EPA-R2-73-231
JUNE 1973                Environmental Protection Technology Series
     Pressure Filtration of Waste Water
     Sludge with Ash Filter Aid
                                 \
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                                  Office of Research and Monitoring
                                  U.S. Environmental Protection Agency
                                  Washington, D.C. 20460

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            RESEARCH REPORTING SERIES
Research reports of the  Office  of  Research  and
Monitoring,  Environmental Protection Agency, have
been grouped into five series.  These  five  broad
categories  were established to facilitate further
development  and  application   of   environmental
technology.   Elimination  of traditional grouping
was  consciously  planned  to  foster   technology
transfer   and  a  maximum  interface  in  related
fields.  The five series are:

   1.  Environmental Health Effects Research
   2.  Environmental Protection Technology
   3.  Ecological Research
   U.  Environmental Monitoring
   5.  Socioeconomic Environmental Studies

This report has been assigned to the ENVIRONMENTAL
PROTECTION   TECHNOLOGY   series.    This   series
describes   research   performed  to  develop  and
demonstrate   instrumentation,    equipment    and
methodology  to  repair  or  prevent environmental
degradation from point and  non-point  sources  of
pollution.  This work provides the new or improved
technology  required for the control and treatment
of pollution sources to meet environmental quality
standards.

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                                             EPA-R2-73-231
                                             June  1973
  PRESSURE FILTRATION OF WASTE WATER SLUDGE
                WITH ASH FILTER AID
                        by

              James W. Gerlich, P.E.
                        and

                M.  Daniel Rockwell
              Assistant Superintendent
            Water Pollution Control Plant
             City of Cedar Rapids, Iowa
               Program No. 11060 EZX
                   Project Officer

               Ralph G.  Christensen
       U.S. Environmental Protection Agency
               1 North Wacker Drive
               Chicago, Illinois 60606
                       for the

     OFFICE OF RESEARCH AND MONITORING
    U. S. ENVIRONMENTAL PROTECTION AGENCY
            WASHINGTON, D. C.  20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
             Price $2.10 domestic postpaid or $1.75 QFO Bookstore

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               EPA REVIEW NOTICE
This report has been reviewed by the Environmental
Protection Agency and approved for publication.
Approval does not signify that the contents necessarily
reflect the views and policies of the Environmental
Protection Agency, nor does the mention of trade names
or commercial products constitute endorsement or
recommendation for use.
                       ii

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                             ABSTRACT

Cedar Rapids, Iowa used pilot plant studies as an effective approach to
an economic  solution to dewatering secondary digested sludge. After
piloting several dewatering processes, the pressure filter system was
selected and a full scale plant was constructed as the first major instal-
lation in the  United States.  During the course of the dewatering studies
it was observed that fly ash was an effective filter aid which cut  chemi-
cal conditioning costs from about $20 per ton to $4 per ton dry solids.

The full scale plant was designed to utilize sludge ash from incinerated
filter cake, as well as power plant fly ash. The  design of this facility
was based upon data collected in the operation of the pilot plant, which
data at that time represented nearly all of the  analytical data available
on the operation of the pressure filter in the United States.  The design
capacity is 28 tons of dry sewage solids for 16 hour operation at 48 per-
cent dry  solids cake.

Performance data from the full scale plant was evaluated over a period of
approximately nine months.  This data indicates  that the full  scale plant
is capable of operation at a greater capacity and  efficiency than that pro-
jected from pilot plant data.   Both fly ash and sludge ash were evaluated
as a filter aid, with and without chemicals.  Economic evaluations were
made of operation  and equipment.

This report was  submitted in fulfillment of Project No. 11060 EZX  under
the partial sponsorship of the office of Research  and Monitoring,  United
States Environmental Protection  Agency.
                                iii

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                        CONTENTS

SECTION                                              PAGE

  I       CONCLUSIONS                                1

  II      RECOMMENDATIONS                           3

  III      BACKGROUND AND INTRODUCTION               5

  IV      VACUUM FILTRATION PILOT PLANT
          STUDIES                                     9

  V      PRESSURE FILTRATION OF SECONDARY
          DIGESTED SLUDGE WITH FLY ASH
          FILTER AID                                   29

  VI      BASIS OF DESIGN OF FULL SCALE PRESSURE
          FILTER DEWATERING FACILITIES                 55

  VII      FULL SCALE PRESSURE FILTER DEWATERING
          FACILITY                                    69

  VIII     EVALUATION METHODS                         83

  IX      RESULTS                                    95

  X      PROCESS EVALUATION                        137

  XI      ACKNOWLEDGMENTS                         151

  XII      GLOSSARY OF TERMS                         153

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                              FIGURES

Figure No.                                                  Page No.

    1     Vacuum Filter Leaf Tests - Correlation of the            11
          Form Filtration Rate vs. Cake  Formation Time

    2     Vacuum Filter Leaf Tests - Form Filtrate Rate            12
          vs. Form Time

    3     Vacuum Filter Leaf Tests - Full Scale Filtration          12
          Rate vs. Cycle Time

    4     Vacuum Filter Leaf Tests - Full Scale Filtrate            13
          Rate Curve

    5     Vacuum Filter Leaf Tests - Cake Moisture vs.           14
          Correlating Factor

    6     Pilot Scale Vacuum Filter                               16

    7     Flow Schematic Pilot Scale Vacuum Filter               17

    8     Test Results Pilot Scale Vacuum Filter                  24
          1/1 Fly Ash to Sludge Ratio

    9     Test Results Pilot Scale Vacuum Filter                  25
          1.4/1 Fly Ash to  Sludge Ratio

   10     Test Results Pilot Scale Vacuum Filter                  26
          Filtration Rate vs. Fly Ash to Sludge Ratio

   11     Test Results Pilot Scale Vacuum Filter Cake             27
          Moisture vs.  Fly  Ash to Sludge Ratio

   12     Laboratory Model Pressure Filter and Accessories        30

   13     Laboratory Model Pressure Filter                       31

   14     Removing Cake from Laboratory Model Pressure          32
          Filter

   15     6" Diameter Filter Cake                                32
                               vii

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Figure No.                                                 Page No.

   16     Filtrate from Laboratory Model Pressure Filter           33

   17     Schematic Flow Diagram Pilot Plant Pressure            36
          Filter

   18     Typical Filtrate Rate vs. Filter Time Curve              43

   19     Typical Filtrate Suspended Solids vs. Filter             44
          Time Curve

   20     Sample Data Sheet for Pressure Filter                   47
          Performance

   21     Filtration Rate vs. Filter Time for 4.5%                 48
          Sludge Solids

   22     Filtration Rate vs. Filter Time for 5.0%                 49
          Sludge Solids

   23     Filtration Rate vs. Filter Time for 5.5%                 50
          Sludge Solids

   24     Filtration Rate vs. Fly Ash/Sludge Ratios                51
          for 4.5% Sludge Solids

   25     Filtration Rate vs. Fly Ash/Sludge Ratios for            51
          5.5% Sludge Solids

   26     Percent Cake Moisture vs. Filtration Time for           52
          4.5% Sludge Solids

   27     Percent Cake Moisture vs. Filtration Time for           53
          5.5% Sludge Solids

   28     Aerial Photograph of the Water Pollution                 70
          Control Plant

   29     Section View of Pressure Filter Plate                    75

   30     Process Schematic and Plant Photographs                78

   31     Photograph of Specific Resistance Meter                89

                             viii

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Figure No.                                                Page No.

   32     Diagram of Specific Resistance Meter                 89

   33     Resistance Meter Test Example                       93

   34     Sludge Ash Particle Size - Coulter Counter            100
          Model T

   35     Fly Ash and Sludge Ash Particle Size Analysis         103
          Bahco Analyzer

   36     Comparison of Sludge Ash vs. Fly Ash                106
          Digested Sludge

   37     Comparison of Sludge Ash vs. Fly Ash                106

   38     Comparison of Sludge Ash vs. Fly Ash                107

   39     Comparison of Sludge Ash vs. Fly Ash,               107
          Raw Sludge

   40     Comparison of Sludge Ash vs. Fly Ash,               108
          Digested Sludge Domestic Wastes

   41     Comparison Photographs of Fly Ash  and               110
          Sludge Ash 200X

   42     Comparison Photographs of Fly Ash  and               111
          Sludge Ash 500X

   43     Comparison Photographs of Fly Ash  and               112
          Sludge Ash 1000X

   44     Ash and Chemical Conditioning Chart                116

   45     Ash/Sludge Ratio Conditioning Chart                 118

   46     Chemical Costs vs. Percent Feed Solids              118

   47     Filter Performance Curves 2.5% Solids               119

   48     Filter Performance Curves 3.0% Solids               119
                                ix

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Figure No.                                                Page No,

   49     Filter Performance Curves 3.5% Solids                120

   50     Filter Performance Curves 4.0% Solids                120

   51     Filter Performance Curves 4.5% Solids                121

   52     Filter Performance Curves 5.0% Solids                121

   53     Filter Performance Curves 5.5% Solids                122

   54     Filter Performance Curves 6.0% Solids                122

   55     Filter Performance Curves 6.5% Solids                123

   56     Filter Performance Curves, Composite                123
          2.5 to 5.5% Solids

   57     Typical Filter Pressure Chart                         127

   58     Sludge Cake Densities vs. Feed Solids and           128
          Cycle Time

   59    Phosphate in Filtrate                                130

   60    Yield vs. Cycle Time - Raw Primary Sludge            136

   61    Process Costs 4j% Solids                           140

   62    Process Costs 5|% Solids                           140

   63    Process Costs 6j% Solids                           141

   64    Chemical Costs, Comparison of                      143
         Recommended and Actual
                              x

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                             TABLES

Table No.                                                 Page No,

   I       Typical Vacuum Filter Leaf Test Results                10
          Test No.  3

   II      Average Characteristics Unconditioned Sludge          96

   III     Specific Resistance at 1012 cm'2                      97

   IV      Comparative Ash Particle Size at Various Plants       101

   V       Chemical  Analysis of Sludge Ash                     101

   VI      Classification of Fly Ash Particles                   103

   VII     Chemical  Analysis of Fly Ash                        104

   VIII    Comparison of Chemical Analysis of                 109
          Fly Ash and Sludge Ash

   IX      Average Filtrate Quality                             125

   X       Phosphate Removal with Fly Ash                      129
          and Sludge Ash

   XI      Phosphate Removals by Pressure  Filtration            131
          and Chemicals

   XII     Summary of Raw Sludge Filtration Data                134

   XIII    Comparative Values Dewatering Raw                 135
          and Digested Sludge

   XIV     Process Costs                                      141
                              xi

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                            SECTION I

                          CONCLUSIONS

1.  Pressure filtration of waste water sludges is an effective and eco-
nomical process.

2.  Ash filter aid increases dewatering production and decreases chemical
costs.

3.  Sludge ash available from incineration of sewage solids can be recy-
cled to the process as an effective filter aid.

4.  Fly ash from a coal burning power plant is a more effective ash ad-
mixture than is sludge ash, however, haul and ultimate disposal is an
additional inconvenience,

5.  A detailed pressure filter pilot plant program is of great value in
preparing for the process design.  Good pilot plant data is reproducible
in a full scale plant which has been confirmed  by this report.  This data
may also serve as an operating aid for trouble-shooting problems that may
occur during operation of the full scale plant.

6.  Some chemicals in combination with ash filter aid further improve
dewatering efficiencies at  Cedar Rapids.  Chemicals alone without ash
filter aid are relatively ineffective in the  range of $20 per ton dry solids
versus $4 to $6 per ton in combination with ash at design conditions.

7.  The full scale  plant performed at greater capacity and efficiency than
was projected from the pilot plant  studies.

8.  The pressure filter is very effective in dewatering raw primary sludges,
This program is based upon secondary digested sludges, however, con-
siderable experimentation was done with raw primary sludge at the
termination of the  basic program.  More experimentation should be done
with raw primary,  as well as raw secondary sludge dewatering,  by an
extension of this program.

9.  Filtrate from the pressure filter is low in suspended solids and phos-
phates and can be  recycled without creating a significant load to the
treatment process.

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                            SECTION II

                       RECOMMENDATIONS

I.  This program covered dewatering digested solids.  An extracurricular
investigation of raw primary sludge was conducted which consisted of
extensive bench work and 60 pressure filter runs. The results were  ex-
tremely encouraging. A more detailed investigation of the performance of
the pressure filter on raw sludge should be undertaken.  This investiga-
tion should include raw primary, raw  secondary and combined raw primary
and secondary.

2.  The physical and chemical mechanism of ash should  be further studied.
Some fly ash works much better  than others.  In some cases  fly ash  is a
better conditioner  than sludge ash, while in other studies, the exact
opposite has been reported.

3.  An  optimization of sludge thickening as this relates to sludge de-
watering in a pressure filter should be undertaken.

4.  An  expanded investigation of polymer sludge conditioning should be
made to better define the limitations under pressure filtration.  Consider-
able bench work was done at Cedar Rapids using many polymers.  Some
showed satisfactory sludge conditioning characteristics  in preparation
for filtration, but failed under actual  pressure filtration.  Favorable  per-
formance was experienced with polymer C-7 on the vacuum filter, as an
example, but unfavorable experiences resulted when applied to the pressure
filter.  Apparently polymer experiences related to low differential pressure
vacuum filters are not applicable to high differential pressure filters.

5.  Some field work is necessary to measure the true demand of the
pressure filter.  Experimentation at Cedar Rapids would indicate that
the true demand of the filter, particularly during early cake formation
is not well understood.  It has been reported that some European pressure
filter processes slowly apply pressure and during initial cake formation
limit the rate of conditioned sludge applied.  Experience at Cedar Rapids
indicated that better filter performance could be expected with extremely
high initial rate of application.

6.  A better definition of the value of precoat would be of considerable
benefit as well as an evaluation of materials other than ash.  The effect
of quantity and quality of precoat material, along with the effect of
pressure should be studied.  The possibility of no precoat should also
be explored in more detail.

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7.   Deterioration of sludge conditioning in both the mix tank and the
contact tank could  have a direct relationship to filter performance.
Some earlier bench work at Cedar Rapids indicated a sludge deterioration
curve starting about 4  to 6 hours after conditioning,  however, later bench
work could not confirm this for the  actual filter operation.  It is believed
this work could be  directly related  to improved design of the mix tank and
contact tank sludge conditioning phase.

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                           SECTION III

                 BACKGROUND AND INTRODUCTION

In 1966, the City of Cedar Rapids began testing ways of dewatering
primary and secondary digested sludge from its Water Pollution Control
Plant.  At the outset of the dewatering investigations, laboratory work
soon confirmed that the  sludge from the Cedar Rapids plant would be
difficult to dewater and  that the filterability was unpredictable.  Samples
drawn from the digesters indicated that the sludge characteristics varied
considerably.  This was attributed to the complex and varied industrial
wastes which entered the  plant along with other wastes from the City.
In terms of numbers, the BOD5 population equivalent at the time of this
program development was  about 800,000 whereas  the actual population
was about 113,000.

To evaluate the sludge dewatering equipment available and to apply it to
these complex wastes, numerous  pilot plants were operated including two
sludge centrifuges, a gravity (plug) filter,  vacuum filter and a pressure
filter.  In addition, sludge samples were treated at a wet air oxidation
process representative laboratory.  Some of these programs were a near
failure from the point  of sludge conditioning and/or sludge dewatering.
The centrifuge showed some promise in the percent of total solids dis-
charged, however, the percent solids captured and the percent solids in
the concentrate was very poor. Following the centrifuge pilot programs
was a belt-type vacuum filter.

Preceding the vacuum filter pilot study, detailed laboratory work on di-
gested sludges was undertaken using the Buchner funnel tests to obtain
optimum chemical dosages for sludge conditioning.  This work was
followed by leaf tests to determine the best yield and most suitable
filter media. These laboratory investigations indicated that the Cedar
Rapids sludge would not dewater at any reasonable cost.  Chemical
costs alone were  in the  range of $20 per ton dry weight.  Chemicals
used were combinations of ferric chloride,  lime and every polyelectrolyte
available.  Combinations of digested and raw sludge were also investi-
gated and this did not affect performance.

In January 1967 experimentation with some conventional and unconven-
tional filter aids  was  begun in an effort to reduce the chemical costs.
Some of  the admixtures investigated  are:

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          1. Volcanic clay

          2. Lime waste slurry from municipal water
            softening plant

          3. Clay fines frpm gravel Dlant 'sooil.jbile

          4. Waste activated carbpn from*a local
            industrial process

          5. Pulverized recycled dried.grit cake from
            waste treatment plant

          6. Recycled grit and .fibrous material from
            plant grit chambers'

          7. Fibrous wastes from packing industry

          8. Commercial diatomaeeous earth

          9. Heated  sludge to 140F.

        10. Fly ash from two power generation plants

Some of these materials were a  complete failure.  Others showed some
promise,  but only fly ash performed satisfactorily.  The fly ash used was
from a fossil fuel fired steam power generation plant where coal is fired
as a pulverized fuel.  Fly,ash is in reality the total ash product of sus-
pended pulverized fuel (rather  than only the precipitated ash product of
the stack as from many furnaces utilizing a traveling "grate for the com-
bustion area).  Fly ash as a filter aid reduced the chemical costs to
approximately $4 per ton dry weight (compared to $20 per ton) and
delivered 28 percent cake solids from a vacuum filter.  Details of the
vacuum filter study with fly ash sludge conditioning  appears later in
this report.

In October 1967, a pressure filter was installed and  operated for a five
month period using fly ash as a filter aid.  No chemical conditioning
was used.  The pressure filter-may be generally described as a plate
type consisting of 6 plates, 24 inches' in diameter having approximately
36 square feet  of filtration area ,; operated at .a shut-off pressure of about
260 pounds per  square inch (psi). The pressure-filter has  been in service
in the United States for fifty plus years in industrial  processes, but it
has only been in recent years-that its application.tp dewatering sewage

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sludge has been seriously considered.  At this time the apparent attrac-
tion to the pressure filter was its success in Europe and England operating
without the use of chemicals in numerous installations and its ability to
produce a relatively dense sludge cake with extremely clear filtrate.
Analytical data on the performance of the pressure filter was nearly non-
existent at that time in this country and therefore the  Cedar Rapids pilot
program was undertaken with very limited guidelines for evaluating the
process.  After several early filter cloth failures, a cloth with suitable
characteristics was obtained and considerable operational data was
accumulated.  It was recognized early during the pilot study that the
pressure filter offered numerous advantages which qualified this process
for  consideration in the design of the proposed Water  Pollution Control
Plant improvements for Cedar Rapids.  Details of the pressure filter
pilot  study and evaluation of a full scale process plant are included in
this report.

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                            SECTION IV

             VACUUM FILTRATION PILOT PLANT STUDIES

Bench Scale - Vacuum Filter Leaf Test

The Cedar Rapids Water Pollution Control Plant is a two-stage high rate
trickling filter plant with separate sludge digestion.  Incoming wastes
are high-strength from numerous wet industries such as meat packing,
metals, wet milling,  and grain processing.  The treatment plant covers
about 30 acres with all of the facilities under roof using thin-shell con-
crete domes.  Flow averages about 22 million gallons per day with maxi-
mum treatment at 64 million gallons per day.  The population equivalent
of organic loadings is approximately 800,000  persons.  The plant has
been  operating at a design overloaded condition for organic and solids
quantities for several years.

The feasibility of using fly ash as a  filter aid in dewatering secondary
digested sludge at Cedar Rapids was  confirmed through a sequence of
laboratory bench work and pilot plant studies. The pilot studies using
fly ash filter aid began with vacuum filtration and finished with pressure
filtration.  A full scale sludge dewatering plant,  using pressure filtration
was constructed as the result of these pilot studies.

The purpose of the first test was to study the  feasibility of vacuum filtra-
tion of digested sludge from the second stage digester.  A bench scale
leaf test was made  on a mixture of secondary  digested'sludge mixed with
chemicals and fly ash.  The bench scale test  equipment consisted of a
circular "leaf" with exactly 0%1 square foot area, upon which was supported
a piece of appropriate filter cloth.  The test leaf was connected through
a filtrate receiver to a source of vacuum. During the test, the leaf was
immersed in the sludge mixture and filtration carried out for a timed
period.   The leaf was then withdrawn from the sludge sample and per-
mitted to drain under  vacuum for another timed interval.  The cake formed
on the leaf was then taken off, and weighed both wet and after drying.
By making individual  runs at a series of filtration times and drying times,
the relationship between cake formation time and yield was obtained.
From  the  same set of  data, the effect of drying times on  the final moisture
content was also established.

The sludge mixture consisted of 4000 ml of 5 percent by weight secondary
digested  sludge, mixed with 500 grams of fly ash, 100 ml of 10  percent
by weight ferric chloride and 20 ml of 2 percent by weight C-7 polymer.

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Temperature was ambient, fly ash used had approximately 18 percent
moisture content.  Mild agitation was performed to maintain a uniformly
suspended slurry.  Results of this typical run No.3 are  shown in Table I.

Filtration Results

The sludge  mixture filtered fairly well, the cake discharged from the
media  cleanly. The  filtrate contained suspended solids that rapidly
settled (5 minutes) in a graduated cylinder.  Figure No. 1 shows a
correlation of the Form Filtration Rate vs.  Cake Formation Time. This
curve is from data  obtained on test No. 3, Table I.  Tests No.  1 and 2
were not plotted.  On test No.  2 there was blinding and on test No. 1,
the medium was too open.  Ideally,  the slope of the curve shown on Fig.
No.l should be -0.5.  The plot shows -1.0.  It is believed that the
openings in the cake, after a short interval, were sealed off by the fines
in the  sludge mixture.  Essentially, the weight of the cake was the  same
for all form times, because of this  sealing off process.  This doesn't
indicate that the mixture  is not filterable but that faster cycle time should
be used, as the cake formation occurs only for a  short period of time for
this sludge mixture.
                              TABLE I
Test Run No. 3 - Digester No. 7
Vacuum Filter 0.1 sq.ft., leaf type
Cloth - Nytex 413
Sludge Solids - 5%
Temperature - ambient
                                                4000 ml. sludge
                                                500 grams wet fly ash
                                                410 grams dry fly ash
                                                100 ml. Fe C13 at 10%
                                                 20 ml. polymer C-7 at 2%
Test         Filtering   Filtering
No.  % Solids  7ac.In.Hg  Time Min.
    Feed     Form Dry   Form Dry
                                              Cake Wt.
                              Filtrate     Cake    Grams    Percent  Dry Sludge Solids
1
2
3
4
5
6
7
10.8
10.6
11.5
11.1
11.0

10.8
25.5
26
25
24.5
24

24.5
24
25
24
24
24

24
,5 3 161
1 2 177
2 1 174
1 1 143
3 .5 1 68
Void
1.5 1.5 139
51.3
28.0
13.8
22.6
8.8
Run
1,7
1/4
1/4
5/16
3/16
3/8

3/16
64.2
68.4
91.8
65,0
93.6

58.4
24.2
25.9
30.5
23.2
29.4

21.2
62.3
62.1
66.8
64.3
70.2

63.8
20,
11.
6.
9.
4.

6.
5
1
1
8
2

0
                                 10

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Figure No. 2 is a plot of Form Filtrate Rate vs. Form Time and the curve
was obtained in a  manner similar to the plot for Figure No. 1.  The slope
of this line is the  same as the  slope of the Form Filtration curve.

Figure No. 3 is a plot of the Full Scale Filtration Rate vs. Cycle Time.
This curve was obtained from Figure No.  1.  Using a 0.8 scale-up
factor and air effective submergence of the filter drum of 25  percent, a
point on the full scale filtration curve was obtained as follows:

      Form time                         = one (1) minute
      Yield at one (1) minute form time = 38#/hr/sq.ft.

At 25 percent effective submergence, 25 percent of the filter area  is
available to form a filter  cake, therefore, the  cycle time is:

      1. minute    = 4 minutes per revolution (MPR)
        0.25 "

Full scale yield  =  38 x 0.25 x 0. 8= 7. 6#/hr/sq.ft.

The slope of the full scale filtration curve is the  same as the slope on
the form filtration rate curve Figure No. 1.

                       FORM  FILTRATION  V3  i-'O^M TIME
                               FLOCCULANTS & FLYASH INCLUDED
             2
             oc.
             o
             li.
               10
       iosU'fl Sluci.K' f.r. PKnU
  Sliui i.' Solids Mixod JOOU n.I with
ii)i) .iidn.s fly ash. 100 ir,I 1'orri; Chlnndo
10 dr. wt.  sfijution dud JO n.I - ?
ury w[. C-7  Pulyn Jr
                              1.0
                            FORM TIME
                         2.0
                    (minutes)
3.0
4.0
                                Figure 1
                                 11

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                 FORM  FILTRATION  VS  FORM  TIME
                          FLOCCULANTS  ft  FLYASH  INCLUDED
            S.';-ond,\iv |ii.ii-st,'ci Sluil.i.1 :'.. . I'l.u )  O
            >.' Skull.' Sn'ids MiXi-il -lODll i; I wl:-.
            fillc) .|I,HV,S ll> .is! , IUO n:l IV'
            10' iliy wl . su'uui	ml W ir.l - .'
            dry wt . ;j - '' IVl\HUM
                 1.0                2.0
             FORM TIME    (minutes)

                  Figure  2

FULL  SCALE  FILTER  RATE  VS  CYCLE TIME
                                                                   4.0
       Secondary Digested Sludg
       5% Sludge Solids Mixed '
       410 cjrams fly ash, 100 m
       10% dry wt. solution and
       dry wt. C-7 Polymer
         NO  CHEMICALS  OR  FLYASH
\L  2
                         2          3       4      56
                      CYCLE  TIME    (minutes/rev.)

                             Figure  3

                                12
                                                7   8  9  10

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In order to size the filter station, it is necessary to know the full scale
filtration rate of the secondary digested sludge.  This was obtained by
calculating the proportion by weight of the digested sludge in the mixture
and assuming the filter cake followed the same weight proportion. This
was calculated as follows:

      4000 ml 5% by weight sludge    200 grams
      410 grams fly ash              410 grams
      100 ml FeCla 10% by weight     10 grams
                                    620 grams
      % digested sludge = 200 = 32%
                          620

The secondary digested sludge without additives was then plotted as
32 percent of the secondary digested sludge with additives.

The Full Scale Filtrate Rate Curve (Figure No. 4) was calculated and
plotted in a manner similar to Figure No.  3.  The purpose of  this curve
is to estimate the filtrate flow.

                FULL SCALE FILTER  RATE VS  CYCLE TIME
~ 14
j: 13
o
 10
& 9
~ 8
u
cc.
LE FILTRAT
o> c
o 4
tf>
_i
^ 3
1
-
-
: \
V
\
x
\
~
Tested 12-16-66
~ Basis Lab Test No. 3
Medium Mytex 413
Vacuum 25 Inch Hg
Test Leaf 0.1 sq. ft.
- Secondary Digested Sludge T.F. Plant
5% Sludge Solids Mixed 4000 ml with
500 grams fly ash, 100 ml Ferric Chloride
10% dry wt. solution and 20 ml - 2%
dry wt. C-7 Polymer
5 23
CYCLE TIME (minutes
Figure 4







\
i i
4 5
/ rev. )

                               13

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Figure No.  5,  Cake Moisture vs. Correlating Factor is a plot of percent
cake  moisture  versus drying time divided by cake weight per unit area.
A good correlation was obtained.  The purpose of this curve is  to deter-
mine  optimum moisture content relative  to operating conditions.
           8 Or-
         13
         U)
         'o
         6 75
         O
         o
           65
         I
         O
         UJ
60o
                 CAKE  MOISTURE  VS  CORRELATING . FACTOR    ( 9 d/w)
                             ,, ,     I.,,: T .( Si . i
                             M--ti|.ji:   \yt--X -ill
                             .\I~LI tl:.   2 j iu_w!*;IJJ
                             .il l.-'cll  U. I rig. fl.

                             .!..;.inu.ir, I .'i :- -,L..". Sim. :  f.i. I'l.u.
                             . Sim: ;. SMiu.i Mi;0'U -Tl >> ;; 1 wit-.
                             '.'I'J jl.n: s f! .- .isn, MI) n I I'-'m;? (.; li
                             IT iir, wl. solution HIK! 2'J r:.l - >
                             'ir, wl. () -i I'olyn .-[
              0
1,0          2.0          3,0
CORRELATING  FACTOR   (.6 d/w)

     Figure 5
                                                             4.0
Summary

I.  The mixture of digested sludge, fly ash and flocculants filtered very
well, giving a good discharge,  with filter yields ranging to 20 Ibs. per
hour per square foot for short form times.  The cake had a comparatively
low moisture content ranging  from 62 to 70 percent. The filtrate appeared
to be fairly high in suspended solids by observation.  It was suggested
that these fines  be separated from the  filtrate before the filtrate is return-
ed to the  treatment system. The purpose of this is to prevent any possible
build up of non-separable  substance in the treatment system. The material
in the filtrate settled out very rapidly.
2.  Full scale filtration of the sludge mixture tested is feasible. It is
our opinion that further leaf tests be run before a  pilot plant study  is
made. When optimum leaf  test filtration results are obtained, based
upon an economic study of the various factors, then a  short tern pilot
plant study would be  feasible.  Further leaf test studies of the  fines in
the sludge mixture should  be made.

3.  The fines in the sludge mixture caused  suspended solids in the fil-
trate and while not detrimental  to filtration, limited the cake formation
by sealing off the openings in the cake. Further leaf tests on varying
dosages of fly ash and flocculants might indicate ways to lessen the
effect of the fines.
                                  14

-------
4.  It was noted that on Test 1 data  sheet, using Nytex 415, the back
calculated feed solids are about 8, percent,  Test 3 data sheet,  Table I,
using Nytex 413, the  back calculated, feed solids are about 11  percent.
The Nytex 413 has a slightly closer  weave than the Nytex 415.  This
would indicate the effect of fines or the size distribution of the fines.

Percent solids in the sludge admixture' was.probably 15 or 16 percent.
The Nytex 413 captured more of the finds  than the Nytex 415 as indicated
by the back  solids feed calculation.

5.  It was not determined what proportion of fines in the filtrate originated
from the fly  ash  conditioner or the sewage sludge.

6.  We  believe that the filtration tests, .as reported herein, were well
wor th running.  The material-is filterable but further bench  scale tests
are in order  to evaluate certain points.  -This work was done in conjunc-
tion with the pilot plant study.

Pilot Scale Vacuum Filter

Following the, bench 'Scale vacuum- filtration leaf testing program reported
in the previous: section-of this report;'a pilot scale vacuum filter was
obtained jand operated for a period of 10 weeks.  The basic objectives of
the pilot scale,vacuum,iilter6, and the flow schematic in
Figure 7.  The vacuum filter was a continuous belt filter, 3 ft. diameter
by 1 ft. face. Two nylon belts were selected from previous laboratory
evaluations.  Both were monofilame.nt nylon.  One was a fine weave
designated by the manufacturer as Nytex 413 .  The other a coarser
weave,  designated as Nytex 415.  The Nytex 413 was used exclusively
during the "test run as a convenience to data evaluation.  Other filter
media may warrant consideration.'
                                15

-------
                                      Vacuum and Filtrate
                                     Pumps and Receivers
                                      Operating Controls
"locculator Mixer
                                      Sludge Mixing Tank
                                      and Variable Speed
                                          Feed Pump

-------
  CHEMICAL
    FEED
    PUMPS
                     SCHEMATIC DIAGRAM OF PILOT PLANT
                             Figure 7

The sludge pump was  a rotating cavity positive displacement pump, with
a variable speed motor drive.  The pump was calibrated both by measur-
ing the displacement at the discharge side and the drawdown in the 225
gallon sludge tank. The displacement of the pump at the setting used
(No. 2) was 4.00+ 0.05 gallons per minute.

Chemicals were added by positive displacement piston type pumps with
variable stroke.

Chemicals and sludge were mixed in a flocculating tank by means of an
air diffuser.  This  method proved very satisfactory and causes the  least
amount of shearing of floe while still maintaining adequate mixing.

Filtration Aids and Chemical Conditions

Fly Ash

The addition of fly ash to the sludge presented a problem and various
methods were used in an attempt to prepare a homogeneous mixture of
digested  sludge and fly ash.  The last method used,  proved to be the
most successful.   This method involved screening the fly ash through a
                                17

-------
 coarse mesh window screen over the vortex described by the mixer in the
 250 gallon mixing tank. Although this method has been most successful,
 a combination of this coarser mesh screen and a finer mesh probably
 would have yielded  better results.  The fly ash used consisted of many
 basic oxides  which  upon addition to  the digested sludge raised the pH
 to approximately 8.4.  Since  the fly  ash was stored out-of-doors,  the
 moisture content varied from 15 to 35 percent.

 Polymeric Flocculants

 The flocculant used in  this test was  a cationic polymer marketed by
 Rohm and Haas  as "Primafloc C-7" .  This polymer had demonstrated
 superior effectiveness  as a flocculating agent in the laboratory over
 other polymers used.  Other polymers are now available which may be
 more effective or as effective as the one  used, but considering the
 number of parameters which needed  to be evaluated,  it was felt that  the
 introduction of another parameter would not be justifiable considering the
 length of time the pilot plant  would  be available. The polymer, a dry
 powder, was  dissolved in water before use.  Stock solutions of 2 percent
 by weight,  were prepared.  Dilutions of 0. 6, 0.4, 0.3,  0.2 and 0 .1 per-
 cent were prepared from this solution for introduction into the sludge.
 It was noted that the dilutions of polymer in the range of 0.1  to 0.01
 percent are more desirable and full scale facilities should include chemi-
 cal  pumps  capable of delivering the needed quantities of polymer at these
 lower dilutions.

 Other Chemical Additives

 Ferric sulfate was used to a lesser extent in this  study than the polymer
 and was always used in conjunction  with the polymeric flocculant. The
 apparent reason for using ferric sulfate was  to lower the dosages of
 polymer needed to form an effective  floe and not to replace  it.  Ferric
 sulfate is not usually considered to  be as effective as ferric chloride,
 however, at increased  dosages, it is still more economical considering
.the initial cost of 3  cents per pound versus approximately 10  cents per
 pound for ferric chloride.

 Ferric sulfate was purchased as "Ferrifloc", a dry powder,  and  dissolved
 in water to form a 5  percent by weight solution, which was  also the con-
 centration used as the  feed solution.  Both ferric  sulfate and  the cationic
 polymer form acidic  solutions and all tanks and feed  lines,  and fittings,
 should  be coated to  protect the metal.  Sodium hydroxide was added to
 the sludge on one occasion,  to determine the dependence of sludge fil-
 tration  on pH. The pH which was 8.4 was raised to 9.4 and filtration
 immediately ceased, that is,  no dischargeable cake was obtained.

                                 18

-------
 Test Procedure

 The vacuum filtration was performed on a batch basis.  The holding tank
 held approximately 225 gallons of sludge, which was enough for a  56 min.
 run at a constant four gallons per minute feed rate.  The fly  ash was added
 immediately and the run was started. The time between the  drawing of
 the sludge to the beginning of the run was never more than one hour. Dur-
 ing the run, the cycle time was varied from approximately one minute to
 four minutes depending upon cake dischargeability. Each cycle time was
 timed by a stop watch. Samples of the filter cake were taken using a ring
 exactly 0.1 square foot of area .  The samples were then dried in the lab-
 oratory at 102 C. to determine dry weight.  The dry weight  in grams was
 then converted to pounds per square foot per hour by the following  formula:

      Cake Wt. in Grams x 10 sq.ft. x 60 Min/Hr.  - Ibs./sq.ft./hr.
      453.6 Grams/Lb.              Min./Cycle

 Many duplicate samples were taken at the same cycle time.  This was
 done because at times, the filter  cake was not uniform across the filter
 media .  This condition is aggravated by the fact the filter face is only
 one foot long, secondly, the digested sludge contained bits  of matter
 not representative of the whole which could give erroneous weights for a
 Particular cycle time.  Finally this method gives the operator a good esti-
 mate of the consistency of  operation and  the precision of his technique.
Summary - Operating Procedure
General
      Cloth

      Length of Run



      Condition of Cloth


      Sludge
     Filtering Minutes
     Per Revolution

     Va cuum
Nytex 413

56 minutes at 4 gallons per minute,
sludge wasted over overflow weir
on filter.

Good - three small holes developed
during la st week of run.

4 to  7% solids, 40-50% volatility
pH 7.0 + 0.2
Rate  of filter feed - 4 gallons per min,
0.73-6.00 depending upon cake quality

22^25 inches Hg
                               19

-------
         Percent Submergence    25% constant

         Number of Wash        5
         Headers Used

         Chemicals              Polymer C-7 only, dilution 0.1 to 0.6%
                                Ferric Sulfate 5% feed (when used)
                                Fly Ash Ratio 0.25 to 1.90 sludge
                                Fly Ash 15-35% moisture

         Filtrate Clarity          The suspended solids in the filtrate
                                ranged from 0.2% solids to 0.64%
                                with the average very nearly 0.5%
                                solids (5000 mg/1).

Cake thicknesses were measured by the metric system and the English
system.  The thickness of the cake is not of great importance in itself/
because of its dependence upon  moisture content and fly ash concentra-
tion.

Explanation of Data Sheet

         Date                   Date of filter run

         Percent Solids          Dry solids (at 102C.) content of
                                digested sludge

         Fly Ash to Sludge        Ratios of weight of dry solids of fly
         Ratio                   ash to weight of dry  solids of digested
                                sludge.

         Lbs./Ton Polymer        Pounds of polymeric  flocculant per ton
                                dry weight digested sludge solids only.

         Lbs./Ton Ferric Sulfate   Pounds of ferric sulfate per ton dry
                                weight digested sludge  solids only.

         Percent Moisture        Percent moisture of filter cake.

         Lbs/Hr/Ft.2/Total       Filter yield of all solids including fly
                               ash

         Lbs/Hr/Ft.2/Sludge     Filter yield in terms  of sludge only
                                (fly ash not included)
                               20

-------
Sludge %
Date Solids
1967
4-12 7.71


4-18 7.40




4-18 7.74



5-17 fi.40

5-25 6.30




5-31 6.90




6-13 5.80




6-15 5.75






6-15 5.75




6-15 5.70








6-21 5.0






6-21 5.0







Fly Ash to Pounds/Ton Pounds/Ton Cycle
Sludoe Ratio Polymer Fej(SO4) j Tune _

0.706 6.90 - 4.00
2.62
1.59
0.253 11.00 - 6.40
3.75
2.75
1.66
1.16
0.359 9.33 - 6.50
4.92
2.67
1.42
0.531 8.64 44 3.00
1.71
0.974 4.34 44 6.50
3.93
2.62
2.06
1.66
0.760 2.16 54 3.92
2.7S
2.61
2.16
1.66
1.00 4.76 80 3,92
2.50
1.83
1.50
1.50
1.412 3.12 64.3 3.70
2.50
2.50
1.61
1.61
1.40
1.10
1.42 3.93 64.3 3.58
2,25
1,93
1.53
1,25
0.96 5.88 48.3 4.08
4.08
3.76
3.03
2.75
2,75
1.92
1.92
1.63
1.32
1-55 5. SI 3.05
3.05
2.13
2.13
1.72
1.4S
1.46
1.55 3.67 3.91
3.91
3 as
3. IB
2.15
2. IS
1.43
1.16
*
Moisture

65
67
68
77
76
72
76
76
75
75
73
73
72
72
64
64
62
63
61
66
57
59
58
60
67
68
66
66
66
_
-
-
-
-
.
-
_
-
-
-
-
70
71
71
68
72
72
67
68
67
70
57
S7
55
62
59
57
57
55
55
54
52
62
53
54
41
Lbs/hr/ft2
Total

4.5
6.52
10i96
7.12
8.19
9.05
13.21
11.44
4.46
4.69
4.84
10.10
6.64
9.36
4.02
5.04
5.47
7.77
7.92
2.17
2.61
2.97
3.18
4.21
6.13
7.97
9.97
10.80
10.80 "*
7.69
8.52
9.52
11.80
13.76
15.40
15,71
8.78
10.46
12.10
11.76
16,00
6.11
6.30
5.95
5.74
7.69
7.80
6.08
9.07
11.15
10.46
6.27
6.02
6.80
6.62
10.50
12.80
12.47
3.82
3.63
4.81
4.73
4.76
6.67
8.91
9.51
Lbs/hr/ft2
Sludae

2.63
3.82'
6.42
5.69
6.55
7.24
10.57
9.15
3.27
3.45
3.55
7.42
4.33
6.11
2.03
2.55
2.77
3,93
4.01
1.23
1.48
1.68
1.80
2.39
3.06
3.98
4.98
5.40
5,40
3.18
3.52
3.94
4.68
5.69
6.37
6.50
3.63
4.33
5.01
4.86
6,62
3.12
3.22
3.04
2.93
3.93
3.98
4.13
4.63
5.87
S.3S
2.45
2.35
2.66
2. 59
4.12
S.01
4.88
1.49
1.42
1.88
1.85
1.86
2.61
3.49
3.72
Discharge

C
C
C
B
B
C
C
C
B
B
C
C
A
A
A
B
B
B
C
B
B
B
C
C
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
C
C
C
C
C
C
D
D
B
B
B
B
C
C
C
B
B
0
C
C
C
C
C
Thickness
	 Inches




1/2
3/8
3/8
-
-
1/4
1/B
3/16
1/B
1/2
1/4
13/64
5/32
1/8
1/8
7/64
9/64
7/64
7/64
7/64
5/64
7/32
13/64
5/32
1/8
1/8
11/64
1/8
1/8
7/64
7/64
5/64
5/64
5/32
1/8
1/8
7/64
7/64
7/32
7/32

5/32
11/64
11/64
9/64
9/64
1/B
1/B














Dollar
Cost

6.90


11.00




9.33



9.96

5.66




3.78




7.16




5.04






5.85




7,33








5.51






3.67







21

-------
Sludge % Fly Ash to Pounds/Ton
Date Solids Sludge Ratio Polymer
6-22 6.6 0.951 4.17










6-28 5.20 1.02 6.93







6-29 6.6 1.07 4.09







6-29 6.6 1.07 2.73



6-30 4.0 1.89 8.03







6-30 4.0 1.89 5.3S




7-3 S.O 1.20 6.34







7-5 S.4 1.36 5.00







Pounds/Ion Cycle
Fej(SOj) 3 Time
3.05
3. OS
1.43
1.43
1.26
1.03
1.03
0.83
O.B3
0.73
0.73
3.20
3.20
2.45
2.20
2.20
2.13
1.50
1.45
3.07
3.07
2.17
2.17
1.47
1.47
1.47
1.27
3.08
3.08
2.13
1.45
3.03
3.03
2.10
2.10
1.43
1.43
1.03
1.03
3.05
3.05
2.10
1.46
1.46
4.43
2.52
2.16
2.16
1.50
1.50
1.07
1.07
3.00
3.00
2.17
2.17
1.50
1.50
1.03
1.03
% Lbs/hr/ft2 Lbs/hr/ft2
Moisture Total Sludge
58
60
59
64
60
58
58
sa
58
59
59
66
67
65
56
62
63
62
62
59
59
57
57
60
59
60
59
55
52
54
-
58
58
57
59
54
58
59
56
56
56
46
56
-
60
61
63
61
61
57
62
63
55
51
53
52
53
53
51
51
4.72
4.44
10.31
9.26
12.9
14.3
14.9
14.86
14.62
15.22
15.25
8.46
B.27
9.76
10.42
9.37
10.31
13.68
11.19
6.79
6.97
8.72
9.13
10.15
10.64
11.70
11.89
3.95
3.95
S.21
No Discharge
6.36
6.49
8.08
7.84
12.80
11.30
13.28
13.53
4.34
4.68
S.74
S.91
S.43
4.31
6,83
7.76
7.53
9.79
10.99
10.88
11.93
6.75
6.67
8.21
8.27
11.27
11.20
13.4
13.7
2.42
2.27
S.28
4.7S
6.62
7.33
7.64
7.60
7.49
7.79
7.82
4.23
4.13
4.88
5.12
4.63
5.15
6.84
5.59
3.27
3.36
4.21
4.40
4.90
5.13
5.65
5.74
1.90
1.90
2,51

2.20
2.24
2.79
2.71
4.42
3.91
.59
.68
.50
.62
.98
2.04
1.68
1.96
3.10
3.53
3.42
4.45
5.00
4.95
5.42
2,86
2.82
3.48.
3.50
4.77
4.74
5.68
5.80
Thickness
Discharge Inches
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
C
C
B
B
G
C
C
C
C
C
C
C
C

B
B
B
B
C
C
C
C
B
B
G
D
D
B
B
B
B
B
B
C
G
B
B
B
B
B
B
B
B
1/8
1/8
7/64
7/M
5/64
5/64
5/64
1/16
1/16
3/64
3/64
5/32
S/32
5/32
5/32
5/32
1/B
3/32
3/32
5/32
5/32
3/32
3/32
1/16
1/16
1/16
1/16
1/16
1/16


1/8
1/8
3/16
3/16
3/16
3/16
1/16
1/16
1/16
1/16
1/16


1/8
t/e
3/32
3/32
3/32
3/32
3/64
3/64
None
None
None
None
None
None
None
None
Dollar
Cost
4.17










6.93







4.09







2.73



8.03







5.35




6.34







5.00







22

-------
      Discharge
         A
         B
         C
         D
         F

      Thickness
      Chemical Costs
                           Since cake dischargeability is a critical
                           factor in filtration studies, cake dis-
                           charge was given a rating from A to F
                           and entered on the data sheets of the
                           valid filter runs.

                           Very good discharge - thick cake
                           Very good discharge
                           Satisfactory discharge,  clean cloth
                           Incomplete discharge
                           No discharge

                           Thickness of filter cake in millimeters
                           or inches ,25.4 millimeters = 1 inch

                           Costs were evaluated on the following:
                             $1.00 per pound - Polymer
                             $0.03 per pound - Ferric Sulfate
    A-SlUo_Sludge Ratio  1/1
        iliustrates the results of four runs with an approximate ratio of
       ash to sludge.  The actual conditions are:
Line

  1
  2
  3
  4
          Fly Ash    Ferric Sulfate    Polymer
         Sludge Ratio   Lbs /Ton	Lbs/Ton
                                     Chemical
                                      Cost/Ton
6-29
6-28
6-13
5-25
1.02
1.02
1.00
0.974
80
44
4.09
6.93
4.76
4.34
$4.09
 6.93
 7.16
 5.66
fc, e effectiveness of ferric sulfate is not apparent.  These plots show
  e range of yields and costs which may be found over a period of time.
                                 23

-------
               FILTRATION   RATE   VS  CYCLE   TIME
                          2            3       45.67
                       CYCLE  TIME    MIN./REV.
                                                   8
                               Figure 8
Fly Ash to Sludge Ratio 1.4/1
Figure 9 shows the result of four runs where the fly ash to sludge ratio
was approximately 1.4 to 1.  The actual operating dosages are given
in the following table.
Line   Date
Fly Ash/Sludge  Ferric Sulfate    Polymer
    Ratio	Lbs/Ton	Lbs/Ton
Cost
1
2
3
4
7-5
6-21
6-15
6-15
1.36
1.55
1.412
1.412
_
-
64.3
64.3
5.00
5.51
3.12
3.93
$5.00
5.51
4.44
5.25
Figure 9 also shows the apparent effectiveness of ferric sulfate in increas-
ing filter yields at approximately the same cost as polymer only at this
higher fly ash to sludge ratio.  Line 2 is characteristic of a high dose
fly ash.  The steep slope shows the effect of fines sealing off the cake.
                                24

-------
                   FILTRATION   RATE  VS   CYCLE   TIME
                        (FLYASH  SLUDGE  RATIO I.4M)
              8

              7
                      X'V.
            rr
            a
            04
            x
            en
            CO
                   (Sewage  Solids)
                         CYCLE  TIME
    3       4
MIN./REV
                               Figure 9
Sludge discharge at highest filtration rate (shorter cycle time) is apparent
also.  Normally the thinnest dischargeable cake at the'shortest cycle time
will give the highest  yield, however, a point of diminishing returns can be
reached at very fast cycle times, when the hourly yield will actually lower
somewhat.  Practically the fastest cycle  time is one minute per revolution.

Sludge Filtration vs.  Fly Ash Ratio

Figure 10 illustrates the sludge filtration rate as a function of the fly ash
to sludge ratio.  The  points shown  are filtration rates taken from their
respective  graphs of filtration rate  vs. cycle time.  The cycle time of
2.00 minutes was chosen because it is a  medium cycle time and repre-
sents a point usually well defined on all  filtration  curves.  The points
used for this curve are as follows.
                                 25

-------
      Line
    Date
          Yield
Fly Ash/Sludge
    Ratio
 Cost
 per Ton
      1
      2
      3
      4
      5
      6
      7
      8
     7-  5
     6-29
     5-31
     6-13
     6-21
     6-15
     6-15
     6-22
           3.80
           4.34
           3.60
           4.50
           3.55
           4.70
           4.-60
           3.85
   1.36
   1.07
   0.760
   1.00
   1.55
   0.956
   1.415
   0.951
 $5.00
  4,
  3,
  7,
  5,
  7.
09
62
16
51
33
  5.07
  5.60
The optimum fly ash  to sludge ratio appears to be approximately 1.0 to
1.1/1.  At this point, the sludge filters well, and gives agood dis-
chargeable cake of moderate moisture content, above this point (higher
fly ash ratio) filtration of sludge may improve, but the economic advan-
tages are  diminished.
                 FILTRATION KATE  VS  FLYASH  RATIO
       6 r
     I  5
     H
          J3.62
       0
       0.7
0.8
0.9    I.O    I.!    I.2   1.3    1.4
    FL Y ASH/SLU DGE  SOLIDS  RATIO

         Figure 10
            1.5
1.6
Cake Moisture
Moisture content in the filter cake is a function of chemical dosages
and fly ash concentration.  Figure 11 represents the range in moisture
content encountered during the test runs.  It can be  readily noted that
the addition of fly ash causes a significant  decrease in moisture con-
tent of the filter cake.
                                26

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                  CAKE MOISTURE  VS  FLYASH  RATIO
     30
            0,2
0.4   0.6    0,8    1,0    1.2    1.4
    FLYASH/SLUDGE SOLIDS  RATIO

            Figure 11
1,6
1.8
2,0
Conclusions
1.  Fly ash is a valuable sludge conditioner. Preliminary studies indi-
cate that its  use lowered the total chemical  costs of sludge conditioning
from $20 to $4 per ton dry sludge solids.

Optimum fly ash to sludge ratio for dewatering performance lies between
0.9 and 1.4.  However,  the optimum economic fly ash to sludge ratio may
be different due to costs of trucking and materials handling of ash and
chemicals, and other operating costs.

2.  The value of ferric sulfate is not clear-cut.  It appears to be an effec-
tive flocculant at higher (1.4/1) fly ash to sludge ratio, but its benefit is
not clearly defined at lower fly ash ratios.  Additional investigation is
warranted in a full scale plant.

3.  It has become clear that an efficient vacuum filtration system must be
under close control by the laboratory.  Any automatic equipment which
might be incorporated in final plans for the close surveillance of sludge
characteristics should be evaluated.
                                 27

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                           SECTION V

       PRESSURE FILTRATION OF SECONDARY DIGESTED SLUDGE
                     WITH FLY ASH FILTER AID

 Introduction

 Prior to the pressure filter program the City of Cedar Rapids investigated
 various other methods of dewatering bottom sludge from their secondary
 digesters.  A laboratory model pressure filter and a pilot scale pressure
 filter were operated for a period of five months using fly ash as a filter
 aid.  The information collected during the initial study and presented in
 the following report represented nearly all of the analytical  data avail-
 able on the operation of the pressure filter in the United States.

 In October 1967, a pressure filter with a filtration  area  of 36 square
 feet was installed with the necessary accessories  at the Cedar Rapids
 Water Pollution Control Plant to conduct a pilot plant study  for dewater-
 ing digested sludge. Fly ash from a local steam generation  plant burning
 fossil fuel was used as a filter aid.  Operation extended over a five
 month period with daily observations recorded. For a  seven week period,
 a two  shift operation was in effect to obtain more data in the limited
 time the equipment was available.

 The pilot scale pressure filter may be generally described  as a  plate
 type, consisting of a series of plates about 24 inches in diameter,
 covered with a synthetic filter media. Sludge is center  fed to  the plates
 at an operating pressure of about 260 pounds  per square inch (psi) for a
 Period of one  to two hours. At the  end of this period,  the  plates are
 Parted and the filter cake is discharged by gravity and the cycle is
 repeated.  No conditioning chemicals were used, the only filter aid
 being waste fly ash.

At the  time the pilot scale pressure filter study was undertaken, the
Pilot scale vacuum  filter study had been completed. The results of  the
 vacuum filter  study have been presented earlier in this report.   The
Purpose of the pressure filter study was  to evaluate the pressure filter
as a practical method of sludge  dewatering in light of the earlier pilot
Plant study using a 3 foot diameter prototype vacuum filter.  The
vacuum filter study was considered successful and indicated that the
Cedar Rapids digested sludge  could be economically dewatered by that
process.  The basic objectives of the pressure filter study were as
follows:
                             29

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      1.  To determine the filterability of digested sludge at
          Cedar Rapids.

      2.  To determine the fly ash filter aid requirements.

      3.  To evaluate the pressure filter performance versus the
          vacuum filter performance  reported earlier at Cedar Rapids.

      4.  To establish an estimate of cost for capital  investment
          and  operation of a full scale sludge filtration facility.

The pilot plant equipment was obtained from  the Beloit-Passavant
Corporation and was operated by personnel from the City Water Pollution
Control Plant, and the Howard R. Green Company, Consulting Engineers,
Cedar Rapids, Iowa.

Laboratory Model Pressure Filter

To determine the possibility of dewatering digested sludge using a
pressure filter,  a laboratory model filter was first used as shown in
Figure 12.

                            Figure 12
                               30

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The unit consists of a  pressure  tank (1),  which is a CO2 gas cylinder.
The gas cylinder, as a  source of pressure for a laboratory scale experi-
ment, is  merely a convenience.

Unit 2, the precoat tank, is used to mix  fly ash and water,  and then to
transfer it  under pressure to the filter media at the beginning of the  cycle.
This mixture  precoats the filter media for protection and improved dis-
chargeability prior to applying sludge for filtration.

The feed tank,  Unit 3,  contains the sludge/fly ash mixture  to be filtered.
Immediately  after precoat,  this mixture is applied  to the filter at a  pr
sure of about 230 psi.  The feed will continue for a predeterminec
of one-half to two hours.  The cycle time is of little concern, as the
purpose of this  experiment  is to demonstrate whether a particular sludge
is filterable.

The pressure filter,  Unit 4, Figure 12 and Figure 13, consists  of a  split
Plate, about 6  inches  in diameter, with an elliptical shaped void
surrounded by filter cloth.  Sludge is forced into this void under pressure
of 230 psi and  builds up on the filter cloth. Filtrate is remove
machined grooves behind the filter cloth.

Figure 13 shows the laboratory pressure  filter in operation at Cedar Rapids
                                                   Figure 13
                                        Laboratory Model Pressure Filter
                               31

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                              Figure 14
             Removing Cake From Laboratory Model Filter
When the cycle is completed,  pressures are reduced and the split plate
is opened exposing the cake for removal as shown in Figure 14.
                              Figure 15
                      6" Diameter Filter Cake
A typical cake about 6 inches in diameter is shown in Figure 15.  The
center thickness is about 3/4 inches tapering to less near the edges.
                              32

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Note the Clarity of the Filtrate in the Beaker

  Sludge Left Beaker - Filtrate Right Beaker
                Figure 16
                   33

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Pilot Model Pressure Filter

Description and Operation

At Cedar Rapids the source of sludge was from the secondary digesters.
Sludge from these units was  conditioned with fly ash obtained as a by-
product from a fossil fuel power generation station.

The filter is a plate type,  consisting of numerous plates in a vertical
position,  operating on a carrier.  Each plate face is covered with a filter
media, similar to materials used on a conventional vacuum filter.  The
filter media used for this  study was a synthetic fabric, monofilament
nylon. At the beginning of the filter cycle, all plates are closed and
held as a  pressure  seal.  Feed to the filter media is through the center
of the plates.

Initially,  the filter media is  precoated with fly ash.  The rate  of appli-
cation used at Cedar Rapids  was 14 pounds wet fly ash per 36 square
feet filter area.   Ash moisture ran about 30 percent due to wet-down at
the power generating station prior to open  truck transportation and out-
door storage at the Water Pollution Control Plant.  The dry rate of
precoat was:

      14 pounds ash x 70% solids = 0.27 pounds ash/square foot
         36 square feet filter

This precoat is an arbitrary figure empirically determined on the basis
of protecting the  filter media and improving dischargeability.  Precoat
was applied at a  pressure  of 40 psi.

Sludge was Placed directly from the digesters into a mixing tank and
conditioned with  fly ash.  This tank contained a mechanical agitator
and a sloped bottom which held the mixture in a homogeneous  state
during the filtering cycle.  Fly ash in the desired quantity to obtain a
given sludge/fly  ash ratio was fed  into this tank by a  screw conveyor.
This mixture is pumped to  the filter feed tank to begin a filtering cycle.
Air pressure applied to the filter feed tank transfers the mixture to the
filter.

The sludge filtering cycle  begins simultaneously with  the  ending of the
precoat cycle and runs for a  predetermined time or pressure.  The mix-
ture of digested sludge and fly ash was applied at an initial pressure
of 100 psi and increased to 260 psi where  this pressure is held until
the sludge filtering cycle is  terminated. The time lapse from start (100
psi)  to beginning of maximum operating pressure (260 psi) was
                              34

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 approximately 20 minutes  (+ 2 minutes).  The filtering cycle was termina-
 ted at various time intervals,  usually increments of one hour.

 Filter pressure was developed by applying compressed air to  the filter
 feed  tank, which in turn,  forced the sludge/fly ash mixture into the
 filter press at the established pressure.  The same air source was  used
 to maintain the filter tank  mixture in a homogeneous state during the
 filtering cycle by means of agitation from a diffused air header  on the
 bottom of the tank.

 To complete a  cycle, the sludge feed line is closed, and the  wet sludge
 filter core is blown back to the storage tank. A reversing hydraulic
 operation of the filter plate carrier causes the filter plates to part, thus
 releasing the cake by gravity.  The filter cake was caught in  a wheel-
 barrow and weighed.  In a  full scale plant,  the  cake would be discharged
 to a conveyor. Following  the removal of the sludge cake, the plates
 are closed, the filter is precoated, and the cycle is repeated.

 A schematic flow diagram of the Cedar Rapids pilot plant is shown in
 Figure 17.
Pilot Filter
Filter
Operating Pressure

Design Pressure

Capacity
Closure

Hydraulic Power Pack

Precoat Tank
24" diameter nominal, carbon steel plates,
gasketed with Teflon  impregnated, braided
asbestos.  Dressed with polypropylene fil-
ter medium. (This can be changed to any
other desired medium.)

225 to 260  psig (pounds square inch gauge)

300 psig

Six filter chambers
Filtration Area: 36 square feet
Cake Space:    2.25  cubic feet,  if"
               thick cake

Hydraulic cylinder 7" x 30 strokes at4500 psi

Operated by compressed air at 100 psig

60 gallon capacity - 22" diameter by
48" high,  200 psig water pressure
                              35

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FLY ASH FEED
SCREW  CONVEYOR
               FLY ASH
               MIX TANK
                 CJD
                   fi
            WEIGH  FILTRATE
                                          ASH *
                                          ~HtO
A i-*
 1 J, ' J FitTta
.^i .
TRANSFER
PUMP
4 	 FILTRATI

FIIC 	 



F



ftESSUM
FILTER


i i
E



1 H
i      J
0      0
                                                                                  	CAKl
                                                                                    DIICMAROC
                                                                    WtlflM SLUDOC CAHC
                   SCHEMATIC  FLOW   DIAGRAM  OF  PILOT  PLANT PRESSURE  FILTER
                                       Figure  17
                                          36

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Feed Tank                  300 gallon capacity - 36" diameter by
                            72" high, 300 psig water pressure

Sludge Mix Tank            48" diameter by 42" high open top tank
                            polyester fiberglass with agitator,
                            30 RPM, 1 horsepower, gear motor

Air Compressor              2  stage, 2 cylinder, with 10 horsepower
                            motor.  Air receiver 120  gallon capacity
                            for 300 psi water pressure.

Sludge Transfer Pump        Centrifugal, horizontal
Operational Experiences

No mechanical problems of significance occurred during the four months
pilot plant testing period.

Considerable difficulty was encountered with the first two filter media
due to extremely high grease content of the digested sludges.  The high
grease content of the digesters occurred during a period of internal
reconstruction of the grease recovery  system at a major meat processing
plant.  This problem was not immediately  obvious until filter clogging
began to occur. The early media was a yarn  type, multifilament.  Micro-
scopic examination of the media, even after washing with grease cutting
detergents, indicated that the grease particles could not be removed from
the twists of the yarn configuration. The  first media lasted forty cycles
(80 hours).  The second media of similar physical characteristics lasted
twenty-six cycles (52  hours). When clogging occurred, it happened
almost instantaneously.

       First Media - No. PP1Y1362, plain weave; multifilament
       polypropylene, thread count of 60  (warp) and 27  (woof),
       weight of 9.78 oz.  per sq. yd., porosity  of 8.5  cfm

       Second Media - No. PP2Y2362, Oxford weave, multifilament
       polypropylene, thread count of 72  (warp) and 22  (woof),
       weight of 11.8 oz.  per sq. yd. , porosity  of 10-14 cfm

For the third media, it was decided to use a monofilament nylon which
then performed satisfactorily the remainder of the testing period of
160 cycles, or in excess of 200  operating hours.  At the end of the
                              37

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testing period, the m on of i lament media had no evidence of clogging or
deterioration.

      ' Third Media - No. NY1S9611, plain weave; monofilament
       nylon, thread count of 107 (warp) and 101  (woof),  weight
       of 3 .3 8 oz.  per sq . yd., porosity of 582.7 cfm

Sludge Characteristics

Sludge to be dewatered is bottom sludge from heated digesters and
originates as thickened sludge from primary settling tanks, and second-
ary settling tanks following two-stage trickling filters (intermediate and
final).

Sludge was removed directly from the digesters and conveyed to the
pilot plant pressure filter site in a three compartment tank.  Each com-
partment held  approximately  150 gallons which was sufficient for one
cycle, or three cycles per load.  Sludge was used  from all eight digesters,
however, the majority of the sludge used for testing came from a second-
ary digester operated in series with a primary digester.  In the future,
all digesters will be operated in series as primary and secondary units,
or stage  digestion.

General Sludge Characteristics

       Total Solids           4 - 7%

       Volatile Solids       47 - 57%

       pH                  6.8-7.3

       Volatile Acids        200 mg/1 as Acetic Acid

       Alkalinity            2800 - 3500 mg/1 as  CaCO3

An extremely high ratio of industrial wastes to domestic wastes exists
at Cedar Rapids.  This  is reflected in the range of raw sludge charac-
teristics which often is also reflected in digester  operation and its
digester bottom sludge. This ratio of industrial to domestic wastes  is
in the range of 5 to 1 as measured in  (BOD) population equivalent.  In
addition  to domestic wastes  from a population of approximately  110,000
persons, there are industrial wastes from meat processing; grain proc-
essing to starch, syrup, and cereal products; dairy operations; metal
plating and over 100 miscellaneous smaller industries.
                               38

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An example of extreme changing sludge characteristics during the pilot
plant evaluation, which would not be normally anticipated,  came from
the meat processing industry.  Municipal ordinances restrict the grease
content of wastes discharged to the sewer to 200 mg/1.  To  comply with
this ordinance, a major meat processing firm undertook extensive in-plant
modifications, at which time considerable quantities of grease were  by-
passed to the waste treatment plant.  This resulted in poor settling in
the primary clarifiers and poor sludge thickener operation. The digester
operation was frequently disrupted creating a low quality bottom sludge
with high grease  content.  The early pressure filter runs  were somewhat
erratic because of this, and the filter media blinded due  to high grease
content.  Blinding of the media was a very sudden occurrence, rather
than a progressive one.  However, some progressive blinding character-
istics were observed in the precoat operation.

        Filter Run            Grease Percent
           No.            (% Total Dry Solids)

           34                   5.21
           37                   8.16
           39                 13.3
           41                 16.3
           50                   8.0
           70                 16.6
         100                   3.1
         106                   1.9
         108                   1.7
         110                   1.0

The grease content which  caused so much trouble was reduced by improve-
ments at the industrial source, and thereafter the filter operation was
more uniform and  dependable.

Fly Ash Characteristics

Fly ash used as sludge conditioner in this pilot plant study was obtained
from a local fossil fuel fired,  steam power generating station.  At this
plant, coal is pulverized and blown into the furnace for suspended com-
bustion. The total ash is principally in the form of fly ash removed
through cyclone collectors. This material is  spherical in shape with
about 95 percent passing a 200 mesh sieve.

At the present time,  and during the pilot plant study, fly ash was trans-
ported in open dump  type trucks  from overhead bunkers.  To  move this
                              39

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 material through the City, it was necessary to wet down the load.  Conse-
 quently, the fly ash was received with varying moisture values. Outdoor
 storage also affected the moisture content.  This varied from 23 to 35 per-
 cent moisture as it was applied to the sludge mixture.  Fly ash, in dry
 condition as it is recovered at the power plant,  has a tendency to "float"
 and is difficult to mix into the sludge mass.

 For pilot plant operation, the  moisture content was not critical  because
 proper allowances could be made in  the calculations, however, in a full
 scale plant, consideration should be given to receiving the ash in a dry
 condition.

Ash was  hand fed through a screw conveyor to the sludge conditioning tank
 The quantities were pre-calculated on a dry solids  ratio after total solids
 determinations were made on both ash and sludge.

Automation of material handling facilities for a full scale plant would
minimize pre-calculations and ash handling.

Laboratory Control

Primary consideration is given in laboratory control to the accurate and
 speedy evaluation of the dry solids content of the sludge,  fly ash and
 cake.  From this  information most operational control data are determined.
The principal  control data are  the fly ash to sludge ratio and specific
 sludge yield.

To accomplish the above evaluations  it was necessary to divide the
laboratory control program into two phases, on site and laboratory anal-
ysis.  To determine sludge dry solids concentrations  and fly ash moisture
content on  the site, an Ohaus  moisture determination balance was used.

The moisture balance is a ten  gram balance with optical projection read
out to a hundredth (0.01) of a  gram.   Drying is done by an infrared lamp
which has time and heat level controls to adjust for differing samples.
Operation of the balance consists of measuring 10 grams of the  sample
in a  tared aluminum dish and setting the heat and timer controls. The
timer shuts off the heater after a predetermined length of time.  When
the sample has been dried, solids content  is read directly to the nearest
tenth of a percent (0.10%).  Drying time is determined by plotting weight
against time at various heat levels until a  time and heat level is found
which gives the fastest drying  time without burning the sample. By using
this  balance it was possible to obtain the solids content of a  sludge in
20 minutes and the solids content of fly ash in 5 minutes.
                               40

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   Infrared
   Heater
Timer
                                                      Heat Control
                                                     Tare Control
                    OHAUS MOISTURE BALANCE

The accuracy of the  moisture balance was +_ 4.0 percent of laboratory
results.  However, laboratory data  are necessary to completely evaluate
the facilities.

Two samples of the sludge were taken for each pilot run.  These were
composited in the  laboratory and triplicate analyses for solids were
performed.  The sludge was also analyzed for grease content.  Grease
was  determined by drying with magnesium sulfate monohydrate and
extracting in a  Soxhlet apparatus with petroleum ether.

A sample of fly ash and sludge admixture was  taken and analyzed in  the
same manner as the sludge. This enabled back calculation of the fly ash
and sludge contents. These samples  confirmed the  fly ash to sludge
ratios obtained by  dividing the dry fly ash weight by the weight of the
dry solids in the sludge.  This confirmed that there  was adequate mixing
in the sludge conditioning tank and  that the sludge and fly ash samples
were representative.
                                41

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Two cake samples were taken by quartering the entire cake and completely
mixing one quarter.  The two samples of the mixed cake were .then com-
posited in the laboratory and analyzed for moisture in triplicate.  All
moisture samples were analyzed as residue by evaporation at 102 C-
after drying 16 to 24 hours in a  forced draft oven to constant weight.

Filtrate samples were grab samples taken during the run to determine the
relative suspended solids concentrations.  Suspended solids were deter-
mined by filtration through a glass fiber filter mat in  a Gooch crucible.
The  suspended solids present in the filtrate appeared to be essentially
precoat which was forced through the filter  under the initial filtering
pressure.                                                 

The filtrate was analyzed also for BOD and  COD. The BOD samples were
seeded to ensure an active aerobic bacterial population.  Both the BOD
and COD tests were performed according  to the 12th Edition of Standard
Methods. Good correlation was obtained between the BOD and COD,
where the BOD equaled 0.6 COD.

Filtrate Characteristics

The filtrate flow rate was monitored by using a container and a  stop watch.
The filtrate quantity was then measured in a graduated cylinder and the
flow rate computed.

Filtrate flow rate observation began at the end of one minute and contin-
ued to the end of the cycle.  At  one minute  for the higher fly ash ratios
(2.00 to 1), the flow rate is approximately 0.15 gallons per minute per
square foot.  At lower fly ash ratios of approximately 1.5, the filtrate
rate is 0. 10 to 0.12 gallons per minute  per  square foot.  At the end of
the cycle, the filtrate rate is about 0.007 gallons per minute per square
foot. A typical filtrate flow rate versus time is given in Figure 18.

The filtrate from the pressure filter is nearly free of suspended  solids.
At the start of each cycle, the filtrate contains approximately 400 mg/1
suspended solids but this rapidly diminishes to less  than 10 mg/1 sus-
pended  solids.  It was determined by the  physical characteristics and
microscopic analysis that the small amount of  suspended solids present
primarily originated from the fly ash precoat and were not fines from the
sludge itself.
                               42

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                         FILTRATE  RATE   VS TIME
         0.12
         0.12
      c
      1
         0.08
      o
      0>
         0.06
      UJ

      cc

      UJ
      CC
      I-
        0.04
        0.02
                               40        60       80       100
                             TIME    (minutes)

                             Figure 18
Figure 19 illustrates the suspended solids content at varying times during
the filter run.  Note the rapid decrease in suspended solids to nearly zero
milligrams per liter,  Because of the low  suspended solids content of the
filtrate,  it can be discharged directly to the biological treatment unit.
                                43

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   400[
FILTRATE  SUSPENDED SOLIDS  VS.  FILTER  TIME

                 i	'1
8  9
TIME
_1_.
10 II  12 13  14-30
 (minutes)
                                                      40
                                            50    60
General Filtrate Analysis

       PH
       Suspended Solids
       Total Solids
       BOD
       COD
       Color
                              Figure 19
                8.9 - 10.2
                <10 to 400 mg/1
                2000 to 3600 mg/1
                180 to 1200 mg/1
                312 to 1700 mg/1
                Pale Yellow
The high pH of the filtrate results from the fly ash which contains several
soluble alkaline metallic oxides  such  as calcium, magnesium, potassium,
and sodium oxides.

Since the pressure filter acts as  an efficient solids-liquid separator,  the
BOD in the filtrate is dissolved organic material. The BOD of the filtrate
varies with the general conditions of the digesters.  During  the test period
the BOD of the filtrate remained at the lower values of 180 to 300 mg/1
for extended periods of time. At intervals the BOD of the filtrate increased
upward to 1200 mg/1.  This value in itself is  not significant, however,
                              44

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 the range of values is significant and probably reflects increased hydrau-
 lic and organic loadings to the digesters during periods of unusual
 industrial wastes loads to the treatment plant.  The pounds of filtrate
 BOD to secondary treatment at the highest observation is such a small
 part of the total that it is insignificant to this study.

 Test Results

 The evaluations and conclusions in this report are based  upon 222 complete
 pressure filter runs on Cedar Rapids digester sludge.  Sludge was with-
 drawn  from all digesters and every effort made to obtain representative
 samples for dewatering.

 Considerable  operating difficulties were experienced for a time due to
 unusual and varying characteristics of industrial wastes received at
 this plant.  Particular problems occurred when wastes extremely high in
 animal fats adversely affected the digester operation and in turn reduced
 the filterability of the digester bottom sludge.  Because of these problems
 some of the early filter data were erratic and did not classify well. This
 was frustrating to the operation of the pilot plant, however,  it did reflect
 the potential problems characteristic  of a plant operating  with complex
 industrial wastes.  These pressure filter operating problems  resulting
 from sludge with high grease content  were very similar to earlier ones
 experienced with the pilot vacuum filter at this plant. Fly ash used as
 a filter aid was obtained from a  local power generation plant and was
 considered to be of uniform quality during the period of study.  This was
 confirmed by analysis  by the power company's laboratory  at the beginning
 and end of the study.

 Most work was performed by a two-man team. After the initial startup,  it
 required about 20 minutes of cycle preparation, which consisted of pre-
 coating the filter, transferring the ash/sludge mixture, and charging the
 pressure filter.  During the cycle, the second batch was prepared by one
man, while the other man prepared laboratory data, or was free to perform
assignments elsewhere.

 The filter was large enough to render  an approximate  evaluation of the time
required to operate a full scale facility.  This equipment did not require
 constant operating attention.  The total time spent at operation was dic-
 tated by the laboratory evaluations desired.  It is assumed that a full
 scale plant would  shut down automatically at the end of a predetermined
 cycle.   The  cake would be discharged and a  new cycle started at the
 convenience of the operating personnel.
                               45

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An operating data sheet was prepared for each filter run and was processed
through the  laboratory to include all of the desired data for that particular
run.  A copy of the data sheet is shown in Figure  20.

The initial step was to precoat the filter for protection  of the media and
to improve cake discharge.  The precoat rate is an arbitrary figure and was
determined to be 0.27 pounds ash per  square foot as previously calcu-
lated in this report.

Filter runs were made over a  range of digester solids varying from 3 .2  to
7.6 percent, with  the majority determinations in the range of 4.5 to 5.5
percent.  It  has been observed at Cedar Rapids  that it is difficult to
consistently produce digester bottom sludge above 5.5 percent.  Sludge
thickening should  be considered in a full scale  plant facility on  digested
sludge preparatory  to dewatering.

Fly ash to sludge ratios are all  expressed on the dry sludge solids basis.
These ratios were  varied over the range of sludge characteristics.  This.
was done  in an attempt to demonstrate the optimum fly  ash/sludge ratio
irrespective of material handling economics.  The ratio was carried
considerably above that which would be economical for municipal sludges.

Filter runs were classified into  groups having similar characteristics of
percent digester solids and fly ash/sludge ratios.  These values were
plotted to establish performance curves for Cedar Rapids' sludge and to
ultimately develop design criteria to evaluate a  full scale pressure filter
sludge dewatering facility.  This objective and  procedure was  similar to
the earlier evaluation of sludge  dewatering using a pilot plant  vacuum
filter.

The primary  objective was to obtain a filtration  rate (yield)  in terms of
pounds of dry sludge solids, per hour,  per square foot  of filter media.

Performance curves were established plotting the filtration  rate (yield) in
terms of pounds per hour,  per square foot, versus time in hours for various
sludge solids densities.  Figures 21, 22 and 23 are for 4.5, 5.0 and 5.5
percent sludge  solids.

A general  observation indicates  that as the fly ash/sludge ratio is increased
to a given level, the filtration rate decreases.  The optimum yield is the
highest yield at the lowest fly ash/sludge ratio consistent with acceptable
cake discharge and mechanical  protection of the filter media.  For this
reason, a considerable amount of field  interpretation is necessary and
the plotted performance values are  not an answer in themselves.
                               46

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                   ;EDAR RAPIDS PRESSURE FILTER PILOT PLANT
Date
Digester #
PH
Quantity of Sludge
Run# Time Start
% Solids
% Volatile
Gallons Vol. Acids
Time End
Avg.
Avg.
mg/1
Remar cs:
ly Ash:  % Solids	Avg..
         Weight Wet	x (%S	) =	Weight Dry




       + Sludge: %Solids	% Volatile	




                 Fly Ash/Sludge	pH	




Precoat:  Lbs. Fly Ash (Wet)	x (%S	)  =	     Lbs. Dry




Filtering: Lbs. Pressure  High	to	low.  Length of run	rrrs .




Fjjtrate;  Turbidity	TS	mg/1        VS	%




        COD	ppm.    BOD	ppm.  Susp. Solids	ppm.




        pH	G aliens	Gal. /SF/Hr.	




Filler Cake;  % Solids	% Volatile	




             Weight Wet	x (%S	) = Weight Dry




             Dry Cake Density -^2.25cf.	Ibs./cf. Total Gallons Filtered	




             Yield (Lbs. Dry Cake-^-36 SF)	Ibs./SF Media




             Filter Rate	Ibs./SF/Hr.




Remarks:	
                                     Figure  20
                                       47

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                  FILTRATION  RATE  VS  CYCLE  TIME
                        (45% SLUDGE  SOLIDS)
 2 0 r
--'  i o
u.
o  9
w 0.8


 0 6

cri 0 5
CD
_J
  O 4

Q

ill 0 3
>-
 0 2
   0 0
                         -1.8:1  FLYASH /SLUDGE RATIO
        A 1.5:1

        O 1.8:1

         2.1:1
               0 5
I 0
I 5
2 0
2 5
                     3 0
                           TIME
                                         HOURS
                               Figure 21
Cake discharge characteristics were observed at the end of each filtering
cycle and evaluated to the fly ash/sludge ratio.  As the filter plates are
opened,  the formed cake should break clean from the filter media and drop
away as  a  single mass.

It was observed that satisfactory cake discharge characteristics could be
maintained at fly ash/sludge ratios of 0.5/1 and with adequate precoat on
the filter media satisfactory cake discharge should be maintained at much
lower fly ash/sludge ratios.
                              48

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    CO
    CD
        :.o i-
    o
    CO
    x
    tr
 1.0
0.9

0.8
    00,7
    I
      0,6
      0.5
    Q O.4
      0,3
      0.2
             FILTRATION  RATE  VS   CYCLE  TIME

           AT VARYING  F L Y A S H / S LU D G E   RATIOS

                    (5.0% SLUDGE  SOLIDS)
                  '5
                     1.0

                       TIME
1.5       2.0

HOURS
                                                       2,5
3.0
                               Figure 22


Plotting filtration rate versus fly ash/sludge ratio for a given percent
sludge solids develops  a reverse curve.  As the fly ash/sludge ratio is
increased the filtration  rate is increased until an optimum ratio is reached,
A further increase in the fly ash/sludge ratio causes a decrease ui the
filtration rate (yield).
                               49

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    2.0
  - 0,9 i-
  CO
  "- 0.7 -
  o:

  go.e
   0.4
  UJ
  >-
   0.3
   0.2
                       FILTRATION RATE VS  CYCLE TIME
                    AT VARYING FLYASH / SLUDGE  RATIOS
                            (5.5 % SLUDGE SOLIDS )
           -2.1.1
O 1.2:1

A 1.5:1

 .81

A 2.1:1
               0.5
                         1,0
                         1,5        2.0
                     TIME  (HOURS)
                                                      2.5
                                                                3.0
                               Figure 23

The optimum fly ash/sludge ratio appears to be less than 1.8/1, as
shown in Figure 24 for 4.5 percent solids and Figure 25 for 5.5 percent
solids for varying filter runs from 0.5 to 2,0 hours.

At the 0.5 hour (30 minutes) filter interval it was difficult to obtain a
truly  representative sample for analysis because the filter cavity was
not completely filled with cake before beginning a new cycle.  From an
operational point of view the filter cavit should be completely filled
with cake before beginning a new cycle and therefore,  any filtering
period of less than 0.5 hour (30 minutes) would not  be  practical for
design purposes.
                                50

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 FILTRATION   RATE   VS  FLYASH/SLUDGE   RATIO
         AT  VARYING   FILTRATION TIME
             (4,5%   SLUDGE  SOLIDS)
1 . C.
. 1.0
6
n
v <8
LC
O
^ 0.6
./i
m
0.4
a
_j
bJ
> 0 2








,^^-



0.0 1



/

. 	 '(




 r

y^


f^~


(
C

o *c
'0 c
-o  -c
\

) 0.75 HOUR
) 1 . 0 HOUR

) 1 ,5 HOUR
)2,0 HOUR
}3,0 HOUR

1




i
1 J
     1.00   1.25    1,50   1.75   2.00  2.25   2.50
           FLYASH / SLUDGE   RATIO

                     Figure  24

FILTRATION  RATE   VS  F L Y AS H / S L U D G E   RATIO
         AT  VARYING  FILTRATION  TIME
           (5 ,5 %  SLUDGE  SOLIDS)

  0.8
a:
o
-
co
CD
Q
_J
LJ
  0.6
  0.4
  0.2
  0.0
    1,00    1.25   1,50   1.75   2,00   2.25   2,50
           FLYASH / SLUDGE   RATIO

                    Figure  25
                       51

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Percent cake moisture decreased at a rapid rate with an increase in
filtration time up to approximately one hour.  A very slight decrease
in cake moisture values was experienced after about one hour of
filtration.

This data is given as Percent Cake Moisture vs. Filtration  Time in
Figure 26 for 4.5 percent  sludge and Figure 27 for 5.5 percent  sludge.
Note the similarity of curves for varying fly ash/sludge ratios.

The rate of development of cake quality is about the same for all fly
ash/sludge ratios, however, the cake moisture value improves with
an increase of fly ash/sludge ratio.  Yield is expressed as percent
of sludge solids only.

          % CAKE  MOISTURE  VS   FILTRATION  TIME
            AT  VARYING   PL YASH / SL UDGE  RATIOS
                       (4,5 % SLUDGE  SOLIDS )
              0.0    0,5    1.0    1.5    2.0   2.5    3.0
                              Figure 26
                               52

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% CAKE  MOISTURE  VS  FILTRATION  TIME
  AT  VARYING  FLYASH / SLU DGE   RATIOS
            ( 5.5 % SLUDGE  SOLIDS }
 90
 30
    ,0    0.5
1,0    1,5    2.0
  TIME  HOURS
2,5   3,0
               Figure 27
                  53

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                            SECTION VI

                  BASIS OF DESIGN FOR FULL SCALE
              PRESSURE FILTER DEWATERING FACILITIES
Introduction
Pilot test data compiled during the pilot scale pressure filter program
was interpreted as a basis of design for a full scale pressure filter
plant.  During the period of study and of in-plant evaluations to plan
solids handling facilities at Cedar Rapids, Iowa, numerous  systems
were evaluated and cost  estimates prepared.  The primary objective of
these investigations was to determine the most economical system  for
dewatering sewage sludge at Cedar Rapids.

At the  time of the original submittal to Research and Development for
grant consideration in July 1967, pilot work had been completed using
a vacuum filter and fly ash filter aid. The grant offer was made in
March  1968 and during that time a pilot model pressure filter had become
available and was operated at Cedar Rapids.  Economic evaluations of
handling power plant fly ash were less favorable than those of incinerated
sludge ash and it was determined to install sludge incineration and re-
cycle sludge ash  as a filter aid.

For the application at Cedar Rapids the pressure filter and the multiple
hearth  incinerator was the most economical combination.  Evaluating
capital investment and full operating  costs, the pressure filter-inciner-
ator combination was  estimated to be  10.3 percent less cost per ton of
dry sewage solids dewatered and incinerated over the vacuum filter-
incinerator combination.  These cost  comparisons are given later in this
section.   Other dewatering systems considered were even less favorable
in cost comparison.  Factors other than cost which influenced the selec-
tion of the pressure filter over the vacuum filter are also stated later in
this section. Some of these favoring  the pressure filter selection are:
less building space, less operator attendance, closed system with  fewer
odor problems,  drier cake, greater capture and clear filtrate, less power
requirements, expandable capacity of filter unit.

The full scale pressure filter sludge dewatering facility was designed on
the basis of experiences  with the pilot scale pressure filter and  with the
attitude that the performance of the pressure filter could be improved
upon.  The equipment and the process was intended to be sufficiently
flexible to allow a wide range of ash/sludge ratio  to be evaluated,
                              55

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and also to perfect other approaches to sludge conditioning for improved
dewatering such as chemical admixtures.

Any deviation from the intended program or process which may be con-
ceived, developed or demonstrated, and which would improve the
performance, should be  considered and undertaken in both the plant
design and in the plant study to follow construction.  Modifications to
the process, if successful, might accomplish one or more of the
following:

      1. Decrease operating time

      2. Improve the quality of the product

      3. Decrease the unit operating cost

      4. Encourage the development of new methods and new
         processes

      5. Give additional meaning to the Research and Development
         program

Filter Loadings

As discussed in  Section V, Pilot Scale Pressure Filter,  there appears
to be an optimum fly ash/sludge ratio for maximum filter yield.  As
evidenced by Figures 21 and 22, this optimum ratio appears to be less
than 1.8 parts fly ash to 1 part sludge at the  lower sludge solids con-
centrations of 4.5-5.0 percent.  However, at 5.5 percent sludge solids
concentration, the optimum ratio of fly ash to sludge appears to be 1.5
to 1.  (Figure 23). The  apparent optimum ash/sludge ratio determined by
the pressure filter performance may not be the optimum ash/sludge
design ratio when evaluated with the total sludge dewatering and dis-
posal process.  Factors influencing the selection of the ash/sludge
design ratio which are not equated in the pressure filter performance
data include:

      1. Material handling  facilities

      2. Volume of feed ash

      3. Relative moisture in the cake product

      4. Cycle time
                              56

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      5.  Volume of ash/sludge product to be dried or
          incinerated

      6.  Unit heat requirements of mixture for drying

      7.  Unit heat value of mixture for incineration,
          and others

To provide continuity to the pilot plant study and this report, we sized
the pressure filter and accessories on the basis of the performance at
a fly ash/sludge ratio of 1.5 to 1, and reported out alternate performance
at 1 to 1 fly ash/sludge ratio,  and also 1 to 1 organic ash/sludge ratio.

Referring  to either Figure 23 or Figure 25, at a  fly ash/sludge ratio of
1.5  to 1,  yield is found to be 0.75 pounds per  hour per square foot of
dry sludge solids at 1.5 hours' filtration time.   Figure 27 indicates that
the cake moisture would be 52  percent (cake solids 48 percent) which
by normal sludge dewatering standards is a very dry cake.  Further
reference to Figure 27 shows that it would be impractical to extend the
filtration time beyond 1.5 hours because the moisture content of the
cake would not further diminish in proportion to the time used.  On the
other hand, the filtration time might well be decreased to about one
hour and not appreciably increase the moisture content of the filter cake.
For basic design,  a 1.5 hour filtration time was used.

Between filtration cycles, during which time the cake will be discharged
and preparations made for a new cycle of operation, an allowance of
30 minutes was made.  This is  a generous amount of time, however, it
does not greatly affect  the total cost of operation if it is estimated to
be 10 minutes or 30 minutes.  Unlike the vacuum filter,  the pressure
filter does not require constant attendance.  It is,  therefore, possible
for the operator responsible for the pressure filter to be performing other
duties between filter cake discharges, and he may  elect to delay recy-
cling immediately after the predetermined 1.5 hour  filtering cycle.  Or,
the operator may quickly discharge cake and recycle in five or  ten
minutes and return to other duties to be delayed on the next cycle. The
total hours of filtration would remain  the same at 16 hours per  24 hour
day.

Sake Production and Handling

It was planned to have  the  pressure filter and accessories as completely
automated as was  practical and to monitor most functions from  a central
console.  Based upon operating experiences with the pilot scale plant,
the method of controlling the filtration cycle would be:
                               57

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 1.  At preset time interval of filtration.  At the end of this time, filtration
 would cease and the operator would discharge the cake and recycle the
 process.

 2.  At preset pressure value.  Theoretically, pressure should build up at
 an increasing rate as the filter cake is formed and could be used to deter-
 mine the end of the  filtration cycle when the filter cavity is filled.  This
 theory of progressive pressure build-up might work in the filtration of a
 quality controlled manufactured product, however,  it does not appear to
 be a useful control in the filtration of sewage sludges.

 3.  A preset rate of  filtration.  By  this method the filtrate discharged
 from the pressure filter is metered and the filtration cycle is terminated
 at a given  rate of filtration,  such as 0.01 gallons per minute per square
 foot of filter media.  A  variation of this would be to meter the total volume
 of filtrate and the filtration cycle would terminate when a given value of
 total gallons was registered.  This facility should be provided on all
 installations.  Where it is not used to control filtration time, it may be
 used to monitor the physical condition of the filter media.

 Time,  pressure, and metered flow  information will be desirable  in the
 operation of the equipment and, therefore, all three methods might be
 readily available.  This installation will plan to utilize the first and
 third methods.   The  time interval will be adjustable to changing condi-
 tions or  requirements of sludge, ash,  cake moisture and other admixtures.
 The rate of filtration will be  metered to determine the end of the cycle.
At the end  of the filtration cycle,  the  operator would normally wish to
 discharge the cake promptly  and recycle. However, with the pressure
 filter it will not be necessary for the operator to do this promptly if he
is engaged in other activities.  Unlike other filtration equipment, the
pressure filter may be turned off at any time during the filtration process
and left  unattended for an indefinite period with no deleterious effect on
 operation or cleanup.

At the completion of filtration cycle, the operator standing at the control
panel will prepare to discharge the cake.  The procedure will be similar
 to that of the pilot filter. Sludge pumping to the filter will be terminated
 (if this was not done automatically as indicated above),  the unfiltered
 sludge remaining in  the filter core  and feed lines will  be discharged to
 the feed tank, conveyor equipment will be made to start, and the operator
will proceed to the pressure  filter.

Cake discharge is to be observed by the  operator after activating a con-
 troller to open the filter plates.  The cake will discharge  freely and the
                                58

-------
 large disc-shaped sludge cake will be sheared into small pieces by a
 series of steel cables strung across the receiving hopper. The broken
 filter cake will be transported by a conveyor system to the incinerator
 and then to ultimate disposal.

 If additional sludge is to be dewatered, the operator will close the filter
 plates and recycle the process from an operating console. The sequence
 of'filter precoat,  sludge filtration, and cake discharge will then be
 repeated.

 Filtrate Disposal

 Disposal of pressure filter filtrate should present no problem.  Filtrate
 from the pressure  filter is nearly free of suspended solids and those
 present originate from the fly ash, rather than fines from the  sludge  solids.
 For a short period of time at the beginning of the filtration cycle filtrate
 rate may be relatively high.  For the size of this facility, the filtrate rate
 may be about 650  gallons per minute for a  few minutes.  Within 10 minutes
 this value may decrease to about  25 percent of this higher rate and then
 continue to decrease to less than 10 percent of the maximum  rate when
 the filtration cycle is half completed.  Refer to Figure  19 for  a  typical
 filtrate rate of flow curve versus time.  Filtrate may be handled in one of
 several ways.

 1.  Filtrate may be discharged to a relatively small holding tank having
 10 to 30  minutes detention and then discharged at a lower rate directly
 to the biological (secondary) treatment units.  The first 10 minutes of
 pressure filtration will produce approximately 33 percent of the total
 filtrate volume  from  the cake and at 20 minutes approximately 58 percent
will be removed. (Figure  18.)  Thus, with about a  7,000 gallon detention
 tank, the filtrate may be discharged at a rate of about  100 gallons.per
minute to the plant for secondary treatment.

 2.  Filtrate may be recycled back into the dewatering process as makeup
water for the filter precoat with excess quantities discharged through the
plant for secondary treatment.

3.  Filtrate may be discharged to sludge thickening facilities where these
are available and where the high pH of the filtrate might be an aid to
 sludge thickening.

Filtrate quantities should be metered on a continuous basis as an instru-
ment of filter operating control.  Filtrate rate of flow and total filtrate
Per time  interval information can be used to adjust the  filtration cycle
and ash/sludge ratio.
                                 59

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Ash Requirements and Handling

Fly ash was available from two sources.  Both  sources are steam genera-
ting stations firing fossil fuel and are located approximately three miles
from the treatment plant.

Fly ash should be hauled in closed trucks and blown into storage.  Stor-
age facilities should be provided for a minimum of 2 to 4 days' ash
requirements.  This will provide an adequate supply of ash to continue
operation until other solids disposal arrangements can be made, should
either ash delivery or cake disposal systems be temporarily out of
service . Ash feed may be fed to the  slurry mixing tank using volumetric
feeders or controlled volume materials handling equipment.  An adjustable
speed screw conveyor may be calibrated with sufficient accuracy to
handle the large volume of ash required.  For this installation, gravimetric
feeders were used to measure ash to the screw conveyor.

Solids Preconditioning and Thickening

Solids  should be preconditioned in a sludge thickening facility prior to
pressure filter feed conditioning.  The pressure filter pilot study was
conducted without sludge thickening because this facility was not avail-
able.  However,  sludge receiving facilities are necessary to control the
application  to the sludge dewatering equipment and these facilities may
well serve a dual purpose. Any degree of sludge thickening will further
improve the performance of the pressure filter.   It was anticipated that
the filter yield would be materially increased with thickened sludge.

Detention times in the digested  sludge thickeners should be relatively
long,  in the range of 2 to 4 days1  detention.  With 5.5 percent solids
entering the thickener, it might  be possible to  consistently  come out
with 8  to 10 percent solids.  Small quantities of chemicals might further
reduce the ash/sludge ratios required.  It was  planned to do research  at
this facility to determine the minimum ash/sludge ratio to obtain the
maximum filter yield at the least cost.

Filter Cake Disposal

Cake discharged from the pressure filter having a moisture content of
only 50 percent can be transported and disposed of as landfill or soil
conditioner,  providing it is pulverized for the latter use.  Cake disposal
in this condition would be limited to the City landfill, and sparsely
populated areas for bulk dumping. Considerable quantities of pulverized
cake might be used as a soil conditioner and fill material by the Parks
                               60

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Department operating many municipal parks/ golf courses and,recreational
areas.  However, even at maximum utilization of this material by the
Parks Department, a large volume will  have to be disposed of by other
programs.

To reduce the volume of material to be  handled for disposal, and to
provide an in-plant source of ash for conditioning, incineration facilities
were installed.  The proposed process  will provide for disposal of solids
at one or more points in the dewatering process to best serve the  interest
of both the process operator and ultimate disposal as follows:

1.  Filter cake may be discharged in part or in total volume where soil
conditioning might require maximum nutrient value or where landfill
might require maximum bulk.

2.  Filter cake may be dried in part or  in total volume where the product
might be stored for  future use, or where wet filter cake might be offen-
sive, and in this manner maintain maximum nutrient value and minimum
bulk handling.

3.  Filter cake may be incinerated in part or in total  volume,  thus mini-
mizing the bulk handling requirements  and utilizing the resultant organic
ash as a sludge  conditioner and filter aid.  This will minimize ash
handling requirements as well as solids handling requirements.

It was planned to both dry and incinerate by using a  multiple hearth
incinerator with  provisions for reverse  air flow when only drying is
desired.  This type of equipment affords the greatest flexibility of
operation and with a reasonably low unit product  cost. More detailed
cost analysis appears later in this report.  Modern facilities include
gas cleaning devices to meet the local air pollution codes.  The minimum
operation and maintenance requirements of a multiple hearth incinerator
are compatible with the minimum operating attention  demanded of the
pressure filter used as the primary equipment for  sludge dewatering ahead
of incineration.  This combination will demand the minimum operator
attention and may be operated by the same personnel.

Other systems evaluated and cost comparisons made for Cedar Rapids
include:

       1.  Vacuum filter and rotary drier

      2.  Vacuum filter and flash drier
                               61

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      3. Vacuum filter and furnace incinerator

      4. Vacuum filter and multiple hearth incinerator

      5. Wet air oxidation and vacuum filter

      6. Pressure filter and multiple hearth incinerator
         a .  Fly ash filter aid
         b.  Organic ash filter aid

Summary of Design Data

Digester Solids

      Total Solids                       56,000 Ibs ./day

      Volatile Solids                    26,400 Ibs./day

      Percent Solids in Sludge           5.5

      Volume of Sludge Withdrawn       122,000 gals./day

Pressure Filter Loadings

                                           Fly Ash	Organic Ash
      Ash/Sludge Ratio                 1.5:1     1:1      1:1

      Filtration Time per Cycle Hours    1.5       1.0     1.0

      Total Cycle Time Hours            2.0       1.25    1.25

      Total Filtration Time Hrs./Day     16        16.7    14.4

      Cake Moisture Percent            52        60      50

      Yield Filter Cake Lbs./Sq.Ft./Hr. 0.75      0.65    0.75

      Filtrate Suspended Solids          Nil       Nil      Nil

      Chemical Required                None     None    None
                              62

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Other Plant Data

      Population            1968 Present          113,000
                           1988 Future           172,000

                           BOD Equivalent        810,000

      Plant Flow            Average, MGD         28.6

                           Wet Weather,MGD     64.4

Estimate of Costs

The original estimates of cost of construction were based upon the
August 1968 costs and were limited to the solids handling facilities
directly related to sludge dewatering and incineration.   For the conven-
ience of this report,  those  1968 costs have been scaled upward to
August 1972 costs based upon  the Engineering News-Record Cost Index
of cost trends  for construction and increased equipment costs.  During
the period since the original cost estimate (1968) and the present (1972),
construction costs have increased about 54 percent and equipment costs
about 20 percent. Other costs, such as operation, have inflated at
least 4 percent per year.

The cost of operating the proposed pressure filter and incinerator was
estimated from the power, fuel, materials and labor required at the
design quantity and quality of sludge to be processed.   These costs
were determined with an electrical rate of 1.8 cents per kilowatt hour;
off-peak natural gas  rates of 4.8 cents per therm for the first 500 therms
and 3 .85 cents per therm over. Labor was determined on the  basis of
continuous attendance.  Maintenance costs were based upon  known cost
of repair and replacement with consideration given to the equipment manu-
facturer's stated minimum estimates.
                              63

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      Ash/Sludge Ratio

      Hours of Operation
      Power Pressure Filter
      Power Incinerator
      Fuel Incinerator
      Fuel Deodorize Stack Gases
      Material Handling:

      Fly Ash to Plant

      Incinerator Ash Disposal

      Filter Cloth Replacement

      Equipment Maintenance

      Operating Labor
Total Estimated Operating Costs
                                     Cost per Ton Dry Solids (Sewage)

                                          Fly Ash    Organic Ash
   1.5:1
   16
  0.48
   1.46
  0.98
  1.89*
  1.25
  0.15
  0.52
  2.75
 1:1
 1:1
16.7     14.4
$0.60    $ 0.70  $  0.54
0.40
0.18
1.68     1.70
1.06
0.24
1.26*
0.97    0.34
0.16
0.56
2.75
0.14
0.56
2.75
$10.08    $  9.54  $ 6.45
      * The Power Company did not charge for the delivered fly ash,
        however, a minimum transportation charge likely would be
        charged in most cases.  To compare processes on an equal
        basis an estimated  charge is added for transportation.

Assuming amortization of bonds at 4f percent interest for 20 years, the
capital cost per ton of dry sewage solids would be $20.00 based upon
the estimated 1972 construction cost of $2,570,000.   The total cost
per ton dry sewage solids, including capital investment and operation,
would be as follows:
                              64

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                          Fly Ash/Sludge Ratio    Organic Ash Ratio

 Ash/Sludge Ratio            1.5:1      1:1            1:1

 Capital Investment        $20.00      $20.00        $20.00

 Operation & Maintenance    10.08        9.54          6.45
                          $30.08      $29.54        $26.45

 Valuations of Other Solids Handling Systems

 During the period of planning solids handling facilities at Cedar Rapids,
 numerous systems were evaluated and cost estimates prepared. For
 the application at Cedar Rapids, the pressure filter and multiple hearth
 incinerator combination is the most economical and offers many additional
 advantages which are difficult to equate to a dollar value.  Because of
 the great amount of extra work done in the plant operation,  in the  labora-
 tory, and in design calculations, it is warranted to at least report briefly
 on the  systems considered.  It should be clearly understood that these
 evaluations are based upon sewage solids conditions at Cedar Rapids,
 and it is not intended to be a recommendation for other installations,
 nor does it imply  criticism of those existing.

At the time of the original submittal to Research and Development  for
 grant consideration, pilot plant work had been completed using a  vacuum
 filter.  This was in July 1967.  The grant offer was not made until March
 1968.  During that time a pilot model pressure filter became available
and was operated at Cedar Rapids and is the main subject of this  report.

 To effect a fair and equal basis for comparison  of costs for several
 Sludge dewatering systems,  all previous cost estimates are projected
 to 1972 construction costs based upon the Engineering News-Record
Index of Cost trends as previously stated.  All  sludge thickening  facil-
 ities were omitted from these comparisons, and all other accessories
were made to be exactly equal.  As an example, all product haul was
 limited to three miles  whether it be wet  cake or incinerator ash. All
 costs of both the  vacuum filter and the pressure filter were  based  upon
performance at a 1:1 ash/sludge ratio because most of  the vacuum filter
Work was  conducted at this optimum ratio.

The estimated costs per ton of dry sewage solids were determined from
 the following sources.
                               65

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1. All equipment costs are budget costs quoted by various manufacturers.

2. Building costs to house the equipment are estimated by the Engineer
and are minimum building requirements.

3. Fuel and electrical costs are based upon equipment requirements and
Cedar Rapids rates.

4. Labor costs are at plant rates.

5. Labor for operation and maintenance was determined by the Engineer
and was considered approximately equal for all installations.  This is
conservative as it is reasonable  to suspect that the vacuum filter main-
tenance costs are considerably greater.

Many advantages and disadvantages have been proposed by many authors
and equipment representatives for each and every item of equipment.  The
merits of these are too of ten shaded by prejudice, and at best have little
real value.  Conclusions and observations from actual pilot plant perform-
ance using both a vacuum  filter and a pressure filter at Cedar Rapids  are
shown on the following page.
                             66

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                                           Operating Cost
                              Amortize   Dewater Dewater and  Total
               Construction 20yrs,@4|% Only    Dry or Incin. Cost
	Costs	Per Ton    Per Ton  Per Ton	Per Ton

Vacuum Filter
and Rotary Drier  $2,135,000   $16.55   $11.25   $25.70*      $42.25*
                                                             Dry

Vacuum Filter &
Flash Drying
Furnace           2,155,000    16.75    13.05   23.00*       39.75*
                                                             Dry

Vacuum Filter &
Flash Drying
Incinerator        2,600,000    20.40    13.05   20.05**      40.45**

Vacuum Filter &
Multiple Hearth
Furnace           2,080,000    16.15    11.25   16.80**      32.95**

Pressure Filter &
Multiple Hearth
Furnace
(Fly Ash Aid)      2,570,000    20.00     5.00     9.54**      29.54**

Pressure Filter &
Multiple Hearth
Furnace
(Organic Ash Aid)   2,570,000    20.00     4.12     6.45**      26.45**
 * Dry cake approximately 12% moisture
** Incinerated ash
                             67

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 Near full time attendance required for
 one operation
                                         Va cuum
                                          Filter
   Yes
 Is a cleanup shift required?                  Yes

 Can one operator perform all shift duties?     Yes

 Odors produced from operation            Considerable
Odor control accessories required
due filter

Chemical required

Ash additive required

Cake, percent solids

Filtrate, suspended solids, mg/1

Cloth life,  estimated hours

Power requirements, with
accessories,  KWH

Can filtration area be increased?

Operating Costs:
      Capital investment for equipment
      Capital investment for building

Incinerated costs per ton dry sewage
solids
   Yes

   Yes

   Yes

  33-38

2000-6400

2000-4000


   1430

   No
 50% more
 $32.95
                 Pressure
                   Filter
    No

    No

    Yes

    Nil


    No

    No

    Yes

  40-55

 10-400

6000-8000


   1080

   Yes


 20% more



 $26.45
Obviously, there are many other items of comparison, some significant
to treatment requirements, and many of little importance.  The most
important consideration, and the one which is often disguised by report-
ing it in part rather than in total,  is how much will the total cost be for
both capital investment and  complete operation.  As reported earlier,
the total cost per ton of dry  sewage solids disposed of strongly favors
the pressure filter.
                              68

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                            SECTION VII

         FULL SCALE PRESSURE FILTER DEWATERING FACILITY

 Treatment Plant Description

 The full scale Solids Handling Facility at Cedar Rapids was planned in
 1968, bids were received and contracts awarded in December 1969, and
 construction was completed in the spring of 1971.  During the remainder
 of 1971 the sludge dewatering and incineration processes were checked
 out.  The full scale  Research and Development program for sludge de-
 watering with ash filter aid was carried out during the calendar year
 1972, and is the subject of this report.

 To better understand the size and operation features of the sludge de-
 watering facility,  it would be well to briefly discuss the basic water
 Pollution control plant from which the  sewage sludges originate.  An
 aerial view of the treatment plant is shown in Figure 28.  The original
 plant construction was done in 1929 and at that time the plant served
 only that portion of the Cedar Rapids population served by sanitary sewers.
 Now the plant serves all of the metropolitan area, including Cedar Rapids,
 Parts of the Town of Hiawatha, and numerous  suburban residential areas.
 Primary and secondary biological treatment and separate sludge digestion
 was completed in 1933, with chemical-mechanical separate industrial
 waste treatment also constructed at that time. Expansion of all  phases
 of the treatment processes, along with the  addition of intermediate stage
 trickling filters  was  made in 1955.  Separate sludge thickening and super-
 natant oxidation facilities  were completed in 1965.  A second major plant
 expansion, whereby  the plant capacity was doubled to the present cap-
 acity, was completed in 1966-67.

 The entire facility, except the final clarifiers, is covered with thin-shell
 domes and the exhaust air ozonated for atmospheric control.  Over 8 ac.
 (378,500 sq.ft.) of process units are under cover to assure high effi-
 ciency during cold weather and to control the potential source of odors.
Five separate air handling facilities evacuate  the air from under the
domes at a rate of over 120,000 cfm and deliver this air to a detention
chamber after ozonation.  The ozone installation represents the largest
installation of its kind on waste water  treatment with a capacity of
 150 pounds per day.  The largest  domes are 167 feet in diameter.
                                69

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                              Figure 28
The plant processes are:
      Primary
      Secondary
Screen chambers
Raw lift pumping
Grit removal
Preaeration
Primary clarification

Plastic media roughing filters (first stage)
Intermediate clarifiers
Final  rock  filters (second stage)
Final  filters
Separate sludge digestion, raw sludge thickening, supernatant oxidation,
digested sludge dewatering and incineration.
      Flow
28.6 MGD average
64.4 MGD maximum
                                 70

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      Organic        Raw BOD      135,000 pounds per day
                     Effluent BOD     8,500 pounds per day

      Population     Approximately  113,000 persons
                     Equivalent BOD approximately 800,000 persons

The digested sludge dewatering and  incineration facility was completed
in 1971.  It consists of parallel trains of sludge dewatering facilities
followed by multiple hearth sludge cake  incineration. The  two sludge
dewatering trains consist of 50 foot  diameter sludge holding tanks,
sludge pumping  and metering, ash feeders, sludge-ash mix tank, sludge-
ash contact tank, conditioned sludge feed pumping, pressure filter,
filtrate removal, cake storage bunker, followed by a multiple hearth
sludge incinerator.

General Description of the PressjjreFUter Process

Digested solids are periodically pumped from the secondary digesters to
the solids holding tanks.   Bottom solids from the sludge holding tanks  are
removed by pump on a demand basis and sent through preconditioning
facilities  to the pressure filters where the sludge is dewatered.

The sludge preconditioning facilities consist of sludge grinders; mix tanks
where ash and/or chemicals are added as a filter aid; contact tanks where
slow mix of filter aids and flocculation occur; and variable  rate sludge
pumps to feed the filter.

The pressure filters consist of a series of plates covered with a nylon, or
similar, filter cloth.  Sludge is pumped through the filter leaving a solids
deposition on the filter cloth.  This  solids formation/ or filter cake,  is
periodically discharged at the end of the filter  cycle.  The filter cycle
may be 1 to 2 hours depending upon  numerous variables.

Cake discharged from pressure filter is broken  and  then fed to the multiple
hearth furnace for drying or complete incineration.  Filtrate removed from
the solids in the pressure filter is discharged back through  the plant for
further treatment.

The multiple hearth furnace may be operated as a cake drier, or as an
incinerator.  As a drier,  the cake will be removed from the bottom and
used as a soil conditioner at about 12 percent moisture.  As an  incinerator
the combustible materials in the cake will be destroyed and the ash remain-
ing will be disposed of as a low grade soil conditioner.   The volume of
                                 71

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incinerated ash will be a small fraction of the volume of dried cake. The
complete process is observed from a panelboard, and operating records
established.  The basis of design is:

      Total solids to digester-lbs/day           92,000
      Total solids from digester-lbs/day         56,000
      Volatile solids from digester-lbs/day      26,400
      Percent solids in sludge                  5.5
      Gallons sludge withdrawn per day        122,000
      Period  operation-hrs/week               112
      Ash to  sludge ratio                       1.5 to 1
      Filtration time per cycle-hours            1.5
      Cake moisture -  percent                 52
      Yield filter cake-lbs/sq.ft./hr           0.75
      Cake production  with fly ash:
        Sludge-lbs/day                         56,000
        Fly ash-lbs/day                        84,000
        Total solids-lbs/day                   140,000
        Moisture 52%-lbs/day                 152,000
        Total cake-lbs/day                    292,000
        Operating schedule-hrs/week          112 (plus startup and
                                                   shutdown)
        Incinerator feed rate-Ibs/hr             18,250

Pressure Filtration Technology

It is not the purpose of this report to  expound upon the  theory of filtra-
tion,  or specifically pressure filtration. There are volumes of literature
published on the theory and practice of sewage sludge filtration, most
all of which relates to  vacuum filtration, with an occasional comment on
pressure filters or variable volume filters.   In this  country there is a Very
limited reference to pressure filtration theory and practice. This project
was researched and developed by the use of pilot scale filters, starting
with bench work and progressing to a small sized,  production model
pressure filter pilot plant as previously described.  At that time there
was no readily available analytical data in  this country on the operation
of the pressure filter.  However, if there had been  other detailed per-
formance data available, the pilot plant approach for the development
of the dewatering facility would still have been the recommended
approach.

Pressure filter knowledge was not sufficient to provide  information on
which to predict or calculate performance.   It is still a fact that the
                                72

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state of present knowledge is very limited and it is necessary to examine
each material in the laboratory and, if possible on the pressure filter
pilot plant, to determine its filtration characteristics, to obtain some
indication of filter performance, and to provide basis to calculate a full
scale plant design.  A brief review of the pressure filtration process may
be of interest. The object of any  sludge dewatering process is to recover
the solid  content in a concentrated form suitable for disposal or further
processing such as incineration, and to dispose of a filtrate relatively
free from  suspended solids.  There are different kinds of cake filters,
however,  they can be conveniently grouped under four headings.

      1.  Vacuum filters, batch or  continuous.

      2.  Pressure vessel filters, vessels operated
         under pressure containing elements upon
         which the cake is formed.

      3.  Pressure filters, filter presses,  plate and
         frame, or recessed plate.

      4.  Variable volume filters, variable chamber,
         tube press.

The pressure filter which is the subject of this report is item three (3).
It may have been described by  numerous names, such as plate and frame
press, or filter press.  Literature  has reported that about 1900,  a  number
of plants in England  installed  sludge filter presses, principally those
plants using  chemically precipitation type of treatment processes. Plants
were also reported at Providence,  R.I. and Worcester, Mass, in the
United States. These units were reported to consist of a number of cast
iron recessed plates fitted in a frame between two head blocks which
were forced together by screw jacks.  The plates were 36" in diameter
with corrugated faces for drainage channels and covered with filter cloth.
Sludge was fed to these cloth covered recesses (chambers) through a 6"
center opening by pump pressure or through an ejector using compressed
air.  Filtrate passing through the cloth was drained from individual spigots
from each chamber.

Houston,  Texas installed two 120  plate presses in 19X9  to dewater acti-
vated sludge which operated at a maximum 100 pound pressure.  Operation
Was discontinued after a short  period of time due to cloth problems and
high costs.
                               73

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Literature indicates that the early pressure filter installations were not
very successful due to premature blinding of the media, excessive leak-
age contributing to poor  housekeeping, low capacities, high labor re-
quirements to remove the cake, long filtration periods, and poorly fabri-
cated plates which frequently failed.  European interests stimulated in the
late  1950's  has lead to the  present more efficient pressure filter.  This
has been followed in the past few years with  interest in the United States
for further development and improvements.  An example of more recent
improvements is the design to provide a separate plate gasket to assure
longer media life and less leakage.  The earlier pressure filters, and
some of the present units, use a filter cloth sack which covered the entire
plate and also served as the gasket on tho joints of the plate.  The filter
media life was often controlled by the mechanical wear, or pressure wear
as the plates  were  held together.  Most often the cloth failed as a gasket
long before  it deteriorated as a filtering media.   Separate plate gaskets
control leakage if properly installed and adjusted and minimizes house-
keeping.  This gasket arrangement also greatly extends the useful  life  of
the filter media.  Details of a typical pressure filter plate are shown in a
cross-section view in Figure 29.

There are a number of other forms of  solids-liquid separation systems,
some of which have a wide range of application, and some of which have
often been used  in  domestic and industrial sludge dewatering.  In the
case of waste treatment  sludges where the material contains a high pro-
portion of fine particles,  the separation of the solids-liquid phase is
extremely difficult, and  the force required to  displace the liquid phase
and replace  it with sludge particles  to produce a cake formation increases.
In this application  the use of applied pressure as in the use  of  the pres-
sure filter to develop sufficient force to cause the  separation of solids-
liquid offers a distinct advantage.  The proper sludge preconditioning,
selection of an efficient  filter media, controlled pressure development
will improve the efficiency of sludge dewatering beyond that normally
found in other systems.  The product of pressure filtration is an increase
in solids concentration in the  cake with a decrease in suspended solids
content of the filtrate.  Compared to other systems for  sludge dewatering
the advantages for pressure filter may be summarized as follows.

      1.  The  quantities  of  sludge conditioners are usually reduced.
Conditioning materials may  be of the least expensive variety  (ash)
as was the case at Cedar Rapids.
                                74

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                                         UrmUerd Sludge

                                         Sludge  Cake

                                       J Filtrate
     RHClH >i I  View
,MUj !: ' .  
 i)\ pl.M'1
             Figure 29
                  75

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      2.  Filtration efficiency is maintained even when there is a  sub-
 stantial range of sludge characterises, and is improved over most
 systems at normal filterability conditions.

      3.  Increased solids content of the filter cake.  The  low  moisture
 content is important where cake is incinerated and often will assure
 autocombustion of the incinerator feed, thus saving supplemental fuel
 cost.

      4.  The filtrate is considerably lower in  suspended solids and
 BOD, and thus minimizes the loadings back to the plant for treatment,
 or other problems of ultimate disposal.

      5.  Operation and manpower requirements are minimal, and con-
 siderably less than most systems.  The attending operators may perform
 other duties during most of the  period of filtration.  This is not true of
 several other systems.  Operators not skilled in mechanics can easily
 be trained to operate this filtration process.

      6.  Maintenance is minimal due to relatively few moving parts and
 standard supporting equipment such as pumps and mix mechanisms.

      7.  Filter cakes are more easily disposed of due to lower moisture
 content and smaller volumes.  The cake may  be conveniently stored in
 bunkers under the filter and removed by standard conveyor  equipment and
 at continuous discharge rate thus overcoming any small effect of the
 batch process.

      8.  The costs based upon amortized capital costs are competitive
 with other processes.  The initial capital costs may be higher than some
other systems, however, the minimum maintenance  costs extended over
 a significant period of useful life, and the low operating costs  make the
 pressure filter system highly attractive.

 The usual methods for evaluation of the filterability of a sewage sludge
 is the Buchner funnel test and the vacuum leaf test in the laboratory.
 The data is then applied to the  standard filtration equations and established
theory.  This conventional laboratory investigation and filtration theory
does not lend itself to pressure filtration because the majority of the
pressure filter designs provided for a fixed volume cake formation. The
cake is of a fixed  thickness and occupies a fixed volume between  the
plates.
                                76

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Cake formation also takes place under several varying conditions.  At the
beginning of the filtration cycle cake is formed at a high rate at relatively
constant flow with the pressure increasing as the cake formation is
developed.  This is followed later in the cycle with a decreasing flow rate
to a very small flow rate at a constant pressure drop.  There are other
variances from the standard filtration process which encourages the inves-
tigation using a pilot filter study to establish design criteria.

The present day pressure filter equipment offers  many advantages and an
efficient and economical system for dewatering sewage sludge.  With a
minimum of investigative study and pilot plant work, meaningful data can
be obtained to size and evaluate a full scale pressure filter dewatering
system.

Detailed Description of the Process

The process schematic and photographs of the facility are shown on Figure
30.  Digested solids are removed daily from one or more of the  secondary
digesters and alternately transferred to one of the two 50 foot diameter
sludge holding tanks.  Each tank has about 1\ days' detention  and sludge
thickening mechanism.  There was no previous experience that  digested
sludge thickening could be assured at this plant, however,  bottom sludge
removal mechanism was necessary so gravity sludge thickening with the
picket design was installed  in anticipation of some degree of sludge
thickening.  These holding tanks may be  overflowed to a scum well for
scum removal and may be decanted if thickening occurs.  The scum may
be preconditioned with chemicals and sent through dewatering,  or it may
be pumped back  through the  plant for further treatment.

Sludge from the bottom of the holding tank is pumped to the  mix tank
which is the beginning of the dewatering  process.  In the pump discharge
line is a nuclear density gauge to measure and record the  sludge density
in terms of percent total  solids. Also included in this line is a magnetic
flow meter to register the volume of liquid flow pumped to the mix tank.
With these two instruments a total accounting of the sludge pumped to the
process can be accurately determined at all times.  Both the nuclear den-
sity gauge and the magnetic flow meter are registered on the instrument
panel in the operating room.  Installed in the pump discharge line to the mix
tank is a sludge grinder.  The purpose of this grinder is to emulsify the
sewage sludge solids to a uniform consistency and thus improve precon-
ditioning and filterability of the sludge liquid applied to the pressure
filter.  The sludge grinders have replaceable cutting plates and blade
assemblies and may be externally cleaned.  Periodic cleaning may be
necessary and may be detected by observing the pressure drop across

                                 77

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             SCHEMATIC SLUDGE DEWftTERING 8 INCINERATION
Control Center
Pressure Filter
                         Figure 30
                            78

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the grinders.  Bypass valving and piping has been provided to bypass the
nuclear density gauge,  the sludge grinders and the magnetic flow meters,
should the units be taken out of service for repair.

The mix tank is designed to blend the incoming sewage sludge with the
filter aid which may consist of incinerated sewage sludge ash, fly ash
from a  local power plant,  chemicals, or any combination of these. The'
mix tank is a horizontal steel tank, approximately 120 cu. ft. capacity,
equipped with two speed agitators designed to thoroughly mix the sludge
slurry and  the ash and chemical admixture.  Ash is fed to the mix tank by
screw conveyor and is accurately measured by gravimetric feeders. Chem-
icals are added to the mix tank from a metered ferric chloride feed pump
and from a lime slaker using  a magnetic flow meter for control.  The ad-
mixtures are controlled  from the instrument control panel at the operators'
room.  Ash dust is controlled with spray showers in the mix tank and the
operator may observe the slurry mixing process through a  plexiglass
observation door in the  top of the tank.

Slurry overflow from the mix tank is gravity fed into the contact tank
where it is further mixed at a very slow rate to provide ample contact
between the sludge and the admixtures of ash and/or chemicals. This
tank is also of horizontal steel construction with a slow speed agitator
to maintain uniform consistency of the slurry.  The tank has a large volume
of about 600 cubic feet  to assure an adequate supply of slurry feed to the
pressure filter during each cycle of the filter operation.  The contact tank
is equipped with liquid  level controls for  starting and  stopping sludge
pumping and the mixing system.  When the liquid level in the contact tank
fails to a predetermined low level, the mix tank operation will start and
a supply of slurry will be fed into the contact tank.  As the pressure
filter feed is  satisfied,  the liquid level in the contact tank will  rise to
a predetermined high level and shut the mix tank operation off.  Slurry
from the contact tank is pumped to the pressure filter by two filter feed
pumps.  These pumps are of a special design and are variable speed,
variable capacity, variable pressure and are automatically controlled to
decrease the  output capacity as the pumping head  increases  until it
reaches a maximum stall pressure of approximately 225 pounds per square
inch, which is the normal high operating pressure  of the pressure filter.
This pump is  hydraulically driven from a power pack which furnishes  the
hydraulic pressure necessary to stroke the piston.

The filter feed pump discharges  to a  surge tank to  smooth the flow and to
minimize pulsating pressures in the feed line to the pressure filter.
                                79

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The pressure filter operates on a process cycle of about one to two hours
duration depending on the density of the sewage  sludge  and the dewater-
ing characteristics of the  sludge slurry fed to the pressure  filter. Prior
to the beginning of the filter cycle the pressure filter is precoated to pro-
tect the filter cloth from blinding or clogging due to possible grease
content or fine particles, and to form a shear plane between the  cloth and
cake  to assure free and clean discharge of the  cake.  Filter precoat is a
mixture of ash carried to  the filter  cloth in recycled filtrate water which
is stored during each filter cycle for this purpose. Ash  is charged into the
precoat tank by a  gravimetric feeder and conveyor system.  All of these
operations are sequenced automatically from the operator's control station
by selecting the filtration cycle  button.

Immediately after  precoat the sludge feed to the pressure filter is auto-
matically applied  through motorized control valves to begin the filtration
cycle,  Sludge feed to  the filter  is from both the front and the rear ends
and the rate of feed at  the beginning of the cycle is very high and dimin-
ishes as the cake is formed.  To accommodate  this high initial rate of
feed at the  beginning of the filtration cycle,  an equalization tank which
was charged with  conditioned sludge during the previous filtration cycle,
and two filter feed pumps will discharge into the  pressure filter.  As  the
filtration cycle continues and the filter feed rate  decreases, first the
equalization will drop out of the system, and later one of the feed pumps
will trip out of service. The  second filter feed pump will continue to
supply the feed to the filter at a decreasing rate until the filtration cycle
has been completed.

The duration of the filtration cycle may be determined by time, pressure,
or rate of filtrate flow. When the filtration cycle  is determined by time,
this may be one to three hours duration based upon previous filtration
experience.  It is not anticipated that the filtration cycle will be governed
by pressure, however,  should this be the choice, the filtration cycle
would continue until a predetermined pressure level was developed, at
which time filter feed would cease or would continue for a predetermined
number of additional minutes.

Usual operation will be controlled  by rate of filtrate wherein the  filtration
continues until a predetermined rate of filtrate flow is observed across
a V-notch weir. An audio or visual signal is operated by a  liquid level
sensor at the  weir.

At the end of'the filtration cycle, the filter core feeding the individual
filter plates remains filled with wet,  soft sludge  slurry.  This soft core
must  be removed prior to opening the filter plates  to discharge the cake.
                                80

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 This is accomplished by applying compressed air to the core at one end
 of the filter thus blowing the wet sludge  core back to the contact tank
 through an energy dissipating cyclone.  The energy dissipating cyclone
 is called a core expansion tank which discharges the wet slurry to the
 contact tank and vents the air to the atmosphere. At the completion of
 the core blow operation, the filter plates may be opened at any time con-
 venient to the operator. Core blow and opening the filter plates for cake
 discharge is controlled from the hydraulic control console located near the
 filter.  When the filter is opened each  plate is progressively moved by a
 mechanical plate shifter located at the top of the filter and which con-
 tinues to operate automatically  unless  stopped by the operator.  The filter
 cake is discharged to the bunker below the pressure  filter and is  sheared
 into smaller particles as it passes shear cables stretched across the open-
 ing below the filter.

 It is desirable that the  operator be in attendance during the time the filter
 cake is being discharged.  The operator should be certain that the cake
 has dropped free of the filter plates and should observe the condition of
 the filter cloth media, the plate gaskets, and other physical  features of
 the filter. Should any large  cake  fragments hang up on the filter media
 the operator should remove these using a wooden paddle or similar
 approved device.  The total  time required to discharge the filter cake
 and recycle the  process is in the range of 5 to 10 minutes.  This is the
 time the operator spends at the filter.

 Filter cake is discharged to a cake bunker under the filter and is conveyed
 to the multiple hearth incinerator by a series of cake conveyors and ele-
 vators.   The cake bunker conveyor has  variable speed control which  serves
 as a cake feed rate control for the incinerator.  All other conveyors moving
 cake to the incinerator are constant speed. If the incinerator is not in
 operation and  it is still desirable to dewater sludge,  an alternate discharge
 is provided to a  truck for conveyance of wet sludge cake to a suitable
disposal area.  Conveyors are controlled  from the operating console and
 are  interlocked for proper sequence of operation.

 The incinerator is a multiple  hearth type, having nine hearths of 25 ft.,
 10 inch O.D.  Temperature is maintained  with automatic temperature con-
 trols by self combustion of the sludge cake and by auxiliary fuel fired
 burners  using  either digester or natural gas.  Incinerated ash is discharged
 through  a grinder to elevated ash storage  hoppers. Combustion gases are
removed  through a high-energy wet Venturi scrubber operating at about
 40 in. W.C.   The system also has a deodorizing afterburner system.  The
 incinerator is  totally automatic and monitored at the operator's control
Panel.
                                 81

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Two (2) pressure filters are installed, each having 83 chambers of
approximately 2.5 cubic feet volume per chamber, for a total of 207
cubic feet volume.  The filters have 3400 square feet of filter area
each and are equipped with an expanded frame for a future increase to
approximately 100 chambers.  The filter plates are carbon steel con-
struction, epoxy coated.  The filter media is a monofilament polypropylene
with a stainless steel mesh backup screen and a carbon steer drainage
screen.  The filter operates at approximately 225 psig filtration pressure.
The plates,  during operation,  are held closed by hydraulic cylinders
operating with an oil  pressure at approximately 5,000 psig.  The pressure
filter plates are gasketed with asbestos Teflon gasket material and the
filter media is caulked in place with cotton caulking cord.
                                 82

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                             SECTION VIII

                         EVALUATION METHODS

Sampling

There were five basic sampling points for the evaluation of a pressure
filter:

        1.  Sludge feed
        2.  Ash feed
        3.  Sludge-ash mixture
        4.  Filter cake
        5.  Filter filtrate

Sludge Feed

A sludge feed grab sample was collected by the operator from the discharge
of the transfer pumps  twice per operating cycle.  The operator determined
the sludge solids content of each sample using an Ohaus moisture bal-
ance  in order to adjust the ash feed. The remainder of the sample was
composited equally with the other sample for  that cycle, refrigerated,
and sent to the laboratory for analysis.

Ash Feed

Ash feed was analyzed on an infrequent basis.  Samples were obtained
from the ash feeders.  A device was fabricated to collect the ash from
a predetermined area of the gravimetric feeder belt.  This ash sample
was weighed and  the quantity checked against the calibrated feed rate
for the feeder belt. The device cut an ash sample one foot long in size
and functioned on  the principle of an ordinary cookie cutter.

Sludge-Ash Mixture

Sludge-ash mixture was sampled at the intake of the filter feed pumps.
These samples were handled in the same  manner as  the sludge feed.
Solids analysis on these by the operator confirmed the ash feeder  settings.
(ash/sludge ratio).

       Ash/Sludge Ratio = %  Sludge Mix  -1
                         %  Sludge Feed
                               83

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Example:

       Sludge           =  5%   Ash + Sludge = 17%

       Ash/Sludge Ratio =  17%   -1=3.4-1 = 2.4
                          5%

These results were later confirmed by the laboratory.

Filter Cake

The filter cake was sampled by grabbing representative pieces of cake
when the cake was discharged and broken on the shear cables at the
top of the cake storage bunker.  These samples were sealed in glass
jars.

Filtrate
Filtrate samples were collected from the weir box adjacent to each filter.
Three grab samples of filtrate were taken during each cycle, composited
and refrigerated for laboratory analysis.

Plant Measurements

All important measurements, such as pressures, ash weight, sludge vol-
ume,  sludge density, time, etc. were monitored continuously by appropriate
instrumentation.  All measurements were recorded and integrated in the
operators' control room.

Sludge Flow

Sludge flow was monitored by integrating magnetic flow meters.

Sludge Density

The percent of total solids of the sludge feed was monitored by nuclear
density meters.  The output (0-16 percent solids) was displayed on the
chart as the sludge flow.

Lime Feed

Gallons of lime feed were measured by integrating magnetic flow meters.
The concentration of lime was maintained at 0.4 pound CaO per gallon
of water by appropriate adjustment of feed water rotameters and lime feed
transmission at the slaker.

                               84

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Therefore, pounds of lime feed equals gallons (from integration) times
0.4 pounds per gallon.

Ferric Chloride

Ferric chloride feed was by positive displacement feed pumps with cali-
brated discharge integrated from time of operation.

A 40 percent ferric chloride solution was used.  Therefore, pounds of
ferric chloride equals gallons solution times 4.75 pounds ferric chloride
per gallon.

Ash Feed (Precoat and Admixture)

Ash quantities were metered by use of gravimetric belt feeder equipped
with totalizers.

Filtrate

Total filtrate from the plant was measured by a 60 V-notch weir on the
discharge of the filtrate storage tank. The depth over the crest was
monitored by a bubble tube sensor.

Instrument Maintenance

All instrumentation was routinely calibrated by a qualified instrument
technician.

Gravimetric feeders were calibrated monthly (minimum) by either weighing
a timed discharge (Lime Feeders) or by removing material from a measured
area of  the weigh belt and checking against theoretical discharge  (Ash
Feeders).

Laboratory Analysis

The following routine laboratory analysis was performed.

      Sludge Feed
      Analysis                   Method

      Total Solids               Dried at 104 C.  overnight

      Volatile Solids             Ignition of dried residue at 550 C.
                                85

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 PH

 Grease


 Volatile Acids



 Alkalinity


 Ammonia


 Organic Nitrogen


 Sludge +Ash (Mixture)

 Total Solids

 Volatile Solids

 PH

 Filter Cake
Total Solids

Volatile Solids

Nitrogen

Filtra te

PH

Total Solids

Volatile Solids

Turbidity
Glass electrode

Dried using magnesium sulfate extracted
with hexane in a Soxhlet extractor

Determined by column chromotography
according to the 13th Edition of Standard
Methods

Titration of the sample to pH of 4.5
using 0.5N H2SO4

Analyzed by distillation into boric acid
and titration with standard acid

Kjeldahl method using mercuric oxide
as catalyst
Same as above
Same as above
Same as above
Same as above
Same as above
Same as above
Same as above

Same as above

Same as above

Determined using a Hach  Model 2100A
turbidimeter
                          86

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      BOD,
      Total Alkalinity
      Color
      Conductivity
      COD
      Total Phosphate
Dilution method (three dilutions per
sample) initial and final dissolved
oxygen concentrations measured with
a Weston and Stack Model 350  dissolved
oxygen meter with stirrer

Titrated with 0.03N H2SO4 to a pH  of
4.5

Determined using a Hach color compar-
itor

Measured using a Labline  Lectromho
conductivity  bridge with a cell constant
of 1.0

Measured basically the same as Standard
Methods and except samples were auto-
claved for two hours at a pressure of
15 psig and titrated with 0.25N FeSO4

Determined according to Standard Meth-
ods.  Potassium persulfate was used as
the oxidant.  Stannous chloride and
ammonium molbdate were used for color
development
Determination of Specific Resistance

The filterability of a sludge can be defined as the ease at which the sludge
9ives up water. While it is possible to talk about the filterability of
sludge using subjective terms such as difficult, average, etc., it is
desirable to be capable of objectively describing a numerical value to a
sludge to give us a  meaningful value for operation.
    method of evaluating a sludge is by determining its specific resist-
ance.  Specific resistance can be viewed as the reciprocal of filterability
Wiere a high value of resistance means a poor value for filtration and
vice versa.  The theory and development of specific resistance and its
relationship to pressure dewatering have been developed by Dr. Oswald
      of the Passavant Werk, Germany.  This background  material may
                               87

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be found in the dissertation "Entwicklung Eines Wirtschaftlichen Verfahrens
Fur Die Beseitigung Von Abwasserschlammen" by Dr. Oswald Busse.

Specific resistance has proved valuable at Cedar Rapids in day to day
operation.  By using the specific resistance an operator can:

      1.  Determine whether a  sludge is conditioned properly

      2.  Evaluate and  optimize new methods of conditioning such
          as organic polymers

      3.  Evaluate the filterability of the unconditioned  sludge

All these evaluations can be accomplished using a  bench scale filter.

Specific resistance is measured by using a  small pressure filter which is
generally referred to as a resistance meter.

Mechanical Description of the  Resistance Meter

The resistance meter is essentially a pressurized Buchner funnel.  The
filter is composed of a  stainless steel body and support screen for a
standardized filter paper that serves as the filter media.  Meter acces-
sories required are  a 250 psig compressed air supply, a  100 milliliter
buret and stand,  stop watch, and  convenient glassware as indicated in
the photograph,  Figure  31.  A functional diagram of the resistance meter
is shown in Figure 32.
A standardized, or calibrated, filter paper is necessary to assure a uniform
filter media .   This is wetted and placed in the meter on the support screen.
A 100 milliliter sample of the sludge to be tested is measured and placed
in the resistance meter.  The top of the unit is secured and 250 psig pres-
sure is applied.  The effluent from  the meter then drains into the 100 mil-
liliter buret,  which was initially filled to the lower 100 ml mark before
the test.  As the filtrate is collected  in the buret, readings of filtrate
quantity and time are  noted and recorded.

In the case of an unconditioned  sludge which  has a high resistance to
filtration the total filtrate is read after two minutes and at one minute
intervals thereafter for a period  of twenty minutes,  or until the filtration
has ceased and air blow-by occurs with air passing through the filter.
                                88

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 r
RESISTIVITY
MT IEH
                Y
                                         Figure 31
PRESSURE
RELEASE VALVE
    100ml
    BURETTE
                                o
                                o
         BEAKER

                 it
                           REGULATOR
                                                  COMPRESSED
          RESISTIVITY   METER  SCHEMATIC
                      Figure 32
                         89

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In testing a condtioned sludge which yields water quite readily if the
filtrate volume reaches the 75 ml mark (25 ml filtrate) within one minute,
filtrate volume readings are then taken every 10 seconds.  These readings
are taken until blow-by occurs or an elapsed time of 200 seconds passes.

Following are two examples of resistance meter tests, one with an uncon-
ditioned  sludge and another with a  conditioned sludge.

Unconditioned Sludge  pH  = 7.4   % Solids  = 6.0   T = 28.0C.
      Filtration Time
    Minutes  Seconds
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0
60
120
180
240
300
360
420
480
540
600
660
720
780
840
900
960
1020
1080
1140
1200
 Volume
Milliliters

   0
   1.3
   2.3
   3.3
   4.1
   4.9
   5.6
   6.2
   6.8
   7.4
   8.0
   8.5
   9.0
   9.5
 10.0
 10.4
 10.9
 11.4
 11.8
 12.2
 12.5
Seconds/
Milliliter
                                                 46.2
                                                 52.2
                                                 54.5
                                                 58.5
                                                 61.2
                                                    ,3
                                                     7
  64,
  67,
  70.6
  73.0
  75.0
  77.6
  80.0
  82.1
  84.0
                                                 86
                                                 88
                                                 89
                                                 91
                                                 93.4
                                                 96.0
                               90

-------
 Conditioned Sludge   pH =9.5   A/S =1.5   CaO =0.5%
                      Ferric Chloride = 0.5%  % Solids = 6.0%
                      T = 29.0C.                    '

       Filtration Time            Volume            Seconds/
         Seconds              Milliliters           Milliliter
            0                    0
           10                    8.6                1.2
           20                   16.5                1.2
           30                   22.6                1.3
           40                   27.7                1.4
           50                   32.4                1.5
           60                   36.6                1.6
           70                   40.4                1.7
           80                 Blow-by

Analysis of Data

The equation for specific filter resistance is
      r = 2 x p x Fj  x b
               n x c

Where r = specific resistance in
       p- differential pressure2 in dynes/cm2
      Fi = filtration area in cm 2
       b = slope of line obtained in test by plotting seconds/milliliter
          versus milliliters
       n - filtrate viscosity in poises
       c = grams  of dry solids/grams liquid for 1 gram sludge

In practice the above equation reduces to

      r = k x b/c

Where k = 2 x p x Fi2
              n

When the pressure and filtration area are held constant, k then becomes
^ function variable only with temperature which governs the filtrate
viscosity.
                               91

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Therefore the constant k can be obtained from a  table of viscosities of
water versus temperature.

The remaining factors b and c are obtained by determining the density of
sludge and the slope of the line obtained in the  test by plotting seconds
per cc versus  cc.

In the case of an unconditioned sludge, Figure 33 shows a graphical
solution of the slope b of 4.4.  For this sludge the percent solids was
6.0, c = 0.06/0.94 = 0.064,  temperature = 28.0 C. for a k  of 5.315,
therefore,  r equals k x b/c = 5.315 x 4.4/0.064 = 366 x 1012cm~2.

Also in the case of the conditioned sludge  as  shown in Figure 33,
b = 0.020,  k again is 5.315 and c = 0.064. Therefore, r = 5.315 x
0.020/0.064 = 1.66 x 1012cm-2.

Interpretation  of Specific Resistance

In the two previous examples  the  specific resistance of an unconditioned
sludge 366 x 10l2cm~2,  and a conditioned sludge 1.66x 1012cm~2 have
been determined.  This data becomes meaningful when it becomes known
what value of  specific resistance a sludge must have to filter well.
Generally  speaking,  at Cedar Rapids  a conditioned sludge must have a
specific resistance of less than 20 x  10l2cm~2 and preferably less than
10 x 10^2cm"2 to filter well.  Knowing this limit we can optimize chemi-
cal or ash dosages using the bench model pressure filter much more
easily and rapidly than a trial and error method on the full scale filter.
                                92

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lOOi-
             84, 10
             60,4.6
                        UNCONDITIONED  SLUDGE
                       SLOPE   *  4,4
                           366xlOIZcirfz
 30
                   I	I
v-
 2.0
                   CONDITIONED SLUDGE
                                           SLOPE     . 0.020
                                    1.7, 40.4
                                          "r"  1,66 x I012 cm2
  1.0
                                  I	I
                  20     30     40     50
                      Volume    (cm3)
                      Figure  33
                          93

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                              SECTION IX

                                RESULTS

Sludge Characteristics

The source of digested sludges for dewatering was eight heated digesters
operated in parallel.  Each digester has a capacity of approximately
600,000  gallons.  Six digesters have some sort of gas mixing while two
gas holders have none.  Hydraulic retention time of the digesters is
approximately 12 to 16 days and the  organic loading is approximately
0.08 to 0.10 pounds volatile solids per 1000 cubic feet.

Sludge feed to the digesters consists of raw sludge from primary clarifiers
and secondary sludge from plastic media roughing filters and rock final
filters.

Digested sludge is pumped more or less continuously to sludge holding
tanks at  the dewatering  site.  These holding tanks tend to stabilize the
input to the dewatering process and provide some thickening.

Table II shows the average characteristics  of the underflow of the sludge
holding  tanks to the process for the months of February through August
1972.  The table  shows  that the solids concentration of the digested
sludge was reasonably stable  except for the months of June and July
where large amounts of waste  lime sludge from the water purification
Plant were received.  During this period, the solids content increased
to 7 percent and the organic content  expressed as percent volatility
decreased from 45 to 53 percent to 33 percent  volatile.

Other parameters of digestion  remained relatively constant.  An increase
in volatile acids was  experienced in April due to a temperature recovery
in the digesters after an extremely cold winter.

Filterability of Sludge

Digested sludge at Cedar  Rapids is unusually  difficult to dewater.  As
mentioned previously, European experience has indicated that a sludge
With a specific resistance value of 150 x 10l2cm-2 is an average sludge
as far as the "degree  of difficulty" in dewatering. Values  of specific
resistance for digested sludge at Cedar Rapids are all considerably
higher than this value, typically possessing specific resistance values
of 300 to 700 x 10l2cm~2. Higher resistance  values indicate greater
difficulty in, or resistance to, dewatering.
                               95

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PH
                 Table II

Average Characteristics Unconditioned Sludge
           February to August 1972

   Feb.  Mar.   Apr.    May    June   July   Aug.   Av.

  7.1    7.2    7.2    7.3    7.3    7.4   7.5    7.28
Total Solids   3.6    3.5  3.6
Percent
                       3.5    7.0    6.0   5.0    4.6
Volatile
Solids
Percent
 53.8   51.9 53.2     45.0    33.6   36.9  44.7  45.6
Volatile Acids  49
mg/1 as
Acetic Acid
         41   410
62
50
77   73    108
Total Alka-
linity mg/1
as CaCO3

Ammonia
mg/1 as N

Organic
Nitrogen
as N

Grease- %
of Total
Solids
  4000   2900 2700     3600   3850   3800  4050 3557
  846    594   604
838    944    993  1049  838
  1464   1411  1338     1123   1272    1424  1493  1360
              9.32
       5.1    4.0  4.6   6.6
                              96

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Complicating the picture is the fact that sludges at Cedar Rapids are not
consistent. Digested sludge is always difficult but can be more or less
difficult at various times.

One of the problems arises from the fact that the sludge varies consider-
ably in  the solids content.  Because the value for  specific resistance is
inversely  proportional to solids  content, sludges with low  solids content
are generally more difficult to dewater than the same sludge which has
been thickened.  For example,  if a 2 percent sludge, which has an
r= 600 x 1 0*2 cm"2 were thickened to 6  percent it would then have an r
value of less than 200 x  lO^cm"2 providing, of course, that the basic
nature of the sludge was not modified.

The basic pattern could be observed at Cedar Rapids as indicated in Table III.

                             Table III
                  Specific Resistance at 10

      Percent Solids           r (Specific Resistance) x
          2.43                          772
          3.33                          571
          5.34                          342
          6.68                          236

As can be noted by the above table,  the specific resistance is inversely
proportional to the solids content.  Unfortunately, however,  in practice
a 2 percent sludge cannot be handled as a mere  volumetric dilution of a
4 to 6 percent sludge.  Generally speaking, low solids sludge (2-3 per-
cent) appears  to contain  a higher proportion of fines and requires a greater
proportional amount of conditioning chemicals than a higher solids content
sludge of 5.5  percent or  greater.

Also it has been found that specific resistance of sludge may vary con-
siderably at Cedar Rapids even though the percent solids remain relatively
constant as  shown in the following continuation  of Table III.

     Percent Solids            r (Specific Resistance) x 10I2cnT2
          5.8                           366
          5.3                           342
          5.5                           308
          5.7                           333
          5.6                           443
          6.0                           361
                               97

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In the above table it can be seen that for the samples shown, while the
percent solids are relatively uniform  the specific resistance varied
appreciably. For example,  a  5.5 percent sludge had an r = 308 x 1012cm"2
while a 5.6 percent sludge had an r = 443 x lO^cm"2, a variance of 44
percent.

This variance in filterability is not unexpected considering the widely
varying nature of the wastes as  far as quality and quantity received at
Cedar Rapids.

The most important ramifications of this varying nature of the sludge is
that conditioning must

1.  Be dependent on the solids content  of the sludge,  i.e., quantities
(ash and chemicals) must vary as the percent solids vary.

2.  Conditioning must  be adjusted to  the worst possible condition.

Because the conditioning charts are based on conditioning the'worst
possible sludge/ a large quantity of sludge  is often over-conditioned.
In the data that will be presented later  in this report,  there will be in-
stances where two sludges with the same solids content and essentially
identical conditioning  will have widely  varied yields.  The problem can
be traced to the  varying filterability as  shown by specific resistance
value in the table above.

Sludge Ash Characteristics

The source of sludge ash for dewatering was the ash from the multiple
hearth incinerator used for the incineration of the cake from the pressure
filter. Initial startup of the pressure  filters was made with fly ash from
a local power plant,  consequently the initial characteristics of the sludge
ash probably were a composite of fly  ash and sludge ash, however,  with
the continued wasting  of ash the sludge ash characteristics stabilized
within a short period.

The physical and chemical properties  of sludge ash are in all probability
quite unique from installation to installation, depending upon the  non-
volatile portions of the sludge dewatered, chemical additives (lime,
ferric chloride and/or polymers), temperature of combustion and the method
of combustion (multiple hearth or fluidized bed).
                               98

-------
At Cedar Rapids the sludge has at times a  high inorganic fraction repre-
 senting the discharge of waste lime from the lime soda softening process
at the municipal water purification plant.  No alum is used or discharged
from the water purification plant.

Chemical additives in addition to sludge ash used for conditioning sludge
are ferric chloride and lime.

The temperature of combustion ranges from 1200 F. to 1400 F.

Physical Properties of Sludge Ash

The physical properties of sludge ash are as follows:

      Specific Gravity           2.63 - 2.78
      Bulk Density               50 Ibs. per cu. ft.
      Color                     Yellow
      Mean Particle Size 29 mm   (22 - 40}

The particle size was determined by Process Measurements Section,
Research Laboratory Branch, Control  System Division of the Environmental
Protection Agency with the cooperation of the Taft Center, Cincinnati.
Particle size analyses were made using two different techniques, one
being the electronic sensing of particle volume with the Coulter Counter
Model T,  and the second, aerodynamic sizing with the Bahco Micro-
particle Classifier.

In the  following table are listed the average particle  sizes of sludge ash
as determined by the Coulter Counter Model T for nine samples of sludge
taken from January 3,  1972 to May 10,  1972.

% of Particles       Size in Millimeters x 10~3
less than Given            Average of
    Size	        	   Nine Samples     	  Range mm x 10"3
25
50
75
90
14.5
29.1
50.7
74.3
9.5
22
38
60
- 18
- 40
- 72
- 100
In Figure 34 is shown the results of a  typical particle size determination
by the Coulter Counter Model T .
                               99

-------
                         PARTICLE  SIZE  ANALYSIS




N
to
Q
1 01
T. <
o S
UJ H
? W
z
> H-
< en
3 J
5 .
3 ^
O u
o
tr
LU




100

90

80


70


60

5 0

40


30
20
10

w>l_UL/\JL_ I->JII
COULTER COUNTER
MODEL T OX
X
O
X
rr
/^






/
O


0/
X
o^
c^pS
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
3 4567 8910 20 30 4050 100
MICRON DIAMETER ( u )
                               Figure 34

Particle size analysis performed by the Bahco Microparticle Size Analyzer
are given in Table IV.  Along with ash from the Cedar Rapids installation
the table includes data on ashes from Mill Creek/ Lake Tahoe, and
Kansas City.

This table shows that for the incinerator ashes from Mill Creek/  Lake
Tahoe and Kansas City approximately 70 percent of the particles  were
28 microns or less, for Cedar Rapids only 47.5  percent of the particles
were less than 28 microns.
                               100

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                             Table IV

                  Classification of Ash Particles
               by Bahco Microparticle Size Analyzer

Effective .Particle    Cedar Rapids   Kansas City*  Lake Tahoe*  MiUCreek*
^Size (10-3 mm)	%	%	%	%

  0.00 -  0.92                      1.49           1.38       2.22
  0.92 -  1.60         3.0         2.95           2.71       3.70
  1.60 -  2.97         5.0         5.70           7.53       7.81
  2.97-  7.71         12.5         9.05           9.83      21.86
  7.71-12.19         9.5        14.56          10.95      26.52
12.19-20.75         10.5        22.41          16.20      22.46
20.75-24.72         4.0         9.21          10.13       6.83
24.72-27.30         3.0         8.83           4.28       8.60
28.38                 52.5        25.8           36.99

*  Sludge Conditioning With Incinerator Ash
   Smith, Hathaway,  Farrell, Dean, AWTR1
   EPA -Cincinnati, Ohio

Chemical Analysis of Sludge Ash

In Table V is shown the chemical analysis of sludge ash samples taken
in January, May and September 1972. The samples are somewhat similar,
however, the September ash sample had more calcium which would indicate
the use of a greater amount of lime and less of the R2O3 group of oxides.

                             Table V
Chemical Analysis                         May 24. 1972

Phosphate pentoxide,
Silica, SiO 2
Ferric oxide,  Fe2O3
Alumina, A12O3
Titania, TiO2
Lime, CaO
Magnesia, MgO
Sulfur trioxide, SO3
Potassium oxide, K2O
Sodium oxide,
Undetermined
                              101

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Silica Value - 27.77
T250 Poises: *1950 F.
*  less Than
Loss on Ignition at  1000 C.

Metals  by Atomic Absorption -%

      Ca
      Fe
      Mg
      Zn
      Cy
      Cr
      Cd
      Na
      K

pH of 1% Solution

Fly Ash Characteristics
             16.50% wt. dry

            January 1972

             12.10
              3.63
              0.70
              0.61
              0.10
              0.04
             <0.01
              0.27
              0.40

              9.5
             September 1972

                20.30
                 3.09
                 0.07
                 0.57
                 0.09
                 0.04
                 0.00
             Not determined
             Not determined

                10.4
Fly ash for the project was obtained from a local power plant.  The fly
ash was the residue from combustion of pulverized coal.  This residue
consisted of a combination of ash collected by cyclone type dust collec-
tors and electrostatic precipitators.  The ash was transported dry from the
power plant to the solidshandling plant by truck and pneumatically con-
veyed into the ash storage bins.

The physical properties of the ash are:
      C olor
      Sp'^ctfic gravity
      Bulk density
Black
2.3
43 .6 Ibs.  per cu. ft.
Analysis of the particle size of fly ash was performed in the same manner
as sludge ash using both the Coulter Counter and the Bahco type analysis.
Coulter analysis of the particle size is shown below, and in Figure 35.
Percent Less Than Given Size
      25
      50
      75
      90
             10
                                      -3
mm
              11
              20
              33
              42
                                102

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                           PARTICLE SIZE ANALYSIS
                               BANCO ANALYZER
             to
             N
             tn
            > I-
                80
                70
                60
                50
                40
                30
            2 .
            D z
            3  20
             CE
                10
                              _L
                                 I
                                   I
                                           I
                                              I
                                                I
                                                   I
                                                      I
                 0 2
8  10 12  14 16  18 20 22 24 26  28 30 32
 MICRON  DIAMETER (u)
                               Figure 35

Fly ash particle size was also determined by using the Bahco apparatus.
Results are  shown in Table VI.
                               Table VI
                     Classification of Fly Ash Particles
                    by Bahco Microparticle Size Analyzer
Effective Particle Size (10"3 mm)
         0.00 -  1
         1.60 -  2
60
97
         2.97 -   7.71
         7.71 -  12.19
        12.19 -  20.75
        20.75 -  24.72
        24.72 -  28.30
        28.30
                   Percent
                      2.5
                      7.5
                     20.5
                     16.5
                     17.5
                      5.5
                      5.0
                     25
Chemical analysis of fly ash is shown in the following table.
                                 103

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                             TABLE VII

                   Chemical Analysis of Fly Ash -

          Phosphate pentoxide, P2O5              0.85
          Silica, SiO2                           53.90
          Ferric oxide, Fe203                    17.00
          Alumina, A12O3                        11.93
          Titania, TiO2                           0.57
          Lime, CaO                             6.80
          Magnesia,  MgO                        1.65
          Sulfur trioxide, SO3                     2.30
          Potassium oxide, K2O                   4.32
          Sodium oxide, Na2O                    0.39
          Undetermined                           0.29
                                               100.00

          Silica Value -   67.93
          T250 Poises:     2355 F.
          Loss on Ignition at 1000C.             14.5%

As can be noted from Table VII, fly ash is mainly composed of silica, iron
oxide and alumina.

Comparison of Sludge Ash and Fly Ash as a Conditioning Agent for Sludge

Extensive testing was performed using the resistance meter in comparing
the effects of sludge ash and fly ash as a conditioning aid for the pressure
filtration of sludge, both digested sludge and raw sludge.

The reason for the initial testing was to attempt to explain the apparent
lack of correlation between the pilot plant study and the performance of
the full scale filter.  The pilot plant had operated without the use of any
other conditioning agent except fly ash, the full scale plant could not
successfully operate at any reasonable sludge/ash ratio without using
ferric chloride and lime.

Consequently a program of study was set up comparing the two different
ashes.  Experimentally the ashes were compared by securing a sample of
digested sludge, determining the solids content of the sludge and then
dosing liter samples with varying amounts of ash, up to an ash ratio of
3 to 4.
                               104

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In Figures 36 to 40 are shown an example of the testing. Figure 36 repre-
sents a sample of digested sludge with a solids content of 2.5  percent.
The initial specific resistance of the sludge was 772,  At an ash/sludge
ratio of 1.3 with fly ash  the specific resistance was lowered to 78 x 10*2
cm"2 while at an ash/sludge ratio of 3.95 for sludge ash the specific
resistance was 67 x 1012cm~2 comparable values.

Figure  37  represents a sample of digested sludge with a solids content of
5.5 percent and an initial specific resistance of 308 x  lO^cm-Z.  in this
case an ash/sludge ratio of 1.5  for fly ash gave a  specific resistance of
12.3 x lO^cm"2 while an ash/sludge ratio of 4.0  for sludge ash gave a
specific resistance of 14.1 x lO^cm"2,  again essentially  the same values.

Figure  38  represents a sample of digested sludge with a solids content of
5.6 percent and a  specific resistance of 443 x 10^2cm~2.  Here a value
of 1.0  ash/sludge ratio for fly ash has a  specific resistance of 63.2  x
   179                                   199
lO-^cm   , comparable to a value of 61.0 x lO^cm"^ for an ash/sludge
ratio of 2.5 for sludge ash.

The above data indicates quite strongly that fly ash is  2.5  to 3.0 times
more effective for  conditioning digested sludge  per unit weight of ash
than sludge ash.

Additional work was performed in comparing the conditioning effect of the
two ashes with other sludges. If the ashes performed in the same manner
with other sludges then any individual  peculiarities of  the digested sludge
at Cedar Rapids would be ruled out.

Figure  39  shows a comparison of fly ash and sludge ash as conditioning
agents for raw sludge at Cedar Rapids. Here again it can be seen that
fly ash was consistently  better than sludge ash.

Figure  40  shows a comparison of fly ash and sludge ash as conditioning
agents for a digested  sludge obtained from a waste water treatment plant
operated by the City of Cedar Rapids which handles strictly domestic
sewage.   Here again fly ash performs better than sludge ash.

In summary, the fly ash obtained from a local power plant at Cedar Rapids
performs significantly better as a sludge conditioning agent than ash
obtained from the incineration of sludge at Cedar Rapids.
                              105

-------
LU
O
 300


 200




  I 0 0




   5 0
tr


u
ul

a   I 0
LJ
O.
01
             DIGESTED  SLUDGE

                MAIN  PLANT

           "r" WITH NO  ASH =772
                 % S = 2.5

                    i
                          _L
                                 _L
                                          O
J_
            0,5    i.O    1.5     2.0    2 . 5
                       ASH/SLUDGE   RATIO

                         Figure  36
                                              3,0    3.0    4.0
  2 0 0 r
    0 0
 LU
 O
 z

 I-
 co
 01
 LJ
 cr
 o

 i
 u
 LJ
 a.
 tn
    5 0
0
             DIGESTED  SLUDGE

                MAIN PLANT

           "r" WITH  NO ASH =308

                 %  S * 5.5

            .....!..  _ _j	i	i
                                            " UDGE  ASH
      0,0    0.5     1,0    1.5    2.0    2,5    3,0

                        ASH/SLUDGE  RATIO
                                                     3.5   4,0
                          Figure 37
                             106

-------
OU
100
UJ
u
2 50
h-
cn
UJ
cr
o
u.
u 1 0
UJ
a.
CO
,5
It
;-


0.0
. **Sw /SLUDGE ASH
\ A<
\^^^^ A
^.
"
^FLYASH
\;
v
X
DIGESTED SLUDGE ^
MAIN PLANT
"r" WITH NO ASH =443
% S = 5. 6
i i i i i i . .. . j
0,5 1.0 1.5 2,0 2,5 3,0 3.5
ASH/SLUDGE RATIO
200
1 0 0
(STANCE
en
O
CO
UJ
a:
o
iZ
o | 0
UJ
Q_
,, 5
k_

0
0
Figure 38
-
-
X^^^X*^ .r-SLUDGE ASH
X^ "^-S^/^
^VQ ^^^^A
x.
>,. /-FLYASH
^o
RAW SLUDGE- MAIN PLANT
"r" WITH NO ASH = 196
% S = 3,1
l i i i l i l l
0 0,5 1.0 1.5 2,0 2.5 3.0 3,5 4.0
ASH / SLUDGE RATIO
Figure 39
107

-------
         2 0 0 r
         I 0 0
          5 0
       UJ
       o
       en
       to
       LJ
       o:
       o
       U.
       o  1 0
       UJ
       Q.
0
0.0
                                SLUDGE  ASH
                   DOMESTIC  SLUDGE
                  INDIAN CREEK  PLANT
                  "r" WITH  NO ASH= 142
                         % = 7. 0
                                                   FLYASH
                    l
        l
                                       i
  I
                                                   i
0.5    1.0    1.5    2.0
           ASH/SLUDGE
 2.5    3.0
RATIO
                                                        3.5
4.0
                               Figure 40

As previously stated, a substantial difference exists between the perform-
ance of fly ash and sludge ash as a conditioning agent for sludge. Signi-
ficantly this  is a problem which is unique at Cedar Rapids.  Studies by
others have found differences between sludge ash and fly ash but in reverse
order, i.e.,  fly ash having little effect or sludge ash being much better.
At Cedar Rapids the fly ash is a good conditioning agent significantly
better than sludge ash, therefore, it appears that the fly  ash and possibly
the sludge ash at Cedar Rapids are unique and results cannot be directly
applied to other cities.

Fly ash and sludge ash can differ in three fundamental ways, chemically,
by size, and by shape.  Chemical analyses have been presented for both
sludge ash and fly ash but are given in Table VIII for comparison.
                                 108

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                            Table VIII

      Comparison of Chemical Analysis of Fly Ash and Sludge Ash

             Results % by Weight on Ignition at 1000 C.

                                   Fly Ash         Sludge Ash
Phosphorous pentoxide  ?2O5         0.85             4.41
Silica                 SiC>2         53.90            22.96
Ferric oxide            Fe2C>3       17.00             7.49
Alumina                A12O3        11.93             6.27
Titania                 11203         0.57             0,31
Lime                   CaO          6.80            50.60
Magnesia              MgO          1.65             1.62
Sulfur trioxide          SOs           2.30             4.42
Potassium oxide        K2O           4.32             0.78
Sodium oxide           Na2O         0.39             0.59
Undetermined                        0.29             0.55
                                   100.00           100.00

Chemically fly ash and sludge ash are quite dissimilar.  Fly ash is
approximately 50 percent silica and sludge ash is approximately 50 per-
cent calcium oxide.  Other components such as ferric oxide and alumina
are present in differing quantities.  The effect of these higher levels
of iron and aluminum salts in fly ash is not known and deserves further
research.

Particle Size

The range of particle sizes for fly ash and sludge ash have been given
in previous sections. Sludge ash particle analysis is shown in
Figure 34, and fly ash analysis is  shown in Figure 35.  Fly ash has a
smaller particle  size than does sludge ash.  As has  been shown in
previous  sections, ash fractions of smaller size (<45pt) are significantly
far more  effective for sludge conditioning than ash of larger size(>45/x).
Further research is needed to further clarify the effect of particle size
on sludge filtration.

Particle Shape

Photo micrographs of sludge ash and fly ash were obtained by a  scanning
electron microscope.
                             109

-------
Sludge Ash 200X
Fly Ash 200X




  Figure 41
    110

-------
Sludae Ash 500X
Fly Ash 50OX
 Figure 42
   111

-------
Sludge Ash 1000X
Fly Ash 1000X
   Figure 43
     112

-------
A small amount of each sample was vacuum coated with a thin conductive
layer of first carbon,  then gold. The coated samples were then examined
by a scanning electron microscope.  On the preceding pages are compar-
ison photographs of fly ash and sludge ash taken at magnifications of
200x, 500x, and 1000 x.  Figures 41, 42 and 43.

From these photomicrographs it appears that fly ash tends to be more
spherical than sludge ash.  The sludge ash is more irregular and appears
to agglomerate.  In comparing particles of each ash at  equal magnifications,
fly ash  appears larger than sludge ash although both Coulter Counter and
Bahco Microparticle Classifier show otherwise.  This apparent particle
size problem may be attributed to the fact that the  sludge ash agglomerates.
In laboratory work the sludge ash tends to clump in water and resists
dispersion much more than does fly ash. This tendency would be charac-
teristic of a material which agglomerates.

Effect of Particle Size on Specific Filter Resistance

Concern has been  expressed that, in sludge filtration processes using
recycled sludge  incinerator ash as a filter aid,  that the recycling of ash
through the incinerator may result in a shift of ash particles to the smaller
particle sizes.  This concern is based upon the premise that the fine
fraction of ash is not effective in sludge conditioning and may in fact be
deleterious.

Consequently sludge ash samples from Cedar Rapids were split up on an
air jet sieve into the following fractions.

                       0-15 microns
                      15-32 microns
                      32-63 microns
                      63-90 microns

These fractions were subsequently used to condition a  digested sludge
from Wiesbaden  Biedrich which had a specific resistance of  159x 10^2cnr2.
All the samples were mixed with ash equivalent to  a 1.0 ash/sludge ratio,
or 42.1 grams ash per liter,  for this 4,21 percent solids sludge.  These
ash/sludge mixtures were then tested for specific filter resistance.  The
following  results were obtained.

      Fraction (microns)      A/S       r x 1012cm"2
          0-15             1.0          29.4
          15-32             1.0          46.5
          32 - 63             1.0          62.4
          63 - 90             1.0          67.0
Original mixtureallfractions   1.0          60.4
                              113

-------
Here it was observed that a decreasing ash particle size enhances the
ability of the ash to condition the sludge.  Sample conditioned with
fractions of ash below 32 microns had less filter resistance than the
original mixture and the most effective fraction of ash was  that fraction
below  15 microns.

The preceding data  was obtained in Germany by Dr. O. Busse, Pas savant
Werk,  during an earlier evaluation of the Cedar Rapids process.

In Cedar Rapids the sludge incinerator ash was mechanically separated
into three fractions.

                       0-45 microns
                      45 - 106 microns
                   > 106      microns

These fractions of sludge ash were used to condition digested sludge
from Cedar Rapids.  Ash  to sludge ratios  of 2.1 were prepared and then
tested  for specific resistance.

     Type of Ash          Size        A/S          r

     None                             0           467
     Sludge ash        0- 45 microns  2.1          77.1
     Sludge ash       45-106 microns  2.1         110
     Sludge ash     :> 106 microns     2.1         112
     Sludge ash        All sizes
                        (Original)       2.1          88.3
     Fly ash            All sizes       2.1          44.7

These results at Cedar Rapids  verify .the work done in Germany.  Again
the smaller fractions of ash are more effective in conditioning sludge.
However,  the smallest fraction of sludge ash is still not as effective
as fly ash as indicated above.  Unfortunately, equipment was not avail-
able for fractionating sludge ash below 45 microns. It may be possible
that smaller fractions of  sludge ash may have performed as  well as fly
ash since 78 percent of fly ash has a particle size  less than 32 microns.

It appears that diminution of ash with the subsequent increases in the
fraction of fines with recycle through an incinerator is not a problem.
The opposite condition of the fines being eliminated might pose a real
problem.  Operation with recycle of incinerator ash at Cedar Rapids  has
not apparently had a significant effect upon ash particle sizes.
                              114

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Conditioning Digested Sludge with Sludge Ash and Chemicals

Quite early in the operation of the pressure filter it became apparent
that the digested sludge could not be conditioned satisfactorily with
sludge ash alone.  Consequently, in January of 1972 Dr. Oswald Busse
of the Passavant Werk, West Germany, came to Cedar Rapids to assist
in a series of tests to determine how the digested sludge could be con-
ditioned  for satisfactory filter performance.

Dr. Busse's testing program consisted of dosing various samples of
sludge with ash/ ferric chloride,  and lime and determining the specific
resistance of the conditioned  sample.  Adequate conditioning was reached
when the conditioned sludge sample  had a specific resistance below
r = 20 x 1012cm~2.

Based upon this testing program and  previous experience,  Dr. Busse
made the following recommendations for ash, ferric chloride and lime
dosages which are still basically in  use today.

Figure  44 shows the dosage chart used for operation.  Dosages are based
upon pounds of conditioning agent (lime, ferric chloride or ash) per
100 gallons. The dosages are presented in this manner because the
sludge is  generally pumped from the  holding tanks through conditioning
to the contact tank at 100 gallons per minute.

Ferric chloride dosages range  from 2.2 pounds liquid (0.88 pounds actual)
per 100 gallons at 2 percent solids to 3.3 pounds liquid (1.32 pounds
actual) per 100 gallons at 15 percent solids.  These dosages on a pound
of dry ferric chloride per ton dry solids  basis range from 103 pounds per
ton at 2 percent solids to 21 pounds  per ton at 15 percent  solids. Ferric
chloride costs at Cedar Rapids are $0.0625 per pound delivered.

Lime dosages range from 6.2 pounds  per 100 gallons at 2 percent solids
to 7.5 pounds per  100 gallons at 15 percent solids. This is  equivalent
to 743 pounds per ton dry solids at 2 percent  to 120 pounds per ton dry
solids at 15 percent.  Lime costs at  Cedar Rapids are  $0.0114 per pound.

Another way of  expressing the recommended ash quantities for properly
conditioned sludge is shown in Figure 45. It is apparent that below a
sludge  concentration of about 5.5 percent the ash/sludge ratio must be
increased in relation to the decreased sludge concentration to assure a
Properly conditioned sludge for filtration,
                               115

-------
UJ
   190


   180


   170


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  en
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  >  2.5
    2.0

en
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     ,5
                           CEDAR RAPIDS

                SLUDGE  CONDITIONING  RECOMMENDATIONS
                                                 V1-
                                                              7.5
                                                                   UJ
7 n w 
'' O   (_)

   ^
   ^ fO

65 
6<52 uJ
   >. s
                                                               6.0
                                                                   CD
                                                               5,5
              2   3   4   5   6  7   8  9  10  11  12  13  14 15  16

                       PERCENT OF SLUDGE SOLIDS
                              Figure 44
                                116

-------
Equivalent costs of these chemical dosages are shown in Figure 46.
Chemical costs  range from $14.89 per ton at 2  percent solids to $2.69
per ton at 15 percent solids.  The optimum solids concentration appears
to be near 6 percent. At this  point the cost per ton for chemicals is
$5.48 and the ash/sludge ratio is at a minimum 1.5/1.   These  dosages
and costs are essentially theoretical and will be compared to actual
costs later in the report.

These costs also are not necessarily typical for sludge dewatering with
pressure filtration but are dependent upon specific resistance of the
sludge and the nature of the sludge ash.

Filter Performance Curves

Filter performance curves were established plotting the filtration rate
(yield) in terms  of pounds per hour per square foot, versus time in hours
at various sludge solids concentrations. Figures 47 to 56  show filter
Performance for  feed sludge solids ranging from 2.5 percent to  6.5 per-
cent.

Sludge conditioning requirements were determined with bench studies
using the resistivity meter prior to actual filtration in the pressure filter.
It was obvious that proper sludge conditioning was necessary to assure
good filter operation and a satisfactory cake product. On the other hand,
if good sludge conditioning was obtained then the process  performed very
well at a given level of production and that production was fairly predict-
able over a broad range of raw solids  input.  Referring to the filter per-
formance curves, Figures 47 to 55, it is  apparent that production (yield)
has an optimum ash/sludge ratio for each sludge density.  The  optimum
yield is at the lowest ash/sludge ratio which will assure good  sludge
conditioning as determined on the  bench  using the resistivity meter. As
an example, at 5.5 percent solids, an ash/sludge ratio of  1.1/1 is
slightly better than an ash/sludge ratio of  1.5/1 as shown  in Figure 53.
However, as the sludge density is decreased to 2.5 percent solids,  the
ash/sludge ratio is necessarily increased to 4.1/1 as shown in Figure 47
and the optimum ash/sludge ratio is still the lowest ratio which will pro-
vide good sludge conditioning.

Maintaining good sludge conditioning, the  yield, expressed as pounds
Per hour  per square foot of filter media increases as the  sludge solids
concentration increases as indicated by Figure  56, which is a best-fit
composite curve from all of the computer curves of Figures  47 to 55.
                                117

-------
  5.0
     r
    O



  4,0 A


       O
cc

LU
o
Q
n

(ft
~^
i
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  3.0
      \
2.0
  1.0
         \
            V
          -I    -  I...
                       DIGESTED SLUDGE
                            O-
                                  J	L
                                                  J	I	!	L
                                                                          .0
                             6789

                            PERCENT OF SLUDGE SOLIDS
                                                   10     II     12     13     I*
                                 Figure 45
                              AVERAGE  CHEMICAL  COSTS
                             EQUIP  MFGRS. RECOMM.  DOSAGE
                             46      8      10     12

                              PERCENT  SLUDGE  SOLIDS

                                 Figure  46
                                    118

-------
                           FILTER  PERFORMANCE
                             ~r
.3
    0 4:1 Log  |Q YIELD= -0,1837 (TIME) t(-0.04573)

   A 4,5:1  Log IftjYIELD= -0.1342 
-------
                          FILTER  PERFORMANCE
.3
.2
A/S RATIO

O  3:1  Log |Q

A  3.5: 1 Log   YIELD=- 0.1761 ,(TIME)+0.02527 :
                Yl ELD =-0.1725 (TIME) tO.05403.
                .0
                         1.5           2.0
                          TIME    (hours)

                         Figure  49


                       FILTER  PERFORMANCE
2.5
	j
 3.0
*- \(
?! o
..0^
*- a
3 -O
o
j: 7
^ '
 4
3
.2
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A/S RATIO
0 2.5:1 Log|0
A 3.5:1 Log|0
> 1

O
5,^0
k.'_ ^^^^^
^>^^
n y **'
u / 4
3.5: I-/


YIELD = -0.1700
YIELD = -O.I648
0 1

SOLIDS= 4%



D^_
k^*^O_
"^-^
8 N\

(TIME)t0.08548
(TIME)tO.OI589
5 2
FIME (hours)




2.5: \-j
[<
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5
	 ^

5 3.
                            Figure  50
                               120

-------
                       FILTER PERFORMANCE
                         Figure 51
                       FILTER  PERFORMANCE
A  1.51  Log
0  2:1  Log
|Q
          |0
YIELD =-0,2127

YIELD =-0,1894
TIME)+0.27987

(TIME)+0,I8048
           1.0
                         1,5
                         TIME
                           2.0
                      (hours)
                                      2,5
                                                   3.0
                        Figure  52
                           121

-------
                        FILTER PERFORMANCE
o
-C
                          Figure 53


                       FILTER PERFORMANCE

C
t
t
.9
.8
.7
.6
.5
.4
.3
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A/S RATIO
0 |:l Log|0
A |.5:l Log

~-
<"*<*?'i A C
/ '"*''"* t
^-1.5:1




YIELD = ~O.I57I
jQ YIELD = -0.1803
0
T
SOLIDS = 5.5%
3^^O
k.  ^>^***1**- r


i.
^*V-. f^***1*11*-
A "*^



(TIME)t0.22520
(TIME)t 0.18972
^"^^ ^C
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^^


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IME (hours)
i



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SA A











A/S RATIO
A 1.5:1 Log|0
0 2.1 Log|0


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A ""^Hfc.
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0





YIELD = -0.237^
YIELD = -0.1003

/ O"OJ*^

2.)



t (TIME) + 0.3635
(TIMEJ+0.05876

i II ^.














^*^^ A ~m C
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5 1.0 1.5 2.0 2.5 3.
TIME (hours)
Figure 54
122

-------
            FILTER  PERFORMANCE
                          2.0
                     (hours)
3.0
              Figure 55
FILTER  PERFORMANCE-VARYING SLUDGE DENSITY
                                                  3.0
              Figure 56

               123

-------
Filter yield increases in almost direct proportion to increased raw sludge
solids density at a ratio of about 1.8/1,  that is, if the sludge density is
increased times 2, the resulting filter yield is increased times  1.8.

It is apparent that properly conditioned sludge filters satisfactorily at
both low and high sludge densities and that yield is principally influenced
by time to completely fill the cavity forming the predetermined cake vol-
ume. In practice it became apparent that the limiting factor to improving
yield was the ability to pump conditioned solids  to the  pressure filter in
sufficient capacity to develop the maximum rate of filtration. Each system
consisted of two pumps and some experimentation was performed using
three and four pumps simultaneously on one system.

Filtrate Characteristics

The pressure filter is a  very  efficient solids liquid separator with effi-
ciencies of separation practically at 100 percent.  During the months of
February to August 1972  the filtrate averaged  74 mg/1 suspended solids
while the sludge solids averaged 4.60 percent solids giving a percent
removal of suspended solids  of  99.99984 percent.

During the pilot study it was noted that during a  run the suspended solids
in the filtrate were relatively high initially (400 mg/1 approx.) and then
decreased  to very  low values during the run.  No variance in suspended
solids could be noted during  a run on the full scale filter.  Possible
explanations for this difference  may lie in two areas, the initial pressure
on the pilot plant was a full  225 psig while on the full  scale filter the
pressure started at a low value  (approximately 40 psig) and gradually
increased to 225 psig.  The full 225 psig pressure may  have forced sus-
pended solids through the thin precoat on the pilot plant while this did
not occur on the full scale filter.

The BOD and COD of the filtrate are due primarily to dissolved organics
such as volatile acids and are not necessarily affected by the filtering
process.  It is possible that  some dissolved COD may be removed by
the process of absorption, however, this was not investigated on the
full scale filter and deserves  further research. Limited bench work
indicates that conditioning with  fly ash alone may remove as much as
50 percent of the dissolved COD.  This removal was not noted in samples
conditioned with sludge ash alone.

Filtrate quality is shown in Table IX.
                                124

-------
                            Table IX
                     Average Filtrate Quality
Suspended
Solids
mg/1 '

Total
Solids
mg/1

Volatile
Percent
Solids

Turbidity
Feb.  Mar.  Apr.  May  Tune   July  Aug.   Average
  25    94   103     60     84     65    88    74
5000  4900  5600   6200   6400   6000  6800  5842
11.2  15.6  23.7   20.8   26.0   30.2  25.1  21.8
  18
20    29    19    17     22     27   21.7
Color Cobalt     62    61    69    89    107    123    132  91.8
Platinum
Units
Conductivity
Micromhos/
7883  7520  7914  7483   6570   5800  5600  6967
cm-
COD           623   706  1310   847   1014    720   780   857
mg/1
BOD 5
mg/1

PH
 329
    901
387   398
504
12.1  12.4  12.4   11.9   11.7   11.5  11.6  11.9
                              125

-------
Sludge Cake Characteristics

Sludge cake is the discharge from the pressure filter and is about 58 inches
diameter, 1^ inches  thick, weighs about 200 pounds.  Each  pressure filter
produces 83 cakes per cycle which are discharged to a storage bunker.
Shear cables  are positioned across the top of the bunker to break these
large cakes into smaller pieces.  Cake is conveyed through  a  series of
drag conveyors and elevators to the incinerator.   At normal operation
cake moisture content is in the range of 36-38 percent and the appearance
is dense, dry and textured.  Extreme care should be exercised in select-
ing proper materials  handling equipment to transport this cake to disposal.
It is essential that minimum transfer and handling of the cake  be performed
from the  filter to ultimate  disposal.  Excessive shear,  breaking, dropping,
grinding, or any form of handling results in plasticizing the  cake to the
consistency of dense putty or clay-like material. At these properties the
plasticized cake material  tends to be extremely sticky even  at 62-64 per-
cent solids and causes many problems due to poor release from drag and
screw conveyors.  The crumbly, dry,  textured character of the original
sludge cake can be maintained with minimum transfer and handling.
Grinders  were originally installed at Cedar Rapids in the incinerator feed
system, however,  these have been removed and are undesirable with this
type of cake.  By the time the cake has traveled through the materials
handling  system to the incinerator it has broken into small pieces of
i inch to 2 inch size.  These pieces are generally in the form of balls
due to the rolling action of the incinerator feed screw conveyor.  Balls
probably  would not develop if this final conveyor were  of a different
design.   However, the shape of the feed particles is not significant to
the incinerator performance.

The specific weight of  the discharged filter cake varies from about 107
to 114 pounds per cubic foot dependent upon moisture content  and ash
ratio. Under normal operating conditions the moisture  content of the
sludge cake varies only a  few percentage points, therefore,  the greatest
influence on the specific weight of the cake is the ash content (ash/sludge
ratio). The specific gravity of organic ash is  in the range of 2.6 to 2.8.
Laboratory  determinations of specific gravity using water medium was
2.63; using hexane medium, was 2.78.

Cake discharged from the  filter to the cake storage bunker is sheared by
a series  of cables which tends to bulk the  broken cake.  However, the
impact of additional  cake  falling on the pile of previously discharged
cake tends to cause  recompaction due to the plastic properties of the
sludge cake.  Bulked filter cake was determined to be about 47 pounds
per cubic foot, but this was for cake not exposed to impact and  compres-
sive forces due to dropping and  stacking.  That low figure might be valid
                                126

-------
for some conditions of limited cake storage where bulking is the major
influence. Cake discharged from the bottom of the hopper under com-
pressed conditions of superimposed loads of 10  to 12 feet depth had a
specific weight of 83 pounds per cubic foot. This was determined by
weighing many fixed volume  samples  in the bunker conveyor.

Conditioned sludge applied to the pressure filter forms a cake structure
which almost immediately serves as the principal filter media.  The
filter cloth has served only as a base structure for this development, and
after the cake has formed to  1/16 or 1/8 inch thickness, the influence of
the filter cloth is negligible  to the continued filter performance.  As the
cake continues to form and the filter void is  filled, the total resistance
increases rapidly developing full pressure differential in possibly 30-40
rriinutes as shown by a typical filter pressure chart,  Figure 57.

                    Typical Filter Pressure Chart
                           0    l5     I
                           TIME  (HOUR)
                             Figure 57
                                127

-------
Continued full pressure differential is desirable to assure that most of
the free water has the time to travel through the cake and underdrainage
system to be disposed of as filtrate.  The noteworthy point after the
development of the full cake formation is that prolonged high pressure
does very little to further dewater the cake. The moisture content may
range only a few  percentage points over a relatively wide span of sludge
feed densities; and the cake moisture content is even less influenced by
prolonged dewatering time.  An  example of this is found in Figure 58
where  sludge cake densities are shown for different percent feed  solids
and cycle time.   The curve is a best fit line using arithmetic probability
points shown as dark points of average solids  groupings.
  75 r-
  70
 g 65
 O
 en
  60
  55
 o
 ir
 ui
 o.
  50
  45
                                                                  o o
  o  o

    0             o
PILOT PLANT DESIGN CONDITION 48% TOTAL SOLIDS
         1    1
                               1
                                    1
                                             1
        2.0   2.5   3.0   3.5   4.0   4.5   5.0   5.5   6.0   6.5   7.0   7.5   8.0   8.5
                            PERCENT OF TOTAL SOLIDS
                               Figure 58

Grease and nitrogen determinations were made on composite-samples for
each shift of operation.  This consisted of three composite samples per
day made up  from each filter run per shift.  The average monthly analysis
is as follows:

                 April    May    June    Tulv     August
Nitrogen mg/1     1.83    1.58    1.16   0.86    0.87
Grease mg/1       2.40            1.36   1.80    1.70
                                 128

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Phosphate Removal

There has been increasing emphasis upon the removal of phosphorous
from waste.  It then becomes very important that the dewatering of
phosphate rich sludges should produce a filtrate low in phosphorous.
Phosphorous generally is removed by precipitation using three commonly
available cations, A1+3,  Fe+2, or Fe+3, and Ca+2 .

In the process of sludge dewatering Fe+3 and Ca+2 are present as coag-
ulants.  Presumably because of the negative oxidation reduction potential
of the sludge a portion of the iron is reduced to Fe+2.  Consequently
three precipitates of phosphorous could be possible; ferrous phosphate
(Fe3(PO4)2),  ferric phosphate (FePC^), and calcium phosphate or more
properly calcium hydroxyapatite
Phosphorous removal through the pressure filter was investigated by
using the resistance meter on the bench and spot checking actual
phosphorous removal through the full scale filter.

Bench Scale Studies

Since most of the data from the pressure filter was obtained when the
sludge was conditioned with lime and ferric chloride,  some bench work
was also performed using the resistance meter and conditioning only
with ash.  In this test a portion of the same sample was conditioned
with fly ash using ash/sludge ratios of 0, 1.0,  1.5,2.0, 2. 5 and  3.0.
Separate portions were also  conditioned with  sludge ash using the  same
ash/sludge ratios.  The samples were then filtered through the resistance
meter and the filtrates were analyzed for total phosphate.  The results
are graphically displayed in Figure 59 and listed in Table X.

                               Table X

                      Fly Ash _ Sludge Ash
Phosphate
A/S Ratio as P
O(noash) 25.
1.0
1.5
2.0
2.5
3.0
12.
10.
10.
8.
7.
Percent
Removed
PH
6 mg/1
3
4
0
6
6
51.
59.
60.
66.
70.
9
4
9
4
3
8.
8.
8.
8.
8.
1
4
4
5
6
Phosphate Percent
as P Removed
25.
12.
10.
8,
6.
6.
PH
6 mg/1
3
4
3
8
2
51.
59.
67.
73.
75.
9
4
6
4
8
8.3
8.3
8.5
8.6
8.7
                               129

-------
The removal of phosphate was nearly identical for samples treated with
fly ash and the  samples treated with sludge ash.
                           PHOSPHATE  IN  FILTRATE
                                      A  Samples conditioned with SLUDGE ASH
                                      O  Samples conditioned with FLYASH
                0.5
1.0          1.5        2.O
   ASH/SLUDGE  RATIO
2.5
3.0
                                  Figure 59
Full Scale Studies
During the course of the study several runs on the full scale filter were
made using only fly ash and no chemicals  (ferric chloride and lime).

The phosphate levels in the raw sludge and the removals are:
Run No.
1 Sludge
Filtrate
2 SJudge
Filtrate
Total
Phosphorous as P
516
8.75
528
18.5
Soluble
Phosphorous as P
100.5
2.5
73.5
4.5
Percent
Removed
91.3
74.8
                               130

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The percent phosphorous removal was based upon the  soluble phosphorous
in the sludge and total phosphorous remaining'in the filtrate. Presumably
the insoluble phosphorous in the sludge is principally discharged with
the sludge cake.

Percent Removal = Soluble P Sludge - Total P Filtrate = 100.5-8.75 = 91.3%
                         Soluble P Sludge              100.5

Additional data on phosphorous removal was obtained when ferric chloride
and lime were also used for conditioning along with ash. This data is
presented in Table XI.

                              Table XI

              Phosphate Removals by Pressure Filtration
      Using Lime, Ferric Chloride and  Sludge Ash as Coagulants
              Sludge
Sludge
Date

7-20-72
7-24-72
7-25-72
7-26-72
7-27-72
7-27-72
7-27-72
8- 3-72
8- 7-72
8- 9-72
8-10-72
Unconditioned
Total
500 mg/1
592
564
786
520
546
514
442
980
446
1150
Soluble
*
*
*
*
*
*
it
96
91.5
104
142
Conditioned
Total
1204
1596
1448
1752
1124
1256
1528
1732
1348
1656
2890
Soluble
*
*
*
*
*
*
*
96
131
134
172
Filtrate
Total
2.4
16.8
16.8
11.8
16.8
14.8
19.8
13.2
12.3
8.75
3.65
Soluble
*
*
*
*
*
*
*
< 0.1
< 0.1
*
*
*  Not determined

The percentage reduction of phosphorous realistically should be based
upon the soluble phosphorous in the unconditioned sludge versus the
total phosphorous in the filtrate.  This will give a  somewhat lower
percentage of removal than a percentage removal based upon total phos-
phate in the conditioned sludge versus total phosphate in the filtrate.

Therefore,  the comparison of the two processes of  phosphate removal ,
ash only conditioning versus ash plus chemical conditioning, may be
difficult unless the  basis of comparison is the same.  That is, the
comparison of total  and soluble phosphate.
                               131

-------
Percentage removals of phosphorous through the pressure filter averaged
86.7 with the soluble phosphorous in the sludge averaging 108.4 mg/1,
and the filtrate total phosphorous averaging 14.4 mg/1.  On a total
phosphate basis the percent removal was 97.8.

Filtration with Fly Ash

During the course of the study a program was developed to operate the
full scale pressure filters using only fly ash as a digested sludge condi-
tioner.  The limited work that was done affirmed the fact that fly ash
alone (without chemicals) was sufficient to dewater digested  sludge.
This also confirmed the conclusions of the original pilot plant report
wherein only fly ash was used as a  sludge conditioner.

However, the use of fly ash posed many problems in materials handling
and conveying.  Fly ash tends to fluidize and with the ash handling equip-
ment used at Cedar Rapids the accurate control of admix quantities was
impossible.   Our experience would indicate that gravimetric feeders are
not adequate for feeding materials which fluidize.

Another problem which occurred was conveying the filter cake.  Although
the percent cake solids averaged 55.5 percent for all  runs, the fly ash
cakes were virtually impossible to convey.  After being conveyed from
the bottom of the bunker the cake turned into a sticky mass which refused
to fall off the cake elevator flights.  Therefore,  only limited work was
performed using  the full scale filter.

Below is the  summary of the best filter runs using fly  ash as a conditioning
agent.

    Percent Solids  = 5 percent
    Ash/Sludge           2.57
    Cycle Time           1.80 hours
    Yield                0.57
    No. Run             4

Other runs which were made at lower percent solids in the feed sludge
were dosed with extreme  amounts of ash (A/S =4.5 to 6.2) because of
the fluidizing of the  fly ash.

Filtrate

Filtrate from  dewatering, using fly ash, showed great differences from the
filtrate using sludge ash  and chemicals. Not enough  runs were made to
draw any generalizations.

                                132

-------
Following is a comparison of filtrate from one day of operation.
                          Fly Ash .
                        Conditioned
pH
Suspended Solids
Color
COD
mg/1
Units
mg/1
mg/1
                                  9.3
                                148
                                 50
                                480
                                250
Sludge Ash and. Chemicals
     Conditioned	
       10.4
       43
      150
      830
      470
It can be seen that the filtrate appears to have a lower COD, BOD and
color when the sludge is conditioned with fly ash rather than sludge ash
and chemicals .

Dewaterinq of Raw Primary Sludge

The filterability  of raw sludge as compared  to digested sludge at Cedar
Rapids is generally much better.  Specific resistance values for raw sludge
range from values of 120 to 300 x lQl2cm-2/ with an average value of
approximately 150 x  lol2cm-2 .

Because of better filterability of raw sludge, a program was set up to
develop chemical and ash dosage requirements so that raw sludge could
be run on the full scale  filter.

Bench testing and testing using the six inch pilot pressure filter indicated
that satisfactory performance could be obtained using the chemical dosages
for digested sludge and  using a 1/1 ash/sludge ratio at all solids concen-
trations.

Accordingly,  in-plant arrangements were made to empty one storage tank
and to pump raw sludge  from a gravity thickener to the empty storage  tank.
First filter runs with  raw sludge were made  November 28, 1972, and during
the month of December over 60 filter runs were made on raw sludge.   The
runs  were generally quite satisfactory.

During the month of December, the raw sludge pumped to the sludge hold-
ing tanks averaged 3.7 percent solids. During this same period, sludge
from  the holding  tanks to the process averaged 5.78 percent solids, and
much clear  supernatant could be decanted from the holding tanks.
                                133

-------
                             Table XII

                 Summary of Raw Sludge Filtration Data

     5%
     Average Ash/Sludge                 1.20 range   1.56 to 1.00
     Chemical Cost       $/ton           6.26range  $7.01to4.67
     Yield                lb,/sq.ft./hr.  0.68 range   0.97 to 0.30
     Cycle Time          hrs .            2.03 range   1.33 to 3. 75
     Number of runs - 27

     6%
     Average Ash/Sludge                 1.19 range   1.44 to 0.68
     Chemical Cost       $/ton           5.17 range  $5. 61 to 4.71
     Yield                Ib./sq.ft./hr.  0.90 range   1.41 to 0.29
     Cycle Time          hrs.            1.75 range   0.83 to 3.83
     Number of runs - 22

     7%
     Average Ash/Sludge                 1.13 range   1.47 to 0.91
     Chemical Cost       $/ton           4.97 range  $5.29 to 4.05
     Yield                Ibs./sq.ft./hr.  0.92 range   1.30 to 0.39
     Cycle Time          hrs.            1.69 range   1.0 8 to 3.25
     Number of runs - 8

In Table XII, the results for the filtration of raw sludge are  given.  Data
are given  for 5, 6 and 7 percent solids. Other data  f or 3,  4, 8 and 9 per-
cent solids are not given.  These were insufficient filter cycles to interpret
a curve.  As can be noted, yield, cycle time and chemical costs all im-
proved as the percent feed solids increased from 5 to 7 percent.

Yield versus cycle time is shown  in Figure 60 for raw sludge. Yields are
all quite  good compared to digested sludge,  basically due to low ash
ratios of approximately  1.2.

Filter Cake

Pressure  filter cake averaged 54,  56 and 58 percent  solids for filter feed
solids  concentrations of 5, 6 and 7  percent solids respectively.  Volatile
solids  content of the cake was higher than for digested sludge, averaging
25,  27 and 29 percent for  5, 6 and 7 percent feed  solids respectively.
These values compare to 11-13 percent volatile solids at comparable de-
watering  conditions with digested solids.  These levels of volatility were
much higher than that necessary  (approximately 11 percent)  to provide
                                134

-------
autocombustion in the incinerator.  No difficulty was experienced in
conveying the raw sludge filter cake.

Filtrate

The most  significant difference in filtrate quality caused by filtration of
raw sludge was the very high BOD's and COD's of the filtrate.  In
Table XIII is given comparative values of filtrate COD's and BOD's for
digested sludge and raw sludge during the two month period of November
and December 1972.  These high BOD's are the result of the formation of
soluble volatile acids in the sludge. These volatile acids depressed the
pH of the raw  sludge to values of approximately 5.0 (+0.2).  The filtrate
pH averaged 11.6 due to sludge conditioning with lime.

Filtrate suspended solids showed no basic difference than the filtrate from
digested sludge.  Filtrate samples from all  runs averaged 79 mg/1. During
this same period  suspended solids from the digested sludge filtrate aver-
aged 90 mg/1. As shown in Table XIII there was no  significant difference
between filtrates.

                              Table XIII
        Comparative Values Dewatering Raw and Digested Sludge

               COD mg/1          BQD5 mq/1        BOD5/COD (%)

Sludge     Digested  Raw      Digested   Raw    Digested  Raw

Maximum     1080     13,500      740     10,300     70     '94
Minimum      270     2,000      140     1,700      41      72
Mean         510     7,080      300     5,700      56      81
Median       470     6,600      250     5,200      58      81

          Comparative Values of Suspended Solids in Filtrate

Type of Sludge        Maximum    Minimum   Mean     Median

Raw                 430 mg/1    22 mg/1   79 mg/1    53 mg/1
Digested             345 mg/1    20 mg/1   90 mg/1    61 mg/1
                               135

-------
u.  I
o  .9
>?  .8
5  .7

-------
                             SECTION X

                       PROCESS EVALUATION

 Economic Evaluation

 Basis for Economic Evaluation

 The labor requirements were determined for sludge dewatering and sludge
 incineration for both operation and maintenance.  These labor costs were
 based upon actual wages paid for 1972 including an allowance for fringe
 benefits such as  insurance, vacation, workmen's compensation,  retire-
 ment, etc.  Hourly rates were $4.58 for the operator and maintenance
 shift foremen and $4.17 for assistant. Management or laboratory costs
 are not included.

 The sludge dewatering plant was bid in December 1968.  To be consis-
 tent throughout this report, all costs have been interpreted to 1972 costs.
 Earlier in this report the pilot plant evaluation costs predictions made  in
 1968 were interpreted to current 1972  costs, therefore the capital costs
 for the construction of the  sludge dewatering facility  for which bids were
 received in December 1968 have also  been interpreted to 1972 using the
 Engineering News-Record index of cost trends. The equipment costs in-
 crease for that period has been interpreted to be 20 percent.  The capital
 costs used in  this evaluation are the actual bid costs for December 1968
 scaled upward as outlined to reflect what these costs might have been  for
 1972.

 Chemical costs are those actually paid for pebble lime and liquid ferric
 chloride.  Power  costs were assumed  to be $0.02/KWH for filtration
although it may actually be less due to a favorable power demand-power
 consumed ratio.  Power consumed for  each filter is separately metered,
 however,  total power for the dewatering and incineration processes is
also metered and  is the basis for determining power company billings.
High power demand charges for the incinerator high energy off-gas
 scrubber distort the power billings and make it necessary to interpret
 the filtration power charge as indicated above. Power consumed for
filtration was measured by  the separate watt-hour meters during observed
filtration cycles and was determined to be 40 KWH per hour of actual
filtration.

Cedar Rapids has two  pressure filters  and all labor costs are determined
for the operation of two units on a full time basis.  All costs were reduced
                                 137

-------
to dollars per filter hour operation so that the final cost would be evaluated
to filtration capacity regardless of the operating capacities experienced
during the study.
Operation Foreman
      Assistant
       Man Hours/Hr. Filtration  $/Hr.  Filtration
        0.90                   4.13
        0.33          1.23     1.37     5.50
Maintenance Foreman   0.17
      Assistant         0.33
                      0.50
0.76
1.37
2.13
                                     1.73
Labor costs per filter ( -*- 2 units)

Power costs 40 KWH/hr.  filtration x $0.02
                                      $7.63

                              $3.82/hr.  filtration

                               0.80/hr.  filtration
Chemical costs  ferric chloride ($130.00/ton)    0.065  /pound
                lime           ($  22.75/ton)    0.0114/pound

Capital costs    20 years at 4|%
               5^ days per week - 286 working days/year
Building    $  417,000
Equipment   1,255,000
           $1,672,000 = $19.10/hr+
                                                     units = $9.55/
                                                       hr. filtration
No cost allowance has been made for fly ash haul to the plant or product
ash from the plant.  This cost may vary considerably from a small cost
as in the case at Cedar Rapids, to a significant cost if the fly ash was
purchased and the haul distance was great.  The basic operation is with
incinerated sludge cake ash recycled for sludge conditioning. However,
as a part of this report and to follow up on the original pilot plant study
wherein fly ash was used as a sludge conditioner, power plant fly ash
was  transported and used as a sludge conditioner.  The fly ash was
obtained from the same power generating plant as was used in the pilot
plant study.  The ash was obtained at no cost.  Transportation was ar-
ranged with a contract hauler using a bulk cement transport trailer truck.
Fly ash was loaded by gravity chute at the power plant, transported
about three miles distant, unloaded by blowing into the ash storage bins.
The cost for transporting fly ash filter aid was $4.00 per ash ton.  The
filter performance using fly ash filter aid averaged about $6 to $8 per ton
dry sewage solids as  the cost of hauling fly ash to the process. Product
                                138

-------
 ash haul to disposal would vary considerably with local conditions and
 would be less significant with organic ash than with fly ash filter aid.
 At Cedar Rapids it was possible to dispose of the excess ash on the site,
 therefore haul-out costs were not accounted.

 Process Costs

 The total cost for pressure filtration is composed of four separate costs.
 These costs are labor  for maintenance and operation, power, chemicals
 for conditioning, and capital investment.

 Labor costs were determined, as described before,  to be $3.82  per hour
 per filter.  Power costs were determined in the same manner to be $0.80
 per hour per filter.  Likewise capital costs were determined to be $9.55
 per hour.   In the compilation of the data each cost  was determined as
 follows:

      Cost/ton  = cycle  time (hours) x cost factor
                    tons dry solids/filter

 Cycle time represents  the time to complete a filter  run.  Added to filtra-
 tion time,  approximately 0.4 hour is required to discharge the cake,
 refill the filter with filtrate,  and precoat, and is referred to as turn
 around  time.

      Cycle time (hours) = filtration time (hours)  +  0.4  hours turn around

 The  cost facfor represents the dollar per hour value placed upon each cost.

 Chemical costs  were determined for each cycle by measurement  of actual
 quantities  of lime and  ferric chloride  used to condition  the sludge.
     i>
 Figures 61, 62 and 63  show cost data for 4|%, 5|%, and 6|% solids
 respectively.  Only operating costs,  which  includes both power and
 labor, capital costs and  total costs are shown.

These three cost curves were determined by  using costs for each run
which were determined by a computer, then taking these values and
performing  a linear regression analysis based upon the relationship log
cost versus log yield.
                                139

-------
        PERCENT OF TOTAL SOLIDS*
l.D
 1.4
qr
< 1.2
1 i.o
X.
A 8
""" .6
3 -4
UJ
> -2
0
(
\
\
\
\
\

OPER
	 COST
	
3
i
%
\
\
V

ATING ^
S 	

\
\

CAPIT



\r
[~ COSTS -7
\. V
^
^>^^

-c



n /


' 	 c



TOTAL

*c
>-....c
i-mm^


COSTS If

:*^,^r
c
^~  5


iJCL. CHEI

J^^ri
_ I

r
^ICAL CO




C!TC:




i  :&=saaa
3 5 10 15 20 25 30 35 40 45 5
              COSTS   (8/Ton.)
             Figure 61
        PERCENT OF TOTAL SOLIDS
                       TOTAL COSTS INCL. CHEMICAL COSTS
    CAPITAL 	rn_
\1  COSTS
             20    25    30

              COSTS   (B/Ton.)
             Figure 62
                140

-------
                       PERCENT  OF  TOTAL SOLIDS
   1,6



 l.4

 cr
 4J 1,2


 2 1.0

         .... __
       \
V  j     fv.
 \   i     I   x
... %,    CAPITA I	J*
9-	t-	
      i
                            \	'	1	
              -TOTAL COSTS  INCL. CHEMICAL COSTS-	
        OPERATING
        COSTS
                             20    25    30

                             COSTS   (fi/Ton,)
                              Figure 63


The following correlation coefficients were obtained.


       Percent Solids   Operating Cost   Capital   Total
                                              45
4*
5*
6*
-0.985
-0.991
-0.988
-0.985
-0.991
-0.988
-0.875
-0.925
-0.865
The range of cost experience is shown in Table XIV.


                              Table XIV

                           Process Costs
                                Maximum   Minimum   Average
Operating
Capital
Total (including chemical)
Operating
Capital
Total
6i%
Operating
Capital
Total
11.89
24.05
43.64
9.32
19.27
34.05
6.85
14.17
28.44
2.84
5.87
17.72
2.65
4.54
14.80
1.47
3.04
11.22
5.83
12.05
26.83
4.69
9.71
21.69
3.83
7.91
18.20
                                           50
                                141

-------
Negative correlation coefficients mean an inverse correlation.  The cor-
relation  coefficient can have values from -1 to +1 where an absolute value
of 1 means a perfect correlation whether the correlation be direct (+1) or
inverse (-1).  A value of zero indicates no correlation or  simply a random
scattering of points.

All correlation coefficients shown above are negative and  closely approach
1 in absolute value.  As can bo expected, costs related to hours operation
decrease as productivity or yield increase, and as can be seen from the
graph, these costs decrease in a predictable manner.

The correlation coefficient for total costs are lower because of the seem-
ingly lack of correlation of chemical dosages (costs) with yield.  Correla-
tion coefficients for chemical costs were developed in  the same manner
as the other costs. The correlation coefficients were  0.122, 0.217 and
0.005 for 4|, 5| and  6| percent solids respectively.  These are very low
and poor correlation coefficients.

Chemical costs  showed poor correlation for several reasons,  they are:

       1.  Varying specific resistance of digested sludge.

       2.  Lack of sufficient conditioned sludge pumping
           capacity to the filter.

       3.  Over-ranging of chemical feeders.

The first two reasons  are interrelated.  As has been brought out in previous
sections, chemical (lime and ferric chloride) dosages  were set up to properly
condition sludge with high resistance.  Since the  sludge  varied consider-
ably in resistance, some sludge was over-conditioned, While over-condi-
tioning the sludge should have resulted in a higher yield by giving a higher
filtration rate, the maximum filtration  rate of the filter  is mechanically
limited by the capacity of the two filter feed pumps.  Later it was demon-
strated that using all four filter feed pumps for one filter  significantly
decreased filtration time (up to 50%),  by meeting the true  capacity of the
filter for sludge.  Because of this mechanical limit, chemical conditioning
was only partially effective in raising yields.

The other problem  which occurred was 'the oversizing of the chemical feeders
specifically the  ferric chloride feeder. Because of this oversizing, the
feeder operated  in the lower 20 percent of its capacity  where accuracy
was at a minimum.
                                 142

-------
                        CHEMICAL  COSTS   (AVER.)
         I 5

         I 4
      o
      5
      LJ
      I
      O
         8
           /ACTUAL OPERATING COSTS
 X:
             	1  '  i '  '   i    i i  i  t i    i	i
          0      2      4      6      8      10 '    12     1415
                        PERCENT  SLUDGE  SOLIDS

                             Figure 64

Figure 64 shows average chemical costs per ton of dry solids versus
percent solids. Also shown is. the chemical cost curve for the proposed
costs developed from work on the resistance meter by Dr. O. Busse.  As
can be seen from the graphs, operating costs were somewhat higher than
theoretical yet followed the cost curve very closely.

Optimization of Costs

From the figures present it becomes quite apparent that real cost savings
can be made by pre-thickening of sludge solids.  Thickening the incoming
sludge from 3 to 6 percent decreases chemical costs at Cedar Rapids  from
$10.00 per ton dewatered to $6.00 per ton dewatered. Ash requirements
also decrease from 3 pounds per pound of sludge solids to li pounds  per
pound of dry solids.  See Figure 45.  Obviously the less volume of the
filter taken up by ash means more capacity for sludge. Cost advantages
for pre-thickening decrease for influent solids in excess of 6 percent.
                               143

-------
All costs developed in this section, especially those for chemicals, are
specifically for Cedar Rapids and do not have general application,  and
do not form a sound basis of comparison with other processes.  Reference
is made to the previous section of this report wherein chemical  costs with-
out ash on a vacuum filter at Cedar Rapids could not be lowered below
$20.00 per ton because of the high specific resistance of the sludge.

Process Evaluation
Having operated the pressure filter pilot plant at an early date when no
analytical data was available for evaluation and guidance, and having
developed the design criteria for a full scale plant from those pilot
studies,  Cedar Rapids has observed the process performance without
bias to other installation performances.  As stated previously, the pilot
plant performed reasonably well with good data correlation, however, the
pilot process was hand controlled and should reasonably be expected to
perform well.  The full scale plant process is highly automated and there-
fore, reflected some of the malfunctions associated with equipment failure
from numerous interrelated units.

Some unsatisfactory process performance  was experienced at Cedar Rapids
during the early days of start-up and check-out.   Much of this was related
to inadequate  training of the operating personnel, particularly those of  the
equipment manufacturer.   This is mentioned here  because so many visitors
to this facility during more than a year of start-up and check-out prior to
acceptance were aware of this problem.  It is understandable that the first
major installation would offer some degree of challenge on start-up and that
experience would make the next one easier.

Looking backward to the expectations of the pilot plant study,  and to the
process performance observed  in the full scale plant, it is obvious that
the pressure filter process is dependable, economical, and offers distinct
advantages over other forms of sludge dewatering  based upon Cedar Rapids
sludge.  The process has  achieved a higher quality product with greater
capacity  in the full  scale  plant than was predicted by the pilot plant study.
Cake quality is maintained over a broad range of  sludge solids concen-
tration, particularly at the low  solids level  (2.5-4.5%) which were not
previously experienced in the pilot studies.
      

Incineration of Sludge Cake

The primary subject of this report is dewatering of digested sludge by
pressure  filtration.   The process at Cedar Rapids is dependent  upon
                               144

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recycled organic sludge ash as a sludge conditioner and filter aid.  It
would, therefore, seem appropriate to briefly review the incineration of
the pressure filter cake and the recycle process.  As previously described,
cake is discharged from the pressure filter to a storage bunker and trans-
ported through a  series of conveyors  to the incinerator.

The  cake incineration process  starts  with the bunker feed conveyor which
is a variable speed drag conveyor. This speed control on the bunker dis-
charge serves to regulate the rate of  cake feed to the incinerator and is
manually set by the operator.  Automatic temperature controls regulate
the auxiliary fuel feed, if required, to maintain hearth temperatures.
The  incinerator is a conventional multiple hearth furnace previously
described.

When pressure filter  performance is maintained close to design ash/sludge
ratios of 1.5/1, it has  been observed that furnace performance can be
controlled  so that the cake feed is autocombustible with 11 to 13 percent
volatile solids content. Under normal filter operation the moisture con-
tent of the cake falls in a narrow range of values  regardless of the raw
sludge feed density,  therefore, the major factor in controlling the per-
centage of volatile solids in the cake is the ash/sludge ratio. As the
ash/sludge ratio increases, the volatile content of the cake decreases.

Numerous  heat balances have been programmed on the furnace operation,
however, these are not the subject of this report and it will suffice to
summarize that the filter cake from digested sludge will burn without
auxiliary fuel when furnace feed conditions are held within a reasonable
range (+ 50%)  of the design ash/sludge ratio and cake feed  rate.

The internal conditions  of the furnace are significantly different while
burning cake with recycle ash.  The internal atmosphere is  extremely
dusty even under minimum excess air control and  good balance.  A con-
siderable amount of ash is picked up  in the gas stream in the upper drying
hearths and carried with the off-gas.  This added  particulate volume
makes an adequate scrubber system even more important.   Cedar Rapids'
incinerator off-gases  are directed to a wet venturi high energy scrubber
operating at about 30  inch water column pressure drop. Scrubber water
is returned to the treatment plant and contains 4,000 - 10,000 mg/1
suspended  solids.  This seemingly large quantity  of fine ash carried
out in the  scrubber water was of considerable concern for some time.
The general philosophy  for successful pressure filtration with ash filter
aid was that the fine  ash particles were extremely important to develop-
ing a good dewatering cake.  The Germans  had experienced  and reported
                               145

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this requirement.  In the early periods of check-out the apparent shortage
of finer ash in the recycled conditioning ash was considered a factor of
poor filter performance.  After considerable bench work with and without
the fine ash carried out in the scrubber water it was determined that the
absence of this fine  ash did not influence sludge conditioning and filter-
ability.

Incinerator ash passes through a hammer mill grinder which originally
contained 1/16 inch  screens.  These were later enlarged and ultimately
increased in size to  f inch.  It has been demonstrated at Cedar Rapids
that,  contrary to the European practices,  grinding recycled ash  is not
necessary, and for ordinary protection of the process equipment simple
screening for foreign and oversized material would be satisfactory.

Considerable difficulty  has been encountered with incinerated ash in
storage and feeding equipment.   Both organic ash and fly ash readily
fluidizes and compacts  over a narrow range of external influence.
Elevated ash storage bins were equipped with customary bin vibrators to
assure even flow to the outlet feeders.  It was necessary to employ a
trial and error adjustment of both frequency and magnitude of vibration to
get the correct adjustment of the external influence to assure satisfactory
flow of ash to the  feeder.  Until  the correct combination of magnitude of t
impact and frequency was determined it was unpredictable whether the
output ash from the storage bin to the feeder would fluidize or compact.
At times it would fluidize at one  set of conditions and at other times  it
would compact.  Temperature of ash leaving the incinerator is normally
about 250 F. and this causes no problems in the elevated storage bins.
However, due to process upsets  the ash temperature  may go to 500-700
degrees F. for short  periods.  These high ash temperatures into a partially
filled bin apparently caused a temperature turbulence and a fluidized con-
dition which would progressively work through  the ash in storage to the
outlet and thus upset the feeder performance.

Proper precoat is essential to satisfactory filter performance and precoat
pressure is an important control for assuring good cake  formation.  Normal
precoat pressure for  a clean filter media starts at about 25 psig and ranges
upward to 40 psig. Above this operating range it appears that gradual
deterioration of filter performance often occurs.  To operate within the
precoat pressure range it is necessary to  have  properly  conditioned raw
sludge applied to the filter and to have had the proper quantity of ash
precoat.
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Improperly conditioned raw sludge will cause some buildup on the media
and thus gradually raise the successive precoat pressures.  Sometimes
this problem can be corrected  by dropping the quantity of precoat from
150 to 50 pounds thus forming a new cleavage plane encouraging  the
discharged cake to remove some accumulated precoat. Improper quanti-
ties of precoat may cause a gradual buildup whereby the excess ash
creates a shear plane distant from the filter media so  that when the cake
breaks away excess precoat remains on the media.

An  operating experience which has reduced the  precoat pressure,  or at
least prolonged the need for a complete filter wash is that of extending
filter fill cycles.  During the fill cycle filtrate  water is pumped into the
filter and by continuing recirculation of the overflow it is sometimes
possible to  perform some degree of washing. This practice is not totally
successful, but it may reduce the precoat pressure by 10 psig which may
extend the need for a  complete filter wash for a few days.

Estimated filter runs between media wash is about 100 runs, or 150 to
200 filter hours of operation.  The method of washing  a pressure filter
could stand much improvement.  The wash rod with a series of high
pressure nozzles originally furnished is no longer used.  A more  success-
ful  system has been a single high pressure nozzle with a broad discharge
operating at about 750 pounds pressure.  Commercial  grade detergent is
used in the  washing solution,

Equipment Evaluation

From the beginning of check-out considerable equipment difficulties were
experienced with the full scale plant.  These are equipment problems
beyond those normally expected or experienced during start-up and check-
out.  Many of these unusual problems related to poor  workmanship and
fabrication details of the pressure filter and accessories equipment.  These
problems were greatly compounded by manufacturers' personnel inexperienced
in detailed process and  equipment performance. Cedar Rapids became a
training center for these people which accounts for the fact that it took
almost 18 months to approach a reasonable performance.  Numerous
modifications to the mechanical features of the equipment have been
made, along with the electrical and process controls.  Some of the poor
fabrication quality control, such as poor machining and assembly,
resulted in failures and  were subsequently repaired or replaced with
improved equipment.
                               147

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Some of the major problems experienced, or which are still of concern
are mentioned here for purposes of objective reporting. It is not the
intent to criticize the equipment supplier at this late date, because in
fact, periodic improvements are still being made to the equipment at
Cedar Rapids by Beloit-Passavant as of January 1973. The equipment
supplier has continued to respond to the problems  as they have occurred
and we have every reason to tnink that this  cooperation will continue.

Pressure filter plate warpage  has been a major problem from day one.
Some of this has been attributed to the early days  of operation when the
plates were inadequately shimmed, however, adequate shimming of the
stay bosses has not totally eliminated this problem.  Warpage occurring
in the plate diaphragm transfers bending to the  plate frame which in turn
accelerates plate gasket deterioration due to warped gasket seating plane.
Plate alignment also affects gasket life wherein poor alignment, or reposi-
tioning of the plates each cycle, causes a reshaping of the  plate gasket
leading to premature failure.   Plate gasket life  is  short from whatever the
cause, and is still unsatisfactory in our opinion. .

Cake and incinerated ash grinders were originally  installed  because the
German process recommendations stressed this requirement.   However,
experience at Cedar Rapids confirmed that cake grinding was undesirable
and that incinerated ash grinding was unnecessary.  Filter cake broke and
crumbled sufficiently in the conveying system for delivery to the incinerator.
Cake passing through the grinder tended to knead to a plastic like ball and
under high moisture content of about 55 percent cake into the  grinder  came
out more as an extrudable paste. The cake  grinder screen was first removed,
then the rotor as well, so that in effect no grinder exists.  The incinerated
ash grinder was originally a 1/16 inch screen.  This fine  screen had a
tendency to clog even though the ash particles  were much smaller. At
present the screen is f inch and serves as a trap metal safety device with
the rotor operating to assure through-put. No measurable effect has been
detected in ash particle  size.

Sludge nuclear density meters are of considerable  value to this process to
have an immediate sludge density reading to guide trie operator in setting
ash feed for proper ash/sludge ratio.  The density meters originally in-
stalled did not hold calibration.  This seems to be the major complaint of
most sludge nuclear density meters.  New meters are on order, and some
calibration techniques developed in the field by plant personnel will be
used  which have proven  more dependable than those practiced by the  meter
manufacturers.
                                148

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 Ash feeders used at Cedar Rapids are of the gravimetric type.  These
 were installed for accurate proportioning of ash.  Experience in handling
 both fly ash and organic ash would suggest that this refinement is not
 necessary and that volumetric feeders would be satisfactory and probably
 more trouble free.  Feeder problems are associated with the  unpredictable
 bulking and compaction characteristics in  storage.  It is difficult to cover
 both extreme properties of ash in the performance  range of the  storage
 bin activators and ash feeders.

 Ferric chloride feeders were very inconsistent throughout the program,
 however, this was primarily due to improper sizing and not really a fault
 of the equipment.  The feeders were oversized and operated on the low
 range where repeatability was poor.

 Materials handling equipment, particularly filter cake conveyors were
 the cause of many problems.  The primary  cause of nearly all conveyor
 problems was  the fact that no experience had existed in discharging
 filter cake to bunker storage.  The bulk density  of broken filter cake was
 greatly underestimated at about 48 pounds  per cubic foot.  This is
reasonable for filter cake as it is  sheared and bulked.  However,  in
bunker storage the cake recompacted dropping into the bunker so that the
bulk density was about 83 pounds per cubic foot at the  drag conveyor
discharge.

Filter feed pumps are hydraulic driven ram  pumps having a  variable cap-
acity, variable head characteristic. Pumps having these characteristics
are desirable as the filter cycle pressure develops and  the input dimin-
ishes, however, a more suitable primary pump should be provided to
meet the early demands of the filter, particularly on a filter installation
as large as Cedar Rapids.  Considerable experimentation was carried
out by the plant operating personnel wherein filter performance was
greatly improved by placing all four filter feed pumps on one filter rather
than the normal two units.  Prolonged slow feed rates to the  pressure
filter to form satisfactory cake development is uncertain from our observa-
tions with sewage  sludges and ash precoat. Where a precoat system is
not used, it may be desirable  and  even necessary  to  slowly develop  a
cake on the cloth to provide a protective zone so that the solids are  not
driven into the cloth upon increasing pressure.   With a  precoat system
the protective zone is already established  and through-put should be as
rapid as possible.   Increased  filter supply  could also be accomplished
by increased equalization tank capacity, but in  that system some prob-
lems of maintaining air volume, level controls,  solids suspension, and
mechanical controls offset the advantages  of additional direct pumpage
to the filter..

                               149

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The mix tanks have mechanically failed in many ways from broken shafts,
bearing failures, gear misalignment, stuffing boxes, blades falling off
and others.  Many of these failures have been attributed to poor shop
workmanship and probably could happen to any manufactured product at
some time.  Modifications and repairs  have  been made and are contin-
uing and hopefully will be eliminated with better design and fabrication.

The pressure filter has been in service in this country for fifty plus years
in industrial processes.  It has only been in recent years that its applica-
tion to sewage sludge dewatering has been seriously considered in Europe
and more recently in the United States.

The pressure filter is nearly the  same age as mechanical type waste water
treatment yet it has taken almost as many years to develop a  serious in-
terest to its application to sewage sludge dewatering.

Equipment and process improve with experience of people in specific
applications, therefore, it is reasonable to have confidence that the
pressure filter application to sewage sludge dewatering will be  improved
with time and that specific experience.

At Cedar Rapids the pressure filter process has  performed well,  with many
of its problems solved by the human beings who created those problems
in the first place.
                               150

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                             SECTION XI

                         ACKNOWLEDGMENTS

The participation and contributions of Mr. George W. Milligan, P.E.,
Water Pollution Control Plant Engineer, and Mr. Sabry Kamhawy,
Chief Chemist, are particularly noted and were vital to the preparation
of this report.

A special thank you is extended to the entire staff of plant operators,
chemists, and  maintenance personnel who contributed and cooperated
in the success of this program.

The Honorable  Mayor and City Council have been active in this program
from the preliminary planning through the final report as has the
Director of Public Services.  These dedicated and concerned people
authorized the  program which otherwise would never have been under-
taken.

                  Donald J.  Canney, P.E., Mayor

                         Commissioners
     John D. Oberthien                     Richard L. Phillips
    Stanislavs Reinis                     Harold G. Schaefer
       Richard  H. Jensen, P.E., Director of Public Services

Appreciation is extended to the Iowa Electric Light and Power Company,
Cedar Rapids, for their civic  spirit in providing fly ash at no cost to
the City.  Particular appreciation is extended to Mr. Lou Stolba, Super-
intendent of the 6th Street Power Station for his generous cooperation,
often  at great inconvenience to him and his staff to make the fly ash
deliveries possible.

Mr. Ralph G. Christensen, Project Officer, EPA Region V,  Chicago,
and Mr. James  E. Smith, Jr., Technical Assistant, EPA, AWTRL,
Cincinnati,  during numerous program conferences offered experienced
advice and technical support and for this, a sincere thanks.
                              151

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                            SECTION XII
A/S
BOD 5
CC (cc)
CFM (cfm)
COD
Cu. Ft.
Ft.
Gals/Min/Sq.Ft.
Gals/SF/Hr.
In.
In. W.C.
KWH
Lbs. (or#)
Lbs./Cu. Ft.
Lbs./SF/Hr.
(*
mg/1
ml
mm
MPR
%
PH
Psig
r
r
Rev.
S.S.
Sq. Ft.(S.F.)
T.F.
T.S.
V.S.
Wt.
   GLOSSARY OF TERMS

Ash to Sludge Ratio
5 Day Biochemical Oxygen Demand
Cubic Centimeter
Cubic Feet per Minute
Chemical Oxygen Demand
Cubic Feet
Feet
Gallons per Minute per Square Foot
Gallons per Square Foot per Hour
Inches
Inches Water Column
Kilowatt Hour
Pounds
Pounds per Cubic Foot
Pounds per Square Foot per Hour
Micron
Milligrams per Liter
MillilUer
Millimeter
Minutes per Revolution
Percent
Hydrogen Ion Concentration
Pounds per Square Inch Gauge
Correlation Coefficient
Specific Filter Resistance  (10l2Cm-2)
Revolution
Suspended Solids
Square Feet
Trickling Filter
Total Solids
Volatile Solids
Greater than
Less than
Weight
                               153
                    AU.S. GOVERNMENT PRINTING OFFICEH973 546-308/4 1-3

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i
Accession Number
W
2

Subject Field &. Group
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
      I Organization
       City of Cedar Rapids, Iowa
       Title
        Pressure Filtration of Waste Water Sludge With Ash Filter Aid
]Q Authors)
James W. Gerlich, P.E.
Howard R. Green Company
Consulting Engineers
Cedar Rapids, Iowa
16

21

Project Designation
11060
EZX
Afofe
     edar Rapids, Iowa
   22  Citation
           Environmental Protection Agency report number,
           EPA-R2-73-231, June 1973.
       Descriptors (Starred First)

        Sludge Dewatering
        Ash Filter Aid
        Pressure Filtration
       Identifiers (Starred First)
   27
Abstract
       Cedar Rapids, Iowa used pilot plant studies as an effective approach to an eco-
nomic solution to dewatering secondary digested sludge.  After piloting several dewatering
Processes, the pressure filter system was selected and a full scale plant was constructed.Dur-
In9 the course of the dewatering studies it was observed that fly ash was an effective filter aid.
      The full scale plant was designed to utilize sludge ash from incinerated filter cake, as
     as power plant fly ash. The design  capacity is 28 tons  of dry sewage solids for 16 hour
       n at 48 percent dry  solids cake.
      Performance data from the full  scale plant was evaluated over a period of approximately
 lne months.  Both  fly ash and sludge ash were evaluated as a filter aid, with and without chemr
  ^s- Economic evaluations were made of operation and equipment.
      Some of the conclusions are: (1) Pressure filtration of waste water sludges  is an effective
    economical process. (2) Ash filter aid increases dewatering production and decreases chemi-
Cal costs.  Sewage sludge ash from an incinerator can  be recycled to process; power plant fly
ash is even more effective sludge conditioner. (3) A detailed pilot plant program is of great
value in design of a full scale plant. (4) Some chemicals in combination with ash filter aid
urther improve dewatering efficiencies.
  Abstractor
         I. W. Gerlich, P.E,
                               Institution
                                Howard R. Green Company
        (F*EV
                       SEND. W.TH COPV Or DOCUMENT. TO, WATER. RESOURCED NT |F ^NFC-MAT 'ON CENTER

                                              : WASHINGTON. D. C. 30240

                                                                       * r.POI I 970-360-831

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