United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-84-084 Sept. 1984
Project  Summary
Development  of  a  New Gravity
Sedimentation  Process for
Dewatering  Flue  Gas  Cleaning
Wastes
A. R. Tarrer
  This report gives results of a project to
develop  and test a novel system for
dewatering flue gas  cleaning  (FGC)
wastes at the pilot plant level.  In this
new system,  the clarification and
thickening functions are conducted in
separate, but interconnected, pieces of
equipment. The new system consists of
a lamella clarifier and a conventional
thickener that is smaller in diameter,
but deeper (than the thickener/clarifier
typically used to dewater FGC wastes),
connected by a recycle stream between
the two units to obtain a high degree of
flexibility  and control of  operating
conditions. Preliminary economic eval-
uation of this system indicates potential
savings of 10 perent of the total capital
costs and 6 percent of  annual operating
costs for the FGC waste management/
disposal  system.
  In pilot testing of this system, a
completely new  concept in thickener
operation, known as "bang-bang"
operation, evolved, in  which the thick-
ener underflow rate is set  as low as
possible without plugging the  under-
flow lines. Periodically, the underflow
rate is increased briefly to remove ad-
ditional solids from the system at the
concentration established by the (pre-
viously set) low underflow rate. This
mode of operation appears to make it
possible  to maintain the  maximum
solids concentration in the underflow.

  This Project Summary was developed
by EPA's Industrial  Environmental
Research Laboratory. Research Triangle
Park, NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title  (see Project Report ordering
information at back).

Introduction
  In the  mid-1970's, EPA's Industrial
Environmental Research Laboratory in
Research Triangle Park, NC, initiated
several  projects to  solve  problems
associated with slowly settling, difficult-
to-dewater flue gas cleaning (FGC) wastes.
These problems were primarily attributed
to small, thin "platelet" crystals of
calcium sulfite (CaS03-1/2H20) in these
wastes.  When present in significant
quantities (more than a few percent), it
was difficult to achieve, greater than
about 30 percent solids  in a gravity
settler, or about 50 percent solids with
vacuum filtration. The approaches used
to solve  these problems  included at-
tempting  to make larger calcium sulfite
crystals, oxidation of the calcium sulfite
to calcium sulfate (CaSO4-2H2O), and
improving dewatering equipment design.
The project reported  here involves
equipment design improvements, and
covers the period May 1, 1976, through
June 15,  1983.
  Phase  1 - A  preliminary evaluation
  determined the direction of the pro-
  gram; i.e.,  limited sedimentation and.
  filtration were studied to disclose
  which offered the better potential for
  improving dewatering efficiencies.
  Phase 2 - A detailed process synthesis
  and development effort was conducted;
  i.e., fundamental concepts in gravity
  sedimentation  and process dynamics

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  were applied to develop a new system
  for dewatering FGC wastes.
  Phase 3  - A feasibility  analysis was
  done to estimate the maximum savings
  possible  with the  use of the  new
  dewatering system. Also,  equipment
  designs, layouts, etc. were made for a
  larger scale pilot evaluation of the new
  dewatering system.
  Phase 4  -  A pilot-scale system was
  constructed, transported, and installed
  at the EPA/TVA pilot FGC facility at the
  Shawnee Steam Plant near Paducah,
  Kentucky. Shakedown studies revealed
  needs for  equipment improvement.
  Equipment  was  modified in  coopera-
  tion  with TVA,  and final pilot-plant
  tests  were  performed. Objectives of
  these tests were to determine if the
  clarification function of the unit could
  be decoupled from  its  thickening
  function, to determine if the unit was
  capable of concentrating unoxidized
  FGC wastes to a  high solids content, to
  identify design limitations, and to make
  operational recommendations.


Phase 1 Results
  In Phase 1, settling data were collected,
showing that  the  FGC  wastes  tested,
containing  a  significant  percentage of
calcium sulfite (CaSO3-1 /2H2O), exhibit a
settling behavior similar to that of weakly
flocculating materials. Assuming that
FGC wastes do form  weak  floes while
settling, the floes appear to  have a low
yield strength, and the gravitational force
(the weight) exerted by overlying layers of
solids in the settling medium compresses
underlying  layers.  This  means that the
operating depth of the blanket of settling
waste  in the thickener  is an important
process parameter: the deeper the blan-
ket of  settling wastes at the bottom of
the thickener, the higher  the solids
concentration  in the underflow;  or, the
longer the residence time of solids in the
compression zone  of the thickener, the
greater the dewatering.  However, in
conventional  FGC  waste  thickeners, a
deep blanket of wastes usually resulted in
a turbid overflow (insufficient  clarifica-
tion). These findings showed promise that
the application of past studies  in our
laboratories could  lead to a  less expen-
sive gravity sedimentation  system for
dewatering FGC wastes.
  Limited filtration studies involved a
0.09 m2 (1 ft2) plate and frame filter press
(see Figure 1). The filtration data collected
did not show much promise that contin-
ued study of the filtration operation itself
would yield much of an improvement over
conventional  filtration. Therefore,  an
Water
                 Inlet Pressure
                 to Filter Press
    Air
                                       Filter Press
                                 Filtrate
                                  Out
                                                Plate
                                                                   Frame
Figure  1.    Filter press apparatus.
alternate strategy was chosen: to improve
the dewatering efficiency of the gravity
sedimentation system,  thereby  increas-
ing the  solids concentration  of the
influent stream to the filters and lowering
the filter dewatering load.

Phase 2 Results
  In Phase  2, the major thrust of the
bench-scale part of this research effort
project was conducted. A new, more effi-
cient  gravity sedimentation  system for
dewatering FGC wastes was conceived,
designed, and tested on a laboratory pilot
scale. The system consisted of two units:
a laminar clarifier (more specifically,  a
tube settler) and a smaller (in diameter)
but deeper,  conventional thickener. The
units were  connected uniquely,
decouple the clarification and the thi
ening functions of the system as much
possible. A recycle stream between t
two units was used to obtain  a grea
degree  of flexibility  and  control
operating conditions.  The importa
operating parameters for the syslet
were investigated by a series of batch ai
continuous settling experiments. Tl
parameters included: the location of tl
level of the solid/liquid interface in tl
clarifier, the  angle of inclination of tl
clarifier, and the flow rates of the fe<
and recycle streams.
  The dewatering efficiency of the ne
system  was  evaluated on  a labors^
pilot scale (see Figure 2). The pilot trnl

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 ener was 15.24 cm (6 in.) in diameter and
 about 4.6 m (15 ft) long. Tube settlers
 (clarifiers) were used; these were 4.445
 cm (1.75 in.) in inside diameter, 1.98 m
 (6.5 ft) long, and inclined at an angle of
 30°. The pilot tests showed the new sys-
 tem  to  be effective in  obtaining  high
 underflow concentrations (30 to 60
 percent)  and a clear overflow  at  high
 throughput rates  while requiring a
 smaller settling than typical conventional
 thickeners used in this application.
            Phase 3 Results
              In  Phase  3,  efforts were directed
            toward an eventual  scale-up of the
            system. Sizing studies, based on a flux
            plot  developed to help  estimate the
            settling area  required for a desired level
            of final concentration, showed that the
            new  dewatering system would require
            only about 59 percent of the area required
            by conventional thickeners to achievethe
            same amount of separation and concen-
            tration. Alternately, for about the same
                          settling area, the degree of concentration
                          achieved is about 60 percent greater.
                            Preliminary economic studies were
                          also performed by TVA at the request of
                          EPA; these studies showed reductions in
                          capital costs of  about 10 percent, and
                          reductions in annual operating costs of
                          about 6 percent, for an entire FGC waste
                          management/disposal system (not just
                          the dewatering portion of the system).
                            To evaluate the feasibility of the system
                          so that it could eventually be used on  a
     Feed
     Feed Tank
           Underflow
            receiving
              tank
Underflow
  pump
Sample

 point Recycle Pump


    Splitter
                                       Column Settler
                                                            Mixer
                                                                               Clarifier
                                                          Thickener
Figure 2.   Laboratory pilot scale clarifier/ thickener system.

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commercial scale,  equipment design,
layout, etc. were made for a large pilot
scale version  evaluation,  and the pilot
dewatering  system was  fabricated. In
addition, studies were performed on the
effects of shear on the settling behavior of
the sludge. It was hypothesized and ex-
perimentally established  that mixing of
the FGC wastes causes a more rapid set-
tling and  a  greater final  concentration.
However, it was also established that vig-
orous mixing  could fracture the solid
waste crystals,  resulting  in  many fine
particles  which would  settle slowly.
Based on these results,  a positive dis-
placement pump was selected for the re-
cycle stream  from  the clarifier to the
thickener.

Phase 4 Results
  In Phase 4, the pilot dewatering system
was fabricated, shipped to TVA's Shawnee
Steam  Plant  near  Paducah, Kentucky,
and installed at  the EPA/TVA pilot FGC
facility  for shakedown testing and final
pilot plant evaluation testing. The FGC
waste introduced to the system contained
a significant amount of calcium sulfite.
  After  the shakedown  test, several
modifications were made to the pilot unit.
The main modification was adding more
settling plates  in  the  clarifier.  Other
modifications  had to do with operational
considerations such as safety and ease of
operation. Figure 3  is a schematic of the
system.
   The initial goal of the pilot tests  was to
establish a baseline for comparison. At
baseline  coditions, the underflow con-
centration was  about 45  percent solids;
the underflow rate was set at about 10.6
to  11.4  l/min  (2.8  to 3.0 gpm).  The
average feed  rate was about 49.2 to 53
l/min  (13  to 14 gpm),  with  a  solids
concentration of about 15 percent solids.
   Prior to start-up, the  thickener  was
filled with solids  from the  existing
dewatering unit; the clarifier  was  not
prefilled with solids. As a result,  during
the initial  period  of operation,  solids
 accumulated  in the clarifier.
   As the clarifier began to fill with solids,
 it  became necessary to increase  the
 thickener underflow  rate periodically to
 maintain a clear clarifier overflow. It was
 discovered that the feed rate  to  the
 system was being periodically increased
 and corresponding increase in thickener
 underflow rate was necessary to keep the
 velocity in the clarifier below about 0.037
 m/min (0.12ft/min). By adding a splitter
 in the feedstream, the feed rate was held
 fairly constant at about 49.2 to 53 l/min
 (13 to 14 gpm),  and   the thickener
 underflow rate could then be set as low as
possible without plugging the underflow
lines.
  Each time the thickener underflow rate
was decreased, the underflow concentra-
tion increased  significantly. When the
thickener underflow rate was decreased
from the baseline value of —10.6 l/min
(—2.8 gpm) to less than 3.8 l/min (1 gpm),
the underflow concentration increased to
as high as 56 percent solids.
  No significant changes  in the lamella
clarifier concentration was observed when
its  underflow rate was decreased from
—76 l/min (—20 gpm) to  as low  as 5.7
l/min (1.5  gpm). However, a significant
response in separation efficiency (percent-
age of water in feedstream that  left in
clear overflow) was observed; it increased
significantly each  time the clarifier
underflow rate was decreased.
  While operating at a very low thickener
underflow rate (—5.3 l/min or —1.4 gpm)
and a high underflow solids content (—53
percent), the underflow rate was increased
significantly  for about 2  hours. During
this time, about  1.2 m (4 ft) of concentra-
ted sludge blanket was dumped. The
underflow  rate  was  then reset   at its
original low setting. This  period of rapid
   Slurry
  Discharge
dumping, followed by  resetting  the
underflow rate to its original low level,
had little effect on the underflow solids
content, because of the relatively constant
solids concentration in the large compres-
sion zone in the thickener.
  On the basis of this observation and the
observed strong inverse dependence of
underflow concentration on steady-state
(continuous) underflow rate, the concept
of "bang-bang" thickener operation  was
proposed, in which the thickener under-
flow rate is set as low as possible without
the underflow lines plugging. Periodically,
the underflow rate is stepped up briefly to
maintain an overall solids flowrate in the
thickener. Using this mode of operation it
should be possible to maintain a maxi-
mum underflow concentration.
  The dewatering system was observed
to be operationally versatile. Any possi-
ble plugging in the  clarifier due to its
overdesign, for example, should be easily
avoided by simply increasing the clarifier
underflow  rate. This  could be done
without  having  to  lower  the  overall
system underflow concentration because
further concentration would occur in the
thickener.
                                       Flow
                                       Meter

Stream No.
Description
Rate, Ib/hr
Rate, gpm
% Solids
Spec. Gravity
1
Slurry
Discharge
2186
4.00
15.0
1.093
2
Thickener
Overflow
3923
7.18
14.9
1.092
3
Clarifier
Underflow
2332
4.00
25.0
1.165
4
Thickener
Underflow
595
0.82
55.0
1.451
5
Clarifier
Overflow
1591
2.18
—
1.000
 Figure 3.   Large pilot dewatering unit.

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Conclusions
  The  FGC waste dewatering  system
developed in this work in effect decouples
the clarification function from the
thickening function. Using this system, it
is possible to concentrate FGC wastes
containing significant quantities (greater
than a few percent) of calcium sulfite to a
high solids concentration (—56 percent)
without the addition of additives or vacuum
filters and with reasonably sized equip-
ment.  Because the clarification and
thickening functions are effectively
separated, but at the same time intercon-
nected by "internal recycle," the system
offers  flexibility for wide  variations in
waste  dewatering behavior created by
changes in FGC system operating condi-
tions. The system operates most effec-
tively in the "bang-bang" mode, with the
thickener underflow set, for most of the
"bang-bang"  cycle, as low as possible
(without  plugging the underflow lines).
On reaching the maximum  feasible
concentration,  the underflow  rate is
increased significantly for a brief period,
then the cycle is  repeated. Preliminary
economic  evaluation of this  system
indicates potential savings of 10 percent
of the total capital costs and 6 percent of
annual operating costs for the FGC waste
management/disposal system.
A. R.  Tarrer is with Auburn University, Department of Chemical Engineering,
  Auburn, AL 36849.
Julian W. Jones is the EPA Project Officer (see below).
The complete report, entitled "Development of a New Gravity Sedimentation
  Process for Dewatering Flue Gas Cleaning Wastes, "(QrderNo. PB 84-231448;
  Cost: $20.25, subject to change) will be available only from:
        National Technical Information Service
        5285 Port Royal Road
        Springfield, VA 22161
        Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
        Industrial Environmental Research Laboratory
        U.S. Environmental Protection Agency
        Research Triangle Park, NC27711
                                                                        . S. GOVERNMENT PRINTING OFFICE: 1984/759-102/10689

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