United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-146  Oct. 1984
Project Summary
Sewage  Sludge  Incinerator
Fuel  Reduction,  Hartford,
Connecticut

Albert J. Verdouw, Eugene W. Waltz, and Paul F. Gilbert
  A field demonstration  project was
conducted to reduce fuel consumption
in municipal sludge  incinerators  by
using a  more fuel-efficient operating
mode.  The  Hartford  Metropolitan
District Commission in  Hartford,
Connecticut, demonstrated  a  new
operating mode at its Hartford Water
Pollution  Control  Plant  using three
conventional multiple-hearth sludge
incinerators.
  The more fuel-efficient incinerator
operating mode was developed from an
extensive  program of  combustion
engineering measurement, testing, and
operational analysis.  Incinerator
operators were then given on-the-job
training  in the  new operating mode
during  a 14-day demonstration  test
period.  After 12 months of routine
operations with  the new operating
mode, a fuel reduction of 61% was
achieved, representing an annual fuel
cost savings  of approximately
$250,000.
  The  Hartford  Plant had  just
completed a conversion of its sludge
dewatering  equipment from  vacuum
filters to continuous-belt filter presses
when  this  project  was  initiated.
Average specific fuel consumption had
already been reduced by more than 65%.
Together, the annual fuel cost savings
from dewatering and improved
incinerator  operations amounted to
$1.3 million.
  Comparisons   of fuel  reductions
achieved in four major cities by means
of new incinerator operating modes are
also reported.
  This Project Summary was developed
by  EPA's  Municipal  Environmental
Research Laboratory. Cincinnati, OH,
to  announce key  findings  of the
research project  that is  fully
documented in a separate report of the
same title fsee Project Report ordering
information at back).
Introduction
  The  disposal  of  municipal  sewage
sludge  by  incineration  most  often
requires the use of large amounts of
auxiliary fuel. Increasing energy costs in
recent years have made incinerator fuel
consumption a major problem for many
municipal  operations. To reduce these
costs, the  Hartford Metropolitan District
Commission instituted  two  major
operational changes at its Hartford Water
Pollution   Control Plant  to  drastically
reduce incinerator fuel consumption. The
first step was a conversion to continuous-
belt filter presses; and the second was the
adoption  of a  more fuel-efficient
operating mode.
  The Hartford Plant was one of the first
in the United  States to  convert to
continuous-belt filter presses for sludge
cake  dewatering.This  conversion  took
place between 1979 and 1982 with the
installation of four belt filter presses. The
conversion to belt filter presses resulted
in dramatic fuel savings  of 65% and
increased  solids  production.
  In  late  1981, the  Hartford  plant
engineering  staff and  incinerator
operators  adopted a new, more fuel-
efficient   operating   mode  with the
technical assistance of the Indianapolis
Center for Advanced Research. The new
incinerator operating  mode has been
exclusively used in routine  operations

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since January 1982. The new operating
mode resulted in an additional reduction
in the specific fuel consumption of 51 %.
  The combined result of these two major
operational   changes  was  an   83%
reduction   in  the  incinerator  fuel
consumption. These results are reviewed
here  as  an example of how improved
dewatering  equipment and  incinerator
operating technology offer new options to
those  municipal  operations that  are
currently bound to sludge disposal by
incineration.

Past Operations
  The  Hartford Water Pollution Control
Plant performs primary and secondary
wastewater treatment for more than 45
million gallons of wastewater per day and
generates in excess of 200 wet tons of
filtered sludge cake per day. The sludge-
handling facility was originally designed
in 1968 with four dissolved-air flotation
thickeners,   five   drum-type  vacuum
filters, and three multiple-hearth incine-
rators.
  In 1978, before conversion to belt filter
presses,  the vacuum filters averaged
13.8% cake solids.  Production required
continuous operation of three of the five
vacuum filters, with two  of the  three
incinerators operating around the clock.
The  plant  operation  experienced the
typical  production  and   maintenance
problems associated with  handling an
extremely wet sludge cake. In addition,
the  Hartford plant started  to  receive
sludge from satellite  plants  in   East
Hartford and Rocky Hill.
  The  incinerator operations were also
plagued  with the  common  operating
problems of handling the very wet cake
and  were consuming large amounts of
fuel. The Hartford incinerators (Figure 1)
are  equipped  for  either  gas  or oil
operation and have a maximum capacity
of 12.5 wet tons per hour. No common
operating procedure was being used by
the incinerator operators. Each operator
had certain practices and techniques for
maintaining  temperatures  on  various
hearth  levels  and  for   managing
incinerator   airflow.  The  operators'
preoccupation with just burning the very
wet  sludge  cake  resulted  in   many
inefficient operating practices such as
high  exhaust   gas  temperatures,
combustion  occurring too high  m the
furnace,  too much draft and  auxiliary
airflow,  misuse of heated-rabble-arm
cooling air^nd less than opliingm burner
use  patterns. The degree  of  remote
instrumentation  and the controls also
handicapped operator performance.
                                       Cooling Air Exhaust
                                                        fieturn Damper
Sludge Fet
Gas/Oil
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21 Damper/ Hearth
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 Figure 1.    Hartford incinerator system schematic.
  In view of these operating problems
and the ever-increasing cost of fuel, the
Hartford Metropolitan District Commis-
sion initiated a major program to find new
methods of plant  operations and  pro-
cesses to regain control of operating costs.
Conversions to the belt filter press and
development of the new inci nerator oper-
ating  mode  were  two  of  the major
projects that were  undertaken.
  To provide an accurate baseline for
comparing the fuel reduction achieved by
converting to belt  filter presses and in
improving  the  incinerator operating
mode, a statistical  analysis was made of
key operating performance data for past
operations during  each of the  years in
which changes were made. In  addition,
the correlation of specific fuel consump-
tion (measured in  gallons of oil per dry
ton) with  the absolute  sludge  cake
moisture to sludge  volatile solids ratio by
weight (M/V) was computed to provide a
more comprehensive measure of change
for comparison.
  The average specific fuel consumption
for the  incinerator  operations in 1978
was  125 gallons of oil per  dry ton. The
sludge cake solids averaged 13.8%, and
the volatiles averaged 77.1 %. The sludge
cake M/V ratio, which is directly  related
to and principally determines the specific
fuel consumption demand, averaged 8.6,
which is high.

Belt Filter Press Conversion
Testing and Operational
Experience
  Hartford began pilot testing belt filter
presses in the spring of 1978. Test results
showed that significantly  drier  sludge
cake was produced  at higher production
rates compared with the performance of

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'the  existing vacuum filters.  The plant
 staff  then  conducted  side-by-side
 performance tests of the most qualified
 machines to select  the first press for
 procurement and  installation. The first
 press  was  installed   in  1979  with
 a carefully monitored start-up and shake-
 down  period.  Despite  numerous
 mechanical   problems  and  excessive
 downtime  (25%),  the  belt filter press
 quickly proved to be  so economically
 beneficial that the approval for acquiring
 the  second press was granted only  4
 months after the first one was installed.
 The payback period for the first press was
 only 6 weeks since the savings were $1
 million per year.
  When the second  press was selected,
 performance tests were again conducted
 to   evaluate  overall  performance,
 mechanical design,  and maintenance
 features of the presses to  incorporate
 these requirements in bid specifications.
 The second press, which was supplied by
 a different manufacturer from the first,
 was installed in December 1979, just 8
 months after the first one
  During the first half of 1980, the Hart-
 ford plant operated with two belt presses
 and one vacuum  filter.  The  mechanical
 reliability sought  with the second press
 was realized along with continued fuel
 savings and inprovements in sludge cake
 production.  With   this  continued
 operational  success,  the   District
 purchased a third press from this same
 supplier and installed  it in  December
 1980. During 1981, the plant was able to
 operate exclusively with belt presses. A
 fourth press was placed into operation in
 June  1982  to increase  capacity and
 operational  flexibility  by pairing two
 presses to each operating incinerator.
  Since the Hartford plant was one of the
 first to try the belt filter presses, the early
 operational  experiences were problem-
 atic, as  expected.  The initial press
 operation suffered from numerous me-
 chanical  problems with  the  bearings,
 spray water pump, filter screen cleaning,
 filter screen tracking, and filter screen
 seam closures. With  assistance from the
 manufacturer,  the  first  press  was
 retrofitted and upgraded for more reliable
 operation. The  second, third, and fourth
 presses  were from a different supplier.
 They have had few mechanical problems
 and have been satisfactory in operation.
  As more  operational  experience was
 gained,  improvements  were  made in
 several  key operating  conditions.  The
 filter  screen  seam closure  wearing
 problem was reduced  by using higher-
 molecular-weight plastic scraper blades,
which resulted in an increase in  filter
screen operating life from an average of
500 hours to an average of 1500 hours.
Proper polymer conditioning of the sludge
cake  on  all  the  presses  has been  a
problem.  A  two-component  liquid
polymer mix was developed from experi-
ments with  the  polymer  supplier  to
reduce the dosage  requirements to the
same  level  as  that required for the
vacuum  filters. Changes in the sludge
conditioning  tank to improve  polymer
sludge mixing have  also helped reduce
dosage requirements and increased the
flexibility to  adjust to  varying  sludge
characteristics. Maintaining a constant
blend in  the  mixing of the  raw primary
and waste-activated sludges from three
plants requires close .operator control.
Sludge blend variations of only 5% to 10%
can cause the press screens to plug and
the sludge to squeeze out the ends of the
rollers, with  a resulting  loss in percent
solids and production. In spite of these
operating  problems associated   with
reducing a new operating technology to
routine  production  line practice,  the
operational   improvements and  cost
savings  achieved with  the  belt  filter
presses in Hartford have been  dramatic.

Fuel Reduction Results
  Figure 2 illustrates the  reduction  of
specific fuel consumption for the Hartford
incinerator over the period 1978 through
1981  at  the average sludge cake M/V
ratio recorded for each year The same
information appears in Table 1 along with
percent solids  and incineration rates.
  The  savings resulting  from the belt
filter presses  is reflected in the sharp
reduction in the sludge cake M/V ratio,
particularly in  1980, when the major fuel
reduction was  achieved.  The  net
reduction of  an average of nearly 82
gallons of oil per dry ton would translate
into  a  savings of  more  than  848,000
gallons  of oil  at the  1982 dry  ton
production level  of  10,351  dry tons
Coupled with the drastic reduction in fuel
consumption, there also occurred a 57%
gain in the volatile solids incineration rate
per operating  equipment  hour, which is
the  key  production   performance
parameter. Furthermore,  the average
hours  of  incinerator operation per  day
also dropped from  46.5 in 1978 to 35.7
in 1981 — a 23% decrease.
New Incinerator
Operating Mode
  These substantial improvements were
accomplished  after  considerable time
and effort were invested by the Hartford
plant management, staff, and operating
personnel. The experience at Hartford
with the  belt filter presses serves as an
 1
 ^
        125 r
        100
         75
         50
         25
                    125
                                   116
                                                                         65%
                                                  60.5
                                                                  43.5
                    1978
                                   1979           198O
                                          Year
                                                                  1981
Figun 2.   Reduction of specific fuel consumption as a result of conversion to belt filter presses

                                        3

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Table 1.   Key Operating Performance Variables for 1978 through 1981
Variable
                                                                     Percent
                                                   Year               Change
                                       1978    1979    1980    1981    78-81
Percent solids
Sludge cake (M/Vj
Fuel consumption (gals/dry ton)
Incineration rate (volatile tons per
incinerator /hr)
13.8
8.6
125.2
0.7
14.5
8.1
116.1
0.7
18.5
5.8
60.5
1.0
19.5
5.4
43.5
1.1
+41
-37
-65
+57
example of the opportunities that exist in
many plants  throughout the country to
achieve internal improvements with the
adoption   and   modification  of   new
operating  technologies. At this  point,
most  operations  would   have  been
contented with  the lower fuel consump-
tion levels that had been achieved.  But
the dramatic  success with the belt filter
presses encouraged the Hartford Metro-
politan District  Commission and staff to
pursue other  innovative operating
technologies  to improve their operations
further.
  One of  the technologies  adopted was
an   improved  incinerator operating
technique that had been developed by the
City of Indianapolis Department of Public
Works with the Indianapolis Center for
Advanced  Research  (ICFAR)  under  the
sponsorship  of the U.S. Environmental
Protection Agency (EPA). A more fuel-
efficient operating mode was developed
for the Hartford  incinerator operations
from an extensive program of combustion
engineering measurement, testing,  and
operational analysis.  The  development
and testing of the new operating mode
was a cooperative effort by the Hartford
engineering and operating personnel and
combustion engineers  from ICFAR.  The
new operating  mode  was  derived from
refined operating technology developed
by  ICFAR through a  major operational
research   project  conducted  on  the
Indianapolis   incinerators.   On-the-job
instruction and training in the use of the
new operating mode was also performed
to   demonstrate  the  potential   fuel
reduction   and  upgrade  operator
performance.


Operational Testing
and Analysis
  An operational analysis  was made of
the  Hartford  incinerator  operations; it
included airflow measurements, exhaust
gas analysis,  assessments of key instru-
mentation and controls,  existing  oper-
ator-specific practices, load-rate manage-
ment, incineration and dewatering modes.
airflow management, burner use profiles,
combustion  zone location  and control
techniques, hearth temperature profiles,
etc. A kinetic incinerator analytical model
was also used to determine the optimum
load rate and plant operating mode that
would result in  the least possible fuel
consumption.
  Preliminary analysis of the  operator-
specific operating modes found several
common practices that were contributing
to excessive fuel consumption such as:
(1) combustion occurring too high  in the
incinerator,  (2)  high  exhaust gas
temperatures, (3) high draft settings and
too much  auxiliary air,  (4) misuse of
heated-rabble-arm cooling return air, (5)
improper burner use  profiles, and (6)
improper techniques for controlling the
combustion zone location. Also contribu-
ting to the high fuel consumption were
other problem areas associated with the
lack  of  remote  operator  controls for
airflow dampers and burners.
  Results of the airflow measurement
were used to help correct problems found
in the airflow management approaches
being used and to  assess the relative
impacts  of various  previously proven
techniques on  the  Hartford incinerator
set-up. A preliminary  analysis indicated
that through optimum airflow manage-
ment alone, fuel consumption could be
reduced  70% with a  sludge cake M/V
ratio  of  5.0 and an incinerator loading
rate of 6 wet tons per hour. Furthermore,
the   kinetic  rate analysis for  these
conditions  predicted that the potential
fuel  consumption  for  the  Hartford
operation with such a dry cake was zero1
This  analytical   result  agreed  with the
empirically based preliminary estimate
drawn   from   airflow  management
techniques, since an additional 30% fuel
reduction could be reasonably expected
from  improved combustion zone location
control,  optimum  burner use profiles,
improved load rate management, and the
synergistic  effect of  these  operating
mode techniques on fuel consumption.
Based on  these results  and previous
ICFAR experience from similar programs
in Indianapolis, Buffalo, and Nashville,
periods of autogenous combustion were
expected with the new operating mode.
Autogenous combustion  was achieved
several times during the operational trial
and demonstration test for as long as 8
hours. During the period when the new
operating mode was routinely used, there
were many days that no fuel was used for
over a 24-hour period.
  Based on the operational trial tests and
analyses,  a new  operating  mode with
specific  instructions  and   operating
settings  was  developed.  The  new
operating mode was then demonstrated
in full plant operation for a 2-week per-
formance   demonstration.   On-the-job
operator training in the use  of the new
mode was also accomplished at the same
time. After the successful performance
test,  the operating mode was further
refined for routine operational use.
  The  new   operating  mode  was
characterized  by the following  general
operating guidelines (refer to Figure  1):

  • Make maximum use of the heated-
    rabble-arm cooling air return.

  • Use  the  lowest possible  draft to
    minimize  air leakage.

  • Maintain combustion on hearth Nos.
    7 or 8 to maximize the drying area.

  • Replace the cold auxiliary air supply
    with heated cooling air return.

  • Minimize excess air.

  • Use   lower   hearth  burners   to
    maximize drying temperature.

  • Eliminate  airflow to top  hearth
    burners.

  • Control  combustion   location with
    burner use profile.

  • Slow center shaft speed to improve
    sludge drying.

  • Discontinue  use of  hearth  No.  5
    burners.

  The  incinerator  operators  received
specific operating instructions that con-
stituted the new operating mode. These
instructions  included  procedures  for
sludge load  management,  operational
control  of  the   incinerator,  general
operating settings, specific settings for
normal  operations,  combustion zone
location  control,  standby and  start-up

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operations,  and techniques to control
sludge cake burn-outs. The trial tests
indicated  that  the  most  effective
incineration and dewatering mode was
two belt filter  presses  paired to each
operating  incinerator.  The   optimum
incinerator loading rate was 6  wet tons
per  hour  per  incinerator,  based on
analysis  and trial tests of  load  rates
between 4.5 and 7 wet tons. The 6-ton-
per-hour load rate was  the lowest rate
possible to keep up with the overall plant
loading  rate and still  minimize  fuel
consumption,  considering  average
sludge  cake M/V ratio  of 4.5.  The
improved operating mode also enabled a
further  reduction  in the  M/V  ratio,
because  the new  mode  allowed the
presses to be slowed down, resulting in a
small increase in cake solids.
 Fuel Reduction Results
  The  new incinerator operating  mode
 was placed into routine operational use
 directly  following  the  2-week
 performance test conducted in January
 1982. The new operating mode has now
 been in routine use by Hartford for more
 than a year. The 1982 operational data for
 the  Hartford  plant were  analyzed  to
 measure and compare the fuel reduction
 achieved.   Figure  3   illustrates  the
 computed least squares correlation of the
 average specific fuel consumption versus
 the sludge cake M/V ratio for both the
 baseline period (1978 to 1981) and for
 1982,  when only the  new mode was
 used.
  The  improved  thermal  operating
 efficiency achieved with the new opera-
 ting mode is reflected in the change of the
 slope of the relationship. This result was
 quite similar to  those that occurred  in
 Indianapolis, Nashville, and Buffalo when
 these operations implemented the ICFAR
 operating techniques. Figure 4 illustrates
 the average specific fuel consumption for
 the  Hartford  operations  from   1978
 through 1982. The average specific fuel
 consumption for 1982 was 21.1 gallons
 per dry ton compared with 43.5 gallons
 for 1981  —a 51.1% reduction. With this
 improvement,  the total fuel reduction
 achieved by Hartford between 1978 and
 1982 amount to  104 gallons per dry ton,
or 83%. At the 1982 production level, this
figure  represented  a  savings of  1.08
 million gallons of No. 2 fuel oil compared
with  1978.
 In addition to reducing direct fuel  con-
sumption,  the  new  operating  mode
provided  increased  operating flexibility
with  the  equipment, since  the
     125
     100

  .
to
      75
      50
     25
        4            5            6             78            9

                         Sludge Cake Moisture to Volatile Ratio flb/fb)

 Figure 3.    Specific fuel consumption versus sludge cake M/ V ratio before and after the change
            in incinerator operating mode.
  .§
  t
  .JjD
        125
        100 -
         75 -
         50 -
         25 -
                   125
-


116


60.5

I
83
43.5
21.1 i

1978 1979 1980 1981 1982
Year
Figure 4.   Average specific fuel consumption for 1978 to 1982.
incinerators could  now  be efficiently
operated at loading rates of 50 to 60
percent of capacity. Such  operation was
not possible  before  without  paying a
tremendous  penalty  in  excess  fuel
consumption.  For  example, compared
with  past operational  averages,  the
results of the 2-week demonstration test

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recorded a 76% fuel  reduction for the
entire test period when the specific fuel
consumption was corrected for load rate.
Incinerator  operation  is   also  now
characterized  by  cooler  maximum
operating  temperature,  more  steady-
state  control,  fewer  particulate
emissions, and reduced maintenance on
internal incinerator parts.
  In spite of the significant fuel reduction
achieved with the new operating  mode
during  routine  operational  use, the
Hartford incinerator operation has still
further  fuel  reduction potential to be
realized from the new operating mode.
Several  equipment-related  conditions
are currently limiting further gains. The
most critical of these has to do with the
inadequacy  of the  original  rabble-arm
raking pattern. The latter was originally
set up for a wetter sludge cake (M/V =
8.6) than the current average sludge cake
(M/V = 4.5). The drier cake commands a
far different number  of  rabble teeth,
depending  on  the hearth  being
considered. During the operational tests,
an uneven-sludge-distribution  problem
was  encountered  several   times.  A
distorted burning  pattern resulted, with
sludge  combustion  occurring  on  three
hearths  simultaneously   and  uneven
burning taking place on one side of these
hearths. This condition also creates an
increase  in  incinerator  burn-outs,  in
which dry sludge cake gets to a point of
uncontrollable autogenous combustion
and the incinerator steady state thermal
equilibrium is lost in the  process.  Plans
are under way to  adjust the  rabbling
pattern for the drier  sludge cake. To a
smaller extent, some improvement in the
instrumentation  and  remote operating
controls would  also  permit  improved
operator performance.


Cost Savings
  The nominal cost savings  from  redu-
cing incinerator fuel consumption on an
annual  basis was estimated from the
change in the specific fuel consumption
from 125 to 21 gallons of oil per dry ton.
Based on the 1982 production of 10,351
dry tons, the savings would be more than
$ 1.08 million per year using an estimated
cost of $1.00 per gallon for No. 2 fuel oil.
  In  addition to  the incinerator fuel
savings,  other energy  savings  were
realized  from  the   belt filter  press
conversion. Since the  plant started up in
1972, the activated  sludge mixed  liquor
suspended solids (MLSS)  level to a more
desirable  2,000-mg/L  range.  The
resulting  decrease   in  the dissolved
oxygen  demand reduced the  daily air
usage to about 55 million ft3 per day. This
reduction in turn reduced the electrical
energy requirements of a  3,000-hp air
compressor by 20%, which amounted to a
$200,000-per-year savings on electricity
costs.  Also,  each  vacuum filter  had
a 71.5-hp requirement, as opposed to 22
hp for each belt press.  This reduction in
electrical use resulted in an estimated
savings of $25,000 per year. Together,
these additional savings totaled  more
than $231,000  per year.
  The  total  operating  costs  saved  by
converting to belt filter presses and the
new incinerator operating mode is  esti-
mated to be more than $1,3 million per
year.

Conclusions

  1. Fuel  consumption  in  multiple-
    hearth  incinerators   can  be
    significantly reduced by improving
    dewatering  equipment   and  by
    optimizing existing  incinerator
    operating  modes  with more fuel-
    efficient operating techniques.

  2. A conversion from vacuum filters to
    continuous belt filter  presses
    resulted in a 65% fuel  reduction.

  3. The use of a new  incinerator oper-
    ating mode in routine operations
    achieved  an  additional 51%  fuel
     reduction  and  a  reduction   in
     equipment maintenance problems.

  4. Reduction  in  fuel  use  from
     dewatering and incinerator opera-
     ting  improvements  resulted   in
     annual cost savings of approximate-
     ly $1.3 million.


   For more information, contact:

     Paul F. Gilbert, Plant Engineer
     Hartford Metropolitan District
       Commission
     555 Main Street
     Hartford, CT 06101
     (203) 524-5901

     or

     Eugene W. Waltz, Associate
       Director
     Energy Engineering and Re-
       search Division
     Indianapolis Center for
        Advanced  Research
     2442 City-County Building
     Indianapolis,  IN 46204
     (317)236-4546
  The  full  report  was  submitted  in
fulfillment of Contract No. 68-02-3169 by
Battelle Columbus  Laboratories under
the sponsorship of the U.S. Environment-
al Protection Agency.
   Albert J.  Verdouw and Eugene W. Waltz are with the Indianapolis Center for
    Advanced Research. Indianapolis, IN 462O4; and Paul F. Gilbert is with the
    Hartford Metropolitan District Commission, Hartford, CT06101.
   Howard O.  Wall is the EPA Project Officer (see below).
   The complete  report,  entitled "Sewage Sludge  Incinerator Fuel Reduction,
    Hartford,  Connecticut," (Order No.  PB 84-243 O96; Cost: $10.00. 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:
          Municipal Environmental Research Laboratory
          U.S. Environmental Protection Agency
          Cincinnati, OH 45268
                                     - U S GOVERNMENT PRINTING OFFICE, 1984 - 559-016/7838

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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
     BULK RATE
POSTAGE & FEES PAID
        EPA
   PERMIT No. G-35
Official Business
Penalty for Private Use $30O

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