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
350°P and 400°F 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 300°F and 350°F. 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 350°F and
400°F) is retained for a period of time in the reactor and is
subsequently returned through the heat exchanger to be cooled to
approximately 120°F (about 60°F 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 300°F
and 320°F. Steam is injected directly into the reactor to
increase the sludge/air mixture temperature to between 330°F and
350°F. 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 10°F due to the
heat of reaction of the sludge, contributing to an overall
temperature increase from reactor inlet to reactpr outlet of
approximately 40°F. 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
110°F and 130°F. 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 120°F and
130°P in order to assure proper settling and reduce odor
potential. Should a processed sludge temperature of less than
120°F 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 350°F and 400°F,, 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
330°F and 350°F. 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 220°F 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 350°F and 400°F) 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.
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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.
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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.
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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 180°F for greater
solubilization of the sulfate scale. Because hydrochloric acid
will pit stainless steel and nitric acid will destroy carbon
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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.
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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
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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
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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.
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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
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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)
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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
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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 180°F. 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
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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
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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 1,400°F, 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:
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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
33
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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
102°F 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.
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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.
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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 350°F and 400°F, 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.
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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.
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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.
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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: 1985—484-844/32849
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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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