules. Although the unit has
a much smaller filter area than the filter leaf press, reason-
able yields are possible because of the reduced cycle time.
The pressures which may be applied to a sludge for re-
moval of water by filter presses now available range from
5,000 to 20,000 times the force of gravity. In comparison,
a solid bowl centrifuge provides forces of 700 to 3,500 times
the force of gravity and a vacuum filter, 1,000 times the
force of gravity. As a result of these greater pressures, ,
filter presses may provide higher cake solids concentrations
X-2
-------
(30 to 50 percent solids) at reduced chemical dosage. In
some cases, ash from a downstream incinerator is recycled as
a sludge conditioner.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Typical loading rates and results from pressure filtra-
tion of various sludges are shown in Table X-l (see following
page). This data was developed from "Process Design Manual
for Sludge Treatment and Disposal", (EPA 625/1-74-006).
Typical performance criteria are the pressing cycle length,
the solids content of the cake, and the quality of the
filtrate. Performance of filter press on various sludges
will vary widely, but the data in Table X-l are typical.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
manual filter presses are shown in Table X-2. The require-
ments are based on the displacement (size) of the press and
continuous operation with a two hour cycle. The labor
requirements were developed from actual plant operation
experience.
TABLE X-2. FILTER PRESS LABOR REQUIREMENTS
Filter press Labor, hr/yr
volume, cu ft Operation Maintenance Total
Less than 50 6,000 1,500 7,500
100 6,500 1,600 8,100
200 8,400 2,100 10,500
400 13,600 3,400 17,000
800 26,400 6,600 33,000
X-3
-------
TABLE X-l. TYPICAL RESULTS PRESSURE FILTRATION
Sludge type
Primary
Primary + FeCl3
Primary + 2 stage
high lime
Primary + WAS
Primary + (WAS
FeCl3)
(Primary + FeCl3)
+ WAS
WAS
WAS + FeCl3
Digested Primary
Digested Primary
+ WAS
Digested 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, 15% Lime
250% Ash
5% FeCl3, 10% Lime
6% FeCl3, 30% Lime
5% FeCl3, 10% Lime
100 % Ash
5% FeCl3, 10% Lime
10% Lime
None
Typical % solids
Feed cycle filter cake
solids, % length, hr solids, %
5
4*
7.5
8*
8*
3.5*
5*
5*
8
6-8*
6-8*
4*
8*
2
1.5
4
1.5
2.5
2.0
3
4
2.5
2.0
3.5
2
2
1.5
3
6
1.5
45
50
40
50
45
50
45
40
45
50
45
40
45
50
40
35
55
* Thickening used to achieve this solids concentration.
X-4
-------
MONITORING
5 TOTAL SOLIDS
O
5
z
i BOD
o
£ SUSPENDED
SOLIDS
LU
O
C3
3 FLOW
N
CO
Y^
Z Q
< O
Q. —
ALL
ALL
ALL
ALL
O
"Z.
UJ
^3
. Q
co u-1
11 1 ft"
1- U-
1/D
2/W
1/D
R
O
Z
1 — |
<
5 co
G
G
G
R
1 —
_ CO
^- 1 1 1
O i—
CO
< oc
LU O
EC LL.
P
p(U
P
P
^FILTRATE
'RECYCLE TO
PLANT
INFLUENT
SLUDGE
FEED
•SLUDGE
CAKE
A. TEST FREQUENCY
R RECORD CONTINUOUSLY
D DAY
W WEEK
B. LOCATION OF SAMPLE
S SLUDGE FEED
C SLUDGE CAKE
F FILTRATE
C. METHOD OF SAMPLE
G
R
GRAB SAMPLE
RECORD CONTINUOUSLY
D. REASON FOR TEST
P PROCESS CONTROL
E. FOOTNOTES:
1 FOR CONTROL OF PROCESS
RECEIVING THIS FLOW.
X-5
-------
NORMAL OPERATING PROCEDURES
Startup
4.
5.
Routine Operations
1.
2.
Check the plates to make sure they are aligned properly,
that fabric is in place, and that there are no obstruc-
tions to operation.
For safety, filter press installations are usually
equipped with a photo-electric light that surrounds the
press and stops the closing mechanism if the light beam
is interrupted. This system must be checked for proper
operation:
a. Check that the light curtain is illuminated after
switching it on.
b. Check that the closing motor stops when the light
beam is blocked.
Close the press. The press is usually closed by advanc-
ing the control handle forward. The press will continue
to run until it is fully closed and disengages itself
from the driving gear box. At this point a change in
pitch of the motor whine will be heard.
Several manufacturers suggest the following procedure
to insure that the press is completely closed and safe
before sludge feed. If cast iron plates are used, pull
back the control handle to the central position, cutting
off the motor momentarily, then restart again by advanc-
ing the handle forward just once for a short burst.
This will insure that the press is completely home. If
rubber trays are used repeat this closing procedure two
or three times.
Start sludge feed pump(s).
Start up chemical conditioning .
Inspect system regularly as the cycle progresses.
Carry out maintenance as required including cleanup
and washdown.
3. Take samples as outlined in MONITORING section.
X-6
-------
Cake Discharge
Monitor the cycle progress to determine when the cycle
is complete and the cake is ready for discharge from
the press.
Before opening the press, shutdown the feed to that
press and the chemical conditioning if necessary. Be
sure that all valves are closed, and there is no pressure
indicated on the pressure gauge.
Move the operating level to the "open" position. This
displaces the moving end back to its final position and
starts the tray or plate moving mechanism. The plates
are opened one at a time allowing the operator to observe
the cake discharge.
If there is any cake remaining on a cloth the operator
may step into the light curtain which stops the press
and allows him to clean that cloth. The press will re-
start when the operator moves out of the light curtain.
Care should be taken during discharge to ensure that no
cake sticks to the gasket area of the cloth and that the
cloths are not wrinkled on the gasket area.
After the last plate has been moved and the cake dis-
charged, close the press as outlined under "startup" in
preparation for the next cycle.
When discharging the press, check each plate feed port
to be sure it is not plugged. A blocked feed port will
starve the plate(s) resulting in uneven pressures and
possible damage to the mechanism.
Shutdown
3.
4.
Run the press through a "cake discharge cycle". For
press shutdown ensure that all feed lines are rinsed,
that all filter cloths and plate gaskets are clean and
that the plate feed eyes are clear. Turn off light
curtain and power to press.
Wash down all plates, fabric, and parts of the press
carefully.
Rinse out feed lines .
Turn off light curtain and power to the press .
X-7
-------
CONTROL CONSIDERATIONS
Physical Control
Instrumentation is usually minimal, however, it is
possible to completely automate the operation of the filter
press if desired. Pressure gauges should be provided to
monitor the feed pressures and the filtrate flow must be
monitored either visually or with a flow indicator.
Process Control
moisture
control
filtrate
flow
precoat
cloth
character-
istics
sampling
analysis
If the filter press is operated as recommended with
sufficient washing and air drying time between cycles, the
cake should have the highest possible solids content. It
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 relation-
ship 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.
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 pre-
coat acts as an additional filtration membrane and also aids
in a clean removal of sludge from the cloth. If the invest-
ment 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. This is unlikely to happen, however, other cloth
types can be tried until the desired performance is obtained.
The addition of precoat may also aid in performance.
Sampling should be performed as outlined under
MONITORING. These samples may be obtained through valves
provided in the respective piping or directly from the
process. If sampling points are not provided, they should
be installed to facilitate operation and control of the
process.
Samples should be analyzed according to procedures
specified in Standard Methods.
X-8
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
A loss of power will not adversely affect the perform-
ance of the filter press except to interrupt its operation.
During opening or closing of the press return the control
lever to its neutral position until power is restored then
continue operation.
Loss of Other Treatment Units
The operation and performance of other treatment units
has little affect on the operation of filter press. Perform-
ance however, may be somewhat affected by a reduced solids
concentration of the incoming sludge.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Gravimetric Ash
Feeders Installed -
bulking problems
with ash.
Solution
1. Install Volumetric Feeders.
2. Cake transport
system inadequate
(screw conveyors
plug; belt conveyor
limited to 15°
slope).
3. Mechanical Ash
Conveyor Installed -
noise and mainte-
nance problems.
4. Improper media
specified - poor
cake discharge,
difficult to clean.
2. Install heavy-duty
flight conveyor.
3. Install pneumatic ash
conveying system.
4. Change media; usually
a relatively coarse
monofilament media is
used on municipal
sludges.
X-9
-------
TROUBLESHOOTING GUIDE
PRESSURE FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Plates fail to seal.
la. Poor alignment.
Ib. Inadequate shimming.
la. Alignment
Ib. Stay bosses.
la. Realign plates.
Ib. Adjust shimming of stay bosses.
2. Cake discharge is
difficult.
2a. Inadequate precoat.
2b. Improper condition-
ing.
2a. Prevent feed.
2b. Conditioner type and
dosage.
2a. Increase percoat, feed @ 25-40
psig.
2b. Change conditioner type on
dosage based on filter leaf
tests.
3. Filter cycle times
excessive.
3a. Improper condition-
ing.
3b. Feed solids too low.
3a. Chemical dosage.
3b. Operation of thicken-
ing processes.
3a. Change chemical dosage.
3b. Improve solids thickening to
increase solids concentration
in press feed.
4. Filter cake sticks
solids conveying
equipment.
4a. Change chemical con-
ditioning by using
more inorganic
chemicals.
4a. Conditioning dosage.
4a. Decrease ash, increase inorgani
conditioners.
5. Precoat pressures
gradually increase.
5a. Improper sludge
conditioning.
5b. Improper precoat
feed.
5c. Filter media plugged.
5d. Calcium buildup in
media.
5a. Conditioning dosages.
5b. Precoat feed.
5c. Filter media.
5a. Change chemical dosage.
5b. Decrease precoat feed substan-
tially for a few cycles, then
optimize.
5c. Wash filter media.
5d. Acid wash media (inhibited
muriatic acid).
-------
TROUBLESHOOTING GUIDE
PRESSURE FILTBATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
6. Frequent media
binding.
6a. Precoat inadequate.
6b. Initial feed rates
too high (where no
precoat used).
6a. Precoat feed.
6a. Increase precoat.
6b. Develop initial cake slowly.
7. Excessive moisture
in cake.
la. Improper condition-
ing.
7b. Filter cycle too
short.
7a. Conditioning dosage.
7b. Correlate filtrate
flow rate with cake
moisture content.
7a. Change chemical dosage.
7b. Lengthen filter cycle.
x
i
Sludge blowing
out of press.
8a. Obstruction, such
as rags, in the
press forcing sludge
between plates.
8a. Shutdown feed pump, hit press
closure drive, re-start feed
pump - clean feed eyes of
plates at end of cycle.
Leaks around lower
faces of plates.
9a. Excessive wet cake
soiling the media
on lower faces.
9a. Cake moisture
content.
9a.
See item 7.
-------
MAINTENANCE CONSIDERATIONS
mechanical
cloth
washing
rubber
A good preventive maintenance program will reduce
breakdowns which could be not only costly, but also very
unpleasant for operating personnel. Plant components
including the following should be inspected semi-annually for
wear, corrosion, and proper adjustment.
1. Drives and gear reducers
2. Drive chains and sprockets
3. Closing mechanism
4. Bearing brackets
5. Electrical contacts in starters and relays
6. Suction lines and sumps
Occasionally it may be necessary to wash the cloths in
place. When this is done the cloths or media should be pulled
square and free of any creases. A hand-held, high pressure,
single jet (about 750 psi) is usually effective for cleaning
of the media. A plastic cover draped over the filter will
be needed to confine spray during the cleaning cycle.
Mechanized washing arrangements are available for some
filters. Where acid washing is provided, a recirculating
system provides both a scrubbing and acid effect as opposed
to merely soaking the media in acid.
The rubber surfaces of the plates should be scraped only
with soft plastic or wood to avoid damage.
SAFETY CONSIDERATIONS
Filter presses are normally equipped with a light curtain
which automatically shuts down the plate shifting cycle if
someone falls or reaches into the machine. Face shields and
rubber gloves should be worn during acid cleaning of media.
The feed pumps develop high pressures and the press should
not be opened until these pressures are relieved.
REFERENCE MATERIAL
References
Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
WPCF Manual of Practice No. 17 (WPCF MOP No. 17),
Paints and Protective Coatings for Wastewater
Treatment Facilities.
X-12
-------
Glossary of Terms and Sample Calculations
1. Solids content, also called percent total solids, is
the weight of total solids in sludge per unit total
weight of sludge, expressed in percent. Water content
plus solids content equals 100 percent. This includes
all chemicals and other solids added to the sludge.
2. Sludge concentration is the weight of solids per unit
weight of sludge. It can be calculated in percent as
follows:
weight of dry sludge solids
concentration = r——— 7—r^ x 100
weight of wet sludge
X-13
-------
XI
BELT FILTRATION
-------
CONTENTS
Process Description XI-1
Typical Design Criteria and Performance XI-1
Staffing Requirements XI-5
Monitoring XI-6
Normal Operating Procedures XI-7
Startup XI-7
Routine Operations XI-7
Shutdown XI-7
Control Considerations XI-8
Physical Control XI-8
Process Control XI-8
Emergency Operating Procedures XI-9
Loss of Power XI-9
Loss of Other Treatment Units XI-10
Common Design Shortcomings XI-10
Troubleshooting Guide XI-11
Maintenance Considerations XI-13
Safety Considerations XI-13
Reference Material XI-13
References XI-13
Glossary of Terms and Sample Calculations XI-13
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PROCESS DESCRIPTION
process
design
differences
filtrate
return to
process
Several types of dewatering devices are included in
this section:
Moving screen concentrators
Belt pressure filters
Capillary dewatering systems
Rotary gravity concentrators
These systems attempt to overcome the sludge pick-up
problem occasionally experienced with rotary vacuum filters.
A combination of sludge conditioning, gravity dewatering and
pressure dewatering is utilized to increase the solids con-
tent of either digested or undigested sludge. Sludge con-
ditioning, usually with polymers, may not be necessary with
some types of easily dewatered sludges such as raw primary.
In all these units, the influent mixture of solids and
polymer (or other chemical) is placed onto a moving porous
belt or screen. Dewatering occurs as the sludge moves
through a series of rollers which squeeze the sludge to the
belt or squeeze the sludge between two belts much like an
old washing machine wringer. The cake is discharged from the
belt by a scraper mechanism.
Many physical differences exist between various belt
filters. For example, the type of filtration belt used for
each unit varies in size, porosity and material. The opera-
tor should note the specific operation and maintenance re-
quirements of the equipment at his plant. Flow schematics
for some of the various designs are shown in Figures XI-1
through XI-4 (see following pages).
Filtrate from the belt filtration unit is usually re-
turned either to the primary or secondary treatment process
and normally causes no problem to process operation.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Belt filtration units are usually designed on the basis
of the sludge feed rate. Most manufacturers offer several
unit sizes that will handle various sludge feed rates.
Typically, the plant requirements, depending on the quantity
and type of sludge, are matched to one or more of the unit
XI-1
-------
X
H
I
to
Figure XI-1. Moving screen concentrator.
-------
FINAL
COMPRESSION
ZONE
FREE WATER
DISCHARGE ZONE
CAPILLARY
DEWATERING
ZONE BELT
DEWATERING
ZONE
Figure XI-2. Capillary dewatering system.
INFLUENT
SLUDGE
GUIDE
ROLLER
CONTINUOUS
PRESSURE BELT
PRESSURE
ROLLERS
DRIVE
JDLLER
CONTINUOUS
FILTER BELT
SUPPORT
ROLLERS
GUIDE
ROLLER
SCRAPER
DRAINING ZONE PRESS ZONE SHEAR ZONE
Figure XI-3. Belt pressure filter.
XI-3
-------
GUIDE WHEEL
CONVEYOR
Concentrator
CAKE DISCHARGE
SLUDGE INLET
EFFLUENT
Multiroll press
Figure XI-4. Dual cell gravity concentrator.
-------
sizes available from the manufacturer.
Reported performance from actual installations for belt
filtration units are shown in Table XI-1.
TABLE XI-1. BELT FILTRATION UNIT PERFORMANCE
Influent Sludge
sludge cake
solids, solids,
Sludge type % %
Moving screen concentrator
Activated 0.5-1.0 8-10
Primary 20-30
Belt pressure filters
Primary 5.7 19
Capillary dewatering systems
Activated 1.0-1.5 15-18
Dual cell gravity with multiroll press
Raw primary 3 20-23
Digest 0.5-4.0 16-20
WAS 1.9-3.0 18-23
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of belt
filtration units are shown in Table XI-2. The requirements
are based on the number of units in use at the plant and in-
clude periodic operational adjustments and minor routine
maintenance. Removal of sludge is not included. Labor re-
quirements are based on experience at actual installations.
TABLE XI-2. BELT FILTRATION UNIT LABOR REQUIREMENTS
Number
of
units
1
2
3
4
5
Operation
265
530
795
1,060
1,325
Labor, hr/yr
Maintenance
100
200
300
400
500
Total
365
730
1,095
1,460
1,825
XI-5
-------
MONITORING
SLUDGE
FEED
ooo
ooo
SF
l-
FILTRATE
RETURNED TO
PLANT
DEWATERED
SLUDGE CAKE
DS
ID
1 TOTAL SOLIDS
5
S BOD
\-
LU SUSPENDED
0 SOLIDS
FLOW
O
ULJ
^
i — tj
00 HI
LU CC
1- LL
1/D
2/W
1/D
R
LU
0 Q.
f— 2
^ ^
0 w
O LL
-J O
SF, DS
F
F
F
LL
0
Q LU
0_J
1 «
^- ^
LU <
G
G
G
R
H-
co
O H
00
< CE
LU O
DC LL.
P
P
P
P
A. TEST FREQUENCY
R = RECORD
CONTINUOUSLY
D= DAY
W= WEEK
B. LOCATION OF SAMPLE
SF=SLUDGE FEED
DS=DEWATERED
SLUDGE
F = FILTRATE
C. METHOD OF SAMPLE
G = GRAB SAMPLE
R = RECORD
CONTINUOUSLY
D. REASON FOR TEST
P = PROCESS CONTROL
XI- 6
-------
NORMAL OPERATING PROCEDURES
Startup
1. Belt filtration equipment should be inspected and
operated to make certain that installation is correct,
proper clearance and adjustments have been made, and
that belt tracks properly.
2. Check chemical tank and mixing apparatus to assure
that enough chemical is available for completion of
the run.
3. Set sludge and chemical pumps to the predetermined rate.
4. Start dewatering unit.
5. Start spray water.
6. Start sludge and chemical pumps.
7. Adjust speed of belts or screens, chemical pumping rate
and sludge pumping rate as necessary to establish a
balance of flow.
Routine Operations
1. Inspect system at least twice per shift.
2. Check tracking of filter belt, adjust if necessary.
3. Carry out maintenance as required including cleanup
and washdown.
4. Take samples as outlined in MONITORING Section.
Shutdown
1. Inspect system at least once per shift.
2. Shutdown sludge and chemical pumps.
3. Shutdown the dewatering unit.
4. Turn off water sprays.
5. Drain sludge and water from unit and clean thoroughly
with hose.
XI-7
-------
CONTROL CONSIDERATIONS
Physical Control
Correct tracking of the filter belt is very important
to assure minimum wear and damage to the belts. Some units
belt are equipped with automatic adjusting devices designed to
tracking correct roller alignment automatically. Other units require
a periodic check and adjustment, if necessary, to be made by
the plant operator.
Correct adjustment of spray nozzles used to clean the
spray underside of the belt or screen is also important. Sludge
adjustment buildup on the underside of the belt creates a tracking
problem. Just enough spray should be used so that the under-
side of the belt remains clean.
Process Control
inspection
sampling
analysis
sludge
conditioning
The filtrate should be relatively clear and no exces-
sive sludge buildup should be occurring anywhere along the
belt or rollers. Once the operator is familiar with this
equipment it should be possible to judge the operation of
the belt filtration unit by visual appearance.
Sampling should be performed as outlined under MONITOR-
ING. Influent sludge and filtrate samples may be obtained
through valves provided. Dewatered sludge samples may be
obtained after the sludge has been removed from the belt by
the scraper mechanism.
Samples should be analyzed according to procedures
specified in Standard Methods.
Proper sludge conditioning is an important step in
any sludge dewatering process. Sludge conditioning re-
sults in flocculation of the small sludge particles into
larger particles which have enough size and strength to
bridge the openings in the filter belt and, thus, be re-
tained on that belt.
In order to determine the best chemicals and chemical
dosages to use, jar testing should be performed on several
sludge samples. The optimum dosage will be that above which
little or no increase in floe size or supernatant clarity
is noted.
In addition to the chemicals, the following parameters
will affect the final percent solids concentration obtained
by the belt filtration unit:
XI-8
-------
1. Incoming sludge percent solids
2. Loading or application rate (Ib/hr) of sludge to belt
filtration unit
solids
loading
rate
3. Operating speed of belt filtration unit
4. Compression of the pressure rollers
In general, a thicker incoming sludge will produce a
drier cake. However, varying the initial solids concentra-
tion is not normally used as a process control variable.
It is customary, unless special conditions apply, to deliver
as thick a sludge as practical to the belt filtration unit.
The sludge loading rate or application rate has a
significant affect on the performance of the belt filtration
unit.
A loading rate that is too high will cause poor perfor-
mance. The ideal loading rate is the highest rate at which
the system can be run without a drop in the desired perfor-
mance. This rate is dependent on the rate of travel of the
filter belt.
belt rate
of travel
compression
The speed of the filter belt should be increased along
with a corresponding increase in the rate of sludge feed.
The exact speed at which the unit should be operated depends
on the results desired in terms of sludge cake dryness, of
percent sludge retained on the filter belt, and the dewater-
ing rate of the sludge. This speed can only be determined
by trial and error operation. Once this setting has been
determined, infrequent minor adjustments should be required.
If the unit is to be shut down, these settings should be
noted for use when restarting.
Determination of the best compression of the pressure
rollers may require a certain amount of experimentation
through actual operation to set properly. Once set they
should require little adjustment.
EMERGENCY OPERATING PROCEDURES
Loss of Power
Belt filtration units will not operate during power
interruptions. During a power loss, shutdown procedures
should be followed, including washing down of equipment
to prevent any clogging from dried sludge. When power is
restored, normal startup procedure should be followed.
XI-9
-------
Loss of Other Treatment Units
Loss of other treatment units should not greatly affect
the operation of the belt filtration unit. Performance may
be affected and process readjustment required if the malfunc-
tioning process causes a decrease in the percent solids of
the sludge pumped to the belt filtration unit.
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Corrosion of steel
components.
Solution
2. Filter belt creeps
off rollers, will
not track properly.
Coat surfaces with proper
paint. Industrial paint
suppliers and appliers can be
located in the yellow pages
of large city telephone direc-
tories. These suppliers can
furnish complete recommenda-
tions on proper coating sys-
tems for various application.
See also WPCF MOP No. 17.
Check automatic tracking
device., if one exists, for
proper operation. Check
bottom of filter belt and
surface of drive roller for
sludge buildup. If buildup
occurs, water spray is not
adequate. Increase spray
pressure or install new
spray heads.
XI-lo
-------
TROUBLESHOOTING GUIDE
BELT FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Dewatered sludge not
thick enough.
la. Sludge application
rate too high.
Ib. Belt speed too high.
Ic. Incorrect polymer
dose.
la. Check sludge pumping
rate.
Ib. Check belt speed.
Ic. Check polymer mixing
and dose.
la. Adjust influent sludge pumping
rate.
Ib. Adjust belt speed.
Ic. If polymer dose is much less
or much greater than the
ideal dose, performance will
decrease. Use jar test pro-
cedure to determine optimum
dose.
Excessive belt wear.
2a. Improper alignment of
rollers.
2b. Sludge buildup on
bottom of belt or on
rollers causing im-
proper alignment.
2a. Check tracking of
belt to see if it
creeps off to one
side.
2b. Check operation of
automatic belt adjust-
er.
2c. Check bottom of belt.
2a. Adjust alignment of rollers.
2b. Replace or repair faulty adjust
or mechanism.
3. Solids in filtrate.
3a. Incorrect polymer
dose.
3b. Solids running off
the edge of the
filter belt.
3a. Check polymer mixing
and dose.
3b. Check influent sludge
pumping rate.
3c. Check belt rate of
travel.
3a. Use jar test to determine op-
timum dose.
3b. Reduce sludge pumping rate
accordingly.
3c. Adjust belt rate of travel as
required.
4. Oil leak.
4a. Oil seal failure.
4a. Check oil seal.
4a. Replace seal.
-------
TROUBLESHOOTING GUIDE
BELT FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. Noisy or hot bearings
or universal joint.
5a. Excessive wear.
5b. Improper alignment.
5c. Lack of lubrication.
5a. Alignment.
5b. Lubrication.
ba. Replace, lubricate, or align
joint or bearing as required.
x
H
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MAINTENANCE CONSIDERATIONS
A good preventative maintenance program will reduce
breakdowns which could be not only costly, but also very
unpleasant for operating personnel. Plant components in-
cluding the following, should be inspected semiannually for
wear, corrosion, and proper adjustment.
1. V-belts, drives, and gear reducers
2. Porous filter belts
3. Rollers, shaft bearings, and bores
mechanical 4. Bearing brackets
5. Baffles
6. Electrical contacts in starters and relays
7. Suction lines and pumps
8. Chemical mixing pumps and tanks
SAFETY CONSIDERATIONS
Hands and arms should be kept away from moving belts
and rollers. Loose clothing is a hazard and may get caught
in these rotating parts. Always be certain protective
guards and covers are in place unless mechanical/electrical
equipment is locked out of operation. Work areas and walk-
ways should be kept free of grease, oil, leaves, snow, and
sludge.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater.
American Public Health Association
1015 Eighteenth Street, N.W.
Washington, D.C. 20036
2. WPCF Manual of Practice No. 17
(WPCF MOP No. 17), Paints and Protective Coatings for
Wastewater Treatment Facilities.
3. WPCF Manual of Practice No. 11, Chapter 8
Operation of Wastewater Treatment Plants, Sludge
Conditioning.
Glossary of Terms and Sample Calculations
Solids content, also called percent total solids,
is the weight of total solids in sludge per unit
total weight of sludge, expressed in percent. Water
content plus solids content equals 100 percent.
XI-13
-------
2. Solids loading is the feed solids to the belt filter
on a dry weight basis including chemicals per unit
time.
3. Filtrate is the effluent or liquid portion of a sludge
removed by or discharged from a filter.
XI-14
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XII
SLUDGE DRYING BEDS
-------
CONTENTS
Process Description XII-1
Typical Design Criteria and Performance XII-3
Staffing Requirements XII-4
Normal Operating Procedures XII-4
Initial Inspection XII-4
Startup XII-5
Routine Operations XII-5
Control Considerations XII-5
Physical Control XII-5
Process Control XII-6
Emergency Operating Procedures XII-7
Common Design Shortcomings XII-7
Troubleshooting Guide XII-9
Maintenance Considerations XII-10
Safety Considerations XII-10
Reference Material XII-10
References XII-10
Glossary of Terms and Sample Calculations XII-11
-------
PROCESS DESCRIPTION
general
construction
drying
operation
construction
variations
sidestream
Drying beds are generally used for dewatering of well
digested sludges. Attempts to air dry raw sludge usually
results in odor problems.
Sludge drying beds consist of perforated or open joint
drainage pipe laid within a gravel base. The gravel is
covered with a layer of sand. Partitions around and between
the drying beds may be of concrete, wood or earthen embank-
ment. Drying beds are generally open to the weather but may
be covered with ventilated green-house type of enclosures
where it is necessary to dewater sludge in wet climates.
The drying of sludge on sand beds is accomplished by
allowing water to drain from the sludge mass through the
supporting sand to the drainage piping and natural evapo-
ration to the air. As the sludge dries, cracks develop in
the surface allowing evaporation to occur from the lower
layers which accelerates the drying process.
Typical sludge drying bed construction is shown in
Figure XII-1 (see following page).
Many design variations are used for sludge drying beds
including the layout of the drainage piping, thickness and
type of materials in the gravel and sand layers, and con-
struction materials used for the partitions. The major
variation is whether or not the beds are covered. Any cov-
ering structure must be well ventilated. In the past, some
beds were constructed with flat concrete bottoms for drain-
age without pipes, but this construction has not been very
satisfactory.
The only sidestream is the drainage water. This water
is normally returned to the raw sewage flow to the plant or
to the plant headworks. The drainage water is not normally
treated prior to return to the plant.
XII-1
-------
SLUDGE
COLLECTION;
SYSTEM "• '
DRAINAGE ^
-LI
SIDE WALL
"SPLASH SLAB
Figure XII-1. Typical sludge drying bed construction.
XII-2
-------
TYPICAL DESIGN CRITERIA & PERFORMANCE
The following data was developed from "Process Design
Manual for Sludge Treatment and Disposal", (EPA 625/1-74-006)
and "WPCF Manual of Practice No. 20".
Design
Gravel layer depth, inches,
typically 3-inch layers
graded from coarse at
bottom to fine at top
Sand layer depth, inches,
typically 0.55 mm size
Drainage pipe spacing,
feet (typically 6-inch
diameter)
Sludge depth, inches
Typical module size, feet:
Length
Width
Typical design values
12-18
6-12
8-20
8-12
20-100
20-25
open beds
covered beds
Bed sizing, sq ft/capita,
from WPCF, 1959:
Primary digested
sludge
Primary and humus
digested sludge
Primary and acti-
vated digested
sludge
Primary and chemi-
cally precipitated
digested sludge
Performance
1.0 - 1.5
1.25 - 1.75
1.75 - 2.5
2.0 - 2.5
0.75 - 1.0
1.0 - 1.25
1.25 - 1.5
1.25 - 1.5
Solids Loading Rate,
Ib/yr/sq ft
Moisture Content of Dried
Sludge, percent
up to 25
50 - 60
up to 40
50 - 60
XI I-3
-------
Sidestream
The flow from the drainage piping consists primarily of
the initial percolation of water from the sludge plus some
periodic percolation after rain storms (assuming open beds).
Percolation from initial drainage of sludge assuming a
10-inch layer of 5 percent solids sludge and initial drainage
to 18 percent solids will be:
10 inches - ^| (10) = 7.2 inches water
U. lo
OR
4.5 gal/sq ft of bed area drainage over the first
2 or 3 days
Drainage water BOD = 200 to 400 mg/1
Suspended solids = 50 to 100 mg/1
STAFFING REQUIREMENTS
Labor requirements shown in Table XII-1 include removal
of dried sludge from beds, sand maintenance, and weeding as
necessary.
TABLE XII-1. SLUDGE DRYING BEDS, LABOR REQUIREMENTS
Total bed area,
sq ft<*>
1,000
5,000
10,000
50,000
100,000
Operation
300
400
500
1,500
2,900
Labor, hr/yr
Maintenance
100
180
220
710
1,500
Total
400
580
720
2,210
4,400
(* )
Assuming dry solids loading rate of 20 Ib/yr/sq ft of
bed area.
NORMAL OPERATING PROCEDURES
Initial Inspection
1. All lines should be clear of debris and valves checked
for free operation.
2. The sand surface should be level and all irregularities
raked smooth.
XII-4
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3. Clear all debris from surface of bed.
4. Install stoplogs or other blocking device at vehicle
entrance to drying bed (if provided).
5. Make sure a splash plate or other diffusion device is in
place where the sludge enters the bed.
6. Check drainage return system and piping.
Startup
1.
2.
3.
Routine Operations
1.
2.
3.
4.
5.
Start flow of liquid sludge into bed. Stop flow when
the liquid is approximately 8 to 12 inches deep through-
out the bed.
If bed is enclosed, open the ventilation openings.
Do not apply new sludge on top of a layer of dry sludge.
Inspect the beds every few days noting any odors or
insect problems.
Remove any weed growth.
When sludge is dry (normally 3 weeks or longer depending
on weather and depth of sludge) remove the sludge taking
care not to damage the sand and gravel layers. Remove
as little sand with the sludge as possible.
Vehicles and equipment should not be operated directly
on the sand but should be operated on planks or plywood
laid on top of the bed if permanent vehicle treadways
are not provided.
After the sludge is removed, inspect the bed, rake the
surface of the sand to level it and to remove any debris,
and add makeup sand if necessary.
6. The bed is ready for the next application of sludge.
CONTROL CONSIDERATIONS
Physical Control (Instrumentation)
Instrumentation is normally not provided for drying
beds except in some cases sludge and drainage flow rates
may be measured.
XII-5
-------
Process Control
application
to bed
drying
removal
Experience is the best guide in determining the depth
of sludge to be applied, however typical application depth is
8 to 12 inches. The condition and moisture content of the
sludge, the sand bed area available, and the need to draw
sludge from digesters are factors to consider. Do not apply
fresh sludge on top of dried sludge in a bed.
A thinner layer will dry more rapidly permitting quick
removal and reuse of the bed. An 8-inch layer should dry in
about 3 weeks in the open during reasonably dry weather. A
10-inch 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 inches. The best operation
can only be determined by trial and error and may also vary
seasonally.
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 availabil-
ity, and, of course, the desired moisture content of the
dried sludge. Sludge can be removed by shovel or forks at a
moisture content of 60 percent, but if it is allowed to dry
to 40 percent moisture, it will weigh only half as much and
is still easy to handle. If the sludge gets too dry (10 to
20 percent moisture) it will be dusty and will be difficult
to remove because it will crumble as it is removed. Many
operators of smaller treatment plants use wheelbarrows to
haul sludge from drying beds. Planks are often laid on the
bed for a runway so that the wheelbarrow tire does not sink
into the sand. Wheelbarrows can be kept close to the worker
so that the shoveling distance is not great. Most plants use
pickup trucks or dump trucks to transport the sludge from the
drying bed. Dump trucks have the advantage of quick unload-
ing and most municipalities have dump trucks available.
Where trucks are used, it is best to install concrete tread-
ways in the sludge drying bed wide enough to carry the dual
wheels since the drying bed can be damaged if the trucks are
driven directly on the sand. The treadways should be in-
stalled so that good access is provided to all parts of the
beds. If permanent treadways have not been installed, heavy
planks may be placed on the sand. Large plants will normally
utilize mechanical equipment for handling the dried sludge.
Some communities have encouraged public usage of the dried
sludge. In some cases users are allowed to remove the sludge
from the beds, but this may not be satisfactory in many
cases. Local regulations should be reviewed before attempt-
ing to establish a public utilization program.
XII-6
-------
Two basic approaches are available to control or
counteract odors: chemicals sprayed into the atmosphere or
chemicals added to the sludge as it is being placed on the
beds. Chemicals are available which may be sprayed into the
atmosphere in the vicinity of the odor to counteract or mask
odors the odor. Such chemicals are described in the February, 1977
issue of Water and Wastes Engineering magazine. Odors may
also be controlled effectively by adding chloride of lime to
the sludge as it is discharged to the drying beds. Hydrated
lime sometimes is used for odor control, but tends to clog
the sand. These chemicals can be obtained from industrial
chemical suppliers.
Flies may be a problem in certain areas and seasons and
should be controlled by destruction of breeding and use of
traps and poisons. The fly may be controlled most effec-
tively in the larva stage and borax or calcium borate will
kill the larvae. Neither chemical is dangerous to man nor to
domestic animals. These chemicals can be sprinkled on the
flies sludge, especially in the cracks of the drying cake. Other
chemicals sometimes used are chloride of lime and sulfate of
iron. The adult fly can be killed by spraying. Fly trapping
is particularly suited for outdoor conditions. A satisfactory
form of trap consists of a conical, gauze-covered structure
leading into a larger space in which a sugary, poisoned bait
is placed.
EMERGENCY OPERATING PROCEDURES
The only emergency that would affect the operation of
the drying beds is the loss of the sludge digestion process.
Undigested or poorly digested sludge applied to drying beds
is likely to result in odor problems and should not be
attempted.
COMMON DESIGN SHORTCOMINGS
Shortcomings Solution
1. Inadequate drying la. This is a common problem and
bed capacity. typical design criteria are
inadequate for many areas.
Ib. Construct more beds.
Ic. Try to apply drier sludge
to beds.
Id. Remove sludge as soon as it
is dry or remove it in a
wetter state.
XII-7
-------
Shortcomings
2.
Poor or no
drainage system.
3.
Inadequate access
for removal of
dried sludge.
Solution
2a. Best solution is to add a
drainage system as this type
of drying bed is rarely
satisfactory for the intended
purpose.
3a. Cut an opening for vehicle
access into one wall of bed.
3b. Cast concrete treadways
within drying bed supported
from bottom of bed and
extending to surface of sand.
3c. Use planks and plywood laid
on top of beds for access.
XII-8
-------
TROUBLESHOOTING GUIDE
SLUDGE DRYING BEDS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Odors from drying
beds.
1. Incomplete digestion
of sludge.
1. Digestion process.
la. Provide complete digestion.
Ib. Feed chemicals to sludge as it
is applied to bed.
x
H
H
I
(£>
2. Sludge will not dry
(in good weather).
2a. Poor drainage.
2b. Too much sludge
applied to bed.
2a. Drainage piping for
plugging.
2b. Sand and gravel for
plugging.
2a. Completely clean and rake
surface of bed before applying
new sludge.
2b. Repair drainage piping.
2c. Replace sand and gravel if
necessary.
2d. Reduce depth of sludge applied
to bed.
2e. Add 1 pound of alum per 100
gallons of sludge as the
sludge is applied to the bed.
3. Sludge is dusty and
crumbles.
3. Excessive drying.
3.
Moisture content.
3. Remove sludge from bed when it
dries to 40 to 60 percent
moisture content.
-------
MAINTENANCE CONSIDERATIONS
bed
surface
weeding
drainage
sludge
lines
partitions
Some sand is removed during each sludge removal cycle.
The amount depends on the method of removing the dried sludge.
The sand depth should be checked periodically from an
established reference point such as the top of the bed wall
until a pattern is established. Sand should be added when
the depth has decreased to 3 or 4 inches. The surface of the
sand should be levelled and raked prior to each sludge
application.
Plants such as tomatoes and weeds may sprout and grow
in the drying sludge. These growths should be controlled
either by spraying with weed killer or hand pulling.
The drainage system should be inspected and maintained
so that free drainage takes place from the drying beds. It
can be inspected for proper operation shortly after new sludge
is placed in a bed.
Sludge lines and valves should be regularly inspected
and maintained as leaky valves may allow wet sludge to enter
a bed during the drying process. Sludge lines must be drain-
ed after use in winter to prevent freezing.
The partitions between and around the beds should be
tight so that sludge will not flow from one compartment to
another nor outside the beds. If earth beds are used, grass
and other vegetation should be kept cut. Stop logs or other
provisions for closing vehicular access cutouts in drying
bed walls should be kept well maintained to minimize leakage.
SAFETY CONSIDERATIONS
Since anaerobic digestion of sewage sludge produces
combustible gases, smoking or open fires should be pro-
hibited when discharging anaerobically digested sludge to the
drying beds.
REFERENCE MATERIAL
References
1. WPCF Manual of Practice No. 11, Chapter 9, Operation of
Wastewater Treatment Plants, Sludge Dewatering.
2. WPCF Manual of Practice No. 20, Chapter 3, Sludge
Dewatering, Land Method.
XII-10
-------
Glossary of Terms and Sample Calculations
Solids loading rate is the weight of solids on a dry
weight basis applied annually per square foot of drying
bed area. As an example, assume that 10 inches of 5
percent solids anaerobically digested sludge is applied
to a drying bed five times per year. The weight of
solids will be calculated for one square foot of bed.
Solids Loading Rate =
/dry weight of solids) . /square feet ofj
V year / . \^ drying bed /
/cubic feet of sludge) / Ibs\ /% solids\ /Number of \
V square feed of bed / V> ft3/ V 100 J \ applications/
(5)
13 Ibs
year-sq ft
2. Sludge moisture content is the weight of water in a
sludge sample divided by the total weight of the sample.
This is normally determined by drying a sludge sample
and weighing the remaining solids. Total weight of the
sludge sample equals the weight of water plus the weight
of the dry solids. As an example, assume that 100 grams
of sludge is evaporated and produces 5 grams of residue.
solids content = —- x 100 = 5%
moisture content = X— x 100 = 95%
3. 8005 (biochemical oxygen demand) is the amount of oxygen
required for the biological oxidation of degradable
organic content in a liquid, in a specific time, and at
a specified temperature. Results of the standard test
assessing wastewater strength usually are expressed in
mg/1 as 5-day 20°C BOD (BOD5).
4. Suspended solids are solids that either float on the
surface of, or are in suspension in, water, wastewater,
or other liquids, and which are largely removed by
laboratory filtering.
XII-11
-------
5. mg/1 is an expression of the weight of one substance
within another. Commonly it is used to express weight
of a substance within a given weight of water and waste-
water. It is sometimes expressed as parts per million
(ppm) which is equal to mg/1. If there is one pound of
a substance mixed in one million pounds of water the
resulting concentration is one mg/1.
weight of carrying substance
concentration, mg/1 = (water or wastewater )
weight of substance x 10b
XII-12
-------
XIII
LAGOONS
-------
CONTENTS
Process Description • XIII-1
Typical Design Criteria and Performance XIII-1
Staffing Requirements XIII-1
Monitoring XIII-2
Control Considerations XIII-2
Emergency Operating Procedures XIII-3
Common Design Shortcomings XIII-3
Troubleshooting Guide XIII-4
Maintenance Considerations XIII-5
Safety Considerations XIII-5
Reference Material XIII-5
References XIII-5
-------
PROCESS DESCRIPTION
Sludge lagoons are similar to sand beds in that sludge
is periodically drawn from a digester, placed in the lagoon,
removed after a period of drying, and the cycle repeated.
Drying lagoons are not typically provided with an underdrain
system as most of the drying is accomplished by decanting
supernatant liquor and by evaporation. Plastic or rubber
fabrics may be used as a bottom lining, or they may be natural
earth basins. Supernatant liquor and rainwater drain off
points are usually provided with the drain off liquid returned
to the plant for further processing.
TYPICAL DESIGN CRITERIA S PERFORMANCE
Design parameter
Solids loading rate
Area required:
Examples:
Dry climate, primary
sludge
Wet climate, acti-
vated sludge
Dike height
Sludge depth after de-
canting (depths of 2-4
ft have been used in
very warm climates)
Drying time for depth
of 15 in or less
Range
2.2 - 2.4 Ib/yr/cu ft of
lagoon capacity
1 sq ft/capita
3-4 sq ft/capita
2 ft
15 in
3-5 mon
STAFFING REQUIREMENTS
Labor requirements shown in Table XIII-1 (see following
page) include application of sludge to the lagoon, periodic
removal of solids and minor maintenance requirements such as
dike repair.
XIII-1
-------
MONITORING
TABLE XIII-1. SLUDGE LAGOON LABOR REQUIREMENTS
Labor, hr/yr
tons/year
100
1,000
10,000
50,000
Operation
30
55
120
450
Maintenance
55
90
300
1,500
Total
85
145
420
1,950
Monitoring of sludge lagoons generally consists of
sensory observations and interpretations by the plant
operator. However, records may be kept on the sludge loading,
quantity, depth, date, drying time and weather conditions.
This will provide the operator with the information necessary
to determine the optimal time of sludge removal from the
lagoon by correlating sludge moisture content with time of
drying for particular climatic conditions.
CONTROL CONSIDERATIONS
weeds
sludge
depth
dewatering
drying
rate
Weeds and other vegetation should always be removed
from the lagoon area before filling with sludge.
Sludge depth should not exceed 15 inches after excess
supernatant has been drawn off. Unless the lagoon is
situated in an arid climate, depths of over 15 inches will
require excessive drying time.
Sludge will generally not dewater in any reasonable
period of time to the point that it can be lifted by a fork
except in an extremely hot, arid climate. If sludge is
placed in depths of 15 inches or less, it may, typically,
be removed with a front-end loader in 3 to 5 months. When
sludge is to be used for soil conditioning, it may be
desirable to stockpile it for added drying before use. One
approach utilizes a 3-year cycle in which the lagoon is
loaded for 1 year, dries for 18 months, is cleaned, and
allowed to rest for 6 months.
There are few operational variables under the control
of the operator other than pretreatment and thickening of
sludge prior to discharge to the lagoon. Once discharged
to the lagoon, the drying rate is largely dependent upon
weather conditions.
XIII-2
-------
The operator should promptly remove supernatant liquor
and rainwater so that the sludge cake is exposed to oxygen
supsrna ca/21 . .
, in the air and can dry rapidly. Supernatant is normally
returned to the main plant treatment processes.
EMERGENCY OPERATING PROCEDURES
The only emergency that may directly affect the operation
of the sludge lagoon is the loss of the sludge digestion
process. Undigested or poorly digested sludge applied to
lagoons is likely to result in an odor problem and should be
avoided.
COMMON DESIGN SHORTCOMINGS
Adverse weather conditions may prolong drying of sludge,
however, short rainy periods followed by sunny conditions
should pose no problems. Problems of too little lagoon area
may be minimized by always removing the sludge when it is
dry enough and removing supernatant as it forms.
XIII-3
-------
TROUBLESHOOTING GUIDE
LAGOONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Odors from lagoons.
la. Inadequately digest-
ed sludge.
Ib. Excess water in
lagoon.
la. Operation of
digestion process.
la. Establish correct digester
operation (see appropriate
section of manual); apply
lime to surface of lagoon.
Ib. Decant supernatant and rain-
water promptly.
2. Insect breeding
problems in lagoons.
2. Excess water in
lagoon.
2. Decant supernatant and rainwater
promptly; apply insecticides.
3. Supernatant decanted
from lagoon is
upsetting treatment
process when
recycled.
3a. Broken dikes between
lagoons causing
freshly drawn sludge
to enter super-
natant .
3b. Supernatant being
drawn prematurely.
3c. Excessive sludge
depths applied
causing supernatant
drawoff to be below
sludge interface.
3a. Dike condition.
3a. Repair broken dikes.
3b. Suspended solids of
supernatant.
3c. Sludge application
depths.
3b. Delay drawing of supernatant
until sludge has settled.
3c. Apply shallower sludge depths.
-------
MAINTENANCE CONSIDERATIONS
Maintenance requirements are very low for sludge
lagoons. Repairing of broken dikes and decanting of excess
water from rain or snow require minimal operator time. If
odor and fly control become a problem, see the section on
maintenance in the SLUDGE DRYING BED manual for solutions.
SAFETY CONSIDERATIONS
Since anaerobic digestion of sewage sludge produces
combustible gases, smoking or open fires should be prohibited
when discharging anaerobically digested sludge to the lagoon.
Fencing of lagoons may be desirable to prevent trespassing.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
2. WPCF Manual of Practice No. 20, Chapter 3, Sludge
Dewatering.
XIII-5
-------
XIV
HEAT DRYING
-------
CONTENTS
Process Description XIV-1
Typical Design Criteria and Performance XIV-3
Staffing Requirements XIV-4
Monitoring XIV-5
Normal Operating Procedures XIV-6
Startup XIV-6
Routine Operations XIV-6
Shutdown XIV-6
Control Considerations XIV-7
Physical Control XIV-7
Process Control XIV-7
Emergency Operating Procedures XIV-9
Loss of Power XIV-9
Loss of Other Treatment Units XIV-9
Common Design Shortcomings XIV-9
Troubleshooting Guide XIV-11
Maintenance Considerations XIV-12
Safety Considerations XIV-12
Reference Material XIV-13
References XIV-13
Glossary of Terms and Calculations XIV-13
-------
PROCESS DESCRIPTION
process
Heat drying raises the temperature of the incoming
sludge to 212 F (100 C) to remove moisture which reduces total
volume, yet retains the nutrient properties of the sludge.
The end product is odor free, contains no pathogenic organ-
isms, and contains soil nutrients.
types
Sludge has been heat dried in flash drying equipment
and rotary kilns as shown in Figures XIV-1 and XIV-2 (see
following pages) respectively.
flash
drying
Before introduction into the flash dryer, the sludge
must undergo thickening and dewatering. The degree of
dewatering depends on the dewatering process used. The in-
coming dewatered sludge is blended with a portion of the
previously heat dried sludge in a mixer. Hot gases from the
furnace at approximately 1,200° to 1,300°F (650 to 700 C)
then are mixed with the blended sludge before drying in the
cage mill. Agitation in the cage mill dries the sludge to
approximately 2 to 10 percent moisture and reduces the
temperature to approximately 300 F (150 C) before cyclone
separation of the solids from the gases. A portion of the
dried solids are recycled to the cage mill and the rest are
stored for use or incinerated. The gases from the cyclone
separators are conveyed by the vapor fan to the deodoriza-
tion preheater in the furnace where the temperature is raised
to approximately 1,200 to 1,400 F (650 to 760 C). The
deodorized gases release a portion of the heat to the in-
coming gases and release more heat in the combustion air
preheater. The temperature is reduced to approximately
500 F (260 C) before the gas is scrubbed for particulate
removal and conveyed to the stack by the induced draft fan.
If the dried solids are not used in the furnace as a fuel,
then auxiliary fuel such as gas, oil, or coal is necessary.
rotary kiln
The rotary kiln is a cylindrical steel shell mounted
with its axis at a slight slope from the horizontal. De-
watered sludge is fed continuously into the upper end. A
portion of the dried sludge is mixed with the feed sludge
to reduce moisture and disperse the cake. These vanes pick
up the material, then steadily spill it off in the form of
a thin sheet of falling particles as the dryer rotates. This
action is intended to provide contact between sludge and
gases to promote rapid drying. The dried sludge from such
XIV-1
-------
.RELIEF VENT
CYCLONE
COMBINATION AIR FAN
,i.1.1.1.1.i.in.1.1.1.1.i.iii.1.1.1.i.,.1.1.i :
HOT GAS DUCT
REFRACTORY
HOT GAS TO DRYING SYSTEM
DRYING SYSTEM
j SLUDGE
COMBUSTION AIR
DEODORIZED GAS
Figure XIV-1. Flash dryer system.
XIV-2
-------
PRODUCT
differences
Figure XIV-2. Rotary kiln dryer.
a unit will consist of varied sizes of particles that may
require grinding before use. Deodorization of the exhaust
gases by afterburning at approximately 1,200 to 1,400 F
(650 to 760 C) is necessary if odors are to be avoided.
Also, scrubbers must be used to remove particulates from the
exhaust gases.
Generally, the normal operating conditions of the flash
dryer are applicable to the rotary dryer. The differences
arise in that the rotary dryer is direct fired, the tempera-
ture around the cake being controlled at approximately 700 F
(370 C). The dryer rotates at approximately 4 to 8 percent
per minute to ensure mixing as opposed to the rapid mixing
provided in the cage mill on a flash dryer.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Flash dryers and rotary kilns are sized on the basis of
the solids loading rate and heating requirements. The
principles that apply are similar to those used in designing
sludge incinerators. Flash dryers and rotary kilns are
usually available in several module sizes with sludge burn-
ing capacities typically ranging from 40 pounds per hour to
2,400 pounds per hour of sludge feed.
The expected performance from a flash dryer or rotary
kiln is a dried sludge with a moisture content ranging from
2 to 10 percent.
XIV-3
-------
Heat drying in general, produces an exhaust that contains
unacceptable quantities of air pollutants. Therefore, the
system design usually includes equipment necessary to reduce
the emissions to acceptable levels. This may require partic-
ulate collection efficiencies as high as 96 to 97 percent.
STAFFING REQUIREMENTS
Labor requirements for operation and maintenance of
heat drying systems include sludge conveyors, control center
and the enclosing structure. The requirements are based on
tons of dry solids processed per year and are shown in
Table XIV-1.
TABLE XIV-1. HEAT DRYING OF SLUDGE LABOR REQUIREMENTS
Dry solids processed, Labor, hr/yr
tons/year Operation Maintenance Total
50 500 120 620
100 750 180 930
500 1,300 300 1,600
1,000 2,080 441 2,520
5,000 3,000 600 3,600
10,000 5,200 1,040 6,240
XIV-4
-------
MONITORING
Fuel/Air
Mixture
Stack Gas
Vapor
Pneumatic
Conveyance
Line
Dewatered Sludge
Dried Sludge Return
Cyclone
Dried
Sludge *^'r Pollution Control District
Suggested Minimum
rH
Ifl
C
•H
4-1
a
o
Percent Solids
Temperature
Sludge Feed
Rate
Oxygen
Particulates
S,02, NOX,
CO, CO 2
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
I/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.
XIV-5
-------
NORMAL OPERATING PROCEDURES
Startup
Normal operation of heat drying equipment varies from
one installation to another. Large installations may operate
continuously while smaller facilities may operate one 8-hour
shift per day or less. With operation that is less than
continuous a warmup period of one hour or so is necessary
to allow the system to reach operating temperatures before
drying begins.
1. After initial settings and inspection, ignite furnace
burner.
2. Adjust fuel and air flows, damper and other control to
obtain desired flame.
3. Start exhaust fans and cooling fans if necessary.
4. Allow sufficient warmup period for system to reach
operating temperatures.
5. Turn on mixer, cage mill, and conveyor.
6. Start sludge feed.
Routine Operations
1. Inspect system periodically during shift.
2. Check system temperature, pressure, fuel flow, air
flow, etc., to insure safe and proper operation.
3. Carry out maintenance required including clean up and
washdown of conveyors and wet sludge handling equipment.
4. Take samples as outlined in MONITORING section.
Shutdown
1. Shutdown sludge feed and conveyors.
2. Turn off mixer and cage mill.
3. Shutdown furnace,
4. Adhere to manufacturer's recommendations for cooling of
furnace with blowers. This will minimize the possibility
of damage to the equipment.
XIV-6
-------
5. If shutdown is only for a few hours it may be more
practical and more economical to reduce the furnace
flame, i.e., idle the system. This will minimize
warmup time when operation is resumed.
CONTROL CONSIDERATIONS
Physical Control
Typically, the flow through the heat drying equipment
should be set for as constant a rate as possible. This will
result in the most efficient operation.
Normally, the exhaust gases from heat drying equipment
are afterburned to prevent odors and scrubbed to remove par-
pollution ticulates. Several types of equipment are used for this
equipment operation. The operator should familiarize himself with the
operation of the particular equipment at his plant to insure
compliance with local air pollution codes.
Some of the heat drying equipment such as pneumatic
sludge conveyors may be subject to caking or blockage if the mixture
caking of the cake and dried sludge becomes too wet. If this occurs,
adjust flow to provide a drier mixture.
A horn alarm should sound if unsuitable temperature
conditions exist. In this case, the operator should determine
temperature the cause immediately and correct the situation or shutdown
the operation.
Process Control
Efficient and consistent operation of heat drying equip-
ment depends on frequent monitoring, both sensory and
analytical.
The drying equipment should be inspected periodically
during the shift. The sludge should be dried to the desired
percent moisture and be free of odors. Operating tempera-
tures, pressures, flow rates, etc., should be noted to detect
any irregularities. After gaining some operating experience
it should also be possible to recognize any strange sounds or
changes in pitch that may indicate a problem.
Sampling should be performed as outlined under
sampling MONITORING. Provisions should be made for grab sampling to
minimize any safety hazard from hot piping or equipment.
Samples should be analyzed according to procedures
analysis specified in Standard Methods.
XIV-7
-------
solids
The following major variables affect the operation of
the heat drying equipment and are discussed in this section.
1. Percent solids in wet sludge feed
2. Ratio of dried sludge/wet sludge mixture
3. Quantity of hot combustion gases used for drying
4. System temperatures
To produce a product at the desired moisture content
ranging from 2 to 10 percent, control must be exercised on
the sludge dewatering process preceding the heat drying
equipment. Efficient operation depends on a consistent
percent solids concentration in the sludge feed. The percent
solids in the feed is also critical to the economy of opera-
tion of heat dryers. The higher the moisture content of the
incoming sludge the more fuel that must be burned to evaporate
the added moisture.
dried sludge
to wet sludge
ratio
hot
combustion
gas
system
tempera-
tures
As shown in Figures XIV-1 and XIV-2 the incoming sludge
is mixed with previously dried sludge to create the proper
consistency for pneumatic conveying equipment. A significant
change in the incoming percent solids concentration will
change the ratio required for proper operation. This ratio
is variable depending on the sludge used and the incoming
percent solids. It must be determined through actual trial
and error.
For efficient operation the quantity of hot combustion
gases used for drying should be just enough to dry the cake
to the desired percent solids. This depends on the ratio
of the dried to wet sludge mixture and the sludge flow rate.
This should be determined through operational experience.
System operating temperatures should be maintained as
suggested in the manufacturer's manuals. Operating tempera-
tures that are too low will not properly dry the sludge
mixture. High operating temperatures are inefficient and
costly.
XIV-8
-------
EMERGENCY OPERATING PROCEDURES
Loss of Power
Power interruptions will affect the heat drying process
since electrical equipment will not operate. Without con-
veyors, fans and blowers the process will not convey sludge
mixtures through the system. The manufacturer's manual for
the furnace equipment must be checked to determine if over-
heating will result with no cooling fans operable. If this
is not a problem, it may be possible to "idle" the furnace
by turning the flame down until power is regained and the
process resumed.
Loss of Other Treatment Units
The loss of the sludge dewatering process prior to the
heat drying operation will greatly affect the economics of
the heat drying equipment, if sludge with a higher than usual
moisture content is fed to the system. In this case it may
be desirable to store the sludge until the dewatering process
is regained. In case of a prolonged problem it may be
necessary to haul sludge to another treatment facility or
disposal.
COMMON DESIGN SHORTCOMINGS
Shortcoming
3.
"Clinkers" or
clumps develop
in the dried
sludge.
Mixers, air locks,
cage mills, dryers,
metal equipment
subject to
excessive wear and
corrosion.
Serious air pollu-
tion such as par-
ticulates and odor
from stack gas.
Solution
Install grinding equipment to
pulverize sludge before final
use. This is a common problem
of rotary kiln heat dryers.
This is a common problem due
to the corrosive nature of the
dried sludge. Set up frequent,
periodic maintenance schedule
on parts, including coating
of metal surfaces where
applicable.
Install afterburner and
scrubbers as suggested or
required by air pollution
control departments.
XIV-9
-------
Shortcoming Solution
4. Stockpiling of 4. Find a market or outlet for
dried sludge at dried sludge so stockpiling
2 to 10 percent is minimized, or unnecessary.
moisture
susceptible to
spontaneous
combustion fires.
XIV-10
-------
TROUBLESHOOTING GUIDE
HEAT DRYING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Sludge not properly
dried.
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.
la. Furnace temperature.
Ib. Moisture content of
wet/dry sludge
mixture.
Ic. Hot gas flow.
Id. Percent solids of
feed sludge.
la. Increase temperature as re-
quired .
Ib. Change ratio to provide drier
mixture.
Ic. Increase flow of combustion
gases.
Id. Check operation of dewatering
equipment preceding heat drying
equipment. Increase percent
solids output.
2. Decreased sludge
flow in pneumatic
lines.
2a. Caking or blockage
of line with wet
mixture of sludge.
2a. Moisture content of
wet/dry sludge
mixture.
2a. Change ratio to provide drier
mixture.
3. Decreased flow in
fans & ductwork.
3a. Grease accumulation.
3a. Visually inspect
ducting, fans.
3a. Steam clean equipment as re-
quired.
4. Excessive particu-
lates in stack gas.
4a. Faulty or poorly
operating pollution
control equipment.
4a. Pollution control
equipment.
4a. Correct operation of pollution
control equipment - see manu-
facturer 's manual.
5. Excessive odors in
stack gas.
5a. Temperature of after
burner too low.
5a. Afterburner tempera-
ture.
5a. Operate after burner between
1,200-1,400°F (650-700°C)
-------
MAINTENANCE CONSIDERATIONS
A good preventive maintenance program will reduce break-
downs which could be not only costly, but also very unpleasant
for operating personnel. All maintenance should be geared
to provide smooth operation and prevent potential total
outages or disasters. Plant components including the follow-
ing should be inspected periodically for wear, corrosion, and
proper adjustment:
1. Drives and gear reducers
2. Sludge belt conveyors
3. Pneumatic conveyor system
mechanical 4. Bearing brackets
5. Mixers and cage mills
6. Electrical contacts in starters and relays
7. Burners
8. Furnace and related equipment
Cleaning of heat exchangers and other components should
be done on a regular bases as determined necessary through
operational experience or as recommended by the manufacturer.
heat Items that have a predictable service life as determined
exchangers through operation, should be replaced on a uniform basis.
If shutdown is required to replace these items, this is a
good time to inspect the entire system.
SAFETY CONSIDERATIONS
The complex mechanical equipment and extremely high
temperatures associated with the heat drying equipment require
a conscientious safety effort. Hazardous areas should be
marked with warning signs including cautions against contact
with hot surfaces. Any equipment that creates a hazard
upon a malfunction should be equipped with sensors and data
recorded to evaluate operating conditions and signal any
breakdowns.
General safety considerations also apply. At least two
persons should be present when working in enclosed tanks or
in elevated areas not protected by handrails. Walkways and
work areas should be kept free of grease, oil, leaves and
snow. Protective guards and covers must be in place unless
mechanical/electrical equipment is locked out of operation.
XIV-12
-------
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and Waste-
water. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
2. WPCF Manual of Practice No. 17 (WPCF MOP No. 17),
Paints and Protective Coatings for Wastewater
Treatment Facilities.
3. WPCF Manual of Practice No. 11, Chapter 22, Operation
of Wastewater Treatment Plants, Heat Drying.
Glossary of Terms and Calculations
1. Caking is the blocking of pneumatic conveying equipment
or other equipment by sludge that is enough to form a
plug or mud ball in the line.
2. Pneumatic Conveyor is a system that uses air pressure
to move the sludge mixture through pipes from one piece
of equipment to the next.
3. Solids Concentration is the weight of the solids per
unit weight of the sludge. It can be calculated in
percent as follows:
weight of dry sludge solids
concentration = weight of wet sludge X 10°
4. Solids Loading is the feed solids applied in pounds
per hour.
XIV-13
-------
XV
MULTIPLE HEARTH INCINERATION
-------
CONTENTS
Process Description * . . . . XV-1
Typical Design Criteria and Performance XV-3
Staffing Requirements XV-6
Monitoring XV-7
Normal Operating Procedures XV-8
Initial Inspection XV-8
Drying Out Process XV-8
Startup XV-9
Routine Operations XV-10
Shutdown XV-11
Control Considerations XV-11
Physical Control XV-11
Process Control XV-13
Emergency Operating Procedures XV-16
Loss of Power XV-16
Loss of Fuel XV-17
Loss of Other Treatment Units XV-17
Common Design Shortcomings XV-17
Troubleshooting Guide XV-19
Maintenance Considerations XV-22
Safety Considerations XV-22
Reference Manual XV-22
References XV-22
Glossary of Terms and Sample Calculations XV-23
-------
PROCESS DESCRIPTION
process
design
mechanical
features
operation
A multiple hearth furnace consists of a circular steel
shell surrounding a number of solid refractory hearths and a
central rotating shaft to which rabble arms are attached.
The operating capacity of these furnaces is related to the
total area of the enclosed hearths. They are available in out-
side diameters ranging from 6.7 feet to over 22 feet with four
to twelve hearths as shown in Table XV-1 (see following page).
Capacities of multiple hearth furnaces vary from 200 to 8,000
pounds per hour of dry sludge with operating temperatures of
1,400 to 1,700 F. The dewatered sludge enters at the top
through a flapgate and proceeds downward through the furnace
from hearth to hearth moved by the rotary action of the
rabble arms. The hearths are constructed of high heat duty
fire brick and special fire brick shapes.
Two doors are typically provided in the wall of each
hearth. They are fitted to cast iron frames and have ma-
chined faces to provide reasonably tight closures. An obser-
vation port with closure is provided in each door. Since the
o
furnace may operate at temperatures up to 2,000 F, the
central shaft and rabble arms are effectively cooled by air
supplied from a blower which discharges into a housing at
the bottom of the shaft. The shaft is motor driven and the
rotational speed is adjustable from about one-half to one
and one-half revolutions per minute. Two or more rabble arms
are connected to the shaft at each hearth. Each rabble arm
is constructed with two internal air passages. One passage
conducts air from the cold air tube of the central shaft to
the end of the rabble arm and the other returns this air back
to the hot air tube of the central shaft. The air may be
discharged to atmosphere or returned to the bottom hearth of
the furnace as preheated air for combustion purposes.
The rabble arms provide mixing action as well as rotary
and downward movement of the sludge. The flow of combustion
air is countercurrent to that of the sludge. Gas or oil
burners are provided on some of the hearths for furnishing
heat for start-up or supplemental use as required. Sludge
is constantly turned and broken into smaller particles by the
rotating rabble arms which exposes the sludge surface to hot
furnace gases. This facilitates rapid and complete drying
as well as burning of sludge.
XV-1
-------
TABLE XV-1. STANDARD SIZES OF MULTIPLE HEARTH FURNACE UNITS
Effective
hearth
area,
sq ft
85
98
112
125
126
140
145
166
187
193
208
225
256
276
288
319
323
351
364
383
411
452
510
560
575
672
760
845
857
944
Reference :
design
variations
Effective
Outer
diameter,
ft
6.75
6.75
6.75
7.75
6.75
6.75
7.75
7.75
7.75
9.25
7.75
9.25
9.25
10.75
9.25
9.25
10.75
9.25
10.75
9.25
10.75
10.75
10.75
10.75
14.25
14.25
14.25
16.75
14.25
14.25
Number
hearths
6
7
8
6
9
10
7
8
9
6
10
7
8
6
9
10
7
11
8
12
9
10
11
12
6
7
8
6
9
10
US EPA, Computerized Design
Hearth Sludge Incinerators,
Other
selection.
vacuum fil
hearth
area,
sq ft
988
1041
1068
1117
1128
1249
1260
1268
1400
1410
1483
1540
1580
1591
1660
1675
1752
1849
1875
1933
2060
2084
2090
2275
2350
2464
2600
2860
3120
Outer
diameter, Number
ft
16.75
14.25
18.75
16.75
14.25
18.75
16.75
20.25
16.75
18.75
20.25
16.75
22.25
18.75
20.25
16.75
18.75
22.25
20.25
18.75
20.25
22.25
18.75
20.25
22.25
20.25
22.25
22.25
22.25
and Cost Estimation for
17071 EBP 07/71
hearths
7
11
6
8
12
7
9
6
10
8
7
11
6
9
8
12
10
7
9
11
10
8
12
11
9
12
10
11
12
Multiple
variations are related to dewatering equipment
Dewatering may be accomplished by centrifuge,
ter, or filter nr«RR. nnava-t--;/-,!-, =v,^ ~_j_j
jr-~~"- <^j^tij.ti>_j.vjji emu uid.Lntenance
of these units is described in sections VIII, IX, & X.
There are two options for handling ash from the furnace.
One is to provide a storage hopper and unload dry ash to
trucks. The other is to add water and handle ash as a slurry,
with the slurry being pumped to a lagoon.
XV-2
-------
A cross section of a typical multiple hearth furnace is
shown on Figure XV-1 (see following page). A typical system
schematic is shown on Figure XV-2 (see following pages).
There are few variations in the furnace design other than
hearth diameter and number of hearths. The two major manu-
facturer's equipment is very similar, therefore, this manual
will be more specific than some of the other manuals.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Loading rates for several types of sludge are shown in
Table XV-2.
TABLE XV-2. MULTIPLE HEARTH FURNACE LOADING RATES
Type of sludge
1.
2.
3.
4.
5.
6.
7.
8.
9.
Primary
Primary + FeCl3
Primary + low lime
Primary + WAS
Primary + (WAS +
FeCl3)
(Primary + FeCl3)
+ WAS
WAS
WAS + FeCl3
Digested primary
Solids
%
30
16
35
16
20
16
16
16
30
Volatile
solids,
%
60
47
45
69
54
53
80
50
43
Chemical
concentration*
mg/1
N/A
20
298
N/A
20
20
N/A
20
N/A
Typical
wet sludge
loading rate ,
Ib/hr/sg ft.
7
6
8
6
6
6
6
6
7
.0-12.0
.0-10.0
.0-12.0
.0-10.0
.5-11.0
.0-10.0
.0-10.0
.0-10.0
.0-12.0
* Assumes no dewatering chemicals.
** Low number is applicable to small plants, high number is applicable to
large plants.
The data in this table developed from manufacturers' information.
XV-3
-------
FLUE GASES OUT
DRYING ZONE
COMBUSTION ZONE
COOLING AIR DISCHARGE
FLOATING DAMPER
SLUDGE INLET
RABBLE ARM
'AT EACH HEARTH
COMBUSTION
AIR RETURN
COOLING ZONE
ASH DISCHARGE
COOLING AIR FAN
Figure XV-1. Cross section of a typical multiple hearth
incinerator.
XV-4
-------
CENTRATE
TO PRIMARY
CLARIFIER
INFLUENT
CHANNEL
SCRUBBER
REVERS IBLE'
FEED CONVEYOR
ASH SCREW
CONVEYOR
AND WATER
SPRAY FOR
DUST
CONTFOL
l\
COMBUST I ON
AIR BLOWER
Figure XV-2. Typical system schematic.
XV-5
-------
The volume reduction by sludge incineration is over
90 percent when compared to the volume of dewatered sludge.
The ash from the incineration process is free of pesticides,
ash viruses and pathogens. Metals will be converted to the less
soluble oxide form or volatilized. The ash can be transport-
ed in the dry state to appropriate landfill sites or used as
a soil conditioner.
The critical sidestream treatment requirement is the
flue gas treatment. The scrubbed gases should meet the most
sidestream stringent air quality requirements. A comparison of scrubbed
gas quality with South em California Air Pollution Control
District Rules is shown in Table XV-3.
TABLE XV-3. STACK SAMPLING RESULTS, MULTIPLE HEARTH INCINERATOR
WITH COMBINATION LIME-ORGANIC SOLIDS FEED
Test A Test B
Combustion contaminants ,
grains/SCFM at 12% CO2 .026 .016
Oxides of sulfur:
(as SO2) , ppm 2.2 2.3
Oxides of nitrogen
(as N02) , ppm 52 65
Test C SCAPGD
.014 0.1
(Rule 473)
3.2 2000
(Rule 53)
300
(Rule 474)
Tests made at South Lake Tahoe Public Utility District, CA, on November
10, 1970.
STAFFING REQUIREMENTS
The labor requirements shown in Table XV-4 are based on a
high degree of automation of this process and include opera-
tion of the furnace, scrubber, and ash handling units.
TABLE XV-4. MULTIPLE HEARTH FURNACE LABOR REQUIREMENTS*
Number of
units
1
3
5
Operation
2,920
8,760
14,600
Labor , hr/yr
Maintenance
1,460
4,380
7,300
Total
4,380
13,140
21,900
*Assuming full-time operation 7 days per week, 52 weeks per
year
XV-6
-------
MONITORING
FEED
4 r
A
A
A
A
— -
A -^~~
A
L
HEARTHS
-PRODUCT
h- Z
Z 3
uj 5
5 - TEMPERATURE
K £
UJ 2 TOTAL
<3C Q VOLATILE
H £ SOLIDS
UJ co '
g "J TOTAL
5 § SOLIDS
O
2
UJ
D
(/} UJ
UJ 1C
h- U-
Mn
1 /D' 1 '
,/om
U_
O
o
< s!.
81
A
F
P
F
P
0
Q
0^
X Q.
"j<
5 co
Mn
G
G
-_ CO
g LU
O i-
to
< a:
UJ o
1C u.
P
P
P
A. TEST FREQUENCY
Mn - MONITOR CONTINUOUSLY
D = DAY
B. LOCATION OF SAMPLE
F FEED
P PRODUCT
A FURNACE ATMOSPHERE
(AT EACH HEARTH)
C. METHOD OF SAMPLE
Mn - MONITOR CONTINUOUSLY
G - GRAB SAMPLE
D. REASON FOR TEST
P PROCESS CONTROL
E. FOOTNOTES:
1 WHEN FURNACE IS OPERATING.
XV-7
-------
NORMAL OPERATING PROCEDURES (For six hearth furnace)
Initial Inspection
4.
5.
Look in feed chute access door above furnace for left-
over or caked feed material, obstructions, or debris.
Look in each hearth to see that the rabble arms and
teeth are in good condition, that clearance is being
maintained between teeth and hearths, and that no
obstructions exist. See that the hearth refractory is
in good condition.
Check that each burner shutoff valve is closed and each
individual air butterfly valve in lines to each burner
is closed. Do not adjust air valve to burner pilot
because it has already been reset for proper operation.
See that all burner tiles are free of slag accumulations.
Check that bottom gas seal around center shaft is
filled with sand.
Drying Out Process
3.
Following furnace inspection, the furnace should be
dried out. The purpose of drying out is to remove mois-
ture from the refractory lining of the furnace and inter-
connecting flues. Long refractory life is dependent
upon proper removal of this moisture as slowly as possi-
ble. The best way of doing this is to maintain a low
heat throughout the entire furnace during the drying
stage.
The operator should monitor temperatures frequently to
insure that the heating takes place as uniformly as pos-
sible; always remember that the heat from the burners
travels up through the furnace and is distributed over
the upper hearths. The temperature of the gases leaving
the furnace should not exceed 400°F for the first 48
hours and should not exceed 500 F at any time during the
drying out operation.
Temperatures on the upper hearth must be high enough to
avoid condensation of moisture. The drying out period
should be approximately 5 days.
The furnace draft during the drying operation should be
high enough only to prevent smoke from passing from the
furnace into the room. This provides maximum efficiency
during the drying operation since the hot gases will not
be drawn out of the furnace too fast.
XV-8
-------
4. Normally, this drying operation should be the first step
of a continuous procedure for heating up and putting the
furnace into operation.
Startup
1. Turn on center shaft lubrication.
2. Turn on scrubber water supply and adjust to proper flow.
3. Open water supply valve to pre-cooler.
4. Start furnace induced draft fan.
5. Start combustion air fan. Regulate scrubber inlet
damper to maintain slightly negative draft on furnace.
6. Start shaft cooling air fan.
7. Check furnace center shaft drive for proper lubrication,
proper sand seal, and shear pin position.
8. Turn on manual main safety gas valve. Check to make sure
bypass is closed. Check gas meter reading and record
reading in plant log.
9. During initial startup and furnace dry out, close slide
gate to cooler.
10. Start furnace center shaft drive.
11. The purge cycle should start. Check panel to determine
when purge is complete.
12. Open automatic main safety shut off gas valve. This
should energize indicator light on furnace control panel.
13. Furnace is now ready for burner to be lit.
14. Light burner according to manufacturer's procedure.
15. For initial startup and dry out, temperature should be
increased slowly as described previously. Additional
burners could be lit if needed to maintain temperatures
and temperature distribution throughout furnace.
16. During initial dry out, the furnace can be operated with
the scrubber bypassed and induced draft fan shutdown.
The furnace is now operating under natural draft con-
ditions. Draft gauge should indicate a slightly negative
pressure (0.08 to 0.10 inches of water column).
XV-9
-------
17. When furnace is dried out and ready for sludge feed, the
scrubber and induced draft fan should be started up.
18. When bringing furnace up to temperature leave burner on
minimum fire until temperature rises at rate of 50 F per
hour or less. Then place temperature controller in oper-
ation with the set point 50°F above the actual tempera-
ture. Continue to increase the set point in 50 F incre-
ments until hearths 4 and 5 reach desired temperature.
19. Light burners on hearths numbered 2 and 3. a_s required.
Bring hearths number 2 & 3 to operating temperature
with rate of increase in temperature not exceeding 5_0°F
per hour. Light burners on hearth number 6 to obtain
temperature of 750 F^
20. Adjust scrubber inlet damper controller on induced draft
fan outlet to maintain a furnace draft of -0.15 inches
water column.
21. Be sure that no burner flame is impinging on any
stationary part of the interior of the furnace.
22. Start up sludge conveying system.
23. Begin sludge feed to furnace.
Routine Operations
1. Every two hours, inspect the furnace operation.
a. Check instrument panel readings. If any change has
occurred since previous inspection, reason for
change should be determined and corrective action
taken if necessary. Enter data in inspection log.
b. Check top of furnace for squeeling top bearing
(lubrication needed) or sludge feed blockage indi-
cated by a buildup in the feed chute.
c. Look in each burner port for slagging of tiles or
other unexpected burner condition.
d. Look into each hearth on which burners are lit to
see that flame characteristics are normal.
e. Look into hearth number 6 for signs of discharge
chute blockage.
XV-10
-------
f. Check for signs of sludge leakage around center
shaft evidenced by a pile of sludge on center shaft
drive gear.
g. Monitor the temperature of the ash discharge
from the cooler and temperature of cooling
water discharge.
h. Check that center shaft cooling fan is running.
i. See that all instrument readings are at desired con-
trol points and that none exceed safe limits.
Shutdown
1. The furnace should be shut down and the burners shut off
only when necessary. If the sludge feed is to be inter-
rupted temporarily, the furnace should be kept at oper-
ating temperature.
2. Stop sludge feed.
3. Twenty minutes later, turn off all burners on hearths
numbers 2, 3, and 4. Be sure to close butterfly valves
at unused burners.
4. Adjust the exhaust damper to a condition of zero draft.
5. When all sludge has rabbled out of the furnace, the
furnace temperature will slowly decline. Keep the center
shaft running at all times.
6. Shut off burners at hearths number 5 and 6.
7. When all furnace temperatures are below 500 F, turn off
center shaft drive, center shaft cooling fan, and induced
draft fan.
CONTROL CONSIDERATIONS
Physical Control (for typical automated system)
Automatic temperature controllers are provided to modu-
late each bank of burners to a set point temperature. All
burners on each fired hearth are controlled through the use
of one temperature controller. A low fire start interlock is
incorporated into the system.
A temperature controller senses the furnace gas outlet
temperature temperature and controls this temperature to a preset set
control point by modulating a control valve on the auxiliary combus-
tion air fan. On increasing temperature, the valve opens to
XV-11
-------
draft
control
draft
gauges
flame
safety
temperature
recording
shaft
rotation
admit more air to the furnace. When the valve is fully
opened, a second motor is activated to modulate a lower inlet
on the furnace and admit room air on increasing temperature.
A manual override is provided to permit adjustment from the
control panel on a manual basis.
Automatic indicating draft control is provided to main-
tain a pre-set pressure at the gas outlet from the furnace.
The controller senses the pressure and positions a damper
in the incinerator induced draft fan outlet. The damper can
normally be controlled manually if desired.
Draft pressure indication is provided for the
following points:
1. Furnace gas outlet
2. Scrubber inlet
3. Differential across scrubber
4. Scrubber outlet
The draft pressure is indicated in inches of water, with
a range to suit the application.
A complete flame safety system is provided to assure safe
operation of the furnace. The system includes a draft switch,
shaft cooling air sensor, purge timer and necessary interlocks
for fuel, air pressure, and other critical parameters. The
system includes low fire start, interrupted ignition, ultra-
violet scanners, and similar features. Individual burner
panels are normally provided adjacent to each burner or set
of burners for a hearth and contain the controls for that
burner(s).
Temperatures are normally recorded on a multipoint
recorder for the following points:
1. Each hearth
2. Quencher inlet
3. Scrubber inlet
4. Scrubber outlet
5. Incinerator central shaft cooling air outlet
6. Incinerator exhaust stack outlet
Shaft rotation is monitored by a "telltale" device to
signal any interruption of the shaft rotation. The conveyor
system is normally interlocked to center shaft rotation so
sludge feed is stopped if the center shaft stops.
XV-12
-------
scrubber
temperature
ash
alarms
The scrubber outlet temperature is normally indicated and
usually a high alarm switch stops the induced draft fan and
opens the emergency vent.
Ash level is monitored in the storage bin by a device to
sound an alarm on high level.
An annunciator system is normally furnished to accom-
modate all the necessary alarm points.
The furnace feed rate is normally indicated and/or
recorded.
Process Control
solids
handling
system
effect
of
moisture
sludge
fuel
value
An incinerator is usually part of a sludge treatment
system which includes sludge thickening, macerations dewater-
ing (such as vacuum filter, centrifuge, or filter press), an
incinerator feed system, air pollution control devices, ash
handling facilities, and the related automatic controls. The
operation of the incinerator cannot be isolated from these
other system components. Of particular importance is the
operation of the thickening and dewatering processes because
the moisture content of the sludge is the primary variable
affecting the incinerator fuel consumption.
The relationship between auxiliary fuel required and
feed sludge solids concentration is shown in Figure XV-3 (see
following page) for typical primary sludges and primary plus
waste activated sludges. Typically, incineration is self
sustaining at sludge solids concentrations of about 26 per-
cent for primary sludge and 23 percent for primary plus WAS.
Incineration will always require some fuel because of startup
requirements. Fuel requirements will be substantially higher
if afterburner operation is required.
As shown in Figure XV-3, incineration is self sustaining
(no fuel required) when the sludge contains less than 75 per-
cent moisture when no afterburner is used.
The fuel value of the sludge itself is also important
in determining fuel consumption. Typical heat values of
various sludges are:
XV-13
-------
(0
2
"5
io
0)
0)
1
10
.£
**
JO
O
m
Q
LU
DC
3
O
HI
tr
lil
z
SELF
SUSTAINING
80
SLUDGE SOLIDS. % by weight
- Assumed heat value of sludge: 10,000 Btu/lb of volatile solids
- Curve assumes that afterburner is not used.
Figure XV-3. Auxiliary heat required to sustain combustion of sludge.
XV-14
-------
Type of sludge
Raw primary
Activated
Anaerobically digested primary
Raw (chemically precipitated) primary
Biological filter
Grease and scum
Fine screenings
Ground garbage
High organic grit
Heating value,
(Btu/lb of dry solids)
10,000-12,500
8,500-10,000
5,500
7,000
8,500-10,000
16,700
7,800
8,200
4,000
As the percentage of volatiles increases, the auxiliary
fuel consumption decreases for a given sludge. The volatile
content of a sludge may be maximized by removing sludge
inorganics such as grit, by avoiding the use of inorganic
chemicals such as ferric chloride and lime in the dewatering
process, and by avoiding biological processes such as diges-
tion before incineration.
In normal operation, a multiple hearth furnace provides
three distinct combustion zones:
hearths
1. Two or more upper hearths on which most of the free
moisture is evaporated.
2. Two or more intermediate hearths on which sludge
volatiles burn at temperatures exceeding 1,500 F.
3. A bottom hearth that serves as an ash cooling zone by
giving up heat to the cooler incoming air.
excess
air
During evaporation of moisture in the first zone the
sludge temperature is not raised higher than about 140 F.
At
this temperature no significant quantity of volatile matter
is driven off, and hence no obnoxious odors are produced.
Distillation of volatiles from sludge containing 75 percent
moisture does not occur until 80 to 90 percent of the water
has been driven off and, by this time, the sludge is down far
enough in the incinerator to encounter gases hot enough to
burn the volatiles which could cause odors. Generally, when
fuel is required to maintain combustion in a multiple hearth
furnace, a gas outlet temperature above 900 F indicates too
much fuel is being burned.
Practical operation of an incinerator requires that air
in excess of theoretical requirements for combustion be
supplied to the combustion chamber. This increases the
opportunity of contact between fuel and oxygen which is nec-
essary if combustion is to proceed. When the amount of excess
XV-15
-------
stack
gas
sensory
furnace
shutdown
air is inadequate, only partial combustion occurs, resulting
in the formation of carbon monoxide, soot, and odorous hydro-
carbons in the stack gases. Multiple hearth incineration is
typically operated at 75 to 100 percent excess air. Excess
air in the 100 to 200 percent range is undesirable because it
wastes fuel. A closely controlled minimum excess air flow is
desirable for maximum thermal economy.
Analysis of stack gas composition is typically used to
control excess air. Oxygen, carbon dioxide, and carbon mon-
oxide may be monitored automatically in the stack and compared
with target levels. If the carbon monoxide level increases,
this indicates that incomplete combustion is occurring and
more excess air may be needed. However, if the oxygen level
is within proper range, either the mixing of sludge and com-
bustion air is inadequate or the temperature has been reduced
by the addition of cake that is wetter than normal.
The stack gas appearance can indicate problems with the
scrubber or furnace operation. If the stack gas contains
excessive particulate concentrations there will be a brown or
black plume. If odors are emitted, the combustion process is
not complete.
Another sensory indicator is the color of the ash. If a
change occurs, there may have been a chemical change in the
raw sewage entering the plant or the combustion process may
not be properly adjusted. Similarly, odors produced by the
ash may indicate incomplete combustion.
A furnace should not be shut down and cooled unless
absolutely necessary because of the stresses placed on the
refractory. If cooling of the furnace is necessary it should
be done slowly and strict startup procedures should be fol-
lowed .
EMERGENCY OPERATING PROCEDURES
Loss of Power
Emergency generation or auxiliary engine drives must be
provided for at least the shaft cooling air fan and center
shaft drive. It is desirable to provide adequate standby
facilities so the furnace can be kept on line and up to
temperature.
XV-16
-------
Loss of Fuel
The natural gas service to the treatment plant may be
interruptible. That is, in times of high demand in the area
for natural gas, the gas service can be temporarily interrupt-
ed. Ordinarily, notice would be given a day or so in advance
by the gas company, who might also give an estimate of the
probable duration of the shutdown.
Propane gas may be used as backup to the natural gas
supply. It is wise to have a backup fuel system because of
the time required to bring the furnace up to temperature
after it has shut down and cooled.
Loss of Other Treatment Units
The most critical treatment unit prior to incineration
is dewatering. Normally, multiple units allow continued
dewatering with one unit out of service. If more than one
unit is out of service the sludge moisture content may in-
crease. It may be possible to increase chemical addition to
the overloaded units to improve dewatering performance. If
not, the furnace auxiliary fuel feed will be higher because of
the larger volume of water.
If the scrubber is out of service, the furnace should not
be operated, but may be kept up to temperature, if desired.
If the scrubber is not functioning properly, the excess air
can be increased to reduce particulate concentrations.
COMMON DESIGN SHORTCOMINGS
Shortcoming Solution
1. Poor dewatering la. Try various chemical
efficiency. (See conditions.
Table XV-2 for
optimum Ib. Add additional dewatering
solids units.
concentration)
Ic. Accept lower solids, con-
centration, increase fuel.
2. Reactor under- 2a. Operate incinerator for
sized. longer period of time.
2b. Improve sludge dewatering
prior to incineration.
XV-17
-------
Shortcoming Solution
3. Inadequate storage 3a. Store sludge in clarifiers
for raw sludge or thickeners (temporary).
when reactor is
out of service. 3b. Haul sludge to landfill.
XV-18
-------
TROUBLESHOOTING GUIDE
MULTIFILE HEARTH INCINERATION
INDICA TORS/OBSERVA TIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Furnace tempera-
ture too high.
la. Excessive fuel feed
rate.
Ib. Greasy solids.
Ic. Thermocouple burned
out.
la. Fuel feed rate.
Ib. If fuel is off and
temperature is ris-
ing, this may be
the cause.
Ic. If temperature indi-
cator is off scale,
this is the likely
cause.
la. Decrease fuel feed rate.
Ib. Raise air feed rate or reduce
sludge feed rate.
Ic. Replace thermocouple.
2. Furnace temperature
too low.
2a. Moisture content of
sludge has increas-
ed.
2b. Fuel system mal-
function .
2c. Excessive air feed
rate.
2a. Moisture content
and dewatering sys-
tem operation.
2b. Check fuel system.
2c. If oxygen content
of stack gas is
high, this is likely
the cause.
2a. Increase fuel feed rate until
dewatering system operation
is improved.
2b. Establish proper fuel feed
rate.
2c. Reduce air feed rate or in-
crease feed rate.
3. Oxygen content of
stack gas is too
high.
3a. Sludge feed rate
too low.
3b. Air feed rate too
high.
3a. Check for blockage
of sludge feed
system and check
feed rate.
3b. Air feed rate.
3a. Remove any blockages and
establish proper feed rate.
3b. Decrease air feed rate.
4. Oxygen content of
stack gas is too
low.
4a. Volatile or grease
content of sludge
has increased.
4a. Sludge composition.
4a. Increase air feed rate or de-
crease sludge feed rate.
-------
TROUBLESHOOTING GUIDE
MULTIPLE HEARTH INCINERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4b. Air feed rate too
low.
4b. Check for malfunc-
tion of air supply
and check feed rate.
4b. Increase air feed rate.
5. Furnace refracto-
ries have
deteriorated.
Furnace has been
started up and shut-
down too quickly.
5. Operating records.
5. Replace refractories and ob-
serve proper heating up and
cooling down procedures in
future.
Unusually high
cooling effect from
one hearth to
another.
Air leak.
6. Hearth doors, dis-
charge pipe, center
shaft seal, air
butterfly valves in
inactive burners.
6. Stop leak.
Short hearth life.
Uneven firing.
Check all burners
in hearth.
7. Fire hearths equally on both
sides.
Center shaft drive
shear pin fails.
Rabble arm is drag-
ging on hearth or
foreign object is
caught beneath arm.
Inspect each hearth.
Correct cause of problem and
replace shear pin.
9. Furnace scrubber
temperature too
high.
Low water flow to
scrubber.
Scrubber water flow.
Establish adequate scrubber
water flow.
10. Stack gas tempera-
tures too low (500-
600°F) and odors
noted.
10. Inadequate fuel feed
rate or excessive
sludge feed rate.
10. Fuel and sludge
feed rates.
10. Increase fuel or decrease
sludge feed rates.
-------
TROUBLESHOOTING GUIDE
MULTIPLE HEARTH INCINERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
11. Stack gas tempera-
tures too high
(1,200-1,600 F).
11. Excess heat value
in sludge or ex-
cessive fuel feed
rate.
11. Sludge character-
istics and fuel rate,
11. Add more excess air or
decrease fuel rate.
12. Furnace burners
slagging up.
12.
Burner design.
12. Consult with
manufacturer.
12. Replace burners with newer
designs which minimize
slagging.
13. Rabble arms are
drooping.
13. Excessive hearth
temperatures or
loss of cooling
air.
13. Operating records;
is grease or scum
being injected into
the hearth.
13. Maintain temperatures in
proper range and maintain
backup systems for cooling air
in working condition; dis-
continue scum injection into
hearth.
-------
MAINTENANCE CONSIDERATIONS
mechanical
A good preventive maintenance program will reduce break-
downs which could be not only costly, but also very unpleasant
for operating personnel. A good preventative maintenance
program is very important for an incinerator because of the
large drives and the need to minimize incinerator shutdowns.
The following are the major elements which should receive
regular attention for wear, corrosion, proper adjustment, and
lubrication according to manufacturer's guidelines.
1. Drives and gear reducers
2. Chains and sprockets
3. Burners
4. Air blowers
5. Sludge conveying equipment
6. Ash conveying equipment
7. Furnace seals
8. Draft controller
9. Temperature controllers
10. Any standby engine drives or generators
11. Scrubber
SAFETY CONSIDERATIONS
1.
2.
3.
4.
REFERENCE MATERIAL
References
Safety measures should include the following:
No smoking should be allowed around the natural gas lines
or when checking the system for leaks.
Protective clothing and face shields should be worn when
repairing or lighting the furnaces.
A colored plate should be used when looking into an oper-
ating hearth to protect the eyes from the bright flame.
Open hearth access doors with caution, do not stand in
front of them when they are initially opened, and close
them as soon as possible.
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
XV-2 2
-------
Computerized Design and Cost Estimation for Multiple
Hearth Sludge Incinerators by Unterberg, et al. (U.S.
EPA, 17070 EBP 07/71). Superintendent of Documents,
U.S. Government Printing Office, Washington, D.C. 20402.
Glossary of Terms and Sample Calculations
1. Excess Air is the amount of air required beyond the
theoretical air requirements for complete combustion.
This parameter is expressed as a percentage of the
theoretical air required.
Sample calculation for excess air:
(actual air rate - theoretical rate) x 100
excess air =
theoretical air rate
= (1,500 - 1,000) x 100
1,000
= 50%
2. Sludge loading rate is the weight of wet sludge fed to
the reactor per square foot of reactor bed area per hour
(Ib/sq ft/hr).
Sample loading rate:
Ib sludge/hr ( 100 \
loading rate = TT I :— —— I
Ttd^ \% moisture content/
4
440 (100
3.14 (20)2 ^ 2°
4
= 7.01 Ib/sq ft/hr
3. Solids concentration is the weight of solids per unit
weight of sludge. It is calculated as follows:
weight of dry sludge solids x 100
concentration = :—— :—
weight of wet sludge
25 x 100
120
= 20.8%
XV-23
-------
Moisture content is the amount of water per unit weight
of sludge. The moisture content is expressed as a per-
centage of the total weight of the wet sludge. This
parameter is equal to 100 minus the percent solids con-
centration or can be computed as follows:
moisture content =
(weight of wet solids)-(weight of dry solids) x 100
weight of wet solids
= (120 - 25) x 100
120
= 79.2%
XV-24
-------
XVI
FLUIDIZED BED INCINERATION
-------
CONTENTS
Process Description XVI-1
Typical Design Criteria and Performance XVI-4
Staffing Requirements XVI-5
Monitoring XVI-5
Sensory Observations XVI-6
Normal Operating Procedures XVI-8
Startup XVI-8
Routine Operations XVI-10
Shutdown XVI-11
Control Considerations XVI-12
Physical Control XVI-12
Process Control XVI-16
Emergency Operating Procedures XVI-17
Loss of Power XVI-17
Loss of Fuel XVI-17
Common Design Shortcomings XVI-17
Troubleshooting Guide XVI-19
Maintenance Considerations XVI-22
Safety Considerations XVI-22
Reference Material XVI-23
References XVI-23
Glossary of Terms and Sample Calculations XVI-23
-------
PROCESS DESCRIPTION
operation
The fluidized bed incinerator is a vertical cylindrical
vessel with a grid in the lower section to support a sandbed.
Dewatered sludge is injected above the grid and combustion air
flows upward at a pressure of 3.5 to 5.0 psig and fluidizes
the mixture of hot sand and sludge. Supplemental fuel can be
supplied by burners above or below the grid. In essence, the
reactor is a single chamber unit where both moisture evapora-
tion and combustion occur at 1,400 to 1,500°F in the sandbed.
All the combustion gases pass through the 1500°F combustion
zone with residence times of several seconds. Ash is carried
out the top with combustion exhaust and is removed by air
pollution control devices.
The quantities of excess air are maintained at 20 to 25
percent to minimize its effect on fuel costs. The heat reser-
voir provided by the sandbed enables reduced start-up times
when the unit is shut down for relatively short periods
(overnight). As an example, a unit can be operated 4 to 8
hours a day with little reheating when restarting, because
the sandbed serves as a heat reservoir.
design
di fferences
Exhaust gases are usually scrubbed with treatment plant
effluent and ash solids are separated from the liquid in a
hydrocyclone, with the liquid stream returned to the head of
the plant and the ash further dewatered mechanically or in a
lagoon.
There are two major variations in the reactor design.
These are the actual point of sludge feed and use of a pre-
heater or heat exchanger. The sludge feed point can be at
a bed level or at the top of the reactor. Most models pro-
duced now include a heat exchanger which preheats incoming
sludge thus reducing fuel requirements. This manual is
written with the sludge feed point at bed level and with the
heat exchanger included.
Other variations are found in the types of dewatering
devices used prior to incineration. Operation and mainte-
nance of these devices have been discussed in earlier sec-
tions (centrifuge, vacuum filter, and filter press).
A cross section of a typical fluid bed reactor is shown
on Figure XVI-1 (see following page). A schematic of a
typical complete system is shown on Figure XVI-2 (see following
page). This manual applies to all those unit processes
XVI-1
-------
SIGHT GLASS
EXHAUST * P
SAND FEED
PRESSURE
TAP
PREHEAT BURNER
ACCFSS
DOORS
THERMOCOUPLE
SLUDGE INLET
FLUIDIZING
AIR IN LEJ
FRtlM WINDBOX
Figure XVI-1. Cross section of a fluid bed reactor.
XVI-2
-------
H
I
U)
Raw
Sludge
— +i
Grit* Disintegrator Thickener Dew
Removal De
Auxiliary Fuel
|
s 1
J
atering Feeder
vice
1
[ Plant Effluent |
i '
__^J • L____teJ -
t
Fluidizing Air Dewatering
Blower Devices
/
Recycle
* If not included in plant headworks
Figure XVI-2. Fluidized bed furnace system schematic
-------
sidestreams
shown on Figure XVI-2 except the grit removal, disintegrator,
thickener, and dewatering device.
There are several sidestreams from the thickening and
dewatering units. These sidestreams and their treatment
methods are presented in other sections. The material leav-
ing the furnace consists of ash and gas. This mixture is
treated by a scrubber which separates the two products.
Gas is then vented through a stack.
The scrubber produces a mixture of water and ash. The
ash is then concentrated, dewatered, and hauled to disposal.
The water is recycled to the headworks.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Typical loading rates for various types of sludge are
shown on Table XVI-1. The loading rates are a function of
the moisture content of the feed sludge.
TABLE XVI-1. LOADING RATES
Type of sludge
Primary
Primary + FeCl3
Primary + low lime
Primary + WAS
Primary + (WAS + FeCl3)
(Primary + FeCl3) + WAS
WAS
WAS + Fed 3
Digested primary
Solids,
%
30
16
35
16
20
16
16
16
30
Vol.
solids,
%
60
47
45
69
54
53
80
50
43
Chemical
concentration , *
mg/1
N/A
20
298
N/A
20
20
N/A
20
N/A
Wet sludge
loading
rate,
Ib/sq ft/hr
14
6.8
18
6.8
8.4
6.8
6.8
6.8
14
*Assumes no dewatering chemicals.
Using the loading rates shown, the ash and gas products
characteristics should be fairly consistent. These products
are mainly a function of the combustion temperature. In
order to deodorize the stack gas a temperature of 1,350 to
1,400°F must be maintained. At these temperatures the
sludge is completely burned assuming the furnace is not
XVI-4
-------
overloaded. Therefore, the measure of performance is the
stack gas quality. The gas quality is measured in terms
of particulates, metals, gaseous pollutants, and organic
compounds. The scrubber is designed to remove particulates
with the ash. Most metals present in municipal sludges are
converted to oxides which appear in the particulates re-
moved by the scrubber. Lead and mercury are two exceptions.
These two metals vaporize and will appear in the stack gas
if present in the sludge. Carbon monoxide is present in the
stack gas only if the furnace is improperly designed or
operated. Another gaseous indicator of furnace performance
is the presence of toxic substances, such as pesticides or
PCB's. Proper operation at temperatures above 1,100°F should
destroy PCB's.
STAFFING REQUIREMENTS
The staff requirements are small due to the automation
of this process. Labor requirements for operation and main-
tenance of the reactor, air pre-heater, fluidizing air blower,
scrubber, and ash dewatering units are shown on Table XVI-2.
TABLE XVI-2. LABOR REQUIREMENTS FLUIDIZED BED REACTOR*
Labor, hr/yr
Number of reactors
1
3
5
Operation
2,920
8,760
14,600
Maintenance
1,460
4,380
7,300
Total
4,380
13,140
21,900
Assuming full-time operation 7 days per week, 52 weeks per year.
MONITORING
Most of the furnace process control monitoring is
automatic. That is, those critical parameters for furnace
operation such as temperature maximum and minimum in the
bed and maximum exhaust temperature are monitored continuously
with built-in thermocouples. Also, critical to furnace
operation is the percent of excess air which is determined by
continuous monitoring of the percent oxygen in the stack gas
from the scrubber.
Other monitoring requirements are those related to
determining process performance and optimization of pre-
processing equipment. The monitoring points include incom-
ing sludge to the thickener, dewatering unit, furnace,
scrubber, ash concentrator, and ash dewatering unit. The
XVI-5
-------
auxiliary/startup fuel line is metered. Sidestreams return-
ing to the plant headwaters from concentration and dewater-
ing units are monitored.
There are also monitoring requirements for regulatory
agencies. These include stack gas and ash for disposal.
These monitoring points are shown on Figure XVI-3 (see
following page).
The analyses required and their frequency shown on
Table XVI-3 for each monitoring point is identified on
Figure XVI-3.
TABLE XVI-3. MONITORING __
Monitoring point
Analysis
Frequency
1
2
3
3
4
5
5
5
5
5
5
5
5
6
6
7
7
Solids content
Solids content
Solids content
Volatile solids
Fuel quantity
Oxygen content
Particulate
concentration
Carbon monoxide
Lead
Mercury
Hydrogen chloride
Sulfur dioxide
Oxides of nitrogen
BOD5
Suspended solids
Metals content
Moisture content
Weekly
Weekly
Weekly
Weekly
Continuous
Continuous
Weekly
Monthly
Semiannual
Semiannual
Semiannual
Semiannual
Semiannual
Weekly
Weekly
Semiannual*
Weekly
*If ash used for soil conditioner.
Those tests taken for evaluating process performance
are accomplished weekly. Those required for regulatory
requirements are accomplished less frequently. The regula-
tory requirements may change depending on the particular
jurisdiction.
Sensory Observations
The stack gas appearance can be indicative of a problem
with the scrubber or proper operation of the furnace. If
XVI-6
-------
H
I
Grit*
Removal
Disintegrator
I Auxiliary Fuelj
s~~-
4
Furnace
Air Preheater
Fluidizing Air
Blower
Dewatering
Device
*If not included in plant headworks.
Ash and Gases
j Plant Effluent |
I
Scrubberf'
I
Dewatering
Devices
>~
Recycle
r
Figure XVI-3. Fluidized bed furnace system monitoring points
-------
the stack gas has excessive particulate concentrations
there will be a brown or black plume appearing. If there
is an odor present then the combustion process is not being
completed.
Another sensory indicator is the color of the ash. If
a change occurs, there may have been a chemical change in
the raw sewage entering the plant or the combustion process
may not be complete. Similarly, odors produced by the ash
may indicate incomplete combustion.
NORMAL OPERATING PROCEDURES
Startup
Part I - Operating the Preheat Burner
1. Check utilities, power, fuel, water, correct position-
ing of valves in purge air system and water system.
2. Set all controls at zero or "MANUAL" position.
3. Start oxygen analyzer sampler for the reactor exhaust
system.
4. Begin water flow to scrubber trays, venturi throat
and water seal.
5. Adjust burner atomizing air and combustion air valve.
Adjust oil metering valve on preheat burner to a low-
fire position. Open manual valves in pilot fuel piping
and burner oil piping.
6. Start preheat burner blower.
7. Ignite preheat burner.
8. After a few minutes, gradually increase fuel rate to
burner until at full fire.
9. Shut manual valves in pilot fuel line.
10. Continue heating and "bumping" bed to slightly above
1,150°F.
11. Open manual valves in bed gun fuel line.
12. Start fluidizing blower and set air rate at 5,450 SCFM.
Check that exhaust oxygen is 4 percent or higher.
13. Shutdown preheat burner.
XVI-8
-------
14. Stop preheat burner blower.
15. Shut manual valves in preheat burner fuel piping.
16. Heat bed to temperature for sludge incineration.
Part II - Reactor Startup When Bed Temperature is Above
1,15QQF
1. Check utilities, power, fuel water, correct positioning
of valves in purge air system and water system.
2. Set all controls at zero or "MANUAL" position.
3. Start oxygen analyzer sample from reactor exhaust system.
4. Start flow of water to scrubber trays, venturi throat
and water seal.
5. Light bed gun burner.
6. Start fluidizing blower. Set air rate at 5,450 SCFM.
7. Start injector purge air blower.
8. Raise gun fuel rate to maximum allowable.
9. Heat bed above auto-ignition temperature of sludge,
say 1,300°F.
10. Reduce fuel rate and start sludge feed system. Increase
air rate as required.
Part III - Feed System Operation (Reactor has been started
and is at least 1,250°F)
1. Set valves for flow of sludge from concentration tank
to dewatering device.
2. Check that feed gun valves are in feed position.
3. Check that supply of chemicals is ample and set valves
for flow of chemicals.
4. Start dewatering devices and transfer screw conveyors.
5. Start macerator.
6. Start dewatering device feed pumps and adjust speed.
7. Start chemical pumps and dilution water.
XVI-9
-------
8. When dewatering devices deliver sludge in chutes,
start reactor feed pumps. DO NOT RUN PUMP DRY:
9. Observe oxygen analyzer. Keep oxygen content at 4 to
6 percent. Adjust sludge and fuel rates as reactor
warms up.
Routine Operations
Instructions in this section apply when the reactor
has already started, and the bed temperature is above the
auto-ignition temperature of the auxiliary fuel. It is
assumed that the bed guns are in use and the oxygen analyzer
is operating.
Before starting the reactor feed system, the operator
should check that the following items are ready:
1. See that the dewatering system is ready for operation
and that there is sufficient sludge in the concentration
tank to permit a normal operating cycle.
2. Check valves in the sludge lines for correct position.
3. Valves between the concentration tank and the dewatering
system inlets should be open.
4. If daily use of polymers is required, check supply of
stock solution.
5. Check that the valves are open in the chemical system.
a. Start the dewatering system.
b. Start the reactor feed pumps after the following
conditions are met:
(1) Dewatering device feed pumps are running.
(2) Fluidizing air flow is above 3,900 SCFM.
(3) Bed temperature is between 1,200°F andl,600°F.
(4) Reactor exhaust temperature is below 1,800°F.
Note: Under no circumstances should reactor feed
pump be allowed to run dry!
c. As soon as the dewatered sewage sludge enters the
reactor, demand for oxygen will increase and it
will be necessary to lower the auxiliary fuel rate.
Unless the auxiliary fuel rate is adjusted at
this time, the maximum heat release from the sludge
XVI-10
-------
Shutdown
will not be achieved and fuel may be wasted.
Daily operating experience will soon show exact
auxiliary fuel settings at start-up.
d. During the next hour, a series of adjustments can
be made as follows:
Gradually increase sludge feed rate to
the reactor to the maximum indicated by
experience. When changes are made in
sludge or auxiliary fuel rate, wait 5
minutes and check oxygen analyzer.
Note: Adjustment of auxiliary fuel and
sludge feed rates should be done
gradually since reaction time for
the bed temperature change can be
as long as 15 to 30 minutes.
(1) Reduce fuel rate to the minimum which is
necessary to maintain bed temperature in
the 1,250 to 1,300°F range.
(2) Chemical feed rate is adjusted to the minimum
required for desired feed sludge dewatering.
(3) Reactor exhaust oxygen should be in the range
of 4 to 6 percent for good combustion. This
can be done by adjusting the fluidizing air
flow rate in very gradual steps. Increasing
the air flow rate will increase exhaust oxygen
content. Decreasing the air flow rate will
decrease exhaust oxygen content. The reaction
time for these adjustments is several minutes.
e. At this point, all equipment required for sludge
incineration is operating. Maintain hourly read-
ings on log sheet.
Normal shutdown will follow the three groups of steps
listed:
1. Reactor feed system shutdown.
2. Heating bed to the maximum allowed by the bed tempera-
ture interlocks.
3. Reactor shutdown and scrubber shutdown.
XVI-11
-------
As soon as the reactor feed system is stopped, it is
possible to heat the reactor with bed guns for overnight
shutdown. The scrubber is then shutdown and a general
cleanup started.
1. Shutdown sludge dewatering equipment. While the reactor
is still operating. Pump a mixture of thin sludge
and water through the reactor feed system to displace
the heavier sludge present in the reactor feed hose.
As the water enters the bed, there will be a rise in
freeboard pressure. By slowly pumping thin sludge
through the feed hose, the reactor sludge gun is
cleaned for the next start-up.
2. Close valves on feed guns. Blow out feed guns with
compressed air to clear sludge remaining in the feed
nozzle.
3. Pumping thin sludge in Step 1 will tend to lower the
bed temperature slightly. Continue heating bed until
"bed high temperature" alarm sounds.
4. Stop fuel flow. Leave fluidizing blower running and
proceed.
5. Close fluidizing air control valve and when the air
flow rate is nil, stop fluidizing blower. A solenoid
valve will automatically close on the scrubber quench
sprays when the blower is stopped.
6. Stop injector purge air blower.
7. Shutdown scrubber by stopping the process water flow,
ash pump, ash classifier, and water to the water seal,
then drain scrubber.
8. Shutdown oxygen analyzer sample system.
9. Check that manual valves are shut off in the bed gun
fuel system and preheat burner fuel system. Check that
water valves are closed.
CONTROL CONSIDERATIONS
Physical Control
The fluid bed furnace is furnished with a semi-automatic
process control system and a mechanical electrical protection
system, which free the operator from continuous supervision.
The process is maintained in balance at the required excess
XVI-12
-------
reactor
oxygen
analyzer
air and operating temperatures by normal adjustments in air
rate and sludge feed rate, and automatic control of auxiliary
fuel rate. The process parameters and physical conditions
are kept in check by means of a multi-point alarm system which
warns the operator of impending imbalances in the process or
mechanical equipment.
The main control panel comes with a two-pen recorder
which gives two important indications of how well the plant
is operating:
Bed temperature - When the reactor is operating in
equilibrium the bed temperature will vary within a
narrow range and neither rise nor fall. A rising
or falling bed temperature trend indicates that an
adjustment of sludge or fuel rates is required.
Oxygen content - If the correct amount of air is being
supplied to the reactor, the exhaust oxygen reading
will be in the 4 to 6 percent range.
Other process parameters and electrical interlocks are
incorporated in the system to prevent starting equipment out
of sequence or to automatically shutdown or stop various com-
ponents of the system, preventing damage to the equipment.
The purpose of the analyzer is to measure the amount of
oxygen in the reactor exhaust gas. The amount of oxygen re-
maining after combustion is a direct measure of the quantity
of excess air being supplied to the reactor. The excess air
during sludge incineration should vary between 20 to 40 per-
cent during normal operation, with an absolute minimum of
10 percent. Good combustion results when the analyzer
indicates 4 to nearly 6 percent oxygen. Operating at less
than 2 percent oxygen (10 percent excess air) must be avoided.
When low oxygen readings occur, the situation is cor-
rected by increasing the air rate slightly. If the fluidiz-
ing air blower is already operating at its design capacity,
low oxygen readings are corrected by decreasing the fuel rate
to the reactor. Remember that sludge is a fuel also, slight
adjustments of sludge feed rate and/or auxiliary fuel rate
can be made. When all three conditions listed below are
satisfied the reactor should be operating efficiently:
1. Reactor exhaust oxygen content between 4 and 6 percent.
2. Bed temperature is steady.
3. Auxiliary fuel rate is at a minimum and sludge feed
rate is at maximum.
XVI-13
-------
The reactor needs several hours after startup to
warm up and approach the above ideal conditions.
When exhaust oxygen readings are high during sludge
incineration, the sludge feed rate should be increased.
The auxiliary fuel rate should be adjusted accordingly to
maintain a steady bed temperature.
A lapse period of 3 to 5 minutes exists between the
oxygen analyzer and any change made in the fuel rate.
An alarm point on the annunciator warns the operator
of low exhaust oxygen readings.
The bed temperature is one of the most important in-
bed strument readings in the entire fluidized bed furnace system.
temperature A series of electrical interlocks prevents operation of the
supervision sludge feed system to the reactor when the bed is not within
the correct temperature range.
There are usually three thermocouples inserted in the
bed. Each is encased by a stainless steel protection tube
(called a "thermowell"). The lead wires of two thermocouples
are connected to temperature indicators. The sole purpose
of these thermocouples is to provide the operator with a
direct reading of bed temperature. This reading is a check
of the control thermocouple.
The third thermocouple is used for input to the bed
temperature controller which controls the fuel rate to the
bed guns and the following interlock circuits:
1. To prevent the flow of fuel to the bed gun when the
bed temperature is below 1,150°F.
2. To prevent the feed of sludge to the bed when the
temperature is below 1,200°F.
3. Low bed temperature alarm, actuated at 1,250°F.
4. High bed temperature alarm. Actuated at 1,550°F.
5. To prevent feed of sewage sludge or auxiliary fuel to
the bed when the temperature is above 1,600°F.
Aside from bed temperature supervision the system nor-
mally has temperature alarms as follows.
1. High temperature switch in the reactor exhaust set for
1,800°F, which prevents feed of sewage sludge to bed
and fuel to either bed guns or preheat burner when
activated.
XVI-14
-------
2. High temperature switch in the scrubber inlet set for
550°F. This activates an alarm.
Air rate measurement is obtained by measuring pressure
difference across an orifice plate installed in the inlet
air pipe to the fluidizing air blower. As the air rate increases,
flow so does the pressure difference across the orifice plate and
pointer on the SCFM scale then moves upward on the indicator
scale. For decreasing air flows, the reverse is true.
It is undesirable to operate the reactor with too low
an air rate because poor bed fluidization and incomplete
fuel combustion will result. At low air flow the pressure
difference across the orifice plate will stop fuel flow to
the fuel guns.
Reactor pressure is measured at 3 points as follows:
Freeboard pressure tap - This pressure tap indicates
the pressure of any point in the reactor freeboard,
as compared to outside atmospheric pressure.
Windbox pressure - During normal operation such as
sludge incineration or bed reheating, the windbox
pressure should be in the vicinity of 100 to 120
inches of water.
Bed pressure tap - The bed pressure tap centerline is
located 12 inches above the surface of the constriction
plate. For practical reasons, it is not possible to
locate the pipe much lower.
normal pres- Due to the pulsations of the fluid bed, it is perfectly
sure tap normal for the pressure readings to bounce slightly in
readings rhythm with the bed.
The freeboard pressure should be approximately 40
inches of water when sludge is being incinerated at the
design feed rate of the reactor. When the sludge feed is
freeboard stopped, the freeboard pressure will decrease to say, 5 to
10 inches of water. This is because the large volume of
evaporated water vapor carried by the sludge is no longer
present.
If the freeboard pressure is unusually high, it may
indicate partial blockage of the exhaust gas ducts or the
scrubber. Investigation is required.
The water seal expansion joint has a low flow switch
which will deactivate the fluidizing blower and the preheat
burner blower upon sensing a low flow to the water seal.
XVI-15
-------
bed
depth
scrubber
water
control
scrubber
high inlet
temperature
The total depth of the fluid bed should be maintained
at 60 inches. A fluidized bed of sand resembles a container
of boiling water. By coincidence the density of a cubic foot
of the fluidized sand-air mixture is nearly the same as the
density of a cubic foot of water. Therefore, the pressure
reading at the bottom of a fluidized sand bed, 60 inches deep,
is approximately the same as if the bed was filled with a
stationary "bed" of water also 60 inches deep. For this
reason, when the bed pressure tap reads, say 48 inches of
water, there is approximately 48 inches of fluidized sand
above the tip of the pressure tap pipe within the bed.
Since the tip of the pressure tap is normally 12 inches above
the bottom of the bed, the total bed depth should be 60
inches of fluidized sand (48 inches plus 12 inches).
The flow to the quench sprays is manually set with the
use of the flow indicator in the water line to the sprays.
A solenoid valve in the line opens with the starting of the
fluidizing blower or preheat burner blower.
The water make-up to the scrubber recirculation system
is accomplished by a liquid level control in the base of
the scrubber. The recirculation rate of ash water can best
be measured by the pressure drop which should be in the 20
to 35 psig range. The pressure drop can be correlated
directly with flow rate.
A temperature switch installed in the scrubber inlet
duct warns the operator of high gas temperature by sounding
an alarm.
Process Control
combustion
air
requirements
The quantity of fluidizing air injected into the reactor
is an important variable. An excessive quantity of air would
blow sand and incomplete products of combustion into the
flue gases and would result in needless fuel consumption.
Insufficient air results in unburned combustibles in the
exhaust gases. Fluidized bed systems are typically operated
with 20 to 40 percent excess air. In practice, this rate is
controlled by measuring the oxygen in the reactor exhaust
gases and adjusting the air rate to maintain 4 to 6 percent
oxygen.
Since the theoretical amount of air is never enough for
complete fuel combustion, "excess air" must be added. The
extra air is expressed as a percentage of the theoretical
air requirement. For example, if a fuel requires 1,000
standard cubic feet of air per minute (SCFM), based on
theoretical air requirements and the actual air rate is
1,200 SCFM, the percent excess air is:
XVI-16
-------
(100) (Actual air rate - theoretical)
Theoretical
(1,200 - 1,000) x 100 on
- - — — — - = 20 percent excess air
X f UUu
Auxiliary fuel is used during startup to raise the sand
bed to about 1,200°F. As soon as sludge feed to the furnace
begins, the auxiliary fuel rate must be adjusted downward to
achieve the maximum heat release from the sludge and to avoid
wasting fuel. This is done by gradually reducing the fuel
feed rate to the minimum that is necessary to maintain bed
temperatures in the 1,250 to 1,300°F range.
EMERGENCY OPERATING PROCEDURES
Loss of Power
Loss of Fuel
Upon a momentary or extended loss of power, the preheat
burner bed gun and all electrically driven equipment will
shut down. Sludge may be stored in the thickener or hauled
to a landfill by truck until the power supply is restored.
If the reactor must be operated continuously and the
primary fuel supply is interruptible, an auxiliary fuel
source should be provided. For instance, LPG can be pro-
vided as a standby to an interruptible natural gas supply.
COMMON DESIGN SHORTCOMINGS
Shortcoming Solution
1. Inadequate dewater- la. Add more chemical aids to
ing, solids content dewatering device.
low.
Ib. Try varying type of chemical.
Ic. Increase fuel to reactor
as temporary measure.
2. No provision for 2a. Store sludge in clarifiers
handling sludge dur- (temporarily).
ing power outage
or reactor downtime. 2b. Haul sludge to landfill.
3. Reactor undersized. 3a. Increase hours of operation.
3b. Improve dewatering
performance.
XVI-17
-------
Shortcoming Solution
4. Scrubber discharge 4.a.Keep pipe short and accessible
water piping plugs for cleaning or replacement.
with scale and ash.
b.Additives may also be effective
in controlling or reducing the
effects of this problem.
XVI-18
-------
TROUBLESHOOTING GUIDE
FLUIDIZED BED INCINERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Bed temperature is
falling.
la. Inadequate fuel
supply.
Ib. Excessive rate of
sludge feed.
Ic. Excessive sludge
moisture.
Id. Excessive air flow.
la. Fuel system
operation.
Ib. Sludge feed system.
Ic. Dewatering system.
Id. Oxygen content of
exhaust gas should
not exceed 6 percent.
la. Increase fuel feed rate or re-
pair any fuel system malfunc-
tions.
Ib. Decrease sludge feed rate.
Ic. Improve dewatering system opera-
tion (see appropriate section
of this manual).
Id. Reduce air rate.
2. Low (<4%) oxygen in
exhaust gas.
2a. Low air flow.
2b. Fuel rate too high.
2a. Air flow rate.
2b. Fuel rate.
2a. Increase air blower rate.
2b. Decrease fuel rate.
3. Excessive (>6%)
oxygen in exhaust gas.
3a. Sludge feed rate too
low.
3a. Sludge feed rate.
3a. Increase sludge feed rate and
adjust fuel rate to maintain
steady bed temperature.
4. Erratic bed depth
readings on control
panel.
4a. Bed pressure taps
plugged with solids.
4a. Tap a metal rod into pressure
tap pipe when reactor is not
in operation.
4b. Apply compressed air to pres-
sure tap while the reactor is
in operation after reviewing
manufacturer's safety
instructions.
-------
H
I
NJ
O
TROUBLESHOOTING GUIDE FLUIDIZED BED INCINERATION
INDICATORS/OBSERVATIONS
5. Preheat burner fails
and alarm sounds.
6. Bed temperature too
high.
7. Bed temperature reads
off scale.
8. Scrubber high
temperature.
*
PROBABLE CAUSE
5a. Pilot flame not
receiving fuel.
5b. Pilot flame not
receiving spark.
5c. Pressure regulators
defective.
5d. Pilot flame ignites
but flame scanner
malfunctions.
6a. Fuel feed rate too
high through bed
guns.
6b. Bed guns have been
turned off but tem-
perature still too
high due to greasy
solids or increased
heat value of sludge.
7a. Thermocouple burned
out.
7b. Controller malfunc-
tion.
8a. No water flowing in
scrubber.
8b. Spray nozzles
plugged.
CHECK OR MONITOR
5a. Fuel pressure and
valves in fuel line.
5b. Remove spark plug
and check for spark;
check transformer.
5d. Scanner operation.
7a. Check the entire
control system.
8a. Water pressure and
valve position^.
8b. Check nozzles by re-
moving and connecting
them to external wate
source .
SOLUTIONS
5a. Open appropriate valves and
establish fuel supply.
5b. Replace defective part.
5c. Disassemble and thoroughly
clean regulators.
5d. Clean sight glass on scanner;
replace defective scanner.
6a. Decrease fuel flow rate
through bed guns.
6b. Raise air flow rate or
decrease sludge feed rate.
7a. Repair as necessary.
8a. Open valves.
8b. Clean nozzles and strainers.
-------
TROUBLESHOOTING GUIDE
FLUIDIZED BED INCINERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8c. Ash water not
recirculating.
8c. Pump operation and
scrubber pluggage.
8c. Return pump to service or
remove scrubber pluggage.
9. Reactor sludge feed
pump fails.
9a. Bed temperature in-
terlocks may have
shut pump down.
9b. Pump is blocked.
9a. Bed temperature.
9b. Sludge too
concentrated.
9a. (See items 1 and 6).
9b. Dilute feed sludge with water.
LO. Poor bed
fluidization.
LOa. During shutdowns,
sand has leaked
through support
plate.
lOa. Once per month, clean windbox.
-------
MAINTENANCE CONSIDERATIONS
Sand from the reactor bed is gradually lost through
addition the exhaust as individual sand particles are gradually
of make- worn into finer and finer particles. When it has been de-
up sand termined that the bed level is getting low, proceed as
follows:
1. Bed temperature should be at least 1,400°F before any
sand is added to the reactor bed. This is to avoid
cooling the bed below 1,150°F, and being forced to
light the preheat burner.
2. Be sure that the fluidizing blower is completely stopped.
3. Remove the blind flange on the sand feed nozzle. Attach
sand feed chute.
reactor
windbox
bed
fuel
guns
bed
pressure
tap
4. Add sand in 10 bag batches. If more than 10 bags are
required, replace the blind flange on the same feed
nozzle and reheat the bed to 1,400°F before adding
second 10 bag batch.
There may be slight leakage of sand down into the wind-
box. About once a month (when the reactor is not operating),
open the windbox manhole and rake out any accumulation.
Occasionally a carbon deposit may form near the tip
of the fuel burner. If this happens, fuel flow to the bed
will be restricted. When the reactor is shutdown, clean the
burner. If available, a slight flow of compressed air will
aid inserting the gun back in the bed.
From time to time, check that the nut on the packing
gland is just tight enough to prevent loss of cooling air.
At times this pressure tap pipe may become partially
plugged. Refer to manufacturer's manual for cleaning
instructions.
gaskets
SAFETY CONSIDERATIONS
Keep gasketed surfaces on the reactor tight to avoid
a fly ash nuisance.
1.
Safety measures should include the following:
No smoking should be allowed around the fuel lines or
when checking the system for leaks.
2. Protective clothing should be worn when repairing or
XVI-22
-------
lighting the furnace.
3. Protective goggles should be used when lighting the
furnace.
REFERENCE MATERIAL
References
1. Standard Methods For The Examination of Water and
Wastewater.
American Public Health Association
1015 Eighteenth Street, N.W.
Washington, D.C. 20036
2. Dorr-Oliver FS Disposal System Operating Instructions.
3. Copeland Systems
Glossary of Terms and Sample Calculations
1. Excess air is the amount of air required beyond the
theoretical air requirements for complete combustion.
This parameter is expressed as a percentage of the
theoretical air required.
Sample calculation for excess air: Assume 1,200 SCFM
actual, and 1,000 SCFM theoretical
Excess air =
(actual air rate - theoretical air rate) x 100
theoretical air rate
= (1,200 to 1,000) x 100 = 20%
1,000
2. Sludge loading rate is the weight of wet sludge fed to
the reactor per square foot of reactor bed area per
hour (Ib/sq ft/hr).
Sample loading rate: Assume 20 foot dia. reactor, 20 per-
cent feed sludge moisture content and 440 pounds dry
sludge per hour
Loading rate = (Ib dry sludge/hr)(100)
(% moisture content)(area)
440 x 100
20% x 3.14 (20)
XVI-23
-------
= 7.01 Ib/sq ft/hr
3. Solids concentration is the weight of dry solids per
unit weight of wet sludge. It is calculated as follows;
Assume 120 Ib wet sludge with 25 Ib of dry solids.
Concentration = weight of dry sludge solids x 100
weight of wet sludge
= 25 x 100
120
= 20.8%
4. Moisture content is the amount of water per unit weight
of sludge. The moisture content is expressed as a
percentage of the total weight of the wet sludge. This
parameter is equal to 100 minus the solids concentra-
tion or can be computed as follows:
Same assumptions as paragraph 3.
Moisture content =
(weight of wet solids - weight of dry solids)
x 100
weight of wet solids
= 120 - 25 x 100
120
= 79.2%
XVI-24
-------
XVII
COMPOSTING
-------
CONTENTS
Process Description XVII-1
Typical Design Criteria and Performance XVII-4
Loading Rates XVII-4
Expected Performance XVII-6
Sidestream XVII-6
Staffing Requirements XVII-6
Monitoring XVII-8
Sensory Observations XVII-9
Normal Operating Procedures XVII-9
Windrow Composting XVII-9
Forced Air Static Pile Composting XVII-11
Control Considerations XVII-14
Physical Control XVII-14
Process Control XVII-14
Emergency Operating Procedures XVII-16
Composting Site Shutdown XVII-16
Odor Generation XVII-16
Common Design Shortcomings XVII-17
Troubleshooting Guide XVII-18
Maintenance Considerations XVII-20
Safety Considerations XVII-20
Reference Material XVII-20
References XVII-20
Glossary of Terms and Sample Calculations XVII-20
-------
PROCESS DESCRIPTION
process
methods
equipment
Raw wastewater sludge (and sometimes digested sludge)
requires processing before disposal or use in order to reduce
the possibility of problems with odors, flies, disease, and
other nuisances. This processing is commonly called stabili-
zation. Composting is one means of stabilizing raw or digest-
ed sludge through biological action (bacterial organisms).
Heat is produced during the composting process and is general-
ly sufficient to produce temperatures above 55 to 60 C within
the compost. These temperatures are high enough to kill most
pathogenic organisms, therefore, composting is capable of
reducing disease-producing organisms to very low levels.
Two methods have been used for composting wastewater
sludge; windrow and forced air static pile. Various contained
composting methods have been used for solid waste, but have
not been used for wastewater sludge. Generally, the Windrow
method, shown in Figure XVII-1 (see following page) is used
with digested sludge and the forced air static pile method,
shown in Figure XVII-2 (see following page) is used with
either raw or digested sludges.
The equipment and methods used are somewhat different
for each composting method and each is covered in this manual.
The type and size of equipment required also depends on the
quantity of sludge to be composted, however, certain minimum
sized equipment is required for any sized operation. Most
composting operations use mobile type equipment, but it is
also possible to use fixed type equipment for certain oper-
ations. The type of equipment required is shown in Table
XVII-1 (see following page).
A schematic of the two typical composting operations
is shown in Figure XVII-3 (see following page). These steps
are described in detail under NORMAL OPERATING PROCEDURES.
The descriptions in this manual are typical for process-
ing an annual sludge input up to approximately 3,500 dry tons
per year.
XVII-1
-------
Figure XVII-1. Windrow composting at Beltsville, Maryland.
XVII-2
-------
AIR IN,
DEODORIZED
EXHAUST
AIR
10
FT
COMPOST
PILE
40 TO 50 FT
GENERAL LAYOUT
BULKING AGENT AND
SLUDGE MIXTURE
COMPOST COVER
UNSCREENED COMPOST
OR BULKING AGENT
PERFORATED
PIPE
15
CROSS SECTION
SCREENED
COMPOST
(5 cu yd)
SUBSEQUENT PILES
FOR EXTENDED PILE
METHOD
Figure XVII-2. Typical forced aeration compost pile.
XVII-3
-------
TABLE XVII-1. COMPOSTING EQUIPMENT
Windrow
Forced air
static pile
Specialized windrow turner
Dump truck (*)
Rubber tired front
loader, 4 cu yd
Drum screen
Rubber tired front loader,
4 cu yd
Dump truck (*)
Aeration blower assemblies
and pipe
Drum screen
Composting machine (**)
sidestreams
* Requirement will depend on site and operation
** May be helpful for mixing on larger applications
Process sidestreams consist of storm runoff water from
the site and excess water released from the piles during the
composting and curing processes. The site should be designed
so this water is collected, normally in a lagoon. In some
cases this runoff can be placed directly into a sewer if it
will not overload its treatment plant hydraulically or bio-
logically. The collected runoff should be disposed of in a
manner acceptable to local conditions. One means of disposal
is to spread it on adjacent land at a. controlled rate which
may or may not require some degree of treatment.
TYPICAL DESIGN CRITERIA & PERFORMANCE
Loading Rates
Dewatered sludge
Sludge - Bulking agent
Mix ratio
Bulking agent
20 to 25 percent solids (1 dry ton
solids is equal to approximately 7
cubic yards of dewatered sludge)
2.5 to 3.0 parts bulking agent to
1 part dewatered sludge by volume
Requires 17 to 21 cubic yard per
dry ton of sludge. Typical bulking
agents:
wood chips
bark chips
shreaded tires
compost
XVI1-4
-------
Sludge ^
Sludge
Delivery
Bulking
-
Mixing
i
Agent
***
Bulking
Agent
Storage
Pile
formation
& compost-
ing
Recycled
"^
Screening
*
1
-
Curing
**
\
Screening
*
bulking agent
Product
Storage
Use
Forced Air Static Pile Composting
H
H
I
U1
Sludge
Sludge
Delivery
Bulking
-
Mixing
i
* —
Agent
***
Bulking
Agent
Storage
-
Windrow
formation
Windrow
turning
Screening
*
Recycled Bulking
Screening
*
Agent
Product
Storage
Use
Windrow Composting
Figure XVI1-3. Composting operations.
* Screening will normally be accomplished either prior to or just after the curing step.
** The purpose of curing is provide storage time for the compost at elevated temperature for additional pathogen kill and stabilization.
*** The purpose of the bulking agent is to add porosity to the sludge so air can pass through more readily.
-------
Composting period
Curing period
Expected Performance
Compost production
Unscreened
Screened (h inch
screen)
Minimum composting
temperature
Finished compost
Moisture content
Volatile solids
Bulking agent recovery -
Sidestream (Runoff water)
14 to 21 days
30 days
26 cubic yard per dry ton sludge
10 to 12 cubic yard per dry ton
sludge
55 to 60 C - Forced air static pile;
50 to 55°C - Windrow
40 to 50 percent
40 percent
Variable depending on type of agent,
degree of screening, but in range of
60 to 80 percent following screening.
Data are not available on runoff water characteristics
except that the quantity may vary from 6 to 20 gallons per
day per pile containing 50 cubic yards of sludge during dry
weather.
STAFFING REQUIREMENTS
Staffing includes personnel for materials handling at
the site, mixing, composting, monitoring and screening. It
does not include hauling materials to or from the composting
site. The equipment operators should be competent on heavy
equipment such as front loaders and trucks. Typical staffing
for two sized operations is shown in Table XVI1-3 (see
following pages). This table is based on two actual
operations.
XVII-6
-------
TABLE XVII-3. COMPOSTING LABOR REQUIREMENTS
Labor, hr/yr
350 3,000
dry tons dry tons
sludge sludge
annually (*) annually(**)
Administration &
supervision 720 1,800
Equipment operator 1,260 7,200
Laborer 360 1,800
(*) Based on sludge delivery to site once a week
(**) Based on sludge delivery to site five days a week
XVII-7
-------
MONITORING
TEMPERATURE
OXYGEN
TOTAL COLIFORM
FECAL CO LI FORM
SALMONELLA
MOISTURE
NITROGEN
O uj
H 0
UJ -1
Q. CO
O co
fS i
1
lil >-
N CC
co Q
ALL
ALL
ALL
ALL
ALL
^_
O
LU
^
1 c^
CO LLI
lil CC
1- LL
D. 2W*
D, 2W*
0
0
0
i—
co H
"o5
lil
OQ.
i- s
^ ^
Oco
0 DC
-1 O
W
w
c, s
W, S, B
D
Q lil
O co
•Z. y_
.'
O Lil
lil 0.
cr >
LL h-
-
-
G
G
G
Lil
CO
O
CO CC
lil D
HQ.
P
P
H
P
H
A. TEST FREQUENCY
D = DAILY
2/W = TWICE/WEEK
O = ONCE/PROCESS CYCLE
B. LOCATION
W =
C =
S =
B =
D =
WINDROW OR COMPOST PILE
CURING PILE
STORAGE ( PRIOR TO SCREENING)
BULKING AGENT ( PRIOR TO USE)
FINISHED COMPOST PRIOR TO DISTRIBUTION
C. FREQUENCY & TYPE OF SAMPLING
G = GRAB
D, REASON FOR TEST
H = HISTORICAL DATA
P = PROCESS CONTROL
*Daily until temperature reaches 50 to 55° C, then twice/week, test for temperature
and oxygen concurrently.
XVII-8
-------
Sensory Observations
1. Odor
2. Visual
a. Steam (heat)
b. Color (moisture)
c. Uniformity
NORMAL OPERATING PROCEDURES
Operating procedures will vary from operation to oper-
ration depending on size, type of bulking agent, and personnel
and equipment. These procedures are general and should be
applicable to many operations or easily adaptable to specific
cases. Some of the operations are common to both windrow and
forced air static pile (FASP) composting. The procedure is
outlined for the windrow method first. The forced air static
pile (FASP) is then covered using referenced back to the
windrow method for common steps.
Windrow Composting
1. Sludge Mixing
a. Lay down a base or bases of bulking agent in the
mixing area in preparation for sludge delivery.
This base should be 18 to 24 inches deep and the
volume related to the volume of each load of sludge
to be delivered and the desired mixing ration. For
example, if sludge is delivered in 7 cubic yard
loads and the desired mix ratio is 3 parts bulking
agent to one part sludge, each bulking agent base
should contain 21 cubic yards. This would be a
pile 2 feet deep and approximately 10 feet by 28
feet for forced air static pile composting. For
windrow composting the base should be laid in strips
approximately 12 inches deep and 8 to 15 feet wide
depending on the type of composter.
b. The sludge should be dumped on top of the prepared
base of bulking agent. The sludge is then back
dragged over the bulking agent to form a reasonably
uniform layer. Another possibility is to place
only half the required bulking agent down before
dumping the sludge and then place the other half of
the bulking agent over the top of the spread sludge
to form a sandwich.
XVII-9
-------
c. The sludge and bulking agent is thoroughly mixed
to form a homogeneous mixture. This mixing can be
performed with a front loader, a combination of
front loader and grader, a composting machine, or
other equipment that can provide a relatively
uniform mixture.
2. Windrow Formation
The mixed sludge and bulking agent is formed into
windrows with a triangular cross section. The windrow
should be a convenient size for the type of composting
machine to be used but, typically, the windrow will be
6 to 8 feet wide and 5 to 6 feet high.
3. Composting
a. The windrow should be turned daily except when it is
raining. The windrow should not be turned or moved
during rainy weather.
b. The interior temperature should increase steadily
to above 50 C within a few days. The temperature
should remain above 50 C for several days.
c. The turning cycle should continue for two to three
weeks and then the row should be flattened for
further drying.
4. Windrow Removal
a. The compost process requires approximately 21 days.
b. At the end of the compost period the compost pile
or windrow should be torn down and placed in curing.
This can be done with a front loader and the forced
aeration pipe may or may not be salvaged as desired.
c. The entire pile contents are placed in the curing
pile.
d. When the extended pile method is used only the
portion of the pile is removed corresponding to 21
days of composting.
5. Curing
The compost should remain in the curing pile for
at least 30 days prior to distribution.
XVII-10
-------
6. Screening
a. Screening is desirable, but not always needed.
The reason for screening is to remove the coarser
bulking agent from the compost to produce a finer
product and/or so the screened out bulking agent can
be reused. Wood product bulking agent draws nitro-
gen available for fertilization purposes.
b. If screening is used, it can be accomplished either
as the compost pile is torn down (prior to curing)
or after curing. For best screening the moisture
content should be below 40 or 50 percent.
c. Generally, screening is difficult to carry out in
freezing weather or in the open during rain.
Forced Air Static Pile Composting
1. Aeration Pipe
a. Lay out parallel sections of perforated pipe (4
inches plastic drainage type, 4 inches schedule 40
steel, or similar) with each section approximately
7 feet apart.
b. Plug the ends with cans or similar closure.
c. The other ends are connected with "Y" fittings to
a common solid pipe leading out of the pile.
2. Blower Equipment
a. Connect the aeration pipe to the suction of the
blower. Provide some means of removing water from
the suction pipe either by making a hole at the
lowest part of the pipe or by installing a moisture
trap. This is extremely important because water is
collected in the aeration piping and must be re-
moved. For locations which experience periods of
below freezing weather, water removal is very
important to prevent moisture from freezing in the
blower during the off cycle and burning out the
motor on attempted restart.
b. Connect a pipe to the blower discharge and cover
the other end with a pile of bulking agent or
compost (5 cubic yards) for deodorization.
XVII-11
-------
3. Base for Pile
Lay down a 6-to 12-inch thick layer of bulking
agent over the aeration piping with the outside dimen-
sions the same as the base of the proposed finished pile
(Some operations omit this step and place the sludge
bulking agent mixture over the piping.).
Caution: The moisture content of the bulking agent
is critical to good composting performance. Bulking
agent containing more than 40 or 50 percent moisture
should not be used. Also, moisture will be picked up
if mixing is carried out during precipitation. If it is
raining it is best to postpone pile construction to
another day. If it must be done, the bulking agent
should be spread just before the sludge arrives, mixing
should proceed as fast as possible, and the mixture
placed on the pile immediately.
4. Sludge Mixing
This step is the same as for step 1 of Windrow
Composting.
5. Pile Formation
a. The mixture is piled on the composting pile over
the aeration piping and base. The pile should be
a convenient height of 7 to 10 feet and should
extend from the "Y" in the aeration piping to about
5 feet beyond the capped ends. The sludge should
be mixed and piled as soon as it is delivered to
the site to make best use of labor and equipment.
b. The compost pile is then covered with 1 to 2 feet
of previously composted material or bulking agent
to provide insulation and help in odor control.
The depth of cover required is somewhat dependent
on ambient air temperature. Generally, more cover
should be used during extreme cold.
6. Blower Operation
Check blower timer settings and activate blower.
Check for proper operation.
7. Extended Pile Modification
If the extended pile configuration is to be used
the procedures for pile formation are the same as for the
individual pile except as follows:
XVII-12
-------
a. Instead of forming new piles each time sludge is
mixed, the new mixture should be piled against one
side of the previous pile.
b. It may be that less cover is needed on the side
that is to be extended especially if sludge is
added each day.
8. Composting
a. Operations should be monitored as outlined in the
monitoring section and as developed for the
specific application.
b. The interior pile temperature should increase stead-
ily to at least 60 C within a few days. See
Troubleshooting Guide if the temperature does not
come up.
c. Check all equipment at least once a day and be sure
that blower is operating properly.
d. The blower operating cycle should be adjusted
based on interior oxygen levels. This is not a
precise method for adjusting timer settings, but
is a guideline method. Ideally, the oxygen level
should be 5 to 15 percent. If the oxygen is around
5 percent the blower "on" time should be increased
and if the oxygen is above 15 percent, the "on"
time should be decreased. Some operations have
found interior temperature measurements to be a
more reliable indicator of composting operation
because of the difficulty of sampling interior
oxygen levels reliably.
e. During the composting period the pile temperature
should rise rapidly to 60 C or above and should
remain at that level for several days. The temper-
ature should rise rapidly to 60 C or above and
should remain at that level for several days. The
temperature may drop during the latter part of the
cycle.
9. Pile Removal
This step is the same as for step 4 of Windrow
Composting.
10. Curing
This step is the same as for step 5 of Windrow
Composting.
XVII-13
-------
11. Screening
This step is the same as for step 6 of Windrow
Composting.
CONTROL CONSIDERATIONS
Physical Control
There is no permanent process instrumentation required.
Process monitoring can be performed with the following
portable instrumentation:
1. Portable oxygen analyzer with a probe.
2. Probe type temperature indicator.
Process control can be accomplished by adjustments to
the aeration blower timer for the forced aeration static pile
method or in modifications to the turning schedule with the
Windrow method.
Process Control
monitoring
moisture
nutrients
and
pathogens
Typically, the composting process can be monitored
based on temperature, oxygen, and moisture analysis.
Measurement of moisture content of bulking agent, compost
at the end of the composting period, and spot checks of the
cured compost will provide useful information. Typically,
wood or wood product bulking agent should not be used if the
moisture content is above 45 to 55 percent. Compost at the
end of the composting period should have a moisture content
of 40 to 50 percent, and cured compost should have a moisture
content less than 40 to 45 percent for best screening. The
moisture content of screened compost is not too important.
Moisture content can be determined according to the procedure
for determination of sludge residue in Standard Methods.
Pathogen and nutrient monitoring may be practiced as a
general check on process performance but, because of the
complexity of the test procedures will not normally be econom-
ical or practical as a basis for day-to-day process control.
This is a very specialized procedure and must be performed by
a properly equipped laboratory.
Some sensory observations are helpful in carrying out
composting operations.
XVII-14
-------
sensory
observations
1. Visual observations are helpful in carrying out the
sludge-bulking agent mixing. The mixed material should
be relatively homogeneous without large lumps of sludge.
2. Visual observations can help to detect materials
(especially bulking agent) with too high a moisture
content. These observations should be followed up
with tests.
3. Noticeable odors are a good indication of problems in
the composting process. If odors are noted, a problem
probably exists and the trouble should be isolated.
4. Personnel should watch for excessive water buildup in
aeration piping and during freezing weather watch for
freezing of aeration piping or freezing of blowers due
to ice accumulation.
temperature
oxygen
Temperature is the most important indicator for the
first several days of operation. The interior temperature
should increase rapidly and reach 50 C for Windrows and 60 C
for forced aeration static piles within 3 or 4 days. Typi-
cally, the temperature should remain above 50 C to 60 C for a
period of time and may drop a little toward the end of the
composting cycle. It is recommended that temperature readings
be taken once a day until the pile temperature reaches 50 C
to 60 C and then every two to three days thereafter. Temper-
ature readings should be taken at three or four locations
within the pile; one near the center, one near the outside
(just under the cover), and one at either end about 2 feet in
from the surface and at several locations within the windrow.
All readings should be recorded on permanent log sheets. It
is also helpful to plot the temperature readings to give a
graphic indication of performance.
Oxygen analysis should be made for each temperature
reading at the same time and location. Theoretically, the
oxygen readings serve as a basis for changing the blower
timer settings and for evaluation of windrow turning. Inte-
rior oxygen levels should be maintained between 5 to 15 per-
cent. If the oxygen falls to 5 or below the blower "on" time
should be increased and if it rises above 15 percent the
blower "on" time should be decreased. In practice, this
control may not work out as a direct relationship between
oxygen and timer setting, but some modification of this ideal
control should provide an adequate basis for timer changes.
In the windrow process, if the oxygen level falls below 5 per-
cent the windrow should be turned more often.
XVII-15
-------
In some cases, if too much aeration is being provided,
it may show up as a decrease in pile temperature and the
aeration should be modified.
One static pile forced air installation found that
reversing the blower connections (blowing air into the pile)
when pile temperatures started dropping part way through the
composting period helped to maintain temperatures for a
longer period.
Another static pile forced air installation located in a
very cold climate found that two modifications helped during
winter operations:
1. Warm compost taken directly from a finished compost pile
could be used as bulking agent for mixing with new
sludge to form a new pile. They were able to reuse
compost as a bulking agent for two or three cycles
before placing it into curing. This decreased the
bulking agent requirements, but more importantly,
provided a warm material to help accelerate the com-
posting action in a new pile. This may be helpful with
both forced air static pile and windrow methods.
2. Exhaust air from an existing hot forced aeration
composting pile is blown into a new pile for the first
few days to help accelerate the composting action. This
should only be done for the first few days or excessive
moisture will build-up in the new pile.
EMERGENCY OPERATING PROCEDURES
Composting Site Shutdown
Sludge should not be delivered to the site if it can
not be mixed and formed into compost piles or windrows either
due to weather, labor, or equipment problems. The best
procedure is to store it at the plant. Some provision must
be available for short term sludge storage or alternate means
of disposal because problems will develop from time to time.
Odor Generation
The best procedure is to carry out careful operations so
odors are not generated. If unusual odors are detected the
cause of the problem should be determined and then resolved.
As a temporary measure, chemicals can be used to mask the
odor. Some odor control chemicals are listed in the
February 1977 issue of Water and Wastes Engineering magazine.
XVII-16
-------
COMMON DESIGN SHORTCOMINGS
Shortcoming Solution
Water accumulates 1. Install a moisture trap in
in aeration piping the suction piping. If blower
to blower and in still freezes, the blower
very cold weather housing should be heated.
blower freezes
during "off"
cycle. (FASP)
XVII-17
-------
TROUBLESHOOTING GUIDE
COMPOSTING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Compost pile does not
reach 50 C to 60°C
in a few days after
construction.
la. Poor mixing of
sludge & bulking
agent.
Ib. Bulking agent too
wet.
Ic. Too much aeration.
la. Pile oxygen.
Ib. Moisture content of
bulking agent.
Ic. Depth and uniformity
of pile cover (in-
sulation) . (FASP)
H
HI
I
M
00
la. If oxygen levels are above 15%,
reduce operating time of
blower. (FASP)
Ib. Pipe hot exhaust air from an
adjacent pile into this pile
to try to bring it up to
temperature. (FASP)
Ic. If the pile does not come up to
temperature within a couple of
days after taking these steps,
the pile should be torn down,
remixed, and reconstructed. If
the bulking agent is too wet
(above 45 to 55% moisture) it
must be dried or, perhaps,
drier bulking agent can be
found deeper within the bulking
agent storage pile.
2. Temperature does not
remain above 50 C to
60 C more than a day
or two, then drops.
2a.
2b.
Poor mixing of
sludge and bulking
agent.
Bulking agent too
wet.
2a. Pile temperatures.
2b. Pile oxygen.
2c.
Improper aeration.
(FASP)
2c. Moisture content of
bulking agent.
2a. Adjust blower cycle to main-
tain oxygen between 5 and 15%.
(FASP)
2b. Pipe hot exhaust air from an
adjacent pile into this pile
to try to maintain it at
temperature. This should only
be done for a few days at a
time because of moisture
accumulation. (FASP)
2c. If temperature does not come
back up it probably isn't
serious enough to justify re-
mixing, but steps should be
taken to correct the problems
for__the_nex^composting cycle.
-------
TROUBLESHOOTING GUIDE
COMPOSTING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
2d. Aeration blower
operation. (FASP)
2e. Depth and uniformity
of pile cover
(insulation). (FASP)
3. Odors are emitted
from a composting
pile.
3a. Poor mixing of
sludge and bulking
agent.
3b. Poor air distribu-
tion in pile. (FASP)
3a. Pile temperatures.
3b. Pile oxygen.
H
H
I
3a. Check blower for proper opera-
tion and that aeration pipes
are not plugged. (FASP)
3b. Generally the odors develop
from anaerobic conditions in
the pile resulting from lack
of air. Probably the best
procedure is to increase the
blower "on" cycle or run it
continuously until the odors
disappear (FASP), or turn the
windrow more often.
4. Blower does not
operate. (FASP)
4a. Timer failure.
4b. Power failure.
4c. Motor failure.
4d. Fan is "frozen"
from corrosion or
ice and will not
turn.
4a. Power supply.
4b. Turn fan by hand.
4c. Inspect timer or
try a new one.
4a. Check each probable cause until
trouble is found.
-------
MAINTENANCE CONSIDERATIONS
Regular maintenance as outlined by the manufacturers
should be performed on all equipment. A protected area should
be provided for equipment service during inclement weather.
There are no special maintenance considerations related
to a composting operation other than the blowers and timers.
Spare units should be kept in stock for these items so that
inoperative units can be replaced.
SAFETY CONSIDERATIONS
The safety considerations are standard for the use of
the mobile and/or fixed equipment. The operations require
that the equipment (especially the front loader) be moved
around the site rapidly with many changes of direction.
Therefore, it is well to have the operating areas separated
from public areas and the access limited.
The site can get very slippery and appropriate caution
should be exercised when operating the equipment.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association, 1015
Eighteenth Street, N.W., Washington, D.C. 20036.
2. Epstein, E., et al. A Forced Aeration System for
Composting Wastewater Sludge. Journal WPCF, Vol. 48,
No. 4, April 1976, page 688.
Glossary of Terms and Sample Calculations
1. Volatile solids are determined from total residue, and
is the ratio of weight lost after heating to 600 C to
the weight of total residue prior to heating to 600 C.
Volatile content, expressed in percent, generally, is
assumed to indicate approximate organic content. Test
procedures are outlined in Standard Methods.
2. Temperature can be measured using one of many types of
portable instruments. In some cases a simple dial
thermometer with an extended probe 3 to 4 feet long has
proven satisfactory. In any case the probe must be 3
to 4 feet long and strong enough to be pushed into the
pile against large pieces of bulking agent. For refer-
ence the following conversion may be helpful:
XVII-20
-------
50 122
55 131
60 140
Oxygen can be measured with a portable oxygen analyzer.
The instrument should have a probe 3 to 4 feet long that
can be inserted into the compost. Typically, these
instruments are calibrated to air and are relatively
easy to operate.
XVII-21
-------
XVIII
LAND APPLICATION
-------
CONTENTS
Process Description XVIII-1
Typical Design Criteria and Performance XVIII-3
Staffing Requirements XVIII-6
Monitoring XVIII-7
Sensory Observations XVIII-7
Normal Operating Procedures XVIII-7
Startup XVIII-7
Routine Applications XVIII-8
Shutdown XVIII-10
Control Considerations XVIII-10
Emergency Operating Procedures XVIII-10
Loss of Power and/or Fuel XVIII-10
Loss of Other Treatment Units XVIII-11
Troubleshooting Guide XVIII-12
Common Design Shortcomings XVIII-15
Maintenance Considerations XVIII-15
Safety Considerations XVIII-16
Reference Material XVIII-16
References XVIII-16
Glossary of Terms and Sample Calculations XVIII-16
-------
PROCESS DESCRIPTION
process
operation
transport
spreading
The land application operation and maintenance infor-
mation in this chapter applies to controlled application of
liquid wastewater sludge to cropland by subsurface injection
or surface spreading. Injection can be accomplished by
truck or tractor mounted injectors. Tank trucks are normally
used for surface spreading. Dewatered sludge can also be
applied, but this is not as common as liquid application and
will not be covered. This manual is tailored to use of
sludge for crop production, the farm operation (beyond appli-
cation) being accomplished by the farmer rather than the
sludge producing agency, and transport of the sludge to the
farm by truck. Obviously, there are many field variations
to this assumed operation, but certain limits must be es-
tablished to limit the scope of this manual.
Sludge is digested, concentrated to 6 to 8 percent
solids, and then pumped into transfer trucks which haul
sludge to the land application site. Sludge is then trans-
ferred to another specialized truck for application. In
some cases, especially smaller operations, the transfer
truck may be used for application.
Sludge transportation can be accomplished by truck,
rail, barge, or pipeline. Barge transport is limited to
areas with navigable waterways and large volumes of sludge
to be moved. Pipeline transport is limited to large volumes
of dilute liquid sludge (less than 4 percent solids). Local
conditions will normally have a significant effect on selec-
tion of a specific or a combination of transport modes. For
example, if there are several application locations or farms,
the sludge may be transported to a central location and
then transferred by another mode to the several application
locations.
Variations in the characteristics of sludge application
equipment are related to local conditions and sludge solids
content. Dewatered sludge (greater than 10 percent solids
concentration) is not practical for injection and would
normally be spread on the surface. Liquid sludge can be
spread on the surface by special irrigation equipment or
tanker truck. Subsurface injection of sludge can be
accomplished by tank truck or tractor mounted injectors.
Tractor mounted injectors require a sludge feed from a close
XVIII-1
-------
following tank trailer or from a hose connected to a storage
system. Ridge and furrow or flooding methods of application
are not recommended unless there is a means of covering the
applied sludge because nuisances may result.
Typical injector trucks are shown in Figure XVIII-1.
Transfer of sludge from the highway truck to the application
truck is shown in Figure XVIII-2.
Figure XVIII-1. Injector truck. (Courtesy of Big Wheels, Inc.)
Figure XVIII-2.
Loading injector truck from transport truck
(Courtesy of Big Wheels, Inc.)
XVIII-2
-------
sidestreams
There are no sidestreams from this process as long as
application rates are not excessive. If the application
rates or methods are not controlled, the sludge may pond
on the surface and runoff thereby creating an unwanted
sidestream.
TYPICAL DESIGN CRITERIA AND PERFORMANCE
application
rate
determination
The loading rate or application rate is a function of the
sludge and soil characteristics, crop, crop end use and crop
nutrient requirements. Sludge and soil characteristics vary
even for a given situation and crop requirements vary widely.
For this reason and for changing nutrient requirements based on
crop rotation, there is no one general application rate.
The sludge application rate is primarily based on the
sludge nitrogen content and the nitrogen requirements of the
crop. Nitrogen is present in aerobically digested sludge in
the organic, ammonium, and nitrate forms. The nitrate form
is not present in anaerobically digested sludge. Nitrogen
is available for immediate plant use in the ammonium (NH4+)
or nitrate (NO3~) forms. The availability of organic
nitrogen to the crop depends on the mineralization rate
and will normally be available over a period of several years.
Usually, the application rate is first determined based
on nitrogen requirements and this rate is then checked for
excessive heavy metals or phosphorus accumulation. The
critical heavy metals are lead, zinc, copper, nickel, and
cadmium.
The procedure for determining the application rate is
as follows:
1. Determine the crop nitrogen requirement from Table
XVIII-1 (see following page).
2. Calculate sludge application rate to meet the nitrogen
requirement.* (All sludge weight based on dry solids.)
a. Available N in sludge:
Ib available N/ton of sludge =
(Ib NH4~N/ton) + (Ib N03-N/ton) +
(Ib organic N/ton x mineralization rate -
Table XVIII-2) (see following page)
b. Residual nitrogen in soil (after first year of
application). The residual nitrogen can be
determined from Table XVIII-2.
* Sludge should be incorporated as soon as practical due to
surface volatization.
XVIII-3
-------
TABLE XVIII-1. APPROXIMATE UTILIZATION OF NUTRIENTS BY SELECTED CROPS*
Plant
Alfalfa**
Orchard grass
Clover grass
Corn (grain)
Sorghum (grain)
(stover)
Corn silage
Oats (grain)
(straw)
Soybeans (grain) **
(straw)
Wheat (grain)
(straw)
Barley (grain)
(straw)
Yield,
per acre
8 tons
6 tons
6 tons
180 bu
8,000 Ib
8,000 Ib
32 tons
100 bu
60 bu
7,000 Ib
80 bu
6,000 Ib
100 bu
Nitrogen,
Ib/acre
370
300
300
240
120
130
240
80
35
242
84
144
42
110
40
P205,
Ib/acre
80
100
90
44
60
30
100
25
15
49
16
44
10
40
15
K20,
Ib/acre
480
375
360
199
30
170
300
20
125
87
58
27
135
35
115
*Better Crops With Plant Food, Copyright 1972 by the Potash Institute
of North America.
**These numbers include a credit of 80 Ib N/acre for alfalfa and 10 Ib
N/acre for soybeans for nitrogen fixation.
TABLE XIII-2. MINERALIZATION OF SLUDGE ORGANIC NITROGEN
Years after
sludge
application
1
2
3
4
5
Mineral!- Annual nitrogen
zation
rate, %
15
10
5
5
5
available during yr, Ib
% Organic nitrogen in sludge,
2.
6.
3.
1.
1.
1.
0
0
4
5
5
4
2.5
7.5
4.2
1.9
1.8
1.7
3.0
9.0
5.1
2.3
2.2
2.1
3.5
10.5
6.0
2.7
2.5
2.4
4.0
12.0
5.8
3.1
2.9
2.8
4.5
13.5
7.6
3.4
3.3
3.1
N/ton
sludge
% by weight
5.0
15.0
8.5
3.8
3.6
3.4
5.5
16.5
9.4
4.2
4.0
3.8
6.0
18.0
10.2
4.6
4.4
4.1
c. Annual application rate:
Tons sludge/acre = (crop N requirement)-(residual N)
Ib available N/ton sludge
3. Calculate the maximum allowable sludge application rate.
This determination is for the total accumulated heavy
metals rather than the annual application. The total
XVIII-4
-------
metals that can be applied are shown in Table XVIII-3.
Table XVIII-3 is valid provided the soil pH does not
fall below 6-5.
TABLE XVIII-3. MAXIMUM ALLOWABLE METALS APPLICATION ON
AGRICULTURAL LAND
Soil cation exchange capacity (meg/100 g)
Metal
Lead (Pb)
Zinc (Zn)
Copper (Cu)
Nickel (Ni)
Cadmium (Cd)
Maximum
0-5
500
250
125
50
5
applied metal
5-15
1,000
500
250
100
10
(Ib/acre)
>15
2,000
1,000
500
200
20
Using the information from Table XVIII-3, the maximum
total sludge application rate is the lowest of the following
five computations:
Pb: Tons sludge/acre = Ib Pb/acre
ppm Pb x .002
Zn: Tons sludge/acre = Ib Zn/acre
ppm Zn x .002
Cu: Tons sludge/acre = Ib Cu/acre
ppm Cu x .002
Ni: Tons sludge/acre = Ib Ni/acre
ppm Ni x .002
Cd: Tons sludge/acre = Ib Cd/acre
ppm Cd x .002
4. Phosphorus Balance
Tons of sludge/acre x Ib P/ton sludge - Ib P required/
acre = excess Ib P/acre (or if negative, the Ib/acre
P needed).
After five years the phosphorus level in the soil should
be determined and sludge application be reduced or
ceased if the phosphorus content in the soil, as deter-
mined by the Olsen Bicarbonate Test, exceeds 400 pounds
per acre.
5. Potassium required
Ib K required/acre - tons of sludge/acre x Ib K/ton
XVIII-5
-------
simplified
application
rate
determination
performance
sludge = Ib K/acre needed. There is no specific limit
on K and, generally, there will be a K deficiency unless
more K is added over that contained in sludge.
Table XVIII-4 has been prepared to simplify the sludge
application rate determination. This table shows application
rates for various crops based on the nitrogen requirements,
and assuming 70 Ib organic nitrogen/ton of sludge, with a
mineralization rate of 15-10-5, and an ammonium nitrogen con-
tent of 30 Ib/ton of sludge.
TABLE XVIII-4. RATE DETERMINATION (tons/acre)
Application yr
Crop*
Alfalfa
Orchard grass
Clover grass
Corn (grain)
(stover)
Sorghum (grain)
(stover)
Corn silage
Oats (grain)
( straw)
Soybeans (grain)
(straw)
Wheat (grain)
(straw)
Barley (grain)
(straw)
1
9.1
7.4
4.2
1.7
3.0
3.2
5.9
2.0
0.86
6.0
2.1
3.6
1.0
2.7
1.0
2 3
Tons/acre of
7.8
6.3
3.6
1.5
2.5
2.7
5.0
1.7
0.74
5.1
1.8
3.0
0.89
2.3
0.84
7.4
6.0
3.4
1.4
2.4
2.6
4.8
1.6
0.70
4.8
1.7
2.9
0.84
2.2
0.80
4
sludge
7.0
5.6
3.2
1.3
2.2
2.4
4.5
1.5
0.66
4.6
1.6
2.7
0.79
2.1
0.75
5
6.6
5.4
3.0
1.2
2.1
2.3
4.3
1.4
0.62
4.3
1.5
2.6
0.75
2.0
0.71
*Same yields as in Table XVIII-1.
This system should provide safe sludge disposal as well
as providing nutrients for crop growth. In general, sludge
application rates computed by nitrogen balance will result
in adequate crop phosphorus but inadequate potassium.
STAFFING REQUIREMENTS
Staff requirements are shown on Table XVIII-5 (see fol-
lowing page). These requirements were based on the system
having several disposal sites with sludge transported to
each site by truck. The sludge is then transferred to the
application truck. The labor does not include any farming
operations, but includes application of sludge to the land.
XVIII-6
-------
TABLE XVIII-5. STAFF REQUIREMENTS
Sludge quantity, tons/year
Description
Truck drivers , number
Lab technicians, number
Operator, number
250
2
1*
1*
1,250
2
1
1
2,500
4
2
1
5,000
8
2
1
*Half-time
MONITORING
The monitoring program consists of the analyses shown
in Table XVIII-6 (see following page). Sampling and monitor-
ing must be performed by qualified personnel or outside
laboratories.
Sensory Observations
Sensory observations can detect many problems before
environmental monitoring tests. When injecting sludge, the
application rate should be such that sludge does not surface.
If sludge surfaces, the injector speed should be increased
or the sludge flow decreased so that the quantity injected
per unit area decreases. If the injector travel speed is
excessive, soil may be thrown away from the shank creating
an open trench.
If sludge is spread on the surface, the rate should be
low enough to prevent excessive ponding or runoff. Excessive
ponding is when the liquid is still above the surface several
hours after application. Either excessive ponding or runoff
indicates excessive application rates for the soil. This
will vary widely from soil to soil.
NORMAL OPERATING PROCEDURES
Startup
The startup procedures include a daily check of trucks
for oil level, fuel level, battery condition, radiator water
level, lights, and turn signals. The injector(s) should be
checked for flushing and lubrication after the previous use.
Solids content of the sludge should be determined in
order to set the application rate. The total application
rate should be determined and provided to operating personnel
along with an application plan.
XVIII-7
-------
TABLE XVIII-6. MONITORING REQUIREMENTS(site specific*)
Sample Test
Sludge TDS
COD
TKN
NH3-N
NO3-N
P
Fecal coliform
Fecal strep
Salmonella
Cysts
PH
Cl
Alkalinity
Metals (Cd,Zn,Cu,Pb,Ni)
Na
K
B
Soil TKN
NH4-N
P
K
Alkalinity
Cation exchange capacity
Salmonella
Cysts
Chloride
B
pH
Frequency
Weekly
Weekly
Weekly
Weekly
Weekly
Weekly
Monthly
Monthly
Monthly
Monthly
Daily
Monthly
Weekly
Monthly
Monthly
Weekly
Monthly
Annually
Annually
Annually
Annually
Annually
Annually
Annually
Annually
Annually
Annually
Monthly
Wells, nearby
streams Coliform
Fecal coliform
NO^-N
^Monitoring schedules should be tailored
Monthly
Monthly
Monthly
to individual
installations.
If the sludge has a very high moisture content, the site
may have to be covered more than once with rest periods be-
tween applications to prevent ponding.
Routine Applications
Sludge is transferred to the site(s) and applied
according to the predetermined plan.
The operator should be alert for ponding or other signs
of problems. A record of the application should be prepared
as shown in Figure XVIII-3 (see following page). The operator
XVIII-8
-------
DATE
CROP Number of Previous Years of Application
Required Sludge Loading Rate tons/acre
Sludge Solids Concentration %.
Figure XVIII-3. Site map record
XVIII-9
-------
should show the areas he has covered each day as well as the
number of passes, and resulting application rate. These
records will enable the farmer to determine additional fer-
tilizer requirements, future application rates, and provide
plant personnel with a record of the application.
Shutdown
At the end of the day the truck and applicator should
be washed to remove any remaining solids and serviced. If
the sludge was surface applied rather than injected, the
farmer should be notified so that he can disc the area the
following day in order to mix soil and sludge.
CONTROL CONSIDERATIONS
Control of this process involves determination of appli-
cation rate by close monitoring of sludge and soil conditions
and determination of crop nutrient requirements. The opera-
rate tion may change substantially after each year of operation;
adjustment for example, application rates may be lower each year due \to
residual nitrogen. The rate may be reduced after several
years of application due to phosphorus or heavy metal buildup.
Added to this variability is crop rotation which the farmer
may practice.
Control steps required are proper rate setting, as
described previously, and daily control of actual quantities
applied. The actual application rate is varied by changing
the number of passes made by the truck over the site. The
field should be marked with numbered stakes to aid the
equipment operators in proper application.
EMERGENCY OPERATING PROCEDURES
Loss of Power and/or Fuel
Loss of electrical power will not affect the field or
transport operations. There will be an impact on the char-
acteristics of the sludge. The nature of this impact depends
on the type of processes involved. Most likely the solids
content will decrease. Under these circumstances the solids
concentration should be determined for each load of sludge.
Nitrogen content and forms will change so the organic, NH~
and N03 nitrogen should be checked for each load.
Adequate provisions must be made to pump sludge from
the holding tank to the transport truck at the plant.
If the trucks are equipped with diesel engines and the
fuel runs out, the entire fuel system must be bled to remove
air prior to starting the engine.
XVIII-10
-------
Loss of Other Treatment Units
Other treatment units which will directly impact the
land application operation are those required for stabili-
zation and concentration or dewatering. If the stabilization
process is not operating properly the sludge characteristics
in terms of nitrogen forms and concentrations will change.
If the concentration or dewatering process is not operating
properly the sludge moisture content will be high and a
greater volume must be handled. In either case, the sludge
application rate must be changed to account for the change
in the sludge characteristics.
XVIII-11
-------
TROUBLESHOOTING GUIDE
LAND APPLICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
Crop suddenly dies or
shows signs of poor
health.
la. Downward shift in
soil pH.
Ib. (1) Excessive nitro-
gen application.
(2) Insufficient
nitrogen.
Ic. Excessive heavy metal
concentration.
Id. (1) Excessive phos-
phorus applica-
tion causing
nutrient
imbalance.
(2) Insufficient
phosphorus.
le. Insufficient
potassium.
la.
Ib.
Ic.
Id.
Soil pH should be
maintained above
6.5, preferably 7.0.
Determine nitrogen
applied and consult
with agricultural
extension service.
Heavy metal content
of sludge and crop.
Determine phosphorus
application and con-
sult with agricultur-
al extension service.
la. Add lime to soil.
Ib. (1) Reduce loading rate.
(2) Increase loading rate.
Ic. Reduce loading rate or reduce
heavy metal content through
enforcement of pretreatment
requirements.
Id. (1) Reduce application rate.
le,
Determine potassium
application and con-
sult with agricultur-
al extension service.
(2) Add phosphorus.
le. Add potassium.
2. Surface runoff of
sludge.
2a. Excessive application
rate or poor soil
p erco lat ion.
2b. Ground saturated
with rainfall.
2a. Reduce application.
2b. Discontinue application until
soil has dried out.
-------
TROUBLESHOOTING GUIDE
LAND APPLICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3. Aerosols drifting out
of disposal area.
3a. Wind carrying
aerosols.
3a. Visual signs of
blowing or drifting
mist.
3a. Discontinue spraying during
windy periods.
3b. Convert spray nozzles to
larger openings.
3c. Reduce spray pressure.
3d. Increase buffer area.
Trucks getting stuck
in fields.
4a. Application of sludge
during wet periods.
4a. Alternative applica-
tion methods.
4a. Acquire a portable "rain gun"
which can spray sludge over
200-300 ft diameter circle.
4b. Use large tires on trucks.
4c. Use tractor.
Mosquitoes breeding
on site.
5a. Ponding of sludge.
5a. Stagnant ponds of
sludge.
5a. Grade site to eliminate pond-
ing, reduce application rate.
Leachate causing pol-
lution of ground or
surface waters.
6a. Excessive liquid
application.
6a. Application rates
and leachate quality.
6a. Intercept and treat leachate.
6b. Reduce liquid applied by improv-
ing sludge dewatering or re-
ducing application rates.
Nitrate pollution of
groundwater.
7a. Excessive nitrogen
applications.
7a. Nitrogen application
rates and nature of
cover crops.
7a. Reduce application.
7b. Replace crop with one of
higher nitrogen uptake.
Coverage of sludge in
subsurface plow in-
jection system not
adequate.
8a. Plow is being pulled
at excessive speed
and soil is thrown
away from shank.
8a. Plow speed.
8a. Pull plow at 1 mph or less.
-------
TROUBLESHOOTING GUIDE
LAND APPLICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
Drying of soil -
sludge mixture is
slow in subsurface
injection system.
9a. Sludge being in-
jected too deeply.
9a. Injection depth.
9a. Inject at 4 inches or less.
10. Plugging of subsur-
face injectors.
lOa. Large sludge solids.
lOa. Check sludge for
large particles.
lOa. Install sludge grinder.
H
H
H
-------
COMMON DESIGN SHORTCOMINGS
Shortcoming
1. Farm operations
can't take all of
the sludge.
Field too damp for
injection, but not
too wet to receive
sludge.
Inadequate land
acreage.
4. Inadequate trans-
port and/or injec-
tor truck capacity.
5. Inadequate storage
capacity.
MAINTENANCE CONSIDERATIONS
Solution
la. Store sludge in lagoon.
Ib. Move to alternate disposal
site.
2. Disconnect injectors and
spray sludge on surface
(monitor carefully).
3a. Buy land, discontinue crop
growth, and convert to
dedicated land disposal site
(higher application rates
with no crops).
3b. Change to crop with higher
nitrogen utilization rate.
3c. Dewater sludge and apply
to surface.
3d. Improve industrial pretreat-
ment enforcement if land
limitation is due to metals
concentrations.
4a. Purchase additional equipment
4b. Increase operating hours.
5a. Add storage lagoon.
5b. Improve dewatering or con-
centration performance.
Maintenance requirements are mainly cleaning and equip-
ment service. The cleaning operation include daily flushing
of the injectors and periodic flushing of the tanks. Truck
and equipment preventative maintenance schedules will be
specified in manufacturer's data.
XVIII-15
-------
SAFETY CONSIDERATIONS
Safety is related to vehicle and equipment operation.
Generally, the highest potential for accidents is when equip-
ment is being backed or trailers are being connected or dis-
connected from tractors. All drivers should be given a
thorough drivers' training course including classroom and
practice operation. All should be required to pass a driver's
test specially designed for this operation.
The only safety measures necessary beyond the usual
common sense is to require a spotter to assist drivers when
backing trailers at the plant and to insure that truck tires
are at adequate pressure and not excessively worn.
REFERENCE MATERIAL
References
1. Standard Methods for the Examination of Water and
Wastewater.
American Public Health Association
1015 Eighteenth Street, N.W.
Washington, D.C. 20036
2. Knezek, B.D. and Miller, R.H. (ed.) Application of
Sludges and Wastewaters on Agricultural Land: A
Planning and Educational Guide, North Central Regional
Research Publication 235, available through:
Cooperative Extension Service
The Ohio State University
1885 Neil Avenue
Columbus, Ohio 43210
Glossary of Terms and Sample Calculations
1. Sample Calculations to Determine Sludge Application
Rates (taken from Reference #2).
Sludge: 2% NH4-N, 0% NO3-N, 5% total N, 2% P, 0.2% K
Zn, 10,000 ppm; Cu, 1,000 ppm; Ni, 50 ppm;
Pb, 5,000 ppm; Cd, 10 ppm
Soil: Silt loam, CEC = 20 meq/100 g; fertilizer
recommendations from soil tests are 25 Ib
of P per acre and 100 Ib of K per acre
Previous applications: 10 tons of sludge per acre for
2 previous years
XVIII-16
-------
Crop requirements (from Table XVIII-1):
180 bu corn requires 240 Ib N, 44 Ib P, and 199 Ib K
A. Calculate annual rate based on N and Cd
(1) Available N in sludge
2% NH4-N + 0% N03-N = 2% initially available
nitrogen
5% total N - 2% =3% organic nitrogen
Lb available N/ton sludge = 20 x 2% + 4 x 3%
=40+12
= 52
52 Ib available N/ton sludge
(2) Residual N (from Table XVIII-2) for 3% organic N
(a) Sludge added 1 year earlier
(available between first and second year)
(10 tons/acre)x(5.1 Ib N/ton)= 51 Ib N
(b) Sludge added 2 years earlier
(available between second and third year)
(10 tons/acre)x(2.3 Ib N/ton)= 23 Ib N
(c) Residual N = 74 Ib
(3) Sludge Application Rate
(a) 240 Ib needed - 74 Ib residual = 166 Ib
from sludge
(b) 166 Ib N =3.2 tons/acre
52 Ib N/ton sludge
(c) Calculate application rate for 2 Ib Cd/acre
2 Ib Cd/acre = tons/acre = 100 tons/ac
10 ppm CD x .002
(4) The lower amount is applied =3.2 tons sludge/ac
B. Calculate total sludge amount which may be applied
(based on Table XVIII-3), maximum amounts are
calculated as follows:
XVIII-17
-------
Metal
(a) Pb
(b) Zn
(c) Cu
(d) Ni
(e) Cd
Maximum
amount,
Ib/acre
2,000
1,000
500
200
20
Cone, in
sludge ,
ppm
5,000
10,000
1,000
50
10
Application
rate , tons
sludge/acre
200 =
50_ =
250 =
2,000 =
1,000 =
Calculation
2,000 Ib Pb/acre
5,000 ppm Pb x .002
1,000 Ib Zn/acre
10,000 ppm Zn x .002
500 Ib Cu/acre
1,000 ppm Cu x .002
200 Ib Ni/acre
50 ppm Ni x .002
20 Ib Cd/acre
10 ppm Cd x .002
The lowest application rate is limited by Zn at 50
tons/acre.
C. Calculate P and K balances
(1) P
3.2 tons/acre x 2% P x 20 = 128 Ib P/acre avail.
Recommendation is 25 Ib P/acre.
No additional P needed
(2) K
3.2 tons/acre x 0.2% K x 20 = 12.8 Ib K/acre
available.
Recommendation is 100 Ib K/acre.
K needed = 87.2 Ib/acre
2. Application Rate - Field Measurement
The application rate determined for the particular
application equipment.
3. Nitrogen Mineralization Rate is the rate at which
organic nitrogen is converted to nitrate nitrogen.
This is also referred to as decay rate.
4. Residual Nitrogen is the mineralized nitrogen remaining
in the soil from previous sludge applications.
5. Pounds N/ton of sludge is determined by multiplying
the percent N by 20.
6. meg/100 q (milliequivalents per 100 grams) is the stan-
dard measure for soil cation exchange capacity.
XVIII-18
-------
7. Olsen Bicarbonate Test - This test is outlined in
Methods of Soil Analysis, Part 2, Published by
American Society of Agronomy, 677 South Segoe Road,
Madison, Wisconsin 53771, 1965, Page 1044.
XVIII-19
-------
XIX
LANDFILL
-------
CONTENTS
Process Description XIX-1
Typical Design Criteria and Performance XIX-4
Staffing Requirements XIX-4
Monitoring XIX-5
Sensory Observations XIX-6
Normal Operating Procedures XIX-6
Startup XIX-6
Routine Operations XIX-6
Shutdown XIX-6
Emergency Operating Procedures XIX-7
Common Design Shortcomings XIX-7
Maintenance Considerations XIX-7
Safety Considerations XIX-8
Reference Material XIX-8
References XIX-8
Glossary of Terms and Sample Calculations XIX-8
-------
PROCESS DESCRIPTION
process
operation
design
differences
leachate
natural
drainage
The landfill operation and maintenance program described
in this section applies to one method of landfilling of de-
watered sludge (15 to 20% solids). The example landfill system
described consists of trenches filled with alternate layers of
sludge and earth. Variations will be required for local con-
straints but the procedures described are applicable to any
site.
The dewatered sludge is transported to the site. The
sludge is stockpiled or dumped directly into a 20-foot deep
trench. A power shovel is used to place two-foot layers of
sludge with one~foot intermediate layers of fill material.
The final cover layer of fill is 3 to 5 feet. Figures XIX-1
and XIX-2 (see following pages) show a cross section and site
plan, respectively, for this example landfill.
System modifications are made to compensate for certain
climatic or soil characteristics. Equipment selection is
based on site specific constraints. Transport to the land-
fill site may be accomplished by truck, rail, or barge.
Pipelines may be used for liquid sludge transport with de-
watering operations at the site. Trench depths and widths
are variable. A wide, shallow trench may be excavated with a
bulldozer and filled with a scraper. There are a large num-
ber of options available, but the basic system is the same.
The major concern for landfill operation is control
of leachate so that groundwater supplies are not contamin-
ated. Groundwater supplies are protected by careful site
selection. A landfill must be located well above and/or
away from any aquifers. An impervious layer should be
located between the bottom of the fill and groundwater.
When filling trenches, leachate water will often appear.
This water should be pumped out to a tanker and returned
to the treatment plant for treatment and disposal.
Natural drainage should be left undisturbed as much as
possible. As shown on Figure XIX-2, fill trenches are
arranged so that they are at least 30 feet from the drainage
ditch. Farm tiles running through the site must be inter-
cepted and routed to the nearest drainage ditch.
XIX-1
-------
Cover with
fill material
28'-0"
-l*-^-
3'- 5fl
12'-0'
-Sludge
Fill
Next trench same
dimensions as
before
SCALE: 1" = 10'-0"
Figure XIX-1. Landfill trench section.
XIX-2
-------
MONITORING WELLS
30' —0 Minimum distance
Office and storage
building
SITE PLAN
DETAIL
Figure XIX-2. Landfill site plan
XIX-3
-------
TYPICAL DESIGN CRITERIA AND PERFORMANCE
Typical design criteria are shown on Table XIX-1.
These criteria should be adjusted for local soil conditions,
Sludge and fill layer depths should not be less than those
value s shown.
TABLE XIX-1. DESIGN CRITERIA
Trench width
Bottom 12 ft
Top 28 ft
Trench length 50 ft
Sludge layer 2 ft
Intermediate fill layer 1 ft
Top fill cover 3-5 ft
Distance between
trenches 15 ft
Distance between cells 15 ft
Distance from property
line 150 ft
Distance from
drainage ditch 30 ft
The proposed system can be operated such that no odors
or vector habitats are produced. There is no runoff, and
natural drainage is not interrupted. Leachate is controlled
by not trenching to an elevation less than 15 to 20 feet
above the impervious layer. Leachate quantity will be mini-
mized by pumping excess from the trench to the tanker truck.
STAFFING REQUIREMENTS
Staffing requirements are shown on Table XIX-2. These
requirements are shown for actual tonnage of sludge hauled
to landfill (based on 30 mile roundtrip).
TABLE XIX-2. LANDFILL LABOR REQUIREMENTS
Sludge hauled Labor, hr/yr
tons/day (wet) Operation Maintenance Total
5
25
50
100
2,800
5,200
10,400
20,800
1,200
2,080
4,160
8,320
4,000
7,280
14,560
29,120
XIX-4
-------
MONITORING The following example is site specific. Each disposal
site should have its own monitoring schedule . There are
eight monitoring wells located just inside the site
boundaries (see Figure XIX-2). Each well is 30
feet deep. The wells consist of 6-inch PVC pipes fitted
with a threaded cap on top and a well screen at the bottom.
These pipes are placed in 16-inch borings, which are packed
with 3/4 to 1-1/2-inch gravel. Sampling is accomplished
through the use of a portable pump and a 20-foot, 4-inch
PVC pipe with a foot valve at the bottom. These wells are
sampled prior to startup of the landfill.
The drainage ditch should be monitored during flow
periods. The ditch monitoring points are at the two points
where the ditch crosses the site. The difference between
the upstream and downstream locations will show if there is
an increase due to the landfill operation.
Depending on Topography domestic wells less than % mile
from the site should be monitored. Testing prior to land-
filling provides background constituent levels.
Table XIX-3 shows a list of constituents to be included
in the monitoring well and domestic well analysis.
TABLE XIX-3. WELL ANALYSIS
Boron TDS
Cadmium pH
Copper Chloride
Iron Phosphorus
Lead NO2-NO3
Mercury Total coliform
Zinc Fecal coliform
Fecal streptococcus
Table XIX-4 shows the tests to be done on the drainage
ditch flows.
TABLE XIX-4. DRAINAGE DITCH ANALYSIS
Fecal coliform NO3
Coliform NH3
Suspended solids Phosphorus
BOD pH
XIX-5
-------
The monitoring wells are sampled monthly. The drainage
ditch is sampled when rainfall occurs but no more than once
per week. The domestic wells are sampled quarterly.
Sensory Observations
If proper visual observations are made, many potential
problems can be avoided. Excessive odors and insect and
vector habitats will result if sludge is not properly covered
or leachate is allowed to stand in the trench. Visual ob-
servation of the sludge covering operation will show if
sludge is properly covered at the end of each day.
Control of runoff to and from the site or trench is
monitored primarily by visual means. Runoff due to rainfall
should not be allowed to enter the site from neighboring
property or conversely leave the site except through the
drainage ditch. On-site runoff from rainfall should be
prevented from entering a trench.
Completed landfill areas are seeded and should be
observed to insure that grass distribution is adequate and
that there are no exposed soil areas.
NORMAL OPERATING PROCEDURES
Startup
The trench site has been marked. The trench is excavated
with material stockpiled nearby for later fill. The trench is
protected from runoff by building a berm with some of the
excavated material on three sides of the trench. The crane
is then positioned for placing fill over sludge dumped into
the trench.
Routine Operations
Shutdown
The end-dump trailers are backed up to the edge of the
trench. After the sludge is dumped into the trench the power
shovel is used to spread the sludge evenly along the bottom
to a depth of two feet. The power shovel is used to sprinkle
fill material over the sludge. If fill is dumped or bull-
dozed into the trench, sludge may be displaced rather than
covered.
This operation is usually accomplished during day-
light hours only. Every night when the operation is shut-
down all sludge should be covered with fill material.
XIX-6
-------
After the sludge is covered, a berm is built on the open
side of the trench.
EMERGENCY OPERATING PROCEDURES
Emergency situations for the landfill operation are of
a different nature than mechanical sludge treatment processes.
Interruptions to normal operation are related to weather and
soil condition related problems. Mechanical problems are
related to vehicles rather than stationary equipment.
Although the site can be served by an all weather road,
traffic adjacent to a trench may be impossible during heavy
rainfall periods. If the rainfall period lasts longer than
inclement the period anticipated in design of the storage capacity
weather (at the plant), then a temporary gravel road can be placed
from the paved road to the trench being used. Covering the
sludge will be more difficult but it can be done. If the
crane is on tracks, it can be moved the relatively short
distances required.
One standby tractor-trailer unit may be provided so that
equipment mechanical failure of one will not hinder hauling except the
failure remaining units will be used for longer hours. Use of all
trucks provides more capacity than necessary so that routine
maintenance will not interfere with disposal operation.
The most critical piece of equipment is the crane used
to fill the trench. However, filling can be accomplished
with a dozer. Efficiency is somewhat reduced (more fill re-
quired due to displacement of sludge) but the sludge can be
covered.
COMMON DESIGN SHORTCOMINGS
The most common design shortcoming is failure to detect
an isolated area of permeable soil such as a sand lens. If,
in the process of excavating a trench, some permeable soil
is found, this area should be isolated by covering with at
least 5 feet of clayey soil.
Another design shortcoming can be found in setting the
slopes of the trench. If the slope is too steep, cave-ins
are possible. Conversely, excessively low slopes result in
inefficient trenching.
MAINTENANCE CONSIDERATIONS
Vehicle maintenance should include preventative main-
tenance in accordance with manufacturer's guidelines and
daily inspection. The manufacturer's guidelines come with
XIX-7
-------
the equipment so will not be repeated here.
Daily inspection should include the following:
Fuel level
Oil level
Battery
Tires (or tracks)
Hydraulic systems (where applicable)
Grease crane and crawler
Turn signals and brake lights on trucks
SAFETY CONSIDERATIONS
The safety considerations are listed below:
1. Maintain 5 feet from edge of trench and equipment
(except for dumping or dozing into trench).
2. Provide traffic direction, especially spotter for
backing trucks.
3. Provide fire extinguishers on all vehicles.
4. Provide equipment operation training sessions.
REFERENCE MATERIAL
References
Lukasik, G.D., and Cormack, J.W., "Development and
Operation of a Sanitary Landfill for Sludge Disposal",
paper presented at EPA 208 Seminar, Reston, Virginia,
March 16, 1977.
G. D. Lukasik
Northshore Sanitary District
301 West Washington Street
Waukegan, Illinois 60085
Standard Methods for the Examination of Water and
Wastewater.
American Public Health Association
1015 Eighteenth Street, N.W.
Washington, D.C. 20036
Glossary of Terms and Sample Calculations
1. Lechate is the liquid remaining in the sludge which
is usually high in BOD and suspended solids concentra-
tions. This material will contaminate water supplies
if not controlled.
XIX-8
-------
2. Impermeable Soil usually consists of a clayey or
hardpan soil or solid rock. Water will not pass
through an impermeable soil layer.
3. Determination of Trench Capacity. First the trench
volume is computed by multiplying the length by width
by depth of each layer and summing the layer volumes.
The trench capacity is determined by multiplying the
sludge unit weight (65 Ib/cu ft) by the trench volume.
Using Table XIX-1 design criteria and Figure XIX-1, a
sample calculation is shown below:
The bottom layer contains 3 feet of sludge.
The next layer contains 3 feet, the next - 2
feet, the next 3 feet, and the last layer has
3 feet.
Volume, cu ft =
(12 ft x 50 ft x 3 ft) + (16 x 50 x 3)
+ (20 x 50 x 2) + (24 x 50 x 3) + (28 x 50 x 3)
Volume = 14,000 cu ft
This volume is equivalent to the following sludge
weight:
Sludge Weight = 14,000 cu ft x 65 Ib/cu ft
Sludge Weight = 910,000 Ib or 455 tons
4. Fill Time Period. For scheduling purposes, the capacity
of each trench should be expressed as days of operation.
This is determined by simply dividing the sludge weight
capacity by the daily haul rate. Assuming 50 tons/day,
the following fill time results:
Fill time, days = 455 tons = 9.1 days
50 tons/day
This means that a new trench must be prepared every 9
days.
XIX-9
------- |