EPA-450/3-84-010
Second Review of Standards of
Performance for Sewage Sludge
Incinerators
Emission Standards and Engineering Division
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
March 1984
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This report has been reviewed by the Emission Standards and Engineering Division, Office of Air Quality
Planning and Standards, Office of Air and Radiation, Environmental Protection Agency, and approved for
publication. Mention of company or product-names dqes not constitute endorsement by EPA. Copies are
available free of charge to Federal employees, current contractors and grantees, and non-profit
organizationsas supplies permitfrom the Library Services Office, MD-35, Environmental Protection
Agency, Research Triangle Park, NC 27711; or may be obtained, for a fee, from the National Technical
Information Service, 5285 Port Royal Road, Springfield, VA 22161.
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TABLE OF CONTENTS
Chapter ' Page
1.0 EXECUTIVE SUMMARY 1-1 ,
1.1 BACKGROUND 1-1
1.2 EMISSIONS CHARACTERISTICS OF SLUDGE INCINERATORS . . 1-2
1.3 CURRENT EMISSION LEVELS ACHIEVABLE 1-3
1.4 COSTS OF EMISSIONS CONTROL 1-4
1.5 COINCINERATION WITH MUNICIPAL REFUSE . 1-4
2.0 DESCRIPTION OF THE INDUSTRY 2-1
2.1 INTRODUCTION 2-1
' 2.2 CHARACTERIZATION OF THE INDUSTRY .' .2-1
2.2.1 Number and Location of Sewage Sludge
Incinerators. 2-2
2.2.2 Amount of Sludge Incinerated 2-4
2.2.3 Prevalence of Alternative Incineration .
Techniques. 2-4
2.2.4 .Number and Type of. Incinerators Installed
Since 1978. . .'. -.'.... 2-5
2.2.5 Growth in the Use of Incineration as a
Sludge Disposal Technique ; 2-8
2.3 -PROCESS DESCRIPTIONS . -. . 2-8
2.3.1 Process Overview 2-8
2.3.2 'Multiple-Hearth Incinerators . 2-11
2.3.3 Fluidized-Bed Incinerators 2-17
2.3.4 Electric Incinerators 2-20
2^3.5 Other Incinerator Designs 2-20
2.4 REGULATORY BACKGROUND. ' . . 2-22
2.4.1 Selection of Sewage Sludge Incinerators for
NSPS 2-22
2.4.2 Current NSPS for Sewage Sludge Incinerators . 2-23
2.4.3 State Regulations 2-25
3.0 EMISSION CHARACTERISTICS, STATUS OF CONTROL TECHNOLOGY,
AND COMPLIANCE STATUS OF SEWAGE SLUDGE INCINERATORS . . 3-1
3.1 INTRODUCTION AND SUMMARY OF FINDINGS 3-1
3.1.1 Introduction ' 3-1
3.1.2 Summary of Findings 3-2
3.2 EMISSION CONTROLS APPLIED TO SEWAGE SLUDGE
INCINERATORS 3-4
3.2.1 Control Technologies Applied Prior to 1978. . 3-4 .
3.2.2 Control Technologies Applied After 1978 . . . 3-6' '
3.-2.3' Venturi/Impingement-Tray Scrubber
Description 3-6
m
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TABLE OF CONTENTS (Continued)
Chapter
Page
3.3 UNCONTROLLED EMISSIONS FROM SEWAGE SLUDGE
INCINERATORS 3-9,
3.3.1 Uncontrolled Emission Characteristics of
Sludge Incinerators 3-9
3.3.2 Factors Affecting Uncontrolled Particulate
Emission Rates from Sewage Sludge
Incinerators 3-14
3.4 ACHIEVABILITY OF THE STANDARD 3-18
3.4.1 Compliance Experience of Incinerators
Installed Prior to 1978 . 3-18
3.4.2 Compliance Experience of Incinerators
Installed After 1978 3-24
3.5 EMISSIONS OF TRACE ELEMENTS FROM SEWAGE SLUDGE
INCINERATORS . . . 3-33
3.5.1 Data Sources and Methods of Analysis 3-33
3.5.2 Uncontrolled Emissions of Trace Elements. . . 3-34
3.5.3 Controlled Trace Element Emissions 3-36
3.5.4 Control Efficiencies for Trace Element
- Emissions 3-38
3.6 NATIONAL EMISSIONS FROM SEWAGE SLUDGE INCINERATORS . 3-38
4.0 CONTROL COSTS . . ' - .
4.1 INTRODUCTION AND SUMMARY 4-1
4.1.1 Introduction 4-1
4.1.2 Summary of Findings 4-1
* 4.2- COST COMPONENTS 4-1
4.3 CAPITAL AND ANNUALIZED COSTS . . . . 4-6
4.3.1 Multiple-Hearth Incinerator Control Systems . 4-10
4.3.2 Fluidized-Bed Incinerator Control Systems . . 4-13
4.4 COST EFFECTIVENESS OF CONTROLS 4-13
5.0 COINCINERATION OF SEWAGE SLUDGE AND MUNICIPAL REFUSE. . . 5-1
5.1 INTRODUCTION AND SUMMARY OF FINDINGS 5-1
5.1.1 Introduction 5-1
.5.1.2 Summary of Findings 5-1
5.2 DESCRIPTION OF COINCINERATION TECHNOLOGIES 5-3
5.2.1 Incineration of Dewatered Sludge in a
Conventional Refuse Incinerator 5-3
5.2.2 Coincineration of Pre-Dried Sludge in a
Conventional Refuse Incinerator 5-3
5.2.3 Combustion of Refuse in a Multiple-Hearth-
Sludge Incinerator 5-6
5.2.4 Combustion of RDF in a Fluidized-Bed Sludge
Incinerator 5-6
5.2.5 Starved-Air Combustion (Pyrolysis) 5-7
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TABLE OF CONTENTS (Continued)
Chapter ' -. paqe
' 3
5.3 REVIEW OF COINCINERATION PROJECTS IN THE U.S .... 5-8
5.4 ECONOMIC AND INSTITUTIONAL CONSIDERATIONS. ..... 5-9,
5.4.1 Costs for Coincineration 5-9
5.4.2 Institutional Factors Affecting
Coincineration 5-13
5.5 PROSPECTS FOR GROWTH OF COINCINERATION 5-14
5.6 EMISSION CHARACTERISTICS OF COMBINED SLUDGE
AND REFUSE INCINERATION 5-14
5.6.1 Control Technologies Used 5-15
5.6.2 Emission Test Data 5-15
5.7 REGULATORY ISSUES 5-18
5.7.1 'NSPS'Applied to. Former, Existing, and
Planned Coincineration Facilities ...'.. 5-21
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LIST OF TABLES
Table
2-1
2-2
2-3
2-4
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
4-1
Sewage Sludge Incinerators Installed in the U.S. Since
1978
Incineration Facilities Currently Under Construction in
the U.S
Summary of Emissions Testing of Sewage Sludge
Incineration for NSPS Development
Summary of State Monitoring and Reporting Requirements .
Distribution of Emission Control Technologies Applied
To Selected Sewage Sludge Incinerators Prior To 1978 .
Distribution of Emission Control Technologies Applied
To Sewage Sludge Incinerators After 1978
Uncontrolled Emission Data for Sewage Sludge
Incinerators . .
. .-».*
Summary of Emissions Data fo'r Incinerators Reviewed in
1978 ;
Summary of Emissions Data for Incinerators 1 and 2 at
the Merrimack Site
Compliance Status of Sludge Incinerators That Have
Begun Operating Since 1978 '
Summary of Emissions Tests on Incinerator in Providence,
Rhode Island
Uncontrolled Trace Element Emissions from Sewage Sludge
Incinerators
Controlled Trace Element Emissions from Sewage Sludge
Incinerators ,
Control Efficiencies for Trace Element Emissions from
Sewage Sludge Incinerators
Equipment Specifications for Venturi/ Impingement-Tray
Scrubber Control System
Page
2-6
2-7
2-24
2-26
3-5
3-7
3-11
*
3-19
3-22
3-25
3-30
3-35
3-37
3-39
4-3
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LIST OF TABLES (Continued).
Table " . Page
4-2 Capital Cost Components 4-4-
4-3 Equipment Installation Factors 4-5
4-4 Operating and Maintenance Cost Components 4-7
4-5 Unit Costs Used in O&M Cost Calculations 4-8
4-6 Annualized Cost Components -4-9
4-7 "Operating Parameters for Model Multiple-Hearth Sewage
Sludge Incinerators and Control Systems 4-11
4-8 Capital Costs for Model Multiple-Hearth Incinerator
Control Systems (January 1983$) 4-12
4-9 Actualized Costs for Model Multiple-Hearth Incinerator
Control Systems'(January 1983$) " 4-14
4-10 Capital, Operating-,,, and. Annual ized Costs for Multiple-
Hearth Sludge Incinerator Control Systems
(January 1983$). .-...: 4-15
4-11 Operating Parameters for Model Fluidized-Bed Sewage
Sludge Incinerators and Control Systems 4-16
4-12 Capital Costs for Model Fluidized-Bed Incinerator
Control Systems (January 1983$) ' 4-17
4-13 Annualized Costs for Model Fluidized-Bed Incinerator
Control Systems (January 1983$) 4-18
4-14 Capital, Operating, and Annualized Costs for Fluidized-
Bed Sewage Sludge Incinerator Control Systems 4-19
4-15 Cost Effectiveness of Multiple-Hearth Sewage Sludge
Incinerator Control Systems. 4-21
4-16 Cost Effectiveness of Fluidized-Bed Sewage Sludge
Incinerator Control Systems 4-22
5-1 Summary of Coincineration Facilities in the U.S 5-10
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LIST OF TABLES (Continued)
Table . . Page
5-2' Waste Feed Rates and Method During Tests on Contra Costa
Multiple-Hearth Incinerator 5-16
5-3 Current Basis for Determining the Applicability of the
NSPS to Incinerators 5-20
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LIST OF FIGURES
Figure Page
1-1
2-1
2-2
2-3
2-4
3-1
3-2 '
5-1
5-2 '
5-3
Cost Effectiveness of Sludge Incinerator Particulate
Control at a Pressure Drop of 40 Inches W.G
Location of Currently Operating Sewage Sludge
Incineration Facilities in the U.S
Cross-Sectional View of a Multiple-Hearth Sewage Sludge
Incinerator
Cross-Sectional View of a Fluidized-Bed Sewage Sludge
Incinerator "
Cross-Sectional View of an Electric Sewage Sludge
Incinerator
Cross-Sectional View of a Venturi/Impingement-Tray
Scrubber
Multiple-Hearth Incinerator Emissions Versus Scrubber
Pressure Drop ^
Cross-Sectional View of a Mass--Burriing Municipal Refuse
Incinerator
Summary of Emissions Tests on the Contra Costa
Multiple-Hearth Incinerator
Summary of Particulate Emissions from Two Municipal
Refuse Incinerators
,
1-5
2-3
2-12
- 2-18
2-21
3-8
3.r23
5-5
5-17
5-19
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1.0 EXECUTIVE SUMMARY
The objective of this report is to review the New Source Performance^
Standard (NSPS) of 1.3 pounds of particulates/ton dry sludge input and the
opacity standard of 20 percent for the incineration of sewage sludge
(Subpart 0 40 CFR 60). This standard is reviewed by gathering and
summarizing information for sewage sludge incinerators built since the
standard was last reviewed in 1978. The achievability, applicability, and
need for revision of the standard is evaluated in light of these data.
Selected data for incinerators built prior-to 1978 are also presented and
discussed in this report. '
1.1 BACKGROUND .
In 1982, the actual wastewa.ter input into sewage treatment, plants was
just under 27,000 million gallons per day (MGD); Approximately 15 percent
(4,525 MGD) of this wastewater flow entered plants capable of incinerating
the sludge generated in the-process of treating these wastewate'rs'. It is
estimated that between 1.1 and 1.5 million dry tons of sludge is incinerated
annually in the U.,S.
Since 1934 when incineration was fi'rst used 'as a sludge disposal
. technique, it is estimated-that over 400 sludge incinerators have been
built. Current estimates -show that there are approximately 150 wastewater
treatment plants capable of incinerating all, or part, of their sludge
production. Since many facilities use more than one incinerator, a
substantially greater number of "individual incinerators are likely to exist.
Since the last NSPS review in 1978, it is estimated that at least 23
new sludge incineration facilities have been installed. Approximately
70 percent of these facilities use multiple-hearth incinerators, 15 percent
r use fluidized-bed incinerators, and 15 percent use electric incinerators.
Between 1984 and 1989, it is estimated that 18 new installations will come
on line and be subject to the provisions of the NSPS.
1-1
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Twenty-two states treat sewage sludge incinerators as a distinct source
category. In these states, the federal NSPS is applied. Most other states
have general standards that encompass incineration of all types of municipal
wastes and refuse. These standards are usually less stringent than the
existing NSPS for sludge incinerators. Of 11 states surveyed, only
Massachusetts and Connecticut have existing monitoring and reporting
requirements for sewage sludge incinerators.
1.2 EMISSIONS CHARACTERISTICS OF SLUDGE INCINERATORS
Uncontrolled emissions from sewage sludge incinerators can vary from
less than 10 Ib/ton dry sludge to over 400 Ib/tdn dry sludge. In general,-
uncontrolled emission characteristics are a function of the incinerator
type, sludge characteristics, and the operating practices used at individual
incinerators. Uncontrolled emissions from multiple-hearth incinerators are
typically about 50 Ib/ton while uncontrolled emissions from fluidized-bed
incinerators average about 88-lb/ton.- For individual incinerators, actual
uncontrolled emissions can vary substantially from these v-a-lues depending on
the sludge quality a'nd operating practices" used.. However, no quantitative
correlation has been identified between specific operating parameters and
uncontrolled particulate emissions.
* \ t *
Sewage sludge incinerators also emit potentially toxic trace elements.
Data for 12 incinerators indicate that emissions of trace elements are
highly variable. Controlled emissions of cadmium range from 0.003 Ib/dry
ton sludge to 0.06 Ib/dry ton. The highest controlled trace element
emissions were for lead, which range from 0.002 to 0.16 Ib/ton, and average
0.05 Ib/dry ton sludge. Data on uncontrolled trace element emission rates
from six incinerators average 0.03 Ib/ton for cadmium, 0.18 Ib/ton for
chromium, 0.08 Ib/ton for nickel, 0.45 Ib/ton for lead, and 0.02 Ib/ton for
arsenic. The efficiency of control devices in reducing trace element
emissions from sludge incinerators is generally less than that for total
particulates.
For multiple-hearth and fluidized-bed incinerators built since 1978,
the predominant control technology for particulate emissions are combination
1-2
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venturi/impingement-tray scrubbers. These devices were applied prior to
1978, but their use has become more widespread in recent years. For the 17
multiple-hearth incinerators built since 1978, scrubber pressure drops range
from 10 to 45 inch W.G. Of 'the four new ,f"Iuidized-bed incinerators
installed since 1978, three are equipped with combination venturi/
impingement-tray scrubbers. New electric incinerators are most often
equipped with individual venturi scrubbers. Pressure drops for scrubbers
used on the four electric incinerators built since 1978 are less than
10 inch W.G.
1.3 . CURRENT EMISSION LEVELS ACHIEVABLE
New sewage sludge incinerators, when correctly operated and equipped
with an appropriate control device, can achieve the existing New Source
Performance Standards. Of the 17 multiple-hearth incinerators that have
begun operating in the past five year.s, 11 are officially in compliance with
the NSPS. Four new units have not yet been tested. The remaining multiple-
hearth incinerator, located in Providence, Rhode Island^ has demonstrated
the capability to meet the NSPS, but has not-yet-'official!y complied with
the standard. All of the four flui-di zed-bed sludge incinerators installed
since 1978 are in compliance with the standard. Of the four electric
incinerators installed since 1978, two were unable to achieve the NSPS.
However, neither of these units is equipped with a scrubber capable of being
operated at a pressure drop considered to represent Best Available Control
Technology. One electric incinerator is officially in compliance, while
another has not yet been tested.
For the 17 multiple-hearth incinerators that have been affected by the
NSPS since 1978, the average emission rate is 0.76 Ib/dry ton sludge input.
If the Providence, Rhode Island incinerator is excluded, the average
emission rate for multiple-hearth incinerators in compliance with the NSPS
is 0.67 Ib/dry ton. This is approximately one-half of the allowable
emission rate. The data indicate that many multiple-hearth incinerators are
capable of reducing emissions to well below the current NSPS level. The
average emission rate achieved by fluidized-bed incinerators affected by the
1-3
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NSPS since 1978 is 0.74 Ib/dry ton sludge. Emission rates for new electric
incinerators average .2.22 Ib/dry ton sludge.
1.4 COSTS OF EMISSIONS CONTROL
The cost effectiveness of controlling particulate emissions from sewage
sludge incinerators is estimated to range from $191 to $1743 per ton
removed. These costs are based on conservative capital cost estimates for
venturi/impingement-tray scrubbers operating at pressure drops of from 20 to
40 inches W.6. Cost effectiveness is most sensitive to incinerator size.
Scrubber pressure drop has a small impact on overall cost effectiveness.
For each 10 inch W.G. change in pressure 'drop, a change in cost effectiye-
; * * " *
ness on the order of $10 is,achieved. Figure 1-1 summarizes the estimated
cost effectiveness of controlling particulate emissions from both fluidized-
bed and multiple-hearth sewage sludge incinerators at a scrubber pressure
drop of 40 inches W.G.
* .
1.5 COINCINERATION WITH MUNICIPAL REFUSE
At the present time, there -is no-explicit statement in either Subpart 0
or Subpart E that defines which standard is to be applied in cases where
sewage sludge is coincinerated with municipal refuse. Although about 23
facilities have coincinerate'd sewage sludge and municipal refuse at one time
or another in the U.S., only 3 facilities have been identified as being
operational over the next 5 years. In each case, sewage sludge will be
coincinerated in a conventional refuse incinerator. Electrostatic
precipitators will be employed to control particulate emissions at all three
of these coincineration facilities. Insufficient data are available to
indicate how coincineration will affect the particulate emissions from these
incinerators.
1-4
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2000-t
1750
1500
01
o
* 1250
1000-
0
'
750: 7
UJ
o
o
500'
250
Multiple-Hearth Incinerator
control system
O Fluidized-bed incinerator-
control system
j
0.25 0.5
.1.0 2.0 3.0
INCINERATOR CAPACITY (Ibs dry sludge/hr)
4.0
Figure 1-1. Cost Effectiveness of Sludge Incinerator Particulate
Control at a Pressure Drop of 40-inches W.6.
1-5
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2.0 DESCRIPTION OF THE INDUSTRY
2.1 INTRODUCTION
Sewage sludge incinerators are subject to particulate emission limits
of 1.3 pounds per ton of dry sludge input and limited to a maximum stack
opacity of 20 percent, as promulgated under subpart 0 of the New Source
Performance Standards (NSPS). As part of the review of the NSPS, this
chapter provides background information on the number and location of sewage
sludge incinerators in the U.S., the types of incineration technologies
employed, as well as on the. initial development of the standard. State
regulations applicable to sludge incinerators are also reviewed.
In Section 2.2, the indus.try is broadly characterized. Information on
the number and location of sludge incinerators, as well as an estimate of
the- amount of sludge incinerated, is presented. The prevalence of alterna-
tive incineration techniques, and the growth in the use of incineration, are
also discussed in the first section.
Section 2.3 provides detailed descriptions of the major technologies
employed to incinerate sewage sludge. Both design and operating charac-
teristics of these incinerators are discu-ssed.
In Section 2.4 of this chapter, background information on the
development of the NSPS for sewage sludge incinerators is presented. The
technical basis of the original standard is reviewed, as are the subsequent
revisions made to the standard. State regulations, particularly those
relating to monitoring and reporting requirements, are also reviewed in this
section.
2.2 CHARACTERIZATION OF THE INDUSTRY
Over 33,000 publicly owned sewage treatment works are currently
operating in the U.S. These plants have a combined capacity to treat over
35,000 million gallons of municipal wastewaters each day. In 1982, the
actual wastewater input into sewage treatment plants was just under
2-1
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27,000 million gallons per day (MGD), 'representing a capacity utilization of
76 percent. Approximately 15 percent of the total 1982 wastewater flow
entered plants that are capable of incinerating all, or a portion of the
sludge generated in the process of treating these wastewaters. However,
nearly all plants employ more than'one sludge disposal technique, and some
incinerators are not currently operating.
2.2.1 Number and Location of Sewage Sludge Incinerators
There are two main sources of information on the number and locations
of sewage sludge incinerators in the U.S. The first is the NEEDS survey
conducted biennially by EPA in compliance with Sections 205(a) and 516(b)(2)
1
of the Clean Water Act. The survey encompasses more than 32,0.00 existing
and planned publicly owned treatment works (POTW) in the U.S. Second, a
survey of incineration facilities has recently been completed as part of
work conducted by EPA's Sludge Task Force.
The EPA Sludge Task Force utilized the NEEDS survey as a starting
point.2 ' However, the Sludge Task Force validated the NEEDS data through
contacts with all of the regional offices of EPA, with state and local
agencies, vendors'of sludge incinerators, as-well as individual 'plants. The
EPA Sludge Task Force continues to update their data on a regular basis, and
is considered to be the most reliable source of information on the number
and location of sludge incinerators currently operating in-the U.S.
The latest update (July, 1983) of the Task Force survey lists 153
treatment plants that are incinerating all, or part, of their sludge
production. Neither the NEEDS data, nor the update prepared by the EPA
Sludge Task Force, list the number of individual incinerators. Since many
treatment plants in the U.S. utilize more than one incinerator (for example,
the wastewater treatment plant in Indianapolis, Indiana, operates 8
incinerators), a substantially greater number of individual incinerators is
implied.
The locations of the plants employing incineration are shown in
Figure 2-1. The largest concentrations of sludge incineration facilities.
are found in the Northeast and along the Great Lakes.
2-2
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2-3
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2.2.2 Amount of Sludge Incinerated
No precise data are available on the amount of sludge incinerated
annually in the U.S. As part of their project, the EPA Sludge Task Force has
estimated that seven million dry tons of sludge are produced annually by
3 '
wastewater treatment plants in the U.S. Of this total, the Task Force
estimates that between 15 and 22 percent is disposed of through
incineration. On this basis, the total amount of sludge incinerated
annually is between 1.1 and 1.5 million dry tons.
This estimate can be confirmed on the basis of the amount of wastewater
entering plants that employ incineration as a disposal technique. The
.corresponding flow of wastewaters into the incineration facilities listed in
the EPA Sludge Task Force survey is 4,525 MGD. Although the amount of
sludge generated per gallon of wastewater treated can vary greatly as a
function of the specific treatment processes employed, an average value of
0.65 dry tons of sludge per million gallons of wastewater was deriv.ed from
35 POTW's that employ incineration. Applying this value to the wastewater
in-flow given by the Sludge Task Force, yields about 1.1 million dry tons of
sludge incinerated annually.
Since it is not known precisely how much of the sludge generated at
treatment plants that are.equipped with incinerators is actually disposed of
in this manner, the lower end of the range estimated above (1.1, million
tons/year) is considered the most reliable.
2.2.3 Prevalence of Alternative Incineration Techniques
A variety of different technologies are available for incineration of
municipal sewage sludge. By far the most common is the multiple-hearth
furnace (MHF). Of the 153 incineration plants listed by the Sludge Task
Force, 120 (78 percent) employ multiple-hearth incinerators. Flu'idized-bed
furnaces (FBF) account for most of the additional incinerators currently
operating in the U.S. The Sludge Task Force lists 24 treatment plants that
employ fluidized-bed incinerators (about 16 percent of the total). Electric
(infrared) incinerators are also sometimes used for disposing of sewage
s.ludge, particularly in-smaller rural communities. EPA's Sludge Task Force
identified six treatment plants that utilize electric furnaces. The Sludge
2-4
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Task Force data also list'one plant that employs a rotary kiln incinerator.
The individual technologies available for incinerating sewage sludge are
discussed in more detail in Section 2.3.
2.2.4 Number and Type of Incinerators Installed Since 1978
Since this report focuses on the compliance status of incinerators
installed after the last review of the standard in 1978, a survey was
conducted to identify all sludge incinerators that have either commenced
operation over the past five years, or that are under construction.
Incineration facilities affected by the NSPS that were installed prior to
1978 were discussed in the previous review. The survey was conducted in
three stages.
First, a questionaire was sent to all ten regional offices of EPA.
Information was requested for incinerators built since 1978 on the location,
capacity, and design of each incinerator as well its associated emissions
control equipment. In addition, the regional. EPA offices were requested to
provide emissions data for these units. Responses were obtained from eight
of the ten regional EPA offices, identifying a total of 16.incinerators
« *
built since 1978 and 7 under construction. Complete information was
available for only a few of these units, however.
Therefore, follow-up telephone contacts were made to regional, state,
and local air pollution control agencies. In all; over 40 individuals were
contacted during the second stage of the survey. Further information was
collected on the incinerators identified in the written survey and an
additional nine new (i.e. operating since 1978) incinerators were
identified. All 23 new plants identified in the third and final stage of
the survey were contacted in order to obtain more detailed information on
actual operating parameters at these facilities.
The results of the survey are presented in Tables 2-1 and 2-2. Of the
25 new incinerators listed in Table 2-1, nearly 70 percent utilize
multiple-hearth furnaces. This is consistent with.data for the total U.S.
population of sludge incinerators. Also, the incinerators installed since
1978 are concentrated geographically in the Northeast and Great Lakes
regions of the U.S. Of the 7 incinerators listed in Table 2-2 as currently
under construction in the U.S., 4 utilize the multiple-hearth design.
2-5
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TABLE 2-1. SEWAGE SLUDGE INCINERATORS INSTALLED IN THE U.S.
SINCE 1978
Location
Petersburg, Alaska
Wrangell, Alaska
Marietta, Georgia
Oahu, Hawaii
Cedar Rapids, Iowa
Kansas City, Kansas
Cynthiana, Kentucky
Kenton County, Kentucky
Attleboro, Massachusetts
Battle Creek, Michigan
'Bay County, Michigan
St. Paul, Minnesota
Independence, Missouri
Atlantic City, New Jersey
Amherst, New York
Hamburg, New York
N. Tonawanda, New York
Niagra County, New York
Rocky Mount, N.. Carolina.
Cleveland, Ohio
Youngstown, Ohio
Providence, Rhode Island
Arlington, Virginia
Design Type
Electric
Electric
Multiple-
Multiple-
Multipie-
Flu id-Bed
Electric
Multiple-
Multiple-
Multipie-
Electric
Multiple-
Fluid-Bed
Mu'ltiple-
Multiple-
Fluid-Bed
Multiple-
Multiple-
Multiple-
Multiple-
Multiple-
Multiple-
Multipie-
Hearth
Hearth
Hearth
Hearth
Hearth
Hearth
Hearth (?.}
Hearth
Hearth
(2). .
Hearth
Hearth
Hearth
Hearth
Hearth
Hearth
Hearth
2-6
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TABLE 2-2. INCINERATION FACILITIES CURRENTLY UNDER CONSTRUCTION IN
THE U.S.
Location Design Type
Decatur, Georgia Electric
Gainsville, Georgia Electric
Fall River, Massachusetts Multiple-Hearth
Lynn, Massachusetts Multiple-Hearth
St. Louis, Missouri Multiple-Hearth
Watertown, New York Fluid-Bed
Cranston, Rhode Island Multiple-Hearth
2-7
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For incinerators which have begun operations since 1978, the results of
the survey are considered to be relatively complete. However, there is less
certainty with respect to what percentage of incinerators currently under
construction in the U.S. were identified in the survey since less emphasis .
was given to this question.
2.2.5 Growth in the Use of Incineration as a Sludge Disposal Technique
Since 1934, when incineration was first used as a sewage sludge
c
disposal technique, over 400 incinerators have been constructed. Over half
of these were built between 1965 and 1975. The rate of growth of sewage
sludge incineration declined sharply beginning in the mid 1970's, however.
The only source of information on the future growth of incineration of
sewage sludge is' the NEEDS'data base. Since the main objective of the NEEDS
survey is to quantify ongoing and future construction programs at wastewater
treatment plants, the growth projections provided in the NEEDS data files
are assumed to be reasonably accurate. ,
On the basis -of plant surveys and demographic., projections, NEEDS
estimates that 63 sludge incineration facilities will be constructed between
1982 and 2000. Assuming a linear rate of growth, 18 incineration facil-ities
would begin operating over the next five years. This would be roughly
consistent with the rate of growth witnessed in both the 1973 to 1978, and
1978 to 1983, five year 'time frames. An estimate of approximately 18 new
sludge incineration facilities over the next five years is also reasonably
consistent with the available data on current construction programs.
2.3 PROCESS DESCRIPTIONS
2.3.1 Process Overview
Incineration is only one method of disposing of sludge generated by a
system for treating municipal wastewater. The major processes involved in
this treatment include sedimentation, filtration, digestion, chemical
conditioning, and dewatering. 'From the standpoint of incineration, the most
important aspect of these related treatment processes is their impact on the
moisture aijd energy content of the sludge. Many of the processes which
reduce the moisture content of'wastewater sludge can also reduce the
2-8
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proportion of volatile elements to inert materials. Secondary treatment
processes, such as anaerobic digestion, can significantly lower the energy
content of the sludge. Most sewage sludges undergo a variety of individual
treatments prior to the final conditioning and dewatering steps. Since
sludge conditioning and dewatering are integral to the overall incineration
process, they are briefly described below.
2.3.1.1 Sludge conditioning. Pre-thickened primary or combined
primary and secondary sludges are chemically treated to enhance their
dewatering characteristics. Chemical conditioning changes the colloidal
structure of the sludge, causing particles to coagulate.6 Absorbed water is
released as voids are created fay the coalescing particles.
A wide variety of chemicals have been used for conditioning sewage
sludge. The most popular agents are ferric chloride, lime, aluminum
chlorohydrate, and organic polymers. Depending on the level and type of
pretreatment that the sludge has received, conditioners are added at a rate
of between 1 and 12 percent 'of the dry sludge weight.
2.3.1.2 Sludge dewatering techniques. Dewatering is a critical step
in the process" of sludge incineration, since it reduces the thermal demand
on the incinerators. Vacuum filtration, filter presses, belt filters and
centifugation are the most widely used sludge dewatering technologies,
alth*ough; numerous other processes are available. The NEEDS data base lists
nearly 1,200 vacuum filters, 242 centrifuges, 151 filter presses, arid 36
"other" dewatering devices as currently in use at sewage treatment plants.
Of the 23 incineration- facilities installed since 1978, 11 employ vacuum
filters, 5 are equipped with horizontal belt presses, 3 use centrifuges, and
4 have installed filter presses. Although these data are limited, tine use
of the relatively new belt press systems appears to be increasing.
Vacuum filtration is a technique that is applicable to all types of
sewage sludge. The major equipment component is a cylindrical drum filter.
Natural and synthetic cloth,- coil springs, or wire mesh fabrics can all be
used as the filter material. The drum is suspended above a vat of sludge
and periodically dips into it. As the drum slowly rotates, part of the
circumference is subjected to an internal vacuum. The vacuum draws water
2-9
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out through the filter medium. Prior to the next submergence into the
sludge vat, the filter cake is scraped from the drum and deposited on a
conveyor. The moisture content of the sludge cake is normally 70 to
80 percent. A range of from 60 to 86 percent final moisture content was
reported for new facilities that use vacuum filters.
In a filter press, dewatering is accomplished by forcing the water from
the sludge under elevated pressures. Various designs are available. The
most common consists of a series of rectangular plates supported in a
vertical position. Filters are placed over the recessed plates. Sludge is
pumped into the space between the plates and the plates are then pressed
against each other (60 to 225 Ib/sq. in.) by hydraulic rams. The entire .
batch cycle takes from 1 to 3 hours to complete. Filter presses are capable
of reducing the moisture content of the sludge to as low as 55 percent. In
the survey of new plants, sludge cake moisture contents of from 65 percent
to 75 percent were reported.
Horizontal belt filters are a relatively new approach to dewatering .
sewage sludge. One variant of these filters consists of two continuous
belts placed one above the other. Chemically conditioned sludge is-
continuously fed between the two belts. Dewatering is accomplished in three
separate zones. In the first, water is removed by the force of gravity. In
the second zone, pressure is.applied by a series of rollers located above
the upper belt. Shear forces are applied in the final zone. The dewatered
sludge cake is then removed from the belt by a scraper. Belt filters are
designed to. achieve approximately the" same level of moisture removal as
vacuum filters. However, the extent to which any dewatering technique can
remove moisture from sewage sludge depends, in part, on the specific
physical, chemical, and biological characteristics of the sludge. For
example, a belt press filter recently installed at the incineration facility
in Merrimack, New Hampshire, achieved a 78 percent final moisture content in
,the sludge compared to the 85 percent that was obtained from the vacuum
filter system that had previously been used. The results of the survey of
new facilities gave a range of from 60 to 82 percent moisture in the sludge
after being dewatered in a belt press.
2-10
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Centrifuges are available in a variety of different design
configurations including the horizontal, conical, solid bowl, basket, and
disc types. Sludge "is fed continuously into the centrifuge where it is
subjected to centrifugal forces of up to 300 gravities. The sludge cake is'
discharged by a screw conveyor. Although centrifuges are capable of
producing a sludge having a moisture content as low as 60 percent, this
Q
level of dewatering is usually not economically feasible. The three
facilities in the survey that employ centrifuges reported final moisture
contents of from 60 to 70 percent.
2.3.2 Multiple-Hearth Furnaces
The basic multiple-hearth furnace design is nearly a century old,
having been initially developed for roasting of mineral ores. ' An air-cooled
variant of the original Herreshoff design has been used for incinerating
sewage sludge since the 1930's.
2.3.2.1 Design characteristics Figure 2-2 illustrates the overall
design of a multiple-hearth furnace. Multiple-hearth furnaces -are
cylindrically shaped and oriented vertically. The outer shell is
constructed of steel and surrounds a series of horizontal refractory
hearths. A hollow cast iron rotating shaft runs through the center of the
hearths. Cooling air is introduced into the shaft by a fan located at. its
base. Attached to the central shaft are rabble arms, which extend above the
hearths. Each rabble arm is equipped with a number of teeth", approximately
6 inches in length, and spaced about 10 inches apart. The teeth are shaped
to rake the sludge in "a spiral motion, alternating in direction from the
outside in, to the inside out, between hearths. Either 2 or 4 rabble arms
extend into each hearth. Typically, the upper and lower hearths are fitted
with 4 rabble arms, while only two are placed within the middle hearths.
Burners, providing auxilliary fuel, are located in the sidewalls of the
hearths.
The size of MHF's used for incineration of sewage sludge typically,
range from 6 hearth furnaces having a.n outer diameter of ^ 6 ft. and a total
effective hearth area of 85 sq. ft., to 12 hearth, 22 ft. diameter furnaces
Q
with hearth areas of over 3000 sq. ft. Hearth loading rates range from
2-11
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FLUE GASES OUT
DRYING ZONE
COMBUSTION ZONE
COOLING AIR DISCHARGE
FLOATING DAMPER
SLUDGE INLET
HASSLE ARM
AT EACH HEARTH
COMBUSTION
AIR RETURN
COOLING ZONE
ASH DISCHARGE
COOLING AIR FAN
Figure 2-2. Cross-Sectional View of a Multiple-Hearth
Sewage Sludge Incinerator
2-12
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between 7 to 12 pounds of wet sludge per hour, per square foot. This
corresponds to.furnace capacities of from 600 pounds of wet sludge per hour
up to 18- tons per hour.
2.3.2.2 Operating characteristics Partially dewatered sludge"is fed '
into the periphery of the top hearth. The motion of the rabble arms rakes
the sludge toward the center shaft where it drops through holes located near
the edge of the hearth. In the next hearth the sludge is raked in the
opposite direction. This process is repeated in all of the subsequent
hearths. The effect of the rabble motion is to break up solid material to
allow better surface contact with heat and oxygen, and is arranged so that a
sludge depth of about one inch is maintained in each .hearth at the desicjn
sludge flow rate. . '
Ambient air is first ducted through the central shaft ,and its
associated rabble arms. A portion, or all, of this air is then taken from
the top of the shaft and recirculated into the lowermost hearth as preheated
combustion air. Shaft cooling air which is not circulated back into the
furnace is ducted into the .stack downstream of the air pollution control
devices. The combustion air flows upward through the drop ho.les in. the
hearths, countercurrent to the flow of the sludge, before being exhausted
from the top hearth. Provisions are usually made to- inject ambient air
directly into one of the middle hearths as well.
From the standpoint of the overall incineration process, multiple-
hearth furnaces can be divided into three zones. The upperhearths comprise
the drying zone where most of the moisture in the sludge is evaporated. The
temperature in the drying zone is typically between 800 and 1400°F.
Combustion occurs in the middle hearths (second zone) as the temperature is
increased to about 1700°F. The combustion zone, can be further subdivided
into the upper-middle hearths where the volatile gases and solids are
burned, and the lower-middle hearths where most of the fixed carbon is
combusted. The third zone, made up of the lowermost hearth(s), is the
cooling zone. In this zone the ash is. cooled .as its heat is transferred to
the incoming combustion air. . .
2-13
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Under proper operating conditions, 50 to 100 percent excess air must be
added to a MHF in order to ensure complete combustion of the sludge.
Besides enhancing contact between fuel and oxygen in the furnace, these
relatively high rates of excess air addition are necessary in order to
compensate for normal variations in both the organic characteristics of the1
sludge feed and the rate at which it enters the incinerator. When an
inadequate amount of excess air is available, only partial oxidation of the
carbon will occur with a resultant increase in emissions of carbon monoxide,
soot, and hydrocarbons. Too much excess air, on the other hand, can cause
increased entrainment of particulates and unnecessarily high fuel
consumption..
Another important parameter in -the operation of a multiple-hearth
sewage sludge incinerator is the rate of feed of the sludge cake. Any
sudden increase or decrease in load to the furnace can severely affect the
performance of the incinerator. A sharp increase in the rate of feed has
been shown to lower the combustion zone- in the furnace. This can
subsequently lead to a decrease in temperature within the combustion zone
and the potential for the fire to.be extinguished.- Conversely, a. sudden
decrease in furnace load can cause excessively high temperatures in the
furnace with the attendant risk of damage to the refractories and rabble
castings. The moisture content of the sludge feed must also be kept
relatively constant for the same reasons.
Maintaining a uniform rate of feed into a MHF can be difficult,
however. First, mechanical sludge dewatering devices are not capable of .
producing a sludge cake of perfectly uniform moisture content. Second, at
most incineration plants, the sludge is fed directly from the treatment
facility to the dewatering device, and then directly into the incinerator.
Holding tanks are not usually available to independently control the rate of
sludge input into the furnace. A related problem is that it may take up to
an hour (or more) for the sludge to descend from the drying zone to the
combustion zone in a nurl-tipie-hearth incinerator. Thus, a change in the
furnace load may not be noticed by the furnace operators in time to take
corrective action. Moreover, there will be an additional delay, before the
2-14
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incinerator responds to these corrective measures and operations become
stable.
' The speed at which the rabble arms are rotated can also have a critical
impact on the operation of a multiple-hearth incinerator. Typically, the '.
t
rotational speed can be varied between 0 and 3 revolutions per minute. As
the speed of the rabble mechanism is increased, the rate of drying in the
upper hearths is increased and the combustion zones tends to rise.
Combustion will also tend to take place in a greater number of hearths.
Experimental data have also demonstrated that the temperature of the hottest
hearth will decrease as the speed of the rabble arm rotation is increased.12
. The opposite effects are observed when the speed of the rabble motion is
decreased.
However, changes in the speed of rotation of the rabble arms will
initially have just the opposite effects of those described above. For
example,_an increase in the rabble arm speed will initially create an
internal increase in the load to the combustion zone. This will cause a
temporary decline of the burning zone and an overall decrease in the
. temperature of the lower hearths-. From 1 to 3 hours are required for a MHF
to stabilize after the speed of the rabble arms is changed. Because of the'
transient furnace instabilities caused by such changes, in the speed of the.
rabble motion, adjustment of rabble arm speed is not 'an effective means of
controlling the process of combustion in a multiple-hearth incinerator.13
Rather, the speed of the rabble movement should be set slow enough to form
good furrows in the sludge, but fast enough to avoid crusting of the sludge
in the upper hearths. The optimum speed is a function of the sludge
moisture content and loading rate.
For optimum performance, the temperature profile within the furnace
should be controlled by adjusting the firing rate of the burners. Ideally,
only those burners located immediately above and below the combustion zone
should be used (depending on the number of hearths, and the capacities of
the available burners). This allows a greater sludge residence time in the
drying zone and can decrease turbulence in the upper hearths.
Theorectically, combustion can become self sustaining in a MHF when
sludges having a heating value of at least 10,000 Btu/lb, a moisture content
2-15
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of less than 75 percent, and a volatile solids fraction of at least 60 to
65 percent are incinerated. However, under autogenous conditions the
highest temperature in the furnace may only be about 900°F, which is
insufficent to completely destroy odor causing organics. Even at minimum^
excess air rates, some auxilliary fuel must be burned in MHF's in order to
maintain a minimum temperature of 1350°F for destruction of odoriferious
materials.
As discussed above, the operation of multiple-hearth sludge
incinerators is complicated by the number of process variables involved, as
well as by the transient nature"of some of the responses observed when these
variables are altered. As a means to establish,guidelines for the operation
of MHF incinerators, particulary for reducing the amount of fuel consumed, a
substantial amount of both tjieorectical and empirical research has recently
been conducted by the Indianapolis Center for Advanced Research (ICFAR).
Although the best mode of operating any incinerator is a function of
numerous site-specific conditions, a number of general procedures"have been
established as the result of the ICFAR work. These operational guidelines
include: ' -
1. Utilization of shaft cooling air as combustion air;
2. Maintenance of sludge combustion on the lower burning Dearths;
3. Use of only those burners located on, or immediately adjacent to,
the combustion hearth(s);
4. Maintenance of rabble arm speed as slow as possible;
5. Minimization of air leakage into the incinerator;
6. Maintenance of sludge loading rates at, or below, design capacity,
and;
7. Maintenance of excess air at 25 to 50 percent.
At incinerators where these procedures have been put into practice,
17 1 ft
fuel savings of from 30 to 70 percent have been attained. ' Moreover,
there are some indications that the operational procedures which result in
reductions in fuel use also result in decreased emissions of particulates.
The relationship between operating procedures and particulate emissions is
discussed in more detail in Chapter 3.
2-16
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2.3.3 FTuidlzed-Bed Incinerators
Since its original development as a method for recovering catalysts in
the oil refining industry, fluidized-bed technology has been applied to a
wide range of industrial processes. The first fluidized-bed reactor,
j
designed specifically for incineration of sewage sludge, was installed in
1961 in Lynwood, Washington.
2.3.3.1 Design characteristics. Figure 2-3 depicts the cross-section
of .a fluidized-bed sludge incinerator. Like multiple-hearth furnaces,
fluidized-bed incinerators (FBF) are cylindrically shaped and oriented
vertically." The outer shell is constructed of steel and is lined with
refractory. Tuyeres are located at the base of the furnace within a-
refractory lined grid. A bed of sand, approximately 2.5 feet thick, rests
upon the grid.
Two general configurations can be distinguished on the basis of how the
fluidizing air is injected into the furnace.. In the "hot windbox" design
the air is-first passed through a heat exchanger where heat is -recovered
from the hot flue gases. Alternatively, ambient air can be injected
directly into the furnace.
The physical dimensions of FBF units range from diameters of 6 to
25 feet. The corresponding range in the freeboard area is 30 to 525 square-
feet. Fluidized-bed incinerators have sludge loading rates of between 30 to
60 wet Ib/hr/sq. ft. (roughly 5 times higher than multiple-hearth furnaces).
Burning capacities of FBF units range from one-half to 15 tons of wet sludge
per hour.
2.3.3.2 Operating characteristics. Partially dewatered sludge is fed
into the lower portion of the furnace. Air injected through the tuyeres at
pressures of from 3 to 5 psig, simultaneously fluidizes the bed of hot sand
and the incoming sludge. Temperatures of 1400 to 1700°F are maintained in
the bed. Residence times are on the order of 2 to 5 seconds. As the sludge
burns, fine ash particles are carried out the top of the furnace. Some sand
is also removed in the air stream; sand make-up requirements are on the
order of 5 percent for every 300 hours of operation.
2-17
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SIGHT GLASS
N,
EXHAUST < 1
SAND FEED
£
FLUIDI ZED
SAND
PRESSURE
TAP
ACCESS
DOORS
C *.*. c*/ ' ""^ , ' "T"? *\*" 1^3 , *f t j'.»-'-:^. i^i
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SLUDGE INLET
FLUIDIZING
AIR INLET
TUYERE
WINDBOX
Figure 2-3. Cross-sectional View of a Fluid-bed Sewage
Sludge Incinerator.
2-18
-------
The overall process of combustion of the sludge occurs in'two zones.
Within the bed itself (zone 1) evaporation of the water and pyrolysis of the
organic materials occur nearly simultaneously as the temperature of the
sludge is rapidly raised. In the second zone, (freeboard area) the
remaining free carbon and combustible gases are burned. The second zone
functions essentially as an after-burner.20
From the standpoint of combustion, fluidization of the sludge has a
number of advantages. First, the turbulence in the bed facilitates the
transfer of heat from the hot sand particles to the sludge. Similarly,
-nearly ideal mixing is achieved between the sludge and the combustion air°as
a result of the greatly increased surface areas available.- Finally, the
sand provides a relatively uniform source of heat within the bed.
The most noticable impact of the better burning atmosphere provided by
a fluidized-bed incinerator is seen in -the amount of excess air required for
complete combustion of the sludge. Fluidized-bed sludge incinerators can
achieve complete combustion with 20'to 50 percent excess air. This is about
half the,amount of excess air typically required for incinerating sewage
sludge, fn multiple-hearth furnaces. As a consequence, FBF incinerators have
generally lower fuel requirements compared to MHF incinerators.
Controlling the rate of feed of the sludge into the incinerator is the
most critical operating variable. There is an upper limit on the rate of
heat transfer that can be achieved for a given quantity of sand. If the
rate of sludge feed exceeds the burning capacity of the sand bed, combustion
will not be.complete. Similarly, either a rapid increase in the overall
furnace load or in the total moisture content of the sludge will lead to
coagulation of the sludge into heavy masses, depress the bed, and halt
combustion. It is also important, for the same reasons, to ensure that an
adequate residence time is available for the sludge to burn completely.
However, due to their excellent-mixing characteristics, as well as their
short residence times, fluidized-bed sludge incinerators are less vulnerable
than MHF's to fluctuations in the rate of sludge, and total moisture input
into the furnace. Moreover, any disruption of combustion will occur almost
2-19
-------
immediately, and can be more easily detected and corrected by-the operators
of the furnace.
2.3.4 Electric Incinerators
The electric furnace is the newest of the technologies currently in
commerical use for the incineration of sewage sludge. Most of these units'
were installed in the middle and late 1970's. The capacities of existing
units are less than one ton of wet sludge per hour.
2.3.4.1 Design characteristics. Electric incinerators consist of a
horizontally oriented, insulated furnace. A belt conveyor extends the
length of the furnace. Infrared heating elements are located In the roof
above the conveyor belt. Combustion air is preheated by the flue gases and
is injected into the discharge end of, the furnace. Electric incinerators
consist of a number of pre-fabricated modules, which can be linked together
to provide the necessary furnace length. A schematic of an electric sludge
incinerator is provided in Figure 2-4.
2.3.4.2 Operating characteristics. The dewater'ed sludge cake is
conveyed into one end of the incinerator. An internal roller mechanism
levels the sludge into'a continuous layer approximately one inch thick
across the width of the belt. The sludge is dryed and then burns as it
moves beneath the infrared heating elements. Ash is discharged into a
hopper at the opposite end of the furnace. -
The preheated combustion air enters the furnace above the ash hopper
and is further heated by the outgoing ash. The direction of air flow is
countercurrent to the movement of the sludge along the conveyor. Exhaust
gases leave the furnace at the feed end.
2.3.5 Other Incinerator Designs
A number of other technologies have been used for incineration of
sewage sludge including cyclonic reactors, rotary kilns, and wet oxidation
reactors. These incinerators are no longer in widespread use, and will be
only briefly described.
2.3.5.1 Cyclonic reactors. The cyclonic reactor is designed for small
capacity applications. It is constructed 'of a cylindrical chamber that is
lined with refractory. Preheated combustion air is introduced into the
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chamber tangentially at high velocities. The sludge is sprayed radially
toward the hot refractory walls. Combustion is rapid: the residence time
21
of the sludge in the chamber is on the order of 10 seconds. The ash is
removed with the flue gases.
2.3.5.2 Rotary kilns. Rotary kilns also have limited capacities
(vl200 Ib/hr). The kiln is inclined slightly to the horizontal plane, with
the upper end receiving both the sludge feed and the combustion air. A
burner is located at the opposite end of the kiln. The kiln rotates at a
speed of about 6 inches per second. Ash is deposited into a hopper located
below the burner.
.2.3.5.3 Wet ox-idation reactors. This process is not strictly one of
incineration, but of oxidation at elevated temperature and pressure in the
presence of water. Untreated sludge is first ground and mixed with a
specified amount of compressed air. The mixture is then circulated through
a series of heat exchangers before entering a pressurized reactor. The
temperature of the reactor is held at from 350 to 600°P; Steam is usually
used for auxilliary heat. The water and remaining ash are circulated out of
the reactor and are finally-separated in a tank or lagoon.-
2.4 REGULATORY BACKGROUND
2.4.1 Selection of Sewage Sludge Incinerators for NSPS
Sewage sludge incinerators were originally selected for NSPS
development in 1973 on the basis of their potential to emit significant
quantities of particulate matter into the atmosphere. It was noted that
less emphasis was given to retention of ash in sludge incinerators compared
to other types of incineration units. Moreover, concern was expressed over
the potential of sludge incinerators to emit "significant concentrations" of
22
mercury and other toxic materials. Although prior to 1973 all sludge
incinerators in the U.S. were controlled with wet scrubbers, nearly all of
these operated at low pressure drops (2 to 8 in. W.G.) with attendant low
removal efficiencies. In addition, existing state and local regulations did
not explicity apply to incinerati.on of sewage sludge.
2-22
-------
Prior to proposal of a NSPS for sewage-sludge incineration, 15 plants
having visible emissions of less than 10 percent opacity wjere visited. Each
of these facilities were evaluated as to the feasibility of performing
emissions measurements. Five locations were subsequently selected for
testing: three multiple-hearth and two fluid-bed units. Four of the
selected incinerators were controlled by low energy (2.5 to 6.0 in. W.6.)
impingement-type scrubbers; one of the fluid bed units was equipped with a
venturi scrubber operating at a pressure drop of 18 inches of water. The
results of these stack tests are presented in Table 2-3. On the basis of
these tests, a particulate emissions standard of 0.031 gr/dscf was proposed
in 1973 for new sewage slude incinerators. An opacity limitation of
10 percent was al(so proposed.
2.4.2 Current NSPS for Sewage Sludge Incinerators
On February 28, 1974, the proposed standard was amended. It was felt
that a standard based on the concentration of the particulate matter in the
flue gases would lead to unacceptable error due to the difficulties in
distinguishing between combustion air as opposed to dilution air in
multiple-hearth furnaces.. Thus, the promulgated standard for'particulate
matter was expressed on a mass basis, and set at 1.3 Ib/ton of dry sludge
input. The opacity standard was also changed from 10 percent to 20 percent.
Sewage "sludge incinerators are also subject to federal emission limits for
mercury of 3200 grams per day.
The revised NSPS promulgated as Subpart 0, Standards of Performance for
Sewage Treatment Plants, applies to incinerators built or modified after
June 11, 1973. Any incinerator that burn wastes consisting of more than
10 percent sewage sludge (dry), or charges more than 1000 kg of sewage
sludge per day, ts subject to the standard.
A facility is considered to have commenced construction on the date
that a continuous program of construction starts, or on the date that a
contractual agreement, including economic penalties for cancellation, is
signed. Existing facilities that are modified in any way which increases
the amount of particulate matter emitted, also become subject to the NSPS.
2-23
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A device must be installed to measure the amount of sludge charged into
the incinerator to within 5'-percent accuracy. Access must also be provided
for taking grab samples of the sludge. No provision is made in the existing
standard for monitoring either particulate emissions .(through periodic stack
testing), or stack opacity,' from sludge incinerators.
2.4.3 State Regulations
State regulations affecting sewage sludge incinerators were surveyed
through written requests to the regional offices of EPA as well as by
telephone contacts to State offices. No changes in these regulations were
identified since the last NSPS review was conducted in 1978. The applicable
State regulations are briefly discussed beTow.
Twenty-two states treat sewage sludge incinerators as a distinct source
category. In these states, the federal NSPS is applied. Most other states
have general standards that encompass incineration of all types of municipal
wastes and refuse. These standards are usually less stringent than .the
existing NSPS for sTudge incinerators.
In order-to assess state requirements for monitoring emissions- from
sewage sludge incinerators, the regulation's in 11 states were surveyed.
Over 70 percent of all facilities currently incinerating sewage sludge in
the U.S. are located in these 11 states. The results of this survey are
provided in Table 2-4.
In six of the states, some provision is made to monitor either
particulate emissions (through periodic stack tests) or opacity. In most of
these six, however, the facilities affected by the monitoring requirement
are to be determined on a case-by-case basis at the discretion of the
Administrator. There is no indication, however, that this discretionary
authority has ever been applied to sewage sludge incinerators in these
states. The cut-off of 100 tons of particulate/year applied.in California
would exclude virtually all sludge incinerators.
Connecticut has a statutory requirement for the installation of a stack
opacity recorder on all incinerators with a waste reduction capacity of more
than 2000 pounds per hour. Opacity readings must be summarized and
submitted to the Administrator on a quarterly basis. Connecticut does not,
2-25
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however, treat the incineration of sewage sludge as a distinct source
category. Thus, the applicability of the requirements to sludge
incinerators is not entirely clear.
Massachusetts is the'only other State in the sample that has a
monitoring requirement that would likely affect facilities incinerating
sewage sludge. Massachusetts requires that a "Standard Operating Procedure"
be developed prior to the granting of an operating permit. Although the
exact content of the Operating Procedure is determined on a plant-specific
basis, the procedure should detail how specific operating practices will
minimize emissions. Operators of affected facilities are required to show
conformity with these practices in an annual summary report to the
Administrator.
Since only Connecticut and Massachusetts appear to have existing
monitoring programs for sludge incinerators, personnel at both the state and
local level were contacted in these states for further information. EPA's
Region I office was also contacted for further details. The information
obtained from these contacts is presented below.
Although Connecticut could legally require an operator of a sludge
incinerator to install a device to continuously monitor and record stack
opacity, this requirement is generally not excercised on sludqe
23'
incinerators. Opacity monitors have been found to not operate properly
when placed in the stack of an incinerator. The major problem encountered
in monitoring opacity is the moisture content of the incinerator flue gas.
At typical incinerator stack gas temperatures of approximately 120°F, all of
the moisture (10 to 30 percent) in the gas is condensed. A related problem
is that the lens of the transmissometer can be easily fouled by solids and
o n
oils in incinerator flue gases. The major manufacturer of opacity
monitors confirms that they will not operate properly in such environments
without prior dehumidification and reheating of the flue gas.25 For these
reasons, installation of opacity monitors on sewage sludge incinerators is
generally not required, and no enforcement action has ever been taken in '
Connecticut on the basis of opacity recordings.
2-27
-------
The Standard Operating Procedure (SOP) required by the State of
nc
Massachusetts is general, and does not follow any specific format.
Normally, only such information, as shut-down procedures in case of scrubber
malfunction, maintenance procedures and schedules, and operator training
programs would be required. Each incinerator would, however, be treated on
a case-by-case basis and more specific information on operating -practices
could potentially be required in certain instances. For example, if an
incinerator fails an initial compliance test, and the reason for such
failure can be correlated to specific operating parameters, the State may
27
require that these parameters be monitored.- However, no specific instance
could be identified where an incinerator was required to maintain and
monitor a specific operating parameter within a specified range, or where an
enforcement action'has been initiated on the basis of an SOP report.
There is nonetheless, some interest in both Connecticut and
Massachusetts to require more detailed monitoring of incinerator operating
practices. Sludge moisture content and scrubber pressure drop have been
28
cited as two variables that might be more closely monitored. The primary
objective in strengthening these requirements would be to improve inspection
procedures. There are, however, no formal plans to institute a scheme to
more closely monitor operating conditions at sludge incineration facilities.
2-28
-------
REFERENCES FOR CHAPTER 2
1. Office of Water Program Operations. The 1982 Needs Survey: Conveyance,
Treatment, and Control'of Municipal Wastewater, Combined Sewer
Overflows, and Stormwater Runoff. U.S. Environmental Protection
Agency, EPA/43019/83-002, June 1983. . .
2. Telecon. R. M. Dykes, Radian Corporation, with J. Smith, Center for
Research Information and Technology Transfer, U.S. Environmental
Protection Agency. January 13, 1984. Source of Sludge Task Force
Survey data.
3. Reference 2.
4. Office of Solid Wastes. Environmental Impact Statement, Criteria- for
Classification of Solid Waste Disposal Facilities and Practices.
U.S. Environmental Protection Agency, EPA/SW-821, 1979.
5. Gordian Associates. Assessment of the Use of Refuse Derived Fuels in..
, Municipal Wastewater Sludge Incinerators. U.S. Environmental
Protection Agency, EPA Contract No. 68-01-4227, 1977.
6. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. -McGraw-Hill Book Company, N.Y., N.Y., 1979.
7. .Trip Report - Visit to the Sewage Sludge Incinerator at Merrimack, New
Hampshire, S. D. Piccott, Radian Corporation, to Naum T. Georgieff,
Office of Air Quality Planning and Standards, U.S. EPA.
8. Reference 6.
9. Unterberg, W., R. J. Sherwood, and G. R. Schneider. Computerized
Predesign and Costing of Multiple-Hearth Furnace Sewage Sludge
Incinerators. AIChE Symposium Series, Vol. 69, No. 129, 1972.
10. Verdouw, A. J. and E. W. Waltz. Sewage Sludge Incinerator Fuel
Reduction at Nashville, Tennessee. Indianapolis Center for Advanced
Research. U.S. Environmental Protection Agency, Contract No.
68-02-3487, 1982.
11. Richards, D. and H. Gershman. The Conversion of Existing Sludge
Incinerators for Codisposal. U.S. Environmental Protection Agency,
SW-743, 1979.
12. Ottman, R. D., et. al. Coincineration of Sewage Sludge with Coal or
Wood Chips. Metropolitan Waste Control Commission of Saint Paul,
Minnesota, MWCC Project No. 75-05, 1979.
2-29
-------
13. Reference 10.
14. Ferrel, G.A. 1973. Sludge Incineration. Pollution Engineering,
Vol. 5, No. 3, March 1973.
15. Reference 9.'
16. Verdouw, A. J., Eugene W. Waltz, and W. Bernhardt. Plant Scale
Demonstration of Sludge Incineration Fuel Reduction. Indianapolis
Center for Advanced Research; U.S. Environmental Protection Agency,
Contract No. S 306248010, 1982.
17. Reference 10.
18. Reference 16.
19. Reference 16. _ -
20. Liao, P. B. Fluidized-Bed Sludge Incinerator Design/ Journal of the
Water Pollution Control Federation, Vol. 46, No. 8, August, 1974.
21. Reference 14.
22. Office of'Air Quality Planning and Standards. Background Information
for Proposed New Source Performance Standards. U.S. Environmental
, Protection Agency, AAPTD-1352a, 1973.
23, Telecon. R. M. Dykes, Radian Corporation, with A. Conklin, Air
Compliance Branch, Connecticut Department of Environmental Protection.
December 29, 1983. Monitoring requirements for sewage sludge
incinerators.
24. Telecon. R. M. Dykes, Radian Corporation, with J. Royce, Air
Compliance Branch, Connecticut Department of Environmental Protection.
December 29, 1983. Use of opacity monitors in sludge incinerator
stacks.
25. Telecon. R. M. Dykes, Radian Corporation, with A. Hudson,
Lear-Siegler, Inc. Use of transmissometers in sludge incinerator
stacks.
26. Telecon. R. M. Dykes, Radian Corporation, with D. Squires, Division of
Air Quality Control, Massachusetts Department of Environmental Quality,
December 29, 1983. Monitoring programs for sewage sludge incinerators.
27. Telecon. R. M. Dykes, Radian Corporation, with T. Parks, Massachusetts
Department of Environmental Quality Engineering.' December 29, 1983.
Monitoring programs for sewage sludge incinerators.
2-30
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28. Telecon.. R. M. Dykes, Radian Corporation, with C. McNair, Control
Technology and Air Compliance Section, U.S. Environmental Protection
Agency (Region I). January 17, 1984. Monitoring requirements for
sewage sludge incinerators in Region I.
2-31
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-------
3.0 EMISSION CHARACTERISTICS, STATUS OF CONTROL TECHNOLOGY,
AND COMPLIANCE STATUS OF SEWAGE SLUDGE INCINERATORS
3.1 INTRODUCTION AND SUMMARY OF FINDINGS
3.1.1 Introduction
The objective of this chapter is to investigate the emission
characteristics of sewage sludge incinerators and to evaluate their ability
to meet the existing NSPS of 1.3 pounds of particulate per ton of dry sludge
input. This evaluation is. focused o>n the compliance-experience of incinera-
tors which have begun operating during the past five years. The compliance
experience of sludge incinerators installed prior to 1978 has been
previously reviewed.
The types of technologies emplpyed to control particulate emissions
from sludge incinerators are identified and discussed in Section 3.2. '
Trends-over the'past ten years in the types of control technologies most
wi-dely used are discussed. The type of coptrol devtce most widely used
since 1978 is described in detail.
Section 3.3, discusses uncontrolled emission characteristics of sewage
sludge incinerators. The-impact" that the quality of the sludge feed, as
well as the manner in which the incinerator is operated, can have on
uncontrolled emission rates is also assessed in this section.
In Section 3.4- the capability of sewage sludge incinerators to comply
with the existing NSPS is addressed. First, the results of the review
conducted in 1978 are briefly summarized. Second, the compliance experience
of incinerators installed since 1978 are presented and evaluated.
The potential of sewage sludge incinerators to emit toxic substances is
briefly reviewed in Section 3.5.
In the final section of this chapter an estimate is made of the
national emissions of participates from incinerators that are expected to be
installed between 1985 and 1990.
3-1
-------
3.1.2 Summary of Findings
Multiple-hearth and fluidized-bed incinerators that have begun
operating over the past five years commonly employ combination
venturi/impingement-tray scrubbers to control participate emissions. In
most cases, these scrubbers are operated at total pressure drops of
approximately 30 inches of water. The standard could likely be achieved at
pressure drops of less than 30 in. W.G., although higher pressure drops are
commonly employed to account for typically wide variations in the
particulate loading at the scrubber inlet.
Over the past ten years there has been a distinct trend toward the
nearly exclusive use of combination venturi/impingement-tray scrubbers to
control emissions from multiple-hearth incinerators. Prior to- 1978 only ,
about 20 percent of multiple-hearth incinerators were equipped with venturi/
impingement-tray scrubbers. All but three of the 17 multiple-hearth
incinerators installed after 1978 utilize this technology, however. Three
of the four new fluidized-bed incinerators are also equipped with
combination venturi/ impingement-tray control devices. Although all
electric incinerators installed ^since 1978 utilize a-venturi, only one. of
these is followed by an impingement-tray scrubber.
The average pressure drop for all scrubbers installed after 1978 is
approximately 25 in. H.G. This is higher than the average pressure drop of
19 in. VI.6. for the control devices in use when the NSPS was reviewed in
1978. The trend toward increasing pressure drops for scrubbers applied to
sludge incinerators reflects the wide variability in the amount of particu-
lates that potentially may enter the scrubber, rather than widespread
difficulties in meeting the NSPS, Emissions from most of the incinerators
installed after 1978 are well under the NSPS limit. Moreover, several
incinerators equipped with control systems operating at considerably lower
pressure drops have achieved the NSPS.
Uncontrolled rates of particulate emissions from sewage sludge
incinerators are highly variable. On the basis of the available data,
uncontrolled emissions can range from less than 10 Ib/dry ton input to over
400 Ib/dry ton. Variability in the quality of the sludge feed, as well as
3-2
-------
the manner in which an incinerator is operated, are responsible for the
variability observed in-uncontrolled emissions from sludge incinerators.
There is some evidence to suggest that uncontrolled emissions can be
decreased by improving incinerator operating practices. However,"no
quantitative correlation has been identified between any specific operating
parameter(s) and uncontrolled particulate emissions.
New sewage sludge incinerators, when correctly operated and equipped
with an appropriate-control device, can achieve the existing New Source
Performance Standards. Of the 17 multiple-hearth incinerators that have
begun operating in the past five years, 12 are officially in compliance with
the NSPS. Four new units have not yet been tested.. The remaining
multiple-hearth incinerator, located in Providence, Rhode Island, has
demonstrated the capability to meet the NSPS during unofficial tests, but
has not yet officially complied with the standard. All of the four
fluidized-bed sludge incinerators installed since 1978 are in compliance
with the standard. Of the four electric incinerators installed since 1978,
two were unable to achieve the NSPS. However, both of these units are
'-equipped wi-th scrubbers operated at very low pressure drops of 8 to 10
inches W.G. One electric incinerator is officially in compliance, while
another has not yet been tested.
Sewage sludge incinerators also emit potentailly toxic trace elements.
Data for 12 sewage sludge incinerators indicate that emissions of trace
elements are highly variable. For example, controlled emissions of cadmium
ranged from 0.003 Ib/dry ton sludge to 0.06 Ib/dry ton. Overall, the
highest controlled trace element emissions were for lead, which can range
from 0.002 up to 0.16 Ib/dry ton sludge, and averaged 0.05 Ib/ton. Data on
uncontrolled trace element emission rates from 6 incinerators averaged 0.03
Ib/ton for cadmium, 0.18 Ib/ton for chromium, 0.08 Ib/ton for nickel, 0.45
Ib/ton for lead, and 0.02 Ib/ton for arsenic. The efficiency of control
devices in removing trace elements from incinerator flue gases ,is generally
less than that for total particulates. For the six incinerators tested,
control efficiencies were lowest for lead (average = 63 percent) and for
3-3
-------
cadmium (average'= 83 percent). There is no apparent correlation between
the pressure drop of the control devices and their corresponding removal
efficiencies for trace elements.
It is estimated that an additional 245,000 dry tons of sludge will be -
incinerated annually at 18 new wastewater treatment plants by the year 1990.
Assuming a maximum particulate emission rate of 1.3 Ib/dry ton sludge, the
increase in national particulate emissions from sewage sludge incinerators
would be 160 tons in 1990.
3.2 EMISSION CONTROLS APPLIED TO SEWAGE SLUDGE INCINERATORS
Particulate emissions from sewage sl.udge incinerators have historically
been controlled by wet scrubbers. The most obvious reasons for this are
that a sewage treatment plant provides a relatively inexpensive source of
scrubber water (plant effluent is used) and a system for treatment of the
scrubber effluent is available (spent scrubber water is fed to the head of
the treatment plant for solids removal). In addition, a long history of
scrubber applications has demonstrated success in meeting pollutio.n control
s-tandards'for particulate matter. This-section identifies the types of
particulate matter emission controls applied to sludge incinerators and
focuses on the controls which are currently most widely used.
3.2.1 Control Technologies Applied Prior to 1978
Table 3-1 shows the estimated distribution of emission controls applied
to sludge incinerators prior to 1978. As Table 3-1 indicates, a wide
variety of emission controls were applied to all types of incinerators prior
to 1978. The types of controls shown in Table 3-1 range from low pressure
drop spray towers and wet cyclones (pressure drops from 4 to 9 inch W.G.),
to higher pressure drop venturi scrubbers and venturi/impingement-tray
scrubbers (pressure drops from 12 to 40 inch W.G.). In general, the lowest
pressure drop scrubbers were utilized prior to proposal of the NSPS in the
early seventies. The most widely used type of control device applied to
multiple-hearth incinerators was the impingement-tray scrubber. Combination
venturi/impingement-tray scrubbers were most widely applied to fluidized-
bed incinerators. Most electric incinerators used venturi scrubbers.
3-4
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3.2.2 Control Technologies Applied After 1978
Table 3-2 shows the distribution of emission control technologies
applied to sewage sludge incinerators built since 1978. The data presented
in this table were collected as part of the survey described in Chapter 2.
The control device installations included in Table 3-2 represent all 25 of
the new incinerators identified in this study as being built since 1978.
Table 3-2 shows that most of the sewage sludge incinerators installed
since 1978 are equipped with venturi/impingement-tray scrubbers. Before
1978, only 20 percent of the multiple-hearth incinerators used venturi/
impingement-tray scrubbers, but after 1978, this number increased to nearly
90 percent. Three of the four new fluidized-bed incinerators are a-lso
equipped with combination venturi/impingement-tray scrubbers. New el-ectric
incinerators are controlled predominantly by individual venturi scrubbers.
Pressure drops for the venturi/impingement scrubbers shown in Table 3-2
range from 10 to 45 inch W.6. In general, this represents an increase in
pressure drop over the"same type of scrubber-used prior to 1978. The
following section presents a brief process description for a typical
venturi/impirigemeht-tray scrubber system.-
3.2.3 Venturi/Impingement-Tray Scrubber Description
Figure 3-1 presents a simplified diagram of a typical venturi/
impingement-tray scrubber. As the figure shows, hot gas exits the
incinerator and enters the preceding or quench section of the scrubber.
Spray nozzles in the quench section cool the incoming gas and the quenched
gas then enters the venturi section of the control device.
Venturi water is usually pumped into an inlet weir above the quencher.
The venturi water enters the scrubber above the throat and floods the throat
completely. This eliminates build-up of solids and reduces abrasion.
Turbulence created by high gas velocity in the converging throat section
deflects some of the water travelling down the throat into the gas stream.
Particulate matter carried along with the gas stream impacts on these water
.-
particles and on the water wall. As the scrubber water and flue'gas leave
the venturi section, it passes into a flooded elbow where the stream
velocity decreases allowing the water and gas to separate. Most venturi
3-6
-------
TABLE 3-2. DISTRIBUTION OF EMISSION CONTROL TECHNOLOGIES APPLIED
TO SEWAGE SLUDGE INCINERATORS AFTER 1978
Range of ,
Pressure Drops
Control Type Total Number Percent of Total (in. w.g.)
Multiple Hearth Incinerators
Venturi/Impingement-Tray 15 88 10 - 45
Fabric Filter 16-
Impingement Tray l 6 10
Total I7~
Fluidized Bed'Incinerators
Venturi/Impingement-Tray 3 75 42
Venturi 1 25 DNRa
' Total 4
Electri c Incinerators
Venturi 3 , 75 .. 8-10
Venturi/Impingement-Tray 1' 25 10
Total 4 .
* i
aData Not Recorded
3-7
-------
GAS EXIT
*
Gas from Incinerator
ater frorr, Treatment
lant Outflow .
QUENCHER
SECTION
MIST ELIMINATOR
Water from Ireatmeni
Outflow
FLOODED PERFORATED
IMPINGEMENT TRAYS
Figure 3-1.
Cross-Sectional View of a Venturi/Impingement-
Tray Scrubber
3-8
-------
sections come equipped with variable throats. By restricting the throat
area within the venturi, the linear gas velocity is increased and the
pressure drop is subsequently increased. Up to a"certain point, increasing
the venturi pressure drop increases the removal efficiency.
At the base of the flooded elbow, the gas stream passes through a
connecting duct to the base of the impingement-tray tower. Gas velocity is
further reduced upon entry to the tower as the gas stream passes upward
through the perforated impingement trays. Water usually enters the trays
from inlet ports on opposite sides and flows across the tray. As gas passes
through each perforation in the tray, it creates a jet which bubbles up the
wa'ter and further entrains solid particles. At the top of the tower, is a
mist eliminator to reduce the carryover of water droplets in the stack
effluent gas. The impingement section can contain from 1 to 4 trays, but
most systems for which data are available have 2 or 3 trays.
Although pressure drop information for individual components of the
venturi/impingement-tray scrubber system is limited, available data show
that the impinger sectio.n usually accounts for -just under -one-third of the
t *
overall scrubber pressure drop. As shown in Table 3-2, overall pressure
drops range from 10 to 45 inch W.G. Individual impingement-tray tower
pressure drops range .between 5 and 10 inch W.G.
As noted earlier, scrubber water consists of effluent from the water
treatment plant. The total solids content of the inlet scrubber water
depends on the.performance of the water treatment plant. Design data for
one plant built after 1978 indicates a permissible total solids content of 1
to 5 percent.
3.3 UNCONTROLLED EMISSIONS FROM SEWAGE SLUDGE INCINERATORS
The following section describes the uncontrolled emission characteris-
tics of sewage sludge incinerators. The discussion focuses on (1) the
differences in emission characteristics for' the three major types of'
incinerators, and (2) the factors affecting uncontrolled emissions.
3.3.1 Uncontrolled Emission Characteristics of Sludge Incinerators
Uncontrolled particulate emission rates can vary widely depending on'
the type of incinerator, the volatiles and moisture content of the sludge,
3-9
-------
and the. overall operating practices employed. Generally, uncontrolled
participate, emissions from fluidized-bed incinerators are the highest
because suspension burning results in most of the ash being carried out of
the incinerator with the flue gas. Uncontrolled emissions from multiple-
hearth and fluidized-bed incinerators are extremely variable, however.
Electric incinerators appear to have the lowest rates of uncontrolled
particulate release.
Since particulate loadings at the scrubber inlet are not normally
measured during compliance testing, uncontrolled emissions data are
limited. The available data are presented in Table 3-3. Both relatively
new, as well as older, incinerators are represented in the table.
For the 21 multiple-hearth incinerators listed in Table 3-3
uncontrolled particulate emission rates range from about 5 Ibs/dry ton
sludge input to over 450 Ib/ton. The average emission rate .for the 21
multiple-hearth -incinerators is 89 Ib/ton. Both of the incinerators with
the highest uncontrolled emi-ssion rates burn a sludge having relatively
low percentage of volatile solids. Nonetheless, in order to emit
450,Ib/ton, a.large percentage of the inert materials would have to be-
discharged with the furnace exhaust. As is discussed below, much of the ash
from these incinerators was probably being suspended by incoming air and
emitted with the Tlue gas. If the two incinerators having the highest
emission rates, Indianapolis #2 and MERL D, are excluded, the average
uncontrolled emission rates for the multiple-hearth incinerators listed in
Table 3-3 decreases to 51 Ib/dry ton sludge input.
Uncontrolled emission rates for the 12 fluidized-bed incinerators
listed in Table 3-3 range from 18 to 342 Ib/dry ton input with an average of
approximately 88 Ib/ton. The results obtained from the incinerators in
Lynwood and Edmonds, Washington, are notable in that they demonstrate the
wide fluctuations in uncontrolled emissions that can occur from a single
incinerator, burning a sludge of relatively constant volatiles and moisture
content, at a relatively constant loading rate.
The data available for electric incinerators indicate a range of
uncontrolled pafticulate emissions of from 3 to 17 Ib/ton with an average
of 11.2 Ib/ton.
3-10
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3.3.2 Factors Affecting Uncontrolled Particu'late Emission Rates From
Sewage Sludge Incinerators
There are numerous factors that may influence- the amount of particulate
matter that is discharged from a'sludge incinerator including characteris- ,
tics of the sludge and operating practices. Since 1978 attempts have been
made to identify correlations between these factors and emission rates, but
for the most part, no quantitative correlations have been found. It is
important to note that the operating variables of a sludge incinerator are
very closely inter-related. With the data presently available, it is
virtually impossible to delineate precisely individual cause and effect
relationships. Nonetheless, although the relationships between operating
"parameters and uncontrolled emission rates, cannot not be quantified, they
are discussed here in a qualitative manner.
3.3.2.1 Sludge characteristics. The two major characteristics of
sewage sludge which may, directly or indirectly, affect the rate of
uncontrolled emissions from sltrdge incinerators are the moisture content -and
the percent of volatile solids in the sludge feed. As the moisture content
of the sludge increases, or as the volatile soltds content decreases, more
fuel is required to burn the sludge. As more fuel is consumed, the amount
of air flowing through the incinerator is also increased. Higher air flow
rates increase the opportunity for particulate: matter to be entrained within
the exhaust gases. Sludges"having a low percentage of volatile solids
compound this problem by also increasing the quantity of inert materials
present.
The moisture content of the sludge can also have an indirect effect on
particulate emissions by making it more difficult to obtain a correct drying
profile within a multiple-hearth incinerator. As mentioned in the
preceding chapter, too rapid drying can lead to severe turbulence in the
Upper hearths. A high degree of turbulence in the drying hearth.(s) might
also increase the amount of solids that become entrained in the exhaust
gases.
Although the moisture content and volatile solids content of the sludge
can affect uncontrolled emissions, no direct correlation is clearly evident
3-14
-------
between these parameters. For example, the two incinerators located in
Merrimack, New Hampshire, listed in Table 3-3, burn a sludge having a very
high moisture content. The uncontrolled emissions from these furnaces,
however, were less than the average for the total number of multiple-hearth'
furnaces listed. Similarly, uncontrolled emissions from the ME-RL 8
incinerator were very low, although this unit burns a relatively high
moisture'content sludge.
3.3.2.2 Ash discharge system. One aspect of the design of multiple-
hearth incinerators that has been clearly related to uncontrolled emission
12
rates is the ash discharge system. In some multiple-hearth incinerators
air is allowed to enter into the'ash drop hole at the base of the furnace.
This allows virtually all of the fines in the ash to be suspended and drawn
back into the incinerator. The unusually high uncontrolled particulate
emission rates measured at both the Indianapolis #2 and MERL D incinerators
were probably caused by this problem. '
3.3.2.3 Operating practices. A number of incinerator operating
practices have the potential to impact uncontrolled emissions from sewage
. sludge incinerators.. These include, bu-t are -not limited to, (1) sludge feed
rate,- (2) excess air rate, (3) rabble arm speed, (4) combustion zone loca-
tion, (5) burner use profile, and (6) combustion air flow geometry. The
delivery of a consistent quality and quantity of sludge is key-in main-
taining steady state incinerator operations. However, no single operating
variable can have a totally independent effect on emissions. For example,
combustion zone location is influenced strongly by both the sludge feed rate
and the rabble arm speed, as well as by the burner use profile. Achieving
optimum operating conditions within a sludge incinerator requires an
optimization of many individual and closely inter-related parameters.
Operating practices have only been indirectly implicated as a factor -.
that may affect uncontrolled emission rates. There are only two documented
cases (discussed below) where changes in operating procedures have led to
reduced emissions from sewage sludge incinerators. However, in both of
these instances, emissions measurements were made at the outlet, rather than
3-15
-------
the inlet, of the control devices. Thus, it is not absolutely certain that
the emission reductions achieved were due entirely to decreases in the
amount of particulate being discharged from the furnace.' Operational
changes could also potentially lead to reduced emission rates by improving -
the efficiency of the scrubber. Scrubber efficiency will be affected by the
particle size distribution, the velocity of the furnace exhaust gas, as well
as by the concentration of the particulate matter in the exhaust gas.
However, it is unlikely that changes in operating practices could result" in
major decreases in controlled emissions by increasing the efficiency of the
control device alone; any major decrease in the controlled emission rate
would imply a corresponding decrease in the total quantity of particulates
entering the scrubber.
The first case where operational modifications have led to reduced
emissions was at the Indianapolis incinerators. The operational changes
were performed by the Indianapolis Center for Advanced Research (ICFAR) and
were primarily directed toward reducing the fuel consumption of the
incinerators.
"Ehe program instituted by ICFAR was based on theoretical analyses of
combustion kinetics, parametric data, and on data obtained from operational
trial runs. The result of these analyses was specific operating ranges for
key incinerator operating variables. For the Indianapolis "incinerators
(eight identical multiple-hearth furnaces with eight hearths each) the
following operating conditions were specified:
1. Maintain excess air at 25 to 50 percent.
2. Utilize cooling air from the center shaft for combustion air.
3. Maintain sludge combustion on hearth 6.
4. Utilize burners on hearth 6 only; if additional fuel is required
utilize hearth 4 burners.
5. Maintain sludge cake loading to design rates (7 tons/hour).
6. Employ slowest possible shaft speed (0.6 rpm).
7. Maintain furnace draft of .02 to .04-inches of water.
In addition, the program instituted by ICFAR called for installation of
instruments to monitor sludge flow'rate, oxygen levels in'the furnace
3-16
-------
exhaust, and fuel flow rates. Control systems were also installed to
remotely control fuel and air supply, into the incinerators. -Finally, a
detailed operating manual was devised and used in conjunction with on-site
operator training in the new operating mode.
Over an eight month, full scale, plant demonstration, fuel use was
reduced by 34 percent after the new operating program was begun. Moreover,
subsequent testing showed particulate emissions to have decreased by
approximately 70 percent compared to those measured before the fuel saving
program was instituted. In more detailed follow-up studies on incinerator
#2, an attempt was made to find direct correlations between emissions and
individual incinerator operating parameters. No consistent correlations
were found, however, although the lowest emissions overall occurred at the
slowest rabble arm speed. ICFAR concluded that additional tests were
required to fill the void that, exists in the analytical and operational
understanding of how incinerator operating modes affect particulate
emissions.
ICFAR instituted a similar operating program for the multiple-^hearth
incinerator located in Providence, Rhode Island.- As will be discussed- in
the following section, the Providence incinerator failed to meet the NSPS
during initial compliance testing; controlled emissions averaged 3.20 Ib/dry
ton sludge input during the first test in October 1980. The objective in
initiating the new operating mode at Providence was to reduce both fuel
consumption and particulate loadings to the scrubber. After the ICFAR
procedures were initiated in the spring of 1982, along with general improve-
ments in the condition of the plant, fuel consumption decreased by about
70 percent. Controlled particulate emissions were reduced by nearly
85 percent on the basis of an unofficial test conducted in July 1982, and by
. 50 percent on the basis of an official test performed in August 1982.
Similar to the Indianapolis incinerators, emission reductions of this
magnitude suggests that the rate of uncontrolled particulate release was
significantly reduced as the.result of changes in incinerator operating
practices. The experience at Providence will be-discussed in more detail in
the following section.
3-17
-------
Some additional insights into the relationship between operating
parameters and uncontrolled emission rates have been obtained from work
carried out by EPA's Municipal Environmental Research Laboratory (MERL). In
tests conducted on ten sewage sludge incinerators, reductions in gas
velocity were shown to reduce the amount of particulate discharged from the
furnace. The average particle size also decreased with decreasing gas
velocity, however. Although rabble arm speed adjustments were not shown to
have any effect on the amount of particulate discharged from the furnace,
there were some indications that decreasing rabble arm speed may result in
increases in the average particle size. Thus, lowering the speed of the
rabble arms may serve to compensate for the lower average particle sizes
obtained when steps ark taken (by lowering excess air or sludge feed rates)
to reduce gas velocities.
3.4 ACHIEVABILITY OF THE STANDARD
In*this section the achievability of the current NSPS for sewage sludge
incinerators is assessed on the basis of the experience that facilities
affected by the NSPS have "had in compl-ying with the -standard". First, the
compliance experiences of facilities installed prior to 1978 will be briefly
summarized. In addition, some follow-up studies performed in response to
the results obtained from the earlier review of the standard in 1978 will 'be
summarized. Second, the compliance experience of incinerators installed
after 1978 will be addressed.
3.4.1 Compliance Experience of Incinerators Installed Prior to 1978
The compliance experience of 26 incinerators was addressed in the 1978
review of the NSPS. Table 3-4 lists these incinerators, and provides
information on sludge characteristics, the types of control devices
employed, as well as the emission levels achieved.
Of the 26 incinerators, 4 multiple-hearth units were unable to meet -the
standard. Of these four, however, only the failure of the Merrimack,
New Hampshire 12 incinerator to meet the standard could not be reasonably
explained, although some evidence impl-ied that the high moisture content of
the sludge burned at Merrimack might be responsible. Between 1977 and
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1979 a number of modifications were made'to the scrubber. These included
alteration of the separator system to increase gas velocity.through the mist
eliminator, decrease of the venturi throat diameter, and the addition of
four impingement plates within the cyclonic separator housing. The success'
of these modifications in reducing emissions from the Merrimack incinerators
is shown in Table 3-5. As can be seen in Table 3-5, both incinerators
eventually were able to demonstrate compliance with the NSPS once the
pressure drops of the control devices were increased to 40 to 42 in. W.6.
On the basis of the data collected during the 1978 review, no
quantitative correlation could be established between controlled emission
rates and either the pressure drop of the scrubber or the moisture content
of the sludge.- As can be-seen in Table 3-4, some incinerators were able to
achieve the standard at relatively low pressure drops. For example, the
three incinerators in Cincinnati, Ohio, demonstrated compliance while using
impingement-tray scrubbers operating at pressure drops of less than 10 in.
W.G. In addition, some plants burning a sludge of relatively high moisture
content (Maryville, TN, for example) easily met the standard with control
devices operating at moderate pressure drops."
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and controlled emissions would become apparent if a larger data base were
used, a follow-up study was initiated.20 ..For this study, detailed data were
collected on 60 sludge incinerators. However, no strong correlations could
be found between emissions and either scrubber pressure drop, sludge
moisture content, or sludge loading rate. Figure 3-2'shows the relationship
found between emissions from multiple-hearth incinerators and the pressure
drop of the control devices. The moisture content of the sludge is also
shown. As can be seen, the relationship is highly scattered and no
correlation is apparent.
Although no quantitative correlation has been generally established
between emissions and scrubber pressure drop, this does not imply that the
pressure drop of a control device has no impact on particulate emissions.
For any given incinerator, and any given scrubber, emissions will increase
as the pressure drop is decreased-. Inversely, emissions can be decreased by
3-21
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3-23
-------
increasing the pressure drop of the scrubber (as seen in Table 3-5), at
least until the particle size cut-off for the scrubber is reached. The
analysis discussed in the preceeding paragraphs only indicates that there is
no specific emission rate (Ib/dry ton sludge) that can be universally
correlated with a specific pressure drop.
3.4.2 Compliance Experience of Incinerators Installed After 1978
Sewage sludge incinerators that have come under the NSPS since 1978 are
listed in Table 3-6. Information is provided on the types of control
devices used, sludge characteristics, and emission rates achieved by these
incinerators.
Of the 17 multiple-hearth incinerators listed in Table 3-6, 12 are _in
compliance with the NSPS. Four new units have not yet been tested. Only
the incinerator located in Providence, Rhode Island, has failed to achieve
the NSPS. The Attleboro incinerator did, however, fail to meet the standard
during initial compliance tests. All four of the fluidized-bed incinerators
installed since 1978 have achieved the NSPS. -Of the four electric
incinerators listed in Table 3-6, only one is in compliance with the
standard. One electric incinerator has not .yet been tested. Incinerators '
that have failed to meet the standard are discussed in more detail in
Section 3.4.2.1.
The average particulate emission rate achieved by all new multiple-
hearth incinerators is 0.76 Ib/dry ton of sludge input. If the Providence
incinerator is excluded, the average emission rate for facilities in
compliance is 0.67 Ib/ton. This is approximately one-half of the allowable
standard. The pressure drops of the scrubbers employed to meet the standard
ranged from 10 to 45 in. W.G. The average pressure drop for all 17
multiple-hearth incinerators is about 28 in. W.G. Apparently, however, many
of the multiple-hearth incinerators listed in Table 3-6 could have achieved
the standard at lower pressure drops. In one instance, Youngstown, Ohio,
the standard is being met with a scrubber operating at a pressure drop of
only 10 in. W.G.
The average emission rate for the fluidized-bed incinerators that have
come under the NSPS since 1978 is 0.74 Ib/ton-. The pressure drops of the
3-24
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3-26
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scrubbers employed to meet the'Standard were between 30 and 42 in. W.6. As
is the case for multiple-hearth incinerators, however, the standard could
evidently be achieved by these units at more moderate pressure drops.
For the three new electric incinerators that have been tested, the
average emission rate is 2.22 Ib/ton. However, the control devices in use
at each of these facilities operate at low pressure drops (average =
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considered be Best Available Control Technology {BACT) for sewaae sludge
21
incinerators.
3.4.2.1 Discussion of incinerators that have failed to achieve the
NSPS. In this-section the experience at the Providence incinerator is
addressed in more detail. Since the Attleboro incinerator failed to pass
its original compliance tests, the experience there will also be briefly
reviewed. Because neither of the two electric incinerators that have failed
to meet the standard are equipped with BACT, these units will not be
considered in an assessment of the achievability of the standard.
The incinerator located in Attleboro, Massachusetts, is a seven hearth
unit with a rated sludge capacity of 3,350 Ib/hour-(dry basis). The
incinerator burns both sludge and scum from primary and secondary wastewater
treatment processes. The maximum design feed rate of the scum is
1.4 gallon-s/minute. Operation of the incinerator is controlled by a
computer, and is designed for completely automatic operation.
The Attleboro incinerator failed to achieve the NSPS when first tested
in December of 1981. Failure to meet the standard was due to a breakdown of
oo
the computer while the test was in progress. Once the problem with the
computer control system was rectified, a second series of tests were
conducted in February of 1983. The incinerator was also unable to achieve
compliance during this second series of tests, however. The main problem
encountered during this test was in the scum feeding mechanism.23
Originally, the scum was atomized and injected into hearths four and six.
Besides a number of mechanical problems associated with the scum atomization
and injection systems, the scum injected into hearth four was not being
completely combusted. At the time of the test, it was determined that if
3-27
-------
the scum were injected into hearth six only, without atomization, then the
24
incinerator would be able to achieve the standard. The incinerator was
retested in this mode in May 1983, and demonstrated compliance with the
NSPS.
The incinerator located in Providence, Rhode Island, is multiple-hearth
design (nine hearths) and a rated capacity of from 2.2 to 2.8 dry tons of
sludge per hour. Design specifications call for a sludge moisture content
of 72 to 78 percent with a volatile solids content of 55 to 75 percent. The
incinerator was originally constructed in 1959. Extensive renovations were
made to the incinerator beginning in the late 1970's. These modifications
were severe enough to bring the unit under the NSPS at that time.
Compliance testing was initially conducted in October 1980. Measured
emissions averaged 3.20 Ib/dry ton input during these tests. None qf the
three separate test runs showed emissions to be within the NSPS. During
subsequent investigations, a number of problems were identified which could
have contributed to the failure of the incinerator- to achieve the standard.
Many of the burners were not functioning properly, and some were not
operating at'all. In addition, numerous'instrumentation and control systems
were out of order, including the center shaft speed alarm, the temperature
25
recorder, and the high/low alarms for the ash slurry tank. The volatiles
content of the sludge,burned during the test was only 50 to 55 percent,
which was slightly lower than the minimum design specifications. The sludge
loading rate was also only about 65 percent of design capacity. Most likely
as a result of the poor sludge quality and the reduced loading rate, combus-
tion was not occurring on the proper hearths. The scrubber inlet water was
also very dirty due to either a dirty watar strainer or to an exceptionally
high total solids content in the treatment plant effluent (no quantitative
measurements of the scrubber water solids content were made at the time of
nc
the test). It also was noted in the test report that the paint used to
coat the interior of the stack was peeling off during testing and collecting
27
in the sampling train. Finally, during the 1980 tests an oil-fired
afterburner was in use which could have also contributed to excessive
28
particulate emissions.
3-28
-------
As noted previously In the section on uncontrolled emission
characteristics, subsequent to the initial compliance testing, a number'of
incinerator modifications and operational changes were made in an effort to
improve control of the incineration system and to reduce-emissions. Many of
these changes were performed in consultation with ICFAR. The following
system modifications and operational changes were initiated after the 1980
test:
1. A temporary oxygen monitor was installed upstream of the control device
to aid operators in controlling the furnace.
2. Inoperative burners were replaced, and other burners adjusted.
3. A sludge feed rate indicator was installed in the control room.
4. The sludge'dewatering process was changed to a 24 hour cycle to
increase the dewatering efficiency of the filter presses and to ensure
a more consistent quality feed to the incinerators.
5. All existing instruments ami controls were repaired and calibrated.
6. An operating procedure was developed to ma-intain combustion on the
proper hearths.
7. An on-site operator training.program was instituted.
8. The scrubber system was inspected and all necessary maintenance carried
out.
After these changes were performed, the Providence incinerator was
unofficially tested. Emissions during those tests averaged 0.65 Ib/dry ton
sludge input; one-half of the allowable NSPS limit and an 80 percent
reduction from the results obtained in the 1980 test. On the basis of these
results, an official compliance test was conducted three weeks later in
August 1982. However, during the August test the incinerator failed to meet
the allowable emission limitation.
The results of all three separate emission tests performed on the
Providence incinerator are summarized in Table 3-7. Important operating
variables as measured during these tests are also provided. The most
noticeable difference between the 1980 tests and those performed in 1982 is
the quantity of fuel consumed. Prior to initiation of the fuel conserving
operational mode, the Providence incinerator burned, on average, 34 gallons
3-29
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of No. 2 fuel oil per ton of sludge feed. After the operational .changes
were made, average fuel use declined to less than 10 gal/ton. Flue gas flow
rates decreased as less fuel was required to burn the sludge. The average
gas flow rate during the 1982 test was 70 percent of what it was in 1980. '
The data in Table 3-7 also indicate that the changes made in the dewatering
cycle were successful in reducing the moisture content of the sludge. The
average sludge moisture content during the 1980 test was 76.4 percent,
compared to average moisture contents of 67.8 percent and 69.9 percent
during the July and August, respectively, 1982 tests. Finally, during the
1982 tests, the sludge feed rate was within design capacity while in the
1980 test sludge was being fed to the incinerator at less than minimum
design loadings. - .
The Providence incinerator failed to meet the NSPS during the August
1982 test, owing to the sharp increase in emissions during Run No. 2. There
were, however, -repeated upsets in the operation of the incinerator while the
test was in progress. These upsets were, for the most part, caused by
externa-1 factors that ultimately interfered w.ith the operation of the
incinerator* One impact o.f these upsets on the operation of .the incinerator
is reflected in the air flow rates. As discussed earlier, an increase in
the air flow rate through the incinerator should result in some increase in
the amount of particulate discharged from the furnace. During run No. 2 of
the August 1982 test, the average flue gas flow rate was nearly 20 percent
higher than it was during the first run, and emissions increased
substantially. The problems encountered during the August test are reviewed
below, and are based on the conclusions drawn by the operators of the
Providence incineration facility.29
Prior to the test, the incinerator had been operated only inter-
mittently due to continuing mechanical problems with the incinerator and
associated equipment. On the day of the test, sludge was first fed to the
incinerator only five hours before the test was scheduled to begin. Plant
personnel felt that this was insufficient time for the incinerator to
stabilize, especially at the required feed rates.
3-31
-------
Approximately one minute after the start of the first run an electrical
problem caused all of the burners in the incinerator to go out of service.
In the ten minutes it took to correct the problem, the operation of the
incinerator became even less stable. .A further failure of the electrical >
system occurred approximately one-half hour later. While the problem was '
being repaired, all power to the circuit was shut-off. Many of the
incinerator control systems were tied to this circuit, including the oxygen
analyzer and the sludge scale. The operators of the incinerator, however,
were not informed that the power to these instruments had been cut. As a
result, a number of incorrect control responses were made, further.
disrupting the overall operation of the furnace.
t
Problems were also experienced with the dewatering system during the
first test run. Due to a problem with a sludge feed pump, two of the vacuum
filters went off-line causing the sludge feed to be reduced by about
50 percent for a short period of time. While the sludge feed was
interrupted, there was a noticeable increase in stack gas opacity.
The various problems experienced while the first run was. in progress
c'arried over to the second test run. Several of the burners .were not
operating which limited the operator's control of the furnace. Continuing
problems with the sludge feed pumps led to an unsteady rate of feed to the
furnace. The major problem occurring in the second run, however, was caused
by waste oil which had inadvertently been stored in one of the sludge
holding tanks. Just prior to the start of run Mo. 2, the operators began to
feed the furnace with the sludge from this tank. The presence of the waste
oil caused periodic flare-ups in the furnace. Sludge began to burn in the
drying hearths. In order to control these flare-ups, the airflow through
the incinerator was increased by opening up the access doors on the upper
hearths. This was estimated to cause a three- to five-fold increase in the
air flow through the system. When the air flow was increased, carbon
particles which had built up inside the exhaust system were dislodged.
* *
Whenever the system air flow was increased, there was a corresponding
. ..-, 30
increase in visible emissions.
3-32
-------
In summary, the Providence incinerator and its associated systems were
not in good working condition when first tested in 1980 for compliance with
the NSPS. A program was subseqently carried out to upgrade the equipment at
the plant and to improve the overall operation of the facility. This
program was successful in reducing fuel consumption and in reducing
particulate emissions, as evidenced by the unofficial test conducted in
July, 1982. During the August compliance test, a series of equipment
failures prevented the incinerator from reaching a stable level of
operation. For the most part, these failures were unusual and can not be
considered as representative of typical operating conditions at the
facility. . In absence of the problems -experienced during the August test,
the Providence-incinerator could be reasonably expected to achieve the NSPS.
3.5 EMISSIONS OF TRACE ELEMENTS FROM SEWAGE SLUDGE INCINERATORS
As noted in Chapter 2, one of the original basis of the development of
the NSPS for sewage sludge incinerators was their potential to emit toxic
trace elements .into the atmosphere. .In this section,.data on trace.element
emissions from sludge incinerators are presented and briefly discussed.
3.5.1 Data Sources and Methods of Analysis
Relatively limited data are available on trace element emissions from
sludge incinerators. The most complete set of data available is that
assembled by EPA's Municipal Environmental Research Laboratory (MERL) from
31
tests on ten incinerators. In these tests sampling was conducted at both
the inlet and the outlet of the control device. Thus, the MERL data
includes measurements of both controlled and uncontrolled trace element
emissions. Complete data are available for only 6 of the 10 incinerators
tested, however.
MERL employed a Source Assessment Sampling System (SASS). Since only
one sampling train was available, measurements at the scrubber inlet and
outlet were not made simultaneously. This could potentially introduce
significant error into the data, because release of trace elements'from
sludge incinerators can be highly variable over relatively short periods of
3-33
-------
time. The particles collected in the front end of the SASS train were
separately digested and analyzed for trace element content using a spectro-
photometer system with an inductively coupled argon, plasma source.
Another set of data on trace element emissions from sludge incinerators
was developed from tests conducted by EPA's Environmental Sciences Research
op
Laboratory (ESRL). Four sludge incinerators were tested by ESRL. A
standard EPA Method 5 sampling train was employed. Samples were collected
at the outlet of the control device only. Analysis of the particulates
(probe and filter catch) for trace element composition was performed through
X-ray fluorescence spectrophotometry.
The final data presented .in this section are for the incinerator
installed in Atlantic City,'New Jersey. The Atlantic City unit is
relatively ftew, having been installed after 1978. The trace element content
(cadmium and lead only) of the particulates collected during compliance
testing v/ere measured using standard X-ray fluorescent techniques.
3.5.2 Uncontrolled Emissions of Trace Elements
Table 3-8 summarizes-the data collected by MERL on uncontrolled
emissions of trace elements from six sewage- sludge incinerators.
The uncontrolled rate of particulate emissions are provided for
reference. The highest uncontrolled trace element emissions from the six
incinerators', we re for lead (Pb). Lead emissions ranged from 0.03 Ib/ton to
1.77 Ib/ton. Average uncontrolled Pb emissions (0.45 Ib/ton) were more than
double those for chromium. Uncontrolled emissions of cadmium (Cd) ranged
from 0.002 Ib/dry ton sludge to 0.07 Ib/ton. The average rate of
uncontrolled Cd emissions was 0.03 Ib/dry ton. Uncontrolled chromium (Cr)
emissions were higher, ranging from 0.007 Ib/ton to 0.63 Ib/ton (average =
0.18 Ib/dry ton). Uncontrolled emissions of Nickel (Ni) averaged 0.08
Ib/dry ton, but were as high as 0.33 from incinerator D. Arsenic (As)
emissions were generally negligible, with the exception of incinerator A
which emitted 0.64 Ib/dry ton sludge. Four of the six incinerators,
however, showed uncontrolled arsenic emissions of less than 0.001 Ib/ton.
Overall, incinerators A and D had the highest rates of uncontrolled trace
element emissions.
3-34
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3.5.3 Controlled Trace Element Emissions
* Data on controlled emissions of trace elements from sewage sludge
incinerators are presented in Table-3-9.
As in the case of the data on uncontrolled emissions, controlled lead .
emissions are the highest of all-the trace elements analyzed. Controlled
lead emissions ranged from 0.002 Ib/ton to 0.16 Ib/ton, and averaged
0.05 Ib/ton for all 12 tests. Controlled emissions of cadmium averaged
approximately 0.01 Ib/dry ton sludge for the 12 incinerators listed. This
is about one-third of the average uncontrolled emission rate for cadmium.
Average controlled emissions of chromium are four percent of the average
uncontrolled rate. Average controlled emissions of nickel-are also about
four percent of the average uncontrolled rate. In all cases, controlled
emissions of arsenic were negligible. The arsenic emission rate of
0.02 Ib/ton reported for incinerator MERL' C is probably in error since the
uncontrolled As emission rate reported for this incinerator was
0.0008 Ib/ton. As mentioned earlier, however, the inlet and outlet samples
were not collected simultaneously. Thus, the arsenic emission rate during
the outlet sampling could have conceivably been higher than it was when the
measurements were made at the scrubber inlet.
The various data sources are in relatively good agreement for
controlled cadmium emissions. 'Controlled cadmium emissions ranged from
0.003 to 0.06 Ib/ton, with an average emission rate of ,0.01 Ib/ton. The
cadmium data from the Atlantic City incinerator illustrate the variability
in emission rates that can occur from an individual incinerator.
The data for controlled chromium and nickel emissions are also
reasonably consistent. Chromium emissions range from 0.0002 Ib/ton to
0.03 Ib/ton; nickel emissions range from 0.0002 Ib/ton to 0.008 Ib/ton.
The lead emissions rates reported for both the ESRL incinerators and
the Atlantic City incinerator are generally lower than those given in the
MERL data. The highest reported controlled emission rate for lead is
0.16 Ib/dry ton. . .
3-36
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3.5.4 Control Efficiencies for Trace Element Emissions
Using the MERL data, the efficiency of control devices in reducing
emissions of trace elements from sewage sludge incinerators can be
estimated. In Table 3-10 both the type, and the operating pressure drops, ,
of control devices in use on the MERL incinerators are listed. The overall
control efficiencies for particulate emissions are shown, and these can be
compared to the control efficiencies calculated for the five trace elements.
Based on average data only, control efficiencies for trace metals are
less than those for total particulates. The lowest control efficiency is
for lead emissions, which'average 63 percent. . The next lowest removal
efficiency, 83 percent, is for cadmium.
There are, however, significant variations among individual
incinerators. In some cases, trace elements are controlled more efficiently
than total 'particulates. For example, the calculated removal efficiency of
nickel for incinerator MERL C, 95 percent, is higher than the 91 percent
calculated'for particulates as a whole. There is also significant
variability within individual trace element categories. The removal
'efficiency of both cadmium and chromium'ranges from about 55 percent to
about 90 percent. An even greater variability is seen in the percent
reduction in lead emissions that can be achieved by typical control devices.
There is no apparent correlation between scrubber pressure drop and either
control of trace elements or control of total particulates.
3.6 NATIONAL EMISSIONS FROM SEWAGE SLUDGE INCINERATORS
Estimates from the NEEDS Survey discussed in Chapter 2 indicate that an
increase of 3,713 million gallons per day of wastewater will flow into new
treatment plants equipped with incinerators between 1982 and the year 2000.
Assuming a linear increase, a 1,031 million gallons per day increase is
estimated to occur for 18 new incineration facilities between 1984 and 1989.
For an average sludge production of 0.65 dry tons per million gallons of
was,tewater (see Section 2.2.2), the flow of sludge into new incineration
facilities is estimated to be 245,000 dry tons in the year 1989. Assuming
all new'incinerators produce particulate emissions at a rate equal to the
3-38
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current standard (1.3 Ib/dry ton sludge), "national particulate emissions
from all new incinerators are estimated to be 160 tons in 1990. Based on a
weighted average uncontrolled particulate emission rate for. all incinerators
of 52 Ib/dry ton sludge, national emissions from new sludge incinerators
would be approximately 6,400 tons if the NSPS were not in place.
3-40
-------
REFERENCES FOR CHAPTER 3
1. Hefland,- R. M. A Review of Standards of Performance for New Stationary
Sources - Sewage Sludge Incinerators. U.S. Environmental Protection
Agency, Research Triangle Park, N. Carolina, EPA-450/2-79-010,
March 1978.
2. Shelton, R. and A. Murphy. Particulate Emission Characteristics of
Sewage Sludge Incinerators - NSPS Review. Acurex Corporation. U. S.
Environmental Protection Agency, Research Triangle Park, North
Carolina. EPA Contract No. 68-02-3064, Final Draft. March 1980.
3. Hobbs, B. Testing and Evaluation of Sewage Sludge Incinerator at Fields
Point Wastewater Treatment Facility Providence, Rhode Island. GCA
Corporation. (Prepared for the Narragansett Bay Water Quality
Management District Commission Providence, Rhode Island.) August,
1982.
t
4. Envirotech Corporation. Multiple Hearth Furnace Test, Hampton Road
Sanitation District Williamsburg Sewage Treatment Plant Williamsburg,.
Virginia. EIMCO-BSP Job No. 5815, January, 1976.
5. Office of Air Quality Plannina and Standards. National Emissions Data
System (NEDS). Unpublished data of the National Air Data"Branch,
Monitoring and Data Analysis Division,. U:S. Environmental Protection
Agency, Research Triangle Park, N. Carolina, April 19, 1983.
6. Wall, H. and J. B. Parrel!. Air Polution Discharges from Ten Sewage
Sludge Incinerators. Municipal Environmental Research Laboratory, U.S.
Environmental Protection Agency, Cincinnati, Ohio, Draft Report,
February, 1981.
7. Mogul Corporation. Emission Evaluation for Merrimack Wastewater
Treatment Plant, Merrimack, New Hampshire. (Prepared for Envirotech
Corporation) September, 1977.
8. Liao, P. B. and M. J. Pilat. Air Pollutarvt Emissions from Fluidized
Bed Sewage Sludge Incinerators. Water and Sewage Works,
February, 1972.
9. Letter from F. K. McGinnis, Shirco Incorporated, to A. Briggs, Accurex
Corporation. January 17, 1980. Emissions Data for Infrared Municipal
Sewage Sludge Incinerators.
10. -Shirco Incorporated. Source Emissions Survey, North Texas Municipal
Water District, Rowlett Creek Plant, Piano, Texas. May, 1978.
11. Reference 2.
3-41
-------
12. Memo from H, Wall, Municipal Environmental Research- Laboratory, EPA, to
R. Meyers, Emissions Standards and Engineering Division, EPA.
May 15, 1980. Review of Report, "Participate Emission Characteristics
of Sewage Sludge Incinerators, NSPS Review."
13. Memo from A. J. Verdouw, Indianapolis Center for Advanced Research, , y
Inc., to E. W. Waltz, Indianapolis Center for Advanced Research, Inc.
June 21, 1979. Incinerator Air Polution Test Reports.
14. Reference 6.
15. Verdouw, A. J., E. W. Waltz, and W. Bernhardt. Plant-scale
Demonstration of Sludge Incinerator Fuel Reduction. Indianapolis
Center for Advanced Research. Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio, EPA
Contract No. S306248010, 1981.
16. Reference's.
17. Reference 1.
18. Memo from R. Myers, Emissions Standards and Engineering Division, EPA,
to S.T. Cuffe, Emissions Standards and Engineering Division,.EPA, May
13, 1981. Review of NSPS for Sewage Slu'dge Incinerators.
19. Piccot, S. D., Radian Corporation, to N. T. Georgieff, EPA. Vi.sit to
the Sludge Incinerator at Merrimack, 'New Hampshire (Draft- Trip'Report),'
November, 1983.
20. Reference 2.
21. Office of Air Quality Planning and Standards. Background Information
* for Proposed New Source Performance Standards. U.S. Environmental
Protection Agency, Research Triangle Park, N. Carolina, AAPTD-1352a,
March 1973.
22. Telecon. S. Piccot, Radian Corporation, with J. Hanley, Attleboro
Advanced Waste Water Treatment Facility. September 30, 1983. Site
visit to the Attleboro Incinerator.
23. Telecon. R. M. Dykes, Radian Corporation, with J. Winkler,
Massachusetts Department of Environmental Quality Engineering.
September 1, 1983. Compliance Status of the Attleboro Sludge
Incinerator.
24. David Gordon Associates, Inc. Stack Sampling Report, Compliance Test #3
at Attleboro Advanced Wastewater Treatment Facility, Attleboro,
Massachusetts. (Prepared for Envirotech Corporation) May 31, 1983.
3-42
-------
25. Letter from J.
Moniz, Field's
Morenzi, Charles
Point Wastewater
Outline of Reasons
Compliance Test.
26. Reference 3.
J. Krasnoff and Associates, Inc.."to R.
Treatment Facility. March 27, 1981.
Why the Sludge Incinerator Failed to Pass the
27. Recon Systems, Inc. Stack Sampling Report, Municipal
Incinerator #1, Providence, Rhode Island. (Prepared
Engineering and Research Corporation). November 11,
Sewage Sludge
for Nichols
1980.
28. Telecon. R. M. Dykes, Radian Corporation, with E. Waltz Indianapolis
Center for Advanced Research. January 18, 1984. Factors affecting
emissions from sludge incinerators.
29. Letter from E. R. Jankel, Narragansett Bay Water .Quality Management
District Commission, to H. F. Laing, U.S. Environmental Protection
Agency (Region I). October 4, 1982. Review of Operational Upsets
During August, 1982 Compliance Test.
30. McCabe M., R. A. Graziano, and H. F. Schiff. Testing and Evaluation
Sewage Sludge Incinerator at Field's Point Wastewater Treatment
Facility, Providience, Rhode Island. GCA Corporation. (Prepared for
Narragansett Bay Water Quality Management District Commission,
Providence, Rhode Island) No. 5-631-001, September, 1982.
31. Reference 6.
32. Bennett, R. L. and K. T. Knapp. Characterization of Particulate
Emissions from Municipal Wastewater Sludge Incinerators. Environment
Science and Technology, Vol. 16, Mo. 12, 1982.
33. Letter from F. W. Giaconne, U.S. Environmental Protection Agency
(Region II), to B. F. Mitsch, Radian Corporation. March 21, 1983.
Results of Stack Tests on Atlantic City Incinerator.
of
3-43
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4.0. CONTROL COSTS
4.1 INTRODUCTION AND SUMMARY
4.1.1 Introduction
This chapter presents the costs of controlling participate emissions
from sewage sludge incinerators. Section 4.2 presents the details of the
cost components. The capital and annualized costs for a range of
multiple-hearth and fluidized-bed sewage sludge incinerator sizes are
presented in Section 4.3. Cost effectiveness of controlling particulate
emissions from these incinerators is presented and discussed in Section 4.4.
4.1.2 Summary of Findings
The cost effectivness of controlling particulate emissions from sewage
sludge incinerators is estimated to range from $191 to $1743 per ton
removed. These costs are based on conservative capital cost estimates for
venturi/impingement-tray scrubbers operating at pressure drops of from
2"0 in. W.G. to 40 in. W.6. Cost effectiveness is most sensitive to
incinerator size. The highest cost effectiveness was calculated for a
500 dry Ib/hr fluidized-bed incinerator equipped with a venturi/impingement-
tray scrubber operating at a pressure drop of 40 in. W.G. The total cost'
impact of operating at lower pressure drops is very small, on the order of
$10 per ton of particulate removed for each 10 in. change in pressure drop.
For equivalent size incinerators, the cost effectiveness of reducing
particulate emissions from fluidized-bed incinerators is less than that from
multiple-hearth incinerators.
4.2 COST COMPONENTS
The estimated control costs for sewage sludge incinerators are based on
the most prevalent control system applied to incinerators built since 1978;
the venturi/impingement-tray scrubber. Flue gas from the incinerator is
first ducted to a variable throat venturi scrubber. In the venturi the dust
particles agglomerate with the scrubbing liquid. The gas stream then passes
4-1
-------
through a flooded elbow which agglomerates the larger, heavy droplets. An
impingement scrubber cools the gas, further reduces particulates, and
eliminates mist with a mist eliminator. The gas then passes onto the stack.
A more detailed description of a typical venturi/impingement scrubber
system was presented in Chapter 3.
The equipment specifications for the control system costed in this
Chapter are shown in Table 4-1. Equipment and materials specifications were
based on information supplied by vendors of these control devices, and on
specifications for actual plants. For the range of pressure drops assumed
for the variable throat venturi (12 to 22 in. W.G.), 3/16 inch type 316
stainless steel was recommended. Data supplied by another vendor of these
systems confirmed that 3/16 inch type 316 stainless stee-1 would be used for
o
both the venturi and impingement-tray scrubbers. Material specifications
for circulation tanks, piping, fans, pumps, and ducting are consistent with
design data for actual plants using similar control systems. ' '
The individual capital cost-components and the general methodology used
for calculating total capital costs are presented in Table 4-2. Direct
capital costs cpns-ist of the basic and auxiliary equipment-costs in addition-
to the labor and material required to install the equipment. Indirect costs
are those costs that are not attributable to specific equipment items.
Contingencies are also included in total capital costs to compensate for
unpredicted construction costs and other unforeseen expense's.
Equipment costs for the venturi scrubber, flooded elbow and the fan
were calculated using cost-equations from ."Capital and Operating Costs of
Selected Air Pollution Control Systems" (6ARD). The equipment 'costs for
the impingement scrubber, scrubber water circulation tanks, ducting, piping,
and pumps were calculated using information contained in the EPA report
"Costs of Uncontrolled Non Fossil Fuel-Fired Boilers and PM Controls Applied
to these Boilers". The installation factors for all equipment were also
based on this report, and are presented in Table 4-3. These factors are
multiplied by the equipment cost to yield the installation cost for each
incinerator.
4-2
-------
TABLE 4-1. EQUIPMENT SPECIFICATIONS FOR VBNTURI/IMPINGEMENT-TRAY
SCRUBBER CONTROL SYSTEM
EQUIPMENT
SPECIFICATIONS
Venturi Scrubber
Impingement Scrubber
Venturi Scrubber
Circulation Tank
Fan & Auxiliaries
External Scrubber
Water Piping .
Impingement Scrubber
Water Pump
Ducting
3/16" inch thick 316 stainless steel, automatic
variable throat venturi
Includes: Venturi, elbow, pumps, controls,
quencher
3/16" inch thick 316 stainless steel
Includes: Impingement scrubber, mist eliminator
5 minute liquid holdup time, 304 stainless
steel storage tanks
Includes: Storage tanks
Radial tip centrifugal fan (60 hp), carbon steel
Includes: Fan motor, and starter, dampers,
V-be'lt drive
100 ft length of pipe, 304 stainless steel, .
Schedule 40
20 ft piping height; centrifugal, open, drip
proof, Stainless steel.
Includes: Pump, motor, and starter
(and a spare)
30-40 ft of straight ducting; 10 gauge stainless
steel.
Includes: Ducting
4-3
-------
TABLE 4-2. CAPITAL COST COMPONENTS
(1) Direct Costs
Equipment
+ Installation
Total Direct Costs
(2) Indirect Costs
Engineering - 10% of direct costs
Construction and Field Expense - 10% of direct costs
Construction Fees - 10% of direct costs
Start Up Costs - 2% of direct costs
f Performance Costs - 1% of direct costs
.Total Indirect Costs - 33% of direct costs
* 4
(3) Contingencies - 20% of (Total indirect- Costs + Total Direct Costs)
(4) .Total Capital Cost = Total Indirect Costs + Total Direct Costs &
Contingencies
4-4
-------
TABLE 4-3. EQUIPMENT INSTALLATION FACTORS9
Equipment Item Installation Cost Factor
Wet Scrubber '0.68
Circulation Pump 1.49
Circulation Tank 0.93
Fan 1.18
Ducting 1.6
External Piping , " O.lb
... ... ...... .T.__ . . . i.i ^ *
Items included in installation cost are the following:
(1) freight and taxes
(2) foundations and supports
(3) erection and handling
(4) electrical
. (5) internal piping
(6) insulation
(7) painting
Estimated from Guthrie, "Process Plant Estimating, Evaluation and Control",
p. 462.
4-5
-------
The- operating and maintenance (O&M) cost components are listed in
Table 4-4. Direct O&M costs include operating and maintenance labor,
supervision, spare parts, and electricity used for pumps, fans, and
controls. Indirect operating costs include payroll and plant overhead which
are based on some key O&M cost components (direct labor, supervisory .labor,
maintenance labor, and spare parts).
Telephone contacts with each of the 23 incineration facilities
installed since 1978 indicated that capacity utilization ranges from about
60 to 120 percent. A mid-point value of 80 percent (7008 hours/year) was
assumed to calculate annual operating costs. Direct labor was assumed to be
8
2 man-hours/shift and maintenance labor 1 man-hour/shift. Supervisory
labor was estimated to be 15 percent of the direct labor costs. The unit
costs used for O&M cost calculations are shown in Table 4-5.
Total annualized costs are the sum of the annual O&M costs and the
annualized capital charges. The annualized capital charges include the
piayoff of the capital investment (capital recovery), general and
administrative costs, taxes, and insurance.
Table 4-6 presents the methods used to calculate the individual
annualized capital charges. The capital recovery cost is determined by
multiplying the capital recovery factor, which is based on the real before
tax interest rate and the equipment life, by the total capital cost.3 For"
this analysis a 10 percent real interest rate and a 15 year equipment life
are assumed. This translates into a capital recovery factor of 13.15
percent. The real interest rate of 10 percent was selected as a typical
constant dollar rate of return on investment to provide a basis for
calculation of capital recovery charges.. Table 4-6 also presents the
methods used to calculate the other annualized capital charges.
4.3 CAPITAL AND ANNUALIZED COSTS
This section presents the capital and annualized costs for the control
system discussed in Section 4.2. The costs for controlling particulate
emissions from multiple-hearth incinerators of various sizes are discussed
in Section 4.3.1. In Section 4.3.2 the control costs for fluidized-bed
incinerators of various sizes are presented.
4-6
-------
TABLE .4-4. .OPERATING AND MAINTENANCE COST COMPONENTS
(1) Direct Operating Costs
Direct Labor
Supervision
Maintenance Labor
Spare Parts
+ Electricity
Total Direct Operating Costs
(2) Indirect Operating Costs
. Payroll - 30% of (Direct Labor + Supervision Labor +
Maintenance Labor)
+ Plant - 26% of (Direct Labor + Supervision + Maintenance
_ Labor + Spare Parts) _ _
Total Indirect Operating Costs
(3) . Total. Annual Operating and Maintenance Costs
= "Total Direct +' Total Indirect Operating Costs
4-7
-------
TABLE 4-5. UNIT COSTS USED IN O&M COST CALCULATIONS
*
' ' January 1983 $
Utilities
Electricity^ $0.0503/kwh
Labor h
Direct Labor $11.75/man-hr
Supervision Labor0 $15.28/man-hr
Maintenance Labor $14.34/man-hr
aMontH1y Energy Review, April 1983.
Average of Chemical & Allied Products and Petroleum direct labor wages,
Monthly Labor Review, April 1983.
cEstimated at 30 percent over direct labor rate.
Estimated at 22.percent over direct labor rate.
4-8
-------
TABLE 4-6. ANNUALIZED COST COMPONENTS
(1) Total Annualized Cost = Annual Operating Costs + Capital Charges
(2) ' Capital Charges = Capital recovery + miscellaneous (G&A, taxes and
insurance)
(3) Calculation of Capital Charges Components
A. Capital Recovery = Capital Recovery Factor (CRF) x Total Capital
Cost
CRF-1 <1+1>
i = interest rate
n = number of years of useful life of control system
Item n_ j_ CRF
Control System 15 10 0.1315
B. G&A, taxes and insurance = 4% of total capital cost
4-9
-------
"4.3.1 Multiple-Hearth Incinerator Control Systems
Table 4-7 shows the operating parameters for the model multiple-hearth
incinerators, and associated control systems. Capital and annualized costs "
were calculated for model multiple-hearth incinerators of sizes ranging from
0.5 dry ton sludge/hour to 4.0 dry ton sludge/hour. This size range
represents the majority of multiple-hearth incinerator sizes. Operating
parameters for the model control systems for multiple-hearth incinerators
were developed from design data for actual plants, from contacts with
equipment vendors, and from theoretical calculations. The moisture content
and volatile solids content of the sludge represent typical values for
currently operating incinerators (see Chapter 3). Excess air rates, flue
gas flow rates, and liquid flow rates are based on design data for two
recently installed incinerators as well on information provided by
n -in in 19
vendors. 3»±U»-IA»-L£- Operating parameters were initially developed for the
1.0 dry ton/hr model plant, and scaled linearly up or down.
*"As discussed in Chapter 3, scrubber pressure drops for the sewage
sludge incinerators built since 1978 range from 10 to 45 inch W.G. Costs
were calculated for three"cases of scrubber pressure drop: '20, 30, and
40 inches W.G. However, since a variable throat venturi is assumed here,
capital costs were estimated for a fan capable of operating at the highest
pressure drop for any given incinerator size. Although the capital costs
for a fan with a maximum operating capability corresponding to a pressure
drop of 20 in. W.G. would cost about 30 percent less than a fan designed for
a 40 in. pressure drop, this cost difference would have a negligible impact
on total annualized costs. The fan power requirements do vary according to
pressure drop, however.
Table 4-8 shows the details of the capital cost estimates. Attempts
were made to verify the capital cost estimates with vendor quotes and costs
for actual systems. For the 1.0 ton/hr model plant, one vendor quote was
13
about $60,000. However, some of the equipment components included in the
estimates (instrumentation and contrql systems, ducting, piping, etc.) were
not included in the vendor quote. Another vendor quote for the 1.0 ton/hour
model plant was 5125,000 (also exclusive of instrumentation and control).
The capital cost of a similar control system installed at an actual plant
4-10
-------
TABLE 4-7. OPERATING PARAMETERS FOR MODEL MULTIPLE-HEARTH SEWAGE
SLUDGE INCINERATORS AND CONTROL SYSTEMS
Incinerator Capacity
(dry ton sludge/hr)
Excess Air (%)
Sludge Moisture
Content (%}
% Volatiles
in Sludge Solids
Gas Flow to
Venturi (acfm)
Gas Flow out of
Impingement (acfm)
Temperature into
Venturi (°F)
Temperature out of
Impingement (°F)
Liquid Flow
into Precooler(gpm)
Liquid Flow into
Venturi (gpm)
Liquid Flow into
Impingement (gpm)
0.5 .
75
70
70
6,000
t
2,250
800
120
20
25
88
1.0
75
70
70
'12,000
4,500
800
120
40
50
175
2.0
75
70
70
24,000
9,000
800
120
80
100
350
4.0
75
70
70
48,000
18,000
800
120
160
200 .
700
4-11
-------
TABLE 4-8. CAPITAL COSTS FOR MODEL MULTIPLE-HEARTH INCINERATOR
CONTROL .SYSTEMS (JANUARY 1983 $)
Incinerator Capacity
(Dry Tons Sludge/Hr)
Venturi Scrubber
Impingement Scrubber
Venturi Circulation Tank
Fan & Auxiliaries
Ducting
Piping
Pump
Total
Total Direct Cost
(Equipment + Installation)
Indirect Cost
n
Contingencies
Total Installed Capital Cost
0.5
75,900
15,500
4,000
17,000
5,500
2,200
3,800
123,900
224 ",500
.-«
74, "100
44,900
343,500
1.0
83,800
26,000
5,900
17,500
7,300
3,100
4,100
147,700
266,500
87,900
53,300
407,700
2.0
97,800
43,800
8,600
18,500
10,000
5,300
4,900
188,900
339,000
111,900
67,800
518,700
4.0
113,900
73,700
'12,600
30,900 f
13,800
7,900
7,800
250,600
470,900
155,400
94,200
720,500
4-12
-------
was $128,000. Thus, the cost estimates presented in Table 4-8 are
considered to be reasonable, but somewhat conservative.
Table 4-9 presents the details of the annualized costs. The capital,
operating, and annualized costs for all incinerator sizes are summarized in'
Table 4-10. As seen from the table, the annualized cost increases as the
incinerator size increases. However, for a given incinerator size,
annualized costs change very little as the scrubber pressure drop increases.
The small increase in annualized costs is due to the increased fan energy
and pumping requirements associated with increasing pressure drop.
4.3.2 Fluidized-Bed Incinerator Control Systems
Cost a-nalysis was performed for five fluidized-bed incinerator sizes.
The incinerator sizes range from 0.25' dry ton/hour to 4.0 dry ton/hour. The
model plant parameters for the fluidized-bed incinerators are shown in
Table 4-11. An excess air rate of 35 percent, and an exit gas temperature
of 150.0°F was assumed for the model fluidized-bed incinerator. The flue gas
flow rates are the same as those developed for the multiple-hearth furnaces,
but have been-.-adjusted to reflect the lower excess air rates, and higher
furnape exhaust temperatures, typical of fluidized-bed sludge Incinerators.
Capital and annualized costs were calculated for scrubber pressure
drops of 20, 30, and 40 inches. The details of the capital costs .are
presented in Table 4-12. The annualized costs are shown in Table 4-13. The
capital, operating, and annualized costs for all fluidized-bed incinerator
sizes are presented in Table 4-14. Once again, the annualized costs
increase with incinerator size, but remain relatively constant for a given
incinerator size as the pressure drop changes.
4.4 COST EFFECTIVENESS OF CONTROLS
Cost effectiveness of the control system was calculated for
multiple-hearth and fluidized-bed incinerators at 20, 30 and 40 inches of
pressure drop. The controlled particulate emission rate was assumed to be
the NSPS limit (1.3 Ib/dry ton sludge). The uncontrolled particulate
emission rate (51 Ib/dry ton) for multiple-hearth incinerators is the
average of 19 incinerators (see Table 3-3). The uncontrolled emission rate
4-13
-------
TABLE 4-9. -ANNUALIZED COSTS FOR MODEL MULTIPLE-HEARTH INCINERATOR .
CONTROL SYSTEMS (JANUARY 1983 $)
Incinerator Capacity (dry tons/hr) .
0.5 1.0 2.0 4.0
Electricity
Fan:
AP - 20"
AP = 30"
AP « 40"
Pumps:
Total Labor Cost
Total Direct Operating Cost
(including spare parts)
AP - 20"
AP s 30"
*AP s .40"
Indirect-.Operating Costs
Total Annual Operating Costs
AP = 20"
AP « 30"
AP « 40"
Capital Recovery
G&A, Taxes and Insurance
Total Capital Charges
Total Annualized Costs
AP » 20"
AP = 30"
AP = 40"
.
2,800
4,200
5,600
700
41,200
57,100
58,600
-66,100
26,300
" «
83,500
84,900
86,300
45,200
13,700
58,900
142,400
143,800
145,200
5,600
8,500
11,300
. .1,100
41,200
60,400
63,300
66,100
26,300
86,700
89,600
92,400
53,600
16,300
69,900
156,600
159,500
162,300
11,300
16,900
22,600
1,800
41,200
66,800
72,400
78,100
26,300
93,100
98,700
' 104,400
68,200
20,700
88,900
182,000
187,600
193,300
'
22,600
33,900
45,200
3,200
41,200
79,500
90,800 .
102,100 '
26,300
105,800
117,100 '
128,400
94,700
. 28,300
123,500
229,300
240,600
251,900
4-14
-------
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-------
TABLE 4-11. OPERATING PARAMETERS FOR MODEL FLUIDIZED-BED SEWAGE
"SLUDGE INCINERATORS AND CONTROL SYSTEMS
Incinerator Capacity
(.Dry Ton Sludge/Hr)
Excess Air (%]
Sludge Moisture
Content (%)
0.25
35
70
0.5
35
70
1.0
35
70
2.0
35
70
4.0
35
70
Sludge Volatiles
Content (%,Solids) 70 70 70 70 70
Gas Flow to Venturi (acfm) "3,375 6..750 13,500 27,000 54,000
Gas Flow out of Impingement 875 1,750 3,500 7,000 14,000
(acfm)
Temperature into Venturi 1,500 1,500 1,500 1,500 1,500
(°F)
Temperature out of 120 120 120 120 ' 120
Impingement (°F)
LiqDid Flow ' 10 .' '20 40 80." 160
into precooler (gpm) .
Liquid flow into venturi 13 25 50 100 200
(gpm)
Liquid flow into impinger (gpm) 44 88 175 350 700
4-16
-------
TABLE 4-12. CAPITAL COSTS FOR MODEL FLUIDIZED-BED INCINERATOR
CONTROL SYSTEMS (JANUARY 1983 $)
Incinerator Capacity
(Dry Tons Sludge/Hr) .
Venturi Scrubber
Impingement Scrubber'
Venturi Circulation Tank
Fan & Auxiliaries
Ducting
Piping
Pump
Total
Total Direct Cost
(Equipment + Installation)
Indirect Cost.
Contingencies
0.25
66,400
8,600
2,600
17,000
. 4,800
1,700
3,000
104,100
190,000
62,700
38,000
0.5
87,900
14,400
4,000
17,100
5,500
2,200
3,400
134,500
-242,100
79,900'
48,400
1.0
90,000
24,300
5,900
17,500
7,300
3,100
4,100
152,200
274,200
90,500 .
54,800
2.0
105,000
40,900
8,600
18,500
10,000
5,300
5,000
193,3.00
346,300
114; 300
69,300
4.0,
136,000
68,700
12,600
30,900
13,800
7,900
7,800
277,700
449,600
164,900
99,900
Total Installed Capital
Cost
290,700 370,400 419,500 529,900 764,400
4-77
-------
TABLE 4-13. ANNUALIZED COSTS FOR MODEL FLUIDIZED-BED INCINERATOR
CONTROL SYSTEMS (JANUARY 1983 $)
Electricity
Fan:
P = 20"
P = 30"
P = 40"
Pumps :
Total Labor Cost
Total Direct Operating
(including spare parts)
p = 20"
P - 30"
P = 40"
0.25
1,100
1,600
2,200
500
41,200
Costs
55,300
55,800
56,400
Indirect Operating Costs 26,300
Total Annua.l Operating
P * 20"
P = 30"
P = 40"
Capital Recovery
G&A, Insurance, Taxes
Total Capital Charges
Total Annual ized Cost
P . 20"
P - 30"
P - 40"
Costs '
81,600
82,100
82,700
38,200
11,600
49,800
131,400
131,900
132,500
Incinerator Capacity
0.5 1.0
2,200
3,300
4,400
700
41,200
56,600
57,700
58,800
' 26,300
82,900'
84,000
85,100
48,700
14,300
63,500
146,400
147,500
148,600
4,400
6,600
8,800
1,100
41,200
59,200
61,400
' 63,600
26,300
85,500
87,700
89,900
55,200
16,800
72,000
157,500
159,700
161,900
(Dry Tons/Hr)
2.0 4.0
8,800
13,200
17,600
1,800
41,200
.
64,300
68,700
73,100
26,300-
90,600
95,000
99,400
' 69,700
21,200
90,900
181,500
185,900
190,300
' t
. 17,600
26,400
35,100
3,200
41,200
74,500
83,200
92,000
26,300
100,800
109,600
118,300
100,500
30,600
131,100
231,900
240,700
249,400
4-18
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for fluidized bed incinerators (88 Ib/dry ton) is the average of 11 emission
tests (see Table.3.-3).
The cost effectiveness was calculated by dividing the annulized costs
($/yr) by the emission reduction achieved (tpy) by the control system to
yield the cost to remove one ton of'particulates. The cost effectiveness
increased slightly with increasing pressure drop and decreased significantly
with increasing incinerator capacity as shown in Table 4-15 for
multiple-hearth control systems,and Table 4-16 for fluidized-bed control
systems.
At equivalent incinerator capacities and pressure drops, fluidized-bed
control systems have lower cost effectiveness.- Multiple-hearth cost
' effectiveness ranged from $329 to $1669 per ton removed and fluidized-bed
cost effectiveness ranged from $191 to $1743 per ton removed. The 0.25 dry
ton/hr fluidized-bed control system had the highest cost effectiveness since
emission reduction is strongly influenced by incinerator size.
4-20
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REFERENCES FOR CHAPTER 4
1. Telecon. R. M. Dykes, Radian Corporation, with F. R. Insinger, Peabody
Corporation. December 14, 1983. Material specifications for .venturi
and impingement scrubbers.
2. Letter from R. H. Hosier, W. W. Sly Manufacturing Co., to C.'
Jamgochian, Radian Corporation. January 10, 1984. Proposal: Sly Wet
Scrubber for Multiple-Hearth Incinerator.
3. Buck, Seifert, and Jost, Consulting Engineers. Advertisement,
Instructions to Bidders, Specifications for the Construction of the
Williamsburg System Sewage Treatment Plant Sludge Dewatering and
Incineration Facilities. January 1973.
4. Installation, Operation and Maintenance Instructions for Scrubber and
Separator.. A'irpol Job No. 3058, City of Providence, Rhode Island^
Wastewater Treatment Plant.'
5. Letter from S. R. Gates, Camp, Dresser & McKee, to C. Jamgochian,
Radian Corporation. January 13, 1984. Cost and design data for the
Arlington, Virginia, Wastewater Treatment Plant.
6. Neveril, R. B. (GARD). Capital and Operating Costs of Selected Air
.Pollution Control Systems. December.1978. EPA-450/5-80-002.
7. Barnett, Keith'W., et al."(Radian Corporation).' Costs" of Uncontrolled
Nonfossil Fuel Boilers and PM Controls Applied to these Boilers.
December 31, 1982. EPA Report (to be published).
8. Reference 6. '
9. Letter from J. Mitchell, Georgia Department of Natural Resources, to
R. M. Dykes, Radian Corporation. October 16, 1983. Design and
Operating Specifications for the Cobb County Wastewater Treatment
Plant.
10. Reference 5.
11. Reference 2.
12. Wall, C. J., Air Pollution and Energy Recovery Aspects of Fluid Bed
Incineration of Sewage Sludge and Solid Wastes. Dorr-Oliver, Inc.
Technical Reprint #6017.
13. Reference 2.
4-23
-------
14. Telecon. C. Jamgochian, Radian" Corporation, with F. R. Insinger,
Peabody Corp. January 16, 1984. Costs for venturi/impingement-tray
scrubbers.
15. Reference 5. .
4-24
-------
5.0 COINCINERATION OF SEWAGE SLUDGE AND MUNICIPAL REFUSE
5.1 INTRODUCTION AND SUMMARY OF FINDINGS
5.1.1 Introduction
At the present time, an NSPS has not been developed specifically for
incinerators that coincinerate sewage sludge and municipal refuse. A
procedure has been developed by EPA for use in determining whether
facilities coincinerating are subject to Subpart E (municipal incinerators)
or Subpart 0 (Sewage sludge incinerators) of the New Source Performance.
Standards.
In this chapter the technologies available for coincinerating sludge.
and solid wastes are described. Former, as well as current coincineration
projects in the U.S. are reviewed. In addition, the technical, economic,
and institutional factors most likely to affect the growth of'coincineration
are overviewed. The effect that coincineration has on particulate emission-s
is also addressed.
5.1-.2 Summary of Findings
No technology has ever been developed for the express purpose of
combined incineration of sewage sludge and municipal refuse. Four different
approaches to co'incineration ,can be distinguished: 1) combustion of
dewatered sludge in a refuse incinerator; 2) combustion of pre-dried sludge,
in a refuse incinerator, and; use of prepared municipal refuse (refuse
derived fuel); in either 3) a multiple-hearth sludge incinerator or 4) a
fluidized-bed sludge incinerator. All of the major techniques for combined
incineration have been tried in the U.S.
Over the past 30 years, 23 facilities in the U.S. have coincinerated
refuse and sewage sludge. Only one facility is currently operating on a
regular commercial basis, 18 have been shut down, and the remaining 4 have
reverted to single purpose incineration. In addition, 6 coincineration
facilities were being planned during the mid 1970's; Of these, only one has
started up. One is still being considered, but plans for the remaining four
facilities-have been dropped.
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*A variety of operational and maintenance problems have plagued
virtually every coincineration facility in the U.S. It has proved difficult-
to maintain combustion in refuse incinerators when partially dewatered
sludge is added. Although thermal drying of the sludge mitigates
combustion-related problems, the dryers themselves are subject to plugging,
corrosion, and odors, as well as fire and explosion. Technical obstacles to
burning refuse-derived fuel in conventional multiple-hearth or fluidized-bed
sewage sludge incinerators include the reliability of refuse preparation
systems and control of combustion.
The process of planning and implementing new coincineration projects in
the U.S. is often hampered by organizational differences between those
groups responsible for disposing of sludge and refuse. Whereas sewage
sludge management authority is ve.sted in centralized, public bodies, the
collection, transport, and disposal of municipal refuse is usually managed
by a combination of decentralized public and private bodies. These
organizational differences detract- from the achievement of the level of
integration in municipal waste management programs necessary for the
implementation of coincineration facilities. Moreover, the criteria
employed in siting a sludge treatment and disposal plant are essentially
different from those used in locating a refuse incinerator.
Very little data are available on particulate emissions from combined
incineration of sewage sludge an.d municipal reiuse. Some evidence indicates
that uncontrolled emissions from refuse incinerators may increase when
sludge is coincinerated. Operating a multiple-hearth unit in a pyrolysis
mode does not appear to offer any significant reduction of uncontrolled
emissions when prepared municipal refuse is used for fuel. It is doubtful
that all of the various approaches to coincineration will have- similar
emission characteristics, although this is a topic deserving further
investigation.
Despite the general lack of technical success with coincineration
.
projects, the costs of combined incineration of sewage sludge and municipal
refuse are still attractive when compared to the costs of burning these
wastes separately. Coincineration is also attractive from the standpoint of
5-2
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energy conservation. Thus, the incentives to coincinerate are still clear.
Yet until the various technical problems and uncertainties are overcome,
little growth in the use of coincineration can be expected over the'next
five years.
A number of regulatory issues have been identified. First, the
separate NSPS's for municipal and sludge incinerators are not expressed in
the same units, and the conversion from a concentration- to a mass-based
standard is not straight-forward. Second, in the existing proration
procedure, a discontinuity exists when an incinerator is burning equal
amounts of sludge and refuse. Third, neither standard addresses the case
where an incinerator is operated in a pyrolysis mode. Finally^ Subpart E
includes a minimum size cut-off, while Subpart-0 applies'to all incinerator
sizes.
5.2 DESCRIPTION OF COINCINERATION TECHNOLOGIES .
Any incinerator that is capable of burning either sewage sludge or
municipal refuse separately could feasibly burn both wastes, simultaneously.
No technologies have been, "or are being, developed specifically for purposes
of combined incineration, however. Thus, coincineration technology can be
classified, at the most general level, according to whether the incinerator
was originally designed^ for "burning refuse or sludge. On the other hand, '
successful coincineration sometimes requires modification of the incinerator
itself, and/or a pretreatment of the waste beyond that which would be
required if the waste were burned separately. When these additional
considerations are taken into account, four distinct categories of
coincineration technology emerge. An additional (fifth) classification can
be made on the basis of whether or not the incinerator is operated in a
pyrolysis, or starved-air combustion mode. These categories are discussed
below.
5'2'1' Incineration of Dewatered Sludge in a Conventional Refuse
Incinerator
The oldest, simplest, and most direct method of achieving combined
incineration is.to burn partially dewatered sludge (i.e., 70 to 80 percent
5-3
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moisture content) in a conventional municipal refuse incinerator.
Figure 5-1 depicts a typical mass-burning refuse incinerator. This approach
can be further subdivided on the basis of the type feeding mechanism
employed. The sludge can be fed separately into the furnace by either
spraying it into the combustion chamber or by dumping it onto the grate.
Alternatively, the sludge can be mixed with the refuse prior to entering the
incinerator.
Although this approach has the advantage of simplicity, it has not
proved to be very successful. The major problem encountered with this
technique relates to combustion. Conventional incinerators usually provide
insufficient residence time for the sludge to burn completely. In addition,
too little heat is generated from the burning refuse to evaporate the
moisture and combust the sludge. These problems are compounded by
.difficulties in distributing the sludge evenly within the furnace. For the
most part, this approach has proved unsuccessful both in this country and in
Europe, although two future projects in the U-.S. are expected to.use it.
5.2.2. Coincineration of Pre-dried Sludge in a Conventional Refuse
Incinerator
As a means to overcome the problems associated with burning sludge
directly in a refuse incinerator, a number of facilities have installed
systems to dry the sludge to less than 20 percent moisture content before it
enters the furnace. A wide variety of different drying systems have been
employed. Flue-gas heated direct contact dryers, steam heated rotary
dryers, flash evaporaters, spray dryers, and multi-effect evaporators have
all been utilized in the past. The dried sludge is then mixed with the
refuse at a ratio of approximately 10 parts refuse to each part sludge and
fed into the incinerator.
This method has been relatively successful. Pre-drying mitigates the
combustion problems associated with the use of only partially dewatered
sludges. Also, separation of the drying process from the combustion process
simplifies furnace operations. Nonetheless, this technique has not been
'entirely devoid of problems. A major difficulty has been th.e prevention of
rapid, corrosion in the dryers. Clogging and general handling problems have
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also been encountered with'the dried sludge. Odors given off by the dryers
(particularly direct contact dryers) have been another obstacle. Flash
evaporators are unattractive because of the potential for explosions to
occur. Nonetheless, the majority of the facilities currently coincinerating
in Europe, as well as the only commercially operating plant in the U.S., can
be classified within this category.
5.2.3. Combustion of Refuse in a Multiple-hearth Sludge Incinerator
In this arrangement, prepared municipal refuse is used in place of
fossil fuels for burning sludge in a multiple-hearth furnace (MHF).
Preparation of the raw refuse entails the mechanical separation of
non-combustibles and subsequent shredding of the remaining organic portion
into uniform particle'sizes. The refuse derived fuel (RDF) thus obtained
can be further treated chemically to produce a fine powder or pressed into
briquettes or pellets. The RDF is then either mixed with the sludge and fed
together into the top of the incinerator, or fed separately into one of the
lower hearths. "
Although at least three units of this type operate in Europe, it has
not been fully demonstrated in the U.S. Some testing has 'been done at a
demonstration facility in Contra Costa County, California. Based on limited
operating data, the major problem with this design is controlling the rate
of combustion .in the incinerator. -Localized overheating caused by periodic
intense heat release from the RDF can lead to structural failures in the
rabble shaft castings. To compensate for the higher heat release rate
associated With co-burning RDF, a greater volume of cooling air is required.
At higher than design air flow rates, the movement of the sludge and refuse
through the hearths could be impeded. Besides installation of all of the
facilities required to produce the RDF, substantial modifications to the
incinerator itself are necessary in order to coincinerate.
The major benefit associated with this type of system would be the
reduction in fuel costs for sludge incineration. Fuel costs represent the
largest share of the total, annualized costs of operating MHF incinerators.
5.2.4. Combustion of RDF in a Fluidized-Bed Sludge Incinerator.
This approach is analogous to that described above, except that
coincineration would take place in a fluidized-bed sewage sludge incinerator
5-6
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.(FBF). The RDF can either be.introduced into the furnace as dry pellets,
fluff, or powder, or alternatively the refuse can be pulped into a slurry
having a 40 to 45 percent moisture content and sprayed into the furnace
along with the sludge. Both the wet and dry systems have been demonstrated
in the U.S.
Compared to coincineration in a multiple-hearth incinerator, use of a
FBF offers a number of advantages. Foremost is that combustion is more
easily controlled in a fluid-bed, and furnace operation is less vulnerable
to changes in the sludge feed rate or moisture content, due to both the
excellent mixing characteristics and longer residence time typical of these
incinerators.
* * *
As in the case of MHF's, however, significant modification has to be
made to the incinerator in order to burn RDF. Beside the addition of a
feeding mechanism, a system for separating inert RDF materials that build up
in the sand be.d is required. The interior of the furnace shell must also .be
protected from the corrosive condensation of HC1 and HF gases evolving frem
combustion of plastic materials.
5.2.5. jitarved-AIr Combustion (Pyrolysis) .
With the exception of fluidized-bed furnaces, all of the incineration
techniques reviewed above can be operated in a starved-air or pyrolysis
mode.: Thus, this category represents not so much a distinct technology type
as it does a' general operating technique, applicable to a number of
alternative technology configurations. Four incinerators, specifically
designed to operate as pyrolytic reactors, are presently under development.
These incinerators include the PuroxR (Union Carbide), TorraxR
\
(Carborundum), Landgard (Monsanto), and the Flash Pyrolysis (Occidental)
systems. Both the Purox and Torrax processes are based on a vertical shaft
reactor design; the Landgard system utilizes a rotary kiln. All of these
technologies are being developed primarily as municipal refuse incinerators.
Each, however, has also been considered as a possible coincineration
technology and some testing has been conducted on them in this mode.
In a conventional refuse incinerator, combustion under starved-air
conditions is the most common operating technique. Generally, however,
5-7
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combustion air is added at only slightly less than stoichiometric rates.
The off-gases from the furnace are then combusted in an afterburner: The
smaller, modular refuse incinerators that have been widely utilized since
the early 1970's are almost always designed to operate under starved-air
conditions.
Operating a multiple-hearth sewage sludge incinerator in a pyrolysis
mode is a technique that was developed specifically for purposes of
coincinerating refuse. As described earlier, the major problem encountered
in coincinerating in these furnaces is controlling the rate of combustion.
By operating the furnace as a pyrolysis reactor, these problems are
effectively overcome. During a series of comprehensive tests conducted on a
MHF at the Contra Costa County demonstration project, operating the furnace
in a pyrolysis mode emerged as the preferred means of co-burning refuse with
* P
sewage sludge. The major manufacturers of multip.le-hearth furnaces also
recommend that the unit be operated in a pyrolysis mode when coincinerating
municipal refuse.3 Other benefits associated with this approach are an
increased furna'ce capacity and the capability for pyrolysis to become
autogenous with sludges having a low solids content.
For all types of pyrolysis, the major disadvantage is the greatly
»
increased complexity of the system. The furnaces must be well sealed
against air infiltration, the interior linings must be highly corrosion
resistant, and additional controls and instrumentation are required.
Moreover, to be economically, viable these systems must be able to recover
and utilize the energy content of the off-gases. Heat recovery systems add
to the overall complexity and capital costs-of the facility. Finally, a
greater volume of residual ash and char is produced when wastes are
processed fay pyrolysis rather than incineration.
5.3 REVIEW OF COINCINERATION PROJECTS IN THE U.S.
All of the available techniques for combined incineration of sewage
sludge and municipal refuse have been, at one time or another, tried in the
U.S. in either commercial- or pilot-scale plants. No single approach has
emerged as a definitively "best" technique, although burning pre-dried
sludge in a conventional refuse incinerator has been attempted most often.
5-8
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A comprehensive listing of former, present, and planned coincineration
projects in the U.S. is provided in Table 5-1. Only one facility., at
Stamford, Connecticut, is currently coincinerating on a regular commercial
basis. The facility in Glen Cove, New York, is in a start-up phase. Out of
the total 32 facilities listed, 18 units that formerly were coincinerating
have been shut-down or abandoned completely and four facilities have
reverted to single-purpose incineration. Of the six major coincineration
projects being considered during the mid 1970's, only the Glen Cove facility
is currently operative. The municipal incinerator in Harrisburg,
Pennsylvania, plans to begin burning sludge sometime in the next year.
In quite a few cases, plants that have shut down have done so for
technical reasons. Operating problems have plagued some of the new, as.well
as the older, coincineration facilities. The Ansonia, Connecticut, Duluth,
Minnesota and Holyoke, Connecticut, plants have each experienced equipment
failures. Even the Stamford plant has been.unable to coincinerate on a
continuous basis since -the facility began operating in 1975." New pyrolysis .
reactors have yet to demonstrate a sufficient level of operating reliability
when processing refuse alone,-and t-he feasibility of coincinerating in these-'
units is still open to question.
5.4 ECONOMIC AND INSTITUTIONAL CONSIDERATIONS
From the standpoint of annualized operating costs, coincinerating
sludge and refuse appears to be an attractive waste management approach in
situations where landfilling or other disposal options are unavailable. In
contrast, there are numerous institutional barriers to coincineration,that
can mitigate the economic incentives for co-disposal,
5.4.1 Costs for Coincineration
The most comprehensive assessment of the costs of coincineration was
conducted in 1976. In this study, the costs for separate incineration of
slu'dge and refuse were compared to the costs of four combined incineration -
systems. Costs for non-thermal disposal options are also used for
comparison. The coincineration designs considered included a
multiple-hearth unit burning RDF, a Torrax pyrolysis shaft furnace, and two
5-9
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