MMiU WATER POLLUTION CONTROL RESEARCH SERIES • H010 EVE 01/71
Evaluation of Conditioning
and Dewatering Sewage
Sludge by Freezing
ENVIRONMENTAL PROTECTION AGENCY • RESEARCH AND MONITORING
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Series describes
the results and progress in the control and abatement
of pollution in our Nation's waters. They provide a
central source of information on the research, develop-
ment, and demonstration activities in the Environmental
Protection Agency, through inhouse research and grants
and contracts with Federal, State, and local agencies,
research institutions, and industrial organizations.
Inquiries pertaining to Water Pollution Control Research
Reports should be directed to the Head, Publications
Branch, Research Information Division, Research and
Monitoring, Environmental Protection Agency, Washington,
D. C. 20^4-60.
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EVALUATION OF CONDITIONING
AND DEWATERING SEWAGE SLUDGE BY FREEZING
Sewerage Commission of the City of Milwaukee
Milwaukee, Wisconsin 53201
for the
ENVIRONMENTAL PROTECTION AGENCY
Program #11010 EVE
Grant #WPRD 71-01-6J
January, 1971
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EPA Review Notice
This report has been reviewed by the Water
Quality Office, EPA, and approved for publication.
Approval does not signify that the contents
necessarily reflect the views and policies of
the Environmental Protection Agency, nor does
mention of trade names or commercial products
constitute endorsement or recommendation for
use.
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ABSTRACT
It is evident that research into new sludge handling processes has
great potential for increasing the efficiency and decreasing the cost
of sludge handling and disposal. Of special interest, is the investi-
gation of methods which significantly reduce secondary treatment sludge
volumes for subsequent treatment and disposal. This project was initiated
to investigate, develop, and evaluate a prototype system for the condi-
tioning and dewatering of waste activated sludge by freezing. The
activated sludge used was that from the treatment plant of the Sewerage
Commission of the City of Milwaukee, Milwaukee, Wisconsin.
The project work scope was divided into two phases: (1) the laboratory
investigation phase and (2) the engineering design and evaluation phase.
The purpose of the laboratory investigation phase was to obtain the basic
process and design parameters for a freeze conditioning system. Emerging
from the laboratory investigation and carried forward to the engineering
design phase was a freeze-conditioning system consisting of the following
process elements: (1) Flotation thickening prior to freezing, (2) Freezing
in thin sheets, (3) Thawing of the frozen product, and (4) Dewatering of
the thawed sludge. The objective of the engineering design phase was the
development of a freeze-conditioning system to handle three tons dry solids
per day of activated sludge.
It was concluded that the freeze conditioning concept, from a technical
standpoint, has definite merit as a sludge conditioning process. However,
an evaluation of the freeze conditioning process and comparison with the
existing chemical conditioning method presently employed showed that the
equipment capital costs, operating costs, and space requirements were
appreciably greater for the freeze conditioning process than for the
existing chemical conditioning method. Attempts to reduce freeze-condition-
ing operating costs by reducing the refrigeration work required were
found to be technically feasible, but would result in appreciably increas-
ing the equipment capital costs above that for the freeze conditioning
system presently envisioned, thereby aggravating an unsatisfactory situation.
It was recommended that the objectives of the project be redirected to the
investigation of alternate sludge conditioning and sludge dewatering means.
This report was submitted in fulfillment of Grant No. WPRD 71-01-68.,
Program No. 11010 EVE, between the Federal Water Quality Administration
and the Sewerage Commission of the City of Milwaukee, Wisconsin.
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CONTENTS
Section Page
I Conclusions and Recommendations 1
II Introduction 3
III Grant Objectives 7
IV Work Scope and Plan of Operation 9
V Discussion of Results 11
Determination of Process and Design Parameters
Engineering Design and Evaluation Study
VI Acknowledgements 45
VII References 47
VIII Appendices 49
Appendix A - Belt Freezing System Design
Appendix B - Thawing System Design
Appendix C - Dewatering System Design
Appendix D - Chemical Treatment System Design
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FIGURES
Number Page
1 Effect of Ferric Chloride on Cake Solids 30
2 Schematic Diagram of Three Ton Dry Solids 33
per Day Freeze-Conditioning System
3 Two Methods Investigated for Freezing 3^
Thickened Sludge
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TABLES
Number Page
1 Effect of Initial Sludge Solids Concentration and 13
Screen Solids Loading on the Dewatering Characteristics
of Frozen-Thawed Activated Sludge
2 Effect of Sludge Storage Time Before Freezing on 18
the Dewatering Properties of the Thawed Sludge
3 Effect of Thawed Sludge Storage Time on the Sludge 20
Dewatering Properties
4 Effect of Storage Time and the Frozen State on the 21
Dewatering Properties of the Thawed Sludge
5 Effect of Freezing Time on the Dewatering Properties 23
of the Thawed Sludge
6 Effect of the Addition of Ferric Chloride Before Freezing 26
on the Sludge Dewatering Properties
7 Effect of the Addition of Ferric Chloride After Freezing 27
and Thawing on the Sludge Dewatering Properties
8 Summary of the Equipment Costs and Floor Space 40
Requirements for the Three Ton Solids per Day
Freeze-Conditioning Demonstration Plant
9 Comparison of Equipment Costs and Floor Space 42
Requirements for the Freeze-Conditioning and
Conventional Conditioning Methods
10 Comparison of Operating Costs for Ferric Chloride and 44
Freeze-Conditioning of Milwaukee Waste Activated
Sludge (238 Ton per Day Basis)
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SECTION I
CONCLUSIONS AND RECOMMENDATIONS
The following conclusions were drawn from the work done on the
subject grant project:
1. The freeze-conditioning concept, from the standpoint
of technical process efficiency, has definite merit
as a means of conditioning waste activated sludge
for subsequent dewatering.
2. Engineering evaluation of the freeze-conditioning
process and comparison with the conventional chemical
(FeClo) conditioning method presently employed showed
that the equipment capital costs, operating costs and
space requirements were appreciably higher for the
freeze method than for the chemical method.
3. Attempts to reduce freeze-conditioning operating costs
by reducing the refrigeration work required were found
to be technically feasible, but would result in appre-
ciably increasing the equipment capital costs and space
requirements above those for the freeze system presently
envisioned, thereby aggravating an already unsatisfactory
economic situation.
On the basis of these conclusions, it was recommended that the
objectives of the project be redirected to the investigation of
alternate sludge conditioning and dewatering means.
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SECTION II
INTRODUCTION
Residual sludges are by-products arising from the treatment of
wastewaters and contain, in more concentrated form, the objection-
able pollutants removed in the wastewater treatment process.
Although the volume of residual sludges obtained is relatively
small, usually 2% to 3% of the wastewater volume treated, sludge
handling and disposal is complex, troublesome, and represents
up to 25% to 50% of the capital and operating costs of a waste
treatment plant (1). Moreover, the problem is growing. With the
expansion of the economy and the population and with the greater
degree of treatment required, it is expected, within the next 10
to 15 years, that the volume of sludge requiring handling and dis-
posal will increase by 60% to 70% (2). By far, the major portion
of the increased sludge volume expected will be obtained from
secondary treatment sludges, which are less concentrated than
primary sludges, and which are most difficult and expensive to
treat. For example, in 1980 it is anticipated that 140 million
gallons per day of secondary sludge (2% solids) will be produced,
whereas only about 10 million gallons per day of primary sludge
(6% solids) are expected in that year (2).
Sludge handling for ultimate disposal consists of a series of
dewatering steps in which the volume of sludge is progressively
reduced by removal of the water associated with the sludge solids.
The process steps usually associated with sludge handling and
disposal include the following:
1. Thickening
a. Sedimentation
b. Flotation
2. Digestion
a. Aerobic
b. Anaerobic
3. Dewatering
a. Sand beds
b. Lagoons
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c. Vacuum Filtration
d. Centrifugation
4. Ultimate Disposal
a. Landfill
b. Discharge to sea
c. Drying for soil conditioning
d. Oxidation
(1) Incineration
(2) Wet oxidation
(3) Atomized suspension technique
For a particular location, the combination of the sludge handling
and disposal steps to be used are integrated in such a manner as
to arrive at an optimum economical solution. Wherever possible,
advantage is taken of new developments by incorporating them into
the process scheme.
It is apparent from the above brief discussion that research into
sludge handling has potential value for increasing process effi-
ciency and decreasing the cost of the total waste treatment process.
It is further evident that the greatest benefit which may be derived
from sludge handling research lies in the investigation of means
for efficiently and economically handling secondary treatment
sludge volumes for subsequent treatment and disposal.
Recently, attention and emphasis has been placed upon physical
methods for conditioning sludges to enhance dewatering and disposal
without the aid of chemicals (3). Freezing of sludge and subsequent
thawing is a physical method which shows promise as a sludge
conditioning means (4). Freezing of activated sludge causes the
suspended solids to agglomerate and form relatively large floe
particles. Upon thawing, the separation and compaction properties
of the solids suspension are significantly improved over those
observed before freezing. Furthermore, dewatering of sludge is
appreciably enhanced by the freeze conditioning process. For
example, dewatering of freeze conditioned activated sludge using
gravity drain on screens varying in size from 24 to 40 U.S. mesh
has been reported (5). The mass loadings used during the dewatering
of the freeze conditioned activated sludge was on the order of 60
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pounds per hour of dry solids per square foot as compared to 1 to
5 pounds per hour per square foot for conventional vacuum filtration.
The applicability of utilizing the freezing process for conditioning
waste activated sludge was demonstrated to the City of Milwaukee
by the Technical Center of Rex-Chainbelt Inc. and the results of
the studies performed (5) served as a basis for the grant obtained
(WPRD 71-01-68) to conduct a program for demonstrating, on a
relatively large scale, this dewatering technique. The grant was
obtained by the Milwaukee Sewerage Commission, Milwaukee, Wisconsin
from the Federal Water Pollution Control Administration. The work
performed under this grant was done by the Technical Center, Rex
Chainbelt Inc., Milwaukee, Wisconsin under contract to the Milwaukee
Sewerage Commission.
The waste activated sludge handling and disposal facilities of the
Milwaukee Sewerage Commission's Water Pollution Control Plant
include (1) gravity thickening, (2) chemical conditioning, (3)
vacuum filtration, and (4) heat drying. The dried sludge is market-
ed as the fertilizer, Milorganite. The Commission uses ferric
chloride to condition its waste activated sludge, and the annual
chemical cost is about $750,000, based on a dose level of about
250 Ib FeCl3 per dry ton of solids.
The purpose of the investigation conducted under this grant was
to evaluate the freeze conditioning and dewatering technique on a
scale large enough to obtain meaningful data for direct comparison
with the conditioning and dewatering methods presently employed
at Milwaukee.
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SECTION III
GRANT OBJECTIVES
The objectives of the project conducted under this grant were as
follows:
1. To evaluate the effect of freeze conditioning of waste
activated sludge at the Milwaukee Sewerage Commission
Water Pollution Control Plant on solids dewatering
characteristics. It was felt that the freeze con-
ditioning technique, which beneficially alters
the characteristics of the sludge, offers a new and
promising method for solids dewatering.
2. To reduce or eliminate the need for chemical sludge
conditioners in the vacuum filtration dewatering process.
At the present time, the Milwaukee Sewerage Commission
spends about $750,000 each year for chemicals to condi-
tion sludge prior to vacuum filtration. If, through the
freeze conditioning technique, the need for chemicals
could be eliminated or reduced, operational costs for
sludge dewatering could be reduced.
3. To Investigate and evaluate new techniques for sludge
dewatering through the design and demonstration of a
continuous gravity screen filter. In addition, modifica-
tions of a conventional vacuum filter would be made to
evaluate the feasibility of using conventional dewatering
equipment for handling freeze conditioned sludge. For
example, at Milwaukee, modifications of existing conven-
tional vacuum filters to handle the anticipated increased
solids dewatering loadings from freeze conditioning would
minimize the future need for additional conventional
vacuum filtration equipment.
4. To study and evaluate the physical and chemical changes
that occur as a result of freeze conditioning and de-
watering and to relate these changes to the fertilizer
properties and characteristics of the dried sludge.
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SECTION IV
WORK SCOPE AND PLAN OF OPERATION
It was originally proposed that the objectives of the project be
met through performance of a work scope encompassing two phases
over a period of two years. The following discussion of the work
scope and plan of operation identifies each of these phases,
outlines the scope of each one, and defines their interrelationships
with respect to the proposed total effort. The primary thrust
of this investigation was to be directed to the development and
evaluation of a three ton dry solids per day demonstration system
for conditioning and dewatering waste activated sludge by freezing.
Phase I - Design, Fabrication and Evaluation of Batch Operation
Investigations of freezing rates and techniques and their related
effect on the dewatering process were to be conducted. From the
work performed by the Technical Center of Rex Chainbelt Inc. (5)(6) ,
it was seen that freezing rates and techniques play an important
role in the floe characteristics of the freeze conditioning process.
Optimum freezing rates were to be determined to provide maximum
utilization of the freezing system, consistent with favorable
sludge conditioning. Moreover, methods of freezing were to be
evaluated to provide the most effective heat transfer characteristics
for a continuous system. The results of these investigations
will have a significant bearing upon the design of the freezing
and dewatering equipment to be used.
Following design, construction and installation of the system at
the Milwaukee Jones Island plant, evaluation of batch freezing
for sludge conditioning and dewatering of the conditioned sludge
on a continuous prototype gravity filter was to be made. Operation
of the freeze conditioning and dewatering system side by side
with a conventional chemical conditioning and vacuum filter
dewatering system would provide a means for evaluating the effect-
iveness of the freeze conditioning and dewatering technique.
Operation of the freeze process with and without chemical aids
would provide information as to the need for chemicals and to
determine the interaction of variables which may occur as a result
of chemical conditioners. The data obtained during operation would
allow comparison of the two dewatering techniques from the stand-
point of mass loadings, hydraulic loadings, cake and filtrate
solids contents, and the physical and chemical properties of the
filter cakes.
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After completion of the above evaluations, modifications to the
conventional vacuum filter were to be made to allow the handling
of freeze conditioned sludge on the vacuum filter. The modifica-
tions would include the design and fabrication of a feed system for
depositing of sludge on the vacuum drum and substitution of a
screen filter media in place of the conventional filter cloth.
These modifications would be required because of the different
floe characteristics of freeze conditioned sludge which would
require a different feed system and filter media for maximum
utilization of this conditioned sludge. Dewatering evaluations
would be made on the modified vacuum filter to illustrate the
potential increased operational efficiencies using freeze conditioned
sludge. The data taken during this evaluation would allow direct
comparison of conventional vacuum dewatering using chemical condi-
tioners with that of vacuum dewatering of freeze conditioned
sludge. Important information such as mass loadings, water through-
put rates, solids concentration in the filtrate and filter cake
and physical and chemical properties of the cake solids would
then be available for direct comparison of the two dewatering
techniques.
Phase II - Semi-Continuous System Operation and Evaluation
Using the results obtained in operation and evaluation of Phase I,
a semi-continuous freezing and a continuous filtration using
energy recovery methods with the prototype system were to be investi-
gated. To optimize the freezing costs for sludge conditioning,
careful recovery of the heat removed during freezing is required.
The semi-continuous freezing and thawing system would use the heat
removed during the freezing in one tank to thaw the frozen sludge
in a second tank.
Evaluation of the dewatering process, using the sludge conditioned
by the semi-continueus method outlined above, were to be demonstrat-
ed on the prototype gravity screen filter and the modified vacuum
filter as described previously. Operational data obtained during
these evaluations would enable comparison of the two dewatering
techniques using freeze conditioned sludge as well as the evalua-
tion of the semi-continuous freezing and dewatering system.
Sufficient information which would allow the economic evaluation
of the freeze conditioning process was also intended to be gathered
during this phase.
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SECTION V
DISCUSSION OF RESULTS
Actually, the above outlined plan of operation was not fully imple-
mented. Only the laboratory investigations and a portion of the
prototype engineering evaluation task in Phase I were completed.
During the engineering design phase of the project, it became
increasingly evident that the equipment capital costs and space
requirements for the freeze conditioning process were appreciably
greater than for the conventional chemical conditioning process.
Further work on the project was suspended pending a thorough review
of the process, engineering design, and economic aspects of the
freeze conditioning project. This review substantiated the above
conclusion. It was recommended that investigation of freeze
conditioning be terminated and that the project objectives be
redirected to the investigation of alternative sludge conditioning
and dewatering means. This section of the report describes the
work performed and results obtained prior to termination of the
freeze conditioning investigation and redirection of the project
objectives.
The work performed during the completed portion of the freeze
conditioning investigation included (1) Determination of the
process and design parameters and the (2) Engineering design and
evaluation study.
Determination of the Process and Design Parameters
A literature search was made in the specific area of sludge
conditioning by freezing methods (3)(4)(5)(6)(7)(8)(9)(10).
The search, in conjunction with laboratory studies, was made to
obtain the basic process and design parameters for prototype
component design.
The literature search showed that the freeze conditioning method
was applicable for various sewage plant sludges, such as, primary,
activated and digested sludges. It was found that to obtain a
significant conditioning action by the freezing method, the sludge
must be completely frozen at relatively slow rates. Slush freez-
ing, for example, produced little or no conditioning action.
Addition of flocculating chemicals prior to freezing enhanced
the dewatering rates after thawing.
Although the results obtained by the freeze-conditioning method
were encouraging, development of the process was limited because
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of (1) high cost and consumption of power, (2) high capital and
operating costs when using conventional vacuum filter dewatering
following freeze-conditioning, and (3) the need for frequent
media washing when vacuum filtration was used following freeze-con-
ditioning. Moreover, it was felt that the development of the freeze-
conditioning method suffered from the necessity for batch operation.
It was felt that the disadvantages may be overcome by utilizing
gravity drain dewatering of freeze-conditioned sludge through
relatively open-mesh media and by utilizing recent advancements in
refrigeration design to produce a continuous freeze-conditioning
operation. It appeared, therefore, that the successful applica-
tion of the process was dependent upon pulling together an inte-
grated engineering system which would be both economically
feasible and practical.
Laboratory bench scale studies were conducted concurrently and
in conjunction with the literature search. All tests were per-
formed utilizing activated sludge from the Jones Island facilities
of the City of Milwaukee Sewerage Commission.
Effect of Thickening
Activated sludge wasted from the Milwaukee Jones Island Treatment
Plant is currently thickened by gravity to a concentration of
about 1.5% to 2.0%. This waste activated sludge can be readily
thickened to 4% solids or higher using dissolved-air flotation
thickening at a solids loading of 10 lb/(day)(sq. ft.). It was
necessary to determine whether the use of sludge with higher solids
content would interfere in some unexpected manner with the observed
improvement in dewaterability brought about by freezing. It was
also felt that such prior thickening (from 1.5% to 4%+) would
reduce the size and capital/operating costs of equipment required
in subsequent freezing, thawing, and dewatering operations.
A series of tests was performed to determine the effect of initial
sludge solids concentration on the dewatering characteristics of
freeze-conditioned activated sludge. Return activated sludge
samples having initial total solids concentrations of 1.1%, 2.1%,
3.3% and 5.3% were frozen overnight in 500 ml portions in a commer-
cial chest type freezer. These sludges were then filtered through
an 80 mesh screen after thawing at screen loadings of 0.25, 0.5
and 0.75 Ib/sq ft. Finally, after filtration was completed, the
filter cakes were mechanically pressed to remove additional free
water. The results of the tests performed appear in Table 1.
The following conclusions were drawn from an analysis of the data
in Table 1.
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TABLE 1
EFFECT OF INITIAL SLUDGE SOLIDS CONCENTRATION AND
SCREEN SOLIDS LOADING ON THE DEWATERING CHARACTERISTICS
OF FROZEN - THAWED ACTIVATED SLUDGE
Initial
Sludge Dewatering Property
Solids
(%)
1.1 Filter Cake Solids (%)
Filter Cake Solids (gm)
Screen Solids Loading (Ib/sq ft)
Filtrate Suspended Solids (mg/1)
Filtrate Suspended Solids
Drain Time (Sec)
Drain Rate (ml/sec)
2.1 Filter Cake Solids (%)
Filter Cake Solids (gm)
Filtrate Suspended Solids
Filtrate Suspended Solids
Drain Time (sec)
Drain Rate (ml/sec)
3.3 Filter Cake Solids (%)
Filter Cake Solids (gm)
Filtrate Suspended Solids
Filtrate Suspended Solids
Drain Time (sec)
Drain Rate (ml/sec)
5.3 Filter Cake Solids (%)
Filter Cake Solids (gm)
(gm)
(mg/1)
(gm)
(mg/1)
(gm)
Filter Suspended Solids (mg/1)
Filtrate Suspended Solids
Drain Time (sec)
Drain Rate (ml/sec)
(gm)
0.25
17.2
6.02
176
0.10
12
44
17.3
6.75
377
0.10
12
21
16.9
5.41
607
0.10
10
14
18.4
4.23
1190
0.12
6
12
0.50
16.1
13.7
127
0.14
75
14
17.7
15.9
348
0.18
40
12
18.9
10.4
437
0.15
17
21
17.8
13.5
1175
0.20
17
8
0.75
14.2
21.3
98
0.16
780
1.7
18.1
24.1
198
0.16
35
22
17.4
20.5
343
0.16
25
18
21.9
19.5
560
0.16
15
17
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1. Freeze-conditioning sludges having higher initial solids
concentrations did not adversely affect the dewatering
properties such as filter cake solids concentration,
filtrate suspended solids, and filtrate drain rate.
Although, for a given screen solids loading, the fil-
trate suspended solids concentration increased with
increasing initial sludge solids concentration, the
weight of filtrate suspended solids was relatively
constant.
2. There was no apparent correlation between the above-
mentioned dewatering properties and screen solids
loadings.
3. Thickening of the plant sludge prior to freezing is
feasible and will result in a reduction in the size of
the refrigeration and dewatering equipment required as
well as in a reduction in the capital and operating
costs of refrigeration.
Therefore, flotation thickening of the feed sludge from the plant
was incorporated as a process element in the freeze-conditioning
system to be designed.
Freezing
Bench scale and field tests were performed to investigate the
effect of the following freezing parameters on the dewatering
properties of the thawed sludge.
1. Freezing Method
The use of direct and indirect refrigeration means for
freezing activated sludge were investigated. Direct
refrigeration means (in which the refrigerant is in
direct contact with the sludge), if successful, would
have the advantage of eliminating the costly heat
exchange surfaces associated with indirect refrigeration
means. However, direct refrigeration means using such
refrigerants as liquid nitrogen, methyl chloride,
butane, isobutane, and freon were unsuccessful in that
difficulties were encountered in freezing the sludge
completely or the dewatering properties of the thawed
sludge obtained were unsatisfactory. For example,
a. Liquid Nitrogen
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Two portions of waste activated sludge were rapidly
frozen (within 20 seconds) using a liquid nitrogen
bath. One portion was thawed immediately for de-
watering tests on an 80 mesh screen. It did not
dewater. The other frozen portion was stored in
a 0° F freezer for 24 hours before thawing. This
thawed portion also did not dewater on the 80 mesh
screen. The appearance of both portions of freeze-
conditioned sludges was identical to that of the
unfrozen sludges.
b. Methyl Chloride (Boiling point, -11° F; liquid
specific gravity, 0.998)
A hard porous ice structure having the appearance
of a sponge formed at the interface between the sludge
and the methyl chloride. Below the porous ice,
which was one inch to 1.5 inches in depth, the sludge
froze in a solid mass from the top down. When
thawed, the porous ice structure did not dewater
on the 80 mesh screen. The sludge below the porous
ice dewatered poorly.
c. Freon 114 (boiling point 38.4° F; liquid specific
gravity, 1.57)
The boiling point of Freon 114 was lowered to about
29° F by applying a vacuum of about 6.5 inches of
mercury. Because Freon was heavier than the sludge,
the Freon vapor passed through the sludge during
the freezing process. This resulted in a porous
ice structure which did not dewater well after
thawing.
d. Butane (boiling point, 31° F; liquid specific
gravity, 0.58)
Tests using butane were performed at reduced pressure
to lower the boiling point. The sludge and butane
were not mixed during the freezing process. Freez-
ing took place from the top down, forming solid ice.
As a result, great pressure was exerted by the
advancing ice on the unfrozen sludge at the bottom.
Many containers were ruptured before freezing was
complete. Containers used in the methyl chloride
tests broke in a similar fashion. Tests which were
successful in completely freezing the sludge produced
sludge which either did not dewater or dewatered
poorly when thawed.
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Freezing using direct refrigeration means was abandoned
as the dewatering properties of the thawed sludge obtained
were unsatisfactory.
Indirect refrigeration means (conventional vapor compres-
sion refrigeration cycles in which the refrigerant does
not come into direct contact with the sludge) were
successful in producing a frozen product, which when
thawed, exhibited the desired dewatering properties.
In the discussion which follows, freezing of the sludge
was performed using indirect refrigeration means.
2. Effect of Sludge Storage Time before Freezing on Dewatering
Properties
The effect of sludge storage time before freezing on
the dewatering properties of the thawed sludge was evaluat-
ed using the data shown in Table 2. Sludge storage
times of 0, 11, and 24 hours before freezing were investi-
gated. The data shown in Table 2 were arranged into two
groups, Tables 2A and 2B, in accordance with the number
of days the sludge remained in the frozen state. (This
variable significantly affects the thawed sludge dewater-
ing properties and will be discussed separately, later.)
The sludges for Table 2A remained in the frozen state
for 4 days before thawing; those in Table 2B, 11 days.
Dewatering tests were performed immediately after thawing.
It is evident from Tables 2A and 2B that cake solids
concentration is adversely affected by storing the sludge
before freezing. Moreover, the data show that drain time,
drain rate, and filtrate suspended solids are not affect-
ed by sludge storage up to eleven hours, but may be detri-
mentally affected by storage of the liquid sludge for 24
hours before freezing.
Filtrate soluble phosphates and soluble COD increase
with increased storage time before freezing. Although
the data is not presented, most of this increase was
observed to have occurred before freezing took place.
It was indicated from the results of these tests, there-
fore, that continuous feeding of flotation thickened
sludge to the freezing operation would be desirable.
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TABLE 2
EFFECT OF SLUDGE STORAGE TIME BEFORE FREEZING
ON THE DEWATERING PROPERTIES OF THE THAWED SLUDGE
TABLE 2A
Age before freezing (hr) 0 11 24
Days frozen 444
Volume filtered (ml)* 475 490 495
Screen loading (Ib/sq ft) 0.42 0.42 0.42
Drain time (sec) 19 18 35
Drain rate (ml/sec) 20 22 11
Cake solids (After pressing) (%) 18.3 17.5 14.9
Filtrate suspended solids (mg/1) 460 318 368
Filtrate soluble P04 (mg/1) 180 212 204
Filtrate COD (mg/1) 2644 3505 2971
TABLE 2B
Age before freezing (hr) 0 11 24
Days frozen 11 11 11
Volume filtered (ml)* 505 490 490
Screen loading (Ib/sq ft) 0.42 0.42 0.42
Drain time (sec) 15 20 23
Drain rate (ml/sec) 27 20 17
Cake solids (After pressing) (%) 19.4 18.9 17.8
Filtrate suspended solids (mg/1) 200 200 260
Filtrate soluble P04 (mg/1) — 162 182
Filtrate COD (mg/1) — 3970 3833
* 80 mesh screen media used for gravity filtration.
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3. Effect of Thawed Sludge Storage Time on Dewatering Properties
The effect of thawed sludge storage time on the sludge
dewatering properties was evaluated using the data shown
in Table 3. Thawed sludge storage times of 0, 8, and
24 hours were investigated.
The data in Table 3 show that cake solids concentration
decreased with increasing storage time after thawing.
In addition, filtrate soluble phosphates and soluble
COD increased with increased age after thawing. However,
filtrate suspended solids and drain rate appeared to
be unaffected by thawed sludge storage up to 24 hours.
It is indicated, therefore, that it would be advantageous
in full-scale operation to dewater the sludge as quickly
as practicable after thawing.
4. Effect of Storage Time in the Frozen State on the Dewatering
Properties
The effect of storage time in the frozen state on the
sludge dewatering properties was evaluated using the
data presented in Table 4. Data for storage times up
to 28 hours are shown in Table 4. The sludge was frozen
in 1/4" thick sheets (Table 4A) and in relatively large
pint blocks (Table 4B).
The data in Tables 4A and 4B show that cake solids con-
tents and drain rates are significantly benefited by
frozen state storage times up to 16 hours.
It was indicated, therefore, that storage of sludge in
the frozen state enhanced the dewatering properties of
the thawed sludge. However, although frozen sludge
storage is desirable from a process standpoint, it was
felt to be impractical on a full scale basis (for example,
at Milwaukee, about three million gallons of waste
activated sludge are processed daily).
5. Effect of Freezing Time on the Dewatering Properties
Times for complete freezing were varied from a matter of
seconds to 16 hours. Fast freezing, such as with liquid
nitrogen, produced no improvement in the dewatering
properties after thawing.
-18-
-------
TABLE 3
EFFECT OF THAWED SLUDGE STORAGE TIME ON THE
SLUDGE DEWATERING PROPERTIES
Time after thawing (hr) 0 8 8 24 24
Days frozen 44646
Volume filtered (ml)* 475 485 482 485 475
Screen loading (Ib/sq ft) 0.40 0.41 0.41 0.41 0.41
Drain time (sec) 19 22 21 26 23
Drain rate (ml/sec) 20 18 18 15 17
Calse solids (After pressing) (%) 18.3 — 17.4 — 16.7
Filtrate suspended solids (mg/1) 460 381 185 518 445
Filtrate soluble PO/ (mg/1) 180 212 210 288 246
Filtrate COD (mg/1) 2644 3223 3452 4029 3884
* 80 mesh screen media used for gravity filtration.
-19-
-------
TABLE 4
EFFECT OF STORAGE TIME IN THE FROZEN STATE ON
THE DEWATERING PROPERTIES OF THE THAWED SLUDGE
A. Freezing in trays (V thick layers)
Time to freeze (hr) 2 1.5 3 3
Storage after freezing (hr) 0 4 16 24
Feed sludge solids (%) 2222
Volume filtered (ml)* 474 491 479 482
Screen loading (lb/sq ft) 0.33 0.35 0.33 0.34
Drain time (sec) — 43 36 29
Drain rate (ml/sec) — 8 10 12
Cake solids (After pressing) (%) 10.8 12.2 14.4 14.3
B. Freezing in pint boxes (3V x 3V x 2V)
Time to freeze (hr) 7 8 8.5
Storage after freezing (hr) 4 16 28
Feed sludge solids (%) 222
Volume filtered (ml)* 495 497 478
Screen loading (lb/sq ft) 0.35 0.35 0.33
Drain time (sec) 48 24 24
Drain rate (ml/sec) 6 16 17
Cake solids (After pressing) (%) 13.5 15.2 15.5
* 80 mesh screen media used in gravity filtration.
-20-
-------
The effect of freezing time on the thawed sludge dewatar-
ing properties was evaluated from data such as that
presented in Table 5. The data shown were generated
from tests performed by freezing sludge in pint blocks.
The data in Table 5 show that, from the standpoint of
drain time, drain rate, and cake solids, a freezing
time of ten hours was optimum for a storage period
after freezing of one day.
In general, it was indicated that the dewatering pro-
perties of the thawed sludge improved as the freezing
time increased. From a practical standpoint, however,
the size of the freezer, and, therefore, the volume
requirements and capital costs of the freezing equipment
increase with increasing freezing time. In this regard,
a sacrifice in dewatering quality may have to be con-
sidered in order, to minimize equipment capital costs.
6. Partial Freezing
Bench tests and field tests showed that, in order to be
effective, the freezing of the sludge must be complete.
Any small volume of unfrozen sludge (a few milliliters
in 500 ml, for example) was sufficient to cause a marked
adverse effect on the dewatering properties of the thawed
sludge. These observations appeared to eliminate the
use of slush ice machines, which are readily available
on the market.
Moreover, partially freezing the sludge and discarding
the unfrozen portion prior to thawing was not effective.
The partially frozen portion of the sludge dewatered
poorly, if at all, upon thawing. This eliminated those
commercial machines capable of producing frozen sludge
in this manner.
7. Mobility of Sludge During Freezing
A field trip was made to the Turbo Refrigerating Company
in Denton, Texas for the purpose of field testing their
refrigeration equipment in the freezing of activated
sludge. The operating principle of the Turbo machine
consists of flowing a sheet of the liquid to be frozen
down a vertical cold plate. Waste activated sludge from
the nearby Denton, Texas Water Pollution Control Plant
was used in the field test. The results of the field
test, both visual and from analysis of samples obtained,
-21-
-------
TABLE 5
EFFECT OF FREEZING TIME ON THE DEWATERING
PROPERTIES OF THE THAWED SLUDGE
Time to freeze (hr) 3 4 9 10 11 16
Storage after freezing (hr) 24 24 24 24 24 24
i Feed sludge solids (%) 2.46 2.46 2.46 2.23 2.23 2.46
K Volume filtered (ml)* 495 492 493 502 440 502
1 Screen loading (Ib/sq ft) 0.43 0.43 0.43 0.40 0.35 0.44
Drain time (sec) 300 57 28 20 25 33
Drain rate (ml/sec) 1 6 13 20 14 11
Cake solids (After pressing) (%) 11.7 12.1 13.9 14.1 14.1 12.6
* 80 mesh media used in gravity filtration.
-------
showed that essentially only the water associated with
the feed sludge was frozen on the vertical cold plates.
The unfrozen sludge flow from the plates, of course,
was more concentrated (2% solids) than the feed sludge
(1% solids).
The results of the field test conducted indicated that
there should be no relative motion between the sludge to
be frozen and the freezing medium. It appeared, there-
fore, that ice machines operating on the Turbo principle
(liquid sludge flowing over a stationary cold plate) or
ice machines in which the freezing medium travels through
the liquid sludge (such as a cold drum rotating in a
vat of liquid sludge) would not be applicable for freeze-
conditioning of sludges.
8. Chemical Additives
Chemical additives were investigated to determine their
effect as adjuncts to the freeze-conditioning method.
Ferric chloride was the additive found to give the most
pronounced beneficial effect in this study. The effect
of ferric chloride was investigated when added to the
sludge before freezing and when added to the thawed sludge
after freezing. The data obtained from the tests per-
formed are presented in Tables 6 and 7.
In Table 6 are shown the results obtained from the
addition of about 28 and 55 Ib/ton of ferric chloride
before freezing. Note the appreciable decrease is soluble
phosphate before freezing caused by the addition of ferric
chloride. This was expected. After freezing, thawing
and gravity filtration, it is evident that from Table 6 the
filtrate soluble phosphate contents obtained were consistently
and appreciably greater than the corresponding soluble
phosphate contents before freezing. It is speculated
that the increase observed was a result of cell rupture
during the freezing process and subsequent release of
phosphorus into solution. However, the filtrate soluble
phosphate contents of the chemically dosed samples were
appreciably less than that for the control sample (zero
ferric chloride).
Referring to Table 6, the addition of the amounts of
ferric chloride shown improved the screened cake concen-
tration, drain rate, and filtrate suspended solids.
Moreover, the soluble COD in the filtrate was reduced
as a result of chemical addition.
-23-
-------
In Table 7 are shown the results of the tests performed from
the addition of about 27 and 54 Ib/ton of ferric chloride
after freezing and thawing and before gravity filtration.
Note that the filtrate soluble phosphate contents for
the control (zero ferric chloride) and 27.1 Ib/ton ferric
chloride samples were greater than the corresponding
soluble phosphate contents before freezing. However,
the ferric chloride dosage of 54.2 Ib/ton was sufficient
to appreciably reduce the filtrate soluble phosphate
content (90 mg/1) from that observed in sludge prior to
freezing (140 mg/1). Again, the filtrate soluble
phosphate content in the chemically treated samples
were lower than that observed in the control sample.
Referring to Table 7, the addition of about 27 and 54
Ib/ton ferric chloride after thawing adversely affected
the drain rate, drain time and filtrate suspended
solids. The cake solids concentration was not improved,
but lower filtrate soluble COD's were obtained as a
result of the chemical treatment.
In summary, ferric chloride was found to beneficially
affect the sludge dewatering properties when introduced
into the sludge before freezing. Introducing ferric
chloride into the sludge after thawing adversely
affected the dewatering properties of the thawed sludge.
Shape of Configuration of the Sludge to be Frozen
Shapes and configurations investigated included relatively
large blocks, small cubes, and relatively thin sheets.
Bench scale and field tests were performed to evaluate
the effect of these shapes.
Most of the data presented thus far was obtained from
tests performed by freezing the sludge in relatively
large blocks. In general, freezing in large blocks or
cubes in inherently slow and was felt to adversely
affect production rate. For example, as the freezing of
blocks progresses, the area for heat transfer and the
overall heat transfer coefficient decrease, resulting
in a progressive decrease in the freezing rate. Further-
more, the rate of thawing is slow because of the relatively
low surface area to volume ratio involved.
On the other hand, when freezing in relatively thin
sheets, the area for freezing and the overall heat
transfer coefficient remain essentially constant during
-24-
-------
TABLE 6
EFFECT OF THE ADDITION OF FERRIC CHLORIDE BEFORE
FREEZING ON THE SLUDGE DEWATERING PROPERTIES
Ferric chloride dosage (Ib/ton)
Days sludge frozen
Feed sludge solids (%)
Soluble P04 before freezing (mg/1)
Screen loading (Ib/sq ft)*
Drain time (sec)
Drain rate (ml/sec)
Cake solids (After pressing) (%)
Filtrate suspended solids (mg/1)
Filtrate soluble PO^ (mg/1)
Filtrate soluble COD (mg/1)
0
2
2.36
140
0.44
37.5
8.1
14.3
90
202
3220
0
7
2.36
140
0.44
23
16.3
16.3
270
—
5303
28.3
2
2.36
84
0.42
30
11.4
16.1
75
152
2440
28.3
7
2.36
84
0.44
21
18
16.6
240
—
4837
54.8
2
2.36
46
0.43
23.5
15.2
17.2
60
100
2407
54.8
7
2.36
46
0.44
26
14.4
16.3
280
—
5056
* 80 mesh screen media used for gravity filtration.
-25-
-------
TABLE 7
EFFECT OF THE ADDITION OF FERRIC CHLORIDE AFTER
FREEZING AND THAWING ON THE SLUDGE DEWATERING PROPERTIES
Ferric chloride dosage (Ib/ton) 0 27.1 54.2
Days sludge frozen 222
Feed sludge solids (%) 2.36 2.36 2.36
Soluble PO, before freezing (mg/1) 140 140 140
Screen loading (Ib/sq ft)* 0.44 0.43 0.45
Drain time (sec) 37.5 40 90
Drain rate (ml/sec) 8.1 7.6 3.3
Cake solids (After pressing) (%) 14.3 14.6 13.2
Filtrate suspended solids (mg/1) 90 190 105
Filtrate soluble P04 (mg/1) 202 152 90
Filtrate soluble COD (mg/1) 3220 3054 2805
* 80 mesh screen media used for gravity filtration.
-26-
-------
the freezing process. Thin sheet freezing also has the
following advantages over freezing in blocks: (1)
Thin sheets lend themselves more readily to a continuous
freezing process, (2) By breaking the frozen sheets into
small chards, more surface area per unit volume for
faster thawing may be obtained, and (3) Faster feedback
information is available for process quality control,
(4) Faster freezing times per unit weight of sludge
frozen may be used, resulting in lower equipment costs.
As a result of the above analysis, it was felt that, if
the freezing cycle could be shortened and storage after
freezing eliminated, the economics of freeze conditioning
would be improved. Therefore, an investigation into the
various aspects of thin sheet freezing was launched.
Bench scale sheet ice tests were performed by placing the
sludge in shallow pans and freezing the sludge in an
upright freezer having a forced air draft and suitable
control over freezer temperature to permit varying
freezing times. Freezing time was accurately measured
using recording thermocouples, strategically placed in
the sludge layer. A time-temperature record was obtained
for each test, and the freezing time was obtained from
the constant temperature (32° F) portion of the curve.
The ice layer depths investigated included 1/8", 1/4",
1/2" and 1" and greater. It was noted that freezing in
one inch and greater ice depths resulted in pan container
deformation. Therefore, these ice depths were eliminated
from consideration for prototype design.
Thawed sludges derived from relatively fast freezing
were sensitive to handling, and this sensitivity to
handling increased with decreasing freezing times. Good
sludge dewatering properties were observed at the fast
freezing times when handling of the thawed sludge prior
to gravity filtration was reduced to a minimum. In
contrast, control samples, which were handled in the
normal manner (inverting the thawed sludge twice in a
graduate prior to pouring into a gravity drain filter
apparatus), did not dewater at all.
Dewatering of the thawed sludge was carried out by allowing
it to gravity drain on an 80 mesh screen filter medium
followed by further dewatering on the screen using either
a 5" Hg vacuum or by mechanical pressing. Vacuum was
selected as the further dewatering means because of its
-27-
-------
relative success in handling the wide range of cake
characteristics obtained in the investigation.
Shown in Figure 1 are typical results obtained using
relatively fast freezing in thin sheets. Waste activated
sludge was used. Freezing times and screen loadings
after thawing for the ice layer depths shown in Figure
1 were:
Ice Layer Freezing Time Screen
Depth (in) (minutes) Loading
(Ib/sq ft)
1/8 15 0.15
1/4 40 0.15
1/2 90 0.14
1 180 0.13
The results shown in Figure 1 were obtained from tests
in which the frozen sludge was not stored prior to
thawing, a minimum of handling was exercised after
thawing, and a 20 second vacuum at 5 in Hg was used
following gravity drain on an 80 mesh screen.
Referring to Figure 1, at zero ferric chloride dosage
(no chemicals), relatively good dewatering properties
were observed at fast freezing times and minimum handling
after thawing. In contrast, control samples, which
were handled in the normal manner after thawing (invert-
ing the thawed sludge twice in a graduate) did not dewater
at all. It is noted in Figure 1 (no chemicals) that a
significant increase in cake solids content was obtained
when the ice layer depth was increased above one-eighth inch.
Presented also in Figure 1 is the effect of adding
nominal amounts of ferric chloride on cake solids
content. (Note: Ferric chloride added before freezing).
In general, it was observed that, for a given ice layer
depth, an increase in ferric chloride dosage resulted
in an increase in cake solids content. It was also
observed that the addition of ferric chloride also
permitted rougher handling of the sludge after thawing.
-28-
-------
18
Ice Layer
Depth (in)
20 40 60 80
FERRIC CHLORIDE DOSAGE pounds/ton solids
FIGURE I. EFFECT OF FERRIC CHLORIDE ON
CAKE SOLIDS
-29-
-------
From Figure 1, cake solids content increased with
increasing ice layer depth up to the one-half inch depth.
A significant difference in performance is not indicated
between the one-half and one inch depths. In addition,
the one inch and greater ice depths resulted in pan
container deformation upon freezing, which is to be
avoided in practice. Based upon the results shown in
Figure 1 and many other similar test results, a one-half
inch ice layer depth was selected for prototype system
design.
The screen loadings tabulated above for the test
results shown in Figure 1 were optimum for the sludge
tested and corresponded to production rates of about
10 to 20 Ib/hr/sq ft. Control box samples (3-1/4" x
3-1/4" x 2-1/4") using slow freezing resulted in similar
production dewatering rates.
From a process performance standpoint, the technical
feasibility of freeze conditioning of activated sludge
in relatively thin sheets was established. The variables
affecting filter cake concentration included ice layer
depth, screen loading, freezing time, freezing rate, and
ferric chloride dosage. A regression analysis of the
sheet freeze data obtained from 46 tests was made.
Sixteen regression equations using various combinations
of the above variables affecting filter cake concentra-
tion were investigated. The regression equation giving
the best fit is shown below in Equation (1). An index
of determination of 0.88 was obtained at the 99% confi-
dence level for Equation (1). The term "index of determi-
nation"is identical to and synonymous with the square of
the multiple correlation coefficient (12).
X = 15.0 - 0.026 - 24.6 z2 + 0.0082 w
y2
- 182 s + 0.028 c + 4193 z2s2c (1)
-30-
-------
Definitions of the variables used in Equation (1)
and the ranges of those variables are listed below.
X = cake concentration, %
y = ice layer depth (1/8 in - 1 in)
z = screen loading (0.1-0.37 Ib/sq ft)
w = freezing time (20 - 336 min)
s = freezing rate (0.0015 - 0.0122 in/rain)
c = ferric chloride dosage (0 - 104 Ib/ton)
The dewatering operating conditions implied for Equation
(1) include minimum handling of the sludge after thawing,
gravity drain on an 80 mesh screen medium, and a 20
second vacuum at 5 in Hg following gravity drain.
Similarly, an expression was developed for calculating
the required gravity drain time. The sheet freeze
data from 46 tests were used in the regression analysis.
The regression equation yielding the best fit is shown
in Equation (2), below.
t - 8.5 - 19.6y + 393z2 - 0.39c + 2280s + 0.044 c. (2)
where, t = gravity drain time, sec.
The other variables in Equation (2) are as defined for
Equation (1). The dewatering operating conditions
implied for Equation (2) include minimum handling of the
sludge after thawing, and gravity drain on an 80 mesh
screen medium.
Equations (1) and (2) may be used in selecting the process
and design parameters for a prototype thin ice-sheet freeze-
conditioning system. The variables affecting cake
solids concentration and gravity drain time included
ice sheet depth, screen loading, freezing time, freezing
rate, and ferric chloride concentration.
-31-
-------
Engineering Design and Evaluation Study
Emerging from the preliminary investigation described above and
carried forward to the engineering design phase of the project
was a freeze-conditioning system consisting of the following
process elements:
1. Flotation thickening prior to freezing
2. Freezing in thin sheets
3. Thawing of the frozen product
4. Dewatering of the thawed sludge
5. Optional use of ferric chloride prior to freezing.
A schematic diagram of the system is shown in Figure 2.
The freeze conditioning demonstration pilot plant, to be installed
in the Machinery Bay building of the Milwaukee Water Pollution
Control Plant, was intended to handle three dry tons activated
sludge solids per day using a feed sludge solids concentration of
1% to 2%.
Design of the 3 Dry Ton/Day Pilot Plant Flotation Thickener
The flotation thickening step was incorporated into the freeze-
conditioning process for the purpose of concentrating the feed
sludge from 1% to 2% to a thickened sludge concentration of 4%.
Thickening was to be accomplished using the dissolved-air flotation
principle. The process elements associated with flotation thickening
include (1) pressurization (40 psig) of plant final effluent or
recirculated thickener effluent, (2) air introduction (about one
standard cubic foot per 100 gallons of pressurized flow), (3) air
solution at the elevated pressure (4) pressure reduction and tiny
(about 100 micron) bubble formation, (5) blending of air-charged
stream and feed sludge flow, (6) air-bubble-solids attachment, (7)
solids-liquid separation, (8) separated solids compaction or
thickening, and (9) thickened sludge and clarified effluent removal.
The thickening step described above was expected to result in the
removal, prior to freezing, of 50% or more of the water associated
with the feed sludge solids. It was felt that the operating cost
of removing this quantity of water by flotation thickening was
significantly less than the cost of freezing the water removed by
thickening. Moreover, the savings in freezer system and thawing
system capital costs brought about by the thickening step were
also expected to be significant.
-32-
-------
PRESSURIZED FLOW SOURCE
(lOOgpm)
WASTE
SLUDGE
|-2%SOLJDS
(25-50 flpm)
FLOTATION
THICKENER
i
u>
V
I
4% SOLIDS
(12.5 gpm)
CHEMICAL
TREATMENT
(UPT050lb/fon)
FERRIC CHLORIDE
14-15% SOLIDS
DE WATERING
WATER TO
HEAT RECOVERY
FILTER
CAKE
TO DRIERS
FIGURE 2. SCHEMATIC DIAGRAM OF THREE TON DRY SOLIDS PER DAY FREEZE-
CONDITIONING SYSTEM
-------
The design of the flotation thickener was based upon a net solids
loading of 10 lb/(day)(sq ft) without the use of chemical flotation
aids (Rex Chainbelt Inc. experience). Based on a net thickener
capacity of 3 dry tons per day, an effective thickener surface
area of 600 sq ft would be required. The cost of the thickener,
without site or housing, was estimated at $46,500 (1968 prices).
Freezing System
The criteria used in the design of the equipment to carry out the
thickened sludge freezing operation included the following:
1. Sludge ice-sheet depth of one-half inch
2. Freezing time of 100 minutes
3. Conventional indirect refrigeration means (vapor
compression cycle) to be used for freezing.
Two methods were investigated for freezing the thickened sludge in
thin sheets (one-half inch depth). Both methods had the advantage
of being capable of batch or continuous operation.
1. One method involved the use of a serpentine pan-type end-
less conveyor employing a cold air blast as the cooling
medium. As visualized (See Figure 3(a)), the pan conveyor
would enter the cold room at the top and vertically wind
down to the bottom of the cold room in serpentine fashion.
Freezing of the sludge in each pan was to take place during
its travel through the cold room. The ends of the pans
were to be attached to two strands of conveyor chains, and
the vertical turns were to be negotiated by the pans by
pivoting means at the points of chain attachment, much in
the same manner that ferris wheel seats are attached.
2. The other freezing method involved the use of an endless
steel belt, employing chilled brine as the indirect
cooling medium. This method is shown, schematically,
in Figure 3(b).
The freezing system utilizing the pan conveyor and cold air blast
as a cooling medium, Figure 3(a), although technically feasible, had
several disadvantages. Inquiry in the marketplace, refrigeration
consultants and contractors, showed that the pan conveyor equipment
was not commercially available to suit the project purposes. It
is evident that the pan conveyor equipment would have to be custom
designed, shaken down, and modified, at the cost of time and
expense, to meet the project needs. One of the prominent inherent
disadvantages of the pan conveyor freezing system, however was
the relatively high rate of evaporation (5 to 10% by weight) which
would be obtained in operation, requiring too frequent defrosting
-34-
-------
SLUDGE FEED
*
en
m
:
CHAIN STANDS
(a) PAN CONVEYOR
SLUDGE
FEED
L
STEEL BELT
A
T
SHEET-ICE
PRODUCT OUT
BRINE COOLANT
SLUDGE LAYER
BRINE COLLECTING
GUTTERS
COOLANT
(b) BELT CONVEYOR
SECTION A-A
FIGURE 3.
TWO METHODS INVESTIGATED FOR FREEZING
THICKENED SLUDGE
-35-
-------
of the refrigeration coils and would result in significantly
increasing operating costs and equipment costs (standby air-blast
equipment during defrosting).
On the other hand, the belt freezing system, Figure 3(b) was
available commercially in unitized sections, allowing great flexibi-
lity in system design and installation. Moreover, the belt freezing
system had been commercially applied to a broad range of products in
the form of solids, slurries, sheets, granules, and powders.
Furthermore, experimental pilot freezing and processing units were
available for trial research and investigation in obtaining the
appropriate prototype design and process parameters.
Therefore, the pan conveyor freezing concept was abandoned, and
the endless steel belt freezing system was selected for freezing
the sludge in thin sheets.
The major elements comprising the steel belt freezing system,
Figure 3(b), were the refrigeration equipment, brine chillers,
and steel belt assembly. The steel belt assembly consisted of,
1. Complete end assembly systems including two pulleys,
drive, take-ups, tensioners, safety equipment, etc. at
a cost of $8600 (1968 prices).
2. Mild carbon steel belting in standard 12 foot sections
including brine coolant pans, piping, etc. at a cost of
$2100 (1968 prices).
3. The maximum recommended belt width available was four
feet. Items 1 and 2, above, reflect the costs associated
with four foot wide mild carbon steel belt assemblies.
For a given application, the freezing system may consist of (a) a
single long steel belt assembly with one complete end assembly or
(b) short, multiple steel belt assemblies with each short steel
belt conveyor requiring a complete end assembly. Comparing the
merits of Items (a) and (b), above, it is apparent that a single
long steel belt assembly, Item (a), is less costly (only one com-
plete end assembly required) but requires more floor space. Con-
versely, the short, multiple steel belts, Item (b), are more
costly (multiple complete end assemblies required at $8600 each)
but will take up less floor space if they are "stacked" one on
top of the other. Both options, Items (a) and (b), are considered
in the following discussion.
The major equipment costs as well as the floor space requirements
for the two alternative belt freezing systems handling three dry
-36-
-------
tons of sludge solids per day at a thickened feed sludge solids
content of 4% are presented below. The major equipment costs
include erection but exclude site and housing costs because these
were available at the Milwaukee Plant. Details of the calculations
used in arriving at the tabulated figures are presented in Appendix
A.
Effective Major
Belt Conveyor Floor Space Equipment Cost
System Required (sq ft) ($)
Single Belt 4713 551,000
Multiple Belt 1227 883,900
Thawing System
The frozen sludge obtained from the belt conveyor freezer system
was to be melted on a suitable thawing conveyor prior to further
dewatering. The frozen sludge sheet from the freezing system was
to be broken up into small shards and discharged onto the thawing
conveyor consisting of a relatively coarse (40 x 60 mesh openings)
porous medium. The porous medium permits the melted ice water
to drain off and be utilized as a refrigeration condenser cooling
source and/or heat exchanged with the incoming thickened feed
sludge to the sludge freezing system.
The source of heat for thawing the frozen sludge was to be that
recovered from the refrigeration system (heat of vaporization and
heat of compression) and from auxiliary heat sources, as required.
The heat required for thawing was to be transferred to the frozen
sludge, using air as the medium, by heater blowing equipment.
Provision for washing the conveying medium was also to be made.
The major components comprising the thawing system included the
conveyor medium, drives, pulleys, take-ups, blowers, wash pump,
ice water collection through, and insulation. The major equipment
costs as well as the floor space requirements for the thawing
system are tabulated below. The equipment costs include erection
costs but exclude the cost of site and housing as these were
available at the Milwaukee Plant. Details of the calculations
used in designing the thawing system are presented in Appendix B.
Effective floor area required = 1632 sq ft
Thawing system equipment cost « $65,000
-37-
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Thawed Sludge Dewatering System
The thawed sludge obtained was expected to contain substantial
amounts of free water and the major portion of this water was to
be removed by a final dewatering step. It was intended to carry
out the dewatering of the thawed sludge by allowing it to gravity
drain on an 80 mesh screen filter medium followed by further
dewatering on the screen using a 5" Hg vacuum.
The equipment required to carry out the final dewatering step was
commercially available in the form of a horizontal belt filter.
The major equipment costs and floor space requirements for the
dewatering system are shown below. The equipment costs include
erection costs but exclude the cost of site and housing as these
were available at the Milwaukee Plant. Details of system design
for the dewatering system are presented in Appendix C.
Effective floor area required = 24 sq ft
Dewatering system equipment costs = $16,800
Chemical Treatment System
Chemical additives were investigated for their effect as adjuncts
to freezing on the dewatering properties of the thawed sludge.
Ferric chloride was the additive found to give the most pronounced
beneficial effect in this study. Up to 50 Ib per ton ferric
chloride was used effectively when introduced into the sludge
before freezing. Therefore, provision for the optimal use of
ferric chloride prior to sludge freezing was to be incorporated
in the freeze-conditioning demonstration plant.
The major components comprising the chemical treatment system
included the chemical stock storage tank and supporting base,
mixer, and chemical feed pump. The major equipment costs and the
floor space requirements for the chemical treatment system are
listed below. Again, site and housing are not included in the
cost listed as these were available at the Milwaukee Plant.
Details of the design of and cost breakdown for the chemical
treatment system are presented in Appendix D.
Effective space requirements = 16 sq ft
Equipment costs = $1100
-38-
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Summary of the Equipment Costs and Floor Space Requirements for
the Three Ton Solids Per Day Freeze-Conditioning Demonstration Plant
A summary of the major equipment costs and floor space require-
ments for the freeze-conditioning demonstration plant is presented
in Table 8. It is evident from Table 8 that the alternative
utilizing a single freezing belt conveyor is less costly but
requires greater floor space than the alternative using the
multiple belt freezing conveyor system.
Comparison of Freeze-Conditioning and Conventional Chemical
Conditioning Methods
At this point, it was deemed appropriate to compare the freeze-
conditioning process with the conventional chemical conditioning
method from the standpoint of equipment costs and floor space
requirements. The chemical conditioning process used as a basis
for comparison is the one presently employed by the Milwaukee
Water Pollution Control Plant, namely, ferric chloride conditioning
of waste activated sludge followed by vacuum filtration dewatering.
Cost figures include the cost of the dewatering step, which is
carried out using horizontal screen belt filters for the freeze-
conditioning method and using rotary vacuum filters for the conven-
tional chemical conditioning method.
The estimated effective space requirements and equipment costs for
the major components of the freeze-conditioning (from Table 8) and
conventional chemical conditioning systems, handling 3 tons dry
solids per day of waste activated sludge are shown in Table 9.
The equipment costs shown for freeze-conditioning do not include
site or housing whereas those shown for conventional chemical
conditioning include the cost of site and housing. The equipment
costs for conventional chemical conditioning were obtained from
Smith (11).
A comparison of the estimated effective space requirements for the
two sludge conditioning methods presented in Table 9 shows that
the space requirements for the freeze-conditioning system (utiliz-
ing the single belt conveyor freezer) are about 130 times that
required for the existing conventional chemical conditioning
system. The ratio of 130:1 may be reduced to 65:1 by utilizing a
multiple belt conveyor freezer in the freeze-conditioning system.
It is concluded, therefore, that from a space requirement stand-
point, the freeze-conditioning process does not compare favorably
with the existing conventional chemical conditioning process.
-39-
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TABLE 8
SUMMARY OF THE EQUIPMENT COSTS* AND FLOOR SPACE
REQUIREMENTS FOR THE THREE TON SOLIDS PER DAY FREEZE-
CONDITIONING DEMONSTRATION PLANT
Process Element
Space
Requirements
(sq ft)
1. Flotation Thickener
2. Freezing System
a. Single Belt Conveyor
b. Multiple Belt Conveyor
3. Thawing System
4. Thawed Sludge Dewatering
5. Chemical Treatment System
6. Totals
a. With Single Belt Conveyor
Freezing System
b. With Multiple Belt Conveyor
Freezing System
600
4,713
1,227
1,632
24
16
Equipment
Cost
($)
46,500
551,000
883,900
65,000
16,800
1,100
6,985
3,509
680,400
948,300
*Not including site or housing
-40-
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Examination of the demonstration site showed that it was physically
impractical to install a 3 ton/day freeze conditioning system at
the Milwaukee Water Pollution Control Plant. Moreover, for the
space available, it would be necessary to reduce the capacity of
the pilot plant from 3 ton/day to 0.25 tons/day in order to fit
in a freeze conditioning demonstration pilot plant at Milwaukee.
A comparison of the estimated equipment costs for the two sludge
conditioning methods presented in Table 9 shows that the costs
for the freeze-conditioning system (utilizing the multiple belt
conveyor freezer) are about ten times the costs for the existing
conventional chemical conditioning system. The cost ratio of 10:1
may be reduced to 7:1 by utilizing a single belt conveyor freezer
in the freeze conditioning system. The cost ratios cited, in
favor of the conventional chemical conditioning process should
actually be greater because the freeze-conditioning system costs
listed in Table 9 do not include the cost of site and housing.
It is concluded, therefore, that the major equipment costs are
appreciably greater for the freeze-conditioning process than for
the conventional chemical conditioning process.
It was estimated that the lowest cost of a 3 ton/day freeze-
conditioning pilot plant (without site or housing) (Table 9)
would be $680,400. This cost is greater than the total estimated
project cost, which was $348,344. Furthermore, if the design
capacity of the freeze conditioning pilot plant were reduced to
0.25 tons/day, the cost of the pilot plant would be about $140,900.
At the 0.25 tons/day capacity, it may be shown that additional
monies over and above those originally granted would be required
to complete the project work and to achieve the objectives of the
project.
Application and Evaluation of the Freeze-Conditioning Process to
Large Scale Plants
Extrapolation of data for 1/4, 1/2, 1, and 3 tons/day plants
indicates that the cost of a freeze-conditioning plant handling
200 tons solids per day (roughly that presently handled by the
Milwaukee Plant) would be about 80.0 million dollars (multiple
belt conveyor freezer). The above costs cited are without site
or housing and reflect 1968 prices.
On the other hand, it is estimated that a conventional chemical
conditioning plant, handling 200 tons solids per day, would cost
5.1 million dollars (11). This cost reflects 1968 prices and
includes housing and installation.
-41-
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TABLE 9
COMPARISON OF EQUIPMENT COSTS AND FLOOR SPACE
REQUIREMENTS FOR THE FREEZE-CONDITIONING AND CONVENTIONAL
CHEMICAL CONDITIONING METHODS*
Conditioning System Equipment Costs Space
($) Requirements
(sq ft)
1. Freeze-Conditioning (Table 8)
a. Single Belt Conveyor
Freezing System 680,400 6,985
b. Multiple Belt Conveyor 948,300 3,509
2. Conventional Chemical
Conditioning System** 95,000 54
Based on handling 3 dry tons solids/day of waste activated sludge
**Vacuum filtration at a yield of 1.5 lb/(hr)(sq ft)
Filter dimensions = 9 foot diameter x 6 foot face
Equipment costs include site and housing
-42-
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It is concluded that the capital or construction costs for both
small and large plants are appreciably greater for the freeze-
conditioning process than for the conventional chemical conditioning
process.
Comparison of Operating Costs for the Freeze Conditioning and
Chemical Conditioning Processes
An examination of the operating costs for the two sludge condition-
ing processes showed that the operating costs for the freeze-
conditioning process were appreciably greater than those for the
chemical conditioning process. For example, the operating costs
for the two processes were investigated for the existing Milwaukee
plant handling 238 tons solids per day. The results of this com-
parison are shown in Table 10. It is noted in Table 10 that the
detailed operating costs shown include labor maintenance, chemical
costs, power, and amortized capital costs (20 year life, 3%). The
operating costs shown for the ferric chloride conditioning process
were the actual costs incurred by the Milwaukee Plant in the year,
1966, whereas the freeze-conditioning costs were on an estimated
basis. The operating costs obtained varied from about $77 to $79
per ton of dry solids processed.
Investigation of the Possibility of Reducing the Operating Costs
of Freeze-Conditioning
Evaluation of the freeze-conditioning process and comparison with
the existing conditioning method presently employed at Milwaukee
showed that the capital and operating costs and space requirements were
appreciably greater for the freeze-conditioning process than for the
existing chemical conditioning method.
An aspect of the engineering design study that was investigated was
.the reduction of the freeze-conditioning process operating costs by
appreciably reducing the amount of refrigeration work required. In
this regard, a technical audit of the freeze-conditioning system was
conducted with the Massachusetts Institute of Technology, Cambridge,
Massachusetts. The purpose of the consultation, particularly, was
to investigate the aspect of energy recovery and energy conservation.
As a result of this audit, it was concluded that other refrigeration
methods could be applied that require less power per unit of sludge
frozen, but that these methods will likely prove to have substantially
higher capital costs.
-43-
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TABLE 10
COMPARISON OF OPERATING COSTS FOR FERRIC CHLORIDE AND
FREEZE-CONDITIONING OF MILWAUKEE WASTE
ACTIVATED SLUDGE (238 TON PER DAY BASIS)
1. Acid House
Labor
Unloading
FeCl3
Maintenance
Ms cellaneous
2. Machinery Bay
Operating Labor
Supplies
Power
Mis cellaneous
3. Filter House
Operating Labor
Supplies
Maintenance and Repair
Air
Power
Miscellaneous
4. Amortized Capital Costs*
5. TOTAL
6. Cost/Ton Solids
Fed 3 Condi-
tioning Costs
1966 Costs
$
38,000
8,000
775,000
15,000
13,000
79,000
1,000
35,000
17,000
85,000
2,000
150 ,000
12,000
11,000
12,000
650,000
1,903,000
21.88
Freeze Conditioning
Annual
Estimated Costs
$
28,000
4,000
230,000
15,000
10,000
79 ,000
1,000
9,000
17,000
85,000
2,000
50,000
10,000
840,000
5,376.000
6,756,000
77.77
*Based on 20 year life, 3%
-44-
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SECTION VI
ACKNOWLEDGMENTS
The support of the Milwaukee Sewerage Commission and its personnel
is acknowledged with sincere thanks. Their cooperation and
assistance were of considerable aid in facilitating the work
performed. In particular, the interest and cooperation of Mr.
R. Leary, Chief Engineer and General Manager, and Mr. L. Ernest,
Project Director, are gratefully acknowledged.
This investigation was carried out by the Ecology Division of
Rex-Chainbelt, Inc. Mr. Anthony Geinopolos directed the work and
is the author of this report. The bench-scale studies, analytical
determinations, and statistical investigations were conducted by
Mr. R. Wullschleger and his staff. Their efforts and diligence
were essential to the collection and evaluation of the data pre-
sented. Their contribution is sincerely acknowledged.
The support of the project by the Environmental Protection Agency,
formerly the Federal Water Quality Administration, and the willing
assistance and helpful advice of Project Officer Dr. Joseph B.
Farrell in the conduct of the project and in the presentation of
this report is acknowledged with sincere thanks.
-45-
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SECTION VII
REFERENCES
(1) Burd, R. S., A Study of Sludge Handling and Disposal. U. S. Dept.
of Interior, FWPCA Publication WP-204, May 1968.
(2) McCarty, P- L., Journal, Water Pollution Control Federation, 38,
1966, p. 493.
(3) Lewin, V. H. and El-Sharkey, M. H., "Sewage Sludge Dewatering",
Water and Waste Treatment, May, June 1966, p. 19.
(4) Clements, G. S., Stephenson, R. J., and Regan, C. J., "Sludge
Dewatering by Freezing with Added Chemicals", Journal of the
Institute of Sewage Purification, Part 4, 1950, p. 318.
(5) Katz, W. J. and Mason, D. G., Freezing Method for Conditioning
Activated Sludge, Presented at the 16th Southern Water Resources
and Pollution Control Conference, Duke University, April 6 and 7,
1967.
(6) Internal Rex Chainbelt Inc. Project Report No. OP-3(J-20789)-1,
Dated December 8, 1966.
(7) Babbit, H. E. and Schlenz, H. E., University of Illinois Engineering
Experiment Station, Bulletin 198, December 1929, pp. 48-54.
(8) Clements, G. S. and Stephenson, R. J., The Water and Sanitary
Engineer, 1953, pp. 257-260.
(9) Downes, J. R., Proceedings, New Jersey Sewage Works Association,
24th Annual Meeting, 1939, pp. 41-43.
(10) Doe, P. W., Benn, D., and Bays, L. R., Journal of the Institute of
Water Engineering, December 1965, pp. 251-275.
(11) Smith, R., Journal, Water Pollution Control Federation, 40, (1968),
p. 1567.
-47-
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SECTION VIII
APPENDICES
-49-
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APPENDIX A
BELT FREEZING SYSTEM SPACE REQUIREMENTS
AND EQUIPMENT COST ESTIMATES
A. Design Basis
1. System to process 3 dry tons solids per day
2. Sludge to be thickened to 4% solids prior to freezing and
dispensed onto the belt conveyor to a liquid depth of 1/2 inch
3. Sludge detention time on belt conveyor i<= 115 minutes (from bench
scale tests)
a. Sensible heat removal (60° F to /32° F) = 15 min
b. Latent heat of fusion and sensible heat removal
(32° F to 20° F) - 100 min
4. Conventional indirect refrigeration equipment to be used.
a. Compressor
b. Condenser
c. Coolant chiller
B. Equipment Design and Costs
1. Refrigeration Equipment
Thickened Sludge Flow Rate, Q (Ib wet/min)
= 6.000 Ib D.S. g Ib wet x day = 1Q4.1 Ib
day 0.04 dry 1,440 min
Weight of Sludge on Belt Conveyor, W (Ib wet)
W « Q x detention time
= 104.1 lk_ x 115 min
min
= 11,971 Ib
Tons of Refrigeration Required
1. Sensible Heat Removed
•nrrtTT
(60° F-32° F) = 11,971 Ib X~^I^F x 28° F x
60 min ton-hr = ,, c c .
* —hr— * 12,000 BTU U'55 tt>ns
-51-
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1
(32° F-20° F) - 11,971 Ib x °-f FU x
60 min ton-hr
x hr x 12,000 BTU = 3-12 tons
2. Latent Heat
144 BTU _, 1 60 min
11,971 Ib x — £b - x 115 min X ~hr~
' 74'94 tons
3. Heat Losses
Assume 20% of Items 1 and 2 = 18.52 tons
4. Total Tons Refrigeration Required
Total Tons = 111.13
Refrigeration Power Requirements
2_H
ton
Sa 2_HP x 111>13 tons = 222.26 horsepower
3 '
Refrigeration Equipment Required
Integrated refrigeration equipment packages are commercially available
which contain the required compressors, condensers, and coolant chiller tanks.
The following information was obtained from Vilter Manufacturing Corporation,
Milwaukee, Wisconsin (Bulletin No. 548).
1. Refrigeration power requirement = say, 225 HP
2. Use 2 - 100 HP units and 1 - 25 HP unit
3. Dimensions
a. 100 HP unit - 147" 1 x 35.5" w x 86" h
b. 25 HP unit - 140" 1 x 33" w x 73" h
4. Floor space requirements
a. 100 HP unit (two required)
b. 25 HP unit (one required)
. 32 sq ft
c. Total floor space = 105 sq ft
5. Refrigeration equipment costs (Vilter Mfg. Corp., Milwaukee, Wis.)
a. Compressors = $15,500
b. Coolant Chillers = $13,000
c. Erection = $19.000
d. Total =
-52-
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2. Belt Conveyor Equipment
The steel belt conveyor assembly is comprised of,
a. Complete end assembly systems including two pulleys, drive, take-ups,
tensioners, safety equipment, etc. at a cost of $8600 (1968 prices).
b. Mild carbon steel belting in standard 12 foot sections including
brine coolant pans, piping, etc. at a cost of $2100 (1968 prices).
c. The maximum recommended belt width available is four feet.
Items (a) and (b), above, reflect the costs associated with four foot
wide mild carbon steel belt assemblies.
For a given application the belt freezing system may consist of (a) a
single long belt assembly or (b) short, multiple belt assemblies
"stacked" one on top of the other. Both options are considered in the
following discussion.
a. Multiple Belt Conveyor Assemblies
Each conveyor in the multiple belt system would consist of a flexible
steel belt with rubber sides bonded to the upper side to retain the
sludge and rubber V-belts bonded to the underside to provide a means
for driving the belt. The belt would float or ride in a coolant
bath (see Figure 3b in text).
Assume the effective dimensions of each belt to be 48 in wide and
24 ft long and carrying 1/2 in depth of sludge. Then, the volume of
sludge on each conveyor is,
M x 24 x 1/2 x 1/12 = 4.0 cu ft
and the effective weight of sludge on each conveyor will be,
4.0 cu ft x 62-4b
The total weight of sludge undergoing freezing on all conveyors in
the system is 11,971 Ib (see Item B.I, above). The number of
conveyors required, therefore, is,
^g conveyors
The pulley diameters per belt, according to the manufacturer,
Sandvik Steel, Inc., Skokie, Illinois, should be a minimum of 800
times the belt thickness. Assuming an available belt thickness of
0.032 in, the minimum pulley diameter is
800 x 0.032 = 25.6 in
-53-
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The maximum height available at the Milwaukee Plant is 40 feet.
Assuming the height of each conveyor is 4 feet, then the 48
conveyors may be erected in 6 tiers containing 8 conveyors per tier.
Shown in Figure A-l is the structure designed to contain 16 conveyors
(one dry ton solids per day capacity) in 2 tiers. Three such
structures would be required for the three ton solids per day pilot
demonstration plant. The effective floor space requirements for the
3 ton solids per day plant are,
3 x 34 x 11 = 1,122 sq ft
The material costs for the structures to contain the 48 belt conveyors
(see Figure A-l, three required) are presented in Table A-l. The
estimated erection cost for the structures required may be broken out
of Table A-2.
The belt conveyor material costs, including pulleys, drives, take-ups,
tensioners, safety equipment, belting, coolant tanks, piping, etc.
are,
48 x $8,600 + 96 x $2,100 = $614,400
The estimated erection cost for installing the belt conveyor equipment
may be broken out of Table A-2.
Additional equipment required to round out the breezing system include
conveyor sludge feeders, conveyor product ice-breakers, conveyor
product ice unloaders and insulation. The equipment cost of these
items is tabulated below.
Sludge feeders $19,560
Ice breakers $ 6,000
Unloaders $13,600
Insulation (installed) $47,600
(11,424 sq ft @ $7/sq ft)
The cost of installing the feeders, breakers and unloaders may be
broken out of Table A-2.
A summary of the freezer system capital costs, including erection, is
shown in Table A-3. The total cost shown in Table A-3 is $883,900.
The effective floor space requirements for the multiple belt conveyor
freezing system are approximated below.
Refrigeration equipment = 105 sq ft
Belt conveyor system = 1,122 sq ft
(3 x 11' x 34')
TOTAL = 1,227 sq ft
-54-
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A=IO"WF COLUMN @ IOOLB/FT OREQ'D)
B= I2"CHANNELBEAM@ 30LB/FT(I8REQ'D)
C-I2"I-BEAM @ 50LB/FT OREQ'D)
D = 6"I-BEAM @) 17. 25 LB/FT (12REQ'D)
FIGURE A-l. BELT CONVEYOR STRUCTURE
I TON SOLIDS/DAY CAPACITY
-55-
-------
TABLE A-l
MATERIAL COSTS FOR THE STRUCTURE OF THE MULTIPLE BELT CONVEYOR
SYSTEM TO HANDLE THREE DRY TONS SLUDGE
Item* Description
"A" Columns 10" wide flange
"B" Beams 12" channel
"C" Beams 12"-I beam
"D" Beams 6"-I beam
Material Cost
Fabricated Cost (@ $0.30/lb)
Column Bases (@ $100 each, 27
Weight
Length
(lb/ft)
100
30
50
17.25
= 219,572 x
required)
TOTAL COST
Say,
SOLIDS PER DAY
Length Number
Required Required
(ft)
34 27
34 54
34 27
11 36
Sub-Total
Plus 10% Misc.
TOTAL
0.3 = $65,872
= $ 2,700
= $68,572
= $69,000
Total
Weight
(lb)
91,800
55,080
45,900
6,831
199,611
19,961
219,572
*Refer to Figure A-l for nomenclature
-56-
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TABLE A-2
ESTIMATED ERECTION COSTS FOR THE MULTIPLE BELT CONVEYOR
SYSTEM TO HANDLE THREE DRY TONS SLUDGE SOLIDS PER DAY
Man
Hours
Required
1 Prepare column bases 54
2 Erect columns 108
3 Assemble beams and square structure 468
4 Install brine pans 1,536
5 Install head and tail pulleys 768
6 Install belts 384
7 Install drives 288
8 Piping of brine pans 288
9 Install weirs 132
10 Piping of conveyor feeders 132
11 Install conveyor unloaders 480
12 Install ice breakers 96
13 Crane operation 480
14 Moving material and machinery into building 720
15 Sub-Total 5,934
16 De-bugging (10% of above sub-total) 593
17 Total hours required 6,527
18 Labor cost Ǥ $8.50/hour) $55,480
19 Erection machinery rental 10,800
20 Total Erection Cost $66,280
Say, $66,300
-57-
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TABLE A-3
Item
SUMMARY OF MULTIPLE BELT CONVEYOR FREEZING SYSTEM
CAPITAL COSTS (3 DRY TON SOLIDS/DAY)
Description
Cost
($)
1 Refrigeration Equipment (compressors, condensers, chillers, 47,500
Erection)
2 Structure (Table A-l) 69,000
3 Belt Conveyors 614,400
4 Sludge Feeders 19,500
5 Ice Breakers 6,000
6 Unloaders 13,600
7 Insulation (installed) 47,600
8 Erection (Table A-2) 66,300
9 TOTAL $883,900
-58-
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b. Single Belt Conveyor Assembly
The single belt conveyor would have a length equivalent to the sum
of the effective lengths of the conveyors comprising the multiple
belt conveyor system described above, or
Length = 48 x 24 = 1,152 ft
The effective floor space requirements would be,
1,152 ft long x 4 ft wide = 4,608 sq ft
It is recognized that the single belt concept for this application
may be impractical because of the inordinate length of the conveyor.
However, this concept is presented for comparison purposes with the
multiple belt concept previously described.
Equipment costs for the single belt conveyor were calculated in a
manner similar to that used for the multiple belt conveyor assembly.
A summary of the estimated freezer system capital costs, including
erection, is shown in Table A-4. The total cost shown in Table A-4
is $551,500.
The effective floor space requirements for the single belt conveyor
freezing system are approximated below.
Refrigeration equipment = 105 sq ft
Belt conveyor system = 4608 sq ft
TOTAL = 4713 sq ft
-59-
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TABLE A-4
Item
SUMMARY OF SINGLE BELT CONVEYOR FREEZING SYSTEM
CAPITAL COSTS (3 DRY TON SOLIDS/DAY)
Description
2 Structure
3 Conveying Equipment
4 Sludge Feeder
5 Ice Breaker
6 Unloader
7 Insulation (installed)
8 Erection
9 TOTAL
Cost
($)
Refrigeration Equipment (compressors, condensers, chillers, 47,500
Erection)
69,000
210,200
6,500
2,000
4,500
145,500
66,300
$551,500
-60-
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APPENDIX B
THAWING SYSTEM SPACE REQUIREMENTS
AND EQUIPMENT COSTS
A. Thawing Process Description
The frozen sludge obtained from the belt conveyor freezer system will be
melted on a suitable thawing conveyor prior to further and final dewatering
on a horizontal dewatering conveyor.
The frozen sludge sheet is to be broken into small shards, to increase the
available surface area for heat exchange, and discharged to a thawing
conveyor containing a coarse porous medium. The porous medium will allow
the melted ice water to drain off and be utilized as a refrigeration
condenser cooling source and/or heat exchanged with the incoming thickened
feed sludge to the sludge freezing system.
The source of heat for melting the frozen sludge will be that recovered from
the refrigeration system (heat of vaporization and heat of compression) and
from auxiliary heat sources, as required. The thawing conveyor is to be
insulated to minimize losses. Provision for washing the medium will be
made.
B. Design Basis
1. The thawing conveyor will be capable of handling frozen sludge containing
3 dry ton solids per day.
2. Assume ice to be delivered to the thawing conveyor at about 5° F.
3. Frozen sludge to be completely thawed at discharge end.
4. Melted ice water to be collected and suitably disposed of.
C. Calculations
1. Heat requirements to thaw frozen sludge from 5° F to ice water at 32° F
a. Ice production rate
150,000 Ib ice x day = 6250 Ib/hr
day 24 hr
b. Sensible heat requirements to raise the temperature of the ice
from 5° F to 32° F
6250 x 0.5 x 27 = 84,375 BTU/hr
c. Latent heat requirements
6250 x 144 = 930,000 BTU/hr
-61-
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d. Total theoretical heat required = 1,014,375 BTU/hr
2. Surface area per unit weight of broken ice
a. Assume ice sheet is broken up into shards having the effective
dimensions, 1/4" x 1/4" x 3/8"
Shard area = (2 x 1/4" x 1/4") + (4 x 1/4" x 3/8") = 0.5 sq in
Shard volume = 1/4" x 1/4" x 3/8" = -?- cu in
l/o
b. Surface area per unit shard weight
.
3. Rate of actual heat transfer required
a. Assume only 50% of apparent surface area (Item C2b, above) is
available for effective heat transfer
0.5 x 4.1 sq ft = 2.05 sq ft
Ib ice Ib ice
b. Rate of actual heat transfer required
(1) Assume inlet air temperature of 150° F
(2) Assume exit air temperature of 70° F
(3) Assume straight line temperature drop, then average
temperature is 110° F and average temperature drop is
110-32 = 78° F
(4) Overall heat transfer coefficient
From Chemical Engineers' Handbook, J. H. Perry, Ed., 3rd Edi-
tion, p. 481, Table 4, the overall heat transfer coefficient,
U, for hot air blowing over and under a stainless steel
conveyor, may vary from 5 to 7 BTU/(hr)(sq f t) (°F) . A value
of U of 5 BTU/(hr)(sq f t) (°F) is assumed for this application.
(5) Rate of actual heat transfer required
q = UA dt
where q = rate of heat transfer, BTU/hr
U = overall heat transfer coefficient, BTU/(hr) (sq ft)(°F)
A = heat transfer area, sq ft
dt = temperature difference, °F
q = 5 x 2.1 sq ft x 6250 Ib ice x 78 = 5,118,750 BTU/hr
Ib ice
4. Thawing Conveyor Effective Area
a. It was observed in the laboratory that the bulk density of ice
shards having individual effective dimensions, 1/4" x 1/4" x 3/8",
is 23 Ib/cu ft.
-62-
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b. The bulk ice rate to be handled by the thawing conveyor is,
6250 Ib x cu ft = 272 cu ft/hr
hr 23 Ib
c. Assuming the prototype conveyor has a detention time of one hour,
the volume of ice on the conveyor at any time is 272 cu ft.
Assuming a 4 ft wide conveyor having a bulk ice depth of 2 in,
then the length of the conveyor is,
L „ 272 * 12 = 4Q8 feet long
*L X *T
d. Effective floor area required
A = 408 x 4 = 1632 sq ft
5. Thawing Conveyor Equipment Costs
The major components comprising the thawing conveyor include the
conveyor medium (40 x 60 mesh openings), drives, pulleys, take-ups,
blowers, wash pump, ice water collection trough, and insulation. The
estimated cost of the thawing conveying equipment, including erection
but excluding site and housing is $65,000.
-63-
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APPENDIX C
DEWATERING SYSTEM EQUIPMENT COSTS
AND SPACE REQUIREMENTS
1. Design Criteria
a. Production Rate = 10 lb/(hr)(sq ft)
b. Screen Loading =0.15 Ib/sq ft
c. Detention Time =1.0 min
d. Vacuum = 5" Hg
e. Provision for washing of media
2. Equipment Selection
Suitable equipment was commercially available from various vendors. The
dewatering equipment selected had the following salient features.
a. Effective dimensions = 2'w x 12"1
b. Variable belt speed = 2 to 12 fpm
c. Variable vacuum levels, up to 22" Hg
d. Continuous media washing
e. Compartmentalized vacuum boxes capable of independent operation
3. Equipment Costs
a. Horizontal filter belt structure, constructed with mild structural
frame, type 304SS contact parts and segmented belt $12,000
b. Vacuum receiver $ 850
c. Vacuum pump $ 2,700
d. Filtrate pump $ 725
e. Erection $ 500
f. Total $16,775
Say $16,800
4. Space Requirements
Effective floor space required = 2' x 12' = 24 sq ft
-65-
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APPENDIX D
CHEMICAL TREATMENT EQUIPMENT COSTS
AND SPACE REQUIREMENTS
1. Design Basis
a. Chemical additive - Ferric Chloride
b. Ferric chloride concentration = 20% FeCl3
c. Ferric chloride design dosage = 50 Ib/ton
2. Chemical Treatment Equipment Costs
a. Stock storage tank (215 gal) $ 85
b. Structure including erection $ 250
c. Storage tank mixer $ 255
d. Chemical feed pump $ 500
e. Total $1,080
Say $1,100
3. Space Requirements
A = 4' x 4' = 16 sq ft
-67-
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1
Accession Number
w
5
ry Subject Field & Group
05D, 05E
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Sewerage Commission of the City of Milwaukee
Milwaukee, Wisconsin 53201
Title
EVALUATION OF Conditioning and Dewatering Sewage Sludge by Freezing
10
Authors)
Geinopolos, Anthony
16
Project Designation
FWQA Program #11010 EVE, Grant #WPRD 71-01-68
21
Note
Rex Chainbelt Inc. Ecology Division performed
work for grant recipient, the City of Milwaukee
(Wisconsin) Sewerage Commission
22
Citation
Available from Government Printing Office, Report No. 11010EVE01/71
23
Descriptors (Starred First)
*Water pollution control, *Wastewater treatment, *Research and development,
*Capital costs, *0perating costs, *Sludge disposal, *Sludge dewatering, *Sludge
conditioning
25
Identifiers (Starred First)
27
Abstract
.aboratory investigations into feasibility of controlled freezing and thawing of waste
activated sewage sludge as a conditioning process prior to dewatering provided data
for the engineering design of a freeze conditioning system consisting of the following
process elements: (1) flotation thickening prior to freezing; (2) controlled freezing
in thin sheets; (3) thawing of the frozen product; and (4) dewatering of the thawed
sludge on a horizontal screen belt filter. It was concluded that the freeze-condition-
ing concept, from a technical standpoint, has definite merit as a treatment process.
However, evaluation of the system and comparison with the existing chemical conditioning
method showed that equipment capital costs and space requirements for the freeze
method are appreciably higher. Operating costs for the two methods are about equal.
Reduction of freeze conditioning operating costs by minimizing refrigeration requirements
appears feasible, but would result in appreciably increased capital costs over conven-
tional refrigeration methods, thereby aggravating an already unsatisfactory economic
situation.
Abstractor
W.C. Sh.lnlr
Institution
Hhainbeli
T?98&yE
WR:102 (REV. JULY 1969)
WRSIC
SEND WITH COPY OF DOCUMENT, TO: W*YE R~R~E"s7FU'R"C'E*S SCIENTIFIC INFORMATION CENTER
' U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 20240
* GPO! 1970-389-930
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