430978001
FIELD MANUAL FOR
PERFORMANCE EVALUATION AND TROUBLESHOOTING
AT MUNICIPAL WASTEWATER TREATMENT FACILITIES
by
Gordon L. Gulp
Nancy Folks Heim
Culp/Wesner/Culp
Clean Water Consultants
EPA Contract No. 68-01-4418
EPA Project Officer
Lehn Potter
January, 1978
Prepared for
Environmental Protection Agency
Office of Water Program Operations
Washington, D. C. 20460
Environmental Protection Agency
Re^nn V I: • '<-,r
£>3fK ; >,; > ^ , ":"' i- -n^ir-m-
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For sale 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 Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publi-
cation. Approval does not signify that the contents necessarily reflect
the views and, policies of the U.S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endorsement
or recommendation for use.
11
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CONTENTS
Figures v
Tables x
Acknowledgment xiii
1. Introduction 1
Purpose and scope of manual 1
Manual format 1
2. General Procedures for Evaluating and Troubleshooting ... 3
Collection of basic data 3
Facility inspection 4
Evaluation of performance 8
Definition of problems 9
Reporting 9
Useful references 10
3. Overall Systems Considerations 11
4. Unit Process Evaluation and Troubleshooting Information . . 14
Raw sewage pump stations 14
Screening 22
Shredding and grinding 29
Grit removal 34
Primary clarification 42
Activated sludge 55
Trickling filters 77
Activated biofilter (ABF) process 96
Lagoons 100
Rotating biological contactors 110
Secondary clarifier '. 118
Chlorination 129
Ozonation 142
Filtration 152
Microscreening 164
Activated carbon adsorption 171
Nitrification 182
Denitrification 194
Ammonia stripping 199
Chemical feeding and conditioning 207
Rapid mixing and flocculation 229
Recarbonation 235
Land treatment 242
Flow measurement 256
Sludge pumping . . . 264
Thermal treatment of sludges 270
111
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CONTENTS (continued)
Gravity thickening 281
Flotation thickening 287
Anaerobic digestion 295
Aerobic digestion 316
Centrifugation 325
Vacuum filtration 335
Pressure filtration 344
Sludge drying beds 350
Sludge drying lagoons 356
Incineration-Multiple hearth 358
Incineration-Fluidized bed 369
Lime recalcining 377
Carbon regeneration 385
Application of sludges to land 392
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FIGURES
Number Page
1 Classification of wastewater treatment plants by
unit operations 5
2 Centrifugal pump 15
3 Screw pump 16
4 Air lift ejector 17
5 Bar screen 23
6 Mechanically cleaned screen 24
7 Fine screen mounted on a drum 25
8 Mechanically cleaned bar screen 26
9 Typical rotating comminutor 30
10 Typical bar screen comminutor unit 31
11 Chain and flight grit collector section 35
12 Typical cross section of an aerated grit chamber 36
13 Cyclone degritter 37
14 Rectangular basin, chain sludge collector 43
15 Rectangular basin, traveling bridge collector 43
16 Circular basin 44
17 Estimated removals of suspended solids and BOD in primary
basins at various hydraulic loadings 45
18 Conventional activated sludge 56
19 Step aeration 56
20 Complete mix activated sludge 56
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FIGURES (continued)
Number Page
21 Contact stabilization 56
22 Oxidation ditch 58
23 Schematic diagram of multistage oxygen aeration system. . . 58
24 Typical air diffuser system 59
25 Aeration basin with diffused aeration 60
26 Floating surface, mechanical aerator 61
27 Platform mounted surface aerator 62
28 Oxidation ditch aeration system 63
29 Typical trickling filter in cross section 78
30 Fixed nozzle distribution system 79
31 Typical one and two-stage trickling filter systems 81
32 Redwood lath media 82
33 Plastic media 83
34 Trickling filter effluent quality - two Texas plants. ... 86
35 Effluent quality trickling filters 87
36 Activated biofilter process schematic 97
37 Treatment processes in lagoons 101
38 Typical RBC process schematic Ill
39 Rotating biological media for secondary treatment 112
40 Effect of BOD concentration and hydraulic load on
nitrification in the RBC process 113
41 Effluent quality - Gladstone, Michigan 114
42 Secondary sedimentation tank 119
43 Typical clarifier configurations 120
vi
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FIGURES (continued)
Number
44 Chlorine contact chamber with end - around baffles
and vanes 130
45 MPN coliform vs. chlorine residual 131
46 Alternative ozonation systems 143
47 Typical ozone contact basin using porous diffusers 144
48 Ozone generator types 146
49 Typical gravity filter 153
50 Typical pressure filter 153
51 Schematic of typical microscreen 165
52 Gravity contactor 172
53 Pressurized contactor capable of using upflow or downflow
operation 173
54 Required aeration time for varying SRT values 183
55 Effect of organic load of nitrification efficiency of
rock media trickling filters 184
56 Design criteria for single stage nitrification with
rotating biological discs 187
57 Temperature effects on single stage nitrification with
rotating biological discs 187
58 Design relationships for a 4-stage RED process treating
secondary effluent 188
59 Process for removal of excess methanol 196
60 Typical ammonia stripping tower 200
61 Typical screw-type volumetric feeder 209
62 Positive displacement pumps 210
63 Typical positive-negative pneumatic conveying system. . . . 212
vii
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FIGURES (continued)
Number Page
64 Illustrative lime feed systems for wastewater
coagulation 213
65 Typical lime feeder and lime slaker 214
66 Typical dry feed system 216
67 Alternative liquid feed systems for overhead storage. . . . 217
68 Alternative liquid feed systems for ground storage 217
69 Typical caustic soda feed system 220
70 Lime dosage as related to wastewater alkalinity 223
71 Feedforward control of methanol based on flow and nitrate
nitrogen 223
72 Mechanical rapid-mixing device 230
. 73 Typical paddle-type flocculator 230
74 Schematic of rapid mix, flocculation and sedimentation
system using separate basins 231
75 Typical recarbonation system using stack gas 237
76 Recarbonation basins at Orange County Water District
Facility 239
77 Methods of land application 243
78 Irrigation techniques 244
79 Propeller meter 257
80 Magnetic flow meter 257
81 Venturi tube meter 257
82 Positive displacement diaphragm meter 259
83 Typical pipe and weir installation 259
84 Parshall flume 259
vnx
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FIGURES (continued)
Number Page
85 Kennison or parabolic nozzle 260
86 Rotameter 260
87 Heat treatment system schematic 271
88 Gravity thickener 282
89 Dissolved air flotation unit 288
90 Dissolved air flotation system 288
91 Low rate digester 296
92 Two-stage anaerobic digestion 297
93 Fixed or floating covers digestion 298
94 Schematic of aerobic digestion system 317
95 Effect of SRT on reduction of biodegradable solids by
aerobic digestion 321
96 Continuous countercurrent solid bowl conveyor discharge
centrifuge 326
97 Cutaway view of a rotary drum vacuum filter 336
98 Side view of a filter press 345
99 Cross section of typical sand drying bed 351
100 Cross section of a typical multiple hearth incinerator. . . 359
101 Auxiliary heat required to sustain combustion of
sludge * 363
102 Cross section of a fluid bed reactor 370
103 Fluidized bed system with air preheater 371
104 The lime recalcining system at South Lake Tahoe 378
105 Typical carbon regeneration system schematic 386
IX
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TABLES
Number Page
1 Aerated Grit Removal 38
2 Typical Activated Sludge Design Parameters 65
3 Trickling Filter Design and Performance Parameters 84
4 Typical Activated Biofilter Design Criteria 98
5 Design Parameters for Stabilization Ponds 103
6 Typical Design Parameters for Secondary Clarifiers 121
7 Chlorine Dosage Ranges 133
8 Effect of Pretreatment of Cl :NH -N Breakpoint Ratio . . . 134
9 Ozone Generator Design Criteria 148
10 Design Criteria for Orange County Water District Open
Gravity, Mixed Media Filter System 155
11 Expected Filter Performance for Activated Sludge Plants . . 157
12 Expected Filter Performance for Trickling Filter Plants . . 157
13 Design Specifications of Some IPC Plants 175
14 Design Specifications of Some AWT Plants 176
15 The Effects of Pretreatment of Carbon Dosage and Carbon
Column Effluent Quality 177
16 Typical Water Quality Before and After Granular Activated
Carbon Treatment at South Tahoe 177
17 General Design Parameters for Nitrification of Domestic
Wastewater with ABF Process 186
18 Design Data for Orange County Water District Ammonia
Stripping Tower 201
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TABLES (continued)
Number Page
19 Design Data for Tahoe Ammonia Stripping Tower ....... 202
20 Chemicals Commonly Used in Wastewater Treatment ...... 208
21 Types of Chemical Feeders ................. 221
22 Design Criteria for Orange County Water District
Recarbonation Process .................. 240
23 Typical Design Criteria for Irrigation, Infiltration-
Percolation, and Overland Flow Systems for Municipal
Wastewater ........................ 246
24 Accuracy of Various Flow Measuring Devices ......... 262
25 Types of Sludge Pumps ................... 266
26 Typical Design Criteria and Performance Data for
Gravity Thickening .................... 283
27 Operating Data for Plant Scale DAF Units .......... 289
28 Air Flotation Thickening Performance Data ......... 291
29 Typical Design Criteria for Low Rate and High Rate
Digesters ........................ 299
30 Supernatant Characteristics From Anaerobic Digesters. . . . 303
31 Aerobic Digestion Design Parameters ............ 318
32 Batch-Type Aerobic Sludge Digestion Operating Data for
Mixtures of Primary and Waste Activated Sludge ......
33 Typical Sludge Quantities ................. 327
34 Typical Solid Bowl Centrifuge Performance ......... 329
35 Typical Design and Performance Data for Vacuum Filtration
Systems ......................... 337
36 Filtering Area of Drum Filters in Square Feet ....... 338
37 Typical Results - Pressure Filtration ........... 346
38 Standard Sizes of Multiple-Hearth Furnace Units ...... 360
XI
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TABLES (continued)
Number Page
39 Multiple Hearth Furnace Operation 361
40 Fluidized Bed Reactor Performance Data 373
41 Typical Temperature Profile in Six Hearth Furnace 380
42 Temperature Profile in Eleven Hearth Furnace 380
XI1
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ACKNOWLEDGEMENTS
This manual was prepared for the office of Water Program Operations
of the United States Environmental Protection Agency. Development and
preparation of the manual was carried out by the firm of Gulp/Wesner/Culp-
Clean Water Consultants, El Dorado Hills, California by Gordon L. Gulp
and Nancy Folks Heim. The understanding and encouragement of Lehn J. Potter,
Project Officer, Water Program Operations of the EPA is greatly appreciated.
Although many references were used for the preparation of this manual,
the principal references include the following:
1. EPA Training Course 179.2, "Troubleshooting Municipal Waste-
water Treatment Plants," Course Manual.
2. "Operation of Wastewater Treatment Plants," Manual of Practice
No. 11, Water Pollution Control Federation, 1976.
3. "Process Control Manual for Aerobic Biological Wastewater
Treatment Facilities," EPA Municipal Operations Branch, EPA-
430/9-77-0066, March 1977.
4. "Operations Manual, Anaerobic Sludge Digestion," EPA-430/9-
76-001, February, 1976.
5. "Orange County Water District Operation & Maintenance
Manual for Water Factory 21," Culp/Wesner/Culp - Clean Water
Consultants, 1974.
6. "Operation & Maintenance Manual for Metropolitan Denver
Sewage Disposal District", Culp/Wesner/Culp - Clean Water
Consultants, 1977.
7. "Manual of Wastewater Operations," Texas Water Utilities
Association, 1976.
8. Manufacturer's O&M data on specific equipment.
Xlll
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SECTION 1
INTRODUCTION
PURPOSE AND SCOPE OF MANUAL
The purpose of this manual is to provide a technical field guide or
reference document for use in improving the performance of municipal waste-
water treatment plants. The main purpose of the manual is to provide a
troubleshooting guide for:
identifying problems
analyzing problems
solving problems
Another purpose of the manual is its use in EPA and State training
courses for plant inspectors and performance evaluators. Consulting
engineers, designers, plant operators, educators, and students may also
find the manual useful.
The manual describes general procedures for evaluating the perfor-
mance of treatment processes and equipment commonly used in municipal
wastewater facilities. The procedures also cover other items related to
the effective operation of municipal wastewater treatment plants.
Troubleshooting and performance evaluation material is provided for
each unit process commonly used in wastewater treatment facilities.
MANUAL FORMAT
It is assumed that the manual user has a general understanding of
both typical wastewater treatment plant design and operation. The style,
language, and format of the manual are directed to the level and technical
knowledge of a technician with some experience in plant operation, design,
inspection or performance evaluation. It is recommended that performance
evaluators complete a training program which includes the evaluation of
several wastewater treatment plants using this manual and other evaluation
methods.
The manual is organized into four major sections:
Section 1 INTRODUCTION. The purpose, scope and format of the
manual are described in this section.
Section 2 GENERAL PROCEDURES FOR EVALUATION AND TROUBLESHOOTING.
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This section contains a step-by-step procedure for organ-
izing information before a plant is visited and for per-
forming the on-site evaluation.
Section 3 OVERALL SYSTEM CONSIDERATIONS. This section discusses
different unit processes and how their operations can
affect other processes. It also presents information on
safety, staffing, monitoring, emergency procedures, and
maintenance considerations which are common to the unit
processes in Section 4.
Section 4 UNIT PROCESS EVALUATION AND TROUBLESHOOTING INFORMATION.
For each unit process, this section contains the follow-
ing information:
Description of the Process
Typical Design Criteria & Performance Evaluation
Control Considerations
Design Shortcomings and Ways to Compensate
Troubleshooting Guide
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SECTION 2
GENERAL PROCEDURES FOR EVALUATING AND TROUBLESHOOTING
Procedures for evaluating and troubleshooting at municipal wastewater
treatment facilities include a detailed study of the following:
Plant performance
Operating personnel
Laboratory facilities
Testing and sampling program
Costs and budgets for O & M
Operational problems
Information and data for each element may be gathered and analyzed in
five basic steps:
1. Collection of basic data before site visit
2. Facility inspection
3. Total plant evaluation
4. Identification of problems
5. Analysis and reporting
The detail of the evaluation will be greater if:
Data collected at the plant are questionable
Discharge quality does not meet standards
O & M costs are very different than would be expected
There are important O & M problems
COLLECTION OF BASIC DATA
Preparation for the on-site inspection should include the collection
and review of information describing the plants layout and design, operat-
ing personnel, any available plant performance records/ and previous in-
spection reports. This data collection will help the evaluator to under-
stand the treatment plant he will inspect. Preparation for the site visit
will also help the evaluator see possible problem areas which may need more
detailed attention during the facility inspection. The evaluator should
gather the following information:
General Information
Physical location of plant - (with attention to possible effects
of climate on plant performance. Extremes of both temperature
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and rainfall should be noted.)
Population and area served by the plant (domestic, industrial,
etc.)
Prior evaluation or inspection reports
Information Related To The Plant And Staff
Type of treatment system (identify plant by type of treatment
it provides; primary, secondary, or advanced waste treatment, as
shown in Figure 1).
Size of system (design, average daily flows, peak flows).
Types of unit operations and basic plant components.
Plant documents (including schematic and design drawings, plant
plans and specifications, O s. M manuals, and operating reports) .
Historical operating data (including complaint record).
Plant discharge requirements (this might include allowable SS,
pH, BOD5, etc., or any special controlling conditions such as
minimum DO and chlorine residuals).
Characteristics of the plant's influents and effluents - this
should include physical and chemical measurements taken at the
plant (BOD5, pH, COD, temperature, etc.), and flow quantity and
variations with time. If the official data are not readily
available from the plant, then local and/or state health depart-
ments and various pollution control agencies may be able to
supply the needed information. These data should then be com-
pared to discharge requirements.
Size of staff, their qualifications, education and experience,
as well as the amount of time spent on each unit operation.
This information should be compared to the number of people at
other treatment plants as discussed in Section 3 of this manual.
Above average skills and motivation may allow good operation at
lower levels of manpower than shown in the references in Section
3. References 2 and 3 listed at the end of this section provide
more detailed guidance for evaluating staffing requirements for
each unit process.
FACILITY INSPECTION
Visual Observations
Problem identification begins with an on-site tour of the facility.
The plant engineer or operator should provide a complete tour of the
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PRIMARY TREATMENT
Removal or reduction of
suspended and floating
solids, but little or no
colloidal and dissolved
matter
I Screens
Clarification
[Sludge Disposal]
SECONDARY TREATMENT
Removal or reduction of fine
suspended colloidal material,
dissolved solids and organic
matter by biological oxidation
Biological
Filters
DISINFECTION
1
1
| Chlorine)Ozone]
ADVANCED WASTE TREATMENT
Removal or reduction in nutrients,
residual organics, residual solids,
and pathogenic organisms
Activated
Sludge
I
[clarification]
1
Physical/chemical
Treatment
Membrane
Processes
| Land Application]
{Sludge Disposal]
* Not all operations are used in all plants
Figure 1. Classification of wastewater treatment plants by unit operations*
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facilities. Watch for and ask about:
Excessive solids passing over overflow weirs
Excessive grease and scum buildup
Any unusual equipment such as special pumps, chemical feeders,
temporary construction on systems which are being used to cor-
rect problems (or possibly cause them)
Diurnal flow variations
Evidence of flow in by-pass channels because of problems in
normal operating units
Flow splitting between units
Excessive odors
Abnormal color of wastewater in various process stages
Treatment units out of service - why and how long
Sludge handling problems
Evidence of severe corrosion problems
Use of proper safety precautions
If any special changes in the plant were made, determine their:
Purpose
Physical makeup
Effect on the other treatment processes by comparing old
operational data with data after modification
Page 5 of EPA Form 7500-5 may serve as a useful guide in the visual
inspection of the plant.
Available Manpower
During the on-site inspection, the inspector should ask how much time
is usually spent on the 0 & M of each unit process and a list of plant per-
sonnel should be completed. The inventory should include:
Numbers and qualifications of all plant personnel
Their certification
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Special training and skills also should be noted and compared to
the personnel requirements usually needed for effective unit
process operation. (EPA publications on estimating staffing may
be use ful.)
Training programs available to plant personnel also should be examined
during this part of the inspection. Training programs may be:
Short courses conducted by universities
Vocational schools, and high schools
Advanced training in laboratory skills, treatment methods,
electrical skills, or mechanical skills
Special training programs as part of local, state, and national
certification programs
Correspondence courses
Analytical Techniques
The following elements should be studied during the laboratory
evaluation:
Tests used in various unit operations (including type,
frequency and location of test)
Testing procedures and analyses used
Equipment used (including type, quantity and general condition
of equipment)
Laboratory personnel
Review of test records
This information should be compared with recommended sampling and
testing for each unit process. Flow measurement records and techniques
should be reviewed for:
Agreement with the design flow and population served
Over and under-loading of each treatment unit
Meter calibration
Budgets for O & M
Costs and budgets for operation and maintenance of the plant should
be reviewed and evaluated. The following data should be identified, with
unusually high or low costs being noted:
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Preventive maintenance
Corrective maintenance
Major repairs
Alteration
Unit costs ($/hr for labor, cents/kwh, $/Btu, etc.)
The plant records should show a breakdown of costs for materials used
and purchased, as well as labor hours spent for the maintenance item.
EVALUATION OF PERFORMANCE
Total evaluation of plant performance should include the following:
Treatment Efficiency
Actual plant performance should be compared with the expected design
performance and typical performance as described in Section 4 of this manual,
Actual plant discharges also should be evaluated and compared to applicable
discharge requirements. EPA Form 7500-5 will be useful in this evaluation.
Operating and Maintenance Procedures
The plant's maintenance program can best be evaluated by comparing
maintenance records at the plant with manufacturer's and 0 & M manual
maintenance schedules for components. The plant maintenance system should
be examined for the following:
Preventive maintenance records
The preventive maintenance schedule for each piece of equipment
Specifications on each major piece of equipment, the supplier,
and where spare parts can be purchased
Spare parts list, and
Instructions for operation and maintenance of each item of major
equipment
Operating procedures checked should include:
Sampling and analysis data logs (process control testing data
sheets and trend charts)
Daily operating logs
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Staffing Levels
As part of the preparation for the on-site inspection, plant staffing
data should have been gathered. As shown on EPA Form 7500-5, actual per-
sonnel (and their classification) should be compared with the number of
persons budgeted and recommended for various types of treatment plant
operations.
Costs
An analysis of costs and budgets should be included as part of the
total plant performance evaluation.
DEFINITION OF PROBLEMS
In general, the problems detailed in Section 4 of the manual are those
most often found for each unit process. However, the following procedures
can be used for any kind of problem evaluation. The first step in trouble-
shooting is to determine if the plant is meeting design performance stand-
ards. This is done by comparing the plant effluent quality and overall
efficiency with that listed on the design and/or discharge permit. If the
plant does not always operate correctly, the problem usually falls under
one of the following causes:
Overloads (organic or hydraulic)
Poor O & M procedures
Poor staffing
Poor laboratory control
Mechanical/electrical failure
Lack of money for 0 & M
Outdated equipment
Once the problem area has been defined, the cause of the problem should
be identified. For example, overload problems may be caused by infiltration,
combined sewers, industrial growth, rapid population growth, increased ser-
vice area, or some other cause.
The troubleshooting guide in Section 4 of this manual will help to
identify and solve common problems with each unit process. For problems
not covered in the manual, and if the evaluator is not able to solve the
problem, a consultant should be hired.
REPORTING
To make sure the evaluation is complete and to help report the plant
inspection, EPA Form 7500-5 may be used. Also, a final report should
be prepared and should contain the following:
Summary of on-site visit
A list of problems found
Solutions recommended
Proposed action and other recommendations
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USEFUL REFERENCES
The following reports, or other similar references may provide the
plant evaluator or troubleshooter with more complete and detailed infor-
mation.
1. "NPDES Compliance Evaluation Inspection Manual", EPA (Office
of Water Enforcement, Compliance Branch) July,1976. De-
tailed information on EPA procedures and policies regarding
plant inspections.
2. "Estimating Costs and Manpower Requirements for Conventional
Wastewater Treatment Facilities", EPA Project 17090 DAN,
October, 1971.
3. "Estimating Staffing for Municipal Wastewater Facilities",
EPA Contract 68-01-0328, March, 1973.
4. "Considerations for Preparation of Operation and Maintenance
Manuals", EPA 430/9-74-001, 1974.
5. "Maintenance Management Systems for Municipal Wastewater
Facilities", EPA 430/9-74-004, October, 1973.
6. "Estimating Laboratory Needs for Municipal Wastewater Treatment
Facilities", EPA 430/9-74-002, June, 1973.
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SECTION 3
OVERALL SYSTEMS CONSIDERATIONS
The next Section of this manual presents process evaluation and
troubleshooting information for each unit process. However, in evalu-
ating plant performance, it must be understood that:
Operating each unit process at its best does not always result
in the best overall plant performance.
The performance of one unit process is often related to other
processes, and
Failure or poor performance of one unit process can have bad
effects on either upstream or downstream processes.
* Because the number of potential combinations of unit processes in plant
design is very large, it is not practical to discuss all potential plant
designs. However, each unit process section contains a discussion of how
a specific unit process performance can affect the overall plant performance.
Although the information on optimum unit process performance presented
later is useful in evaluating a process, it is important to remember that
each unit process must be operated to meet the needs of the other processes
used in the plant. For example, if a sludge will burn without the need for
added fuel at 30% solids, there is no need to operate a filter press feed-
ing an incinerator to provide a 40% solids concentration - even though the
filter press might easily reach such a concentration. Added costs would
be associated for sludge conditioning and longer filter runs for the drier
cake. It is possible to operate a sludge thickener so well that it will
produce a sludge so thick that it will not readily pass through a down-
stream centrifuge. This is another example of how the operation of one
process must be based on the other processes used in a plant.
The relationship between unit processes must be carefully considered
in plant evaluation and troubleshooting. For example, a sudden increase
in final effluent suspended solids may result from the failure of a chemi-
cal feed pump in the sludge conditioning system. Because of improper
conditioning, a large amount of solids may be recycled to the treatment
plant causing poor effluent quality. The source of the problem would be
unrelated to the operation of the biological system or to the final clari-
fier. It requires understanding how one process affects another to trace
the problem to a small pump in a part of the plant remote from where the
operator first notices the problem. Jamming of mechanical surface aerators
11
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may be unrelated to the maintenance or operation of the aerators; for
example, the cause could be rags and debris passing through the upstream
screening process.
Failure or poor performance of a unit process may result in upstream
or downstream problems - or both. The supernatant from a poorly operated
anaerobic digester, for example, could cause odor problems when recycled
to the upstream wastewater processes. The sludge withdrawn from the diges-
ter could also cause problems in the downstream sludge handling processes.
The above examples show the need to consider the relationships be-
tween processes in tracing problems^and in plant evaluation. The location
and amounts of flows recycled to the main wastewater treatment process is
often the source of important information. The next section includes in-
formation on how each unit process can affect the overall system. It is
important to consider the overall system when reviewing individual unit
processes.
There are several aspects of the unit processes discussed in Section
4 which are covered in other available references or which are common to
several unit processes. These aspects include staffing requirements, moni-
toring programs, emergency operating procedures, maintenance system con-
siderations, and safety considerations. The following paragraphs discuss
each of these aspects.
Staffing Requirements
An inadequate or improperly qualified staff can be a source of major
operational problems. Staffing requirements for the operation and mainten-
ance of various unit processes are presented in detail in the following two
references:
"Estimating Costs and Manpower Requirements for Conventional
Wastewater Treatment Facilities", EPA Project 17090 DAN, October,
1971.
"Estimating Staffing for Municipal Wastewater Facilities", EPA
Contract 68-01-0328, March, 1973.
Because of the many differences in personnel from plant to plant,
the manpower estimates in these references should be used only as general
guides. Should the troubleshooter see equipment out of operation or per-
forming poorly due to lack of maintenance or poor housekeeping, he should
investigate the level and quality of staffing. A high rate of personnel
turnover may be caused by poor pay scales and poor management. Poor
training programs will also contribute to operational problems.
Monitoring Programs
The troubleshooter should refer to the EPA publication "Estimating
Laboratory Needs for Municipal Wastewater Treatment Facilities", (EPA
12
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430/9-74-002, June, 1973) for information on monitoring programs for various
unit processes as well as information on appropriate laboratory space,
equipment and manpower.
Emergency Operating Procedures
The discussions of control considerations presented for each unit
process in Section 4 tell how failure of one unit process can upset the
control of upstream and downstream processes. Detailed mechanical/elec-
trical problems depend on the equipment used at the plant and the trouble-
shooter should review the plant operations and maintenance manual to de-
termine if it has detailed emergency instructions. Suggestions on plant
operation during an emergency may be found in "Design Criteria for
Mechanical, Electric, and Fluid System and Component Reliability", (EPA
430/9-74-001). Guidance on emergency planning may also be found in
"Emergency Planning for Municipal Wastewater Treatment Facilities", (EPA
430/9-74-013).
Maintenance System Considerations
A sound maintenance management system can be a major 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 bud-
gets and in planning the needed supply of replacement parts. Where poor
maintenance programs are a source of problems, the troubleshooter should
refer to the following references for detailed guidance:
"Maintenance Management System for Municipal Wastewater Facilities",
EPA 430/9-74-004, October, 1973.
"Considerations for Preparation of Operation and Maintenance Manuals",
EPA 430/9-74-001, 1974.
"Operation of Wastewater Treatment Plants", Manual of Practice No.
11, Water Pollution Control Federation (1976) (Chapter 30).
Safety Considerations
Although safety problems do not usually cause poor process performance,
the troubleshooter should look for hazardous conditions for both his pro-
tection and the protection of the operating staff. The following referen-
ces will be useful:
"Safety in Wastewater Works", Manual of Practice No. 1, Water Pollu-
tion Control Federation (1969).
"Operation of Wastewater Treatment Plants", Manual of Practice No. 11,
Water Pollution Control Federation (1976) (Chapter 31).
13
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SECTION 4
UNIT PROCESS EVALUATION AND TROUBLESHOOTING INFORMATION
RAW SEWAGE PUMP STATIONS
Process Description
Many wastewater treatment plants need pump stations to provide flow of
the wastewater through the treatment plant.
Where practical, grit removal and screening facilities are located
ahead of the raw wastewater pumps in order to protect the pump. Since
screening equipment, either of the bar screen or comminutor type, may be
easily built in pumping station influent channels, such equipment is
normally included in the station design. As a result, most raw sewage
pumping facilities include the pumping station structure, pumping unit,
screening facilities, controls, and discharge piping.
The most commonly used systems for raw sewage pumping include centri-
fugal pumps, screw lift pumps, and air lift ejectors. Centrifugal pumps
can deliver a wide range of flows depending on their design, speed and
total dynamic head. Most centrifugal pumps used for raw sewage pumping
have smooth channels and impellers with large openings to allow a free-
flowing condition and reduce the chances of clogging. Figure 2 shows how
the centrifugal pump works.
Because of its simple open design, the screw lift pump is most often
used for pumping unscreened raw wastewater. As shown in Figure 3, the
general design is a semi-circular channel that is open at the top, with a
revolving screw inside the channel. The screw normally rotates at speeds
ranging from 20 to 110 rpm, and can produce 30 to 50 feet of head. Depend-
ing on the design and size of the screw, it is possible to reach pumping
capacities up to 80,000 gpm.
Air lift ejectors are very useful for pumping raw, unscreened waste-
water, since there are no moving parts to clog. As shown in Figure 4, the
ejector has a sealed chamber with a cone-shaped floor, inlet and discharge
lines with check valves, a compressed air inlet, an automatic air shut-off
valve, and a vent valve for releasing the pressure in the chamber. As the
raw wastewater fills the chamber and touches the electrode level detector,
air is forced into the container and the wastewater is ejected through the
discharge line.
Typical Design Criteria
Because of the many kinds of pump designs and capacity, the manufac-
turer's performance curves should be used to provide information on dis-
charge, power requirements, and head characteristics for a specific pump.
14
-------
CENTRIFUGAL PUMP
Figure 2. Centrifugal pump.
15
-------
•DRIVE
•UPPER BEARING
SCREW
CONCRETE TROUGH
ANGLE'
•LOWER BEARING
Figure 3. Screw pump.
-------
VALVE
R LINE->v
k
ECK
r^>
>
j
LEVEL
DETECTOR
DISCHARGE
CHECK
VALVE
RECEIVING
CONTAINER
Figure 4. Air lift ejector.
17
-------
To evaluate the performance of the pump station, the following should
be checked:
1. Determine the physical conditions affecting pump operation.
Height of influent above pump (suction head) = 10 ft
Height of pump effluent above pump (discharge head) = 20 ft
Friction loss (f) = 1 ft
Pump efficiency = 90%
Motor efficiency = 80%
Plant flow (mgd) = 5 mgd
gal/min = mgd = 3472 gal/min
24 x 60
2. Determine the head characteristics that the pump is operating
under.
Static head (ft) = Difference in elevation between discharge
and suction
20-10 ft
10 ft
Total head, H (ft) = Static head + friction loss
10 + 1 ft
11 ft
3. Determine the horsepower characteristics of the pump and motor.
Required pump HP = (flow in gal/min) (total H in ft)
3960
(3472 (11)
3960
9.64 HP
Required motor HP = (pump HP)
(pump efficiency) (motor efficiency)
9.64
(.9) (.8)
13.4 HP
4. Compare the actual computed head and power characteristics of the
pump with those provided by the pump manufacturer. Check to see
that the pump is not being forced to operate under conditions be-
yond its design.
5. The Shortcomings and Troubleshooting guide also may be useful
in providing a visual inspection of proper pump operation.
Control Considerations
In many cases, raw sewage pump stations have screens or racks to pro-
tect pumps from abrasive material and objects that can plug the suction
lines.
18
-------
Pumping stations have an important effect on the overall performance
of the treatment plant, since the flow through the plant depends on the
pump capacity. Intermittent pumping during periods of low flow and fre-
quent changes in pumping rates can cause process upsets. As a result, it
is desirable for the pump capacity and storage capacity to be designed so
that frequent stops and starts of the lead pump are avoided, and changes
in pumping rates are minimal.
Flow rate in piping generally should range from 4 to 6 ft/sec.
Velocities in excess of 10 ft/sec result in high lead loss, and velocities
less than 2 ft/sec allow solids to settle out in the pipes.
Because pump stations are important in overall plant operation, spare
parts should be kept on-site so that the pumps can be quickly repaired.
Common Design Shortcomings and Ways to Compensate
Shortcoming
1. Difficulty in handling high
peak flows.
2. No preliminary screening or
shredding provided prior to
pumping may result in clogging.
3. In wet-pit installations -
explosive and corrosive gases
are close to electrical
motors and equipment.
4. Float control systems for
measuring levels in wet
wells may become greasy.
Solution
1. Provide standby pumps with
automatic control.
2. Install screen in wet well
prior to pumping.
3. Construct a separate compart-
ment, (dry well) for this
equipment.
4. Install air bubbler control
system.
19
-------
TROUBLESHOOTING GUIDE
RAW SEWAGE PUMP STATIONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Black and odorous
septic wastewater.
la. Improper operation
of lift station.
Ib. Flat grades in col-
lection system.
la. Inspect lift station.
Ib. Velocity in collect-
ing lines.
la. Repair lift station.
Ib. Flushing program to maintain
correct velocities.
2. Intermittent flow or
surging.
2a. Improper wet well
sensor adjustment.
2b. Hydraulic capacity
of station is
exceeded.
2c. Illegal connections
to the system.
2a. Check sensor adjust-
ment.
2b. Check designed
capacity.
2c. Check sanitary sewer
system to determine
the source.
2a. Adjust level sensors.
2b. Install surge tank.
2c. Remove and prevent illegal
connections.
3. Intermittent flow or
surging during heavy
rainfall.
3a. Flooded streets and
water entering
through manholes.
3b. Broken lines.
3a. Check seals on
manholes.
3b. Inspect for broken
lines.
3a. Seal manholes and repair
cracks in manhole structures.
3b. Repair broken lines.
4. Pump not running.
4a. Defective control
circuit.
4b. Defective motor.
4a. Use a meter to check
switching circuits.
4b. Motor operation.
4a. Replace defective part.
4b. Replace motor.
Pump not running,
circuit breaker will
not reset.
5a. Clogged pump or
closed valve.
5a. Inspect pump for
obstruction.
5a. Remove obstruction.
6. Pump is running, but
reduced discharge.
6a. Pump air-bound.
6b. Clogged impeller.
6c. Wearing rings.
6a. Air bleed pipe.
6b. Inspect for
obstructions.
6c. Check clearance.
6a. Remove obstruction.
6b. Remove obstruction.
6c. Replace worn rings.
-------
TROUBLESHOOTING GUIDE
RAW SEWAGE PUMP STATIONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. Clogged pump or pump
suction line.
7a. Grease accumulations,
7a. Check grease accumu-
lation on walls of
wet well.
7a. Frequent cleaning of wet well
or removal of grease by dewater-
ing the well, and scraping the
bottom.
8. Rising power consump-
tion per gallon.
8a. Clogged pump.
8b. Misaligned belt
drives.
8a. Total daily pumpage
and maximum and
minimum flow rates.
8b. Alignment.
8a. Remove obstruction in pump.
8b.
Realign belt drive.
9. Improper liquid levels
9a. Coating on liquid
high probes.
9b. Hang-ups in float
level detectors.
9c. Leaks in bladders.
9d. Fouling in bubbler
controls.
9a. Probe.
9b. Float detector.
9c. Bladder.
9d. Bubbler.
9a. Clean probe.
9b. Remove obstruction, release
float.
9c. Repair or replace bladder.
9d. Clean bubbler.
0. Excessive wear or
damage to pumps.
.Oa. Sand accumulations
in wet well.
Ob. Grease accumulations
in the wet well.
lOa. Inspect for eroding
action, corrosion,
and solids build up.
lOb. Inspect wet well
walls.
lOa. Remove sand from wet well.
lOb. Clean wet well. (see 7a.
solution)
-------
SCREENING
Process Description
As wastewater enters a plant for primary treatment, it flows through
a screen which removes the large floating objects such as rags, sticks,
and other items that may clog pumps and small pipes. Screens may be coarse
or fine. Coarse screens are typically made of parallel steel or iron bars
with spacings ranging from 2 to 4 inches. Fine screens, on the other hand,
consist of perforated plates, woven wire cloth or closely spaced bars. When
fine bar screens are utilized, the spacing may range from 0.75 to 2.0 inches.
As shown in Figure 5, screens are often placed in a chamber or channel in a
slanted position in order to make the cleaning process easier. The debris
is caught on the upstream surface of the screen and can be raked out manu-
ally or mechanically. Most wastewater treatment plants utilize mechanical
cleaning rather than manual cleaning to reduce labor costs and provide
better flow conditions (see Figure 6).
The three most commonly used fine mesh screen systems are the travel-
ing water screen, the static screen and the rotating drum. The traveling
screen has wire mesh panels attached to a conveyor belt system with steps
or shelves located at the lower edge of each panel, where heavier material
is allowed to collect. The panels of the screen usually are set in the
vertical position. The channels usually range in width from 2 to 10 ft.
As the panels are raised to the top of the unit, a high pressure spray
washes the debris from the screen and drops it into a collecting bin where
it can be easily removed.
Like bar screens, static screens are slanted at an angle, but with
openings ranging from 0.09 to 0.25 inches.
In the rotating drum system (Figure 7), a large drum is covered with
a fine mesh screen and partially submerged in the wastewater stream. The
screening material usually is a layer of wire cloth; however, heavy backup
screens sometimes are used to provide extra strength. As the drum rotates,
the wastewater filters through the mesh and allows material to accumulate
on the surface. The accumulated solids are then continuously removed from
the unsubmerged portion of the drum using a high pressure spray of water.
Typical Design Criteria and Performance Evaluation
Figure 8 shows typical design data for various classifications of
screens. These data may be used by the plant evaluator to compare actual
plant screening performance to typical design performance. Since screens
are primarily used to protect downstream equipment from damage, the
22
-------
FLOW
FLOW
I
SECTIONAL ELEVATION
PLAN
Figure 5. Bar screen.
23
-------
Figure 6. Mechanically cleaned screen.
24
-------
Screenings
discharge
trough
Spray pipes
Influent
Drive
Screen—covered drum
Figure 7. Fine screen mounted on a drum.
25
-------
O
z
UJ
HI
UJ
(9
UJ
13
13
12
11
10 -
8 -
7 -
6 -
5 '
4 '
2-
1 •
'/J
21A
2V»
OPENINGS BETWEEN BARS, inches
Figure 8. Mechanically cleaned bar screen. (Cubic feet of screenings
per million gallons sewage).
26
-------
evaluator should check equipment repair records to see if poor screen opera-
tion has resulted in damage to other plant equipment. The evaluator should
also check the shortcomings and troubleshooting guide for visual inspection
of screen performance.
Control Considerations
Screens are used to collect material which will block flow. There-
fore, if a screen is not cleaned often enough, water will back up in the
channel leading to the screen, resulting in surges of high flow after the
screen has been cleaned. These conditions can cause problems in clarifier
and aeration tanks which may follow the screening process.
The velocity of flow ahead of and through the screen affects its
operation. Generally, flows of 2 to 4 ft/sec are acceptable.
The efficient design and operation of screening facilities are essen-
tial to overall plant performance, especially where no shredding or grind-
ing units are provided. Poorly screened debris will damage equipment and
will interfere with good operation of other processes.
Common Design Shortcomings and Ways to Compensate
Shortcomings Solution
1. Debris suddenly accumulates
on screen and clock-operated
timers set for automatic
timed operation of screen
rakes do not remove debris
fast enough.
2. Storage and separate dispo-
sal of screened debris is
causing problems.
3. Front cleaned bar screens
become jammed at the
bottom.
Pressure-sensitive bubbler systems
may be installed in parallel with
the timer in order to activate the
raking mechanism when unusual quan-
tities of debris accumulate.
2. Install a comminutor.
3. Provide back cleaning device.
27
-------
TROUBLESHOOTING GUIDE
SCREENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Obnoxious odors, flies
and other insects.
la. Accumulation of rags
and debris.
la. Method and frequency
of debris removal.
la. Increase frequency of
removal and disposal.
2. Excessive grit in bar
screen chamber.
2a. Surging in chamber
due to increased
water level.
2b. Flow velocity too
low.
2a. Depth of grit in
chamber, irregular
chamber bottom.
2b. Flow velocity.
2a. Remove bottom irregularity,
or reslope the bottom.
2b. Increase flow velocity in
chamber or flush regularly
with a hose.
NJ
oo
3. Excessive screen
clogging.
3a. Unusual amount of
debris in
wastewater.
3a. Screen size and
velocity of waste-
water through
screen.
3a. Use a coarser screen, or
identify source of waste
causing the problem so its
discharge into the system can
be stopped.
4. Mechanical rake inop-
erable, circuit
breaker will not reset
4a. Jammed mechanism.
4a. Screen channel.
4a. Remove obstruction.
5. Rake inoperative, but
motor runs.
5a. Broken chain or
cable.
5b. Broken limit switch.
5a. Inspect chain.
5b. Inspect switch.
5a. Replace chain or cable.
5b. Replace limit switch.
6. Rake inoperative, no
visible problem.
6a. Defective remote
control circuit.
6b. Defective motor.
6a. Check switching
circuits.
6b. Check motor
operation.
6a. Replace circuit.
6b. Replace motor.
-------
SHREDDING AND GRINDING
Process Description
Shredders and grinders are used to reduce the size of objects in a
wastewater stream before treatment. Some plants use a comminutor which
combines the functions of a screen and grinder at the same time. In this
system, debris is shredded and then returned to the wastewater stream as
shown in Figure 9.
There are two general types of shredders and grinders. In one de-
sign, the whole wastewater stream flows through the unit and all material
is automatically shredded. In the second type, called a barminutor,
shredders and grinders are used with coarse bar screens (Figure 10).
Comminutors of this type are generally one of two of the following designs:
(1) the shredder or grinder may be attached to the top of the mechanically
cleaned bar screen where large objects are automatically dropped into the
hopper of a shredder, and (2) revolving cutters may be employed which
slide up and down the face of the screen, shredding objects which accumu-
late.
Typical Design Criteria and Performance Evaluation
Comminutors generally have capacities ranging from 0.35 to 25 mgd.
They are typically capable of handling 650 to 5,200 Ibs of waste per hr.
A simple evaluation of shredders and grinders should include:
1. Observe flow and cutting action of shredder or grinder. Plug-
ging may occur if rags and debris are not properly cut up.
2. Check maintenance records for regular sharpening and adjustment
of teeth.
3. Read the shortcomings and troubleshooting guide for any prob-
lems that may appear.
Control Considerations
If screenings are not properly ground or shredded, serious problems
may occur in downstream treatment processes. For example, grit chambers
could become clogged or subject to odors as a result of submerged rags
and debris. In addition, pumps and suction lines could become clogged,
and pump impellers damaged by poorly shredded material. Mechanical sur-
face aerators also may become clogged with rags and debris.
29
-------
-MOTOR
HEAD CASTING
SHAFT
DRUM CASTING
SLOT
INFLUEN
EFFLUENT
VALVED DRAIN FOR
DEWATERING COMMINUTOR
CHANNEL
Figure 9. Typical rotating comminutor.
30
-------
HOSING ASSEMBLY.
MACHINE FRAME
AIR TUBING
GEAR HOUSING
ELECTRIC CABLE
ELECTRIC MOTOR
BAR SCREEN
COMMUNITING UNIT
TRAVEL UNIT MAY ( /
BE RAISED TO HIGH- / !
EST POSITION AND / /
SWUNG OUT TO / /
FLOOR LEVEL FOR / /
SERVICING-i
\l
(--COUNTERWEIGHT
AIR TUBING
•UPSTREAM SENSING PIPE
DOWNSTREAM
SENSING PIPE
FRONT ELEVATION
SIDE ELEVATION
Figure 10. Typical bar screen coiraninutor unit.
-------
Failure of shredders and grinders can be minimized by proper, regular
maintenance. Regular resharpening and readjustment of cutting teeth and
cutting edges is one maintenance item that can reduce the chances of prob-
lems occuring in downstream treatment processes.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. No provision made for by- 1. Install by-pass channel with
passing malfunctioning manually cleaned screen.
comminutor - sewage
overflows channel.
32
-------
TROUBLESHOOTING GUIDE
SHREDDING AND GRINDING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. System inoperable,
circuit breaker will
not reset.
la. Jammed mechanism.
la. Check for obstruc-
tions.
la. Remove obstruction.
2. System inoperable,
but motor runs.
2a. Broken coupling.
2a. Check coupling.
2a. Replace coupling.
3. Receiving chute
clogged.
3a. Insufficient wash
3b. Broken swing hammer
blades.
3a. Wash water feed rate.
3b. Inspect blades.
3a. Open valve to provide more water
3b. Remove broken pieces and replace
blades.
co
co
4. Output coarser than
usual.
4a. Dull blades.
4b. Broken teeth.
4a. Check blades.
4b. Check teeth.
4a. Sharpen or replace blades.
4b. Replace broken teeth.
-------
GRIT REMOVAL
Process Description
After wastewater has been screened, it usually passes into a grit
chamber where sand, grit, cinders, small stones and other material settle
out. Grit removal is an important process for a number of reasons: (1)
to prevent clogging in pipes and problems in pumping sludge, (2) to prevent
cementing effects on the bottom of sludge digesters and primary sedimenta-
tion tanks, (3) to protect moving mechanical equipment and pumps from un-
necessary wear and abrasion, (4) to reduce accumulations of materials in
aeration tanks and sludge digesters which would result in loss of usable
volume.
Grit removal equipment can be velocity controlled, aerated, or of the
cyclone degritter type. Velocity controlled grit removal systems have
chambers which can be either manually or mechanically cleaned. As shown in
Figure 11, these systems usually are square or rectangular with a flow con-
trol device and a chain-and-flight mechanism to move the grit into a sump
for later disposal. The grit is then removed from the sump using a pump or
bucket elevator.
Aerated grit removal systems (Figure 12) inject air into a chamber to
produce a spiral-type flow in the wastewater. The air flow is adjusted to
a low enough velocity to allow the grit to settle out.
Cyclone degritters use centrifugal force in a cone-shaped unit to
separate grit from the rest of the wastewater. In this system, the waste-
water is fed into the degritter at the edge of the upper end. This feed
creates a vortex that forces the heavier grit particles to the outside of
the rotating flow stream. The grit stream then falls into a grit washer
and the degritted flow leaves the cyclone degritter through an opening
near the top of the unit where it is channeled for further treatment.
Figure 13 shows a cyclone degritter and the location of the feed inlet,
degritted effluent, and grit removal drain. Since there are no moving parts
in the cyclone, the pump which feeds the wastewater into the system creates
the vortex flow. As a result, characteristics of the slurry, particularly
its viscosity, determine how well the process works. The size of the upper
and lower orifices may require changing after the system has been installed.
Removed grit or gravel is usually washed clean to remove organics
prior to its disposal in a landfill.
34
-------
-CHAIN &
DRIVE UNIT
EFFLUENT
Figure 11. Chain and flight grit collector section.
35
-------
Figure 12. Typical cross section of an aerated grit chamber.
36
-------
DEGRITTED OVERFLOW
TANGENTIAL
FEED INLET
GRIT REMOVAL
Figure 13. Cyclone degritter.
37
-------
Typical Design Criteria and Performance Evaluation
Typical loading and performance data for air injected grit removal
systems are presented in Table 1. The plant evaluator can use this infor-
mation as a general guide for evaluating the performance of the actual
grit removal system he is evaluating. The shortcomings and troubleshoot-
ing guide also should be read if any problem is noted during the plant
evaluation.
Control Considerations
Inadequate grit removal causes wear on pipes and other downstream
equipment. In determining how to best operate the grit removal unit, the
fact that the grit removal equipment wears out faster at an increased grit
removal efficiency, must be weighed against the improved downstream equip-
ment life.
When operating velocity-controlled degritters, the scrapers should be
operated at a low speed while the bucket elevator should function at a rate
fast enough to remove the collected grit. Improper speed control will re-
sult in damage, due to grit packing in the basin.
In aerated grit removal systems, the operator must control the air
flow into the grit chamber. The air rate must be adjusted to create a
velocity near the floor of the chamber, low enough to allow the grit to
settle, but high enough to prevent or:j \r material from being removed
with the grit.
TABLE 1. AERATED GRIT REMOVAL
Parameter Typical operating ranges
Transverse velocity at surface 2-2.5 ft/sec
Depth to widtn ratio 1.5/1 - 2/1
Air supply 3 - 5 ft /min/ft of length
0.04 - 0.06
Detention time 3-5 min
Quantity of grit 1 - 10 ft /MG
Quantity of scum (skimmings) 1 - 6 ft / MG
38
-------
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1.
2.
10.
Aerated grit chamber may be
subject to short-circuiting.
Increased removal of grit
may result in increased re-
moval of organic materials
which cause objectionable
odors.
Corrosion of metal work
and concrete.
Grit storage hoppers
equipped with slide gates
may become clogged.
Excessive wear on the
bucket elevator of
mechanically cleaned
chambers may occur.
Excessive wear on shoes
and rails of mechanically
cleaned chambers may
occur.
Bottom scour may reduce
grit chamber efficiency.
Jamming and clogging
of equipment.
Excessive deposition of
grit in inlet distribu-
tion flumes.
Unpleasant duties of
handling grit.
1.
2.
4.
5.
10.
Install submerged baffles next
to diffusers or along the wall
opposite the diffusers.
Install and properly operate
grit washer equipment.
Install blowers and gas scrubbers
for oxidizing gases and reducing
corrosive atmosphere.
Provide hopper with flushing
water.
Wear can be reduced by one or
more of the following modifications:
a) Use cast nylon or other
strong and light-weight buckets.
b) Install a jet to spray water to
wash the emerging chain of grit
which may be lodged in links.
Attach polyurethane floats to
flights in order to reduce their
weight and which would otherwise
increase wear.
In non-mechanical chambers, re-
movable partitions or floor grat-
ings between storage section and
flow-through section may be instal-
led.
Install screens and/or grinders
ahead of grit chamber.
Aerate inlet flume.
Install mechanically-cleaned grit
chambers, and automatic removal of
grit into storage hoppers to reduce
handling.
39
-------
TROUBLESHOOTING GUIDE
GRIT REMOVAL
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Grit packed on
collectors.
la. Collector operating
at excessive speeds.
Ib. Bucket elevator or
removal equipment
operating at slow
speeds.
la. Collector speed.
Ib. Removal system
speed.
la. Reduce collector speed.
Ib. Increase speed of grit re-
moval from collector.
2. Too much vibration
of cyclone degritter.
2a. Obstruction in the
lower port.
2b. Obstruction in the
upper port.
2a. Lower port.
2b Too much flow in
lower end.
2a. Remove obstruction.
2b. Reduce flow.
3. Rotten egg odor in
grit chamber.
3a. Hydrogen sulfide
formation.
3b. Submerged debris.
3a. Sample for total and
dissolved sulfides.
3b
Inspect chamber for
debris
3a. Wash chamber and dose with
hypochlorite.
3b. Wash chamber daily.
4. Corrosion of metal
and concrete.
4a. Inadequate
ventilation.
4a. Ventilation
4a. Increase ventilation
(also see Shortcomings).
5. Removed grit is grey
in color, smells and
feels greasy.
5a. Improper pressure on
cyclone degritter.
5b. Inadequate air flow
rate.
5c. Grit removal system
velocity too low.
5a. Discharge pressure
on cyclone degritter.
5b. Check air flow rate.
5c. Use dye releases to
check velocity.
5a. Keep pressure between 4 and 6
psi by governing pump speed.
5b. Increase air flow rate.
5c. Increase velocity in grit
chamber.
6. Surface turbulance in
aerated grit chamber
is reduced.
6a. Diffusers covered by
rags or grit.
6a. Diffusers.
6a. Clean diffusers.
-------
TROUBLESHOOTING GUIDE
GRIT REMOVAL
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. Low recovery rate of
grit.
7a. Bottom scour.
7b. Too much aeration.
7c. Not enough retention
time.
7a. Velocity.
7b. Aeration.
7c. Retention period.
7a. Maintain velocity near
1 ft/sec.
7b. Reduce aeration.
7c. Increase retention time.
8. Overflowing grit
chamber.
8a. Pump surge problem.
8a. Pumps.
8a. Adjust pump controls.
9. Septic waste with
grease and gas
bubbles rising in grit
chamber.
9a. Sludge on bottom of
chamber.
9a. Grit chamber bottom.
9a. Wash chamber daily.
*>.
-------
PRIMARY CLARIFICATION
Process Description
With screening and grit removal finished, the wastewater still con-
tains large amounts of settleable and floatable solids. Primary clarifiers
are tanks used to remove or reduce suspended solids and organic loading
from the wastewater before it goes to secondary treatment units.
The principal objectives of primary clarification are:
1. Removal of settleable solids.
2. Removal of floatable solids.
Clarifiers may be rectangular, square, or circular in shape. In rec-
tangular tanks, the wastewater flows from one end to the other and the
settled sludge is moved to a hopper at one end, either by flights set
on parallel chains (Figure 14), or by a single bottom scraper set on a
traveling bridge (Figure 15). Floating materials such as grease and oil,
are collected by a surface skimmer and then removed from the tank.
In circular tanks (Figure 16), the wastewater usually enters in the
middle and flows toward the outside edge. Settled sludge is pushed to a
hopper that is in the middle of the tank bottom, and floating material
is removed by a surface skimmer connected to the sludge collector.
A number of factors affect the performance of sedimentation tanks,
including the following:
1. Rate of flow through the clarifier tank expressed in terms of
gallons per day per square foot of surface area of the tank.
2. Wastewater characteristic (wastewater strength, freshness, and
temperature; types and amount of industrial waste; and the
density, shapes, and sizes of particles).
3. Pretreatment operations (carryover of grit and screenings).
4. Nature and amount of any in-plant wastes recycled to the plant
ahead of the primary clarifier.
Typical Design Criteria and Performance Evaluation
Figure 17 shows how much BOD and suspended solids can be removed from
42
-------
Adjustable
weir
Sludge pipe-
Sludge hopper-
Figure 14. Rectangular basin, chain sludge collector.
•Influent mixing chamber
Scum trough
• Surface skimmer blade
-Traveling bridge and traction drive
/ Effluent weir plate
• Sludge well
-Sludge scraper blade
Sludge drawoff pipe'
Figure 15. Rectangular basin, traveling bridge collector.
43
-------
Effluent Pipe
Effluent.
Drive Contro with Load Indicator
Scum Pipe
Blades & Adjustable Squeegees
Sludge Drawoff Pipe
Figure 16. Circular basin.
44
-------
100
Overload
Range of basin performance
SS removal
ange of basin performance
BQD removal
0.2 0.5
1.0
1.5
2.0
2.5
3.0
4.0
Hydraulic Loading Factor
Actual Plant Loading
Design Loading
Figure 17. Estimated removals of suspended solids and BOD in primary
basins at various hydraulic loadings.
45
-------
wastewater in a primary clarifier using the ratio of actual to design
flow. The figure shows a design overflow rate of 800 gpd/sq ft. Be-
cause of the fixed size of the tank, the overflow flow rate and detention
time will change with flow, resulting in variable removal efficiencies.
Design overflow rates usually expected for primary clarifiers are in the
range of 600-1,000 gpd/sq ft. The clarifier depth (usually 10-15 ft)
should be set to provide a detention time of 90 to 150 minutes.
Procedures for evaluating the performance of a primary clarifier are
illustrated in the following example:
1. Determine clarifier configuration, dimensions and design
criteria.
Circular
Diameter, dia. = 100 ft
Depth, D = 12 ft
Clarifier area, A = (ir/4) dia = 7,850 sq ft
Clarifier volume,
V=A x D x 7.48 = 704,616 gal
Design surface loading rate = 800 gpd/sq ft
2. Determine total clarifier flow.
Total = Influent Flow + Recycle Flow
Daily Average: 6 mgd + 0.5 mgd = 6.5 mgd
Peak Hour: 8.5 mgd + 0.5 mgd = 9.0 mgd
3. Determine actual hydraulic surface loading rate (both daily
average and peak hour) for clarifier.
Hydraulic Surface Loading Rate = flow in gal/day
surface area in sq ft
Daily Average Loading Rate = 6,500,OOP _
7,850
Peak Hour Loading Rate = 9,000,000 , , ,,. -, /
77850 = 1,146 gpd/sq ft
4. Determine actual detention time for clarifier.
Detention Time = (volume in gal) x 24 hr/day x 60 min/hr
flow in gal/day
= 704,616 x 24 x 60
6,500,000
= 156 minutes
5. Compare actual clarifier operational data with typical design
criteria.
46
-------
Parameters Actual Typical Design
Average daily hydraulic
loading, gpd/sq ft 828 600 - 1,000
Peak hour hydraulic loading,
gpd/sq ft 1,146 1,200 - 2,500
Detention Time, min 156 90 - 150
6. Determine the ratio of actual plant loading to design loading.
Use this ratio to predict removal efficiencies expected under
average and peak conditions.
Loading factor ratio = actual loading
design loading
Average conditions = 828 _
800 ~
Peak conditions = 1,146 _
~ -
Referring to Figure 17, a loading factor of 1.0 (average condi-
tion) should result in 50 to 65% SS removal and 25 to 35% BOD
removal. Under peak loading conditions, a factor of 1.4 should
provide 40 to 50% SS removal, and 20 to 30% BOD removal.
7. If the clarifier does not provide acceptable treatment, the
shortcomings and troubleshooting sections of this manual
should be read.
It is important for the clarifier to be designed properly and have
the ability to handle the actual average and peak flows. In some cases,
especially in combined collection systems, the peak flow conditions should
govern the design. It should be noted, however, that clarifier efficiency
at peak flows is dependent on both magnitude and duration. Therefore, it
is important to examine past flow data and the characteristics of the
collection system in order to assure that the design is not based on a
short duration peak which may have little effect on clarifier efficiency.
Control Considerations
As with most unit processes, primary clarification is related to
other plant processes, both upstream and downstream. Upstream, some of
the factors that will affect settling tank operation include recycling
of waste sludge and supernatant, and carryover of grit and screenings
from pretreatment. The frequency and duration of sludge pumping determines
the solids concentration in the sludge which in turn has a major effect
on downstream sludge thickening and dewatering processes. Pumping of
thin sludge may lower plant digester capacity; cause hydraulic overloads
to sludge thickening processes; and use too much fuel for sludge heating
47
-------
purposes. If sludge becomes septic, downstream sludge handling processes
may be affected and odor problems made worse. Laboratory "spin" tests
(small centrifuge) are often used by operators to determine when it is
time to pump out the sludge. Primary sludge concentrations of 5-7% are
often found with proper operation of the sludge pumping system.
A properly operated primary clarifier will do much to provide smooth
and efficient operation of downstream unit processes. For example, impro-
per control of primary tank operations may cause solids and BOD overloading
problems for secondary processes. Solids not removed in primary treatment
result in greater amounts of secondary sludge being produced, and grease
carryover can upset aeration tank and trickling filter operations. This
often results in a poor quality effluent.
For good operation, clarifier flows must be distributed evenly among
all available tanks. Uneven flows to the various tanks results in a poor
overall reduction of SS and BOD.
Common Design Shortcomings and Ways to Compensate
Shortcoming
1. Poor flotation of grease. 1.
2. Scum overflow.
3.
4.
5.
7.
8.
Sludge hard to remove from
hopper because of exces-
sive grit.
Short circuiting of flow
through tank causing poor
solids removal.
Heavy wear and frequent
breakage of scrapers and
shear pins due to grit.
Grease particles adhered to
wooden flights.
Inadequate removal of heavy
grease loading.
Low grease removal due to
inadequate particle
agglomeration.
2.
Solution
Pre-aeration of wastewater to
increase grease buoyancy.
Move scum collection system away
from outlet weir.
Install grit chamber or eliminate
sources of grit entering the
system.
Modify hydraulic design and install
appropriate bciffles to disperse
flow and reduce inlet velocities.
Install grit chamber.
Lower return wooden flights to
below the water surface or install
water sprays to remove grease
into scum troughs.
Install flotation or evacuator
equipment.
Provide chlorine contact tanks
with grease removal equipment to
enhance agglomeration.
48
-------
9.
Shortcoming
Septic conditions resulting
from overloading.
Solution
9. Divert or provide alternate dis-
posal for other plant process
wastes (i.e. centrates and super-
nates) which are normally
recirculated into the sedimentation
tank.
10. Excessive corrosion due to
septic wastewater.
10. Coat all surfaces with proper
paint and/or install sacrificial
anode.
11. Consistent problems with
thermal currents in
clarif ier.
11. Install flow equalization and
mixing basin ahead of clarifier.
12. Poor scum removal due to
wind.
12. Install a wind barrier to protect
tank from wind effects.
49
-------
TROUBLESHOOTING GUIDE
PRIMARY CLARIFICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
L. Floating sludge.
la. Sludge decomposing in
tank.
Ib. Scrapers worn or
damaged.
Ic. Return of well nitri-
fied waste activated
sludge.
Id. Sludge withdrawal line
plugged.
le. Damaged or missing in-
let baffles.
Ib. Inspect scraper.
Ic. Effluent nitrates.
Id. Sludge pump output.
le. Baffles.
la. Remove sludge more frequently
or at a higher rate.
Ib. Repair or replace as necessary.
Ic. Vary age of returned sludge, or
move point of waste sludge
recycle.
Id. Clear line by reversing flow.
le. Repair or replace baffles.
2. Black and odorous
septic wastewater.
2a. Sludge collectors worn
or damaged.
2b. Improper sludge remov-
al pumping cycles.
2c. Inadequate pretreat-
ment of organic indus-
trial wastes.
2d. Sewage decomposing in
collection system.
2e. Recycle of excessively
strong digester super-
natant.
2f. Sludge withdrawal line
plugged.
2a. Inspect sludge collec-
tors.
2b. Sludge density.
2c. Pretreatment practices
2d. Retention time, and
velocity in collection
lines.
2e. Digester supernatant
quality and quantity.
2f. Sludge pump output.
2a. Repair or replace as necessary.
2b. Increase frequency and duration
of pumping cycles until sludge
density decreases to undesirably
low value.
2c. Pre-aerate waste.
2d. Chlorinate in collection system.
2e. Provide treatment before recycl-
ing, or reduce rate of return.
2f. Clean line by reversing flow.
-------
TROUBLESHOOTING GUIDE
PRIMARY CLARIFICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3. Erratic operation of
sludge collection
mechanism.
3a. Broken shear pins,
damaged collector.
3b. Rags and debris entang
led around collector
mechanism.
3c. Excessive sludge
accumulation.
3a. Shear pins and sludge
collector.
3b. Sludge collector.
3c. Sound bottom of tank.
3a. Repair or replace damaged parts.
3b. Remove debris.
3c. Increase frequency of pumping
sludge from tank.
4. Scum overflow.
4a. Frequency of removal
inadequate.
4b. Heavy industrial waste
contributions.
4c. Worn or damaged scum
wiper blades.
4d. Improper alignment of
skimmer.
4a. Scum removal rate.
4b. Influent waste.
4c. Wiper blades.
4d. Alignment.
4a. Remove scum more frequently.
4b. Limit industrial waste contri-
butions .
4c. Clean or replace wiper blades.
4d. Adjust alignment.
5. Broken scraper chains
and frequent shear pin
failure.
5a. Improper shear pin
sizing and flight
alignment.
5b. Ice formation on walls
and surfaces.
5c. Excessive loading on
mechanical sludge
scraper.
5a. Pin sizing and flight
alignment.
5b. Inspect walls and
surfaces.
5c. Sludge loading.
5a. Realign flights and change shear
pin size.
5b. Remove or break up ice formation
5c. Operate collector for longer
period and/or remove sludge more
often.
-------
TROUBLESHOOTING GUIDE
PRIMARY CLARIFICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
&. Sludge hard to remove
from hopper.
6a. Excessive grit, clay
and other easily com-
pacted material.
6b. Low velocity in with-
drawal lines.
6c. Pipe or pump clogged.
6a. Operation of grit
removal system.
6b. Sludge removal velocity
6a. Improve operation of grit removal
unit.
6b. Increase velocity in sludge
withdrawal lines.
6c. Backflush clogged pipe lines and
pump sludge more frequently.
7. Undesirably low solids
content in sludge.
01
to
7a. Hydraulic overload.
7b. Short circuiting of
flow through tanks.
7c. Over-pumping of sludge
7a. Influent flow rate.
7b. Dye or other flow
tracers.
7c. Frequency and duration
of sludge pumping; SS
concentration.
7a. Provide more even flow distribu-
tion in all tanks, if multiple
tanks.
7b. (See 8a and 8b).
7c. Reduce frequency and duration of
pumping cycles.
8. Short circuiting of
flow through tanks.
8a. Uneven weir settings.
8b. Damaged or missing
inlet line baffles.
8a. Weir settings.
8b. Damaged baffles.
8a. Change weir settings.
8b. Repair or replace baffles.
9. Surging flow.
9a. Poor influent pump
programming.
9a. Pump cycling.
9a. Modify pumping cycle.
10. Excessive sedimentationjlOa. Velocity too low.
in inlet channel.
lOa. Velocity.
lOa. Increase velocity or agitate
with air or water to prevent
decomposition.
-------
TROUBLESHOOTING GUIDE
PRIMARY CLARIFICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
11. Excessive growth on
surfaces and weirs.
lla. Accumulations of
wastewater solids and
resultant growth.
lla. Inspect surfaces.
lla. Frequent and thorough cleaning
of surfaces.
12. Excessive corrosion
on unit.
12a. Septic wastewater.
12a. Color and odor of
wastewater.
12a. Paint surfaces with corrosion-
resistant paint.
13. Noisy chain drive.
13a. Moving parts rub
stationary parts.
13b. Chain does not fit
sprockets.
13c. Loose chain.
13d. Faulty lubrication.
13e. Misalignment or im-
proper assembly.
13f. Worn parts.
13a. Alignment.
13d. Lubrication.
13e. Alignment and assem-
bly.
13a. Tighten and align casing and
chain. Remove dirt or other
interfering matter.
13b. Replace with correct parts.
13c. Maintain taut chain at all
times.
13d. Lubricate properly.
13e. Correct alignment and assembly
of drive.
13f. Replace worn chain or bearings.
Reverse worn sprockets before
replacing.
14. Rapid wear of chain
drive.
14a. Faulty lubrication.
14b. Loose or misaligned
parts.
14a. Lubrication.
14b. Alignment.
14a. Lubricate properly.
14b. Align and tighten entire drive.
15. Chain climbs sprockets.
15a. Chain does not fit
sprockets.
15b. Worn out chain or
worn sprockets.
15c. Loose chain.
15a. Replace chain or sprockets.
15b. Replace chain. Reverse or
replace sprockets.
15c. Tighten.
-------
TROUBLESHOOTING GUIDE
PRIMARY CLARIFICATION
INDICATORS/OBSERVATIONS
16. Stiff chain.
17. Broken chain or sproc-
kets in chain drive
system.
18. Oil seal leak.
19. Bearing or universal
joint failure.
20. Binding of sludge
pump shaft.
PROBABLE CAUSE
16a. Faulty lubrication.
16b. Rust or corrosion.
16c. Misalignment or im-
proper assembly.
16d. Worn out chain or
worn sprockets .
17a. Shock or overload.
17b. Wrong size chain or
chain that does not
fit sprockets.
17c. Rust or corrosion.
17d. Misalignment.
17e. Interferences.
18a. Oil seal failure.
19a. Excessive wear.
19b. Lack of lubrication.
20a. Improper adjustment
of packing.
CHECK OR MONITOR
16a. Lubrication.
16c. Alignment and assem-
bly.
17a. Influent flow rate.
17d. Alignment.
18a. Oil seal.
19b. Lubrication.
SOLUTIONS
16a. Lubricate properly.
16b. Clean and lubricate.
16c. Correct alignment and assembly
of drive.
16d. Replace chain. Reverse or
replace sprockets.
17a. Avoid shock and overload or
isolate through couplings.
17b. Replace chain. Reverse or re-
place sprockets.
17c. Replace parts. Correct corro-
sive conditions.
17d. Correct alignment.
17e. Make sure no solids interfere
between chain and sprocket
teeth. Loosen chain if neces-
sary for proper clearance over
sprocket teeth.
18a. Replace seal.
19a. Replace joint or bearing.
19b. Lubricate joint and/or bearings.
20a. Adjust packing.
-------
ACTIVATED SLUDGE
Process Description
The activated-sludge process is a treatment technique in which waste-
water and biological sludge (microorganisms) is mixed and aerated. The bio-
logical solids are then separated from the treated wastewater and returned to
the aeration process as needed.
In the activated-sludge process, microorganisms are mixed thoroughly with
the organics so that they grow by using the organics as food. As the micro-
organisms grow and are mixed by the agitation of the air, the individual or-
ganisms clump together (flocculate) to form an active mass of microbes called
"activated sludge". The wastewater flows continuously into an aeration tank
where air is injected to mix the activated sludge with the wastewater and
to supply the oxygen needed for the microbes to break down the organics
(Figure 18). The mixture of activated sludge and wastewater in the aeration
tank is called "mixed liquor". The mixed liquor flows from the aeration
tank to a secondary clarifier where the activated sludge is settled. Most
of the settled sludge is returned to the aeration tank to maintain a high
population of microbes to permit rapid breakdown of the organics. Because
more activated sludge is produced than can be used in the process, some of
the return sludge is diverted or "wasted" to the sludge-handling system for
treatment and disposal. Air is introduced into the aeration basins either
by injecting it into diffusers near the bottom of the aeration tank (Figures
24 and 25) or by mechanical mixers located at the surface of the aeration
tank (Figures 26 and 27). The volume of sludge returned to the aeration
basin is typically 20-50% of the wastewater flow. There are many variations
of this conventional system as described in the following paragraphs.
In long, narrow aeration tanks, the demand for oxygen is much greater at
the inlet to the aeration basin where the wastewater enters. As a result, the
"tapered aeration" process can be used. In this process, a greater portion
of the air is injected at the inlet end than at the outlet end of the aera-
tion basin. The total amount of air used is the same, but its distribution
is tapered along the aeration tank. Another variation is one in which the
wastewater flow is introduced at several points rather than all at once. The
process is known as "step aeration" (Figure 19). Multiple feed points spread
the oxygen demand over more of the aeration basin, which results in more
efficient use of the oxygen. Existing conventional plants are often modified
to the step aeration process to increase their capacity. To extend even
further the benefits achieved with step aeration, the "complete mix"
activated-sludge concept may be used (Figure 20). In this system, the in-
fluent wastewater is fed as evenly as possible along the entire length of
the aeration basin, so that the oxygen demand is uniform from one end to the
other.
55
-------
INFLUENT
PLUG FLOW
"^ AERATION TANK
SLUDGE RETURN
INFLUENT
INFLUEh
\ V i
PLUG FLOW
AERATION TANK
SLUDGE RETURN
»MMM
'Tr ttttttt
I1UIJI
AERATION TANK
4444444
SLUDGE RETURN
INFLUENT _ , CONTACT
ALTERNATE
WASTE SLUD
DRAWOFF PO
•^ TANK
STABILIZATION SL
GE
INT
^^^
^/SFTTI iNril EFFLUENT Figure 18.
^A TANK J Conventional acti-
X^^X vated sludge.
WASTE SLUDGE
p/s1rTU^G\ EFFLUENT Fiaure lg_
I TANK r Step aeration.
^*****—~**^
\
WASTE SLUDGE
©EFFLUENT
^ figure ^u.
Complete mix
activated sludge
(CMAS) .
\
WASTE SLUDGE
AC-TTC mA EFFLUENT
^JSFTTLIMr.X ^
*l TANK ] ^ Figure 21.
x.^/ Contact
stabilization .
JDGE RETURN
1
WASTE SLUDGE
56
-------
Another variation of activated sludge is the contact stabilization
process (Figure 21). In this system, the incoming wastewater is mixed
briefly (20-30 mins) with the activated sludge - just long enough for the
microbes to absorb the organic pollutants from solution but not long enough
for them to actually break down the organics. The activated sludge is then
settled out and returned to another aerated basin (stabilization tank), in
which it is aerated for 2-3 hrs to allow the microbes to break down the
absorbed organics. Because the settled volume of the activated sludge is
much smaller than the total wastewater flow, the total size of the plant
is reduced.
Many small activated-sludge plants use the "extended aeration" form of
activated sludge. The process flow diagram is essentially the same as in
the complete mix system, except that these small plants usually have no
primary treatment and aerate the raw wastewater for a 24-hr period rather
than the 6-8 hrs used in conventional plants. The long aeration time
allows the activated sludge to be partially digested within the aeration
tank so that it can be dewatered and disposed of without the need for
large sludge digestion capacity.
A variation of the conventional process, called the oxidation ditch,
has been very popular (Figure 22). A surface-type aerator is used that
both aerates and circulates the wastewater through the ditch (Figure 28).
The process is commonly designed with a 24-hr aeration period and is
considered a form of the extended aeration process.
Since 1970, there has been interest in systems using pure oxygen
instead of air. To provide efficient use of the oxygen, the aeration
tanks are often covered and the oxygen is recirculated through several
stages (Figure 23). When the tanks are covered, very pure oxygen (over
90%) enters the first stage of the system and flows through the oxygenation
basin together with the wastewater being treated. Pressure under the
tank covers maintains control and prevent backmixing from stage to stage.
This system allows for efficient oxygen use at low power requirements.
Surface aerators or submerged rotating-sparge systems are used for mixing.
Instead of using covered basins, special oxygen diffusers can be used in
open basins. The number of stages and the type of mixing device selected
depends on waste characteristics, plant size, land availability, treatment
requirements, and other similar considerations. Pure oxygen allows the
use of much smaller aeration tanks (1.5-2 hrs aeration rather than 6-8 hrs).
The oxygen used in the process is typically generated onsite. For larger
plants, air is liquified and then distilled in "cryogenic" oxygen production
units. For smaller plants, air separation is achieved by selectively
adsorbing nitrogen from the air and allowing the oxygen to pass through a
"sieve" or "pressure swing adsorption" unit. It is generally more
economical to use cryogenic processes for 10 to 2,000 tons/day oxygen
production and adsorption processes for 0.5 to 36 tons/day. The choice of
a process for the 10 to 36 tons/day range depends on many factors, but
adsorption is the simplest process.
57
-------
INFLUENT
WASTE SLUDGE
• SLUDGE-CONCENTRATING HOPPER
DIVIDING STRIP
TO SECONDARY
CLARIFIER
AERATION ROTOR
Figure 22. Oxidation ditch.
CONTROL
VALVE
OXYGEN
FEED GAS
INFLUENT
RECYCLE
SLUDGE
^AERATION TANK
COVER
AGITATOR
EXHAUST
GAS
MIXED LIQUOR
TO SECONDARY
CLARIFIER
Figure 23. Schematic diagram of multistage o;xygen aeration system.
58
-------
Figure 24. Typical air diffuser system.
59
-------
Figure 25. Aeration basin with diffused aeration.
60
-------
Figure 26. Floating surface, mechanical aerator.
61
-------
Figure 27. Platform mounted surface aerator.
62
-------
Figure 23. Oxidation ditch aeration system.
63
-------
Typical Design Criteria and Performance Evaluation
Typical design criteria for variations of the activated sludge process
are presented in Table 2.
The activated sludge process can convert nearly all influent soluble
organic matter to solids. Solids must be removed in order to have high
quality effluents in terms of organics. Unfortunately, plain sedimentation
of flocculant solids is not easily predicted. When there are large amounts
of solids, density currents, and thickening considerations, careful
operational control of solids is needed to produce consistently good
effluent quality. When properly designed and operated, an activated sludge
plant should consistently produce effluent suspended solids and BOD of
20-30 mg/1.
Many small extended aeration plants do not practice good sludge inventory
and wasting management and sometimes discharge high solids concentrations.
The oxidation ditch extended aeration process has performed very well and
very reliably when solids are managed properly.
The following terms are important in evaluating activated sludge
systems.
Mixed Liquor Suspended Solids (MLSS)—
This is a very important measurement and shows the amount of activated
sludge inventory. The MLSS is determined often - several times per day at
large plants and daily at smaller plants. Typical MLSS concentrations for
various activated sludge modifications are shown in Table 2.
Mixed Liquor Volatile Suspended Solids (MLVSS)—
This test indirectly shows the active biological fraction of mixed
liquor solids and directly tells the amount of inert solids. For example,
MLVSS will typically be 70-80% of the total MLSS. However, during times
of heavy infiltration of the sewer system, the carryover of silt into the
aeration basins may decrease the MLVSS to 55-60%. When the percent of
MLVSS decreases, the total MLSS must be increased to maintain the same
level of active organisms.
Sludge Density Index (SDI)—
The rate that activated sludge solids settle to the bottom of a final
settling tank depends on the settling characteristics of the sludge. These
characteristics are determined by a simple settling test, the results of
which can be used to determine the SDI. A 1,000 ml sample is collected
from the aeration tank and allowed to settle for 30 mins in a 1,000 ml
graduated cylinder. The volume of settled sludge is read at the end of
the 30 mins.
_, , _ ., _ , MLSS (mg/1)
Sludge Density Index = ml of settled sludge after 30 min settling x 10
A good Sludge Density Index is about 1.0. A sludge with an index of 1.5 is
64
-------
TABLE 2. TYPICAL ACTIVATED SLUDGE DESIGN PARAMETERS
cn
en
Process
Modification
Flow regime
Sludge
retention
time
(days)
Food to
microorganism
ratio-#BOD5/
MLVSS/day
Aerator
loading #BOD5/
1,000 ft3
tank volume
Mixed liquor
suspended
solids (mg/1)
Detention Recirculaticn
time (hr) ratio
Conventional
Complete mix
Step aeration
Plug
Complete mix
Plug
Contact
stabilization Plug
Extended
aeration
Pure oxygen
systems
Complete mix
Complete mix
reactors in
series
5-15
5-15
5-15
5-15
20-30
8-20
0.2 -0.4
0.2 -0.6
0.2 -0.4
0.2 -0.6
0.05-0.15
0.25-1.0
20-40
50-120
40-60
30-75
10-15
100-250
1500-3000
3000-6000
2000-3500
1000-4000*
4000-10000+
*
2000-6000
4000-8000
4-8 0.25-0.5
3-5 0.25-1.0
3-5 0.25-0.75
0.5-1.5* 0.5 -1.5
24 0.5 -2.0
2-5 0.25-0.5
* Contact unit
+ Stabilization tank
-------
dense and settles quickly. An index of less than 1 means a lighter sludge
which settles slowly.
Sludge Volume Index (SVI)—
This index is also used to reflect the settling characteristics of
activated sludge, but is defined as:
_ ml of settled sludge after 30 min settling x 1,000
~ MLSS (mg/1)
In this case, the lower the SVI, the more dense the sludge. An SVI of 100
or less is generally considered a good settling sludge.
Food to Microorganism (F/M) Ratio—
This parameter is used to express the total loading of organics on the
biological system. It is the ratio of Ibs of BOD entering the aeration
basin per day to the Ibs of MLVSS in the aeration basin and the secondary
clarifier.
A high F/M reflects a high loading on the activated sludge system which
will result in more waste activated sludge generated per pound of BOD
removal. A very high F/M (above 0.5) indicates a more unstable system.
A low F/M at normal MLSS concentrations (less than 0.1) indicates a
lightly loaded activated sludge plant. The waste sludge should be stable
and may not require any added digestion.
Solids Retention Time (SRT)—
The Solids Retention Time is the average length of time that the
activated sludge solids are kept in the process.
The following steps are examples of how to calculate the above terms in
evaluating plant performance.
1. Determine the operating conditions:
Flow = 2 mgd
MLSS = 2500 mg/1
MLVSS = 2000 mg/1
SS, secondary effluent =20 mg/1
BOD, primary effluent =120 mg/1
BOD, secondary effluent = 15 mg/1
Return Activated Sludge Flow = 700 gpm = 1 mgd
Waste Activated Sludge Flow = 10,000 gpd =0.01 mgd
Waste Activated Sludge Concentration = 9200 mg/1
Aeration Tank Size = 10 ft deep x 125 ft long x 40 ft wide
Secondary clarifier = 65 ft diameter x 12 ft deep
Volume of Settled Sludge in 1000 ml MLSS sample after 30 min
settling = 200 ml
66
-------
The secondary effluent quality is good and there are no apparent major
problems.
2. Calculate volume of aeration tank and secondary clarifier
Aeration tank volume = 10 ft x 125 ft x 40 ft x 7.48 gals/cu ft
= 374,000 gals
Clarifier volume = 11(65) x 12 ft x 7.48 gals/cu ft
4
= 24,808 gals
3. Calculate Sludge Volume Index
SVI = 200 x 1000 = 80
2500
A sludge volume index of less than 100 indicates a good settling sludge.
4. Calculate F/M Ratio
F = 120 mg/1 x 2 mgd x 8.34 lbs/mg/mg/1 = 2000 Ibs/day
M = MLVSS in aeration tank + MLVSS in clarifier
= 2000 mg/1 x 8.34 lbs/mg/mg/1 x (0.374 + 0.025) = 6655 Ibs
F/M = 2000 = 0.30
6655
As can be seen from Table 2, the F/M value is within the range normally
used in conventional activated sludge systems.
5. Determine the amount of activated sludge being wasted. The
amount of sludge wasted from a conventional plant is normally
about 0.5-0.6 Ibs of activated sludge per Ib of BOD removed.
If greater amounts are being wasted, the MLSS will decrease and
eventually reach levels that are too low. If smaller amounts
are wasted, then MLSS will increase to the point where they will
eventually spill over the secondary clarifier weirs.
Lbs BOD Removed/Day = (120-15) x 8.34 lbs/mg/mg/1 x 2 mgd
= 1751 Ibs/day
Lbs Activated Sludge Wasted/Day = 0.01 mgd x 9200 mg/1 x 8.34
=767 Ibs/day
= 0.44 Ibs/lb BOD removed
This wasting rate is somewhat lower than normal. If the MLSS have been
increasing, the wasting rate should be increased about 20%.
67
-------
6. Calculate Sludge Retention Time (SRT)
SRT = MLSS in aeration tank and final clarifier
solids wasted + solids lost in effluent/day
= 2000 mg/1 x 8.34 x (0.374 + 0.025 mil gal)
9200 mg/1 x 8.34 x 0.01 mgd + 20 x 8.34 x 2.0 mgd
6655
767 + 334
= 6.0 days
As shown in Table 2, this is within the normal range of 5-15 days
found in conventional activated sludge plants.
7. Calculate Sludge Recirculation Rates
Recirculation Rates = Return Sludge Flow Rate
Influent Plow Rate
= 1 mgd
2 mgd
= 0.5
This is at the high end of the range usually found for conventional
activated sludge plants. With the good settling sludge at this plant, it
should be possible to use a lower return rate without problems of solids
carryover from the secondary clarifiers.
Control Considerations
Dissolved Oxygen in Aeration Tank—
With conventional aeration systems, dissolved oxygen (DO) in the mixed
liquor should be maintained in the 1-3 mg/1 range. With pure oxygen systems,
higher levels of DO are maintained with minimum levels being 2 mg/1. Good
mixing is also very important, but it is a fixed parameter with some aeration
systems. The operator should monitor the aeration tank DO levels and air
flow rates periodically (every 2 hrs is suggested) to make appropriate air
rate adjustments as required. If DO monitoring instrumentation is provided,
it is imperative that it be properly maintained and calibrated to provide
values that are valid and reliable.
In varying the suspended solids concentration in the aeration basins,
oxygen demand will tend to increase as the solids concentration is increased.
The dissolved oxygen level should be watched as the suspended solids con-
centration is increased. The MLSS should be carried at a level to maintain
the desired Solids Retention Time (SRT), but carefully controlled by frequent
sludge wasting as discussed later.
68
-------
Return Activated Sludge Flow Rate—
To properly operate the activated sludge process, a good settling mixed
liquor must be achieved and maintained. The MLSS are settled in a clarifier,
and then returned to the aeration tank as the Return Activated Sludge (RAS).
The RAS makes it possible for the microorganisms to be in the treatment
system longer than the flowing wastewater. For conventional activated sludge
operations, the RAS flow is generally about 20 to 50% of the incoming waste-
water flow. Changes in the activated sludge quality will require different
RAS flow rates due to settling characteristics of the sludge.
There are two basic approaches that can be used to control the RAS
flow rate. These approaches are based on the following:
Controlling the RAS flow rate independently from the influent flow.
Controlling the RAS flow rate as a constant percentage of the
influent flow.
Most activated sludge operations perform well and require less attention
when the constant RAS flow rate approach is used. However, setting the RAS
at a constant flow rate results in a continuously varying MLSS concentration
because the MLSS are flowing into the clarifier at a higher rate during
peak flow but they are being removed at a constant rate. At minimum influent
flow rates, the MLSS are being returned to the aeration tank at a higher rate
than they are flowing into the clarifier. The clarifier acts as a storage
reservoir for the MLSS, and the clarifier has a constantly changing depth of
sludge blanket as the MLSS moves from the aeration tank to the clarifier.
The advantage of using this approach is simplicity, because it minimizes
the amount of effort for control. Many plants do not have the controls
necessary to control RAS flow rates at a constant percentage of influent
flow.
Sludge Blanket Depth in Secondary Clarifier—
Checking the depth of the sludge blanket in the clarifier is the most
direct method for determining the RAS flow rate. The location of the sludge
blanket may be found by several types of devices. Some are commercially
available while others must be made by the operator. The following are some
of the different types of blanket finders:
A series of air lift pumps mounted within the clarifier at
various depths.
Gravity flow tubes located at various depths
Electronic sludge level detector-a light source and photo-electric
cell attached to a graduated handle or drop cord. The photo-
electric cell actuates a meter, buzzer, light, etc.
Sight glass finder-a graduated pipe with a sight glass and light
source attached at the lower end.
Plexiglass core sampler
Some type of portable pumping unit with a graduated suction pipe
or hose
69
-------
The blanket depth should be kept to less than one-fourth of the
clarifier sidewall water depth. The operator must check the blanket depth
on a routine basis, making adjustments in the RAS to control the blanket
depth. If the depth of the sludge blanket is increasing, the increase
may result from having too much activated sludge in the treatment system,
and/or, because of a poorly settling sludge, or plugging of the sludge
removal system. Long-term corrections must be made that will improve the
settling characteristics of the sludge or remove the excess solids from the
treatment system.
Measurements of the sludge blanket depth in the clarifier should be
made at the same time each day. The best time to make these measurements
"is during the period of maximum daily flow, because the clarifier is operat-
ing under the highest solids loading rate. The sludge blanket should be
measured daily,and adjustments to the RAS rate can be made as necessary.
Adjustments in the RAS flow rate should only be needed occasionally if
the activated sludge process is operating properly.
Waste Activated Sludge Flow Rate—
The objective of wasting activated sludge is to maintain a balance
between the microorganisms and the amount of BOD. When the microorganisms
remove BOD from wastewater, the amount of activated sludge increases. The
rate at which these microorganisms grow is called the growth rate and is
defined as the increase in the amount of activated sludge that takes place
in one day. The objective of sludge wasting is to remove just that amount
of microorganisms that grow. When this is done the amount of activated
sludge formed by the microorganism growth is just balanced by that which is
removed from the process. This allows the total amount of activated sludge
in the process to remain nearly constant. This condition is called "steady-
state" which is a desirable condition for operation. However, "steady-state"
can only be approximated because of the variations in the nature and quantity
of the food supply (BOD) and of the microorganism population.
Wasting of the activated sludge is normally done by removing a portion
of the RAS flow. The waste activated sludge is either pumped to thickening
facilities and then to a digester, or to the primary clarifiers where it is
pumped to a digester with the raw sludge.
An alternate method for wasting sludge is from the mixed liquor in the
aeration tank. There is much higher concentration of suspended matter in
the RAS than there is in the mixed liquor. When wasting is done from the
mixed liquor, larger sludge handling facilities are required. However,
wasting from the mixed liquor has the advantage of not wasting excessive
amounts of sludge because of the large quantity of sludge involved. The
extra security of wasting from the mixed liquor should not be underestimated.
Many plants do not have the flexibility to waste from the mixed liquor nor
are there sufficient sludge handling facilities to handle the more dilute
sludge.
Wasting of the activated sludge can be done on an intermittent or con-
tinuous basis. The intermittent wasting of sludge means that wasting is
70
-------
conducted on a batch basis from day to day. Intermittent wasting of sludge
has the advantage that less variation in the suspended matter concentration
will occur during the wasting period, and the amount of sludge wasted will
be more accurately known. The disadvantages of intermittent wasting are
that the sludge handling facilities in the treatment plant may be loaded at
a higher hydraulic loading rate and that the activated sludge process is
out of balance for a period of time until the micoorganisms regrow to replace
those wasted over the shorter period of time.
The simplest and most commonly used approach in controlling the amount
of sludge wasted is to waste enough to maintain a nearly constant MLVSS.
This technique usually produces good quality effluent as long as the in-
coming wastewater characteristics are fairly constant with minimal variations
in influent flow rates. The operator tries to maintain a constant MLVSS
concentration in the aeration tank to treat the incoming wastewater
organic load. To put it in simple terms, if it is found that a MLVSS concen-
tration of 2000 mg/1 produces a good quality effluent, the operator must
waste sludge from the process to maintain that concentration. If the MLVSS
level increases above the desired concentration, more sludge is wasted until
the desired level is reached again. Other approaches that have been used
involve wasting sludge so as to maintain a constant F/M or a constant SRT.
Microscopic Examination—
Microscopic examination of the MLSS can be a significant aid in the
evaluation of the activated sludge process. The presence of various micro-
organisms within the sludge floe can rapidly indicate good or poor treatment.
Protozoa play an important role in clarifying the wastewater and act as
indicators of the degree of treatment. The protozoa eat the bacteria and
help to provide a clear effluent. The presence of rotifers is also an
indicator of effluent stability. A predominance of protozoa (ciliates)
and rotifers in the MLSS is a sign of good sludge quality. The presence of
filamentous organisms and a limited number of protozoa is characteristic
of a poor quality activated sludge. This condition is commonly associated
with a sludge that settles poorly.
Process Control References—
There have been many articles, reports, manuals, and books written
on the control of activated sludge systems. It is not practical to
summarize all of this information in this manual. The evaluator should
read the following reference (from which portions of this section were
drawn) for detailed information on process control:
"Process Control Manual for Aerobic Biological Wastewater
Treatment Facilities", U.S. EPA, Municipal Operations
Branch, Office of Water Programs, Washington, D.C. 20460
March, 1977, EPA 430/9-77-006.
Another useful EPA document is:
West, Alfred W., Operational Control Procedures for the
71
-------
Activated Sludge Process, Parts I, II, IIA, and IIIB,
U.S. EPA National Training & Operational Technology
Center, May 1974.
Common Design Shortcomings and Ways to Compensate
Shortcoming
1. Diffusers plug from
dirty air.
2. Inadequate screening of
raw wastes causing
mechanical aerators and
sludge return system to
plug.
3. No or inadequate system
to measure and control
rate of return or waste
activated sludge flow.
4. Grit buildup in aeration
basin.
1.
2.
3.
Solution
Install air cleaners on blower.
Install finer bar screen or
comminutor.
Install flow measuring devices
on RAS and WAS systems.
4. Install grit removal system.
5. Aeration system not
adequate to maintain DO
at peak flows.
6. Surface aeration systems
throwing spray onto walk-
ways causing slippery
conditions.
5. Place more aeration basins in
service (if available); install
flow equalization facilities;
install more aeration equip-
ment. Convert to oxygen system.
6. Install shields.
7. Inadequate return sludge
flow capacity.
8. Inadequate process
flexibility.
Install added return sludge
pumps or increase pump size.
Modify system to operate in
plug, step, or contact
stabilization.
9. Inadequate metering to
balance flows between
multiple units.
Install appropriate flow
metering system.
72
-------
TROUBLESHOOTING GUIDE
ACTIVATED SLUDGE
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Sludge floating to
surface of secondary
clarifiers.
la.
Filamentous organisms
predominating in
mixed liquor ("bulk-
ing sludge")
la. SVI - if less than
100, l(a) is not
likely cause; micro-
scopic examination
also can be used to
determine presence of
filamentous organisms
Ib.
Denitrification
occurring in second-
ary clarifiers;
nitrogen gas bubbles
attaching to sludge
particles; sludge
rises in clumps
Ib. Nitrate concentration
in clarifier influent;
if no measureable NO ,
than l(b) is not the
cause
la. (1) Increase DO in aeration
tank if less than 1 mg/1
(2) Increase pH to 7
(3) Supplement deficiency of
nutrients so that BOD to
nutrient ratio is no more
than 100 mg/1 BOD to 5 mg/1
total nitrogen; to 1 mg/1
phosphorus; to 0.5 mg/1
iron
(4) Add 5-60 mg/1 of chlorine
to return sludge until SVI
<150
(5) Add 50-200 mg/1 of hydrogen
peroxide to aeration tank
until SVI <150
(6) Increase SRT
(7) Increase sludge return rate
Ib. (1) Increase sludge return rate
(2) Increase DO in aeration tanJ<
(3) Reduce SRT
2. Pin floe in secondary
clarifier overflow -
SVI is good but
effluent is turbid
2a. Excessive turbulence
in aeration tanks
2b. Overoxidized sludge
2c. Anaerobic conditions
in aeration tank
2d. Toxic shock load
2a. DO in aeration tank
2b. Sludge appearance
2c. DO in aeration tank
2d. Microscopically
examine sludge for
inactive protozoa
2a. Reduce aeration agitation
2b. Increase sludge wasting to
decrease SRT
2c. Increase DO in aeration tank
2d. Re-seed sludge with sludge from
another plant if possible;
enforce industrial waste
ordinances
-------
TROUBLESHOOTING GUIDE
ACTIVATED SLUDGE
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3. Very stable dark tan
foam on aeration tank:
which sprays cannot
break up
3a. SRT is too long
3a. If SRT greater than
9 days, this is
probable cause
3a. Increase sludge wasting so as
to reduce SRT
4. Thick billows of white
sudsy foam on aeration
tank.
4a. MLSS too low
4a. MLSS
4a. Decrease sludge wasting so as
to increase MLSS
5. Aerator contents turn
dark-sludge blanket
lost in secondary
clarifier
5a. Inadequate aeration
5a. Aeration basin DO
5a. (1) Increase aeration by
placing another blower in
service
(2) Decrease loading by
placing another aeration
basin in service
(3) Check aeration system
piping for leaks
(4) Clean any plugged diffuser;
6. MLSS concentrations
differ substantially
from one aeration
basin to another
6a. Unequal flow distri-
bution to aeration
6a. Flow to each basin
6a. Adjust values and/or inlet
gates to equally distribute
flow
7. Sludge blanket over-
flowing secondary
clarifier weirs uni-
formly throughout
basin
7a. Inadequate rate of
sludge return
7a. Sludge return pump
output
7a. (1) If return pump is malfunc-
tioning, place another pump
in service & repair
(2) If pump is in good condi-
tion increase rate of
return and monitor sludge
blanket depth routinely.
Maintain 1-3 foot deep
blanket. When blanket in-
creases in depth, increase
rate of return
(3) Clean sludge return line if
plugged
-------
TROUBLESHOOTING GUIDE
ACTIVATED SLUDGE
INDICATORS/OBSERVATIONS
8. Sludge blanket over-
flowing secondary
clarifier weirs in
one portion of
clarifier.
9. In diffused aeration
basin, air rising in
very large bubbles
or clumps in some
areas.
10. pH of mixed liquor
decreases to 6.7 or
lower. Sludge becomes
less dense.
PROBABLE CAUSE
7b. Unequal flow distri-
bution to clarifiers
causing hydraulic
overload.
7c. Peak flows are over-
loading clarifiers.
7d. Solids loadings are
too high on
clarifier.
8a. Unequal flow dis-
tribution in
clarifier.
9a. Diff users plugged.
LOa. Nitrification
occurring and waste-
water alkalinity is
low.
LOb. Acid wastewater
entering system.
CHECK OR MONITOR
7b. Flow to each
clarifier.
7c. Hydraulic overflow
rates at peak flows
if >1,000 gpd/sq ft,
this is a likely
cause .
7d. Loadings should not
exceed 1.25 Ib/sq ft/
hr.
8a. Effluent weir.
9a. Visual inspection.
LOa. Effluent NH ; in-
fluent and effluent
alkalinity.
LOb. Influent pH
SOLUTIONS
7b. Adjust valves and/or inlet
gates to equally distribute
flow.
7c. Install flow equalization
facilities or expand plant.
7d. Reduce MLSS concentration
by increased wasting.
8a. Level effluent weirs.
9a. Clean or replace diff users
Check air supply-install air
cleaners ahead of blowers to
reduce plugging from dirty air.
lOa. (1) Decrease sludge age by in-
creased wasting if nitri-
fication not required.
(2) Add source of alkalinity -
lime or sodium bicarbonate.
lOb. Determine source and stop flow
into system.
-------
TROUBLESHOOTING GUIDE
ACTIVATED SLUDGE
INDICA TOM/OBBEHVA TIONC
11. Sludge concentra-
tion in return
sludge is low
(<8,000 mg/1)
PROVABLE CAUSE
lla. Sludge return rate
too high.
lib. Filamentous growth.
Lie. Actinomycetes
predominates.
CHECK OR MONITOR
Lla. Return sludge con-
tration, solids
balance around
final clarifier,
settleability test.
lib. Microscopic exami-
nation, DO, pH,
nitrogen concentra-
tion.
lie. Microscopic
examination,
dissolved iron
content.
lib.
lie.
SOLUTIONS
la.
Reduce sludge return rate.
Raise DO, raise pH, supplement
nitrogen, add chlorine.
(see item 1).
Supplement iron feed if
dissolved iron less than
5 mg/1.
12. Dead spots in
aeration tank.
12a Diffusers plugged.
12b. Underaeration
resulting in low DO.
12a. Visual inspection.
12b. Check DO and RAS
rate.
12a. Clean or replace diffusers -
check air supply - install air
cleaners ahead of blowers to
reduce plugging from dirty air
L2b. Increase rate of aeration to
bring DO concentration up to
1 to 3 mg/1.
-------
TRICKLING FILTERS
Process Description
A trickling filter consists of a bed of coarse media (such as rock,
plastic or other material) covered with microorganisms. Wastewater is
applied at a controlled rate, using moving distributors or fixed nozzles.
As the wastewater trickles down through the openings of the media, organic
matter is removed by contact with the microorganisms. The treated waste-
water then is collected by an underdrain system.
Figure 29 shows a trickling filter and its principal components which
include:
The distribution system which applies the wastewater to the
filter media
The filter media which provides surface area for the micro-
organisms to grow
The underdrain system which supports the media and provides
drainage of the waste flow to a collection channel while
permitting air circulation
The trickling filters usually are built with a tank which contains
the media. They also may be built by placing the media on an underdrain
system and allowing the natural slope of the media at the edges to form
their outer boundary. The system may be either square, rectangular or
circular in shape, although the circular filter is the most popular.
Two kinds of distribution systems are in use: fixed nozzle and
rotary distributors. In a fixed nozzle system, the wastewater is applied
using a pipe grid that ends in nozzles evenly spaced over the bed. To
distribute the sewage evenly, the water pressure on the nozzles must change
during each dosing period. This is accomplished by using a dosing tank
and siphon which automatically starts to discharge when full, and stops
when empty (Figure 30). The filter rests while the dosing tank refills.
In recent years, the rotary distributor (Figure 29) has become more
popular than the fixed type nozzle.
Most of the distributors are rotated by the reaction of the water
jets pushing against the air, however, they also may be motor driven. In
order to get even distribution of the wastewater over the surface of the
bed, the nozzle openings are smaller and spaced farther apart near the
center of the bed. The nozzles discharge the wastewater in a thin sheet
or a fine spray.
77
-------
ROTARY DISTRIBUTOR/OUTLET BOX
CD
Figura 29. Typical tricklina filter in cross section.
-------
DISCHARGE LEVEL
-VENT PIPE
-BELL
rBLOW -OFF
TRAP
'o j / AI R VENT
idL
^
DEFLECTOR
TYPICAL DOSING SIPHON
FIXED NOZZLE DISTRIBUTORS
iXW
•*°<.
&••<
'%}
;&•<
.V*
n n."n.n-nn n7
O
Q. ^>
n.n-n n n
:»e
^^.^g^s^^M^m^^t^^^
FIXED NOZZLE DISTRIBUTION SYSTEM
Figure 30. Fixed nozzle distribution system.
-------
Trickling filters are not primarily a filtering or straining process
as the name implies (the rocks in a rock filter are 1-4 inches in diameter,
too large to strain out solids), Instead, these filters provide large
amounts of surface area where the microorganisms cling and grow in a slime
on the rocks as they feed on the organic matter. Excess growths of micro-
organisms wash from the rock media and would cause high levels of suspended
solids in the plant effluent if not removed. Thus, the flow from the
filter is passed through a secondary clarifier to allow these solids to
settle out.
There are several ways to prevent the biological slimes from drying
out and dying when wastewater flows are too low to keep the filter wet.
One method is to recycle filter effluent. Recirculation reduces odor
potential and improves filter efficiency as it provides another opportunity
for the microbes to attack any organics that escaped the first pass through
the filter. Another approach to improving performance or handling strong
wastewaters is to use two filters in series, referred to as a "two-stage"
trickling filter system. Figure 31 shows a schematic of typical one and
two-stage trickling filter systems.
Although rock trickling filters have performed well for years, certain
limitations have been found. Under high organic loadings, the slime
growths can be so great that they plug the void spaces between the rocks,
causing flooding and failure of the system. Also, the volume of void
spaces is limited in a rock filter, which limits the circulation of air in
the filter and the amount of oxygen available for the microbes. This
problem, in turn, limits the amount of wastewater that can be processed.
To overcome these limitations, synthetic media for trickling filters have
recently become popular. These materials include modules of corrugated
plastic sheets, redwood slats, and plastic rings. These media offer
larger surface areas for slime growths (typically 27 sq ft of surface area
per cu ft as compared to 12-18 sq ft per cu ft for 3-in rocks) and greatly
increase void ratios for increased air flow. The materials are also much
lighter than rock (by a factor of about 30), so that the trickling filters
can be much taller without structural problems. While rock in filters is
usually not more than 10 ft deep, synthetic media depths are often 20 ft
or more, reducing the overall space requirements for the trickling filter
portion of the treatment plant.
One type of manufactured media consists of vertical stacks of redwood
laths constructed into 4 x 4 ft racks, with spacer rails between the layers
to allow for air circulation and water flow. (Figure 32).
The molded plastic media uses pieces of interlocking corrugated sheets
of plastic that look like a honeycomb. The sheets of media are stacked
so that they interlock and fit inside the filter structure (Figure 33).
Typical Design Criteria and Performance Evaluation
Trickling filters are classified on the basis of the hydraulic and
organic loads they are designed to treat. Table 3 summarizes the loading
80
-------
Recycle
Influent
Primary
Settling
Tank
1
Filter
Clarifier
Effluent
Influent
Primary
Clarifier
\
Recycle Recycle
First— stage
Filter
i
Second-stage
Filter
Clarifier
Effluent
Figure 31. Typical one and two-stage trickling filter systems.
81
-------
REDWOOD LATH
REDWOOD RAILS
STACKED
Figure 32. Redwood lath media.
82
-------
Figure 33. Plastic media.
83
-------
TABLE 3. TRICKLING FILTER DESIGN AND PERFORMANCE PARAMETERS
CD
Opening characteristics
Hydraulic loading:
MG/Acre
gpm/sf
gpd/sf
Organic loading:
Ib BOD/ac ft
Ib BOD/1000 cu ft
Recirculation ratio
Depth, ft
BOD removal, %
Low -rate
rock media
1-4
0.02-0.06
28.8-86.4
500-1000
12-22
NONE
6-10
85
High -rate
rock media
10-40
0.2-0.6
288-864
2000-4000
45-90
1:1-4:1
3-8
85
High -rate
plastic media
40-190*
0.6-3.0
864-4320
870-13000+
40-200
0.5:1-2:1
15-40
40-80+
* Recirculation included
+ Recirculation not included
-------
rates and general operating characteristics for low-rate rock media,
high-rate rock media, and high-rate plastic media.
Typical overall efficiency of a trickling filter treatment plant is
about 85% removal of BOD and suspended solids for municipal wastewaters,
or a concentration of about 30 mg/1 of suspended solids and BOD in the
final effluent. The actual effectiveness of the trickling filter process,
however, depends on the following factors:
Growth of biological organisms
Raw wastewater concentration
Dissolved oxygen
Temperature
pH and/or toxic conditions
Trickling filter plant effluent data for 2 plants are presented in
Figure 34. Figure 35 shows a guideline summary relating approximate
effluent quality to organic loading.
Performance evaluation for trickling filters should consist of
BOD removal efficiencies in relationship to:
Type of media
Loading parameters
Plant recirculation flows
The following will serve as an example of step-by-step procedures
for evaluating the performance of trickling filters:
1. Define the design and operating mode of the trickling filter.
High Rate Rock Media
Depth, D = 6 ft
Diameter, dia = 133 ft
Surface Area, A = (TT/4) dia = 13,893 ft
Volume, V = A x D = 83,358 ft
Flow
Raw wastewater = 2.5 mgd
Recirculated = 5.0 mgd
Total, Raw + Recirculated = 7.5 mgd
BOD, influent = 200 mg/1
BOD, clarifier effluent = 30 mg/1
2. Determine the combined efficiency of BOD removal for the trickling
filter and secondary clarifier.
% BOD = BOD (primary effluent)-BOD (clarifier effluent) x 100
removal BOD (primary effluent)
= (200-30) 100
200
= 85%
85
-------
80
70
t
in
Q
§
Z
III
_J
LL
U.
OA
on
10
/
/
AVE
50#/1
^
3AGE I
,000 Cl
/
/
/
/
.OAD
JBIC F
/
EET
jS
'
~)
/
/
,/
/^
/
/
/
S*
^S^
S
/
/
/
AVERAGE LOAD
"^-20#
/1 ,000 CU
BIC FE
/
= T
0 "
2 5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 34. Trickling filter effluent quality - two Texas plants.
86
-------
130 -
120
110
100
90
-- 80
f
I
in
0 70
O
CD
K
? 60
LL
U.
IU
50
40 '
30
20 •
10-
10 20 30 40 50 60 70
80
90 100
BOD5 - pounds per 1,000 cu ft/day
Figure 35. Effluent quality trickling filters.
87
-------
3. Compare this removal efficiency with the expected removal for
high rate rock media shown in Table 3.
Determine the hydraulic loading rate for the filter.
Hydraulic load = Total Flow in gpd
Surface Area in ft
= 7.5 x 10 gpd
13,893 ft*
= 540 gpd/ft2
As shown in Table 3, this loading is within the acceptable range of
288-864 gpd/ft2.
4. Determine the organic loading rate for the filter.
Organic Load = (Flow in mgd) (BOD mg/1) (8.34 Ib/gal)
(Vol in ft ) r 1000
= (2.5) (200) (8.34) 1000
83,358
= 50 Ib BOD/1000 cu ft
For high rate rock media trickling filters, the organic loading
should be between 45 and 90 Ib BOD/1000 cu ft as shown in Table 3.
5. Calculate the recirculation ratio for the trickling filter.
Recirculation ratio = Recirculated flow, mgd
Raw wastewater flow, mgd
= 5.0
2.5
= 2/1
Usually, the recirculation ratio has a greater effect on filter
performance than the recirculation flow scheme. To maximize filter per-
formance and minimize operational problems, recirculation ratios should
be controlled within the ranges given in Table 3.
Control Considerations
In the treatment of domestic wastewater, the trickling filter is
usually preceded by a primary clarifier and followed by a final clarifier.
Where primary sedimentation is not provided, some pretreatment such as that
provided by a shredder, grinder or screens may be necessary to prevent
filter clogging.
The efficiency of treatment attained by trickling filter plants is
greatly affected by the operation of the final settling tanks. It is
88
-------
essential that sludge be removed from the final settling tank before it
rises to the surface and is carried out with the final effluent. Sludge
from final settling tanks can either be pumped back to the primary settling
tanks or directly to a thickener for further sludge treatment and disposal.
The operation of final settling tanks is especially important in the case
of high rate trickling filters. In this case, sludge becomes septic
much faster than the sludge from standard rate filters; consequently, it
should be removed more rapidly. Sludge from a low-rate trickling filter
is relatively stable, and periodic removal at 3 to 24 hr intervals,
depending upon operational conditions, is usually sufficient. During
warm summer weather and periods of heavy sloughing, removal at 3 to 6 hr
intervals may be required. Sludge from a high-rate trickling filter has
a higher oxygen demand, and therefore, it must be removed from the sedi-
mentation tank within a short time, preferably on a continuous basis.
In the trickling filter process, wastewater contacts the micro-
organisms attached to the filter media, and the organic material is
oxidized. As a result, it is very important to keep a healthy popu-
lation of microorganisms which can continue to do the job.
Because the organic materials in the wastewater are the food source
for the organisms, the characteristics of the raw wastewater are important
process factors. Changes in the organic strength of the wastewater will
cause changes in the way the microorganisms grow. This change in growth
will then affect treatment efficiencies. Recirculation of filter or
final effluent is one plant operation which can be used to lower the
strength of the wastewater applied to the filter. Some of the advantages
of recirculation include the following:
Maintains biological growth throughout media depth.
May improve operation of primary and final sedimentation units
during low flow periods by reducing septicity.
Dilutes high strength or toxic wastes to make them treatable.
Minimizes hydraulic and organic loading variations.
Improves distribution of the wastewater over the filter surface.
Minimizes odors, ponding, and filter fly breeding by increasing
hydraulic loading to encourage continuous sloughing and reduce
slime thickness.
Prevents biological growth from drying out during low flows.
When selecting the recirculation rate, the hydraulic loading on the
filter and affected clarifiers must be considered as well as the hydraulics
of the distribution and underdrain systems. As a rule of thumb, the under-
drain conduits and effluent channels should not flow more than one-half
full. Although some experimentally based equations have been developed to
calculate the amount of recirculation needed, it is recommended that
operational control be based on filter response and process performance.
In high-rate trickling filters, recirculation ratios usually range from 0.5
to 4.0 with higher ratios considered to be economically unjustifiable.
Common engineering practice is to design the high-rate trickling filter
process for ratios of 0.5 to 2.0. Trickling filters utilizing synthetic
89
-------
media employ recirculation as a means of maintaining a hydraulic loading
(gpm/sq ft) which will maintain biological growth throughout the media
depth.
As an aerobic system, it is important that the trickling filter
have enough dissolved oxygen to satisfy the oxygen needed for biological
oxidation of the organic material. These oxygen requirements are
determined by the BOD loading, the quality of the microorganisms,
temperature, and hydraulic loading. Except for changing recirculation,
operators have little control over temperature variations. Temperature
increases cause more biological activity and greater oxygen utilization,
while decreases result in lower activity and less oxygen utilization.
This means that winter treatment efficiency may be lower than efficiency
in the summer. The effects of temperature changes may be reduced by
adjusting the recirculation ratio.
Extreme changes in pH can reduce the filter's efficiency and, in
severe cases can even kill biological growth. pH control may be needed
if the pH often goes outside the 6.5 to 8.5 range.
For more detailed information on process control, the evaluator
should read:
"Process Control Manual for Aerobic Biological
Wastewater Treatment Facilities", U.S. EPA,
Municipal Operations Branch, Office of Water
Programs, Washington, D.C. 20460 (March, 1977)
Common Design Shortcomings and Ways to Compensate
Shortcomings Solution
1. Fly nuisance caused by 1. Modify ends of distributor arms
alternately wet-and dry to maintain continuously wet
filter walls. filter walls. (Flies cannot
survive on walls which are kept
wet.)
2. Odors, resulting from 2. (a) Increase ventilation by
poor ventilation of forcing air into filter
filter drain system.
(b) Cover filter and deodorize
the off gases.
3. Ice build up on filter 3. (a) Construct wind screen to
media. protect filter from
prevailing wind.
(b) Cover pump sumps and dosing
tanks.
(c) Cover filter.
90
-------
Common Design Shortcomings and Ways to Compensate (Cont'd)
Shortcomings Solution
4.
4.
5.
6.
7.
Clogging of distributor
orifices caused by
inadequate primary
treatment.
Filter subject to
clogging with leaves
from nearby trees.
Excessive sloughing
from filter due to
excessive organic
loading.
Recirculation of
secondary clarifier
effluent is causing
high flows through
the clarifier which
are carrying solids
over the clarifier weir.
Improve grease and SS removal
in primary clarifier.
Removal of nearby trees.
Decrease loading by using
more filters or expanding
the plant.
Modify recirculation
system so that trickling
filter effluent (secondary
clarifier influent) is
recirculated directly.
91
-------
TROUBLESHOOTING GUIDE
TRICKLING FILTERS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Filter Ponding
la. Media too small or
not uniform in size.
Ib. Rock media broken
due to temperature
extremes.
Ic. Improper operation of
primary treatment
units.
Id. Excessive sloughing
excessive biological
growth.
le. Excessive organic
loading
If. Accumulation of
leaves, debris, etc.
Ig. Snails, moss, roaches
la. Check size of media
for uniformity.
Ib. Fines clogging filter
voids.
Ic. Excessive SS in
filter influent
Id. Slime growth clogging
filter voids.
le. Check loading rates.
If. Inspect filter
Ig. Visual inspection
la. Replace media
Ib. Replace media
Ic. Correct improper operation
of primary treatment units.
Id. Flush media with high pressure
stream of water and/or dose
with chlorine to control
slime growth.
le. Increase recirculation or
flood the filter to loosen
and remove surface
accumulations.
If. Remove debris from filter media
Ig. Flush filter and/or chlorinate
to produce a residual of 0.5 -
1.0 mg/1
2. Filter Flies
2a. Excessive biological
growth on filters.
2b. Plant grounds provide
breeding ground for
flies.
2a. Visual inspection.
2b. Inspect grounds.
2a. Remove excessive growth as
described in Id.
2b. Maintain grounds so as not to
provide a sanctuary for flies.
-------
TROUBLESHOOTING GUIDE
TRICKLING FILTERS
INDICATORS/OBSERVATIONS
2. Filter Flies (cont'd)
3 . Odors
PROBABLE CAUSE
2c. Hydraulic loading
too low to wash
filter of fly larvae.
2d. Poor distribution of
wastewater especially
along filter walls
3a. Excessive organic
loading.
3b. Poor ventilation
due to clogged vent
pipes or filter
drain .
3c. Poor ventilation
due to excessive
biological growth
filling media voids.
CHECK OR MONITOR
2c. Hydraulic loading
should be greater
than 200 gpd/sq ft
2d. Visual inspection.
3a. Check organic
loading.
3b. Check vent pipes and
filter drain.
3c. Inspect media voids.
SOLUTIONS
2c. Prevent completion of fly
life cycle by the following
remedies .
1. Increase recirculation
rate.
2. Flood filter for several
hrs at regular intervals
3. Chlorinate to produce a
residual of 0.5-1.0 mg/1.
4. Apply an insecticide to
filter walls and areas
breeding flies.
2d. 1. Unclog spray orifices or
nozzles .
2. If alternating wet and dry
environment exists, see
shortcomings .
3a. 1. Maintain aerobic conditions
in all treatment units by
adding forced ventilation
equipment .
2 . Chlorinate filter influent
when plant flow is low.
3. Increase recirculation rate
to dilute organic strength
and improve oxygen transfer
3b. 1. Clear vents and drain
system of obstructions.
2 . If underdrain system is
flowing more than half
full reduce hydraulic
loading.
3c. Increase recirculation rate to
filter.
-------
TROUBLESHOOTING GUIDE
TRICKLING FILTERS
INDICATORS/OBSERVATIONS
3. Odors (cont'd)
4. Ice build up on
filter media.
5. Uneven distribution
of flow on filter
surface .
6. Snails, Moss and
Roaches.
PROBABLE CAUSE
3d. Poor housekeeping.
3e. Septic filter
influent .
4a. Climate.
4b. Uneven distribution
during freezing
weather.
5a. Clogging of
distributor orifices.
5b. Inadequate hydraulic
load on filter.
5c. Seal leaks.
6a. Climatic conditions
and geographical
location.
CHECK OR MONITOR
3d. Visual inspection.
3e. Check influent for
4a. Air and wastewater
temperature .
4b. Visual inspection.
5a. Ponding in some areas
with concurrent
drying in other areas.
5b. Hydraulic loading
rate.
5c. Seal.
6a. Visual inspection.
SOLUTIONS
3d. Remove debris from filter media
and wash down distributor
splash plates and walls above
media .
3e. Correct upstream system by
aeration or controlled
prechlorination .
4a. 1. Decrease recirculation
2. When used, operate two-
stage filters in parallel.
3. Adjust orifices and splash
plates for coarse spray.
4. Partially open dump gates
at outer end of distributor
arms to provide a stream
rather than a spray.
5. Break up and remove ice
formation Calso see
shortcomings) .
4b. Adjust distributors for more
even flow (remove debris if it
has clogged orifices) .
5a. Remove and clean distributor
nozzles and flush distributor
piping (also see shortcomings) .
5b. Maintain adequate hydraulic
load.
5c. Replace seal.
6a. 1. Chlorinate to produce
residual of 0.5-1.0 mg/1.
2. Flush filter with maximum
recirculation rate.
10
-------
TROUBLESHOOTING GUIDE
TRICKLING FILTERS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. Increase in
clarifier effluent
suspended solids.
7a.
7b.
7c.
7d.
7e.
7f.
7g.
Excessive sloughing
from filter due to
seasonal change.
Excessive sloughing
due to organic
loading.
Excessive sloughing
due to pH or toxic
conditions.
Denitrification in
clarifier.
Final clarifier
hydraulically
overloaded.
Final clarifier
equipment
malfunction.
Temperature currents
in final clarifier.
7a. Seasonal changes
affecting micro-
organisms.
7b. Organic loading.
7c. pH, toxic substances.
7d. Check effluent for
nitrification and
see if sludge floats
in clumps.
7e. Clarifier overflow
rate (should not
exceed 1200 gpd/sq ft
7f. Check for:
1. Broken sludge
collection
equipment.
2. Broken baffles.
3. Uneven flows over
effluent weirs.
7g. Temperature profile
of clarifier.
7a.
7b.
7c.
7d.
7e.
7f.
7g.
Polymer addition to clarifier
influent.
Increase clarifier underflow
rate (also see shortcomings)
Maintain pH between 5.5 and 9.0
and preferably between 6.5 and
8.5.
Increase clarifier underflow
rate.
If due to recirculation, reduce
recirculation rate during peak
flow periods.
Repair or replace broken
equipment; adjust weirs to an
equal elevation.
Install baffles to stop short-
circuiting (see "Secondary
Sedimentation Process," Trouble
shooting Guide and
Shortcomings).
-------
ACTIVATED BIOFILTER (ABF) PROCESS
Process Description
This process uses a combination of both fixed growth systems and
suspended microorganisms as in the conventional activated sludge process.
(Figure 36). The ABF process recirculates settled sludge from the
secondary clarifier. The fixed growth in this system occurs on redwood
racks stacked on top of each other. Through sludge recirculation, a
population of suspended microbes is developed, in addition to the
fixed growth population on the wooden racks. Oxygen is supplied by
the splashing of the wastewater between layers of the wooden racks and
by the movement of the wastewater across the microbial layer of the
racks. The racks usually are stacked to a depth of 14 ft.
An aeration tank like the one described in the activated sludge section
of this manual, is sometimes used between the filter and the secondary
clarifier to provide secondary treatment. This tank is smaller than the
one used for activated sludge and it provides 1-3 hr detention. Part of
the bio-cell underflow passes to the aeration basin and the rest is
returned to the wet well. The short-term aeration basin is a completely
mixed activated sludge unit that helps to oxidize organics and allow
them to settle better in the final clarifier.
The combination of fixed microbial growth and high concentration of
suspended growths provides good operation and few system upsets. The
process has sometimes been added ahead of existing activated sludge basins
to increase plant capacity or efficiency. The ABF process takes up less
room than a trickling filter plant and is less affected by cold temper-
atures.
Typical Design Criteria and Performance Evaluation
Table 4 shows the most common design criteria for the ABF process.
Activated biofilters using an aeration basin can provide treatment as good
as the activated sludge process. The same procedures used in the acti-
vated sludge section of this manual should be used to evaluate the per-
formance of an ABF system.
Control Considerations
The control of the ABF process is much like the control of the acti-
vated sludge process, with the bio-cell serving as a mixed liquor aerator.
The ABF process is usually characterized by good settling activated
sludges (SVI = 70-90). The activated sludge control considerations section
of this manual should be reviewed for information relevant to the ABF process.
96
-------
Process Influent^
Bio-cell
lift station
Fixed film
Bio-cell
Aeration
Flow control
& splitting
Clarifier
, Process
Effluent
Return Sludge
Waste
Sludge
Figure 36. Activated biofilter process schematic.
97
-------
TABLE 4. TYPICAL ACTIVATED BIOFILTER DESIGN CRITERIA
Media depth
Biofilter loading
Aeration basin size
Aeration basin F/M
Aeration basin
organic loading
Bio-cell recycle
Sludge recycle
MLSS
Return sludge
concentration
14 ft
200 Ibs BOD/1,000 cu ft at peak month loading
(1.5 - 5.5 gpm/sq ft)
0.5 - 2 hrs detention
(2.5 - 5 hrs if nitrification required)
0.2 - 0.9
(0.1 - 0.2 if nitrification required)
50 - 225 Ibs BOD/1,000 cf/day
(20 - 40 if nitrification required)
0.5 - 2Q
0.3 - 1Q
2000 - 5000 mg/1
1-3%
98
-------
The amount of recycled flow to the bio-cell lift station is usually
adjusted to account for influent and return sludge flow variations in
order to keep the bio-cell flow rate constant. This constant flow is
maintained to: (a) insure enough aeration of the mixed liquor in the bio-
cell under different organic concentrations, (b) maintain uniform flow
distribution on the media surface, (c) insure consistent sloughing of
fixed film growth, and (d) simplify lift station pump design and operation.
The bio-cell liquid and air flow rates are important process variables
because they affect particle-microorganism mixing, reaeration, biofilm
shear forces, and reactor temperature change. At low hydraulic rates,
reaeration and shear forces are low, which can limit aerobic metabolism
and affect uniform biomass sloughing. Also, at very low hydraulic loadings,
there is the chance for solids settling and accumulating in the reactor.
If hydraulic loadings are less than 0.75 gpm/sq ft, the bio-cell recycle
rate should be increased until this minimum rate is reached. Very high
liquor loading rates, can affect biofilm performance by washing microbes
from the slats. Mixed liquor oxygenation and temperature changes are
affected by air and liquid flow rates. Usually, enough oxygen is supplied
by natural air flow, since only 75 to 100 scfm air/1,000 cu ft media are
required for biologic metabolism during extreme peak demands. At these
low air flow rates, the temperature loss across the bio-cell is small.
Return sludge rates of 50% of the average flow rate and bio-cell
recycle rates of 50% of the average flow rate are most often used.
Common Design Shortcomings and Ways to Compensate and Troubleshooting Guide
Because of the similarity to the activated sludge process, the
shortcomings and troubleshooting guide from the activated sludge section
should be referred to for guidance on ABF systems.
99
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LAGOONS
Process Description
Lagoons, or stabilization ponds, are man-made impoundments that treat
wastewater through the use of sunlight, wind algae, and oxygen. They are
one of the most commonly employed secondary systems and account for about
one-third of all secondary plants in the United States. About 90 percent
of the ponds are used in towns with less than 10,000 people (1-mgd capa-
city) .
Stabilization ponds are simple basins usually built entirely of earth;
they may be dug into natural soil or built above grade by enclosing the
area with earthen dikes after removing the natural topsoil. Usually, raw
wastewater enters the pond at a single point, either in the middle or at
the edge of the pond. The ponds should be deep enough to prevent weed
growth, taut shallow enough to allow mixing by wind currents.
Figure 37 is a simple illustration of how a stabilization pond works
as a wastewater treatment system. Algae grow by taking energy from the
sunlight and using up the carbon dioxide and inorganics released by the
bacteria in the pond. The algae, in turn, release oxygen needed by the
bacteria to add to the oxygen introduced into the pond by wind action.
It is important to make sure that there is enough oxygen in the pond for
aerobic conditions.
As described in the following paragraphs, there are two main kinds
of lagoons that operate almost the same, but differ in depth and in the
type of biological life within the lagoon.
An unaerated or facultative lagoon is usually 3-5 feet deep, with
the organics broken down by aerobic and facultative bacteria. The oxygen
in the pond is furnished by oxygen transfer between the air and the water
surface and by the algae. In general, a facultative pond has an aerobic
zone at the surface, an aerobic and anaerobic zone in the middle depth,
and an anaerobic zone at the bottom.
Aeration equipment is sometimes installed in the pond to provide the
needed oxygen supply. Such a system is called an "aerated lagoon". Air
can be supplied by a compressor that injects air into the pond through
tubing installed on the pond bottom or by mechanical aerators installed
at the surface of the pond. Aerated ponds are usually about one-fifth
the size of a conventional lagoon and are 10 to 15 feet deep.
100
-------
Raw wastes
Settleable
solids
Algae
Oxygen
V
Carbon dioxide, Ammonia, Phosphate, Water
Bacteria
Anaerobic
Figure 37. Treatment processes in lagoons.
101
-------
Typical Design Criteria & Performance Evaluation
Ponds are sometimes designed with several cells in parallel to dis-
tribute the wastewater better. This design also avoids localized zones
of high oxygen demand caused by uneven deposits of sludges. Several
smaller parallel cells also reduce the wave problems that can occur in
larger ponds. Ponds are sometimes placed in series for strong wastes or
to permit use of the last pond in a series as a polishing step to provide
higher removals of suspended solids. Pond effluent is sometimes recircu-
lated to improve mixing and reduce odors.
Typical design parameters for various types of lagoons are shown in
Table 5.
Facultative lagoons can often produce a monthly average effluent
8005 concentration of less than 30.0 mg/1 during most of the year. However,
effluent BOD concentrations tend to be higher during the winter months,
particularly where ice covers the pond creating anaerobic conditions.
Spring overturns can cause increases in effluent BOD5 concentrations.
Well designed, maintained, and operated facultative ponds can pro-
duce a high quality effluent. Although these systems are subject to
seasonal upsets, they are capable of producing a low Biochemical Oxygen
Demand (6005) effluent that can be polished by several different processes.
In general, the effluent suspended solids concentrations of faculta-
tive lagoons follow a seasonal pattern; concentrations are high (40-80
mg/1) during summer months when algal growth is high, and also during the
spring and fall overturn periods when settled suspended solids are resus-
pended from bottom sediments due to mixing. Most of the suspended solids
discharged from a facultative lagoon are algal cells which may not be
particularly harmful to receiving streams. In areas where effluent sus-
pended solids standards are low, some type of polishing process can be
used to reduce suspended solids concentrations to acceptable levels.
Aerated lagoons which are properly designed, operated and maintained
can produce an effluent that is low is SS, and which consistently has a
BOD5 concentration of less than 30 mg/1. Aerated lagoon effluent SS con-
centrations are variable (20-100 mg/1), but BOD5 concentrations are not
affected much by changes in seasons.
In evaluating the performance of a lagoon, the evaluator should
check the system records to see that effluent BOD^ and SS concentrations
are generally within the expected ranges described previously.
If the performance is not as good as expected, the following steps
should be taken:
1. Determine the design criteria for the pond system.
102
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TABLE 5. DESIGN PARAMETERS FOR STABILIZATION PONDS (Metcalf and Eddy, 1972).
o
OJ
Parameter
Detention time, days*
Depth , ft
PH
o
Temperature range, C
o
Optimum temperature , C
BOD5 loading,
Ib/a ere/day
8005 conversion,
percent
Principal conversion
products
Algal concentration,
mg/liter
Oxidation
Pond
10-40
3-4
6.5-10.5
0-40
20
60-120
80-95
Algae , C02 /
bacterial
cell tissue
80-200
Facultative
7-30
3-6
6.5-9.0
0-50
20
15-50
70-95
Algae, CO^ ,
CH^ bacter-
ial cell
tissue
40-160
Type of Pond
Me chan i ca 1 ly
Aerated
Facultative
7-20
3-8
6.5-8.5
0-50
20
30-100
80-95
C02, CH4
bacterial
cell tissue
10-40
Mechanically
Aerated
Lagoons
3-10
6-20
6.5-8.0
0-40
20
80-95
CO2, bacterial
cell tissue
*Depends on climatic conditions and in cold weather areas detention times as great as 120
days are used for facultative ponds.
-------
Type of Pond, Facultative
Pond Dimensions
Area, A = 872,356 ft or 20 acres
Depth, D = 3 ft
Volume, V = A x D = 60 acre-ft
Flow = 1.0 mgd
BOD = 300 mg/1
2. Determine the detention time of the wastewater within the pond.
Detention Time = (V in acre-ft) (7.48 gal/ft3) (43,560 ft2/ac)
(Flow in gpd)
= (60) (7.48) (43,560)
(1 x 10b)
= 20 days
For a facultative lagoon, Table 5 shows that a 20 day detention
time is within the acceptable 7 to 30 day range.
3. Determine the organic loading for the pond.
Organic Loading = (Flow in mgd)(BOD, mg/1)(8.34 Ib/gal)
(Area in Acres)
= (1) (300) (8.34)
(20)
= 125 Ibs BOD/day/acre
By checking Table 5, it is evident that the organic loading of
the lagoon is higher than normally expected. If the pond is
not performing as desired, the design shortcomings and trouble-
shooting guide which follow should be read for possible solutions
to this problem.
Control Considerations
Properly designed and operated lagoons usually are capable of produc-
ing high removal of organic material, solids and bacteria. Primary
treatment is sometimes used as pretreatment, but the added cost is usually
not justified. Aerated lagoons may be followed by settling tanks and
sludge recirculation to the lagoon influent, much like the activated sludge
process. The lagoons may be designed and operated to provide complete
treatment of the wastewater by allowing complete evaporation of the inflow.
To achieve best results, lagoons must be operated to provide enough
mixing to distribute the influent and settleable solids throughout the
pond. In unaerated ponds, mixing is provided by wind blowing across the
water surface as well as inlets and outlets.
104
-------
For the light to moderately loaded lagoon, sludge usually does not
accumulate in large quantities, although there may be small deposits
near the inlet and deposits in cold weather over wider areas. For mod-
erate to heavily loaded lagoons, sludge accumulation may be more signi-
ficant and may need removal and disposal. The accumulation of sludge must
be carefully controlled since the performance of the pond will be reduced,
as measured by the SS content of the effluent.
Common Design Shortcomings and Ways to Compensate
Shortcomings
1. Poor effluent quality due
to organic overloading.
2.
3.
Ice formation resulting in
poor effluent quality.
Short-circuiting of the
flow resulting in poor
effluent quality.
Solutions
1. a) Install aeration equipment.
b) Provide mechanical mixing
equipment.
2. a) Install diffused air system.
b) Provide for winter storage.
3. a) If short-circuiting is wind-
induced, install wind screen.
4.
5.
6.
Loss of lagoon volume caused 4.
by sludge deposits.
Excess turbidity from
storm flows, may interfere
with light penetration and
affect pond performance.
Anaerobic conditions.
7. Dike erosion. 7.
J. Animals burrowing into the 8.
dikes.
b) Revise piping arrangements.
If accumulation is due to excessive
land and street drainage, reduce
loading via maintenance and repair
of sewer system.
Remove storm flow from sanitary
sewer lines by disconnecting storm
inlets.
a) In the case of unaerated ponds,
provide aeration.
b) Divert flow from another aero-
bic pond to it, or pump high
D. O. make-up water to it.
Plant proper grass cover or add
rip rap.
a) Place a layer o^ sand or fine
gravel on the inner slope
(coarse gravel shall not be
used since it tends to breed
mosquitos).
1.05
-------
9.
10.
Shortcomings
Continual accumulations of
scum and floating material
collecting in pond corners.
Solutions
8. b) If 8-a) fails, trapping may
be required.
9. Install a spray with a small
pump to supply a spray of water
to dispose stagnant accumulations.
A single point of entry into 10.
a pond tends to overload the
pond in the feed zone, re-
sulting in odors.
Utilize multiple entry and single
exit approach to distribute the
organic load evenly throughout
the pond.
11.
Thin surface layer of scum
forms on calm days.
11. Provide surface agitation to
break up layer.
106
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TROUBLESHOOTING GUIDE
LAGOONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Poor effluent quality.
la. Organic overloading.
Ib. Low temperature.
Ic. Toxic material in
influent.
Id. Loss of lagoon volume
caused by sludge
accumulation.
le. Aeration equipment
malfunction.
If. Mixing/agitation
equipment malfunction.
Ig. Excess turbidity
from scum and algal
mats.
Ih. Blockage of light by
excessive plant growth
on dikes.
la.
Ib. Air temperature,
lagoon brown-colored.
Ic. Brown colored lagoon.
Id.
le. Inspect aeration
equipment (if used).
If. Inspect mixing/agita-
tion equipment.
Ig. Turbidity.
Ih. Visual inspection.
la. Add sodium nitrate to lagoon in
order to provide more oxygen or
recirculate pond effluent.
Ib. When 2 or more cells provided,
operate in series.
Ic. Identity and control at the
source.
Id. Remove sludge more frequently.
le. Repair or replace damaged and
worn parts.
If. Repair or replace damaged and
• worn parts.
Ig. (1) Break up scum mats.
(2) Operate transfer pipes less
than *z full so that unob-
structed water surface is
maintained between channels
and ponds.
Ih. Remove plant growth at regular
intervals.
2. Inability to maintain
sufficient liquid
level.
2a. Leakage.
2b. Excessive evaporation
or percolation.
2a. Seepage around dikes.
2b. Detention time in pond
is probably long.
2a. Apply bentonite clay to the pond
water to seal leak.
2b. Divert land drainage or stream
flow into lagoon.
-------
TROUBLESHOOTING GUIDE
LAGOONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3. Odors.
3a. Anaerobic conditions.
3a. D.O., total and dis-
solved sulfides.
3b. Algal blooms.
3b. Check for blue-green
algal growth.
3a. (1) Break up and re-suspend
septic sludge and scum.
(2) If available, use extra cell
to provide a rest for odor-
ous cell.
(3) Add sodium nitrate to pond.
(4) Prechlorinate pond influent.
(5) Recirculate pond effluent.
3b. Add CuS04 at regular monthly
intervals: 10 Ib/MG for alkalin-
ity >50 mg/1; 5 Ib/MG for alkal-
inity <50 mg/1.
4. Foaming and spray in
aerated lagoon.
4. Windy conditions.
4. Visual inspection.
4. Construct a wind barrier around
lagoon.
oo
5. Insect generation.
5a. Layers of scum and
excessive plant growth
in sheltered portions
of the lagoon.
5b. Shallow pools of
standing water out-
side lagoon.
5a. Visual inspection.
5b. Visual inspection.
5a. (1) Weed and scum removal.
(2) Application of insecticides.
5b. Cut vegetation outside lagoon,
and fill in potholes that
collect standing water nearby.
6. Groundwater contamina-
tion.
6a. Leakage through bottom
and/or sides of
lagoon.
6a. Seepage around lagoon
dikes.
6a. Apply bentonite clay to pond
water to seal leak.
7. Animals burrowing into
the dikes.
7a.
7a. Visual inspection.
7a. Alter lagoon level several times
in rapid succession.
(Also see shortcomings).
-------
TROUBLESHOOTING GUIDE
LAGOONS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8. Excessive weeds and
tule growth.
8a. Pond too shallow.
8b. Inadequate maintenance
program to control
vegetation.
8c. Poor circulation.
8a. Visual inspection mos-
guitos in the area.
8b. Maintenance program.
8c. Visual inspection of
flow characteristics.
8a. Deepen all pond areas to at
least 3 feet.
8b. (1) Correct program deficiency.
(2) Install pond lining.
8c. Fluctuate pond level.
9. Low dissolved oxygen
content in pond.
9a. Low algal growth.
9b. Hydrogen sulfides in
pond influent.
9c. Detention time.
9a. Pond grey in color.
9b. Hydrogen sulfide odor.
9c. Detention time is low.
9a. Remove floating weeds and other
debris to increase light pene-
tration.
9b. (1) Chlorinate influent.
(2) Eliminate septic inflow.
9c. Increase detention time.
-------
ROTATING BIOLOGICAL CONTACTORS
Process Description
A bio-disc or Rotating Biological' Contactor (RBC) uses a biological
slime of microorganisms which grow on a series of thin discs mounted side
by side on a shaft (Figure 38). The discs are rotated slowly and partially
submerged in the wastewater. The discs usually are made of lightweight
plastic. The RBC is covered to protect the process from low temperatures and
from bad weather. When the process is first started, the microbes in the
wastewater begin to stick to the disc surfaces and grow there until all
the discs are covered with a 1/16 - 1/8-in layer of biological slime. A
thin film of wastewater and the organisms on the disc get oxygen from the
air as the disc rotates. This film of wastewater then mixes with the rest
of the wastewater, adding oxygen to the treated and partially treated
wastewater. The excess growth of microbes break off from the discs and
flow to the clarifier to be separated from the wastewater. The discs
provide media for the buildup of attached microbial growth, bring the
growth into contact with the wastewater, and aerate the wastewater and
growths in the wastewater reservoir. The speed of rotation can be changed.
The attached growth is like the growth in a trickling filter, except that
the microbes are passed through the wastewater rather than the wastewater
being passed over the microbes. Some of the advantages of both the
trickling filter and activated sludge processes are shared. The process
can achieve secondary effluent quality or better. By placing several sets
of discs in series, it is possible to achieve even higher degrees of
treatment - including biological conversion of ammonia to nitrates
(nitrification).
Typical Design Criteria and Performance Evaluation
The design of RBC systems is based on the disc surface area and the
percent BOD and/or ammonia removal efficiency (see Figures 39 and 40).
Common loading rates for secondary treatment of municipal wastewaters are
2-4 gpd/sq ft of effective media area. At temperatures above 15 C, 90%
nitrification can be obtained at loadings of 1.5 gpd/sq ft. Systems
designed for secondary treatment produce effluent quality like the quality
expected from properly designed and operated activated sludge systems.
Performance data from one RBC system is shown in Figure 41.
The following will serve as an example of a simple step-by-step
procedure for evaluating the performance of an RBC system.
110
-------
SECONDARY CLARIFIER
PRIMARY TREATMENT
SOLIDS DISPOSAL
Figure 38- Typical RBC process schematic.
-------
30 -,
INFLUENT SOLUBLE BOD. mg/l
150 1?0 100
25
20 .
01
d
o
m
ID
_i
00
D
_l
O
to
z
UJ
13
UL
UJ
15 .
10 •
BIO-SURF PROCESS DESIGN CRITERIA
DOMESTIC WASTEWATER TREATMENT
Wastewater Temperature s 1ITC
4-Stage Operation
0 0.5 1.0 1 5 2.0 2.5 3.0 3.5 4.0 4.5
HYDRAULIC LOADING, gpd/sq It
Figure 39. Rotating biological media for secondary treatment.
112
-------
100
95 -
s.
o
m
oc
(9
O
e
o
s
<
85
80
75
70
•2501-150 T120-100-T80
INLET BOD5 CONCENTRATION,
mg/l
MAXIMUM AMMONIA
NITROGEN CONCENTRA-
TION, mg/l
REGION OF
UNSTABLE
NITRIFICATION
gpd/sq ft=41& /m2/day)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
HYDRAULIC LOADING, gpd/sq ft
TEMPERATURE > 13" C
Figure 40. Effect of BOD concentration and hydraulic load on nitrification
in the RBC process.
113
-------
2.0
cnUI
^°
Su.1.0
.CC
ig
LL
UJ
50
40
30
0
1-
z
UJ 20
_l
u.
LL
Ul
10
71
SUSPENDED SOLIDS
BOD,
10 20 30 40 50 60 70 80 90 95
PERCENT OF TIME VALUE WAS LESS THAN
Figure 41. Effluent quality - Gladstone, Michigan.
98
99
114
-------
1. Define the design and operating data for the system.
4 stage system
Effective surface area (single stage) = 39,062 ft
Flow = 0.5 mgd
Influent BOD = 100 mg/1
Effluent BOD = 20 mg/1
2. Determine the hydraulic loading for the RBC system
Hydraulic loading = Total Flow in gpd
(No of stages)(area/stage)
= 5 x 105
(4)(39,062)
= 3.2 gpd/ft2
3. Determine the efficiency of BOD removal for the system.
% BOD removal = (Influent BOD-effluent BOD) x 100
Influent BOD
= (100 - 20) 100
100
= 80%
4. The calculations determined in steps 2 and 3 should be compared
to the expected values shown in Figure 39. As shown in this
Figure, a loading rate of 3.2 gpd/ft with an influent BOD of
100 mg/1 should produce an effluent BOD around 23 mg/1.
In the example problem, the effluent BOD was 20 mg/1, slightly
better than the expected valve. If the removal were much less
than the expected value the troubleshooting guide should be
read.
Control Considerations
Very few decisions must be made by the operator to control the process.
No sludge or effluent recirculation is practiced, so there is no need for
decisions on recycle rates. Sludge should be pumped from the secondary
clarifier at a rate high enough to keep the clarifier from going septic,
but low enough to avoid very dilute sludge. Where multiple units are used
in each stage, flow distribution should be monitored to keep loading
uniform. If actual loadings are much less than design loadings, it may be
possible not to use some of the equipment, in order to reduce operating
costs. Idle units should not be filled with wastewater. A plant designed
for secondary treatment will have DO of 0.5 to 1.0 mg/1 in the first stage
increasing to 1-3 mg/1 in the last stage. Nitrification systems often
range from 1-3 mg/1 in the first stage to 4-8 mg/1 in the last stage. Sec-
ondary systems usually have a gray, shaggy appearing bio-mass while nitrify-
ing systems have much thinner, less shaggy, and a browner growth.
115
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TROUBLESHOOTING GUIDE
ROTATING BIOLOGICAL CONTACTORS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Decreased treatment
efficiency
la. Organic overload.
Ib. Hydraulic overload.
Ic. pH too high or too
low.
Id. Low wastewater
temperatures.
la. Check peak organic
loads - if less than
twice the daily ave.,
should not be the
cause.
Ib. Check peak hydraulic
loads - if less than
twice the daily ave.,
should not be the
cause.
Ic. Desired range is 6.5
- 8.5 for secondary
treatment; 8-8.5
for nitrification.
Id. Temperatures less
than 55°F will reduce
efficiency.
la. Improve pretreatment or expand
plant.
Lb. Flow equalization: eliminate
source of excessive flow;
balance flows between reactors
Ic. Eliminate source of undesirable
pH or add acid or base to adjust
pH. When nitrifying, maintain
alkalinity at 7 times the
influent NH concentrations.
Id. If available, place added
treatment units in service.
Excessive sloughing
of biomass from disci
2a. Toxic materials in
influent.
2b. Excessive pH
variations.
2a. Determine material
and its source.
2b. pH below 5 or above
10 can cause
sloughing.
2a. Eliminate toxic material if
possible-if not, use flow
equalization to reduce varia-
tions in concentration so
biomass can acclimate.
2b. Eliminate source of pH
variations or maintain control
of influent pH.
Development of white
biomass over most of
disc area.
3a. Septic influent or
high H S concentra-
tions.
3b. First stage is
overloaded organic-
ally.
3a. Influent odor.
3b. Organic loading on
first stage.
3a. Preaerate wastewater or add
sodium nitrate or hydrogen
peroxide.
5b. Adjust baffles between first and
second stages to increase
fraction of total surface area
in first stage.
-------
TROUBLESHOOTING GUIDE
ROTATING BIOLOGICAL CONTACTORS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Solids accumulating
in reactors.
4a. Inadequate
pretreatment.
4a. Determine if solids
are grit or organic.
4a. Remove solids from reactors and
provide improved grit removal
or primary settling.
5. Shaft bearings
running hot or
failing.
5a. Inadequate mainten-
ance.
5a. Maintenance schedules
and practices.
5a. Lubricate bearings per manufac-
turers instructions.
6. Motors running hot.
6a. Inadequate
maintenance.
6b. Chain drive
alignment improper.
6a. Oil level in speed
reducer and chain
drive.
6b. Alignment.
6a. Lubricate per manufacturers
instructions.
6b. Align properly.
-------
SECONDARY CLARIFIER
Process Description
A secondary clarifier (Figure 42} is constructed and operated very much
like a primary clarifier, except that the secondary tank follows the bio-
logical treatment process (i.e. trickling filter, activated sludge, etc.).
The function of secondary clarifiers varies with the method of biological
treatment used. Clarifiers following trickling filters are used to separate
biological solids which have broken away from the filter media. Clarifiers
in an activated sludge system, however, serve two purposes; Besides pro-
viding a clarified effluent, they also provide a concentrated source of
return sludge for process control.
Like primary clarifiers, secondary tanks may be round or rectangular
in shape. These tanks may be designed for natural settling or chemically
aided settling with the tank size being related to one of the following:
Surface loading rates
Solids loading rate
Flow-through velocities
Weir placement and loading rates
Retention time of settled sludge
There are several designs of secondary clarifiers, some of which are
shown in Figure 43. Hydraulic systems use inlet suction nozzles connected
to single hollow collector pipes that sweep the entire tank bottom in a
single revolution. This method has the advantage of fast sludge removal
and reduces the chances of getting anaerobic sludge. Floating material
usually is moved to the skimmer by a surface blade attached ~o the sludge
collector.
Typical Design Criteria and Performance Evaluation
Preceding ^sections on activated sludge and trickling filters presents
information on evaluation of the secondary clarifier portion of these
procedures.
Typical design data for secondary clarifiers following different kinds of
treatment processes are presented in Table 6.
For clarifiers following trickling filters, the design is based on
hydraulic overflow rates like those described for primary clarifiers. Design
overflow rates must include recirculated flow where clarified secondary
118
-------
Secondary sedimentation tank.
119
-------
EFFLUENT
SLUDGE
j,— INFLUENT
(a) CIRCULAR CENTER-FEED CLARIFIER WITH A SCRAPER
REMOVAL SYSTEM
INFLUENT
EFFLUENT
SLUDGE
(b) CIRCULAR RIM-FEED, CENTER TAKE-OFF CLARIFIER
WITH A HYDRAULIC SUCTION SLUDGE REMOVAL SYSTEM
3« INFLUENT
EFFLUENT
SLUDGE
(C) CIRCULAR RIM-FEED, RIM TAKE-OFF CLARIFIER
Figure 43. Typical clarifier configurations.
120
-------
TABLE 6. TYPICAL DESIGN PARAMETERS FOR SECONDARY CLARIFIERS
Overflow rate Solids loading
Type of treatment Average Peak Average Peak Depth
gpd/sq ft Ib solids/day/sq ft ft
Settling following
trickling filtration 400-600 1,000-1,200 - - 10-12
Settling following
activated sludge 400-800 1,000-1,200 20-30 <50 12-15
Settling following
oxygen-activated
sludge with primary
settling 400-800 1,000-1,200 25-35 <50 12-15
Allowable solids loadings are governed by sludge settling characteristics.
121
-------
effluent is used for recirculation. Because the influent SS concentrations
are low, tank solids loadings need not be considered.
Clarifiers in activated sludge systems must be designed not only for
hydraulic overflow rates, but also for solids loading rates. This is
because both clarification and thickening are needed in activated sludge
clarifiers. When the MLSS concentration is less than about 3,000 mg/1, the
clarifier size is normally based on hydraulic overflow rates. At higher
MLSS values, the ability of the clarifier to thicken solids becomes more
important, and the solids loading rate becomes more critical in determining
tank size. As a result, the design of clarifiers following the activated
sludge process should be based on average and peak overflow rates and
solids loadings. The combination that gives the largest surface area should
be used, so that good quality may always be obtained.
The performance of secondary wastewater treatment systems is determined
by comparing the quality of the overflow from secondary clarifiers to that
of the incoming wastewater. The biological treatment unit converts some of
the soluble and insoluble organics to suspended organic solids. However,
the treatment process is successful only if these organic solids are removed
in the secondary clarifiers. Secondary clarifier design variables have the
most critical effect on overall plant performance. Therefore, in order to
have good quality plant effluent, the secondary clarifier must be properly
designed.
Control Considerations
As with most wastewater treatment equipment, the efficient operation
of secondary clarifiers depends on the proper operation of other plant
processes. For example, in many treatment plants, the MLSS concentration
will be changed to achieve a desired operating condition in the aeration
tanks without considering the possible adverse effects on the secondary
clarifiers. It is common practice among many operators to carry high MLSS
concentrations to increase SRT, so that more organic matter is oxidized and
sludge mass is reduced. In many cases, the high solids loading rate
associated with high MLSS concentrations will cause the sludge blanket to
rise to a level where solids will be swept over the effluent weirs. Thus,
it is most important that a proper solids balance be kept between the
aeration tank and clarifier.
Stability can be achieved by providing a large enough aeration basin
to reduce changing oxygen demands and unusual shifts in solids inventory.
When the recycle rate is constant, and plant inflow rates are low, the
system solids will tend to shift to the aeration basin since the solids
going to the clarifier is low. However, when the peak flows occur, the
solids will shift to the clarifier. The most important thing is to keep
the solids from filling the final clarifier and spilling over into the
effluent.
In conventionally designed plants, the solids inventory can best be
controlled during maximum and minimum flows, by varying the recycle rate
122
-------
to balance the shift in solids which occurs during changes in hydraulic
load.
The secondary clarifier should not be used as a storage basin for
activated sludge ; the sludge should be removed and a portion of it
returned to the aeration tanks as quickly as possible. Best return rates
are often in the range of 25 to 30% of the secondary inflow rate for most
systems. The sludge level in each clarifier should not be greater than
k the final basin depth. The sludge level is controlled by the sludge
removal rate. Some of the removed solids are wasted from the system and
the remainder is returned to the aeration basins. Excessive sludge
inventory in the secondary clarifiers can lead to loss of sludge over
the clarifier effluent weirs, causing'high effluent solids. The earlier
activated sludge section discusses control of the rate at which sludge
should be removed from the secondary clarifier.
Scum should be properly removed from the secondary clarifier, since
poor scum removal can have bad effects on plant performance. Excessive
skimming will result in too much water being carried over with the scum.
If insufficient scum is removed, it will flow around or under the baffle
and leave the tank in the effluent.
Equal flow distribution should be provided among all available
secondary settling tanks. Even with equal distribution of flow, some
differences in efficiencies may be found between two or more units. With
unequal flow, however, less SS and BOD will be removed overall.
In many cases, the addition of iron, aluminum salts, or polymers can
greatly improve secondary clarifier performance. This depends on the amount,
where it is added in the system, and the flocculant nature of the biomass.
For example, in certain processes such as trickling filtration and extended
aeration, solids may not flocculate and settle well in the secondary
clarifier. In this case, the addition of iron or aluminum salts into the
secondary clarifier may improve overall plant performance. Whenever
chemicals are used in secondary processes, however, the dosage should be
carefully controlled. When too much or not enough coagulant is added,
poor clarification occurs. The addition of iron or aluminum salts can
greatly increase the amount of sludge, so that other ideas for improved
sedimentation should be studied first.
Good control of secondary clarifier operations is also important to
the operation of downstream processes. An effective secondary clarifier
allows better disinfection, reduces the frequency of cleaning chlorine
contact tanks, and provides a clear effluent.
123
-------
Common Design Shortcomings and Ways to Compensate
Shortcomings
Clarifiers in activated
sludge systems are
sensitive to sudden
changes in flow rate
due to influent pumping.
Poor hydraulic distribu-
tion of influent into
several clarifier tanks
will cause some tanks to
become overloaded.
Inability to capture
settleable solids at
high overflow rates.
Poor sludge removal
with conventional
sludge scraping
mechanism.
Solutions
Where flow equalization is not
provided, install multispeed
pumps for in-plant lift stations.
Provide sufficient head loss in
the ports feeding the tanks, in
order to eliminate the effect of
water level variations in the
distribution channel.
Use tube settlers to decrease
the depth of settling or feed
settling aid at high flows.
Use suction-type units which
remove sludge from entire
tank bottom in one revolution.
5. Clarifier too shallow,
side walls less than
10 ft.
5. Use increased rates of return
activated sludge to control
sludge blanket.
124
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TROUBLESHOOTING GUIDE
SECONDARY CLARIFIER
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Sludge floating to
surface of secondary
clarifiers.
la.
Ic.
Id.
"Bulking Sludge" -
Filamentous organisms
predominating in
mixed liquor.
la.
"Rising Sludge" -
Denitrification
occurring in second-
ary clarifiers;
nitrogen gas bubbles
attaching to sludge
particles; sludge
rises in clumps.
Broken or warped
wooden flights.
Sludge collectors
operating too slowly
(septic sludge).
lb.
SVI - If less than
100, l(a) is not
likely cause; micro-
scopic, examination
also can be used to
determine presence of
filamentous
organisms.
Nitrate concentration
in clarifier influent
if no measureable NO
then 1(b) is not the
cause.
Ic. Visual inspection.
Id. Frequency and speed
of sludge collection
(sludge black with
septic odor).
la. (1) Increase DO in aeration
tank if less than 1 mg/1.
(2) Increase pH to 7.
(3) Supplement deficiency of
nutrients so that BOD to
nutrient ratio is no more
than 100 mg/1 BOD to 5
mg/1 total nitrogen; to
1 mg/1 phosphorus; to 0.5
mg/1 iron.
(4) Add 5-60 mg/1 of chlorine
to return sludge until SVI
<150.
(5) Add 50-200 mg/1 of
hydrogen peroxide to
aeration tank until SVI
<150.
(6) Increase SRT.
(7) Increase sludge return
rate.
lb. (1) Increase sludge return rate
(2) Increase DO in aeration
tank.
(3) Reduce SRT.
Ic. Repair or replace flights.
Id. (1) Increase speed or frequenc
of operation of sludge
collectors.
(2) Install skimming baffles
to keep sludge from
entering effluent weirs.
-------
TROUBLESHOOTING GUIDE
SECONDARY CLARIFIER
INDICATORS/OBSERVATIONS
1. Sludge floating to
surface of secondary
clarifiers (cont'd)
2. Pin floe in secondary
clarifier overflow -
SVI is good but
effluent is turbid.
3. Fouling of weirs.
4. Plugging of sludge
ports .
PROBABLE CAUSE
le. Over-aerated sludge
2a. Excessive turbulence
2b. Long SRT
2c. Anaerobic conditions
in aeration tank.
2d. Toxic shock load.
2e. Short-circuiting of
flow allowing solids
to pass over weirs.
2f. Anaerobic side
streams recycled.
3a. Accumulation of
wastewater solids
and/or aquatic plant
growth on weirs.
4a. High content of
heavy compacted
material.
4b. Low velocity in with-
drawal lines.
CHECK OR MONITOR
2b. MLSS
2c. DO in aeration tank.
2d. Microscopically
examine sludge for
inactive protozoa.
3a. Visual inspection
4a. Visual inspection.
4b. Sludge withdrawal
rate and resulting
velocity.
SOLUTIONS
le. Reduce turbulence in
aeration tank.
2a. Reduce aeration agitation.
2b. Increase sludge wasting to
decrease SRT
2c. Increase DO in aeration tank.
2d. Re-seed sludge with sludge
from another plant if possible;
enforce industrial waste
ordinances.
2e. Level weirs to prevent short-
circuiting.
2f. Identify and correct sources
of anaerobic conditions.
3a. 01)- More frequent and thorough
cleaning on surfaces.
(2) Pre-chlorination in
addition to more frequent
and thorough scrubbing.
4a. Loosen compacted material
manually or with liquid or air
pressure jetting.
4b. 01) Backflush clogged lines.
02} Pump sludge more
frequently .
(3\ Revise sludge piping.
-------
TROUBLESHOOTING GUIDE
SECONDARY CLARIFIER
INDICA TORS/OBSERVA TIONS
5. Short-circuiting of
flow through
clarifier
6. Excess torque on
rake mechanism.
7. Deflocculation in
clarifier.
PROBABLE CAUSE
5a. Excessive hydraulic
loading.
5b. Weir not level.
5c. Equipment malfunc-
tion.
5d. Reduced detention
time resulting from
large solids and
grit accumulation.
6a. Excessive load on
sludge scraper.
7a. Toxic or acid wastes
7b. Anaerobic conditions
in aeration tank.
7c. Aeration tank over-
loaded .
7d. Inadequate nitrogen
or phosphorus supply
CHECK OR MONITOR
5b. Visual inspection.
5c. Visual inspection.
5d. Visual inspection.
6a. Scrapers stop; motor
overloading; torque
meter indicating
excessively high
torque reading.
7a. Supernatant above
settled sludge is
uniform in turbidity.
SOLUTIONS
5a. Place more units in service.
5b. Level weir.
5c. Replace or repair damaged
scrapers etc.
5d. (1) Remove excessive solids
accumulation .
(2) Operate grit chamber.
6a. (1) Inspect for and repair/
replace defective and
worn parts.
(2) Remove or break up ice
formation on walls and
surfaces.
(3) Operate collector for
longer period or pump
sludge more often at a
higher rate.
(4) Drain and check basin for
objects which may have
fallen into basin.
7a. Remove source of industrial
waste .
7b. Increase DO in aeration tank.
7c. Place more basins in service.
7d. Supplement deficiency in
nutrients by chemical addition.
-------
TROUBLESHOOTING GUIDE
..'.~wu__.,..>,w ...^v vv.~_ SECONDARY CLARIFIER
INDICATORS/OBSERVATIONS
7. Deflocculation in
clarifier (cont'd)
8. Sludge blanket over-
flowing secondary
clarifier weirs uni-
formly throughout
basin.
9. Billowing sludge.
PROBABLE CAUSE
7e. Excessive shear
caused by turbulence.
8a. Inadequate rate of
sludge return.
8b. Unequal flow distri-
bution to clarifiers
causing hydraulic
overload .
8c. Peak flows are over-
loading clarifiers.
9a. Hydraulic surges.
9b. Density currents.
9c. Stirring by sludge
scrapers.
CHECK OR MONITOR
8a. Sludge return pump
output, or depth of
sludge blanket.
8b. Flow to each
clarifier.
8c. Hydraulic overflow
rates at peak flows
if >1,000 gpd/sq ft
this is a likely
cause.
9a. Visual inspection of
sludge conditions.
SOLUTIONS
7e. Reduce agitation.
8a. (1) If return pump is malfunc-
tioning, place another
pump in service & repair.
(2) If pump is in good condi>
tion increase rate of
return and monitor sludge
blanket depth routinely.
Maintain 1-3 ft depth
blanket. When blanket
increases in depth, in-
crease rate of return.
(3) Clean sludge return line
if plugged.
8b. Adjust valves and/or inlet
gates to equally distribute
flow.
8c. Install flow equalization
facilities or expand plant.
9a. Eliminate hydraulic surges.
9b. Keep sludge depth as low as
possible.
9c. Reduce scraper speed.
to
00
-------
CHLORINATION
Process Description
The most common use of chlorine in sewage treatment is for disinfection,
which usually is the last treatment step in a secondary plant. Where the
treated effluent is fed into a stream to be used for water supply or for
recreational purposes, chlorination is effective in destroying the disease-
producing organisms (called "pathogens") found in treated wastewater.
Other principal uses of chlorine are odor control and control of bulking
in activated sludge.
Chlorine may be fed into the wastewater automatically, with the dosage
depending on the degree of treatment previously given the sewage. The waste-
water then flows into a tank, where it usually is held for about 30 mins to
allow the chlorine to react with the pathogens (Figure 44). Chlorine
often is used either as a gas, or a solid or liquid compound containing
hypochlorite. Hypochlorite has been used mostly in small systems (less
than 5,000 persons), or in large systems, where safety concerns related
to handling chlorine gas outweigh economic concerns. The use of chlorine
has proven to be a very effective means of disinfection.
Chlorine is also used in advanced wastewater treatment (AWT) for
nitrogen removal, through a process known as "Breakpoint Chlorination". For
nitrogen removal, enough chlorine is added to the wastewater to convert all
the ammonium nitrogen to nitrogen gas. To do this, about 10 mg/1 of
chlorine must be added per mg/1 of ammonia nitrogen in the wastewater -
about 40 or 50 times more chlorine than normally used in a wastewater
plant for disinfection only.
The facilities required for the process are simple. Wastewater (after
secondary or tertiary treatment) flows into a mixing tank where the chlorine
is added and complete mixing is provided. Because a large amount of
chlorine i^ used and has an acidic effect on the wastewater, alkaline
chemicals (such as lime) may be added to the same chamber to balance this
effect. The nitrogen gas which is formed is then released to the
atmosphere. The amount of chlorine used for nitrogen control provides
very effective disinfection. Because the process is just as effective in
removing 1 mg/1 as 20 mg/1 of ammonium, breakpoint chlorination often
is used as a polishing step downstream of other nitrogen removal processes.
Typical Design Criteria and Performance Evaluation
Figure 45 shows how residual chlorine effects coliform number. The
curves show the most probable number (MPN) of coliforms remaining
129
-------
PLAN VIEW
WATER DEPTH 10 ft
FLOW
Figure 44 . Chlorine contact chamber with end - around baffles and vanes.
130
-------
100,000-4
10.000-
1000_
100-
10
J I I i L
CONTACT TIME OF 30 MINUTES
o - PRIMARY EFFLUENT
v - SECONDARY EFFLUENT
0123456
CHLORINE RESIDUAL, MG/L MODIFIED STARCH - IODIDE METHOD
Figure 45. MPN Coliform vs. chlorine residual.
131
-------
after 30 mins of chlorine contact in a well-designed chlorine contact
tank. These results should not be considered as being exact.
Table 7 lists chlorine dosages often used for disinfection of raw and
partially-treated sewage.
In breakpoint chlorination, about 10 mg/1 of chlorine must be added
for each mg/1 of ammonia nitrogen present in the wastewater. Studies
show that better pretreatment will reduce the amount of chlorine needed
to reach breakpoint. Table 8 shows how different pretreatment processes
affect the chlorine to ammonia-nitrogen ratio needed for breakpoint
chlorination. The breakpoint process can result in 99+ percent removal of
ammonium nitrogen, reducing concentrations to less than 0.1 mg/1 (as N).
To evaluate the performance of a chlorination system, the evaluator
should check the contact time, chlorine residual and MPN of coliform
organisms after chlorination. This can be done easily in the following
steps:
1. Obtain design and typical operating data for the chlorination
system being studied.
Example:
Type of effluent Activated sludge
Peak plant flow 5.0 mgd
Volume of Cl contact tank, V 13,926 cu ft
Chlorine dosage 6.0 mg/1
Chlorine residual 1.0 mg/1
2. Determine the contact time for the chlorine contact tank
based on peak flow.
Contact time, hrs = V in cu ft x 7.48 gal/cu ft x 24 hrs/day
Flow in gpd
= (13,926) (7.48) (24)
5 x 106
= 0.45 hrs or 27 min
3. Examine the daily disinfection log sheet for chlorine feed
rates and chlorine residual patterns. Compare both contact
time and chlorine residual with those required by the proper
regulatory agency. As a general rule, residuals between 0.2 and
1.0 mg/1 after 15 to 30 min contact times provide good dis-
infection. As shown in the example the 27 min contact time and
1.0 mg/1 residual should be generally sufficient.
4. If the chlorination system does not perform as expected, the
shortcomings and troubleshooting guide should be studied.
132
-------
TABLE 7. CHLORINE DOSAGE RANGES
Waste Chlorine dosage
mg/1
Raw sewage 6 to 12
Raw sewage (septic) 12 to 25
Settled sewage 5 to 10
Settled sewage (septic) 12 to 40
Chemical precipitation effluent 3 to 10
Trickling filter effluent 3 to 10
Activated sludge effluent 2 to 8
Sand filter effluent 1 to 5
133
-------
TABLE 8.
EFFECT OF PRETREATMENT ON Cl- rNfT-N BREAKPOINT RATIO
2 4
Sample
Breakpoint
PH
Initial
NH+-N
(mg/1)
Final
NH+-N
Irreducible
minimum
residual
(mg/1 as Cl )
Breakpoint
ratio
Cl :NH+-N
(weight basis)
Buffered water
Raw wastewater
Lime clarified
raw wastewater
Secondary effluent
6-7
6.5-7.5
6.5-7.5
6.5-7.5
20
15
11.2
8.1
Laboratory Tests
0.1
0.2
0.1
0.2
0.6
7
7
3
8:1
9:1-10:1
8:1-9:1
8:1-9:1
Lime clarified
secondary effluent 6.5-7.5
Ferric chloride
clarified raw
wastewater-
carbon adsorption 3. 2
Filtered secondary
effluent 6-8
Lime clarified raw
wastewater-filtered 7.0-7.3
Alum clarified
oxidation pond
effluent-filtered 6.6
9.2
0.1
10.2 0.1 20
Pilot Plant Tests
12.9-21.0 0.1
9.7-12.5 0.4-1.2
2-8.5
20.6
0.1
7.6
8:1
8.2:1
8.4:1-9.2:1
9:1
9.6:1
-------
Control Considerations
In general, the better the treatment plant is operated, the easier it
will be to disinfect the effluent. Any failure to provide adequate treat-
ment will increase the bacterial count and the chlorine requirement.. High
solids content and soluble organic loads increase the amount of chlorine
needed.
Effective chlorine disinfection is dependent upon the combined effect
of chlorine dosage, mixing and contact time with the wastewater. Enough
disinfectant should be added to always meet the bacterial quality required
by the regulatory agency. Control of the disinfection process is
accomplished by measurement of the chlorine residual.
Proper mixing is one of the most important factors in chlorine dis-
infection. Applying chlorine to wastewater in a well mixed system produces
a much better effluent than a system where chlorine is fed without rapid
mixing, even with adequate residual and contact time. However, sufficient
contact time (usually 30 min) between the chlorine and the wastewater
also is needed to provide good disinfection. Usually, longer contact
times are more important than higher residuals in wastewater treatment.
In breakpoint chlorination, the system must be able to meet quick
changes in ammonia nitrogen concentrations, chlorine demand, pH, alkalinity
and flow. Failure to properly control chlorine dosage can result in
poor nitrogen removal, and chlorine overdoses. Overdoses of chlorine are
a direct waste of this chemical and cause problems in adjusting the
operation of the dechlorination equipment. Overdoses also can cause the
direct discharge of high concentrations of chlorine residuals to the
receiving water, and can result in the undesirable formation of NCI .
Usually, a base chemical is added to the breakpoint process to
neutralize some of the acidity resulting from the chlorine addition. The
base requirements depend on wastewater alkalinity, individual treatment
processes used before breakpoint chlorination as well as effluent pH or
alkalinity restrictions by regulatory agencies.
Another consideration in breakpoint chlorination is dechlorination
to remove the chlorine residual from the final effluents before it is
discharged. Very often, dechlorination using sulfur dioxide or activated
carbon may be needed when the breakpoint chlorination process is used.
In most cases, control of breakpoint chlorination requires the use
of accurate and reliable automatic equipment to reduce the need for manual
process control by operators. However, the operator must give special
attention to this equipment and monitoring devices in order to insure their
proper operation.
135
-------
Common Design Shortcomings and Ways to Compensate
Shortcomings Solution
1. Big changes in effluent
chlorine residual when
chlorine flow proportion-
ing control device is
operating properly.
2. Short-circuiting in
chlorine contact
tank.
3. High residual chlorine
concentrations in the
effluent toxic to
aquatic life.
4. Sodium hypochlorite can-
not be stored for long
periods of time without
deteriorating.
5. Lack of mixing.
Install a continuous chlorine
residual analyzer to control
the feed rate automatically,
or use a closed loop system.
2. Make channels very narrow or
provide thorough baffling in
channel to insure complete
mixing and a sufficient
contact time.
3. Install dechlorinating systems
(activated carbon, hydrogen
peroxide, sulfur dioxide,
sodium metabisulfite).
4. If long storage periods cannot
be avoided, dilute the sodium
hypochlorite to slow down the
rate of deterioration, or use
liquid (gas) chlorine as an
alternate source.
5. Install mechanical mixer.
136
-------
TROUBLESHOOTING GUIDE
CHLORINATTON
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Low chlorine gas
pressure at chlorina-
tor.
la. Insufficient number
of cylinders con-
nected to system.
Ib. Stoppage or flow
restriction between
cylinders and chlor-
inators.
la. Reduce feed rate and
note if pressure
rises appreciably
after short period
of time. If so, la
is the cause.
Ib. Reduce feed rate and
note if icing and
cooling effect on
supply lines contin-
ues.
la. Connect enough cylinders to the
system so that chlorine feed
rate does not exceed the with-
drawal rate from the cylinders.
Ib. Disassemble chlorine header
system at point where cooling
begins, locate stoppage and
clean with solvent.
No chlorine gas
pressure at chlorina-
tor.
2a. Chlorine cylinders
empty or not connect-
ed to system.
2b. Plugged or damaged
pressure reducing
valve.
2a. Visual inspection.
2b. Inspect valve.
2a. Connect cylinders or replace
empty cylinders.
2b. Repair the reducing valve after
shutting off cylinder valves,
and decreasing gas in the
header system.
Chlorinator will not
feed any chlorine.
3a. Pressure reducing
valve in chlorinator
is dirty.
3b. Chlorine cylinder
hotter than chlorine
control apparatus.
3a. Visual inspection
3b. Cylinder area tem-
perature.
3a. 1. Disassemble chlorinator and
clean valve stem and seat.
2. Precede valve with a filter-
sediment trap.
3b. 1. Reduce temperature in
cylinder area.
2. Do not connect a new
cylinder which has been
sitting in the sun.
4. Chlorine gas escaping
from chlorine pres-
sure reducing valve
(CPRV).
4a. Main diaphragm of
CPRV ruptured due to
1. Improper assembly
or fatigue.
2. Corrosion
4a. Place ammonia bottle
near termination of
CPRV vent line to
confirm leak.
4a. 1. Disassemble valve and
diaphragm.
2. Inspect chlorine supply
system for moisture
intrusion.
-------
TROUBLESHOOTING GUIDE
CHLORINATION
INDICATORS/OBSERVATIONS
5. Inability to main-
tain chlorine feed
rate without icing
of chlorine system.
6. Chlorination system
unable to maintain
water-bath tempera-
ture sufficient to
keep external CPRV*
open.
7. Inability to obtain
maximum feed rate
from chlorinator.
PROBABLE CAUSE
5a. Insufficient evapora-
tor capacity.
5b. External CPRV car-
tridge is clogged.
6a. Heating element
malfunction.
7a. Inadequate chlorine
gas pressure.
7b. Water pump injector
clogged with
deposits.
7c. Leak in vacuum
relief valve.
7d. Vacuum leak in
joints, gaskets,
tubing, etc. in
chlorinator system.
CHECK OR MONITOR
5a. Reduce feed rate to
about 75% of evapora-
tor capacity. If this
eliminates problem 5a
is the cause.
5b. Inspect cartridge.
6a. Evaporator water-
bath temperature .
7a. Gas pressure.
7b. Inspect injector
7c. Disconnect vent line
at chlorinator; place
hand over vent con-
nection to vacuum
relief valve, observe
if this results in
more vacuum and high-
er chlorine feed
rate.
7d. Moisten joints with
ammonia solution, or
put paper containing
orthotolidine at each
joint in order to
detect leak.
SOLUTIONS
5b. Flush and clean cartridge.
6a. Remove and replace heating
element.
7a. Increase pressure - replace
empty or low cylinders.
7b. Clean injector parts using
muriatic acid. Rinse with
fresh water and replace in
service .
7c. Disassemble vacuum relief
valve and replace all springs.
7d. Repair all vacuum leaks by
tightening joints, replacing
gaskets, replacing tubing and/
or compression nuts.
u>
CO
*Chlorine Pressure Reducing Valve
-------
TROUBLESHOOTING GUIDE
INDICATORS/OBSERVATIONS
8. Inability to maintain
adequate chlorine
feed rate.
9. Wide variation in
chlorine residual
produced in effluent.
10. Chlorine residual
analyzer recorder
controller does not
control chlorine
residual properly.
PROBABLE CAUSE
8a. Malfunction or de-
terioration of water
supply pump.
9a. Chlorine flow propor-
tion meter capacity
inadequate to meet
plant flow.
9b. Malfunctioning auto-
matic controls.
9c. Solids settled in
chlorine contact
chamber.
9d. Flow proportioning
control device not
zeroed or spanned
correctly.
lOa. Electrodes fouled.
lOb. Loop- time too long.
CHECK OR MONITOR
8a. Inspect pump.
9a. Check chlorine meter
capacity against
plant flow meter
capacity.
9b.
9c. Solids in contact
chamber .
9d. Check zero and span
of control device on
chlorinator.
lOa. Visual inspection.
lOb. Check loop-time.
CHLORINATION
SOLUTIONS
8a. Overhaul pump (if turbine pump
is used, try closing down
needle valve to maintain proper
discharge pressure) .
9a. Replace with higher capacity
chlorinator meter.
9b. Call manufacturer's field
service personnel.
9c. Clean chlorine contact chamber.
9d. Re-zero and span the device in
accordance with manufacturer's
instructions .
lOa. Clean electrodes.
lOb. Reduce loop time by doing the
following:
1. Move injector closer to
point of application .
2. Increase velocity in sample
line to analyzer cell.
3 . Move cell closer to
sample point.
4. Move sample point closer to
point of application.
-------
TROUBLESHOOTING GUIDE
INDICATORS/OBSERVATIONS
10. Chlorine residual
analyzer recorder
controller does not
control chlorine
residual properly
(Cont'd)
11. Coliform count fails
to meet required
standards for
disinfection .
PROBABLE CAUSE
lOc. Insufficient potas->
slum iodide being
added for amount of
residual being
measured.
lOd. Buffer additive
system malfunctioning
lOe. Malfunctioning of
analyzer cell.
lOf. Poor mixing of
chlorine at point of
application.
lOg. Rotameter tube range
is improperly set.
lla. Inadequate chlorina-
tion equipment
capacity.
lib. Inadequate chlorine
residual control.
lie. Short-circuiting in
contact chamber.
CHLORI NATION
CHECK OR MONITOR
lOc. Potassium iodide
dosage.
lOd. See if pH of sample
going thru cell is
maintained.
lOe. Disconnect analyzer
cell and apply a
simulated signal to
recorder mechanism.
lOf. Set chlorine feed
rate at constant
dosage and analyze a
series of grab
samples for consist-
ency.
lOg. Check tube range to
see if it gives too
small or too large
an incremental change
in feed rate.
lla. Check capacity of
equipment .
lib. Continuously record
residual in effluent.
lie. Contact time.
SOLUTIONS
lOc. Adjust potassium iodide feed to
correspond with residual being
measured.
lOd. Repair buffer additive system.
lOe. Call authorized service per-
sonnel to repair electrical
components .
lOf. Install mixing device to cause
turbulence at point of
application.
lOg. Replace with a proper range of
feed rate.
lla. Replace equipment as necessary
to provide treatment based on
maximum flow through plant.
lib. Use chlorine residual analyzer
to monitor and control the
chlorine dosage automatically.
lie. 1. Install baffling in contact
chamber .
2 . Install mixing device in
contact chamber.
-------
TROUBLESHOOTING GUIDE
CHLORINATION
INOICA TORS/OBSERVA TIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
11. Coliform count fails
to meet required
standards for
disinfection (Cont'd)
lid. Solids build-up in
contact chamber.
lie. Chlorine residual too
low.
lid. Visual inspection.
lie. Chlorine residual.
lid. Clean contact chamber to reduce
solids build-up.
lie. Increase contact time and/or
increase chlorine feed rate.
12.
,hlorine residual
too high in plant
effluent to meet
requirements.
12a. Chlorine residual too
high.
12a. Determine toxicity
level by bioassay
procedures.
12a. Install dechlorination facility
(See Shortcomings).
-------
OZONATION
Process Description
Ozone has been used for disinfection of water since 1900 and has
recently found increased use for disinfection of wastewaters. Some of
the advantages of using ozone rather than chlorine include:
its high germicidal effectiveness, even against resistant
organisms such as viruses and cysts;
on decomposition, the only residual material is more
dissolved oxygen;
no dissolved solids, such as chlorides, are added;
its disinfecting power is not affected by pH or ammonia
content.
Ozone has also been used to control odors from treatment units.
In these cases, ozone is applied to the exhaust air, given a short period
of contact, and then discharged to the atmosphere. The reaction takes place
very quickly, and is effective in destroying organic odors. Doses
usually are between 1 and 2 mg/1 by volume, with a contact time ranging
from a few seconds to half a minute.
There are three basic ways to generate and use ozone in waste-
water treatment: (1) generation from air, (2) generation from oxygen and
recycle oxygen to the ozone generation system, and (3) generation from
oxygen used for oxygen activated sludge system and recycle oxygen to the
activated sludge system. Figure 46 pictures these different methods.
The once-through air approach uses conventional air drying techniques
such as compression and refrigeration, followed by desiccant drying.
Ozone generated from air is usually 0.5 to 2.0% by weight, but is usually
produced at 1.0 weight percent. Ozone is typically mixed with waste-
water in a contact basin as shown in Figure 47. Fine bubble diffusers
are used to feed the ozone into the basin. Packed beds have also been
used as contactors. Following treatment in the covered ozone contactor,
the gas is decomposed to prevent high concentrations of ozone from
being released to the atmosphere.
The oxygen recycle approach may be used to recover valuable oxygen-
rich off-gas from the contractor when very pure oxygen is fed to the
ozone generator. High-purity oxygen gas sometimes is used since it
142
-------
CHILLED
WATER
CLEAN AIR
DISCHARGE
WASTE WATER
COMPRESSOR
T
OZONE
DECOMP.
DEVICE ->
DRYER]
ONCE-THROUGH AIR PROCESS
TREATED
WATER
PURGE
OXYGEN RECYCLE LINE
OZONE DECOMPOSITIO
DEVICE
OXYGEN
GENERATOR
WASTE
WATER
CHILLED
COMPRESSOR WATER
OXYGEN RECYCLE PROCESS
T
WATER
CONTACTOR
DRYER
T
TREATED
WATER
OXYGEN
GENERATOR
j— n U-OPTIONAL
> X COMPRESSOR
OZONE
GENERATOR
OZONE DECOMPOSITION
DEVICE
INFLUENT
WASTE »
WATER
OXYGEN
ACTIVATED
SLUDGE
REACTOR
SECONDARY
CLARIFIER
• EFFLUENT
I WASTE
| WATER
OZONE
CONTACTOR
I J L I
ONCE-THROUGH OXYGEN PROCESS
Figure 46. Alternative ozonation systems.
143
-------
Influent
3-
To ozone
destruct
Upward velocity
< 0.5 fps
, Entrance
baffle
HTTTTT
TTTTTTTTTT
TTT
/
/
Downward
velocity
< 0.5 fps
•Exit baffle
Figure 47. Typical ozone contact basin using porous diffusers.
144
-------
enables the generator to produce two to three times as much ozone per
unit time, and uses only about half as much generator power per pound
compared to ozone produced from air.
Once-through oxygen is the simplest method of ozone wastewater dis-
infection. Dry oxygen is produced on-site by a cold air separation
process or by pressure-swing adsorption and then fed to the ozone generator.
Wastewater is next treated by ozonation in the contactor and, after
destruction of the unreacted ozone, the off-gas is used elsewhere in the
plant. It may, for example, be used in a biological reactor or fed
into incinerators for sludge disposal.
There are three basic types of commercially available ozone generators:
The Otto plate, the tube and the Lowther plate (Figure 48). While there
may be some differences in these units, the following describes the basic
systems.
1. Otto Plate Type: The ozonator has several parts arranged as
follows: a cast-aluminum, water-cooled block that acts as the ground
electrode, a glass plate dielectric, an air space, another glass dielectric
and a high voltage stainless steel electrode. A complete "unit" would
include the mirror image of dielectrics, air space and a grounded,
water-cooled electrode.
Air is blown into the ozonator and enters the discharge gap
where the air turns into ozone. The ozonized air is drawn through a man-
ifold pipe with holes cut in the center of each of the electrodes and
dielectrics. One drawback of the Otto plate-type generator is that
operation is limited to low pressure.
2. Tube Type: The tube type generator consists of several tubular
units. The outer electrodes are stainless steel tubes fastened into
stainless steel tube spacers and surrounded by cooling water. Centered
inside the stainless steel tubes are tubular glass dielectrics whose
inner surfaces are coated with a conductor which acts as the second
electrode. The stainless steel outer tubes are arranged in parallel and
are sealed into a cooling water distribution system.
3. Lowther Plate-Type: This generator is very different from the
Otto plate-type. The Lowther generator is air cooled, and operates on
either air or oxygen feed.
The basic unit is a gas-tight "sandwich" made up of an aluminum
heat dissipator, a steel electrode coated with a ceramic dielectric, a
glass spacer to set the discharge gap, a second ceramic coated steel
electrode with an oxygen inlet and an ozone outlet which exists through a
second aluminum heat dissipator. These basic units are pressed together
in a frame and manifolded for oxygen and ozone flow. Cooling is
accomplished by a fan moving outside air across the heat dissipators.
This is the most commonly used system in the U.S.
145
-------
Glass dielectrics
Water cooled aluminum
block ground electrode •
Air-
•* 1^)
I
.^Discharge gap
k - ^°3
Stainless steel high voltage electrode
High voltage
electrode
Discharge gap
OTTO PLATE GENERATOR
Water cooled stainless
steel ground electrode
*-03
GlaSS tube
TUBE TYPE GENERATOR
High voltage
steel electrode
— .
Aluminum heat
dissipator
Ceramic dielectric
*^
Ground steel
electrode
Glass separator
Ceramic dielectric
coated steel
electrode
LOWTHER PLATE TYPE GENERATOR
Figure 48. Ozone generator types.
146
-------
Thus, the equipment required for ozonation falls into three major
parts: air preparation, ozone generation, and ozone injection.
Typical Design Criteria and Performance Evaluation
Coliform standards of 200 fecal coliform/100 ml can usually be met in
secondary effluents by ozone dosages of about 5 mg/1. Standards calling
for almost complete coliform removal (to 2/100 ml total coliform) may
require dosages of 15 mg/1 or more. As with any disinfectant, exact
dosages are a function of secondary effluent quality, efficiency of
mixing, and contact time.
Key design variables related to ozone generators are summarized in
Table 9. Ozone contact systems are typically designed for a minimum
ozone utilization of 90% and disinfection contact time of about 15 min.
Methods used for feeding ozone into water include porous diffusers,
emulsion turbines, and injectors working on the venturi principal.
Besides being used as a disinfectant, ozone may also produce an
effluent with less color and turbidity.
As with chlorine disinfection, the performance evaluation of ozonation
systems should involve contact time and MPN of coliform in the effluent
just before discharge. The best indicator of performance is the MPN
of coliforms in the effluent as it compares to requirements of the
regulatory agency. If the system does not provide acceptable treat-
ment, the troubleshooting guide on ozonation should be reviewed.
Control Considerations
Ozone is a highly toxic gas and the manufacturer's safety instructions
should be carefully followed.
To get the most ozone from the ozone generator:
1. The voltage should be kept relatively low while reasonable oper-
ating pressures are maintained. Keeping voltage low protects the dielectric
and/or the electrode surfaces from the high voltage failure.
2. High frequency a-c should be used. High frequency is less
damaging to the dielectric surfaces than high voltage. This decreases
maintenance requirements and increases the useful life of the machine,
while producing increased ozone yields.
3. Heat removal should be as efficient as possible.
Ozone generation rates are determined by the feed gas oxygen content,
the feed gas flow rate, and the corona power. The feed gas oxygen content
usually cannot be changed. Gas flow rate can usually be varied within a
pressure range of about 8-15 psig. The corona power is determined by the
147
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TABLE 9. OZONE GENERATOR DESIGN CRITERIA
Comparison of Commercial Ozona tors- Typical Ozonator Operating Characteristics
Type
Otto
Tube
Low the r
Feed
air
air
oxygen
air
oxygen
Dew Point
of feed,°F
-60
-60
-40
Pressure
Cooling psig
water 0
water' 3-15
air 1-12
Discharge
gap , in .
0.125
0.10
0.05
Voltage
kv, peak
7.5-20
15-19
8-10
Frequency
Hz
50-500
60
2000
Dielectric
thickness, in.
0.12-0.19
0.10
0.02
Ozone Generator Power Requirements*
Type
Otto
Tube
Low the r
*for 1 percent ozone/
per Ib.
Air feed
kwhr/lb
10.2
7.5-10
6.3-8.8
higher ozone
Oxygen feed
kwhr/lb
3.75-5.0
2.5-3.5
concentrations require greater power
148
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frequency of the electrical pulses which trigger the corona cells. A
balance must be kept between ozone concentration and operating costs.
The loser ozone concentrations (1-2%) are less costly to produce, but
higher concentrations usually give the best results.
The volts, amps, cell temperature, line pressure, gas flow rate
should be recorded each shift with daily determination made of ozone
production using calibrated ozone monitors and dew point of feed gas.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. No scum removal or foam 1. Install skimmer.
control installed in
contact basin.
2. No self-contained 2. Purchase self-contained
breathing apparatus breathing apparatus for
« provided near but safety - do not use chemical
outside ozone generator cartridge respirators.
area.
3. Ozone concentration 3. Install indicator/recorders
indicator/recorder not with range of 0-6% ozone.
provided for gas
exiting the ozone
generators and on off-
gas from contactor.
149
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TROUBLESHOOTING GUIDE
OZONATION
INDICATORS/OBSERVATIONS
1. Ozone generator
overheats and shuts
down.
2 . No voltage or current
to generator.
3. Full voltage, no
current.
4. Low ozone production.
PROBABLE CAUSE
la. Fan or cooling system
malfunction.
2a. Silicon controlled
Rectifier (SCR) fuse
blown.
2b. Control circuit
blown.
2c. Interlock circuit
failure.
2d. No main power.
3a. Master oscillator
malfunction.
3b. Control circuit
malfunction.
4a. Feed gas dew point
high.
4b. Decreased oxygen
purity of feed gas.
4c. Significant number
of cell fuses blown.
CHECK OR MONITOR
la. Fan louvers for
obstruction.
Ib. Fan for free rotation
Ic. Fan belts for tight-
ness and condition.
2c. 1. Make sure all
doors or panels
with interlocks
are closed.
2 . Check interlock
switches such as
gas flow.
2d. Remote, main
breakers.
3a. Fuses.
3b. 1. Relays & heaters.
2. Fan rotation.
4a. Check dew point.
4b. Feed gas oxygen
content.
4c. Check fuses.
SOLUTIONS
la. Clean louvers.
Ib. Lubricate fan bearings or
remove obstructions hampering
rotation.
Ic. Tighten or replace belts.
2a. 1. Replace SCR fuse.
2. Replace surge arrestor.
3. Replace SCR's.
2b. 1. Find and repair fault.
2. Replace control fuses.
2c. 1. Replace panel or door inter-
lock switches.
2. Check reset mechanisms,
establish proper gas flow.
2d. Reset main breaker.
3a. 1. Locate fault & repair.
2. Replace fuses.
3b. 1. Reset relays.
2. See Ib.
4a. Find leak in feed gas system
and repair.
4b. Find leak in feed gas system
and repair .
4c. Check fuses.
Ul
o
-------
TROUBLESHOOTING GUIDE
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Low ozone production
(Cont'd)
4d. Cell exteriors dirty.
4e. Low gas flow.
4d. Cell modules and
cleanliness of
cooling air.
4e. Check gas flow rate
and pressure.
4d. Clean cell modules in accor-
dance with manufacturers
instructions; insure cooling
air is clean.
4e. Establish proper feed pressure.
Ozone odor
detectable.
5. Ozone leak.
5a. Manifold and flange
connections - paper
towels soaked with
potassium iodide will
turn purple when held
next to ozone leak.
Tighten or repair faulty
connection.
Low voltage which
falls to zero when
operation starts.
6. Blown rectifier
fuses.
6. Fuses.
6a. Find/repair fault.
6b. Replace fuses.
7. Full voltage, 1/2
current, increased
noise level.
7. Master oscillator
malfunction.
7. Shut down at once and replace
with spare units.
8. Inadequate disinfec-
tion achieved.
8a. Low ozone dosage.
8b. Secondary effluent
quality has
degraded.
8c. Diffusers partly
plugged.
8a. Ozone generator
output.
8b. Secondary effluent
turbidity.
8a. Increase dosage (see item 4).
8b. Improve operation of secondary
plant.
8c. Clean diffusers or replace as
necessary.
-------
FILTRATION
Process Description
In the filtration process, wastewater is passed through a filtering
medium, such as fine sand or coal, in order to remove suspended or colloidal
matter. The main purpose of filtration in tertiary treatment is to remove
suspended solids from a secondary effluent or from effluent following the
coagulation - sedimentation process. Filtration reduces turbidity and
improves the chlorine disinfection process.
The two most commonly used types of filters in wastewater treatment
are the gravity and pressure filters. Figures 49 and 50 show these two
types of filters and their major parts.
Usually, wastewater is passed downward through the filter medium.
After sometime, the filter becomes plugged with material removed from the
wastewater, and the filter must be cleaned by reversing the flow ("back-
washing"). The upward backwash rate must be high enough that the media
particles are suspended and the wastewater solids are washed from the bed.
These backwash wastewaters (usually less than 5% of the wastewater flow
treated) must be recycled to the wastewater treatment plant for processing.
Filter beds usually are 30-36 in deep and made of relatively small
particles (less than 1.5 millimeters in size). However, some filters use
deeper beds and more coarse materials. Modern wastewater filters usually
are made up of a mixture of two or three different media (coal, sand, and
garnet are commonly used) of varying sizes and specific gravities. These
materials form a filter (called a "multimedia" or "mixed media" filter),
which is coarse at the top of the filter and becomes more fine with depth.
This coarse to fine mixture allows solids removed from the water to be
stored throughout the bed, rather than only at the top surface. Also,
this design requires less frequent backwashings than single media beds.
Typical Design Criteria and Performance Evaluation
The major design variables are:
Filter configuration (gravity or pressure)
Media type, size, and depth
Filtration rate
Backwash system
Closed type filters allow influent pressures above atmospheric,
while open filters have only the water pressure over the bed to overcome
152
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WASH TROUGHS
FILTER SAND
CAST-IRON
MANIFOLD
Fl LTER
TANK
FILTER FLOOR
Figure 49. Typical gravity filter.
AIR RELEASE
INFLUENT a
BACKWASH WASTE
SURFACE WASH
EFFLUENT &
BACKWASH
7>-"
FILTER DRAIN-
Figure 50- Typical pressure filter.
153
-------
filter headlosses. Pressure units are generally used where high final
headlosses are expected or where the added head will allow effluent to pass
through downstream units without repumping. They are most often used in
small-to-medium-sized treatment plants where steel-shell package units are
economical.
Filter media designs usually consist of dual media filters with 15 in
of coal (about 1.8 mm in diameter) over 15 in of sand (0.55 mm) or mixed
media with about 16 in of coal, 9 in of sand, and 4 in of garnet. Design
criteria used for a typical mixed media system are shown in Table 10.
The most common filtration rates used in wastewater treatment range
from 3 to 6 gpm/sq ft of filter area.
Backwash systems for filters usually are operated at rates of about
15 gpm/sq ft for 5-10 min. To insure complete cleaning, filters often
have rotary surface wash devices which are operated for 1 to 2 min before
the backwashing flow is started. Many surface wash systems have a revolving
pipe with several nozzles attached. These units are set 1 to 2 in above the
normal surface of the filter. Washwater under 50 to 100 psi pressure is
fed through the nozzles, causing the pipe to rotate during the backwash
cycle. The amount of water needed for these devices usually is in the range
of 0.75 to 1.0 gpm/sq ft. Rather than use surface wash devices, some systems
inject air into the backwashing system.
Some of the major items that determine actual filter performance
include:
maximum available headloss
filtration rate
influent characteristics
media characteristics
design of backwash system
Of these, the single most important factor is the quality of the
influent to the filter. When filtering secondary effluent, if the bio-
logical system always operates very well, good filter performance can be
expected. However, if the biological system often is upset, filtration
will be much more difficult. Because filter performance is affected so
much by the quality of its influent, a simple performance evaluation may
be determined as follows:
1. Calculate the filter area and maximum filter rate: (using the
filters in Table 10).
Maximum flow rate = 15 mgd = 10,417 gpm
1440 min
Filter Area = 4 x 22 ft x 24 ft = 2112 sq ft
Max Filter Flow Rate = 4.93 gpm/sq ft
This filter rate falls within the normal range of 3-6 gpm/sq ft.
154
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TABLE 10. DESIGN CRITERIA FOR ORANGE COUNTY WATER DISTRICT
OPEN GRAVITY, MIXED MEDIA FILTER SYSTEM
Dimensions:
Bed construction:
Media
Anthracite Coal
Silica sand
Garnet sand
Garnet gravel
Garnet gravel
Silica gravel
Silica gravel
Silica gravel
4 filters each
22 ft x 24 ft (plan area)
media depth = 30 in
Depth
(in)
16.5
9
4.5
1.5
1.5
2
2
2
Specific
gravity
1.6
2.6
4.0
4.0
4.0
2.6
2.6
2.6
Grain size
range (mm)
0.84-2.00
0.42-0.84
0.18-0.42
1
2.00-4.76
3.18-6.36
6.36-12.72
12.72-19.08
Leopold blocks
Surface hydraulic
loading rate: 4.93 gpm/sq ft at 15 mgd (2604 gpm per filter)
Max operating
headless: 10 ft
155
-------
2. Calculate the amount of backwash water that is being used.
Four filters in operation, each at 2640 gpm
One backwash/filter/day of 7 min duration at 7920 gpm rate/filter
Volume of backwash water = 7920 gpm x 7 min x 4 filters
= 221,760 gal
Filter Throughput = (1440 min - 7 min) x 2640 gpm x 4
= 15.13 mg
Percent backwash water = 221,760 x 100 = 1.5%
15,130,000
A backwash water percentage of greater than 3% indicates that the
system performance is questionable and that either:
1) The solids being applied to the filter are excessive,
2) Filter aid dosages are excessive, reducing filter runs,
3) The filter surface wash system is not working or not
being operated long enough per backwash cycle, or
4) Excessively long backwash is being used.
3. Identify the type of process (activated sludge, trickling
filters or chemical coagulation) that comes before filtration.
4. From operational data, determine the average BOD and SS con-
centrations in the filter influent and effluent.
5. Using the information gathered in steps 1 and 2, refer to Table 11
for activated sludge plants, and Table 12 for trickling filter
plants. Compare the expected filter performance shown in these
tables with the actual filter data collected in step 2. When
treating chemically coagulated and settled effluent, the filter
effluent should be less than 1 turbidity unit. If the filter
does not provide acceptable treatment, the shortcomings and
troubleshooting guide on filtration should be checked for more
details.
Control Considerations
Modern filter systems usually have equipment for feeding polymers as
filter aids, and instruments that continuously monitor and record the
turbidity of the filter effluent. Filter operation depends on the flow
rate through the filter, which, in turn, is related to the filtered waste-
water pressure differential. Control should be based on effluent quality
and differential pressure.
Filter Aid Dosage Control—
Filter performance can be improved by adding filter aids such as
polymer and/or alum. The process is improved by:
156
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TABLE 11. EXPECTED FILTER PERFORMANCE FOR ACTIVATED SLUDGE PLANTS
Good biological treatment
Filter influent Filter effluent
Type of activated BOD SS BOD SS Run time
sludge process mg/1 mg/1 mg/1 mg/1 hr
Conventional and
extended aeration 12-15 15-25
Contact stabilization 15-20 15-25
2-5 1-4 16-24
5-10 1-5 12-20
Conventional and
extended aeration 20-35
Contact stabilization 30-45
Fair biological treatment
30-50
25-50
5-10
20-25
5-10
5-10
6-12
6-10
TABLE 12. EXPECTED FILTER PERFORMANCE FOR TRICKLING FILTER PLANTS
% soluble BOD removed in secondary process
85%
Filter Filter
influent effluent
BOD SS BOD SS
80%
Filter Filter
influent effluent
Run time BOD SS BOD SS
mg/1 mg/1 mg/1 mg/1 hr
30-40 30-^0 20-30 15-20 6-11
mg/1 mg/1 mg/1 mg/1
40-50 35-45 30-40 20-25
Run time
hr
5-9
157
-------
Strengthening the floe
Controlling the depth of floe penetration into the beds
Improving the clarity of the filtered water
Increasing the maximum allowable flow rate through the bed
The amount of filter aid needed increases with lower water temperature,
higher flow rates through the filters, and higher turbidities. The optimum
dose of filter aid should be based on the desired filter headloss when
turbidity breakthrough is about to happen. Too much filter aid shortens
the length of filter runs by increasing the rate of headloss too quickly.
If not enough filter aid is added, turbidity breakthrough occurs before
maximum allowable headloss is reached.
Filter aid coats the grains of fine filter media. In placing a new
filter into service, it may take 3 or 4 filter runs to get the best coating.
Also, 3 or 4 filter backwashes may be needed to reduce the coating in a
filter if the dosage is reduced. The operator should be careful in
reducing filter aid dosages because of the left-over effects of previous
coats on the bed, and the time lag before the full effect of the reduced
dosage is noticed.
Turbidity, Flow and Headloss Monitoring—
The effluent turbidity of each individual filter should be continuously
recorded, and the results used to control the rate of filter aid added to
each unit. Also, if the effluent turbidity is too high, the filter should
be taken out of service and backwashed. However, this should not happen
too often since high headloss through a filter bed usually is the basis
for backwashing.
The rate of flow through each filter unit should be monitored, and the
total plant flow divided equally among all filter units in service. It is
good practice to use all available filters regardless of flow. Sudden
changes in filter flow should be avoided, since hydraulic surges tend to let
particles pass through the bed. As a result, changes in flow should be
made slowly.
Filter Backwashing—
Effective cleaning of the filter media during backwash is very important
to successful plant operation. If the bed is not cleaned well, a large
collection of biological organisms could cause plugging problems. Good
backwashing aided by surface wash or air water backwash can prevent plugging.
The length of time needed for backwashing usually is about 5 to 8 mins.
Because all filter backwash wastewater must be reprocessed, the percentage
of recycled backwash water is checked to see how well the system operates.
When the backwash water percentage is greater than 3%, system performance is
not good and the troubleshooting guide should be checked to identify and
correct the problem.
158
-------
Common Design Shortcomings and Ways to Compensate
1.
2.
Shortcoming
Air introduced into the
gravel support bed can
overturn the gravel and
disrupt filter operation.
This may occur when air
collects in the pump
column between back-
washings.
Operating difficulties
resulting from surging
wastewater flows.
Solution
Air can be eliminated by:
1) Starting the pump against a
closed backwash valve and
releasing the air through a
pressure release valve in the
backwash line.
2) Placing an air release valve
at the high point in the
washwater line with a separate
pressure water connection to
the washwater line at the
high point to keep it full of
water and expel the air.
Provide flow equalization among
all available filters.
3. When a single-media sand ;
filter is used, more
frequent backwashing
is required than for multi-
media filters.
Replace sand filter bed with
multi-media bed.
4. Inadequate filter cleaning 4.
results in plugging.
Provide adequate backwashing
assisted by surface wash or
air-water backwash.
5. Significant hydraulic
surges result if filter
backwash wastewater is
recycled directly back
to the rapid mix basin.
Collect flows in a backwash
water receiving basin and
recycle flows at controlled
rate.
159
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TROUBLESHOOTING GUIDE
FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Effluent turbidity
too high.
la. Filter needs back-
washing.
Ib. Inadequate upstream
chemical coagulation
la. Turbidity in excess
of 1 JTU.
Ib. Run jar tests, deter
mine proper coagulan
dosage,
la. Remove filter from service and
backwash it.
Ib. Feed proper coagulant dosage.
High headless through
filter bed.
2. Filter needs back-
washing.
2. High headless
through filter.
2. Remove filter from service
and backwash it.
High headloss through
a filter just
backwashed.
3a. Insufficient back-
wash time to
thoroughly clean
the filter media.
3b. Inoperative surface
wash arm or air
scouring system.
3a. Initial headloss
greater than normal
(1-2 ft).
3b. Visually inspect
surface wash arm.
3a. Increase the setting on the
backwash timer to provide
longer backwash period.
3b. Repair surface wash arm or
air scouring system.
4. Percentage of back-
wash water recycled
exceeds 5%.
4a. Solids carryover to
filter basin is
too high.
4b. Filter aid dosages
too high.
4c. Surface wash system
not working.
4d. Surface wash system
not being operated
long enough per
backwash cycle.
4e. Backwash used is
too long.
4a. SS concentration.
4b. Filter aid dosage.
4c. Visual inspection of
surface wash system.
4d. Length of surface
wash cycle.
4e. Length of backwash
4a. Provide better treatment in
settling tanks by improving
solids settling characteristics
4b. Reduce filter aid dosage.
4c. Repair surface wash system.
4d. Increase surface wash system
time.
4e. Reduce length of backwash cycle
-------
TROUBLESHOOTING GUIDE
INDICATORS/OBSERVATIONS
5. Clogging of filter
surface indicated by
very rapid increase
in headloss after
backwash.
6. Short filter runs.
7. Filter effluent
turbidity increases
suddenly but filter
headloss is low.
PROBABL :. CAUSE
5. Rapid accumulation
of solids on the
top surface of the
media due to :
a. Inadequate prior
clarification for
single-media sand
filters.
b. Excessive filter
aid dosages in
dual or mixed -
media filters.
c. Surface wash or
backwash is in-
adequate .
6a. High headloss
caused by surface
clogging.
6b. (See item 4a)
7a. Inadequate dosage
of polymers as
filter aid.
7b. Coagulant feed
system malfunction.
7c. Change in
coagulant demand.
CHECK OR MONITOR
5. Rate of headloss
buildup.
6. Visual inspection,
headloss through
filter.
7a. Excessive turbidity.
7b. Chemical feeders.
7c. Run jar tests.
SOLUTIONS
5a. Improve pre treatment or change
to dual or mixed-media filters
to provide greater porosity at
the top of the filter.
5b. Reduce or eliminate filter aid
dosage to allow particles to
penetrate deeper into the bed.
5c. Provide adequate surface wash
and backwashing.
6a. Replace sand media, with dual
or mixed- media.
6b. Reduce filter aid dosage.
6c. Use polymer as a filter aid
to control rate of headloss
buildup.
6d. Be sure adequate surface wash
and backwash is provided.
7a. Increase polymer dosage.
7b. Repair feeders.
7c. Adjust coagulant dosage.
-------
TROUBLESHOOTING GUIDE
FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8. Mud ball formation.
Inadequate backwash
flow rate and
surface wash.
8. Visual inspection.
Provide backwash flow rate
up to 20 gpm/sq ft, and
maintain proper auxiliary
scour (surface wash).
9. Gravel displacement.
Introduction of air
into filter under-
drain in the back-
wash water.
9. Visual inspection of
gravel bed.
9. If displacement is severe,
filter media may have to be
replaced. Limit the total
flow and head of water avail-
able for backwash (also see
shortcomings).
10. Loss of media during
backwashing.
cr>
KJ
lOa. Excessive flows
used for backwashing
lOb. Auxiliary scour
excessive.
lOc. Air bubbles attach-
ing to coal causing
it to float. - (See
item 12).
lOa, Backwash rate.
lOb. Backwash program.
lOa. Reduce rate of backwash flow.
lOb. Cut off the auxiliary scour
1 to 2 rain before the end of
the main backwash.
11. Difficult to clean
filter adequately
in warm weather at
normal backwash
rates.
11. Decreased viscosity
of backwash water
due to higher
temperatures.
11. Increase backwash rate until
desired bed expansion during
backwash is achieved.
-------
TROUBLESHOOTING GUIDE
FILTRATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
12. Air binding causing
headloss to increase
prematurely.
en
10
12. Air bubbles accumu-
late within bed due
to:
a) Filter influent
containing dis-
solved oxygen at
or near satura-
tion levels being
subjected to less
than atmospheric
pressure in
filter.
b) Lowered water
level and stop-
page of flow
through filter
during prepara-
tion for back-
washing reduces
pressure and
releases gases.
12. Rapidly increasing
headloss through
filter.
12a. Provide more frequent back-
washing to prevent bubbles
from accumulating as quickly.
12b. Maintain maximum water depths
above the beds.
-------
MICROSCREENING
Process Description
The most common type of microscreen used in wastewater plants is the
horizontal rotating drum. The variable speed drum is shaped like a
cylinder and made of finely woven stainless steel or synthetic cloth. It
is mounted in a tank so that wastewater enters the inside of the drum from
one end and flows outward through the straining fabric, as shown in
Figure 51.
The drum is usually about two-thirds underwater. Solids are stored
on the inner surface of the drum as the flow passes through the drum.
These solids are then backwashed with screened effluent into a trough
inside the drum, where the solids are then returned to the head of the works.
Most meshes are woven type 316 18/8 stainless with openings ranging
from 23 to 60y. The microscreen usually is used to remove suspended solids
from secondary effluent. They also have been used successfully on combined
sewer overflows. Microscreens are not effective for removal of chemical
floe nor are they used on raw sewage due to problems with grease buildup
on the screen.
Typical Design Criteria and Performance Evaluation
The hydraulic capacity of the microscreen depends on the rotational
speed (50-150 fpm peripheral) of the drum, the area of which is submerged;
head applied across the screen (about 12 in in tertiary treatment); rate
of mesh clogging; and backwash efficiency. Generally, loading rates in
tertiary applications are 5-10 gpm/sq ft of submerged area with 6-12 in of
headloss. When applied to combined sewer overflows, rates as high as 20-40
gpm/sq ft with heads of 48 in are used. Backwash flow requirements are
usually about 5% of the influent flow, but may range from 3-25%. Detailed
design information may be found in the article "Designing Microscreens?
Here's Some Help" in the April, 1976 issue of Water and Wastes Engineering.
Information also may be obtained from the manufacturers of these systems.
Where the influent is less than 35 mg/1, microstraining can produce
an effluent with less than 10 mg/1 of solids. The throughput rates are
very dependent on changes in influent solids. Increases from 35 mg/1 to
200 mg/1 would probably reduce the throughput rate by a factor of 4 or 5.
Thus, it is important to keep a fairly constant concentration of SS in
the influent. The performance of the screen will be affected by the
following:
164
-------
Shower header and nozzles -v I
Head
differential
Solids collection trough
Outlet structure
Solids lift paddle
Figure 51. Schematic of typical microscreen.
165
-------
1. Biological Loading of the Upstream Process - Highly loaded plants
(for example, in activated sludge, a sludge retention time (SRT) of three
days or less) would provide less removal at the screen and higher screen
effluent suspended solids concentration.
2. Final Clarifier Hydraulic Loading - Higher clarifier loadings
often result in poorer screen effluent quality, but higher percentage
removal of solids at the screen.
3. Hydraulic Head Across Screen Media - As the head increases
from hydraulic or solids loading, removal efficiency and effluent quality
suffer. The additional head forces some of the particles through the media
that otherwise would have been strained out.
4. Upstream Hydraulics - Excessive headless or velocity in the hydrau-
lic passages just before the screen may result in the breakup of agglomerated
particles, causing effluent quality and removal efficiency to be reduced.
5. Effectiveness of Solids Collection System - Since many of the
screened particles are composed of several smaller particles that have been
agglomerated, the force of the spray water may break up some of the particles
small enough to let them pass through the media. If these particles go to
the drum pool rather than the collection trough, they may pass through the
media causing a poor effluent quality.
6. Media Aperture - A small amount of the total suspended solids are
within the size range of 20 to 36y, and because of this,effluent quality
is reduced slightly.
7. Screen Peripheral Speed - Increases in speed often cause decreases
in head across the media, and improve the effluent quality and removal
efficiency. Beyond a certain speed however, the head across the media
increases because of reduction in efficiency of the solids collection system
at the higher speed. Also, floe breakup becomes more of a problem at the
higher speed. Best effluent quality is possible at speeds ranging from
70 to 140 fpm.
Control Considerations
Except for drum speed there are not many things the operator can
control with the microscreening process. If the drum travels too fast,
it will not do an effective screening job and may cause excess wear on the
mechanical parts. If it runs too slowly, the fabric will become clogged
and the water level difference between the influent and the effluent may
become so great that the fabric breaks. A clogged screen may also cause
the influent to bypass the microscreen. There are units which automatically
increase drum speed as the headloss across the screen increases. General
guidelines for good control of operation are:
Operate the microscreens at the slowest possible speed or at
a rate with the manufacturers' recommendations for water level
166
-------
differential between influent and effluent.
Ensure that the microscreen receives the best quality influent
that can be produced by secondary treatment.
Provide good preventive maintenance on mechanical equipment.
Provide daily inspection of the operating units.
Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Accumulation of settled 1.
solids in influent chamber
which can become anaerobic
and float to the surface
temporarily overloading
unit.
2. Grease frequently plugging 2.
the screen.
3. No flow measurement 3.
installed on shower water
line making it difficult
to determine effective-
ness of screen cleaning.
4. Screen capacity marginal. 4.
5. High pressure (60-120 psi)
shower system installed
causing shortened media
life.
6. Constant speed drive
motor used on screen
making it difficult to
cope with variations in
influent solids.
7. Large objects inadvert-
ently entering and
damaging screen.
Reduce size of influent chamber.
Clean screen according to manufacturers
instructions and improve scum removal
in primary settling tank.
Install flow meter on shower line.
Increase available head to maximum
allowable for specific screen.
Increases in screen opening size or
increases in submerged depth usually
do not help significantly.
Operate system normally at 30 psi,
but once per week operate at 60-120
psi.
Convert to variable speed to
allow speeds of 80-150 fpm to be
used.
Install one-inch square mesh
ahead of microscreen.
167
-------
Shortcomings
8. Solids loading to 8.
screens too high to
achieve desired
throughput.
9. Persistent problem with 9.
slime accumulations on
screen surface.
Solution
Install added secondary clarifier
capacity or system to use chemical
coagulation/flocculation/sedimentation
ahead of screen.
Install ultra-violet light irradiation
unit directed at screen exposed after
backwashing. Special filters are
used to eliminate ozone-producing
wavelengths which could cause metal
corrosion.
168
-------
TROUBLESHOOTING GUIDE
MICROSCREENING
INDICATORS/OBSERVATIONS
1 . Gradual decrease in
throughput rate due
to slime growths on
screen.
2. Leakage at ends of
drum causing poor
efficiency.
3. After shutdown perioc
screen capacity is
low.
4. Drive system running
hot or noisy.
5. Drum rotation
erratic.
6. Sudden increase in
screen effluent
solids.
PROBABLE CAUSE
1. Inadequate cleaning.
2. Seals leaking.
3. Screen fouled.
4. Inadequate lubrica-
tion.
5. Drive belts not
adjusted properly
or worn out.
6a. Hole in screen or
fabric securing
screws loose.
6b. Solids collection
trough overflowing.
CHECK OR MONITOR
la. Backwash pressure
low (<_ 30 psi) .
Ib. Spray nozzles partly
plugged.
2. Seals.
4. Oil level.
5. Drive belt tension
and condition .
6a. Surface of screen.
SOLUTIONS
la. Increase backwash pressure to
60 to 120 psi until cleaned or
add hypochlorite ahead of
screen.
Ib. Clean nozzles and convert to
automatic, self-cleaning
nozzles.
2. Increase tension on sealing
bands gradually until leakage
is eliminated; replace bands
if work excessively.
3 . Never allow screen to dry out
in dirty condition; clean with
sodium hypochlorite solution
to restore capacity.
4. Fill to required level with
recommended oil.
5. Adjust drive belt to proper
tension.
6a. Repair screen fabric per manu-
facturer's instructions or
tighten screws as appropriate.
6b. Reduce wastewater flow rate.
-------
TROUBLESHOOTING GUIDE
MICROSCREENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. Screen capacity can-
not be restored by
high pressure flush-
ing or hypochlorite
treatment.
7. Film of iron or
manganese oxide has
accumulated.
7. Screen surface.
7. Clean screen with inhibited
acid cleanser per manufacturers
instructions.
-------
ACTIVATED CARBON ADSORPTION
Process Description
The activated carbon process is almost always a part of some larger
wastewater treatment operation. The main purpose of activated carbon adsorp-
tion is the removal of soluble organics. There are two ways to use
activated carbon: (1) as advanced wastewater treatment (AWT), or (2) as
independent physical-chemical treatment (IPC).
As an AWT process, activated carbon is used after secondary treatment,
and sometimes after coagulation, sedimentation and filtration, in order to
remove soluble organic material which is difficult to remove biologically.
These "difficult to remove" materials are often called "refractory organics"
and are measured by the COD (chemical oxygen demand) test.
In the IPC processes, biological secondary processes are not used at
all, and the carbon is the only way of removing soluble organics. In the IPC
system, the raw wastewater is usually coagulated and settled (and sometimes
filtered) before it is passed through the carbon system. This process
removes more organics than biological secondary, but not as much as bio-
logical secondary followed by carbon adsorption.
Wastewater treatment with activated carbon involves two process
operations, a contact system, and a regeneration system. The water passes
through a container filled either with carbon granules or with a carbon
slurry. Impurities are removed from the water by adsorption when there is
enough contact time. The carbon system usually has several columns or
basins used as contactors. Most contact columns are either open concrete
gravity-type systems, or steel pressure containers applicable to either
upflow or downflow operation (Figures 52 and 53 ).
After a while, the carbon loses its adsorptive capacity. The carbon must
then be regenerated by taking the contactor out of service and connecting
it to the regeneration system. Fresh carbon is sometimes added to the
system to replace carbon lost during hydraulic transport and regeneration.
The regeneration process is explained in a later section of this manual.
Activated carbon used for wastewater treatment may be either in a
granular form (about 0.8 millimeter in diameter, the size of a fairly coarse
sand) or in a powdered form. Granular carbon adsorption is used by passing
the wastewater through beds of the carbon housed in columns. These carbon
beds usually provide 20-40 mins contact between the carbon and the wastewater.
Because powdered carbon is very fine, it is not used in columns, but
171
-------
Eflluent
Sand
Gravel
Filter blocks
Drain
Figure 52. Gravity contactor.
172
-------
CARBON INFLOW
SURFACE OF CARBON
/U -^V^-'Y^-'^r-X^ -PRESSURE VESSEL
'/.': :•...':;.• --•:•'• :-v :'^££:££:':#£^
OUTLET SCREENS
(8)
TANGENTIAL NOZZLES (4)
INLET SCREENS (8)
CARBON OUTFLOW
Figure 53. Pressurized contactor capable of using upflow or downflow
operation.
173
-------
instead is added to the wastewater and removed by coagulation and settling.
Because of difficulties in powdered carbon regeneration and recovery, it is
not used in wastewater treatment as often as granular carbon.
Typical Design Criteria and Performance Evaluation
Table 13 and 14 show design parameters from several activated carbon
systems. (Table 13 shows design specifications for IPC plants (secondary
plants, reblacing biological processes) and Table 14 shows the same type
of data for AWT plants (tertiary plants).
The most important things that affect activated carbon performance
are contact time and pretreatment. Longer contact times and greater amounts
of pretreatment usually result in higher effluent quality and lower carbon
dosage requirements. Table 15 compares dosages and carbon effluent quality
for various kinds of pretreatment. Performance data from one plant designed
to treat secondary effluent is shown in Table 16.
COD removals in the carbon columns may vary from 40 to 75% from day
to day. The absolute value of the effluent COD in milligrams per liter is
a more important measure of process efficiency, however. The Ibs of COD
removed per Ib of carbon (ranges from 0.5 to 2.0 Ibs) is the most useful
measure of efficiency because it shows the whole operation of the carbon
adsorption and regeneration systems.
Principals for evaluating the performance of activated carbon systems
should involve contact time, hydraulic loading, and pretreatment effects.
The following will serve as an example of step-by-step procedures for
evaluating the performance of an activated carbon adsorption system:
1. Define the dimensions and design data for the carbon system.
Upflow Contactor
Pretreatment: Filtered Secondary Effluent
Column Flow Rate = 650 gpm
Column Diameter, dia = 12 ft
Column Area, A = (TT/4) dia = 113 sq ft
Carbon Depth = 26'2"
Column Volume, V=A x Depth = 22,140 gal
2. Determine the contact time of the carbon column and compare it
to the designed contact time.
. . . Carbon column volume in gal
Contact time (mm) = — : —
Flow rate in gpm
= 22,140
650
= 34 min
In general, contact times should range between 20 and 40 min.
Longer contact times may produce a lower effluent COD, color and
perhaps, a lower carbon dosage. However, contact times can
174
-------
Ul
Contactor size
Carbon size
Carbon inventory
Backwash rate
Surface wash
Air scour
Vessel type
Regeneration rate
Furnace*
Corrosion protection
TABLE 13. DESIGN SPECIFICATIONS OF SOME IPC PLANTS
(Secondary plants, replacing biological plants)
Carbon requirements
Plant size
Hydraulic loading
Contact time
Bed depth
Rocky River,
Ohio
500 Ibs/MG
10 MOD
4 . 3 gpm/sq ft
26 minutes
15 ft
Owosso,
Michigan
600 Ibs/MG
6 MGD
6.2 gpm/sq ft
36 minutes
30 ft
Garland,
Texas
1800 Ibs/MG
30 MGD
2 . 4 gpm/sq ft
30 minutes
10 ft
Niagara Falls,
New York
750 Ibs/MG
60 MGD
1.5 gpm/sq ft
40 minutes
8 ft
16 ft dia x 25.3 ft 12 ft dia
8 x 30 mesh
736,870 Ibs
15-20 gpm/sq ft
stainless steel,
rotating spray
none
pressure-downflow
500 Ibs/hr
72 OD 8
rubber lining
12 x 40 mesh
246,480 Ibs
N/A
N/A
N/A
pressure-upflow
416 Ibs/hr
54 ID 6
20 x 47.5 ft
2,600,000 Ibs
N/A
N/A
N/A
gravity-upflow
concrete and
stainless steel
40 x 20 x 18 ft
gravity-downflow
concrete
After burner
Wet scrubber
yes
no
no
yes
yes
no
*72 OD 8 means 72" outer diameter and an 8-hearth furnace
-------
TABLE 14. DESIGN SPECIFICATIONS OF SOME AWT PLANTS
(Tertiary plants/ upgrading biological plants)
Colorado Springs,
Colorado
Pomona,
California
South Lake Tahoe,
California
Carbon requirements
Plant size
Hydraulic loading
Contact time
Bed depth
Contactor size
Carbon size
Carbon inventory
Backwash rate
Surface wash
Air scour
Vessel type
Regeneration rate
Furnace*
Corrosion protection
After burner
Wet scrubber
250 Ibs/MG
3 MOD
5 gpm/sq ft
30 minutes
20 ft
20 ft dia x 20 ft
8 x 30 mesh
250,000 Ibs
20 gpm/sq ft
pressure-downflow
75 Ibs/hr
30 ID 6
yes
350 Ibs/MG
0.3 MGD
7 gpm/sq ft
40 minutes
38 ft
6 ft dia x 16 ft
12 x 40 mesh
12 gpm/sq ft
pressure-downflow
110/hr
30 ID 6
coal tar epoxy
yes
yes
250 Ibs/MG
7.5 MGD
6.2 gpm/sq ft
17 minutes
14 ft
12 ft dia x 14 ft
8 x 30 mesh
500,000 Ibs
N/A
N/A
N/A
pressure-upflow
250 Ibs/hr
54 ID 6
coal tar epoxy
available
yes
*30 ID 6 means 30" inner diameter and a 6-hearth furnace.
-------
TABLE 15. THE EFFECTS OF PRETREATMENT ON CARBON DOSAGE
AND CARBON COLUMN EFFLUENT QUALITY
Pretreatment
Carbon
contact
Carbon dosage Ib/MG
SS, mg/1
BOD, mg/1
COD, mg/1
TOC, mg/1
Color, units
Turbidity, JU
Primary
Downflow series
2 beds 4 beds
15 min 30 min
1,200 800
10 5
20 10
65 45
20 10
__-_ — __
Secondary, plus-
plain
filtration
Downflow
4 series bed
20 min
500
<1
<1
12
3
4
1.5
Chemically
flocculated and
filtered secon-
dary effluent
Up flow
countercurrent
17 min
250
<1
<1
12
3
4
0.5
TABLE 16. TYPICAL WATER QUALITY BEFORE AND AFTER GRANULAR
ACTIVATED CARBON TREATMENT AT SOUTH TAHOE
Quality parameter
Carbon column
Influent Effluent
BOD (mg/1)
COD (mg/1)
TOC (mg/1)
MBAS (mg/1)
Color (units)
3
24
12
0.85
15
12
3
0.13
4
177
-------
3.
become so long as to cause anaerobic conditions (no oxygen) in
the carbon columns. Anaerobic conditions in a carbon contactor
can cause odors.
Determine the hydraulic loading rate and note if the calculated
loading is within the typical range of 2 to 8 gpm/sq ft.
Hydraulic loading rate
4.
_ Flow rate in gpro
Surface area of column
650
113
= 5.75 gpm/sq ft
Numerous tests have shown that the efficiency of the carbon is
not affected by hydraulic loading rate (at a given contact time)
for rates in the range of 2 to 8 gpm/sq ft.
Review available effluent quality data and, if needed, collect
samples from the carbon column influent and effluent, and analyze
the samples for the following:
TOC
Soluble Organic Carbon
SS
BOD
COD
Color
Turbidity
The results should be compared to those shown in the following
table or Table 15, taking into account pretreatment considerations.
Description
TOC Removed, percent
Soluble Organic Carbon
Removal, %
Soluble Organic Carbon
Removed per Ib Active
Carbon, Ib
Filtered Secondary
Effluent
45 - 55
40 - 45
0.19 - 0.20
Unfiltered Secondary
Effluent
50 - 60
45 - 50
0.22 - 0.23
5. If carbon effluent quality is not acceptable, the troubleshooting
and shortcomings section should be read.
Control Considerations
The rate of adsorption of organics found in municipal wastewater usually
increases as the pH of the water decreases. Adsorption is very poor at pH
values above 9.0. High pH wastewaters should be neutralized before carbon
adsorption, and the influent pH should be kept fairly constant. A sudden,
178
-------
upward shift in pH can lead to desorption of organics and an increase in
effluent COD.
Treating water that has high turbidity or high organic content will
plug carbon pores and result in the loss of carbon capacity. Thus, special
attention should be given to the control of processes upstream of the carbon
columns. The adsorptive capacity and service life of the carbon can be
maximized by applying to the carbon, water that has been carefully pretreated
to the highest practical clarity.
The regeneration process requires careful operator control.
section describes carbon regeneration in detail.
Common Design Shortcomings and Ways to Compensate
A later
Shortcoming
1. BOD removal is poor.
2.
3.
Carbon adsorptive capacity 2.
is poor and carbon must
be regenerated often.
Effluent suspended solids
are too high when column
is operated in expanded,
upflow mode.
3.
Solution
Provide biological pretreatment
prior to activated carbon
application.
Pretreat the water to the
highest practical quality.
Operate as upflow, packed bed
or provide filtration to remove
solids downstream of carbon process.
179
-------
TROUBLESHOOTING GUIDE
ACTIVATED CARBON ADSORPTION
INDICATORS/OBSERVATIONS
1. Excessive headloss.
2. Hydrogen sulfide in
carbon contactor.
PROBABLE CAUSE
la. Highly turbid water
being applied to
carbon .
Ib. Growth and accumu-
lation of biological
solids in contactor.
Ic. Carbon deteriorat-
ing during handling
and large amounts
of carbon fines
accumulating .
Id. Inlet or outlet
screens plugged.
2a. Low concentration
or absence of DO
and nitrate in
carbon contactor
influent.
2b. High BOD concentra-
tions in influent.
CHECK OR MONITOR
la. Turbidity, SS
concentration .
Ib. Visual inspection,
hydrogen sulfide
(rotten egg) odor
in effluent.
Ic. Run gradation
analysis of carbon
and compare with
original specifica-
tion.
2a. DO of influent.
2b. BOD in influent.
SOLUTIONS
la. Vigorously backwash to restore
headloss. Improve turbidity of
applied water by improved
pretreatment .
Ib. Operate carbon contactor as
expanded upflow beds so solids
are flushed continuously;
backwash downflow beds more
frequently and improve upstream
removal of soluble BOD.
Ic . Remove carbon and wash fines
from system. It may be nec-
essary to replace carbon with
a different brand of greater
hardness.
Id. Backflush screens.
2a. Add oxygen, air or sodium
nitrate to influent, in case
of upflow operation.
2b. If possible, maintain aerobic
conditions in carbon column to
prevent H S formation by
addition of ozone, nitrates,
or oxygen to carbon influent.
Improve upstream soluble BOD
removal to remove sul fides
already formed, precipitate
with iron or add chlorine.
00
o
-------
TROUBLESHOOTING GUIDE
ACTIVATED CARBON ADSORPTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
2c. Long detention
time in carbon
columns.
CHECK OR MONITOR
2c. Detention time of
30-60 mins is a
typical maximum.
SOLUTIONS
2c. 1. Reduce the detention time
by removing some of the
carbon contactors from
service.
2. Backwash columns more fre-
quently and violently by
use of air scour or surface
wash.
Large decrease in
COD removed per Ib
of carbon
regenerated.
3. Carbon fouled and
losing efficiency.
3. COD removal
efficiency (COD re-
moved/lb carbon).
3. Evaluate and adjust regenera-
tion system to provide
increased efficiency (see
carbon regeneration section).
oo
Corrosion of metal
and damage to
concrete in carbon
contactors.
4a. H S present in
carbon contactors
(anaerobic condi-
tions) .
4b. Holes in metal
coatings permitting
partially dewatered
carbon to contact
metal.
4a. H S concentration,
low DO.
4a. See solution 2.
4b. Recoat metal surfaces.
-------
NITRIFICATION
Process Description
The biological processes (activated sludge, trickling filters, ABF
process, and rotating biological contactors) described in earlier sections
may all be used to convert ammonia nitrogen to nitrate nitrogen. Basic
process descriptions and troubleshooting guidance for each process presented
earlier will not be shown again here. This section will present some added
guidance directly related to the nitrification process.
The form of nitrogen that is in raw sewage may be biologically oxidized
to nitrate after the carbonaceous oxygen demand is met. But this can only
happen if the proper aerobic conditions are maintained in the process.
Nitrification may be done in either one or two stages. In single-stage,
the carbon and nitrogen oxidation steps are combined in a single unit.
In two-stage systems, the carbonaceous oxidation is first carried out in
a separate unit, followed by nitrification in another unit.
Typical Design Criteria and Performance Evaluation
Activated Sludge—
In the activated sludge process, the degree of nitrification depends
on the sludge retention time (SRT - see activated sludge chapter). Figure
54 shows the effects of SRT on nitrification, oxygen requirements, and the
aeration time needed to obtain different SRT values. For single-stage
nitrification, temperatures of 18 C and a sludge age of at least 10 days
or an aeration basin loading ratio less than 0.25 Ibs of BOD /day/lb of
MLVSS will usually work. In two-stage systems, the first stage activated
sludge system often is designed to produce an effluent BOD of less than
50 mg/1. Detention times of 4-5 hrs are common for the second stage basins.
Trickling Filters—
Nitrification in trickling filters depends on organic loading. Results
from several rock trickling filters are pictured in Figure 55. For good
nitrification, loadings should be less than 5 Ibs BOD/1000 cu ft/day.
Oxidation-nitrification can be done in a single-stage system using
synthetic-media trickling filters. BOD loading is the limiting factor
in this application. The following are common design and operating data for
this type of system:
Influent Ammonia-Nitrogen = 25 mg/1
Media Area = 27 sq ft/cu ft (94% void volume)
182
-------
REQUIRED AERATION DETENTION TIME, hrs
LB OXYGEN DEMAND. Ib BOD5
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1 kg/ma/day =62.4 Ib BOD5/ 1000 cu ft/day
10
20
30
40
50
BC05 LOAD, lb/1000 cu ft/day
Figure 55. Effect of organic load on nitrification efficiency of rock-
media trickling filters.
184
-------
Forced Draft Provided
Maximum BOD loading = 25 lb/1,000 cu ft/day
Recycle to maintain minimum flow of 0.5-1 gpm/sg ft
to keep media from drying out
For nitrification of secondary effluents in a two-stage system, the
contact time in the filter becomes most important. For this type of
system, following are typical criteria:
Influent BOD = 50 mg/1
Influent Ammonia-Nitrogen =25 mg/1
Media Area = 27 sq ft/cu ft (94% void volume)
Recycle to maintain constant flow
for media depth of 21.5 ft
The following flow rates may be used to obtain the desired performance:
Nitrification
Performance Flow Rate @ Wastewater Temp. Shown
65°F 44°F
90% 0.5 gpm/sq ft 0.5 gpm/sq ft
85% 0.75 0.65
80% 1.0 0.75
75% 1.5 0.85
The ABF system can provide complete nitrification at average loadings
of 200-250 Ibs BOD/1,000 cu ft/day (with peaks of 450 Ibs BOD/1,000 cu ft/day)
with an aeration basin detention time of 3-4 hrs. Common design data are
listed in Table 17.
Rotating biological contractors (RBC) also may be used in single-stage
or two-stage nitrification systems. As the rotating discs operate in
series, organic matter is removed in the first disc stages and following
stages are used for nitrification. Figure 56 shows design based on hydraulic
loading. When the ammonia concentration exceeds the maximum ammonia con-
centration on the appropriate BOD curve, the curve for the ammonia concentra-
tion is used. The nitrifying ability of the discs is relatively constant
when the temperature is between 15 and 26 C. Temperature correction factors
are used to adjust the hydraulic loadings in Figure 57 for any wastewater
temperature lower than 13 C. The RBC process may also be used to nitrify
secondary effluents. Figure 58 shows design and performance for nitrifi-
cation systems using 4 stages of discs. This is the most common type of
system used for nitrification of secondary effluents.
The final clarifier overflow rate usually is kept at a peak hourly rate
of no more than 1,000 gpd/sq ft with a minimum depth of 12 ft and equipped
with a surface skimmer. Sludge return usually should not be greater than
100% of the average daily flow. Rising sludge caused by denitrification has
185
-------
TABLE 17. GENERAL DESIGN PARAMETERS FOR NITRIFICATION OF
DOMESTIC WASTEWATER WITH ABF PROCESS
Parameter
Units
Typical
value
Range
Effluent criteria
5 -day BOD
Suspended solids
mg/1
mg/1
mg/1
15
20
1.0
5-30
15-30
0.5-2.5
Bio-cell parameters
Organic load
Media depth
BOD removal
Hydraulic parameters
Bio-cell recycle
Sludge recycle
Bio-cell flow
Bio-cell hydraulic
load
Ib BOD /day/1000 cu ft
ft
gpm/sq ft
200
14
65
1.5 Q
0.5 Q
3.0 Q
3.5
100-350
5-22
55-85
0.5-2.0 Q
0.3-1.0 Q
2.3-4.0 Q
1.5-5.5
Aeration parameters**
Detention time*
Organic load
Ammonia load
F/M
MLVSS concentration
MLSS concentration
Carbonaceous oxygen***
Clarifier parameters
Overflow rate
Solids loading
Return sludge
concentration
hr 3.5 2.5-5.0
Ib BOD /day/1000 cu ft 25 20-40
Ib NH -N/day/1000 cu ft 10 5.0-15.0
Ib BOD5/day/lb MLVSS 0.13 0.1-0.2
mg/1 3000 1500-4000
mg/1 4000 2000-5000
IJD O /Ib BOD 1.4 1.2-1.5
gpd/sq ft 600 300-1200
Ib/hr/sq ft 1.0 0.5-2.0
% 1.5 1.0-3.0
Sludge production
Ib VS/lb BOD removed
0.45
0.30-0.55
* Based on design average flow and secondary influent BOD = 150 mg/1.
** Based on aeration BOD loading after bio-cell removal.
*** Total oxygen utilization = carbonaceous oxygen + 4.6 Ib O /Ib NH -N
oxidized.
186
-------
§
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SL
Ul
cc
tu
o
o
a
I
100
95
90
85
80
75
70
•2501-150
INLET BODS CONCENTRATION,
tng/l
MAXIMUM AMMONIA
NITROGEN CONCENTRA-
I, mg/l
TEMPERATURE > 13° C
REGION OF
UNSTABLE
NITRIFICATION
gpd/»q M=41£ /m2/day)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
HYDRAULIC LOADING, gpd/sq ft
Figure 56. Design criteria for single-stage nitrification with
rotating biological discs.
2.5
EC
O
O
< 2.0
O
H
o
UJ
OC
S 1-5
o
UJ
oc
D
UJ 1.0
0.
S
Ul
• 90
T 1 r
AMMONIA NITROGEN REMOVAL,PERCENT
1
6
11
12
13
7 8 9 10
WASTEWATER TEMPERATURE, C
Figure 57. Temperature effects on single-stage nitrification with rotating
biological discs.
187
-------
100
INLET AMMONIA NITROGEN
CONCENTRATION, mg/1
65
HYDRAULIC LOADING, gpd/sq ft
RELATIVE CAPACITIES
FOR STAGED OPERATION
(90-95% Nitrification)
No. Stages
1
2
3
4
6
Relative Capacity
0.60
0.80
0.90
1.00
1,07
Conditions
Temperature > 13°C
BOD5 <20mg/l
Figure 58. Design relationships for a 4-stage RED process treating
secondary effluent.
188
-------
sometimes been a problem in nitrification systems, especially with warm
wastewaters. The following are control measures for preventing floating
sludge:
provide rapid sludge removal in clarifier tank design to
prevent bubbles from forming,
provide flexibility in influent feed points (e.g., change from
step aeration to plug flow in warm weather periods),
chlorinate the return activated sludge to control sludge bulking
and to reduce the sludge volume index. This will allow more
rapid sludge removal from the clarifier.
The following is an example of how to evaluate the performance of
different nitrification systems using the information just provided:
1. Define the type of biological system used at the treatment
plant and any necessary design information.
Type of Plant Rock Media Trickling Filter
No. of Stages 1
BOD Loading 10 lb/1000 ft /day
Ammonia Nitrogen Content
Influent 25 mg/1
Effluent 10 mg/1
2. Determine the nitrification efficiency of the system.
(Influent NH_ - Effluent) x 100
% Nitrification =
Influent NH
= (25-10) x 100
25
= 60%
3. Compare the 60% removal with the average performance curve in
Figure 55. The actual nitrification is very close to the expected
value. This would mean that the nitrification system is working
very well.
The evaluator should refer to the section of this manual about the
process used for nitrification (activated sludge section, for example) for
detailed information on process evaluation.
Control Considerations
There are several very important factors controlling the nitrifica-
tion process.
pH has a major effect on the rate of nitrification, with values of
189
-------
about 8.5 usually best. At pH 7, nitrification rates are often 50% of those
at pH 8.5. Sudden decreases in pH also produce bad effects on nitrification.
The nitrification process itself can lower the pH to undesirable levels.
About 7.14 mg of alkalinity as CaCO is removed per rag of ammonia nitrogen
oxidized. In many wastewaters there is not enough alkalinity to leave a
good residual for buffering the wastewater during the nitrification process.
Because of the effect pH has on nitrification, it is very important to
provide enough alkalinity in the wastewater to balance the acid produced
by nitrification. Caustic or lime addition may be needed to help add
alkalinity to moderately alkaline wastewaters.
The rate of nitrification is also affected by temperature. The rate
at 10 C is about one-half of the nitrification rate at 20 C.
The concentration of dissolved oxygen (DO) also has a major effect on
the rates of nitrification. Nitrification can occur at DO levels of 0.5
mg/1, but at much lower rates than at higher DO levels.
The following heavy metals and organic compounds are toxic to nitrifying
bacteria:
Organics Inorganics
Thiourea Zn
Allyl-thiourea OCN
8-hydroxyquinoline ClO
Salicyladoxine Cu
Histidine Hg
Amino acids Cr
Mercaptobenzthiazole Ni
Perchloroethylene Ag
Trichloroethylene
Abietec acid
Common Design Shortcomings and Ways to Compensate
Shortcomings Solution
1. Aeration system not sized 1. Install flow equalization facilities
for maximum hourly ammon- to moderate peak conditions or
ium load with resultant install added aeration capacity
loss of nitrification to meet peak hourly BOD and
efficiency during peak nitrogenous load.
load periods.
2. Drop in pH because no 2. Install lime or sodium bicarbonate
means provided for addition feeders and pH probes in aeration
of alkalinity to aeration basin.
basins to offset loss of
alkalinity from nitrifi-
cation reaction.
190
-------
Shortcomings Solution
No means provided to 3. Add line to return skimmings
return secondary as required to maintain adequate
clarifier skimmings nitrifier population.
to aeration (nitrification)
basin.
Inadequate sludge return 4. Increase sludge return
capacity to prevent capacity.
denitrification in final
clarifier with resultant
rising sludge.
191
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TROUBLESHOOTING GUIDE
NITRIFICATION
INDICATORS/OBSERVATIONS
1. Decrease in nitrifi-
cation unit pH with
loss of nitrification
2. Inability to com-
pletely nitrify.
3. In 2-stage act.
sludge system, SVI of
nitrification sludge
is very high (>250) .
PROBABLE CAUSE
la. Need more alkalin-
ity to offset nitri-
fication acidic
effects .
Ib. Addition of acidic
wastes to sewer
system.
2a. Oxygen concentra-
tions are limiting
nitrification .
2b. Cold temperatures
are limiting nitri-
fication.
2c. Increases in total
daily influent nit-
rogen loads have
occurred.
2d. Biological solids
too low in nitrifi-
cation unit.
2e. Peak hourly ammoni-
um concentrations
exceed available
oxygen supply .
3. Nitrification is
occurring in first
stage .
CHECK OR MONITOR
. la. Alkalinity in efflu-
ent from nitrifi-
cation unit.
Ib. Raw waste pH and
alkalinity.
2a. Minimum DO in nitri-
fication unit should
be 1 mg/1 or more .
2c. Current influent
nitrogen concentra-
tions.
2d. SRT should be
greater than 10 days
and in cold tempera-
tures may need to be
greater than 15
days.
3 . Nitrates in first
stage effluent.
SOLUTIONS
la. If alkalinity less than 30 mg/1
start addition of lime or
sodium hydroxide to nitrifi-
cation unit.
Ib. Initiate source control.
2a. Increase aeration supply or
decrease loading on nitrifi-
cation unit.
2b. Decrease loading on nitrifica-
tion unit or increase biologi-
cal population in nitrification
unit.
2c. Place added nitrification units
in service or modify pre-
treatment to remove more
nitrogen.
2d. 1. Decrease loading on nitrifi
cation unit and decrease
wasting or loss of sludge
from nitrification unit.
2 . Add settled raw sewage to
nitrification unit to gen-
erate biological solids.
2e. Install flow equalization sys-
tem to minimize peak concen-
trations or increase oxygen
supply.
3. Transfer sludge from first
stage to second and maintain
lower SRT in first stage .
-------
TROUBLESHOOTING GUIDE
NITRIFICATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Loss of solids from
final clarifier.
4. .(See activated sludg
and secondary clarifier s(
ctions)
5. Loss of solids from'
trickling filter or
RBC.
5. (See trickling filte
: and RBC sections)
-------
DENITRIFICATION
Process Description
If bacteria come in contact with a nitrified element, and there is no
oxygen, nitrates are reduced to nitrogen gas ("denitrification"). In this
form, the nitrogen will escape as a gas from the wastewater.
Denitrification can be done either in an anaerobic activated-sludge
system (suspended growth system) or in a columnar system (fixed-film system).
With good biological treatment upstream, there is not much oxygen-demanding
material left in the wastewater by the time it reaches denitrification. The
denitrification will occur only if there is an oxygen demand when no oxygen
is present in the wastewater. Usually, an oxygen-demand source must be
added to reduce the nitrates quickly. The most common method of supplying
the needed oxygen demand is to add methanol. About 3 mg/1 of methanol are
added per 1 mg/1 of nitrate-nitrogen.
Suspended growth reactors used for denitrification are mixed mechan-
ically only enough to keep the bio-mass from settling. Too much mixing
will add unwanted oxygen.
Submerged filters using different kinds of media may be used for
denitrification. Usually, a fine media (2-4 mm) is used in a downflow,
packed bed. This system not only denitrifies the wastewater, but also
filters the effluent. Another system uses a fluidized sand bed where waste-
water flows upwards through a bed of small media such as activated carbon
or sand at a rate which causes the bed to fluidize. The small media provides
a large surface for the denitrifying bacteria to grow on.
Typical Design Criteria and Performance Evaluation
Suspended growth reactors are usually designed as plug-flow units to
prevent short circuiting. Submerged mechanical mixers (0.25-0.5 hp/1000
cu ft) are usually used for mixing. Detention times often range from 1-3
hours, depending upon cold weather conditions and nitrogen concentrations.
About 3 mg/1 of methanol usually is fed per mg/1 of nitrate-nitrogen.
Other materials also have been used, but some cause greater sludge production
than others. For example, about twice as much sludge is produced per mg
of nitrogen reduced when saccharose is used instead of methanol. On the
other hand, acetone and acetate produce about the same amount of sludge as
methanol. Low-nitrogen industrial wastes (such as brewery wastes) also
have been used when available. The amount of methanol fed (or other oxygen
demand source) must be carefully controlled since too much would result in a
194
-------
residual BOD in plant effluent. The feed rate should be automatically
controlled by the incoming nitrate concentration. Flow pacing usually
doesn't work because of changes in nitrate concentrations. Unless methanol
removal is provided, excess methanol doses will cause methanol to appear in
the effluent. It may be necessary to provide a methanol removal system as
backup to provide fail-safe operation. Figure 59 shows one system used for
removal of excess methanol. After denitrification, the mixed liquor passes
to an aerated stabilization tank. In this tank, facultative organisms
use up any remaining methanol. In fixed-film denitrification systems, this
type of system would not work because there are not enough organisms in the
column effluent to accomplish the biological oxidation.
There is a fixed-film filtration system using 6 ft of uniformly graded
sand 2 to 4 mm in size. To remove 20 mg/1 NO" nitrogen, filtration rates
of 2.5 and 1.0 gpm/sq ft are needed for minimum wastewater temperatures
of 21 C and 10 C respectively. Mixed-media filters (coal, sand, garnet)
have also been used as downflow, packed beds for denitrification. Using
a 36-inch mixed-media filter (3 inches of 0.27 mm garnet, 9 inches of 0.5
mm sand, 8 inches of 1.05 mm coal, and 16 inches of 1.75 mm coal), almost
complete denitrification is possible at 1.5 gpm/sq ft at temperatures of 10°C,
and at 3 gpm/sq ft at temperatures o£ 20° C.
Settling basins downstream of denitrification units usually are
designed with overflow rates less than 1,000 gpd/sq ft at peak hour. They
are equipped with surface skimmers that return the scum to the denitrifi-
cation tank. Waste sludge quantities depend on the oxygen-demand source
fed to the system. For methanol, waste sludge quantities will be about
0.2 Ibs/lb of methanol fed.
Control Considerations
Like nitrification, pH and temperature will effect the rate of
denitrification. Denitrification rates are much less when pH is below
6.0 or above 8.0. Highest rates occur between 7.0 and 7.5. Denitrification
rates at 15 C are often about 50% of those at 20 C. Fixed film beds using
2-4 mm sand are backwashed with one or two minutes of air agitation followed
by ten to fifteen minutes of air and water scouring and finally five minutes of
water rinse. Nitrogen gas may accumulate in such a filter during a filter
run. These gas bubbles must be removed at certain times to prevent too
much loss of head on the filter. A "bumping" procedure may be used
consisting of a short backwash cycle lasting one or two minutes, after four to
twelve hours of operation. When several filters are used, backwashed filter
effluents are blended with other operating filters in order to reduce the
effects of any initial nitrate leakage.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. No provision for removal 1. Add aerated stabilization unit.
of excess methanol from
195
-------
Methanol
Nitrified
^mfmf^m
effluent ' ,
ANOXIC MIXED
DENITRIFICATION REACTOR
t
T=50 min
Return denitrified sludge
^ 1 ^•-^••m
^- Mildly aerated physical" •
conditioning channel
Denitrified
effluent ^
AERATED
STABILIZATION
DENITRIFICATION
CLARIFIER
Figure 59. Process for removal of excess methanol.
-------
Shortcoming
1. (continued)
mixed denitrification
reactor effluent.
2. Short circuiting in
mixed reactor is causing
nitrate leakage.
3. Coarse rock-media in
downflow fixed-film
reactor cannot be
backwashed.
Solution
Install baffles in mixed
reactor to provide plug
flow conditions.
Replace media with media
finer than 0.5 inches.
Methanol feed pumps
erratic due to pump
materials not being
compatible with
methanol.
Replace pump heads (or
pumps) with appropriate
materials.
Packed bed denitrifiers
backwashed with nitrate
containing effluent
causing high nitrate leak-
age at start of run.
Inadequate methanol
storage provided causing
outages due to shipping
delays.
Change source of backwash
water to denitrified effluent.
Install added storage to
provide a total of at least
two weeks storage.
197
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TROUBLESHOOTING GUIDE
DENITR1FICATION
INDICATORS/OBSERVATIONS
1. Effluent BOD shows
sudden increase.
2. Effluent nitrates
show sudden increase
3. High headless across
packed bed denitri-
fication units.
4. Packed bed denitri-
fier which has been
out of service blind
•imnuaH- Vinnn sfart-UD
PROBABLE CAUSE
1. Excessive addition
of methanol (or
other oxygen demand-
ing material used) .
2a. Inadequate methanol
addition .
2b. pH has drifted out-
side 7-7.5 range due
to low pH in nitri-
fication stage .
2c. Loss of solids from
denitrifier due to
failure of sludge
return.
2d. Excessive mixing is
introducing DO
3a. Excessive solids
accumulation in
filter.
3b. Nitrogen gas
accumulating in
filter.
4. Solids have floated
to top of bed and
j blind filter surface
CHECK OR MONITOR
1. Methanol to nitrate-
nitrogen ratio
should be 3:1.
2a. Methanol feed sys-
tem malfunction^
(See Nitrification S«
2c. Denitrifier unit
solids and clari-
fier effluent
2d. Denitrifier DO,
should be as near
zero as possible
(<0.5 mg/1) .
3a. Length of filter
run - if 12 hrs or
more, this is the
probable cause .
3b. Run times or less
than 12 hrs indi-
cates this may be
the cause.
SOLUTIONS
la. Reduce methanol addition.
Ib. Install automated methanol
feed system.
Ic. Install aerated stabilization
unit for removal of excess
methanol per Figure 59.
2a. Correct malfunction
ction)
2c. Increase sludge return; de-
crease sludge wasting; transfer
sludge from carbonaceous unit
to denitrifier.
2d. Turn some mixers off or reduce
speed.
3a. Initiate full backwash cycle.
3b. Backwash bed for 1-2 mins and
return to service.
4. Backwash beds before removing
them from service and immediate-
ly before starting them.
co
-------
AMMONIA STRIPPING
Process Description
The ammonia stripping process removes gaseous ammonia from water by
agitating the water-gas mixture in the presence of air. Ammonium in
secondary effluent can be converted to ammonia gas by raising the pH of
the wastewater to 10.8 to 11.5. The gaseous ammonia can then be released
by passing the high pH effluent through a stripping tower. In the tower,
large amounts of air are mixed with the water flowing through the tower
which releases the ammonia. Lime used in coagulation-sedimentation
process not only removes suspended solids and phosphorus, but also raises
the pH for the stripping process.
The three basic steps in ammonia stripping are (1) raising the pH of
the water to at least 10.8 with the lime added in the chemical clarifier,
(2) cascading the water down through a stripping tower to release the
ammonia gas, and (3) forcing large amounts of air through the tower to
carry the ammonia gas out of the system.
The towers used for ammonia stripping (Figure 60) look much like
cooling towers. The Orange County facility in Figure 60 pictures a common
ammonia stripping tower design. In this facility, clarifier effluent is
pumped to the top of the towers where the flow is distributed through
spray nozzles to the cells. Each tower has cells equipped with sections
of fill material. These sections are separated by inside and outside access
areas. Water flows down through the fill bundles into catch basins where
the effluent may flow to recarbonation basins.
Each cell in each tower has a two speed fan. Air is drawn into the
tower through the separate cooling sections which are located on the out-
side faces of the towers. The air passes up through the packing, (opposite
in direction to water flow) stripping the ammonia from the water droplets
as they are formed by the splash bar packing. The air leaves through the
fan stacks on top of the tower structure.
Typical Design Criteria and Performance Evaluation
Tables 18 and 19 show the major design data for the ammonia stripping
towers at the Orange County Water District Facility, and at the South Tahoe
Facility.
Air temperature is the major element which can effect the performance
of the ammonia stripping process. As the air temperature drops, efficiency
also drops. For example, stripping removes about 95% of the ammonia in warm
199
-------
AIR OUTLET
FAN STACK
2 SPEED MOTOR
WATER INLET FLOW
CONTROL VALVE
AIR OUT
" HOIST'
INTERIOR
ACCESS
AREA
AMMONIA
REMOVAL
FILL
DOLLY
WATER
DRIFT
ELIMINATOR -7
r—~^
V3"NOZZLE'
WATER
IN
-AMMONIA
REMOVAL
FILL BUNDLES
EXTERIOR
ACCESS
AREA
I \
AVCOOLING FILL
AND AIR INLET
WASTEWATER-
COLLECTION
CHANNEL -
COOLED PROCESS
WATER OR BRINE
COLLECTION CHANNEL'
Fiqure 60. Typical ammonia stripping tower.
200
-------
TABLE 18. DESIGN DATA FOR ORANGE COUNTY WATER DISTRICT
AMMONIA STRIPPING TOWER
Number of towers:
Dimensions:
Capacity:
No. of fans:
Air capacity:
Hydraulic loading rate:
Cooling fill:
Overall length = 207 ft, width = 61 ft,
height of main tower structure = 55 ft,
depth of packing = 25 ft; plan area of
ammonia stripping fill material = 4752
sq ft per tower; fill bundles are
removable for cleaning.
5200 gpm each tower
6 per tower., 18 ft diam, two speed 125/155
hp, electric motors (1800/1200 rpm).
350,000 cfm per fan at static pressure of
1.1 in water (400 cfm/gpm @ design loadings)
1.1 gpm/sq ft of packing plan area @ 15 mgd
9 ft high, 200 ft long per tower at air
inlet plenum.
weather (70°F air temperature) but only about 75% of the ammonia when the
temperature falls to 40°F. The process becomes impossible because of frost
inside the stripping tower when the air temperature falls very far below
freezing. Performance also is affected by pH, hydraulic loading and the
quantity of air circulating through the tower. These factors are described
in more detail under the section on "Control Considerations".
The following steps can be used to check performance:
1. Determine influent pH - it should be above 10.8.
2. Check the tower hydraulic loading rate:
Area covered by tower packing = 4752 sq ft
Flow to tower = 5200 gpm
5200 gpm _ , / f.
Hydraulic loading 4752 sq ft= 1'1 gpm/Sq ft
Loadings should be less than 2 gpm/sq ft to keep water from
moving through the tower in sheets rather than in drops.
201
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TABLE 19. DESIGN DATA FOR TAHOE AMMONIA STRIPPING TOWER
Capacity: Nominal, 3.75 mgd
Type: Cross flow with central air plenum and
vertical air discharge through fan
cylinder at top of tower.
Fill: Plan area, 900 sq ft
Height, 24 ft
Splash bars:
material, roug-sawn treated hemlock
size, 3/8 x 1-1/2 in
spacing, vertical 1.33 in
horizontal 2 in
Air flow: Fan, two-speed, reversible, 24-ft diameter,
horizontal
Water Rate Air Rate
gpm gpm/ft cfm cfm/gpm
1,350 1.0 750,000 550
1,800 2.0 700,000 390
2,700 3.0 625,000 230
Tower structure: Redwood
Tower enclosure: Corrugated cement asbestos
Air pressure drop: 1/2 in of water at 1 gpm/ft
3. Check the tower packing to make sure it is not coated with
calcium carbonate.
4. Calculate the removal of ammonia in the tower and check the
air temperature:
Influent = 20 mg/1
Effluent = 2 mg/1
Removal = 20-2 = 90%
20
Air Temperature = 65°F
At 65°F, 90% removal is good. For every °F decrease in
temperature, the efficiency will drop about 0.5%.
If performance is poor, refer to the troubleshooting guide.
202
-------
Control Considerations
Influent pH—
The pH of the tower influent (chemical clarifier effluent) must be kept
above 10.8 for efficient ammonia removal by air stripping. The pH usually
is controlled in the chemical clarifier using lime addition.
Only the ammonia gas can be removed by stripping. At 20°C and pH 7.0
almost all of the ammonia nitrogen is present as ammonium ion and no ammonia
can be removed by stripping. At a pH between 10.8 and 11.0 about 95 to 98%
of the ammonia is present as dissolved gas and can be removed by stripping.
It is important then to maintain the proper pH (at least 10.8) in the tower
influent at all times for efficient ammonia removal.
Temperature—
The temperature of the air entering the tower is also a factor in
ammonia removal efficiency. The removal efficiency is increased
about 0.5% for each degree Fahrenheit increase in inlet air temperature.
In most cases, the operator has no control over the temperature of
the air entering the tower. It is not practical to heat the large volumes
of air needed for the stripping process.
Hydraulic Loading—
Removal efficiencies also vary with the hydraulic loading of the towers.
Ammonia removals generally increase with decreased hydraulic loading. For
best operation, this means that all sections of the tower should be used
whenever possible. Loadings greater than 2 gpm/sq ft will probably result
in sheets or streams of water flowing through the tower rather than in
droplets. This will cause efficiency to drop.
The amount of droplets that form depends on the height and spacing of
the packing and how evenly the flow is distributed. Spray inlet nozzles
should be adjusted to give the best flow distribution possible.
Air Quantity—
Ammonia removal efficiency usually increases with increasing air flow
in the stripping tower. Where the tower has variable speed fans, the lower
speed can be used during low flow periods to reduce power consumption.
Scale Control—
When stripping lime-treated wastewater, high calcium and pH values make
the water unstable, allowing the formation of calcium carbonate scale.
Excessive scale must not be allowed to gather on the"tower fill because
air flow through the tower will be reduced and ammonia removal efficiency
will drop. When scale first forms at pH values of 10.8 to 11.0, it is
usually soft (the scale may be harder when formed at pH values of 11.5 to
12.0) and usually can be removed by hosing with a stream of water. If
hard scale forms, it may be necessary to remove the fill bundles from the
tower for cleaning.
203
-------
Polymer Feed to Stripping Tower Influent—
There are polymers such as Cyanamer P-38 and Nalco Polyol-Ester, which
are made especially to prevent calcium carbonate scale from forming at
high pH. Polymer dosages of 0.5 to 5.0 mg/1 are used to prevent scale from
forming. Regular use of a polymer will depend on how bad the scale problem
is, and how much the polymer costs. Sometimes it may be cheaper to manually
remove the scale instead of adding polymer.
Common Design Shortcoming and Ways to Compensate
Shortcoming Solution
1. Continuous formation of 1. Install a system of water sprays
soft scale deposits on for automatic cleaning to be
tower requiring frequent operated at a specific frequency.
maintenance.
2. Noise complaint. 2. Reduce fan speed to minimum speed
compatible with removal of ammonia.
204
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TROUBLESHOOTING GUIDE
AMMONIA STRIPPING
to
o
Ul
INDICATORS/OBSERVATIONS
1. Scale build-up on
ammonia stripping
packing.
2. Scale build-up on
pumping units feeding
water into the
tower.
3. Ice build-up on
outside of tower.
4. Fan inoperable.
5. Loss of ammonia re-
moval efficiency.
PROBABLE CAUSE
la. Insufficient clean-
ing of tower
(removal of scale) .
2. Insufficient back-
washing of pumping
units .
3. Freezing weather.
4a. Loose or damaged
blade.
4b. Drive bearings over-
heating.
5a. Scale build-up on
fill material.
5b. pH of tower influent
too low.
5c. Excessive hydraulic
loading.
5d. Insufficient air
flow through tower.
CHECK OR MONITOR
la. White coating accum-
ulating on packing
and reduction of
air flow through
the tower.
2 . Backwashing
frequency .
3. Air temperature.
4a. Visual inspection.
5a. (See Item 1)
5b. pH
5c. Sheets or streams of
water flowing througl
tower rather than in
droplets .
5d. Air flow rate.
SOLUTIONS
la. Clean by hosing with a spray
of water.
Ib. Add a descaling polymer to the
tower in f luen t .
Ic. Clean with a mixture of dilute
sulfuric acid and an organic
dispersant.
2. Backwash pumps 2 or 3 times
per day.
3 . Reverse draft fan to blow warm
inside air to the frozen area
to melt the ice.
4a. Repair or replace damaged or
worn parts.
5a. (See Item 1)
5b. Increase pH to at least 10.8 by
lime addition in chemical
clarifier.
5c. (See Item 6)
5d. Increase air flow rate by oper-
ating fans at higher speed, or
recycle air flow back through
the tower.
-------
TROUBLESHOOTING GUIDE
AMMONIA. STRIPPING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
6. Sheets or streams
of water rather than
droplets flowing
through tower.
6a. Excessive hydraulic
loading rate.
6b. Non-uniform flow
distribution.
6c. Scale build-up may
be blocking a
portion of the fill,
6a. Flow rate should be
less than 2 gpm/ft2.
6b. Spray inlet nozzle
adjustment.
6c. (See Item 1)
6a. 1. Decrease the flow rate or
2. Increase the number of
units in service.
6b. Adjust or clean spray inlet
nozzles to provide even flow
distribution.
6c. (See Item 1)
to
o
en
-------
CHEMICAL FEEDING AND CONDITIONING
Process Description
Many different chemicals are used to treat municipal wastewaters.
Some of the most often used chemicals are lime, aluminum sulfate (alum),
ferric chloride, soidum hydroxide, and powdered activated carbon. The
chemical used depends on what purpose is to be served, the quality of
the wastewater, the type of handling and feeding equipment available, and
chemical costs. Good feed control is important because of chemical costs,
and to prevent excess chemicals from reaching the receiving stream. Table
20 lists some of the chemicals used in wastewater treatment, as well as
the properties of each, and information on the feeding of each chemical.
Three basic types of chemical feed equipment are used: volumetric,
belt gravimetric, and loss-in-weight gravimetric. The feeder must be
chosen carefully, especially in the small facilities where a single feeder
may be used for more than one chemical.
Volumetric feeders usually are used only where low feed rates are
required. These feeders deliver a constant, preset amount of chemical.
There are many types of volumetric feeders, but the screw type feeder is
most often used (Figure 61). Gravimetric feeders, however, are more
accurate and generally are used in larger plants. Both types of feeders
can be used to feed in proportion to the flow of wastewater.
The loss-in-weight feeder should be used where accuracy or cost of
chemical is important. This feeder only works for feed rates up to a
rate of 4,000 Ib/hour. The loss-in-weight type feeder has a material hop-
per and feeder set on enclosed scales. The feed rate controller is used
to deliver the dry chemical at the desired rate.
Belt-type gravimetric feeders have a wide capacity range and usually
can be sized for any use in a wastewater treatment plant. Belt-type gravi-
metric feeders use a basic belt feeder with a weighing and control system.
Feed rates can be changed by adjusting the weight per foot of belt, the
belt speed, or both.
Liquid feeders usually are metering pumps or orifices. These meter-
ing pumps usually are positive displacement, plunger, or diaphragm type
pumps. The type of liquid feeder used depends on the viscosity, corrosi-
vity, solubility, suction and discharge heads, and internal pressure-
relief requirements. Some examples are shown in Figure 62. In some cases,
control valves and rotameters may be all that is needed. For uses like
lime slurry feeding, however, centrifugal pumps with open impellers are used.
207
-------
CHEMICALS COMMONLY USED IN WASTEWATER TREATMENT
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
Name
Formula
Trade name
Use
Aluminum
sulfate
A12(S04)3
14 H20
Filter alum
Coagula-
tion
sludge con-
ditioning
Calcium
oxide
CaO
Quicklime
Coagulation,
pH adjust-
ment sludge
conditioning
Calcium
oxide
CaO
Quicklime
Coagulation,
pH adjust-
ment sludge
conditioning
Ferric
chloride,
Fed 3
solution
FeCl3, 6H20
crystal
FeCl 3-
anhydrous
Liquid ferric
chloride
Crystals,
ferric
chloride
Coagulation,
sludge con-
ditioning ,
odor control
(H2S)
Methanol
CH3OH
Methanol
Denitrifica-
tion
Sodium
hydroxide
NaOH
Caustic
soda
pH adjust-
ment, acid
neutraliza-
tion
Available
forms and
appearances
Light tan to
gray-green
lumps , gran-
ules, or
powde r
Liquid alum
also avail.
White powder
White powder
or porous
white to
light gray
lumps
Dark brown
solution
Yellow,
brown
crystals
Green, black
granules
Colorless
liquid
Liquid
Commercial
strength
Powder - 17%
A1203
Minimum
Liquid - 8.3%
A1203, 49%
dry alumi-
num sulfate
Ca(OH)2
97-99%
CaO 71-74%
90-96% CaO
below 88%
is poor
quality
Solution -
37-47%
FeCl3
Crystal -
60% FeCl3
Anhydrous -
96-97%
FeCl3
99.9%
CH3OH
50% NaOH
73% NaOH
Properties
likely
to cause
trouble
Dusty
Dusty
Dusty,
slakes
upon
standing
long,
causing
expansion
Solution -
acid, cor-
rosive.
stains
Crystal -
melts at
37 °C, hy-
groscopic ,
corrosive.
stains,
Anhydrous -
deliquescent
corrosive,
stains
Vapors toxic
Toxic liquid
Flammable
Skim irrita-
tion
Alkali
burns
Storage
container
materials
Concrete
Steel
Wood
Moisture-
proof
Concrete
Steel
Wood
Concrete
Steel
Rubber-
lined
steel ,
wood, or
concrete
f
Steel
tanks
Steel
Strength of
solution or
Best feed suspension (%)
requlation and properties
Dry or so- 0.25-6.0% -
lution Acid and
feed corrosive
Dry feed Up to 10% -
or slurry alkali and
prone to
encrustment
Thin, milk- Up to 25%,
like slurry then dilute
to 10% after
slaking -
alkali and
prone to
encrustment
Solution Up to 45%
feed only PeClg-
acid and
corrosive
Solution
feed only
Solution Up to 20%
feed only
Suitable
handling
material for
solutions
Lead
Rubber
Acid-resisting
tile
Duriron
Plastics
Stainless steel
316
Asphalt
Cypress
Rubber'
Iron
Concrete
Rubber
Iron
Cement
Acid-proof brick
Ceramics
Stoneware
Rubber
Glass
Plastics
Asbestos
Teflon
Steel
Stainless
steel - 316
Rubber
Nickel
Nickel alloys
Plastic
208
-------
O
VO
MOTOR
GEAR REDUCER
FEED RATE REGISTER AND
FEED ADJUSTING KNOB
SOLUTION CHAMBER
HOPPER
ROTATING AND
RECIPROCATING
FEED SCREW
JET MIXER
DWG. NO. 1643
Figure 61. Typical screw type volumetric feeder
(courtesy of Wallace & Tiernan, division of Pennwalt Corporation)
-------
DISCHARGE
VALVE
PLUNGER-
SUCTION VALVE
PLUNGER PUMP
DISCHARGE VALVE
DIAPHRAGM
SUCTION VALVE
DIAPHRAGM PUMP
Figure 62. Positive displacement pumps.
210
-------
Lime—
Lime can be added either before primary treatment or after secondary
treatment (as part of an AWT process). As a coagulant used in primary
treatment, lime helps to remove SS, phosphorus, heavy metals, grease and
viruses. Lime also may be used to adjust the pH of the wastewater, or
for sludge conditioning.
Although lime comes in many forms, quicklime and hydrated lime are
used most often for wastewater coagulation. Quicklime (unslaked lime)
is almost all calcium oxide (CaO), and first must be converted to the
hydrated form (Ca(OH)2). Hydrated or slaked lime is a powder obtained by
adding enough water to quicklime to satisfy its affinity for water.
Lime may be unloaded using screw or bucket conveyors, or pneumatically
as shown in Figure 63.
Lime is never fed as a solution because of its low solubility in water.
Also, quicklime and hydrated lime usually are not applied dry directly to
the wastewater for the following reasons:
they are transported more easily as a slurry;
the lime mixes better with the wastewater;
pre-wetting the lime in the feeder with rapid mixing helps to
make sure that all particles are wet and that none settle out
in the treatment basin.
A schematic drawing of a lime feed system is shown in Figure 64 with
details of gravimetric feeders and slakers shown in Figure 65. Quicklime
feeders usually must be the belt or loss-in-weight gravimetric types be-
cause bulk density changes so much. Feed equipment usually has an adjust-
able feed range of at least 20:1 to match the operating range of the slaker.
The main parts of a lime slaker for wastewater treatment usually include one
or more slaking bins, a dilution bin, a grit separation bin, and a contin-
uous grit remover.
Hydrated lime is slaked lime and needs only enough water to form
milk of lime. Wetting or dissolving tanks usually are designed for 5
minutes detention with 0.5 Ib/gal of water or 6% slurry at the highest
feed rate. Hydrated lime often is used where maximum feed rates are less
than 250 Ib/hr.
Dilution is not too important in lime feeder, therefore, it is not
necessary to control the amount of water used in feeding. Hydraulic jets
may be used for mixing in the wetting chamber of the feeder, but the jets
should be the right size for the water supply pressure.
Alum—
Aluminum sulfate may be added to wastewater for coagulation or phos-
phorus removal. It may be used as the primary coagulant instead of lime,
211
-------
Dust collector.
NJ
I-1
N)
Filter/receiver
Rotary
air lock
Typical
Blower and motor
•Material unloading nozzle
Figure 63. Typical positive-negative pneumatic conveying system.
-------
pH RECORDER
CONTROLLER
FLOW RECORDER
WITH PACING
TRANSMITTER
Dosage
adjuster
[FLEXIBLE
\CONNECTO
| r-
Variable speed/
meter —-^
Level
control
SLURRY
HOLDING
TANK
Solenoid valve
-Slaking water
Transfer pump
_^r= _J J
Flow
Primary
Device
REACTION TANK
Treated
waste
Figure 64. Illustrative lime feed system for wastewater coagulation.
213
-------
MATERIAL
ENTERS FROM
k LIME STORAGE
BIN
I
ELECTRIC I 1
CLUTCH FEED SECTION
SECTION > I
BEAM SECTION
GATE
POSITIONING
CAM
{REAR
STATIONARY)
DECK
SCALE BEAM-}
CONTACTS
WEIGH BELT-)
-VERTICLE /
&SGA TE x^yxyafe:::
FRONT
STATIONARY
DECK
'WEIGHING PLATFORMS*'
SLAKING
WATER
TORQUE CONTROLLED
WATER VALVE- / AO
DUST SHIELD
WATER SPRAY
SLAKING COMPARTMENT
DILUTION CHAMBER
SLURRY DISCHARGE
SECTION
DISCHARGE PORT
GRIT DISCHARGE
LIQUID LEVEL
WATER FOR GRIT
WASHING
GRIT ELEVATOR
CLASSIFIER
Figure 65. Typical lime feeder and lime slaker.
214
-------
or along with lime. Alum may be added as a filter aid to the influent of
a mixed-media filter. It may also be added at several other points in the
wastewater treatment process including the primary influent, rapid mix
basin, or first stage recarbonation basin.
Alum is available in either dry or liquid form. As a dry chemical,
alum may be in the granular, powder, or lump form in bag, barrel or carload
quantities. When the granular form is used, it is dry fed, and weighs 60
to 75 Ib/cu ft. A common type of storage and feeding system for dry alum
is pictured in Figure 66. Dry alum in bulk can be unloaded with screw
conveyors, pneumatic conveyors, or bucket elevators made of mild steel.
Pneumatic conveyor elbows should have a strong backing since the alum
can contain harsh impurities.
The feed system for dry alum has all of the parts needed for good
preparation of the chemical solution.
To prepare alum for feeding, dissolving tanks should be made of a
non-corrosive material. Dissolvers should be the right size to get the
desired solution strength. Most solution strengths are 0.5 Ib of alum to
1 gal of water, or a 6% solution. The dissolving tank should be designed
for a minimum detention time of 5 minutes at the maximum feed rate.
Dissolvers should have water meters and mixers so that the water/alum mix-
ture can be controlled.
Liquid alum is shipped in rubber-lined or stainless steel, insulated
tank cars or trucks. The liquid form is delivered at a solution strength
of about 8.3% as A12O3, and contains about 5.4 Ibs dry alum (17% A12C>3)
per gal of liquid. Figures 67 and 68 show two common feed systems for
liquid alum. The rotodip-type feeder or rotameter often is used for
gravity feed and the metering pump for pressure feed systems. The pres-
sure at the point of application often determines the type of feeding sys-
tem used. Overhead storage can be used to gravity feed the rotodip as
shown in Figure 67. A centrifugal pump may also be used, but needs an
excess flow'return line to the storage tank, as shown in Figure 68. Alum
usually is fed by positive displacement metering pumps. Positive displace-
ment pumps can be set to feed over a wide range by adjusting the pump
stroke length. Dilution water usually is added to an alum feed pump dis-
charge line to prevent line plugging, to reduce delivery time to the point
of application, and to help mix the alum with the water being treated.
The output of the pumps can be controlled automatically in proportion to
plant flow. This is done by setting the alum dosage for the maximum flow
rate. The controls are then set to automatically adjust the off-on cycle
and the amount of alum pumped to the actual flow.
Polymers—
Polymers are used as an aid to flocculation, where a light or fine
floe settles too slowly. The normal dose for this use is about 0.10 to
0.25 mg/1 of polymer. It is almost impossible to overdose, but this may
occur in the range of 1.0 to 2.0 mg/1. Polymers are also very useful as
filter aids (common doses are 0.01 to 0.1 mg/1). By adding the right
215
-------
^DUST COLLECTOR
y/2-FILL PIPE (PNEUMATIC)
BULK STORAGE
BIN
DAY HOPPER
FOR DRY CHEMICAL
FROM BAGS OR DRUMS
BIN .GATE
FLEXIBLE
CONNECTION
ALTERNATE SUPPLIES DEPENDING
ON STORAGE
DUST COLLECTOR
^
BAG FILL
-SCREEN
WITH BREAKER
DUST AND VAPOR REMOVERy
WATER
SCALE OR SAMPLE CHUTE
, DRAIN
SOLENOID VALVE
CONTROL
PRESSURE REDUCING J?
VALVE
WATER SUPPLY
GRAVITY TO
APPLICATION
PUMP
TO APPLICATION
Figure 66. • Typical dry feed system.
216
-------
OVERHEAD
STORAGE
TANK
Control
valve
,_&&_.#—
-Rotodip-type
feeder
Gravity feed
Gravity feed
Relief
valve i
Back pressure
valve
/• Bac
V va
_&5>
Metering pump
Gravity feed
Figure 67. Alternative liquid feed systems for overhead storage.
Pressure feed
'Rotodip—type feeder
'Control valve
'Rotameter
GROUND
STORAGE
TANK
Gravity feed
Pressure
feed
Metering pump--1
Back pressure
valve
Figure 68. Alternative liquid feed systems for ground storage.
217
-------
amount of polymer at the right point in treatment, both turbidity and
phosphorus removal can be improved.
When polymers are used as a dry powder, complete wetting is necessary
using a funnel-type aspirator. After wetting, warm water must be added and
gently mixed for about 1 hr. Polymer feed solution strengths are usually
in the range of 0.2 to 2.0%. Stronger solutions are often too viscous to
feed. The solution usually is fed using positive displacement metering
pumps. Dilution water is added to the feed discharge to reduce line plug-
ging and to aid mixing. Like alum pumps, polymer feed pumps can be paced
automatically for plant flow.
Polymer solutions above 1% in strength should not be used because
they are very viscous and difficult to handle. Most powdered polymers are
very stable, but even in cool, dry conditions, they should not be stored
as powders for more than 1 yr. Once polymers are dissolved, they may
become unstable within 1 or 2 weeks.
Ferric Chloride—
Ferric chloride is available in dry, liquid, or crystalline (hydrated)
form. The crystalline form weighs 60 to 64 Ib/cu ft and the anhydrous form
weighs 85-90 Ib/cu ft. Most liquid forms contain 35 to 45% ferric chloride
and weigh 11.2 to 12.4 Ib/gal. The liquid should be handled in rubber,
glass, ceramics, or plastic. If iron salts are used for wastewater coagu-
lation in softwaters, a small amount of base (such as sodium hydroxide or
lime) is needed to neturalize the acidity of these strong acid salts.
Like alum and lime, ferric chloride is an effective coagulant used to
remove phosphorus and to lower suspended solids. Ferric chloride also can
be used as an oxidant to control odor problems coming from hydrogen sulfide.
Since ferric chloride is always fed as a liquid, it is normally obtained as
a liquid and unloaded pneumatically. The ferric chloride content of the
solution is about 35 to 45% FeCl3. It contains about 3.95 to 5.58 Ib FeCl3/
gal.
Feeding systems for ferric chloride are much like those for liquid
alum. The differences are the materials of construction, and the use of
glass tube rotameters. Because of hydrolysis, it may not be a good idea
to dilute the ferric chloride solution from its shipping concentration to
a weaker feed solution. Ferric chloride solutions may be transferred
from underground storage to day tanks with rubber-lined self-priming cen-
trifugal pumps having teflon rotary and stationary seals. Because liquid
ferric chloride can stain or deposit, glass-tube rotameters are not used
for metering. Instead, rotodip feeders and diaphragm metering pumps made
of rubber-lined steel and plastic often are used for ferric chloride feeding.
Methanol—
In the biological nitrification-denitrification process, an oxygen-
demand source such as methanol often is added to the wastewater in order
to reduce the nitrates quickly. Methanol may be used either in a column
or in a 3-stage reaction basin. (See Denitrification Section of this manual)
218
-------
Methanol is a colorless liquid, and non-corrosive (except to aluminum
and lead) at normal atmospheric temperature. Transfer pumps should always
have positive suction pressure and should be protected by a strainer. There
are three basic pumping arrangements which can be used: (1) diaphragm
chemical feed pumps using an adjustable stroke for volume control; (2)
positive displacement pumps with variable speed drives controlled by
either a flow meter or by counting revolutions; (3) centrifugal or re-
generative turbine pumps with variable speed drives controlled by a flow
meter.
Sodium Hydroxide—
Sodium hydroxide (NaOH) is a strong base used to neutralize an acidic
wastewater. Without proper neutralization, acid wastewaters can damage
treatment facilities and biological treatment processes. Sodium hydroxide
also is used for pH adjustment along with other chemicals used in the
treatment process.
Liquid caustic soda (sodium hydroxide) comes in two concentrations,
50% and 73% NaOH. The 50% solution contains 6.38 Ib/gal NaOH and the 73%
solution contains 10.34 Ib/gal NaOH. Sodium hydroxide comes in bulk ship-
ments, transferred to storage, and diluted just before feeding. Dilution
of liquid caustic soda below the storage strength may be good when volu-
metric feeders are used. Feeding systems for caustic soda are about the
same as for liquid alum except for materials of construction. A typical
feeder system schematic is shown in Figure 69. Feeders often are made of
ductile iron, stainless steels, rubber, and plastics.
Powdered Carbon—
Activated carbon most often is used to remove soluble organics. When
the powdered form is used, it is mixed directly with the wastewater, fed as
slurry, and removed by coagulation and settling. The carbon slurry is
transported by pumping the mixture at a high velocity to keep the particles
from settling and collecting along the bottom of the pipe. The velocity
of the slurry should not be greater than 10 ft/sec in order to
keep the pipe from wearing out,
keep from losing carbon particles during transport,
keep pressure at a good level.
Most plant feeding slurry concentrations around 10.7% or 1 Ib carbon/
gal water use either centrifugal pumps or a combination of centrifugal
pumps and eductors. Diaphragm slurry pumps or double-acting positive dis-
placement pumps are used for transporting higher concentrations. The carbon
slurry may be fed using a rotodip feeder.
Typical Design Criteria
Table 21 lists several types of chemical feeders often used in waste-
water treatment. The table also shows the use and limitations of each
feeder. Chemical feed systems must be very reliable. The design of the
system should depend on:
219
-------
TRUCK FILL LINE
DILUTION
WATER
SODIUM HYDROXIDE
STORAGE TANK
VENT. OVERFLOW
AND DRAIN
VENT, OVERFLOW
AND DRAIN
MIXER
SAMPLE TAP
SODIUM HYDROXIDE
FEEDER
POINT OF
APPLICATION
Figure 69. Typical caustic soda feed system.
220
-------
Type of Feeder
Dry feeder:
Volumetric:
Oscillating plate
Oscillating throat (universal)
Rotating disc
Rotating cylinder (star)
Screw
Ribbon
Belt
Gravimetric:
Continuous-belt and scale
Loss in weight
Solution feeder:
Nonpositive displacement:
Decanter (lowering pipe) ...
Orifice
Rotameter (calibrated valve)
Loss in weight (tank with
control valve).
Positive displacement:
Rotating dipper
Proportioning pump:
Diaphragm
Piston
Gas feeders:
Solution feed
Direct feed.
TABLE 21. TYPES OF CHEMICAL FEEDERS
Use
Any material, granules or
powder.
Any material, any particle
size.
Most materials including NaF,
granules or powder.
Any material, granules or
powder.
Dry, free flowing material,
powder or granular.
Dry, free flowing material,
powder, granular, or lumps.
Dry, free flowing material up
to IVMnch size, powder or
granular.
Dry, free flowing, granular
material, or floodable
material.
Most materials, powder,
granular or lumps.
Most solutions or light slurries
Most solutions
Clear solutions
Most solutions
Most solutions or slurries
Most solutions. Special unit
for 5% slurries.1
Most solutions, light slurries. ,
Chlorine
Ammonia
Sulfur dioxide
Carbon dioxide
Chlorine
Ammonia
Carbon dioxide
General
Use disc un*
loader for
arching.
Use hopper
agitator to
maintain
constant
density.
No slurries
No slurries . .
No slurries . .
Limitations
Capacity
cu ft/hr
0.01 to 35
0.02 to 100
0.01 to 1.0
8 to 2,000
or
7.2 to 300
005 to 18
0.002 to 0.16
0 1 to 3 000
0 02 to 2
0.02 to 80
0.01 to 10
0.16 to 5
0.005 to 0.1 6
or
0.01 to 20
0.002 to 0.20
0.1 to 30
0.004 to 0.1 5
0.01 to 170
8000 Ib/day max
2000 Ib/day max
7600 Ib/day max
6000 Ib/day max
300 Ib/day max
1 20 Ib/day max
1 0.000 Ib/dav max
Range
40 to 1
40 to 1
20 to 1
10 to 1
or
100 to 1
20 to 1
10 to 1
10 to 1
or
100 to 1
100 to 1
100 to 1
100 to 1
10 to 1
lOtol
30 to 1
100 to 1
100 to 1
20 to 1
20 to
20 to
20 to
20 to
10 to
7 to
20 to
1 Use special heads and valves for slurries.
2P1
-------
the form of chemical being fed,
properties of the chemical,
maximum waste flows,
reliability of the feeder.
In both storage and feeding, the capacity of a chemical feed system
is important. Chemicals must not be allowed to deteriorate with time,
and feeders must be able to work within the range of feeding rates re-
quired. Chemical feeder control can be manual, automatically proportioned
to flow, dependent on some form of process feedback, or a combination of
any two of these methods.
Most feeders use a small dissolving tank with a nozzle system and/or
a mixer. It is essential that the surface of each chemical particle be-
come completely wetted before entering the feed tank. This will avoid
clumping, settling or floating and make sure the chemical is well mixed.
The dissolver should be designed for the right capacity, detention
times, water requirements, and chemical properties.
Chemical Dosages and Performance Evaluation
Coagulants—
The amount of coagulant needed for good coagulation depends on the
properties of the wastewater. Aluminum and iron salt dosages for good
phosphorus removal are generally proportional to the phosphorus concentra-
tion, and lime dosages depend mostly on the alkalinity of the wastewater.
Magnesium hydroxide, (formed when lime is added to hard waters containing
the magnesium ion) also helps remove colloidal material. It may cause
problems in sludge dewatering. Most coagulant doses are aluminum sul-
fate (alum), 75 to 250 mg/1; ferric chloride, 45 to 90 mg/1; and lime 200
to 400 mg/1. At these dosages, lime, alum, and ferric chloride can re-
move 85 to 95% phosphorus by sedimentation; and 98-99% when sedimentation
is followed by filtration.
Sometimes extra lime may be needed to form a fast-settling floe.
Lime also is used along with aluminum or ferric coagulants for pH control.
Figure 70 shows how lime dosage affects alkalinity to get a pH of 11
(commonly used for ammonia stripping).
Lime coagulation can reduce heavy metal concentrations because many
metals form insoluble hydroxides at pH 11. With the exception of mercury,
cadmium, and selenium, 90 percent or more of most heavy metals may be re-
moved by lime coagulation at pH 11. Also, at this pH, lime coagulation
removes viruses, but disinfectants such as chlorine or ozone also must be
used for complete disinfection.
Alum coagulation can remove 95 to 99% of viruses, and ferric chloride
coagulation can remove 92 to 94%. With both alum and ferric chloride,
good virus removal depends on good floe formation. With filtration, coagu-
lation using alum or ferric chloride can remove 99% of viruses.
222
-------
O
n
O
500
400
I 300
£
o
u.
Q
UJ
E 200
5
o
UJ
E
UJ
O 1000
Q
Ul
Figure 70.
100 200 300 400 500
WASTEWATER ALKALINITY, mg/l as CaCO3
Lime dosage as related to wastewater alkalinity.
Nitrification
Effluent
r
KEY
FT = Flow transmitter
FFIK = Ratio station
AIT = Wet chemical analyzer
HIK = Auto/Manual control
station
X = Analog multiplier
To Denitrificatlon
tanks
Figure 71. Feedforward control of methanol based on flow and
nitrate nitrogen.
223
-------
The following illustrates calculation of lime feed rates for a plant
where lime recovery by recalcining of lime sludges is practiced:
Plant flow
Total CaO dosage =
Ca(OH)2
380 mg/1 Cao
Makeup lime =
Recalcined lime =
Makeup lime dosage =
Recalcined dosage =
Makeup lime dosage =
95 x 8.34
791 x 100
92
860 x 15
15 mgd
380 mg/1
1.32 x CaO; therefore
1.32 x 380 = 500 mg/1 Ca(OH)2
92% CaO
70% CaO
25% of total
75% of total
0.25 x 380 mg/1
95 mg/1 as Cao
791 Ib of CaO/MG
860 Ib makeup lime at 92% purity/MG
12,900 Ibs for 15 MG, Ib/day of makeup
lime
12,900
24
Recalcined lime
dosage
285 x 8.34
2,377 x 100
70
3,396 x 15
= 536 Ib/hr of makeup lime (makeup lime
feeder setting)
= 0.75 x 380 mg/1
= 285 mg/1 CaO
= 2,377 Ib of CaO/MG
= 3,396 Ib of recalcined lime at 70%
purity/MG
= 50,940 Ib for 15 MG (Ib/day of recalcined
lime)
50,940
24
= 2,123 Ibs/hr of recalcined lime (recalcined
lime feeder setting)
The following illustrates the calcuation of alum feed rates for a
piston feed pump system:
Three alum feed pumps; two dual head and one single head.
Dual head pumps 0-50 gpm at 100% stroke/head
Single head pump 0-11.5 gph at 100% stroke
Plant flow
Liquid alum
strength
Alum dosage
Alum dosage
lb/15 mgd
= 15 mgd
= 5.4 Ib dry alum/gal
= 20 mg/1
= (20 mg/1) x (8.34 Ib/gal)
= 167 lb/MG
= (167 lb/MG) x (15 mgd)
= 2,505 Ib/day
224_
-------
Ib/hr = 2,505 Ib/day
24 hr/day
= 104 Ib/hr
gal/hr = 104 Ib/hr
5.4 Ib/gal
= 19 gal/hr
One pump = (19 gal/hr)
(2 heads) (100 gal/hr)
= 19 (use 20% stroke setting)
Methanol—
For methanol in the nitrification-denitrification process, the
methanol feed to nitrate-nitrogen ration often is about 3 to 1.
Powdered Carbon—
The design and performance of powdered activated carbon systems
depends mostly on contact time and pretreatment of the wastewater. The
effect of these items on carbon dosage is discussed in the Activated
Carbon section of this manual.
Control Considerations
pH Adjustment—
Lime coagulation often raises the pH to 10 or 11. At this pH, the water
is unstable and calcium carbonate floe will precipitate readily. This floe
will cause problems by encrusting downstream pumps, piping, filters, and
carbon particles. The pH must be lowered below 8.8 to stabilize the water.
For good carbon adsorption and disinfection, it is also usually necessary
to reduce the pH to less than 7.5. In large plants, pH usually is lowered
by injecting carbon dioxide gas ("recarbonation process") into the waste-
water. In small plants, acid is used to lower the pH.
Control of Coagulant Dosage—
Proper control of coagulant dosage is the jar test. A sample of waste-
water is collected, taken to the laboratory, and divided into several
beakers. Different coagulant dosages are then added to each beaker.
After mixing, the floe is allowed to settle. The sample having the most
clarity then is used to determine the best coagulant dose.
Methanol Feed Control—
Because methanol is expensive and too much can cause high effluent
it is important to accurately pace methanol dosage with the nitrogen load.
Pacing methanol dose with plant flow does not work well since it does not
account for changes in the nitrate concentration. Feed forward control
using plant flow and nitrification effluent nitrate-nitrogen is shown in
Figure 71. Feed ratio is about three parts methanol to one part of
nitrate-nitrogen (by weight) . This method requires continuous nitrate
measurement using an automatic wet chemistry analyzer. The wet chemistry
analyzer (AIT) output is proportional to the nitrate concentration in
the effluent. The dependability of this system depends on how reliable
225
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the automated wet chemical analyzer is. These analyzers require very
careful regular maintenance and calibration.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Use of iron or aluminum
salts adds significant
quantities of dissolved
solids - such as sulfate
and chloride to the
treated water.
2. Inadequate equipment for
monitoring coagulation
process installed.
Lime added to hard waters
containing magnesium may
form MgOH which is a
gelatinous precipitate that
may adversely affect sludge
dewatering.
Lack of flexibility in
points at which chemicals
can be added to wastewater
processes.
Dry feed chemicals deposit
in feeder.
6. Corrosive properties
of some chemicals.
1. If possible, use lime instead
of alum or ferric chloride as
coagulant.
2. Run frequent jar tests; install
continuous turbidity monitoring
equipment on effluent from
clarifiers or filters.
3. Reduce operating pH to 10.5 or
less.
Run hoses from chemical feeders
to desired points of chemical
addition until suitable piping
can be installed.
Provide mechanical mixers for
dissolving solids and maintain-
ing them in suspension prior to
delivery to feeder.
Use proper materials for trans-
port and handling of chemicals.
226
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TROUBLESHOOTING GUIDE
CHEMICAL FEEDING AND CONDITIONING
INDICATORS/OBSERVATIONS
1. High turbidity in the
settling tank
effluent.
2. Air slaking occurring
during storage of
quicklime.
3. Feed pump discharge
line clogged.
4. Grit conveyor or
slaker inoperable.
5. Paddle drive on
slaker is overloaded.
6. Lime deposits in
lime slurry feeder.
7. "Downing" or incomplete
slaking of quicklime.
PROBABLE CAUSE
la. Improper chemical
dosage.
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 interferring
with paddle action.
6a. Velocity too low.
7a. Too much water is
being added.
CHECK OR MONITOR
la. Jar test.
Ib. Visual inspection.
2a. Humidity, storage
facility not air-
tight.
3a. Visual inspection.
4a. Broken shear pin.
5a. Visual inspection.
5b. Visual inspection.
7a. Hydrated particles
coarse due to rapid
formation of a
coating.
SOLUTIONS
la. Correct dosage according to
results of jar test.
Ib. Repair failure in feed system.
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
materials or use a better grade
of lime.
6a. Maintain continuously high
velocity by use of a return
line to the slurry holding
tank.
7a. Reduce quantity of water added
to quicklime (detention slakers
water to lime ratio = 3-1/2:1
paste slaker ratio = 2:1)
to
to
-------
TROUBLESHOOTING GUIDE
CHEMICAL FEEDING AND CONDITIONING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8. "Burning" during
quicklime slaking.
'8a. Insufficient water
being added, result-
ing in excessive
reaction temperatures.
8a. Some particles left
unhydrated after
slaking.
8a. Add sufficient water for
slaking (See Solution 7).
9. Chemical feed lines
ruptures.
9a. Positive displacement
pump has been started
against a closed
valve.
9a. Valve positions.
9a. Open valves in feed lines
before pump is started.
to
oo
-------
RAPID MIXING AND FLOCCULATION
Process Description
When coagulants are added to wastewater, good mixing is necessary
for effective treatment. Rapid mixing basins are used to completely
mix the coagulant particles with the wastewater. Figure 72 shows a
rapid mix basin with a mechanical mixer.
After the rapid mix process, the coagulated wastewater flows to a
flocculation basin where slow-mixing can occur. The flocculation pro-
cess stirs the water slowly to allow large particles to form which can
then be removed by gravity settling. Mechanically driven paddles (Fig-
ure 73) or air mixing may be used for stirring. When flocculation is
carried out in separate multiple basins, two or three basins usually
are placed in series, each one providing more gentle stirring as
the water moves down the line.
Figure 74 shows a schematic of the coagulation and flocculation pro-
cesses and how they are related to one another.
Typical Design Criteria and Performance Evaluation
Rapid Mixing—
Rapid mixing basins for dissolving the coagulants usually have
high-speed mixers that can operate at 300 fps/ft with detention times
of 15-60 sec. Power requirements for mechanical mixers are 0.25 to 1 hp/
mgd. The following shows design data for a rapid mixer that can handle
a flow of 10 mgd:
Detention time at maximum flow, in minutes 1.1
Width, in feet 11.0
Water depth, in feet 8.5
Volume, in cubic feet 1,030
Propeller diameter, in inches 38
Propeller capacity, in cubic feet per minute 2,060
Shaft speed, in revolutions per minute 100
Motor horsepower 5
Flocculation—
Flocculators usually are operated at 0.6 fpm, with velocity gradients
of 30-100 fps/ft. Flocculator detention times of 15-60 minutes are commonly
used.
When flocculation is carried out in a separate basin from the clari-
fier, the flow from the floe basin to the clarifier should be kept between
229
-------
MOTOR
OVERFLOW -*
FEED
Figure 72. Mechanical rapid-mixing device.
Figure 73. Typical paddle-type flocculator.
230
-------
Coagulant
Wastewater
Polymer (optional)
Rapid mix
To Disposal
Figure 74. Schematic of rapid mix, flocculation and sedimentation
system using separate basins.
231
-------
0.5 and 1.0 fps to keep the floe from breaking up.
For a flow of 10 mgd, the following dimensions are typical:
Width, in feet 30
Depth, in feet 10
Length, in feet 140
Volume of tank, in cubic feet 41,778
Detention period, in minutes 45
Performance Evaluation
The flocculation and rapid mix processes can only be considered
efficient operations if the downstream clarifier produces a good quality
effluent. If the flocculation basin produces a good floe that does not
break up and which settles quickly in the clarifier, the process can be
considered to operate effectively. If it does not produce a good floe,
the troubleshooting guide should be read for possible solutions to the
problem.
Control Considerations
Rapid Mixing—
Good mixing of the coagulant and wastewater is important to ensure
efficient use of the coagulant. An overfeeding of chemicals usually is
needed if the system is poorly mixed.
The mixing must be rapid enough to obtain uniformly dissolved chem-
icals throughout the wastewater before the floe begins to form. Too
much rapid mixing will cause floe break-up into fine particles that
settle at a very slow rate.
Flocculation—
The velocity of flocculator mixers must be carefully controlled.
Very high velocity gradients will prevent a settleable floe from form-
ing. To produce compact floe particles, the highest velocity gradient
should be used which still produces a strong floe.
Since flocculated particles are quite fragile, velocities in and
after the flocculating unit generally should not be greater than 1.2
fps.
In wastewater treatment, a blanket should not be allowed to form
in the clarifier, because the sludge contains organics that may quickly
turn anaerobic. This will lower phosphorus removal and clarifier effi-
ciency.
232
-------
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
Mechanical flocculators
fouled with rags and
debris.
2. Poor floe formation.
Remove rags and screened debris
from wastewater stream and dis-
pose separately. Do not run
debris through a comminuter and
return debris to the treatment
process.
Increase solids concentration by
recirculation of chemical solids
from clarifier sludge removal
line to rapid mix or flocculator
influent line.
233
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TROUBLESHOOTING GUIDE
RAPID MIXING AND FLOCCULATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Poor floe formation
and settling charac-
teristics.
la. Chemicals not suffi-
ciently dispersed
during rapid mixing.
Ib. Prolonged rapid mix-
ing.
Ic. Improper coagulant
dosage.
Id. Flocculator agitators
operating at excessive
speeds.
la. Chemicals not uniformly
distributed in rapid
mix basin.
Ib. Floe formation break-
up in rapid mix basin,
rapid mix detention
time.
Ic. Jar test.
Id. Agitator speed.
la. Increase speed of rapid mixing
device.
Ib. Reduce rapid mixing time (usually
15-60 seconds is sufficient).
Ic. Correct coagulant dosage as per
jar test results.
Id. Reduce flocculator agitator
speed.
NJ
OJ
2. Good floe formation in
flocculation basin but
poor settleability in
clarifier tank.
2a. Excessive velocity
between flocculation
unit and clarifier
unit.
2a. Velocity between
flocculation and
clarifier units.
2a. Reduce velocity (usually veloc-
ity should not exceed 1.2 fps).
3. Clarifier sludge turn-
ing anaerobic.
3a. Sludge blanket pres-
ent in clarifier.
3b. Excessive carryover of
organic solids from
secondary process to
chemical clarifier.
3a. Visual inspection.
3b. Secondary effluent
quality.
3a. Increase sludge removal rate to
remove sludge blanket and pre-
vent its formation.
3b. Correct problems in secondary
process (see appropriate section
of this manual).
-------
RECARBONATION
Process Description
The main purpose of recarbonation is to inject carbon dioxide gas
into the wastewater to lower the high pH resulting from lime treatment.
When carbon dioxide is added, the pH is lowered and the hydroxides are
converted to carbonates and bicarbonates. Recarbonation prevents cal-
cium scale formation.
Recarbonation can be carried out in either one or two stages of
treatment. In single-stage recarbonation, the pH of the water is usually
reduced from 9-11 down to 7-8 by applying all of the carbon dioxide at
one point. This results in the calcium (which is added with the lime)
being dissolved and being present in the plant effluent. In two-stage
recarbonation, the pH is lowered to about 9.3 in the first stage. At
this pH, calcium carbonate floe readily forms, and allows the calcium
to be removed easily from the wastewater making it available for recov-
ery of lime. The floe often needs a coagulant or settling aid, and
enough CO is then added to further reduce the pH to the desired value.
The second stage of recarbonation to pH 7 does several things:
It prepares the water for filtration
It lowers the pH to a value which aids carbon adsorption of
organics; provides effective disinfection by chlorine; and
a pH value good for dischage
It keeps scale from forming in pipelines
The usual source of carbon dioxide for recarbonation is stack gas
either from sludge incineration or lime recalcining furnaces. When
stack gas is not available, special CO9 generators, underwater burners,
or liquid CO may be used.
Typical Design Criteria and Performance Evaluation
Stack Gas as a Source of CO2—
If stack gas is used as a source of CO2, the gas should be passed
through a wet scrubber to cool the gas and to remove solids from the
gas. Scrubbers remove air pollutants from the exhaust gas, and help
to protect the CO2 compression equipment against plugging or scaling.
With 8 percent CO2 in the stack gas about 270 CFM of compressor
capacity is needed per mgd of water for recarbonation. A water-sealed
235
-------
compressor should be used when dirty stack gas from sludge or lime fur-
naces is used.
A typical recarbonation system using stack gas at atmospheric pres-
sure is shown in Figure 75. As shown in the figure, automatic pH control
of the recarbonated effluent can be provided by the system.
Pressure Generators—
Pressure or forced-draft generators produce CC>2 by burning natural
gas, oil, or other fuels in a pressure chamber. The fuel and excess
air first are compressed, and then burned at a high enough pressure to
feed directly into the water to be recarbonated. The compressors can
use only dry gas or dry air at ambient temperatures, so there are no
corrosion problems resulting from hot, moist stack gases. Because of
limited capacity range (usually 3-1), two or more units may be needed
for flexibility and process control.
Submerged Underwater Burners—
In this system, air and natural gas are compressed and then burned
underwater in the recarbonation basin. Automatic underwater ignition
equipment is used to start the burning process. Like pressure generators,
underwater burners have a limited capacity range (about 2-1) so that it
is necessary to have enough burners to get the desired range of CC>2
dosages.
Liquid Carbon Dioxide—
Liquid carbon dioxide is available in tank trucks ranging in size
from 10-20 tons. For smaller plants, 20-50 Ib cylinders are more common.
Liquid CC>2 may be fed using either liquid or gas feed systems.
When CC>2 is removed from the storage tank, the pressure usually is re-
duced in two stages before feeding at 20 psi. This reduces the chances
for dry ice formation which could occur if the expansion takes place
too quickly. Vapor heaters also may be used just ahead of the pressure
reducing valves.
For CO2 gas feed, flow may be measured using a manometer in the
feed line. A manual valve may be placed downstream to control the amount
of CC>2 applied. The CC>2 feed also can be made fully automatic using pH
control. In this case, an electrode is used to measure the pH of the
recarbonated water. The signal is sent through a control valve on the
feed line which closes at low pH and opens it at high pH.
Except for materials of construction, equipment for solution feed
of CC>2 is much like solution feed chlorinators. When used to feed CC>2,
chlorinator capacity is reduced about 25 percent. About 60 gal of
water are needed to dissolve 1 Ib of CO2 at room temperature and atmos-
pheric pressure. With solution feed of CO2 almost 100 percent absorption
is possible.
Design of Diffusion Systems and Reaction Basins—
Carbon dioxide distribution grids usually are pipes with holes and
23-6
-------
-FURNACE STACK
-STACK GAS SUPPLY LINE
-WET SCRUBBER, WATER JET IMPINGEMENT TYPE
EXPANSION JOINTS
OR FLEXIBLE
COUPLINGS
THREE WATER
SEALED C02 -
COMPRESSORS
EXPANSION JOINTS
OR FLEXIBLE
COUPLING
WATER SEPARATOR
DRAIN LINE
PNEUMATIC SIGNAL TO
/• VALVE POSITIONER
WATER SEAL SUPPLY
,-LINE TO COMPRESSORS
STRAINER
SOLENOID
VALVE
ORIFICE UNION
C02
DISTRIBUTION GRIDS
BASIN INFLUENT,
HIGH pH WATER
PRIMARY
RECARBONATION
BASIN
BASIN EFFLUENT,
RECARBONATED WATER
SECONDARY
RECARBONATION
BASIN
Figure 75. Typical recarbonation system using stack gas.
237
-------
submerged in the wastewater at least 8 feet. This will make sure that
the CC>2 bubbles dissolve before they reach the surface. Well designed
systems put 85-100 percent of the CC>2 into solution.
Figure 76 shows the two-stage recarbonation basins being used at
the Orange County Water District Facility. Table 22 describes the de-
sign data for the system.
Control Considerations
Using pH measurements, control of CC>2 dosages may be determined by
trial and error. In two-stage systems, the CO2 flow first should be set
to reduce the pH in the second-stage recarbonation basin. Then, this
flow should be split between the two stages to get a pH of 10 in the
center of the intermediate settling basin. Once the CO2 valves are set
for a proper split, they should need very little other adjustment for
changes in total C02 flow. At average wastewater temperatures, about
15 min are needed for all CO2 bubbles to react completely to form cal-
cium carbonate. This is why pH is measured at the middle of the settling
basin rather than at the entrance.
Common Design Shortcomings and Ways to Compensate
Shortcomings Solution
1. No backup or auxiliary 1. Install liquid CO2 feed system
system to supplement for use when stack gas is not
stack gas as CO2 source. available.
2. C02 compressor equipment 2. Improve operation of wet scrubber
becomes plugged or scaled to remove particulates before gas
by particulates from stack enters compressor.
gas.
3. Pressure generators and 3. Install 2 or more units in order
underwater burners have a to provide more flexibility.
limited capacity range.
238
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SECOND STAGE
RECARBONATION
Effluent to
filtration (Main
building)
NJ
00
ID
INTERMEDIATE SETTLING
FIRST STAGE
RECARBONATION
CO2 Distribution
lines
Ammonia
Tower effluent
CO2 Distribution
lines
Figure 76. Recarbonation basins at Orange County Water District Facility.
-------
TABLE 22. DESIGN CRITERIA FOR ORANGE COUNTY WATER
DISTRICT RECARBONATION PROCESS
Basins
Number
2 (parallel)
1st Stage Recarbonation Basin (each basin
divided into two compartments, equipped
with oscillating flocculators and C0_
distributors)
Length, ft
Width, ft
Depth, ft
Water depth, ft
Detention time, min @ 15 mgd
16
36.75
10
9
15
Intermediate Settling
Length, ft
Width, ft
Sidewall depth, ft
Sidewater depth, ft
Overflow rate, gpd/ft @ 15 mgd
Weir loading, gpm/ft @ 15 mgd
Detention time, min @ 15 mgd
70.75
73.5
12.17
10.
3000
18.
40
2nd Stage Recarbonation Basin (three of
four compartments equipped with C©2
distributors)
Length, ft
Width, ft
Depth, ft
Water depth, ft
Detention time, min @ 15 mgd
7.17
36.75
12
10
15
240
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TROUBLESHOOTING GUIDE
RECARBONATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Inadequate CC>2 supply
to recarbonation
basins.
la. CC>2 compressor mal-
function.
Ib. Plugging of CO2
diffusers.
Ic. Improper valving.
la. Visual inspection.
Ib. Visual inspection.
Ic. Valve position.
la. Connect auxiliary liquid CC>2
source and inspect compressor
for damage.
Ib. Clean or replace diffusers.
Ic. Open valves.
2. Sludge scraper in
intermediate settling
basin stops.
2a. Object jammed under
scraper arm.
2a. Visual inspection.
2a. Remove object jamming system.
White scale is deposit-
ing in pipes or other
treatment units down-
stream of recarbona-
tion.
3a. Calcium carbonate
deposits.
3a. pH of recarbonation
effluent should be less
than 8.8 to prevent
scale deposition.
3a. Increase CO2 additions to recar-
bonation basin to lower effluent
pH to less than 8.8(see Item 1).
4. In two stage system,
calcium carbonate floe
is being formed in
first stage but does
not settle well in
intermediate basin.
4a. Floe size too small.
4a. Milky appearance in
intermediate settling
basin.
4a. 1. Add coagulants such as FeCl3
or settling aids such as
polymers.
2. Recirculate settled calcium
carbonate sludge to first
stage basin to provide nuclei
for floe formation.
-------
LAND TREATMENT
Process Description
In land treatment, effluent usually is pretreated and applied to
land by conventional irrigation methods. With this process, wastewater and
its nutrients are used as a resource rather than considered as a disposal
problem. Treatment is provided by natural processes as the effluent flows
through the soil and plants. Part of the wastewater is lost by evapotrans-
piration, and the rest goes back to the hydrologic cycle through overland
flow or the groundwater system. Most of the groundwater finally returns,
directly or indirectly, to the surface water system. Land treatment of
wastewater may be done by one of the following methods (Figure 77):
Irrigation
Overland flow
Infiltration-percolation
In the irrigation method, the wastewater is applied to the land by
sprinkling or by surface spreading (Figure 78). Sprinkling systems may
be either fixed or moving. Fixed sprinkling systems, often called solid
set systems, may be either on the ground surface or buried. Both types
usually have sprinklers set on risers that are spaced along pipelines.
These systems can be used on many kinds of terrain and may be used for
irrigation of either cultivated land or woodlands. There are several
kinds of moving sprinkling systems, but the center pivot system is the most
widely used for wastewater irrigation.
The two main types of surface application systems are ridge-and-furrow
and flooding techniques. In ridge-and-furrow irrigation, the effluent
flows by gravity through furrows where it seeps into the ground.
Typical removals of pollutants from secondary effluent by irrigation
are:
BOD 98%
COD 80%
Suspended solids 98%
Nitrogen "85%
Phosphorus 95%
Metals 95%
Micro-organisms 98%
In an overland flow system, the wastewater is sprayed over sloping
terraces where it flows slowly down the hill and through the vegetation.
242
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EVAPORATION
CROP
SPRAY OR
SURFACE
APPLICATION
ROOT ZONE
SUBSOIL
SLOPE
VARIABLE
•DEEP
PERCOLATION
(a) IRRIGATION
SPRAY OR SURFACE APPLICATION
':.>£• PERCOLATION THROUGH '-.^^
if•'•'::•: UNSATURATED ZONE
ORIGINAL
WATER TABLE
LAND APPLICATION
SLOPE 2-4%
(b) INFILTRATION-PERCOLATION
EVAPORATION
GRASS AND VEGETATIVE LITTER
SHEET FLOW
C RUNOFF
OLLECTION
(c) OVERLAND FLOW
Figure 77. Methods of land application.
243
-------
Rain Drop Action
\\ \\\\\\\\\ \\\\ \
(a) SPRINKLER
Completely flooded
(b) FLOODING
(c) RIDGE AND FURROW
Figure 78. Irrigation techniques.
244
-------
Although the soil is not the primary filter, treatment efficiencies are high
in a well-run system. Typical removals are:
BOD 92%
Suspended solids 92%
Nitrogen 70-90%
Phosphorus 40-80%
Metals 50%
Soils best suited for this approach are clays and clay loams with even,
moderate slopes (2-6%). Grass usually is planted to provide a habitat for
biota and to prevent erosion. As the effluent flows down the slope, some
flows into the soil, a small amount evaporates, and the rest flows to col-
lection channels. As the effluent flows through the grass, the suspended
solids are filtered out and the organic matter is oxidized by the bacteria
living in the vegetative litter.
In infiltration-percolation systems, the groundwater is recharged by
percolation of wastewater (after secondary treatment) using spreading
basins. There is not much difference between treatment and disposal in
this process. Wastewater applied to the land for the purpose of disposal
is also being treated by infiltration and percolation. Removals by this
system for secondary effluent are:
BOD 85-99%
Suspended solids 98%
Nitrogen 0-50%
Phosphorus 60-95%
Metals 50-95%
Infiltration-percolation is mostly a groundwater recharge system, and
does not recycle nutrients through crops.
Land application systems usually include the following parts:
Preapplication treatment
Transmission to the land treatment site
Storage for the wastewater during the non-irrigation season
Distribution over the irrigated area
A system to recover the renovated water
The crop system
Typical Design Criteria and Performance Evaluation
There are no two land application systems exactly alike, and design
depends on many factors, especially the site and project objectives.
Table 23 briefly lists some of the major design differences. This table
can be used as a guide for performance evaluation of actual land treat-
ment systems. If a system does not perform as shown in the table, the
troubleshooting guide should be checked. Each of the major parts of a
land treatment system that must be carefully designed and operated are
discussed below.
245
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TABLE 23. TYPICAL DESIGN CRITERIA FOR IRRIGATION, INFILTRATION-PERCOLATION, AND OVERLAND FLOW
SYSTEMS FOR MUNICIPAL WASTEWATER
Liquid loading
rate , in/wk
Annual applica-
tion , f t/yr
Land required
for 1 mgd flow-
rate , acresa
Application
techniques
Vegetation
required
Crop production
Soils
Climatic
constraints
Wastewater lost
to:
Expected treatment
performance
BOD and SS removal
Nitrogen removal
Phosphorus removal
Irritation
Low rate High rate
0.5 to 1.5 1.5 to 4.0
2 to 4 4 to 18
280 to 560 62 to 280
Spray or surface
Yes Yes
Excellent Good/Fair
Moderately permeable soils
with good productivity
when irrigated
Storage often needed
Evaporation and
percolation
98+1
85+la
80 to 99%
Infiltration-percolation
4 to 20
18 to 500
2 to 62
Usually surface
No
Poor/none
Rapidly permeable soils,
such as sands, loamy
sands, and sandy loams
Reduce loadings in
freezing weather
Percolation
85 to 99%
0 to 50%
60 to 95%
Overland flow
2 to 9
8 to 40
28 to 140
Usually spray
Yes
Fair/Poor
Slowly permeable soils,
such as clay loams and
clays
Storage often needed
Surface runoff and
evaporation with some
percolation
92+1
70 to 90%
40 to 80%
Dependent on crop uptake.
-------
Preapplication Treatment—
The type and level of treatment will have an effect on:
The loading rate of different contaminants
The methods of application to be used
The type of crop or vegetative cover to be grown
Before wastewater is applied to the land, it is usually treated by
oxidation ponds, aerated lagoons, or secondary treatment processes, followed
by disinfection.
Transmission—
The three main ways of transferring the wastewater to the point of
application are:
Gravity pipe
Open channels
Force mains
The method of transmission depends mostly on terrain. When open
channels are used, however, the chances of public contact with the waste-
water must be carefully considered.
Storage—
Requirements for storage may range from 1 day to 6 months of flow.
Storage capacity should be based on the local climate and the time period
of operation for the system. System backup and flow equalization also
should be considered.
To prevent percolation to the groundwater, some storage reservoirs
must be lined.
Distribution—
Solid set spraying—Solid set spraying using buried pipe is used mostly
for spray irrigation systems. It may also be used for infiltration-percola-
tion and overland flow systems. Designs may differ in sprinkler spacing,
application rate, nozzle size and pressure, depth of buried pipe, pipe
materials, and type of control system.
Sprinkler spacing - May range from 40 to 60 ft to 100 by 100 ft.
Spacing may be rectangular, square, or triangular.
Application rate - May range from 0.10 to 1 in/hr or more, with
0.16 to 0.25 in/hr being most common. Annual rates depend on
climate, soil type, and crop requirements and may range from 1
to 15 ft/yr.
Nozzles - Openings range in size from 0.25 in to 1 in. The dis-
charge per nozzle often is 8 to 25 gpm, with discharge pressures
ranging from 50 to 60 psi.
247
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Depth of buried laterals and mainlines - Where freezing is not
a problem, a depth of 18 in for laterals and 36 in for mainlines
is common. Aluminum surface piping may be 40 to 50% less costly
than buried piping, but it is also less reliable.
Pipe materials - May be any type used for standard pressure
pipe; however, asbestos-cement and plastic (PVC) pipes are
most common.
Center pivot spraying—For moving sprinkling systems, center pivot
systems are most often used.
Sizes - System laterals may be 600 to 1,400 ft long. The lateral
is supported by wheels which are free to rotate. Each unit can
irrigate areas of 35 to 135 acres.
Propulsion - May be by means of either hydraulic or electric
drive. One rotation may take from 8 hrs to as much as 1 wk.
Pressures - Usually 50 to 60 psi at the nozzle, which may re-
quire 80 to 90 psi at the pivot. Standard sprinkler nozzles
or spray heads are used.
Surface flooding—For surface flooding using border strips, there are
several different designs:
Strip dimensions - Depend on type of crop, type of soil, and
slope. Border widths are usually 40 to 60 ft, and slopes may
range from 0 to 0.4%. The steeper slopes are used with relatively
permeable soils. Strip length may range from 600 to 1,400 ft.
Method of distribution - Generally by a concrete-lined ditch with
slide gates at the head of each strip, or underground pipe with
risers and valves.
Application rates - At the head of each strip, rates depend
mostly on soil type. Rates may range from 10 to 20 gpm/ft
width of strip for clay, and 50 to 70 gpm/ft width of strip
for sand. The period of application for each strip depends on
strip length and slope.
The major design variables are:
Application - Usually by gated aluminum pipe. Short runs of
pipe (80 to 100 ft) are used to minimize pipe diameter and head-
loss and to provide maximum flexibility. Surface standpipes
are used to provide the 3 to 4 ft of head necessary for even
distribution.
Topography - Relatively flat land (less than 1% slope) with
furrows running down the slope, or on moderately sloped land
with furrows running along the contour.
248
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Dimensions - Furrow lengths usually range from 600 to 1,400 ft.
Furrows are usually spaced between 20 and 40 in apart, depending
on the crop.
Overland flow distribution—General practice is as follows:
Sprinkler application - May be by either fixed sprinklers or
rotating boom-type sprays. Sprinklers are spaced from 60 to 80
ft apart on the laterals.
Surface application - May be by flooding or by gated pipe.
Works best with wastewater low in organic solids.
Slopes - Slopes may range from 2 to 8%, but a 2 to 4% slope
is best for adequate detention time. Lengths of slope may
range from 150 to 300 ft.
Application cycles - Commonly 6 to 8 hrs of wetting and 16 to
18 hrs of drying to keep the microorganisms active on the soil.
Infiltration basins —
Application rates - Rates range from 4 to 120 in/wk, with the
most common rate being 12 to 24 in/wk. Loading cycles may be
from 9 hrs to 2 weeks of wetting, followed by 1 day to 3 weeks
of drying.
Basin size - Usually depends on flow, and the wetting and drying
periods. Basins may range in size from less than 1 acre to
more than 10 acres. Usually at least two separate basins are
provided.
Height of dikes - Depends on the depths of water applied. For
depths of 1 to 2 ft, about 4 ft dikes are common.
Recovery of Renovated Water—
Systems that may be used to recover renovated water include underdrains,
runoff collection followed by chlorination and discharge, and recovery
wells.
Underdrains—An underdrain system normally has a number of drainage
pipes (4 to 8 inches in diameter) and buried 4 to 10 ft. These pipes
empty at one end of the field into a ditch. The distance between pipes
can range from 100 ft for clayey soils to 400 ft for sandy soils.
Tailwater return—A tailwater return system is used with surface irri-
gation to collect and return excess applied water from the bottom of the
strip or furrow. The system usually has collection ditches, a small reser-
voir, a pump, and piping to the nearest distribution line.
Recovery wells—Recovery well systems are used for reducing ground-
water levels to make sure that treatment is effective or to collect and
249
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reuse renovated water. Design variables include well location and spacing,
depth, type of packing, and flowrate. Each of these variables depends on
the geology, soil, and groundwater conditions at the site, application
rates, and the amount of renovated water to be recovered.
Crops—
Except for infiltration-percolation systems, crops or vegetative cover
are an important part of all land application systems. In selecting the
type of crop to be grown, the following should be considered:
Water requirement and tolerance
Nutrient requirements, tolerances, and removal capabilities
Sensitivity to inorganic ions
Public health considerations relating to the use of the crop
Ease of cultivation and harvesting
Length of growing season
Value of crop (marketability)
Control Considerations
The best operation of irrigation systems requires good crop management
and proper wastewater pretreatment. Personnel must have a working know-
ledge of farming practices, and principles of wastewater treatment. Seasonal
(often weekly) changes in operation must respond to changing crop require-
ments for nutrients and water; monitoring must be done to determine removal
efficiencies and to forecast buildups of toxic compounds; and the system
must be continuously watched to avoid problems of ponding, runoff, or
mechanical breakdowns.
There are several different ways land treatment systems can be managed:
Managed and operated by the wastewater agency
Managed by the wastewater agency and operated by a private
party through contract or crop sharing
Managed and operated by agreement with one private party
Managed by agreement with a private party and operated by a
subcontract agreement with another private party.
Close cooperation between the treatment system management and the
farm operation is always needed. Irrigation must be scheduled with farm
operations such as planting, tilling, spraying and harvesting, for success-
ful operation. Farm specialists can be helpful in setting up the manage-
ment of the crops, soil, and irrigation portions of the operation.
Proper cropping is also important for good nitrogen removal to prevent
pollution of groundwater. Nitrate-nitrogen can be removed by growing and
removing from the area a crop which takes up nitrogen. Nitrogen removal by
crops is dependent on the length of growing season, crop type, and the
availability of nitrogen.
250
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Crop requirements for nitrogen during the growing season are about
the same as evapotranspiration demand. Thus, applications based on
seasonal changes in evapotranspiration may be better than constant appli-
cation.
Crops normally used with land application can be divided into three
broad groups and removal rates. A forage crop, such as grasses, will re-
move 150-160 Ibs/acre or more of nitrogen, field crops, such as corn, will
remove 75-150 Ibs/acre, and forests will remove 20-100 Ibs/acre. Some crops
do not fit under these general removal groups because of differences in the
length of the growing season.
Common Design Shortcomings and Ways to Compensate
3.
4.
5.
Shortcoming
Water ponding during project
start-up because of poor
site choice.
Plastic laterals installed
above ground are breaking
because of cold weather or
deterioration from sun-
light.
Spray nozzles plugged be-
cause of no screens on in-
let side of irrigation pumps.
Aerosols are drifting onto
neighbors property because
of inadequate buffer area.
Excessive wear on irrigation
pumps due to sand in waste-
water.
Solution
Improve drainage by installing
drainage wells or drain tiles
or decrease application rate
to level compatible with soils
while expanding total site area.
Bury plastic laterals.
3. Install screens.
Do not operate sprays on windy
days; enlarge buffer area.
Improve pretreatment or install
sand trap ahead of pumps.
251
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TROUBLESHOOTING GUIDE
LAND TREATMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Water ponding in
irrigated area where
ponding normally has
not been vised.
la.
Ib.
Application rate is
excessive.
If application rate
is normal, drainage
may be inadequate.
Ic.
Broken pipe in dis-
tribution system.
la. Application rate.
lb(l) Seasonal variation
in groundwater
level.
lb(2) Operability of any
drainage wells.
lb(3) Condition of drain
tiles.
Ic. Leaks in system.
la. Reduce rate to normal value.
lb(l) Irrigate portions'of the site
where groundwater is not a
problem or store wastewater
until level has dropped.
lb(2) Repair drainage wells or
increase pumping rate.
lb(3) Repair drain tiles.
Ic. Repair pipe.
2. Lateral aluminum
distribution piping
deteriorating.
2a. Effluent permitted
to remain in alumi-
num pipe too long
causing electro-
chemical corrosion.
2b. Dissimilar metals
(steel valves and
aluminum pipe).
2a. Operating techniques.
2b. Pipe and valve
specifications.
2a. Drain aluminum lateral lines
except when in use.
2b. Coat steel valves or install
cathodic or anodic protection.
3. No flow from some
sprinkler nozzles.
Nozzle clogged with
particles from waste-
water due to lack of
screening at inlet
side of irrigation
pumps.
3. Screen may have de-
veloped hole due to
partial plugging of
screen.
3. Repair or replace screen.
4. Wastewater is running
off of irrigated
area.
4a. Sodium adsorption
ratio of wastewater
is too high and has
caused clay soil to
become impermeable.
4b. Soil surface sealed
by solids.
4a. Sodium adsorption
ratio (SAR) should
be less than 9.
4b. Soil surface.
4a. Feed calcium and magnesium to
adjust SAR.
4b. Strip crop area.
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TROUBLESHOOTING GUIDE
LAND TREATMENT
INDICATORS/OBSERVATIONS
5. Irrigated crop is
dead.
6. Growth of irrigated
crop is poor.
PROBABLE CAUSE
4c. Application rate
exceeds infiltration
rate of soil.
4d. Break in distribution
piping.
4e. Soil permeability has
decreased due to con-
tinuous application
of wastewater.
4f. Rain has saturated
soil.
5a. Too much (or not
enough) water has
been applied.
5b. Wastewater contains
excessive amount of
toxic elements.
5c. Too much insecticide
or weed killer
applied.
5d. Inadequate drainage
has flooded root
zone of crop.
6a. Too little nitrogen
(N) or phosphorus (P)
applied.
6b. Timing of nutrient
application not con-
sistent with crop
need. (Also, see
5a - 5c)
CHECK OR MONITOR
4c. Application rate.
4d. Leaks in distribution
piping.
4e . Duration of contin-
uous operation on the
given area.
4f. Rainfall records
5a. Water needs of spe-
cific crop versus
application rate.
5b. Analyze wastewater
and consult with
county agricultural
age nt .
5c. Application of in-
secticide or weed
killer.
5d. Water ponding.
6a. N and P quantities
applied - check with
county agricultural
agent .
6b. Consult with county
agricultural agent.
SOLUTIONS
4c. Reduce application rate until
compatible with infiltration
rate.
4d. Repair breaks.
4e. Each area should be allowed to
rest (2-3 days) between applica-
tions of wastewater to allow
soil to drain.
4f. Store wastewater until soil has
drained.
5a. Reduce (or increase) application
rate.
5b. Eliminate industrial discharges
of toxic materials.
5c. Proper control of application
of insecticide or weed killer.
5d. (See Item 1)
6a. If increased wastewater applica-
tion rates are not practical,
supplement wastewater N or P
with commercial fertilizer.
6b. Adjust application schedule to
meet crop needs.
to
Ul
OJ
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TROUBLESHOOTING GUIDE
LAND TREATMENT
INDICATORS/OBSERVATIONS
7. Irrigation pumping
station shows normal
pressure but above
normal flow.
8. Irrigation pumping
station shows above
average pressure but
below average flow.
9. Irrigation pumping
station shows below
normal flow and
pressure.
10. Excessive erosion
occuring.
11. Odor complaints.
PROBABLE CAUSE
7a. Broken main, lateral,
riser, or gasket.
7b. Missing sprinkler
head or end plug.
7c. Too many laterals on
at one time.
8. Blockage in distribu-
tion system due to
plugged sprinklers,
valves, screens, or
frozen water.
9a. Pump impeller is worn.
9b. Partially clogged in-
let screen.
lOa. Excessive applica-
tion rates .
lOb. Inadequate crop
cover.
lla. Sewage turning sep-
tic during transmis-
sion to irrigated
site and odors being
released as it is
discharged to
p re t r e a tmen t .
lib. Odors from storage
reservoirs.
CHECK OR MONITOR
7a. Inspect distribution
system for leaks.
7b. Inspect distribution
system for leaks.
7c. Number of laterals
in service.
9a. Pump impeller.
9b. Screen.
lOa. Application rate.
lOb. Condition of crop
cover.
lla. Sample sewage as it
leaves transmission
system.
lib. DO in storage
reservoirs.
SOLUTIONS
7a. Repair leak.
7b. Repair leak.
7c. Make appropriate valving
changes .
8. Locate blockage and eliminate.
9a. Replace impeller (See Design
Shortcoming No. 5 also) .
9b. Clean screen.
lOa. Reduce application rate.
lOb. (See Items 5 and 6)
lla. Contain and treat off-gases
from discharge point of
transmission system by covering
inlet with building and passing
off-gases through deodorizing
system.
lib. Improve pretreatment or
aerate reservoirs.
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TROUBLESHOOTING GUIDE
LAND TREATMENT
INDICA TORS/OBSERVA TIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
12. Center pivot irri-
gation rigs stuck
in mud.
12a. Excessive applica-
tion rates.
12b. Improper tires or
rigs.
12c. Poor drainage.
12a. Reduce application rates.
12b. Install tires with higher
flotation capabilities.
12c. Improve drainage (See Item Ib)
13. Nitrate concentra-
tion of groundwater
in vicinity of
irrigation site is
increasing.
10
13a. Application of
nitrogen is not in
balance with crop
needs.
13b. Nitrogen being
applied during
periods when crops
are dormant.
13c. Crop is not being
harvested and
removed.
13a. Check Ibs/acre/yr of
nitrogen being
applied with needs
of crops.
13b. Application
schedules.
13c. Farming management.
_3a. Change crop to one with higher
nitrogen needs.
13b. Apply wastewater only during
periods of active crop growth.
13c. Harvest and remove crop.
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FLOW MEASUREMENT
Process Description
Flow measurement is necessary for good operation and control of a
wastewater treatment plant. Reasons for measuring flow of wastewater
include:
1. To provide operating and performance data concerning the
treatment plant.
2. To compute costs of treatment, where such costs are based
on wastewater volume.
3. To obtain data for long term planning of treatment plant
capacity versus actual capacity used.
There are many methods of measuring flow; some for open channel
flows, and others to measure flow in pipelines. The most commonly used
flow measurement devices will be described in the following paragraphs.
Propeller Meter—
The propeller meter (Figure 79 ) operates on the principle that
liquid hitting the propeller will cause the propeller to rotate at a
speed that is proportional to flow rate. The meter is self-contained
and requires no other energy or equipment other than a mechanical total-
izer to obtain a reading of cumulative flow. Equipment may be added to
the meter to produce a flow rate reading, pace chemical feed equipment,
and control telemetering equipment for remote readout.
Magnetic Flow Meter—
If a liquid conductor (such as wastewater or sludge) moves through
a magnetic field, a voltage is induced. This voltage in turn, is
directly proportional to the velocity of the liquid moving through the
field. This is the basis for the operation of a magnetic flow meter
(Figure 80)• The magnetic flow meter does not restrict flow and normally
does not need any flushing or cleaning to maintain good operation. The
meter may be provided with recorders, and totalizers using electric or
pneumatic transmission systems.
This type of flow meter is useful at sewage lift stations, to mea-
sure total raw wastewater flow, or to measure raw or recirculated sludge.
Venturi Tube—
This meter only can be used in pipes where the wastewater flow is
under pressure. With the Venturi Tube (Figure 81 1, the wastewater flows
256
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Reduction
gears
Direct reading
totalizer
Straigtening \ Bevel gears
vanes V Propeller
Figure 79. Propeller meter.
Power supply
Process
liquid
E lectromagnet
Signal electrodes
Measuring tube
Figure 80. Magnetic flow meter.
Vent and drain
Inspection hole
Pressure
connections
Figure 81. Venturi tube meter.
25"
-------
through a constriction of known dimensions that will cause a pressure
drop at the constricted area. The difference between the inlet and exit
pressures is proportional to the flow rate. Venturi meters may be used
for nearly all pipe flows including raw wastewater, settled wastewater,
plant effluent, raw sludge, digested sludge, mixed liquors, and air.
Positive Displacement Diaphragm Meter—
This meter (Figure 82) operates on the principle of alternately
filling and discharging a definite volume of gas from either side of a
stroking diaphragm. The motion turns a direct reading register. It
is a low pressure, wide range device and is used to meter digester gas.
Weirs—
Weirs (Figure 83) consist of a vertical plate with a sharp crest,
placed in a stream, channel or partly filled pipe. The top of the plate
may be straight, v-notched, or trapazoidal in shape, suitable for the
quantity of flow passing over it. To determine the flow rate, it is
necessary only to measure the head (height) of water above the crest of
the weir. In order for this device to be accurate, the crest of the
weir must be kept clean, sharp, and close to original dimensions and
elevation, and particles-clinging to it must be removed.
When a continuous flow record is needed, permanent or "bubbler"
meters may be used with a weir. Mechanical float and cable gauges also
may be used for water height measurements.
In wastewater treatment plants, weirs often are used to measure
flow recirculation, return sludge, and mixed liquor flow.
Parshall Flume—
The Parshall Flume (Figure 84 ) operates on the principle that open
channel flow, when passing a constriction in the channel, will pass
through a minimum (critical), depth. This will produce a hydraulic head
at a certain point upstream of the constriction that is proportional to
the flow.
The Parshall flume is good for measuring open channel waste flow
because there is no difficulty with sand or suspended solids since the
flume cleans itself; is simple; and is accurate.
Kennison or Parabolic Nozzle—
This nozzle (Figure 85 ) is much like a Parshall flume. The waste-
water flows through a partially filled pipe with a known constriction,
and produces a certain hydraulic head at a point upstream of the con-
striction. By choosing a particular shape for the constriction, the
flow can be calculated from the head, as long as the nozzle has free
discharge. These nozzles, like the flume, are self-cleaning and can
handle liquids high in solids quite well.
Rotameter—
The rotameter (Figure 80 or variable area meter operates as a flow
tube, with float position dependent on viscous differential head or
253
-------
Stroking arm
of register
Valve
Spring
Inlet
Diaphragm
Meter housing
Figure 82. Positive displacement diaphragm meter
Figure 83. Typical pipe and weir installation.
Float cable to
transmitter
Float stop--
Parshall flume
To drain
Figure 84. Parshall flume.
259
-------
FLOAT STOP
TEST
CONNECTION
TO DRAIN
FLOAT CABLE TO
TRANSMITTER
FLOAT PIPE
CAP
FLOAT PIPE
FLOAT
KENNISON
NOZZLE
SEDIMENT TRAP
Figure 85. Kennison or parabolic nozzle.
TAPERED
METERING TUBE
INLET
FLOW
OUTLET FLOW
•PLUMMET OR FLOAT
Figure 86. Rotameter.
260
-------
pressure. The rotameter is most often used for measuring chemical flows,
but cannot be used for measuring wastewater or other plant flows which
may clog the system.
Typical Design Criteria and Performance Evaluation
Not all flow measurement devices can be used for all purposes at a
wastewater treatment plant. Some designs, such as the propeller meter,
are affected by high solids concentrations and cannot be used for measur-
ing wastewater flows with excessive solids. Some units require frequent
or continuous backflushing when the meters are used to handle raw waste-
water or sludge. The range of flows, accuracy, space available, particular
application and maintenance, are important design considerations which
must be evaluated for each type of flow measuring device. Table 24 shows
the normal accuracy for each type of meter already described.
Control Considerations
Because most wastewater treatment processes require accurate flow
measurement for effective operation, measuring devices must be checked
at regular intervals to make sure that the flow is unobstructed and that
the meter is discharging freely. When weirs are used, careful attention
should be paid to level setting, low approach velocity, and depth of
flow above and below the weir.
Common Design Shortcomings and Ways to Compensate
Shortcomings
1. Weirs must be kept clean and
particles clinging to it
must be removed periodi-
cally for proper operation.
2. Most flow measuring devices
in pipelines involve some
constriction in flow.
3. Magnetic flow meters re-
quire an auxiliary power
source.
3.
Solution
Replace weir with a Parshall
flume which scours itself clean
or place a screen before the weir.
Replace with a magnetic flow meter
where no flow constriction is
necessary for operation.
If possible, replace with a Venturi
meter which requires no power and
can be used for nearly all pipeline
flows.
Magnetic flow meters can-
not be used for measuring
flows in partially filled
pipelines.
Vent holes in Venturi tube
pressure chambers are
subject to clogging.
Install a weir or Parshall flume
which would be more practical for
open channel flow or flow in
partially filled pipelines.
Provide hand-operated vent cleaners
(special rods with handles) which
can be pushed into the vent to
remove accumulations.
261
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TABLE 24. ACCURACY OF VARIOUS FLOW MEASURING DEVICES
Type of flow meter
% Accuracy
Propeller meter
Magnetic meter
Below 3 fps
3-30 fps
Venturi tube
Flow tube
Positive displacement
and diaphragm meter
Weirs
Parshall flume
Kennison or parabolic nozzle
Rotameter
+2% of actual flow rate over
a range of 7:1 for small meters
and up to 12 : 1 for large meters
+1% of maximum scale reading
+2% of maximum scale reading
+3 to 4% of flow rate
of flow rate
+1% of flow rate
+5% of flow rate
+5% of flow rate
+2% of flow rate over flow
range of 10:1
+2% of maximum scale
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TROUBLESHOOTING GUIDE
FLOW MEASUREMENT
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Drop or sharp increase
in indicated flow.
la. Obstruction on float
(if float operated).
Ib. Improper air flow, or
bubbler tube damaged
(if operated on
bubbler).
Ic. Grease build-up on
magnetic flow meter
coils.
Id. Weir plates clogged
with foreign material.
la. Visual inspection.
Ib. Check air pressure
gauge.
Ic. Visual inspection.
Id. Visual inspection.
la. Remove obstruction. Keep float
clean and free from grease.
Ib. Clean bubbler and keep free from
grease.
Ic. Remove grease build-up.
Id. Clean foreign matter off weir.
to
en
00
2. Inconsistent or inaccu-
rate weir flow measure-
ment readings.
2a. Weir not level.
2a. Visual inspection
using a leveling
device.
2a. Level the weir.
3. Propeller meter shows
inaccurate reading.
3a. Improper calibration.
3b. Meter clogged with
debris.
3b. Visual inspection.
3a. Recalibrate meter.
3b. Remove debris and consider
replacing meter with a self-
scouring type.
-------
SLUDGE PUMPING
Process Description
Sludge pumps have many uses in a municipal wastewater treatment
plant. Settled primary sludge must be moved regularly; activated sludge
must be returned continuously to aeration tanks, with the extra sludge
wasted; scum must be pumped to digestion tanks; and sludge must be recir-
culated and transferred within the plant in processes such as digestion,
trickling filter operation, and final disposal. The type of pumping
station used at the plant depends on the characteristics of the sludge
itself.
Pumps used for handling sludges may be centrifugal, air lift and
ejectors, grinding, Archimedes screw lift, and positive displacement
types.
Typical Design Criteria and Performance Evaluation
Centrifugal and Screw Lift Pumps—
Centrifugal and screw lift pumps are used to handle large volumes
of flow that have a low solids content, and when precise control of the
flow rate is not required. Centrifugal pumps often are used to return
activated sludge and waste unthickened solids from primary and secondary
treatment processes. This pump also is used for recirculation of diges-
ter contents (with less than 4 or 5 percent TS), and for scum and skim-
mings removal.
Positive Displacement Pumps—
The most common types of positive displacement pumps used for sludge
include the plunger, rotary pump and diaphragm pump.
The plunger pump is the most popular pump for handling high viscosity
sludges containing large and abrasive solids.
Of the rotary positive displacement pumps, only the progressing
cavity pump has widespread use in pumping sludges. This pump is self-
priming and delivers a smooth flow in contrast to the plunger-type pump.
Not only is the rotary displacement pump able to handle very thick sludges,
but it also may be used to transport sludge cake; it can pump centrifuge
and filter cakes having 15 to 40 percent TS. When the progressing cavity
pump is made of the right materials, it also may be used for handling
chemical slurries. The diaphragm-type pump may be used for the same
applications as a plunger pump, except that it has no problems with
abrasion. This pump also may be used for handling strong or toxic chemi-
cals when leakage of the chemicals is a major concern.
264
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Sludge Grinding Pumps—
There are two main types of sludge grinding pumps. The first type
is a comminuting device which produces only enough head to pass solids
through the grinder itself. This unit is mostly used for comminuting
thickened sludges, scum, and screenings that may cause clogging in de-
watering systems.
The second type of pump combines both grinding and pumping of the
liquid and comminuted solids. The unit may be used to grind scum and
screenings, handle sludge flows, and break up relatively large trash
particles.
Table 25 lists various types of sludge pumps, their capacities,
and delivered pressure. This table may be used as a general guide to
evaluating the performance of sludge pumps at a treatment plant. For a
very precise evaluation, the actual operating characteristics of the
pump should be checked against manufacturers design data for the pump.
Pumps cannot be expected to operate beyond their designed capacity and
intended use.
Control Considerations
To be effective, sludge pumping systems must be flexible under
different plant operating conditions. The overall piping, valves, and
pumping system must be set up to allow bypassing and provide standby
pumping capacity when problems occur.
The most important control considerations which must be understood
by the operator are 1) the total quantity of sludge per day to be handled,
and 2) the rate at which solids build up and must be removed. Unless the
system and the operator are prepared to handle grit and other solids dur-
ing times of heavy solids inflow, the operator may find all the sludge
lines plugged and overloaded. The operator also must be aware of the
effects of overpumping and underpumping from different unit operations.
For example, solids removed at too high of a rate will result in thin
sludge and overpumping of the thickener.
Sludge removal rates may depend on downstream operations such as
dewatering and combustion. For these processes, a uniform rate of solids
delivery is necessary. On the other hand, sludge flow is not so critical
in downstream units like aerobic or anaerobic digestion.
If a sludge concentrator is used, time is needed to accumulate the
solids (known as a "solids inventory"). If the removal rate is higher
than the stocking rate of the inventory, then the solids concentration
will be lowered. Whenever possible, the concentration of the withdrawn
sludge should be used to determine the sludge pumping rate.
When multiple units are used, it is best to withdraw thickened
sludge from two or more units at the same time using a multiple pump
arrangement. This practice will result in a much more uniform and highly
265
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TABLE 25. TYPES OF SLUDGE PUMPS
Type of pump Capacity (gpm) Delivered
pressure (psi)
Plunger pump up to 500 100 - 150
Rotary positive displacement
(progressing cavity pump) up to 400 up to 500
Diaphragm up to 100 up to 100
Sludge grinding pumps
(comminuting and pumping type) 25 - 300
Screw lift pump up to 80,000
266
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concentrated sludge than doubling the pumping rate of a single unit.
Another way is to pump at a higher rate, but for a much shorter time.
Sludge removal from an aerobic or anaerobic digester is more effi-
cient when the pumping schedule is based on solids accumulation. The
pumping program should tend to underpump the thickener or secondary
settling tank so that a daily manual check on the sludge inventory can
be made, and the pumping schedule adjusted to remove any accumulated
inventory. Sludge removal considerations also are discussed in each
unit process section of this manual.
267
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TROUBLESHOOTING GUIDE
SLUDGE PUMPING
INDICA TORS/OBSERVA TIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Overpumping.
la. Long suction line with
high head loss on
suction side of pump.
la. Sludge dilutes as it
breaks through sludge
blanket if pump is
operating at too high
a rate.
la.(1) Pump sludge more frequently.
(2) Reduce speed of pump.
2. Unwanted (dilute) flow
of sludge through pump.
2a. Improper location of
pump.
2b. Ball valve too light
or ball hung up on
trash accumulations.
2b. Visual inspection.
2a. Relocate pump.
2b. Change the weight of the ball
check to prevent it from lifting
and allowing dilute sludge to
flow through the pump.
to
CN
CD
3. Water hammer.
3a. High suction head and
high discharge
pressure.
3a. Check pressures.
3a. (1) Be sure suction and discharge
air chambers are filled with
air.
(2) Change ball checks and seatinc
arrangement.
(3) Modify pumping rate.
4. Pump inefficiency at
high suction.
4a. Air leakage through
pump seals or valve
stem seals.
4a. Pour water around seal
and visibly inspect
sealing check. You
may also hear the leak
4a. Check seating and seals on valve,
valve covers, valve stems, and
piston on plunger pump (repair
or replace damaged and worn
parts.)
5. Grease build-up in raw
sludge line.
5a. Sludge characteristics
5a. (1) Fill raw sludge line with
digested sludge and let sit
overnite.
(2) Recirculate warm sludge
through raw sludge line to
"melt" grease.
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TROUBLESHOOTING GUIDE
SLUDGE PUMPING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
6. Excessive wear on
pumps.
6a. Plunger pump operating
on a short stroke for
long periods of time.
6b. Improper clearance
adjustment on grinder
pump.
6b. Cutter clearance.
6a. Run pump on a longer stroke at
slower speed.
6b. Properly adjust clearance of
cutters.
7. Excessive leakage
around seals on shafts
and plungers.
7a. Excessive wear on
shaft or cylinder.
7a. Packing rapidly
destroyed.
7a. Replace shaft or plunger, re-
place mechanical seals with
water-lubricated seals.
to
10
8. Progressive cavity
pump unable to trans-
port sludge.
8a. Slippage occurring in
pump due to wear on
stators and rotors.
8b. Pump operating at
excessive speeds.
8a. Stator and rotor
condition.
8b. Excessive wear.
8a. Replace stator and/or rotor.
8b. Reduce operation to 200 to 300
rpm.
-------
THERMAL TREATMENT OF SLUDGES
Process Description
There are two basic types of high temperature, high pressure treat-
ment of sludges. One - "wet air oxidation" - involves the flameless
oxidation of sludges at 450-550 F at pressures of about 1200 psig. The
other type - "heat treatment" - is more common and is carried out at
lower temperatures and pressures (350-400 F at 150-300 psig) to improve
sludge dewatering. Because the equipment for both processes is almost
the same, this section reviews both approaches.
Water escapes from the sludge as the sludge is heated. Heat treat-
ment systems release bound water from the sewage sludge to improve de-
watering and thickening.
A typical heat treatment process is shown in Figure 87. Sludge is
ground and pumped to a pressure of about 300 psi. Compressed air is
fed into the system and the mixture is brought to an operating tempera-
ture of about 350 F. The heated, conditioned sludge is cooled by heat
exchange with the incoming sludge. The treated sludge is separated by
settling before the dewatering step. Gases released at the separation
step are passed through a catalytic afterburner at 650-750 F.
The same basic system can be used for sludge reduction by wet air
oxidation, except that higher temperatures (450-650 F) and higher pres-
sures (1200-1600 psig) are used. The wet air oxidation (WAO) process
is based on the fact that any material that can be burned can also be
oxidized in the presence of liquid water at temperatures between 250 F
and 700 F. Wet air oxidation does not require preliminary dewatering
or drying, like other burning processes. However, the oxidized ash
must be separated from the water by vacuum filtration, centrifugation,
or some other solids separation process.
Unfortunately, heat treatment converts suspended solids to dissolved
or dispersed solids. These dissolved solids cause a highly polluted
liquid from the dewatering process which must be recycled to the waste-
water treatment plant for reprocessing.
Typical Design Criteria and Performance Evaluation
The size of heat treatment units depends on the expected sludge
flow rate (gpm). The detention time in the heat exchanger is usually
30-60 minutes.
The degree of wet oxidation - low, intermediate, or high - refers
270
-------
Sludge
GRINDER
GROUND SLUDGE
HOLDING TANK
HEAT
EXCHANGER
PUMP
POSITIVE
DISPLACEMENT
SLUDGE PUMP
AIR COMPRESSOR
To Incinerator
OXIDIZED
SLUDGE
TANK
-4
REACTOR
Exhaust gas
PCV
VAPOR
COMBUSTION
UNIT
FILTER
PUMP
Treated
boiler
water
BOILER
Figure 87. Heat treatment system schematic.
-------
to how much the COD of the sludge is reduced. Higher temperatures are
needed to obtain higher degrees of oxidation. The higher temperatures,
in turn, require higher pressures in order to prevent flashing to
steam or burning.
The operating temperature and pressure ranges for the three degrees
of oxidation are given below:
Reduction in
sludge COD,
Oxidation percent Temp. , F Pressure, psi
Low 5 350-400 300-500
Intermediate 40 450 750
High 92-98 675 1,650
With high oxidation, the amount of sludge ash is about the same as
with incineration.
If the detention time in the thermal reactor is increased, COD and
color of the liquor increase. For example, in low oxidation at 350 to
400 F, the color of the 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.
It is much easier to dewater a sludge that has been heat treated
than one which has been chemically conditioned (sludge solids of 30-40%
as opposed to 15-20% with chemical conditioning). The dewatering also
can handle relatively high loading rates for heat treated sludges (2-3
times the rates with chemical conditioning). The process also provides
effective disinfection of the sludge.
Control Considerations
Four important factors control the performance of wet oxidation
units: temperature, air supply, pressure, and feed solids concentration.
The degree and rate of sludge solids oxidation are greatly influenced
by the reactor temperature. Much higher degrees of oxidation and shorter
reaction times are possible with increased temperatures. Like conven-
tional incinerators, an external supply of oxygen (air) is needed for
almost complete oxidation. The amount of air needed for the process
depends on the heat value of the sludge and the degree of oxidation.
Thermal efficiency and process economy depend on air input, so it is
important that the right amount be used. Because the input air becomes
saturated with steam in the reactor, it is important to control the air
also to prevent too much water from evaporating.
The reactor temperature and pressure affect the amount of recycle
BOD and how easily the sludge can be dewatered. Temperatures usually
should be kept as low as possible, consistent with adequate condition-
ing of the sludge. Higher temperatures breakdown the sludge particles,
produce more BOD in the liquor and breakdown the fibrous material in
272
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the sludge which is needed for high filtration rates and thick cakes.
The following lists some of the substances found in thermal treat-
ment liquor.
Substances in Concentration range,
strong liquor mg/1 (except as shown)
SS 100 - 20,000
COD 100 - 17,000
BOD 3,000 - 15,000
NH3-N 400 - 1,700
Phosphorus 20 - 150
Color 1,000 - 6,000 units
These high concentrations show the potential impact that liquor
recycle can have on the wastewater treatment processes. It is important
that the operator know and understand the importance of the recycle
load in overall plant operation.
"Over-cooking" can breakdown fibrous material which would otherwise
aid filtration. Filterability also is affected by the pH of the sludge.
Low pH values are much more effective, but corrosion problems are in-
creased. Odor can be reduced if the conditioned sludges are cooled
before being exposed to the atmosphere. Increases in the solids content
of heat treatment process feed lowers operating costs, but makes dewater-
ing of the conditioned sludge more difficult. This results in increasing
the dissolved COD, nitrogen, and phosphorus in the liquor. If the heat
treatment temperature is increased, soluble nitrogen increases, and
suspended solids decrease in the recycle liquor.
Thermal treatment units should always have an operator in attendance
when the units are running. The lead operator should be able to do
routine preventive maintenance on the thermal conditioning equipment.
Each hour, the operator should:
1. Record all instrument readings on log sheet. Compare with pre-
vious readings and check any unusual changes. Rapid changes in
temperature or pressure may be the first indication of trouble.
2. Adjust pumping system to maintain proper sludge flow rate.
3. Adjust oxidation system to maintain proper temperatures.
4. Examine each operating piece of equipment. Check lubrication,
cooling water, operating temperatures, leakage, sound, and
vibration. Any unit which may not be operating properly should
be closely watched, and the cause of the problem discovered
and corrected quickly. The operator should shut down the system
if operating problems continue without obvious cause, or become
worse.
273
-------
5. Take samples as required.
Common Design Shortcomings and Ways to Compensate
1.
2.
3.
Shortcoming
Effects of recycled
liquors on wastewater
process were not ade-
quately considered and
plant is upset.
Off-gases from decant
tanks, thickeners, or
dewatering system sub-
ject to odors.
Backup support systems
(boiler, feed pumps,
grinders, air compressors,
etc) not provided.
High temperatures and pre-
sence of calcium, sulfates,
or chlorides in the sludge
can create scaling and
corrosion in heat exchang-
ers & reaction 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 re-
cycled (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 consul-
tant) .
3. Install backup components.
4. Use SS 316L or Titanium for
materials of construction.
274
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TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Odors.
la. Odors being released
in decant tanks,
thickeners, vacuum
pump exhaust or in
dewatering.
Ib. Odors being released
when recycle liquors
enter wastewater
treatment tanks.
la.(l) Cover units, collect air
and deodorize it before
release by use of inciner-
ation, adsorption, or
scrubbing.
la.(2) Cover open tank surface with
small floating plastic balls
to reduce evaporation and
odor loss.
Ib. Pre-aerate liquors in covered
tank and deodorize off gases.
to
~j
Ul
2. Raw sludge grinder re-
quires very frequent
maintenance.
2. Excessive grit in raw
sludge.
2. Operation of raw sludge
degritting system and
raw sewage grit re-
moval .
2. Maintain and properly operate the
raw sludge and raw sewage de-
gritting systems.
3. Scaling of heat ex-
changers.
3a. Calcium sulfate
deposits.
3b. Operating temperatures
too high - causing
baking of solids.
3a. Efficiency of heat
transfer - difficult
to maintain reactor
temperatures.
3a. Provide acid wash in accordance
with manufacturers instructions.
3b. Operate reactor at temperatures
below 390 F for heat condition-
- ing of sludge.
3c. Use hydraulically driven clean-
ing bullet to clean inner tubes.
4. Heat treatment system
down time is substan-
tial.
4. Inadequate operation
and maintenance skills.
4. Contract for maintenance of sys-
tem and institute training pro-
gram for operators.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
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 re-
versing automatically
when overloaded.
5a. Establish flow of seal water.
5b. Remove obstruction.
6. Feed pumps are over-
heating.
6a. Inadequate lubrication
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 concentration
to heat treatment unit
is low.
7. Sludge concentration.
7. Operate thickener to maintain
6% solids if possible; 3%
minimum.
NJ
-J
8. Solids dewater poorly.
8a. Anaerobic digestion
prior to heat treat-
ment.
8b. Temperatures not main-
tained high enough.
8b. Reactor temperatures.
8a. Discontinue anaerobic digestion
of sludge to be heat treated.
8b. Temperature should be at least
350°F.
9. High system pressure.
9a. Blockage in reactor.
9b. Pressure ^ntroller
set too high.
9c. Block valve closed.
9a.(1) If relief valves
are blowing, shut
down unit.
9a.(2) If relief valves
are not blowing,
blockage was
temporary.
9b. Pressure controller
setting.
9c. Block valve.
9a.(1) Remove blockage.
9a.(2) Check pressures and tempera-
tures to note any discrep-
ancies from normal.
9b. Reduce set point on pressure
controller.
9c. Check system for proper valving,
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
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.
lOa. Adjust control setting.
lOb. Repair or replace check valves.
lOc. Bleed off air.
11. System pressure is
dropping.
lla. Pressure contoller
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 temperature
is rising.
12a. Inlet temperature too
high.
12b. Sludge feed rate is
too slow.
12c. Improper control
setting.
12d. Pump stopped or
slowed.
12e. Volatile matter such
as gas or oil being
pumped through the
system.
12 f. Pneumatic steam valve
not functioning
properly.
12a. Should not exceed
310°F for sludge
conditioning.
12b. Operation of sludge
feed pumps and feed
rate.
12c. Temperature control.
12d. Pump operation.
12f. Valve 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
12e. Switch from sludge to water
and stop the process air com-
pressor.
12f. Repair malfunctioning valve.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
to
-j
CO
13. Oxidation temperature
is falling.
13a. Heat exchanger
fouled.
13b. Reactor inlet temper-
ature is too low,
because of low
density sludge.
13c. High flow rate being
pumped through sys-
tem.
13d. Improper temperature
control setting.
13e. Pneumatic steam
valve not functioning
properly.
13f. No signal air to the
temperature control
valve.
13g. Boiler not function-
ing 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.
14. Scoring of air com-
pressor cylinder 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 difficult
to feed into inciner-
ator.
15a. Filter cake too dry.
15a. Reduce temperature (and pres-
sure) of the treatment system.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
INDICA TORS/OBSERVA TIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
16. Low system pressure.
to
--j
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 parti-
ally 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.
17c. Intercooler and/or
jackets plugged.
17d. Poor lubrication.
17a. Water flow.
17b. Cylinder valves.
17c. Visual inspection.
17d. (1) Low oil level.
17d. (2) Malfunctioning
lubricator.
17d. (3) Loose or worn
belt
17a. Adjust water flow.
17b. Repair and/or clean or replace.
17c. Clean intercooler and/or replace
17d. (1) Add oil.
17d. (2) Repair lubricator.
17d. (3) Tighten loose belt, or
replace if worn.
-------
TROUBLESHOOTING GUIDE
THERMAL TREATMENT OF SLUDGES
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
18. Air compressor safety
valve relieving.
18a. Pressure controller
set too high.
18b. No signal air pres-
sure 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.
NJ
oo
o
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GRAVITY THICKENING
Process Description
Gravity thickening is a type of sedimentation process for sludge
which usually is operated continuously, with flow dependent on plant
flow.
A conventional gravity thickener is shown in Figure 88. The thick-
ener looks like a circular clarifier, but the bottom has more slope.
Sludge enters at the middle of the thickener and the solids settle into
a sludge blanket at the bottom. The thickened sludge is very gently
mixed by the moving rake which releases gas bubbles. This prevents bridg-
ing of the sludge solids, and keeps the sludge moving to the center
where it is removed. Supernatant liquor passes over an effluent weir
around the outside of the thickener.
Supernatant flow from the thickener is usually returned to either
the primary settling tank or the secondary process. Thickened sludge
usually is pumped either to a blending, holding, or surge tank or directly
to a sludge dewatering process.
Typical Design Criteria and Performance Evaluation
Gravity thickeners are designed on the basis of hydraulic and solids
surface loadings. Most thickeners are designed for hydraulic overflow
rates of 400-800 gpd/sq ft. Table 26 shows common design data and expected
performance. In evaluating the performance of a gravity thickener the
following steps should be taken:
1. Identify the type of sludge being thickened.
2. Determine the percent solids in the feed, the loading rate, and
the thickened sludge concentration.
The loading rate may be calculated as follows:
Primary sludge solids concentration to thickener = 4.7%
1% solids = 10,000 mg/1 = 83,400 Ibs/MG
4.7% solids = 47,000 mg/1 = 392,000 Ibs/MG
Flow to thickener = 115,000 gpd = 0.115 mgd
Solids to thickener = 0.115 mgd x 392,000 Ibs/MG =
45,080 Ibs/day
Thickener Diameter = 50 ft
Thickener Area = 1963 sq ft
281
-------
to
CD
NJ
Effluent weir
Raised position
of truss arm
Scraper blades
Hopper plow
Effluent
Elevation
Figure 88. Gravity thickener.
-------
TABLE 26. TYPICAL DESIGN CRITERIA AND PERFORMANCE DATA FOR GRAVITY THICKENING
K)
00
Feed solids
concentration
Sludge type (percent)
Primary
Trickling filter
Primary + FeCl3
Primary + low lime
Primary + high lime
Primary + WAS*
WAS
Primary + (WAS + Fed 3)
(Primary + FeCl3) + WAS
Digested primary
Digested primary + WAS
Digested primary + (WAS + Feds)
Tertiary, 2 stage high lime
Tertiary, low lime
5.0
1.0
2.0
5.0
7.5
2.0
1.0
1.5
1.8
8.0
4.0
4.0
4.5
3.0
Typical loading
rate
(lb/sq ft/day)
20-30
8-10
6
20
25
6-10
5-6
6
6
25
15
15
60
60
Thickened sludge
concentration
(percent)
8.0-10
7-9
4.0
7.0
12.0
4.0
2-3
3.0
3.6
12.0
8.0
6.0
15.0
12.0
*WAS = Waste Activated Sludge
-------
Loading = 45,080 Ibs/day _, ., ., . .
1963 sq ft = " lbs/d*Y/sct ft
3. Check the values obtained in Step 2 against the performance
data shown in Table 26.
4. If the system does not operate within or near the values shown
in the Table, the troubleshooting guide should be checked for
possible solutions to the problem.
CONTROL CONSIDERATIONS
Fresh liquid should be kept from entering the thickener so that
septic conditions and odors do not develop. This can be done with over-
flow rates of 600 to 800 gpd/sq ft. To get hydraulic loadings in this
range, secondary effluent is sometimes blended with the sludge feed to
the thickener. Another control is the sludge volume ratio (SVR) which is
the volume of the sludge blanket divided by the daily volume of sludge
pumped from the thickener. This ratio has the units of days and is used
to measure the average retention time of solids in the thickener. A long
SVR gives a very thick sludge, but may cause too much biological decom-
position. Values for SVR usually are kept between 0.5 and 2 days, with
the lower values being used during warmer weather. The sludge blanket
depth may be changed with differences in solids production to achieve good
compaction. During peak conditions, the detention time may have to be
shortened to keep the sludge blanket from flowing over the weirs.
The quality of supernatant from a well operated thickener is usually
about the same as the quality of raw municipal wastewater, and does not
cause problems when recycled to the plant. However, if the thickener
operates poorly, large amounts of solids can be lost over the thickener
weir and create problems when returned to the plant.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Scum overflow. 1. Move scum collection system
away from outlet weir.
2. Sludge hard to remove 2. Improve grit removal operation
because of too much grit. or eliminate sources of grit
entering system.
3. Short circuiting of flow 3. Modify hydraulic design and
through tank causing poor install baffles.
solids removal.
284
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TROUBLESHOOTING GUIDE
GRAVITY THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Septic odor, rising
sludge.
la. Rate of thickened
sludge pumping is too
low.
Ib. Thickener overflow
rate is too low.
la. Depth of sludge blan-
ket too high (>4 ft) .
Ib. Maintain minimum over-
flow rate at 600
gpd/sf.
la. Increase pumping of thickened
sludge.
Ib. Increase influent flow to
thickener - a portion of the
secondary effluent may be
pumped to thickener.
Ic. Chlorinate influent to thickener
to maintain 1 mg/1 residual in
thickener effluent.
Id. Add air 1-2 ft below surface or
in wet well.
to
CD
Ul
2. Thickened sludge too
thin.
2a. Overflow rate is too
high.
2b. Underflow rate is too
high.
2c. Short circuiting of
flow through tank.
2a. If overflow rate ex-
ceeds 800 gpd/sf, may
be the cause.
2b. Maintain a minimum
sludge depth of 3
feet.
2c. Visual observation of
tank surface; uneven
discharge of solids
over effluent weir.
2a. Decrease influent raw sludge
pumping rate.
2b. Decrease pumping of thickened
sludge.
2c. Change weir settings: repair or
replace baffles.
3. Torque overload of
sludge collecting
equipment.
3a. Heavy accumulation of
sludge following a
period of equipment
shut down.
3b. Heavy foreign object
jamming the scraper.
3a. Probe along front of
collector arms.
3a. Agitation of sludge blanket in
front of collector arms with
rods or water jets.
3b. Remove foreign object with
grappling device if possible;
if not, drain basin and remove
object.
-------
TROUBLESHOOTING GUIDE
GRAVITY THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Excessive growth on
weirs.
4. Accumulations of waste-
water solids and
resultant growth.
4. Inspect surfaces.
4. Frequent and thorough cleaning
of surfaces.
5. Sludge lines and pump
plugging.
5. Attempting to pump
sludge at too high a
concentration.
5. Flush line with water; make sure
all valves are fully open. (See
sludge pumping section also).
oo
-------
FLOTATION THICKENING
Process Description
The flotation thickening process feeds air into the sludge under pres-
sure (40-80 psi), so that a large amount of air can be dissolved. The
sludge then flows into an open tank (Figure 89) where, at atmospheric pres-
sure, much of the air comes out of solution as small air bubbles which attach
themselves to sludge solids particles and float them to the surface. Flo-
tation works very well on activated sludge, which is difficult to thicken
by gravity. The sludge forms a layer at the top of the tank; this layer
is removed by a skimmer for further processing.
Figure 90 shows a typical air flotation system. Part of the effluent
from the flotation unit is pumped to a retention tank at 60-70 psig. Air
is fed into the pump discharge line at a controlled rate and mixed by the
reaeration pump. The flow through the recycle system is controlled by a
valve. Effluent recycle ratios can range from 30-150 percent of the in-
fluent flow. The recycle flow and sludge feed are mixed in a chamber at
the entrance to the unit. If flotation aids are used, they usually are
fed into this mixing chamber. The sludge particles are floated to the
sludge blanket and the clarified effluent flows over a weir. The thickened
sludge is removed by a skimmer. Bottom sludge collectors are used to re-
move any settled sludge or grit.
The sludge is thickened in the sludge blanket, which usually is 8-24
inches thick. The floating sludge and air bubbles force the blanket to
settle above the water level so that the water from the sludge particles
drains out. Detention time in the flotation basin is not too important,
as long as the particles rise quickly and the sludge blanket is not broken.
Typical Design Criteria and Performance Evaluation
1. Check the solids loading.
Solids loading often is designed at 2 Ib/hr/sq ft. This rate is pos-
sible using flotation aids, with or without auxiliary recycle. Many flo-
tation thickeners are operated at 3.0 lb/hr/sq ft, although built-in capaci-
ties of 4.0-5.0 lb/hr/sq ft are common (Table 27) and provide flexibility
in operation. There are times when flotation can be done without flotation
aids, and auxiliary recycle is used instead. Without flotation aids, load-
ing rates are about 50 percent and solids removal may be less.
2. Check the float solids concentration.
287
-------
SLUDGE REMOVAL MECHANISM-
• RECYCLE FLOW
BOTTOM SLUDGE COLLECTOR
. .r SLUDGE
FEED
Figure 89. Dissolved air flotation unit.
UNIT EFFLUENT
AUX. RECYCLE CONNECT.
(PRIMARY TANK
OR PLANT
EFFLUENT)
AIR FEED-
FLOTATION UNIT
RECIRCULATION PUMP
REAERATION PUMP
THICKENED SLUDGE
•DISCHARGE
UNIT FEED
SLUDGE
K-RECYCLE FLOW
RETENTION TANK
(AIR DISSOLUTION)
Figure 90. Dissolved air flotation system.
288
-------
TABLE 27
OPERATING DATA FOR PLANT SCALE DAF UNITS
Location
Bernardsville, N.J.
Bernardsville, N.J.
Abington, Pa.
Hatboio, Pa.
Morristown.N.J.
Omaha, Nebr.
Omaha, Nebt.
w BeUevUle, 111.
oo Indianapolis, Ind.
<£>
Warren, Mich.
Frankenmuth, Mich.
Oak mo nt, Pa.
Columbus, Ohio
Levittown.Pa.
Nassau Co., N.Y.
Bay Park S.T.P.
Nassau Co., N.Y.
Bay Park S.T.P.
Nashville, Tenn.
Feed
M.L."
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/t
3,600
17,000
5,000
7,300
6,800
19,660
7,910
18,372
2,960
6,000
9,000
6,250
6^00
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
tt
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/hi/fr
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
jpm/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
Remark*
Standard0
Standard
Flotation Aid**
After 1 2 hours holding
Flotation Aid
Standard
Flotation Aid
After 24 hours holding
Flotation Aid
Flotation Aid
Flotation Aid
After 1 2 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.
^Standard - Indicates no flotation aid and no holding before sampling.
''Flotation Aid - Indicates use of coagulant-flotation aid.
-------
A 4 percent minimum float solids concentration by weight is normally
used for design purposes. However, a 5-6 percent float solids concentra-
tion can be expected. Flotation without chemical aids usually results in
a solids concentration that is about 1 percent less than with flotation
aids.
Typical maximum hydraulic loading or overflow rate is 0.80 gpm/sq ft
at minimum solids concentration of 5,000 mg/1. Lower solids levels or
higher hydraulic loadings result in lower efficiencies and/or float solids
concentrations. Using flotation, at least 95 percent of suspended solids
can be removed with flotation aids, and 50-80 percent without flotation
aids. Tables 27 and 28 can be used as a guide in evaluating the performance
of flotation thickening systems. The gravity thickening section shows how
to calculate solids loadings.
Control Considerations
The primary operating variables for flotation thickening are:
Pressure
Recycle ratio
Feed solids concentration
Detention period
Air-to-solids ratio
Type and quality of sludge
Solids and hydraulic loading rates
Use of chemical aids
Air pressure used in flotation is important because it determines the
size of the air bubbles, and can affect the solids concentration and the
subnatant (separated water) quality. Either increased pressure or air
flow produces greater float (solids) concentrations and a lower effluent
suspended solids concentration. There is an upper limit, however, because
too much air breaks up floe.
Recycle ratio and feed solids concentration are related. Additional
recycle of clarified effluent does two things:
1. It allows more air to be dissolved because there is more liquid.
2. It dilutes the feed sludge.
Dilution reduces the effect of particle interference on the rate of
separation. Concentration of sludge increases and the effluent suspended
solids decrease as the sludge blanket detention time increases.
The air-to-solids ratio is also important because it affects the
sludge rise rate. The air-to-solids ratio needed, depends mostly on
sludge characteristics such as SVI. The most common air-to-solids ratio
used for an activated sludge thickener is 0.02.
290
-------
TABLE 28. AIR FLOTATION THICKENING PERFORMANCE DATA
Type of sludge
Waste activated
Waste activated
Waste activated
Waste activated
Waste activated
Waste activated
Waste activated
Sludge
loading
ps f /day
12-18
24-48
13.9
7.1
19.8
26.2
28.8
Feed
solids
percent
0.5-1.5
0.5-1.5
0.81
0.77
0.45
0.80
0.46
Float
solids
percent
4.0-6.0
4.0-5.0
4.9
3.7
4.6
6.5
4.0
Solids
recovery
percent
85-95
95-99
85
99
83
93
88
Combined primary and
waste activated
24-30
1.5-3.0 6.0-8.0
85-95
Combined primary and
waste activated
21
0.64
8.6
91
Combined primary and
waste activated
46.6
40.7
2.30
1.77
7.1
5.3
94
88
3-6 Ib polyelectrolyte/ton dry solids.
Chemical flotation aids (polymers) improve thickening and solids cap-
ture. The dosage must be determined for each specific sludge, but dosages
of 5-15 Ibs/ton of sludge are common. To determine the right polymer dos-
age, take a 1,000 ml sample of sludge, and using a pipette, measure the ml
of polymer solution (from mix tank) needed to produce a firm, well defined
floe. Set the polymer feed pump at about 1.5-2.0 times this ratio when the
system is started. With experience, reduce the chemical feed to a minimum
that will produce good results.
Experience will allow most operators to judge the performance of their
flotation thickeners. The rise test performed as follows, is useful to
compare test results with those conditions that have produced good operat-
ing results in the past. On most units, a valve is provided to sample from
the inlet mixing chamber. When the unit is in operation, a quart jar sample
291
-------
is taken and the time for the sludge to rise to a clear surface is measured.
Normal rise times are 10-25 seconds, and an optimum time can be found for
each particular plant. The relative depth of the blanket, subnatent clarity
and general appearance of flocculated sludge particles are also good visual
indicators.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Feed pumps run on an on-off 1. Install a flow indicator and
cycle causing uneven feed flow control system to provide
to dissolved air flotation even, controllable inflow rate.
unit.
2. Only primary effluent avail- 2. Install line so that secondary
able for auxiliary recycle. effluent can be used for recycle
during times when primary eff-
luent has more than 200 mg/1
solids or contains unusual
amounts of stringy materials.
3. Wide variations in feed 3. Move feed point to sludge re-
solids concentrations occur aeration tank if available or
due to direct feed of DAF install a mixing-storage tank
from final clarifier. to minimize fluctuations.
292
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TROUBLESHOOTING GUIDE
FLOTATION THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
to
VO
W
1. Floated sludge too
thin.
la. Skimmer speed too
high.
Ib. Unit overloaded.
Ic. Polymer dosages too
low.
Id. Excessive air/solids
ratio.
le. Low dissolved air.
la. Visual inspection.
Ib. Rise rate.
Ic. Proper operation and
calibration of poly-
mer pumps.
Id. Float appears very
frothy.
le. (see Item 2)
la. Adjust as required.
Ib. Turn off feed sludge and
allow unit to clear or purge
the unit with auxiliary recycle,
Ic. Adjust as required.
Id. Reduce air flow to pressuriza-
tion system.
2. Low dissolved air.
2a. Reaeration pump off,
clogged, or
malfunctioning.
2b. Eductor clogged.
2c. Air supply
malfunctioning.
2a. Pump condition.
2b. Visual inspection.
2c. Compressor, lines,
and control panel.
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 formation
with solids settling.
3c. Adjust speed.
3d. Increase air flow to pressuriza
tion system.
-------
TROUBLESHOOTING GUIDE
FLOTATION THICKENING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Skimmer blade leaking
on beaching plate.
4a. Skimmer wiper not
adjusted properly.
4b. Hold-down tracks
too high.
4a. Visual inspection.
4b. Visual inspection.
4a. Adjust as required.
4b. Adjust as required.
5. Skimmer blade binding
on beaching plate.
5. Skimmer wiper not
properly adjusted.
5. Visual inspection.
5. Adjust as required.
10
<£>
£>.
6. High water level in
retention tank.
6a. Air supply pressure
low.
6b. Level control
system not operat-
ing.
6c. Insufficient air
injection.
6a. Compressor and
air lines.
6b. Level control system.
6c. Compressor and air
lines.
6a. Repair as required.
6b. Repair as required.
6c. Repair as required.
7. Low water level in
retention tank.
7a. Recirculation pump
not operating or
clogged.
7b. Level control system
not operating.
7a. Pump operation.
7b. Level control.
7a. Inspect and clean as required.
7b. Repair as required.
8. Low recirculation
pump capacity.
8. High tank pressure.
8. Back pressure.
8. Adjust back pressure valve.
9. Rise rate too slow.
9a. Unit overloaded.
9b. Low dissolved air.
9c. Polymer dosages too
low.
(see Item Ib)
(see Item le)
(see Item le)
-------
ANAEROBIC DIGESTION
Process Description
EPA has published "Operations Manual - Anaerobic Sludge Digestion"
(EPA 430/9-76-001) which provides very detailed information on the pro-
cess and troubleshooting. This manual should be read for more complete
information than given here. In this process, the organic matter in the
sludge is broken down without oxygen. Anaerobic digesters may be "low
rate" or "high rate". In the low rate, one-stage digestion process
(Figure 91) fresh sludge is fed into the system two or three times daily.
As decomposition occurs, three separate layers form. A scum layer is
formed at the top of the digester, and below it are supernatant and sludge
layers. The sludge zone has an actively decomposing upper layer and a
relatively stablized bottom layer. The stabilized sludge settles at the
base of the digester and supernatant is usually returned to the plant
influent. Most modern systems are "high rate" systems utilizing one or
two stages. A typical two-stage process is shown in Figure 92. The
sludge stabilizes in the first stage, while the second stage provides set-
tling and thickening. In a single-stage system, the secondary digester is
replaced by some other thickening process. The digester is headed to 85-
95°F and usually provides 10-20 days detention of the sludge.
The process has been successful when fed primary sludge or combina-
tions of the primary sludge and small amounts of secondary sludge. With more
efficient systems than simple sedimentation, large amounts of activated
sludges are produced at many plants. This additional sludge, when placed
in a two-stage anaerobic digestion process, does not settle well or dewater
well after digestion.
The process converts about 50 percent of the organic solids to liquid
and gas, greatly reducing the amount of sludge to be disposed. About
two-thirds of the gas produced in the process is methane, with a heat value
of 600 BTU/standard cubic foot (scf). About 15 scf of gas is formed per
pound of volatile solids destroyed. Anaerobic digester gas has been used
in wastewater treatment plants for many years to heat digesters and build-
ings and as fuel for engines that drive pumps, air blowers and electrical
generators.
Typical Design Criteria and Performance Evaluation
Table 29 summarizes design criteria for low and high rate digestion
systems. Digesters have either fixed or floating covers, as shown in
Figure 93.
295
-------
GAS OUTLET
SLUDGE INLET
SUPERNATANT
ACTIVELY
DIGESTING SLUDGE
SLUDGE OUTLET
SCUM REMOVAL
SUPERNATANT
REMOVAL
Figur^ 91. Low rate digester.
296
-------
Gas release
Sludge
Inlet
ZONE OF
ACTIVELY DIGESTING
SLUDGE
Mixed
Liquor
Sludge drawoff
SUPERNATANT
DIGESTED SLUDGE
Supernatant
removal
To further
processing
Figure 92. Two-stage anaerobic digestion.
297
-------
Fixed cover
Pressure vacuum relief
Supernatant
overflow
FIXED COVER
Gas
PV relief valve
Weights
Floating cover
Supernatant
FLOATING COVER
PV relief valve
Gas holder cover
Lowest
position —
Supernatant
GAS HOLDER COVER
Figure 93. Fixed and floating digestion covers.
298
-------
TABLE 29. TYPICAL DESIGN CRITERIA FOR LOW RATE AND HIGH RATE DIGESTERS
Parameter Low rate High rate
Solids retention time (SET), days 30-60 10-20
Solids loading, Ib VSS/cu ft/day 0.04-0.1 0.15-0.40
Volume criteria, cu ft/capita
Primary sludge 2-3 1.33-2.0
Primary sludge + trickling
filter sludge 4-5 2.66 - 3.33
Primary activated + waste
activated sludge 4-6 2.66 - 4.0
Fixed-covers are made of concrete or steel and may be flat, conical,
or domed. It is difficult to make concrete covers gas-tight because con-
crete tends to develop cracks. Sludge must be removed from fixed-cover
units without letting air into the system and possibly forming an explosive
mixture. For this reason, fixed-cover digesters have WT+-er-level controls
to make the overflow equal to inflow.
Floating covers may be either: (a) the floating cover type
resting directly on the liquid with limited gas storage; or (b) the
gas-holder type provided with side skirts and resting on a cushion of gas.
Floating covers are the safest digesters to operate since there is little
chance of creating an explosive mixture under the cover.
The gas-holder type is used to store gas as it is produced. The pres-
sure developed inside the tank causes the cover to lift as much as six feet
or more above the minimum height.
Digesters can be heated by:
1. Hot-water coils within the digester,
2. Recirculating sludge through an external heat exchanger,
3. Direct contact of hot gas with sludge, and
4. Steam injection.
Hot water coils inside the digester have been used widely in the past.
Their main disadvantage is that they corrode and cake with sludge.
299
-------
The external heat exchanger with recirculation of the sludge is the
most often used method of heating. This method provides good scum control
with no pipes inside the digester.
Direct flame heating has been used where gas is mixed into the sludge
in small heating tanks. Steam injection has been used in only a few cases.
Mixing can be provided by:
1. Recirculating sludge through an exterior heat exchanger,
2. Mechanically mixing or pumping the sludge within the digester,
and
3. Releasing compressed wastewater gas through diffusers near the
bottom of the digester, through several pipes discharging above
the top of the cone.
In evaluating the performance of an anaerobic digester, the following
steps are useful:
1. Check the digester temperature. It should be 90-95°F and should
be held constant.
2. Check the digester pH and alkalinity. The pH should be 6.5-7.5
and preferably 6.8-7.2. The bicarbonate alkalinity should be
1,000 mg/1. If the alkalinity has been dropping but the pH is
still good, it is an indication of future trouble.
3. Check the digester gas production. The digester should produce
13 to 18 cu ft of gas per pound of volatile solids destroyed and
the gas should be at least 50% methane.
If any of these values are outside these ranges, the Control Considera-
tions and Troubleshooting sections should be read. The EPA publica-
tion, "Operations Manual - Anaerobic Sludge Digestion" (EPA 430/9-76-001)
also is useful.
Control Considerations
Anaerobic primary sludge digestion works in two steps. In the first
step, facultative and anaerobic bacteria, (called acid-forming bacteria)
convert the organic material in the sludge to organic acids. In the first
step, some carbon dioxide is formed, and some stabilization occurs. In the
second step, the organic acids are converted to carbon dioxide and methane
by anaerobic bacteria called methane-forming bacteria. Most of the sta-
bilization occurs in this step as the organics are converted into gas, water,
and a small amount of biological mass.
The anaerobic process is mostly controlled by the methane-forming
bacteria. These bacteria grow slowly and have generation times which
range from just less than 2 days to about 22 days. Methane formers are
300
-------
very sensitive to pH, sludge composition, and temperature. If the pH drops
below 6.0, methane does not form and the organics in the sludge do not
decrease. The methane bacteria are very active in the mesophilic range
between 80°F and 110°F, and in the thermophilic range between 113°F and
149°F. Almost all digesters in the United States operate within the meso-
philic temperature range.
Proper control of anaerobic sludge digestion is based on:
Food supply
Time and temperature
Mixing
pH and alkalinity
Gas production
Food Supply—
Digester organisms are most effective when food (raw sludge feed) is
provided in small amounts at frequent intervals or on a continuous basis.
If too much sludge is added too quickly to the primary digesters, the
first (acid-forming) step may produce acid faster than the organisms needed
for the second (gas-forming) step can break them down. This results in in-
complete digestion, along with causing bad odors.
The sludge fed to the digester should be as thick as possible without
clogging pumps and piping. Thin sludge takes up too much digester space
and adds excess water which must be heated.
Time and Temperature—
Less detention time usually is needed for complete digestion as
temperature increases. Most digesters are designed to operate in the 90-
95°F temperature range. If the temperature falls much below this range,
more time is needed for digestion. Complete digestion usually occurs in
about 15 days in a well mixed, properly heated digester. A temperature
change of 2 or 3 degrees can be enough to disturb the balance between
the acid and methane formers.
Mixing—
Raw sludge feed should be well mixed with the contents of the primary
digester. This helps to assure that the organisms have their food supply,
and that the digester temperature is even. The mixing system operation
should be closely monitored.
pH and Alkalinity—
Anaerobic digestion is relatively effective within the pH range of
6.5-7.5; however, the optimum range is 6.8-7.2. Outside these ranges,
digestion efficiency drops rapidly. Bicarbonate alkalinity should be kept
at a minimum level of 1,000 mg/1 as calcium carbonate (CaCC^) for good pH
control. To determine the bicarbonate alkalinity, both the volatile acid
concentration and the total alkalinity must be measured. The bicarbonate
alkalinity is then calculated as shown:
Bicarbonate Alkalinity = (Total Alkalinity - 0.8 Volatile Acids)
301
-------
The 0.8 factor in the above equation is needed to convert the volatile
acid units from mg/1 as acetic acid to mg/1 as CaCOo, the equivalent alka-
linity unit. The volatile acid to total alkalinity ratio should be kept
below 0.5 for good digester operation.
If digester volatile acid concentration increases, pH is lowered un-
less bicarbonate alkalinity is added. If alkalinity drops, pH problems can
be expected. Alkalinity can be added in many chemical forms. Two of the
most popular are lime and sodium bicarbonate. Lime additions beyond a
bicarbonate alkalinity of 500-1,000 mg/1 will react with carbon dioxide,
form a precipitate, and have little effect on digester pH. Sodium bicar-
bonate does not react with carbon dioxide, and although it is more expen-
sive than lime, smaller amounts are needed because it does not precipitate
out of solution.
Chemicals can be added to the digestion system as several points. It
is best to feed the chemicals with metering pumps for good control.
Chemicals can be added directly to the digester to make big changes in
bicarbonate alkalinity. The EPA operations manual on anaerobic digestion
contains detailed guidance on chemical addition.
Gas Production—
Gas production is one of the most important measurable digestion
parameters. Overall digester performance is reflected by the total volume,
rate, and composition of gas produced. Generally, the gas production
should be between 13-18 cu ft of digester gas/lb VS destroyed.
Differences in average gas production at a plant usually mean a change
in the degree of digestion or a change in the character of the sludge being
fed.
Gas from a properly operating digester contains about 65 percent
methane and 30 percent carbon dioxide. If more than 35 percent of the gas
is carbon dioxide, there is probably something wrong with the digestion
process.
As noted in Figures 91 and 92, supernatant (the liquid above the
sludge zone) is displaced as sludge is added to the digester. Usually,
supernatant is returned to the head of the plant; however, this recycle
stream may greatly increase the BOD, SS, and ammonia nitrogen loading on
the plant. Table 30 presents typical digester supernatant quality data.
When operations permit, it is good to return supernatant to other
plant units where it will have the least bad effect. Usually, it is best
to do this when the raw wastewater flow to the plant is at its daily low.
It is not good to add the supernatant load during peak flows. Inadequate
digestion can result in poor quality supernatant which can lower overall
plant performance when recycled.
302
-------
TABLE 30. SUPERNATANT CHARACTERISTICS FROM ANAEROBIC DIGESTERS
Activated
Primary plants Trickling filters* sludge plants*
(mg/1) (mg/1) (rng/1)
Suspended solids
200-1,000
BOD5 500-3,000
COD 1,000-5,000
Ammonia as NH3 300- 400
Total phosphorus as P 50- 200
500- 5,000
500- 5,000
2,000-10,000
400- 600
100- 300
5,000-15,000
1,000-10,000
3,000-30,000
500- 1,000
300- 1,000
*Includes primary sludge.
Common Design Shortcomings and Ways to Compensate
1.
2.
3.
4.
5.
Shortcoming
Gas recirculation system
for mixing undersized with
only 5-10 cfm/1,000 cu ft
of digester volume.
No provisions for controlled
addition of chemicals for
alkalinity control.
Pressure relief valves ex-
posed to cold weather are
freezing.
Sludge metering system in-
accurate or unreliable.
High air temperatures
cause mechanical mixers
to kick out.
Solution
1. Increase capacity of compressors
to provide 20 cfra/1,000 cu ft.
2. Install chemical storage tank
and metering pump.
3. Place a barrel over the valve
with an explosion-proof light
bulb inside it.
4. Measure the distance that float-
ing cover travels when pumping
in and not removing supernatant.
Calculate volume of sludge by
this method.
5. Protect motor by covering with
ventilator housing.
303
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TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
1. A rise in the vola-
tile acid/alkalinity
(VA/Alk.) ratio.
PROBABLE CAUSE
la. Hydraulic overload
caused by storm
infiltration, acci-
dental overpumping,
withdrawing too
much sludge .
Ib. Organic overload.
Ic. Discharge of toxic
materials to diges-
ters such as heavy
metals, sulfides,
ammonia.
CHECK OR MONITOR
la. Monitor the follow-
ing twice daily
until problem is
corrected:
volatile acids
alkalinity
temperature
Ib. Monitor sludge pump-
ing volume , amount o:
volatile solids in
feed sludge; check
for increase in sep-
tic tank sludge dis-
charged to plant or
industrial wastes.
Ic. Volatile acids, pH,
gas production;
check industrial
wastes at source;
check for inadequate
sludge pumping gener-
ating sulfides.
SOLUTIONS
la. If ratio increases to 0.3:
(1) add seed sludge from
secondary digester (or)
(2) decrease sludge withdrawal
rate to keep seed sludge
in digester (and/or)
(3) extend mixing time.
(4) check sludge temperatures
closely and control heat-
ing if needed.
Ib. See la.
Ic. Use 'any or combination of the
following:
(1) solids recycle.
(2) liquid dilution.
(3) decrease feed concentraticr
(4) precipitate heavy metals
with sulfur compound. Be
sure pH in digester is
greater than 7.0.
(5) Use iron salts to pre-
cipitate sulfides.
(6) institute source control
program for industrial
wastes.
-------
TROUBLESHOOTING GUIDE
.-- ANAEROBIC DIGESTION
INOICA TORS/OBSER VA TIONS
2. CO2 in gas starts to
increase.
3. pH starts to drop
and CO2 increases to
the point (42-45%)
that no burnable gas
is obtained.
4. The supernatant qual-
ity returning to
process is poor,
causing plant upsets.
PROBABLE CAUSE
2. VA/Alk. ratio has
increased to 0.5.
3a. VA/Alk. ratio has
increased to 0.8.
4a. Excessive mixing
and not enough
settling time.
4b. Supernatant draw-
off point not at
same level as super-
natant layer.
4c. Raw sludge feed
point too close to
supernatant draw-
off line.
4d. Not withdrawing
enough digested
sludge.
CHECK OR MONITOR
2a. Waste gas burner.
2b. Gas analyzer.
3a. Monitor as indicated
above .
3b. Hydrogen sulfide
(rotten egg) odor.
3c. Rancid butter odor.
4a. Withdraw sample and
observe separation
pattern.
4b. Locate depth of
supernatant by samp-
ling at different
depths .
4c. Determine volatile
solids content.
Should be close to
value found in well
mixed sludge and
much lower than raw
sludge.
4d. Compare feed and
withdrawal rates -
check volatile
solids to see if
sludge is well-digesl
SOLUTIONS
2. See Item 1 and start adding
alkalinity using the volatile
acids to calculate the amount.
3a. Add alkalinity.
3b. Decrease loading to less than
0.01 Ib vol. solids/cu ft/ day
until ratio drops to 0.5 or
below.
4a. Allow longer periods for
settling before withdrawing
supernatant.
4b. Adjust tank operating level or
draw-off pipe.
4c. Schedule pipe revision for
soonest possible time when
digester can be dewatered.
4d. Increase digested sludge with-
drawal rates. Withdrawal
should not exceed 5% of
digester volume per day.
ed.
U)
o
171
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4e. Review feasibility of adding
powdered carbon to digesters
with consultant or regulatory
agency.
5. Supernatant has a
sour odor from either
primary or secondary
digester.
5a. The pH of digester
is too low.
5b. Overloaded digester
("rotten egg odor").
5c. Toxic load (rancid
butter odor).
5a. See Item 3.
5b. See Item 3.
5c. See Item Ic.
5a. See Item 3.
5b. See Item 3.
5c. See Item Ic.
co
O
6. Foam observed in
supernatant from
single stage or
primary tank.
6a. Scum blanket break-
ing up.
6b. Excessive gas
recirculation.
6c. Organic overload.
6a. Check condition of
scum blanket.
6b. 20 CFM/1,000 cu ft
is adequate.
6c. Volatile solids
loading ratio.
6a. Normal condition but should
stop withdrawing supernatant
if possible.
6b. Throttle compressor output.
6c. Reduce feeding rate.
7. Bottom sludge too
watery or disposal
point too thin.
7a. Short-circuiting.
7b. Excessive mixing.
7c. Sludge coning, allow
ing lighter solids
to be pulled into
pump suction.
7a.
7b.
• 7c.
Draw-off line open
to Supernatant Zone.
Take sample and ched?
how it concentrates
in setting vessel.
Total solids test or
visual observation.
7a. Change to bottom draw-off line.
7b. Shut off mixing for 24-48 hours
before drawing sludge.
7c.(1) "Bump" the pump 2 or 3 times
by starting and stopping.
(2) Use whatever means available
to pump digester contents
back through the withdrawal
line.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7c. (continued)
(3) If available, attach a water
hose to the pump suction
line and force water through
it. (Water source must be
nonpotable.) Run for no mor(
than 2 or 3 min to avoid
diluting the digester
8. Sludge temperature
is falling and can
not be maintained at
normal level.
8a. Sludge is plugging
external heat
exchanger.
8b. Sludge recirculation
line is partially or
completely plugged.
8a.
8b.
Check inlet and out-
let pressure or
exchanger.
Check pump inlet and
outlet pressure.
8c. Inadequate mixing.
8d. Hydraulic overload.
8c.
8d.
Check temperature
profile in digester.
Incoming sludge
concentration.
8a. Open heat exchanger and clean.
8b.(1) Backflush the line with
heated digester sludge.
(2) Use mechanical cleaner.
(3) Apply water pressure. Do
not exceed working line
pressure.
(4) Add approx. 3 lb/100 gal
water of trisodium phos-
phate (TSP) or commerical
degreasers. (Most conven-
ient method is to fill scum
pit to a volume equal to
the line, add TSP or other
chemical, then admit to the
line and let stand for an
hour.)
8c. Increase mixing.
8d. See Item la.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8e. Low water feed rate
in internal coils
used for heat ex-
change .
8f. Boiler burner not
firing on digester
gas.
8g. Heating coils inside
digester have coat
ing.
8e.(1) Air lock in
line.
(2) Valve partially
closed.
8f.(1) Low gas pressure.
(2) Unburnable gas
due to process
upset.
8g. Temperature of inlet
and outlet water is
about the same.
8e. (1) Bleed air relief valve.
(2) Upstream valve may be
partially closed.
8f . (1) Locate and repair leak.
(2) See Item 3.
8g. (1) Remove coating, may require
draining tank .
(2) Control water temperature to
130°F maximum.
u>
o
Sludge temperature
is rising.
9. Temperature control-
ler is not working
properly.
9. Check water tempera
ture and controller
setting.
9. If over 120 °F, reduce tempera
ture . Repair or replace
controller.
10. Recirculation pump
not running; power
circuits O.K.
10. Temperature override
in circuit to prevent
pumping too hot water
through tubes.
10. Visual check, no
pressure on sludge
line.
lOa. Allow system to cool off.
lOb. Check temperature control
circuits .
11. Gas mixer feed lines
plugging.
lla. Lack of flow
through gas line.
lib. Debris in gas lines.
11. Identify low tempera-
ture of gas feed
pipes or low pressure
in the manometer.
lla. Flush out with water.
lib. Clean feed lines and/or valves.
lie. Give thorough service when tank
is drained for inspection.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DICESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
12. Gear reducer wear
on mechanical mixers
12a. Lack of proper
lubrication.
12b. Poor alignment of
equipment.
12a. Excessive motor
amperage, excessive
noise and vibration,
evidence of shaft
wear.
12b. See Item 15
12a. Verify correct type and amount
of lubrication from manufac-
turer's literature.
12b. Correct imbalances caused by
accumulation of material on
the internal moving parts.
13. Shaft seal leaking
on mechanical mixer.
13. Packing dried out
or worn.
13. Evidence of gas leak
age (evident odor of
gas) .
U)
o
ID
13a. Follow manufacturer's instruc-
tions for repacking.
13b. Replace packing any time the
tank is empty if it is not
possible when unit is operat-
ing.
14. Wear on internal
parts of mechanical
mixer.
14. Grit or misalignment
14. Visual observation
when tank is empty,
compare with manu-
facturer 's drawings
for original size.
Motor amperage will
also go down as mov-
ing parts are worn
away and get smaller.
14. Replace or rebuild - experience
will determine the frequency
of this operation.
15. Imbalance of internal
parts because of ac-
cumulation of debris
on the moving parts
of mechanical mixers
(large-diameter im-
pellers or turbines
would be affected
most).
15. Poor comminution
and/or screening.
15. Vibration, heating
of motor, excessive
amperage, noise.
15a. Reverse direction of mixer if
it has this feature.
15b. Stop and start alternately.
15c. Open inspection hole and
visually inspect.
15d. Draw down tank and clean mov-
ing parts.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
16. Rolling movement of
scum blanket is
slight or absent.
16a. Mixer is off.
16a.
Mixer switch or
timer.
16b. Inadequate mixing.
16c. Scum blanket is too
thick.
16c.
Measure blanket
thickness.
16a. May be normal if mixers are
set on a timer. If not and
mixers should be operating,
check for malfunction.
16b. Increase mixing.
16c. See Items 18 and 19.
17. Scum blanket is too
high.
17. Supernatant overflow .
is plugged.
17. Check gas pressure,
it may be above nor-
mal or relief valve
may be venting to
atmosphere.
17- Lower contents through bottom
drawoff then rod supernatant
line to clear plugging.
18. Scum blanket is too
thick.
18. Lack of mixing, high
grease content.
18. Probe blanket for
thickness through
thief hole or in gap
beside floating covei
18a. Break up blanket by using
mixers.
.18b. Use sludge recirculation pumps
and discharge above the
blanket.
18c. Use chemicals to soften
blanket.
18d. Break up blanket physically
with pole.
18e. Tank modification.
19. Draft tube mixers
not moving surface
adequately.
19. Scum blanket too high
and allowing thin
sludge to travel
under it.
19. Rolling movement on
sludge surface.
19a. Lower sludge level to 3-4"
above top of tube allowing
thick material to be pulled
into tube - continue for 24-
48 hours.
19b. Reverse direction (if possible)
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
1 ND ICA TORS/O BSER VA T IONS
20. Gas is leaking
through pressure re-
lief valve (PRV) on
roof.
21. Manometer shows
digester gas pressure
is above normal.
22. Manometer shows
digester gas pres-
sure below normal.
23. Pressure regulating
valve not opening as
pressure increases.
PROBABLE CAUSE
20. Valve not seating
properly or is
stuck open.
21a. Obstruction or
water in main burner
gas line.
21b. Digester PRV is
stuck shut.
21c. Waste gas burner
line pressure con-
trol valve is closed
22a. Too fast withdrawal
causing a vacuum in-
side digester.
22b. Adding too much lime
23a. Inflexible diaphragm
23b. Ruptured diaphragm.
CHECK OR MONITOR
20. Check the manometer
to see if digester
gas pressure is
normal.
21a. If all use points
are operating and
normal, then check
for a waste gas line
restriction or a
plugged or stuck
safety device.
21b. Gas is not escaping
as it should.
21c. Gas meters show ex-
cess gas is being
produced, but not
going to waste gas
burner.
22a. Check vacuum breaker
to be sure it is
operating properly.
22b. Sudden increase in
CO in digester gas.
.23a. Isolate valve and
open cover.
23b. Visual inspection.
SOLUTIONS
20. Remove PRV cover and move
weight holder until it seats
properly. Install new ring
if needed. Rotate a few times
for good seating.
2 la. Purge with air, drain conden-
sate traps , check for low
spots. Care must be taken
not to force air into
digester.
21b. Remove PRV cover and manually
open valve, clean valve seat.
21c. Relevel floating cover if gas
escapes around dome due to
tilting.
22a. Stop supernatant discharge
and close off all gas outlets
from digester until pressure
returns to normal .
22b. Stop addition of lime and
increase mixing.
23a. If no leaks are found (using
soap solution) diaphragm may
be lubricated and softened
using neats-foot oil.
23b. Ruptured diaphragm would re-
quire replacement.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
24. Yellow gas flame
from waste gas
burner.
24. Poor quality gas with
a high CO content.
24. Check CO2, content
will be higher than
normal.
24. Check concentration of sludge
feed - may be too dilute. If
so, increase sludge concentra-
tion. See Items 2 and 3.
25. Gas meter failure
(propeller or lobe
type).
25a. Debris in line.
25b. Mechanical failure.
25a. Condition of gas
line.
25b. Fouled or worn
parts.
25a. Flush with water, isolating
digester and working from
digester toward points of
usage.
25b. Wash with kerosene or replace
worn parts.
26. Gas meter failure
(bellows type).
OJ
M
ro
26a. Inflexible diaphragm
26b. Ruptured diaphragm.
26a. Isolate valve and
open cover.
26b. Visual inspection.
26a. If no leaks are found (using
soap solution) diaphragm may be
lubricated and softened using
neats-foot oil.
26b. Replace diaphragm.
26c. Metal guides may need to be
replaced if corroded.
27. Gas pressure higher
than normal during
freezing weather.
27a. Supernatant line
plugged.
27b. Pressure relief
stuck or closed.
27a.
Supernatant over-
flow lines.
27b.
Weights on pressure
relief valves.
27a. Check every two hours during
freezing conditions, inject
steam, protect line from
weather by covering and in-
sulating overflow box.
27b. If freezing is a problem, apply
light grease layer impregnated
with rock salt.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
28. Gas pressure lower
than normal.
28a. Pressure relief
valve or other pres-
sure control devices
stuck open.
28b. Gas line or hose
leaking.
28a. Pressure relief
valve and devices.
28b. Gas line and/or hose.
28a. Manually operate vacuum relief
and remove corrosion if present
and interferring with operation
28b. Repair as needed.
29. Leaks around metal
covers.
29. Anchor bolts pulled
loose and/or sealing
material moved or
cracking.
29. Concrete broken arounc
anchors, tie-downs
bent, sealing mater-
ials displaced.
oo
M
U)
29. Repair concrete with fast seal-
ing concrete repair material.
New tie-downs may have to be
welded onto old ones and re-
drilled. Tanks should be
drained and well ventilated for
this procedure. New sealant
material should be applied to
leaking area.
30. Suspected gas leaking
through concrete
cover.
30. Freezing and thawing
causing widening of
construction cracks.
30. Apply soap solutions
to suspected area and
check for bubbles.
30. If this is a serious problem,
drain tank, clean cracks and
repair with concrete sealers.
Tanks should be drained and
well ventilated for this pro-
cedure.
31. Floating cover tilt-
ing, little or no
scum around the edge;
31a. Weight distributed
unevenly.
31a. Location of weights.
31a. If moveable ballast or weights
are provided, move them around
until the cover is level. If
no weights are provided, use
a minimal number of sand bags
to cause cover to level up.
(Note: pressure relief valves
may need to be reset if signi-
ficant amounts of weight are
added.)
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
32. Floating cover tilt-
ing, heavy thick scum
accumulating around
edges .
33. Cover binding even
through rollers and
guides are free .
PROBABLE CAUSE
31b. Water from conden-
sation or rain water
collecting on top of
metal cover in one
location .
32a. Excess scum in one
area, causing excess
drag.
32b. Guides or rollers
out of adjustment.
32c. Rollers or guides
broken.
33. Internal guide or guy
wires are binding or
damaged (some covers
are built like umbrel-
las with guides
attached to the cen-
ter column) .
CHECK OR MONITOR
31b. Check around the
edges of the metal
cover. (Some covers
with insulating
wooden roofs have
inspection holes for
this purpose.)
32a. Probe with a stick
or some other method
to determine the con-
dition of the scum.
32b. Distance between
guides or rollers
and the wall.
32c. Determine the normal
position if the sus-
pected broken part
is covered by sludge.
Verify correct loca-
tion using manufac-
turer's information
and/or prints if
necessary.
33. Lower down to corbels.
Open hatch and using
breathing apparatus &
explosionproof light,
if possible, inspect
from the top. If
cover will not go all
SOLUTIONS
31b. Use siphon or other means to
remove the water. Repair roof
if leaks in the roof are con-
tributing to the water problem.
32a. Use chemicals or degreasing
agents such as Digest-aide or
Sanfax to soften the scum,
then hose down with water.
Continue on regular basis ever;
two to three months or more
frequently if needed.
32b. Soften up the scum (as in 32a)
and readjust rollers for
guides so that skirt doesn't
rub on the walls.
32c. Drain tank if necessary taking
care as cover lowers to cor-
bels not to allow it to bind
or come down unevenly. It may
be necessary to use a crane or
jacks in order to prevent
structural damage with this
case.
33. Drain and repair, holding the
cover in a fixed position if
necessary.
-------
TROUBLESHOOTING GUIDE
ANAEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
33. (continued)
the way down, it may
be necessary to secure
in one position with a
crane or by other means
to prevent skirt dam-
age to sidewalls.
w
H
U1
-------
AEROBIC DIGESTION
Process Description
Aerobic digestion is separate aeration of waste primary sludge, waste
biological sludge, or a combination of these in an open or closed tank.
It is usually used to stabilize excess activated sludges, but has also been
used to stabilize both primary and activated sludge solids. Figure 94
is a schematic of an aerobic digestion system.
Some plants use a separate sedimentation tank (Figure 94) and others
use a one-tank, batch-type system, where the sludge is aerated and mixed
for a long time, then settled and decanted in the same tank. Aerobic
digesters are often designed as rectangular aeration tanks and use con-
ventional aeration systems. In the past, aerobic digestion has been
used mostly at small plants on either contact stabilization sludge or
on mixtures of primary and activated sludge. Aerobic digesters in small
package plants often are segments of the circular package plant.
Recently, pure oxygen aeration has been used in aerobic digester
design. In conventional digesters, influent sludge VSS concentrations
must be no more than 3 percent for retention times of 15 to 20 days.
Above this percentage, oxygen from atmospheric air cannot be dissolved
into the digesting sludge fast enough to keep the biological reaction
going. However, pure oxygen can dissolve in sludge nearly five times
as fast as can oxygen from the air. As a result, pure oxygen aeration
allows a more concentrated sludge feed. By feeding a more concentrated
sludge, either the sludge retention time (SRT) can be longer or the total
pounds of sludge digested per day can be increased. Pure oxygen digesters
usually are closed so that oxygen is not lost to the atmosphere.
Typical Design Criteria and Performance Evaluation
Table 31 summarizes typical design criteria, and Table 32 shows
operating data for batch-type digestion of mixtures of primary and waste
activated sludge.
Usually, about 15 days of detention time is provided for excess
biological sludge, and more time when primary sludge is included.
Loadings normally are from 0.1 to 0.2 Ib VSS/ft /day, with volatile
suspended solids being reduced by 40-50 percent. The supernatant may
contain as little as 10-30 mg/1 BOD, 10 mg/1 ammonia nitrogen, and
from 50-100 mg/1 nitrate-nitrogen. When nitrification occurs, both
pH and alkalinity are reduced.
316
-------
PRIMARY SLUDGE
EXCESS ACTIVATED
OR TRICKLING
FILTER SLUDGE
CLEAR
OXIDIZED
OVERFLOW
TO PLANT
Settled sludge returned to aerodigester
Figure 94. Schematic of aerobic digestion system.
-------
TABLE 31.
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/1,009 cu ft
10-15
15-20b
3-4
0.024-0.14
20-35a
>60b
1.0-1.25
Minimum DO, mg/1 1.0-2.0
Temperature, C >15
VSS reduction, percent 35-50
Tank design
Power requirement,
BHP/10,000
Population Equivalent
8-10
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.
Excess activated sludge alone.
Primary and excess activated sludge, or primary sludge alone.
318
-------
TABLE 32. BATCH-TYPE AEROBIC SLUDGE DIGESTION
OPERATING DATA FOR MIXTURES OF PRIMARY
AND WASTE ACTIVATED SLUDGE
Detention
Time
days
5
10
30
60
5
10
15
30
60
5
10
Temperature
deg C
15
15
15
15
20
20
20
20
20
35
35
VSS
Reduction
percent
21
32
40.5
46
24
41
43
44
46
26
45
pH
7.6
7.6
6.6
4.6
7.6
7.6
7.8
5.4
5.1
7.9
8.0
Alkalinity
mg/1
510
380
81
23
590
390
560
31
35
630
540
NH3-N
mg/1
54
3.2
4.0
38
54
4.9
7.0
28
7.0
14
10.0
NO -N
mg/1
Trace
1.28
0.36
0.23
Trace
0.59
2.27
0.19
0.51
0.18
0.08
NO -N
mg/1
None
64
170
835
None
60
29
275
700
None
None
319
-------
To evaluate the performance of the aerobic digester:
1. Check the dissolved oxygen. It should be at least 1 mg/1.
2. Check the sludge retention time (see Activated Sludge Chapter
for example). It should be 10-15 days for waste activated
sludge and 15-20 days for primary and waste activated sludge.
3. Check data on VSS reduction. It should be in the range of
40-50 percent. If temperatures are low (<15 C), reductions
may fall to 35-40 percent unless very long detention times
are provided.
4. Check digester pH, it should be above 6.5.
If the above checks show unusual values, refer to the Troubleshooting
Guide.
Control Considerations
The rate of aerobic digestion is affected by temperature and SRT.
The rate of a biological reaction will increase as the temperature
increases. A rule of thumb is that the reaction rate doubles for each
o
10 C rise in temperature. Unfortunately, actual aerobic digestion plant
experience does not support fully this rule of thumb. Because of long
detention times and tank sizes, aerobic digestion usually occurs at
ambient temperatures. However, the energy released by the process can
cause temperatures to rise if the digester is covered. The effects of
SRT on the digestion process are shown in Figure 95.
The normal operating procedures for a batch-feed digester are as
follows:
1. Turn off aeration equipment and allow the solids to settle.
This solid-liquid separation should not be longer than
several hours or else air diffusers may become clogged.
2. Decant as much supernatant as possible. Sample a portion
of the supernatant for quality control.
3. Remove the thickened, digested sludge. The removed sludge
should be sampled for quality control.
4. Add the sludge feed volume. Sludge feed should be added over
a period of time, if possible. The volume and concentration
of sludge added each day should be as uniform as possible.
Sludge settling and removal may be performed once a week,
while sludge addition or feed is practiced daily. The sludge
volume in the digester therefore will increase each day until
the next decanting and removal period.
320
-------
BIODEGRADABLE SOLIDS
16
Figure 95. Effect of SRT on reduction of biodegradable solids by
aerobic digestion.
321
-------
For continuous feed digesters, the operator should adjust the rate of
settled sludge return to get the best return sludge concentration and
supernatant quality. He should watch carefully the settling tank inlet
and outlet flow to prevent short-circuiting and turbulence.
Aerobic digestion is a self-regulating process, except when the
process is overloaded or the equipment does not operate. Routine checks
should include periodic looks at electrical and mechanical equipment
such as seals, packings, timers, and relays.
Common Design Shortcomings and Ways to Compensate
Shortcoming
No provisions made for pH :
control resulting in undesir-
ably low pH in aerobic digester.
Air diffusers plug frequently. 2.
Solids depositing and accumu-
lating in digester due to poor
mixing.
No provision for solids
separation.
Solution
Install system to feed sodium
bicarbonate to digester influ-
ent or alkaline materials such
as sodium hydroxide or lime to
digester.
Replace diffusers with sock-
type devices or course bubble
diffusers.
Improve operation of grit
chamber, install grit separa-
tor on sludge feed stream,
or increase mixing in digester
by adding mechanical mixers to
supplement aerators.
Add clarifier downstream or
use batch operation as described
in Control Consideration Section.
322
-------
TROUBLESHOOTING GUIDE
AEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Excessive foaming.
la. Organic overload.
Ib. Excessive aeration.
la. Organic load.
Ib. Dissolved oxygen.
la. (1) Reduce feed rate.
(2) Increase solids in digester
by decanting and recycling
solids.
Ib. (1) Use surface sprays.
(2) Reduce aeration rate.
(3) Use a defearning agent.
U)
N3
U)
2. Low dissolved oxygen.
2a. Diffusers clogging.
2b. Liquid level not
proper for mechanical
aeration.
2c. Blower malfunction.
2d. Organic overload.
2a. Decant digester, with-
draw 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 objection-
able odor.
3a. Inadequate SRT.
3b. Inadequate aeration.
3a. SRT.
3b. D.O. should exceed
1 mg/1.
3a. See la.
3b. Increase aeration or reduce feed
rate.
4. Ice formation damaging
mechanical aerators.
4. Extended freezing
weather.
4. Check digester surface
for ice block forma-
tion.
4. Break and remove ice before it
c aus e s damage.
-------
TROUBLESHOOTING GUIDE
AEROBIC DIGESTION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. pH in digester has
dropped to undesirable
level.
5a. Nitrification is
occurring and waste-
water alkalinity is
low.
5b. In covered digester
CO is accumulating
in air space and is
dissolving into sludge
5a. Alkalinity and
nitrates in digester
supernatant.
5a. Add sodium bicarbonate to feed
sludge or lime or sodium
hydroxide to digester.
5b. Vent and scrub CO,
00
to
-------
CENTRIFUGATION
Description
Solids-liquid separation occurs in a centrifuge by rotating the liquid
at high speeds to cause separation by gravitational forces.
There are many types of centrifugal equipment, for different uses in
industry. However, the solid bowl centrifuge is the most often used type
for dewatering of sewage sludge. The solid bowl-conveyor sludge dewater-
ing centrifuge assembly (Figure 96) has a rotating unit with a bowl and
conveyor. The bowl, or shell, is supported between two sets of bearings.
The unit has a conical section at one end that acts as a drainage deck.
The conveyor screw pushes the sludge solids to outlet ports and then to
a sludge cake discharge hopper. Sludge slurry enters the rotating bowl
through a feed pipe leading into the hollow shaft of the rotating screw
conveyor. The sludge is distributed through ports into a pool inside the
rotating bowl.
As the liquid sludge flows through the hollow shaft toward the over-
flow devices, finer solids settle to the rotating bowl wall. The screw
conveyor pushes the solids to the conical section where the solids are
forced out of the water, and free water drains from the solids back into
the pool.
Most solid bowl centrifuges use countercurrent flow as pictured in
Figure 96 and are called "countercurrent" centrifuges. Recently, a
"concurrent" centrifuge design has also been introduced in which the solids
and liquid flow in the same direction. These units are much like the
countercurrent design except there are no effluent ports in the bowl head.
Instead, the clarified effluent is removed by a skimmer near the bowl and
drainage deck.
In addition to dewatering sludges, centrifuges have been used to
separate impurities from the lime sludges resulting from some phosphorus
removal processes. Centrifuges allow efficient recovery and reuse of
the lime.
Typical Design Criteria and Performance Evaluation
Loading Rates - The sizing of centrifuge equipment depends on sludge
feed rate, solids characteristics, temperature and condition processes.
Sludge feed rate is the parameter most often used for sizing centrifuges.
Single centrifuge capacities range from 4 gpm to about 250 gpm. Typical
quantities of sludge are shown in Table 33.
325
-------
COVER
DIFFERENTIAL
SPEED GEAR
BOX-*
ROTATING CONVEYOR
n
MAIN DRIVE SHEAVE
kw| [
f UUW
FEED PIPES
(SLUDGE AND
^•BEARING CHEMICAL)
CENTRATE
DISCHARGE
SLUDGE CAKE
DISCHARGE
Figure 96. Continuous countercurrent solid bowl-conveyor
discharge centrifuge.
326
-------
TABLE 33. TYPICAL SLUDGE QUANTITIES
—
1
Sludge type
Primary
Primary + FeCl
Primary + low lime
Primary + high lime
Primary + WAS
Primary + (WAS + FeCl3)
(Primary + FeCl ) + WAS
WAS
WAS + FeCl
Digested primary
Digested primary + WAS
Digested primary
?otal solids
(wt percent
of sludge)
5
2
5
7.5
2
1.5
1.8
1.0
1.0
8.0
4.0
4.0
Sludge solids
(Ib/mil gal)
total solids
1151
2510
4979
9807
2096
2685
3144
945
1535
806
1226
1817
Sludge volume
(gpm/mgd)
1.9
11.5
8.3
10.9
8.7
14.9
14.6
7.9
12.8
0.8
2.6
3.8
+ WAS + FeCl
Tertiary alum
Tertiary high lime
Tertiary low lime
1.0
4.5
3.0
700
8139
3311
5.8
15.0
9.2
327
-------
Table 34 shows expected performance for centrifugation based on sludge
type.
There are several variables that affect the performance of solid bowl
centrifuges. Bowl speed is one of the most important since centrifugal
force speeds up the separation process. At any given pool depth, an
increase in bowl speed provides more gravity-settling force, providing
greater clarification. For many years, G values for a solid bowl machine
were about 3,000. In recent years, units which operate at G=700 have been
developed. These "low" speed units work just as well as the high speed
units only with less power consumption.
The use of polymers, has allowed more materials to be dewatered by
centrifuges. The degree of solids recovery can be controlled over wide
ranges depending on the amount of coagulating chemical used. When floc-
culation aids are used, wetter sludge cake usually result
Heat treatment also is used for conditioning before centrifuging.
Heat treated sludges will dewater to 35-45% solids with no polymer re-
quired for 85% capture. Recovery of 92-99% of the solids from heat
treatment (primary) sludges is possible with polymer dosage of 2-5 Ibs
per ton of dry solids. Dewatering of heat treated mixtures of activated
sludge and raw primary sludge can produce cake solids of 40% at 95%
recovery without chemicals. Dewatering of heat treated activated sludges
alone has achieved 35% cake solids at 95% recovery without chemicals.
In evaluating performance:
1. Check the solids recovery against that shown as typical
in Table 34. Solids recovery is the ratio of cake solids
to feed solids for equal sampling times. It can be cal-
culated 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, since
Recovery =
/wet cake flow, lb\ (cake solids, %) (100)
_\ _ hr/ _
Cyet feed flow, lb\
hr/
(feed solids, %)
_ (cake solids, %) (feed solids, % - centrate solids, %) (100)
(feed solids, %) (cake solids, % - centrate solids, %)
The centrate is diluted by the extra water from, the chemical
and chemical dilution water. The measured centrate solids,
therefore, are less than the actual solids would be without
the added water from the chemical feed. The correction for
nh°™i cal addi tion is performed as follows:
328
-------
TABLE 34. TYPICAL SOLID BOWL CENTRIFUGE PERFORMANCE
Sludge cake characteristics
Wastewater sludge type
Raw or digested primary
Raw or digested primary, plus
trickling filter humus
Raw or digested primary, plus
activated sludge
Activated sludge
Oxygen activated sludge
High-lime sludges
Lime classification
Solids
(%)
28-35
20-30
15-30
8-9
8-10
50-55
40
Solids
recovery
(%)
70-90
80-95
60-75
80-95
50-65
80-85
80-85
90
70
Polymer
addition
no
5-15 Ibs/ton
no
5-20 Ibs/ton
no
5-10 Ibs/ton
3-5 Ibs/ton
no
no
329
-------
correction _ (feed rate, gpm)+(chemical flow, gpm)+(dilution water, gpm)
factor feed rate, gpm
corrected centrate solids = (measured centrate solids) (correction factor)
Solids feed rate is the dry solids feed to the centrifuge.
(feed flow, gpm)
/8.33 Ib) /J
V gai / V
feed solids, %
100
X60 min\
hr /
2. Check the solids concentration of the cake against the typical
values shown in Table 34. If the performance is not adequate,
refer to the Troubleshooting Section.
Control Considerations
High centrate turbidity may result in fine solids build-up in the
treatment process and'loss of solids in the plant effluent (see Trouble-
shooting Guide, Item 1). Low turbidity usually results in lower cake
solids since the fine solids are now being kept in the cake. As a result,
there must be a careful balance between centrate clarity and cake solids.
Common Design Shortcomings and Ways to Compensate
Shortcoming
1. Improper materials used and
corrosion problems result.
3.
1.
Solution
Replace components affected with
proper materials.
2. Washwater supply not strained, 2. Install washwater strainer.
plugs nozzles.
No means for removal of
conveyor.
3.
Install overhead hoist.
4. Rigid piping connections to
centrifuge.
5. Lack of adequate degritting
causes excessive wear.
4. Install flexible connections.
5. Install degritting system.
330
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
U)
u>
1. Centrate clarity
inadequate.
La. Feed rate too high.
Lb. Wrong plate dam
position.
Ic. Conveyor flights worn.
Ld. Speed too high.
Le. Feed solids too high.
If. Chemical conditioning
improper.
la. Flow records.
Ib. Setting of dam.
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 feed sludge.
If. Change chemical dosage.
2. Cake dryness
inadequate.
2a. Feed rate too high.
2b. Wrong plate dam
position.
2c. Speed too low.
2d. Chemical conditioning
too high.
2e. Influent too warm.
2a. Flow records.
2b. Setting of dam.
2c. Pulley setting.
2d. Chemical feed rate.
2e. Influent temperature.
2a. Reduce flow.
2b. Decrease pool depth to improve
dryness.
2c. Change pulley setting for higher
speed.
2d. Decrease chemical dosage.
2e. Reduce influent temperature.
3. Centrifuge torque
control keeps
tripping.
3a. Feed rate too high.
3b. Feed solids too high.
3a. Flow records.
3b. Spin test on feed
sludge - should be
<40% by volume.
3a. Reduce flows.
3b. Dilute feed sludge.
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATTON
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3c. Foreign material such
as tramp iron in
machine.
3d. Gear unit is mis-
aligned.
3e. Gear unit has faulty
bearing, gear, or
spline.
3c. Inspect interior.
3d. Vibration.
3e. Inspect gear unit.
3c. Remove conveyor and remove
foreign material.
3d. Correct alignment.
3e. Replace faulty parts.
4. Excessive vibration.
w
to
4a. Improper lubrication.
4b. Improper adjustment of
vibration isolators.
4c. Discharge funnels may
be contacting centri-
fuge.
4d. Portion of conveyor
flights may be plug-
ged with solids
causing imbalance.
4e. Gear box improperly
aligned.
4f. Pillow block bearings
damage.
4g. Bowl out of balance.
4h. Parts not tightly
assembled.
4a. Check lubrication
system.
4b. Vibration isolators.
4c. Position of funnels.
4d. Interior of machine.
4e. Gear box alignment.
4f. Inspect bearings.
4a. Correct lubrication.
4b. Adjust isolators.
4c. Reposition slip joints at
funnels.
4d. Flush out centrifuge.
4e. Align gear box.
4f. Replace bearings.
4g. Return rotating parts to manu-
facturer for rebalance.
4h. Tighten parts.
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATTON
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4i. Uneven wear of
conveyor.
4i. Inspect conveyor.
4i. Resurface, rebalance.
5. Sudden increase in
power consumption.
5a. Contact between bowl
and accumulated 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 wear.
5b. Clear effluent pipe.
u>
OJ
10
6. Gradual increase in
power consumption.
6a. Conveyor blade wear.
6a. Conveyor condition.
6a. Resurface blades.
7. Spasmodic, surging
solids discharge.
7a. Pool depth too low.
7b. Conveyor helix rough.
7c. Feed pipe (if adjust-
able) too near
drainage deck.
7d. Machine vibration
excessive (see Item 4)
7a. Plate dam position.
7b. Improper hard
surfacing or
corrosion.
7a. Increase pool depth.
7b. Refinish conveyor blade areas.
7c. Move feed pipe to effluent end.
8. Centrifuge shuts down
(or will not start).
8a. Blown fuses.
8b. Overload relay
tripped.
8c. Motor overheated,
thermal protectors
tripped.
8a. Fuses.
8b. Overload relay.
8c. Thermal protectors.
8a. Replace fuses, flush mahcine.
8b. Flush machine, reset relay.
8c. Flush machine, reset thermal
protectors.
-------
TROUBLESHOOTING GUIDE
CENTRIFUGATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
8d. Torque control
tripped.
8e. Vibration switch
tripped.
8d. See Item 3.
8e. See Item 4.
10
00
-------
VACUUM FILTRATION
Process Description
The vacuum filter is a device for separating solid matter from liquid.
A vacuum filter has a round drum which rotates partially submerged in a
tank of sludge. The filter drum is divided into sections by seal strips.
A vacuum is applied between the drum deck and filter medium causing filtrate
to be removed, and filter cake to stay on the medium. In the drum filter
pictured in Figure 97, the cake of dewatered sludge is removed by a fixed
scraper blade. There are other designs which use different methods for
sludge removal.
Typical Design Criteria and Performance Evaluation
Table 35 lists design data and expected performance for vacuum filtra-
tion of different types of sludges. In evaluating performance:
1. Check the yield of the vacuum filter.
The performance of vacuum filters may be evaluated by the yield, the
efficiency of solids removal and the cake characteristics. Yield is the
most common measure of filter performance. The yield describes the filter
output and is expressed in terms of pounds of dry total solids in the
filter cake per square foot of filter area, per hour. Table 36 can be
used to determine the filter area knowing the diameter of the drum and
the length of the filter face.
2. Check the solids capture.
The second measure of filter performance is the efficiency of solids
removal (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 cause a load on the plant treatment units, and should
normally be kept as low as possible. It may be necessary to reduce the
filter efficiency to have a higher filter output in order to keep up with
sludge production.
3. Check the filter cake solids content.
The filter cake quality is another measure of filter performance,
depending upon cake moisture and heat value. Cake solids content varies
from 20-40 percent by weight, depending on the type of sludge and the
filter cycle time. Producing a very dry cake does not always mean good
filter performance. Cake moisture should be adjusted to the method of
final disposal. It is not efficient to dry the cake more than is neces-
335
-------
CLOTH CAULKING
STRIPS-
AUTOMATIC VALVE
AIR AND FILTRATE
LINE
DRUM
FILTRATE PIPING
CAKE SCRAPER
SLURRY AGITATOR
VAT
SLURRY FEED
AIR BLOW-BACK LINE
Figure 97. Cutaway view of a rotary drum vacuum filter.
336
-------
TABLE 35. TYPICAL DESIGN AND PERFORMANCE DATA FOR VACUUM FILTRATION SYSTEMS
Sludge type
Primary
Percent
solids
Design assumptions to VF
Thickened to 10% solids 10
Typical
loading
rates ,
(psf/hr)
8-10
Percent
solids
VF cake
25-38
Primary + FeCl.
Primary +
low lime
Polymer conditioned
85 mg/1 Fed dose 2.5
Lime conditioning
Thickening to 2.5% solids
300 mg/1 lime dose 15
Polymer conditioned
Thickened to 15% solids
1.0-2.0
15-20
32-35
Primary +
high lime
600 mg/1 lime dose
Polymer conditioned
Thickened to 15% solids
15
10
28-32
Primary + WAS
Primary +
(WAS + FeCl )
(Primary + FeCl )
+ WAS
Waste activated
sludge (WAS)
WAS + FeCl,
Digested primary
Digested primary
+ WAS
Thickened to 8% solids
Polymer conditoned
Thickened to 8% solids
FeCl & lime conditioned
Thickened primary sludge
to 2.5%
Flotation thickened WAS
to 5%
Dewater blended sludges
Thickened to 5% solids
Polymer conditioned
Thickened to 5% solids
Lime + FeCl conditioned
Thickened to 8-10% solids
Polymer conditioned
Thickened to 6-8% solids
Polymer conditioned
4-5
3.5
8-10
6-8
1.5
2.5-3.5
1.5-2.0
7-8
3.5-6
16-25
20
15-20
15
15
25-38
14-22
Digested primary
+ (WAS + FeCl3)
Tertiary alum
Thickened to 6-8% solids 6-8
FeCl + lime conditioned
Diatomaceous earth
precoat
0.6-0.8
2.5-3
0.4
16-18
15-20
337
-------
TABLE 36. FILTERING AREA OF DRUM FILTERS IN SQUARE FEET
0)
4J
•H 12 34567
Face (in ft)
8 9 10 11 12 13 14 15 16 17 18 19 20
3' 9.4 18.8 28.2 37.7 47.1 56.5
4V 42.4 56.5 70.6 84.8 98.9 103 127
94.2 113 132 151 170 188 207 226
176 201 226 251 277 302 327 352 377 402
10' 283 314 345 377 409 440 471 502 534 565 596 628
12' 377 415 452 490 528 565 603 641 679 716 754
UJ
00
00
-------
sary. When incineration is practiced, a raw sludge cake having a high
moisture content can be burned without auxiliary fuel because of the
higher volatile content. On the other hand, a digested sludge cake will
have to be dryed to burn well without make-up heat, since the sludge has
low volatile solids content.
The effect of heat treatment before vacuum filtration is to make all
types of sludges about equally dewaterable. Heat treatment produces a
sludge that dewaters very easily. 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 that are heat treated should
produce yields well over 10 psf/hr.
Control Considerations
The operator should determine the best conditioning chemical for the
feed sludge. If the character of the feed sludge changes much, the condi-
tioning agents should be evaluated after each change.
Once an effective conditioner has been selected, the next task is to
see how performance is affected by machine variables and chemical dosage
rates. One or more of the most sensitive variables should be held constant
and the others changed one-by-one to develop a series of performance curves.
The performance curves are checked to obtain proper operating procedures
for the best sludge dewatering. These relationships should be checked
periodically to see if small changes in the operating procedures are needed.
Usually, filtrate is sent to the primary treatment process where the
most solids will be retained. When filtrate quality is poor, large amounts
of fine particles build-up in the plant and reduce treatment efficiencies.
In a plant with an activated sludge process, the filtrate may be directed
to a flotation or thickener process.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Improper filter media specified 1. Run filter leaf tests with
resulting in (a) filter blinds different media. Replace
or (b) inadequate solids capture media with best one.
occurs.
2. Improper chemical conditioning 2. Run filter leaf tests to
system specified. determine proper condition-
ing chemical and dosage.
3. No provisions for cleaning of 3. Install unions in filtrate
filtrate lines. line to permit ready clean-
ing.
4. Cake does not release well from 4. Add doctor blade to supple-
belt-type filter. ment discharge roll.
339
-------
Shortcoming Solution
Filtrate pumps are easily 5. Install an equalizing line
air bound. from high point of receiver
to the eye of the pump.
340
-------
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 blinding.
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 system,
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 vibrating.
5a. Filtrate pump is clog-
ged.
5a. Filtrate pump output.
5a. Turn pump off and clean.
-------
TROUBLESHOOTING GUIDE
VACUUM FILTRATION
IND ICA TORS/O BSE R VA T IONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. Receiver is vibrating.
(Cont'd)
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. Drum face.
5f. Drum.
5b. Tighten bolts and align gasket.
5c. Replace ball check.
5d. Seal leaks.
5e. Clean face with pressure hose.
5f. Replace seal strips.
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
^ACUUM 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.
9. Scale buildup on vacuun
pump seals.
9a. Hard, unstable water.
9a. Vacuum pump seals.
9a. Add sequestering agent.
-------
PRESSURE FILTRATION
Process Description
The filter press is a batch device used to process sludges that are
difficult to dewater. There are several types of presses available but
the most common type is shown in Figure 93. This type of press has ver-
tical plates held in a frame and pressed together between a fixed and
moving end as pictured in Figure 98. A filter cloth covers each plate.
The 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 need a precoat material (incinerator ash or diatomaceous
earth) to reduce blinding and to help release the cake. The water passes
through the cloth, while the solids stay on the cloth and form a cake.
Sludge feeding is stopped when the sections between the trays are com-
pletely filled. There are drainage ports at the bottom of each press
section where the filtrate is collected, taken to the end of the press,
and discharged to a common drain. Filtrate quality should be very good
(less than 100 mg/1 suspended solids) if the system is properly operated.
At the end of a cycle, the drainage from a large press can be 2,000 to
3,000 gallons per hour. This rate drops quickly to about 500 gallons
per hour as the cake begins to form. When the cake completely fills the
section, the rate is almost nothing. The dewatering step is finished
when the filtrate 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 press is opened electrically and each plate
is moved to let the filter cake 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 set of plates is then pushed back by the
electrical closing gear. The valve to the press is then opened, the
sludge feed pump started, and the next dewatering cycle begins.
Filter presses are usually installed well above floor level, so that
the cakes can drop onto conveyors or trailers set under the press. Filter
presses can be operated at pressures ranging from 5,000 to 20,000 times
the force of gravity. In comparison, a solid bowl centrifuge provides
forces of 700-3,500 g and a vacuum filter, 1,000 g. Because of these
greater pressures, filter presses may provide higher cake solids concen-
trations (30-50% solids) at lower chemical dosages. In some cases, ash
from a downstream incinerator is recycled as a sludge conditioner.
Typical Design Criteria and Performance Evaluation
Table 37 lists normal loading rates and expected performance for
pressure filtration systems. Loading rates depend on the length of the
344
-------
Fixed end
^Travelling end
OJ
£>
Ul
QQQQQQQQQQQQQ
OO
Operating handle
o
{Electric
closing gear
Figure 98. Side view of a filter press.
-------
TABLE 37. 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 + Fed 3)
Tertiary alum
Tertiary low lime
Percent
solids to
pressure
Conditioning filter
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
5
4*
7.5
8*
8*
3.5*
5*
5*
8
6-8*
6-8*
4*
8*
Typical cycle
length
2
1.
4
1.
2.
2.
3
4
2.
2.
3.
2
2
1.
3
6
1.
hrs
5
5
5
0
5
0
5
5
5
Percent
solids
filter cake
45
50
40
50
45
50
45
40
45
50
45
40
45
50
40
35
55
*Thickening used to achieve this solids concentration.
-------
cycle described above.
In evaluating performance:
1. Check the filter cake solids concentrations against typical
values in Table 37.
2. Check the length of filter cycle against Table 37.
3. Check the quality of the filtrate. With proper operation
and conditioning, the suspended solids concentration should
be less than 100 mg/1.
Refer to the Troubleshooting section if unusual values are found.
Control Considerations
For any given filtrate flow rate, a certain filter cake concentration
can be expected. Whether or not to precoat depends on operation. The
purpose of precoat is to protect the filter media from early blinding
and to reduce the frequency at which the filter media needs, cleaning.
If the investment in a precoat system has been made, its use should re-
duce manpower requirements for media cleaning. The influent sludge to
the press should be thickened as greatly as possible.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Gravimetric Ash Feeders
Installed - bulking
problems with ash.
2. Cake transport system
inadequate (screw con-
veyors plug; belt con-
veyor limited to 15°
slope).
3. Mechanical Ash Conveyor
Installed - noise and
maintenance problems.
4. Improper media specified -
poor cake discharge,
difficult to clean.
1. Install Volumetric Feeders.
2. Install heavy-duty flight
conveyor.
3. Install pneumatic ash conveying
system.
4. Change media - usually monofila-
ment, relatively coarse media
are used on municipal sludges.
347
-------
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 conditioning.
2a. Prevent feed.
2b. Conditioner type and
dosage.
2a. Increase precoat, feet @ 25-40
psig.
2b. Change conditioner type on
dosage based on filter leaf
tests.
CO
*.
oo
3. Filter cycle times
excessive.
3a. Improper conditioning.
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 concentra-
tion in press feed.
4. Filter cake sticks to
solids conveying
equipment.
4. Change chemical con-
ditioning by using
more inorganic
chemicals.
4. Conditioning dosages.
4. Decrease ash, increase
inorganic conditioners.
5. Precoat pressures
gradually increase.
5a. Improper sludge
conditioning.
5b. Improper precoat feed.
5c. Filter media pluggd.
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).
6. Frequent media
blinding.
6a. Precoat inadequate.
6b. Initial feed rates
too high (where no
precoat used).
6a. Precoat feed.
6a. Increase precoat.
6b. Develop initial cake slowly.
-------
TROUBLESHOOTING GUIDE
PRESSURE FTT.TRATTON
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
7. Excessive moisture
in cake.
7a. Improper conditioning.
7b. Filter cycle too
short.
7a. Conditioning dosages.
7b. Correlate filtrate
flow rate with cake
moisture content.
7a. Change chemical dosage.
7b. Lengthen filter cycle.
Sludge blowing out
of press.
8. Obstruction, such as
rags, in the press
forcing sludge be-
tween plates.
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.
9. Excessively wet cake
soiling the media
on lower faces.
9. Cake moisture content,
See Item 7.
-------
SLUDGE DRYING BEDS
Process Description
Sand drying beds are often used for dewatering digested sludges.
Digested and/or conditioned sludge is discharged onto a drying bed and allowed
to dewater and dry under natural conditions. After digested sludge is applied
to the sand bed, dissolved gases are released and rise to the surface, float-
ing the solids and leaving a layer of liquor at the bottom. The liquor drains
through the sand, is collected in the underdrain system and usually returned
to a plant unit for further treatment. Drying beds drain very slowly in the
beginning, but after about 3 days, the rate increases. After maximum drain-
age is reached, the dewatering rate gradually slows down and evaporation
continues until the moisture content is low enough to permit sludge removal.
Dry sludge may be removed from the beds manually, by special conveyors,
or with other loading equipment. Some users of sludge are allowed to remove
the dried cake directly from the bed. Sludge cake is finally disposed of by
land application as a soil conditioner.
Typical Design Criteria and Performance Evaluation
Figure 99 shows a sand bed cross-section.
Drying beds usually have 4 to 9 inches of sand placed over 8 to 18
inches of graded gravel or stone. The sand usually is 0.3 to 1.2 mm in size
and a uniformity coefficient less than 5.0. Gravel is normally graded from
1/8 to 1.0 inches. Drying beds have underdrains that are spaced 8 to 20 feet
apart. Underdrain piping is often vitrified clay laid with open joints, has
a diameter no less than 4 inches, and a slope of at least 1 percent. Collect-
ed filtrate is usually returned to the treatment plant. Several beds are
usually provided so that one will be free to accept sludge while the others
are in different stages of drying.
Sandbeds are sometimes enclosed by glass or covered with a fiber glass
or glass roof to protect the drying sludge from rain, control odors and
insects, reduce the drying periods during cold weather, and improve the
appearance of a waste treatment plant. Enclosed beds usually need only 67
to 75 percent of the area needed for an open bed. Good ventilation is im-
portant to control humidity and provide a good evaporation rate.
The area needed for open sand beds depends on climate, but typical
criteria are:
350
-------
Sludge
Tile collection
system
Drainage
Figure 99. Cross-section of typical sand drying bed.
351
-------
Sludge loading
Area dry solids
Type of digested sludge (sq ft/capita) (Ib/sq ft/yr)
Primary 1.0 27.5
Primary and standard
trickling filter 1.6 22.0
Primary and activated 3.0 15.0
Chemically precipitated 2.0 22.0
In evaluating performance:
1. Check the length of time needed for the sludge to dry to the point
that it can be removed. In good weather, the sludge should be
ready for removal in 2-3 weeks and a cake of 45 percent solids may
only need six weeks using a well digested sludge. As with any de-
watering method, the performance of a sand bed may be improved by
chemical conditioning and the dewatering time may be reduced by
more than 50 percent. With proper chemical conditioning, some
sludges can be removed in less than one day. Sand beds can pro-
duce sludges with 85-90 percent solids content.
2. Check for odors. Odors indicate poor sludge digestion.
3. Check to see how much sand is left in the beds. If there is less
than 4 inches, makeup sand should be added to replace sand hauled
away with sludge.
Refer to the Control Considerations and Troubleshooting sections for
guidance on solving any problems.
Control Considerations
The operator has less control over the sludge drying bed operation than
with mechanical dewatering systems. Sludge drying bed performance is affect-
ed by weather, sludge characteristics, system design (including depth of
fill), chemical conditioning, and drying time.
The type of sludge and its moisture content affect the drying
process. Sludges containing grit dry rapidly; those containing grease more
slowly; aged sludge dries slower than new sludge; primary sludge dries faster
than secondary sludge; and digested sludge dries faster than fresh sludge.
It is important that wastewater sludge be well digested for good drying. In
well digested sludge, gases tend to float the sludge solids and leave a clear
liquid layer, which drains through the sand.
Chemicals are especially useful for sludges that are hard to dewater or
when drying beds are overloaded. The conditioning chemicals most commonly
used are alum, ferric chloride, chlorinated copperas, and organic poly-
electrolytes.
352
-------
The useful capacity of the sand beds can be maximized by always re-
moving sludge as soon as it has reached the desired dryness.
Sludge should never be added to a bed that contains partially dried
sludge. Any weeds and other vegetation that might be present should be
removed. Herbicides might be needed if there is much weed growth.
The sand bed should be leveled, and raked to make sure that it can drain
the sludge properly. When necessary, sand should be added to keep at least
4 inches of depth.
After the sludge is applied to the beds, lines should be drained and
flushed with water to prevent plugging and high pressures caused by gases
resulting from the decomposing sludge.
Common Design Shortcomings and Ways to Compensate
Shortcoming Solution
1. Beds undersized. 1. Condition sludge with polymers.
2. Sludge distribution system 2. Modify distribution piping.
results in uneven loading
of beds.
3. Walls dividing beds are made 3. Replace with heavy creosoted
of untreated wood and warp lumber; pour concrete walls;
rapidly. build concrete block walls.
4. Beds sometimes built on 4. Relocate beds on higher ground.
flood plains.
353
-------
TROUBLESHOOTING GUIDE
SLUDGE DRYING BEDS
INDICATORS/OBSERVATIONS
1. Excessive dewatering
time.
2. Sludge feed lines
are plugged.
3. Very thin sludge
being drawn from
digester.
PROBABLE CAUSE
la. Applied sludge depth
is too great.
Ib. Sludge applied to
improperly cleaned
bed.
Ic. Underdrain system
has plugged or lines
are broken.
Id. Beds undersized.
le . Weather conditions .
2 . Accumulation of grit
and solids in lines.
3. "Coning" occurring in
digester with water
being pulled out and
sludge left behind.
CHECK OR MONITOR
la. Typically, 8 inches
of applied sludge
is satisfactory.
Ib. Note condition of
any empty beds.
Id. Effects of adding
polymer.
le . Temperature ,
precipitation .
SOLUTIONS
la. When bed has dried, remove
sludge and clean. Apply a
smaller depth of sludge and
measure the draw down over a
3 day period. Next applica-
tion, apply twice the 3 day
draw down.
Ib. After sludge has dried, remove
sludge and dirty sand and
replace with 0.5-1 inch of
clean sand.
le. Backflush beds slowly by hook-
ing clean water source to
underdrain piping. Check sand
bed and replace media as
needed. Drain underdrain lines
during freezing weather to keep
them from freezing.
Id. Normally 5-30 Ibs/ton of dry
solids of cationic polymer
provides improved dewatering
rates.
le. Cover or enclose bed to protect
from weather.
2. Open valves fully at start of
sludge application to clean
lines; flush lines with water
if necessary.
3. Reduce rate of withdrawal from
digester.
w
U1
-------
TROUBLESHOOTING GUIDE
SLUDGE DRYING BEDS
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Flies breeding in
sludge beds.
4. Break sludge crust and use
larvicides such as borax, or
calcium borate or kill adult
flies with suitable insecticide
5. Odors when sludge is
applied.
Inadequate digestion
of sludge.
Operation of diges-
tion process (see
appropriate section
of this manual).
5a. Establish correct operation of
digestion process.
5b. As temporary solution, add
lime to sludge. Lime may
control odors but may tend to
clog sand.
w
in
ui
-------
SLUDGE DRYING LAGOONS
Process Description
Sludge lagoons are like sand beds in that sludge is removed from a
digester, placed in the lagoon, removed after a period of drying, and the
cycle repeated. Unlike sand beds, drying lagoons do not have an underdrain
system since most of the drying occurs by decanting supernatant liquor and by
evaporation.
Typical Design Criteria and Performance Evaluation
Solids loading rates often used for drying lagoons are 2.2 to 2.4
Ib/yr/cu ft of lagoon capacity. Other designs used are 1 sq ft/capita for
primary digested sludges in a dry climate, and 3 to 4 sq ft/capita for acti-
vated sludge plants where the annual rainfall is 36 inches. A 2 foot high
dike with a sludge depth of 15 inches (after decanting) often is used. Sludge
depths of 2.5 to 4 feet have been used in warmer climates with longer drying
periods. Draw off points are provided to remove supernatant and rainfall.
Except for treatment in very hot, arid climates, sludge will generally
not dewater to the point that it can be lifted by a fork. If sludge is
placed in depths of 15 inches or less, it may be removed with a front-end
loader after 3 to 5 months. When sludge is to be used for soil conditioning,
it may be good to stockpile it for added drying before use. One approach is
to use a 3 year cycle where the lagoon is loaded for 1 year, dries for 18
months, is cleaned, and allowed to rest for 6 months.
Control Considerations
There is not much the operator can control in this process except for
pretreatment and thickening of sludge prior to lagoon drying. Once dis-
charged to the lagoon, the drying rate mostly depends on weather conditions.
However, the operator should quickly remove supernatant liquor and rainwater
so that the sludge cake is exposed to the air and can dry rapidly. Weeds and
other vegetation always should be removed from the lagoon area before filling
with sludge.
356
-------
TROUBLESHOOTING GUIDE
SLUDGE DRYING 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 rain-
water promptly; apply
insecticides.
U)
Ul
3. Supernatant decanted
from lagoon is up-
setting treatment
process when recy-
cled.
3a. Broken dikes between
lagoons causing
freshly drawn sludge
to enter supernatant.
3b. Supernatant being
drawn prematurely.
3c. Excessive sludge
depths applied
causing supernatant
drawoff to be below
sludge interface.
3a. Dike condition.
3b. Suspended solids of
supernatant.
3c. Sludge application
depths.
3a. Repair broken dikes.
3b. Delay drawing of supernatant
until sludge has settled.
3c. Apply shallower sludge depths.
-------
INCINERATION - MULTIPLE HEARTH
Process Description
Multiple hearth furnaces are the most often used furnace for municipal
wastewater sludge incineration. As shown in Figure 100, a multiple hearth
furnace has a steel shell around several solid refractory hearths and a cen-
tral rotating shaft with rabble arms. The dewatered sludge enters at the top
through a flapgate and drops down through the furnace from hearth to hearth
by the action of the rabble arms. The hearths are made of high heat, heavy-
duty fire brick. The capacity of these furnaces depends on the total area
of the enclosed hearths. They are designed with diameters ranging from 54
inches to 21 feet 6 inches and with four to eleven hearths. Table 38 shows
the effective hearth area for different size furnaces. Capacities of multiple
hearth furnaces range from 200 to 8,000 Ib/hr of dry sludge with operating
temperatures of 1,700 F.
Each hearth usually has 2 doors, fitted to cast iron frames and designed
to close reasonably tight. Each door has an observation port which can be
opened. Since the furnace may operate at temperatures up to 2,000 F, the
shaft and rabble arms are cooled by air supplied in controlled amounts from
a blower located at the bottom of the shaft. The shaft is motor driven and
speed can be adjusted from about 0.5 to 1.5 rpm. Two or more rabble arms are
connected to the shaft at each hearth. Each rabble arm has a central tube for
conducting air from the central shaft to the end of the rabble arm. The air
may be discharged to atmosphere or returned to the bottom hearth as preheated
air, for combustion.
The rabble arms provide mixing action as well as rotary and downward
movement of the sludge. Combustion air flow is countercurrent to that of the
sludge. Combustion air flow is countercurrent to that of the sludge. Some
hearths have oil or gas burners to provide start-up or supplemental heat.
Sludge is constantly turned and broken into smaller particles by the rotating
rabble arms. This exposes the sludge surface to the hot furnace gases so that
rapid and complete drying as well as burning of sludge occurs.
The multiple hearth system usually has an instrumentation system which
sends operating data to a control panel. Temperature can be recorded for each
hearth, cooling, and exhaust, and scrubber inlet gas. The temperature on each
hearth can be controlled to within +_ 40 F. Breakdowns such as burner shut-
down, furnace overtemperature, draft loss and feed belt shutdown can be
monitored. If there is a power or fuel failure, the furnace should be shut
down automatically and the shaft cooling air fan should be run on standby
power to keep the shaft from melting.
358
-------
^
FLUE GASES OUT>
^COOLING AIR DISCHARGE
-FLOATING DAMPER
DRYING ZONE
COMBUSTION ZONE
COOLING ZONE
ASH DISCHARGE
SLUDGE INLET
RABBLE ARM AT
EACH HEARTH
•COMBUSTION
AIR RETURN
RABBLE ARM
DRIVE
COOLING AIR FAN
Figure 100. Cross section of a typical multiple hearth incinerator.
359
-------
TABLE 38. 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
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
Effective
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
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
Number
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
Typical Design Criteria and Performance Evaluation
Table 39 lists typical loading rates for several types of sludges.
Combustion temperatures of about 1400 F are commonly used.
To avoid damaging the brick lining, the furnace must be carefully
heated and cooled. Heat-up times usually are:
-------
Effective hearth area,
sq ft
less than 400
400 - 800
800 - 1400
1400 - 2000
greater than 2000
Heatup time,
hr
18
27
36
54
108
TABLE 39. MULTIPLE HEARTH FURNACE OPERATION
Typical
Type of sludge
1.
2.
3.
4.
5.
6.
7.
8.
9.
Primary
Primary +
Primary +
Primary +
Primary +
Fed 3)
(Primary
+ WAS
WAS
FeCl
low lime
WAS
(WAS +
+ FeCls)
WAS + FeCl3
Digested
Primary
Percent
solids
30
16
35
16
20
16
16
16
30
Percent
VS
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/hr/sq
7.0 -
6.0 -
8.0 -
6.0 -
6.5 -
6.0 -
6.0 -
6.0 -
7.0 -
12
10
12
10
11
10
10
10
12
ft
.0
.0
.0
.0
.0
.0
.0
.0
.0
* Assumes no dewatering chemicals
** Low number is applicable to small plants, high number is applicable to
large plants.
In some plants, the stack gases are passed through a heat exchanger to
remove heat. This heat is used for preheating the incoming furnace air,
for sludge conditioning by heat treatment, or for other uses in the plant.
A boiler-generator system fueled by stack gas can be used to produce
electricity.
To control air pollution, the stack gases often are passed through a
wet scrubber.
Sludge incineration can reduce sludge volume by more than 90 percent.
The ash from the incineration process is free of pesticides, viruses and
pathogens. The metals in the ash are about the same as in the raw sludge;
except that now, the metals are less soluble. The ash can be finally
disposed in a landfill.
The following steps may be useful in evaluating performance:
361
-------
1. Check temperature records to make sure that temperatures on each
hearth are kept uniform and are within the range specified by the
manufacturer .
2. Check carbon monoxide data for stack gas. Carbon monoxide indi-
cates incomplete combustion.
3. Check maintenance records to see how often hearth and refractory
repairs have been needed. Check shutdown and startup procedures
if repairs are frequent.
4. Check for odors which indicate poor combustion.
5. Check furnace loading and fuel use:
6 hearth, 14.25 ft outer diameter
Primary + WAS feed, 18% solids, 69% volatile
18000- Ibs/day/dry solids
Effective furnace area = 575 sq ft (Table 38)
Wet feed rate = 18000 Ibs/day/dry = 100,000 Ibs/day wet solids
0.18% solids = 4167 lbs/hr
Loading rate = . = 7.25 ^s/hr/sq ft
c
From Figure 101, approximate heat needed = 0.5 x 10 BTU/ton VS
Volatile solids = 18,000 Ibs/day x 0.69 = 12,420 Ibs/day
=6.2 tons/day
Fuel needed = 3.1 x 10 BTU/day or 93 x 10 BTU/month
If the furnace is shut down and started up during the month, fuel
will also be needed for startup. If fuel used is drastically
different than above, consult troubleshooting quide.
Control Considerations
An incinerator is usually part of a sludge treatment system which
includes sludge thickening, a disintegrating system, a dewatering device
(such as a vacuum filter, centrifuge, or filter press), an incinerator feed
system, air pollution control devices, ash handling facilities, and related
controls. The operation of the incinerator cannot be separated from these
other parts of the system. The thickening and dewatering processes are very
important because the moisture content of the sludge has a major effect on
the amount of fuel needed for incineration. The amount of fuel needed for
different sludge solids contents is shown in Figure 101. Incineration
usually 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 need some fuel for startup operations. Fuel also may be needed
for air pollution control equipment.
However, as shown in Figure 101, incineration of high solids sludge can
produce more heat than is needed.
362
-------
3
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a
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The fuel value of the sludge itself is also important in determining
fuel consumption. Typical heat values of various sludges and their con-
stituents are:
Heating value
Type of sludge (Btu/lb of dry solids)
Raw primary 10,000-12,500
Activated 8,500-10,000
Anaerobically digested primary 5,500
Raw (chemically precipitated) primary 7,000
Biological filter 8,500-10,000
Grease and scum 16,700
Fine screenings 7,800
Ground garbage 8,200
High organic grit 4,000
Auxiliary fuel requirements are lower for sludges with a high percentage
of volatiles. The volatile content of a sludge may be maximized by
removing sludge inorganics such as grit,
avoiding the use of inorganic chemicals such as ferric chloride
and lime in the dewatering process, and
avoiding biological processes such as digestion before incineration.
When burning a normal load of sludge, a multiple hearth furnace provides
three separate zones:
1. Two or more upper hearths where most of the moisture is evaporated.
2. Two or more middle hearths where the sludge burns at temperatures
greater than 1500°F.
3. A bottom hearth that cools the ash by giving up heat to the cooler
incoming air.
During moisture evaporation in the first zone, the sludge temperature is
not raised higher than about 140 F. At this temperature not much volatile
matter is driven off, so there are no bad odors. Distillation of volatiles
from sludge containing 75 percent moisture does not occur until 80-90 percent
of the water has been driven off. By this time, the sludge is down far
enough in the incinerator to reach hot gases that burn the volatiles and
could cause odors. Generally, when fuel is needed to maintain combustion in
a multiple hearth furnace, a gas outlet temperature above 900 F means that
too much fuel is being burned.
For good incinerator operation, more air must be supplied than the cal-
culated amounts. This provides better contact between fuel and oxygen which
is necessary for burning to occur. When there is not enough excess air, only
J164
-------
partial combustion occurs. This results in the formation of carbon monoxide,
soot and odorous hydrocarbons in the stack gases. Multiple hearth incinera-
tion normally is operated at 75 to 100 percent excess air; more than 100
percent excess air only wastes fuel. For best thermal economy, excess air
flow should be controlled, based on stack gas composition. Oxygen, carbon
dioxide, and carbon monoxide may be monitored automatically in the stack and
compared with preset values. If the carbon monoxide level increases, this
means that incomplete burning is occurring, and more excess air may be needed.
However, if at the same time, the oxygen level agrees with the preset value,
then either mixing of sludge and combustion air is poor, or the temperature
is low because the sludge cake is wetter than normal.
-------
TROUBLESHOOTING GUIDE
INCINERATION - MULTIPLE HEARTH
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Furnace temperature
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.
w
-------
TROUBLESHOOTING GUIDE
INCINERATION - MULTIPLE HEARTH
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Oxygen content of
stack gas is too
low.
4a. Volatile or grease
content of sludge
has increased.
4b. Air feed rate too
low.
4a. Sludge composition.
4b. Check for malfunc-
tion of air supply
and check feed rate.
4a. Increase air feed rate or
decrease sludge feed rate.
4b. Increase air feed rate.
5. Furnace refractories
have deteriorated.
Furnace has been
started up and shut-
down too quickly.
5. Operating records.
Replace refractories and
observe proper heating up and
cooling down procedures in
future.
OJ
(Ti
6. Unusually high cool-
ing effect from one
hearth to another.
Air leak.
Hearth doors, dis-
charge pipe, center
shaft seal, air
butterfly valves in
inactive burners.
Stop leak.
7. Short hearth life.
Uneven firing.
Have hearths been
operated with only
one burner on.
1. Fire hearths equally on both
sides.
8. 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 failure 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
INCINERATION - MULTIPLE HEARTH
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
11. Stack gas tempera-
tures too high
(1200-1600°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 de-
crease fuel rate.
12. Furnace burners
slagging up.
12.
Burner design.
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; discon-
tinue scum injection into
hearth.
W
cr>
CD
14. Stack gases contain
excessive air
pollutants.
14a. Incomplete combus-
tion due to insuf-
ficient air.
14b. Inoperative air
pollution control
equipment.
14a. Monitor stack gases
for unburned HC, NO
CO.
x
14a. Increase air fuel ratio to
provide for more complete
combustion.
14b. Repair or replace damaged
equipment.
15. Flashing or
explosion.
15.
Scum or grease.
15. Sludge character-
istics.
15. Provide for scum and grease
removal before incineration.
-------
INCINERATION-FLUIDIZED BED
Process Description
The fluidized bed incinerator is a cylindrical vessel with a grid near
the bottom to support a sandbed. A typical fluid bed reactor used for
combustion of wastewater sludges is shown in Figure 102. Dewatered sludge
enters above the bottom grid, and air flows upward at a pressure of 3.5-5.0
psig to fluidize the mixture of hot sand and sludge. Extra fuel can be
supplied by burners located above or below the grid. The reactor is a
single chamber unit where moisture evaporation and combustion occur at
1,400-1,500 F. Because of the large heat reservoir in the bed and a rapid
distribution of fuel and sludge throughout the bed, there is good contact
between fuel and oxygen. The sand bed keeps the organic particles until
they are reduced to mineral ash. The motion of the bed grinds up the ash
material which is constantly stripped from the bed by the upflowing gases.
The heat reservoir provided by the sandbed also allows faster start-up
when the unit is shut down for short periods (overnight). As an example,
a unit can be operated 4-8 hours a day with little reheating after
restarting, because the sandbed is such a large heat reservoir.
Exhaust gases are usually scrubbed with treatment plant effluent and
ash is separated from the liquid in a hydrocyclone, with the liquid stream
returned to the head of the plant. An oxygen analyzer in the stack controls
the air flow into the reactor. The auxiliary fuel feed rate is controlled
by a temperature recorder.
As shown in Figure 103, an air preheater can be used with a fluidized
bed to lower fuel costs.
Typical Design Criteria and Performance Evaluation
In evaluating performance:
1. Check data on oxygen content of stack gas. It should
be 3-6%.
2. Check fluidized bed temperatures. They should be
1250-1300°F.
3. Check bed temperature records to see if temperatures
are constant or show major changes.
4. Check records on depth of sand to see if sand is being
lost from the system.
369
-------
Sight glass
Exhaust
Preheat burner
Thermocouple
Sludge inlet
Access doors
Fluidizing
air inlet
Figure 102. Cross section of a fluid bed reactor.
370
-------
Hot gas in
1500* F
REACTOR
AIR
PREHEATER
Gas out to
scrubber
BLOWER
Figure 103. Fluidized bed system with air preheater.
371
-------
Table 40 lists typical design criteria and field performance for several
operating fluidized bed systems. These data and the Control Consideration
Section can be used as a guide for evaluating fluidized bed performance.
If the system does not operate near its expected performance, the Trouble-
shooting Guide should be read for possible solutions to the problem.
Control Considerations
The amount of air fed into the reactor is very important. Too much
air would blow sand and unburned sludge into the fuel gases and would
result in needless fuel consumption.
Since the theoretical amount of air is never enough for actual fuel
combustion, "excess air" must be added. The extra air is expressed as a
percentage of the theoretical air requirement, e.g., if a fuel requires
1000 standard cubic feet of air per minute (SCFM) based on theoretical air
requirements, and the actual air rate is 1200 SCFM, the percent excess air
is:
Actual Air Rate - Theoretical _ (1200 - 1000) x 100% = 20%
Theoretical 1000 excess air
Fluidized bed systems usually are operated with 20-40% excess air.
In practice, this rate is controlled by measuring the oxygen in the
reactor exhaust gases and adjusting the air rate to maintain 3-6% oxygen.
If not enough air is added, unburned particles leave the reactor, and if
too much air is added, fuel is wasted.
Auxiliary fuel is used during start-up to raise the sand bed tempera-
ture to about 1200 F. As soon as sludge is fed into the furnace, the
auxiliary fuel rate must be lowered to get the most heat from the sludge
and to avoid wasting fuel. This is done by slowly reducing the fuel feed
rate to the minimum that is necessary to keep bed temperatures between
1250 and 1300°F.
J3.72
-------
TABLE 40.
FLUIDIZED BED REACTOR PERFORMANCE DATA
Stack
Plant
Liberty. N.Y
Ocean City, Md.
Barstow, Cal.
Northwest Bergen.
New Jersey
Upper Menon
Twp. Penn.
Port Washington,
New York
Arlington, N.Y.
New Windsor,
N.Y.
Bath, N.Y.
Loram, Ohio
Somerset-Raritan
Typ«
or
Sludge
Prim +
T.F.
Prim
Prim
Prim +
WAS
Prim +
T.F.
Prim +
T.F.
Prim +
WAS
Prim -f
T.F.
Prim +
WAS
Prim +
WAS
Prim +
WAS
PS
Reactor
Dia.
6'
6'
T
12'
9'
9'-6"
9-
7'
9'-6"
14'
12'
Heat
Re-
covery
No
No
No
Yes
Yes
No
No
No
No
No
Yes
Capacity
#/Mr. D.S.
Design
282
350
500
1100
865
860
700
570
605
1400
1170
Actual
338
445
552
1169
918
865
742
666
657
1635
1376
Aux. Fuel
O»l. /Ton O.S.
Design
102.8
48.0
36.0
41.5
18.4
64.5
—
56.6
113.9
40.0
55.0
Actual
53.3
22.9
31.9
57.0
14.4
85.5
—
75.5
85.5
32.2
23.8
Power
KWH/Ton D.S.
Design
-
—
239
267
—
252
—
—
400
274
247
Actual
—
—
210
243
—
261
—
—
344
181
247
% Volatile
In Ash
Design
3.0
3.0
—
4.0
—
3.0
3.0
3.0
3.0
3.0
3.0
Actual
0.31
0.85
—
0.59
—
0.4
0.3
0.4
0.4
0.7
0.5
Emissions
Grains/SCF
Design
—
—
0.1
0.1
—
0.1
—
—
0.1
_
0.2
Actual
—
—
0.025
0.018
—
0.025
—
—
0.044
—
0.047
373
-------
TROUBLESHOOTING GUIDE
INCINERATION-FLUIDIZED BED
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%.
la. Increase fuel feed rate or repair
any fuel system malfunctions.
Ib. Decrease sludge feed rate.
Ic. Improve devatering system opera-
tion (see appropriate section of
this manual).
Id. Reduce air rate.
2. Low (<3%) 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.
3. Sludge feed rate too
low.
3. Sludge feed rate.
3. Increase sludge feed rate and
adjust fuel rate to maintain
steady bed temperature.
4. Erratic bed depth
readings on control
panel.
4. 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 pressure
tap while the reactor is in
operation after reviewing manu-
facturer's safety instructions.
5. Preheat burner fails
and alarm sounds.
5a. Pilot flame not
receiving fuel.
5a. Fuel pressure and
valves in fuel line.
5a. Open appropriate valves and
establish fuel supply.
-------
TROUBLESHOOTING GUIDE
INCINERATION-FLUIDIZED BED
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. Preheat burner fails
and alarm sounds.
(Cont'd)
5b. Pilot flame not
receiving spark.
5c. Pressure regulators
defective.
5d. Pilot flame ignites
but flame scanner
malfunctions.
5b. Remove spark plug and
check for sparks;
check transformer.
5d. Scanner operation.
5b. Replace defective part.
5c. Dissemble and thoroughly clean
regulators.
5d. Clean sight glass on scanner;
replace defective scanner.
6. Bed temperature too
high.
6a. Fuel oil 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.
6a. Decrease fuel oil flow rate
through bed guns.
6b. Raise air flow rate or decrease
sludge feed rate.
7. Bed temperature reads
off-scale.
7. Thermocouple burned
out.
7. Replace thermocouple.
8. Scrubber inlet shows
high temperature.
8a. No water flowing in
scrubber.
8b. Spray nozzles
plugged.
8c. Ash water not
recirculating.
8a. Water pressure and
valve positions.
8b. Check nozzles by
removing and con-
necting them to
external water source.
8c. Pump operation,
clogged scrubber.
8a. Open valves.
8b. Clean nozzles and strainers.
8c. Return pump to service or remove
clogging from scrubber.
-------
TROUBLESHOOTING GUIDE
INCINERATION-FLUIDIZED BED
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
9. Reactor feed pump
fails.
9a. Bed temperature
interlocks may have
shut pump down.
9b. Pump is blocked.
9a. Bed temperature.
9b. Sludge too concen-
trated .
9a. See Items 1 and 6.
9b. Dilute feed sludge with water.
10. Poor bed fluidization.
10. During shutdowns, sand
has leaked through
support plate.
10. Once per month, clean windbox.
CTi
-------
LIME RECALCINING
Process Description
Lime often is used as a coagulant either as a tertiary step or ahead
of the primary clarifier for removal of phosphorus from wastewaters. In
recalcining, the dewatered calcium-containing sludge is heated to about
1,850 F. This drives off water and carbon dioxide leaving calcium oxide
(or quicklime) : 1850% ^ +
In municipal wastewater treatment, multiple hearth furnaces have been
used for recalcining. Although fluidized bed furnaces have been used for
industrial and water treatment purposes, these furnaces have not been used
for municipal wastewater recalcining.
In wastewater treatment, the calcium carbonate sludge is mixed with
the wastewater solids and other chemical precipitates. After recalcining,
these other solids leave an inert ash mixed with CaO. The plant design
must provide a method for removing these inert solids from the system.
One way is to use a centrifuge to classify the phosphate and other inert
solids into the centrate and leave the calcium carbonate in the dewatered
cake sent to the recalcining furnace. A second centrifuge is used to
dewater the centrate from the first machine producing about 50% solids
in the dewatered sludge. This cake is sent to a separate sludge disposal
system. By operating the first centrifuge at only 70-75 percent solids
capture, it is possible to remove most of the inert solids from the lime
recovery system. Figure 104 shows the lime recalcining system using a
multiple hearth furnace and series centrifuges at South Lake Tahoe,
California. In tertiary treatment, centrifugal systems usually remove
enough inert solids. Where lime is added to the raw wastewater, however,
there are more inert solids, and other classification may be needed.
This may be done after recalcination by passing the ash through a dry
classification device. Using these two techniques in series, enough
inert material is removed to allow lime reuse when coagulating raw waste-
water.
The recalcined lime usually is discharged into a hammermill to break
up any lumps that might form in the furnace. The material is forced
against a grinding plate by the rotating hammers. The lumps are broken
up until they are small enough to fit through the openings in a metal
screen.
After the hammermill, the material goes to a thermal disc cooler.
The cooler has a series of hollow discs mounted in tubular shafts , inside
377
-------
NO. 1 CENTRIFUGE^J^I
/SPENT LIME PUMPS
SLUDGE -> _- ^\
FOR L.ME SLUDGE/rREVERg|BLE ^ ± ] J '
MA.N STACK-^* ]\SCM
TO CO2 COMPRESSORS \ TCON)
* * /lr\
j ^
jtfr
INDUCED //
DRAFT &
FAN— -^^
rnrr-
I T
SCRUBBER -f U ^
DRAIN
1 IMF
RECALCINING
FURNACE — «•*
FURNACE
DRIVE *^n-nr^3
IL^ I^^^^Jy
^ — ^Wv 1 1
/
^BSYTACSKS/-HOTA,Rl * )
aiaiy RETURN/^YT
-FEED INLET
Jm
=W . »* _»»! Z Jl Z
4jCTjc__ T/VT
'^tfTaCEMTR,-?^
/f\ K FUGE FOR f ••
Mb) LIME OR *l i :
^dNw^v/ SEWAGE SLUDGE
'XTS-r~1 1 ' . L UIF Ml in
1 II H J" THICKENER
1 M i1^ >M •
U 1,1 f - —
fift f FROM CHEMICAL
]7_ REVERSIBLE FEED>I 1 1 «7°ArP ^^CLARIFIER SLUDGE
%*— I CONVEYOR^/ 1 1 SEWAGE PUMP
__ —BYPASS
1 DAMPER
^ HEARTH
2 i III
i^
3 oO
4. . "si
i}aU
5 ¥^;
XJ/THERMAL y
6 /DISC COOLER
\VWYVI.
p^"*^k T / W
V^ > SLUDGE FURNACE
f^BYPASS DEWATERED LIME SLUDGE
TRUCK LOADING ST°RAGE BIM
I1 *" "1 1 FROM LIMb
f £tt£ DELIVERY TRUCK
BJ** ' 1
i ^!s L>J
\ Siz /'^
|l \J/ ^PNEUMATIC
\/ UNLOADING AND
- CONVbYING bQUIP.
_ ^
^— LIME FEEDER
1 *^-LIMESLAKER
>J< -^ CAIR LOCK
kLUINtU FRESH LIME
t COMBUSTION SHAFT \ 'S*V*. "-IME TO TQ SPLITTER
AIR BLOWER COOLING ) (J) SPLITTER BOX
AIR FAN^ *» BOX
RECALCINED
LIME BLOWER
Figure 104. The lime recalcining system
at South Lake Tahoe.
378
-------
a rectangular housing. Cooling water passes through the shafts and discs,
while a chain and sprocket motor rotates the discs. A variable height
weir at the discharge is used to control the operating depth. The thermal
disc cooler usually moves cooled recalcined lime to storage bins using a
rotary air lock.
Typical Design Criteria and Performance Evaluation
The capacity of a multiple hearth furnace depends on solids loading
per unit of hearth surface area. Sizing also depends on the nature of
the sludge cake, including moisture, volatile solids, inerts content,
and heat value. Loading rates of 7-12 Ib/hr/sq ft are common based on
wet sludge. Rates for recalcining are generally in the low end of this
range.
Tables 41 and 42 list common temperature profiles for both six and
eleven hearth furnaces.
The furnace usually has a wet scrubber for air pollution control.
In evaluating performance of a recalciner:
1. Check CaO content of recalcined lime. It should be at
least 70%.
2. Check actual feed rates against the design rate.
3. Check hearth temperatures using Tables 41 and 42 as guide.
4. Check slaking characteristics of recalcined lime (see
Control Considerations Section).
5. Check for odors which may indicate incomplete combustion.
6. Check amounts of make-up lime being used.
Control Considerations
With tertiary lime systems, the recalcined lime should contain at
least 70 percent CaO. If the percentage is less, the classification
processes need adjustment. When raw sewage is lime coagulated, the
recalcined lime content usually is 60-70%. Classifying centrifuges
should be operated to provide only about 70-75 percent solids capture;
higher solids capture allows too much inert material to stay with the
lime.
There are two important characteristics of the recalcined lime
which must be controlled.
1. Activity, and
2. Slaking characteristics
379
-------
TABLE 41. TYPICAL TEMPERATURE PROFILE
IN SIX HEARTH FURNACE
Hearth No.
1 (Top)
o
.)
4
5
('» (Bottom)
Temperature
C
427
(549
899
788
G49
149
F
800
1,200
1,650
1,450
1,200
,'K)0
TABLE 42. TEMPERATURE PROFILE IN ELEVEN
HEARTH FURNACE
Hearth No.
1 (Top)
2b
3
4
5
P
7
S
9
10
11 (Bottom)
Temperature
C
76 Oa
427
677
899
1,010
1, 010
1,010
1,010
1,010
677
399
F
l,400a
800
1,250
1,650
1,850
1,850
1,850
1,850
1,850
1,250
750
a.
op hearth used as afterburner
Feed hearth
3QO
-------
Recalcined lime may be classified according to the AWWA Standard for
quicklime (CaO) and hydrated lime (Ca(OH) ) (AWWA B202-65):
o
Time for 40 rise Time for complete
in temp., (min) reaction (min)
High-reactive, 3 or less 10
soft burned lime
Medium-reactive, 3-6 20
medium burned lime
Low-reactive, More than 6 More than 20
hard burned lime
It is possible to produce quicklime with the same CaO content, but
with very different slaking properties.
The most important variables in the operation of the recalcining
furnace are temperature and feed rate. The rabble arm rate in a
multiple hearth furnace has little effect on recalcined lime if it is
within 1.5-2 rpm.
At South Lake Tahoe with a six hearth furnace, increasing the
temperature on hearth 5 from 1600°F to 1900°F resulted in an increase
of CaO from 76 percent to 86 percent while still maintaining an accep-
table slaking rate. Also, at 1600°F, there was some unburned organic
material in the recalcined lime. Thus, temperature is very important
in providing good operation of the recalcining system.
There are a number of critical mechanisms involved in the handling
of recalcined lime discharge from a furnace system. In order of flow,
these are:
Lime Grinder - If the lime grinder stops, this usually
means that some object such as a wrench or a piece of
firebrick has fallen into the grinder and must be removed.
Thermal Disc Cooler - When it is operating, this unit must
have a supply of cooling water at all times. The system
could be damaged by steam if there is no cooling water.
Safety valves are usually provided to limit pressures.
Temperatures greater than 600°F will damage the rotary
air lock which discharges cooled reclaimed lime to the
conveying system.
Rotary Air Lock - This unit must never get hotter than
600 F and must never be operated unless the air seal
system is working. The rotary air lock bearings must
be kept clear with clean air at all times.
381
-------
Common Design Shortcomings and Ways to Compensate
(Also refer to Lime Feeding Section of this manual)
Shortcoming Solution
1.
2.
The area around the recal- 1. Install bag-type air filters
cining system gets coated on exhaust air streams from
with lime dust. lime storage area.
Recalciner making too
much noise.
2. Install acoustic enclosures on
fan and blower assembly; add
sound absorbing materials to
building walls.
382
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TROUBLESHOOTING GUIDE
(NOTE: Incineration - Multiple Hearth Section should also be reviewed for guidance
related to furnace & chemical feeding section for lime handling). LJME RECALCINING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Recalcined lime has
less than:
. 70% CaO for tertiary
applications.
. 60% CaO for raw waste-
water applications.
la. Furnace temperatures
too low.
Ib. Classification of lime
sludges inadequate
permitting too many
inerts to enter recal-
cining furnace.
la. Temperature on fired
hearths should be
1850°F.
Ib. Solids capture in
first stage centrifuge
too high.
la. Increase temperatures.
Ib. Reduce solids capture in first
stage centrifuge (see Centrifuge
Section), if not adequate,
evaluate dry classification of
recalcined lime with your
consultant.
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2. Large clinkers forming
in furnace.
Inadequate air.
Excess air is usually
70-100% for recal-
cining.
Increase air flow.
3. Recalcined lime tends
to agglomerate into
soft particles \ inch
in diameter or larger
rather than having
desired flour-like
appearance.
Furnace temperatures
too low.
See la.
4. Furnace shaft stops.
4a. Foreign material
caught between rabble
arm and hearth or arm
may have come loose
or sagged.
4b. Excessive clinker
formation.
4a. Inspect hearths.
4a. Repair arm or remove obstruction.
4b. See Item 2.
-------
TROUBLESHOOTING GUIDE
LIME RECALCINING
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
5. Lime grinder stops.
5. Foreign object stuck
in grinder.
5. Inspect grinder.
5. Remove obstruction.
6. Thermal disc cooler
overheats.
6. Lack of cooling water.
6. Cooling water flow.
6. Open cooling water valves and
establish adequate flow.
7. Cooling air temperature
too high.
7. Cooling air fan
malfunction.
7. Fan operation.
7. Repair fan.
8. Cooling air pressure
too low.
8. See Item 7.
CO
00
9. Furnace temperature
too high.
9. Firing rate of fired
hearths too high.
Is temperature rising
or falling?
9. If temperature is still rising,
reduce firing rate.
10. Furnace scrubber
temperature too high.
10. Low water flow to
scrubber.
LO. Scrubber water flow.
10. Establish adequate scrubber water
flow.
11. Offensive odor in
recalcining area.
11. Insufficient com-
bustion air.
LI. See Item 2.
-------
CARBON REGENERATION
Process Description
After the adsorptive capacity of activated carbon has been exhausted,
the carbon can be restored by high temperature heating to drive off the
adsorbed organics. Keeping oxygen at very low levels in the furnace prevents
the carbon from burning. The organics are passed through an afterburner to
prevent air pollution. In small plants where the cost of an onsite regenera-
tion furnace is too high, the spent carbon may be shipped to a central regen-
eration facility for processing. Today, only granular carbon is regenerated,
and this is accomplished in multiple hearth furnaces. Multiple hearth
furnaces already have been described in the sludge incineration section of
this manual.
As shown in Figure 105, the basic steps for regenerating carbon are:
The granular carbon is pumped in a water slurry to the regeneration
system for dewatering by gravity drainage.
After dewatering, the carbon is fed to a furnace and is heated to
1,500 - 1,700 F to volatilize and oxidize the impurities.
The hot regenerated carbon is quenched in water.
The cooled carbon may be washed to remove fine material and then
transported to the adsorption equipment or to storage.
The furnace off-gases are scrubbed, (the scrubber water is returned
to the plant for processing) and also may pass through an after-
burner.
Typical Design Criteria and Performance Evaluation
Multiple hearth furnaces used for regenerating carbon have a hearth
area of about one sq ft per 40 Ibs of carbon to be regenerated per day. An
allowance also should be made for 10-40 percent downtime. Usually, about 1
Ib of steam per Ib of carbon is fed to the furnace to make the temperatures
within the furnace uniform. Maximum operating temperatures are usually 1650-
1700 F. Air pollution control equipment is included in the design of the
carbon regeneration furnace. Some systems have an afterburner for removal of
smoke and odors, and a wet scrubber or bag filter for removal of particulates.
A variable-throat Venturi-type scrubber with 20 inch pressure drop can be
used to meet strict air quality standards.
The total amount of fuel needed for regeneration is about 3,000-3,500
385
-------
Makeup carbon
•CXHr*
Water back
«
to process
rf-CX}
Carbon slurry'
pumps •
Carbon fines
back to proces!
x
jr Spent carbon
drain and feed
tanks
x:
Screw conveyors
-j
A
Y
Carbon
slurry pumps^
Kx><>hJx^
Spent carbon
1 from carbon
columns
Scrubber and air
pollution control
equipment
Carbon
regeneration
furnace-
Steam
supply
-Quench tank
V. Regenerated carbon defining
and storage tanks
+H-
Regenerated
carbon to
carbon columns
Figure 105. Typical carbon regeneration system schematic.
386
-------
Btu/lb of carbon for furnace heat, about 1,250-1,600 Btu/lb of carbon for
steam generation, and about 2,400 Btu/lb for afterburner fuel.
The results of plant-scale regeneration of granular carbon at South Lake
Tahoe, California are as follows:
Carbon Virginal Spent Carbon after
property carbon carbon regeneration
Apparent
density 0.48 0.52-0.59 0.47-0.50
Iodine
number 935 550-843 743-969
Ash (%) 5.0 4.5 4.7-8.2
Although these values cannot be used as an absolute measure or standard
for performance at all plants, the figures can be used as a general guide
for performance evaluation. Through laboratory tests, the apparent density,
iodine number and ash content of the carbon should be determined after each
regeneration. If a system does not perform as expected, the troubleshooting
guide should be checked.
Control Considerations
The quality of regenerated carbon is controlled by measuring the
apparent density (A.D.) of the regenerated carbon. A typical A.D. of virgin
granular carbon is about 0.48 gm/cc. As carbon becomes saturated with
adsorbed organics, the A.D. may increase to 0.50 or more. As the carbon is
regenerated, organics are removed, and the carbon loses weight. If properly
regenerated, the A.D. will return to 0.48. The A.D. is quickly and easily
determined by weighing a known volumn of carbon.
If the apparent density of the regenerated carbon is greater than 0.49,
the carbon is not getting enough heat to remove much of the organic material.
If the apparent density is less than 0.48, the carbon is getting too much
heat, and carbon is being burned in the furnace.
The A.D. of the regenerated carbon can be changed by changing the
following process conditions:
1. Temperature is the most important factor in carbon regeneration; a
higher temperature will give a lower A.D. and a lower temperature will
give a higher A.D.
2. Carbon feed rate is the next most important in carbon regeneration.
Increasing the carbon feed rate will increase the carbon depth on the
furnace hearths and will reduce the amount of heat supplied to the
carbon. This change will give the carbon a higher A.D. Lowering the
carbon feed rate will decrease the depth of carbon and increase the
amount of heat supplied to the carbon. This will lower the A.D.
3&7
-------
3. Increasing the steam feed rate will decrease the A.D.
steam feed rate will increase the A.D.
Decreasing the
4. Furnace drive speed regulates the contact time in the furnace. Increas-
ing the drive speed will reduce the contact time. This is not as
important as temperature and carbon feed rate.
The ash content of carbon may be used to detect any buildup of calcium
or other unwanted material. The ash content of virgin carbon usually is
about 5.2 percent. Just before the first regeneration, the ash content may
increase to about 5.7 percent, and after the first regeneration, to about
6.4 percent. In later regenerations, there should not be much change in the
values for spent and regenerated carbon. Greater ash buildups can be expect-
ed for lesser degrees of pretreatment.
Common Design Shortcomings and Ways to Compensate
Shortcoming
1. Regeneration furnace too
small.
2. Carbon storage and
dewatering equipment corrodes
because it is not properly
coated.
2.
Solution
Contract for carbon regeneration
with commercial carbon supplies
while expanding regeneration
facility.
Partially dewatered carbon is
very corrosive. Recoat equipment.
3. No way to remove carbon
fines from regenerated
carbon, and packed beds
show excessive headloss.
3. Remove some carbon from packed
bed and vigorously backwash to
remove carbon fines.
388
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TROUBLESHOOTING GUIDE
CARBON REGENERATION
INDICATORS/OBSERVATIONS
1. Apparent density
of regenerated
carbon is greater
than virgin carbon.
2 . Apparent density of
regenerated carbon
is less than virgin
carbon .
3. Carbon losses
exceed 5%.
PROBABLE CAUSE
1. Insufficient heat
being applied to
carbon .
2. Too much heat being
applied to carbon .
3. Substantial carbon
loses due to frequ-
ent startup and
CHECK OR MONITOR
1 . Furnace temperature
should be 1650-1700°
F on fixed hearths .
2. Furnace temperature
should be 1650-
f\
1700 F on fixed
hearths .
3 . Operating schedule
for furnace .
SOLUTIONS
la. Increase furnace temperatures
by 50 F increments until virgin
carbon AD is achieved.
Ib. If temperature is in correct
range , decrease carbon feed
rate.
Ic. Increase steam feed rate if
less than 1 Ib/lb of carbon.
Id. Increase furnace drive speed.
2a. Decrease furnace temperatures
by 50 F increments until virgin
carbon AD is achieved.
2b. If temperature is in correct
range, increase carbon feed
rate.
2c. Decrease steam feed rate.
2d. Decrease furnace drive speed.
2e. If above adjustments don't
solve problem, check for air
leaks at feed entry, discharge
pipe , hearth doors , bottom
shaft seal, etc.
2f. Change fuel/air ratio to
produce more CO at burners
but not more than 4% CO.
3 . Store enough spent carbon to
prevent more continuous opera-
tion of furnace at less
Ul
00
-------
TROUBLESHOOTING GUIDE
CARBON REGENERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
3. (Cont'd)
shutdown of furnace
and loss of carbon
in furnace.
3. (Cont'd)
frequent intervals.
4. Center shaft drive
shear pin fails.
4. Rabble arm is drag-
ging on hearth or
foreign object is
caught beneath arm.
Inspect each hearth.
4. Correct cause of failure and
replace shear pin.
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5. Furnace temperature
too high on upper,
unfired hearths.
5a. Too much heat being
applied to fired
hearths.
5b. Steam rate low.
5c. Carbon feed rate
too low.
5a. Fired hearth temp.
5b. Steam rate.
5c. Carbon feed rate.
5a. Reduce firing rate on uppermost
fired hearth or turn it off.
5b. Increase steam rate on upper-
most fired hearth.
5c. Increase feed rate.
6. Furnace temperature
too high on lower,
fired hearths.
6a. Firing rate too
high.
6b. Steam rate low.
6c. Air leaks.
6a. Fired hearth temp.
6b. Steam rate.
6a. Reduce firing rate.
6b. Increase steam rate on lowest
fired hearth.
6c. Repair air leaks.
7. Furnace temperature
too low.
(Reverse procedures
.n Items 5 and 6)
8. Furnace refractor-
ies have
deteriorated.
8. Furnace has been
started up and shut-
down too quickly.
8. Operating records.
8. Replace refractories and ob-
serve proper heating up and
cooling down procedures in
future.
-------
TROUBLESHOOTING GUIDE
CARBON REGENERATION
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
9. Unusually high
cooling effect from
one hearth to
another.
Air leak.
Hearth doors, dis-
charge pipe, center
shaft seal, air
butterfly valves in
inactive burners.
Stop leak.
10.
Short hearth life.
10. Uneven firing.
10. Have hearths been
operated with only
one burner on.
10. Fire hearths equally on both
sides.
-------
APPLICATION OF SLUDGES TO LAND
Process Description
Some of the section on land treatment of wastewater has troubleshooting
guidance which will be useful in sludge applications. The Land Treatment
Section should be reviewed because it will not be repeated in this section.
It should be cautioned that there are two Acts which may affect the
application of sludges to land; the Resource Conservation & Recovery Act,
Section 4004; and the Clean Water Act, Section 405. EPA will be issuing
guidelines through these Acts, and these guidelines should be carefully
checked before using this treatment method.
Before being applied to the land, sludge is stabilized to reduce odors
and health hazards. The most common method of treatment is either anaerobic
or aerobic sludge digestion.
The sludge application site usually is not very close to the treatment
plant, so the sludge must be transported by truck, barge, railroad or pipe-
line. For large plants, pipeline transportation of sludge is usually the
cheapest, while smaller plants use trucks.
Sludge must be stored between treatment and land disposal because
treated sludge is generated at nearly a constant rate while the sludge
disposal rate depends on weather, field conditions, and the application
method. Many plants use the second-stage anerobic digester for storage.
Lagoons are also used for storage.
The main methods for applying sludge to the land are:
Principal sludge form
Sanitary landfill Dewatered cake or ash
DISPOSAL -
Sites dedicated to Liquid or dewatered
sludge disposal
Cropland application Liquid, cake dried, or
compost.
REUSE -
Land reclamation Liquid or dewatered
Sludge may be applied to land, (1) which will be used for growing crops,
parkland or forests, or (2) which will be used for sludge disposal with no
attempt to grow crops. Managing crop growth using sludge is more difficult
because the needs of the crop must be carefully balanced against sludge
disposal considerations.
392
-------
Sludge may be applied to the land in several ways. Small plants may
spread liquid sludge directly from tank trucks- In some cases, shallow
trenches may be dug, filled with sludge, and covered. Sludge may also
be applied using sprinkler systems with large spray nozzles where the
site is isolated enough. In some cases, sludge has been injected into
the soil under pressure. Ridge and furrow systems have also been used
and this solves the problem of aerosols from spray systems. The method
used usually depends on the amount of sludge to be disposed and whether
crops are to be grown on the site.
Typical Design Criteria and Performance Evaluation
The sludge application rate is one of the most important design
criteria. The effect of viruses, organics, cysts, and parasites in
sludge are a concern in land application. However, they do not usually
limit the application rate of sludge to the land. Application rate
mostly depends on the amount of water, nitrogen, and heavy metals in
the sludge.
If surface runoff is to be prevented, the application of water to
land cannot exceed the amount of water lost by percolation, evaporation,
and transpiration. This is not a problem with dewatered sludges, but
many systems apply liquid sludge to the land. The amount of water which
may be applied depends on the climatic conditions, the type of soil,
whether vegetation grows on the disposal site, and the type of vegetation
which may be grown on the disposal site.
If nitrogen pollution of the groundwater is a concern, the amount of
nitrogen in the sludge may limit the annual sludge application rate. The
nitrogen content of sludges should be measured for each sludge. Usually,
the total nitrogen in each sludge varies, and can range from 60 to 120
pounds nitrogen per ton of dry weight sludge solids. To avoid nitrate
pollution of the ground waters, there must be a balance between the
nitrogen in the sludge and the amount removed by the crop.
In a single growing season, crop uptake of nitrogen may range from
50-450 Ibs/acre/yr depending on the type of crop. When the nitrogen
balance is not a concern, sludge loading rates may be 100 dry tons/acre/yr,
while loading rates as low as 5 tons/acre/yr may be needed if nitrogen is
a problem.
Heavy metals also may affect the application rate of sludge to land.
Cadmium, copper, molybdenum, nickel, and zinc can accumulate in plants
and may be a hazard to plants, animals, or humans. High metal concen-
trations from industrial sources may limit application rates to less
than 5 tons/acre/yr.
393
-------
In actual practice, sludge application rates to cropped areas often
range from 10-30 tons/acre/yr.
Other important design variables are runoff control, crop selection,
groundwater control, monitoring systems, and land application equipment.
For landfill operations, the landfill should have limited access and
the waste should be spread evenly in layers not over 2 feet thick, followed
by compaction. The compacted wastes should have at least six inches of
compacted earth cover at the end of each working day. When each portion
of the landfill is completed, at least 2 feet of earth cover should be
placed over it, and grasses planted to prevent erosion.
Adequate monitoring of any land application or landfill site is most
important. This plan must be designed for local conditions and should
include monitoring groundwater observation wells, surface water, sludge
and soils for heavy metals, persistent organics, pathogens, and nitrates.
Human food chain products grown in soils with sludge should also be
monitored for heavy metals, persistent organics, and pathogens.
Because some pathogens can survive the sludge digestion process,
liquid sludges usually should not be applied to root crops or crops
intended for humans in the raw form.
Pastureland and farmland used to grow forage crops are often used
as land disposal sites. There is not much problem of disease transmission
via livestock grazing on these fields.
Control Considerations
As noted above, it is important to keep a balance between nitrogen
applications and nitrogen uptake by crops in order to prevent nitrate
pollution of groundwaters.
It is important to keep the soil pH near 7.0, since a metal content
which is safe at pH 7 can be lethal to most crops at pH 5.5. Land appli-
cation may lower the soil pH because of nitrification. Proper soil
amendments can be used to take care of this problem.
Adequate monitoring of a landfill site is vital, as discussed above.
Leachate and runoff from a sanitary landfill should be minimized and
when necessary collected and treated to prevent pollution of ground and
surface waters.
Where crops are grown, close cooperation is needed between the treat-
ment system management and farming operation. Scheduling of sludge appli-
cation with farm operations such as planting, tilling, spraying and
harvesting is most important to successful managment.
394
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TROUBLESHOOTING GUIDE
APPLICATION OF SLUDGES TO LAND
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
1. Odors from sludge
storage lagoons.
la. Improperly digested
sludge.
la. pH, alkalinity, and
volatile acids of
anaerobically digested
sludge.
Ib. VSS content of aero-
bically digested
sludge.
la. (1) Correct digester operation.
(2) Apply lime to lagoon surface.
(3) Flood lagoon with heavily
chlorinated water.
Ib. (1) Correct digester operation.
(2) Install temporary floating
aeration in lagoon.
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2. Crop suddenly dies or
shows signs of poor
health.
2a. Downward shift in soil
PH.
2b. Excessive nitrogen
application.
2c. Excessive heavy metal
concentrations.
2d. Excessive phosphorus
application causing
nutrient imbalance.
2a. Soil pH should be
maintained above 6.5,
preferably 7.0.
2b. Determine nitrogen
applied and consult
with agricultural
extension service.
2c. Heavy metal content
of sludge and crop.
2d. Determine phosphorus
application and
consult with agri-
cultural extension
service.
2a. Add lime to soil.
2b. Reduce loading rate.
2c. Reduce loading rate or reduce
heavy metal content through
enforcement of pretreatment
requirements.
2d. Reduce application rate.
3. Surface runoff of
sludge.
3a. Excessive application
rate.
3b. Ground saturated with
rainfall.
3a. Reduce application rate.
3b. Discontinue application until
soil has dried out.
-------
TROUBLESHOOTING GUIDE
APPLICATION OF SLUDGES TO LAND
INDICATORS/OBSERVATIONS
PROBABLE CAUSE
CHECK OR MONITOR
SOLUTIONS
4. Aerosols drifting out
of disposal area.
4. Wind carrying aerosols*
4a. Discontinue spraying during windy
periods.
4b. Convert spray nozzles to larger
openings.
4c. Reduce spray pressure.
4d. Increase buffer area.
5. Trucks getting stuck in
fields.
5. Need to apply sludge
during wet periods.
5. Acquire a portable "rain gun"
which can spray sludge over
200-300 ft. diameter circle.
to
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6. Mosquitoes breeding on
site.
6. Ponding of sludge.
6. Stagnant ponds of
sludge.
6. Grade site to eliminate ponding,
reduce application rate.
7. Plies breeding and/or
odors at landfill.
7. Landfill operation
inadequate.
7. Is fill being covered
at end of day?
7. Cover fill with at least 6 inches
of compacted soil at the end of
each day.
8. Leachate from landfill
causing pollution of
ground or surface
waters.
8. Excessive liquid
application.
8. Application rates and
leachate quality.
8a. Intercept and treat leachate.
8b. Reduce liquid applied to landfill
by improving sludge dewatering or
reducing application rates.
9. Nitrate pollution of
groundwater occurring.
9. Excessive nitrogen
applications.
9. Nitrogen application
rates and nature of
cover crops.
9a. Reduce application.
9b. Replace crop with one with higher
nitrogen uptake.
-------
TROUBLESHOOTING GUIDE
APPLICATION OF SLUDGES TO LAND
INDICATORS/OBSERVATIONS
10. Coverage of sludge in
subsurface plow
injection system not
adequate.
PROBABLE CAUSE
10. Plow is being pulled
at excessive speed
and soil is thrown
away from shank.
CHECK OR MONITOR
10. Plow speed.
SOLUTIONS
10. Pull plow at 1 mph or less.
11. Drying of soil -
sludge mixture is
slow in subsurface
injection system.
11. Sludge being injected
too deeply.
11. Injection depth.
11. Inject at 4-inches or less.
12. Subsurface injection
plugging.
12. Large sludge solids.
12. Install grinder on sludge.
u>
ID
(See Land Treatment Sectici for irrigation equipment
related information and oth
r troubleshooting guidance.)
------- |