United States

Environmental Protection

Agency

Office of Municipal

Poilition Control (WH546)

Washington DC 20460

September 1985

430 /

9-85-001

&EPA Heat Treatment/Low Pressure

Oxidation Systems:

Design and Operational

Considerations
I

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

-I
I

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 r~view. 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 wastewater treatment facilities. EPA assumes no responsibility

for use of this information in a particular situation.

Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

ii

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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 condi tioning of sludge to improve dewaterability

was first used in municipal wastewater treatment facilities in

the late 1960s. Over the years, however, thermal condi tioning

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 condi tioning in a sludge treatment train, or who

are concerned wi th optimizing the performance of an existing

thermal condi tioning 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.

iii

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I

CONTENTS

EPA Review Notice .. Foreword . Acknowledgements .•. ....

......

• • • • •1.

ii

iii

vi

1

1

1

3

4

4

7

15

16

20

20

34

37

39

39

40

41

42

·.

·.

..

..;... .....

...

......

Introduction.

Purpose .. Background.

Process Problems

Process and Equipment .. Process Description •.

Equipment Description •..••••

Operational Characteristics••...•.

Process Selection and Application •.•••••

Common Problems and Solutions•.

Design Problems .••..•

Equipment Problems •.....•.

Operations Problems•••.•••

Summary of Design and Operational Considerations•• Improving System Design •.•.••••.•••••

Improving Existing Systems..••••.•...•••.••

Desirable Operating Characteristics•••.•••.

Improving Plant Operation and Maintenance.

2.

1.

3.

4.

References

English to Metric Units Conversion Table •. ·.

44

45

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I

ACKNOWLEDGEMENTS

This report was prepared for the

Protection Agency by Metcalf & Eddy, Inc.,

setts under Contract No. 68-03-3208.

u.S. Environmental

Wakefield, Massachu-

-I

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 Moni tor, 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
I

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

dewater ing 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 condi tioning systems. It is based on an investigation

which evaluated the causes for operations and maintenance

problems experienced wi,th 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 descriptions, 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 information, 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 condi tioning is a continuous flow process

in which sludge is heated to temperatures in the range between

350°F and 400°F in a reactor under pressures of 250 to 400 psig

1

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I

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 characteris~ics.

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 & Templewood 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 capaci ties

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 organization 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 limi ted to intermediate and high

pressure oxidation installations.

Seventy ei.ght 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

2

.1
I

reported not operating. Of the known operating LPO facili ties,

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 limi ted successful and costeffective 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 conditioning systems have been shut down as a result of high energy costs.

These costs can be directly related to improper ~esign 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 benefi ts such as

the potential for gas production by anaerobic digestion of decant

liquors, should all be included in such considerations.

3

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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

thiokened by thermal conditioning before dewatering and ultimate

disposal. How the process conditions and stabilizes sludge and

the equipment that compr ises a thermal condi tioning 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 SO

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 S-day biochemical oxygen demand (BODS). The

BODS produced is of primary concern in the recycle of

sidestreams, as discussed in Section 3. The solubilization of

4

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VSS and resultant BODS production for HT systems may be estimated

as follows (2):

VSS = 0.1 PS + 0.4 WAS

BODS = 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

BODS = S-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 BODS

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 BODS production are expected

to be 10 and S percent greater, respectively, than the above

estimates for HT systems.

Thermally condi tioned 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 3S0oF. 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 dewater ing device. Process off-gases can be treated by

various odor control methods as discussed in Chapter 3.

s

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I .1

RAW SLUDGE

CONTROL

VALVE

REACTOR

STEAM

LEVEL

SENSOR

SLUDGE

SLUDGE

WATER

CI RCULATING

PUMP

SLU DG E I....L-~ --.~

FEED

PUMP

GRINDER

STORAGE OR

BLENDING

TANK

BOILER TREATED

OFF-GAS BOILER

WATER

DECANl

TANK

OFF-GAS

_-"""! DEWATERING

L1aUOR

CAKE

SOURCE: EPA PROCESS DESIGN MANUAL: SLUDGE TREATMENT

AND DISPOSAL, EPA 625/1-79·011, SEPT. 1979 (MODIFIED)

FIGURE 1. HEAT TREATMENT PROCESS FLOW DIAGRN\l

6
I

Low Pressure Oxidation

A schematic diagram of the LPO system is shown in

Figure 2. Raw sludge is first. passed through a gr inder 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 betw~en 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, contiibuting to an overall

temperature increase from reactor inlet to reactpr outlet of

approximately 40°F. Detention time or "cook time" i~ 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 gr inder, 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.

7

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I

COMPRESSED AIR

RAW SLUDGE

REACTOR

SLUDGE

STANDPIPE

SLUDGE r...L-I-__-..:L..~

FEED

PUMP

GRINDER

STORAGE OR

BLENDING

TANK

SLUDGE - TOSLUDGE HEAT

EXCHANGER

STEAM

CONTROL

VALVE

SLUDGE

BOILER TREATED

..

OFF-GAS

DECANT

TANK

OFF-GAS

,.....-.... DEWATERING

BOILER

WATER

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

8
Figure 3 is an illustration of a typical LPO system which shows

most of this equipment and its relative location within a thermal

conditioning facili ty. ~his general ar r"angement 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

9

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o

BOILER SYSTEM:

HEAD TANK ----H--+.

SOFTENER

BRINE TANK

BOILER

SCHEMATIC SOURCE: ZIMPRO. INC.

ROTHSCHILD, WI

DECANT

TANK

OXIDIZED SLUDGE

TO DEWATERING

STORAGE OR

BLENDING

TANK

-

FIGURE 3. LOW PRESSURE OXIDATION SYSTEM CONFIGURATION
I I

c=J- HYDRAULIC

OIL

~SLUDGE

INLET CHECK

~ALVE

----y .,

•

I

SLUDGE I

DIAPHRAGM

BAGS --I~-

PRESSURE

VESSEL

STROKE

CONTROL

CYLINDER

PRESSURE

VESSEL

HYDRAULIC

PUMP

OUTLET CHECK

VALVE -<:>-=. SLUDGE TO

SYSTEM

_ ......._DIAPHRAGM

BAGS

STROKE

CONTROL

CYLINDER

r-<r-

•I

I SLUDGE

SOURCE: ZIMPRO, INC.

ROTHSCHILD, WI.

(MODIFIED)

flGURE4. HYDRAUUCEXCHANGEPUMP

11
I

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-watersludge 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, sludgeto-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 condi tioned 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 abras'ion from coarse

particulate mater ial at these upper bends can result in rapid

wear. Normal design velocities are:

-I

inside tube inlet

inside tube outlet

annular space inlet

annular space outlet

6 to 8 feet per second

10 to 12 feet per second

12 to 14 feet per second

8 to 9 feet per second.

Processed sludge temperature should be between 120°F and

130°F 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
I

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 requi red for proper sludge condi tioning.

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 400oF~ 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 condi tioning 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 ps~g. In the deaerator, dissolved oxygen is removed from the

13

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"'-_----=8==- SLUDGE AND AIR

MANWAV

STEAM

OFF·GAS--_-r

LINE

LEVEL SENSOR

i STANDPIPE

l ( I

STANDPIPE

)

l

1

I 1 f-'

I MANWAY

) MANWAY_ (

TO SHELL INLET OF - HEAT EXCHANGE R

STEAM

- - TO HEAT EXCHANGER

HEAT TREATMENT REACTOR LOW PRESSURE OXIDATION REACTOR

FIGURE 5. HEAT TREATMENT AND LOW PRESSURE OXIDATION REACTORS
I

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 0 P and 400°F) that will allow acceptable

dewatering. Dewaterability can be moni tored 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 temperaturf

can be achieved by two methods. One method is to increase '0

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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I

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 conditioning 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 dewaterabili ty, wi th 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 proces~ 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 condi tioning 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 addi tion of a thermal conditioning step would be an

economic asset in their sludge train.

16

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complete

excessive

thorough

I

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 condi tioning can be greatly

reduced. In general, thermal condi tioning 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

inventory of spare parts to reduce

downtime. Also, both systems require a

pre~entive maintenance program.

The unit capital cost of thermal conditioning systems

is in the range of $350 to $500 per dry ton of annual

sludge production when processing over 10,000 dry tons

per year (10 to 20 mgd of plant capacity) due to use

of multiple treatment units and standby units rather

than larger sized individual units (4). At lower

loadings, processing costs increase significantly, and

the comparatively high cost of support systems such as

boilers, air compressors, and decant tanks, makes

HT/LPO systems more costly to build than other sludge

conditioning facilities.

System Comparison

Low pressure oxidation and heat treatment offer two

alternative methods of thermally conditioning sludge. The major

difference between the two processes is that air is added to the

LPO system, offering a slight potential for reduced odor

production. Both systems may be purchased today, although HT

systems are not actively marketed.

Low pressure oxidation systems have been more widely

utilized than HT systems. This wider use is probably the result

of a more aggressive marketing strategy for the LPO system, and a

perceived reduction in the odor production potential of this

system. Aside from odor potential, neither system would appear

to have any particular technical advantage over the other.

17

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I

Advantages and Disadvantages of HT!LPO Conditioning

Previous Iiterature 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 wi th excellent dewater ing

characteristics. Cake solids concentrations of 30 to

50 percent are obtained wi th 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:

-I

The process has

complexity and

materials, such

exchangers.

high capi tal cost due to mechanical

the use of corrosion-resistant

as stainless steel, in the heat

The process .requires careful supervision, skilled

operators, and a good preventive maintenance program.

The process produces an odorous gas stream that must

be collected and treated before release.

18
I

The process produces darkly colored sidestreams with

high concentrations of organics and ammonia nitrogen.

Scale formation in heat exchangers, pipes, and reactor

requires cleaning by difficult and/or hazardous

procedures.

Subsequent centrifugal dewatering may require

continuous or intermittent polymer dosage to control

recycle of fine particles.

The daily sludge throughput of the process cannot be

adjusted by a significant amount without incurring

high energy and/or labor costs.

19
I

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

20

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I

TABLE 1. SUMMARY OF COMMON PROBLEMS

WITH THERMAL CONDITIONING SYSTEMS

Design Problems

Inappropriate materials of construction

Process sizing inconsistent with solids produced in the

wastewater treatment train

Improper sizing of storage, blending, and decant tanks

Poor physical layout of the process

Inadequate or poorly located system control instrumentation

Under/oversizing of support systems such as boilers,

circulation water pumps, and air compressors

Inadequate grit and rag handling or removal

Inadequate odor control provisions

Inadequate handling of high strength sidestreams

Equipment Problems

Loss of grinder seals

Wear of sludge feed pumps

Corrosion, plugging'and scaling of heat exchangers

Unreliable level probes and off-gas control in reactors

Operations Problems

Lack of system understanding by senior management personnel

Poorly qualified O&M staff

Insufficient operator training

Improper process control operation and system maintenance

steel, a bimetallic (carbon steel/stainless steel) system cannot

be descaled without damage to one of the metals.

The cleaning system for heat exchangers should be designed

to accommodate the range of cleaning options normally used and

should be constructed of materials compatible with those used in

the HT and LPO systems. This criterion ensures compatibility

between the acid and metals in the heat exchanger and related

piping.

21
I

Selection of materials of construction is also constrained

by the potential for corrosion in heat exchangers. The LPO

system may require more corrosion-resistant metals than the HT

system due to the possible production of more organic acids which

lowers the pH, especially where primary sludge is processed.

Aftercooler sections of sludge-water-sludge double pipe

heat exchangers with stainless steel tubes and carbon steel

shells have experienced some corrosion and leakage problems due

to use of effluent water as a cooling medium. Effluent water

with fairly high levels of dissolved oxygen at elevated

temperatures speeds up the rate of corrosion of metal.

Another corrosion concern applicable to both HT and LPO

systems is the presence and concentration of chloride in the

wastewater and sludge. "High chloride concentrations have

contributed to or caused corrosion of heat exchangers in numerous

installations. Where chloride levels are expected to exceed 300

to 400 mg/L in the raw wastewater, the possibility of developing

problems from chloride stress corrosion exists, and corrosion

resistant metals should be used. Available material types

include 316L stainless steel, nickel based alloys, higher alloyed

iron based alloys, and titanium, in order of increasing corrosion

resistance and cost. The capital cost for nickel based alloys

and higher alloyed iron based alloys is about 5 to 7 percent

greater than 316L stainless steel. The capital cost for titanium

is about 15 percent greater than stainless steel. To keep costs

as reasonable as possible, titanium or higher nickel based alloy

construction only needs to be used in the heat exchanger bundle

nearest the reactor where temperatures are highest and corrosion

potential is greatest.

Process Sizing

The capacity of HT/LPO systems should be based on a

careful estimate of sludge production rates. A low sludge

production estimate will result in an undersized system that

cannot process the total sludge produced without continuous

operation, leaving no time for preventive maintenance. A high

estimate leads to an oversized system that is only operated

several hours a week, requiring a large amount of fuel to reheat

the reactor contents. These sizing problems are due to

insufficient or inadequate design data and to lack of

consideration of operating conditions in the initial and design

years.

To minimize the problem 'of over/undersizing, thermal

conditioning systems should be designed with enough flexibility

to accommodate initial, design, and future sludge production at

minimum and maximum rates. These production rates can be

estimated using existing sludge production data if available,

mass balances using historical data and/or past treatment

22

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I

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 fl~xibility 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 addi tional uni ts 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 uni ts 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 costeffective 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 condi tioning 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: (I) 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

capaci ty. Good design includes low level alarms and automatic

23

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I

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 condi tioning and dewater ing 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 dewaterabili ty 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/ft 2/day for combined

sludges

Floor slopes greater than 2.75 inches per foot

Proper access for maintenance

Proper sealing of covers to minimize ooors

.1

Continuous

conditions.

sludge withdrawal to prevent septic

The sludge depth in decant tanks should be moni tored to

assure proper thickening. Use of a portable gauge allows easy

measurement of sludge depth. One improvement to the decant tank

is installation of a 4-inch pipe that extends down through the

roof of the tank to 4 to 5 inches below the liquid level. The

pipe is extended below the liquid level to prevent the escape of

gases. This improvement allows the operator to monitor the

sludge level without opening roof hatches and allowing odors to

escape. Installation of the pipe is not possible on tanks

equipped with surface skimming arms. On these tanks, a hinged

cap or valve can be placed on the pipe to prevent odors from

escaping when sludge levels are not being measured and the end of

the pipe can be located above the liquid level. Measurements are

made after passage of the slowly rotating skimmer arms.

24
Physical Layout of the Process

For effective operation of thermal conditioning systems,

operators should be able to moni tor 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 ar r angemen t 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 capi tal 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 con~itioning system to provide the operator with

25

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I -II

TABLE 2. RECO~iENDED 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

Ci~culation tank, pressure

Circulation water, flow

Instrument air press~re! high and low

Deaerator level, high and low

Decant tank temperature, high (optional)

Recorded Instrument Readings

Reactor pressure

Sludge flow

Steam flow

Reactor level

Steam pressure

Reactor inlet and outlet temperature

Circulation water crossover temperature (optional)

Process air flow (LPO only)

Digital Readout and/or Recorder

Sludge inlet temperature to heat exchanger

Circulation water inlet and outlet temperature

Heat exchanger, sludge outlet temperature

Deaerator temperature

Additional

Flow meter for reactor off-gas, for economic control to

monitor steam losses from reactor (more critical on larger

units)

Reliable flow and density meters for feed sludge

Needle valve assembly on air compressor (LPO only)

26
I

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 instrumentation. 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 proportionalintegral-derivative (proportional-reset-rate action) control loop

should be supplied. For the LPO system, a 2-mode proportionalintegral (proportional-reset) control loop shodld be supplied.

Sizing of Support Systems

HT/LPO system boilers should be sized to account for less

than optimal heat exchanger efficiencies. In addi tion, 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 boile'r

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 e'nsure

27

.1
I

that they do not limi t 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 condi tion 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

uni ts. Because these pumps require frequent maintenaI?-ce 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 airto-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 uni t wi th a 5 percent

nitric acid solution heated to l80 o P. The intermittent use of

this support system makes equipment duplication unnecessary.

Normally, one system will be adequate for multiple LPO system

installations.

Process control valves (PCV's) in LPO systems should be

large enough to release the process byproducts through one valve.

An additional valve of the same size should be provided as

standby. Because these valves frequently become restricted with

28

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residue from the process, LPO systems cannot be operated reliably

without a standby PCV.

Multiple LPO system installations may still require a

certain amount of standby support system capacity. However, with

adequate cross connection of equipment, much of the duplication

can be avoided. In either single or multiple LPO unit

installations, O&M personnel should not allow standby equipment

to remain idle for long per iods 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-watersludge 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-tosludge heat exchanger installed in a vertical position to

facilitate the turbulent flow of sludge and air. Stabilizers are

installed in the annulus between the two pipes to prevent

vibration of the inner pipe. Rags and other foreign materials

that have not been sufficiently ground up will accumulate on

these stabilizers and lead to plugging problems.

In plants where rags pass through influent screens and

comminuting devices and are left in the flow stream, the problem

of plugged equipment is extremely severe. Shredded rags can

reconstitute into long rope-like formations that are pumped with

the sludge into the thermal conditioning system. Rags should be

removed from the process flow stream to avoid the costly and time

consuming process of later removing them from HT/LPO system

pipes, valves and heat exchangers.

As with most wastewater treatment process equipment, the

passage of grit through piping, pumps, valves and internal

equipment components is never a good practice due to abrasion on

these components. In the HT system, the abrasive grit problem is

29

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I

compounded by the high veloci ties exper ienced 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 gr it

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.

Degr i tting 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 gr'i t 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 wi th thermal condi tioning 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 o P, 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

scrubbing or a multiple chemical system,

some combination of the following:

30

include hypochlorite

which will consist of
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 wi th 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 dewa tering 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 capi tal 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).

31

-II
I

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 ni trogen. '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 condi tioning

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 compar ison wi th

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 wi th data from past treatment exper ience. 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

32
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 dur ing 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 sui table for

fueling the conditioning processes. The use of anaerobic filters

has also been tested w~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 4.6 6,660 14,640 7.12 1,110 4,590 83 80

Minimum 4.2 4,870 8,680 6.99 700 2,660 86 69

Average 4.4 5,883 11,171 7.06 930 3,145 84 72

The average feed to the anaerobic digester was between

50,000 and 60,000 gallons per day. Mixed liquors were between

33

-I
1,200 and 1,500 mg/L. The mass was continuously circulated and

no problems developed with accumulating bottom debr is. 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 wi thin 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 wi th a piston pump produces a flowchart of

peaks and valleys reflecting pump pulsations, the record is

useful in estimating average flow and in monitoring dropoff

trends in flow rate.

34

-II
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 gr i t 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 cavi ty 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 shoulp 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 wi th these substances.

Polymer dosages should be carefully monitored and controlled to

35

.11
I

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 hav~ hard' water or certain

industrial contributions (5,7). The inverse solubility of

calcium sulfate wi th temperature can be a serious problem wi th

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 tqe 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 systemspecific cleaning procedure and frequency should be developed for

each plant by plant personnel in conjunction wi th the design

engineer and system manufacturer. Operations personnel should be

thoroughly trained prior to implementatiQn of these procedures.

36
I

Reactors

Reactor problems are generally associated with the level

control, steam injection, and off-gas systems. Use of

capaci tance 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 limi t 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

-I

Problems associated with thermal

operations are in the areas of management,

and process operation.

conditioning system

staffing, training,

Management personnel should

complexities of HT/LPO systems.

knowledge of system O&M needs,

methods to evaluate performance,

accountability.

be aware of the technical

Management should have a

including expected results,

and a system of personnel

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

maintain, and

training is required for personnel who operate,

manage these systems. The training normally

37
I

provided by the manufacturer dur ing 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

-I

Support system

troubleshooting.

O&M, control, fine tuning, and

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. Al though 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

cook temperatures

dewaterability through adjustment of

Control of energy consumption through proper system

maintenance and control of cook temperatures

Control of clogging problems through scheduled, thorough

cleaning of the system

Control of odors and sidestream strengths through control

of sludge feed characteristics and cook temperatures, and

through proper system maintenance.

Thermal conditioning systems include a number of hazardous

operations. Chief among these are system backwashing and acid

cleaning. The use of concentrated acids in acid cleaning

requires careful attention to safety precautions and should not

be taken lightly by plant personnel.

38
SECTION 4

SUMMARY OF DESIGN AND OPERATIONAL CONSIDERATIONS

As with most processes in wastewater treatment facilities,

the selection, design, and operation of a thermal condi tioning

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

operatiqnal factors to review when considering thermal

condi tioning 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

condi tion 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
I

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 permi t a uniform sludge mixture to be

fed to the system.

.1

The materials of construction for

should be carefully selected in

characteristics of the sludge to

extreme operating conditions.

should also be compatible with the

will be used.

the heat exchangers

recognition of the

be treated and the

Material selection

cleaning system that

Maximizing heat exchanger area,

maintaining a reasonable pressure

exchanger, should be considered.

effectiveness of heat transfer

consumption.

consistent with

drop across the

Increasing the

lowers energy

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 addi tion, the

following key factors should be given careful consideration as

means to improve system effectiveness:

40
I

Install equipment to improve gri t 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 threemode 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.

-I

Investigate the feasibility of energy

reuse systems, such as using digester gas

from incineration for boiler heating,

energy costs.

recovery and

or waste heat

to decrease

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
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 stearn production

rate. The specification of both variable speed sludge

feed pumps and stearn 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

condi tioning is a complex process wi th very specialized

equipment. These complexi ties 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 wi th solids handling, condi tioning, 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.

-II

The training

presented to

concepts and

turnovers.

program should be routinely updated and

the operators to reinforce essential O&M

to minimize the impact of personnel

42
I

All treatment plants with thermal conditioning systems

-I

should conduct a detailed evaluation of

solids handling train to identify

modifications can be made to improve

operation and reduce O&M costs.

43

the complete

areas where

the overall
I

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.

I

4. U.S. Environmental

Estimating Sludge

October 1985.

Protection

Management

Agency,

Costs,"

"Handbook for

EPA/625/6-85/010,

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," EPA600/2-78-073, 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 A073-LA, Iowa State Water Resources Research Institute, Ames,

Iowa, March 1983.

9. San Mateo Water Quality Control Plant, San Mateo, California,

June 1984 Monthly Log Sheet.

44
I

ENGLISH TO METRIC UNITS CONVERSION TABLE

-II

English Multiplier

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, of 0.555 (OF - 32)

Feet per second, ft/sec 0.305

Foot, ft 0.305

Gallons per day, gpd 0.00379

Gallons per minute, gpm 0.063

Inch, in 25.40

Million gallons per day, 43,800

mgd

Pound, lb 0.4536

Pounds per gallon, 0.119

lb/gal

Pounds per square foot 0.057

per day, lb/sq ft/d

Pounds per square inch, 6895

psi

Ton, ton 907.2

45

Metric

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, rom

Milliliters per

second, mL/s

Kilogram, kg

Kilograms per liter,

kg/L

Grams per square ~eter

per second, g/m .S

Pascals, Pa

Kilogram, kg

trU.S. Government Printing Office: 1985-484-844/32849
&EPA

WH-595

United States

Environmental Agency Protectiol

Washington, DC 20460

0pfficial Busine.ss

$300 enalty for

Pr1vate Use

•