625478012A
           Technology Transfer
           Sludge Treatment
           and Disposal
           Sludge Treatment
           This document has not been
           submitted to NTIS, therefore it
           should be retained.

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                                    EPA-625/4-78-012
                                     October 1978
Sludge Treatment and Disposal

         Sludge Treatment
             Volume 1
           Environmental Research Information Center
               Cincinnati, Ohio 45268
                        CL

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NOTICE
  The  mention of trade names or commercial products in this publication  is for illustration
purposes, and does not constitute endorsement or recommendation for use by the  U.S.
Environmental Protection Agency.

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         Acknowledgments
  This seminar publication contains  material prepared for the  U.S. Envi-
ronmental Protection Agency Technology Transfer Program. It has been
presented at  Technology Transfer Design Seminars held at various loca-
tions throughout the United States.  The information in this publication was
prepared by the following:

Volume I, Sludge Treatment

  Introduction. Donald J. Ehreth, Office of Air, Land and Water Use, EPA,
    Washington,  D.C.; and  Dr. Joseph  B. Farrell, Municipal  Environmental
    Research Laboratory and Dr. J. E. Smith,  Jr., Environmental Research
    Information  Center, EPA, Cincinnati, Ohio.
  Lime Stabilization of Wastewater Treatment  Plant Sludges.  Richard F.
    Noland and  James D. Edwards, Burgess & Niple,  Limited, Columbus,
    Ohio.
  Anaerobic Digestion of Municipal  Wastewater Sludges. N. A. Mignone,
    Envirex Inc., Waukesha, Wis.
  Aerobic Digestion of Municipal Wastewater Sludges. N. A.  Mignone,
    Envirex Inc., Waukesha, Wis.
  Thermal Treatment for  Sludge Conditioning.  Dr. G.  M. Wesner, Gulp/
    Wesner/Culp, Santa  Ana, Calif.
  Thickening  of  Sludge. Richard  F.  Noland and Ronald B. Dickerson,
    Burgess & Niple, Limited, Columbus,  Ohio.
  Developments  in Dewatering Wastewater Sludges.  J.  R. Harrison, Con-
    sulting Environmental Engineer,  Hockessin, Del.

Volume II, Sludge Disposal

  Incineration-Pyrolysis of Wastewater Treatment Plant Sludges.  Ronald B.
    Sieger and  Patrick M. Maroney, Brown and Caldwell, Walnut Creek,
    Calif.
  Sewage Sludge Composting. Dr. G.  M. Wesner, Culp/Wesner/Culp,
    Santa Ana,  Calif.
  Principles and Design Criteria for  Sewage Sludge Application  on Land.
    Dr. L. E.  Sommers,  Purdue University, West Lafayette, Ind.; R. C.
    Fehrmann, H. L. Selznick and C. E. Pound, Metcalf and Eddy, Palo
    Alto, Calif.
  Sludge Landfilling. James J.  Walsh and Wayne M. Coppel, SCS Engi-
    neers, Reston, Va.

  In addition, G. Kenneth Dotson and Dr. J. A. Ryan, Municipal Environ-
mental Research Laboratory, EPA, Cincinnati,  ONo; and Dr. Ronald Lofy,
SCS Engineers,  Long Beach,  Calif.,  assisted in preparation of the "Land
Utilization" section of "Principles  and  Design  Criteria for Sewage  Sludge
Application on Land" chapter.  David Sussman, Office of Solid Waste,
EPA, Washington, D.C.,  was a contributor to the "Incineration-Pyrolysis of
Wastewater Treatment Plant Sludges" chapter. Donald  J. Ehreth, Office of
Air, Land and Water Use, Robert K. Bastian,  Office of Water Program
Operations, and  Bruce Weddle, Office of Solid Waste, EPA, Washington,
D.C., provided technical review of Volume II. Dr. J. E. Smith,  Jr.,  Environ-
mental Research Information Center, EPA, Cincinnati, Ohio,  provided over-
all direction, guidance in preparation, as well  as review  of the document.

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        Contents
                                                                Page

Volume 1

Introduction  	   vii

Chapter 1. Lime   Stabilization  of  Wastewater  Treatment  Plant
    Sludges  	    1

Chapter 2. Anaerobic Digestion and Design of Municipal Wastewater
    Sludges  	   35

Chapters. Aerobic  Digestion  and Design of  Municipal Wastewater
    Sludges  	   57

Chapter 4. Thermal Treatment for Sludge Conditioning	   69

Chapter 5. Thickening of Sludge   	   79

ChapterG. Review  of   Developments  in   Dewatering  Wastewater
    Sludges  	  101


Volume 2

Chapter 7. Incineration-Pyrolysis  of  Wastewater  Treatment Plant
    Sludges  	    1

Chapter 8. Sewage Sludge Composting  	   35

Chapter 9. Principles and Design Criteria  for Sewage Sludge Applica-
    tion on Land 	   57

Chapter 10.  Sludge Landfilling 	  113

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                             Introduction
Of the utilization  and disposal options available for
sludge, each has its own specific set  of environmental
problems. In order to implement any policy,  resolution to
a number of problems that  presently inhibit sludge man-
agement  must be developed. These problems can be
summarized and categorized into four  general  areas:

  •  Public  health issues.
  •  Technological factors.
  •  Intermedia issues.
  •  Social/economic/institutional  factors.
  This section briefly discusses the  seminar  publication's
contents, their evolution, and the U.S. Environmental
Protection Agency's  (EPA) sludge management research
and  development program.
  This publication is the culmination of two years of
intensive work and eight seminars that were presented
around the  nation in an  effort to develop and  dissemi-
nate the  latest information on the design of  sludge  treat-
ment and disposal systems. It presents in great detail
technical information for the following  sludge treatment
and  disposal processes:
     Lime Stabilization.
     Anaerobic Digestion.
     Aerobic Digestion.
     Thermal Sludge Conditioning.
     Thickening.
     Dewatering.
     Incineration  and Pyrolysis.
     Composting.
     Land Utilization.
     Landfilling.

  The discussion of  each process includes where possi-
ble a presentation of performance data for existing  oper-
ations as well as operation and maintenance experiences
and  energy and cost information.  Each chapter includes
one  or more design  examples to illustrate step-by-step,
the philosophy, rationale, and methodology behind the
design of the particular  process.
  The "Lime Stabilization" discussion  gives information
for determining lime  requirements  as well as a detailed
case history for a 1.0 Mgal/day (.04 m3/s) plant. Com-
parative  designs and cost information  are presented for
both the "Lime Stabilization" and "Anaerobic  Digestion"
processes in 4 (.18  m3/s) and  40 (1.175 m3/s) Mgal/day
plants. A list of installations employing lime stabilization
is  included.
  Both the  "Anaerobic  Digestion" and "Aerobic Diges-
tion"  chapters thoroughly review the pertinent parame-
ters for such biological processes and  include a design
relationship  between the percent reduction  in volatile
solids and sludge age and digestion temperature. The
"Thickening" chapter provides design examples for two
different  plant sizes which include a detailed  cost  effec-
tive analysis for choosing the alternative techniques of
gravity,  dissolved air flotation, centrifugation,  and no
thickening.  The "Dewatering" discussion includes various
schemes for designing and operating drying beds,  the
continuous belt filter presses, and both the plate and
frame and recessed  chamber pressure  filters. Standard
as well  as membrane and diaphragm pressure filters are
discussed. The electric or infrared furnace  is explained
in  the chapter on "Incineration  and  Pyrolysis," and per-
formance data are presented. The plant scale partial
pyrolysis (starved air combustion) work done at  the Con-
tra Costa County Sanitation  District  is described in de-
tail. Considerable discussion  is devoted to the use of
alternative fuels  and energy  recovery.
  The chapter on "Composting" discusses  at length the
work  on forced  aeration static pile  composting at  Belts-
ville, Md.; Bangor, Maine; and Durham, N.H.  European
developments with mechanical systems are  also  covered.
Very detailed design information applicable  to any size
system is presented  in both  the  "Land Utilization" and
"Landfilling" chapters  along  with step-by-step examples.
  EPA sludge management research and development
program encompasses four major technical  areas:  proc-
essing and treatment,  utilization, disposal, and health and
ecological effects. The primary objective of the program
is  to  develop new and improved technology and man-
agement schemes which will  enable communities to solve
problems associated with the residues or byproducts of
wastewater treatment in  a cost effective and environmen-
tally  acceptable  manner.
  The present state-of-the-art provides  adequate (but ex-
pensive) capability to dewater sludges. Incineration prac-
tice is well established with  exception of the potential
impact of air emissions on health and ecology. However,
coincineration (e.g., sludge plus  solid waste)  and pyroly-
sis technology is just emerging. Controversy continues
both within and  outside the  Agency with  regard  to the
environmental  acceptability of applying  municipal sludges
to the land. This is especially true for agricultural  uses.
Heavy metals (especially cadmium),  complex  organics,
and microbiological contaminants are the constituents of
primary concern.
  Specific examples  of technological gaps that presently

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exist are:

  •  Cost of sludge processing and disposal is a major
     factor in wastewater treatment.
  •  Methods of converting  sludge to beneficial  byprod-
     ucts are in the embryonic stages.
  •  Limited confidence exists in  the efficacy of local
     industrial pretreatment programs for metals removal
     and methods  for monitoring  their effectiveness.
  •  Relative risks associated with land application  need
     to  be  established with greater precision.
  •  Varying  climatic and soil conditions as well as vary-
     ing sludge composition require evaluation for a vari-
     ety of sludges with  optimum combinations of soil
     and vegetation.
  •  Methods for removing toxicants at the treatment
     plant are in the development stage; application is
     impeded because of economics of technology.
PROCESSING AND  TREATMENT

  Sludge must  undergo some processing or treatment to
prepare it for ultimate disposition.
  The goal of processing  and treatment research and
development  is  to produce technology alternatives which
can be  used  to prepare the  sludge for application to the
land or  for one of the conversion  processes so that the
total cost of  handling or disposal is minimized.
  Implementation of the program is focused on the fol-
lowing objectives:

  •  Evaluate the efficacy of pretreatment as  an option
     to  minimize toxicants in sludge.
  •  Characterize the nature of, and the dewatering
     properties  of, "new"  sludges  using existing,  up-
     graded and new technology.
  •  Develop hardware capable of producing  a substan-
     tially drier  sludge cake.
  •  Develop and define performance of existing and
     new processes for stabilizing  sludge  (anaerobic di-
     gestion,  auto thermal thermophilic aerobic digestion,
     composting, etc.).
  •  Investigate ways to minimize energy consumption
     while simultaneously maximizing fuel production (ac-
     tivated carbon enhancement,  solar heating, etc.).
  •  Determine  cost and environmental impact of sludge
     processing systems.
  •  Provide  guidance on technology for disinfection (up
     through  sterilization) of sludge.

CONVERSION PROCESSES

  This part of the research  program has been divided
between efforts devoted to  upgrading conventional  incin-
eration  and tasks oriented toward  development of new
processes.
  Current program objectives directed  to meeting these
needs include several  projects, ongoing and planned  to:

  •  Develop techniques for  substitution of more abun-
     dant, less  costly supplemental fuels such as coal
     and solid wastes (incineration  and co-incineration).
  •  Develop processes and  hardware for pyrolysis. co-
     pyrolysis and starved-air combustion.
  •  Characterize emissions to  determine levels  of  poten-
     tial pollutants (gaseous,  liquid, solid) contained in
     emissions from sludge conversion facilities.
  •  Establish  the "least cost"  approaches to sludge
     conversion  to the satisfaction  of administrators,
     technologists and the general  public.
  •  Evaluation of cementation  processes and other ben-
     eficial use alternatives.

LAND APPLICATION—MANAGEMENT

  The objective  relating to land application management
is to develop methods and technology to control the
transformation and/or movement of pollutants through
the soil, plants,  groundwater, and human food chain.
The  function of  research and development associated
with  the health and ecological  area is to analyze, evalu-
ate,  and  interpret the data for  purposes of establishing
safe loading rates.
  It is anticipated that accomplishment of the primary
objectives will result in the establishment of management
schemes for a variety of sludges with optimum  combina-
tions of soil and vegetation.  Practices can then be de-
fined for applying sludge to the land  for purposes  of
reclaiming marginal or sub-marginal land, determining ag-
ricultural  uses for both food  and fiber,  and landfill  dis-
posal.
HEALTH EFFECTS

  The difficulty in  resolving this issue is that data which
will permit a  definitive evaluation and decision  regarding
the significance  of sludge in  the human food chain im-
pact do not exist to the satisfaction of the several sci-
entific disciplines involved.  EPA is,  therefore, working
cooperatively with other  Federal agencies, particularly
USDA and FDA, to develop the information required to
resolve the issue.  Information developed by others, nota-
bly universities, State agencies,  and municipalities also is
being obtained.
  Some current  work directed to this issue includes:
  •  Evaluation of current knowledge of potential  health
     effects.
  •  Determine viral  contamination  of ground and  surface
     water of a  land  reclamation site.
  •  Developing  methods for isolating viruses and chemi-
     cals.
  •  Characterize type, quantity and biological  persist-
     ence of biologicals, trace  metals, and  other organic
     and inorganic substances  in the environs  of a
     sludge  disposal site.
  •  Determine the potential of  biologicals,  metals, and
     organic  substances entering the human food  chain
     when digested sludge is used  as a fertilizer.
  •  Study heavy metal uptake in beef  animals grazed
     on sludge amended pasture.

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                            Chapter 1
                            Lime  Stabilization  of  Wastewater  Treatment
                            Plant  Sludges
INTRODUCTION

Sludge constitutes the most significant  byproduct of
wastewater treatment; its treatment and disposal is per-
haps the most complex  problem which  faces  both the
designer and operator. Raw sludge contains large quan-
tities of microorganisms, mostly fecal in origin, many of
which are pathogenic and potentially hazardous to hu-
mans. Sludge processing is further complicated by its
variable properties and relatively low solids concentra-
tion. Solutions have long been sought for  better stabili-
zation and  disposal methods which are reliable and eco-
nomical and able to render sludge either inert or stable.
  Lime stabilization has  been shown  to be an effective
sludge disposal alternative when there  is a need to:

  A. Provide alternate means of sludge treatment during
     the period when existing  sludge handling facilities,
     e.g., anaerobic or  aerobic digesters,  are out of
     service for cleaning or repair.
  B. Supplement  existing sludge handling  facilities, e.g.,
     anaerobic or aerobic digesters,  incineration or heat
     treatment, due to the loss of fuel supplies or be-
     cause of excess sludge quantities above design.
  C. Upgrade existing facilities or construct new facilities
     to improve odor, bacterial, and  pathogenic orga-
     nism control.
  Lime stabilization has  been demonstrated to effectively
eliminate odors.  Regrowth of pathogens following  lime
stabilization is minimal. Of the organisms studied,  only
fecal streptococci have  a potential for  remaining viable.
  Lime stabilized  sludges are suitable for  application to
agricultural land;  however, lime stabilized sludges  have
lower soluble phosphate, ammonia nitrogen,  total Kjel-
dahl nitrogen, and total  solids concentrations than  anaer-
obically digested  primary/waste activated  mixtures  from
the same plant.
  The purpose of this chapter is to  present a review of
stabilization and disinfection of municipal wastewater
treatment plant sludges  using lime stabilization, including
specific design considerations. Two design examples in-
corporating lime stabilization into a 4 and 40  Mgal/d
(0.18 and 1.75 m3/s) wastewater treatment plant have
been included to demonstrate the design  procedure.
Comparisons of the performance, capital and annual op-
eration and maintenance costs for lime stabilization and
anaerobic digestion were included for each design exam-
ple. To further illustrate the application of lime stabiliza-
tion techniques to small plants and/or facilities in need
of an emergency sludge-handling process, an actual
case history of lime stabilization  at a  1  Mgal/d (0.04
m3/s) facility was also included.  The case history in-
cludes capital and annual operation and maintenance
costs; chemical, bacterial, and pathological properties;
and land application techniques.

LIME STABILIZATION  PROCESS
DESCRIPTION

Background

  Historically, lime has been used to treat nuisance con-
ditions resulting  from  open pit privies  and  from the
graves of domestic animals. Prior to  1970. there was
only a small amount of quantitative information  available
in the literature on the reaction  of lime  with sludge  to
make a more stable material. Since that time, the litera-
ture contains numerous references concerning  the effec-
tiveness of lime  in reducing microbiological hazards in
water and wastewater.1'3  Information is also available on
the bactericidal value of  adding  lime to sludge. A report
of operations at the Allentown, Pa., wastewater treat-
ment plants  states that conditioning an  anaerobically di-
gested sludge with lime to pH 10.2 to 11, vacuum filter-
ing and storing the cake  destroyed all odors and patho-
genic enteric bacteria.4 Kampelmacher and Jansen5 re-
ported similar experiences. Evans6 noted that lime addi-
tion to sludge released ammonia and  destroyed bacillus
coli and that the sludge  cake was a good source of
nitrogen and lime to the  land.
  Lime stabilization of raw sludges has  been conducted
in the laboratory and  in full-scale plants. Parrel et al.7
reported, among other results, that lime stabilization of
primary sludges  reduced  bacterial hazard  to a negligible
value, improved  vacuum filter performance, and providec
a satisfactory means of stabilizing sludge  prior to ulti-
mate disposal.
  Paulsrud and Eikum8 reported  on the  effects of long-
term storage of  lime stabilized sludge. Their research
included  laboratory investigations of pH and microbial
activity over periods up to 28 days.
  Pilot scale work by C.  A. Counts et al.9 on lime stabil
zation showed significant reductions in pathogen popula
tions and  obnoxious odors when the sludge pH was
greater than 12. Counts conducted growth studies on
greenhouse  and outdoor  plots which indicated that the

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Table  1-1.—Lime required for stabilization to pH 12  for  30 minutes
Sludge type
Primary sludge0
Waste activated sludge
Septage 	
Anaerobic 	

Percent
solids
3-6
1-1 5
1-45
6-7

Average lbsa
Ca(OH)2/lbs
dry solids
012
030
020
019

Range lbsa
Ca(OH)2/lbs
dry solids
006-017
0 21-043
009-0 51
014-025

Totalb
volume
treated
1 36 500
42000
27500
23500

Average
total
solids,
mg/l
43276
13143
27494
55345

Average
initial
pH
67
7 1
73
72

Average
final
PH
127
126
127
124

  Numerically equivalent to kg Ca(OH)2 per kg dry solids.
  "Multiply gallons x 3.785 to calculate liters.
  Includes some portion of waste activated sludge.
disposal of lime stabilized sludge on cropland would
have no detrimental effects.
  A research and demonstration contract was awarded
to Burgess &  Niple, Ltd. in  March 1975 to complete the
design, construction, and  operation of full-scale  lime sta-
bilization  facilities for  a 1  Mgal/d (0.4 m3/s)  wastewater
                                    treatment plant,  including land application of treated
                                    sludges.  The contract also  included funds for cleaning,
                                    rehabilitation, and operating an existing anaerobic sludge
                                    digester.  Concurrent with the research and  demonstration
                                    project, a considerable amount of full-scale  lime stabili-
                                    zation work was completed by  cities in Ohio and Con-
   I3.0
   I20-
    10 . .
   IOO- -
   9.0-
    8D- •
    7.0. -
    6.0
L^«dA^^A*^±^Al*'l^ii^£*.^_—» ^^^^x^_^_
1»7^K»^»*^K^^^^K^^     ^^^^»    ——^^—
                           	AVERAGE
                           •:W:¥:::::":¥ RANGE  OBSERVED
                                                                                                       35%PRIMARY SLUDGE

                                                                                                    3% PRIMARY SLUDGE
                                                                             jl  /  *-S% PRIMARY SLUDGE
                                                                         3% PRIMARY SLUDGE
                                                               	3.5% PRIMARY SLUDGE

                                                               	4% PRIMARY SLUDGE

                                                                    	  4.5% PRIMARY SLUDGE
                                                                    	  5% PRIMARY SLUDGE
                                                                 	  6V.PRIMARY SLUDGE
               1,000      2JDOO      3POO      4,000

                      DOSAGE Ca IOH>2 MG/L
                             5,000
                                                                 6.0
1,000      2000      3JOOO      4/300

       DOSAGE  Ca (OH)2 MG/L
                                                                                                                    spoo
Figure 1-1.—Combined lime dosage versus  pH for all       Figure 1-2.—Lime dosage versus pH  primary sludge.
sludges.

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necticut. Wastewater treatment plant capacities which
were  representative ranged from  1 to 30  Mgal/d (0.04
to 1.31  m3/s). A summary of these results has previously
been  reported.10

Lime  Requirements

  The lime dosage required to exceed pH 12 for at
least  30 minutes was found to be affected by the type
of sludge, its chemical composition, and percent solids.
As  an operational procedure,  a target pH of 12.5  was
selected to  insure that the final pH would be greater
than 12. A summary of the lime dosage required for
various  sludges  is shown in table 1-1. Of the total
amount  of lime which was  required, an excess of  0 to
50  percent was  added after pH 12 was reached  in
order to maintain the pH. Figure  1-1  shows  the com-
bined lime dosage versus pH  for primary, anaerobically
digested, waste  activated, and septage sludges. Figures
1-2 to  1-5  describe the actual lime dosages which were
required for each sludge type.
  Table 1-2 compares the Lebanon full-scale test re-
sults,  which are  described  later in the case history, with
the data previously presented  by Farrell et al., Counts,
et al., and  Paulsrud and Eikum for raw primary sludges.
In general,  excellent correlation was achieved.
  Counts9 has proposed the following equation for pre-
dicting the  lime dosage required for primary and  secon-
dary sludges from the  Richland, Wash., trickling filter
plant:

  Lime dose = 4.2+ 1.6  (TS)
  When:  Lime dose is  expressed in grams Ca(OH)2 per
    liter of sludge TS is the total  solids fraction in the
    sludge.

  Table 1-3 compares the values  predicted by the
Counts equation to the Lebanon data for raw primary,
waste activated, anaerobically digested, and septage
sludges.
  With increasing solids concentrations, the Counts
equation results in lower than actual lime dosages.

pH  Versus Time

  Previous  research has attempted to determine the
magnitude of pH  decay versus time and  to quantify the
   I30r
   I20--
   no-
   IO.O
   6.0
             2,000     4POO     6,000     8,000     IOPOO

                    DOSAGE  Ca(OH)2 MG/L
                                                           3.0 r
                                                           I2.0
             1,000      ZjOOO      3jOOO     4,000     5.00C

                    DOSAGE Ca (OH>2 MG/L
Figure 1-3.—Lime dosage versus pH anaerobic digested   Figure 1-4.—Lime dosage versus pH waste activated
sludge.                                                  sludge.
                                                                                                        3

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   i3.o r
   130
   110-
   100
 CL
   90
   8.0
   6.0
                                                         Table 1-3.—Comparison of lime dosages predicted by
                                                         the counts equation to actual data at Lebanon,  Ohio
                                              -1.5%
                                              -3%
                                              -4%
                                              -4.5%
             1,000      3,000     3,000      4,000     5,000

                    DOSAGE  Co (OH)2 MG/L


Figure 1-5.—Lime dosage versus pH septage.
Table 1-2.—Comparison of lime  dosages required to
treat raw primary sludge
               Investigator
   Lime dose,
kg lime/kg sludge
   dry solids
Burgess & Niple, Ltd (Lebanon).
Farrell et al	
Counts et al	
Paulsrud et al	
    a0.120
    b 0.098
    c 0.086
    a0.125
  "Based on pH 12.5 for sludges reported.
  bBased on pH 11.5 for sludges reported.
  cBased on 4.78% solids.
 variables which affect pH decay. Paulsrud8 reported that
 negligible pH decay occurred when the sludge  mixture
 was  raised to pH 12 or  greater or when  the lime dose
 was  approximately five times the dose to reach pH 11.
 In  either case, for raw primary sludge,  Paulsrud's dose
 was  in the range of 0.100 to 0.150 kg lime/kg dry
Sludge type
Percent
solids
Actual
lime dose,
kg lime/kg D.S.
Counts'
lime dose,
kg limeykg D.S.
Raw primary 	
Waste activated.
Anaerobically digested . . .
Septage 	

4.78
1 37
6.40
2.35

0.120
0.300
0.190
0.200

0.086
0.305
0.065
0.180

solids, which was approximately the dosage used at
Lebanon.
  Counts9 hypothesized that pH decay was caused  by
the sludge chemical demand which was exerted on the
hydroxide ions supplied in the lime slurry. He further
concluded  that the degree of decay  probably decreased
as the treated sludge  pH increased because of the ex-
tremely large  quantities of lime required to elevate the
pH to 12  or above. However, this pH phenomenon is
probably because pH  is an exponential function, e.g.,
the amount of OH~ at  pH 12  is ten times  more than the
amount of  OH~ at pH  11.
  In the full-scale work at Lebanon, all sludges were
lime stabilized to pH 12 or above and held for at least
30  minutes with the addition of excess lime. All treated
sludges had less than a 2.0 pH unit  drop after six
hours. Limed  primary sludge was  the  most stable with
septage being the  least stable.  During the full-scale pro-
gram,  only the pH  of  limed primary sludge was meas-
ured  for a period greater than 24 hours, which showed
a gradual  drop to  approximately 11.6 after 18 hours
beyond which  no further  decrease was observed.
  The total mixing  times from start through the 30  min
contact time at Lebanon were as follows:
                                                    Hours
Primary sludge	    2.4
Waste activated sludge	    1.7
Septic tank sludge	    1.5
Anaerobic digested sludge	    4.1
   Mixing time was a function of lime  slurry feed  rate
and was not  limited by the agitating capacity of  the
diffused air system. Mixing time may have been reduced
by increasing the capacity of the lime slurry tank.
   To further examine  the effects  of excess lime addition
above the levels necessary to reach pH  12, a series of
laboratory tests were  set up  using a  standard jar test
apparatus.  The tests were  made  on  six one-liter  portions
of  primary sludge with 2.7 percent total  solids. The pH
of  each of the samples was  increased to 12 by  the
addition of 10 percent hydrated lime  slurry. One sample
was  used  as a control. The remaining samples had 30
percent, 60 percent, 90 percent,  120 percent,  and 150
percent by weight of  the  lime dose added to the con-
trol.  The  samples were  mixed continuously for 6 hours
and then again 10  minutes prior  to each additional pH
measurement. There was a negligible drop in pH over a

-------
10-day period for those tests where excess lime was
added.
  A second laboratory scale test was completed using a
5 gal (19 I) raw primary sludge  sample which was lime
stabilized to pH  12.5 and  allowed to stand at 18°C.
Samples were withdrawn weekly and analyzed for pH
and bacteria concentration. The results of the pH and
bacteria studies  are shown on figures 1-6 and  1-7,
respectively. After 36 days, the pH had dropped to 12.0.
  In conclusion,  significant pH decay should  not occur
once sufficient lime  has been added to raise the sludge
pH to 12.5  and  maintain that value for  at least  30 min.

Odors

  Previous work9 stated that the threshold odor  number
of raw primary and  trickling filter sludges was approxi-
mately  8,000, while  that of lime  stabilized sludges usually
ranged from 800 to 1,300. By retarding bacterial re-
growth, the deodorizing effect can be  prolonged. Fur-
ther,  it was concluded  that by incorporating the stabi-
lized  sludge into the soil,  odor  potential should  not be
significant.
  During the full-scale  operations at Lebanon, there was
   I30p-
   e.o
   no
   O.O
   9.0
   8.0

60



> I0 20 30 40 50
                           DAYS

Figure 1-6.—Lime stabilized primary sludge pH versus
time.
an  intense odor when raw sludge was first  pumped to
the lime stabilization mixing  tank, which increased when
diffused air was applied for  mixing. As the  sludge pH
increased, the sludge odor was masked by  the odor of
ammonia which was being air stripped from the sludge.
The ammonia odor  was most intense with anaerobically
digested sludge and was strong enough to  cause nasal
irritation.  As mixing  proceeded, the treated  sludge ac-
quired a  musty humuslike odor,  with the exception of
septage which did not have a significant odor reduction
as  a  result of treatment.

Sludge Characteristics

  Several authors have previously attempted to summa-
rize the chemical  and bacterial compositions of sewage
sludges.11"13 Recent  data on  the nutrient concentrations
for various sludges  have  been  reported by  Sommers.12
Chemical and  pathogenic data  on  raw and  lime stabi-
lized  raw primary, waste activated,  septage, and anaero-
   100,000,0001

    io,ooopoo|
     ipoopoojl

      too poo I

       10,000 I
        1,000  N.

         100
          0

   100,000,000

    lopoopoo
     ipoo.ooo

      100,000
       10,000

        1,000

         100

          0
   loopoopoo

    iopoo,ooo
     IPOO.OOO

      100,000

       10,000
        I POO
         100

          0
                                                                                      FECAL STREP

                                                                                      ECAL COLIFORM

                                                                                      oTAL COLIFORM
                                                                  20f

                                                                  10!

                                                                   °I
                                                                  501

                                                                  40J

                                                                  301

                                                                  201

                                                                  104
                            t/PS. AERUGINOSA
                                                                                     ^SALMONELLA
                                                                             10
                                                                                     20
                                                                                              30
                                                                                                      40
                                                                                                               5C
                                                                                      TIME . DAYS
Figure 1-7.—Bacteria concentration versus time labora-
tory  regrowth studies.

-------
bically digested  sludges from the Lebanon, Ohio full
scale project have  been summarized below and are in-
cluded in more  detail in the case history.
  The addition  of lime  and mixing  by diffused air altered
the chemical characteristics of each sludge.  In all sludg-
es, lime stabilization resulted in an  increase  in alkalinity
and  soluble COD and a decrease  in soluble  phosphate.
Total COD and  total  phosphate decreased for all sludg-
es except waste activated. Ammonia nitrogen and total
Table 1-4.—Volatile solids  concentration of raw and lime
stabilized sludges
     Sludge type
   Raw sludge
  volatile solids,
solids concentration,
      mg/l
Lime stabilized sludge
   volatile solids,
 solids concentration,
       mg/l
Privary 	
Waste 	
Septage 	
Anaerobically digested .
732
806
695
49.6
544
542
506
37.5
Table 1-5.—Nitrogen  and phosphorus concentrations in
anaerobically digested and lime stabilized sludge
Sludge type
Lime stabilized primary 	
Lime stabilized waste activated.
Lime stabilized septage 	
Anaerobic digested

Total
phosphate
as P, mg/l
283
263
134
580

Total
Kjeldahl
nitrogen
as N, mg/l
1,374
1,034
597
2731

Ammonia
nitrogen
as N, mg/l
145
53
84
709

Kjeldahl nitrogen decreased  for all sludges except waste
activated.
  The  volatile solids concentrations of raw and lime sta-
bilized sludges are shown in table 1-4. The  actual vola-
tile solids concentrations following lime stabilization are
lower than those which  would result only from the addi-
tion  of lime.  Neutralization, saponification, and hydrolysis
reactions  with the lime probably result in the lower vola-
tile solids concentrations.
  In terms of the agricultural value, lime stabilized sludg-
es had lower soluble phosphate,  ammonia nitrogen, total
Kjeldahl  nitrogen, and total  solids concentrations  than
anaerobically digested primary/waste  activated mixtures
from the  same plant, as shown in table  1-5. The signifi-
cance of these  changes is discussed in the  section on
land disposal.
  Considerable  research has been conducted on  the de-
gree of bacterial reduction which can be achieved by
high lime doses.14'15 In general, the degree of pathogen
reduction increased as  sludge pH increased  with  consist-
ently high pathogen reductions occurring only after the
pH reached  12.0. Fecal streptococci  appeared to resist
inactivation by lime treatment particularly well in the low-
er pH values; however,  at pH 12, these organisms were
also inactivated after  1  hour of contact time.9
   In all lime stabilized sludges, Salmonella and Pseudo-
monas aeruginosa concentrations were reduced to near
zero. Fecal and total  coliform concentrations were re-
duced greater than 99.99 percent in  the primary  and
septic sludges.  In waste activated sludge, the total and
fecal coliform concentrations decreased 99.9 percent
and 99.94 percent,  respectively. The  fecal streptococci
kills were as follows:  primary sludge, 99.93  percent;
waste activated  sludge, 99.41 percent; septic sludge,
99.90  percent;  and anaerobic digested,  96.81  percent.
   Pathogen  concentrations  for the lime stabilized sludges
are  summarized in table 1-6.
   Anaerobic digestion is currently an acceptable  method
of sludge stabilization.16 For reference,  lime  stabilized
sludge pathogen concentrations at Lebanon have been
compared in table 1-6  to those  observed for well di-
gested sludge from the same plant.
Table 1-6.—Comparison of  bacteria in  anaerobic digested  versus  lime
stabilized sludges

Anaerobically digested . .
Lime stabilized3
Primary
Waste action
Seotaae

Fecal
conform
#/100 ml
1.450X103
4x103
16X103
265

Fecal
streptococci
#/100 ml
27X103
23X103
61 x 1 03
665

Total ,
coliform
#/100 ml
27,800 X103
27.6 X 1 03
212 X103
2,100

Salmonella
#/100 ml
6
b3
3
3

Ps.
aeruginosa
#/100 ml
42
3
13
3

  aTo pH equal to or greater than 12.0.
  bDetection limit = 3.

-------
  Pathogen concentrations in lime stabilized sludges
range from  10 to  1,000 times less than  for anaerobically
digested sludge.
  A pilot scale experiment was  completed in  the labora-
tory to  determine  the viability and regrowth potential of
bacteria in lime stabilized primary sludge over an ex-
tended  period of time.
  The test was  intended to simulate storing stabilized
sludge in a holding tank or lagoon when weather condi-
tions prohibit  spreading.  In the  laboratory test, 5 gal (19
I) of 7 percent raw sludge from the Mill  Creek sewage
treatment plant in  Cincinnati  were lime stabilized to pH
12.0. Lime was  added until equivalent to 30 percent of
the weight of  the  dry solids  which  resulted in a final pH
of 12.5. The sample was then covered with foil and kept
at room temperature 65° F (18.3°C) for the remainder  of
the test. The  contents were  stirred before samples  were
taken for bacterial analysis.
  The results, shown on figure  1-7, indicate that  a hold-
ing period actually increases the bacteria kill.  Salmonella
in  the raw sludge  totaling 44 per 100 ml were reduced
to the  detection limit by lime stabilization. Pseudomonas
aeruginosa totaling 11  per 100  ml in the raw sludge
were reduced to the detection limit  by lime stabilization.
The initial fecal  coliform  count of 3.0xi07was reduced
to 5xi03after lime stabilization, and after 24 hours was
reduced to  less than 300.  The raw sludge contained
3.8 X108 total  conforms, but  24  hours  after lime stabiliza-
tion the  coliform total was less  than 300. The fecal strep
count in the raw sludge  was 1.8 x 108 which decreased
to 9.6 X104 after lime stabilization. After 24 hours,  the
count was down to 7.0xl03and after 6 days reduced
to less  than 300.  The count  increased to 8 x 105 after  40
days.
     20 r
     I5--
     IO--
                  10
                             20

                           TIME-DAYS
                                  -H-
                                   25
                                         30
                                               35
                                                    40
Figure 1-8.—Dewatering characteristics of various
sludges on sand drying beds.
Sludge  Dewatering Characteristics

  Farrell et  al.7 have previously reported on the dewater-
ing characteristics of ferric  chloride and alum treated
sludges which were  subsequently treated with lime.  Trub-
nick and Mueller17 presented, in detail, the procedures to
be  followed in conditioning  sludge  for filtration,  using
lime with and without ferric chloride. Sontheimer18 pre-
sented information on the improvements in sludge filtera-
bility produced by lime  addition.
  Standard  sand drying beds, which were located at the
Lebanon,  Ohio wastewater treatment plant,  were used
for  sludge dewatering comparisons. Each bed was 30
ft x 70 ft (9.2 x 21.5  m). For the study,  one bed was
partitioned to form two, each 15 ftx70 ft (4.6x21.5 m).
Limed  primary sludge was applied  to one bed with  limed
anaerobically digested sludge being applied to the other
side. A second  full-sized bed was  used  to dewater  un-
limed anaerobically digested sludge. The results of the
study are summarized on figure 1-8.
  Lime stabilized sludges generally dewatered at a lower
rate than  well digested  sludges. After 10 days,  lime sta-
bilized primary sludge had dewatered to approximately
6.5 percent  solids as opposed  to 9 percent for lime
stabilized anaerobically  digested sludge, and 10 percent
for  untreated anaerobically digested sludge.
  The anaerobically digested sludge cracked first and
dried more rapidly than either of the lime stabilized
sludges. Initially, both of the lime  stabilized sludges mat-
ted, with the digested sludge cracking  after approximate-
ly 2 weeks. The lime stabilized primary sludge did not
crack which hindered drying and  resulted in the lower
percent solids values.

Land Application
  Numerous references  are available regarding the appli-
cation of anaerobically  digested sludges to agricultural
land.11'12'16'19 The application of sewage  sludge on  land
has generally been viewed from two standpoints,  either
as a rate of application consistent with the utilization of
nutrients in sludge by growing plants (i.e., agricultural
utilization),  or as the maximum amount of sludge  applied
in a minimum amount of time (i.e., disposal only).
USEPA  guidelines16 generally favor the  former approach.
The successful  operation of a program utilizing  the appli
cation of sewage sludge on  land  is dependent upon  a
knowledge  of the particular sludge,  soil, and crop char-
acteristics.

-------
  Organic matter content, fertilizer nutrients, and trace
element concentrations are generally regarded as being
vital to the evaluation of the applicability of land applica-
tion  of sewage sludge. The range of nitrogen, phospho-
rus,  and  potassium concentrations for sewage sludges
have been reported by Brown et al.11
  Sommers12 has also summarized fertilizer recommenda-
tions for  crops based primarily on the  amount of major
nutrients  (nitrogen, phosphorus, and  potassium)  required
by a crop and on the yield desired.
  Counts9 conducted greenhouse  and test plot studies
for  lime stabilized  sludges which  were  designed  to  pro-
vide information on the response of  plants grown in
sludge-soil mixtures ranging in application rate from 5 to
100  tons/acre (11 to 220 Mg/ha). Counts concluded
that sludge addition to poor, e.g., sandy,  soils would
increase  productivity,  and therefore would be beneficial.
The  total  nitrogen and phosphorous  levels in plants
grown  in greenhouse  pots, which contained sludge-soil
mixtures, were consistently lower than  plants which were
grown  in control  pots. The control set,  which  contained
only soil  with  no sludge  additions, received optimum
additions of chemical fertilizer during the actual  plant
growth phase  of  the studies. Calcium concentration in
plant tissues from the  sludge-soil pots  were higher  than
those for the  controls. The  pH values  of the various
sludge-soil mixtures were lower after plant growth than
before. Counts attributed the decrease  to carbon dioxide
buildup in the soil which resulted from  biological activity.
  Land application studies at  Lebanon, Ohio, were  con-
ducted by spreading liquid sludge on agricultural land
and on controlled test plots. Winter  wheat, soybeans,
and  hay  were grown  on  fields which were in normal
agricultural production. Corn, swiss chard,  and soybeans
were grown on 22 test plots,  each with an  area of
0.021 acre (0.0085 ha). A preliminary report on  the re-
sults of the land  application studies  will be  published in
1978.29
  Sludge application was accomplished by spreading as
a liquid using  a four-wheel drive  vehicle which was
equipped  with a 600  gal (2.3  m3) tank.  The width of
sludge spread per pass  was approximately 24 in. (60
cm).
  The  lime stabilized  sludge formed  a  filamentous mat
1/8-1/4  in (0.3-0.6 cm) thick which, when dry, partly
choked out the wheat. The mat partly  deteriorated  over
time, but significant portions remained  at the time of
harvest. There was no matting on the  fields where  the
lime stabilized sludge was incorporated into the soil be-
fore planting.
  Spontaneous growth of tomatoes was significant  in the
fields which had  lime stabilized sludge  incorporated into
the soil before planting.  Seeds were contained in the
sludge and were not  sterilized by the lime. These plants
were absent at the site where the sludge was not  incor-
porated,  even though no herbicide was applied, probably
because  of frequent frosts and the lack of sludge  incor-
poration  into the soil.  During the next  year's growing
season, an increase in insect concentration was noticed
on  the fields which had  received lime  stabilized sludge.
LIME STABILIZATION DESIGN
CONSIDERATIONS

Overall Design Concepts

  Lime and  sludge are  two of the most difficult materials
to transfer,  meter, and  treat in any wastewater treatment
plant. For these reasons,  design  of stabilization facilities
should emphasize simplicity, straightforward piping layout,
ample space for  operation and maintenance of equip-
ment, and gravity flow wherever  possible. As discussed
in more detail in  the following sections,  lime transport
should be by auger with the slurry or slaking operations
occurring at the  point  of  use. Lime slurry pumping
should be avoided with transport being by gravity in
open channels. Sludge  flow to the tank truck and/or
temporary holding lagoon should also be by gravity if
possible.
  Figures 1-9,  1-10, and 1-11 show conceptual designs
for lime stabilization facilities at wastewater treatment
facilities  with 1, 5, and 10 Mgal/d (0.04, 0.22 and 0.44
m3/s) throughputs. The 1  Mgal/d (0.04 m3/s) plant, as
shown on figure  1-9, utilizes hydrated  lime and a simple
batch mixing tank, with capability to  treat all sludges  in
less than one shift per day. Treated  sludge  could be
allowed to settle for several hours before hauling in
order to thicken, and thereby reduce the volume hauled.
Alternately,  the sludge  holding  lagoon could be used for
thickening.
  Figure 1-10 shows the conceptual  design for lime
stabilization facilities of a 5 Mgal/d (0.22 m3/s) waste-
water treatment facility. Pebble lime is  utilized in this
installation.  Two  sludge mixing tanks  are provided, each
with the capacity to treat the total sludge production
from two shifts. During the remaining shift,  sludge could
be thickened and hauled  to the  land disposal site. Alter-
nately, the temporary sludge lagoon could be used  for
sludge thickening.
   Figure 1-11  shows the conceptual  design for lime
stabilization facilities of a 10 Mgal/d (0.44  m3/s) waste-
water treatment plant.  A continuous lime treatment tank
with 2 hours detention  time is  used to raise the sludge
pH to 12. A separate  sludge thickening tank is provided
to increase the treated sludge solids content before land
application. Sludge transport is assumed to  be  by  pipe-
line to the land disposal site. A  temporary  sludge hold-
ing  lagoon was assumed  to be necessary,  and would
also be  located at the land disposal  site.

Lime Requirements
  The quantity  of lime which will be required to raise
the pH of municipal wastewater sludges to pH greater
than 12 can be estimated  from the data presented in
table 1-1 and from figures 1-2 to 1-5. Lime dosages
have been shown as 100 percent Ca(OH)2and should be
adjusted for the  actual type of lime used.  Generally, the
lime requirements for primary and/or waste  activated
sludge will be in the range of 0.1 to 0.3 Kg 100 percent
Ca(OH)2 per Kg of dry  sludge solids. Laboratory jar test-
ing can confirm the dosage required  for existing sludges.

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w~-
                                       DUST COLLECTOR
             FILL
             PIPE
                                                                      TAMK- TRI irkr
                                                                      TANK TRUCK
                                                                                LAGOON
                                                   SLUDGE FROM LAGOON
Figure 1-9.—Conceptual design for lime stabilization facilities for a
1 Mgal/d  (0.04 rrvVs) treatment plant.
                                             DUST COLLECTOR
                                      WATER J TURBINE AGITATORS
                                                                   TREATED SLUDGE
                                                                   TO LAGOON   ^
                                                                                   TANK TRUCK
                                                                                        LAGOON
                                                                   SLUDGE FROM LAGOON
Figure 1-10.—Conceptual design for lime stabilization facilities for a
5 Mgal/d (0.22 m3/s) treatment plant.

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INFLUENT
SLUDGE
                     PEBBLE
                    I LIME
                     STORAGE
                     BIN
                             =0--
                                     •DUST COLLECTOR
^BUILDING


 AUGERS



 • LIME SLAKERS/FEEDERS

 MECHANICAL TURBINE AGITATOR

 MIX TANK WITH 2 HOUR
 DETENTION TIME
DECANT TO
PRIMARY
INFLUENT
                                                                                            TANK TRUCK
00	00
                                                                           TREATED SLUDGE
                                                                           TO LAGOON  *-
                                                                                               LAGOON
                                                                                SLUDGE FROM LAGOON
Figure 1-11.—Conceptual design for lime stabilization facilities for a
10  Mgal/d (0.44  m3/s)  treatment plant.
Types of  Lime  Available

  Lime  in its various forms, as quicklime and hydrated
lime,  is  the  principal, lowest  cost alkali. Lime is a gener-
al term,  but  by strict definition,  it only embraces burned
forms of lime—quicklime,  hydrated  lime, and hydraulic
lime.  The two forms of particular interest  to lime stabili-
zation,  however, are quicklime and hydrated lime. Not
included are carbonates (limestone or precipitated calci-
um carbonate) that are occasionally but erroneously re-
ferred to as "lime."20

Quicklime
  Quicklime  is the product resulting from  the calcination
of limestone and to  a lesser extent shell. It  consists
primarily of  the oxides  of calcium and magnesium.  On
the basis of their chemical analyses, quicklimes may be
divided  into  three  classes:
  1.  High calcium quicklime—containing less than 5 per-
      cent magnesium oxide,  85-90 percent CaO
  2.  Magnesium quicklime—containing 5 to 35 percent
      magnesium oxide, 60-90 percent CaO
  3.  Dolomitic quicklime—containing 35 to  40 percent
      magnesium oxide, 55-60 percent CaO
                       The magnesium quicklime is relatively rare in the Unit-
                     ed States and, while available in a few localities, is not
                     generally obtainable.
                      ' Quicklime is available  in a number of more or less
                     standard sizes, as follows:

                       1.  Lump lime—the product with a maximum size of 8
                          in (20 cm) in  diameter down to 2 in (5.1 cm) to 3
                          in (7.6 cm)  produced in vertical kilns.
                       2.  Crushed or  pebble lime—the most common form,
                          which  ranges  in size from about 2-1/4 in  (5.1-0.6
                          cm), produced in most  kiln types.
                       3.  Granular lime—the product obtained from Fluo-Sol-
                          ids kilns that has a particulate size range of  100
                          percent passing a  #8 sieve and  100 percent re-
                          tained on a #80  sieve (a dustless  product).
                       4.  Ground lime—the  product resulting  from  grinding
                          the larger sized material and/or passing  off the  fine
                          size. A typical  size is substantially all passing a  #8
                          sieve and 40 to 60 percent passing  a  #100 sieve.
                       5.  Pulverized lime—the  product resulting from a more
                          intense grinding that is used to produce  ground
                          lime. A typical size is substantially all passing a
                          #20 sieve  and 85 to 95 percent  passing a  #100
                          sieve.
     10

-------
  6. Pelletized  lime—the product made by compressing
     quicklime fines into about 1-inch size pellets or
     briquettes.

Hydrated Lime

  As defined by the American Society for Testing and
Materials, hydrated lime is: "A dry  powder obtained by
treating quicklime  with water enough to satisfy its chemi-
cal  affinity for water under the conditions of its hydra-
tion."
  The  chemical composition of hydrated lime  generally
reflects the composition of the quicklime  from which it is
derived. A high calcium quicklime will produce a high
calcium hydrated  lime obtaining  72  percent to 74 per-
cent calcium oxide and 23 percent to 24 percent water
in chemical  combination with the calcium oxide.  A do-
lomitic quicklime will produce  a dolomitic hydrate. Under
normal hydrating  conditions, the calcium  oxide fraction
of the  dolomitic quicklime completely hydrates, but gen-
erally only a small portion  of the magnesium oxide hyd-
rates (about  5  to  20  percent). The composition of a
normal dolomitic hydrate will be 46  percent to 48 per-
cent calcium oxide, 33  percent to 34 percent magnesium
oxide,  and 15 percent to 17 percent water in chemical
combination with  the  calcium oxide. (With some  soft-
burned dolomitic  quicklimes, 20  percent to 50 percent of
the  MgO  will hydrate.)
  A "special" or  pressure  hydrated  dolomitic  lime is also
available.  This lime has almost all (more  than 92 per-
cent) of the  magnesium oxide hydrated;  hence,  its water
content is higher  and its oxide content lower than the
normal dolomitic hydrate.
  Hydrated lime is packed in  paper bags weighing 50 Ib
(22.7 kg)  net; however, it is also shipped in bulk.
  Quicklime  is obtainable in either bulk carloads or tank-
er trucks  or  in 80 Ib  (36.4 kg) multiwall  paper bags.
Lump,  crushed, pebble, or pelletized lime, because of
the  large  particle sizes, is  rarely  handled  in bags and is
almost universally  shipped  in bulk. The finer sizes of
quicklime, ground, granular, and  pulverized, are readily
handled in either  bulk or bags.

Lime Storage and Feeding
  Depending on the type of lime, storage and feeding
can be either in bag  or bulk.  Bagged lime will probably
be  more economical for treatment plants  less than one
Mgal/d (0.04 m3/s) and for temporary or emergency
feed systems, e.g., when a digester is out of  service for
cleaning  and repair.  In  new facilities, bulk storage will
probably be cost  effective. Storage  facilities should  be
constructed  such  that dry lime is conveyed to the  point
of use and then mixed  or slaked. Generally, augers are
best for transporting either hydrated or pebble lime. Au-
ger  runs should be horizontal  or  not exceeding  an in-
cline of 30°.27
  The  feeder facilities, i.e., dry feeder and slaking or
slurry tank, should be located adjacent to the stabiliza-
tion mixing tank such  that lime slurry  can flow by gravity
in open channel troughs to the point  of mixing. Pumping
lime slurry should  be avoided.  Slurry transfer distances
should be kept to a minimum.  Access to feeder, slaker
and/or slurry  equipment should be adequate for easy
disassembly and maintenance.
Mixing

  Lime/sludge  mixtures can be mixed either with  me-
chanical  mixers or with diffused air. The level of agita-
tion should be  great enough to keep sludge solids sus-
pended and dispense the lime slurry evenly and rapidly.
The principal difference between the resultant lime stabi-
lized sludges in both cases is that ammonia will be
stripped  from the sludge with diffused air mixing.  Me-
chanical  mixing has  been used by previous researchers
for lime stabilization  but only  on the pilot scale.
  With diffused  air mixing, adequate ventilation  should
be provided to dissipate odors generated during mixing
and stabilization. Coarse bubble diffusers should be used
with  air supplies in the range of 150-250 ftVmin  per
1,000 ft3 (150-250 rrvVmin per 1,000 m3) of mixing tank
volume. Diffusers should be mounted such that  a  spiral
roll  is established in the mixing tank away from the point
of lime slurry application. Diffusers should  be accessible
and piping should be kept against the tank wall to  mini-
mize the collection of  rags, etc. Adequate piping  support
should be provided.
  With the design of mechanical mixers, the bulk  velocity
(defined  as the turbine agitator pumping capacity divided
by the cross sectional area of the mixing vessel)  should
be in the range of 15 to 26 ft/min  (4.6 to  7.9  m/min).
Impeller Reynolds numbers should exceed  1,000 in order
to achieve a constant power  number.21 The mixer  should
be specified according  to the standard motor horsepow-
er and AGMA  gear ratios in order to  be commercially
available.
  For convenience, table  1-7 was completed which
shows a series  of tank and mixer combinations which
should be adequate  for mixing sludges up to 10 percent
dry solids,  over a  range of viscosity, and Reynolds  num-
ber combinations which were  as follows:

  Max. Reynolds number  10,000 at  100  cp sludge vis-
    cosity
  Max. Reynolds number   1,000 at  1,000 cp sludge  vis-
    cosity

  Table 1-7 can be used to select  a  mixer horsepower
and standard AGMA gear combination depending  on the
volume of sludge to  be stabilized. For example, for  a
5,000 gal (1.9  m3) tank, any of the  mixer-turbine combi-
nations should  provide adequate mixing. Increasing tur-
bine  diameter and decreasing shaft  speed  results  in a
decrease in horespower requirement as shown.
  Additional assumptions were that the bulk fluid velocity
must exceed  26 ft/min  (7.9 m/min), impeller Reynolds
number must exceed 1,000, and that power requirements
                                                                                                         11

-------
Table 1-7.—Mixer specifications for sludge slurries
Tank
size,
liters
18925


56 775



1 1 3 550



283 875 	



378 500


Tank Prime mover,
diameter, hp/shaft
meters speed, r/min
2.9 7.5/125
5/84
3/56
4.2 20/100
15/68
10/45
7.5/37
5 3 40/84
30/68
25/56
20/37
7.2 100/100
75/68
60/56
50/45
8.0 125/84
100/68
75/45
Turbine
diameter,
centimeters
81
97
109
114
135
160
170
145
155
168
206
157
188
201
221
183
198
239
ranging from 0.5-1.5 hp/1,000  gal (0.5-1.5  hp per 3,785
I) are necessary.  The mixing  tank configuration assumed
that the liquid depth equals tank diameter and that baf-
fles  with a width  of 1/12 the tank diameter were placed
at 90° spacing. Mixing theory and equations which were
used were after Badger,21 Hicks,22 and Fair.23
Raw  and Treated  Sludge Piping, Pumps,
and  Grinder

  Sludge piping design should include allowances for
increased  friction  losses due to the non-Newtonian pro-
perties of  sludge.  Friction  loss calculations  should be
based  on  treated  sludge solids concentrations and
should allow for thickening in the mixing  tank after stabi-
lization. Pipelines  should not be less than 2 in (5.1 cm)
in diameter and should  have tees in major  runs at each
change in direction to permit rodding, cleaning, and
flushing the lines.  Adequate drains should be provided. If
a source of high  pressure water is  available (either non-
potable or noncross-connected potable),  it  can  be used
to  flush and clean lines.
  Spare pumps should be provided and mounted  such
that they can be  disassembled easily. Pump impeller
type materials of  construction  should be  adequate for
the sludge solids  concentration and pH.
  Sludge grinding equipment should be used to make
the raw sludge homogenous.  Sticks, rags, plastic, etc.,
will be broken up prior  to lime stabilization to improve
the sludge mixing and flow characteristics and to elimi-
nate unsightly conditions at the land disposal site.
A CASE HISTORY OF LIME
STABILIZATION

Background
  Facilities for lime stabilization of sludge were incorpo-
rated  into an existing 1.0  Mgal/d (0.04  m3/s) single
stage activated sludge wastewater treatment plant locat-
ed at Lebanon, Ohio. Lebanon  has a population of
about 8,000 and is located in southwestern Ohio, 30 mi
(48 km) northeast  of Cincinnati. The surrounding area is
gently rolling farmland  with a small number of light in-
dustries,  nurseries, orchards,  and truck  farms.
  Major unit processes at the wastewater treatment plant
include influent pumping, preaeration, primary clarifica-
tion, conventional activated sludge, and anaerobic sludge
digestion. Average influent BOD5 and suspended solids
concentrations are 180 and  243 mg/l,  respectively. The
treatment plant flow schematic  is shown on figure 1-12.
  Prior to completing the  sludge  liming  system, the exist-
ing anaerobic  sludge digester was inoperative and was
being used as a sludge holding tank. The digester pH
was approximately 5.5 to  6.0. Grit and  sand accumula-
tions had reduced its effective  volume to 40-50 percent
of the total. Waste activated sludge was being returned
to the primary clarifiers and resettled with the primary
sludge. Combined  primary/waste  activated sludge was
being pumped to the digester and ultimately recycled to
the primary clarifiers via the  digester supernatant.  Typi-
cal supernatant suspended solids concentrations were in
the range of 30,000 to 40,000  mg/l. When possible,
sludge was withdrawn  from  the digester and  dewatered
on sand  drying beds.
   USEPA made the  decision to utilize  lime stabilization
at Lebanon not only as a full scale  research and  dem-
onstration project, but also as  a  means of solids han-
dling during the period while the anaerobic digester  was
out of service for  cleaning and repair.

Revisions to  the Existing Wastewater
Treatment Plant

Lime Stabilization
   The  lime stabilization process was designed to treat
raw primary, waste activated, septic tank, and  anaerobi-
cally digested sludges. The  liming system was integrated
with the  existing treatment plant  facilities, as shown  on
figure 1-13. Hydrated  lime was stored  in a bulk storage
bin and  was augered into a  volumetric feeder. The feed-
er transferred dry lime at a constant rate into a 25  gal
(95 I) slurry tank which discharged an  8-10 percent lime
slurry by gravity into an existing  6,500  gal (25 m3) tank.
The lime  slurry and  sludge were  mixed with diffused air.
A flow schematic  for the  lime stabilization facilities is
shown on figure 1-14.  Design  data  are shown in table
1-8.

Septage Holding Facilities
   Because the Lebanon wastewater treatment plant  rou-
tinely accepted septic tank  pumpings,  a 5,000 gal (18.4
     12

-------
                PREAERATION
PRIMARY
CLARIFER





AERATION





FINAL
CLARIFIER
_J
FINAL
                                   CLARIFIER
                                                 J
                                                       AERATION
                                                  RETURN SLUDGE
                                        WASTE ACTIVATED SULDGE

SUPERNATANT
REAERATION
TANK
\J"@
SLUDGE
WELL
SUPERNATANT





ANAEROBIC
DIGESTER
                                                                       CLARIFIER
                                                                                    FINAL
                                                                                    EFFLUENT \
                                                                                              CREEK
Figure 1-12.—Treatment plant  flow  schematic prior to incorporating lime
stabilization.
                           VOLUMETRIC FEEDER

                           LIME SLURRY TANK
                           WATER
                              PRIMARY AND/OR
                              WASTE ACTIVATED
                              SLUDGE
Figure 1-13.—Treatment plant  flow schematic after incorporating  lime  sta-
bilization.
                                                                                                                13

-------
                                                                       ANAEROBIC
                                                                       DIGESTED SLUDGE
                                                                        PRIMARY
                                                                        SLUDGE
                                                                        WASTE
                                                                        ACTIVATED SLUDGE;
                                               -VOLUMETRIC FEEDER
                                                                       SLUDGE
                                                                       WELL AND
                                                                       PUMP
                                                                                   TREATED

                                                                                   SLUDGE
                                                                                   TANK TRUCK FOR
                                                                                   LAND DISPOSAL
                                                                                                      CO	00
Figure 1-14.—Lime stabilization process flow diagram.
m3) tank was installed to hold septic tank sludges prior
to lime treatment. The tank was equipped with a transfer
pump which could be used to either feed the lime stabi-
lization process or transfer septage to the primary tank
influent  at a controlled rate.

Ultimate Sludge Disposal
  Treated sludges were applied  to sand drying beds, to
test plots, and to three  productive agricultural sites.
Land  spreading operations began in early March and
continued through October. The sludge hauling vehicle
was a four-wheel drive truck  with a  600 gal  (2.3 m3)
tank.

Anaerobic  Digester
  As  previously described,  the existing  single stage an-
aerobic  sludge digester was  inoperative and  was being
used  as  a sludge holding tank.  The  digester and auxilia-
ry equipment were completely renovated and returned to
good  operating condition which  allowed a comparison of
anaerobic digestion and  lime stabilization. The digester
was cleaned, a new boiler and  hot water circulating
system were  installed, and all necessary repairs were
made to piping,  valves,  pumps,  and  electrical equipment.
  The anaerobic digester design data are shown in  table
1-9.
Operation and Sampling
  Raw sludge, e.g., primary, waste activated, septage or
digested sludge, was  pumped  to  the mixing tank where
it was mixed by diffused air. Four coarse bubble diffus-
ers were mounted approximately  1  ft (30.5 cm) above
the top of the tank hopper  and 1.25 ft (38 cm) from the
tank wall. This  location permitted mixing to roll sludge
up and across  the tank at which point lime slurry was
fed.  Lime which was  used for the stabilization  of all
sludges was industrial grade hydrated  lime with CaO and
MgO contents of 46.9 percent and 34 percent, respec-
tively. All lime requirements have been converted and
are expressed as 100 percent Ca(OH)2 except  as  noted.
Samples were taken from the  untreated, but thoroughly
mixed, sludge for chemical,  pH, bacteria, and parasite
analyses.
  After  the initial pH  determination, the lime slurry addi-
tion  was started. Hydrated lime was augered from the
lime storage bin to the volumetric feeder which was
located directly  above the sludge mixing tank.  The lime
was slurried by  the tangential  injection of water into  a
     14

-------
Table 1-8.—Design data for lime stabilization facilities
                                                          After 30 minutes, samples were  taken for chemical, bac-
                                                          teria, and parasite  analyses. Air mixing was then  discon-
                                                          tinued, allowing the limed sludge to concentrate.  The
Mixing tank                                                 sludge then flowed by  gravity to a sludge well from
  Total volume	  30 m3 (8,000 gal)                     wnicn it  was pumped to  the  land  disposal truck.
  working volume	  25 m (6.500 gal)                       Samples of raw and  treated Lebanon sludges were
                       3.05 mX3^66 mx2.38 m (10'x12 X7.8)  ^ *      egch       t|    da  of tne |ime stabilization
                       co9ars: Si£       '                operations. Anaerobically dFgested sludge samples were
  Number of diffusers      4                                  taken at the same  time and analyzed  for use in  compari-
  Air supply	"""  14-34 m3/min (500-1,200 ft3/min)        sons of chemical, bacterial,  and pathogen properties.
Bulk lime storage                                               Sample preservation  and chemical analysis techniques
  Total volume	  28 m3 (i.ooo ft3)                      were performed in  accordance with procedures as  stated
  Diameter	  2.74 m (9')                          in  "Methods for Chemical Analysis of  Water and  Wastes,
  vibrators	  2 ea Syntron v-41                     USEPA,"24 and "Standard Methods for the Examination
  FIN system	  Pneumatic                           of  Water and Wastewater.' '25
  Discharge system	  15 cm (6") dia. auger                   Salmonella species and Pseudomonas aeruginosa were
  Material of construction..  steel        . .  _ „                 determined by EPA staff using the method developed by
voLmethc feeder     e"'                                     Kenner  and Clark.26 Fecal coliform, total coliform, and
  Total volume            028 m3(io cu ft)                     fecal streptococcus were  determined according to meth-
  Diameter              71 cm (28")                         ods specified in Standard Methods for Examination of
  Material of construction.,  steel                               Water and Wastewater.
  Type and manufacturer..  Vibrascrew LBB 28-10
  Feed range	  45-227 kg/hr (100-500 Ib/hr)
  Average feed rate	  78 kg/hr (173 Ib/hr)
Lime slurry tank                                                Chemical data for Lebanon, Ohio  raw primary, waste
  Total volume	  94.6 i (25 gal)                       activated, anaerobically digested, and  septage  sludges
  Diameter.......        0.61 m (2')                          hgve been summarizecj  in table  1-10.  Data for each
    toM^JSSkT9                                      parameter  include the average and range of the values
  Total volume	  18.4 m3 (650 ft3)                      observed.
  working volume	  15 m3 (4,000 gal)                       Analyses for heavy metals were conducted on grab
  Dimensions	  3.66 mxi.92 mx2.62 m               samples  of raw primary, waste activated,  and anaerobi-
                       O2'x6.3'x8.6')                      cally digested sludges.  These data have been  reported
  Mixing	  Coarse bubble                       in  table 1-11  as mg/kg on  a dry weight basis and
  Number of diffusers	  1                                  include  the average and  range of values.
  Air supply	  2.8-8.4 m3/min (100-300 ft3/min)          Pathogen data for  Lebanon, Ohio raw primary,  waste
Transfer  pumps                                              activated, anaerobically digested, and  septage sludges
  Raw and treated sludge.  1 136  I/mm (300 gpm)                 nave been summarized jn table  -,_12.  |n general,  the
  Septage transfer pump...  379 I/mm (100 gpm)	  ^ ^ jn agreement  wjth  the  va|ues reported by
                                   "                      Stern,14 with the exception of  Salmonella and  Pseudomo-
                                                          nas aeruginosa, which  are lower than the reported val-
Table 1-9.—Anaerobic digester  rehabilitation design data  ues-
Tank dimensions	
Total volume	
Actual volatile solids loading..
Hydraulic detention time	
Sludge recirculation rate	
Boiler capacity	
                           15 m (50') dia. x 6.1  m (20') SWD
                           1,223 m3 (43,200 ft3)
                           486 g VSS/m3 (0.03  Ib VSS/ft3)
                           36 days
                           757 1/min (200 gpm)
                           2.53 X108 Joules/hr (240,000 Btu/hr)
25  gal (95 I) slurry  tank. The lime solution (8-10 percent
by  weight) then flowed  by  gravity into an  open channel
with three feed points into the sludge mixing tank.
  The sludge pH  was checked every 15 minutes  as the
lime slurry was added until the sludge reached a pH of
12, at which time it was held for 30 minutes.  During the
30  minute period, lime slurry continued to be  added.
Lime Stabilized Sludges
  Chemical  and bacterial data for lime  stabilized sludges
have previously been summarized  in the general discus-
sion on lime stabilization. Specific data from the Leba-
non, Ohio full scale project  have been  summarized in
tables  1-13 and 1-14. Lime stabilized sludges had lower
soluble phosphate, ammonia nitrogen, total Kjeldahl nitro-
gen, and total  solids concentrations than anaerobically
digested  primary/waste  activated mixtures from the same
plant.
  In all lime stabilized sludges, Salmonella and  Pseudo-
monas aeruginosa concentrations were  reduced to  near
zero.  Fecal  and total coliform concentrations were  re-
duced greater  than  99.99 percent in the primary and
septic  sludges.  In waste activated sludge, the total and
fecal coliform concentrations decreased 99.99 percent
                                                                                                            15

-------
Table  1-10.—Chemical  composition of raw sludges at
Lebanon, Ohio
Table 1-11.—Heavy metal concentrations in  raw sludges
at Lebanon,  Ohio
Parameter
(mg/l)
Alkalinity 	
Alkalinity range 	
Total COD 	
Total COD range 	
Soluble COD 	
Soluble COD range 	
Total phosphate, as P 	
Total phosphate range, as P 	
Soluble phosphate, as P 	
Soluble phosphate range, as P 	
Total Kjeldahl nitrogen 	
Total Kjeldahl nitrogen range 	
Ammonia nitrogen 	
Ammonia nitrogen range 	
Total suspended solids 	
Total suspended solids range 	
Volatile suspended solids 	
Volatile suspended solids range ....
Volatile acids 	
Volatile acids range 	
Alkalinity 	
Alkalinity range 	
Total COD 	
Soluble COD 	
Soluble COD range 	
Total phosphate, as P 	
Total phosphate range, as P 	
Soluble phosphate, as P 	
Soluble phosphate range, as P 	
Total Kjeldahl nitrogen 	
Total Kjeldahl nitrogen range 	
Ammonia nitrogen 	
Ammonia nitrogen range 	
Total suspended solids 	
Total suspended solids range 	
Volatile suspended solids 	
Volatile suspended solids range ....
Volatile acids 	
Volatile acids range 	


Raw
primary
sludge
1,885
1 ,264-2,820
54,146
36,930-75,210
3,046
2,410-4,090
350
264-^96
69
20-150
1 ,656
1 ,250-2,470
223
19-592
48,700
37,520-65,140
36,100
28,780-43,810
1 ,997
1,368-2,856
3,593
1 ,330-5,000
66,372
39,280-1 90,980
1,011
215-4,460
580
379-862
15
6.9-34.8
2,731
1,530-4,510
709
368-1,250
61,140
48,200-68,720
33,316
27,000-41,000
137
24-248


Waste
activated
sludge
1,265
1,220-1,310
12,810
7,120-19,270
1,043
272-2,430
218
178-259
85
40-119
711
624-860
51
27-85
12,350
9,800-13,860
10,000
7,550-12,040
NA
NA
1,897
1,200-2,690
24,940
10,770—32,480
1,223
1 ,090-1 ,400
172
123-217
25
21 .6-27.9
820
610-1,060
92
68-116
21,120
6,850-44,000
12,600
3,050-30,350
652
560-888


Metal
(mg/kg)
Cadmium, average 	
Cadmium, range 	
Total chromium, average . .
Total chromium, range 	
Copper, average 	
Copper, range 	
Lead, average 	
Lead, range 	
Mercury, average 	
Mercury, range 	
Nickel, average 	
Nickel, range 	
Zinc, average 	
Zinc, range 	


Table 1-12. — Pathogen
non, Ohio


Parameter
(#/100 ml)
Salmonella average 	
Salmonella range 	
Ps. aeruginosa average 	
Ps. aeruginosa range 	
Fecal coliform average MF...
Fecal coliform range MF 	
Fecal coliform average MPN.
Fecal coliform range MPN . . .
Total coliform average MF . . .
Total coliform range MF 	
Total coliform average MPN .
Total coliform range MPN....
Fecal streptococci average . .
Fecal streptococci range 	


Parameter
(#/100 ml)
Raw Waste Anaerobic
primary activated digested
sludge sludge sludge
105 388 137
69-141 119-657 73-200
633 592 882
287-979 133-1,050 184-1,580
2,640 1 ,340 4,690
2,590-2,690 670-2,010 4,330-5,050
1,379 1,624 1,597
987-1 ,770 398-2,850 994-2,200
6 46 0.5
0.4-11 0.1-91 0.1-0.9
549 2,109 388
371-727 537-3,680 263-540
4,690 2,221 7,125
4,370-5,010 1,250-3,191 6.910-7,340


data for raw sludges at Leba-



Raw Waste
primary sludge activated sludge
62 6
11-240 3-9
195 5.5 X103
75-440 91-1. 1X104
NA 2.65 X107
NA 2.0X107-3.3X107
8.3 X108 NA
. 1.3X108-3.3X109 NA
NA 8.33 X108
NA 1.66X10*-1.5X109
2.9 X109 NA
. LSXI^-S.SXIO9 NA
3.9X107 1.03X107
. 2.6X107-5.2X107 5X105-2X107


Anaerobically Septage
digested sludge sludge
and  99.47 percent,  respectively. The fecal streptococci
kills  were as follows:  primary sludge, 99.93  percent;
waste  activated sludge, 99.41  percent; septic  sludge,
99.90  percent; and  anaerobic  digested sludge, 96.81
percent. Pathogen concentrations in  lime stabilized
sludges range  from 10 to 1,000 times less than for
anaerobically digested sludges.

Economic  Analysis
Lebanon Facilities
  As previously described, the  anaerobic sludge diges-
tion  facilities at Lebanon were  essentially  inoperable at
     16
Salmonella average	
Salmonella range	
Ps. aeruginosa average	
Ps. aeruginosa range	
Fecal coliform average MF...
Fecal coliform range MF	
Fecal coliform average MPN.
Fecal coliform range MPN ...
Total coliform average MF ...
Total coliform range MF	
Total coliform average MPN .
Total coliform range MPN....
Fecal streptococci average ..
Fecal streptococci range	
       6
     3-30
      42
     3-240
    2.6 X105
s^xioM.exio5
   1.45X106
   2.42 X107
1.3X105-1.8X108
   2.78 X107

    2.7 X105
       6
      3-9
      754
  14-2.1 X107
    1.5X107
1.0X107-1.8X107
      NA
      NA
   2.89 X108
 1,
      NA
      NA
    6.7X105
3.3X105-1.2X106

-------
Table 1-13.—Chemical composition of lime stabilized sludges at Lebanon,  Ohio
Parameter
(mg/l)
Alkalinity 	
Alkalinity range
Total COD
Total COD range
Soluble COD
Soluble COD range
Total phosphate 	
Total phosphate range
Soluble phosphate
Soluble phosphate range 	
Total Kjeldahl nitrogen
Total Kjeldahl nitrogen range
Ammonia nitrogen
Ammonia nitrogen range 	
Total suspended solids 	
Total suspended solids range 	
Volatile suspended solids
Volatile suspended solids range . . .

Raw
primary
sludge
4,313
3 830-5 470
41 180
26 480-60 250
3556
876-6 080
283
164-644
36
17-119
1 374
470-2510
145
81-548
38,370
. . 29,460-44,750
23,480
19,420-26,450

Waste
activated
sludge
5,000
4 400-5 600
14700
10880-20800
1 618
485-3010
263
238-289
25
17-31
1,034
832-1 ,430
64
36-107
10,700
10,745-15,550
7,136
6,364-8,300

Anaerobically
digested
sludge
8,467
2,600-13,200
58,690
27 190-107,060
1,809
807-2,660
381
280-460
29
1 .4-5.0
1,980
1,480-2,360
494
412-570
66,350
46,570-77,900
26,375
21 ,500-29,300

Septage
sludge
3,475
1,910-6,700
17,520
5,660-23,900
1,537
1,000-1,970
134
80-177
2.4
1.4-4.0
597
370-760
110
53-162
23,190
14,250-29,600
11,390
5,780-19,500

Table 1-14.—Pathogen data  for lime stabilized sludges at Lebanon, Ohio
Parameter
(#/100 ml)
Salmonella average
Salmonella range 	
Ps aeruginosa average
Ps. aeruginosa range 	
Fecal coliform MF average 	
Fecal coliform MF range
Fecal coliform average MPN...
Fecal coliform range MPN 	
Total coliform average MF 	
Total coliform range MF 	
Total coliform average MPN . . .
Total coliform range MPN 	
Fecal streptococci average ....
Fecal streptococci range 	

Raw
primary sludge
a3
a3
a3
a3
NA
NA
5.93 X103
560-1 .7 X104
NA
NA
1.15X105
640-5.4 X105
1.62X104
4.0X103-5.5X104

Waste
activated sludge
a3
a3
a3
a3-26
1.62X104
S.SxlC^-S^XIO4
NA
NA
2.2 X105
3.3X10M.2X105
NA
NA
6.75X103
I.SXIOM.SSXIO3

Anaerobically
digested sludge
a3
a3
a3
a3
3.3 X103
3.3 X103
18
18
NA
NA
18
18
8.6 X103
3.3X103-1.4X104

Septage
sludge
a3
a3
a3
a3
2.65 X102
2 x 102-3.3 X102
NA
NA
2.1 X103
200-4 X103
NA
NA
665
3.3X102-! X103

  "Detectable limit = 3.
the start of the lime stabilization project. Funds were
allocated to construct lime  stabilization facilities, as  well
as to rehabilitate the anaerobic digester. In both cases,
the existing structures,  equipment,  etc.,  were utilized to
the maximum extent possible. Table 1-15 includes the
actual amounts paid to contractors, following competitive
bidding, and does not include engineering  fees, adminis-
trative costs, etc.
  The cost of  the lime stabilization facilities was
$29,507.45 compared to $32,134.59 for cleaning and
repair of the anaerobic sludge digester.
Capital  Cost of New Facilities

  Capital  and annual operation and maintenance costs
for  lime stabilization and anaerobic sludge digestion fa-
cilities were estimated assuming new  construction as a
part of a  1.0  Mgal/d (0.04 m3/s) wastewater treatment
plant with primary clarification and single stage conven-
tional activated  sludge treatment processes.
  The capital  costs for lime stabilization facilities includ-
ed  a bulk lime storage bin  for  hydrated  lime, auger,
volumetric feeder and lime  slurry tank, sludge mixing and
                                                 17

-------
 Table 1-15.—Actual cost of digester  rehabilitation and
 lime  stabilization  facilities  construction
                                                                   475 Ib/day (215 kg/day)
                                                                                                   3 hrs/day
                                                                                                   1,000 ft3 (28 m3)
                                                                  Daily lime requirement as 100
                                                                    percent Ca(OH)2
	   Treatment period
~                      ~~~Bulk lime storage bin volume min-
Anaerobic digester cleaning                                            imum
  Cleaning contractor	    $5,512.12   Bulk lime storage bin detention     34 days
  Temporary sludge  lagoon	     2,315.20     time
  Lime for stabilizing digester contents	       514.65   Lime feeder and slurry tank ca-    5-15 ft3/hr (0.14-0.42 m3/hr)
  Temporary pump rental	       300.30     pacity (spared)
      Subtotal digester cleaning	          864227   I"/'"6"1 sludge grinder capacity     20° gpm (12'6 "?>
                                                                  Sludge mixing tank volume         15,000 gal  (57 m3)
Anaerobic digester rehabilitation                                      Sludge mixing tank dimensions      14 ftxl4 ftxlO ft SWD (4.3
  Electrical equipment, conduit, etc	     1,055.56                                      mx4.3 mx3 m)
  Natural  gas piping	       968.76   Sludge mixer horsepower          15 HP (11.2 kW)
  Hot water boiler, piping, pump, heat exchanger repair...     7,472.26   Sludge mixer turbine  diameter      53 in  (135  cm)
  Control  room rehabilitation	     1,465.00   Turbine speed                    68 rpm
  Sludge recirculating pump  repair	       771.00   Sludge transfer pump capacity      400 gpm (25.2 l/s)
  Piping and valve rehabilitation	     8,587.30     (spared)
  Floating cover  roof repair	     1,014.04   Treated sludge percent solids      4 percent
  Repair utilities,  drains	       211.52   Sludge holding  lagoon volume      100,000 ft3  (2,860 m3)
  Miscellaneous	     1,946.88   Sludge holding  lagoon maximum    60 days
      Subtotal digester rehabilitation	    23 492 32     Detention time
                                                         '         Treatment building floor  area       150 ft2 (14m2)
Lime stabilization  process                                            Treatment building construction     Brick  and block
  Electrical equipment, conduit, etc	     1,692.00   Instrumentation                   pH record treated sludge volume

  4" sludge crossover1 'pipfTvaives  and^ttings  ' m9s""     'MO? 48     CaPital  costs for the  lime stabilization facilities were
  1-1/2" air line  and diffusers	'	'.'.'.'.'.'.'.'.'.'.'.'.     U10.00   based on  JulV 1- 1977> bid  date-  and nave been SUmma
  3/4" water lines and hose bibbs	       865.00   r'zec'  'n *ab'e 1-16.
  Lime bin, auger, vibrators	     7,229.44     ^me stabilization  operation assumed  one man, 2  hours
  Volumetric feeder,  trough and gate	     3,460.00   Per day,  365 days  per year,  at $6.50 per  hour, including
  Existing pump repairs	     3,399.00   overhead. Maintenance  labor and materials assumed 52
  Miscellaneous metal	     1,200.00   hours per year labor at $6.50  per  hour and $800 per
  Relocate sanitary service ime	       200.00   year for maintenance  materials. The total  quantity of
  Repair utilities	       134.00   46.8 percent CaO  hydrated lime required was  141 tons
  Miscellaneous..      	       934.34   (128 Mg)  per year  at $44.50 per ton ($49.06/Mg)
  contractors overhead	     1.842.00     Tne total  annual cost for  lime stabilization,  excluding
      Subtotal lime  stabilization	    29,507.45   land application  of treated  sludge,   has been summarized
Septage holding tank                                               in  table 1-17.
  Septage holding tank and pump                          617470     Tne bas's  f°r design  of a single  stage anaerobic
      subtotal septage holding tank	     e!i 74^70   fudge  digester for the  same treatment plant  was as
      Total cost  for digester  cleaning and rehabilitation,                TOIIOWS.
        lime stabilization,  and septage facilities	    67,816.74   Daily primary sludge dry solids      1,250  Ib/day (568 kg/day)
	production
thickening tank  with a  mechanical  mixer,  sludge grinder,
all  weather treatment building, electrical  and instrumenta-
tion, interconnecting piping  and  transfer  pumps,  and  60-
day detention treated sludge  holding lagoon. The basis
for  design is as  follows:
Daily primary sludge dry solids
  production
Average primary sludge volume
  @ 5 percent solids
Daily waste activated dry solids
  production
Average waste activated sludge
  volume @ 1.5  percent solids
Average lime dosage required per
  unit dry solids
1,250 Ibs/day (568 kg/day)

2,910 gal/day (11  rrrVday)

1,084 Ibs/day (493 kg/day)

8,580 gal/day (32  m3/day)

0.20  kg/kg
                                  Table  1-16.—Capital  cost of lime  stabilization facilities
                                  for  a new  1  Mgal/d  (0.04 m3/s) wastewater treatment
                                  plant
                                                                 Site work, earthwork and yard piping	
                                                                 Lime storage bin and feeders	
                                                                 Treatment tank,  pumps,  sludge grinders, and building
                                                                   structure	
                                                                 Electrical and instrumentation	
                                                                 Sludge holding lagoon	
                                                   $6,OOC
                                                   30.00C

                                                   52.00C
                                                   10.00C
                                              ..    20,00(
Subtotal construction cost	   118,000
                                                                 Engineering 	

                                                                       Total capital cost	
                                                                 Amortized  cost at 30  yrs., 7% int. (CRF = 0.081).
                                                                 Annual capital cost per ton dry  solids	
                                                   12.00C
                                                  130.00C
                                                   10.50C
                                                    24.66
     18

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 Table 1-17.—Total annual cost for lime stabilization ex-
 cluding  land  disposal for  a 1  Mgal/d (0.04 m3/s) plant
Item
Operating labor 	
Maintenance labor and materials 	
Lime 	
Laboratory 	
Capital 	

Total annual cost

Total
annual
cost
$4,700
1,100
6,300
500
10,500

23 100

Annual
cost
per Kkg
dry solids
$12.14
2.84
16.20
1.29
27.11

5958

Annual
cost
per ton
dry solids
$1 1 .03
2.58
14.74
1.17
24.65

54.17

Average primary sludge volume
   @ 5 percent solids
Daily waste activated dry solids
   production
Average waste activated sludge
   volume @  1.5 percent solids
Daily volatile solids production
Volatile solids loading

Digester hydraulic  detention time
Digester gas production
Average volatile solids reduction
Digested sludge dry solids pro-
   duction
Digested sludge percent solids
Digester net heat requirement
Mechanical mixer horsepower
Sludge  recirculation pumps (2 ea)
                        2,910 gal/day (11 m3/day)

                        1,084 Ib/day (493 kg/day)

                        8,580 gal/day (32 m3/day)

                        1,634 Ib/day (743 kg/day)
                        0.05 Ib VSS/ft3/day (0.8
                           kg/m3/day)
                        21 days
                        13 ft3/lb VSS (0.8 m3/kg) feed
                        50 percent
                        1,515 Ib/day (690 kg/day)

                        6 percent
                        186,000 Btu/hr (54,500 W)
                        15 HP (11.2 kW)
                        350 gal/min ea (22 l/s)
  Capital cost for the anaerobic sludge digestion facili-
ties, including the control  building,  structure, floating
cover, heat exchanger, gas  safety  equipment, pumps,
and interconnecting  piping, assuming July 1, 1977, bid
date, and  engineering,  legal,  and administrative costs are
summarized in table 1-18.
  Digester operation  assumed one  man, 1  hour per day,
Table  1-18.—Capital cost for single stage  anaerobic
digestion facilities for a  1  Mgal/min (0.04 m3/s) waste-
water  treatment  plant
Site work, earthwork, yard piping	    $44,000
Digester	    233,000
Control building	    133,000
Electrical and instrumentation	    47,000

      Subtotal construction cost	    457,000

Engineering	
        	    46,000

Total capital cost	   503,000
365 days per  year at $6.50 per hour, including over-
head.  Maintenance labor and  material assumed 52 hours
per year at  $6.50 per hour and $1,500  per year for
maintenance materials.
  The cost of anaerobic  digester operation  was offset
by assuming a value of $2.10 per million Btu  ($1.99  per
million kJ) for all digester gas produced above the net
digester heat requirement.
  The total annual  cost for anaerobic sludge digestion,
excluding land  application has been summarized  in table
1-19.
  Both the  lime stabilization and  anaerobic  digestion  al-
ternatives were assumed  to  utilize land  application of
treated sludge  as a liquid hauled by truck.  The capital
cost for a sludge hauling vehicle was assumed to be
$35,000, which was depreciated on  a straight  line basis
over a 10-year period. Alternatively,  a small treatment
plant could  utilize an existing vehicle which could be
converted for  land  application at a somewhat lower cap-
ital cost.
  The assumed hauling distance was 3  to 5 miles (5  to
8 km), round trip. Hauling time assumed 10 minutes  to
fill,  15 minutes to empty, and  10 minutes driving, or  a
total of 35  minutes per round  trip. The  truck volume  was
assumed to  be 1,500  gal (5.68 m3) per  load. The cost
of truck operations, excluding  the driver and deprecia-
tion, was assumed  to  be $8.50 per operating  hour. The
truck driver  labor rate was assumed to  be $6.50 per
hour, including  overhead.
  Truck  operation time was  based on hauling an aver-
age  of 6,860 gal (1.812 m3) of lime  stabilized sludge,
i.e., five loads and  2,940 gal (0.777 m3) of anaerobically
digested sludge,  i.e.,  two loads per  day. The reduced
volume of anaerobically digested  sludge resulted from
the volatile  solids reduction during digestion and the
higher solids concentration  compared to lime stabilized
sludge.
  Although  it may be  possible to obtain the use  of farm-
land at no cost,  e.g.,  on  a voluntary basis,  the land
application economic analysis assumed that land  would
                                                       Table 1-19.—Total  annual cost for single stage  anaero-
                                                       bic  sludge digestion excluding land disposal for  a 3,785
                                                       rrrVday plant
Amortized cost at 30 yrs, 7% int. (CRF = 0.081).
Annual capital cost per unit feed dry solids	
                                               40,700
                                                95.54


Item


Operating labor . . .
Maintenance labor and materials...
Laboratory 	
Capital 	
Fuel credit 	

Total annual cost 	


annual
cost

$2400
1,800
500
40700
(2 900)

42.500
Annual

cost
per Kkg
dry solids
$620
4.65
1 29
10509
(749)

109.74
Annual

cost
per ton
dry solids
$563
4.23
1 17
9554
(6811

99.76
                                                                                                               19

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Table 1-20.—Annual  cost for land application of lime stabilized and an-
aerobically digested sludges  for a 3,785 rrvVday plant
Lime stabilization
Item
Amortized cost of land 	
Truck depreciation
Truck driver 	
Truck operation. .
Laboratory
Fertilizer credit .
Land credit . .
Total annual cost 	
Total
annual
cost
$2,600
3,500
7,100
9,300
500
(3,100)
(2,200)
17,700
Annual
cost
per Kkg
solids
$6.75
9.04
18.35
24.03
1.29
(8.05)
(5.68)
45.73
Annual
cost
per ton
solids
$6.14
8.22
16.67
21.83
1.17
(7.30)
(5.16)
41.57
Anaerobic digestion
Total
annual
cost
$1,700
3,500
2,800
3,600
500
(2,000)
(1 ,400)
8,700
Annual
cost
per Kkg
solids
$4.39
9.04
7.24
9.30
1.29
(8.05)
(3.62)
19.59
Annual
cost
per ton
solids
$3.99
8.22
6.57
8.45
1.17
(7.30)
(3.29)
17.81
be purchased at  a cost  of $750 per acre ($1850 per
ha).  Sludge application rates were  assumed  to be  10 dry
tons per acre per year (22.4 Mg/ha/year). Land costs
were amortized at 7 percent interest over a 30-year
period.
  To offset  the land cost, a fertilizer credit of $7.30 per
ton ($8.05  per Mg) of dry sludge solids was  assumed.
This rate was arbitrarily  assumed to be  50 percent of
the value published by Brown11 based  on medium fertiliz-
er market value and low fertilizer content. The reduction
was  made to  reflect resistance to accepting sludge as
fertilizer. The  land cost was further  offset by assuming a
return of $50  per acre ($124 ha), either as profit after
farming  expenses, or  as the rental  value of the land.
  Capital and annual  operation and maintenance costs
for land application of lime stabilized and anaerobically
digested sludges  have been summarized  in table 1-20.
  For each  item  in table 1-18, the total annual cost was
calculated and divided by the total raw  primary plus
waste activated sludge quantity,  i.e., 426 tons/year (386
Mg). Anaerobically digested sludge land requirements
were less than for lime stabilized sludge because of the
volatile solids reduction during  digestion. Truck driving
and  operation costs were similarly  less for digested
sludge because of the volatile  solids reduction and more
concentrated sludge (6 percent versus 4 percent) which
would be hauled.  Fertilizer credit was less for digested
sludge because of the lower amount of  dry  solids  ap-
plied to the land.  Land credit was  based on  the amount
of sludge applied and was,  therefore,  less for digested
sludge.
  The total  annual capital and  annual operation and
maintenance costs for lime stabilization and  single  stage
anaerobic sludge digestion,  including land application for
a 1  Mgal/day (0.04 m3/s) wastewater treatment  plant,
are summarized  in table 1-21.
Table 1-21.—Comparison of total annual capital and
annual O. &  M. cost for lime stabilization and anaerobic
digestion including  land disposal for a 3,785  mVday
plant
Lime stabilization

Facilities 	
Land application . . .
Total annual cost ..
Total
annual
O. & M.
cost
$23,100
17,700
40,800
Annual
cost per
Kkg dry
solids
$59.58
45.70
105.28
Anaerobic digestio
Total
annual
O. & M.
cost
$42,500
8,700
51,200
Annual
cost pe
Kkg dr
solids
$109.7-
19.5!
129.3:
Lime Stabilization by Others
  A considerable amount of  lime stabilization work has
occurred in Connecticut. A number of incinerators have
been  shut down and  replaced  by lime stabilization. A
total of 27 plants with capacities from 0.3 to 29 mgd
(0.01  to 1.27 m3/s) are utilizing lime  stabilization either
on a  full-  or part-time basis. The following tabulation anc
comments for nine plants are typical  and summarize the
current situation.  Lime stabilized  sludges are  either used
as landfill  cover or are composted. These methods  have
been  satisfactory. Most of the  communities have indicat-
ed that they will continue with lime stabilization. Typical
plants in Connecticut which are utilizing lime  stabilization
are shown at the top  of the facing page.
     20

-------
                        Design
                        plant            Incinerator
                        size,
                        mgd    Installed   Used    Hours
Stratford3	
West Bridgeport".
Stamford0	
Middletownd	
Willimatic6	
Glastonburg*	
Torrington9	
Naugatuckh	
Enfield1	
11
29
20
 7
5.5
3.2
 7
 7
10
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
Yes
No
 24
 24
 No
N/A*
N/A
N/A
N/A
N/A
Used

Yes
Yes
N/A
Yes
Yes
Yes
Yes
Yes
                                             Lime stabilization

                                            Hours     Ultimate disposal
  8
  8
N/A
 16
N/A
N/A
N/A
N/A
Landfill cover
Landfill cover
Lagoon
Landfill cover
Land and landfill
Landfill cover
Landfill
Landfill cover
Yes   1/4 of year   Yes   3/4 of year   Land
  *N/A denotes data not available at the time of writing.
  "Incinerator abandoned  in favor of  lime stabilization to pH 12. Composted and used as final cover at landfill.
  bStabilized cake used as final cover at landfill.
  °Centrifuged with lime sludge. Haul away and lagooned.
  dPreviously plagued with odors; now all sludge processed in two shifts, 5 days per week with no odors. Lime
stabilized and final cover at landfill.
  "Began lime stabilization in 1973. Screened sludge and leaf material used on parks as fertilizer and final cover
for landfill.
  'Final cover for landfill and composted with leaves.
  9Lime stabilization used when incinerator out of service.
  hLime stabilized sludge  used as final cover at two landfills.
  'Incineration is used in winter during inclement weather. Lime stabilized sludge stockpiled and spread on corn
land  during remainder of year.
LIME  STABILIZATION DESIGN  EXAMPLES

Statement  of  Problem
  The problem is to provide lime stabilization facilities
for  two communities,  both of which have existing con-
ventional  activated sludge wastewater treatment  plants.
  The smaller community has existing wastewater treat-
ment facilities capable of treating 4.0 Mgal/day  (0.18
m3/s). The facilities consist of  screening, grit removal,
primary settling,  conventional activated sludge aeration,
final settling, cnlorination, and  sludge lagooning.  Present
flow to the plant is 3.5 million  gallons  per day (0.15
m3/s); the 20-year projected flow is  4.0  million gallons
per day (0.18  m3/s).  The plant meets its proposed  dis-
charge permit  requirements,  but the  city has  been or-
dered to  abandon the sludge lagoons (which are periodi-
cally  flooded by the receiving  stream). Sludge disposal
alternatives include the following:
  1. Lime stabilization followed by liquid application to
     farmland.
  2. Anaerobic digestion  followed by liquid application to
     farmland.
  The larger community  has existing wastewater treat-
ment facilities capable of treating 30 million  gallons per
day (1.31  m3/s). Present  flow to the plant is 35 million
gallons per day  (1.53 m3/s); the 20-year projected flow
is 40 million  gallons per day (1.75 m3/s). The existing
treatment  system consists of screening,  grit removal,  pri-
mary settling, conventional activated sludge aeration,  fi-
nal  settling, chlorination, aerobic sludge  digestion, sludge
dewatering, and  landfilling of dried sludge solids. The
                                   existing treatment scheme will meet proposed permit re-
                                   quirements.  As a part of the treatment plant expansion
                                   planning and in view of future electric power costs, the
                                   following solids handling  alternatives  were proposed:
                                     1.  Lime stabilization followed  by pipeline transportation
                                        to the  land application site.
                                     2.  Anaerobic digestion followed by  mechanical dewa-
                                        tering and land application.
                                     The  design  logic which will be followed to develop
                                   and evaluate the sludge  handling  alternatives is summa-
                                   rized on figure  1-15.

                                   Wastewater  Characteristics
                                     The  wastewater characteristics and removal efficiencies
                                   of the  various  treatment  units are required to determine
                                   the  basis for design of the sludge stabilization and  ulti-
                                   mate disposal  processes.  This information may be ac-
                                   quired  from plant records or from sampling  programs  at
                                   the  existing facilities. When  these data are not available
                                   (such as in  the case of new wastewater treatment plants
                                   for new service areas), assumptions  based on sound
                                   engineering  judgment and previous experience are nec-
                                   essary.  For  the sake of simplicity, the wastewater char-
                                   acteristics and treatment unit removal efficiencies for the
                                   example plants were assumed to  be  equal.  Raw waste-
                                   water characteristics for  the  example  plants  are  given in
                                   table 1-22.

                                   Treatment  Unit Efficiencies
                                     Both  plants in this example will meet  their proposed
                                   permit  requirements by utilizing  the existing  treatment
                                                                                                              21

-------
 1.    Establish regulatory constraints for effluent and sludge disposal

 2.    Determine WWTP influent loads

 3.    Deterimme WWTP unit process scheme

 4.    Determine raw sludge loads

 5.    Establish cost effective constraints and sludge solids concentrations
      for ultimate sludge disposal processes

 6.    Set sludge thickening requirements

 7.    Select stabilization alternatives


 Figure 1-15.—Process alternative design logic.
                             7a.   Develop capital cost

                             7b.   Develop 0 & M requirements and cost

                             7c.   Develop environmental constraints

                             7d.   Evaluate supernatant impact on plant

                             7e.   Evaluate estimated total sludge handling costs

                             8.    Screen alternatives

                             9.    Select final stabilization process

                             10.   Prepare final flow sheets and cost estimates
Table 1-22.—Raw wastewater  characteristics
                   Parameter
               Concentration
                  (mg/l)
BOD5	
Suspended solids.
Organic nitrogen...
Ammonia nitrogen .
Phosphorus	
Grease	
                   200
                   240
                    15
                    25
                    10
                   100
Table  1-23.—Treatment unit efficiencies
              Unit
Parameter
Removal efficiency
    (percent)
Primary settling 	
Aeration and final settling . . .

BOD*
SS
BODK
SS
30
65
60
25
                  processes.  Nitrification and phosphorus removal  are not
                  required. Removal  efficiencies based on  percentages of
                  the raw "domestic" wastewater characteristics are pre-
                  sented  in table  1-23.

                  Sludge Characteristics
                    The characteristics  of  sludge discharged to the sludge
                  stabilization  facilities may vary  considerably depending
                  on the  type and amount of industrial waste treated, the
                  sludge  origin (which particular  treatment unit) and the
                  sludge  age.  Ideally, samples of sludge  would  be  avail-
                  able for analysis. The assumed sludge  characteristics for
                  each  example plant are  as follows:
                                                                              Sludge type
                                                               Thickened raw primary	
                                                               Thickened waste activated.
                                                                       Design percent solid
                                                                               7.0
                                                                               2.5
                                                                 Thickening facilities for primary and  waste  activated
                                                              sludge were assumed to be cost effective for both  the A
                                                              and 40  Mgal/d (0.18 and 1.75 m3/s) wastewater  treat-
                                                              ment plants. Waste activated sludge production was 0.5
                                                              pound of  volatile  solids  per pound of  BOD5 reduced.
                                                                 Preliminary studies have  indicated  that  anaerobic
     22

-------
sludge  digestion will not be  adversely affected  by the
inclusion  of  thickened  waste activated sludge.
  The sludge quantities for the 4 Mgal/d (0.18 m3/s)
wastewater treatment plant were developed as  follows:
Influent BOD5
     Influent 4.0 Mgal/d x 8.34X200  mg/l = 6,672  Ib/day
      (3,033 kg/day)
     Primary  removal = 6,672x0.3 = 2,002 Ib/day (910
      kg/day)
     BOD5 remaining  in settled  sewage = 4, 670 Ib/day
      (2,123 kg/day)
Influent suspended solids
     Influent 4x8.34x240  mg/l = 8,006 Ib/day (3,639
      kg /day)
     Primary  removal = 8,006x0.65 = 5,204  Ib/day  (2,365
      kg /day)
     Suspended solids remaining  in settled  sew-
      age =2,802 Ib/day (1,274 kg/day)
Waste activated solids
     Biological = 6,672x0.60x0.5 Ib  VSS/lb
      BOD5 = 2,002  Ib  VSS/day
     Suspended solids = 8,006x0.25 = 2,002 Ib/day
     Total biological  solids  produced = 4,004 Ib/day
      (1,820 kg/day)
Net  daily sludge quantities
     Primary: 5,204 Ib/day  (2,360 kg/day) at 7  percent
      following thickening

          5'2°4
= 8'740
                                      (33 m'/day)
    8.34 X102X 0.07
Waste activated sludge

    0.025X48°3044X1.01=19'014
Net sludge produced (5,204 + 4,004) = 9,208 Ib sol-
ids/day (4,185  kg/day)
    Volume = (8,740 + 19,01 4) = 27,754 gal/day (105
      m3day)
    Percent solids = 3.9 percent

  Design  sludge quantities were  developed  for the 40
Mgal/d (1.75 m3/s)  facility  in an identical manner. The
design sludge quantities are summarized as follows:
Primary sludge solids, Ib/day	
Primary sludge volume at 7 percent,
  gal/day	
Biological sludge solids, Ib/day	
Biological sludge volume at 2.5 percent,
  gal/day	
Total sludge solids, Ib/day	
Combined sludge volume, gal/day	
Combined sludge percent solids	
                4.0 Mgal/d
                  WWTP

                   5,204

                   8,740
                   4,004

                  19,014
                   9,208
                  27,754
                   3.9
40 Mgal/d
  WWTP

  52,040
  87,400
  40,040

 190,140
  92,080
 277,540
   3.9
  For simplicity,  the  design examples for the 4  and 40
Mgal/d (0.18 and 1.75  m3/s) treatment plants will  be
presented separately. Each example  will  include the de-
sign basis  for each alternative stabilization and  ultimate
disposal process, final sludge volumes, capital and annu-
al  operation  and maintenance costs.
                                      Process Alternatives—-4  Mgal/d
                                      (0.18  mVs) WWTP

                                        As previously discussed, process alternatives for the 4
                                      Mgal/d  wastewater  treatment  plant will be as follows:
                                        1.  Lime stabilization followed by liquid  application to
                                           farmland.
                                        2.  Anaerobic  digestion followed by liquid application to
                                           farmland.

                                      Lime Stabilization
                                        A flow diagram  for the  proposed lime stabilization fa-
                                      cilities is shown on  figure 1-16. Significant process
                                      equipment includes  a bulk lime storage bin for pebble
                                      quicklime, auger, lime slaker and feed slurry  tank, sludge
                                      mixing and thickening tank with a mechanical mixer,
                                      sludge  grinder,  all weather treatment building, electrical
                                      and instrumentation, interconnecting piping and transfer
                                      pumps,  and a sludge holding  lagoon with 60 days deten-
                                      tion time. The basis for design is  as follows:
Total sludge solids

Sludge volume

Raw sludge percent solids
Overall lime dosage required per unit dry
  solids, as  100 percent Ca(OH)2
Daily lime requirement as Ca(OH)2

Treatment period
Bulk lime storage bin volume minimum
Bulk lime storage bin detention time
Lime slaker and slurry tank capacity (2
  ea)
Influent sludge grinder capacity (spared)
Sludge mixing tank volume
Sludge mixing tank dimensions

Sludge mixer horsepower
Sludge mixer turbine diameter
Turbine  speed
Sludge transfer pump capacity (spared)
Treated sludge volume
Treated sludge percent solids
Sludge holding lagoon total volume (4
  cells)
Sludge holding lagoon maximum detention
  time
Treatment building floor  area
Treatment building construction
Instrumentation
9,208 Ib/day (4,185
  kg/day)
27,754 gal/day (105
  m3/day)
3.9
0.20 Ib/lb

1,826 Ib/day (830
  kg/day)
6 hrs/day
1,000 ft3 (28 m3)
34 days
200-300 Ib  CaO/hr (91-
  136 kg/hr)
200 gal/min (12.6 l/s)
25,000 gal (95 m3)
18 ftxl8 ftxlO ft SWD
  (5.5 mX5.5 mx3 m)
15 HP (11.2 kW)
53 in (135 cm)
68 rpm
400 gal/min (25.2 l/s)
24,050 gal (91 m3)
4.5
240,000 ft3 (6,800 m3)

60 days

250 ft2 (23.2 m2)
brick and block
pH record treated sludge
  volume
  With the exception  of the  lime storage  bin detention
time and  pump capacities, the reasons  for selecting the
particular design quantities have been discussed in previ-
ous sections. Lime storage bin capacity was based on  a
minimum detention time of 30 days to allow  capacity for
a standard 20-ton  (18 Mg) lime shipment. The pump
capacity was based on convenient transfer times be-
tween  units.
                                                                                                              23

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                                                                                                         TREATED
SECONDARY



CHLORINATION
EFFLUENT
TO DISCHARGE
                                                                                         LIQUID SLUDGE
                                                                                       TO LAND APPLICATION
Figure 1-16.—4 Mgal/d (0.18 m3/s) lime stabilization/truck haul and  land
application.
  Capital costs for  the  lime stabilization  facilities were
based on January 1,  1978, bid date and have been
summarized in table 1-24.
  Lime stabilization operation assumed one man, 8 hours
per day, 365  days  per  year, at $6.50 per hour, including
overhead.  Maintenance  labor  was assumed to be 156
hours per  year labor  at $6.50 per hour  and $2,400 per
year  for maintenance materials. The total quantity  of  85
percent  CaO  quicklime  required was 297 tons (269 Mg)
per year at $40 per ton ($44/Mg).
  The total annual  cost for lime stabilization, excluding
land  application of  treated sludge, has been  calculated
as follows and is summarized  in table  1-25.

Lime Stabilization  Operating Costs
Labor: 8hr/dayx365 day/yrx$6.50/hr = $18,980 say
$19,000
Table 1-24.—Capital  costs of lime  stabilization facilities
for a 4 Mgal/d wastewater treatment plant
Site work, earthwork, yard piping	    $26,00
Lime storage bin and feeders	     84,00
Treatment tank, pumps, sludge  grinders, and building
  structure	
Electrical and instrumentation	
Sludge holding lagoon	

      Subtotal construction  cost	
Engineering.
                                              142,00
                                               29,00
                                          ..     54,00

                                          ..    335,00

                                               36,00

Total capital cost	    371,00
Amortized cost at 30 yrs., 7 percent int. (CRF = 0.081)	
Annual capital cost  per ton dry solids	
                                               30,10
                                                17.9
     24

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Table 1-25.—Total annual cost for lime stabilization ex-
cluding  land  disposal for a 4 Mgal/d plant


Item


Operating labor 	
Maintenance labor and materials
Lime . 	
Laboratory . . 	
Capital . .

Total annual cost

Total

annual


	 $19,000
. . . 3,400
	 12,000
	 1 ,500
	 30,100

66000

Annual


per ton
dry solids
$11.31
2.02
7.14
0.89
17.91

3927

Maintenance labor: 156 hr/yrx$6.50 = $1,014 say $1,000
Maintenance materials: $2,400/yr lump sum
Lime primary: 5,204 Ib/dayx0.12 Ib Ca(OH)2/lb = 624
Ib/day (283 kg/day)
Waste  activated: 4,004 Ib/day x 0.3 Ib Ca(OH)2/lb = 1,201
Ib/day (545 kg/day)
Total lime = (624 + 1,201 ) = 1,825lb Ca(OH)2/day (828
kg/day)
    1,825 lb/day/0.85) x 56/74 = 1,625 Ib/day CaO (737
      kg/day)
    1,625x365/2,000 = 297 ton/yr  (269 Mg/yr)
    say 300 ton/yr x$40/ton = $12,000/yr
Laboratory: $1,500/yr lump sum
Capital: $371,000X0.081 =$30,100/yr

  Both the lime  stabilization and anaerobic digestion al-
ternatives were assumed to utilize land application of
treated sludge as a liquid hauled by truck. The capital
cost per sludge  hauling vehicle was assumed to be
$35,000, which was depreciated on a straight-line basis
over a 5-year  period.
  The assumed  hauling distance was 3 to 5  miles (5-8
km), round trip.  Hauling time assumed 10 minutes to fill,
15 minutes to  empty,  and 10 minutes driving, or a total
of 35 minutes  per round trip. The truck volume was
assumed to be 1,500  gallons (5.7  m3) per load. The cost
of truck  operations, excluding the driver  and deprecia-
tion, was assumed to  be $8.50 per operating hour. The
truck driver labor rate was assumed to be $6.50  per
hour, including overhead.
  Truck operation time was based on hauling on  a
5-day per week  basis, approximately 10 months per
year, which results in  the  assumed  215  hauling days per
year. The average volume hauled is 40,800 gallons
(154.4  m3)  per day. Two trucks were assumed to  be
required, with  a  combined total of 28 loads per day.
  Although it may be  possible to obtain the  use of farm-
land at  no cost, e.g.,  on a voluntary basis, the land
application economic  analysis assumed that  land would
be purchased  at a cost of $750 per acre ($1850/ha).
Sludge application rates were assumed to be 10  dry
tons per acre per year. Land  costs were amortized at 7
percent interest over a  30-year period.
  To offset the land cost, a fertilizer credit  of $7.30 per
ton ($8.05 Mg) of dry sludge  solids was assumed. This
rate was arbitrarily assumed to be 50 percent of the
value  published  by Brown11 based on  medium fertilizer
market value and  low fertilizer content.  The reduction
was made to reflect resistance to accepting sludge as
fertilizer. The land cost  was further offset by assuming a
return of $50 per  acre  ($123/ha),  either as profit after
farming expenses  or as the rental  value of  the land.
  Capital and annual operation and maintenance costs
for land application of lime stabilized  sludge were  calcu-
lated as follows and have been  summarized in  table 1-
26.

Lime Stabilization Land Application Costs

Land: 9,208 Ib solids/day x 365  days/2,000
lb/ton = 1,681  ton/yr (1,525 Mg/yr)
    1,681 ton/yr/10 ton/acre = 168 acres (68.0 ha)  say
      200 (80.9 ha)
    200 acres x $750/acre = $150,000
    $150,000x0.081 =$12,150/yr say $12,200
Truck depreciation: $35,000 x 2 = $70,000 capital
    $70,000/5  yrs = $14,000/yr
Truck driver: 40,800 gal/day/2,571  gal/truck/hr = 15.9
hr/day
    say 2 trucks at 8 hr/day
    $6.50x2 menxS hr/day = $104/day
    $104X215 = $22,360 say  $22,400/yr
Truck operation: 2 trucks x 8
hr/day x $8.50/hr = $136.00/day
    $136.00x215 = $29,240 say $29,200/yr
Laboratory:  $1,500/yr lump sum
Fertilizer credit: 1,681 ton/yrx$7.30/ton = $12,271 say
$12,300/yr
Land  credit: 168 acres x$50/acre = $8,400/yr
Table 1-26.—Annual cost for land application of lime
stabilized sludge for a  4 Mgal/d plant
Item
Amortized cost of land 	
Truck depreciation 	
Truck driver 	
Truck operation 	
Laboratory 	
Fertilizer credit 	
Land credit. . . .

Total annual cost

Total
annual
cost
	 $12,200
	 14,000
	 22,400
	 29,200
	 1,500
(12300)
(8 400)

58600

Annual
cost
per ton
dry solids
$7.26
8.33
13.33
17.38
0.89
(7.30)
(5.00)

3489

                                                                                                        25

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Anaerobic  Digestion
  A flow diagram  for the  proposed anaerobic sludge
digestion facilities  is shown on figure 1-17.  Two-stage
anaerobic digestion  was assumed  with stabilized  sludge
being  hauled to farmland. Sludge storage  was  allowed in
the digester design  and no lagoon was  included. The
basis for design for the anaerobic digesters for the 4
Mgal/d  (.18 m3/s) treatment plant was as follows:
First Stage
  Feed solids loading
  Feed volume
  Feed percent solids
  Feed percent volatile solids
  Digester dimensions
  Digester volume
  Mixers
  Hydraulic detention  time
  Loading rate
  Digester bulk temperature
  Average feed temperature
  Volatile solids reduction
  Overall total solids reduction
  Sludge heaters
9,208 Ib/day (4,185 kg/day)
27,754 gal/day (105 m3/s)
39
65
60 MX25 ft SWD (18.3 mX7.6 m)
529,000 gal (2,002 m3)
2 ea at 3,500 gpm  (221  l/s)
19 days
0.085 lb/VSS/ft3/day (1.36
  kg/m3/day)
95° F (35° C)
55° F (13°C)
50 percent
32 percent
3 ea at 500,000 Btu/hr (14,650 W)
                               Second  Stage
                                 Digester dimensions
                                 Digester volume
                                 Hydraulic detention time
                                 Digester gas production
                                 Digester gas heat value
                                 Digested sludge dry solids pro-
                                   duction
                                 Digested sludge percent solids
                                 Sludge  recirculation pumps (2
                                   ea)
                              60 ft x 25 ft SWD (18.3  mx7.6 m
                              529,000 gal (2,002 m3)
                              19 days
                              10 ft3/lb VSS (0.6 rrvVkg) feed
                              500 Btu/ft3 (18,625 kJ/m3)
                              6,261  Ib/day (2,846 kg/day)

                              6.5 percent
                              500 gpm ea (31.5 l/s)
  Design conditions were based on  the  criteria  enumer-
ated in Ten States' Standards28 and  assumed  installation
in  the  Midwestern  United States.
  Capital cost for the anaerobic sludge  digestion facili-
ties, including the control building, structures,  floating
cover, heat  exchanger,  gas safety equipment,  pumps,
and interconnecting piping,  assuming January  1, 1978,
bid date, and engineering, legal, and administrative cost:
is  summarized in table  1-27.
  Digester  operation assumed one man, 3  hours per
day, 365 days  per year at $6.50 per hour, including
overhead. Maintenance  labor  and  material  assumed 416
                                                        ACTIVATED
                                                         SLUDGE
                                                                                                             TREATED
SECONDARY



CHLORINATION
EFFLUENT
TO DISCHARGE
                                                          RETURN  SLUDGE
                                                   IASTE  ACTIVATED SLUDGE
                                                RECYCLE  SUPERNATANT
THICKENED SLUDGE v

^^-

C
/


0 C
11 STAGE
VNAEROBIC
DIGESTER
— ^

0
                                                                                                LIQUID SLUDGE
                                                                                               TO  LAND APPLICATION
                                                                                                        TANK TRUCK
Figure 1-17.—4 Mgal/d  (0.18 m3/s) anaerobic digestion/truck haul and
land application.
     26

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Table  1-27.—Capital cost of two-stage anaerobic diges-
tion  facilities for a 4 Mgal/d wastewater treatment plant
Site work, earthwork, yard piping and pumps	   $151,000
Digesters	    675,000
Control building	    251,000
Electrical and instrumentation	    125,000

      Subtotal construction  cost	   1,202,000

Engineering	
        	   107.000

Total capital cost	  1,309,000
Amortized cost at 30 yrs., 7 percent int. (CRF = 0.081)
Annual capital cost per unit  feed dry solids	
                                            106,000
                                              63.08
hours per year at $6.50 per hour and $7,000 per year
for maintenance materials.
  The cost  of anaerobic digester operation was  offset
by assuming a value of $2.70 per million Btu ($2.56 per
million  kJ) for all digester  gas produced above the  net
digester  heat requirements.
  The total  annual  cost for anaerobic sludge digestion,
excluding land  application  was calculated as follows and
has been summarized in table 1-28.

Anaerobic Digester O  & M Cost

Operator labor:  3 hr/dayx365
day/yrx$6.50/hr = $7,118/yr say $7,100/yr
Maintenance labor: 416 hr/yrx$6.50/hr = $2,704 say
$2,700/yr
Maintenance materials:  $7,000/yr lump sum
Laboratory:  $1,500/yr lump sum
Capital $1,309,000X0.081  =$106,000
Fuel credit:  9,208 Ibx 0.65 = 5,985 Ib VSS feed/day
    5,985 IbX 10 cf/lb VSS = 59,850  cf/day (1,695
      m3/day) gas
    59,850  ft3 X 500 Btu/ft3 = 29.9 X106 Btu/day
      (31.6xl06kJ/day)
              475,000 Btu/hrx24 hr/day/0.5 eff = 22.8xl06
                Btu/day  (24.1 xi06kJ/day) required for digester
                heat
              29.9 X106-22.8X106 = 7.1 X106 Btu/day (7.5 X106
                kJ/day) excess gas
              7.1 X106 X $2.70 XIO^X 365 = $6,997 say $7,000/yr

            Land  application  costs  were developed for the anaero-
          bic  digestion alternative in a manner similar to that  pre-
          viously described for lime stabilization. Anaerobically di-
          gested sludge land requirements were  less than for  lime
          stabilized  sludge  because of the volatile  solids reduction
          during digestion.  Truck driving and operation costs  were
          similarly less for digested sludge because of the volatile
          solids reduction and  more concentrated  sludge (6.5  per-
          cent versus 4.5 percent) which would be hauled. The
          total fertilizer credit was based on $7.30 per ton
          ($8.05/Mg) of dry solids,  but was  lower  because of the
          lower amount of  dry  solids  applied to the land. The total
          land credit was less  because land requirements were
          based on  the total  amount  of  sludge solids applied.
          Land application  costs for the anaerobic digestion alter-
          native were calculated in  a manner similar to those  for
          the  lime stabilization  alternative and are  summarized in
          table 1-29.
            The total annual  capital and annual operation and
          maintenance costs  for lime  stabilization and two-stage
          anaerobic sludge digestion, including land application for
          a 4 Mgal/d (0.18 m3/s) wastewater treatment plant, are
          summarized in table 1-30.

          Process alternatives—-40 Mgal/d
          (1.75  m3/s) WWTP

            As previously discussed, process alternatives for the
          40  Mgal/d wastewater treatment plant will  be as follows:

            1. Lime stabilization followed by pipeline transportation
               to  the  land application  site.
            2. Anaerobic digestion followed by mechanical dewa-
               tering and land  application.
Table 1-28.—Total annual cost for two-stage anaerobic
sludge digestion excluding land disposal for a 4 Mgal/d
plant
                 Item
Operating labor	
Maintenance labor and materials.
Laboratory	
Capital	
Fuel credit	
                                   Total
                                  annual
                                   cost
 Annual
  cost
 per ton
dry solids
     Total annual cost
$7,100
9,700
1,500
106,000
(7,000)
117,300
$4.23
5.77
0.89
63.08
(4.16)
69.81
                                                    Table  1-29.—Annual cost for land application of anaero-
                                                    bically digested sludges for a 4 Mgal/d plant
                                                                            Item
 Total
annual
 cost
                                                    Amortized cost of land .
                                                    Truck depreciation 	
                                                    Truck driver	
                                                    Truck operation	
                                                    Laboratory	
                                                    Fertilizer credit	
                                                    Land credit	
Annual
 cost
per ton
 solids
                                                               Total annual cost
$8,200
7,000
11,200
14,600
1,500
(8,300)
(5,700)
28,500
$4.88
4.17
6.66
8.69
0.89
(4.94)
(3.39)
16.96
                                                                                                           27

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Table 1-30.—Comparison  of  total annual  capital and
annual O. &  M. cost  for lime stabilization and  anaerobic
digestion including land disposal  for a  4 Mgal/d plant
                          Lime stabilization   Anaerobic digestion
                           Total    Annual
                          annual   cost per
                         O. & M.   ton dry
                           cost     solids
                   Total    Annual
                  annual    cost per
                  O. &  M.   ton dry
                   cost     solids
 Facilities
  Amortized capital	
  Operating labor	
  Maintenance labor and
    materials	
  Lime	
  Laboratory	
  Fuel credit	
      Subtotal facilities ...
 Land application
  Amortized cost of land..
  Truck depreciation	
  Truck drivers	
  Truck operations	
  Laboratory	
  Fertilizer credit	
  Land credit	
      Subtotal land appli-
        cation 	
       Raw sludge percent solids
       Overall lime dosage required per
         unit dry solids as 100 percent
         Ca(OH)2
       Daily lime requirement as Ca(OH)2
       Treatment period
       Bulk lime storage bin volume min-
         imum
       Bulk lime storage bin detention
         time
       Lime slaker & slurry tank capacity
         (2 ea)
       Influent sludge grinder max. ca-
         pacity
       Sludge mixing tank volume at  1
         hr detention time (2 ea)
       Sludge mixing tank dimensions
       Sludge mixer  horsepower (2 ea)
       Sludge mixer  turbine diameter
       Turbine speed
       Sludge thickener dimensions (2
         ea)
       Thickened sludge volume
       Thickened sludge percent solids
       Sludge transfer  pump capacity (2
         ea)
       Intermediate pump station pumps

       Treatment building floor  area
       Treatment building construction
       Instrumentation
 58,600    34.89     28,500    16.96   Lagoon volume at application site
$30,100
19,000
3,400
12,000
1,500
N/A
$17.91
11.31
2.02
7.14
0.89
N/A
$106,000
7,100
9,700

1,500
(7,000)
$63.08
4.23
577

0.89
(4.16)
 66,000    39.27    117,300
69.81
12,200
14,000
22,400
29,200
1,500
(12,300)
(8,400)
7.26
8.33
13.33
17.38
0.89
(7.30)
(5.00)
8,200
7,000
1 1 ,200
14,600
1,500
(8,300)
(5,700)
4.88
4.17
6.66
8.69
0.89
(4.94)
(3.39)
 Total annual cost facilities
    and land application..
124,600    74.16    145,800
86.77
  The design  logic which will be  followed to develop
and evaluate the sludge handling alternatives has previ-
ously been summarized on figure 14. Wastewater char-
acteristics,  treatment unit efficiencies, and sludge charac-
teristics have  also  been  previously summarized.

Lime  Stabilization
  A flow diagram for the proposed lime stabilization fa-
cilities  is shown on figure 1-18. Significant process
equipment  includes a bulk lime storage bin for pebble
quicklime, augers,  lime slakers and feed slurry  tanks,
sludge  mixing tanks, sludge thickeners, sludge  grinders,
all  weather treatment building, electrical and  instrumenta-
tion,  interconnecting piping, and sludge pump stations.
  The sludge  pipeline was assumed  to  be 10 miles (1.6
km) long with two  intermediate pump stations.  One land
application farm site was assumed. A sludge storage
lagoon  with 60-days holding  capacity was provided at
the land  application site.
  The basis for design  is as  follows:
        Pipeline length
        Pipeline diameter
        Pipeline working pressure
        Land application trucks
                                                                   3.9
                                                                   0.20 Ib/lb dry solids
18,250 Ib/day (8,295 kg/day)
24 hrs/day
2 ea 4,260 ft3 (120.6 m3)

30 days

500-750 CaO/hr

2 ea 200 gpm  (12.6 l/s)

12,000 gal  (45.4 m3)

10 ftxiO ft SWD (3 mx3 m)
10 HP (7.5 kW)
5 ft (1.5 m)
45 rpm
65 ft dia. X12 ft SWD (19.8
  mx3.7 m)
240,500  gal/day (910.3 m3/day)
4.5
250 gpm at 200 psi (15.8 l/s at
  14.1 kg  km2)
4 ea 250 gpm  at 200 psi (15.8
  l/s at 14.1 kg/cm2)
600 ft2 (55.7 m2)
brick and block
pH record/control raw sludge vol-
  ume treated  sludge volume
  pipeline  pressure control
10,000,000  gal  (37,850 m3) (20
  cells)
53,000 ft (16,154 m)
6 in (15  cm)
200-250 psig (14.1-17.6 kg/cm2)
12 at 1,500 gal (5.7 m3) ea
Total sludge solids
Sludge volume
     92,080 Ib/day (41,855 kg/day)
     227,540 gal/day (861.2 m3/day)
         The reasons for selecting the particular design quanti-
       ties have been discussed in  previous sections. Sludge
       pump capacities were selected  to permit reasonable
       pipeline pressure drops and velocities. The sludge lagoon
       was divided into  several  cells to  permit  convenient with-
       drawal  of all  sludge and to prevent  solids accumulation.
         Capital costs for the lime stabilization facilities,  based
       on January 1, 1978,  bid  date, excluding  final  sludge
       pumping, pipeline, application trucks, lagoon,  and land,
       are summarized in  table  1-31.
         Lime  stabilization operation assumed  two men, three
       shifts per day, 365 days per year at $6.50 per  hour,
       including  overhead. Maintenance  labor was assumed to
       be 1,664 hours per year at $6.50 per hour and $7,500
       per year for maintenance materials.  The  total  quantity of
       85 percent  CaO quicklime required was 2,966 tons
       (2691 Mg)  per year at $40 per ton  ($447Mg).
         The total annual  cost for lime stabilization,  excluding
       land application of treated sludge, was calculated  in a
       manner to that previously shown  on the  4  Mgal/d (0.18
       m3/s) example and has been summarized in table  1-32.
         Ultimate sludge  disposal was  assumed  to  be as  a
       liquid on  farmland with truck  spreading.  The total land
      28

-------
                                                                                                                   TREATED
SECONDARY



CHLORINATION
EFFLUENT
TO DISCHARGE
Figure  1-18.—40  Mgal/d (1.75 m3/s)  lime stabilization/pipeline  transport
and land application.
Table 1-31.—Capital  cost of lime  stabilization facilities
for a 40 Mgal/d wastewater treatment plant
Site work, earthwork and yard piping	
Lime storage, slakers, and feed	
Lime treatment tanks, mixers, grinders and building.
Sludge thickeners	
Electrical and instrumentation	
      Subtotal construction cost	
Engineering	
      Total capital cost	
Amortized cost at 30 yrs.,  7% int. (CFR = 0.081).
Annual capital cost per unit feed dry solids	
  90,000
1,077,000

  87,200
     5.19
           Table 1-32.—Total annual cost  for lime stabilization ex-
           cluding land  disposal for a  40 Mgal/d plant
Item
Total
annual
cost
Annual
cost per
ton dry
solids
 $95,000
 106,000
 155,000
 529,000
 102,000

 987,000   Operating labor	   $114,000
Maintenance labor and materials.
Lime	
Power	
Laboratory	
      Total annual cost	
 18,300
118,600
  2,000
  4,500
$6.78
 1.09
 7.06
 0.12
 0.27
257,400     15.32
                                                                                                                        29

-------
spreading operation will require  2,000 acres (810 ha).
Land cost was assumed  to  be $1,250 per acre
($3088/ha) to reflect the more urban setting  than  the  4
Mgal/d (0.18 m3/s) case. The capital  cost  per sludge
hauling vehicle was assumed to be $35,000,  with 12
being required. The vehicles were depreciated over a  7-
year period. The sludge holding lagoon was  located at
the farm site and was  sized  to hold 60-days  sludge
production.  The lagoon was  partitioned into 500,000 gal-
lon (1,892 m3) cells to  permit access and efficient utiliza-
tion  of the storage volume.
  The  assumed  hauling time was 10 minutes  to fill, 20
minutes to haul,  empty and  return,  for a total of 30
minutes per round trip. The truck volume was assumed
to be 1,500 gallons (5.7  m3) per load. The  cost  of truck
operations,  excluding the driver  and depreciation, was
assumed to be $8.50 per operating hour. The truck
driver labor rate was assumed to be  $6.50 per hour,
including  overhead.
  Truck operating time was based on hauling on a 215-
day-per-year schedule,  12 hours per day.
  To offset the  land cost, a fertilizer  credit of $7.30 per
ton ($8.05/Mg)  of dry  sludge solids was assumed.  This
rate was  arbitrarily assumed to be 50 percent of the
value published  by Brown11 based on medium fertilizer
market value and low fertilizer content. The reduction
was made to reflect resistance to accepting  sludge as
fertilizer. The land cost was  further offset by  assuming a
return  of  $50 per acre ($124/ha), either as profit after
farming expenses, or as the rental value of the land.
  Easements for the sludge pipeline were assumed to
cost $2.50 per foot ($8.20/m). Two intermediate booster
stations were provided  to maintain a  reasonable pressure
profile along the line. Progressive cavity pumps were
used for both the treatment  plant and intermediate pump
stations. Allowance was assumed to permit regular
cleaning of the  line by utilizing pipeline "pigs."
Table 1-33.—Capital cost of lime stabilization land appli-
cation facilities for a 40 Mgal/d wastewater treatment
plant
Site work, earthwork	
Sludge transfer pumps	
Sludge pipeline	
Booster station	
Sludge lagoon	
Electrical and  instrumentation.
      Subtotal construction cost
Engineering	

      Total capital cost pipeline, pump stations and
       lagoon	
Amortized cost at 30 yrs., 7 percent int. (CFR = 0.081)
Annual capital cost per unit feed dry solids	
 $17,000
  45,000
 675,000
 104,000
 569,000
  19,000

1,429,000

 124.000


1,553,000

 125,800
    7.49
         Table 1-34.—Annual  cost  for transportation and land
         application  of  lime stabilized sludge for a 40 Mgal/d
         plant
Item
Land 	
Easements 	
Pipeline, pump stations and
lagoon 	
Truck depreciation
Truck drivers .
Truck operation
Power 	
Pipeline operation and mainte-
nance 	
Laboratory 	
Fertilizer credit 	
Land credit 	

Total annual cost

Capital
cost
$2 500 000
1 32 000
1 553 000
420 000








4 605 000

Total
annual
cost
$202 500
10700
1 25 800
60000
201 200
263 200
35000
17000
4500
(122700)
(84 000)

713200

Annual
cost pe
ton dry
solids
$1205
064
749
357
11 97
1566
208
1 01
027
(730
(500

42 44

  Capital costs for the lime stabilization land application
site, based  on January 1, 1978, bid date, have been
summarized in table 1-33.
  Annual operation and maintenance costs for transpor-
tation and land application of lime stabilized sludge were
calculated in a manner similar to that previously summa-
rized  and have been shown in table  1-34.

Anaerobic Digestion

  A flow diagram for the proposed anaerobic  diges-
tion/vacuum filtration alternative is shown on figure 1-
19. Significant process equipment includes two-stage
standard  rate anaerobic sludge digestion,  bulk lime  and
ferric chloride  storage, lime slakers, vacuum filtration,
sludge conveyors, and sludge storage bin. All  facilities
were  assumed to be housed in an all weather brick-
block type building and included  all electrical, instrumen-
tation, interconnecting piping,  and sludge  pumps. The
existing sludge dewatering  equipment was assumed not
to be capable of functioning over the project  life and
was replaced.  Similarly, the existing filter building and
chemical  feed  facilities were replaced.
  Design data for the anaerobic  digester alternative are
as  follows:
Primary digesters

Secondary digesters

Vacuum filtration
Vacuum filter loading rate

Lime storage bin
3 ea 110 ftx30 ft (33.5 mX9.1
  SWD
3 ea 110 ftx30 ft (33.5 mx9.1
  SWD
3 ea at 400 ft2 ea (37.2 m2)
3.5 Ib dry solids/ft2/hr (17.1
  kg/m2/hr)
1  ea  4,000 ft3 (113.3 m3)
m)

m)
     30

-------
SECONDARY



CHLORINATION
EFFLUENT
TO DISCHARGE *"
                                               -r—WASTE ACTIVATED  SLUDGE
THICKENED
SLUDGE
^•-
c
t

o o
•!• STAGE
ANAEROBIC
DIGESTER
(3 EACH)
— ,
35
STAGE
ROBIC
STER
ACH)

	 • 	
OIOESTED


4-
>


VACUUM
FILTER
(3 EACH)



                                                                                                 TRUCK  TO
                                                                                            LAND APPLICATION SITE
                                                                                  -LJL
                                                                                     oo  oo
                                                       SLUDGE
Figure 1-19.—40  Mgal/d  (1.75  rrrVs) anaerobic digestion/vacuum filtration
and land application.
Lime slaker/feeders

Ferric chloride storage tanks
Dewatered sludge storage bin
Filter building
Digester loading—1st stage
Hydraulic detention time—1st
  stage
Digester gas production
Digester gas heat value
Volatile solids reduction
Overall solids reduction
Sludge mixers
Digester heat requirement
  (primary only)
Gas production
Net gas available
3 at 250-500 Ib CaO/hr (113.6-227.3
  kg/hr)
2 ea at 5,000 gal ea (18.9 m3)
1 ea at 2,000 ft3 (56.6 m)
3,000 ft2 (278.7 m2) w/basement
0.07 Ib VSS/ft3/day (1.1  kg/m3/day)
23 days

10 ft3/lb VSS feed (0.6 m3/kg)
500 Btu/ft3 (18,625 kJ/m3)
50 percent
32 percent
4 at 5,000 gpm ea (315.4 l/s)
22.7X107  Btu/day (2.8X106W)

30.0 x 107  Btu/day (3.7 X106 W)
7.3 x 107 Btu/day (0.9 X106 W)
  Design conditions were  based on the criteria enumer-
ated in Ten  States'  Standards28 and assumed  installation
in  the  midwest.
  Annual capital costs operation and  maintenance for
the anaerobic digestion facilities were based on the Jan-
uary 1, 1978 bid date  and have been summarized in
table 1-29.  Capital costs  included  the digesters, control
buildings, covers, heat exchangers,  gas safety equip-
ment,  interconnecting  piping, engineering, legal  and  ad-
ministrative costs.  Capital  costs are summarized in table
1-35.
  Digester operation assumed one man,  two shifts per
day, 365 days per  year, at  $6.50,  including overhead.
Maintenance labor  and material assumed 4,160 hours
per year  at  $6.50  per hour  and $30,000 per year for
maintenance materials.
  The  cost of anaerobic digester operation was offset
by assuming a value  of $2.70 per  million Btu ($2.56 per
million kJ) for all digester  gas  produced above the  net
digester heat requirement. Total annual  operation and
maintenance  cost for the digestion facilities is  summa-
rized  in table 1-36.
                                                                                                                31

-------
Table 1-35.—Capital  cost of  two-stage anaerobic diges-
tion  facilities for a 40  Mgal/d (1.75 m3/s)  wastewater
treatment plant
                                                        Table 1-38.—Vacuum filtration capital  and annual  opera-
                                                        tion and maintenance costs for a 40 Mgal/d  (1.75  m3/s]
                                                        plant
Site work, earthwork, yard piping	   $688,000
Digesters and control building	   7,222,000
Pumping	     35,000
Electrical and instrumentation	    745,000

      Subtotal construction cost	
                           	   8,690,000

                           	    649.000

Total capital cost	   9,339,000
Engineering.
Amortized cost at 30 yrs., 7 percent int. (CFR = 0.081)
Annual capital cost per unit feed dry solids	
                                               756,500
                                                 45.02
Table  1-36.—Total annual cost for two-stage anaerobic
sludge digestion excluding vacuum  filtration and land
disposal for a 40 Mgal/d (1.75 m3/s)  plant
Item
Operating labor 	
Maintenance labor and materials 	
Laboratory 	
Capital 	
Fuel credit 	

Total annual cost 	

Total
annual
cost
$38 000
. . 57,000
6000
	 756 500
	 (71 ,900)

	 785,600

Annual
cost per
ton dry
solids
$226
3.39
0.36
45.02
(4.28)

46.75

   Capital costs for the filtration  facilities are  summarized
 in table 1-37
   Vacuum filtration costs were estimated  as summarized
 in table 1-38.
   Land application costs were calculated based on  haul-
 ing 20 miles (32  km) round  trip. A sludge transfer site
Table 1-37.—Capital cost for vacuum filtration facilities
for a 40 Mgal/d (1.75 m3/s) wastewater treatment plant
Site work, earthwork, yard piping.
Chemical storage and feed	
Filtration equipment	
Filter and chemical building	
Sludge loading pad	
Electrical and instrumentation	

      Subtotal construction cost.
Engineering	

      Total capital cost	

Amortized cost at 30 years., 7 percent int. (CFR =
Annual capital cost per unit feed dry solids	
                                     < 0.081).
$297,000
 177,000
 546,000
 230,000
  78,000
 322.000

1,650,000

 140,000

1,790,000

 145,000
    8.63
                                                                          Item
                                                     Total
                                                    annual
                                                     cost
                                                   Annual
                                                   cost pe
                                                   ton dry
                                                    solids
Variable cost
Electric power 	
Chemicals
Lime 	
FeCI3 	
Maintenance materials. ... 	
Maintenance labor
Laboratory 	

Subtotal variable cost
Operator labor ... 	
Supervision 	
Capital 	

Subtotal fixed cost
Total annual cost

	 $7,100
91 400
52 000
. . 7800
25800
	 6000

190100
47000
15000
	 145000

207000
397100

$0.42
544
309
046
1 54
036

11 31
280
089
863

1232
2363

was assumed to be  located  at  the  land application site.
Sludge  transfer trucks were  assumed to be equipped
with 8 yd3 (6.1 m3) dump beds. A total of four trucks
were required, operating 8 hours per day, 215 days per
year. The  loader and land spreading  vehicle were as-
sumed to operate 8  hours per  day. Land application
vehicles were assumed to  have 17  yd3 (13 m3) capacity.
Sludge  application rate assumed 7  dry tons  (6.4  Mg) pei
hour, including loading  time. The land application vehicle
was depreciated on  a straight-line basis over a 7-year
period.  Sludge hauling was based  on current rental
costs for equipment.  Dewatered sludge was  assumed to
be  22 percent dry solids.
  Anaerobically digested sludge land  requirements were
less than for lime stabilized  sludge  because  of the vola-
tile solids  reduction  during digestion.  The fertilizer value
and land rental return credits were taken as  previously
described  in the 4 Mgal/d (0.18 m3/s) design case.
Table 1-39 summarizes the  total land application cost.
  To summarize, the total  cost for  the  lime stabilization
and anaerobic digestion alternatives,  including ultimate
disposal, is shown in table 1-40.
  In the 4 Mgal/d case (0.18  m3/s),  the total annual
cost for the lime  stabilization alternative is $74.16 per
dry ton  ($81.75/Mg) compared to $86.77  per dry ton
($95.65  Mg) for  anaerobic digestion.  Each of these  alter
natives assumed liquid  application  to farmland, with  a 3-
5 mile (5-8  km) round  trip hauling  distance. With in-
creasing haul distances, lime stabilization will be  de-
creasingly cost effective because of the greater volume
of sludge  which  must be transported.
  In the 40  Mgal/d  case (1.75 rrrVs), the total annual
cost for lime stabilization alternative is $62.94 per dry
     32

-------
Table  1-39.—Annual cost for land application of dewa-
tered anaerobically digested sludges for a 40 Mgal/d
(1.75 m3/s) plant
Item
Amortized cost of land 	
Truck depreciation (spreader only)
Truck drivers
Truck and loader operation
Laboratory 	
Fertilizer credit 	
Land credit

Total annual cost 	

Total
annual
cost
	 $202,500
12100
67100
260,600
	 4,500
	 (83,400)
	 (57,000)

	 406,400

Annual
cost per
ton dry
solids
$12.05
072
3.99
15.51
0.27
(4.96)
(3.39)

24.19

  ton  ($69.38/Mg) compared  to  $94.56 per dry ton
  ($104.23/Mg) for anaerobic digestion. The cost of pipe-
  line  transportation/land  application of the liquid sludge is
  $42.44 per dry ton ($46.78/Mg) compared to $47.82 per
  dry  ton ($52.71 /Mg) for dewatering and land application.
  The pipeline  alternative  also has the disadvantage of
  being inflexible for long-term implementation. With the
  dewatered sludge and truck hauling system,  sites could
  be changed  with  little difficulty.


  REFERENCES
  1. Riehl, M. L. et al, "Effect of Lime Treated Water on Survival of
     Bacteria," Journal American Water Works Assn., 44,466 (1952).
  2. Grabow, W. O. K. et al., "The Bactericidal Effect of Lime Floccu-
     lation Flotation as a Primary Unit Process in a Multiple System for
     the Advanced Purification of Sewage Works Effluent,"  Water Re-
     sources 3,  943 (1969).
  3. Buzzell, J.  C., Jr., and Sawyer, C. N., "Removal of Algal Nutrients
     from Raw Wastewater with Lime," Journal WPCF, 39, R16, 1967.
Table 1-40.—Comparison of total annual  capital and  annual O. & M. cost
for lime stabilization and anaerobic digestion including land disposal for  a
40 Mgal/d (1.75 m3/s) plant
                                       Lime stabilization
Anaerobic digestion
Total
annual
O. & M.
cost
Annual
cost per
ton dry
solids
Total
annual
O. & M.
cost
Annual
cost per
ton dry
solids
Facilities
Amortized capital lime stabilization 	
Amortized capital digesters 	
Amortized capital filtration 	
Operating labor 	
Maintenance labor and materials 	
Chemicals 	
Laboratory 	
Fuel credit 	
Power 	
Subtotal facilities 	
Land Application
Amortized cost of land, facilities and
easements 	
Truck depreciation 	
Truck drivers 	
Truck operations 	
Pipeline O. & M 	
Power 	
Fertilizer credit 	
Land credit 	
Laboratory 	
Subtotal land application 	
Total annual cost facilities and land
application 	

$87,200
N/A
N/A
114,000
18,300
118,600
4,500
N/A
2,000
344,600


339,000
60,000
201,200
263,200
17,000
35,000
(122,700)
(84,000)
4,500
713,200

1 ,057,800

$5.19
N/A
N/A
6.78
1.09
7.06
0.27
N/A
0.12
20.51


20.17
3.57
11.97
15.66
1.01
2.08
(7.30)
(5.00)
0.27
42.43

62.94

N/A
a $756,000
a1 45,000
a1 00,000
a90,600
a 143,400
a12,000
a(71 ,900)
a7,100
a1,182,700


a 202,500
a 12,1 00
a67,100
a 260,600
N/A
N/A
a(83,400)
a(57,000)
a 4,500
3 406,400

a 1,589,1 00

N/A
$45.02
a8.63
"5.95
a5.39
a8.53
a0.71
a(4.28)
a042
"70.37


a 12.05
a0.72
a3.99
a15.51
N/A
N/A
a(4.96)
a(3.39)
a0.27
a24.19

a94.56
  "Includes cost for digestion and vacuum filtration.
                                                                                                               33

-------
 4.  "How Safe is Sludge?" Compost Science 10 March-April  1970.
 5.  Kempelmacher,  E. H. and Van Noorle Jansen,  L. M., "Reduction
    of Bacteria in Sludge Treatment," Journal WPCF 44, 309 (1972).
 6.  Evans, S. C., "Sludge Treatment at Luton," Journal Indust  Sew-
    age  Purification 5,  381, 1961.
 7.  Farrell, J. B., Smith, J. E., Hathaway, S. W., "Lime  Stabilization of
    Primary Sludges," Journal  Water Pollution Control Federation, vol.
    46, No. 1, January 1974, pp. 113-122.
 8.  Paulsrud, B  and Eikum, A. S., "Lime Stabilization of Sewage
    Sludges," Norwegian Institute for Water Research, volume 9, pp.
    297-305, 1975.
 9.  Counts, C A., Shuckrow, A. J., "Lime Stabilized Sludge:  Its Sta-
    bility and Effect on Agricultural Land," EPA-670/ 2-75-012, April
    1975.
10.  Noland, R  F., Edwards, J.  D., "Stabilization and Disinfection of
    Wastewater Treatment Plant Sludges," USEPA Technology Trans-
    fer Design Seminar Handout,  May 1977.
11.  Brown, R. E. et al., "Ohio Guide for Land Application of Sewage
    Sludge," Ohio Agricultural Research  and Development Center,
    Wooster, Ohio,  1976.
12.  Sommers, L. E., "Principles of Land  Application of Sewage
    Sludge," USEPA Technology  Transfer Design Seminar Handout,
    May 1977.
13.  Sommers, L. E., et al., "Variable Nature of Chemical Composition
    of Sewage Sludges," Journal of Environmental Quality 5:303-306.
14.  Stern,  Gerald, "Reducing the Infection Potential of Sludge Dispos-
    al," presented at Northwest Regional Physical Chemical Wastewater
    Treatment Short Course at University of Washington, Seattle, March 25,
    1975
15.  U.S. Environmental Protection Agency, "Process Design  Manual
    for Sludge Treatment and Disposal," USEPA Technology  Transfer,
    1006, Oct 1974
16.  U.S. Environmental Protection Agency, "Municipal Sludge Manage-
    ment: Environmental Factors," Federal Register, vol. No. 41, No.
    108, p. 22533.
17. Trubnick,  E.  H., Mueller, P.  K., "Sludge Dewatering Practice,"
    Sewage and  Industrial Wastes 30, 1364 (1958).
18. Sontheimer, H., "Effects of Sludge Conditioning with Lime on De-
    watering," Proc. 3d  Int'l Conference, Water Pollution Research,
    Munich, 1966, in Advances in Water Pollution  Research.
19. Zenz, D.  R.,  Lynam, B. T., el al., "USEPA Guidelines on Sludge
    Utilization and Disposal—A Review of Its  Impact Upon Municipal
    Wastewater Treatment Agencies," presented at the 48th Annual
    WPCF Conference, Miami Beach, Fla.,  1975.
20. National  Lime Association, "Lime Handling Application and Stor-
    age in Treatment Processes  Bulletin  213," National Lime Associa-
    tion, Washington, D.C.,  pp. 1-3.
21. Badger and Banchero,  "Introduction  to  Chemical  Engineering,"
    page 614, McGraw-Hill, 1955.
22. Hicks, R. W. et al.,  "How to Design Agitators for  Desired Process
    Response," Chemical Engineering, April 26, 1976,  pp. 103-106  ff.
23. Fair, G. M. and Geyer, J. C., "Water Supply and Wastewater
    Disposal," John Wiley & Sons, New  York,  1956.
24. USEPA, "Methods for Chemical Analysis of Water and Wastes," USEPA,
    Technology Transfer 625/6-74-003a, Cincinnati, Ohio, 1974.
25. Standard  Methods for Examination of Water and Wastewater, 13th
    and 14th  Editions, AWWA, APHA, WPCF,  American Public Health
    Association, Washington, D.C.
26. Kenner and Clark, "Enumeration  of Salmonella and Pseudomonas
    aeruginosa,"  Journal WPCF,  vol.  No. 46.,  No.  9, September 1974,
    pp. 2163-2171.
27. USEPA,  "Lime Use in Wastewater Treatment:  Design and  Cost
    Data," Office of Research and Development, U.  S. EPA-600/2-75-
    038, October 1975.
28. Recommended Standards for Sewage Works,  Health Education
    Service,  Albany, N.Y. 1971,  pp.  57-64.
29. U.S. EPA, "Full Scale Demonstration of Lime  Stabilization," Office
    of Research and Development, U.S. EPA, 600/2-78-171, Cincinnati, Ohio
      34

-------
                             Chapter 2
                             Anaerobic  Digestion  and  Design
                             of  Municipal  Wastewater  Sludges
The claimed  advantages of the anaerobic digestion pro-
cess are:1'2
  • Low sludge production.
  • The  production of a useful gas of moderate caloric
     value.
  • A high kill  rate of pathogenic organisms.
  • Production  of a solids residue suitable for use  as a
     soil conditioner.
  • Low operating  cost.
  Table 2-1  indicates the  kinds of sludges which  have
been  studied on  a full-scale basis.
  In the  past 50 years municipal wastewater sludge has
changed from simple primary sludge of purely domestic
origin to complex sludge mixtures (primary, secondary,
chemical) of  domestic and industrial origins.
  At first  when design engineers only had to consider a
primary sludge, the  developed rules of thumb22 were ade-
quate. As the sludge generated became more complex,
more  and more  systems failed and the  process devel-
oped  a "bad reputation." The use of steady state mo-
dels in the 1960's,23~2s dynamic models in the 1970's,26"31
and research into the basic biochemical processes32"35
has led to significant improvements both in the design
Table 2-1.—Type  and reference of full-scale studies on
high rate anaerobic digestion of municipal wastewater
sludge
                                Reference on
                                 mesophilic
Reference on
 thermophilic
Primary and lime	  3,4
Primary and ferric chloride	  5
Primary and alum	  5
Primary and trickling filter	  7,8
Primary, trickling filter and alum	  9
Primary and waste activated	  10,11,12       11,13,14
Primary, waste activated and lime	  15,16
Primary, waste activated and alum	  15,17,18
Primary, waste activated and ferric
  chloride	  15
Primary, waste activated and sodium
  aluminate	  17,18
Waste activated only (pilot plant only)  19,20,21       19,20,21
and  operation of the process. Still the transfer of data
from the laboratory to the real world can be difficult.

GENERAL  PROCESS DESCRIPTION

  Anaerobic digestion of municipal wastewater sludge  is
a two-step, very complex biochemical process, depend-
ent on many physical (temperature, solids concentration,
degree of mixing, organic loading, detention time) and
chemical (pH, alkalinity, volatile acid level,  nutrients, tox-
ic  materials) factors.  Probably the easiest way to visual-
ize what is taking place is to think in terms of a two-
step process.
  In  the  first step, facultative microorganisms (sometimes
called acid forming bacteria) convert complex organic
waste sludge substrate  (proteins,  carbohydrates,  lipids)
into  simple organic fatty acids by hydrolysis and  fermen-
tation. The principal end products, with sludge as sub-
strate, are acetic acid,  approximately 70 percent, and
propionic acid, about 15 percent.36"38 The microorganisms
involved  can  function over a wide environmental  range
and  have doubling times normally measured in hours.
  In  the  second step, strictly anaerobic microorganisms
(sometimes called methane-forming bacteria) convert the
organic  acids  to methane,  carbon dioxide and  other
trace gases. The bacteria involved are much more sensi-
tive to environmental  factors than step one bacteria and
normally  have doubling  times measured  in days. Because
of this, step two  bacteria control the overall process.
  Figure  2-1  gives  an overview of the entire process.
For a more  complete review the  reader is  referred to
either Kirsch35 or  Toerien.32

MESOPHILIC—THERMOPHILIC DIGESTION

  Temperature can  be considered one of the  most im-
portant factors in the anaerobic  digestion process. Even
though the total temperature range  for operation  of the
process is very broad, specific microorganisms often
have relatively narrow temperature ranges in which they
can grow.
  For the purpose of classification,  the following  three
temperature  zones of bacterial action will be used
throughout this chapter:
Cryophilic zone	  Liquid temperature below 10°C (50° F)
Mesophilic  zone 	  Liquid temperature between 10°C to 42° (
                       (50°F to 108°F)
Thermophilic zone	  Liquid temperature above 42° C (108° F)
                                                                                                       35

-------
Raw sludge
Complex
substrate
Carbohydrates,
fats and
proteins


Micro-
+ organisms
"A"
Principally
acid formers





K1 Non-
products
C02, H2O

Stable and
intermediate
degradation
products
Cells
Reactive
products
Organic acids

Cellular and
other inter-
mediate
degradation
products
Micro- K2
organisms 	 ^- CH^
"B"
Methane
fermenters





Other
i- C02 end
products
~H2O, H2S

Cells and stable
degradation
products


Figure 2-1.—Summary of anaerobic digestion  process.
                                                    39
  In the  past, the vast  majority of  lab, pilot, and  full-
scale research has been  done in the  mesophilic range
and some in the thermophilic range. The reason for this
is that thermophilic digestion did not seem economical
because  of  the  higher energy  requirements and the gen-
eral feeling  that operation at the higher temperature
would  be highly unstable.  Recently, the literature  indi-
cates that there is a renewed  interest  in thermophilic
digestion40 because of its elimination of pathogens,  high
reaction  rates and possibly higher  gas yields and better
dewaterability.

VOLATILE SOLIDS  REDUCTION

  One of the main objectives of the anaerobic digestion
process  is  to reduce the amount of solids that need to
be  disposed. This reduction  is normally assumed  to take
place only  in the  volatile content of the sludge and it  is
probably safe to assume only  in the biodegradable vola-
tile  fraction  of the sludge. Research into the area of the
biodegradable fraction  is quite limited  but the following
generalities  can be used:
  1. Approximately 20  to 30 percent of the influent sus-
     pended solids of a typical domestic wastewater is
     nonvolatile.41  Of  the  remaining suspended solids that
     are volatile, approximately 40 percent are nonbiode-
     gradable organics consisting chiefly of lignins, tan-
     nins, and other  large complex molecules.
  2. For waste  activated sludges generated from sys-
     tems having  primary treatment, approximately 20 to
     35  percent of the volatile solids  produced are non-
     biodegradable.42'43
  3. For waste  activated sludges generated from the
     contact-stabilization  process (no  primaries—all in-
     fluent flow into  aeration tank), 25 to 35 percent of
     the volatile suspended solids  are
     nonbiodegradable.44

  Though it  is realized that  only the biodegradable frac-
tion can actually  be  destroyed, all past research and
most of  the  present  day  work report  on volatile solids
destroyed without making any distinction between biode-
gradable and  nonbiodegradable. Because of  lack of
data, all reference  here  to solids destruction will be
based on volatile solids  only.
  Figures 2-2, 2-3, and 2-4 show the effect of sludge
age  and temperature on volatile solids reduction for
three common sludges.
   70 -
O

0
UJ
EC

>
I-

LU
O
   60
   50
   40
             A A
                               • PI LOT PLANT43
                               * PI LOT PLANT44
        200 400  600  800 1000 1200 1400 1600 1800
              TEMP <°C) X SLUDGE AGE (DAYS)

Figure 2-2.—Volatile  solids versus reduction  versus tem-
perature x sludge age for anaerobically digested primary
sludge.
UJ
CJ
(T
UJ
D.
   70


   60


   50


   40


   30 -
       /A
                                     10
                         •  FULL SCALE
                         A  PI LOT PLANT47
                           FULL SCALE
                                     13
                                       1
        200 400  600  800 1000 1200 1400 1600
           TEMP (°C) X SLUDGE AGE (DAYS)


Figure 2-3.—Volatile solids versus reduction versus tem-
perature X sludge age for anaerobically digested mixture
of primary and waste-activated  sludge.
     36

-------

z
o
h-
o
O
LU
DC
C/3
1-
z
L1J
O
oc
LU
Q.


60


50

40

30

20

10

-


. A ' • ' '
A A . " •
A • *m m •

" ' •*•
. . • A PILOT PLANT19
* • PI LOT PLANT20
• PI LOT PLANT20

i i I I I i i I I i
200 400 600 800 1000 1200 1400 1600 1800 2000


•









1
2200
                                                         Table 2-2.—Concentration—organic loading—time  pa-
                                                         rameters for several  full-scale  anaerobic  digestion facili-
                                                         ties
Figure 2-4.—Volatile  solids versus  reduction versus tem-
perature x sludge age for anaerobically digested waste-
activated sludge.
  Though the data is somewhat scattered, the following
generalizations seem valid.
  1. For all three  sludges the practical upper limit of
     volatile solids  reduction seems  to be 55 percent.
     Though  it was noted that approximately 60 percent
     of the volatile solids are biodegradable, figures  2-2,
     2-3, and 2-4  suggest  that practically all the biode-
     gradable fraction  is being consumed.
  2. The data seem to indicate that under the same
     design conditions primary sludge  will degrade faster
     than a mixture of primary and waste activated,
     which in turn degrade  faster than straight waste
     activated. This implies  that adjustments must be
     made in  design depending on the type  of sludge to
     be processed.

SOLIDS CONCENTRATION—
ORGANIC LOADING—SLUDGE  AGE
  Considerable capital cost  savings  could be realized if
the anaerobic digestion process could be operated at
higher organic loadings and  shorter detention times than
commonly used  today.
  There  have been several  pilot plant  studies48"51 which
have  been able to  operate  at levels approaching  4-5
days  residence times,  organic loadings approaching 0.5
Ib volatile solids/cu ft/day (8.0 kg/m3/day) and solids
concentrations up to 12-15  percent solids. Unfortunately,
pilot plant digesters are  ideally mixed  and environmental-
ly controlled, and scaling up the results  can be difficult.
  Nevertheless, over the years there have been several
full-scale facilities which  were and still are being oper-
ated successfully at short detention  times, high  organic
loadings and high  solids concentrations.  Some of  these
plants are listed in  table 2-2.
  Solids concentration.—It must be remembered that the
solids concentration within the digester affects the vis-
cosity which  in turn affects  the ability  of  the mixing
Feed solids
concentration,
percent
6.0
6.6
69
4 6-52
5.0
63
80

Organic load
Ibs VS/ftVday
016
017
0 15-038
0.28
0 13-0 17
03
0 16
015

Hydraulic
retention
time3 days
150
144
117 159
8.0
14
10
165
21 0

Reference
10
52
53
54
12
12
55
56

                                                          aAII data based on primary digester only. Digester equipped with
                                                         mixing and sludge heating.
equipment (see section on mixing). Also,  because of the
solids reduction  taking place,  the  solids concentration
within the digester is less than the feed solids concen-
tration. Though it depends on sludge type,  the practica-
ble upper limit on the feed  solids  concentration is in the
range of 8 to 9 percent. With a properly designed mix-
ing system this will not cause any operational problems
within the digester.
  Organic loading rate.—The  organic loading rate is a
function of the solids concentration within the digester
and system  sludge age. These two parameters are im-
plicit when one speaks of a loading rate  of pounds
volatile solids per cubic foot per day.  As is shown in
table 2-2 designing  a digestion  system to operate at
0.15 to  0.20 Ib VS/cu ft/day  (2.4 to 3.2  kg/m3/day) is
no problem.
  Sludge age.—At present,  high-rate (mixed and heated)
primary  digesters to  recycle concentrated digested solids
are not  practiced; therefore, hydraulic residence  time
and sludge  age  are  almost  synonymous. As noted in
table 2-2, a minimum  time of  15 days  in  the primary
digester is very  practicable. It should be  remembered
though that this time is also related to sludge type and
tank temperature, as was shown in figures  2-2, 2-3,
and 2-4.
  There seems to be an important relationship between
the above design parameters.  In a study  conducted  by
Clark,51 involving solids concentration, organic loading
rate, and  sludge  age,  the curve shown in figure  2-5 was
developed.
  The shape of  the  probable  digestion  limit  curve, i.e.,
higher organic loadings as the sludge age decreases, is
a reflection  of the accumulation of various  system by-
products which may reach inhibitory concentration levels.
If for a  given digester volume and organic  loading rate
the sludge age is increased (only  possible if influent
                                                                                                         37

-------
 15
 si
 st
§3
1.0


0.8


0.6


0.4


0.2


0.0
                     PROBABLE DIGESTION LIMIT'
                     10   15    20    25    30

                         SLUDGE AGE (DAYS)
                                       35    40
 Figure 2-5.—Relationship between solids concentration—
 organic loading—sludge age limits for anaerobic diges-
 tion.51
sludge concentration is increased), then the chance for
potential inhibitory byproduct  concentration levels also is
increased.
  An  engineer designing a high-rate  primary digester
might, e.g.,  determine that an organic loading rate  of 0.5
Ib VS/cu ft/day (8.0 kg/m3/day)  and a sludge age (hy-
draulic detention time) of 15 days is possible  (figure 2-
5, point 1).  If the detention time were doubled (point 2)
by doubling the influent solids concentration (volume of
tank stays  the same), the digester would fail.  If instead,
the tank volume is doubled (point 3) rather than dou-
bling  the influent solids concentration, the unit would still
be operating on the failure  boundary and nothing would
be gained. As a third alternative,  halving the loading
rate  by doubling the tank volume (point 4),  assuming the
influent solids concentration is halved, would be accept-
able.  Finally, if the loading rate is to be maintained at
0.5 Ib VS/cu ft/day (8.0  kg/m3/day), the sludge age
(hydraulic detention  time) should  be  decreased since the
tank volume is fixed  to allow  a lower influent  solids
concentration.

MIXING

  Mixing in  an anaerobic digester that treats municipal
wastewater  sludge of domestic  origin is considered  to
have  the following benefits. (Note: It is  assumed that a
favorable environment exists to allow development of an
anaerobic digestion  system.)

  •  It keeps the  food supply uniformly  dispersed and in
     constant contact with the growing  cells to promote
     maximum utilization of  the system.
  •  It keeps the  concentration of biological end prod-
     ucts at their lowest value by dispersing them uni-
     formly  throughout the digester.
  •  It provides environmental  uniformity (temperature,
     nutrients, etc.)  throughout the digester allowing best
     possible cell  development.
  •  It allows fairly fast dispersion of any toxic  material
     entering the system, thus, possibly minimizing  its
     effect on the anaerobic process.
  •  It assists in the prevention of a scum layer  buildup
     at  the top  of the digestion tank.

  At the present time not  many in the environmental
engineering field would dispute the  advantages of mixing
in  an anaerobic digester; however, problems arise with
such questions as what  is adequate mixing, how do you
define mixing, how do you specify mixing, etc.
  Before any discussion about mixing can be developed,
some time must be  spent  discussing what and where
this mixing is to take place.

Defining Mixing

  In  recent years  it  has become popular to use the terrr
"complete mix"  when discussing biological process  reac-
tors. Unfortunately, engineers associate this term on a
time scale as applied to activated sludge systems when
talking about mixing an  anaerobic digester.
  The term "complete mix"  means that the  time  for
dispersion of the feed stream  is  short  in relation to  the
total hydraulic residence time  in  the reactor. It is also
defined  as sufficient mixing so that concentration  gra-
dients of chemical and biological  ingredients are uniform
for the  particular reaction  rates that exist in the  basin.
  Present-day "complete mix" activated sludge systems
have hydraulic residence times of approximately 3 hours
based on plant  influent flow.  Generally a "turn over
rate" of 15 to 20 minutes is  considered sufficient to
achieve "complete mix" conditions within the aeration
basin. This would give a turnover rate to hydraulic de-
tention  time ratio of 0.08.  Present-day high-rate primary
digesters have hydraulic detention times of  12 to  17
days. This would seem to imply that a "turnover rate"
of about 1 day  would provide complete mix conditions
within the system.
  Mixing within  the  anaerobic  digestion tank occurs on
two  levels: macromixing  and micromixing.35 Macromixing
deals with the bulk  mass flow within the digester, while
micromixing deals with  the degree of intermingling of the
system  molecules. In biological theory, "complete mix"
assumes micromixing.
  The actual mixing of the sludge within the digester
can  be  by gas  recirculation, mechanical, or a combina-
tion  of  the two. Malina and  Miholites60 describe all
present-day systems.
  No matter  what type of device  is utilized  the intent  is
to achieve mixing through a pumping action.  Because  of
this  relationship, engineers have  come to use the term
horsepower/unit volume as some type of parameter to
define mixing  in an  anaerobic digester. Unfortunately,
this term by  itself has no  meaning. For mechanical  type
mixers the wide variation in  impeller diameters and
speeds  can result in similar  horsepower  but widely differ-
ent pumping  capacities.  For gas mixing systems, gas
flow, depth, and bubble size  can also result in similar
horsepower but widely different pumping capacities.
  Probably the  best way to  evaluate  mixing  is from the
standpoint of  zone influence (figure 2-6): energy is  dissi-
pated with movement horizontally away from the energy
source.  The loss due to friction  between the  fluid mole-
     38

-------
                        TOP VIEW
                  PROFILE VIEW
                                           LIQUID HEIGHT
      D., = EFFECTIVE ZONE DIAMETER FOR MICROMIXING.

      D2 = EFFECTIVE ZONE DIAMETER FOR MACROMIXING.


Figure 2-6.—Shear-stress  relationship for a thixotropic
pseudo plastic material.
cules is a function of  liquid density, temperature, and
solids concentration. Within a certain  area of the point
source there is sufficient energy to achieve micromixing.
There is also  a larger area where bulk flow (macromix-
ing)  still takes place even though there  is insufficient
energy for micromixing.
  Presently, the only published work that  could  be  found
discussing this concept  in the sanitary engineering  field
was  done by  the  EPA.61'62 This concept  indicates that the
width of the micromixing zone  in  water  is no more  than
twice the  liquid depth, with the  liquid  depth being a
function of the type of mixing device  utilized and not
necessarily the tank liquid depth.  It is probably safe to
assume that for thickened, anaerobically digested sludg-
es, the zone of influence for any  given energy  input is
smaller than for mixing plain water.
 CHARACTERISTICS OF AN
 ANAEROBIC DIGESTER
   The existing trend in wastewater treatment  is to re-
 move  more and  more material from the main  liquid  proc-
 essing stream. This is done through the use of secon-
 dary biological treatment schemes, chemical addition,
 and filters. The sludge produced can vary widely and
 change rapidly even on an hour-to-hour basis.
   Table 2-3 gives specific gravity and  particle size dis-
 tribution on two  common  type sludges: plain  primary and
 plain  waste activated.
   There  is little data on the rheology of municipal waste-
 water  sludge  and even less on anaerobically  digested
 sludge.48'59 One of the main problems is that it is ex-
 tremely difficult to do such studies correctly.58
   Even though the  majority of raw wastewater sludges
 behaves as a thixotropic (time dependent),  pseudo  plas-
 tic, material (figure  2-7), it may not be correct to as-
 sume that the sludge within the anaerobic digester  has
                                                          Table 2-3.—General characteristics  of raw primary and
                                                          waste activated sludge57
                                                                                     Primary sludge
                                           Waste activated
                                               sludge
                                                          Specific gravity	  1.33-1.4         1.01-1.05
                                                          Particle size	  20% <  1 pm     40%  1-50 ^m
                                                                                    35%  1-100 urn   60%  50-180 fim
                                                                                    45% >  100 turn
                                                          Physical appearance	  Fibrous          Slimy, gelatinous
 CO
 CO
 LU
 DC

 CO
 oc
 <
 in
 I
 CO
                       RATE OF STRESS
Figure 2-7.—Schematic of zone of mixing  influence for
energy source  in fluid with only fixed upper and  lower
boundaries.
                                                                                                          39

-------
the same general properties. The liquid  within the tank is
normally at a  higher temperature where there is entrap-
ment of gas bubbles  and a general reduction in particle
size, all of which affect fluid viscosity.
  At the present time  anaerobic digestion tanks have a
ratio of inside tank diameter to average liquid depth of
1:1  to  5:1. This imposes some restrictions on the  ability
to develop a mixing regime.

SUPERNATANT

  Poor quality  anaerobic digester supernatant is a major
operational problem at many municipal wastewater treat-
ment plants. The supernatant will most likely contain
high concentrations of carbonaceous organic materials,
dissolved and suspended solids, nitrogen, phosphorus,
and other materials63 to impose extra loads on other
treatment processes and effluent receiving waters. An
analysis  of 20  high-rate, mesophilic, two-stage, anaerobic
digestion  systems63 showed a range of supernatant sus-
pended solids from 100  to 32,400  mg/l with an average
value of 5500 mg/l and for  BOD5from  100  to 6,000
mg/l with an average value  of  875 mg/l.  Table 2—4
indicates the effects at one  midwestern treatment facility
where  anaerobic digester supernatant from a high-rate
system was returned  to the  plant influent.
  Many supernatant treatment  alternatives have been
tried,65 some working  with a  certain degree of success.
The question that  really  needs to be asked  is why even
expect a clean  supernatant stream when digesting mixed
primary and waste  activated sludges.
  The  concept of  obtaining high quality supernatant  de-
veloped  during the early days  of separate anaerobic
digestion  systems.  During this  time period the only
sludge being digested was primary sludge, which had
excellent settling properties (table 2-3).
  Modern-day sludges are much more  complex. They
contain not only primary sludge but sludges  generated
from secondary treatment, predominately activated sludge
systems. Waste activated sludge tends to have fragile
floe and is difficult to concentrate by gravity thickening.
Because of this, waste activated sludges are thickened
by dissolved air flotation thickeners.
  Also present-day, high-rate  anaerobic digesters are
mixed. This constant  mixing of the sludge tends to re-
duce  particle size. At  the same time the process itself is
reducing particle size  through biological destruction.
  Finally anaerobic digestion systems generate gas
throughout the entire tank under a slightly  positive pres-
sure (6-  to  15-inches (15-38  cm) water column). Thus,
the system  becomes supersaturated  with digester gas.
  When the digested sludge is finally pumped to the
secondary digester, it  contains many fines  or sludge that
is difficult to gravity thicken and is supersaturated with
gas. The  gas is then liberated in  the form  of small gas
bubbles which  tend to attach themselves to the sludge
particles,  thus  promoting a flotation  effect.  The combina-
tion of these events is very detrimental to  gravity  con-
centration. It has been estimated  that at least 30  or
more  days12 would be  required in a secondary  digester
to obtain a clear supernatant from high-rate systems
digesting  sludges containing waste activated sludge.
  In many cases, it would be better  to take all  digester
contents directly to mechanical  dewatering and eliminate
provisions for gravity solids-liquid separation. This  would
give a constant, predictable centrate stream having  low
suspended solids content.


ENERGY

Energy Production

  One of the advantages of anaerobic digestion of mu-
nicipal wastewater sludge is that energy is produced
rather than  consumed and could go  a long way in meet-
ing energy requirements at wastewater plants.66 One
problem  encountered with this energy source is in pre-
dicting how much energy will  be produced for any given
plant. This variability  in possible  production is indicated
in  table 2-5.
  Figure 2-8 shows temperature effects on anaerobic
digestion. Schwerin71 reviewed the literature and plotted
Table 2-4.—Effect of returning supernatant from high  rate anaerobic
digester  to plant influent64

Suspended solids 	
Total phosphorus 	

To
primaries
Ib/day
	 15,969
"(36,801)
	 914
0,304)
To
secondaries
Ib/day
9,501
(15,306)
803
(991)
Final
effluent
Ib/day
2,836
(3,467)
500
(435)
Primary
sludge
Ib/day
13,249
(19,626)
156
(299)
Waste
activated
sludge
Ib/day
9,593
(14,645)
287
(453)
  "Data in parentheses were obtained when untreated anaerobic digester supernatant was
discharged to head of plant. Data not in parenthesis were obtained when no supernatant
was discharged to head of plant. Data shown is average for the entire time period of study.
     40

-------
Table  2-5.—Cubic feet digester gas produced per
pound of organic matter destroyed
Material
Fats
Scum
Grease
Crude fibers 	
Protein ... 	
Carbohydrate 	
Fat 	
Insoluble soap
Protein 	
Percent CH4
Pure compounds67
62-72
70-75
68
	 45-50
	 73
Pure compounds39



Ft3 gas/lb digested
18-23
14-16
17
13
12
14.2
24.6
22.3
9.4
                     Municipal sludges68
  "The volume of gas produced per pound of volatile solids digested
is reported as 17 to 18 cu. ft./lb at the larger and better instrumented
plants. Smaller plants report lesser values, sometimes as low as 6 cu.
ft./lb. volatile solids destroyed, but these values  are probably due to
poor measurement techniques."

                     Municipal sludges69
  ".. .maximum gas production of approximately 11 to 12 cu. ft. of
gas per pound of total solids destroyed."

                     Municipal sludges70

  "In terms of solids digested, the average yield.. .is about 15 cu. ft.
of gas per pound of volatile solids destroyed."
    	FT3/LBVS ADDED
           90
100     110     120
 TEMPERATURE -°F
                                          130
                                                  140
Figure 2-8.—Effect of digestion temperature on gas pro-
duction based  on data from  23 studies.71
                                        reported gas  production values as a function  of diges-
                                        tion temperature.  The results show the potential effect of
                                        digestion temperature on gas production.
                                          Since  the basis of all cost analysis depends on the
                                        value of gas  produced  per  mass of solids  destroyed,
                                        and since there is no existing  data, it is suggested  that
                                        a range of 12 to  17 cu ft/lb (0.75-1.06  m3/kg) volatile
                                        solids destroyed be used.
                                          Note:  As was noted in the section on  Volatile Solids
                                        Reduction, the amount  of solids destroyed  is  a function
                                        of sludge type and solids retention time  (figures 2-1,
                                        2-2,  and 2-3).
                                          The heating value of  the  gas can also range from 550
                                        to 650 Btu/cu ft  (20,500-24,200 kJ/m3). Based on  an
                                        average of 50 plants72 a value of  600 is  suggested.

                                        Hazards of  Digester Gas

                                          Explosion.—Sludge gas becomes violently explosive in
                                        mixtures of 1 volume gas to 5-15 volumes air. There
                                        are  many  case histories which  have  shown just how
                                        violent and explosive it can be.
                                          Burning.—When the ratio of gas to air is higher than
                                        the  above values,  a "burning mixture is  encountered."
                                        Such a  mixture is not as dangerous as an explosive
                                        mixture, since it can  be extinguished  if encountered.
                                        However, sewage  plant workers have been seriously
                                        burned by an instantaneous flame  "puff."
                                          Toxicity.—Of the minor constituents of sewage  gas,
                                        hydrogen sulfide (H2S)  is the  most important.  Table 2-6
                                        shows the effects at  various concentrations.
                                          Suffocation.—Man works  best and breathes easiest
                                        when the  air  contains about 21 percent  oxygen.  Men
                                        breathing  air  that  has as little  as 15 percent  of oxygen
                                        usually become dizzy, have a  rapid heart beat, and suf-
                                        fer from headache.
                                          Though over 30 years old,  two publications by  Lang-
                                        ford73'74 on gas  safety design considerations are still rec-
                                        ommended reading for  design  engineers. Figure 2-9
                                        shows a schematic of a modern-day gas piping system.75

                                        Digester Gas Utilization
                                          Since digester  gas was first used  in the United States
                                        in 191576for  heating  and cooking, the use of digester
                                        gas has increased, decreased in the  1950's  and  1960's
                                        because of cheap power alternatives, and  presently in-
                                        creasing again  because of  the energy situation.77 Several
                                        recent publications have described  not only operating
                                        experience with conventional  utilization methods,  power
                                        generation, and heating72'77'79 but also potential new
                                                           Table 2-6.—Effects of various concentrations  of  H2S
Immediate death	   Greater than 2,000 ppn
Fatal in 30 minutes or less	   600 to 1,000 ppm
Severe illness caused 1 /2 to 1 hour	   500 to 700 ppm
No severe effects if exposed 1/2 to 1 hour   50 to 100 ppm
                                                                                                             41

-------
                                          SERVICE
                                          OR HEATER
                                  DIGESTER

                               WASTE
                                                  FLAME TRAP

                                              PRESSURE CONTROL LINE
                             LOW PRESSURE
                             CHECK VALVE
                                                                              CONTROL PANEL
                               DIGESTER HEATER AND
                                                                             HEAT EXCHANGER
                                  WASTE
                                                                                                WASTE
                                                                                                GAS
                                                                                                BURNER
PILOT LINE
COMBINED PRESSURE
RELIEF AND FLAME TRAP
                                                                                                         FLAME
                                                                                                         CELL
                                       VENTTO OUTSIDE
                                         ATMOSPHERE
V
\
p
PRESSURE GAGE
@
DRIP TRAP
o
GAS METER
Figure 2-9.—Gas piping schematic of a  modern anaerobic digestion sys-
tem.75
uses.77 One piece of important operating information
which has come to light  is the amount  of hydrogen
sulfide permissible for operation  of engine generators.72'79
  Because  of its  potential corrosive action early uses of
digester gas as engine fuel tried to keep H2S levels
under 60 grains  per 100 ft3 (200 grains/m3).80'81 This was
done  by  incorporating some type of  dry gas scrubber or
wet type bubbling scrubber. Recently a new simple
method82 of removal has  been developed.
  A recent publication72 describing  the operating results
of several plants  noted that even though levels of 1,000
to 3,000  mg/l of H2S were in the  gas no adverse ef-
fects  had been seen on  the engines.
Digester  Heat Requirements

  In  calculating digester heat requirements the  two par-
ameters of concern are (see figure 2-10):

  1.  Heat required to raise the temperature of  the  in-
     coming sludge flow to digester operating tempera-
     ture.
  2.  Heat required to maintain  the  digester operating
     temperature  (radiation heat loss).

  Heat Required  for Raw  Sludge.—It is often necessary
          to raise the temperature of the incoming sludge stream.
          The amount of heat  required is given by equation 1.
                  gal of sludge  8.34 Ibs  (T2 - T,)  1 day
                               X	X	 X
                      day
      gal
hrs
                                 (D
          where:
            Qs = Btu/hr required to  raise incoming sludge stream
                 from temperature T, to  T2
            T, = temperature of raw sludge stream
            T2 = temperature desired within the digestion tank
            hrs = length of time  raw  sludge  is pumped through
                 the heat exchanger.

            Heat Required for Heat Losses.—Digesters have radia-
          tion  heat losses which must be controlled to maintain
          digester operating temperatures within ±1°F otherwise
          the system could go into thermal shock. The amount of
          heat loss depends on  the tank shape, materials of con-
          struction, and external temperatures.
            The  general design equation for heat flow through
          compound  structures  is:
                            Q = U X A X (T2 - T3)
                                 (2)
     42

-------
Figure 2-10.—A heater  and heat exchanger.
where:
  Q = heat loss Btu/hr
  A = area of material  normal to  direction of  heat  flow
      in ft2
  T2 = temperature desired within  the digestion tank
  T3 = temperature outside the digestion tank

              1      -                              (3)
  U =
       EC,
            1 +
Table 2-7.—"U" factors for various anaerobic digestion
tank materials75
                      Material
                                                      U
Fixed steel cover (1/4" plate)	
Fixed concrete cover (9" thick)	
Floating cover (wood composition)	
Concrete wall (12" thick) exposed to air	
Concrete wall (12" thick),  1" air space and 4" brick	
Concrete wall or floor (12" thick) exposed to wet earth
  (10' thick)	
Concrete wall or floor (12" thick) exposed to dry earth
  (10' thick)	
                                                    0.91
                                                    0.58
                                                    0.33
                                                    0.86
                                                    0.27

                                                    0.11

                                                    0.06
where:
  C, = conductance  for a certain  thickness of material
       Btu/hr-ft2-°F
  x,  = thickness of material—inches
  k.  = thermal conductivity of material Btu - (inch)/hr-
       ft2-°F

  Values of C, and k, can be found in various
handbooks.82
  Various values of  U for different digester covers, wall
construction, and  floor conditions are given in  table 2-7.

NUTRIENTS
  In  general, it is commonly assumed that municipal
wastewater sludge is not nutrient deficient.  It has been
extremely difficult to  conduct research on optimum nutri-
ent requirements of  anaerobic bacteria on sewage
sludge.84 To date, the literature has shown85 that, like
aerobic bacteria, nitrogen and phosphorus are required
in  the  highest amount (12 and 2  percent, respectively,
based  on the weight of biological solids present in the
system).  It is suggested that a minumum C:N:P ratio of
100:15:1  be used for design purposes.
  Several researchers have also shown that  the  addition
of certain trace  materials,  iron86 and sulfur,84 could be
very beneficial to the process.

pH  CONSIDERATIONS

  As was noted  under General Process Description,  an-
aerobic digestion  is  a two-step process consisting of an
"acid forming" and  "methane forming"  step. During the
first step the production of volatile acid tends  to reduce
the pH. The reduction  is normally countered by  destruc-
tion of volatile acids by methane  bacteria and the sub-
sequent production of bicarbonate.
  Past research87"89 has shown that the optimum pH val-
ue for methane  producing bacteria is in the  range of 6.4
to 7.5  and that  these bacteria are very sensitive to pH
change.  Recent research though90 now seems to indicate
that the  pH tolerance of methane producing bacteria is
greater than previously expected. The study also indicat-
ed that high and  low pH values were only bacteriostatic
and not  bactericidal as  previously thought. Because of
the importance of this finding to  system control, more
research is needed  in this area to verify these results.
  pH  is related  to several different acid-base chemical
equilibria. In the  anaerobic  digestion  process the range
of interest  is between 6.0  to  8.0, which for all practical
purposes makes  the  carbon dioxide-bicarbonate  relation-
ship the  most important. In an anaerobic  digestion sys-
tem the amount of carbon dioxide is dependent  only on
the law of partial pressure. Since soluble  carbon dioxide
depends primarily on the CO2 gas content and since at
any given time the composition of digester gas is rela-
tively fixed, pH is a function of the bicarbonate  concen-
tration as shown in  figure  2-11.
  This relationship is very  important  from  a process con-
trol standpoint.92 Also, it points out the importance of
                                                                                                           43

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                         LIMITS OF
                         NORMAL
                         ANAEROBIC
                         TREATMENT
      250
             500    1000     2500   5000   10,000

            BICARBONATE ALKALINITY (mg/l AS CaC03)
                                            25,000
Figure 2-11.—Relationship between pH and bicarbonate
concentration near 95° F.91
analyzing for bicarbonate alkalinity instead of total alka-
linity as  is commonly done  today. The relationship be-
tween  the  two is given in equation  4.
                   BA = TA - 0.71  VA
                                              (4)
where:

  BA
  TA

  VA
  0.71
= bicarbonate alkalinity as mg/l  CaCO3
= total alkalinity as mg/l  CaCO3 determined by
  titration to pH 4.0.
= volatile acids measured as  mg/l acetic acid
= a combination of two factors (0.83)(0.85). 0.83
  converts volatile acids as acetic to volatile acid
  alkalinity CaCO3 and 0.85 from the fact  that in
  a titration to pH 4.0, about 85 percent of the
  acetate has been converted to the acid  form.
  It has  been suggested92 that the only way to increase
digester  pH is  by the  addition of sodium bicarbonate.
Other materials such as caustic  soda,  soda ash, and
lime cannot increase bicarbonate alkalinity  without  react-
ing  with  soluble carbon dioxide,  which in turn causes a
partial  vacuum  within the system. Also above  pH 6.3,
lime may react with bicarbonate to form insoluble calci-
um  carbonate,  thus promoting scale formation or encrus-
tration.
  Sodium can  be toxic at certain concentrations (see
section on Toxicity—light metal cations), and  it  is re-
commenced to  keep sodium levels  below 0.2M (approxi-
mately 4,600 mg/l), which may require dilution of the
digester  contents  as part of the corrective measures.

TOXICITY

  Kugelman and  Chin93 have noted  that much of the
published data  on  toxicity in  anaerobic digestion systems
is erroneous and  misleading  because of inadequate ex-
perimental techniques and  general  lack of understanding.
Therefore, before  any discussion of toxicity takes place
a  review  of several fundamentals must  be made.
   First of all for any material to be biologically "toxic  it
must be in solution. If any substance is not in solution,
it  is not possible  for it to pass through the cell wall and
therefore  cannot affect the organism.
   Second, toxicity is a relative  term. There are many
organic and  inorganic materials  which,  depending if they
meet condition one above, can be  either stimulatory or
toxic. A good example of this is the effect of ammonia
nitrogen on anaerobic digestion—table  2-8.
   Acclimation is a third  consideration.  When potential
toxic materials are slowly increased within the environ-
ment, many biological organisms  can rearrange their
metabolic resources, thus overcoming the  metabolic
block produced by the toxic  material.  Under shock load
conditions there is not sufficient time for  this rearrange-
ment to take place.
   Finally,  there is  the  possibility of  antagonism and syn-
ergism.  Antagonism is defined as a reduction of the
toxic effect of  one substance by the presence of anoth-
er. Synergism is defined  as an  increase in  the toxic
effect of  one substance by the presence of another.
This is an important relationship for cation  toxicity.
   Though there are many potential  toxic materials, this
section  will  only concern itself with the following:  volatile
acids; heavy metals; light metal  cations; oxygen; sulfides;
and ammonia.
   Volatile acids.—Up until the 1960's it was commonly
believed that volatile acid concentrations  over 2,000
mg/l were toxic to an anaerobic digester. There was
also considerable  controversy on whether or not alkaline
substances should be added  to  maintain adequate buffer
capacity.
   In the early 1960's,  McCarty and his  coworkers pub-
lished results from very  carefully controlled studies in
this  area.94'96'97 Their results showed the following:
   1. Studies  clearly indicate that volatile acids, at  least
     up to 6,000-8,000  mg/l, were not toxic  to methane
     bacteria. Therefore,  as long as there was adequate
     buffer capacity to maintain  the system pH in the
     range of 6.6  to 7.4, the system would function.
  2. Control  of pH by the addition  of an alkaline materi-
     al  is a valid  procedure as long as the cation of  the
                                                   Table  2-8.—Effect of ammonia nitrogen on anaerobic
                                                   digestion94'95
                                                                 NH3-N
                                           Effect
                                                   50-200	   Beneficial
                                                   200-1,000	   No adverse effects
                                                   1,500-3,000	   Inhibitory at pH over 7.4-7.6
                                                   Above 3,000	   Toxic
    44

-------
     alkaline material does not cause toxicity. It was
     found that  the addition  of sodium,  potassium, or
     ammonium  compounds is detrimental but magnesium
     or calcium  alkaline compounds are not.

  Heavy metals—Heavy metal toxicity has frequently
been  cited as the cause of many anaerobic digestion
failures. Even though trace amounts  of  most  heavy met-
als  are necessary for maximum  biological development,98
the concentrations existing in raw wastewater sludges
could cause potential problems.
  Since heavy metals tend to attach themselves to
sludge particles,99'100 even low influent concentrations can
be  concentrated significantly in  the  sludge-handling proc-
ess. Table 2-9—column 2 gives the range of influent
concentrations of some heavy metals. The range is quite
wide  with the higher values normally being attributed to
a local industrial polluter.
  Column 3 of table 2-9 gives the typical  range of
removal that can  be expected through a standard sec-
ondary treatment  system. Published data seem to indi-
cate  that the percent removal, without chemical addition,
is a function of  influent concentration. The higher the
influent concentration the higher the percent  removal.
  Column 4 of table 2-9 shows  expected removals with
lime additions at a pH  of 11.0.  In fact it has  been
noted106 that treatment  systems which add lime or other
chemical coagulations for phosphate removal  can expect
significant  amounts of influent heavy metals to also be
removed.
  Because of the dependence of inhibition on naturally
occurring reagents, such as  carbonate  and sulfide, it is
Table  2-9.—Influent concentrations and expected re-
movals of some heavy metals in wastewater  treatment
systems


                                          Removal
Heavy metal
Cadmium 	
Chromium +3 ....
Chromium +6 . . . .
Coooer 	 	
Mercury
Nickel
Lead
Zinc
Arsenic
Iron 	
Manganese 	
Silver 	
Cobalt 	
Barium 	
Selenium 	
innueni
concentrations
(mg/l)
<.008-1.142101'104
<.020-5.8101'104
<.020-5.8101'104
< 020-9 6101'104
< 0001 -068101'104
< 1-880101'104
< 05-1 2 2101'104
< 02-1 8 OO101'104
< 002- 0034102
<.1-13104
<.02-95102
<.05-6104
Below detection104

	
Secondary
treatment,
(percent)
20-45101
40-80101
0-10101
0-70101
20-75101
15-40101
50-90101
35-80101
28-73102
72105
25105


47105
7g105
Lime— pH 11,
(percent)
95103,109
95106
2Q106
gQl03,109
40106
gg103,106

90103,106
70106
gg103
95103,106
96103

75106

Table 2-10.—Total concentration of  individual metals
that  have been found  to cause severe inhibition in
anaerobic digesters107
                                      Concentration of metal
                                       in digester contents
                .. i ,                   (dry sludge solids)
                Metal
                                      Percent
mM Kg"
CoDDer 	
Cadmium ... 	
Zinc 	
Iron 	
Chromium +6 	
Chromium +3 . 	

	 0.93
	 1.08
	 0.97
	 9.56
	 2.20
	 2.60

150
100
150
1,710
420
500

not possible to  define precise total toxic concentrations
for any heavy metal.107 Table 2-10 gives some concen-
trations of individual metals  required to cause severe
inhibition. Table 2-11  gives  an  indication of the differ-
ence  between total and  soluble  concentrations that may
exist  in an anaerobic  digester.
  The problem of heavy metal toxicity may not  necessar-
ily be reduced with strict enforcement of industrial point
sources. For example, the normal  digestion and excre-
tion of zinc is approximately 10 mg  per person.109 Anoth-
er nonprofit source is the paved street. Table 2-12
gives the results of a study on  heavy metal pollution
from  paved road surfaces of several  large cities.109 In
another extensive study,110 based on 9,600 analyzed sam-
ples,  it was shown that  if all industry in  metropolitan
New  York had zero discharge,  there would only be a 9
percent reduction in  copper, 20 percent in chromium, 6
percent in zinc, 16 percent  in cadmium,  and 62 percent
in nickel.
   Except for chromium,  heavy metal toxicity in  anaerobic
digesters can be prevented  or  eliminated  through precip-
itation with sutfides.108'111"113 Hexavalent chromium  is nor-
mally reduced to trivalent chromium which under  normal
anaerobic digester pH levels is  relatively insoluble and
not very toxic.114
                                                          Table 2-11.—Total and soluble heavy metal content of
                                                          digesters108
                                                                       Metal
                                   Total        Soluble
                                concentrations  concentrations
                                   (mg/l)         (mg/l)
Chromium +6 	
Copper ...
Nickel .
Zinc 	
420
196
70
	 341
30
07
1 6
0.1
                                                                                                           45

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Table 2-12.—Heavy  metal from  paved-curb  streets109
Metal
Zinc
Copper 	 	
Lead. .
Nickel .... 	
Mercury .
Chromium 	 	

Arithmetic
mean
1075
	 21
68
060
080
.12

Range
10 062-2 1
.020-. 59
.03 -1.85
011-.19
01 9-. 2
0033-.45

Table 2-13.—Stimulatory and  inhibitory concentrations of
light metal cations117'118
  1Data given in pounds/mile of paved street.


  The  reason for using sulfide precipitation  is the ex-
treme insolubility of  heavy metal sulfides.115 Approximately
0.5 mg of sulfide is  required  to precipitate 1.0 mg of
heavy  metal. If insufficient sulfide is  not available  from
natural  sources, then it must  be added in the form of
sulfate which  is reduced to sulfide under anaerobic con-
ditions.
  One potential drawback of  using the sulfide saturation
method is the possible production of hydrogen sulfide
gas or  sulfuric acid  due to excess amounts of sulfide in
the digester. Because of this, it  is recommended that
ferrous sulfate be used as a  source of sulfide.93 Sulfides
will be  produced from the biological breakdown of sul-
fate, and the  excess will be held out of solution  by the
iron. However, if heavy metals enter the digester, they
will draw the  sulfide preferentially from the iron  because
iron sulfide  is the most soluble heavy  metal sulfide.
  Two other methods  of controlling excess sulfide addi-
tions have  been proposed.112-116 One  method would be to
continuously analyze the digester gas for  hydrogen sul-
fide.103 When there are detectable levels of H2S, sulfate
addition would be terminated; when  the level becomes
undetectable,  additions would start. A second method116
was the use of a silver-silver sulphide electrode to
measure very low levels of soluble sulphides. The elec-
trode is calibrated in standardized solutions of sodium
sulphide of  known value to yield  a parameter, pS, de-
fined  in  a manner similar to pH, as the negative com-
mon logarithm of the divalent sulphide ion concentration.
For example,  when  S~2 is 10~5M, pS  would be 5.
  Light metal cations—Only recently93'117'118 has the signif-
icance  of the light metal cations  (sodium, ammonium,
potassium,  magnesium, calcium)  in anaerobic digestion
started to be unraveled. Normally, domestic wastewater
sludges  have  low concentrations  of these cations but
significant contributions, enough  to cause toxicity, can
come from  two sources.

  1.  Industrial operations.
  2.  The addition of alkaline  material for pH control.

  Not  only  can each of these cations be  either stimula-
tory or toxic depending on concentration  (table 2-13)
but when combined with each other will produce  either
an antagonism or synergism  relationship.
Cation
Calcium 	
Magnesium 	
Potassium 	
Sodium

Stimulatory
(mg/l)
	 100-200
	 75-150
	 200-400
1 00-200

Moderately
inhibitory
(mg/l)
2,500-4,500
1 ,000-1 ,500
2,500-4,500
3,500-5 500

Strongly
inhibitory
(mg/l)
8,000
3,000
12,000
8,000

  Based on current knowledge whenever inhibition is
being caused by an excess of a certain cation, the
cation can be  antagonized by the addition of one or
more of the cations listed in table 2-14.
  Oxygen.—Engineers have always been  concerned  with
air  getting into anaerobic digesters since a mixture of
one volume digester gas with 5 to 15 volumes of air is
an  explosive mixture.
  Many engineers have  also expressed concern over the
possibility of oxygen toxicity when using dissolved air
flotation thickeners for sludge thickening. In  1971   Fields
and Agardy119 showed  "...  that small additions of air (up
to 0.01  volume per volume of digester contents) ap-
proaching one percent by volume, will not significantly
affect anaerobic  digester performance." This value is
several  magnitudes higher than the amount of air that
would be generated from a dissolved air  thickening sys-
tem.
  Sulfides.—By itself soluble sulfide  concentrations over
200 mg/l are toxic to anaerobic  digestion systems.111'120
The soluble sulfide concentration within the digester  is a
function  of the incoming source of sulfur, the pH, the
rate of  gas production,  and the amount  of heavy  metals
to act as complexing  agents. The high  levels of soluble
sulfide can be reduced  by  the  addition of iron salts, or
gas scrubbing.
  Ammonia.—Whenever there are high concentrations of
protein  waste,  which  is  possible in some  systems  with
highly concentrated feed sludges, ammonia toxicity must
be  considered.94'118 Ammonia can  be in two forms, ammo-
nium iron NH4 +  or ammonia gas. Both forms are always
 Table 2-14.—Cation antagonists
         Inhibiting
          cation
Antagonist
  cation
Ammonium	   Potassium
Calcium	   Sodium, potassium
Magnesium	   Sodium, potassium
Potassium	   Sodium, potassium, calcium, ammonium
Sodium	   Potassium
     46

-------
in  equilibrium, the concentration of each depending  on
pH. Equation  5  shows the relationship.
                              Table 2-17.—Pathogenic organisms in mesophilic  anaer-
                              obically digested sludge123'124
                   NH,
NH
(5)
  When the pH is 7.2 or lower,  equilibrium  is shifted
toward the ammonium ion and inhibition  is possible  at
certain concentrations. At pH values over 7.2,  the reac-
tion shifts toward  the gas phase which is inhibitory  at
low values.
  Analysis for ammonia toxicity is done by analyzing the
total ammonia concentration.  If the  total ammonia con-
centration is between 1,500 to 3,000  mg/l and the  pH is
above 7.4-7.6, there are possible inhibitory effects due
to ammonia gas. This can be controlled by the addition
of enough  HCI to maintain the pH between 7.0 to 7.2. If
total ammonia levels are  over 3,000 mg/l, then the
NH4 + ion will  become toxic no  matter what pH level.
The only solution  is to  dilute  the incoming waste sludge.

BACTERIAL EFFECTS

  Pathogenic  organisms in wastewaters consist of bacte-
ria, virus,  protozoa,  and parasitic worms. Many of these
Table 2-15.—Human enteric pathogens occurring in
wastewater and the diseases associated with the patho-
gens125
          Pathogens
            Diseases
Vibrio cholera	  Cholera
Salmonella typhi	  Typhoid and other enteric fevers
Shigella species	  Bacterial dysentery
Coliform species	  Diarrhea
Pseudomonas species	  Local infection
Infectious hepatitus virus	  Heptatitis
Poliovirus	  Poliomyletis
Entamoeba histolytica	  Amoebic dysentery
Pinworms (eggs)	  Aseariasis
Tapeworms	  Tapeworm infestation
Table 2-16.—Pathogenic organisms in sludge123'124
Type
Raw primary
Trickling filter 	
Raw WAS 	
Thickened raw
WAS 	

Salmonella
(No./ 100 ml)
460
62
93
74
2,300
6
9300

Pseudomones
aeruginosa
(No./100 ml)
46X103
195
110X103
1.1 X103
24X103
5.5 X103
2X103

Fecal coliform
(No. X106/100 ml)
11.4
11.5
2.8
2.0
26.5
20


Primary only
WAS only
Mixture
Primary and WAS....
Salmonella
(No./100 ml)
29
7.3
6
Pseudomonas
aeruginos
(No./ 100 ml)
34
103
42
Fecal coliform
s 106
(No./lOO ml)
0.39
0.32
.26
     organisms,  especially enteric viruses,122 have a strong
     tendency to bind themselves to sludge solids.
       Table 2-15  lists the human enteric pathogens that
     have been  found in  wastewater sludges along with the
     diseases normally associated with them. Table 2-16 lists
     some data  on bacterial concentrations  found in  raw
     sludges from two studies.123'124
       The reduction of  pathogenic organisms  under  meso-
     philic, anaerobic digestion has been studied by  various
     researchers.122'126"129 Though some early research indicat-
     ed die off  may be due to bactericidal effects,126'127 cur-
     rent  research  supports that die off is strictly related to
     natural die off. Data from two  studies are given in table
     2-17 for mesophilic  anaerobically digested sludge.
       No reported work on pathogen destruction for thermo-
     philic anaerobic digestion could be found.

     ACTIVATED CARBON

       The first  reported  studies on the addition of activated
     carbon to anaerobic digesters  treating  municipal waste-
     water sludges was  in  1935, at Plainfield, N.J.,131 and in
     1936 in U.S. Patent 2,059,286.132 At this time the addi-
     tion  of activated carbon was claimed to have the  follow-
     ing benefits:

       1.  Enhanced the  rate of digestion.
       2.  Increased the  total amount of gas  produced.
       3.  Produced clear supernatants.
       4.  Enhanced the  drainability of the digested sludge.
       5.  Increased temperatures within the digester.
       6.  Gave  higher volatile solids reductions.
       Until recently no  other  reported work in this area
     could be found. In  1975  Adams133'134 discussed the re-
     sults of studies carried out by ICI. In  his  discussion he
     pointed  out the following  advantages based on full-scale
     studies carried out at  Cranston, R.I.135 and  Norristown,
     Pa.136

       1.  Promoted sludge  settling and clear supernatants
          due to the  high carbon density.
       2.  Catalyzes the breakdown of sludge solids,  thereby
          reducing  the amount of sludge to be handled.
       3.  Increase gas production per mass of solids added
          plus producing  a  gas with  higher methane  content.
       4.  Can absorb certain substances such  as pesticles,
          heavy metals, grease, scum, and detergents.
                                                                                                          47

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  5. Reduction in odors.
  6. Possible improvement in mechanical dewatering op-
     eration at least for vacuum filtration.

  Even  though several  full-scale studies have been con-
ducted,  they have not been  done scientifically but more
of a general "add some carbon and see what happens"
attitude. Though  improved  operating  results  have been
shown, the  real mechanism for  these results have not
yet been clearly  identified. At the present  time EPA has
awarded a grant to  Batelle to study  the effects  of acti-
vated carbon addition on anaerobic digesters.

TANK LAYOUT

  Essentially four basic types of anaerobic digestion sys-
tems are available to stabilize municipal wastewater
sludges. The four systems are discussed below  in order
of their complexity.
  Conventional low rate anaerobic digestion.—Figure
2-12  shows what is typically thought of as  a convention-
al,  low  rate, anaerobic digestion system.  Essentially, this
system  is nothing more than a  large storage tank and
no  attempt  to control  the  environment  or  accelerate the
process is made.
  Conventional high rate anaerobic digestion.—Figure
2-13  shows what is typically considered a conventional,
high rate, anaerobic digestion system and is the most
commonly used system in  the United States today. In
this system  attempts are made  to  control  the environ-
ment  (through  thickening,  heating and  mixing) and  accel-
erate the  process.  Essentially, all digestion  takes place
in the first tank.  This tank is normally  maintained  at
   RAW SLUDGE
                                           SUPERNATANT
                                        DIGESTED SLUDGE
 NO SUPPLEMENTAL HEATING

 NO SUPPLEMENTAL MIXING
                                                         95°F  (34°C) and mixed with some type of gas mixing
                                                         system.  Hydraulic detention times are normally 15-25
                                                         days.  The majority of designs also provide a so-called
                                                         "secondary  digester" for solids-liquid separation (dotted
                                                         line tank in  figure  2-13) but this practice  is being chal-
                                                         lenged as not being useful  in many applications and that
                                                         going direct to mechanical  dewatering  can have several
                                                         significant advantages.137
                                                           Anaerobic contact.—The  advantage of sludge recycle
                                                         in  the anaerobic digestion  process has not only been
                                                         discussed but applied138"141 in treating high strength
                                                         waste and  has been indicated  to be worthwhile  in treat-
                                                         ing waste sludges.142 Nevertheless,  this process alterna-
                                                         tive is rarely considered in  municipal anaerobic sludge
                                                         digestion.
                                                           Figure 2-14 shows a typical schematic  of the process.
                                                         The essential feature of this system is that positive sepa-
                                                         ration through the use  of a centrifuge  biomass is uti-
                                                         lized.  Part  of this  biomass is recycled  back to the an-
                                                         aerobic  digester where it is mixed with the incoming
                                                         sludge.  This recycling  of the sludge  thus  allows for ade-
                                                         quate cell retention  to  meet kinetic requirements yet
                                                         significantly reduces hydraulic detention time.
                                                           Phase separation.—As was noted  under the general
                                                         process  section, the anaerobic digestion  process con-
                                                         sists  of  two distinct phases. The previous three  systems
                                                         attempted to do this in one reactor. As early as 1958143
                                                         the possible value of actually separating  the  two proc-
                                                         esses was discussed.  Work in 1968144 using dialysis se-
                                                         paration techniques clearly showed "—that the hydrol-
                                                         ysis-acid production sludge is the  rate limiting process in
                                                         anaerobic digestion of  sewage sludge. Furthermore,  the
                                                         acid  formers in a  digester must  operate at below  opti-
                                                         mum  conditions in  order to maintain a healthy population
                                                         of methane forming  bacteria."  During the  past several
                                                         years considerable research has been  conducted in  this
                                                         area  which  was summarized by Ghosh145 and has also
                                                         led to a patented  process.146 Figure 2-15  shows a sche-
RAW ^n«rn _
SLUDGE
^ ex I
1

_. 	

<;
POSIT
	 —SOLIDS
SEPAR
IVE
LIQUID 	
4TION
CLARIMED^LIQUIP
DIGESTED SLUDGE
Figure 2-12.—Schematic of conventional low rate anaer-
obic digestion system.
                                                         Figure 2-14.—Schematic of anaerobic contact process.
RAW



^
*-t
1
L.__

-^
1
^"•^ DIGESTED SLUDGE
DIGESTED SLUDGE
Figure 2-13. — Schematic  of conventional high rate an-
aerobic digestion system.
                                                               ACID DIGESTER
                                                                                    METHANE DIGESTER
                                                           RAW
                                                          SLUDGE
                                                                                                  POSITIVE
                                                                                               -•-SOLID LIOUID-v-EFFLUENT
                                                                                                 SEPARATION
                                                          Figure 2-15. — Schematic of phase separation anaerobic
                                                          digestion of sludge.145
     48

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matic of this multistage system as conceived by
Ghosh.145
  The  phase separation process  has several potential
benefits when compared to the other  processes. These
are:145

  1.  Capability of  maintaining the optimum environment
     for each group of digester organisms.
  2.  Substantial reduction in total reactor volume and
     the  consequent savings in capital  and operating
     costs.
  3.  Higher rates of solids stabilization and increased
     production rate and methane content of the final
     product gases.
  4.  Decreased heat  requirement and increased  thermal
     efficiency.
  5.  Suitable for incorporation into existing treatment
     plants with minimum  capital  investment.
  6.  Reduction of  nitrogen content of the  system effluent
     by simultaneous  liquefaction and denitrification of
     waste feeds in the acid digester.

GENERAL  OPERATIONAL CONTROL
PROCEDURES

  It should be noted  that there is no one  test or control
parameter that will signify good or bad anaerobic  diges-
tion operation. Control or operation  of  an  anaerobic dig-
estion  system should  be done through  a combination of
several analyses, the  results plotted as a function  of
time. In this way an unbalanced  digester would be de-
fined as  one which starts to radically deviate from  past
norms. Note  that the  norm at one plant can be  failure
conditions at another.
  At the  present time it is suggested that  a minimum of
four  different tests  be performed on  a  regular basis. The
four  proposed  tests are: pH, bicarbonate alkalinity, vola-
tile acids and percent carbon dioxide (CO2) in the  di-
gester gas.
  pH.—As  was discussed under  pH Considerations, opti-
mal pH is between 6.4 to 7.5. Unfortunately,  the pH test
by itself  is  not a good control procedure92 because:
  1.  It is a  logarithmic function and is  not very sensitive
     to large fluctuations  in the alkalinity concentration.
     For example,  a change in alkalinity from 3,600 to
     2,200  mg/l would only change the pH from 7.1 to
     6.9  which is within the error involved in pH meas-
     urement.
  2.  It does  not provide adequate warning. A low pH
     only informs the operator that an  upset  has oc-
     curred.

  Bicarbonate  alkalinity.—The importance of measuring
bicarbonate alkalinity  rather than total alkalinity was dis-
cussed in the section entitled "pH Considerations." The
bicarbonate alkalinity  and volatile  acid test  are used
together  to develop the ratio  of volatile acid  to bicar-
bonate alkalinity. In order to insure good operation (that
is  good buffering capacity), this ratio should be  below
0.7.
  Note: A fast, simple method for differentiating bicar-
bonate and  volatile acid alkalinity without  using distilla-
tion has  been developed  by DiLallo and Albertson.147
  Volatile acids.—By itself this analysis means nothing.
Only when plotted as a function of  time or used  in
conjunction  with the volatile acid-bicarbonate ratio can
impeding operation problems  be interpreted early enough
to allow  some type of correctional procedures.
  Carbon dioxide content.—Under good operation the
CO2 content in digester gas will  be  between 35-45 per-
cent.  As an unbalance condition starts to occur,  there
will be an increase in the percentage of  CO2 as the
methane producers become incapable of functioning.
  When the control  parameters indicate an unbalance
condition, the following  steps of action have been rec-
ommended:91

  1. Maintain  pH near neutrality
  2. Determine cause of  unbalance
  3. Correct cause of unbalance
  4. Provide pH control  until  treatment returns to nor-
     mal.

Maintaining  the pH near neutrality can be done two
ways. The first is to reduce the waste feed. A second
way is through the addition of some type neutralizing
material (see section on pH Considerations and Toxici-
ty—Light Metal Cations).
  Determining the cause  of unbalance  can be difficult.
Some of the easier things to  check are hydraulic wash-
out,  heat exchanger not capable of providing  sufficient
heat,  mixing system  not operating, sudden change in the
amount of sludge  pumped to the digester and extreme
drop in pH. If nothing shows up after the above  prelimi-
nary analysis,  then testing for ammonia, free sulfides,
heavy  metal and  light metal concentrations will have to
be  made.
  Once it has been  determined what is causing the
problem, corrective measures can be  taken to  put the
digester  back on line. Depending on the cause of unbal-
ance, the length of time  required to bring a digester
back  to normal operating condition  may take from 2 to
3 days to 4 to 6 months.


BASIC SIZING CRITERIA—ANAEROBIC
DIGESTION SYSTEMS
Operating temperature for optimiz-
  ing gas production
Hydraulic detention time (no recy-
  cle) of primary digester to
  achieve max. volatile solids de-
  struction
    primary sludge only

    primary plus waste activated
     sludge
    waste activated sludge only
Mesophilic 35-37.8° C (95-100° F)

Thermophihc 54.4-57.2° C (130-
    135°F)
8.5-10 days at 35° C
5.5-7 days at 54.4° C
15-17 days at 35° C
11-13 days at 54.4 °C
25-27 days at 35 °C
16-18 days at 54.4 °C
                                                                                                         49

-------
Feed solids concentration, degnt-   Mixing difficulties start to develop
  ted sludge                     at feed solids concentrations
                              over 8-9 percent.
Organic loading rate             Function of hydraulic detention
                              time and feed solids concentra-
                              tion. Many present-day facilities
                              are operating from 0.15 to 0.25
                              Ib VS/ft3/day (2.4^1.0
                              kg/m3/day).
  Note: When dealing with anaerobic digestion of waste
activated sludges or mixtures of,  should not assume any
solids-liquid  separation within secondary digester.

DESIGN  PROBLEM

  Two designs, a 4 Mgal/d (0.18  m3/s) and  40 Mgal/d
(1.75  m3/s), are evaluated. Influent is typical  domestic
wastewater of 200 mg/liter biochemical oxygen demand
(BOD5) and  200 mg/liter suspended solids (SS) with no
heavy  industrial contributors.  Liquid treatment  consists of
grit removal,  primary treatment, secondary treatment (ac-
tivated sludge), and chlorination.  No chemicals are add-
ed  to liquid  treatment portion.

Sludge Type and  Amount

  Every million gallons (3,785 m3)  of raw  plant influent
will generate approximately 1,000  Ib (453.6 kg) of pri-
mary  sludge and  1,000  Ib (453.6 kg) of waste-activated
sludge.148 This can be further broken down as  in  table
2-18.
  Based on table 2-18, the total sludge generated for
the design  examples would be 8,000 Ib (3,636 kg) for
the 4-Mgal/d  (0.18 m3/s) design  and 80,000 Ib (36,364
kg) for the 40 Mgal/d (1.75  m3/s) design.

Temperature
  Operating  temperature in a high-rate digester would
be:

  • 35°C (95° F) for a 4-Mgal/d  (0.18 m3/s) design
     based on  mesophilic conditions
  • 54.4°C (130°F) for a 40-Mgal/d (1.75 m3/s) design
     based on  thermophilic conditions
  The  coldest ambient air temperature for  both designs
is assumed to be 12.2°C (10°F).  The coldest  raw
Table 2-18.—Breakdown of inert and volatile suspended
solids per mg of  plant influent (Ibs)
                             Inert     Inert    Biodegradable
                          Nonvolatile  volatile     volatile
sludge temperature for both designs is assumed to be
4.5° C  (40° F).
Required Hydraulic Residence  Tii
Organic Loading—Influent  Solids
Concentration for  High-Rate Digester
  For both designs maximum volatile solids destruction
desired. Figure 2-3 shows that for this particular type
sludge, a  practical upper limit of 55 percent volatile
solids destruction is  possible and  can be obtained  in
600 degree-days.
  Thickened sludge recycle will  not  be used in  either
design; therefore, sludge age will  equal hydraulic resi-
dence time (HRT) in  a high-rate digester.
4 Mgal/d design
  600°C  - days^-35°C = 17 days minimum HRT.

CD
                       SLUDGE AGE (DAYS)

 Figure 2-16.—Relationship between solids concentra-
 tion—organic loading—sludge  age for anaerobic diges-
 tion.
1.0
0.8
0.6
0.4
0.2
0.0
(
—
h^~ PROBABLE DIGESTION LIMIT
L F
I
: I
,
) 5 10
s.
s.

j^
>>
^
^


i i l 1
15 20 25 30 35 40
Primary sludge 	
Waste-activated sludge 	

Totals ....

250
300

550

300
210

510

450
490

940

                                                        Figure 2-17.—An  anaerobic  digester floating cover with
                                                        a gas mixing system.
     50

-------
40  Mgal/d design
  600°C - days-5-54.4°C = 11  days minimum HRT.
  For both designs, a three-day storage capacity also is
desired.  This dictates that floating covers will be utilized
with minimum hydraulic detention time  based on  when
the cover rests  on landing corbels and maximum deten-
tion  time  based on when the cover is  floating at maxi-
mum liquid level.
  Figure 2-16 indicates the possible safe range of or-
ganic loading for a given  HRT.
  The  practical  upper limit on feed solids concentration
is 8 to 9 percent.
  Within the  constraints given, the designer has consid-
erable latitude for selection of digester tank volume (see
figure 2-17 for  example) and, to a certain point, selec-
tion  of necessary  thickening equipment. For the designs
given,  the following organic loading has been selected:

4 Mgal/d design—0.15 Ibs VS/ftVday
 5,800 Ibs VS/day   7.48 gal         1	
0.15 Ib VS/ft3/day      ft3      17 day minimum
                                   = 17,014  gallons/day

 8,000 Ibs solids/day
(17,014  gal/day)(8.34)
X100
                      = 5.64 percent feed solids required

   Minimum tank volume:
     17  day x 17,014 gal/day = 289,238 gal
       (38,668 ft3).
   Maximum tank volume:
     20  day x 17,014 gal/day = 340,280 gal
       (45,492 ft3).

   Use one digester,  45 ft (13.7 m) diam, 5.7 ft (1.7 m)
 deep cone, 28.7 ft (8.7 m)  side wall depth with 4.3 ft
 (1.3 m)  cover travel.

 40 Mgal/d design—0.20 Ibs VS/ft3/day
            58,000  Ibs VS/day   7.48  gal          1	
             0.2  Ib VS/ft3/day      ft3     11  day minimum

                                                  = 197,200 gal/day

             80,000 Ibs solids/day    1QQ
             (197,200 gal/day)(8.34)

                                  = 4.87 percent feed solids required

              Minimum tank volume.
                 11  day x 197,200 gal/day = 2,169,200 gal
                  (290,000 ft3).
              Maximum tank volume.
                 14  day x 197,200 gal/day = 2,760,800 gal
                   (369,091 ft3).
              Use two digesters, each 95 ft diam, 11.9 ft deep
            cone, 24.5 ft side wall depth with  5.6  ft  cover travel.
              Table  2-19 gives various calculated results for volatile
            suspended solids destruction in an anaerobic digester.

            Expected Energy Production
              Depending on sludge composition (oil, grease, fiber,
            protein), gas production can range from 12 to 18  ft3/lb
            (0.75-1.12 m3/kg) VS destroyed,  with  the higher values
            indicating high grease content.
              Depending on methane  content, each cubic foot of
            digester gas has an  energy value  between 550 to  650
            Btu  (580-685 kJ).
              4  Mgal/d design at 55  percent  VS destruction.
              Ibs VS         Cu ft         Total cu ft
             destroyed    produced per    produced per
             per day    Ib VS destroyed       day
            3,190	        12           38,280
                                                    15           47,850
                                                    18           57,420
                               Btu
                               per
                               cu ft
                               550
                               600
                               650
                               550
                               600
                               650
                               550
                               600
                               650
  Total Btu
produced per
  dayxW6
   21.054
   22.960
   24.862

   26.317
   28.710
   31.102

   31.581
   34.452
   37.323
Table  2-19.—Various calculated results for volatile suspended solids de-
struction in anaerobic digester
                                                4 Mgal/d
                                                 design
                                          40 Mgal/d
                                            design
Lbs volatile suspended solids (VSS) destroyed
  per day 	
Percent of TS destroyed	

Percent of biodegradable VS destroyed.

Percent original inlet feed VSS/TS	

Percent final VSS/TS	
                   0.55 (2,040+ 3,760) = 3,190
                   3.190
                   8,000
                   3,190,
                                              X100 = 39.9
                   3,760
                   5,800
                   8,000
                   5,800 - 3,190
X100 =72.5
                                             8,000
                              X100 = 32.6
31,900

 39.9

 84.8

 72.5

 32.6
                                                                                                            51

-------
  40 Mgal/d design at 55 percent VS destruction.
  Would be same as 4 Mgal/d except  10 times greater.
  Note: 1 hp-hr = 2,545 Btu; electrical  energy conversion
32 to 37 percent.

Sludge Heat  Requirements
4 Mgal/d design.—40 hrs/wk.
17,014 gal  7 day.
   day       wk
                     1 wk   8.34 Ibs
                           x-
                    '40 hrs     gal
                              (95-40) °F

                              = 1,365,756 Btu/hr
40 Mgal/d design.—12  hrs/day-
197,200 gal
    day
    x
         1
      2 units
                    x
. 8.34 Ibs.
   gal
                      -x
-7 day/wk.
(130-40) °F
  12  hrs
  = 6,167,430 Btu/hr/unit
  For both designs, the designer has selected a floating
cover with wood composition  roof,  12-in. (30.5 cm) thick
concrete wall with  air space and 4-in. (10.2 cm) brick,
and  12-in.  (30.5 cm) thick concrete floor exposed  to  wet
earth (table 2-20).

Summary of Heat Requirements
               Btu/hr                 4 Mgal/d   40 Mgal/d
Expected max.  winter output per unit	   1,491,584   6,738,006
Expected max.  winter input  per unit (heat
  ex. only 80 percent efficient)	   1,864,480   8,422,508
Expected total max. winter Btu requirement   14.9 X106   219.3 X106
Expected total min. summer (air at 75° F,
  sludge at 50° F) Btu requirement	   10.5X106   182.3X106
Expected min. summer input per unit
  (heat ex. only'80 percent efficient)	   1,482,643   7,224,150

Matching  Output With Requirement
                         4 Mgal/d
                            Average   Max. req.   Min,  req.
                             hourly      winter     summer
                            production,  conditions,  conditions,
                             Btu/hr      Btu/hr     Btu/hr
  Expected'
  total Btu
produced per
  dayxw6
 21.054..
 22 960.
 24 862 .
 26.317..
 28.710..
 31 102.
 31 581.
 34.452..
 37.323..
                      877,250
                      956,666
                    1,035,916
                    1,096,541
                    1,196,250
                    1,295,916
                    1,315,875
                    1,435,500
                    1,555,125
                                                   Table 2-20.—Maximum winter—full tank heat radiation
                                                   loss (Btu/hr)

                                                                                         4 Mgal/d  40 Mgal/d
Roof/tank
Wall/tank 	
Floor/tank 	
Total

44,61 1
	 71,162
	 10,055
	 125,828

251,925
252,840
65,811
570,576

                                                                    If^J
                                                   RAW
                                                                                       A
•— 1
1
L

1
1
-^ 	 ^•'-DIGESTED SLUDGE
DIGESTED SLUDGE
                                                   Figure  2-18.—Schematic of conventional  high-rate anaer-
                                                   obic  digestion system.
                                                   digester at lower temperature, (2) increase  heat ex-
                                                   changer operating time, and (3) provide some type of
                                                   gas storage,  either a low pressure gas holder (12-24 hr
                                                   capacity) or  high pressure gas holder (several weeks'
                                                   capacity).
                                                     An example of a system for heating anaerobic diges-
                                                   ters is  shown in figure  2-10.
                                                            Expected
                                                            total Btu
                                                          produced per
                                                            dayxw6
                                                          210.54...
                                                          229.60..
                                                          248.62...
                                                          263.17...
                                                          287.10...
                                                          311.02 ..
                                                          315.81 .
                                                          344.52...
                                                          373.23...
                                                             40 Mgal/d

                                                                 Average
                                                                  hourly
                                                                production,
                                                                  Btu/hr
                                                                 8,772,500
                                                                 9,566,660
                                                                10,359,160
                                                                10,965,410
                                                                11,962,500
                                                                12,959,160
                                                                13,158,750
                                                                14,355,000
                                                                15,551,125
                                                                 Max. req.   Min. req.
                                                                  winter     summer
                                                                conditions,   conditions.
                                                                  Btu/hr     Btu/hr
                                                                 8,422,508   7,224,150
                                                            Figure 2-18 shows the  general  system layout pro-
                                                          posed for  both designs.

                                                 1,482,643  SIZING  GAS SAFETY  EQUIPMENT
                                       1,864,480
   Note that the maximum hourly requirement is above
the expected  hourly production  and that even the mini-
mum  just makes it,  even though total  maximum require-
ments are below minimum total  expected gas production.
There are three actions which can be taken: (1)  Operate
                                                     The objective is  to  remove moisture and convey  di-
                                                   gester gas from digester to gas utilization, storage  or
                                                   flaring device.
                                                     Since hourly production fluctuates greatly each day, it
                                                   is common to size piping to  handle 2.5  times the hourly
                                                   average.
     52

-------
4 Mgal/d Design
  Assume  all gas is  produced in  one digester.
  Possible to produce 57,420 ft3/day.
                                2.5 = 5.963 ff/hr
               ,h
               24  hrs/day
                         5,963 ftVhr
                       Ax 3,600  sec/hr

A = Cross sectional  area  inside pipe

    2  in. pipe A = 0.022 ft2       4 in. pipe A = 0.088 ft2
    3  in. pipe A = 0.049 ft2       6 in. pipe A = 0.1 96 ft2

  With a 4-in. (10.2  cm)  pipe,  the maximum velocity is
almost 19 ft/sec (5.8 m/s), well in excess  of the 11  to
12  ft/sec (3.4-3.7  m/s) recommended for successful
condensate removal. Rather than increase the line size
to 6  in  (15.2  cm),  it is  recommended that oversize  accu-
mulators be used.
  Gas safety  piping  specifications are  as follows  (see
figure 2-9):

  1. All gas lines must  be tight, sloped (1/4  in./ft)(2.1
     cm/m) toward condensate  traps and accumulators,
     have ample capacity and be protected against
     freezing.
  2. Lines leading  to gas burners or gas engines must
     be protected against flashbacks by flame traps.
     Trap should be located near point of  combustion
     with  a maximum allowable  distance of  30 ft  (9.1  m)
     from point  of  gas  combustion.
  3. Bypasses are needed to  permit flexibility of  opera-
     tion, but flame  traps are  never  bypassed.
  4. Total  pressure  loss through the appurtenances  and
     gas lines from the digester to use should be only
     2.0 in. (5.1  cm) W.C. at maximum gas flow rate.

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88.  Barker,  H. A., "Studies Upon the Methane Fermentation  Proc-
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89.  Mylroie, R. L., and Hungate,  R. E., "Experiments on Methane
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90.  Clark, R.  H., and Speece,  R. F.,  "The pH Tolerance of Anaero
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91.  McCarty,  P  L., "Anaerobic Waste Treatment Fundamentals -
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-------
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 95.   Albertson,  O. E.,  "Ammonia Nitrogen and  the  Anaerobic  Environ-
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100.   Neufeld,  R. D., and Hermann, E. R., "Heavy Metal Removal by
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101.   Cohen, J.  M., "Trace Metal Removal by Wastewater Treatment"
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102.   Johnson, W. F., and Hinden,  E.,  "Bioconcentration of Arsenic  by
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103.   Argo, D. G., and Clup, G.  L., "Heavy Metals Removal in Waste-
      water Treatment Processes: Part 1" Water and Sewage  Works,
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104.   Mytelka,  A. I., et al.,  "Heavy Metals in Wastewater and Treat-
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105.   Esmond, S. E., and Petrasek, A. C., "Removal of Heavy Metals
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106.   Maruyama,  T., et al.,  "Metal  Removal by  Physical and Chemical
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107.   Mosey, F.  E., "Assessment of the Maximum Concentration  of
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108.   Barth, E. F., et al., "Interaction  of Heavy  Metals  in Biological
      Sewage  Treatment Processes" U.S. Department of Health,  Edu-
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109.   Montague,  A., "Urban Sludge Disposal or Utilization Alterna-
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110.   Klein, L.  A., et al.,  "Sources  of  Metals in New York City Waste-
      water" Journal WPCF, vol. 46,  No. 12, p. 2653 (1974).
111.   Lawrence,  A. W., and McCarty,  P. L., "The Role of Sulfide in
      Preventing  Heavy Metal Toxicity  in Anaerobic  Treatment" Journal
      WPCF, vol. 37, p. 392 (1965).
112.   Masselli,  J. W., et al., "Sulfide Saturation  For  Better Digester
      Performance" Journal WPCF, vol. 39, p.  1369 (1967).
113.   Regan, T.  M., and Peters,  M. M., "Heavy Metals in Digesters:
      Failure and Cure" Journal  WPCF, vol. 42, p. 1832 (1970).
114.   Moore, W.  A., et al.,  "Effects of Chromium on the Activated
      Sludge Process"  Journal WPCF,  vol. 33, p.  54 (1961).
115.   Lang's Handbook of Chemistry (1973).
116.   "Inhibition  of Anaerobic Digestion by Heavy Metals" abstract
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117.   Kugelman,  I. J., and McCarty, P. L., "Cation Toxicity  and Stimu-
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118.   Kugelman,  I. J., and McCarty, P. L., "Cation Toxicity  and Stimu-
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119.  Fields, M., and Agardy, F. J., "Oxygen Toxicity  in Digesters"
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120.  Lawrence, A. W., and McCarty, P. L., "Effects of Sulfides  on
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121.  Process Design Manual for Land  Treatment of Municipal Waste-
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122.  Ward, R. L., "Inactivation of Enteric Viruses  in Wastewater
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123.  Kenner,  B.  A.,  et al., "Simultaneous Quantitation of Salmonella
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124.  "Stabilization and Disinfection of Wastewater Treatment Plant
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125.  Love, Gary J.,  et  al., "Potential Health  Impacts  of Sludge Dis-
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126.  Lund, E., and Ronne, V.,  "On The Isolation of Virus From  Sew-
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127.  Palf, A., "Survival of Enteroviruses During Anaerobic Sludge  Di-
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128.  McKinney, R. E., et al., "Survival of Salmonella  lyphosa During
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      1469 (1958).
129.  Leclerc,  H, and  Brouzes, P. "Sanitary Aspects  of Sludge Treat-
      ment" Water Research, vol. 7, p.  355 (1973).
130.  Drnevich, R. F., and Smith,  J. E.,  Jr.,  "Pathogen Reduction In
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      48th  WPCF Conference, Miami Beach, October  (1975).
131.  Rudolfs, W., and Trubnick, E. H.,  "Activated Carbon in Sewage
      Treatment" Sewage Works Journal, vol.  7, p. 852 (1935).
132.  Statham,  N., "Method of Sewage  Disposal" U.S. Patent
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133.  Adams, A. D.,  "Activated Carbon:  Old Solution to an Old Prob-
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134.  Adams, A. D.,  "Improved  Anaerobic Digestion with  Powered Acti-
      vated Carbon"  Presented at Central State Water Pollution  Con-
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135.  Ventetuolo, T.,  and Adams,  A.  D.,  "Improving Anaerobic  Digester
      Operation  with  Powdered Activated Carbon"  Deeds and Data-
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136.  Hunsicker, M.,  and Almeida,  T., "Powdered Activated Carbon
      Improves  Anaerobic Digestion" Water and Sewage  Works,  p.  62,
      July (1976).
137.  Mignone,  N. A., "Elimination of  Anaerobic  Digester Supernatant"
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138.  Schroepfer, G.  J., and Ziemke,  N.  R.,  "Development of The
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139.  Steffen, A. S.,  and Bedker,  M.,  "Operations of a Full Scale
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                                                                                                                               55

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145.  Ghosh,  S., et al., "Anaerobic Acidogenesis of Sewage Sludge"      147.  DiLallo,  Ft., and Albertson, O. E.,  "Volatile Acids by Direct Titra-
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146.  Ghosh,  S., and Klass, D. L, "Two Phase Anaerobic  Digestion"      148.  Kormanik, R. A., "Estimating  Solids Production For Sludge Han-
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      56

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                              Chapter 3
                              Aerobic  Digestion  and  Design
                             of  Municipal  Wastewater Sludges
Aerobic digestion of municipal  wastewater sludges is
based on the principle that with inadequate external
food sources, biological cells will consume their own
cellular material.
   The claimed advantages of aerobic digestion are:1

   • Volatile solids reduction approximately equal to that
     obtained anaerobically.
   • Low BOD5 concentrations  in the supernatant liquor.
   • Production of an odorless, humuslike,  biologically
     stable  end product that can be disposed of easily.
   • Production of a sludge with excellent dewatering
     characteristics.
   • Recovery of more of the basic fertilizer value in the
     sludge.
   • Few operational problems.
   • Low capital cost.

   Table 3-1 indicates the types of  sludges that  have
been studied on a full-scale basis.  Operating results from
operating installations indicate that  the third and fourth
items above are not correct. Some aerobically digested
sludges, even at very long detention times  are not bio-
logically stable and aerobically  digested  sludge does not
dewater very readily with  mechanical equipment.
   Today most aerobic digesters are designed using rules
of thumb developed from  past  experience  (table 3-2)
and as the  literature has noted19"22  do not  always per-
                          Table 3-2.—Typical present day aerobic digestion de-
                          sign  criteria23
                                             Parameter
                                                               Value
                         Hydraulic detention time, days at 20° Ca
                           Activated sludge only	
                           Activated sludge from plant operated without primary
                             settling	
                           Primary plus activated or trickling filter sludge	
                         Solids loading, Ibs volatile solids/ft3/day	
                                                               12-16

                                                               16-18
                                                               1&-22
                                                                0.1-
                                                                0.20
             Oxygen requirements, Ib/lb cell destroyed	     b2.0
             Energy requirements for mixing
               Mechanical aerators, hp/1,000 ft3	   0.5-1.0
               Air mixing, scfm/1,000 ft3	    20-30
             Dissolved oxygen level in liquid, mg/l	     1-2


               'Detention times should be increased for temperatures below 20° C.
             If sludge cannot be withdrawn during certain periods (e.g., weekends,
             rainy weather) additional storage capacity should be provided.
               "Ammonia produced during carbonaceous oxidation oxidized to ni-
             trate.
                         form as  intended. This chapter presents the most up-to-
                         date design criteria available.  Whenever possible full-
                         scale operating data are presented.
Table 3-1.—Type and reference of full-scale studies on
aerobic digestion  of municipal wastewater  sludge
                                 Reference on
                                  mesophilic
             Reference on
             thermophilic
Primary sludge only	
Waste activated only	
Mixed primary and waste activated
  sludge	
Waste activated sludge from contact
  stabilization	
Primary and lime	
Trickling filter only	
Mixed primary and trickling filter	
Sludges containing iron or alum	
2,3,4,5,18
7,8

7,8,11


14,15
16
2
2
17,18
6
9,10


10,12,13
CRYOPHILIC—MESOPHILIC—
THERMOPHILIC DIGESTION

  For purposes of classification  the following  three tem-
perature zones of bacterial action will be used through-
out this chapter:

  •  Cryophilic zone—liquid temperature below 10°C
     (50° F)
  •  Mesophilic zone—liquid temperature  between 10°C
     to 42°C (50° F to 108°F)
  •  Thermophilic zone—liquid temperature  above 42° C
     (108°F)

  The  effect  of temperature on the effectiveness of aer-
obic digestion  is still an area of considerable controver-
sy,24 especially in the areas of solids  reduction, dewater-
ability and settleability. The data shown in subsequent
sections should help clarify some of the controversy.
                                                                                                           57

-------
  At the present time considerable research is  being
undertaken in the design and operation of thermophilic
aerobic systems,13'24'31 especially auto-thermophilic aerobic
systems.13'27'29'31 Claimed advantages  of  the thermophilic
aerobic system are:13'30'31

  • Higher rates of organic stabilization that allow
     smaller volume requirements.
  • Higher maintenance energy  requirements and higher
     microbial decay rates that give smaller amounts of
     sludge for  disposal.
  • Digestion  in this temperature range should make
     liquid essentially pathogen free.
  • All weed seeds should be destroyed.
  • Total oxygen demand should be 30 to  40 percent
     less  than  mesophilic  since few, if  any,  nitrifying
     bacteria exist in this temperature  range.
  • Improved  solids-liquid separation due to decreasing
     liquid viscosity.
  • Possible  improved  oxygen transfer rates because of
     the significantly higher coefficient  of diffusivity of
     oxygen.

VOLATILE  SOLIDS REDUCTION

  One of  the main objectives  of aerobic digestion is to
reduce the amount of solids that need  to be disposed.
This reduction is normally assumed to take place only
with the volatile content of the sludge,  though some
studies24'32 have shown  that  there can be destruction of
the nonorganics as  well. In  this  discussion solids  reduc-
tion will pertain  only to  the  volatile content.
  The change in volatile content  is normally represented
by  a first  order biochemical reaction,
                      dx/dt = -KdX
0)
where
  dx/dt = rate of change of  volatile suspended solids
           per unit of time
  Kd    = reaction rate constant - day'1
  X     = concentration of volatile suspended  solids at
           time t in  aerobic digester

  The  time t in  equation (1)  is actually the sludge age
in the  aerobic digester and, depending on how the aero-
bic  digester is being operated (continuous flow without
recycle or with  recycle, batch with supernatant decant),
can be considerably greater  than  the theoretical hydrau-
lic residence time (HRT).
  A distinction must  be made between biodegradable
volatile suspended solids and nonbiodegradable volatile
suspended solids. Research in this area is quite limited
but the following  generalities can  be  used.

  •  Approximately 20 to  30  percent  of the  influent sus-
     pended  solids of a typical  domestic  wastewater is
     inert.33 Of the remaining  suspended solids that are
     volatile,  approximately 40 percent  are inert organics
     consisting chiefly of lignins, tannins,  and other large
     complex  molecules.
        .40
      -8 -35
        .30
      < .20
      cc
         A-Pilot Plant Ref (28)
         • - Pilot Plant Ref (36)
         X- Full Scale Ref (10)
         O- Pilol
         D- Pilot
         • - Piloi
         a- Piloi
         +- Piloi
t Plant Ref (10)
.1 Plant Ref (11)
.1 Plant Ref (27)
.t Plant Ref (37)

-------
   50
 § «
 k-
 CJ
 o 30
 > 20


   10


    0
          O ODD
                           x- Pilot Plant Ref (16)
                           • - Full Scale Ref (15)
                           o- Pilot Scale Ref (7)
                           A- Full Scale Ref (10)
                           + - Pilot Plant Ref (36)
                           A- Pi lot Plant Ref (38)
                           • - Pilot Plant Ref (39)
                           o- Full Scale Ref (37)
         200   400  600   800  1000  1200  1400  1600

               TEMPERATURE (°C) x SLUDGE AGE (days)
                                        1800  2000
Figure 3-3.—Volatile suspended solids  reduction as a
function of digester liquid temperature and  digester
sludge  age.
this time there is  not enough data to allow segregation
of Kd by sludge type; therefore,  the line drawn through
the data points represents  an overall average (Rvalue.
  Figure 3-2  indicates  the  results from one study15 on
the effects of aerobic digester solids concentration on
the reaction  rate,  Kd. Figure 3-3 shows the effect of
temperature and sludge age on  total  volatile suspended
solids  reduction.

OXYGEN  REQUIREMENTS

  Activated sludge biomass is most often represented by
the empirical  equation C5H7NO2.  Under prolonged periods
of aeration, typical of the aerobic digestion process, the
biochemical equation for oxidation is  represented by
equation (2).

        C5H7NO2 + 7O2 -+ 5CO2 + 3H2O + H + + NO3'    (2)
LU Q-

gi
Q. 
D >
Z 5
UJ o
O O
>- o
X ^

2°
O UJ
ai I-
o. <
to cc
    8.0


    6.0



    4.0


    2.0
         TEMPERATURE RANGE >10C

^ \°                         <10C
        o
          20      60      100     140      180
                       SLUDGE AGE (DAYS)
                                           220
Figure 3-4.—Effects  of sludge age and liquid termpera-
ture on oxygen  uptake rates  in aerobic digesters.19
  Theoretically,  this reaction states that  1.98  pounds of
oxygen are required  per pound of cell mass oxidized.  In
pilot36 and  full-scale10'15 studies where this value has been
evaluated,  the range was from 1.74  to 2.07 pounds of
oxygen required per  pound of volatile solids destroyed.
For mesophilic systems, a design value of 2.0 is recom-
mended. For thermophilic systems where  nitrification
would  not  exist,13'30'31 a value  of 1.4  is recommended.
  The  actual specific oxygen utilization rate, pounds  of
oxygen per 1,000 pounds volatile solids  per hour, is  a
function of total sludge  age and  liquid temperature.19'24'38
In one study,  Ahlberg and Boyko19 visited several  operat-
ing installations  and developed the relationship shown  in
figure  3-4. Field studies19 have indicated that  a minimum
value of 1.0 mg of oxygen should be maintained in the
digester at all times.

MIXING

  Mixing in an aerobic digester, treating municipal
wastewater sludge of domestic origin,  is considered to
have the following benefits. (Note: It is assumed that a
favorable environment exists to allow development of an
aerobic digestion system.)

  •  It continues to bring deoxygenated  liquid to the
     aeration device.
  •  It keeps  the food supply uniformly dispersed and  in
     constant  contact with the growing cells to promote
     maximum utilization  of the system.
  •  It keeps  the concentration of biological end pro-
     ducts at  their lowest value  by dispersing them uni-
     formly throughout the digester.
  •  It provides environmental uniformity (oxygen,  tem-
     perature, nutrients, etc.) throughout the digester to
     allow  the best possible cell development.
  •  It allows  fairly fast dispersion of any toxic material
     entering the system,  thus, possibly minimizing its
     effect on the aerobic process.

  There is general agreement  that mixing is an important
criterion  in the  aerobic digestion  process. The problem
arises  when one tries to evaluate, define or specify a
mixing system.
  In  recent years it has become  popular  to use the term
"complete  mix"  when discussing  biological process reac-
tors. The term "complete mix" means that the time for
dispersion  of the  feed stream  is short in  relation to the
total hydraulic residence time  in the reactor. It is also
defined as sufficient  mixing so that concentration  gra-
dients  of chemical and biological ingredients are uniform
for the particular reaction rates that exist in the basin.
  Mixing within  the aerobic digestion tank occurs on two
levels:  macromixing and micromixing.43 Macromixing deals
with  the  bulk  mass flow within the digester, while micro-
mixing deals with the degree of intermingling  of the
system molecules. In biological theory, "complete mix"
assumes micromixing.44
  The  actual mixing can be performed by a gas system,
mechanical system or a combination of the two.
                                                                                                           59

-------
  No  matter what type device is utilized, the intent is to
achieve mixing through a pumping action. Because of
this relationship, engineers have come to use the term
horsepower/unit volume as some  type of parameter to
define mixing  in an aerobic digester. Unfortunately, this
term by itself  has no meaning. For mechanical  type mix-
ers the wide  variation in impeller  diameters and speeds
can result in  similar horsepower but widely different
pumping capacities. For gas mixing systems gas flow,
depth, and bubble  size can also result in similar horse-
power but widely different pumping capacities.  In addi-
tion, tank geometry and solids concentrations can signifi-
cantly affect  power requirements.
  Probably the best way to define mixing is from the
                       TOP VIEW
                 PROFILE VIEW
                                          LIQUID HEIGHT
           D, = EFFECTIVE ZONE DIAMETER FOR MICROMIXING.

           D2 = EFFECTIVE ZONE DIAMETER FOR MACROMIXING.


Figure 3-5.—Shear-stress  relationship for  a thixotropic
pseudo plastic  material.
standpoint of zone of influence of an energy source
(figure 3-5).  Essentially the zone  of  influence  states that
energy is dissipated as one  moves horizontally away
from the energy source. This loss is due to friction
between the  fluid  molecules  which is a  function of liquid
density, temperature, and solids concentration. Within a
certain area of the point source there is sufficient en-
ergy to achieve micromixing.  There is also a larger area
where bulk flow (macromixing) still takes place even
though there is  insufficient energy for micromixing.
  Studies45'46 done with point  energy  sources,  in clean
water and with no side boundaries (only surface and
floor boundaries) have indicated that the width of the
micromixing zone  is no more than twice the liquid depth,
with the  liquid depth being  a function of the type of
mixing device utilized and not necessarily the  tank  liquid
depth.
  The effect  of tank  geometry47 on mixing (as measured
by oxygen transfer rates  in  clean water) for various
aeration  devices (high and low speed mechanical aera-
tors, submerged turbines, oxidation ditch aerator and
diffused aeration)  in tanks from several  thousand to 1
million gallons (~10 to 3,800 m3), was  shown to fall  into
three general categories (figure 3-6).
  Category 1 is represented by basin geometry A in
figure 3-6. This is the idealized case in which geometry
has no effect on the liquid  flow pattern. Each  increment
of power into this specific volume has a corresponding
increase in the oxygen supplied.
  Category 2 is represented by basin geometry B in
figure 3-6 and has been termed  the "flywheel effect."
                                                                BASIN GEOMETRY A
                                  BASIN GEOMETRY B
                                                            ENERGY INPUT-
                                                                                          ENERGY INPUT-
                                                                            BASIN GEOMETRY C
                  ENERGY INPUT-*-


Figure 3-6.—Schematic of zone of mixing  influence for
energy source  in fluid with only fixed upper and lower
boundaries.
     60

-------
    140
~S.  120
E
trt
Q
=i   100
8
o
B   80
5   60
DC
O

1   40


8
S   20
                                   Theoretical form
Table  3-3.—General characteristics of raw primary and
waste  activated sludge40
                I
                                   I
               10        20         30

                 POWER LEVEL, hp/1 mg
Figure 3-7.—Effects of tank geometry on mixing in
clean water as measured by oxygen transfer rates.
Here  tank  constraints, represented,  for example, by a
channel aeration tank, cause a rapid  increase in oxygen
supply for  small inputs of energy. As  the  energy per unit
volume increases, the geometry of tanks causes a level-
ing off in transfer.
  Category 3 is represented by basin geometry  C in
figure 3-6  and has been termed the "choke flow ef-
fect." Here tank geometry interferes with  the mixing pat-
tern until a certain energy level is reached. At this point
there is sufficient energy to  override the constraint and
allow for complete mixing in the tank contents.
  No published  studies on field evaluation of the effect
of suspended solids on  mixing in  aerobic  digesters are
available. There have  been  several  such studies48-50 con-
ducted in lagoons with suspended solids  in the  range  of
100 to 400 mg/l and  figure  3-7 shows the results. In
general, increased solids concentrations required in-
creased power levels, though the tank geometry50 and
interaction  effects of  other aerators49 also  influenced mix-
ing patterns.
CHARACTERISTICS OF  AEROBIC
DIGESTERS
  The  existing  trend in wastewater treatment is to re-
move more  and more  material from the main liquid pro-
cessing stream. This is frequently done through the use
of secondary biological treatment schemes, chemical
treatment and filtration. The  sludge  produced can  vary
widely  and change rapidly even on  an hour-to-hour ba-
sis.
  Table 3-3 gives specific gravity and particle size dis-
                               Primary
                               sludge
               Waste activated
                   sludge
Specific gravity
Particle size	
Physical appearance.
   1.33-1.4
 20%  <1 ju
35%  1-100
45%  <100
   Fibrous
   1.01-1.05
 40%  1-50 fim
60%  50-180 jam

Slimy, gelatinous
                                                         C/3
                                                         111
                                                         X
en
<
LU
   0                 RATE OF SHEAR


Figure 3-8.—Power level versus suspended solids.50
tribution  on two common type sludges:  plain primary and
plain waste activated.
  There  is little data on the rheology of municipal waste-
water sludge,40 and none could be found on strictly  aero-
bically digested sludge.  One of the main problems with
collecting data is that such studies are  extremely difficult
to perform correctly.41
  Even though the majority of raw wastewater sludges
behave as a thixotropic (time dependent), pseudo plastic
material  (figure 3-8), it may not be correct  to  assume
that the  sludge within the aerobic  digester has the same
general  properties.  The  liquid will have  a variable solids
concentration and there is a general reduction in particle
size and  shape,38'43 both of which affect fluid viscosity.
  Another characteristic of present-day  designs is that
the tanks tend to  have large  surface area to liquid
depth ratios.

SUPERNATANT

  It is common practice in most aerobic digestion facili-
ties not to prethicken the sludge but to concentrate it
                                                                                                          61

-------
Table 3-4.—Characteristics of mesophilic aerobic
digester supernatant
                    Reference 9a   Reference 19   Reference 52C
Turbidity 	
NO3-N 	
TKN 	
COD 	
PCvP 	
Soluble 	
BOD5 	
Filtered BOD5 	
Suspended solids 	
AIK 	
S04 	
Silica 	
pH 	
120
40
115
700
70

50

300



6.8
_

2.9-1,350
24-25,500
2.1-930
.4-120
5-6,350
3-280
9-41 ,800



5.7-8.0

30


35

2-5

6.8
150
70
26
6.8
  " Average of 7 months of data.
  bRange taken from 7 operating facilities.
  °Average values.
  The drop in pH  is caused by an increased concentra-
tion  of  nitrate ions and a corresponding loss of alkalinity
due  to  the conversion of NH3-N  to NO3-N commonly
called nitrification.  Though at one  time, the low  pH was
considered inhibitory to the process, it  has been shown
that  over time the system will  acclimatize and perform
just  as  well at the  lower pH values.7'38'51
  It  should be noted  that if nitrification does not take
place, there  will  be very little,  if any, pH  drop.  This
could happen at low  liquid  temperatures and short
sludge  ages  or in  thermophilic operation.31 Nitrifying bac-
teria  are sensitive  to  heat and do not exist  in tempera-
tures over 45°C.52

BACTERICIDAL  EFFECTS

  Pathogenic organisms  in wastewaters consist of bacte-
ria, virus, protozoa and parasitic worms; a good  current
review  on the subject can be  found in  Kenner  et al.56
Many of these organisms, especially enteric  viruses,54
have a strong tendency  to  bind  themselves to sludge
solids.
after digestion.  This  is done by sending  the  flow to a
clarifier-thickener  or  by turning off the aeration device
within  the  digester for 12 to 18 hours. When this is
done,  a  digester supernatant is taken off which is  nor-
mally returned to  the head  end of the treatment plant.
Table  3-4 gives supernatant characteristics from several
full-scale facilities operating in  the mesophilic tempera-
ture range.

pH Reduction
  Figure 3-9 shows  the effect of sludge age on digester
pH  for mesophilic operation.
   8.0 r-
   7.0
   6.0
   5.0
   4.0
   3.0
                                  Liquid temp at 5° C
               Liquid temp at 20° C
                  I
                              I
            10           30           50

                 SLUDGE AGE IN AEROBIC DIGESTERS
                                                70
Table  3-5.—Human  enteric pathogens occurring in
wastewater and  the  diseases associated with the patho-
gen"
,56
           Pathogens
                                     Diseases
Vibrio cholera	   Cholera
Salmonella typhi	   Typhoid and other enteric fevers
Shigella species	   Bacterial dysentery
Coliform species	   Diarrhea
Pseudomonas species	   Local infection
Infectious hepatitus virus	   Hepatitis
Poliovirus	   Poliomyelitis
Entamoeba  histolytica	   Amoebic dysentery
Pinworms (eggs)	   Aseariasis
Tapeworms	   Tapeworm infestation
                                                            Table 3-6.—Pathogenic organisms in sludge1
                                                                                                        ,57
Type
Raw primary
Trickling filter 	
Raw waste activated
sludge 	

Salmonella
(No./ 100 ml)
460
62
93
74
2,300
6
Pseudomonas
aeruginosa
(No./ 100 ml)
46X103
195
110X103
1.1 X103
24X103
5.5 X103
Fecal
coliform
(No. x
106/100 ml
11.4
11.5
2.8
2.0
26.5
                                                            Thickened raw waste
Figure 3-9.—Effects of sludge age on  pH for mesophilic     activated sludge...
aerobic digestion.                                           	
                          9,300
                                   2X103
20
     62

-------
 Table 3-7.—Thermophilic aerobic digestion time required
 for reduction of  pathogenic organisms below minimum
 detectable level61

Temper-
Type ature
°C

Time required
for lowest
detectable limit
of salmonella
hours
Time required
for lowest
detectable limit
of pseudomonas
aeruginosa hours
Mixture of primary and
waste activated




45
50
55
60

24
5
1
0.5

24
2
2
0.5


UNSTABILIZED
SOLIDS

>•»


*

AEROBIC DIGESTER #1


AEROBIC DIGESTER #2


1
T
^




                                                                                           STABILIZED SOLIDS
                                                          Figure 3-10.—Tank configuration for a  batch operated
                                                          aerobic digester.
  Table 3-5 gives a  listing of human enteric pathogens
occurring in  wastewater sludges along with the diseases
associated with them. Table 3-6 gives some data on
bacterial concentrations of various types of raw sludges.
  Researchers have studied pathogenic organism reduc-
tion  in both  mesophilic56'58'59 and thermophilic digestion.60
Under mesophilic operation, the bactericidal effects  ap-
pear to be  related  to natural die-off with time. For ther-
mophilic  operation,  the time required for reduction of
pathogenic organisms below minimal detection  level  is  a
function  of basin liquid temperature (table  3-7).

DEWATERING

  One of the supposed  benefits of  aerobic digestion is
the production of a sludge with excellent dewatering
characteristics.1 Much of the published literature on  full-
scale operations has indicated  this  is not true,3'4'17'26'61
though there are some published reports of excellent
operating systems.15
  Although most recent  investigators agree that there  is
a deterioration in dewaterability with increasing sludge
age,2'16'17'27'62 there  is  still debate as to the  cause; lack  of
sufficient oxygen26'27 reduction in particle size16'17 or con-
centration of biological anionic  polymers.63
  At this time it can  only  be recommended that conser-
vative design be used for  designing mechanical sludge
dewatering facilities  unless pilot plant data indicate  oth-
erwise.

TANK  LAYOUTS AND  OPERATION

  Originally  aerobic  digesters were  operated as strictly a
batch operation and  this concept is still  used at many
facilities  (figure  3-10).
  Solids  are pumped directly from the clarifiers into the
aerobic digester. Eventually, the digester fills up, and the
time required depends not only on  the waste sludge flow
but on the amount  of precipitation or evaporation. When
the tank is full,  the aeration device is turned off for
several hours to allow solids-liquid separation,  then  a
decant operation takes place. After decanting,  thickened
stabilized solids of about 2 to 4  percent  in concentra-
tion, can be removed and more waste  sludge can be
added.
  Many  engineers tried  to make the process  more con-
tinuous by installing stilling wells  in part of the digester.
This has proved not to  be effective20'64'65 and  should  not
be  incorporated into the design.
  The next step was then to provide the aerobic di-
gester with its own clarifier-thickener (figure 3-11).
  Solids are still pumped directly from  the clarifiers into
the aerobic digester. In this case the aerobic digester
operates at a  fixed level with the overflow going to  a
solids-liquid separator.  Thickened solids are normally re-
cycled back to the digestion tank but when required  can
also be  removed from  the system.
  Though initially  more  costly than a batch operated
system,  much  of the manual  work involved with aerobic
digestion is eliminated.
  A third type of system would  involve prethickening
before aerobic digestion. This is  employed in the cur-
rently being  researched auto thermophilic  aerobic diges-
tion system (figure 3-12).
  In this system, sludge from  the  clarifiers would go to
some type of  thickening device  to produce a concentra-
tion greater  than 4 percent solids then into the digester.
When  operating in this  mode, one should  not expect any
  UNSTABILIZED
    SOLIDS
                                  STABILIZED SOLIDS
Figure 3-11.—Tank configuration for  a continuous oper-
ated aerobic digester.
                                                                                                           63

-------
                    CENTRATE
                                AEROBIC DIGESTER
    SOLIDS
                                                STABILIZED
                                                 SOLIDS
Figure 3-12.—Tank configuration for an auto thermophil-
ic aerobic digestion system.
Figure 3-13.—A floating  low speed  aerator in an aero-
bic digester.
Figure 3-14.—A diffused air system in an aerobic diges-
tion  tank.
further gravity solids-liquid  separation to take place after
digestion (see figures 3-13 and 3-14).

SUMMARY
   The basic design  criteria for aerobic  sludge digestion
systems presented  in  the previous sections are summa-
    64
Table  3-8.—Criteria for design of aerobic  digestion sys-
tems
                                   Days    Liquid temperature
Sludge age required to achieve
  40 percent volatile solids reduction.
                                                              55 percent volatile solids reduction.
                                   108        4.4°C(40°F)
                                    31       15.5°C(60°F)
                                    18       26.7° C (80° F)
                                   386        4.4° C (40° F)
                                   109       15.5°C (60°F)
                                    64       26.7° C (80° F)
Oxygen requirements	   Liquid temperature 45° C or
                                    less; 2.0 IDS. oxygen/Ib
                                    volatile solids destroyed
                                   Liquid temperature greater
                                    than 45° C;  1.4 Ibs.
                                    oxygen/lb volatile solids
                                    destroyed
Oxygen residual	   1.0 mg O2/l at worst condi-
                                    tions
Expected maximum solids concentration   2.5 to 3.5 percent solids
  achievable with decanting	    (degritted sludge)
Mixing horsepower	   Function of tank  geometry
                                    and type of aeration
                                    equipment utilized
rized in table  3-8.  Obviously, operational  criteria will
vary with the  quantity and  biodegradability of material to
be stabilized,  as  well as temperature and other critical
parameters.

DESIGN PROBLEM
  Two  designs, a 4 Mgal/d (.18 m3/s) and  40 Mgal/d
(1.75 m3/s),  are evaluated. Influent is typical domestic
wastewater of 200  mg/liter biochemical oxygen demand
(BOD5) and  200 mg/liter suspended solids (SS) with no
heavy  industrial contributors.  Liquid  treatment consists of
grit removal, primary treatment,  secondary treatment (ac-
tivated  sludge) and  chlorination. No chemicals are  added
to liquid treatment  portion.

Sludge Type and Amount
  Every million gallons (3,785 m3) of raw  plant influent
will generate approximately 1,000  Ibs. (453.6 kg) of dry
primary sludge and  1,000 Ibs. (453.6 kg)  of waste-acti-
vated sludge solids.67 Table 3-9 shows how this can be
further  broken down.
  Based on  table 3-9 the sludge  generated for the two
design  examples  would be
Inert nonvolatile	
Inert volatile	
Biodegradable volatile.
      Total	
                            4 Mgal/d design
                                 (Ibs)

                             4X550 = 2,200
                             4X510 = 2,040
                             4X940 = 3,760   	

                             4X940 = 8,000    40x940 = 80,000
40 Mgal/d design
     (Ibs)

40X550 = 22,000
40X510 = 20,400
40X940 = 37,600

-------
 Table 3-9.—Breakdown of inert and volatile suspended
 solids per mg of plant  influent (Ibs)
                             Inert     Inert   Biodegradable
                           nonvolatile  volatile    volatile
Primary sludge 	
Waste activated sludge 	

Totals

250
300

550

300
210

510

450
490

940

Temperature Effect

  Temperature in the  aerobic digestion process:

  • Affects oxygen transfer capabilities.
  • Affects volatile destruction capabilities.

  Temperature in aerobic digester is a function of:

  • Feed solids concentration.
  • Geographical location  of treatment facility.
  • Tank location and  material of construction.
  • Type of aeration device utilized.

  For this design example the following assumptions will
be  made:

  • Thermophilic or auto-thermophilic aerobic digestion
     will  not be  considered. This  implies  average inlet
     feed solids  to digester under 3.5 percent solids
     concentration.
  • Lowest liquid temperature expected  during winter  is
     10°C (50° F). During the summer  25.5° C (78° F) is
     expected.
                             quire a temperature-sludge age combination of 475 days.
                             At the minimum liquid temperature of 10°C., this would
                             imply a sludge age of 47.5 days. If the system  is de-
                             signed to maintain a 47.5-day sludge age, then during
                             the summer this combination  would be 47.5x25.5 =
                             1211°C-days. This would give a 49 percent reduction.
                             Table 3-10 gives  various ratios which could be devel-
                             oped.
                             Expected Suspended Solids  Concentration
                             in Aerobic  Digester Underflow

                               This is  a function of overall detention  time, local evap-
                             oration rate and  type of aerobic digestion system em-
                             ployed (batch or continuous).
                               Aerobically digested sludge, typically degritted  with no
                             chemical  addition, can be gravity thickened to 2.5 to 3.5
                             percent. For this design  a maximum of 3.0 percent is
                             assumed.
                               If there is no prior thickening of  the raw sludges so
                             that the average inlet feed solids concentration is under
                             3.0  percent, then gravity thickening is possible. For this
                             example,  the inlet feed solids concentration for the com-
                             bined sludge is assumed to  be  1.5 percent solids (based
                             on 4 percent sludge from the primary clarifier  and 1
                             percent sludge from the  secondary clarifier).


                             Oxygen Requirements

                               Since it is assumed that these design  examples would
                             not  be  designed  for thermophilic aerobic digestion, nitrifi-
                             cation oxygen  demand must be met. From previous dis-
                             cussions  and for design  purposes,  2.0 Ibs of oxygen  will
                             be considered  as the amount required to oxidize a
                             pound of cell mass (table 3-11).
Expected Type of Volatile Solids
Destruction
  Figure 3-3 showed a plot of volatile suspended solids
destruction as a function of liquid temperature and
sludge age. A minimum of  40 percent VSS reduction  has
been  chosen for the design example which would re-
                             Minimum Tank Volume  Necessary To
                             Achieve Desired  Results

                               It was previously noted that a minimum volatile sus-
                             pended solids  reduction of  40 percent was required at
                             the 10°C  liquid level.  Based on figure 3-3 this would
                             imply a minimum sludge age ot 47.5 days.
                               Sludge age in aerobic digester can be approximated
                             as follows:
                Sludge age =
      total Ibs SS in aerobic digester

total Ibs SS lost per day from aerobic digester
                                                total Ibs SS in aerobic digester
                           (total Ibs SS lost per day in supernatant) + (total Ibs SS wasted per day from system)

                           	(SS cone, in digester)(8.34)(digester tank volume)	

                           [(SS cone, in supernatant)(1 - f) + (SS cone, in underflow)(f)] (8.34)(influent flow)
where:
         (influent SS cone.)(percent solids not destroyed)

                     thickened SS cone.
                               SS cone, in  supernatant—if good solids liquid  separa-
                             tion takes place  can expect about 300 mg/l SS  in su-
                             pernatant.
                                                                                                       65

-------
Table 3-10.—Various  calculated results for volatile suspended solids de-
struction in aerobic  digester
                                               4 Mgal/d
                                                design
                         40 Mgal/d
                          design
Lbs volatile suspended solids (VSS) destroyed
  per day
   Winter	
   Summer	
Percent of total solids destroyed
 0.4 (2,040+ 3,760) = 2,320     23,200
0.49 (2,040+ 3,760) = 2,842     28,420
Winter
Summer
Percent of biodegradable VS destroyed
Winter
Summer
Original inlet feed VSS/TS
Final VSS/TS
Winter
Summer

2'320XlOO-29%
8,000
X 1 00 35 5%
8,000 J?-^
o oon
X 100 — 61 2%
3,760 01. -A.
X 100 -75 5%
3,760 's.jTo
5'8°°X100 725%
'" 8,000
5,800-2,320
8 000
5,800-2,842
8,000 Jo.u«
29%
355%
61 2%
755%
725%
435%
369%

Table 3-11.—Average pounds of oxygen  required per
day for aerobic digestion system


                           4 Mgal/d design   40 Mgal/d design


Winter	   2.0X2,320 = 4,640      46,400
Summer	   2.0X2,842 = 5,684      56,840
  SS cone, in digester—can  range from a minimum
equal to the influent SS concentration to a maximum
equal to the thickened concentration (assume no evapo-
ration). Assume that on the average SS cone, equal to
70  percent of the thickened concentration.
  Digester tank volume—million  gallons.

 For 4 Mgal/d design
   Sludge age             =47.5  days
   SS  cone,  in digester    =(0.7)(30,000 mg/l)
   SS  cone,  in supernatant = 300 mg/l
   SS  cone,  in underflow   =30,000 mg/l
                   47.5 -
                                  (0.7)(30,000)(tank vol)
   f
               Influent flow =
                                3.0%

                                8,000
= 0.35
                             (0.15)(8.34)

                           = 63,950 GPD

                           = 0.06395 Mgal/d
       (300X1 - .35) + (30,000)(.35)(0.06395)
                                     _ (21.000 tank vol.)
                                             697

      Digester tank volume =   21 000    1-576 m9

  Tank geometry function of site location and type of
aeration equipment to be utilized.

For 40 Mgal/d design
  Everything  the  same except  for influent flow which =
0.6395.
  Tank volume = 15.76 mg.
  In addition to the tank volume calculated, additional
volume may be required depending on local weather
conditions and type of  downstream sludge-handling facili-
ties.

Tank Layout
  For the mesophilic aerobic  digestion system being con-
sidered, there are two types of  systems  to choose from:
the batch operated  system  (figure 3-10) or the continu-
ous flow through system (figure  3-11).
  The original  aerobic digestion  systems were batch op-
erated; this is  still the most prevalent design  (figure 3-
10).
  Solids  are  pumped directly  from the clarifiers into the
     66

-------
aerobic digester. The time required for the tank  to fill up
depends not only on the  waste  sludge flow but the
amount of precipitation  or evaporation. When the tank  is
full,  the aeration device is turned  off  for  several  hours  to
allow  solids-liquid separation, then  a decant operation
takes  place. After decanting, thickened stabilized  solids,
about 3 percent, can then be removed or more waste
sludge would be added.
  In the past,  many engineers have tried  to make this
design more continuous by installing stilling wells  in  part
of the  tank.  This has proved not to be effective20'64'65 and
should not be incorporated into the design.
  For  the continuously operated system, solids are
pumped directly from the clarifiers into the aerobic di-
gester. In this case,  the aerobic digester  operates at a
fixed liquid  level  with  the overflow going  to a solids-liq-
uid separator.  Thickened  solids are normally  recycled
back to the  digestion tank but when  required can also
be  removed  from  the system.
  Though initially  more  costly  than a  batch operated
system, much  of the manual work  involved with aerobic
digestion  is eliminated.
  Another consideration when sizing the aerobic  diges-
tion tank  is the  relationship  between the  tank geometry
desired, the  type of aeration equipment being utilized,
and the mixing pattern  that  will  develop.  An  example of
an  aerobic digester with a mechanical aerator is  shown
in figure  3-13 and one  with diffused  aeration equipment
is shown  in figure 3-14. Figure  3-6 shows the effect of
tank geometry on mixing  as measured by oxygen  trans-
fer rates.
  Assume power cost at  $0.03/kwh ($0.83/mJ),  no  pac-
ing device on the aeration equipment  and that oxygen
demand is uniform over 24 hours  per  day.
  Design  to  handle  peak  conditions (summer  conditions).
  For 4 Mgal/d (.18 m3/s) was 5,684 Ibs oxygen/day
(236.8 Ibs O2/hr) (107.4 kg/hr). For optimum tank geom-
etry power  bill would amount  to $23,225/year. For non-
optimum design power  bill could get  as high  as  $38,-
700/year.
  Note that winter conditions  use less oxygen, 4,640
Ibs/day (193.4 Ibs/hr) (87.7 kg/hr). Using a  pacing  de-
vice, savings of $3,500  to $5,900/year in power  cost
could  be  realized.
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 1. Smith, A. R., "Aerobic Digestion Gains Favor" Water and Waste
    Engineering, vol. 8, Fe., p. 24 (1971).
 2. Aerobic Digestion of Organic Sludges by Oklahoma State Universi-
    ty NTIS Publication PB-211-204, 170070 DAV.
 3. Burton, H. N., and Malina, J. F., Jr., "Aerobic Stabilization of
    Primary Wastewater Sludge" Proceedings 19th Purdue Ind. Waste
    Conference, No. 117,  p. 716 (1964).
 4. Loehr, R. C., "Aerobic Digestion—Factors  Affecting Design," Wa-
    ter and Sewage Works, vol. 112, R169 (1965).
 5. Cook, E.  E. et al., "Detention Time  and Aerobic Sludge  Diges-
    tion" Public Works, November,  p. 69 (1971).
 6. Raw Sludge Coagulation and Aerobic Sludge Digestion, U.S. EPA
    600/2-75-049.
 7.  Lawton, G. W., and Norman, J. D., "Aerobic Sludge Digestion
    Studies" Journal WPCF, vol. 36, No. 4, p. 495 (1964).
 8.  Drier, D. E., "Aerobic Digestion of Solids" Proceedings 18th Pur-
    due Ind. Waste Conference, No. 116 (1963).
 9.  "Aerobic Stabilization of Waste Activated Sludge—An Experimental
    Investigation,"  EPA  Technology Series, EPA 600/2-75-035 (1975).
10.  Gay, D. W., et al., "High Purity Oxygen Aerobic Digestion Experi-
    ences at Speedway Indiana."
11.  Jaworski,  N., et al., "Aerobic Sludge Digestion" 3d Conference on
    Biological Waste Treatment, Manhattan College, April  (1960).
12.  Hamilton,  Ohio "Full Scale  Conversion  of Anaerobic Digesters to
    Heated  Aerobic Digesters"  EPA Technology Series, EPA R2-72-
    050 (1972).
13.  Personal communication with Dr. William J. Jewell, Dept. of Agri-
    cultural Engineering, Cornell University.
14.  Reynolds, T. D., "Aerobic Digestion of Waste Activated Sludge"
    Water and Sewage  Works,  vol. 114, p. 37 (1967).
15.  Reynolds, T. D., "Aerobic Digestion of Thickened Waste Activated
    Sludge" Proceedings 28th Purdue Ind.  Waste Conference, p. 12
    (1973).
16.  Hamoda, M. F., and Ganczarczyk, J., "Aerobic  Digestion of Sludg-
    es Precipitated From Wastewater by Lime Addition," Journal
    WPCF,  vol. 49, No. 3, p. 375  (1977).
17.  Ganczarczyk, J. and Hamoda.  M. F., "Aerobic Digestion of Organ-
    ic Sludges Containing Inorganic Phosphorus Precipitates—Phase I"
    Research Report No. 3, Canada—Ontario Agreement  on Great
    Lakes Water Quality, Environment Canada, Ottawa (1973).
18.  Eikum, A. S., et al., "Aerobic  Stabilization of  Primary  and Mixed
    Primary—Chemical (Alum) Sludge" Water Research, vol. 8, p. 927
    (1974).
19.  Ahlberg, N.  R., and Boyko, B. "Evaluation and  Design of Aerobic
    Digesters" Journal WPCF, vol. 44, No. 4,  p. 634 (1972).
20.  Folk, G., "Aerobic Digestion of Waste  Activated Sludge" WPCF
    Deeds and Data, July (1976).
21.  Marino, K. and Bologna, A., "Determining Stability of  Sludge From
    Aerobic Digesters" WPCF Deeds and Data, October (1976).
22.  Paredes, M. "Supernatant Decanting of Aerobically Digested
    Waste Activated Sludge" WPCF Deeds and Data, October (1976).
23.  Metcalf and Eddy Inc., Wastewater Engineering: Collection, Treat-
    ment and Disposal, McGraw-Hill, Inc., p. 611  (1972).
24.  Randall, C. W., et al., "Temperature Effects on Aerobic Digestion
    Kinetics"  Journal EED ASCE,  vol. 101,  October, p. 795 (1975).
25.  Kambhu, K., and Andrews, J.  F., "Aerobic Thermophilic Process
    For the Biological Treatment of Wastes," Journal WPCF, vol. 41,
    p.  R127 (1969).
26.  Shindala,  A., and Parker, J. E., "Thermophilic Activated Sludge
    Process"  Water and Wastes Engineering, vol. 7, p. 47 (1970).
27.  Andrews,  J. F., and Kambhu,  K., "Thermophilic Aerobic Digestion
    of Organic Solid Wastes" Clemson University Final Report, May
    (1970).
28.  Popel, F.  V., and Ohnmacht, C., "Thermophilic  Baterial Oxidation
    of Highly Concentrated Substrates" Water Research,  vol. 6, p.
    807 (1972).
29.  Matsche,  N. F., and Andrews, J, F., "A Mathematical Model For
    the Continuous Cultivation  of Thermophilic Microorganisms" Bio-
    technology Bioengineering, Symposium No. 4, p. 77 (1973).
30.  Surucu, G. A., et al., "Aerobic Thermophilic Treatment of High
    Strength Wastewaters" Journal WPCF, vol. 48,  No. 4, p. 669
    (1976).
31.  Matsch, L. C., and Drnevich, R. F., "Autothermal Aerobic Diges-
    tion" Journal  WPCF, vol. 49, No. 2, p. 296 (1977).
32.  Randall, C.  W., et al., "Aerobic Digestion of Trickling Filter Hu-
    mus" Proceedings 4th Environmental Engineering and Science
    Conference, University of Louisville, Louisville, Ky. (1974).
33.  McKinney, R.  E.,  "Design and Operational Model for  Complete
    Mixing  Activated Sludge Systems" Biotechnology and Bioengineer-
    ing, vol. 16, p. 703 (1974).
34.  Kountz, R. R.  and Forney,  C.  Jr., "Metabolic Energy  Balances in
    a Total Oxidation Activated Sludge System" Sewage  and Industrial
    Wastes, vol. 31, July, p. 819 (1959).
                                                                                                                       67

-------
35. McKinney,  R.  E., Advances In Biological Waste Treatment, Perga-   52.
    mon Press, N.Y. (1963).
36. Smith, J. E., Jr., et al., "Biological Oxidation and  Disinfection of    53.
    Sludge"  Water Research, vol. 9,  p. 17 (1975).
37. "Aerobic Sewage Digestion Process" U.S. Patent  4,026,793         54.
    (1977).
38. Koers, D. A.,  and  Mavinic, D. S.,  "Aerobic Digestion of Waste
    Activated Sludge At Low Temperatures" Journal WPCF, vol.  49,    55.
    March, p. 460 (1977).
39. Evans, R.  R.,  "Sludge Treatment Process Offers Flexibility, Low
    Cost"  Chemical Engineering, p. 86, December  5 (1977).             56.
40. Vesilind,  P. A., "Sludge  Characteristics"  Treatment and Disposal
    of Wastewater Sludges,  Ann Arbor Press (1974).
41. Dick, R.  I., and Ewing, B. B., "The Rheology of Activated Sludge"  57.
    Journal WPCF, vol. 39 (1967).
42. Laubenberger, G., and Hartman,  L.,  "Physical Structure of Activat-
    ed  Sludge in  Aerobic  Stabilization" Water Research, vol.  5, p. 335  58.
    (1971).
43. Kalinske, A. A., "Turbulence in Aeration Basins" Industrial Water    59.
    Engineering, vol. 8, No.  3, p. 35 (1971).
44. Tsai, et al., "The Effect of Micromixing on Growth Processes"      60.
    Biotechnology and Bioengineering, vol. 11, No.  2,  p. 181  (1969).
45. Optimum  Mechanical Aeration Systems for River and Ponds,  Water
    Pollution Control Research Series EPA 16080 DOO 7/70.            61.
46. Induced  Air Mixing of Large Bodies of Polluted Water, Water
    Pollution Control Research series EPA 16080 DWP  11/70.
47. Rooney,  T. C., and Mignone, N.  A., "Influence of  Basin Geometry   62.
    On Different Generic Types of Aeration Equipment" Proceedings
    33d Purdue Ind.  Waste  Conference (1978).
48. "Performance of The  Aerated Lagoon Process" Design Guides for  63.
    Biological  Wastewater Treatment Processes,  Tech. Report EHE-
    70-22, CRWR-71,  University of Texas Center for Research in Wa-
    ter Resources (1971).                                            64.
49. Price, K.  S. et al., "Surface Aerator Interactions"  Journal Environ-
    mental Engineering Division ASCE, vol. 99, No. 3, p. 283 (1973).
50. "Mixing  Characteristics of Aerated Stabilization Basins" Tappi, Oc-  65.
    tober, p.  1664 (1971).
51. Stankewich, M. J., Jr., "Biological Nitrification  With  The High Pu-
    rity Oxygenation Process" Proceedings 27th Purdue Ind.  Waste     66.
    Conference, p. 1 (1972).
Brock, T.  D., and  Darland, G. K.,  "Limits of  Microbial Existence
Temperature and pH" Science, vol. 169, p.  1316 (1970).
Process Design  Manual for Land Treatment of Municipal Wastewa
ter published by EPA Technology  Transfer, 1008, October 1977.
Ward, R. D.,  "Inactivation of Enteric Viruses in Wastewater
Sludge,"  Proceedings 3d National Conference on Sludge Manage-
ment, Disposal and Utilization, p. 138,  December (1976).
Love, G.  J., et al., "Potential  Health Impacts of Sludge Disposal
on the Land," Municipal Sludge Management and Disposal, Augus
(1975).
Kenner, B. A., et al., "Simultaneous Quantitation of Salmonella
Species and Pseudonomas Aerginosa," USEPA  National Environ-
mental Research Center, Cincinnati, OH  (1971).
"Stabilization and  Disinfection of Wastewater Treatment Plant
Sludges" EPA Technology Transfer Sludge Treatment and Dispos-
al Seminar (1977).
Lund, E.,  "The Oxidation  Potential Concept of Inactivation of Po-
liovirus in Sewage" Amer. Jour. Epidemiol, vol. 81, p. 141  (1965).
Leclerc, H., and Brouzes, P.,  "Sanitary Aspects of Sludge Treat-
ment" Water Research, vol. 7, p.  355  (1973).
Drnevich,  R. F.,  and Smith,  J. E.,  Jr.,  "Pathogen Reduction  In the
Thermophilic Aerobic Digestion Process" Presented at the 48th
WPCF Conference, Miami Beach,  October (1975).
Hagstrom,  L. G., and Mignone.  N. A.,  "Operating Experiences
With a Basket Centrifuge  on Aerobic Sludges" Water and Wastes
Engineering, February (1978).
Bisogni, J. J., and Laurence,  A. W., "Relationship Between Biolog
ical Solids Retention Time and Settling Characteristics of  Activatec
Sludge,"  Water  Research, vol. 5,  p. 753 (1971).
Novak, J. T., et al., "Factors  Influencing Activated  Sludge Proper-
ties"  Journal Environmental  Engineering  Division ASCE, vol.  103,
No. 5, October  p. 815 (1977).
Paredes,  M.,  "Supernatant Decanting of Aerobically Digested
Waste Activated Sludge"  Journal  WPCF Deeds and Data,  October
(1976).
Ritter, L., "Design and  Operating  Experiences Using  Diffused Aer-
ation for Sludge Digestion"  Journal WPCF, vol.  42, No. 10,  p.
1782 (1970).
Kormanik,  R. A., "Estimating Solids Production for  Sludge Han-
dling" Water and  Sewage Works,  December (1972).
      68

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                              Chapter 4
                              Thermal  Treatment  for  Sludge  Conditioning
INTRODUCTION

The  purpose of this chapter is to consider  thermal  treat-
ment of sludge as a conditioning process to improve
sludge  dewaterability by subsequent processes such as
vacuum filter, centrifuge or filter press.  Thermal condi-
tioning  (also often called heat treatment) involves heating
sludge,  with or without  the addition of air or oxygen, to
temperatures of 300 to 500° F (150 to 260° C) in a
reactor under  pressures of 150 to 400  psig (10.5 to
28.1  kgf/cm2)  for periods  of 15  to  40 minutes. Thermal
conditioning causes the  release of water and organic
material from sludge in  the form of a dark  brown fluid
or "cooking liquor."
  Other thermal treatment processes  not discussed
herein include: (1) pasteurization, which operates  at low-
er temperatures, in the  range of 160°F, and (2) wet air
oxidation, which operates  at higher temperatures  and
pressures for more complete  oxidation of sludge solids.
  The EPA Technology Transfer manual on sludge  treat-
ment1 describes thermal  conditioning,  or heat treatment,
as follows:

  In  heat treatment, temperatures of from 300 to 500° F and pressures
of 150 to 400 psig are attained for protracted periods. Significant
changes  in the nature and composition of wastewater sludges result.
The effect of heat treatment  has been ideally likened to syneresis, or
the breakdown of a gel into water and residual solids. Wastewater
sludges are essentially cellular material. These cells contain intracellular
gel and extracellular zoogleal slime with equal amounts of carbohy-
drate and protein. Heat treatment breaks open the cells and releases
mainly proteinaceous protoplasm. It also breaks down  the protein and
zoogleal  slime, producing a dark brown liquor consisting of soluble
polypeptides, ammonia nitrogen, volatile acids, and carbohydrates. The
solid  material left  behind is mineral matter and cell wall debris.
  Dewatering is improved by the solubility and hydrolyzing of the
smaller and more  highly hydrated sludge particles which then end up
in the cooking liquor. While analysis of this liquor from domestic
wastewater sludges indicates the breakdown products are mostly or-
ganic acids, sugars, polysaccharides, amlno acids, ammonia, etc., the
exact composition of the liquor is not well defined.
  A review of reported analyses of liquor from the heat treatment of
sludge gives the range of values shown: BOD5 = 5,000 to 15,000 mg/l,
COD = 10,000 to 30,000 mg/l, Ammonia = 500  to 700 mg/l, and Phos-
phorus as P = 150 to 200 mg/l. About 20 to 30 percent of the  COD Is
not biodegradable in a 30-day period. The volume of cooking liquor
from  an activated sludge plant with heat treatment amounts to 0.75 to
1.0 percent of the wastewater flow. Based on BOD5 and solids  load-
ings,  the liquor can represent 30 to 50 percent of the loading to the
aeration  system. The pH of cooking liquors is normally in the range of
4 to 5, which necessitates chemical neutralization and/or corrosion
resistant  equipment.

  Figure 4-1  is a flow  diagram for a typical heat treat-
ment system. Major components in the  system are  a
heat exchanger and  a reaction  vessel. Heat treatment
                                                 STEAM
DECANT
LIQUOR
OFR GAS
_. .


DEWATER-
ING
LIUU
	 »•
i
                                                      CAKE
Figure 4-1.—Typical  heat  treatment system.


may be used to condition  raw or digested sludges  and
thus location of the system in the overall treatment train
may vary.  If a treatment plant employs anaerobic diges-
tion, heat  treatment is more commonly used to condition
the digested sludge. Heat  treatment before anaerobic
digestion  to  improve degradability and energy  production
was pilot tested by LA/OMA in  Los Angeles.2'3 Heat
treatment  may be  used in  conjunction with incineration  ir
a system that  recycles waste heat to  minimize energy
requirements. These variations in the use of heat treat-
ment in sludge management systems are illustrated  in
figure 4-2.
  The effect of heat treatment on the chemical composi-
tion of sludge was investigated by Sommers and  Curtis.4
Heat treated sludges  from plants in  Speedway and  Terrc
Haute, Indiana  were tested to obtain information  on the
forms of nitrogen,  phosphorus, copper, zinc, nickel, cad-
mium and lead.  In general, heat treatment produced
greater than 50 percent reductions in total nitrogen with
essentially no  change or a slight increase in phosphorus
and metals concentrations.
                                                                                                              69

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CONVENTIONAL

SYSTEM
                    PRIMARY a/OR
                   WASTE BIOLOGICAL SLUDGE
                                                                                                         CAKE
                                                                                      DECANT LIQUOR
                                                DECANT LIQUOR
LA / OMA

SYSTEM
' PRIMARY 8/OR _
WASTE BIOLOGICAL SLUDGE
HEAT
THICKEN
1
1 THERMAL
\ TREATMENT



ANAEROBIC
DIGESTION



DEWATER
I
                                                                                                 CAKE
                                             DECANT
                                             LIQUOR
                                     DECANT LIQUOR
ENERGY

RECOVERY

SYSTEM
Figure  4-2.—Heat treatment in sludge  management systems.
RAW a/OR
WASTE BIOLOGICAL SLUDGE
THICKEN
DECANT
LIQUOR
                                 WASTE   HEAT
                                                                           DECANT
                                                                           LIQUOR
                                                                                         ASH
PROCESS DESCRIPTION

  Equipment for thermal conditioning of sludge is manu-
factured and  supplied in the United States by Envirotech
BSP (Porteous  System), Zimpro (wet oxidation), Zurn,
and  Nichols. Almost  all of the equipment for  thermal
conditioning of  sludge in the  United States has been
supplied by Zimpro or Envirotech.  Mayer and Knopp4
reported in January 1977, that 70  thermal conditioning
plants were operating in the United States and Canada
and  43 others were  under construction.
                   Table 4-1.—Size  and status of largest thermal condition-
                   ing  installations
             Type of plant
With air addition
  Operating	
  Under construction
Without air addition
  Operating	
  Under construction
Number of installations
       45
       35

       25
        8
  A partial list of thermal  conditioning installations is
shown in table 4-1.

Zimpro Process

  The Zimpro system is similar to the process illustrated
in figure 4-1 except that  air is also  added to the reac-
tor. Basic features of the  Zimpro process are (1) air
addition to the reactor for oxidation, improvement of
heat exchange characteristics and reduction of fuel re-
quirements,  and  (2) use of sludge-to-sludge  heat  ex-
changer. Some of the equipment used  in this process is
shown in figures 4-3 and  4-4.
  In the continuous process, the sludge is passed
Location
Toronto, Ontario (Ashbridges Bay)...
Cleveland Ohio (Southerly) .
Louisville Ky 	
Cincinnati, Ohio (Mill Creek) 	
Flint Mich
Green Bay, Wis 	
Columbus Ohio (Southerly)
Suffolk Co NY
Toronto, Ontario (Lakeview)
Springfield, Mass 	
Kalamazoo, Mich 	
Columbus Ohio
Toronto, Ontario (Highland Creek) . . .
Chesapeake-Elizabeth, Va 	
Hopewell, Va 	
York Pa
Billings Mont
Escondido Calif

I
Status
uca
UC
Operating
(1976)
Operating
Operating
Operating
(1975)
Operating
(1976)
UC
Operating
(1975)
UC
Operating
(1971)
Operating
(1972)
UC
UC
UC
UC
UC
UC

Number
of
units
7
5
4
4
3
4
3
2
3
2
3
1
3
1
3
2
2
1

Unit
capacity
(gal/min)
250
280
250
280
250
150
200
205
125
200
125
200
125
150
150
125
100
100

                      Under construction.
     70

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Figure 4-3.—Reactor (left),  heat exchangers  (center),
waste  heat recovery boiler (right).
Figure 4-4.—Pump (left), grinder (right).
through a grinder which reduces the size of sludge par-
ticles to about one-fourth  inch. Sludge and air are then
pumped into the system and the mixture is passed
through heat exchangers and brought to the initial reac-
tion  temperature. As oxidation takes place in the reac-
tor, the temperature increases.  The oxidized products
leaving the reactor  are cooled  in the heat exchanger by
the entering cold  sludge and air.  The gases are sepa-
rated from the liquid carrying the residual oxidized sol-
ids,  usually in  a decant tank, and  released through an
odor control unit. The oxidized liquid and  remaining sus-
pended solids  are separated in a decant tank. The de-
cant tank underflow may be further dewatered by sev-
eral  methods; the overflow cooking liquor  is recycled to
the main plant or treated  by a separate system  such as
activated sludge, rotating  biological disk or anaerobic
filter.

Envirotech BSP Process
  This system  was formerly called the Porteus process.
The  Porteous process was purchased by Envirotech and
various changes  have been made  in the system. The
basic system components  and operation of the BSP sys-
tem  are similar to the Zimpro  process as illustrated in
figure 4-1. One basic difference is that air is  not inject-
ed into the reactor  in the BSP system. The  BSP systems
also normally employ a water-to-sludge heat exchanger.

Other Processes
  The Nichols  heat  treatment system was previously
marketed as the Dorr-Oliver Farrer system.  The  Nichols
process is  used at  a plant serving York, Pa., and there
are five installations  of the Farrer system in  the  United
States: San Bernardino, Calif.; Elkhart, Ind.;  Port Huron,
Mich.; Glouster, N.  J.; Norwalk, Conn. There is a Zurn
heat treatment system in Mentor, Ohio which serves an
area of Lake County, Ohio.

Thermal Treatment Process Sidestreams

  There are both  liquid  and gas byproducts from any
thermal conditioning system. These sidestreams must be
considered in planning for an  accurate comparison  with
other processes and in design  for a properly operating
system.

Gas Sidestreams
  There are four principal sources of odor resulting  fron
thermal sludge treatment:  (1) vapors from treated sludge
storage (decant tank or thickener), (2) mechanical dewa
tering system exhaust,  (3) exhausted  air from working
atmosphere in filter and loading hopper areas, and (4)
vapors from strong  liquor  pretreatment devices.  The
odorous gases produced are simple, low molecular
weight, volatile organic substances, consisting  of aldeh-
ydes, ketones, various sulphurous  compounds, and  or-
ganic acids. The odor  level  source associated with ther-
mal  sludge conditioning is dependent to a high  degree
                                                                                                       71

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on the total hydrocarbon  content. The odor  level and
hydrocarbon content are highest in off-gases from  the
heat treated sludge solids separation units, i.e., decant
tank or thickener and  mechanical dewatering systems.
  Off-gases are best controlled  by use of  incineration,
adsorption, or scrubbing (or some combination of these
processes).
  Water scrubbing plus incineration.—For high hydrocar-
bon airstreams, the highest degree of odor control can
be obtained by water  scrubbing followed by incineration.
The scrubbing  portion  of  this  system consists of a
packed bed unit which uses  plant effluent water at rates
of 20 to 30 gpm (1-3  to 1.9  l/s) per 1,000 ftVm (472.0
l/s).  The incineration portion of this system can be ei-
ther direct flame incineration at 1,500°F (815°C) or cat-
alytic  incineration at 800° F (427° C). The oxidation  cata-
lysts that  are commonly used in  catalytic incineration are
supported platinum or  palladium materials.
  Water scrubbing plus adsorption.—In scrubbing meth-
ods, the odorous substances are removed by solubiliza-
tion, condensation,  or  chemical  reaction with the scrub-
bing medium. Scrubbing media that  are  commonly  used
for odor control are potassium permanganate, sodium
hydroxide,  or sodium hypochlorite. Two to four pounds
of potassium permanganate  are required per  pound of
hydrocarbon removed.  In  the adsorption method, sub-
stances are removed from the odorous gas stream  by
adsorption on activated carbon  or silica gel. The activat-
ed carbon or silica gel must be capable of regeneration,
usually by steaming. High hydrocarbon sources can be
treated in an odor  control system composed of a water
scrubber followed  by an activated carbon  adsorption
unit. The water scrubber  is  the same as that described
above. The carbon adsorption unit is a  multiple bed
adsorber that  is sized  to  minimize the required number
of steam regenerations. Normally, the carbon bed would
be sized so that only  one steam regeneration per  day
would  be required. Treating a 1,000  ffVmin  (472.0 l/s)
gas stream would  require a dual bed carbon system
containing  1,800 pounds (816 kg) of carbon per bed.
This sizing would permit an adsorption cycle of 24
hours.  After a  24-hour adsorption time, the second  car-
bon bed would be  placed in the adsorption  cycle and
the spent bed  would be steam regenerated. The regen-
eration cycle requires  low pressure steam  at a maximum
of 50  psig (3.5 kgf/cm2) for  a period of one hour.  The
steam  and desorbed organic compounds from the  bed
are condensed and collected. The aqueous condensate
is returned to the  head of the treatment plant and the
liquid organic  phase is incinerated.
  Multiple scrubbers.—A third option for treating high
hydrocarbon sources is a multiple scrubber system. The
multiple scrubber system would  contain at least two and
preferably three scrubbing stages. In all cases, the  first
scrubbing stage of the system should be water scrub-
bing using plant effluent at a rate of about 27 gal/min
(1.7 l/s) per 1,000 ftVmin (472.0 l/s). The second  and
third stages should be chemical scrubbing using a com-
bination of scrubbing media selected from 5 percent
sodium hydroxide, 3 percent sodium  hypochlorite, and  3
percent potassium permanganate. The potassium perman
ganate solution effects the highest degree of hydrocar-
bon  reduction and, hence, the highest odor reduction.
One of the  most effective multiple scrubber systems con
sists of three  stages  utilizing plant effluent, 5 percent
sodium hydroxide and 3 percent potassium permanga-
nate.

Liquid  Sidestreams

  The liquid (cooking liquor) containing  materials solubi-
lized during heat treatment  of sludge may be separated
from the solids (1) during storage in decant tank,  thick-
ener, or  lagoon,  and  (2) in  the  dewatering step using a
vacuum filter,  centrifuge, filter press, sand drying  bed or
other method.
  The following  tabulation shows  some of the substances
present in thermal treatment liquor and  the general
ranges of concentration.
               Constituent
Suspended solids	
Chemical oxygen demand
Biochemical oxygen demand.
Ammonia nitrogen	
Phosphorus	
Color, units	
Concentration range
mg/l (except color)

    100-20,000
  10,000-30,000
   5,000-15,000
    500-  700
    150-  200
   1,000- 6,000
  The composition of thermal treatment  liquor varies
widely depending upon sludge composition  and reactor
operating conditions.  For a given  reactor temperature,
increasing the reactor detention time will generally in-
crease the concentration of soluble organic material in
the cooking liquor.  Heat treatment can normally be ex-
pected to solubilize from 40 to 70 percent  of the sludge
biomass. As much as 60 to 70 percent  of  the suspend-
ed solids in waste activated sludge were solubilized in
heat treatment pilot tests in Los Angeles.6
  The character of the cooking liquor is somewhat un-
certain and  the subject of some debate. The EPA
Sludge Manual1 states: "About 20 to 30 percent of the
COD is  not biodegradable in  a 30-day period." Based
on pilot scale heat treatment  studies of  mixed primary
and waste activated sludge, Erickson  and Knopp7 esti-
mated that the soluble nonbiodegradable COD was 7
percent of the total cooking liquor COD. Laboratory
tests by  Stack, et al.,8 indicated that about 40 percent of
organics in the cooking  liquor from heat treatment of
waste activated sludge were resistant to biological oxida-
tion.
  The EPA Sludge Manual further states: "Based on
BOD5and solids loadings, the  liquor can represent 30  to
50 percent of the loading to the aeration system."  Boyle
and Gruenwald9 reported that the  heat treatment recycle
liquor BOD contributed approximately 21 percent  of the
BOD entering the Colorado Springs, Colorado plant. Stu-
dies by Haug, et al.,6 indicated that recycle of cooking
liquor in the  Hyperion plant at Los Angeles would in-
crease the oxygen  demand on the aeration system by
about 30 percent.
    72

-------
  Thermal treatment liquor can  be treated by  recycle to
the main treatment plant or by  separate treatment sys-
tems such as activated sludge,  rotating biological disks
or anaerobic filters.
  Recycle to main plant.—Thermal treatment liquor often
is recycled through the main treatment plant, being add-
ed to the raw sewage or primary effluent. This places
an additional load upon the system principally in the
form of oxygen  demand, suspended solids and color. In
most cases  the  color and  COD of the final  effluent will
increase. The effects of recycle can be mitigated to
some extent  by  storing thermal  treatment liquor and re-
turning it to the treatment plant at a uniform rate or
during  off-peak  hours.
  Separate treatment and  disposal.—Another method  for
handling  liquor is to treat the sidestreams separately
with  no return of any liquor to  the main treatment plant.
Sometimes digester supernatant and waste activated
sludge are combined with  the thermal treatment liquor
for separate  processing; one example of this method  is
the installation at Indio, Calif, where aerated lagoons
with  long  retention  provide satisfactory results. Lagoon
effluent is blended with plant effluent for discharge.
  Separate treatment prior to recycle.—In order to re-
duce the load on the main treatment plant and maintain
final  effluent  quality, cooking liquor  is often  treated sepa-
rately prior to recycle to  the main plant.  Plain aeration,
extended aeration, and activated  sludge treatment have
been used for pretreatment of cooking liquors. BOD
reductions by conventional activated sludge  pretreatment
of liquors have  been reported as high as  90 percent.  It
may  be necessary to collect and  deodorize  aeration
basin off-gases.


THERMAL CONDITIONING COSTS

  Thermal conditioning of sludge  affects the cost of
other treatment  plant processes, decreasing  some  and
increasing others. Total cost includes direct  capital, op-
erating, and maintenance  costs  for sludge handling plus
or minus the indirect net  cost effect of sludge handling
on other treatment  plant processes. Added costs result-
ing from  heat treatment include: (1) cooking liquor treat-
ment, and (2) treatment of odorous off-gases. Potential
cost  savings include reduction in  subsequent sludge
treatment  and disposal costs because of improved
sludge dewatering.
  An EPA10 report presents detailed cost estimates  for
thermal conditioning and sidestream treatment. Costs
were based on  data  from  several sources including op-
erating plants, published literature, manufacturers data
and engineering estimates. The  following cost information
was  developed for thermal conditioning systems (does
not include costs for sidestream treatment):

  1.  Capital costs for thermal systems vary  from about
     $50,000 per gal/min ($790,000 per l/s) of thermal
     treatment system capacity  for a 10 gal/min (.6 l/s)
     system to $10,000 per gal/min ($159,000 per l/s)
     for a 200 gal/min (12.6 l/s) system.
  2.  Typical fuel  requirements are 900 to  1,000 Btu per
     gallon (249 to 277  kJ/l) for systems  that do  not
     employ air addition and 300 to  600 Btu per gallon
     (83 to 166 kJ/l) with air addition.
  3.  Average electrical  energy consumption  averaged 22
     kWh per 1,000 gallons (209 J/l) for plants with air
     addition and  10 kWh per 1,000 gallons (95 J/l)
     without air addition.
  4.  Operation and maintenance labor constitutes  a sig-
     nificant fraction of overall costs, ranging from 6,000
     hours per year for  a 10 gal/min (.6 l/s) system to
     20,000 hours per year for  a 200 gal/min (12.6 l/s)
     system.
  5.  Costs for  materials and supplies range  from $5,000
     per year for a 10  gal/min  (.6 l/s) system  to
     $20,000 per year for a  200 gal/min (12.6  l/s) sys-
     tem.

  The following cost information is related to indirect
costs for treating sidestreams:

  1.  Increased capital costs  primarily result  from the
     need to increase aeration tank  volume  and air sup-
     ply capabilities.
  2.  Increased energy is required for aeration capacity
     required to treat the recycled liquor.
  3.  Increased labor is required for maintaining  and op-
     erating the additional aeration capacity and related
     settling and  pumping systems.

  Costs  for treating the  off-gas from the thermal treat-
ment system  typically constitutes 5 to 10 percent  of the
total cost for thermal treatment. Carbon adsorption is
the most costly technique for odor control.  Incineration
is  most economical in smaller plants and chemical  scrub-
bing in  larger plants.
  Based on unit costs of $7 per hour for labor, $0.03
per kWh for  electricity, and $2.80 per million Btu  and
amortization of capital costs over 20  years at 7 percent
interest,  the following typical costs for thermal  condition-
ing were determined (all  costs are dollars per ton of dry
solids processed):
            Construction costs
O. & M. cost
  Sludge
 ton/day   Direct  Indirect  Total   Direct  Indirect  Total   Total

  1....
  5....
 10....
 50....
100....
97.53
30.79
21.45
12.20
10.96
4.11
3.18
2.93
1.83
1.98
101.64
33.97
24.38
14.03
12.94
150.14
46.46
32.52
19.10
16.58
4.93
3.67
3.50
2.99
2.87
155.07
50.13
36.02
22.09
19.45
256.7"
84.11
60.4(
36.1 1
32.3!
  The  March 1975 national average construction costs
for  thermal conditioning  are shown on figure 4-5. These
costs include feed  pumps; grinders; heat exchangers;
reactors; boilers; gas separators;  air  compressors where
applicable; decanting tanks; standard  odor control sys-
tems; and piping, controls, wiring and installation serv-
ices usually furnished by the equipment or system manu-
facturer.  Not included in  the basic thermal treatment
costs are buildings: footings; piping; electrical work and
utilities not supplied by the equipment manufacturer;
                                                                                                          73

-------
sludge storage and  thickening prior to thermal treatment;
sludge dewatering, incineration  or  diposal;  land; and  en-
gineering  fees. In escalating costs for  later dates, it
should be considered that the escalation determined
from the  EPA-STP index may not  adequately  reflect the
increased costs for  high  temperature, equipment-domi-
nated processes such as thermal treatment.
  A second curve (curve B) is  shown  on figure 4-5  and
includes  the  costs for typical building,  foundation  and
utility needs  for thermal treatment  systems. The building
costs represent single-story, concrete or masonry  con-
struction  with built-up roofing,  insulation and heat  and
vent systems, and assume that reactors and decant tank
will be located outside of the building. The costs  also
include  piping and wiring within the building, foundations
for internal and external  equipment, and limited amount
of site work. Building sizes provide for easy access to
equipment and control room. For larger installations,
where multiple units  are anticipated, space for some
standby equipment is included.  Typical building sizes
range from 1,500 square feet (139 m2) for a 10 gal/min
(.6 l/s) plant to  5,250 square feet (488 m2) for a  200
gal/min (12.6 l/s) plant.  The construction  cost of  the
building was estimated to be $36/ft2 ($387/m2).
  The curves show  a rapid  rise in unit construction
costs for  plants smaller than about 20 gal/min (1.3 l/s).
The minimum direct cost of a  thermal  treatment plant is
estimated to be approximately $350,000 regardless of
plant size. For plants above about 150 gal/min (9.5  l/s)
the increased use of multiple treatment units and of
standby units results in a  lower  limit for unit cost per
gal/min of capacity. This lower  limit appears to  be  in
the range of $9,000 to $12,000  per gal/min ($143,000
to $190,000  per  l/s). Data for these larger plants are
sparse, however, and some plants reported lower unit
costs.
  The  annual fuel requirements based  on 8,000 hours ol
operation  are shown in figure 4-6.  Fuel is used  chiefly
as a source  of heat to produce steam. The amount of
fuel used  is  influenced  by the  reaction  temperature, effi-
ciencies of the boiler and heat exchange systems, insu-
lation or heat losses from the  system and  the  degree of
heat-producing oxidation which takes place in  the reac-
tor. Some reduction in  the unit heat requirement for
increase in plant size is reported. This  is believed to
result from more uniform  and constant  operation of the
system, greater  heat transfer and insulation efficiencies
and possibly a greater  amount  of oxidation in  the larger
units. Plants  adding air to heat  exchangers and  reactors
experiencing  some oxidation have lower fuel require-
ments.
  Typical  fuel requirements averaged 900 to 1,000 Btu
per gallon (249  to 277 kJ/l) for plants not practicing  air
addition and  300 to 600  Btu per gallon (83 to 166  kJ/l)
depending on the degree  of oxidation  obtained,  for
plants  practicing air addition. Curves in this paper are
based  on fuel requirements  of 900  Btu per gallon (249
kJ/l) for thermal conditioning without air and  500 Btu
e
7
6
5
4
3
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,000
9
8
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SEATMENT SYSTEM ONL
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9 2 3456783
                                                           i,ooo,ooo
                     10
                                     100
                                                     1,000
                  THERMAL TREATMENT CAPACITY, GPM
Figure 4-5.—Direct construction costs for  thermal condi-
tioning.
                                                           ,100,000
                                                                    CURVE A-THERMAL CONDITION! G
                                                                    CURVE B- AIR ADDITION
                                                                            THERMAL TREATMENT CAPACITY, GPM
 Figure 4-6.—Annual direct fuel  requirements  for thermal
 conditioning.
     74

-------
 per gallon (139 kj/l), corresponding to about five per-
 cent oxidation,  plants with air addition. These fuel re-
 quirements do not include allowances  for treatment of
 off-gas.
   Annual electrical energy usages for  the two types of
 plants (with  and without air addition) are shown in  figure
 4-5. A separate curve is included on  figure 4-7 for
 estimating the energy requirements for building needs.
 Electrical energy  requirements are determined by sizes
 and efficiencies of machinery such as sludge and boiler
 water pumps, grinders,  thickeners and, in plants where
 air addition  is practiced, air  compressors. Electrical ener-
 gy is also required  for lighting  and other building uses.
 Average unit energy requirements are  22 kWh per 1,000
 gallons (209 J/l)  for plants practicing  air addition and
 10 kWh per 1,000 gallons (95 J/l) for plants without air
 addition.
   Operation and maintenance labor requirements are
 shown  in figure 4-8. In this  paper operation  comprises
 time spent collecting and logging data on the process,
 controlling and  adjusting the various systems and com-
 ponents, and laboratory  work. The functions  covered by
 maintenance include cleaning and repairing process com-
 ponents, general  upkeep of the process area, checking
 and repairing of controls and instrumentation, and per-
 forming  preventative maintenance. Maintenance in figure
 4-8 does  not include major  overhauls which  will be re-
 quired periodically.  In some  plants these operation  and
 maintenance functions  may vary  or may overlap.
 100,000
    9
    a
    7
    6
    5
    4
 10,000
    c
    £
    7

s   I
   100
                        OPERATION -—,
                                 MAINTENANCE
              4 56789
                                5 6 7 B9
                                    100
                                              4 56789
                                                    1,000
                 THERMAL TREATMENT CAPACITY, GPM
Figure 4-8.—Operating and maintenance  labor require-
ments for thermal conditioning.
       CURVE A-AIR ADDITION

       CURVE B-THERMAL CONDITIONING
       CURVE C- BUILDING AND SITE NEEDS
                                                    1,000
                   THERMAL TREATMENT CAPACITY, GPM
Figure 4-7.—Annual direct electrical energy requirements
for  thermal conditioning.
  In general, maintenance labor is approximately one-
fourth of operating labor, ranging from the equivalent of
one maintenance man for one shift at a  50 gal/min (3.2
l/s) plant to one and  one-half men for one shift at a
200 gal/min (12.6 l/s) plant.  The amount of maintenance
required depends  greatly on the design and operation of
the plant, particularly  on equipment and  materials used
for  construction. It is  also dependent on the skill and
knowledge of the  maintenance  personnel and the design
of,  and  adherence to, a preventative  maintenance pro-
gram.
  Annual costs for materials and supplies are shown in
figure 4-9.  Curve  A shows the normal annual costs for
materials and supplies required to operate and  maintain
the thermal conditioning system. These costs are plotted
against  thermal treatment plant capacity  and include  ma-
terials and parts such as seals,  packing, coatings,  lamps,
bearings, grinder blades, and other items used  in sched-
uled and normal maintenance. They also include operat-
ing  supplies  such  as lubricants,  cleaning chemicals, boil-
er feed  water, and water treating chemicals. These costs
vary from about $5,000 per year for a 10  gal/min  (.6
l/s) plant to approximately  $20,000 per year for a  200
gal/min (12.6 l/s) plant.
  Besides normal, periodic  maintenance required for  a
plant shown by curve A, additional costs for major over-
haul work are incurred.  This work includes such items
as  motor rewinding; major overhauls of pumps and com-
                                                                                                           75

-------
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                                                          10,000
                                                                             TREATMENT PLANT FLOW, MGD
                                                                             10    20     50   100
                      10               100


                   THERMAL TREATMENT CAPACITY, GPM
                                                     1,000
Figure 4-9.—Materials  and supplies for thermal condi-
tioning.
                                           3 456789
                                                   1,000
                                                                          THERMAL TREATMENT LOADING, TONS/DAY
Figure 4-10.—Direct and indirect costs for thermal  con-
ditioning.
pressors;  major non-routine rehabilitation or replacement
of heat exchanger tubing piping and controls; and refit-
ting  of boilers. This type of work is required at an
average interval of about 6 to 7 years, depending on
the conditions  at  a  particular plant. Because labor for
this  type  of major work is often contracted, labor costs
are treated as part  of  the overhaul and  included  in its
cost under this section. Curve B shows  the combination
of these costs with  those included under curve A to
give the total annual cost for the materials and supplies.
The  inclusion of major  overhaul work increases the an-
nual materials  cost  by  about 45 percent over that re-
quired for routine and  preventative maintenance mate-
rials.
  There was considerable variation among the costs for
materials  in seemingly  similar plants and it appeared that
three factors tended to govern  the costs.


  1.  Preventative  maintenance program.  In plants  where
     a good program was practiced, overall costs for
     parts supplies generally were  lower.  Where mainte-
     nance  was neglected,  more major failures were
     found to occur with a need for greater expenditure
     for parts.
  2.  Design of the plant and selection of materials of
     construction. If a  higher grade of materials and
     equipment were selected for initial construction  and
     if the plant were designed with  ease of mainte-
     nance  in  mind, less maintenance  and better  mainte-
     nance  were found and hence  less need  for replace-
     ment was noted.
  3. Quality of the water supply. In areas with  high
     hardness and high mineral  contents  in their water
     supplies, more scaling and corrosion were noted in
     equipment, particularly in heat exchangers. Scaling,
     along with  the increased amount of  cleaning re-
     quired,  resulted in both  an increase  in replacement
     parts for boilers and heat exchangers and an in-
     creased amount of chemicals  for boiler water treat-
     ment and heat exchanger cleaning.

  Total costs for thermal conditioning systems,  with air
addition, including  costs for treatment of  cooking liquor
and odorous gas  sidestreams are shown  in figure 4-10.
Costs  in figure 4-10 are based  on  the following:

  1. Cooking liquor treated in the main plant by increas-
     ing the size of activated sludge system.
  2. Capital costs  include an allowance for engineering,
     legal and administrative  and interest during con-
     struction and  amortized  over 20 years at 7 percent
     interest.
  3. Electrical energy cost =  $0.03/kWh ($0.83/mJ).
  4. Fuel cost = $2.80/million  Btu ($2.65/GJ).
  5. Labor  cost = $7.00/hour.

  Using the above criteria, total costs for thermal condi-
tioning  range from $257/ton  ($283/Mg) in a  1  ton/day
(0.9 Mg/day) capacity  plant  to $32/ton ($35/Mg) in a
100 ton/day (91 Mg/day) plant.
     76

-------
DESIGN EXAMPLE

  The design example considered herein is a 4 Mgal/d
standard activated sludge plant with the following sludge
characteristics:
                                              Flow
     Sludge
      type

Primary	
Secondary	
      Total	
Total solids
   (Ib)
Volatile solids
    (Ib)
(gal/min)  (Mgal/d)
5,200
4,000
3,120
3,200
5.4
8.3
0.008
0.012
  9,200
   6,320
  13.7
0.020
  These sludge quantities were determined with the fol-
lowing assumptions:
  1. Raw wastewater suspended solids = 240 mg/l;
     BOD = 200 mg/l.
  2. Suspended solids removal = 65 percent in primary
     treatment and 90 percent overall;  BOD removal = 30
     percent in primary treatment and 90  percent overall.
  3.  One-half pound activated sludge produced per
     pound  BOD removed.
  4.  Primary sludge  is 4 percent solids and  is gravity
     thickened  to 8  percent solids.
  5.  Waste  activated sludge  is 1 percent solids and  is
     thickened  to 4  percent solids.
  A  process and materials flow  diagram is shown in
figure 4-11  for a thermal conditioning system of primary
and  secondary sludge. The example system  utilizes air
addition and assumes that the recycle liquor will be
treated  in the  main activated sludge plant. Other fea-
tures of this system  include the following:

  1.  One  thermal conditioning reactor required.
       Flow = 20 gpm (1.3 l/s)
       Operating pressure = 350 psig (24.6 kgf/cm2)
       Operating temperature = 370° F (225° C)
       Operating schedule: 24 hours/day, 7  days/week
       Installed  horsepower = 85 (63.5 kW)
                                                                                   BOILER
                                                                             TO ATMOSPHERE
                                                                                ODOR
                                                                                CONTROL
      TO MAIN PLANT
PRo'rSr® VACUUM ^
PROCESSING ys FILTER
^ AND OR /
DISPOSAL
LOCATION

1 . Primary Sludge
2. Secondary Sludge
3. Recycled Sludge
4. Total Sludge
5. Conditioned Sludge
6. Decant Underflow
7. Vacuum Filter Cake
8. Decant Supernatant
9. Vacuum Filter Filtrate
10 Total Liquid Recycle
11 Decant Tank Exhaust— 81 scfm
12 Vacuum Filter Exhaust-2400 scfm
13 Air to Reactor— 32 scfm
14 Steam to Reactor— 8,000 Ib/day
15. Boiler Feed Water-0 001 Mgal/d (0.7 gpml
16 Vacuum Filter Wash Water— 0.007 mgd {5 gpm)
Mgal/d

0.008
.012
.002
.022
.023
.009
_
.015
013
028







gmp

5.5
8.4
1.4
15.3
16.0
6.3
_
10.4
90
19.4







Ton/day

32
50
11
93
98
36
10
61
56
117






-« 	

Total
Solids
Ib/day
5,230
4,040
830
10,100
9,760
8,015
7,200
1,730
840
2,570






{°J

Percent
Solids

8.0
4.0
3.6
5.4
5.1
11.1
36.0
_
-
1 1








BOD5

Ib/day mg/l
_
_ _
_ _
_ _
_ _
_ _
_ _
875 7,000
370 3,400
1245 5,300






Figure 4-11.—Thermal conditioning example 4 Mgal/d  activated sludge plant.
                                                                                                         77

-------
       Building area required = 1,115 square feet
          (103.6 m2)
  2. One decant tank required.
       Design loading = 50 Ib/sq ft/day  (244 kg/rrrVday)
       Diameter = 15  feet  (4.57  m)
       Side water  depth = 10 feet (305 m)
  3. Scrubber-afterburner  system  to treat 81  scfm
     (38.2  l/s) odorous gas from  decant tank.
       Installed  horsepower = 3  (2.2 kW)
       Building area required = 32 square feet (3.0 m2)
  4. Multi-stage scrubber  to treat 2,400 scfm  (1130  l/s)
     odorous  gas  from vacuum  filter.
       Installed horsepower = 13 (9.7  kW)
       Building area required = 144  square  feet  (13.4 m2)
  In this example,  the assumed BOD  loading without
thermal  conditioning is  6,670 pounds  (3025  kg)  per day
in the raw wastewater  and 4,670 pounds (2118 kg)  per
day to the aeration basins. The BOD  in the decant tank
supernatant and  the vacuum filter filtrate are estimated
to increase the main  treatment  plant loading as follows:
                                    Decant     Vacuum    Total
                                     tank       filter    recycle
                                  supernatant   filtrate    flow
BOD5,  Ib/day	      875       370     1,245
Percent BOD5 in raw wastewater ....      13.1       5.6     18.7
Percent BOD5 to aeration  basins	      18.7       7.9     26.7
REFERENCES
 1.  "Process Design Manual for Sludge Treatment and Disposal," EPA
    Technology Transfer, EPA 625/1-74-006, pp. 6-14—6-16, October
    1974.
 2.  Haug, R. T., "Sludge Processing to Optimize Digestibility and
    Energy Production," Journal WPCF, pp. 1713-1721, July 1977.
 3.  Haug, R. T., et al., "Effort  of Thermal  Pretreatment on  Digestibility
    and Dewaterability of Organic Sludges," Journal WPCF, pp. 73-
    85, January 1978.
 4.  Sommers, L. E. and Curtis, E. H., "Wet Air Oxidation: Effect on
    Sludge Composition," Journal WPCF, pp. 2219-2225, November
    1977.
 5.  Mayer, M. R. and Knopp, P. V.,  "The  Cost Effectiveness of Ther-
    mal Sludge Conditioning," paper presented at the Annual Confer-
    ence,  New York Water  Pollution  Control Association,  January
    1977.
 6.  Haug, R. T., et al., "Anaerobic  Filter Treats Waste Activated
    Sludge," Water and Sewage Works, pp. 40-43,  February 1977.
 7.  Erickson, A. H. and Knopp, P. V., "Biological Treatment of Ther-
    mally  Conditioned Sludge Liquors," Advances in Water  Pollution
    Research, Pergamon Press, pp.  11-3311-11-3315,  1972.
 8.  Stack, V. T., Jr., et al., "Pressure Cooking of Excess Activated
    Sludge," paper presented at the National Industrial Solid Wastes
    Management Conference, University of  Houston, March  1970.
 9.  Boyle, J. D. and Gruenwald, D.  D., "Recycle of Liquor from Heat
    Treatment of Sludge," Journal WPCF,  pp.  2482-2489, October
    1975.
 10.  Ewing, L. J., Jr., et al.,  "Effects of Thermal Treatment  of Sludge
    on Municipal Wastewater Treatment Costs," U.S. EPA,  Cincinnati,
    Ohio  EPA-600/2-78-073.
     78

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                             Chapter 5
                             Thickening  of  Sludge
 INTRODUCTION

 Sludge thickening is defined as increasing the total sol-
 ids concentration  of a dilute sludge from its  initial value
 to some higher value, up to a limit of  about  10-12
 percent total solids. Thickening is  contrasted with "de-
 watering" which increases the total solids concentration
 to the range of 15-30 percent. Thickening operations
 are intended to reduce the volume of sludge to be
 further processed  and normally constitute  an intermediate
 step preceding dewatering or stabilization.
  The unit processes most  commonly associated  with
 wastewater sludge thickening are gravity thickening,  dis-
 solved air flotation, and  centrifugation.  Some of the
 heavier sludges, such as raw primary and combinations
 of raw primary and some biological sludges,  may be
 readily thickened with gravity thickeners. Other, more
 flocculent sludges, such  as those from activated sludge
 processes, may require more elaborate  methods.  The
 most  frequent applications of the common processes are
 summarized in  table 5-1.
  The selection and design of a sludge thickening sys-
 tem is dependent  upon many factors including the
 sludge characteristics, sludge processing following thick-
 ening, and the type and size of  wastewater treatment
 facility.  Each thickening  situation will be somewhat differ-
 ent. Applications other than  those  shown in table 5-1
 are possible and,  in some cases, may  provide the de-
 sired  results.
  This paper will discuss in detail  the processes of grav-
 ity thickening, dissolved  air flotation,  and centrifugation.
 Other newer methods will also be  mentioned. First,
 sludge characteristics and sludge handling methods will
 be discussed. This will be followed by  a discussion of
 the thickening processes, performance  data, and  recom-
 mended design standards. This material will then  be
Table 5-1.—Frequent applications of thickening proc-
esses
   Process description
         Sludge applications
Gravity thickening
Dissolved air flotation..
Centrifugation	
Primary sludge
Combined primary and secondary sludges
Secondary sludges
Secondary sludges
                                  used in a design example  which will illustrate the gener-
                                  al  approach necessary  in thickening alternative  evalu-
                                  ation and selection.  Bench scale or pilot studies are
                                  frequently required for determining  applicability  of,
                                  and/or design parameters  for,  the  various thickening
                                  processes. Examples of these will be presented with the
                                  design example. Additionally, equipment capital, opera-
                                  tion, and  maintenance cost data will necessarily be pre-
                                  sented to aid  in screening the  alternatives. As the exam-
                                  ple is developed, the methodology  for determining  the
                                  most reliable and cost effective process for a given
                                  sludge will be shown.

                                  SLUDGE  CHARACTERISTICS  AND
                                  HANDLING
                                    Separation of  solid matter from wastewater in a  set-
                                  tling tank results in  a clarified  tank effluent and a  watery
                                  mass of  solids known as  "sludge."  Many  different  sludge
                                  types and variations in  sludge  concentration are encoun-
                                  tered in  wastewater  treatment.  The  characteristics  of a
                                  sludge prior to thickening will generally  depend upon the
                                  type of wastewater treated, the sludge origin (which par-
                                  ticular  wastewater treatment process), the degree of
                                  chemical addition for improved settling  or phosphorus
                                  removal,  and the sludge age. Additionally, the sludge
                                  produced by a specific settling  tank will also depend
                                  somewhat upon  the  design  and operation  of the unit.
                                  Typical  "as removed"  sludge concentrations are pre-
                                  sented in table 5-2.

                                  Table 5-2.—Typical  sludge  characteristics "as  removed"
                                  from treatment processes
                                                                        Sludge type
                                                                            Range   Typical
                                                                           percent   percent
                                                                            solids    solids
Primary (PRI)	    2-7      4
Waste activated (WAS)	   0.5-1.5    1
Extended aeration (EA)	    1-3      2
Trickling filter (TF)	    1-4      2
Rotating biological disc  (RBD)	    1-3.5    2
Combinations:
  PRI+WAS	   2.5-4      3
  PRI + TF	    2-6      3.5
  PRI + RBD	    2-6      3.5
  WAS + TF	   0.5-2.5    1.5
                                                                                                         79

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          THICKENING
                            STABILIZATION
                                                   DEWATERING
                                                                        STABILIZATION
                                                                        REDUCTION
                                                                                      HEAT DRYING
                                                                                                   ULTIMATE DISPOSA
Figure 5-1.—Alternative primary sludge disposal process trains.
  The  lower figures  in the range of expected results
may be indicative  of settling units processing lighter,
more flocculent sludges or of units operating above their
design capacity. The higher values may be indicative of
the results from units processing easily settled solids or
of units operating  below their design capacity. Chemical
additions may result in higher or lower concentrations
depending  upon the chemical and dosage utilized. The
"typical" percent solids are indicative of the results ob-
tained  from settling tanks operating  at  design capacity
and  treating normal  "domestic wastewater."
  Treatment and disposal of sludges represent two of
the major problems associated with  wastewater treat-
ment. Thickening of  the sludge  represents but one step
of a total disposal scheme which  may  include thickening,
stabilization, dewatering,  stabilization  reduction,  or heat
drying  prior to ultimate disposal. Figures 5-1  and 5-2
show various  primary and secondary sludge disposal al-
ternatives and how sludge thickening may fit  into the
total treatment and disposal  scheme.
  In general, the required degree of  thickening is directly
related to the sludge processing method(s) downstream
of the thickener (see figures 5-1  and 5-2). The stabili-
zation stage, in  particular,  will  normally be more suc-
cessful if the solids  concentration is  kept within the
range that  optimizes the rates  of biological and chemical
stabilization. Likewise,  ultimate  disposal of liquid sludge
by land application will generally be  less costly when the
solids concentration  is maximized but kept within the
range dictated by pumping  equipment. Suggested opti-
mum percent dry solids operating ranges for  various
sludge handling  processes following thickening  are
shown  in table 5-3.
THICKENING PROCESSES

Gravity Thickening
  Gravity thickening of sludges, probably the most com-
mon unit process in use, is relatively simple in  principle
     so

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             THICKENING
                               STABILIZATION
                                                       DEWATERING
                                                                              STABILIZATION
                                                                              REDUCTION
                                                                                             HEAT DRYING
                                                                                                           ULTIMATE DISPOSAL
r> o
O I
0 S
I?
2
O





VACUUM
FILTER

SECONDARY
SLUDGE
O 0
O I
2 rn
2 I
5?
z
o








 Figure 5-2.—Alternative secondary sludge disposal process trains.
 Table 5-3.—Post thickening  process operating ranges
                 Process type
Operating ranges
 optimum sludge
 solids, percent
 Stabilization
   Aerobic digestion	
   Anaerobic digestion	
   High pressure wet  oxidation..
   Low pressure heat treatment.
   Lime treatment	
 Other
   Land application	
     2-4
     4-6
     4-6
     4-6
     6-8

     6-8
 and operation and  low in cost.  Gravity thickening is
 basically a sedimentary process carried out  in a  unit
 which resembles a  wastewater settling basin. A typical
 unit is shown in figure 5-3. Solids settle to  the thickener
                                       SCRAPER BLADES

                                      SLOPE I 4 MINIMUM

                                    UNDERFLOW
                          ELEVATION

Figure  5-3.—Gravity thickener.


bottom, are then raked to a sludge hopper,  and are
periodically removed and  discharged to the next proc-
ess. Water separated from the sludge (supernatant) rises
as  the  sludge settles. This supernatant or overflow con-
taining  some solids and probably a  high biochemical
                                                                                                                   81

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Table 5-4.—Existing gravity thickener performance data

Location
Rumford Mexico, Me 	
Kokomo Ind 	
York, Nebr 	
Salem Ohio
Middletown, Ohio ...


Feed
	 WAS
	 Heat treat3
Combined b
PRI
WAS

Sludge solids
Unthickened
1.2
4-6


0.9

, percent
Thickened
2.7
14-18
6-7
6
3.8

Solids
loading
(Ibs/ft2/day)
5
18

6
1.5

  "Contains heat treated primary and waste activated (equal portions).
  bContains primary, intermediate (trickling filter), and final (biodisc), proportions unknown.
oxygen demand should be  returned to the plant  for fur-
ther treatment. Several existing gravity thickener  installa-
tions were recently contacted. Data, indicative of equip-
ment performance at that time, are presented in  table
5-4.
  Gravity thickeners are  normally circular in  shape and
have a side water depth of about 10 feet (3.0 m). The
tank diameter is  a function of the required surface area.
The required surface area  is determined by  applying
either  pilot tested or average  recommended  solids load-
ing rates to the total solids that the unit  will receive
each day. Tank side water depth is influenced by the
desired retention time and  equipment availability.  Sludge
solids  concentrations obtainable by gravity thickening  de-
pend upon the sludge  type, thickener  overflow rate, and
solids  retention time. Average recommended solids load-
ing  rates and the possible  performance for some sludges
are  presented  in table  5-5.
  The  values  are average  ranges only and may  or may
not  be indicative of the  possible results for  the  particular
sludge in question. A case in point is a community
Table  5-5.—Gravity thickener loading  rates and  per-
formance
                         Sludge solids, percent

Sludge type
Primary (PRI)
Waste activated (WAS) 	
Extended aeration (EA) 	
Trickling filter (TF) 	
Biodisc (RBD)
Combinations.
PRI+WAS 	
PRI+TF 	
PRI + RBD
WAS + TF .


Unthickened
2-7
0.5-1.5
1-3
1-4
1-35
25-4
2-6
2-6
05-25


Thickened
5-10
2-3
1.5-4
3-6
2-5
4-7
5-9
5-8
2-4

Solids loading
(Ibs/ft2/day)
20-30
4-8
4-8
8-10
7-10
8-16
12-20
10-18
4-8

which  gravity thickens a  0.9  percent  dry  solids waste
activated  sludge to 3.8 percent  with  solids capture of
over 90 percent. The solids loading is 2  to 4  Ibs/ft2 (.91
to 1.81 kg) per day  and  the  hydraulic loading ranges
from 50 to  100 gal/ft2/day (2.0 to 4.1  m3/m2/d). This
plant treats a high percentage of  paper mill waste whicl
results in  significant  concentrations of inorganic  solids
escaping  the  primary tanks. These solids, when  com-
bined with the biological  sludge, form a floe that has
much better settling  characteristics than most  waste acti
vated sludges. This results in a better than average
thickened product.
  Although  the  solids loading usually governs  gravity
thickener  design, the hydraulic loading  should  also  be
checked.  Hydraulic loadings  in the range of 600 to 800
gal/ft2/day  (24.4 to 32.6 m3/m2/d) have  been reported
as optimum.1 Also,  loadings below 400 gal/ft2/day (16.3
m3/m2/d) have  been reported as possibly resulting  in
odor problems;  recycling  of secondary  effluent to main-
tain  the higher  rates has been recommended.1 Much low
er rates,  as low as 100 to 200  gal/ft2/day (4.1  to 8.1
rrrVmVd), will often  be more  applicable.  Recycling  of
secondary effluent to control  odor will dilute the influent
solids. The  overall solids thickening performance of the
unit  may  not  deteriorate,  however,  since  dilution will elu-
triate fine solids and reduce  the interference  between
the settling  particles. Polyelectrolyte addition may have
the effect of  improving solids capture and thus  reducing
solids overflow  in the supernatant,  but  may have little
effect  on  improving the solids concentration in the  un-
derflow. To achieve  maximum sludge concentration, a
sludge retention time of one-half to 2 days is  normally
required.


Dissolved  Air  Flotation

  Dissolved  air  flotation is presently the  most widely
used method  of thickening  waste activated sludge. The
system uses air buoyancy to  literally  float solids  to  the
surface of a tank to be collected. The  main advantage
of this method over gravity thickening is  that  very light
particles,  such as waste-activated  sludge solids,  can be
removed more completely in  less time. A typical  dis-
     82

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solved air flotation  system is shown in figures 5-4 and
5-5. The units  physically range from small steel package
units to  custom designed large units with concrete tanks.
Recycle  flow may consist of either underflow from the
unit or recycled plant effluent. It  is returned at rates of
up to five times the feed sludge  rate, combined with air,
and then pressurized to approximately 60-70 Ibs/in.2
(4.2-4.9  kg/cm2). Since the solubility  of air in water
increases with  increasing pressure, large quantities of air
      ADJUSTABLE FLOAT SKIMMER
                                 CHAIN TENSIONER
                                       \
  \    INFLUENT

BACK PRESSURE VALVE
                                 REDWOOD SCRAPER
Figure 5-4.—Dissolved air  flotation unit.
UNIT EFFLUEN1 	
AUX RECYCLE CONNECTION
(PRIMARY TANK OR 1
PLANT EFFLUENT) T
(AIR FEED Cl-*-
AITERNATE' 	 ^ ^Xl

FLOTATION UNIT

TH
-*•
RECIRCULATION PUMP
11 TO 5 1

1 A^



                                          THICKENED SLUDGE
                                              DISCHARGE (FLOAT)
                                                UNIT FEED
                                               "SLUDGE (WAS I
                                               RECYCLE
                                               FLOW
                  REAERATION PUMP
                                      RETENTION TANK
                                      (AIR SOLUBIIIZATION/
                                      003 TO 005 LB'S  DISSOLVED
                                      AIR PER LB OF SOLIDS
Figure 5-5.—Dissolved air flotation system.
go  into solution. Later, this recycle flow is allowed to
depressurize as  it  is mixed with the influent sludge. De-
pressurization  releases the excess air out of the recycle
liquid in the form of tiny air bubbles (80 microns). These
air  bubbles attach themselves to the sludge solids and
float them to the surface. Thickened sludge is scraped
off  the  liquid surface by  a skimmer  mechanism  consisting
of a series of paddles. Liquid that is  not  contained in
the thickened  sludge or recycled is discharged  from the
system  as subnatant. Subnatant may contain high solids
and biochemical oxygen  demand,  and thus should be
returned to the plant for further treatment.
  Data  from existing operating full-scale dissolved air
flotation units  have been presented  in other
publications.2'3 Some of the same installations  were re-
cently contacted. Updated performance data for these
and other dissolved  air flotation units are presented  in
table 5-6.
  The effluent sludge  (float) percent  solids will  depend
on  many variables including the type  and quality of the
feed sludge, recycle ratio,  detention time, air  to solids
ratio, system pressure, the solids and hydraulic loading
rates, and the amount  of chemical aids used. Some
general statements that have  been made  regarding dis-
solved air flotation thickening  of the  "average"  waste-
activated sludge are as follows:2

  1. Increased air pressure or flow will  yield higher float
     solids and lower  effluent suspended  solids concen-
     tration.
  2. Polymer usage  will yield  higher float  solids concen-
     tration  and improve the  subnatant quality.
  3. Detention time  in the  flotation zone is not  critical.

  Since  there  are  so many variables and  each  sludge
will react somewhat differently to the  dissolved  air flota-
tion thickening process, these  "general  rules of thumb"
Table  5-6.—Recent data for some plant scale DAF units
Location
Indianapolis, Ind 	
Warren, Mich 	
Frankenmuth Mich
Columbus Ohio 	
Nashville, Tenn . .
Xenia, Ohio 	

Feed
.... WAS3
.... WAS"
WAS
.... WASC
WASd
.... PS.WAS"
.... WAS

Influent
SS (mg/l)
10,000
1 1 ,000
8,000
6,000
8,000
35,000;5,000
4,000

Subnatant
SS (mg/l)
100-1,000
200
90
800
150
100

Float
percent
solids
3.5-4.2
5
3.5-5.5
3.2
3
6
2.5-3.0

Polymer used
Ibs/ton
dry solids
30
40
bO-26
0
0
30

  "Contains some primary sludge—proportions unknown.
  bMajor flow to plant is brewery waste. Polymer sometimes used to keep sludge from
adhering to skimmers. Sometimes thicken anaerobically digested sludge—similar results with
no polymers required (influent SS 10,000 mg/l).
  cJackson Pike facility.
  "Southerly facility—units are being used as gravity settlers since they get better results
this way.
  'Primary and  waste activated are handled by separate units—combined product is 6
percent solids.
                                                                                                                83

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Table 5-7.—Dissolved  air flotation design  parameters and expected results


Sludge type


Feed
solids,
percent

Solids
loading
(Ib/ft2/hr)

Air to
solids
ratio
Float solids,
„ , percent
Recycle K
ratio,
percent with™ ,t
Solids capture,
percent

With Withni it
                                                              polymer  polymer  polymer  polymer
Waste activated	   0.5-1.5     2-3
Primary and waste activated...    3—4      2-4
0.03-0.05  100-500
5-6
4-5
5-8
95-100
85-95
85-95
  a Limited experience prohibits listing typical numbers.
may not apply in all  cases. Additionally,  when  the  guide-
lines are valid, it is generally only within certain  ranges
of the  variable parameters. The  ranges are typically 40-
70  Ibs/in.2 (2.8-4.9 kg/cm2) for air pressure and 0-40
Ibs (0-18.1  kg) for polymer dosage. Likewise,  the  deten-
tion time may not be critical  once a minimum  value of
1.5-3 hours  has been attained.
  System design is based primarily on a solids loading
rate and the desired air to solids  ratio. Additionally,
maximum hydraulic loading rates  are usually checked to
avoid exceeding manufacturers' recommendations.  If any
flow other  than the dissolved air  flotation thickener un-
derflow is  used for recycle, it must be included  in  the
unit's total hydraulic  loading calculation.
  Pilot studies are  recommended to determine  the  appli-
cability  of  the dissolved air flotation process to the
sludge and to  optimize some of the variables.  When  pilot
studies  are undertaken,  the full-scale design is based  on
the study findings.  Since data and sludge  samples are
lacking  at  new wastewater treatment plants, thickener
design must  be based on sound  engineering judgment
backed up with past experience.  Commonly used design
parameters and expected  unit performance are present-
ed  in table 5-7. It  must be emphasized  that these are
general guidelines only.

Centrifugation

  Centrifugal thickening  of sludge is a process which
uses the force developed by  fast  rotation  of a cylindrical
drum or bowl to separate the sludge solids and  liquid.  In
the basic process,  when a sludge slurry is introduced to
the centrifuge, it is forced against the bowl's interior
walls, forming a thin  slurry layer or "pool." Density dif-
ferences cause the sludge solids and  the  liquid  to sepa-
rate into two distinct layers. The sludge solids "cake"
and the  liquid  "centrate"  are then drawn from the unit
separately and discharged. The three  types of centri-
fuges—basket, disc-nozzle, and  solid bowl—all operate
on  the basic principles described  above. They are differ-
entiated  by the method  of sludge feed, applied centrifu-
gal force,  method of solids and liquid  discharge, and  to
some extent performance.
  The  basket centrifuge, as shown in  figure 5-6, is a
                                       FEED
                             POLYMER -i
                SKIMMINGS
                              CAKE    '   CAKE

              Figure  5-6.—Schematic diagram of a basket centrifuge.
              batch type thickening  unit. As slurry is fed to the unit,
              the  sludge solids form a cake on the bowl walls,  while
              the  centrate is discharged over a weir or baffle. Slurry
              feed is continued until the centrate solids reach the
              maximum  tolerable  limit.  At this point, the unit stops  and
              a knife wipes  the sludge cake off  the walls.  The sludge
              is then discharged  from  the  system through the unit's
              open bottom.  Of the three centrifuge types, the basket
              unit has the capability of producing  the driest sludge
     84

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cake since there is a  minimum  of disturbance to the
depositing solids. Its use, however,  is generally  restricted
to smaller plants because of its intermittent operation
and resultant lower capacity.
  The  disc-nozzle centrifuge, as shown  on figure 5-7, is
a continuously  operating unit. It is composed  of a series
of conical plates which are stacked together to form a
series  of narrow channels. Sludge slurry enters  the  unit
and is dispersed to these channels. The centrate tends
to rise and  is discharged from  the top of the cones,
while  the sludge cake is discharged downward and
through small nozzles  in the bowl wall at the  cone  bot-
toms.  High sludge throughput and good  solids capture
are possible with these units. Their  solids concentrating
capability is limited,  however, by the small diameter
(0.05-0.10  in.)  (0.13-0.25 cm) orifices through which the
sludge cake must discharge. Additionally, depending
upon  the sludge type  and previous  treatment, degritting
and screening  prior to the disc centrifugation  may be
mandatory  to avoid plugging these  sludge discharge ori-
fices and to reduce wear on  the machine.
  Like the  disc centrifuge, the  continuous solid  bowl
centrifuge is a continuously operating unit. It consists  of
a horizontal cylindrical bowl  containing a screw type
conveyor. At one end, the bowl necks down to a coni-
cal section  that acts  as a beach plate for the screw
conveyor. In operation, sludge solids are forced to  the
                                  FEED
                                 EFFLUENT
                                 DISCHARGE
                                              SLUDGE
                                              DISCHARGE
                 RECYCLE

Figure 5-7.—Disc-nozzle centrifuge.
bowl  surface and are moved toward the beach plate  by
the conveyor where they are discharged from  the unit.
The sludge pool level is controlled by adjustable skim-
mers or weir plates.  These also function as discharge
points for  the centrate. A typical countercurrent solid
bowl  centrifuge is shown in figure 5-8. Sludge slurry
enters the unit just before  the  conical section  and
distributes itself along the bowl surface. Sludge solids
are discharged  at the cone end while centrate is dis-
charged at the  opposite end. Sludge solids do not  travel
the full length of the bowl. A second variation of the
solid  bowl centrifuge is the concurrent  model.  In this
unit, sludge slurry is  introduced at the  far end of the
bowl. Turbulence and interference present at  the slurry
inlet point in the countercurrent machine  are  reduced
with this variation.  Also, the slurry must travel the full
length of the  bowl before discharge.  This  may result  in
a  drier sludge cake.
   Centrifuge performance  is measured  by  the  percent
solids of  the sludge  cake and  the centrate quality or
total  solids captured. Several existing centrifuge  installa-
tions were recently contacted.  Data,  indicative of equip-
ment performance at that time, are presented  in table
5-8.  The performance of a particular centrifuge unit will
vary with the inlet  sludge type  and solids  characteristics,
the sludge feed rate, and the degree of chemical addi-
tion.  Centrifuge performance on a particular sludge will
also vary with bowl design, bowl  speed, pool  volume,
and conveyor (if present) design.  In practice,  bowl  and
conveyor design are set  by the manufacturers. Pool
depth is variable on  solid bowl units. Increasing  the pool
depth will normally result in a  wetter sludge cake but
better solids recovery.  Bowl speed is normally variable
on most centrifuge models. Difficulty  involved  in  chang-
ing speeds varies with the manufacturers.  An  increase in
bowl speed normally results in  a  drier  sludge  cake  and
better solids recovery.  Conveyor differential speed is  nor-
mally variable on continuous solid bowl centrifuges. In-
creasing  the differential normally results in a  wetter
sludge cake and poorer  solids recovery. Varying these
parameters will probably result in significant solids
                                                          DIFFERENTIAL
                                                          SPEED
                                                          GEAR
                                                          BOX
                       ROTATING
        COVER

          MAIN
          DRIVE
          SHEAVE
                                                                                                        FEED
                                                                                                        PIPES
                                                                CENTRATE
                                                                DISCHARGE
                "ROTATING
                 CONVEYOR
                                  -"I   V-T>- (SLUDGE
                                     ^UJIHJ     AND
                                     BEARING  CHEMICAL)
          BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
     T
Figure 5-8.—Continuous  countercurrent solid  bowl con-
veyor discharge centrifuge.
                                                                                                           85

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 Table 5-8.—Existing solid bowl centrifuge  performance data
               Location
                                      Feed
                                             Sludge solids, percent
                                            Unthickened  Thickened
         Solids
        recovery,
        percent
Great Northern Paper, Millinocket, Maine . . . .
Kendall Co, Griswoldville, Mass 	
Miller Brewing Co., St. Louis, Mo 	
Dubuque, Iowa

. WAS3
WAS3
WASb
WASb

4
3
075-1
115

10-12
7
5-7
6

90

80-85


  'Polymers used—quantity unknown.
  bPolymers not used.
Table 5-9.—Centrifuge mechanical characteristics  and
performance data
                                    Centrifuge type
          Parameter
                             Basket  Disc-nozzle  Solid bowl
Operation method
Bowl diameter (inches) 	
Max centrifugal force (G) 	
WAS feed solids, percent . . . .
WAS cake solids, percent 	
Solids recovery, percent 	
Batch
12-60
2,000
0.5-1.5
8-10
80-90
Continuous
8-30
12,000
0.5-1.5
4-6
80-90

6-60
3,200
0.5-1.5
5-8
70-90
changes only  within limited  ranges. Each performance
improvement must be compared with the additional costs
required to  produce it.
  Centrifuges  have seen more service in dewatering ap-
plications than in thickening applications. When utilized
for  thickening,  their use is normally limited to the thinner
biological or industrial sludges that cannot be thickened
by  less expensive methods. Data on  the three centrifuge
types and their possible performance on waste activated
sludge are presented in table 5-9.
  Polymers  may be  required to meet the stated perfor-
mance. The required dosage  depends upon  the manufac-
turer and may  range from  0-8 Ib/ton (0-4.0 kg/Mg) of
dry solids.
  Polymer addition  generally improves both the percent
solids  and the  solids recovery. It must  be emphasized
that the  tabular values  are  representative of possible
results from an "average"  waste activated sludge. Num-
erous sludge and machine variables make consultation
with manufacturers  mandatory and  pilot tests highly rec-
ommended for  each installation.

Other  Methods

  Thickening of sludge  is often a  secondary benefit of a
sludge treatment  unit having an entirely different pur-
pose. Decanting facilities are  provided  in aerobic and
anaerobic digesters to remove excess liquids which ha\
risen above the solids layer. In  such facilities, sludge
solids concentrations  may increase as much as one  pe
cent over inlet feed solids concentrations.
  New sludge thickening methods are being marketed
each year. One such method  is the sludge filter bag
system. In  this process,  sludge  is mixed with polymer
and then held  in suspended porous bags. The weight c
the sludge forces  water  out the bag sides  and bottom.
Sludge is held from four to eight  hours depending upor
the desired dryness and is  then released through a bot
torn opening.  Bag life should  be about 2 years.  This
method has not been in existence long  enough to have
been proven reliable.

DESIGN EXAMPLE

Statement  of  Problem

  The  problem is to provide sludge thickening facilities
for two communities,  both of  which have existing con-
ventional activated sludge wastewater treatment  plants.
  The  smaller community has  existing wastewater treat-
ment facilities capable of treating 4.0 million gallons pe
day (.18 m3/s). The facilities consist of  screening, grit
removal,  primary settling, conventional activated  sludge
aeration, final settling, chlorination, and  sludge lagoonin
Present flow to the plant is 3.5 million gallons per day
(.15 nf/s); the 20 year projected flow  is 4.0 million
gallons per day (.18 rrvVs).  The plant meets its propose
discharge permit requirements, but the city has been
ordered to abandon  the sludge  lagoons (which are peri
odically flooded by the receiving stream) and in  their
place construct digestion facilities and  devise a  plan fo
disposal  of the digested sludge. The digested sludge  w
be dewatered on sand drying beds or  hauled  as a liqu
to nearby farms. Thickening facilities are required to
reduce the size of the required  anaerobic digester, to
insure  efficient digester operation, and  reduce hauling
costs.
  The  larger community  has existing wastewater treat-
ment facilities capable of treating 30 million gallons per

-------
day (1.31  m3/s).  Present flow to the plant is 35 million
gallons  per day (1.53 m3/s); the 20-year projected flow
is 40  Mgal/d (1.75 m3/s).  The existing treatment system
consists of screening, grit  removal,  primary settling, con-
ventional activated sludge  aeration,  final settling,  chlorin-
ation, aerobic sludge digestion,  sludge dewatering, and
landfilling  of dried sludge solids. The existing treatment
scheme will meet proposed permit requirements. There-
fore, all treatment units will be  expanded to handle the
20-year flow  projections. Anaerobic digestion has been
determined to be more cost-effective than the aerobic
sludge digestion. The aerobic digesters will  be  aban-
doned as such (will  become part of expanded  aeration
tank facilities).  Thickening  facilities are required to re-
duce  the size of the required anaerobic digesters, to
insure efficient digester operation, and to improve the
dewatering operation.

Wastewater  Characteristics
  The wastewater characteristics and  removal efficiencies
of the various treatment units are required  to determine
the possible solids loading  on the  thickeners. This infor-
mation  may be acquired from plant records or  sampling
programs at  existing facilities. When these  data are  not
available (such as in the case of new wastewater treat-
ment  plants for new service areas), assumptions based
on  sound engineering judgment and previous experience
are necessary. For the sake of simplicity, the wastewater
characteristics  and treatment unit removal efficiencies for
the example  plants  are assumed equal. Raw wastewater
characteristics  for the example  plants are given in table
5-10.

Treatment Unit Efficiencies
  Both  plants in this example will meet their proposed
permit requirements by utilizing  the existing treatment
processes. Nitrification  and phosphorus removal are not
required.  Removal efficiencies based on percentages of
the raw "domestic" wastewater characteristics  are pre-
sented  in table 5-11.

Sludge  Characteristics
  The characteristics of sludge discharged to the thick-
ening facilities may  vary considerably depending upon
 Table 5-10.—Raw wastewater characteristics
            Table 5-11.—Treatment unit efficiencies
                  Parameter
Concentration
   (mg/l)
 BOD5	
 Suspended solids.
 Organic nitrogen..
 Ammonia nitrogen
 Phosphorus	
 Grease	
    200
    240
     15
     25
     10
    100
                            Unit
                                                Removal
                                      Parameter   efficiency,
                                                percent
            Primary settling	   BOD5        30
                                                  SS          65
            Aeration and final settling	   BOD5        60
                                                  SS          25
the type and amount of industrial wastes treated, the
sludge origin (which particular treatment unit), the de-
gree of chemical addition for improved settling or phos-
phorus  removal, and the sludge  age.  Ideally, samples of
the sludge will  be available for analysis. In lieu of this,
the ranges and typical  concentrations shown in
table 5-2 may  be utilized.
  Existing plant operating data at the example plants
have shown that the unthickened primary sludge  con-
tains four percent dry  solids;  the waste activated sludge,
one percent dry solids. Field  experiments at both plants
were conducted by returning  the waste activated sludge
to the  primaries.  This did not seriously alter their opera-
tional characteristics and an unthickened primary sludge
containing 3  percent dry solids resulted. Additionally,
data at these plants have  shown that for every pound of
5-day biochemical oxygen  demand removed in aeration,
0.5 pound of volatile suspended  solids is produced.

Sludge  Handling Following Thickening

  The  required degree  of thickening is directly related to
the sludge processing method(s) following thickening.
Suggested optimum  percent dry  solids operating  ranges
for some sludge  handling processes following thickening
were presented in table 5-3. In  the examples, anaerobic
digestion is to  follow the thickening step.  Hence, sludge
delivered to the digester should  have a solids concentra-
tion between 4 and 6 percent.
   For any sludge thickening problem, there will be sever-
al alternative solutions  which will result in a sludge  prod-
uct in  the desired solids range.  However, since each
solution will probably not result  in the same  "guaranteed
average" percent dry  solids,  the design of the sludge
processing facilities following thickening will also be af-
fected.  Consequently,  these facilities will also have to be
included  in the cost analysis.

Process Alternatives

Gravity Thickening

   In the example, a primary (4  percent) and waste  acti-
vated sludge (1  percent),  or combined sludge (3 per-
cent) is obtained, and  a sludge  concentration for the
anaerobic digester of  4 to 6 percent is needed.  Table
5-5 and past  experience indicate that gravity thickening
                                                                                                           87

-------
of "normal" waste activated sludge alone will not yield
the required 4 percent solids.  Gravity thickening may
yield  reasonable  results for the combined sludges.  Addi-
                    SETTLING CHARACTERISTICS-8' COLUMN
                    WASTE ACTIVATED SLUDGE


                    W.A.S. SUSPENDED SOLIDS = 10,000 Mgal/d

                    MAXIMUM SOLIDS CONCENTRATION = 2.8%
                                       1/3 VOLUME
                120
     180   240  300   360

    SETTLING TIME (min)
                                           420  480
Figure 5-9.—Settling characteristics—8 foot column
waste-activated  sludge.
tionally, gravity thickening primary sludge alone and
waste activated alone,  and later mixing the two, is a
possibility.  At this point in an actual problem at  an  exisl
ing treatment plant, bench or pilot studies would be
performed  to  determine the applicability  of gravity thick-
ening to the sludge and  to determine design parameters
  Examples of results of typical 8-foot column bench
scale tests are shown  on figures 5-9 and 5-10. Both
the undiluted  and elutriated activated sludges reached
their maximum solids concentrations of 2.8 percent  and
2.3 percent,  respectively, in less than 3  hours. A similar
test would be made on primary only and combinations
of primary and waste  activated sludge.
  For the example plants, assume the results of the
tests showed  that gravity thickening the  sludges will re-
sult in the following: primary sludge, nine percent; waste
activated sludge, 2.8 percent;  combined  primary and
waste activated sludge, 5 percent.

Dissolved  Air Flotation

  Reviewing  the  example problem, there is primary (4
percent) and waste activated sludge (1  percent) or com
bined sludge (3 percent), and a sludge concentration fo
    10 •
                                                                               SETTLING CHARACTERISTICS-8' COLUM
                                                                               WASTE ACTIVATED SLUDGE
                                                                               ELUTRIATED (1:1 DILUTION)


                                                                               INITIAL S.S. OF W.A.S. = 11,600 Mgal/d
                                                                               S.S. AFTER DILUTION = 5,800 Mgal/d
                                                                               MAX. SOLIDS CONCENTRATION = 2.3%
           30
60    90   120   150    180   210   240   270   300   330   360   390   420   450  480


                        SETTLING TIME (MINUTES)
Figure 5-10.—Settling characteristics—8 foot column elutriated waste-
activated sludge.
     88

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the anaerobic digesters of  4 to  6 percent is  needed. If
primary sludge is  to be thickened alone,  gravity thicken-
ing is generally utilized since the costs would be much
less than for dissolved air  flotation.  Likewise, in the  case
of the primary-waste activated combined  sludge, gravity
thickening will  yield similar  results at less cost. This
leaves thickening  the  waste activated sludge  alone by
dissolved air flotation  thickening as  a possible option.
Dissolved air flotation thickening  of  the waste activated
sludge,  coupled with either unthickened or  gravity thick-
ened primary  sludge,  represents a viable alternative  and
will be  considered. At existing plants,  pilot  tests should
be performed to aid in  process  selection and equipment
design.
   Assume a pilot  study was completed using dissolved
air flotation thickening on the waste activated sludge.
The variables studied included recycle  ratio,  air to solids
ratio, solids loading rate, and amount of polymer used.
The system pressure  was kept constant.  The results,
shown graphically in figures 5-11, 5-12, 5-13, and
5-14, were as follows:
   1. Increasing the recycle rate generally yielded higher
     percent float solids but also higher  effluent sus-
     pended  solids.  A compromise rate was selected  for
     use in later tests.
  2.  A concentrated sludge of 4  percent solids could be
     consistently achieved with a unit  loading of 2
     Ib/ft2/hr (9.8 kg/m2/hr) and an air to  solids ratio of
     0.04. Increasing the solids loading reduced the float
     concentration and increased the effluent suspended
     solids concentration with and without polymer  us-
     age.
  3.  At the recommended loading, an  effluent suspended
     solids concentration of 50 milligrams per liter with-
     out the  use of  polymers and 20 milligrams per liter
     with polymer addition  was consistently achieved.
     Polymer  usage, however, resulted  in no  clearly
     identifiable improvement in the float solids concen-
     tration.
  4.  Very rapid deterioration in the effluent quality  oc-
     curred when the air to solids ratio fell below 0.020.
     Increasing  the  air to solids rates  from 0.040 to
     0.250 resulted  in only slight reduction  in effluent
     suspended solids.
  As seen from the  results, the waste activated sludge
differed somewhat from the experience of others2 and an
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              CONCENTRATION
                                               EFFLUENT
                                               SUSPENDED
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                                                                                                             800
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                                            SOLIDS LOADING (LB/SQ FT/HR)
Figure 5-11.—Float concentration and  effluent  suspended solids versus
solids  loading—without polymers.
                                                                                                            89

-------
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                                                                    FLOAT
                                                                    CONCENTRATION
                                             /"EFFLUENT
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                                                 3              4

                                            SOLIDS LOADING (LB/SQ FT/HR)
Figure 5-12.—Float concentration and effluent suspended  solids versus
solids loading—with polymers.
"average" waste activated  sludge. A  4 percent float was
obtained with or without  polymers.
  For the example plants, it will be assumed that dis-
solved air flotation thickening is applicable to the waste
activated  sludge and that a thickened sludge of 4 per-
cent solids will  be produced at solids loadings of 2
Ib/ft2/hr (9.8  kg/m2/hr) and an air to solids ratio of
0.04.


Centrifugation
  The  problem at the example wastewater plants  is to
produce a 4 to 6 percent dry solids sludge for anaero-
bic digestion from  primary sludge (4 percent) and waste
activated  sludge (1  percent),  or combined sludge (3  per-
cent).  Past experience indicates that  thickening the pri-
mary or the combined  sludge by  centrifugation would be
a more costly alternative than gravity thickening. These
alternatives  are eliminated from further consideration.
Centrifugal thickening of  the waste activated sludge,
however,  combined with  either  unthickened or  gravity
thickened primary sludge does  represent a viable alter-
native  and will be  considered. As  in the case of gravity
and dissolved air  flotation thickening,  sludge treatability
and variable  optimization make pilot studies highly  desir-
able when possible.
  For the example, assume a  pilot  study using a solid
bowl centrifuge  was performed as part of the sludge
thickening  study on the  waste activated sludge. Some
typical data from this  pilot test are shown  in table 5-12
In the  pilot study, the feed rate of  the sludge, bowl
speed, and pond setting were varied to determine  the
optimum combination to yield a  5 percent sludge. Minor
pond  setting  changes  had little effect on the unit's per-
formance.  Operation at  3,200  G produced  a sludge
much thicker (12 percent) than needed, while operation
at 1,150 G produced  a  wet  sludge and poor  solids
removal efficiency.  A force of 2,100 G was selected  as
an optimum.  At  the selected  bowl speed,  solids recover
and percent  solids of  the cake were analyzed  for differ-
ent sludge feed rates. The data indicated that although
the centrifuge could thicken  the sludge to  the  required
percent,  the  percent solids could drop from 5  percent
down to 2 percent or increase up to 15 percent, with
only minor feed rate changes. Consistently obtaining  the
required  5 percent solids concentration was difficult.
Based on  the pilot test  data, solid  bowl centrifuge thick
ening of the  waste activated  sludge was not consistent.
     90

-------
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FLOAT CONCENTRATION
                                       EFFLUENT SUSPENDED SOLIDS
                                                                                                         900
                                                                                                         800
                                                                                                          700
                                                                             600
                                                                             500
CO
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111
Q
                                                                             400   I
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                                                                                  LU
                                                                             300   3
                                                                                                         200
                                                                                                         100
     0     .02     .04     .06     .08     .10     .12     .14     .16     .18

                                              AIR SOLIDS RATIO


Figure 5-13.—Float  concentration and  effluent suspended solids versus
air-solids ratio—without polymers.
                                                                          .20
                                                                                 .22
                                                                                        .24
                                                                  .26     .28
  For the example plants, however, it will be assumed
that centrifugation is applicable to the waste activated
sludge.  Also, based on available equipment reliability,
plant operator  preference, desired performance, minimum
supportive equipment requirements,  and past experience,
the solid  bowl  continuous centrifuge is selected over the
basket and disc centrifuge for the examples. Available
data from equipment manufacturers and data in table 5-
9 indicate that a product sludge of 6 percent solids may
be  reasonably  expected.

Other  Methods

  Decanting  may result in some  thickening in the diges-
ters. It is not,  however, a reliable, consistent method
and does not normally result  in appreciable thickening.
Thus, it will not be considered as one of the process
alternatives for the example plants.
  New methods, such as the  sludge filter bag system,
have not  been in  existence long enough to have been
proven reliable. Thus, they will not be considered as
thickening process alternatives for the example plants.
                             Alternative Evaluation

                             Preliminary Screening

                               The preliminary screening of sludge thickening alterna-
                             tives for the example plants was performed  in the previ-
                             ous section. The remaining alternatives at this point  are
                             presented in table 5-13.
                               The general approach to use, at this point,  is to first
                             determine if any of the remaining  alternatives can  be
                             eliminated without performing  a detailed cost-effective-
                             ness analysis.  A detailed cost-effectiveness analysis ex-
                             amining capital and operation and  maintenance costs
                             would then be performed on the remaining alternatives.
                             Capital costs to be considered may normally include
                             thickener and  supportive equipment costs, land costs,
                             building or protective structure costs,  and, in certain
                             cases,  post thickening treatment unit costs.  Other  costs
                             to be considered include power costs, chemical costs,
                             manpower  costs, and maintenance costs. The cost-effec-
                             tiveness analysis will show which alternative  has the low-
                             est annual  equivalent cost.
                                                                                                          91

-------
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800
700



600



500


400

300




200

100
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.02      .04     .06     .08     .10      .12     .14     .16

                                      AIR SOLIDS RATIO
                                                                         .18
                                                                                 .20
                                                                                         .22
                                                                                                .24
                                                                                                        .26     .28
Figure  5-14.—Float concentration  and effluent suspended solids versus
air-solids ratio—with polymers.
Secondary Screening Analysis

  Since alternative numbers 1 and 2  both utilize gravity
thickening only, elimination of one of them should be
relatively simple. Wastewater characteristics and  settling
tank performance data presented previously will  be  used
in determining  loadings on the required thickeners.  For
the examples,  differences  in density  of  the sludges  are
assumed insignificant and  the density is taken  as equal
to water. Thickener  designs will  be based on loading
rates proposed in tables 5-5, 5-7, and 5-9. Designs will
be  conservative to assure the desired performance. A
total of two thickeners will be provided with each alter-
native to assure that some thickening will be obtained  if
one unit fails.  Calculations required for the  4.0  million
gallons  per  day wastewater plant gravity  thickener de-
signs follow:

Alternative  No. 1
  Definition—Gravity thicken primary sludge;  gravity thicken waste acti-
vated sludge.

Primary sludge
Quantity:  4x240x8.34x0.65 = 5,204 Ibs/day (2360 kg/day)
Volume.  5,2047(0.04X8.34) = 15,600 gals/day (59,050  I/day)
Required thickener: 5,204/20 Ib/ft2/day = 260 ft2 (24.2 m2) or an 18.2
  ft (5.55 m)  dia. unit
                                                  Recommended thickener: one 20 ft (6.10 m) dia., 10 ft (3.05 m) deep
                                                    unit
                                                  Thickened  product: 5,2047(0.09x8.34) = 6,933 gals/day (26,240 I/day
                                                  Thickener cost: $64,000

                                                  Waste  activated sludge
                                                  Nonbiological- 4X240X8.34X0.25 = 2,002 Ibs/day (908 kg/day)
                                                  Biological:  4x200x0.60x8.34x0.5 = 2,002 Ibs/day (908  kg/day)
                                                  Total quantity: 4,004 Ibs/day (1816 kg/day)
                                                  Volume:  4,004/(0.01 x 8.34) = 48,010 gals/day  (181,740 I/day)
                                                  Required thickener: 4,004/4  Ibs/ft2/day = 1,001 ft2 (93.0 m2) or a 35.'
                                                    (10.88 m)  dia. unit
                                                  Recommended thickener: one 35 ft (10.67 m)  dia., 10 ft (3.05 m) de«
                                                    unit
                                                  Thickened  product: 4,004/(0.028 X 8.34) = 17,146 gals/day (64,905
                                                    I/day)
                                                  Thickener cost. $98,000

                                                  Combined  product

                                                  [(6,933X9)+ 17,146 (2.8)]/(6,933 + 17,146) = 4.59
                                                  24,079 gals/day (91,950 I/day) of 4.59 percent sludge

                                                  Alternative No. 2
                                                     Definition—Gravity thicken combined sludge.

                                                  Combined  sludge
                                                  Nonbiological: 4x240x8.34x0.9 = 7,206 Ibs/day (3269 kg/day)
                                                  Biological: 2,002 Ibs/day (908 kg/day)
                                                  Total quantity: 9,208 Ibs/day (4177 kg/day)
                                                  Volume: 9,208/(0.03x8.34) = 36,803 gals/day (139,315 I/day)
     92

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Table 5-12.—Pilot centrifuge results

Run
No.
1
2
3
4
5
6 ...
7 .. .
8 ....
9 ...
10 ....
11 ....
12 ....
13 ....
14 ....
15 ....
16
17
18
19
20 . . . .
21 ....
22 . . . .
23 ....
24 . . . .
25 . . . .
26 ....
27 ....
28 . . . .
Feed
Rate
gpm
136
108
168
177
183
. 25.2
. 25.7
. 21.6
. 23.0
. 35.4
. 23.6
. 12.2
. 10.7
. 11.1
. 22.2
27.3
27.3
28.3
44.6
. 59.0
—
. 23.0
. 25.4
. 44.0
. 44.5
. 40.4
—
. . 63.2
sludge
Concen-
tration
percent
SS
0.799
.859
.817
.925
.918
.833
.845
.809
.813
.809
.782
.790
.699
.757
.757
.779
.737
.793
.777
.760
.786
.760
.750
.751
.745
.701
.487
.725
Centrate
Rate
gpm
12.5
6.8
15.8
16.2
10.0
24.0
240
13.0
17.2
22.2
22.2
10.0
10.0
9.7
20.0
26.1
26.1
26.1
42.8
42.8
23.1
17.6
24.0
42.8
42.8
27.3
42.8
42.8
Concen-
tration
percent
SS
0.027
.018
.077
.034
.020
.230
.072
.024
.027
.039
.136
.018
.015
.014
.026
.191
.152
.039
.236
.034
.032
.023
.078
.349
.165
.030
.040
.061
Cake
Concen-
tration
percent
SS
9.7
2.3
11.5
10.7
2.0
12.7
10.9
2.0
3.1
2.1
11.5
4.3
10.5
5.8
7.5
13.6
13.3
10.2
15.1
2.8
—
3.2
12.8
14.7
12.1
2.1
1.2
2.1
Mechanical conditions
Percent Bowl
solids speed,
recovered rpm
97
99
91
96
99
73
92
98
96
97
86
98
98
98
97
63
79
95
70
97
—
98
90
55
78
97
98
94
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3.250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
Bowl-
Pond conveyor
setting differential
(rpm)
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
4.0
9.2
3.1
4.2
73
3.3
4.8
6.3
2.8
2.8
4.1
59
4.1
5.4
5.4
2.7
4.0
5.4
4.4
6.0
5.6
8.0
4.0
4.0
5.6
7.1
7.1
6.1
Table 5-13.—Example  sludge thickening alternatives
        Alternative
                                Sludge thickening method
                           Primary  Waste activated  Combined
Number 1 . . . .
Number 2 	
Number 3
Number 4
Number 5
Number 6

	 Gravity

None
Gravity
None
Gravity

Gravity

DAF
DAF
Centrifuge
Centrifuge


Gravity





Required thickener: 9,208/8 Ib/ft2/day = 1,151 ft2 (106.9 m2) or two
  27.1 ft (8.26 m) dia. units
Recommended thickener: two  30 ft (9.14 m) dia., 10 ft (3.05 m) deep
  units
Thickened product: 9,208/(0.05x 8.34) = 22,082 gals/day (83,590 I/day)
Thickener cost: $160,000
  The analysis has shown that capital costs for alterna-
tive No.  2 are slightly less  than those for alternative No
1 ($160,000 versus $162,000).  Additionally, a thicker
sludge would be  obtained with alternative No. 2  (5  per-
cent versus  4.6 percent). This  would result in additional
cost savings in the digestion facilities. A similar analysis
for  the 40 million gallons (1.75 m3/s) per day plant
resulted  in a $167,000  unit (60-foot (18.29 m) diameter)
for  the primary sludge,  and a $489,000  unit (110-foot
(33.53 m) diameter) for the waste activated sludge  (tota
cost $656,000), or  two  $305,000 units (85-foot (25.91 m
diameter) for the  combined  sludge (total cost $610,000)
Thus, on the basis of capital costs, alternative No.  1  is
deleted from further consideration.
  Alternative No.  6 appears to be a viable solution for
our example plant.  However, an initial check of the
thickened sludge  product should be made since  a
sludge that is  too concentrated can actually cause  mor<
problems in  the anaerobic  digestion facilities than a
sludge which is too thin.
                                                   93

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Alternative  No.  6
  Definition—gravity thicken primary sludge; thicken waste activated
sludge  by centrifugation. Three shifts (24 hours) 7 day  per week oper-
ation of gravity thickeners at both plants and of centrifuges at 40
Mgal/d (1.75 m3/s) plant;  two shifts (15 hours) 5 day per week opera-
tion of  centrifuges at 4 Mgal/d (.18 m3/s)  plant.

Primary sludge

                     4 Mgal/d (.18  m3/s)   40 Mgal/d (1.75 m3/s)

Quantity (Ibs/day)    5,204 (2360 kg/day)    52,040 (23,605 kg/day)
Volume (gals/day)    15,600 (59,050  I/day)   156,000 (590,500 I/day)

Recommended  thickener
     4  Mgal/d—one 20 ft (6.10 m) dia., 10 ft (3.05 m) deep  unit
     40 Mgal/d—one 60 ft (18.29 m)  dia., 12 ft (3.66 m) deep unit
Thickened  product
     4  Mgal/d—6,933 gals/day (26,240 I/day)  of 9 percent sludge
     40 Mgal/d—69,330 gals/day (262,400 I/day) of 9 percent sludge
Thickener cost
     4  Mgal/d—$64,000
     40 Mgal/d—$167,000

Waste activated sludge

                    4 Mgal/d (.18 m3/s)
40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day)   4,004 (1816 kg/day)    40,040 (18,161 kg/day)
Volume (gals/day)   48,010 (181,740 I/day)  480,100 (1,817,400 I/day)

Recommended thickener
     4 Mgal/d—one 75 gpm  unit (4.73 l/s)
     40 Mgal/d—one 667 gpm unit (42.08 l/s)
Thickened product (daily average based on 7 day week)
     4 Mgal/d—8,002 gals/day (30,290 I/day) of 6 percent sludge
     40 Mgal/d—80,020 gals/day (302,900 I/day) of 6 percent sludge
Thickener  cost (based on one unit)
     4 Mgal/d—$89,000
     40 Mgal/d—$280,000

Combined product
4 Mgal/d—[(6,933 X9) + (8,002 X 6)]/(6,933 + 8,002) = 7.39
14,935 gals/day (56,535  I/day) of 7.39 percent sludge
40 Mgal/d—149,350 gals/day (565,360 I/day) of 7.39 percent sludge

   The calculations show that a 7.4 percent solids  sludge
would result. This exceeds the  4 to 6 percent  solids
recommended for  efficient digester operation. Thus,  alter-
native No.  6 is  eliminated  from further  consideration.
Detailed cost  analyses are required  for  screening the
remaining alternatives.

Cost-Effectiveness  Analysis
   Design of the thickener units (based on data previous-
ly presented in  this  paper) and capital costs for those
units will be presented  first for the remaining alterna-
tives. Other costs will then be analyzed.

Alternative  No. 3
   Definition—thicken waste activated  sludge with  dissolved air flota-
tion;  no thickening  of primary sludge;  two shifts (15 hours) 5 days per
week operation of  DAF units at 4 Mgal/d (.18 rrvVs) plant; three shifts
(24 hours) 7 days  per week  operation of units at 40  Mgal/d (1.75
m3/s) plant.
Waste activated sludge
                     4 Mgal/d (.18 m3/s)
Required DAF equipment
     4 Mgal/d—<4,004X7)/(15X5X2.0 Ib/ft2/hr) = 187 ft2 (17.4 m2)
     40 Mgal/d—40,040/(24x 2.0 Ib/ft2/hr) = 834 ft2 (77.5 m2)
Recommended  DAF equipment
     4 Mgal/d: two 100 ft2 units (9.3 m2)
     40 Mgal/d: two 400 ft2 units (37.2 m2)
Thickened product (daily average based on 7-day week)
     4 Mgal/d—(4,004/0.04x8.34) = 12,002 gals/day (45,430 I/day)
     40 Mgal/d—120,020 gals/day (454,330  I/day)
Thickener cost
     4 Mgal/d—$82,000
     40 Mgal/d—$205,000

Combined product (unthickened primary-t-thickened
WAS)

4 Mgal/d—15,600 + 12,002 = 27,602 gals/day (104,490 I/day) of 4 per-
  cent sludge
40 Mgal/d—276,020 gals/day (1,044,900 I/day) of 4 percent sludge

Alternative  No.  4
  Definition—gravity thicken primary  sludge; thicken waste activated
sludge with dissolved  air flotation. Three shifts (24 hours)  7 days per
week operation of gravity thickener at both plants and of  DAF unit at
40 Mgal/d  (1.75  rrrvs) plant;  two shifts (15 hours) 5 days per week
operation of DAF unit at 4 Mgal/d (.18 m3/s) plant.

Primary sludge
40 Mgal/d (1.75 m3/s)
 Quantity (Ibs/day)  4,004 (1816 kg/day)     40,040 (18,161  kg/day)
 Volume (gals/day)  48,010 (181,740 I/day)   480,100 (1,817,400 I/day)
                     4 Mgal/d (.18 m3/s)   40 Mgal/d (1.75 m3/s)

Quantity (Ibs/day)     5,204 (2360 kg/day)    52,040 (23,605 kg/day)
Volume (gals/day)     15,600 (59,050 I/day)   156,000  (590,500 I/day)

Recommended thickener
     4 Mgal/d—one 20 ft (6.10 m) dia., 10 ft (3.05 m)  deep unit
     40 Mgal/d—one 60 ft (18.29 m) dia., 12 ft (3.66 m) deep unit
Thickened product
     4 Mgal/d—6,933 gals/day (26,240 I/day) of 9 percent sludge
     40 Mgal/d—69,330 gals/day (262,400 I/day) of 9  percent sludge
Thickener  cost
     4 Mgal/d—$64,000
     40 Mgal/d—$167,000

Final  sludge

                    4 Mgal/d (.18  m3/s)     40 Mgal/d (1.75 m3/s)

Quantity (Ibs/day)   4,004 (1816  kg/day)    40,040 (18,161  kg/day)
Volume (gals/day)   48,010 (181,740 I/day)  480,100 (1,817,400 I/day)

Recommended thickener
     4 Mgal/d—one 200 ft2 unit  (18.6 m2)
     40 Mgal/d—one 800 ft2 unit (74.3 m2)
Thickened product (daily average based on 7-day week)
     4 Mgal/d—12,002 gals/day (45,430 I/day) of 4 percent sludge
     40 Mgal/d—120,020 gals/day (454,300 I/day) of 4 percent
       sludge
Thickener cost
     4 Mgal/d—$55,000
     40 Mgal/d—$91,000 (built-in-place unit, equipment only)

Combined  product

4 Mgal/d—[(6,933 x 9) + (12,002 x 4)]/(6,933 +12,002) = 5.83
18,935 gals/day (71,680 I/day) of 5.83 percent sludge
40 Mgal/d—189,350 gals/day  (716,800 I/day) of 5.83 percent sludge

Alternative No.  5
   Definition—thicken waste activated sludge by centrifugation; no
thickening of primary sludge. Two shifts (15 hours) 5 days per week
operation  of centrifuge units at 4 Mgal/d (.18 m3/s) plant; three shifts
(24  hours) 7  days  per week operation of units at 40  Mgal/d (1.75
m3/s) plant.
      94

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 Table  5-14.—Thickener product and  anaerobic digester requirements
                             Digester influent sludge
                                                       Digester volume (ft3)       Digester cost
       Alternative
Volume (gals/day)
                         Percent
                         solids
                                 4 Mgal/d  40 Mgal/d  4 Mgal/d  40 Mgal/d  4 Mgal/d  40 Mgal/d
Number 2
Number 3
Number 4 	
Number 5 	

50
40
	 5 83
4 68

22082
27602
18,935
23602

220 820
276 020
189,350
236 020

58034
71 938
49,683
61 661

580 340
71 9 380
496,830
616610

$789 000
877 000
742,000
806,000

$4 074 000
5310000
3,425,000
4,361 ,000

   Notes: If thickeners were not used, digester influent sludges would be as follows:
         Alternative No. 2—     4 Mgal/d, 36,803 gals/day of 3.0%
                            40 Mgal/d, 368,030 gals/day of 3.0%
         All other alternatives— 4 Mgal/d, 63,610 gals/day of 1.74%
                            40 Mgal/d, 636,100 gals/day of 1.74%
         Digester design is based on the thickened sludge, 85° F. temperature, 20 days detention, 75 percent
 sludge volatile content,  2,302  pounds of dry solids  per million gallons of wastewater and the volatile sludge
 loading factor method. Digester costs are based on two high rate units for each plant.
Final sludge
                  4 Mgal/d (.18 m3/s)   40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day)  4,004 (1816 kg/day)    40,040 (18,161 kg/day)
Volume (gals/day)  48,010 (181,740 I/day)  480,100 (1,817,400 I/day)
Recommended thickener
     4 Mgal/d—two 38 gpm units (2.40 l/s)
     40 Mgal/d—two 334 gpm units (21.07 l/s)
Thickened product (daily average based on 7 day week)
     4 Mgal/d—4,004/(0.06x 8.34) = 8,002 gals/day (30,291 I/day) of
      6 percent sludge
     40 Mgal/d—80,020 gals/day (302,910 I/day) of 6 percent sludge
Thickener cost
     4 Mgal/d—$116,000
     40 Mgal/d—$324,000
Combined product
4 Mgal/d—[(15,600 X 4) + (8,002 X 6)]/(15,600 + 8,002) = 4.68
23,602 gals/day (89.340 I/day) of 4.68 percent sludge
40 Mgal/d—236,020 gals/day (893,440 I/day) of 4.68 percent sludge
  The design calculations  for the  various  alternatives
indicate that  they will result  in different sludge moisture
contents  and sludge  volumes. These  data and the resul-
tant required anaerobic digester volumes  and costs are
summarized  in table  5-14. As shown by the  data,  con-
siderable digester cost savings  are possible with the
thicker  sludges.
  The example plants are  located  in  the  Midwest.  There-
fore, the  problem of  possible freezing temperatures
needs to be addressed.  Except for icing  of weirs and
possibly a thinner product  sludge,  exposed gravity thick-
ener operation  should  not  be seriously affected in  freez-
ing weather. Flotation and  centrifuge equipment,  how-
ever, should be  located in heated  enclosures to  prevent
freezing  of the exposed  piping and to  protect corrodible
components from the elements.  Besides housing  the
                           thickening equipment, the structure should  also provide
                           space for polymer feed equipment, and for polymer stor-
                           age if polymers are to be used. At the example plants,
                           assume  that existing building space  is  fully utilized and,
                           thus, any thickener building  would be new construction.
                           The required building space and  associated costs for
                           alternatives utilizing flotation or centrifugal  thickening  are
                           presented in  table 5-15.  Polymers are  required with al-
                           ternatives Nos. 4  and 5.  Storage space for a 30-day
                           supply has been included in the  required  building area.
                             All capital costs for the alternatives have been summa-
                           rized in  table  5-16.
                             Power requirements and associated costs vary  with
                           the type and size of thickeners utilized. Gravity thicken-
                           ing systems require power for the operation of  raw and
                           Table 5-15.—Required  thickener building  space
                                             Thickener description
Building description
Alternative

#3-4 Mgal/d.. .
#3-40 Mgal/d..
#4-4 Mgal/d....
#4-40 Mgal/d ..
#5-4 Mgal/d ..
#5-40 Mgal/d ..

Type
DAF
DAF
DAF
DAF
Centrifugal
Centrifugal

Unit
size
2-100 ft2
2-400 ft2
1-200 ft2
1-800 ft2
2-38 gpm
2-167 gpm

Area
(ft2)
1,520
2,750
1,150
2,050
770
1,000

Height
(ft)
12
14
14
10
10
10

Building
cost
$84,000
136,000
75,000
"181,000
49,000
58,000
'includes concrete tankage.
                                                                                                                  95

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Table  5-16.—Capital costs
         Alternative
         description
         (Mgal/d)
Thickeners
 Supportive
 equipment
    Building
                              Anaerobic
              Total
Number 2.
Number 3.
Number 4.
Number 5.
                                4 Mgal/d plant
 $160,000
   82,000
  119,000
  116,000
  $18,000
   28,000
   46,000
   28,000
    $84,000
     75,000
     49,000
   $789,000
    877,000
    742,000
    806,000
                               40 Mgal/d plant
     $967,000
    1,071,000
      982,000
      999,000
Number 2 	
Number 3 	
Number 4
Number 5

	 610,000
	 205 000
258 000
324 000

24,000
44,000
68000
44000

_ „ _
136,000
181 000
58000

4,074,000
5,310000
3 425 000
4 361 000

4,708,000
5,695,000
3,932 000
4 787 000

thickened sludge pumps and the sludge collector drive.
Dissolved air flotation  systems also require power for
raw  and thickened  sludge pumps, but additionally for a
recirculation pump,  reaeration pump (if present),  bottom
collector drive, skimmer drive, air compressor,  polymer
feed system (if present), and heating and lighting of the
thickener building. Centrifugal thickening  systems require
power for  the raw thickened sludge pumps, bowl drive,
conveyor drive (if present),  polymer feed system  (if pre-
sent), and heating and lighting of the thickener building.
Since the  required  anaerobic digester volume differs with
the four alternatives, the yearly  sludge heating require-
ments will  also vary. These  sludge heating costs need to
be included in  the thickener cost-effectiveness analysis
                                since they  are directly related  to  thickening  process.
                                Total operating  horsepower,  thickener  building heating
                                requirements,  and associated power costs for the vari-
                                ous alternatives,  excluding digester heating costs, are
                                presented in table 5-17. Building  lighting costs were
                                determined insignificant and  are not presented. Operating
                                horsepower figures include influent  and effluent sludge
                                pump motors  which total  as  follows: Alternate No. 2: 4
                                million  gallons per day (.18 rrrVs)—1 horsepower (.75
                                kW), 40 million gallons per day (1.75 m3/s}—5 horse-
                                power  (3.73 kW); Alternate No. 3: 4 million  gallons  per
                                day (.18  m3/s)—1-1/2 horsepower  (1.12  kW), 40 million
                                gallons per day (1.75 m3/s)—4-1/2  horsepower (3.36
                                kW); Alternate No. 4: 4 million gallons per day (.18
Table 5-17.—Thickening power requirements  and costs
           Alternative
           (Mgal/d)
                                   Power requirements
                                 Equipment     Heating
                                (operating hp)  (Btu/year)
                                     Yearly power costs
                              Equipment    Heating
                                            Total
Number 2, gravity thickener	
Number 3, DAF thickener	
Number 4, gravity thickener	
Number 4, DAF thickener	
Number 5, centrifugal thickener.
 Number 2, gravity thickener	
 Number 3, DAF thickener	
 Number 4, gravity thickener	
 Number 4, DAF thickener	
 Number 5, centrifugal thickener.
      5
     50
      2.5
     40
     42.5
     11
    140
      4
    110
    106
                                        4 Mgal/d plant
 42.5
1.85X108

1.63X108
8.60 X107
                                       40 Mgal/d plant
114
$1,306
 5,817
  653
 4,653
 4,944
   —      2,874
3.91 X108  36,581
   —      1,045
2.08 X108  28,743
1.12 X108  27,697
5,306
                         29,788
$765

 675
 355
                                 1,620
                                  855
$1,306
 6,582
  653
 5,328
 5,299
                        2,874
                       38,201
                        1,045
                       29,598
5,981
                      30,643
                                  465   28,162
     96

-------
m3/s)—2-1/2  horsepower (1.87 kW), 40 million gallons
per day (1.75 m3/s)—6-112 horsepower (4.85 kW); Alter-
nate No. 5: 4 million gallons per day  (.18 m3/s)—1-1/2
horsepower (1.12 kW),  40  million gallons per day (1.75
m3/s)—4-1/2  horsepower (3.36 kW). Power costs for
equipment  operation are based on a rate  of  $0.04  per
kilowatt-hour ($1.11/mJ). Power costs for  heating the
building are based  on using  fuel oil at a cost of $0.45
per gallon  ($.12/1).  The cost associated with heating the
sludge in the  anaerobic digesters and the total power
costs for each alternative are  presented in table 5-18.
In  developing  heating costs for the  digester,  it was as-
sumed that  auxiliary fuel (fuel  oil at a cost of $0.45 per
gallon ($.12/1)) would be required 50  percent of the
time.
  Polymers  are required for dissolved  air flotation thick-
ening and may be  required for centrifugal thickening of
the waste activated sludge. Polymer requirements quoted
by the various equipment manufacturers vary consider-
ably for the same type  process. Average polymer re-
quirements based on several submittals and data from
existing installations and the associated  costs are pre-
sented in table 5-19.
  Labor associated with operating and maintaining the
thickening equipment varies with the complexity of the
process.  The continuously  operating gravity thickener re-
quires a visual inspection only once a shift, whereas the
more complex dissolved air flotation and centrifuge sys-
tems should be checked every 2 or 3 hours. The in-
spections should  be visual checks on  the  product quality
and  also on the operating  conditions  of all system com-
ponents.  Additional  time for startup  and shutdown of
either the dissolved air  flotation or centrifuge systems
must be included if they are not operated on a continu-
ous 24-hour basis (Alternatives Nos. 3, 4,  and  5 for the
4 Mgal/d  (.18 m3/s) plant). Startup  and shutdown time
Table 5-18.—Digester heating costs and alternative total
power costs
Table 5-19.—Polymer requirements and costs
Alternative
(Mgal/d)
Number 2
Number 3
Number 4 	
Number 5 	
Number 2
Number 3
Number 4
Number 5 	

Digester heating
(Btu/year) (Cost/year)
4 Mgal/d plant
1 6831 X109 $6749
2 0820 X 1 09 8 61 5
1.4563X109 6026
1 7875 X109 7397
40 Mgal/d plant
1 5628 X1010 64 668
1 9415X1010 80338
1 3426 X1010 55556
1 6851 X1010 69729

Table 17
power
costs
(cost /year)
$1 306
6582
5,981
5,299
2874
38201
30643
28,162

Total
yearly
power
costs
$8055
15197
12,007
12,696
67542
118539
86199
97,891

Polymer Polymer cost
renuireri
Alternative
Number 3 and 4- 4 Mgal/d DAF) ..
Number 3 and 4—40 Mgal/d DAF)
Number 5- 4 Mgal/d centrifugal)
Number 5—40 Mgal/d centrifugal) . .

(Ib/ton
of dry Unit
solids) ($/lb)
35 008
35 008
6 1 80
6 1 80

Yearly
total
$2,046
20,460
7,892
78.92C
probably amounts to  a total  of about 1  hour per day.
Routine sampling and testing of the thickener influent
sludge,  effluent sludge, and  supernatant is  required for
any type thickener. The tests involved are  essentially the
same regardless  of thickener type  or size.  Testing must
be done more frequently, however, on DAF and centri-
fuge systems than on gravity systems. Routine mainte-
nance includes such  things as lubricating equipment and
daily washdown  or cleanup operations. At  least once a
year,  all thickeners should be dewatered, throroughly
inspected, and repaired, as necessary. Painting  of cor-
rodible  components will  probably be  necessary at 5-year
intervals. Solid bowl centrifuge conveyors may  have  to
be resurfaced or replaced after 5,000-10,000 hours  use,
depending upon the amount  of grit in the sludge and
conveyor construction. A summary of the yearly opera-
tion and maintenance time and the associated  costs for
each  alternative are presented in  table 5-20.
  Maintenance materials costs were developed from  in-
formation provided by equipment manufacturers and  date
from existing installations. The materials costs shown in
table  5-20 are estimates and, hence, may  not  be indica-
tive of the  costs associated  with any one particular  man
ufacturer's  equipment. These costs may be described as
percentages of the thickener system  capital costs as
follows:  gravity thickening, 0.3 percent for  small installa-
tions  and 0.2  percent for larger installations; dissolved
air flotation, 1 percent for small installations and 0.9
percent for larger installations; centrifugation, 5.2 percen
for  small installations and 3.1 percent for larger installa-
tions.
  Power, chemicals, and operation and  maintenance
yearly costs have been  summarized in table  5-21. Since
the  power requirements  for the gravity thickening alter-
native (alternative 2)  are low and chemicals are not
required, it has the lowest yearly  operating cost of all
the  alternatives.  Although the centrifugation alternative
(alternative 5) has power costs similar to those of the
dissolved air flotation  alternative  (alternative 4),  the year
ly operating cost is considerably higher  due to  much
higher chemical  and  operation and maintenance costs.
  The alternatives' total  capital costs and total yearly
costs previously derived in tables 5-16  and 5-21, re-
spectively, are repeated in table 5-22. The data show
                                                                                                         97

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Table 5-20.—Operation  and maintenance time and  costs
Alternative
description
Operator's time
(hrs/year)a
($/year)6
Maintainer's time
(hrs/year)a
($/year)b
Material
cost
($/year)
Total cost
($/year)
Number 2, gravity ....
Number 3, DAF	
Number 4, DAF	
Number 4, gravity	
Number 5, centrifugal.
                                     4 Mgal/d plant
    483
   1,416
    868
    373
   1,659
    $2,415
     8,496
     5,208
     1,865
     9,954
    252
    586
    293
    126
    264
   $1,260
    3,516
    1,758
      630
    1,584
     ,$535
     1,100
       830
       245
     C6,000
$4,210
13,112
 7,796
 2,740
17,538
10,536
                                     40 Mgal/d plant
Number 2
Number 3
Number 4,
Number 4
Number 5,
qravitv
DAF
DAF 	
Qravitv
centrifugal . . .
483
2496
1,408
373
2,739
2415
14 976
8448
1 865
16,434
440
804
402
220
445
2200
4 324
2412
1 100
2,670
1 260
2240
1 215
380
C1 0,000
5875
22 040
12075
3 345
29,104


15420


   Time variances are due to equipment and operating time differences noted in the alternative defini-
tions.
  "Costs are based on $5/hr wage for gravity thickener operators/maintainers; $6/hr wage for DAF or
centrifuge operators/maintainers.
  cCosts are based on replacing conveyor after 7,500 operating hours.
Table 5-21.—Yearly operating  cost summary
                                        Table 5-22.—Cost summary and  rank
     Alternative                            Operation
     description        Power   Chemicals      and       Total
      (Mgal/d)                            maintenance
Number 2.
Number 3 .
Number 4..
Number 5.
 4 Mgal/d  plant
$8,055      —
15,197
12,007
12,696
$2,046
 2,046
 7,892
                       40 Mgal/d  plant
$4,210
13,122
10,536
17,538
$12,265
 30,355
 24,589
 38,126
Number 2 	
Number 3
Number 4 	
Number 5 	
67 542
1 1 8 539
86,199
	 97,891
	
20460
20,460
78,920
5875
22040
15,420
29,104
73,417
161 039
122,079
205,915
                                              Alternative
                                              description
                                                       Capital
                                                        costs
                                                               Yearly
                                                     Ranking   operating   Ranking
                                                                costs
                       4 Mgal/d plant
Number 2	   $967,000     1       $12,265      1
Numbers	   1,071,000     4       30,355      3
Number 4	    982,000     2       24,589      2
Numbers	    999,000     3       38,126      4

                       40 Mgal/d plant
Number 2
Number 3
Number 4 	
Number 5

4 708 000
5 695,000
	 3,932,000
4 787,000

2
4
1
3

73,417
161,039
122,079
205,915

1
3
2
4

that for the 4 million gallons  per day (.18 m3/s) plant,
the least expensive option in  terms of both  capital  and
operating  costs is  gravity thickening of the  combined
sludge followed by anaerobic digestion (alternative  2).
Note that  there is  only  3 percent  difference between the
capital cost of alternative 2 and the third most expen-
sive alternative (in  terms of capital costs—alternative 5).
  The results  for the 40 Mgal/d (1.75 mVs) plant are
somewhat different than those for the 4 Mgal/d (.18
m3/s)  plant. In this case, the  least costly  alternative in
                                        terms of capital costs does not correspond with the
                                        least costly one in terms of yearly operating costs.  Addi-
                                        tionally, for the 40 Mgal/d (1.75 m3/s) plant, the  least
                                        costly alternative (capital costs) is not alternative  No. 2
                                        (as was  the case  for the 4 Mgal/d (.18 m3/s)  plant)  but
                                        alternative No.  4.  Also,  in this case, there  is a  22 per-
                                        cent difference between the  capital cost of the least
                                        expensive  and  the third most expensive alternative.  Since
                                        the lowest capital  cost and lowest operating cost alter-
                                        natives do not  correspond, a present worth analysis
     98

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Table 5-23.—Present  worth analysis 40 Mgal/d alterna-
tives No. 2 and No. 4

Construction cost 	
Project cost8 . . 	
Project contingency 	 	
Total cost for facilities 	
Constant 1-20 O & M. costs 	
Salvage value . . . 	
Present worth
Initial project 	
PW. constant O. & M 	

Subtotal 	
P W salvage value

Total present worth 	
Average annual equivalent cost

Alternative
No. 2
	 $4,708,000
5,273,000
527,300
	 5,800,300
	 73,400
1,527,200
	 5,800,300
	 800,800

	 6,601,100
423,300

6,177,800
566500

Alternative
No. 4
$3,932,000
4,482,500
448,300
4,930,800
122,100
1 ,247,000
4,930,800
1,332,100

6,262,900
345,700

5,917,200
542,600

  a Includes costs associated with engineering, legal and administrative,
inspection, surveying, soil borings, start-up and generation and mainte-
nance manual, and interest  during construction.
would  be required to make the final selection. Although
the alternative capital cost rankings varied with plant
capacity,  the yearly  operating cost rankings did not.
Gravity thickening  of the combined  sludge followed by
anaerobic digestion had the  lowest  operating  costs;  cen-
trifugation of the waste activated  sludge or thickening  of
primary sludge, followed  by anaerobic digestion,  had the
highest. A present worth analysis is presented in  table
5-23. This analysis shows that alternative No.  4 (gravity
thickened primary sludge  and DAF thickened waste acti-
vated sludge) has  the  lowest average annual equivalent
cost for the  40 Mgal/d (1.75 m3/s) facility.

SUMMARY

  The purpose  of this  paper has  been to describe, in
detail, those thickening methods which are currently uti-
lized, and to  present the  general  approach  necessary in
evaluation of thickening alternatives by means  of  a de-
sign example. The methods presented can be  used to
analyze a thickening problem at any wastewater treat-
ment plant,  regardless of  its size  or  complexity. The
results of the design example are valid  for the assump-
tions made. Any change in problem  definition could
mean a different solution.
  Recommendation of  a particular process should be
geared  to available operation and  maintenance person-
nel.  Considerably more skill is required to operate and
maintain dissolved  air  flotation and centrifuge equipment
than gravity  thickeners. The final  recommended alterna-
tive  process will be one that is agreed  upon by the
owner, the engineer, and  the regulatory agency.

REFERENCES
1.  USEPA, "Process Design Manual for Upgrading Existing Wastewa-
   ter Treatment Plants," USEPA Technology Transfer EPA-625/1-74-
   004a, October 1974.
2.  USEPA, "Process Design Manual for Sludge Treatment and Dispos-
   al," USEPA Technology  Transfer, EPA-625/1-74-008, October 1974.
3.  Water Pollution  Control Federation, "Operation of Wastewater
   Treatment Plants,"—Manual  of Practice No. 11, WPCF, Washington
   D.C., 1976.
4.  Metcalf and Eddy, Inc.,  "Wastewater Engineering," McGraw-Hill,
   New York,  1972.
                                                                                                             99

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                            Chapter 6
                            Review of  Developments  in   Dewatering
                            Wastewater  Sludges
INTRODUCTION

This chapter  reviews the sludge dewatering operating
experiences which have occurred over the past 4 to 6
years and assesses the impact of these results on future
designs. Particular emphasis is placed  on  innovative  con-
cepts and equipment.
  It should be noted that practically all of the innovative
development  of new dewatering equipment in the last 4
to 6 years occurred first in Europe (particularly in the
Federal Republic  of Germany) and Japan, and has only
begun to  be  transferred and utilized in the United States
very recently. It will also become apparent that older,
previously dominant equipment and  concepts  have, in
many instances,  been replaced as a direct consequence
of operating  results.
  In attempting to incorporate the latest and  best equip-
ment and  concepts into current design, the U.S. design
engineer must be aware of plant operational  results  with
various alternate systems.
  Alternative  dewatering  equipment  and procedures can-
not be evaluated in isolation,  but only  as  part of an
overall system conceptual design. The  interrelationship
between sludge processing and liquid stream processing
should always be considered.  Previous works1'2 graphical-
ly illustrate the adverse effects of equipment  or systems
that provide  less  than  90 percent capture of  influent
solids, and thereby illustrating the profound effect of the
choice of a dewatering system on the  operability and
cost-effectiveness of the liquid processing system. Events
of the past 4 to 6 years have further verified this princi-
ple.
  There have been indications that the selection of the
type of  activated sludge system  could have  a strong
effect on the relative severity of associated  sludge proc-
essing problems.
  Table  6-1 lists the effect of various activated sludge
process modifications on yields of excess biomass and
on sludge processability.
  While  this table is a summary  estimate,  the trends and
principles involved are accurate. Given the above infor-
mation,  it is understandable why some states have
banned  the  "High Rate"  version of the  activated  sludge
process. Regardless of statutory positions, results at
plants incorporating "High Rate  Activated Sludge" are
sufficient to deter its use if the  resultant sludge is to be
disposed of in other than liquid  form.
  In  selecting a  dewatering system, an item of real con-
cern is  the choice of the final or ultimate disposal meth-
od for the sludge or its residue. Indeed, the available
options  for final  disposal should  be known prior to se-
lection of the dewatering system. Fortunately,  some of
the new dewatering equipment, by virtue of producing
higher solids content dewatered  cake and by  offering
the capability to eliminate inorganic conditioning solids in
dewatered cakes, provides  considerably more flexibility
than was previously available in  matching up a dewater-
ing process and an ultimate disposal process.
  Dewatering is  essentially  always preceded by thicken-
ing and conditioning, and frequently by  stabilization.  The
essential role of dewatering  is to transform  a  dilute wa-
ter slurry into a  damp, moist cake form for  either direct
final disposal or for drying  as a final product, or  for
reduction via an incineration or  other combustion  proc-
ess prior to final disposal.
  In evaluating  dewatering processes it  is essential to
Table 6-1.—Excess biomass  production and sludge processability from
various activated sludge processes
Process
variation
High rate 	
Conventional 	
Extended aeration 	
Pounds biochemical
oxygen demand
per 1,000 ft3
100-1 000
20-40
10-25
Food to
microorganism
ratio
0.4-1 .5
0.2-0.4
0.05-0.15
Pounds W.A.S.
pound biochemical
oxygen demand
removed (typical)
1.07
0.4
0.15
Sludge
processability
Poor
Good
Variable
                                                                                                      101

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Table 6-2.—Autothermic combustion4
                Parameter
                                      Case A  Case B
Gross calorific value	
Percent combustible matter in solids .
Percent solids for autothermicity	
                                       17,400
                                        60
                                        41.8
29,100
 75
 18.5
consider more than the direct operating costs, the  pro-
duction rate, and the dry solids content of the dewater-
ed cake. The evaluation should  include complete materi-
al  balances (Quantified  Flow Diagram 1,  2) around  the
dewatering process with a concurrent evaluation of the
effect of all recirculation streams on other preceding  unit
processes, and the effect of all  dewatered cake proper-
ties on  the  processes subsequent to dewatering, includ-
ing final disposal.
  To illustrate this point, note in table 6-2 that the
percent dry solids level at which autogenous incineration
occurs  is a function of  the  calorific value of dry solids
in  dewatered cake, which in turn varies with the chemi-
cal composition  of the solids. The requisite dry solids
level for self sustaining  combustion varies from 18.5 to
41.8 percent depending on  these factors  which are in
turn materially affected  by the unit processes to which
the sludge has been subjected prior to dewatering.
ANALYSIS OF RECENT PLANT  OPERAT-
ING RESULTS AND IMPLICATIONS  FOR
DESIGN

  What lessons should the past 5 years of  plant operat-
ing results bring to bear on current and future designs?
The following  five  points which bear consideration:
  1. The  effect of choice of type of biological process
     on sludge processing, and vice versa.
  2. The  effect of the inclusion of biomass  on the
     sludge processing system.
  3. The  effects of processing discontinuity on biomass
     or mixed sludge processability.
  4. The  importance of painstaking analysis of plant  re-
     sults.
     Relative  operability and maintainability  of various
5.
     sludge processing systems or units.
Type of Biological Process Chosen

  As  previously noted  in table 6-1, the selection of the
High  Rate activated sludge  process variation can  result
in a plant  having to process a mixed sludge with  65
percent or greater  biomass  content. Further, that  particu-
lar type of biomass is  normally much more  difficult to
process than other types. While imposition of other de-
sign constraints may have resulted in utilization of the
High Rate process  in certain cases, it is apparent that a
current  overall  system  evaluation of alternate conceptual
designs, particularly in the light  of  operating experiences,
would usually not support the use  of the  High  Rate
system.
  Results have also shown that  the extended aeration
process, unless kept within certain food to  microorga-
nism (F/M)  and solids retention  time (SRT) ranges can
cause sludge processing problems. These factors further
strengthen the  need for adequate testing  of sludges from
alternate biological  processes prior to selection of same.


Effects of  Inclusion of Biomass

  The results of the past  5 years are reflected  in the
following list:

  1. Gravity thickening of mixtures of primary and ex-
     cess biomass sludges is usually ineffective (unless
     flocculants are used).
  2. Recycling  of biomass to primary clarifiers  is nearly
     always  a self-defeating process which  causes more
     problems than  it cures.
  3. Inclusion of biomass  in a mixture with  primary
     sludge  causes  settling problems in conventional
     two-stage  anaerobic  digestion systems. This, plus
     the need to maximize gas production frequently
     makes  single stage complete mix anaerobic diges-
     tion the process of choice  for stabilization prior  to
     dewatering in plants  where sludge stabilization is
     required prior  to dewatering.
  4. Biomass causes poor settleability in elutriation
     tanks. These tanks can  be  modified to serve as
     post digestion  thickening tanks (with use of floccu-
     lants). This is  essential for  economic dewatering.
  5. Inclusion  of biomass  makes the careful selection  of
     dewatering systems,  including  pretreatment process-
     es  such as conditioning and thickening, essential to
     successful design.


Processing  Discontinuity and Sludge
Storage Effects

  The following list delineates the pitfalls  inherent in ex-
cessive  accumulation  of sludge within a plant brought on
by  discontinuity in removal by the dewatering process,
either by excessive unplanned down time,  or by design.

  1. Development of septicity.
  2. Destruction of some  of the  bioflocculation  of the
     biomass.
  3. Partial  solubilization through prolonged aqueous
     contact.
  4. Increased  hydration and more sensitivity to shear
     (pumping,  etc.).
  5. Deterioration of processability occasioned by all
     four of the preceding.
     102

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Methods  of Analyzing  Plant Operating
Results

  In  considering the significance of plant results and
relevance to design decisions, the following four con-
cepts bear consideration:

  1.  The use of single static numbers as bench marks
     for a dynamic, interrelated system can be seriously
     misleading.
  2.  Appreciation of the "Inertia"  inherent in moderate
     and large  plant processing  systems is necessary.
  3.  There is a paramount need to maintain  "Steady
     State"  conditions as much  as possible.
  4.  Recycle or sidestreams should be minimized within
     reasonable ranges.

  In  developing design criteria, it is frequently assumed
that  dewatering equipment can be sized using steady
state flow conditions for the overall system with some
allowance for peaking. These  assumptions are reason-
able  as long as reliable conditioning,  thickening, and
dewatering equipment are installed. However, if sludge
removal operations are  interrupted for lengthy periods or
fundamental changes  are  made,  then  the standard fac-
tors  may not be applicable in terms of order of magni-
tude. While  some properly aerated sludge storage ca-
pacity  is beneficial, storage usage should be minimized
and  septicity avoided whenever  dewatering is used.
  The  length of time required  to reestablish equilibrium
or steady state conditions in moderate or large  size
plants  with significant inventories of sludge is much
longer  than  would normally be anticipated. This  "Inertia
Factor" is calculable  through  the  use of  mathematical
models. From experience, in large plants, it can take
several months to fully evaluate  the effect of changes.
  The  need to  maintain a "Steady State" or equilibrium
removal rate of sludge sufficient to prevent overaccumu-
lation within a  plant is paramount. Once  an accumulation
problem  develops, rapid  resolution via accelerated re-
moval  rate procedures will prevent further difficulty.
  Particular attention  must be paid to processes which
inherently cause heavy  recycle loads.  Processes or
equipment which  cause heavy recycle loads can have a
negative effect  on sludge removal rate. If large  quantities
of sludge have accumulated in a plant either because of
heavy  recycle loads or  from a shut down period, normal
operating schedules will  require  alteration. In order to
clean out such an accumulation the "Sludge Removal
Rate"  during the transition "Clean Out" period prior to
reestablishment of a normal equilibrium must be much
greater than the normal  rate.  Unfortunately, if the over-
accumulation is due to  processes or  equipment  which
cause  a significant recirculation  load  of biomass, the
aeration  system of the plant will, during the "Clean  Out"
period  of higher than normal sludge removal rate, be
extremely overloaded and will also produce more excess
activated sludge than normal.  Another effect is that
sludge storage  renders  biomass more difficult to process
and  results  in a much greater amount of recirculation
than normally  would be predicted by "standard condi-
tion" testing figures and criteria.

Relative Operability and  Maintainability of
Various  Dewatering Systems and  Units
  The  reliability and maintenance characteristics associ-
ated with various types of conditioning-dewatering proc-
esses,  equipment,  and brands is  very  important to the
municipality and its personnel, and ultimately to the  pub-
lic who pays the  bill. In addition  to  the need  to keep
units operating to  prevent sludge accumulation and  its
attendant bad effects, maintenance costs are  a very
important factor in  overall system costs.
  The  only truly accurate source of reliability  and main-
tenance  cost data  is actual plant operational  results. To
justify  professional  process and equipment selection, the
design engineer should acquaint  himself thoroughly with
reliability and  maintenance parameters by visiting existing
installations and obtaining accurate  information from op-
erating personnel.  It is also necessary to sort out when
problems are due  to poor plant maintenance  practices
and when they are due to inherent  process or equip-
ment characteristics.  If performance data are  not avail-
able then they should be specified and a guarantee
provided by the supplier.
  The  current methodology of bidding and selection of
suppliers to equipment municipal  plants has been, in
some cases,  a cause of some of the  reliability and  main-
tenance  problems  now being experienced. The bidding
documents or plans and specifications should include
cost factors for maintenance and life cycle, and should
be sufficiently complete to ensure that truly equal equip-
ment specified is provided. If this is not done, and the
job is  awarded on  a strictly lowest price basis, inferior
processes and equipment can be selected.
CONDITIONING FOR DEWATERING

  The following list delineates the normal functions of
conditioning for dewatering:
  1.  Flocculation  of suspended solids (particularly fines).
  2.  Washing out the alkalinity of anaerobically digested
     sludge (the original purpose of elutriation).
  3.  Promotion of rapid formation of a  stable drainable
     cake.
  4.  Promotion of cake release from filtration support
     media.
  5.  Enhancement of cake fuel value.
  6.  Prevention of scale formation and  corrosion inhibi-
     tion.

  The methods used to accomplish  the above  functions
are as follows:

  1.  Chemical addition (inorganic).
  2.  Chemical addition (organic flocculants).
  3.  Elutriation (new function).
  4.  Heat treatment (conversion).
                                                                                                        103

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  5.  Ash addition (cake release).
  6.  Coal addition (fuel value).
  7.  Polyphosphonate addition (scale inhibition).

Chemical  Conditioning
  In  organic  chemical conditioning the most notable oc-
currence has been the increase in total availability of
metal salts, such as  ferric chloride, due to the entry into
the market of firms recovering the products from waste
acids.
  The dosage and costs for chemical  conditioning  vary
substantially for activated sludge plants depending  on
the type of biological system employed and the overall
sludge processing system. For this reason, unless other-
wise noted, all quotations of typical  dosage figures as-
sume that plants are well designed,  not involving proce-
dures or systems which are known to materially increase
conditioning demand and costs. An example of the latter
would be to  pipeline sludge for several miles  prior to
dewatering.
  In  the organic polyelectrolyte  flocculant area, there
have been several developments of consequence to de-
watering processes:
  1.  New high charge density, high  molecular weight
     materials in dry powder form, which  are  more effi-
     cient in  conditioning the  difficult sludges,  have be-
     come available  and are used widely.
  2.  A  new class of compound, the "Mannich" cationic
     products,  which have different performance charac-
     teristics, have been introduced, almost entirely as
     liquid products. These  materials produce a floe and
     drainage characteristic  more akin to that  produced
     by ferric chloride.
  3.  Emulsion form cationic products of  high  charge
     density  and molecular weight have been developed
     and are used.

Elutriation
  This process had  been  applied successfully to  digest-
ed primary sludge, was misapplied to  mixtures of  primary
and  biomass sludges, and then  adapted very successful-
ly as a flocculant  aided postdigestion  thickening  process
to facilitate cost-effective dewatering.1

Heat  Treatment
  This type process, sometimes called "Thermal Condi-
tioning," is covered  in detail in  Chapter  4.
  City utilities which have had dewatering experiences of
note, some written up in the literature and some not,
are:
     Kalamazoo, Mich.
     Colorado  Springs,  Colo.
     Chattanooga, Tenn.
     Chicago,  III.
     Columbus, Ohio
     Perth, Scotland
  •  Ft. Lauderdale, Fla.
  •  Port  Huron,  Mich.
  •  Flint, Mich.
  •  Lakeview,  Ontario
  •  Green Bay, Wis.

  In  Great Britain, where the most and earliest  installa-
tions of the Porteous and Farrer heat treatment process-
es were made,  the heat treatment process has  been
largely abandoned. In one case,  a new plant, never
used, has been offered for sale.
  British  water  authorities detected significant quantities
of refractory  organic material in the effluent  from  plants
dewatering heat treated sludges. The authorities conse-
quently banned recycle of cooking liquors into biological
treatment  systems which discharge into rivers subse-
quently used  as sources of  drinking water, since the
biological  systems are incapable  of removing the refrac-
tory  organic  material.
  An additional development in dewatering heat treated
sludges has  been the need  to chemically condition
sludges in a  number of cases,  either on a spasmodic or
regular basis. In the case of Port Huron, Mich.  (Farrer
System),  which employs centrifuges for dewatering,  rou-
tine use of flocculants at the rate of $8/ton ($8.82/Mg)
of sludge dewatered has been  found necessary. Other
heat treatment  plants have found  flocculants necessary
to promote cake  formation to obtain  reasonable solids
capture.
  To help alleviate scaling  problems,  Grand  Rapids has
found  it necessary to condition  heat  treated  sludge with
$3/ton ($3.31/Mg) of polyphosphonat.es.
  Various other chemicals have  been found  necessary to
raise the  pH  of sludges, to  condition boiler feed water,
and  to solvent  wash scale from heat exchangers.


DEWATERING EQUIPMENT TRENDS

  The  following is a list of the types of dewatering
equipment or processes normally  used in municipal
wastewater sludge processing:

  1.  Drying beds.
  2.  Rotary vacuum filters.
  3.  Horizontal solid bowl centrifuges.
  4.  Pressure filters.
  5.  Continuous belt filter presses.
  6.  Rotating cylindrical devices.
  7.  Imperforate  basket (batch)centrifuges.
  8.  Lagoons.

  Drying beds  are widely used at a large number  of
plants, particularly moderately sized plants in sunny
climes, but not restricted to same. As will be seen, they
have been the subject  of recent developmental improve-
ment activity, both with regard to improved  capacities
and  mechanical removal facilities.
  Whereas rotary vacuum filters  were once the common
of mechanical dewatering systems, their incidence of
selection  has rapidly decreased due  to energy  costs, the
     104

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problem of cake pick up with certain sludges, and lack
of ability to provide  as  dry  a dewatered cake as  various
other devices.
  Horizontal solid  bowl  centrifuges, particularly of the
new low speed type, are still popular where a very high
solids cake is not essential.  Their popularity has dwin-
dled to some  extent due to  energy considerations.
  Pressure filters of  the ordinary recessed chamber type
have been installed  in a few U.S. plants. Results  have
been mixed insofar as overall performance  is concerned,
despite the attainment of somewhat higher  total cake
solids levels (without necessarily improving  the ratio of
sewage solids to water) compared to  Rotary Vacuum
Filters  or  Solid Bowl Centrifuges.  Major problems are
cost, maintenance, and the frequent need to use  high
percentages of inorganic conditioners.
  The  new continuous belt  filter presses have become
the most  widely selected dewatering devices for munici-
pal sludge dewatering.  Their rapid growth in popularity  is
due to  ease of operation,  low energy  consumption, and
the ability (in  some  models) to  produce dewatered cakes
with solids contents much  greater than  obtainable with
Rotary  Vacuum  Filters,  Centrifuges, or  conventional Pres-
sure Filters.
  Rotating cylindrical devices,  such as the Pernutit DCG,
have been installed  in some plants. Their use has been
primarily at small  plants and as the first stage of a dual
system which  includes  an  inclined multiroll  press  (MRP)
for further cake dewatering.
  Imperforate  basket batch centrifuges have been in-
stalled  at a few small plants where a  low solids,  relative-
ly fluid cake is tolerable.
   Lagoon  drying is  now frequently applied.


DEWATERING METHODOLOGY

  Wastewater sludges all form cakes during the dewater-
ing process which are  compressible to some  degree  and
by virtue  of this fact and their  inherent water binding
nature  tend to require  application of conditioning proc-
esses  to  facilitate a reasonable dewatering  rate.
  The various sludges  may be  indexed  or characterized
by determination of  the "Specific Resistance to  Filtra-
tion."  They may also be characterized by being subject-
ed to  standardized  bench  scale dewatering test proce-
dures  (Filter leaf or Buchner funnel tests).
  An important  facet for design consideration is that
dewatering of wastewater  sludges is a "Cake Filtration"
process.  The  cake which forms during dewatering is the
primary filtration media and relative cake structure and
form throughout the dewatering process will largely  de-
termine the efficacy of the system.
   In assessing the cost-effectiveness of the pretreatment
methods  aimed  at improving dewatering it  is essential
that the effect of these processes on the type of cake
formed be considered. In  most municipal wastewater
treatment plants,  if  the following  steps are effected, a
mixed primary and biological sludge will result which is
amenable to  a cost-effective dewatering process yielding
a dewatered cake suitable for either reduction or direct
ultimate disposal  in an economic fashion:
  1.  Maximization of solids  capture  in well-designed pri-
     mary basins so as to provide as much  typically
     easy to process "Primary" sludge as possible. This
     precludes high  recycle loads of W.A.S.  or thickener
     overflows or heat treat cooking liquors  to the pri-
     mary basins.
  2.  Selection  of biological  process variation with rea-
     sonable assessment  of the amount and type of ex-
     cess biomass which  will be produced and will  have
     to be processed. This  usually precludes use of
     "High  Rate  Activated Sludge"  processes and some
     Extended Aeration designs.
  3.  Use of gravity sludge thickeners only for straight
     primary sludge, or if this is not possible, provision
     of flocculant dosage capability to ensure reasonable
     solids capture and underflow thickened sludge sol-
     ids contents when mixed primary-biological sludge
     is being  thickened.
  4.  Use of dissolved air flotation or  centrifugal thicken-
     ing for excess  activated  sludge prior to mixed
     sludge anaerobic digestion, or prior to  dewatering  if
     stabilization  is not to be included.
  5.  If anaerobic  digestion  of mixed sludge  is employed,
     use of a single stage  complete mix process and a
     post digestion thickening process, either gravity  set-
     tling  or dissolved air flotation (DAF).
  6.  Use of a conditioning  process which does not re-
     sult in creation of a heavy recycle load,  either in
     the form of  suspended or  dissolved solids or in  the
     form of BOD5 or  chemical  oxygen demand (COD) or
     refractory organics.  Likewise the conditioning pro-
     cess should  not destroy any significant  amount of
     the matrix forming material in the sludge solids
     which  will form the cake in the dewatering process,
     and should  not alter other  cake properties requisite
     to the succeeding processes.
  7. Selection and use of a dewatering device which is
     of rugged design, readily maintainable  and will pro-
     vide a minimum solids capture of 90 percent and a
     cake  solids  content  amenable  to succeeding proc-
     esses. It is, for all practical purposes,  always  nec-
     essary to condition  municipal sludges prior to  de-
     watering.
 DRYING  BEDS
   Sludge drying beds are frequently referred to as
 "Sand Beds."  In most cases except instances  wherein
 "paved drying  beds"  or wedge water screens  are used,
 sand is the primary drainage and cake  support medium.
 The recent  and continuing development of various types
 of Drying Beds prompts the use of that term, rather
 than Sand Bed.
   Drying  Beds are still the most common  method  of
 municipal wastewater sludge dewatering. The only reasor
 they are  not widespread  in use is that  they have  not
                                                                                                         105

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been the subject of  any significant degree of develop-
ment and improvement. This situation  is changing as
municipalities become  more cognizant of their viability
and relatively low cost of construction, operation, and
maintenance when properly designed.  An additional pre-
vious deterrent to their use has been  the frequent lack
of inclusion of  mechanical sludge removal capability  and
an understandable dislike by operating personnel, occa-
sioned  by a need for  manual removal.  This deterrent can
be and has been removed  in many cases by relatively
minor design modifications to facilitate mechanical re-
moval.
  An additional  previous deterrent  to selection of the
drying  bed alternative  is that the "Ten State  Standards"
do not reflect the application of conditioning  to  sludges
prior to dewatering.  The use of "Ten  State Standards"
criteria, which assume no sludge conditioning, can result
in excessive land requirements and the resultant acquisi-
tion costs  artificially inflate  cost estimates for the drying
bed alternative.
  A well-designed and properly operated drying  bed  can
produce  a drier sludge than any mechanical  device.
They are also less sensitive to the influent solids con-
centration.
  On the negative side,  drying beds are generally appli-
cable only to digested or stabilized solids. Though they
are particularly suitable for  small installations and the
"Sun Belt," drying beds are used  successfully in treat-
ment plants of  all sizes and in widely varying climates
(i.e., Chicago southwest treatment  plant, the
largest plant in  the world).
  Drying beds  may be roughly categorized  as follows:

  1.  Conventional rectangular beds with side walls, lay-
     ers of sand and  then  gravel with  under  drainage
     piping to carry away the  liquid. They are built ei-
     ther with or without provision for  mechanical remov-
     al and with or  without either  a roof or a green-
     house type covering.
  2.  Paved rectangular drying  beds with a  center sand
     drainage strip with or  without heating  pipes buried
     in the paved section and with or  without covering
     to prevent incursion of rain.
  3. "Wedge-Water" drying beds which include  a wedge
     wire septum incorporating provision for  an  initial
     flood with  a thin  layer of water,  followed by intro-
     duction of liquid  sludge on top of the water layer,
     controlled  formation of cake,  and provision for  me-
     chanical  cleaning.
  4. Rectangular vacuum assisted  sand beds with provi-
     sion for application of vacuum as  a motive force to
     assist gravity drainage.

Mechanism

  On drying beds,  the dewatering  initially proceeds  by
drainage and then continues by evaporation. The propor-
tion and absolute amount achieved by drainage will  vary
depending on whether or not  the  cake has been condi-
tioned, and its  overall drainage characteristics. An impor-
tant  consideration is the relative time period  required for
the cake to develop cracks which expose additional
sludge to evaporation  effects.  Since  one of  the main
functions of conditioning is to  flocculate  and immobilize
the smaller "fines" particles in the sludge cake it is
immediately apparent why  a conditioned  sludge slurry
dewaters in a fraction of the time required for an uncon-
ditioned sludge. The completion  of the drainage period
is  substantially delayed in  an unconditioned  sludge by
migration of the fines  to the sludge cake sand interface
resulting in some  plugging of the uppermost  layer of
sand. Maintenance of  porous,  relatively open structure
within the cake is also essential to evaporation rate.


Conventional  Rectangular Beds

  Drying bed  drainage media normally consists approxi-
mately as follows:
  1.  The top  layer is  6 to 9 inches (15.2 to  22.9 cm) of
     sand,  usually with an effective size  of 0.3 to 1.2
     mm and  a uniformity  coefficient less than 5.
  2.  About 8  to 18 inches (20.3 to 45.7 cm) of gravel
     with size gradation of 1 /8 to 1.0 inch (0.3 to 2.3
     cm). The top three inches (7.6  cm) of  the gravel
     layer are preferably 1/8 to 1/4 in.  (0.3 to .6 cm)
     size.
  3.  Underdrain piping with  a  minimum diameter of 4
     inches (10.2  cm) is often vitrified clay with open
     joints spaced 8 to 20 feet  (2.44 to 6.10 cm) apart.
     Recently, plastic  pipe is being used to prevent pos-
     sible cracking when front end loaders are run
     across the bed for sludge removal. If a gridwork of
     concrete runways is  provided for the front end
     loader, the selection  of pipe is not critical.

   Drying beds are frequently enclosed by glass.  The
glass enclosures can  materially improve  the performance
of the beds, particularly in cold or wet climates. Experi-
ence has shown that  in some cases only 67 percent of
the  area required for  an open bed is  required with en-
closed  beds.  The degree  to which, at specific locations,
the  space requirement could be reduced and  the sludge
loading increased by  use  of translucent roofing or total
glass enclosure is a function of site rainfall,  temperature,
and sunlight prevalence.
   Unfortunately,  mechanical removal methods have  not
normally, in the past,  been used with  glass enclosed
beds.  Obviously the adaptation  would not be either diffi-
cult or expensive.
   Table 6-3  describes the typical design criteria  for
open drying beds.
   The combination of the use of chemical conditioning
plus design to permit mechanical  sludge removal coupled
with the use  of either a translucent roof or  complete
glass enclosure with  ventilation  louvers dramatically low-
ers  the space requirement for conventional drying bed
use and should  be the first alternative considered for
dewatering in most plants.
   The sidestream from drying bed operation consists  of
     106

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Table 6-3.—Criteria for design of open conventional dry-
ing  beds
Type digested
sludge
Primary and humus
Primary and activated...
Primary and activated...

Pre-
treatment
None
None
Chemically
conditioned
Area
(sq.ft./cap.)
1.6
3.0
0.64

Sludge loading
dry solids
(Ib/sq. tt./yr.)
22
15
55

the drainage liquor which may be augmented  by rainfall
in  the case of open beds. The additional drainage water
is  not normally a  problem. The drainage water is usually
relatively innocuous and can be recyled  into the plant
with impunity.
  Drying times in  open beds also vary due to climate,
type of  sludge,  and whether or not it has  been condi-
tioned. In good weather, an average of 45 days is rea-
sonable for unconditioned sludge. This period can be
reduced  to 5-15 days or less via conditioning.
  A typical case study of the use of conventional drying
beds follows.
TAMPA,  FLA.—CURRENT  PLANT

  The current Tampa  plant is a primary treatment facility
featuring anaerobic digestion and sand drying beds for
sludge dewatering. The plant is designed for a flow of
36 Mgal/d  (1.58 m3/s) and is  normally treating 40
Mgal/d  (1.75 nrvVs). On occasion, alum  and  polyelectro-
lyte are used in  the liquid treatment phase to meet the
current  interim effluent standards.

Drying Bed Details and Operations—
Existing  Tampa Primary Plant

  Thirty-three beds, each 125  by 60 feet (38.10 by
18.29 m) are employed. The rectangular beds employ a
drainage medium of two  sizes  of graded sand above
two layers  of differently sized stone or gravel. The beds
are usually refurbished every 2 to 3 years, at most.
Current  anaerobically digested  primary sludge production
is estimated to be 56,000 gallons (211,980 I) of 3.0
percent  dry solids content per day. This is equivalent to
14,000 pounds/day (6350 kg/day)  of dry solids. With 33
beds of 7,500 square  feet (696.8 m2) area each, the
total available drying area is 247,500 square feet (22,993
m2).
  The 33 older drying beds at Tampa are not covered
so the drying cycle varies somewhat  due to  rainfall vari-
ation. Nonetheless, the operation has been so successful
that the new expanded advanced waste treatment (AWT)
plant which will be in  operation shortly is also equipped
with  drying beds for sludge dewatering.  Tampa has  for
about 3 years regularly used polyelectrolytes for condi-
tioning the sludge on  its way into the drying beds. Dry-
ing time to liftable cake conditions without conditioning
used to run 30 days minimum. With chemical condition-
ing, the drying time varies from 8 to 15 days depending
on rainfall pattern.
  Tampa features front end loader mechanical  removal
of dried sludge cake from the beds.  One man  can easily
empty one bed in 6-8 hours. Previous  removal methods
involved use of 5 men for 1-1/2-2 days to remove
sludge from one bed.
  Figure 6-1  is a photograph of the mechanized sludge
removal equipment used at Tampa on the  drying beds.
  Current  operating procedure involves pumping about
55,000 to  60,000  gallons (208,200 to 227,130 I) of di-
gested sludge onto  a  bed with in-line dosing of cationic
liquid  polyelectrolyte at a dosage  rate of about 50
pounds per ton (25 kg/Mg). The price  of the liquid
cationic polymer is $0.13 per pound  (0.29  per  kg) on an
as is,  liquid  basis making the conditioner cost  $6.50 per
ton ($7.17 per Mg)  of dry solids.
  Taking the estimated bed loading  volume of  56,000
gallons (211,980 I) of  3.0 percent sludge and an aver-
age drying time of 11.5 days, the solids loading rate on
the current Tampa beds is  60 pounds/square foot/year
(292.8 kg/m2/year). It should be noted that current
practice is to produce a very dry cake as shown  in
figure 6-2.


TAMPA,  FLA.—NEW ADVANCED WASTE
TREATMENT PLANT

  Tampa has installed and  is now starting  up a new
plant which features biological nitrification  and  denitrifica-
tion with  chemical addition  for phosphorous removal. The
Figure 6-1.—Mechanized sludge removal  at Tampa.
                                                                                                     107

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                                                           Table 6-4.—Tampa AWT plant, estimated annual aver-
                                                           age  unstabilized byproduct solids production
                                                                                                           Year
                                                                             Item
Figure 6-2.—Dried  cake appearance on  bed at Tampa
before removal.
new plant  is designed for a treatment capacity of 60
million gallons  per day (2.63 m3/s).
  Aerobic  digestion for advanced  waste treatment plant
(AWT) sludges and anaerobic digestion for primary
sludge plus 32 new sand  drying beds (each 100 by 140
feet 30.48  by 42.67 m) were included in  the  new facility.
  While the original concept of the new facility was to
aerobically digest the excess biological solids and to
dewater them separately  on the new drying beds, along
with the AWT chemical solids, considerable flexibility was
designed into the plant and the eventual  process config-
uration to  be utilized will  be selected  on  an empirical
basis. There is some apprehension regarding the energy
costs for aerobic digestion which  was designed into the
plant as an option prior to the surge  in energy  prices. If
aerobic digestion proves  too costly, anaerobic digestion
of mixed sludges will be  evaluated.

DESIGN EXAMPLE—DRYING BEDS—
6O MGAL/D  (2.63  m3/s)  PLANT

  The design of  the new  Tampa  AWT plants'  drying
beds serves  as an example of the design of  this type
system for a large plant  in a subtropical  climate.
  Estimates of quantities of unstabilized sludge solids  to
be  encountered  in the  new plant  are summarized in
table 6-4.
  Faced  with processing  the daily volumes of sludges
shown and considering the  acceptable results previously
achieved at Tampa with anaerobic digestion,  further cal-
culations of the  amounts of sludges which  would result
from anaerobic digestion of primary solids  and aerobic
digestion of AWT solids were carried  out and results are
listed in table 6-5.
                                                                                                      1976
                                                    1985
Primary solids slurry
    Ibs/day (dry)	   37,000
    percent solids	    5.0
    gals/day	   89,000
AWT solids slurry
  Biological solids
    Ibs/day (dry)	
  Chemical solids
    Ibs/day (dry)	   31,000
      Total	   75,000
    percent solids	    3.0
    gals/day	  300,000
Combined solids slurry
    Ibs/day (dry)	  112,000
    percent solids	    3.5
    gals/day	  389,000
                                                                                                               37,000
                                                                                                               5.0
                                                                                                               89,000
                                                                                                      44,000    71,000
         48,000
        119,000
          3.0
        476,000

        156,000
          3.3
        565,000
Table 6-5.—Tampa AWT plant, estimated annual aver-
age  stabilized byproduct solids production
                                                Year
                  Item
                                           1976
         1985
Primary solids slurry
    Ibs/day (dry)	    14,000
    percent solids	     3.0
    gals/day                                 56,000
AWT solids slurry
  Biological solids
    Ibs/day (dry)	
  Chemical solids
    Ibs/day (dry)	    31,000
      Total	    69,500
    percent solids	     5.0
    gals/day	   169,000
Combined solids slurry
    Ibs/day (dry)	    83,500
    percent solids	     4.4
    gals/day	   225,000
        14,000
         3.0
        56,000
38,500   57,500
        48,000
        105,500
         5.0
        253,000


        119,500
         4.6
        309,000
  A  series of sludge solids  stabilization, dewatering, and
disposal options were then  reviewed for reliability, envi-
ronmental impact, and capital plus operating and mainte-
nance costs. Table  6-6 summarizes these cost results.
  Based on the comparative costs shown and on other
     108

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Table 6-6.—Tampa AWT plant, alternative byproduct
solids systems total cost comparison
Estimated com-
parative costs —
$1 ,000,000
Rank Description

Capital Avera9,e
annual

Average
annual
cost per
ton raw
solids3
  1   Air dry—with chemicals—cake

2
3

4
5

6

7
8
9
10
to user 	
Air dry — cake to user 	
Air dry — with chemicals — cake
to landfill 	
Air dry — cake to landfill 	
Kiln dry — without anaerobic
digestion 	
Kiln dry — with anaerobic
digestion 	
Mechanical dewatering 	
Liquid spray 	
Liquid slurry to user 	
Incineration 	
$11.67
14.14

11.67
14.14

15.18

16.07
15.87
23.79
23.65
21.47
$2.75
2.84

3.31
3.40

b3.44

"3.50
3.84
4.32
4.38
4.71
$96.52
99.81

116.16
119.46

"120.68

b1 22.76
134.85
151.78
153.79
165.49
  aBased on 78 tons per day (dry) raw byproduct solids.
  bNet after revenue deduction from sale of product.
Table
beds
6-7.—Tampa AWT plant, design criteria—drying
                                        Design year—1985
             Air drying beds
                                        Annual
                                       average
                                          Maximum
                                           month
Volume each drying bed (gals at 12" fill
    depth)	    65,000
Area each drying bed (ft2)	    8,690
Number of drying beds	     140
Total area (ft2)	   1,216,600
Drying time (days)	
Solids loading (Ibs/ft2/yr).
Dried solids
  Ibs/day (dry)	
  percent solids	
  Ibs/day (wet)	
  tons/day (wet)	
  cu ft/day (wet)	
                                  29.5
                                  35.85

                                 119,500
                                  40.0
                                 298,800
                                  149
                                  3,900
 65,000
 8,690
  140
1,216,600
  19.6
 53.79

 179,300
  40.0
 448,300
  224
 5,800
evaluation factors,  the  alternate of air drying (drying
beds)  with use of flocculants was chosen as the  most
cost effective.
  The total estimated capital cost for the air drying sys-
         tem being installed at Tampa currently, including all pip-
         ing, auxiliaries such as equalizing  storage, site work,
         engineering,  underdrainage system, etc., was $4,671,000
         including  $941,000 contingency.
           The drying  bed operational design criteria are as
         shown in  table 6-7.

         PAVED  RECTANGULAR DRYING  BEDS
         WITH CENTER DRAINAGE

           A good  example of this type of system is that at
         Dunedin,  Fla. Figure 6-3 is  a photograph of the Dunedin
         beds.
           As can be  seen, the two  beds  in the left portion of
         the photo contain  previously loaded sludge which  is dry-
          ing. The two empty beds on the  right are ready to be
          loaded.
            The Dunedin plant is of interest due to  use of a
          unique heated drying bed system.
            Plant process features:
            1. An average flow of 2.5 Mgal/d (.11 m3/s) of pri-
               marily domestic wastes.
            2. Liquid treatment via primary  sedimentation followed
               by conventional activated sludge.  The plant original-
               ly used  a contact  stabilization system but was con-
               verted to conventional  activated sludge with positive
               results.
            3. Primary  sludge is subjected  to two stage anaerobic
               digestion with a Pearth gas  recirculation  system.
            4. The excess activated sludge is thickened in a DAF
               unit and most of  the thickened WAS then goes  into
               the  anaerobic digester system. Some of the WAS  is
               subjected to  aerobic digestion, but no  more than
               necessary due to the energy consumption of same.
                                                   Figure 6-3.—Paved rectangular  heated drying  beds,
                                                   Dunedin, Fla.
                                                                                                          109

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     (The operation of the DAF unit is well managed, as
     is the entire plant, and the plant  is a good refer-
     ence point  for the proper application of  DAF  thick-
     ening in a smaller plant.)
  5.  The digested sludges are processed in three  differ-
     ent  ways. A portion  is dried on the  heated drying
     beds prior to use as a soil additive. Some of the
     sludges are dewatered on an  existing rotary vacu-
     um  filter when this is required. An additional portion
     is disposed of in liquid form via tanker.
  6.  The digester gas is burned in a hot water heating
     system. The heated water is circulated through  pip-
     ing  in the  paved portion  of the drying beds.

  The Dunedin  plant  has  four drying beds (75x25 feet
each) (22.86 x 7.62 m) or 7500 ft2 (696.8  m2)  of evapora-
tive drying area. The drainage drying area, due to the
type of construction is only a fraction of  the  evaporative
area. The beds  are heated, as noted,  but are not  cov-
ered and the Tampa  Bay area has a high average annu-
al  rainfall. Polyelectrolytes are used to  condition the
sludge.
  Sludge drying time  (averages) to liftable condition  is 5
days normally and 12 days in rainy periods.  The beds
are charged with 5,000 gallons (13,930 I) of a 2.6 per-
cent dry  solids content sludge at a time.  Thus the load-
ing rate  varies  from 18 to 43 pounds  (6.35 to 19.50 kg)
of dry solids sludge per square foot (.09 m2) per year.
  With a 5-day drying period  the 4 beds are capable of
dewatering  about 13  dry  tons (11.8 Mg)  per month.
Certainly the capacity of  43 pounds per square foot per
annum (209.8 kg/m2/year) achieved at Dunedin is sever-
al  times  greater than the Ten States  Standards for con-
ventional open  beds.

WEDGEWATER  DRYING BEDS

  Wedgewater  "Filter Beds" or drying  beds  were de-
signed to introduce sludge slurry onto  a  horizontal rela-
tively open  drainage  media in a fashion which would
yield a clean filtrate and  also give  a reasonable drainage
rate.
  The Wedgewater Filter  Bed (figure 6—4) consists of a
shallow rectangular watertight basin fitted with a false
      CONTROLLED DIFFERENTIAL HEAD IN VENT
     . BY RESTRICTING RATE OF DRAINAGE
      -VENT
  WEDGEWATER SEPTUM-
                          OUTLET VALVE TO CONTROL
                                RATE OF DRAINAGE
 Figure  6-4.—Cross section  of a wedgewater drying bed.
floor of wedgewater panels. These panels have slotted
openings of 1/4  MM and produce a total open area  of
8 percent.  The boundary of this false floor is  made
watertight with caulking  where the panels abut the walls,
An  outlet valve is fitted  in one wall of the bed to com-
municate with the underside  of  the wedgewater decking.
  The  controlled  drainage rate is obtained by  first intro-
ducing a layer of water  into  the wedgewater unit to a
level above the septum.  The sludge is then  slowly intro-
duced  and in effect,  under the  proper conditions,  floats
on  the water  layer. After the  proper  amount of sludge
has been  introduced, the initial  separate water layer and
drainage water is allowed to  percolate away at a con-
trolled  rate. The  exact procedure varies somewhat with
different types of sludges. It is  apparent that for  this
concept to perform as intended  the sludge and the ini-
tial  water  layer must  be  relatively immiscible.
  The  wedgewater technique  is designed to permit con-
trolled  formation  of a cake at the crucial sludge/support
media  interface before any significant quantity of fines
migrates to the interface or into the  openings of  the
septum or  escapes in the filtrate. Since polyelectrolyte
flocculants promote rapid cake  formation and  bind up
fines they are now used in conjunction with Wedgewater
Filter  Bed  installations processing municipal  sludges.
  Each square foot (0.9 m) of wedgewater can normally
dewater between 1/2 Ib (.23  kg) and 1 Ib (.45 kg) of
dry matter per charge. The loading rate depends on  the
initial  solids concentration of  the waste sludge applied.
Most sludges can be dewatered to a handleable  condi-
tion of 8 to  12 percent  solids within 24 hours. This
process is most  practical for  the smaller treatment plant
which  has  an average daily flow of 500,000 gal/day  (.02
m3/s)  or less. Sludge loading rates of 182-365 Ib/ft2 per
year (882.2-1781.2 kg/m2/year) are  normal.
  Results with Wedgewater units at 2  U.S. plants are
described  in the  following paragraphs.

ROLLINSFORD,  N.H.
  This plant  produces an excess biological  sludge at the
rate of 150 gallons per  day  (567 I) at 2 percent  dry
solids  content. A wedgewater unit, as shown  in figure
6-5, is used  to dewater the sludge to a solids content
of  8 percent, which  is liftable.
  A polyelectrolyte conditioner  is used in the  process.
Calculations  from the data in the reference  cited show
that conservatively assuming  2  drying cycles per day for
the 15' by 6' (4.57x1.83 m) unit, the production  rate
could  be  1.1  Ib/hr/sq ft (5.4 kg/m2/hr), or  570 Ib/sq ft
year (2780 kg/m2/year) which is, of course, an order of
magnitude greater than  the dewatering rates normally
associated with conventional  drying beds. These  results
are tempered by the fact that 8 percent, while a liftable
condition  for this sludge, is not a particularly  high solids
content. It is apparent,  however, that higher than 8 per-
cent solids would be readily  obtainable with increased
drying times while still maintaining a very high annual
solids loading, if  such a higher  solids content were re-
quired.
     110

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                                               WEDGEWATER PANELS
                                               15'0" X MULTIPLES
                                               OF 3' 0"
                                                                                         x 3" X 3" WOODEN STRIPS

                                                                                                (4) SIDES
                                       PLAN VIEW OF UNIT
                                                                  Q
                                     SIDE ELEVATION


Figure  6-5.—Wedgewater drying bed, Rollinsford,  N.H.
                                                                                                 END ELEVATION
                                                                                          3" VENT PIPE
DUNEDIN,  FLA.

  Additional results on the wedgewater system are re-
ported from work at Dunedin, Fla. At that location, the
biological sludge was dewatered to a solids content of
10.4 percent in  22  hours through the mechanism of the
wedgewater element,  use of  support water, and the re-
stricted drainage procedure,  without the use of polymer
flocculants.
  There  are 18  U.S. installations of the wedgewater sys-
tem. Several are industrial applications  but  most are
installed  at small plants of the contact  stabilization type.
  A tillable  unit, more or less similar  to the lift and
dump mechanism of a dump truck, is available to facili-
tate removal of  sludge when slightly fluid cake can be
tolerated or when  removal by rake is feasible. The sup-
plier,  Hendricks  Manufacturing Co. of Carbondale,  Pa.,
also supplies design recommendations for mechanical
removal  via small front end loader when  indicated. A
1 square foot (0.09 m2)  bench scale test model is avail-
able for  test purposes.
  The  stainless steel wedgewire septum  in the 15- by
6-foot  (4.57 x 1.83 m) Rollinsford unit would  cost $4,500
at today's  prices.
VACUUM ASSISTED DRYING BEDS
  At the 4.5 Mgal/d (.20  nrrVs)  Sunrise City, Fla., con-
tact stabilization plant, a  vacuum assisted
drying  bed system has been  used for the past 18
months to dewater the 2  percent dry solids sludge.
  Principal components of the system are:

  1.  A rigid multimedia filter  top surface.
  2.  An intermediate void filled  with stabilized aggregate.
  3.  A low impermeable barrier, consisting of reinforced
     concrete. (It would alternatively be pre-fabricated
     fiberglass.)
  Figure 6-6 is a photograph of one of the two drying
bed units showing the sludge being fed onto the surface
of the  upper multimedia in one  of the beds.
                                                                                                        m

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Figure  6-6.—Rapid sludge  dewatering beds, Sunrise
City, Fla.
  The following sequence of operations  is used:

  1. Sludge  is fed  onto  the filter surface by gravity flow
    at a rate of 150 gallons (567 I) per minute to a
    depth of 12 to 18 inches (.30 to .46 m).
  2. Filtrate  is drained through the interconnected voids
    of the stabilized aggregate to a sump, from which
    it is pumped back to the plant by a self-actuated
    submersible pump.
  3. As  soon as the entire  surface of the rigid filter is
    covered with sludge, the vacuum system  is turned
    on  to maintain a vacuum of  one to  10 inches (2.54
    to 25.4 cm) of mercury on the intermediate void
    area.
 Figure 6-7.—Vacuum assisted  drying beds, Sunrise City,
 Fla.
  Under favorable weather conditions, this  system dewa-
ters the 2 percent solids aerobically digested contact
stabilization  sludge (a difficult  high  bound water content
sludge) to a 12  percent solids level in 24 hours without
polymer use, and to  the same level  in 8 hours if floccu-
lant is used. The 12  percent condition is liftable. The
sludge will further dewater to  about 20 percent  solids  in
48 hours.
  The sludge cake is removed from the filter surface
either manually,  mechanically by a small  hydrostatic  drive
front-end loader such as a Melroe  Bobcat  520,  or by  a
vacuum  truck.
  Controlled tests of this type system have shown that a
sludge loading rate of 306 pounds  per square foot year
(1490 kg/m2/year) is attainable.
  At Sunrise City plant (figure 6-7), the  two  20 feet by
40 feet (6.10x12.19  m) vacuum drying beds are proc-
essing a substantial  portion of the  total plant load. The
photograph  below shows the appearance of  a bed at
the end of the drying period and also shows the proxim-
ity to a local athletic field.
  The vacuum assisted drying bed  system  at Sunrise
City  is a proprietary  system now designed  and supplied
by International  Sludge Reduction  Co.

DESIGN EXAMPLE—DRYING BED FOR
4 MGAL/D (.18 m*/s)  PLANT

Basic Assumptions

  These  assumptions are  as follows:

  1. The sludge to be processed  is an anaerobically
     digested mixture of primary and WAS at 4 percent
     dry solids content. It  is a mixture of 60 percent
     primary sludge  and 40 percent WAS with the WAS
     originating  from  a conventional activated sludge
     system.
  2. Ultimate disposal is to be by  hauling  to a  sanitary
     landfill, or  to farmland or other horticultural use.
  3. Equilibrium sludge removal rate of 2.5 tons (2.3
     Mg)  of dry solids per day to  be maintained.
  4. The  plant is located in the Middle-Atlantic  section
     of the  United States.

Alternate Units for  Consideration or Eval-
uation
   For a  plant of this size, depending on site limitations,
either conventional enclosed drying beds or  vacuum as-
sisted enclosed drying beds should be considered. The
economics and  other constraints of final disposal, such
as length of truck haul and final solids content require-
ments would bear consideration. Land area  availability
would materially affect the choice  between gravity or
vacuum  assisted drying beds. If excess  methane was
available  from  anaerobic digestion,  consideration could
be given  to  use for heating the enclosed  bed air space
during the winter.
     112

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   For the purposes of this example it is assumed that
sufficient  land area is available for either gravity or vac-
uum assisted drying beds.

Evaluation  Procedure

   The  general sequential  procedure recommended to be
followed would be similar to that fully described on page
37 in the RVF  design example. The only variation would
be that bench scale and/or pilot plant tests on the
drying  bed dewatering characteristics of the sludge
would  probably have  to be planned and carried out
entirely by the  consulting  engineering firm and the  client
for the  conventional enclosed bed  option.  On the vacu-
um assisted bed  option the suppliers have developed
small scale testing procedures and could be involved in
the work.

Pilot  Scale Tests

   Since temperature conditions could affect the sizing of
enclosed  beds  it  is suggested that,  in the absence of
available data from existing plants  in the same  general
area with equivalent sludges, a small greenhouse type
test installation would  be  in order.  Ready-made unitized
small greenhouse enclosures intended for the homeowner
are now available at  modest prices and could  be adapt-
ed to enclose a small drying bed  for test work on both
options.


Design  Calculations

  It is assumed that the test work  has  shown that  by
enclosing  the beds and using  in line flocculant condition-
ing the  average bed loading for the conventional gravity
system is  55  Ib/ft2 per year (268 kg/m2/year) and for
the vacuum assisted option is 110  Ib/ft2 per year (537
kg/m2/year).

  1. Since drying bed operation is  a batchwise proce-
     dure  a sludge storage or surge vessel should  be
     provided to contain  the thickened  digested sludge
     and serve  as a feed  tank for the drying beds.
  2. Sludge volume rate would be  14,000 gallons/day
     (53,000 l/d)  or 98,000 gallons (371,000 I) per week,
     so a 100,000 gallon  (378,500  I) surge vessel would
     be required as a feed tank.
  3. Assuming tests showed a 12  inch  (0.30 m) bed fill
     level  to be practical,  for the conventional  gravity
     beds loaded at a conservative loading of  47 Ib/ft2
     per year (229 kg/m2/year), five beds, each 65 feet
     by 120 feet (19.81 x  36.58 m)  would be adequate.
  4. The use of five beds would permit the bed filling
     procedure to average less than two  per week on
     an annual  basis.
  5. For the vacuum  assisted  bed option using  a con-
     servative design  loading of 91  Ib/ft2 per year (444
     kg/m2/year)  would result in selection of four 50
     feet by 100 feet  (15.24 by 30.48  m) drying  beds.
Additional Considerations

  The system should include for  either  of the two op-
tions, mechanical sludge removal via a  front end loader.
  An important point in evaluating the two options would
be  a determination of the energy requirements involved
in operating the vacuum system  in that option.

FUTURE  OF DRYING BEDS

  An objective review of past results and consideration
of the developments of the  past  5 to 7 years in modify-
ing and increasing  the dewatering capacity and improv-
ing the mechanical removal  capabilities of drying beds
must lead to the conclusion that they should  be much
more widely used  than at present.
  It seems clear that a judicious combination of the
following aspects would in many locations make drying
beds the dewatering system of choice:

  1. Provision in the bed  design  for mechanical removal
     via front  end  loaders a la Tampa,  etc.
  2. Provision for conditioning of the sludge on its way
     into the bed with polyelectrolytes or  equivalent as
     needed.
  3. Inclusion  in the design  of a  translucent roof, or  a
     total greenhouse type enclosure with adequate ven-
     tilation  and odor control systems.
  4. Where  required for capacity purposes some form of
     vacuum assistance (a la Sunrise City, Fla.) for in-
     creasing  the drainage rate and enhancing evapora-
     tion where indicated.

  If these aspects were included In  conceptual designs,
the design criteria in  terms of square footage of bed
area required  would be many times  less than the figures
listed in the Ten State Standards. As a result of this an
overall system evaluation of  cost-effectiveness would
surely result in more widespread  use of drying beds than
is currently the case.

ROTARY  VACUUM  FILTERS

  There are three  normal  types of rotary  vacuum filters
and  they are described in table  6-8.
  The first (drum) type was  largely displaced  by the
latter two due to cloth plugging  problems associated
Table 6-8.—Types of rotary vacuum filters
  Type
  Support media
      Discharge mechanism
Drum.
Coil...
Belt...
Cloth
Stainless steel coils
Cloth
Blowback section/doctor blade
Coil layer separation/tines
Small diameter roll, flappers, doctor
  blades
                                                                                                       113

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with the use of lime and ferric chloride/lime conditioning
systems. The  drum  type filter  does not exhibit cloth
plugging problems with polyelectrolyte flocculants.
  The coil filter has been widely used and does  have a
positive release mechanism. Care  must be exercised with
coil  filters  to  ensure a sufficiently  rapid  rate of cake
formation to prevent loss of fines  through the more open
media involved during the initial phase of cake forma-
tion. This is a relatively infrequent problem and if the
fines problem does occur it is usually symptomatic of
predewatering processes which have destroyed a sub-
stantial portion of the matrix forming material in the
sludge(s) or of inadequate conditioning. Such pretreat-
ment processes will  be detrimental in some  manner to
any  dewatering device.
  Belt type filters were introduced to permit continuous
washing of the cloth and  ostensibly overcome effects of
plugging by lime or fines. This concept  was erroneous  in
most cases since the belt washes were not particularly
effective in removing lime.  In  several plants  which had
early installations of the Drum type  filter and later instal-
lations  of Belt filters side  by side, the purported  advan-
tages of the  Belt filters proved to be illusory.  Belt type
filters are  particularly prone to cake discharge  problems.
  Rotary vacuum filters produce typical results  when  in-
organic chemicals are  used for conditioning.  The  results
appear in  table 6-9.
  While the data  in this table above and the following
one are representative, they should  not be used  for
design  purposes if  the actual sludges to be dewatered
are  available  for lab and/or pilot  test work. It should
also be noted that  the cake solids figures shown in this
table include  the  significant amounts of ferric chloride
and lime used so the actual sewage solids content is
lower than  what is  shown. For instance, the correction
would typically bring the net sewage solids  of a  22
percent cake down to a correct figure of 18 percent.
  There are instances  where  a combination of  ferric
chloride and  polyelectrolyte is employed to maximize  ro-
Table 6-9.—Typical rotary vacuum filter results for
sludge conditioned with inorganic  chemicals
                             Table 6-10.—Typical rotary vacuum filter results for
                             polyelectrolyte conditioned  sludges
                             Chemical dose
                               (percent)
        Type sludge
                             Ferric
                            chloride
              Yield    Cake
             (Ib/hr/   solids
              ft2)    percent
       Lime
Raw primary	
Anaerobically digested primary .
Primary and humus	
Primary and air activated	
Primary and oxygen activated ..
Digested primary and air
  activated	
1-2
1-3
1-2
2-4
2-3

4-6
6-8    6-8
6-10   5-8
6-8    4-6
7-10   4-5
6-8    5-6
6-19
                                            4-5
25-38
25-32
20-30
16-25
20-28

14-22
                                       Type sludge
                                                      Chemical
                                                        cost
                                                       ($/ton)
                                                   Yield     Cal
                                                 (Ib/hr/ft2)   soli
                             Raw primary	
                             Anaerobically digested primary	
                             Primary and humus	
                             Primary and air activated	
                             Primary and oxygen activated	
                             Anaerobically digested primary and
                               air activated	
                                                       1.5-3
                                                         3-6
                                                         4-8
                                                         5-18
                                                         5-15

                                                         6-22
                                                    8-10
                                                    7-8
                                                    4-6
                                                    4-5
                                                    4-6


                                                   3.5-6
                                                    25-
                                                    25-
                                                    20-
                                                    16-
                                                    20-

                                                    14-
tary vacuum filter production rate. This is frequently the
case where the sludge has a high grease content and
tends to stick to the filter  cloth on belt type filters.
  Aluminum chloride or aluminum  chlorohydrate  are  alsi
effective inorganic  conditioning agents  and where plant
have existing  rotary vacuum  filters, the availability of
such materials as waste  byproducts of industrial plants
worth exploration.
  Typical results for polyelectrolyte conditioned  sludges
are described in table 6-10.
  In point  of fact,  more of the sludge  processed in
plants  equipped with rotary vacuum filters is conditione
with polymer flocculants than with inorganic conditioner
The chemical cost is normally about  the  same for the
use  of polyelectrolytes or inorganic conditioners. The u
of polyelectrolytes  largely prevails because of more  coi
venient handling, less extensive preparation  facilities, ai
freedom from corrosion  problems, plus the elimination c
significant  quantities of inorganic solids in the dewatere
cake.
  On the other hand, some plants must use inorganic
conditioners to  obtain cake release, provide matrix forr
ing material in the cake, or to facilitate lime addition fc
ultimate  disposal.
  With a digested  mixture  of primary and excess active
ed  sludge, in most  plants,  rotary vacuum  filters will  prc
duce dewatered cakes with cake  solids contents within
the 18-22  percent range, which is almost always too
wet for autogenous incineration or some composting
processes. These facts, plus energy costs have causec
the selection  rate for rotary  vacuum  filters to wane  coi
siderably.
  The  sludge feed to rotary  vacuum  filters should neve
be  below 3 percent dry solids content and  preferably
should be  greater  than 4 percent if reasonable  produo
tion rates  are to be attained.

AUXILIARY DEVICES FOR ROTARY
VACUUM FILTERS
  To obtain  higher  solids cakes from  rotary vacuum  fil-
ters (RVF), three companies have developed devices
     114

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which can further dewater the filter cake. These devices
are,  in some cases, specifically designed as add-ons to
existing filters or in others, supplied as integral parts of
the rotary vacuum  filter.
  The items of reference are:

  1.  The Eimco Hi-Solids filter.
  2.  The Parkson Magnum Press high pressure section.
  3.  The Komline Sanderson  Unimat high pressure sec-
     tion.


Eimco Hi-Solids  Filter

  This device  combines normal rotary vacuum filtration
with  a batch type adjunct pressure filter. The cake while
still on the rotary vacuum filter belt feeds into a small
co-joined stage where  it  is subjected on  one side to
pressure from a rubber diaphragm (50-150 Ib/in.2 g or
3.5-10.5 Kg/cm2) while on the other side (below the
belt) a vacuum is applied to facilitate drainage. Since
this is a batch procedure with the rotation of the rotary
vacuum filter being  momentarily interrupted while the
pressure and vacuum are applied in the  pressure cham-
ber section, some lowering of production occurs.
  Eimco supplies this unit as an integral  system and
also  supplies the press portion as an add-on device for
existing conventional rotary vacuum filters. This device
was  tested on pilot scale at Washington, D.C.,  and in-
creased the cake solids content from a normal 17 per-
cent up to a level  of  25 percent. The sludge tested was
a rather difficult to  process mixture of primary  and sec-
ondary sludges.


Parkson  Magnum Press

  This unit (more  fully  described in the section on Con-
tinuous  Horizontal Belt Filters) was evaluated on pilot
scale at Washington, D.C., for dewatering filter cake
from the existing rotary vacuum filters. Filter cake of 18
percent dry solids content was further dewatered to
35—40 percent dry solids with no further  conditioning
employed.
  Commercial  availability of this unit hinges on  success-
ful conclusion of development work required to enable
design of a mechanical method of transmitting  filter cake
from the rotary vacuum filter  to the auxiliary press sec-
tion without degrading  the processability  of the cake.


Komline  Sanderson Unimat

  A pilot model of the medium and high  pressure sec-
tions  of the Unimat was  evaluated at Washington,  D.C.,
on the cake from the  rotary vacuum filters and produced
a cake  of 38  percent  dry solids.  Once again mechanical
development work  would be required to  facilitate an
installation.
  In  summation, the three devices briefly described
above offer real promise for providing a  means to fur-
ther  dewater sludge cake from existing rotary vacuum
filter installations where such a procedure is  in order.
DESIGN  EXAMPLE—ROTARY  VACUUM
FILTRATION 4  MGAL/D (0.18 mVs)
PLANT

Basic System Assumptions:

  The sludge is an  anaerobically digested mixture of
primary and excess activated sludge which has been
thickened to 4  percent solids via  a flocculant aided
post-digestion thickening process. System design has
been such  that the  sludge  mixture is about 60  percent
primary and 40 percent secondary sludge. The  sludge is
available for testing.
  The ultimate  disposal  method for the sludge is to be
by hauling  dewatered cake to either a sanitary  landfill,
or for disposal  on farmland, or for composting  and  horti-
cultural  use.
  The sludge removal rate  required is to average 2.5 dry
tons (2.3 Mg)  per day and the cake must possess suffi-
cient dimensional stability to preclude flow out  of a
truck.

Alternate Units for Consideration and/or
Evaluation

  1.  A Coil filter.
  2.  A Belt type  filter.
  3.  A Drum type filter.

Evaluation   Procedure

  The sequence to  be followed in the  evaluation and
design is planned as follows:

  1.  Verification of  the amounts and relative degree of
     uniformity  of the flow  of sludge to be dewatered.
     This is to  be obtained by review of plant  operating
     data.
  2.  Diagnostic bench scale dewatering tests of the
     sludge, repeated several  times during different oper-
     ational periods to assure uniformity. It is absolutely
     essential that these tests and any  pilot tests be
     done  on site with fresh sludge.
  3.  Review of the  above  results with  interested candi-
     date suppliers  and then repetition of the bench
     scale  tests in conjunction with suppliers personnel.
  4.  A pilot dewatering test series should then  ensue,
     particularly if there  is  any doubt about any facet of
     the dewatering operation.  This should be carried
     out with at  least two  of the potential suppliers.
  5.  Summation of  design  data should be prepared by
     the consulting  engineer. Each potential supplier
     should be asked to prepare and transmit a report
     of the bench and pilot test work  including their
     design recommendations, including equipment re-
     quired, sizing,  delivery time,  etc.,  together with
     "budget price  quotes" and estimates of annual op-
     eration maintenance costs, and life cycles of the
     various items of equipment.
  6. A detailed design should then be prepared and
     plans, specifications, conditions of contract,  etc.,
                                                                                                       115

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     forwarded to  those suppliers whose equipment and
     performance have  qualified them to enter a firm
     price quotation.
  7.  From the design and overall  system cost data  avail-
     able, and with full  consideration of relative equip-
     ment reliabilities, a selection  of the supplier can
     then be made.

Bench Scale Tests

  The "Buechner Funnel" test procedure is well docu-
mented  and all suppliers of rotary vacuum filters are
very familiar with it.  The "Filter Leaf" test procedure is
likewise readily available.
  Normally the Buechner Funnel test, employing a cake
support media identical  to  that to be employed will  sup-
ply  all the required information needed. However,  if the
dewatered cake shows  real signs  of sticking to the filter
media, then a leaf test  to check this property  may  be  in
order.
  In the Buechner Funnel test it  is important to:

  1.  Determine dewatering rate, time to vacuum break
     and resultant cake solids after  a simulated cycle.
  2.  Analyze the filtrate for suspended  solids, BOD5,
     COD, and total dissolved solids.
  3.  The data from (2), along with analogous sludge
     feed data should be used to determine exactly
     what total solids capture is  being  obtained.
  4.  The cake release characteristics should be carefully
     assessed. If a problem is indicated, a  leaf test can
     be run to observe  whether or not  the  cake falls
     freely from a vertically held  leaf. If it doesn't, then
     a Belt filter will cause  release problems.

Pilot Tests

  Most  suppliers have packaged pilot units  which can
be wheeled  in for testing. This is  advisable,  in most
cases.
  It  is important that the sludge quality during the com-
parative pilot plant tests be reasonably comparable. This
can  be  verified by concurrent "Buechner Funnel" test-
ing.

Design Calculations

  1.  Operating cycle to be  35 hours per week (7
     hours/day). This permits start-up and wash-up  times
     within an 8 hour shift
  2.  One filter, with  adequate supply of key spare parts
     to  be maintained.
  3. Size of vacuum filter.—Production  rate  has been
     determined via  pilot testing to be  5 Ib/hr/ft2 (24
     kg/m2/hr),  but  to  provide a margin of  safety, 4
     Ib/hr/ft2 (20 kg/m2/hr), will be  used. Steady  state
     sludge  removal  rate requirement is 35,000 pounds
     (15,870  kg) per week.  With  a 35 hour-per-week
     schedule, weekly filter  capacity at  4 pounds  per
     hour per square foot (20 kg/m2/hr) is 140 pounds
     (63.5 kg) perft? 35,000 pounds/week  (15,870
     kg/wk) -s-140 pounds (Ibs/ft2/wk) per square foot
     per week  (685  kg/m2/wk) = 279 square feet (26 m
     of  filter  area  required.  The nearest standard size
     filter is 300 square feet (28 m2),  so a single unit c
     this size is chosen.
  4.  Sizing of auxiliary  equipment.—In  each case the
     details of  sizes of  vacuum  equipment, conveyors o
     other system  required to get the dewatered cake
     into the truck for hauling, and the chemical dosing
     equipment for sludge conditioning must be devel-
     oped, and priced.
  5.  Sludge storage capability.—The one shift per day-
     five day per week mode of operation  plus the use
     of  a single filter will require provision  of several
     days storage capacity  for the digested sludge.  Thii
     could potentially be  provided  by  a combination  of
     the inherent surge capacities  of the digestion tank;
     and post digestion thickening  tanks, or by provisio
     of  a separate storage  tank equipped to ensure  ho-
     mogeneity of feed  to the RVF.

  For a sludge of the type  described, a cationic poly-
electrolyte flocculant would  probably be used for condi-
tioning.  The testing and selection of suitable conditionin
agents would necessarily be carried out in conjunction
with  the series of  bench scale and pilot test programs
used to select  and size the rotary  vacuum filters. As
part  of  the selection process for suitable conditioners,
data should be obtained and reviewed on:

  1.  Price, dosage rate, and availability of  both  polyele<
     trolytes  and inorganic conditioners in the  particular
     locale.
  2.  The system required for solution  preparation and
     application, and its cost.
  3.  The storage stability (shelf life) of the conditioner i
     its  form  as supplied  and in stock solution for use.
  4.  Handling characteristics, safety aspects and corro-
     sion  properties  of  the material in dry and liquid
     forms.
  5.  Previous experience with the same materials at  oil-
     er  plants with similar sludges.

DEWATERING SYSTEM CONSIDERATIONS
  Auxiliary equipment such as sludge  conveyor or  re-
moval facilities, chemical  mixing and feed  equipment, ar
sludge feed pumps are usually available from the rotary
vacuum filter supplier.
  Polymer solution preparation  and dosing  equipment is
also  frequently  available from the polymer supplier or
from an equipment supplier  other than the  rotary vacuu
filter supplier.
  An Energy Audit should be a part of every system
evaluation. The Energy  Audit should include not only ar
estimate of the power consumption of  the dewatering
equipment and  its immediate auxiliaries, but also the
impact of the particular dewatering system on  the  overs
treatment process system. In this regard, the assessmer
should specifically include the impact  of the
conditioning/dewatering system on  both the post dewa-
    116

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tering portion of the system and the pre-dewatering por-
tion of the system. The latter facet makes preparation
and consideration of  "Quantified Flow Diagrams" for
both the conditioning/dewatering system and the overall
treatment  system mandatory to cost effective design.
  For purposes of  comparison, the rotary vacuum filter
in this design example would  require a vacuum  pump of
30 horsepower (22 kW), and filtrate pump of 3  horse-
power (2.2 kW).  To make  a complete  energy audit, all
the  auxiliary equipment data, and the other points men-
tioned in the previous paragraph would have to be as-
sessed.

DESIGN EXAMPLE—ROTARY VACUUM
FILTRATION—4O MGAL/D (1.75 mVs)
PLANT

Basic System Assumptions

  These would be  the same as in the preceding design
example for a 4 Mgal/d (0.18 m3/s) plant except that
the  required removal  rate would  be 25 tons of dry solids
per day (22.7 Mg/day).

Other Considerations

  The following parts of the Design Example would be
the  same  as for the 4 Mgal/d plant (0.18 m3/s) in the
preceding example:

  1.  Alternate units for consideration and evaluation.
  2.  Evaluation procedures.
  3.  Bench  scale testing.
  4.  Pilot tests.

Design  Calculations

  1.  Operating cycle.—To be either a seven day per
     week, 24 hour per day operation or five day per
     week, 24 hour per day operation depending on
     reduction and final disposal processes chosen.
  2.  Size  and number of  Rotary Vacuum  Filters re-
     quired.—Production rate to be conservatively taken
     at 4  pounds/hour/sq ft (20 kg/hr/m2). At  350,000
     pounds (159 Mg) per week the weekly capacity  of
     a square foot of filter area  for a seven day opera-
     tion (allowing  2  hours/day downtime average for
     clean up and  maintenance) is 4 pounds/hour/sq ft
     (20  kg/hr/m2) x 154  hours per week or 616
     pounds/week/sq ft Dividing 350,000 pounds per
     week by 616  (3) gives a  filtration  area requirement
     of 568 square feet (53 m2). A similar calculation for
     a five day operation gives a filtration area  require-
     ment of 793 square  feet (74 m2). In  either  the
     seven day/week or five day/week options, two 500
     square foot (46  m2) rotary vacuum filters would
     normally be specified to  provide sufficient capacity
     and redundancy.
  3.  All of the other  facets of the design procedure
     would be the  same as in the 4 Mgal/d (0.18 m3/s)
     example.
General  Comment—Rotary Vacuum Fil-
ters

  The RVF was, for many years, the common device
for dewatering municipal sludges. Their frequency of use
had persisted longer in the United States than in the
rest of the world.
  Operating problems such as the cake pick-up difficul-
ties,  poor cake release from belt filters with sticky sludg-
es, and the maintenance  requirements associated with
vacuum producing  equipment have existed in  numerous
cases.  Solids capture problems associated with either
the effect of less than adequate cake formation rate in
some relatively  open media filter installations or with
cake recycle due to  sticking problems have also oc-
curred. While these problems could  be moderated  in
many cases by revision of conditioning methodology or
mechanical changes, they  are deterrents to widespread
continued usage.
  More  universal deterrents to the continued selection  of
RVF's are:

  1.  The energy and maintenance costs associated with
     operating vacuum systems.
  2.  The inability to produce nearly  as dry a  cake as
     other  newer devices.

  These  comments are made to encourage the design
engineer to review current operating and cost experi-
ences at existing plants prior to making a design deci-
sion.

CONTINUOUS  BELT FILTER PRESSES

  This general type of device, which employs single
and/or double moving  belts  to  continuously dewater
sludges through one or more phases of dewatering was
originally developed,  and  in subsquent years modified
and  improved, in West Germany. The  earliest concurrent
U.S.  development was  under the aegis of the late Brian
Goodman,  at Smith and Loveless Division of Ecodyne.
  The scope and depth of development of this newer
type  device has been much  more pronounced in Europe
than  in the  United  States  until the past 3 to 4 years.
Within those past 3 to 4  years, many  different models of
the same type device,  differing  in configuration and  ca-
pability, have been introduced into the U.S. market.
  While there is general agreement  that the Continuous
Belt  Filter Press (CBFP) materially extends capabilities
for improved dewatering of sludges, the U.S.  design en-
gineer is faced with a  real task in selecting the optimum
device from the many which are now  available. But  that
task  must be dealt with if advantage is to be taken of
this technological breakthrough.
  U.S. installations of the  latest and best models are just
now  coming onstream. To review actual operating  per-
formance on particular sludges, usage of available mo-
bile pilot test units, coupled  with site visits is in order.
There is considerable operating experience available at
existing European sites. The old conundrum that Europe-
an sludges are different and results  are not applicable
                                                                                                     117

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 should be treated with the contempt it deserves,  since it
 is inaccurate.

Original Concept and  Evolutionary Devel-
opments—Continuous Belt  Filter Presses

  Figure 6-8 illustrates the single level device  originally
marketed by Klein of Germany and their U.S. licensee,
R. B. Carter.
  Practically concurrent with this development  was Brian
Goodman's work with  the  Smith & Loveless  Concentrator
which  is described later.
  This type unit was successful with  many normal mixed
sludges. Typical dewatering results for digested mixed
sludges with initial feed solids of 5 percent are to give a
dewatered cake of 19 percent solids at a rate of 6.7
Ib/hr/sq ft (32.8 kg/hr/m2) with  a chemical  conditioning
cost of $4.10/ton ($4.52/Mg). In general, most of the
results with these units closely parallel  those achieved
with rotary vacuum filters.  They do have advantages in
that there  is no sludge pickup problem which sometimes
occurs with rotary vacuum filters, and they have a lower
energy consumption.
  These results  are  satisfactory for many installations
and  the Continuous  Belt Filter Press  of this  first type or
its immediate successor, a two-level unit of the same
basic design and concept  (see figure 6-9) has in the
past 5 years become the most frequently selected dewa-
tering  device around the world.
  There have been additional developments  of the basic
principles of the Continuous Belt Filter  Press and  several
third generation units from various companies are now
available. In a broad sense these latest improvements
may be described as:

  1. The  addition of some form of continuous  mechani-
     cal thickening device  as the initial stage of a Con-
     tinuous Belt Filter Press.
  2. The  addition of additional medium and/or high
     pressure press  sections to the Continuous Belt  Fil-
     ter Press, and variations in  the cake shearing
     mechanisms to  obtain additional dewatering.
                          PRESS BELT
SLUDGE
FEED
                     SLUDGE FEED
 CHEMICAL
FLOCCULANT
 ADDITION
(• \-
FILTERj
BELT

f f 
                                                                                          PROCESSED
                                                         DIRTY WASH WATER,
                                                         FILTRATE, AND
                                                         RECYCLE POLYMER
                                                                                          DISCHARGE
                                                                                   PATENTS APPLIED FOR
Figure 6-8.—Original concept: continuous belt filter
press.
Figure 6-10.—Conceptual schematic: R.B. Carter  series
31732—CBFP.
     118

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                                             DISCHARGE
                                             END
                                10" APPROX.1

 Figure 6-11.—R.B. Carter series 31/32—CBFP.
  3.  In the second or high pressure zone (4 atmo-
     spheres)  the sludge is sandwiched between two
     sieve belts. Large mesh openings are possible be-
     cause the sludge has developed structural integrity
     at this  point.
  4.  A serpentine  configuration makes up the Shear
     Zone at the end  of the  second pressure zone
     wherein by stretching the  belts and sludge cake
     over smaller rollers, a squeezing action expels more
     water from the cake.

  As will be noted  subsequently in more detailed de-
scriptions of each  unit, the advanced third generation
CBFP's  give cake  dry solids  contents equivalent to those
achieved with  pressure filters.
  In  addition to the Carter Series  31/32 device, other
suppliers of  similar third generation type devices are:
              Company
Unit
Komline Sanderson	  Unimat
Parkson Co	  Magnum Press
Ashbrook Simon Hartley	  Winklepress
Carborundum	  Sludge Belt Filter Press
Tait Andritz	  SDM

  There are also other  Continuous Belt Filter  Presses
which are more  advanced than the original first genera-
tion type units. These are also described later.

Categorization of  Continuous Belt Filter
Presses

  Only units which  have at  least  two phases built into
their operation, and which yield cakes which are truly
dewatered and dimensionally stable (nonflowable) can
logically be classified as Continuous Belt Filter Presses.
The Dual Cell Gravity (DCG) Concentrator as supplied
by Permutit when used  in series  with the Permutit multi-
ple roll press (MRP) is  a system  which performs as a
continuous dewatering device in a fashion analogous to
the first generation  CBFP.
  All of the variations start with a gravity drainage zone
followed  by various combinations of shear and different
levels of pressure  (or vacuum) applied to the gravity
drained cake.  Rather than attempting to lump presses of
different  configuration into rigid  categories, each will be
described and results listed.

SMITH AND  LOVELESS  (S & L) SLUDGE
CONCENTRATOR
  This  device, as described in reference 10,  was devel-
oped and is marketed  by the  Smith and Loveless Divi-
sion of Edodyne. It is essentially a "Gravity-Pressure"
filtration  unit which uses an endless,  variable  speed,
relatively open mesh filter screen to retain flocculated
solids while the  bulk of liquid  passes through the screen.
Solids  from the  gravity drainage stage pass into the
second or  pressure stage where three sets of compres-
sion rollers fjrther dewater the  cake. The pressure in-
creases with each set  of rollers. The dewatered sludge
falls off the belt into a discharge chute for removal.
  The  S  & L Concentrator is  offered in two  models of
varying size. Typical dewatering capacities claimed are
described in table 6-11.
  As will be noted this device does not give  as dry a
cake as  some of the other more complicated machines.
It has  found usage at certain  plants which can utilize
cake solids levels  as shown.  The unit uses only 5 horse-
power  versus a  normal 40 horsepower for a rotary vac-
uum filter.

PERMUTIT DCG—MRP

  This  system consists of a dual cell gravity unit fol-
lowed  in series  by a multiple  roll press.  In reference to
the schematic cross section of the DCG, this first drain-
age section forms a plug of fluid sludge in the first fine
mesh nylon cell  and then the  plug  is further  dewatered
in  cake form in  the second  cell (see figures  6-12 and
6-13).
  The  relatively  moist cake from the  DCG  is conveyed
to  the  MRP, an  inclined  dual  continuous spring loaded
belt which  further  dewaters the  sludge cake.
Table 6-11. — S & L sludge concentrator
estimate
Type of sludge
Anaerobic digester primary
Aerobic digester W.A.S. . . .
W.A.S 	

Estimated
dewatering rate
(Ib/hr)
Model Model
40 80
250 500
250 500
225 450
performance
Polymer
dosage
Ib/ton
15
10
10
Cake
solids,
percent
12
10
10
                                                                                                        119

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                    DRIVE ROLL
                    AND SPROCKET
                    ASSEMBLY
          NYLON FILTER
          CLOTH
                                            DEWATERING
                                            CELL
                                                SLUDGE
                                                INLET
CON
VEYOR
FILTRATE DISCHARGE
 Figure 6-12.—Cross section of a Permutit DCG.
                                              CAKE

                                              DISCHARGE
 SLUDGE INLET
                                              J
 Figure 6-13.—Cross  section of a Permutit MRP.
   Typical performance on  the DCG-MRP (Caldwell, N.J.)
 indicates dewatering  of an anaerobically digested mixture
 of primary and humus sludge from a feed solids of 4-5
 percent  yielding a dewatered cake of 15 percent dry
 solids with polymer costs of $8 to $10 per ton ($9 to
 $11  per Mg).
   The DCG-MRP has worked reasonably well at small
 plants with noncontinuous  dewatering schedules. Some
 problems have been  noted with maintainability of the
 early units and some modifications are in process.

 INFILCO  DEGREMONT FLOC-PRESS

   This is a two stage unit of French origin featuring a
 horizontal belt gravity drainage area  on a woven synthet-
ic  fiber belt followed by a  press section. The partially
dewatered  cake is sandwiched between the lower belt
and a  rubber pressure belt (adjustable hydraulic loading)
to  provide  cake solids levels similar to that which is
obtained in rotary vacuum  filters or centrifuges (see fig-
ure 6-14).
  There are 46 world-wide Floe-Press installations and
there were five in the United States as of  January 1976.
A  notable U.S.  installation  is at Medford, N.J.11 At Med-
ford, a 0.9 Mgal/d (0.04 m3/s) contact stabilization
plant, a two meter wide Floe-Press replaced  an existing
rotary  vacuum filter which  has been shut down. The
results are shown in table  6-12.
  The  horsepower consumption is 6.25 (4.7 kW) for the
Floe-Press  versus 22 (16.4 kW) for the previously used
rotary  vacuum filter. The RVF had provided similar cake
solids  but poorer solids capture. Polyelectrolyte costs
are in  the $11-15/ton ($12-16/Mg) range. The filter bell
is  still  in excellent condition after almost a year of oper-
ation. The  wash water rate is 22 gpm (1.4  l/s) at 50
Ib/in.2 (3.5  kg/cm2) and plant effluent water is  used.
  The  Floe-Press system includes a mounted sludge con-
ditioning chamber and other  auxiliaries such as chemical
conditioner and sludge feed systems, conveyors for
sludge removal and automated control panels.
  Output in pounds per foot of belt width  per  hour is
quoted at 134-268 (200-400 kg/m) for an  anaerobically
digester mixture of  primary and W.A.S. at  a feed solids
of  3.5  to 9 percent, the Medford,  N.J.,  Floe-Press is  16
                                                                        WOVEN SYNTHETIC
                                                                        FIBER BELT
                                                                PRESSURE BELT
                                                                HYDRAULIC JACK
                                                                 ROTARY DRUM
                                                                 FLOCCULATOR
                                                         DISTRIBUTION
                                                         BOX
                                AIR
                                ACTUATED
                                PINCH 	
                                ROLLERS
                                                            BOTTOM
                                                            DRAIN PAN
                                                         PRESSURE BELT
                                                                         FLEXIBLE
                                                                         SCRAPER
                                                                         OR DOCTOR
                                                                         BLADE
                                            SAFETY
                                            SHUT
                                           .DOWN
                                            MONITOR
                                                      BELT WASH '
                                                "**""    SPRAY
                                             SUPPORT  NOZZLES   RUBBER COVERED
                                             ROLLERS            DRUM
                                Figure 6-14.—Infilco Degremont Floe-Press.
                                Table 6-12.—Floe press  results—Medford, N.J.
                                Feed solids, percent	
                                Cake	
                                Filtrate suspended solids (PPM).
                                Percent solids capture	
                                               Averages


                                                 3-4
                                                17-19
                                                 100
                                                  98
     120

-------
feet  1-1/4  inches long,  10 feet 4-3/8" wide, and 10 feet
6 inches (4.9 mx3.1  mX3.2 m)  tall.
  The  Floe-Press is available in belt widths varying from
a nominal 3 feet (0.9 m) to a nominal 10 feet (3 m) with
effective belt areas of 32.28 square feet to 96.84 square
feet  (3-9 m2). For the larger units,  only additional width
must be provided for.

PASSAVANT  VAC-U-PRESS
  This is a German development  which features the fol-
lowing:

  1.  A continuous press utilizing  gravity and vacuum
     drainage followed by a pressure zone.
  2.  Conditioned sludge  is evenly distributed on a mov-
     ing belt which initially drains by  gravity and then  by
     virtue  of vacuum boxes beneath the  belt.
  3.  The compression belt is applied  on top of  sludge
     on the lower  belt to form  a  sandwich.
  4.  The two belts are subjected to pressure by going
     under tension around large dewatering cylinders.
     Pressure is then applied to  alternating sides of the
     belt by smaller  pressure rolls.
  5.  Dewatered sludge is discharged  and belts  are con-
     tinuously back-washed.
  6.  The Vac-U-Press is enclosed in  a fiberglass rein-
     forced polyester housing to  control noise and odor.

  Typical sizing data are as shown in table 6-13.
  There  are five U.S. installations of the Vac-U-Press,  all
of the BFP-200 model. Indications are that  it gives a
dewatered  cake slightly drier than a rotary  vacuum filter.
  A  mobile test unit is available for rental.

TAIT ANDRITZ  SDM  and  SDM-SM

  Andritz, an Austrian equipment  firm, first developed a
continuous double belt filter dewatering device for use
on various industrial sludges. In the past  two years Tait
Andritz of  Lubbock,  Tex., has sold  and installed 43 of
these devices at 28 total U.S. locations for dewatering
of various  industrial  and municipal sludges.  The 1977
world-wide installation list shows  68 locations where
these devices are in use. Twenty of these locations are
on municipal sludges. The industrial installations are in
some cases on straight 100 percent biomass sludges.
  The dewatering  in the Tait Andritz  unit(s) is achieved
Figure 6-15.—Tait Andritz—SDM-SM model.
by passage of the sludge through a gravity dewatering
zone, into a wedge zone for pressure dewatering, fol-
lowed by higher pressure  dewatering in a module zone.
The  module zone can  be  either an S configuration (off-
set rolls), or a press configuration (pressure loaded rol-
lers).
  Main design features are: variable speed  drive for
belts and conditioning  drum; pneumatic belt tensioning
and  pressure adjustment during operation;  pneumatic belt
tracking; and in  the industrial SDM model, use the end-
less  belts.
  Figure 6-15 shows  the  SDM-SM  model (seamed belts)
designed for municipal operation where unattended
round-the-clock operation  is  not necessary.
  Table 6-14 summarizes reported  operating results.
  The results shown in table 6-14  tend to indicate that
the Tait Andritz  CBFP's will  normally produce a cake
solids content somewhat higher than that obtainable  in a
rotary vacuum filter.  Further, more  definitive results on
the two versions  (either the  "Press  Module" or the "S"
Module  equipped) of the basic device will  be forthcom-
ing during  1978.  In this vein, it is understood that Bur-
lington,  Wis. (an  installation discussed later) has recently
ordered several  units.
  The Tait Andritz SMD device (Industrial) has an excel-
lent  performance record (ease of maintenance,  etc.)  in
dewatering  biological  and mixed sludges in the paper
industry.12'13
  Data on  the size of  the  three SDM-SM models  avail-
able are shown  in table 6-15.
Table 6-13.—Passavant Vac-U-Press—sizing data
Model
Number
BFP075
BFP125 	
BFP200 	
Belt
width
26-1/2
43-1/2
... 72-1/2
Length
(ft/in.)
14-9
14-9
14-9
Width
(ft/in.)
4-1
5-8
8-2
Height
(ft/in.)
5-3
5-3
5-3
Drive
motor
(hp)
1.5
3
3
Active
belt area
(Ib/ft2)
90
150
250
Nominal
capacity
(gal/hr)
1,500
2,500
4,200
                                                                                                       121

-------
Table 6-14.—Tait Andritz—SDM-SM results
Percent
dry solids
rype sludge
Feed Cake
Throughput3
gal/min
Dry solids
(Ib/hr)
Polymer
cost
($/ton
dry
solids)
Raw primary	
Primary and W.A.S.
Unox ext. aer	
5-7  22-26   10-14
3-5  20-25   15-20
1-2  18-23   20-25
300-500    4-7
200-350    4-8
200-250    8-10
  aPer 20 inches of working belt width.

Table 6-15.—Tait Andritz SDM-SM—machine sizing data
Size and type
SDM 40
SDM 60
SDM 80 	

Working
belt
width
(in.)
40
60
80
Overall dimensions
Length
(in.)
152-1/2
186
186
Width
(in.)
75
114
134
Height3
(in.)
75
83
83
Weight
(Ib)
5,513
14,333
17,640
Conn
H.P.
load
3-1/2
5-3/4
5-3/4
Belt spray
consumption
(gal/min)
18-24
30-37
35-45
  "Height will vary according to drive system used.
ASHBROOK SIMON-HARTLEY  WINK-
LEPRESS
  The Winklepress was developed by Gebr.  Bellmer KG.
of Germany. Simon-Hartley of the  United Kingdom  mar-
kets U.S. units through a subsidiary, Ashbrook Simon-
Hartley of Houston, Tex.
  Figure  6-16 is a schematic conceptual drawing which
shows that the device employs two endless  synthetic
fiber mesh  sieve belts to convey and dewater condi-
tioned sludge. After an initial gravity sandwich drainage
stage, the primary belt meets the  second belt and forms
a vertical sandwich drainage section. The two belts,
which are under tension,  then carry the sludge along an
arrangement of staggered rollers where multiple shear
force action areas squeeze  out  remaining free water.
The sieve belts are continuously washed.
  While there are a number of operational installations  in
Europe, as of November  1,  1977 none of the U.S. instal-
lations  under construction had started operation. (See
tables 6-16 and 6-17.)

KOMLINE  SANDERSON  UNIMAT GM,H-7
CONTINUOUS BFP

  Komline Sanderson  manufactures its version of the
German Unimat under license from Mull-Abwasser-Trans-
portanlagen-GMBH, Elversberg,  West Germany.
                         HORIZONTAL
                         DRAINAGE
                         SECTION
                                         ROTARY DRUM
                                         CONDITIONER
                                        REAGENT
                                            FEED
                              VERTICAL
                              DRAINAGE
                              SECTION
                                                  FINAL
                                                  DEWATERING
                                                  SECTION
X . ' , . . « ,
y,..,..


BAND
WASH


                     Figure 6-16.—Schematic of an Ashbrook Simon-Hartley
                     Winklepress.
     122

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 Table 6-16.—Winklepress test results (from supplier)
                                 Dry solids
                          Capacity feed


Digested primary and humus

Digested primary and WAS


Feed
38
5.7
35
4.8

Cake
362
36.3
363
38.5
Filtrate
(mg/l)

85
95
90
75
Polymers
(kg/m3)

0.182
0.165
0.165
0.182
m3/h
meter
7.5
6.5
7.5
7.5

gal/min
33.0
28.6
33.0
33.0
Table 6-17.—Winklepress size and capacity data
                            Input width
      Winklepress
         size
           Nominal
         capacity of
        digested sludge
                           mm
inches   m3/h   gal/min
0 	
1 	
2
3 	
4 	

	 200-300
	 500-800
1 000-1 300
	 1,500-1,800
	 2,000-2,300

8-12
20-32
39-51
59-71
79-91

2-3
5-8
10-13
15-18
20-23

8.8-13
,22-35
44-57
,61-79
,88-101

  The  most advanced model of the modularized Unimat
(figure 6-17) which  is designed for maximum cake dry-
ness and  throughput is the GM2H-7. This press consists
of four stages:

  1.  Gravity drainage (actually a thickening stage)
  2.  A mild pressure stage
  3.  A medium pressure stage
  4.  A high pressure stage

  The  initial gravity  drainage stage is a continuous belt
of pockets which are formed  by folding a  rectangular
piece of cloth. This  is a separate  belt. After thickening
in this first stage the sludge dumps into  a different belt
which  moves over a gravity drainage  tray prior to dump-
ing onto another belt on  a succeeding tray (and a dif-
  FLOCCULATED
  SLUDGE
                                              •GRAVITY DRAINAGE
                                               STAGE
                                   SLUDGE DUMPS TO NEW BELT:
                                   INTERNAL WATER INCREASED
SLUDGE DUMPS
TO NEW BELT-
INTERNAL WATER
RELEASED

MILD
PRESSURE
STAGE
         MEDIUM PRESSURE STAGE
Figure 6-17.—Komline Sanderson Unimat GM2H-7.
                    —- GRAVITY DRAINAGE BELT
                    —} PRESSURE BELTS

                     @ ADJUSTABLE PRESSURE ROLLS
                                                                                                          123

-------
ferent belt) where a small amount of pressure is added
by small  diameter rollers. Then  it is moved to the third
tray of the mild pressure section (and back on the origi-
nal pressure belt) and subjected to  slightly more pres-
sure  before going into the medium pressure stage. All
the rollers in  the medium pressure stage are adjustable
for pressure optimization. While passing over the medium
pressure  rolls, the cake  sandwich between the belts  is
flexed from one side to  the  other.  Each of the large
diameter  drums has smaller  diameter rolls which apply
pressure  as the sandwich passes over the drums. Every
other roll is perforated for water removal. Pressure is
applied to the cake by tension  on the belts as the belts
go around the drums and by the small diameter rollers.
The belt  tension is,  however, relatively low and all syn-
thetic media is used instead of stainless steel in the
long  axis.
  The cake now goes to the high  pressure stage which
can be thought of as two caterpillar tractors standing
upright with the tracks butting together. As in the medi-
um pressure section the  pressure is adjustable through
springs.
  In  applications where  a very  high dry solids in the
cake  is not imperative, the unit is available without the
high  pressure  section.
  In  addition  to the previously mentioned  nomenclature
and  model system the Unimat series is  available in three
models:
  Model  S Gravity stage
  Model  SM  Gravity and medium pressure stages
  Model  SMH Gravity,  medium  and high pressure stages
Table 6-18.—Active filtration surface areas and  retention
times
Machine
Machine model width
(meter)

S



M



1
2
3

1
2
3
Active
filtration Retention time
surface area (min)
(ft3)
S
68
136
204
5 roll
101
203
305
L S
104 1.2 to 6
208
312
7 roll 5 roll
190 5 to 19
380
570
L
2 to 9


7 roll
10 to 36


                           ALL

                           32.9
                           65.6
                           98.4
 ALL

2 to 6
  Note: When using 2 or more sections, the retention time and active
 surface areas are cumulative
  Table 6-18 lists the design features of this series.
  There were 69  European locations employing the Uni-
mat as of November 1976, with practically all  of them
processing  municipal sludges of some type, including
straight 100 percent biomass.
  Table 6-19 lists reported results.
  While at  the time of writing this, no Unimat  systems
are yet operating in the United States, 16  units have
been sold and some will be operative by  early 1978.
  A mobile test unit is available  and considerable  U.S.
test work was carried out on site during 1977.

Performance of Unimat on  Washington,
D.C., Mixed  Sludge
  At Blue Plains the Unimat GM2H-7 dewatered a  sludge
mixture of  1 part primary  plus 2 parts W.A.S.  to a dry
solids content of  27 to  33 percent at rates of 196 to
206 Ib/hr/ft (292 to 307 kg/hr/m) width.  Polymer costs
were mostly between $8.76 to $9.20 per dry ton  ($9.66-
$10.14/Mg) with a solids  capture of 95-98 percent. On
the existing rotary vacuum filters a total dry cake  solids
of 22-24 percent (including solids resulting from use  of
5-7 percent ferric chloride and 15-20 percent lime) is
normally obtained. Because of the large variation of the
sludge quality, the  lime  dosage for the rotary  vacuum
filters reaches 30-40 percent on occasion.
  At Blue Plains, the dewatered vacuum filter  cake was
fed to the  M2H sections of the Unimat and the cake
solids were increased to 37-40 percent at a feed rate
of 366 Ib/hr/ft (545 kg/hr/m) width with  no auxiliary
conditioner dosage.

Performance  of Unimat  on  Columbus,
Ohio, Southerly Plant Sludge
  At Columbus Southerly  plant, the anaerobically digest-
ed  mixture of primary and W.A.S. was dewatered  to a
cake  solids content of 36-39 percent at a rate of 228-
305 Ib/hr/ft (341-455  kg/hr/m) width. Solids capture
was 90-95 percent and polymer costs $8-$14/ton
($9-$15/Mg). Feed solids were  3-4 percent dry solids.
Thus  an autogenous cake is feasible with  this difficult
sludge.
  It is quite apparent that the K.S. Unimat press is one
of the CBFP's newly introduced into the U.S. from Ger-
many which has  the capability to effectively dewater
mixtures of primary and W.A.S.  sludges to a dry solids
content  high enough to be in the autogenous incinera-
tion range.

PARKSON MAGNUM PRESS
  This device,  of  Swedish origin, is  manufactured and
sold in the United States by the Parkson  Corporation of
Ft. Lauderdale,  Fla.
   The Magnum  Press is an advanced or  third generation
type CBFP designed to maximize dry solids content of
dewatered cake. The Magnum Press has three stages
and can best be  described by reference  to the cross
sectional side-view of figure 6-18.
     124

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Table 6-19.—Dry solids of cake and polymer dosage
                     Unimat
   Model S
Model SM
Model SMH
Type of sludge feed cone.
(percent D.S.)
Fresh-primary — raw (4-6 percent) 	
Fresh primary and trickling filter (3—5 percent)
Fresh primary and activated (3-5 percent)
Anaerobically digested primary and activated (4-9 percent)
Activated — 100 percent W.A.S. (0.5-1.0 percent) 	

After gravity
stage
(percent D.S.)
12-18
10-15
10-15
14-24
8-12

After gravity
and medium
pressure
(percent D.S.)
25-35
22-32
17-27
25-35
17-20

After gravity
and medium
and high
pressure
(percent D.S.)
30-45
28-40
25-35
30-45
17-23

Typical polymer
dosage
(Ibs/ton D.S.)
60-85
60-100
60-100
5.0-8.5
70-100

             UPPER SCREEN
                          HYDRAULIC CONTROLLED
                          SQUEEZE BELL TENSION
HIGH PRESSURE
SQUEEZE BELTS
     PNEUMATIC CONTROLLED
     SCREEN GUIDANCE
                                                                          PNEUMATIC CONTROLLED
                                                                          SCREEN TENSION
  DOCTOR
  BLADE
DOCTO
BLADE
                                                                                           PNEUMATIC CONTROLLED
                                                                                           SCREEN TENSION
                                               VARIABLE SPEED
                                               DRIVE MOTOR
                                                                         SCREEN WASH
                                                                         SPRAY NOZZLES
                                                                           SCREEN WASH
                                                                           COLLECTING TRAY
                                                                     PLOUGH AND ROLLER
                                                                     SECTION (OPTIONAL)
SCREEN
WASH
SPRAY
NOZZLES
              DRAINAGE
              ROLLER
                            COLLECTING TRAY


Figure 6-18.—Cross section of a Parkson Magnum Press.
   LOWER SCREEN
                     PNEUMATIC CONTROLLED
                     SCREEN GUIDANCE
  The initial stage is a unique gravity drainage section.
In addition to normal dewatering occurring by gravity
from  a properly conditioned sludge, the sludge can be
subjected to a light pressure involved by  rollers and be
turned by plows (both optional). The partially formed
cake  then proceeds to the low pressure stage where  the
second polyester screen belt comes into play on the top
forming a sandwich  that is fed  into the second or low
          pressure stage. In the low pressure stage perforated
          press rolls  of decreasing diameter  subject the cake to
          continuously increasing pressures. In the last or high
          pressure stage  the cake is subjected to very high pres-
          sure that is adjustable, depending on the application.
          The high pressure is  generated by a series  of 1  inch
          wide flat belts that press the screens against a  perforat-
          ed roll uniformly from  side to side. This feature  allows
                                                          125

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Table 6-20.—Magnum press size data
Model
MP-20 	
MP-40 ..
MP-60
MP-80 . .

Screen
width
(nominal)
	 20"
40"
60"
... . 80"

Weight
(tons)
3.8
4.4
4.8
6.0
Overall dimensions
A-width
4'
5'-8"
7'-4"
9'
B-height C-length
7-9" 14'-10"
7'-9" 14-10"
7'_9" 14'-10"
7<_9" 14'-10"
Screen wash
water flow
rate @ 100
gal
12 gal/min
24 gal/min
36 gal/min
48 gal/min
the sludge to  be subjected to high pressure for a long
period of  time without producing an excessive  load on
the screens. The pressure is adjustable through the  use
of two hydraulic cylinders.
  This final high pressure stage of the Magnum Press
can also be employed in a modular fashion to further
dewater filter cake from existing  Rotary Vacuum Filter
installation.
  The Parkson Magnum Press  is available in four  sizes
as shown in table 6-20.
  As  of December 1977, nineteen Magnum  Presses had
been  sold worldwide. There are seven Japanese installa-
tions, nine in Europe, and three in the United States.
The first U.S.  unit (at Mobil Oil Co.) processing straight
excess biological sludge  is just  now commencing opera-
tion.
  Parkson has a mobile Magnum  Press and a  smaller
pilot unit,  both of which  have been used  to carry  out
on-site tests at various U.S. locations.

Performance of Magnum  Press at
Washington,  D.C.

  A 0.25  meter  pilot unit was evaluated on the various
sludges at Blue Plains plant. The  following  two figures
show the  results obtained with various mixtures of pri-
mary  and excess activated sludges (including phosphorus
removal sludges resulting  from iron salt use).
  In assessing  results of dewatering work at Blue  Plains
it is important to note the following:
  1. The  normal mix  is 32 percent raw primary/68 per-
     cent raw secondary sludges  (on a weight percent
     dry solids basis). The primary is gravity thickened
     to 9.5 percent and  the secondary is DAF thickened
     to 5.5 percent.  The resulting 6.8 percent  solids  mix
     is filtered on RVF's to about 18 percent (without
     lime).
  2. The  Blue  Plains plant has  an abnormally large
     amount of  a difficult to process excess activated
     sludge due primarily to the use  of a high rate acti-
     vated sludge biological treatment system.  This sys-
     tem  was apparently chosen because of certain site
     and  capacity constraints.
  As  can be seen in figure 6-19, the Magnum Press
produced a dewatered cake of 30 percent dry solids
     126
                                                                  PRESSED
                                                                  VACUUM
                                                                  FILTER
                                                                  CAKE
                                                             0   10  20   30   40   50  60  70   80   90  100
                                                                PERCENT PRIMARY (WT. PERCENT DRY SOLIDS)
                                                            100  90  80   70   60   50  40  30   20   10   0
                                                               PERCENT SECONDARY (WT. PERCENT DRY SOLIDS)


                                                         Figure 6-19.—Magnum Press results, Blue Plains.
                                                         content  at a rate of 244 Ib/hr/ft (364 kg/hr/m) belt
                                                         width.
                                                           It should also  be noted that a straight interpolation of
                                                         the data in figure 6-20 indicates that at a more normal
                                                         sludge ratio of 60 percent primary and  40 percent sec-
                                                         ondary,  even with the  high rate  W.A.S., the production
                                                         rate would be 17 percent greater and the cake solids
                                                         would be  34  percent. As shown in figure 6-20, polymer
                                                         dosages varied from 5.5 to 1.6 pounds per ton (2.8 to
                                                         0.8 kg/Mg) of dry solids and solids  recoveries varied
                                                         from 95 to 98 percent.
                                                           The Magnum Press was also tested for dewatering the
                                                         filter cake from the existing RVF's.  Cake  solids of  35-42
                                                         percent were obtained at rates of 244 to 853  Ib/hr/ft
                                                         (364 to 1273 kg/hr/m) belt width. There is mechanical
                                                         development  work required to design equipment to trans-
                                                         fer the  filter cake to such a press.

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    0    10   20   30   40  50   60   70  80   90   100
        PERCENT PRIMARY (WT. PERCENT DRY SOLIDS)

   100   90   80   70   60  50   40   30  20   10    0
        PERCENT SECONDARY (WT. PERCENT DRY SOLIDS)


Figure 6-20.—Magnum Press results II, Blue Plains.
Magnum Press Performance  at Los
Angeles/Orange County  Metropolitan  Ar-
ea (LAOMA)
  The Magnum Press mobile unit was  evaluated  on sev-
eral mixtures of the sludges being studied in this major
research  and  development project.
  While the results in table 6-21  are impressive  and
may well  be acceptable for the system, it is also appar-
ent that the dewatering devices' performance is  penal-
ized by attempting to dewater an unthickened sludge. It
is strongly suspected that  if the LAOMA sludges were
thickened a much higher capacity and  cake solids  would
                                                       be realized, in  addition to being operable at a much
                                                       lower polymer dosage.

                                                       Magnum Press Performance—Other
                                                       Locations
                                                         A bench scale Magnum Press has been evaluated at
                                                       various other locations in table 6-22.
                                                         It is significant to note that  the Magnum  Press will
                                                       function with inorganic conditioning  agents  to extend the
                                                       flexibility of the unit and  to  reduce  polymer costs.
CARBORUNDUM  SLUDGE BELT FILTER
PRESS

  Carborundums' Pollution  Control Division at Knoxville,
Tenn., manufactures and sells a unit called the Sludge
Belt Filter Press (SBFP). This unit is based on  the de-
sign of Rittershaus and Blecher of Germany who devel-
oped the "Dreibandpresse."
  The Carborundum unit incorporates two unique fea-
tures:  stainless steel wire supported  belts and oscillating
pressure  rollers.
  As can be seen  in figure 6-21, the gravity drainage
section of the  SBFP includes two phases involving a
dumping  of the partially  drained sludge  from  the initial
belt onto a second drainage belt prior to the incidence
of the upper sandwiching belt. The two belt cake sand-
wich then proceeds around a large diameter  roll into  a
further pressurizing section involving smaller diameter  off-
set pressure rollers in a two level configuration. Thus, in
effect, the Carborundum SBFP has a two stage gravity
drainage  section plus two additional pressureshear
stages to successively expose the cake to  increasing
degrees of shear and  pressure.
  Carborundum is also  bringing  out a newer  model  with
a "Pre-Concentrator" stage in the same vein as the
Unimat and R. B. Carter Series 31/32 devices.
  The current  Carborundum SBFP is available in 2  mod-
els. Table 6-23 shows the dimensions.
  This unit was introduced into  the United States in
1977 so  no U.S. commercial scale operating  data are
yet available.  A pilot unit is available for testing and the
supplier quotes the results as shown  in table 6-24.
  Additional field U.S. results are now available from
Table 6-21.—Performance of magnum press—Los Angeles/Orange County
metropolitan area


70
30
Sludge mixture
(digested mix)
Prim-30 WAS 	
Prim-70 W.A.S 	
Dry solids,
percent
Feed Cake
1 .8 29
	 2.1 21
Capacity-
dry solids
(Ib/hr/m)
360
320
Polymer
($/ton
dry solids)
12.60
21.40
Percent
solids
recovery
96
88
                                                                                                    127

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 Table  6-22.—Performance of Magnum Press—various  sludges8


Dry
solids,
percent
Location


Blue Lake, St. Paul, Minn. . .

Lake Charles, La.
Richardson, Tex.

Industry 	

Sludge mixture


45-Prim.b
55-W.A.S.
Prim + W A S
Digested prim +
W.A.S. + alum
W.A.S.


Feed
5.3

2 9
4 1

3.5


Cake
35

29-34
26-27

d 22-23

Capacity-
dry solids
(Ib/hr/m)

1,260

580
615

500

Flocculant
($/ton
dry solids)

14

12
C11

d17

Percent
solids
recovery

98

95
95

95
  "All results from 0.25 meter bench scale press.
  bConcentrations by volume.
  °Costs using 75 Ib/ton FeCI3 plus 5 Ib/ton polymer: straight polymer = $16/ton.
  dValues shown are for 100% polymer usage: use of 30-55 Ibs/ton FeCI3 will increase cake solids to
net of 27% at slightly lower capacity.
                                                         Table 6-24.—Carborundum SBFP results
Figure 6-21.—Carborundum  sludge  belt filter press.
Table 6-23.—Carborundum SBFP
            Model
        Approximate overall
 Belt     dimensions (inches)
width  	
 (in.)
      Length   Height   Width
135.
215.
 39     160
 70     160
96     69
96    100
Carborundum and German full-scale installations have
been  in operation for several years.

R.  B.  CARTER SERIES  30 PRESSES
  R. B. Carter  of Hackensack, N.J., is  the U.S. licensee
of Klein of Germany,  the  developers of three successive
Type sludge
Capacity
(gal/hr)
Feed
solids
(percent)
Cake
solids
(percent)
Polymer
cost
($/ton
dry solids]
Primary + W.A.S
Anaerobically digested
primary + W.A.S 	
W.A.S 	

900
1,300
1 100

4-6
4-9
4

34-37
26-40
16-20

9
10
11

generations of continuous belt filter presses, each of
increasing  capability in  either capacity or cake solids
content realized.
  The  original single level Klein device which was intro-
duced  in Germany  in about 1969, the Carter Series 30
(a two level unit),  and the lastest multistage unit, the
Carter  Series 31/32 CBFP (based  on the Klein "S"
press)  were described in  a  preceding section dealing
with  the  evolution  of the CBFP. The early single  level
device has been superseded by the two level  Series 30
and  the  multistaged Series 31/32.

R. B. Carter  Series 3O Installations,
Dimensions and Results

  As of July 1976  there were 21  U.S. installations of the
Carter  Series 30 CBFP that  were either operating or
were on order. The 21  installations involved 36 units. Of
these installations,  8 were for industrial sludges and  13
municipal.
  The  series 30 units are available  in 3 sizes  as  shown
in table 6-25.
    128

-------
 Table 6-25.—Carter series 30—overall dimensions
                             Table  6-27.—Single level press—R. B. Carter type,
                             Lenham  Works, East Kent,  U.K.

5/30
10/30
15/30
Table
Model




6-26. — Performance
width
(inches)
53
	 73
	 93

data — Carter series 30
Weight
(ibs)
2,500
3,500
4,500
CBFP
Type sludge
Primary + humus +
W.A.S 	
Straight humus..

Dry solids,
percent
Feed
4.5
45

Cake
22
18
Capacity8
(Ibs dry
solids/ hr)
72
49
Polymer
($/ton)
5.64
8.00
Percent
solids
capture
96-99
96
        Type sludge
   _ .. .                         "0.5 meter belt width x 3.0 meter length—Wm. Jones, Chem. Eng.
   Solids                       LW
  content     _   ..   Polymer
  •—Q     as?   «r
              sq.ft)       f*     Table 6-28.—Lenham Works—cost analysis, first  genera-
Feed  Cake                >    tion CBFP
Primary + W.A.S	   4-5   20-30   6.5-12    4-8
Anaerobically digested primary +
  WAS	   6-8   20-30   10-20    4-8
Extended aeration (no primary
  treat)	   2-4   16-24    6-10    2-6
  The  Series 30 is typically about 12 feet long and five
feet tall. Quoted typical results for the  Carter Series 30
model  are  shown in table 6-26.
  A mobile pilot unit of the Series 30 has been used in
onsite  test work.

Performance of a CBFP  of the  Carter
Series 30  Type in the  U.K.

  In addition to the quoted typical results  above addi-
tional insights into the capabilities of the Carter Series
30  units can be gained by study of references  14 and
15. The  latter reference is an exhaustive study  by the
U.K. Department of  the Environment (D.O.E.) on an in-
stallation of the British version of the first  generation
Carter type press. This study was carried  out over many
months by the D.O.E., an agency of the government,  at
Lenham  Works in  East Kent.
  Different mixtures  of sludges were processed to deter-
mine applicability of the single level first generation
CBFP, including operability, maintainability, and  all cost
factors as  well as dewatering capacity.
  Typical results are shown in  table 6-27.
  As will be noted  the normal  mixed  sludge  is not a
difficult one and results were essentially equivalent to
dewatering with an RVF. However, it is doubtful that an
RVF would have achieved  results on  straight secondary
sludge similar  to those shown.
  The  Lenham plant is a small plant designed  to treat a
dry weather flow  of  0.11 Mgal/d (0.005 m3/s)  and actu-
                                                                           Item
                                                                        $/ton dry solids
                             Polymer	
                             Wash water	
                             Power	
                             Operating labor (inc. super.).

                                   Total operating	

                             Capital costs	

                                   Total (ex. maint.)a	
 4.90
 1.94
 0.66
12.00
19.50

46.00
65.50
                               aMaintenance estimate+ 3/4 hour/1,000 hours operation.
                             ally processing about one half of design flow. The plant
                             includes  primary, trickling filter and activated sludge op-
                             eration. Though the  normal  sludge  mixture is a  relatively
                             easy to process material, the performance of the first
                             generation CBFP was viewed as highly successful.
                               The  cost analysis  (table 6-28) showed a total operat-
                             ing and capital cost of  $65.50 per ton ($72.20/Mg) of
                             dry solids dewatered. Maintenance costs were low.

                             Performance of an R. B.  Carter Series 3O
                             CBFP——Hutchinson, Minn.

                               At Hutchinson, Minn., a Series 30 Carter CBFP has
                             been operating for many months on a municipal sludge
                             from an activated sludge plant.  Figure 6-22 is a photo
                             of the  unit.
                               At Huntchinson,  the waste activated sludge is fed to
                             the CBFP at a solids concentration of 1-1.5 percent
                             resulting  in a cake solids content of 13-15 percent and
                             dry solids throughput of 340 pounds per hour (155
                             kg/hr). While this performance is satisfactory it  could be
                             greatly improved by  prethickening to a solids content
                             more logical for maximum dewatering capability.
                                                                                                        129

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Figure 6-22.—Carter series 30  CBFP.
R.B. CARTER SERIES 31 CBFP

  The basic  design characteristics of this unit have been
delineated in ealier sections.  Essentially  it consists of an
initial  "Reactor Conditioner" system which performs the
dual  function of conditioning  and prethickening followed
by two successive pressure zones and a shear zone
under pressure.
  The Series 31 device also  comes in 3 sizes, 5/31,
10/31, and 15/31 which differ  in widths. The largest
unit,  the  15/31, is designed for a nominal feed of 85
gal/min (5.4 l/s) of typically  a  5 percent mixed sludge.
Complete systems, including the chemical feed system,
pumps, controls and erection costs  are  usually priced at
slightly less than $2,000/gal/min ($31,160/l/s) or
$170,000 for an 85 gal/min (5.4  l/s) Series  15/31 unit.
Solids capture in the Series 31  normally averages 95
percent plus. Connected electrical power, including
sludge pumps and conditioner system pumps totals not
more  than 15 horsepower (11.2 kW).
  Sizing of a building  or space for  a  two unit Series 31
system, including polymer preparation system, and con-
veyor sludge removal system indicates a floor space
requirement  of about 36 feet by  18 feet (11.0 mx5.5
m). Height requirement is 13 feet 6 inches (4.1  m) mini-
mum.
  While there are quite a few operating installations of
the Series 31 type unit (Kleins or Wm. Jones "S" Press)
around the world, U.S. commercial  units were just com-
ing on stream during 1977.

Performance of  R. B. Carter Series  31
CBFP at  Hamilton, Ontario

  The Carter Series 31 mobile pilot unit has been tested
at several North American locations including Hamilton,
Ontario,  among others.
  On a digested mixed primary and W.A.S.  sludge at
Hamilton, a  27  percent dry solids cake  was obtained
     130
which compared very favorably with a 16 percent cake
being obtained at the same time  on the existing Rotary
Vacuum Filters.  Hamilton was experiencing some problem
with  fines  recirculation and accumulation within the sys-
tem at the time and no doubt even more favorable
results would be realized in a situation with  normal
sludge conditions.

Performance of R. B. Carter Series 31
CBFP at  Parkersburg,  W. VA.
  At the Borg Warner Co., two 15/31  Carter units are
dewatering a pure excess biological sludge.  Feed solids
are 0.5 to 2.0 percent with  a  cake solids content of
25-33 percent.  Capacity averages 1500 pounds (682 kg)
of dry solids per  hour per  machine.

Performance of R. B. Carter Series 31
CBFP at  Scituate, Mass.
  A  Carter Series 31 unit equipped with a Reactor-
Thickener  was evaluated on the difficult aerobically di-
gested extended aeration sludge  at the Scituate, Mass.
plant. Results are shown in table 6-29.
  In  a cost  comparison, the engineers involved estimated
that  a production level  of 3 dry tons (2.7 Mg) per day
for a 5-day  week either 2 Carter Series 31  CBFP's  (60
inches wide) with Reactor-Thickener first stages: or two
250  ft2 (23 m2) DAF units plus two 200  ft2 (19 m2) RVF's
would  be  required. Equipment costs for the  CBFP option
were estimated  at $222,000 and  for the second option
at $425,000. Horsepower requirements were estimated at
26 hp (19 kW)  and 200 hp (149  kW) respectively for the
two  systems.

DESIGN EXAMPLE—CONTINUOUS BELT
FILTER  PRESS
Basic Assumptions
  These assumptions are identical to those  used in  the
example for design of a Rotary Vacuum Filter System:

  1.  Anaerobically digested mixture of primary and
     W.A.S. at 4  percent solids content, 60  percent  pri-
     mary and 40 percent  W.A.S.


 Table 6-29.—Carter CBFP—Model 5/31, aerobically dig-
 ested extended  aeration sludge scituate,  Mass.
Test

1 	
2 	

Percent
dry solids
Feed Cake
2 18
3 16

Sludge
feed
(Ibs/dry
solids/hr)
88
255

Solids
capture,
percent
91
98

Polymer
cost
($/ton
dry solids)
a26
"11

   aCationic polymer A used.
   bCationic polymer B used.

-------
  2. Ultimate disposal by hauling to either a sanitary
     landfill, or to farmland, composting  or other horticul-
     tural use.
  3. Equilibrium sludge removal rate of 2.5 tons (2.3
     Mg) of dry solids per day required.

Alternate  Units for Consideration  or
Evaluation

  Any of the  twenty or so varieties of continuous  BFP's
available from 11  different companies. Depending on the
length of the  truck haul and the cake dryness require-
ments for final disposal the design  engineer would pre-
screen the many alternates  and select perhaps three
companies to work with in proving  specific devices and
carrying out bench and pilot scale  qualification
trials.
  For the purposes  of this example it  will be  assumed
that a dry solids content  cake of at least 28  percent is
required. Accordingly,  units  such as the  R. B. Carter
Series 31, Komline Sanderson  Unimat, Parkson Magnum
Press, Ashbrook-Simon Hartley Winklepress, and Carbo-
rundum  Sludge Belt Filter Press would certainly be con-
sidered. Certain models of the Tait Andritz,  Infilco De-
gremont Floe-Press and Passavant Vac-U-Press would
require at least preliminary consideration with  further
study dependent on estimates  of capabilities from  the
supplier firms.

Evaluation Procedure
  The systematic procedure for evaluation would be
identical to that described in the RVF design  example.

Bench Scale Tests
  Most of the equipment suppliers have  laboratory or
bench scale test equipment and procedures which indi-
cate general acceptability of their units.  In most cases,
unless the sludge  to be dewatered  is an unusually easy
one, pilot scale testing will yield  much more accurate
design criteria and should be pursued. Most companies
have mobile pilot or full size units.

Design Calculations
  1. Operating cycle to be 35 hours per week (7
     hours/day), permitting start-up and  wash  down
     times within 8 hour shift.
  2. One CBFP with adequate spare  parts to  be main-
     tained.
  3. Size of CBFP.—Production rate proves to be 50
     GPM (3.2 l/s) of 3-4 percent  feed  sludge giving
     rate of 228-305  Ib/hr/ft (341-455 kg/hr/m) width
     (from pilot test runs). Solids capture is an accept-
     able 93-98 percent in all  tests. Cake solids with
     complete press (all sections, including high pressure
     stage) in  use is 38 percent. Without high pressure
     section, cake solids are 30 percent. Polymer  dos-
     age is consistent. Design  Engineer must then as-
     sess added capital and  O/M costs  for high pres-
     sure section and  effect  of 8 percent drier cake on
    haulage costs to determine which unit is to be
    chosen. A single CBFP of two meter width would
    be adequate if several days sludge storage surge
    capacity were provided. Alternatively  2  one meter
    wide units could be chosen.
  4. Sizing  of auxiliary equipment.—Same  as described
    in RVF design example. If,  for example, a  Komline
    Sanderson  Unimat were the selected  unit, the basic
    machine is just under 24 feet (7.3 m) long, width
    requirement is 5 feet  2 inches (1.6 m)  at base with
    the upper drive motor making upper  width need  just
    under  8 feet (2.4 m).  Height of the Unimat is 10
    feet 2  inches (3.1 m). The  same considerations
    apply to selection of a suitable flocculant system,
    sizing  of conditioning  system and overall "Dewater-
    ing System  Considerations"  as noted in the RVF
    design example.
DESIGN  EXAMPLE—CONTINUOUS BELT
FILTER  PRESS—40  MGAL/D (1.75  mVs)
PLANT

Basic Assumptions

  1.  Anaerobically  digested mixture of primary and
     W.A.S. at 4 percent dry solids content,  60 percent
     primary and 40  percent W.A.S.
  2.  Ultimate disposal by either composting or incinera-
     tion, both systems requiring'a minimum  cake solids
     content of 30 percent.
  3.  The sludge removal rate to be an average of 25
     dry tons (22.7 Mg) of solids  per day.


Alternate Units  for  Consideration

  Same comment as  in 4 Mgal/d  (0.18 m3/s) example
preceding.


Evaluation  Procedure
  The same procedure as  described in the RVF design
example could be  used, except:
  1.  Determination of the calorific value of the dewater-
     ed cake produced in  pilot tests would  be essential
     for evaluating efficacy of  incineration and to  ensure
     whether or not autogenous incineration  would be
     achieved in burning periods (there is no such thing
     as totally autogenous incineration since  startup and
     shutdown procedures require fuel usage regardless
     of  cake characteristics). Nonetheless, self-sustaining
     combustion would at  least minimize fuel consump-
     tion.
  2.  Review of  the suitability for composting could be
     carried out with experts in that field.


Bench Scale and Pilot  Tests

  Same as  in 4 Mgal/d (0.18 m3/s) example.
                                              131

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Design  Calculations

  1.  Pilot  results show that  50 gal/min  (3.2 l/s) of 3-4
     percent sludge will yield a cake solids of 38  per-
     cent  at a production rate of 228-305 Ib/hr/ft (341-
     455 kg/hr/m) width,  with adequate 93-98 percent
     solids capture and usage of polymer at $10 per ton
     ($11/Mg) of dry solids.
  2.  Operating cycle.—To be  based on 3 shifts/day, 7
     days  per week and 22 hours/day unit  operating
     time  since incineration  requires continuous operation
     to  minimize fuel consumption.
  3.  Sizing of CBFP.—50,000  pounds/day (22,730  kg/
     day).

                  Meter               Daily production/unit
                  width                     (pounds)
                                                                                     FILTER CLOTHS
                                                                       FIXED END
                                            16,500
                                            33,000
                                            49,500

                                               two
     On  the above basis 4  one meter units or 2
     meter units would  be  chosen.
  4. Summation.—All other  facets of the design proce-
     dure would be  similar to the 4 Mgal/d (0.18 m3/s)
     RVF design example.

PRESSURE FILTERS

  The  original main focal point for the development of
the plate and  frame, and recessed chamber types of
pressure filters was Stoke-on-Trent,  United Kingdom.  The
slurriers  incident to the manufacture of pottery and china
are particularly difficult  to dewater and as a result pres-
sure filters were employed.
  These  types of pressure  filters,  particularly the re-
cessed chamber type have  been frequently designed into
the U.K.  wastewater treatment plant sludge dewatering
systems.
  A few  U.S. installations of pressure filters have also
been made in the  past  few years.
  Pressure filters are batch devices and to some extent
because  of the level of development of feed and chemi-
cal dosage systems  normally use substantial quantities of
metal salt and lime for  conditioning. These chemicals
require relatively extensive  handling  systems requiring
considerable maintenance. This is one of the factors
which  has slowed  acceptance of pressure  filters outside
the United Kingdom.
  Essentially, a pressure filter consists of  a series of
vertical plates, usually recessed, covered with cloths  to
support and contain the cake,  mounted in  a framework
consisting of head supports connected by  two heavy
horizontal and parallel bars  or  an overhead rail. Figure
6-23 shows a cross section of a  pressure filter.
  Conditioned  sludge is pumped into the pressure filter
at increasing pressure.  Presses are  normally supplied to
operate at either a nominal  100 lb/in.2g (7 kg/cm2)  or
225 lb/in.2g (16 kg/cm2). Cake building time or sludge
feed time is normally 20 to  30 minutes followed by a 1
to 4 hour pressing period.  The press is then opened
and the filter cake falls off  into the  removal system.

    132
                                                        SLUDGE IN
                                                                             FILTRATE DRAIN HOLES

                                                        Figure 6-23.—Cross section  of a  partial pressure filter.
                                                          While pressure filters will  generally produce a cake
                                                        solids content 10-20 percent  points drier than a rotary
                                                        vacuum filter,  some  portion  of these total cake solids
                                                        are lime and metal salt rather than sewage solids. Ca-
                                                        pacities of pressure  filters are usually about 10 to 20
                                                        percent of the loadings achieved on  rotary vacuum fil-
                                                        ters.
                                                          Significant developments in  Pressure Filter technology
                                                        are the diaphragm press  and  other membrane type
                                                        presses which are discussed  later.
                                                          Since an  excellent survey of three  operating U.S. in-
                                                        stallations was available,  a review of those case histories
                                                        is the most applicable way  to present a perspective on
                                                        conventional recessed chamber type  presses.

                                                        CASE  HISTORY—KENOSHA, WISCONSIN

                                                          This is a  26 Mgal/d (1.1  m3/s) plant with a primary
                                                        and  activated  sludge system.

                                                          1.  The sludges are mixed, gravity thickened, anaerobi-
                                                             cally digested,  and then  dewatered in  Nichols (Ed-
                                                             wards  &  Jones) pressure filters. The dewatered
                                                             cake is given to farmers who land spread from
                                                             manure spreaders.
                                                          2.  Chemical dosage is  3  percent ferric chloride  and
                                                             25 percent lime (both on a dry  solids sludge basis)
                                                          3.  Digested  sludge at 3-7 percent solids is  dosed in
                                                             line with  ferric  chloride and lime is added in  a
                                                             subsequent mix tank with slow speed  mixing.
                                                          4.  Two Moyno pumps feed  the two presses simulta-
                                                             neously.  The Moynos have worked very well. Fil-
                                                             trate is returned to head of plant.

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  5.  Cycle includes maintenance  of 100 lb/in.2g (7
     kg/cm2) for  30 minutes  and total cycle  time is
     2-1 /3-2-112 hours.  Operate 16 hours per day, 7
     days per week to produce 12 tons (10.9 Mg) per
     day of dry solids cake at 35-38 percent solids.
     Cake  thickness is one inch.
  6.  Two Nichols-Edwards & Jones pressure filters, with
     80—4 feet by 4 feet (1.2 m x 1.2  m) plates (rubber-
     coated steel) used.
  7.  One operator in continuous  attendance.

Results

  Table 6-30 shows the good handleable press cake
and  clear filtrate.

Problems

  High chemical dosage  and costs have been experi-
enced. Cake is actually about 25  percent added chemi-
cal so analysis is really about 65  percent water, 26
percent sewage sludge and 9 percent inorganic chemi-
cal.  Net  sludge production must be reduced  by 25 per-
cent to get actual figures.  Excessive  wear in cloths and
stay  bosses causes serious maintenance problems. Filter
cloths were replaced 3 times  in 2 years ($3,000 per
press per change).  Severe ammonia odor problems have
occurred in press room (effect of lime  and high pH).

Comment

  Despite problems noted above  there  have  no extensive
forced downtime  periods in the 2 years of operation.
Much of the chemical consumption might be eliminated if
the alkalinity of the digested  sludge were washed out in
a properly designed and operated elutriation system us-
ing  flocculants. Why use pressure filters when the  wet
cake is disposed  of on land  by a manure spreader?

BROOKFIELD,  WIS.

  This plant design includes  a primary  and activated
sludge system and  contact stabilization. Flow is 2
Mgal/d (0.09 m3/s). 80 percent Primary Sludge-I-20 per-
cent Secondary Sludge is mixed, pumped through  a
grinder, diluted with recycled  incinerator ash (0.5  Ib/lb
sludge),  conditioned with lime (15-18 percent) and Ferric
Chloride (5-7 percent), pressed and fed to  a 5 hearth
Table 6-30.—Costs—pressure filtration, Kenosha,  Wis.
                      Costs
$/ton
Labor	   $7.43
Chemicals	   20.17
Power	'.	    1.71
Maintenance	    3.25
     Total.
32.56
incinerator. 95 percent of incinerator  ash  is recycled.
The incineration is not autothermic and uses natural gas.
Pressure filters are standard  Passavant design  with forty-
six 52"  (1.3 m) diameter plates of steel and have been
operated for  1-1/2 years.

Results
  Plant  personnel  state that no major operating problems
have been encountered.  There have only  been two
"Sludge  Blowing Incidents"  in the 1-1/2 years of opera-
tion. Press cloths  have had to be replaced every 6
months  at a  cost of $3,600 per shot.  The press  cake,
which contains a large amount of inorganic conditioning
agents and recycled ash averages 45 percent  total sol-
ids. The press cake is only 30-40 percent volatile so
the ratio of water/sewage solids is quite  high.

Comments
  1.  The mixed sludge being  processed is a relatively
     easily dewaterable material which is  high  (80 per-
     cent) in  primary content  and high in fibrous materi-
     al.  Indeed the high  fiber content has caused prob-
     lems in  the press cake breaking  operation.
  2.  No  records are available on  natural  gas consump-
     tion and no cost data on the system have been
     made available.
  3.  The system appears to be  a complex high capital
     and high operating  and maintenance cost one
     which is difficult to rationalize, particularly at a
     plant with such an easily processable sludge.
  4.  The plant has two components of interest to other
     potential press  filter designs: the wet sludge grinder
     and the slow speed cake breaker.

Conclusions on U.S.  Results to Date
  Reference  16 from which the above results  came, is
an  excellent  review  of the current U.S. installations.
  The conclusions from  reference 9 are as follows:

  1. In  looking at the two types of presses, we  found
     some advantages with the  lower pressure design.
     Essentially, it is a much  simpler operation. The re-
     cycling  of incinerator  ash seemed to provide few
     benefits, particularly because  it  only complicated
     the operation with additional material handling
     equipment.
  2. In  general, we  found that filter presses are  an ac-
     ceptable method for dewatering sludge. Theoretical-
     ly, they  should always produce  an autocombustible
     sludge cake.  But, practically,  we know of no instal-
     lation anywhere that can achieve this. The ash re-
     circulation is  probably  the limiting factor.  (The  inor-
     ganic conditioning agents also contribute to the
     problem.)
  3. Filter presses seem to be quite  capable of  handling
     different sludge concentrations and different types
     of  sludge feed.  Proper conditioning,  especially  with
     lime, is  the key to good operation. Vacuum filters
     are not  quite so adaptable.
                                                                                                        133

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  4. The necessity of using high lime for conditioning
     could be a drawback. Lime handling is always diffi-
     cult.
  5. Prior to a large scale installation, pilot  plant work
     should always be performed to evaluate the dewa-
     tering characteristics and chemical requirements.
  6. Filter presses have  a  higher capital cost than vacu-
     um filters. The presses also usually have a  higher
     operational cost.  Their real advantage is in  greatly
     reducing the costs  of final  disposal for the  sludge
     cakes. A detailed economic analysis of the  total
     system is needed before deciding for or against
     filter presses.
POLYELECTROLYTE CONDITIONING FOR
PRESSURE FILTERS

  Due to the more prevalent previous incidence of the
use of filter  presses in continental Europe and the Unit-
ed Kingdom, and also due to innovative work there, the
successful use of certain polyelectrolytes in conditioning
sludges  for dewatering in pressure filters has been real-
ized  at a number of locations.
Farnham  Pollution Control Works,
Thames Water Authority, U.K.
  This  plant  is a primary and  trickling filter installation.
Humus sludge is recirculated to the primaries,  the  mixed
sludge  gravity thickened, and  then dewatered on two
filter presses. Operating pressures are 85-100 lb/in.2g
(6-7 kg/cm2).
  Initially the plant used aluminum chlorohydrate for
sludge  conditioning. Figure 6-24  is a flow diagram of
the dewatering system.
  The  Farnham plant experienced severe filter cloth
blinding problems and proceeded to carry out diagnostic
trails with various conditioning agents to rectify the
problem.  They found that  by converting the system to
use Allied Colliods Zetag  63 polyelectrolyte the cloth
blinding problems were alleviated sufficiently for the two
presses to cope with the  sludge  load. (See Table  6-31
for dewatering results.)

CASE HISTORY—THORNBURY STP, U.K.
  Reference 17 describes exhaustive test work on the
use of polymers for conditioning  sludge for dewatering
via recessed chamber pressure filters.
                                                                                     FILTRATE
                                                                                                 CAKE
                                                                                                 DISCHARGEE
                                                                                            FILTER PRESSES
   CHEMICAL
   STORAGE
   TANK
     CHEMICAL
  j   DILUTION
-h^TANK
                 i

              .j
                                                                                      FILTRATE
                                                                                                 CAKE
                                                                                                 DISCHARGEE
 KEY


  »  ALUMINIUM CHLOROHYDRATI
     BATCH CONDITIONING

-->-ZETAG 63 IN-LINE
     CONDITIONING
Figure 6-24.—Farnham Plant dewatering system.
     134

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Table 6-31.—Farnham dewatering results
Conditioning
agent
Aluminum chlorohydrate (batch) . . .
Aluminum chlorohydrate (in-line)...
Zetag 63 (batch)
Zetag 63 (in-line) . ..
Ferric chloride and lime (batch)...
Ferric chloride and lime (in-line)...
Dose
(% on
dry solids)
2.5
2.5
0.2-0.3
0.2-0.3
3
25
3
25
Cost
($/ton
dry solids)
22.00
22.00
6.70-10.10
6.70-10.10
14.80
14.80
CST range
during cycle
(seconds)
10-65
10-32
8-14
8-45
8-15
Pressing cycle
time range
(hours)
6-18
6-12
6-9
3-6
3-13
3-5
  aResults not available.
  By virtue of using  in-line conditioning and observing
logical procedures the results shown in table 6-32 were
achieved.
  The Thornbury works processes a mixture of 45 per-
cent primary  sludge  and 55 percent of  mixed sludges
from adjacent secondary treatment  plants. In addition to
illustrating successful use of polyelectrolytes, the article
delineates other significant facts relative to pressure filter
design.

MEMBRANE USE—PRESSURE FILTERS

  References 18 and 19 describe the successful upgrad-
ing of the production rate in conventional recessed
chamber pressure  filters by equipping same with alter-
nate "membrane"  plates.  This retrofitting process causes
each of the chambers formed between  the standard re-
cessed  plate and the membrane plate  to be subject to
the squeezing action of a  membrane at will during the
press cycle. The membrane plate is a  steel reinforced
rubber plate  in which the rubber membrane is inflatable
by air pressure. After the initial  filling period in a press
cycle, when the filtrate rate falls off, the sludge feed
Table 6-33.—Conventional versus membrane press
Severn Trent Water Authority18
Table 6-32.—Thornbury, U.K.—pressure
filtration
Conditioner
Aluminum chlorohydrate
Polyelectrolyte (Zetag 94) 	
dry solids
Feed
46
4.6
(Primary + secondary
Cake
38
37
sludge)
Conditioner
cost
($/ton
dry solids)
23.40
4.60
Press
cycle
(hrs.)
4.9
4.9
Type press
used
Conventional....
Membrane 	
Cycle
(minutes)
390
87
Cake
thickness
(inches)
1.25
0.7
Dry
solids
(percent)
28
27
Weight
(Ibs)
1,227
558
Output
(Ib/hr/
press)
186
385
 (Raw feed sludge—3.9% dry solids—2.0% alum chlorohydrate cond.)
pump is stopped  and the  membrane inflated to give a
pressure  up to 150 lb/in.2(10.5  kg/cm2) to squeeze the
partially formed cake and  obtain quick dewatering.
  As can  be seen in table 6-33 following, though a
thinner cake results, the overall  filtration cycle is so
much shorter that the total throughput doubles or even
triples in some cases.
  The suppliers of the rubber membrane plates claim
that  new installations of the membrane type unit are less
expensive overall due to the  increased capacity of  the
membrane  units.
  A somewhat analogous  but  different type of variable
volume pressure filter is described in the following  sec-
tion.

DIAPHRAGM  TYPE  PRESSURE FILTERS

  As described in reference 20,  a new type of pressure
filter,  employing  flexible rubber diaphragms between the
chambers of a pressure filter, has recently been intro-
duced into the United States. This type device was de-
veloped in Japan and there are  several operating instal-
lations there.
  At least  two versions of this new type of  pressure
filter have  been  tested and are  available in the  United
                                                                                                       135

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States. The earliest one  was supplied  by NGK Insulators
Ltd., of Nagoya who  have now licensed Envirex  division
of Rexnord  for U.S. sale of their device. Ingersoll Rand
has the U.S. rights to the  Lasta automatic diaphragm
pressure filter. There  are indications that Dart  Industries
and Industrial Filters OMD of  Chicago have devices
based  on similar principles.
  Figure 6-25 is a diagram of the  I.  R.  Lasta  press  that
illustrates the operating  principles.
  As will be noted in figure 6-25  the  feed slurry enters
the top of the chamber  between the  filter cloths and
gradually fills the chamber. After a cake is formed the
diaphragm is expanded by water under  pressure to 250
lb/in.2g (17.6 kg/cm2) which  squeezes and  dewaters the
cake. The  filter  plates are then automatically opened and
the cake discharged.  Cloth washing ensues before an-
other pressing cycle.
  It is  claimed that the length of the cycle is shorter
than for conventional presses because of the improved
control of the relationship  between cake formation and
pressure build-up.
  Table 6-34 lists dimensional data on the I. R.  Lasta
press.
  The  most detailed report on these  devices is Refer-
ence 20 which describes the  extensive pilot work at
Blue Plains with the Envirex-NGK  Locke diaphragm
                      press. This Envirex unit is highly automated  and in  wort
                      at Blue  Plains (mixture of primary and W.A.S.  sludges),
                      it produced  a 40 percent total dry solids cake using 20
                      percent  lime  and 10 percent ferric chloride dosages. Th
                      only problem is  that when the  correction is  made for th
                      inorganic conditioning solids present in the dewatered
                      cake, the percentage of  dry sewage solids in  the cake
                      relative  to water content is only about 28 percent.
                        This new type pressure filter does offer much im-
                      proved capabilities  over conventional pressure filters for
                      extremely difficult to dewater sludges.  Pricing  figures
                      available indicate that the  units will be priced  about
                      eight times the price of a  conventional pressure filter,  si
                      the need must be clear and obvious.


                      CENTRIFUGES  FOR DEWATERING
                        Horizontal  solid  bowl decanter type  centrifuges have
                      been  used for wastewater sludge dewatering for a  num-
                      ber of years. They were popular for primary sludges wit
                      low grit content in  coastal resort  areas  with large  swing
                      in loadings because of ease of operation, quick startup
                      and shutdown and  ease  of odor control. Attempts to
                      adapt these  relatively high speed  devices (g forces of
                      1000 + ) to heavy  duty operation in large  cities or for
                      use with mixed  sludges containing significant quantities
          FILTERING I
                                 COMPRESSION II
                                                            CAKE DISCHARGE
                                                       WASHING OF
                                                       FILTER CLOTHS IV
 FILERING
 CHAMBER
PRESSATE
           DIAPHRAGM
           HIGH
           PRESSURE
           WATER
          O     O
o    o
o
                                                                                              SHOWERS
Figure 6-25.—I. R. Lasta diaphragm pressure filter.
     136

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Table 6-34.—I. R. Lasta automatic  filtering press

600 IT
800 FT
1,000
1,250
1,500
Size of
filtering
plate
im (24")
im (32") ..
mm (40") 	
mm (50")
mm (60") 	

Number of
filtering
chambers
8
14
20
14
20
26
20
26
32
26
32
38
32
38
44
"area9 He"ht Length
m2
4
7
10
13
19
24
30
39
48
62
77
91
112
113
154
ft2 mm
43 2,050
75
108
140 2,485
204
258
323 2,845
420
516
667 3,200
829
979
1,205 3,620
1,431
1,657
ft. mm
7 2,660
3,650
4,640
8 3,490
4,930
5,920
9 5,240
6,230
7,220
10 6,555
7,545
8,535
12 8,205
9.225
10,245
ft.
9
12
15
13
16
19
17
20
24
22
25
28
27
30
34
Width
mm ft.
1,610 5
1,800 8
2,100 7
2,600 9
3,050 10
of biomass were previously plagued by two problems:

  1.  Erosion of the surfaces exposed  to high speed im-
     pingement of  abrasive materials caused mainte-
     nance problems.
  2.  Prior to the development  of polyelectrolytes capable
     of  providing a reasonable clean centrate and avoid-
     ing serious fines  recirculation  problems, solids cap-
     ture was  inadequate.

  In  the past 5 years or so six steps  were taken which
have helped this type  device gain a wider use:
  1.  Development and use  of new  high molecular weight
     cationic shear  resistant polyelectrolytes.
     Use of lower  rotational speeds to reduce turbu-
2.

3.
     lence, power costs, and erosion wear problems.
     Use of a concurrent flow pattern for sludge and
     centrate to minimize turbulence.
  4.  Adjustable variation of speed differential  between
     the bowl and the sludge removal scroll.
  5.  Use of longer bowls with smaller diameters.
  6.  Provision of extremely large units at plants with
     large sludge removal  needs producing  an economy
     of scale.
  Various  manufacturers have combined some of the
above features in their newer models. This  resulted  in a
surge of popularity about 4 years ago. Since  the energy
crisis the degree of popularity of centrifuges,  even  with
the above mentioned improvements,  has slackened  be-
cause of energy costs.
  Once again, the pioneering development work on
these devices was carried out primarily in West Ger-
many. The most practical description of these develop-
ments is contained in references 21 and  22 which are
excerpted in the following section.


CASE HISTORY—CENTRIFUGAL DEWA-
TERING—WUPPERTAL-BUCHENHOFEN,
GERMANY

  Reference 21 is a  comprehensive article relating re-
sults obtained  at Wuppertal-Buchenhofen  plant with  a
low speed concurrent flow type unit. This is a combined
municipal-industrial treatment plant treating 1,200,000
population equivalent. After primary and biological treat-
ment the mixed sludges are  thickened to 3-4 percent
and anaerobically digested, followed by sludge settle-
ment and decantation,  thence dewatering.
  After initial trial work the authority asked for competi-
tive tenders from various suppliers of centrifuges with
performance requirements as follows:

  1.  Capacity  of each  centrifuge:  40-60 rrvVhour  of
     sludge with feed of  2.5-3 percent dry solids.
  2.  Minimum  cake solids: 20 percent.
  3.  Centrate  maximum suspended solids of 0.2 percent.
  4.  Maximum polyelectrolyte dosage permissible of 3.6
     kg/Mg of dry solids (100 gm/m3).
  5.  Maximum permissible power consumption of 1 kWh
     per cubic meter of sludge  feed including ancillary
     equipment such as pumps, flocculant metering  sta-
     tions, etc.
  6.  Guaranteed life  of screw conveyor = 10,000 hours.
  7.  Provision  of a package plant  with a minimum ca-
     pacity of  40 m3/h for a 4-month  trial period  under
     a  leasing  agreement.
                                                                                                     137

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Table 6-35.—Effect of  speed differential  on throughput
and  dry solids
    Speed differential
Table 6-36.—Side  by side comparison—process results
Flocculant dosage (g/m3)
Dry solids carried by dis-
charge, percent 	
Dry solids carried by cen-
trate (undissolved solids)
Ideal throughput (m3/h)....
60

26

0.35
33
80

285

0.25
37
60

24

017
43
80

23

0.07
45
60

205

0.12
40
80

20

0.07
48
  KHD Industrieanlagen AG Humboldt-Wedag of  Cologne
(U.S.  Licensee—Bird Machine) won  the  contract and
initially installed two S3-2 type low speed concurrent
flow centrifuges with capacities of 20-30 m3/h each.
These units met the agreed performance guarantees but
when the full  civil installation was completed they were
replaced, as planned, by  two  of the larger  S4-1  units (of
the same basic type) but with  capacities of 40-60 mVti
each.
  Power consumption for the  complete dewatering plant
was 0.9-0.95  kWh/m3 with S3-2  units  and improved to
0.75-0.8 with  the  larger S4-1  units.  Disage of Zetag 92
polymer  (Allied Colloids) averaged 60-80 gm/m3.
  The article contains much data on the effect of centri-
fuge dewatering variations on  overall process perform-
ance  and sludge disposal costs.
  A significant factor studied was that of the effect of
the differential in speed between the scroll  and the bowl
(see table  6-35).
  As  can be seen in table 6-35, a 28.5 percent dewa-
tered cake at a reasonable throughput of 37 nrrVhour
and centrate suspended solids of 0.25 percent can  be
obtained with  flocculant dosage of 80 g/m3 by using a
speed differential of 2 instead  of 6.
  The paper claims  and purports to show that very large
capacity  centrifuges of  the improved  low speed-concur-
rent flow type, when operated  in a lower differential
speed mode can offer significant capital and O/M cost
savings where  large volumes of sludge are  to be proc-
essed.
  Unit costs are given as follows:

    Operating—Deutsche  mark 36.40/ton (DM
      40.12/Mg) dry solids
    Annual Capital—Deutsche  mark  47.60/ton (DM
      52.477 Mg) dry solids

CASE  HISTORY—CENTRIFUGAL DEWA-
TERING—STOCKHOLM, SWEDEN

  Stockholm has operated three high  speed centrifuges
for  a  3 year period  and also has operated  a new low
speed concurrent  flow unit on  the same sludge for
1-1/2 years.
  Table  6-36  shows the results obtained with the two
different  types of centrifuge.
                                                                Centrifuge design
                                                                sludge identification
                               Anaerobically digested primary
                                 plus waste activated with
                                      alum sludge
                                                                                         Low speed
                                              High speed
No. of operation units 	
Flow rate per unit 	
Percent feed consistency
Percent cake solids
Percent solids recovery
Polymer type . .
Polymer dosage

1
190 gal/min
3
16-18
95-98
Allied Colloids
Cationic
6 Ibs/ton

3
90 gal/min
3
16-18
95-98
Percol # 72f
12 Ibs/ton

  While table  6-36 only shows the improvement realized
by reduction in polyelectrolyte costs by  about $9/ton
($10/Mg) (which  is a considerable savings), table 6-37
illustrates the  additional advantages for the low speed
design.
  Wear played an important  part in displacing the high
speed  centrifuges in favor of the low speed centrifuges
at this particular  plant.  The low speed centrifuge was
inspected after 2,000 hours of operation and found to
have only 1/18 of the wear  of the high speed  alterna-
tive.  The abrasive protection on the low speed machine
conveyor blades  is tungsten  carbide, while  the  protectior
on the high speed  machine is equivalent to an alloy
called  Stellite  1016.  The Stellite material is  considered
inferior to the tungsten  carbide  hardness values ap-
proach Rc-69. Experience shows that  if both materials
had  been similar  that the wear rate would  still  have
favored the low speed design by as much  as a five to
one  ratio.
  Summarized in  table 6-38  is the annual  cost analysis
of the  operation  of these two types of centrifuges in-
stalled side by side. The  low speed unit clearly has the


Table 6-37.—Side  by side comparison machine parame-
ters
        Centrifuge design
Low speed
High speed
Bowl diameter	   36"
Bowl length	    96"
Centrifugal force	   511 x G
Unit flow rate	   190 gal/min
Unit pool volume	   196 gallons
Sigma factor	   1.15X107 cm2
Unit motor size rating	    100 hp
Absorbed horsepower	   0.3 gal/min
Noise level at 3 ft	   80-85 dBa
Wear at 2,000-hour inspection	   1/2 mm
            25"
            90"
            1,878 x G
            90 gal/min
            73 gallons
            5.3 X107 cm
            180 hp
            0.6 gal/min
            95-100 dBa
            9 mm
    138

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Table 6-38.—Side  by side comparison annual cost—
profile
Table 6-40.—Basket centrifuge operation—Burlington,
Wis., WWTP
Centrifuge design
Tons/year per unit 	
Power expenditure 	
Polymer expenditure 	
Maintenance expenditure 	
Amortized equipment 	
Total annual cost 	

Low speed
	 12,483
	 $0.06/ton
	 $9.00/ton
	 $1 .21 /ton
	 $1 .50/ton
	 $12.33/ton

High speed
5.913
$1.19/ton
$16.00/ton
$8.30/ton
$2.44/ton
$27.93/ton
Feed rate (gal/min)
Dewater rate (Ibs D.S./hr) 	
Hours required/week 	







23
	 104
	 168
17R
147
0
R— ft f\\ T ^
^0
f\9

88
397
44
99
48
30
13-15 (T)
14
47
Table 6-39.—Dimensional data—low speed centrifuge
Model No.
HB 2500
HB 3700 	
HB 6400

Overall
length
(in.)
138
	 139
276

Overall
width
(in.)
80
72
150

Overall
height
(in.)
36
41
71

Weight
(Ibs)
6500
9,400
3440

edge  in all categories. Power consumptions are one-half
(1/2)  that of high speed unit. With respect to polymer
consumption, the low speed  centrifuge  in this particular
case  utilized  44 percent less cationic polymer than the
high speed centrifuge. With respect to  conveyor mainte-
nance, we  have modified the high speed centrifuge fig-
ure to reflect a ratio  of  conveyor resurfacings more in
the category of 5 to  1  than  the  18 to  one margin
indicated by  the actual side  by side installation. The
category entitled "Amortized Equipment" includes the
cost of centrifuge, the motor, and the starter, and is
expressed on a tonnage basis  and reflects an amortiza-
tion rate of 7 percent interest  over  a 20-year period.
Electrical usage rate  was assumed to  be 0.02/kWh and
polymer (Allied Colloids Percol  728) was figured at
$1.50/lb ($3.30/kg).
  While the larger size of the low speed unit would
account for a minor portion of the above noted superior-
ity, it  is abundantly clear that the lower speed concur-
rent flow unit is superior from  a  cost-effectiveness stand-
point.

Dimensional Data—Centrifuges
  Table 6-39 shows dimensional  data  for one brand of
the newer low  speed centrifugals.

CASE HISTORY—CENTRIFUGAL DEWA-
TERING—BURLINGTON,  WIS., WWTP
  The experiences at Burlington  are described (in an
outstanding fashion) in reference  23.
  The Burlington plant treats an  average flow of com-
bined municipal and industrial wastes at DWF  level  of
1.5 Mgal/d  (0.06 m3/s) and  a wet weather flow of  2.0
Mgal/d (0.09  m3/s).
  The  treatment plant  employs contact stabilization  (12
hour aeration  time, 25 percent return rate,  MLSS of
2000 mg/l). The F/M  ratio is 0.2 to 0.5. A sludge age
varying from 5 to 12 days is employed, including aera-
tion and aerobic digestion time.
  The  above described liquid treatment system results in
sludge disposal requirements of 160,000 gallons (606 m3)
of W.A.S. per week or 3400 pounds (1545 kg) per  day,
about 27,000  gallons/day (102 mVday).
  The  plant was designed for ultimate liquid sludge dis-
posal by lagoon. When this disposal option was cur-
tailed,  dewatering studies ensued. Needless to say,  the
sludge dewatering problems  are significant. It is a clas-
sic example of the problems which result when a plant's
liquid treatment system is designed for liquid sludge dis-
posal and then  dewatering is required.
  A batch, cycling, basket centrifuge was  tested, pur-
chased, installed and has been operated for some time.
The essence of the  results of the full scale performance
is  listed in table 6-40.
  As can  be seen, despite the high polymer cost, the
overall cost analysis showed the  total  operation  to be
more cost effective with  polymer use.
  It should be noted that if the city could start again
from square one, it  is  certain that, now having to dewa-
ter sludge, and  knowing  the  overall energy costs of the
type  total system involved, a different  liquid treatment
system would  be chosen.
  Additional valuable insights  in the referenced paper
relate to the correlations  between activated sludge sys-
tem operating parameters and resulting sludge processa-
bility.

REFERENCES
 1. Process Design Manual Sludge Treatment and Disposal, Technolo-
   gy Transfer,  U.S. EPA,  Washington, D.C. (1974).
 2. Camp Dresser & McKee, Report No. PB-255-769, NTIS, Spring-
   field, Va. June 1976.
 3. Jones, Edgar R., P.  E., "Sludge Production  Rates, District of
   Columbia," Ecoletter of Chesapeake WPCF and Water and Waste
   Operators of Md., Del., and D.C., vol. 3, No. 2, p. 4., Spring
   1977.
 4. Gale, R. S.,  "Recent Research on Sludge Dewatering." Filtr. Se-
   par. (September-October,  1971), pp. 531-538.
                                                                                                        139

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 5.  Corrie, K. D.,  "Use of Activated Carbon  in the Treatment of  Heat-    15.
    Treatment Plant Liquor," Water Pollution Control (U.K.)  1972, pp.
    629-635.
 6.  Stack, V. T., Jr., Marks,  P. J., and Garvey,  B.  T.,  "Pressure          16.
    Cooking of Activated Sludge," paper  by  Roy F. Weston, Inc.
 7.  Reports by Greeley & Hansen to the  city of Tampa.                 17.
 8.  Crockford,  J.  B., Sr., and Sparham, V., "Developments to Upgrade
    Settlement  Tank Performance, Screening,  and  Sludge Dewatering
    Associated with Industrial Water Treatment," Purdue Industrial
    Waste Conference,  May  1975, pp. 1072-1083.                       18.
 9.  Personal Communication, Dr. Dan  Swett  and Mr. Mike Riise of G.
    C. One Ltd., Suite 605, 2700 N.E. 135th  St., North Miami, Fla.
    33181.                                                            19.
10.  Bell, J. A., Higgins,  R., and Mason, Donald  G., "Dewatering, a
    New Method Bows," Water and Wastes Engineering,  April 1977,
    pp. 33-41.
11.  Eichmann,  Bruce W., "Dewatering Machine  Solves Sludge Drying     20.
    Problems," Water and Sewage Works,  October 1977, pp.  99-100.
12.  Creek, John, "Tait Andritz SDM Sludge Dewatering Machine,"        21.
    WWEMA Conference paper, April 20, 1977,  Altanta, Ga.
13.  Keener, Phillip M., and Metzger, Larry R., "Startup and Operating    22.
    Experience With a Twin  Wire Moving-belt Press for Primary
    Sludge," vol.  60, No. 9,  September 1977, TAPPI, pp. 120-123.       23.
14.  Grove, G. W.,  Exxon Research &  Engineering,  "Use Gravity  Belt
    Filtration for Sludge Disposal,"  Hydrocarbon Processing, May 1975.
Department of the Environment,  U.  K., Directorate General Water
Engineering, R  & D Division,  Project Report No. 4, Sewage
Sludge Dewatering by Filter Belt Press.
Cassel, A. F., "Review of U.S. Filter Press Operations," paper
presented at Chesapeake Section, WPCF, June 1976.
White,  M. J. D., and Baskerville, R. C., "Full Scale Trials of
Polyelectrolytes for Conditioning  of  Sewage  Sludges for Filter
Pressing," Journal of Institute of Water Pollution Control,  No. 5,
1974.
Heaton, H. M.,  "The Practical Application of the Membrane Filter
Plate to Increase Filter Press Productivity and Overall  Economics
Filtech 77, September 20-22, 1977, Olympia, London.
White,  M. J. D., Bruce,  A. M., and  Baskerville, R.  C.,  "Mechanic:
Dewatering of Municipal  Sludge  in the U.K.—Laboratory to  Full
Scale," presented at conference, Theory,  Practice,  and Process
Principles for Physical Separations,  California,  10/10 to 11/4/77.
Cassel, A. F., "Update on Filter  Press Operations," paper presen
ed at Chesapeake Section, WPCF,  June 1977.
Reimann, D.,  Kommunalwirtschaft, No.  9, September 1974, pp.
343-352.
Guidi, E. J., "Why Low Speed Centrifugation," Presented at Ohio
WPCF, Columbus, June 16, 1976.
Pietila, K. A., and Zacharias, D.  R., "Full  Scale Study  of  Sludge
Processing and Land Disposal Utilizing Centrifugation for  Dewater
ing," Paper presented at  Central States WPCF, May 1977.
      140
                                                                                 - U. S. GOVERNMENT PRINTING OFFICE. 1978-760-566/8 Region No. 5-11

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