United States
            Environmental Protection
            Agency
            Office of Municipal
            Pollition Control (WH-546)
            Washington DC 20460
September 1985
430/9-85-001
vvEPA
Heat Treatment/Low Pressure
Oxidation Systems:
            Design and Operational
            Considerations

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HEAT TREATMENT/LOW PRESSURE OXIDATION SYSTEMS:
     DESIGN AND OPERATIONAL  CONSIDERATIONS
                      by
             Metcalf & Eddy, Inc.
       Wakefield, Massachusetts   01880
               Project Officer
             Francis L. Evans III
         Wastewater Research Division
    Water Engineering  Research Laboratory
           Cincinnati, Ohio   45268
    OFFICE OF MUNICIPAL POLLUTION CONTROL
               OFFICE OF WATER
     U.S. ENVIRONMENTAL PROTECTION AGENCY
           WASHINGTON, D.C.   20460

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     This  document  is condensed from an EPA  research report
entitled "Improving Design and Operation of Heat Treatment/Low
Pressure Oxidation  Systems",  which will be available in late
1985. That report  has  been  subjected  to  the United States
Environmental  Protection Agency's peer review.  This document
has undergone  an EPA administrative review and has been found
to be  consistent  with  the  EPA  research  report referenced
above.  The information in this  document  is  made  available
for the  use  of  the  technical  community.  The  information
contained  herein  does not constitute EPA policy, guidance or
directive.  Design  engineers, municipal officials, and others
are cautioned  to  exercise  care  in  applying  this  general
information  to  particular circumstances of individual waste-
water  treatment  facilities.  EPA  assumes  no responsibility
for  use  of  this  information  in  a  particular  situation.
Mention of trade names or commercial products does not consti-
tute endorsement or recommendation for use.
                             11

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                             FOREWORD
       The construction grants  program of the U.S.  Environmental
Protection Agency (EPA) has provided financial assistance to many
municipalities to construct new and expanded wastewater treatment
facilities.  As  more  municipal  wastewaters are treated to higher
levels, there  has been an  accompanying increase in  the amount of
sewage  sludge   that  must  be  treated  and  disposed.   This  has
stimulated the search  for  improved technologies to  treat  these
sludges in a cost-effective way.

       Thermal  conditioning  of  sludge  to  improve  dewaterability
was  first  used  in  municipal wastewater  treatment  facilities in
the  late  1960s.   Over  the years,  however,  thermal conditioning
processes  have  experienced  a   variety  of   serious  design  and
operational  problems  that  have compromised  process  performance
and  raised questions  as to the suitability  for use in municipal
wastewater   treatment   facilities.      EPA's   Water  Engineering
Research Laboratory in Cincinnati, Ohio has undertaken a study of
thermal conditioning processes  to identify the nature and extent
of these problems, to identify possible problem solutions, and to
determine the  applicability of  the process for  use  as  part of a
sludge  treatment  system  in a  municipal  wastewater  treatment
facility.

       This  summary  document is  based  on that  EPA  study and is
intended to provide a basic understanding of thermal conditioning
processes,   as   well   as   concise   information   on   design
considerations,  operational  characteristics,  and  process  and
equipment problems and  possible solutions.   The document will be
useful to design engineers, governmental agency review personnel,
municipal officials,  operators,  and  others  who are  considering
using  thermal  conditioning in  a  sludge treatment  train,  or  who
are  concerned  with  optimizing  the performance  of  an  existing
thermal  conditioning  system.   Whether  for  design  or  operating
decisions,  the  information  in  this  summary  will  supplement
detailed  guidance  available  elsewhere.    As  in all  comparative
analyses, process  applicability  to  a particular wastewater  and
effective integration  into  a total  treatment  system  should be
considered,  along with the associated  costs,  before  any thermal
conditioning  process is selected.
                                 111

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                             CONTENTS
                                                           Page

EPA Review Notice	 ii
Foreword	 iii
Acknowledgements	... vi

    1.  Introduction	 1
            Purpose	 1
            Background	 1
            Process Problems	 3
    2.  Process and Equipment	 4
            Process Description	 4
            Equipment Description	j	 7
            Operational Characteristics	 15
            Process Selection and Application	 16
    3.  Common Problems and Solutions	 20
            Design Problems	 20
            Equipment Problems	 34
            Operations Problems	 37
    4.  Summary of Design and Operational Considerations.. 39
            Improving System Design	 39
            Improving Existing Systems	 40
            Desirable Operating Characteristics	 41
            Improving Plant Operation and Maintenance	 42

References	 44
English to Metric Units Conversion Table	 45

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                         ACKNOWLEDGEMENTS
       This  report  was  prepared  for  the  U.S.  Environmental
Protection Agency  by Metcalf & Eddy,  Inc.,  Wakefield,  Massachu-
setts under Contract No. 68-03-3208.

       Mr.  Francis  L.  Evans  III,  EPA  Project  Officer,  was
responsible for  overall project direction.   Other  EPA  staff who
contributed to this work included:

  Dr.  Harry   E.  Bostian,  Technical   Project   Monitor,   Water
    Engineering Research Laboratory
  Dr. Joseph B. Farrell, Water Engineering Research Laboratory
  Mr. Walter Gilbert, Office of Municipal Pollution Control

       Metcalf   &  Eddy  staff   participating  in  this  project
included:
       Allan F. Goulart, Project Director
       Thomas K. Walsh, Project Manager
       Norman E. Renaud, Operations Specialist
       Richard Lansdown, Operations Specialist
       Elizabeth M. Gowen, Project Engineer


       Other contributers were  Mr.  Peter  Owre and Mr.  Herb Filer
who provided special consultant services to the project.
                                  VI

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                            SECTION 1

                          INTRODUCTION
       In  recent  years  the  quantities  of  sludge  produced  in
wastewater  treatment  plants  have  increased  substantially  as  a
result of improved treatment efficiency.  As  the volume of sludge
production has  increased,  the need  for more efficient  means  of
dewatering  sludges  has  become  a  growing concern.    Wastewater
sludges are  difficult to  dewater,  at  best.    Both the  rate and
extent  of   sludge   dewatering  may  be  enhanced   by   thermal
conditioning of the  sludge before the dewatering process.

PURPOSE

       This design  information summary report presents  data and
best practices  relating  to the  design and operation  of thermal
sludge conditioning  systems.    It  is  based  on  an investigation
which  evaluated  the  causes   for   operations   and  maintenance
problems  experienced  with  thermal  sludge conditioning  systems.
The  data  contained  in  the report were obtained  from technical
literature,   discussions   with  manufacturers,    and   telephone
inquiries  and  site  visits  to  municipal  wastewater  treatment
plants.   This document presents process and equipment  descrip-
tions,   operational   characteristics,   process    selection   and
application  information,   common  problems  and  solutions,  and
design and operational considerations  relative  to heat treatment
and low pressure oxidation of sludge.

       The  purpose  of  this  report   is  to  concisely  summarize
available  thermal  conditioning  design and operational  informa-
tion,  thereby  providing  a  general  understanding  of  the thermal
sludge conditioning process as well  as its proper application in
a  sludge  handling  system.    The  report  is  not  intended  as  a
detailed design guide  or  as a replacement for other design guides
available from  manufacturers  or technical information contained
in  published  literature.    Detailed  discussions of  particular
topics are  contained  in  the original  investigation  document (1)
and in the references  cited within the text.

BACKGROUND

       Thermal sludge conditioning  is  a  continuous  flow process
in which  sludge  is  heated  to temperatures in the range between
350P and 400F in  a  reactor  under pressures of  250  to  400 psig

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for 15  to  40  minutes.   There are two  basic  modifications of the
thermal  conditioning  process  employed in wastewater  treatment.
One  modification,   Heat  Treatment  (HT),  does  not  include  the
addition of air  to the process.  In  the  other modification, Low
Pressure Oxidation (LPO),  air  is  added  to  the process.   Both
thermal  conditioning  processes  produce  a  biologically  stable
sludge with excellent dewatering characteristics.

Heat Treatment

       The  heat  treatment  process  was  developed by  William K.
Porteous  in England  during  the  period  of  1932  to  1953.   The
process  went  through  several  design  modifications during  this
period and  ultimately  was  patented  by the firm Norstel & Temple-
wood Hawksley as a continuous flow process using steam injection.
Norstel &  Templewood Hawksley (N.T.H.) continued to develop this
technology  in the  early 1960's and installed  several  continuous
flow  systems  in  Europe.   The  Envirotech  Corporation,  through
their BSP  Division, acquired  an exclusive license from N.T.H. to
market  the Porteous process  in  the  United  States.   The first
full-scale  Porteous process  heat treatment  system  in  the  U.S.
went into  operation in 1969  at Colorado  Springs,  Colorado.   The
heat treatment product  line was sold  to the  Lurgi Corporation in
1980.

       Approximately 31 wastewater treatment plants in the United
States have heat  treatment facilities.   Of  these, 13  facilities
are  reported  to  be operating,  9  facilities  are reported  not
operating, and the operating  status of the remaining 9 facilities
is unknown.  Of the 13 operating  HT facilities, 8 incinerate the
thermally  conditioned   sludge  after   dewatering  using  either
multiple-hearth or  fluid bed furnace  incineration.   The startup
dates  for   the  operating  HT facilities  are  from 1971  through
1977.  The  sludge  processing  capacity of these facilities ranges
from  25 to  150 gallons per  minute   at  plants  with  capacities
ranging  from   2.1  to  50  million   gallons   per   day  (mgd),
respectively (1).

Low Pressure Oxidation

       The low pressure oxidation system, along with intermediate
and high pressure systems, was developed by Fred J. Zimmerman and
his associates at  Rothschild, Wisconsin.   The business organiza-
tion established to develop  and  sell  commercial applications of
these processes was called  Zimpro, Inc.  The first LPO system was
installed  in  Levittown, Pennsylvania  in 1967.    Prior  to 1967,
thermal oxidation  systems- were limited to intermediate  and high
pressure oxidation installations.

       Seventy  eight   municipal  wastewater   treatment  plants
nationwide are known  to have low pressure  oxidation  facilities.
Of these,  75  facilities are  reported  to  be  operating,  and 3 are

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reported not  operating.   Of the  known operating LPO facilities,
32  facilities incinerate the thermally  conditioned  sludge after
dewatering.    Startup dates  are  from  1969  to  1980.    Sludge
processing capacities  range from 6 to  280  gallons per  minute at
plants with capacities from 1.5 to 200 mgd, respectively (1).

PROCESS PROBLEMS

       Although thermal  conditioning  of  sludge is an established
and proven process,  these  systems  have had problems  in  the areas
of  design  and operation that  have limited successful  and cost-
effective operation.   Design problems have related  primarily to
sizing of thermal  conditioning  and support systems,  materials of
construction,   odor   control,   and   treatment   of   sidestreams
generated  by  use  of  the   process.    Operational problems  have
included  excessive  energy  consumption,  insufficient   operator
training or skills, and high maintenance requirements.

       In some  cases,  these problems  have  been serious  enough to
cause abandonment of the process.  In general, thermal condition-
ing systems have been shut down as a result of high energy costs.
These costs  can be directly related  to improper design  and/or to
improper operation of the thermal conditioning systems.

       The potential for  operational problems,  the ability to
minimize or  avoid  these  problems through proper  design features
or operational controls, and a  careful analysis of operation and
maintenance costs associated with the specialized requirements of
a thermal conditioning system should all be considered before the
process is selected  over other  sludge conditioning alternatives.
Both  the  benefits  and  the potential  problems  attributed  to
thermal conditioning  systems,  as well  as  side benefits  such as
the potential for gas production by anaerobic digestion  of decant
liquors, should all be included in such considerations.

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                            SECTION 2

                      PROCESS AND EQUIPMENT


       The  thermal  conditioning process enhances  the dewatering
characteristics  of   sludge  through  simultaneous application  of
heat and  pressure.   This process  is  one step in a  total sludge
handling  system  in  which sludge is  conditioned,  stabilized,  and
thickened by  thermal conditioning  before dewatering and ultimate
disposal.   How the  process conditions  and  stabilizes sludge and
the  equipment that   comprises  a thermal conditioning  system are
described  in  this   section.    Operational  characteristics  and
guidelines  for  process  selection   and  application  are  also
discussed.

PROCESS DESCRIPTION

Thermal Conditioning

       Wastewater sludge  contains water  and cellular  and inert
solids  which  form   a gel-like  structure.    The  water  portion
consists  of  bound water, which surrounds each  solids particle,
and  water of  hydration,  which  is  inside  the  cellular  solids.
Thermal conditioning improves sludge dewaterability by subjecting
the  sludge  to elevated  temperature and  pressure  in  a confined
reactor vessel  to coagulate solids  and break down  the gel-like
structure of  the sludge.   As  the  temperature and  pressure  are
increased, particle  collisions increase.  These collisions result
in the  breakdown of  the  gel-like  structure, allowing  the bound
water  to separate   from  the  solids  particles.    In  addition,
hydrolysis of protein material in the sludge occurs.   Cells break
down and  water is released,  resulting in coalescence  of solids
particles.    In  its  conditioned  state,  the  sludge  is  readily
dewatered on  most  commonly used  dewatering  devices to 30  to  50
percent solids without the addition of chemicals.

       A portion of  the volatile suspended solids (VSS) in sludge
is  solubilized  as   a  result  of  the  breakdown of  the  sludge
structure.      The    solubilization   of   VSS   increases   its
biodegradability.  Although this solubilization does  not change
the total organic carbon content of the sludge,  it  does result  in
an increase in  the  5-day biochemical oxygen demand  (BOD^).   The
BODg  produced   is   of  primary   concern   in  the   recycle   of
sidestreams,  as  discussed  in  Section  3.   The solubilization  of

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VSS and resultant BOD5 production for HT systems may be estimated
as follows (2):

       VSS  = 0.1 PS + 0.4 WAS

       BOD5 = 0.07 PS +0.3 WAS

       where:  VSS  =  Volatile suspended solids solubilized,
                         dry Ib
               PS   =  Primary sludge, dry Ib
               WAS  =  Waste activated sludge, dry Ib
               BODc =  5-day biochemical oxygen demand produced
                         by VSS solubilization, Ib

       Using  these  rule-of-thumb procedures,  one  would estimate
22 pounds  of  VSS  solubilization   and  16.2 pounds   of   BOD^
production by heat  treatment of 100  pounds  of a  typical mixture
of 60 percent primary and  40 percent  waste  activated sludge.  In
LPO systems,  VSS  solubilization  and  BOD5 production are expected
to be  10  and 5  percent  greater, respectively,  than  the  above
estimates for HT systems.

       Thermally  conditioned sludge   can  be dewatered  on  vacuum
filters,  belt  filter  presses,  recessed plate filter  presses,
centrifuges,  or  sand drying beds.  Ultimate disposal of dewatered
solids  can   be  by   incineration,   landfill,  or   other   land
application methods.

Heat Treatment

       A  schematic  diagram  of  a typical HT system  is  shown in
Figure  1.   In this  continuous  process,  raw sludge  is  ground to
reduce  particle  size to  less than 1/4  inch and  is  then  pumped
through a  heat  exchanger  and  into a reactor.   Normal  discharge
pressure from the sludge feed pump is approximately 250 psig.  In
the heat  exchanger,  the temperature  of  the  sludge  is raised from
ambient to between  300F and 350F.   The heated sludge exits the
heat exchanger and enters  a  reactor feed standpipe where steam is
injected  through  a nozzle and  the sludge  is  turbulently  mixed.
The steam and  sludge  proceed upward through the  standpipe and
enter  the reactor at the top.   The hot  sludge (between 350F and
400F)  is  retained for a  period of   time  in the  reactor and is
subsequently  returned through  the  heat  exchanger  to be cooled to
approximately  120F  (about  60F  greater  than   the  incoming
sludge).   From  the discharge  side  of  the  heat  exchanger,  the
conditioned sludge  flows through a control  valve,  which controls
reactor sludge  level  and  pressure, and into a  decant tank.   The
decant  tank  permits rapid  settling and  compaction of the  sludge
particles and the release  of gas.  The  settled  sludge  is  pumped
to a  dewatering  device.   Process off-gases  can  be  treated by
various odor control methods as discussed in Chapter 3.

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         RAW SLUDGE
STORAGE OR
BLENDING
TANK
   GRINDER
   SLUDGE
   FEED
   PUMP
                                                                     REACTOR
           WATER
           CIRCULATING
           PUMP
SLUDGE -WATER
 SLUDGE HEAT
  EXCHANGER
DECANT
LIQUOR
                                                                        CAKE
   SOURCE: EPA PROCESS DESIGN MANUAL: SLUDGE TREATMENT
          AND DISPOSAL, EPA 625/1-79-011, SEPT. 1979 (MODIFIED)
              FIGURE 1.  HEAT TREATMENT PROCESS FLOW DIAGRAM

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Low Pressure Oxidation

       A  schematic  diagram  of  the  LPO  system  is  shown  in
Figure 2.   Raw sludge  is  first, passed  through a  grinder  where
particles are  reduced  to  less than 1/4 inch.   The  ground sludge
is then  pumped at  approximately 400  psi  through a  heat exchanger
followed by  an LPO reactor.   High pressure air from  the system
air compressor is introduced into  the sludge flow upstream of the
heat  exchanger.    The  air  improves  heat  transfer and  converts
sulfur products in the sludge to sulfate, slightly reducing odors
from off-gases.   The  resulting turbulent flow  of  sludge  and air
proceeds through the heat exchanger where the sludge is preheated
by processed  sludge  returning from the LPO  reactor.   The sludge
and air mixture enters the reactor at a temperature between 300F
and  320F.    Steam  is  injected  directly  into the   reactor  to
increase the  sludge/air mixture temperature  to  between 330F and
350F.   The combined products  rise  slowly  in  the  reactor  and a
slight  heat  of  reaction  or oxidation  occurs  producing  a  small
amount of heat.  From the reactor  midpoint to the reactor  outlet,
the  sludge  temperature increases  approximately 10F  due  to the
heat  of  reaction of  the  sludge,  contributing  to   an  overall
temperature  increase  from  reactor  inlet  to  reactpr outlet  of
approximately 40F.  Detention time or  "cook time"  rn the  reactor
is based on  the  volume of  the reactor,  by the  height  of the
discharge pipe  (standpipe  or downcomer line),  and  is controlled
by the air, steam, and sludge flow rates to the reactor.

       After  leaving   the  LPO  reactor,  the partially  oxidized
product  flows  back  through the heat exchanger  and  releases heat
to the  incoming  sludge/air  mixture.  When  the  partially  oxidized
product reaches the control valve,  the  temperature ranges  between
110F  and  130F.    This  valve  controls  the   pressure  in  the
reactor.   From the valve,  the thermally conditioned  sludge and
exhaust  gases  flow to the  decant  tank where the  sludge  settles
and  exhaust  gases are  released.    The  settled solids are  then
pumped to a  dewatering device prior to  final disposal.   Process
off-gases from the LPO system also can  be treated by various odor
control methods as discussed in Chapter 3.

EQUIPMENT DESCRIPTION

       The  equipment   for   both types  of  thermal  conditioning
processes  is  similar.    Both processes  include a  grinder,  high
pressure pump, heat exchanger, reactor, boiler  system, and decant
tank.   HT/LPO systems  should also have a  blending tank  to mix
sludges prior to thermal conditioning.

       In addition to the above equipment,  the HT system includes
a circulating  water  system  for its  heat exchanger,  and  the LPO
system includes  a  compressed air  system for process  air  supply.
Descriptions  of  this   equipment   and  differences  in  equipment
between  the  HT and LPO systems are presented  in  this  section.

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              RAW SLUDGE
      STORAGE OR
      BLENDING
      TANK
        GRINDER
        SLUDGE
        FEED
        PUMP
                                                      STANDPIPE-
                                          COMPRESSED AIR
                                     SLUDGE-TO-
                                     SLUDGE HEAT
                                     EXCHANGER
                                                                  T
                                                      SLUDGE	I
                                                       SLUDGE
REACTOR
STEAM
     DECANT
     LIQUOR
                                                                            CAKE
SOURCE: EPA PROCESS DESIGN MANUAL: SLUDGE TREATMENT
       AND DISPOSAL, EPA 625/1-79-011, SEPT. 1979 (MODIFIED)
                FIGURE 2. LOW PRESSURE OXIDATION PROCESS FLOW DIAGRAM

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Figure 3 is an  illustration of a typical LPO  system which shows
most of this equipment and its relative location within a thermal
conditioning  facility.    This  general arrangement  may also  be
utilized for a HT system,  except  that  space  must  be provided for
a horizontal heat exchanger.

Blending Tank

       A blending tank  should be provided  to receive  and blend
primary and waste activated sludges.   The tank  can  also act as a
storage unit before  the sludge is released into the HT/LPO system
for processing.   It should have  a  paddle-type mixing  mechanism
that creates sufficient agitation to blend the dissimilar sludges
into a homogeneous feed without entraining air in the mixture.

Grinder

       Sludge from  the blending  tank passes  through  a  grinder
which shreds foreign objects  to a particle  size of  approximately
1/4 inch.  This  prevents  objects  from plugging the  high pressure
pump, the heat exchanger piping, and the control valve.

High Pressure Pump

       After  grinding,   the  sludge  is  brought  up  to  system
pressure using a high pressure pump.  Positive displacement pumps
such  as  piston  pumps ^or  progressive  cavity pumps  are normally
used  for  this  purpose.     A  hydraulic  exchange  "bag"  pump,
illustrated in Figure 4,  has  been specifically developed for LPO
systems and is often found in these  systems.

       The hydraulic exchange  pump  is  unique to LPO systems, and
has been operated successfully in this rigorous application.  The
pump  forms  part  of   a   pumping  system  that  consists  of   a
conventional  type feed pump  (either  positive displacement  or
centrifugal)  and the  hydraulic  exchange pump.    The  hydraulic
exchange  pump   itself  is  a  hydraulically  operated  positive
displacement  diaphragm  pump.    The  hydraulic  exchange  pump
utilizes hydraulic fluid  for  power.   The  fluid is pumped through
a  hydraulic  system   to  diaphragm bags inside  pressure vessels.
The hydraulic fluid  is completely isolated from the sludge by the
diaphragm bag and the stroke control cylinder.

Heat Exchanger

       A heat exchanger is  used to  preheat  feed sludge  with heat
recovered from the  treated sludge.    Double-pipe  heat exchangers
are used  for  both HT and LPO processes.   In this  type  of heat
exchanger, two pipes  are  mounted concentrically, one inside the
other.  The inner pipe  is  often referred  to  as the  tube, and the

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                                                                                  DECANT
                                                                                  TANK
                   BOILER SYSTEM
                   HEAD TANK
                   SOFTENER
                   BRINE TANK
                   BOILER
MOTOR CONTROL CENTER
AND INSTRUMENT PANEL
                         HEAT
                         EXCHANGER
                                                             - OXIDIZED SLUDGE
                                                              TODEWATERING
                                                                                              STORAGE OR
                                                                                              BLENDING
                                                                                              TANK
INSTRUMENT
AIR COMPRESSOR
                                ODOR
                               CONTROL
                                SYSTEM
                                                                      \    RAW SLUDGE
                                                                        SLUDGE
                                                                        FEED
                                                                        PUMP
                                                                  HIGH
                                                                  PRESSURE
                                                                  PUMP
                                                         AIR
                                                         COMPRESSOR
SCHEMATIC SOURCE: 2IMPRO. INC.
                ROTHSCHILD. Wl
                        FIGURE 3.  LOW PRESSURE OXIDATION SYSTEM CONFIGURATION

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             -PRESSURE
              VESSEL
             -DIAPHRAGM
              BAGS

               STROKE
               CONTROI	
               CYLINDER

       A OUTLET CHECK
   x   f   >LVALVE
 -O--1	
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outer pipe as  the  shell.   In  the wastewater treatment field, the
double-pipe heat  exchanger is termed  a  tube-in-shell exchanger,
although  in  the heat  exchange field  this  term is  reserved for
another common  type of  exchanger  where an entire bundle of tubes
is  placed inside  one  large cylindrical  vessel,  commonly termed
the shell.

       The HT   system  normally uses  a  horizontal  sludge-water-
sludge  double-pipe  heat  exchanger  with  a  water  circulation
loop.   The sludge  flows through the inner  tube,  while the water
to  heat  and  cool  the  sludge  flows   through  the annular  space
between  the   tube   and  shell.    In  different  sections  of  the
exchanger, heat is transferred from the hot treated sludge to the
water,  and then from the  water  to the  feed sludge.   The water
circulates in  a closed loop.   Although  sludge-water-sludge heat
exchangers are  used in  the majority of HT installations,  sludge-
to-sludge exchangers,  described below, have also been used in the
HT process.

       The LPO system  always uses  a vertical  sludge-to-sludge
double-pipe heat   exchanger.    In  this  type  of exchanger,  the
thermally conditioned  sludge   flows  in the  annular  space,  while
the  cool  feed  sludge  flows  in  the  inside   tube.    Heat  is
transferred directly  from  the thermally  conditioned  sludge  to
preheat the feed sludge.

       Sludge velocities through  the  heat exchanger  are designed
to  be  high  enough  to  maintain a  scouring  action  sufficient  to
move  debris  through the  system, yet  not  so  high  as  to induce
damage  from  grit  and  loose scale  from  mineral deposits  at the
180 degree upper  turns in  the LPO system.   The combination  of
cavitation  caused  by   gas release,   and  abrasion  from  coarse
particulate material  at these upper   bends  can result in  rapid
wear.  Normal  design velocities are:

       inside  tube inlet           6 to 8 feet per second

       inside  tube outlet         10 to 12 feet per  second

       annular space inlet         12 to 14 feet per  second

       annular space outlet        8 to 9 feet per second.

       Processed sludge  temperature should be  between 120F and
130P  in  order  to  assure  proper  settling  and  reduce  odor
potential.  Should a  processed  sludge temperature of  less than
120F be  required, an  aftercooler  section  may  be  added  to the
heat exchanger.  An aftercooler is a  separate  heat  exchanger  in
which nonpotable water  is utilized for cooling.
                               12

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Circulation Water System

       In  sludge-water-sludge  heat  exchangers,  the  circulation
water  system  circulates  water   through   the  heat  exchanger,
transferring heat  from  treated to  raw  sludge.  In  this  system,
water is pumped through  a closed,  pressurized circulation loop in
the heat exchanger.  The circulating water  tank is  pressurized to
100 psig by compressed nitrogen.   The tank  has a safety pressure
relief valve,  a  vent valve,  a pressure  indicator,  and  a  level
indicator.  Treated boiler feed water is used in the circulating
water system as initial  fill  and as makeup  water when needed.

Reactor

       The reactor for  both  the heat treatment  and  low pressure
oxidation  processes  is  a cylindrical  pressure vessel  that  is
sized to provide sufficient holding  time to achieve  the physical
and  chemical  changes required for  proper   sludge  conditioning.
The operating pressure  in the vessel can be varied  depending on
the characteristics of the sludge being  treated.  The differences
between the HT and LPO reactors are described below and are shown
in Figure 5.

       Sludge enters the  HT  reactor through  a central standpipe
that runs to the top of  the reactor.  Steam is injected directly
into the sludge at the base of the standpipe, heating the sludge
between 350F and 400F,,  After exiting  the standpipe, the sludge
moves  down  through   the  reactor   and  is   discharged  from  the
bottom.   The  level   in   the  reactor is  controlled  by  a  level
control  valve which  is  activated  by  a   level sensor  in  the
reactor.   Reactor  detention  time of approximately  40 minutes is
controlled by sludge  flow rate.

       In the LPO reactor, a  mixture of  sludge and  air enters the
bottom of  the  reactor.    Steam  is  injected  directly into  the
reactor to  raise the sludge/air  mixture  temperature  to  between
330F and 350F.   The mixture  slowly rises  in the  reactor and is
discharged through a standpipe or  downcomer line.   The detention
time, which could  vary  between 15  to 40 minutes,  is established
by controlling the influent sludge flow rate.

Boiler System

       Both HT and LPO systems have a boiler system that supplies
steam to  raise the temperature of the reactor  feed  sludge.   The
boiler system includes a  deaerator  which removes oxygen from the
water,  a   water  conditioning   system,   a   single-pass  steam
generator, piping, and  a pump.  Feed water  is first conditioned
with sulfite and its pH  is adjusted  with caustic soda to prevent
system corrosion.  The  water   then passes into a deaerator which
is a covered  tank  where  the  water is heated to  220F at  5 to 10
psig.   In the deaerator, dissolved oxygen is  removed  from the

                               13

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LEVEL SENSOR
                                                                     MANWAV

                     \
^;
                 rfi
    SLUDGE
                               -STANDPIPE
                              -MANWAY
                                                      MANWAY . --
                                                  STEAM
                              . TO HEAT EXCHANGER
                      STEAM

        HEAT TREATMENT REACTOR
                                                                                 STANDPIPE
                                                                                        TO SHELL INLET OF
                                                                                        HEAT EXCHANGER
                                                                                  SLUDGE AND AIR
                                             LOW PRESSURE OXIDATION REACTOR
          FIGURE 5. HEAT TREATMENT AND LOW PRESSURE OXIDATION REACTORS

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water  as  it   reacts   with  the  sulfite  conditioner  to  form
sulfate.   From  the deaerator, the water  is  pumped to the boiler
where the  temperature  is further raised,  and  steam is generated
and injected into the sludge stream.

Decant Tank

       In  the  decant   tank,   oxidized  sludge   is  thickened  to
between  8  and 12 percent solids,  water  released as  a result of
the conditioning  process is decanted, and gases produced in the
process are released.   The  tank  functions similarly to a gravity
thickener  and  is similarly equipped.   It is covered to prevent
the escape of odorous gases.  Thickened sludge is normally pumped
to  dewatering while  decant  liquor  is  either  recycled  to  the
mainstream processes or treated separately.  As noted before, the
gases released in  the  tank  should  be  vented  through the cover to
an odor control system, as will be discussed in Section 3.

OPERATIONAL CHARACTERISTICS

       The  primary  operating  variables  used  to  control  the
thermal  conditioning  process are  sludge  temperature  and  flow
rate.   In LPO  systems,  process  air flow  rate  is  also a  key
operating variable.  Given  a proper  solids  feed,  monitoring and
control of these variables will produce a stable, sterile (though
easily  reinoculated)  end product  that  is  easily  thickened  and
dewatered.  The  process  will  also  produce odorous gases and high
strength sidestreams that must be treated.

       The proper temperature for sludge conditioning is directly
related  to  sludge  dewatering   characteristics  and  the  fuel
required  to   maintain   "cook   temperature".    The  optimum  cook
temperature is  the lowest  temperature in the  recommended range
(between   350F   and    400F)    that  will   allow   acceptable
dewatering.   Dewaterability  can  be  monitored  by performing  a
Specific  Filtration  Resistance  Analysis   (3)   on  the  solids
entering the  decant tank.

       Since  a  few degrees difference  in  cook  temperature  can
significantly affect sludge dewaterability and can have a major
impact  on boiler  fuel  consumption, it  should  be  constantly
monitored  and  adjusted.    Lower   cook  temperatures  result  in
savings   through   reduced    fuel   consumption   and   decreased
solubilization and recycle of BOD.

       In the HT process, changing the reactor outlet temperatun
can be  achieved  by two  methods.   One method  is  to  increase b
decrease the sludge feed  rate by  adjusting  the  speed  of the hig
pressure feed pumps.  The second method  is  to  regulate the stea
production  rate  by  automatic  adjustment  from  a  temperatur
control setpoint in the reactor outlet.


                                  15

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       Similarly, in the LPO process, the reaction temperature is
controllable  by  increasing  or  decreasing the  sludge  feed  and
steam production  rates.   In addition,  the  process  air flow rate
affects  the temperature  and  can  be changed  by, using  a needle
valve  on  the  air  compressor  system.   Limiting  the  amount  of
process air also  prevents  slug  flow and back  surging in the heat
exchanger.   Process air  flow  rates greater than  0.15 pounds of
air per gallon  of sludge  cause  a back surging  condition in which
the sludge and air  cannot flow  through  the  piping  at the same
time.   In order to prevent this  condition, the  process air flow
rate should be controlled between 0.08 and 0.15 pounds of air per
gallon of  sludge.

       The  operational  characteristics of  the  thermal  condi-
tioning process  are not  solely  dependent  on  process design  and
operation.    External   influences   that   include  the  quality,
characteristics,  and  particularly  the continuity   of  the  raw
sludge  feed  can directly  impact  the operational characteristics
of the process.   The solids cc .tent of  the feed  sludge should be
thickened  to  3  to  5 percent solids in order to minimize fuel
consumption.  The proportion of waste activated sludge  in the raw
sludge  feed  will impact dewaterability, with  higher percentages
decreasing dewatered cake solids.  These variables should be kept
as stable  as  possible,  and should  be carefully  monitored by the
thermal conditioning  system operator  in  order  to  fully assess
process control needs.

PROCESS SELECTION AND APPLICATION

       The  thermal  conditioning  process  can  be   successfully
applied to nearly any  combination  of primary,  waste activated,
digested,  or  trickling  filter  sludges.   Selection  of the HT/LPO
processes and their applicability in a particular treatment plant
are largely dependent  upon  the  plant  process  flow  scheme  and
total system  size  and  cost.    The   thermal conditioning process
must  be  utilized as  part of  a  treatment system   designed  to
incorporate  its  operational   characteristics  and  performance
features.     In   addition,  for   effective performance  of  the
treatment  plant,  allowances  must be made  for  handling  the high
strength sidestreams and odorous gases produced in the process.

Plant Size and Costs

       The increase in  the cost  of natural gas  and fuel oil since
the  early  1970's   has   significantly  changed  the  economic
feasibility  of   new  thermal   conditioning   systems  for  small
plants.   Larger  installations (greater  than 10 mgd)  that utilize
dewatering  and  incineration  with  energy  recovery may determine
that the  addition  of  a  thermal  conditioning  step  would  be  an
economic asset in their  sludge  train.
                                16

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       Several  factors  must  be  considered  regarding  the  cost
effectiveness of  a  thermal conditioning system  as  a function of
plant size.

           Present-day energy costs dictate some form of resource
           recovery  to  make   the   thermal  conditioning  process
           competitive  with other  conditioning  processes.    In
           plants  with  waste  heat  recovery   from  incineration,
           energy costs  for thermal conditioning can  be greatly
           reduced.    In  general,   thermal  conditioning  is  more
           economical  where   waste  heat  recovery   for  steam
           generation  is possible from sludge incineration.

           Thermal conditioning  systems  require  well-trained and
           skilled  supervisors  and operators  to  optimize  the
           operation and maintenance of  the system.  Maintenance
           and   instrumentation   personnel   also   must   have
           specialized  skills  that are  not normally  present  at
           small (1 to 5 mgd)  plants.

           Both  systems  should  be supported  with  a  complete
           inventory   of   spare   parts   to   reduce   excessive
           downtime.    Also,   both  systems require  a  thorough
           preventive maintenance program.

           The unit capital cost  of thermal conditioning systems
           is in the range  of  $350  to  $500  per dry  ton of annual
           sludge production when processing over 10,000 dry tons
           per year  (10  to 20  mgd of plant capacity)  due to use
           of multiple treatment units and standby units rather
           than  larger  sized  individual  units (4).    At  lower
           loadings, processing costs increase significantly, and
           the comparatively high cost of support systems such as
           boilers,  air  compressors,  and  decant  tanks,  makes
           HT/LPO systems more  costly  to build than other sludge
           conditioning facilities.

System Comparison

       Low  pressure  oxidation  and  heat  treatment  offer  two
alternative methods of thermally  conditioning  sludge.   The major
difference between the two  processes is  that  air is added to the
LPO  system,  offering  a   slight   potential   for  reduced  odor
production.   Both  systems  may be  purchased  today, although  HT
systems are not actively marketed.

       Low  pressure  oxidation  systems  have  been more  widely
utilized than HT systems.   This  wider  use  is  probably  the result
of a more aggressive marketing  strategy for the LPO system, and a
perceived  reduction in  the odor  production  potential  of  this
system.  Aside  from odor potential, neither  system would appear
to have any particular technical advantage over the other.

                                 17

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Advantages and Disadvantages of HT/LPO Conditioning

       Previous literature  on HT/LPO  provides  a summary  of  the
advantages  and   disadvantages  of  using   these   processes   to
condition wastewater sludges (5).

       Advantages cited include:

           Except  for  straight   waste   activated   sludge,   the
           process produces  a sludge  with  excellent  dewatering
           characteristics.   Cake  solids  concentrations  of 30 to
           50 percent  are  obtained with  conventional  mechanical
           dewatering equipment.

       -   The  processed   sludge   does  not   normally   require
           chemical  conditioning  to dewater  well  on  mechanical
           equipment.

           The process  stabilizes  the sludge  and destroys  all
           living organisms  including pathogens.

           The process provides  a  sludge  with a heating  value of
           11,000 to  13,000  Btu/lb of volatile solids,  suitable
           for incineration  or  anaerobic  digestion with  energy
           recovery.

           The process is suitable for many types of sludges that
           cannot  be  stabilized  biologically   because   of  the
           presence of toxic materials.

           The process is effective on feed  sludges with a broad
           range of characteristics and is relatively insensitive
           to changes in sludge characteristics.

           Continuous  operation  is   not   required   as   with
           incineration,  since the system can easily be placed on
           standby.

       Disadvantages cited  include:

           The process  has  high  capital  cost  due  to  mechanical
           complexity   and    the  use  of   corrosion-resistant
           materials,  such   as  stainless  steel,  in  the  heat
           exchangers.

           The  process  requires  careful  supervision,   skilled
           operators, and a  good  preventive maintenance program.

           The process produces  an odorous gas  stream that  must
           be collected and  treated before release.
                                 18

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The process  produces darkly colored  sidestreams  with
high concentrations of organics and ammonia nitrogen.

Scale formation in heat exchangers, pipes, and reactor
requires  cleaning   by  difficult   and/or   hazardous
procedures.

Subsequent   centrifugal    dewatering    may   require
continuous or  intermittent  polymer dosage  to control
recycle of fine particles.

The daily sludge  throughput of the process  cannot be
adjusted  by  a  significant  amount without  incurring
high energy and/or labor costs.
                      19

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                            SECTION 3

                  COMMON PROBLEMS AND SOLUTIONS
       The  problems  commonly  associated  with  thermal  sludge
conditioning and  some  practical solutions to  these  problems are
discussed  in  this   section.    The  problems,   as  summarized  in
Table 1,  can  be   grouped  into  three   categories:     design,
equipment,  and   operations.     They   can  inhibit   operational
efficiency,  degrade  performance,   and  increase  the  costs  of
thermal conditioning  systems  and the  other  associated treatment
plant processes,  as well  as  increase  the safety risk  to plant
personnel.   Solutions  to  these problems  exist  and have  been
successfully implemented  in plants  throughout  the country.   The
solutions presented  can  be used as  guidelines for designing and
operating  thermal  conditioning systems,  keeping  in mind  that
specific  solutions  may have to be  modified to  fit  a particular
plant.

       This  information,  gathered  as  part  of a  research study
conducted for  EPA,  is drawn  from  a number  of sources including
telephone  inquiries  and  site   visits  to  wastewater  treatment
plants, and discussions with manufacturers and consultants.

DESIGN PROBLEMS

       Design problems are those which affect the construction,
sizing, and control  of the thermal  conditioning process.

Materials of Construction

       The design of HT  and LPO systems should  avoid  the  use of
dissimilar  metals   in  heat    exchangers  and  related   process
piping.    Use  of  dissimilar  metals  provides  a  potential  for
corrosion due to  galvanic  action and increases the difficulty of
acid washing the  system.   Galvanic  action does not  take  place,
however, between stainless steel and the more corrosion resistant
nickel  based alloys  and  titanium.    Carbon steel heat exchangers
are normally cleaned with hydrochloric acid that has an inhibitor
added  to  prevent the  acid  solution  from attacking  the  steel.
Stainless  steel   heat  exchangers  are always  cleaned  using  a
5 percent  nitric  acid  solution heated   to   180F  for  greater
solubilization  of the  sulfate  scale.   Because hydrochloric  acid
will pit  stainless   steel  and  nitric  acid  will  destroy  carbon

                                20

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                TABLE 1.   SUMMARY OP COMMON PROBLEMS
                 WITH THERMAL CONDITIONING  SYSTEMS
Design Problems

     Inappropriate materials of construction

     Process sizing inconsistent with solids produced in the
       wastewater treatment train

     Improper sizing of storage, blending, and decant tanks

     Poor physical layout of the process
     Inadequate or poorly located system control instrumentation

     Under/oversizing of support systems such as boilers,
       circulation water pumps, and air compressors

     Inadequate grit and rag handling or removal

     Inadequate odor control provisions

     Inadequate handling of high strength sidestreams

Equipment Problems

     Loss of grinder seals

     Wear of sludge feed pumps

     Corrosion, plugging'and scaling of heat exchangers

     Unreliable level probes and off-gas control in reactors

Operations Problems

     Lack, of system understanding by senior management personnel

     Poorly qualified O&M staff

     Insufficient operator training

     Improper process control operation and system maintenance
steel, a bimetallic  (carbon  steel/stainless  steel)  system cannot
be descaled without damage to one of the metals.

       The cleaning system for heat exchangers should be designed
to accommodate  the range of  cleaning  options  normally  used and
should be constructed of  materials  compatible with  those used in
the HT  and LPO  systems.   This  criterion ensures  compatibility
between the  acid and metals  in the  heat exchanger  and related
piping.
                                21

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       Selection of materials of construction is also constrained
by  the potential  for corrosion  in  heat  exchangers.   The  LPO
system may  require more  corrosion-resistant metals  than  the HT
system due to the possible production of more organic acids which
lowers the pH, especially where primary sludge is processed.

       Aftercooler  sections  of  sludge-water-sludge  double  pipe
heat  exchangers  with  stainless  steel  tubes  and  carbon  steel
shells have  experienced  some corrosion and  leakage  problems  due
to  use  of effluent  water as a  cooling medium.   Effluent water
with  fairly  high   levels   of  dissolved   oxygen   at  elevated
temperatures speeds up the rate of corrosion of metal.

       Another  corrosion  concern applicable  to both  HT and  LPO
systems  is  the  presence and  concentration  of  chloride  in  the
wastewater  and   sludge.     High  chloride  concentrations  have
contributed to or caused corrosion of heat exchangers in numerous
installations.  Where chloride  levels are  expected to exceed 300
to  400 mg/L  in  the  raw wastewater,  the possibility of developing
problems  from chloride  stress  corrosion  exists,  and corrosion
resistant  metals  should be used.    Available  material  types
include 316L stainless steel, nickel based alloys, higher alloyed
iron based alloys, and titanium, in order of increasing corrosion
resistance and  cost.   The capital  cost for  nickel  based  alloys
and  higher  alloyed  iron  based alloys  is about  5 to  7  percent
greater than 316L stainless  steel.  The capital cost for titanium
is  about 15 percent greater  than  stainless  steel.   To keep costs
as  reasonable as  possible, titanium  or  higher nickel based alloy
construction only needs  to  be used  in  the  heat exchanger  bundle
nearest the  reactor where temperatures  are highest and corrosion
potential is greatest.

Process Sizing

       The  capacity  of   HT/LPO  systems  should  be  based on  a
careful  estimate  of  sludge  production  rates.   A   low  sludge
production  estimate will  result  in an  undersized  system  that
cannot  process   the  total   sludge   produced  without  continuous
operation, leaving  no  time  for  preventive maintenance.   A high
estimate  leads  to  an oversized  system  that  is only  operated
several hours a week, requiring a large  amount  of fuel to  reheat
the  reactor  contents.    These   sizing  problems  are  due  to
insufficient  or   inadequate  design  data  and   to   lack   of
consideration of  operating conditions in the initial  and  design
years.

       To  minimize  the   problem  of  over/undersizing,  thermal
conditioning systems  should  be designed with enough flexibility
to  accommodate  initial,  design,  and  future  sludge  production at
minimum  and  maximum  rates.    These production  rates  can  be
estimated using  existing sludge  production  data  if  available,
mass  balances  using  historical  data   and/or  past  treatment

                               22

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experience, or textbook values.  Once sludge production rates are
developed,  system  operation  times   should  be  established  for
average and peak conditions.  In general, these systems should be
designed  with  sufficient   flexibility  to  satisfy  initial  year
sludge production  rates  with a standby  system  in  place to allow
up  to double  the  initial  year  solids  loading,  and  sufficient
floor  space  to  install  additional units  to satisfy  later  year
requirements.

       Although installation of large units  rather than multiple
small  units  may  involve a  lower initial  capital  cost,  larger
units can require more effort to maintain due to the construction
and  size  of  the equipment.    Larger units require the  use  of
special rigging and hoisting equipment plus a considerably larger
work area.  The alternative, multiple small units with sufficient
cross  connections,  should  be  considered and  may  be  more cost-
effective if full capacity is not  needed within the near future.

Sludge Blending and Storage Tanks

       To assure  a  homogeneous feed   to  the  thermal conditioning
process,  the  various   sludges   to   be   conditioned  should  be
uniformly blended.   To assure  continuous  operation of  both the
sludge  generating and  thermal conditioning  systems,   a  storage
tank  should be  provided  to dampen surges in sludge  flow.   Where
sludge production variations are  not expected  to  be major,  both
of  these  functions  can  be  accomplished   in  a  single  sludge
blending  tank.     Where  major variations   in  sludge  flow  are
expected, off-line sludge  storage  in  combination with  a separate
blending tank(s) may be preferable.

       Sludge  storage tanks hold peak sludge flows.  Their volume
must  be  large  enough  to  store  the peak  flows  yet  minimize
detention  time   to   avoid   septicity and   odor   problems.     A
recommended sizing  criteria for an off-line storage tank  is  to
provide a volume  equal  to three days average  sludge production.
Off-line  storage  tanks  should  be  aerated to  prevent  septicity.
Should the sludge  become  septic,  thermal conditioning  can cause
the following operational  problems:   (1) poor  thickening  of the
feed  sludge,  which  increases  the  cost  of conditioning due  to  a
decrease  in  tons  of  solids processed  per  hour;   (2)  increased
solubilization of VSS during the  HT/LPO processes  and resulting
high levels of BOD in the  sidestreams recycled  to  the  plant; (3)
decreased  settleability  of  thermally conditioned  sludge  which
also causes excessive recycle  of BOD  and suspended solids  to the
treatment plant;  and (4)  decreased  dewaterability  of  thermally
conditioned sludge  which compounds solids  backlog  problems  and
also increases sidestream loadings to the treatment processes.

       Sludge  blending  tanks  should  be  sized  to  hold not  less
than 12 hours  nor more than  24 hours  of  the  HT/LPO system design
capacity.   Good  design includes  low  level  alarms  and  automatic

                               23

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low level pump  shut  off  for  the tank to prevent feed sludge pump
damage.   Blending tanks that  hold less than  12  hours  of design
capacity are  operator-intensive and there  is a potential for the
feed sludge pumps  to run dry,  causing considerable damage to the
pumps.   An  oversized blending  tank  can  result in septic sludge,
increased energy costs for  mixing the large volume, and increased
maintenance costs for parts in  these larger tanks.

Decant Tanks

       In addition  to thickening,  a  decant tank  functions  as a
storage  tank  to  permit the  operating  schedules or  production
rates  of thermal  conditioning  and  dewatering  to differ.   The
sizing,  septicity,   and  thickening  problems  in  storage  and
blending tanks  can  also occur in  decant  tanks.   If  the decant
tank holding  time is  too  long  or  if the  operator  draws sludge
from  a  tank  on  an  infrequent  basis,   its  contents can become
reinoculated  with  bacteria.    This  condition  can  cause  odor
problems  and  will  affect   the  dewaterability  of the  thickened
sludge.   The  decant  tank  should be sized  to  be  compatible with
dewatering operations,  such  that  the  decant tank is  empty when
the dewatering operation shuts down at the end of the week or the
thermal conditioning system is put on standby.  Other decant tank
design guidelines include:

        Solids  loadings less   than  40  Ib/ft2/day  for  combined
        sludges

        Floor slopes greater than 2.75 inches per foot

        Proper access for maintenance

        Proper sealing of covers to minimize odors

        Continuous   sludge    withdrawal   to   prevent   septic
        conditions.

       The sludge  depth  in decant  tanks should  be  monitored  to
assure proper thickening.   Use  of a portable  gauge  allows easy
measurement  of  sludge depth.   One  improvement to  the decant tank
is installation  of  a 4-inch pipe  that  extends down  through the
roof of  the  tank  to 4 to  5  inches  below the liquid  level.   The
pipe is extended below the liquid  level  to  prevent the  escape  of
gases.    This  improvement  allows  the  operator  to  monitor  the
sludge level  without  opening roof hatches  and  allowing odors  to
escape.   Installation  of   the  pipe is  not  possible  on  tanks
equipped with surface skimming arms.   On these  tanks,  a hinged
cap or  valve  can  be placed on the pipe  to prevent odors  from
escaping when sludge levels are not being measured and the end of
the pipe can be  located above the liquid level.  Measurements are
made after passage of the slowly rotating skimmer arms.


                                24

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Physical Layout of the Process

       For effective  operation  of thermal  conditioning  systems,
operators should  be  able  to monitor  the  total process  easily.
This is facilitated by  installation of  equipment on  one  level in
one building.   Installation of  a HT/LPO system in  a new  or an
existing  building  in  a complex  multifloor arrangement  greatly
increases the difficulty of  system operation and maintenance.  A
multifloor arrangement  substantially  increases piping,  wiring,
foundation,  rigging,  erection,  lighting, and  building  structure
costs as  well as  creating  operation  and maintenance  problems.
This arrangement  should be  avoided if  at all  possible,  because
the  initial  capital  savings in  construction  costs  may  soon be
lost due  to  the increased  cost of operation  and  maintenance or
equipment replacement due to damage caused by inadequate operator
attention to  the system.  Generally,  a system that  is spread over
several floors and rooms of  a building  will  not be monitored and
maintained as well as a system that is contained on one level.

       The placement  and  elevation  of  HT/LPO system tanks  and
sludge withdrawal pumps are  also important  considerations in the
layout  of the  process.     To   avoid  cavitation  problems,  the
withdrawal pumps should be  located below the liquid  level in the
tank.  As an  example, at  one plant the  vacuum filter  feed pumps
were several  hundred  feet  removed and  at a  higher  elevation than
the decant tank water surface.   Cavitation  routinely occurred in
these filter   feed pumps due to the high suction lift  created by
elevation, friction  losses,  and the elevated  temperature of the
feed sludge.

Inadequate or Poorly Located System Instrumentation

       The  instrumentation   for  process  control   of   thermal
conditioning    systems  should   be   state-of-the-art,   properly
located,  and   adequate   to provide   needed  process   control
information.   Analyzers and  other instrumentation  must  be placed
where  a  true   representation   of  process conditions   can  be
obtained, with  indicators  located  where  operators  can  readily
observe and respond  to them.   Otherwise, the  instrumentation is
of little use.

       All mechanical gauges for high vibration areas  should be
anti-shock types,  mounted  on  external  gauge boards  to  avoid
continual vibration.  The gauges should have flexible connections
to the monitoring elements.  In one plant where this modification
was made, the  life  expectancy of Bourdon tube  type  gauges is in
excess of four years compared to a few weeks life expectancy when
the gauges were mounted directly on pipes and components.

       Adequate  numbers of  process  monitoring points   must  be
provided.  The instrumentation  listed  in Table 2  should be built
into the thermal conditioning system to provide the operator with

                                25

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     TABLE 2.   RECOMMENDED HT AND LPO SYSTEM INSTRUMENTATION

Alarms - no local readout
     Reactor level, high and low
     Reactor pressure, high and low
     Sludge flow, low
     Circulation tank, water level
     Circulation tank, pressure
     Circulation water, flow
     Instrument air pressure, high and low
     Deaerator level, high and low
     Decant tank temperature, high (optional)
Recorded Instrument Readings
     Reactor pressure
     Sludge flow
     Steam flow
     Reactor level
     Steam pressure
     Reactor inlet and outlet temperature
     Circulation water crossover temperature (optional)
     Process air flow (LPO only)
Digital Readout and/or Recorder
     Sludge inlet temperature to heat exchanger
     Circulation water inlet and outlet temperature
     Heat exchanger, sludge outlet temperature
     Deaerator temperature
Additional
     Flow meter for reactor off-gas,  for economic control to
       monitor steam losses from reactor (more critical on larger
       units)
     Reliable  flow and density meters for feed sludge
     Needle valve assembly on air compressor (LPO only)
                               26

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 the  necessary information and/or  control  to operate the  process
 effectively  and  efficiently.    Additional  instrumentation  that
 could  improve LPO  systems  is  a  process  air  flow  meter,  which
 allows  the operator to monitor the proper ratio of  air-to-sludge
 volume.   A  needle  valve assembly should  be provided on  the  air
 compressor   discharge,  prior  to  the  airflow  meter,   to  allow
 adjustment  of the  air-to-sludge  ratio.    This instrumentation/
 control  is needed because excessive  air  addition to the  process
 can  cause back  surges in the  heat  exchanger  and  damage to  the
 process  air  compressor  safety valve.

       The  quality  of  control  instrumentation must be  carefully
considered  during design.  The available data  indicate  that  over
 the  long run, sophisticated, heavy-duty control systems  are  more
 cost-effective   and   trouble-free  than  lighter-duty   instrumen-
 tation.    An HT/LPO  system  should   have  an  integrated  control
 system supplied  by  one  manufacturer that can perform all  required
 control  functions.    For  the  HT   system,  a  3-mode  proportional-
 integral-derivative  (proportional-reset-rate action)  control  loop
 should  be supplied.   For the  LPO system,  a  2-mode  proportional-
 integral (proportional-reset) control  loop shou-ld be supplied.

 Sizing of  Support Systems

       HT/LPO system boilers should  be sized to account  for  less
 than  optimal heat  exchanger efficiencies.   In addition,  single
 LPO system  installations  should have  100 percent standby  capacity
 for  the  design  steam production rate  to  the system.  Scaling  in
 the heat exchanger  will  lower heat transfer  efficiency  requiring
 a  higher steam production rate  to maintain system  temperatures.
 An operator may  discover  that  there   is  insufficient  steam
 production  to maintain system temperatures  or  to meet the  total
 system  needs when  trying to  recover from  a process  upset.    A
 properly sized boiler  and frequent cleaning  of  the heat  exchanger
 to minimize  scaling are recommended.

       Insufficient  potable  water pressure  to the boiler  system
 may  appear  as  a boiler  sizing  problem.    The  installation  of
 potable  water booster pumps  should be  considered and will  benefit
 plants   that  experience   large  fluctuations   in  potable water
 pressure.

       The   size  and  location  of boiler   feed  water  pumps  in
 relation to  deaerators has caused problems.   To avoid cavitation
 and ensure a flooded  suction, boiler  feed water pumps should  be
 located  close to and  below  the deaerator.   Locating the  boiler
 feed  water pumps on  a floor below  the  deaerator may  be a  good
 design  in  this   case.    The use  of  booster pumps  between  the
 deaerator  and the  boiler feed water  pumps   is  another  means  to
 ensure  that  feed water  pumps  maintain  a  flooded  suction.    If
 booster  pumps are used,  the  friction  losses  between  the  deaerator
 and boiler  water feed  pump  must  be  carefully assessed to ensure

                                27

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 that  they  do not  limit  the  necessary  flooded  suction  of the
 boiler  feed water pumps.  To avoid cavitation, feed water  return
 should  be to the  deaerator  and not  to the suction  side of the
 feed water  pump.

        When   sizing  the  circulation  water  pumps  in  the heat
 exchanger  circulation  water  system of HT  systems,  both minimum
 and maximum  flow  requirements should be considered and allowances
 should  be made  for loss  of efficiency  due  to  pump wear.   As
 circulation  pump efficiency  drops, the  cost  of  operation will
 increase  because  the  HT  system  depends  on a  countercurrent
 (water-to-sludge) flow to  transfer  heat between processed and raw
 sludge  using  the  water  as  an   exchange  medium.      If  the
 circulation   water   pressure  is   allowed   to  drop  below  the
 saturation  point for  the  corresponding temperature  in  the heat
 exchanger,  the  water will  flash to steam.   This condition will
 cause water  hammer that  may  in turn cause extensive damage  to the
 heat exchanger piping.   Proper  pump sizing, along with  attentive
 pump maintenance, can minimize these problems.

        High  pressure sludge  feed   pumps  are one  of  the hardest
 worked  pieces of equipment in the  LPO  system  due to the various
 foreign materials  contained  in  the sludge (rags, grit,  plastics,
 etc.) and  to the high pressure  and vibrations produced by  these
 units.    Because these  pumps require  frequent  maintenance and
 routine cleaning of ball  check  valves  for rag  removal,  a full
 sized standby pump is necessary.

        High  pressure air  compressors  in  the  LPO  system  should
 have  the  capability  to  supply  0.08  to  0.15  pounds of air per
 gallon  of  sludge.   If the air  compressor output  is much  higher
 than 0.15 pounds per gallon, back surging will occur.  If the air
 compressor  output  is  too high,  the compressor discharge pressure
 will increase until  the  safety  valve  actuates to release  system
 pressure.     The  valve  will  close   after  the  excess  pressure  is
 relieved and will  reopen if  pressure  builds up again.  This will
 continue until system operation is  adjusted.   To control the air-
 to-sludge ratio,  the air compressor discharge should be equipped
 with a  flow  meter  and  needle valve bypass for  diverting air away
 from the process.

       Every  LPO  system  is equipped with an acid cleaning  system
 that should be  capable  of  flushing  the  unit with  a  5 percent
 nitric  acid  solution heated  to  180F.   The  intermittent use  of
 this  support  system  makes   equipment  duplication  unnecessary.
 Normally,  one system  will be adequate  for multiple  LPO  system
 installations.

       Process control  valves (PCV's)  in LPO systems  should  be
 large enough to release the process byproducts through one  valve.
An  additional valve  of  the same size  should  be  provided  as
 standby.  Because  these  valves  frequently become restricted with

                               28

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residue from the process, LPO systems cannot be operated reliably
without a standby PCV.

       Multiple  LPO  system  installations  may  still  require  a
certain amount of standby support system capacity.  However, with
adequate cross  connection  of equipment, much  of  the duplication
can  be  avoided.     In  either   single  or  multiple   LPO  unit
installations,  O&M  personnel should not  allow standby equipment
to remain  idle  for  long periods of  time.   Specifications should
require that equipment O&M manuals correctly detail the frequency
of rotation of support components.

Inadequate System for Grit  and Rag Removal

       Grit and  rag  removal  from the feed  sludge to the thermal
conditioning  system  is essential  to insure minimal plugging  of
the heat  exchanger,  pumps,  and  valves.   Plugging  due  to grit,
rags,   or  both  can  be so  severe as  to require  frequent system
shutdown  to  clean  out  these  materials.    These shutdowns  are
costly because  they  waste  energy and cause  unnecessary delays  in
processing sludge.

       Grit and rags are less of a problem in the HT process than
in the LPO process  due to  the design  of  the  HT sludge-water-
sludge  heat exchanger.    In  this  heat  exchanger,  sludge flows
through  a  single pipe  with  no  obstructions,   and  the  design
permits pigging  of  the sludge pipes  to  remove built-up deposits
of sludge, grit and rags.

       The  configuration of  the LPO  heat exchanger can  lead  to
plugging.   The  LPO  system utilizes  a concentric pipe  sludge-to-
sludge  heat  exchanger  installed  in  a  vertical  position  to
facilitate the turbulent flow of sludge  and air.  Stabilizers are
installed  in   the  annulus  between  the two  pipes   to  prevent
vibration of  the inner pipe.   Rags and  other foreign materials
that  have   not  been  sufficiently  ground up  will accumulate  on
these stabilizers and lead to plugging problems.

       In plants  where  rags  pass  through  influent screens  and
comminuting devices  and are  left in the flow stream, the problem
of plugged  equipment  is  extremely  severe.   Shredded  rags  can
reconstitute into long rope-like formations that are pumped with
the sludge  into  the  thermal  conditioning system.   Rags should  be
removed from the process flow stream to  avoid the costly and time
consuming  process of later  removing  them  from HT/LPO  system
pipes, valves  and heat exchangers.

       As with  most  wastewater  treatment process equipment,  the
passage  of grit  through  piping,   pumps,  valves   and  internal
equipment components  is never  a  good practice due to abrasion  on
these components.  In the HT system, the abrasive grit problem is


                                29

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compounded  by  the high  velocities  experienced in some  parts of
the  system.   One of  the areas  most  often affected  is the 180
degree  elbows  on heat exchangers.   If  excessive  amounts of grit
are  allowed  to  enter   the  thermal  conditioning system,  these
elbows will wear away to  the point where leakage occurs.

       Careful  consideration  should be  given  to grit  removal
early   in   the  design  stage  of  wastewater  treatment  plants
utilizing   thermal  conditioning  systems  to  ensure  that  this
removal process  achieves maximum efficiency.  The use of a grit
removal  system  (e.g.,   cyclone  separator)  in  the  sludge  flow
stream  in   addition   to  a   grit   chamber   may  be  warranted.
Degritting  of  primary   sludge,  however,  should  be  followed  by
thickening   to   keep   primary   sludge   pumping  and   thermal
conditioning  operation  costs  low  due  to  processing  of higher
solids  feed sludge.    The grit  levels  in feed sludge to HT/LPO
systems  should  be  monitored  routinely  and  corrective  actions
taken if levels  trend upward.   These actions may include the use
of  a cyclone  separator  to  handle  increased  grit levels during
storm events, or because of seasonal variations.   Redesign of the
plant's grit chamber may be needed if the problem is chronic.

Odor Control

       The  odors  associated with thermal  conditioning of sludge
are some of the most problematic found in wastewater treatment in
terms of intensity and available control  methods.  The main odor
producing or  releasing areas are the decant tanks and  dewatering
areas.  Odors  in an  LPO system may be less than in a HT system
because the addition  of  air oxidizes hydrogen sulfide and other
sulfur  compounds to  sulfate.    However,  this  difference  is not
significant  to  the   extent  that  odor  control   measures  become
unnecessary in an LPO system.   Odor  control must be  addressed in
the design  of  both  processes  and must  include the collection of
all gases in order to be effective.  Treatment of these off-gases
typically  constitutes 5  to 10  percent  of  the  total  costs for
thermal conditioning(6).

       Many  methods  exist   for  handling  these  gases.    One
effective treatment method,  if the  option  exists,  is  to collect
the off-gases  and include them  with  incinerator  combustion air.
As long as  furnace  temperatures  remain above  1,400F, good odor
destruction  will occur.    Off-gases  can  also be collected and
sparged into  activated  sludge aeration basins where the soluble
odorous constituents  are adsorbed and absorbed.   A backup method
is  necessary  for treatment  of  the collected   gases when  the
primary  process  unit,   such   as  an   incinerator,   is   not  in
operation.

       Other   odor    treatment   methods   include   hypochlorite
scrubbing or  a multiple  chemical  system,  which   will  consist of
some combination of  the following:

                                30

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        Chlorinated water scrubbing  (countercurrent)

        Sodium hydroxide (countercurrent)

        Potassium permanganate  (countercurrent)

        Carbon columns (direct  flow  through)

        Fume incineration (flow through).

Other  attempts  to control  odors  using either ozone  or  a single
chemical system have not been successful.

       For plants  under  10  mgd,  total  life  cycle costs  for odor
control  are  generally  the  least  for  incineration  and  chemical
scrubbing.    As   plant   size  increases,   the  total  cost  for
incineration  rapidly   increases,   and  chemical  scrubbing  in
conjunction  with  carbon   columns  becomes  a  more  economical
alternative.    Detailed  cost  comparisons  for  odor  control are
given  in  the U.S. EPA  publication  entitled  "Effects of Thermal
Treatment of Sludge on Municipal Wastewater Treatment Costs" (6).

       Odors  generated  during  flushing  and  cooling  of  heat
exchangers  prior  to  temporary  hot  shutdown  of   the  reactor
(bottling) can  be  reduced  through judicious  selection of the
cooling water source and piping design.  Normally, nonchlorinated
effluent  water   is  used   for   flushing  and  cooling,   because
chlorinated  effluent contains chlorine which  interacts  with the
metal  heat exchanger.   However,  the discharge of nonchlorinated
water  into  the  decant  tank  can  result  in  contamination  with
sulfur reducing organisms and production of odors.  To avoid this
problem, flushing water  piping  should  bypass the decant  tank and
direct all flushing  water  to the aeration basins  or  to  the head
of the  plant.   This  piping can be  installed between the process
control valve and the inlet to the decant tank.

High Strength Sidestreams

       The handling  and  treatment  of  sidestreams  from thermal
conditioning systems must be a major design consideration.  These
sidestreams  include  supernatant  liquor from  decant  tanks,  and
liquors withdrawn  from  sludge in dewatering  processes.   Recycle
liquor  can  increase  plant   influent   BOD  loading  by  15  to
30 percent.  If not properly accounted for in design, these loads
can pass through the plant,  causing permit violations.  Treatment
of these  sidestreams will  increase plant capital  and operating
costs.  Total costs  for  treatment of thermal conditioning liquor
will  be  a  small percentage  of  total plant  cost, but may  be  as
much as 20 percent of the costs for  thermal conditioning (6).

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       In  general,  the  composition  and  strength  of  thermal
conditioning  liquor  are  a  function  of  sludge  type  and  age,
volatile  solids  content,   reactor  detention  time,  and  reactor
temperature and pressure (5).  These sidestreams have high levels
of  BOD  and chemical oxygen demand (COD) as  well  as significant
levels of  total phosphorus  and  total nitrogen.   As reported in
the  literature,  the  sidestream  can  have  a  BOD  of  5,000  to
15,000 mg/L,  suspended  solids of 100  to  20,000 mg/L,  and COD of
10,000 to 30,000 mg/L (5).

       Assessment  of the  sidestream  characteristics to  use for
design  is  difficult.    At plants  where  sludge  is  available,
sampling  and  testing  for  processed sludge   filterability  and
recycle characteristics  can be  performed.   Thermal  conditioning
pilot plants  are  available from system  manufacturers  and may be
used  to  generate  samples  of  conditioned  sludge  and  recycle
streams.  Where sampling and testing  are not feasible,  ranges of
sludge characteristics  should be identified.   A comparison with
similar  sludge  conditioning  experiences  at   other  plants  can
provide a  basis  for  design.  Where  no  sludge  is available for
analysis,  solids  balances  and a  determination of  the  range of
primary/waste  activated   sludge  mixtures   should  be  used  in
conjunction with  data from past treatment  experience.   General
characteristics of  recycle  streams from  thermal conditioning are
published  in  the  U.S. EPA  publication entitled  "Process Design
Manual for Sludge Treatment and Disposal" (5), and can be used as
guidelines.   These  data  indicate that the upper end  of parameter
concentration  ranges for   LPO  sidestreams  are  higher than for
HT.    Addition  of  oxygen  to  the   LPO . process  increases  the
production of organic acids and carbon dioxide, depressing the pH
to about 4.5  to 5,  compared to a pH  of 5 to 6 with HT (7).  This
change  in  pH  may  add  to  the corrosiveness  of the sludge and
sidestreams.

       Thermal  conditioning  sidestreams carry high  levels  of
solubilized  BOD  caused  by the  breakdown   of  volatile  matter.
Factors that  tend to increase the solubilization of  BOD during
thermal conditioning are:

    -   Septic feed sludge  to  the process

    -   A high  proportion  of  waste  activated  sludge  to  primary
        sludge

        Improper control of cook temperature

        Excessive retention time in the decant tank.
       Thermal conditioning liquors  may  be treated using several
methods.   Since  the  liquors are  biodegradable,  the  preferred
treatment methods utilize  biological processes.   The liquors may

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be treated  by  recycle to the treatment plant  headworks,  or  to a
biological  process  such as  activated  sludge,  or  by  separate
treatment.   Recycle  systems can either  involve  direct  full  time
recycle, or  can  involve  off-line  storage  with  return feed during
off-peak hours.

       Where   thermal   conditioning   liquors   are   recycled,
mainstream  processes must  be  sized  accordingly.    For  example,
where  recycled to  activated sludge  systems,  these sidestreams
will   require    increased   aeration   tank  size,   air   supply
capabilities,  and  return and  waste  sludge  pumpage.  This  will
increase  the capital  cost  of  the  aeration system,  as  well  as
increasing operating power and labor costs.

       In  order   to  minimize   capital   costs  and  peak  power
requirements,  thermal conditioning  liquors  can be  stored  in a
holding  tank during  the daytime  when plant flows and  loads are
high,  then  returned  to  mainstream processes  at night  when the
load is down.

       Thermal   conditioning  liquors  may  also  be  separately
treated.  Biological systems have  been exclusively used for  this
purpose.  Although physical-chemical  treatment  may  be  possible,
there  are  no  known systems  in  use.    One  separate  treatment
alternative  is  to  anaerobically  digest  the  separated  liquors
while  stabilizing  the  mixture  and providing  gas  suitable for
fueling the conditioning processes.  The use of anaerobic filters
has also been tested wi^th apparent success (8).

       The   wastewater   treatment   facilities  at  San   Mateo,
California have been anaerobically digesting their  LPO thickening
tank  supernatant  and dewatering  process  filtrate  for  several
years.   The following data,  taken from their June  1984 monthly
log  sheet (9),  indicate an  average reduction in  sidestream BOD
and COD loadings of  84 and 72 percent, respectively.


            Sidestream to                             Percent
              Digesters           Supernatant         Removal

          pH    BOD,    COD      pH     BOD,   COD
                mg/L   mg/L            mg/L   mg/L    BOD   COD
Maximum
Minimum
Average
4.6
4.2
4.4
6,660
4,870
5,883
14,640
8,680
11,171
7.12
6.99
7.06
1,110
700
930
4,590
2,660
3,145
83
86
84
80
69
72
       The  average  feed  to  the  anaerobic  digester  was  between
50,000 and  60,000  gallons per  day.   Mixed  liquors  were  between


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1,200 and  1,500  mg/L.   The mass  was  continuously circulated and
no  problems  developed  with  accumulating  bottom  debris.    Gas
production was about 40,000 cubic feet per day.

       The controlling factor for success at the plant is keeping
the  temperature  of  the sludge feed to  the  anaerobic digester at
102F or less.   This  practice maintains the digestion process in
the  mesophilic temperature  range,  optimizing biological activity
and  gas  production.   This  temperature  is maintained  by running
the  supernatant  and  filtrate piping  from the  LPO system through
an aeration basin prior to discharge  to the digester.

EQUIPMENT PROBLEMS

       Equipment  problems  are  associated   with  grinders,  high
pressure feed pumps, heat exchangers,  and reactors.

Grinders

       The major  problem with grinders  in  the HT/LPO  system is
failure  of  the  heavy  duty  internal  upper  and lower  seals and
bearings  after  only  a  few  hundred  hours  of  operation.    The
grinders  can  be modified  by  changing  the   factory  installed
internal seals to a less  expensive external standard seal having
a packing  gland  with  a  lantern  ring and water  seal.   Although
such a  change requires modification  of the grinder  housing and
relocation of  the prime  mover, it can  greatly increase the life
of the bearings and seals for these components.

High Pressure Feed Pumps

       The most common problems experienced with piston pumps are
high wear  of the pistons  and piston  rods due  to grit, internal
recirculation  of  sludge  within  the  pump due  to failure  of the
cylinder  liner  seal,  and  breakage  of oil  lubrication  system
piping which can result  from equipment  vibration.   Removal of
grit in  the  headworks  and, when necessary,  from  the sludge feed
to a thermal conditioning system  will  minimize  wear  of pistons
and piston rods.

       Cylinder  liner  seal   failure  of   piston  pumps  is  of
particular concern  where  a stroke  counter  is  used  instead  of a
sludge flow meter.  With a stroke  counter,  the problem of sludge
recirculation within the pump will go undetected for long periods
due  to  the  inability  to  detect  loss  of  flow  and/or  pump
efficiency.   This flow reduction to  the HT/LPO system can cause
plugging problems in  the  heat exchanger.  Although using a flow
meter and  recorder  with  a piston pump  produces a  flowchart of
peaks  and   valleys  reflecting  pump  pulsations,   the   record  is
useful  in   estimating  average  flow  and  in monitoring  dropoff
trends in flow rate.
                                34

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       The  problem  of  breakage  in the  oil  lubrication  system
piping can be solved by installing flexible tubing connections to
pumps  and  by  locating  hard  piping  away  from  the pump  which
isolates it from equipment-generated vibrations.

       The  most  common  problem  experienced  with  progressive
cavity pumps  is excessive  wear of  the  rotor and stator  due to
high  grit  content of  the feed  sludge or the  pump  running dry.
Removal of  grit  from the  feed sludge  is the best  way  to avoid
rotor  and  stator wear.   Stator  life  can be extended  up  to 50
percent by  reversing the stator  when  pump efficiency  begins to
drop  off.   If  grit wear occurs, the stator will  wear out on the
suction  side  first.    By  reversing  the   stator,   the  intact
discharge side will  deliver  the required efficiency.  Again, the
use of flowmeters is a major benefit in early detection of stator
wear.   It should  be emphasized  that  a  progressive cavity pump
should not  be  considered as  a true positive displacement pump,
and  a  pump revolutions  meter,  calibrated  to  cubic  feet  per
minute, should  not be used  as a true  indication  of actual flow
rate.   If  flowmeters are not  installed,  systematic  and frequent
sludge blending  tank drawdown measurement should  be employed to
determine actual pump flow rates.

       The  hydraulic exchange  pump  does not have  the  problems
observed  with   the  positive  displacement  and  the  progressive
cavity  pumps.     The  design  of  the  hydraulic  exchange  pump
completely  isolates   the  pump  components  and hydraulic  system
(except for  inlet  and outlet ball check  valves)  from the sludge
being  pumped  by  means  of  a  positive  mechanical   seal  and  a
flexible  diaphragm.     It  was   designed    for   and   has   been
successfully used in LPO systems.

Heat Exchangers

       Problems with heat exchangers include corrosion,  clogging,
and   scaling.      In  some  HT   system  heat  exchangers   with
aftercoolers,  corrosion and  leakage  in  the  outer tubes  have
occurred.   These  problems  are caused  by  using  effluent  water as
the  cooling medium.    One  solution  is  to  use  noncorrosive
materials in the  aftercooler;  however, the initial  capital cost
may  be excessive.   A  more  cost-effective   solution may  be  to
locate the aftercooler  externally  to the  heat exchanger  for easy
access  to   the  tube  sections.   The  small  aftercooler  can  be
repaired without removing the  massive  insulation  around  the main
heat exchanger.

       High concentrations of grease, oil, polymers,  tar,  fibers,
rags  and  metal  particles  may  clog  the  heat exchanger  in  a
relatively short period of time.   Removal of  scum and screenings
prior  to  thermal conditioning  (preferably  from the  liquid flow
train) will  reduce  problems  associated  with  these substances.
Polymer dosages  should  be  carefully monitored  and controlled to

                                35

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minimize clogging associated  with  polymers.  Cleaning methods for
these   substances   include   steam,   polypigging,   backflushing,
alternating forward-back flushing,  hot  water or steam/cold water
shock,  acid  cleaning,  and  combinations, of   these  procedures,
depending on  the  material  of construction of  the  HT/LPO system.
The  specific  cause of  clogging must be  identified  in  order to
determine the cleaning procedure likely to be most effective.

       Formation of hard calcium sulfate scale on heat exchangers
is a  problem reported in  areas that have hard  water or certain
industrial  contributions  (5,7).     The  inverse  solubility  of
calcium sulfate with  temperature  can be  a  serious  problem with
the   thermal   conditioning   process.       Sludge   'with   high
concentrations  of  calcium,   sulfate  and   phosphate  normally
precipitates  a  scale  over  a  relatively  long  period  of  time.
Regular acid  washing  removes the  scale and  prevents its initial
build-up (5).   The  acid wash solution  used to  clean the HT/LPO
system  is  diluted with  nonpotable  water  and bled  back  into the
mainstream process.

       Scale  accumulation  usually  is  a problem and is  often  a
serious one in  the  LPO system.   During the  cool down cycle when
conditioned  sludge  passes  through  the  shell  side  of  the heat
exchanger, scale  is deposited on the outer  surface  of the inner
pipe (the tube).  When the  unit is put on standby or is shutdown,
the  cold  effluent  water  fed  through  the  heat  exchanger  for
flushing  loosens  and  fractures  some  of  this scale   from  the
surface of the  tubes and is flushed out of  the system.   Some of
the larger pieces  of scale  can become trapped in the tee sections
at the  shell bottom crossover.   Prior  to  cleaning  of  the heat
exchanger, this  trapped scale should be  removed from the shell by
high  pressure/high flow  backflushing  of   the  heat  exchanger.
After backflushing,  the system should  be  flushed with  acid to
remove any scale deposits still on the tubes.  Following the acid
cleaning  procedure,   the  operator  should   again  backflush  the
annular space between  the tube and shell of the heat, exchanger to
remove all remaining scale  deposits.

       The  frequency   of  backflushing  and  acid and  mechanical
cleaning can  vary from  a  few weeks  to a few  months,  depending
upon the sludge characteristics.   Cleaning  costs  for scale have
exceeded  manufacturers  estimates   by  several  times  in  some
facilities,    resulting  in   reduced   cleaning   frequency   or
abandonment  of  the  thermal  conditioning process.    A  system-
specific cleaning  procedure and frequency should be developed for
each plant  by  plant  personnel  in  conjunction  with  the  design
engineer and system manufacturer.   Operations personnel should be
thoroughly trained prior  to implementation of these procedures.
                               36

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Reactors

       Reactor problems  are  generally associated with  the  level
control,  steam   injection,   and   off-gas   systems.     Use   of
capacitance probes for reac'tor  level  detection should be avoided
due to the tendency of these probes to become clogged and to send
false signals.  A more dependable level control system is nuclear
source level detection.

       The  steam  injection system,  in multi-unit  systems  where
steam  supply  is  controlled  by a  single  manual  valve,  allows
preferential steam loading to occur.   To  avoid this problem,  the
use of constant pressure  regulators is recommended.

       Plugging  of  the  off-gas  line  in  an  HT reactor  is  an
operational problem  rather than a design  or  equipment problem.
The solution to this problem is for the operator to  use the steam
clean-out system on the reactor as  frequently as required to keep
the  off-gas  line  clear.   The  frequency of  cleaning  will vary
depending on system and process conditions.

       The  installation  of a  flowmeter  and throttling  valve  to
the  reactor off-gas  system  to limit  the  amount  of  hot  gases
released  from  the  system would   be an  improvement.    Manual
throttling  would  assure  sufficient  steam  injection to sustain
treatment  and  limit  the  amount   of   energy  wasted  to  the
atmosphere.

OPERATIONS PROBLEMS

       Problems  associated   with   thermal   conditioning  system
operations  are  in the areas  of management,  staffing,  training,
and process operation.

       Management  personnel   should  be  aware  of  the  technical
complexities  of  HT/LPO  systems.     Management  should  have  a
knowledge  of  system  O&M  needs,   including  expected  results,
methods  to  evaluate  performance,  and   a  system  of  personnel
accountability.

       Because  of  the complexity  of  the  thermal  conditioning
process, having an O&M staff  with  the right qualifications is an
absolute necessity.  This  staff must  have a working knowledge of
the process, the ability to troubleshoot process problems, and be
motivated   to  learn.      Salaries   commensurate   with   these
qualifications  are  also  needed  to  attract   and  retain  highly
skilled personnel.

       Quality  training  is required  for  personnel  who operate,
maintain,  and  manage these   systems.   The  training  normally

                                37

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provided by  the manufacturer during  startup is,  in  most  cases,
not sufficient  to  ensure  that  long term personnel training needs
are met.  Specialized training is required and should include:


        Basic knowledge  of routine system  startup,  shutdown and
        standby procedures

        Preventive maintenance requirements and procedures

    -   System process control theory and testing procedures

        Process troubleshooting techniques

        System economic considerations

    -   Process data collection, documentation, and evaluation

        Support   system   O&M,    control,   fine   tuning,   and
        troubleshooting.


       A number  of other  problems associated with  operation of
thermal conditioning systems, and a number of potential solutions
to  the  problems,  were  discussed  under  Design  Problems  and
Equipment  Problems in  this  section.    Although  many of  these
problems  are  related  to  the   design   of   the   system  and  the
equipment  provided,  they  can often  be  eliminated  or at  least
mitigated  by  proper  operation  of   the  thermal  conditioning
process.   In  many  instances,   properly designed systems  have
functioned poorly  due to improper operations.   Areas in  which
improved operations can enhance system performance include:


        Control of  sludge  dewaterability  through adjustment of
        cook  temperatures

        Control  of  energy  consumption  through  proper   system
        maintenance and control of cook temperatures

    -   Control of clogging  problems  through scheduled,  thorough
        cleaning of the system

        Control of odors and sidestream strengths through  control
        of sludge feed characteristics and cook temperatures, and
        through proper system maintenance.


       Thermal conditioning systems include a number of hazardous
operations.   Chief .among  these  are  system  backwashing and acid
cleaning.    The  use   of   concentrated   acids  in  acid  cleaning
requires careful attention to safety precautions  and  should not
be taken lightly by plant personnel.

                               38

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                            SECTION 4

         SUMMARY  OF  DESIGN AND OPERATIONAL CONSIDERATIONS
       As with most processes in wastewater treatment facilities,
the  selection,  design,  and  operation  of a  thermal  conditioning
system must  take  into account not only  its  integration  into the
overall  treatment process,  but  also  the   complexities  of  the
system.   Failure  to  do  this  has led  to many  of  the past  and
current  problems  that   have plagued  the  thermal  conditioning
process.   Under  the  right circumstances, it can  be  an effective
part of the sludge processing train.

       This  section   summarizes  the  more  important  design  and
operational   factors    to   review   when   considering   thermal
conditioning as  part  of  a  new or  upgraded treatment plant,  or
when optimizing the performance of an existing  facility.

IMPROVING SYSTEM DESIGN

       When planning  a thermal conditioning system,  the following
process and equipment  design factors  should be  considered:


          The characteristics of  the feed sludge to  the  thermal
          conditioning  process  are   extremely  important.     It
          should be a uniform 3  to  5  percent  solids, contain  a
          minimum of  grit and rags,  and  be  a homogeneous  mixture
          of primary,  waste  activated,   digested,  or  trickling
          filter sludges.  Waste  activated  sludge alone does not
          condition well.

          The physical layout of  the system should provide  easy
          access  to   all  equipment   components,   controls,   and
          instrumentation.    The  installation   of   all   system
          equipment on one  floor  of  a single  building is  highly
          desirable.

          Equipment selection  and   sizing  should  be carefully
          matched  to  the  rate  and  mass  of  sludge  production
          expected  from  the  mainstream   treatment   processes.
          Multiple and  standby units  should  be  provided  where
          needed  to  allow  efficient  operation   of  the  thermal
          conditioning system.


                               39

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          Instrumentation   that   ensures   continuous   process
          monitoring and  control  should  be carefully located and
          properly  installed.   Where  vibrations are  a problem,
          gauges  should   be   remotely  mounted,  with  flexible
          connections  to   the  monitoring  element,   to  extend
          instrument life.

          A blending tank  with  a  mechanical mixing system should
          be provided  to permit  a  uniform sludge mixture  to be
          fed to the system.

     -    The materials  of construction for  the heat exchangers
          should  be carefully  selected   in  recognition of  the
          characteristics  of  the  sludge   to  be  treated  and  the
          extreme  operating   conditions.     Material  selection
          should also be compatible with the cleaning system that
          will be used.

     -    Maximizing   heat  exchanger   area,   consistent   with
          maintaining  a  reasonable  pressure  drop  across  the
          exchanger,  should  be   considered.     Increasing  the
          effectiveness   of    heat    transfer   lowers   energy
          consumption.

          The collection  and treatment of  odorous off-gases from
          decant tanks and  dewatering  areas must  be  included in
          all facility designs.   The ability to  control odors is
          an  important  consideration  in  the  selection  of  a
          thermal conditioning system.

          Sidestreams  must be  fully  characterized   to  evaluate
          accurately     recycle    and     separate     treatment
          alternatives.   Pilot plant  testing may be warranted.
          If sidestreams are returned to the mainstream treatment
          train, their impact on  plant loading and capacity must
          be taken into account.

          Energy  recovery  systems  to  reduce plant-wide  energy
          consumption  should   be   incorporated  into  the  solids
          handling system design where feasible.


IMPROVING EXISTING SYSTEMS

       To improve the operation and increase the efficiency of an
existing  thermal   conditioning   system,   the   design   factors
discussed above should be  used  as a  guide  for determining needed
plant modifications  or  operational  changes.   In addition,  the
following key  factors  should  be  given careful  consideration as
means to improve system effectiveness:
                                40

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          Install equipment  to improve  grit  and rag  removal  at
          the plant headworks  and/or  in the feed sludge  flow  to
          the  thermal  conditioning  system.   This  will  reduce
          problems  of  clogging  and abrasive  wear  in the  heat
          exchanger and high pressure pumps.

          Modify or replace  existing sludge thickening equipment
          to  provide  a  uniform feed  sludge  solids  of  3  to  5
          percent.

          Consider  the installation  of hydraulic exchange  pumps
          in lieu of other high pressure  feed  pumps  if pump wear
          is excessive.

     -    Upgrade system  instrumentation to  provide  the  proper
          number, types,  and locations  of analyzers,  gauges,  and
          other instrumentation to obtain representative  process
          readings  and  to  facilitate   operator   use.     Where
          necessary, consider upgrading to provide two- or three-
          mode control  loops.

          Undertake sampling and characterization of  sidestreams
          to  determine  treatment  alternatives.    If  necessary,
          perform pilot plant  testing  of treatment  alternatives
          to determine  improved sidestream treatment.

          Investigate  the   feasibility  of  energy  recovery  and
          reuse systems,  such as using  digester gas  or waste heat
          from  incineration  for   boiler heating,   to  decrease
          energy costs.

          Maintain  the original  heat transfer efficiency of  the
          heat  exchanger   by  establishing   and   implementing   a
          routine mechanical or acid cleaning program.


DESIRABLE OPERATING CHARACTERISTICS

       The key variables for achieving  successful performance  of
HT/LPO systems include  temperature,  sludge feed,  and,  for  the LPO
system,  process  air.     These  parameters   are   an  important
consideration  in  designing  a  new  system  or  in optimizing  the
performance of an operating facility.


          Sludge dewatering  characteristics  are  directly  related
          to the temperature to which  the sludge is subjected  in
          the  reactor.     This reaction  temperature  should  be
          monitored   by    performing   a   Specific   Filtration
          Resistance  Analysis  on   the   solids   leaving   the
          process.  The cook temperature in  the reactor should  be
          kept between 350F and  400F,  and  as  low  as  possible

                               41

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          within this range,  to  reduce fuel consumption  and the
          solubilizing of BOD.

          The reaction temperature can  be  controlled by changing
          the sludge  feed rate  or the  boiler steam  production
          rate.   The  specification of  both variable speed sludge
          feed  pumps  and  steam  production rate  controls  with
          adequate operating  ranges must be  provided to properly
          control reactor temperatures.

          Process air  rates in LPO systems  should be between 0.08
          and 0.15 pounds of  air per gallon  of sludge.   A needle
          valve assembly on the  compressor discharge will permit
          this  control   and  prevent  back   surging   in  the  heat
          exchanger.


IMPROVING PLANT OPERATION AND MAINTENANCE

       As with any wastewater treatment process, efficient, safe,
and cost-effective operation  of  a  thermal  conditioning system is
dependent  upon  having  well-qualified  and  trained  personnel
working  within  a  well-managed   system   with  adequate  budget
support.     It   must  be   recognized,  however,   that  thermal
conditioning  is   a   complex  process  with   very   specialized
equipment.   These  complexities  demand that  special  attention be
given to  the staffing,   training,  and  management of  any  thermal
process to ensure safe and efficient  operation.

       Wastewater treatment plants with HT/LPO systems must also
have  an  effective  process  control  program  to  balance  solids
production with  solids   handling,  conditioning, dewatering,  and
disposal operations.   Such a program  can  minimize  startups and
shutdowns of the HT/LPO processes,  decreasing operating costs for
existing  thermal  conditioning   systems  by  at  least   10  to  20
percent of current costs.

       Special attention should be given to the following areas:


     -    A  thorough   training   program should be  provided  to
          operators of HT/LPO systems,  with  emphasis on hands-on
          training.   Instructors should have practical, hands-on
          operating experience with thermal conditioning systems.

          The training  emphasis  should be  on process control,
          special maintenance requirements, and safety.

          The training  program should  be  routinely updated and
          presented to the  operators  to reinforce  essential O&M
          concepts  and   to  minimize   the   impact  of  personnel
          turnovers.

                               42

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All treatment plants with  thermal  conditioning systems
should conduct  a detailed  evaluation of  the  complete
solids   handling  train   to   identify    areas   where
modifications  can  be  made  to  improve  the  overall
operation and reduce O&M costs.
                     43

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                            REFERENCES
1.  U.S. Environmental  Protection Agency,  "Improving  Design  and
    Operation of Heat Treatment/Low  Pressure Oxidation Systems,"
    EPA Contract No. 68-03-3208, In preparation.

2.  Orris E. Albertson,  Enviro Enterprises, Inc., Correspondence,
    March 12, 1985.

3.  Adams Jr, Carl  E.,  Davis L. Ford, and W.  Wesley Eckenfelder
    Jr.,  Development  of  Design   and  Operational  Criteria  for
    Wastewater Treatment,   Enviro Press,  Inc.,   Nashville,  TN,
    1981.

4.  U.S.   Environmental   Protection   Agency,   "Handbook   for
    Estimating   Sludge   Management   Costs,"   EPA/625/6-85/010,
    October 1985.

5.  U.S. Environmental Protection  Agency,  "Process Design Manual
    for   Sludge   Treatment   and   Disposal,"   EPA-625/1-79-011,
    September 1979.

6.  U.S.  Environmental  Protection  Agency,  "Effects of  Thermal
    Treatment of  Sludge  on  Municipal  Wastewater  Costs,"  EPA-
    600/2-78-073, June 1978.

7.  U.S. Environmental  Protection Agency,  "Municipal  Wastewater
    Treatment Plant Sludge and Liquid Sidestreams," EPA-430/9-76-
    007, June 1976, National Technical Information Service, Order
    No. PB 255-769.

8.  Dague,  Richard R., "Treatment  of  Recycle Streams from Thermal
    Sludge Conditioning," U.S. Department of Interior,  Project A-
    073-LA,  Iowa State  Water Resources Research Institute, Ames,
    Iowa, March  1983.

9.  San Mateo Water Quality Control Plant, San Mateo, California,
    June 1984 Monthly Log Sheet.
                               44

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English
ENGLISH TO METRIC UNITS

              Multiplier
CONVERSION TABLE

        Metric
British Thermal Unit,     1.055
  Btu

British Thermal Unit,     2.326
  per pound, BTU/lb

Cubic feet per day,       28.3
  cu ft/d

Degrees Fahrenheit, F    0.555  (F  -  32)
Feet per second, ft/sec   0.305
Foot, ft

Gallons per day, gpd
              0.305

              0.00379
Gallons per minute, gpm   0.063

Inch, in                  25.40

Million gallons per day,  43,800
  mgd

Pound, Ib                 0.4536

Pounds per gallon,        0.119
  Ib/gal

Pounds per square foot    0.057
  per day, Ib/sq ft/d

Pounds per square inch,   6895
  psi
Ton, ton
              907.2
                                Kilojoule, kJ
                                Kilojoule per
                                  kilogram, kJ/kg

                                Liters per day, L/d
        Degrees Centigrade,
          C

        Meters per second,
          m/s

        Meter, m

        Kiloliters per day,
          kL/d

        Liters per day, L/d

        Millimeter,  mm

        Milliliters  per
          second,  mL/s

        Kilogram,  kg

        Kilograms  per liter,
          kg/L

        Grams per  square meter
          per second, g/m.S

        Pascals,  Pa
        Kilogram,  kg
                                45
                                           *U.S. Government Printing Office: 1985484-844/32849

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oEPA
      WH-595
    United States
    Environmental Protectiot
    Agency
    Washington, DC 20460


    Official Business
    Penalty for Private Use
    $300

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