United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600/2-80-096
Laboratory August 1980
Cincinnati OH 45268
Research and Development
Evaluation of Hot
Acid Treatment for
Municipal Sludge
Conditioning
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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ADDENDUM
EVALUATION OF HOT ACID TREATMENT FOR
HOT ACID TREATMENT FOR MUNICIPAL SLUDGE CONDITIONING
(EPA-600/2-80-096)
After completion of this report, the U.S. Environmental
Protection Agency (EPA) had an engineering assessment made of
the Wai den hot-acid process for removal of heavy metals from
municipal sludge. The results of this assessment will
eventually be available as an EPA report. In the interim, an
abstract of the report can be obtained from:
R. V. Villiers, Project Officer
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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EPA-600/2-80-096
August 1980
EVALUATION OF HOT ACID TREATMENT FOR
MUNICIPAL SLUDGE CONDITIONING
by
Kenneth J. McNulty
Ann T. Malarkey
Robert L. Goldsmith
Wai den Division of Abcor, Inc.
Wilmington, Massachusetts 01887
Henry A. Fremont
Champion International Corporation
Knightsbridge
Hamilton, Ohio 45020
Contract No. 68-03-2459
Project Officer
Roland V. Villiers
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publi-
cation. Approval does not signify that the contents necessarily reflect
the views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the'American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention, treat-
ment, and management of wastewater and solid and hazardous waste pollutant
discharges from municipal and community sources, for the preservation and
treatment of public drinking water supplies, and to minimize the adverse
economic, social, health, and aesthetic effects of pollution. This publica-
tion is one of the products ,of that research; a most vital communications
link between the researcher and the user community.
Land application is an attractive method for final disposal of sludge
solids. However, this method has not been fully exploited because of the
potential environmental risk certain concentrations of heavy metals in
sludge present. In some cases, the concentrations of heavy metals must be
reduced to acceptable levels prior to land disposal. This report presents
an investigation of a process designed to remove heavy metals from sludge.
The process involves both acidification and heating of the sludge. Results
show that the process has potential for good solubilization and removal of
toxic heavy metals and, in addition, destroys essentially all pathogens and
improves the dewaterability of the sludge. A preliminary economic analysis
of the process indicates that it is quite cost-competitive with alternative
stabilization/conditioning processes.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
m
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ABSTRACT
Bench-scale tests were conducted to evaluate the technical and economic
feasibility of the hot acid process for stabilization/conditioning of
municipal sewage sludge. This process involves acidification of the sludge
(pH 1.5-3) and heating to temperatures below boiling (^95°C). Test results
indicate that the process improves the dewaterability of the sludge, destroys
essentially all pathogens, and preferentially solubilizes certain heavy
metals relative to nitrogen and organics. The process demonstrated the
potential for good solubilization and removal of toxic heavy metals including
cadmium, zinc, and nickel with minimal solubilization of nitrogen. Thus the
hot acid process improves the desirability of sludge solids for land
application. A preliminary economic analysis of the process indicates that
it is quite cost-competitive with alternative stabilization/conditioning
processes.
Thts report was submitted i;n fulfillment of Contract No. 68-Q3-2459
by the Walden Division of Abcor, Inc. under the sponsorship of the U.S.
Environmental Protection Agency. This report covers the period of Sept-
ember 26, 1976 to February 11, 1979 and work was completed as of April 6,
i y / y *
IV
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CONTENTS
Foreword .. iii
Abstract '. iv
Fi gures vi
Tables ix
Acknowl edgment . xi i
,1. Introduction 1
2. Conclusions 4
3. Recommendations 7
4. Bench-Scale Process Studies 8
5. Comparison with Alternative Conditioning Processes 87
6. Optimization for Removal of Heavy Metals 105
7. Specification of Solids-Separation Equipment ' 123
8. Specifications for Pilot System 126
9. Energy and Economic Analyses 133
References 150
Appendi ces
A. Verification of Selected Analytical Results 153
B. Laboratory Report on Centri fugation Tests 156
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FIGURES
Number
Page
1 Digestion apparatus 10
2 Filtration test system 11
3 Centrifugation apparatus 12
4 Effect of stirring speed during digestion on the
sett!ing rate of hot-aci d treated WAS 18
5 Effect of oxygen sparging during digestion on the
sett!ing rate of hot-acid-treated WAS 19
6 Effect of the concentration of added acid on the
settling rate of hot-acid-treated WAS 21
7 Variation of WAS pH with time for various storage
conditions 23
8 Variation of WAS TOC with time for various
storage conditions 24
9 The effect of pH on the settling rate of WAS
digested at 95°C for 30 minutes 27
10 The effect of pH on the filtration rate of WAS
digested at 95°C for 30 minutes , 28
11 The effect of pH on the centrifugation rate of
WAS di gested at 95°C for 30 mi nutes 29
12 Acid demand vs. pH for various sludges (sludge
solids basis) 32
13 Acid demand vs. pH for various sludges (wet sludge
basis) 33
14 Effect of treatment conditions on settling rate for
Brockton WAS 39
15 Effect of treatment conditions on filtration rate
for Brockton WAS • o 49
16 Effect of treatment conditions on centrifugation rate
for Brockton WAS 41
17 Effect of treatment conditions on settling rate for
Fitchburg WAS 44
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FIGURES (Continued)
Number
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Effect of treatment conditions on filtration rate for
Fitchburg WAS
Effect of treatment conditions on centrifugation rate
for Fitchburg WAS
Effect of treatment conditions on settling rate for
Brockton pri mary pi us WAS •
Effect of treatment conditions on filtration rate for
Brockton primary pi us WAS •
Effect of treatment conditions on centrifugation rate
for Brockton primary pi us WAS
Effect of treatment conditions on settling rate for
Fi tchburg primary pi us WAS • • • •
The effect of time and temperature on the degree of
suspended solids solubilization for Fitchburg WAS
Effect of treatment conditions on the degree of COD
solubilization for Fitchburg WAS
Effect of treatment conditions on the degree of zinc
solubilization for Fitchburg WAS
Effect of treatment conditions on the degree of COD
solubilization for Brockton primary plus WAS
Effect of treatment conditions on the degree of COD
solubilization for Fitchburg primary plus WAS
Effect of ferric chloride dosage on the filtration rate
of hot-acid-treated WAS . ... •
Effect of ferric chloride dosage on the centrifugation
rate of hot-acid- treated WAS •
Effect of polymer dosage on the filtration rate of hot-
acid-treated WAS
Effect of polymer dosage on the centrifugation rate of
hot-acid-treated WAS
Comparison of filtration rates for WAS treated by
various conditioning processes •
Comparison of centrifugation rates for WAS treated by
various conditioning processes •
Schematic diagram of digestion apparatus used to re-
evaluate heavy metals solubilization ...
Cadmium solubilization as a function of pH .-
Cadmium solubilization as a function of acid usage .....
Zinc solubilization as a function of acid usage
Page
46
47
50
I" T
51
53
56
64
65
67
70
75
. . . 89
, , , 90
•• f**t '
91
••
92
f\f\
93
f\ r~ "
95
....-' , -
•'/.,. 106
112 :
,., 114
115
VI1
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FIGURES (Continued)
Number
39 Nickel solubilization as a function of acid usage ,
40 Chromium solubilization as a function of acid usage . „ ,
41 Solids solubilization as a function of acid usage ,
42 COD solubilization as a function of acid usage „ ,
43 Process flow schematic for 5 gpm hot acid treatment
44 Simplified flow schematic of hot acid sludge treatment
process .•
45 Material balance results for full-scale hot acid treatment
system
46 Energy balance results for full-scale hot acid treatment
system „
47 Operating and maintenance cost as a function of plant capa-
city for various sludge treatment alternatives
Page
116
117
118
119
127
134
137
138
148
vm
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TABLES
Number
Page
1 Assays and Procedures . ^
2 Solids Concentrations as a Function of Sludge
Age and Storage Conditions •• "
3 Acid Requirements as a Function of pH for
• Brockton and Fitchburg Sludges 6l
4 Test Matrix for Evaluation of Primary Variables 35
5 Effect of Treatment Conditions on Solid-Liquid
Separation Rates for Brockton WAS 36
6 Analysis of Variance Results fipr Solids Separation
Tests (Brockton Waste Activated Sludge) 37
7 Effect of Treatment Conditions on Solid-Liquid
Separation Rates for Fitchburg WAS 4^
8 Analysis of Variance Results for Solids Separation
Tests (Fitchburg Waste Activated Sludge) • • • 43
9 Effect of Treatment Conditions on Solid-Liquid
Separation Rates for Brockton Primary PI us WAS 48
10 Analysis of Variance Results for Solids Separation
Tests (Brockton Primary PI us WAS) 49
11 Effect of Treatment Conditions on Solid-Liquid
Separation Rate for Fitchburg Primary PI us WAS 54
12 Analysis of Variance Results for Solids Separation
Tests (Fitchburg Primary PI us WAS) 55
13 Effect of Treatment Conditions on Solubilization of
Sludge Constituents for Brockton WAS 58
14 Analysis of Variance Results for Solubilization
Tests (Brockton WAS) • • 60
15 Effect of Treatment Conditions on Solubilization
of SIudge Constituents for Fitchburg WAS 6^
16 Analysis of Variance Results for Solubilization
Tests (Fitchburg WAS) 63
17 Effect of Treatment Conditions on Solubilization of
Sludge Constituents for Brockton Primary Plus WAS 68
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TABLES (Continued)
Number
18
Analysis of Variance Results for Sol utilization Tests
(Brockton Primary Plus WAS) 59
Effect of Treatment Conditions on Sol utilization of
Sludge Constituents for Fitchburg Primary Plus WAS «••• 72
Analysis of Variance Results for Solubilization Tests
(Fitchburg Primary PI us WAS) 73
Summary of Metals Solubilization for Brockton WAS 76
Bacterial Assays for Raw and Hot-Acid-Treated Sludges 78
Summary of Optimization Tests for Solid-Liquid Separation.. 81
Summary of Solubilization Results for Optimization
Tests 82
Effect of Time and Temperature on the Survival of
Typical Pathogens Found in Sludge 85
Potential Advantages and Disadvantages of Hot Acid
Treatment 86
Solubilization of Solids and Organics for Various
Treatment Processes 96
Comparison of Solubilization for the Hot-Acid and
Thermal Conditioning Processes 97
Estimated Chemical Conditioning Dosages for
Vacuum Filtration 100
Comparison of Supernatant Quality for Various Sludge
Treatment Techniques „ 101
Comparison of Sludge Conditioning Alternatives 104
Effect of Treatment Conditions on the Degree of Solubil-
ization of Heavy Metals and Other Sludge Constituents 108
33 Solubilizations obtained for Sludges from Various Cities
at Preferred Operating Conditions 121
34 Specification for Centrifuges to Dewater Brockton WAS
FolTowing Hot Acid Treatment 125
35 Specification of Pilot System Components 130
36 Assumptions and Design Bases for Economic Analysis 135
37 Purchased Equipment Cost for Hot Acid Treatment 140
38 Capital Costs for Hot Acid Treatment 141
39 Operating and Maintenance Costs for Hot Acid Treatment
($/Dry Metric Ton Sol ids) 142
40 Capttal and Operating Costs for Anaerobic Digestion 144
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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TABLES (Continued)
Number
Page
41 Capital and Operating Costs for Aerobic Digestion 145
42 Capital and Operating Costs for Lime Treatment 146
43 Capital and Operating Costs for Heat Treatment 147
XI
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the following contributors to this
program.
-- Leah B. Daniel of the Walden Division of Abcor, Inc. for conducting
many of the bench-scale experiments and for providing technical
input on the experimental aspects of the program.
-- Tim Murphy and John Tanzi of the Brockton Wastewater Treatment
Plant, Rich Willians of the Fitchburg Wastewater Treatment Plant,
and Richard Manthe of the Milwaukee Sewage Commission for providing
samples of sewage sludge.
— Adam Nisbet, Jim Gibbs, and staff of Bird Machine Company for
conducting bench-scale and pilot-scale centrifugation tests.
— John R. Harland of the Walden Division of Abcor, Inc. for providing
engineering assistance on the plant design and economic analysis
for the process.
Financial support for this program was provided through the Municipal
Environmental Research Laboratory of the U.S. Environmental Protection
Agency. The support and technical assistance of the Project Officer,
Mr. R.V. Villiers, and the Director of the Ultimate Disposal Section,
Dr. J.B. Parrel!, are gratefully acknowledged.
xn
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SECTION 1
INTRODUCTION
The disposal of sludge from municipal wastewater treatment plants is a
problem of increasing complexity and magnitude. The amount of sewage sludge
produced in the United States is expected to increase over the next several
years because of increases in the sewered population and the upgrading of
existing wastewater treatment plants. Simultaneously, environmental re-
strictions are decreasing the options available for sludge disposal. At
present, four techniques for sludge disposal are in general use 0-):
—• sanitary landfill,
— ocean disposal,
— incineration with
-- land application.
landfill of ash and scrubber sludge, and
Of these, ocean disposal is being phased out by 1981.
Of the various options listed above, land application is the only option
that utilizes sludge in a beneficial manner. When applied to land, sludge
provides an excellent soil conditioner and results in the conservation and
reuse of organic matter, nitrogen, phosphorus, and certain trace elements,
all of wfifdT are necessary plant nutrients. At present, only about 25% of
the sludge produced is applied to land(2), and only part of that is applied
to cropland. On a national basis there is more than enough cropland available
to accomodate all of the sludge produced; however in the Northeast the
percentage of the cropland required for land application of sludge is higher
than the national average. (In New Jersey, for example, it is projected
appears
to be the most promising technique presently available for utilization/
disposal of sludge.
The land application of sludge, however, does have some potential
limitations particularly where the land is to be used for food-chain crops.
Of particular concern is the uptake of heavy metals such as cadmium, zinc,
nickel, copper, and molybdenum which can accumulate in plants and may pose a
hazard to plants, animals, or humans^2). Of these potentially hazardous
metals, cadmium is the metal of greatest environmental concern. The daily
dietary intake of cadmium for U.S. adults approximates the total tolerable
daily intake proposed by the Food and Agriculture Organization and the World
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Health OrganizationC3). Because of the potential of certain food-chain crops
to accumulate significantly increased cadmium levels from sewage sludge applied
to land, rules have been proposed(^) for limiting and monitoring the land
application of cadmium bearing sludges to food-chain crops.
In addition to heavy metals, the presence of pathogens and pesticides
(and other persistant organics) in the sludge are also of concern(4).
Pathogens may create a public health hazard, and pesticides and heavy metals
could be ingested directly if the sludge is applied in such a way as to
adhere to the edible portion of the crops. While lead is not a potentially
hazardous metal with respect to plant uptake, the direct ingestion of lead
(in addition to cadmium) should be avoided because of the proximity of the
current daily intake to the proposed tolerable level(3).
While there are many municipal sludges that, when stabilized, are suit-
able for direct application to land, many others contain high concentrations
of cadmium or other heavy metals. None of the sludge stabilization
processes in current use (e.g. anaerobic digestion, aerobic digestion,
thermal treatment (190°C), Time treatment, pasturization (70°C), etc.) is
capable of removing significant quantities of heavy metals. Thermal treat-
ment, for example, has been shown to remove approximately 72% of the nitrogen
content from sludge solids but it does not remove significant quantities of
heavy metals (5,6,7). Thus thermal treatment is detrimental to the soil
conditioning value of the sludge solids.
The objective of this program was to evaluate and develop a new process
for the treatment of municipal wastewater sludge. This process was invented
by Champion International Corporation (patent applied for ), and Abcor, Inc.
has an option to negotiate an exclusive liscense for its commercialization.
In brief, the process consists of acidifying the sludge (pH approximately 1.5
to 3) and heating it to a temperature below boiling (80 to 100°C) for a
relatively short time (10 to 60 minutes). This "hot acid treatment"
process:
1. improves the dewaterability of the sludge,
2. destroys essentially all pathogens, and
3. preferentially solubilizes heavy metals relative
to nitrogen and organics.
The preferred application for this new process is for stabilization/
conditioning of thickened waste activated sludge (WAS) or mixtures of primary
sludge and thickened WAS. Following hot acid digestion, the solids would be
separated from the liquor using appropriate dewatering equipment. Neutrali-
zation of solids (possibly by impregnation with anhydrous ammonia to simultan-
eously increase nitrogen levels) may be required prior to land application.
The liquor from the hot acid digestion would be treated with lime to pre-
cipitate the solubilized heavy metals before recycling the liquor to secondary
treatment. The heavy metal sludge would be dewatered and disposed of in a
secured landfill or by other appropriate techniques.
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Ideally, the hot acid process would be similar in concept and application
to the thermal conditioning process. Both would stabilize the sludge and
condition it for dewatering in a single operation. However, the unique
advantage of the hot acid process would be its ability to solubilize heavy
metals to a much greater degree than organics and nitrogen. This process, if
successfully developed to commercialization, could provide economical sludge
stabilization while upgrading the quality of solids for disposal by land
application, and could make land application a viable utilization/disposal
option for many municipalities where high levels of heavy metals currently
necessitate disposal by other alternatives.
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SECTION 2
CONCLUSIONS
Bench-scale tests were conducted to evaluate the effects of the primary
process variables, pH, temperature, and time, on the performance character-
istics of the hot acid process. These variables were investigated over the
ranges of: pH 2 to 3, temperature 80 to 95°C, and digestion time 10 to 60
minutes. Four sludges, two waste activated sludges and two primary-WAS
mixtures, were evaluated during these tests. The following conclusions are
based on the results of these tests.
1. Within the ranges investigated, the pH had the most significant
influence on the rate of solid-liquid separation following hot
acid treatment. For all sludges the optimum pH for solids sep-
aration was 2.5. The preferred temperature level was 95°C, but
digestion time, within the range investigated, did not have a
pronounced influence on the solids separation rate.
2. The amount of concentrated sulfuric acid required to reduce the
pH of the sludge to 2.5 was 70-100 kg/dmt (dry metric ton). At
an acid usage of 100 kg/dmt the cost for acid would be $4.95/dmt
($4.50/dst (dry short ton)) of sludge solids. The acid costs for
the process are therefore quite reasonable and operation at even
lower pH's cannot be ruled out on the basis of acid cost.
3. The primary process variables did not appear to have a significant
influence on the solubilization of suspended solids. Over the
range of variables investigated the average solubilization of
suspended solids ranged from 6.2% to 10.6% for the four sludges
tested.
4. The pH and temperature had a significant effect on the degree of
organics solubilization, as determined by COD. Lower pH and
higher temperature promoted greater COD solubilization. Within
the range of variables investigated the degree of COD solubili-
zation was generally <10%.
5. The pH was the only variable which had a significant influence on
zinc solubilization. Lower pH's resulted in greater solubili-
zation. Solubilizations as high as 80% were determined for zinc
during these tests. On the other hand, solubilizations of copper
at the conditions investigated during these tests were negligible.
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6. Bacterial assays confirmed the expectation that the hot acid
process destroys essentially all pathogens.
, Bench-scale tests were also conducted with WAS to directly compare the
hot acid treatment process with thermal treatment and chemical conditioning
(ferric chloride and polymer addition). Thermal treatment produced the
greatest improvement in sludge dewaterability. The improvement in sludge
dewaterability for hot acid treatment was at least as great as for chemical
conditioning. The hot acid process solubilized significant quantities of
zinc, cadmium, and nickel while thermal treatment produced no significant
solubilization of heavy metals, with the possible exception of nickel. On
the other hand, the hot acid treatment solubilized only 8% of the COD
compared to 27% for thermal treatment and solubilized only 10% of the
nitrogen compared to 76% for thermal treatment. Thus the sludge from hot
acid treatment should be much more desirable for land application than
sludge from a thermal treatment process.
Additional bench-scale tests were conducted to define the factors re-
sponsible for solubilization of heavy metals and to optimize the hot acid
process for heavy metals removal. Consistent correlations were obtained
between the degree of solubilization and the acid usage in kg of concentrated
H2S04 per dmt. These correlations indicate a rapid increase in the degree
of solubilization of cadmium, zinc, nickel, and chromium over the range of
100 to 200 kg/dmt and-of copper (based on limited data) over the range of
250 to 300 kg/dmt. The correlation of metals solubilization with acid usage
was found to be more consistent than correlation with pH.
Tests conducted with fresh WAS samples from various municipalities
indicated excellent solubilizations of cadmium (88-100%), zinc (82-100%), and
nickel (73-100%) at acid usages >200 kg/dmt. Only moderate solubilization
of chromium (M5%) was obtained, and appreciable copper solubilization (^80%)
required a higher acid usage (>300 kg/dmt). For these tests, average solids,
COD, and TKN solubilizations were 24, 16, and 28%, respectively.
An acid usage of 200 kg/dmt was selected as the optimum for heavy metals
solubilization. This is two to three times as great as the acid usage re-
quired for optimum dewatering (70-100 kg/dmt to achieve pH 2.5). However,
the cost for acid at a usage of 200 kg/dmt is still <$10/dmt ($9/dst) and is
considered to be quite acceptable for the levels of solubilization obtained.
Bench-scale and pilot-scale centrifugation tests were conducted in order
to specify full-scale dewatering equipment for the hot acid process. The
pilot tests indicated good cake solids concentrations (18-36% solids), but
solids recoveries were low. The use of a flocculant was recommended to
improve the recovery of solids. :
Preliminary designs and specifications were developed for a pilot-scale.
and various-capacity, full-scale, hot-acid-treatment plants. For a full-
scale plant with a feed capacity of 20 dmtpd (dry metric tons per day) of
5
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solids at a consistency of 3%, the total capital investment is estimated at
$250,000 for hot acid treatment alone and at $647,000 for hot acid treatment
plus solids separation and liquor treatment. Depending upon the amount of
acid added, the process can be optimized either for dewaterability (MOO kg/
dmt) or for metals sol utilization (^200 kg/dmt). The operating and mainten-
ance costs, including the cost of capital, for a 20 dmtpd plant are:
Hot Acid
Treatment Alone
Optimum Dewatering
Optimum Metals Removal
$/dmt
19.00
24.00
($/dst)
(17.25)
(21.79)
Hot Acid Treatment,
Solids Separation, &
Liquor Treatment
$/dmt ($/dst)
29.90
37.71
(27.15)
(34.24)
Total operating and maintenance costs, including cost of capital, were
compared for various stabilization/conditioning processes: hot acid treat-
ment, thermal treatment, anaerobic digestion, aerobic digestion, and lime
treatment. The comparison did not include the cost of dewatering and
supernatant (or liquor) treatment. The estimated treatment costs for a
20 dmtpd plant decreased in the order: anaerobic digestion ($74/dmt),
aerobic digestion ($47/dmt), thermal treatment ($39/dmt), hot acid treatment
($19-247dmt), and lime treatment ($16/dmt).
Based on the results of this program it is concluded that the hot acid
process is a highly promising technique for the treatment of municipal sludge
It has been shown that this process:
— substantially improves sludge dewaterability,
— destroys essentially all pathogens,
— has the potential to solubilize significant quantities
of heavy metals including cadmium,
~ preferentially solubilizes heavy metals rather than
nutrients such as organics and nitrogen, and
~ is highly cost-competitive with alternative stabilization/
conditioning processes.
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SECTION .3
RECOMMENDATIONS
The bench-scale tests have indicated that the hot acid process is a
highly promising technique for sludge stabilization/conditioning, and further
development of the process is recommended. The next step in development of
the process should be the design, fabrication, and testing of continuous-
flow, transportable, pilot system similar to that described in Section 8.
This system should be installed and operated at various municipal treatment
plants in order to evaluate the process under dynamic flow conditions with
fresh sludge.
Rather than designing and building an entire pilot system at the outset,
it is recommended that only the hot acid treatment portion be.built initially.
Field operation of this portion of the pilot plant will permit an evaluation
of the operating characteristics and control methodology for hot acid treat-
ment. In addition, an assessment should be made of alternative techniques
for dewatering the hot-acid-treated sludge and for treating the liquor
remaining after separation of the solids. When the preferred techniques
have been identified, the pilot system should be modified to include all unit
processes and the performance of the integrated system should be,-demonstrated.
Sufficient information should be obtained from operation of the pilot system
to permit an accurate assessment of the process economics.
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SECTION 4
BENCH-SCALE PROCESS STUDIES
Bench-scale tests were conducted in order to characterize and optimize
the performance of the hot acid process. Specific objectives of these tests
were:
— to evaluate the process with various secondary sludges and
and various mixtures of primary and secondary sludge;
— to determine the acid requirements for the various sludges;
— to optimize the process with respect to pHs
temperature, and reaction time; and
— to evaluate alternative solid-liquid separation techniques.
METHODS AND MATERIALS
Sludge Identification
Tests were conducted,with sludge from two municipalities in eastern
Massachusetts: Brockton and Fitchburg. At both locations samples of primary
sludge and waste activated sludge (WAS) were obtained. Tests were conducted
either with WAS alone or with an equal-volume mixture of primary and WAS.
The individual sludges were stored separately and the primary WAS mixtures
were prepared just before the tests in which they were used. Samples were
obtained in 5-gal polyethylene carboys and refrigerated immediately upon
arrival at Walden. The samples were used within 7 days of the date on which
they were obtained.
Samples of Brockton waste activated sludge were obtained after dissolved-
air-flotation thickening. No chemicals were added prior to the thickener.
The Fitchburg waste activated sludge was obtained following dissolved-air-
flotation thickening which was preceeded by the addition of polymer (Nalco
7120 or Calgon 2620). The total solids (TS) concentrations of the sludges as
received were:
Range of Total
Sludge Solids "
Brockton WAS
Brockton primary & WAS
Fitchburg WAS
Fitchburg primary & WAS
2.5
4.0
4.0
4.2
4.9
4.5
4.1
6.8
8
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Bench-Scale Test System
Digestion—
The system used for hot acid digestion of the sludge is shown in
Figure 1. The acidified sludge was contained within a one-liter graduated
cylinder wrapped with heating tape, and the power input to the heating tape
was controlled by a variable transformer. The contents of the cylinder was
agitated at a speed of about 60 rpm to provide good heat transfer from the
walls of the container. Agitation was provided by a Phipps and Bird stirrer
(Model 7790-300) which was modified for use with graduated cylinders as shown
in the "Stirrer Detail" of Figure 1. A dial thermometer was attached to the
shaft of the stirrer to indicate the sludge temperature. Three one-liter
samples of the sludge were treated simultaneously.
Settling--
Settling tests were conducted in the same graduated cylinders as used
for digestion (Figure 1). Following the digestion period the stirrers were
removed from the cylinders, the power to the heating tapes was turned off,
and the sludge-water interface was recorded as settling occurred.
Filtration—
The filtration test system is shown in Figure 2. Samples of treated
sludge were filtered through Whatman #1 filter paper which was supported in
a Buchner funnel. A,tubular plexiglass extender was used to permit the
entire 45 ml sludge sample to be poured into the funnel at one time.
Filtrate was produced under a controlled vacuum of 51 cm (20 inches) of
mercury and was collected in a.250 ml graduated cylinder-
Centrifugation— , ,
The apparatus^ shown in Figure 3 was used to centrifuge samples of the
treated sludge. The centrifuge tube was a plastic graduated cylinder (10 ml)
with the base removed, and a slot was machined in the metal tube shield. A
stroboscope was used to "stop" the rotation of the centrifuge so that the
interface height could be recorded as a function of time without stopping
and removing the tubes. The centrifuge (Clay Adams Model 0101) was
operated at 2,600 rpm which provided an average centrifugal acceleration of
750 G's.
Experimental Procedures
The refrigerated carboy containing the sludge was agitated to mix the
sludge before removing samples. Three one-liter samples were placed in
beakers for pH adjustment and a fourth sample was set aside for analysis.
The three one-liter samples (generally adjusted to different pH's) were
treated simultaneously. The pH was adjusted to the desired level by addition
of concentrated (96%) reagent-grade sulfuric acid. The samples were then
poured into the graduated cylinders of the apparatus shown in Figure 1 and
heated to the desired temperature. The time required to heat the sludge was
generally in the range of 10-20 minutes with the longer times required for
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samples exhibiting a greater tendency to foam. Samples were held at the
desired temperature for the desired time by manually controlling the power
input to the heating tapes.
Following digestion, 45 ml of the sludge from each of the three
cylinders was poured into each of the three corresponding filters, and 10 ml
from each of the three cylinders was poured into the three corresponding
centrifuge tubes. The settling tests were conducted with the remaining
sludge (~ 900 ml in each cylinder) by recording the interface height as a
function of time over a 1-hour period. The filtration and centrifugation
tests were conducted simultaneously as soon as possible after placing the
samples in the respective test equipment. For filtration, the volume of
filtrate was recorded as a function of time over a period of one hour, while
for centrifugation, the interface height was recorded as a function of time
over a period of 10 minutes.
Sampling and Analysis
Following the settling test, the contents of each graduated cylinder was
thoroughly mixed and divided into two portions. One of the two portions was
retained for analysis; the other was filtered through Whatman fl filter paper,
and the filtrate was retained for analysis. Similar samples (unfiltered and
filtrate) of the raw sludge, which had been set aside at the outset of the
test, were also retained for analyses.
The assays performed and the analytical procedures used are listed in
Table 1.
Samples of sludge and sludge filtrate which had been subjected to the
hot acid treatment were preserved by refrigeration prior to analysis. It
was believed that the low pH of the samples (generally 2-3) along with
refrigeration would sufficiently arrest any biological activity. On the
other hand samples of the raw sludge were chemically preserved in addition to
refrigeration. Samples for total solids analyses were preserved by the
addition of formaldehyde (to 1% by weight). The formaldehyde was assumed to
vaporize during evaporation of the sample prior to the gravimetric determin-
ation of total solids. Samples for COD and metal analyses were preserved by
reducing the pH to 1.0 with I-^SO^..
Data Reduction
In general the calculational procedures used in reducing the data were
straight forward and require no special explanation. However, the rationale
and the procedures used to calculate the percent of various sludge con-
stituents solubilized by the hot acid process are not directly obvious and
deserve further clarification. For each set of three hot-acid-treated
samples, a sample of raw sludge was obtained and analyzed as the control to
which the hot-acid-treated samples were to be compared. Thus for a hot-
acid test at given conditions the analytical results obtained for sludge
constituent M (e.g. a heavy metal or COD) were:
13
-------
TABLE 1. ASSAYS AND PROCEDURES
Assay
Total solids
Volatile solids
BOD
COD
TOC
Zinc
Copper
Cadmi urn
Nickel
Chromium
Lead
Total phosphorus
TKN
Fecal coli form
Fecal streptococcus
Procedure
Gravimetric - total residue
dried at 103°C - 105°C
Gravimetric - at 550°C
5 day incubation, electrode
Di chroma te reflux
Combustion - membrane detection
Dohrmann Envirotech T.O.C.
Analyzer
Atomic absorption
Atomic absorption
Atomic absorption
Atomic -absorption
Atomic absorption
Atomic absorption
Persulfate digestion
Ascorbic Acid Method
Kjeldahl, selective ion electrode
Fecal coli form membrane
filter procedure
Membrane filter technic
Reference*
SM208A
SM208E
SM507
SM508
EPA, p.
SM301A
SM301A
SM301A
SM301A
SM301A
SM301A
SM425C,
EPA, p.
SM909C
SM910B
236t
425F
175t
Standard Methods for the Examination of Water and Wastewater. Fourteenth
edition. American Public Health Association, Washington, D.C. 1976.
Methods for Chemical Analysis of Water and Wastes. EPA-625/6-74-003. U.S.
Environmental Protection Agency, Office of Technology Transfer, Washington,
D.C., 1974.
14
-------
1. Total concentration ' of M -in raw sludge (unfil te red) = (TM)raw
2. Total concentration of M in raw sludge filtrate == (DM).,.',..,, '..."'
i dw
3. Total concentration of M in treated sludge (unfiltered) =
(TM) treated ..... '
4. Total concentration of M in treated sludge filtrate =
^treated
The concentration of suspended constituent M in the raw and treated
samples is then:
(SM>
= <™
aw
treated
treated ' (DM)treated
The simplest procedure for calculating the percent of constituent M
solubilized by the treatment process is:
% Solubilized =
raw
treated
(SM)
x'100%
(1)
raw
The use of Equation (1) to calculate the percent solubilized resulted in
numerous inconsistent and widely scattered results. For example, for one
set of analyses the percent COD solubilized was -7% as calculated by
Equation (1) even though the COD of the treated filtrate was nearly 8 times
as great as the COD of the raw filtrate.
A careful inspection of the data indicated that the results calculated
by Equation (1) are strongly influenced by inaccuracies in obtaining
representative samples of sludge. Because of the heterogeniety of the raw
sludge, two samples taken from the'same container of sludge could have
significantly different concentrations of suspended constituents (solids,
COD, metals) although the dissolved constituents would be expected to be
homogeneously distributed for all samples. These sampling-related differences
in suspended constituents for two samples (one,to be treated, one to remain
untreated) can have a large influence on the percent solubilization
calculated byEquation (1). ,
A second source of inaccuracy in the use of Equation (1) is that any
constituents that are volatilized by the hot acid process (e.g. carbonates
-»• carbon dioxide) are calculated as being solubilized. Thus in many, cases
the calculated degree of solubilization using Equation (1) indicates much
higher dissolved concentrations in the filtrates than are actually present.
In order to reduce the effects of sampling inaccuracies and volatili-
zation on the calculated percent solubilization, the amount of suspended
constituent in the raw sludge was calculated from the treated sample:
(SM>
raw
treated ' (DM)raw
(2)
15
-------
Then (SM)*aw was used in Equation (1) in
procedure gave quite consistent results,
calculated.
place of (SM)raw. This
and no negative solubilizations
calculational
were
The degree of suspended solids sol utilization was calculated by a similar
procedure except that Equation (2) was modified to take into account the
sulfate added with the sulfuric acid during acidification of the sludge. Thus
Equation (2) becomes:
(SS>raw = ^treated - (DSVaw ' Padded
(3)
where:
(SS)
raw
treated
(TS(Vadded
calculated suspended solids concentration of raw
sludge
total solids concentration of treated sludge
dissolved solids concentration of raw sludge
total concentration of sulfate added to the raw
sludge
The amount of sulfate added was calculated from the known volume of concen-
trated H?S04 added to the sample, the concentration of the acid (96%), its
density (1.83 g/cc), and its SOzj: content. The concentration of suspended
solids in the treated sludge was calculated by:
(SS>
treated
= ^
treated
treated
(4)
No correction for sulfate is required since both the total and dissolved
solids analyses for the treated sample include the added sulfate, which is
assumed to remain dissolved. Equations (3) and (4) were then used in
Equation (1) to calculate the percent sol utilization of suspended solids.
RESULTS AND DISCUSSION
Tests were conducted to characterize and optimize the hot acid process
with respect to the important performance criteria for sludge conditioning.
Results are presented below for the acid requirements of the process, the
solid-liquid separation rates, the solubilization of various sludge
constituents, and the destruction of pathogens.
Preliminary Tests
Preliminary tests were conducted at the outset of the program in order to
evaluate the effect of several process variables believed to be of minor im-
portance and to determine the range of interest of the process variables
believed to be of primary importance (pH, temperature, and digestion time).
The minor process variables investigated were:
1. Stirring speed during digestion
2. Aeration during digestion
3. Concentration of ^SCvj. added to the raw sludge
16
-------
4. Sludge age and storage conditions
5. Ferric chloride or polymer addition to the treated sludge.
The ranges of major process variables investigated were:
pH
Digestion Temperature
Digestion- Time
1.5 - 4.0
80 - 95°C
10-60 minutes
All preliminary tests were conducted with Brockton WAS using the
procedures described above. Unless otherwise specified, the minor process
variables were investigated at a pH of 2, a temperature of 95°C, and a
digestion time of 30 minutes.
Stirring Speed During Reaction—
Different stirring speeds were investigated during digestion to determine
if the stirring speed affects the solid-liquid separation rate or the degree
of TOC solubilization. Two samples of sludge were acidified to pH 2 and
digested at 95°C for 30 minutes using a stirring speed of 60 rpm. One of
the samples was then subjected to high shear by mixing in a Waring blender
for two minutes.
Settling curves for the two samples are shown in Figure 4. The height
of the interface (as indicated by the ml graduations on the graduated
cylinder) is plotted as a function of settling time. The sample subjected
to high shear settled much slower than the sample subjected to low shear.
This suggests that floes formed during digestion may have been broken up
by the intense mixing in the blender.
Since homogenization is used in many biological investigations to
induce lysis'of cellular materials, it was anticipated that high-shear
mixing would increase the TOC of hot-acid-treated sludge. However, the TOC
of the filtrate of the high-shear sample (1140 mg/1) was slightly lower
than the TOC of the filtrate of the low-shear sample (1250 mg/1). This
indicates that cell lysis was not significant at the stirring speeds used.
It was concluded that the stirring speed should be adjusted to the minimum
speed that maintains a reasonably uniform temperature profile throughout
the sludge during digestion. In all subsequent tests with this apparatus a
stirring speed of 60 rpm was used. (Tests at a higher stirring speed
using a different apparatus are described in Section 6).
Aeration During Reaction--
Aeration during digestion was investigated to determine whether or not
it had any effect on the solids separation rate or the degree of TOC
solubilization. Two samples were digested at identical conditions except
that oxygen was sparged into one of the samples during digestion.
The settling curves are shown in Figure 5 for both the oxygen-sparged
sample and the control. The oxygen-sparged sample settled more slowly than
the control. This may be the result of oxygen-entrainment in the sparged
sludge which would reduce its density and inhibit settling.
17
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19
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There was no significant difference in the concentrations of soluble
TOG for the two samples (1500 mg/1 for the oxygen-sparged sample, 1540 mg/1
for the control).
Based on these results, air sparging would appear to have no beneficial
effects on the hot-acid process. The evaluation of air sparging to improve
the sol utilization of heavy metals is considered in Section 6.
Concentration of Sulfuric Acid--
The effect of acid concentration of process performance was investigated
using three different acid concentrations:
1. concentrated H2S04 (96% by weight)
2. concentrated 1^04 diluted with an equal volume of water (50:50)
3. concentrated H2S04 diluted with three times its volume of
water (25:75)
Settling curves are shown in Figure 6 for digested samples acidified with
the three different concentrations of acid. The samples treated with the
two higher acid concentrations gave similar settling curves,, and, except for
the last 15 minutes of the test, both settled more rapidly than the 25:75
samp!e.
The effect of the concentration of added acid on the solubilization of
organics (here, measured as soluble TOC) was investigated. The TOC values
of the filtrates for the concentrated 50:50 and 25:75 concentrations were
1760, 1850, and 2120 mg/1, respectively. This is interesting, since it was
expected that the higher concentrations would solubilize more organics.
Although the explanation of this behavior is not apparent, it can be con-
cluded that concentrated acid should be used in the process.
Sludge Age and Storage Conditions--"1
Sludge age and storage conditions were investigated in order to
determine whether a new supply of sludge was required each day, or whether
sludge could be stored for a certain period of time without significant
deterioration with respect to evaluating process performance.
The three storage conditions examined were: 1) room temperature
(biologically active sludge), 2) formaldehyde treatment, 1% by volume
(biologically inactive), and 3) refrigeration (biologically arrested).
Each working day during a six-day period, a sample of each of the three
sludges was acidified and digested, and settling curves, as well as TOC and
solids data, were generated. The interface height after 30 minutes of
settling time is shown below as a function of sludge age and storage
condition. (The interface height is given, in terms of the volume of sludge
remaining out of an initial volume of TOGO ml.)
20
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21
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Time
(days)
0.25
1.1
4
5
6
Sludge volume remaining after 30 minutes of settling (ml)
Room temperature
810
760
810
820
820
Refrigerated
810
700
820
790
790
Formal dehyde
920
815
960
930
930
From these data it is apparent that the room temperature and refrigerated
samples settle at approximately the same rate after hot acid treatment, but
the formaldehyde-treated sample settles significantly slower following hot
acid treatment. This result rules out the use of formaldehyde for sludge
preservation. Comparing the settling rates observed on the first day with
those observed on the sixth day, the results appear to lie within the
experimental precision of the test.
The pH of the sludge was measured each day during storage and is shown
in Figure 7. The curves all have the same general shape and indicate a
decrease in pH with storage time. The pH change — from 6.0 to 5.5 over
six days — is considered tolerable.
The soluble TOC concentrations of the three sludge samples are shown
in Figure 8 as a function of storage time. The soluble TOC of the refriger-
ated sludge remained fairly constant throughout the six-day period (160 mg/1
± 25), while both the HCHO-treated and room-temperature sludges showed
constant increases in soluble TOC (80 and 60 mg/l-day, respectively). TheSe
increases may be due either to cell autolysis which occurs during endogenous
respiration (in the case of room-temperature sludge) or to the loss of cell
membrane semi-permeability (in the case of the HCHO-treated sludge). The
TOC values for the HCHO-treated sludge are much larger than the values for
the other two sludges because the formaldehyde contributes to the total
organic carbon concentration.
The total solids and dissolved solids concentrations of the three
sludge samples are shown as a function of sludge age in Table 2. Although
there is some variability in the total solids analyses, there is no
consistent trend to higher or lower total solids concentrations with storage
time. The scatter in the total solids data is probably the result of the
heterogeniety of the sludge which made it difficult to obtain a truely
representative sample.
The dissolved solids in the raw sludge account for only about 5% of the
total solids. There appears to be a consistent increase in dissolved solids
during the first day of storage, but thereafter the dissolved solids concen-
trations remain reasonably constant. The scatter in the dissolved solids
data could be the result of analytical errors since only a rather small
volume of sample was submitted for analysis.
22
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3000
O Room Temperature
Q Formaldehyde-Treated
/\ Refrigerated
2500
2000
1500
1000
500
L..
-.-.O
•-:-'-.4 A
TIME, DAYS
Figure 8. Variation of WAS TOC with time for various
storage conditions.
24
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Ferric Chloride or Polymer Addition—
Ferric chloride and various polymers were evaluated to determine whether
or not their use in conjunction with the hot acid process would produce a
substantial improvement in the solid-liquid separation rates. For each set
of tests three samples were compared: the first was hot acid treated without
additives, the second was hot acid treated with the additive added before
digestion, the third was hot acid treated with the additive added after
digestion. Ferric chloride was added as a 10%-by-weight solution in
sufficient quantity to reduce to pH of the raw sludge to 3.0. Several types
of acrylami de-based polymers were evaluated (Allied Colloids' Percol 720,
725, 726, and 728) each at a dosage of 50 ppm based on the weight of the wet
siudge.
It was concluded on the basis of settling, filtration, and centri-
fugation tests that the addition of ferric chloride or polymer at the
conditions investigated did not substantially improve the solid-liquid
separation rates for the hot acid process. Although minor improvements were
observed in some cases, the improvements were not considered sufficient to
justify the additional cost of the additive.
Selection of Ranges for Major Process Variables--
Tests were conducted to identify the ranges of pH, temperature, and time
for which the hot acid process produced the best performance. The pH was
studied over a range of 1.5-4.0; the temperature, over a range of 80-95°C;
and the time, over a range of 10-60 minutes. For the tests at various pH 's
all samples of Brockton waste activated sludge (2.7% solids) were digested at
95°C for 30 minutes. The settling, filtration, and centrifugation curves
are shown in Figures 9, 10, and 11, respectively. For settling the interface
height (calculated as the percent of initial sludge volume remaining) is
plotted against settling time. The sludge samples digested at pH 2.5 settled
most rapidly followed by the sludge settled at pH 2.0. For filtration
(Figure 10) the volume of filtrate collected out of an initial volume of
45 ml is plotted against filtration time. The sludge samples digested at
pH 2.5 and 3.0 filtered the most rapidly. For centrifugation the interface
height (calculated as the volume of sludge remaining from an initial volume
of 10 ml) is plotted against the centrifugation time. The sample adjusted to
pH 4 centrifuged least rapidly, but all of the other samples appeared to
centrifuge at about the same rate. Based on these tests it was concluded
that the optimum pH range for the hot acid process is 2.0 to 3.0.
Tests conducted at temperatures of 80, 90, and 95°C (at pH 1.5 and
45 min digestion time) indicated better solid-liquid separation at a
digestion temperature of 95°C. Since higher temperatures are ruled out by
the boiling point of water and lower temperatures give lower solid-liquid
separation rates, it was concluded that the optimum temperature range for
solid-liquid separation is 80-95°C.
26
-------
10A 1VI1INI JO % '1H 3DWH31NI
27
-------
pH 1.5
pH 1.8
A pH 2.0
10
20 30 40
TIME, MINUTES
50
60
Figure 10. The effect of pH on the filtration rate of WAS
digested at 95°C for 30 minutes.
28
-------
()
3.5
3;0
2.5
§2.0
1.5
0
O pH 1.5
Q pH 1.8
A PH 2.0
O PH 2.5
O PH 3.0
0 PH 4-°
-JO
10
TIME, MINUTES
I
Figure 11. The effect of pH on the centrifugation rate of
WAS digested at 95°C for 30 minutes.
29
-------
Two different sets of tests were conducted to evaluate the variable of
time. In the first, three samples were digested at pH 1.5, a temperature of
95°C and times of 30, 45, and 60 minutes, respectively. In the second set,
three samples were digested at pH 2.5, a temperature of 95°C, and times of
10, 20, and 30 minutes, respectively. From these tests the solid-liquid
separation rates did not appear to be strongly dependent on digestion time,
and a range of 10 to 60 minutes was selected as the preferred range of
digestion times for the hot acid process.
Acid Requirements
Concentrated sulfuric acid was added to the sludge before heating, and
the volume of acid required to adjust the sludge to the desired pH was
recorded for each test. Table 3 summarizes the acid requirements determined
during the program for the tests of primary importance. A series of tests
were conducted with the first four sludges listed in Table 3 resulting in 9
separate tests for each sludge at each pH. The acid requirements Were cal-
culated both on a solids basis (kg acid per metric ton of solids) and on a
wet sludge basis (kg acid per metric ton of wet sludge). For each sludge at
a given pH, the acid requirement should be the same for each of the 9 tests.
In general, there is some scatter about the mean as indicated by the
standard deviation, but the standard deviation (relative to the mean) is con-
sistently less when the acid requirement is calculated on a wet sludge basis
rather than a sludge solids basis. This implies that the liquid fraction of
the sludge exerts a greater acid demand than the solid fraction. On the other
hand, comparison of the acid requirements for sludges obtained on different
dates (e.g. results for Brockton WAS at pH 1.5) suggests a better correlation
on a sludge solids basis.
Figures 12 and 13 present the average acid demand as a function of pH
on a sludge solids basis and a wet sludge basis, respectively. The points
in each figure indicate the average of the means listed in Table 3 for the
indicated pH. (In calculating this average all means of Table 3 were
weighted equally even through some means are more uncertain than others.)
The relationship between acid demand and pH is approximately linear over the
range of pH 2 to 3 but begins to increase rapidly below a pH of 2. It is of
interest to note that at pH 2.0 the acid demand of all the sludges are about
the same. At this pH the approximate acid requirement is 110 kg/dmt solids
or 4.4 kg/ wmt sludge. Of the sludges tested the Fitchburg WAS required
the most acid to achieve the desired pH levels.
Solid-Liquid Separation
For each of the four sludges, a full-factorial experimental design was
used to evaluate the effects of the major-process variables (pH, temperature,
and time) on solid-liquid separation and on solubilization of various sludge
constituents. In the full-factorial design, all combinations of the three
"factors" (pH, temperature, and time) at each of three levels (27 tests per
sludge) were tested. The levels investigated were:
30
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TABLE 3. ACID REQUIREMENTS AS A FUNCTION OF PH FOR
BROCKTON AND FITCHBUR6 SLUDGES
Sludge , >
sample la)
B-2° (2-28)
F-2° (3-15)
B-l°+2° (3-28)
F-10«° (4-11)
B-2° (2-14)
B-2° (2-22)
8-2° (4-26)
F-2° (5-10)
8-10*20 (5-3)
•6-2° (5-19)
8-2° (5-26)
B-l°+2° (5-27)
' F-2° (6-3)
F-10+20 (6-7)
B-2° (8-17)
Total solids(b)
(t by weight)
n Mean std. dev. pH
9 4.21
9 4.02
9 4.53
9 4.22
6 3.98
3 2.53
3 3.53
2 4.06
2 4.11
3.21
2.51
4.00
4.06
6.78
4.90 ,
a. Sludge Identification; example
b. Total solids concentration of
Std. Oev. - standard deviation
c. Gives the
j «_j_i
0.51 2.0
2.5
3.0
0.16 2.0
2.5
3.0
0.42 2.0
2.5
3.0
1.18 2.0
2.5
3.0
0.19 1.5
2.0
2.5
0.22 1.5
1.8
2.0
2.5
3.0
4.0
0.95 2.0
2.3
2.5
2.6
2.8
3.0
0.05 2.0
2.3
2.5
2.6
2.8
3.0
0.24 2.0
2.3
2.5 9
2.6
2.8
3.0
2.5
2.5
2.5
2.5
2.5
1.0
1.2
1.5
2.0
2.5
: B-lo+2° (3-28) stands
raw si udge . n = number
" of6' (m
tests Hean
9
9
9
9
9
9
9
9
9
9
9
9
7
10
1
2
1
1
4
1
1
1
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2.38
1.68
0.87
2.57
1.94
1.07
2.48
1.90
0.90
2.43
1.91
0.82
4.23
?.26
2.0
2.2
2.4'
1.8
1.2
0.7
0.5
2'.7
2.2
1.65
1.3
1.0
0.9
2.8
2.6
2.4
2.2
2.0
•1.8
2.5
2.0
1.6
1.3
'1.1
0.9
1.8
1.8
1.9
2.2
2.3
13.5
10.3
5.5
3.2
2.3
Std. dev.
0.15-
0.26
0.09
0.07
0.05
0.17
0.04
0.00
0.00
0.07
0.03
0.07
0.21
0.14
—
0.00
—
..
0.00
._
—
_.
~_ '
0.07
..
..
—
._
..
..
..
- ._
—
J. •
..
..
..
—
—
—
—
r..
..
._
..
. '•
Acid required * '
(kg/dmt solids)
Hean Std. dev.
100
70.9
36.6
112
84.8
46.6
98.8
74.1
35.1
112
87.9
38.7
186
101
82.5
219
156
117
92.2.
53.8
38.4
15S
128
78.7
781Q
60 .ft
54.0
120
112
103
96.2
87.4
78.7'
111
89.2
71.4
53.4
45.2
36.9
98.5
12.6
83.4
95.2
59.6
484
369
197
115
82.5
'15.2
15.7
5.5
5.5
3.8
7.6
7.4
6.3
3.0
43.5
34.2
10.2
2.1
3.4
—
7.0
—
..
0.0
..
—
..
.-
14.2
.-
—
._
..
.•.
._
—
._
._
._
„_
._
—
—
—
„
..
..
._
—
Acid required ^ '
(kg/writ sludge)
Hean Std. dev.
4.18
2.95
1.52
4.51
3.42
1 .88
4.35
3.34
1.58
4.28
3.36
1.44
7.43
3.97
3.51
5.80
4.22
2.99
2.11
1.23
0.88
4.74
3.86
2.90
2.28
1.76
1.58
4.92
4.57
4.22
3.86
3.51*
3.16
4.39'
3.51
2.81
2.28
1.93
1.58
3.16
3.16
3.34
3.86
4.04
23.7
18.1
9.66
5.63
4.04
for Brockton primary plus secondary sludge (WAS) obtained on February 28,
of independent sanple/analyses on the sludge, mean = arithmetic average,
0.26
0.46
0.15
0.12
0.09
0.29
0.07
0.00
0.00
0.12
0.06
0.12
0.37
0.24
—
0.00
__
0.00
—
—
: .
.-
0.13
—
—
..
.;_
..
"_;.
—
„ .
._
—
•
__
._
—
1977.
ml. of 96% HjSOfl added to 1 liter of sludge. • '
d. Acid requirement calculated on a. 100% H2S04 basis per dry mstric ton (dmt) or wet metric ton (wmt).
31
-------
600
500
400
CO
o
300
o
UJ
o
HH
o
200
100
O Brockton WAS
A Fitchburg WAS
D Brockton Primary plus WAS
Fitchburg Primary plus WAS
1.0
2.0
PH
3.0
4.0
5.0
Figure 12. Acid demand vs. pH for various sludges (sludge
solids basis).
32
-------
30
25
20
cs
o
=>
CO
CD
15
o
«=c
O Brockton WAS
A Fitchburg WAS
^ Brockton Primary plus WAS
Fitchburg Primary plus WAS
1.0
2.0 3.0
•- pH ^
4.0
5.0
Figure 13.
Acid demand vs. pH for various sludges (wet sludge
basis).
33
-------
PH
Temperature
Time
2.0, 2.5, and 3.0
80, 90, and 95°C
10, 30, and 60 minutes
These levels were selected on the basis of the preliminary tests described
above. For the most part, all of the 27 tests for a particular sludge were
completed within one week in order to limit the extent of changes in the
nature of the sludge. In addition, the order in which the variables were
investigated was randomized so that if the age of the sludge had an effect
on performance, the effect would not be misinterpreted as arising from the
variation of one of the factors being investigated. The test matrix for the
optimization tests is given in Table 4. In general, a fresh sample of sludge
was obtained on Monday morning, and the first set of tests was conducted in
the afternoon. On subsequent days, two sets of tests were conducted. For
each set, three samples, adjusted to the three different pH levels, were
digested simultaneously at the temperature and time indicated in Table 4.
Brockton Waste Activated Sludge--
Table 5 gives the results for solid-liquid separation rates following
hot-acid treatment of Brockton WAS. As indicated in Table 3, the average
total solids content of this sludge was 4.21 % by weight. Because of the
large number of tests, it is not feasible to present rate curves for each
test and each solid-liquid separation technique. Instead, the rates of
solid-liquid separation are indicated in Table 5 by a single parameter for
each technique. These parameters are:
for settling:
for filtration:
the volume of sludge remaining after 30 minutes
settling time (calculated as a percentage of the
initial volume of sludge which was ~ 900 ml).
the volume of filtrate collected after 20 minutes
of filtration (from an initial volume of 45 ml).
for centrifugation: the volume of sludge remaining after 10 minutes
of centrifugation (from an initial volume of
10 ml).
In order to determine which factors have a significant influence on the
solids separation rate, a 3-way, 3-level analysis of variance was performed
using standard statistical techniques (" '. The "F-Test" was used to
determine, with 95% confidence, whether or not there was a difference in
solids separation rates at different levels of a given factor. The results
of the analysis of variance for solids separation are shown in Table 6.
The first column gives the main effect or interaction for which the variance
of test results is to be analyzed. The three-way interaction between time,
temperature, and pH was assumed to be negligible and was therefore used as
an estimate of the experimental error. For each solids separation technique
the test F, calculated from the experimental data, and the tabulated F for
the 95% confidence level (Fn^) are given. If the test F is greated than
Fg.95 the main effect or interaction is significant. The third column
under each solids separation technique indicates whether the factor (or
interaction) is significant ("yes") or not ("no").
34
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The results of Table 6 indicate that the temperature and pH have a
significant effect on the rate of solids separation for all three solids
separation techniques. Of the various main effects and interactions
evaluated, the pH had the most significant effect on solids separation rates.
The effect of time, on the other hand, was not significant. The results of
Table 6 also indicate that there were no significant two-way interactions
between the factors of time, temperature, and pH.
Since the time was not a significant variable in these tests, results
obtained at different times can be averaged to give a more precise indication
of the effects of temperature and pH. The results for settling, filtration,
and centrifugation are shown in Figures 14, 15, and 16, respectively. In
these three figures, each data point represents the average of three tests
at the three different digestion times for the indicated pH and temperature.
The experimental error for these data is indicated by the standard deviation
bar which was calculated in accordance with standard statistical procedures
by taking the square root of the mean square of the time x temperature x pH
interaction.
As shown in Figure 14, the settling rate increased (30-min sludge volume
decreased) with digestion temperature, and at the highest temperature,
appeared to go through a maximum (minimum) at pH 2.5. However, the absolute
rate of settling was low. At the optimum conditions, the sludge settled to
only about 87% of its initial volume in 30 minutes.
The filtration rate (shown in'Figure 15) also increased with digestion
temperature, and at all temperatures, the maximum filtration rate occurred
at a pH of 2.5.
The centrifugation rate (Figure 16) also increased (10-min sludge volume
decreased) with digestion temperature, and the best centrifugation rates were
again obtained in the vicinity of pH 2.5.
Based on the settling, filtration, and centrifugation results, the
optimum treatment conditions appear to be pH 2.5 and 95°C. Since time was
not a significant variable over the range investigated, the minimum time of
10 minutes would be selected as the optimum digestion time.
Fitchburg Waste Activated Sludge-
Similar tests were conducted for Fitchburg WAS (4.0% solids), and results
of the solid-liquid separation tests are presented in Table 7.
The results of the analysis of variance for the solid-liquid separation
data are given in Table 8. For filtration the results indicate that pH is the
only significant variable. For settling all three variables (time, temper-
ature, and pH) and all three interactions (time x temperature, time x pH, and
temperature x pH) are significant. For centrifugation, all three variables
and one interaction (time x temperature) are significant.
For settling, in which all three variables and interactions are signifi-
cant, Figure 17 gives the volume of sludge remaining after 30 minutes
settling time (calculated as a percentage of the initial 900 ml volume of
sludge) as a function of pH for various digestion times and temperatures.
38
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95
90
85 i—
80 L
2.0
Digestion Temperature
80°C
O 95°C
~T~ Estimate of
_J_ Standard Deviation
2.5
3.0
PH
Figure 14.
Effect of treatment conditions on settling rate for
Brockton WAS.
39
-------
40
35
30
z 25
o
20
o
CM
0£
t
w 15
10
2.0
Digestion Temperature
D 80°C
A go°c
O 95°C
2.5
pH
3.0
Figure 15. Effect of treatment conditions on filtration rate
for Brockton WAS.
40
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4.0
«=c
CD
LU
o
3.5
5 3.0
t
«=c
2,5
CD
O
2.0
2.0
Digestion Temperature
D 80°C
A 90°C
O 95°C
I
95°C
Estimate of
Standard Deviation
2.5
PH
3.0
Figure 16. Effect of treatment conditions on centrifugation rate
for Brockton WAS.
41
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In examining Figure 17 for the optimum treatment conditions with respect
to settling, it can be seen that the point at pH 2.5y90°C/60 minutes gave the
minimum interface height after 30 minutes. The next lowest point is at
pH 2.5/95°C/60 minutes, so that the optimum conditions for settling appear to
be pH 2.5/90-95°C/60 minutes.
For filtration, only one variable --pH-- was significant. The filtration
results (volume of filtrate collected after 20 minutes from an initial volume
of 45 ml) is shown in Figure 18. Each of the points shown in Figure 18 was
determined by averaging the filtrate volume over time and temperature at each
pH (average of nine points). The optimum filtration rate occurs at a pH of
2.5.
For centrifugation, all three variables (time, temperature, and pH) were
significant, as well as the time x temperature interaction. The centrifu-
gation results (sludge volume remaining after 10 minutes of centrifugation
from an initial volume of 10 ml) are shown in Figure 19 as a function of pH
for'various times and temperatures. From these data it appears that the
optimum conditions for centrifugation are pH 2.5/95°C/60 minutes.
Based on the above solid-liquid separation tests the preferred treatment
conditions for Fitchburg WAS are a pH of 2.5, a temperature of 95°C, and a
digestion time of 60 minutes.
Brockton Primary Plus Waste Activated Sludge--
A 50:50 mixture of Brockton primary and WAS containing 4.5% total solids
was tested using the same full-factorial experimental design as for the
secondary sludges.- Results for the solid-liquid separation rates as a
function of treatment conditions are given in Table 9.
The analysis of variance for the solid-liquid separation data is given
in Table 10. For settling, all three of the factors (time, temperature, and
pH) are significant and one interaction (time x temperature) is significant.
However, the time factor and the interaction are only marginally significant
since the Test F is close to the Fo.95. For filtration, only the pH has a
significant influence on the rate, and for centrifugation, only pH and
temperature have a significant effect on the solid-liquid separation rate.
Results for settling at various treatment conditions are shown in
Figure 20. In general the settling rate passed through a maximum at pH 2.5,
and at this pH, the most rapid settling occurred for a digestion time of
10 minutes and a temperature of 90 or 95°C. The digestion time had an
inconsistent effect on settling rate giving rise to the significant time x
temperature interaction: at 80°C a digestion time of 60 minutes gave the
best settling rate while at 95°C it gave the worst settling rate.
For filtration, only pH was significant, and at each pH the data for
different times and temperatures were averaged (9 tests at each pH) and
plotted in Figure 21. The filtration rate passes through a maximum at a
pH of 2.5.
45
-------
45
40
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LU
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LU
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25
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2.5
PH
3.0
Figure 18.
Effect of treatment conditions on filtration
rate for Fitchburg WAS.
46
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Estimate of Standard Deviation
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Effect of treatment conditions on filtration
rate for Brockton primary plus WAS.
51
-------
Figure 22 gives the results for centrifugation as a function of the
significant variables (pH and temperature). Results at pH 2.0 and 2.5 are
essentially identical. The centrifugation rate appears to pass through a
maximum at 90°C, but the results at 90 and 95°C are nearly, the same. In fact,
considering the magnitude of the standard deviation, there is little
difference between the three temperatures. This is consistent with the
results of the analysis of variance (Table 10) which indicated that temper-
ature is only a "marginally significant" variable at the 95% confidence level.
Fitchburg Primary Plus Waste Activated Sludge-
Similar tests were conducted with a 50:50 mixture of Fitchburg primary
and WAS. As shown in Table 3, the mean total solids concentration of this
sludge was 4.22% with a rather large standard deviation of 1.18%. The 95%
confidence interval on the mean is from 3.3% to 5.1% total solids. The
problem of obtaining a representative sample of sludge was particularly
evident during these tests. The primary sludge contained some rather large
clumps and the distribution of clumps between samples could not be controlled.
The centrifugation results were particularly erratic, probably because of the
small sample size. The sludge centrifuged quite rapidly reaching its
maximum compaction within 30 seconds. However the volume of sludge remaining
after maximum compaction was reached was much more dependent on the distri-
bution of solid "clumps" between centrifuge tubes than on the hot-acid
treatment conditions to which the samples were subjected. After reviewing the
data it was decided that the centrifugation results were not amenable to a
detailed analysis.
Table 11 gives the results for settling and filtration as a function of
treatment conditions, and the analysis of variance results are given in
Table 12. For filtration, no variables or interactions were found to be
significant. This could be the result of uncontrolled solids distribution
when the treated samples are poured into the filters. Since the filtrate
volume was only 45 ml, these tests could be subject to the same type of
errors as the centrifugation tests.
For settling, a large volume was
homogeneous distribution of suspended
of the analysis of variance indicate
and the time-temperature interaction
and time-temperature interactions are
confidence level, and considering the
producible results with this sludge,
for testing significance may be justi
significant.
used and could have resulted in a more
solids between samples. The results
that pH and time are significant factors
is also significant. However, the time
only marginally significant at the 95%
problems encountered in obtaining re-
the use of a higher confidence level
fied. In this case, only the pH would be
Figure 23 gives the settling rate for Fitchburg primary plus WAS as a
function of pH. Each point indicates the average of nine tests conducted at
various times and temperatures at the indicated pH. The rate of settling is
greatest following treatment of this sludge at pH 2.5.
52
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oc
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OpH 2.0
ApH 2.5
QpH 3.0
90
°C
80 85
TEMPERATURE,
Figure 22. Effect of treatment conditions on
centrifugation rate for Brockton
primary plus WAS.
95
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Effect of treatment conditions on settling rate
for Fitchburg primary plus WAS.
56
-------
Solubili'zation of Sludge Constituents
One of the important aspects of the hot acid process is the extent to
which it solubilizes various constituents of the sludge. Using the
sampling, analysis, and data reduction procedures described above (Methods
and Materials), the degree of solubilization was determined for suspended
solids, suspended COD, and suspended metals. More detailed and better
controlled experiments to specifically evaluate the solubilization of heavy
metal were subsequently conducted, and results from these tests are presented
and discussed in Section 6.
Effect of Process Variables on Degree of Solubilization—-
In order to determine the effect of the major process variables on the
degree of solubilization of various sludge constituents, samples were obtained
and analyzed during the full-factorial experiments described above. Samples
were analyzed for total solids, COD, zinc, and copper. Because of the large
number of tests, analyses were not performed for other metals during the full-
factorial experiments. Zinc and copper were selected for analysis in order to
indicate the range of expected heavy-metals solubilization: zinc is rela-
tively difficult to solubilize. The results of these tests confirm the
difficulty of copper solubilization: under all conditions covered by the full-
factorial experiment, there was virtually no solubilization of copper. There-
fore only the zinc results are presented below for these tests. While cadmium
would have been a more interesting metal than copper to follow, the cadmium
concentrations in some samples of the sludges was judged to be too close to
the detection limit (0.2 mg/1) to give meaningful results in all cases.
Brockton Waste Activated Sludge—The solubilization results for
Brockton WAS are shown as a function of treatment conditions in Table 13.
All of the sludge samples listed in Table 13 were obtained from the same
batch of Brockton waste activated sludge. With each set of three tests, a
sample of raw sludge (indicated in Table 13 by "no treatment") was analyzed.
The mean concentrations and standard deviations for the nine raw sludge
samples are given below.
Total Solids (mg/1)
Dissolved Solids (mg/1)
Total COD (mg/1)
Dissolved COD (mg/1)
Total Zinc (mg/1)
Dissolved Zinc (mg/1)
Mean Concentration
42,100
1,500
53,600
2,790
60
0.39
Standard Deviation
5,100
490
5,440
1,550
14
0.26
The largest standard deviations (relative to the mean) were obtained for
dissolved solids, dissolved COD and dissolved zinc. If the samples are
placed in chronological order according to Table 4» there is a general
increase in the dissolved constituents with sludge age. For example the
dissolved COD of the fresh sludge was 325 mg/1, but after 4 days of re-
frigerated storage it h*ad increased to 4,480 mg/1. Because of the low
57
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concentrations of dissolved zinc, it is likely that experimental errors in
the analysis also added to the scatter of the data. These changes in dis-
solved concentrations with time were assumed to have no effect on the ease
of solubilization of suspended material, and, if an effect did occur, the
randomization of the tests was designed to preclude misinterpretation of
aging effects for the effects of the primary variables being investigated.
In order to determine the factors which have a significant influence on
the degree of solubilization., an analysis of variance was performed on the
solubilization data of Table 13. The results are given in Table 14. For
suspended solids, none of the factors investigated had a significant in-
fluence on the degree of solubilization within the ranges investigated.
Furthermore, none of the interactions between factors were significant.
Therefore all 27 data points were averaged to give a mean percent solids
solubilized. The mean solubilization and 95% confidence interval are:
Percent Solids Solubilized = 10.6% ± 1.6%
For COD, the results of Table 14 indicate that, of the factors and
interactions evaluated, only the pH has a significant effect on the degree
of solubilization. Thus the results at different times and temperatures
can be averaged to give the percent COD solubilized at each pH. The mean
percent solubilization (average of 9 tests at each pH) as a function of
pH is:
fiH
3.0
2.5
2.0
Mean Percent
COD Solubilized
3.9
5.2
8.0
95% Confidence
Interval
± 0.5
± 1.0
± 3.7
It is apparent from these data that the percent solubilization of COD
increases as the pH decreases.
Results for zinc sol utilization are also shown in Table 14. As was the
case for COD solubilization, the pH is the only factor which influences the
degree of zinc solubilization. The analysis of variance also indicates a
significant interaction between time and temperature. However, neither the
main effect for time nor the main effect for temperature is significant, and
the Test-F for the interaction is only slightly greater than FQQ^. There-
fore, the interaction was neglected, and the mean percent solubinzation of
zinc (average of 9 tests at each pH) as a function of pH is:
3.0
2.5
2.0
Mean Percent
Zinc Solubilized
5.4
23.9
55.2
95% Confidence
Interval
± 3.1
± 7.4
±15.1
As for COD, the degree of zinc solubilization increases with decreasing pH.
59
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Fitchburg Waste Activated Sludge—Table 15 gives the analytical and
solubilization results as a function of treatment condition for Fitchburg
WAS. The average concentration and standard deviation for the nine raw
sludge samples are given below.
Total Solids (tng/1)
Dissolved Solids (mg/1)
Total COD (mg/1)
Dissolved COD (mg/1)
Total Zinc (mg/1)
Dissolved Zinc (mg/1)
Mean Concentration
39,600
2,240
41,600
4,910
25
0.7
Standard Deviation
1,600
250
2,600
930
3
0.2
Compared to the Brockton WAS, this sludge exhibited much less change in
dissolved solids concentrations with storage time as evidenced by the much
smaller standard deviations (relative to their respective means) for the
dissolved constituents.
Table 16 gives the results of the analysis of variance for the
Fitchburg WAS. For suspended solids, none of the factors investigated
appeared to have a significant effect on solubilization within the
ranges investigated. However, the interaction between temperature and time
was determined to be significant. The temperature-time interaction is shown
in Figure 24 where the percent suspended solids solubilization is plotted
against temperature for various digestion times. Each data point represents
the mean of the results obtained at the three different pH levels. The fact
that the curves are not parallel indicates an*interaction between temperature
and time, and the analysis of variance indicates that this interaction is
statistically significant. The interaction appears to indicate that the
degree of solubilization is relatively low either at low temperature and low
digestion times or at high temperature and high digestion times. This result
is unexpected. The significance of the interaction depends largely on the
high solubilities obtained during the set of three tests conducted at 80°C
and 60 minutes. Further tests should be conducted to reproduce this result
before concluding that it is real.
The mean solubilization and 95% confidence interval for the Fitchburg
WAS are: ... -x
Percent Solids Solubilized =10.1% ± 1.8%"
This value is in very good agreement with the corresponding value (10.6%)
determined for the Brockton WAS.
For COD, the results of Table 16 indicate that all of the factors
investigated had a significant effect on the degree of solubilization and
that the interaction between temperature and pH was significant. Figure 25
shows the percent COD solubilization as a function of pH for various times
61
-------
TABLE 15. EFFECT OF TREATMENT CONDITIONS ON SOLUBILIZATION OF SLUDGE CON-
STITUENTS FOR FITCHBUR6 WAS
Treatment conditions
Time
(min)
10
-- No
-- No
-- No
30
-- No
— No
— No
60
-- NO
— No
Temp
(°C)
80
treatment
90
treatment
95
treatment
80
treatment
90
treatment '
95
treatment
80
treatment
90
treatment
95
Concentration
Solids*
PH
2.0
2.5
3.0
--
2.0
2.5
3.0
--
2.0
2.5
3.0
—
2.0
2.5
3.0
--
2.0
2.5
3.0
—
2.0
2.5
3.0
—
2.0
2.5
3.0
—
2.0
2.5
3.0
—
2.0
2.5
3.0
-- No treatment —
Tot.
38,700
35,600
39,000
38,800
44,100
43,600
43,400
41,400
46,400
45,900
41 ,600
42,100
39,600
37,900
39,100
40,100
40,800
39,337
36,900
37,900
44,600
41 ,300
41 ,000
37,200
45,400
38,100
42,300
38,800
43,000
39,900
40,700
40,500
37,500
36,500
32,700
39,800
Dis.
7,510
7,740
6,040
1 ,872
10,270
8,570
8,320
2,480
13,560
30,520
9,000
2,270
8,740
6,420
6,340
1 ,890 ,
8,800
8,090
7,050
2,040
12,020
10,250
8,670
. .2,200
16,770
10,070
9,090
2,400
9,730
8,210
7,140
2,480
10,480
8,310
7,260
2,510
in wet sludge (mq/£)
COD
Tot.
28,700
31 ,600
31,500
,39,900
45,400
46,300
46,700
43,300
54,800
45,600
46,600
44,400
41 ,600
40,900
43,700
41,300
38,800
40,700
37,300
,37,600
46,600
50,300
42,800
43,400
47,900
36,400
42,400
44,400
31 ,200
28,600
27,800
42,500
27,400
24,700
27,000
38,000
Dis.
5,170
4,600
4,830
3,760
7,950
7,260
7,100
6,010.
9,740
8,150
6,810
5,560
6,040
5,520
6,150
4,760
5,630
4,980
5,020
3,690
9,450
8,680
7,510
5,960
8,540
7,390
6,980
5,550
7,130
6,460
5,900
4,9.90
7,150
5,820
5,510
3,950
Zinc
Tot.
32
25
22
26
26
27
26
,26
23
22
19
20
30
30
30
.30
28
30
27
25
27
27
25
25
20
19
22
21
22
22
18
24
28
27
24
26
Dis.
27
14
•9
0.8
17
15
14
0.5,
19
18
10
0.7
19
14
8
1
16
15
11
(13)
18
13
8
0.5
19
15
14
0.5
18
13
7
0.6
18
14
8
1
Solubilization (%)
Sus.
solids
1.5
8.5
4.6
8.9
7.0
10.5
17.2
11. '9
12.9
7.6
3.8
7.7
7.1
8.2
9.9
14.1
12.9
12.8
25.7
13.1
13.0
8.1
7.2
8.1
11.9
8.2
10.6
Sus.
COD
5.7
3.0
3.9
4.9
3.1
2.7
8.5
6.5
3.0
3.5
2.1
3.6
5.5
3.5
4.0
8.6
6.1
4.2
7.1
6.0
3.9
8.2
6.2
4.0
13.6
9.0
6.8
Sus.
zinc
84^0
54.5
38.9
64.7
54.7
52.9
82.1
81.2
50.8
62.1
44.8
24.1
56.0
52.4
37.7
66.0
47.2
30.6
94.9
78.4
62.8
81.3
57.9
36.8
63.0
50.0
30.4
-• Treated samples include sulfate solids added as i.^ou*.
( ) Indicates questionable value. Average value for other untreated samples
was used in calculating percent solubilized.
62
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80° 90°
TEMPERATURE, °C
95C
Figure 24.
The effect of time and temperature on
the degree of suspended solids solubilization
for Fitchburg WAS.
64
-------
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65
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and temperatures. In general the degree of COD solubilization increased
with increasing time and temperature and increased with decreasing pH.
The degree of solubilization ranged from 2% at 30 minutes, 80°C, and pH 2.5
to 13.7% at 60 minutes, 95°C, and pH 2.0.
The analysis of variance results for zinc solubilization, also shown in
Table 16, indicate that pH, and time are significant variables and the time-
temperature interaction is also significant. Figure 26 illustrates the
dependence of zinc solubilization on pH and time. Each point is the average
of the results at the three temperature levels for the indicated time and
pH. The degree of solubilization increased linearly with decreasing pH and
reached a level of about 80% solubilization at pH 2.0. The data appear to
indicate lower solubilization at 30 minutes digestion time than at either
10 or 60 minutes digestion time.
Brockton Primary Plus Waste Activated Sludge--Solubilization results for
the 50:50 mixture of Brockton primary and WAS are shown in Table 17. The
average concentrations and standard deviations for the nine raw sludge samples
are given below.
Total Solids (mg/1)
Dissolved Solids (mg/1)
Total COD (mg/1)
Dissolved COD (mg/1)
Total Zinc (mg/1)
Dissolved Zinc (mg/1)
Mean Concentration
45,300
1,290
54,500
2,590
78
0.2
Standard Deviation
4,200
330
3,320
940
7
0.05
The ratio of the standard deviation to the mean gives a measure of the
relative degree of scatter about the mean. As noted for the other sludge
samples, the scatter of results is noticably greater for dissolved species
(ratios of 0.25 to 0.36) than for total species (ratios of 0.06 to 0.09).
The greater standard deviations for dissolved constituents can be attributed
to significant changes in dissolved concentrations with sludge age. However
the changes were less than for the Brockton WAS discussed above.
Results of the analysis of variance for the Brockton primary plus WAS
mixture are shown in Table 18. Within the ranges investigated, none of the
variables or interactions had a significant effect on solubilization of
suspended solids. The mean percent solubilization (average of 27 data points)
and 95% confidence interval are:
Percent Suspended Solids Solubilized = 6.2% ± 1.1%
Thus the degree of suspended solids solubilization is significantly less for
the primary-WAS mixture than for unmixed WAS (10.6%).
As indicated in Table 18, all three variables (time, temperature, and
pH) and time-temperature interaction had a significant effect on the degree
of COD solubilization. Figure 27 shows' the percent COD solubilized as a
66
-------
100
80
60
DO
o
OO
20
OL
O 10 min
A 30 min
60 min
2.0
I Estimate of
Standard Deviation
2.5
PH
3.0
i
Figure 26. Effect of treatment conditions on the degree
of zinc solubilization for Fitchburg WAS.,
67
-------
TABLE 17. EFFECT OF TREATMENT CONDITIONS ON SOLUBILIZATION OF SLUDGE
CONSTITUENTS FOR BROCKTON PRIMARY PLUS WAS
Treatment conditions
Time Temp
(min) (°C) pH
Concentrations in wet sludge (mg/&)
Solids* COD Zinc
Tot. Dis. Tot. Dis. Tot. Dis.
Sol utilization (%)
Sus. Sus. Sus.
solids COD zinc
10 80 2.0
2.5
3.0
— No treatment ~
90 2.0
2.5
3.0
— No treatment —
95 2.0
2.5
3.0
— - No treatment —
30 80 2.0
2.5
3.0
— No treatment —
90 2.0
2.5
3.0
— No treatment —
95 2.0
2.5
3.0
— No treatment ~
60 80 2.0
2.5
3.0
-- No treatment --
90 2.0
2.5
3.0
~ No t reatment —
95 2.0
2.5
3.0
— No .treatment —
f
50,300
50,300
43,400
43,200
44,300
43,200
54,600
53,200
50,400
52,000
50,900
41,100
43,900
46,800
41 ,400
44,600
44,800
47,900
50,900
51 ,200
43,300
41 ,500
42,400
46,100
43,800
41,500
42,300
41 ,700
46,400
44,800
40,500
42,800
50,200
54,500
40,900
44,000
7,750
7,326
4,170
711
8,580
6,800
5,220
1,740
9,300
7,140
5,280
1,330
8,820
6,360
5,410
1,260
10,410
8,190
4,930
1,100
6,550
6,980
5,060
1,360
7,960
5,080
4,190
1,770
9,060
6,960
5,680
1,220
10,530
8,860
5,320
1 ,090
51 ,400
53,700
55,600
53,100
55,400
54,200
51 ,800
51 ,000
57,700
57,500
59,400
61 ,400
60,500
55,500
'53,400
52,900
53,700
54,300
59,700
57,500
63,700
51 ,800
51 ,400
53,200
59,400
56,500
56,700
56,700
56,900
55,900
54,200
52,300
59,000
62,500
47,100
52,700
3
1
2
5
4
4
3
5
4
4
3
4
3
4
3
4
4
3
1
4
5
5
3
5
4
4
3
5
4
4
2
6
5
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1
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,480
800
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,130
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,060
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,850
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,010
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,180
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,270
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70
72
73
74
77
76
83
79
84
86
90
72
82
86
88
89
70
73
84
71
71
75
71
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82
90
79
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85
82
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79
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0.2
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0.3
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0.7
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4.8
1.6
3.1
4.3
2.1
3.1
5.2
3.4
3.1
2.1
0.6
1.7
5.9
4.5
3.3
2.5
3.3
3.6
3.7
2.2
2.5
5.0
3.0
3.0
8.3
5.3
5.1
38.5
27.7
1.0
17.9
1.1
0.4
10.5
0.7
0.0
16.9
• 0.3
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22.6
8.0
0.1
0.0
0.7
0.3
15.6
0.4
0.1
8.0
0.4
0.0
24.3
7.2
0.0
* Treated samples include sulfate solids added as
,68
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function of pH for different digestion times and temperatures. These data do
not indicate a consistent monotonic variation of COD sol utilization with any
of the three factors investigated, and the significance of all the factors
plus one interaction may be attributable to the small estimated error
calculated for this set of tests. It would be advisable to repeat the
analysis of variance using the two-way interactions in addition to the three-
way interaction to estimate the experimental error before testing the
significance of the main factors.
For zinc, only the pH was observed to have a significant effect on
solubilization. Therefore the results obtained at each pH for the nine
different time and temperature combinations were averaged to give the
following mean solubilization and 95% confidence intervals.
EH
3.0
2.5
2.0
Mean Percent
Zinc Solubilized
0.2
5.2
17.1
95% Confidence
Interval
± 0.25
± 6.9
± 8.4
The 95% confidence intervals are quite large, reflecting the large range over
which the degree of solubilization varied at each pH. For example, the degree
of solubilization at pH 2.0 ranged from zero to 38.5 percent. It is note-
worthy that the degree of zinc solubilization was substantially less than for
the unmixed Brockton WAS (which gave 55% solubilized at pH 2.0).
Fitchburq Primary Plus Waste Activated Sludge—Solubilization results for
the 50:50 mixture of Fitchburg primary and WAS are shown in Table 19. The
average concentrations and standard deviations for the nine raw sludge samples
are:
Mean Concentration Standard Deviation
Total Solids (mg/1)
Dissolved Solids (mg/1)
Total COD (mg/1)
Dissolved COD (mg/1)
Total Zinc (mg/1)
Dissolved Zinc (mg/1)
42,200
2,030
36,400
2,920
24
1
.11,800
620
11,000
710
7
0.5
With the exception of dissolved zinc, the standard deviation relative to the
mean is approximately the same for the total and dissolved constituents
(ratios from 0.24 to 0.30). The scatter in results for dissolved con-
stituents can be attributed to changed during storage. The scatter in results
for total constituents can be attributed largely to sampling inaccuracies
resulting from the presence of relatively large clumps of solids in this
sludge sample. These sampling errors can occur each time a portion of a
sample of sludge is transferred from one container to another.
The results of the analysis of variance for the Fitchburg primary plus
secondary sludge mixture are given in Table 20. For suspended solids, none
of the factors or interactions had a significant effect on the degree of
71
-------
TABLE 19. EFFECT OF TREATMENT CONDITIONS ON SOLUBILIZATION OF SLUDGE CON-
STITUENTS FOR FITCHBURG PRIMARY PLUS WAS
Treatment conditions
Time Temp.
(min) (°C) pH
10
— No
~ No
— No
30
~ No
-- No
-- No
60
— No
-- No
-- No
80
2.0
2.5
3.0
treatment —
90
2.0
2.5
3.0
treatment —
95
2.0
2.5
3.0
treatment —
80
2.0
2.5
3.0
treatment —
90
2.0
2.5
3.0
treatment —
95
2.0
2.5
3.0
treatment —
80
2.0
2.5
3.0
treatment —
90
2.0
2.5
3.0
treatment — -
95
2.0
2.5
3.0
•treatment —
Concentrations in wet sludge
Solids* COD
Tot. Dis. Tot. Dis.
31 ,200
16,500
31,500
50,400
30,500
39,900
34,900
50,700
33,900
35,400
34,300
49,300
40,600
43,200
21 ,200
50,200
43,600
21,100
26,500
28,900
41 ,800
50,600
24,800
52,200
28,700
24,500
19,700
32,600
15,900
14,400
20,200
20,300
41,900
28,300
38,000
45,100
12,170
6,320
5', 220
1,470
6,680
6,820
4,120
2,170
7,380
5,860
3,480
1,760
6,360
6,160
3,400
1 ,820
20,600.
10,830
3,680
1,270
8,040
6,890
5,890
2,220
7,520
8,010
4,380
3,360
6,620
6,700
4,740
2,410
9,090
6,470
5,040
1,760
42,500
42,000
43,600
39,200
31,300
22,100
24,800
52,900'
39,700
33,200
52,200
43,600
51 ,800
40,700
25,300
37,100
29,200
24,400
26,400
36,400
35,200
30,000
30,900
45,800
'26,300
20,500
26,700
31 ,800
22,500
21 ,700
33,400
21,700
51,200
38,000
29,000
18,800
3,450
2,880
2,740
2,040
4,690
4,390
4,640
3,770
4,250
3,850
4,450
3,240
3,980
3,500
3,140
2,840
2,900
2,720
2,540
1,960
5,480
5,280
5,120
3,990
3,900
3,860
3,110
2,910
3,700
3,770
3,790
3,180
4,380
4,060
3,000
2,360
(mg/a)
Zinc
Tot. Dis.
25
20
20
26
22
15
15
34
18
21
21
25
29
21
15
27
18
17
16
20
28
22
19
34
20
20
13
23
19
15
15
14
24
20
14
15
18
12
10
1
12
10
2
1
12
9
2
2
11
12
2
0.7
13
12
6
0.5
19
12
3
0.8
15
14
5
1
11
7
2
1.8
16
14
4
0.9
Solubilization (%)
Sus. Sus. Sus.
solids COD zinc
24.6 .
12.1
7.1
----
1.6
4.0
1.8
5.3
2.7
1.1
1.2
2.8
3.9
39.4
37.9
4.4
— — — -
4.7
3.1
10.8
.'-- —
0.1
7.7
0.0
.
0.9
11.7
5.8
8.9 '
6.2
5.5
3.5
2.1
1.7
-- —
3.3
3.4
4.1
2.8
2.0
2.5
2.3
1.7
1.3
3.5
3.4
2.4
— — — —
4.8
5.0
4.2
— —
4.2
5.4
0.8
— - — —
2.7
3.2
2.0
4.1
4.8
2.5
70.8
57.9
52.6
52.4
64.3
7.1
_: —
62.3
36.8
0.0
36.4
55.7
9.1
71.4
69.7
35.5
— ~ - —
66.9
52.8
12.1
— — — —
73.7
68.4
33.3
_____
53.5
• 9.1
1.5
65.4
65.6
23.7
* Treated samples include sulfate solids added as
72
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sol utilization. The mean sol utilization for suspended solids (average of 27
data points) and 95% confidence interval are:
Percent Suspended Solids Sol utilized = 8.0% ± 4%
The large confidence interval reflects the wide range of values obtained and
implies a higher level of experimental error associated with the tests on this
siudge.
For COD sol utilization, temperature, pH, and toth the time-temperature
and the time-pH interactions were significant. Figure 28 shows the percent
COD solutilized as a function of pH for various digestion times. The degree
of COD sol utilization was atout the same for toth pH 2.0 and 2.5, while
pH 3.0 gave relatively low COD sol utilizations. In general, COD solutili-
zation increased with increasing temperature. However, with the exception
of pH 3 and 80°C, the mean degree of COD solutilization was tetween 3 and
4% for all conditions tested. Thus, the selection of conditions does not
appear to have a dramatic influence on COD solutilization.
For zinc solutilization, pH was the only significant variable, and the
time-temperature interactions was the only significant interaction. The
results at each pH (nine tests) were averaged to give the mean solutilization
and 95% confidence intervals shown telow.
3.0
2.5
2.0
Mean Percent
Zinc Solutilized
19%
53%
61%
95% Confidence
Interval
± 14%
± 15%
± 9%
The degree of zinc solutilization for this sludge sample is approximately
the same as for the sample of Fitchturg WAS at pH 2.0 and 2.5 and is
significantly greater than for the primary-WAS mixture of Brockton sludge.
Summary of Metals Sol utilizations at Various pH's—
In addition to the matrix of tests described atove, metals sol utili-
zations were determined for various other tests. Most of these other tests
were conducted with Brockton waste activated sludge. As shown i.n Tatle 14,
pH is the only factor that is significant at the 95% confidence level for
solubilization of zinc bound by this sludge. Therefore the results of all
the tests can te presented as a function of pH only. Tatle 21 summarizes
the metals solutilization results for Brockton WAS. These results include
the full factorial experiments descrited above, averaged over time and
temperature at the indicated pH.
Of the various metals assayed, zinc was the easiest to solubilize,
chrome and lead were more difficult to solubilize, and the solubilization of
cadmium was highly variable. In general the degree of solubilization
increased with decreasing pH, but the range of solubilization at each pH
was quite broad. The variatility of results, particularly for cadmium led
74
-------
D
CO
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t/o
Q
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Estimate of Standard
Deviation
2.0
2.5
PH
3.0
Figure 28. Effect of treatment conditions on the
degree of COD solubilization for
Fitchburg primary plus WAS.
75
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76
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to questions on analytical accuracy. A number of samples were therefore
submitted to the Municipal Environmental Research Laboratory of EPA for an
independent check on the accuracy of Wai den analyses. The results from the
two analytical laboratories are compared in Appendix A. It is concluded that
there is, in general, good agreement between the two laboratories and that
the Wai den results are at least as consistent as those obtained by EPA. The
broad range in the degree of solubility at a given pH is probably attributabl
to a difference between the bulk pH and the acidity at specific reaction,
sites (see Section 6.)
Destruction of Pathogens
Several analyses were conducted to assess the extent of pathogen
destruction during hot acid treatment. Following treatment at pH 2.5, 95°C,
and 10 minutes digestion time, samples of the treated sludge cake for each
of the four sludges tested were submitted for bacteria analyses. Samples
of each of the raw sludges were also submitted. Fecal coliform and fecal
streptococcus were selected as indicators of pathogenic activity. The
analytical results, shown in Table 22, indicate essentially complete de-
struction of both fecal coliform and fecal streptococcus
ASSESSMENT OF BENCH-SCALE OPTIMIZATION TESTS
The bench-scale process studies described above are useful for deter-
mining general process characteristics and estimating the technical and
economic feasibility of the hot acid process. However, because of the bio-
chemical changes that can occur in sludge, it is dangerous to draw un-
qualified conclusions on the basis of bench-scale tests. In addition, the
process may behave differently under on-site, dynamic-flow conditions as
opposed to the remote, static conditions investigated during this program.
Thus the objective of bench-scale tests should be to indicate the potential
of the process rather than to provide definitive process design data.
One of the major cost elements for the hot-acid process is the cost of
acid required to reduce the pH of the sludge. Based on the results of
Figure 12 and on a sulfuric acid cost of $49.50/metric ton ($45/short ton)
for concentrated (98%) acid, the approximate acid cost as a function of pH ,
would be: ,
2IL
3.0
2.5
2.0
1.5
Acid Cost
($/dmt solids!
2.40
4.35
5.80
10.10
Acid Cost
($/dst solids)
2.20
3.95
5.30
9.20
77
-------
TABLE 22. BACTERIAL ASSAYS FOR RAW AND HOT-ACID-TREATED SLUDGES
Sludge sample
Fecal Streptococcus,
counts/100 ml
Fecal Coliform,
counts/100 ml
Brockton WAS
Raw
Hot acid treated
Brockton primary plus WAS
Raw
Hot acid treated
Fitchburg WAS
Raw
Hot acid treated
Fitchburg primary plus WAS
Raw
Hot acid treated
3.3xl06
<100
9.6xl07
<100
3.5xl07
<100
4.2xl08
<100
2.8xl08
<100
78
-------
The acid cost for operating at pH 2.0 is quite reasonable and even at pH 1.5
the cost for acid is not highly excessive. The economics of the hot acid
process will be considered in detail in Section 9, but based on the above
costs, operation at pH's as low as 1.5 cannot be ruled out.
The question of relative acid demand of the liquid and solid fractions
of the sludge was not directly addressed during the tests covered in this
section. However, during the tests described in Section 6, a sample of
Milwaukee WAS containing 1.7% solids was acidified to pH 1.9 and a sample of
filtrate from the same sludge was acidified to the same pH. Based on the
relative amounts of acid added, three-fourth of the acid demand was in the
liquid fraction. For the raw sludge sample the acid demand calculated on a
wet sludge basis (6.4 kg/wmt) is in good agreement with the data of Figure 13,
but the acid demand calculated on a dry solids basis (375 kg/dmt) is nearly
three times as great as the data of Figure 12 would indicate. These results
indicate an appreciable acid demand for the liquid fraction and suggest that
the acid requirement for the process can be significantly reduced by im-
proved thickening of the sludge prior to hot acid treatment. During the
tests described in this section, the solids concentration of the sludges
generally ranged from 2.5 to 4.5%. Significantly higher solids concentrations
have been reported (10) for both activated (6.5%) and mixtures of activated
and primary (8.6%) sludges produced by flotation thickening without chemical
additives.
A second technique that could be used to reduce acid costs is the use of
waste acid rather than commercial sulfuric acid. Tests were conducted with
a waste pickle liquor obtained from a local galvanizing shop. Two samples of
Brockton waste activated sludge were adjusted to pH 2.5: one with concen-
trated sulfuric acid, the other with waste pickle liquor. The samples were
digested at 95°C for 10 minutes prior to determining the settling, filtration,
and centrifugation rates. Because of the very low solids concentration for
the sludge sample tested (-1% solids) solid-liquid separation rates for the
H2S04~treated sample were quite rapid; however the waste pickle liquor appear-
ed to significantly hinder the rate of solid-liquid separation. In addition
the waste pickle liquor contained high concentrations of some heavy metals,
e.g. 61,000 mg/1 zinc. Considering the quantity of waste pickle liquor
required for pH adjustment (23.5 ml/liter of sludge), it is possible to show
by calculation that the amount of zinc remaining in the dewatered sludge
(in the bound and interstitial water) is greater than the amount of zinc
bound in the raw sludge even if the treated sludge is dewatered to impossibly
high solids levels. Thus it would be necessary to either wash the cake
solids or to select waste pickle liquors, containing low concentrations of the
more toxic heavy metals. Furthermore, heavy metals added with the waste
pickle liquor would increase the cost for metals removal from the liquid
stream prior to recycle to secondary treatment. Other sources of waste acid
or low-cost acid .were neither identified nor evaluated; however the results
with waste pickle liquor suggest a cautious approach to the selection of
substitute acids.
79
-------
The solid-liquid separation tests provided preliminary information on
the feasibility of three solids separation techniques (settling, filtration,
and centrifugation) and on the effects of the primary process variables
(pH, temperature, and time) on the rates of solid-liquid separation. Because
of the difficulties inherent in using bench-scale tests to predict the per-
formance of full-scale equipment, the bench-scale tests were used to select
the perferred solid-liquid separation technique. Pilot scale tests were then
conducted to develop quantitative design information for the selected techni-
que. These results are presented and discussed in Section 7.
Table 23 summarizes the effects of th'e primary process variables on
solid-liquid separation rates for the four sludges tested. For each sludge
the variables having a significant effect, at the 95% confidence level, on
solids separation rate are listed in order of decreasing significance. In
general, pH is the variable with greatest influence on solid-liquid
separation followed by temperature and then time. It is also of interest to
note that the same variables are, in general, significant for both settling
and centrifugation suggesting that the mechanism of solid-liquid separation
is similar for these techniques and that their difference lies merely in-the
different gravitational accelerations used. The preferred levels of the
operating variables for solids separation are also given in Table 23. For
all sludqes, the optimum pH is 2.5. The preferred temperature level is 90-
95°C, with 95°C more generally appropriate, and the preferred time, which is
not a critical variable, is 10-60 minutes, with 30 minutes representing a
suitable compromise.
During the full-factorial optimization tests, the effect of the major
process variables on solubilization of various sludge constituents was
evaluated. Table 24 summarizes the variables found to be significant at the
95% confidence level. The variables are listed in order of decreasing
significance for each sludge.
None of the variables investigated appeared to have a significant effect
on the solubilization of suspended solids. This result is somewhat sur-
prising since the suspended solids include both organics and metals, for
which the variables investigated were determined to significantly affect
solubilization. That is, as the pH is lowered, the solubilization of both
organics and metals increase, and one would therefore expect an increase in
suspended solids solubilization as the pH is lowered. Analyses conducted on
the Fitchburg secondary sludge indicated that an average of 62.5% (range of
9 analyses: 60.1% - 64.6%) of the total solids in the raw sludge were
volatile solids. Assuming the volatile solids consisted primarily of organics,
the effect of pK on suspended organics and suspended solids should be similar.
A possible explanation of this apparent anomalie lies in the assumption that
the sulfate added as H2S04 remains dissolved. If a portion of the sulfate
becomes bound to the solids, then the percent solubilization as calculated by
Equations (1), (3), and (4) would be too low, and the size of the error would
increase with the amount of I^SO^ added. This would have the effect of
canceling out the pH dependence for suspended solids solubilization. The lack
of a pH dependence for suspended solids solubilization could also be explained
by postulating that a decrease in pH increases either the volatilization or
80
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the fixation by solids of dissolved inorganics other than sulfate. However,
from a practical point of view, it is really the solubilization of organic
solids rather than total soli'ds that is important in assessing the additional
loading imposed on the treatment system by recycle. Thus the solubilization
of inorganics could be viewed as being of little importance.
As shown in Table 24 the pH had a significant effect on the solubiliza-
tion of COD and zinc for all sludges tested. The temperature was a
significant variable for COD solubilization but not for zinc solubilization,
and the time was not generally significant for either COD or zinc
solubilization.
For optimum results it is desirable to minimize COD solubilization and
maximize zinc solubilization. However, since the solubilization of both COD
and zinc increases with decreasing pH, some compromise must be reached.
Determination of the preferred pH level would require an economic optimi-
zation which would contain some very site-specific elements (such as the
levels of metals in the sludge and the cost penalty for organics recycle).
From Table 24 it would appear that operation at low temperature could
be used to minimize COD solubilization without affecting zinc solubilization.
Although this approach may be of some use, the solid-liquid separation
becomes more difficult as the temperature is decreased below 90°C. Again,
an economic optimum would have to be defined to properly balance tne
opposing effects.
The bench-scale tests were useful in demonstrating that the hot acid
process has the potential for solubilizing significant quantities of certain
heavy metals. As shown in Table 21, solubilization ranged up to 100% for
zinc, 90% for cadmium, 44% for chrome, and 45% for lead at pH's of 1.5 or
above. In addition, results to be presented in Section 5 indicate good
solubilization of nickel. While these results demonstrate the potential of
the process for heavy metals removal, the degree of removal obtained in
these tests was highly variable, e.g., cadmium solubilization at pH 2.5
varied from 0 to 90%. This suggests that either the characteristics of the
particular sludge sample had a large influence on solubilization or there was
some uncontrolled process variable (e.g. pH localization) responsible for the
inconsistent results. Further tests were conducted to specifically evaluate
"heavy metals solubilization, and the results of these tests are presented and
discussed in Section 6. In brief, high and consistent solubilization of
heavy metals were obtained with various sludge samples during these latter
tests. These results confirm the conclusion that the hot acid process has
the potential for significant heavy metals removal from sludge.
Although the number of bacterial assays performed during the bench-
scale tests were minimal, they confirmed the expectation that the hot acid
process effectively destroys all pathogens. Pasteurization (exposure to
high temperature for an adequate period of time) has long been recognized
83
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and used as an effective disinfection technique. The generally accepted
conditions for sludge pasteurization (11)are a temperature of 70°C and
holding times of 30 minutes to 1 hour. Table 25 (12)gives the holding times
at temperatures of 70°C and below required to completely eliminate the
various types of pathogens listed. It is evident from these data that the
temperatures and times employed in the hot acid process would be more than
sufficient to completely destroy all pathogens. In addition, the use of low
pH should increase the rate of pathogen destruction.
Based on the bench-scale optimization tests and the above discussion of
the state of advancement of the hot acid process, the potential advantages and
disadvantages of the process are listed in Table 26. The unique potential
advantage of the process is its ability to remove heavy metals from the sludge
solids. The trade-off between advantages and disadvantages must eventually
be reduced to an economic assessment of the hot acid process relative to
other acceptable alternatives. A preliminary analysis of the process eco-
nomics is given in Section 9.
The next step in the development of the process should be the design,
installation, and operation of a transportable pilot system at various
municipal wastewater treatment plants. Operation of a pilot system will
provide data under dynamic flow conditions similar to a full-scale plant and
will provide data unaffected by sludge age. The operation of a pilot system
is considered essential to the further development, optimization, and
verification of the hot acid process.
84 '
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.TABLE 25. EFFECT OF TIME AND TEMPERATURE ON THE SURVIVAL OF
TYPICAL PATHOGENS FOUND IN SLUDGE*.'(iz)
Organism
Cysts of Entamoeba histolytica
Eggs of Ascaris lumbricoides
Bruce! la abortus
Corynebacterium diphtheria
Salmonella tyhposa
Escherichia coli
Micrococcus pyrogene var. aursus
Mycobacteri urn tubercul os i s var . promi xi s
Viruses
Temperature °C
50 55 60 65 70
5
60 7
60 3
45 4
30 4
60 5
20
20
25
*Pathogens completely eliminated at indicated time and temperature.
85
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TABLE 26. POTENTIAL ADVANTAGES AND DISADVANTAGES OF HOT ACID TREATMENT
Potential advantages
Potential disadvantages
1.
2.
3.
4.
Improves solid-liquid separation 1
rate and cake dryness
Solubilizes some organics
Removes toxic heavy metals from
sludge solids
Destroys all pathogens
2. Requires heat and chemical addition
Requires corrosion-resistant
materials of construction
Produces solids suitable for land
application or animal-feed supp-
1ement
86
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SECTION 5
COMPARISON WITH ALTERNATIVE CONDITIONING PROCESSES
It is of interest to compare the performance of the hot acid process to
the performance of alternative sludge conditioning processes. Bench-scale
tests were conducted to compare the hot acid treatment with thermal treat-
ment, ferric chloride treatment and polymer treatment. In addition* a
literature review was performed to identify the characteristics of aerobic
and anaerobic stabilization relative to the hot acid treatment process.
BENCH-SCALE COMPARISON OF CONDITIONING ALTERNATIVES
Bench-scale tests were conducted to directly compare various alternatives
for conditioning the same sample of waste activated sludge. The various
conditioning alternatives were compared both with respect to solid-liquid
separation rates and to solubilization of sludge constituents.
Methods and Materials^
The experimental apparatus and procedures for the hot acid treatment
process were essentially identical to those described in Section 4. A
one-liter sample of the sludge was treated at conditions of pH 2.5, 95°C,
and 10 minutes digestion time.
For thermal conditioning, a 150-ml sample of sludge was placed in a
high-pressure, stainless-steel, gas sampling cylinder fitted with a
pressure gauge and thermocouple. The sealed cylinder was wrapped with
heating tape and placed on a wrist-action shaker to provide mild agitation.
The temperature during treatment was maintained within the range of 180-
190°C and the time at temperature was 30 minutes. These conditions were
selected on the basis of typical commercial practice (7 ) (30-40 minute
detention time at temperatures of 170-205°C).
Ferric chloride was added as a 10% solution to a one-liter sample of
raw sludge. In order to select the preferred dosage, preliminary tests
were conducted with the same sludge to determine the effect of dosage on
the solid-liquid separation rate.
A cationic polymer concentrate (Calgon Corp. WT-2860) was diluted to
10% of its initial concentration (concentrate = 15% solids) and added to a
one-liter sample of sludge. As for ferric chloride, the polymer dosage
was selected on the basis of preliminary tests at various dosages.
87
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All tests were conducted on a single sample of Brockton waste activated
sludge. The sample was obtained in the morning, and all tests were con-
ducted during that same working day. The total solids concentration of the
raw sludge was 2.5%.
The same apparatus and procedures as described in Section 4 were used to
determine filtration and centrifugation characteristics. The sampling and
analysis procedures were also similar to those described previously. Because
of the small volume of sludge subjected to thermal treatment, the rates of
settling after treatment were not compared.
Results and Discussion
Selection of Chemical Dosages-
Figures 29 and 30 show the rates of filtration and centrifugation,
respectively for various dosages of ferric chloride. The dosages ranged from
0.016 to 0.80 g FeCls/g solids. Over this range, the rates of both filtration
and centrifugation increased with dosage. However, the improvement in solid-
liquid separation rate diminished beyond a dosage of 0.16 g/g solids, which
was therefore selected as the preferred dosage for this sludge.
Similar tests were conducted with various polymer dosages ranging from
0.008 to 0.4 g of polymer concentrate per g solids. Filtration and
centrifugation results for the various dosages are shown in Figures 31 and
32, respectively. The rate of solid-liquid separation for both filtration
and centrifugation passed through a maximum at a dosage of 0.08 g/g solids.
This dosage was therefore selected for subsequent tests.
It is of interest to note the chemical cost for the selected dosages of
ferric chloride and polymer. Assuming a cost for ferric chloride (13) Of
$110/metric ton ($100/ton) the chemical cost corresponding to the selected
dosage would be $17.60/metric ton of solids ($16/ton solids). Based on a
book price for the polymer concentrate (personal communication with Calgem
Corp., Pittsburgh, PA, June 1978) of $1.06 kg ($0.48 Ib) in tank truck lots,
the cost for polymer at the selected dosage would be $85/dmt solids ($77/dst
solids). Because of the high cost, polymer treatment, at least with the sel-
ected polymer and dosage, would not be considered a viable alternative for
the conditioning of this particular sludge.
Comparison of Solid-Liquid Separation Rates--
The rates of filtration following application of the various sludge
conditioning treatments are shown in Figure 33. All of the conditioning
processes gave a substantial increase in filtration rate relative to the
untreated sludge. The thermal conditioning process produced very rapid
filtration rates while the filtration rates for the hot acid, ferric
chloride, and polymer treatments were roughly equal* but significantly less
* It is noteworthy that the filtrate of the hot-acid-treated sample produced
foam during this particular filtration test. This phenominon occurred
sporadically from time to time and was accompanied by a significant re-
tardation in filtration rate. It is postulated that, in the absence of
foaming, the hot-acid sample would have filtered at least as rapidly as
the polymer-treated sample.
88
-------
40
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o
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30
20
10
O 0.016 g FeCl3/g Solids
A 0.032 g FeC13/Q Solids
Q 0.080 g FeCl3/g Solids
Q 0.160 g FeCl3/g Solids
Q0.320 g FeCl3/q Solids
f~] 0.800 g FeCl3/q Solids
10
20 30 40
TIME, MINUTES
50
60
Figure 29. Effect of ferric chloride dosage on the filtration rate of
hot-acid-treated WAS.
89
-------
10
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t—I
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\-
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O 0.016 g FeClg/g solids
0.032 g FeCl3/g Solids
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0 0.320 g FeCl3/g solids
0
0.800 g FeCl3/g solids
4 6
TIME, MINUTES
8
10
Figure 30. Effect of ferric chloride dosage on the centrifugation
rate of hot-acid-treated WAS.
90
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O 0.008 g Polymer/g Solids
A 0.024 g Polymer/g Solids
D 0.08 g Polymer/g Solids
O 0.16 g Polymer/g Solids
Q 0.40 g Polymer/g Solids
\ 0s A
10
TIME, MINUTES
Figure 32. Effect of polymer dosage on the centrifugation rate
of hot-acid-treated WAS.
92
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93
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than for thermal treatment.
Rates of centrifugation following the various sludge treatments are
shown in Figure 34. Again, all of the conditioning techniques produced a
substantial improvement in centrifugation rate relative to the untreated
sludge. Thermal treatment gave the best centrifugation rate followed by hot
acid, polymer, and ferric chloride treatment in that order.
From these results, it can be concluded that the hot acid process
produced solid-liquid separation rates which are at least as good as those
produced by chemical conditioning. For centrifugation, the hot acid process
can produce a sludge having dewatering characteristics superior to those of
chemically conditioned sludge.
Solubilization of Sludge Constituents—
In addition to the rate of. solid-liquid separation, the degree of
Solubilization of various sludge constituents is important in comparing
alternative sludge conditioning processes. For sludge disposal by land
application, the conditioning process should, ideally, solubilize the toxic
heavy metals without significantly solubilizing organics, phosphorus, and
nitrogen.
Table 27 gives the suspended solids and organics solubilizations
obtained with the sample of Brockton waste activated sludge. The ferric
chloride and polymer treatment produced no appreciable Solubilization of
suspended solids. The thermal treatment solubilized one-third of the solids
while the hot acid process solubilized about one-tenth of the solids.
Since the supernatant from the sludge conditioning process must be
treated, either directly or by recycle to the secondary treatment system,
the organic loading of the filtrate is an important parameter in judging the
merits of alternative conditioning processes. Both the BOD and COD of the
various filtrates are shown in Table 27. Of the treated samples, the organic
loading for ferric chloride treatment was the lowest. The higher BOD for
the polymer-treated filtrate could have resulted from some residual polymer
that was not removed with the solids. The BOD of the hot-acid-treated
filtrate was only slightly higher than the BOD of the polymer-treated
filtrate. Thermal treatment resulted in substantially greater organics
Solubilization than for the other processes. The high level of organics in
the filtrate from the thermal process represents a significant additional
load on the secondary treatment system. The recycle of thermal treatment
liquors generally results in an increase inorganic loading of approximately
20% on the secondary treatment system (7,14).
Table 28 further compares the solubilizations obtained with the hot acid
and thermal processes. (The additional analyses of Table 28 were not per-
formed for the ferric chloride and polymer treated samples since little, if
any, Solubilization was anticipated.) Comparing the results for heavy metals
Solubilization, the hot acid process solubilized significant quantities of
zinc, cadmium and nickel but only small amounts of lead, chromium, and copper.
The thermal process resulted in no significant Solubilization of heavy metals
except for nickel. The high nickel Solubilization for the thermal process is
questionable in the light of the low Solubilization for other metals and in
94
-------
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A Fed 3
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O Untreated
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Figure 34. Comparison of centrifugation rates for WAS treated by
various conditioning processes.
95
-------
TABLE 27. SOLUBILIZATION OF SOLIDS AND ORGANICS FOR VARIOUS
TREATMENT PROCESSES
Process
No treatment
Hot acid
Thermal
Ferric chloride
Polymer
Suspended solids
solubilization
(%)
__
11
34
1.5
1.7
Filtrate
BOD
(mg/D
310
1,300
5,600
610
1,000
Fi 1 trate
COD
(mg/D
540
2,940
11,300
830
—
COD
solubilization
(X)
__
7.8
27
0.9
—
96
-------
TABLE 28. COMPARISON OF SOLUBILIZATIONS FOR THE HOT-ACID AND
THERMAL CONDITIONING PROCESSES
Sludge Process SS COD Zn Cd Ni Cu Cr Pb P TKN
Brockton Hot acid 11 7.8 81 (90) 36 0 7 10 45 10
WAS
Thermal 34 27 0.6 0 (69) 0 3 1 13 76
Milwaukee Hot acid 11 5 47 5 79 0 1.3 0 20 8.6
WAS
* ~~~ ~~~~~~
Treatment conditions for hot acid process = pH 2.5, 95°C, and
10 min. digestion time. Treatment conditions for thermal
process = 180-190°C for 30 min. digestion time.
( ) indicates questionable or uncertain results.
97
-------
the light of previous research (5) which showed no significant metals
sol utilization (including nickel) for the thermal process. The high
cadmium sol utilization for the hot acid process is particularly noteworthy
since cadmium is of considerable environmental concern. However, the 90%
cadmium solubilization was not reproducible in these tests as noted in
Section 4 (see Table 21).
The results for phosphorus and nitrogen (total Kjeldahl nitrogen)
solubilizations are also shown in Table 28. For nitrogens the hot acid
process solubilized only 10% compared to 76% for the thermal process. The
degree of phosphorus solubilization for the hot acid process (45%) was
significantly greater for this sample than for other WAS samples tested and
was considerably greater than the degree of phosphorus solubilization for the
thermal process.
Table 28 also shows the solubilization results for hot acid treatment of
Milwaukee WAS. The solubilizations of suspended solids and COD were essen-
tially identical to those obtained for the Brockton sludge. Good
solubilizations of zinc and nickel were obtained for the Milwaukee sludge,
but the solubilization of cadmium was low. Copper, chromium, and lead
were not solubilized. The 20% phosphorus solubilization was more typical
than for the Brockton sludge, and the nitrogen solubilization was about the
same as for the Brockton sludge.
These solubilization results indicate that the hot acid process exhibits
a number of advantages over thermal treatment. Relative to thermal treat-
ment, the hot acid process:
— produces lower solubilization of solids,
— produces lower solubilization of organics,
— produces lower solubilization of nitrogen, and
— has better potential for removal of toxic heavy metals.
As a result of these advantages, the sludge produced by the hot acid process
should be much more desirable for land application than sludge from a
thermal treatment process.
CHARACTERISTICS OF BIOLOGICAL STABILIZATION
Of the various stabilization processes, anaerobic and aerobic digestion
are the most widely used. Although experiments were not conducted with
these processes, it is possible to make some general comparisons between
these processes and the hot acid and thermal treatment processes.
Solid-Liquid Separation
Both anaerobic and aerobic digestion improve the dewaterability of the
treated sludge (generally a mixture of primary and waste activated sludge),
but the degree of improvement is not sufficient to permit direct dewatering
of the digested sludge. Chemical conditioning of digested sludge, with
lime, ferric, and/or polymer is generally required prior to dewatering.
98
-------
1 g
Estimates of chemical requirements ( ) (based on experience at different
treatment plants in the United States) are shown for raw and anaerobically
digested sludge in Table 29. The chemical cost for conditioning
anaerobically digested sludge can considerably exceed that for raw sludge
because the high concentration of carbonates produced during anaerobic
digestion inhibits coagulation of the solids with ferric chloride. Similarly,
aerobically digested sludg.es generally have been found to have poor de-
watering characteristics ('6).
Reduction of Solids
Both anaerobic and aerobic digestion reduce the volume of solids
requiring disposal. In anaerobic digestion organic solids are converted to
volatile organic acids which are, in turn, converted primarily to methane
and carbon dioxide. Thus a portion of the raw sludge solids is converted
to a gas mixture which can be recovered and used for fuel (heating valued5)
5,300-7,100 kcal/m3 [600-800 Btu/ftJ]). Anaerobic digestion can typically
achieve (on a dry basis) a 25% reduction in sludge total solids and 35%
reduction in sludge volatile solids (15).
Aerobic digestion also reduces the volume of solids requiring disposal.
The primary mechanism-for solids reduction is endogenous respiration in
which cellular matter is biologically oxidized. Aerobic digestion can
achieve (on a dry basis) total solids reductions of 25%-35% with corresponding
volatile solids reductions of 40%-50% (15).
Pathogen Destruction
Both aerobic and anaerobic digestion reduce the concentration of patho-
genic organisms. For anaerobic digestion, digestion conditions are not
lethal to most pathogens, but are not conductive to multiplication. Thus
pathogen destruction appears related to a natural die-off with time 05 ).
Anaerobic digestion has been rated (11)as "fair" (1 to 3 logs reduction) for
destruction of pathogenic bacteria (Salmonella) and "poor" (less than 1 log
reduction) for viruses. Aerobic digestion has been rated as "poor" for
pathogen removal; however the potential for virus removal was unknown.
Supernatant Quality
The quality of the supernatants from anaerobic and aerobic treatment 15)
are compared in Table 30. The variability in the parameters listed results
primarily from differences in the digester designs and the efficiency of
solids separation. For comparison purposes, the supernatant quality from
thermal treatment is also shown in Table 30. In general, aerobic digestion
produces the highest quality supernatant followed in order by anaerobic
digestion and thermal treatment. Undoubetedly, much of the organic loading
of the supernatants from biological digestion arises from the presence of
biological solids. On the other hand, the organics loading of the thermal
treatment liquor is largely dissolved (compare results of Table 27) and
represents a much more difficult treatment problem than the biological
supernatants.
99
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Q.
to
o
a.
.
re
o
101
-------
Heavy Metals Removal
Anaerobic and aerobic digestion do not release any significant
of heavy metals from the sludge solids. In fact, it has been found O7/
soluble metals are incorporated into the biomass during anaerobic digestion.
Measurements have been madeUS) on the extractability of heavy metals from
anaerobically and aerobically digested sludges using water and EDTA. The
water extractability of cadmium, lead, and zinc remained uneffected by
anaerobic digestion, while nickel and copper became less extractable. The
water extractability of chromium, copper, and zinc increased following
aerobic digestion, but that of cadmium, nickel, and lead remained unchanged.
However, in all cases the water extractability of metals remained low, i.e.,
at a level of less than 10 percent of the EDTA extractability of the metals.
Therefore it is concluded that neither anaerobic nor aerobic digestion result
in any significant removal of heavy metals from sludge.
DISCUSSION AND CONCLUSIONS
The sludge treatment processes considered above fall into four main
categories:
— chemical treatment (polymer or inorganic chemicals),
— biological digestion (anaerobic or aerobic),
— thermal treatment, and
— hot acid treatment.
These four categories are discussed below with respect to process perfor-
mance characteristics. The economics of the various treatment alternatives
are considered in Section 9.
Considering current recommendations for the disposal of wastewater
sludges (l ), it appears to be necessary to stabilize the sludge (with
treatment equivalent to anaerobic digestion) prior to disposal by land
application or sanitary landfill. Since ocean dumping is being phased out,
incineration is the only major disposal technique which does not require
prior sludge stabilization. However, incineration is a volume reduction
technique rather than an ultimate disposal technique and, as such, should be
considered as a stabilization rather than a disposal process. Thus the need
for stabilization prior to ultimate disposal appears to be quite general.
Chemical treatment, as generally practiced for dewatering, will not produce
a disposable sludge since the process does not stabilize the sludge solids.
As noted above, biological digestion produces a stabilized sludge, but
the sludge cannot generally be dewatered without chemical conditioning.
Thus biological stabilization requires chemical conditioning, and chemical
conditioning requires biological (or other) stabilization. On the other hand!
both thermal treatment and, ideally, hot acid treatment produce a stabilized,
dewaterable sludge in a single processing step. Therefore, in comparing
various sludge conditioning alternatives, three categories may be considered:
— biological stabilization with chemical conditioning,
— thermal treatment, and
— hot acid treatment.
102
-------
The performance characteristics of the above alternatives are compared
in Table 31. While this type of comparison can highlight general character-
istics of the alternative processes, the final selection must be based on an
economic assessment which includes consideration of ultimate disposal.
Nevertheless it can be concluded from'Table 31 that the hot acid process
compares quite favorably to the other two alternatives.
103
-------
TABLE 31. COMPARISON OF SLUDGE CONDITIONING ALTERNATIVES
Performance characteristic
Biological/
chemical
treatment
Thermal
treatment
Hot
acid
treatment
Solids separation " Good
Pathogen destruction Fair
Putrefaction potential Good
Supernatant organics (dissolved) Good
Heavy metals removal Poor
Solids nitrogen Good
Excellent
Excellent
Fair
Poor
Poor
Poor
Good (?)
Excellent
Fair (?)
Fair
Good
Good
104
-------
SECTION 6
RE-EVALUATION OF HEAVY METALS SOLUBILIZATION
As discussed in Section 4 and 5 the hot acid process has several
potential advantages over alternative conditioning techniques. The most
unique advantage is its potential for sol utilization and removal of toxic
heavy metals. Of particular importance is the removal of cadmium because
of its tendency to accumulate in certain agricultural crops. The degree of
cadmium sol utilization obtained in previous tests (see Table 21) was quite
erratic, ranging from 90% in one test at pH 2.5 to essentially zero in other
tests conducted at the same pH.
Additional tests were undertaken to focus primarily on heavy metals
solubilization and to attempt to resolve the erratic results observed in
previous tests. Several variables were investigated for their effect on the
degree of solubilization. These included: amount of acid added, stirring
speed, heating before acidification vs. acidification before heating, di-
gestion time, and source of sludge. A different digestion apparatus was
used for these tests which permitted the use of larger sludge samples, better
control of operating conditions, and wider variability of stirring speed.
METHODS AND MATERIALS
A schematic diagram of the digestion apparatus is shown in Figure 35.
The digestion was conducted in a 14-liter fermenter constructed of pyrex
glass with a stainless-steel top plate and internals. The reactor con-
tained four vertical baffles., a turbine mixer, a single orifice sparger,
pH/reference electrodes, a thermocouple well, a sample withdrawal tube,
and two 1000-watt Vycor immersion heaters. With two heaters each operated
at 110 volts, the time required to heat 8 liters of sludge to 95°C was
about 35 minutes, and rapid stirring (600 rpm) was used to prevent local
overheating of the sludge. All tests were conducted at 959C.
In previous tests the pH of the sludge was always adjusted to the
desired level prior to heating; however in some of the tests reported in
this section, acid was added after heating to the digestion temperature.
This necessitated the measurement of pH at elevated temperatures, which
proved to be somewhat inaccurate. For these samples the pH values reported
in this section were measured after the digested sample had cooled to room
temperature.
Following digestion a sample of the treated sludge was obtained for
105
-------
Stirrer Motor
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NOT SHOWN
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Figure 35. Schematic diagram of digestion apparatus used to
evaluate heavy metals sol utilization.
106
-------
analysis. The sample was allowed to cool and was divided into two portions.
One portion was submitted for analysis without further treatment (treated
sludge sample). The other sample was centrifuged and the centrate passed
through a 0.45u Millipore filter before submission for analysis (treated
sludge filtrate sample). A similar treatment was applied to a sample of
the raw sludge to give a raw sludge sample and a raw sludge filtrate sample.
All samples were preserved by refrigeration until analysis, and analytical
work on the samples was initiated within 24 hours. The same analytical
techniques and calculational procedures as described in Section 4 were used
for these tests.
Fresh samples of WAS were obtained for all tests (with the exception
of a few tests conducted on Milwaukee WAS). The samples were obtained in
the morning and the tests were conducted on the afternoon of the same day.
RESULTS AND DISCUSSION
The experiments conducted to characterize heavy metals solubilization
were divided into two groups. For the, first group, all tests were conducted
with fresh Brockton WAS to evaluate the effects of various operating condi-
tions. For the second group of tests, the preferred operating conditions
were fixed and sludges from different cities were evaluated. The detailed
test data, including test conditons, analytical results, and calculated
solubilizations, are given in Table 32.
A number of operating variables were investigated in an attempt to
define the factors controlling cadmium (and other constituent) solubil-
izations, these included:
--apparatus (14-liter fermenter, designated 14-LF vs. one-liter
graduated cylinders, designated 1-L6C and described in Section 4);
--stirring speed (600 rpm vs. 60 rpm using 14-LF; stirring speed for
1-LGC was 60 rpm);
--order of acid heat addition (acid added before heating vs. heating
before acid addition);
--digestion time (30 min.vs. 60 min. not including heat-up time); and
--amount of acid added.
Tests were first conducted to compare the 14-liter fermenter (operated
at 600 rpm with acid addition after heating and a digestion time of 30
minutes) to the one-liter graduated cylinders (operated at 60 rpm with
acid addition before heating and a digestion time of 30 minutes). These
tests were somewhat inconclusive because of the difficulty in properly
adjusting the pH of hot sludge. Therefore, in the first two tests the
amount of acid added per unit weight of sludge was different for the two
digesters. This variable was determined to strongly influence the degree
of solubilization and could have hidden the effects of the other variables
such as digester configuration, degree of mixing, order of acid-heat ad-
dition, etc. In the third test the amount of acid added to the two samples
was the same but was insufficient to produce any significant solubilization
of cadmium. The solubilization of zinc was greater for the 14-liter fer-
107
-------
TABLE 32. EFFECT OF TREATMENT CONDITIONS ON THE DEGREE OF SOLUBILIZATION
OF HEAVY METALS AND OTHER SLUDGE CONSTITUENTS.
Line
1
2
3
4
5
6
7
8
9
10
n
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Date
2/28/78
2/28/78
9/5/78
9/11/78
9/19/78
9/25/78
10/2/78
10/10/78
10/16/78
10/23/78
10/30/79
11/6/78
11/14/78
Raw
* solids
Conditions (wt %)
Raw sludge f
14-LF, 600, HBA
Raw sludge
1-LGC, 60, ABH
14-LF, 600, HBA
Raw si udge
1-LGC, 60, ABH
14-LF, 600, HBA
Raw sludge
1-LGC, 60, ABH
14-LF, 600, HBA
Raw si udge
14-LF, 60 HBA
14-LF, 600, HBA
Raw sludge
14-LF, 600, ABH
14-LF, 600, HBA
Raw sludge
14-LF, 600, HBA
14-LF, 600, HBA
14-LF, 600, HBA
Raw sludge
14-LF, 600, HBA
14-LF, 600, HBA
14-LF, 600, HBA
14-LF, 600, HBA
Raw sludge
Optt( Brockton WAS)
Raw sludge
Optt(Fitchburg WAS)
Raw s 1 udge
Optt (Lawrence WAS)
Raw s 1 udge
OptJ (Milwaukee WAS)
Optf (Milwaukee WAS)
3.42
3.42
2.50
2.50
2.50
4.03
4.03
4.03
3.75
3.75
3.75
3.28
3.28
3.28
3.40
3.40
3.40
3.27
3.27
3.27
3.27
3.57
3.57
3.57
3.57
3.57
3.62
3.62
1.59
1.59
4.87
4.87
1.70
1.70
1.70
H2S04
added
(ml/1)
_
4 188
_
3.00
5.63
_
2.50
2.94
„
2.20
2.20
_
2.50
2.50
.
2.28
2.28
_
1.1
2.30
2.30
-
3.28
3.74
4.29
4.88
4.67
.
2.74
.
7.65
2.94
2.50
Acid
usage
kg/dmt
„
261
_
220
412
_
112
132
_
107
107
_
140
139
_
122
122
_
62
129
129
-
168
192
220
250
236
_
315
-
288
317
269
pH G
25°C
6.01
1.90
6.51
2.20
1.65
6.08
2.14
2.04
6.39
2.15
2.44
6.37
2.07
2.05
6.58
2.10
2.10
6.00
. 3.50
2.00
2.00
6.70
1.75
1.55
1.57
1.33
6.36
1.47
6.25
1.80
6.10
1.78
6.85
1.96
2.40
Digestion
time
(min)
„
30
-
30
30
.
30
30
.
30
30
-
30
30
-
30
30
-
30
30
60
-
30
30
30
30
60
-
60
-
60
60
60
Condition Code: 14-LF « 14-liter ferraenter; 1-LGC - one-liter graduated cylinder;600 or 60
stirring speed in rpm; HBA = heat before acid; ABH = acid before heat.
^ptiraum conditions for sludges from various cities were: use of 14-liter fermenter, 600 rpm
agitation, acid addition before heating, and 60 minute digestion time.
108
-------
TABLE 32. EFFECT OF TREATMENT CONDITIONS ON THE DEGREE OF SOLUBILIZATION
OF HEAVY METALS AND OTHER SLUDGE CONSTITUENTS (continued)
Line
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25 ,
26
27
28
29 .
30
31
32
33
34
35
Solids**
Total Dis
34,200
34,400
25,000
39,183
30,117
40,800
41 ,053
39,786
37,500
39,953
41 ,574
32,800
49,453
37,053
34,000
39,043
37,043
32,700
36,287
35,600
- -
35,700
33,405
-
-
-
36,200
42,576
15,900
15,040
48,700
49,098
•17,000
17,888
"*
1,000
8,840
510
5,183
2,717
790
6,353
7,786
700
5,953
8,474
550
11,653
7,853
640
5,143
8,043
760
4,617
6,900
-
628
7,605
-
-
—
-624
8,776
268
4,370
1,050
14,098
1,340
4,688
"
COD
Tot 01s
-
45,100
59,700
54,100
46,700
5,200
54,100
47,700
51 ,400
56,800
55,200
57,000
53,600
40,000
45,000
46,000
38,000
42,000
43,000
-
43,000
41 ,000
-
-
•*
34,000
51 ,000
14,000
13,000
36,000
65,000
18,000
17,000
-•"
-
640
6,750
10,900
500
5,050
7,350
415
5,090
8,050
460
8,430
6,950
140
3,500
6,000
450
4,300
6,400
-
660
72,000
-
-
"*
420
7,900
79
2,500
1,800
10,000
900
3,800
"•
Concentrations in wet sludge (mg/1 )
Cadmium Zinc Nickel
Tot Dis Tot Dis Tot Dis
0.88
0.92
0.6
0.8
0.6
1.0
1.2
1.1
1.2
1.1
0.9
0.9
1.2
1.0
0.89
0.86
0.74
0.76
0.76
0.79
0.81
0.85
0.74
O.S7
0.66
0.77
0.87
0.97
<0.1
oil
0.34
0.38
2.3
2.3
3.2
<0.2
0.90
<0.2
0.3
0.7
<0 1
o'.2
0.5
<0.2
<0.2
O.2
0.2
0.5
0.4
O.2
0.7
O.2
;
-------
TABLE 32. EFFECT OF TREATMENT CONDITIONS ON THE DEGREE OF SOLUBILIZATION
OF HEAVY METALS AND OTHER SLUDGE CONSTITUENTS, (concluded)
Line
1
£
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Sol Ids
_
23.5
_
12.1
7.5
_
13.8
17.9
..
13.4
19.0
_
22.7
20.0
_
11.7
20.3
_
10.9
17.6
-
_
21.3
-
-
~
19.4
_
27.8
«
27.2
_
20.2
"
COO
_
-
_
10.3
19.2
_
8.8
12.8
_
9.2
13.5
_
14.1
12.2
_
7.5
12.8
„
9.3
14.0
-
_
21.3
-
-
-
14.8
_
18.7
-
13.0
_
18.0
"
Cadmium
.
97.8
_
37.5
100.0
_
12.5
41.8
_
0
0
_
41.7
40.0
_
81.4
0
.
0
0
60.5
.
81.1
64.9
100
90.9
87.6
~1ff(Jt
BDL
_
100
_
91.3
87.5
% Solubilization
Zinc
_
97.0
_
77.8
78.7
_
50.4
59.7
_
25.4
65.4
_
73.5
67.3
_
100
560
_
1.6
31.7
-
_
76.2
64.9
72.3
73.9
84.6
_
82.1
_
100
_
100
93.2
Nickel
_
100.0
_
73.9
-
_
0
0
„
68.8
0
.
81.3
80.0
_
0
62.5
_
0
0
-
_
BDL
-
-
••
72.7
_
85.5
_
100
.
90.1
74.2
Copper
.
-
_
0
9.1
..
0
0
_
0
0
.
0
0
_
0
0
.
0
0
-
_
0
•
-
~
3.1
_
81.5
.
48.3
_
78.0
0
Chromium
»
33.6
_
11.8
34.5
_
5.2
11.3
_
5.9
3.1
.
6.6
5.8
_
4.4
6.2
-
1.0
11.0
-
_
19.3
-
-
••
.
44.3
_
41.9
_
80.4
-
44.0
TKN
_
••
_
-
-
„
-
-
_
-
-
_
-
-
_
-
-
_
-
.
-
_
_
-
.
-
_
26.7
_
24.7
_
42.0
.
17.4
BDL » Below Detectable Limit
110
-------
fermenter, but the solubilization of nickel was greater for the one-liter
graduated cylinder (see lines 10 and 11 of Table 32).
Tests were conducted with the 14-liter fermenter to determine the
effect of stirring speed on the degree of solubilization. Two samples of
the same sludge were digested sequentially under identical conditions except
that a stirring speed of 60 rpm was used for one sample and 600 rpm was
used for the other. The results, shown in lines 13 and 14 of Table 32,
indicate nearly identical solubilizations for the two samples. It is
therefore concluded that the effect'of stirring speed on solubilization is
not significant.
The effect of the order in which acid and heat are applied to the
sludge is shown by lines 16 and 17 of Table 32. Both cadmium and zinc
were solubilized to a greater extent when the acid was added before heating
(811 and 100%, respectively) rather than after heating (0% and 56%, res-
pectively). This could be due in part to the longer exposure to acid when
it is added before heating.
Comparison of lines 20 and 21 of Table 32 indicate a substantial in-
crease in the amount of cadmium solubilized when the digestion time was
increased from 30 to 60 minutes. A sample taken after 30 minutes of
digestion indicated no cadmium solubilization while the sample taken after
an additional 30-minute digestion of the same batch of sludge indicated 60%
solubilization. In general, however, the digestion time was much less
significant than this result would indicate.
The above results suggest that the conditions most conducive to heavy
metals solubilization are:
—use of the 14-liter fermenter for digestion,
—use of 600 rpm stirring speed (primarily to promote good heat
transfer and uniform conditions):,
—acid addition before heating, and ;
, —60 minutes digestion time.
These conditions were used in tests with sludges obtained from various
municipal sources.
The test results of Table 32 indicate that the variable which exerts the
greatest influence on metals solubilization is the amount of acid added.
This is in agreement with the findings reported in Section 4. An attempt
was made to correlate the degree of solubilization as a function of pH.
The results for cadmium are shown in Figure 36. This correlation indicates
a general trend toward higher solubilization of cadmium as the pH is
decreased. At pH's greater than 2.5, there is essentially no solubilization
of cadmium while at pH's below about 2.0, typically 80 to 100% of the cad-
mium is solubilized. Between pH 2 and 2.5, the data are widely scattered,
ranging from zero to nearly 90%. These results are consistent with those
presented in sections 4 and 5 for cadmium solubilization and in particular
with the 0 to 90% solubilization range previously observed at pH 2.5.
Ill
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112
-------
A somewhat better correlation was obtained by plotting the degree of
cadmium solubilization against the .acid usage in kg of concentrated sulfuricr
acid (97%) per dry metric ton of solids. This plot is shown in Figure 37.
Below an acid usage of about 125 kg/dmt there is no significant solubil-
ization of cadmium, while above about 175 kg/dmt, 80 to 100% of the cadmium
is solubilized. Thus the critical acid usage for good solubilization of
cadmium is around 150 kg/dmt.
The fact that a better correlation was obtained on the basis of the
amount of acid added rather than pH may indicate that the bulk pH of the
sludge is not really representative of the hydrogen ion concentration at
the reaction sites. This concept of a localized pH along with the steep
slope of the curve in Figure 37 could explain the "erratic" solubilizations
of cadmium noted in previous tests.
The solubilization of zinc as a function of acid usage is shown in
Figure 38. The shape of the curve is similar to that for cadmium. The
solubilization increases very rapidly over the range of 100 to 150 kg acid
per dmt. Above this level the solubilization is generally in the range of
75 to 100%.
Results for nickel solubilization as a function of acid usage are
shown in Figure 39. The degree of solubilization increases very sharply
at about 125 kg/dmt. At higher acid usages the degree of solubilization
varies between about 70 and 100%.
The results for chromium solubilization are shown in Figure 40. The
degree of solubilization for chromium increases more gradually than for
cadmium, zinc, and nickel and reaches a plateau of about 40% solubilized at
higher acid usages.
In addition to heavy metals it is also of interest to know how the
solubilization of solids and COD vary with acid usage. The plot for
solubilization of solids is shown in Figure 41. The degree of solubilization
appears to gradually increase up to an acid usage of about 150 kg/dmt but
remains approximately constant thereafter at about 25%. The COD solubiliza-
tion increased over the same range as shown in Figure 42. Above an acid
usage of 150 kg/dmt the COD solubilization leveled off at 15-20%.
The results of Figures 37-40 indicate that good solubilization of the
heavy metals can be obtained at acid usages of about 200 kg/dmt and above.
This level was therefore specified for tests conducted with sludges obtained
from various municipal sources. However, in order to add a fixed amount of
acid per unit of solids, it is necessary to determine the solids content
of the sludge prior to the test. The amount of acid to be added was cal-
culated on the basis of the solids concentration as estimated by the various
municipalities at the time the sludge was obtained. The actual solids con-
tent was not determined until after the tests were completed. Therefore,
the acid usages for the sludges obtained from different cities varied-from
225 to 325 kg/dmt.
113
-------
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Table 32 (lines 27-35) shows the degree of solubilization for various
sludge constituents as a function of acid usage at the preferred operating
conditions (14-liter fermenter, 600 rpm, acid addition before heating, 60
minute digestion time, and acid usage > 200 kg/dmt). Tests were conducted
with samples of sludge from Brockton, MA, Fitchburg, MA, Lawrence, MA, and
Milwaukee, WI (two tests on same sample). The mean solubilizations and
ranges obtained for the various sludges are given in Table 33.
Excellent solubilizations were obtained for cadmium, zinc, and nickel
and moderate solubilizations were obtained for chromium and copper. With
the exception of copper the range of solubilizations of heavy metals was
reasonably narrow indicating that at an acid usage of >200 kg/dmt the
solubilization curve has reached its plateau. For copper, the results of
Table 32 (lines 27-35) indicate that the solubilization increases from zero
to 80% over a range of acid usage of 270 - 315 kg/dmt. Thus copper is more
difficult to solubilize than other heavy metals and requires an acid usage
of >300 kg/dmt for good solubilization.
Reasonably good agreement (except for copper as noted above) was ob-
tained for solubilization of a given constituent from the various sludges
as shown by the range of solubilizations in Table 33. The sample of
Lawrence, MA sludge exhibited the best solubilizations of heavy metals (100%
for cadmium, zinc, and nickel; 80% for chromium) but also exhibited the
highest solubilization of nitrogen (42%). This may indicate that a sig-
nificantly lower acid usage could be applied to the Lawrence sludge to
achieve acceptable solubilizations. In general, however, the results indi-
cate that the acid usage criteria of >200 kg/dmt for solubilization of cad-
mium, zinc,- nickel, and chrome, and >300 kg/dmt for solubilization of copper
are applicable to sludges from various municipalities
It is of interest to compare the costs of the hot acid process op-
timized for heavy metals removal to the same process optimized for de-
watering. The major difference in operating cost will result from differ-
ences in acid usages for the two cases. Based on the results of Section
4, optimum dewatering is obtained at a digestion pH of about 2.5. From
Figure 12, the acid requirement to achieve a pH of 2.5 ranges from 70 to
100 kg/dmt. For optimum metals removal (except copper) the acid require-
ment is approximately 200 kg/dmt which is a factor of two to three times
greater than for optimum dewatering.
Based on an acid cost of $49.50 per metric ton ($45 per short ton)
for concentrated sulfuric acid, the costs per ton of solids treated are:
Process
Optimized for Dewatering
Optimized for Metals Removal
Acid usage
kg/dmt
100
200
Acid cost
$/dmt $/dst
4.95 4.50
9.90 9.00
120
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TABLE 33. SOLUBILIZATIONS OBTAINED FOR SLUDGES FROM VARIOUS
CITIES AT PREFERRED OPERATING CONDITIONS
Sludge
constituent
Mean
sol utilization
Range of
sol utilizations
Cadmium
Zinc
Nickel
Chromium
Copper
Solids
Organics (COD)
Nitrogen (TKN)
92
92
84
53
53
24
16
28
88
82
73
42
3
19
13
17
100
100
100
80
82
28
19
42
121
-------
Thus, for the hot acid process, the cost differential for acidification
between optimized dewatering and optimized metals removal is only about
$5/dmt which is considered to be quite reasonable.
CONCLUSIONS
Additional bench-scale tests were conducted to define the factors
responsible for solubilization of heavy metals and to optimize the hot acid
process for heavy metals removal. Consistent correlations were obtained
between the degree of solubilization and the acid usage in kg of concentrated
H?S04 per dmt. These correlations indicate a rapid increase in the degree
of solubilization of cadmium, zinc, nickel, and chromium over the range of
100 to 200 kg/dmt and of copper (based on limited data) over the range of
250 to 300 kg/dmt. The correlation of metals solubilization with "acid
usage" was found to be more consistent than correlation with "pH".
Tests conducted with fresh WAS samples from various municipalities
indicated excellent solubilization of cadmium (88-100%), zinc (82-100%),
and nickel (73-100%) at acid usages >200 kg/dmt. Only moderate solubilization
of chromium (^45%) was obtained, and appreciable copper solubilization
(^0%) required a higher acid usage (>300 kg/dmt). For these tests, average
solids, COD, and TKN solubilizations were 24, 16, and 28%, respectively.
An acid usage of 200 kg/dmt was selected as the optimum for heavy'metals
solubilization. This is two to three times as great as the acid usage
required for optimum dewatering (70-100 kg/dmt to achieve pH 2.5). However,
the cost for acid at a usage of 200 kg/dmt is still <$10/dmt ($9/dst) and is
considered to be quite acceptable for the levels of solubilization obtained.
122
-------
SECTION 7
SPECIFICATION OF SOLIDS - SEPARATION EQUIPMENT
Three techniques for solid-liquid separation: settling, vacuum
filtration, and centrifugation were investigated during bench-scale tests.
The solid-liquid separation tests were conducted for the purpose of iden-
tifying the effects of changes in the process variables on solids separation
rates and were not intended for generation of design data for full-scale
dewatering equipment. The rationale for concluding that the hot acid
process produces a sludge with adequate dewatering characteristics is based
on:
1) the substantial improvement in dewatering characteristics
relative to the untreated sludge, and
2) the results of Figures 33 and 34 which indicate that filtration
and centrifugation rates are as favorable as for chemically
conditioned sludge.
Of the three solid-liquid separation techniques evaluated, settling
appeared to have the least potential applicability to the hot acid
treatment process. Thickening of the sludge prior to hot acid treatment
is very important in reducing the treatment costs. However, for properly
thickened sludge, the hot acid process does not produce rapid additional
settling. As shown in Figure 14, the volume of treated sludge was reduced
by only about 10% during 30 minutes of settling following hot acid treat-
ment. The rate and extent of settling observed during the bench-scale tests
appeared inadequate to justify the added cost and complexity of settling
before final dewatering of the sludge.
The bench-scale filtration data can be analyzed to give the specific
resistance of the filter cake which can then be used to calculate the filter
yield'8''. The specific resistance for raw Brockton WAS (see filtration
curve of Figure 33) was approximately 1.5x10'° sec^/g. The hot acid process
(or chemtcal conditioning) reduced the specific resistance to approximately
1.5x109 sec2/g. This resistance is higher than the level at which good
filtration results are normally obtained (108 sec2/g (15) ).
A number of attempts were made to obtain filtration data using a filter
leaf apparatus. However none of the filter materials tried gave satis-
factory results: some filter materials plugged allowing no passage of
123
-------
filtrate while others permitted near complete passage of both solids and
liquid. Consequently the filter leaf tests were abandoned.
Of the solid-liquid separation techniques evaluated, centrifugation was
selected as the preferred technique for sludge dewatering following hot
acid treatment. This selection was based on several considerations:
1. The centrifugation curve for hot acid treatment
(see Figure 34) fell approximately mid-way between the curves for chemical
treatment and thermal treatment. This was taken as an indication that
centrifugation rather than filtration was better suited for dewatering
the hot-acid-treated sludge.
2. The high specific resistance and problems with the filter
leaf test indicated potential problems for dewatering by vacuum filtration.
3. In the initial process design, the digested sludge was
dewatered before cooling (by heat exchange with the feed) in order to take
advantage of the lower viscosity of water at elevated temperatures.
Vacuum filtration was not considered well-suited for dewatering hot sludge
because of potential odor problems and potential heat loss from the
filtrate by vacuum evaporation. (However, this process design was later
abandoned when economic calculations indicated a large energy-cost penalty
associated with failure to recover heat from the sludge solids.)
4. Centrifugation appeared to have certain other advantages
over vacuum filtration such as lower costs and operating .labor (&).
Based on these advantages bench-scale and pilot-scale centrifugation
tests were conducted at Bird Machine Co., South Walpole, MA. The test
results are given in Appendix B. On the basis of the bench-scale tests it
was concluded that hot-acid-treated sludge would be a good application for
a continuous solid bowl centrifuge, and pilot-scale tests were recommended.
However, during the bench-scale tests, lime, ferric chloride, and various
polymers were used to improve the solid-liquid separation. Because of the
cost of these additives and their potential for adsorption or precipi-
tation of solubilized heavy metals, the pilot-scale tests were conducted
without chemical additives. For these tests good cake solids concentra-
tions (18-36% solids) were obtained, but solids recoveries were low. The
use of flocculents was recommended to improve the recovery of solids as
cake. Based on these tests, the full-scale dewatering equipment specified
by Bird Machine Co. for hot-acid-treated Brockton WAS is shown in Table 34.
While centrifugation can be used to dewater hot-acid-treated sludge,
other solid-liquid separation devices may offer significant advantages over
centrifugation and should be evaluated. Of particular interest is the
filter belt press which can generally produce higher cake consistencies
than centrifugation and operate with significantly lower power and
maintenance requirements
124
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TABLE-34, SPECIFICATION FOR CENTRIFUGES TO DEWATER BROCKTON WAS
FOLLOWING HOT ACID TREATMENT
Centrifuge Capacity
Solids Liquid
kg/hr (Ib/hr) 1pm (gpm)
Description*
68 (150) 38 (10) One Bird Machine Co. HB1400 or equivalent
680 (1,500) - 380 (100) One Bird Machine Co. HB3700 or equivalent
6,800 (15,000) 3,800 (1,000) Three Bird Machine Co. HB64000 or equivalent
All machines are horizontal, solid-bowl, conveyor-type centrifuges. Materials
of construction are 300-series stainless steel. Centrifuges are of the low-
speed, sludge-dewatering type which operate in the range of 500-800 x gravity.
125
-------
SECTION 8
SPECIFICATIONS FOR PILOT SYSTEM
The process flow schematic for a 5 gpm, pilot-scale, hot-acid-
treatment system is shown in Figure 43. Primary sludge (which has been
screened to remove gross particulates) and waste activated sludge are mixed
in the desired proportions and pass through in float control valve that
maintains a constant level in the feed tank. The feed tank is agitated by a
0.19kw (0.25 h.p.) mixer to insure complete mixing of the two sludges, and
the 10 minutes residence time in the 0.2 m3 (50 gal) tank smoothes any
sudden changes in feed composition.
A progressive cavity pump circulates sludge through the pilot plant.
This type of pump provides a constant flow rate with varying upstream
pressure and can pump abrasive solids and viscous sludges. The flow rate
is controlled by varying the quantity of sludge returned to the pump suction
through the bypass valve. The ultrasonic flow meter and total flow counter,
which uses the Doppler principle to measure velocity, is attached to the
outside wall of the pipe and does not contact the sludge.
The sludge is heated in two stages. In the first stage, the feed sludge
is heated to approximately (75°C) by exchange with the hot digested sludge;
in the second stage steam heats the sludge to the desired digestion temper-
ature C90-95°C). A co-axial double pipe heat exchanger is used for the first
stage since both sludge and concentrate contain solids which may settle out
unless a high velocity is maintained in both streams. A shell and tube heat
exchanger with an area of 2.5 m2 (27 ft2) (based on an estimated overall heat
transfer coefficient, U, of 270 W/m2 °C (50 Btu/hr-ft2-°F) is used in the
second stage with the sludge on the tube side to avoid settling. A 68 kW
botler, capable of generating 95 kg sjteam/hour (210 Ib steam/hour), supplies
steam to the second stage at 450 kN/m (50 psig), and the rate of steam supply
Is controlled by a temperature controller.
The pH of the feed is adjusted to the desired level (pH 2-3) in a
0.1 m3 (25 gal) mixing tank by adding concentrated sulfuric acid at a rate of
approximately 0.1 kg acid/kg sludge solids (2 1/hr of 95% actd for sludge
With 3% solids). A pH probe with a feedback control system regulates the
acid metering pump. The pH is adjusted after the heat exchanger rather than
before In order to reduce corrosion and recover heat from the solids stream.
126
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The hot acidified feed flows from the mixing tank over a weir to the
digester where it is held for the desired residence time. The digester has
a total volume of 0.57 m3 (150 gallons) to provide a maximum residence time
of 30 minutes at the design flow rate of 19 1/min (5 gpm). The tank is con-
structed of fiberglass with a 30°-slope conical bottom, a height of 2 m, and
a diameter of 0.65 m. The volume is divided into three equal stages by two
30° conical baffles with 10-cm-diameter holes in the center of each. This
configuration reduces axial mixing which for an unbaffled tank would result
In a poor residence time distribution.
A variable speed mfxer (20-350 rpm) with a turbine agitator in each stage
can be used to determine the effect of agitation on performance. The mixer
ha.s §n Independent support structure to avoid stresses in the fiberglass
digester tank. A manhole in each digester section allows assembly of the
agitators to the mixer shaft and cleaning between the baffles (should this be
necessary).
The level (residence time) in the digester is maintained by a differ-
ential pressure level transducer and controller that maintains a constant
preset pressure differential across the sludge by adjusting a proportional
flow control valve at the bottom of the digester. A small air blower main-
tains a slightly reduced pressure in the digester and pH mixing tank to avoid
venting untreated gases past the agitator shaft and unsealed tank connections.
The gases from the blower are vented through a condenser and carbon adsorp-
tion column.
Following cooling (heat recovery) in the first stage heat exchanger, the
treated sludge is centrifuged to remove solids. (Other dewatering devices
could also be evaluated). The solid cake is discharged; and the concentrate
flows into a 0.075 m3 (20 gallon) surge tank where the pH is adjusted by the
addition of sodium hydroxide (NaOH) or lime to precipitate heavy metals. The
pH can be varied to determine the optimum level for maximum metals removal.
The precipitated metals are settled in a clarifier. with a surface area of
approximately 0.7 m2 (7 ft2). It is anticipated that other suspended solids
will coagulate and settle with the metal hydroxides. The clarified overflow
is pumped back to the secondary treatment system. Batch tests can be
conducted to evaluate various dewatering techniques for the settled heavy
metal sludge, but it is anticipated that the flow of sludge will be too small
to allow continuous dewatering.
Temperature (T) and pressure (P) are monitored at strategic locations
as Indicated in Figure 43. Automatic vacuum lift samplers are used to obtain
samples of untreated sludge, dewatered sludge, and centrate. Either time-
averaged composites or sequential samples can be collected.
128
-------
Specifications and approximate costs of the equipment required to build
the pilot plant are shown in Table 35. The approximate total purchased
equipment cost, not including sludge dewatering, is $40,000. To this cost
must be added the cost of sludge dewatering equipment, the cost of system
design and engineering, and the cost of site preparation and installation.
129
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132
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SECTION 9
ENERGY AND ECONOMIC ANALYSES
A simplified flow schematic of the hot acid sludge conditioning system
is shown in Figure 44. Thickened sludge is pumped through a heat exchanger
in which the sludge temperature is increased by heat recovered from the
treated sludge. The sludge is then acidified with concentrated sulfuric add
(H2S04) in a flash mixing tank before entering the digester. In the digester
the sludge is brought to digestion temperature by steam circulated through
heating coils and is retained for the desired residence time.
After digestion the sludge is pumped back through the heat exchanger
(for energy recovery) to the centrifuge where the sludge is dewatered.
Because of the low pH, it may be necessary to neutralize the solid cake prior
to land application. This can be accomplished by the addition of anhydrous
ammonia which will also increase the nitrogen value of the sludge. The
liquid concentrate is returned to the secondary treatment system after pH
adjustment and settling to precipitate heavy metal hydroxides.
Cost estimates have been developed for hot acid sludge conditioning
systems to treat 200, 20 and 2 dmtpd (dry metric tons per day) of sludge
solids. These are the approximate quantities of primary plus waste activated
sludge generated by wastewater treatment plants of 500, 50, and 5 m3/min
(200, 20 and 2 million gallons per day) capacity, respectively (20). The
assumptions and design bases used to develop the cost estimates are given
in Table 36.
The flow rates of the various streams in the system are shown in
Figure 45. It is assumed that the sludge is thickened to 3% solids prior to
conditioning, and after conditioning the Centrifuge produces a sludge cake
with 20% solids. From Figure 12 the acid demand is taken as 0.1 kg H?S04 per
kg of sludge solids which should reduce the pH to 2.0 - 2.5. As pointed out
in Section 6, optimum metals removal would require about twice this amount
of acid-
The energy balance around the system is shown in Figure 46. The
thickened sludge is initially heated to 80°C in the heat exchanger by energy
recovered from the treated sludge (approximately 80% of the heat is re-
covered). The sludge is further heated to 95°C in the digester by steam
passed through the heating coils. Following digestion, during which heat
losses are assumed to be negligible, the temperature is reduced to 35°C in the
133
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TABLE 36. ASSUMPTIONS AND DESIGN BASES FOR ECONOMIC ANALYSIS
Sludge solids treated, dmtpd
Feed sludge solids concentration, percent
Feed sludge flow rate, m3/hr (gpm)
Feed sludge temperature: Entering H/E, °C
Leaving H/E, °C
Treated sludge temperature: Entering H/E, °C
Leaving H/E, °C
Overall coefficient of heat transfer- for
H/E, W/m2 °C (Btu/hr ft2 °F)
Surface area of heat exchanger, m2 (ft2)
Digestion pH
Sludge acid demand, kg acid/kg sludge solids
Acid flow rate, kg/hr
Acid price, 66°Be H2SO,(3 $/kg ($/short ton)^
Acid cost, $/hr
Overall coefficient of heat transfer
of coils in digester, W/m2 °C(Btu/hr ft2 °F)
Sludge temperature in digester, °C
Steam pressure, kN/m2 (psig)
Steam flow rate to digester, kg/hr (Ib/hr)
Energy to generate steam, :kW
Cost of No. 4 fuel oil, $/kW-hr ^
Sludge residence time in digester, min
Total pump head, m
pump efficiency, %
Miscellaneous electric power, kW
Electric power cost, $/kW-hr'b'
Labor man-days/year (conditioning)
Labor rate $/man-day (incl. fringe benefits)
Annual capital cost (20-year life, 7% per
a.nnum interest) as % of capital investment
Maintenance materials, % of total O&M cost
2
3
2.8(12.2)
20
80
95
35
575(100)
23(250)
2.0
0.1
8.3
0.05(45)
0.415
575(100)
95
450(50)
82.5(182)
48.5
0.009
30
30
70
2
0.05
200
100
9.4
5
20
3
28(122)
20
80
95
35
575(100)
230(2,500)
2.0
0.1
83
0.05(45)
4.15
575(100)
95
450(50)
825(1820)
485
0.009
30
30
70
12
0.05
275
100
9.4
5
200
3
280(1220)
20
80
95
35
575(100)
2300(25,000)
2.0
0.1
830
0.05(45)
41.5
575(100)
95
450(50)
8250(38,200)
4850
0.009
30
30
70
72
0.05
360
100
9-4
5
(continued)
135
-------
TABLE 36. CONTINUED
Sludge solids treated, dmtpd
20
200
Centrifuge power, kVr '
Additional labor for centrifuge, man-day/year
Lime demand, kg lime/m3 supernatant^0'
Lime flow rate kg/hr
Lime cost, $/kg ($7100 Ibs) ^
n
20
3
7.2
0.0325(1.50)
30
25
3
72
0.0325(1.50)
246
40
3
720
0.0325(1.50)
(a) Quotation, May 1978.
(b) Private communication with supplier.
(c) Based on experimental dosage required to raise pH from 1.85 to 9.0.
(d) Reference 12
136
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heat exchanger before entering the centrifuge. The sludge is dewatered after,
rather than before, heat recovery so that heat can be recovered from the
sludge solids. The lower sludge temperature in the centrifuge increases^
viscosity and hence the required centrifuge capacity; however, the additional
heat recovered more than compensates for the added centrifugation costs.
Cost estimates or quotations for all major pieces of process equipment
are given in Table 37. All costs are adjusted to an EPA - Sewage Treatment
Plant Construction Cost Index (EPA-STPCCI) of 288 (fourth quarter 1977).
Land costs are not included since these costs are site specific, and in an
integrated plant it is difficult to segregate the component of land cost
attributable to sludge conditioning from that attributable to other wastewater
and sludge treatment processes.
Capital costs are shown in Table 38. Equipment installation is estimated
to cost 40% of the purchased equipment cost. Installation is defined as site
preparation (assuming reasonable subsoil conditions), foundations, and
transportation and positioning of equipment. It does not include equipment
hook-up which is included with piping, electrical and instrumentation.
Engineering, legal and contingency is estimated at 37.5% of the total plant
cost (21) and includes all costs not directly attributable to materials and
labor for plant construction, such as construction financing, contractor
fees, construction management, etc. These cost factors are obviously
generalizations of costs which will vary with local conditions. The overall
capital investment computed using these factors is approximately 2.4 times the
purchased equipment cost, which is in close agreement with other estimates
given in the literature (21»22).
The capital costs are shown for both the hot acid treatment process
alone, and hot acid treatment in conjunction with centrifugal dewatering and
supernatant treatment. In this way, costs can be compared directly either
with other conditioning techniques or with complete conditioning and de-
watering systems.
Operating and maintenance cost estimates are shown in Table 39. It is
assumed that energy to heat the sludge is provided by No. 4 fuel oil at a
cost of $0.009/kWhr, and that all other power would be electrical at a cost
of $0.05/kWhr. Operating labor was estimated assuming a fully automatic
system requiring only periodic routine checks. The labor rate was estimated
at $100 per man-day and includes all salary related costs and supervision.
No attempt is made to differentiate between operating and maintenance labor.
The capital-recovery-factor method (23) was used to determine investor
capitalization. In essence, this technique calculates the equal annual pay-
ment which must be made to yield the same sum at the end of the plant life
as if the capital had been invested at compound interest when the plant was
constructed. It is assumed that the plant has a 20 year life with no resale
value, and the annual interest rate is 7%. Under these circumstances the
yearly cost of financing the construction is equal to 9.4% of the total
fixed capital investment.
139
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TABLE 38. CAPITAL COSTS FOR HOT ACID TREATMENT
Plant size, dmtpd
20
200
103,300
41,300
576,300
230,500
36,200 201,700
CASE I; HOT ACID TREATMENT ONLY
Purchased equipment cost (PEC), $ 25,800
Equipment installation (40% PEC^a')»- $ 10,300
Piping, electrical, instrumentation
(35% PECU)), $ 9,000
Total plant cost (TPC), $ 45,100
Engineering, legal, contingency
(37.5% TPdb)) 16,900
Total capital investment, $ 62,000
Capital investment, $/dmtpd 31,000
(Capital investment, $/dstpd) (28,100)
CASE II: HOT ACID TREATMENT WITH DEUATERIN6 AND SUPERNATANT TREATMENT
180,800 1,008,500
67,800 378,200
248,600 1,386,700
12,400 6,900
(11,200). (6,300)
Purchased equipment cost (PEC),$ 113,800
Equipment installation (40% PEC^), $ 45,500
Piping, electrical, instrumentation
(35% PEC(a)), $ 39,800
Total plant cost (TPC), $ 199,100
Engineering, legal, contingency
(37.5% TPC), $ 74,700
Total capital investment, $ 273,800
Capital investment, $/dmtpd 136,900
(Capital investment, $/dstpd) (124,300)
280,300 1,681,300
112,100 672,500
98,100 588,500
490,500 2,942,300
183,900 1,103,400
674,400 4,045,700
33,700 20,200
(30,600) (18,300)
(a) Reference 23
(b) Reference 21
141
-------
TABLE 39. OPERATING AND MAINTENANCE COSTS FOR HOT ACID TREATMENT
($/DRY METRIC TON SOLIDS)
Plant
CASE
CASE
size, dmtpd
I: HOT ACID TREATMENT ONLY
Capital cost
Labor
Electricity
Fuel oil
Chemicals, acid
Maintenance materials
Total operating and maintenance cost
II: HOT ACID TREATMENT WITH DEWATERING AND
Capital cost
Labor
Electricity
Fuel oil
Chemicals, acid
lime
Maintenance materials
Total operating and maintenance cost
2
7.98
27.39
1.40
5.18
5.00
2.37
49.23
20
3.20
3.77
0.91
5.18
5.00
0.91
18.97
200
1.79
0.50
0.63
5.18
5.00
0.66
13.76
SUPERNATANT TREATMENT
35.25
30.13
8.00
5.18
5.00
2.80
4.39
90.75
8.68
4.11
2.71
5.18
5.00
2.80
1.43
29.91
5.21
0.55
2.1
5.18
5.00
2.80
1.04
21.88
142
-------
The total operating and maintainance cost for hot acid treatment is
$13.76, $18.97, and $49.23 per dmt solids ($12.50, $17.22, and $44.75 per dst
solids) for the 200, 20, and 2 dtntpd plant capacities. Inclusion of sludge
dewatering and supernatant treatment increases the costs to $21.88, $29.91
and $90.75/dmt for the respective plant capacities.
To compare the economics of hot acid treatment with other sludge treat-
ment techniques, cost estimates have been developed for: anaerobic digestion,
aerobic digestion, lime treatment, and heat treatment. The cost estimates,
shown in Tables 40-43, respectively, are based upon capital costs (adjusted
to EPA-STPCCI of 288) and recently published operating and maintainance
requirements (21). Since many operating costs (such as for energy) have
increased at a more rapid rate than the STPCCI, care was taken to adjust all
operating and maintainance costs to the same bases used to estimate the
costs of hot acid treatment given in Table 39.
The cost estimates of Tables 40-43 are for the treatment or stabilization
process only and do not include the cost of sludge dewatering, sludge
disposal, or supernatant treatment. However, heat treatment and hot acid
treatment both condition sludge for dewatering whereas anaerobic digestion,
aerobic digestion and lime treatment are principally stabilization processes
that do little to improve dewaterability. To bring the costs to the same
basis, the cost of chemicals which must be added to the latter group to
prepare the stabilized sludge for dewatering has been included in the cost
estimate. The chemical demand of the sludges was based upon generalized
estimates given in the literature, and it should be noted that the chemical
demand may vary considerably from these estimates depending upon the type
of sludge and the wastewater treatment process.
Cost comparisons for various plant capacities are shown in Figure 47.
Lime stabilization potentially provides the lowest treatment cost per ton
of sludge because of the low capital and operating cost. However, the high
chemical cost to prepare the sludge for dewatering makes this process only
slightly less expensive than the hot acid treatment process. In addition,
sludge disposal costs (not included in this analysis) are increased by the
large volume of lime present in the final sludge. In addition, the lime
process does not provide metals removal.
The principle economic advantage of hot acid treatment over heat treat-
ment is the much lower capital cost and operation at atmospheric pressure.
The advantage of heat treatment is that no chemical addition is required.
Anaerobic digestion is more costly than aerobic digestion because of
the higher capital cost resulting from the need for odor control. Aerobic
digestion costs are approximately the same as those for hot acid treatment
for smaller plant capacities, but the economic advantage of hot acid treat-
ment is greater for larger plants. The main differences are the need for
more chemicals to condition the aerobic sludge, and the higher capital costs
for aerobic digestion equipment.
143
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148
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For optimum removal of heavy metals, the costs presented in Table
39 and Figure 47 should be adjusted to reflect an acid usage of 200 kg/dmt
rather than 100 kg/dmt. This will add $5/dmt for hot acid treatment only
and $7.80/dmt for hot acid treatment with dewatering and supernatant treat-
ment. The dashed curve of Figure 47 shows the cost as a function of plant
capacity for hot acid treatment when the process is optimized for heavy
metals removal. Total costs, including dewatering and liquor treatment and
including capital amortization, for optimized metals removal at plant capa-
cities of 2, 20, and 200 dmtpd would be $98.55/dmt, $37,71/dmt, and $29.687
dmt, respectively.
Based on the above cost comparisons it is concluded that the hot acid
treatment process is highly cost competitive with alternative stabilization/
conditioning techniques and is particularly attractive where land application
of sludge is limited by the concentration of heavy metals in the sludge.
149
-------
REFERENCES
1. Federal Register, 42 (211): 57420-57427, Wed., Nov. 2, 1977.
2. Council for Agricultural Science and Technology. Application of Sewage
Sludge to Cropland: Appraisal of Potential Hazards of the Heavy Metals
to Plants and Animals. EPA 430/9-76-013, U.S. Environmental Protection
Agency, Washington, D.C., 1976. 63 pp.
3. Jelinek, 6.F., Health Perspectives - Sludge Use on Land. In: Proceed-
ings of 1977 National Conference on Composting of Municipal Residues and
Sludges, Information Transfer, Inc., Rockville, MD, 1978. pp 27-29.
4. Federal Register, 43(25): 4942-4955, Mon. Feb. 6, 1978.
5. Everett, J.6. The Effect of Heat Treatment on the Sol utilization of
Heavy Metals, Solids, and Organic Matter from Digested Sludge. Water
Pollution Control, 73: 207-209, 1974.
6. Brooks, R.B. Heat Treatment of Activated Sludge. Water Pollution
Control, 67(5): 592, 1968.
7. Marshall, D.W. and F.C. Fiery. Investigations of Heat Treatment for
Paper Mill Sludge Conditioning. EPA-600/2-78-015, U.S. Environmental
Protection Agency, Cincinnati, Ohio, 1978. 73 pp.
8. Vesilind, P.A. Treatment and Disposal of Wastewater Sludges. Ann
Arbor Science Publishers, Inc., Ann Arbor, Michigan, 1974. 236 pp.
9. Andersen, L.B. Factorial Design of Experiments. Chemical Engineering,
70(18) :99-104, 1963.
10. Katz, W.J. and A. Geinopolos. Concentration of Sewage Treatment Plant
Sludges by Thickening. In: Proceedings of the Tenth Sanitary
Engineering Conference, Dept. of Civil Engineering, University of
Illinois, Urbana, Illinois, 1968. pp. 33-45.
11. Stern, G. and J.B. Farrell. Sludge Disinfection Techniques. In:
Proceedings of the 1977 National Conference on Composting of Municipal
Residues and Sludges, Information Transfer Inc., Rockville, Maryland,
1977. pp. 142-148.
150
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12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Black, Crow, and Eidshess, Inc., Process Design Manual for Sludge Treat-
ment and Disposal. EPA 625/1-74-006, U.S. Environmental Protection
Agency Technology Transfer, Washington, D.C., 1974. (Original source
cited as reference 40 of Chapter 5).
Chemical Marketing Reporter, 213(24), June 12, 1978.
Co., Inc., New York, N.Y.
Schnell Publishing
Boyle, J.D. and D.D. Gruenwald. Recycle of Liquor from Heat Treatment
of Sludge. J. Water Pollution Control Fed., 47(10): 2482-2489, 1975.
Black, Crow, and Eidsness, Inc., Process Design Manual for Sludge Treat-
ment and Disposal. EPA 625/1-74-006, U.S. Environmental Protection
Agency Technology Transfer, Washington, D.C. 1974.
Cameron, J.W. Aerobic Digestion of Activated Sludge to Reduce Sludge
Handling Costs. Presented at 45th Annual Conference of the Water
Pollution Control Federation, Atlantic, Georgia, October 1972.
Hayes, T.D. and T.L. Theis. The Distribution of Heavy Metals in
Anaerobic Digestion. J. Water Pollution Control Fed. 50(l):61-72,
1978.
Blumfield, C. and G. Prudens. The Effects of Anaerobic and Aerobic
Incubation on the Extract!bilities of Heavy Metals in Digested Sewage
Sludge. Environmental Pollution 8:217-232, 1975.
Miner, R.A., D.W. Marshall, and I. Gellman. Pilot Investigation of
Secondary Sludge Dewaterfng Alternatives. EPA-600/2-78-014, U.S.
Environmental Protection Agency, Cincinnati, Ohio, 1978. 115 pp.
Metcalf and Eddy, Inc.
Disposal. McGraw-Hill
Wastewater Engineering:
Book Co., New York, N.Y.,
Collection, Treatment,
1972. 782 pp.
Metcalf and Eddy, Inc. Assessment of Technologies and Costs for
Publically Owned Treatment Works. Report to National Commission on
Water Quality, Washington, D.C. Available from National Technical
Information Service, PB-250 690-01 through PB-250 690-03, 1976.
1401 pp.
Campbell, H.W., R.J. Rush, and R. Tew. Sludge Dewatering Design
Manual. Research Report No. 72, Training and Technology Transfer
Division (Water), Environmental Protection Service, Ottawa, Ontario,
1978.
151
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23. Peters, M.S. and K.D. Tinmerhaus. Plant Design and Economics for
Chemical Engineers. Second Edition. McGraw-Hill Book Co., New York,
N.Y., 1968. 850 pp.
24. Garber, W.F., G.T. Ohara, I.E. Colbaugh, and S.K. Rakset. Thermophilic
Digestion at the Hyperion Treatment Plant. J. Water Pollution Control
Fed., 47(5):950-961, 1975.
25. Notevaert, F.F., A.A. Van Haute, and D.A. Wilms. Conditioning of
Aerobically Stabilized Sludge. Water Research 9: 1037-1046, Pergamon
Press. New York, N.Y. 1975.
152
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APPENDIX A
VERIFICATION OF SELECTED ANALYTICAL RESULTS
During the course of the experimental program, questions arose concern-
ing the accuracy of analytical results, particularly those for heavy metals.
Therefore a number of samples were submitted to the Municipal Environmental
Research Laboratory of EPA (Mr. Vincent B. Sal otto) for analytical verifi-
cation. Samples submitted to EPA were refrigerated prior to shipment and
were shipped by air freight.
Analytical results are compared for the various samples in Table A-l.
The columns headed by "W" are the results from the Wai den Analytical
Laboratory; those headed by "E" are the results from the EPA analytical
laboratory. In general, there is reasonably good agreement between the
results of the two laboratories.
The cadmium results are particularly noteworthy because of the environ-
mental importance of cadmium. One of the specific objectives of this
comparison was to verify the high cadmium solubilization (90%) reported by
the Wai den lab for Brockton WAS (first line of Table A-l). The EPA results
do verify the high cadmium solubillzation for this test. For the Fitchburg
and Milwaukee waste activated sludges the EPA results Indicate 50% and 84%
solubllizatlon, of cadmium respectively, while the Walden results indicate
zero percent for both- For the sample of Brockton WAS injected at various
pH's, good agreement was obtained between the Walden and EPA results. It
should be noted that, for this latter sample of sludge, the heavy metals
proved to be quite difficult to solubllize, and the results should not be
considered characteristic of the potential of the hot acid process to
solubilize heavy metals.
From a comparison of all the results of Table A-l it is concluded that
the Walden analytical results are at least as consistent as those obtained
by EPA. For cadmium, the EPA results suggest that solubilizations could be
greater than those reported on the basis of the Walden analytical results.
153
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155
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APPENDIX B
LABORATORY REPORT ON CENTRIFUGATION TESTS
BIRD MACHINE COMPANY, INC.
SOUTH WALPOLE, MASSACHUSETTS
LABORATORY REPORT NO. 8876
Customer: Abcor, Inc.
Wilmington, MA
Material: Brockton Waste Activated Sludge
1599240
Problem: Clarification
Test: Preliminary
Test date: July 20, 1977
Witnesses: Dr. Kenneth McNulty
Ms. Ann Malarkey
Sample No. 129 (Rec'd 7/20/77)
Six (6) liters of a waste activated sludge were received in the laboratory
for preliminary centrifugal testing.
The sludge was obtained by the witnesses from the Brockton, MA STP and pre-
treated before arriving at the laboratory. The sludge was identified only
as WAS. A more exact definition of the sludge would be helpful; i.e., where
the sample was collected, time of sampling.
The sludge received was still hot. The sample contained 3.59% total sol Ids bv
weight, 2.16% dissolved solids in the mother liquor.
Problem
A new sludge treatment process has been developed by Dr. Fremont of Champion
International. Further studies to more fully refine and design a treatment
system are being carried out by Abcor, Inc.
Thts new process is an acid/heat treatment for raw sludge. Acid is added to
156
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the sludge to lower the pH to 2-3 and the sludge is heated to 90-100°C. Many
of the heavy metal compounds dissolve providing a solids product which has
better soil conditioning characteristics. Improved settling, filtration, and
centrifugation have been noted in laboratory tests with treated sludge.
Vacuum filtration has been ruled out as a means of dewatering since the sludge
is to be processed hot and cooling would be needed prior to filtration.
The objective of this test is to demonstrate the ability of a centrifuge to
dewater this new-type waste activated sludge.
A large scale test will be run once the sludge characteristics are defined.
Test
' Spin tests were run on the sludge as received at ?0-1006C. The new test tube
spinner was used to allow for variations in centrifugal force. The data is
included on pages 1-1 and 1-2. (Note: Huch of the data was given in cryptic
notation and has not been included in this appendix).
Increased centrifugal force shows a direct effect in settling out suspended
soltds and producing a firmer cake. The settled solids do not have much
body (bulk) since a glass rod readily penetrates into 50% of the cake.
flocculants including alum, ferric chloride and lime were used to agglomerate
the solids to aid in clarification. All of the polymers tested on the as
received sludge showed only minimal, if any, agglomeration. The polymers used
included Allied Colloids, Hercules, and American Cyanamid. Polymers used in
conjunction with others of opposing charge showed no improvement in agglomer-
ation. A combination of lime and polymer seemed to work best in providing
clarity and ftrm cake. These spin tests are also included on pages 1-1 and
1-2. (Not included herein).
Both lime and polymer used together would produce the best results as noted
in the spin tests.
The batch pulp centrifuge was also run at varying gravitational forces on as
received sludge and flocced slurry.
Cake solids are low and recovery poor without pretreatment. The best results
are obtained where lime and polymer are used together. This data is attached
on pages 2-1 and 2-2. (Not included herein).
Conclusions
1. This waste sludge or one similar to that tested would be a good appli-
cation for a Bird Continuous Solid Bowl Centrifuge.
2. Since the cake product will be sold as soil conditioner and will probably
need to be neutralized to reduce residual acid content, lime should be added.
157
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This will both add bulk to the cake, to produce a more easily centrifuged
material, and neutralize the acid content of the final product. The final
cake material would also have improved soil conditioning properties. The
lime dosage will have to be defined more exactly in large scale tests but
would be in the range of 500-1000 IDS lime/ton dry solids of sludge.
3. Polyelectrolytes would be required for optimum clarification. The pre-
liminary test indicated about 1 Ib/ton dosage for +90% solids recovery.
4. The expected cake product would be directly related to the amount of
lime addition. The more lime, the drier the cake product that would result.
5. A large scale test would be recommended. The amount of lime to be
allowed should be determined prior to testing.
MMangion:nt/22
NOTE:
Allied Colloids Percol 763
Cost = $1.50/lb in bulk
$2.50/50 Ib bag for one bag only
1 Ib/ton dosage = $2.50/ton max. cost
Lime
Cost = 2<£/lb
500 Ib/ton dosage = $10/ton cost
158
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BIRD MAGHINE COMPANY, INC.
SOUTH WALPOLE, MASSACHUSETTS
Addendum to: Laboratory Report No. 8876
Customer: Abcor, Inc.
850 Main Street
Wilmington, MA
Material: Brockton Waste Activated Sludge
Problem: Clarify and Dewater
Test: 6" Bird Continuous Solid Bowl Centrifuge
Test date: April 20, 1978
Witness: Mr. John Harland - Engineer for Abcor
Sample No. 127 (Rec'd April 20, 1978)
Two 55-gallon drums containing Brockton waste activated sludge were delivered
to the laboratory for large scale testing purposes.
Sample as received contained approximately 2.92% total solids by weight and
had a spectftc gravity of 1.01 @ room temperature. Slurry was prepared for
testing by heating to 95° then acidifying with sulfuric acid to a pH of 1.94.
Problem
Abcor is studying a new process for waste treatment described as the "Hot
Acid Process", the principle objective being to dissolve and remove heavy
metal compounds making a solids product which has better soil conditioning
characteristics.
Pilot plant operation will require a deliquoring device which will handle 5
to 10 gallons per mtnute.
As dry a cake as possible is desired, and lime or flocculants cannot be used
as they might precipitate the solubilized metals.
Tests
Five test runs were completed, for data obtained. Please note data sheet
159
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appended to this report.
Variables investigated were machine speeds of 6000 and 3500 RPM, equivalent
to 3000 x 6 and 1000 x G, respectively.
u Pu°LVOl^mn us?d dun'ng Run #1 was ^creased to the maximum for
Runs n through #4 and Run #4 was made with increased cake retention time in
an effort to produce a drier cake.
Discussion
Test results show that acidified Brockton waste activated sludge would be a
possible application for a Bird Continuous Solid Bowl Centrifuge. However,
the restriction on the use of flocculants limits ability to obtain good
recovery of feed solids as cake product.
Samples of feed, cake and effluent from each run were taken by the witness
for analysis.
160
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TABLE B-l. SOLID BOWL CENTRIFUGAL TEST DATA FOR BROCKTON WAS AT pH 1.94
SOLID BOWL CENTRIFUGAL TEST DATA
Customer:
Run no.
Feed: % solids
Sp. gr.
GPM
PPH solids in feed
Temp., °C
Cake: % solids
PPH wet
PPH dry
Effluent: % solids
Percent recovery
Machine RPM
Force x gravity
Materi al
Date:
1
3.64
1.0
0.73
13.29
95
36.25
2.0
0.72
2.84
23.84
3500
1000
.
2
3.
1.
P.
12.
95
17.
15.
2.
2.
25.
Abcor - Wilmington, MA
W.A.S.
4/20/78
3
64 3.64
0 1.0
70 0.695
75 12.65
95
89 25.81
0 9.33
68 2.4
85 2.47
81 35.54
6000
3000
4
3.64
1.0
0.559
10.18
95
26.88
9.33
2.5
3.0.9
17.07
161
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-80-096
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Evaluation of Hot Acid Treatment for
Municipal Sludge Conditioning
5. REPORT DATE
August 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Kenneth J. McNulty, Ann T. Malarkey
Robert L. Goldsmith, and Henry A. Fremont
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Wai den Division of Abcor,
850 Main Street
Wilmington, MA 01887
Inc.
10. PROGRAM ELEMENT NO.
C36B1C Decision Unit B-121
11. CONTRACT/GRANT NO.
68-03-2459
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory Cin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Roland V. Villiers
(513) 684-7664
16. ABSTRACT ~~ ~—
Bench-scale tests were conducted to evaluate the technical and economic
feasibility of the hot acid process for stabilization/conditioning of municipal
sewage sludge. This process involves acidification of the sludge (pH 1.5-3) and
heating to temperatures below boiling (^95°C). Test results indicate that the
process improves the dewaterability of the sludge, destroys essentially all
pathogens, and preferentially solubilizes certain heavy metals relative to nitrogen
and organics. The process demonstrated the potential for good solubilization and
removal of toxic heavy metals including cadmium, zinc, and nickel with minimal
solubilization of nitrogen. Thus the hot acid process improves the desirability of
sludge solids for land application. A preliminary economic analysis of the process
indicates that it is quite cost-competitive with alternative stabilization/
conditioning processes.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I field/Group
Municipal Sewage Sludge
Sludge Conditioning
Heavy Metals Removal
Hot Acid Treatment
Metals Solubilization
Solid-Liquid Separation
Stabilization
17B
8. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
174
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
162
U.S. GOVERNMENT PRINTING OFFICE: 1980--657-165/0105
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