MUNICIPAL WASTEWATER SLUDGE
MANAGEMENT ALTERNATIVES
PREPARED FOR THE
ENVIRONMENTAL PROTECTION AGENCY
TECHNOLOGY TRANSFER
NATIONAL CONFERENCE ON
208 PLANNING AND IMPLEMENTATION
GORDON L. CULP
and
DANIEL J. HINRICHS
CULP / WESNER / CULP
Clean Water Consultants
El Dorado Hills, California
-------
MUNICIPAL WASTEWATER SLUDGE MANAGEMENT
ALTERNATIVES
BY
GORDON L. CULP
AND
DANIEL J. HINRICHS
CULP, WESNER, CULP
CLEAN WATER CONSULTANTS
EL DORADO HILLS, CALIFORNIA
PREPARED FOR THE
ENVIRONMENTAL PROTECTION AGENCY
TECHNOLOGY TRANSFER
NATIONAL CONFERENCE ON
208 PLANNING AND IMPLEMENTATION
-------
TABLE OF CONTENTS
Page
Table of Contents i
List of Tables iii
List of Figures v
I. INTRODUCTION 1-1
II. SIGNIFICANCE OF SLUDGE DISPOSAL ASPECT
OF WASTEWATER MANAGEMENT II-1
III. BASIC DISPOSAL ALTERNATIVES III-l
IV. SLUDGE CHARACTERISTICS IV-1
V. SLUDGE PROCESSING TECHNOLOGY V-l
1. SLUDGE CONDITIONING V-l
a. Chemical Conditioning V-l
b. Elutriation v-3
c. Heat Treatment V-4
d. Freezing V-6
e. Hydrolysis With Sulfur Dioxide V-8
f. Radiation Treatment V-8
2. SLUDGE THICKENING V-9
a. Gravity Thickening V-9
b. Flotation Thickening V-l3
c. Centrifugal Thickening V-16
3. SLUDGE DEWATERING V-16
a. Drying Beds V-16
b. Vacuum Filtration V-20
c. Centrifugation V-23
d. Pressure Filtration V-27
e. Drying Lagoons V-30
4. INCINERATION V-33
a. Multiple Hearth Incineration V-43
b. Fluidized Bed Incineration V-43
c. Wet Air Oxidation V-47
d. Lime Recalcining V-47
e. Pyrolysis V-48
i
-------
TABLE OF CONTENTS (CONT'D)
V. SLUDGE PROCESSING TECHNOLOGY (cont'd)
5. DRYING OF SLUDGE
a. Flash Drying
b. Solvent Extraction
6. DISPOSAL AND LAND APPLICATION
a. Sanitary Landfill
b. Cropland Application
7. STABILIZATION
a. Anaerobic Digestion
b. Aerobic Digestion
c. Composting
(1) Windrow Composting
(2) Static Pile Composting
(3) The Product
(4) The Economics
VI. SLUDGE TRANSPORT
1. TRUCK TRANSPORT
2. BARGE TRANSPORT
3. RAILROAD TRANSPORT
4. PIPELINE TRANSPORT
VII. ALTERNATIVE SYSTEMS
REFERENCES
Page
V- 54
V-54
V-58
V-60
V-62
V-63
V-69
V-70
V-73
V-75
V-77
V-78
V-80
V-80
VI-1
VI-2
VI-3
VI-6
VI-8
VI I-1
1-10
ii
-------
LIST OF TABLES
Page
1. Major Components of Sludge IV-2
2. Summary of Direct and Indirect Cost For Thermal
Treatment V-7
3. Typical Results - Gravity Thickening V-12
4. Gravity Thickening Costs V-14
5. Typical Results - Flotation Thickening V-17
6. Flotation Thickening Costs V-18
7. Typical Results - Vacuum Filtration V-22
8. Estimated Costs For Dewatering by Vacuum Filter V-24
9. Typical Solid Bowl Centrifuge Performance V-26
10. Estimated Costs For Dewatering By Centrifuge V-28
11. Typical Results - Pressure Filtration V-31
12. Estimated Costs For Dewatering By Filter Press V-32
13. High Heat of Combustion of Sludges
(Total Dry Solids Basis) V-34
14. Estimated Costs of Sludge Incineration V-45
15. Estimated Costs of Lime Recalcining V-49
16. Municipal Solid Waste and Sewage Sludge
Pyrolysis Processes V-52
17. Estimated Costs of Drying By Solvent Extraction
(B.E.S.T. Process) For Primary + WAS at 11% Solids V-61
18. Heavy Metals Content of Sewage Sludges V-67
19. Aerobic Digestion Costs Utilizing Conventional
Air System V-76
iii
-------
LIST OF TABLES (CONT'D)
Page
20. Truck Transport Costs VI-4
21. Barge Transport Costs VI-7
22. Railroad Transport Costs VI-9
23. Pipeline Transport Costs VI-12
24. Comparison of Processes for Primary + WAS Sludges VII-3
iv
-------
LIST OF FIGURES
Page
1. Basic Sludge - Handling Alternatives III-2
2. Zimpro LPO System V-5
3. Effect Of Increasing Sludge Solids On The
Final Sludge Volume*^' V-10
4. Gravity-Thickener V-ll
5. Dissolved Air Flotation System V-15
6. Cutaway View Of a Rotary Drum Vacuum Filter V-21
7. Continuous Countercurrent Solid Bowl Conveyor
Discharge Centrifuge V-25
8. Side View Of a Filter Press V-29
9. The Effects Of Sludge Moisture and Volatile
Solids Content On Gas Consumption V-36
10. Impact Of Excess Air On The Amount Of Auxiliary
Fuel For Sludge Incineration V-37
11. Potential Heat Recovery From Incineration Of
Sludge V-38
12. Heat Required To Sustain Combustion Of Sludge V-40
13. Combustion of Sludge and Solid Waste V-41
14. Cross Section of a Typical Multiple Hearth
Incinerator V-44
15. Cross Section of a Fluid Bed Reactor V-46
16. Flash Dryer System V-55
17. Sludge Drying System Using The Jet Mill Principle V-57
18. B.E.S.T. System Schematic V-59
-------
LIST OF FIGURES (CONT'D)
Page
19. Two-stage Anaerobic Digestion V-71
20. Anaerobic Digester Gas Utilization System V-72
21. Aerobic Digestion System V-74
22. Static Pile Composting As Developed By The
Agricultural Research Service At Beltsville,
Maryland V-79
23. Typical Sludge Handling (Primary + WAS) System
Costs Per Ton of Dry Solids VII-4
vi
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SECTION I
INTRODUCTION
The following paper describes alternatives for the treatment and
disposal or reuse of sludges resulting from wastewater treatment. There
are several factors to keep in mind when planning a sludge handling system.
The oft-quoted, "There is no such thing as a free lunch" is certainly
applicable to sludge disposal or reuse. There has been no system demon-
strated which produces useful by-products such as fertilizer or cattle
feed that produces revenues which exceed processing costs. The revenues
from by-products can recover portions of the processing costs.
Another important factor to consider is the variability of sludge
characteristics between geographical areas and often within the same
system. For example, two wastewater treatment systems in different
locations may use identical processes but the sludge produced can vary
in nature. Seasonal factors such as industrial waste loads from a cannery,
can cause variations in a given system. Because of variability in char-
acteristics, there is no universal solution to treating and disposing of
sludges. The economics change from location to location as well as the
environmental acceptance of alternative processes.
Another important factor relating to land disposal is the complica-
tion of adding one highly variable substance (sludge) to another (soil).
Both substances are highly variable in terms of chemical characteristics,
moisture contents and drainability, and trace element contents. Crops
grown on sludge amended soil must be compatible with the soil and sludge
used.
Additionally, the planner must carefully consider compatibility of
various processes. For example, some chemical sludges may not be satis-
factorily treated by anaerobic digestion.
The planner must have a basic understanding of the individual unit
processes and the environmental restraints of his community and region
to properly evaluate sludge management plans.
1-1
-------
SECTION II
SIGNIFICANCE OF SLUDGE DISPOSAL ASPECT OF WASTEWATER MANAGEMENT
Implementation of wastewater management plans which result in higher
degrees of wastewater treatment will result in improvements in water
quality. Unfortunately, these improvements will be accompanied by the
production of increasing quantities of increasingly difficult-to-handle
sludges. For example, primary treatment of municipal wastewaters typi-
cally produces 2,500-3,000 gallons of sludge per million gallons (MG) of
wastewater treated. When treatment is upgraded to secondary with acti-
vated sludge, the sludge quantities increase by 15,000-20,000 gallons per
MG treated. Use of chemicals for phosphorus removal can add another
10,000 gallons per MG. The sludges withdrawn from primary treatment are
as much as 97% water. Secondary and many chemical sludges have higher
water contents and are much more difficult to dewater than primary sludges.
A recent projection^ of sludge trends in the U.S. indicates the magnitude
of the problem:
1972
1985
Sludge Type
Primary
Secondary
Chemical
TOTAL
Population
(Million)
145
101
10
Tons/Year
(Million)
3.20
1.50
0.09
4.8
Population
(Million)
170
170
50
Tons/Year
(Million)
3.7
2.5
0.5
6.7
The cost of sludge handling and disposal is often greater than the
cost of treating the wastewater itself. For example, the cost (capital,
operation, and maintenance) of providing secondary treatment of 10 MGD
of municipal wastewater may be 20-25 cents/1000 gallons, while the cost
of disposing of the resulting sludges may be 30 to 100% (or more) of
this amount. Another significant consideration is that although there
may be several environmentally acceptable, technically feasible, and
economically competitive methods for wastewater treatment in a given area,
there may be only a few - perhaps only one - such sludge disposal alter-
natives. Thus, sludge disposal considerations are an important element
in the selection of an overall wastewater management plan.
Regionalization of several smaller wastewater systems into a larger
system may favorably affect the relative economics of some sludge dis-
posal alternatives to the degree that they become economically feasible,
whereas, they were not for any of the individual, smaller plants. For
example, heat drying of sludge for use as a fertilizer decreases in cost
by a factor of nearly 2 as plant capacity increases from 10 mgd to 50
mgd, while the cost of the anaerobic digestion process decreases only
about 10%.
II-l
-------
In some cases, integration of solid waste and wastewater sludge dis-
posal plans may offer a useful, synergistic relationship. For a given
population, the volume of solid wastes is about 10 times the volume of
wastewater sludge. Thus, inclusion of sludge with solid wastes from an
area may not significantly alter the volume of material to be handled in
the solid waste system. For example, under certain conditions, disposal
of dewatered, stabilized sludge may be readily compatible with an existing
solid waste landfill practice. On the other hand, a sludge handling system
would have to be altered drastically to have capacity for solid wastes.
Such a plan could unfavorably affect an otherwise acceptable sludge hand-
ling plan. For example, a local market for compost might be ample for
composted sewage sludge but might be overwhelmed by the much larger volume
of a composting operation handling both sludge and solid wastes.
II-2
-------
SECTION III
BASIC DISPOSAL ALTERNATIVES
Although a large number of alternative combinations of equipment
and processes are used for treating sludges, the basic alternatives
are fairly limited . The ultimate depository of the materials con-
tained in the sludge must either be land, air (by-products of inciner-
ation) , or water. Current policies discourage practices such as ocean
dumping of sludge as long term solutions. Air pollution considerations
necessitate air pollution facilities as part of the sludge incineration
process. Incineration results in a residual ash which must be disposed
of. Thus, sludge in some form will eventually be returned to the land.
There are two basically different philosphical approaches in handling
the sludges from wastewater treatment: reuse as opposed to disposal.
The reuse approach is based upon recycling the sludges so that nutrients
and organics contained in the sludges are beneficially reused. The goal
of sludge treatment in this case is to make the sludge compatible with
the proposed reuse system (i.e., stabilize the sludge so that it will
not cause nuisance conditions, eliminate pathogens to prevent disease
problems, etc.). The organic solids which make up 60-80 percent of the
solids in a typical municipal sludge also are a potential source of
energy (typically about 10,500 BTU/lb). Some processes discussed later
in this paper convert these solids so that this energy can be beneficially
reused. The disposal philosphy considers the sludge a waste material.
In some cases, such as ocean dumping, limited pretreatment of sludge is
provided prior to disposal. However, most disposal systems incorporate
treatment techniques to provide maximum reductions in volume of sludge
prior to disposal with little or no regard for the potentially beneficial
components of sludge.
The choice between disposal and reuse approaches must be based upon
the evaluation of the many factors (economics, environmental impacts,
energy consumption, etc.) associated with each of the processes involved
(as discussed in detail later in this paper). For example, the feasibil-
ity of recycling of sludges to the land for agricultural reuse is depend-
ent upon the quality of the sludge, availability, location, use, nature,
and cost of land. These factors may be a problem in some urban areas.
Chicago transports a portion of its sludges to a site 160 miles downstate
for application to previously strip-mined land indicating that under some
circumstances transport to even relatively distant locales is practical.
EPA report
Figur?8i)
summarizes general sludge handling alternatives. A recent
summarizes the current use of these various alternatives
III-l
-------
SLUDGE
USE AS
FERTILIZER
*• ASH TO
LANDFILL
USE AS SOIL
CONDITIONER
USE AS FERTILIZER
DISINFECT
DRY
HEAT
DRY
DEWATER
THICKEN
LANDFILL
DIGEST
(STABILIZE)
BURN
(OXIDIZE)
DEWATER
COMPOST
Figure 1
BASIC SLUDGE-HANDLING ALTERNATIVES
III-2
-------
in the U.S. Subsequent portions of the paper discuss the available tech-
nology, costs, implementation considerations, and environmental impacts
of the available alternatives.
III-3
-------
SECTION IV
SLUDGE CHARACTERISTICS
The composition of sewage sludges from municipal systems varies
widely from one locale to another depending on a variety of factors.
The presence or absence of industrial wastes can have a profound effect
on the quantity and quality of wastewater sludges. The chemical quality
of the community's raw water supply will effect the chemical composition
of the wastewater sludges. The presence or absence of stormwater in the
system will also affect the sludge composition (i.e., by the amount of
grit carried into the plant). Table 1 summarizes typical components of
sewage sludges in 150 treatment plants in north central and eastern U.S.
Because the nature of sludges resulting from the treatment of muni-
cipal wastewaters varies so greatly from one locale to another, general-
ized statements about their nature are of limited value. However, some
observations which are generally true follow.
Raw primary sludges almost universally settle, thicken and dewater
with relative ease compared to secondary biological sludges due to their
coarse, fibrous nature. Generally, at least 30% of the solids are larger
than 30 me^h in size. These coarse particles permit rapid formation of
a sludge cake with sufficient structural matrix to permit good solids
capture and rapid dewatering. Contrary to some reports in the literature,
anaerobic digestion of primary sludges frequently makes them somewhat
more difficult to thicken and dewater. However, the dewatering results
are still generally good at relatively low costs.
Activated sludges show much greater variation in dewatering charac-
teristics than do primary sludges. These variations may even be substan-
tial from day to day at the same plant. The sludges are much finer than
primary sludges and are largely cellular organic material with a density
very nearly the same as water. They are much more difficult to dewater
than primary sludges.
The nature of sludges resulting from the chemical coagulation of
sewage depends on the nature of the coagulant used. Generally, alum and
iron coagulants produce gelatinous floe which is difficult to dewater.
Lime coagulation produces a sludge which readily thickens and dewaters
in most cases. Estimates of sludge quantities and characteristics from
a variety of wastewater processesbe found in EPA's design manual
for sludge treatment and disposal
IV-1
-------
TABLE 1
(87)
MAJOR COMPONENTS OF SLUDGE
Sample
Component
Type
Number
Range
Median
Mean
Organic C, %
Total N, %
NH4~N, ppm
NO i ppm
Total P, %
Total S, %
Anaerobic
Aerobic
Other
All
Anaerobic
Aerobic
Other
All
Anaerobic
Aerobic
Other
All
Anaerobic
Aerobic
Other
All
Anaerobic
Aerobic
Other
All
Anaerobic
Aerobic
Other
All
31
10
60
101
85
38
68
191
67
33
3
103
35
8
3
45
86
38
65
189
19
9
28
18-
27-
6.5-
6.5-
0.5-
0.5-
<0.1-
<0.1-
39
37
48
48
17.6
7.6
10.0
17.6
120- 67,600
30- 11,300
5- 12,500
5- 67,600
2- 4,900
7- 830
2- 4,900
0.5-
1.1-
<0.1-
<0.1-
0.8-
0.6-
0.6-
14.3
5.5
3.3
14.3
1.5
1.1
1.5
26.8
29.5
32.5
30.4
4.2
4.8
1.8
3.3
1,600
400
80
920
79
180
140
3.0
2.7
1.0
2.3
1.1
0.8
1.1
27.6
31.7
32.6
31.0
5.0
4.9
1.9
3.9
9,400
950
4,200
6, 540
520
300
780
490
3.3
2.9
1.3
2.5
1.2
0.8
1.1
IV-2
-------
TABLE 1 (CONT'D)
Component
K
Sample
Type
Number
Anaerobic
Aerobic
Other
All
86
37
69
192
Range
0.02-
0. 08-
0.02-
0.02-
2.64
1.10
0.87
2.64
Median
0. 30
0. 38
0.17
0. 30
Mean
0.52
0.46
0.20
0.40
Na
Anaerobic
Aerobic
Other
All
73
36
67
176
0.01-
0.03-
0.01-
0.01-
2.19
3.07
0.96
3.07
0.73
0. 77
0.11
0.24
0.70
1.11
0.13
0. 57
Ca
Anaerobic
Aerobic
Other
All
87
37
69
193
1.9
0.6
0.1
0.1
20. 0
13.5
25.0
25.0
4.9
3.0
3.4
3.9
5.8
3.3
4.6
4.9
Mg
Anaerobic
Aerobic
Other
All
87
37
65
189
0.03-
0.03-
0.03-
0.03-
1. 92
1.10
1.97
1.97
0.48
0.41
0.43
0.45
0.58
0.52
0. 50
0.54
Ba
Anaerobic
Aerobic
Other
All
27
10
23
60
<0.01-
<0.01-
<0.01-
<0.01-
0.90
0.03
0.44
0.90
0.05
0.02
<0.01
0.02
0.08
0.02
0.04
0.06
Fe
Anaerobic
Aerobic
Other
All
96
38
31
165
0.1
0.1
<0.1
<0.1
15. 3
4.0
4.2
15.3
1.2
1.0
0.1
1.1
1.6
1.1
0.8
1.3
A1
Anaerobic
Aerobic
Other
All
73
37
23
133
0.1
0.1
0.1
0.1
13.5
2.3
2. 6
13. 5
0. 5
0.4
0.1
0.4
1.7
0.7
0.3
1.2
IV-3
-------
TABLE 1 (CONT'D)
Sample
Component
Type
Number
Mn
Anaerobic
Aerobic
Other
All
81
38
24
143
Anaerobic
Aerobic
Other
All
62
29
18
109
As
Anaerobic
Aerobic
Other
All
7
10
Co
Anaerobic
Aerobic
Other
All
9
13
Mo
Anaerobic
Aerobic
Other
All
9
3
17
29
Hg
Anaerobic
Aerobic
Other
All
35
20
23
78
Range
Median
Mean
58- 7,100
55- 1,120
18- 1,840
18- 7,100
mg/kg
280
340
118
260
400
420
250
380
12-
17-
4-
4-
760
74
700
760
36
33
16
33
97
40
69
77
10-
230
116
119
6-
6-
18
230
9
10
11
43
3-
18
7.0
8.8
1-
1-
11
18
4.0
4.0
4.3
5.3
24-
30-
5-
5-
30
30
39
39
30
30
30
30
29
30
27
28
0.5-10,600
1.0- 22
2.0- 5,300
0.5-10,600
5
5
3
5
1,100
7
810
733
IV-4
-------
TABLE 1 (CONT'D)
Component
Type
Sample
Number
Pb
Anaerobic
Aerobic
Other
All
98
57
34
189
Zn
Anaerobic
Aerobic
Other
All
108
58
42
208
Cu
Anaerobic
Aerobic
Other
All
108
58
39
205
Ni
Anaerobic
Aerobic
Other
All
85
46
34
165
Cd
Anaerobic
Aerobic
Other
All
98
57
34
189
Cr
Anaerobic
Aerobic
Other
All
94
53
33
180
Range
Median
Mean
58- 19,730
13- 15,000
72- 12,400
13- 19,700
mg/kg
540
300
620
500
1,640
720
1,630
1,360
108- 27,800
108- 14,900
101- 15,100
101- 27,800
1,890
1,800
1,100
1,740
3, 380
2,170
2,140
2,790
85- 10,100
85- 2,900
84- 10,400
84- 10,400
1,000
970
390
850
1,420
940
1,020
1,210
2-
2-
15-
2-
3,520
1,700
2,800
3, 520
85
31
118
82
400
150
360
320
3-
5-
4-
3-
3,410
2,170
520
3,410
16
16
14
16
106
135
70
110
24- 28,850
10- 13,600
22- 99,000
10- 99,000
1,350
260
640
890
2,070
1,270
6,390
2,620
IV-5
-------
SECTION V
SLUDGE PROCESSING TECHNOLOGY
It is not the purpose of this section to provide detailed design
guidance. Other readily available references (4-7) provide such informa-
tion. This section will briefly describe available sludge processing
technology. The discussion of each of the processes will center upon the
aspects of major concern in developing regional plans such as results,
costs, and implementation considerations. The costs and energy data used
in this report were developed by Culp/Wesner/Culp under EPA Contract
68-03-2186 unless otherwise referenced. All cost and energy data pre-
sented in this report are based on detailed studies for a given set of
conditions. The summary information presented herein will serve to put
the various process in perspective but should not be used for a given
project where conditions may vary significantly from those assumed for
illustrative purposes. Technical conditions selected for illustrative
costs are based on handling a mixture of primary and waste activated
sludges - the most likely mixture to be encountered.
SLUDGE CONDITIONING
The purpose of sludge conditioning is to increase the rate and/or
extent of dewatering achievable for a given sludge. A wide variety of
physical and chemical techniques are used. The use of sludge condition-
ing prior to dewatering has become standard practice and their combined
result permits the moisture content of sludges to be reduced form 95 to
98% to 60-75%.
Chemical Conditioning
The most frequently encountered conditioning practice in the U.S.
today^is the use of ferric chloride either alone or in combination with
lime although the use of polymers is rapidly gaining widespread accept-
ance. Although ferric chloride and lime are normally used in combination,
it is not unusual for them to be applied individually. Lime alone is a
fairly popular conditioner for raw primary sludge and ferric chloride
alone has been used for conditioning activated sludges. Lime treatment
to high pH value has the added^advantage of providing a significant degree
of disinfection of the sludge
Organic polymeric coagulants, and coagulant aids that have been
developed in.the past 20 years, are rapidly gaining acceptance for sludge
conditioning . These polyelectrolytes are of three basic types:
V-l
-------
1. Anionic+^negative+cj:harge) - serve as coagulant aids complementing
inorganic A1 and Fe coagulants by increasing the rate of fioccu-
lation, size, and toughness of particles.
2. Cationic (positive charge)- serve as primary coagulants or in
conjunction with inorganic coagulants.
3. Nonionic (equal amounts of positively and negatively charged
groups in monomers)- serve as coagulant aids in a manner similar to
that of both anionic and cationic polyelectrolytes.
The popularity of polymers is primarily due to their ease in handling,
small storage space requirements, and their effectiveness. All of the
inorganic coagulants are difficult to handle and their corrosive nature
can cause maintenance problems in the storing, handling, and feeding sys-
tems in addition to the safety hazards inherent in their handling. Many
plants in the U.S. have abandoned the use of inorganic coagulants in favor
of polymers.
The facilities for chemical conditioning are relatively simple and
consist of equipment to store the chemical(s), feed the chemical(s) at
controlled dosages, and mix the chemical(s) with the sludge. The cost
of chemical conditioning is primarily a function of the quantity of chemi-
cal required which is affected by factors such as:
1. Solids concentration.
2. Sludge particle size.
3. Proportion of volatile matter in sludge.
4. Reducing agents in sludge, i.e. H2S.
5. Alkalinity.
The chemical requirements for any given sludge can be determined
accurately onl^b^j tests on the specific sludge involved. Typical values
are as follows ' :
FeCl^ Lime
Polymer
lbs/dry ton lbs CaQ/dry
solids ton solids
lbs/dry ton
solids
Raw Primary + Activated
Sludge
40-50
200
15-20
Digested Primary +
Activated Sludge
80-100
160-370
30-40
Elutriated Primary +
Activated Sludge
40-125
20-30
V-2
-------
Chemical costs also vary widely from one locale to another. Typical
chemical conditioning costs are $10-$25 per dry ton solids. The use of
the inorganic chemical conditioning chemicals can increase the weight of
sludge by 10-20%.
Energy consumed in the feeding and mixing of conditioning chemicals
is negligible in terms of overall wastewater treatment plant energy con-
sumption. However, the energy required to produce the chemicals consumed
(secondary energy requirement) may be significant and may be summarized
as follows:
~Indicates principal type of energy used in production.
Elutriation
Elutriation is a washing operation which removes sludge constituents
that interfere with thickening and dewatering processes. The process of
elutriation was originally developed, and its use was justified, as an
aid in the reduction (by one-fourth to one-half) of the inorganic chemical
requirements for vacuum filtration of digested sludge. Digestion substan-
tially reduces the organic fraction in sludge solids while increasing bio-
chemical products such as ammonium bicarbonate which in turn significantly
increase the flocculent demand exerted by a sludge.
Elutriation operations consist of "washing" the sludge solids with
water or plant effluent in continuous or batch units. The washing oper-
ation flushes the bicarbonates from the sludge. The elutriate (or spent
wash water) is recycled to the treatment plant and the sludge is pumped
to the next solids handling process.
Elutriation may reduce the cost of chemical conditioning, but often
causes a problem due to elutriate solids recycle. Recycling uncaptured
elutriate solids can overload aeration facilities at activated sludge
plants.
Estimates of the capital and operating cost of elutriation are typi-
cally less than $5 per ton of dry solids. Generally, savings in sludge
conditioning chemicals will exceed this cost. However, any economic
evaluation must also consider the added cost of properly handling the
recycled elutriate. Energy requirements are 1,500-3,000 Kwh/ton of dry
solids.
Many consulting engineers in the U.S. do not now consider elutriation
when designing new waste treatment facilities. They believe the loss of
solids in the elutriate is unavoidable and, therefore, the process is
unsatisfactory even if high chemical costs for mechanical dewatering are
required as a substitute.
6 Fuel
10 BTU/ton
10
5.5*
3*
Electricity
Kwh/lb
Ferric Chloride
Lime (CaO)
Polymer
0.5*
0.3
0.1
V-3
-------
Heat Treatment
There are two basic types of high temperature-high pressure treatment
of sludges. One - "wet air oxidation" - involves the flameless oxidation
of sludges at 450-550 F at pressures of about 1200 psig. Wet air oxidation
is discussed in the subsequent section on sludge reduction processes.
The other type - "heat treatment" is carried out at lower temperatures and
pressures (350-400 F at 150-300 psig) to improve the dewaterability of
sludges and is the subject of this section.
When colloidal gel systems are heated, thermal activity causes water
to escape from the structure. It is the goal of heat treatment systems
to release bound water form the sewage sludge to improve the dewatering
and thickening characteristics of the sludge. Unfortunately, the physical
effect of heat treatment also ruptures the cell walls of biological sludges,
releasing bound organic colloidal material, solubilizes previously insol-
uble organic material, aid creates fine particulate debris. This solubili-
zation process means that a principal result of heat treatment is the
conversion of suspended solids to dissolved or dispersed solids, facili-
tating dewatering, but simultaneously creating a separate problem of
recycling of highly polluted liquid from the dewatering process to the
wastewater treatment plant. This recycling must be recognized when
assessing the feasibility of heat treatment.
Heat treatment is a conditioning technique which has had increasing
use by consulting engineers although its use is still limited when com-
pared to the large number of plants using other techniques for sludge
conditioning. Several operating heat treatment plants have had signifi-
cant operational problems from (1) the increased loadings of BOD, COD,
nitrogen, phosphorus, and suspended solids on the secondary plant and
(2) the refractory nature of a portion of the recycled load which will
pass through the secondary plant as COD and can cause taste and odor
problems in downstream water plants. The COD refractory to the biological
processes has also been found difficult to remove by advanced wastewater
treatment processes such as activated carbon adsorption. The refractory
nature of the recycled organics may limit the applicability of this con-
ditioning process in areas where downstream water uses dictate very low
effluent COD values. Odor problems in the vicinity of some heat treat-
ment facilities have been a problem.
An advantage of the heat treatment of sludges is that it produces a
more readily dewaterable sludge than chemical conditioning. Dewatered
sludge solids of 30-40% (as opposed to 15-20% with chemical conditioning)
have been achieved with heat treatment at relatively high loading rates
on the dewatering equipment (2-3 times the rates with chemical condition-
ing) . The process also provides effective disinfection of the sludge.
A typical heat treatment process (Zimpro LP0) is shown in Figure 2.
Sludge is ground and pumped to a pressure of about 300 psi. Compressed
air is introduced into the sludge and the mixture is brought to an oper-
ating temperature of above 350 F by heat exchange and direct steam injec-
tion and flows to the reactor. The heated, conditioned sludge is cooled
V-4
-------
GROUND
SLUDGE
HOLDING
TANK
HEAT
EXCHANGER
Sludge
GRINDER
POSITIVE
DISPLACEMENT
SLUDGE PUMP
PUMP
REACTOR
AIR COMPRESSOR
Exhaust Gas
VAPOR
COMBUSTION
UNIT
OXIDIZED
SLUDGE
TANK
PRESSURE
CONTROL
VALVE
TO INCINERATOR
T reated
Boiler
Water
PUMP
BOILER
FILTER
FIGURE 2.
Zimpro LPO system.
-------
by heat exchange with the incoming sludge. The treated sludge is separated
by settling before the dewatering step. Gases released at the separation
step are passed through a catalytic afterburner at 650-750 F. The system
is more mechanically complex than many unit processes in municipal waste-
water treatment plants and some installations have encountered significant
maintenance problems.
An extensive analysis of the costs of heat treatment sludges under
EPA Contract 68-03-2186 was summarized in a recent paper . Analyses
of costs are complicated by the fact that an accurate estimate must reflect
the indirect costs associated with treatment of the recycled polluted
liquors tc^gjjie treatment plant. For the typical conditions described in
the paper , the illustrative costs presented in Table 2 were developed.
It is apparent that heat treatment costs become very high at smaller capa-
cities. In evaluating costs, the total system (conditioning, dewatering,
disposal) costs must be considered. The fact that heat treatment produces
a more readily dewaterable sludge produces economies in downstream dewater-
ing processes. It may produce a sludge dry enough to burn without auxiliary
fuel, providing some downstream savings.
Electricity requirements are 100-150 Kwh/ton of dry solids with fuel
requirements of about 3 x 10 BTU/ton of dry solids (for primary + waste
activated sludge at 4.5% solids).
Freezing
A number of people have observed that sludge frozen and later thawed
in sand drying beds or lagoons had good dewatering and fertilizer or
soil-conditioning characteristics. Thawed sludge stable and dewatered
rapidly if provisions were made for water drainage . These observations
encouraged researchers, particularly in Great Britain, to evaluate artifi-
cial freezing of sludge as a means to promote rapid dewatering.
The City of Milwaukee, Wisconsin has studied the process^12'13^ for
application to activated sludge. These studies found that vacuum filter
rates of 55 psf/hour were achievable; the filtrate and filter cake quality
were equivalent or better than that produced from conventional vacuum
filter operation; freeze conditioned sludge could be dewatered by a wire
screen cloth (40-80 mesh) by gravity draining; and that freezing rate was
an important variable.
Early engineering studies at Milwaukee revealed that equipment costs
and space requirements would be substantially higher than for chemical
conditioning techniques currently employed, and that space requirement
for freeze-conditioning would be 65 to 130 times that required for the
conventional chemical dewatering system. Equipment costs for the freeze-
conditioning system were estimated at 7 to 10 times those for the chemical
conditioning process. The estimated annual operating costs for freeze-
conditioning process was 3 to 4 times that of the chemical conditioning
approach.
Coupled with the high capital cost, very high operating cost has
been the major reason why freezing has not been adopted as a conditioning
V-6
-------
TABLE 2
(10)
SUMMARY OF DIRECT AND INDIRECT COST FOR
THERMAL TREATMENT
Sludge
Ton/Day
1
5
10
50
100
Construction Cost
Direct Indirect
2 3 4
97.53
30.79
21.45
12.20
10.96
4.11
3.18
2.93
1.83
1.98
Total
101.64
33.97
24.38
14.03
12.94
O & M Cost
Direct Indirect
150.14
46.46
32.52
19.10
16.58
4.93
3.67
3.50
2.99
2.87
Total
155.07
50.13
36.02
22.09
19.45
Total Cost
256.71
84.10
60.40
36.12
32.39
(1) Basis
a) 1.1 tons solids per mgd-includes recycle
b) 4 gpm to thermal treatment per mgd @ 4^% solids
c) 8000 hrs/yr operating time
(2) Ammortized 20 years-7%
(3) All costs in dollars/ton
(4) Not including odor control
-------
technique for wastewater sludge.
Hydrolysis with Sulfur Dioxide
This process consists of heating activated sludge in the presence
of water and small amounts of sulfur dioxide to improve its dewatering
characteristics. This treatment increases soluble solids and produces
a filtrate which can be concentrated to produce a molasses-type syrup
which could be of value as an animal feed(14-16)_
A small scale study has been made to preliminarily evaluate the
feasibility of the hydrolysis process applied to sewage sludge. It was
found that the filtration rate of activated sludge was increased by a
factor of six when S02 was added to the activated sludge before heat
treatment while heat treatment alone increased the rate by a factor of
only three. The amount of moisture retained in the filter cake was also
reduced by the S02 treatment. It was found that heat treatment alone
increases the soluble solids content of activated sludge by about 90%
and that an additional 20% was obtained by the addition of SO2 (0.5%
sulfurous acid, 140 C for one hour). Evaporation of the filtrate to a
syrup with 60% solids produced a molasses which was 82% organic.
Preliminary cost estimates indicated that the sale of the molasses
resulting from this process could recover about 20% of the cost of the
hydrolysis treatment. The SO2 hydrolysis process has not yet been tried
on sewage sludges on a plant-scale or continuous basis. Thus, the
economics of the process and the marketability of the resulting molasses
are yet to be demonstrated.
Radiation Treatment
Radiation of sludges produces charged and oxidizing species which
affect colloidal systems and may improve the thickening and dewatering
characteristics of the sludges . A limited amount of data are avail-
able on the feasibility of using radiation treatment for sludge condition-
ing (1^-20)_ A preliminary analysis of the economic feasibility indicated
that if a dosage of 10^ rads would enable a doubling of the vacuum filtra-
tion rate, that the costs of radiation treatment would be of the same order
of magnitude as the potential savings.
A study of the effects of gamma irradiation on the settleability and
filterability of digested activated sludge indicated that a dose of 5 x 10^
rad showed essentially no effect on the settling properties of the sludge
but, in combination with ferric chloride conditioning to approximately
one-third of the optimum conditioning dosage, was able to effect about a
three-fold increase in the dewaterability. The filterability of undigested
activated sludge was not increased by the same treatment.
(19)
Another study found that irradiation produced a marked effect on
the filterability of sludge but that this effect saturated at a dose of
approximately 10 rads at a level of specific resistance too high to permit
the sludges to be filtered at a useful rate on a rotary vacuum filter. The
V-8
-------
range of specific resistance needed for effective vacuum filtration could
not be reached with the use of ionizing radiation alone.
The use of radiation for sludge conditioning is not currently prac-
ticed in the U.S. and it does not appear likely that it will be in the
near future.
The disinfection of sewage sludge by bombardment with an electron
beam is being tried on a large scale at the Deer Island sewage treatment
plant in Boston Harbor. The facility uses a 50-kilowatt electron generator.
A beam of electrons sweeps back and forth across a four-ft. wide, two-
millimeter deep stream of sludge that is passing over a metal drum at the
rate of more than six ft. per second. In one hundredth of a second, the
water receives a radiation dose of 4000,000 rads.
SLUDGE THICKENING
The purpose of sludge thickening is to reduce the sludge volume to
be stabilized, dewatered, or disposed of. Figure 3 illustrates the impact
that thickening can have on sludge volume. Thickening a 1% sludge to
6% solids reduces the volume of sludge to be handled by a factor of over
5. This reduction can provide significant savings in the cost of dewater-
ing, digestion, or other downstream facilities. There are three commonly
used methods for sludge thickening: gravity, flotation, and centrifugation.
Gravity Thickening
Thickening by gravity is the most common concentration process in
use at wastewater treatment plants. It is simple and inexpensive. Gravity
thickening is essentially a sedimentation process similar to that which
occurs in all settling tanks. But, in comparison with the initial waste
clarification stage, the thickening action is relatively slow. The
theoretical aspects of gravity thickening have been the subject of many
studies and are well summarized in a few recent papers(21-25).
Figure 4 illustrates a typical, circular gravity thickener. The
units have a typical side water depth of 10 feet. Loading rates are
expressed in terms of pounds of dry solids in the sludge applied to the
thickener surface area per day (lbs/day/sf). Table 3 summarizes typical
results achieved with gravity thickening.
The degree to which waste sludges can be thickened depends on many
factors; among the most important are the type of sludge being thickened
and its volatile solids concentration. Bulky biological sludge, particu-
larly that from the activated sludge process, will not concentrate to the
same degree as raw primary sludge. Activated sludges, if thickened
separately, are usually thickened by the flotation process. The degree
of biological treatment and the ratio of primary to secondary (biological)
sludge will affect the ultimate solids concentration obtained by gravity
thickening. Hydraulic and surface loading rates are also of importance.
Current practice calls for the use of overflow rates of 400-800 gpd/sq.ft.
Excessively low flow rates can lead to odor problems. If the sludge flow
V—9
-------
100
CO
PERCENT SOLIDS IN
RAWSLUDGE
UJ
75
UJ
UJ
50
CO
LU
25
0.5
UJ UJ
UJ
o.
SOLIDS IN CONCENTRATED SLUDGE, (%)
FIGURE 3 Effect of increasing sludge solids on the final sludge volume (4)
V-10
-------
Courtesy Link Ht l!
XKT~l
EFFLUENT
WEIR
influent
RAISED POSITION
OF TRUSS ARM
WATER LEVEL
¦vvvyi^
EFFLUENT ^
NFLUtNT
BAFFLE
FLOW
PIPE SHAFT
SCRAPER BLADES
HOPPER PLOW
ELEVATION
) UNDERFLOW
FIGURE 4 Gr avity thickener
-------
TABLE 3
TYPICAL RESULTS -
GRAVITY THICKENING
<
H
to
Sludge Type
Primary
Trickling Filter
Primary + FeCl^
Primary + Low Lime
Primary + High Lime
Primary + WAS*
WAS
Primary + (WAS + FeCl^)
(Primary + FeCl3) + WAS
Digested Primary
Digested Primary + WAS
Digested Primary + (WAS + FeCl^)
Tertiary, 2 stage high lime
Tertiary, low lime
Feed Solids
Concentration
(Percent)
5.0
1.0
2.0
5.0
7.5
2.0
1.0
1.5
1.8
8.0
4.0
4. 0
4.5
3. 0
Typical Loading
Rate
(lb/sgft/day)
20-30
8-10
6
20
25
6-10
5-6
6
6
25
15
15
60
60
Thickened Sludge
Concentration
(Percent)
8.0-10
7-9
4.0
7.0
12. 0
4.0
2-3
3.0
3.6
12. 0
8.0
6.0
15.0
12.0
*WAS = Waste Activated Sludge
-------
to the thickener is far below the design rate, pumping of secondary efflu-
ent to the thickener may be practiced to minimize odors.
(26)
There is some evidence that activated sludges from systems using
pure oxygen gravity thicken more readily than those sludges from conven-
tional air systems, at that, concentrations of 4-6% may be achieved at
rates of 10-20 lbs/day/sf.
The quality of the overhead liquid removed from the sludge solids is
important in any thickening operation because this liquid is usually
returned to the treatment processes. Generally, the overhead quality is
similar to that of raw sewage, 150 to 300 mg/1 suspended solids and a BOD
of about 200 mg/1. A well-operated thickener should have a minimum of
anaerobic decomposition and a solids capture exceeding 90 percent. Thus,
the overflow returned to the treatment process should not present an
operational problem.
Table 4 summarizes the estimated cost of gravity thickening based
on a loading rate of 20 lbs/day/sf. The electrical consumption of the
process is low - on the order of 1-1.5 kwh/ton of solids.
Flotation Thickening
Flotation thickening units are becoming increasingly popular for
sewage treatment plants in the U.S., especially for handling waste acti-
vated sludges where they have the advantage over gravity thickening tanks
of offering higher solids concentrations and lower initial cost for the
equipment. The objective of flotation-thickening is to attach a minute
air bubble to suspended solids and cause the solids to separate from the
water in an upward direction due to the fact that the solid particulates
have a specific gravity lower than water when the bubble is attached.
Figure 5 illustrates the basic considerations involved in the pro-
cess. A portion of the unit effluent, or plant effluent, is pumped to
a retention tank (a pressurization tank) at 60-70 psig. Air is fed into
the pump discharge line at a controlled rate and mixed by the action of
an educator driven by the reaeration pump. The flow through the recycle
system is metered and controlled by a valve located immediately before
the mixing with the sludge feed. The recycle flow and sludge feed are
mixed in a chamber at the unit inlet. If flotation aids (such as polymers)
are employed, introduction is normally in this mixing chamber. The
sludge particles are floated to the sludge blanket and the clarified
effluent is discharged under a baffle and over an adjustable weir. The
thickened sludge is removed by a variable speed skimming mechanism. In
practice bottom sludge collectors are also furnished for removal of any
settled sludge or grit that may accumulate. Sludge thickening occurs in
the sludge blanket, which is normally 8" to 24" thick. The buoyant sludge
and air bubbles force the surface of the blanket above the water level,
inducing drainage of water from the sludge particles.
Similar to gravity thickening, the type of quality of sludge to be
floated affects the unit performance. Flotation thickening is, as stated
V-13
-------
TABLE" 4
GRAVITY THICKENING COST!
I tern
Construction Cost
Engr., Legal, Adm. , S. Interest
During Construction (19'M
Total Capital Cost
0 & M Labor @ $lU/Hr.
Power @ $0.025/Kwh
Maintenance Materials
Annual 0 & M Costs
Annual Capital Cost ( x .0944)
Total Annual Cost
Cost per ton of dry solids
Tons P<-
r Hav o f
Dry Solid:
s
i_o
25
SO
100
$120,000
$160,000
$ 320,000
$540,000
22,R00
30,4 00
60,800
102,600
$142,800
$190,400
$ 380,800
$642,600
3,600
4, 400
6, 000
10,000
100
300
son
800
400
700
1 ,300
2,200
4 ,100
5,400
7 ,800
13,000
13,500
18,000
36,000
60,700
$ 17,600
$ 2 3,4 00
$ 4 3,800
$ 73,700
o
cc
$2.60
$2. 40
$2.00
Assumption: 20 lbs/day/sf loading rate.
V-14
-------
Courtcsv Komline-Sanderson
UNIT EFFLUENT
AUX. RECYCLE CONNECTION
(PRIMARY TANK OR
PLANT EFFLUENT)
FLOTATION UNIT
THICKENED SLUDGE
- DISCHARGE
UNIT FEED
RECIRCULATION PUMP
AIR FEED
y
SLUDGE
RECYCLE
FLOW
^ RETENTION TANK
(AIR DISSOLUTION)
REAERATION PUMP
FIGURE 5 Dissolved air flotation system.
-------
before, most applicable to activated sludges but hiqhor float concentra-
tions can be achieved by combining primary with activated sludge. However,
equal or greater concentrations may be achieved by combining sludges in
gravity thickening units. A high Sludge Volume Index (SVI), representing
a bulky sludge, results in poor thickener performance. Table 5 presents
typical results from flotation thickening.
Many different chemicals have been used in various air flotation sys-
tems The overall effect is to increase the allowable solids loadings,
increase the percentage of floated solids, and increase the clarity of
the effluent. Cationic polyelectrolytes have been the most successful
chemical used in sewage sludge thickening<27 30> with dosages of 8-12 lbs/
ton reported as typical. There is some evidence*26' that activated sludges
from pure oxygen systems are more amenable to flotation thickening than
activated sludges from conventional air systems. Pilot tests at Louisville,
Kentucky indicate that with polymer doses of only about 3 pounds per ton
of dry solids, an influent solids of 1.7-2% was increased to 6-7% solids
at loading rates of 6-10 psf/hour with 200% recycle and 97 99% solids
recovery. Similar results have been observed on a plant scale at the
Westgate plant in Fairfax County, Virginia (6-8% solids with 3 pounds of
polymer per ton)(31)•
Table 6 presents typical costs for flotation thickening at a loading
rate of 40 lbs/day/sf not including any chemical feed costs. Polymer
costs could add from $>$15/ton of solids. Electrical consumption is
about 100 kwh/ton of solids.
Centrifugal Thickening
Although centrifuges have been used widely for dewatering (See page
V-23) they have had limited use for thickening because of relatively
hiah cost. They have been used for thickening of MAS where space limita-
tions or sludge characteristics make other methods unsuitable . WAS
concentrations of 5-8% have been typically produced by centrifugal thick-
ening.
SLUDGE DEWATERING
Drying Beds
The most widely used dewatering method in the United States is drying
* rrn ouen or covered sandbeds. Over 6,000 wastewater treat-
I 1 this method(4)• They are especially popular in small
SanM sandbeds possess the advantage of needing little operator skill.
Air drying is normally restricted to well digested sludge, because raw
sludge is odorous, attracts insects, and does not dry satisfactorily when
applied at reasonable depths. Oil and grease discharged with raw sludge
cloq sandbed pores and thereby seriously retard drainage. The design and
use of drying beds are affected by many parameters They include weather
conditions, sludge characteristics, land values and proximity of residences,
and use of sludge conditioning aids, climatic conditions are most important.
Factors such as the amount and rate of precipitation, percentage of sunshine.
V-16
-------
TABLE 5
TYPICAL RESULTS
FLOTATION THICKENING
SIudge Type
Primary + WAS
Feed Solids
Concentration
(Percent)
Typical Loading Rate
Without Polymer
(Ib/sqft/day)
Typical Loading Rate
With Polymer
(Ib/sqft/day)
Float Solids
Concentration
(Percent)
2.0
20
60
5.5
Primary + (WAS + FeCl^)
1.5
15
45
3.5
(Primary + FeCl-j) + WAS
1.8
15
45
4.0
WAS
1.0
10
30
3.0
WAS + FeCl3
1.0
10
30
2.5
Digested Primary + WAS
4.0
20
60
10.0
Digested Primary + (WAS +
FeCl3) 4.0
15
45
8.0
Tertiary, Alum
1.0
8
24
2.0
-------
tahi.e: 6
FI.OTATTON 'It TOKENING COSTS
Ton:;
For Day of
Dry_ id;
;
Item 10
25
50
100
Construction Cor:t $200,000
$290,000
$300,000
$500,000
Engr. , Legal, Aclrn. , ;; Intor^st
During Constructi on {1 rJH ) 38,000
55,100
72,200
95,000
Total Capital Cost $2 38,000
$345,100
$452,200
$595,000
0 & M Labor @ $8/Hr. 9,600
22,400
44,800
89,600
Power at $0.025/Kwh 10,000
20,000
40,000
75,000
Maintenance Materials 2u0
300
500
1,000
Annual 0 & M Costs 19,800
42,700
85,300
165,600
Annual Capital Costs { x .0944) 22,500
32,600
42,700
56,200
Total Annual Cost $ 4 2,300
$ 75,300
$128,000
$221,800
Cost per ton of dry solids $11.60
$8. 30
$7. 00
$6.10
Assumption: 40 lbs/day/sf loading rate.
V-18
-------
air temperature, relative humidity, and wind velocity determine the effect-
iveness of air drying. It is important that wastewater sludge be well
digested for optimum drying. In well digested sludge, entrained gases
tend to float the sludge solids and leave a layer of relatively clear
liquid, which can readily drain through the sand. Typical design criteria
are:
Sludge Loading
Area Dry Solids
Type of Digested Sludge (sq ft/capita) (Ib/sq ft/yr)
Primary 1.0 27.5
Primary and standard trickling filter 1.6 22.0
Primary and activated 3.0 15.0
Chemically precipitated 2.0 22.0
Sandbeds can be enclosed by glass. Glass enclosures protect the
drying sludge from rain, control odors and insects, reduce the drying
periods during cold weather, and can improve the appearance of a waste
treatment plant. Experience has shown that only 67 to 75 percent of area
required for an open bed is needed for an enclosed bed. Good ventilation
is important to control humidity and optimize the evaporation rate. As
expected, evaporation occurs rapidly in warm, dry weather. Adaptation of
mechanical sludge removal equipment to enclosured beds is more difficult
than to open drying beds.
Mechanical removal of sludge from drying beds has been practiced for
many years at some large treatment plants, but now it is receiving more
attention as the need to minimize problems with labor costs. Mechanical
devices can remove sludges of 20 to 30 percent solids while cakes of 30
to 40 percent are generally required for hand removal. Small utility
tractors with front end loaders are often used for mechanical removal.
Drying times typically range from 4-12 weeks, depending upon the
weather. Especially adverse weather can result in drying times as long
as 6 months .
A major disadvantage in the larger plants likely to be involved in
regional systems is the space required. For a 10 mgd activated sludge
plant, about 11 acres of drying beds would be required for the primary
and WAS at a loading rate of 15 lbs/yr/sf. The space requirements plus
dependency on uncontrollable weather factors are severe restrictions on
the use of drying beds in large, regional plants. The limited cost data
available for large drying beds (adequate for 10-25 mgd activated
sludge plants) indicate total costs would be $70-$90/ton of dry solids.
V-19
-------
Vacuum Filtration
A vacuum filter basically" consists of a cylindrical drum (see Figure
6) which rotates partially submerged in a vat of sludge. The filter drum
is divided into compartments by partitions or seal strips. A vacuum is
applied between the drum deck and filter medium causing filtrate to be
extracted and filter cake to be retained on the medium during the pickup
and cake drying cycle. In the drum filter shown in Figure 6, the cake
of dewatered sludge is removed by a fixed scraper blade. There are alter-
native designs which use other methods for sludge removal.
The performance of vacuum filters may be measured by various criteria
such as the yield, the efficiency of solids removal and the cake charac-
ertistics. Each of these criteria is of importance, but one or the other
may be particularly significant in a given plant. Typical results are
shown in Table 7.
Yield is the most common measure of filter performance. The yield
expresses the filter output and is expressed in terms of pounds of dry
total solids in the cake discharged from the filter, per square foot of
effective filter area, per hour.
The second measure of filter performance is the efficiency of solids
removal. Basically, the vacuum filter is a device used for separating
solid matter from liquid, and the actual efficiency of the process is
the percentage of feed solids recovered in the filter cake. Solids re-
movals on vacuum filters range from about 85 percent for coarse mesh media
to 99 percent with close weave, long nap media. The re-cycled filtrate
solids impose a load on the plant treatment units, and should normally
be kept to a practical minimum. However, it may be necessary to reduce
the filter efficiency in order to deliver more filter output and thus
keep up with sludge production.
The filter cake quality is another measure of filter performance,
depending upon cake moisture and heat value. Cake solids content varies
from 20 to 40 percent by weight, depending upon the type of sludge handled
and the filter cycle time and submergence. Delivery of a very dry cake
does not necessarily indicate good filter performance. Cake moisture
should be adjusted to the method of final disposal, and it is inefficient
to dry the cake more than is required. When incineration is practiced,
a raw sludge cake having a fairly high moisture content can be burned
without auxiliary fuel because of the higher volatile content, while a
digested sludge cake will have to be dryer to burn successfully without
make-up heat. One approach to improving the filtration and incineration
characteristics of primary - WAS mixtures is to feed powdered coal as a
conditioning agent prior to the dewatering step^34^. It was found that
a coal dose of about 0.3 lbs/lb dry sludge solids produced a sludge cake
which permitted autogenous combustion with no effect on filter yield.
The effect of heat treatment prior to vacuum filtration on various
municipal sludges is to make all types dewaterable to approximately the
V- 20
-------
CLOTH CAULKING
STRIPS
DRUM
AUTOMATA *AL_vfE
FILTRATE PIPING
CAKE SCRAPER
AIR AND FILTRATE
LINE
X SLURRY AGITATOR
VAT
-==/ SLURRY FEED
AIR BLOW-BACK LINE
FlGURt 6 CutawtV v'ew a rotary drum vacuum filter.
-------
TABLE 7
TYPICAL RESULTS
VACUUM FILTRATION
SIudqe Type
Primary
Primary + FeCl ^
Primary +
Low Lime
Primary +
High Lime
Primary + WAS
Primary +
(WAS + FeCl3)
(Primary-+ FeCl.,)
+ WAS J
Design Assumptions
Thickened to 10% solids
polymer conditioned
85 mg/1 FeCl^ dose
Lime conditioning
Thickening to 2.5% solids
300 mg/1 lime dose
Polymer conditioned
Thickened to 15% solids
600 mg/1 lime dose
Polymer conditioned
Thickened to 15% solids
Thickened to 8% solids
Polymer conditioned
Thickened to 8% solids
FeCl3 & lime conditioned
Thickened primary sludge
to 2.5%
Flotation thickened WAS
to St
Dewater blended sludges
Percent
Soli ds
To VF
10
2.5
15
15
8
3.5
Typical
Loading
Rates,
(PAf/M
8-10
1.0-2.0
10
4-5
1.5
Percent
Solids
VF Cake
25-38
15.20
32-35
28-32
16-25
20
15-20
Waste Activated
Sludge (WAS)
WAS + FeCl,
Digested Primary
Digested Primary
+ WAS
Digested Primary
+ (WAS + FeCl3)
Tertiary Alum
Thickened to 5% solids
Polymer conditioned
Thickened to 5% solids
Lime + FeCl3 conditioned
Thickened to 8-10% solids
Polymer* conditioned
Thickened to 6-8% solids
Polymer conditioned
8-10
6-8
Thickened to 6-8% solids 6-8
FeCl^ + lime conditoned
Diatomaceous earth
precoat
0.6-0.8
2.5-3.5
1.5-2.0
7-8
3.5-6
2.5-3
0.4
15
15
25-38
14-22
16-18
15-20
V- 22
-------
(33)
same degree . Heat treatment provides a sludge that is readily de-
waterable from primary or secondary sludges. Raw primary sludges have
been dewatered at rates as high as 40 psf/hr and waste activated sludges
at 7 psf/hr. Mixtures of raw primary and secondary sludges subjected to
heat treatment should produce yields well over 10 psf/hr.
Illustrative costs based on an average loading rate of 4 psf/hr are
shown in Table 8. Typical electrical consumption is 40-60 kwh/ton of
solids.
Centrifugation
There are many types of centrifugal equipment available, for a variety
of specialized applications in industry'36). However, the solid bowl
centrifuge is the most widely used type for dewatering of sewage sludge.
The solid bowl-conveyor sludge dewatering centrifuge assembly (Figure 7)
consists of a rotating unit comprising a bowl and conveyor. The solid
cylindrical-conical bowl, or shell, is supported between two sets of
bearings and includes a conical section at one end to form a dewatering
beach or drainage deck over which the helical conveyor screw pushes the
sludge solids to outlet ports and then to a sludge cake discharge hopper.
Sludge slurry enters the rotating bowl through a stationary feed pipe
extending into the hollow shaft of the rotating screw conveyor and is
distributed through ports into a pool within the rotating bowl.
As the liquid sludge flows through the cylindrical section toward
the overflow devices, progressively finer solids are settled centrifugally
to the rotating bowl wall. The helical rotating conveyor pushes the
solids to the conical section where the solids are forced out of the
water, and free water drains from the solids back into the pool.
There are several variables which affect the performance of solid
bowl centrifuges. Bowl speed is one of the prime variables since centri-
fugal force speeds up the separation of solids from liquids. At any
given pool depth, an increase in bowl speed provides more gravity-settling
force, providing greater clarification. Typical G values for a solid
bowl machine for many years were about 3,000. In recent years, units
which operate at G=700 have been developed. These "low" speed units
provide comparable results at lower power consumption.
The introduction of polymers, has increased the range of materials
that can be dewatered satisfactorily by centrifuges. The degree of
solids recovery can be regulated over rather wide ranges depending on
the amount of coagulating chemical used. Wetter sludge cake usually
results from the use of flocculation aids because of the increased cap-
ture of fines.
Table 9 presents data on typical results with solid bowl centrifuga-
tion. Heat treated sludges will dewater to 35-45% solids with no polymer
required for 85% capture. Recovery of 92-99% of the solids from heat
treatment (primary) sludges have been reported(33) with polymer dosage
V-23
-------
iMH.V, 8
PSTIMATED COST.1" FOR PEWATEKINO HY VACUUM I' H.lhK
I torn
Construction Cost
Engr. , Legal, A dm. , & Interest
During Construction (19*)
Total Capital Cost
Labor, Operations at $1O/Hr.
Labor, Maintenance at $10/Hr.
Maintenance Materials, Chemical
Maintenance Materials;, Other
Annual 0 & M Costs
Annual Capital Costr, ( x .094-1)
Total Annual Cost
Cost per ton of dry solids;
Tons E< ¦ r O.iy nl lay O'olids
JO 2 r> r>()
100
$270,000 $<170, 000 $ 7 B0 , 000 $1,400,000
rj ) , 300 09 , VM) ] 48 , 200
266,000
$321,300 $'359,300 $929,200 $1,666,000
4 7,000 00,000 1.60,000 270,000
0,000 17,000 28,000 51,000
46,000 100,000 L 80,000 310,000
_ 24,000 4 r> , () 00 72,000 120,000
$125,000 $ 2 S 2,000 $440,000 $ 751,000
_J0,300 5.\ 80:) 37,600 157, 300
$155,300 $304,800 $477,600 $ 908,300
$42.60 $3 3.40 $26.20 $24.90
Assumptions: Primary + WAS
4 lbs/hr/sf loading rate
V-24
-------
, COVER
DIFFERENTIAL SPEED
GEAR BOX
ROTATING BOWL
MAIN DRIVE SHEAVE
CENTRATE
DISCHARGE
BEARING
ROTATING
CONVEYOR
FEED PIPES
(SLUDGE AND
CHEMICAL)
BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
+
FIGURE ? Continuous countercurrent solid bowl conveyor discharge centrifuge.
-------
TABLE 9
TYPICAL SOLID BOWL CENTRIFUGE PERFORMANCE
Wastewater Sludge Type
Raw or digested primary
Raw or digested primary, plus trickling
filter humus
Raw or digested primary, plus activated
sludge
Activated Sludge
Oxygen Activated Sludges
High-Lime Sludges
Lime Classification
Sludge Cake Characteristics
Solids
(%)
28-35
20-30
15-30
8-9
8-10
50-55
40
Solids
Recovery
(%)
70-90
80-95
60-75
80-95
50-65
80-85
80-8 5
90
70
Polymer
Addition
no
yes(5-15 lbs/ton)
no
yes (5-20 lbs/ton)
no
5-10 lbs/ton
3-5 lbs/ton
no
no
V-26
-------
of 2-5 lbs per ton of dry solids. Dewatering of heat treated mixtures
of activated sludge and raw primary sludge have produced cake solids of
40% at 95% recovery without chemicals. Dewatering of heat treated acti-
vated sludges alone has achieved 35% cake solids at 95% recovery without
chemicals. The use of 4 lbs per ton of polymers in this latter case^33)
enabled a 50% increase in centrifuge capacity while producing cake solids
of 28%.
In addition to dewatering sludges, centrifuges have been used to
separate impurities ("classify") from the lime sludges resulting from
some phosphorus removal processes to enable efficient recovery and reuse
of the lime.
Typical costs for centrifugation are presented in Table 10 based
upon 4% influent solids concentration. Electrical requirements are a
function of the bowl speed but are typically 200-400 kwh/ton of solids.
Centrifuges have the advantages of being a totally enclosed process
and requiring less space than vacuum filters. They have the disadvantage
of requiring more maintenance and more highly skilled maintenance.
Pressure Filtration
During the last five years, there has been a substantial increase
in use of pressure filtration systems in U.S. wastewater plants. Improve-
ments in the equipment involved coupled with increasing quantities of
difficult-to-dewater sludges account for the increase.
The filter press is a batch device, which has been used in industry
and in European wastewater plants for many years to process difficult-to-
dewater sludges. There are several variations in mechanical design and
operating pressures. For purposes of illustrating the concept, a vertical
plate filter press system will be described. Such a press consists of
vertical plates which are held rigidly in a frame and which are pressed
together between a fixed and moving end as illustrated in Figure 8. On
the face of each individual plate is mounted a filter cloth. The sludge
is fed into the press and passes through the cloth, while the solids are
retained and form a cake on the surface of the cloth. Sludge feeding
occurs at pressures up to 225 psi and is stopped when the cavities or
chambers between the trays are completely filled. Drainage ports are
provided at the bottom of each press chamber. The filtrate is collected
in these, taken to the end of the press, and discharged to a common drain.
At the commencement of a processing cycle, the drainage from a large press
can be in the order of 2,000 to 3,000 gallons per hour. This rate falls
rapidly to about 500 gallons per hour as the cake begins formation and
when the cake completely fills the chamber, the rate is virtually nothing.
The dewatering step is completed when the filtrate is near zero. At this
point the pump feeding sludge to the press is stopped and any back pressure
in the piping is released through a bypass valve. The electrical closing
gear is then operated to open the press. The individual plates are next
moved in turn over the gap between the plates and the moving end. This
allows the filter cakes to fall out. The plate moving step can be either
V-27
-------
TAHLK 10
KSTI MAT'1
•:» copt. khr
DKWATKRITir; ItY
ct:ntrt fi;
;k
Tons T
or Day of
iif
v Solids
I tein
K)
2 r>
50
100
Construction Cost
$360,000
$5B0,000
86 0,000
$1 ,400,000
Erwr. , Loqal, Adm.,
f. Int.nros 1
16 3,400
266,000
During Construction (19%)
68 ,400
110,200
Total Capital Cost
$428,400
$690,200
$1,
02 3,400
$1,666,000
Labor, Operation:; at
5 1.0/Hr .
25,600
50,400
96,000
184,000
Labor, Maintenance a
t $ 10 /H r .
6, 200
13,600
22,400
45,600
Choni i c,\ 1
o
o
o
80,000
160,000
270,000
Maintenance; Materia I
" ( •!
27,000
52. ,000
8 5,000
140,000
Annual 0 & M Costs
94,800
196,000
36 3,400
6 39,600
Annual Capital Costs
( x .0044)
40,400
65,200
96,600
157,300
Total Annual Cost
$135,200
$261,2.00
$
46 0,000
$ 796,900
Cost per ton of dry
solids
$37.00
$28.60
$25.20
$21.80
Assumed: 4 % solids in influent
V-28
-------
ELECTRIC
CLOSING GEAR
FIXED END
TRAVELLING END
OPERATING HANDLE
otoldolololnk^olotora
FIGURE 8
Side view of u filter press [47].
-------
manual or automatic. When all the plates have been moved and the cakes
released, the complete pack of plates is then pushed back by the moving
end and closed by the electrical closing gear. The valve to the press
is then opened, the sludge feed pump started, and the next dewatering
cycle commences.
Filter presses are normally installed well above floor level, so
that the cakes can drop onto conveyors or trailers positioned underneath
the press. The pressures which may be applied to a sludge for removal
of water by the filter presses now available range from 5,000 to 20,000
times the force of gravity. In comparison, a solid bowl centrifuge pro-
vides forces of 700-3,500 g and a vacuum filter, 1,000 g. As a result
of these greater pressures, filter presses may provide higher cake solids
concentrations (30-50% solids) at reduced chemical dosages. In some
cases, ash from a downstream incinerator is recycled as a sludge condi-
tioner.
Table 11 presents typical results from pressure filtration. As
readily apparent, the process produces a drier cake than either vacuum
filtration or centrifugation. Table 12 presents illustrative costs for
the assumed conditions as defined in the Table. Although the dewatering
costs are higher than for vacuum filters or centrifuges under comparable
conditions, the drier cake produced may result in savings in the down-
stream process which are more than adequate to offset the higher costs.
Electrical consumption is primarily a function of influent solids concen-
tration and ranges from 100-200 kwh/ton of solids at 4% influent solids
to 50-100 kwh/ton at 8% influent solids.
Drying Lagoons
Lagoon drying is a low cost, simple system for sludge dewatering
that has been commonly used in the United States. Drying lagoons are
similar to sandbeds in that the sludge is periodically removed and the
lagoon refilled. Lagoons have seldom been used where the sludge is never
removed, because such systems are limited in application to areas where
large quantities of cheap land are available. Sludge is stabilized to
reduce odor problems prior to dewatering in a drying lagoon. Odor problems
can be greater than with sandbeds, because sludge in a lagoon retains
more water for a longer period than does sludge on a conventional sand
drying bed.
Other factors affecting design include consideration of groundwater
protection and access control. Major design factors include climate,
subsoil permeability, lagoon depth, loading rates, and sludge character-
istics.
(4)
Solids loading rates suggested for drying lagoons are 2.2 to 2.4
lb/yr/cu ft of lagoon capacity. Other recommendations range from 1 sq
ft/capita for primary digested sludges in an arid climate to as high as
3 to 4 sq ft/capita for activated sludge plants where the annual rainfall
is 36 inches. A dike height of about 2 feet with the depth of sludge
after decanting of 15 inches has been used. Sludge depths of 2.5 to 4
V- 30
-------
TABLE 11
TYPICAL RESULTS
PRESSURE FILTRATION
Sludge Type
Prima ry
Primary +
Primary +¦ 2 stage
high lime
Primary + /JAS
Primary + (WAS + FeCl^)
(Primary + FeCI^) + WAS
WAS
WAS + FeCl3
Digested Primary
Digested Primary + WAS
Digested Primary +
(WAS + FeCl3)
Tertiary Alum
Tertiary Low Lime
Conditioning
5% FeCK,
100% Ash
10% Lime
None
10% Lime
5% FeCl-j, 10% Lime
150% Ash
5% FeCl^, 10% Lime
10% Lime
7.5% FeCl3, 15% Lime
250% Ash
5% FeCl3# 10% Lime
6% FeC13> 30% Lime
5% 10% Lime
100% Ash
5% FeCl3> 10% Lime
10% Lime
None
Percent
Sol ids
To Pressure Filter
5
4*
7.5
8*
8*
3.5*
5*
5*
8
6-8*
6-8*
4*
8*
Typical Cycle
Length
2 hours
1.5
4
1.5
2.5
2.0
3
4
2.5
2.0
3.5
2
1.5
3
6
1.5
Percent
Solids
Fi1ter Cake
45
50
40
50
45
50
45
40
45
59
45
40
45
50
40
35
55
*Thickening used to achieve this solids concentration
-------
oao:,*: 12
i'ost:; ; SO 100
Corn-it ru'.:t ion >00 J , 08 5,000 2,170,';00 3,498, GOO
ICnqr., Logal, Aflm. , u;.d
Tr.t crest Dnrinq Con:--t . (191) 1 %, 000 412, 300 664,700
Total Capita] Cost $9 76,'>00 1 ,291,200 2,582,300 4,163,300
Labor, 0 f, M at Slo/Iir. 76,000 120,000 180,000 310;000
Chemical;; 46,000 100,000 100,000 310,000
Electric Power 3 $0. 025/Kwli .3,500 6,500 10,500 16,800
Siileiitreair; Treatment, $R.ho/T ^2,00<< HO, 000 160,000 320,000
Maintenance Materi al r, at 3'J> .jf
Capital Costs/Yr. .22j_±)0 _'*;S? 77,500 124,900
Annual 0 & M Costa; 186,800 345,200 608,000 1,081,700
Annual Capital Cost.*-; (x .00-14; 121 ,900 24 3,800 393,000
Total Annua 1 Cost ^2>9,090 Slu7,]00 $851,800 $1,474,700
Cog t Per Ton of Dr.''/ Solid:-,
$76.40 $51.20 $46.70 $40.40
Assumed: 5% FeCl3, 10% Lime For Conditioning
2.4 hour cycle time
Cake solids = 40%
4% influent solids
V-32
-------
feet may be used in warmer climates where longer drying periods are
possible.
Sludge will generally not dewater in any reasonable period of time
to the point that it can be lifted by a fork except in an extremely hot,
arid climate. If sludge is placed in depths of 15 inches or less, it
may be removed with a front-end loader in 3 to 5 months. When sludge is
to be used for soil conditioning, it may be desirable to stockpile it
for added drying before use. One proposed approach utilizes a 3-year
cycle in which the lagoon is loaded for 1 year, dries for 18 months, is
cleaned, and allowed to rest for 6 months. Definitive data on lagoon
drying are scarce. Sludge may be dewatered from 5 percent solids to
40 to 45 percent solids in 2 to 3 years using sludge depths of 2 to 4
feet.
(32)
Limited cost data on large lagoon systems indicate costs in the
range of $10-20/ton exclusive of land costs.
INCINERATION
An incinerator is usually part of a sludge treatment system which
includes sludge thickening, a macerating or disintegrating system, a
dewatering device (such as a vacuum filter, centrifuge, or filter press),
an incinerator feed system, air pollution control devices, ash handling
facilities, and the related automatic controls. Important considerations
in evaluating incineration methods include the composition of the sludge
feed and the amount of auxiliary fuel reguired. Air pollution constraints
and resultant equipment and treatment requirements as well as ash disposal
are also important.
Of major interest from the standpoint of sludge incineration is the
heat value of the sludge which is summarized in Table 13. The combustible
portion of sewage sludge has a BTU content approximating that of lignite
coal.
Incineration is a two-step process involving drying and combustion.
In addition to fuel and air; time, temperature, and turbulence are necessary
for a complete reaction. The drying step should not be confused with pre-
liminary dewatering; dewatering is usually by mechanical means and precedes
the incineration process in most systems. When a sludge with a moisture
content of about 75 percent is delivered to the incinerators, (3 pounds
of water for each pound of dry solids), the heat required to evaporate
the water nearly balances the available heat from combustion of the dry
solids.
Drying and combustion may be done in separate units or successively
in the same unit. Manufacturers have developed diversified types of
equipment. The two major incineration systems employed in the United
States are the multiple hearth furnace and the fluidized bed incinerator
(discussed later in this section). The drying and combustion process
consists of the following phases: (a) raising the temperature of the
feed sludge to 212 F, (b) evaporating water from the sludge, (c) increasing
V-33
-------
TAP-LE 13
HIGH MEAT OF COMBUSTION OF $1 UDGES* (TOTAl DRY SOLIDS BASIS)
From Reference (t?)
Material
Combustibles
(%)
Ash
(%1
Average
RTU/Pound*
Grease and scum
88.5
11.5
16,750
Raw sewage solids
74.0
26.0
10.285
Fine screenings
86.4
13.6
8,990
Ground garbage
84.8
15.2
8,245
Digested sewage solids
and ground garbage
49.6
50.4
8,020
Digested sludge
59.6
40.4
5,290
Grit,
33.2
69.8
4,000
~Moisture free basis
V-2
-------
the water vapor and air temperature of the gas, and (d) increasing the
temperature of the dried sludge volatiles to the ignition point. Prac-
tical operation of an incinerator reguires that air in excess of theoreti-
cal reguirements be supplied for complete combustion of the fuel. The
introduction of excess air has the effect of reducing the burning tempera-
ture and increasing the heat losses from the furnace.
Heat is emitted by the burning of sludge in a furnace. Some of this
heat is absorbed by the furnace and lost by radiation. A large portion
of the emitted heat is lost with the stack gases, while a small portion
is lost with the ash. The heat lost in the stack gas is available for
recovery and reuse for purposes such as heating the incoming sludge and
air.
There are a number of variables which influence the amount of fuel
required and the resulting cost for sludge incineration. Principal
variables are the moisture and volatile solids content of the sludge.
Their effect on the amount of fuel required for incineration is shown by
Figure 9. Temperatures of 1350-1400 F are generally accepted as necessary
to insure deodorization of the stack gases of a conventional incinerator.
To insure complete thermal oxidation, it has been found necessary to
maintain 50 to 100 percent excess air over the stoichiometric amount of
air required in the combustion zone. This excess air is undesirable
because it pirates 12 to 24 percent of the input BTU's for heating of
the excess air. If excess air is not supplied, it is difficult to main-
tain the minimum deodorizing temperature. Therefore, a closely controlled
minimum excess air flow is desirable for maximum thermal economy. The
amount of excess air required varies with varies with the type of incin-
eration equipment, the nature of the sludge to be incinerated, and the
disposition of the stack gases. A closely controlled minimum excess air
flow is desirable for maximum thermal economy. The impact of use of
excess air on fuel required for sludge incineration is shown in Figure 10.
The stack gases leaving the incinerator represent a potential source
of energy. By passing the gases through a heat exchanger, it is possible
to extract heat for use in preheating the incoming furnace air, in sludge
conditioning by heat treatment, or for other uses in the plant. Electri-
city can be generated by use of a boiler-generator system fueled by the
stack gas heat. Figure 11 presents the potential for net recovery of heat
by heat exchange equipment installed on a sludge incinerator. This
analysis of heat recovered is independent of the type of incinerator used
for combustion of sludge because only the combustion products or flue
gases are considered. The potential for energy recovery from stack gases
is significant. A detailed analysisof a 30 mgd activated sludge
plant showed that all of the electrical needs of the plant (about 8,300,000
kwh/yr) could meet by generating electricity form the incinerator stack
gases (1400°F initial flue gas temperature) from incineration of a 16%
solids primary + WAS sludge. Whether or not this would be a cost-effective
source of electrical energy would be dependent upon local conditions.
A heat treatment-incineration system has been proposed (and is being
installed in 3 U.S. plants) which eliminates the need for any auxiliary
V-35
-------
q 1,600
1,400
UJ
LLI
LLI
o
I—
1,200
1,000
zi
o
o
H
Q.
3
CO
O
o
c/j
<
O
<
ac
3
h-
<
800
600
400
200
Sludge heat content - 10,000 Btu/lb
volatile solids
4'/
75 76 77 78 79 80 81
MOISTURE CONTENT OF FEED (%)
82
83
FIGURE 9. The effects of sludge moisture and volatile solids content on gas consumption.
V-36
-------
10,0
o.o •— L
0 20 40 60 80 100
EXCESS AIR, percent
Assumptions:
Solids: 30%
Exhaust Temp: 1500*F
Vola tiles: 70%
IMPACT OF EXCESS AIR OH THE AMOUNT
OF AUXILIARY FUEL FOR SLUDGE INCINERATION
V-37
FIGURE 10
-------
4500
primary* was
3750
3000
2250
1500
PRIMARY
750
1000
500
1500
INITIAL FLUE GAS TEMPERATURE, *F
Assumptions:
Final Stack Temp s 500° F
50% excess air
(To convert Btu to kwh: 1 kwh = 10,500 Btu)
POTENTIAL HEAT RECOVERY FROM INCINERATION OF SLUDGE
FIGURE 11
V- 38
-------
fuels for the heat conditioning-incineration system . The heat treat-
ment conditioning enables an autogenous sludge cake to be achieved. As
can be seen from Figure 12, heat production exceeds that required for
combustion as typical primary + WAS concentrations exceed 25% solids (69%
volatile). Solids concentrations of 30-40% are often achieved following
heat treatment. Thus, there may be sufficient heat available in the stack
gases to provide the heat needed for heat treatment which results in a
self-sustaining sludge incineration system.
One approach to supplying the supplementary fuel needed for sludge
incineration that has been suggested is to use solid wastes as fuel. The
amount of solid waste required to sustain combustion of sludges is shown
in Figure 13based on 25% moisture in the solid waste and 4,750 BTU/lb
of solid waste. Sludge with 5% solids and 70% volatile solids would
require 28% refuse to sustain combustion.
(85)
The Kansas City metropolitan area is considering incineration
of 750-1,000 tons/day of shredded, air classified refuse and dried
sludge (85% solids) for generation of electricity. Ferrous metals and
possibly aluminum would be recovered. Use of a suspension-fired water
wall incinerator and sale of the electricity provides a potential economic
savings of about 28% over separate refuse disposal in a landfill.
Air pollution concerns must be addressed when considering incinera-
tion. Detailed data have been presented for several municipal sludge
installations^'"^ . The major categories of concern are: particulates,
metals, gaseous pollutants, and organic compounds.
National air pollution standards for discharges from municipal sludge
incinerators have been promulgated which limit emissions of particulates
(including visible emissions) from incinerators used to burn wastewater
sludge as follows'
1. No more than 0.65 g/kg dry sludge input (1.30 lb/ton dry sludge
input).
2. Less than 20 percent opacity.
Available data indicate that on the average, uncontrolled multiple
hearth incinerator gases contain about 0.6 grain of particulate per
standard cubic foot of dry gas . Uncontrolled fluid bed reactor gases
contain about 1.0 grain of particulate per standard cubic foot^D . For
average municipal wastewater sludge, this corresponds to about 33 pounds
of particulates per ton of sludge burned in a multiple hearth, and about
45 pounds of particulates per ton of sludge burned in a fluid bed incinera-
tor. Particulate collection efficiencies of 96 to 97 percent are required
to meet the standard, based on the above uncontrolled emission rate. Ven-
turi scrubbers have the demonstrated capability to meet the particulate
discharge requirement without a significant increase in electrical power
requirements
Most metals present in municipal sludges are converted to oxides
which appear in the particulates removed by the scrubber or in the ash.
V-39
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0
20
40
60
SLUDGE SOLIDS, percent
HEAT REQUIRED TO SUSTAIN
COMBUSTION OF SLUDGE
FIGURE 12
V-40
-------
Assumptions:
Heat value of sludge:
Heat value of solid waste:
Moisture in solid waste:
Heat required to evaporate
water in furnace:
10,000 Btu/lb VS
4,750 Btu/lb
25 percent
2,100 Btu/lb water
30% VS
100
80 60
SLUDGE MOISTURE CONTENT, PERCENT
COMBUSTION OF SLUDGE AND SOLID WASTE
FIGURE 13
V-41
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Lead and mercury are the only two metals which vaporize to an extent that
the stack gas concentrations would be increased. However, it has been
found that less than 15% of the lead and 2% of the mercury appear in the
flue gas at the Palo Alto, Calif, sludge incinerator. EPA has set
a standard of 3200 gms/day of mercury for discharge from a sewage sludge
incinerator. The Palo Alto incinerator discharge was only 6 gms/day.
The per capita lead discharge was equivalent to the lead discharged from
driving an auto using unleaded gas a distance of 200 ft./day. Metal dis-
charges should not present a limitation as properly designed and operated
municipal systems have met all air pollution standards for metals.
Gaseous pollutants could be released by sludge incineration are
hydrogen chloride, sulfur dioxide, oxides of nitrogen, and carbon monoxide.
Carbon monoxide is no threat if the incinerator is properly designed and
operated. Hydrogen chloride, which would be generated by decomposition
of certain plastics, is not a significant problem at concentrations currently
observed. Consideration of the possibility of SC>2 and N0X pollution is
aided by examination of the sulfur and nitrogen content of sludges. Sul-
fur content is relatively low in most sludges. In addition, much of this
sulfur is in the form of sulfate, which originated in the wastewater.
Sulfur dioxide is not expected to be a serious problem. Sludge typically
has a high nitrogen content from proteinaceous compounds and ammonium ion.
Limited data are available for predicting whether a high proportion of
these materials will be converted to oxides of nitrogen from sludge incin-
eration should be less than 100 ppm from a properly operated incinerator
and were observed to be less than 10 ppm from one facility tested by
EPA^40^. Considering this low concentration, the production of oxides
of nitrogen will probably not limit the use of incineration for disposing
of sludge in most cases. The amount of NOx per capita generated by a
sludge incinerator has been equated to that generated by driving an auto
less than 0.1 mile under the 1975 Federal N0X Standards^43'.
Toxic substances could be discharged from the organic substances -
such as pesticides and PCB's - in the sludge. However, tests 1^9) ^ave
shown that total destruction of PCB's was possible when oxidized in com-
bination with sewage sludge and with exhaust gas temperatures of 1100°F.
Ninety-five percent destruction of PCB's was achieved in a multiple hearth
furnace with no afterburning at exhaust temperatures of 700°F.
(42)
The EPA Sewage Sludge Incineration Task Force concluded that it
has been adequately demonstrated that existing well-designed and operated
municipal wastewater sludge incinerators equipped with an adequate scrub-
bing system are capable of meeting the most stringent particulate emission
control regulation existing in any state or local control agency. This
observation coupled with the fact that the newly promulgated federal
standards are based on demonstrated performance of an operating facility
indicates that use of proper emission controls and proper operation of
the incineration system will enable a facility to meet all existing air
pollution regulations.
The volume reduction by sludge incineration is over 90% when compared
to the volume of dewatered sludge. The ash from the incineration process
V-42
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is free of pesticides, viruses and pathogens. The metals in the ash are
approximately at the same ratio as in the raw sludge; however, the
metals are now in the less soluble oxide form. The ash can be readily
transported in the dry state to appropriate landfill sites.
Multiple Hearth Incineration
The multiple hearth furnace is the most widely used wastewater sludge
incinerator in the United States today, because it is simple, durable,
and has the flexibility of burning a wide variety of materials even with
fluctuations in the feed rate. A typical multiple hearth furnace is
shown in Figure 14 and consists of a circular steel shell surrounding a
number of solid refractory hearths and a central rotating shaft to which
rabble arms are attached. The operating capacity of these furnaces is
related to the total area of the enclosed hearths. They are designed
with diameters ranging from 54 inches to 21 ft 6 inches and from four to
eleven hearths. Capacities of multiple hearth furnaces vary from 200 to
8,000 lb/hr of dry sludge with operating temperatures as high as 1,700 F.
The dewatered sludge enters at the top through a flapgate and proceeds
downward through the furnace from hearth to hearth through the rotary
action of the rabble arms.
The estimated costs for incineration of 20% solids, (primary + WAS)
are shown in Table 14. Fuel consumption is 8-10 x 106 BTU/TON of dry
solids and electrical consumption is 50-90 kwh/ton of dry solids. Fuel
consumption can be reduced by feeding a drier cake. The cost of achieving
the drier cake must, of course, be balanced against the savings in incin-
eration cost.
Fluidized Bed Incineration
The first fluidized bed wastewater sludge incinerator was installed
in 1962, and there are now several units operating. They range in size
from 220 to 5,000 lb/hr dry solids. A typical section of a fluid bed
reactor used for combustion of wastewater sludges is shown in Figure 15.
The fluidized bed incinerator is a vertical cylindrical vessel with a
grid in the lower section to support a sandbed. Dewatered sludge is
injected above the grid and combustion air flows upward at a pressure of
3.5 to 5.0 psig and fluidizes the mixture of hot sand and sludge. Supple-
mental fuel can be supplied by burners above or below the grid. In
essence, the reactor is a single chamber unit where both moisture evapora-
tion and combustion occur at 1,400 to 1^500°F in the sandbed. All the
combustion gases pass through the 1,500 F combustion zone with residence
times of several seconds. Ash is carried out the top with combustion
exhaust and is removed by air pollution control devices.
The quantities of excess air are maintained at 20 to 25 percent to
minimize its effect on fuel costs as was illustrated by Figure 10. The
heat reservoir provided by the sandbed enables reduced start-up times
when the unit is shut down for relatively short periods (overnight).
As an example, a unit can be operated 4 to 8 hours a day with little
reheating when restarting, because the sandbed serves as a heat reservoir.
V-43
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COOLING AIR DISCHARGE
FLOATING DAMPED
SLUDGE INLET
FLUE GASES OUT
RABBLE ARM
AT EACH HEARTH
DRYING ZONE
COMBUSTION
AIR RETURN
COMBUSTION ZONE
COOLING ZONE
RABBLE ARM
DRIVE
ASH DISCHARGE
COOLING AIR FAN
FIGURE 14 Cross section of a typical multiple hearth incinerator.
V-44
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TABLE 14
ESTIMATED COSTS OF SLUDGE INCINERATION
Tons Per Day of Dry Solids
Item
10
25
50
100
Construction Cost
$1
,800,000
$2
,200,000
$3,
000,000
$5,000,000
Engr., Legal, Adm., & Interest
During Construction (19%)
342,000
418,000
570,000
950,000
Total Capital Cost
$2
,142,000
$2
,618,000
$3,
570,000
$5,950,000
Labor, At $10/Hr.
40,000
55,000
95,000
140,000
Fuel @ $ 3/M Btu
90,000
210,000
420,000
840,000
Electric Power @ $0.025/Kwh
8,000
16,500
27,500
47,500
Maintenance Materials
8,000
15,000
25,000
42,000
Annual O & M Costs
$
146,000
$
296,500
$
567,500
$1,069,500
Annual Capital Costs ( x .0944)
202,200
247,100
337,000
562,000
Total Annual Cost
$
348,200
$
543,600
$
904,500
$1,631,500
Cost Per Ton of Dry Solids
$95.39
$59.57
$49.56
$44.70
Assumption:
20% solids, Primary + WAS
Combustion Temperature = 1400°F
Furnace Operated 70% of the time (Start-up fuel included)
Does not include cost of dewatering prior to incineration
V-45
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SIGHT GLAS:
EXHAUST ~
PREHEAT BURNER
SAND FEED
FLUIDIZED
SAND v
THERMOCOUPLE
PRESSURE
TAP
SLUDGE INLET
ACCESS
DOORS"
FLUIDIZING
AIR INLET
FIGURE 15 Cross section of a fluid bod reactor.
V-46
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Exhaust gases are usually scrubbed with treatment plant effluent
and ash solids are separated from the liquid in a hydrocyclone, with
the liquid stream returned to the head of the plant.
Actual field cost data on fluidized bed systems for municipal
sludges are limited. They are often competitive with multiple hearth
systems on capital costs but typically have somewhat higher 0 & M costs.
Electrical requirements are substantially higher than multiple hearth
furnaces.
Wet Air Oxidation
The heat treatment, sludge conditioning system illustrated in Figure
2 can be used for sludge reduction of oxidation by operation at higher
temperatures (350-400 F) and higher pressures (1200 psig). The wet air
oxidation (WAO) process is based on the fact that any substance capable
of burning can be oxidized in the presence of liquid water at temperatures
between 250 F and 700 F. Wet air oxidation does not require preliminary
dewatering or drying as required by conventional combustion processes.
However, the oxidized ash must be separated from the water by vacuum
filtration, centrifugation, or some other solids separation technique.
Air pollution is minimized because the oxidation takes place in water at
low temperatures and no flyash, dust, sulfur dioxide or nitrogen oxides
are formed.
The problems noted earlier in the heat treatment discussion related
to recycle of high-strength liquors to the wastewater treatment plant,
the presence of refractory materials, high maintenance, and odor control
also exist for the WAO application. The high pressure-temperature
system also introduces some significant safety concerns. The cost of
the system for sludge reduction is usually higher than competitive reduc-
tion systems(45). Use treatment for sludge conditioning is more
widespread than use of WAO as a reduction process.
Lime Recalcining
Lime if often used as a coagulant either as a tertiary step or ahead
of the primary clarifier in either a biological or a physical-chemical
plant for removal of phosphorus form wastewaters. There is, of course,
considerable experience around the world with the successful recalcining
and reuse of lime used in water treatment plants and these techniques
may also be used to recalcine and reuse lime in wastewater applications.
The process of recalcining consists of heating the dewatered calcium-
containing sludge to about 1#850°F which drives off water and carbon
dioxide leaving only the calcium oxide (or quicklime). Either multiple
hearth or fluidized bed furnaces may be used for recalcining. Recovery
and reuse of the lime reduces the amount of chemical sludge requiring
disposal by a factor of about 20. The significant savings in sludge dis-
posal costs may offset enough of the costs of lime recalcining costs to
make the economics attractive. The economic feasibility of lime recalcining
must be carefully evaluated for each locale, however. In cases where there
V-47
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are acceptable sludge landfill sites, it has proven to be cost effective
to dewater the lime sludges and bury them rather than recover them by
recalcining.
Table 15 presents illustrative recalcining costs. Costs do not
include dewatering since dewatering costs would be common with disposal
alternatives.
Fuel costs, as can be seen from Table 15, have a major impact on
recalcining costs and can indeed be the determining factor in recalcining
feasibility. In some parts of the country, fuel oil costs exceed the
$3/MBTU used in Table 15, while in others, natural gas may be available
at $1.50/MBTU. The impact of the $1.50/MBTU fuel costs is illustrated
by the last line in Table 15. Makeup lime costs have increased sharply
as a result of recent fuel cost increases. A cost of $51.50/ton of lime
was quoted for delivery in the Los Angeles area on December 27, 1976.
As indicated in Table 15, the cost of recalcined lime approaches that of
fresh lime at 100 tons/day of chemical sludge.
For a situation where there are 25 tons/day (dry solids) of chemical
sludges generated, the following calculations illustrate some consider-
ations in evaluating the break-even point for costs of recalcining:
Daily Costs With Recalcining: at $3/MBTU at $1.50/MBTU
Incineration Costs, 25 Tons/Day $1,457.50 $1,169.84
Make Up Lime, 3.75 Tons/Day @ $50/Ton 187.50 187.50
$1,645.00 $1,357.34
Daily Costs Without Recalcining:
Lime, 18.8 Tons/Day @ $50/Ton - $ 940.00 $ 940.00
Break Even Costs for Sludge Disposal $ 705.00 $ 417.34
t 25T/DAY = $28.20/TON $16.70/TON
In a real-life case, there may be other elements affected by the
recalcining decision. For example: dewatering costs may differ; other
chemical feed costs (i.e., polymers) may differ, etc. The cost effects
on these other elements would, of course, be included in an analysis
of recalcining economic feasibility.
For the illustrative example, if the 25 tons/day (dry basis) of 50%
solids chemical sludge can be disposed of for $28.20/TON or less, economics
would not favor recalcining at $3/MBTU. At $1.50/MBTU, the breakeven
point is $16.70/TON for disposal of the dewatered sludge.
A detailed discussion of lime recalcining systems and costs is avail-
able <46>.
Pyrolysis
There is a recent surge of interest in pyrolysis as a means of disposing
V-48
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TABLE 15
ESTIMATED COSTS OF LIME RECALCINING
Tons Per Day of Dry Solids
Item
10
25
50
100
Construction Cost
$1
,400,000
$2
,000,000
$2
,800,000
$4,000,000
Engr., Legal, Adm. , &
Interest During Constr. (19%)
266,000
380,000
532,000
760,000
Total Capital Cost
$1
,666,000
$2
,380,000
$3
,332,060
$4,760,000
Labor @ $10/Hr
40,000
70,000
120,000
200,000
Fuel @ $3/MBTU
95,000
210,000
450,000
810,000
Power @ $0.025/kwh
7,500
15,000
25,000
50,000
Maintenance Materials
8,000
12,000
20,000
30,000
Annual 0 & M Costs
$
150,500
$
307,000
$
615,000
$1,090,000
Annual Capital Costs (x .0944)
157,000
225,000
315,000
449,000
Total Annual Cost
$
307,500
$
532,000
$
930,000
$1,539,000
Cost Per Ton of Dry Solids
$84.24
$58.30
$50.95
$42.16
Cost Per Ton of Recovered CaO
$135.00
$93.28
$81.53
$67.46
Cost Per Ton of Recovered CaO of
Fuel = $1•50/MBTU
$113.98
$74.87
$61.80
$49.70
Assumptions: Coagulation of Secondary Effluent
7 psf/hr (wet) loading rate
20% downtime
50% solids in furnace feed
Multiple Hearth Furnace
Lime Dosage = 400 mg/1 as CaO
Lime Recovery = 80%
Does not include cost of dewatering prior to recalcining
V-49
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of sewage sludge. This has come about as a result of the apparent need
for new and improved processes and equipment in the practice of sludge
disposal and the possibility that pyrolysis may offer an alternative to
incineration which may be lower in cost, use less fuel, provide improved
air pollution control, and afford greater heat recovery, under certain
conditions.
Pyrolysis is a process in which organic materia] is decomposed at
high temperature in an oxygen-deficient environment. The action, causing
an irreversible chemical change, produces three types of products: gas,
oil and char (solid residue). Water vapor is also produced, usually in
relatively large amounts depending on the initial moisture content of the
materials being pyrolysed. Residence time, temperature and pressure in
the reactor are controlled to produce various combinations and composi-
tions of the products. Two general types of pyrolysis processes may be
used. The first, true pyrolysis, involves applying all required heat
external to the reaction chamber. The other, sometimes called partial
combustion and gasification, involves the addition of small amount of
air or oxygen directly into the reactor. The oxygen sustains combustion
of a portion of the reactor contents which in turn produces the heat
required to dry and pyrolyse the remainder of the contents.
Pyrolysis of municipal refuse and of sewage sludge has been con^47 5Q>
sidered as a means for ultimate disposal of wastes for several years
The results of various studies and pilot programs indicate that if the
moisture content of a sludge is below 70 to 75 percent, enough heat can
be generated by combustion of the oil and gases produced from the pyrolysis
of sludge for the process to be thermally sustaining. Pyrolysis of
municipal refuse, and combinations of refuse and wastewater sludges^ijM
provide energy in excess of that required in the pyrolytic process '
Laboratory, pilot and demonstration systems for pyrolysis of waste-
water sludges have been tested but no full-scale systems are in operation.
Therefore, data presented must be considered preliminary. Pryolysis
systems are in the developmental stages and additional information will
become available as research and development work and the operation of
full-scale plants progresses.
The BSP Division of Envirotech Corporation and others have conducted
research and development work on using multiple hearth furnaces, similar
in design to conventional sludge incinerators, for pyrolysis of waste-
water sludges mixed with municipal solid wastes. A 100 ton/day unit is
being installed at Concord, California. Shredded and classified
solid wastes and dewatered sludge are fed to the furnace either in a
mixture or separately with the wetter sludge fed higher in the furnace.
Recirculated hot shaft cooling air and supplemental outside combustion
air are fed to the lower hearths to sustain partial combustion of the
wastes circulating down through the furnace. Fuel gas produced through
the pyrolysis reaction is then burned in a high temperature afterburner.
The resulting heat can be used in a waste heat boiler to produce high
pressure steam. It may also be possible to burn the fuel gases directly
V- 50
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in a boiler. Char from the process is not used but, because it has some
fuel value, it may be usable as an industrial fuel.
The multiple hearth process offers the following advantages: (1)
usable in much smaller plants than most other pyrolysis systems, (2) em-
ploys modifications of well developed sludge incineration equipment,
(3) produces high temperature gases without raising temperatures in the
solid phase to the slagging point, and (4) conversion from existing con-
ventional sludge incineration systems is a relatively simple procedure.
Disadvantages include: (1) fuel value of the char is not used, (2) high
temperature fuel gases must be used on-site, and (3) incoming solid wastes
must be well classified.
It is estimated that this process will produce between 2 and 2.5 tons
of steam from one ton of a 2:1 mixture of municipal solid waste and sludge.
Table 16 summarizes the status of the several independent efforts
in the area of pyrolysis (solid waste) or co-pyrolysis (sludge plus solid
waste) now underway. The following paragraphs describe some of the more
fully developed systems.
Landgard Process - In this process, shredded waste materials are heated
indirectly by combusting a portion of the pyrolytic gases produced in a
rotary kiln reactor. The remaining gases are burned to produce steam in
a utility boiler. The char is not combusted and requires disposal, how-
ever, it does have characteristics similar to some activated carbons and
eventually may be usable. Residue discharged from the kiln is water-
quenched and then treated by flotation to separate the char from metal
and glass wastes. The off-gases from the reactor are drawn into a waste
gas burner where they are burned in air. The hot exhaust gases from the
burner than pass through a water-tube boiler and then through a final
cooler and air pollution control equipment. Operating on municipal solid
wastes, the process will produce slightly less than 2.5 tons of steam
per ton of waste.
Purox System - The Union Carbide Purox system is a gasification or partial
combustion process which maximizes gas production. Unshredded waste
materials are charged into the top of a vertical shaft furnace. The
char is combusted by injecting pure oxygen at the bottom of the furnace.
Hot combustion gases, essentially free of oxygen, rise through furnace
and pyrolyse the descending wastes into fuel gas, oil and additional
char. The resulting gaseous mixture rises further, drying the incoming
wastes. Water and oil are condensed from the gaseous stream which is
then cleaned for use. The condensed oil is returned to the furnace for
combustion and further production of gas. The end result is a clean-
burning fuel with a heat value of about 300 to 500 Btu/scf produced at
a rate of about 7.5 million Btu/ton of solid waste. This system will
receive unprocessed trash and, as a result of the high combustion temper-
ature of the char, produce a molten metal and glass slag. The slag is
water-quenched and reportedly is suitable for use as a construction fill
material.
V-51
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TAK. f 16
MUNICIPAL SOI 10 WASTC AND SEWACC SlUWif
PVc>0LYSIS PROCESSES
Developer
Products
Pilot Plant Scale
F i rs t
Major Demonstration Plant
Monsanto Fnvirocnem Systems
Inc., St. Louis, Mo.
(landgard)
Occidental Research Corp.
(formerly Garrett),
La Verne, Calif.
Union CarDide Corp.,
New York, N. Y. (Purox)
Carborundum Environmental
Systems, Inc., Niagara Falls
N. Y, (Torrax)
BSF Division tnvitctech
tielmont, California
Jet Propulsion Laboratory,
California institute of
Technology
Pasadena, California
Battelle Pacific Northwest.
Laboratories, Richland,
Washi ngton
Pyrolytic Systems, Inc.
Riverside, Calif.
DEVCO Management, Inc.
New York, N.Y .
Pol 1ution Control, Ltd.
Copenhagen, Denmark
Urban Research & Develop nent
Corp., East Granby, Conn.
Fuel Gas or Steam, Ferrous
Metal, Wet Char, Glass
Aggregate
Pyrclytic Oil. Char, Glass,
Ferrous Metal, Nonferrous
Metal, Organics in
Condensa te
Fuel Gas, Slag
Steam (or fuel Gas), Slag
Steam (fuel Gas)
Activated Carbon and fuel
Steam (or Fuel Gas)
Fuel Gas ur Electric
Power
fuel Gas
Fuel Gas
Slag, Fuel Gas
36 ton/day
H ton/day
200 ton/day
75 ton/day
100 ton/day
Initial pilot plant
operated at 10,000
gpd - sf'v.aye
I ton/day; 150 ton/day
demonstration plant
under consideration
50 ton/<1ay by late 1976
y) ton/(My
5 ton/day
1?0 urn/day
1000 ton/day solid wastes (Baltv
more, MD) co-pyrolysis con-
s i dered
200 ton/day solid wastes; start-
up schedule for late 1976 (San
Diego, CA)
Solid waste; scheduled for co-
pyrolysis. Pilot plant still in
operation late 1976 (S. Charles-
ton, UV)
?00 ton/day commercial plant
under construction in Europe
(Andco, Inc.)
Co-pyrolysis (Concord, CA)
1 nqd pilot plant in operation
(Fountain Valley, CA)
V-52
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Occidental Process - Occidental Research Corporation has developed a pro-
cess that produces oil as its main product. A finely divided, organic
feed is supplied to the pyrolysis reactor. Dividing is accomplished in
a two-stage shredding operation which also reduces the inorganic content
of raw refuse through air classification and screening to less than 4
percent by weight. The process, using the finely divided feed, permits
flash pyrolysis at atmospheric pressure for maximum oil production. Dis-
charge from the reactor goes first to a char separator and then to a
gas-liquid separator where gases and water are separated from the oil.
The relatively small amount of char and gases produced are recycled
to produce heat for the reaction. The pyrolytic oil produced has a
heating value of about 10,5000 Btu/lb and about 0.2 tons of oil are
produced per ton of solid waste processed. This oil is best utilized
by blending with No. 6 fuel oil for use in utility boilers and has the
advantage of being storable and transportable.
Torrax System - The Carborundum Torrax system is somewhat similar in
concept to the Purox system. Char is combusted to provide the heat
necessary for pyrolysis. However, air, not pure oxygen, is supplied to
support combustion. The result is diluted fuel gas with a low heating
value (120-150 Btu/scf) best utilized by combustion on-site to produce
steam.
Estimates of the potential energy production from pyrolysis of
refuse and sludge combined have been made for the Landgard and Purox
systems(38). These systems should be representative of most pyrolysis
systems since the main interest is in a heat balance for the overall
concept and not in the unit heating values for an individual product.
Process differences result in variations in the composition and quanti-
ties of fuel produced, but should result in relatively minor variations
in net heat output.
The estimates made indicate that the pyrolysis process would be
self-sustaining from an energy standpoint with mixtures of sludge and
solid waste containing 25%-40% sludge and 60%-75% solid wastes. The
refuse to sludge ratio for a typical residential community is in the
range of 10:1 to 15:1 on a dry solids basis and 3:1 to 8:1 on a wet
solids basis, indicating that more than enough refuse is generally avail-
able for mixing with sludge to operate the process without the need for
an external energy source.
Pyrolysis appears to have several advantages over incineration.
For example, some pyrolysis processes can convert wastes to storable,
transportable fuels such as fuel gas or oil while incineration only
produces heat that must be converted to steam. Pyrolysis gives a 50
percent greater reduction in volume of residue over incineration and
the residue is a more readily usable by-product. Air pollution is not
as severe a problem in pyrolysis systems because the volume of stack
gases and the quantity of particulates in the stack gases are less.
V-53
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On the other hand, pyrolysis is essentially still in the develop-
mental stage and, with few exceptions, viable commercial systems are
not readily available. Most of the pyrolytic fuel gases have relatively
low heat values and the pyrolytic oil is corrosive, requiring it to be
mixed with other fuel oil for best results.
The construction and operating costs for most pyrolysis systems are
much more uncertain than for incineration. Reliable cost data for
pyrolysis systems will not be available until significant operating exper-
ience is developed from the ongoing and planned demonstration projects.
DRYING OF SLUDGE
Flash Drying
Flash drying is the instantaneous removal of moisture from solids
by introducing them into a hot gas stream. This process was first
applied to the drying of wastewater sludge at the Chicago Sanitary District
in 1932. A flow diagram of the process is shown in Figure 16^2^. Origin-
ally, units were designed to dry sludge for fertilizer and burn only the
excess. The system is based on three distinct cycles which can be combined
in different arrangements. The first cycle is the flash drying cycle,
where wet filter cake is blended with some previously dried sludge in a
mixer to improve pneumatic conveyance. The blended sludge and the hot
gases from the furnace at 1,300°F are mixed ahead of the cage mill and
flashing of the water vapor begins. The cage mill mechanically agitates
the mixture of sludge and gas and the drying is virtually complete by
the time the sludge leaves the cage mill. The sludge, at this stage, is
at a moisture content of 8 to 10 percent and dry sludge is separated from
the spent drying gases in a cyclone. The dried sludge can be sent either
to fertilizer storage or to the furnace for incineration.
The second cycle is the incineration cycle. Combustion of fuel is
essential to provide heat for drying the sludge and the fuel may be gas,
oil, coal, or wastewater sludge. Primary combustion air, provided by
the combustion air fan, is preheated and introduced at a high velocity
to promote complete sludge combustion.
The third cycle is the effluent gas cycle or induced draft cycle
consisting of the deodorizing and combustion air preheaters, dust collec-
tor, induced draft fan, and stack. Heat recovery is practiced to improve
economy. The effluent gases then pass through a dust collector (dry
centrifuge or wet scrubber) and the induced fan discharges the effluent
gases through a stack into the atmosphere.
Perhaps the most notable current United States usage of this process
is that by the City of Houston, Texas^52 primarily for drying sludge
for use as a fertilizer. The dry product after complete processing has
a moisture content of around 5.5 percent. From analysis at the time of
sales of fertilizer in January, 1972, the moisture content was 5.0 per-
cent; ash 34.76 percent; nitrogen 5.34 percent; and available phosphoric
acid, 3.93 percent. The ash content fluctuates; the lowest on record is
V- 54
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CYCLONE
AUTOMATIC OAMPfcRS
INOUCfcO
STAC
DRAFT FAN
EXPANSION
XMNT
EXPANSION JOINT
EXPANSION JOINT
DOUBLE FLAP VALVE
MANUAL
DRY DIVIDER
REMOTE
COMBUSTION AIR
PREHEATER
DAMPERS
DRY PRODUCT
CONVEYOR
WET SLUDGE
CONVEYOR
DEODORIZING
PREHFATER
MIXER
DtSCHARGt SPOUT
FURNACE
BURNERS
AUTOMATIC DAMPERS
COMBINATION AIR FAN
CAGE MILL
HOT GAS DUCT
REFRACTORY
•/ /•/\ MOT GAS TO DRYING SYSTEM
3 DRYING SYSTEM
I V ] SLUDGE
Y////A COMBUSTION Al ft
V77771 DEODORIZED GAS
FIGURE 16. Flash dryer system(52)
V-55
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26.4 percent and highest, 44.3. Throughout the experience with this oper-
ation, the city's marketing arrangements have been scheduled on the basis
of competitive bidding. The successful bidder is committed to placing
orders with the city for its entire production for the contract period
of five years. The material is shipped in bulk by railroad car lots or
sometimes by barge. It is bagged for resale at the point of arrival.
The present contractor has been handling it for about 10 years, disposing
of about 80 percent of the production in the citrus groves of Florida.
There has never been a time when it was not possible to dispose of the
entire sludge production by sales.
The use of the flash drying systems for incineration alone has not
proven attractive. The Metropolitan Denver Sewage Disposal District No.
1 plant (approximately 100 mgd in capacity) abandoned a system of this
type due to air pollution and problems of continuing explosions in the
units. As an incineration unit, the flash drying system has the disad-
vantages of complexity, potential for explosions, and potential for air
pollution by fine particles. An advantage is the flexibility it offers
for drying a portion of the sludge for fertilizer.
Flash drying is relatively expensive because of fuel costs (contrasted
to incineration - no heating value is realized from the sludge) and because
pretreatment needs for production of sludge, which must have some reasonable
nutrient balance, are also expensive. Fuel consumption for production
of dried sludge is about 8,000 BTU/lb for flash drying.
Many flash drying installations have been abandoned due to high
costs (typically twice or more the cost for incineration and heat re-
covery) , odor problems, and the problems (air pollution and explosions)
associated with the fine particulates.
Another approach to heat drying of wastewater sludges for use as
fertilizers has been studied at the Blue Plains plant in Washington,
D.C.^51^. A schematic of the system is shown in Figure 17. Drying is
achieved in a jet mill in this case. The mill has no moving parts and
offers the ability to dry and classify solids simultaneously.
Sludge is dried and sterilized at a temperature of 1100°F. In
order to obtain a fertilizer with the desired nutrient balance at the
Blue Plains plant, it has been necessary to supplement the nitrogen
content of the sludge. The resulting product being marketed under the
trade name, OrganaGro, contains 6 percent nitrogen, 4 percent phosphoric
acid, and no potash.
During the system's break-in phase, more than 15,000 tons of sludge
were processed, at a running rate 200-270 wet tons of sludge per day,
with production of dry product per day being 40-54 tons*51). The solids
content of the feed sludge averaged 22 percent. Operation and maintenance
problems have resulted in the temporary shut-down of this unit at the
Blue Plains plant. Trash and fibrous material from the primary clari-
fiers has caused problems of fires and materials handling. Very serious
V-56
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AIR & WATER
VAPOR
WET SLUDGE
FROM WASTEWATER
PLANT
AiR
DUST
FILTER
WET
SLUDGE
STORAGE
PRODUCT
COLLECTOR
AIR
POLLUTION
CONTROL
PRODUCT
IN BAGS
BLOWER
PRODUCT
AIR
HEATER
AIR INLET
FERTILIZER
FINISHING
TOROIDAL
DRYER
FIGURE 17. Sludge drying system using the jet mill principle
-------
erosion of the drying unit has been encountered. In addition, the product
has been extremely dusty, thereby limiting its marketability.
Capital cost data for the "Organo" system are unavailable at this
time. The cost of operating the unit at Blue Plains has been as follows,
adjusting fuel oil costs to current levels of $0.50/gallon:
Operating Costs Only
Per Dry Ton
No. 2 Fuel Oil $30.00
Electricity 9.08
Labor 6.81
Nitrogen Supplement 10.67
TOTAL $56.56
Unit costs for power, labor, and nitrogen were not reported so
these cost items may not be on a basis comparable to costs presented for
other processes in this report.
Preliminary data from the Blue Plains plant indicate that with a
feed sludge at 78 percent water, approximately 60 gallons of number 2
fuel oil (8,500,000 BTU) are required per ton of dry sludge processed
(13 gallons per wet ton processed).
Solvent Extraction
A system (Basic Extractive Sludge Treatment of B.E.S.T.) for drying
of sludge to 95% solids is under development by Resources Conservation
Company(51' 53-55) _
In addition to drying the sludge, greases and oils are recovered for
possible use as an energy source or commercial byproduct. Sludge is
introduced into the B.E.S.T. system, stabilized at about 50 F and mixed
with cold recycled solvent. The system uses triethylamine ("TEA") as the
solvent in the primary dewatering step (See Figure 18). The solvent and
water are completely miscible at temperatures below 65 F and are immiscible
above that point. When the solvent and sludge are mixed below 65°F, the
solids are easily separated by either centrifuge or a filter. Because
the solvent-sludge mixture is warmed to about 60°F by the addition of heat
of solution, it is chilled to 50°F before being fed to a solid bowl cen-
trifuge. After the centrifuge separates the solvent-water-solids mixture,
solids go to a dryer and the iiquid fraction to a decanter. The closed-
cycle dryer removes solvent and produces dry solids (95% solids, sterile);
the solvent driven off is condensed and returned to the system. The
water-solvent fraction is heated and moved to a decanter where the solvent
and water form two layers. This solvent is recycled to the system, and
the water layer is fed to a steam stripping distillation column where
the remaining fraction of solvent is recovered and returned to the system.
The sterile dry solids produced by this process can be used as a fertil-
izer, a soil conditioner, or as a raw material depending upon the composi-
tion of the original sludge.
V-58
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CHEMICAL
PRETREAT
r-dUD
J \ SOLVENT
T" RECYCLE
V/ m PUMP
OIL
DISCHARGE
HX
SOLIDS
DISCHARGE
SOLVENT
STILL
STEAM
WATER
STILL
VENT
DRYER
STEAM
CENTRIFUGE
WATER
DISCHARGE
NOTE: HX= Heat Exchange
Figure 18
B.E.S.T. System Schematic
-------
Heat and electricity requirements are a function of the solids fed
and may be summarized as follows:
% Solids Kwh/lb BTU/lb
in Feed Solids Feed Solids Feed
7 0.52 3,514
11 0.38 3,266
20 0.26 3,088
For a typical sludge containing 16% oil, about, 2,000 BTU will be
recovered in the form of oil per pound of dry solids. The recycle
stream from a primary-WAS mixture is expected to have the following
quality:
BOD = 2760 mg/1 COD = 7335 mg/1 SS = 400 mg/1
The estimated costs for the B.E.S.T. process for a primary-WAS
mixture centrifuged to 11% solids are shown in Table 17 (based upon
data from manufacturer gathered in pilot scale tests - yet to be con-
firmed on plant scale). If a potential value of $40/ton for the recov-
ered fertilizer could be achieved, it is apparent that the costs would
be very attractive.
DISPOSAL AND LAND APPLICATION
The three major alternative modes for application of sludge to
the land and their constraints have been definedas:
Disposal
Sanitary Landfill
Principal Sludge Form
Dewatered cake or ash
Main Constraints
Gas, leachate, and
runoff control;
land availability.
Sites Dedicated to
Sludge Disposal
Liquid or Dewatered
Leachate, runoff
control, land
availability.
Cropland Application
Liquid, cake dried,
or compost.
Application rate,
unsatisfactory
sludge.
Land Reclamation
Liquid or Dewatered
Application rate,
unsatisfactory
sludge, availabil-
ity of land.
EPA has recently published a Technical Bulletin on Sludge Disposal
Methodsfor public review and comment which state that: "Because
land application of sludge conserves organic matter, nitrogen, phosphorus,
and certain essential trace elements, such utilization is encouraged
when it is supported by environmental assessment and, if necessary, an
V-60
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TABLE 17
ESTIMATED COSTS OF DRYING BY
SOLVENT EXTRACTION (B.E.S.T. PROCESS)
FOR PRIMARY + WAS AT 11% SOLIDS
10 TON/ 25 TON/ 50 TON/ 100 TON/
ITEM DAY DAY DAY DAY
Amortized Capital ( x 0.0944) $38.80 $28.97 $22.76 $18.88
Electric ($0.025/KWH) 19.00 19.00 19.00 19.00
Fuel ($ 3.00/MBTU) 19.60 19.60 19.60 19.60
Maintenance 12.33 9.21 7.23 6.00
Operations 8.63 4.60 3.45 2.30
Solvent (TEA at $0.89/lb) 2.78 2.78 2.78 2.78
Lime 2.10 2.10 2.10 2.10
Recycle Stream Treatment 4.06 3.68 3.44 3.34
Subtotal $107.30 $89.94 $80.36 $74.00
Fuel Credit(Recovered Oil) (12.30) (12.30) (12.30) (12.30)
TOTAL $95.00 $77.64 $68.06 $61.70
V-61
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environmental impact statement".
"Specifically, stabilization of sludge and subsequent land applica-
tion for enhancement of parks and forests and reclamation of poor or
damaged terrain should be considered for the utilization of sludge. Appli-
cation of stabilized sludge to agricultural lands on which crops entering
the human food chain will not be grown may also be regarded as an environ-
mentally acceptable method of sludge disposal. However, application of
sludge to lands on which crops entering the human food chain will or may
be grown must be examined closely in terms of protection of human health
and future land productivity. Priority consideration should be given to
non-agricultural uses".
The draft Bulletin does not yet represent EPA policy as it may be
revised as a result of the public review process.
Sanitary Landfill
(62)
The draft Bulletin states, "Sanitary landfill of sludge, either
separately or along with municipal solid waste, is acceptable when
supported by the environmental assessment and, if necessary, an environ-
mental impact statement".
The Bulletin also states that the landfill must be designed and
operated in accordance with EPA Guidelines for Land Disposal of Solid
Wastes (40 CFR 241 Appendix III); that the sludge must be stabilized
prior to landfill; and that daily soil cover must be provided.
A survey of 176 landfill operatigijis in 1972 found that only 30%
permitted disposal of sewage sludge^ . Despite this rather low per-
centage, stabilized sludge in landfills is recognized as an acceptable
method of disposal^. EPA estimates^ that 40% of wastewater
sludged are disposed of in landfills and dumps currently and it was pro-
jected that this percentage will be maintained in 1985.
The landfill site's geology, hydrology, and soil conditions should
be considered relative to the need for adequate protection of groundwater,
conformation of area land use planning, and provision of an adequate
quantity of earth cover'1'' .
(62)
Adequate monitoring of the landfill site is essential . The
monitoring plan must be specifically designed for applicable local con-
ditions and should include monitoring groundwater observation wells,
surface water, sludge and soils for heavy metals, persistent organics,
pathogens, and nitrates. Leachate and runoff from a sanitary landfill
should be minimized and when necessary collected and suitably treated
to prevent pollution of ground and surface waters.
Although past practice has emphasized disposal of ash or dewatered
(i.e., 20% solids) sludge, a recent study'64' indicates that liquid
digested sludge (4% solids) can be successfully disposed of in a landfill.
V-62
-------
With use of proper sludge spreading techniques, the municipal solid waste
had sufficient absorptive capability to retain the associated sludge
moisture and prevent leachate generation. The entire liquid sludge pro-
duction from Oceanside, Calif, has been disposed of in a landfill since
1972.
(4 64)
The costs for landfill of sludge are reported to be $l-5/ton
at the landfill site. Transportation costs are discussed separately in
a later section of this paper. Total costs at Oceanside for truck
transport, unloading, and landfill disposal of liquid (4% solids) sewage
sludge was $25-32/ton - economically competitive with other alternatives
there.
Of course, landfilling is a disposal technique which makes no use
of the nutrients in the sludge. The following section discusses land
application which does reuse these nutrients.
Cropland Application
It would be impossible in this brief paper to completely address
the factors involved in land application of sludges and to review past
experiences. Literally hundreds of references are available on the topic -
many of which are listed in references 69 and 75. Current practices
and much information is contained in two seminar proceedings.
Design guidance is also presented in reference 6. Major cities such as
Denver^^) and Chicago^ ) are applying their sludge to the land now.
San Francisco(67) initiating a study of such a plan. There are
numerous, successful land application systems utilizing liquid or dewatered
sludge throughout the U.S. which accounted for disposal of 20 percent of
the sludges in 1972(-*-). EPA projects the percentage will increase to
25% in 1985^).
Prior to applying sludge to the land, sludge stabilization is re-
quired to avoid nuisance conditions and minimize health hazards. Diges-
tion (anaerobic or aerobic) is the most commonly used stabilization
technique and is discussed in detail in the next section of this paper.
Typically, the land application site is remote from the plant site.
The sludge may be transported by truck, barge, railroad or pipeline as
discussed in detail in a subsequent portion of this paper. Transport
costs may comprise a very significant portion of the overall costs of
land application.
Storage of sludge between treatment and land application is usually
required because the application rate of the sludge to the land is usually
not the same as the rate at which sludge is generated. Treated sludge
will be generated at nearly a constant rate; whereas, the sludge disposal
rate will depend on weather conditions, field conditions, and the appli-
cation method.
The critical factor for determining the volume of the storage facility
is the length of time the disposal area cannot be used. The influences
V-63
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of the method of application, the climatic conditions, and the site may-
require very small storage volume or storage for several months. For
example, in Shreveport, Louisiana, it was determined that 150 day storage
would be necessary''7'1" . Rainfall data, by days, for the past 10 years
were reviewed and the operation of land disposal was synthesized upon
this period. By methods similar to a mass hydrograph analysis, a maximum
storage period of 150 days were derived.
Where storage requirements are minimal, a second stage anaerobic
digester may be used for storage. A covered digester is well suited for
sludge storage because it will contain odorous gases which may be a
problem in open basins or lagoons.
There are two philosophies concerning land application operations:
(1) apply the sludge to a plot of land which will be used for growing
agricultural products or other vegetation (parkland, forests, etc.),
(2) to dedicate the area to the disposal of sludge with no attempt to
grow crops. Overall management of a system using sludge for crop growth
is more complex because the needs of the crop must be carefully balanced
against sludge disposal considerations.
The advantage of an agricultural operation in conjunction with sludge
disposal is the beneficial use of nutrients in the sludge and the removal
of nitrogen, heavy metals, etc., from the soil. It has been estimated
that some 480,000 tons of phosphorus could be recycled if land application
were practiced for all sludges when secondary treatment is applied to
all U.S. wastewaters. The EPA Bulletin(62) recognizes this advantage but
also expresses a concern over possible human food chain effects:
"Although utilization of sewage sludges as a resource to re-
cover nutrients and other benefits has been encouraged by PL 92-500
and the EPA Science Advisory Board, the workgroup members and others
involved in developing this Technical Bulletin have received con-
flicting opinions concerning the overall merits vs hazards of apply-
ing sludges to cropland. Possible adverse effects upon the human
food chain (e.g., potential for increasing human cadmium intake) has
remained a major concern expressed whenever this practice is con-
sidered. The relative risks of applying sewage sludges to croplands,
when compared to ether routes through which these contaminants enter
the human diet, have yet to be determined".
Sludge constituents such as viruses, organics, cysts, and parasites
are of concern from the standpoint of their ultimate fate and effect on
the environment. However, they do not usually limit the rate at which
sludge is applied to the land. Those constituents of sludge which poten-
tially may limit the application rate of the sludge to the land are the
amount of water in the sludge, the amount of nitrogen in the sludge, and
the quantity of heavy metals in the sludge.
If surface runoff is to be prevented, the application of water to
land obviously cannot exceed the amount of water lost by percolation,
evaporation, and transpiration. While not of concern with dewatered
V-64
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sludges, many systems apply liquid sludge to the land. The amount of
water which may be applied will vary depending on the climatic conditions,
the type of soil, whether vegetation grows on the disposal site, and the
type of vegetation which may be grown on the disposal site. Although
the water application rate to the land should be considered, for sludges
having a dry solids content of greater than 2 percent, water content
usually does not limit the rate at which sludge may be applied to the
land. For example, a 2 percent sludge applied at a liquid application
rate of 1 inch per week will result in a solids loading of 120 tons per
year per acre. At this application rate, normally some other constituent
(such as nitrogen) in the sludge will control the application rate.
If nitrogen pollution of the groundwater is a concern, the amount
of nitrogen in the sludge may limit the annual sludge application rate.
The nitrogen concentration of sewage sludges should be measured for each
sludge. For an anaerobically digested raw sludge, the total nitrogen
is typically 50 to 70 pounds of nitrogen per ton of dry sludge solids.
The amount of nitrogen (as N) found in waste activated sludges or aerobic-
ally digested sludges is generally higher than that of raw or anaerobically
digested sludges and typically ranges from 100 to 120 pounds of nitrogen
per ton of the dry weight sludge solids.
The amount of nitrogen contained in the sludge is a concern because
of the potential for nitrogen to leach to the ground water in the form
of nitrate. The concentration of nitrate is limited in potable water
supplies to 10 mg/1 (as N). The fate of nitrogen in soils and in ground
water is difficult to predict with accuracy because of the many processes
which can affect the fate of nitrogen in the soil system. There is no
doubt that excessive applications of nitrogen will lead to passage of
nitrogen into the ground water. High nitrate contents are observed in
Illinois and Washington in ground waters below agricultural areas util-
izing commercial fertilizers. To avoid nitrate pollution of the ground
waters, a balance between nitrogen applied in the sludge and that removed
in the crop or by other mechanisms must be struck.
In a single growing season, crop uptake of nitrogen may vary from 50 to
600 lbs/acre/yr depending upon the specific crop growth. Typical ranges are
(lbs/acre/yr): forest crops - 20-60; field crops - 50-150; forage crops -
75-600. Consideration of the nitrogen balance may reduce the permissable
sludge loading rate from values of 100 dry tons per acre per year exper-
ienced in average conditions without concern for a nitrogen balance to
as low as 5 tons per acre per year. The Chicago, Illinois "Prairie Plan"
proposes initial application rates of 75 dry tons per acre per year to
previously strip-mined land, which will taper to 20 tons of dry sludge
per acre per year and an associated 1,000 pounds of nitrogen per acre
per year (50 lbs N/dry ton). The sludge application rate for the north-
east water pollution control plant at the City of Philadelphia, Pennsylvania,
may be limited to 25 dry tons per acre per year in order to prevent
nitrate-nitrogen leaching to the ground water in excess of 10 mg/1
Added discussion of nitrogen balances in crop systems is presented in
documents on wastewater application to the land but is applicable to
sludge systems as well 74,76,77)_
V-65
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The third constituent of sewage sludge which may affect the appli-
cation rate of sludge to land is the heavy metals content. Elements in
sludge that are potential hazards to plants or the food chain are: B,
Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn. The quantity of heavy metals in
sewage sludge is highly variable and depends to a great extent on the
types of industry connnected to the sewage collection system and the
degree of emphasis and enforcement to which the operating agency imposes
on limiting the heavy metals which enter the sewage collection system.
Table 18 presents heavy metals concentrations for several sludges.
Among the factors that affect the toxicity of metals to plants are:
The amount of toxic metals present in the soil.
The toxic metals present. Diffent metals differ in their
toxicity to specific plants and in specific soils.
The pH of the amended soil. The toxic metal content safe
at pH 7 can easily be lethal to most crops at pH 5.5.
Land application may lead to a lowering of the soil pH due
to nitrification of the NH4-N added. Properly selected soil
amendments can readily be used to overcome this potential
problem.
The organic content of the amended soil. Organic matter
chelates the toxic metals and makes them less available
to injure plants.
The phosphate content of the amended soil. Phosphate is
well known for reducing Zn availability to plants and
decreasing the stunting injury caused by excessive levels
of toxic metals.
The cation exchange capacity (C.E.C.). The C.E.C. of the
soil is important in binding all cations, including the
toxic metal cations. A soil with high C.E.C. is inherently
safer for disposal of sludge than a soil with low C.E.C.
The plant grown on sludge treated soil. Plant species vary
widely in tolerance to heavy metals, and varieties within a
species can vary three to tenfold.
/ go \
A recent EPA report presents a detailed review of the potential
hazards associated with specific heavy metals. The report concludes
that, with correct management practices, manganese, iron, aluminum,
chromium, arsenic, selenium, antimony, lead, and mercury pose relatively
little hazard to crop production and plant accumulation when sludge is
applied to soil. Cadmium, copper, molybdenum, nickel, and zinc can
accumulate in plants and may pose a hazard to plants, animals, or humans
under certain circumstances.
V-66
-------
TABLE 18
HEAVY METALS CONTENT OF SEWAGE SLUDGES(86^
(mg/kg, ppm)
Ag, Silver
As, Arsenic
B, Boron
Ba, Barium
Be, Beryllium
Cd, Cadmium
Co, Cobalt
Cr, Chromium
Cu, Copper
Hg, Mercury
Mn, Manganese
Ni, Nickel
Pb, Lead
Sr, Strontium
Se, Selenium
V, Vanadium
Zn, Zinc
Range Mean
nd - 960 225
10-50 9
200 - 1430 430
nd - 3000 1460
nd nd
nd - 1100 87
nd - 800 350
22 - 30,000 1800
45 - 16,030 1250
0.1 - 89 7
100 - 8800 1190
nd - 2800 410
80 - 26,000 1940
nd - 2230 440
10 - 180 26
nd - 2100 510
51 - 28,360 3483
Median
90
8
350
1300
nd
20
100
600
700
4
400
100
600
150
20
400
1800
V- 67
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Cadmium is a nonessential element which can be a serious hazard to
animals and humans if dietary levels are increased substantially. Cad-
miums lability in soil is reduced by organic matter, clay, hydrous iron
oxides, high pH, and reducing conditions. Annual cadmium application
rates, soil pH, and crop species and varieties have a major influence
on the cadmium concentration in plant tissue. One study (78) foun(j that
35 years of sludge application resulted in large accumulations of Cd
and other trace elements in the soil but no significant accumulation
in the grain of corn plants. Concentrations in the leaves and roots
were significantly higher than normal. The following management options
are available to limit cadmium accumulation in the food supply to a
relatively low level on sludge-treated land: (1) maintain soil pH at or
above 6.5; (2) grow crops which tend to exclude cadmium from the whole
plant or from reproductive tissue; (3) apply low annual rates of cadmium,
and use sludges which have a low cadmium concentration; and (4) grow
nonedible crops.
Copper, although essential to plants, can become toxic to them at
high concentrations. Sludges often contain appreciable levels of
copper, but application of sludge to soil results in only slight to
moderate increases in the copper content of plants. Under good manage-
ment practices, copper in sludges will seldom be toxic to plants and
should not present a hazard to the food supply.
Molybdenum is not particularly toxic to plants, even when applied
at relatively high levels. As a result, molybdenum may accumulate in
plants at concentrations sufficient to cause molybdenosis in ruminant
animals without prior warning from plant behavior. The practice of
maintaining the soil pH at 6.5 or higher results in greater solubility
and availability of the molybdenum than would occur at lower pH values.
However, since sludges are usually very low in molybdenum, it is doubt-
ful that molybdenum in sludge would present a serious hazard to the health
of grasing animals except for the unusual circumstances in which forages
from sites receiving high-molybdenum sludge form the major part of the
animal diet.
Sludges often contain substantial quantities of nickel, which appears
to be more readily available from sludges than from inorganic sources.
Nevertheless, toxicity of nickel to plants occurs only on acid soils.
If the soil pH is maintained at 6.5 or above, nickel should not cause
toxicity to plants or pose a threat to the food supply.
Zinc, an essential element for both plants and animals, is often
found in sludge at relatively high concentrations. Additions of sludge
to soil may cause substantial increases in the zinc content of plants,
but toxicity seldom occurs. In general, if the pH of sludge-treated
soils is maintained at 6.5 or greater, zinc should not be a hazard to
plants or to the food supply unless exceptionally high amounts are added
in the sludge.
Heavy metal concentrations may restrict application of sludges in
the New York-New Jersey Metropolitan Area to 2 dry tons/acre/year.
V-68
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Sludge may be applied to the land in a variety of ways. Small plants
may spread liquid sludge directly form tank trunks. In some cases,
shallow trenches may be dug, filled with sludge, and covered. Sludge may
also be applied through sprinkler systems using large diameter spray
nozzle openings in cases where aerosol transport can be controlled by
adequate isolation of the site. In some cases, sludge has been injected
into the subsoil under pressure. Ridge and furrow systems have also
been used successfully. The method used is generally related to the
quantity of sludge to be disposed and whether crops are to be grown on
the site.
The proper management of the land application system is the key to
the success of the system. The economical and technical success of the
project depends on intelligent decisions, firm and established project
goals and proper monitoring of results. Monitoring of ground water and
leachate (percolate) is information necessary to assure protection of
the ground water. Where crops are grown, close cooperation between the
treatment system management and farming operation is required. Scheduling
of sludge application with farm operations such as planting, tilling,
spraying and harvesting is vital to successful management.
It is difficult to generalize on the cost of land disposal of
sludges because of the tremendous number of variables which affect cost
such as land costs, climate, soil types, distance to disposal area from
treatment plant site, allowable loading rates (may range from 2-100
dry tons/acre/year) Chicago was experiencing costs of $70+ per dry ton
in 1972 with a lengthy barge haul involved. Of this total, about
$20/ton was related to the land application portion of the project with
the remainder resulting from digestion, concentration, and transport.
If the barging operation were replaced by a pipeline, total costs were
projected to drop to $35/ton. Past reports of costs dD at other projects
range from $8 to $50/ton. A recent report^1) estimated the costs for
several alternative land application approaches for a large city for
sites 20-100 miles from the treatment plant. Estimated costs were
$39-$57/ton including, in some cases, dewatering to 20% solids. Trans-
port costs, as discussed in a later section, are often the major cost
item in a land application system. Costs for the New York Metropolitan
Area(6°) were estimated at $110-$185/ton but were adversely affected by
long transport distance (100 miles), low application rates (5-10 tons/
acre/year), and high land costs ($6000/acre). Where adverse factors such
as these exist, land application may not be cost effective. However,
there are many municipalities where conditions are such that land appli-
cation is cost effective.
STABILIZATION
The principal purpose of sludge stabilization are to render the
sludge less putrescible, to reduce the pathogenic content, and to reduce
the sludge quantity. Processes commonly used are: anaerobic and aerobic
digestion and composting.
V-69
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Anaerobic Digestion
In this process, the organic matter in the sludge is stabilized in
an anaerobic environment (oxygen devoid). Most modern systems are "high-
rate" systems utilizing one or two stages (see reference 79 for detailed
information on the process). A typical two-stage process is shown in
Figure 19. The stabilization of the sludge occurs in the first stage,
mixed and heated unit with the second stage digester providing settling
and thickening. In a single stage system, the secondary digester is
replaced by some other thickening process. The digester is heated to
85-95°F and typically provides 15 days or less detention of the sludge.
The process has been successful when primary sludge or combinations
of primary sludge and limited amounts of secondary sludge constitute
the system's feed. With the advent of wastewater treatment systems that
are more efficient than simple sedimentation, large quantities of acti-
vated sludges are produced at the plants. This additional sludge, when
placed in a two-stage anaerobic digestion process, can cause high
operating costs and poor plant efficiencies. The basic cause of the
problem is that the additional solids do not readily settle or dewater
after digestion.
The process converts about 50% of the organic solids to liquid and
gaseous forms - providing a substantial reduction in the quantity of
sludge requiring disposal. A major component of the gaseous by-products
(usually about two-thirds) is methane. The resulting gas has a typical
heat value of 600 BTU/scf with about 15 scf of gas formed per pound of
volatile solids destroyed.
The use of anaerobic digester gas has been practiced to some extent
in wastewater treatment plants for many years. Digester gas is currently
being used at several wastewater treatment plants to heat digesters and
buildings and as fuel for engines that drive pumps, air blowers and elec-
trical generators.
The following criteria give estimates for gas and heat available from
anaerobic digestion^38^:
Primary Waste Acti-
Sludge vated Sludge Total
Gas Produced, scf per
million gallons treated. 5,175 5,670 10,845
Heat Available, Btu per
million gallons treated. 3,105,000 3,402,000 6,507,000
A schematic of a typical system to utilize digester gas in an inter-
nal combustion (IC) engine is shown in Figure 20. As indicated in this
figure the engine could be coupled to a generator, blower or pump. Typi-
cal IC engine efficiency is 36.4% (7,000 BTU/hp-hr). An IC engine-generator
typical efficiency is 30% (11,400 BTU/hp-hr). The electrical energy
V-70
-------
GAS
RELEASE
GAS
RELEASE
GAS
GAS
SUPERNATANT
REMOVAL
ZONE OF
MIXED
LIQUOR
SLUDGE
INLET
SUPERNATANT
MIXING
ACTIVELY
DIGESTING
SLUDGE
DIGESTED SLUDGE
SLUDGE DRAWOFF-^
TO FURTHER PROCESSING
FIGURE 19. Two-stage anaerobic digestion.
-------
STEAM OR
HOT WATER
TO DIGESTER
OR OTHER USE
STORAGE
45 psi
EXCESS GAS j
BURNER i
INTERNAL
COMBUSTION
ENGINE
HEAT
RECOVERY
UNIT
ALTERNATE
FUEL
SYSTEM
ELECTRICAL GENERATOR
AIR BLOWER
WATER PUMP
ANAEROBIC DIGESTER GAS
UTILIZATION SYSTEM'38'
-------
which can be generated from anaerobic digestion of primary and WAS could
supply about 85% of the electrical energy required for an activated
sludge plant while also providing over 50% of the heat for the digestion
process itself^®) .
The process also provides substantial reductions of pathogenic
bacteria (85-100%) ^ .
The process disadvantages include: (1) process control requires
considerable operator expertise and time to achieve optimum solids reduc-
tions and gas production (2) the supernatants from the process are often
high in BOD, solids, and ammonia and impose an added load when recycled
to the wastewater treatment system.
(32)
Updating previously published costs for two-stage anaerobic
digestion to the same basis used for other processes in this report
results in the following costs per dry ton:
Tons Per Day Dry Solids to Digester
Sludge Source 10 25 50 100
Primary $18.80 $15.50 $14.30 $13.70
Primary + WAS $46.80 $42.70 $42.30 $40.80
No credit for the fuel value of the digester gas is reflected in
the above estimates. At $3/M BTU, the value of the fuel could be $16/
ton of dry solids for primary sludge digestion and $18.60/ton for pri-
mary + WAS.
Aerobic Digestion
Aerobic digestion consists of separate aeration of waste primary
sludge, waste biological sludge, or a combination of waste primary and
biological sludges in an open tank. It is usually used to stabilize
excess activated sludges or the excess sludges from small plants which
do not have separate primary clarification. Figure 21 is a schematic
diagram of an aerobic digestion system. The advantages that the system
offers over anaerobic digestion include: simpler operation, less capital
cost, and better supernatant quality. Disadvantages are: higher oper-
ating cost, poor sludge dewatering characteristics, and net energy con-
sumption rather than energy production.
Current practice is to provide 10-15 days of detention time for the
stabilization of excess biological sludges, additional time is required
when primary sludge is included^ .
The destruction of solids is a function of temperature (the process
is not heated). Volatile solids reductions of 35-50% have been achieved.
Pure oxygen rather than air can be used in the digester to enable higher
loading rates. The oxygen system also has the advantage of generating
heat from the biological reaction which increases the sludge temperature
and a corresponding increase in the rate of solids destruction.
V-73
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PRIMARY SLUDGE
EXCESS
ACTIVATED OR
TRICKLING FILTER
SLUDGE
CLEAR
OXIDIZED
OVERFLOW
TO PLANT
V
rr
SETTLED SLUDGE RETURNED TO AERODIGESTER
(4)
FIGURE 21. AEROBIC DIGESTION SYSTEM
-------
Based upon criteria presented recently , Table 19 presents costs
calculated for the aerobic digestion process for primary + WAS (exclusive
of solids separation an/or thickening). Capital costs are significantly
less than for two-stage anaerobic digestion partly because total sludge
detention time is about one-third that of two-stage anaerobic digestion.
O & M costs are significantly higher for the aerobic process due to power
consumption. In order to properly compare these two systems, differences
in thickening, dewatering and supernatant treatment costs must be added
to each of the digestion system costs.
Composting
Composting is a method of biological oxidation of organic matter in
sludge by thermophilic organisms. Composting, properly carried out, will
dewater, destroy objectionable odor producing elements of sludge, destroy
or reduce disease organisms because of elevated temperature, and produce
an aesthetic and useful organic product.
Composting of wastewater sludge differs significantly from processing
and composting of solid waste; therefore, past poor publicity related to
composting of solid waste need not discourage the use of composting in
processing of wastewater sludge. There are many differences between the
two(56):
Composting of solid waste is proceeded by complex materials
handling and separation process.
Solid waste varies widely in composition which makes processing
more difficult.
Many past solid waste composting operations were operated and
evaluated on the basis of profit making potential rather than
as an alternative disposal means.
For a given population, the volume of solid waste compost is
several times the volume of wastewater sludge compost, there-
fore, solid waste creates a much greater marketing or disposal
task.
Composting systems generally fall into three categories: (a) pile,
(b) windrow, and (c) mechanized or enclosed systems. The pile (static
aerated pile) and windrow systems have been used almost exclusively in
composting sewage sludge because of their low cost and demonstrated per-
formance. In general, the windrow process has been used in composting
digested primary and waste activated sludge in various combinations.
The static pile method has been used more recently for composting raw
primary and waste activated sludge in various combinations. The windrow
process was found to be unsuitable for composting raw sludge because of
odor problems. Thus, at this time, the windrow process has been demon-
strated on digested sludges, the static pile method on raw sludges, and
mechanized or enclosed systems have not been used to any extent recently
V-75
-------
TABLE 19
AEROBIC DIGESTION COSTS
UTILIZING CONVENTIONAL AIR SYSTEM
Tons Per Day of Dry Solids (Primary + WAS)
Item
10
25
50
100
Construction Cost
$800,000
$1,400,000
$2,200,000
$3,300,000
Engr., Legal, Adm., &
During Construction(19%)
152,000
266,000
418,000
627,000
Total Capital Cost
$952,000
$1,666,000
$2,618,000
$3,927,000
0 & M Labor @ $10/Hr.
32,000
50,000
80,000
105,000
Power @ $0.025/Kwh
37,500
75,000
150,000
300,000
Maintenance Materials
7,000
10,000
16,000
20,000
Annual 0 & M Costs
76,500
135,000
246,000
425,000
Annual Capital Cost ( x .0944)
90,000
157,000
247,000
371,000
Total Annual Cost
$166,500
$292,000
$493,000
$796,000
Cost Per Ton of Dry Solids
$45.61
$32.00
$27.00
$21.80
V-76
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in the U.S. on sewage sludge. Thus, only windrow and static pile pro-
cesses will be discussed in detail.
The general composting method is very similar for both processes.
The dewatered sludge (typically 20 percent solids) is delivered to the
site and is usually mixed with a bulking agent. The purpose of the
bulking agent is to increase the porosity of the sludge to assure aerobic
conditions during composting. If the composting material is too dense
or wet it may become anaerobic thus producing odors or if it is too porous
the temperature of the material will remain low. Low temperatures will
delay the completion of composting and reduce the kill of disease organisms.
Various bulking materials can be used and suitable low cost materials
include wood chips, bark chips, rice hulls, and cubed solid waste. Un-
screened finished compost has also been used. Generally, one part sludge
(20% solids) is mixed with three parts bulking agent although this mixture
can be varied depending on moisture content of sludge, type of bulking
agent, and local conditions. The sludge-bulking agent mixture is then
formed into the windrow or static pile as applicable.
Following composting, the product is removed from the windrow or
static pile and cured in storage piles for 30 days or longer. This
curing provides for further stabilization and pathogen destruction. Prior
to or following curing, the compost may be screened to remove a portion
of the bulking agent for reuse or for applications requiring a finer
product. The compost can also be used without screening. Removal of the
bulking agent also reduces the dilution of the nutrient value of the
compost.
The compost is then ready for distribution.
Windrow Composting - The sludge-bulking agent mixture, (2-3 parts of
bulking agent by volume to one part of sludge) is spread in windrows with
a triangular cross section. The windrows are normally 10 to 16 feet wide
and 3 to 5 feet high. An alternative method of mixing the bulking agent
and sludge and forming the windrow consists of laying the bulking agent
out as a base for the windrow. The sludge is dumped on top of the bulking
agent and spread. A composting machine (similar to a large rototiller)
then mixes the sludge and bulking agent and forms the mixture into a
windrow. Several turnings (about 8 to 10 times) are necessary to adequately
blend the two materials.
The windrow is normally turned daily using the composter; however,
during rainy periods turning is suspended until the windrow surface layers
dry out. Temperatures in the windrow interior under proper composting
conditions range from 55 to 65°C. Turning moves the surface material to
the center of the windrow for exposure to higher temperatures. The higher
temperatures are needed for pasteurization and kill of most pathogenic
agents. Turning also aids in drying and increases the porosity for
greater air movement and distribution.
V-77
-------
The windrows are turned for a two week period or longer depending
on the weather and efficiency of composting. The compost windrow is
then flattened for further drying. The compost is moved to curing when
the moisture content has decreased to approximately 30 to 40 percent.
Proper windrow composting should produce a relatively stable product
with a moisture content of 30 to 40 percent which has been exposed to
temperatures of at least 50 C for a portion of time during the composting
process.
The composting process requires longer detention times in cold or
wet weather, therefore, climate is a significant factor with the windrow
process in open spaces. Covering the composting area would significantly
reduce the effects of cold weather and nearly eliminate the problems of
wet weather. In any case, the curing area should be covered if operations
are to be carried out during precipitation.
(57 58)
Static Pile Composting - The static pile composting method ' as
applied to raw sludge requires a forced ventilation system for control
of the process. The pile then remains fixed, as opposed to the constant
turning of the windrow, and the forced ventilation system maintains
aerobic conditions.
A base is prepared for the pile consisting of a 1 foot thick layer
of bulking agent or previously composted unscreened product. A 4-inch
diameter perforated pipe is installed in the base as an aeration header.
The base in constructed with a typical plan dimension of approximately
40 by 20 feet. The sludge-bulking agent mixture is piled on this base
to a heighth of approximately 8 feet to form a triangular cross section.
The pile is capped with a 1 foot layer of screened compost product.
This top layer extends down the sides to help absorb odors and to act
as a shield or roof against penetration of precipitation. A typical
static pile is illustrated in Figure 22. An alternative configuration
is the extended static pile method where subsequent piles are "added"
to the initial static pile. This configuration saves space compared to
a number of separate static piles.
The perforated underdrain pipe is attached to a blower by pipe and
fittings. The other side of the blower is piped to a smaller, adjacent
pile of screened compost product. Air and gases are drawn by the blower
from the static compost pile and discharged through the small pile of
product compost. The small pile effectively absorbs odors. The oper-
ating cycle of the blower is adjusted to maintain oxygen levels in the
exhausted gases and compost pile within a range of 5 to 15 percent.
Temperatures within the compost pile will vary somewhat with monitoring
location in the pile, but should reach 60-65 C. Normally the blower is
operated on an on-off cycle to maintain proper oxygen levels and temper-
atures within the pile.
After an average composting period of 3 weeks, the compost is moved
to the curing area.
V-78
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WATER
REMOVAL
COMPOST
PILE
FAN SCREENED
O* COMPOST
LONGITUDINAL SECTION
WOOD CHIPS AND SLUDGE
EXTENDED PILES
SCREENED COMPOST
UNSCREENED COMPOST
PERFORATED PIPE
CROSS SECTION
Figure 22
STATIC PILE COMPOSTING AS DEVELOPED BY
THE AGRICULTURAL RESEARCH SERVICE AT
BELTSVILLE, MARYLAND
V-79
-------
Outdoor temperatures as low as -7°C and rain totaling 7 inches per
week has not interferred with the successful outdoor operation of exposed
static pile composting. Temperatures produced during static pile com-
posting are generally above 55 C and often exceed 70 to 80 C.
The? Product - The compost product has a slight musty odor, is moist, dark
in color, and can be bagged. The texture of the compost varies depending
upon the degree of screening. Compost is valuable as a soil conditioner
and low grade fertilizer and varies widely in content. Typical compost
contains an average of 1.5 percent nitrogen and 1.0 percent phosphorus.
Agricultural Research Service Personnel indicate that proper static pile
composting should reduce total and fecal coliform and salmonella below
detectable limits. Compost produced by the windrow process is likely
to contain detectable pathogens because lower temperatures are produced.
Composting has little effect on total heavy metal content of the sludge,
but there is some dilution and also some indication of a lower uptake
rate after composting. Content and effect of heavy metals must be consid-
ered for each individual application.
The Economics - The economics of composting are determined by two factors;
the cost of producing the compost and the cost of (or income from) dis-
posal of the compost product.
The marketing of the end product is a key to the success of a com-
posting effort. A recent market study(^) found several successful muni-
cipal sludge composting operations where all of the end product was sold
or otherwise successfully used. The study concluded that the upper price
limit for bulk sewage sludge compost would be $4-$10/ton and for packaged,
bagged, sewage sludge compost, $60/ton. Bagging costs could approach
$ 30/ton.
Those municipal sludge compost marketing operations that have been
successful have generally(59) :
had favorable local publicity.
had the product available for pick-up (or make deliveries).
offered guidelines for its use, or at least suggestions,
offered the product at no cost or inexpensively,
given the product a trade name.
The cost of producing compost includes the following elements:
Amortization of land, capital site improvements, and structures.
Amortization of major mobile equipment costs.
Operation and maintenance costs.
V-80
-------
Land requirements are affected by several factors but are typically
0.2-0.4 acres/dry ton for the static pile technique. Windrow techniques
require 2-3 times more area.
The required site improvements and structures will vary depending
on process used, availability of existing facilities, degree of mechani-
zation of the process, and to a degree, the demands of the climatic region.
Site improvements related to composting will generally include site access
and improvements, bulking agent storage, bulking agent-sludge mixing area
or mechanical fixed equipment, composting pads and appurtenances such as
blowers, screening area, compost storage area, support facilities such
as electrical, and fixed materials handling equipment.
Major mobile equipment includes screens, front loaders, trucks, and
testing equipment. The number and size of major equipment will depend
on the capacity and type of operation.
Operation and maintenance costs normally include:
Labor for constructing the compost piles, handling materials,
and screening the compost.
Labor for regular inspection of operations and performing
tests on the piles.
Electrical energy for blowers, lighting, and other miscellaneous
uses.
O & M costs for the equipment including front loader and screen.
Costs of transport of materials as required.
Cost of bulking agent, typically $2 to $4 per cubic yard.
As compared to many other sludge handling processes, the amount of
information available for estimating costs for sludge composting is
rather limited, and the accuracy of estimates of costs is likely to be
much less.
A studyof the sludge disposal alternatives for the New York-
New Jersey Metropolitan area developed a cost of $40-45 per dry ton for
composting large quantities of dewatered sludge without any hauling or
land costs included.
The USDA and MES estimate the total cost for static pile composting
of approximately 600 wet tons per day of sludge (20 percent solids) would
be $40 drY ton excluding land and hauling. Camp, Dresser, and
McKee estimated a cost of $45 per dry ton including land, but excluding
hauling, to windrow conpost 600 wet tons per day of sludge. They indi-
cate that the cost would be less for static pile composting.
V-81
-------
Preliminary studies indicate that total costs to a municipality
for static pile composting should be in the range of $30 to $40 per
dry ton of sewage sludge solids excluding dewatering and hauling, but
including land at $10,000 per acre. This cost varies with local condi-
tions and with the size of the operation. Windrow costs would be
expected to be somewhat higher.
V-82
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SECTION VI
SLUDGE TRANSPORT
It is becoming increasingly common to transport solids in liquid or
dewatered form from one location to another as part of the treatment,
disposal, or reuse steps. Significant technical and cost considerations
must be evaluated in planning a transport system to achieve satisfactory
results. The costs associated with transport can be very substantial.
This section will discuss general aspects of solids transportation
systems by truck, barge, railroad, and pipeline.
A significant EPA sponsored sewage sludge transport cost study was
completed by CWC(82) _ purpose of this study was to develop a method
of calculating transport costs for each mode using basic parameters such
as gallons of fuel, operator manhours, operating miles, and similar
factors. Therefore, the information developed in the study would not
grow out of date with inflation and current unit costs could be used in
making calculations at any future date. Formats are set up in the study
for both manual and computer calculation of transport costs and methods
of escalation. This section represents a very general summary of the
information in the EPA study. Time and space do not permit a presenta-
tion of the total calculation procedure nor complete breakdown of cost
estimates, so only total cost typical, current information is provided.
A copy of the EPA study, Contract No. 68-03-2186, should be obtained if
greater detail is needed.
The total costs for sludge transport consist of:
1. Point to point transport costs including capital and 0 & M.
2. Facilities capital and 0 &
M costs.
(In case of
truck, ]
and railroad. Facilities
include:
Transport
Mode
Liquid
Truck
Railroad
Barge
Loading Storage
No (2)
Yes
Yes
Loading Equipment
Yes
Yes
Yes
Dispatch Office
Yes
Yes
Yes
Dock and Control Bldg.
N/A
N/A
Yes
Railroad Siding(s)
N/A
Yes
N/A
Unloading Equipment
Yes
Yes
Yes
Unloading Storage (1)
No
No
No
VI-1
-------
Dewatered
Truck
Railroad
Barge
Loading Storage
Yes (3)
Yes
N/A
Loading Equipment
Yes
Yes
N/A
Dispatch Office
Yes
Yes
N/A
Dock and Control Bldg.
N/A
N/A
N/A
Railroad Siding(s)
N/A
Yes
N/A
Unloading Equipment
Yes
Yes
N/A
Unloading Storage (1)
No
No
N/A
(1) Storage assumed to be a part of another unit process.
(2) Storage required for one or two truckloads is small
compared with normal plant sludge storage.
(3) Elevated storage for ease of gravity transfer to trucks.
Pipeline facilities consist of pipeline and pumping stations.
The forms of sludge studied and the transport modes are:
Transport Mode
Truck
Barge
Railroad
Pipeline
Form of Sludge
Liquid Dewatered
X X
X
X X
X
The most common liquid sludge concentration is 1 to 4 percent solids
although liquid sludge up to 10 percent solids can be handled with relative
ease. Dewatered sludges are normally 15 to 20 percent solids and can be
moved with belt conveyors or similar handling systems.
TRUCK TRANSPORT
Truck is widely used for transport of both liquid and dewatered
sludges. This mode offers flexibility because the terminal points and
route of haul can be changed readily and at low cost. Investment in
terminal facilities can be minimal. Many truck configurations are
available ranging from standard tank and dump bodies to very special-
ized equipment for hauling and spreading sludges. Trucks can be pur-
chased or leased or the hauling contracted to a private operator. The
generalized cost curves presented are based on the following criteria
and assumptions:
1. Most economical type truck from selection of standard frame
or semi-trailer mounted bodies; tanks for liquid and dump
or ram type for dewatered.
2. Eight hours of trucking operation per day.
VI-2
-------
3.
Fuel cost at $0.60 per gallon.
4. Amortization of truck capital cost over 6 years at 7 percent.
5. Truck O & M cost, excluding fuel and operator, $0.20 to 0.30
per mile depending on type of truck.
6. Truck loading time 30 minutes and unloading time 15 minutes.
7. Truck average speed 25 mph for first 20 miles one way and
35 mph for rest.
8. General and administrative costs 25 percent of total 0 & M
cost.
9. Sludge densities: Liquid - 62.4 lbs/cf; Dewatered - 55 lbs/cf;
Ash - 50 lbs/cf.
In general, the total cost of truck transport will be decreased (per
unit of material hauled) if the daily period of truck operation is in-
creased. Restrictions may be placed on any significant truck operations
such as specific routes or daylight hours for operations. The larger
trucks are the most economical except for one way haul distances less
than ten miles and annual sludge volumes less than 3,000 cubic yards for
dewatered sludge and for less than one million gallons per year for liquid
sludge. Generally, diesel engines are used in the larger trucks and are
the economical choice for small trucks when operated at high annual mileage.
Table 20 summarizes truck transport costs for a variety of sludge types
and haul distances.
BARGE TRANSPORT
Barge transport has been used in the past for ocean disposal of
sludges, but barge can be used for transport of sludges between land
points that are connected by navigable waterways. The use of barges is
limited to those locations in reasonable proximity to suitable waterways.
Barges have been used in the past for transport of liquid sludges
and no applications for dewatered sludges are known. Barges can be
leased or purchased or the barging can be performed by an outside pri-
vate operator. In most cases, the towing is subcontracted to a tug
operator. Self propelled barges have been used in New York City for
many years but, except for special cases, separate tugs and barges offer
more flexibility.
In general, the large barges are much more cost effective than
smaller barges. Larger barges have deeper drafts and, therefore, may
not be practical for many inland waterways. The major factor in barging
is the cost of tyg (towing) services and the larger barges minimize
this cost.
VI-3
-------
TABLE 20
TRUCK TRANSPORT COSTS
10
Tons Dry Solids
Per Day
COST,
$/TON (DRY
SOLIDS)
SLUDGE
TYPE
ONE
WAY HAUL, MILES
% SOLIDS
5
10
20
40
80
4
$35.60
$46.60
$74.00
$120.50
$200.00
LIQUID
7
21.90
30.10
46.60
71.20
123.30
11
17.50
21.40
35.60
49. 30
79.50
DEWATERED
20
15.90
19.70
24.10
35.60
49. 30
40
11.50
13.40
15.90
19. 20
26.30
DRIED
95
9.00
10.40
11. 20
12.90
15.90
ASH
100
7.10
8.00
9.00
10.10
11.80
25
Tons Dry Solids Per Day
COST
, $/TON (DRY
SOLIDS)
SLUDGE
TYPE
ONE
WAY HAUL, MILES
% SOLIDS
5
10
20
40
80
4
$29.60
$41.60
$68.00
$116.50
$198.20
LIQUID
7
18.60
24.10
41.60
68.00
120.60
11
14.30
18.60
28.50
47.10
77.80
DEWATERED
20
10.10
14. 30
19.70
28.50
48. 20
40
6.90
8. 80
11.00
17.50
25.20
DRTFD
95
4.70
5.60
6.50
8.10
10.70
ASH
100
4.05
4.40
4.90
5.70
7.20
VI-4
-------
80
60
70
10
60
40
50
)
,80
90
,00
,60
,60
.10
.20
TABLE 20 (Cont'd)
50 Tons Dry Solids Per Day
COST, $/TON (DRY SOLIDS)
ONE WAY HAUL, MILES
% SOLIDS 5 10 20 40
4 $26.90 $39.50 $65.80 $109.60
7 16.40 23.00 38.40 76.70
11 11.50 15. 30 26. 30 44.90
20 8.80 10.40 15.90 26.90
40 4.90 7.10 9. 30 14. 30
95 3. 20 4.00 4.90 7.10
100 2.40 2.70 3.20 4.10
100 Tons Dry Solids Per Day
POST, $/TON (DRY SOLIDS)
ONE WAY HAUL, MILES
% SOLIDS 5 10 20 40
4 $27.40 $33.80 $54.80 $104.10
7 15.40 23.00 35.60 65.80
11 10.40 15.10 24.40 46.60
20 6.30 8.50 14.30 26.30
40 4.40 5.20 8.00 13.40
95 2. 30 3. 30 4.40 6.00
100 1.60 2.00 2.50 3.60
VI-5
-------
The information in this section is based on barges up to 850,000
gallon capacity, but barges are available in sizes to two million
gallons and greater. These larger sizes will substantially reduce the
cost of transport for medium to large installations, but the larger
barges may be too large for some inland waterways. As an example, for
an annual sludge volume of 150 million gallons and a one way haul distance
of 150 miles, the total annual cost using two million gallon capacity
barges would be half of the total annual cost using 850,000 gallon barges.
The generalized cost curves were based on the following criteria
and assumptions:
1. Most economical barge size up to 850,000 gallons.
2. Single barge per tow.
3. Towing services contracted to outside tug operator.
4. Amortization of barge cost over twenty years at 7 percent.
5. Barge loading and unloading time five hours each.
6. Barge average towing speed 4 mph.
7. Barges not manned during tow.
8. General and administrative costs 25% of total 0 & M cost.
Barge transit times will be variable depending on traffic, draw
bridges, locks, tides, currents, and other factors. The 4 mph speed
is an average and speeds in open water may exceed 7 mph. Barges are
normally unmanned during transit.
Loading can be accomplished by either a gravity pipeline or pump(s)
and pipeline from a storage tank. A barge is normally filled in 2-5 hrs.
Unloading requires a pump(s) for transfer of sludge to a storage
system. The pump can be barge or dock mounted and can be diesel or
electric.
The use of barge was limited to liquid sludge because of the diffi-
culty of unloading dewatered sludge from a barge and because of lack of
full scale experience.
Table 21 summarizes barge transport costs.
RAILROAD TRANSPORT
It is hard to obtain information on railroad transport for gener-
alized cases. Most rail companies prefer to deal in specific cases.
There are very few actual cases of rail transport of sludges at present,
so there is little experience from which to draw information.
VI-6
-------
4
7
11
4
7
11
4
7
11
4
7
11
TABLE 21
BARGE
TRANSPORT COSTS
10
TONS DRY
SOLIDS
PER DAY
COST,
$/TON C DRY
SOLIDS)
ONE-WAY HAUL,
MI LES
20
40
80
160
320
$82.20
$93.20
$1 17.80
$161.60
$268 .
68.50
82 .20
98.60
123.30
189.
63.00
74 .00
87. 70
106.90
139.
25
TONS DRY
SOLIDS
PER DAY
$48.20
$60.30
$ 87.70
$ 142.50
$230.
38.40
43 . 80
57. 00
85.50
153.
32 .90
37 . 30
48.20
64. 70
93.
50
TONS DRY
SOLIDS
PER DAY
$34.00
$49.30
$ 76.70
$120.50
$2 19.
25.80
31. 80
46.60
71.20
142 .
2 1. 36
25 . 80
35. 10
50.40
93.
100
TONS DRY
SOLIDS
PER DAY
$27.40
$46.60
$ 71.20
$115 . 10
$216.
18.90
27.40
43. 80
68.50
137.
14.50
19. 50
30 . 10
46.6 0
90.
50
00
70
00
40
20
20
50
20
40
00
40
VI-7
-------
Rail cars can be leased from manufacturers on a full maintenance
basis. This would be the best method to assure a continuous supply of
cars in good running condition. Rail companies provide a rebate of
approximately $0.06 to $0.20 per loaded mile (depending on condition of
the car) to compensate the shipper for providing his own cars. The
number of cars required is related to the round trip transit time. Transit
times have a significant effect on the number of rail cars needed and,
hence, on capital or lease costs. Even with careful planning it would
be difficult to reduce rail transit time, even between close points, to
less than three days round trip because of train make-up, switching, and
weighing. Round trip transit time typically will be four to eight days
for one way haul distances of 20 to 320 miles.
Rail rates vary widely, but in general rates in various parts of the
country vary according to the following table:
Approximate Railroad
Area Rate Variation
North Central and Central - Average
Northeast 25% Higher than Average
Southeast 25% Lower than Average
Southwest 10% Lower than Average
West Coast 10% Higher than Average
The following rates were used in preparing costs:
One Way Rate,
Distance, Miles $/Net Ton
20 2.10
40 3.00
80 4.10
160 6.50
320 12.20
Table 22 presents typical costs.
PIPELINE TRANSPORT
There are many choices to be made in the design of a sludge pipeline
system. The following assumptions were made for purposes of this paper
and are representative of design criteria used in actual designs. The
liquid sludge was assumed to be reasonably free of grit and grease,
similar to anaerobically digested material. Raw sludge can also be
transported by pipeline, but the grease may require additional main-
tenance procedures. The solids content does not affect the calculations
within the range of 0-4 percent solids. The minimum pipeline size is
4 inch. The literature describes installations with smaller pipelines,
but these small pipelines represent special design cases.
Sludge pumps are of the dry pit, horizontal or vertical, non-clog,
centrifugal type which are widely used for sludge pumping applications.
Because of the high friction loss in 4 and 6 inch pipelines, pumping
stations for these lines contain more than one pump in series in order
to develop higher pumping heads and minimize the number of stations.
Two pumps are operated in parallel for the 16, 18, and 20 inch pipelines
because of the high flows. Each pumping station contains facilities for
pipeline cleaning using plastic pigs and macerators.
VI-8
-------
TABLE 22
RAILROAD TRANSPORT COSTS
10 Tons Dry Solids Per Day
COST,
$/TON (DRY
SOLIDS)
SLUDGE
ONE
WAY HAUL, MILES
TYPE
% SOLIDS
5
10
20
40
80
4
$87.70
$104.10
$134.30
$205.50
$356.20
LIQUID
7
54.80
65.80
82. 20
197.30
350.70
11
43.80
52.10
65.80
85.00
137.00
DEWATERED
20
25. 20
35.60
41.10
49. 30
82. 20
40
17.30
19.70
22.50
27.10
46.60
DRIED
95
_
••
_
—
ASH
100
_
__
25
Tons Dry Solids Per Day
COST,
, $/TON (DRY
SOLIDS)
SLUDGE
ONE
WAY HAUL, MILES
TYPE
% SOLIDS
5
10
20
40
80
4
$84.40
$101.90
$131.50
$197.30
$350.90
LIQUID
7
49. 30
59.20
74.50
120.60
208.20
11
34. 00
40.60
52.60
81.10
131.50
DEWATERED
20
18.60
21.90
28.50
39.50
68.00
40
12.10
15.30
18.60
21.90
38.40
DRIED
95
6.80
8.00
9.20
10.90
18.60
ASH
100
__
_
VI-9
-------
TAB LI' 22 (Cont'd)
50 Tons Dry _Sol_ids Per Day
sludge
TV PE
Lipurn
DEWATHRED
DRIED_
ASH
4
7
_1_1
20
40
$82.20
46.60
31. 20_
13. 20
8.80
OS
5.10
100
3.50
COST, ?/TON (DRY SOLIDS)
ONE WAY HAUL, MILES
10
$98.60
54.80
38. 40
19. 20
11.00
7.10
4.10
20
24.10
14.30
8. 20
4.60
40
35.60
19.70
9.90
6.00
80
$126.00 $191.80 $339.70
71.20 115.10 202.70
48.20 76.70 128.80
65.80
34.00
15. 90
9. 30
100 Tons Dry Solids__P££_
COST, $/TON (DRY SOLIDS)
SLUDGE
TYPE
LIQUID
nr. WATERED
DRIED
ASH
% SOT,IDS
4
7
__ 1J
2d
_40_
C)
l.t,M
ONE WAY HAUL, MILES
10
$79.50
43.80
_ 30.10
12. 90
7.40
4_.10
2.70
$98.63
49.30
37.00
18.40
_9. 90
4.90
3.60
20
23.80
12.30
6. 30
4.40
40
34.30
18.60
8.20
4.90
80
$123.30 $189.00 $328.80
65.80 101.40 186.30
46.£0 74.00 126.00
63. 00
32.90
14.50
8.80
VI-10
-------
The pipeline is cement lined cast iron or ductile iron pipe which
is typical for sludge pipelines. The cement lining provides long life
and a smooth interior surface. Installation is assumed to be in normal
soil conditions with average shoring and water problems typical to shallow
force main installations. Installation is assumed to be above hard rock.
The pipeline cost included one major highway crossing per mile and one
single track railroad crossing per five miles plus a number of driveway
and several minor road crossings per mile. These costs should be typical
for average installations to be expected for sludge pipelines.
The pipelines in this study were designed based on an operating
velocity of 3 fps. The depth of the pipeline will not affect the capital
cost within the range of 3 to 6 feet of burial in normal soil. Facil-
ities at the discharge end of the pipeline such as lagoons, dewatering
equipment, or spreading equipment are assumed to be a part of other unit
processes. Table 23 presents typical costs.
The following general conclusions on sludge transport costs may
be reached:
Liquid Sludges of 4% Solids
At 10 Tpd (dry solids) capacity, pipeline is the cheapest
method for distances up to 30 miles, and barge is the
cheapest for greater distances.
For 4 percent solids at a 25 Tpd (dry solids) capacity,
pipeline is the choice for distances up to 55 miles, and
barging at greater distances.
For plant capacities of 50 Tpd (dry solids) or more, pipeline
transport is the most economical method for all transport
distances.
Liquid Sludges of 7 and 11 Percent Solids
Truck hauling is typically lowest in cost for distances up
to 20 to 30 miles.
For distances greater than 20 miles and for plant capacities
exceeding 10 Tpd (dry solids), barge or rail transport is
less expensive than trucking.
Sludge Cake and Ash
At sludge solids concentrations greater than 20 percent,
usually the only alternates considered are truck and rail
transport.
For plant capacities of 10 Tpd (dry solids) or less, truck
transit is cheaper.
For plant capacities greater than 10 Tpd (dry solids), truck
and rail transport are competitive for hauls between 20 and 80
miles; below 20 miles, trucking is cheaper; and over 80 miles,
rail transit is cheaper.
VI-11
-------
TABLE 23
PIPELINE TRANSPORT COSTS
SLUDGE
% SOLIDS
0 TO 4
10 TONS DRY SOLIDS PER DAY
COST. $/TON (DRY SOLIDS)
DISTANCE, MILES
10
20
40
80
160
320
$14.10 $28.20 $56.^0 $112.80 $225.60 $451.20 $902.40
0 TO 4
2 5 TONS DRY SOLIDS PER DAY
$6.30 $12.60 $25.10 $ 50.20 $100.50 $200.90 $401.9C
0 TO 4
50 TONS DRY SOL IDS PER DAY
$ 3.30 $ b.CO $13.10 $ 26.30 $ 52.50 $ 105.00 $210. 10
0 TO 4
10 0 TONS DRY SOL^MJS PER DAY
$ 1.70 $ 5.50 $ 7.00 $ 13.90 $ 27.80 $ 55.60 $110.20
VI-12
-------
SECTION VII
ALTERNATIVE SYSTEMS
There are many combinations (some of which are summarized by Figure
1) of the unit processes for sludge handling that may be worthy of con-
sideration in a specific region. Among the factors which must be consid-
ered for each alternative system are:
Costs
Energy Requirements
Environmental Impacts
Availability (current and future) of consumables required
(chemicals, fuel, electricity).
Land Requirements
Operation Skills Required
Potential for resource recovery and resulting economic benefits
from the sludge or from sludge processing by products (i.e.,
digester gas, waste heat from incinerator).
Potentials for implementation delays (acquisition of land,
rights-of-ways for pipelines, sensitive environmental issues
involved).
Status of Technology Involved (demonstrated at plant scale,
pilot scale, etc.)
Experience with process(es) at other locales.
Compatibility with existing local, state, and federal guidelines
and regulations.
Energy implications may be particularly significant in the future.
One recent study^® concluded that relatively high energy requirements
for aerobic digestion, sludge (heat) drying, and incineration of sludges
with low solids content (<_ 20-25 percent solids), will make these options
economically less competitive (but not necessarily non-competitive) in
the future.
VII-1
-------
The preceding sections represent a brief summary of the major consid-
erations associated with many unit processes that may be combined into a
sludge handling system. Although there are several factors to be considered,
costs must be a major consideration. Table 24 presents a summary of factors
for major unit process alternatives. Ten and 100 ton/day systems are
tabulated to illustrate typical costs and the effects of scale on the
various processes. As noted earlier, costs must be carefully evaluated
for local conditions and these illustrative costs are offered to merely
provide an overall perspective of relative costs. The reader should refer
to the earlier sections of this paper and associated references for dis-
cussion of the factors that may effect costs.
Individual unit process costs in themselves do not reflect the overall
economic competitiveness of a unit process. For example, filter press
costs are typically higher than vacuum filtration or centrifugation costs;
however, the higher degree of dewatering achieved by the filter press may
result in significant economies in the downstream disposal process whether
it be incineration or landfill. Typical system costs are presented in
Figure 2 3 to provide still another perspective on costs. There are many
other possible combinations but these illustrate some of the most often
considered alternatives.
VI I-2
-------
TABLE M.
COMPARISON OF PROCESSES FOR PRIKARY PLUS VAS SLUDGES
CONDITIONING
THICKENING
DEVATERING
INCINERATION
DIGESTION
HEAT
DRYING
(41
I
DRYING BV COWPOSTING 1 TRANSFORT
SOLVENT Mi
EXTRACTION .
HEAT
TREATMENT
CHEMICAL
GRAVITY
FLOTATION
DRYING
BEDS
VF
(3)
CENTRIFUGE
(3)
FILTER
PRESS (3)
LAGOONS
SOL
20
OS %
40
AEROBIC
ANAEROBIC
Typical Costs
(J/Ton)
10 T/Day
100 T/Day
75
42
10-25
10-25
5
2
11.S0
C
70-90
N/A
43
25
37
22
76
40
10-20
10-20
95
45
54
31
46
22
47
41
100
63
95
62
40 I 10-300
30 1 per ton
Typical Energy
Requirements
kwh/Ton
Btu/Ten
100-150
3x106
Miniaal
1-1.5
100
-
40-40
200-400
50-200
-
50-90
8-10x106
1200-1500
150-200
200- 300
8-16x11)6
500-70U
6-7x106
Dependent on
Compost
Method
Can oe stg-
see Transport
Section
Land
Requiieaents
Miniaal
Mioiaal
Miniaal
Minimal
Appro*. 1
acre/ton/
day
Minimal
Minimal
Minimal
Appro*. 15
acres/ton /
day
Minimal
Minimal
Minimal
Minimal
Minimal
0.2-1
acre/ton
.
Opwatoc Skills
Required (*)
10
2
2
4
1
4
7
4
1
8
4
8
10
8
4
Resource
Recovery
Potential
Heat
Recovery
None
None
None
Dewatered Sludge to Land
Heat Recovery
None
Energy From
Gas
(1)
Dried Fertilizer
Soil Conditioner
-
Potential
Foi Delays
-
-
-
-
Land
-
-
-
Land
-
-
-
-
Agency Approval
Compost Site
Rights ol way
Status of
TocMofy
fide Use but
Prableas
Commi
Vide Use
Wide Use
Vide Use
Vide Use
Vide Use
Vide Use
Vide Use
Vide Use
Vide Use
Wide Use
Vide Use
Very limited
Use
In Development
Stage
Limited use to
Date
-
Compatibility
With
Reflations
Impact on
Effluent
Quality Odors
Must include
Provisions
for Air Poll.
Control
Air Pollution
Standards may
not be met
Some Agencies Do
Not Yet have Reg-
ulations Developed
(1) Can produce about 6*106 BtiVTon.
(2) 10- very hichly skilled; 1 - ainiaal skills - includes maintenance Mid operation considerations.
(3) Costs include cheaiui conditional.
(4) Costs do not include credit foe potential sale of resulting product.
-------
Figure 23
TYPICAL SLUDGE HANDLING (PRIMARY* WAS)
SYSTEM COSTS PER TON OF DRY SOLIDS
TYPICAL SYSTEM COSTS
10 TONS/DAY
100 TONS/DAY
CHEMICAL
VACUUM
20%
INCINERATE
CONDITION
FILTER
SOLIDS
$136
$71
CHEMICAL
FILTER
40%
INCINERATE
CONDITION
PRESS
SOLIDS
154
71
4%
DIGESTION
ANAEROBIC
TRANSPORT
PIPELINE
SPRAY
APPLICATION
TO LAND
5 miles (one-way)
20 miles
80 miles
64
103
259
46
51
71
ANAEROBIC
4%
TRUCK
LAND
DIGESTION
SOLIDS
TRANSPORT
APPLICATION
5 miles (one-way)
20 miles
80 miles
82
110
202
59
79
142
FLOTATION
THICKENING
4%
SOLIDS
UJ
AEROBIC
DIGESTION
PRIMARY
SPRAY
APPLICATION
TO LAND
PIPELINE
TRANSPORT
5 miles (one—way)
20 miles
80 miles
76
34
119
39
288
60
VII-4
-------
TYPICAL SYSTEM COSTS
10 TONS/DAY
100 TONS/DAY
HEAT
TREATMENT
VACUUM
40%
TRUCK
LAND
FILTER
HAUL
APPLICATION
SOLIDS
5 miles (one—way)
20 miles
80 miles
$ 135
140
150
74
77
93
CHEMICAL
20%
HAUL TO
STATIC
CONDITION
COMPOST
PILE
VACUUM
FILTER
SOLIDS
SITE
COMPOST
0 miles
5 miles (one—way)
10 miles
80
96
100
56
62
65
(Does not Include potential
income Irom sale of compost)
CHEMICAL
CONDITION
FILTER
40%
HEAT
PRESS
SOLIDS
DRY
95%
SOLIDS
180 104
(Does not Include potential
Income of $30/ton from sale
of dried product)
VI I-5
-------
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-1-
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-------
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-3-
-------
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56. Ettlich, W. F., "Composting as an Alternative", presented at the
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57. Epstein, E.; Willson, G. B.; Burge, W. E.; Mullen, D. C.; and Enkiri,
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60. Kalinske, A. A.; Pincince, A. B.; Klegerman, M. H.; Flynn, T. F. X.,
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63. Manson, R. J., and Merritt, C. A., "Land Application of Liquid Munici-
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65. Stone, R., "Practices in Disposal of Sewage Sludge by Landfill",
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66. Schwiny, J. E., and Puntenney, J. L. , "Denver Plan: Recycle Sludge
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67. Hyde, H. C., "Utilization of Wastewater Sludge for Agricultural
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68. Dalton, F. E., and Murphy, R. R., "Land Disposal IV: Reclamation and
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69. "Agricultural Utilization of Sewage Effluent and Sludge - An
Annotated Bibliography", Federal Water Pollution Control Admin-
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70. Hinesly, T. P., "Sludge Recycling: The Most Reasonable Choice?",
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71. Benjes, H. H., Jr., "Liquid Sludge Disposal on Land", presented at
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73. "Recycling Treated Municipal Wastewater and Sludge Through Forest
and Cropland", Pennsylvania State University Press (1973).
74. Powell, G.M., "Land Treatment of Municipal Wastewater Effluents, Design
Factors - Part II", presented at U.S. EPA Technology Transfer Seminars(1975).
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75. "Land Application of Sewage Effluents and Sludges: Selected Abstracts",
U.S. EPA Environmental Protection Technology SEries, EPA - 660/2-74-042
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76. Pound, C. E., and Crites, R. W. , "Wastewater Treatment and Reuse
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78. Kirkham, M. B., "Trace Elements in Corn Grown on Long-Term Sludge
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83. Jones, J. L.; Bomberger, D. C., Jr.; Lewis, F. M., "The Economics
of Energy Usage and Recovery in Sludge Disposal", presented at the
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84. Sieger, R. B., and Pracken, B. D., "Sludge, Garbage May Fuel California
Sewage Plant", American City and County, p.. 37 (January, 1977).
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of Their Potential Use as Fertilizers", Purdue University Agricultural
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88. "Application of Sewage Sludge to Cropland: Appraisal of Potential
Hazards of the Heavy Metals to Plants and Animals", EPA 430/9-76-013
(November, 1976).
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