United States
Environmental Protection
Agency
Technology Transfer
c/EPA Sludge
Sludge
*_
Treatment
and
Disposal
Thickening
Dewateri
Reduction
•*• Dewateri
SEMINAR HANDOUT
May 1978
I
\
Disposal
Disposal
Heat Drying
PART I Introduction and Sludge Processing
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SLUDGE TREATMENT AND DISPOSAL
SEMINAR HANDOUT
MAY 1978
INTRODUCTION
AND
SLUDGE PROCESSING
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO 45268
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SUPPLEMENTAL INFORMATION FOR
PRESENTATION BY JOSEPH B. FARRELL
AT
TECHNOLOGY TRANSFER DESIGN SEMINAR
"SLUDGE TREATMENT AND DISPOSAL"
March 30-31, 1978
Philadelphia, Pennsylvania
iii
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SUMMARY OF PROPOSED CRITERIA FOR
SOLID WASTE DISPOSAL FACILITIES*
ACTION: Proposed Rule
COMMENTS: Received until May 8, 1978
257.1 Scope and Purpose
257.2 Definitions
257.3 Criteria for classification
257.3-1 Environmentally sensitive areas**
-2 Surface water**
-3 Ground water**
-4 Air
-5 Application to land for food-chain crops**
-6 Disease vectors
-7 Safety
** Discussed with respect to sludge in the following section.
To be put into effect: 30 days after final publication.
257.3-1 Environmentally sensitive areas
(a) Wetlands - use for sludge disposal or utilization highly unlikely.
(b) Floodplains - sludge may be applied for beneficial use as soil
conditioner or fertilizer.
(c) Permafrost - not applicable.
(d) Critical habitats - unlikely for sludge disposal or use.
(e) Sole source aquifers - possible if other options are limited.
257.3-2 Surface water
(a) For point source, an NPDES permit is needed.
(b) Non-point sources are controlled or prevented.
Excerpted from Fed. Register, 43_, No. 25, Feb. 6, 1978, pp. 4942-4955.
iv
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257.3-3 Ground water
(a) Case 1. If aquifer is a designated water supply or IDS less
than 10,000 mg/1, the quality of groundwater at
boundary must not be "endangered." To assure no
endangerment, use liners and treat leachate, or use
other means (e.g., soil attenuation, prevent in-
filtration). Monitoring'must continue as long as
endangerment can occur, and a current contingency
plan is required.
(b) Case 2. If a use other than groundwater is designated by
the State, the quality of groundwater shall be
maintained at such quality as specified by the
State.
257.3-5 Application to land used for production of food chain crops
(a) Cadmium (either Case 1 or Case 2)
Case 1: Maximum annual Cd rate present to 12/31/81, 2 kg/hectare
1/1/82 to 12/31/85, 1.25 kg/hectare
1/1/86 ,0.5 kg/hectare
Maximum cumulative Cd C.E.C. Max. Cd (kg/ha)
less than 5 5
5-15 10
greater than 15 20
If Cd is greater than 25 mg/kg dry weight, sludge may not be
applied to sites growing tobacco, leafy vegetables, or root
crops for human consumption.
Maintain soil pH greater than 6.5.
Case 2: If Cd levels in crops and meats raised on sludge-amended
soil are comparable to levels in similar crops produced
locally, land application of sludge is acceptable.
Contingency plan is needed, which should include safe-
guards from alternative use after closure of the site.
Facility operator must demonstrate capability to manage
and monitor their operation.
(b) Pathogens
Sludge applied to the surface must be stabilized. At least a
year must pass before land to which sludge has been applied
is used for production of human food crops normally eaten raw
(except orchard fruits).
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(c) Pesticides and persistent organics
Residues in or on crops must be below FDA limits.
(d) Direct ingestion
Sludge must be applied in a manner such that freshly applied
sludge is not directly ingested by humans or by animals
raised for milk.
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USEFUL DOCUMENTS ON SLUDGE
1. EPA 430/9-77-004, Technical Bulletin, "Municipal Sludge Management:
Environmental Factors," O.W.P.O. Pub. MCD-28, Oct. 1977 (Bulletin
without appendices appeared in Federal Register, 42, No. 211, Nov. 2,
1977, pp. 57420-57427).
2. Federal Register,43_, No. 25, Feb. 6, .1978, pp. 4942-4955, "Solid
Waste Disposal Facilities, Proposed Criteria for Classification."
3. Federal Register, 43_, No. 31, Feb. 14, 1978, pp. 6560-6573,
"Electroplating Point Source Category, Pretreatment for Existing
Sources."
4. Farrell, J. B., "Interim Report on Task Force on Phosphate Removal
Sludges," U.S. EPA, National Environmental Research Center,
Jan. 1975, NTIS No. PB 238317.
5. SCS Engineers, "Review of Techniques for Treatment and Disposal of
Phosphorus-Laden Chemical Sludges," Contract 68-03-2432, to be
published by EPA (est. pub. date July 1978).
6. Di Gregorio, D., Ainsworth, J. B., and Mounteer, K. J., "Chemical
Primary Sludge Thickening and Dewatering," Contract 68-03-0404,
to be published by EPA (est. pub. date July 1978).
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EPA No.
February, 1978
REVIEW OF TECHNIQUES FOR TREATMENT
AND DISPOSAL OF PHOSPHORUS-LADEN
CHEMICAL SLUDGES *
Contract No. 68-03-2432
R. V. Villiers
Project Officer
Wastewater Research Division
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
by
SCS ENGINEERS
4014 Long Beach Boulevard
Long Beach, California 90807
•* DRAFT COPY-
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TABLE 2-1. RESULTS OF PLANT SURVEY *
State
(Name)
Cal ifornia
Colorado
Illinois
Indiana
Michigan
Minnesota
New York
Ohio
Pennsylvania
Wisconsin
Texas
Canada
Identified Plants
in State (No. )
2
1
22
26
91
12
8
34
13
59
1
92
Plants Responding
to Survey (No. }
1
1
5
7
59
4
4
15 + 1
9
26
1
41
TOTAL 361 174
P/anis retevrinj qu&s tic waives * S~£ furrr? e>f
es of Phffthcfuj - Lad en
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TABLE 3-1. PREVALENCE OF PHOSPHORUS REMOVAL METHODS (CHEMICALS AND POINTS OF ADDITION)
AMONG PLANTS RESPONDING TO QUESTIONNAIRE SURVEY
Polnt(s) of
Addition
Primary
Secondary
Tertiary
Primary and
Secondary
Primary and
Tertiary
Secondary and
Lime
6
0
5
0
0
0
Iron
Salt
29
46
5
6
0
1
Al umi num
Salt
9
50
2
0
0
0
Iron and
Aluminum
Salts
0
0
0
0
0
0
Lime and
Aluminum
Salt
0
0
0
2
1
0
Lime and
Iron
Salt
0
0
0
0
0
0
Total
Plants
44
96
12
8
1
1
Percentage of
Total Plants
Using
Polymer
48
32
50
62
100
inn
Tertiary
Total Plants
11
87
61
0
162
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11. STATE-OF-THE-ART APPRAISAL
It Is evident that the addition of chemical sludges to the
regular sewage treatment plant primary and secondary sludges
often has a significant impact upon subsequent sludge handling.
The volume and mass of the sludge increase and the percentage
of volatile solids decreases. Thickening and dewatering effi-
ciencies are often adversely affected. Chemical conditioning
requirements change. Where incineration is used, an increased
need for supplemental fuel is reported. The extent and serious-
ness of these and other effects varies greatly between treatment
plants. Thus, neither the problems nor their solutions are
universal, and each treatment plant is unique. However, certain
generalizations can be made from the data obtained during this
investigation.
Of the three chemicals normally considered for phosphorus
removal, lime, iron salts, or aluminum salts, iron salts
generally appear to have the least overall adverse effect upon
subsequent sludge handling. This conclusion is based primarily
upon two factors:
1. The addition of lime generates a much greater mass of
sludge than does the addition of iron or aluminum salts
2. The chemical sludge generated by the addition of
aluminum salts is usually more difficult to thicken
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and/or dewater than the sludge generated by the
addition of iron salts.
Obviously, for many plants these two advantages are over-
ridden by other considerations or else iron salts would be
routinely used by all plants. Other considerations might
include wastewater treatment efficiency, chemical cost (there
are large geographical variations), and the relatively high
corrosiveness of iron salts.
The next decision to be made is where in the sewage treat-
ment chain to apply the phosphorus removal chemical. For a
typical activated sludge treatment plant, there appears to be
some advantage to adding iron or aluminum salts to the mixed
liquor at a point where good mixing is achieved prior to dis-
charge to th« secondary clarifier. The waste activated sludge
can then be pumped to the primary clarifier influent for
settling vith the primary sludge. TMs scheme generally
results in the least total volume of combined
sludge to be treated.
If lime is the chemical used, it is added to the primary
treatment step or occasionally to a special tertiary treatment
process. Lime is never added to the secondary biological
process.
It has become common practice to thicken raw sludge in a
gravity thickener prior to further sludge treatment. In virtu-
ally all cases, chemical sludges containing iron salts thicken
much better than sludge containing aluminum salts. Best results
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with either iron or aluminum sludges are obtained with the
addition of a polymer (dosage range 0.5 to 1.0 mg/1). It has
also been found that the lower the dosage of iron or aluminum
salt used the easier the resulting sludge is to thicken. For
this reason, and to save chemical costs, it is recommended that
chemical feed equipment be automatically controlled to prevent
overdosage of more chemical than required to achieve the phos-
phorus reduction needed.
If it 1s necessary to thicken secondary biological sludge
separately, experience indicates that air flotation thickening
is superior to gravity thickening. Again, the addition of
polymers substantially improves performance.
Centrifuge dewatering of primary or combined chemical
sludges is greatly enhanced by polymer addition. Sludges con-
taining iron salts dewater much better than sludges containing
aluminum salts. Lime sludges dewater very well.
Anaerobic and aerobic sludge digestion is reported to be
essentially uninhibited by the addition of chemical sludges,
there being no toxic effects from the presence of the chemical
precipitates or the pH of the sludge. However, there is fre-
quently the need for more volume to handle the increased sludge
mass while maintaining proper retention time. When iron and
aluminum sludges are added to anaerobic digesters, there is also
commonly an adverse effect on supernatant quality and digested
sludge solids concentration because of poor solids-liquid
separation.
10
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Vacuum filtration of chemical sludges presented problems
at a number of plants due to increased solids mass, sludge
volume, and/or poorer sludge dewatering characteristics. Experi-
mentation with chemical conditioning, i.e., polymer dosages,
lime addition, etc., generally led to improved vacuum filter
performance. In addition, changes in filter media were reported
helpful. The city of Milwaukee, WI, found in pilot tests that
top feed vacuum filtration of iron sludges was more effective
than conventional bottom feed filters. As was the case with
centrifuges, iron sludges are generally reported easier to
dewater than aluminum sludges.
Thermal conditioning of chemical sludges is generally
reported successful prior to vacuum filtration or centrifugation
Sludge cakes of 35 percent TS and above are routinely achieved.
Potential negative aspects are similar to those for non-chemical
sludges: sidestreams have high dissolved organic strength, an!
operation and maintenance costs are high. One plant reported
excessive corrosion and erosion of the thermal conditioning
unit components, but it is not known if the problem was aggra-
vated by the chemical component of the sludge.
Because of the impacts of phosphorus removal on sludge
production and treatment, plants are now hauling more sludge
in liquid and cake form to land disposal sites than before.
Hauling sludge as a liquid or cake rather than dewatering or
incinerating has been a common solution to many of the diffi-
culties experienced by plants in dewatering and incinerating
11
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chemical sludges. In some cases, phosphorus removal has shifted
the economics of sludge processing in favor of hauling rather
than dewatering and/or incineration. Especially in the case of
lime sludges, it is being found that land application of cake
is preferable to lagoon storage or incineration. In other
cases, hauling simply provides interim solution to problems,
although it Is not necessarily the most cost-effective alterna-
tive. It is frequently relied upon in this manner by plants
which have inadequate capacity to handle the additional sludge
generated by phosphorus removal with existing facilities.
In view of the large amounts of chemical sludges being
applied to land, it is important that the negative or beneficial
effects on plants and animals be considered. Chemical sludges
contain nutrients and other elements which are beneficial to
plant growth. Lime sludges can improve low pH, low calcium, or
low phosphorus soils. Chemical sewage sludges must be charac-
terized on an individual basis to determine If possibly hazardous
concentrations of heavy metals or other contaminants exist.
12
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UfE STABILIZATION OF
WASTEWATER TREATOfT PLANT SLUDGES
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI., OHIO 45268
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
RICHARD F, NOLAND, P,E,
JAMES D, EDWARDS, P,E,
BURGESS X NIPLE, LIMITED
CONSULTING ENGINEERS AND PLANNERS
5085 REED ROAD
COLUMBUS,, OHIO 43220
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TABLE OF CONTENTS
INTRODUCTION 1
LIME STABILIZATION PROCESS DESCRIPTION 3
Background 3
Lime Requirements 4
pH Versus Time 8
Odors 10
Sludge Characteristics 10
Sludge Dewatering Characteristics 16
Land Application 17
LIME STABILIZATION DESIGN CONSIDERATIONS 21
Overall Design Concepts 21
Lime Requirements 25
Types of Lime Available 25
Quicklime 25
Hydrated Lime 27
Lime Storage and Feeding 28
Mixing 28
Raw and Treated Sludge Piping, Pumps,
and Grinder 30
A CASE HISTORY OF LIME STABILIZATION 31
Background 31
Revisions to the Existing Wastewater
Treatment Plant 33
Lime Stabilization 33
Anaerobic Digester 33
Septage Holding Facilities 37
Ultimate Sludge Disposal 37
Operation and Sampling 38
Raw Sludges 39
Lime Stabilized Sludges 45
Economic Analysis 45
Lebanon Facilities 45
Capital Cost of New Facilities 49
Lime Stabilization by Others 57
iii
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TABLE OF CONTENTS (continued)
Page
LIME STABILIZATION DESIGN EXAMPLES 58
Statement of Problem 58
Wastewater Characteristics 59
Treatment Unit Efficiencies 61
Sludge Characteristics 61
Process Alternatives - 4 MGD Wastewater Treatment Plant 63
Lime Stabilization 54
Anaerobic Digestion 70
Process Alternatives - 40 MGD Wastewater Treatment
Plant 76
Lime Stabilization 7g
Anaerobic Digestion 22
iv
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LIST OF FIGURES
Figure No. Description Page
1 Combined Lime Dosage vs. pH for All Sludges 5
2 Lime Dosage vs. pH Primary Sludge Appendix
3 Lime Dosage vs. pH Anaerobic Digested Sludge Appendix
4 Lime Dosage vs. pH Waste Activated Sludge Appendix
5 Lime Dosage vs. pH Septage Appendix
6 Lime Stabilized Primary Sludge pH vs Time n
7 Bacteria Concentration vs. Time Laboratory 12
Regrowth Studies 12
8 Dewatering Characteristics of Various Sludges
on Sand Drying Beds 18
9 Conceptual Design for Lime Stabilization
Facilities for a 3,785 Cu M/Day Treatment
Plant 22
10 Conceptual Design for Lime Stabilization
Facilities for an 18,925 Cu M/Day Treatment
Plant 23
11 Conceptual Design for Lime Stabilization
Facilities for a 37,850 Cu M/Day Treatment
Plant 24
12 Treatment Plant Flow Schematic Prior to In-
corporating Lime Stabilization 32
13 Treatment Plant Flow Schematic After Incor-
porating Lime Stabilization 34
14 Lime Stabilization Process Flow Diagram 35
15 Process Alternative Design Logic 60
16 4 MGD Lime Stabilization/Truck Haul & Land
Application 65
17 4 MGD Anaeorbic Digestion/Truck Haul &
Land Application 71
18 40 MGD Lime Stabilization/Pipeline Transport
& Land Application 77
19 40 MGD Anaerobic Digestion/Vacuum Filtration
& Land Application 83
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LIST OF TABLES
Table No. Description Page
1 Lime Required for Stabilization to
pH 12 for 30 Minutes 6
2 Comparison of Lime Dosages Required to
Treat Raw Primary Sludge 7
3 Comparison of Lime Dosages Predicted by
the Counts Equation to Actual Data at
Lebanon, Ohio 8
4 Volatile Solids Concentration of Raw
and Lime Stabilized Sludges 13
5 Nitrogen and Phosphorus Concentrations
in Anaerobically Digested and Lime
Stabilized Sludge 14
6 Comparison of Bacteria in Anaerobic
Digested Versus Lime Stabilized Sludges 15
7 Mixer Specifications for Sludge Slurries 29
8 Design Data for Lime Stabilization
Facilities 35
9 Anaerobic Digester Rehabilitation Design
Data 33
10 Chemical Composition of Raw Sludges at
Lebanon, Ohio 40
11 Heavy Metal Concentrations in Raw Sludges
at Lebanon, Ohio 42
12 Pathogen Data for Raw Sludges at
Lebanon, Ohio 43
13 Chemical Composition of Lime Stabilized
Sludges at Lebanon, Ohio 45
14 Pathogen Data for Lime Stabilized Sludges
at Lebanon, Ohio 47
15 Actual Cost of Digester Rehabilitation and
Lime Stabilization Facilities Construction 45
16 Total Annual Cost for Lime Stabilization Ex-
cluding Land Disposal for a 3,785 Cu/M
Day Plant 51
17 Total Annual Cost for Single Stage Anaerobic
Sludge Digestion Excluding Land Disposal for
a 3,785 Cu M/Day Plant 53
vi
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TABLE OF CONTENTS (continued)
Table No. Description Page
18 Annual Cost for Land Application of Lime
Stabilized and Anaerobically Digested
Sludges for a 3,785 Cu.M/Day Plant 55
19 Comparison of Total Annual Capital and
Annual O&M Cost for Lime Stabilization
and Anaerobic Digestion Including Land
Disposal for a 3,785 Cu M/Day Plant 56
20 Raw Wastewater Characteristics 59
21 Treatment Unit Efficiencies 61
22 Total Annual Cost for Lime Stabilization
Excluding Land Disposal for a 4 MGD Plant 67
23 Annual Cost for Land Application of Lime
Stabilized Sludge for a 4 MGD Plant 69
24 Total Annual Cost for Two-Stage Anaerobic
Sludge Digestion Excluding Land Disposal
for a 4 MGD Plant 73
25 Annual Cost for Land Application of
Anaerobically Digested Sludges for a
4 MGD Plant 74
26 Comparison of Total Annual Capital and
Annual O&M Cost for Lime Stabilization
and Anaerobic Digestion Including Land
Disposal for a 4 MGD Plant 75
27 Total Annual Cost for Lime Stabilization
Excluding Land Disposal for a 40 MGD
Plant 79
28 Annual Cost for Transportation and Land
Application of Lime Stabilized Sludge
for a 40 MGD Plant 82
29 Total Annual Cost for Two-Stage Anaerobic
Sludge Digestion Excluding Vacuum Fil-
tration and Land Disposal for a 40 MGD Plant 85
30 Vacuum Filtration Capital and Annual Operation
& Maintenance Costs for a 40 MGD Plant 86
31 Annual Cost for Land Application of Dewatered
Anaerobically Digested Sludges for a 40
MGD Plant 87
32 Comparison of Total Annual Capital and
Annual O&M Cost for Lime Stabilization
and Anaerobic Digestion Including Land
Disposal for a 40 MGD Plant 88
vii
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INTRODUCTION
Sludge constitutes the most significant by-product of wastewater
treatment; its treatment and disposal is perhaps the most complex prob-
lem which faces both the designer and operator. Raw sludge contains
large quantities of microorganisms; mostly fecal in origin, many of
which are pathogenic and potentially hazardous to humans. Sludge pro-
cessing is further complicated by its variable properties and relatively
low solids concentration. Solutions have long been sought for better
stabilization and disposal methods which are reliable and economical and
able to render sludge either inert or stable.
Lime stabilization has been shown to be an effective sludge dis-
posal alternative when there is a need to:
A. Provide alternate means of sludge treatment during the period
when existing sludge handling facilities, e.g., anaerobic or
aerobic digesters, are out of service for cleaning or repair.
B. Supplement existing sludge handling facilities, e.g., anae-
robic or aerobic digesters, incineration or heat treatment,
due to the loss of fuel supplies or because of excess sludge
quantities above design.
C. Upgrade existing facilities or construct new facilities to
improve odor, bacterial, and pathogenic organism control.
Lime stabilization has been demonstrated to effectively eliminate
odors. Regrowth of pathogens following lime stabilization is minimal.
Of the organisms studied, only fecal streptococci have a potential for
remaining viable.
Lime stabilized sludges are suitable for application to agricul-
tural land; however, lime stabilized sludges have lower soluble phos-
phate, ammonia nitrogen, total Kjeldahl nitrogen, and total solids
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concentrations than anaerobically digested primary/waste activated
mixtures at the same plant.
The purpose of this report is to present a review of stabilization
and disinfection of municipal wastewater treatment plant sludges using
lime stabilization, including' specific design considerations. Two de-
sign examples incorporating lime stabilization into a 4 and 40 MGO
wastewater treatment plant have been included to demonstrate the design
procedure. A comparison of the performance, capital and annual opera-
tion and maintenance costs for lime stabilization and anaerobic digester
was included for each design example. To further illustrate the appli-
cation of lime stabilization techniques to small plants and/or facilities
in need of an emergency sludge handling process, an actual case history
of lime stabilization at a 1 MGD facility was also included. The case
history includes capital and annual operation and maintenance costs;
chemical, bacterial, and pathological properties; and land application
techniques.
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LIME STABILIZATION
PROCESS DESCRIPTION
Background
Historically, lime has been used to treat nuisance conditions
resulting from open pit privies and .from the graves of domestic animals.
Prior to 1970, there was only a small amount of quantitative information
available in the literature on the reaction of lime with sludge to make
a more stable material. Since that time, the literature contains num-
erous references concerning the effectiveness of lime in reducing micro-
biological hazards in water and wastewater. ^z^ ' Information is
also available on the bactericidal value of adding lime to sludge. A
report of operations at the Allentown, Pennsylvania wastewater treatment
plant states that conditioning an anaerobically digested sludge with
lime to pH 10.2 to 11, vacuum filtering and storing the cake destroyed
all odors and pathogenic enteric bacteria. Kampelmacher and Jansen^ '
reported similar experiences. Evans^ ' noted that lime addition to
sludge released ammonia and destroyed bacillus coli and that the sludge
cake was a good source of nitrogen and lime to the land.
Lime stabilization of raw sludges has been conducted in the labora-
tory and in full scale plants. Parrel 1 et ar ' reported, among other
results, that lime stabilization of primary sludges reduced bacterial
hazard to a negligible value, improved vacuum filter performance, and
provided a satisfactory means of stabilizing sludge prior to ultimate
disposal.
Paulsrud and Eikunr J reported on the effects of long-term storage
of lime stabilized sludge. Their research included laboratory investi-
gations of pH and microbial activity over periods up to 28 days.
Pilot scale work by C.A. Counts et aP ' on lime stabilization
showed significant reductions in pathogen populations and obnoxious
odors when the sludge pH was greater than 12. Counts conducted growth
-------
studies on greenhouse and outdoor plots which indicated that the dis-
posal of lime stabilized sludge on cropland would have no detrimental
effects.
A research and demonstration contract was awarded to Burgess &
Niple, Limited in March 1975 -to complete the design, construction, and
operation of full scale lime stabilization facilities for a 3,785 cu
in/day (1 MGD) wastewater treatment plant, including land application of
treated sludges. The contract also included funds for cleaning, reha-
bilitating, and operating an existing anaerobic sludge digester. Con-
current with the research and demonstration project, a considerable
amount of full scale lime stabilization work was completed by cities in
Ohio and Connecticut. Wastewater treatment plant capacities which were
representative ranged from 3.785 to 113,550 cu m/day (1 to 30 MGD). A
summary of these results has previously been reported.
Lime Requirements
The lime dosage required to exceed pH 12 for at least 30 min was
found to be affected by the type of sludge, its chemical composition,
and percent solids. As an operational procedure, a target pH of 12.5
was selected to insure that the final pH would be greater than 12. A
summary of the lime dosage required for various sludges is shown in
Table 1. Of the total amount of lime which was required, an excess of
0 to 50 percent was added after pH 12 was reached in order to maintain
the pH. Figure 1 shows the combined lime dosage vs. pH for primary,
anaerobically digested, waste activated, and septage sludges. Figures
2-5 have been included in the Appendix and describe the actual lime
dosages which were required for each sludge type.
Table 2 compares the Lebanon full scale test results, which are
described later in the case history, with the data previously presented
by Farrell, et. al, Counts, et. al, and Paulsrud and Eikum for raw
primary sludges. In general, excellent correlation was achieved.
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AVERAGE
RANGE OBSERVED
1,000 2JDOO 3POO 4,000 5,000
DOSAGE Co IOH)2 MG/L
Rgure I. Combined Lime Dosage vs. pH For All Sludges
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Table 1
LIME REQUIRED FOR STABILIZATION
TO pH 12 FOR 30 MINUTES
Sludge Type
Primary sludge 3-6 0.12
Waste activated
sludge 1-1.5 0.30
Septage 1-4.5 0.20
Anaerobic 6-7 0.19
Average Lbs Range Lbs
Percent Ca2/Lbs
Solids Dry Solids Dry Solids
0.06-0.17
0.21-0.43
0.09-0.51
0.14-0.25
_ Average
Total Total Average Average
Volume Solids, Initial Final
Treated mg/1 pH pH
136,500 43,276 6.7 12.7
42,000 13,143 7.1 12.6
27,500 27,494 7.3 12.7
23,500 55,345 7.2 12.4
.Includes some portion of waste activated sludge
fNumerically equivalent to Kg Ca(OH)2 per kg dry solids
Multiply gallons x 3.785 to calculate liters
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Table 2
COMPARISON OF LIME DOSAGES
REQUIRED TO TREAT RAW PRIMARY SLUDGE
Lime Dose,
Investigator kg lime/kg sludge dry solids
Burgess & Niple, Limited
(Lebanon) 0.120
Parrel 1, et al 0.098(c)
Counts, et al 0.086(a'
Paulsrud, et al 0.125(b)
(a) Based on 4.78% solids
(b) Based on pH 12.5 for sludges reported
(c) Based on pH 11.5 for sludges reported
has proposed the following equation for predicting the
lime dosage required for primary and secondary sludges from the Rich-
land, Washington trickling filter plant:
Lime Dose =4.2+1.6 (TS)
When: Lime dose is expressed in grams
Ca(OH)2 per liter of sludge
TS is the total solids fraction
in the sludge.
Table 3 compares the values predicted by the Counts equation to the
Lebanon data for raw primary, waste activated, anaerobically digested,
and septage sludges:
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Table 3
COMPARISON OF LIME DOSAGES PREDICTED
BY THE COUNTS EQUATION TO ACTUAL DATA AT LEBANON, OHIO
Sludge Type
Raw primary
Waste activated
Anaerobically
digested
Septage
Percent Actual Lime Dose,
Solids kg lime/kg D.S.
4.78 0.120
1.37 0.300
6.40 0.190
2.35 0.200
Counts'
Lime Dose,
kg lime/kg D.S.
0.086
0.305
0.065
0.180
With Increasing solids concentrations, the Counts equation results
In lower than actual lime dosages.
pH Versus Time
Previous research has attempted to determine the magnitude of pH
decay versus time and to quantify the variables which affect pH decay.
(8}
Paulsrud^ ' reported that negligible pH decay occurred when the sludge
mixture was raised to pH 12 or greater or when the lime dose was approx-
imately five times the dose to reach pH 11. In either case, for raw
primary sludge, Paulsrud's dose was in the range of 0.100 to 0.150 kg
lime/kg dry solids, which was approximately the dosage used at Lebanon.
(9}
Countsv ' hypothesized that pH decay was caused by the sludge
chemical demand which was exerted on the hydroxide ions supplied in the
lime slurry. He further concluded that the degree of decay probably
decreased as the treated sludge pH increased because of the extremely
large quantities of lime required to elevate the pH to 12 or above.
However, this pH phenomenon is probably because pH is an exponential
function, e.g., the amount of OH" at pH 12 is ten times more than the
amount of OH* at pH 11.
-------
In the full scale work at Lebanon, all sludges were lime stabilized
to pH 12 or above and held for at least 30 mln with the addition of
excess lime. All treated sludges had less than a 2.0 pH unit drop after
six hours. Limed primary sludge was the most stable with septage being
the least stable. During the full scale program, only the pH of limed
primary sludge was measured for a period greater than 24 hours, which
showed a gradual drop to approximately 11.6 after 18 hours beyond which
no further decrease was observed.
The total mixing times from start through the 30 min contact time
at Lebanon were as follows:
Primary sludge 2.4 hours
Waste activated sludge 1.7 hours
Septic tank sludge 1.5 hours
Anaerobic digested
sludge 4.1 hours
Mixing time was a function of lime slurry feed rate and was not
limited by the agitating capacity of the diffused air system. Mixing
time may have been reduced by increasing the capacity of the lime slurry
tank.
To further examine the effects of excess lime addition above the
levels necessary to reach pH 12, a series of laboratory tests were set
up using a standard jar test apparatus. The tests were made on six one-
liter portions of primary sludge with 2.7% total solids. The pH of each
of the samples was increased to 12 by the addition of 10% hydrated lime
slurry. One sample was used as a control. The remaining samples had
30%, 60%, 90%, 120%, and 150% by weight of the lime dose added to the
control. The samples were mixed continuously for six hours and then
again ten minutes prior to each additional pH measurement. There was a
negligible drop in pH over a ten day period for those tests where excess
lime was added.
-------
A second laboratory scale test was completed using a 19 1 (5 gal)
raw primary sludge sample which was lime stabilized to pH 12.5 and
allowed to stand at 18° C. Samples were withdrawn weekly and analyzed
for pH and bacteria concentration. The results of the pH and bacteria
studies are shown on Figures 6 and 7, respectively. After 36 days, the
pH had dropped to 12.0.
In conclusion, significant pH decay should not occur once suffi-
cient lime has been added to raise the sludge pH to 12.5 and maintain
that value for at least 30 min.
Odors
(9)
Previous work stated that the threshold odor number of raw
primary and trickling filter sludges was approximately 8,000, while that
of lime stabilized sludges usually ranged from 800 to 1,300. By re-
tarding bacterial regrowth, the deodorizing effect can be prolonged.
Further, it was concluded that by incorporating the stabilized sludge
into the soil, odor potential should not be significant.
During the full scale operations at Lebanon, there was an intense
odor when raw sludge was first pumped to the lime stabilization mixing
tank, which increased when diffused air was applied for mixing. As the
sludge pH increased, the sludge odor was masked by the odor of ammonia
which was being air stripped from the sludge. The ammonia odor was most
intense with anaerobically digested sludge and was strong enough to
cause nasal irritation. As mixing proceeded, the treated sludge ac-
quired a musty humus like odor, with the exception of septage which did
not have a significant odor reduction as a result of treatment.
Sludge Characteristics
Several authors have previously attempted to summarize the chemical
and bacterial compositions of sewage sludges. '^ '* ' Recent data
on the nutrient concentrations for various sludges have been reported by
10
-------
13.0
C.O
11.0
10.0
9.0 •
ao •
7O
6.0
^
LEBANON, OHIO DATA
-• DATA BY RMJLSRUD
,(8)
10
30
40
50
DAYS
Figure 6. Lime Stabilized Primary Sludge pH vs Time
11
-------
o
o
I
o
<
CD
100,000,000
10,000 poo
1000,000
100 poo
10.000
1.000
100
0
100.000.000
(OPOOPOO
ipoo.ooo
100,000
10,000
1,000
100
0
lOOOOOpOO
10,000,000
1,000,000
IQOPOO
10,000
I POO
100
0
20
10
0
50
40
30
20
10
0
FECAL STREP
-FECAL COLIFORM
-TOTAL COLIFORM
AERUGINOSA
^•SALMONELLA
10
—I—
20
—I—
30
40
SO
TIME , DAYS
Rgure 7. Bacteria Concentration vs Time Laboratory Regrowth Studies
12
-------
Sommersv . Chemical and pathogenic data on raw and lime stabilized
raw primary, waste activated, septage, and anaerobically digested sludges
from the Lebanon, Ohio full scale project have been summarized below and
are included in more detail in the case history.
The addition of lime and mixing-by diffused air altered the chemical
characteristics of each sludge. In all sludges, lime stabilization
resulted in an increase in alkalinity and soluble COD and a decrease in
soluble phosphate. Total COO and total phosphate decreased for all
sludges except waste activated. Ammonia nitrogen and total Kjeldahl
nitrogen decreased for all sludges except waste activated.
The volatile solids concentrations of raw and lime stabilized
sludges are shown in Table 4. The actual volatile solids concentrations
following lime stabilization are lower than those which would result
only from the addition of lime. Neutralization, saponification, and
hydrolysis reactions with the lime probably result in the lower volatile
solids concentrations.
Table 4
VOLATILE SOLIDS CONCENTRATION OF
RAW AND LIME STABILIZED SLUDGES
Raw Sludge Lime Stabilized Sludge
Volatile Solids Volatile Solids
Solids Concentration, Solids Concentration,
Sludge Type
Primary
Waste activated
Septage
Anaerobically digested
mg/1
73.2
80.6
69.5
49.6
mq/1
54.4
54.2
50.6
37.5
13
-------
In terms of the agricultural value, lime stabilized sludges had
lower soluble phosphate, ammonia nitrogen, total Kjeldahl nitrogen, and
total solids concentrations than anaerobically digested primary/waste
activated mixtures at the same plant, as shown in Table 5. The signifi-
cance of these changes are discussed in the section on land disposal.
Table 5
NITROGEN AND PHOSPHORUS CONCENTRATIONS
IN ANAEROBICALLY DIGESTED AND LIME
STABILIZED SLUDGE
Sludge Type
Lime Stabilized Primary
Lime Stab. Waste Activated
Lime Stabilized Septage
Anaerobic Digested
Total
Phosphate
as P, mg/1
283
263
134
580
Total
Kjeldahl
Nitrogen
as N, mg/1
1,374
1,034
597
2,731
Ammonia
Nitrogen
as N, mg/1
145
53
84
709
Considerable research has been conducted on the degree of bacterial
reduction which can be achieved by high lime doses.^ ^ ' In general,
the degree of pathogen reduction increased as sludge pH increased with
consistently high pathogen reductions occurring only after the pH
reached 12.0. Fecal streptococci appeared to resist inactivation by
lime treatment particularly well in the lower pH values; however, at pH
12, these organisms were also inactivated after one hour of contact
time.
(9)
In all lime stabilized sludges, Salmonella and Pseudomonas aerug-
inosa concentrations were reduced to near zero. Fecal and total coli-
form concentrations were reduced greater than 99.99% in the primary and
septic sludges. In waste activated sludge, the total and fecal coliform
concentrations decreased 99.97% and 99.94%, respectively. The fecal
14
-------
streptococci kills were as follows: primary sludge, 99.93%; waste
activated sludge, 99.41%; septic sludge, 99.90%; and anaerobic digested,
96.81%.
Pathogen concentrations for the lime stabilized sludges are sum-
marized in Table 6.
Anaerobic digestion is currently an acceptable method of sludge
stabilization. For reference, lime stabilized sludge pathogen
concentrations at Lebanon have been compared in Table 6 to those ob-
served for well digested sludge from the same plant.
Table 6
COMPARISON OF BACTERIA IN ANAEROBIC
DIGESTED VERSUS LIME STABILIZED SLUDGES
Fecal Fecal Total Ps.
Coliform Streptococci Coliform Salmonella Aeruginosa
#/100 ml #/100 ml #/l_QP_jnl__ #/100 ml 0/100 ml
Anaer. digested
Lime stabilized*
Primary
Waste act.
Septage
*To pH equal to or greater than 12.0
**Detection limit = 3
Pathogen concentrations in lime stabilized sludges range from 10 to
1,000 times less than for anaerobically digested sludge.
A pilot scale experiment was completed in the laboratory to deter-
mine the viability and regrowth potential of bacteria in lime stabilized
primary sludge over an extended period of time.
1, 450x1 O3
4x1 03
16xl03
265
27x1 O3
23x1 O3
61xl03
665
27, 800x1 03
27.6xl03
212xl03
2,100
6
3**
3
3
42
3
13
3
15
-------
The test was intended to simulate storing stabilized sludge in a
holding tank or lagoon when weather conditions prohibit spreading. In
the laboratory test, 19 1 (5 gal) of 7% raw sludge from the Mill Creek
sewage treatment plant in Cincinnati were lime stabilized to pH 12.0.
Lime was added until equivalent to 30% of the weight of the dry solids
which resulted in a final pH of 12.5. The sample was then covered
with foil and kept at room temperature 18.3 C. (65° F.) for the remainder
of the test. The contents were stirred before samples were taken for
bacterial analysis.
The results are shown on Figure 7, and indicate that a holding
period actually increases the bacteria kill. Salmonella in the raw
sludge totaling 44 per 100 ml were reduced to the detection limit by
lime stabilization. Pseudomonsas aeruginosa totaling 11 per 100 ml
in the raw sludge were reduced to the detection limit by lime stabili-
zation. The initial fecal coliform count of 3.0 x 10 was reduced to
3
5 x 10 after lime stabilization, and after 24 hours was reduced to less
o
than 300. The raw sludge contained 3.8 x 10 total coliform, but 24 hours
after lime stabilization the total coliform were less than 300. The
fecal strep count in the raw sludge was 1.8 x 10 which decreased to
A
9.6 x 10 after lime stabilization. After 24 hours, the count was down
to 7.0 x 10 and after six days reduced to less than 300. The count
increased to 8 x 10 after 40 days.
Sludge Oewatering Characteristics
Farrell et ar ' have previously reported on the dewatering char-
acteristics of ferric chloride and alum treated sludges which were
subsequently treated with lime. Trubnick and Mueller^17^ presented, in
detail, the procedures to be followed in conditioning sludge for fil-
tration, using lime with and without ferric chloride. Sontheimer^18^
presented information on the improvements in sludge filterability pro-
duced by lime addition.
Standard sand drying beds, which were located at the Lebanon, Ohio
wastewater treatment plant, were used for sludge dewatering comparisons.
16
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Each bed was 9.2 x 21.5 m (30' x 70'). For the study, one bed was
partitioned to form two, each 4.6 x 21.5 m (151 x 70'). Limed primary
sludge was applied to one bed with limed anaerobically digested sludge
being applied to the other side. A second full sized bed was used to
dewater unlimed anaerobically digested sludge. The results of the study
are summarized on Figure 8.
Lime stabilized sludges generally dewatered at a lower rate than
well digested sludges. After ten days, lime stabilized primary sludge
had dewatered to approximately 6.5% solids as opposed to 9% for lime
stabilized anaerobically digested sludge, and 10% for untreated anae-
robically digested sludge.
The anaerobically digested sludge cracked first and dried more
rapidly than either of the lime stabilized sludges. Initially, both of
the lime stabilized sludges matted, with the digested sludge cracking
after approximately two weeks. The lime stabilized primary sludge did
not crack which hindered drying and resulted in the lower percent solids
values.
Land Application
Numerous references are available regarding the application of
anaerobically digested sludges to agricultural land.
The application of sewage sludge on land has generally been viewed from
two standpoints, either as a rate of application consistent with the
utilization of nutrients in sludge by growing plants (i.e., agricultural
utilization), or as the maximum amount of sludge applied in a.minimum
amount of time (i.e., disposal only). USEPA guidelines^ ' generally
favor the former approach. The successful operation of a program uti-
lizing the application of sewage sludge on land is dependent upon a
knowledge of the particular sludge, soil, and crop characteristics.
Organic matter content, fertilizer nutrients, and trace element
concentrations are generally regarded as being vital to the evaluation
17
-------
20 i I i I i I i i I i I I I I I I i I I I I I I I i I I I M I I I i I I I I M
15-•
in
o
(T
LJ
0.
10--
5- -
20 25 30 35 40
TIME-DAYS
Rgure 8. Dewotering Characteristics of Various Sludges on Sand
Drying Beds
18
-------
of the applicability of land application of sewage sludge. The range of
nitrogen, phosphorus, and potassium concentrations for sewage sludges
have been reported by Brown et al .
Sommersv ' has also summarized fertilizer recommendations for
crops based primarily on the amount' of major nutrients (nitrogen, phos-
phorus, and potassium) required by a crop and on the yield desired.
Counts^ ' conducted greenhouse and test plot studies for lime
stabilized sludges which were designed to provide information on the
response of plants grown in sludge-soil mixtures ranging in application
rate from 11 to 220 metric tons per hectare (5 to 100 tons/acre).
Counts concluded that sludge addition to poor, e.g., sandy, soils would
increase productivity, and therefore would be beneficial. The total
nitrogen and phosphorus levels in plants grown in greenhouse pots, which
contained sludge-soil mixtures, were consistently lower than plants
which were grown in control pots. The control set, which contained only
soil with no sludge additions, received optimum additions of chemical
fertilizer during the actual plant growth phase of the studies. Cal-
cium concentration in plant tissues from the sludge-soil pots were
higher than those for the controls. The pH values of the various sludge-
soil mixtures were lower after plant growth than before. Counts attrib-
uted the decrease to carbon dioxide buildup in the soil which resulted
from biological activity.
Land application studies at Lebanon, Ohio were conducted by spread-
ing liquid sludge on agricultural land and on controlled test plots.
Winter wheat, soybeans, and hay were grown on fields which were in
normal agricultural production. Corn, swiss chard, and soybeans were
grown on 22 test plots, each with an area of 0.0085 ha (0.021 acre).
Sludge application was accomplished by spreading as a liquid using
a four-wheel drive vehicle which was equipped with a 2.3 cu m (600 gal)
tank. The width of sludge spread per pass was approximately 60 cm
(24 in).
19
-------
The lime stabilized sludge formed a filamentous mat 0.32 to 0.64 cm
(1/8-1/4 in) thick which, when dry, partly choked out the wheat. The
mat partly deteriorated over time, but significant portions remained at
the time of harvest. There was no matting on the fields where the lime
stabilized sludge was incorporated into the soil before planting.
Spontaneous growth of tomatoes was significant in the fields which
had lime stabilized sludge incorporated into the soil before planting.
Seeds were contained in the sludge and were not sterilized by the lime.
These plants were absent at the site where the sludge was not incorpor-
ated, even though no herbicide was applied, probably because of frequent
frosts and the lack of sludge incorporation into the soil. During the
next year's growing season, an increase in insect concentration was
noticed on the fields which had received lime stabilized sludge.
20
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LIME STABILIZATION DESIGN CONSIDERATIONS
Overall Design Concepts
Lime and sludge are two of the most difficult materials to transfer,
meter, and treat in any wastewater .treatment plant. For these reasons,
design of stabilization facilities should emphasize simplicity, straight-
forward piping layout, ample space for operation and maintenance of
equipment, and gravity flow wherever possible. As discussed in more
detail in the following sections, lime transport should be by auger with
the slurry or slaking operations occurring at the point of use. Lime
slurry pumping should be avoided with transport being by gravity in open
channels. Sludge flow to the tank truck and/or temporary holding lagoon
should also be by gravity if possible.
Figures 9, 10 and 11 show conceptual designs for lime stabiliza-
tion facilities at wastewater treatment facilities with 3,785, 18,925,
and 37,850 cu m/day (1, 5 and 10 MGD) throughputs. The 3,785 cu m/day
(1 MGD) plant, as shown on Figure 9, utilizes hydrated lime and a
simple batch mixing tank, with capability to treat all sludges in less
than one shift per day. Treated sludge could be allowed to settle for
several hours before hauling in order to thicken, and thereby reduce the
volume hauled. Alternately, the sludge holding lagoon could be used for
thickening.
Figure 10 shows the conceptual design for lime stabilization
facilities of an 18,925 cu m/day (5 MGD) wastewater treatment facility.
Pebble lime is utilized in this installation. Two sludge mixing tanks
are provided, each with the capacity to treat the total sludge produc-
tion from two shifts. During the remaining shift, sludge could be
thickened and hauled to the land disposal site. Alternately, the temp-
orary sludge lagoon could be used for sludge thickening.
Figure 11 shows the conceptual design for lime stabilization
facilities of a 37,850 cu m/day (10 MGD) wastewater treatment plant. A
21
-------
OUST COLLECTOR
M MFLUENT SLUDGE
o—
SLUDGE
GRINDER
/" ^V-TANIC TRUCK
( Y—TANK TRUCK
i*^ uf\
00—00
^TREATED SLUDGE TO LAGOON
N.
\
LAGOON
SLUDGE FROM LAGOON
Figure 9. Conceptual Design For Lime Stabilization Facilities For A
3,785 cu. meter/day Treatment Plant
-------
U>
.TANK TRUCK
LAGOON
SLUDGE FROM LAGOON
Figure 10. Conceptual Design For Lime Stabilization Facilities For An
18,925 cu. meter/day Treatment Plant
-------
DUST COLLECTOR
.-BUILDING
AUGERS
LIME SLAKERS / FEEDERS
MECHANICAL TURBINE AGITATOfl
MIX TANK .
DETENTION TIME
SLUDGE
THICKENER
DECANT TO
PHIMAffY INFLUENT
r
TANK TRUCK
80—00
TR ATEO SLUDGE
LAGOON
LAGOON
SLUDGE FROM LAGOON
Figure II. Conceptual Design For Lime Stabilization Facilities For A
37,850 cu. meter/day Treatment Plant
-------
continuous lime treatment tank with two hours detention time is used to
raise the sludge pH to 12. A separate sludge thickening tank is pro-
vided to increase the treated sludge solids content before land appli-
cation. Sludge transport is assumed to be by pipeline to the land
disposal site. A temporary sludge holding lagoon was assumed to be
necessary, and would also be located at the land disposal site.
Lime Requirements
The quantity of lime which will be required to raise the pH of
municipal wastewater sludges to pH greater than 12 can be estimated from
the data presented in Table 1 and from Figures 2-5. Generally, the
lime requirements for primary and/or waste activated sludge will be in
the range of 0.1 to 0.3 Kg per Kg (Ib per Ib) of dry sludge solids.
Laboratory jar testing can confirm the dosage required for existing
sludges.
Types of Lime Available
Lime in its various forms, as quicklime and hydrated lime, is the
principal, lowest cost alkali. Lime is a general term, but by strict
definition, it only embraces burned forms of lime - quicklime, hydrated
lime, and hydraulic lime. The two forms of particular interest to lime
stabilization, however, are quicklime and hydrated lime. Not included
are carbonates (limestone or precipitated calcium carbonate) that are
occasionally but erroneously referred to as "lime.
Quicklime. Quicklime is the product resulting from the calcination
of limestone and to a lesser extent shell. It consists primarily of the
oxides of calcium and magnesium. On the basis of their chemical analyses,
quicklimes may be divided into three classes:
1. High calcium quicklime - containing less than 5% magnesium
oxide, 85-90% CaO
2. Magnesium quicklime - containing 5 to 35% magnesium oxide, 60-
90% CaO
25
-------
3. Dolomitic quicklime - containing 35 to 40% magnesium oxide,
55-50% CaO
The magnesium quicklime is relatively rare in the United States
and, while available in a few localities, is not generally obtainable.
Quicklime is available in a number of more or less standard sizes,
as follows:
1. Lump lime - the product with a maximum size of 20.3 cm (8") in
diameter down to 5.1 cm (2") to 7.6 cm (3") produced in ver-
tical kilns.
2. Crushed or pebble lime - the most common form, which ranges in
size from about 5.1 to 0.6 cm (2" to 1/4"), produced in most
kiln types.
3. Granular lime - the product obtained from Fluo-Solids kilns
that has a particulate size range of 100% passing a #8 sieve
and 100% retained on a #80 sieve (a dustless product).
4. Ground lime - the product resulting from grinding the larger
sized material and/or passing off the fine size. A typical
size is substantially all passing a #8 sieve and 40 to 60%
passing a #100 sieve.
5. Pulverized lime - the product resulting from a more intense
grinding that is used to produce ground lime. A typical size
is substantially all passing a #20 sieve and 85 to 95% passing
a #100 sieve.
6. Palletized lime - the product made by compressing quicklime
fines into about one inch size pellets or briquettes.
26
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Hydrated Lime. As defined by the American Society for Testing and
Materials, hydrated lime is: "A dry powder obtained by treating quick-
lime with water enough to satisfy its chemical affinity for water under
the conditions of its hydration."
The chemical composition of hydrated lime generally reflects the
composition of the quicklime from which it is derived. A high calcium
quicklime will produce a high calcium hydrated lime obtaining 72% to 74%
calcium oxide and 23% to 24% water in chemical combination with the
calcium oxide. A dolomitic quicklime will produce a dolomitic hydrate.
Under normal hydrating conditions, the calcium oxide fraction of the
dolomitic quicklime completely hydrates, but generally only a small
portion of the magnesium oxide hydrates (about 5 to 20%). The compo-
sition of a normal cfolomitic hydrate will be 46% to 48% calcium oxide,
33% to 34% magnesium oxide, and 15% to 17% water in chemical combination
with the calcium oxide. (With some soft-burned dolomitic quicklimes,
20% to 50% of the MgO will hydrate.)
A "special" or pressure hydrated dolomitic lime is also available.
This lime has almost all (more than 92%) of the magnesium oxide hy-
drated; hence, its water content is higher and its oxide content lower
than the normal dolomitic hydrate.
Hydrated lime is packed in paper bags weighing 23 kg (50 Ib) net;
however, it is also shipped in bulk.
Quicklime is obtainable in either bulk carloads or tanker trucks or
in 36.3 kg (80 Ib) multiwall paper bags. Lump, crushed, pebble, or
pelletized lime, because of the large particle sizes, are rarely handled
in bags and are almost universally shipped in bulk. The finer sizes of
quicklime, ground, granular, and pulverized, are readily handled in
either bulk or bags.
27
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Lime Storage and Feeding
Depending on the type of lime, storage and feeding can be either in
bag or bulk. For small or intermittent applications, bagged lime will
probably be more economical. In new facilities, bulk storage will
probably be cost effective. -Storage facilities should be constructed
such that dry lime is conveyed to the point of use and then mixed or
slaked. Generally, augers are best for transporting either hydrated or
pebble lime. Auger runs should be horizontal or not exceeding an in-
cline of 30°.
The feeder facilities, i.e., dry feeder and slaking or slurry tank,
should be located adjacent to the stabilization mixing tank such that
lime slurry can flow by gravity in open channel troughs to the point of
mixing. Pumping lime slurry should be avoided. Slurry transfer dis-
tances should be kept to a minimum. Access to feeder, slaker and/or
slurry equipment should be adequate for easy disassembly and main-
tenance.
Mixing
Lime/sludge mixtures can be mixed either with mechanical mixers or
with diffused air. The level of agitation should be great enough to
keep sludge solids suspended and dispense the lime slurry evenly and
rapidly. The principal difference between the resultant lime stabilized
sludges in both cases is that ammonia will be stripped from the sludge
with diffused air mixing. Mechanical mixing has been used by previous
researchers for lime stabilization but only on the pilot scale.
With diffused air mixing, adequate ventilation should be provided
to dissipate odors generated during mixing and stabilization. Coarse
bubble diffusers should be used with air supplies in the range of 150-
250 cu m/min per 1,000 cu m (150-250 cfm per 1,000 cu ft) of mixing tank
volume. Diffusers should be mounted such that a spiral roll is estab-
lished in the mixing tank away from the point of lime slurry applica-
tion. Diffusers should be accessible and piping should be kept against
the tank wall to minimize the collection of rags, etc. Adequate piping
support should be provided.
28
-------
With the design of mechanical mixers, the bulk velocity (defined as
the turbine agitator pumping capacity divided by the cross sectional
area of the mixing vessel) should be in the range of 4.6 to 7.9 m/min
(15 to 26 fpm). Impeller Reynolds Numbers should exceed 1,000 in order
(21")
to achieve a constant power number., ' The mixer should be specified
according to the standard motor horsepower and AGMA gear ratios in order
to be commercially available.
For convenience, Table 7 was completed which shows a series of
tank and mixer combinations which should be adequate for mixing sludges
up to 10% dry solids, a range of viscosity, and Reynolds number combi-
nations which were as follows:
Max. Reynolds number 10,000 @ 100 cp sludge viscosity
Max. Reynolds number 1,000 @ 1,000 cp sludge viscosity
Table 7
MIXER SPECIFICATIONS FOR SLUDGE SLURRIES
Tank
Size,
liters
18,925
56,775
113,550
283,875
378,500
Tank
Diameter,
meters
2.9
4.2
5.3
7.2
8.0
Prime Mover, HP/
Shaft Speed, RPM
7.5/125
5/84
3/56
20/100
15/68
10/45
7.5/37
40/84
30/68
25/56
20/37
100/100
75/68
60/56
50/45
125/84
100/68
75/45
Turbine
Diameter,
centimeters
81
97
109
114
135
160
170
145
155
168
206
157
188
201
221
183
198
239
29
-------
Table 7 can be used to select a mixer horsepower and standard AGMA
gear combination depending on the volume of sludge to be stabilized.
For example, for a 18,925 1 (5,000 gal) tank, any of the mixer-turbine
combinations should provide adequate mixing. Increasing turbine diameter
and decreasing shaft speed results in a decrease in horsepower requirement
as shown.
Additional assumptions were that the bulk fluid velocity must
exceed 7.9 m/min (26 ft/min), impeller Reynolds number must exceed
1,000, and that power requirements range from 0.5 to 1.5 HP per 3,785 1
(0.5-1.5 HP/1,000 gal) is necessary. The mixing tank configuration
assumed that the liquid depth equals tank diameter and that baffles with
a width of 1/12 the tank diameter were placed at 90° spacing. Mixing
(21) (22)
theory and equations which were used were after Badgerv ', Hicksv
and Fair.(23)
Raw and Treated Sludge Piping, Pumps, and Grinder
Sludge piping design should include allowances for increased fric-
tion losses due to the non-Newtonian properties of sludge. Friction
loss calculations should be based on treated sludge solids concentrations
and should allow for thickening in the mixing tank after stabilization.
Pipelines should not be less than 5.08 cm (2 in) in diameter and should
have tees in major runs at each change in direction to permit rodding,
cleaning, and flushing the lines. Adequate drains should be provided.
If a source of high pressure water is available (either nonpotable or
noncross-connected potable), it can be used to flush and clean lines.
Spare pumps should be provided and mounted such that they can be
disassembled easily. Pump impeller type and materials of construction
should be adequate for the sludge solids concentration and pH.
Sludge grinding equipment should be used to make the raw sludge
homogenous. Sticks, rags, plastic, etc., will be broken up prior to
lime stabilization to improve the sludge mixing and flow characteristics
and to eliminate unsightly conditions at the land disposal site.
30
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A CASE HISTORY OF LIME STABILIZATION
Background
Facilities for lime stabilization of sludge were incorporated into
an existing 3,785 cu m/day (1.0 MQD) single stage activated sludge
wastewater treatment plant located at Lebanon, Ohio. Lebanon has a pop-
ulation of about 8,000 and is located in southwestern Ohio, 48.27 km
(30 mi) northeast of Cincinnati. The surrounding area is gently rol-
ling farmland with a small number of light industries, nurseries, or-
chards, and truck farms.
Major unit processes at the wastewater treatment plant include in-
fluent pumping, preaeration, primary clarification, conventional acti-
vated sludge, and anaerobic sludge digestion. Average influent BOD5 and
suspended solids concentrations are 180 and 243 mg/1, respectively. The
treatment plant flow schematic is shown on Figure 12.
Prior to completing the sludge liming system, the existing anae-
robic sludge digester was inoperative and was being used as a sludge
holding tank. The digester pH was approximately 5.5 to 6.0. Grit and
sand accumulations had reduced its effective volume to 40-50 percent of
the total. Waste activated sludge was being returned to the primary
clarifiers and resettled with the primary sludge. Combined primary/
waste activated sludge was being pumped to the digester and ultimately
recycled to the primary clarifiers via the digester supernatant. Typical
supernatant suspended solids concentrations were in the range of 30,000
to 40,000 mg/1. When possible, sludge was withdrawn from the digester
and dewatered on sand drying beds.
USEPA made the decision to utilize lime stabilization at Lebanon
not only as a full scale research and demonstration project, but also
as a means of solids handling during the period while the anaerobic di-
gester was out of service for cleaning and repair.
31
-------
INFLUENT
PUMP
STATION
10
hO
PRIMARY
CLARIFIER
PRIMARY
CLARIFIER
AERATION
J
AERATION
RETURN SLUDGE
WASTE ACTIVATED SLUDGE
CREEK
DIGESTED
-
SLUDGE
DRYING
BEOS
Rgure 12. Treatment Plant Flow Schematic Prior to Incorporating Lime Stabilization
-------
Revisions to the Existing Wastewater Treatment Plant
Lime Stabilization. The lime stabilization process was designed to
treat raw primary, waste activated, septic tank, and anaerobically
digested sludges. The liming system was integrated with the existing
treatment plant facilities, as shown on Figure 13. Hydrated lime was
stored in a bulk storage bin and was augered into a volumetric feeder.
The feeder transferred dry lime at a constant rate into a 94.6 1 (25
gal) slurry tank which discharged an 8-10% lime slurry by gravity into
an existing 25 cu m (6,500 gal) tank. The lime slurry and sludge were
mixed with diffused air. A flow schematic for the lime stabilization
facilities is shown on Figure 14. Design data are shown in Table 8.
Anaerobic Digester. As previously described, the existing single
stage anaerobic sludge digester was inoperative and was being used as a
sludge holding tank. The digester and auxiliary equipment were com-
pletely renovated and returned to good operating condition which allowed
a comparison of anaerobic digestion and lime stabilization. The digester
was cleaned, a new boiler and hot water circulating system was installed,
and all necessary repairs were made to piping, valves, pumps, and elec-
trical equipment.
The anaerobic digester design data are shown in Table 9.
Table 9
ANAEROBIC DIGESTER REHABILITATION DESIGN DATA
Tank dimensions 15 m (50') dia. x 6.1 m (201) SWD
Total volume 1,223 cu m (43,200 cu ft)
Actual volatile solids «.
loading 486 g VSS/cu m (0.03 Ib VSS/ft1^
Hydraulic detention time 36 days
Sludge recirculation
rate 757 1/min (200 gpm)
p
Boiler capacity 2.53 x 10 Joules/hr (240,000
BTU/hr)
33
-------
CO
WASTE ACTIVATED SLUDGE
SEPTAGE
HOLDING
TANK
VOLUMETRIC FEEDER
LIME SLURRY TANK
WATER
FOR MIXING
PRIMARY AND/OR
ANAEROBIC
DIGESTER
WASTE ACTIVATED SLUDGE
DIGESTED SLUDGE
TREATED SLUDGE TO
TRUCK FOR
LAND DISPOSAL
Figure 13. Treatment Plant Flow Schematic After Incorporating Lime Stabilization
-------
Ui
-VOLUMETRIC FEEDER
—LIME SLURRY TANK
DIFFUSED AIR
FOR MIXING
TREATED SLUOCE
ANAEROBIC DIGESTED SLUDGE
PRIMARY SLUOCE
WASTE ACTIVATED SLUDGE
TREATED SLUDGE T
SLUDGE
WELL A
PUMP
TANK TRUCK FOR LA
DISPOSAL
00 00
Figure 14. Lime Stabilization Process Flow Diagram
-------
Table 8
DESIGN DATA FOR LIME STABILIZATION FACILITIES
Mixing Tank
Total volume 30 cu m (8,000 gal)
Working volume 25 cu m (6,500 gal)
Dimensions 3.05 m x 3.66 m x 2.38 m
(10' x 12' x 7.8')
Hoppered bottom 0.91 m (31) @ 27° slope
Type of diffuser Coarse bubble
Number of diffusers 4
Air supply 14-34 cu m/min (500-1,200 cf/min)
Bulk Lime Storage
Total volume 28 cu m (1,000 cu ft)
Diameter 2.74 m (9')
Vibrators 2 ea Syntron V-41
Fill system Pneumatic
Discharge system 15 cm (6") dia. auger
Material of construction Steel
Type & manufacturer Columbian Model C-95
Volumetric Feeder
Total volume 0.28 cu m (10 cu ft)
Diameter 71 cm (28")
Material of construction Steel
Type & manufacturer Vibrascrew LBB 28-10
Feed range 45-227 kg/hr (100-500 Ib/hr)
Average feed rate 78 kg/hr (173 Ib/hr)
36
-------
Table 8 (continued)
Lime Slurry Tank
Total volume 94.6 1 (25 gal)
Diameter 0.61 m (21 )
Septic Tank Sludge Holding Tank (septage tank)
Total volume 18.4 cu m (650 cu ft)
Working volume 15 cu m (4,000 gal)
Dimensions 3.66 m x 1.92 m x 2.62 m
Mixing Coarse bubble
Number of diff users 1
Air supply 2.8-8.4 cu m/min (100-300 cf/min)
Transfer Pumps
Raw and treated sludge 1,136 1/min (300 gpm)
Septage transfer pump 379 1/min (100 gpm)
Septage Holding Facilities. Because the Lebanon wastewater treat-
ment plant routinely accepted septic tank pump ings, an 18.4 cu m (5,000
gal) tank was installed to hold septic tank sludges prior to lime treat-
ment. The tank was equipped with a transfer pump which could be used to
either feed the lime stabilization process or transfer septage to the
primary tank influent at a controlled rate.
Ultimate Sludge Disposal. Treated sludges were applied to sand
drying beds, to test plots, and to three productive agricultural sites.
Land spreading operations began in early March and continued through
October. The sludge hauling vehicle was a four-wheel drive truck with a
2.3 cu m (600 gal) tank.
37
-------
Operation and Sampling
Raw sludge, e.g., primary, waste activated, septage or digested
sludge, was pumped to the mixing tank where it was mixed by diffused
air. Four coarse bubble diffusers were mounted approximately 30.5 cm (1
ft) above the top of the tank hopper and 38 cm (1.25 ft) from the tank
wall. This location permitted mixing to roll sludge up and across the
tank at which point lime slurry was fed. Lime which was used for the
stabilization of all sludges was industrial grade hydrated lime with CaO
and MgO contents of 46.9% and 34%, respectively. All lime requirements
have been converted and are expressed as 100% Ca(OH)2 except as noted.
Samples were taken from the untreated, but thoroughly mixed, sludge for
chemical, pH, bacteria, and parasite analyses.
After the initial pH determination, the lime slurry addition was
started. Hydrated lime was augered from the lime storage bin to the
volumetric feeder which was located directly above the sludge mixing
tank. The lime was slurried by the tangential injection of water into a
94.6 1 (25 gal) slurry tank. The lime solution (8-10% by weight) then
flowed by gravity into an open channel with three feed points into the
sludge mixing tank.
The sludge pH was checked every 15 min as the lime slurry was added
until the sludge reached a pH of 12, at which time it was held for 30
min. During the 30 min period, lime slurry continued to be added.
After 30 min, samples were taken for chemical, bacteria, and parasite
analyses. Air mixing was then discontinued, allowing the limed sludge
to concentrate. The sludge then flowed by gravity to a sludge well from
which it was pumped to the land disposal truck.
Samples of raw and treated Lebanon sludges were taken during each
operating day of the lime stabilization operations. Anaerobically
digested sludge samples were taken at the same time and analyzed for use
in comparisons of chemical, bacterial, and pathogen properties.
38
-------
Sample preservation and chemical analysis techniques were performed
in accordance with procedures as stated in "Methods for Chemical Anal-
ysis of Water and Wastes, USEPA,"^24^ and Standard Methods for the
( 25}
Examination of Water and Wastewater. '
Salmonella species and Pseudom'onas aeruginosa were determined by
( 26}
EPA staff using the method developed by Kenner and Clark. ' Fecal
coliform, total coliform, and fecal streptoccocus were determined ac-
cording to methods specified in Standard Methods for Examination of
Water and Wastewater.
Raw Sludges
Chemical data for Lebanon, Ohio raw primary, waste activated,
anaerobically digested, and septage sludges have been summarized in
Table 10. Data for each parameter include the average and range of the
values observed.
Analyses for heavy metals were conducted on grab samples of raw
primary, waste activated, and anaerobically digested sludges. These
data have been reported in Table 11 as mg/kg on a dry weight basis and
include the average and range of values.
Pathogen data for Lebanon, Ohio raw primary, waste activated,
anaerobically digested, and septage sludges have been summarized in
Table 12. In general, the data are in agreement with the values re-
ported by Stern, with the exception of Salmonella and Pseudomonas
aeruginosa. which are lower than the reported values.
39
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Table 10
CHEMICAL COMPOSITION OF RAW SLUDGES
AT LEBANON, OHIO
Raw Waste
Primary Activated
Parameter Sludge Sludge
Alkalinity, mg/1 1,885 1,265
Alkalinity Range, mg/1 1,264-2,820 1,220-1,310
Total COD, mg/1 54,146 12,810
Total COD Range, mg/1 36,930-75,210 7,120-19,270
Soluble COD, mg/1 3,046 1,043
Soluble COD Range, mg/1 2,410-4,090 272-2,430
Total Phosphate, mg/1 as P 350 218
Total Phosphate Range, mg/1 as P 264-496 178-259
Soluble Phosphate, mg/1 as P 69 85
Soluble Phosphate Range, mg/1 as P 20-150 40-119
Total Kjeldahl Nitrogen, mg/1 1,656 711
Total Kjeldahl Nitrogen Range,
mg/1 1,250-2,470 624-860
Ammonia Nitrogen, mg/1 223 51
Ammonia Nitrogen Range, mg/1 19-592 27-85
Total Suspended Solids, mg/1 48,700 12,350
Total Suspended Solids Range,
mg/1 37,520-65,140 9,800-13,860
Volatile Suspended Solids, mg/1 36,100 10,000
Volatile Suspended Solids Range,
mg/1 28,780-43,810 7,550-12,040
Volatile Acids, mg/1 1,997 NA
Volatile Acids Range, mg/1 1,368-2,856 NA
40
-------
Table 10 (continued)
Parameter
Alkalinity, mg/1
Alkalinity Range, mg/1
Total COD, mg/1
Total COD Range, mg/1
Soluble COD, mg/1
Soluble COD Range, mg/1
Total Phosphate, mg/1 as P
Total Phosphate Range, mg/1
as P
Soluble Phosphate, mg/1 as P
Soluble Phosphate Range,
mg/1 as P
Total Kjeldahl Nitrogen, mg/1
Total Kjeldahl Nitrogen Range,
mg/1
Ammonia Nitrogen, mg/1
Ammonia Nitrogen Range, mg/1
Total Suspended Solids, mg/1
Total Suspended Solids Range,
mg/1
Volatile Suspended Solids, mg/1
Volatile Suspended Solids Range,
mg/1
Volatile Acids, mg/1
Volatile Acids Range, mg/1
Anaerobically
Digested
Sludge
3,593
1,330-5,000
66,372
39,280-190,980
1,011
215-4,460
580
379-862
15
6.9-34.8
2,731
1,530-4,510
709
368-1,250
61,140
48,200-68,720
33,316
27,000-41,000
137
24-248
Septage
Sludge
1,897
1,200-2,690
24,940
10,770-32,480
1,223
1,090-1,400
172
123-217
25
21.6-27.9
820
610-1,060
92
68-116
21,120
6,850-44,000
12,600
3,050-30,350
652
560-888
41
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Table 11
HEAVY METAL CONCENTRATIONS IN
RAW SLUDGES AT LEBANON, OHIO
Cadmium, average mg/kg
Cadmium, range mg/kg
Total Chromium, average mg/kg
Total Chromium, range mg/kg
Copper, average mg/kg
Copper, range mg/kg
Lead, average mg/kg
Lead, range mg/kg
Mercury, average mg/kg
Mercury, range mg/kg
Nickel, average mg/kg
Nickel , range mg/kg
Zinc, average mg/kg
Zinc, range mg/kg
Raw
Primary
Sludge
105
69-141
633
287-979
2,640
2,590-2,690
1,379
987-1,770
6
0.4-11
549
371-727
4,690
4,370-5,010
Waste
Activated
Sludge
388
119-657
592
133-1,050
1,340
670-2,010
1,624
398-2,850
46
0.1-91
2,109
537-3,680
2,221
1,250-3,191
Anaerobic
Digested
Sludge
137
73-200
882
184-1,580
4,690
4,330-5,050
1,597
994-2,200
0.5
0.1-0.9
388
263-540
7,125
6,910-7,340
42
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Table 12
PATHOGEN DATA FOR RAW SLUDGES
AT LEBANON, OHIO
Parameter
Salmonella avg. #7100 ml
Salmonella range, 0/100 ml
Ps. aeruginosa avg., 0/100 ml
Ps. aeruginosa range, 0/100 ml
Fecal coliform avg. MF, 0/100 ml
Fecal coliform range MF, 0/100 ml
Fecal coliform avg. MPN, 0/100 ml
Fecal coliform range MPN,
#/100 ml
Total coliform avg. MF, 0/100 ml
Total coliform range MF, 0/100 ml
Total coliform avg. MPN, 0/100 ml
Total coliform range MPN,
0/100 ml
Fecal streptococci avg.
0/100 ml
Fecal streptococci range,
0/100 ml
Raw
Primary
Sludge
62
11-240
195
75-440
NA
NA
8.3 x 10
8
1.3xl08-3.3xl09
NA
NA
2.9 x 109
1.3xl09-3.5xl09
3.9 x 10'
2.6xl07-5.2xl07
Waste
Activated
Sludge
6
3-9
5.5 x 103
91-1.1 x 1
2.65 x 10 ,
2.0x10-3.3x10'
NA
NA
8
8.33flx 10 q
1.66x10 -1.5x1 CT
NA
NA
1.03 x 10'
5xl05-2xl07
43
-------
Table 12 (continued)
Parameter
Salmonella avg. #1/100 ml
Salmonella range, 0/100 ml
Ps. aeruginosa avg., 0/100 ml
Ps. aeruginosa range, 0/100 ml
Fecal coliform avg. MF, 0/100 ml
Fecal coliform range MF, 0/100
ml
Fecal coliform avg. MPN,
#/100 ml
Fecal coliform range MPN,
0/100 ml
Total coliform avg. MF, 0/100 ml
Total coliform range MF,
#/100 ml
Anaerobically
Digested
Sludge
6
3-30
42
3-240
2.6 x 105
3.4xl04-6.6xl05
1.45 x 10°
1.9xl05-4.9xl06
2.42 x 107
1.3xl05-1.8xl08
Total coliform avg. MPN, -
0/100 ml 2.78 x 10'
Total coliform range MPN,
0/100 ml
Fecal streptococci avg. 0/100 ml 2.7 x 105
Fecal streptococci range, 0/100 ml
Septage
Sludge
6
3-9
754 ?
14-2.1 x 10'
1.5 x 107
1.0xl07-1.8xl07
NA
NA
2.89 x 10
8
1.8xl07-7xl08
NA
NA
6.7 x 105 ,
3.3x10-1.2x10°
44
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Lime Stabilized Sludges
Chemical and bacterial data for lime stabilized sludges have prev-
iously been summarized in the general discussion on lime stabilization.
Specific data from the Lebanon, Ohio full scale project have been sum-
marized in Tables 13 and 14. Lime stabilized sludges had lower soluble
phosphate, ammonia nitrogen, total Kjeldahl nitrogen, and total solids
concentrations than anaerobically digested primary/waste activated mix-
tures at the same plant.
In all lime stabilized sludges, Salmonella and Pseudomonas
aeruginosa concentrations were reduced to near zero. Fecal and total
coliform concentrations were reduced greater than 99.99% in the primary
and septic sludges. In waste activated sludge, the total and fecal
coliform concentrations decreased 99.99% and 99.47%, respectively. The
fecal streptococci kills were as follows: primary sludge, 99.93%; waste
activated sludge, 99.41%; septic sludge, 99.90%; and anaerobic digested
sludge, 96.81%. Pathogen concentrations in lime stabilized sludges
range from 10 to 1,000 times less than for anaerobically digested
sludges.
Economic Analysis
Lebanon Facilities. As previously described, the anaerobic sludge
digestion facilities at Lebanon were essentially inoperable at the start
of the lime stabilization project. Funds were allocated to construct
lime stabilization facilities, as well as to rehabilitate the anaerobic
digester. In both cases, the existing structures, equipment, etc., were
utilized to the maximum extent possible. Table 15 includes the actual
amounts paid to contractors, following competitive bidding, and does not
include engineering fees, administrative costs, etc.
45
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Table 13
CHEMICAL COMPOSITION OF LIME STABILIZED SLUDGES
AT LEBANON, OHIO
CM
Parameter
Alkalinity, rag/1
Alkalinity range, mg/1
Total COD, mg/1
Total COD range, mg/1
Soluble COD, mg/1
Soluble COD range, mg/1
Total Phosphate, mg/1
Total Phosphate range, mg/1
Soluble Phosphate, mg/1
Soluble Phosphate range, mg/1
Total Kjeldahl nitrogen, mg/1
Total Kjeldahl nitrogen range,
mg/1
Ammonia nitrogen, mg/1
Ammonia nitrogen range, mg/1
Total suspended solids, mg/1
Total suspended solids range,
mg/1
Volatile suspended solids, mg/1
Volatile suspended solids range,
mg/1
Raw
Primary
Sludge
4,313
3,830-5,470
41,180
26,480-60,250
3,556
876-6,080
283
164-644
36
17-119
Waste
Activated
Sludge
5,000
4,400-5,600
14,700
10,880-20,800
1,618
485-3,010
263
238-289
25
17-31
Anaerobically
Digested
Sludge
8,467
2,600-13,200
58,690
27,190-107,060
1,809
807-2,660
381
280-460
2.9
1.4-5.0
Septage
Sludge
3,475
1,910-6,700
17,520
5,660-23,900
1,537
1,000-1,970
134
80-177
2/4
1.4-4.0
1,374
470-2,510
145
81-548
38,370
29,460-44,750
23,480
1,034
1,980
832-1,430 1,480-2,360
64
36-107
10,700
10,745-15,550
7,136
19,420-26,450 6,364-8,300
494
412-570
66,350
46,570-77,900
26,375
21,500-29,300
597
370-760
110
53-162
23,190
14,250-29,600
11,390
5,780-19,500
-------
Table 14
PATHOGEN DATA FOR LIME STABILIZED SLUDGES
AT LEBANON, OHIO
Parameter
Salmonella avg. , 0/100 ml
Salmonella range, #7100 ml
Ps. aeruginosa avg., 0/100 ml
Ps. aeruginosa range, 0/100 ml
Fecal coli form MF avg.,
0/100 ml
Fecal coli form MF range,
0/100 ml
Fecal coli form avg. MPN,
0/100 ml
Fecal coli form range MPN,
0/100 ml
Total coli form avg. MF,
0/100 ml
Total coli form range MF,
0/100 ml
Total coli form avg. MPN,
0/100 ml
Total coli form range MPN,
0/100 ml
Fecal streptococci avg. ,
0/100 ml
Raw
Primary
Sludge
< 3*
< 3*
< 3*
< 3*
NA
NA
0
5.93 x 10J
/\
560-1. 7x1 0H
NA
NA
c
1.15 x 103
c
640-5.4 x 10
A
1.62 x 10^
Waste
Activated
Sludge
C 3*
C 3*
13
< 3*-26
A
1.62 x 10H
3.3xl02-3.2xl04
NA
NA
c
2.2 x 10s
•} c
3.3x10 -4. 2xl03
NA
NA
0
6.75 x 10J
Anaerobical ly
Digested
Sludge
< 3*
< 3*
< 3*
< 3*
•3
3.3 x 10J
3.3 x 103
18
18
NA
NA
18
18
•3
8.6 x 1015
Septage
Sludge
< 3*
< 3*
< 3*
< 3*
o
2.65 x 10^
2xl02-3.3xl02
NA
NA
•)
2.1 x 10J
•5
200-4 x 10J
NA
NA
665
Fecal streptococci range,
0/100 ml
,0xl03-5.5xl04
1.5x103-1.35x1O3
3.3xl03-1.4xl04
3.3xl02-lxl03
*Detectable limit = 3
-------
Table 15
ACTUAL COST OF DIGESTER REHABILITATION
AND LIME STABILIZATION FACILITIES CONSTRUCTION
Anaerobic Digester Cleaning
Cleaning contractor $5,512.12
Temporary sludge lagoon 2,315.20
Lime for stabilizing digester contents 514.65
Temporary pump rental 300.30
Subtotal Digester Cleaning $8,642.27
Anaerobic Digester Rehabilitation
Electrical equipment, conduit, etc. $1,055.56
Natural gas piping 968.76
Hot water boiler, piping, pump, heat
exchanger repair 7,472.26
Control room rehabilitation 1,465.00
Sludge recirculating pump repair 771.00
Piping and valve rehabilitation 8,587.30
Floating cover roof repair 1,014.04
Repair utilities, drains 211.52
Miscellaneous 1.946.88
Subtotal Digester Rehabilitation $23,492.32
Lime Stabilization Process
Electrical equipment, conduit, etc. $ 1,692.00
3" & 4" sludge lines, supports, valves,
and fittings 6,140.19
4" sludge crossover pipe, valves, and
fittings 1,101.48
1 1/2" air line and diffusers 1,310.00
3/4" water lines and hose bibbs 865.00
Lime bin, auger, vibrators 7,229.44
Volumetric feeder, trough and gate 3,460.00
Existing pump repairs 3,399.00
Miscellaneous metal 1,200.00
Relocate sanitary service line 200.00
Repair utilities 134.00
Miscellaneous 934.34
Contractor's overhead 1.842.00
Subtotal Lime Stabilization $29,507.45
Septage Holding Tank
Septage holding tank and pump $ 6,174.70
Subtotal Septage Holding Tank $ 6,174.70
Total Cost for Digester Cleaning &
Rehabilitation, Lime Stabilization,
and Septage Facilities $67,816.74
48
-------
The cost of the lime stabilization facilities was $29,507.45 com-
pared to $32,134.59 for cleaning and repair of the anaerobic sludge
digester.
Capital Cost of New Facilities. Capital and annual operation and
maintenance costs for lime stabilization and anaerobic sludge digestion
facilities were estimated assuming new construction as a part of a 3,785
cu m (1.0 MGD) wastewater treatment plant with primary clarification and
single stage conventional activated sludge treatment processes.
The capital costs for lime stabilization facilities included a bulk
lime storage bin for hydrated lime, auger, volumetric feeder and lime
slurry tank, sludge mixing and thickening tank with a mechanical mixer,
sludge grinder, all weather treatment building, electrical and instru-
mentation, interconnecting piping and transfer pumps, and 60 day deten-
tion treated sludge holding lagoon. The basis for design is as follows:
Daily primary sludge dry solids
production
Average primary sludge volume
@ 5% solids
Daily waste activated dry solids
production
Average waste activated sludge
volume @ 1.5% solids
Average lime dosage required per
unit
dry solids
Daily lime requirement as 100%
Ca(OH)2
Treatment period
Bulk lime storage bin volume
minimum
568 kg/day (1,250
Ibs/day)
11,015 I/day (2,910
gal/day)
493 kg/day (1,084
Ibs/day)
32,470 I/day (8,580
gal/day)
0.20 kg/kg/ (0.20
Ib/lb)
215 kg/day (475 Ib/day)
3 hrs/day
28 cu m (1,000 cu ft)
Bulk lime storage bin detention time 34 days
Lime feeder and slurry tank
capacity (spared)
0.14-0.42 cu m/hr
(5-15 cu ft/hr)
49
-------
Influent sludge grinder capacity
Sludge mixing tank volume
Sludge mixing tank dimensions
Sludge mixer horsepower
Sludge mixer turbine diameter
Turbine speed
Sludge transfer pump capacity
(spared)
Treated sludge percent solids
Sludge holding lagoon volume
Sludge holding lagoon maximum
detention time
Treatment building floor area
Treatment building construction
Instrumentation:
757 1/min (200 gpm)
57 cu m (15,000 gal)
4.3 m x 4.3 m x 3 m
(14'xl4'xlO' SWD)
15 HP
135 cm (53")
68 rpm
106 1/min (400 gpm)
4%
2,860 cu m (100,000
cu ft)
60 days
13.9 m2 (150 ft2)
Brick and block
pH record
Treated sludge volume
Capital costs for the lime stabilization facilities were based on
July 1, 1977 bid date, and were as follows:
Site work, earthwork & yard piping $ 6,000
Lime storage bin and feeders 30,000
Treatment tank, pumps, sludge
grinders, and building structure 52,000
Electrical and instrumentation 10,000
Sludge holding lagoon 20.000
Subtotal Construction Cost $118,000
Engineering 12,000
Total Capital Cost $130,000
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081) $ 10,500
Annual Capital Cost per unit feed
dry solids $ 24.65
50
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Lime stabilization operation assumed one man, two hours per day,
365 days per year, at $6.50 per hour, including overhead. Maintenance
labor and materials assumed 52 hours per year labor at $6.50 per hour
and $800 per year for maintenance materials. The total quantity of
46.8% CaO hydrated lime required was 141 tons per year at $44.50 per ton.
The total annual cost for lime stabilization, excluding land ap-
plication of treated sludge, has been summarized in Table 16.
Table 16
TOTAL ANNUAL COST FOR LIME STABILIZATION
EXCLUDING LAND DISPOSAL FOR A 3,785 CU M/DAY PLANT
Annual Annual
Total Cost Cost
Annual Per Kkg Per Ton
Item Cost Dry Solids Dry Solids
Operating labor $4,700 $12.14 $11.03
Maintenance labor
and materials 1,100 2.84 2.58
Lime 6,300 16.20 14.74
Laboratory 500 1.29 1.17
Capital 10.500 27.11 24.65
Total Annual Cost $23,100 $59.58 $54.17
The basis for design of a single stage anaerobic sludge digester
for the same treatment plant was as follows:
Daily primary sludge dry solids 568 kg/day (1,250
production Ib/day)
Average primary sludge volume 11,015 I/day
@ 5% solids (2,910 gal/day)
Daily waste activated dry solids 493 kg/day (1,084
production Ib/day)
51
-------
Average waste activated sludge
volume @ 1.5% solids
Daily volatile solids production
Volatile solids loading
Digester hydraulic detention time
Digester gas production
Average volatile solids reduction
Digested sludge dry solids
production
Digested sludge percent solids
Digester net heat requirement
Mechanical mixer horsepower
Sludge recirculation pumps (2 ea)
32,470 I/day (8,580
gal/day)
743 kg/day (1,634
Ib/day)
0.81 kg/cu m/day
(0.05 Ib VSS/ft3/day)
21 days
0.37 cu m/lb VSS feed
(13 cu ft/lb VSS
feed)
50%
689 kg/day (1,515
Ib/day)
6%
186,000 BTU/hr
15 HP
1,234 1/min ea. (350
gpm ea.)
Capital cost for the anaerobic sludge digestion facilities, in-
cluding the control building, structure, floating cover, heat exchanger,
gas safety equipment, pumps, and interconnecting piping, assuming July 1,
1977 bid date, and engineering, legal, and administrative costs is as
follows:
Site work, earthwork, yard piping $ 44,000
Digester 233,000
Control building 133,000
Electrical and instrumentation 47.000
Subtotal Construction Cost $457,000
Engineering 46.000
Total Capital Cost $503,000
Amortized cost @ 30 yrs, 7% int.
(CRF = 0.081) $ 40,700
Annual Capital Cost per unit
feed dry solids $ 95.54
52
-------
Digester operation assumed one man, one hour per day, 365 days per
year at $6.50 per hour, including overhead. Maintenance labor and
material assumed 52 hours per year at $6.50 per hour and $1,500 per year
for maintenance materials.
The cost of anaerobic digester operation was offset by assuming a
value of $2.10 per million BTU for all digester gas produced above the
net digester heat requirement.
The total annual cost for anaerobic sludge digestion, excluding
land application has been summarized in Table 17.
Table 17
TOTAL ANNUAL COST FOR SINGLE STAGE ANAEROBIC SLUDGE
DIGESTION EXCLUDING LAND DISPOSAL FOR A 3,785 CU M/DAY PLANT
Annual Annual
Total Cost Cost
Annual Per Kkg Per Ton
Item Cost Dry Solids Dry Solids
Operating labor $ 2,400 $ 6.20 $ 5.63
Maintenance labor
and materials 1,800 4.65
Laboratory 500 1.29
Capital 40,700 105.09
Fuel credit (2.900) (7.49)
Total Annual Cost $42,500 $109.74
Both the lime stabilization and anaerobic digestion alternatives
were assumed to utilize land application of treated sludge as a liquid
hauled by truck. The capital cost for a sludge hauling vehicle was
assumed to be $35,000, which was depreciated on a straight line basis
over a ten year period. Alternatively, a small treatment plant could
utilize an existing vehicle which could be converted for land applica-
tion at a somewhat lower capital cost.
53
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The assumed hauling distance was three to five miles, round trip.
Hauling time assumed 10 minutes to fill, 15 minutes to empty, and 10
minutes driving, or a total of 35 minutes per round trip. The truck
volume was assumed to be 5,680 liters (1,500 gal) per load. The cost of
truck operations, excluding the driver and depreciation, was assumed to
be $8.50 per operating hour. The truck driver labor rate was assumed to
be $6.50 per hour, including overhead.
Truck operation time was based on hauling an average of 1,812 1
(6,860 gal) of lime stabilized sludge, i.e., five loads and 777 1 (2,940
gal) of anaerobically digested sludge, i.e., two loads per day. The
reduced volume of anaerobically digested sludge resulted from the vola-
tile solids reduction during digestion and the higher solids concentra-
tion compared to lime stabilized sludge.
Although it may be possible to obtain the use of farmland at no
cost, e.g., on a voluntary basis, the land application economic analysis
assumed that land would be purchased at a cost of $750 per acre. Sludge
application rates were assumed to be ten dry tons per acre per year.
Land costs were amortized at 7% interest over a 30 year period.
To offset the land cost, a fertilizer credit of $7.30 per ton of
dry sludge solids was assumed. This rate was arbitrarily assumed to be
50% of the value published by Brown^ based on medium fertilizer
market value and low fertilizer content. The reduction was made to
reflect resistance to accepting sludge as fertilizer. The land cost was
further offset by assuming a return of $50 per acre, either as profit
after farming expenses, or as the rental value of the land.
Capital and annual operation and maintenance costs for land appli-
cation of lime stabilized and anaerobically digested sludges have been
summarized in Table 18.
54
-------
Table 18
ANNUAL COST FOR LAND APPLICATION OF LIME STABILIZED AND
ANAEROBICALLY DIGESTED SLUDGES FOR A 3,785 CU M/DAY PLANT
Lime Stabilization
Anaerobic Digestion
en
en
Item
Amortized cost
of land
Truck depreciation
Truck driver
Truck operation
Laboratory
Fertilizer credit
Land credit
Total Annual Cost
Total
Annual
Cost
$ 2,600
3,500
7,100
9,300
500
(3,100)
(2.200)
$17,700
Annual
Cost
Per
Kkg
Solids
$ 6.75
9.04
18.35
24.03
1.29
(8.05)
(5.68)
$45.73
Annual
Cost
Per
Ton
Solids
$ 6.14
8.22
16.67
21.83
1.17
(7.30)
(5.16)
$41.57
Total
Annual
Cost
$1,700
3,500
2,800
3,600
500
(2,000)
(1,400)
$8,700
Annual
Cost
Per
Kkg
Solids
$ 4.39
9.04
7.24
9.30
1.29
(8.05)
(3.62)
$19.59
Annual
Cost
Per
Ton
Solids
$ 3.99
8.22
6.57
8.45
1.17
(7.30)
(3.29)
$17.81
-------
For each item in Table 18, the total annual cost was calculated and
divided by the total raw primary plus waste activated sludge quantity,
i.e., 387 kkg/year (426 tons/year). Anaerobically digested sludge land
requirements were less than for lime stabilized sludge because of the
volatile solids reduction during digestion. Truck driving and operation
costs were similarly less for digested sludge because of the volatile
solids reduction and more concentrated sludge (6X vs. 4%) which would be
hauled. Fertilizer credit was less for digested sludge because of the
lower amount of dry solids applied to the land. Land credit was based
on the amount of sludge applied and was, therefore, less for digested
sludge.
The total annual capital and annual operation and maintenance costs
for lime stabilization and single stage anaerobic sludge digestion,
including land application for a 3,785 cu m/day wastewater treatment
plant, are summarized in Table 19.
Table 19
COMPARISON OF TOTAL ANNUAL CAPITAL AND ANNUAL
O&M COST FOR LIME STABILIZATION AND ANAEROBIC DIGESTION
INCLUDING LAND DISPOSAL FOR A 3,785 CU M/DAY PLANT
Lime Stabilization Anaerobic Digestion
Facilities
Land Application
Total Annual Cost
Total
Annual
O&M
Cost
$23,100
17.700
$40,800
Annual
Cost
Per
Kkg Dry
Solids
Total
Annual
O&M
Cost
$ 59.58 $42,500
45.70 8.700
Annual
Cost
Per
Kkg Dry
Solids
$109.74
19.59
$105.28 $51,200 $129.33
56
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Lime Stabilization by Others
A considerable amount of lime stabilization work has occurred in
Connecticut. A number of incinerators have been shut down and replaced
by lime stabilization. In each case, the time required to process the
sludge produced was greatly reduced. The following tabulation and
comments reflect and summarize the situation in December 1976. This
summary shows that eight of nine communities had either wholly or par-
tially abandoned incineration. While no chemical or bacterial data are
available, qualitative observations indicate that disposal is satis-
factory. Most of the communities have indicated that they will continue
with lime stabilization and disposal in landfills. Plants in Connec-
ticut which abandoned incineration in favor of lime stabilization:
Plant
Size,
Incinerator
Lime Stabilization
Stratford^JL
Bridgeport^ .,>
Stamford }.i
Middletown^
Willimatic^ ;
Glastonburg^v
Torrington,!u'
Naugatuckxqx
Enfield l ;
mgd
6
N/A*
N/A*
N/A
N/A
N/A
N/A
5
N/A
Installed
Yes
Yes
Yes
Yes
N/A
Yes
Yes
Yes
Used
Yes
Yes
No
No
N/A
No
No
Yes
Hours
24
24
N/A
N/A
N/A
N/A
1/3 of
year
Used
Yes
Yes
N/A
Yes
Yes
Yes
Yes
Yes
Yes
Hours
8
8
N/A
16
N/A
N/A
N/A
2/3 of
year
Ult. Disp.
Landf i 1 1
Landfill
Landfill
Land &
Landfill
N/A
N/A
Landfill
Landfill
*N/A denotes data not available at the time of writing
(1) Incinerator abandoned in favor of lime stabilization to pH 12.
Two shifts of labor no longer required.
(2) Stabilized coke used as final cover at landfill. Labor problems
when incinerator shut down because labor force reduced.
(3) Centrifuged with lime sludge.
(4) Previously plagued with odors; now all sludge processed in two
shifts, five days per week with no odors.
(5) Began lime stabilization in 1973. Screened sludge and leaf
material used on parks as fertilizer.
(6) Mix dewatered raw sludge and lime before disposing in landfill.
(7) Fluid bed reactor broke down; reluctant to go to lime stabilization.
(8) Incinerator too expensive to operate; lime stabilized sludge used
as final cover at landfill.
(9) Incineration is used in winter during inclement weather.
57
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LIME STABILIZATION
DESIGN EXAMPLES
Statement of Problem
The problem is to provide lime stabilization facilities for two
communities, both of which have existing conventional activated sludge
wastewater treatment plants.
The smaller community has existing wastewater treatment facili-
ties capable of treating 4.0 million gallons per day. The facilities
consist of screening, grit removal, primary settling, conventional acti-
vated sludge aeration, final settling, chlorination, and sludge la-
gooning. Present flow to the plant is 3.5 million gallons per day; the
20 year projected flow is 4.0 million gallons per day. The plant meets
its proposed discharge permit requirements, but the city has been or-
dered to abandon the sludge lagoons (which are periodically flooded by
the receiving stream). Sludge disposal alternatives include the
following:
1. Lime stabilization followed by liquid application to farm-
land.
2. Anaerobic digestion followed by liquid application to farm-
land.
The larger community has existing wastewater treatment facilities
capable of treating 30 million gallons per day. Present flow to the
plant is 35 million gallons per day; the 20 year projected flow is 40
million gallons per day. The existing treatment system consists of
screening, grit removal, primary settling, conventional activated sludge
aeration, final settling, chlorination, aerobic sludge digestion,
sludge dewatering, and landfilling of dried sludge solids. The exist-
ing treatment scheme will meet proposed permit requirements. As a
part of the treatment plant expansion planning and in view of future
electric power costs, the following solids handling alternatives were
proposed:
58
-------
1. Lime stabilization followed by pipeline transportation to
the land application site.
2. Anaerobic digestion followed by mechanical dewatering and
land application.
The design logic which will be followed to develop and evaluate
the sludge handling alternatives is summarized on Figure 15.
Wastewater Characteristics
The wastewater characteristics and removal efficiencies of the
various treatment units are required to determine the basis for design
of the sludge stabilization and ultimate disposal processes. This
information may be acquired from plant records or from sampling programs
at the existing facilities. When these data are not available (such as
in the case of new wastewater treatment plants for new service areas),
assumptions based on sound engineering judgment and previous experience
are necessary. For the sake of simplicity, the wastewater character-
istics and treatment unit removal efficiencies for the example plants
are assumed to be equal. Raw wastewater characteristics for the example
plants are given in Table 20.
Table 20
RAW WASTEWATER CHARACTERISTICS
Parameter Concentration (mg/1)
BOD5 200
Suspended Solids 240
Organic Nitrogen 15
Ammonia Nitrogen 25
Phosphorus 10
Grease 100
59
-------
1. Establish Regulatory Constraints for
Effluent and Sludge Disposal
2. Determine WWTP Influent Loads
3. Determine WWTP Unit Process Scheme
4. Determine Raw Sludge Loads
5. Establish Cost Effective Constraints and
Sludge Solids Concentrations for Ultimate
Sludge Disposal Processes
6. Set Sludge Thickening Requirements
7. Select Stabilization Alternatives
7a. Develop Capital Cost
7b. Develop O&M Requirements and Cost
7c. Develop Environmental Constraints
7d. Evaluate Supernatant Impact on Plant
7e. Evaluate Estimated Total Sludge
Handling Costs
8. Screen Alternatives
9. Select Final Stabilization Process
10. Prepare Final Flow Sheets and Cost
Estimates
Figure 15. Process Alternative Design Logic
-------
Treatment Unit Efficiencies
Both plants in this example will meet their proposed permit re-
quirements by utilizing the existing treatment processes. Nitrification
and phosphorus removal are not required. Removal efficiencies based on
percentages of the raw "domestic" wastewater characteristics are pre-
sented in Table 21.
Table 21
TREATMENT UNIT EFFICIENCIES
Unit Parameter Removal Efficiency
Primary Settling B005 30%
SS 65%
Aeration & Final Settling BODr 60%
SS 25%
Sludge Characteristics
The characteristics of sludge discharged to the sludge stabiliza-
tion facilities may vary considerably depending on the type and amount
of industrial waste treated, the sludge origin (which particular treat-
ment unit) and the sludge age. Ideally, samples of sludge would be
available for analysis. The assumed sludge characteristics for each
example plant are as follows:
Sludge Type Design % Solids
Thickened Raw Primary 7.0
Thickened Waste Activated 2.5
61
-------
Thickening facilities for primary and waste activated sludge were as-
sumed to be cost effective for both the 4 and 40 MGD wastewater treat-
ment plants. Waste activated sludge production was 0.5 pound of vola-
tile solids per pound of BODr reduced.
Preliminary studies have indicated that anaerobic sludge diges-
tion will not be adversely affected by the inclusion of thickened waste
activated sludge.
The sludge quantities for the 4 MGD wastewater treatment plant
were developed as follows:
Influent BOD5
Influent 4.0 MGD x 8.34 x 200 mg/1 =6,672 #/day
Primary removal = 6,672 x 0.3 = 2,002 #/day
BOD5 remaining in settled sewage = 4,670 #/day
Influent Suspended Solids
Influent 4 x 8.34 x 240 mg/1 = 8,006 #/day
Primary removal = 8,006 x 0.65 = 5,204 #/day
Suspended solids remaining in settled sewage = 2,802 #/day
Waste Activated Solids
Biological = 6,672 x 0.60 x 0.5 #VSS/#BOD5 = 2,002 #VSS/day
Suspended solids = 8,006 x 0.25 = 2.002 #/day
Total Biological Solids Produced = 4,004 #/day
Net Daily Sludge Quantities
Primary: 5,204 #/day 0 7% following thickening
5.204 = 8,740 gpd
8.34 x 1.02 x 0.07
62
-------
Waste Activated Sludge
4.004 = 19,014 gpd
0.025 x 8.34 x 1.01
Net Sludge Produced (5,204 + 4,004) = 9,208 #solids/day
Volume = (8,740 + 19,014) = 27,754 gpd
% solids = 3.9%
Design sludge quantities were developed for the 40 MGD facility
in an identical manner. The design sludge quantities are summarized
as follows:
4.0 MGD 40 MGD
WWTP WWTP
Primary Sludge Solids, Ib/day 5,204 52,040
Primary Sludge Volume @ 7%, gal/day 8,740 87,400
Biological Sludge Solids, Ib/day 4,004 40,040
Biological Sludge Volume @ 2.5%, gal/day 19,014 190,140
Total Sludge Solids, Ib/day 9,208 92,080
Combined Sludge Volume, gal/day 27,754 277,540
Combined Sludge Percent Solids 3.9 3.9
For simplicity, the design examples for the 4 and 40 MGD treat-
ment plants will be presented separately. Each example will include
the design basis for each alternative stabilization and ultimate dis-
posal process, final sludge volumes, capital and annual operation and
maintenance costs.
Process Alternatives - 4 MGD WWTP
As previously discussed, process alternatives for the 4 MGD
wastewater treatment plant will be as follows:
1. Lime stabilization followed by liquid application to farm-
land.
63
-------
2. Anaerobic digestion followed by liquid application to farm-
land.
Lime Stabilization. A flow diagram for the proposed lime stabili-
zation facilities is shown on Exhibit 16. Significant process equipment
includes a bulk lime storage bin for pebble quicklime, auger, lime
slaker and feed slurry tank, sludge mixing and thickening tank with a
mechanical mixer, sludge grinder, all weather treatment building, elec-
trical and instrumentation, interconnecting piping and transfer pumps,
and 60 day detention treated sludge holding lagoon. The basis for design
is as follows:
Total sludge solids, Ib/day
Sludge volume, gal/day
Raw sludge percent solids
Overall lime dosage required per unit
dry solids, as 100% Ca(OH)2
Daily lime requirement as Ca(OH)2
Treatment period
Bulk lime storage bin volume minimum
Bulk lime storage bin detention time
Lime slaker and slurry tank capacity
(2 ea)
Influent sludge grinder capacity (spared)
Sludge mixing tank volume
Sludge mixing tank dimensions
Sludge mixer horsepower
Sludge mixer turbine diameter
Turbine speed
Sludge transfer pump capacity (spared)
Treated sludge volume
Treated sludge percent solids
9,208
27,754
3.9
0.20 Ib/lb
1,826 Ib/day
6 hrs/day
28 cu m (1,000 cf)
34 days
200-300 Ib CaO/hr
200 gpm
25,000 gal
IS'xlS'xlO' SWD
15 HP
53"
68 rpm
400 gpm
24,050 gal
4.5
64
-------
Ln
CNLORINATION
TREATED
EFFLUENT
TO DISCHAR6E
LIQUID SLUDGE
TO LAND APPLICATION
lr
TANK TRUCK
LAGOON
SLUDGE FROM LAGOON
Figure 16. 4 MGD Lime Stabilization / Truck Haul 8 Land Application
-------
Sludge holding lagoon total volume
(4 cells) 240,000 cf
Sludge holding lagoon maximum detention
time 60 days
Treatment building floor area 250 sf
Treatment building construction brick and block
Instrumentation pH record
treated sludge
volume
Capital costs for the lime stabilization facilities were based on
January 1, 1978 bid date and were as follows:
Site work, earthwork, yard piping $ 26,000
Lime storage bin and feeders 84,000
Treatment tank, pumps, sludge
grinders, and building structure 142,000
Electrical and instrumentation 29,000
Sludge holding lagoon 54,000
Subtotal Construction Cost $335,000
Engineering 36.000
Total Capital Cost $371,000
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081) $ 30,100
Annual capital cost per ton
dry solids $ 17.91
Lime stabilization operation assumed one man, eight hours per day,
365 days per year, at $6.50 per hour, including overhead. Maintenance
labor was assumed to be 156 hours per year labor at $6.50 per hour
and $2,400 per year for maintenance materials. The total quantity of
85% CaO quicklime required was 297 tons per year at $40 per ton.
The total annual cost for lime stabilization, excluding land ap-
plication of treated sludge, has been calculated as follows and is sum-
marized in Table 22.
66
-------
Lime Stabilization Operating Costs
Labor: 8 hr/day x 365 day/yr x $6.50/hr = $18,980 say $19,000
Maint. labor: 156 hr/yr x $6.50 = $1,014 say $1,000
Maint. materials: $2,400/yr lump sum
Lime Primary: 5,204 #/day x 0.12# Ca(OH)2/# = 624 #/day
Waste Activated: 4,004 #/day x 0.3# Ca(OH)2/# = 1,201 #/day
Total Lime = (624 + 1,201) = 1,825# Ca(OH)2/day
1,825 #/day/0.85) x 56/74 = 1,625 0/day CaO
1,625 x 365/2,000 = 297 ton/yr
say 300 ton/yr x $40/ton = $12,000/yr
Laboratory: $l,500/yr lump sum
Capital: $371,000 x 0.081 = $30,100/yr
Table 22
TOTAL ANNUAL COST FOR LIME STABILIZATION
EXCLUDING LAND DISPOSAL FOR A 4 MGD PLANT
Annual
Total Cost
Annual Per Ton
Item Cost Dry Solids
Operating labor $19,000 $11.31
Maintenance labor and
materials 3,400
Lime 12,000
Laboratory 1,500
Capital 30.100
Total Annual Cost $66,000
Both the lime stabilization and anaerobic digestion alternatives
were assumed to utilize land application of treated sludge as a liquid
hauled by truck. The capital cost per sludge hauling vehicle was as-
sumed to be $35,000, which was depreciated on a straight-line basis
over a five year period.
The assumed hauling distance was three to five miles, round trip.
Mauling time assumed 10 minutes to fill, 15 minutes to empty, and 10
67
-------
minutes driving, or a total of 35 minutes per round trip. The truck
volume was assumed to be 1,500 gallons per load. The cost of truck
operations, excluding the driver and depreciation, was assumed to be
$8.50 per operating hour. The truck driver labor rate was assumed to be
$6.50 per hour, including overhead.
Truck operation time was based on hauling on a five day per week
basis, approximately ten months per year, which results in the as-
sumed 215 hauling days per year. The average volume hauled is 40,800
gallons per day. Two trucks were assumed to be required, with a com-
bined total of 28 loads per day.
Although it may be possible to obtain the use of farmland at no
cost, e.g., on a voluntary basis, the land application economic analysis
assumed that land would be purchased at a cost of $750 per acre.
Sludge application rates were assumed to be ten dry tons per acre per
year. Land costs were amortized at 7% interest over a 30 year period.
To offset the land cost, a fertilizer credit of $7.30 per ton of
dry sludge solids was assumed. This rate was arbitrarily assumed to be
50% of the value published by Brown(11) based on medium fertilizer
market value and low fertilizer content. The reduction was made to re-
flect resistance to accepting sludge as fertilizer. The land cost was
further offset by assuming a return of $50 per acre, either as profit
after farming expenses or as the rental value of the land.
Capital and annual operation and maintenance costs for land appli-
cation of lime stabilized sludge were calculated as follows and have
been summarized in Table 23.
68
-------
Lime Stabilization Land Application Costs
Land: 9,208 #solids/day x 365 days/2,000 #/ton = 1,681 ton/yr
1,681 ton/yr/10 ton/acre = 168 acres say 200
200 acres x $750/acre = $150,000
$150,000 x 0.081 = $12,150/yr say $12,200
Truck depreciation: $35,000 x 2 = $70,000 capital
$70,000/5 yrs = $14,000/yr
Truck driver: 40,800 gal/day/2,571 gal/truck/hr = 15.9 hr/day
say 2 trucks @ 8 hr/day
$6.50 x 2 men x 8 hr/day = $104/day
$104 x 215 = $22,360 say $22,400/yr
Truck operation: 2 trucks x 8 hr/day x $8.50/hr = $136.00/day
$136.00 x 215 = $29,240 say $29,200/yr
Laboratory: $l,500/yr lump sum
Fertilizer credit: 1,681 ton/yr x $7.30/ton = $12,271 say
$12,300/yr
Land credit: 168 acres x $50/acre = $8,400/yr
Table 23
ANNUAL COST FOR LAND APPLICATION
OF LIME STABILIZED SLUDGE FOR A 4 MGD PLANT
Annual
Cost
Total Per Ton
Annual Dry
Item Cost Solids
Amortized cost of land $12,200
Truck depreciation 14,000
Truck driver 22,400
Truck operation 29,200
Laboratory 1,500
Fertilizer credit (12,300)
Land credit (8,400)
Total Annual Cost $58,600
69
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Anaerobic Digestion. A flow diagram for the proposed anaerobic
sludge digestion facilities is shown on Exhibit 17. Two-stage anae-
robic digestion was assumed with stabilized sludge being hauled to
farmland. Sludge storage was allowed in the digester design and no
lagoon was included. The basis for design for the anaerobic digesters
for the 4 MGD treatment plant was as follows:
First Stage
Feed solids loading
Feed volume
Feed percent solids
Feed percent volatile solids
Digester dimensions
Digester volume
Mixers
Hydraulic detention time
Loading rate
Digester bulk temperature
Average feed temperature
Volatile solids reduction
Overall total solids reduction
Sludge heaters
9,208 Ib/day
27,754 gal/day
3.9
65
60* x 25' SWD
529,000 gal
2 ea @ 3,500 gpm
19 days
0.085 #VSS/ft3/day
95° F.
55° F.
50%
32%
3 ea @ 500,000 BTU/hr
Second Stage
Digester dimensions
Digester volume
Hydraulic detention time
Digester gas production
Digester gas heat value
Digested sludge dry solids production
Digested sludge percent solids
Sludge recirculation pumps (2 ea)
60' x 25' SWD
529,000 gal
19 days
10 cf/lb VSS feed
500 BTU/ft3
6,261 Ib/day
6.5%
500 gpm ea
70
-------
INFLUENT
COMBINED SLUDeC
PRIMARY
SLUDGE-*
ACTIVATED
SLUDGE
RETURN SLUDGE
'-WASTE ACTIVATED SLUD0E
RECYCLE SUPERNATANT
GRAVITY
THICKENER
THICKENED SLUDGE
TREATED
EFFLUENT
TO OMCHARtE
LIQUID SLUDGE
TO LAND APPLICATION
TANK TRUCK
Figure 17. 4 MGD Anaerobic Digestion/Truck Haul 8 Land Application
-------
Capital cost for the anaerobic sludge digestion facilities, in-
cluding the control building, structures, floating cover, heat ex-
changer, gas safety equipment, pumps, and interconnecting piping, as-
suming January 1, 1978 bid date, and engineering, legal, and administra-
tive costs is as follows:
Site work, earthwork, yard piping & pumps $ 151,000
Digesters 675,000
Control building 251,000
Electrical and instrumentation 125.000
Subtotal Construction Cost $1,202,000
Engineering 107.000
Total Capital Cost $1,309,000
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081) $ 106,000
Annual capital cost per unit
feed dry solids $ 63.08
Digester operation assumed one man, three hours per day, 365 days
per year at $6.50 per hour, including overhead. Maintenance labor and
material assumed 416 hours per year at $6.50 per hour and $7,000 per
year for maintenance materials.
The cost of anaerobic digester operation was offset by assuming a
value of $2.70 per million BTU for all digester gas produced above the
net digester heat requirements.
The total annual cost for anaerobic sludge digestion, excluding
land application was calculated as follows and has been summarized in
Table 24.
72
-------
Anaerobic Digester O&M Cost
Operator labor: 3 hr/day x 365 day/yr x $6.50/hr = $7,118/yr
say $7,100/yr
Maintenance labor: 416 hr/yr x $6.50/hr = $2,704 say $2,700/yr
Maintenance materials: $7,000/yr lump sum
Laboratory: $l,500/yr lump sum
Capital: $1,309,000 x 0.081 = $106,000
Fuel credit: 9,208# x 0.65 = 5,985 #VSS feed/day
5,985# x 10 cf/#VSS = 59,850 cf/day gas
59,850 cf x 500 BTU/ft3 = 29.9 x 106 BTU/day
475,000 BTU/hr x 24 hr/day/0.5 eff = 22.8 x 106
BTU/day required for digester heat
29.9 x 106 - 22.8 x 106 = 7.1 x 106 BTU/day
excess gas
7.1 x 106 x $2.70 x 10"6 x 365 = $6,997 say
$7,000/yr
Table 24
TOTAL ANNUAL COST FOR TWO-STAGE ANAEROBIC SLUDGE
DIGESTION EXCLUDING LAND DISPOSAL FOR A 4 MGD PLANT
Annual
Total Cost
Annual Per Ton
Item Cost Dry Solids
Operating labor $ 7,100 $ 4.23
Maintenance labor and materials 9,700 5.77
Laboratory 1,500 0.89
Capital 106,000 63.08
Fuel credit (7.000) (4.16)
Total Annual Cost $117,300 $69.81
73
-------
Land application costs were developed for the anaerobic digestion
alternative in a manner similar to that previously described for lime
stabilization. Anaerobically digested sludge land requirements were
less than for lime stabilized sludge because of the volatile solids re-
duction during digestion. Truck driving and operation costs were simi-
larly less for digested sludge because of the volatile solids reduc-
tion and more concentrated sludge (6.5% vs 4.5%) which would be
hauled. The total fertilizer credit was based on $7.30 per ton of dry
solids, but was lower because of the lower amount of dry solids ap-
plied to the land. The total land credit was less because land re-
quirements were based on the total amount of sludge solids applied.
Land application costs for the anaerobic digestion alternative were
calculated in a manner similar to those for the lime stabilization al-
ternative and are summarized in Table 25.
Table 25
ANNUAL COST FOR LAND APPLICATION OF
ANAEROBICALLY DIGESTED SLUDGES FOR A 4 MGD PLANT
Total
Annual
Item Cost
Amortized cost of land $ 8,200
Truck depreciation 7,000
Truck driver 11,200
Truck operation 14,600
Laboratory 1,500
Fertilizer credit (8,300)
Land credit (5.700)
Total Annual Cost $28,500 $16.96
74
-------
The total annual capital and annual operation and maintenance
costs for lime stabilization and two-stage anaerobic sludge digestion,
including land application for a 4 MGD wastewater treatment plant, are
summarized in Table 26.
Table 26
COMPARISON OF TOTAL ANNUAL CAPITAL AND ANNUAL
O&M COST FOR LIME STABILIZATION AND ANAEROBIC DIGESTION
INCLUDING LAND DISPOSAL FOR A 4 MGD PLANT
Lime Stabilization Anaerobic Digestion
Facilities
Amortized capital
Operating labor
Maintenance labor &
materials
Lime
Laboratory
Fuel credit
Subtotal Facilities
Land Application
Amortized cost of land
Truck depreciation
Truck drivers
Truck operations
Laboratory
Fertilizer credit
Land credit
Subtotal Land
Application
Total Annual Cost
Facilities and Land
Application
Total
Annual
O&M
Cost
Annual
Cost
Per
Ton Dry
Solids
Total
Annual
O&M
Cost
Annual
Cost
Per
Ton Dry
Solids
$
30
19
3
12
1
,100
,000
,400
,000
,500
$1
1
N/A
$
66
,000
7.
1.
2.
7.
0.
91 $
31
02
14
89
N/A
$39.
27 $
106
7
9
1
(7
117
,000
,100
,700
,500
,000)
,300
$63.
4.
5.
0.
(4-
$69.
08
23
77
89
16)
81
$ 12,200
14,000
22,400
29,200
1,500
(12,300)
(8,400)
$ 7.26 $
8.33
13.33
17.38
0.89
(7.30)
(5.00)
8,200
7,000
11,200
14,600
1,500
(8,300)
(5,700)
$ 4.88
4.17
6.66
8.69
0.89
(4.94)
(3.39)
$ 58,600 $34.89 $ 28,500 $16.96
$124,600 $74.16 $ 145,800 $86.77
75
-------
Process Alternatives - 40 MGD WWTP
As previously discussed, process alternatives for the 40 MGD
wastewater treatment plant will be as follows:
1. Lime stabilization followed by pipeline transportation to
the land application site.
2. Anaerobic digestion followed by mechanical dewatering and
land application.
The design logic which will be followed to develop and evaluate the
sludge handling alternatives has previously been summarized on Figure
14. Wastewater characteristics, treatment unit efficiencies, and sludge
characteristics have also been previously summarized.
Lime Stabilization. A flow diagram for the proposed lime stabili-
zation facilities is shown on Figure 18. Significant process equipment
includes a bulk lime storage bin for pebble quicklime, augers, lime
slakers and feed slurry tanks, sludge mixing tanks, sludge thickeners,
sludge grinders, all weather treatment building, electrical and instru-
mentation, interconnecting piping, and sludge pump stations.
The sludge pipeline was assumed to be ten miles long with two in-
termediate pump stations. One land application farm site was assumed.
A sludge storage lagoon with 60 days holding capacity was provided at
the land application site.
The basis for design is as follows:
Total sludge solids, Ib/day 92,080
Sludge volume, gal/day 227,540
Raw sludge percent solids 3.9
Overall lime dosage required per
unit dry solids as 100% Ca(OH)2 0.20 Ib/lb dry solids
Daily lime requirement as Ca(OH)2 18,250 Ib/day
76
-------
INFLUENT
COMBINED SLUDGE
PRIMARY
SLUDGE-*
ACTIVATED
SLUDGE
SECONDARY
CHUNUNATION
EFFLUENT
TO DISCHARGE
RETURN SLUDGE
••-WASTE ACTIVATED SLUD6E
THICKENER OVERFLOW
TANK TRUCK
LIQUID SLUDGE
TO LAND APPLICATION
LAGOON
THICKENED
STABILIZED
SLUDGE
Figure 18. 40 MGD Lime Stabilization/Pipeline Transport a Land Application
-------
Treatment period
Bulk lime storage bin volume minimum
Bulk lime storage bin detention time
Lime slaker & slurry tank capacity
(2 ea)
Influent sludge grinder max capacity
Sludge mixing tank volume @ 1 hr
detention time (2 ea)
Sludge mixing tank dimensions
Sludge mixer horsepower (2 ea)
Sludge mixer turbine diameter
Turbine speed
Sludge thickener dimensions (2 ea)
Thickened sludge volume
Thickened sludge percent solids
Sludge transfer pump capacity (2 ea)
Intermediate pump station pumps
Treatment building floor area
Treatment building construction
Instrumentation
Lagoon volume at application site
Pipeline length
Pipeline diameter
Pipeline working pressure
Land application trucks
24 hrs/day
2 ea 4,260 ft3
30 days
500-750 CaO/hr
2 ea 200 gpm
12,000 gal
10' x 10' SWD
10 HP
51
45 rpm
65' dia x 12' SWD
240,500 gal/day
4.5
250 gpm @ 200 psi
4 ea 250 gpm @ 200 psi
600 ft2
brick and block
pH record/control
raw sludge volume
treated sludge volume
pipeline pressure
control
10,000,000 gal (20 cells)
53,000'
6"
200-250 psig
12 @ 1,500 gal ea
Capital costs for the lime stabilization facilities, based on
January 1, 1978 bid date, excluding final sludge pumping, pipeline,
application trucks, lagoon, and land, were as follows:
78
-------
Site work, earthwork & yard piping $ 95,000
Lime storage, slakers, and feed 106,000
Lime treatment tanks, mixers,
grinders & building 155,000
Sludge thickeners 529,000
Electrical & instrumentation 102.000
Subtotal Construction Cost $ 987,000
Engineering 90,000
Total Capital Cost $1,077,000
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081) $ 87,200
Annual capital cost per unit
feed dry solids $ 5.19
Lime stabilization operation assumed two men, three shifts per day,
365 days per year at $6.50 per hour, including overhead. Maintenance
labor was assumed to be 1,664 hours per year at $6.50 per hour and
$7,500 per year for maintenance materials. The total quantity of 85%
CaO quicklime required was 2,966 tons per year at $40 per ton.
The total annual cost for lime stabilization, excluding land ap-
plication of treated sludge, was calculated in a manner to that prev-
iously shown on the 4 MGD example and have been summarized in Table
27.
Table 27
TOTAL ANNUAL COST FOR LIME STABILIZATION
EXCLUDING LAND DISPOSAL FOR A 40 MGD PLANT
Annual
Cost
Total Per
Annual Ton Dry
Item Cost Solids
Operating labor $114,000 $ 6.78
Maintenance labor and materials 18,300 1.09
Lime 118,600 7.06
Power 2,000 0.12
Laboratory 4,500 0.27
Total Annual Cost $257,400 $15.32
79
-------
Ultimate sludge disposal was assumed to be as a liquid on farmland
with truck spreading. The total land spreading operation will require
2,000 acres. Land cost was assumed to be $1,250 per acre to reflect
the more urban setting than the 4 MGD case. The capital cost per
sludge hauling vehicle was assumed to be $35,000, with 12 being re-
quired. The vehicles were depreciated over a seven year period. The
sludge holding lagoon was located at the farm site and was sized to hold
60 days sludge production. The lagoon was partitioned into 500,000
gallon cells to permit access and efficient utilization of the storage
volume.
The assumed hauling time was 10 minutes to fill, 20 minutes to
haul, empty and return, for a total of 30 minutes per round trip. The
truck volume was assumed to be 1,500 gallons per load. The cost of
truck operations, excluding the driver and depreciation, was assumed to
be $8.50 per operating hour. The truck driver labor rate was assumed
to be $6.50 per hour, including overhead.
Truck operating time was based on hauling on a 215 day per year
schedule, 12 hours per day.
To offset the land cost, a fertilizer credit of $7.30 per ton of
dry sludge solids was assumed. This rate was arbitrarily assumed to be
50% of the value published by Brown' ' based on medium fertilizer
market value and low fertilizer content. The reduction was made to re-
flect resistance to accepting sludge as fertilizer. The land cost was
further offset by assuming a return of $50 per acre, either as profit
after farming expenses, or as the rental value of the land.
Easements for the sludge pipeline were assumed to cost $2.50 per
foot. Two intermediate booster stations were provided to maintain a rea-
sonable pressure profile along the line. Progressive cavity pumps
80
-------
were used for both the treatment plant and intermediate pump stations.
Allowance was assumed to permit regular cleaning of the line by utili-
ing pipeline "pigs."
Annual operation and maintenance costs for transportation and land
application of lime stabilized sludge were calculated in a manner sim-
ilar to that previously summarized and have been shown in Table 28.
Capital costs for the lime stabilization land application site,
based on January 1, 1978 bid date, have been summarized as follows:
Site work, earthwork $ 17,000
Sludge transfer pumps 45,000
Sludge pipeline 675,000
Booster station 104,000
Sludge lagoon 569,000
Electrical & instrumentation 19.000
Subtotal Construction Cost $1,429,000
Engineering 124,000
Total Capital Cost Pipeline,
Pump Stations & Lagoon $1,553,000
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081) $ 125,800
Annual capital cost per unit
feed dry solids $ 7.49
81
-------
Table 28
ANNUAL COST FOR TRANSPORTATION AND LAND APPLICATION
OF LIME STABILIZED SLUDGE FOR A 40 MGD PLANT
Item
Land
Easements
Pipeline, pump stations
& lagoon
Truck depreciation
Truck drivers
Truck operation
Power
Pipeline operation & maintenance
Laboratory
Fertilizer credit
Land credit
Total Annual Cost
Capital
Cost
$2,500,000
132,000
1,553,000
420,000
$4,605,000
Total
Annual
Cost
$202,500
10,700
125,800
60,000
201,200
263,200
35,000
17,000
4,500
(122,700)
(84,000)
$713,200
Annual Cost
Per Ton Dry
Solids
$12.05
0.64
7.49
3.57
11.97
15.66
2.08
1.01
0.27
(7.30)
(5.00)
$42.44
Anaerobic Digestion. A flow diagram for the proposed anaerobic
digestion/vacuum filtration alternative is shown on Figure 19. Sig-
nificant process equipment includes two-stage standard rate anaerobic
sludge digestion, bulk lime and ferric chloride storage, lime slakers,
vacuum filtration, sludge conveyors, and sludge storage bin. All fa-
cilities were assumed to be housed in an all weather brick-block type
building and included all electrical, instrumentation, interconnecting
piping, and sludge pumps. The existing sludge dewatering equipment was
assumed not to be capable of functioning over the project life and was
replaced. Similarly, the existing filter building and chemical feed
facilities were replaced.
Design data for the anaerobic digester alternative is as follows:
82
-------
TREATED
EFFLUENT
00
U)
-. -
THICKENED
SLUDGE
_^*
c
—• •- —
o en
fi STAGE
WAEROBIC
DIOESTER
[9 EACH)
~«^
13
STA6E
ROBJC
STER
:ACH)
'
T DIGESTED
C
JL
>
VACUUM
FILTER
(3 EACH)
TRUCK TO
LAND APPLICATION SITE
SLUDGE
Figure 19. 40 MGD Anaerobic Digestion / Vacuum Filtration 8 Land Application
-------
Primary digesters
Secondary digesters
Vacuum filtration
Vacuum filter loading rate
Lime storage bin
Lime slaker/feeders
Ferric chloride storage tanks
Dewatered sludge storage bin
Filter building
Digester loading - 1st stage
Hydraulic detention time - 1st stage
Digester gas production
Digester gas heat value
Volatile solids reduction
Overall solids reduction
Sludge mixers
Digester heat requirement (primary
only)
Gas production
Net gas available
3 ea 110' x 30' SWD
3 ea 110' x 30' SWD
3 ea @ 400 ft2 ea
3.50 dry solids/ft2/hr
1 ea 4,000 ft3
3 @ 250-500* CaO/hr
2 ea @ 5,000 gal ea
1 ea @ 2,000 ft3
3,000 ft2 w/basement
0.07 #VSS/ft3/day
23 days
10 ft3/lb VSS feed
500 BTU/ft3
50%
32%
4 @ 5,000 gpm ea
22.7 x 107 BTU/day
30.0 x 107 BTU/day
7.3 x 107 BTU/day
Annual capital costs operation and maintenance for the anaerobic
digestion facilities were based on the January 1, 1978 bid date and
have been summarized in Table 29. Capital costs included the digesters,
control buildings, covers, heat exchangers, gas safety equipment, in-
terconnecting piping, engineering, legal and administrative costs.
Capital costs are summarized as follows:
Site work, earthwork, yard piping
Digesters & control building
Pumping
Electrical & instrumentation
Subtotal Construction Cost
Engineering
Total Capital Cost
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081)
Annual capital cost per unit
feed dry solids
84
$ 688,000
7,222,000
35,000
745.000
$8,690,000
649.000
$9,339,000
$ 756,500
$ 45.02
-------
Digester operation assumed one man, two shifts per day, 365 days
per year, at $6.50, including overhead. Maintenance labor and material
assumed 4,160 hours per year at $6.50 per hour and $30,000 per year for
maintenance materials.
The cost of anaerobic digester operation was offset by assuming a
value of $2.70 per million BTU for all digester gas produced above the
net digester heat requirement. Total annual operation and maintenance
cost for the digestion facilities is summarized in Table 29.
Table 29
TOTAL ANNUAL COST FOR TWO-STAGE ANAEROBIC
SLUDGE DIGESTION EXCLUDING VACUUM FILTRATION
AND LAND DISPOSAL FOR A 40 MGD PLANT
Annual
Cost
Total Per
Annual Ton Dry
Item Cost Solids
Operating labor $ 38,000 $ 2.26
Maintenance labor and materials 57,000 3.39
Laboratory 6,000 0.36
Capital 756,500 45.02
Fuel credit (71.900) (4.28)
Total Annual Cost $785,600 $46.75
Vacuum filtration costs were estimated as summarized in Table 30.
Capital costs for the filtration facilities were as follows:
85
-------
Site work, earthwork, yard piping
Chemical storage & feed
Filtration equipment
Filter & chemical building
Sludge loading pad
Electrical & instrumentation
Subtotal Construction Cost
Engineering
Total Capital Cost
Amortized cost @ 30 yrs., 7% int.
(CRF = 0.081)
Annual capital cost per unit
feed dry solids
$ 297,000
177,000
546,000
230,000
78,000
322.000
$1,650,000
140.000
$1,790,000
$ 145,000
$ 8.63
Table 30
VACUUM FILTRATION CAPITAL AND ANNUAL
OPERATION & MAINTENANCE COSTS FOR A 40 MGD PLANT
Item
Variable cost
Electric power
Chemicals
Lime
FeCl3
Maintenance materials
Maintenance labor
Laboratory
Subtotal Variable Cost
Operator labor
Supervision
Capital
Subtotal Fixed Cost
Total Annual Cost
Total
Annual
Cost
$ 7,100
Annual
Cost
Per
Ton Dry
Solids
$ 0.42
91,400
52,000
7,800
25,800
6,000
$190,100
$ 47,000
15,000
145,000
$207,000
$397,100
5.44
3.09
0.46
1.54
0.36
$11.31
$ 2.80
0.89
8.63
$12.32
$23.63
86
-------
Land application costs were calculated based on hauling 20 miles
round trip. A sludge transfer site was assumed to be located at the
land application site. Sludge transfer trucks were assumed to be
equipped with eight cubic yard dump beds. A total of four trucks were
required, operating 8 hours per day, 215 days per year. The loader and
land spreading vehicle were assumed to operate eight hours per day.
Land application vehicles were assumed to have 17 cubic yard capacity.
Sludge application rate assumed seven dry tons per hour, including
loading time. The land application vehicle was depreciated on a
straight-line basis over a seven year period. Sludge hauling was based
on current rental costs for equipment. Dewatered sludge was assumed to
be 22% dry solids.
Anaerobically digested sludge land requirements were less than for
lime stabilized sludge because of the volatile solids reduction during
digestion. The fertilizer value and land rental return credits were
taken as previously described in the 4 MGD design case. Table 31 sum-
marizes the total land application cost.
Table 31
ANNUAL COST FOR LAND APPLICATION OF OEWATERED
ANAEROBICALLY DIGESTED SLUDGES FOR A 40 MGD PLANT
Annual
Cost
Total Per
Annual Ton Dry
Item Cost Solids
Amortized cost of land $202,500 $12.05
Truck depreciation (spreader only) 12,100 0.72
Truck drivers 67,100 3.99
Truck and loader operation 260,600 15.51
Laboratory 4,500 0.27
Fertilizer credit (83,400) (4.96)
Land credit (57.000) (3.39)
Total Annual Cost $406,400 $24.19
87
-------
To summarize, the total cost for the lime stabilization and anae-
robic digestion alternatives, including ultimate disposal, are shown in
Table 32.
Table 32
COMPARISON OF TOTAL ANNUAL CAPITAL AND ANNUAL
O&M COST FOR LIME STABILIZATION AND ANAEROBIC DIGESTION
INCLUDING LAND DISPOSAL FOR A 40 MGD PLANT
Lime Stabilization Anaerobic Digestion
Facilities
Amortized capital
lime stabilization
Amortized capital
digesters
Amortized capital
filtration
Operating labor
Maintenance labor &
materials
Chemicals
Laboratory
Fuel credit
Power
Subtotal Facilities
Land Application
Amortized cost of land
facilities &
easements
Truck depreciation
Truck drivers
Truck operations
Pipeline O&M
Power
Fertilizer credit
Land credit
Laboratory
Subtotal Land
Application
Total Annual Cost
Facilities and
Land Application
Total
Annual
O&M
Cost
$ 87,200
N/A
N/A
114,000
18,300
118,600
4,500
N/A
2,000
$ 344,600
$ 339,000
60,000
201 ,200
263,200
17,000
35,000
(122,700)
(84,000)
4,500
$ 713,200
$1,057,800
Annual
Cost
Per
Ton Dry
Solids
$ 5.19
N/A
N/A
6.78
1.09
7.06
0.27
N/A
0.12
$20.51
$20.17
3.57
11.97
15.66
1.01
2.08
(7.30)
(5.00)
0.27
$42.43
$62.94
Total
Annual
O&M
Cost
N/A
$ 756,500
145,000
100,000*
90,600*
143,400*
12,000*
(71,900)
7,100*
$1,182,700
$ 202,500
12,100
67,100
260,600
N/A
N/A
(83,400)
(57,000)
4,500
$ 406,400
$1,589,100
Annual
Cost
Per
Ton Dry
Solids
N/A
$45.02
8.63
5.95*
5.39*
8.53*
0.71*
(4.28)
0.42*
$70.37
$12.05
0.72
3.99
15.51
N/A
N/A
(4.96)
(3.39)
0.27
$24.19
$94.56
Mncludes cost for digestion and vacuum filtration
88
-------
In the 4 MGD case, the total annual cost for the lime stabiliza-
tion alernative is $74.16 per dry ton compared to $86.77 per dry ton
for anaerobic digestion. Each of these alternatives assumed liquid
application to farmland, with a 3-5 mile round trip hauling distance.
With increasing haul distances, lime stabilization will be decreas-
ingly cost effective because of the greater volume of sludge which
must be transported.
In the 40 MGD case, the total annual cost for the lime stabiliza-
tion alternative is $69.24 per dry ton compared to $94.56 per dry ton
for anaerobic digestion. The cost of pipeline transportation/land ap-
plication of the liquid sludge is $62.94 per dry ton compared to $47.82
per dry ton for dewatering and land application. The pipeline alter-
native also has the disadvantage of being inflexible for long-term im-
plementation. With the dewatered sludge and truck hauling system,
sites could be changed with little difficulty.
89
-------
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Flotation as a Primary Unit Process in a Multiple System for the
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943 (1969).
3. Buzzell, J. C., Jr., and Sawyer, C. N., "Removal of Algal Nutrients
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1975.
90
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10. Noland, R. F., Edwards, J. 0., "Stabilization and Disinfection of
Wastewater Treatment Plant Sludges," USEPA Technology Transfer
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11. Brown, R. E. et al, "Ohio Guide for Land Application of Sewage
Sludge," Ohio Agricultural Research and Development Center, Wooster,
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Sewage Sludges," Journal of Environmental Quality 5:303-306.
14. Stern, Gerald, "Reducing the Infection Potential of Sludge Disposal."
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1974.
16. U. S. Environmental Protection Agency, "Municipal Sludge Manage-
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108, p. 22533.
17. Trubnick, E. H., Mueller, P. K., "Sludge Dewatering Practice,"
Sewage and Industrial Wastes 30, 1364 (1958).
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watering," Proc. 3rd Int'l Conference, Water Pollution Research,
Munich, 1966, in Advances in Water Pollution Research.
91
-------
19. Zenz, D. R. , Lynam, B. T., et al, "USEPA Guidelines on Sludge
Utilization and Disposal - A Review of Its Impact Upon Municipal
Wastewater Treatment Agencies," presented at the 48th Annual WPCF
Conference, Miami Beach, Fla., 1975.
20. National Lime Association, "Lime Handling Application and Storage
in Treatment Processes Bulletin 213," National Lime Assoc., Washington
D.C., pp 1-3.
21. Badger and Banchero, "Introduction to Chemical Engineering," page
614, McGraw-Hill, 1955.
22. Hicks, R. W. et al, "How to Design Agitators for Desired Process
Response," Chemical Engineering. April 26, 1976, pp 103-106 ff.
23. Fair, G. M. and Geyer, J. C., "Water Supply and Wastewater Dis-
posal," John Wiley & Sons, New York, 1956.
24. USEPA, "Methods for Chemical Analysis of Wastes," USEPA, Cincinnati,
Ohio, 1974.
25. Standard Methods for Examination of Water and Wastewater, 13th &
14th Editions, AWWA, APHA, WPCF, American Public Health Association,
Washington, O.C.
26. "Enumeration of Salmonella and Pseudomonas aeruginosa," Journal
WPCF, Vol #46, No. 9, Sept. 1974, pp 2163-2171.
92
-------
APPENDIX
93
-------
13.0
12.0-
II.O"
10.0"
X
Q.
6% PRIMARY SLUDGE
3.5% PRIMARY SLUDGE
3% PRIMARY SLUDGE
4.5% PRIMARY SLUDGE
PRIMARY SLUDGE
PRIMARY SLUDGE
3% PRIMARY SLUDGE
3.5% PRIMARY SLUDGE
----- - 4% PRIMARY SLUDGE
4.5% PRIMARY SLUDGE
5% PRIMARY SLUDGE
6%PRIMARY SLUDGE
1,000
2JDOO 3,000
DOSAGE Co (OH)2 MG/L
4,000
5pOO
Figure 2. Lime Dosage vs pH Primary Sludge
-------
13.0
12.0-
11.0-
10.0-
I
o.
9.0
8.0-
7.0 •
6.0
6.5%
7.0%
7.5%
. 0
2,000 4JDOO 6,000 8,000
DOSAGE Ca(OH)2 MG/L
10,000
Figure 3. Lime Dosage vs pH Anaerobic Digested Sludge
95
-------
13.0
X
Q.
1,000 2,000 3,000 4,000
DOSAGE Co (OH)2 MG/L
5,000
Figure 4. Lime Dosage vs pH Waste Activated Sludge
96
-------
13.0
12.0
i i.o-
10.0
X
CL
1,000 2,000 3,000 4,000
DOSAGE Co (OH)2 MG/L
5,000
Figure 5. Lime Dosage vs pH Septage
97
-------
ANAEROBIC DIGESTION
OF
MUNICIPAL WASHWATER SLUDGES
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO 45268
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
N, A, MlGNONE
ENVIREX INC,
WAUKESHA, WISCONSIN 53186
-------
The claimed advantages of the anaerobic digestion process are
(1,2):
1
2
3
4
Low sludge production
The production of a useful gas of moderate caloric
value
A high kill rate of pathogenic organisms
Production of a solids residue suitable for use as
a soil conditioner
Low operating cost.
Table 1 Indicates the kinds of sludge which have been studied
on a full scale basis.
TABLE I: TYPE AND REFERENCE OF FULL SCALE STUDIES ON HIGH RATE
ANAEROBIC DIGESTION OF MUNICIPAL WASTEWATER SLUDGE
Reference On Reference On
Hesoph111c Thermophlllc
Primary and L1me
Primary and Ferric Chloride
Primary and Alum
Primary and Trickling Filter
Primary, Trickling Filter and Alum
Primary and Waste Activated
Primary, Waste Activated and Lime
Primary, Waste Activated and Alum
Primary, Waste Activated and Ferric
Chloride
Primary, Haste Activated and
Sodium Alumlnate
Waste Activated Only (Pilot Plant
only)
3.4
5
6
7,8
9
10,11,12
15,16
15,17,18
15
17,18
19,20.21
11.13,14
19,20,21
In the past 50 years municipal wastewater sludge has changed from
simple primary sludge of purely domestic orgm to complex sludge
mixtures (primary, secondary, chemical) of domestic and Indus-
trial origins.
At first when design engineers only had to consider a primary
sludge, the developed rule of thumb (22) were adequate. AS the
sludge gneerated became more complex, more and more systems failed
and the process developed a "bad reputation." The use of steady
state models In the 1960's (23-25), dynamic models 1n the 1970's
(26-31), and Increasing research Into the basic biochemical pro-
cess (32-35), has led to significant Improvements both In the
design and operation of the process. Still the transfer of data
from the laboratory to the real world can be difficult. It Is
-1.
-------
the purpose of this presentation to show how this knowledge can
be and 1s being applied to present day anaerobic digestion sys-
tems. Whenever possible, full scale operating data 1s presented.
Topics to be covered In this discussion:
General Process Description
MesophlUc • ThermophlUc Digestion
Volatile Solids Reduction
Solids Concentration - Organic Loading - Sludge Age
Mixing
Supernatant
Energy
Nutrients
pH Considerations
Tox1c1ty
Bactericidal Effects
Actlvated Carbon
Tank Layout
General Operational Control
GENERAL PROCESS DESCRIPTION
Anaerobic digestion of municipal wastewater sludge Is a very
complex biochemical process, dependent on many physical (tem-
perature, solids concentration, degree of mixing, organic
loading, detention time) and chemical (pH, alkalinity, volatile
acid level, nutrients, toxic materials) factors. Probably the
easiest way to visualize what 1s taking place Is to think 1n
terms of a two step process.
In the first step, faculatlve microorganisms (sometimes called
add forming bacteria) convert complex organic waste sludge
substrate (proteins, carbohydrates, 11p1ds) Into simple organic
fatty adds by hydrolysis and fermentation. The principle end
products, with sludge as substrate, Is acetic add (approximately
70 percent) and proplonlc add (about 15 percent) (36 - 38). The
microorganisms Involved can function over a wide environmental
range and have doubling times normally measured 1n hours.
In the second step, strictly anaerobic microorganisms (sometimes
called methane forming bacteria) convert the organic adds to
methane, carbon dioxide and other trace gases. The bacteria
Involved are much more sensitive to environmental factors than
step one bacteria and normally have doubling times measured In
days. Because of this, step two bacteria control the overall
process.
Figure 1 gives an overview of the entire process. For a more
complete review the reader Is referred to either Klrsch (35)
or ToeHen (32).
-2-
-------
Micro- Micro- Other
+ organisms K. Nonreactive + Reactive + organisms K_ + end
Raw Sludge "A" Products Products "8" CH4+CO2 Products
Complex Principally
substrate acid formers
Carbohydrates,
Fats and
Proteins
C02, H20
Stable and
intermediate
degradation
products
Organic acids Methane
fermenters
Cellular and
other inter-
mediate
degradation
products
H2O.H2S
Cells and stab
degradation
products
Celts
FIGURE 1: SUMMARY OF ANAEROBIC DIGESTION PROCESS (41)
MESDPHILIC - THERMOPHILIC DIGESTION
Temperature can be considered one of the most Important factors
1n the anaerobic digestion process. Even though the total tem-
perature range for operation of the process Is very broad,
specific microorganisms often have relatively narrow temperature
ranges In which they can grow.
For the purpose of classification, the following three tempera-
ture zones of bacterial action will be used throughout this
presentation:
Cryophilic Zone - liquid temperature below 10°C (50°F)
MesopMHc Zone - liquid temperature between 10°C to
42dC (50°F to 108°F)
ThermophlUc Zone - liquid temperature above 42°C (108°F).
In the past, the vast majority of research (lab, pilot, full scale)
has been done 1n the mesophlUc range and only a little effort In
the thermophlUc range. The reason for this 1s that thermophlHc
digestion did not seem economical because of the higher energy
requirements and the general feeling that operation at the higher
temperature would be highly unstable. Recently though the
literature (135) seems to Indicate a renewed Interest 1n thermo-
phlUc digestion because of Its elimination of pathogens, high
reaction rates and possibly higher gas yields.
-3-
-------
VOLATILE SOLIDS REDUCTION
One of the main objectives of the anaerobic digestion process
1s to reduce the amount of solids that need to be disposed.
This reduction Is normally assumed to take place only 1n the
volatile content of the sludge and It 1s probably safe to
assume only 1n the biodegradable volatile fraction of the
sludge. Research Into the area of the biodegradable fraction
1s quite limited but the following generalities can be used:
I. Approximately 20 - 30 percent of the Influent
suspended solids of a typical domestic waste-
water Is nonvolatile (46). Of the remaining
suspended solids which are volatile, approxi-
mately 40% are Inert organics consisting
chiefly of Hgn1ns» tannins and other large
complex molecules.
2. For waste activated sludges generated from
systems having primary treatment, approximately
20 to 352 of the volatile solids produced are
non-biodegradable (47,48).
3. For waste activated sludges generated from the
contact-stabilization process (no primaries -
all Influent flow into aeration tank), 25 - 35%
of the volatile suspended solids are non
biodegradable (49).
Though 1t 1s realized that only the biodegradable fraction can
actually be destroyed, all past research and most of the pre-
sent day work, report on volatile solids destroyed without
making any distinction on biodegradable and non-biodegradable.
Because of this lack of data, all reference to solids destruc-
tion will be based on volatile solids only.
Figures 2, 3, 4, show the effect of sludge age and temperature
on volatile solids reduction for three common sludges.
-4-
-------
70
O 60
50
40
30
• PILOT PLANT REF. (43)
APILOT PLANT REF. (44)
FIGURE 2
I
200 400 600 800 1000 1200 1400 1600 1800
TEMP (°C) x SLUDGE AGE (DAYS)
VOLATILE SOLIDS REDUCTION VERSUS TEMPERATURE X SLUDGE
AGE FOR ANAEROBICALLY DIGESTED PRIMARY SLUDGE
70
| 60
u
£ 50
0£
l/l
> 40
30
I
I
• FULL SCALE REF. (10)
APILOT PLANT REF. (45)
• FULL SCALE REF. (13)
200 400 600 800 1000 1200 1400 1600
TEMP. (°C) x SLUDGE AGE (DAYS)
FIGURE 3: VOLATILE SOLIDS REDUCTION VERSUS TEMPERATURE X SLUDGE
AGE FOR ANAEROBICALLY DIGESTED MIXTURE OF PRIMARY AND
WASTE ACTIVATED SLUDGE
-5-
-------
z
o
D
O
60
50
40
30
M 20
10
• •
• •
APILOT PLANT REF. (19)
• PILOT PLANT REF. (20)
• PILOT PLANT REF. (20)
_L
I
_L
I
200 400 600 800 1000 1200 1400 1600 1800 2000 2200
TEMP. (°C) x SLUDGE AGE (DAYS)
FIGURE 4: VOLATILE SOLIDS REDUCTION VERSUS TEMPERATURE X SLUDGE
AGE FOR ANAEROBICALLY DIGESTED WASTE ACTIVATED SLUDG
Though the data 1s somewhat
Izatlons seem valid.
scattered, the following general-
For all three (3) sludges the practical upper limit
of volatile solids destruction seems to be 55 per-
cent. It was noted back on page 4 that approximately
60 percent of the volatile solids are biodegradable.
Figure 2, 3 and 4 would suggest that practically all
the biodegradable fraction 1s being consumed.
The data seems to Indicate that under the same design
conditions primary sludge will degrade faster than a
mixture of primary and waste activated which 1n turn
degrades faster than straight waste activated. The
Implications of this 1s that adjustments must be made
1n design depending on the type of sludge to be
processed.
-6-
-------
SOLIDS CONCENTRATION - ORGANIC LOADING - SLUDGE AGE
Considerable capital cost savings could be realized If the
anaerobic digestion process could be operated at higher organic
loadings and shorter detention times than commonly used today.
There have been several pilot plant studies (90,136,142,143)
which have been able to operate at levels approaching 4-5
days residence times, organic loadings approaching 0.5 Ibs.
volatile sol1ds/cu.ft./day and solids concentrations up to 12 -
15 percent solids. Unfortunately, pilot plant digesters are Ideally
mixed and environmentally controlled and scallng-up the results can
be difficult.
Nevertheless, over the years there have been several full scale
facilities which were and still are being operated successfully
at short detention times, high organic loadings and high solids
concentrations. Some of these plants are listed 1n table 2.
TABLE 2: CONCENTRATION - ORGANIC LOADING - TIME PARAMETERS FOR
SEVERAL FULL SCALE ANAEROBIC DIGESTION FACILITIES
FEED SOLIDS
CONCENTRATION
6.0
6.6
6.9
4.6 - 5.2
5.0
6.3
8.0
ORGANIC
LOAD*
Ibs. vs/cu.ft./day
HYDRAULIC
RETENTION
TIME*
DAYS
0.15
0.13
0.16
0.17
- 0.38
0.28
- 0.17
0.3
0.16
0.15
11
15.0
14.4
,7 - 15.9
8.0
14
10
16.5
21.0
REF.
10
137
138
139
12
12
140
141
*A11 data based on primary digester only. Digester equipped
with mixing and sludge heating.
Solids Concentration - It must be remembered that the solids con-
centratlon within the digester effects the viscosity which 1n
turn effects the ability of the mixing equipment (see section on
MIXING). Also, because of the solids destruction taking place,
the solids concentration within the digester 1s less than the
feed solids concentration. Though 1t depends on sludge type, the
practicable upper limit on the feed solids concentration Is 1n the
range of 8 - 9 percent, with a properly designed mixing system,
this will not cause any operational problems within the digester.
• 7-
-------
Organic Loading Rate - The organic loading rate Is a function
of the solids concentration within the digester and system
sludge age. These two parameters are Implicitly Implied when
one speaks of a loading rate of pounds volatile solids per cubic
foot per day. As 1s shown In table 2 designing a digestion
system to operate at 0.15 to 0.20 Ibs. vs/cu.ft./day 1s no
problem.
Sludge Age - In present day, high rate (mixed and heated) primary
digesters, recycle of concentrated digested solids 1s not prac-
ticed, therefore hydraulic residence time and sludge age are
almost synonymous. As noted 1n table 2. a minimum time of 15
days 1n the primary digester Is very practicable. It should be
remembered though that this time Is also related to sludge type
and tank temperature as was shown 1n figures 2, 3 and 4.
There seems to be an Important relationship between the above
design parameters. In a study conducted by Clark (143). Involving
solids concentration, organic loading rate and sludge age, (both
his and other researchers data), the curve shown 1n figure 5 was
developed.
15
o
o
o «
o -«
1.0
0.8
0.6
0.4
0.2
0.0
PROBABLE DIGESTION LIMIT
1
10 15 20 25 30 35 40
SLUDGE AGE - DAYS
FIGURE 5: RELATIONSHIP BETWEEN SOLIDS CONCENTRATION - ORGANIC
LOADING - SLUDGE AGE LIMITS FOR ANAEROBIC DIGESTION (14
The interpretation of this curve 1s as follows.
-8-
-------
The shape of the probable digestion limit curve (shaded area),
that 1s higher organic loadings as the sludge age decreases,
Is a reflection of the accumulation of various system by-
products which may reach Inhibitory concentration levels. If
for a given digester volume and organic loading rate, the
sludge age Is Increased (only possible If Influent sludge con-
centration Is Increased), then the chance for potential
Inhibitory by product concentration levels also Increases.
An engineer designing a high rate primary digester might, e.g.,
determine that an organic loading rate of 0.5 Ibs, vs/cu.ft./
day and a sludge age (hydraulic detention time) of 15 days was
possible (figure 5, point 1). If he then doubled the detention
time (point 2) by doubling the Influent solids concentration
(volume of tank stays the same), the digester would fall. If
Instead, the tank volume was doubled (point 3) rather than
doubling the Influent solids concentration, the unit would still
be operating on the failure boundary and nothing would have been
gained. As a third alternative, halving the loading rate by
doubling the tank volume (point 4), Implies halving the Influent
solids concentration, would be acceptable. Finally, 1f the
loading rate were to be maintained at 0.5 Ibs vs/cu.ft./day,
decreasing the sludge age (hydraulic detention time) would be
better than Increasing and since the tank volume Is fixed, It
would allow a lower Influent solids concentration.
MIXING
Mixing 1n an anaerobic digester, treating municipal wastewater
sludge of domestic origin, 1s considered to have the following
benefits. (NOTE: It 1s assumed that a favorable environment
exist to allow development of an anaerobic digestion system).
1. To keep the food supply uniformly dispersed and In constant
contact with the growing cells to promote maximum utiliza-
tion of the system.
2. To keep the concentration of biological end products at
their lowest value by dispersing them uniformly through-
out the digester.
3. To provide environmental uniformity (temperature, nutrients,
etc.) throughout the digester allowing best possible cell
development.
4. To allow fally fast dispersion of any toxic material
entering the system thus possibly minimizing Us effect
on the anaerobic process.
-9-
-------
5. To assist In the prevention of a scum layer build-up at
the top of the digestion tank.
At the present time not many In the Environmental Engineering
field would dispute the advantage of mixing 1n an anaerobic
digester. The problems arise when trying to answer such ques-
tions as; what 1s adequate mixing, how do you define mixing,
how do you specify mixing, etc.
Before any discussion about mixing can be developed, some time
must be spent discussing what and where this mixing Is to take
place.
CHARACTERISTICS OF AN ANAEROBIC DIGESTER
The existing trend In wastewater treatment Is to remove more
and more material from the main liquid processing stream. This
Is done through the use of secondary biological treatment schemes.
chemical addition and filters. The sludge produced can vary
widely and change rapidly even on an hour to hour basis.
Table 3 shows specific gravity and particle
two common type sludges: plain primary and
(89).
size distribution on
plain waste activated
TABLE 3: GENERAL CHARACTERISTICS OF RAW PRIMARY AND WASTE
ACTIVATED SLUDGE (89)
Specific Gravity
Particle Size
PRIMARY SLUDGE
1.33 - 1.4
20% < 1 urn
352 1 - 100 urn
451 > 100 urn
Physical Appearance Flberous
WASTE ACTIVATED
SLUDGE
1.01 - 1.05
40X 1 - 50 urn
60? 50-180 urn
Sllmey, gelantlnous
There 1s little data on the rheology of municipal wastewater
sludge and even less on anaeroblcally digested sludge (90,91).
One of the main problems 1s the extreme difficulty In doing
such studies correctly (92).
-10-
-------
Even though the majority of raw wastewater sludges behave as
a thlxotropfc (time dependent), pseudo plastic, material
(figure 6), H may not be correct to assume that the sludge
within the anaerobic digester has the same general properties.
The liquid within the tank Is normally at a higher temperature,
there Is entrapment of gas bubbles, and there 1s a general
reduction In particle size all of which affect fluid viscosity.
*/»
Wl
UJ
Of
LU
FIGURE 6:
0 RATE OF SHEAR
SHEAR - STRESS RELATIONSHIP FOR A THIXOTROPIC
PSEUDO PLASTIC MATERIAL
At the present time anaerobic digestion tanks have a ratio of
Inside tank diameter to average liquid depth of 1:1 to 5:1.
This Imposes some restrictions on the ability to develop a
mixing regime.
DEFINING MIXING
In recent
mix" when
engineers
activated
digester.
years 1t has become popular to use the term "complete
discussing biological process reactors. Unfortunately,
associate this term on a time scale as applied to
sludge systems when talking about mixing an anaerobic
-11-
-------
The term "complete mix" Is a relative term. It means that the
time for dispersion of the feed stream 1s short 1n relation to the
total hydraulic residence time In the reactor. It 1s also defined
as sufficient mixing so that concentration gradients of chemical
and biological Ingredients are uniform for the particular reaction
rates that exist 1n the basin.
Present day "complete mix" activated sludge systems have hydraulic
residence times of approximately 3 hours based on plant Influent
flow. Generally a "turn over rate" of 15 - 20 minutes 1s considers
sufficient to achieve "complete mix" conditions within the aeration
basin. This would give a turn over rate - hydraulic detention time
ratio of 0.08. Present day high rate primary digesters have
hydraulic detention times of 12 - 17 days. This would seem to Imply
that a "turn over rate" of about 1 day would provide complete mix
conditions within the system.
Mixing with the anaerobic digestion tank occurs on two levels:
macromlxing and m1crom1x1ng (35). Macrom1x1ng deals with the bulk
mass flow within the digester while m1crom1x1ng deals with the
degree of Intermingling of the system molecules. In biological
theory the assumption of "complete mix" assumes micromlxing.
The actual mixing of the sludge within the digester can be by gas
reclrculatlon, mechanical or a combination of the two. Reference
(93) offers a good description of all present day systems.
No matter what type of device 1s utilized the Intent 1s to achieve
mixing through a pumping action. Because of this relationship,
engineers have come to use the term hp/unlt volume as some type of
parameter to define mixing 1n an anaerobic digester. Unfortunately
this term by Itself has no meaning. For mechanical type mixers thi
wide variation 1n Impeller diameters and speeds can result 1n
similar horsepower but widely different pumping capacities. For
gas mixing systems gas flow, depth and bubble size can also result
In similar horsepower but widely different pumping capacities.
Probably the best way to evaluate mixing 1s from the standpoint of
zone of Influence (figure 7). Essentially, the zone of Influence
says that energy Is dlssapated as one moves horizontally away from
the energy source. This loss Is due to friction between the fluid
molecules which Is a function of liquid density, temperature and
solids concentration. Within a certain area of the point source
there 1s sufficient energy to achieve micromlxing. There 1s also
a larger area where bulk flow (macromlxing) still takes place even
though time Is Insufficient energy for micromlxing.
Presently, the only published work that could be found discussing
this concept In the sanitary engineering field was done by the
EPA (94,95). The work to date Indicates that the width of the
micromlxing zone In water Is no more than twice the liquid depth,
with the liquid depth being a function of the type of mixing
device utilized and not necessarily the tank liquid depth.
Whether or not 1t would be safe to assume that for thickened
anaerobic sludges the zone would be less Is unknown.
-12-
-------
TOP VIEW
ENERGY
SOURCE
PROFILE VIEW
LIQUID SURFACE
Dj = Effective zone diameter for micromixing
^2 = Effective zone diameter for macromixing
FIGURE 7: SCHEMATIC OF ZONE OF MIXING INFLUENCE FOR
ENERGY SOURCE IN FLUID WITH ONLY FIXED
UPPER AND LOWER BOUNDARIES
-13-
-------
SUPERNATANT
Poor quality anaerobic digester supernatant 1s a major opera-
tional problem at many municipal wastewater treatment plants.
The supernatant will most likely contain high concentrations
of carbonaceous organic materials, dissolved and suspended
solids, nitrogen, phosphorous and other materials (39) Impos-
ing extra loads on other treatment processes and effluent
receiving waters. Table 4 shows the effects at one mldwestern
treatment facility where anaerobic digester supernatant from a
high rate system was returned to the plant Influent.
TABLE 4: EFFECT OF RETURNING SUPERNATANT FROM HIGH RATE
ANAEROBIC DIGESTER TO PLANT INFLUENT (40)
Suspended
Solids
Total
Phosphorus
To
Primaries
Ib/day
15,969
(36,801)'
914
( 1,304)
To
Secondaries
Ib/day
9,501
(15,306)
803
(991)
Final
Effluent
Ib/day
2,836
(3,467)
500
(435)
Primary
Sludge
Ib/day
13.249
(19,626)
156
(299)
Waste
Activated
Sludge
Ib/day
9.593
(14,645)
287
(453)
•Data in parentheses were obtained when untreated anaerobic digester supernatant was discharged to head of
plant. Data not in parenthese were obtained when no supernatant was discharged to head of plant. Data
shown is average for the entire time period of study.
Many supernatant treatment alternatives have been tried (42),
some working with a certain degree of success. The question
that really needs to be asked 1s why even expect a clean
supernatant stream.
The concept of obtaining high quality supernatant developed
during the early days of separate anaerobic digestion systems
During this time period the only sludge being
primary sludge which had excellent settling
.„ digested was
properties (table 3)
Modern day sludges are much more complex. They contain not only
primary sludge but sludges generated from secondary treatment-
predomlnately activated sludge systems. Waste activated sludge
tends to have fragile floe and 1s difficult to concentrate by
gravity thickening. It 1s because of this, waste activated
sludges are thickened by dissolved air flotation thickeners.
-14-
-------
Also present day, high rate, anaerobic digesters are mixed.
This constant mixing of the sludge tends to reduce particle
size. At the same time the process Itself 1s reducing
particle size through biological decomposition.
Finally anaerobic digestion systems generate gas throughout
the entire tank and also under a slightly positive pressure
(6 - 15 Inches water column). Thus, the system becomes
supersaturated with digester gas.
When the digested sludge 1s finally pumped to the secondary
digester, It contains many fines, 1t contains sludge that was
difficult to thicken by gravity and It 1s supersaturated with
gas. The gas 1s then liberated 1n the form of small gas
bubbles which tend to attach themselves to the sludge particles,
thus promoting a floatlon effect. The combination of these
events are very detrimental to gravity concentration. It has
been estimated that at least thirty or more days (12) would
be required 1n a secondary digester to obtain a clear super-
natant from high rate systems digesting sludges containing
waste activated sludge.
In many cases It would be better to take all digester contents
direct to mechanical dewaterlng and eliminate provisions for
gravity sol1ds-11qu1d separation. This would give a constant,
predictable centrate stream having low suspended solids
content.
ENERGY
ENERGY PRODUCTION
One of the advantages of anaerobic digestion of municipal
wastewater sludge Is that energy 1s produced rather than
consumed and could go a long way In meeting energy require-
ments at wastewater plants (145). One problem encountered
with this energy source 1s predicting how much energy will
be produced for any given plant. This variability Is
possible production as Indicated 1n table 5.
-15-
-------
TABLE 5: CUBIC FEET DIGESTER GAS PRODUCED PER POUND OF
ORGANIC MATTER DESTROYED
Reference (75) - Pure Compounds
Material
Fats
Scum
Grease
Crude Fibers
Protein
CH4
62 -72
70 - 75
68
45 - 50
73
Cu.Ft. Gas/Lb. Decomposed
18
14
- 23
- 16
17
13
12
Reference (41) - Pure Compounds
Material
Carbohydrate
Fat
Insoluble Soap
Protein
Cu.Ft. Gas/lb. Digested
14.2
24.6
22.3
9.4
Reference (76) - Municipal Sludges
"The volume of gas produced per pound of volatile solids des-
troyed 1s reported as 17 - 18 cu.ft./lb. at the larger and
better Instrumented plants. Smaller plants report lesser
values, sometimes as low as 6 cu.ft./lb. volatile solids des-
troyed, but these values are probably due to poor measurement
techniques."
Reference (77) - Municipal Sludges
"—maximum gas production of approximately 11 to 12 cu.ft.
of gas per pound of total solids destroyed."
Reference (78) - Municipal Sludges
"In terms of solids digested, the average yield — 1s about
15 cu.ft. of gas per pound of volatile solids destroyed."
Figure 8 1s of Interest. As part of his graduate studies on
temperature effects on anaerobic digestion (134) Schwerln
reviewed the literature and plotted reported gas production
-16-
-------
values as a function of digestion temperature. The results
show the potential effect of digestion temperature on gas
production.
FT3/lb VS ADDED
\
FT3/lb VS DESTROYED
I
\
I
I
20
18
16
14
12
UJ
Q
£
^.
n
^
u.
Z
O
^
u
o
o
Q.
U)
O
10
80
90
100 HO 120
TEMPERATURE - °F
130
140
FIGURE 8: EFFECT OF DIGESTION TEMPERATURE ON GAS PRODUCTION
BASED ON DATA FROM 23 STUDIES (134)
Since the basis of all cost analysis depends on the value of gas
produced per pound of solids destroyed, and If there 1s no exist-
ing data, 1t 1s suggested that a range of 12 - 17 cu.ft. per pound
volatile solids destroyed be used.
-17-
-------
NOTE: As was noted 1n the discussion, VOLATILE SOLIDS REDUCTION,
the amount of solids destroyed 1s a function of sludge
type and solids retention time (figures 2,3 and 4).
The heating value of the gas can also vary, the typical range
being 550 to 650 BTU per cubic foot. Based on the average of
50 plants (81) a value of 600 Is suggested.
HAZARDS OF DIGESTER GAS
1. Explosion - Studies have shown that sludge gas becomes
—violently explosive 1n mixtures of 1 volume
gas to 5 - 15 volumes of air and there are
many case histories which have shown just
how violent an explosive 1t can be.
2. Burning
- When the ratio of gas to air 1s higher than
the above values, a "burning mixture 1s
encountered." Such a mixture 1s not as
dangerous as an explosive mixture since 1t
may be extinguished 1f encountered. However,
sewage plant workers have been seriously
burned by an Instantaneous flame "puff."
3. Toxldty - Of the minor constituents of sewage gas,
hydrogen sulflde (H?S) 1s the most Important.
Table 6 shows the effects at various concen-
trations.
TABLE 6: EFFECTS OF VARIOUS CONCENTRATIONS OF H9S
Immediate death
Fatal 1n 30 m1n. or
less
Severe Illness caused
1/2 to 1 hr.
No severe effects
If exposed 1/2 to
1 hour
Greater than 2,000 ppm
600 to 1,000 ppm
500 to 700 ppm
50 to 100 ppm
4. Suffocation -
the
Man works best and breathes easiest when
air contains about 21 percent of oxygen.
Men breathing air that has as little as 15
percent of oxygen usually become dizzy, have
a rapid heart beat and suffer from headache.
. _ » J A.. ._ _..W1J_m»4»«» ku I * *ttlf/\f/\ f flA - RR I
Though over 30 years old, two publications by
on gas safety design considerations are still
1ng for design engineers. Figure 8A shows a
modern day gas piping system (146).
Langford (84,85)
recommended read-
schematic of a
-18-
-------
ii_QPL_ M i 0*3 uiurs MINIMUM o i/a* HER FOOT
i on r>n*iM»ci
SSLSjiffif ' ONTHOl LINES *NP VI NT LINES
TO ill I/K'< PIPI. OB Itfm TUBING
ifRVlCE
OR
riCATLR
TLAMt TRAP
PBCSSUBE CONTRCX. LINE
LOW PRESSURE
CMEC« VALVE
PILOT
LINE
CONTROL
PANEL
DiGCSTER HEATER C MEAT CKCHANGER
WASTE
WASTE
CAS
BURNER
COMBINED
PRESSURE. RELIEF
AND FLAME TRAP
FLAME
CELL
1-
VENT TO OUTSIDE
ATMOSPLRE
P
PRESSURE GAUGE
@
DRIP TRAP
13
GAS METIR
FIGURE 8A; GAS PIPING SCHEMATIC OF A MODERN ANAEROBIC DIGESTION SYSTEM (146)
-------
DIGESTER GAS UTILIZATION
Since digester gas was first used In the United States 1n 1915
(79) for heating and cooking, the use of digester gas has
Increased, decreased 1n the 50's and 60's because of cheap
power alternatives, and presently Increasing again because of
the energy situation (80). Several recent publications have
described not only operating experience with conventional
utilization methods, power generation and heating (80 • 83)
but also potential new uses (80). One piece of Important
operating Information which has come to light Is the amount of
hydrogen sulflde permissible for operation of engine genera-
tors (81,83).
Because of Us potential corrosive action early uses of digester
gas as engine fuel tried to keep H?S levels under 60 grains per
100 cubic feet (86,87). This was done by Incorporating some
type of dry gas scrubber or wet type bubbling scrubber. Recently
a new simple method (88) of removal has been developed.
A recent publication (81) describing the operating results of
several plants noted that even though levels of 1000 to 3000
mg/1 of H2S were 1n the gas no adverse affects had been seen
on the engines.
DIGESTER HEAT REQUIREMENTS
In calculating digester heat requirements the two parameters of
concern are:
1. Heat required to raise the temprature of the
Incoming sludge flow to digester operating
temperature.
2. Heat required to maintain the digester
operating temperature (radiation heat loss).
Heat Required for Raw Sludge - It 1s often necessary to raise
the temperature of the Incoming sludge stream. The amount of
heat required 1s given by equation 1.
Qs » gal of sludge v 8.34 Ibs x (T2 - TI) x 1 day n\
-------
Heat Required For Heat Losses - Digesters have radiation heat
losses which must be controlled to maintain digester operating
temperatures within - 1°F otherwise the system could go Into
thermal shock. The amount of heat loss depends on the tank
shape, materials of construction and external temperatures.
The general design equation for heat flow through compound
structures 1s:
Q = U x A x (T2 - T3) (2)
where:
Q - heat loss Btu/hr
A » area of material normal to direction of heat flow In
ft2
T£ • temperature desired within the digestion tank
13 « temperature outside the digestion tank
u • ! O)
J. * v~ —
C1 *- kj
where:
Of ij
C< a conductance for a certain thickness of material
hr-ft2-°F
x< » thickness of material - Inches .
J Btu - (Inch)
kj » thermal conductivity of material hr-ft*-°F
Values of C^ and kj can be found 1n various handbooks (147).
Various values of U for different digester covers, wall construc-
tion and floor conditions are given 1n table 7.
-21-
-------
TABLE 7:
"UH FACTORS FOR VARIOUS ANAEROBIC DIGESTION TANK
MATERIALS (146)
MATERIAL
'U1
0.91
0.58
0.33
0.86
Fixed steel cover (1/4" plate) -
Fixed concrete cover (9" thick) ---.----------------.
Floating cover (wood composition)
Concrete wall (12" thick) exposed to air —........
Concrete wall (12" thick), 1H air space
and 4" brick 0
Concrete wall or floor (12" thick) exposed to
wet earth (10* thick) 0.11
Concrete wall or floor (12" thick exposed to
dry earth (10' thick) 0.06
27
NUTRIENTS
In general, Its commonly assumed that municipal wastewater sludge
1s not nutrient deficient. It has been extremely difficult to
conduct research on optimum nutrient requirements of anaerobic
bacteria on sewage sludge (131). To date, the literature has
shown (132) that, like aerobic bacteria, nitrogen and phosphorous
are required 1n the highest amount (12 and 2 percent respectively
based on the weight of biological solids present 1n the system).
It 1s suggested that a minimum C:N:P ratio of 100:15:1 be used
for design purposes.
Several researchers have also shown that the addition of certain
trace materials, Iron (133) and sulfur (131), could be very
beneficial to the process.
pH CONSIDERATIONS
digestion
"methane
As was noted under General Process Description, anaerobic
1s a two step process consisting of an "acid forming" and
forming" step. During the first step the production of volatile
add tends to reduce the pH. The reduction 1s normally countered
by destruction of volatile adds by methane bacteria and the sub-
sequent production of bicarbonate.
Past research (124 - 126) has shown that the optimum pH value for
methane producing bacteria 1s In the range of 6.4 - 7.5 and that
these bacteria were very sensitive to pH change. Recent research
though (127) now seems to Indicate that the pH tolerance of methane
-22-
-------
producing bacteria 1s greater than previously expected. The
study also Indicated that high and low pH values were only
bacterlostatlc and not bactericidal as previously thought.
Because of the Importance of this finding to system control,
more research 1s needed In this area to verify these results.
pH Is related to several different acid-base chemical equilibria.
In the anaerobic digestion process the range of Interest Is be-
tween 6.0 - 8.0 which for all practical purposes makes the
carbondloxlde - bicarbonate relationship the most Important. In
an anaerobic digestion system the amount of carbon dioxide Is
dependent only on the law of partial pressure. Since soluble
carbon dioxide depends primarily on the C02 gas content and since
at any given time the composition of digester gas 1s relatively
fixed, pH Is a function of the bicarbonate concentration as shown
1n figure 9.
O
tt
LU
^
CO
LU
O
o
z
cs
O
u
LIMITS OF
NORMAL
ANAEROBIC
TREATMENT
O
V
10 -
250 500 1000 2500 5000 10,000 25,000
BICARBONATE ALKALINITY - mg/l AS CoCO3
FIGURE 9 : RELATIONSHIP BETWEEN pH AND BICARBONATE
CONCENTRATION NEAR 95° F (128)
This relationship 1s very Important from a process control stand-
point (129). Also, It points out the Importance of analyzing for
bicarbonate alkalinity Instead of total alkalinity as Is commonly
done today. The relationship between the two 1s given 1n equa-
tion 4.
-23-
-------
BA « TA - 0.71 VA (4)
where:
BA * bicarbonate alkalinity as mg/1 CaC03
TA « total alkalinity as mg/1 CaCOs determined by
tltration to pH 4.0.
VA » volatile adds measured as mg/1 acetic add
0.71 <* a combination of two factors (0.83) (0.85).
0.83 converts volatile adds as acetic to
volatile add alkalinity CaCOa and 0.85 from
the fact that 1n a tltration to pH 4.0, about
85 percent of the acetate has been converted
to the add form.
It has been suggested (129) that the only way to Increase digester
pH 1s by the addition of sodium bicarbonate. Other materials such
as caustic soda, soda ash and lime can not Increase bicarbonate
alkalinity without reacting with soluble carbon dioxide which 1n
turn causes a partial vacuum within the system. Also above pH 6.3.
lime may react with bicarbonate to form Insoluble calcium carbonatt-
thus promoting scale formation or encrustratlon.
Sodium can be toxic at certain concentration (see section on Tox1-
dty - light metal cations) and It Is recommenced to keep sodium
levels below 0.2 M (approximately 4600 mg/1) which may require
dilution of the digester contents as part of the corrective
measures.
TOXICITY
Kugelman and Chin (96) have noted that much of the published data
on toxldty 1n anaerobic digestion systems Is erroneous and mis-
leading because of Inadequate experimental techniques and general
lack of understanding. Therefore, before any discussion of
toxldty takes place a review of several fundamentals must be
made.
First of all for any material to be biologically toxic 1t must
be In solution. If any substance 1s not In solution, 1t 1s not
possible for 1t to pass through the cell wall and therefore can
not effect the organism.
Second toxldty 1s a relative term. There are many organic and
Inorganic materials whlch.dependlng 1f they meet condition one
above, can be either stlmulartory or toxic. A good example of
this 1s the effect of ammonia nitrogen on anaerobic digestion -
table 8.
-24-
-------
TABLE 8; EFFECT OF AMMONIA NITROGEN ON ANAEROBIC DIGESTION (97.98)
NH3 " N EFFECT
50 - 200 Beneficial
200 - 1000 No adverse effects
1500 - 3000 Inhibitory at pH over 7.4 - 7.6
above 3000 Toxic
Acclimation Is a third consideration. When potential toxic materials
are slowlsy Increased within the environment, many biological
organisms can rearrange their metabolic resources, thus overcoming
the metabolic block produced by the toxic material. Under shock
load conditions there Is not sufficient time for this rearrangement
to take place.
Finally, there 1s the possibility of antagonism and synerglsm.
Antagonism Is defined as a reduction of the toxic effect of one
substance by the presence of another. Synerglsm 1s defined as
an Increase 1n the toxic effect of one substance by the presence
of another. This 1s an Important relationship when designing for
cation toxlclty.
Though there are many potential toxic materials, this section will
only concern Itself with the following:
Volatile Acids
Heavy Metals
Light Metal Cations
Oxygen
SulHdes
Ammonia
Volatile Acids - Up until the 1960's 1t was commonly believed that
volatile add concentrations over 2000 mg/1 was toxic to an
anaerobic digester. There was also considerable controversy on
whether or not alkaline substances should be added to maintain
adequate buffer capacity.
In the early 1960's McCarty and his co-workers published results
from their very carefully controlled studies 1n this area (97,99,
100). Their results showed the following:
1. Studies clearly Indicate that volatile adds, at least up
to 6000 - 8000 mg/1, were not toxic to methane bacteria.
Therefore as long as there was adequate buffer capacity
to maintain the system pH In the range of 6.6 - 7.4, the
system would function.
-25-
-------
2. That pH control by the addition of an alkaline material
was a valid procedure as long as the cation of the
alkaline material did not cause toxldty. It was found
that the addition of sodium, potassium or ammonium com-
pounds was detrimental but magnesium or calcium alkaline
compounds were not.
Heavy Metals - Heavy metal tpxlclty has frequently been cited as
the cause of many anaerobic digestion failures. Even though trace
amounts of most heavy metals are necessary for maximum biological
development (101), the concentrations existing In raw waste water
sludges could cause potential problems.
Since heavy metals tend to attach themselves to sludge particles
(102,103),even low Influent concentrations can be concentrated
significantly 1n the sludge handling process. Table 9 - column 2
gives the range of Influent concentrations of some heavy metals.
The range 1s quite wide with the higher values normally being
attributed to a local Industrial polluter.
Column 3 of table 9 gives the typical range of removal that can
be expected through a standard secondary treatment system. Pub-
lished data seem to Indicate that the percent removal, without
chemical addition. Is a function of Influent concentration. The
higher the Influent concentration the higher the percent removal.
TABLE 9: INFLUENT CONCENTRATIONS AND EXPECTED REMOVALS OF
SOME HEAVY METALS IN WASTEWATER TREATMENT SYSTEMS
HEAVY
METAL
Cadlum
Chromium +3
+6
Copper
Mercury
Nickel
Lead
Z1nc
Arsenic
Iron
Manganese
Silver
Cobalt
Barium
Selenium
<.008
<.020
<.020
<.020
< .0001
< .1
< .05
< .02
.002
< . 1
.02
<.05
below
INFLUENT
CUNC.
mg/1
REMOVAL
SEC.
- 1.142 (104,107) 20 -
- 5.8 (104,107;
40 -
- 5.8 (104,107) 0 -
- 9.6 (104.107) 0 -
- .068 (104,107
- 880 (104,107
- 12.2 (104,107
- 18.00 (104,107
- .0034 (105)
- 13 (107)
- .95 (107
- .6 (107)
detection (107)
...
1 20 -
15 -
50 -
35 -
28 -
-
-
TREAT
%
REMOVAL
LIML - pH 11
X
45 (104) 95 (106.109;
80 (104
10 (106;
70 (104;
75 (104;
40 (104
90 (104
80 (104
73 (105
72 (108
2b (108
--
--
95 (109)
ZU (109)
90 (106.109.
40 (109)
90 (106,109
...
90 (106,109)
70 (109)
i 99 (106)
i 95 (106,109,
96 (106)
...
47 (108) 75 (109)
79 (108)
-26-
-------
Column 4 of table 9 shows expected removals with 11 me additions
at a pH of 11.0. In fact 1t has been noted (109) that treatment
systems which add lime or other chemical coagulations for
phosphate removal can expect significant amounts of Influent
heavy metals to also be removed.
Because of the dependence of Inhibition on naturally occurring
reagents* such as carbonate and sulphide, It 1s not possible to
define precise total toxic concentrations for any heavy metal
(110). Table 10 gives some concentrations of Individual metals
required to cause severe Inhibition. Table 11 gives an Indica-
tion of the difference between total and soluble concentrations
that may exist 1n an anaerobic digester.
TABLE 10: TOTAL CONCENTRATION OF INDIVIDUAL METALS REQUIRED TO
CAUSE SEVERE INHIBITION (110)
CONCENTRATION OF METAL IN DIGESTER
CONTENTS (dry sludge solids)
METAL 1 mM Kg'1
Copper
Cadlum
Zinc
Iron
Chromium
+6
+ 3
0.
1.
0.
9.
2.
2.
93
08
97
56
20
60
150
100
150
171
420
500
0
TABLE 11; TOTAL AND SOLUBLE HEAVY METAL CONTENT OF DIGESTERS (111)
METAL
Chromium +6
Copper
Nickel
Z1nc
TOTAL CONC.
mg/1
420
196
70
341
SOLUBLE CONC.
mg/1
3.0
0.7
1.6
0.1
The problem of heavy metal toxlclty may not necessarily be reduced
with strict enforcement of Industrial point sources. For example,
the normal digestion and excretion of zinc Is approximately 10 mg
per person (112). Another non-point source 1s the paved street.
Table 12 gives the results of a study on heavy metal pollution from
paved road surfaces of several large cities (112). In another
extensive study (113),' based on 9600 analyzed samples, H was shown
that 1f all Industry In metropolitan New York had zero discharge,
there would only be a 9 percent reduction In copper, 20 percent In
chromium, 6 percent 1n zinc, 16 percent In cadlum and 62 percent
In nickel.
-27-
-------
TABLE 12; HEAVY METAL FROM PAVED - CURB STREETS (112)
Data given 1n pounds/mile of paved street
METAL
Z1nc
Copper
Lead
Nickel
Mercury
Chromium
ARITHMETIC
Mean
0.75
0.21
0.68
0.060
0.080
0.12
Range
.062 -
.020 -
.03 -
.011 -
.019 -
.0033 -
2.1
.59
1.85
.19
.2
.45
Except for chromium, heavy metal toxldty 1n anaerobic digesters
can be prevented or eliminated through precipitation with sul-
fldes (111,114 - 116). Hexavalent chromium 1s normally reduced
to trlvalent chromium which under normal anaerobic digester pH
levels are relatively Insoluble and not very toxic (117).
The reason for using sulflde precipitation 1s the extreme Insolu-
bility of heavy metal sulfldes (118). Approximately 0.5 mg of
sulflde 1s required to precipitate 1.0 mg of heavy metal. If
Insufficient sulflde Is not available from natural sources, then
1t must be added In the form of sulfate which 1s reduced to
sulflde under anaerobic conditions.
One potential drawback of using the sulflde saturation method 1s
the possible production of hydrogen sulflde gas or sulfurlc acid
due to excess amounts of sulflde 1n the digester. Because of this,
It 1s recommended that ferrous sulfate be used as a source of
sulflde (96). Sulfldes will be produced from the biological break-
down of sulfate, and the excess will be held out of solution by the
Iron. However, 1f heavy metals enter the digester, they will draw
the sulflde preferentially from the Iron because Iron sulflde 1s
the most soluble heavy metal sulflde.
Two other methods of controlling excess sulflde additions have beer
proposed (115,119). One method would be to continuously analyze
the digester gas for hydrogen sulflde (115). When there are
detectable levels of HgS, sulfate addition would be terminated, whe«
the level became undetectable additions would start. A second
method (119) was the use of a silver - silver sulphide electrode
to measure very low levels of soluble sulphides. The electrode Is
calibrated 1n standardized solutions of sodium sulphide of known
value to yield a parameter, pS, defined 1n a manner similar to pH,
as the negative common logarithm of the divalent sulphide Ion
concentration. For example, when S'2 1s 10'5M, pS would be 5.
Light Metal Cations - Only recently (96,120,121) has the s1gn1f1cai,
of the light metal cations (sodium, ammonium, potassium, magnesium.
calcium) an anaerobic digestion start to be unravelled. Normally,
-28-
-------
domestic wastewater sludges have low concentrations of these
cations but significant contributions, enough to cause toxIcHy,
can come from two sources.
1. Industrial operations
2. The addition of alkaline material for pH control.
Not only can each of these cations be either stimulatory or toxic
depending on concentration (table 13) but when combined with each
other will produce either an antagonist or synerglsm relation-
ship.
TABLE 13: STIMULATORY AND INHIBITORY CONCENTRATIONS OF LIGHT
CATION
Calcium
Magnesium
Potassium
Sodium
STIMULATORY
mg/1
100 - 200
75 - 150
200 - 400
100 - 200
MODERATELY
INHIBITORY
mg/1
2500 - 4500
1000 - 1500
2500 - 4500
3500 - 5500
STRONGLY
INHIBITORY
mg/1
8.000
3,000
12,000
8,000
Based on current knowledge whenever Inhibition Is being caused
by an excess of a certain cation, the cation can be antagonized
by the addition of one or more of the cations listed 1n table 14.
TABLE 14; CATION ANTAGONISTS
INHIBITING ANTAGONIST
CATION CATION
Ammonium Potassium
Calcium Sodium, Potassium
Magnesium Sodium, Potassium
Potassium Sodium, Potassium, Calcium, Ammonium
Sodium Potassium
Oxygen - Engineers have always been concerned with air getting Into
anaerobic digesters since a mixture of one volume digester gas with
5-15 volumes of air 1s an explosive mixture.
-29-
-------
Many engineers have also expressed concern over the possibility
of oxygen toxlclty when using dissolved air flotation thickeners
for sludge thickening. In 1971 Fields and Agardy (122) showed
• that small additions of air (up to 0.01 volumes per
volume of digester contents) approaching one percent by volume*
will not significantly affect anaerobic digester performance."
This value Is several magnitudes higher than the amount of air
that would be generated from a dissolved air thickening system.
Sulfldes - By Itself soluble sulflde concentrations over 200
mg/1 are toxic to anaerobic digestion systems (114,123). The
soluble sulflde concentration within the digester 1s a function
of the Incoming source of sulfur, the pH, the rate of gas pro-
duction and the amount of heavy metals to act as complexlng
agents. The high levels of soluble sulflde can be reduced by
the addition of Iron salts, or gas scrubbing.
Ammonia - Whenever there are high concentrations of protein
waste, which Is possible In some systems with highly concentra-
ted feed sludges, ammonia toxlclty must be considered (97,121).
Ammonia can be 1n two forms, ammonium Iron NH^+ or ammonia gas.
Both forms are always In equilibrium, the concentration of each
depending on pH. Equation 5 shows the relationship.
+ H* (5)
When the pH Is 7.2 or lower, equilibrium 1s shifted towards the
ammonium 1on and Inhibition 1s possible at certain concentra-
tions. At pH values over 7.2,the reaction shifts towards the gas
phase which 1s Inhibitory at low values.
Analysis for ammonia toxlclty 1s done by analyzing the total
ammonia concentrations. If the total ammonia concentration Is
between 1500 to 3000 mg/1 and the pH Is above 7.4 - 7.6, there
1s possible Inhibitory effects due to ammonia gas. This can be
controlled by the addition of enough HCL to maintain the pH
between 7.0 to 7.2. If total ammonia levels are over 3000 mg/1,
then the NH$* 1on will become toxic no matter what pH level.
The only solution Is to dilute the Incoming waste sludge.
BACTERICIDAL EFFECTS
Pathogenic organisms 1n wastewaters consist of bacteria, virus,
protoza and parasitic worms and a good current review on the
subject can be found In reference 66. Many of these organisms,
especially enteric viruses (67), have a strong tendency to bind
themselves to sludge solids.
-30-
-------
Table 15 lists the human enteric pathogens that have been found
1n wastewater sludges along with the diseases normally associ-
ated with them. Table 16 list some data on bacterial
concentrations found 1n raw sludges from two studies (69,70).
TABLE 15: HUMAN ENTERIC PATHOGENS OCCURRING IN WASTEWATER
AND THE DISEASES ASSOCIATED WITH THE PATHOGENS (68)
PATHOGENS
Vibrio Cholera
Salmonella typhl
Shlgella species
Conform species
Pseudomonas species
Infectious hepatltus virus
Pol1ov1rus
Entamoeba hlstolytlca
Plnworms (eggs)
Tapeworms
DISEASES
Cholera
Typhoid and other enteric fevers
Bacterial dysentery
Diarrhea
Local Infection
Heptatltls
Pollomyletls
Amoebic dysentery
Asear1as1s
Tapeworm Infestation
TABLE 16: PATHOGENIC ORGANISMS IN SLUDGE (69.70)
PSEDUDOMONAS
AERUGINOSA
No./lOO ml
46 x 103
195
110 x 103
1.1 x 103
24 x 10?
5.5 x 103
2 x 103
Trickling Filter
Raw WAS
Thickened Raw WAS
SALMONELLA
No./lOQ ml
460
62
93
74
2300
6
9300
FECAL
COLIFORM
No. x 1Q6/1QO ml
11.4
11.5
2.8
2.0
26.5
20
The reduction of pathogenic organisms under mesophlllc. anaerobic
digestion has been studied by various researchers (67,71 - 74).
Though some early research Indicated die off may be due to
bactericidal effects (71.72), current research supports that die
off Is strictly related to natural die off. Data from two studies
1s given 1n table 17 for mesophlllc anaeroblcally digested sludge.
-31-
-------
TABLE 17: PATHOGENIC ORGANISMS IN MESOPHILIC ANAEROBICALLY
DIGESTED SLUDGE (69.70)
PSEUDOMONAS FECAL COLI
SALMONELLA AERUGINOS s 1O6
1/100 ml 1/100 ml ff/100 ml
Primary only 29 34 0.39
MAS only 7.3 103 0.32
Mixture
Primary and WAS 6 42 .26
No reported work on pathogen destruction for thermophlllc
anaerobic digestion could be found. Pilot plant studies on
pathogen destruction for thermophlllc aerobic digestion have
been conducted (table 18) and have found that the time for
reduction of pathogenic organisms 1s a function of basin
liquid temperature
TABLE 18: THERMOPHILIC AEROBIC DIGESTION TIME REQUIRED FOR
REDUCTION OF PATHOGENIC ORGANISMS BELOW MINIMUM
DETECTABLE LEVEL (148)
TIME REQUIRED FOR TIME REQUIRED FOR
TEMP LOWEST DETECTABLE LOWEST DETECTABLE
DEG. LIMIT OF SALOMONELLA LIMIT OF PSEUDOMON,
TYPE
Mixture of
and waste
primary
activated
C
45
50
55
60
HOURS
24
5
1
0.5
AERUGINOSA
24
2
2
0.5
HOURS
ACTIVATED CARBON
The first reported studies on the addition of activated carbon
to anaerobic digesters treating municipal wastewater sludges
was In 1935, at Plalnfleld, NJ (50) and 1n 1936 1n U.S. patent
2,059,286 (51). At this time the addition of activated carbon
was claimed to have the following benefits.
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1. Enhanced the rate of digestion.
2. Increased the total amount of gas produced.
3. Produced clear supernatants.
4. Enhanced the dralnabllUy of the digested
sludge.
5. Increased temperatures within the digester.
6. Gave higher volatile solids reductions.
Until recently no other reported work 1n this area could be found.
In 1975 Adams (52,53) discussed the results of studies carried out
by ICI. In his discussion he pointed out the following advantages
based on full scale studies carried out at Cranston, RI (54) and
NorMstown, PA (55).
1. Promoted sludge settling and clear supernatants
due to the high carbon density.
2. Catalyzes the breakdown of sludge solids, there-
by reducing the amount of sludge to be handled.
3. Increase gas production per pound of solids added
plus producing a gas with higher methane content.
4. Can absorb certain substances such as pesticides,
heavy metals, grease, scum and detergents.
5. Reduction In odors.
6. Possible Improvement 1n mechanical dewateMng
operation at least for vacuum filtration.
Even though several full scale studies have been conducted, they
have not been done scientifically but more of a general "add some
carbon and see what happens" attitude. Though Improved operating
results have been shown, the real mechanism for these results have
not yet been clearly Identified. At the present time EPA has
awarded a grant to Batelle to study the effects of activated
carbon addition on anaerobic digesters.
TANK LAYOUT
Essentially four (4) basic types of anaerobic digestion systems
are available to stabilize municipal wastewater sludges. The
four systems are discussed below In order of their complexity.
Conventional Low Rate Anaerobic Digestion - Figure 10 shows what
Is typically thought of as a conventional, low rate, anaerobic
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digestion system. Essentially, this system Is nothing more than
a large storage tank and no attempt to control the environment
or accelerate the process 1s made.
RAW SLUDGE
SUPERNATANT
DIGESTED
NO SUPPLEMENTAL HEATING
NO SUPPLEMENTAL MIXING
FIGURE 10 : SCHEMATIC OF CONVENTIONAL LOW RATE ANAEROBIC
DIGESTION SYSTEM
- Figure 11 shows
Conventional High Rate Anaerobic Digestion - F1gui
what is typically considered a conventional, high rate,
anaerobic digestion system and 1s the most commonly used system
1n the United States today. In this system attempts are made
to control the environment (through thickening, heating and
mixing) and accelerate the process. Essentially, all digestion
takes place 1n the first tank. This tank Is normally maintained
at 95°F and mixed with some type of gas mixing system. Hydraulic
detention times are normally 15 - 25 days. The majority of
designs also provide a so-called secondary digester for sollds-
Hquld separation (dotted line tank 1n figure 11] but this
RAW
SLUDGE
A
r_ ~.
H
I
I
^ "• ^ -*nT"J_ £19^SJJ
DIGESTED SLU
DIGESTED SLU
»»
FIGURE 11 : SCHEMATIC OF CONVENTIONAL HIGH RATE ANAEROBIC
DIGESTION SYSTEM
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practice 1s being challenged as not being useful 1n many
applications and that going direct to mechanical dewaterlng
can have several significant advantages (56).
Anaerobic Contact - The advantage of sludge recycle in the ana-
eroblc digestion process have not only been discussed but
applied (57 - 60) In treating high strength waste and has been
Indicated to be worthwhile 1n treating waste sludges (61).
Nevertheless, this process alternative Is rarely considered 1n
municipal anaerobic sludge digestion.
Figure 12 shows a typical schematic of the process. The essen-
tial feature of this system 1s that positive separation of the
blomass 1s utilized. Part of this blomass Is recycled back to
the anaerobic digester where It 1s mixed with the Incoming
sludge. This recycling of the sludge thus allows for adequate
cell retention to meet kinetic requirements yet significantly
reduces hydraulic detention time.
RAW
SLUDGE
^1 HEAT
EX
POSITIVE
-SOLIDS LIQUID-
SEPARATION
CLARIFIEDJ.IQUID
DIGESTED SLUDGE
FIGURE 12: SCHEMATIC OF ANAEROBIC CONTACT PROCESS
Phase Separation - As was noted under the general process section,
the anaerobic digestion process consists of two distinct phases.
The previous three systems attempted to do this 1n one reactor.
As early as 1958 (62) the possible value of actually separating
the two processes was discussed. Work 1n 1968 (63) using dialysis
separation techniques clearly showed "---that the hydrolysis-add
production sludge 1s the rate limiting process 1n anaerobic
digestion of sewage sludge. Furthermore, the add formers In a
digester must operate at below optimum conditions In order to
maintain a healthy population of methane forming bacteria." Dur-
ing the past several years considerable research has been conducted
1n this area which was summarized by Ghosh (64) and has also led to
a patented process (65). Figure 13 shows a schematic of this multi-
stage system as conceived by Ghosh (64).
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ACID DIGESTER
METHANE DIGESTER
RAW
POSITIVE
HEAT
DGE
— 1 1 -^3VJLIU LIWVJIV
LJ | SEPARATION
SOLIDS RECYCLE 1 J
EX
5
^^
C
^ —
ILIDS 1
^^
IECY
••*
Cl
m .JWkl
SEP/
i
.E
L* LlVfWIW*
'RATION
POSITIVE
DIGES"
«» SLUDC
FIGURE 13 : SCHEMATIC OF PHASE SEPARATION ANAEROBIC DIGESTION
OF SLUDGE (64)
The phase separation process has several potential benefits when
compared to the other processes. These are (64):
1. Capability of maintaining the optimum environment for each
group of digester organisms.
2. Substantial reduction 1n total reactor volume and the con-
sequent savings In capital and operating costs.
3. Higher rates of solids stabilization and Increased produc-
tion rate and methane content of the final product gases.
4. Decreased heat requirement and Increased thermal efficiency.
5. Suitable for Incorporation Into existing treatment plants
with minimum capital Investment.
6. Reduction of the nitrogen content of the system effluent by
simultaneous liquefaction and denltrlflcatlon of waste feeds
1n the add digester.
GENERAL OPERATIONAL CONTROL PROCEDURES
It should be noted that there 1s no one test or control parameter
that will signify good or bad anaerobic digestion operation. Con-
trol or operation of an anaerobic digestion system should be done
through a combination of several analysts, the results plotted as
a function of time. In this way an unbalanced digester would be
defined as on which starts to radically deviate from past norms.
Note that the norm at one plant can be failure conditions at
another.
At the present time It 1s suggested that a minimum of four (4)
different tests be performed on a regular basis. The four pro-
posed tests are: pH, bicarbonate alkalinity, volatile adds and
percent carbon dioxide (C02) In the digester gas.
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pH - As was discussed under pH CONSIDERATIONS, optimal pH 1s
bTtween 6.4 to 7.5. Unfortunately, the pH test by Itself Is
not a good control procedure (129) because:
1. It 1s a logaMthnlc function and 1s not very sensi-
tive to large fluctuations 1n the alkalinity
concentration. For example, a change 1n alkalinity
from 3600 to 2200 mg/1 would only change the pH
from 7.1 to 6.9 which 1s within the error Involved
In pH measurement.
2. It does not provide adequate warning. A low pH
only Informs the operator that an upset has occurred.
Bicarbonate Alkalinity - The Importance of measuring bicarbonate
alkalinity rather than total alkalinity was discussed In the sec-
tion entitled pH CONSIDERATIONS. The bicarbonate alkalinity and
volatile add test are used together to develop the ratio volatile
acid to bicarbonate alkalinity. In order to Insure good operation
(that Is good buffering capacity), this ratio should be below 0.7.
NOTE: A fast, simple method for differentiating bicarbonate and
volatile acid alkalinity without using d1str11lat1on has
been developed by DILallo and Albertson (130).
Volatile Acids - By Itself this analysis means nothing. Only when
plotted as a function of time or used In conjunction with the
volatile acid-bicarbonate ratio can Impeding operation problems
be Interpreted early enough to allow some type of correctional
procedures.
Carbon Dioxide Content - Under good operation the C02 content In
digester gas will be between 35 - 45 percent. As an unbalance
condition start to occur, there will start to be an Increase 1n
the percentage of CO? as the methane producers become Incapable
of functioning.
When the control parameters Indicate an unbalance condition, the
following steps of action have been recommended (128):
1. Maintain pH near neutrality
2. Determine cause of unbalance
3. Correct cause of unbalance
4. Provide pH control until treatment returns to
normal.
Maintaining the pH near neutrality can be done two ways. The
first Is to reduce the waste feed. A second way Is through the
addition of some type neutralizing material (see section on pH
CONSIDERATIONS and TOXICITY - LIGHT METAL CATIONS.
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Determining the cause of unbalance can be difficult. Some of the
easier things to check are hydraulic washout, heat exchanger not
capable of providing sufficient heat, mixing system not operating.
sudden change 1n the amount of sludge pumped to the digester and
extreme drop 1n pH. If nothing shows up after the above preli-
minary analysis, then testing for ammonia, free sulfldes, heavy
metal and light metal concentrations will have to be made.
Once 1t has been determined what 1s causing the problem, correc-
tive measures can be taken to put the digester back on line.
Depending on the cause of unbalance, the length of time required
to bring a digester back to normal operating condition may take
from 2-3 days to 4 - 6 months.
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Wastewater" Journal WPCF. Vol. 46, #12, pg. 2653, (1974).
114. Lawrence, A.W., and McCarty, P.L., "The Role of Sulfide in
Preventing Heavy Metal Toxicity in Anaerobic Treatment"
Journal WPCF. Vol. 37, pg. 392 (1965).
115. Hasselli, J.W., et.al.. "Sulfide Saturation For Better Digester
Performance" Journal WPCF. Vol. 39, pg. 1369 (1967).
116. Regan, T.M. and Peters, M.M., "Heavy Metals in Digesters:
Failure and Cure" Journal WPCF, Vol. 42, pg. 1832, (1970).
117. Moore, W.A., et.al. , "Effects of Chromium on The Activated
Sludge Process1T~Jo'urnal WPCF. Vol. 33, pg. 54 (1961).
118. Lang's Handbook of Chemistry (1973).
119. "Inhibition of Anaerobic Digestion by Heavy Metals" abstract
from Water Research. Vol. 6, pg. 1062 (1972).
120. Kugelman, I.J. and McCarty, P.L., "Cation Toxicity and
Stimulation in Anaerobic Waste Treatment - I Slug Feed
Studies" Journal WPCF. Vol. 37, pg. 97 (1965).
121. Kuaelman, I.J., and McCarty, P.L., "Cation Toxicity and
Stimulation in Anaerobic Waste Water - II Daily Feed Studies"
Proceedings 19th Purdue Ind. Waste Conference, pg. 667 (1965).
122. Fields, M., and Agardy, F.J., "Oxygen Toxicity in Digesters"
Proceedings 26th Purdue Ind. Waste Conference, pg. 284 (1971).
-47-
-------
123. Lawrence, A.M. and McCarty, P.L., "Effects of Sulfides
on Anaerobic Treatment" Proceedings 19th Purdue Ind.
Haste Conference (1964).
124. Heukelekian, H. and Heinemann, B., "Studies on Methane
Producing Bacteria" Sewage Works Journal , Vol. 11, pgs.
426-453, 571-586, 965^970(1939).
125. Barker, H.A., "Studies Upon the Methane Fermentation
Process" Proceedings of the National Academy of Science
Vol. 29, pg. 184 (1943).
126. Mylroie, R.L. and Hungate, R.E., "Experiments on Methane
Bacteria In Sludge" Canadian Journal of Microbiology,
Vol. 1, pg. 55, (195TT
127. Clark, R.H. and Speece, R.F., "The pH Tolerance of Anaerobic
Digestion" Advances In Water Pollution Research Vol. I ed.
by S.H. Jenkins, Pergamon Press (1970).
128. McCarty, P.L., "Anaerobic Waste Treatment Fundamentals -
Part 2 Environmental Requirements and Control" Pub!ic
Works, Oct., pg. 123, (1964).
129. Brovko, N., et.al ., "Optimizing Gas Production, Methane
Content and Buffer Capcity in Digester Operation" Hater and
Sewage Works, July, pg. 54, (1977).
130. UiLallo, R. and Albertson, O.E., "Volatile Acids by Direct
Titration" Journal WPCF, Vol. 33, April, (1961).
131. Bryant, M.P. et.alI ., "Nutrient Requirements of Methanogenic
Bacteria" Anaerobic Biological Treatment Processes, American
Chemical Society #105 published (1971).
132. Speece, R.E. and McCarty, P.L., "Nutrient Requirements and
Biological Solids Accumulation in Anaerobic Digestion"
Advances In Water Pollution Research, Vol. II ed. by W.W.
Eckenfelder, Pergamon Press (1964).
133. Pfeffer, J.T. and White, J.E., "The Role of Iron In Anaerobic
Digestion" Proceedings 19th Purdue Ind. Waste Conference
(1964).
134. Schwerin, D.J., "The Effect of Temperature on Anaerobic
Digestion" unpublished essay Civil Engineering Dept. in
partial fulfillment for Master of Science Degree, Marquette
University, June (1976).
135. Therkelsen, H.H., and Carlson D.A., "Thermophilic Anaerobic
Digestion of A Strong Complex Substrate" Presented at the
50th WPCF Conference, Philadelphia (1977).
-48-
-------
136. Morgan, P.P., "Studies of Accelerated Digestion of Sewage
Sludge", Sewage and Industrial Wastes. Vol. 26, pg. 462
(1954).
137. Blodgett, J.H., "Discussion on Studies of Accelerated
Digestion of Sewage Sludge" Discussion presented at
Federation of Sewage & Industrial Hastes Association,
Miami, FL., Oct. (1953).
138. Nash, H. and Chasick, A.M., "High Rate Digester Performance
at Jamaica" Journal WPCF. Vol. 32, pg. 526 (1960).
139. Garrison, W.E., et.al.. "Gas Recirculation - Natural,
Artificial" Water Works waste Engr.. Vol. 1, pg. 58 (1964).
140. Zablatzky, H.R., "High Rate Anaerobic Sludge Digesters
Prove Economical, Easy to Operate, In NO Sewer Plant"
Mater and wastes Engineering, Vol. pg. 43 (19 ).
141. Suhr, C.J., "High Rate Digestion Tamed" Water and Wastes
Engineering. Aug. (1964).
142. Sawyer, C.N. and Roy, M.K., A Laboratory Evaluation of
High Rate Sludge Digestion", Sewage and Industrial Wastes,
Vol. 27 pg. 1356 (1955).
143. Clark, R.H. and Orr, V.D., "Digestion: Concentration -
Loading - Time Limits" Journal SEP. ASCE. Vol. 98, SA5,
pg. 809, (1972).
144. Kormanik, R.A., "Estimating Solids Production For Sludge
Handling" Water and Sewage Works. Dec. (1972).
145. Ward R.S., "Digester Gas Helps Meet Energy Needs" Journal
WPCF. Vol. 46, pg. 620 (1974).
146. Courtesy of Envirex Inc.
147. Lang, N.R., Handbook of Chemistry, 10th edition 1966.
148. Drnevich, R.F., and Smith, J.E., Jr., "Pathogen Reduction
In the Thermophilic Aerobic Digestion Process" Presented
at the 48th WPCF Conference, Miami Beach, Oct. (1975).
-49-
-------
ANAEROBIC DIGESTION
OF
MUNICIPAL WASTEWATER.SLUDES
DESIGN PROBLEM
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION
CINCINNATI, OHIO 45268
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
N, A, MlGNONE
ENVIREX INC,
WAUKESHA, WISCONSIN 53186
-------
The claimed advantages of anaerobic digestion are as follows
(1,2)*:
1. Low sludge production.
2. The production of a useful gas of moderate caloric
value.
3. A high kill rate of pathogenic organisms.
4. Production of a solids residue suitable for use as
a soil conditioner.
5. Low operating cost.
*NOTE: ALL REFERENCES ARE LISTED IN BIBLIOGRAPHY OF WRITTEN
DISCUSSION ENTITLED, "ANAEROBIC DIGESTION OF MUNICIPAL
WASTEWATER SLUDGES."
-1-
-------
Table 1 Indicates the type of sludges which have been studied
on a full scale basis.
TABLE 1: TYPE AND REFERENCE OF FULL SCALE STUDIES ON
ANAEROBIC DIGESTION OF MUNICIPAL MASTEWATER
SLUDGE
Sludge Type
Primary and L1me
Primary and Ferric Chloride
Primary and Alum
Prim, and Trickling Filter
Prim., Trickling Filter, Alum
Prim, and Waste Activated
Prim., Waste Activated, L1me
Prim., Waste Activated, Alum
Prim., WAS, Ferric Chloride
Prim., WAS, Sodium Alumlnate
Waste Activated (Pilot Plant only)
Reference on Reference on
MesophlHc ThermophlUc
3,4
5
6
7,8
9
10.11,12 11,13,14
15,16
15,17,18
15
17,18
19,20,21 19,20,21
-2-
-------
TOPICS DISCUSSED IN PAPER:
• GENERAL PROCESS DESCRIPTION
• MESOPHILIC - THERMOPHILIC DIGESTION
• VOLATILE SOLIDS REDUCTION
- SOLIDS CONCENTRATION - ORGANIC LOADING - SLUDGE AGE
• MIXING
• SUPERNATANT
• ENERGY
• NUTRIENTS
• pH CONSIDERATIONS
' TOXICITY
• BACTERICIDAL EFFECTS
• ACTIVATED CARBON
• TANK LAYOUT
• GENERAL OPERATIONAL CONTROL PROCEDURES
-3-
-------
GIVEN
Influent 1s typical American domestic wastewater:
200 mg/1 BODr
200 mg/1 SS
No heavy Industrial contributor
Liquid treatment consists of grit removal, primary treatment,
secondary treatment (activated sludge) and chlorlnatlon.
No chemicals added to liquid treatment portion.
Two flows to be evaluated: 4 MGD and 40 MGD.
-4-
-------
HOW MUCH AND WHAT TYPE OF SLUDGE TO BE AEROBICALLY DIGESTED?
For the typical American wastewater being considered every 1 MG
of raw plant Influent will generate approxlmatly 1000 Ibs. of
primary sludge and 1000 Ibs. of waste activated sludge (144)
This can be further broken down as follows:
TABLE 2: BREAKDOWN OF INERT AND VOLATILE SUSPENDED SOLIDS
PER MG OF PLANT INFLUENT
INERT INERT BIO-DEGRADABLE
NON-VOLATILE VOLATILE VOLATILE
Primary
Sludge 250 Ibs 300 Ibs 450 Ibs
Waste Activated
Sludge 300 Ibs 210 Ibs 490 Ibs
Totals 550 Ibs 510 Ibs 940 Ibs
Based on table 2 the sludge generated for the two design examples
would be
4 MGD DESIGN 40 MGD DESIGN
Inert
Inert
non-volatl le
volatile
B1o-degradable volatile
Total
4
4
4
X
X
X
550 =
510 =
940 =
2
2
3
8
,200
,040
.760
,000
40
40
40
Ibs.
X
X
X
550 =
510 =
940 =
22
20
37
80
,000
,400
,600
,000
-5-
-------
WHAT TEMPERATURES TO BE USED FOR DESIGN?
Temperature 1n high rate digester:
a. For 4 MGD, designer has decided on mesophlUc
conditions - operate at 35°C (95°F).
b. For 40 MGD, designer has decided on thermophl11c
conditions - operate at 54.4°C (130°F).
Coldest ambient air temperature:
For both designs 1t will be assumed to be - 12.2°C (10°F),
Coldest raw sludge temperature:
For both designs It will be assumed to be 4.5°C (40 F).
-6-
-------
WHAT WOULD BE THE REQUIRED HYDRAULIC RESIDENCE TIME - ORGANIC
LOADING - INFLUENT SOLIDS CONCENTRATION FOR HIGH RATE DIGESTER?
For both designs maximum volatile solids destruction 1s desired,
Figure 1 Indicates that for this particular type sludge, a
practical upper limit of 55% VS destruction Is possible and can
be obtained 1n 600 degree-days.
70
g ..
D
g 50
oc
VI
> 40
5?
30
_
• *
\t ' " '"'
1
A
1 1
• FULL SCALE REF.
APILOT PLANT REF
• FULL SCALE REF.
I i i i
(10)
•(45)
(13)
1
200 400 600 800 1000 1200 1400 1600
TEMP. (°C) x SLUDGE AGE (DAYS)
FIGURE 1: VOLATILE SOLIDS REDUCTION VERSUS TEMPERATURE X
SLUDGE AGE FOR ANAEROBICALLY DIGESTED MIXTURE
OF PRIMARY AND WASTE ACTIVATED SLUDGE
Thickened sludge recycle will not be used 1n either design,
therefore sludge age » hydraulic residence time (HRT) 1n high
rate digester.
-7-
-------
4 MGO DESIGN
600°C - days f 35°C • 17 days mfnlmum HRT.
40 MGP DESIGN
60QOC - days - 54.4°C • 11 days minimum HRT.
•
For both designs, a three (3) day stroage capacity 1s also
desired. This dictates that floating covers will be utilized
with minimum hydraulic detention time based on when cover rests
on landing corbels and maximum detention time based on when cover
Is floating at maximum liquid level.
Figure 2 Indicates the possible safe range of organic loading
possible for the given HRT's.
15
o
.3
is
o ^
£ •
o -*
t.o
0.8
0.6
0.4
0.2
0.0
I I i i i i i
—
^* «•
-
—
-
I
X,
1
PROBABLE DIGESTION LIMIT "
> :
%
|
^1
^
^
^^
~ -
i i 1
10 15 20 25 30
SLUDGE AGE - DAYS
35 40
FIGURE 2: RELATIONSHIP BETWEEN SOLIDS CONCENTRATION - ORGANIC
LOADING - SLUDGE AGE FOR ANAEROBIC DIGESTION
-8-
-------
The practical upper limit on feed solids concentration Is 8 - 9%
Within the constraints given, the designer has considerable
latitude for selection of digester tank volume and to a cer-
tain point, selection of necessary thickening equipment. For
the design flows given, the following organic loading has been
selected.
4 MGD DESIGN - 0.15 Ibs VS/cu.f t./day
40 MGD DESIGN - 0.20 Ibs. VS/cu.f t./day
4 MGD DESIGN
x
7*48 gfl1- x 1 - 17 014 GPD
!'•"•* wu
„
0.15 Ib vs/cu.ft./day cu.ft. 17 day m1n.
8.000 Ibs. sol Ids/day x 100 « 5.64X feed solids required
40 MGD DESIGN
58.000 Ibs. vs/day 7.48 gal. 1
0.2 Ib vs/cu.ft./day cu.ft. x 11 day nHn.
-9-
-------
TABLE 3: VARIOUS CALCULATED RESULTS FOR VOLATILE SUSPENDED SOLIDS DESTRUCTION IN ANAEROBIC
DIGESTER _____________
4 MGD DESIGN
40 MGD DESIGN
Lbs Volatile Suspended Solids
(VSS) destroyed per day
.55 (2.040 + 3,760)= 3,190
31.900
% of Total Solids Destroyed
39-9*
39.9*
% of Blo-degradable VS destroyed
84.82
84.8%
Original Inlet Feed VSS/TS
5.800 x 100
8,000
72.5*
Final VSS/TS
5'TiioV90 x
32.6%
-------
WHAT IS EXPECTED ENERGY PRODUCTION?
Depending on sludge composition (oil, grease, fiber, protein) gas production can range
from 12 - 18 cu.ft./lb VS destroyed, with the higher values Indicating high grease con-
tent.
Depending on methane content, each cu.ft. of digester gas has an energy value between
550 to 650 BTU.
4 MGD DESIffN at 55% VS destruction.
IBS. VS CU.FT.
DESTROYED PRODUCED
PER PER
DAY LB VS. PEST.
12
15
3,190
18
40 HGD DESIGN at 55X VS destruction
Would be same as 4 MGD except 10 times greater
NOTE: 1 hp-hr » 2,545 BTU; Electrical energy conversion 32-37X
TOTAL CU.FT.
PRODUCED
PER
DAY
38,280
47.850
57.420
BTU
PER
CU.FT.
550
600
650
550
600
650
550
600
650
TOTAL BTU
PRODUCED
PER .
DAY X 106
21.054
22.960
24.862
26.317
28.710
31.102
31.581
34.452
37.323
-------
WHAT IS EXPECTED SLUDGE HEAT REQUIREMENTS?
In calculating digester heat requirements the two parameters of
concern are:
1. Heat required to raise the temperature of
the Incoming sludge flow to digester operat-
ing temperature.
2. Heat required to maintain the digester
operating temperature (radiation heat loss).
HEAT REQUIRED FOR RAM SLUDGE
Q. « gal of sludge 8.34 Ibs (13 -_Jj) 1 day m
*ay ~~5aT i 24~7r7 '
where:
Qs « Btu/hr required to raise Incoming sludge stream from
temperature T-j to Tg
T] « temperature of raw sludge stream
T2 • temperature desired within the digestion tank
HEAT REQUIRED FOR RADIATION LOSS
Q » U x A x (T2 - 13) 12)
where:
Q B heat loss Btu/hr
A » area of material normal to direction of heat flow
In ft*
T2 « temperature desired within the digestion tank
T3 » temperature outside the digestion tank
U - __ I(3)
~^ '
Iq- *
where:
C1 « conductance for a certain thickness of material
xj . thickness of material - Inches
k. « thermal conductivity of material *tu
J
-12-
-------
Various values of U for different digester covers, wall
construction and floor conditions are given 1n table 4.
TABLE 4: "U" FACTORS FOR VARIOUS ANAEROBIC DIGESTION TANK
MATERIALS (146)
MATERIAL 1!^!.
Fixed steel cover (1/4" plate) 0.91
Fixed concrete cover (9" thick) 0.58
Floating cover (wood composition) 0.33
Concrete wall (12" thick) exposed to air 0.86
Concrete wall (12" thick). 1" air space
and 4" brick 0.27
Concrete wall or floor (12" thick) exposed to
wet earth (101 thick) 0.11
Concrete wall or floor (12" th1ck)exposed to
dry earth(10' thick) 0.06
NOTE: Sludge heat exchangers are normally 75 to 80 percent
efficient. Adjust outlet BTU's by appropriate correc-
tion to find required Inlet BTU's.
-L3-
-------
FIGURE 4: GAS SAFETY PIPING SCHEMATIC
^
VENT TO OUTSIDE
ATMOSPERE
V
PRESSURE GAUGE
0
DRIP 1RAP
0
GAS METCR
NOTES ON DESIGN
1. All gas lines must be tight, sloped (l/4"/ft.) towards condensate traps and accumulators.
have ample capacity and be protected against freezing.
2. Lines leading to gas burners or gas engines must be protected against flashbacks by
flame traps. Trap should be located near point of combustion with a maximum allowable
distance of 30* from point of gas combustion.
3. Bypasses are needed to permit flexibility of operation, but flame traps are never bypassed,
4. Total pressure loss through the appurtenances and gas lines from the digester to use
be onlv 2.0" VI.C. at maximum oas fl"w "••*«.
-------
AEROBIC DIGESTION
OF
MUNICIPAL WASTEWA7ER- SLUDGES
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION
CINCINNATI, OHIO 45268
SEMINAR
SLUDGE TREATS AND DISPOSAL
BY
N, A, MlGNONE
ENVIREX INC,
WAUKESHA, WISCONSIN 53186
-------
Aerobic digestion of municipal wastewater sludges 1s based on
the principal that with inadequate external food sources,
biological cells will consume their own cellular material.
The claimed advantages of aerobic digestion are as follows (1):
1. Volatile solids reduction approximately equal to that
obtained anaerobically.
2. Low 8005 concentrations'in the supernatant liquor.
3. Production of an odorless, humus like, biologically
stable end product that can be disposed of easily.
4. Production of a sludge with excellent dewaterlng
characteristics.
5. Recovery of more of the basic fertilizer value in
the sludge.
6. Few operational problems.
7. Low capital cost.
Table 1 Indicates the type of sludges which have been studied
on a full scale basis.
TABLE 1. TYPE AND REFERENCE OF FULL SCALE STUDIES ON AEROBIC
DIGESTION OF MUNICIPAL WASTEUATER SLUDGE
Reference on Reference on
Mesophilic Thermophilic
Primary sludge only 2,3,4,5,18 6
Waste activated only 7,8 9,10
Mixed primary and WAS 7,8,11 10,12,13
WAS from contact stabilization 14,15
Primary and lime 16
Trickling filter only 2
Mixed primary and trickling
filter 2
Sludges containing Iron or
alum 17,18
Today most aerobic digesters are designed using rules of thumb
developed from past experience (Table 2} and as the literature
has noted (19-22) do not always perform as intended. It is the
intent of this paper to present the most up to date design
criteria available. Whenever possible full scale operating data
is presented.
-1-
-------
TABLE 2. TYPICAL PRESENT DAY AEROBIC DIGESTION DESIGN CRITERIA (2!
Parameter Value
Hydraulic detention time, days at 20°C*
Activated sludge only 12-16
Activated sludge from plant operated
without primary settling 16-18
Primary plus activated or trickling
filter sludge 18-22
Solids loading, Ibs volatile solIds/cu.ft./day 0.1-0.20
Oxygen requirements, Ib/lb cell destroyed 2.0**
Energy requirements for mixing
Mechanical aerators, hp/1,000 cu.ft. 0.5-1.0
A1r mixing, scfm/1,000 cu.ft. 20-30
Dissolved oxygen level In liquid, mg/Hter 1-2
*Detent1on times should be Increased for temperatures below 20°C.
If sludge cannot be withdrawn during certain periods (e.g., week-
ends, rainy weather) additional storage capacity should be
provided.
**Ammon1a produced during carbonaceous oxidation oxidized to
nitrate. .
Topics to be covered 1n this discussion:
Cryophilic - Mesoph1l1c - ThermophlUc Digestion
Volatile Solids Reduction
Oxygen Requirements
Mixing
Supernatant
pH Reduction
Bactericidal Effects
Dewaterlng
Tank Layout
CRYOPHILIC - MESOPHILIC - THERMOPHILIC DIGESTION
For purposes of classification the following three temperature
zones of bacterial action will be used throughout this presen-
tation:
Cryophilic Zone - liquid temperature below 10°C (50 FJ
MesophlUc Zone - liquid temperature between 10°C to 42"C
(50°F to 108°F)
ThermophlUc Zone - liquid temperature above 42°C (108 F)
-2-
-------
The effect of temperature on the effectiveness of aerobic digestion
Is still an area of considerable controversy (24), especially in
the areas of solids reduction, dewaterability and settleability.
The data shown In subsequent sections should help clarify some of
the controversy.
At the present time considerable research 1s being undertaken 1n
the design and operation of thermophllic aerobic systems (13.24-31)
especially auto-thermophll1c aerqbic systems (13,27,29,31). Claimed
advantages of the thermophllic aerobic system are (13,30,31):
1. Higher rates of organic stabilization thus allowing
smaller volume requirements.
2. Higher maintenance energy requirements and higher
microbial decay rates which give smaller amounts
of sludge for disposal.
3. Digestion in this temperature range should make
liquid essentially pathogen free.
4. All weed seeds should be destroyed.
5. Total oxygen demand should be 30 to 40 percent less
than mesophlHc since few, if any, nitrifying
bacteria exist in this temperature range.
6. Improved solids-liquid separation due to decreasing
liquid viscosity.
7. Possible improved oxygen transfer rates because of
the significantly higher coefficient of diffusivity
of oxygen.
VOLATILE SOLIDS REDUCTION
One of the main objectives of aerobic digestion is to reduce the
amount of solids that need to be disposed. This reduction is
normally assumed to take place only with the volatile content of
the sludge through some studies (24,32) have shown that there
can be destruction of the non-organics as well. In this discus-
sion solids reduction will pertain only to the volatile content.
The change in volatile content is normally represented by a first
order biochemical reaction,
dx / dt = -Kd X (1)
where:
dx / dt = rate of change of volatile suspended solids
per unit of time.
Kd = reaction rate constant - day "'
X = concentration of volatile suspended solids
at time t In aerobic digester.
-3-
-------
The time t in equation one is actually the sludge age in the
aerobic digester and depending on how the aerobic digester is
being operated (continuous flow without recycle or with recycle,
batch with supernatant decant) can be considerably greater than
the theoretical hydraulic residence time.
A distinction must be made between biodegradable volatile sus-
pended solids and non-biodegradable volatile suspended solids.
Research 1n this area is quite limited but the following
generalities can be used.
1. Approximately 20 - 30 percent of the Influent
suspended solids of a typical domestic waste-
water is inert (33). Of the remaining suspended
solids which are volatile, approximately 40% are
inert organics consisting chiefly of llgnins,
tannins and other large complex molecules.
2. For waste activated sludges generated from systems
having primary treatment, approximately 20 to 35%
of the volatile solids produced are non-biodegradable
(34,35).
3. For waste activated sludges generated from the
contact-stabilization process (no primaries - all
Influent flow into aeration tank), 25 - 35% of the
volatile suspended solids are non-biodegradable
(15).
The reaction rate term KQ- 1s a function of sludge type, tempera-
ture and solids concentration. It 1s only a psuedo constant,
the term actually being the average results of the many variables
affecting 1t at any one time. Figure 1 shows a plot of various
reported Kj values as a function of the liquid temperature 1n
the aerobic digester. The data shown 1s for several types of
waste sludge which probably is a partial reason for the scatter.
At this time there is not enough data to allow segregation of
Kd by sludge type, therefore the line drawn through the data
points represents an overall average K^ value.
-4-
-------
(A
10
•o
O
U
.40
.35
P .30
i
•
R
.20
.15
.10
.05
A-Pilot Plant
• -Pilot Plant
X- Full Scale
o-Pilot Plant
a- Pilot Plant
• -Pilot Plant
A- Pilot Plant
+- Pilot Plant
Ref(28)
Ref(36)
Ref(10)
Ref(10)
Ref(11)
Ref(27)
Ref(37)
Ref(38)
£
x i
I
/ +
S a
I
•
i
10 20 30 40 50
TEMPERATURE OF LIQUID IN AEROBIC DIGESTOR, °C
Figure 1. Reaction rate Kj versus liquid temperature in digester.
60
Figure 2 Indicates reported effects of solids concentration on
the reaction rate Kj (15).
ra
.7 -
.6
O
.3
_L
J.
_L
J_
6000 10-000 14.000 18,000 22,000
TOTAL SUSPENDED SOLIDS CONCENTRATION IN AEROBIC DIGESTER
Figure 2. Effect of solids concentration of reaction rate Kd constant temperature
(20°) with activated sludge (15)
-5-
-------
Figure 3
volatile
shows the effect
suspended solids
of temperature and
reduction.
sludge age on total
60
50
o
o 30
UJ
> 20
as
10
x - Pilot Plant Ref (16)
• - Full Scale Ref (15)
o- Pilot Scale Ref (7)
A- Full Scale Ref (10)
+ - Pilot Plant Ref (36)
A- Pi lot Plant Ref (38)
• - Pilot Plant Ref (39)
o- Full Scale Ref (37)
I
200 400 600 800 1000 1200 1400 1600
TEMPERATURE (°C) x SLUDGE AGE (days)
1800 2000
Figure 3. Volatile suspended solids reduction as a function of digester
liquid temperature and digester sludge age.
OXYGEN REQUIREMENTS
Activated sludge blomass 1s most often represented by the empiri-
cal equation C5H7N02. Under prolong periods of aeration, typical
of the aerobic digestion process, the biochemical equation for
oxidation 1s represented by equation (2).
702
5C02
3H20
N03
(2)
Theoretically, this reaction states that 1.98 pounds of oxygen 1s
required per pound of cell mass oxidized. In those pilot (36) and
full scale (10,15) studies where this value has been evaluated.the
range was from 1.74 to 2.07 pounds of oxygen required per pound of
oxygen required per pound of volatile solids destroyed. For meso-
phH1c systems a design value of 2.0 1s recommended. For
thermophlllc systems where nitrification would not exist (13,30,
31) a value of 1.4 1s recommended.
-6-
-------
The actual specific oxygen utilization rate, pounds oxygen per
1000 pounds volatile solids per hour, is a function of total
sludge age and liquid temperature (19,24,39). In one study,
Ahlberg and Boyko (19) visited several operating Installations
and developed the relationship shown in figure 4.
£1
8.0
6.0
s 2.0
Temperature Range >10 C
<10C
20
60 100 140
SLUDGE AGE. days
180
220
Figure 4. Effects of sludge age and liquid temperature on oxygen uptake
rates in aerobic digesters (19)
Field studies (19) have Indicated that a minimum value of 1.0 mg
of oxygen should be maintained 1n the digester at all times.
MIXING
Mixing 1n an aerobic digester, treating municipal wastewater
sludge of domestic origin, is considered to have the following
benefits. (Note: It is assumed that a favorable environment
exist to allow development of an aerobic digestion system).
1. To continuously bring
device.
deoxygenated liquid to the aeration
2. To keep the food supply uniformly dispersed and in constant
contact with the growing cells to promote maximum utiliza-
tion of the system.
-7-
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3. To keep the concentration of biological end products at
their lowest value by dispersing them uniformly throughout
the digester.
4. To provide environmental uniformity (oxygen, temperature,
nutrients, etc.) throughout the digester allowing best
possible cell development.
5. To allow fairly fast dispersion of any toxic material
entering the system thus possibly minimizing its effect
on the aerobic process.
There is general agreement that mixing is an Important criteria
in the aerobic digestion process. The problem arises when one
tries to evaluate, define or specify.
CHARACTERISTICS OF AEROBIC DIGESTERS
The existing trend in wastewater treatment is to remove more and
more material from the main liquid processing stream. This 1s
done through the use of secondary biological treatment schemes,
chemical addition and filters. The sludge produced can vary
widely and change rapidly even on an hour to hour basis.
Table 3 shows specific gravity and particle size distribution on
two common type sludges: plain primary and plain waste activated
(41).
IABLE 3. GENERAL CHARACIERISTICS OF RAW PRIMARY AND WASTE ACTIVATEC
SLUDGE
WASTE ACTIVATED
PRIMARY SLUDGE SLUDGE
Specific Gravity 1.33 - 1.4 1.01 - 1.05
Particle bize 20% < 1 urn 40% 1 - 50 urn
35% 1 - 100 urn 60% 50 - 180 urn
45% > 100 urn
Physical Appearance Fiberous Slimey, gelantinous
There is little data on the rheology of municipal wastewater sludge
(42) and none could be found on strictly aeroblcally digested
sludge. One of the main problems 1n obtaining data is the extreme
difficulty 1n doing such studies correctly (43).
Even though the majority of raw wastewater sludges behave as a
thixotroplc (time dependent), pseudo plastic material (figure 5),
1t may not be correct to assume that the sludge within the aerobic
digester has the same general properties. The liquid will have a
variable solids concentration and there is a general reduction 1n
particle size and shape (39,44) both of which effect fluid v1scos1t.
-8-
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RATE OF SHEAR
Another
tend to
Figure 5. Shear-stress relationship for a thixotropic,
psuedo plastic material
characteristic of present day designs 1s that the tanks
have large surface area to liquid depth ratios.
DEFINING MIXING
In recent years It has become popular to use the term "complete
mix" when discussing biological process reactors. The term
"complete mix" 1s a relative term. It means that the time for
dispersion of the feed stream Is short 1n relation to the total
hydraulic residence time In the reactor. It 1s also defined as
sufficient mixing so that concentration gradients of chemical
and biological Ingredients are uniform for the particular reac-
tion rates that exist 1n the basin.
Mixing within the aerobic digestion tank occurs on two levels:
macromlxlng and m1crom1x1ng (45). Macromlxlng deals with the
bulk mass flow within the digester while m1crom1x1ng deals with
the degree of Intermingling of the system molecules. In bio-
logical theory the assumption of "complete mix" assumes micro-
mixing (46).
The actual mixing can be performed by
or a combination of the two.
a gas system, mechanical
No matter what type of device 1s utilized, the Intent 1s to
achieve mixing through a pumping action. Because of this
relationship, engineers have come to use the term hp/un1t volume
as some type of parameter to define mixing In an aerobic digester.
Unfortunately, this term by Itself has no meaning. For mechanical
type mixers the wide variation 1n Impeller diameters and speeds
can result in similar horsepower but widely different pumping
capacities. For gas mixing systems gas flow, depth,and bubble
size can also result in similar horsepower but widely different
-9-
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pumping capacities. In addition tank geometry and solids con-
centration can s1gn1f1dantly affect power requirements.
Probably the best way to define mixing is from the standpoint
of zone of Influence of an energy source (figure 6). Essen-
tially the zone of influence states that energy is dissipated
as one moves horizontally away from the energy source. This
loss is due to friction between the fluid molecules which is a
function of liquid density, termperature and solids concentra-
tion. Within a certain area of the point source there is
sufficient energy to achieve micromixlng. There is also a larger
area where bulk flow (macomlxing) still takes place even though
there is Insufficient energy for mlcromlxing.
Studies (47,48) done with point energy sources, in clean water
and with no side boundrles (only surface and floor boundries)
have Indicated that the width of the micromixlng zone is no more
than twice the liquid depth, with the liquid depth being a
function of the type of mixing device utilized and not necessar-
ily the tank liquid depth.
The effect of tank geometry (49) on mixing (as measured by oxygen
transfer rates in clean water) for various aeration devices (high
and low speed mechanical aerators, submerged turbines, oxidation
ditch aerator and diffused aeration) in tanks from several thou-
sand to one million gallons, was shown to fall into three general
categories (figure 7).
Category 1 is represented by basin geometry A in figure 7. This
is the Idealized case in which geometry has no effect on the
liquid flow pattern. Each increment of power into this specific
volume has a corresponding Increase in the oxygen supplied.
Category 2 1s represented by basin geometry B 1n figure 7 and
has been termed the "flywheel effect". Here tank constraints
represented for example by a channel aeration tank, causes a
rapid increase In oxygen supply for small inputs of energy. As
the energy per unit volume increases, the geometry of tanks
cause a leveling off in transfer.
Category 3 is represented by basin geometry C 1n figure 7 and
has been termed the "choke flow effect." Here tank geometry
interferes with the mixing pattern until a certain energy level
is reached. At this point there is sufficient energy to over-
ride the constraint thus allowing for complete mixing of the
tank contents.
-10-
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PROFILE VIEW
('<
ENERGV
X
SOURCE
y
\~ - °1
.D2
LIQUID HEIGHT
= Effective zone diameter for micromixing
* Effective zone diameter for macromixing
Figure 6. Schematic of zone of mixing influence for energy source in fluid with
only fixed upper and lower boundaries.
-11-
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T
BASIN GEOMETRY A
BASIN GEOMETRY B
ENERGY INPUT
ENERGY INPUT-
BASIN GEOMETRY C
ENERGY INPUT
Figure 7. The effects of tank geometry on mixing in clean water as measured by oxygen transfer rates.
No published studies on field evaluation of the effect of
suspended solids on mixing 1n aerobic digester could be found
at this time. There have been several such studies (50-52)
conducted 1n lagoons with suspended solids 1n the range of
100 to 400 mg/1. Figure 8 shows the results from reference 50.
In general, Increasing solids concentrations required increased
power levels though the tank geometry (52) and interaction
effects of other aerators (51) also influenced mixing patterns.
-12-
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Ol
E
140 -
120
CO
§ 100
8
Q
UJ
O
in
33
ce
O
O
80
60
40
20
I
Theoretical form
_L
_L
10 20 30
POWER LEVEL, hp/1 mg
Figure 8. Power level versus suspended solids (50)
SUPERNATANT
It is common practice in most aerobic digestion facilities not
to prethicken the sludge but to concentrate after digestion.
This is done by sending the flow to a clarifier-thickener or
by turning off the aeration device, within the digester for
12 - 18 hours. When this is done, a digester supernatant is
taken off which is normally returned to the head end of the
treatment plant. Table 4 gives supernatant characteristics
from several full scale facilities operating in the mesophilic
temperature range.
-13-
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TABLE 4. CHARACTERISTICS OF MESOPHILIC AEROBIC DIGESTER
SUPERNATANT
Turbidity
N03 - N
TKN
COD
PDA - P
Soluble
BODe
Filtered BODc
Suspended Solids
AIK
S04
Silica
PH
Ref. (9)*
120
40
115
700
70
50
300
6.8
Ref. (19)**
_ m i
2.9 -
24 -
2.1 -
.4 -
5 -
3 -
9 -
5.7 -
»
1350
25,500
930
120
6,350
280
41,800
8.0
Ref. (65)***
• MB
30
35
2 - 5
6.8
150
70
26
6.8
*Average of 7 months of data
**Range taken from 7 operating facilities
***Average values
pH REDUCIION
Figure 9 shows the effect of sludge age on digester pH for
mesophilic operation.
8.0
7.0
6.0
5.0
4.0
3.0
Liquid temp at 5° C
Liquid temp at 20° C
I
I
10 30 50
SLUDGE AGE IN AEROBIC DIGESTERS
70
Figure 9. Effects of sludge age on pH for mesophilic aerobic digestion.
-14-
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The drop in pH 1s caused by an increased concentration of nitrate
ions and a corresponding loss of alkalinity due to the conversion
of NH3-N to N03-N commonly called nitrification. Though at one
time, the low pH was considered inhibatory to the process, It has
been shown, that over time, the system will accllmize and perform
just as well at the lower pH values (7,39,65).
It should be noted that if nitrification does not take place,
there will be very little, if any, pH drop. This could happen
at low liquid temperatures and short sludge ages or 1n thermo-
philic operation (31). Nitrifying bacteria are sensitive to
heat and do not exist 1n temperatures over 45 degree centrlgrade
(66).
BACTERICIDAL EFFECTS
Pathogenic organisms 1n wastewaters consist of
protoza and parasitic worms and a good current
subject can be found in reference 55. Many of
especially enteric viruses (56), have a strong
themselves to sludge solids.
bacteria, virus,
review on the
these organisms,
tendency to bind
Table 5 gives a listing of human enteric pathogens occurring in
wastewater sludges along with the diseases associated with them.
Table 6 gives some data on bacterial concentrations of various
type raw sludges.
TABLE 5. HUMAN ENTERIC PATHOGENS OCCURRING IN WASTEWATER AND
THE DISEASES ASSOCIATED WITH THE PATHOGEN (57)
PATHOGENS
Vibrio Cholera
Salmonella typhi
Shigella species •
Coliform species
Pseudomonas species
Infectious hepatitus virus
Poliovirus
Entamoeba histolytica
Plnworms (eggs)
Tapeworms
DISEASES
Cholera
Typhoid and other enteric fevers
Bacterial dysentery
Diarrhea
Local infection
Heptatitls
Poliomyletis
Amoebic dysentery
Aseariasis
Tapeworm Infestation
-15-
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TABLE 6. PATHOGENIC ORGANISMS IN SLUDGE (54.58)
TYPE
Raw Primary
Trickling Filter
Raw WAS
Thickened Raw WAS
SALMONELLA
No./TOO ml
460
62
93
74
2300
6
9300
PSEUDOMONAS
AERUGINOSA
No./TOO ml
46 x 103
195
110 x 103
1.1 x 103
24 x 103
5.5 x 103
2 x 103
FECAL
COLIEORM
No. x 10°/100 ml
11.4
11.5
2.8
2.0
26.5
20
Researchers have studied pathogenic organism reduction 1n both
mesophilic (54,59,60) and thermophH1c digestion (53). Under
mesophilic operation the bactericidal effects appear to be re-
lated to natural dies off with time. For thermophlUc operation,
the time required for reduction of pathogenic organisms below
minimal detection level 1s a function of basin liquid tempera-
ture (Table 7).
TABLE 7. THERMOPHILIC AEROBIC DIGESTION TIME REQUIRED FOR
REDUCTION OF PATHOGENIC ORGANISMS BELOW MINIMUM
DETECTABLE LEVEL (53)
TYPE
Mixture of primary
TIME REQUIRED FOR
LOWEST DETECTABLE
LIMIT OF SALOMONELLA
HOUR?
TIME REQUIRED FO
LOWEST DETECTABLE
LIMIT OF PSEUDOM"
AERUGINOSA HOU
and waste activated
45
50
55
60
24
5
1
0.5
24
2
2
0.5
DEWATERING
One of the supposed benefits of aerobic digestion is the produc-
tion of a sludge with excellent dewatering characteristics (1).
Much of the published literature on full scale operations have
indicated this 1s not true (3,4,17,26,61) though there are pub-
lished reports of excellent operating systems (15).
-16-
-------
Although most recent Investigators agree that there 1s deteriora-
tion with Increasing sludge age (2,16,17,27,62) there is still
debate as what 1s the cause; lack of sufficient oxygen (26,27)
reduction 1n particle size (16,17) or concentration of biological
anionlc polymers (63).
At this time 1t can only be recommended that conservative design
be used for designing mechanical sludge dewaterlng facilities
unless pilot plant data Indicates'otherwise.
TANK LAYOUT
Originally aerobic digesters were operated as strictly a batch
operation and this concept 1s still used at many facilities
(Figure 10).
UNSTABILIZED
SOLIDS
AEROBIC DIGESTER
AEROBIC DIGESTER #2
SUPERNATANT
STABILIZED SOLIDS
Figure 10. Tank configuration for a batch operated aerobic digester.
Solids are pumped directly from the clarlfiers into the
digester. Eventually, the digester fills up, the time
depending not only on the waste sludge flow but amount
cipitation or evaporation. When the tank is full, the
device is turned off for several hours to allow solids-
separation, then a decant operation takes place. After
ing, thickened stabilized solids, about 2-4 percent,
be removed or more waste sludae can be added.
aerobic
required
of pre-
aeratlon
liquid
decant-
can then
Many engineers tried to make the process more continuous by
installing stilling wells 1n part of the digester. This has
proven not to be effective (20,67,68) and should not be incor-
porated into the design.
-17-
-------
The next step was then to provide the aerobic digester with Its
own clarlfler-thlckener (Figure 11).
UNSTABILIZED
SOLIDS
AEROBIC
DIGESTER
SUPERNATANT
RECYCLE
CLARIFIER
THICKENER
STABILIZED SOLIDS
Figure 11. Tank configuration for a continuous operated aerobic digester.
Solids are still pumped directly from the clarlflers Into the
aerobic digester. In this case the aerobic digester operates
at a fixed level with the overflow going to a solids-liquid
separator. Thickened solids are normally recycled back to the
digestion tank but when required can also be removed from the
system.
Though Initially more costly than a batch operated system.
much of the manual work Involved with aerobic digestion 1s
eliminated.
A third type of system would Involve prethlckenlng before
aerobic digestion. This 1s essentially an auto thermophlUc
aerobic digestion system (Figure 12).
CENTRATE
i
UNSTABILIZED
SOLIDS
THICKENER
AEROBIC DIGESTER
STABILIZED
SOLIDS
Figure 12. Tank configuration for an auto thermophilic aerobic digestion system.
-18-
-------
In this system sludge from the clarlfiers would go to some
type of thickening device to produce a concentration greater
than 4 percent solids Into the digester. When operating 1n
this mode, one should not expect any further gravity solids-
liquid separation to take place after digestion.
-19-
-------
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-20-
-------
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-21-
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-22-
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-23-
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-24-
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67. Paredes, M., "Supernatant Decanting of Aerobically Digested
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Handling" Water and Sewage Horks, Dec. (1972).
-------
AEROBIC DIGESTION
OF
MUNICIPAL WASTEWATER SLUDGES
DESIGN PROBLEM
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO 45268
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
N, A, MlGNONE
ENVIREX INC,
WAUKESHA, WISCONSIN 53186
-------
The claimed advantages of aerobic digestion are as follows (1)*
1. Volatile solids reduction approximately equal to
that obtained anaerobically.
2. Low 0005 concentrations in the supernatant liquor.
3. Production of an odorless, humus like, biologically
stable end product that can be disposed of easily.
4. Production of a sludge with excellent dewatering
character!sties.
5. Recovery of more of the basic fertilizer value in
the sludge.
6. Few operational problems.
7. Low capital cost.
*NOTE: ALL REFERENCES ARE LISTED IN BIBLIOGRAPHY OF WRITTEN
DISCUSSION ENTITLED, "AEROBIC DIGESTION OF MUNICIPAL
WASTEWATER SLUDGES."
-1-
-------
Table 1 Indicates the types of sludges which have been studied
on a full scale basis.
TABLE 1: TYPE AND REFERENCE OF FULL SCALE STUDIES ON AEROBIC
DIGESTION OF MUNICIPAL WASTEWATER SLUDGE
Reference On Reference On
Sludge Type Mesophilic Thehnophi He
Primary Sludge only 2,3,4,5,18 6
Waste Activated only 7,8 9,10
Mixed Primary and Waste Actlved
Sludge 7,8,11 10,12,13
WAS from Contact Stab. 14,15
Prim.andLlme 16
Trickling Filter only 2
Mixed Prim, and TF 2
Sludges Containing Iron and Alum 17,18
-2-
-------
TOPICS DISCUSSED IN PAPER:
• CRYOPHILIC - MESOPHILIC - THERMOPHILIC DIGESTION
• VOLATILE SOLIDS REDUCTION
• OXYGEN REQUIREMENTS
• MIXING
• SUPERNATANT QUALITY
• pH REDUCTION
• BACTERICIDAL EFFECTS
• DEWATERING EXPERIENCE
• TANK LAYOUT
-3-
-------
GIVEN
Influent is typical American domestic wastewater:
200 mg/1 BOD5
200 mg/1 SS
No heavy industrial contributor
Liquid treatment consists of grit removal, primary treatment,
secondary treatment (activated sludge)and chlorination.
No chemicals added to liquid treatment portion.
Two flows to be evaluated: 4 MGD and 40 MGD.
-4-
-------
HOW MUCH AND WHAT TYPE OF SLUDGE TO BE AEROBICALLY DIGESTED?
For the typical American wastewater being considered every 1 MG
of raw plant influent will generate approximately 1000 Ibs. of
primary sludge and 1000 Ibs. of waste activated sludge (69).
This can be further broken down as follows:
TABLE 2: BREAKDOWN OF INERT AND VOLATILE SUSPENDED SOLIDS
PER MG OF PLANT INFLUENT
Primary
SI udge
Waste Activated
Sludge
INERT
NON-VOLATILE
250 Ibs
300 Ibs
INERT
VOLATILE
300 Ibs
210 Ibs
BIO-DEGRADABLE
VOLATILE
450 Ibs
490 Ibs
Totals 550 Ibs 510 Ibs 940 Ibs
Based on table 2 the sludge generated for the two design examples
would be
4 MGD DESIGN 40 MGD DESIGN
Inert non-volatile 4 x 550 = 2,200 40 x 550 = 22,000
Inert volatile 4 x 510 • 2,040 40 x 510 = 20,400
Bio-degradable volatile 4 x 940 = 3.760 40 x 940 = 37.600
Total 8,000 Ibs. 80,000 Ibs
-5-
-------
WHAT WILL BE THE WARMEST AND COLDEST WATER TEMPEARTURE IN THE
AEROBIC DIGESTER?
Temperature plays Important roles in the aerobic digestion
process:
• Effects oxygen transfer capabilities
• Effects volatile destruction capabilities
Temperature in aerobic digester is a function of
Feed solids concentration
' Geographical location of treatment facility
• Tank location and material of construction
* Type of aeration device utilized
For this design example the following assumptions will be made
1. Thermophilic or auto-thermophi11c aerobic digestion
will not be considered. This implies average inlet
feed solids to digester under 3.5% solids concentra-
tion.
2. Lowest liquid temperature expected during winter is
10°C (50°F). During the summer 25.5°C (78°F) is
expected.
-6-
-------
WHAT TYPE OF VOLATILE SOLIDS DESTRUCTION CAN BE EXPECTED?
Figure 1 shows a plot of volatile suspended solids destruction
as a function of liquid temperature and sludge age. A minimum
of 40% VSS reduction has been chosen for the design example
which would require a temperature - sludge age combination of
475 days. At the minimum liquid temperature of 10°C., this would
Imply a sludge age of 47.5 days. Assuming that the system will
be designed to maintain a 47.5 day sludge age, then during the
summer this combination would be 47.5 x 25.5 = 1211 °C-days.
This would give a 49% reduction. Table 3 gives various ratios
which could be developed.
60
50
40
o
Q 30
UJ
CO
> 20
10
0
OfO
A'
* - Pilot Plant Ref (16)
• - Full Scale Ref (15)
D- Pilot Scale Ref (7)
A- Full Scale Ref (10)
+ - Pilot Plant Ref (36)
A- Pi lot Plant Ref (38)
• - Pilot Plant Ref (39)
o- Full Scale Ref (37)
200 400 600 800 1000 1200 1400 1600 1800 2000
TEMPERATURE (°C) x SLUDGE AGE (days)
FIGURE 1: VOLATILE SUSPENDED SOLIDS REDUCTION AS A FUNCTION
OF DIGESTER LIQUID TEMPEARTURE AND DIGESTER SLUDGE
AGE
-7-
-------
TABLE 3- VARIOUS CALCULATED RESULTS FOR VOLATILE SUSPENDED SOLIDS DESTRUCTION IN AEROBIC
DIGESTER
4 MGD DESIGN 40 MGD DESIGN
Lbs Volatile Suspended Solids
(VSS) destroyed per day
Winter 0.4 (2.040 + 3,760) = 2,320 23,200
Summer 0.49 (2,040 + 3,760) = 2,842 28,420
% of Total Solids Destroyed
Winter 2.320 x ]00 = 29% 29%
8,000
Summer 2.842 x 100 = 35.5% 35.5%
8,000
% of Bio-degradable VS destroyed
Winter 2.320 x 100 = 61.2% 61.2%
3,760
„ n.« x 100 = 75.5% 75.5%
Summer 2.842
3,760
Original inlet feed VSS/TS 5.800 x 100 = 72.5% 72.5%
8,000
Final VSS/TS
Winter 5.800 - 2.320 x 10Q = 43.5% 43.5%
8,000
Summer 5,800-2,842 x 100 = 36>95; 36s9%
o, UUU
-------
WHAT IS THE EXPECTED SUSPENDED SOLIDS CONCENTRATION IN THE
AEROBIC DIGESTER UNDERFLOW?
Function of overall detention time, local evaporation rate and
type of aerobic digestion system employed (batch or continuous).
Typically degritted, no chemical addition, aerobically digested
sludge can be gravity thickened to 2.5 to 3.5 percent. For
this design a maximum of 3.0 percent is assumed.
It is assumed that there is no prior thickening of the raw
sludges so that the average inlet feed solids concentration is
under 3.0 percent and gravity thickening is possible. For
this example, the inlet feed solids concentration for the
combined sludge is assumed to be 1.5% solids.
-9-
-------
WHAT WILL BE THE OXYGEN REQUIREMENTS?
It was assumed that these design examples would not be designed
for thermophilic aerobic digestion therefore must meet nitrifi-
cation oxygen demand.
Theoretically, equation (1) states that 1.98 pounds of oxygen
is required per pound of cell mass oxidized.
C5H7N02 + 702 > 5C02 + 3H20 + H + + NOT (1)
In those pilot (36) and full scale (10,15) studies where this
value has been evaluated, the range was from 1.74 to 2.07
pounds of oxygen per pound of volatile solids destroyed. Use
2.0 for design purposes.
TABLE 4: AVERAGE POUNDS OF OXYGEN REQUIRED PER DAY FOR AEROBIC
DIGESTION SYSTEM
4 MGD DESIGN 40 MGD DESIGN
Winter 2.0 x 2,320 = 4,640 46,400
Summer 2.0 x 2,842 = 5,684 56,840
-10-
-------
WHAT IS THE MINIMUM TANK VOLUME NECESSARY TO ACHIEVE DESIRED
RESULTS? _^____
It was previously noted that a minimum volatile suspended solids
reduction of 40% was required at the 10° C liquid level. Based
on figure 1 this would imply a minimum sludge age of 47.5 days.
z
o
60
50
40
o
Q 30
to
to
20
10
I
o ceo
0*0
x - Pilot Plant Ref (16)
• - Full Scale Ref (15)
0- Pilot Scale Ref (7)
A- Full Scale Ref (10)
+ - Pilot Plant Ref (36)
A- Pilot Plant Ref (38)
• - Pilot Plant Ref (39)
o- Full Scale Ref (37)
I
200 400 600 800 1000 1200 1400 1600 1800 2000
TEMPERATURE (°C) x SLUDGE AGE (days)
FIGURE 1. VOLATILE SUSPENDED SOLIDS REDUCTION AS A FUNCTION OF
DIGESTER LIQUID TEMPERATURE AND DIGESTER SLUDGE AGE
-11-
-------
SLUDGE AGE IN AEROBIC DIGESTER CAN BE APPROXIMATED AS FOLLOWS:
SLUDGE AGE =
Total Ibs. SS In Aerobic Digester
Total Ibs. SS Lost per Day From Aerobic Digester
Total Ibs. SS In Aerobic Digester
'Total Ibs. SS Wasted
(2)
(3)
Total Ibs. SS Lost per>
Day In Supernatant
per day from system
(SS Cone. In Pigester)(8.34)(Pigester tank Volume)
... (4)
-/SS Cone. 1n\ /SS Cone. 1n>
] (1-f) + HO
\Supernatanty \Underflow
(8.34) (influent flow)
where:
f = (influent SS cone) (X solids not destroyed)
'thickened SS cone.
SS Cone, in Supernatant - 1f good solids liquid separation takes place can
expect about 300 mg/1 SS in supernatant.
SS Cone, in Digester
- can range from a minimum equal to the influent
SS concentration to a maximum equal to the
thickened concentration (assume no evaporation)
Assume that on the average SS cone equal to 70
percent of the thickened concentration.
Digester Tank Volume - million gallons
-------
FOR 4 MGD DESIGN
Sludge Age = 47.5 days
SS Cone, in Digester = (0.7) (30,000 mg/1)
SS Cone, in Supernatant = 300 mg/1
SS Cone, in Underflow = 30,000 mg/1
f = (1.5%) (.71) . n 35
3.0% °'Jb
influent flow = Q^WQ 34) * 63'950 GPD = O-063" MGD
47.5 = (0.7)(30tOOO) (Tank Vol) = (21.000)Tank Vol
(30QH1-.35) + (30.DOOM.35) (0.06395) 6T7
Digester Tank Volume = (697)(47.5) = 1>576MGD
FOR 40 MGD DESIGN
Everything the same except for influent flow which = .6395 MGD
Tank volume = 15.76 MG
In addition to the tank volume calculated, additional volume
may be required depending on local weather conditions and type
of down stream sludge handling facilities.
-13-
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TANK LAYOUT
For the mesophlUc aerobic digestion system being considered,
there are two type of systems to choose from: the batch
operated system (figure 2) or the continuous flow through
system (figure 3).
SUPERNATANT
UNSTABILIZEO
SOLIDS
AEROBIC DIGESTER
AEROBIC DIGESTER 02
STABILIZED SOLIDS
FIGURE 2: TANK CONFIGURATION FOR A BATCH OPERATED AEROBIC
DIGESTER
UNSTABILIZED
SOLIDS
AEROBIC
DIGESTER
SUPERNATANT
RECYCLE
CLARIFIER
THICKENER
STABILIZED SOLIDS
FIGURE 3: TANK CONFIGURATION FOR A CONTINUOUS OPERATED AEROBIC
DIGESTER _
-14-
-------
The original aerobic digestion systems were batch operated
system and this is still the most prevalent design.
Solids are pumped directly from the clarifiers into the aerobic
digester. Eventually, the tank f.ills up, the time required
depending not only on the waste sludge flow but the amount of
precipitation or evaporation. When the tank is full, the
aeration device is turned off for several hours to allow solids-
liquid separation, then a decant operation takes place. After
decanting, thickened stabilized solids, at a 3 percent, could
then be removed or more waste sludge would be added.
In the past many engineers have tried to make this design more
continuous by installing stilling wells in part of the tank.
This has proven not to be effective (20,67,68) and should not be
incorporated Into the design.
For the continuous operated system solids are again pumped
directly from the clarifiers into the aerobic digester. In
this case, the aerobic digester operates at a fixed liquid
level, with the overflow going to a solids-liquid separator.
Thickened solids are normally recycled back to the digestion
tank but when required, can also be removed from the system.
Though initially more costly than a batch operated system, much
of the manual work involved with aerobic digestion is eliminated,
-15-
-------
Another consideration when sizing the aerobic digestion tank
1s the relationship between the tank geometry desired, the
type of aeration equipment being utilized and the mixing
pattern that will develop. Figure 4 shows the effect of tank
geometry on mixing as measured by oxygen transfer rates.
01
BASIN GEOMETRY A
BASIN GEOMETRY B
ENERGY INPUT
ENERGY INPUT-
BASIN GEOMETRY C
ENERGY INPUT —^
FIGURE 4- THE EFFECTS OF TANK GEOMETRY ON MIXING IN CLEAN WATER
AS MEASURED BY OXYGEN TRANSFER RATES (49)
-16-
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SUMMARY
There are times when biological sludge stabilization using
aerobic digestion will be cost effective. A method of
design logic was presented showing common design parameters
that must be considered when designing such a system. It
should be remembered though, that each design project has
its own peculiarities which must be incorporated. Failing
to do so may lead to serious operational problems.
-17-
-------
THERMAL TREATOfT
FOR
SLUDGE CONDITIONING
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO 452R8
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
G, M, WESNER
CULP/WESNER/CULP
SANTA ANA, CALIFORNIA
-------
TABLE OF CONTENTS
Page
INTRODUCTION 1
PROCESS DESCRIPTION 5
ZIMPRO PROCESS 5
ENVIROTECH BSP PROCESS 7
OTHER PROCESSES 7
THERMAL TREATMENT PROCESS SIDESTREAM 7
Gas Sidestreams 7
Liquid Sidestreams 8
THERMAL CONDITIONING COSTS 11
DESIGN EXAMPLE 22
REFERENCES 25
LIST OF TABLES
1. Thermal Conditioning Installations 6
LIST OF FIGURES
1. Typical Heat Treatment System 3
2. Heat Treatment in Sludge Management Systems 4
3. Construction Costs 13
A. Fuel Requirements 15
5. Electrical Energy Requirements 16
6. Operation and Maintenance Labor Requirements 17
7. Costs for Materials and Supplies 19
8. Total Costs for Thermal Conditioning Systems 20
9. Thermal Conditioning Example 23
iti
-------
INTRODUCTION
The purpose of this paper is to consider thermal treatment of sludge
as a conditioning process to improve sludge dewaterability by subsequent
processes such as vacuum filter, centrifuge or filter press. Thermal
conditioning (also often called heat treatment) involves heating sludge,
with or without the addition of air or oxygen, to temperatures of 300 to
500°F in a reactor under pressures of 150 to 400 psig for periods of 15
to 40 minutes. Thermal conditioning causes the release of water and organic
material from sludge in the form of a dark brown fluid or "cooking liquor".
Other thermal treatment processes not discussed herein include: (1)
pasteurization which operates at lower temperatures, in the range of 160°F,
and (2) wet air oxidation which operates at higher temperatures and pressures
for more complete oxidation of sludge solids.
The EPA Technology Transfer manual on sludge treatment1 describes thermal
conditioning, or heat treatment, as follows:
"In heat treatment, temperatures of from 300 to 500°F and pressures
of 150 to 400 psig are attained for protracted periods. Significant
changes in the nature and composition of wastewater sludges result.
The effect of heat treatment has been ideally likened to syneresis,
or the breakdown of a gel into water and residual solids. Wastewater
sludges are essentially cellular material. These cells contain intra-
cellular gel and extracellular zoogleal slime with equal amounts of
carbohydrate and protein. Heat treatment breaks open the cells and
releases mainly proteinaceous protoplasm. It also breaks down the
protein and zoogleal slime, producing a dark brown liquor consisting
of soluble polypeptides, ammonia nitrogen, volatile acids, and carbohy-
drates. The solid material left behind is mineral matter and cell vail
debris.
Dewatering is improved by the solubility and hydrolyzing of the
smaller and more highly hydrated sludge particles which then end up
in the cooking liquor. while analysis of this liquor from domestic
wastewater sludges indicates the breakdown products are mostly organic
acids, sugars, polysacchandes, amino acids, ammonia, etc., the exact
composition of the liquor is not well defined.
A review of reported analyses of liquor from the heat treatment of
sludge gives the range of values shown: BOD$ = 5,000 to 15,000 mg/1
COD = 10,000 to 30,000 mg/1, Ammonia = 500 to 700 mg/1, and Phosphorus
as P = 150 to 200 mg/1. About 20 to 30 percent of the COD is not bio-
degradable in a 30-day period. The volume of cooking liquor from an
-------
activated sludge plant with heat treatment amounts to 0.75 to 1.0
percent of the wastewater flow. Based on BOD$ and solids loadings,
the liquor can represent 30 to 50 percent of the loading to the
aeration system. The pH of cooking liquors is normally in the range
of 4 to 5, which necessitates chemical neutralization and/or corrosion
resistant equipment."
Figure 1 is a flow diagram for a typical heat treatment system. Major
c:r?onents in the system are a heat exchanger and a reaction vessel. Heat
treatment may be used to condition raw or digested sludges and thus location
of the system in the overall treatment train may vary. If a treatment plant
ertloys anaerobic digestion, heat treatment is more commonly used to condition
t-= digested sludge. Heat treatment before anaerobic digestion to improve
d=*radability and energy production was pilot tested by LA/OMA in Los
Ar.g=les.2'3 Heat treatment may be used in conjunction with incineration in
a system that recycles waste heat to minimize energy requirements. These
variations in the use of heat treatment in sludge management systems are
illustrated in Figure 2.
The effect of heat treatment on the chemical composition of sludge was
investigated by Sommers and Curtis.1* Heat treated sludges from plants in
Speedway and Terre Haute, Indiana were tested to obtain information on the
fotjis of nitrogen, phosphorus, copper, zinc, nickel, cadmium and lead. In
general, heat treatment produced greater than 50 percent reductions in total
nitrogen with essentially no change or a slight increase in phosphorus and
metals concentrations.
-------
SLUDGE
STORAGE
HEAT
EXCHANGER
DECANT
LIQUOR
CONTROL
VALVE
OFF GAS
SOLIDS
SEPARATION
PUMP
REACTOR
STEAM
BOILER
OFF GAS
DEWATER-
FNG
LIQUOR
CAKE
TYPICAL HEAT TREATMENT SYSTEM
FIGURE 1
-------
PRIMARY 8/OR
WASTE BIOLOGICAL SLUDGE
HEAT
1
THERMAL
CONDITIONING
THICKEN
DIGESTION
CAKE
DECANT LIQUOR
DECANT LIQUOR
CONVENTIONAL
SYSTEM
PRIMARY 8/OR
WASTE BIOLOGICAL SLUDGE
HEAT
THERMAL
TREATMENT
ANAEROBIC
DIGESTION
DEWATER
CAKE
DECANT
LIQUOR
DECANT LIQUOR
LA /OMA SYSTEM
WASTE HEAT
RAW a/OR
THICKEN
WASTE BIOLOGICAL SLUDGE
I
DECANT
LIQUOR
4
IAL
T ION IN ft
—••I DEWATER
INCINERATE
DECANT
LIQUOR
ASH
ENERGY RECOVERY SYSTEM
HEAT TREATMENT IN SLUDGE MANAGEMENT SYSTEMS
-------
PROCESS DESCRIPTION
Equipment for thermal conditioning of sludge is manufactured and supplied
in the United States by Envirotech BSP (Porteous system), Zimpro (wet oxidation),
Zurn, and Nichols. Almost all of the equipment for thermal conditioning of
sludge in the United States has been supplied by Zimpro or Envirotech. Mayer
and Knopp1* reported in January 1977, that 70 thermal conditioning plants were
operating in the United States and Canada and 43 others were under construction.
With Mr Addition No of Installations
Operating 45
Under Construction 35
Without Air Addition
Operating 25
Under Construction 8
A partial list of thermal conditioning installations is shown in Table 1.
ZIMPRO PROCESS
The Zimpro system is similar to the process illustrated in Figure 1
except that air is also added to the reactor. Basic features of the Zimpro
process are (1) air addition to the reactor for oxidation, improvement of
heat exchange characteristics and reduction of fuel requirements, and (2) use
of sludge-to-sludge heat exchanger.
In the continuous process, the sludge is passed through a grinder which
reduces the size of sludge particles to about one-fourth inch. Sludge and
air are then pumped into the system and the mixture is passed through heat
exchangers and brought to the initial reaction temperature. As oxidation
takes place in the reactor, the temperature increases. The oxidized products
leaving the reactor are cooled in the heat exchanger by the entering cold
sludge and air. The gases are separated from the liquid carrying the
residual oxidized solids, usually in a decant tank, and released through an
odor control unit. The oxidized liquid and remaining suspended solids are
separated in a decant tank. The decant tank underflow may be further dewatered
by several methods; the overflow cooking liquor is recycled to the main plant
or treated by a separate system such as activated sludge, rotating biological
disk or anaerobic filter.
-------
TABLE 1
SIZE AND STATUS OF LARGEST
THERMAL CONDITIONING INSTALLATIONS
Location Status
Toronto, Ontario UC
(Ashbridges Bay)
Cleveland, Ohio UC
(Southerly)
Louisville, Kentucky
Cincinnati, Ohio
(Mill Creek)
Flint, Michigan
Green Bay, Wisconsin
Columbus, Ohio
(Southerly)
Suffolk, Co., New York
Toronto, Ontario
(Lakeview)
Springfield, Massachusetts
Kalamazoo, Michigan
Columbus, Ohio
Toronto, Ontario UC
(Highland Creek)
Chesapeake-Elizabeth, UC
Virginia
No. of Lnits
Unit Capacity
(gpm)
250
280
Operating
(1976)
Operating
Operating
Operating
(1975)
Operating
(1976)
UC
Operating
(1975)
UC
Operating
(1971)
Operating
(1972)
4
4
3
4
3
2
3
2
3
1
250
280
250
150
200
205
125
200
125
200
125
150
, Virginia
nnsylvania
, Montana
>, California
UC
UC
UC
UC
3
2
2
1
150
125
100
100
UC = Under Construction
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ENVIROTECH BSP PROCESS
This system was formerly called the Porteous process. The Porteous
process was purchased by Envirotech and various changes have been made in
the system. The basic system components and operation of the BSP system
are similar to the Zimpro process as illustrated in Figure 1. One basic
difference is that air is not injected into the reactor in the BSP system.
The BSP systems also normally employ a water-rto-sludge heat exchanger.
OTHER PROCESSES
The Nichols heat treatment system was previously marketed as the Dorr-
Oliver Farrer system. The Nichols process is used at a plant serving York,
Pennsylvania, and there are five installations of the Farrer system in the
United States: San Bernardino, California; Elkhart, Indiana; Port Huron,
Michigan; Glouster, New Jersey; Norwalk Connecticut. There is a Zurn heat
treatment system in Mentor, Ohio which serves an area of Lake County, Ohio.
THERMAL TREATMENT PROCESS SIDESTREAMS
There are both liquid and gas by-products from any thermal conditioning
system. These s*de streams must be considered in planning for an accurate
comparison with other processes and in design for a properly operating system.
Gas Sidestreams
There are four principal sources of odor resulting from thermal sludge
treatment: (1) vapors from treated sludge storage (decant tank or thickener),
(2) mechanical dewatering system exhaust, (3) exhausted air from working
atmosphere in filter and loading hopper areas, and (4) vapors from strong
liquor pre-treatment devices. The odorous gases produced are simple, low
molecular weight, volatile organic substances, consisting of aldehydes,
ketones, various sulphurous compounds, and organic acids. The odor level
source associated with thermal sludge conditioning is dependent to a high
degree on the total hydrocarbon content. The odor level and hydrocarbon
content are highest in off-gases from the heat treated sludge solids
separation units, i.e., decant tank or thickener and mechanical dewatering
systems.
Off-gases are best controlled by use of incineration, adsorption, or
scrubbing (or some combination of these processes).
1. Water Scrubbing Plus Incineration - For high hydrocarbon air streams,
the highest degree of odor control can be obtained by water scrubbing followed
by incineration. The scrubbing portion of this system consists of a packed
bed unit which uses plant effluent water at rates of 20 to 30 gpm per 1,000
cfm. The incineration portion of this system can be either direct flame
incineration at 1,500°F or catalytic incineration at 800°F. The oxidation
catalysts that are commonly used in catalytic incineration are supported
platinum or palladium materials.
-------
2. Water Scrubbing Plus Adsorption - In scrubbing methods, the odorous
substances are removed by solubilization, condensation, or chemical reaction
with the scrubbing medium. Scrubbing media that are commonly used for odor
control are potassium permanganate, sodium hydroxide, or sodium hypochlorite.
Two to four pounds of potassium permanganate are required per pound of hydro-
carbon removed. In the adsorption method, substances are removed from the
odorous gas stream by adsorption on activated carbon or silica gel. The
activated carbon or silica gel must be* capable of regeneration, usually by
steaming. High hydrocarbon sources can be treated in an odor control system
composed of a water scrubber followed by an activated carbon adsorption unit.
The water scrubber is the same as that described above. The carbon adsorption
unit is a multiple bed adsorber that is sized to minimize the required number
of steam regenerations. Normally, the carbon bed would be sized so that only
one steam regeneration per day would be required. Treating a 1,000 cfm gas
stream would require a dual bed carbon system containing 1,800 pounds of carbon
per bed. This sizing would permit an adsorption cycle of 26 hours. After
a 24 hour adsorption time, the second carbon bed would be placed in the
adsorption cycle and the spent bed would be steam regenerated. The regenera-
tion cycle requires low pressure steam at a maximum of 50 psig for a period
of one hour. The steam and desorbed organic compounds from the bed are
condensed and collected. The aqueous condensate is returned to the head
of the treatment plant and the liquid organic phase is incinerated.
3. Multiple Scrubbers - A third option for treating high hydrocarbon
sources is a multiple scrubber system. The multiple scrubber system would
contain at least two and preferably three scrubbing stages. In all cases,
the first scrubbing stage of the system should be water scrubbing using plant
effluent at a rate of about 27 gpm per 1,000 cfm. The second and third
stages should be chemical scrubbing using a combination of scrubbing media
selected from 5 percent sodium hydroxide, 3 percent sodium hypochlorite,
and 3 percent potassium permanganate. The potassium permanganate solution
effects the highest degree of hydrocarbon reduction and, hence, the highest
odor reduction. One of the most effective multiple scrubber systems consists
of three stages utilizing plant effluent, 5 percent sodium hydroxide and
3 percent potassium permanganate.
LIQUID SIDESTREAMS
The liquid (cooking liquor) containing materials solubilized during heat
treatment of sludge may be separated from the solids (1) during storage in
decant tank, thickener, or lagoon, and (2) in the dewatering step using a
vacuum filter, centrifuge, filter press, sand drying bed or other method.
The following tabulation shows some of the substances present in
thermal treatment liquor and the general ranges of concentration.
-------
Concentration Range
Constituent mg/1 (except color)
Suspended Solids 100 - 20,000
Chemical Oxygen Demand 10,000 - 30,000
Biochemical Oxygen Demand 5,000 - 15,000
Ammonia Nitrogen 500 - 700
Phosphorus 150 - 200
Color, units 1,000 - 6,000
The composition of thermal treatment liquor varies widely depending
upon sludge composition and reactor operating conditions. For a given reactor
temperature, increasing the reactor detention time will generally increase
the concentration of soluble organic material in the cooking liquor. Heat
treatment can normally be expected to solubilize from 40 to 70 percent of the
sludge biomass. As much as 60 to 70 suspended solids in waste activated
sludge were solubilized in heat treatment pilot tests in Los Angeles.6
The character of the cooking liquor is somewhat uncertain and the subject
of some debate. The statement in the EPA Sludge Manual1 quoted previously
is that, "About 20 to 30 percent of the COD is not biodegradable in a 30-day
period". Based on pilot scale heat treatment studies of mixed primary and
waste activated sludge, Erickson and Knopp estimated that the soluble non-
biodegradable COD was 7 percent of the total cooking liquor COD. Laboratory
tests by Stack, et al., 8indicated that about 40 percent of organics in the
cooking liquor from heat treatment of waste activated sludge were resistant
to biological oxidation.
The EPA Sludge Manual quoted previously, states, "Based on BOD$ and
solids loadings, the liquor can represent 30 to 50 percent of the loading
to the aeration system." Boyle and Gruenwald reported that the heat treat-
ment recycle liquor BOD contributed approximately 21 percent of the BOD
entering the Colorado Springs, Colorado plant. Studies by Haug, et al.,6
indicated that recycle of cooking liquor in the Hyperion plant at Los Angeles
would increase the oxygen demand on the aeration system by about 30 percent.
Thermal treatment liquor can be treated by recycle to the main treatment
plant or by separate treatment systems such as activated sludge, rotating
biological disks or anaerobic filters.
1. Recycle to Main Plant - Thermal treatment liquor often is recycled
through the main treatment plant, being added to the raw sewage or primary
effluent. This places an additional load upon the system principally in
the form of oxygen demand, suspended solids and color. In most cases the
color and COD of the final effluent will increase. The effects of recycle
can be mitigated to some extent by storing thermal treatment liquor and
returning it to the treatment plant at a uniform rate or during off-peak
hours.
-------
2. Separate Treatment and Disposal - Another method for handling liquors
Is to treat the sidestreams separately with no return of any liquor to the
main treatment plant. Sometimes digester supernatant and waste activated
sludge are combined with the thermal treatment liquor for separate processing;
one example of this method is the installation at Indio, California where
aerated lagoons with long retention provide satisfactory results. Lagoon
effluent is blended with plant effluent for discharge.
3. Separate Treatment Prior to Recycle - In order to reduce the load
on the main treatment plant and maintain final effluent quality, cooking
liquor is often treated separately prior to recycle to the main plant. Plain
aeration, extended aeration, and activated sludge treatment have been used
for pretreatment of cooking liquors. BOD reductions by conventional activated
sludge pretreatment of liquors have been reported as high as 90 percent.
It may be necessary to collect and deodorize aeration basin off-gases.
10
-------
THERMAL CONDITIONING COSTS
Thermal conditioning of sludge affects the cost of other treatment plant
processes, decreasing some and increasing others. Total cost includes direct
capital, operating, and maintenance costs for sludge handling plus or minus
the indirect net cost effect of sludge handling on other treatment plant
processes. Added costs resulting from heat treatment include: (1) cooking
liquor treatment, and (2) treatment of odorous off-gases. Potential cost
savings include reduction in subsequent sludge treatment and disposal costs
because of improved sludge dewatering.
An unpublished draft report for EPA" presents detailed cost estimates
for thermal conditioning and sidestream treatment. Costs were based on data
from several sources including operating plants, published literature,
manufacturers data and engineering estimates. The following cost information
was developed for thermal conditioning systems (does not include costs for
sidestream treatment):
1. Capital costs for thermal systems vary from about $50,000
per gpm of thermal treatment system capacity for a 10 gpm
system to $10,000 per gpm for a 200 gpm system.
2. Typical fuel requirements are 900 to 1,000 Btu per gallon for
systems that do not employ air addition and 300 to 600 Btu per
gallon with air addition.
3. Average electrical energy consumption averaged 22 Kwh per 1,000
gallons for plants with air addition and 10 Kwh per 1,000 gallons
without air addition.
It. Operation and maintenance labor constitutes a significant fraction
of overall costs, ranging from 6,000 hours per year for a 10 gpm
system to 20,000 hours per year for a 200 gpm system.
5. Costs for materials and supplies range from $5,000 per year for
a 10 gpm system to $20,000 per year for a 200 gpm system.
The following cost information is related to indirect costs for treating
sidestreams:
1. Increased capital costs primarily result from the need to increase
aeration tank volume and air supply capabilities.
2. Increased energy is required for aeration capacity required l_o
treat the recycled liquor.
3. Increased labor is required for maintaining and operating the
additional aeration capacity and related settling and pumping
systems.
11
-------
4.11
3.18
2.93
1.83
1.98
101.64
33.97
24.38
14.03
12.94
150.14
46.46
32.52
19.10
16.58
4.93
3.67
3.50
2.99
2.87
155.07
50.13
36.02
22.09
19.45
256.71
84.10
60.40
36.12
32.39
Costs for treating the off-gas from the thermal treatment system typically
constitutes 5 to 10 percent of the total costs for thermal treatment. Carbon
adsorption is the most costly technique for odor control. Incineration is
most economical in smaller plants and chemical scrubbing in larger plants.
Based on unit costs of $7 per hour for labor, $0.03 per kwh for electric-
ity, and §2.80 per million Btu and amortization of capital costs over 20 years
at 7 percent interest, the following typical costs for thermal conditioning
were determined (all costs are dollars per ton of dry solids processed):
CONSTRUCTION COSTS 0 & M COST
Sludge
Ton/Day Direct Indirect Total Direct Indirect Total Total
1 97.53
5 30.79
10 21.45
50 12.20
100 10.96
The March 1975 national average construction costs for thermal condition-
ing are shown on Figure 3. These costs include feed pumps; grinders; heat
exchangers; reactors; boilers; gas separators; air compressors where applicable;
decanting tanks; standard odor control systems; and piping, controls, wiring
and installation services usually furnished by the equipment or system manu-
facturer. Not included in the basic thermal treatment costs are buildings:
footings; piping; electricl work and utilities not supplied by the equipment
manufacturer; sludge storage and thickening prior to thermal treatment;
sludge dewatering, incineration or disposal: land: and engineering fees.
In escalating costs to later dates, it should be considered that the escala-
tion determined from the EPA-STP index may not adequately reflect the
increased costs for high temperature, equipment-dominated processes such as
thermal treatment.
A second curve (Curve B) is shown on Figure 3 and includes the costs
for typical building, foundation nnd utility needs for thermal treatment
systems. The building costs represent single-story, concrete or masonry
construction with built-up roofing, insulation and heat and vent systems,
and assume that reactors and decant tank will be located outside of the
building. The costs also include piping and wiring within the building,
foundations for internal and external equipment, and limited amount of site-
work. Building sizes provide for easy access to equipment and control room.
For larger installations, where multiple units are anticipated, space for
some standby equipment is included. Typical building sizes range from
1,500 square feet for a 10 gpm plant to 5,250 square feet for a 200 gpm plant.
The construction cost per square foot of building was estimated to be $36.
The curves show a rapid rise in unit construction costs for plants smaller
than about 20 gpm. The minimum direct cost of a thermal treatment plant is
estimated to be approximately $350,000 regardless of plant size. For plants
above about 150 gpm the increased use of multiple treatment units and of
12
-------
10,000
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THERMAL TREATMENT CAPACITY, GPM
Figure 3. Direct construction costs for thermal conditioning.
13
-------
standby units results in a lower limit for unit cost per gpm of capacity.
This lower limit appears to be in the range of $9,000 to $12,000 per gpm.
Data for these larger plants are sparse, however, and some plants reported
lower unit costs.
The annual fuel requirements based on 8,000 hours of operation are shown
in Figure 4. Fuel is used chiefly as a source of heat to produce steam. The
amount of fuel used is influenced by the reaction temperature, efficiencies
of the boiler and heat exchange systems, insulation or heat losses from the
system and the degree of heat-producing oxidation which takes place in the
reactor. Some reduction in the unit heat requirement for increase in plant
size is reported. This is believed to result from more uniform and constant
operation of the system, greater heat transfer and insulation efficiencies
and possibly a greater amount of oxidation in the larger units. Plants adding
air to heat exchangers and reactors experiencing some oxidation have lower
fuel requirements.
Typical fuel requirements averaged 900 to 1,000 Btu per gallon for plants
not practicing air addition and 300 to 600 Btu per gallon, depending on the
degree of oxidation obtained, for plants practicing air addition. Curves in
this paper are based on fuel requirements of 900 Btu per gallon for thermal
conditioning without air and 500 Btu per gallon, corresponding to about five
percent oxidation, plants with air addition. These fuel requirements do not
include allowances for treatment of off-gas.
Annual electrical energy usages for the two types of plants (with and
without air addition) are shown in Figure 5. A separate curve is included
on Figure 5 for estimating the energy requirements for building needs.
Electrical energy requirements are determined by sizes and efficiencies of
machinery such as sludge and boiler water pumps, grinders, thickeners and,
in plants where air addition is practiced, air compressors. Electrical energy
is also required for lighting and other building uses. Average unit energy
requirements are 22 kwh per 1,000 gallons for plants practicing air addition
and 10 kwh per 1,000 gallons for plants without air addition.
Operation and maintenance labor requirements are shown in Figure 6. In
this paper operation comprises time spent collecting and logging data on the
process, controlling and adjusting the various systems and components, and
laboratory work. The functions covered by maintepance include cleaning and
repairing process components, general upkeep of the process area, checking
and repairing of controls and instrumentation, and performing preventative
maintenance. Maintenance in Figure 6 does not include major overhauls which
will be required periodically. In some plants these operation and maintenance
functions may vary or may overlap.
In general, maintenance labor is approximately one-fourth of operating
labor, ranging from the equivalent of one maintenance man for one shift at
a 50 gpm plant to one and one-half men for one shift at a 200 gpm plant.
The amount of maintenance required depends greatly on the design and operation
of the plant, particularly on equipment and materials used for construction.
It is also dependent on the skill and knowledge of the maintenance personnel
and the design of, and adherence to, a preventative maintenance program.
-------
ANNUAL FUEL REQUIREMENTS, 10° BTU
s I
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THERMAL TREATMENT CAPACITY, GPM
Figure 4. Annual direct fuel requirements for thermal conditioning.
15
-------
10.000 . ,
ANNUAL ELECTRICAL ENERGY REQUIREMENTS, 10JKWH
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THERMAL TREATMENT CAPACITY, GPM
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conditioning.
16
-------
100,000
10,000
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7
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OPERATION
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THERMAL TREATMENT CAPACITY, GPM
Figure 6.. Operating and maintenance laUor requirements for
thermal conditioning.
17
-------
Annual costs for materials and supplies are shown in Figure 7. Curve A
shows the normal annual costs for materials and supplies required to operate
and maintain the thermal conditioning system. These costs are plotted against
thermal treatment plant capacity and include materials and parts such as seals,
packing, coatings, lamps, bearings, grinder blades, and other items used in
scheduled and normal maintenance. They also include operating supplies such
as lubricants, cleaning chemicals, boiler feed water, and water treating
chemicals. These costs vary from about $5,000 per year for a 10 gpm plant
to approximately $20,000 per year for'a 200 gpm plant.
Besides normal, periodic maintenance required for a plant shown by Curve
A, additional costs for major overhaul work are incurred. This work includes
such items as motor rewinding; major overhauls of pumps and compressors;
major non-routine rehabilitation or replacement of heat exchanger tubing
piping and controls; and refitting of boilers. This type of work is required
at an average interval of about 6 to 7 years, depending on the conditions at
a particular plant. Because labor for this type of major work is often
contracted, labor costs are treated as part of the overhaul and included in
its cost under this section. Curve B shows the combination of these costs
with those included under Curve A to give the total annual cost for the
materials and supplies. The inclusion of major overhaul work increases the
annual materials cost by about 45 percent over that required for routine
and preventative maintenance materials.
There was considerable variation among the costs for materials in
seemingly similar plants and it appeared that three factors tended to govern
the costs.
1. Preventative maintenance program. In plants where a good program
was practiced, overall costs for parts supplies generally were lower.
Where maintenance was neglected, more major failures were found to
occur with a need for greater expenditure for parts.
2. Design of the plant and selection of materials of construction.
If a higher grade of materials and equipment were selected for
initial construction and if the plant were designed with ease of
maintenance in mind, less maintenance and better maintenance were
found and hence less need for replacement was noted.
3. Quality of the water supply. In areas with high hardness and high
mineral contents in their water supplies, more scaling and corrosion
were noted in equipment, particularly in heat exchangers. Scaling,
along with the increased amount of cleaning required, resulted in
both an increase in replacement parts for boilers and heat exchangers
and an increased amount of chemicals for boiler water treatment and
heat exchanger cleaning.
Total costs for thermal conditioning systems, with air addition,
including costs for treatment of cooking liquor and odorous gas sidestreams
are shown in Figure 8. Costs in Figure 8 are based on the following:
1. Cooking liquor treated in the main plant by increasing the size of
activated sludge system.
18
-------
1,000,000
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Figure 7. Materials and supplies for thermalconditioning.
19
-------
TREATMENT PLANT FLOW, MGD
10,000
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2. Capital costs include an allowance for engineering, legal and
administrative and interest during construction and amortized
over 20 years at 7 percent interest.
3. Electrical energy cost = $0.03 per kwh.
4. Fuel cost = $2.80 per million Btu.
5. Labor cost = $7.00 per hour.
Using the above criteria, total costs for thermal conditioning range
from $257 per ton in a 1 ton per day capacity plant to $32 per ton in a
100 ton per day plant.
21
-------
DESIGN EXAMPLE
The design example considered herein is a 4 mgd standard activated sludge
plant with the following sludge characteristics:
Flow
Sludge
Type
Primary
Secondary
Total
Total
Solids
5,200
4,000
9,200
Volatile
Solids
(lb)
3,120
3,200
6,320
(gpm) (mgd)^
5.4 0.008
8.3 0.012
13.7 0.020
These sludge quantities v<:re determined with the following assumptions:
1. Raw wastewater suspended solids = 240 mg/1; BOD = 200 mg/1.
2. Suspended solids removal = 65 percent in primary treatment and ,
90 percent overall; BOD removal = 30 percent in primary treatment
and 90 percent overall.
3. One-half pound activated sludge produced per pound BOD removed.
4. Primary sludge is 4 percent solids and is gravity thickened to
8 percent solids.
5. Waste activated sludge is 1 percent solids and is thickened to
4 percent solids.
A process and materials flow diagram is shown in Figure 9 for a thermal
conditioning system of primary and secondary sludge. The example system
utilizes air addition and assumes that the recycle liquor will be treated
in the main activated sludge plant. Other features of this system include
the following:
1. One toer^al conditioning reactor required.
Flow = 20 gpra
Operating pressure = 350 psig
Operating temperature = 370°F
Operating schedule: 24 hours/day, 7 days/week
Installed horsepower = 85
Building area required = 1,115 square feet
2. One decant tank required.
Design loading = 50 Ib/sq ft/day
Diameter = 15 feet
Side water depth = 10 feet
22
-------
SLUDGE
HOLDING
TANK
GRINDER
COMPRESSOR
NJ
U)
TO MAIN
PLANT
LOCATI ON
1 . Primary SI udge
2. Secondary Sludge
3. Recycled Sludge
4. Total Sludge
5- Conditioned Sludge
6. Decant Underflow
7 Vacuu-i Filter Cake
S. Decant Supernatant
9 Vacuum Filter Filtrate
10. Total Liquid Recycle
HIGH
PRESSURE
PUMP
BOILER
TO
ATMOSPHERE
TO ATMOSPHERE
J, HEAT
| EXCHANGER
^PRESSURE
I CONTROL
VALVE
REACTOR
ODOR
CONTROL
ODOR
CONTROL
1
AND OR ./-" NLI LK
_ DISPOSAL y
Total
Solids %
gp>"
5.5
8.4
1 .4
15-3
16.0
6.3
-
10 4
9 0
19.4
Ton/pay
32
50
11
93
98
36
10
61
56
117
Ib/day Solids 1 fa/day1
5.230
4,040
830
10,100
9,760
3,015
7,200
1,730
840
2,570
8 0
4.0
3.6
5.4
5.1
11 . 1
36.0
-
-
1.1
_
-
-
-
-
-
-
875
370
1245
-»»i
' mg/1
_
-
-
-
-
-
-
7,000
3,400
5.300
11.
12.
13.
14.
15.
16.
(C3
Decant Tank Exhaust - 81 scfm
Vacuum Filter Exhaust - 2400 s
Air to Reactor - 32 scfm
Steam to Reactor - 8,000 Ib/da
Boiler Feed Watec - 0.001 mgd
Vacuum Filter Wash Water - 0.0
(0.7 gpm)
7 mgd (5
THERMAL CONDITIONING EXAMPLE
4 mgd ACTIVATED SLUDGE PLANT
FIGURE 9
-------
3. Scrubber-afterburner system to treat 81 scfm odorous gas from
decant tank.
Installed horsepower = 3
Building area required = 32 square feet
4. Multi-stage scrubber to treat 2,400 scfm odorous gas from vacuum
filter.
Installed horsepower = 13
Building area required = 144 square feet
In this example, the assumed BOD loading without thermal conditioning is
6,670 pounds per day in the raw wastewater and 4,670 pounds per day to the
aeration basins. The BOD in the decant tank supernatant and the vacuum filter
filtrate are estimated to increase the main treatment plant loading as
follows:
Decant Tank Vacuun Total
Supernatant Filter Recycle
Filtrate Flow
BOD5, Ib/day 875 370 1245
% BOD5 in raw
Wastewater 13tl 5-6 18-?
% BOD= to Aeration
Basics 18'7 7.9 26.7
24
-------
REFERENCES
1. "Process Design Manual for Sludge Treatment and Disposal", EPA 625/1-74-006,
pp. 6-14 - 6-16, October 1974.
2. Haug, R.T., "Sludge Processing to Optimize Digestibility and Energy
Production", Journal WPCF, pp. pp. 1713-1721, July 1977.
3. Haug, R.T., et al., "Effort of Thermal Pretreatment on Digestibility
and Dewaterability of Organic Sludges", Journal WPCF, pp. 73-85,
January 1978.
4. Sommers, L.E. and Curtis, E.H., "Wet Air Oxidation: Effect on Sludge
Composition", Journal WPCF, pp. 2219-2225, November 1977.
5. Mayer, M.R. and Knopp, P.V., "The Cost Effectiveness of Thermal Sludge
Conditioning", paper presented at the Annual Conference, New York Water
Pollution Control Association, January 1977.
6. Haug, R.T., et al., "Anaerobic Filter Treats Waste Activated Sludge",
Water and Sewage Works, pp. 40-43, February 1977.
7. Erickson, A.H. and Knopp, P.V., "Biological Treatment of Thermally
Conditioned Sludge Liquors", Advances in Water Pollution Research,
Pergamon Press, pp. 11-3311 - 11-3315, 1972.
8. Stack, V.T., Jr., et al., "Pressure Cooking of Excess Activated Sludge",
paper presented at the National Industiral Solid Wastes Management
Conference, University of Houston, March 1970.
9. Boyle, J.D. and Gruenwald, D.D., "Recycle of Liquor from Heat Treatment
of Sludge", Journal WPCF, pp. 2482-2489, October 1975.
10. Culp/Wesner/Culp, "Effects of Thermal Treatment of Sludge on Municipal
Wasteuater Treatment Costs", draft report, Contract No. 68-03-2186,
U.S. EPA, Cincinnati, Ohio (unpublished).
25
-------
REVIEW OF DEVELOPMENTS IN
DEWATERING WASTEWATER SLUDGES
MARCH 1978
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO 45268
SEMINAR
SLUDGE TREATMENT AND DISPOSAL
BY
J, R, HARRISON
CONSULTING ENVIRONMENTAL ENGINEER, P,E,
2 KENT DRIVE, R,n,2
HOCKESSIN, DELAWARE 19707
-------
THICKENING OF SLUDGE
MARCH 1973
PREPARED FOR
U,S, ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER
CINCINNATI, OHIO
SEMINAR
SLUDGE TREATierr AND DISPOSAL
BY
RICHARD F, NOLAND, P,E,
RONALD B, DICKERSON, P,E,
BURGESS s NIPLE, LIMITED
CONSULTING ENGINEERS AND PLANNERS
508^ REED ROAD
COLUMBUS, OHIO 43220
-------
TABLE OF CONTENTS
Page
INTRODUCTION 1
SLUDGE CHARACTERISTICS AND HANDLING 3
THICKENING PROCESSES 7
Gravity Thickening 7
Dissolved Air Flotation 10
Centrifugation 15
Other Methods 23
DESIGN EXAMPLE 24
Statement of Problem 24
Process Alternatives 27
Alternative Evaluation 38
Cost-Effectiveness Analysis 42
SUMMARY 57
iii
-------
LIST OF TABLES
Table No. Description Page
1 Frequent Applications of Thickening
Processes '
2 Typical Sludge Characteristics "As
Removed" From Treatment Processes 3
3 Post Thickening Process Operating Ranges 4
4 Existing Gravity Thickener Performance Data ?
5 Gravity Thickener Loading Rates and
Performance 9
6 Recent Data for Some Plant Scale OAF Units 13
7 Dissolved Air Flotation Design Parameters
and Expected Results '*>
8 Existing Solid Bowl Centrifuge Performance
Data 22
9 Centrifuge Mechanical Characteristics and
Performance Data 22
10 Raw Wastewater Characteristics 25
11 Treatment Unit Efficiencies 26
12 Pilot Centrifuge Results 37
13 Example Sludge Thickening Alternatives 38
14 Thickener Product and Anaerobic Digester
Requirements 46
15 Required Thickener Building Space 45
16 Capital Costs 48
17 Thickening Power Requirements and Costs 49
18 Digester Heating Costs and Alternative Total
Power Costs 50
19 Polymer Requirements and Costs 51
20 Operation and Maintenance Time and Costs 53
21 Yearly Operating Cost Summary 54
22 Cost Summary and Rank 55
IV
-------
LIST OF EXHIBITS
Exhibit No. Description Page
1 Alternative Primary Sludge Disposal Process
Trains 5
2 Alternative Secondary Sludge Disposal Process
Trains 6
3 Gravity Thickener 8
4 Dissolved Air Flotation Unit 11
5 Dissolved Air Flotation System 12
6 Schematic Diagram of a Basket Centrifuge 18
7 Disc-Nozzle Centrifuge 19
8 Continuous Countercurrent Solid Bowl Conveyor
Discharge Centrifuge 21
9 Settling Characteristics - 8' Column Waste
Activated Sludge 28
10 Settling Characteristics - 8' Column Elutriated
Waste Activated Sludge 29
11 Float Concentration and Effluent Suspended
Solids vs Solids Loading - Without Polymers 31
12 Float Concentration and Effluent Suspended
Solids vs Solids Loading - With Polymers 32
13 Float Concentration and Effluent Suspended
Solids vs Air/Solids Ratio - Without Polymers 33
14 Float Concentration and Effluent Suspended
Solids vs Air/Solids Ratio - With Polymers 34
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INTRODUCTION
Sludge thickening is defined as increasing the total solids con-
centration of a dilute sludge from its initial value to some higher
value, up to a limit of about 10-12% total solids. Thickening is con-
trasted with "dewatering" which increases the total solids concentration
to the range of 15-30 percent. Thickening operations are intended to
reduce the volume of sludge to be further processed and normally con-
stitute an intermediate step preceding dewatering or stabilization.
The unit processes most commonly associated with wastewater sludge
thickening are gravity thickening, dissolved air flotation, and centri-
fugation. Some of the heavier sludges, such as raw primary and combi-
nations of raw primary and some biological sludges, may be readily
thickened with stirred gravity thickeners. Other, more flocculant
sludges, such as those from activated sludge processes, may require more
elaborate methods. The most frequent applications of the common pro-
cesses are summarized in Table 1.
Table 1
FREQUENT APPLICATIONS OF THICKENING PROCESSES
Process Description Sludge Applications
Gravity Thickening Primary Sludge
Combined Primary and
Secondary Sludges
Dissolved Air Flotation Secondary Sludges
Centrifugation Secondary Sludges
The selection and design of a sludge thickening system is dependent
upon many factors including the sludge characteristics, sludge process-
ing following thickening, and the type and size of wastewater treatment
facility. Each thickening situation will be somewhat different.
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Applications other than those shown in Table 1 are possible and, in some
cases, may provide the desired results.
This paper will discuss in detail the processes of gravity thick-
ening, dissolved air flotation, and centrifugation. Other newer methods
will also be mentioned. First', sludge characteristics and sludge hand-
ling methods will be discussed. This will be followed by a discussion
of the thickening processes, performance data, and recommended design
standards. This material will then be used in a design example which
will illustrate the general approach necessary in thickening alternative
evaluation and selection. Bench scale or pilot studies are frequently
required for determining applicability of, and/or design parameters for,
the various thickening processes. Examples of these will be presented
with the design example. Additionally, equipment capital, operation,
and maintenance cost data will necessarily be presented to aid in
screening the alternatives. As the example is developed, the method-
ology for determining the most reliable and cost effective process for a
given sludge will be shown.
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SLUDGE CHARACTERISTICS AND HANDLING
Separation of solid matter from wastewater in a settling tank re-
sults in a clarified tank effluent and a watery mass of solids known as
"sludge." Many different sludge types and variations in sludge concen-
tration are encountered in wastewater treatment. The characteristics of
a sludge prior to thickening will generally depend upon the type of
wastewater treated, the sludge origin (which particular wastewater
treatment process), the degree of chemical addition for improved set-
tling or phosphorus removal, and the sludge age. Additionally, the
sludge produced by a specific settling tank will also depend somewhat
upon the design and operation of the unit. Typical "as removed" sludge
concentrations are presented in Table 2.
Table 2
TYPICAL SLUDGE CHARACTERISTICS
"AS REMOVED" FROM TREATMENT PROCESSES
Sludge Type Range % Solids Typical % Solids
Primary (PRI) 2-7 4
Waste Activated (WAS) 0.5-1.5 1
Extended Aeration (EA) 1-3 2
Trickling Filter (TF) 1-4 2
Rotating Biological Disc (RBD) 1-3.5 2
Combinations:
PRI + WAS 2.5-4 3
PRI + TF 2-6 3.5
PRI + RBD 2-6 3.5
WAS + TF 0.5-2.5 1.5
The lower figures in the range of expected results may be indica-
tive of settling units processing lighter, more flocculant sludges or of
units operating above their design capacity. The higher values may be
indicative of the results from units processing easily settled solids or
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of units operating below their design capacity. Chemical additions
may result in higher or lower concentrations depending upon the chemical
and dosage utilized. The "typical" percent solids are indicative of the
results obtained from settling tanks operating at design capacity and
treating normal "domestic wastewater."
Treatment and disposal of sludges represent two of the major prob-
lems associated with wastewater treatment. Thickening of the sludge
represents but one step of a total disposal scheme which may include
thickening, stabilization, dewatering, stabilization reduction, or heat
drying prior to ultimate disposal. Exhibits 1 and 2 show various pri-
mary and secondary sludge disposal alternatives and how sludge thick-
ening may fit into the total treatment and disposal scheme.
In general, the required degree of thickening is directly related
to the sludge processing method(s) downstream of the thickener (see
Exhibits 1 and 2). The stabilization stage, in particular, will norm-
ally be more successful if the solids concentration is kept within the
range that optimizes the rates of biological and chemical stabilization.
Likewise, ultimate disposal of liquid sludge by land application will
generally be less costly when the solids concentration is maximized but
kept within the range dictated by pumping equipment. Suggested optimum
percent dry solids operating ranges for various sludge handling pro-
cesses following thickening are shown in Table 3.
Table 3
POST THICKENING PROCESS OPERATING RANGES
Operating Ranges
Optimum Sludge
Process Type Solids (%)
Stabilization
Aerobic Digestion 2-4
Anaerobic Digestion 4-6
High Pressure Wet Oxidation 4-6
Low Pressure Heat Treatment 4-6
Lime Treatment 6-8
Other
Land Application 6-8
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THICKENING
STABILIZATION
DEWATERING
STABILIZATION
REDUCTION
HEAT DRYING
ULTIMATE DISPOSAL
PRIMARY SLUDGE THICKENIN3
MAY NOT BE REQUIRED
RAW PRIMARY SLUDGE
(FROM PRIMARY TANKS)
—— STABILIZED
DEWATEREO PRIMARY
EXHIBIT I
ALTERNATIVE
PRIMARY SLUDGE DISPOSAL
PROCESS TRAINS
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THICKENING
STABILIZATION
DEWATERING
STABILIZATION
REDUCTION
HEAT DRYING
ULTIMATE DISPOSAL
.3ECOMDAHY
' SLUOOE
•-STABILIZED OEWATEREO
SECONDARY SLUDGE
STABILIZED^ OEWATERED SECOHDARY^SLUDOE
EXHIBIT 2
ALTERNATIVE
SECONDARY SLUDGE DISPOSAL
PROCESS TRAINS
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THICKENING PROCESSES
Gravity Thickening
Gravity thickening of sludges, probably the most common unit pro-
cess in use, is relatively simple in principle and operation, and low in
cost. Gravity thickening is basically a sedimentary process carried out
in a unit which resembles a wastewater settling basin. A typical unit
is shown on Exhibit 3. Solids settle to the thickener bottom, are then
raked to a sludge hopper, and are periodically removed and discharged to
the next process. Water separated from the sludge (supernatant) rises
as the sludge settles. This supernatant or overflow containing some
solids and probably a high biochemical oxyen demand should be returned
to the plant for further treatment. Several existing gravity thickener
installations were recently contacted. Data, indicative of equipment
performance at that time, are presented in Table 4.
Table 4
EXISTING GRAVITY THICKENER PERFORMANCE DATA
Sludge Solids (%) So11ds
Location
Rumford Mexico, Me.
Kokomo, Indiana
York, Nebraska
Salem, Ohio
Middle town, Ohio
Feed
WAS
Heat treat1
Combined
PRI
WAS
Unthickened
1.2
4-6
0.9
Thickened (1
2.7
14-18
6-7
6
3.8
Ibs/ftVday)
5
18
6
1.5
Contains heat treated primary and waste activated (equal portions)
2
Contains primary, intermediate (trickling filter), and final (biodisc)
(proportions unknown)
Gravity thickeners are normally circular in shape and have a side
water depth of at least ten feet. The tank diameter is a function of
the required surface area. The required surface area is determined by
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oo
INFLUENT
BAFFLE*
EFFLUENT WEIR
EFFLUENT
RAISED POSITION
OF TRUSS ARM
HOPPER PLOW
SCRAPER BLADES
SLOPE 1=4 MINIMUM
3 UNDERFLOW
ELEVATION
EXHIBIT 3
GRAVITY THICKENER
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applying either pilot tested or average recommended solids loading
rates to the total solids that the unit will receive each day. Sludge
solids concentrations obtainable by gravity thickening depend upon
the sludge type, thickener overflow rate, and solids retention time.
Average recommended solids loading rates and the possible performance
for some sludges are presented in Table 5.
Table 5
GRAVITY THICKENER LOADING RATES AND PERFORMANCE
Sludge Solids (%) So1ids fading
Sludge Type Unthickened Thickened (Ibs/ft /day)
Primary (PRI) 2-7 5-10 20-30
Waste Activated (WAS) 0.5-1.5 2-3 4-8
Extended Aeration (EA) 1-3 1.5-4 4-8
Trickling Filter (TF) 1-4 3-6 8-10
Biodisc (RBD) 1-3.5 2-5 7-10
Combinations:
PRI + WAS 2.5-4 4-7 8-16
PRI + TF 2-6 5-9 12-20
PRI + RBD 2-6 5-8 10-18
WAS + TF 0.5-2.5 2-4 4-8
The values are average ranges only and may or may not be indicative
of the possible results for the particular sludge in question. A case
in point is a community which gravity thickens a 0.9 percent dry solids
waste activated sludge to 3.8 percent with solids capture of over 90
percent. The solids loading is 2 to 4 pounds per square foot per day
and the hydraulic loading ranges from 50 to 100 gallons per square foot
per day. This plant treats a high percentage of paper mill waste which
results in significant concentrations of inorganic solids escaping the'
primary tanks. These solids, when combined with the biological sludge,
form a floe that has much better settling characteristics than most waste
activated sludges. This results in a better than average thickened
product.
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Although the solids loading usually governs gravity thickener de-
sign, the hydraulic loading should also be checked. Hydraulic loadings
in the range of 600 to 800 gallons per day per square foot have been
reported as optimum. ' Also, loadings below 400 gallons per day per
square foot have been reported as possibly resulting in odor problems;
recycling of secondary effluent to maintain the higher rates has been
recommended.^ Much lower rates, as low as 100 to 200 gallons per day
per square foot, will often be more applicable. Recycling of secondary
effluent to control odor will dilute the influent solids. The overall
solids thickening performance of the unit may not deteriorate, however,
since dilution will elutriate fine solids and reduce the interference
between the settling particles. Polyelectrolyte addition may have the
effect of improving solids capture and thus reducing solids overflow in
the supernatant, but may have little effect on improving the solids
concentration in the underflow. To achieve maximum sludge concentra-
tion, a sludge retention time of one-half to two days is normally
required.
Dissolved Air Flotation
Dissolved air flotation is presently the most widely used method of
thickening waste activated sludge. The system uses air buoyancy to
literally float solids to the surface of a tank to be collected. The
main advantage of this method over gravity thickening is that very light
particles, such as waste activated sludge solids, can be removed more
completely in less time. A typical dissolved air flotation system is
shown on Exhibits 4 and 5. The units physically range from small steel
package units to custom designed large units with concrete tanks.
Recycle flow may consist of either underflow from the unit or recycled
plant effluent. It is returned at rates of up to five times the feed
sludge rate, combined with air, and then pressurized to approximately
60-70 pounds per square inch. Since the solubility of air in water in-
creases with increasing pressure, large quantities of air go into solu-
tion. Later, this recycle flow is allowed to depressurize as it is
mixed with the influent sludge. Depressurization releases the excess
10
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ADJUSTABLE FLOAT SKIMMER
I
FLOAT
STORAGE
SUMP
'INFLUENT
BACK PRESSURE VALVE
CHAIN TENSIONER
X
ADJUSTABLE
WEIR
EFFLUENT
SLUDGE
DISCHARGE
REDWOOD SCRAPER
EXHIBIT 4
DISSOLVED AIR FLOTATION UNIT (2)
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UNIT EFFLUENT
AUX. RECYCLE CONNECTION
(PRIMARY TANK OR
PLANT EFFLUENT)
(AIR FEED
ALTERNATE j-
FLOTATION UNIT
THICKENED SLUDGE
DISCHARGE fFLOAT/
RECIRCULATION PUMP
l:l TO 5:1
AIR FEED •
50 TO 70 PS I
UNIT FEED
'SLUDGE (W.A.SJ
RECYCLE
FLOW
REAERATION PUMP
(ALTERNATE /
RETENTION TANK
(AIR SOLUBILIZATIONj
0.03 TO 0.05 LB'S DISSOLVED
AIR PER IB OF SOLIDS
EXHIBIT 5
DISSOLVED AIR FLOTATION SYSTEM
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Table 6
RECENT DATA FOR SOME PLANT SCALE DAF UNITS
Location
Indianapolis, Ind.
Warren, Mich.
Frankenmuth, Mich.
Columbus, Ohio
Nashville, Tenn.
Xenia, Ohio
Feed
WAS1
WAS
WAS2
WAS3
WAS4
PS, WAS5
WAS
Influent
SS Cmg/1)
10,000
11,000
8,000
6,000
8,000
35,000,5;000
4,000
Subnatant
SS (mg/1)
100-1,000
200
90
800
150
100
Float
% Solids
3.5-4.2
5
3.5-5.5
3.2
3
6
2.5-3.0
Polymer Used
Ibs/ton dry solids
30
40
0-262
0
0
30
Contains some primary sludge - proportions unknown.
y
Major flow to plant is brewery waste. Polymer sometimes used to keep sludge from adhering
to skimmers. Sometimes thicken anaerobically digested sludge - similar results with no
polymers required (influent SS 10,000 mg/1)
3Jackson Pike facility
A
Southerly facility - units are being used as gravity settlers since they get better results
this way
Primary and waste activated are handled by separate units - combined product is 6% solids.
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air out of the recycle liquid in the form of tiny air bubbles (80ji).
These air bubbles attach themselves to the sludge solids and float them
to the surface. Thickened sludge is scraped off the liquid surface by
a skimmer mechanism consisting of a series of paddles. Liquid that is
not contained in the thickened sludge or recycled is discharged from the
system as subnatant. Subnatant may contain high solids and biochemical
oxygen demand, and thus should be returned to the plant for further
treatment.
Data from existing operating full scale dissolved air flotation
units has been presented in other publications. '^ Some of the same
installations were recently contacted. Updated performance data for
these and other dissolved air flotation units are presented in Table 6.
The effluent sludge (float) percent solids will depend on many
variables including the type and quality of the feed sludge, -recycle
ratio, detention time, air to solids ratio, system pressure, the solids
and hydraulic loading rates, and the amount of chemical aids used. Some
general statements that have been made regarding dissolved air flotation
(2)
thickening of the "average" waste activated sludge are as follows:
1. Increased air pressure or flow will yield higher float solids
and lower effluent suspended solids concentrations.
2. Polymer usage will yield higher float solids concentration and
improve the subnatant quality.
3. Detention time in the flotation zone is not critical.
Since there are so many variables and each sludge will react some-
what differently to the dissolved air flotation thickening process,
these "general rules of thumb" may not apply in all cases. Addition-
ally, when the guidelines are valid, it is generally only within certain
ranges of the variable parameters. The ranges are typically 40-70
14
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pounds per square inch for air pressure and 0-40 pounds for polymer
dosage. Likewise, the detention time may not be critical once a minimum
value of 1.5-3 hours has been attained.
System design is based primarily on a solids loading rate and the
desired air to solids ratio. Additionally, maximum hydraulic loading
rates are usually checked to avoid exceeding manufacturers' recommenda-
tions. If any flow other than the dissolved air flotation thickener
underflow is used for recycle, it must be included in the unit's total
hydraulic loading calculation.
Pilot studies are recommended to determine the applicability of the
dissolved air flotation process to the sludge and to optimize some of
the variables. When pilot studies are undertaken, the full scale design
is based on the study findings. Since data and sludge samples are
lacking at new wastewater treatment plants, thickener design must be
based on sound engineering judgment backed up with past experience.
Commonly used design parameters and expected unit performance are pre-
sented in Table 7. It must be emphasized that these are general guide-
lines only.
Centrifugation
Centrifugal thickening of sludge is a process which uses the force
developed by fast rotation of a cylindrical drum or bowl to separate the
sludge solids and liquid. In the basic process, when a sludge slurry is
introduced to the centrifuge, it is forced against the bowl's interior
walls, forming a thin slurry layer or "pool." Density differences cause
the sludge solids and the liquid to separate into two distinct layers.
The sludge solids "cake" and the liquid "centrate" are then drawn from
the unit separately and discharged. The three types of centrifuges -
basket, disc-nozzle, and solid bowl - all operate on the basic prin-
ciples described above. They are differentiated by the method of sludge
feed, applied centrifugal force, method of solids and liquid discharge,
and to some extent performance.
15
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Table 7
DISSOLVED AIR FLOTATION DESIGN PARAMETERS AND EXPECTED RESULTS
Sludge
Type
Waste
Activated
Primary &
Waste
Activated
Feed
Solids %
0.5-1.5
3-4
Solids
Loading
(Ib/ft2/hr)
2-3
2-4
Air to
Solids
Ratio
0.03-0.05
(1)
Recycle
Ratio
(%)
100-500
(1)
Float
With
Polymer
5-6
(1)
Solids (%)
Without
Polymer
4-5
5-8
Solids Capture (%)
With Without
Polymer Polymer
95-100 85-95
(1) 85-95
(1) Limited experience prohibits listing typical numbers.
CTl
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The basket centrifuge, as shown on Exhibit 6, is a batch type
thickening unit. As slurry is fed to the unit, the sludge solids form a
cake on the bowl walls, while the centrate is discharged over a weir or
baffle. Slurry feed is continued until the centrate solids reach the
maximum tolerable limit. At this point, the unit stops and a knife
wipes the sludge cake off the walls'. The sludge is then discharged from
the system through the unit's open bottom. Of the three centrifuge
types, the basket unit has the capability of producing the driest sludge
cake since there is a minimum of disturbance to the depositing solids.
Its use, however, is generally restricted to smaller plants because of
its intermittent operation and resultant lower capacity.
The disc-nozzle centrifuge, as shown on Exhibit 7, is a contin-
uously operating unit. It is composed of a series of conical plates
which are stacked together to form a series of narrow channels. Sludge
slurry enters the unit and is dispersed to these channels. The centrate
tends to rise and is discharged from the top of the cones while the
sludge cake is discharged downward and through small nozzles in the bowl
wall at the cone bottoms. High sludge throughput and good solids cap-
ture are possible with these units. Their solids concentrating capa-
bility is limited, however, by the small diameter (0.05-0.10 inches)
orifices through which the sludge cake must discharge. Additionally,
depending upon the sludge type and previous treatment, degritting and
screening prior to the disc centrifugation may be mandatory to avoid
plugging these sludge discharge orifices and to reduce wear on the
machine.
Like the disc centrifuge, the continuous solid bowl centrifuge is a
continuously operating unit. It consists of a horizontal cylindrical
bowl containing a screw type conveyor. At one end, the bowl necks down
to a conical section which acts as a beach plate for the screw conveyor.
In operation, sludge solids are forced to the bowl surface and are moved
toward the beach plate by the conveyor where they are discharged from
the unit. The sludge pool level is controlled by adjustable skimmers
17
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FEED
POLYMERn
SKIMMINGS
CAKE
CAKE
EXHIBIT 6
SCHEMATIC DIAGRAM OF A BASKET CENTRIFUGE (2)
18
-------
Wash
EXHIBIT 7
DISC - NOZZLE CENTRIFUGE
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or weir plates. These also function as discharge points for the cen-
trate. A typical countercurrent solid bowl centrifuge is shown on
Exhibit 8. Sludge slurry enters the unit just before the conical
section and distributes itself along the bowl surface. Sludge solids
are discharged at the cone end while centrate is discharged at the
opposite end. Sludge solids do not travel the full length of the bowl.
A second variation of the solid bowl centrifuge is the concurrent model.
In this unit, sludge slurry is introduced at the far end of the bowl.
Turbulence and interference present at the slurry inlet point in the
countercurrent machine are reduced with this variation. Also, the
slurry must travel the full length of the bowl before discharge. This
may result in a drier sludge cake.
Centrifuge performance is measured by the percent solids of the
sludge cake and the centrate quality or total solids captured. Several
existing centrifuge installations were recently contacted. Data, in-
dicative of equipment performance at that time, are presented in Table
8. The performance of a particular centrifuge unit will vary with the
inlet sludge type and solids characteristics, the sludge feed rate, and
the degree of chemical addition. Centrifuge performance on a particular
sludge will also vary with bowl design, bowl speed, pool volume, and
conveyor (if present) design. In practice, bowl and conveyor design are
set by the manufacturers. Pool depth is variable on solid bowl units.
Increasing the pool depth will normally result in a wetter sludge cake
but better solids recovery. Bowl speed is normally variable on most
centrifuge models. Difficulty involved in changing speeds varies with
the manufacturers. An increase in bowl speed normally results in a
drier sludge cake and better solids recovery. Conveyor speed is norm-
ally variable on continuous solid bowl centrifuges. Increasing the
conveyor speed normally results in a wetter sludge cake and poorer
solids recovery. Varying these parameters will probably result in
significant solids changes only within limited ranges. Each performance
improvement must be compared with the additional costs required to
produce it.
20
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COVER
DIFFERENTIAL SPEED
GEAR BOX
MAUN DRIVE SHEAVE
j 7- FEED PIPES
J— (SLUDGE AND
CENTRATE
DISCHARGE
/ROTATING
CONVEYOR
CHEMICAL)
BEARING
BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
EXHIBIT 8
CONTINUOUS COUNTERCURRENT SOLID BOWL CONVEYOR DISCHARGE CENTRIFUGE (2)
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Table 8
EXISTING SOLID BOWL CENTRIFUGE PERFORMANCE DATA
Sludge Solids (%)
Location
Great Northern Paper
Millinocket, Me.
Kendall Co.
Griswoldville, Mass.
Miller Brewing Co.
St. Louis, Mo.
Dubuque, Iowa
Feed
i
WAS1
i
WAS1
9
WAS^
WAS2
Unthickened
4
3
0.75 - 1
1 - 1.5
Thickened
10-12
7
5-7
6
90
80-85
Polymers used - quantity unknown
2
Polymers not used
Centrifuges have seen more service in dewatering applications than
in thickening applications. When utilized for thickening, their use is
normally limited to the thinner biological or industrial sludges that
cannot be thickened by less expensive methods. Data on the three cen-
trifuge types and their possible performance on waste activated sludge
are presented in Table 9.
Table 9
CENTRIFUGE MECHANICAL CHARACTERISTICS AND PERFORMANCE DATA
Centrifuge Type
Parameter Basket Disc-Nozzle Solid Bowl
Operation Method Batch Continuous Continuous
Bowl Diameter (inches) 12-60 8-30 6-60
Max Centrif. Force (G) 2,000 12,000 3,200
WAS Feed Solids (%) 0.5-1.5 0.5-1.5 0.5-1.5
WAS Cake Solids (%) 8-10 4-6 5-8
Solids Recovery (%) 80 - 90 80 - 90 70 - 90
22
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Polymers may be required to meet the stated performance. The re-
quired dosage depends upon the manufacturer and may range from 0-8
pounds per ton of dry solids.
Polymer addition generally improves both the percent solids and the
solids recovery. It must be emphasized that the tabular values are
representative of possible results from an "average" waste activated
sludge. Numerous sludge and machine variables make consultation with
manufacturers mandatory and pilot tests highly recommended for each
installation.
Other Methods
Thickening of sludge is often a secondary benefit of a sludge
treatment unit having an entirely different purpose. Decanting facil-
ities are provided in aerobic and anaerobic digesters to remove excess
liquids which have risen above the solids layer. In such facilities,
sludge solids concentrations may increase as much as one percent over
inlet feed solids concentrations.
New sludge thickening methods are being marketed each year. One
such method is the sludge filter bag system. In this process, sludge is
mixed with polymer and then held in suspended porous bags. The weight
of the sludge forces water out the bag sides and bottom. Sludge is held
from four to eight hours depending upon the desired dryness and is then
released through a bottom opening. Bag life should be about two years.
This method has not been in existence long enough to have been proven
reliable.
23
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DESIGN EXAMPLE
Statement of Problem
The problem is to provide sludge thickening facilities for two com-
munities, both of which have existing conventional activated sludge
wastewater treatment plants.
The smaller community has existing wastewater treatment facilities
capable of treating 4.0 million gallons per day. The facilities consist
of screening, grit removal, primary settling, conventional activated
sludge aeration, final settling, chlorination, and sludge lagooning.
Present flow to the plant is 3.5 million gallons per day; the 20 year
projected flow is 4.0 million gallons per day. The plant meets its
proposed discharge permit requirements, but the city has been ordered to
abandon the sludge lagoons (which are periodically flooded by the re-
ceiving stream) and in their place construct digestion facilities and
devise a plan for disposal of the digested sludge. The digested sludge
will be dewatered on sand drying beds or hauled as a liquid to nearby
farms. Thickening facilities are required to reduce the size of the
required anaerobic digester, to insure efficient digester operation, and
reduce hauling costs.
The larger community has existing wastewater treatment facilities
capable of treating 30 million gallons per day. Present flow to the
plant is 35 million gallons per day; the 20 year projected flow is 40
million gallons per day. The existing treatment system consists of
screening, grit removal, primary settling, conventional activated sludge
aeration, final settling, chlorination, aerobic sludge digestion, sludge
dewatering, and landfilling of dried sludge solids. The existing treat-
ment scheme will meet proposed permit requirements. Therefore, all
treatment units will be expanded to handle the 20 year flow projections.
Anaerobic digestion has been determined to be more cost-effective than
the aerobic sludge digestion. The aerobic digesters will be abandoned
as such (will become part of expanded aeration tank facilities).
24
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Thickening facilities are required to reduce the size of the required
anaerobic digesters, to insure efficient digester operation, and to im-
prove the dewatering operation.
Wastewater Characteristics. The wastewater characteristics and
removal efficiencies of the varidus treatment units are required to
determine the possible solids loading on the thickeners. This infor-
mation may be acquired from plant records or sampling programs at ex-
isting facilities. When these data are not available (such as in the
case of new wastewater treatment plants for new service areas), assump-
tions based on sound engineering judgment and previous experience are
necessary. For the sake of simplicity, the wastewater characteristics
and treatment unit removal efficiencies for the example plants are
assumed equal. Raw wastewater characteristics for the example plants
are given in Table 10.
Table 10
RAW WASTEWATER CHARACTERISTICS
Parameter Concentration (mg/1)
BOD5 200
Suspended Solids 240
Organic Nitrogen 15
Ammonia Nitrogen 25
Phosphorus 10
Grease 100
Treatment Unit Efficiencies. Both plants in this example will meet
their proposed permit requirements by utilizing the existing treatment
processes. Nitrification and phosphorus removal are not required.
Removal efficiencies based on percentages of the raw "domestic" waste-
water characteristics are presented in Table 11.
25
-------
Table 11
TREATMENT UNIT EFFICIENCIES
Unit Parameter Removal Efficiency
Primary Settling BOD5 30%
SS 65%
Aeration & Final Settling BOD5 60%
SS 25%
Sludge Characteristics. The characteristics of sludge discharged
to the thickening facilities may vary considerably depending upon the
type and amount of industrial wastes treated, the sludge origin (which
particular treatment unit), the degree of chemical addition for improved
settling or phosphorus removal, and the sludge age. Ideally, samples of
the sludge will be available for analysis. In lieu of this, the ranges
and typical concentrations shown in Table 2 may be utilized.
Existing plant operating data at the example plants has shown that
the unthickened primary sludge contains four percent dry solids; the
waste activated sludge, one percent dry solids. Field experiments at
both plants were conducted by returning the waste activated sludge to
the primaries. This did not seriously alter their operational charac-
teristics and an unthickened primary sludge containing three percent dry
solids resulted. Additionally, data at these plants has shown that for
every pound of five day biochemical oxygen demand removed in aeration,
0.5 pound of volatile suspended solids is produced.
Sludge Handling Following Thickening. The required degree of
thickening is directly related to the sludge processing method(s) fol-
lowing thickening. Suggested optimum percent dry solids operating
ranges for some sludge handling processes following thickening were
presented in Table 3. In these examples, anaerobic digestion is to
follow the thickening step. Hence, sludge delivered to the digester
should have a solids concentration between four and six percent.
26
-------
For any sludge thickening problem, there will be several alterna-
tive solutions which will result in a sludge product in the desired
solids range. However, since each solution will probably not result in
the same "guaranteed average" percent dry solids, the design of the
sludge processing facilities following thickening will also be affected.
Consequently, these facilities will'also have to be included in the cost
analysis.
Process Alternatives
Gravity Thickening. In the example, a primary (four percent) and
waste activated sludge (one percent), or combined sludge (three percent)
is obtained, and a sludge concentration for the anaerobic digesters of
four to six percent is needed. Table 5 and past experience indicates
that gravity thickening of "normal" waste activated sludge alone will
not yield the required four percent solids. Gravity thickening may
yield reasonable results for the combined sludges. Additionally, grav-
ity thickening primary sludge alone and waste activated alone, and later
mixing the two, is a possibility. At this point in an actual problem at
an existing treatment plant, bench or pilot studies would be performed
to determine the applicability of gravity thickening to the sludge and
to determine design parameters.
Examples of results of typical eight foot column bench scale tests
are shown on Exhibits 9 and 10. Both the undiluted and elutriated acti-
vated sludges reached their maximum solids concentrations of 2.8 percent
and 2.3 percent, respectively, in less than three hours. A similar
test would be made on primary only and combinations of primary and
waste activated sludge.
For the example plants, assume the results of the tests showed
that gravity thickening the sludges will result in the following: pri-
mary sludge, nine percent; waste activated sludge, 2.8 percent; combined
primary and waste activated sludge, five percent.
27
-------
_
H
m
2
0
m
X
m
0
X
H
•n
m
m
H
10-
9
8
EXHIBIT 9
SETTLING CHARACTERISTICS - 8' COLUMN
WASTE ACTIVATED SLUDGE
W.A.S. SUSPENDED SOLIDS = 10,000 MG/L
MAX. SOLIDS CONCENTRATION = 2.8 %
i
A
7-
6
5-
4
3
2
\
A
\
\
\
\
\
^k i
\^ J/2 VOLUME
^^^^
^^^.^^^ ^'/3 VOLUME
r
0 30 60 90 120 ISO 180 210 240 270 300 330 360 390 420 450 480
SETTLING TIME - MINUTES
-------
EXHIBIT 10
SETTLING CHARACTERISTICS - 8' COLUMN
WASTE ACTIVATED SLUDGE
ELUTRIATED (Ml DILUTION)
INITIAL S.S. OF W.A.S = II.6OO MG/L
S.S AFTER DILUTION = 5,800 MG/L
MAX. SOLIDS CONCENTRATION = 2.3 %
30 60 90 120 ISO 180 210 240 270 300 330 36O 390 420 450 480
SETTLING TIME - MINUTES
-------
Dissolved Air Flotation. Reviewing the example problem, there is
primary (four percent) and waste activated sludge (one percent) or
combined sludge (three percent), and a sludge concentration for the
anaerobic digesters of four to six percent is needed. If primary sludge
is to be thickened alone, gravity thickening is generally utilized since
the costs would be much less than for dissolved air flotation. Like-
wise, in the case of the primary-waste activated combined sludge, grav-
ity thickening will yield similar results at less cost. This leaves
thickening the waste activated sludge alone by dissolved air flotation
thickening as a possible option. Dissolved air flotation thickening of
the waste activated sludge, coupled with either unthickened or gravity
thickened primary sludge, represents a viable alternative and will be
considered. At existing plants, pilot tests should be performed to aid
in process selection and equipment design.
Assume a pilot study was completed using dissolved air flotation
thickening on the waste activated sludge. The variables studied in-
cluded recycle ratio, air to solids ratio, solids loading rate, and
amount of polymer used. The system pressure was kept constant. The
results, shown graphically on Exhibits 11, 12, 13 and 14, were as
follows:
1. Increasing the recycle rate generally yielded higher percent
float solids but also higher effluent suspended solids. A
compromise rate was selected for use in later tests.
2. A concentrated sludge of four percent solids could be con-
sistently achieved with a unit loading of two pounds per
square foot per hour and an air to solids ratio of 0.04.
Increasing the solids loading reduced the float concentration
and increased the effluent suspended solids concentration with
and without polymer usage.
30
-------
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FLOAT
CONCENTRATION
EFFLUENT
SUSPENDED
SOLIDS-
2345
SOLIDS LOADING (Ibi/sq. ft./hr )
800
700 3
-n
eoom
500
m
o
300
200
100
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FLOAT CONCENTRATION
T 1 1 r
•EFFLUENT SUSPENDED SOLIDS
900
8OO
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600 H
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500
4OO
300
200
100
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.02 04 06 08
10 12 14 16 18
AIR SOLIDS RATIO
20 22 .24 26 28
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02
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TRATION
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0
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) 02 04 06 OS 10 12 14 16 18 2O 22 24 26 21
80O
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600 m
500
400
3OO O
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100
AIR SOLIDS RATIO
-------
3. At the recommended loading, an effluent suspended solids con-
centration of 50 milligrams per liter without the use of poly-
mers and 20 milligrams per liter with polymer addition was
consistently achieved. Polymer usage, however, resulted in no
clearly identifiable improvement in the float solids concen-
tration.
4. Very rapid deterioration in the effluent quality occurred when
the air to solids ratio fell below 0.020. Increasing the air
to solids rates from 0.040 to 0.250 resulted in only slight
reduction in effluent suspended solids.
As seen from the results, the waste activated sludge differed some-
(2)
what from the experience of othersv ' and an "average" waste activated
sludge. A four percent float was obtained with or without polymers.
For the example plants, it will be assumed that dissolved air flo-
tation thickening is applicable to the waste activated sludge and that a
thickened sludge of four percent solids will be produced.
Centrifugation. The problem at the example wastewater plants is to
produce a four to six percent dry solids sludge for anaerobic digestion
from primary sludge (four percent) and waste activated sludge (one per-
cent), or combined sludge (three percent). Past experience indicates
that thickening the primary or the combined sludge by centrifugation
would be a more costly alternative than gravity thickening. These al-
ternatives are eliminated from further consideration. Centrifugal
thickening of the waste activated sludge, however, combined with either
unthickened or gravity thickened primary sludge does represent a viable
alternative and will be considered. As in the case of gravity and
dissolved air flotation thickening, sludge treatability and variable
optimization make pilot studies highly desirable when possible.
35
-------
For the example, assume a pilot study using a solid bowl centrifuge
was performed as part of the sludge thickening study on the waste acti-
vated sludge. Some typical data from this pilot test are shown in Table
12. In the pilot study, the feed rate of the sludge, bowl speed, and
pond setting were varied to determine the optimum combination to yield a
five percent sludge. Minor pond setting changes had little effect on
the unit's performance. Operation at 3,200 G produced a sludge much
thicker (12 percent) than needed, while operation at 1,150 G produced a
wet sludge and poor solids removal efficiency. A force of 2,100 G was
selected as an optimum. At the selected bowl speed, solids recovery and
percent solids of the cake were analyzed for different sludge feed
rates. The data indicated that although the centrifuge could thicken
the sludge to the required five percent, the percent solids could drop
from five percent down to two percent or increase up to 15 percent, with
only minor feed rate changes. Consistently obtaining the required five
percent solids concentration was difficult. Based on the pilot test
data, solid bowl centrifuge thickening of the waste activated sludge was
not consistent.
For the example plants, however, it will be assumed that centrifu-
gation is applicable to the waste activated sludge. Also, based on
available equipment, equipment reliability, plant operator preference,
desired performance, minimum supportive equipment requirements, and past
experience, the solid bowl continuous centrifuge is selected over the
basket and disc centrifuge for the examples. Available data from equip-
ment manufacturers and data in Table 9 indicate that a product sludge of
six percent solids may be reasonably expected.
Other Methods. Decanting may result in some thickening in the di-
gesters. It is not, however, a reliable, consistent method and does not
normally result in appreciable thickening. Thus, it will not be con-
sidered as one of the process alternatives for the example plants.
36
-------
Table 12
PILOT CENTRIFUGE RESULTS
Feed Sludge
Run
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Rate
gpm
13.6
10.8
16.8
17.7
18.3
25.2
25.7
21.6
23.0
35.4
23.6
12.2
10.7
11.1
22.2
27.3
27.3
28.3
44.6
59.0
-
23.0
25.4
44.0
44.5
40.4
-
63.2
Cone
X SS
.799
.859
.817
.925
.918
.833
.845
.809
.813
.809
.782
.790
.699
.757
.757
.779
.737
.793
.777
.760
.786
.760
.750
.751
.745
.701
.487
.725
Centrate
Rate
qpm
12.5
6.8
15.8
16.2
10.0
24.0
24.0
13.0
17.2
22.2
22.2
10.0
10.0
9.7
20.0
26.1
26.1
26.1
42.8
42.8
23.1
17.6
24.0
42.8
42.8
27.3
42.8
42.8
Cone
X SS
.027
.018
.077
.034
.020
.230
.072
.024
.027
.039
.136
.018
.015
.014
.026
.191
.152
.039
.236
.034
.032
.023
.078
.349
.165
.030
.040
.061
Cake
Cone
% TS
9.7
2.3
11.5
10.7
2.0
12.7
10.9
2.0
3.1
2.1
11.5
4.3
10.5
5.8
7.5
13.6
13.3
10.2
15.1
2.8
-
3.2
12.8
14.7
12.1
2.1
1.2
2.1
% Solids
Recov'd
97
99
91
I
96
99
73
92
98
96
97
86
98
98
98
97
63
79
95
70
97
-
98
90
55
78
97
98
94
Mechanical Conditions
Bowl Speed
rpm
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3.250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
Pond
Bowl-Conveyor
Setting Differential (rpm)
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
a
8
8
8
8
8
8
8
8
8
8
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
5/8
4.
9.
3.
4.
7.
3.
4.
6.
2.
2.
4.
5.
4.
5.
5.
2.
4.
5.
4.
6.
5.
8.
4.
4.
5.
7.
7.
6.
0
2
1
2
3
3
8
3
8
8
1
9
1
4
4
7
0
4
4
0
6
0
0
0
6
1
1
1
37
-------
New methods, such as the sludge filter bag system, have not been in
existence long enough to have been proven reliable. Thus, they will not
be considered as thickening process alternatives for the example plants.
Alternative Evaluation
Preliminary Screening. The preliminary screening of sludge thick-
ening alternatives for the example plants was performed in the previous
section. The remaining alternatives at this point are presented in
Table 13.
Table 13
EXAMPLE SLUDGE THICKENING ALTERNATIVES
Sludge Thickening Method
Alternative
No. 1
No. 2
No. 3
No. 4
No. 5
No. 6
The general approach to use, at this point, is to first determine
if any of the remaining alternatives can be eliminated without perform-
ing a detailed cost-effectiveness analysis. A detailed cost-effective-
ness analysis examining capital and operation and maintenance costs
would then be performed on the remaining alternatives. Capital costs to
be considered may normally include thickener and supportive equipment
costs, land costs, building or protective structure costs, and, in
certain cases, post thickening treatment unit costs. Other costs to be
considered include power costs, chemical costs, manpower costs, and
maintenance costs. The cost-effectiveness analysis will show which al-
ternative has the lowest annual equivalent cost.
Primary Waste Activated
Gravity
None
Gravity
None
Gravity
Gravity
DAF
DAF
Centrifuge
Centrifuge
Combined
-
-
-
-
-
38
-------
Secondary Screening Analysis. Since Alternative Nos. 1 and 2 both
utilize gravity thickening only, elimination of one of them should be
relatively simple. Wastewater characteristics, settling tank perform-
ance, and thickener performance presented previously will be used in
sizing the required thickeners. For this example, differences in den-
sity of the sludges are assumed insignificant and the density is taken
as equal to water. Designs will be based on conservative loading rates
to assure the desired performance. Calculations required for the 4.0
million gallons per day wastewater plant gravity thickener designs
follow:
Alternative No. 1
Definition - Gravity thicken primary sludge; gravity thicken waste
activated sludge
Primary Sludge
Quantity: 4 x 240 x 8.34 x 0.65 = 5,204 Ibs/day
Volume: 5,204/(0.04 x 8.34} = 15,600 gals/day
Required Thickener: 5,204/20 Ib/ft2/day = 260 ft2 or an 18.2' dia. unit
Recommended Thickener: 1 - 20' dia., 10' deep unit
Thickened Product: 5,204/(0.09 x 8.34) = 6,933 gals/day
Thickener Cost: $64,000
Waste Activated Sludge
Nonbiological: 4 x 240 x 8.34 x 0.25 = 2,002 Ibs/day
Biological: 4 x 200 x 0.60 x 8.34 x 0.5 = 2.002 Ibs/day
Total Quantity: 4,004 Ibs/day
Volume: 4,004/(0.01 x 8.34) = 48,010 gals/day
Required Thickener: 4,004/4 Ibs/ft2/day = 1,001 ft2 or a 35.7 dia. unit
Recommended Thickener: 1 - 35' dia., 10* deep unit
Thickened Product: 4,004/(0.028 x 8.34) = 17,146 gals/day
Thickener Cost: $98,000
39
-------
Combined Product
[(6,933 x 0.09) + 17,146 (0.028)]/(6,933 + 17,146) = 0.0459
24,079 gals/day of 4.59% sludge
Alternative No. 2
Definition - Gravity thicken combined sludge
Combined Sludge
Nonbiological: 4 x 240 x 8.34 x 0.9 = 7,206 Ibs/day
Biological: 2.002 Ibs/day
Total Quantity: 9,208 Ibs/day
Volume: 9,208/(0.03 x 8.34) = 36,803 gals/day
Required Thickener: 9,208/8 Ib/ft2/day = 1,151 ft2 or 2-27.1' dia. units
Recommended Thickener: 2 - 30' dia., 10' deep units
Thickened Product: 9,208/(0.05 X 8.34) = 22,082 gals/day
Thickener Cost: $160,000
The analysis has shown that capital costs for Alternative No. 2 are
slightly less than those for Alternative No. 1 ($160,000 vs. $162,000).
Additionally, a thicker sludge would be obtained with Alternative No. 2
(5 percent vs. 4.6 percent). This would result in additional cost
savings in the digestion facilities. A similar analysis for the 40
million gallons per day plant resulted in a $167,000 unit (60 foot
diameter) for the primary sludge, and a $489,000 unit (110 foot diam-
eter) for the waste activated sludge (total cost $656,000), or two
$305,000 units (85 foot diameter) for the combined sludge (total cost
$610,000). Thus, on the basis of capital costs, Alternative No. 1 is
deleted from further consideration.
Alternative No. 6 appears to be a viable solution for our example
plant. However, an initial check of the thickened sludge product should
be made since a sludge that is too concentrated can actually cause more
problems in the anaerobic digestion facilities than a sludge which is
too thin.
40
-------
Alternative No. 6
Definition - gravity thicken primary sludge; thicken waste activated
sludge by centrifugation. Three shifts (24 hours) 7 day per week
operation of gravity thickeners at both plants and of centrifuges
at 40 mgd plant; two shifts (15. hours) 5 day per week operation of
centrifuges at 4 mgd plant.
Primary Sludge 4 mgd 40 mgd
Quantity (Ibs/day): 5,204 52,040
Volume (gals/day): 15,600 156,000
Recommended Thickener:
4 mgd - 1 - 20' dia., 10' deep unit
40 mgd - 1 - 60' dia., 12' deep unit
Thickened Product:
4 mgd - 6,933 gals/day of 9% sludge
40 mgd - 69,330 gals/day of 9% sludge
Thickener Cost:
4 mgd - $64,000
40 mgd - $167,000
Waste Activated Sludge 4 mgd 40 mgd
Quantity (Ibs/day) 4,004 40,040
Volume (gals/day) 48,010 480,100
Recommended Thickener:
4 mgd - 1 - 75 gpm unit
40 mgd - 1 - 667 gpm unit
Thickened Product (daily average based on 7 day week):
4 mgd - 8,002 gals/day of 6% sludge
40 mgd - 80,020 gals/day of 6% sludge
Thickener Cost (based on one unit):
4 mgd - $89,000
40 mgd - $280,000
41
-------
Combined Product
4-mgd - [(6,933 x 9) + (8,002 x 6)] / (6,933 + 8,002) = 7.39
14,935 gals/day of 7.39% sludge
40 mgd - 149,350 gals/day of 7.39% sludge
The calculations show that a 7.4 percent solids sludge would result.
This exceeds the four to six percent solids recommended for efficient di-
gester operation. Thus, Alternative No. 6 is eliminated from further
consideration. Detailed cost analyses are required for screening the
remaining alternatives.
Cost-Effectiveness Analysis
Design of the thickener units (based on data previously presented
in this paper) and capital costs for those units will be presented first
for the remaining alternatives. Other costs will then be analyzed.
Alternative No. 3
Definition - thicken waste activated sludge with dissolved air
flotation; no thickening of primary sludge; two shifts (15 hours)
five days per week operation of DAP units at 4 mgd plant; three
shifts (24 hours) 7 days per week operation of units at 40 mgd plant.
Waste Activated Sludge 4 mgd 40 mgd
Quantity (Ibs/day): 4,004 40,040
Volume (gals/day): 48,010 480,100
Required DAP equipment:
4 mgd - (4,004 x 7)/(15 x 5 x 2.0 Ib/ft2/hr) = 187 ft2
40 mgd - 40,040/(24 x 2.0 Ib/ft2/hr) = 834 ft2
Recommended DAP equipment:
4 mgd: 2-100 ft2 units
40 mgd: 2 - 400 ft2 units
Thickened Product (daily average based on seven day week):
4 mgd - (4,004/0.04 x 8.34) = 12,002 gals/day
40 mgd - 120,020 gals/day
42
-------
Thickener Cost:
4 mgd - $82,000
40 mgd - $205,000
Combined Product (unthickened primary + thickened WAS)
4 mgd - 15,600 + 12,002 = 27,602 gals/day of 4% sludge
40 mgd - 276,020 gals/day of 4% sludge
Alternative No. 4
Definition - gravity thicken primary sludge; thicken waste acti-
vated sludge with dissolved air flotation. Three shifts (24 hours)
7 days per week operation of gravity thickener at both plants and
of DAF unit at 40 mgd plant; two shifts (15 hours) five days per
week operation of DAF unit at 4 mgd plant.
Primary Sludge 4 mgd 40 mgd
Quantity (Ibs/day): 5,204 52,040
Volume (gals/day): 15,600 156,000
Recommended Thickener:
4 mgd - 1 - 20l dia., 10' deep unit
40 mgd - 1 - 60' dia., 12' deep unit
Thickened Product:
4 mgd - 6,933 gals/day of 9% sludge
40 mgd - 69,330 gals/day of 9% sludge
Thickener Cost
4 mgd - $64,000
40 mgd - $167,000
Final Sludge 4 mgd 40 mgd
Quantity (Ibs/day): 4,004 40,040
Volume (gals/day): 48,010 480,100
Recommended Thickener:
4 mgd - 1 - 200 ft2 unit
40 mgd - 1 - 800 ft2 unit
43
-------
Thickened Product (daily average based on 7 day week):
4 mgd - 12,002 gals/day of 4% sludge
40 mgd - 120,020 gals/day of 4% sludge
Thickener Cost:
4 mgd - $55,000
40 mgd - $91,000 (built-in-place unit, equipment only)
Combined Product
4 mgd - [(6,933 x 9) + (12,002 x 4)]/(6,933 + 12,002) = 5.83
18,935 gals/day of 5.83% sludge
40 mgd - 189,350 gals/day of 5.83% sludge
Alternative No. 5
Definition - thicken waste activated sludge by centn'fugation; no
thickening of primary sludge. Two shifts (15 hours) 5 days per
week operation of centrifuge units at 4 mgd plant; three shifts (24
hours) 7 days per week operation of units at 40 mgd plant.
Final Sludge 4 mgd 40 mgd
Quantity (Ibs/day): 4,004 40,040
Volume (gals/day): 48,010 480,100
Recommended Thickener:
4 mgd - 2 - 38 gpm units
40 mgd - 2 - 334 gpm units
Thickened Product (daily average based on 7 day week):
4 mgd - 4,004/(0.06 x 8.34) = 8,002 gals/day of 6% sludge
40 mgd - 80,020 gals/day of 6% sludge
Thickener Cost:
4 mgd - $116,000
40 mgd - $324,000
Combined Product
4 mgd - [(15,600 x 4) + (8,002 x 6)]/(15,600 + 8,002) = 4.68
23,602 gals/day of 4.68% sludge
40 mgd - 236,020 gals/day of 4.68% sludge
44
-------
The design calculations for the various alternatives indicate that
they will result in different sludge moisture contents and sludge vol-
umes. These data and the resultant required anaerobic digester volumes
and costs are summarized in Table 14. As shown by the data, consider-
able digester cost savings are possible with the thicker sludges.
The example plants are located in the midwest. Therefore, the
problem of possible freezing temperatures needs to be addressed. Except
for icing of weirs and possibly a thinner product sludge, exposed grav-
ity thickener operation should not be seriously affected in freezing
weather. Flotation and centrifuge equipment, however, should be located
in heated enclosures to prevent freezing of the exposed piping and to
protect corrodible components from the elements. Besides housing the
thickening equipment, the structure should also provide space for poly-
mer feed equipment, and for polymer storage if polymers are to be used.
At the example plants, assume that existing building space is fully
utilized and, thus, any thickener building would be new construction.
The required building space and associated costs for alternatives
utilizing flotation or centrifugal thickening are presented in Table
15. Polymers are required with Alternative Nos. 4 and 5. Storage space
for a 30 day supply has been included in the required building area.
Table 15
REQUIRED THICKENER BUILDING SPACE
Building Description
Alternative
#3-4 mgd
#3-40 mgd
$4-4 mgd
#4-40 mgd
#5-4 mgd
#5-40 mgd
Thickener
Type
DAF
OAF
DAF
DAF
Cent.
Cent.
Description
Unit Size
2-100 ft2
2-400 ft2
1-200 ft2
1-800 ft2
2-38 gpm
2-167 gpm
Area
y
(ft2)
1,520
2,750
1,150
2,050
770
1,000
Height
(ft)
12
14
14
10
10
10
Building
Cost
$ 84,000
$136,000
$ 75,000
$181 .OOO1
$ 49,000
$ 58,000
1
Includes concrete tankage
45
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Table 14
THICKENER PRODUCT AND ANAEROBIC DIGESTER REQUIREMENTS
Digester Influent Sludge Digester -
* Volume (gals/day) Volume (ft3) Digester Cost
Alternative Percent Solids 4 mgd 40 mgd 4 mgd 40 mgd 4_mqd 40 m9d
2 5.0 22,082 220,820 58,034 580,340 $789,000 $4,074,000
3 4.0 27,602 276,020 71,938 719,380 $877,000 $5,310,000
4 5.83 18,935 189,350 49,683 496,830 $742,000 $3,425,000
5 4.68 23,602 236,020 61,661 616,610 $806,000 $4,361,000
Notes: If thickeners were not used, digester influent sludges would be as follows:
Alternative No. 2 - 4 mgd, 36,803 gals/day of 3.0%
40 mgd, 368,030 gals/day of 3.0%
All Other Alternatives - 4 mgd, 63,610 gals/day of 1.74%
40 mgd, 636,100 gals/day of 1.74%
Digester design is based on the thickened sludge, 85° F. temperature, 20 days detention,
75 percent sludge volatile content, 2,302 pounds of dry solids per million gallons of
wastewater and the volatile sludge loading factor method. Digester costs are based on
two high rate units for each plant.
-------
All capital costs for the alternatives have been summarized in
Table 16.
Power requirements and associated costs vary with the type and size
of thickeners utilized. Gravity thickening systems require power for
the operation of raw and thickened sludge pumps and the sludge collector
drive. Dissolved air flotation systems also require power for raw and
thickened sludge pumps, but additionally for a recirculation pump,
reaeration pump (if present), bottom collector drive, skimmer drive, air
compressor, polymer feed system (if present), and heating and lighting
of the thickener building. Centrifugal thickening systems require power
for the raw and thickened sludge pumps, bowl drive, conveyor drive (if
present), polymer feed system (if present), and heating and lighting of
the thickener building. Since the required anaerobic digester volume
differs with the four alternatives, the yearly sludge heating require-
ments will also vary. These sludge heating costs need to be included in
the thickener cost-effectiveness analysis since they are directly re-
lated to thickening process. Total operating horsepower, thickener
building heating requirements, and associated power costs for the var-
ious alternatives, excluding digester heating costs, are presented in
Table 17. Building lighting costs were determined insignificant and are
not presented. Operating horsepower figures include influent and ef-
fluent sludge pump motors which total as follows: Alternate No. 2 - 4
million gallons per day - 1 horsepower, 40 million gallons per day - 5
horsepower; Alternate No. 3 - 4 million gallons per day - 1 1/2 horse-
power, 40 million gallons per day - 4 1/2 horsepower; Alternate No. 4 -
4 million gallons per day - 2 1/2 horsepower, 40 million gallons per
day - 6 1/2 horsepower; Alternate No. 5 - 4 million gallons per day -
1 1/2 horsepower, 40 million gallons per day - 4 1/2 horsepower. Power
costs for equipment operation are based on a rate of $0.04 per kilowatt
hour. Power costs for heating the building are based on using fuel oil
at a cost of $0.45 per gallon. The cost associated with heating the
sludge in the anaerobic digesters and the total power costs for each
alternative are presented in Table 18. In developing heating costs for
the digester, it was assumed that auxiliary fuel (fuel oil at a cost of
$0.45 per gallon) would be required 50 percent of the time.
47
-------
Table 16
CAPITAL COSTS
4 MGD PLANT
Alternative
Descriotion
#2 -
#3 -
#4 -
#5 -
4 mgd
4 mgd
4 mgd
4 mgd
Thickeners
$160,000
82,000
119,000
116,000
Supportive
Equipment
$18,000
28,000
46,000
28,000
Building
—
$ 84,000
75,000
49,000
Anaerobic
Digester
$ 789,000
877,000
742,000
806,000
Total
$ 967,000
1,071,000
982,000
999,000
40 MGD PLANT
#2 - 40 mgd
#3 - 40 mgd
#4 - 40 mgd
#5 - 40 mgd
$610,000
205,000
258,000
324,000
$24,000
44,000
68,000
44,000
$136,000
181,000
58,000
$4,074,000
5,310,000
3,425,000
4,361,000
$4,708,000
5,695,000
3,932,000
4,787,000
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Table 17
THICKENING POWER REQUIREMENTS AND COSTS
4 MGO PLANT
Power Requirements
Alternative Description
#2-4 mgd, grav. thick.
#3-4 mgd, DAF thick.
#4-4 mgd, grav. thick.
4 mgd, DAF thick.
#5-4 mgd, cent, thick.
Equipment
(Operating HP)
5
50
2.5]
40] 42"5
42.5
Heating
(BTU/year)
1.85 x 108
1.63 x 108
8.60 x 107
Yearly
Equipment
$ 1,306
5,817
653]
4.653]5'306
4,944
Power Costs
Heating
$ 765
675
355
Total
$ 1,306
6,582
653]
5,328]5
5,299
,981
40 MGD PLANT
ID
#2 - 40 mgd, grav. thick.
#3 - 40 mgd, DAF thick.
#4 - 40 mgd, grav. thick.
40 mgd, DAF thick.
#5 - 40 mgd, cent, thick.
11
140
4]
1
110]
106
114
3.91 x 10
8
2.08 x 10
1.12 x 10
$ 2,874
36,581
1,045]
28,743]
27,697
29,788
1,620
855
465
$ 2,874
38,201
1,045]
29,598]
28,162
30,643
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Table 18
DIGESTER HEATING COSTS AND ALTERNATIVE TOTAL POWER COSTS
4 MGD PLANT
Digester Heating Table 15 Power Costs Total Yearly
Alternative
#2 -
#3 -
#4 -
#5 -
4
4
4
4
mgd
mgd
mgd
mgd
(BTU/Year)
1.
2.
1.
1.
6831 x
0820 x
4563 x
7875 x
109
109
109
109
(Cost/Year)
$ 6
8
6
7
,749
,615
,026
,397
(Cost/Year)
$ 1
6
5
5
,306
,582
,981
,299
Power
$ 8
15
12
12
Costs
,055
,197
,007
,696
40 MGD PLANT
#2-40 mgd 1.5628 x 1010 $64,668 $ 2,874 $ 67,542
° #3-40 mgd 1.9415 x 1010 80,338 38,201 118,539
#4 - 40 mgd 1.3426 x 1010 55,556 30,643 86,199
#5 - 40 mgd 1.6851 x 1010 69,729 28,162 97,891
-------
Polymers are required for dissolved air flotation thickening and
may be required for centrifugal thickening of the waste activated
sludge. Polymer requirements quoted by the various equipment manu-
facturers vary considerably for the same type process. Average polymer
requirements based on several submittals and data from existing instal-
lations and the associated costs are presented in Table 19.
Table 19
POLYMER REQUIREMENTS AND COSTS
Polymer Required Polymer Cost
Alternative (Ib/ton of dry solids) Unit ($/lb) Yearly Total
No. 3&4 - 4 mgd (OAF) 35 0.08 $ 2,046
40 mgd (OAF) 35 0.08 $20,460
No. 5- 4 mgd (Cent.) 6 1.80 $7,892
40 mgd (Cent.) 6 1.80 $78',920
Labor associated with operating and maintaining the thickening
equipment varies with the complexity of the process. The continuously
operating gravity thickener requires a visual inspection only once a
shift, whereas the more complex dissolved air flotation and centrifuge
systems should be checked every two or three hours. The inspections
should be visual checks on the product quality and also on the operating
conditions of all system components. Additional time for startup and
shutdown of either the dissolved air flotation or centrifuge systems
must be included if they are not operated on a continuous 24 hour basis
(Alternative Nos. 3, 4, and 5 for the 4 mgd plant). Startup and shut-
down time probably amounts to a total of about one hour per day. Rou-
tine sampling and testing of the thickener influent sludge, effluent
sludge, and supernatant is required for any type thickener. The tests
involved are essentially the same regardless of thickener type or size.
Testing must be done more frequently, however, on DAF and centrifuge
systems than on gravity systems. Routine maintenance includes such
things as lubricating equipment and daily washdown or cleanup opera-
tions. At least once a year, all thickeners should be dewatered,
51
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thoroughly inspected, and repaired, as necessary. Painting of corrodi-
ble components will probably be necessary at five year intervals. Solid
bowl centrifuge conveyors may have to be resurfaced or replaced after
5,000-10,000 hours use, depending upon the amount of grit in the sludge
and conveyor construction. A summary of the yearly operation and
maintenance time and the associated costs for each alternative are
presented in Table 20.
Maintenance material costs were developed from information provided
by equipment manufacturers and data from existing installations. The
material costs shown in Table 20 are estimates and, hence, may not be
indicative of the costs associated with any one particular manufac-
turer's equipment. These costs may be described as percentages of the
thickener system capital costs as follows: gravity thickening, 0.3
percent for small installations and 0.2 percent for larger installa-
tions; dissolved air flotation, 1 percent for small installations and
0.9 percent for larger installations; centrifugation, 5.2 percent for
small installations and 3.1 percent for larger installations.
Power, chemicals, and operation and maintenance yearly costs have
been summarized in Table 21. Since the power requirements for the
gravity thickening alternative (Alternative 2) are low and chemicals are
not required, it has the lowest yearly operating cost of all the al-
ternatives. Although the centrifugation alternative (Alternative 5) has
power costs similar to those of the dissolved air flotation alternative
(Alternative 4), the yearly operating cost is considerably higher due to
much higher chemical and operation and maintenance costs.
The alternatives' total capital costs and total yearly costs pre-
viously derived in Tables 16 and 21, respectively, are repeated in
Table 22. The data shows that for the four million gallons per day
plant, the least expensive option in terms of both capital and operating
costs is gravity thickening of the combined sludge followed by anaerobic
digestion (Alternative 2). Note that there is only three percent dif-
ference between the capital cost of Alternative 2 and the third most
expensive alternative (in terms of capital costs-Alternative 5).
52
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Table 20
OPERATION AND MAINTENANCE TIME AND COSTS
4 MGD PLANT
in
10
Alternative
Description
#2 -
#3 -
#4 -
#5 -
#2 -
#3 -
#4 -
#5 -
4 mgd gravity
4 mgd DAF
4 mgd DAF
gravity
4 mgd cent.
40 mgd gravity
40 mgd DAF
40 mgd DAF
gravity
40 mgd cent.
Operator's Time
(hrs/year)
483
1,416
868
373
1,659
483
2,496
1,408
373
2,739
1 ($/year)2
$2,415
8,496
5,208
1,865
9,954
40
$2,415
14,976
8,448
1,865
16,434
Maintainer's Time
(hrs/year)
252
586
293
126
264
MGD PLANT
440
804
402
220
445
($/year)2
$1,260
3,516
1,758
630
1,584
$2,200
4,824
2,412
1,100
2,670
Material
Cost ($/year)
$ 535
1,100
830
245
6.0003
$ 1,260
2,240
1,215
380
10.0003
Total Cost ($/year)
$ 4,210
13,112
7,796]
2.740]10'536
17,538
$ 5,875
22,040
12,075]
3.345]15'420
29,104
Time variances are due to equipment and operating time differences noted in the alternative
definitions.
2
Costs are based on $5/hr wage for gravity thickener operators/maintainers; $6/hr wage for DAF
or centrifuge operators/maintainers
Costs are based on replacing conveyor after 7,500 operating hours
-------
Table 21
YEARLY OPERATING COST SUMMARY
4 MGD PLANT
Alternative
Description
n - 4 mgd
#3 - 4 mgd
#4 - 4 mgd
Power
$ 8,055
15,197
12,007
Chemicals
$ 2,046
2,046
Operation and
Maintenance
$ 4,210
13,112
10,536
Total
$ 12,265
30,355
24,589
#5 - 4 mgd 12,696 7,892 17,538 38,126
40 MGD PLANT
#2 - 40 mgd
#3 - 40 mgd
#4 - 40 mgd
#5 - 40 mgd
$ 67,542
118,539
86,199
97,891
—
$20,460
20,460
78,920
$ 5,875
22,040
15,420
29,104
$ 73,417
161,039
122,079
205,915
54
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Table 22
COST SUMMARY AND RANK
4 MGD PLANT
Alternative
Description
#2
#3
#4
#5
Capital Costs
$ 967,000
1,071,000
982,000
999,000
Ranking
1
4
2
3
Yearly
Operating
Costs
$ 12,265
30,355
24,589
38,126
Ranking
1
3
2
4
40 MGD PLANT
#2
#3
$4,708,000
5,695,000
3,932,000
4,787,000
2
4
1
3
$ 73,417
161,039
122,079
205,915
1
3
2
4
55
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The results for the 40 million gallons per day plant are somewhat
different than those for the four million gallons per day plant. In
this case, the least costly alternative in terms of capital costs does
not correspond with the least costly one in terms of yearly operating
costs. Additionally, for the 40 million gallons per day plant, the
least costly alternative (capital costs) is not Alternative No. 2 (as
was the case for the four million gallons per day plant) but Alternative
No. 4. Also, in this case, there is a 22 percent difference between the
capital cost of the least expensive and the third most expensive alter-
native. Since the lowest capital cost and lowest operating cost alter-
natives do not correspond, a present worth analysis would be required to
make the final selection. Although the alternative capital cost rank-
ings varied with plant capacity, the yearly operating cost rankings did
not. Gravity thickening of the combined sludge followed by anaerobic
digestion had the lowest operating costs; centrifugation of the waste
activated sludge or thickening of primary sludge, followed by anaerobic
digestion, had the highest.
56
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SUMMARY
The purpose of this paper has been to describe, in detail, those
thickening methods which are currently utilized, and to present the
general approach necessary in evaluation of thickening alternatives by
means of a design example. The methods presented can be used to analyze
a thickening problem at any wastewater treatment plant, regardless of
its size or complexity. The results of the design example are valid for
the assumptions made. Any change in problem definition could mean a
different solution.
Recommendation of a particular process should be geared to avail-
able operation and maintenance personnel. Considerable more skill is
required to operate and maintain dissolved air flotation and centrifuge
equipment than gravity thickeners. The final recommended alternative
process will be one that is agreed upon by the owner, the engineer, and
the regulatory agency.
57
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REFERENCES
1. USEPA, "Process Design Manual for Upgrading Existing Wastewater
Treatment Plants," USEPA Technology Transfer, Oct., 1974.
2. USEPA, "Process Design Manual for Sludge Treatment and Disposal,"
USEPA Technology Transfer, Oct., 1974.
3. Water Pollution Control Federation, "Operation of Wastewater
Treatment Plants," - Manual of Practice No. 11, WPCF, Washington,
D.C., 1976.
4. Metcalf and Eddy, Inc., "Wastewater Engineering," McGraw-Hill,
New York, 1972.
58
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INTRODUCTION
The objective of this section of the seminar is to review the sludge
dewatering operating experiences which have occurred over the past 4-6 years
and to assess the impact of these results on future designs, both for grass
roots plants and for retro-fitting existing plants• Particular emphasis will
be placed on innovative concepts and equipment.
It should be noted that practically all of the innovative development
of new dewatering equipment in the last 4-6 years occurred first in Europe
(particularly in the Federal Republic of Germany), and Japan, and has only
begun to be transferred and utilized in the United States very recently.
It will also become apparent that older previously dominant equipment and
concepts have, in many instances, been replaced as a direct consequence of
operating results.
These developments place a particular and unusual burden on the U. S. .
design engineer in attempting to incorporate the latest and best equipment
and concepts in current design. It makes ever more imperative that, prior
to selection of a conceptual design for a sludge processing system, the
design engineer must have kept up to date on plant operational results with
various alternate systems. Also, pilot plant testing on innovative develop-
ments should be carried out whenever the sludge to be process is available.
The careful evaluation of alternate dewatering equipment and procedures
cannot be done in isolation, but rather only as part of an overall system
conceptual design evaluation. The inter-relationship between sludge process-
ing and liquid stream processing should always be considered. Previous
works (1,2) have graphically illustrated the adverse effects of recirculation
of sludge solids from equipment or systems providing less than 90TT capture of
influent solids, thereby illustrating the profound effect of choice of dewater-
ing system on the operability and cost-effectiveness of the liquid processing
system. Events of the past 4-6 years have further verified this principle.
There had been some previous indications that the selection of the type
of activated sludge system could have a strong effect on the relative severity
of associated sludge processing problems. This principle has also been greatly
strengthened by the events of the recent past.
-------
Consideration of Table I which lists the effect of various activated sludge
process modifications on yields of excess biomass and the processability of same
graphically illustrates the profound effect of the activated sludge process
variation chosen on the magnitude of the sludge disposal problem.
TABLE I
EXCESS BIOKASS PRODUCTION AND SLUDGE PROCESSABILITY
FROM VARIOUS ACTIVATED SLUDGE PROCESSES
Process Pounds BOD F/M Pounds E.A.S. Sludge
Variation Per 1000 Cubic Ft. Ratio Pound BOD Removed Processability
(Typical
High Rate
Conventional
Extended Aeration
100-1000
20-10
10-25
0.1-1.5
0.2-0.1
0.05-0.15
1.0?
0.1
0.15
Poor
Good
Variable
While this table is of the summary estimate type, the trends and principles
involved are accurate. Given the above information, it is understandable why
some states have banned the "High Rate" version of the activated sludge process.
Regardless of statutory positions, results at plants incorporating "High Rate
Activated Sludge" are sufficient to deter selection of same, if the resultant
sludge is to be disposed of in other than liquid form.
In selecting a dewatering system, an iteir. of real concern is the choice of
the final or ultimate disposal method for the sludge or its residue. Indeed,
the available options for final disposal should be known prior to selection of
the dewatering system. Fortunately, some of the new dewatering equipment, by
virtue of producing higher solids content dewatered cake and by offering the
capability to eliminate inorganic conditioning solids in dewatered cakes^
provides considerably more flexibility than was previously available in match-
ing up a dewatering process and an ultimate disposal process.
Dewatering is essentially always preceded by thickening and conditioning,
and frequently by stabilization. The essential role of dewatering is to trans-
form a dilute water slurry into a damp, moist cake form for either direct
final disposal or for drying as a final product, or for reduction via an
incineration or other combustion process prior to final disposal.
-------
In evaluating dewatering processes it is essential to consider more than
the direct operating costs, the production rate, and the dry solids content
of the dewatered cake. The evaluation should include complete material bal-
ances (Quantified Flow Diagram 1, 2) around the dewatering process with a
concurrent evaluation of the effect of all 'recirculation streams on other
preceding unit processes, and the effect of all dewatered cake properties
on the processes subsequent to dewatering, including final disposal,
TABLE II
AUTHOTHERMIG COMBUSTION 00
Parameter
Gross Calorific Value
% Combustible Matter
in Solids
% Solids for Autothermicity
Case A
17,^00
60
in. 8
Case B
29,100
75
18.5
To illustrate this point, note in Table II that the percent dry solids
level at which autogenous incineration occurs is a function of the calorific
value of dry solids in dewatered cake, which in turn varies with the chemical
composition of the solids. The requisite dry solids level for self sustaining
combustion varies from 18.5 to 41.8$ depending on these factors which are in
turn materially effected by the unit processes to which the sludge has been
subjected prior to dewatering.
-------
ANALYSIS CH1 33CSIJT PLANT OPERATING R2SULTS
IIIPLIZATIONS FOR DESIGN
Vhat lessons should the past five years of plant operating results bring
to bear on current and future designs?-' The following are five points which
bear considerations
1. The effect of choice of type of biological process on
sludge processins, and vice versa.
2. The effect of the inclusion of biomass on the sludge
processing system.
3. The effects of processing discontinuity on biomass or
nixed sludge processability.
iJ-. The importance of painstaking analysis of plant results.
5. Relative operability and maintainability of various sludge
processing systems or units.
Type of Biological Process Chosen.
As previously noted in Table I, the selection of the "High Rate"
activated sludge process variation can result in a plant having to process
a mixed sludge with off or greater biomass content. Further, that parti-
cular type of biomass is norsnally much more difficult to process than other
types. V/hile imposition of other design constraints may have resulted in
utilization of the High Hate process in certain cases, it is apparent that
a current overall system evaluation of alternate conceptual designs, parti-
cularly in the light of operating experiences, would usually not support
the use of the High Rate system.
Results have also shown that the extended aeration process, unless kept
within certain 7 /I I and SET ranges can cause sludge processing problems. These
factors further strengthen the need for adequate testing of sludges from
alternate biological processes prior to selection of same.
Effects of Inclusion of Biomass.
The effects apparent in the results of the past five years are reflected
in summary fashion in the following list and are self-explanatory:
1. Gravity thickening of mixtures of primary and excess biomass
sludges is usually ineffective (unless flocculants are used).
-------
2. Recycle of biomass to primary clarifiers Is nearly always a
self-defeating process which causes more problems than it cures,
3. Inclusion of biomass in a mixture with primary sludge causes
settling problems in conventional two-stage anaerobic digestion
systems. This, plus the need to maximize gas production fre-
quently makes single stage complete mix anaerobic digestion the
process of choice for stabilization prior to dewatering in
plants where sludge stabilization is required prior to dewater-
ing.
4. Biomass causes poor settleability in elutriation tanks.
These tanks can be modified to serve as post digestion thick-
ening tanks (with use of flocculants) 1. This is essential
for economic dewatering.
5. Inclusion of Komass makes the careful selection of dewater-
ing systems, including pre-treatment processes such as con-
ditioning and thickening, essential to successful design.
Processing Discontinuity and Sludge Storage Effects.
The following list delineates the pitfalls inherent in excessive accumu-
lation of sludge within a plant brought on by discontinuity in removal by the
dewatering process, either by excessive unplanned down time, or by design.
1. Development of septicity.
2. Destruction of some of the bio-flocculation of the biomass.
3. Partial solubilization through prolonged aqueous contact.
U. Increased hydration and more sensitivity to shear (pumping,
etc.).
5. Deterioration of processability occasioned by all four of
the preceding.
Methods of Analyzing Plant Operating Results.
In considering the significance of plant results and relevance to design
decisions, the following four concepts bear consideration:
1. The use of single static numbers as bench marks for a dynamic,
inter-related system can be seriously misleading.
2. Appreciation of the "Inertia" inherent in moderate and large
plant processing systems is necessary.
-------
3. There is a paramount need to maintain "Steady State" conditions
as much as possible.
*(•, Recycle or side-streams should be minimized within reasonable
ranges.
In developing design criteria, it is frequently assumed that dewatering
equipment can be sized using steady state flow conditions for the overall
system with some allowance for peaking. These assumptions are reasonable as
long as reliable conditioning, thickening, and dewatering equipment are installed.
However, if sludge removal operations are interrupted for lengthy periods or
fundamental changes are made then the standard factors can be off in terms of
order of magnitude. While some properly aerated sludge storage capacity is
beneficial, storage usage should be minimized and septicity avoided whenever
dewatering is used*
The length of time required to re-establish equilibrium or steady state
conditions in moderate or large size plants with significant inventories of
sludge is much longer than would normally be anticipated. This "Inertia
Factor" is calculable through the use of mathematical models. From experience,
in large plants, it can take several months to fully evaluate the effect of
changes.
The need to maintain a "Steady State" or equilibrium removal rate of
sludge sufficient to prevent over-accumulation within a plant is paramount.
Further, once an accumulation problem develops, rapid resolution via accelerated
removal rate procedures will prevent increasing difficulty.
Particular attention must be paid to processes which inherently cause
heavy recycle loads. Processes or equipment which cause heavy recycle loads
can have a double-barreled negative effect on sludge removal rate. If large
quantities of sludge have accumulated in a plant either because of heavy
recycle loads or from a shut down period, normal operating schedules will
require alteration. In order to clean out such an accumulation the "Sludge
Removal Rate" during the transition "Glean Out" period prior to re-establish-
ment of a normal equilibrium must be much greater than the normal rate.
Unfortunately, if the over accumulation is due to processes or equipment which
cause a significant recirculation load of biomass, the aeration system of the
plant will, during the "Glean Out" period of higher than normal sludge
-------
removal rate, be extremely over-loaded due to the much greater volume of
recycle load arising from the higher rate of sludge processing and it will
also produce more excess activated sludge than normal. The other barrel of
the effect is that sludge storage renders "biomass more difficult to process
resulting in a much greater amount of re-circulation than normally would be
predicted by "standard condition" testing figures and criteria.
Relative Operability and Maintainability of Various Dewatering Systems and UnitsA
The reliability and maintenance characteristics associated with various
types of condition-dewatering processes, equipment, and brands, is very important
to the municipality and its personnel, and ultimately to the public which pays
the bill. In addition to the need to keep units operating to prevent sludge
accumulation and its attendant bad effects, maintenance costs are a very
Important factor in overall system costs.
The only truly accurate source of reliability and maintenance cost data
is actual plant operational results. To justify professional process and equip-
ment selection, the design engineer should acquaint himself thoroughly with
reliability and maintainability parameters by visiting existing installations
and obtaining accurate information from operating personnel. It is also
necessary to sort out when problems are due to poor plant maintenance practices
and when they are due to inherent process or equipment characteristics. If
performance data is not available then it should be specified and guaranteed
by the supplier.
The current methodology of bidding and selection of suppliers for equip-
ment for municipal plants has been in itself, in some cases, a cause of some
of the reliability and maintenance problems now being experienced. The bidding
documents or plans and specifications should include cost factors for mainten-
ance costs and life cycle, and should be sufficiently complete to ensure that
truly equal equipment is being specified. If this is not done, and selection
is done on a strictly lowest price bid basis, inferior processes and equipment
can be selected.
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CONDITIONING FOH
The following list delineates the normal functions of conditioning for
dewateringi
1. Flocculation of suspended solids (particularly fines).
2, Washing out the alkalinity of anaerobically digested sludge
(the original purpose of elutriation) .
3. Promotion of rapid formation of a stable drainable cake.
k. Promotion of cake release from filtration support media.
5. Enhancement of cake fuel value.
6. Prevention of scale formation and corrosion inhibition.
The methods used to accomplish the above functions are as follows i
1. Chemical addition (inorganic)
2, Chemical Addition (Organic Flocculants)
3, Slutriation (New Function)
b. Heat Treatment (Conversion)
5. Ash Addition (Cake Release)
6. Coal Addition (Fuel Value)
7. Polyphosphonate Addition (Scale Inhibition)
Chemical Conditioning.
In inorganic chemical conditioning the most notable occurrence has been
the increase in total availability of metal salts, such as ferric chloride,
due to the entry into the market of firms recovering the products from waste
acids.
In the organic polyelectrolyte flocculant area, there have been several
developments of consequence to dewatering processes!
1. New high charge density, high molecular weight materials in
dry powder form, which are more efficient in conditioning the
difficult sludges, have become available and are used widely.
2. A new class of compound, the "Mannlch" cationic products, which
have different performance characteristics have been introduced,
almost entirely as liquid products. These materials produce a
floe and drainage characteristics more akin to that produced by
ferric chloride.
3. Emulsion form cationic products of high charge density and
molecular weight have been developed and are used.
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Elutriation.
This process had been applied successfully to digested primary sludge,
was missapplied to mixtures of primary and bioraass sludges, and then adapted
very successfully as a flocculant aided post-digestion thickening process to
facilitate cost-effective dewatering (l).
Heat Treatment.
This type process, sometimes called "Thermal Conditioning", is covered in
detail in another section of the seminar.
Specific cities which have had dewatering experiences of note, some
written up In the literature, and some not, arei
Kalamazoo, Michigan
Colorado Springs, Colorado
Chattanooga, Tennessee
Chicago, Illinois
Columbus, Ohio
Perth, Scotland
Ft. Lauderdale, Florida
Port Huron, Michigan
Flint, Michigan
Lakeview, Ontario
Green Bay, Wisconsin
In Great Britain, where the most and earliest installations of the Porteous
and Farrer heat treatment processes were made, the heat treatment process has
been largely abandoned. In one case, a brand new plant, never used, has been
offered for sale.
British water authorities detected significant quantities of refractory
organic material In the effluent from plants dewatering heat treated sludges.
The authorities consequently banned recycle of cooking liquors into biological
treatment systems which discharge into rivers subsequently used as sources of
drinking water since the biological systems are Incapable of removing the
refractory organic material.
An additional development in dewatering heat treated sludges has been the
need to chemically condition sludges in a number of cases, either on a spasmodic
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or regular basis. In the case of Port Huron, Michigan (Farrer System), which
employs centrifuges for dewatering, routine use of flocculants at the rate of
$8 worth/ton of sludge dewatered has been found necessary• Other heat treat-
ment plants have found flocculants necessary to promote cake formation to
obtain reasonable solids capture.
To help alleviate scaling problems, Grand Rapids has found it necessary
to condition heat treated sludge with $3/ton worth of polyphosphonates.
Various other chemicals have been found necessary to raise the pH of
sludges, to condition boiler feed water, and to solvent wash scale from heat
exchangers•
DEWATERING EQUIPMENT TRENDS
The following list spells out the types of dewatering equipment or
processes normally used in municipal wastewater sludge processing:
1. Drying Beds
2. Rotary Vacuum Filters
3. Horizontal Solid Bowl Centrifuges
b. Pressure Filters
5. Continuous Belt Filter Presses
6. Rotating Cylindrical Devices
?. Imperforate Basket Batch Centrifuges
8. Lagoons
Drying Beds are widely used at a large number of plants, particularly
moderately sized plants in sunny climes, but not restricted to same. As will
be seen, they have been the subject of recent developmental improvement
activity, both with regard to improved capacities and mechanical removal
facilities.
Whereas Rotary Vacuum Filters were once the bellwether of mechanical
dewatering systems, their incidence of selection has rapidly decreased due
to energy costs, the problem of cake pick up with certain sludges, and lack
of ability to provide as dry a dewatered cake as various other devices.
Horizontal Solid Bowl Centrifuges, particularly of the new now speed
type, are still popular where very high solids cake is not essential. Their
popularity has also dwindled to some extent due to energy considerations.
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11
Pressure Filters of the ordinary recessed chanter type have "been installed
in a few U. S. plants. Results have been mixed insofar as overall performance
is concerned, despite the attainment of somewhat higher total cake solids levels
(without necessarily improving the ratio of sewage solids to water) compared
to Rotary Vacuum Filters or Solid Bowl Centrifuges. Major problems are cost,
maintenance, and the frequent need to use high percentages of inorganic con-
ditioners.
The new Continuous Belt Filter Presses have become the most widely selected
dewatering devices for municipal sludge dewatering. Their rapid growth in
popularity is due to ease of operation, low energy consumption, and the ability
(in some models) to produce dewatered cakes with solids contents much greater
than obtainable with Rotary Vacuum Filters, Centrifuges, or conventional
Pressure Filters.
Rotating Cylindrical Devices, such as the Pernutit DCG have been installed
in some plants. Their use has been primarily at small plants and as the first
stage of a dual system which includes an inclined multi-roll press (MRP) for
further cake dewatering,
Imperforate Basket Batch Centrifuges have been installed at a few small
plants where a low solids, relatively fluid cake is tolerable.
Lagoon drying is now infrequently applied.
DEtfATERING METHODOLOGY
Wastewater sludges all form cakes during the dewatering process which are
compressible to some degree and by virtue of this fact and their inherent water
binding nature tend to require application of conditioning processes to facili-
tate a reasonable dewatering rate.
T^e various sludges may be indexed or characterized by determination of
the "Specific Resistance to Filtration". They may also be characterized by
being subjected to standardized bench scale dewatering test procedures (Filter
leaf or Buchner funnel tests).
An important facet for design consideration is that dewatering of waste-
water sludges is a "Cake Filtration" process. The cake which forms during
dewatering is the primary filtration media and relative cake structure and
form throughout the dewatering process will largely determine the efficacy
of the system.
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In assessing the cost-effectiveness of the pre-treatment methods aimed
at improving dewatering it is essential that the effect of these processes
on the type of cake formed be consideredt In most municipal wastewater treat-
ment plants, if the following steps are effected, a mixed primary and biologi-
cal sludge will result which is amenable to a cost-effective dewatering process
yielding a dewatered cake suitable for either reduction or direct ultimate
disposal in an economic fashion:
1. Maximization of solids capture in well designed primary
basins so as to provide as much typically easy to process
"Primary" sludge as possible. This precludes high recycle
loads of E.A.S. or thickener overflows or heat treat cooking
liquors to the primary basins.
2.. Selection of biological process variation with reasonable
assessment of the amount and type of excess biomass which will
be produced and will have to be processed. This usually
precludes use of "High Rate Activated Sludge" processes and
some Extended Aeration designs.
3. Use of gravity sludge thickeners only for straight primary
sludge, or if this is not possible, provision of flocculant
dosage capability to ensure reasonable solids capture and
underflow thickened sludge solids contents when mixed primary-
biological sludge is being thickened.
4. Use of dissolved air flotation or centrifugal thickening for
excess activated sludge prior to mixed sludge anaerobic
digestion, or prior to dewatering if stabilization is not to
be included.
5. If anaerobic digestion of mixed sludge is employed, use of a
single stage complete mix process and a post digestion thickening
process, either gravity or DAF.
6. Use of a conditioning process which does not result in creation
of a heavy recycle load, either in the form of suspended or
dissolved solids or in the form of BOD^ or COD or refractory
organics. Likewise the conditioning process should not destroy
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any significant amount of the matrix forming material in the
sludge solids which will form the cake in the dewatering process,
and should not alter other cake properties requisite to the
succeeding processes.
7. Selection and use of a dewatering device which is of rugged
design, readily maintainable and which will provide a minimum
solids capture of 90f-, and a cake solids content amenable to
succeeding processes. It is, for all practical purposes, always
necessary to condition municipal sludges prior to dewatering.
DRYING BEDS
Sludge drying beds are frequently referred to as "Sand Beds". In most
cases except instances wherein "paved drying beds" or wedge water screens are
used, sand is the primary drainage and cake support medium. The recent and
continuing development of various types of Drying Beds prompts the use of that
term, rather than Sand Bed.
Drying Beds are still the most common method of municipal waste water
sludge dewatering. The only reason they are not still further widespread in
use is that they hitherto have not been the subject of any significant degree
of development and improvement. This situation has changed as municipalities
have become more cognizant of their viability and relatively low cost of
construction, operation and maintenance when properly designed. An additional
previous deterrent to their use has been the frequent lack of inclusion of
mechanical sludge removal capability and an understandable dislike by operating
personnel, occasioned by a need for manual removal. This deterrent can and
has been removed in many cases by relatively minor design modifications to
facilitate mechanical removal.
An additional previous deterrent to selection of the drying bed alter-
native is that the "Ten State Standards" do not reflect the application of
conditioning to sludges prior to dewatering. The use of "Ten State Standards"
criteria, which assume no sludge conditioning, can result in excessive land
requirements and the resultant acquisition costs artifically inflate cost
estimates for the drying bed alternative.
A well designed and properly operated drying bed can produce a drier
sludge than any mechanical device. They are also less sensitive to the
influent solids concentration.
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On the negative side, drying "beds are generally applicable only to digested
or stabilized solids. Though they are particularly suitable for small installa-
tions and the "Sun Belt", drying beds are used successfully in treatment plants
of all sizes and in widely varying climates (i.e. Chicago SMTP, the largest
plant in the world).
Drying beds may be roughly categorized as follows«
1. Conventional rectangular beds with side walls, layers of sand
and then gravel with under drainage piping to carry away the
liquid. They are built either with or without provision for
mechanical removal and with or without either a roof or a
greenhouse type covering.
2. Paved rectangular drying beds with a center sand drainage
strip with or without heating pipes buried in the paved
section and with or without covering to prevent incursion of
rain.
3. "Wedge-Water drying beds which include a wedge wire septum
incorporating provision for an initial flood with a thin layer
of water, followed by introduction of liquid sludge on top of
the water layer, controlled formation of cake, and provision
for mechanical cleaning.
i*. Rectangular vacuum assisted sand beds with provision for
application of vacuum as a motive force to assist gravity
drainage.
Mechanism
On drying beds, the dewatering initially proceeds by drainage and then
continues by evaporation* The proportion and absolute amount achieved by
drainage will vary depending on whether or not the cake has been conditioned,
and its overall drainage characteristics. An important consideration is the
relative time period required for the cake to develop cracks which expose
additional sludge to evaporation effects. Since one of the main functions
of conditioning is to flocculate and immobilize the smaller "fines" particles
in the sludge cake it is immediately apparent why a conditioned sludge slurry*
dewaters in a fraction of the time required for an un'conditioned sludge. The
completion of the drainage period is substantially delayed in an uncondltione'
sludge by migration of the fines to the sludge cake sand interface resultin?
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15
in some plugging of the uppermost layer of sand. Maintenance of a porous,
relatively open structure within the cake is also essential to evaporation
rate.
Conventional Rectangular Beds
Drying bed drainage media normally consists approximately as follows:
1. The top layer is 6 to 9 inches of sand, usually with an
effective size of 0.3 to 1.2 mm and a uniformity coefficient
less than 5«
2. About 8 to 18 unches of gravel with size gradation of 1/8 to
1.0 inches. The top three inches of the gravel layer is
preferable 1/8 to 1/4 in size.
3. Underdrain piping with a minimum diameter of 4 inches is ogten
vitrified clay with open joints spaced 8 to 20 feet apart.
Recently, plastic pipe is being used to prevent possible crack-
ing when front end loaders are run across the bed for sludge
removal. If a gridwork of concrete runways are provided for
the front end loader, the selection of pipe is not critical.
Drying beds are frequently enclosed by glass. The glass enclosures can
materially improve the performance of the beds, particularly in cold or wet
climates. Experience has shown that in some cases only 6? °? of the area
required for an open bed is required with enclosed beds. The degree to which,
at specific locations, the space requirement could be reduced and the sludge
loading increased by use of translucent roofing or total glass enclosure is a
function of site rainfall, temperature, and sunlight prevalence.
Unfortunately, mechanical removal methods have not normally, in the past,
been used with glass enclosed beds. Obviously the adaptation would not be
either difficult or expensive.
Typical design criteria for open drying beds are as follows:
TABLE III
ggiTSRIA FOR DESIGN OF OPEN CONVENTIONAL DRYING BEDS
Type Digested~~AreaSludge Loading
Sludge Pre-Treatment (Sq.Ft./Cap.) Dry Solids
(Ib/Sq. Ft./yr.)
Primary & Humus None 1.6 22
Primary & Activated None 3«0 15
Primary & Activated Chemically Conditioned 0.64 55
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The combination of the use of chemical conditioning plus design to permit
mechanical sludge removal coupled with the use of either a translucent roof or
complete glass enclosure with ventilation louvers dramatically lowers the
space requirement for conventional drying bed use and should be the first
alternative considered for dewatering in most plants.
The sidestream from drying bed operation consists of the drainage liquor
which may be augmented by rainfall in the case of open beds. The additional
drainage water is not normally a problem. The drainage water is usually
relatively innocuous and can be recycled into the plant with impunity.
Drying times in open beds also varies due to climate, type of sludge, and
whether or not it has been conditioned. In good weather, an average of ^5
days is reasonable for unconditioned sludge. This period can be reduced to
5-15 days or less via conditioning.
A typical case study of the use of conventional drying beds follows.
TAMPA. FLORIDA - JURRENT PLANT
The current Tampa plant is a primary treatment facility featuring
anaerobic digestion and sand drying beds for sludge dewaterlng. The plant
is designed for a flow of 36 MGD and is normally treating kO MGD. On
occasion alum and polyelectrolyte are used in the liquid treatment phase
to meet the current interim effluent standards.
Drying Bed Details and Operations - Existing Tampa Primary Plant
Thirty-three beds, each 125 feet by 60 feet are employed. The rectangu-
lar beds employ a drainage medium of two sizes of graded sand above two
layers of differently sized stone or gravel. The beds are usually refurbished
every 2-3 yea rs, at most. Current anaerobically digested primary sludge
production is estimated to be 56,000 gallons of 3.0$ dry solids content per
day. This is equivalent to 14,000 pounds/day of dry solids. With 33 beds
of 7500 square feet area each, the total available drying area is 2^7,500
square feet.
The 33 older drying beds at Tampa are not covered so the drying cycle
varies somewhat due to rainfall variation. Nonetheless, the operation has
been so successful that the new expanded AWT plant which will be in operation
shortly is also equipped with drying beds for sludge dewatering. Tampa has
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regularly used polyelectrolytes for conditioning the sludge on its way into
the drying beds for about 3 years. Drying time to liftable cake conditions
without conditioning used to run 30 days minimum. With chemical conditioning,
the drying time varies from 8 to 15 days depending on rainfall pattern.
Tampa features front end loader mechanical removal of dried sludge cake
from the beds. One man can easily empty one bed in 6-8 hours. Previous
removal methods involved use of 5 men for lj-2 days to remove sludge from
one bed.
7igure I below is a photograoh of the mechanized sludge removal equip-
ment used at Tampa on the drying beds.
Figure I - Mechanized Sludge Removal at Tampa
Current operating procedure involves pumping about 55,000 to 60,000
gallons of digested sludge onto a bed with in-line dosing of cationic liquid
polyelectrolyte at a dosage rate of about 50 pounds per ton. The price of
the liquid cationic polymer is "'.1; per pound on an as is, liquid basis making
the conditioner cost • cr> per ton of dry solids.
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Taking the estimated bed loading volume of 56,000 gallons of 3.0# sludge
and an average drying time of 11.5 days, the solids loading rate on the
current Tampa beds is 60 pounds/square foot/year. It should be noted that
current practice is to produce a very dry cake as shown in Figure II:
Figure II - Dried Cake Appearance on Bed at Tampa Before Removal
TAMPA. FLORIDA - NEW AWT PLANT
Tampa has installed and is now starting up a new plant which features
biological nitrification and denitrification with chemical addition for
phosphorous removal. The new plant is designed for a treatment capacity of
60 million gallons per day.
Aerobic digestion for AWT sludges and anaerobic digestion for primary
sludge plus 32 new sand drying beds (each 100 feet by 1^-0 feet) were included
in the new facility.
While the original concept of the new facility was to aerobically digest
the excess biological solids and to dewater them separately on the new drying
beds, along with the AWT chemical solids, considerable flexibility was designed
into the plant and the eventual process configuration to be utili3ed will be
selected on an empirical basis. There is some apprehension re the energy
costs for aerobic digestion which was designed into the plant as an option
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19
prior to the surge in energy prices. If aerobic digestion proves too costly,
anaerobic digestion of mixed sludges will be evaluated.
DESIGN EXAMPLE - DRYING BEDS - 60 MOD PLANT
The design of the new Tampa AWT plants' drying beds serves as an
example of the design of this type system for a large plant in a sub-tropical
climate.
Estimates of quantities of unstabilized sludge solids to be encountered
in the new plant are summarized in the following table:
TABLE IV
TAMPA AWT PLANT
ESTIMATED ANNUAL AVERAGE UNSTABILIZED BY-PRODUCT SOLIDS PRODUCTION
Year
Item
Primary Solids Slurry -
-
-
AWT Solids Slurry
Biological Solids
Chemical Solids
Total
-
-
Combined Solids Slurry -
-
-
Ibs/day (dry)
percent solids
gals/day
Ibs/day (dry)
Ibs/day (dry)
Ibs/day (dry)
percent solids
gals/day
Ibs/day (dry)
percent solids
gals/day
1976
37,000
5.0
89,000
44,000
31,000
75,000
3.0
300,000
112,000
3.5
389,000
1985
37,000
5.0
89,000
71,000
48,000
119,000
3.0
476,000
156,000
3.3
565,000 -
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2C
Faced with processing the daily volumes of sludges shown and considering
the acceptable results previously achieved at Tampa with anaerobic digestion,
further calculations of the amounts of sludges which would result from anaerobic
digestion of primary solids and aerobic1, digestion of AWT solids were carried
out and results are listed in Table V:
TABLE V
TAMPA AWT PLANT
ESTIMATED ANNUAL AVERAGE STABILIZED BY-PRODUCT SOLIDS PRODUCTION
Year
Item 1976 1985
Primary Solids Slurry - Ibs/day (dry) 1^,000 1^,000
- percent solids 3.0 3.0
- gals/day 56,000 56,000
AWT Solids Slurry
Biological Solids - Ibs/day (dry) 38,500 57,500
Chemical Solids - Ibs/day (dry) 31.000 W,000
Total - Ibs/day (dry) 69,500 105,500
- percent solids 5«0 5«0
- gals/day 169,000 253,000
Combined Solids Slurry - Ibs/day (dry) 83,500 119,500
- percent solids b,k ^.6
- gals/day 225,000 309,000
A series of sludge solids stabilization, dewatering, and disposal
options were then reviewed for reliability, environmental impact and capital
plus operating and maintenance costs. Table VI summarizes these cost
results:
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21
TABLE VI
TAMPA AWT PLANT
ALTERNATIVE BY-PRODUCT SOLIDS SYSTEMS TOTAL COST COMPARISON
Est. Comparative Av. An.
Costs-$l,000,000 Cost Per Ton
Rank
1
2
3
4
5
6
7
8
9
10
(1)
(2)
Description
Air Dry - W/Chems. - Cake to User
Air Dry - Cake to User
Air Dry - W/Chems. - Cake to L'fill
Air Dry - Cake to L'fill
Kiln Dry - W/0 An. Digestion
Kiln Dry - W/An. Digestion
Mechanical Devraterlng
Liquid Spray
Liquid Slurry to User
Incineration
Based on ?8 tons per day (dry) raw
Capital Avg. An. Raw Solids (1
$11.6?
14.14
11.6?
14.14
15.18
16.07
15.87
23.79
23.65
21.47
$2.75
2.84
3.31
3.40
3.44 (2)
3.50 (2)
3.84
4.32
4.38
4.71
$ 96.52
99.81
116.16
119.46
120.68 (2)
122.76 (2)
134,85
151.78
153.79
165.49
by-product solids.
Net after revenue deduction from sale of product.
Based on the comparative costs shown and on other evaluation factors, the
alternate of air drying (drying beds) with use of flocculants was chosen as
the most cost effective.
The total estimated capital cost for the air drying system being installed
at Tampa currently, including all piping, auxiliaries such as equalizing
storage, site work, engineering, underdrainage system, etc., was $4,671,000
including $941,000 contingency.
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The drying bed operational design criteria are as shown in the following
table.
TABLE VII
TAMPA AWT PLANT
DESIGN 'ORITERIA - DRYING BEDS
Item
Air Drying Beds
Volume each drying bed - gals
at 12" fill depth
Area each drying bed - sf
No. of drying beds
Total Area - sf
Drying time - days
Solids Loading - Ibs/sf/yr
Dried Solids - Ibs/day (dry)
- percent solids
- Ibs/day (wet)
- tons/day (wet)
- cu ft/day (wet)
Design
An. Avg.
65,000
8,690
140
1,216,600
29. 1
35.85
119,500
40.0
298,800
149
3,900
Year - 1985
Max. Month
65,000
8,690
140
1,216,600
19;6
53.79
179,300
40.0
448,300
224
5,800
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PAVED RECTANGULAR DRYING BEDS WITH CENTER DRAINAGE
A good example of this type of system is that at Dunedin, Florida. A
photograph of the Dunedin beds appears below:
r
Figure III - Dunedin, Florida, Paved
Rectangular Heated Drying Beds
As can be seen, the two beds in the left portion of the photo contain
previously loaded sludge which is drying. The two empty beds on the right
are ready to be loaded.
The Dunedin plant is of interest due to use of a unique heated drying
bed system.
Plant process features:
1. An average flow of 2.5 MGD of primarily domestic wastes.
2. Liquid treatment via primary sedimentation followed by con-
ventional activated sludge. The plant originally used a contact
stabilization system but was converted to conventional activated
sludge with positive results.
3. Primary sludge is subjected to two stage anaerobic digestion
with a Pearth gas recirculation system.
^. The excess activated sludge is thickened in a DAF unit and most
of the thickened 2 AS then goes into the anaerobic digester system.
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Some of the EAS is subjected to aerobic digestion, but no more
than necessary due to the energy consumption of same. (The
operation of the DAP unit is well managed, as is the entire
plant, and the plant is a good reference point for the proper
application of DAF thickening in a smaller plant).
5. The digested sludges are processed in three different ways. A
portion is dried on the heated drying beds prior to us as a
soil additive. Some of the sludges are dewatered on an exist-
ing rotary vacuum filter when this is required. An additional
portion is disposed of in liquid form via tanker.
6. The digester gas is burned in a hot water heating system. The
heated water is circulated through piping in the paved portion
of the drying beds.
The Dunedin plant has four drying teds (75 feet x 25 feet each) or 7500
square feet of evaporative drying area. The drainage drying area, due to the
type of construction is only a fraction of the evaporative area. The beds are
heated, as noted, but are not covered and the Tampa Bay area has a high average
annual rainfall. Polyelectrolytes are used to condition the sludge.
Sludge drying time (averages) to liftable condition is 5 days normally
and 12 days in rainy periods. The beds are charged with 5,000 gallons of a
2.605 dry solids content sludge at a time. Thus the loading rate varys from
18 to ^3 pounds of dry solids sludge per square foot per year.
With a five day drying period the ^ beds are capable of dewatering about
13 dry tons per month. Certainly the capacity of U3 pounds per square foot
per annum achieved at Dunedin is several times greater than the Ten States
Standards for conventional open beds.
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WEDGEWATER DRYING BEDS
Wedgewater "Filter Beds" or drying beds were designed to introduce sludge
slurry onto a horizontal relatively open drainage media in a fashion which
would yield a clean filtrate and also give, a reasonable drainage rate.
The Wedgewater Filter Bed consists of a shallow rectangular watertight
basin fitted with a false floor of wedgewater panels. These panels have
slotted openings of £ MM and produce a total open area of 8$. The boundary
of this false floor is made watertight with caulking where the panels abut
the walls. An outlet valve is fitted in one wall of the bed to communicate
with the underside of the wedgewater decking.
CONTROLLED DIFFERENTIAL HEAD IN VENT
.BY RESTRICTING RATE OF DRAINAGE
..VENT
.PARTITION TO FORM VENT
WEOGEWATER SEPTUM.
OUTLET VALVE TO CONTROL
RATE OF DRAINAGE
Figure IV - Wedgewater Drying Bed - Cross Section
The controlled drainage rate is obtained by first introducing a layer of
water into the wedgewater unit to a level above the septum. The sludge is then
slowly introduced and in effect, under the proper conditions, floats on the
water layer. After the proper amount of sludge has been introduced, the initial
separate water layer and drainage water is allowed to percolate away at a
controlled rate. The exact procedure varys somewhat with different types of
sludges. It is apparent that for this concept to perform as intended the
sludge and the initial water layer must be relatively immiscible.
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The wedgewater technique Is designed to permit controlled formation of a
cake at the crucial sludge/support media interface before any significant
quantity of fines migrates to the interface or into the openings of the septum
or escapes in the filtrate. Since polyelectrolyte flocculants promote rapid
cake formation and bind up fines they are now used in conjunction with Wedge-
water Filter Bed installations processing municipal sludges.
Each square foot of wedgewater can normally dewater between •§• Ib. and
1 Ib. of dry matter per charge. The loading rate depends on the initial
solids concentration of the waste sludge applied. Most sludges can be
dewatered to a handleable condition of &fo-12fo solids within 2k hours. This
process is most practical for the smaller treatment plant which has an average
daily flow of 500,000 G.P.D. or less. Sludge loading rates of 182-365 pounds
per square foot per year are normal.
Results with Wedgewater units at 2 U. S. plants are described in the
following paragraphs.
ROLLINSFORD. N.H.
This plant produces an excess biological sludge at the rate of 150 gallons
per day at 2$ dry solids content. A wedgewater unit as shown below is used
to dewater the sludge to a solids content of 8^, which is liftable.
Figure V - Rollinsford. N. H. Wedgewater Drying Bed
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A polyelectrolyte conditioner is used in the process. Calculations
from the data in the reference cited show that conservatively assuming 2
drying cycles per day for the 15" by 6* unit, the production rate could be
1.1 lb/hr/sq.ft., or 5?0 lb/sq.ft.year which is, of course, an order of
magnitude greater than the dewatering rates normally associated with conven-
tional drying beds. These results are tempered by the fact that 8%, while a
liftable condition for this sludge, is not a particularly high solids content.
It is apparent, however, that higher than 8fo solids would be readily obtain-
able with increased drying times while still maintaining a very high annual
solids loading, if such a higher solids content were required.
DUNEDIN. FLORIDA
Additional results on the wedgewater system are reported from work at
Ounedin, Florida. At that location, the biological sludge was dewatered to
a solids content of 10.4$ in 22 hours through the mechanism of the wedgewater
element, use of support water, and the restricted drainage procedure, without
the use of polymer flocculants.
There are 18 U.S. installations of the wedgewater system. Several are
industrial applications but most are installed at small plants of the contact
stabilization type.
A tiltable unit, more or less similar to the lift and dump mechanism of
a dump truck is available to facilitate removal of sludge when slightly fluid
cake can be tolerated or when removal by rake is feasible. The supplier,
Hendrichs Manufacturing Company of Carbondale, Pa., also supplies design
recommendations for mechanical removal via small front end loader when indicated.
A one square foot bench scale test model is available for test purposes.
The stainless steel wedgewire septum in the 15 foot by 6 foot Rollinsford
unit would cost $4,500 at todays prices.
VACUUM ASSISTED DRYING BEDS
At the 4.5 MGD Sunrise City, Florida contact stabilization plant, a
purpose built vacuum assisted drying bed system has been used for the past
18 months to dewater the 2/5 dry solids sludge.
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Principal components of the system ares
It A rigid multi-media filter top surface.
2. An intermediate void filled with stabilized aggregate.
3. A low impermeable barrier, consisting of reinforced concrete.
(it would alternatively be pre-favricated fiber-glass).
A photograph of one of the two drying bed units showing the sludge being
fed onto the surface of the upper multi-media in one of the beds appears
below:
Figure VI - Rapid Sludge Dewatering Beds
Sunrise Jity, Florida
The following sequence of operations is used:
1. Sludge is fed onto the filter surface by gravity flow at a rate
of 150 gallons per minute to a depth of 12 to 18 inches.
2. Filtrate is drained through the interconnected voids of the
stabilized aggregate to a sump, from which it is pumped back
to the plant by a self-actuated submersible pump.
3. As soon as the entire surface of the rigid filter is covered with
sludge, the vacuum system is turned on to maintain a vacuum of
one to ten inches of mercury on the intermediate void area.
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Under favorable weather conditions, this system dewaters the 2^ solids
aerobically digested contact stabilization sludge (a difficult high bound
water content sludge) to a 12r' solids level in 2^ hours without polymer use,
and to the same level in 8 hours if flocculant is used. The 12% condition is
liftable. The sludge will further dewater to about twenty per cent solids in
^8 hours.
The sludge cake is removed from the filter surface either manually,
mechanically by a small hydrostatic drive front-end loader such as a Melroe
Bobcat 520, or by a vacuum truck.
Controlled tests of this type system have shown that a sludge loading
rate of 306 pounds per square foot year is attainable.
At Sunrise City plant, the two 20 feet by ^0 feet vacuum drying beds
are processing a substantial portion of the total plant load. The photo-
graph below shows the appearance of a bed at the end of the drying period
and also shows the proximity to a local athletic field.
Figure VII - Vacuum Assisted Drying Beds
Sunrise City, Florida
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:c
The vacuum assisted drying bed system at Sunrise City is a proprietary
system now designed and supplied by International Sludge Reduction Company,
Starlight Towers, 6000 N. Ocean Blvd., Suite l6A, Ft. Lauderdale, Fla., 33308.
DESIGN EXAMPLE
DRYING BED FOR U FO PLANT
Basic Assumptions!
These assumptions are as follows:
1. The sludge to be processed Is an anaerobically digested mixture
of primary and E.A.S. at ^ dry solids content. It is a mixture
of 60$ primary sludge and UO?S E.A.S. with the E.A.S. originating
from a conventional activated sludge system.
2. Ultimate disposal is to be by hauling to a sanitary landfill,
or to farmland or other horticultural use.
3. Equilibrium sludge removal rate of 2.5 tons of dry solids per
day to be maintained.
4. The plant is located in the Middle-Atlantic section of the
United States.
Alternate Units for Consideration or Evaluation
For a plant of this size, depending on site limitations, either con-
ventional enclosed drying beds or vacuum assisted enclosed drying beds should
be considered. The economics and other constraints of final disposal, such
as length of truck haul and final solids content requirements would bear
consideration. Land area availability would materially effect the choice
between gravity or vacuum assisted drying beds. If excess methane was avail-
able from anaerobic digestion, consideration could be given to use for heating
the enclosed bed air space during the winter.
For the purposes of this example it is assumed that sufficient land area
is available for either gravity or vacuum assisted drying beds.
Evaluation Procedure
The general sequential procedure recommended to be followed would be
similar to that fully described on page J1 in the RVF design example. The
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31
only variation would be that bench scale and or pilot plant tests on the drying
bed dewatering characteristics of the sludge would probably have to be planned
and carried out entirely by the consulting engineering firm and the client for
the conventional enclosed bed option. On the vacuum assisted bed option the
suppliers have developed small scale testing procedures and could be involved
in the work.
Pilot Scale Tests
Since temperature conditions could effect the sizing of enclosed beds it
is suggested that, in the absence of available data from existing plants in the
same general area with equivalent sludges, a small greenhouse type test installa-
tion would be in order. Ready made unitized small greenhouse enclosures
intended for the homeowner are now available at modest prices and could be
adapted to enclose a small drying bed for test work on both options.
Design Calculations
It is assumed that the test work has shown that by enclosing the beds and
using in line flocculant conditioning the average bed loading for the conventional
gravity system is 55 pounds per square foot per year and for the vacuum assisted
option is 110 pounds per square foot per year.
1. Since drying bed operation is a batch-wise procedure a sludge
storage or surge vessel should be provided to contain the
thickened digested sludge and serve as a feed tank for the
drying beds.
2. Sludge volume rate would be 1^,000 gallons/day or 98,000 gallons
per week, so a 100,000 gallon surge vessel would be required as
a feed tank.
3. Assuming tests showed a 12 inch bed fill level to be practical,
for the conventional gravity beds loaded at a conservative
loading of ^7 pounds per square foot per year, five beds, each
65 feet by 120 feet would be adequate.
4. The use of five beds would permit the bed filling procedure to
average less than two per week on an annual basis.
5. For the vacuum assisted bed* option, using a conservative design
loading of 91 pounds per square foot per year would result in
selection of four 50 feet by 100 feet drying beds.
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32
Additional Considerations
The system should include for either of the two options, mechanical sludge
removal via a front end loader.
An important point in evaluating the two options would be a determination
of the energy requirements involved in operating the vacuum system in that option.
FUTURE OF DRYING BEDS
An objective review of past results and consideration of the developments
of the past 5-7 years in modifying and increasing the dewatering capacity and
improving the mechanical removal capabilities of drying beds must lead to the
conclusion that they should be much more widely used than at present.
It seems clear that a judicious combination of the following aspects
would in many locations make drying beds the dewatering system of choice:
1. Provision in the bed design for mechanical removal via front
end loaders ala Tampa, etc.
2. Provision for conditioning of the sludge on its way into the
bed with polyelectrolytes or equivalent as needed.
3. Inclusion in the design of a translucent roof, or a total
greenhouse type enclosure with adequate ventilation and
odor control systems.
4. Where required for capacity purposes some form of vacuum
assistance (ala Sunrise City, Florida) for increasing the
drainage rate and enhancing evaporation where indicated.
If these aspects were included in conceptual designs, the design criteria
in terms of square footage of bed area required would be many times less than
the figures listed in the Ten State Standards. As a result of this and over-
all system evaluation of cost-effectiveness would surely result in more wide-
spread use of drying beds than is currently the case.
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ROTARY VACUUM FILTERS
There are three normal types of rotary vacuum filters and they are
described in the table below:
TABLE VJII
TYPES OF ROTARY VACUUM FILTERS
Type
Drum
Coil
Belt
Support Media
Cloth
Stainless Steel
Coils
Cloth
Discharge Mechanism
Blowback Section/Doctor
Blade
Coil Layer Separation/
Tines
Small Diameter Roll,
Flappers, Doctor Blades
The first (drum) type was largely displaced by the latter two due to cloth
plugging problems associated with the use of lime and ferric chloride/lime
conditioning systems. The drum type filter does not exhibit cloth plugging
problems with polyelectrolyte flocculants.
The coil filter has been widely used and does have a positive release
mechanism. Care must be exercised with coil filters to ensure a sufficiently
rapid rate of cake formation to prevent loss of fines through the more open
media involved during the initial phase of cake formation. This is a relatively
infrequent problem and if the fines problem does occur it is usually sympto-
matic of pre-dewatering processes which have destroyed a substantial portion
of the matrix forming material in the sludges (s) or of inadequate condition-
ing. Such pre-treatment processes will be detrimental in some manner to any
dewatering device.
Belt type filters were introduced to permit continuous washing of the
cloth and ostensibly overcome effects of plugging by lime or fines. This
concept was erroneous in most cases since the belt washes were not parti-
cularly effective in removing lime. In several plants which had early installa-
tions of the Drum type filter and later installations of Belt filters side by
side, the purported advantages of the Belt filters proved to be illusory.
Belt type filters are particularly prone to cake discharge problems.
Rotary vacuum filters produce the following typical results when inorganic
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chamicals are used for conditioning:
TABLE IX
TYPICAL ROTARY VACUUM FILTER RESULTS
FOR SLUDGE CONDITIONED WITH INORGANIC CHEMICALS
Chemical Dose (ib/ton) Yield ,
Type Sludge
Raw Primary
Anaerobically Digested Primary
Primary + Humus
Primary + Air Activated
Primary + Oxygen Activated
Digested Primary and Air Activated
Ferric Chloride
1-2
1-3
1-2
2-4
2-3
4-6
Lime (ib/hr/ft*
6-8 6-8
6-10 5-8
6-8 4-6
7-10 4-5
6-8 5-6
6-19 4-5
, Cake
') Solids
25-38
25-32
20-30
16-25
20-28
14-22
(Z)
While the data in this table above and the following one are representa-
tive, they should not be used for design purposes if the actual sludges to be
dewatered are available for lab and or pilot test work. It should also be
noted that the cake solids figures shown in this table include the significant
amounts of ferric chloride and lime used so the actual sewage solids content
is lower than what is shown. For instance, the correction would typically
bring the net sewage solids of a 22£ cake down to a correct figure of 18%.
There are instances where a combination of ferric chlorida and polyelectro-
lyte is employed to maximize rotary vacuum filter production rate. This is
frequently the case where the sludge has a high grease content and tends to
stick to the filter cloth on belt type filters.
Aluminum chloride or aluminum chlorohydrate are also effective inorganic
conditioning agents and where plants have existing rotary vacuum filters, the
availability of such materials as waste by-products of industrial plants is
worth exploration.
Typical results for polyelectrolyte conditioned sludges are as follows:
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TABLE X
TYPICAL ROTARY VACUUM FILTER RESULTS
FOR POLYELECTROLYTE CONDITIONED SLUDGES
35
Type Sludge
Chemical Cost
($/ton)
Yield
Cake
(Ib/hr/ft*) Solids
Raw Primary
Anaerobically Digested Primary
Primary + Humus
Primary + Air Activated
Primary + Oxygen Activated
Anaerobically Digested Primary
and Air Activated
1-2
2-5
3-6
5-12
5-10
6-15
8-10
7-8
4-6
4-5
4-6
3.5-6
25-38
25-32
20-30
16-25
20-28
14-22
In point of fact, more of the sludge processed in plants equipped with
rotary vacuum filters is conditioned with polymer flocculants than with
inorganic conditioners. The chemical cost is normally about the same for
the use of polyelectrolytes or inorganic conditioners. The use of poly-
electrolytes largely prevails because of more convenient handling, less
extensive preparation facilities, and freedom from corrosion problems, plus
the elimination of significant quantities of inorganic solids in the dewatered
cake.
On the other hand, some plants must use inorganic conditioners to obtain
cake release, provide matrix forming material in the cake, or to facilitate
lime addition for ultimate disposal.
Uith a digested mixture of primary and excess activated sludge, in most
plants, rotary vacuum filters will produce dewatered cakes with cake solids
contents within the 18-22^ range, which is almost always too wet for autogenous
incineration or some composting processes. These facts, plus energy costs
have caused the selection rate for rotary vacuum filters to wane considerably,
The sludge feed to rotary vacuum filters should never be below 3# dry
solids content and preferable should be greater than k% if reasonable production
rates are to be attained.
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AUXILIARY DEVICES FOR ROTARY VACUUM FILTERS
To obtain higher solids cakes from rotary vacuum filters, three companies
have developed devices which can further dewater the filter cake. These devices
are in some cases, specifically designed as add-ons to existing filters or in
others, supplied as integral parts of the rotary vacuum filter.
The items of reference are:
1. The Eimco Hi-Solids filter
2. The Parkson Magnum Press high pressure section
3. The Komline Sanderson Unimat high pressure section.
Eimco Hi-Solids Filter
This device combines normal rotary vacuum filtration with a batch type
adjunct pressure filter. The cake while still on the rotary vacuum filter
belt feeds into a small co-joined stage where it is subjected on one side to
pressure from a rubber diaphragm (50-150 PSIG or 3-5-10.5 Kg/sq. cm) while
on the other side (below the belt) a vacuum is applied to facilitate drainage.
Since this is a batch procedure with the rotation of the rotary vacuum filter
being momentarily interrupted whilst the pressure and vacuum are applied in the
pressure chamber section, some lowering of production occurs.
Eimco supplies this unit as an integral system and also supplies the
press portion as an add-on device for existing conventional rotary vacuum
filters. This device was tested on pilot scale at Washington, D. G., and
increased the cake solids content from a normal 11% up to a level of 25?.'.
The sludge tested was a rather difficult to process mixture of primary and
secondary sludges.
Parkson Magnum Press
This unit (more fully described in the section on Continuous Horizontal
Belt Filters), was evaluated on pilot scale at Washington, D. G. for dewater-
ing filter cake from the existing rotary vacuum filters. Filter cake of 18#
dry solids content was further dewatered to 35-^0£ dry solids with no further
conditioning employed.
Commercial availability of this unit hinges on successful conclusion of
development work required to enable design of a mechanical method of trans-
mitting filter cake from the rotary vacuum filter to the auxiliary press
section without degrading the processability of the cake.
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Komline Sanderson Unimat
A pilot model of the medium and high pressure sections of the Unimat was
evaluated at Washington, D, C. on the cake from the rotary vacuum filters and
produced a cake of 38/5 dry solids. Once again mechanical development work
would be required to facilitate an installation.
In summation, the three devices briefly described above offer real
promise for providing a means to further dewater sludge cake from existing
rotary vacuum filter installations where such a procedure is in order.
DESIGN EXAMPLE - ROTARY VACUUM FILTRATION 4 MGD PLANT
Basic System Assumptions;
The sludge is an anaerobically digested mixture of primary and excess
activated sludge which has been thickened to ^ solids via a flocculant
aided post digestion thickening process. System design has been such that
the sludge mixture is about 60& primary and ^0?S secondary sludge. The sludge
is available for testing.
The ultimate disposal method for the sludge is to be by hauling dewatered
cake to either a sanitary landfill, or for disposal on farmland, or for com-
posting and horticultural use.
The sludge removal rate required is to average 2.5 dry tons per day and
the cake must possess sufficient dimensional stability to preclude flow out
of a truck.
Alternate Units for Consideration and or Evaluation
1. A Coil filter
2. A Belt type filter
3. A Drum type filter
Evaluation Procedure
The sequence to be followed in the evaluation and design is planned
as follows:
1. Verification of the amounts and relative degree of uniformity
of the flow of sludge to be dewatered. This is to be obtained
by review of plant operating data.
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33-
2. Diagnostic bench scale dewatering tests of the sludge, repeated
several times during different operational periods to assure
uniformity. It is absolutely essential that these tests and
any pilot tests be done oh site with fresh sludge.
3. Review of the above results with interested candidate suppliers
and then repetition of the bench scale tests in conjunction
with suppliers personnel.
4-. A pilot dewatering test series should then ensue, particularly
if there is any doubt about any facet of the dewatering operation.
This should be carried out with at least two of the potential
suppliers.
5. Summation of design data should be prepared by the consulting
engineer. Each potential supplier should be asked to prepare
and transmit a report of the bench and pilot test work including
Their design recommendations, including equipment required,
sizing, delivery time, etc., together with "budget price quotes"
and estimates of annual 0/M costs, and life cycles of the various
items of equipment.
6. A detailed design should then be prepared and plans, specifi-
cations, conditions of contract, etc., forwarded to those
suppliers whose equipment and performance have qualified them
to enter a firm price quotation.
7. From the design and overall system cost data available, and with
full consideration of relative equipment reliabilities, a
selection of the supplier can then be made.
Bench Scale Tests
The "Buechner Funnel" test procedure is well documented and all suppliers
of rotary vacuum filters are very familiar with it. The "Filter Leaf" test
procedure is likewise readily available.
Normally the Buechner Funnel test, employing a cake support media identi-
cal to that to be employed will supply all the required information needed.
However, if the dewatered cake shows real signs of sticking to the filter
media, then a leaf test to check this property may be in order.
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39
In the Buechner Funnel test it Is important to:
1. Determine dewatering rate, time to vacuum break and resultant
cake solids after a simulated cycle.
2. Analyze the filtrate for suspended solids, BQD-, COD, and total
dissolved solids.
3. The data from (2), along with analagous sludge feed data
should be used to determine exactly what total solids capture
is being obtained.
^. The cake release characteristics should be carefully assessed.
If a problem is indicated, a left test can be run to observe
whether or not the cake falls freely from a vertically held leaf.
If it doesn't, then a Belt filter will cause release problems.
Pilot Tests
Tost suppliers have packaged pilot units which can be wheeled in for
testing. This is advisable, in most cases.
It is important that the sludge quality during the comparative pilot
plant tests be reasonably comparable. This can be verified by concurrent
"Buechner Funnel" testing.
Design Calculations
1. Operating cycle to be 35 hours per week (? hours/day). This
permits start-up and wash-up times within an 8 hour shift.
2. One filter, with adequate supply of key spare parts to be
maintained.
3. Size of Vacuum Filter
Production rate has been determined via pilot testing to be
5 Ib/hr/sq.ft., but to provide a margin of safety, 4 Ib/hr/sq.ft.,
will be used.
Steady state sludge removal rate requirement is 35i000 pounds
per week.
With a 35 hour per week schedule, weekly filter capacity at
4 pounds per hour per square foot is l^J-0 pounds.
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35,000 pounds/week t 1^0 pounds per square foot per week = 279
square feet of filter area required.
The nearest standard size filter is 300 square feet, so a single
unit of this size is chosen.
4. Sizing of auxiliary equipment
In each case the details of sizes of vacuum equipment, conveyors
or other system required to get the dewatered cake into the
truck for hauling, and the chemical dosing equipment for sludge
conditioning must be developed, and priced.
5. Sludge storage capability
The one shift per day - five day per week mode of operation plus
the use of a single filter will require provision of several
days storage capacity for the digested sludge. This could'potent-
ially be provided by a combination of the inherent surge capacities
of the digestion tanks and post digestion thickening tanks, or by
provision of a separate storage tank equipped to ensure homogeneity
of feed to the RVF.
For a sludge of the type described, a cationic polyelectrolyte flocculant
would probably be used for conditioning. The testing and selection of suit-
able conditioning agents would necessarily be carried out in conjunction with
the series of bench scale and pilot test programs used to select and size the
rotary vacuum filters. As part of the selection process for suitable conditioners,
data should be obtained and reviewed on:
1. Price, dosage rate, and availability of both polyelectrolytes and
inorganic conditioners in the particular locale.
2. The system required for solution preparation and application, and
its cost.
3. The storage stability (shelf life) of the conditioner in its
form as supplied and in stock solution for use.
')-. Handling characteristics, safety aspects and corrosion properties
of the material in dry and liquid forms.
5. Previous experience with the same materials at other plants with
similar sludges.
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'41
Dewatering System Considerations
Auxiliary equipment such as sludge conveyor or removal facilities, chemical
mixing and feed equipment, and sludge feed pumps are usually available from
the rotary vacuum filter supplier.
Polymer solution preparation and dosing equipment is also frequently
available from the polymer supplier or from an equipment supplier other than
the rotary vacuum filter supplier.
An Energy Audit should be a part of every system evaluation. The Energy
Audit should include not only an estimate of the power consumption of the
dewatering equipment and its immediate auxiliaries, but also the impact of
the particular dewatering system on the overall treatment process system. In
this regard, the assessment should specifically include the impact of the
conditioning/dewatering system on both the post dewatering portion of the
system and the pre-dewaterinf portion of the system. The latter facet makes
preparation and consideration of "Quantified Flow Diagrams" for both the
conditioning/dewatering system and the overall treatment system mandatory
to cost effective design.
For purposes of comparison, the rotary vacuum filter in this design
example would require a vacuum pump of 30 horsepower, and filtrate pump of
3 horsepower, To make a complete energy audit, all the auxiliary equipment
data, and the other points mentioned in the previous paragraph would have
to be assessed.
DESIGN EXAMPLE - ROTARY VACUUM FILTRATION - UP KGD Plant
Basic System Assumptions:
These would be the same as in the preceding design example for a k KGD
plant except that the required removal rate would be 25 tons of dry solids
per day.
Other Considerations
The following parts of the Design Example would be the same for the KGD
plant as for the 4 KGD plant in the preceding example:
1, Alternate units for consideration and evaluation
2. Evaluation procedures
3. Bench scale testing
IK Pilot tests
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Design Calculations
1. Operating Cyclet to be either a seven day per week, 2k hour per
day operation or five day per week, 2^ hour per day operation
depending on reduction and final disposal processes chosen.
2. Size and number of Rotary Vacuum Filters required.
Production rate to be conservatively taken at ^ pounds/hour/
sq. ft.
At 350fOOO pounds per week the weekly capacity of a square foot
of filter area for a seven day operation (allowing 2 hours/day
downtime average for clean up and maintenance) is b pounds/hour/
sq. ft. x 15^ hours per week or 6l6 pounds/week/sq.ft.
Dividing 350,000 pounds per week by 6l6 gives a filtration area
requirement of 568 square feet.
A similar calculation for a five day operation gives a filtration
area requirement of 793 square feet.
In either the seven day/week or five day/week options, two 500
square foot rotary vacuum filters would normally be specified
to provide sufficient capacity and redundancy.
3. All of the other facets of the design procedure would be the
same as in the **• JIGD example.
General Comment - Rotary Vacuum Filters
The RVF was, for many years, the bellwether device for dewatering muni-
cipal sludges. Their frequency of use had persisted longer in the United
States than the rest of the world.
Operating problems such as the cake pick-up difficulties, poor cake
release from belt filters with sticky sludges, and the maintenance requirements
associated with vacuum producing equipment have existed in numerous cases.
Solids capture problems associated with either the effect of less than adequate
cake formation rate in some relatively open media filter installations or with
cake recycle due to sticking problems have also occurred. While these problems
could be moderated in many cases by revision of conditioning methodology or
mechanical changes, they are deterrents to widespread continued usage.
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Fore universal deterrents to the continued selection of RVF's are:
1. The energy and maintenance costs associated with operating
vacuum systems.
2. The inability to produce nearly as dry a cake as other newer
devices.
These comments are made to encourage the design engineer to review current
operating and cost experiences at existing plants prior to making a design
decision.
CONTINUOUS BKLT FILTER PRESSES
This general bype of device, which employs single and or double moving
belts to continuously dewater sludges through one or more phases of dewater-
ing was originally developed, and in subsequent years modified and improved,
in West Germany. The earliest concurrent U.S. development was under the
aegis of the late Brian Goodman, at Smith and Loveless Division of Ecodyne,
The scope and depth of development of this newer type device has been
much more pronounced in Europe than in the United States until the past 3-V
years. Within those past 3-^ years, many different models of the same type
•'evice, differing in configuration and capability, have been introduced into
the U. S. market.
While there is general agreement that the Continuous Belt Filter Press
materially extends capabilities for improved dewatering of sludges, the U. Bt
design engineer is faced with a real task in selecting the optimum devise from
the many which are now available. But that task must be dealt with if advantage
is to be taken of this technological break-through.
U. 5. installations of the latest and best models are just now coining
on-strean. To review actual operating performance on particular sludges,
usage of available mobile pilot test units, coupled with site visits is in
order. There is considerable operating experience available at existing
i'uropean sites. The old conundrum that ISuropean sludges are different and
results are not applicable should be treated with the contempt it deserves,
since it is inaccurate.
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Original Concept and Evolutionaxy Developments - Continuous Belt Filter Presses
The figure below illustrates the single level device originally marketed
by Klein of Germany and their U.S. licensee, R. B. Carter.
SLUDGE
FEED
1- 4-
FILTeR -J
BELT
T" ^r •»•
"\vi
DOCTOR BLADE
DRAINING ZONE
PMUIONI
•HIAHIONl
Figure VIII - Original Concept - Continuous Belt Filter Press
Practically concurrent with this development was Brian Goodman's work
with the Smith & Loveless Concentrator which is described later.
This type unit was successful with many normal mixed sludges. Typical
dewatering results for digested mixed sludges with initial feed solids of
5=* are to give a dewatered cake of 19£ solids at a rate of 6.? Ib/hr/sq.ft.
with a chemical conditioning cost of $4.10/ton. In general, most of the
results with these units closely parallel those achieved with rotary vacuum
filters. They do have advantages in that there is no sludge pick-up problem
which sometimes occurs with rotary vacuum filters, and they have a lower
energy consumption.
These results are satisfactory for many installations and the Continuous
Belt Filter Press of this first type or its immediate successor, a two-level
unit of the same basic design and concept (see Figure IX following), has in
the past five years become the most frequently selected dewatering device
around the world.
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SLUDGE FEED
CHEMICAL
ROCCULANT
ADDITION
Figure IX - Second Generation - Continuous Belt Filter Press
There have been additional developments of the basic principles of the
Continuous Belt Filter Press and several third generation units from various
companies are now available. In a broad sense these latest improvements may
be described ast
1. The addition of some form of continuous mechanical thickening
device as the initial stage of a Continuous Belt Filter Press.
2. The addition of additional medium and or high pressure press
sections to the Continuous Belt Filter Press, and variations in
the cake shearing mechanisms to obtain additional dewatering.
A schematic conceptual drawing of the R. B. Carter Series 31/32 device,
the design of same being based on the Klein "S" Press ( e. unit widely installed
around the world) typifies the third generation type unit.
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POLYMER
MIXING
—SLUDGE
jl /H j| ^
LUDGE
REACTOR _
SNOmONER"
-p WASH WATER
I (EFFLUENT OR
! CITY WATER)
(-{optional)
CLEAN FILTRATE
DISCHARGE
SOLIDS
BELT PRESS
(LOW PRESS7HIGH PRESS./
SHEAR PRESS.)
'XT ^^^"XCAKE
^ | DISCHARGE
DIRTY WASH WATER,
FILTRATE, AND
RECYCLE POLYMER
M1MTS UnMD fQH
Figure X - R. B. Carter Series 31/32 - CBPP - Conceptual Schematic
Referring to figure X preceding, and Figure XI, next page, this device
functions a.s follows:
1. The reactor conditioner (rotating cylindrical screens) removes
free draining water, usually increasing sludge solids content
from 0.1-0.55 to 3-S£.
2. The sludge then passes into the first or low pressure zone of
the belt press proper with the top belt being solid and the
lower one being a sieve belt. Herein further water removal
occurs and a sludge mat with significant dimensional stability
is forming.
3. In the second or high pressure zone (4 atmospheres) the sludge
is sandiriched between two sieve belts. Large .mesh openings
are possible because the sludge has developed structural integ-
rity at this point.
-------
A serpentine configuration makes up the Shear Zone at the end
of the second pressure zone wherein by stretching the belts and
sludge cake over smaller rollers, a squeezing action expels more
water from the cake.
Figure XI - R. 3. barter Series 31/32 - GBFP
-------
As will be noted subsequently in more detailed descriptions of each unit,
the advanced third generation GBPP's give cake dry solids contents equivalent
to those achieved with pressure filters.
In addition to the barter Series 31/32 device, other suppliers of similar
third generation type devices ares
Company Unit
Komline Sanderson Unimat
Parkson Company Magnum Press
Ashbrook Simon Hartley Winklepress
Carborundum Sludge Belt Filter Press
Tait Andritz SDH
There are also other Continuous Belt Filter Presses which are more advanced
than the original first generation type units. These are also described later.
Categorization of Continuous Belt Filter Presses
Only units which have'at least two phases built into their operation,
and which yield cakes which are truly dewatered and dimensionally stable
(non-flowable) can logically be classified as Continuous Belt Filter Presses.
The DCG (Dual Cell Gravity Concentrator) as supplied by Pennutit when used in
series with the Permutit I-0P (multiple roll press) is a system which performs
as a continuous dewatering device in a fashion analogous to the first generation
CBFP.
All of the variations start with a gravity drainage zone followed by
various combinations of shear and different levels of pressure (or vacuum)
applied to the gravity drained cake. Rather than attempting to lump presses
of different configuration into rigid categories, each will be described and
results listed.
SI1ITH AND LOVSLSS5 SLUDGE CONCENTRATOR
This device, as described in reference (10), was developed and is markete
by the Smith and Loveless Division of Ecodyne. It is essentially a "Gravity-
Pressure" filtration unit which uses an endless, variable speed, relatively
open mesh filter screen to retain flocculated solids while the bulk of liquid
passes through the screen. Solids from the gravity drainage stage past* into
the second or pressure stage where three sets of compression rollers further
-------
iin
dewater the cake. The pressure increases with each set of rollers. The
dewatered sludge falls off the belt into a discharge chute for removal.
The S.
12
10
10
As will be noted this device does not give as dry a cake as some of the
other more complicated machines. It has found usage at certain plants which
can utilize cake solids levels as shown. The unit uses only 5 horsepower vs
a normal 40 horsepower for a rotary vacuum filter.
PISniJTIT DCG - MRP
This system consists of a dual cell gravity DCG) unit followed in series
by a multiple roll press (MRP). Referring to the schematic cross-section of
the DCG, this first drainage section forms a plug of fluid sludge in the first
fine mesh nylon cell and then the plug is further dewatered in cake form in
the second cell.
-------
50
Figure XII - Permutit DOG - Cross Section
The relatively moist cake from the DCG is conveyed to the MRP, an inclined
dual continuous spring loaded belt which further dewaters the sludge cake.
CAKE DISCHARGE
SLUDGE INLET
EFFLUENT
Figure XIII - Permutit KRP - Cross Section
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51
Typical performance on the DCG-KRP (Caldwell, New Jersey) indicates
dewatering of an anaerobically digested mixture of primary and humus sludge
from a feed solids of b-5 per cent yielding a dewatered cake of 15/S dry solids
nith polymer costs of $8 to $10 per ton.
The DCG-MRP has worked reasonably well at small plants with non-continuous
dewatering schedules. Some problems have been noted with maintainability of
the early units and sone modifications are in process.
INFILCO DEGBBMDNT FLOC-PRESS
This is a two stage unit of French origin featuring a horizontal belt
gravity drainage area on a woven synthetic fiber belt followed by a press
section. The partially dewatered cake is sandwiched between the lower belt
and a rubber pressure belt (adjustable hydraulic loading) to provide cake
solids levels similar to that which is obtained in rotary vacuum filters or
centrifuges.
Figure XIV - Infilco Degremont Floe-Press
There are 46 world-wide Floe-Press installations and there were five in
the U. S. as of January 19?6. A notable U. S. installation is at Medford,
New Jersey (ll). At Medford, a 0.9 KGD contact stabilization plant, a two
meter wode Floe-Press replaced an existing rotary vacuum, filter which has been
shut down:
-------
TABLE XII
FLOG PHESS RESULTS - BEDFORD, HEW JH?SEY
Averages
Feed Solids f=
Cake 1?-19
Filtrate Susp. Solids (PPtf) 100
f> Solids Capture 98
The horsepower consumption is 6.25 for the Floe-Press versus 22 for the
previously used rotary vacuum filter. The HVF had provided similar cake solids
but poorer solids capture. Polyelectrolyte costs are in the $11-15/ton range.
The filter belt is still in excellent condition after almost a year of operation.
The wash water rate is 22 gpm at 50 psi and plant effluent water is used.
The Floe-Press system includes a mounted sludge conditioning chamber and
other auxiliaries such as chemical conditioner and sludge feed systems, con-
veyors for sludge removal and automated control panels.
Output in pounds per foot of belt width per hour is quoted at 13^-268
for an anaerobically digested mixture of primary and E.A.S. at a feed solids
of 3,5 to 9#. The Hedford, New Jersey Floe-Press is 16 feet 1-J- Inches long,
10 feet i(~3/8" wide, and 10 feet 6 inches tall.
The Floe-Press is available in belt widths varying from a nominal 3 feet
to a nominal 10 feet with effective belt areas of 32.28 square feet to 96.&J-
square feet. For the larger units, only additional width must be provided for.
PASBAVAHT VAG-U-PRSSS
This is a German development which features the following:
1. A continuous ress utilizing gravity and vacuum drainage
followed by a pressure zone.
2. Conditioned sludge is evenly distributed on a moving belt
which initially drains by gravity and then by virtue of vacuum
boxes beneath the belt,
3, The compression belt is applied on top of sludge on the lower
belt to form a sandwich.
-------
53
4. The two belts are subjected to pressure by going under tension
around large dewatering cylinders. Pressure is then applied to
alternating sides of the belt by smaller pressure rolls.
5. Dewatered sludge is discharged and belts are continuously back-
washed.
6. The Vac-U-Press is enclosed in a fiberglass reinforced polyester
housing to control noise and odor.
Typical sizing data is as shown in the following table:
TABLE XIII
PASSAVANT VAC-U-PRESS - SIZING DATA
Model Belt
No. Width
BFP075 26j
BTT125 43|
BFP200 72|
Length
14-9
14-9
14-9
Width
(ft. -
4-1
5-8
8-2
Height
In.)
5-3
5-3
5-3
Drive
Motor
(HP)
1.5
3
3
Active
Belt Area
(Sq.ft.)
90
150
250
Nominal
Capacity
(Gal./Hr.)
1500
2500
4200
There are five U. S. installations of the Vac-U-Press, all of the BFP-
200 model. Indications are that it gives a dewatered cake slightly drier
than a rotary vacuum filter.
A mobile test unit is available for rental.
TAIT ANDRITZ SDK and SDP-SE
Andritz, an Austrian equipment firm first developed a continuous double
belt filter dewatering device for use on various industrial sludges. In the
past two years Tait Andritz of Lubbock, Texas has sold and installed 43 of
these devices at 28 total U.S. locations for dewatering of various industrial
and municipal sludges. The 1977 world wide installation list shows 68 locations
where these devices are in use. Twenty of these locations are on municipal
sludges. The industrial installations are in some cases on straight
biomass sludges.
-------
The dewatering in the Tait Andritz unit(s) is achieved by passage of the
sludge through a gravity dewatering zone, into a wedge zone for pressure
dewatering, followed "by higher pressure dewatering in a module zone. The
module zone can be either an S configuration (offset rolls), or a press
configuration (pressure loaded rollers).
Main design features are: variable speed drive for belts and conditioning
drum; pneumatic belt tensioning and pressure adjustment during operation;
pneumatic belt tracking; and in the industrial SDK model, use of endless belts.
The following figure shown the SDM-SM model (seamed belts) designed for
municipal operation where unattended round-the-clock operation is not necessary,
Figure XV - Tait Andritz - SDK-SM Model
-------
55
The following table summarizes reported operating results:
TABLE XIV
TAIT ANDRITZ - SDM-SH RESULTS
Type
Sludge
Raw Primary
Primary + E.A.S.
Unox Ext. Aer.
(1) Per 20 inches of
% Dry
Feed
5-7
3-5
1-2
Working
Solids Throughput (l) Polymer ~ost
:ake (GPfi) (D.S.-lb/hr) ($/ton D.S.)
22-36 10-14 300-500
20-25 15-20 200-350
18-23 20-25 200-250
Belt Width
4-7
4-8
8-10
The results shown in the above table tend to indicate that the Tait Andritz
llFP's will normally produce a cake solids content somewhat higher than that
obtainable in a rotary vacuum filter. Further, more definitive results on the
two versions (either the "Press Module" or the "S" Kodule equipped) of the
basic device will be forthcoming during 19?8. In this vein, it is understood
that Burlington, Wisconsin (an installation discussed later) has recently
ordered several units.
The Tait Andritz SDM device (industrial) has an excellent performance
record (eash of maintenance, etc.) in dewatering biological and mixed sludges
in the paper industry. (12,13)
Data on the size of the three SDM-SM models available are as follows:
TABLE XV
TAIT ANDRITZ SDK-SM - MACHINE SIZING DATA
Size & Type
SDH 40
SDK 60
SDM 80"
Working Belt Overall Dimensions t
Width Length Width Height
40"
60"
80"
152|"
186"
186"
75"
114"
134"
75"
83"
83"
t Weight
' (lb)
5513
14333
17640
Conn H.P.
Load
3*
5 3/4
5 3/4
Belt Spray
Consumption
(CPH)
18-24
30-37
35-45
*Height will vary according to drive system used
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56
ASHBROOK SIMON-HARTLEY WINKLEPRESS
The Winklepress was developed by Gehr. Bellmer KG. of Germany. Simon-
Hartley of the United Kingdom markets U.S. units through a subsidiary, Ash-
brook Simon-Hartley of Houston, Texas.
The schematic conceptual drawing which follows shows that the device
employs two endless synthetic fiber mesh sieve belts to convey and dewater
conditioned sludge. After an initial gravity drainage stage, the primary
belt meets the second belt and forms a vertical sandwich drainage section.
The two belts which are under tension, then carry the sludge along an arrange-
ment of staggered rollers where multiple shear force action areas squeeze out
remaining free water. The sieve belts are continuously washed.
ROTARY DRUM
CONDITIONER
REAGENT
HORIZONTAL
DRAINAGE
SECTION
FEED
DISCHARGE
Figure XVI - Ashbrook Simon-Hartley Winklepress Schematic
While there are a number of operational installations in Europe, as of
November 1, 1977, none of the U. S. installations under construction had
started operation.
-------
TABLE XVI
WINKLEPRESS TEST RESULTS (FROM SUPPLIER)
Dry Solids Filtrate Polymers Capacity Feed
———^— ,_ -
Feed Cake (mg/l) (kg/nr) nr/h gpm
meter
Digested Primary
and Humus
M ii i< ii
Digested Primary
and S.A.S.
3.8
5.7
3.5
^.8
36.2
36.3
36.3
38.5
85
95
90
75
0.182
0.165
0.165
0.182
7.5
6.5
7.5
7.5
33.0
28.6
33.0
33.0
The following table shows the range of production units available.
TABLE XVII
WINKLEPR3SS SIZE AND CAPACITY DATA
— .
Vlinklepress
Size
0
1
2
3
4
KOKLINE
Input Width
mm inches
200-300 8-12
500-800 20-32
1000-1300 39-51
1500-1800 59-71
2000-2300 79-91
SANDERSON UNIIIAT Gt'H-7
e.
Nominal capacity of
-digested sludge
nr/h §Pm
2-3 8.8-13
5-8 22-35
10-13 ^-57
15-18 61-79
20-23 88-101
CONTINUOUS BFP
Komline Sanderson manufactures its version of the German Unimat under
license from Kull-Abwasser-Transportanlagen-GMBH, Slversberg, West Germany.
-------
58
The most advanced model of the modularized Unimat which is designed for
maximum cake dryness and throughput is the GMpH-?. This press consists of
four stages:
1, Gravity drainage (actually a thickening stage)
2. A mild pressure stage
3. A medium pressure stage
^. A high pressure stage
A conceptual schematic follows:
ffUIMACI tU6I
TVvvvvvvvv
MjgiTUU MIIUM MUJ
Figure XVII - Komline Sanderson Unimat GI'UH-?
The initial gravity drainage stage is a continuous belt of pockets which
are formed by folding a rectangular piece of cloth. This is a separate belt.
After thickening in this first stage the sludge dumps into a different belt
which moves over a gravity drainage tray prior to dumping onto another belt on
a succeeding tray"(and a different belt) where a small amount of pressure is
added by small diameter rollers. Then it is moved to the third tray of the
-------
59
mild pressure section (and back on the original pressure belt) and subjected
to slightly more pressure before going into the medium pressure stage. All
the rollers in the medium pressure stage are adjustable for pressure optimi-
zation. While passing over the medium pressure rolls, the cake sandwich
between the belts is flexed from one side to the other. Each of the large
diameter drums has smaller diameter rolls which apply pressure as the sand-
wich passes over the drums. Every other roll is perforated for water removal.
Pressure is applied to the cake by tension on the belts as the belts go around
the drums and by the small diameter rollers. The belt tension, is, however,
relatively low and all synthetic media is used instead of stainless steel in
the long axis.
The cake now goes to the high pressure stage which can be thought of as
two caterpillar tractors standing upright with the tracks butting together.
As in the medium pressure section the pressure is adjustable through springs.
In applications where a very high dry solids in the cake is not imperative,
the unit is available without the high pressure section.
In addition to the previously mentioned nomenclature and model system the
Unimat series is available in three models:
Model S - Gravity Stage
I!odel SH - Gravity & Medium Pressure Stages
Model SMH - Gravity, Medium & High Pressure Stages
The following table lists the design features of this series:
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60
TABLE XVIII
ACTIVE FILTRATION SURFACE AREAS & RETENTION TIMES
Machine
Model
S
M
H
Machine
Width
(Meter)
1
2
3
1
2
3
1
2
3
Active Filtration
Surface Area (Sq..Ft.)
S
68
136
204
5 roll 7
101
203
305
ALL
32.9
65.6
98.4
L
104
208
312
roll
190
380
570
Retention
Time (Minutes)
S L
1.2 to 6 2 to 9
5 roll 7 roll
5 to 19 10 to 36
ALL
2 to 6
Note: When using 2 or more sections, the retention time
and active surface areas are cumulative.
There were 69 European locations employing the Unimat as of November, 19?6,
with practically all of them processing municipal sludges of some type, including
straight lOOfT tiomass.
The following table lists reported results:
-------
TABLE XIX
DRY SOLIDS OF GAKS AND POLYMER DOSAGE
UNI NAT:
Type of Sludge
""eed Cone.
(« D.3.)
Fresh-Primary
(Raw)
4 - &
Fr. Prim £• Trickling
Filter
3 - &
Fr. Primary
c" Activated
3 - y
Anaerobically Dig.
Prim. & Act.
4 - 9*
Activated (lOO^ W.A.S, )
0.5 - 1.0^
yodel S
After
Gravity
Stage
(*fc.S.)
12-18
10-15
10-15
14-24
8-12
Model SM
After Gravity
: Medium
Pressure
(T).S.)
25-35
22-32
17-27
25-35
17-20
Model SMH
After Gravity
£•• Medium &
High Pressure
30-45
28-40
25-35
30-45
17-23
Typical
Polymer
Dosage
Its/ton D.S,
6.0 - 8.5
6.0 - 10.0
6.0 - 10.0
5.0 - 8.5
7.0 - 10.0
While at the time of writing this, no Unimat systems are yet operating in
the U. S., 16 units have been sold and some will be operative by early 1978.
A mobile test unit is available and considerable U. S. test work was
carried out on site during 1977.
Performance of Unimat on Washington. P. ?.. Mixed Sludge
At Slue Plains the Unimat Gi\'H-7 dewatered a sludge mixture of 1 part
primary plus 2 parts S.A.S. to a dry solids content of 27 to 33?' at rates of
644 to 677 pounds per hour per meter of width. Polymer costs were mostly
between 'S8.76 to 59.20 per dry ton with a solids capture of 95-98;'. On the
existing rotary vacuum filters a total dry cake solids of 22-24^ (including
solids resulting from use of 5-7T* ferric chloride and 15-20:' lime) is
normally obtained. Because of the large variation of the sludge quality, the
lime dosage for the rotary vacuum filters reaches 30-40?! on occasion.
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62
At Blue Plains, the dewatered vacuum filter cake was fed to the ligH sections
of the Unimat and the cake solids were increased to 3?-**V% at a feed rate of
1200 pounds/hour/meter width with no auxiliary conditioner dosage.
Performance of Unimat on Columbus, Ohio'. Southerly Plant Sludge
At Columbus Southerly plant, the anaerobically digested mixture of primary
and E.A.S. was dewatered to a cake solids content of 36-39?? at a rate of 750-
1000 pounds per hour per meter width. Solids capture was 90-95?5 and polymer
costs $8 - SlV'ton. Feed solids were 3-4# dry solids. Thus an autogenous
cake is feasible with this difficult sludge.
It is quite apparent that the K.S. Unimat press is one of the CBFP's
newly introduced into the U.S. from Germany which has the capability to
effectively dewater mixtures of primary and E.A.S. sludges to a dry solids
content high enough to be in the autogenous incineration range.
PARKSON MAGNUM PRESS
This device, of Swedish origin, is manufactured and sold in the U. S.
by the Parkson Corporation of Ft. Lauderdale, Florida.
The Fagnum Press is an advanced or third generation type GBFP designed to
maximize dry solids content of dewatered cake. The Magnum Press has three
stages and can best be described by reference to the cross sectional side-view
of Figure XVIII following:
-------
CaniuMd
Cotacbng In/
Tij^ure XVIII - Tarkson itacnuR-Press - Cross Section
V.J
-------
The initial stage is a unique gravity drainage section. In addition to
normal dewatering occurring by gravity from a properly conditioned sludge,
the sludge can be subjected to a light pressure provided by rollers and be
turned by plows (both optional). The partially formed cake then proceeds to
the low pressure stage where the second polyester screen belt comes into play
on the top forming a sandwich that is fed into the second or low pressure
stage. In the low pressure stage perforated press rolls of decreasing diameter
subject the cake to continuously increasing pressures. In the last or high
pressure stage the cake is subjected to very high pressure that is adjustable,
depending on the application. The high pressure is generated by a series of
1 inch wide flat belts that press the screens against a perforated roll
uniformly from side to side. This feature allows the sludge to be subjected
to high pressure for a long period of time without producing an excessive
load on the screens. The pressure is adjustable through the use of two
hydraulic cylinders.
This final high pressure stage of the Magnum Press can also be employed
in a modular fashion to further dewater filter cake from existing Rotary
Vacuum Filter installation.
The Parkson Magnum Press is available in four sizes as shown below:
TABLE XX
ilAGNUN PRESS SIZE DATA
Fodel Screen Width Weight Overall Dimensions Screen Wash
(Nominal) A-Width B-Height C-Length Water Flow
Rate 3 100 gal
FP-20
FT-40
FP-60
i:p-8o
20"
1*0"
60"
80"
3.8 Tons
b.b Tons
>4.S Tons
6.0 Tons
V
5 '-8"
7 '-IT
9'
7 '-9"
7' -9"
7 '-9"
7'-9"
IV -10"
1V-10"
IV -10"
IV -10"
12 gpm
2^ gpm
36 gpr.
iJ-8 gpn
As of December, 1977» nineteen I.'agnum Presses had been sold world-wide.
There are seven Japanese installations, nine in Europe, and three in the United
States. The first U. 3. unit (at iobil Oil Company), processing straight excess
biological sludge is just now comnencing operation.
-------
Parkson has a mobile Hagnum Press and a smaller pilot unit, both of which
have been used to carry out on-site tests at various U. S. locations.
Performance of Magnum Press at Washington, D. C.
A 0.25 meter pilot unit was evaluated on the various sludges at Blue Plains
plant. The following two figures show the results obtained with various mixtures
of primary and excess activated sludges (including phosphorous removal sludges
resulting from iron salt use).
In assessing results of dewatering work at Blue Plains it is important to
note the following:
1. The normal mix is 32^ raw primary/68^ raw secondary sludges
(on a weight f' dry solids basis). The primary is gravity
thickened to 9.5?^ and the secondary is DAF thickened to 5.5?'.
The resulting 6.6^ solids mix is filtered on RVF's to about
I8tf (without lime).
2. The Blue Plains plant has an abnormally large anount of a
difficult to process excess activated sludge due primarily
to the use of a high rate activated sludge biological treatment
system. This system was apparently chosen because of certain
site and capacity constraints.
As can be seen in Figure XIX, the :."agnum Press produced a dewatered cake
of 30^' dry solids content at a rate of 800 pounds/hour/39 . 37 inches of belt
width.
It should also be noted that a straight interpolation of the data in
?ipure XX indicates that at a more normal sludge ratio of 60# primary and
bW seconday, even with the high rate 3.A.S., the production rate would be
1?" rreater and the cake solids would be y**4 '. As shown in Figure XX, polymer
dosages varied fro-n 5.5 to 1.6 pounds per ton of dry solids and solids recoveries
varied from 95 to 99"'.
The iiaftnuin Press ;ras also tested for dewatering the filter cake from the
existing RVF's. :ake solids of 35-42'"' were obtained at rates of 800 to 2800
pounds per hour of dry solids per 39.3? inches belt width. There is mechanical
development work required to design equipment to transfer the filter cake to
such a press.
-------
Pressed
Vacuum
Filter
Cake
I i I i I » I i I i
10 20 30 40 50 60 70 80 90 100
%Primary(wt.% dry solids)
100 90 80 70 60 50 40 30 20 10 0
^Secondary (wt. % dry solids)
Figure XIX - I'agnum Press Results, Blue Plains
-------
-.100
10 20 30 40 50 60 70
% Primary
-------
68
Magnum Press Performance at LAOMA
The Magnum Press mobile unit was evaluated on several mixtures of the
sludges "being studied in this major R&D project.
TABLE XXI
PERFORMANCE OF MAGNUM PRESS - LAOMA
Sludge Dry Solids (?-) Capacity-D
Fixture Feed Cake (Ib/hr/m)
DiRested Mix 1.8 29 360
''O Prim-30 3.A.S.
Digested Mix 2.1 21 320
30 Prim-70 R.A.S.
.3. Polymer
($/ton D.S.)
12.60
21.40
^Solids
Recovery
96
88
While the above results are impressive and may well be acceptable for the
system, it is also apparent that the dewatering devices' performance is penalized
by attempting to dewater an unthickened sludge. It is strongly suspected that
if the LAOKA sludges were thickened a much higher capacity and cake solids
would be realized, in addition to being operable at a much lower polymer dosage.
Kagnum Press Performance - Other Locations
A bench scale Magnum Press has been evaluated at various other locations:
TABLE XXII
PERFORMANCE OF MAGNUM PRESS - VARIOUS SLUDGES (l)
Location
Blue Lake,
St. Paul, Finn
Lake Charles,
La.
Richardson ,
Texas
Industry
Sludge
Mixture
45-PrimS2)
.55-s.A.s.
Prim +
2.A.S.
Digested
Prim.+E.A.S.
+ alum
S.A.S.
Dry Solids(f) Capacity-D.S.
Feed
5.3
2.9
4.1
3.5
Cake
35
29-34
26-2?
22-23^
(lb/hr/m;
1260
580
615
500
Flocculant fSolids
(p/ton D.S.) Recovery
14 98
12 95
11<3> 95
17(4) 95
-------
(l) All results from 0.25 meter bench scale presst
(2) Concentrations by volume
(3) Costs using ?5 Ib/ton Fed- plus 5 Ib/ton polymer: Straight Polymer=l6$/ton.
(4) Values shown are for 100$ polymer usagei use of 30-55 Ibs/ton FeCl_ will
increase cake solids to net of 27% at slightly lower capacity.
It is significant to note that the Magnum Press will function with inorganic
conditioning agents to extend the flexibility of the unit and to reduce polymer
costs.
CARBORUNDUM SLUDGE BELT FILTER PRESS
Carborundums' Pollution Control Division at Knoxville, Tennessee manu-
factures and sells a unit called the Sludge Belt Filter Press. This unit is
based on the design of Rittershaus and Blecher of Germany who developed the
"Dreibandpresse".
The Carborundum unit incorporates two unique features: stainless steel
wire supported belts and oscillating pressure rollers.
As can be seen in the following diagram, the gravity drainage section of
the SBFP includes two phases involving a dumping of the partially drained
sludge from the initial belt onto a second drainage belt prior to the incidence
of the upper sandwiching belt. The two belt cake sandwich then proceeds
around a large diameter roll into a further pressurizing section involving
smaller diameter offset pressure rollers in a two level configuration. Thus,
in effect, the Carborundum SBFP has a two stage gravity drainage section plus
two additional pressureshear stages to successively expose the cake to increas-
ing degrees of shear and pressure.
Carborundum is also bringing out a newer model with a "Pre-Concentrator"
stage in the same vein as the Unimat and R. B. Carter Series 31/32 devices.
-------
Figure XXI - Carborundum Sludge Belt Filter Press
The current Carborundum SBFP is available in 2 models with the following
dimensions:
TABLE XXIII
CARBORUNDUM SBFP
Belt Approximate Overall
Model
135
215
ifidth (in,)
Length
39 160
70 160
(inches)
Height
96
96
Dimensions
Width
69
100
This unit was introduced into the U. S. in 1977 so no U. S. commercial
scale operating data is yet available. A pilot unit is available for testing
and the supplier quotes the following results:
-------
TABLE XXIV
3ARBO.RUKDUN SBFP RESULTS
71
Type Sludge
Capacity Feed Solids Cake Solids Polymer
(gal/hr) % $ ^ cost
J/ton D.S.
Primary + 3.A.S.
Anaerobically Digested
900
1300
4-6
4-9
34-37
26-40
9
10
Primary + 3.A.S.
t? A O
_* 9 n i ij »
1100
16-20
11
Additional field U. S. results are now available from Carborundum and
German full scale installations have been in operation for several years.
R. B. CARTER SERIES 30 PRESSES
T. 3. barter of Hackensack, New Jersey is the U. S. licensee of Klein of
Germany, the developers of 3 successive generations of continuous belt filter
presses, each of increasing capability in either capacity or cake solids
content realized.
The original single level Klein device which was introduced in Germany
in about 1969, the Carter Series 30 (a two level unit), and the latest multi-
stage unit, the Garter Series 31/32 CBFP (based on the Klein "S" press) were
described in a preceding section dealing with the evolution of the CBFP. The
early single level device has been superseded by the two level Series 30 and
the nulti-staged Series 31/32.
P. B. Carter Series 30 Installations. Dimensions and Results
As of July, 1976 there were 21 U. S. installations of the Garter Series
30 CBFP that were either operating or were on order. The 21 installations
involved 36 units. Of these installations, 8 were for industrial sludges
and 13 municipal.
The series 30 units are available in 3 sizes as shown:
-------
TABLE XXV
CARTER SERIES 30 - OVERALL DIMENSIONS
i'iodel
5/30
10/30
15/30
Width
(inches)
53
73
93
Weight
(Ibs)
2500
3500
^500
The Series 30 is typically about 12 feet long and five feet tall.
Quoted typical results for the Carter Series 30 model are as follows:
TABLE XXVI
PERFORI-'ANCE DATA - GARTER SERIES 30 CBFP
Type Sludge Solids Content(£) Capacity Polymer
Feed Cake Ibs/hr/sq.ft $/ton D.S.
Primary + 3.A.S. 4-5 20-30 6.5-12 4-8
^naerobically 6-8 20-30 10-20 4-8
Digested Primary
-I- 5? A CJ
1 J • ri • vJ •
Tbctended Aeration 2^- 16-24 6-10 2-6
(No Primary Treat)
A mobile pilot unit of the Series 30 has been used in on-site test work.
Performance of a CBFF of the Garter Series 30 Type in the U.K.
In addition to the quoted typical results above additional insights into
the capabilities of the Carter Series 30 units can be gained by study of
references 14 and 15. The latter reference is an exhaustive study by the U.K.
Department of the Environment on an installation of the British version of
the first generation Carter type press. This study was carried out over
many months by the D.O.S., an agency of the government, at Lenham Works in
Bast Kent.
-------
Different mixtures of sludges were processed to determine applicability
of the single level first generation 3BFP, including operability, maintain-
ability, and all cost factors as well as dewatering capacity.
Typical results are shown in the following table.
TABLE XXVII
SINGLE LEVEL PR3SS - R. B. CARTER TYPE
LSMHAK WORKS - EAST KENT U.K.
Type
Sludge
Primary + Humus
+ E.A.S.
Straight Humus
Dry Solids (f )
Feed Cake
4.5 22
4.5 18
Capacity
(Ibs D.S
72
49
(l) Polymer
./hr) ($/ton)
5.64
8.00
# Solids
Capture
96-99
96
(1) 0.5 P'eter belt width x 3.0 meter length - Wm. Jones, Chem. Eng. Ltd.
As will be noted the normal mixed sludge is not a difficult one and results
were essentially equivalent to dewatering with an RVF. Hoever, it is doubtful
that an RVF would have achieved results on straight secondary sludge similar to
those shown.
The Lenham plant is a small plant designed to treat a dry weather flow
of 0.11 FIGD and actually processing about one half of design flow. The plant
includes primary, trickling filter and activated sludge operation. Though
the normal sludge mixture is a relatively easy to process material, the per-
formance of the first generation C3FP was viewed as highly successful.
The cost analysis showed a total operating and capital cost of $65-50
per ton of dry solids dewatered. ?'aintenance costs were low*
-------
TABLE XXVIII
LENHAM WORKS - COST ANALYSIS
FIRST GENERATION CBFP
Item $Aon D.S.
Polymer ^.90
Wash Water 1.9^
Power 0.66
Oper. Labor 12.00
(inc. super.)
Total Operating 19.50
Capital Costs ^6.00
Total (Ex. Kaint.T1' 65-50
(l) Maintenance Estimate + 3A hour/1000 Hours Operation
Performance of an 3. B. barter Series 30 CBFP - Hutchinson. Minn.
At Hutchinson, Ilinnesota a Series 30 Carter CBFP has been operating for
many months on a municipal sludge from an activated sludge plant. A photo
of the unit appears below:
-------
Figure XXII - Carter Series 30
]3FP
At Hutchinson, the vraste activated sludge is fed to the CBFP at a solids
concentration of 1-1.5:" resulting in a cake solids content of 13-15" and dr.,
solids through-put of j^-0 pounds per hour. While this performance is satis-
factory it could be greatly improved by pre-thickening to a solids content
-K>re logical for maximum dewatering capability.
R.3. :ART'.
The basic design characteristics of this unit have been delineated in
earlier sections. Essentially it consists of an initial "Reactor Conditioner
system which performs the dual function of conditioning and pre-thickening
followed by two successive pressure zones and a shear zone under pressure.
The Series 31 device also cones in 3 sizes, 5/31, 10/31, and lr/31 which
differ in widths. The largest unit, the 15/31 is desj -ned for a nominal feed
of 85 GP!' of typically a 5"" mixed sludge. "ouplete systems, including the
chemical feed system, pumps, controls and erection costs are usually priced
at slightly less than 32,000/GPr: or 1?0,000 for an 85 GP!: Series 15/31 unit.
Solids capture in the Series 31 normally averages 95r' plus. Connected eleet^-
cal power, including sludge pumps and conditioner system pumps totals not
more than 15 horsepower.
-------
Sizing of a building or space for a two unit Series 31 system, including
polymer preparation system, and conveyor sludge removal system indicates a
floor space requirement of about 36 feet by 18 feet. Height requirement is
13 feet 6 inches minimum.
While there are quite a few operating installations of the Series 31
type unit (Kleins or Win. Jones "S" press) around the world, U. S. commercial
units were just coming on stream during 1977.
Performance of R. B. barter Series 31 3BFP at Hamilton. Ontario
The Garter Series 31 mobile pilot unit has been tested at several North
American locations including Hamilton, Ontario, among others.
On a digested mixed primary and 3.A.S. sludge at Hamilton, a Zl% dry
solids cake was obtained which compared very favorably with a 1& cake being
obtained at the same time on the existing Potary Vacuum Filters. Hamilton
was experiencing some problem with fines recirculation and accumulation within
the system at the time and no doubt even more favorable results would be
realized in a situation with normal sludge conditions.
Performance of 3. 3. Carter Series 31 3BFP at Parkersburg. :J. Va.
At the Borg Warner Company, two 15/31 "arter units are dewatering a
pure exn-ss biological sludge. Feed Solids are 0.5 to 2.051 with a cake
solids content of 25-33"". Capacity averages 1500 pounds of dry solids per
hour per machine.
Performance of rt. 3. :arter Series 31 SEFP at Scituate. lass.
A barter Series 31 unit equipped with a Reactor-Thickener was evaluated
on the difficult aerobically digested extended aeration sludge at the Scituate,
Massachusetts plant. Results are shown in the following table:
-------
77
TABLE XXIX
CARTER C3FP - MODEL 5/31
AEROBICALLY DIGESTED EXTENDED AERATION SLUDGE
SGITUATE, 1'ASSACHUSSTTS
f Dry Solids Sludge Feed Solids Capture Polymer Cost
Test Feed Cake (lbs/D.S./hr) t (VtonTJ.S.)
1
2
2
3
18
16
88
255
91
98
26 (1)
11 (2)
(l) Cationic Polymer A Used.
(2) " " B "
In a cost comparison, the engineers involved estimated that at a pro-
duction level of 3 dry tons per day for a five day week either 2 Carter Series
31 CBFP's (60 inches wide) with Reactor-Thickener first stages: or two 250
square foot DA? units plus two 200 square foot RVF's would be required. Equip-
ment costs for the CBFP option were estimated at $222,000 and for the second
option at 5425,000. Horsepower requirements were estimated at 26 HP and 200
HP respectively for the two systems.
DESIGN EXAMPLE - CONTINUOUS BELT FILTER PRESS
Basic Assumptions;
These assumptions are identical to those used in the example for design
of a P.otary Vacuum Filter System:
1. Anaerobically digested mixture of primary and 2.A.S. at k?
solids content. CW T>rimary and UO" S.A.3.
2. Ultimate disposal by hauling to either a sanitary landfill,
or to farmland, composting or other horticultural use.
3. '"Jquilibrium sludge removal rate of 2.5 tons of dry solids
per day required.
-------
Alternate Units for Consideration or Evaluation
Any of the twenty or so varieties of Continuous BFP's available fro"
11 different companies. Depending on the length of the truck haul and the
cake dryness requirements for final disposal the design engineer would pre-
screen the many alternates and select perhaps 3 companies to work with in
proving in specific devices and carrying out bench and pilot scale qualifi-
cation trials.
For the purposes of this example it will be assumed that a dry solids
content cake of at least 29"' is required. Accordingly, units such as the
R. B. Carter Series 31i Komline Sanderson Unimat, Farkson Kagnum Press,
Ashbrook-Simon Hartley Winklepress, and Carborundum Sludge Belt Filter Press
would certainly be considered. Certain models of the Tait Andritz, Infilco
Degremont Floe-Press and Passavant Vac-U-Press would require at least pre-
liminary consideration with further study dependent on estimates of capa-
bilities from the supplier firms.
Evaluation Procedure
The systematic procedure for evaluation would be identical to that
described in the RVF design example.
Bench Scale Tests
Most of the equipment suppliers have laboratory or bench scale test
equipment and procedures which indicate general acceptability of their units.
In most cases, unless the sludge to be dewatered is an unusually easy one,
pilot scale testing will yield much more accurate design criteria and should
be pursued. Most companies have mobile pilot or full size units.
Design Calculations
1. Operating cycle to be 35 hours per week (? hours/day), per-
mitting start-up and wash down times within 8 hour shift.
2. One CIFP with adequate spare parts to be maintained.
3. Size of CBFT
Production rate proves to be 50 GPii of 3-^f' feed sludge Divine,
rate of 750-1000 pounds of dry solids per hour per neter width
(from pilot test runs). Solids capture is an acceptable 93-98"
in all tests. Sake solids with complete press (all sections,
-------
including high pressure stage) in use is 38^. Without high
pressure section, cake solids are 30?' • Polymer dosage is
consistent • Design Engineer must then asses added capital and
0/M costs for high pressure section and effect of QA drier cake
on haulage costs to determine which unit is to be chosen. A
single GBFP of two meter width would be adequate if several
days sludge storage surge capacity was provided. Alternatively
2 one meter wide units could be chosen.
k. Sizing of Auxiliary Equipment
Same as described in EW design example. If, for example, a
Komline Sanderson Unimat were the selected unit, the basic
machine is just under 2^ feet long, width requirement is 5
feet 2 inches at base rith the upper drive motor making upper
width need just under 8 feet. Height of the Unimat is 10 feet'
2 inches.
The same considerations apply to selection of a suitable floccu-
lant system, sizing of conditioning system and overall "Dewater-
ing System Considerations" as noted in the RVF design example.
DESIGN 5XAHPLS - CONTINUOUS BELT FILTER PRESS - kO IIGD PLANT
Basic Assumptions
1. Anaerobically digested mixture of primary and S.A.S. at ty£
dry solids content. 60?' primary and b
-------
80
produced in pilot tests would be essential for evaluating
efficacy of incineration and to ensure whether or not auto-
genous incineration would "be achieved in burning periods (There
is no such think as totally autogenous incineration since start-
up and shut down procedures require fuel usage regardless of
cake characteristics). Nonetheless, self sustaining combustion
would at least minimize fuel consumption.
2. Review of the suitability for composting could be carried out
with experts in that field.
Bench Scale L Pilot Tests
Same as in k MGD example.
Design Calculations
1. Pilot results show that 50 GPH of 3-bf> sludge will yield a-cake
solids of 38T- at a production rate of 750-1000 pounds/hour/meter
width, with adequate 93~98T solids capture and usage of polymer
at ^10 per ton of dry solids.
2. Operating Cycle; to be based on 3 shifts/day, 7 days per week
and 22 hours/day unit operating time since incineration requires
continuous operation to minimize fuel consumption.
3. Sizinr: of CBET
25 tons per day = 50.000 pounds/day.
Meter Daily Production/Unit
Width (pounds)
1 16,500
2 33•000
3 49,500
On the above basis 4 one meter units or 2 two meter units would
be chosen.
Surynation
All other facets of the design procedure would be similar to
the 4 ;1.GD RVF design example.
-------
81
PRSSSURS FILTERS
'The original main focal point for the development of the plate and frame,
and recessed chamber types of pressure fillers was Stoke-on-Trent, United
Kingdom, The slurries incident to the manufacture of pottery and china, are
particularly difficult to dewater and as a result pressure filters were employed.
These types of pressure filters, particularly the recessed chamber type
have "been frequently designed into U.K. wastewater treatment plant sludge
dewatering systems.
A few U. S. installations of pressure filters have also been made in the
past few years.
Pressure filters are batch devices and to some extent because of the
level of development of feed and chemical dosage systems normally use sub-
stantial quantites of metal salt and lime for conditioning. These chemicals
require relatively extensive handling systems requiring considerable mainten-
ance. This is one of the factors which has slowed acceptance of pressure
filters outside the United Kingdom.
Essentially, a pressure filter consists of a series of vertical plates,
usually recessed, covered with cloths to support and contain the cake, mounted
in a framework consisting of head supports connected by two heavy horizontal
and parallel bars or an overhead rail. The diagram below shows a cross section
of a pressure filter:
FILTER CLOTHS
r\
SLUDGE IN
FIL1RATE DRAIN HOLES
Figure XXIII - Section of a Pressure- Filter
-------
:2
Conditioned slud^s is pur.ped into the pressure filter at increasing pressures•
Presses are normally supplied to operate at either a nominal 100 PSIG or 225 PSIG.
Cake building time or sludge feed time is normally 20 to 30 minutes followed by
a 1 to 4> hour pressing period. The press is then opened and the filter cake falls
off into the removal system.
While pressure filters will generally produce a cake solids content
10-20T points drier than a rotary vacuum filter, SOJIG portion of these total
cake solids are liir.e and metal salt rather than sewage solids. Capacities of
pressure filters are usually about 10 to 20;1 of the loadings achieved on rotary
vacuum filters.
Significant developments in Pressure Filter technology are the diaphragm
press and other menbrane type presses which are discussed later.
Since an excellent survey of three operating U. S. installations was
available, a review of those case histories is the most applicable way to
present a perspective on conventional recessed chamber type presses.
CAS3 HISTORY - CflPSHA. WISCONSIN
This is a 26 -:GD plant with a primary and activated sludge system.
1. The sludges axe mixed, gravity thickened, anaerobically
digested, and then dewatered in Nichols (Edwards f Jones)
pressure filters. The dewatered cake is given to farirers who
land spread from nanure spreaders.
2. Iheinical dosage is J' ferric chloride and 2J lime (both on a
dry solids sludge basis).
3. Digested sludge at 3-?' solids is dosed in line with Ferric
Chloride and line is added in a subsequent nix tan'c with slow
speed mixing.
k. Two . loyno runps feed the tuo presses simultaneously. Hie
ioynos have worked very well. Filtrate is returned to head
of plant.
5. ?ycle includes maintenance of 100 FSIG for 30 minutes and total
cycle tine is 2 1/3-2 1/2 hours. Operate 16 hours per day, 7
days per week to produce 12 tons per day of dry solids caite at
35-3ET' solids. 3ake thickness is one inch.
-------
6. Tiro Nichols-Edwards & Jones pressure filters, with 00 - ^ feet
by 4 feet plates (rubbercoated steel) usedi
7. One operator in continuous attendance.
Results:
Good handleable press cake and clear filtrate:
TABLE XXX
20STS - razssuns FILTRATION, KEKOSHA
Costs ^on
Labor i 7-^3
Chemicals 20.1?
?ouer "L. 71
i:aintenance 3.25
032,56
Problems
HiSh cheniical dosage and costs have been experienced. Cake is actually
about 25" added chemical so analysis is really about 65?' water, 2&,' sewage
slufi^e and
-------
EROOKPI.a.D. WIS30II5III
This plant design includes a prinary and activated sludge system and
contact stabilization. Flow is 2 HGD. 80^ Primary Sludge + 20T Secondary
Sludge is mixed, pumped through a grinder, diluted with recycled incinerator
ash (0.5 Ib/lb sludge), conditioned with lime {15-18T) and ferric Chloride
(5-7^)1 pressed and fed to a 5 hearth incinerator. 95f of incinerator ash is
recycled. The incineration is not autothermic and uses natural gas. Pressure
filters are standard Passavant design with forty-six 52" diameter plates of
steel and have been operated for if years.
Results
Plant personnel state that no major operating problems hav been encount-
ered. There have only been two "Sludge Blowing Incidents" in the l£ years of
operation. Press cloths have had to be replaced every 6 months at a cost of
$31600 per shot. The press cake, which contains a large amount of inorganic
conditioning agents and recycled ash averages 45£ total solids. The press
cake is only 30-^Of volatile so the ratio of water/sewage solids is quite high.
Comments
1. The mixed sludge being processed is a relatively easily
dewaterable material which is high (80£) in primary content
and high in fibrous material. Indeed the high fiber content
has caused problems in the press cake breaking operation.
2. No records are available on natural gas consumption and no
cost data on the systen has been made available.
3. The system appears to be a complex high capital and high
operating and maintenance cost one which is difficult to
rationalize, particularly at a plant with such an easily
processable sludge.
b. The plant has two components of interest to other potential
press filter designs: the wet sludge grinder and the slow
speed cake breaker.
Conclusions on U. 3. Results to Date
Reference 16, from which the above results cai.ie, is an excellent review
of the current U. 3. installations.
-------
The conclusions froir. reference 9 axe as follows:
1. In looking at the two types of presses, we found some advantages
with the lower pressure design. Essentiallyt it is a much
simpler operation. The recycling of incinerator ash seem to
provide few benefits, particularly because it only complicated
the operation with additional material handling equipment.
2. Tn general, ire found that filter presses are an acceptable nethod
for deuatering sludge. Theoretically, they should always produce
an autocoiabustible sludge cake. But, practically, we know of no
installation anywhere that can achieve this. The ash recircu-
lation is probably the limiting factor. (The inorganic condition-
ing agents also contribute to the problem).
3. Filter presses seem to be quite capable of handling different
sludge concentrations and different types of sludge feed.
Proper conditioning, especially with lime, is the key to good
operation. Vacuum filters are not quite so adaptable.
M-. The necessity of using high lime for conditioning could be a
drawback. Lime handling is always difficult.
5. Prior to a large scale installation, pilot plant work should
always be performed to evaluate the dewatering characteristics
and chemical requirements.
6. Filter presses have a higher capital cost than vacuum filters.
The presses also usually have a higher operational cost. Their
real advantage is in greatly reducing the costs of final disposal
for the sludge cakes. A detailed economic analysis of the total
system is needed before deviding for or against filter presses.
POLYSL'JCi7>OLYT:J SOITDITIOHIHG FOR PRESSURE FILTZ3S
Due to the more prevalent previous incidence of the use of filter presses
in continental ^rope and the United Kingdom, and also due to innovative work
there, the successful use of certain polyelectrolytes in conditioning aluijes
for dei.-atering in pressure filters has been realized at a number of locations.
-------
86
Farnham Pollution Control '.'forks,
Thames Water Authority. U. K.
This plant is a primary and trickling filter installation. Humus sludge
is recirculated to the primaries, the mixed sludge gravity thickened, and then
dewatercd on two filter presses. Operating pressures are 85-100 PSIG (586-690
kPa).
Initially the plant used aluminum chlorohydrate for sludge conditioning.
A flow diagram of the dewatering system follows:
IBMUMTAMT
ALUMINIUM CMlOOOMVOlUn
IATCN GOIOTIONIN6.
U IN-LMI
CONMTlOMIIG
Figure XXIV - Farnham Plant Dewatering System
The Farnham plant experienced severe filter cloth blinding problems
and proceeded to carry out diagnostic trails with various conditioning agents
to rectify the problem. They found that by converting the system to use
Allied ^ollioc1? Zetag 63 polyelectrolyte the cloth blinding problems was
alleviated sufficiently for the two presses to cope with the sludge load.
-------
TA2LE XXXI
FAHNHAv D3WAT3RIKG RESULTS
Conditioning
Agent
Aluminum
Chlorohydrate
(batch)
Aluminum
Ch 1 or o hydrate
(in-line)
Zetag 63
(batch)
Zetag 63
(in-line)
Ferric Chloride
" Lime (batch)
Ferric Chloride
". Lime (in-line)
CAS3
Dose -Cost
(r' on ds) 0/ton ds)
2.5
2.5
0.2-0.3
0.2-0.3
3
25
3
25
HISTORY -
22.00
22.00
6.70-
10.10
6.70-
10.10
14.80
14.80
THoanajRY STP,
CST 3ange Pressing Cycle
during cycle Time Range
(seconds) (hours)
10-65
Results not
available
10-32
8-14
8-45
8-15
U.K.
6-13
6-12
6-9
3-6
3-13
3-5
Reference 17 describes exhaustive test work on the use of polymers for
conditioning sludge for dewatering via recessed chamber pressure filters.
By virtue of using in-line conditioning and observing logical procedures
the results shown in the following table were achieved:
TA3L3 XXXII
, u. K. - KtEssuas FILTRATION
Conditioner
"* Dry Solids
Feed Cake
Conditioner ^ost Press
(3/ton D.S.) Cycle (hrs.)
Aluminum Chlorohydrate
Polyelectrolyte
(Zetag 94)
4.6 38 23.40
4.6 37 4.60
(Primary + Secondary Sludge)
4.9
4.9
-------
The Thornbury works processes a mixture of bff, primary sludge and 55f-
of mixed sludges from adjacent secondary treatment plants. In addition to
illustrating successful use of polyelectrolytes , the article delineates other
significant facts relative to pressure filter design.
KEI'BRANE USE - PRESSURE FILTERS
References IK and 19 describe the successful upgrading of the production
rate in convention?,! recessed chanber pressure filters by equipping same with
alternate "membrane" plates. This retro-fitting process causes each of the
chambers formed between the standard recessed plate and the membrane plate
to be subject to the squeezing action of a membrane at will during the press
cycle. The nerabrane plate is a steel reinforced rubber plate in which the
rubber membrane is inflatable by air pressure. After the initial filling
period in a press cycle, when the filtrate rate falls off, the sludge feed
pump is stopped and the membrane inflated to give a pressure up to 150 PSI
to squeeze the partially formed cake and obtain quick dewatering.
As can be seen in Table XXXIII following, though a thinner cake results,
the overall filtration cycle is so much shorter that the total throughput
doubles or even triples in some cases.
TA3LE XXXIII
10MVEITIONAL VS. f2!.
1 P
SEVERS! TR3NT if ATS? AUTHORITY
Type Press Sycle Sake Thick. D. Solids Weight Output
Used (iinutes) (inches) (") (ibs) Ib/hr/press
Conventional 390 1.25 28 122? 186
Membrane B? 0.? 2? 558 385
(:Taw 7eed Sludge - 3.9" D. Solids - 2.0"' Alum Zhlorohydrate Cond.)
The suppliers of the rubber menbrane plates claim that new installations
of the menbrane type unit are less expensive overall due to the increased
capacity of the nombrane units.
A somewhat analogous but different type of variable volume pressure filter
is described in the following- section.
-------
DIAFHRAGi: TYPE PRESSURE FILTERS
As described in Reference 20, a new type of pressure filter, employing
flexible rubber diaphragms between the chambers of a pressure filter, has
recently been introduced into the U. S. This type device was developed in
Japan and there are several operating installations there.
At least two versions of this new type of pressure filter have been
tested and are available in the U. S. The earliest one was supplied by
IJGX Insulators Ltd., of Nagoya who have now licensed Envirex division of
"exnord for U. S. sale of their device. Ingersoll rfand has the U. S. rights
to the Lasta automatic diaphragm pressure filter. There are indications
that 'Dart Industries and Industrial Filters 0!D of Chicago have devices
based on similar principles.
A diagram of the I• Tl. Lasta press below illustrates the operating
principles:
figure XXV - I. R. Lasta Diaphragm Pressure Filter
As will be noted in the diagram the feed slurry enters the top of the
chamber between the filter clothes and gradually fills the chamber. After
a cake is for-.ed the diaphragm is expanded by water under pressure to 250
P3IG which squee.es and dewaters the cake. The filter plates are then auto-
matically opened and the cake discharged. 31oth washing ensues before anothei
pressing cycle.
-------
90
It is claimed that the length of the cycle is shorter than for conventional
presses because of the improved control of the relationship between cake forma-
tion and pressure build-up.
The table below lists dimensional -data on the I. R. Lasta press.
TABLE XXXIV
I. 3. LASTA AUTOKATI2 FILTERING FR3SS
Size of
Filtering
Plate
600 mm
(2t")
900 r.m
(32")
1000 mm
(to11)
1250 mm
(50")
1500 mm
(60")
No. of Filtering Area Height
Filtering ^ 2
Chambers ! ft mm ft.
5
It
20
It
20
26
20
26
32
26
32
38
32
35
t
7
10
13
19
2t
30
39
t8
62
77
91
112
133
15t
t3 20^0 7
75
108
ItO 2t85 8
20t
253
323 23t5 9
t20
516
66r- 3200 10
629
979
1205 3620 12
It31
165?
Length
nun ft.
2660
3650
t6tO
39tO
t930
5920
52tO
6230
7220
6555
?5t5
8535
8205
9225
102t5
9
12
15
13
16
19
17
20
2t
22
'^
28
27
30
*
Width
mm ft.
1610 5
1800 8
2100 7
2600 9
3050 10
The most detailed report on these devices is Reference 20 which describes
the extensive pilot work done at Blue Plains with the dnvirex-i!GK Locke
diaphragm press. This Dnvirex unit is highly automated and in work at Blue
Plains (idxture of primary and 2.A.S. sludges), it produced a tO'' total dry
solids cake using 20T lime and 10" ferric chloride dosaces. The only problem
is that when the correction is made for the inorganic conditioning solids
present in the deiiatered cake, the percentage of dry sewage solids in the cake
relative to water content is only about
-------
91
This new type pressure filter does offer much improved capabilities over
conventional pressure filters for extremely difficult to dewater sludges.
Pricing figures available indicate that the units will be priced about eight
times the price of a conventional pressure filter, so the need must te clear
and obvious.
JISETRIFUGES FOR DEHAT3RIHG
Horizontal solid botd decanter type centrifuges have been used for waste-
water sludge dewatering for a number of years. They were popular for primary
sludges with low grit content in coastal resort areas with large swings in
loadings because of ease of operation, quick start-up and shut-down and ease
of odor control. Attempts to adapt these relatively high speed devices ("-J,"
forces of 1000+) to heavy duty operation in large cities or for use with mixed
sludges containing significant quantities of biomass were previously plagued
by two problems:
1. IDrosion of the surfaces exposed to high speed impingement
of abrasive materials caused maintenance problems.
2. Prior to the development of polyelectrolytes capable of
providing a reasonable clean centrate and avoiding serious
fines recirculation problems, solids capture was inadequate.
In the past five years or so six steps were taken which have helped this
type device gain a wider use:
1. Development and use of new high molecular weight cationic shear
resistant polyelectrolytes.
2. Use of lower rotational speeds to reduce turbulence, rower
costs, and erosion wear problems.
3. 'Jse of a concurrent floi-: pattern for sludge and centrate to
ninir.ize turbulence.
k. Adjustable variation of speed differential between the bowl
and the sludge removal scroll.
5. Use of longer bowls with smaller diameters.
6. Trovision of extremely large units at plants with large sludge
removal needs producing an economy of scale.
Various manufacturers have combined some of the above features in their
newer models. This resulted in a surge of popularity about four years ago.
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92
Since the energy crisis the degree of popularity of centrifuges, even with
the above mentioned improvements, has slackened because of energy costs.
Once again, the pioneering development work on these devices was carried
out rarimarily in 'Jest Germany. The most practical description of these
developments is contained in References 21 and 22 which are excerpted in the
following section.
CAS3 HISTORY - rBHTRIFUGAL DSWATSRING - VUFPERTAL-aUCHSHHOFEN. GERMANY
Deference 21 is a comprehensive article relating results obtained at
'.•Juppertal-Buchenhofen plant with a low speed con-current flow type unit.
This is a combined municipal-industrial treatment plant treating 1,200,000
population equivalent. After primary and biological treatment the mixed
sludges are thickened to 3-*f and anaerobically digested, followed by sludge
settlement and decantation, thence dewatering.
After initial trial work the authority asked for competitive tenders
from various suppliers of centrifuges with performance requirements as follows:
<5
1. Capacity of each centrifuge: ^0-60 nT/hour of sludge with
feed of 2.5-3^ dry solids.
2. I-'inimum cake solids: 20?.'.
3. Centrate maximum suspended solids of 0.2f.
I*. Maximum polyelectrolyte dosage permissible of 3-3 kg/ton of
*i
dry solids (100 gm/nr).
$. "axirnum permissible power consumption of 1 KVH per cubic meter
of sludge feed including ancillary equipment such as pumps,
flocculant metering stations, etc.
6. Guaranteed life of screw conveyor = 10,000 hours.
?. Provision of a package plant with a minimum capacity of
UO nrVh f°r a ^ nonth trial period under a leasing agreement.
KHD Industrieanlagen AG Humboldt-Wedag of Cologne (U. S. Licensee -
Bird Machine) won the contract and initially installed two S3-2 type low
speed concurrent flow centrifuges with capacities of 20-30 n /h each. These
units met the agreed performance guarantees but when the full civil installa-
tion was completed they were replaced, as planned, by two of the larger S^-l
units (of the same basic type) but with capacities of lfrO-60 m /h each.
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93
Power consumption for the complete dewatering plant was 0.9-0.95 KWH/ra
with S3-2 units and improved to 0.75-0.8 with the larger S&-1 units. Disage
*3
of Zetag 92 polymer (Allied Colloids) averaged 60-80 gm/m .
The article contains much data on the effect of centrifuge dewatering
variations on overall process performance and sludge disposal costs.
A significant factor studied was that of the effect of the differential
in speed between the scroll and the bowl.
TABLE XXXV
EP7E7T OF SPEED DIFFERENTIAL OK THROUGHPUT AND DRY SOLIDS
Speed Differential
o
Flocculant Eosage (g/n )
Dry Solids carried by
discharge (<0
Dry Solids carried by
centrate (undissolved
solids)
Ideal throughput (m /h)
60
26
0.35
33
2 46
80 60 80 60 80
28.5 24 23 20.5 20
0.25 0.17 0.07 0.12 0.07
37 43 45 40 48
As can be seen, a 28,5^ dewatered cake at a reasonable throughput of
37 ri-yhour and centrate suspended solids of 0.25f can be obtained with floccu-
lant dosage of 80 g/m by using a speed differential of 2 instead of 6.
The paper claims and purports to show that very large capacity centri-
fuges of the improved low speed-concurrent flow type, when operated ir a
lower differential speed mode can offer significant capital and 0/K cost
savings where large volumes of sludges are to be processed.
Unit costs are given as follows:
Operating - DK 36.^0/ton dry solids
Annual Capital - Dll ^7.60/ton dry solids
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HISTORY - asmiyuGAL pgfATsainc - STOCKHOLM. s.fEns:?
Stockholm has operated three high speed centrifuges for a three year
period and also have operated a new low speed concurrent flow unit on the
same sludge for one and one-half years..
Table XXXVI below shows the results obtained with the two different
types of centrifuge:
TABL'J XXXVI
SID:: BY SID*] SOIIPARISDK - PROCESS RESULTS
Centrifuge Design Low Speed High Speed
Sludge Identification Anaerobically Digested Primary Plus
'./aste Activated with \lurn Sludge
No. of Operation Units one (l) three (3)
71ow Rate Per Unit 1?0 GPi: 90 OKI
-- 7eed Consistency J" 3"
^ Hake Solids 16-18" 16-liF
- Solids Recovery 95-98" 95-9^
Polyner Type Allied Colloids Percol #723
^ationic
Polyrer Dosage 6 Ibs/ton 12 Ibs/ton
Jhlle the a'oove table only shows the Improvement realized by reduction
in polyelectrolyte costs by about :9/ton (which is a considerable savings),
the followins Table illustrates the additional advantages for the low speed
design.
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TABLE XXXVII
SIDE BY SID3 COMPARISON MACHINE PARA13TERS
I
Centrifuge Design
Bowl Diameter
Bowl Length
Centrifugal ?orce
Unit Flow 3 ate
Unit Pool Volume
Si^ma 'factor
Unit I.otor Size Rating
Absorbed Horsepower
?:oise Level 3 J ft,
'.'ear 1 ?000-Hour
Inspection
Low Speed
36"
96"
511 x G
190 GPH
196 Gallons
1.15 x 107 en2
100 HP
.3 HP/G?1'
80-85 <*3A
1/2 mm
High Speed
25"
90"
1878 x G
90 GPI,
73 Gallons
5.3 x 107 cm2
180 HP
.6 HP/GPI:
95-100 d3A
9 mm
'..'ear played an important part in displacing the high speed centrifuges
in favor of the low speed centrifuges at this particular plant. The low
speed centrifuge was inspected after 2000 hours of operation and found to
have only 1/13 of the wear of the high speed alternative. The abrasive
protection on the low speed machine conveyor blades is tungsten carbinde,
while the protection on the high speed n-achine is equivalent to an alloy
called Stellite 1016. The Stellite material is considered inferior to the
tunrsten carbinde hardneww values approach Re-69. Experience shows that if
both naterials had been sinilar that the uear rate would still have favored
the low speed design by as nuch as ?. five to one ratio.
3unnurized in Table XXXVII! is the annual cost analysis of the operation
of those two types of centrifuges installed side by side. The low speed
unit clearly has the elpe in all categories. Power consumptions are one-half
'1/2} that of L>.e hi^i speed u.iit. "ith respect to polymer consumption, the
low speerl centrifuge in this particular case utilized W*~' less cationic polymer
than the hifh s^eed centrifuge, i.'ith respect to conveyor maintenance, we
have '-o-'ifie-i the hifh speed cnntriCuge fi^re to reflect a ratio of conveyor
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96
resurfacings more in the category of five to one than tlie 18 to one margin
indicated by the actual side by side installation. The category entitled
"Amortized Equipment" includes the cost of the centrifuge, the motor, and
the starter, and is expressed on a tonnage basis and reflects an amortization
rate of 7,' interest over a 20-year period. Slectrical usage rate was assumed
to be 0.02/r"rH and polymer (Allied Colloids Percol 728) was figured at
*1.50/lb.
TA3L3 XXXVIII
SIDE 3Y SIDE aOf-PASISO!? ANNUAL COST - PROFIL2
Centrifuge Design
Tons/Year Per Unit
Power Expenditure
Polymer Expenditure
J'aintenance Expenditure
Amortized Equipment
Total Annual 3ost
Low Speed
12,483
$0.06/ton
39.00/ton
L1.21/ton
"1.50/ton
'512.33/ton
High Speed
5.913
$1.19/ton
>l6.00/ton
$8.30/ton
>2.^/ton
527.93/ton
',.'hile the larger size of the lou speed unit would account for a minor
portion of the above noted superiority, it is abundantly clear that the lower
speed concurrent FLo:t unit is superior fron a cost-effectiveness standpoint.
Dimensional Data - Centrifuges
The following table shows dimensional data for one brand of the newer
lou speed centrifur^alsj
TAoLJJ XXXIX
DI!3!SIOirAL DATA - LOW SPS3D CENTRIFUGE
"odel Overall Overall Overall :/eif;ht
::o. Length (In.) 1,'idth (In.) Height (in.) (ibs)
>:r 2500 138 80 36 6500
W 3700 139 72 *H 9^00
m &oo 276 150 71 3^o
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9?
CAS? HISTORY - 33NTPDUCAL DS.'AT^RING - BURLINGTON, VISC.. TJTP
The experiences at Burlington are described (in an outstanding fashion)
in Reference 23.
The Burlington plant treats an average flow of combined municipal and
industrial wastes at D'.f? level of 1.5 iiGD and a wet weather flow of 2.0 HGD.
The treatment plant employs contact stabilization (12 hour aeration time,
25T return rate, iiLSS of 2000 mg/l). The F/II ratio is 0.2 to 0.5. A sludge
age varying from 5 to 12 days is employed, including aeration and aerobic
digestion time.
The above described liquid treatment system results in sludge disposal
requirements of 160,000 gallons of 2.A.3. per week or 3^00 pounds per day
(about 27,000 gallons/day).
The plant was designed for ultimate liquid sludge disposal by lagoon.
'/hen this disposal option was curtailed, deuatering studies ensued. Needless
to say, the sludge dewatering problems are significant. It is a classic
example of the problems which result when a plants liquid treatment system is
designed for liquid, sludge disposal and then dewatering is required.
A batch, cycling, basket centrifuge was tested, purchased, installed and
has been operated for some time. The essence of the results of the full scale
performance is listed in Table XXXX following:
TA3LE XXXX
BASKET -CENTRIFUGE OFJSATIOK - BURLINGTON, '.IIS3. , WWTP
7eed Rate (GPi:)
Dewater Hate (ibs D.S./hr)
Hours rJequired/VJeek
Labor + Trucking Cost ( Vwk)
Slectricity Cost ( ?/w&0
Chemical Cost 0/ton)
Cake Solids (")
Skimming Volume (r')
Total Costs
23
1(X*
160
373
1^7
0
6-8
(U.T.)
50
62
88
397
Vi
99
48
30
13-15
(T)
14
47
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98
As can be seen, despite the high polymer cost, the overall cost analysis
showed the total operation to be more cost effective with polymer use.
It should be noted that if the city could start again from square one, it
is certain that, now having to dewater sludge, and knowing the overall energy
costs of the type total system involved, a different liquid treatment system
would be chosen.
Additional valuable insights in the referenced paper relate to the
correlations between activated sludge system operating paremeters and resulting
sludge processability
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99
BIBLIOGRAPHY
1 - Process Design Manual Sludge Treatment and Disposal, Technology Transfer,
U. S. EPA, Wasnington, D. C, (197*0
2 - Gamp Dresser & McKee, Report No. PB-255-769, OTIS, Springfield, Va.
June, 1976
3 - Jones, Edgar R., P.E., "Sludge Production Rates, District of Columbia",
Ecoletter of Chesapeake WPCF and Water and Waste Operators of Md., Del.,
and B.C., Vol. 3, Number 2, p.1*., Spring 1977
^ - Gale, R. S., "Recent Research on Sludge Dewatering". Piltrv Separ.
(Sept.-Oct., 197l)i PP. 531-538
5 - Corrie, K. D., "Use of Activated Carbon in the Treatment of Heat-Treatment
Plant Liquor',1, Water Pollution Control (U.K.) 1972, p.629-635
6 - Stack, V. T. Jr., Marks, P. J., and Garvey, B. T., "Pressure Cooking of
Activated Sludge", paper by Roy F. Weston, Inc.
7 - Reports by Greeley & Hansen to the city of Tampa.
8 - Crockford, J. B,, Sr., and Sparham, V., "Developments to Upgrade Settle-
ment Tank Performance, Screening, and Sludge Dewatering Associated with
Industrial Water Treatment", Purdue Industrial Waste Conference, Hay 1975»
p. 1072-1083.
9 - Personal Communication, Dr. Dan Swett and Mr. Mike Riise of G. C. One Ltd.,
Suite 605, 2700 N.E..135th St., N. Miami, Fla., 33181
10 - Bell, J. A., Higgins, R., and Mason, Donald G., "Dewatering, a New
Method Bows, W. & W.E., April, 1977i p.33-^1
11 - Eichmann, Bruce, W., "Dewatering Machine Solves Sludge Drying Problems",
W. & S.W., October, 1977, pages 99-100
12 - Creek, John, "Tait Andritz SDM Sludge Dewatering Machine", WWEMA
Conference paper, April 20, 1977, Atlanta, Georgia
13 - Keener, Phillip M., and Metzger, Larry R., "Startup and Operating Experience
With a Twin Wire Moving-belt Press for Primary Sludge", Vol. 60, No. 9,
September, 1977, TAPPI, p. 120-123
1^ - Grove, G. W., Exxon Research & Engineering, "Use Gravity Uelt Filtration
for Sludge Disposal", Hudrocarbon Processing, May 1975
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1(
15 - Department of the Environment, U. K. , Directorate General Water
Engineering, R & D Division, Project Report No. ^, Sewage Sludge
Eewatering Icy Filter Belt Press
16 - Cassel, A. P,, "Review of 0. S. Filter Press Operations", paper
presented at Chesapeake Section, WPGF, June, 1976
1? - White, M, J. D.( and Baskerville, R. C,, "Full Scale Trials of
Polyelectrolytes for Conditioning of Sewage Sludges foT Filter
Pressing", Journal of Institute of Water Pollution Control, No. 5i
18 - Heaton, H. M. , "The Practical Application of the Membrane Filter
Plate to Increase Filter Press Productivity and Overall Economics",
Filtech 77, Sept. 20-22, 1977, Olympia, London
19 - White, M. J. D., Bruce, A. M. , and Baskervllle, R. C., "Mechanical
Dewatering of Municipal Sludge in the U. K. - Laboratory to Full
Scale", presented at conference! Theory, Practice, and Process
Principles for Physical Separations, California, 10/10 to 11 A/77
20 - Cassel, A. F., "Update on Filter Press Operations", paper presented
at Chesapeake Section, WPCF, June 1977
21 - Reimann, D., Kommunalwirtschaf t , Wo. 9, Sept., 19?** , pp 3^3-352
22 - Guidi, $. J,, "Why Low Speed Centrlfugation" , Presented at Ohio
WPCF, Columbus, June 16, 1976
23 - Pietila, K. A., and Zacharias, D, R., "Full Scale Study of Sludge
Processing and Land Disposal Utilizing Centrlfugation for Dewatering",
Paper presented at Central States WPCF, May, 1977
S GOVHmttHT PIHKTIHG OFFICE 1978-757-140/6491 Region No. S-(.
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