EPA-600/2-76-288
December 1976
Environmental Protection Technology Series
A REVIEW OF TECHNIQUES FOR
INCINERATION OF SEWAGE SLUDGE WITH
SOLID WASTES
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring 5
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-76-288
December 1976
A REVIEW OF TECHNIQUES FOR
INCINERATION OF SEWAGE
SLUDGE WITH SOLID WASTES
by
W. Niessen, A. Daly,
E. Smith, and E. Gilardi
Roy F. Weston, Inc.
West Chester, Pennsylvania 19380
Contract No. 68-03-0^75
Project Officer
Robert A. Olexsey
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio ^5268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Office of Research and Development,
U.S. Environmental Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and policies
of the U.S. Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation for use.
i i
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our national environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from munici-
pal and community sources, for the preservation and treatment of public drink-
ing water supplies, and to minimize the adverse economic, social, health, and
aesthetic effects of pollution. This publication is one of the products of
that research; a most vital communications link between the researcher and the
user community.
Development of safe and economical methods for disposing of the sludges
produced from wastewater treatment operations is one of the most pressing
environmental needs. This publication provides much needed information on
the feasibility of utilizing an integrated approach to municipal sewage sludge
and solid waste disposal.
Francis T. Mayo, Director
Municipal Environmental
Research Laboratory
!• »
i i
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ABSTRACT
This report discusses the state of the art of co-incineration of Munic-
ipal refuse and sewage sludge. European and American practice is described.
Four co-incineration techniques are evaluated for thermodynamic and economic
feasibility: Pyrolysis, Multiple-Hearth, Direct Drying and Indirect Drying.
This report was submitted in fulfillment of Contract No. 68-03-0^75 by
Roy F. Weston, Inc. under the sponsorship of the U.S. Environmental Protection
Agency. Work began k June 1974 and was completed as of 5 September 1976.
IV
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CONTENTS
Foreword ..... iil
Abstract iv
Figures vii
Tables ix
I Introduction 1
Purpose and Scope 1
Background ....1
Refuse Incinerators 2
Sludge Incinerators ^
References ^
I I Conclusions 7
General 7
Historical 7
Technical 8
Environmental ....9
Economic 9
Ml Recommendations ..11
Technical Development 11
Policy 11
IV Co-Incineration — The State of the Art 12
Information Sources 12
Overview of Applicable Techniques 13
Discussion of Principal Co-Incineration Techniques . . .13
References 37
V Feasibility Study kl
Approach to Evaluation 41
Direct-Drying Co-Incineration k$
Indirect-Drying Co-Incineration 55
Multiple-Hearth Co-Incineration 68
Pyrolysis Co-Incineration 76
VI Air Pollution Aspects 89
Characteristics of Emissions 89
Emission Regulations 90
Control Options — MSS or MMR Systems 91
Impact of Co-Incineration on Emissions ^93
Controls Required for Co-Incineration 95
Final Considerations 95
References S95
VII Economic Considerations 98
Background 98
Basic Cost Calculations 101
Analysis of Co-Incineration Costs 102
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CONTENTS
continued
VIM Circumstances Having Impact on Feasibility 119
Geography 119
Local Political Situations 120
Cooperation between Public Agencies 121
Public- and Private-Sector Factors 122
Government Funding 122
Relative Acuteness of MSS or MMR Disposal Problems . . 123
Local Cost Factors 124
Auxiliary Fuels 12k
Appendices
A. Annotited bibliography 125
B. Plant visits 16A
C. Generation and handling of wastewater treatment sludges . . 195
D. Refuse 212
VI
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FIGURES
Number Page
1 Continuous-feed incinerator 3
2 Multiple-hearth sludge incinerator 5
3 Fluid-bed sludge incinerator 6
k Direct dryer co-incineration — schematic flow sheet 46
5 Direct dryer co-incineration gas temperature
(equivalent population) 50
6 Direct dryer co-incineration gas temperature
(20% excess sludge) 52
7 Indirect dryer co-incineration — schematic flow sheet 56
8 Indirect dryer co-incineration gas temperature
(equivalent population) 61
9 Indirect dryer co-incineration gas temperature
(20% excess sludge) 63
10 Heat-transfer coefficient for sludge drying 65
11 Multiple-hearth co-incineration — schematic flow sheet 68
12 Multiple-hearth co-incineration gas temperature
(equivalent population) 70
13 Multiple-hearth co-incineration gas temperature
(20% excess sludge) 72
14 Multiple-hearth co-incineration using incinerator flue gas —
schematic flow sheet 75
15 Pyrolysis co-incineration — schematic flow sheet 77
16 Pyrolysis (equivalent population) 82
17 Pyrolysis feasibility — 120% equivalent sludge 8k
vil
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FIGURES
continued
Number
18 Pyrolysis co-incineration with direct sludge pre-drying —
process flow sheet ........................ 86
19 Pyrolysis co-incineration with indirect pre-drying —
process flow sheet ........................ 8?
B-1 Water treatment and incinerator flow sheet,
Ansonia, Connecticut ....................... 167
B-2 Longitudinal section through incinerators,
Ansonia, Connecticut ....................... 168
B~3 Flow sheet of sewage sludge and refuse incinerating plant,
Dieppe, France ..................... ..... 1/2
B-k Cross-section of Von Roll Evaporator, Dieppe, France ...... • • 173
B-5 Refuse incinerator at Dordrecht, Netherlands ............ 176
B-6 Flow sheet of combustible waste disposal system,
Eastman Kodak Company ...................... ] 73
B-7 Flow sheet of Franklin (Ohio) environmental control complex . . . . ]82
B-8 Water treatment incinerator flow sheet:
Holyoke, Massachussetts
C-1 Wastewater treatment processes: substitution and
sequence diagram
C-2 Alternative sludge handling processes and systems ......... 200
D-1 Refuse heating value as received .................. 227
VI I 1
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TABLES
Number Page
1 Applications of MMR/MSS Co-Incineration Techniques 1A
2 Status Summary of Co-Incineration 19
3 Refuse and Sludge Composition M
k Input and Output of Analysis of Direct Dryer Co-Incineration
Alternative by INCIN Program — Equivalent Sludge/Refuse
Production ^9
5 Input and Output of Analysis of Direct Dryer Co-Incineration
Alternative by INCIN Program — Sludge/Refuse Rates at
120 Percent of Equivalent Value 51
6 Input and Output of Analysis of Indirect Dryer Co-Incineration
Alternative by INCIN Program — Equivalent Sludge/Refuse
Production 60
7 Input and Output of Analysis of Indirect Dryer Co-Incineration
Alternative by INCIN Program — Sludge/Refuse Ratio at
120 Percent of Equivalent Value 62
8 Input and Output of Analysis of Multiple Hearth Co-Incineration
Alternative by INCIN Program — Equivalent Sludge/Refuse
Production 69
9 Input and Output of Analysis of Multiple Hearth Co-Incineration
Alternative by INCIN Program — Sludge/Refuse Ratio at
120 Percent of Equivalent Value 71
10 Input and Output of Analysis of Pyrolysis Co-Incineration
Alternative by INCIN Program — Equivalent Sludge/Refuse
Production 81
11 Input and Output of Pyrolysis Co-Incineration Alternative by
INCIN Program — Sludge/Refuse Ratio at 120 Percent of
Equivalent Value 83
12 Estimated Air Pollutant Emissions from Refuse, Sludge
and Combined Incineration 96
ix
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TABLES
continued
Number
13 Construction Cost — Modern Refuse Incinerator 103
Ik Total Facility Capital Cost — Modern Refuse Incinerator 103
15 Operating Cost — Modern Refuse Incinerator 10^
16 Construction Cost — Multiple-Hearth Sludge Incinerator ^Q^
17 Total Facility Capital Cost — Multiple-Hearth Sludge
Incineration 105
18 Operating Cost — Multiple-Hearth Incinerator 105
19 Construction Cost — Co-Incineration/Rotary Sludge Dryer . 106
20 Total Facility Capital Cost — Co-Incineration/Rotary
Sludge Dryer 106
21 Operating Cost — Co-Incineration/Rotary Sludge Dryer 107
22 Construction Cost — Co-lncineration/lndirect Sludge Dryer 107
23 Total Facility Capital Cost — Co-Incineration/Indirect
Sludge Dryer 108
2k Operating Cost — Co-Incineration/Indirect Sludge Dryer 108
25 Construction Cost — Co-lnc?neration/Multiple Hearth Furnace. . . . 109
26 Total Facility Capital Cost — Co-Incineration/Multiple
Hearth Furnace 109
27 Operating Cost — Co-lncinerat?on/Multiple Hearth Furnace 110
28 Construction Cost ~ Co-lncineration/Pyrolysis 110
29 Total Facility Capital Cost — Co-lncineration/Pyrolysis 111
30 Operating Cost — Co-lncineration/Pyrolysis 111
31 Summary of Co-Incineration Cost Analysis H2
32 Manpower and Utility Summary ^,-
33 1985 Co-Incineration Cost Analysis
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TABLES
continued
Number Page
C-1 Typical Composition of Domestic Sewage 197
C-2 Typical Air Flotation Design Parameters 203
C-3 Air Flotation Thickening Performance 204
C-4 Typical Rotary Vacuum Filter Results for Sludge
Conditioned with Organic Chemicals 206
C-5 Typical Rotary Vacuum Filter Results for Polyelectrolyte-
Conditioned Sludges 207
C-6 Estimated Chemical Conditioning Dosage for
Vacuum Filtration 205
C~7 Typical Solid-Bowl Centrifuge Performance 208
C-8 Typical Filter Press Production Data 209
C-9 Sludge Quantities 211
D-1 Projected Population and Annual Domestic Refuse
Tonnage: Baltimore Region — 1970 to 1990 214
D-2 Domestic Solid Waste Composition Comparison —
Baltimore Region 215
D-3 Annual Net Waste Generation — Baltimore Region 217
D-4 Annual Net Generation of Special Wastes in the
Baltimore Region 218
D-5 Municipal Refuse Composition 220
XI
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SECTION I
INTRODUCTION
PURPOSE AND SCOPE
This study was undertaken with the following objectives:
—To assess the state of the art of co-incineration, including demonstrated
and experimental techniques, both-foreign and domestic.
—To select four techniques for further study to identify the important
design and operational parameters affecting feasibility.
—To establish the economic feasibility, for a selected area, of
co-incineration compared to separate sludge and refuse incineration.
—To assess the environmental and political impact of co-incineration
as a processing step for refuse and sludge before ultimate disposal
of residues.
BACKGROUND
For the past 30 years, design engineers have been evaluating the technical
feasibility of co-incineration of wastewater sludge and municipal refuse. A
number of prototype co-incineration units reportedly have been successfully
operated, including Holyoke, Massachusetts; Ansonia, Connecticut; Hershey,
Pennsylvania; and Dordrecht, Netherlands. Others have been plagued with major
operating problems which have resulted in discontinuation of the sludge portion
of the feed, including Frederick, Maryland; Whitemarsh, Pennsylvania; Newburgh,
New York; and Altrincham, England. A more complete list of co-incineration
plants, demonstrations, experiments, and proposed facilities appears in Table 1.
There are other circumstances affecting the problem of sludge disposal.
Shortage of land in the vicinity of large metropolitan areas has become a major
concern in the selection of sites for future sanitary landfills. Furthermore,
there continues to be considerable controversy surrounding ocean disposal of
municipal sludges. The alternative, sludge incineration, is costly and makes
demands on fossil-fuel supplies. The potential for savings in investment,
operating expense, and energy consumption has focused renewed attention on the
latest co-incineration technology and economics.
The number of co-incineration installations is small compared to the number
of separate refuse and sludge incinerators. For example, in ^^^k there were
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incinerator plants (most with multiple furnaces) in the United States
burning municipal refuse.' In 1970, there were over 230 sludge incinerators
(municipal or industrial); of this number, 175 were multiple-hearth units
and the remainder were f luidized-bed units. Known domestic sites where
co-incineration was reportedly practiced totaled about a dozen at the start
of the study.
REFUSE INCINERATORS
Domestic mixed municipal refuse (MMR) normally is an autogenous but
highly variable fuel that can be difficult to burn under adverse climatic
conditions when collection practice allows the refuse to become saturated
with water from rain or snow.
Most of the MMR incinerators in the United States are grate-fired,
refractory-wall furnaces that burn as-received refuse. Batch-feed furnaces
are being phased out in favor of continuous-feed units. Excess air is used
to control furnace temperature between 760° and 980°C (1 ,AOO° and 1,800°F).
Figure 1 shows a cross-section of a continuous-feed incinerator, and typical
design criteria appear in the next tabulation (Source: DeMarco et al .3) ;
Grate Loading 2A5-340 kg/sq m/hr
(50-70 Ib/sq ft/hr)
Grate Heat Release Rate 815,000 kg-cal/sq m/hr
(300,000 Btu/sq ft/hr)
Furnace Heat Release Rate 112,500-225,000 kg cal/cu m/hr
(12,500-25,000 Btu/cu ft/hr)
Excess Air 150-200 percent
Furnace Exhaust Gas Temperatures 760~980°C
Grate-fired, steam-generating incinerators are very popular in Europe and
are becoming increasingly popular in the United States and Canada. With a
boiler-incinerator, the boiler surfaces remove sufficient heat from the furnace
gases that the excess air can be cut back to only 50 to 100 percent.
Typical grate-fired incinerators burn as-received refuse, and no refuse
preparation is necessary unless bulky wastes are to be processed. Other
incinerators, however, burn shredded refuse, which does require preparation
of the refuse.
The steam-generating incinerator in Hamilton, Ontario, Canada is designed
to burn shredded refuse in a spreader-stoker incinerator.^ Another approach is
to burn shredded and cleaned refuse in suspension in a utility boiler, 5 the
shredded refuse replacing a portion of the fossil fuel normally burned in the
furnace.
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VjJ
Tipping
Floor
Gas Conditioning Chamber
Receiving Bin
Figure 1. Continuous-Feed Incinerator.
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In addition, several manufacturers are developing and marketing proprie-
tary systems. One shaft furnace (Torrax)6 is being marketed; another shaft
furnace (Purox) is undergoing developmental testing;? and a 900 metric ton/day
(1,000 tpd) rotary kiln system (Landgard) has been installed in Baltimore,
Maryland." There are others In various stages of development.
SLUDGE INCINERATORS
Simple gravity thickening, vacuum filtration, centrifugation, and pressure
filtration have been and are being used to dewater Municipal Sewage Sludge (MSS)
before incineration. Even with this preparation, the sludge is still generally
not autogenous. Dewatered primary treatment plant sludge requires little
auxiliary fuel, since the sludge is readily dewatered and sludge solids have a
significant heating value. Secondary sludges are dewatered with difficulty,
producing a high-moisture sludge cake, containing solids of low heating value.
The addition of large quantities of secondary sludge, resulting from improved
wastewater treatment, will reduce the "combustibility" of the total sludge load,
thereby increasing the auxiliary fuel requirement.
The three most popular sludge-incinerator systems are the multiple-hearth
and fluid-bed furnaces and the flash-drying arrangement used in conjunction
with a fossil-fuel or refuse-fired furnace. Figures 2 and 3 are sketches of
the cross-sections of typical multiple-hearth and fluid-bed furnaces, respec-
tively. All these systems require the use of an auxiliary fuel during operation
to make up for the sludge's heat deficiency. Burd° and Balakrishnan^" have
reviewed sludge-incinerator practices and discuss the three systems mentioned,
plus a number of others. More recently, EPA has published a technology transfer
document on sludge treatment and disposal.
REFERENCES
1. Fenton, R. Present Status of Municipal Refuse Incinerators with Particular
Reference to Problems Related to Non-Residential Refuse Input. Presented
at the American Society of Mechanical Engineers, New York, NY (1975).
2. Sebastian, F. Advances in Incineration and Thermal Processes. Presented
at Advanced Wastewater Seminar, University of California, Berkeley
(September 1971).
3. DeMarco, J., et_ a_t_. Incinerator Guidelines — ^^6^. Public Health Service
Publication No. 2012 (19&9).
k. Sutin, G.L. Solid Waste Reduction Unit Promises to be a Better Mousetrap.
Public Works 100,12 (February 1969).
5. Wisely, F.E., e_£ a_L Use of Refuse as Fuel in an Existing Utility Boiler.
Proceedings of 1972 National Incinerator Conference, ASME (1972).
6. Torrax. Private communication (197*0.
7. Anderson, J.E. The Oxygen Refuse Converter—A System for Producing Fuel
Gas, Oil, Molten Metal and Slag from Refuse. Proceedings of 197*» National
Incinerator Conference, ASME (197*0.
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FLUE GASES OUT
RABBLE ARM AT
EACH HEARTH
DRYING ZONE
COMBUSTION ZONE
COOLING ZONE
COOLING AIR DISCHARGE
FLOATING DAMPER
SLUDGE INLET
ASH DISCHARGE
COMBUSTION
AIR RETURN
RABBLE ARM
DRIVE
COOLING AIR FAN
Figure 2. Multiple-Hearth Sludge Incinerator.
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ALTERNATE SLUDGE INLET
EXHAUST
ALTERNATE SLUDGE INLET
ALTERNATE
PREHEAT BURNER
SLUDGE INLET
ACCESS ™ ^•a'^ft'#'tftt'tf'^^ft'fiiSs*S AIR DISTRIBUTION PLATE
DOORS
ir^wi ai 21 j j^m.
AMBIENT OR PREHEATED
FLUIDIZING AIR INLET
Figure 3. Fluid-Bed Sludge Incinerator.
8. Blelski, E.T., and A.C.J. Ellenberger. Landgard for Solid Wastes.
Proceedings of 197^ National Incinerator Conference, ASME (197*0.
9. Burd, R.S. A Study of Sludge Handling and Disposal. FWPCA (EPA)
Publication WP-20-4, NTIS No. PB-179 5H (May 1968).
10. Balakrishnan, S., et al. State of the Airt Review of Sludge Incineration
Practice. FWQA (EPA) No. 17070DIV (1970).
11. Process Design Manual for Sludge Treatment and Disposal. Environmental
Protection Agency 625.1-7A-006 (October 1974).
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SECTION I I
CONCLUSIONS
GENERAL
1. Co-incineration is a viable technique for the disposal of municipal
refuse and wastewater treatment plant sludge before ultimate disposal.
2. A co-incineration process using direct-contact pre-drying in a rotary
dryer is the best developed technique, although co-incineration by in-
direct contact pre-drying, in multiple-hearth furnaces or by pyrolysis,
is technically feasible.
3. Co-incineration by direct contact pre-drying in a rotary dryer offers
the most attractive economics of the four techniques evaluated; however,
all four co-incineration processes will result in lower total disposal
cost than separate incineration of refuse and sludge.
k. Socio-political factors have been and will continue to be the greatest
impediment to the widespread use of co-incineration.
5. The Environmental Protection Agency and other governmental agencies and
authorities should act to encourage the use of co-incineration as a
sludge and refuse pre-processing step to reduce the environmental,
economic, and energy-usage impact of the total disposal system.
HISTORICAL
6. Co-incineration of refuse and sludge has not been widely practiced in the
U.S. At present, there are only three municipal co-incineration plants
in operation in this country.
7. Previous attempts at co-incineration in the U.S. have generally met with
little success. With few exceptions, most U.S. co-incineration plants
have been designed with sludge incineration retrofitted to existing
refuse incinerators.
8. Ready availability of alternative solids-disposal methods has been the
principal reason for the relative lack of U.S. interest in co-incineration
to date. Landfill areas have been available at reasonable distances.
Simple sludge incineration has been economical with plentiful supplies
of low-cost fuels.
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9. The organizational separation of those responsible for refuse disposal
and those responsible for wastewater treatment and sludge disposal is
another factor inhibiting the development of co-incineration.
10- Co-incineration experiences in Europe have been more widespread and more
sucessful than in the U.S., indicating that interest in co-incineration
is stimulated when land area becomes limited and fuel costs become high.
TECHNICAL
11. The following four co-incineration techniques appear most promising,
and were selected for in-depth study:
a. Direct-contact sludge pre-drying with heat provided by the
co-incineration of refuse and dried sludge.
b. Indirect-contact sludge pre-drying with heat provided by the
co-incineration of refuse and dried sludge.
c. Combined feed of sludge and refuse to a multiple-hearth
incinerator.
d. Combined feed of sludge and refuse to a pyrolysis furnace.
t
12. A heat and material balance for all four systems indicates that sufficient
heat is available to co-incinerate refuse and dewatered sludge (20 percent
solids) at rates consistent with those generated by equivalent population,
i.e., the amount of solid waste and sludge produced per capita.
13. The two most significant variables affecting the co-incineration heat
balance are furnace excess air and total moisture entering the
co-incineration plant (i.e., moisture in refuse plus moisture in sludge).
14. All co-incineration techniques studied but pyrolysis could be designed
and built with available equipment, engineering practices, and standards.
The pyrolysis co-incineration technique is dependent on the development
of new hardware, processes, or technologies.
15. The co-incineration technique involving direct-contact sludge pre-drying
involves the lowest technical risk of any of the four techniques, and
should be considered as in the same risk category as the design/
construction of a refuse incinerator.
16. Co-incineration involving indirect-contact sludge pre-drying also repre-
sents a low-risk technology. Indirect sludge drying has been demonstrated
in Europe. The only risk factor is the potential fouling of the
heat-transfer surface with dry sludge or char. This co-Incineration
technique is useful where high-moisture sludge must be processed.
17. Multiple-hearth co-incineration is a medium-risk technique. Although
plants have been operated in Europe, questions remain. The system requires
a shredder to reliably produce a nominal 2.5 cm (1") refuse size. The
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combustion of refuse a?one in a multiple-hearth furnace has not been
demonstrated. Multiple-hearth co-incineration does offer the lowest unit
capacity, and is thus appropriate for small communities.
18. Co-incineration by pyrolysis must be regarded as a high-risk technology.
Units operating on refuse alone have yet to be commercially demonstrated.
This process does offer several potential advantages, including small
size, potential for energy recovery, and flexibility in the sludge
moisture which is acceptable.
19. The co-incineration plant site should be within pumping distance of the
wastewater treatment plant. Storage, transportation, and odor control
for thickened or dewatered sludge are impractical. Refuse is generally
handled by truck already, and while additional mileage may be incurred,
it is more desirable than transporting both sludge and refuse.
20. Because of the many factors involved in determining the advisability of
co-incineration and in selecting the process, plant site, sludge
dewatering equipment, and sludge-to-refuse ratio, a thorough study must
be made case by case.
ENVIRONMENTAL
21. Volume and weight reductions for co-incineration will be approximately
the same as for separate incineration of both materials. Unburned carbon
levels in the residue should be lower for co-incineration in pyrolysis
and multiple-hearth equipment than for incineration in conventional
incinerators.
22. The adverse impact of co-incineration on air and water quality should be
no greater than for separate incineration of sludge and refuse. Partic-
ulate air pollution control for co-incineration equipment has increased
efficiency requirements over separate incineration facilities, but
proven technology is available.
23. The impact of co-incineration on land use will be beneficial.
Co-incineration provides a technically feasible and economically viable
process for reducing the amount of waste requiring ultimate disposal,
thereby providing an alternative to direct land disposal of unprocessed
refuse and sludge.
ECONOMIC
2k. Each of the four co-incineration techniques evaluated will have lower
overall cost than separate incineration of sludge and refuse. The
improved economics, however, will not bring the costs down to the level
of land or ocean disposal.
25. In terms of capital investment, direct-pre-drying/co-incineration and
indirect-pre-drying/co-incineration are lowest, followed by
multiple-hearth and pyrolysis. Until recently, municipalities could
obtain the capital for major facilities at attractive interest rates.
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As the municipal bond market falters and interest rates climb, however,
total capitalized cost will become a more significant factor.
26. With regard to operating cost, direct-pre-dry?ng/co-incineration and
indirect-pre-drying/co-incineration have the lowest estimated cost,
followed by pyrolysis and multiple-hearth in that order.
27. Total annual cost (cost of owning and operating) is lower for any of the
four co-incineration processes than for separate incineration of sludge
and refuse. The direct-contact-dryer process would have the lowest total
cost, 18 percent lower than separate incineration. The indirect-drying
process would follow closely (17 percent), then pyrolysis (10 percent)
and multiple hearth co-incineration (k percent).
28. Economic advantages of co-incineration result primarily from reductions
in manpower and auxiliary fuel costs.
29. Projection of costs indicates that co-incineration will be more econom-
ically attractive in 1985 than in 1975, because inflation is expected to
have more impact on separate disposal than on combined disposal of sludge
and refuse.
10
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SECTION III
RECOMMENDATIONS
TECHNICAL DEVELOPMENT
1. Study the technical problems of indirect-drying co-incineration through
demonstrations in existing boiler-incinerator systems. Particular prob-
lems include boiler corrosion and slagging in the tube banks and the
effects of fouling on heat transfer rates and abrasion in the dryer.
2. Demonstrate multiple-hearth co-incineration, since this concept is a
viable technology which has special applicability for smaller communities
but which is not now used in the U.S. Include studies to minimize
expense entailed in shredding the refuse and sludge.
3. Study the feasibility of mixing sludge with refuse ahead of shredding
(for multiple-hearth systems). Such an approach would make use of the
lubricating effect of sludge moisture (reducing shredder wear and power
consumption), inhibit fires in the shredder, and bring about homogen-
ization of sludge and refuse. Odor control would have to be considered.
k. Assess the performance, economics, and environmental impact of an oper-
ating co-incineration plant in the U.S. The Stamford, Connecticut plant
appears to be a good candidate.
POLICY
5. Establish guidelines to clarify the dividing line between wastewater
treatment and solid waste management (as it affects the availability of
federal grants for 201 and 208 planning, facilities construction, etc.)
in co-incineration situations.
6. Consider the development of a policy favorable to co-incineration by EPA
(perhaps in conjunction with the Federal Energy Office).
7. Study and test alternative EPA policies which encourage the inter- and
intra-jurisdictional cooperation usually needed for co-incineration
systems to become politically feasible.
8. Actively participate (financial support and information exchange) with
the European technical community now engaged in studies of co-incineration-.
11
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SECTION IV
CO-INCINERATION -- THE STATE OF THE ART
INFORMATION SOURCES
Information on co-incineration practices was acquired from review of
patents and the technical literature, and from operator and vendor contacts,
professional associations, and visits to selected installations in the United
States and overseas. An annotated bibliography of published material is
presented in Appendix A.
Several reportedly successful installations were shut down at the time of
the scheduled visit, had been abandoned, were being phased out of operation,
or had never practiced co-incineration. Where the incinerator operators had
abandoned co-incineration, the reasons were explored insofar as possible. Some
of the incinerator plants where co-incineration was being or had been practiced
had been visited by one or more of Roy F. Weston's professional staff before
this study; others were visited as a part of this study to obtain additional
background information. The incinerator plants visited before or during this
study are as follows:
Co-i ncineration
Technique
(See Table 1)
Altrincham, England* (5)
Ansonia, Connecticut* (k)
Dieppe, France* (1|)
Dordrecht, Holland* (8)
Franklin, Ohio (3)
Holyoke, Massachusetts* (A)
Kodak Park, Rochester, New York* (6)
Nieder-Uzwil, Switzerland* (2)
Orchard Park, New York (Torrax) (7)
Reigate, England* (2)
South Charleston, West Virginia (Purox)* (7)
Waterbury, Connecticut (6)
Whitemarsh Township, Pennsylvania (l)
*Plants visited during this study.
12
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Reports on the visits which were a part of this study are in Appendix B,
as are reports on selected contacts with incinerator operators, manufacturers,
vendors, and others relative to co-incineration facilities and practices. The
visit to the Dordrecht plant was a substitution for the planned visit on an
incinerator at BUlach (Switzerland), which turned out to be permanently shut
down. The manufacturer indicated that the type of equipment at BUlach was being
phased out, and recommended the Dordrecht visit. Weston staff personnel have
also visited many separate MMR (Mixed Municipal Refuse) and MSS (Municipal
Sewage Sludge) incinerators in the course of previous investigative and design
ass ignments.
During the course of this study, we became aware of a joint European in-
vestigation of co-incineration of refuse and sludge. This effort, under the
chairmanship of Dr. R.S. Gale* of the Water Research Center in England, involved
the testing of two co-incinerators: the multiple-hearth unit at Uzwi1, Switzer-
land and the von Roll System in operation at Dieppe, France. The European re-
ports have not yet been published.
OVERVIEW OF APPLICABLE TECHNIQUES
EPA designated seven principal co-incineration techniques for study:
1. Incineration of dewatered sludge filter cake and raw solid waste in a
conventional sol id waste incinerator.
2. Incineration of dewatered sludge filter cake in a multiple-hearth unit
employing raw solid waste as an auxiliary fuel.
3. Combustion of dewatered sludge filter cake with raw solid waste in a
fluidized-bed incinerator.
k. Utilization of waste heat from the combustion of raw solid waste to
evaporate moisture from wet sludge (less than 10 percent solids) be-
fore incineration of sludge with refuse in the same combustion chamber.
5. Utilization of spray techniques to inject wet sludge directly into the
combustion chamber of a refuse incinerator.
6. Utilization of flash evaporation techniques to feed wet sludge into a
solid waste combustor.
7. The use of pyrolysis combustion techniques to co-incinerate refuse and
sludge.
Weston conducted a literature search and made other contacts to identify plants
where co-incineration of Mixed Municipal Refuse (MMR) and Municipal Sewage
Sludge (MSS) has been practiced or planned. Table 1 lists the plants so
*Dr. R.S. Gale, Head of Sludge Technology, Water Research Center, Stevenage
Laboratory, Elden Way, Stevenage, Hertfordshire SG 1TH England, U.K.
13
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TABLE 1. APPLICATIONS OF MMR/MSS CO-INCINERATION TECHNIQUES
Technique
Location of Installation
Remarks
1. Incineration of dewatered sludge
filter cake and raw solid waste
in a conventional solid waste
incinerator.
2. Incineration of dewatered sludge
filter cake in a multiple-hearth
unit employing raw solid waste
as an auxiliary fuel.
3. Combustion of dewatered sludge
filter cake with raw solid waste
in a fluidized-bed incinerator.**
Utilization of waste heat from
the combustion of raw solid waste
to evaporate moisture from wet
sludge (^10 percent solids)
prior to incineration of sludge
with refuse in the same combus-
tion chamber.
Kewaskum, Wisconsin
Whitemarsh, Pennsylvania
Cheneviers, Switzerland
Frederick, Maryland
Reigate, England*
Ebingen, Germany
Blilach, Switzerland
Dlibendorf, Switzerland
Bowhouse, Alloa, Clackmannshire,
Scotland
Uzwil, Switzerland*
Franklin, Ohio*
Menlo Park, California
Lausanne, Switzerland
Great Lakes Paper Co.,
Thunder Bay, Ontario, Canada
Duluth, Minnesota
Ansonia, Connecticut*
Hoi yoke, Massachusetts*
Dieppe, France*
Stamford, Connecticut
Gluckstadt, Holstein,
West Germany
Krefeld, Germany
Harrisburg, Pennsylvania
Nurnberg, West Germany
Hershey, Pennsylvania
Lulea, Sweden
Abandoned
Abandoned
Abandoned
Lurgi System
Lurgi System
Lurgi System; abandoned
Nichols System
Lurgi System
Nichols System
Special
Demonstration Plant
Tests Only
Tests Only
P roposed
Spray Dryer; being modified
Rotary Dryer
von Rol1 System
Rotary Dryer
Drag-Conveyor Dryer
Flash-Drying System(probably)
Proposed
von Roll System; proposed
Inactive
Rotary Dryer
-------
TABLE 1 (continued)
Technique
Location of Installation
Remarks
5. Utilization of spray techniques
to inject wet sludge directly
into the combustion chamber of
a refuse incinerator.
6. Utilization of flash-evaporation
techniques to feed wet sludge
into a solid waste combustor.
V/1
7. Pyroiysis
8. Miscellaneous
Alrincham, United Kingdom
Dickerson Station, Havant,
United Kingdom
Neenah-Menasha, Wisconsin
New Albany, Indiana
Waterbury, Connecticut
Watervliet, New York
Bloomsburg, Pennsylvania
Essen Karnap, West Germany
Newburgh, New York
Eastman Kodak,"
Rochester, New York
Louisville, Kentucky
Trenton, Michigan
South Charleston, West Virginia*
Baltimore, Maryland
Orchard Park, New York
San Diego, California
Kalnudborg, Denmark
Minneapolts/St. Paul, Minnesota
Glouchester City, New Jersey
International Paper Company
Mobile, Alabama
Georgetown, South Carolina
Vicksburg, Mississippi
Manistque Pulp and Paper Co.
Manistique, Michigan
Farberjabriken Baya A.G.,
Laverkusen, Germany
Abandoned
Proposed
Shut down
Shut down
On standby
Shut down
Abandoned
Proposed
Abandoned
Special
Not co-incinerating
Drying only
Purox co-incineration; proposed
Monsanto System; 1/75 startuo
Torrax co-incineration; tested
Garrett System
Titan Thermogen System; inactive
refuse only
Proposed
Abandoned
Sludge & Hogged Fuel
Rotary KiIn
-------
TABLE 1 (continued)
Technique Location of Installation Remarks
Buick Motor Division, GMC, Oily sludge only
Flint, Michigan
Plaine de Rhone, France MacLaren System; proposed
Pleasantvi1le, New Jersey Salt bath
WIBAU Matthias Plant, South Germany
Riegal Paper Company
Mil ford, New Jersey
Cologne, West Germany Rotary Kiln
Dordrecht, Netherlands* Multiple Hearth
Keller-Peukert System Conceptual
*Visited as part of the present study.
**None was identified as using raw solid waste. The listed installations used prepared solid waste
as an auxiliary fuel.
-------
identified under each of the seven principal co-incineration techniques, with
appropriate remarks on status of operation and type of equipment. The listing
on Table 1 also includes cities where serious attempts at co-incineration were
made on an experimental basis, or where co-incineration has been practiced but
abandoned.
The techniques did not always fit the specific requirements, yet were close
enough to provide a satisfactory directory of existing technology in the seven
specified categories of technique. This table also includes a final section,
"Miscellaneous," for other plants where some form of co-incineration had been
or was being practiced but where either the technique did not fit into any of
the other categories, or the refuse and sludge involved were industrial in
origin (and, therefore, with characteristics different from those of MMR and
MSS). None of the approaches listed under Miscellaneous was considered suf-
ficiently promising to justify designation as an additional principal technique.
Sludge Treatment
Another approach to categorizing co-incineration is the methods that have
been used to treat and feed sludge with refuse. Methods I through IV entail
separate injection of the sludge stream into the furnace; the only parameter
that is different is the moisture content of the sludge at the time of injection.
Category
I
II
III
IV
VI
VII
VIM
Type of
Sludge
Thickened
Dewatered
Direct-Contact Dried
Indi rect-Contact
Dried
Th i ckened
Dewatered
Direct-Contact Dried
Indi rect-Contact
Dried
Type
of Feed
Direct Injection
Direct Injection
Injection
Injection
Addition to Refuse before
Feeding
Addition
Feeding
to Refuse before
Addition to Refuse before
Feeding
Addition to Refuse before
Feeding
Items V through VIM entail addition of the sludge to the refuse before feeding
to the furnace, and again each category represents a different degree of resi-
dual moisture content before the sludge stream is added to the refuse stream.
17
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These methods are typical of the primary process steps that have been used
in preparing sludge for incineration, but this broad categorization does not
cover intermediate steps (e.g., sludge heat treatment and/or chemical treatment)
that might precede the physical dewatering step. We have also not differenti-
ated between the dewatering attainable by centrifugal, vacuum, or pressure fil-
tration methods of physical dewatering. Likewise, the drying steps do not ac-
count for ultimate moisture content nor for the method used in drying the sludge
other than differentiating between direct- and indirect-contact drying. This
tabulation of methods of sludge treatment and feeding does not account for ex-
actly how the sludge would be injected into the furnace or added to the refuse
before feeding the furnace. Nevertheless, this list of methods provides the
basis for assessing the impact and importance of the sludge processing steps.
Feed Method
Injection or addition of thickened sludge imposes the maximum thermal load
on the furnace, because a large amount of water must be evaporated and heated
to the furnace temperature. Physically dewatered sludge reduces the water quan-
tity, thus easing the thermal requirements, which in turn permits a high sludge-
to-refuse ratio. Reduction of sludge moisture beyond mechanical dewatering, by
pre-drying the sludge using heat generated by the combustion of dried sludge
and refuse, is one approach which eliminates many of the problems associated
•with the feeding and direct combustion of wet sludge (i.e., dewatered to 20 to
kS percent solids). Pre-drying, however, creates a potential odor pollution
problem: the water vapor or wet gases emitted from the sludge dryer will con-
tain organics distilled from the wet sludge in sufficient quantity to require
odor destruction. The most logical means of such odor destruction is high-
temperature incineration by re-injection of the gases into the furnace. Raising
the temperature of such gases to at least ?60°C (l,itOO°F) or higher, in the
presence of air (oxygen), will result in the destruction of the odorous
materials.
Reheating the dryer gases places an additional load on the incinerator.
In direct-contact sludge dryers, the entire gas volume passing through the dryer,
plus the evaporated moisture, is returned to the incinerator and must be re-
heated to 760°C (l,*tOO°F). The flue gas is then exhausted to the atmosphere
(through pollution control equipment which may require gas cooling), and
significant quantities of heat are lost.
In indirectly heated systems, only the water vapor, plus small amounts of
purge air and leakage, are returned to the furnace for odor destruction. While
the gases are raised to 7&0°C (1,^00°F) as required to destroy odors, a large
portion of the flue gas enthalpy is recovered in the boiler, and the flue gas
leaves the unit at temperatures between 150° and 260°C (300° - 500°F).
The allowable sludge-to-refuse ratio is higher when a drier sludge is fed
to the furnace or dryer circuit. Any inerts or thermally unstable inorganics
will represent a heat loss in the system, because the inerts must be heated,
and because thermal decomposition of unstable inorganics is endothermic.
Table 2 has been prepared to summarize the status of furnace systems
applicable to co-incineration of MMR and MSS. Note that the furnace systems
18
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TABLE 2. STATUS SUMMARY OF CO-INCINERATION
Methods of Sludge
Treatment and Feed
1 II III IV V VI
E - C
E E P
E E P
E E P
E E E
E E E
E * P
E E P
E E P
* E C
E E E
A A A
PC*
P P *
P E
P E
F E
P E
E E
E E
P E
F E
P E
* *
E E
* p
A A
* P
E
E
E
E
E
E
E
E
P
A
E
C
A
P
VII
E
E
E
E
E
E
E
E
E
E
E
A
A
A
A
VIII
E
E
E
E
E
E
E
E
E
E
E
A
A
A
A
Municipal Refuse
Furnace Systems
(Continuous Feed)
A. Raw Refuse
a. Grate firing
b. Rotary Kiln
1. Co-Current
2, Counter-Current
3. Two-Stage
c. Shaft Furnace
1. Air-Blown
2. Oxygen-Blown
d. Combination Furnace
1. Grate Plus Rotary Kiln
B. Shredded Refuse
a. Grate Firing
1 . Spreader Stoker
b. Suspension Firing
1. Utility Furnace
(Fossil- Fuel Fired)
2. Industrial Furnace
3. Cyclonic
c. Multiple Hearth
1. Mixed Feed
2. Dual Feed
d. Fluid Bed
1 . Top Feed
2. Dual Feed
Furnace
Status
Principal
Technique
Energy Recovery
Fuel
None Steam Gas or Oil
(See next subsection)
C
P
P
P
E
E
C
C
P
C
E
C
C
p
P
1,4,5,6
11,5,6,7
4,5
i|
7
7
1,4,5,6
1,4,5,6
4,5,6
4,5,6
-~
2,7
2
7
J
3
C
P
P
P
C
A
C
p
A
A
E
C
C
c
p
c
c
p
p
p
p
c
c
c
c
E
P
P
p
P
A
C
E
f.
P
C
A
*
A
*
*
P
A
c
A
C -- Commercial (one or more installations)
p — Possible (practically similar to commercial, or proposed commercially)
E — Experimental (work needs to be done)
v,- — Unsound, undesirable, unnecessary (redundant), etc.
Source: Estimates prepared by Roy F. Weston, Inc.
-------
are divided into two categories: those which are capable of handling raw re-
fuse as received (exclusive of bulky items), and those which require shredded
refuse for firing. The techniques which could be used with each furnace system
are noted, along with the potential for energy recovery, in terms of whether
the system can be considered commercial, possible, experimental, unsound, un-
desirable, unnecessary, redundant, etc. The advantages and disadvantages of
each of the seven principal co-incineration techniques are discussed in the
next sub-section.
DISCUSSION OF PRINCIPAL CO-INCINERATION TECHNIQUES
Synopsi s
Following is a brief description and experience summary for the seven co-
incineration techniques studied. A more detailed discussion, including reports
on individual plant experiences, is included in the detailed discussions appear-
ing later in this section.
Technique Number 1: Incineration of Dewatered Sludge Filter Cake and Raw Solid
Waste in a Conventional Solid Waste Incineratoi—
This technique involves the direct injection of sludge cake into the active
combustion zone of the incinerator. A number of plants have attempted co-
incineration using this technique, with poor results. Difficulties included
problems with dispersing a high-solids sludge cake throughout the combustion
zone and poor ignition and burn-out of sludge cake. This technique requires a
minimum in capital investment and was the first co-incineration technique to
receive serious consideration; however, all plants that employed this co-
incineration technique have been abandoned.
Technique Number 2: Incineration of Dewatered Sludge Filter Cake in Multiple-
Hearth Incinerator Using Solid Refuse as an Auxiliary Fuel—
The technique employs shredded refuse to improve the "average" heating
value of dewatered sludge cake for incineration in a multiple-hearth unit.
Several successful operations can be found in Europe, although no U.S. instal-
lations are currently in operation. Advantages of this technique include ex-
tended residence time, zoned temperature control in the furnace, excellent
sludge/refuse mixing, and standard equipment design. The major disadvantage
of this system is the extensive refuse shredding and cleansing necessary with
multiple-hearth furnaces.
Technique Number 3: Combustion of Dewatered Sludge Filter Cake and Raw Solid
Waste in a Fluid!zed Bed Incineratoi—
No plants routinely using this co-incineration technique were identified.
A number of experiments have been run using refuse and refuse-like fuels to
provide the additional heat necessary to incinerate sludge. The major ad-
vantages of fluidized bed combustion include the availability of a large heat
sink in the sand bed, thereby maintaining satisfactory operating conditions with
wide variations in sludge feed rate and moisture. In addition, the fluidizing
action provides excellent mixing of the refuse and sludge. As with multiple-
hearth incinerators, extensive refuse preparation in the form of shredding and
air classification will most likely be required for this technique.
20
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Technique Number k: Utilization of Waste Heat from the Combustion of Raw Waste
to Evaporate Moisture from Wet Sludge Prior to Incineration of Sludge with
Refuse in the Same Combustion Chambei—
In this technique, sludge is pre-dried before it is combined with the ref-
use for incineration. The burning sludge and refuse provide much, if not all,
of the heat necessary for pre-drying the sludge. In all cases, the sludge is
pre-dried in a processing unit separate from the actual incinerator. Heat may
be transferred from the incinerator to the dryer by direct use of incinerator
flue gas or by the production of steam which is later used in the sludge dryer.
Co-incineration facilities have been successfully operated using both these
techniques. By pre-drying the sludge, the operation of the incinerator portion
of the process is greatly facilitated. Separation of the drying and inciner-
rating facilities also permits independent operation of either unit. The major
disadvantage is the increased operating and capital cost of two processing
units. This technique appears to be the most widely used and successful co-
incineration process.
Technique Number 5: Utilization of Spray Techniques to Inject Wet Sludge
Directly into the Combustion Chamber of a Refuse Incinerator—
This technique is actually a modification of Technique Number 1. Proved
means of spraying or dispersing the sludge in or above the active combustion
zone are included to improve the ignition and burn-out of sludge solids. There
are no facilities currently using this technique, although one plant was in
operation for an extended period. Spraying a low-solids sludge obtains better
atomization, and the sludge solids are more readily dried and ignited. However,
the need to maintain low solids in the spray system led to the eventual failure
of this technique.
Technique Number 6: Utilization of Flash Evaporation Techniques to Feed Wet
Sludge into a Waste Combustoi—
Technique Number 6 is a special adaptation of Technique Number 4. In-
cinerator flue gas is used as the heat source in pre-drying sludge. This
system has been separated because it has a well-known developer, the Raymond
Division of Combustion Engineering. In addition to drying sludge, the flash
evaporator (normally a cage mill) comminutes the sludge particles for direct
firing into the incinerator or into a boiler. The dried sludge can also be
separated and sold as fertilizer or soil conditioner. A number of units have
been installed and operated, using both refuse and fossil fuels as heat sources.
Of late, the system appears to have lost much of its early popularity. Dis-
advantages of this technique appear to be extensive equipment corrosion and
erosion, critical operating parameters, potential for dust explosion, and for
the generation of extremely fine particulate material which is carried through
the incinerator to collection equipment.
Technique Number 7' Co-incineration by Pyrolysis—
Pyrolysis is a recent development in municipal refuse disposal designed
to recover some of the energy resource in convenient form. Other than demon-
stration plants built by vendors, there are no operating units in the U.S.
Early indications are that co-incineration in such units may be practical with
the loss of some of the potentially recoverable energy in the refuse. Feasi-
bility studies and demonstrations are currently under way.
21
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Technique 1: Incineration of Dewatered Sludge Filter Cake and Raw Solid Waste
in a Conventional Solid Waste Incinerator
Four plants were identified where this technique had been or was being
used. All the domestic sites have abandoned co-incineration, and there is good
reason to believe that this technique has been attempted but unreported at a
great many more plants, because it is one of the simplest methods possible for
the disposal of MSS along with MMR. Lancoud1 has reported on a Swiss inciner-
ator where dewatered sludge was added to the raw MMR and fed to a 180 metric
ton per day (200-tpd) continuous-feed, grate-fired, steam-generating inciner-
ator. The heat-treated MSS was filter-pressed to 40 percent moisture, and the
cakes were broken up before the sludge was added to the refuse. Because there
seemed to be an excessive amount of combustibles remaining in the ash, a series
of tests was run in June 1969. The test data indicated that none of the sludge
was incinerated. Lancoud concluded that the MSS was not satisfactorily de-
stroyed by incineration, for the following reasons:
1. The pieces of sludge cake are larger than refuse and move faster
through the furnace; thus, burning time is reduced.
2. A crust formed on the sludge cake hinders complete combustion.
3. Poor mixing permits sludge cakes to separate from the refuse, with
poor air contact and resulting poor combustion.
Lancoud suggested the following remedial action:
1. Reduce the size of the sludge cake to 2 cm (0.8") or less.
2. Use mechanical action in the incinerator to break up the crusted cakes
(some mechanical action during test sequence).
3. Improve the mixing of the sludge cake and MMR before they are fed to
to the furnace.
Clinton2* has reported on the Kewaskum incinerator, a batch-feed furnace
with a capacity of 23 metric tons/day (25 tpd). The primary and secondary
sludge was vacuum-fi1ter-dewatered and added to the raw refuse before charging
the furnace. Reportedly,** the plant stopped burning sludge in the furnace
after a short time, because the heat supplied by the refuse to burn the de-
watered sludge was insufficient.
Reilly3 has reported on the original Whitemarsh incinerator, which was de-
designed to mix vacuum-filter-dewatered MSS with the refuse prior to feeding the
furnace. The author reports a five-day performance test during which 29 metric
tons (32 tons) of sludge were burned with 3k6 metric tons (38! tons) of refuse,
*The papers referenced in this section are also abstracted in Appendix A.
**Verbal communication with Mr. Peter Alberts.
22
-------
a sludge-to-refuse ratio of 8.4:100. The test was reportedly sucessful, but the
original incinerator failed, and further attempts to burn MSS were abandoned
because of burning problems.
L
Defeche has reported on the combined disposal of refuse and sludges,
specifically referring to the Swiss plant reported on by Lancoud. In addition,
Defeche reports on a series of short-term co-incineration tests that were con-
ducted at Locarno, Sutton Coldfield, Rotterdam, and Issy-Les-Moulineaux. He
reported that the carbon content of the residue at Rotterdam increased with
co-incineration, and that at Issy-Les-Molineaux there were several tests during
which the sludge stuck to the feed hopper and steam production dropped. In
general, Defeche stated that the co-incineration tests were successful, but the
kind of problems that developed in Switzerland could not have been observed,
because al1 the tests were short-term.
Knaak and Kuhl-' have reported on co-incineration experiments at the Gen-
tofte incinerator in Copenhagen and at the incinerator at Bad Godesberg. Both
incinerators are grate-kiln-fired, continuous-feed, steam-generating inciner-
ators. The author reports on four sludges and several dewatering processes and
on four methods of adding the sludge to the furnaces: 1) sludge continuously
added to the refuse fed by a spreader; 2) sludge mixed with the refuse in a
trommel before feeding to the furnace; 3) a full bucket-load of sludge placed
in the feed chute; and k) sludge manually added by shovel into the feed chute.
Knaak and Kuh1 reported that the only problem encountered was the formation of
crusts on the sludge cake, which hindered the drying and burning of these pieces.
Frederick, Maryland" has reportedly mixed dewatered MSS with MMR before
feeding it to the furnace, but contact with plant operators indicated that
the reasons for abandonment were lost in the fog of time. While undoubtedly
one of the simpler systems, the limited experience available would seem to
indicate that intensive mixing of the MSS with the MMR is a prerequisite to
successful implementation of this technique.
Lancoud's conclusion, that improved mixing of the sludge cake and refuse
is a mandatory requirement, seems valid. It is difficult to conceive of a
simple system that would adequately mix dewatered sludge cake with a material
that is as heterogeneous as MMR. We do not consider this technique feasible
without extensive development work and cannot forecast a successful conclusion
of the current developmental work.
Technique 2; Incineration of Dewatered Sludge Filter Cake in a Multiple-Hearth
Unit Employing Raw Solid Waste as an Auxiliary Fuel
Six plants were identified as using this technique, with a slight modifi-
cation; i.e. a prepared (shredded) refuse was used in place of raw refuse. No
U.S. installations were identified, although it is very likely that the tech-
nique has been tried in existing multiple-hearth furnaces intended for drying
or burning MSS. However, tests have been reported? at Envirotech Corporation's
Brisbane, California test facility, and the Metropolitan Waste Control Com-
mission - Twin Cities Aera has received an EPA research grant to study co-
incineration of sewage sludge with refuse and/or coal. The European and English
23
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installations can be divided into two classes: 1) those that feed refuse and
sludge to the top hearth of the furnace; and 2) those that feed sludge to the
top hearth and feed refuse to some mid-point hearth.
The Dubendorf installation has been described in an anonymous^ 1969
article. Here, thickened sludge (90-95 percent water) is mixed with ground
refuse and fed to the top hearth of the furnace. This furnace also accommodates
waste oils, which are separately fed to a mid-hearth of the furnace.
Kiefer° and Burgess have described the Ebingen plant, where the waste
is shredded in a hammermill and magnetically cleaned before being added to the
second and third levels of the multiple-hearth furnace. The sludge is added to
the uppermost level and is dried, before reaching the combustion zone, by the
waste gases from the combustion of MMR. The sludge fed is dewatered to 60-65
percent water, and 36 tons of sludge are fed for every 100 tons of refuse.
stated that the additional energy necessary for incineration at 700°-800°C
(1,300°-1,500°F) is introduced in the third and fourth levels of the furnace,
where the actual incineration takes place.
Contacts with various individuals indicate that there are no odor problems
with multiple-hearth systems where sludge and refuse are introduced in the top
hearth, because the discharge temperatures are relatively stable at approxi-
mately 600°C (1,100°F). The Uzwi1 plant is said to be recognized by the Swiss
Government as a model plant; however, magnetic separation of ferrous metals
from the shredded refuse is essential, because any ferrous metal (particularly
wire) not so removed from the feed would foul the plows on the rabble arm or
plug the hearth drop holes.
The Bulach plant, where sludge was dried on the upper hearths and refuse
was fed to the mid-hearths, had been plagued with odor problems that could not
be technically resolved, because the upper hearths performed a drying function
and the flue gas temperature leaving the unit was about 100° to 200°C (200°-
*tOO°F). Doubling the height of the discharge stack could overcome the problem
by providing significantly greater odor dissipation, but would not attack the
root cause of the problem (low flue-gas temperature). Furthermore, the wide
variability in heat content of the feed material made it difficult to maintain
off-gas temperatures; consequently, even with higher off-gas temperatures, there
were periodic odor episodes. These circumstances could have been significant
in the reported abandonment of the Bulach plant.
At Reigate, a number of solvable problems had been encountered with a
Lurgi multiple-hearth furnace, including the need to re-insulate the central
shaft of the furnace because of problems with local overheating, which could
have been caused by receiving refuse with a high Btu content or by problems in
maintaining the insulating material on the exterior of the drive shaft.
The applicability of multiple-hearth furnaces to co-incineration was dis-
cussed with a representative of a domestic vendor. The general conclusions
were that many of the problems will be associated with the refuse pre-processing
step and that adequate cleansing of the refuse to remove those materials likely
to cause problems within the multiple-hearth furnace is both mandatory and
solvable.
2k
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In conclusion, the dual-feed multiple-hearth furnace (such as Bulach) has
Inherent odor-emission problems. A properly designed, top-feed, multiple-hearth
installation (Uzwil) would seem technically and practically possible, except
that there are built-in size constraints in this type of system.
Technique 3: Combustion of Dewatered Sludge Filter Cake with Raw Solid Waste
in a F1uidized-Bed Incinerator
All the fluidized-bed incinerators that were identified 5n the survey used
a prepared refuse rather than raw refuse.
The Franklin, Ohio system (visited some time ago by Weston personnel) con-
stitutes a very special means of using the fluid-bed incinerator on MMR, in
that extensive refuse preparation and resource-recovery steps precede the feed-
ing of the residual MMR to the fluid-bed unit. A wet pulper is employed at
Franklin, and ferrous metal, heavy solids, etc. are removed from the waste
stream. Liquid sludge is then mixed with the prepared refuse before the
residual material is dewatered in a cone press and pneumatically conveyed and
injected into the furnace above the sand bed level. No problems that could be
attributed to the accumulation of coarse material in the bed (which would tend
to de-fluidlze the bed) have been encountered on this installation; however, it
has been necessary to remove excess bed material periodically, rather than add
to the bed material as is common when fluid-bed incinerators are used to burn
sludge only. The Franklin unit, with a diameter of 7.3 m (2k ft), is one of the
largest in existence and approaches the maximum size available for this type of
equipment. There are severe design constraints associated with the constriction
plate under the sand bed which feeds the fluidizing/combustion air to the fluid
bed unit. Herber^ has reported on the Franklin installation in a paper, "Waste
Processing Complex Emphasizes Recycling."
12
Albrecht has reported on the fluid-bed sludge incinerator in Lausanne
(Switzerland), where the sludge is fed by gravity through a pipe that penetrates
the reactor head into the furnace freeboard. Experiments have been conducted
at this site by adding a variety of solid waste materials to the sludge feed.
The author reported excellent combustion of these materials, which were largely
fine to begin with or were totally combustible.
Limerick^* has reported on the fluid-bed unit at Thunder Bay, Ontario,
which, although not typical of municipal refuse or municipal sludge, is handling
hogged bark and wood debris along with pulp mill sludge, and has been tested on
MMR. Unsorted solid waste from the City of Thunder Bay has been burned along
with sludge at Great Lakes Paper Company, as a demonstration of fluid-bed
furnace operation. Duluth, Minnesota is the proposed site for two fluid-bed
units to burn prepared refuse and sludge. The refuse will be shredded, mag-
netically separated, and air cleaned before being fed to the furnace. The
system will include a waste-heat boiler, and the steam will be used to power
the plant's prime movers.
Fluidized-bed furnaces are considered competitive with multiple-hearth
furnaces for the combustion of MSS. In sewage sludge combustion, a certain
amount of the sand is lost from the bed and must be made up. At Franklin, Ohio
the bed attrition problem probably exists, but, at the same time, excess bed
25
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material must be periodically drained from the unit. It is known that large
materials will accumulate at the bottom of the fluid bed, causing ultimate de-
fluidization of the bed; therefore, it is important that the refuse be shredded
to a uniform size, generally in the 2.5 cm to 7.6 cm (1" - 3") range.
As long as the materials introduced to the bed unit have a maximum size
not greater than the consist of the bed material, there should be very little
problem in burning MMR and MSS in the same unit. In all probability, the
degree of shredding would have to be much greater with the fluidized-bed unit
than with the multiple-hearth unit, and cleansing of the material would appear
important, to prevent excessive down time (for removal of material from the
furnace). Since all the fine ash introduced to a fluidized-bed unit goes out
the overhead, the particulate loading to the control equipment will undoubtedly
be high, but certainly controllable by cyclones and venturi scrubbers normally
used with fluidized-bed incinerators.
Other problems of using a fluidized-bed unit involve size limitations which
are slightly more restrictive than in the multiple-hearth unit. In addition to
size, the density of the shredded refuse must be uniform. Extremely dense
material will drop through the fluidized bed and can cause blockage and disturb
the flow through the air distribution plant under the sand bed.
The refuse preparation and cleansing train is essentially a solvable
problem, with built-in capital and operating cost constraints that are unique
to this type of system.
The final problem involves the best method of feeding the MMR and MSS to
the furnace. Two of the four principal fluid-bed manufactuers introduce sewage
sludge into the fluidized bed; the other two introduce the sludge through the
freeboard of the furnace. There are several other manufacturers who would in-
troduce material through the freeboard in a fashion similar to that used at the
Lausanne installation. If shredded MMR must be introduced through the free-
board it is possible that material will be blown out of the furnace before com-
bustion is complete; this does not appear to be an insurmountable problem, but
none of the manufacturers offers a system of MMR introduction that would
clearly overcome the problem of unburned material leaving the freeboard of the
furnace before combustion is completed.
In summary, the fluidized bed method of co-incineration is reasonably well
established theoretically, with some practical experience to indicate that a
full-scale installation would be wholly practical.
Technique k: Utilization of Waste Heat from the Combustion of Raw Solid Waste
to Evaporate Moisture from Wet Sludge (Less than 10 Percent Solids) Prior to
Incineration of Sludge with Refuse in the Same Combustion Chamber
Ten existing or proposed installations were identified, although not all
of them can be properly classified as handling wet sludge containing less than
10 percent solids. Each of the systems identified can be termed a refuse in-
cinerator; the major variables are the method of drying the sludge and the
method of injecting or adding the dried sludge to the furnace or to the refuse.
26
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The sludge drying methods can be broadly categorized as direct utilization of
the hot flue gases for direct-contact drying of the sludge, and utilization of
steam or an intermediate hot fluid to dry the sludge indirectly.
The Ansonia, Connecticut installation (see Appendix B) utilizes a spray
dryer, and is apparently the only application of the spray drying technique to
sludge drying in incineration practice. The system has undergone several
changes, with one change under way at the present time. The present location
of hot flue gas takeoff from the incinerator ductwork has provided the spray
dryer with inlet gas temperatures below design conditions, thus reducing the
dryer capacity. By relocating the takeoff to the primary combustion chamber,
gas temperatures of 980°C (l,800°F) are expected to restore dryer capacity.
The Holyoke, Massachusetts (' ^ and the Stamford, Connecticut instal-
lations utilize rotary dryers. The Holyoke installation has been in use for
many years, while the Stamford installation was just started up recently (1975).
17
Tanner has reported on the von Roll indirect sludge-drying system to be
used at Dieppe, where steam will provide the heat for a scraped-surface sludge
evaporator. Another system of this type has been proposed for Nurnberg in West
Germany.
1 o
Joachim has reported on two sludge-drying schemes. One of the schemes
uses a direct-contact drag-conveyor dryer, and the other uses an indirect-
contact drag conveyor; both use hot gas as the heating medium. One installation
is planned for Gluckstadt, West Germany.
iq
Pepperman has reported on the planned modification of the Harrisburg,
Pennsylvania incinerator to burn MSS. In a personal communication, Pepperman
indicates that the thickened sludge will be pumped to the incinerator site and
that a sludge-processing building will be built adjacent to the existing incin-
erator. In the processing building, the sludge will be heat-treated, vacuum-
filtered, and steam-dried to a moisture content between 10 and 15 percent using
steam from the refuse incinerator for the heat treatment and drying. After it
has been dried, the sludge will be pneumatically conveyed in a 5 cm (kn) steel
pipe to an elevator installed in the incinerator building. From the elevator,
the sludge will be discharged to one of the two incinerator-charging hoppers
utilizing screw conveyors for that purpose. Since the charging hoppers are
5.5 m (18 feet) long, the sludge will be discharged at the quarter points, to
obtain suitable mixing with the refuse. The spent air from the pneumatic con-
veyors will be discharged to the combustion-air intake chamber on the roof of
the incinerator and then used as over-fire or-under-fI re air. In addition, the
vapor-laden air from the steam dryers or evaporators will be conveyed in ducts
from the sludge processing building to the incinerator, where it will be used
as over-fI re air, thereby eliminating potential odor problems.
The Holyoke, Massachusetts installation, which apparently was the first
application of a rotary dryer to co-incineration of MSS and MMR, was visited in
the course of this study. The plant consists of two batch-fed furnaces with a
combined capacity of 20*1 metric tons per day (225 tpd). The incinerator
operates 8 hours a day, burning 50 metric tons (55 tons) of mixed municipal
refuse (including a significant quantity of paper waste from nearby mills).
27
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The wastewater treatment plant employs primary treatment only, producing about
49 metric tons per day (51* tpd) of thickened (5 percent solids) sludge. The
sludge is dewatered through vacuum filters, producing a 28 percent solids
fi1ter cake.
The cake is premixed with previously dry sludge, increasing solids to
about 50 percent, to reduce deposits on the dryer wall. The sludge is then
dried in a 2.1 m (7 ft) diameter by 12.2 m (kO ft) long direct-fired steel-
shell rotary dryer. Sludge leaving the dryer contains about 15 percent
moisture. The sludge solids are conveyed to either incinerator and injected
into the furnace by a high-velocity air jet.
Hot gas to dry the sludge is drawn from the incinerator flue and tempered
to 650°C (1,200°F). The gases pass in co-current flow through the dryer and
are returned to the incinerator stack breaching. Auxiliary fuel is fired, as
required, at the dryer inlet.
The plant has been co-incinerating sludge and refuse with little diffi-
culty since 1965. Initially, no auxiliary fuel was required, because large
quantities of high-heating-value waste paper were present in the refuse load.
When environmental regulations required a reduction in the quantity of coated
paper incinerated, some auxiliary fuel was required. In addition, the inter-
mittent operation of the plant (8 hours/day, 5 days/week) also increased the
quantity of auxiliary fuel required. Further details are reported in Appendix B.
20
An anonymous publication concerning the Krefeld installation lacks the
detail necessary for positive identification of the method, but describes it as
using furnace flue gases to dry the sludge before blowing it into the main com-
bustion zone. This particular installation may be Technique k or could be
Technique 6.
Gater has reported on the Stamford, Connecticut installation, where the
sludge will be dried in a rotary dryer and then injected into the combustion
chamber of a conventional refuse incinerator.
21
Bergling has reported on an installation in Lulea, Sweden, where the hot
gases from each of two 330 metric ton/day (360 tpd) refuse incinerators pass
through a rotary dryer-incinerator. This installation does not, strictly speak-
ing, fit within the technique being discussed, because the sludge is either
dried or incinerated in the rotary unit rather than introduced to the combustion
chamber of the MMR incinerator.
Zj
Defeche, in reviewing the combined disposal of refuse and sludges, also
referred to the Dieppe furnace, stating that flue gas or steam can be used in
the thin-film evaporator.
22
Samuels has described a system where sludge is dried in a screw conveyor
arranged for heat transfer by means of a jacketed shell and hollow screws. The
system uses 320°C (600°F) oil, and the heat necessary to raise the oil to'that
temperature is obtained by burning the dried sludge in an incinerator.
28
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23
Schlotmann has described a number of processes for co-incineration of re-
fuse and sludge. One of them utilizes the excess heat from refuse incineration
to provide the heat for a thin-layer evaporator; the vapors from the heat ex-
changer go into the incinerator and are heated to 800°C (1,470°F), while the
sludge is further dried in a mill dryer before it is blown into the furnace.
7k
Taylor*- has discussed three methods of burning sludge with refuse, two
of which involve this technique, using either direct gas-phase contact with the
products of combustion, or indirect drying using an intermediate heat transfer
fluid such as steam. The author concluded by recommending, on the basis of
practicality, the indirect drying system using a thin-film evaporator fitted
with a rotor and scraping blades to keep the heat-transfer surfaces clean. The
system the author recommended is the one installed at Dieppe. He further stated
that the vapors and incondensible gases from the evaporator are ducted into the
furnace and heated to 750°C (1,380°F) to prevent odor emissions from the raw
refuse incineration plant. Tanner and Vrenegoor in another paper^-* also have
described the thin-film evaporator, stating that the dried sludge is metered
into the feed hopper of an MMR incinerator and burned on a grate system.
9A
Thompson^ has described several systems, including one in which multiple-
effect evaporation is used to dry the sludge prior to firing. The author stated
that a multiple-evaporation experimental plant has been run for 1,800 hours in
Hamburg, Germany, but there was concern about caking of the sludge on the coils
as a major constraint of this system.
A multiple-effect evaporator has been used to dry MSS at Hershey, Pennsyl-
vania (see Appendix B). The system uses an oil carrier to maintain sludge
fluidity as water is removed through successive evaporation steps. Sludge
solids are then separated from the oil in a centrifuge. Most of the oil is re-
cycled to the evaporators. The oily sludge solid is then burned in a boiler
to raise steam to drive the evaporators. The use of multiple-effect evaporators
improves the energy efficiency of the entire process. The plant experienced
problems with corrosion and erosion in the evaporator system. The corrosion
from one effect to another was reduced by the injection of a small quantity of
ammonia to neutralize organic acids which distilled with the evaporating water.
The erosion problem, as the sludge was dehydrated, was a more serious problem
which was never completely solved. The evaporator is presently shut down and
will probably be abandoned in the near future.
Direct-contact drying of MSS has received considerable attention, and
there are a number of installations both here and abroad. Potential odor
problems seem to mandate return of the spent dryer gases to the hot zone of the
MMR furnace. One reported problem (maintenance of dryer inlet temperatures)
would appear solvable by employing a variation of the Lulea technique, in which
waste oils can be burned in the dryer circuit.
Indirect-contact drying has received some attention, but the only instal-
lation appears to be in Dieppe, France. Domestic installations have been pro-
posed, but none has been implemented. Indirect drying has a very real advantage
over direct-contact drying, because the flue gases need not be reheated. The
major problems are heat-transfer surface fouling and lack of demonstrated
methodology.
29
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On balance, both direct- and indirect-contact drying appear feasible, if
it is assumed that sludge drying is necessary before injecting MSS into the
furnace or before adding the dried sludge to the MMR when feeding the refuse
furnace. The history of Techniques 1 and 5 lend credence to this assumption.
The degree of dryness needed is one variable, as is the degree of physical agi-
tation required during the drying operation. The recycle of dried sludge is
practical in direct-contact drying and has been used in indirect drying to abate
surface foul ing.
Both direct and indirect sludge drying techniques before MSS combustion
are proposed for further study. Techniques 4 and 6 should be examined together,
since either can employ the hot flue gases to dry MSS.
Technique 5: Utilization of Spray Techniques to Inject Wet Sludge Directly
into the Combustion Chamber of a Refuse Incinerator
Only one operating plant and two proposed plants involving this method
were identified. There is one plant in Europe which is conducting a short-term
experiment to evaluate the method.
27
Davies has described the Altrincham incinerator, a 196 metric ton/day
(216 tpd) plant consisting of two continuous-feed, stoker-fired, refractory-wall
furnaces complete with evaporative gas conditioning systems and electrostatic
precipitators. The plan called for raw sludge at 5 percent solids to be sprayed
into the furnace through the end walls of each furnace. The wet sludge feed
rate was to be 76,000 liter/day (20,000 gpd), based on a five-day week. Co-
incineration at this site has been abandoned, for two salient reasons: 1) the
refuse was wetter than anticipated; 2) the injection pipe to obtain the desired
wet sludge rate had to be small and was thus subject to frequent plugging. The
engineers have calculated that the feed pipe would have to be 7.6 cm (3 in.) in
diameter in order to prevent plugging problems and, thus, that the wet sludge
feed rate would far exceed the capacity of the furnace to handle thickened
sludge.
28
Munro has reviewed the disposal of sewage sludges, and concluded with a
description of a full-scale installation for the incineration of sewage sludge
with domestic refuse on a continuous burning grate. The author mentioned
Altrincham, Reigate, and Havant, stating that there are two basic ways that
sludge can be incinerated in a continuous-burning domestic refuse incinerator.
One method is spraying the sludge into the furnace chamber above the refuse bed,
and the other is mixing sludge cake with the refuse fed to the furnace. At the*
Havant incinerator site, it is proposed to spray thickened sludge into'the
furnace using a dual fluid spray nozzle. Limited trials using the spray nozzle
were made, but with 3.5 percent solids sludge rather than the 7-8 percent
solids sludge expected to be used in the full-scale installation. Munro
emphasized the importance of atomization, if drying and solids combustion are
to be achieved, and pointed out that in order to destroy odor the temperature
should be about 830°C (1,530°F). This temperature imposes a limitation on the
amount of wet sludge that can be sprayed into the furnace. The refuse furnace
at Havant is operating, and an addition of the sludge system is scheduled for
late 1975.
30
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29
Herrman, et_ at_. J have discussed the proposed Dickerson Station and its
plan to inject sludge into a coal-fired utility boiler along with prepared
refuse. The contract for the furnace has been let and the basic furnace con-
fuguration has been established, but the plans for the proposed adjacent waste-
water treatment plant have not yet been finished. If the plnat is built on this
site the plan is to inject dewatered sludge at 13 percent solids into the top
of the furnace through two sewage sludge guns. The author stated that the
total heat input from dry solids in the sewage sludge will amount to approxi-
mately 0.5 percent of the unit's maximum continuous rated heat input; this
would be approximately equivalent to the heat required to evaporate the sludge
moisture. In order to avoid possible plugging problems, the sewage must not
contain more than 25 percent of its solids at a size of 100 percent minus 8
mesh and 90 percent minus 30 mesh. The sludge will be injected at a point
where there will be four levels of coal nozzles below the point of sludge in-
jection and three levels of coal nozzles above.
The failure of the Altrincham installation, where no further work is
planned, and the proposed nature of the Havant incinerator and of the Dickerson
Station make it difficult to directly assess the capabilities of this method,
except by analogy to known problems. The major problem would be to sufficiently
atomize the sludge such that it will both dry and burn within the confines of
the furnace. Extremely high pressures and small orifice sizes are necessary
to obtain fine droplets simply by mechanical atomization of water. Dual fluid
spray nozzles utilizing compressed air or steam have the potential of producing
a finer-size consist of sprayed material. The fibrous nature of MSS suggests
that considerable experimentation would be necessary to determine the optimum
combination of dual fluid spray nozzle, nozzle location, and throughput rate.
In spray-drying practice, a rotary disc atomizer has been used. There are
no recorded uses of a disk-type atomizer for sludge disperson inside a refuse
incinerator. Such disk atomizers have been used in experimental spray dryers
(Ansonia, Technique k), with k percent solids. Another approach would be to
use a rotary cup burner assembly, as has been used for high-firing heavy fuel
oil. Again, all of these approaches would require considerable experimentation
to determine if satisfactory atomization could be achieved, and then, within
the confines of a furnace, whether that degree of dispersion would yield drying
and burn-out in the furnace.
Another analogous situation is the method of feed used on certain types
of fluid-bed sludge incinerators where the sludge is introduced from the roof
of the vessel through the freeboard of the furnace to the sand bed. The spray
nozzle in this case is a rather crude assembly, and photographs taken by the
manufacturer showing the dispersion pattern under test conditions in ambient
air would seem to indicate that this type of injection method would not be
suitable for a conventional refuse incinerator. Fluid-bed units of this type
inspected by Weston staff members have not had observation ports, thus making
it impossible to observe the descent of the sludge solids through the freeboard
to the sand bed. As a result, we can speculate no further than is visually
evident from the photographs supplied by one of the manufacturers.
31
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Altrincham has abandoned this approach for operational reasons, but the
thermal load problems should not be overlooked. At Altrincham, they proposed
burning only a portion of the sludge generated in the refuse service area.
Thickened sludge is difficult to feed, and dewatered sludge feed has not been
proposed at either A.ltrincham or Havant. Clearly, dewatered sludge would be
more difficult to atomize; therefore, for the present, it appears impractical
to forecast using this method to feed sludge with a higher solids contents to
reduce the thermal load on the system. While this approach is basically simple,
the available hardware to achieve the desired results when feeding thickened
sludge cannot be extrapolated from existing data without a considerable amount
of experimental work. We therefore consider this technique, within the con-
straints of available hardware, as not a viable approach to co-incineration.
The major problem with direct sludge injection into the furnace can be sum-
marized as the increasing difficulty in atomizing or properly spreading sludges
of increasing moisture content. We know from the experience reported at
Altrincham that the injection of high-moisture-content, easily atomized sludge
is not a viable technique. Further development in this technique should be
directed toward techniques and equipment for the dispersion of high-solids
(greater then 20 percent) sludge cake into a refuse incinerator. High-speed
disk atomizers, high-velocity jet atomizers, sonic atomizers, pre-blending
sludge cake with ash to increase solids, sludge cake comminution before in-
jection, and means of pre-mixing sludge and refuse are techniques for sludge
introduction directly into the combustion zone of the furnace which hold promise
for this technique.
Technique 6; Utilization of Flash Evaporation Techniques to Feed Wet Sludge
into a Solid Waste Combustor
A total of ten plants where co-incineration involving this technique could
have or had been practiced were identified (see Appendix B). Most of these
plants had been abandoned for one reason or another; of the remainder, the
Neenah-Menasha incinerator was shut down around the first of 1975, the shutdown
being dictated by the need to add air pollution control equipment to an already
old incinerator. The Neenah-Menasha job constitutes a special case, since both
the refuse fired and the sludge had a high heating value when compared to normal
MMR and MSS, because large quantities of paper were in the refuse load and
because there were significant amounts of fines (cellulose) in the pulp mill
wastewater treated at the municipal plant.
The Waterbury, Connecticut plant^°»31 js On standby and could be operated,
but the sludge is normally disposed of in a multiple-hearth sludge incinerator.
The flash drying system was not expanded when secondary water treatment greatly
increased the sludge volume. A multiple-hearth furnace is now in use.
At Essen-Karnap,' »",3 siuc|ge has been burned with coal, and the use of
refuse has been proposed. The system incorporates a dryer mill that blows the
dried sludge solids into the furnace.
The Eastman-Kodak installation (see Appendix B) is a recent one and is
special, because the refuse and sludge are industrial rather than municipal in
origin. The installation also is a steam-generating furnace handling shredded
refuse, with the dried sludge injected and burned in suspension.
32
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The Newburgh, New York installation (see Appendix B) is a fairly recent in-
stallation of a Raymond flash-drying system, and personal contact with the plant
indicates that the system was purchased in 1970 and abandoned after a year of
attempts to get the system operational. Newburgh reports that to start the
operation the MMR incinerator temperature was raised to 930°C (l,700°F) by
burning cardboard, and that the hot gases were then directed to the flash dryer;
as long as they were trying to dry sludge the incinerator temperature would drop
drastically—at times reaching temperatures as low as 260° (500°F). As the
temperature got lower, the drying process slowed, and the partially dried sludge
clogged the materials-handling equipment, filling the building with smoke. The
operator reported that the incinerator is a very old one with many leaks and
poor draft control. The superintendent believes that this was the cause of poor
operation and that the system would work with a modern incinerator. They at-
tempted to start up again in 197^, with much the same results as in the original
startup.
While Combustion Engineering has merchandised the Raymond dryer system in
this country for many years, it is surprising to learn that there is only one
operating unit where co-Incineration is practiced—at Eastman-Kodak—a special
situation, because the refuse and sludge are industrial in origin.
The Allegheny County^ sludge incinerator is reported to be one of the best-
operated sludge incinerators of the Raymond type in existence. The support fuel
at this facility is not refuse, but coal or natural gas; nevertheless, some of
the problems encountered would be typical of the Raymond system installed for
co-incineration, with the exception that the support fuel used at Allegheny
County has a controllable heat release whereas refuse does not. At the Alle-
gheny County facility, there were frequent explosions within the furnace until
the operators learned to control the operation of the pug mill more closely and
thus blend dired and wet material before feeding the cage mill of the dryer
circuit. The plant operator did not believe that this pug mill operation could
be automated and felt that is was a critical element in the system. The plant
also experienced dryer-fan unbalance problems because tar vapors condensed on
the fan blades. Odor-emission problems were resolved by preheating the dryer
vent gases and passing them into the hot zone of the furnace. Odors emanating
from the vacuum filter area were deodorized by ducting the foul gases from the
vacuum pumps into the incinerators.
The Raymond system of drying sludge is certainly a feasible one, and the
system hardware is commercially available. A flash-drying system, however, re-
quires careful operator control. It is our conclusion that most of the failures
and abandonments of the system result from poor application of the principle and
from the use of unskilled, inadequate operators.
has reported on the Kel ler-Peukert system, which is similar to the
Raymond system except that hot air is used and the spent dryer air is introduced
into the MMR windbox under the grates, thus serving as both drying and com-
bustion air. While this system appears advantageous, Matsumoto" has pointed
out the problems of early ignition of refuse on the drying-grate section.
This technique is therefore judged acceptable for further study.
33
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Technique 7; Co-Incineration by Pyrolysis
Seven pyrolysis systems were identified as being potentially capable of
handling the co-incineration of MMR and MSS. Each of these systems is proprie-
tary and in an early state of development. Union Carbide's prototype, full-
scale Purox^° system is undergoing a test program in South Charleston, West
Virginia, and Union Carbide has not yet decided whether to offer the system
commercially. Private discussions and communications with Union Carbide
personnel indicate that their desk-top study on co-incineration of MMR and MSS
shows that it is wholly feasible. The Purox system can be described as an
oxygen-blown shaft furnace requiring the purchase of oxygen or the construction
and operation of an oxygen plant along with the incinerator plant. The City
of South Charleston has recieved an EPA grant to test co-incineration using
the prototype "Purox" reactor at Union Carbide's South Charleston plant.
The Monsanto system, accomplishing pyrolysis in a rotary kiln, is also
in a prototype full-scale demonstration status. The installation in Baltimore,
Maryland may be considered as having gone on line in January of 1975; again,
the manufacturer has considered the co-incineration option.
kQ
Carborundum's Torrax system, which was pilot-tested under EPA grant at
Orchard Park, New York, has been deemed a commercial product by Carborundum as
of midsummer, 197^. A Torrax furnace is being built in Luxemburg, and another
has been sold in France. As of this writing, none has been sold in the United
States. Tests charging sludge to the furnace were conducted at Orchard Park,
with reputed success. The Torrax system may be termed an air-blown shaft
furnace.
The Garrett system, planned for San Diego, California, accomplishes
pyrolysis in a flash pyrolysis reactor. Ground has been broken for the full-
scale prototype unit, but it will not be operational for some time.
The Titan Thermogen system may be described as an air-blown shaft furnace.
At present, the manufacturer is not marketing the system, and the sole instal-
lation is at the National Secrity Agency in Ft. Meade, Maryland. Problems in-
clude solidification of the material in the furnace, which shuts down the
operation.
The very early developmental state of all of the pyrolysis units makes it
very difficult to assess the ultimate feasibility, except as concerns some very
basic parameters. As is typical of pyrolysis systems, they operate with less
than the stoichiometric quantity of air and produce an off-gas or oil that
after proper cleansing, may be used as a fuel. They produce either a char'or
a slag residue, depending on the system considered. In the case of Purox,
Torrax, and Titan Thermogen, the furnaces produce a molten slag, which when
fritted in water produces a chemically inert black sand. This is potentially
useful as an aggregate in road building, or can be safely disposed of in a land-
fill. The Monsanto system produces a char, and the flash-reactor Garrett
system probably produces a fine inert ash.
-------
The Purox system, even with the penalty of the attendant oxygen plant, is
an attractive system because the use of oxygen obviates the need to heat copious
quantities of relatively inert nitrogen; this represents a thermal economy that
cannot be achieved with an air system. The Torrax system uses a portion of the
pyrolysis off-gases to preheat the air blown into the shaft furnace. The
pyrolysis technique has sufficient promise to be included among the candidates
warranting further study; the technological obstacles encountered to date
probably will be overcome in one or more of these systems.
None of the co-incineration techniques identified that did not fit within
the techniques already discussed was sufficiently interesting to warrant es-
tablishing another technique category.
Summary
The state-of-the-art of co-incineration is poor, and none of the plants
practicing the air can be considered an outstanding success. The major recent
interest has been abroad, but this may be waning. One European contact states,
"I do admit that there was quite a loud-spoken interest in sludge incineration
a few years ago. The interest has died off since then, however. It is highly
doubtful that there will be any further installations." Nevertheless, Lurgi
reports orders for a number of new units. Perhaps the answer Is that there
are too many approaches and lack of a consensus on the best system for the
future.
We have subjected the various techniques to a subjective analysis, with
the following results (a low ranking is best):
Technique Rank
1. Solid waste incinerator 2
2. Multiple-hearth furnace k
3. Fluid!zed-bed incinerator 5
4. Waste heat evaporation 3
5. Spray injection 1
6. Flash evaporation 3
7. Pyrolysis 1
Twelve factors were considered, each on a scale of zero to four. Each tech-
nique was analyzed both by applying equal weight to each factor, and by
weighting the factors from one to five. These factors were considered:
•The status of the technique—ranging from failure to proven.
•The need for sludge preprocessing—ranging from intensive to simple
sedimentation.
• The need for refuse preprocessing—ranging from intensive plus cleansing
to none.
•The emission potential — ranging from extreme to minimal.
35
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•The odor potential — ranging from extreme to minimal.
•The excess air requi red—ranging from high to low.
•Specific capacity—ranging from minimal to optimal.
• Size limitations—ranging from very small to very large.
• Potential explosion hazards—ranging from probable to none.
•The permissible ratio of sludge to refuse—ranging from marginal to
excel lent.
•Capital cost—ranging from the most to the least.
• Required ancillary equipment—ranging from the most to the least.
Two of our principal investigators performed the analysis independently,
and their rankings (weighted and unweighted) were averaged to yield the final
ranking. Four co-incineration approaches were then selected for further study:
Co-lncineration System
Pyrolysis Furnace
(Technique 7)
Indirect-Contact Sludge Drying
(Techniques k £ 6)
Direct-Contact Sludge Drying
(Techniques ^66)
Multiple-Hearth Furnace
(Technique 2)
Probable Approach
Either the Purox or
Torrax Shaft Furnace.
A grate-fired steam-
generating incinerator
with a thermal screw
type of sludge dryer.
A grate-fired refractory-
wall incinerator with
rotary sludge dryer.
A MHF with the sludge
and shredded-cleaned
refuse fed to the top
of the furnace.
Each of the prime recommendations also identifies a specific approach, i.e.,
Purox, Thermal Screw, Rotary Dryer. We consider the systems as firm recommen-
dations and the specific approaches as tentative selections subject to further
study.
Pyrolysis (Technique 7) ranks high because the sub-stoichiometr!c com-
bustion better utilizes the available heat. The Purox system, using oxygen,
is particularly advantageous since the nitrogen in the air need not be heated,
and because better control of slagging is theoretically possible. Another ad-
vantage is the fuel gas produced, which can be used to produce steam in con-
ventional steam generators. The disadvantages are the developmental stage of
36
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this method and the need for an oxygen supply. The Torrax system, which uses
preheated air, is an alternative to Purox and has the advantage of being com-
mercially available, with one installation being constructed and anothp.r sold.
Techniques k and 6 ranked third but have been selected because once sludge
drying has been taken as necessary it is clearly desirable to make the sludge
drying step controllable. Both direct- and indirect-contact drying are tech-
nically feasible and adaptable to a wide range of MMR furnace designs (see Table
2). Either technique can be used with raw or prepared refuse, with the dried
sludge added to the MMR or burned in suspension. The thermal-screw and rotary-
dryer approaches are suggested as appropriate devices.
Technique 2 has been selected over Technique 3, principally because there
are several multiple-hearth furnaces in use abroad on co-incineration of MSS
and MMR.
Omitted from our recommended list are Techniques 1 and 5, for which de-
watered sludge is added to the refuse fed or thickened sludge is injected into
the furnace. These techniques subjectively rank first or second largely be-
cause they are simple systems representing a very direct approach to co-
incineration; however, both are technologically supect. Technique 1 has failed
in the past because of sludge-burning problems. Techniques for sludge dis-
persion, such as high-speed disk atomization, high-velocity air jet atomization,
sonic atomization, combined shredding of sludge and refuse, blending sludge
with dry ash, and sludge comminution before injection are all techniques which
might provide solutions to the sludge dispersion problem. Technique 5 failed
at Altrincham, and any solution would require test confirmation to lend credence
to the method. Technique 5 may very well succeed, as in the proposed Dickerson
Station, but would constitute a special case applicable only to utility or
large industrial furnaces fired primarily by a fossil fuel.
REFERENCES*
1. Lancoud, F. Combined Disposal of Refuse and Sludges: Technical and Eco-
nomic Considerations. (Presented at 1st International Congress on Solid
Waste Disposal and Public Cleansing, ISWA Praha 1972, Thema V, June-July,
1972.)
2. Clinton, M.O. Experience with Incineration of Industrial Waste and Sewage
Sludge Cake with Municipal Refuse. Proceedings of the 1*tth Purdue In-
dustrial Waste Conference (1959), pp.'155-170.
3. Reilly, B.B. Incinerator and Sewage Treatment Plant Work Together. Public
Works 92,7:109-110 (July 1961).
4. Defeche, Jean. Combined Disposal of Refuse and Sludges: Technical and
Economic Considerations. (Presented at 1st International Congress on
Solid Wastes Disposal and Public Cleansing ISWA - PRAHA '72, Theme V,
3-39, June-July, 1972.)
*These papers are abstracted in Appendix A.
37
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5. Knaak, R. and R. Kuhl. Ergebnisse der Grossversuche mit gemeinsamer
Verbrunnung von Mull und Kla'rschlamm in den Mul1verbrennungsanlagen
Kopenhagen und Bonn-Bad Godesberg. (Results of Large-scale Experiments
with Joint Incineration of Refuse and Sewage Sludge in the Incinerators
of Kopenhagen and Bonn-Bad Godesberg). VGB Kraftwerkstechnik 53,4:210-213
(April, 1973).
6. Zack, S.I. Sludge Dewatering and Disposal. Sewage and Industrial Waste
22,8:975-996 (August 1950).
7. Anon. Solid Waste and Sludge = Energy Self-Sufficiency. Resource Recovery
and Energy Review 2,1:16-17 (January/February 1975).
8. Anon. Kehricht- und Schlammverbrennungsanlage Region Dubendorf. (Inciner-
ation Plant for Domestic Refuse and Sewage Sludge in the Dubendorf Area.)
Schweizerische Zeitschrift fur Hydrologie, 31,2 (1969). (Presented at the
Fourth International IAM- Congress, 1969 in Basel).
9. Kiefer, B. Gemeinsame Aufbereitung von Kla'rschlamm und Mull. (The Joint
Elimination of Sewage Sludge and Waste). Stadtehygiene, 16,8:179-181
(August 1965).
10. Burgess, J.V. Developments in Sludge and Waste Incineration. Process Bio-
chemistry 8,1:27-28 (January 1973).
11. Herbert, W. and W.A. Flower. Waste Processing Complex Emphasizes Re-
Cycling. Public Works 10,6 (June 1971).
12. Albrecht, O.E. Schlarnmverbrennung im Wi rbelschichtofen. (Sludge Inciner-
ation in Fluidized Bed Furnaces.) Chemie Ing. Techn. 41,10:615-619
(May 1969).
13. Limerick, J. McK. Copeland System for Burning Bark, Debris and Sludge
Start-Ups at Great Lakes. Pulp and Paper Magazine of Canada (January
1972).
14. Anon. Sewage Sludge Drying. Bartlett-Snow-Pacific, Inc., Engineering
Bulletin No. SE-2 (April 1, 1969).
15. Bayon. E.J. Sludge-disposal Solution: Thicken, Filter, Dry and Burn.
The American City (June 1966).
16. Gater, D.W. Incinerator is Part of Integrated Waste Disposal System
Public Works 105,5:64-67 (May 1967).
17. Tanner, R. Gemeinsame Verbrennung von Mull und Klarschlamm mit Abwarmever-
wertung zur Schlammtrocknung. (Combined Refuse and Sewage Sludge Inciner-
ation with Waste Heat Utilization for Sludge Drying.) VGB Kraftwerks-
technick 52,2:140-145 (April 1972).
38
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18. Joachim, O.H. Energle aus Mull unter besonderer Betrachtung der Nutzung
zur Klarschlammtrocknung. (Energy from Refuse In Particular Consideration
of its Utilization for Sludge Drying.) Aufbereitungs-Technik, 6,5:279-283
(May 1965).
19. Pepperman, C.M. The Harrisburg Incinerator: A Systems Approach.
Proceedings of the ASME National Incinerator Conference (May 197*0.
20. Anon. Krefelder M'ul Iverbrennungsanlage mit Klarschlammbeseitigung an
VKW/BSH vergeben (VKW/BSH Granted Contract to Build an Incinerator for
Refuse and Sewage Sludge at Krefeld). Stadtehygien 23,8:4 (August 1972).
21. Bergling, S. Combined Treatment of Refuse, Sewage Sludge, Waste Oil and
Nightsoil at Lulel, Sweden. International Solid Waste and Public Cleansing
Association (ISWA) Information Bulletin No. 1, pp. 25-28 (September 1969).
22. Samuels, L.J. New Look at Sewage Disposal. Western Precipitation Group,
Joy Manufacturing Co, Los Angeles, California (No date).
23. Schlotmann, W. Klarschlamm. Seine Behandlung und Beseitigung spexiell
durch gemeinsame M'ul 1 - Klarschlamm-Verbrennung (Sewage Sludge, its Treat-
ment and Disposal Especially Through Combined Refuse - Sludge Inciner-
ation). Net Ingenieursblad 42,10:304-312 (May 1973).
24. Taylor, R. Combined Incineration of Refuse and Sludge. Environmental
Pollution Management 3,2:89-94 (March/April 1973).
25. Tanner, R. and W. Vrenegoor. Sludge and Liquid Wastes Disposal for Com-
bined Incineration Systems. Solid Waste Management and Disposal, The
International Edition of 1971 Australian Waste Disposal Conference, Uni-
versity of New South Wales, pp 105-108 (1971).
26. Thompson, L.H. Sludge Treatment and Disposal — GLC Experience and Investi-
gations in the Field of Sludge Disposal. (Presented at Fourth Public
Engineering Conference, Langhborough University of Technology, 1971.)
27. Davies, G. Altrincham Refuse and Sewage Sludge Incineration Plant. Public
Cleansing 63,5:247-256 (May 1973).
28. Munro, C.S.H. and T.J.K. Rolfe. The Incineration of Sewage Sludge with
Domestic Refuse on a Continuous Burning Grate. Part of a Symposium held
at the University of York, Yorkshire, England, The Institution of Chemical
Engineers Symposium Series No. 41 (April 1975).
29. Herrmann, W., W.A. Stevens, and E.A. Ramspeck. 1700-Mw Dickerson Plant
Design Includes Refuse and Sewage Sludge Firing. American Power Conference
(April-May 1974).
30. Nickerson, R.D. Sludge Drying and Incineration. Journal of the Water
Pollution Control Federation 32,1:90-98 (January 1960).
39
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31. Cross, F.L., Jr., R.J. Drago, and H.E. Francis. Metal and Participate
Emissions from Incinerators Burning Sewage Sludge and Mixed Refuse.
Proceedings and Discussions of ASME 1970 National Incinerator Conference,
PP. 189-195.
32. Weyrauch, H. Die Karnaper Verfahren als Beitrag zur Veroschung von
Siedlungs- und Industrieabfallen (The Karnap Procedure for the Reduction
of Settled Solids and Industrial Wastes). (Presented at the Second Inter-
national Congress of the International Society for Refuse Research Con-
cerning Removal and Disposal of Settled Solids and Industrial Wastes, 22
to 25 May 1962 in Essen, West Germany).
33. Moegling, E. Praxis der zentralen Mlil Iverbrennung am Be! spiel Essen -
Karnap (Experience with Central Refuse Incineration at Essen-Karnap).
Brennst-Warme-Kraft 17,8:383-391 (August 1965).
34. Fife, J.A. Sewage Sludge—Another Waste Disposal Problem. (Presented at
symposium on Solid Wastes, New York Dept. of Health, Albany, N.Y., January
29, 1968).
35. Brinsko, G.A. Sludge Disposal by Incineration at Alcosan. Proceedings of
the National Conference on Municipal Sludge Management, pp. 157~161, June
1974).
36. Rub, F. Moeglichkeiten und Beispiele der kombinierten Verbrennung von MUl 1
und Abwasserschlamm (Possibilities and Examples of a Combined Incineration
of Refuse and Waste Water Sludge). Wasser Luft und Betrieb 14,12:484-488
(December 1970).
37. Matsumoto, K. £t_ aj_. The Practice of Refuse Incineration in Japan—Burning
of Refuse with High Moisture Content and Low Calorific Value. Proceedings
of ASME National Incinerator Conference, pp. 180-197 (1968).
38. Anderson, J.E. Solid Refuse Disposal Process and Apparatus. U.S. Patent
Number 3,729,298 (April 24, 1973).
39. Sussman, D.B. Baltimore Demonstrates Gas Pyrolysis-Resource Recovery from
Solid Waste. U.S. EPA Report SW~75dL (1975).
40. Snyder, N.W. Energy Recovery and Resource Recycling. Chemical Engineer-
ing, pp. 65-72 (October 21, 1974).
41. Anon. Refuse Refineries—A Danish Development* Environmental Pollution
Management 4,4:183-185 (July/August 1974).
40
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SECTION V
FEASIBILITY STUDY
APPROACH TO EVALUATION
It is apparent from the state-of-the-art assessment that co-burning of
wet sludge and refuse in a conventional, grate-fired incinerator is not
feasible. There is little question, however, that dry sludge is a readily
combustible material which could be easily co-burned with refuse. Two of the
four techniques discussed in this section, therefore, place heavy emphasis on
predrying the sludge in equipment external to the refuse incinerator. Both
direct- and indirect-contact drying are explored. The engineering principles
defining these unit operations are well established, and the drying circuits
are combined with conventional refractory or steam-generating incinerators,
whose design and operational basis are also highly developed.
Two additional co-incineration techniques (not involving sludge predrying)
are examined. Multiple hearth (M.H.) co-incineration and pyrolysis co-incin-
eration combine wet sludge and refuse before feeding to the combustion units.
Although the sludge drying is preliminary to actual combustion, it occurs with-
in the incinerator (together with drying of the refuse). In neither case,
however, is wet sludge fed directly to the combustion zone of the unit; In*
stead, the sludge moves through a predrying zone, within the unit, before
entering the active combustion zone.
Sludge Drying
Sludge drying, either external or internal, plays a significant role in
co-incineration. The failure of many attempts to burn wet sludge clearly in-
dicates that some form of sludge predrying is necessary for effective co-in-
cineration.
The emphasis in this feasibility section is therefore placed on the
thermodynamics of sludge drying and its relationship to the combustion of dry
sludge and refuse. A feasible energy balance is a minimum requirement in the
establishment of the viability of a co-incineration process. Before evaluat-
ing any other parameters affecting the feasibility of co-incineration, the
availability of sufficient energy to dry the sludge must be determined.
For practicality, not only must sufficient energy be available, but also
the energy must be available in such a form that the processes and equipment
for heat transfer fall within reasonable bounds of size and cost-effective-
ness. It would be easy to demonstrate that sufficient energy for sludge dry-
ing is available from many sources, including non-combustion sources such as
41
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solar or nuclear energy. Sludge drying using heat from such sources is thermo-
dynamically feasible, but is not currently practical with existing hardware.
Thus, energy transfer and transport also play a major role in determining the
feasibility of a co-incineration process.
Refuse-to-Sludge Ratio
Any attempt at an energy balance must be based on an appropriate mass
balance. It is clear that the refuse-to-sludge ratio is a major factor affect-
ing co-incineration feasibility. If refuse is considered to be an auxiliary
fuel (to provide the additional heat necessary to dry the sludge), it is ob-
vious that sufficient refuse could be fired to make a feasible energy balance.
On a small scale, such an approach appears valid. However, although available
in large quantities, refuse as a fuel is limited; jurisdictional boundaries and
transportation costs for sludge and refuse play a critical role in limiting the
quantity of fuel-refuse available for co-incineration at a single location (See
Section VI I I).
Since this study deals primarily with domestic sludge and refuse, it is
appropriate to base the evaluation on equivalent-population quantities of
sludge and refuse, for these are precisely the quantities that are available.
These quantities and the expected composition of sludge and refuse are devel-
oped in Appendices C and D. The basic quantities are refuse--!.24 kg (2.74 Ib)
per capita per day; sludge—0.09 kg (0.2 Ib) per capita per day (dry solids).
An analysis of co-incineration feasibility based on population equivalents
of sludge and refuse is likely to lead to a conservative estimate of fuel (re-
fuse) availability, because it considers only domestic sources of refuse. Both
commercial/institutional and industrial refuse also require disposal in any spe-
cific area. Many municipalities will not accept refuse from industrial sources,
but commercial/institutional refuse generally is accepted at the municipal in-
cinerator, and would tend to increase the heat available for sludge drying in
a co-incineration facility.
In addition to providing sufficient heat to dry and ignite the sludge,
the refuse fuel must provide sufficient heat to raise the temperature of com-
bustion products to a temperature which insures complete oxidation of organic
materials; a flue gas temperature of 760°C (1,400°F) is generally considered
the minimum necessary for destruction of organic materials, elimination of
odor and minimum emission of carbon monoxide. Consequently, a feasible co-
incineration system must produce a flue gas temperature of at least 760°C
(1,400°F).
Excess Air
A major factor influencing furnace and flue gas temperature is the excess
air encountered in refuse incineration. Excess air insures sufficient oxygen
to complete combustion, and provides some cooling to the furnace system. Typi-
cal refractory incinerators are operated with excess air levels of 150 to 250
percent, although levels as high as 600 percent are encountered in older fur-
naces in which air infiltration often is high. Steam-generating incinerators
of the water-wall configuration operate at excess air levels of 50-150 percent,
42
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because steam generation removes heat normally carried off in the higher flue-
gas volumes of refractory incinerators. Multiple-hearth furnaces are designed
for excess air rates of 75 percent and generally operate in the range of 50-
100 percent excess air. A higher percentage of excess air will be required
when co-burning sludge with refuse in such equipment, and a range of 75-150
percent has been suggested by equipment manufacturers. Pyrolysis equipment,
by definition, uses less than stoichiometric quantities of combustion air;
this lesser amount of air provides partial combustion to produce heat for
pyrolysis of the remaining refuse and sludge volatiles. Combustion air re-
quirements for pyrolysis systems are determined by factors other than the tem-
perature considerations applicable to the other types of co-incineration sys-
tems.
One of the major objectives of this study is the reduction or elimination
of auxiliary fuel requirements in sludge incineration by the substitution of
refuse as the source of heat. While elimination of fuel requirements would be
the ideal, several factors will require the continued use of small quantities
of auxiliary fuel. Both sludge and refuse are highly variable materials; in
particular, moisture variations in sludge can vary from hour to hour, and
moisture in refuse is often variable, depending on weather conditions. The
response time of an incinerator, with its large mass of refractory, is very
slow; consequently, some auxiliary fuel will be necessary to provide system
control.
Miscellaneous Factors
A number of other factors may have impact, to various degrees, on the
feasibility of co-incineration:
Plant Size Reliability
Incinerator/Treatment Plant Location Maintenance
Population Air Pollution
Growth Water Pollution
Administration Ultimate Disposal
These factors are discussed in this section on technical feasibility and in
Section VII—Economic Considerations and Section VIM—Circumstances Having
Impact on Feasibility.
Heat and Material Balance
Since thermodynamics plays such a major role in determining the feasibil-
ity of co-incineration, the heat and material balances for the four chosen
techniques have been studied in detail. A Weston-developed incineration com-
puter model is used .to predict the operating parameters of the system. The
ultimate analyses of sludge and refuse (Table 3) are input to the computer,
along with the capacity and the sludge-to-refuse ratio. Flue-gas enthalpy and
composition are calculated for a range of excess air rates. The program also
predicts steam-generation rates, gas-conditioning tower parameters, and satu-
ration conditions of the combustion products.
-------
TABLE 3. REFUSE AND SLUDGE COMPOSITION
(Percent by Weight)
—•• ~
Refuse Sludge
(as delivered) (Dry, Volatile Fraction)
Carbon
Hydrogen
Oxygen
Inerts
Water
Nitrogen
Chlorine
Sulfur
HHV
k<
U)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
8tu
1b
3~cal
kg
24.7
2.6
21.8
22.0
28.0
0.45
0.30
0.15
4,500
2,500
55-0
5.9
35.0
3.13
0.0
0.93
10,000
5,600
The co~incineration heat balance can best be studied as a system of in-
dependent variables: sludge-to-refuse ratio, sludge solids (assuming fixed
refuse moisture), and excess air; flue-gas temperature is taken as the depend-
ent variable, it is difficult to work with and graphically represent a four-
variable system. As noted, the sludge-to-refuse ratio was fixed at an equiva-
lent population basis. The graphical data presented in the following sections
thus represent only two independent variables: excess air and sludge solids
(4 percent, 20 percent, and 45 percent representing the three common dewater-
ing techniques, as discussed in Appendix C).
In co-incineration practice, however, there may be cases where equivalent
quantities are not available or where it is necessary to determine the minimum
quantity of refuse necessary to support co-incineration. In analysis of such
cases, one should note that dry sludge and dry refuse have essentially the
same heating value. Thus, even though the composition of the composite wastes
will vary with various sludge/refuse ratios, this variation will not materially
affect the thermodynamics of the system. The refuse (dry basts)-to-sludge
(dry basis)-ratio can, therefore, be eliminated as a significant variable The
two remaining independent variables then become excess air and total moisture
in the feed.
By assuming a typical refuse moisture content (note that this is not a
variable which can be controlled), it is possible to determine the minimum
refuse quantity necessary to support co-incineration (i.e. maintain a final
flue-gas temperature at any given excess air rate, from data provided in the
-------
following subsections). Given the maximum quantity of refuse available, it is
also possible to determine the minimum sludge solids concentration of the maxi-
mum sludge rate acceptable for successful co-incineration. These quantities
will be of interest where wastewater treatment is regionalized but refuse col-
lection is not, or where high refuse-hauling costs tend to preclude transporta-
tion of all refuse to a co-incineration disposal site. The data will also be
useful in specifying the type of sludge-dewatering equipment necessary for a
co-incineration facility.
DIRECT-DRYING CO-1 NCINERATION
Direct-contact drying was chosen for detailed study because this technique
has been successfully demonstrated in sewage-sludge drying service and because
it is a well-established drying process widely practiced in many forms in the
industrial sector. Figure k, a block flow diagram, shows the material flows
for such a process. Sludge is predried by contact with the incinerator flue-
gas. Then the dried sludge and moisture-laden gases go to the combustion zone
of the incinerator, where the volatile portion of the sludge solids and any
odorous materials in the moisture-laden gases are destroyed by high-temperature
oxidation.
Direct-contact drying implies intimate contact between the hot drying
medium and the material to be dried, with no barrier between the two. In ap-
plications involving the drying of solids, hot gases (generally heated air or
combustion products) are passed over, around, or through the solids to be dried.
Contact between the hot gases and the solids is good. Inlet temperatures are
high, thus increasing the driving force for evaporation. High heat-transfer
rates and high inlet temperature, 150°-1,650°C (300-3,000°F), account for the
generally favorable efficiency of direct drying of solids, 16-96 kg water/cu m
dryer capacity/hr (1-6 Ib water/cu ft dryer capacity/hr). However, the gas-
solids contact often results in contamination of the gases with pollutants (in-
cluding particulate and vapors), and the consequent need for pollution control
equipment such as electrostatic precipitators, baghouses, or scrubbers.
Direct drying is widely used in industry. Common dryer types include ro-
tary dryers, spray dryers, flash dryers, and fluid-bed dryers.
Rotary Dryers
Rotary dryers and kilns are widely used in industrial applications such
as drying, dehydrating, and calcining. The dryer generally consists of a large,
horizontal, cylindrical drum, rotating at slow speeds on its axis, with hot
gases passing through the shell in either a concurrent or counter-current flow.
In many high-temperature operations, fuel is fired directly into the dryer,
with combustion taking place within the dryer shell; in such cases, the dryer
shell is often lined with refractory.
Lifting flights are generally installed in the dryer to provide increased
gas/solids contact by lifting and then dropping the solids through the gas
stream. The lifting flights and the rotating action of the drum provide gentle
agitation, and prevent the formation of extremely small particles. The agita-
tion also provides a degree of back mixing, resulting in a more uniform dry
45
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Thickened Sludge
1
Sludge
De watering
Dewatered Sludge
Waste water
to Treatment Plant
Hot Combustion
Auxiliary Fuel
Electrostatic
Precipitator
Ash
to Disposal
Figure 4. Direct Dryer Co-Incineration
Schematic Flow Sheet.
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product. The effective agitation and the fact that the means of solids injec-
tion into the dryer generally is not critical make the rotary dryer particular-
ly suitable for heavy sludges. With sticky solids, however, rotary dryers per-
form poorly. Such materials deposit on the walls of the dryer and dry there;
after extended periods, the deposits can significantly reduce the inside dia-
meter of the dryer, resulting in high gas velocities and poor performance.
Although the throughput of a rotary dryer generally is high, the residence
time is long. This extended residence time tends to reduce the effects of vari-
ations in sludge concentrations and in the temperature and the volume of the
incinerator off-gas. The response time of a rotary dryer, however, is slow,
because of the large mass of solids. It can be improved by providing direct
fuel firing at the hot gas inlet.
Spray Dryers
Spray dryers normally consist of large vertical towers into which the ma-
terial to be dried is sprayed. It is always necessary to break the material
into small particles or fine droplets, thus increasing the surface area and re-
ducing drying time. High-pressure spray dual-fluid spray, and high-speed disc
atomization are commonly used. The heat input in spray dryers is pre-heated
air or combustion products. Hot gases enter the dryer in a variety of con-
figurations, but flow is generally co-current. Design parameters of gas flow
and temperature can be varied through only a narrow range. Residence time in
the spray dryer is generally very short, with dried solids separated from the
hot gases immediately after leaving the dryer, normally in mechanical collec-
tors. More efficient secondary pollution control equipment is also required,
to remove the small particles which remain.
Problems associated with the relatively short retention time in the dryer
and with atomizing the thickened sludges are considered major disadvantages to
the use of spray dryers in sludge co-incineration.
Flash Dryers
Flash drying, as applied to sludge, is similar to spray drying. In most
instances, however, flash drying combines the drying with sludge cake size
reduction (by mechanical comminution), thereby eliminating the critical atom-
ization step encountered in spray drying. The solids are generally injected
into a high-velocity gas stream and-pneumatically conveyed to a mill for size
reduction. (Flash dryers are often referred to as pneumatic-conveyor dryers.)
Hammer and cage mills have been widely used for this purpose. With heavy
solids (such as filter cakes or slurries), the solids may be injected directly
into the mill. Drying occurs in the mill and during the transfer to the solids-
separating equipment.
As with spray dryers, residence time is short and operating conditions
must be reasonably stable for satisfactory performance. A major disadvantage
of flash drying is the fine, dusty powder which results. The dust is difficult
to collect and control, and may constitute an explosion hazard when organic
materials are dried. Despite this disadvantage, flash drying has been one of
the more popular sludge-drying processes. The CE/Raymond flash dryer, for
-------
instance, has been used in many sewage treatment plants to dry thickened
sludges. In most cases, the dry sludge has been collected and distributed as
fertilizer or soil conditioner, rather than incinerated.
Fluid-Bed Dryers
In fluid-bed systems, natural gravitational forces acting on the solids
are balanced by the drag force of rising gases. Under proper operating con-
ditions, the net solids flow is zero, and a dense-phase fluidized bed occurs.
As the solids dry, density 5s reduced, and the dry solids are discharged
overhead with the drying gases. Fluid-bed dryers are limited by the velocity
of the air or gas stream fluidizing the solids. Once the air volume is speci-
fied, a limit is placed on the heat transported into the dryer, and control of
the dryer becomes difficult. To provide a heat sink within the dryer, a second
solid (generaly sand) is often added. The particle size of the sand must be
chosen carefully to insure fluid-bed operation. The bed becomes heated to the
temperature of the incoming gases and serves as a reservoir of heat to decrease
the variations in other operating parameters.
The dried solids are separated from the gas stream by mechanical collec-
tors. Because of the high velocities necessary to maintain the fluid bed, the
gas stream is often recirculated, with removal of a bleed stream as an integral
part of the operations. Make-up air and heat are added before the stream is
returned to the dryer.
The major problem experienced with fluid-bed sludge dryers has been the
tendency for large lumps of dense sludge to drop through the sand bed. (The
smaller sludge particles are buoyed up by the sand and fluidizing gas and dry
rapidly.) It is possible that blending wet sludge with predried material would
improve the performance of fluid-bed driers, but back mixing the dry sludge
with incoming wet sludge has not yet been evaluated full scale.
The advantages of fluid-bed drying include no internal moving parts,
excellent mixing, good heat transfer, and close control of average moisture
content of the product. At present, however, problems with sludge feeding
(e.g. large lumps of cake dropping through the bed) and with the variations
experienced with thickened and dewatered sludge limit the uses of this drying
technique.
Heat and Material Balance
Based on the foregoing considerations, a rotary dryer was selected to
represent the direct-drying technique. Before a heat and material balance can
be developed, basic relationships must be established. Sludge and refuse quan-
tities and characteristics, i.e. 0.09 kg (0.2 Ib)/capita/day dry weight for
sludge and 1.2^ kg (2.?4 Ib)/capita/day at 28 percent moisture are based on
those appearing in Appendices C and D, and on an equivalent-population ratio
of sludge to refuse. Sludge drying depends on refuse combustion, but refuse
can be incinerated without drying any sludge. Consequently, there may be
occasions when the sludge drying rate may have to be increased temporarily to
make up for dryer down-time, and the effect of 20 percent excess (above
-------
equivalent population) sludge on process thermodynamics was evaluated to assess
such conditions. The minimum flue gas temperature suitable for odor destruc-
tion is 760°C (1,400°F).
To establish a heat balance, the Weston co-incineration model was run at
three sludge-moisture levels: 4 percent solids, representing thickened sludge;
20 percent solids, representing vacuum-filtered sludge; and 45 percent solids,
representing pressure-filtered sludge. Refuse in all cases was assumed to con-
tain 28 percent moisture. The input and output of the INCIN program run for
direct-dryer co-incineration appear in Table 4; the calculated results also
appear in Figure 5.
TABLE 4. INPUT AND OUTPUT OF ANALYSIS OF DIRECT DRYER
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM -- EQUIVALENT
SLUDGE/REFUSE PRODUCTION
Sludge Total Furnace Outlet Temp. (°F)
Refuse Sludge Solids Moisture Excess Ajr (%)
0/° * * % 0 100 200 300 400
35
71
84
65
29
16
4
20
45
72
43
32
1,010
2,520
2,900
780
1,750
1,950
640
1,350
1,480
540
1,100
1,200
470
930
1,020
With an excess air rate of 150 percent (the minimum acceptable for satis-
factory operation of a refractory incinerator) co-incineration will achieve
the desired flue-gas temperature of 760°C (l,400°F) only if the overall mois-
ture content of the feed (sludge plus refuse) is 50 percent or less. Since
refuse moisture depends on factors beyond the control of the incinerator oper-
ator or designer, control of sludge moisture is the only approach available
for control of the overall moisture content of the mixed feed. A moisture
content of 50 percent in a mixed feed containing typical refuse requires a
sludge containing 13.5 percent solids. Vacuum-filtered sludge generally con-
tains 20 percent solids; when combined with typical refuse, a mixed feed with
a moisture content of 43 percent is produced, well within the feasible opera-
ting range for co-incineration. With such a feed, the incinerator could oper-
ate at 200 percent excess air and still maintain an acceptable flue-gas temper-
ature. Thus, the incremental solids in the sludge (over the minimum required)
provides a margin of safety for overcoming fluctuations in refuse or sludge
moisture.
If the sludge is pressure-filtered (to 45 percent solids), the excess air
rate can be as high as 250 percent without adverse effect. However, pressure
filtration often involves the addition of significant quantities of inert fil-
ter aid, which increases the amount of residue requiring disposal. Consequently,
there is little justification for increasing the solids content of the sludge
to 45 percent.
-------
1800
3000
2500
-------
Based on available heat in the incinerator flue-gas, 760°-980°C (1,400-
1,800°F), approximately 10 to 15 percent of the combustion products will be
directed to the dryer, to overcome the variation in flue-gas temperature and
to provide some cooling so that metal duct work can be used, it is advisable
to dilute the dryer gas with sufficient ambient air to reduce the temperature
to about 650 C (1,200°F). This should be compensated for by further reducing
the excess air in the incinerator, since all gases passing through the dryer
are returned to the furnace's combustion zone for destruction of odorous
gases. The gases returned from the dryer should enter the incinerator through
the high-pressure overfire-air supply system; this will minimize the total air
requirement as well as provide good mixing in the active combustion zone.
The dryer off-gas could be returned with the underfire air (through the
wind box under the grate), but excessive condensation may be experienced, and
this approach is not recommended. In some plants (Ansonia, Connecticut), the
return gas is simply ducted into the incinerator furnace, but such a con-
figuration makes complete mixing of return gas and furnace gas more difficult
and may result in poor odor control.
As indicated in Figure 4, dry sludge solids (at 15 percent moisture) are
conveyed to the incinerator and blown over the active combustion zone; burn-
ing will be in suspension. In addition to reducing the weight and volume of
sludge solids, combustion of the sludge solids provides additional heat for
dryer operation.
The input and output of the INCIN program run for direct-dryer coinciner-
ation (120 percent of equivalent value) appear in Table 5. The calculated
results also appear in Figure 6. Figure 6 is a representation of the same
sludge and refuse as in Figure 5, except that the sludge feed rate has been
increased by 20 percent beyond the "equivalent population" rate. As expected,
the curve falls somewhat below that of the equivalent-population ratio of
sludge and refuse. However, results of combustion modeling indicate that co-
incineration remains feasible at excess air rates of up to 175 percent for
20%-solids sludge and at 225 percent excess air for 45%-solids sludge.
TABLE 5. INPUT AND OUTPUT OF ANALYSIS OF DIRECT DRYER
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM — SLUDGE/REFUSE
RATES AT 120 PERCENT OF EQUIVALENT VALUE
Refuse
%
67
81
Sludge
%
33
19
Sludge
Solids
%
20
45
Total
Furnace Outlet Temp. (°F)
Moisture
%
45
32
2
2
0
,440
,870
1
1
Excess
100
,700
,940
1
1
Air (*)
200
,310
,470
300
1,070
1,200
400
910
1,010
-------
1800
3000
2500
£
2L
I 2000
5=
o
I
.£ 1500
c
1000
500
40 60
Total Moisture. Percent
200
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 6. Direct Dryer Co-Incineration
Gas Temperature (20% excess sludge).
-------
Dryer Design
The heat balance data indicate that co-incineration is thermodynamically
feasible. However, the heat must be conveyed to the dryer and transferred to
the solids (with resulting evaporation) efficiently if direct drying is to be
a practical co-incineration technique.
To determine if equipment size is reasonable and to provide a basis for
later cost estimates, some preliminary design data are necessary. In a direct-
drying system, hot combustion gases are drawn from the incinerator at a temp-
erature close to the incinerator flue-gas temperature, 760°-980°C (1,^00-
1,800 F). Refractory-lined ductwork is necessary at the furnace withdrawal
point.
If the entire ductwork and dryer system is refractory-1ined, no reduction
in temperature will be required for system integrity. However, excessively
high inlet-gas temperatures could result in ignition or explosion of the
sludge in the dryer. If steel equipment is used for ductwork and dryer com-
ponents, the temperature of the inlet gas will have to be reduced to about 650°C
(1,200°F) by dilution with ambient air. Since dryer off-gas will be returned
to the combustion air system, such an approach will not increase the overall
furnace excess air. A water spray system could also be used, but would un-
necessarily increase humidity throughout the gas-handling system.
The dryer should be operated on a co-current flow basis. This minimizes
the possibility of fire hazard in the dryer by contacting the hottest gases
with the wettest sludge. Since there is no need to dry the sludge completely,
maintaining a high temperature differential at the dry end, by using counter-
current flow, is unnecessary. In fact, it is generally undesirable to dry the
sludge solids completely, because dry sludge solids create a flammability and
dust problem. A final, average sludge moisture content of 15 percent is
generally sufficient to minimize dusting and the resultant explosion hazard.
In the design of a dryer system, the temperature and flow rate of the
hot inlet gases, the inlet and outlet sludge moisture content, and the outlet
gas temperatures must all be specified. The inlet temperature for unlined
equipment is generally about 650°C (1,200°F), and the outlet temperature from
the dryer should be at least 150°C (300°F) to minimize condensation. The gas
flow, along with inlet and outlet temperatures, determines the capacity of
the dryer. The shell diameter is determined by gas-velocity considerations;
velocities of 60 to 76 m/min (200 to 250 ft/min), at mean gas temperature, are
typical. These velocities provide adequate turbulence, yet prevent undue
carry-over of solids. The dryer length and pitch are determined by residence-
time requirements. (A residence time of at least 60 minutes is recommended.)
Dryer pitch generally is approximately 1 cm/m (1/8 in./ft).
Dryer rpm will also affect the drying rate and residence time, and rota-
tion speeds of 3-5 rpm are typical. For effective drying, the shell should
also be equipped with lifting flights to tumble the sludge through the hot
gas more effectively.
53
-------
Auxiliary fuel, as required to maintain design operating conditions,
should be fired at the inlet end of the dryer. Experience indicates that
firing auxiliary fuel, at a level of 20 percent of the heat necessary to dry
the sludge, has little effect on the incinerator outlet temperature. However,
a major portion of the auxiliary heat is utilized in sludge drying, rather
than in heating the large quantities of excess air and combustion products in
the incinerator.
After the sludge is dried, the moist dryer gas should be returned to the
incinerator. At this point, the temperature is elevated to a minimum of 760°C
(1,400°F), at which point all odorous material is destroyed. A cyclone, in
the dryer gas return line before the induced-draft fan, is generally needed
to remove particulate material carried over in the gas stream.
A portion of the dried sludge is separated and premixed with incoming
wet sludge solids. Premixing of dry and wet sludge minimizes variations in
dryer inlet moisture content and also reduces handling problems with the wet
sludge. Sufficient dry sludge should be back-mixed with incoming wet sludge
so that the dryer inlet moisture content is approximately 50 percent. Good
agitation will be necessary in the mixer to insure reasonable distribution of
the dry sludge and to break up the wet sludge cake.
Effect of Size
Rotary dryers are available in a wide range of sizes from 0.15 m (6 in.)
to over 7 m (2k ft) in diameter. On a practical basis, however, dryer size is
generally limited to 3-7 to 4.3 m (12-14 ft) in diameter (outside), which is
the largest diameter which can be shop-fabricated and transported over the
road. If this does not provide sufficient capacity, multiple dryers should
be used. A dryer 3.7 m (121) in diameter will dry approximately 180 metric
tons (200 tons) per day of 20%-solids sludge, corresponding to a community of
about 400,000. Thus, dryer size is not a major determinant of technical
feasibility.
The other major part of this co-incineration system is the incinerator
itself. Again, with the use of additional units, there is no maximum overall
plant size. The smallest incinerator generally constructed has a capacity of
about 230 metric tons (250 tons) per day, which corresponds to the refuse-
disposal requirements of a community of 150,000 people.
Although it is not necessary to operate an incinerator 24 hours per day,
intermittent operation of a co-incineration plant, with the dryer described,
may significantly increase auxiliary fuel consumption, because of temperature
control requirements and the heat-up time of the incinerator. If the incin-
erator must be started up daily, at least one hour of start-up time, during
which the dryer cannot be operated without auxiliary fuel firing, will be re-
quired. The incinerator flue gas must reach a temperature of at least 760°C
(1,400°F) before the dryer can be placed in operation.
54
-------
Based on size considerations, co-incineration involving a refractory in-
cinerator and a rotary dryer is feasible for municipalities serving 150,000
or more. The major design consideration will be the incinerator size rather
than the dryer size. For communities of less than 150,000, a direct-dryer/co-
incineration plant could be constructed, but economics would be adversely
affected.
INDIRECT-DRYING CO-INCINERATION
Indirect-contact drying is the second of the two pre-drying processes
evaluated for technical feasibility. While there have been some successful
sludge-drying processes demonstrated in Europe utilizing this technique, the
process as described here (and presented as a block flow diagram in Figure 7)
has never been practiced. There are several potential advantages to this type
of drying process:
1. Heat transport to the dryer should be more efficient than in
direct drying.
2. The total energy available for sludge-moisture evaporation
should be greater than in direct drying.
3. Equipment size should be considerably smaller.
ty. There may also be sufficient heat available at equivalent-
population sludge-to-refuse ratio to make the sale of excess
energy (or its use within the plant) feasible.
In an indirect heat-transfer application, there is no contact between the ma-
terials being heated and cooled; a heat-transfer surface provides a barrier
which eliminates contact between the two materials. The most widely used in-
direct heat-transfer configuration is the shel1-and-tube heat exchanger. With
the presence of a barrier between the two materials, there is no contamination
of the heat source by the materials being heated or dried. This fact is of
significant importance in applications such as sludge drying, where all con-
taminated materials must be later incinerated or further treated in some way.
The same barrier which prevents contamination of the heat transfer fluid also
acts to impede heat transfer by adding additional resistance. However, it
permits the use of a phase change in the heating fluid, with the recovery of
latent heat. The use of a condensing fluid as the heat source maintains high
temperatures uniformly over the entire heat-transfer surface, and permits the
transport of large quantities of heat in a small mass of heat-transfer fluid.
In solids-drying applications employing indirect heat, the wet solids
are distributed on or about the hot surface. Consequently, agitation of the
solids is of major importance in obtaining good heat transfer. If solids are
un-agitated, heat must pass through the solids by conduction and convection.
Often, both the hot surface and the solids are in motion.
55
-------
Sludge
1
Sludge
De watering
ToWW
Treatment Plant
vr.
Water
To Atmosphere
Steam
Vapor
Boiler I
r-1
^•"^* Refuse
Condensate
Incinerator
Indirect
Sludge Drier
Dry
Sludge
Combustion Air
To Disposal
Figure 7. Indirect Dryer Co-Incineration
Schematic Flow Sheet.
-------
The most common heating fluid is steam. Steam is relatively safe, pro-
vides a high latent heat, and is widely available from power and industrial
boilers. High steam temperature is accompanied by high pressure, however,
and this high pressure often becomes the limiting factor in the design of in-
direct heat-transfer equipment. The problem (of high pressure) can often be
overcome by using non-aqueous heat-transfer fluids, which exert a lower vapor
pressure than steam. Other media (hot water, condensate, hot air, or com-
bustion products) can also be used in indirect heat-transfer applications.
The steam or other fluid generally is not the actual source of the heat, but
rather is merely a convenient method of conveying the heat from its source,
generally some form of combustion, to the point of use. Particularly if the
heat must be transferred over great distances, steam or other condensfble
fluids provide a more efficient means of conveying heat than the use of hot
gases, such as combustion products. The use of steam also provides a
convenient means of temperature control. Since steam temperature and pres-
sure are directly related, the use of a pressure-reducing valve on the steam
line can be used to control the temperature at the heat-transfer surface very
accurately. In all further considerations in the ensuing discussion we have
assumed that steam is the heat-transfer fluid.
In heat transfer applications, one measure of effectiveness is the over-
all heat-transfer coefficient DO:
U0, cal Btu
sec sq cm °C hr sq ft °F
with dimensions expressed as a ratio of energy transferred as related to time,
surface area, and temperature. Uo is a measure of the heat passing from the
hot material, through the barrier, and into the colder material. The higher
the heat-transfer coefficient, the more effective the transfer will be. Con-
sideration of the dimensions of U shows that an increase in the quantity of
heat transferred can be obtained by extending the contact time, by increasing
the temperature difference between the hot and cold materials, and/or by ex-
panding the surface area involved. Typical Uo in solids-drying application,
using steam as the heat source, range from 6.2 to \2k cal/sec sq cm °C (5 to
100 Btu/hr sq ft °F), depending on degree of agitation and moisture content
of the sol ids.
Industrial application of indirect drying of solids is not as widespread
as direct drying. In general, it is used in areas where the vapor being re-
moved must be collected or controlled in some manner, where vacuum must be
applied to reduce the drying temperature of the heat-sensitive solids, or in
cases where steam is the only form of heat available. Indirect solids-drying
equipment generally takes the form of jacketed and/or hoilow-flight conveying
equipment, wiped-surface dryers, twin-shell or jacketed rotary dryers, twin-
cone or other enclosed containers, flakers, or deck or shelf dryers.
Jacketed and/or Hollow-Flight Dryers and Conveyors
Jacketed and/or hollow-flight dryers or screw conveyors are frequently
used in heating, drying, and cooling solids. They can often perform the dual
function of heat transfer and solids conveying in one piece of equipment,
57
-------
generally a horizontal, semi-circular trough with a jacket or coil to provide
heat. Such equipment has one or more agitation devices (e.g. screw, flight,
disc, paddle) rotating on the axis through the center of the trough. A signif-
icant degree of agitation is necessary to maintain reasonable heat transfer to
the solids. Simple screw conveyors are notably poor in this regard, because
increasing the speed reduces the residence time in the dryer, by moving the
sludge rapidly through the system. UQ for this type of equipment ranges from
18.6 to 93 cal/sec sq cm °C (15-75 Btu/hr sq ft °F), depending on moisture
content and degree of agitation.
The agitators, paddles, or flights should also be designed to minimize
build-up on the walls of the dryer and on the agitator itself. Generally,
baffles or ploughs should be provided between the flights to improve mixing
and to break up any lumps which form. The rotating flights are often fitted
with small paddles or similar projections to improve agitation and reduce
fouling of the shell surface.
Significant increases in heat transfer can also be obtained if the rotor
is hollow and fitted for steam heating. A hollow heated rotor often provides
one to two times the heat-transfer area available in the shell.
Wiped-Surface Evaporators
Wiped- or scraped-surface evaporators or dryers find limited use in indus-
trial applications for drying viscous liquids, pastes, or materials which tend
to foul heat-transfer surfaces. They are most widely used in vacuum drying
applications. Wiped-surface dryers have been used in Europe for partial dry-
ing of wastewater sludge. Such dryers consist of a small-diameter cylinder,
jacketed for steam heating. A slowly rotating shaft on the axis of the evapo-
rator moves a series of blades which wipe the heat-transfer surface of the
dryer. The knives or doctor blades provide some degree of agitation and re-
duce the fouling on the heat-transfer surface. The material passing through
such a dryer must remain fairly fluid. Such devices would not, for instance,
be effective in drying sludge solids to a moisture content of 15 percent. The
maximum sludge solids content which has been satisfactorily handled in wiped-
surface drying equipment is 50 percent. The available heat-transfer surface
is relatively small. Such units should be considered only for very wet
sludges, where some reduction in moisture (rather than drying to a solid form)
is necessary.
Wiped-film evaporators generally have excellent heat transfer coeffi-
cients, in the range of 60 to 120 cal/sec sq cm °C (50-100 Btu/hr sq ft °F).
Their use at higher moisture levels generally results in a higher transfer
coefficient than would be expected from jacketed/hoilow-fltgKt dryer. The
inability of wiped-surface evaporators to dry sludge to a moisture level much
below 50 percent reduces their usefulness in sludge pre-drying for co-inciner-
ation.
Jacketed or Steam-Tube Rotary Dryers
An indirectly heated rotary dryer is similar in size and configuration
to the direct-fired rotary dryers described previously, but with the shell
58
-------
of the dryer jacketed to permit heating by steam or combustion gases. Steam
tubes are often placed inside such dryers to increase the available heat-trans-
fer surface. Industrial applications of such dryers are generally limited to
cases where direct fire cannot be used, e.g. drying of solids wetted with
flammable solids, drying of materials which are sensitive to carbon dioxide,
or drying under vacuum conditions.
Jacketed rotary dryers are best used with free-flowing solids, but could
be used for partial drying of sludge. Clearly, a high degree of dry sludge
recycle must be provided. Steam-tube rotary dryers, in particular, require
very free-flowing solids.
Heat-transfer coefficients for such dryers are relatively low, about 6.2
to 2k.8 cal/sec sq cm °C (5-20 Btu/hr sq ft °F), since they are generally
used with low-moisture solids. Only a portion of the total available heat-
transfer surface is in contact with the solids, thus reducing the overall
heat transfer coefficient. This fact, plus the free-flowing solids require-
ments, severely limits the potential for jacketed or steam-tube rotary dryers
in co-incinerator applications.
Other Dryer Types
There are other types of solids-drying equipment which use indirect heat
transfer. Most, such as twin-cone or tumble dryers, operate on a batch basis
and are therefore unsuited to continuous sludge drying.
Flakers can also be used as dryers, although they are generally used for
simple heating and cooling. Flakers resemble rotary vacuum filters in appear-
ance. A large, slowly rotating drum is coated with a layer of solids, slurry,
etc. and is steam- or oil-heated from inside. After the material is dried, a
knife scrapes the solids from the drum. When drying very wet solids, Uo is
in the range of 62 to 99 cal/sec sq cm OG (50-80 Btu/hr sq ft °F), but Uo
falls rapidly as the solids approach dryness, because of poor agitation and
the use of only conductive heat transfer.
Vibrating-conveyor dryers employ a steam-heated metal plate or tray to
dry the solids. The tray is heated from beneath and, as the solids are de-
posited, the vibrating action provides agitation to prevent formation of a
large cake. Such dryers are useful only with free-flowing solids.
Multiple-tray dryers, similar in configuration to multiple-hearth fur-
naces, have also been used in indirectly heated solids-drying applications.
The individual trays and the rabble arms are hollow, for steam heating. Wet
solids are charged at the dryer top, and the rabble arms provide some agitation
while moving the solids from tray to tray, down the dryer. While heat transfer
in such equipment is relatively good, this type of dryer is also limited to
free-flowing solids.
Heat and Material Balance
After consideration of the available types of indirect-drying equipment,
the jacketed/hollow flight dryer was selected for the technical feasibility
59
-------
study. The heat-and-material balance is based on sludge and refuse quantities
and characteristics presented in Appendices C and D; an equivalent-population
ratio of sludge to refuse is used. A minimum flue-gas temperature of 760°C
(1,400°F) is required for odor destruction.
To establish a heat balance, the Weston co-incineration model was run at
three moisture levels representing thickened sludge (4 percent solids), vacuum-
filtered sludge (20 percent solids), and pressure-filtered sludge (45 percent
solids), with refuse in all cases assumed to contain 28 percent moisture.
Table 6 gives the input and output of the INC IN program applied to Indirect-
dryer co-incineration. These results are summarized on Figure 8.
TABLE 6. INPUT AND OUTPUT OF ANALYSIS OF INDIRECT DRYER
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM ~ EQUIVALENT
SLUDGE/REFUSE PRODUCTION
SI udge
Refuse
*
35
71
84
Sludge
*
3
7
9
Sol
%
As
Rec'd
4
20
45
ids
As
Burned
85
85
85
Vapor
%
Total
Moisture
%
As
Burned
62
22
8
10
21
25
As
Chg1
72
43
32
Furnace Outlet Temp. ( F)
0
d
1,910
2,750
2,970
Excess Air
100 200
1,480
1,910
2,000
1,220
1,470
1,520
(*)
300
1,030
1,200
1,230
400
900
1,020
1,040
The computer-modeled heat balance indicates that co-incineration is fea-
sible over a broader range of sludge moisture content than is true of direct
drying. At 150 percent excess air, a 760°C (1,400°F) temperature can be main-
tained at an total (sludge and refuse) moisture content of 70 percent, com-
pared with 50 percent for the direct dryer. A 70 percent overall moisture
feed corresponds to a sludge moisture of close to 95 percent, i.e. 5 percent
sludge solids, if 28 percent moisture is assumed in the refuse. Thus, at an
excess air rate of 125 percent, within the normal operating range of a boiler-
incinerator, co-incineration of thickened sludge is feasible. However, with
a 4%-solids sludge at an equivalent population ratio, there is little margin
for the normal fluctuations in sludge and refuse moisture. With a vacuum-
filtered sludge of 20 percent solids, excess air rates can approach 225 per-
cent; and with 45 percent solids sludge, excess air can be as high as 250
percent.
Although there is sufficient heat available to dry a thickened sludge,
vacuum filtration, at least, is recommended, because drying a 4%-solids sludge
would require a vastly larger dryer. The potential for fouling of the heat-
ing surface is also greater with a low-solids feed, and a high level of solids
recycle would be necessary. Starting with a 4%-solids thickened sludge will
60
-------
1800
3000
2500
2L
I 2000
18
o
o
o
2
v
.£ 1500
1000
500
200
40 60
Total Moisture, Percent
I I
100
I
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 8. Indirect-Dryer Co-Incineration
Gas Temperature (Equivalent Population).
61
-------
reduce steam demand to only 15 percent of boiler output, thereby providing
steam for plant use or sale.
The temperature/moisture curves for direct and for indirect dryers have
approximately the same shape. The available heat, as indicated by flue-gas
temperature, drops off more rapidly for direct drying than for indirect as
moisture in the feed is increased. In direct drying, flue-gas leaves the
incinerator at 760°C (1,400°F) (minimum), with no further heat recovery. In
indirect drying, the flue-gas is raised to 76QOC (1,400°F), but the boiler
recovers much of the heat, and flue-gas will actually leave the process at
200° to 260°C (400°-500°F). As the sludge moisture increases, more and more
water leaves the incinerator at 760°C (1,400°F) for the direct-drying process,
but at only 200° to 260°C (400°-500°F) for the indirect process.
The actual indirect drying operation is very much the same as the direct
drying operation. Sludge is first premixed with previously dried solids
(entering the dryer at about 50 percent moisture) and is dried to about 15
percent moisture. Steam generated in the incinerator waste heat boiler pro-
vides the heat. Dry sludge is conveyed to the incinerator, where it is fired
in suspension. Water vapor plus a small amount of purge air is combined with
overfire excess entering the incinerator. As an alternative, the water vapor
could be condensed in a barometric condenser and returned to the water treat-
ment plant, with only a small volume of purge air returned to the incinerator.
Figure 9 is another representation of the indirect process, with sludge
increased to 120 percent of equivalent population ratios. This figure sum-
marizes input and output data from the INCIN program. The data indicate that
co-incineration with a 4%-solids sludge remains feasible, but at a maximum
excess air rate of 100 percent. The operating conditions for 20 percent ex-
cess sludge at 20%- and 45%-solids remains about the same as for equivalent-
population ratios.
TABLE 7. INPUT AND OUTPUT OF ANALYSIS OF INDIRECT DRYER
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM — SLUDGE/REFUSE
RATIO AT 120 PERCENT OF EQUIVALENT VALUE
Sludge
Refuse
32
67
81
Sludge
2
8
10
Sol
As
Rec'd
4
20
45
••«M«H— •^••^.^••-••.^.m
Total
ids Vapor Moisture
As
As
Burned Burned
85
85
85
65
25
9
9
20
24
As
Furnace
0
Outlet Terno. (°F^
Excess Air
100 200
(*)
300
400
Chg'd
75
45
33
1,740
2,700
2,960
1,380
1,890
1,990
1,140
1,460
1,520
980
1,190
1,230
860
1,010
1,040
62
-------
1800
3000
2500
£
1
s.
I 2000
o
It
o
I
I 1500
c
1000
500
200
40 60
Total Moisture, Percent
I
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 9. Indirect-Dryer Co-Incineration
Gas Temperature (20% excess sludge).
63
-------
Dryer Design
Some preliminary design data will be necessary to determine if equipment
requirements for indirect-drying/co-incinerat?on are reasonable. The design
parameters are based on a jacketed/hollow-flight dryer, with heating in both
the jacket and flights.
The major design variable applied to indirectly heated jacketed and/or
hollow-flight equipment is the heat-transfer coefficient. An average value
of 2k cal/sec sq m °C (20 Btu/hour sq ft °F) is typical of such dryers. How-
ever, there are many factors which will affect the heat-transfer rate, and
values of UQ of 12 to % cal/sec sq m °C (10-80 Btu/hour sq ft °F) have been
reported for sludge drying. Generally, the heat-transfer rate decreases as
sludge solids increase; thus, as sludge moves through the dryer, the drying
rate can be expected to decrease. Figure 10 is a plot of a typical relation-
ship between sludge solids and heat-transfer coefficient. The source of the
sludge will also affect the sludge drying rate. Higher heat-transfer coeffi-
cients are obtained with raw primary and secondary sludge than they are with
digested sludges, as indicated in test data supplied by Bethelehem Co.
(Figure 10). Fouling of the heat transfer surface can drastically reduce ef-
fective heat transfer and is most likely the cause of poor operation in an
indirect-drying system.
Recycling dry sludge solids significantly reduces the potential for foul-
ing of the dryer surface. Experience with direct rotary dryers at Holyoke,
Massachusetts indicates that sufficient dry sludge should be returned and
mixed with incoming wet sludge so that the feed to the dryer contains about
50 percent moisture. Although reducing the feed moisture content also reduces
the overall heat-transfer coefficient, the loss is more than offset by the
reduction in surface fouling. The effective working volume of such a dryer is
about kO percent of the total volume available in the empty shell. However,
all drives should be designed to handle a fully loaded shell.
Rotor speed is generally about 20 rpm. Higher rotational speed increases
power consumption and wear on moving parts and reduces the residence time,
with very little increase in heat-transfer coefficient. The pitch of the ro-
tor flights or paddles also affects residence time in the dryer. The pres-
ence of the rotary joint on the agitator shaft limits steam pressure in the
rotor to 11 kg/sq cm (150 psi). Higher pressures can often be accommodated in
the jacket; however, excessive wall temperature in the dryer will result in
increased fouling and possible char formation. Skin temperature in the dryer
should be limited to 180° C (350°F), corresponding to a steam pressure of 9.5
kg/sq cm (135 psi). Where equipment cannot be designed to handle this steam
pressure, non-aqueous heat-transfer agents, with lower vapor pressures, can
be substituted for steam. To maintain high, uniform wall temperatures through-
out the dryer, a condensing-type fluid is preferred. The latent heat of such
fluids is 30-50 percent of the latent heat of steam, and the system must be
designed to accommodate higher gas and liquid flow rates.
-------
V/l
90
80
LEGEND:
• D-275 -
• D-275 •
D 94-0180 •
•fc 94-0172-
V94-0172
A 91-0007
O 91-0007
Run 1 - No Sparge - Secondary Treated Sludge
Run 2 - No Sparge - Secondary Treated Sludge
Run 3 - 45 CFM Sparge Below 39% - Primary Anaerobically Treated Sludge
Run 2 - No Sparge - Cover Open - Mixed Sludge
Run 3-21.5 SCFM Air - Cover Open Air Off Under 29% - Mixed Sludge
Run 4 • 8.6 SCFM Air - Cover Closed - Mixed Sludge
Run 2 - No Sparge - Zimpro Sludge - Cover Open
Run 7 - 45 CFM Air - Zimpro Sludge - Cover Open
25 30 35 40 45 50 55
0 5 10 15 20
Source: Bethlehem Corp. Test Data
Figure 10. Heat-Transfer Coefficient for Sludge Drying.
-------
Some air flow through the dryer is necessary to purge water vapor. The
dryer should be closed or covered, but not vapor-tight. A slight negative
pressure should be maintained throughout the dryer to eliminate vapor leakage
and the resulting odor problem. Available data also indicate that an air
purge or sparge under the solids will substantially improve heat transfer and
drying rate. Air flows of 2 to 10 cu m/min per cu m of dryer working volume
(2-10 cfm/cu ft) have been effective. An air purge is particularly important
as the sludge becomes dry and the heat-transfer coefficient decreases (see
Figure 10).
The moisture content of sludge solids leaving the dryer should be about
15 percent. Attaining complete dryness not only is unnecessary, but would
also significantly increase the dryer size because of the low heat-transfer
coefficient encountered with dry sludge. Dry sludge is also very dusty and
would present a potential explosion hazard.
Water vapor and purge air drawn from the dryer should be returned to the
incinerator for destruction of odorous materials. Where the design calls for
high air-purge rates, a cyclone in the water vapor/air line will reduce the
potential for solids build-up in the return duct work. Insulation of the re-
turn duct work is also advisable to minimize condensation of water vapor and
build-up of solids carried over from the dryer. The water-vapor/purge-air
•should be combined with overfire air and injected into the active combustion
zone of the incinerator.
The dried sludge solids are divided between recycle and solids sent to
the incinerator. The recycle solids and the incoming filter cake should be
adequately mixed in a pug mill to produce a dryer feed with reasonable
moisture distribution and to break up the sludge cake. The remaining dry
sludge solids can be conveyed to the incinerator by belt, screw, or pneumatic
conveyor. Conveying equipment should be closed and reasonably air-tight, to
minimize odor emission. It is generally desirable to spread the sludge
throughout the active combustion zone of the incinerator; a pneumatic injector
will adequately spread the dry solids and result in suspension burning of the
sludge.
Effect of Size on Feasibility
Indirect-contact dryers are available in an almost unlimited range of
heat-transfer area, from several square meters to several hundred square
meters. Where large capacity is needed, multiple units can be installed.
Technical feasibility, therefore, is not affected by dryer size availability.
As with the direct dryer, the sludge predrying section is directly as-
sociated with a refuse incinerator. With a small incinerator plant of 230
metric tons (250 tons) per day, co-incineration is feasible for a community
of 150,000 or more. In smaller communities where the incinerator is operated
less than 2k hours per day, indirect drying offers additional advantages in
requiring less start-up time than does a direct-drying process. Intermittent
operation does reduce the potential steam sale.
66
-------
With the small size of indirect-drying equipment and the low proportion
of total steam consumption, some reduction in manpower may be achieved by
oversizing the dryer and co-incinerating on a one- or two-shift basis.
MULTIPLE-HEARTH CO-INCINERATION
Co-incineration in a multiple hearth furnace has been chosen for detailed
study because this technique has been commercially demonstrated in Europe and
represents equipment familiar to those in the wastewater sludge disposal field.
It is also a technique which does not require a separate sludge dryer, which
tends to simplify the co-incineration process. However, this technique requires
refuse Pre-shredding. A block flow diagram for co-incineration in a multiple-
hearth furnace appears in Figure 11.
All commercial multiple-hearth furnaces have essentially the same config-
uration. The furnace consists of a vertical cylinder, usually lined with re-
fractory, containing a series of horizontal decks or trays (the hearths of the
furnace). Hearth openings alternate at the center and outside. A vertical
shaft extends through the center of the furnace, with rabble arms extending
radially from the shaft at each hearth. These areas provide for agitation and
movement of the solids, as the shaft and rabble arms are slowly rotated. In
high-temperature operation, the shaft and arms are hollow, for the passage of
cool ing air.
Combustion air enters at the base of the furnace. Where high-moisture-
content sludges are dried, warm exhaust air from the center-shaft cooling
system is often used as part of the combustion air supply. Combustion air and
flue-gases move counter-current to the sludge flow. For sludge incinerators,
normal excess air level ranges from 50 to 100 percent.
In sewage sludge incineration, the wet sludge is charged at the top hearth.
By the action of the rabble arms, the sludge moves across the hearths until it
reaches the drop holes, where It falls to the next hearth. Rising combustion
products provide the heat to dry the sludge as it moves across the top two or
three hearths. Once the sludge is dry, combustion starts. Burners are
generally installed to provide ignition of the dry sludge and make-up heat
for sludge drying. Combustion continues on the next few hearths. After burn-
out, the furnace design generally includes one additional hearth, for ash
cooling and for some combustion air preheating. Ash is discharged at the base
of the furnace. Thus there are three zones in a typical sludge Incinerator:
drying, combustion, and cooling. Typical operating temperatures and con-
ditions for a six-hearth furnace are as fn the next tabulation. In larger
furnaces , the same three zones are present, but are spread over more hearths.
Typical multiple-hearth sludge incinerators range from 1.8 to 8.5 m
(6 - 28 ft) in diameter and have from k to 12 hearths. In addition to their
use in sludge disposal applications, multiple-hearth furnaces are used for ore
roasting and reduction, calcining, and char production.
67
-------
Primary
Shredder
Refuse
oo
Magnetic
Separator
Secondary
Shredder
Thickened Sludge
Demister
Water to
Treatment Plant
Air I T Ash to Disposal
Figure 11. Multiple-Hearth Co-Incineration:
Schematic Flow Sheet.
-------
Hearth No.
1
2
3
4
5
6
Gas
Temperature
426
538
635
732
913
799
1,000
1,175
1,350
1,675
Sludge
Temperature
"TO(°py
38
66
82
100
150
180
Condi tion
Drying
Drying
Drying
Flame
Flame
Cool ing
Heat and Material Balance
For heat-balance purposes, the multiple-hearth incinerator can be con-
sidered as a direct-drying process, with all the combustion products passing
over the wet sludge, but without recirculating of furnace off-gas back to the
combustion zone. It is therefore necessary to maintain a gas temperature of
760°C (1,400°F) after the sludge drying zone ?n order to Insure destruction of
odorous materials.
Equivalent population quantities of refuse and sludge were again used as
the input to the co-incineration computer model. The model was run with a
4%-solids sludge, representing thickened sludge; a 20%-solids sludge, repre-
senting vacuum-filtered suldge; and 45%-solids sludge, representing sludge
dewatered by pressure filtration. Table 8 gives the input and output of the
program, Figure 12 shows the results graphically.
TABLE 8. INPUT AND OUTPUT OF ANALYSIS OF MULTIPLE HEARTH
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM ~ EQUIVALENT
SLUDGE/REFUSE PRODUCTION
Refuse
%
35
71
84
Sludge
%
65
29
16
Sludge
Solids
%
4
20
45
Total
Furnace Outlet Temp. (°F)
Moisture
%
72
43
32
0
1,010
2,520
2,900
Excess Air
100
780
1,750
1,950
200
640
1,350
1,480
(*)
300
540
1,100
1,200
400
470
930
1,020
As the typical operating excess air rate of 100 percent, co-incineration
is feasible with overall moisture contents of 57 percent or less. An overall
69
-------
1800
3000
2500
&
I 2000
0
2
co
.£ 1500
o
1000
500
200
40 60
Total Moisture, Percent
j I
100
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 12. Multiple-Hearth Co-Incineration
Gas Temperature (Equivalent Population)
70
-------
moisture of 57 percent corresponds to a sludge consistency of 8.7 percent
solids, which is not a typical solids content available from standard process-
ing equipment. Consequently, solids would have to be dewatered to 20 percent
by the use of vacuum filtration or centrifugation. With a feed consisting of
20%-solids sludge and refuse (equivalent-population basis), excess air levels
could range as high as 175 percent without adverse effect on the required ex-
haust temperature. For co-incineration applications, excess air levels of
100 percent have been recommended by vendors.
Co-incineration with 4%-solids sludge does not meet the prescribed temper-
ature requirements and should be considered unfeasible. Sludges with solids
higher than 20 percent will, of course, meet the 760Q (1,^00°F) minimum temper-
atures at all reasonable excess air rates for multiple-hearth incinerators.
Figure 13 is based on incineration model results (Table 9) with sludge
at 20 percent above the equivalent-population basis. This reduces the
overall moisture limit to 54 percent, which corresponds to a sludge solids
level of 10 percent. However, none of the sludge treatment techniques gener-
ally used produces a sludge near 10 percent solids. Thickened sludge at 4
percent and vacuum filtered sludge at about 20 percent solids are the sludge
treatments producing solids closest to the 10 percent required. Since the
4%-solids sludge does not meet the temperature requirements a 20%-solids,
vacuum-filtered sludge represents the minimum sludge solids level which can
be co-incinerated without the use of significant quantities of auxiliary fuel.
TABLE 9. INPUT AND OUTPUT OF ANALYSIS OF MULTIPLE HEARTH
CO-INCINERATION ALTERNATIVE BY INC IN PROGRAM ~ SLUDGE/REFUSE
RATIO AT 120 PERCENT OF EQUIVALENT VALUE
Refuse
%
67
81
Sludge
%
33
19
Sludge
Solids
%
20
45
Total Furnace Outlet Temp. (°F)
Moisture
% 0
45 2,440
32 2,870
Excess
100
1,700 1
Air
200
,310
1,940 1,470
(*)
300
1,070
1,200
400
910
1,010
From the standpoint of the available heat, a multiple-hearth furnace acts
the same as the direct pre-drying process, but operates at lower excess air
levels than refractory incinerators.
Plant Design
The actual design work for a multiple-hearth incinerator is normally pro-
vided by the equipment vendors.. However, there are some basic parameters
which should be kept in mind when considering a multiple-hearth co-incineration
plant; the mo^t significant parameter is hearth loading. Several factors
affect the loading rate: moisture content, volatiles content, and size and
71
-------
1800
3000
2500
u.
o
I 2000
CD
I
.£ 1500
c
1000
500
200
40 60
Total Moisture, Percent
J 1
_L
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 13. Multiple-Hearth Co-Incineration
Gas Temperature (20% excess sludge)
72
-------
consistency of feed materials. Design loading must be adjusted to provide
sufficient drying time in the drying zone and reasonable heat release in the
active combustion zone of the furnace. For burning sludge alone, typical
hearth loading ranges from 34 to 59 kg of wet sludge per hour per sq m
(7-12 Ib/hr per sq ft) of hearth area. The loading should be reduced, to
29 to 49 kg/hr per sq m (6-10 Ibs/hour/sq ft), when co-incinerating sludge and
refuse, because of the high heat release of the refuse. Small-diameter fur-
naces operate at the lower end of the range, while higher loadings can be
expected with large-diameter units.
Where significant quantities of water are present, it is often useful to
consider loading on a solids basis. On this basis, there is little difference
in loading between simple sludge incineration and co-incineration. Expected
solids loading will be in the range of 15 to 2k kg/hr solids per sq m (3-5
Ib/hr/sq ft).
On occasion, the combustion spaces over the uppermost one or two hearths
of a furnace are used to insure the burn-out of odors in the off-gases; with
auxiliary-fuel firing, these spaces can be used as afterburners, to raise off-
gas temperature. In such cases, the area of the inactive hearths should not
be included when considering hearth loading.
In general, the largest (diameter) furnace consistent with the foregoing
hearth loadings should be provided in the plant design. Large-dfameter fur-
naces have a larger drop-hole opening and more total open area than do the
smaller-diameter hearths. Larger openings will reduce the potential for
drop-hole plugging attributable to oversized refuse or clinkers. In no case,
however, have furnaces with less than six hearths been successful in co-
incineration applications; this is the minimum number of hearths necessary
for adequate burn-out of the refuse/sludge.
Two methods of feeding sludge and refuse to a multiple-hearth furnace
have been tested: combined feed to the top hearth and sludge feed to the top
hearth with refuse feed to a lower hearth. Both feed systems have been used
successfully, but the systems with sludge feed to the top hearth and refuse
feed to a lower hearth have experienced odor-emission problems if off-gas
temperatures are not kept above 760°C (1,400°F). With combined top feed, it
may be necessary to provide some premixing of shredded refuse with the sludge
before charging the furnace; reports from the Uzwi1 (Switzerland) installation
indicate that combustion of refuse occurs in the top hearth if the refuse and
sludge are not mixed. High carbon monoxide concentrations in the off-gas have
also been detected at the Uzwi1 unit. Separate feed is a more complex system,
but offers greater flexibility and control of sludge-to-refuse ratio and may
also produce more efficient sludge drying. It is therefore the recommended
arrangement for co-incineration feed to a multiple-hearth furnace.
Multiple-hearth co-incineration requires the refuse to be shredded and
cleaned of metal before entering the incinerator, to minimize entanglement
problems with the rabble arms and plows. Shredded refuse must be small enough
to fall through the drop-holes in each hearth of the furnace, but there has
been insufficient experience to determine the precise extent of refuse
shredding required. Based on European practice, a refuse maximum size of
73
-------
7.6 cm (3 in.), i.e., 95 percent passes this size of mesh, is recommended by
equipment vendors for furnaces with diameter of 3 m (10 ft) or more.
Shredding to this size is best accomplished in two stages. First, bulky
waste should be separated. Refuse is then rough-shredded to permit magnetic
separation of ferrous metals. At this point, the refuse can be screened to
reduce large, non-combustible pieces by the removal of glass, non-ferrous
metals, etc. The refuse is then fine-shredded to a 6.7 cm (3 in.) max. size.
A certain amount of pieces of refuse as large as 15 cm (6 in.) can be
accommodated in the furnace, but the fraction in the 7.6 to 15 cm (3 In.-6 in.)
range must be small. Significant quantities of material over 7.6 cm (3 in.)
will result in poor rabbling on the hearth and blockage of the drop-holes.
Wire and similar material must also be removed before the refuse is fed to the
furnace, because they may interfere with furnace operation by wrapping around
tines on the rabble arms and on the center shaft of the furnace.
Refuse shredding is a high-maintenance operation with relatively re-
stricted equipment availability. To maintain incinerator operation during
shredder down-time, facilities for storage of shredded refuse equivalent to
at least two days' capacity should be provided. Special care is required In
the design of shredded refuse storage silos to avoid severe compaction and
bridging.
The requirement for shredded refuse and storage will add significantly
to the cost and complexity of a multiple-hearth co-incineration plant. One
European plant4ias avoided the shredded refuse problem by a modification in
the plant design. At Dordrecht, Holland, refuse is incinerated in a con-
ventional refractory incinerator equipped with Martin grates; no refuse
shredding is required. Combustion products are ducted to the normal combus-
tion air inlet of a multiple-hearth incinerator. At this point, there is
sufficient oxygen for combustion of the sludge, because of the high excess
air levels used in the refuse incinerator. Sludge is fed at the top of the
multiple-hearth furnace, where it is dried, and it is incinerated in the
lower section of the multiple-hearth furnace. By this process, heat from
refuse incineration, in the form of preheated combustion air, is used in
sludge drying and incineration. While two separate furnaces are required in
this design, the process is simplified by elimination of the shredding
operation and its related problems. The plant continues to operate with
no auxiliary fuel required in the sludge incinerator. A diagram for this
alternative multiple hearth co-incineration process appears in Figure Ik.
Effect of Size on Feasibility
Multiple-hearth incinerators are available in wide range of sizes, from
0.3 to 9 m (I1 - 30') in diameter and from 2 to 12 hearths. As furnace dia-
meter decreases, drop-hole size also decreases, which in turn increases the
amount of refuse shredding necessary. The ability to shred refuse becomes the
limiting factor in scale-down of a multiple hearth co-incineration unit.
A refuse of 2.5 cm (1 in.) nominal size can be incinerated in a 2.7 m
(9 ft) diameter, 2 m (7 ft) I.D. furnace. With six hearths, this furnace
would have an effective area of 19 sq m (200 sq ft), and could handle about
Ik
-------
Thickened Sludge •
To Atmosphere
Sludge Dewatering
Sludge
Water to
Treatment Plant
To Atmosphere ^W-
Dust Collector
Refuse
Incinerator
Flue Gas as
Combustion Air
I
Multiple Hearth
Sludge Incinerator
i
i
i
i
Incinerator
Shaft
Cooling Air
-Combustion Air
Ash to Disposal
Ash to Disposal
Figure 14. Multiple-Hearth Co-Incineration Using
Incinerator Flue Gas—Schematic Flow Sheet
14 metric tons (15 tons) per day (24-hr operation) of refuse and sludge.
Since the required refuse size is small and it is impractical to install a
shredder whose capacity is less than 1.8 metric tons (2 tons) per hour
(because of the small size of the feed chute and shredder opening relative
to typical raw refuse dimensions), the shredding sub-system in such an
installation would account for a higher proportion of the overall cost than
it would with a larger co-incineration furnace.
The smallest shredder generally used with municipal refuse has a capacity
of about 18 metric tons (20 tons) per hour, which would permit a co-incineration
plant to handle 110 metric tons (120 tons) per day of refuse when operating 6
hours per day. This refuse rate is equivalent to a co-incineration plant of
about 172 metric tons (190 tons) per day (of refuse and sludge) and would
serve a community of about 100,000. Furnace diameter for this capacity would
be 4.9 to 5.5 m (16 - 18 ft).
75
-------
The largest furnace commercially available will provide up to 510 sq m
(5,500 sq ft) of hearth with a rating of about 450 metric tons (500 tons) per
day of combined feed. A single furnace in this size would serve a population
of 250,000. Beyond this point, multiple units would be required.
PYROLYSIS CO-INCINERATION
The use of pyrolysis for processing mixed municipal refuse has been advo-
cated primarily as a means of recovering some of the fuel value in refuse.
Until recently, attempts at energy recovery from refuse combustion have been
based on the generation of steam in a boiler-incinerator. The problems
associated with steam generation have been logistics (use must be near the
incineration), low boiler pressure (not efficient for power generation), and
intermittent operation. Thus, pyrolysis of mixed municipal refuse has a two-
fold objective: 1) incineration of refuse, with all the related advantages;
and 2) production of energy in the form of a fuel product. The same factors
which improve energy recovery should prove useful in co-incinerating sludge
and refuse. Studies thus far have indicated that low excess air rates are re-
quired for co-incineration, primarly to conserve available heat. Pyrolysis,
with its sub-stoichiometric air/oxygen usage should provide a high level of
heat availability. A block flow diagram for co-incineration appears as
Figure 15.
In concept, pyrolysis of an organic substance is a destructive distil-
lation process wherein the solids are heated in a partial or total absence of
oxygen. The solid decomposes, producing a gaseous product and carbonaceous
residue called "char." The gaseous products are often partially condensed,
forming a liquid product as well.
The gaseous products consist of typical combustion products, carbon
dioxide, and water vapor, even though little actual combustion may have
occurred in the pyrolysis equipment. These oxidized compounds are present ?n
combined form in many organic substances, or are formed from oxygen available
in the solids undergoing pyrolysis. The fuel value of the pyrolyttc gas is
derived from the presence of carbon monoxide, hydrogen, methane, and other
light hydrocarbons.
If the gaseous products of pyrolysis are cooled, a liquid product will
condense. The bulk of the liquid product is water, but significant quantities
of organics also condense. These include methanol and higher alcohols,
benzene and other aromatics, and, to a lesser extent, aldehydes, ketones, and
organic acids.
The residue which remains after pyrolysis contains the ash or
non-combustibles of the original material, plus a carbon char. The char
component of the residue also has a fuel value, similar to that of bituminous
coal. In certain pyroJysis processes, this char is combusted within the
furnace (slagging furnaces) to provide the heat necessary to carry on the
reaction. This is an appropriate use of the char component, because the
carbon portion is difficult to separate from ash, and its external use as a
fuel is doubtful.
76
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Thickened Sludge <
1
Sludge Dewatering
Water to ^
Treatment Plant
Preheated Air
or Oxygen
Water
Water to
Treatment Plant
Steam
Sludge
Slag
• Slag Solids to Disposal
Figure 15. Pyrolysis Co-Incineration — Schematic Flow Sheet.
-------
Pyrolysis for the production of organic chemicals has been, and continues
to be, a significant industrial process. Early processes for the production
of methanol were derived from the destructive distillation (pyrolysis) of
wood. Thermal cracking of heavy petroleum stocks to form lighter hydrocarbons
could also be considered a pyrolytic operation, as could the production of fuel
oils and gases from coal.
A variety of pyrolytic equipment is available or has been proposed. One
means of classifying this equipment is the way in which heat to carry on the
reaction is supplied: indirect heating, direct auxiliary fuel firing, and
partial combustion of the refuse.
The pyrolysis reactor can be heated externally as in a retort. This is
the most widely used form of pyrolysis in Industrial applications, where the
purpose of the reaction generally is the production of a new organic chemical.
Much of the experimental work on the pyrolysis of mixed municipal refuse has
also been carried out in such vessels, utilizing externally supplied heat to
initiate and maintain reaction temperature. However, it is unlikely that
commercial-scale co-incineration equipment could be operated using an indirect
heat source.
A second system employs an auxiliary fuel, fired directly into the pyro-
lysis chamber, to provide the energy input to maintain the pyrolysis reaction
temperature. Such a system, employing a rotary kiln, is the basis for Monsanto's
Landgard system. Shredded refuse is ram-fed into the kiln, where auxiliary
fuel is fired to maintain an off-gas temperature of 650° (1,200°F) and a
residue temperature of 820°C (1,500°F). The pyrolytic gases and refuse move
through the kiln countercurrently. The gases are removed and burned tn a
"gas purifier." Steam is recovered from the combustion products, for sale to
a nearby utility; the quantity of steam generated, however, is greater than
could be obtained by firing the auxiliary fuel directly into a boiler—thus,
the economy of operation. There is no attempt to recover a liquid product,
and, since the gas purifier immediately follows the pyrolysis kiln, there is
little loss in sensible heat of the pyrolytic gases. Char product does
represent a significant loss of potential fuel value; in this installation,
the char is removed from the kiln, quenched, and landfilled.
Although it is feasible to co-incinerate in equipment similar to the
Landgard process, significant increases in auxiliary fuel would be required;
virtually all the heat necessary to dry the sludge would have to be input in
the form of residual fuel oil. (A 20%-solids sludge would require a heat
input of 789 cal/g (1,421 Btu/lb) of wet sludge.) Although this represents an
improvement over simple sludge incineration because small quantities of excess
air are fired, the added fuel cost would make co-incineration in this type of
equipment unattractive. In addition, the economics of the Landgard system are
based on the sale of steam to recover the cost of fuel. While such a system
may be feasible, it would be fortuitous to find a major steam customer near
the wastewater treatment plant.
A third type of pyrolysis process employs a refractory-1ined shaft furnace
to carry out the reaction. Refuse is charged at the top of the shaft, providing
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a seal, and as It descends through the furnace, hot pyrolytfc gases from the
slagging and combustion zones move in a counter-current direction, thus pro-
viding pre-ignition and drying of the sludge and refuse. Pre-heated air or
oxygen-enriched air is injected into the combustion zone at the base of the
shaft furnace, where combustion of the unpyrolyzed char occurs.
Preheating and oxygen enrichment serve essentially the same purpose:
maintaining a furnace temperature high enough to form a slag and to produce a
pyrolysis gas with as high a heating value as possible. The intentional forma-
tion of a fluid slag, at temperatures of 1,370° - 1,650°C (2,500-3,000°F), Is
a departure from typical refuse incineration, where slag formation is avoided
wherever possible. The slag formed is virtually free of combustibles, and
therefore represents the beneficial use of the char portion of typical pyro-
lysis products, in that the char supplies heat to the pyrolysis reactor while
producing a sterile, non-leaching, minimum-heat-value residue.
As the cooled pyrolysis products leave the reactor, two options are avail-
able. These gases (low heating value), after preliminary cleaning, can be
directed to a remote facility; or they can be burned in a secondary combustion
chamber immediately following the shaft furnace, with energy recovery in the
form of a waste-heat boiler. Since the primary objective of co-incineration
is the combined disposal of sludge and refuse (not energy recovery) and since
heating values are expected to be lower with co-incineration than with the
pyrolysis of refuse alone, immediate combustion followed by energy recovery,
if economically attractive, should be the type of pyrolysis process best suited
to co-incineration. In explanation of the expectation of lower heating values
for co-incineration pyrolysis, more heat must be provided in sludge addition
than with refuse alone in order to evaporate the additional water in the
sludge. The only source of this heat is increased combustion of burnable
material. Increased combustion will result in higher CC^ in the gas plus
additional moisture in vapor form, thus reducing both the heat content and
heating value of off-gases. A full-scale co-incineration experiment is under
way in a Purox (Union Carbide) system. There is some indication that the
Purox system is affected less by the moisture content than the Torrax system
described in the report. Data available from Torrax (Carborundum) indicates
a reduction of about 25 percent in total heat available from pyrolysis gases
during co-incineration at equivalent population rates of sludge and refuse,
with sludge dewatered to 20 percent solids. The major disadvantage of in-
direct combustion is the expense related to the boiler and to off-gas clean-
ing equipment.
The major shaft furnace systems available, the Purox (oxygen enrichment)
and Torrax (regenerative heat recovery) pyrolysis units, marketed by Union
Carbide Co. and The Carborundum Co., have similar basic operating principles.
The Purox plant maintains a high slag-zone temperature by oxygen enrichment,
and the Torrax process maintains the furnace temperature by recovery of waste
heat and combustion air pre-heating in a system of regenerative towers. The
oxygen enrichment system is designed primarily for the production of high-Btu
fuel gas; thus, the added capital and operating cost of oxygen-generating
equipment may be justified. However, where the objective is the use of waste
energy in refuse to co-incinerate the sludge by a pyrolysis technique, the
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use of oxygen enrichment appears to offer little advantage over air preheating
if the co-incineration plant is adjacent to the wastewater treatment plant.
If an energy consumer can be located nearby, the potential sale of fuel gas or
steam should not be ignored, and the equipment type should be re-evaluated in
that light.
Heat and Material Balance
With Incineration, combustion is assumed to be complete, with well-defined
reactions, end products, and heats of reaction. In pyrolysls, with a uniform
feed of known composition, the kinetics of reaction can be determined, and the
relationship between operating parameters and pyrolytic products can be es-
tablished. Such information has been developed for a variety of materials,
including wood, petroleum products, coal, and even various constituents of
municipal refuse, such as cellulose and polyethylene. However, the wide
variation in refuse composition makes it impossible to define, exactly, the
quality of pyrolytic products generated In the co-incineration reaction
period.
One of the major factors affecting the yield of pyrolysts products is the
operating temperature of the furnace. The pyrolysis reaction does not occur
at any significant rate until the temperature reaches 480°C (900°F). Higher
temperature increases the yield and heating value of the pyrolysis gases, and
decreases the quantity of char. A reaction temperature of about 820°C
(1,500°F) appears to be the minimum operating temperature for reasonable re-
action rates and char burn-out.
SIagging-type pyrolysis furnaces, operating at temperatures of 1,370° to
1,650°C (2,500°-3,000°F), produce a slag with virtually no heating value at
all. Therefore, maintaining a slagging temperature in the hearth area is
a necessary requirement for pyrolysis in a shaft furnace.
Torrax System—
In the system analyzed In the computer model (Torrax), there are two
heat sources: 1) partial combustion in the shaft furnace, and 2) secondary
combustion of the pyrolysis products. Control of the slagging and pyrolysis
temperatures is maintained by controlling the temperature and volume of pre-
heated combustion air injected into the base of the shaft furnace. Secondary
combustion of pyrolysis products, outside the shaft furnace, Is the source of
preheat (through regenerative heat exchangers) for Incoming combustion air.
(In the Purox system, heat is provided only by the partial combustion of the
refuse in the slagging section of the furnace.)
The heat necessary to dry the sludge must be obtained from one or both
of these two sources. The secondary combustion occurs outside the shaft
furnace, however, and the only way that heat generated in the secondary com-
bustion chamber can be used to evaporate sludge moisture is by pre-heating
combustion air through heat exchange in the regenerators. The volume and the
temperature of preheated combustion air are limited by the degree of com-
bustion required in the shaft furnace and by the temperature and size
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limitations of the regenerators; therefore, secondary combustion of pyrolysis
products is a poor source of heat for drying the sludge.
The only remaining feasible source of heat, therefore, is the increased
combustion of the refuse within the furnace. Increases in the degree of com-
bustion can be closely controlled by increases in the amount of combustion
air injected into the base of the shaft. Increasing combustion will provide
additional heat to dry the sludge. The pyrolysis gases drawn from the top of
the shaft will be lower in fuel value and larger in volume than the gas obtain-
ed with refuse alone. The lower fuel value results from additional combustion
in the furnace, and the larger volume is attributable to the water vapor evapo-
rated from the sludge. The wet, low-Btu gas is then burned in the secondary
combustion chamber, and the combustion products are passed through the re-
generative tower (or through a waste-heat boiler) and out to pollution control
equipment.
The limits on the feasibility of co-incineration can be expressed in
terms of the temperature of the gases exiting from the combustion chamber. As
noted in the preamble to this section, a temperature of 760°C (1,400°F) would
normally be considered sufficient to insure complete combustion and destruction
of odorous gases. Because the combustion products must also supply heat to the
regenerative towers, and the temperature of air pre-heated in these towers must
be raised to about 1,040°C (1,900°F), which requires a combustion gas temper-
ature of at least 1,150°C (2,100°F). Thus, the temperature of combustion
products leaving the secondary combustion chamber becomes the limiting factor
in determining the feasibility of co-incineration by pyrolysis.
Figure 16 represents the relationships between combustion gas temperature
and the overall moisture and the sludge moisture content (at equivalent-
population sludge-to-refuse ratio). The combustion calculations were made by
the INCIN model using input described in Table 10.
TABLE 10. INPUT AND OUTPUT OF ANALYSIS OF PYROLYSIS
CO-INCINERATION ALTERNATIVE BY INCIN PROGRAM — EQUIVALENT
SLUDGE/REFUSE PRODUCTION
Refuse
Sludge Total
Sludge Solids Moisture
Furnace Outlet Temp. ( F)
Excess
) 10 20
Air (%)
30
40
50
35 65 4 72 1,200 1,160 1,130 1,100 1,070 1,040
71 29 20 43 2,650 2,530 2,420 2,330 2,240 2,160
84 16 45 32 3,000 2,850 2,730 2,610 2,500 2,400
The calculations were performed with 20 percent of the total air requirement
pre-heated to 1,040°C (1,900°F), representing the preheated air entering the
slagging zone of the furnace. As noted, an overall moisture content of 55
percent (corresponding to sludge solids of 13.5 percent) or less resulted
in acceptable operating conditions, that is, secondary combustion chamber
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1800
3000
2500
Q)
3
re
i
I 2000
8
o
S
2
1500
1000
500
200
20
40 60
Total Moisture, Percent
I I
80
100
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
Figure 16. Pyrolysis (Equivalent Population).
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off-gas temperatures of at least 1,15QOC (2,100°F). Thus, non-dewatered sludge,
at 4 percent solids, cannot be co-incinerated with refuse on an equivalent-
population basis. Vacuum-filtered sludge at 20 percent solids or pressure-
filtered sludge at 45 percent solids results in acceptable performance. In-
creasing sludge quantities by 20 percent, as suggested for design purposes,
also results in satisfactory operating conditions for sludges of 20 percent
solids or higher, as indicated in Figure 17, which is based on data presented
in Table 11.
TABLE 11. INPUT AND OUTPUT OF ANALYSIS OF PYROLYSIS
CO-INCINERATION ALTERNATIVE BY INC IN PROGRAM--SLUDGE/REFUSE
RATIO AT 120 PERCENT OF EQUIVALENT VALUE
Sludge Total . .
Refuse Sludge Solids Moisture Furnace Outlet Temp. (°F)
a, o, °, a. Excess Air (%)
" * * * 10 20 30 40 50
67
81
33
19
20
45
45
33
2,570 2,460 2,360 2,270 2,180 2,100
2,970 2,830 2,710 2,590 2,480 2,390
Purox System—
Analysis of the Purox system is more complex than that of the Torrax
system. Since the only source of heat is partial combustion of the refuse,
the temperature/moisture relationship will approach that of a direct-dryer
process operated at stoichiometric air levels. (Note that this should lead
to a conservative estimate of co-incineration feasibility, since the Purox
furnace is actually operated at sub-stoichiometric oxygen levels and no
diluent nitrogen is present.) If the pyrolytic gas is immediately combusted,
the temperature requirement remains at 760°C (1,400°F) to insure complete
destruction of organics. As indicated in Figure 16, co-Incineration appears
feasible at total moisture contents of 65 percent or less.
All currently available pyrolysis furnaces are based on proprietary de-
signs. Both the Torrax system and the Purox system utilize a shaft furnace.
Refuse and sludge are charged at the top and travel down the furnace. As the
dry refuse approaches the base of the furnace, air or oxygen is injected and
partial combustion occurs. Rising combustion products provide the heat for
pyrolysis of the refuse/sludge. The slag is continuously tapped from the
furnace and quenched in water, producing an inert solid. The pyrolytic gases
are burned immediately, or are cooled to reduce the dew point, and then are
sent to a remote combustion unit.
The major problem encountered thus far has been maintaining proper
slagging conditions at the base of the furnace, where slag freezing and ac-
cumulation result when the bed cools. The principal remedial action is to use
auxiliary fuel to maintain proper temperature in the slag-tap area. Occasion-
ally, residue contamination from unburned refuse has been a problem, because
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1800
3000
2500
£
3
2
2L
I 2000
1500
1000
500
200
20
I
40 60
Total Moisture, Percent
I
45 20 4
Sludge Solids, Percent (with 28 Percent Moisture in Refuse)
100
Figure 17. Pyrolysis Feasibility - 120% Equivalent Sludge.
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refuse dropping from the shaft into the slag bed is difficult to control. At
times the arch collapses, dumping unburned refuse onto the slag pool. Although
temperatures in the pool are high, the oxygen-deficient atmosphere permits
unburned refuse to leave the furnace.
The need for shredded rather than raw refuse must also be determined. At
present, the Torrax process operates on a raw refuse feed, whereas Purox re-
quires a finely shredded feed, with a nominal size of 7.6 cm (3 tn.). A
shredder plant adds significantly to the capital and operating cost of a co-
incineration facility.
There are two plant design alternatives for a pyrolysis co-incineration
facility, involving direct and indirect sludge predrying. Both shaft furnace
systems produce a fuel gas with low heating value. This gas could be used
in a direct-fired rotary dryer as described under Direct-Drying Co-Incineration.
Dry sludge could then be fed into the shaft furnace, with an afterburner to
insure odor destruction In the dryer off-gas. (See Figure 18 for the block
flow diagram.) Although the sludge predrying increases the cost and com-
plexity of the process, it is expected to improve the performance of the
furnace. The same sludge moisture limitations as noted in the discussion of
the heat and material balance apply to sludge predrying/pyrolysis co-incineration,
Sludge can also be predried using indirect techniques as described under
Indirect-Drying Co-Incineration. In such a system, a boiler would be added
to the pyrolysis plant, using the pyrolytic gas as fuel. Steam from refuse
pyrolysis would be used to predry sludge in an indirect type of dryer. (See
Figure 19 for the block flow diagram.) The advantage, over direct predrying,
is the elimination of the afterburner. Water evaporated from the sludge is
condensed in a barometric condenser and returned to the treatment plant. The
small amount of purge air pulled into the dryer will be used as combustion air
In the combustion of the pyrolysis gas.
Effect of Size
The effect of size on the feasibility of the pyrolysis co-incineration
process is difficult to assess, since commercial plants have not yet been con-
structed. Torrax pilot facilities, 68 metric tons (75 tons) per day, have
been successfully operated for extended periods. Thus, a pyrolysis co-
incineration plant could serve a community with a population as small as
55,000. However, there are questions as to whether it would be economically
feasible for a municipal authority to build and operate such a small plant.
The present Purox demonstration plant in South Charleston, W. Virginia has
been operated continuously at 91 metric tons (100 tons) per day. Quite likely,
the minimum practical size for a pyrolysis co-incineration plant will be 227
metric tons per day (250 tpd), i.e., about the same capacity of a refuse in-
cinerator. A unit of this size would serve an area of about 150,000 people.
Heat Recovery
In the co-incineration process, refuse was described as a fuel which is
burned to provide additional heat necessary to dry the sludge and thereby
85
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To Atmosphere
oo
Preheated Air
or Oxygen
Water.
Water to __
Treatment Plant ^*"
Direct Drier
Dry Sludge
Refuse
Shaft Furnace
Slag
Sludge
• Sludge
Sludge Dewatering
• Air
Water to
Treatment Plant
Pyrolysis Gas
Slag Solids to Disposal
Figure 18. Pyrolysis Co-Incineration with Direct
Sludge Pre-Drying — Process Flow Sheet.
-------
Water
Thickened Sludge
oo
Barometric
Condensate
Water to
Treatment Plant
Water to
Treatment Plant
Water to
Treatment
Plant
Oust
Collector | jo
Atmosphere
Slag Solids to Disposal
Figure 19. Pyrolysis Co-Incineration with Indirect
Pre-Drying — Process Flow Sheet.
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permit Incineration to occur. If refuse is considered as an energy resource,
a variety of other uses in addition to co-incineration should be considered.
While it is premature to consider refuse as a "valuable" resource, other
means of utilizing this energy should be considered in the establishment of a
solid waste management program.
A number of systems are available or under development for the recovery
of energy from refuse, including steam-generating incinerators, pyrolysis
plants producing gaseous or liquid fuels, and refuse-derived solid fuels. Heat
recoveries of up to 70 percent of the heating value of the refuse can be
obtained. However, it is unlikely that co-incineration and heat recovery can
be practiced at the same facility. Co-incineration by direct pre-drying,
indirect pre-drying, and multiple-hearth techniques requires a substantial
portion of the fuel value available in the refuse. Pyrolysis, with its low
air rate, may be compatible with energy recovery systems.
In addition to the thermodynamics, the logistics of energy markets, with
regard to the location of a co-incineration plant, present a serious problem.
The co-incineration facility must be located within sludge-pumping distance
of the wastewater treatment plant. It would be fortuitous, indeed, to locate
a viable energy market in the immediate vicinity of a wastewater treatment
plant. Even with new wastewater treatment facilities, conditions other than
energy markets (such as gravity flow and pumping requirements) normally
dictate the location for the treatment plant.
We must therefore conclude that co-incineration and energy recovery are
mutually exclusive, in most situations. A case-by-case study will be necessary
to determine the potential for energy recovery at a co-incineration plant.
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SECTION VI
AIR POLLUTION ASPECTS
CHARACTERISTICS OF EMISSIONS
The potential air pollutant emissions from the incineration of mixed muni-
cipal refuse (MMR) and/or municipal sewage sludge (MSS) fall into three dis-
tinct categories:
Combustible gases or vapors (carbon monoxide, hydrocarbons, odorous
organics).
Particulates (ash, unburned carbon, and metallic fumes or oxides).
Inorganic gases (especially, NOX, S02, HC1, and HF).
___!_ ._•__ ._ __ •• f _ .___ _ _ _ . ._ I _•_ _ I
In any combustion process, it is necessary to design and operate the system to
provide combustion conditions which will assure oxidation of the combustible
gases and vapors to yield an innocuous and essentially odor-free effluent flue
gas. Classically, the design and operating parameters of concern are referred
to as the "three T's"—time, temperature, and turbulence. They relate to as-
suring that combustible and oxidant are intimately mixed at a temperature at
which oxidation reactions are rapid, i.e. above 760°C (1,400°F), long enough
for the reactions to proceed to completion.
The presence of detectable quantities of carbon monoxide in the flue gas
has been proposed as an indicator of combustion efficiency. Niessen and
Sarofinr have stated, "if turbulence above the fuel bed is high enough to pro-
vide perfect mixing, no CO should be found in the exit gases." They also
state, "Based on the well stirred reactor studies, it is also expected that,
because CO burns so much slower than other fuel elements, carbon monoxide will
be detected in larger quantities than hydrocarbons in incompletely-burned flue
gases." Incinerators have often been cited as sources of odorous emissions,
but these are controllable by proper desjgn and operation of the equipment to
insure~burnout of odorous compounds before the exhaust gases are vented.
Proper design of the furnace system and consistent, careful operator control
of the combustion process are the essential elements of the indicated route to
abatement of combustible gas and vapor emissions.
Particulate emissions from an incinerator are primarily a function of the
design and operation of the furnace system. The designer's furnace configura-
tion and use of primary and secondary air establish the incinerator's basic
performance capability. Within the limits set by the designer, the operator
can significantly influence indinerator performance. When less than ideal
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combustion conditions exist because of design constraints or faulty operator
control of the combustion process, the quantity of particulates emitted will
increase (as will the combustible content of the fly ash).
Incinerator particulate emissions are also a function of the character-
istics of the wastes fed to the incinerator. An increase in the quantity of
fine material fed to the incinerator will increase the emission rate for any
given incinerator system. Similarly, an increase in the volatile inorganic
content of the feed will increase the quantity of fumes that are produced
when the volatile materials condense.
The third type of air pollutant emitted from a combustion process in-
cludes the oxides of sulfur and nitrogen, plus the halogen acids. The poten-
tial emissions of sulfur oxides and halogen acids are directly related to the
quantity of incompletely oxidized sulfur (sulfides, elemental sulfur, sulfites
and organo-sulfur compounds) and organic halogens present in the MMR and MSS
being burned. Some of these materials do not exit with the furnace flue
gases, because they may react with the incinerator ash and pass out of the
system with the incinerator residue. The nitrogen oxides are formed in the
flame, from the nitrogen present in the combustion air or in the material
being burned. Lower flame temperatures and low excess-air levels decrease
the amount of nitrogen oxide in the flue gases, but deliberate efforts to
minimize nitrogen oxide formation may contribute to higher quantities of
combustible carbon monoxide and increase the carbon content of the fly ash.
EMISSION REGULATIONS
Separate performance standards controlling the emissions from refuse and
sludge incinerators have been promulgated by the federal government. Current
(1975) federal emission standards limit the discharge of particulates, but do
not regulate the discharge of the combustible or inorganic gases that may be
produced by the incineration of refuse and sludge. Emission limitations for
refuse incinerators, published in the Federal Register on 23 December 1971,3
include "no owner or operator subject to the provisions of this part shall
discharge or cause the discharge into the atmosphere of particulate matter
which is in excess of 0.18 g/NM^ (0.08 gr/scf) corrected to 12 percent C02,
maximum 2-hour average." Federal standards for sludge incinerator emissions
published in the Federal Register on 8 March 197A^ read as follows:
"No operator of any sewage sludge incinerator subject to the provisions
of this sub-part shall discharge or cause the discharge into the atmos-
phere of:
"1. Particulate matter at a rate in excess of 0.65 g/kg dry sludge
input (1.30 Ib/ton dry sludge input).
"2. Any gases which exhibit 20% capacity or greater. Where the
presence of uncombined water is the only reason for failure
to meet the requirements of this paragraph, such failure shall
not be a violation of this section."
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The emission standard for refuse incinerators is based on units of con-
centration, whereas the standard for sludge incinerators is based on units of
mass. The reasons for this difference in national emissions standards are
complex, but basically recognize the operating practices and feasible pro-
cedures involved with separate incineration of refuse and sludge. For in-
stance, the Federal EPA considered setting emission standards for refuse
incinerators on a mass basis, but rejected it because it concluded that there
was no reliable method to determine the actual incinerator firing rate.
While it is conventional practice to weigh MMR upon receipt at the incinerator
plant, no domestic incinerator was at that time equipped to weigh the material
fed to the furnaces on a continuous and reliable basts.
In the case of sludge incineration, the original proposed regulation was
based on units of concentration, but was changed to units of mass for two
principal reasons. First, dilution does occur, and is significant. Second,
the control devices normally used on sludge incinerators—wet scrubbers—
absorb some of the CO, present in the gases discharged to the atmosphere.
This, as well as the 602 contributed by auxiliary fuel, alters the gas com-
position and precludes the relatively simple correction of results to a
reference basis such as 12 percent 062. Because of this, the determination
of the amount of dilution could then prove difficult. Further information on
the rationale behind setting the Federal standards is contained in the back-
ground documents which were published in connection with the Federal new-source
performance standards.
Available information on the species of air pollutants permitted from
refuse incinerators has been summarized in a paper by Smith.5 Similar infor-
mation on sludge incinerator emissions has been summarized in an EPA Technology
Transfer Seminar publication."
Based upon available data,^» the efficiency of particulate control
devices required to meet the Federal emissions standards is 90 to 95 percent
for refuse incinerators, and 96.6 to 99.6 percent to meet sewage sludge in-
cinerator emission standards. The "Standards of performance for New
Stationary Sources" were authorized in Section III of the Clean Air Act
Amendments of 1970: "The overriding purpose of this section is to prevent
the general occurrence of new air pollution problems by requiring the installa-
tion of the best controls during initial construction, when the installation
of such controls is least expensive."7 The term "standard of performance" is
defined as "a standard for emissions of air pollutants which reflects the
degree of emission limitation achievable through the application of the best
system of emission reduction which (taking into account the cost of achieving
such reduction) the administrator determines has been adequately demonstrated."
Under the terms of the Clean Air Act Amendments, a state or local jurisdiction
may adopt more stringent emission limitations where it is necessary, in order
to achieve national ambient air quality standards. Maryland is one juris-
diction that has adopted a more stringent refuse incinerator emission standard,
0.07 g/NM3 (0.03 gr/scf), which is considerably lower than the federally
mandated 0.18 g/NM3 (0.08 gr/scf).
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CONTROL OPTIONS - MSS OR MMR SYSTEMS
High-efficiency control devices are obviously necessary to meet the
Federal standards or the more stringent state or local standards that may be
enacted. The devices that are capable of attaining the performance required
by regulatory action are electrostatic precipitators, wet scrubbers, and
fabric filters (baghouses). All three of these devices have been used to
control the emissior.s from refuse incineration.
By far the most commonly installed on new MMR systems is the electro-
static precipitator. In fact, the Federal emissions standards for refuse
incinerators were based on the degree of control achieved with electrostatic
precipitators.
The most popular high-efficiency collector in use on sludge incinerators
is the wet scrubber. Fife° reports an electrostatic precipitator (ESP) in-
stallation on a municipal sewage treatment plant sludge incinerator system in
the 19^0's; many problems were created by the sticky nature of the sludge fly
ash. Precipitators in such circumstances have since been replaced with wet
scrubbers. In Japan, however, the emission standard is based on ESP-equipped
plants.
Baghouses have been tried on refuse incinerators, both experimentally
and commercially, but never widely applied or advocated. To our knowledge,
they have not been used in sludge incineration practice.
Fife also provides a list of important considerations that have led to
the reported preference for electrostatic precipitators on refuse incinerators:
"1. The equipment does not produce a wastewater stream requiring treat-
ment prior to disposal.
"2. The equipment operates at a lower pressure drop than any other high-
efficiency type of gas cleaning equipment, at a very significant
saving in power. For 20-year life installations, the power saving
may well justify the extra cost of a precipitator in comparison with
other systems.
"3. The equipment does not produce the heavy steam plume normal to wet
scrubbers. This may be a very real aesthetic advantage, or may
eliminate the need for complex and expensive plume suppression
facilities.
"4. The equipment operates dry, and may not require special construction
to resist corrosion, particularly where continuous operation is
contemplated."
Precipitators, however, are ineffective in controlling gaseous pollutants such
as carbon monoxide, the oxides of nitrogen and sulfur, and hydrogen chloride.
The concentrations of these pollutants are low relative to most other combus-
tion sources and the net quantities emitted from refuse/sludge incineration
are usually insignificant in the total emission inventory of a region. Lastly,
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the high stack gas temperatures associated with ESP control, 230°C (450°F),
contribute to rapid atmospheric dilution of the gaseous contaminants.
If gaseous pollutant control is a requirement of a local regulatory
agency (no such requirement now exists In the New Source Performance Standards)
or is felt to be important by the design engineer (e.g. if an unusually high
concentration of PVC or other halogen acid source was expected), the ESP per-
formance goals should be thoroughly discussed.
The primary alternative is a wet scrubber arranged to use an alkaline
solution and designed to remove gaseous pollutants In addition to particulate
pollutants. The experience to date with the use of scrubbers on MMR inciner-
ator applications has been that the acid gases which are present create a very
corrosive environment within the scrubber, thus putting a severe constraint
on material selection. As the collection equipment generally is located up-
stream of the induced-draft fan and discharge stack, it is probably necessary
that all the materials used downstream of the scrubber be constructed of
costly, corrosion-resistant materials. Finally, unless stack reheat is prac-
ticed, there is always the possibility of condensation and the consequent
emission of "acid rain" from the incinerator stack.
Another control option is the fabric filter or baghouse, where it would
be possible and practical to use a dry adsorbent material to remove the acid
gases from the gas stream. Dry adsorption in this fashion may some day be
developed to the point of being commercially applicable; it is not now a
proven control technique.
' Present state-of-the-art considerations indicate that the order of pref-
erence for control of air pollutants from MMR incinerators is 1) electrostatic
precipitators, 2) wet scrubbers, and 3) baghouses. For sewage sludge inciner-
ators the present state of the art favors the use of wet scrubbers, as neither
electrostatic precipitators nor fabric filters have been successfully applied
to this service in the United States. ESP's have been sucessfully applied in
Japan, however, and their performance has formed the basis for setting emis-
sion standards.
IMPACT OF CO-INCINERATION ON EMISSIONS
Of the three emission categories, only particulates are regulated by
Federal emission standards. Stack-gas opacity is an indirect method of
assessing particulate emissions, and only the MSS incinerator regulations
include an opacity requirement. Inorganic gas emissions are a function of
waste characteristics, with the exception of the nitrogen oxides, where com-
bustion conditions do affect the conversion of atmospheric nitrogen to NOX.
The available data,5.6 although inadequate for full assessment of the
impact of co-incineration on the emissions of inorganic gases, seem to indi-
cate that inorganic gaseous emissions from MSS incineration are less than or
equal to comparable emissions from the incineration of MMR alone. Because
inorganic gas emissions from incineration are not now regulated and since the
state of the art regarding control of such emissions is uncertain, further
discussion of this category of emissions is not warranted.
93
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To prevent the release of combustible gases and vapors to the environ-
ment, two conditions must be met. First, all combustion air and all vapors
evaporated from the MMR and MSS must pass into an active combustion zone.
Second, the combustion process must be completed in the active combustion
zone. The design and operating objectives for the incineration of MMR or MSS
and for the co-incineration of MMR and MSS are identical insofar as the poten-
tial emissions of combustible gases and vapors are concerned. In all cases,
the combustion system must be designed to insure completion of the combustion
reactions. (The electrostatic precipitator problem reported by Fife8 can be
attributed to the condensation of tar vapors, indicating that these gases
bypassed the active combustion zone.)
The effect of co-incineration on furnace particulate emissions cannot be
easily generalized, since the furnace design and method of handling the MMR
and MSS are basic variables. Nevertheless, all the co-incineration techniques
reviewed will require the use of high-efficiency control equipment to meet
present Federal standards of performance. Each of the co-incineration tech-
niques discussed in Section IV has a different emission potential.
Where the MSS is first dried and then burned in an MMR furnace, an in-
crease in particulate emissions from the furnace can be expected, because the,
ash remaining after MSS solids are burned will undoubtedly be fine and easily
entrained in the flue gases. If all the MSS ash becomes fly ash, the impact
of co-incineration on furnace emissions can be assessed for a given ratio of
MMR and MSS (see Feasibility Study, Section V). When MMR emissions are
10 kg/metric ton (20 Ibs/ton) of refuse and MSS emissions are 150 kg/metric
ton (300 Ibs/ton) of dry solids, the furnace particulate emission rate would
roughly double. Therefore, in order to meet Federal particulate emission
standards of performance for MMR incinerators, the efficiency of the control
device must be increased from the 90-95 percent range to a 95~97.5 percent
range.
The particulate emission potential of multiple-hearth furnaces burning
sludge, refuse, or both is poorly documented. Similarly, data for assessment
of the emission potential of the pyrolysis technique of co-incineration are
not available. The following discussion, however, indicates the probable
particulate emissions from these types of operation.
In a multiple-hearth unit, the wastes cascade from hearth to hearth.
When they cascade from an in-hearth to an out-hearth (See Figure 2, Section
III), the falling wastes are exposed to a cross current of gas flow. When
the wastes fall through the drop holes from an out-hearth to an in-hearth,
they pass countercurrent to the rising stream of flue gases. The net result
is that the particulate fines are exposed to an elutriating gas stream as the
wastes are transferred from hearth to hearth in the furnace and the flue gases
pass over the wastes. The probable effect on co-incineration furnace emis-
sions is to make the particulate emissions greater than those experienced
with sludge incineration alone. Since collection efficiencies of 96.6 to
99.6 percent were considered necessary to meet Federal standards of perfor-
mance for any sludge incinerators, increase in inlet loading would make it
difficult to comply with promulgated emission limits when a multiple-hearth
furnace is used to co-incinerate MMR and MSS.
-------
The pyrolysis (shaft) furnace is the most difficult to assess, because
so very little information is available. We have to speculate that co-
incineration will increase the quantity of particulate emissions, because all
the combustion and the level of emissions compared to conventional refuse
incinerators are unknown.
In all of these cases, the furnace emission rates are expected to in-
crease over those that have been reported when burning only MMR or MSS. The
actual Increase in particulate emission rates is moot, and establishment of
these levels should be a part of any demonstration project. Estimates of
uncontrolled emission rates for both separate and combined incineration are
shown in Table 12. For demonstration purposes, we suggest that the level of
control attainable with the control equipment already installed be deemed
acceptable. For new projects incorporating co-incineration, we suggest that
the least stringent of present regulations be applied until actual perfor-
mance data can be obtained.
CONTROLS REQUIRED FOR CO-INCINERATION
The Clean Air Act as amended clearly mandates the use of the best practi-
cable and demonstrated control technology that can be economically justified.
Where MMR and MSS are being burned together, on an equal-population basis,
the amount of refuse being processed totally dominates the dry sludge solids
resulting from primary and secondary treatment of municipal sewage. On such
a basis, refuse quantities would account for 93 percent of the incinerator
feed; therefore, where two Federal standards of performance are involved, the
co-incineration device should probably be considered as a refuse incinerator
and be required to meet applicable codes for refuse incinerator emissions.
Federal standards of performance are less stringent for MMR inciner-
ation; however, since the emission potential of joint MMR-MSS incineration is
higher than MMR incineration alone, the collection efficiency of control
devices will have to be Increased in order to meet MMR emission regulations.
If co-incineration plants were required to meet MSS incinerator regulations,
there is reason to believe that the performance required of the control device
can be attained at only a modest increase in capital cost and a negligible
increase in operating cost (for ESP systems).
FINAL CONSIDERATIONS
Concern has been expressed about the emissions from incineration of toxic
organic compounds and potentially hazardous metals. Available data" indicate
that toxic organics can be destroyed during incineration and that the bulk of
the metals, with the exception of mercury, can be removed by the air pollution
control device. The sole exception, mercury, is present in both MMR and MSS;
consequently, potential mercury emissions should be determined in relation to
the allowable ambient concentrations at the time of design.
95
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TABLE 12, ESTIMATED AIR POLLUTANT EMISSIONS
FROM REFUSE, SLUDGE AND COMBINED INCINERATION
Incinerator
Uncontrolled Emission
Factor kg/metric ton (Ib/ton) '
Co-inci nerat ion(est imates)
Control Technology
Typical U.S. Practice (1970)
Pollutant
(Best Available Technology)(1976)
Conventional Multiple Dry Sludge Mult.
Solid Waste Hearth Sludge Injection Hearth Pyrolysis Solid Waste Sludge
Particulate 15.0 (30.0) 16.5 (33.0) 22 (44)
S0v 1.3 ( 2,5) 0.5 ( 1.0)
X
NO 1.0 ( 2.0) 2.5 ( 5.0)
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REFERENCES
1. Niessen, W.R. and A.F. Sarofim. Air Pollution Control for Incinerators.
(Presented at the National Industrial Solid Wastes Management Conference,
University of Houston, Houston, Texas, March 1970.)
2. Kaiser, E.R. The Sulfur Balance of Incinerators. JAPCA 18,171: 171-17*
(March 1968).
3. Environmental Protection Agency. Standards of Performance for New
Stationary Sources. Federal Register, 36, 247, Part II, 2*876-24895
(December 23, 1971).
4. Environmental Protection Agency. Air Programs; Standards of Performance
for New Stationary Sources—Additions and Miscellaneous Amendments.
Federal Register 39, 47, Part II, pp. 9308-9323 (March 8, 197*).
5. Smith, E.M. Incinerator Emissions—Current Knowledge. A.I.Ch.E. Series
70,137:*56-*6* (197*).
6. Anon. Air Pollution Aspects of Sludge Incineration. EPA-625/*-75-009
(June 1975).
7. Jenkins, R.E. and G.D. McCutchen. New Source Performance Standards,
Environmental Science & Technology 6,10:884-888 (October 1972).
8. Fife, J.A. Techniques for Air Pollution Control in Municipal Incineration.
A.I.Ch.E Symposium Series 70,137:465-473 (197*).
97
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SECTION VII
ECONOMIC CONSIDERATIONS
BACKGROUND AND METHODOLOGY
Plant Size and Other Design Considerations
There are several technically feasible co-incineration techniques, each
of which can be expected to have advantages in specific circumstances related
to the plans for a given area. However, selection of the appropriate co-
incineration system also requires careful consideration of the economics in-
volved. System economics will therefore be developed for a given primary-
secondary wastewater treatment plant in the size range of 75,700-378,500
cu m/day (20-100 mgd) capacity.
The wastewater treatment plant selected as the basis for calculation of
MSS and MMR incineration is the Piscataway plant of the Washington Suburban
Sanitary Commission. This 114,000 cu m/day (30 mgd) plant is located south of
Washington, D.C. in Prince Georges County, Maryland of Piscataway Creek, which
discharges to the Potomac River. Piscataway provides primary and secondary
treatment, and has contracted for the construction of tertiary treatment
facilities. For the purposes of this economic analysis, however, only the
sludge generated by primary-secondary treatment steps at design plant capacity
will be considered.
The sludge generated at Piscataway is based upon an influent flow of
114,000 cu m/day (30 mgd) and Biochemical Oxygen Demand and Suspended Solids
loadings of 23,000 kg/day (50,000 Ib/day) each. The sludge is dewatered by
vacuum filtration to a solids concentration of 20 percent. Lime, polymers,
and ferric chloride are used to condition the sludge before dewatering. The
design sludge quantity is 28,000 kg (61,700 Ib) dry solids per day, or
HO,000 kg (308,500 Ib) wet sludge per day at 20 percent solids.
The population of Prince Georges County was 661,192 on 1 April 1970. At
1.24 kg/capita/day (2.74 Ib/capita/day), the domestic MMR generated in the
county is about 820,000 kg/day (1,810,000 Ibs/day). Note that not all the
wastewater sources in the county discharge to the Piscataway treatment plant
at the present time. For a valid assessment of co-incineration economics, it
is inappropriate, therefore, to include al1 the refuse generated by county
residents, when less than half of the sludge quantity generated in the same
region is involved. To provide a more reasonable basis for the economic
analysis, we equate the two unit factors: 1.24 kg (2.74 Ib) of MMR per capita
per day and 379 I/capita/day (100 gpcd) of wastewater. Consequently, with
98
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a wastewater flow of 114,000 cu m/day (30 mgd), the corresponding refuse gener-
ation is 373,000 kg (822,000 lb) of MMR per day.
The refuse-to-sludge ratio, on this basis, is 2.67:1. Since the feasi-
bility study showed that co-incineration was practical at a refuse-to-sludge
ratio of 2.43:1 (or higher) with 20 percent solids in the dewatered sludge,
the proportions are considered satisfactory. Since the county population
will increase by 1980 and the Piscataway plant is not expected to reach full
capacity unitl after 1980, there is clearly sufficient refuse available for
the practice of co-incineration without using the refuse generated by com-
mercial, institutional or industrial sources. The final refuse mix might in-
clude refuse from these sources, but those decisions need not be made for
purposes of this economic analysis.
With a daily sludge generation rate of 140,000 kg (308,000 lb) of de-
watered sludge per day and using 373,000 kg (822,000 lb) per day of MMR to
provide the necessary additional heat, we can compute disposal needs in terms
of a 6-day work week for 50 weeks per year. The MSS disposal rate is thus
170 metric tons/day (187 tpd) using 453 metric tons/day (499 tpd) of MMR for
a total disposal rate of 622 metric tons/day (686 tpd).
For two of the co-incineration techniques, the dewatered sludge is first
processed through direct or Indirect contact dryers, and only the dried solids
are added to the main furnace. The dried sludge, preferably, Is blown into
the furnace and burns largely in suspension. Therefore, the refuse inciner-
ator is required to handle only 453 metric tons/day (499 tpd) of MMR over the
grate surface. Two 227 metric ton/day (250 tpd) furnaces would be a marginal
selection, because refuse is a highly variable fuel. Two 272-metric ton/day
(300 tpd) furnaces, however, would provide an incinerator plant capacity with
a 20 percent safety factor over required MMR capacity. Good practice dictates
designing the incinerators for a high-heat-release refuse.. In this case we
have sized the furnaces for a heat release rate of 71 x 10° kg-cal (280 x 10°
Btu) per hour, or 2,830 kg-cal/kg (5,600 Btu/lb) of refuse. The sludge driers
were also designed with a 20 percent safety factor, over the expected daily
generation rate. Each sludge drying circuit will handle 203 metric tons/day
(224 tpd) of 20% solids sludge.
The two other co-incineration techniques combine the MMR and MSS before
feeding the furnace. In order to make these two systems (Torrax and Multiple-
Hearth) compatible with the first two systems they will be sized for the same
heat release rate of 71 x 106 kg-cal/hr (280 x 106 Btu/hr). The shaft furnace
(Torrax) has been demonstrated on MMR as the basic feed material, but the
multiple-hearth furnace has not been used to Incinerate MMR without feeding
MSS with the MMR. The feasibility of using a multiple-hearth furnace to In-
cinerate MMR alone is thus suspect, but by sizing the various furnace systems
on a common heat release basis, we provide for equivalent air and gas handling
systems.
99
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The design basis used in the following cost estimates fs as follows:
Refuse — 5M metric tons/day (600 tpd)
Sludge — 203 metric tons/day (22k tpd)
Total for Co- Incineration — 7^7 metric tons/day (824 tpd)
Sources of Cost Information
Capital costs reported in this section are based on Weston's experience
in the design and reconstruction of the City of Baltimore Incinerator No. k.
Adjustments in costs have been made, however, to account for items such as
"foundations already in place. Cost figures have been updated to mid-1975.
Cost estimates for various types of equipment were supplied by the following
manufacturers:
Envi ro Tech Corporation
Menlo Park, California
Combustion Engineering, Inc.
Ch i cago , 1 1 1 i no i s
Bethlehem Corp.
Bethlehem, Pennsylvania
Williams, Co.
St. Louis, Missouri
Carborundum Environmental
Hagerstown, Maryland
Multiple-Hearth Furnaces
Rotary Driers
Indirect Driers
Shredders
Systems, Inc.
Pyrolysis Plants
Additions were made to quoted prices, where appropriate, to include cost
of construction and installation, materials-handling equipment, auxiliaries,
and buildings; these additions are based on estimates prepared by the Weston
staff. As with any construction cost estimates made at the conceptual stage,
accuracy will be in the range of + 25 percent.
The accuracy of operating cost estimates is better then that of con-
struction estimates, even when made at an early stage. Manpower requirements
were made with first-hand knowledge of the staffing in Baltimore and Phila-
delphia. Power and water consumption costs were based on design calculations
or on vendor's information. Residual disposal costs were based on Weston's
experience in the design and evaluation of land disposal sites throughout the
U.S. Only maintenance and overhead were calculated as fixed percentages of
equipment, a widely accepted method of estimating maintenance cost for com-
parison of alternative approaches at the feasibility level. Accuracy of the
operating cost estimate should be in the range of + 10 percent.
Discussion of Cost Components
Capital and operating costs have been developed for refuse and sludge in-
cineration separately, and for the four co-incineration techniques examined
100
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in the feasibility study. A figure representing the total disposal cost per
ton of refuse/sludge was also determined. While all facets of the economic
analysis are significant, it is the cost-per-ton of material incinerated which
represents the truest picture of co-incineration cost vs. separate incineration
of sludge and refuse.
Capital Costs-
Capital costs include: equipment, labor and materials for installation,
construction overhead, and contingency. Note that the contingency (at 15
percent) is added on individual equipment modules, rather than as a lump sum
on the total construction cost. Costs for additional capital items are then
added to the direct construction costs. The category designated as Design,
Construction Management and Start-up also includes preparation of manuals,
permits, and other engineering functions, and is estimated at 15 percent of the
direct construction cost. A land cost of $124,000/hectare ($50,000 per acre)
was assumed, noting that the co-incineration plant will be located near the
water treatment plant on prime waterfront property. Legal fees throughout the
entire life of the projected are placed at 3 percent of direct construction
cost. The standard 3 percent bond discount is also included in the total
monies requiring capitalization. Bond life is taken as 20 years, at 7 percent
annual interest.
Operating Costs—
Typical operating costs (mid-1975) were combined to provide the Direct
Operating Cost values used in the comparison. Manpower includes four full
shifts for 7~day per week operation, plus supervision and maintenance.
Salary/overhead ranged from $10,000 to $20,000 per year, with operators and
senior maintenance men at $17,000 per year. An additional 20 percent is added
to the total manpower cost to cover overtime, vacations, holidays, etc. Power
costs are based on 13,000-volt service, at $0.027 per kwh. Water and sewer
costs are combined at $0.10/cu m ($0.37/thousand gal). Fuel costs are based
on No. 2 fuel oil, at $0.10/liter ($0.379/gal), delivered in 15 cu m (4,000
gal) quantities. (The variability of fuel usage eliminates contract delivery
and results in higher fuel prices.) Maintenance cost is estimated at 2.5
percent of installed equipment cost. Plant operating overhead, estimated at
1 percent of installed equipment cost, includes insurance, chemicals, expend-
ibles, janitorial service, etc. Residue disposal cost is based on off-site
disposal, by private contractor. A tipping fee of $2.20 per metric ton ($2/ton)
(solids) and a transportation cost of $0.069 per metric ton-km ($0.10 per
ton-mile) and a 32-km (20 mi) haul bring total residue disposal cost to $4.40
per metric ton ($4.00 per ton).
BASIC COST CALCULATIONS
A series of three cost estimates (Construction Cost, Total Facility
Capital Cost, and Operating Cost) was prepared for each of six equipment
systems:
1. Modern Refuse lncinerator~540 metric ton/day (600 tpd) capacity
2. Multiple-Hearth Sludge lncinerator~203 metric ton/day (224 tpd)
capacity
101
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3. Co-Incineration/Rotary Sludge Drier
A. Co-Incineration/Indirect Sludge Drier
5. Co-Incineration/Multiple-Hearth Furnace
6. Co-lncineration/Pyrolysis.
In each of the co-incineration systems, the refuse capacity was 540 metric
tons (600 tons) and the Sludge capacity was 204 metric tons (224 tons) per
day.
These estimates, presented in Tables 13 through 30, are the basic data
used in the analysis of co-incineration costs.
ANALYSIS OF CO-INCINERATION COSTS
Comparison of Co-Incineration and Separate Incineration
The principal conclusion to be drawn from the economic analysis is that
every cost factor (Capital, Direct Operating, Total Annual Cost) favors co-
incineration, by any of the four techniques; over separate incineration of
sludge and refuse.
Capital Cost--
Capital cost for each of the four co-incineration techniques was lower
than separate incineration by one to two million dollars. While the magnitude
of the cost difference will vary as the capacity of the plant changes from
the capacity used in this analysis, percent differences will remain approxi-
mately the same. Table 31 lists the percent capital cost reduction to be
expected by co-incineration, based on comparison with the combined capital
cost of separate incineration facilities.
The lower capital cost is obtained primarily from the replacement of the
sludge incinerator and related equipment (scrubbers, fans, etc.) by less ex-
pensive sludge-drying equipment in the pre-drying techniques and from incre-
mental capacity (low cost) in the direct sludge incineration techniques.
Sludge pre-drying is low in capital cost compared with sludge incineration.
The dry sludge solids feed to the incinerator represents an increase of only
five percent in the total quantity of solids handled, and an even smaller
percentage on the basis of combustible solids. Since dry sludge solids are
burned in suspension, the actual impact on the refuse incinerator size is
almost negligible. In the direct sludge incineration techniques, on the
other hand, the sludge is actually pre-dried within the incinerator unit.
Direct-drier and multiple-hearth techniques also show lower capital costs than
the indirect-drier and pyrolysis techniques; the latter require the addition
of steam-generating equipment, plus auxiliaries. Of course, all differences
in capital cost are translated directly into annual operating costs (cost of
owning) by application of the capital recovery factor.
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TABLE 13. CONSTRUCTION COST—MODERN REFUSE INCINERATOR
(Design Capacity: 600 tpd; two-tpd units)*
I tern Cost
Two 300-tpd Units:
Furnace & Combustion System $1,462,000
Stack 306,000
Fans/Ductwork 424,000
Piping 469,000
Electrical/Instrumentation 681,000
Conveyors 711.000
Sub-Total $^,053,000
Two Electrostatic Precipitators 2,420,000
Two Bridge Cranes 1,000,000
Building; Including Foundations,
Pit, Office Space, Scales 7.818.000
Direct Construction Cost $15,291,000
Direct Construction Cost
per tpd (Design Cap.) $25,500
* 1 ton = 0.907 metric ton
TABLE 14. TOTAL FACILITY CAPITAL COST—MODERN REFUSE INCINERATOR
(Design Capacity; 600 tpd)*
Item Cost
Direct Construction Cost (DCC) $15,291,000
Design, Construction Management, 2,294,000
Start-up (15% of DCC)
Land ($50,000/acre) 500,000
Legal Fees (3% of DCC) 459,000
Bond Discount Fee (3% of Total Cost) 556.000
Total Facility Cost $19,100,000
Facility Cost per tpd $31,800
(Design Cap.)
* 1 ton = 0.907 metric ton
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TABLE 15. OPERATING COST-"MODERN REFUSE INCINERATOR
(Design Capacity; 600 tpd)
Item
Manpower
(38 Employees)
Power
C845 kwh/hr)
Water (Sewer
420 gpm)
Auxiliary Fuel & Heating
(18,400 gpy)
Maintenance
(2.5% DCC)
Overhead
(U DCC)
Residue Disposal
(150 tpd)
TOTAL OPERATING COST
Cost Per
Ton *
$ 4.16
1.09
0.37
0.04
2.55
1.02
1.20
$10.43
Total Annual
Cost
$ 624,600
164,000
55,400
6,000
382,300
152,900
179,600
$1,564,900
* Based on throughput of 150,000. Divide by 0,907 to obtain
cost per metric ton.
TABLE 16. CONSTRUCTION COST—MULT IPLE-HEARTH SLUDGE INCINERATOR
(Design Capacity! 224 tpd)
Item Cos t
One Multiple-Hearth Incinerator:
Incinerator - 22 ft. diam x 8 hearth, $1,458,000
including venturi scrubbers
Fans/Ductwork 350,000
Piping 383,000
Electrical/Instrumentation 561,000
Conveyors/Ash Handling 583,000
Sub-Total $3,335,000
Building 1,284,000
Oil Storage/Distribution 225,000
Direct Construction Cost $4,844,000
Direct Construction Cost per tpd $21,600
* 1 ton = 0.907 metric ton
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TABLE 17. TOTAL FACILITY CAPITAL COST—MULTIPLE-HEARTR SLUDGE mCtNEMTQR
(Design Capacity: 224 tpd)*
Item Cost
Direct Construction Cost (DCC) $4,844,000
Design, Construction Management 727,000
Start-Up (15% DCC)
Land ($50,000/acre) 150,000
Legal Fees (3% DCC) 145,000
Bond Discount (3% Total Cost) 176.000
Total Facility Cost $6,042,000
Facility Cost per tpd $27,000
(Design Cap.)
* 1 ton = 0.907 metric ton
TABLE 18. OPERATING COST—MULTIPLE-HEARTH INCINERATOR
(Design Capacity: 224 tpd)
Item
Manpower
(24 employees)
Power
(150 kwh/hr)
Water/ Sewer
(275 gpm)
Auxiliary Fuel & Heating
(1,238,400 gal/year)
Maintenance
(2.5% DCC)
Overhead
(n DCC)
Residue Disposal
(IT tpd)
Total Operating Cost
Cost Per
Ton *
$ 6.77
0.52
0.78
8.49
2.06
0.82
0.24
$19.6*4
Total Annual
Cost
$ 379,200
29,400
44,000
475,400
115,400
46,100
13,200
$1,102,700
* Based on throughput of 150,000. Divide by 0.907 to obtain cost
per metric ton
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TABLE 19. CONSTRUCTION COST—CO-1NCINERATION/ROTARY SLUDGE DRYER
(Design Capacity: Refuse 600 tpd; Sludge 22k tpd)*
I tem . Cost
incinerator DCC (as in Table 20) $15,291,000
Drier Circuit:
Rotary Drier, Fan, Cyclone $ 1,477,000
Ductwork 138,000
Conveyors & Pug Mill 278.000
Subtotal 1,893,000
Additional Building 1.370,000
Direct Construction Cost $18,554,000
Direct Construction Cost Per tpd
(Design Cap.) $22,500
* 1 ton = 0.907 metric ton
TABLE 20. TOTAL FACILITY CAPITAL COST—CO-1NCINERATI ON/ROTARY SLUDGE DRYER
(Design Capacity: Refuse 600 tpd; Sludge 224 tpd)*
Item
Incinerator DCC
Dryer DCC
Total Installed Cost
Design, Construction Management,
Start-up (15% of DCC)
Land (50,000 per acre)
Legal Fees (3% DCC)
Bond Discount (3% Total Cost)
Total Facil ity Cost
Facil ity Cost per tpd
(Design Cap.)
Cost
$15,291,000
3?263,000
$18,554,000
2,783,000
500,000
557,000
672,000
$23,066,000
$28,000
* 1 ton = 0.907 metric ton
106
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TABLE 21. OPERATING COST—CO-INC INERATI ON/ROTARY SLUDGE DRYER
Design Capacity; Refuse 600 tpd; Sludge 224 tpd
1 tern
Manpower
(46 employees)
Power
(1265 kwh/hr)
Water/ Sewer
(435 gpm)
Auxiliary Fuel & Heating
(128,800 gpy)
Maintenance
(2.52 DCC)
Overhead
(1* DCC)
Residue Disposal
{161 Ton/day)
Total Operating Cost
Cost Per
Ton *
$ 3.61
1.20
0.29
0.20
2.25
0.90
.94
$ 9.39
Total Annual
Cost
$ 744,000
247,700
59,700
41,200
463,800
185,500
193,200
$1,935,100
* Based on throughput of:
Refuse — 150,000 Tons Per Year
Sludge — 56,000 Tons Per Year
Total — 206,000 Tons Per Year
Divide by 0.907 to obtain cost per metric ton.
TABLE 22. CONSTRUCTION COST—CO-1 NCINERATI ON/INDIRECT SLUDGE DRYER
(Design Capacity; Refuse 600 tpd; Sludge 224 tpd)*
I tern Cost
Incinerator DCC $17,031,000
Dryer Circuit
Two Porcupine Dryers
Model No. (2P-30 X 16) 1,106,000
Ductwork & Blower 87,000
Piping 132,000
Conveyors & Pug Mill 278,000
Subtotal 1,603,000
Additional Building 856.000
Dfrect Construction Cost $19,490,000
Direct Construction Cost per tpd $23,600
(Design Cap.)
* 1 ton = 0.907 metric ton
107
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TABLE 23. TOTAL FACILITY CAPITAL COST—CO-INC I HERAT I ON/INDIRECT SLUDGE DRYER
(Design Capacity; Refuse 600 tpd; Sludge 22k tpd)*
Item Cost
Incinerator DCC $17,031,000
Dryer DCC 2,459.000
Total Installed Cost $19,490,000
Design, Construction Management 2,924,000
Start-up (15% DCC)
Land (50,000/acre) 500,000
Legal Fees (3% DCC) 585,000
Bond Discount (3% Total) 705.000
Total Facility Cost $24,204,000
Facility Cost Per tpd $29,400
(Design Cap.)
* 1 ton = 0.907 metric ton
TABLE 24. OPERATING COST--CO-INCINERATION/INDIRECT SLUDGE DRYER
(Design Capacity: Refuse 600 tpd; Sludge 224 tpd)*
Item
Manpower
(46 employees)
Power
(1265 kwh./hr)
Water/Sewer
(485 gpm)
Fuel
(87,700 gpy)
Maintenance
(2.5* DCC)
Overheat
(U DCC)
Residue Disposal
(161 tpd)
Total Operating Cost
Cost Per
Ton *
$3.66
1.20
.32
.14
2.36
.95
.94
$9.57
Total Annual
Cost
$ 753,600
247,700
65,900
28,800
487,200
194,900
193,200
$1,971,300
-Based on throughput of:
Refuse — 150,000 Tons Per Year
Sludge — 56,000 Tons Per Year
Total — 206,000 Tons Per Year
Divide by 0.907 to obtain cost per metric ton
108
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TABLE 25. CONSTRUCTION COST—CO-tNCMERATtON/MULTtPLE HEARTH FURNACE
(Design Capacity; Refuse 600 tpd; Sludge 224 tpd)*
Item Cost
Shredder:
Two Primary Shredders $ 562,000
Two Screen & Mag. Separator 7*6,000
Two Secondary Shredder 628,000
Conveyors 930,000
SubTotal $ 2,866,000
Pneumatic Conveying System (Shredder to Storage) 366,000
Storage Silo (166,000 cu ft) 1,5*1,000
Four Feed Conveyors (Storage to Furnace) 648,000
Four Multiple-Hearth Furnaces 13,800,000
(22 ft diameter x H ft hearth)
Building A.31*,OOP
Direct Construction Cost $23,535,000
Direct Construction Cost Per tpd $28,600
(Design Cap.)
* 1 ton = 0.907
TABLE 26. TOTAL FACILITY CAPITAL COST—CO-INC INERATI ON/MULTIPLE HEARTH FURNACE
(Design Capacity; Refuse 600 tpd; Sludge 22* tpd)*
Item Cost
Shredding Plant DCC $ 5,*21,000
Incinerator Plant DCC 18,11*,000
Total Installed Cost 23,535,000
Design, Construction Management, 3,530,000
Start-Up (15% DCC)
Land ($50,000/acre) 350,000
Legal Fees (3% DCC) 706,000
Bond Discount (3% Total Cost) 8**,OOP
Total Facility Cost $28,965,000
Facility Cost Per tpd $35,200
(Design Cap.)
* 1 ton = 0.907 metric ton
109
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TABLE 27. OPERATING COST-^CO-INCINERATION/MULT I PIE HEARTH FURNACE
(Design Capacity: Refuse 600 tpd; Sludge 224 tpd)
1 tern
Manpower
(*5 employees)
Power
(2885 kwh/hr)
Water/Sewer
(1310 gpm)
Auxil iary Fuel
(85,300 gal/yr)
Maintenance
(2.5% Incinerator DCC)
(5% Shredder DCC)
Overhead
(U DCC)
Res i due
(161 Ton/day)
Total Operating Cost
Cost Per
Ton *
$ 3.*0
2.72
1.02
0.16
3.20
1.1*
0.9*
$12.58
Total Annual
Cost
$ 699,000
561,200
209, *00
32,300
660,000
235, *00
193,200
$2,591,100
* Based on throughput:
Refuse — 150,000 Tons Per Year
Sludge — 56,000 Tons Per Year
Total — 206,000 Tons Per Year
Divide by 0.907 to obtain cost per metric ton
TABLE 28. CONSTRUCTION COST--CO-INCINERATION/PYROLYSIS
(Design Capacity; Refuse 600 tpd; Sludge 22* tpd)*
Item Cost
Two Shaft Furnaces $18,125,000
Additional Building 1,5*1.000
Direct Construction Cost $19,666,000
Direct Construction Cost (per tpd) $23,900
* 1 ton «• 0.907 metric ton
10
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TABLE 29. TOTAL FACILITY CAPITAL COST—CO-INCINERATION/PYROLYSIS
(Design Capacity: Refuse 600 tpd; Sludge 224 tpd)*
Item Cost
Pyrolysis Plant DCC $19,666,000
Design, Construction Management 2,950,000
Start-Up (\5% DCC)
Land ($50,000/acre) 250,000
Legal Fees (3% DCC) 590,000
Bond Discount (3% Total Cost) 704.000
Total Facility Cost $24,160,000
Facility Cost Per tpd $29,300
(Design Cap.)
* 1 ton = 0.907 metric ton
TABLE 30. OPERATING COST—CO-INCINERATION/PYROLYSIS
(Design Capacity: Refuse 600 tpd; Sludge 224 tpd)
1 tem
Manpower
(42 employees)
Power
(1450 kwh/hr)
Water/Sewer
(300 gpm)
Auxiliary Fuel & Heating
(1,057, 400 gal/yr)
Maintenance
(2.5% DCC)
Overhead
(}% DCC)
Residue
(488 Ton/day)
Total Operating Cost
Cost Per
Ton *
$ 3.15
1.37
0.23
2.14
2.37
.95
0.71
$10.92
Total Annual
Cost
$648,000
282,100
48,000
440,700
491 ,600
196,700
146,500
$2,253,600
» Based on throughput:
Refuse — 150,000 Tons Per Year
Sludge — 56,000 Tons Per Year
Total —• 206,000 Tons Per Year
Divide by 0.907 to obtain cost per metric ton
111
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TABLE 31. SUMMARY OF CO-INCINERATION COST ANALYSIS
N>
Separate Incineration
Refuse M.H. Sludge Rotary
Item Incinerator Incinerator Total Drier
Total Facility Cost 519,100,000 $6,042,000
Annual ized Capital Cost 1,803,000 570,000
Annual ized Capital Cost Per Ton 12.01 10.18
Percentage Savings, based on
Separate Incineration Total
Operating Cost 1 ,564,900 1,102,700
Operating Cost Per Ton 10. 43 19.68
Percentage Savings, based on
Separate Incineration Total
Total Annual Cost 3,368,000 1,673,000
Total Annual Cost Per Ton 22. 45 29.86
Percentage Savings, based on
Separate Incineration Total
Total Annual Cost Per Ton
(Based on Sol ids) 31.18 1*9.37
$25,11(2,000 $23,066,000
2,373,000 2,177,000
11.52 10.57
8.2%
2,667,600 1,935,100
12.95 9.39
27.4*
5,041,000 It, 112, 000
24.47 19.96
18. 4*
42.29 34.50
Co- Incinerat ion
Indirect Multiple
Drier Hearth
$24,204,000
2,285,000
11.09
3.7*
1,971,300
9.57
26.1*
4,256,000
20.66
15.6*
35.70
$23,535,000
2,221,000
10.78
6.4*
2,591,100
12.58
2.9*
4,812,000
23.36
4.5*
40.37
Pvrolysis
$24,160,000
2,280,000
11.07
3.9*
2,253,600 (1,222,600)**
10.92 (5.93)
15.5% (54.2*)
4,534,000 (3,503,000)
21.99 (17.00)
10.0* (30*)
38.04
* Throughput Basis.
** Parenthesis denote credit for stream sale.
+ Divide by 0.907 for cost per metric ton.
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Direct Operating Costs—
The direct operating cost of each of the four co-incineration techniques,
like the capital cost, is less then the corresponding cost of separate inciner-
ation of sludge and refuse. Again, the magnitude of these cost differences
will vary with the size of the incinerator. The percentage differences, as
indicated on Table 31, can be expected to remain nearly constant.
Reductions in operating cost can be accounted for by cost savings in two
major areas: manpower and auxiliary fuel. A total of 62 employees would be
required for independent incineration, while co-incineration requires only
42 to 46 employees, depending on the process. The resulting savings approaches
$300,000 per year, or $1.60 per metric ton ($1.45 per ton) of sludge and
refuse processed. Auxiliary fuel, used primarily in the multiple-hearth
sludge incinerator, also adds to the cost of the separate systems. With the
exception of the pyrolysis plant, co-incineration should result in a fuel
savings of about $450,000 per year, or $2.45 per metric ton ($2.22 per ton)
of throughput. This corresponds to a savings of 4,500,000 liters (1,200,000
gallons) of fuel oil per year. The pyrolysis co-incineration technqiue re-
quires a significant auxiliary fuel input; however, there is a definite
potential for the sale of excess steam from this plant, with the corresponding
benefit of recovery of energy and cost inputs.
Power costs do not have a major impact on cost differential, except in
the case of multiple-hearth co-incineration. Here, power costs are high,
because of the pressure drop in the venturi scrubbers and the power costs of
operating the shredder plant.
Other factors making up the total annual operating cost (water/sewer
service, maintenance, overhead, and residue disposal) do not have a major
impact on the cost differentials between co-incineration and separate in-
cineration of sludge and refuse.
Total Annual Cost—
Total annual cost, in total dollars or in dollars per ton, is the real
indicator of cost differences between separate incineration and the four
co-incineration techniques considered. Table 31 indicates the relative
savings to be expected from co-incineration, as a percentage based on separate
incineration costs. All co-incineration processes show a savings in total
annual cost. The total cost savings is attained as the result of both capital
and direct operating cost reductions associated with co-incineration.
At first glance, it is surprising to note that the cost-per-ton figures
are lower for co-incineration than for typical refuse incineration. It would
be indeed unusual to add complexity to the system and expect to reduce the
processing cost. This anomaly can be accounted for by the water content in
the sludge. Water adds significantly to the tonnage of material processed,
although the water does not add to the actual loading on the incinerator.
Table 31 also indicates co-incineration costs per ton of dry refuse and dry
sludge. As expected, all co-incineration techniques are more costly than
typical refuse incineration, on a strictly solids basis.
13
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Comparison of the Selected Co-Incineration Techniques
Within the four co-incineration technqiues, there are definite cost dif-
ferences. Direct rotary drying represents the lowest-cost option (assuming
no steam recovery for pyrolysis), since it combines the lowest capital and
operating cost of any of the systems, indirect sludge drying follows closely,
with slightly increased capital costs brought about by the required steam
plant. Multiple-hearth co-incineration appears to be the least attractive
alternative from an economic standpoint because of the high operating costs
associated with the plant. Multiple-hearth co-incineration includes high
power costs, high maintenance costs (mostly associated with the shredder
plant), and high water and sewer costs (related to the venturi scrubbers on
the furnaces).
With no credit for steam sales, pyrolysis falls between the pre-drying
techniques and the multiple-hearth co-incineration technique in terms of
processing cost. The high capital associated with this plant is partly
offset by lower operating costs, resulting from reduced manpower requirements.
However, pyrolysis has the potential for sale of the steam generated. If the
steam credit is included, total processing costs could be reduced to about
$19/metric ton ($17/ton), thus making pyrolysis the most economically attrac-
tive co-incineration process. However, since factors other than a steam market
influence the plant location, revenue from steam sales cannot be assured with
any co-incineration process. Therefore, a credit for steam sale should be
taken only when it can be demonistrated that a market for the recovered energy
is avallable within reasonable distances of the feasible co-incineration
plant site.
Cost Trends
In considering cost alternatives for plants which will operate for 10
to 20 years, the effects of expected changes in cost parameters over the
operating period should be considered. The basis for the capital and
operating cost estimates in this Section is mid-1975. The major cost factors
include capital, interest, manpower, fuel and power. Table 32 is a manpower
and utility summary for the four co-incineration techniques.
Of course, It is impossible to predict cost increases for manpower and
commodities with complete accuracy over a 10-year period. The figures
presented below are little more than guesses, necessary to project operating
costs in the future.
Capital and Construction—
There is little doubt that construction and material costs will continue
to rise. However, after the plant- is constructed, increases in capital costs
will not affect the operating cost of the plant.
Increases in materials and construction costs will, however, affect
maintenance costs. For the purpose of this study, we have projected mainte-
nance costs at a constant rate. Actually, these costs will be lower for the
first few years of operation, and then increase sharply as the plant ages.
114
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TABLE 32. MANPOWER S UTILITY SUMMARY
v/i
Separate Incineration
Design Cap. (tpd)*
Throughput, (tpy)*
Employees
Power kwh/hr
Water/Sewer gpm**
Fuel, gallons per year**
Refuse
Incinerator
600
150,000
38
845
420
18,400
Sludge
Incinerator
224
56,000
24
150
275
1,238,400
Total
824
206,000
62
995
695
1,256,800
Direct
Dryer
824
206,000
46
1,265
435
128,800
Co-Incineration
Indirect
Dryer
824
206,000
46
1,265
485
87,700
Multiple
Hearth
824
206,000
45
2,885
1,310
85,300
Pyrolysis
824
206,000
42
1,450
300
1,057,400
* Divide by 0.907 to obtain metric tons per day (year).
** Multiply by 3.79 to obtain liters.
-------
It is often advisable to set up a sinking fund during the early life of the
plant, to offset higher maintenance costs in later stages of operation. On
this basis, increases caused by inflation can be largely offset.
Personnel—
Manpower costs can also be expected to continue their upward spiral into
the foreseeable future. Manpower costs will continue to escalate, for reasons
common to the rest of the economy and as a result of the organizational sup-
port from municipal employees' unions. On the average, we have projected an
increase of 7 percent per year, thus almost doubling the cost of wages,
salaries, and fringe benefits over the next 10 years.
Powei—
Power costs can also be expected to increase over the next few years;
however, the impact of nuclear power plants expected on-stream within the next
decade will partially offset increases in fossil fuel costs. Also, the fact
that incinerators pull a relatively steady load, rather than a sharply peaked
load, will maintain power costs on the low side of the rate schedule. The
substitution of coal for imported oil should also hold down the cost of power
generation. In ten years, we have projected a power cost increase from 2.7$
to 5$ per kilowatt-hour (6.4 percent increase per year).
Fuel —
The cost of fossil fuel is certain to rise as reserves dwindle. Un-
fortunately, auxiliary fuels necessary for incineration and co-incineration
must be petroleum-based products, and there is little opportunity for sub-
stitution of coal for oil or gas in this application. In the absence of major
increases in fuel oil reserves within the United States, fuel oil costs will
continue to be controlled by exporting nations.
Increases in fuel costs will be checked, however, by reduced demand,
substitution of nuclear and third-generation power sources, and the impact of
unchecked fuel costs on the world economy. We have therefore projected an
increase in fuel oil costs to approximately $0.l6/liter ($0.60/gal) during
the next ten years (4.7 percent increase per year).
Water and Sewer Service—
Water and sewer costs should not be a major factor in co-incineration
costs, because the incinerator would be located at or very near a water-
treatment plant, and there should be ample supplies of process cooling water,
at a continued low cost.
Residue Disposal —
The cost estimate has assumed off-site residue disposal by private con-
tractor. With effective incineration, residue should be minimized, and the
actual cost of disposal is not expected to increase sharply. If it does
municipally owned and operated landfills can supplant private operations',
thus maintaining control over residue disposal cost.
Resource Recovery—
On the positive side, new techniques and markets for recovered resources
are expected to reduce refuse processing costs. At present market conditions,
116
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costs could be reduced by as much as one dollar per ton by simple magnetic
separation and sale of ferrous metals. As naturally occurring raw materials
are consumed, and are supplanted by recovered and recycled materials, other
components may find new markets. Table 33 lists 1985 projected processing
costs, based on the inflation factors listed above. There are several
significant conclusions to be drawn from these data:
1. The sensitivity to inflation for all co-incineration techniques
Is approximately the same. Pyrolysis, with steam revenue included,
has the smallest increase in operating cost for the 10-year period.
2. Separate incineration of sludge and refuse is highly susceptible
to inflation, because of the cost contribution of the sludge inciner-
ator. Both fuel and manpower costs contribute heavily to the in-
creases projected for sludge incineration costs.
3. Cost savings expected from co-incineration processes will be greater
in 1985 than at today's costs. Both the actual dollar amount and the
percentage of expected savings will increase as inflation raises the
cost of separate incineration of sludge and refuse at rates faster
than for co-incineration.
k. Inflation does not appreciably affect the relative ranking of the
various co-Sncineration techniques. Thus, a choice of system based
on 1975 economics should remain economically sound in future years.
117
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oo
Item
Operating Cost, $
Manpower
Power
Water/Sewer
Fuel
Maintenance
Overhead
Residue Disposal
Total OOP Cost, $
Debt Service, $
Total Annual Cost, $
1985 Cost Per Ton Of Throughput
1975 Cost Per Ton Of Throughput
% Increase
% Saving Based On Separate
Incineration
Separate Incineration
Incineration M.H. Sludge Combined
1 nc i n .
1,229,000
304,000
55,000
9,000
382,000
153,000
180,000
2,312,000
1,803,000
4,115,000
27.43
22.45
22.2%
746,000
54,000
44,000
753,000
115,000
46,000
13,000
1,771.000
570,000
2,341,000
41.80
29.80
40.3%
1,975,000
358,000
99,000
762,000
497,000
199,000
193,000
4,083,000
2,737,000
6,820,000
33.11
24.47
35.3%
Rota ry
Dryer
1 ,464,000
459,000
60,000
65,000
464,000
186,000
193,000
2,891,000
2,177,000
5,063,000
24.60
19.96
23.3%
25.7%
C o- 1 nc i ne rat i on
Indirect Multiple
Dryer Hearth
1,482,000
459,000
66,000
46,000
487,000
195,000
193,000
2,928,000
2,285,000
5,213,000
25.30
20.66
22.4%
23.6%
1,376,000
1,039,000
209,000
51,000
660,000
235,000
193,000
3,763,000
2,221,000
5.984,000
29.05
23.36
24.4%
12.3%
Pyrolysis
1,275,000
522,000
48,000
698,000
492,000
197,000
146,000
3,408,000
2,280,000
5,688,000(4,211,000)*
27.61 (20
21.99 (17
25.6% (20
16.6% (38
.44)
.00)
.2%)
.3%)
* Parentheses denote credit for steam sale.
+ Divide by 0.907 for cost per metric ton.
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SECTION VIII
CIRCUMSTANCES HAVING IMPACT ON FEASIBILITY
Although the technical and economic feasibility of co-incineration systems
is a basic consideration, there are circumstances, events, and situations of a
less quantifiable type that can affect the desirability or practicality of
implementation:
Geography
Local Political Situations
Cooperation between Public Agencies
Public- and Private-Sector Factors
Government Funding
Relative Acuteness of MSS or MMR Disposal Problems
Local Cost Factors
Auxi1iary Fuels
The following discussions present our qualitative judgments on the impact of
these factors on this implementation of co-incineration in the United States.
"All generalizations, this one included, are no damn good" (Oliver Wendell
Holmes). This quotation is presented not as an escape from responsibility for
the subjective judgments given, but rather to counsel the reader to recognize
that local conditions can cause or prevent actions — even in the face of
"national trends."
GEOGRAPHY
A 1970 EPA study of the air pollution impact of municipal incineration
("Systems Study of Air Pollution from Municipal Incineration," NAPCA Contract
CPA-22-69-23, by W.R. Niessen et^ £]_.) showed that both population and popula-
tion density were significant variables in correlating the incidence of refuse
incinerator construction with local conditions. Further investigation of the
relative impact of these two factors resulted in the derivation of the follow-
ing expression:
y - a(P x P/A)2 + bP
where y is the incidence of incinerator construction
P is population
P/A is population density
19
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Comparison of predicted and actual values produced a correlation coefficient
squared of 0.8. This correlation indicates incinerators (reflecting solid
waste disposal problems so severe as to justify costly volume-reduction prob-
lems) will be constructed where large waste loads (population) and intensive
land use (high population density) occur together. An equally plausible
speculation would equate great economic resources and technical sophistication
to State Economic Areas having large populations. Although no such analysis
for correlating the installation of incinerators for wastewater sludge dis-
posal is available at this time, it appears reasonable to conclude that simi-
lar coincident forces would result in installation of sludge Incinerators.
This suggests that co-Sncineration can be expected to take hold most quickly
in the major metropolitan areas in southern New England, Delaware, New Jersey,
New York, Pennsylvania (Region II); Maryland, Florida, Michigan, Hawaii, and
Wisconsin.
A second geographical factor is the degree of correlation and coincidence
between the jurisdictions responsible for solid waste and those responsible
for sewage treatment. In general, solid waste jurisdictions closely, sometimes
fiercely, follow political boundaries. Sewage treatment systems may follow
political boundaries, but they also reflect the hydrological basins conducive
to gravity flow. Clearly, a congruence of responsibility will encourage co-
incineration, because:
1. A common administrative structure and, often, a common cost dis-
tribution system will obtain.
2. Any economic and/or environmental benefits of joint disposal of MSS
and MMR will more clearly support the collective good.
3. Problems in MSS disposal will be felt by those with MMR responsibility
and vice versa. Thus, seeking after advantageous joint approaches to
resolving the problem will be more obvious and plausible.
Another point relating to geographical Impact arises from the following:
1. Sewage treatment plants (STP) are usually located next to receiving
streams in low-lying areas, which are often poor locations for a
landfill.
2. For a number of reasons (hauling cost, odor in hauling and storage),
the co-incineration plant should-be located adjacent to the STP.
3. For economy, in both work force and transportation expense, it is
desirable to locate the residue disposal site adjacent to the
incinerator.
Thus, co-incineration tends to penalize the MMR disposal function.
LOCAL POLITICAL SITUATIONS
Because of the frequent mismatch of jurisdictions responsible for MMR
disposal and for MSS disposal, it is almost a foregone conclusion that
120
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co-incineration wl11 require interjurisdictional cooperation. Particularly
in the more independent rural states and regions, such cooperation is viewed
with suspicion and open hostility. More than one political figure has been
defeated at the polls because he was "soft" on Independence. Although such an
adverse situation does not hold in all areas of the country, the repeated
failures or, at least, the ponderously slow pace of even regional STP con-
struction is testimony to its impact. Furthermore, co-incineration is capital-
intensive and thus (when landfill is a viable alternative for either MMR or
MSS disposal) will be politically unattractive, especially in hard economic
tImes.
On the positive side, co-incineration may be viewed as "modern," incor-
porating conservation of materials and energy resources. Such positive features
will be useful for political posturing, but, most likely, would not produce a
firm commitment to the concept unless the electorate were unusually enlightened.
On balance, political realities will probably have a net negative impact
on the implementation rate of co-incineration systems.
COOPERATION BETWEEN PUBLIC AGENCIES
In many jurisdictions, the STP agency is organizationally distinct from
the solid waste agency. The former is often an Authority or other regional
organization, whereas the solid waste activity is usually tied closely to
jurisdictions headed by elected officials.
These generalizations are, clearly, more valid when the seat of politi-
cal power is at the state and municipal levels rather than at the county level.
With a strong county government, a STP district and a solid waste organization
to match it are often both under a central control, and, importantly, the
citizens look to a single seat of authority to solve both solids disposal
problems. When this is not the case, or whenever the control or prestige of
elected officials would be perceived to be weakened by joint action, one can
anticipate foot-dragging and "poor cooperation."
The incentive needed for cooperation, and for a resolution of the politi-
cal "costs" previously discussed, may be to identify and publicize a clear-cut
benefit, or at least a lesser cost, associated with a cooperative effort.
Such benefits may be found if the public is we11-enough informed to make non-
support of co-incineration totally unacceptable. Except in rare instances,
however, it is difficult to achieve such a state of public awareness. More
reasonable to expect is the use, by the enlightened official, of a "new land-
fill crisis" to tout the benefits of a solution not requiring landfill. Never-
theless, the official may have to accept some erosion of his power base to
accomplish these goals. Not all are willing to do this.
In summary, needed interagency cooperation can be expected if those re-
sponsible for MMR and MSS both report to a powerful central elected official.
If the senior officials differ, one or the other, or both, will probably fail
to exhibit a strong cooperative spirit.
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PUBLIC- AND PRIVATE-SECTOR FACTORS
In almost all areas, sewage treatment is provided by public agencies
(local, regional, and state). In contrast, many solid waste collection and
disposal services are provided by the private sector, and control of these
private-sector activities is usually limited to environmental protection regu-
lations and zoning control of land use. Generally, refuse collection and
disposal is a very profitable business. The refuse disposal industry is par-
ticularly powerful, both politically and economically, in the urbanized
regions — the same regions where co-incineration generally is both needed
and economical. It is reasonable to expect, therefore, that wherever the
solid waste industry is strong and is deeply involved in solid waste disposal
the industry will exercise its influence to stop, or at least slow the imple-
mentation of co-incineration.
Where the private sector is involved in collection but not in disposal,
its receptiveness to co-incineration will largely relate to the degree that
the hauling time is affected. If, as indicated in the discussion of geographi-
cal factors, the STP/co-incinerator facility is located at some distance from,
say, present landfills, the resultant refuse collection dis-economy will lead
to counterpressure from this group. This situation will be particularly true
if competing firms can make use of more strategically located disposal sites.
Conceptually, such competitive stresses could be balanced by the disposal fee
structure and by exercising control over dumping sites, but such procedures
would undoubtedly be complex and/or legally delicate.
A measure of support from segments of the private sector may be obtained
if co-incineration systems are constructed and/or operated under turnkey or
full-service contracts. Such direct participation will be limited in its in-
fluence, however, and, on balance, the influence of private-sector involvement
in solid waste will have, at best, a null impact and, more likely, will tend
to repress co-incineration.
GOVERNMENT FUNDING
Government funds exert a profound influence over the direction and degree
of capital investment by public bodies. The availability of federal planning
grants often determines the patterns of future development. The federal grant
program, as presently constituted, directly affects co-incineration, because:
•Most early solid waste planning grants were oriented to the development
of state-wide plans. Regional planning was later encouraged, but funds
were cut off before sufficient momentum was attained. Present efforts
in many states are still state- or county-oriented.
•Most sewage and sewage treatment planning has been carried out regionally,
with river basins or other natural drainage boundaries defining the region
and, in time, the sewage service districts.
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•Federal construction grants have been closely limited to wastewater col-
lection and treatment systems, partly through strict Interpretation of
legislation and partly through a lack of interest and a lack of technical
expertise in the needs, disciplines, and problems of non-water media.
At the present time, several major national situations are emerging:
1. Increasing shifts to treatment equivalent to secondary or better
treatment will make MSS disposal a major problem by 1980.
2. Increasingly stringent land disposal requirements (for ground-water
protection) will Increase the difficulty of finding and constructing
landfill disposal sites for MSS. With ocean barging in disfavor and
land spreading an environmental question mark, sludge Incineration
may become the preferred or the only acceptable alternative.
3. Auxiliary fuel for sludge incineration is increasingly costly, diffi-
cult to obtain, and may represent an inferior use to an emerging
national energy policy which emphasizes conservation.
k. MMR landfills, as for MSS, are becoming harder to find and more costly
to operate.
5. Resource recovery, as it is practiced in many co-incineration tech-
nologies, responds to a growing national desire for conservation.
Therefore, if the results of this project and other investigative efforts in-
dicate that co-incineration should play a major role in mitigating the MSS/MMR
disposal problems and in meeting their challenges, a shift in Federal planning
and construction grant policy could be extremely conducive to the rapid,
probably pervasive introduction of co-Incineration as the way to go.
RELATIVE ACUTENESS OF MSS OR MMR DISPOSAL PROBLEMS
Conceptually, a problem with MSS or MMR disposal will, at a minimum,
encourage looking at co-incineration. More likely, however, an MMR problem
will not lead to an investigation of co-incineration unless an MSS disposal
crisis also exists or Is clearly forecast. In contrast, an MSS problem leads
to consideration of incineration, and the projected high fuel costs for MSS
incineration practically demand evaluation of co-incineration. Our judgment,
therefore, realizing the increasing enormity of the MSS disposal problem, is
that the co-incineration option will receive more attention in the future.
Running counter to this forecasted enthusiasm by those with MSS disposal
problems Is the rapidly growing interest in the use of prepared refuse as a
utility boiler fuel or as the feed to a refuse boiler-incinerator which markets
steam. The lower apparent technical risk of the MMR-based energy recovery
approach, the absence of a requirement for interjurisdictional cooperation,
and the political attractiveness of energy recovery make for a powerful com-
petitor for co-incineration, even if the net effect is a not-so-favorable
jurlsdicttonal energy use pattern.
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LOCAL COST FACTORS
Co-incineration is a high-capital-cost, high-operating-cost disposal
method for either MMR or MSS when compared with landfill. To the extent that
there are alternatives, therefore, areas with a high construction cost index
and with powerful public-servant unions will find co-incineration unattractive.
Unfortunately, the same geographical factors which might appear to stimulate
co-incineration often are paralleled with these economic disincentives. An-
other view is that if co-incineration must come its introduction will place
unusually heavy burdens on those with the fewest alternatives.
AUXILIARY FUELS
As mentioned in the preceding discussions and as recommended in recent
EPA reports, incineration, using auxiliary fuels, may become the preferred
method for MSS disposal. The fuels typically used as auxiliaries include gas
and light fuel oil, both of which are in short supply and have rapidly escalated
in cost.
If no major fuel supply crisis (such as the oil boycott of 197*0 occurs
again, it is reasonable to assume that such fuels could be reliably obtained
for MSS disposal, though at a high price. Given the emergence of co-incineration
as a viable technical alternative, if the cost of refuse-energy preparation and
other costs ascribable to co-firing are competitive, co-incineration should take
hold as the preferred alternative. This basic economic decision would probably
be made, albeit more slowly, without the changes in Federal grant policy indi-
cated above. With such changes, however, and with further encouragement by the
Federal energy-related agencies, co-incineration should be expected to achieve
dominance rather quickly, particularly on the eastern seaboard.
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APPENDIX A
ANNOTATED BIBLIOGRAPHY
Reference; Albrecht, O.E. Schlammverbrennung Im Wirbelschichtofen (Sludge
Incineration in Fluidized Bed Furnaces). Chemie Ing. Techn.
A1,10:615-619. May 1969.
Abstract: The fluid bed sludge incinerator in Lausanne, Switzerland was oper-
ational in 1965 and has a capacity of 2,600 kg (5,700 Ib) fi1tercake/hour, with
a peak of 3,120 kg (6,860 Ib) having a maximum water content of 60 percent. The
sludge is fed by gravity through a pipe that penetrates the reactor head into
the furnace freeboard. Besides sludge, it was possible to incinerate other
waste materials, such as oils, coal dust, saw dust, clay, coffee, sludge, soot,
fruit peel, etc. Even refuse incineration experiences were positive. Although
large amounts of solid wood, bottles, slaughter house waste, and other refuse
were incinerated without previous crushing, good combustion was obtained.
Reference: Anderson, J.D. Solid Refuse Disposal Process and Apparatus. U.S.
Patent Number 3,729,298, dated April 2k, 1973.
Abstract; The patent is assigned to Union Carbide Corporation and covers the
oxygen-blown shaft furnace marketed under the tradename of "Purox System."
The inventor discusses the impact of oxygen-enriched combustion air on the
combustion temperature, on the air pollution control equipment, and on the
heating value of the pyrolysis off-gas. There are 25 claims, and STP sludge
is specifically mentioned in the disclosure as disposable along with MMR.
Reference; Anon., Incineration Gobbles up Plant Wastes. Chemical Engineering,
Pages 50-52, October 5, 1959.
Abstract; Dow Chemical Company's industrial solid wastes (including drums),
high- and low-Btu liquid wastes are burned in a k m (131) diameter by 11 m
(351) long rotary kiln incinerator. Combustion is completed in a stationary
secondary combustion chamber. The high-Btu liquid wastes are concurrently
fired to insure burnout of the largely plastic solid wastes and low-Btu liquids.
Refuse provides slag and ash that aids the combustion of the plastics and
liquid chemicals.
Reference; Anon., Sewage Sludge Drying. Bartlett-Snow-Pacific, Inc. Engine-
ering Bulletin No. SE-2, April 1, 1966.
Abstract: At Holyoke, Massachusetts, primary sludge is dewatered in vacuum
filters (with ferric chloride and lime conditioning) to 25~35 percent solids.
Filter cake is mixed with dried sludge in a pug mill and fed to a rotary dryer.
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Flue gases from the MMR incinerator dry the sludge, and the spent gases dis-
charge to the MMR expansion chamber. The dried sludge is screw-conveyed to the
MMR furnace. The dryer can also be fired by #k fuel oil when necessary. The
system was operational in May of 1965.
This Bulletin states, "the dryer was necessary because undried sludge ...
depressed the temperature of the incinerator burning zone too much for proper
operation." The pug mill is also used to prevent "wet cake from forming large
balls in the dryer."
Reference; Anon., Fluid Bed Incinerators Studied for Solid Waste Disposal.
Environmental Science and Technology 2,7:495-^97, July 1968.
Abstract; This article generally reviews the use of fluid-bed incinerators
for disposal of solid refuse. Fluid-bed incinerators offer the following
advantages over conventional grate incinerators: rapid and complete combustion,
minimum excess air (5 percent vs. 250 percent for grate incinerators), smaller
stack-gas cleaning equipment, low concentrations of unburned hydrocarbons and
oxides of nitrogen, large heat sink, compactness, and possible waste heat
recovery (as much as 50 percent of the heating value of the refuse).
During pilot-plant work at West Virginia University, two problem areas
were encountered. Feed of raw solid waste to the incinerator presented some
difficulties, probably because of the small scale of the unit. The largest
problem seems to be removal of residue from the fluidized bed.
Reference: Anon., Kehrict- und Schlammverbrennungsanlage Region DUbendorf
(Incinerator Plant for Domestic Refuse and Sewer Sludge in the
Dubendorf Area). Schweizerische Zeitschrift fur Hydrologie 31, 2,
1969. (Presented at the Fourth Internation 1AM - Congress, 1969,
in Basel, Switzerland, 1969.)
Abstract; The Dubendorf, Switzerland incineration plant is situated on the
property of the sewage purification plant. It has been designed for inciner-
ation of domestic refuse and liquid raw sludge. After treatment in a grinder,
the refuse is mixed with the liquid sludge and the mixture in then incinerated
in a multiple-hearth furnace. Refuse which cannot be ground is burned in an
auxiliary furnace, and the flue gas resulting from this operation is then
burned in the main furnace. The heat resulting from incineration of the refuse
is utilized for combustion of the 1 iquid-s Judge in the same furnace. A wet
process is applied for flue gas purification.
Reference; Anon., Solid Waste Disposal. Chemical Engineering, Pages 155-159,
June 21, 1971.
Abstract; The only reference to co-incineration is the following:
A pyrolytic system for the joint handling of sludge and municipal
refuse was recently put into operation in South Houston, Texas. The
$600,000 system, designed by Waste Control Systems, Inc., an affiliate
of Houston Natural Gas Corp., is capable of handling 100 tons/week.
The sludge is pumped from a tertiary treatment plant to a concentrator
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for dewatering. It is then discharged directly into the pyrolysis
plant. A small tractor is used to push the trash and sludge alter-
nately into the loading devices.
Waste Control Systems, Inc. (a division of Houston Natural Gas), a maker of
rotary incinerators, supplied the unit for South Houston. It was intended to
burn "nearly dry" sludge, not raw sludge. The incinerator can be equipped
with a screw charger for heavier materials such as sludge, and uses waste-heat
recovery to pre-heat incoming combustion air.
Waste Control Systems, Inc. knows of no one co-incinerating refuse and
sludge in any of its equipment.
Reference; Anon., The Reigate Incinerator. Surveyor (London) 138, 130:34-35,
August 6, 1971.
Abstract; In this furnace, which was operational by the end of 1972, the top
hearths are used for drying the sludge, which is introduced to the top hearth.
The shredded refuse is introduced to an intermediate hearth, and the lowest
hearths are used for ash cooling and combustion air preheating. There are four
sludge-drying hearths, two refuse-combustion hearths, and two cooling hearths.
The raw sludge contains 6 to 7 percent solids; no further data are
provided. The article states that oil is used for start-up, and that the com-
bined process is autogenous thereafter. The refuse is shredded, and magneti-
cally separated solids are rejected from the system. Any bulky oversize pieces
that cannot be passed through the pulverizer are placed in an auxiliary furnace,
the exhaust from which goes to the multiple-hearth furnace. The shredded refuse
is charged by a grapple to a conveyor and is fed via chute to an intermediate
hearth on the multiple-hearth furnace. Hot air is also introduced from a
combustor, which is apparently used for start-up only.
Reference; Anon., A 12-Year Record of Achievement in Pollution Control.
Copeland Systems, Inc., Brochure No. CS-14.
Abstract: The Thunder Bay installation features fluidized-bed incineration of
pulp mill wood and bark wastes, capacity 150 metric tons (170 tons) per day,
bone-dry basis. It is located in Thunder Bay, Ontario; The Great Lakes Paper
Company Ltd. scheduled it to start up in 1971.
Designed Feed Rate—Normal Operation
1. Ground wood & sulfite mill sludges
@ 25% solids, tpd, bone-dry basis ... 40
2. Kraft mill sludge § 25% solids,
tpd, bone-dry basis 5
3. Ground wood rejects @ 25% solids,
tpd, bone-dry basis 5
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4. Kraft Mill rejects @ 25% solids,
tpd, bone-dry basis 5
5. Waste wood debris @ kQ% solids,
tpd, bone-dry basis 50
6. Surplus bark @ 35% solids,
tpd, bone-dry basis ......... 20
Total, tpd 120
The Manistique Pulp and Paper Co., Manistique, Michigan installation also uses
fluidized-bed incineration of waste wood and sludge, capacity 27 metric tons
(30 tons) day. The unit was scheduled to start up in 1972:
Design Condi tons
1. Feed rate, tpd 30
2. Feed solids, % 30
3. Combustion temperature, °F . . .1,350-1,500
Reference; Anon., Krefelder Mlil 1 verb rennungsanl age mit KlaYschlammbeseitigung
an VKW/BSH vergeben (VKW/BSH Granted Contact to Build an Incinerator
for Refuse and Sewage Sludge at Krefeld). Staetehygiene 23, 8:4,
August 1972.
Abstract; A new incineration plant is to be constructed in Krefeld, Germany,
initially with two units with a throughput of 12 tons/hour. A third unit of
the same capacity is scheduled for installation at a later date. Sewage sludge
will be incinerated, together with refuse. The sludge will be dewatered in
centrifuges, preheated with flue gases from the furnace, milled, and blown
into the main combustion zone. This will provide complete combustion of the
sludge. The heat will be used for the generation of steam and electricity. An
electrostatic precipitator and a stack 150 m (490 ft) high guarantee the
maintenance of the required emission standards. The plant was scheduled to be
in operation by the end of 1974.
Reference; Anon., Twin Cities to Have Pyrolysis Plant. Public Works Page 56,
October 1973.
Abstract; A plant will be built (1980) in the Twin Cities (Minneapolis-St.
Paul, Minnesota) to dispose of 1,100,000 cu m/day (290 mgd) of sludge and 360
metric ton/day (400 tpd) of refuse by pyrolysis. The useful gas by-products
will be used as fuel and will be sufficient for the complex when run on full-
scale operation. Other by-products will include ferrous and other metals,
fertilizer, and organic chemicals.
Reference: Anon., Refuse Refineries—A Danish Development. Environmental
Pollution Management 4,k: 183-185, July/August 1974.
Abstract: Shredded, cleansed refuse is decomposed in indirectly heated, closed
vertical retorts at 800-1,000°C (1,500-1,800°F). The alkaline portion of the
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refuse reacts with the acid gases, and the pH of the stack gas condensate is
8.5. Liquid wastes can be added to the refuse, but in the prototype plant in
Kalundborg the plans called for simply drying sewage sludge. The solid waste
pyrolysis portion of the system was operation in 1971 and the sludge drying
system in 1972.
Reference: Anon., Solid Waste and Sludge—Energy Self-Sufficiency. Resource
Recovery and Energy Review 2,1:16-17, January/February 1975.
Abstract; Central Costa County, California plans to co-incinerate MMR and MSS
in multiple-hearth furnaces. Ferrous metal and aluminum will be removed from
the MMR and the combustible fraction will be burned. The MSS will be dewatered
by a two-stage centrifuge operation.
Tests were run in November of 197^ at Envirotech's Brisbane, California
Test facility. During these tests, the sludge (primary plus secondary) was
dewatered with some of the refuse and fed to the second hearth on an eleven-
hearth furnace; the balance of the refuse was fed to hearths six and eight.
The furnace was operated under reducing conditions to provide pyrolysis gases
to the after-burner, which raises off-gas temperatures from 430 to 760°C (800 to
1,400°F) to assure complete oxidation of residual wastes. The advantages of
the hybrid system are said to be:
1. The wastewater treatment plant and the solids recource-recovery
facility would be energy self-sufficient in an era of rising energy
prices.
2. There is a direct correlation between wastewater and solid waste
generation. Hence, an adequate future supply of solid waste would
be assured.
3. The integral facility would not be dependent on an outside market
and pricing structure, as is the case with a separate resource-
recovery^plant producing electrical power or prepared fuels.
k. As a combined facility, the solid waste resource-recovery portion
appears eligible in all or part for construction grant funding from
EPA and some State sources.
In addition to co-incineration plans, there are plans to incorporate waste-
heat boilers to power steam turbine-driven aeration blowers and electrical
generators. The balance of the steam produced would be used for plant heating
and air conditioning purposes.
Reference: Bayon, E.J. Sludge-disposal Solution: Thicken, Filter, Dry and
Burn. The American City. June 1966.
Abstract: At Holyoke, Massachusetts, sludge (dewatered in vacuum filters) is
dried in an insulated rotary sludge dryer 2.1 m (7 ft) in diameter and 12 m
(kO ft) long. The dryer uses hot gases from an adjacent 200 metric ton/day
(225 tpd) refuse incinerator to dry the sludge from 70 percent to 20 percent
moisture content before the dried sludge is burned in the refuse incinerator.
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The dried sludge is blown into the incinerator by air jets. The hot gases
from the refuse incinerator are tempered with ambient air to 650°C (1,200°F)
before entering the dryer, and the dryer-vent gases, at 150°C (300°F), are
returned to the incinerator system expansion chamber. One-fourth of the sludge
from the dryer is recycled and mixed with dewatered sludge in a pug mill before
feeding the rotary dryer. The dryer is equipped with an auxiliary oil burner
that is used to supplement or replace the hot incinerator gases when necessary.
Reference; Bergling, S. Combined Treatment of Refuse, Sewage Sludge, Waste
Oil and Nightsoil at LuleS, Sweden. International Solid Wastes
and Public Cleansing Association (ISWA) Information Bulletin
No. 1, pages 25-28, September 1969.
Abstract,; There are two furnace systems at LuleS, Sweden, each consisting of
a 330 metric ton/day (360 tpd) refuse incinerator and a 2.4-meter by 16-meter
(81 x 52') rotary dryer/incinerator capable of handling 15 cu m (530 cu ft) per
hour of thickened sludge. Flue gases from the refuse furnace can be passed
through the rotary drum, which is also equipped with waste oil burners. The
gases from the rotary drum discharge into the refuse furnace flue upstream of
the wet washer air pollution control device. The sludge can be dried or in-
cinerated, but most of the sludge has been incinerated. Odor problems have
been encountered, and waste oil quantities have exceeded projections. The
system was reportedly operational before 1969.
Reference; Burgess, J.V. Developments in Sludge and Waste Incineration.
Process Biochemistry 8,1:27-28, January 1973.
Abstract; The following is quoted from the paper, concerning "combined
incineration";
Whatever methods are used for sludge dewatering and conditioning, the
operation is still a costly part of the disposal process. Removal of
capillary water can be done by thermal means, and if a source of inex-
pensive heat is available, then this can be carried out within the
multiple-hearth furnace. This has led to the development of simultaneous
or combined incineration of high calorific value wastes and sludge. Many
examples of this can now be seen. The Lurgi-Ebingen process combines the
incineration of refuse and sludge in a single multiple-hearth furnace.
In this process household refuse or industrial waste is conveyed directly
to the combustion hearth of the furnace. Liquid sludge with a water con-
tent of 93-95 percent is pumped directly to the top of the furnace and as
the sludge passes through the drying zone, water is evaporated, the heat
being provided by the excess heat from burning of the combustibles in the
solid waste. Waste oils or other organic liquids can be fed direct to the
combustion zone. Many notable examples of this process may now be seen in
Europe. Figure 1 [photograph of plant exterior] shows the installation
now being completed at Reigate which is capable of burning 4-8 t/hr of
sewage sludge and 3-7 t/hr of domestic refuse. A separate furnace for
bulky wastes is provided, the waste gases from this furnace being fed to
combustion zone of the main furnace. The economics of this process are
remarkably attractive if there is a convenient source of solid waste re-
qui ring disposal.
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The Lurgi-Dordrecht process of combined waste incineration permits
simultaneous incineration in a separate furnace. With this arrangement,
the hot waste gases normally leave the solid waste furnace at about 1,000°C
and pass directly to the combustion zone of the multiple-hearth (MH)
furnace. Again, the liquid sludge is fed direct to the top of the MH
furnace and in a similar way to the Ebingen process, the heat from the
refuse passes upwards in the MH furnace, drying the sludge moving down-
wards in countercurrent manner. At Dordrecht, a plant of this type is
now working and will ultimately burn 21 t/hr of refuse and 21 t/hr of
sewage sludge. The process has been used with equal success for industrial
wastes, and an installation at a large chemical works successfully in-
cinerates 1-9 t/hr of biological sludges in combination with 0-5 t/hr of
factory waste.
Reference; Cardinal, T.J., Jr. and P.P. Sebastian. Operation, Control and
Ambient Air Quality Considerations in Modern Multiple Hearth In-
cinerators. Proceedings and Discussions of ASME National Inciner-
ator Conference, 1972.
Abstract^ The authors argue that multiple-hearth sludge incinerators are not
a source of odor (because there is a thermal jump that occurs before odor
evolution and where the sludge to be burned goes from a damp to an ignition
state almost instantaneously).
Further investigation has proved that the sludge cake readily ignites
in a multiple hearth furnace when the moisture content has been reduced
to approximately ^8 percent, thus, staying within the researched ranges.
It is not uncommon, in fact, to see sludge burning vigorously in the
middle of a hearth, and, immediately adjacent to the hot coals, filter
cake from which steam vapor is being evaporated. The average temperature
of this zone would be 1,400°F.
The authors further comment on a proposal that, for the destruction of in-
secticides and polychlorinated biphenyls, EPA may mandate that all incinerators
operate with an exhaust gas temperature of 870°C (1,600°F) and with a residence
time of two seconds. They state, "This requirement can be achieved through the
multiple hearth unit by feeding sludge to the second or third hearths of the
unit and altering the furnace temperature profile and excess air distribution."
The paper further contains considerable information on cost of operation and
maintenance and on aspects of air pollution impact.
Reference; Chapman, R.A. and F.R. Wocasek. CPU-^00 Solid-Waste-Fired Gas
Turbine Development. Proceedings of ASME National Incinerator
Conference,
Abstract:; The authors discuss the combustion of shredded, cleaned, municipal
refuse in a fluid-bed furnace and propose that sludge be disposed of along
with the municipal refuse. "The capacity of each power module would be
significantly increased by using water or sewage sludge instead of excess air
to keep system temperatures below the maximum allowed turbine inlet temperature.
A Solar Centaur turbine is estimated to be capable of consuming 160 tons/day
of solid waste and ^,000 gallons (167,000 1) of water or undewatered sewage
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sludge at 100 percent excess air while generating 3 mw of power..*. Sludge
disposal incomes are site sensitive, and must be evaluated on an individual
basis...."
Reference; Clinton, M.O. Experience wi th Incineration of Industrial Waste
and Sewage Sludge Cake with Municipal Refuse. Proceedings of the
Purdue Industrial Waste Conference, Pages 155-170 (1959).
Abstract.; The Neenah-Menasha incinerator plant (operational: April 1958)
consists of two Combustion Engineering traveling-grate furnaces. The design
capacity of the plant is 1^0 metric ton (150 tons) per day. The sewage from
the treatment plant is vacuum-filtered to a 70 percent moisture content. The
cake is then dried in a flash dryer (Combustion Engineering Raymond flash
drying system) to a 15-20 percent moisture content. It is then separated in
a cyclone and conveyed to the furnace by a belt conveyor and apparently added
to the refuse at the furnace feed hopper. The feed to the furnace is reported
to be: 27 metric ton/day (30 tpd) garbage (80 percent moisture); 76 metric
ton/day (Bk tpd) rubbish (10 percent moisture); and 32 metric ton/day (35 tpd)
sludge (15-20 percent moisture). The gases from the drying circuit are dis-
charged to the breeching before the stack. The hot gases are apparently
drawn from the residue end of the furnaces.
It is reported that the refuse and sludge solids are rich (high heating
value) but that the incinerator will be shut down because of age and the
expense involved in meeting current air pollution control regulations. The
plant has had maintenance problems and uncontrolled combustion because of
variation in the characteristics of the refuse.
The Kewaskum incinerator (operational: October, 195*0 consists of a
Nichols furnace with a capacity of 23 metric ton/day (25 tpd) (batch-feed
presumed). The primary and secondary sludge is dewatered using a vacuum
filter, and the dewatered sludge is added to the raw refuse before charging
the furnace. It is reported that the plant stopped burning sludge in the
furnace after a while, because of insufficient heat supplied by the refuse.
Reference; Cross, F.L., Jr. R.J. Drago, and H.E. Francis. Metal and
Particulate Emissions from Incinerators Burning Sewage Sludge
and Mixed Refuse. Proceedings and discussions of ASME 1970
National Incinerator Conference, pages 189-195, May 1970.
Abstract At the Waterbury, Connecticut plant—two batch-fed MMR furnaces of
140 metric ton/day (150 tpd) capacity each — tests were conducted to determine
the metal and particulate emissions while burning sewage sludge and refuse to-
gether (1:3.5 Ratio), and while burning refuse only. It was found that
emissions from sludge and refuse burning were 1.7 times greater than those
when burning refuse only.
The sludge is dried in a Raymond Flash Dryer system and burned in sus-
pension in the refuse furnace. The APC device is a wetted impingement baffle,
and "the sludge burned is a unique sludge from a highly industrialized area
with a large portion of the industry being metal industry (copper and others)."
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Reference; Davies, G. Altrincham Refuse and Sewage Sludge Incineration Plant.
Public Cleansing 63,5:247-256, May 1973.
Abstract; The 200 metric ton/day (220 tpd) Altrincham incinerator plant in
England(operation in November 1972) consists of two continuous-feed, stoker-
fired, refractory-wall furnaces complete with evaporative gas-conditioning
systems and electrostatic precipitators (ESP's). The furnace is ram-fed, and
the stoker is a two-section Heenan Nichols rocking-grate unit. The raw sludge
at 5 percent solids is sprayed into each furnace through the end walls.
The incinerator plant processes the domestic, commercial, and industrial
refuse from five participating authorities, but sludge from only the Borough
of Altrincham is burned. "The quantities of refuse produced by a given
population is insufficient to generate the heat necessary to burn by direct
spraying all raw primary settled sewage sludge at 5 percent solids content
produced by that same population." The sludge burned is described as "raw
primary settled and humus sludge at the rate of 100,000 gallons per seven-day
week, i.e., 20,000 gpd on a five-day basis, rising to an estimated 24,000 gpd
in the mid 1980's." The refuse population area sludge quantity was estimated
as 4.5 times the 380 cu m/day (100,000 gpd) rate for the Borough.
Landfilling, composting, and incineration were considered, and inciner-
ation chosen. Quotations were requested including three sludge options:
1) dewatering, 2) external thermal drying, and 3) direct injection of thickened
sludge at 5 percent solids. The two low bidders (out of 11) proposed direct
injection as their first choice.
The Authority in 1969 surveyed the refuse heating value; it ranged from
6,100 to 7,000 kg-cal/kg (3,400-3,900 Btu/lb). During acceptance testing
during December/January following start up, the refuse heating value ranged
from 6,700 to 9,400 kg-cal/kg (3,700 to 5,200 Btu/lb), with an everage of 7,700
(4,300). Incinerator design was based upon 8,500 kg-cal/kg (4,700 Btu/lb)
refuse (gross heating value presumably as received). The tested ferrous
metal content of raw refuse was approximately 7 percent, and this was to be
confirmed after several months of operation. (Presumably, these tests were
based upon domestic refuse.)
Design allowable incinerator emissions were 0.1 grains per cubic ft @
N.T.P. Ferrous metals are separated from the residue, baled, and sold. Bulky
refuse is sheared and returned to the pit. Sludge is macerated and kept in
motion prior to injection, to eliminate settlement. Further work on sludge
burning is planned.
Notes; Co-incineration at this site has been abandoned because the small
sludge injection nozzle continually pllugged.
Reference; Defeche, Jean. Combined Disposal of Refuse and Sludges; Technical
and Economic Considerations. 1st International Congress on Solid
Wastes Disposal and Public Cleansing ISWA - PRAHA '72, Theme V,
Pages 3~39, June-July 1972.
133
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Abstract: The paper reports on composting and incineration of MMR and MMS.
European sites are identified and the results of brief tests reported. The
European sites are:
Cheneviers, Geneva (Switzerland)
Dubendorf (Switzerland)
Biilach (Switzerland)
Dieppe (France)
Bienne (Switzerland)
Horgen (Switzerland)
Mannedorf (Switzerland)
The Cheneviers plant in Geneva has two steam-generating incinerators
having a nominal capacity of 180 metric ton/day (200 tpd) of domestic refuse.
Filter-press-dewatered digested sludge is received from the Aire STP and added
to the refuse before burning. "If the proportion of the sludge remains below
20 percent, combustion is stable but difficulties arise with the mixing of the
two products."
At DUbendorf, a multiple-hearth sludge incinerator was used for co-
incineration. "An attempt" was made to use shredded refuse to supply the
required auxiliary heat. There is a similar system in Bulach.
At Dieppe, a von Roll system is being installed. The sludge is dried
using a thin-film evaporator. Flue gas or steam can be used in the
evaporator.
The remaining plants basically were composing systems were a portion of
the wastes could be incinerated.
A series of short term co-incineration tests was conducted at Locarno,
Sutton Coldfield, Rotterdam, Zermatt, and Issy-Les-Moulineaux. At all sites
but Zermatt, the sludge was added to the refuse before burning; at Zermatt,
the 5-10 percent solids mix of primary and secondary sludge was added to the
furnace via a rotating atomizer. Various sludges were tried, and most of
the test results reported few if any problems. The problems encountered were
enumerated as follows:
1. At Rotterdam, the carbon content of the residue increased with co-
incineration.
2. At Issy-Les-Moulineaux, poor combustion, sludge sticking to the
feed hopper, and a loss of steam production was observed during two
tests.
3. At Zermatt, it was difficult to obtain good sludge atomization.
Notes; The Dieppe installation was visited. The Bulach plant was scheduled
for a visitation but could not be visited, because the plant was shut down—
reportedly permanently, since the manufacturer has decided to abandon this
co-incineration approach.
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Reference; Eberhardt, H. European Practice In Refuse and Sewage Sludge Dis-
posal by Incineration. Proceedings of ASME 1966 National Inciner-
ator Conference, Pages 12^-1^3, May 1966.
Abstract; This conclusion is pertinent to co-incineration: "The most economi-
cal solution for the disposal of all waste products of a housing area is the
combustion of sludge together with refuse by using the heat which is released
by the combustion for sludge drying." (This process was apparently not in
practice at that time.) Common American systems for burning sludge (the multiple
hearth, the fluidized bed method, the Raymond flash dryer, and the Passavant
Procedure) are discussed. The European Lurgi process is described as follows;
"By using the Lurgi procedure...drying sludge is accomplished on vacuum drum
filters which are coupled to a centrifuge. Part of the centrifuged substance
is sprayed through the flue gas scrubber; the remainder is mixed with the ash
5n the reaction vessel. The filter cake from the vacuum filter and from the
centrifuge is transported into the furnace by means of belt conveyors."
Reference; Edlin, M. A Refuse - Sewage Treatment Works. The American City,
Pages 89-91, August I960.
Abstract; There are two 73 metric ton/day (80 tpd) continuous-feed, traveling-
grate-fired refractory incinerators and a Raymond flash drying system at New
Albany, Indiana. Sludge conditioned by ferric chloride and lime is vacuum-
filtered to a moisture content of 70 percent, and then dried to a moisture
content of 8 percent. The dried sludge is shipped out or carried to the
refuse incinerators and burned.
Notes; The plant has been shut down.
Reference; Fernandes, J.H. and R.C. Shenk. Solid Waste Fuel Burning in
Industry. American Power Conference, April-May 197^.
Abstract; This paper is a general commentary on waste material firing but
does represent the current thinking of one vendor—Combustion Engineering.
The following is quoted from the paper:
Sludge Drying a_s_ an_ Adjunct fr? a_ Sol id Waste Fi red Boi ler. Of special
interest is the system...which combines in-plant solid waste burning
with plant sludge firing. The shredded solid waste is burned tangentially
in suspension with the larger pieces burning on the dump grate at the
bottom of the furnace. The sludge is dried from 80% to 15% moisture
in a flash drying system and pneumatically conveyed to opposite corners
of the boiler to be burned in suspension. The vent gases from the
drying system are returned to the upper part of the boiler furnace for
deodorization.
Many of the solid wastes found in industry are in the form of sludges
having high moisture contents. The system above is designed to dispose
of these sludges by predrying and burning without causing air or water
pollution or requiring supplemental oil firing.
135
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Reference; Fife, J.A. Sewage Sludge—Another Waste Disposal Problem (Presented
at Symposium on Solid Wastes, New York Department of Health, Albany,
NY, January 29, 1968.)
Abstract: The author presents a brief review of sludge disposal techniques,
including costs and sludge-preconditioning requirements. Improved wastewater
treatment will increase the quantity of sludge. Primary sedimentation produces
1.1 cu m (ifO cu ft) per 1,000 people per day, or 68 kg (150 Ibs) of solids;
secondary sedimentation will increase this to 2.5 cu m (90 cu ft) per 1,000
people per day, or 100 kg (225 Ibs) of solids. Sludge consistency will be
5 to 6 percent.
Pretreatment is required before any disposal. Operations include gravity
settling (most common), flotation, centrifugation, and anaerobic digestion.
Thickened sludge is often dewatered further through the use of mechanical
equipment such as vacuum filters (most common), screens, pressure filters,
and centrifuges. Conditioning chemicals are often used. Sludge cake
moisture from secondary thickening ranges from 55 to 80 percent.
Past methods of sludge disposal have included lagooning, ocean disposal,
landfilling, and composting. The author suggests sludge incineration as the
most desirable means of reducing volume and sterilizing sewage sludge. The
most significant sludge variables affecting incinerator design are calorific
value, percent solids, percent volatile matter, and percent inerts. Moisture
content should be no greater than 75 percent, to insure complete drying and
selt-sustaining combustion. Digested sludge combustion is not self-sustaining.
Temperatures of 650-760°C (1,200-1,^00°F) are required for odor elimination.
Multiple-hearth furnaces, with five to ten hearths, are most widely used.
Rotating rabble arms move the sludge around each hearth until the sludge drops
to the next lower hearth. Burning sludge at the lower hearths provides the
heat input to dry the sludge cake fed at the top. Advantages include moderate
operating cost, durability, flexibility to adapt well to fluctuating loads,
and the capability of handling screenings and grit.
Flash drying in a cage mill, with direct fuel or refuse burning as the
heat source, has also been used. Raw sludge must be dewatered in a vacuum
filter or centrifuge. Sludge is flash-dried with 540°C (1,000°F) gases.
Dry sludge is recycled to reduce moisture and particle size. Moisture con-
tent of sludge from dryer should be no more than 10 percent. This system is
of interest only where dried sludge is co-incinerated with refuse to provide
heat for the dryer.
Fluidized-bed dryers/incinerators can handle sludge with as little as 10
percent solids, with auxiliary fuel required when burning secondary sludges.
Operation should be competitive with multiple-hearth incinerators. Dorr
Oliver and Copeland are equipment suppliers. Units have been installed at
Lynwood, Washington and East Cliff Sanitary District, California.
Spray dryers/incinerators also appear interesting as a sludge disposal
technique, but required feed consistency must be k.5 to 11 percent solids.
Grinding is necessary to reduce particle size to less then 10 microns, for
136
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rapid drying and combustion. Spray units are thermally balanced at 25 percent
solids, but sludge cannot be atomized; sludges of lower solids content require
supplementary fuel.
Co-incineration of sewage sludge with solid refuse appears to be a work-
able solution to the disposal of both materials. Logistics must play an
important role in incinerator site selection. The following plants have
practiced co-incineration.
Refuse
Plant and Location Nature of Sludge Sludge Ratio
Whitemarsh, Pennsylvania 75% water cake 24:1
Frederick, Maryland 70-75? water cake 2.9:1
Waterbury, Connecticut Vacuum filter cake
Neenah-Menasha, Wisconsin 70^ water cake* 4.1
Kewaskum, Wisconsin Vacuum filter cake
Holyoke, Massachusetts Vacuum filter cake
*The 70% water cake is dried to 11% water before firing into traveling grate
furnaces.
Another co-incineration plant has been operated at Hershey, Pennsylvania,
where sludge is ground, separated by adding oil, evaporated, centrifuged,
pressed to 75 percent solids, and then incinerated. A plant in Essen, Germany
flash-dries filter-pressed sludge, which is then co-incinerated with solid
refuse.
Reference; Gater, D.W. Incinerator is Part of Integrated Waste Disposal
System. Public Works 105,5:64-67, May 1974.
Abstract; The 330 metric ton/day (360 tpd) rocking-grate-fired, refractory-
wall incinerator at Stamford, Connecticut will burn dried sewage sludge at
20 percent maximum moisture content. Incinerator capacity is 14 metric tons
(15 tons) per hour, and they plan to burn 1.4 metric ton (1.5 tons) per hour
of dried sludge, i.e. 10 percent of the incinerator capacity. The inciner-
ator is equipped with a spray-cooling chamber and an electrostatic precipitator
designed to remove 95 percent of the particulates, reducing an assumed 3.5:
1,000 ratio of dust to gas (corrected to 50 percent excess air) to 0.175:1,000.
The hot gases will be drawn from the furnace and used to dry the sludge. The
spent gases will be returned to the combustion chamber. The dried sludge will
be injected through the roof arch and suspension burning is expected.
This co-incinerator is now operational.
Reference; Grop, A. Gemeinsame Verbrennung von Klarschlamm und Mull (Common
Incineration of Sewage Sludge and Trash). Brennst Wa'rme Kraft
17,11:262-564, November 1965.
137
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Abstract; The facility in Franklin, Ohio, partially funded by EPA, is a solid
waste plant having a design capacity of 140 metric ton/day (150 tpd). Planned
capacity was 36 to 45 metric ton/day (40-50 tpd) of MMR and 6.3 metric ton/day
(7 tpd) of sludge.
The process features a "Hydropulper" to shred the municipal refuse.
Ferrous and non-ferrous metals, glass, and paper fiber are to be removed a-
head of the press feeding the fluid-bed unit. The. partially dewatered material
from the press is pneumatically conveyed and introduced just above the sand
bed of the fluid-bed incinerator. The municipal sewage sludge is burned with
the residue from the refuse processing plant.
Reference; Herrmann, W., W.A. Stevens, and E.A. Ramspeck. 1700-MW Dickerson
Plant Design Includes Refuse and Sewage Sludge Firing. American
Power Conference, April-May (1974).
Abstract; While the Dickerson plant is not operational (still in the design
stage), i t is of interest since it will be a fossil-fuel-fired steam generator
arranged to burn both shredded refuse and sewage sludge wet solids.
There are two 850-MW gross base load, coal-fired, steam-generating units
side by side. The furnaces are Combustion Engineering tangentially fired units,
14.2 m deep, 29.6 m wide, and 58.2 m high (46i' x 97' x 191'). The furnace
combustion rate is 1,240 kg-cal/cu m (11,080 Btu/cu ft) per hour. The net heat
release rate is 27,000 kg-cal/sq m (72,400 Btu/sq ft) per hour to the vertical
furnace outlet plane. The analysis of the Eastern bituminous coal to be burned
is as follows:
Proximate Analysis (percent) Typical
Moisture 7.0
Ash 14.0
Volatile Matter 21.0
Fixed Carbon 58.0
Total 100.0
Sulfur 1.91 0.5-3.5
Higher heating value, as fired
kg-cal/kg 22,000 20,000-23,000
Btu/lb 12,000 11,000-13,000
Grindabi1ity, hardgrove 60 50-100
Minimum ash softening temperature 1,150°C
H-W (reducing atmosphere) (2,100°F)
138
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The coal Is pulverized in seven bowl mills, each having a capacity of
64,000 kg (140,000 Ib) per hour when grinding the typical coal to a fineness
of 70 percent less than 200 mesh. The pulverized coal is delivered to the
furnace through 56 41-cm (16") fuel pipes and admitted to the furnace located
in the front and rear walls of the divided furnace. Each pulverizer supplies
a single elevation of coal nozzles, one per windbox. The 66-cm (26") wide
windboxwill have an overall height of 12.8 m (42 ft). Shredded municipal
refuse (from which glass, sand, and metals has been removed) may be fired
initially at a rate of up to 10 percent of the maximum continuous Btu input to
the furnace. Typical municipal refuse ranges in heating value from 7,200 to
9,000 kg-cal/kg (4,000-5,000 Btu/lb). The shredded refuse will be pneumatically
introduced to the top level of the eight windboxes. Thus, all the fossil fuel
firing will take place below the level of municipal refuse injection. Each of
the boilers' corners will have a refuse nozzle and a separate refuse conveying
system.
Up to 23,000 kg (50,000 Ib) per hour of wet organic sewage sludge from the
neighboring advanced waste treatment plant can be incinerated. The sludge will
contain approximately 87 percent moisture (13 percent solids) when received by
pipeline at the boiler. The sludge has a heating value of approximately 13,000
kg-cal/kg (7,000 Btu/lb) on a dry basis. Heat input from sewage sludge dry
solids thus will amount to approximately 0.5 percent of the unit's maximum con-
tinuous rated heat input; this will be approximately equivalent to the heat
required to evaporate the sludge moisture. To avoid possible plugging problems,
the sewage must not contain more than 25 percent solids at a size of 100 percent
minus eight mesh and 90 percent minus thirty mesh. The sewage sludge will be
injected into the furnaces through two sewage sludge guns located in opposite
corners of each furnace half and at an elevation of approximately half of the
windbox height. Thus, four coal nozzles will be positioned below the point of
sludge injection, and three coal nozzles above.
Fly ash will be removed by a high-temperature precipitator, and the per-
formance of the hot precipitator is expected to be 99.6 percent removal,
based upon 25 percent ash, 0.7 percent sulfur coal, and one electrical set
out of service in each gas lane. Each gas lane of the precipitator will also
be equipped with an inlet damper so that overall precipitator performance can
be maintained, at reduced load, in the event of a serious electrical failure
in any particular lane.
Sulfur oxide removal is to be accomplished by a wet-scrubber system, to
accomplish 90 percent removal of the S02 in the flue gas with an inlet S02
concentration of 1,000 ppm or more. According to the paper, the specific S02
removal system had not been selected, but the user was looking at the magnesium
oxide process. A news release s.ince that time indicates that the user (Pepco)
has signed a contract with Basic Chemicals for the processing of magnesium
sludge from the S0£ scrubber.
Reference; Hescheles, C.A. Disposal of Wastes from Industrial Plants.
Paper 69 - Pwr 1, ASME-IEEE Power Conference, Charlotte, NC,
*»___*.-. —L. _ _ 1 ft £. rt
139
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Abstract; The author reviews refuse and sludge disposal techniques and presents
two concepts for combined disposal of refuse, sludge, and waste liquids in a
spreader/stoker-fired furnace.
Reference; Hescheles, C.A. and S.L. Zied. Investigation of Three Systems
to Dry and Incinerate Sludge. Proceedings and Discussions of
ASME 1972 National Incinerator Conference, pp. 265-280 (1972).
Abstract.: Three Sludge Incineration Systems are compared: Fludized-Bed,
Multiple-Hearth, and Rotary Kiln Furnaces. The Fluidized-Bed System consists
of a reactor, a dry collector, and a scrubber. The Multiple-Hearth System has
a scrubber, and the third system consists of two rotary kilns: one for sludge
drying and the other for incineration; a scrubber is also used with the kilns.
The three systems were evaluated on industrial and sanitary sludges, and
operating cost estimates were reported, indicating that the Rotary Kiln Furnace
is significantly cheaper than the Fluidized-Bed system and somewhat cheaper than
the Multiple Hearth. Annual costs per 1,000 kg (1,000 Ib) of solids were
estimated as follows:
Rotary Kiln $56.00 ($25.50)
Fluidized Bed $91.00 ($41,50)
Multiple Hearth $65.00 ($29.50)
Reference; Joachim, O.H. Energie aus Mull unter besonderer Betrachtung der
Nutzung zur Klarschlammtrocknung (Energy from Refuse in Particular
Consideration of its Utilization for Sludge Drying). Aufbereitungs
Technik 65: 279-283, May 1965.
Abstracts The author describes two sludge-drying schemes—one of which has
been installed at Gluckstadt, West Germany, using drag-flight conveyors. The
double pass (counter and co-current) direct contact system and the co-current
indirect contact dryer are both heated by the refuse flue gas. Either may be
used or they may be combined, with the off-gas or vapor going back into the
furnace. The author shows a catalytic after-burner in the return gas stream
as an option. He states the dried sludge may be incinerated but provides no
detaiIs.
Reference; Kiefer, B. Gemeinsame Aufbereitung von Kla'rschlamm und Mull (The
Joint Elimination of Sewage Sludge and Waste). Stadtehygien
16,8:179-181, August 1965.
Abstract; At the Ebingen plant, the refuse is first crushed in a hammer mill
and cleaned magnetically, then added at the second and third levels of a Story
(Multiple Hearth) furnace; the sludge is added to the uppermost hearth. The
ratio is 100 tons waste per 36 tons sludge. The sludge is dewatered to 60-
65 percent water. The additional energy necessary for incineration at 700-
800°C (1,300-1,500°F) is introduced in the third and fourth levels (inciner-
ation zone). The sludge is predried in the two upper levels of the furnace;
at the sixth and seventh level the furnace air is preheated and the ash is
cooled. The flue gases, at 600°C (1,100°F), go to a wet scrubber.
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Reference; Knaak, R. and R. Kuhl. Ergebnlsse der Grossversuche mit gemein-
samer Verbrennung von Mull und Klarschlamn in den MUllverbren-
nungsanlagen Kopenhagen und Bonn-Bad Godesberg (Results of Large-
scale Experiments with Joint Incineration of Refuse and Sewage
Sludge in the Incinerators of Kopenhagen and Bonn - Bad
Godesberg). VGB Kraftwerkstechnik 53,A:210-213, April 1973.
Abstract; Experiments were conducted at the Gentofte incinerator in Copenhagen
and at Bad Godesberg to investigate whether sludge can be incinerated jointly
with household refuse, which would reduce disposal cost and obviate the need
to dehydrate the sludge to the degree required by current processing.
The 270 metric ton/day (300 tpd) Bonn-Bad Godesberg plant is a steam-
generating, grate-kiln-fired, continuous-feed incinerator. The Copenhagen in-
cinerator is similar. Four sludges and several dewatering processes were used,
and the sludge was added to the furnaces at ^7"^8 percent moisture. Some sludge
was continuously added to the refuse by a spreader; other sludge was mixed
with the refuse in a trommel and fed to the furnace; at least one full bucket
of sludge was placed in the feed chute; and sludge was also placed manually
(by shovel) into the feed chute during one test sequence. The only problem
reported was the formation of crusts that hindered the drying and burning of
these pieces.
It is also pointed out that waste components such as cans, glass, organics,
etc. help to break up the refuse, similar to the action of the balls in a ball
mill. It is very important to have a good mixture of sludge and refuse.
Reference; Komline, T.R. Sludge Incineration. U.S. Patent No. 3,322,979,
dated May 30, 1967.
Abstract; This patent describes a combined solid waste incinerator and sewage
sludge spray dryer.
Solid refuse is burned in an elongated, traveling-grate incinerator. Above
the incinerator housing is a cylindrical shell, typical of spray-drying towers.
The bottom of this tower opens into the combustion area of the incinerator,
over the traveling grate. Sewage sludge (no consistency reported) is injected
into the top of the tower (most likely through a disc atomizer). The atomizer
must be capable of dispersing the sludge into finely divided particles, to
promote rapid drying. Some of the hot combustion gases are withdrawn from
over the burning refuse through a blower. The hot gases are tangentially
directed into the spray tower; the gases provide the heat input necessary to
dry the sludge. Temperatures should be high enough to permit combustion of
smaller sludge particles in the drying tower. Heavier particles not burned
will evenly deposit over the burning refuse, traveling beneath the drying
chamber. These particles will have been dried and will remove little
additional heat from the burning solid refuse.
Gases from the drying chamber are directed downward and across the burn-
ing refuse. Temperature should be maintained at 760°C (1,^00°F) to ensure
combustion of all odorous materials from sludge drying. A baffle across the
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incinerator solid waste combustion zone helps to divert the drier gases into
the combustion zone. The inventor claims that the baffle also reduces the
particulates entrained in the blower supplying the air to the dryer section.
No operating conditions for sludge and solid waste restrictions were noted
in the patent. No estimate of the sludge to solid waste ratio was recorded.
Comments: This type of facility appears somewhat limited in practice. Sludge
consistency will have to be very low to get good atomization. Even with best
disbursement of sludge, contact time does not appear adequate to achieve good
drying. In view of the rapid cooling of the gases, combustion is not likely
to start in the spray drying section. Deposits of sludge on the walls of the
dryer would be a serious problem. Cooled gases from the dryer pass into the
combustion zone; this cooling may result in poor combustion, and dryer gases
may also be short-circuited back into the dryer with loss of drying capacity.
This unit would appear to have a very low sludge-to-waste-solids ratio.
Reference; Kuchta, H.D. Veraschungsanlage flir olhaltige Schlamme und Rechengot
nach dem Drehrohrofensprinzip (Incinerator for Oily Sludges and
Refuse Using a Rotary Kiln). Brennst. Warme Kraft 18,5:2M-21»7,
May 1966.
Abstract,; A co-current rotary kiln installed in 1962 at Cologne, Germany.
The kiln was designed to burn 65 metric tons/day (72 tpd) of oil sludge and
some small refuse. After installation, the kiln was lengthened in 1963. The
quantity of refuse is not stated, and only small solids are fed. The original
screw feeder was replaced with an inclined chute feed method. The unit was
designed to burn waste oil, but availability problems forced a switch to methane
gas burning. The author reports kiln preheating problems and great difficulty
in maintaining proper combustion temperatures. The author concludes that
uniform, complete combustion is possible only if the refuse is shredded, well
mixed with the sludge, and uniformly fed to the furnace.
Reference: Kurney, W. Die gemeinsame Aufbereitung von Feststaffabfa'l len und
Kla'rschlamm flfr die Rottendeponie, Kompostierung und Verbrennung
(Common Treatment of Solids Waste and Sanitary and Industrial
Sludge for Disposal on Rotting Stockpiles for Composting and In-
cineration). Aufbeitungs-Technik 5:255-260 (1971).
Abstract; The Hazemag impact mill was modified to make possible the reduction
of refuse and industrial waste. The author claims that this permits mixing
solid wastes and sanitary and industrial sludge while comminuting them in the
impact mi 11.
Reference; Lancoud, F. Combined Disposal of Refuse and Sludges; Technical
and Economic Considerations. 1st International Congress on Solid
Waste Disposal and Public Cleansing, ISWA Praha 1972, Thema V.
June-July 1972.
Abstract; The MMR furnace in Geneva, Switzerland was installed in 1967 to
burn refuse and sludge. Raw sludge at 5 percent consistency was thermally
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pretreated at 180°C (356°F) and then filter pressed 6.9 bar (100 psi) producing
sludge cakes containing 40 percent moisture. The cakes were broken to a size
of about 20 sq cm by 4 cm thick (7.9 in. by 3.9 in.).
The sludge was to be burned in a 180 metric tons/day (200-tpd) continuous-
feed, grate-fired, steam-generating incinerator with oscillating knives to
improve combustion. The incinerator manufacturer estimated that input could
contain up to 20 percent pretreated sludge cakes without affecting combustion.
However, there appeared to be an excessive amount of combustibles remaining in
the ash. In June 1969, tests were run on the incinerator burning refuse alone
and refuse containing 10 percent (by weight) sludge cakes. The composition of
the sludge cakes, refuse, and ash from two tests is listed below:
Sludge Analysis
I tern Amount (percent)
Water 41
Ash 40.3
Combustion material 18.7
Fixed carbon 6.5
Volatile substances 12.2
The specific energy of the sludge was 645 kg-cal/kg (358 Btu/lb); of the com-
bustible material, 4,763 kg-cal/kg (2,645 Btu/lb).
Refuse Analysis (percent)
Item First Test Second Test
Water 25.2 28.5
Ash 27.7 34.7
Combustible material 47.1 36.8
Fixed carbon 9.0 6.9
Volatile substances 38.1 29.9
The specific energy of the dried refuse was as follows:
First Test Second Test
Dried Refuse
kg-cal/kg 1,914 1,336
Btu/lb 1,062 2,273
Combustible material
kg-cal/kg 4,392 4,096
Btu/lb 2,438 741
143
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Residue Analysis (percent)
First Test Second Test
Item (No Sludge) 10% Sludge Added
Production of remaining
ashes 40.65 55.85
Water Content 26.3 26.9
Ash 71.6 68.9
Combustible material 2.1 ^.9
Fixed carbon 1.0 2.5
Volatile substances 1.1 2.k
The data indicate that when refuse is burned alone, the ash contained only
2 percent combustibles, but when sludge cakes were included, the residue con-
tained almost 5 percent combustibles. It was assumed that the increased com-
bustible material resulted solely from the sludge. A combustible material
balance indicates that none of the sludge was incinerated. A carbon balance
revealed that only 12.6 percent of the available carbon in the sludge was
burned.
Carbon Analysis (percent)
First Test Second Test
Values (No Sludge) (10% Sludge)
Carbon in refuse 23.73 18.2
Carbon in sludge cakes — 10.^8
Carbon in dried remaining
ashes 2.95 3.91*
Weights of carbon in:
Refuse 21.5 17.3
Sludge cake — 1.03
Dried ashes 0.885 1.61
The tests indicated that the sludge was not satisfactorily destroyed by in-
cineration, because:
1. Larger pieces of sludge cake move faster than refuse through the
furnace; thus, burning time is reduced.
2. A crust formed on the sludge cake, hindering complete combustion.
3. Poor mixing permitted sludge cakes to separate from the refuse,
with poor air contact and resulting poor combustion.
Suggested remedial action:
1. Reduce size of sludge cakes to 2 cm (3 A in.) or less.
2. Employ mechanical action in incinerator to break crusted cakes.
3. Improve mixing of sludge cakes and refuse.
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Reference: Martin, W.S. Sludge and Refuse Disposal. U.S. Patent No. 2,k83,S\B,
dated October 4,
Abstract: The author describes a cylindrical incinerator for the combined
combustion of solid waste, and the drying/combustion of sewage sludge.
The base of this incinerator is designed with a stationary, circular grate
Trash is charged from above through a circular flue. A rotary stoker (10-20
rev/hr) with radial fins continually moves the burning refuse outward, with
ash discharge downward through the grate. Combustion air is preheated in a
waste heat exchanger. The heated combustion air is supplied to the incinerator
through the grate, under the stoker drive, through the stoker fins, and also
through a cone housing mounted above the stoker. The preheated air improves
combustion of the waste with the air brought through the stoker fins, to dry
the refuse before combustion. Hot gases travel upward through the combustion
chamber.
Directly above the combustion chamber is a circular sludge-drying zone.
Sludge with a consistency of 10-50 percent is charged to a rotating, annular
hearth. This sludge is spread and continually agitated by stationary rabble
arms, mounted above the rotating hearth. A portion of the combustion gases is
drawn off below the drying hearth, the remainder passing through the center
opening into the hearth area, where the sludge is dried. In addition to con-
ductive heat transfer, considerable heat is transferred to the sludge by
radiation from the hot firebrick above and around the drying-hearth area. The
rabble arms continually move the sludge towards the center opening. When dry,
it falls from the drying hearth into the solid waste combustion zone.
Vapors escaping from the drying hearth are combined with hot gases from
the burning refuse in a secondary combustion zone, where any noxious materials
in the sludge vapors are destroyed. Hot gases pass through a heat exchanger
to preheat the incoming combustion air, and the hot gases then go out to
the atmosphere.
Notes; Sludge drying in this incinerator appears to be inefficient. There is
very little contact between the wet sludge and hot combustion gases. Radiant
heat transfer from the incinerator wal Is provides surface drying only. Much
of the mechanical equipment inside the incinerator might be a serious mafnte-
nace problem.
Such a furnace was installed in 1950 at Gloucester City, NJ, and was
reportedly operated successfully, but has since been shut down.
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Reference; Matsumoto, K. et al. The Practice of Refuse Incineration in
Japan — Burning of Refuse with High Moisture Content and Low
Calorific Value. Proceedings of ASME National Incinerator
Conference, pp 180-197 (1968).
Abstract; Solid refuse in Japan contains 40 to 70 percent moisture while
typical American or European refuse contains only 10 to 45 percent moisture.
This is due to the significant amounts of garbage (waste food) in Japanese
solid waste. Predrying is required before incineration.
Two predrying methods are used: hot-air and hot-gas, both assisted by
direct-fired surface drying. In hot-air systems, preheated air is forced
through the refuse in a typical grate-fired incinerator. There may be problems
with ignition of some of the refuse on the drying grate. Hot-gas drying re-
circulates part of the combustion gases through the refuse. Since oxygen in
the spent gases is low, ignition temperature is much higher, and early ignition
is not a problem. Higher temperatures also promote more efficient drying.
These are more complex than air-drying incinerators.
The following is taken from the paper:
The difference between these methods is the drying equipment.
In the hot-gas method there is no fear of blow-off (crater)
due to partial burning of the refuse layer, and a conventional
and reliable traveling grate stoker can be employed. Since
the refuse partially burns in the hot-air method, a reciprocating
stoker which properly shakes the refuse layers must be used to
prevent blow-off (crater).
There are various types of reciprocating stokers. As classified
according to type of stoker, the mechanical incinerators in Japan
can be generally classified into following types:
Traveling grate stoker (1)
Chain grate stoker (1)
Reciprocating stokers: (Gas drying = 1)
Stepped grate stokers (1, 2)
Shaking stokers (1, 2)
(von Roll type)
Rocker action grate stokers (1, 2) (Air drying = 2)
Rotating grate stoker (VKW type) (1, 2)
Rotary kiln stoker (1, 2)
The authors state that recirculated gas drying has the following
advantages:
•Flue gas with temperatures higher than air temperatures can be employed
as the drying medium.
•The CO content of the combustion gas can be increased, and the required
capacity of the gas exhaust system can be materially reduced.
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•Greater drying heat can be applied at the same temperatures because the
specific heat capacity of the flue gas with its higher moisture content
is higher than that of air.
•Excessive furnace temperature rise can be prevented by controlling the
amount of recirculating gas.
Reference; McMullen, F.G. Waste Materials Processing System. U.S. Patent
No. 3,769,921, dated November 6, 1973.
Abstract; The Inventor describes a method of incinerating solid waste using
a combination of solid waste combustion on a traveling grate, and suspension
combustion of pulverized solid waste. Pre-dried sewage sludge can also be
burned. A portion of the heat generated is used in the pyrolysis of additional
pulverized solid material, to generate recoverable products.
Trash with a density of 225-255 kg/cu m (H-16 Ib/cu ft) — no moisture
specification — is burned in a typical inclined, moving-grate incinerator.
Pulverized waste material (no pulverizing equipment specified) with a density
of 255-320 kg/cu m (16-20 Ib/cu ft) and a moisture content of 20-25 percent
is pneumatically injected into the combustion zone. Approximately 80 percent
of the pulverized waste is suspension-burned, with combustion air supplied
by the pneumatic injection system. The 20 percent falling to the grate has
combustion air supplied through the grate system. The quantity of pulverizer
trash burned will be regulated by the steam demand on the waste heat boiler.
Sewage sludge, pre-conditioned to a 15 percent moisture content, is also
pneumatically injected into the combustion zone above the grate.
Heat generated during combustion can be used for steam generation or for
pyrolysis of additional pulverized waste material. Gases from the pyrolysis
unit are cleaned in consecutive acid and caustic scrubbers, dried, and light
hydrocarbons are then recovered. Waste heat from the boiler is used in a
flash dryer to dewater the sewage sludge to an 85 percent consistency. Few
details are provided for the dewatering step.
Notes; This patent utilizes processes that are not relatively new; rather,
the patent is a combination of existing incinerator technology. The scheme
for recovery of pyrolysis gases appears questionable. After steam generation
and combustion air preheating, heat remaining in the combustion gases for
evaporation of water from sewage sludge is very limited.
Reference; Moegling, E. Praxis der zentralen Mlilverbrennung am Beispiel
Essen - Karnap (Experience with Central Refuse Incineration at
Essen-Karnap). Brennst WSrme Kraft. 17,8:383-391, August 1965.
Abstract; A plant at Essen-Karnap, W. Germany was designed to burn 2,200
metric tons/day (2,^00 tpd) of refuse and 1,360-1,910 metric tons/day (1,500-
2,100 tpd) of sludge. (See abstract of Weyrauch paper, May 1962.)
Reference; Munro, C.S.H. and T.J.K. Rolfe. The Incineration of Sewage Sludge
with Domestic Refuse on a Continuous Burning Grate. (Part of a
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Symposium held at the University of York, Yorkshire, England);
The Institution of Chemical Engineers Symposium Series No. 41,
pgs. 3.1-3.34, April 1975.
Abstract: The authors review sludge origins, treatment, dewatering, and dis-
posal techniques. They cite two basic methods of introducing sludge into a
refuse incinerator: 1) Spraying thickened sludge into the furnace above the
refuse bed; and 2) by mixture of the sludge cake with the refuse on the grate.
They mention three modern refuse incineration plants in the U.K. where sludge
is being burned or planned for; these are Altrincham, Reigate, and Havant.
The proposed Havant co-incineration system is further described. Tests
were conducted using dual-fluid spray nozzles to atomize thickened sludge.
The tests were conducted with 3.5 percent solids sludge, but thicker sludge is
expected at the site. The thickened sludge would be injected over the refuse
grate to dry and burn in suspension. The sludge will pass through disinte-
grators en route to the spray nozzles. Furnace exhaust temperature will be
maintained at 830°C (1,525°F) to ensure odor destruction and to maintain the
combustion of sludge moisture and low excess air rates used to control furnace
temperatures. The system is expected to be operational late in 1975.
Reference; Nickerson, R.D. Sludge drying and Incineration. Journal of the
Water Pollution Control Federation 32,1:90-98, January I960.
Abstract; The system at Waterbury, Connecticut consists of a circular-type
refuse incinerator and a Raymond flash dryer system. Raw primary sludge is
vacuum filtered. "The sludge is very fibrous. The average grease content on
a dry solids basis has ranged from 23.0 percent in 1956 to 26.3 percent In
1953. It was 23.5 percent in 1957."
The grease and fiber make the material unfit for soil conditioning.
Comments: The original plant was installed in the mid- 1950's. The sludge at
Waterbury contains significant quantities of oils and greases, which are not
characteristic of ordinary municipal sludge. As originally installed, the
sludge being dried was from a primary plant. However, Waterbury has since
gone to secondary treatment, and as a part of the secondary treatment system
multiple-hearth furnaces were installed. Hence, the waste sludge Is not
burned in the multiple-hearth units. Note that the Raymond flash drying
system, while not in use, could be used.
Reference; Novak, R.G. Eliminating or Disposing of Industrial Solid Wastes.
Chemical Engineering, Pages 78-82, October 5, 1970.
Abstract; Dow Chemical Company's industrial solid wastes (including drums and
high and low-Btu liquid wastes) are burned in a 16 x 10^ kg-cal/hr (65 MM
Btu/hr) rotary-kiln incinerator. Combustion is completed In a stationary
secondary combustion chamber. The high-Btu liquid wastes are concurrently
fired to ensure burn-out of the largely plastic solid wastes and low-Btu
1iquids.
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Note; Dow was preparing a second rotary kiln incinerator for operation, with
a target date May of 1975. The new kiln installation was to be very similar
to the present system, and would handle tars, refuse and biological sludge.
Reference; Palm, R. Gedanken zur kombfnierten Schlamm- und MulIverbrennung
in Rostfeuerungen (Ideas on the combined Combustion of Sludge and
Waste Products in Grate Fires). Brennst-Wa'rme-Kraft 18,5:223-226,
May 1966.
Abstract; There has been little progress, to date, in combining the combus-
tion of sewage sludge and solid refuse. This is due to the lack of knowledge:
how the wide variations in sludge composition affect the combustion process;
the characteristics of the slag; and the effects of the products of combustion.
The article discusses the special difficulties encountered from the point
where the charge is prepared, and considers ignition and the combustion process
through to the end product. A number of types of sludge are discussed with
regard to compostion, ignition, and combustion.
Reference; Pepperman, C.M. The Harrisburg Incinerator: A Systems Approach.
Proceedings of the ASME National Incinerator Conference, 1974.
Abstract; The Harrisburg, Pennsylvania Incinerator consists of two 272 metric
ton/day (300-tpd) continuous-feed, stoker-fired, steam-generating incinerators.
Plans are underway to heat-treat, vacuum-filter, and dry the sewage sludge in
an evaporator or steam dryer to a moisture content of about ten percent. The
dried sludge will be pneumatically conveyed and injected into the furnaces and
burned in suspension. "Based on an operating schedule of 120 hours per week
for the dewatering and drying facilities, the refuse incinerator will be
required to burn 3,410 pounds of dry sewage solids at the design sewage flow
of 30.9 mgd. Because of its high heating value, it is estimated that when
dried sludge is burned, it will release sufficient heat to generate 13,600
pounds of steam per hour, which is only slightly less than the quantity of
steam needed to heat-treat and dry the sludge and heat the building."
Reference: Rathgeber, F. MulIverbrennung mit und ohne Energiegewinnung
(Waste Incineration with and without Energy Production). V/asser
Luft und Betrieb 17,9:295-301 (1973).
Abstract: The following is quotation from the translation of the paper, con-
cerning "Combined incineration of refuse and settled sludge":
Anti-pollution centers have the purpose of removing, with respect to
rendering harmless, both refuse and settled sludge. With correct planning,
such plants also provide extensive heat utilization. The use of waste
gases from refuse incineration makes the mechanical removal of water from
previously 'conditioned' sludge more feasible. Subsequently, it can also
be dried by the use of hot waste gases, so that it can be burned.
A thermal process (Seiler-Koppers system) facilitates the drying of
fresh sludge, noxious sludge, and aerobically stablized, biologically
active sludge from community sewage treatment. Under certain conditions,
it is also possible to dry sludge from factory or industrial sewage
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treatment plants. At the outset, the sludge has an Initial water content
of 50-90 percent, but It is only 5-15 percent after drying. The drum
dryer consists of three concentric cylinders. They are equipped with
longitudinal ribs with a saw-tooth-profile cross section. Air, and the
sludge to be dried, are rotated within the drum by 180° twice. The
sludge particles, now dry and, therefore, lighter in weight, are con-
tinuously sucked away. The dried material, as well as the sucked-away
air with the vapors, are brought into a cyclone in which the dried
material is removed from the air and the vapors. After being separated,
the dried material is transported to a storage silo or, possibly, back
to the double shaft mixer installed ahead of the dryer. Here it is mixed
with the wet sludge and recirculated to the dry drum. According to need
for supply, the dried matter can be placed in paper bags or put into a
storage silo.
In combined refuse and sludge incinerators, the waste gases can be
used for thermal drying of the sludge in the installation described
above. The dried sludge can be sold as germ-free soi1-improving material,
or incinerated with the refuse. The vapors form the dryer are also
introduced into the furnace in order to destroy malodorous substances.
Where the local conditions do not permit the use of dried sludge, and
the proportion of settled sludge to refuse is low, an incinerator com-
bining grates with a rotary drum as the next stage is recommended. This
eliminates the sludge dryer. In such cases, the water content of sludge
is reduced 70-80 percent by purely mechanical means, and the sludge is
burned with the refuse. Incineration tests in various plants have shown
that up to 20 percent of merely mechancial-dried sludge can be added to
the refuse. Good combustion is still achieved by these proportions;
the furnace temperature does not drop below 850°C. This temperature
is necessary to ensure the breakdown of malodouous material.
A plant built for an association in Switzerland solved the problem of
sludge removal. Since this plant deals with large quantities of indus-
trial sewage, it was impossible to break the sludge down biologically.
It is therefore dried and burned. Vacuum filters are used for water
removal. After a flocculation agent and lime are added to the sludge,
the filters reduce the water content to approximately 70 percent. The
dried sludge is carried to an incinerator drum which combines a dryer
and a furnace portion lined with refractory material in one unit. In
future expansion plans, the waste gases from a yet-to-be-built refuse
incinerator will be used to dry and incinerate the sludge. Until this
facility is built, the requisite temperatures are achieved by fuel oil.
The resulting vapors are first rendered dust-free in a washer, after
which the malodorous substances are removed in a catalytic after-burner.
A fuel-saving temperature of approximately 300°C is sufficient.
Reference; Reilly, B.B. Incinerator and Sewage Treatment Plant Work To-
gether. Public Works 92,7:109-110, July 1961.
Abstract; The original Whitemarsh Township, Pennsylvania Incinerator was a
refractory-wall furnace with tubes buried in the refractory walls in order to
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cool the walls. The 272 metric ton/day (300-tpd) furnace was a stoker-ffred.
continuous-feed unit, and was equipped with an impingement-tray-type scrubber.
The raw sludge from the adjacent STP was dewatered, using a vacuum filter,
before being mixed with the refuse. The sludge is both primary and secondary
from the trickling filter STP. The sludge cake from the vacuum filter was
mechanically mixed with the refuse before burning. No details on the mixing
device were included in the article.
Design conditions were as follows: "At design loads, the breakdown is
10.9 metric ton/hr (12 ton/hr) of refuse and 0.45 metric ton (0.5 ton)
of sludge. Thus, even though the sludge cake is 75 percent water, only 0.34
metric ton (0.38 ton) of moisture is added to the charge, resulting fn a
total moisture increase of three percent."
Performance test data are as follows:
Interval: Five operating days, September 22-28
Total Refuse Burned: 346.7 metric tons (381.4 tons)
Total Sludge Burned: 29.1 metric tons (32.0 tons)
Total hours operated: 31~5
Combustible material not destroyed: 4.36 percent
Particulate matter in stack gas: 0.0572 kg 1,000 kg of gas, or
69.8 mg per std cu m of gas (0.0305 grains per std cu ft of gas).
Thus the sludge-to-refuse ratio in design was 4.2 percent, and they achieved
8.3 percent under test conditions. As the sewer system was a new one, a high
proportion of mud appeared in the sludge.
Note; The Whitemarsh incinerator was extensively rebuilt and it is understood
they have not attempted to burn sludge since the rebuilding and, in fact,
probably did not use it extensively for sludge originally. The reasons are
unknown.
Reference; RUb, F. Moeglichkeiten und Beispiele der kombinierten Verbrennung
von Mull und Abwasserschlamm (Possibilities and Examples of a
Combined Incineration of Refuse and Waste Sludge). Wasser Luft
and Betrieb 14,12:484-488, December 1970.
Abstract; Four of the five systems briefly described are burning sludge and
refuse. These four are:
1. Multiple hearth, with MSS added to shredded MMR before charging top
hearth (Uzwil, Switzerland)
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2. Multiple hearth, with MSS dried on upper hearths and shredded MMR
charged to mid-hearths; lower hearths cool ash and preheat combustion
air. (Zurich, Switzerland)
3. Keller-Peukert system (similar to Raymond flash drying system), where
the dried sludge is metered into the charge hopper of the refuse
furnace. The dryer uses hot air that passes through a heat exchanger
at the refuse furnace exhaust. The dryer vent gases are introduced
into the windbox under the refuse furnace grate system.
k. Rotary kiln system handling sludge and refuse.
Reference: Salamon, 0. Process for the Concomitant Incineration of Solid
Refuse and of Aqueous Sewage Sludge. U.S. Patent No. 3,552,333,
dated January 5, 1971.
Abstract; The inventor proposes a method of drying sewage sludge in a thin-
film evaporator, utilizing heat generated by the combustion of solid waste.
The dried sludge is then to be incinerated along with the solid waste fuel.
Per capita solid waste is estimated at 0.68-2.0 kg/day (1.5-^.5 Ib/day),
with a heating value of 1,000-2,500 kg-cal/kg (1,800-4,500 Btu/lb). At the
same time, 1.0-2.0 kg/day (2.2-4.4 Ib/day) of 8 percent consistency sewage
sludge per person are generated. The inventor estimates that only 60 percent
of the potential heating value in the solid waste can be used for evaporation
purposes (430-3,020 kg-cal -- 1,700-12,000 Btu — per person per day), due to
the need to maintain a gas temperature of 300°C (570°F). Heat required for
evaporation and superheating of a per diem quantity of sludge is estimated at
600-1,310 kg-cal (2,600-5,200 Btu) per person per day (further reduced to 500-
1,160 kg-cal — 2,000-4,600 Btu — if the solid residue is also burned).
The sludge is to be pre-dried in a thin-film evaporator before combustion,
No details of the evaporator are provided. Combustion gases provide the heat
input into the evaporator. Heat is supplied directly from the hot gases or
through the use of an intermediate waste-heat boiler which produces steam to
drive the evaporator. In addition, multi-effect evaporators are proposed to
increase the quantity of sludge which can be evaporated with same heat input.
The incoming sludge has a consistency of 8 percent. As described, the sludge
is almost completely dried. Sludge solids are then mixed with solid waste
refuse and burned in the incinerator.
Notes; Complete drying of waste sludge, even in a thin-film, wiped-surface
evaporator, would be a serious problem because of charring of the sludge on
the evaporator walls. Multi-effect evaporators operate only when there is a
boiling point elevation in the material being evaporated. In a two-phase
sludge case, this is probably not the case. Even if effective, multi-effect
evaporation leaves the condensed vapor from the second effect, and would
contain noxious materials and would have to be returned to the sewage treat-
ment plant.
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Reference; Samuels, L.J. New Look at Sewage Disposal. Western Precipitation
Group, Joy Manufacturing Co., Los Angeles, California (No date).
Abstract; This equipment supplier suggests predrying sludge in its Holo-
Flite conveying dryer before landfill disposal or incineration. The conveying
dryer has a jacketed shell and hollow screws, which were heated by 320°C
(600°F) oil during their experiments. Digested sludge at 62.7 percent moisture
and vacuum-filtered raw sewage sludge at 75.5 percent moisture were dried in
pilot-scale equipment. Heat transfer coefficients ranged from 170-56.8 W/sq m -
°C (30 to 10 Btu/hr-°F-sq ft) and dropped rapidly with loss of moisture in the
sludge.
On a commercial basis, they suggest using a five-pass unit (7.9 m — 26
ft long) to dry 11,3&0 kg (25,000 lb) of raw sludge from 77 percent to 25
percent moisture. After drying, the sludge can be disposed of in landfill or
incinerated to make up the heat for drying.
Notes: Although they report no deposits in the dryer, test runs were only one
hour. Charring of the surfaces and further loss of heat transfer should be
expected. Evaporation of the water (not counting preheating to 100°C — 212°F)
requires 85 percent of heat value of the sludge solids. While this would appear
to balance, no consideration is given to heat losses from equipment, enthalpy
of stack gas, or heat content of the ashes.
Reference; Schlotmann, W. Klarschlamm, seine Behandlung und Beseitigung
Speziell durch gemeinsame Mul 1-Klarschlamm-Verbrennung (Sewage
Sludge, its Treatment and Disposal Especially Through Combined
Refuse — Sludge Incineration). Met Ingenieursblad, 42,10:304-
312, May 1973.
Abstract; Mostly theoretical and state-of-the-art. Co-incineration is ad-
vantageous. Sludge-to-refuse ratios are given, considering heat values and
moisture content. The process yields sterile ashes which are dumped.
Five combinations of processes to co-incineration refuse and sludge are
discussed:
1. Sludge having a water content of 90-96 percent, coming directly from
the thickener, is delivered directly to the incinerator. The sludge
has to be prepared to incineration with the refuse. This is to be
done with optimal utilization of the excess heat from the refuse in-
cineration. The sludge first goes into a heat exchanger, then into
a thin-layer drier; by then, it has a moisture content of 40-60
percent. The vapors go into the incinerator and are heated to 800°C
(1,470°F). The sludge is further dried in a mill-drier. In the
second drying step the water is reduced to 15-20 percent. The sludge
is then blown into the incinerator.
2. The sludge is mechanically dewatered (60-75 percent), then goes to
the crusher-drier and is dried to a maximum of 20 percent with hot
air from a heat exchanger. Depending on the ratio of sludge-to-
refuse, the dried material is blown directly into the incinerator
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with the drier air and the vapors; otherwise, there would be a
separation of the dried sludge from the arivapor mixture.
3. A mill is used in place of the crusher-drier. This is practical only
for large amounts of sludge.
4. Sludges coming from two different plans are to be processed. Plant 1
is quite close to the incinerator. Plant 2 is 10-14 km (6.3-8.8 mi)
away from the incinerator.
a) The sludge was transported as liquid.
b) The sludge was dewatered mechanically and the filter cake was
transported.
5. Sludge and refuse are co-incinerated. In addition, sludge is also
incinerated by itself in a fluid-bed incinerator, after being
mechanically dewatered.
Reference; Synder, N.W. Energy Recovery and Resource Recycling. Chemical
Engineering, pp. 65-72, October 21, 1974.
Absjract^; The author reviews the status of the various solid-waste disposal
plants, some considered advanced technology and others considered experimental
technology. There is specific mention of the use of dewatered sewage sludge
in the demonstration plant for the Purox process being piloted on a full scale
by Union Carbide Corporation's Linde Division in South Charleston, West
Virginia.
Reference; Stephenson, J.W. Burning Wet Refuse. Proceedings and Discussions
of ASME National Incinerator Conference, pp. 260-264, June 1972.
Abstract; A survey was conducted to investigate wet-refuse burning problems.
The results from fifteen plants of different sizes, types, and ages are dis-
cussed in the paper. The main problems are: start-up, slow burning, and low
heating value of refuse. The author concludes that preheated combustion air,
automatic control of underfire and overfire air, auxiliary fuel burners, and
grate operation are all necessary to burn wet refuse.
Reference; Storck» W.J. Sludge id Beautiful in the Twin Cities. Water and
Wastes Engineering. 11,7:43-46, July 1974.
Abstract: Minneapolis/St. Paul is planning a sludge pyrolysis unit which
will use municipal refuse as the source of fuel for the processing of sludge
and for production of both fuel char and process char, which can be used in
augmenting the overall treatment in sludge disposal systems. Pyrolysis will
produce two important and useful products: off-gases that can be used as fuel
to support the process; and a carbonaceous residue that can provide carbon
suitable for fuel and/or activation, thus providing a source of absorptive
media for use in wastewater purification. The process is described as mfxtng
sludge and refuse, drying, and feeding the mixture to a rotary kiln. The kiln
is externally heated by a furnace that burns the gaseous and liquid fuels
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derived from the pyrolysis off-gases. The mixed material is thoroughly agi-
tated and exposed to a temperature of 680°C (1,250°F) for 30 to 60 minutes,
driving off volatiles and leaving a char material consisting mainly of carbon
and ash. The kiln off-gases are collected and processed in a gas-cleaning
system that condenses the tar and oils. The remaining gas is suitable for fuel
gas, having a heat value of approximately 4,000 kg-cal/st cu m (450 Btu/st cu
ft). The tar and oils condensed from the pyrolysis off-gas stream are stored
in tanks and burned within the plant. The char produced by the pyrolysis kiln
is processed into three fractions: char suitable for processing into activated
carbon; fuel-grade high ash char for delivery to the primary sludge inciner-
ators; and a residue of inert ash to be disposed of in a landfill. The pyro-
lysis system includes equipment for sorting valuable materials from the refuse
stream, including cellulose products, ferrous metals, glass, and aluminum.
Reference; Stovall, J.H. and Berry, D.A. Pressing and Incineration of Kraft
Mill Primary Clarifier Sludge. TAPPI, 52,11, November 1969.
Abstract: The paper describes the disposal of sludge from the paper industry;
dewatered sludge is mixed with bark and burned i-n hogged-fuel-fired boilers.
These installations are at Mobile, Alabama; Georgetown, South Carolina; and
Vicksburg, Mississippi.
Little data are presented on the co-incineration aspects of the oper-
ation, but at the Mobile Mill the sludge is dewatered in a vertical screw press
to 30-35 percent solids, mixed with bark and burned in a small 45,500 kg
(100,000 Ib) per hour boiler. When burning bark and sludge, a 20 percent drop
in steam flow is observed, although sludge feed rate is unknown.
Reference; Sumner, J. and D.H.A. Price. Combined Incineration of Refuse and
Sludge. Proceedings of a Symposium at the University of
Southampton, U.K., January 1972.
Abstract: The authors discuss the pros and cons of co-incineration. Includ-
ed are the comments of attendees. The presentation and discussions are general,
although the authors conclude that further experimental work is necessary and
that co-incineration is likely to prove more economical than separate inciner-
ation methods.
Reference; Sussman, D.B. Baltimore Demonstrates Gas Pyrolysis-Resource Re-
covery from Solid Waste. U.S. EPA Report S.W.-75d.L. (1975).
Abstract: The prototype 909 metric ton/day (1,000-tpd) Monsanto "Landgard"
pyrolysis plant in Baltimore, Maryland is described. The full-scale plant was
based upon a 32 metric ton/day (35~tpd) plant operated in St. Louis by
Monsanto. The pyrolyzer is a 5.8-m (19 ft) diameter by 30.5-m (100 ft) long
rotary kiln. The pyrolysis gases are burned in an afterburner and pass through
a waste heat boiler. The steam is exported for sale, and the residue is
quenched with water. Sewage sludge was pyrolyzed successfully at the pilot
plant, and co-incineration may be further demonstrated in the full-scale unit.
The facility is in start-up and is scheduled to be turned over to the City
in 1975-
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Reference; Tanner, R. A Process of Incineration of Solid Waste, in Particular
Refuse, and Hygienic Destruction of Sewage or Liquid Sewage Sludge
and an Incineration Furnace for Carrying the Process into Effect.
U.K. Patent No. 1,165,349, dated 24 September 1969.
Abstract; This patent describes a basic solid waste incinerator into which
sewage sludge is sprayed to maintain a safe operating temperature and to
dispose sewage sludge. The inventor notes that the feeding value of solid
waste refuse has been steadily increasing from a historical value of 2,600-
4,200 kg-cal/kg (1,440-2,340 Btu/lb) to a present range of 4,900-6,500 (2,700-
3,600). This increase results from an improved standard of living and in-
creased use of "disposables". Higher heating values have resulted in exces-
sively high temperatures within the incinerator combustion zone, which have
limited the capacity of existing incinerators. While excess air could be
used to cool the combustion chamber, the existing units generally do not have
the blower capacity or stack-gas cleaning capacity to reduce the temperature
of the combustion chamber.
As a solution, the inventor suggests spraying raw sewage sludge, at 8
percent solids, into the combustion chamber over the burning refuse. Excessive
heat is thereby consumed in the evaporation and superheating of the water in
the sludge. At the same time, noxious sewage sludge is continually disposed.
Available solid waste was estimated at 250 kg (550 Ib) per person per year.
At the same time, 350 kg (770 Ib) of sewage sludge, at an 8 percent con-
sistency (30 kg — 66 Ib — of solid), are generated per person per year.
Drying, evaporating, and superheating the water in the sludge to 300°C (570°F),
50-66 percent of the available heat output of the burning refuse is consumed.
The raw sewage sludge is simply sprayed into the combustion chamber, above the
burning refuse. The sludge may also be indirectly preheated by combustion
gases before being sprayed into the incinerator. A waste heat boiler can also
be used to recover some of the heating value of the refuse. In this case,
the quantity of sludge to be dried would be reduced to the point of safe
operating temperature in the incinerator.
Notes; The thrust of this patent is the control of temperature in the inciner-
ator, rather than the disposal of sewage sludge. Water could more conveniently
be used for the same purpose.
Reference; Tanner, R. Process and Mechanical Equipment for the Concomitant
Incineration of Solid Refuse and Aqueous Sewage Sludge. U.S.
Patent No. 3,529,558 dated September 22, 1970.
Abstract; The inventor proposes a means of p retreat ing sewage plant sludge
for ultimate co-incineration with solid waste. Heat requirements for pre-
treatment are supplied by the heat of the combustion.
Tanner reports available solid refuse in the range of 0.7-2.0 kg (1.5-
4.4 Ib) per person per day. This waste would contain approximately 30 percent
non-combustibles, 30 percent water, and 40 percent combustible materials. Com-
bustibles would have a heating value of about 13,000 kg-cal/kg (7,200 Btu/lb)
bringing the overall heating value of the solid waste to 45,000 kg-cal/kg
(2,500 Btu/lb).
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Sewage sludge was estimated at one to two kg (2.2-4.4 Ib) per person per
day, with a solids content of 8 percent. Half the sludge solids are combus-
tible, with a heating value of about 13,000 kg-cal/kg (7,200 Btu/lb).
In the process described, solid waste is burned in a typical incinerator.
The combustion gases are used to indirectly heat the sewage sludge, concen-
trating it from a consistency of 8 to 15 percent solids. The thickened sludge
can then be sprayed into the solid waste incinerator and burned. The vapor
from the evaporator is combined with hot gases from the incinerator to destroy
any odorous material.
Alternatively, the evaporator could be of a pressure type. If the sludge
is evaporated at the temperature of 150°C (3009F) or higher (about 5.9 bar ~
85 psi), a conditioning takes place which permits further mechanical dewatering
of the sludge. The inventor reports that 15 percent sludge from the pressure
evaporator can be thickened to 30 percent in a centrifuge or 50-55 percent in
a pressure filter. The semi-solid thus formed is combined with the solid
waste and burned in the incinerator. The liquid from the mechanical dewater-
ing step, containing noxious material, is then eliminated by spraying it into
the hot exhaust gases from the incinerator.
Notes; The secondary dewatering, after high temperature treatment, may be a
useful concept if a satisfactory means of liquor disposal can be found. Fouling
in the evaporator may be a serious problem. Spraying 15 percent consistency
sludge into an incinerator also seems questionable.
Reference; Tanner, R. Method and Combined Furnace for the Simultaneous In-
cineration of refuse or Garbage and Sewage Sludge. U.S Patent
No. 3,533,305 dated October 13, 1970.
Abstract; This patent describes a two-zone incinerator for the combined
drying and incineration of solid waste and pre-conditioned sewage sludge cakes.
In the proposed incinerator, solid waste is burned in a two-zone, travel-
ing grate. Solid waste is deposited on the first grate, where it is dried and
ignited. The burning waste then drops to a second grate, where combustion is
completed. Combustion air is provided by underdraft.
Sewage sludge is dried In the second zone of the incinerator, above the
solid waste combustion zone. Pre-conditioned sludge with a moisture content
of 40-50 percent is spread, at a controlled thickness, onto a moving grate.
A portion of the hot combustion gases is forced through the grate, thereby
drying the sludge to a mositure content of 20 percent. The dried sludge moves
to the second grate, where it is Ignited and burned. Hot air (unspecified
source) is supplied underdraft.
At the end of the second sludge grate, the residue drops onto the center
of the second solid waste combustion grate, where any remaining combustible
materials are consumed. Hot combustion gases from the burning solid waste
pass over the burning sewage sludge, further heating the sludge and promoting
the combustion. Vapors from sludge drying are also combined with the hot com-
bustion gases, and any noxious materials are destroyed.
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Notes; Use is highly limited, due to the need for very high consistency sludge.
References; Tanner, R. and W. Vrenegoor. Sludge and Liquid Wastes Disposal
for Combined Incineration Systems. Solid Waste Management and
Disposal. The International Edition of 1971 Australian Waste
Disposal Conference, University of New South Wales, pp. 105-108.
Abstract: The von Roll, S.A., Switzerland personnel presented the paper, which
describes use of a special high-speed evaporator to concentrate the sludge
solids from 95 percent to 40-^5 percent moisture content. The dried sludge is
then metered into the feed hopper of a MMR incinerator and burned on a grate
system. Steam for the evaporator may be supplied by a steam-generating
incinerator.
References; Tanner, R. Gemeinsame Verbrennung von Mlill and Kla'rschlamm mit
Abwa'rmeverwertung zur Schlammtrocknung (Combined Refuse and Sewage
Sludge Incineration with Waste Heat Utilization for Sludge Drying).
VGB Kraftwerkstechnik, 52,2:l40-H5, April 1972.
Abstract; The von Roll indirect sludge drying system, which will be installed
at Dieppe, France, has a steam-generating, grate-fired incinerator, where the
dried sludge is metered into the furnace charging hopper. The vapors from the
evaporator are ducted to the furnace, and the system is recommended for sludge
quantities of less than 50 liters/hr (13.2 gph).
Reference; Taylor, R. Combined Incineration of Refuse and Sludge. Environ-
mental Pollution Management. 3,2:89-91», March/April 1973.
Abstract; The paper discusses three methods of burning sludge with refuse:
1) direct addition of dewatered sludge into the furnace using mechanical dis-
persion (rotating brush or impeller or a secondary pressurized fluid, such as
compressed air or steam; 2) direct gas-phase contact with the products of
combustion; and 3) indirect drying using an intermediate heat transfer fluid
such as steam.
The recommended system is indirect drying using steam in a special (von
Roll) thin-film evaporator fitted with a rotor and scraping blades to keep
the heat transfer surfaces clean. A full-scale plant is operating in Dieppe,
France. The vapors and incondensible gases from the evaporator are ducted
into the furnace and heated to 750°C (1,380°F) to prevent odor emissions from
the raw refuse incineration plant.
Reference; Thompson, L.H. Sludge Treatment and Disposal - GLC Experience
and Investigations In the Field of Sludge Disposal. (Presented
at the Fourth Public Engineering Conference, Lanhborough
University of Technology, 1971.
Abstract; The following is quoted from the paper:
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"Incineration with Refuse
It will be realized that the Council is responsible not only main drainage
and sewage disposal (or treatment as we now to our operations in this
field) but also for refuse disposal. It is therefore convenient to con-
sider where, if at all, these functions may overlap and facilities or
processes complement each other. For example, it has been simple and
mutually beneficial to both of these sections of the waste disposal
service, for sludge to be used as a top dressing to reinstated tips,
accelerating the growth of luxuriant herbage and providing a cheap, use-
ful method of disposal for the sludge.
"Similarly, as the increasing shortage of refuse tips within economic
reach of London drives the Council deeper into a policy of refuse in-
cineration, it has been possible, indeed logical, to ask: Can the (waste)
heat derived from burning refuse be used to evaporate the unwanted water
from digested sewage sludge? Both the waste heat and the water tend to
be problems, so why not bring them both together to eliminate each other.
"In August 1969 a Working Party under the Chairmanship of the Council's
Scientific Advisor, with representatives of the Sewage Treatment and Ref-
use Disposal Branches and the Mechanical and Electrical Department was
set up by the Director of Public Health Engineering to investigate the
possibilities of joint incineration. At the time of writing, the Working
Party has not yet reported although at least an interim report is imminent.
"Several specialist firms, in this country and abroad, have been consulted
and installations inspected. Two schemes are considered worthy, in the
author's opinion, of further (pilot scale) investigation and development.
The flexible methods, which may allow the heat from the separate inciner-
ation of the refuse to be used for driving off the water from the sludge,
to whatever water-content is desired, are particularly attractive. The
sludge could then be either fully incinerated or withdrawn for utilization
on land in a dry sterilized state, as demand may dictate. These two
methods are: 1) drum-drying, which has been used, and is currently being
further installed, at several plants in Sweden; and 2) multiple evapora-
tion, which is in a relatively early stage of development (in Germany)
so far as sludge is concerned, but which is a well establsihed method for
desalination and other purposes.
"At a plant near Stockholm 800 metric ton/day (880 tpd) of refuse has been
burnt in an incinerator intended to serve 2 drum-dryers, each of which
was intended to dewater 216 metric ton/day (238 tpd) of digested sludge.
However, because the exhaust gases from the refuse incinerator could not
be maintained at the design figure of 900°C (1,650°F) it has been found
necessary to burn oil in the drum-dryers at a rate of 800 litres per 9
metric tons of wet sludge (211 gal/9.9 tons) - at 95 percent water content,
"It would not appear to be an impossible task to design and construct a
refuse incinerator from which exhaust gases at 900°C (l,650°F) could be
consistently obtained. In which case, the drum-dryer seems to be a
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flexible, and perhaps a not unduly expensive tool with which to reduce
the water content of sludge over a wide range. The final product seen
in Sweden resembled Morganic - as produced at Mogden.
"The multiple evaporation experimental plant is said to have run for 1,800
hours at Hamburg but until at least a pilot scale unit had demonstrated
its ability to run on British sludge without caking of the colls and con-
sequently a serious reduction of heat transfer, a major installation
would not be recommended.
"Other methods of sludge incineration where wet sludge is mixed with the
refuse or sprayed into the combustion chamber do not appear to be capable
of dealing in practice with anything approaching the quantity of water
which theoretical calculations indicate should be possible. Seldom has
it been demonstrated that 1 kg of refuse will evaporate more than 1 kg
of water. Consequently, if both sludge and refuse are derived from the
same catchment area, either the sludge has to be dewatered to some extent
prior to joint incineration or one must accept that only a proportion of
the sludge can be dealt with in this way. Either way, costs are increased.
Furthermore, to burn refuse in multiple hearth or fluidized bed furnaces
it is first necessary to mill the refuse — another additional cost.
"The destruction of organic matter which is potentially useful must be
questionable and it is therefore suggested that further development of
methods for the drying of sludge using refuse incineration is preferable
to complete joint incineration.
"This appears to be a suitable point to consider further the basic choice,
which seems to be arising with increasingly important conservationist
connotations."
Reference; Wallace, J.A. Incineration of Refuse in Hong Kong. Proceedings
of ASME National Incinerator Conference
Abstract; The author discusses Hong Kong's experience in deaillng with high-
moisture-content refuse. Most of their systems have been Volund, where a
drying grate feeds a burning grate and combustion is completed in a short
rotary kiln. Based on their experience with these Volund units, they state:
"The wide and frequent rapid variations in the moisture content have
resulting in fluctuations in the time required for drying and combustion, the
net effect being substandard burnout unless the charging rate can be corrected
in time to compensate for such fluctuations. It is evident that the concept
of drying by radiation and convection from the flame is only partially ef-
fective, for little more than the top layers of the refuse bed receive treat-
ment and, under some conditions, the under layers may still be high in moisture
content upon the burning grate prior to the kiln. Thus even with reciprocating
grate, distrubance and turn over of the under layers are difficult when dealing
with refuse containing a high moisture content.
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"A further very noticeable effect is the retardation of both drying and
the combustion of released volatiles above the bed when large quantities of
moisture vapor are contained within the combustion chamber.
From these observations, it was concluded that it was essential in the in-
cineration of Hong Kong refuse to incorporate in any new plants, effective and
flexible means of pre-drying, such treatment taking place in a chamber inde-
pendent of the combustion chamber proper."
They further state that: "The rotary kiln provided on the existing plants
has proved eminently suitable for this prupose by virtue of the available vari-
ation in speed and consequent variation of residence time in the final burnout
stages. It was decided, therefore, that on future plants the overall com-
bustion rating should be decreased on the rotary kiln should be incorporated
unless it could be shown that equal flexibility was obtainable by other means."
The rating was thus to be based on 215 kg/sq m-hr (kk Ibs/sq ft-hr), based
upon the effective grate area and the outline of the proposed plant. Note
that drying of the wet refuse is accomplished on a separate grate system using
hot flue gases as the drying medium. This practice of ventilation-drying is
based on Japanese practice, and the authors further state that the choice of
flue gas as the drying medium was made for the following reasons:
"(a) It minimizes the burning of easily combustible materials during the
drying process by retardation of ignition so that the maximum heating value
of the refuse is retained for the combustion process within the combustion
chamber.
"(b) More drying heat can be applied than with hot air as higher temper-
atures can be obtained.
"(c) The specific heat of flue gas with its higher moisture content is
higher than that of air.
"The advantage in using recirculated flue gas is however slightly affected
by the fact that the grate materials and supporting framework have to be con-
structed from materials capable of withstanding possible corrosion and
de fo rma t i on.
"Sufficient moisture will be removed in the drying process to ensure that,
upon discharge from the drying grate, the refuse will be in a self supporting
condition from a combustion aspect irrespective of the condition at entry."
Reference; Watson, R.H., and J.M. Burnett. Recent Developments and Operating
Experience with British Incinerator Plant. Proceedings and Dis-
cussions of ASME National Incinerator Conference (1972).
Abstract; The authors comment on co-incineration as follows:
"The costs involved in conventional dewatering of sludge to make it auto-
thermic, or the expense of supplementary fuel, and the difficulties in achiev-
ing satisfactory burnout have led to much thought on combined refuse and sludge
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incineration. Trials have been made both on standard refuse plants accepting
sludge cake or sprayed sludge and on special purpose furnaces. Two points
on which there is yet no general agreement are the temperature to which the
gases from the sludge must be heated and the period of time for which the
temperature must be held, to avoid offensive smells at the exhaust.
"The first combined sludge/refuse plant has recently been ordered and is
intended to accept sludge at *f percent solids with provision for supplementary
fi ring of gas oil."
Reference: Wegman, L.S. A Single Project for Refuse Incineration Sewage Treat-
ment and Steam Generation. Paper No. 68-162, APCA Annual Meeting,
St. Paul, Minnesota, June 1968.
Abstract; The only information pertinent to co-incineration is:
"Sludge disposal via mixing in a refuse furnace is more likely. Sewage
sludge moisture content ranges from 97 percent, or higher, for a raw
primary sludge down to about 75 percent for a thickened and filtered
sludge. At the 75 percent content, the daily yield from 1,000,000
persons is 272 metric tons (300 tons) a sizeable fraction of the 2,0^5
metric tons (2,250 tons) of refuse. Some preheating will be needed
but any odor emission can be readily handled—which previously deterred
this process. And adequate heat should not be a problem."
Reference; Weyrauch, H. Die Karnaper Verfahren als Beitrag zur Veroschung
von Siedlungs — und Industrieabfallen (The Karnap Procedure for
the Reduction of Settled Solids and Industrial Wastes). (Pre-
sented at the Second International Congress of the Internation
Society for Refuse Research Concerning Removal and Disposal of
Settled Solids and Industrial Wastes, 22 to 25 May 1962 in Essen,
West Germany)
Abstract; The paper describes a conceptual furnace design based on experience
in burning shredded refuse or dried sludge in pulverized-coal-fired furnaces.
Included is a hammer mill modified to operate as a hot flue gas-swept sludge
dryer discharging to the furnace, where the dried sludge burned in suspension.
Raw refuse burns on the grate system, and the auxiliary fuel is pulverized
coal.
Reference; Zack, S.I. Sludge Dewatering and Disposal. Sewage and Industrial
Waste 22,8:975-996, August 1950.
Abstract: This paper identifies some of the plants reportedly burning sludge
with refuse. These plants are:
1. Frederick, Maryland (Mixed with MMR)
2. Bradford, Pennsylvania (Proposed) (Flash Dryer)
3. Bloomsburg, Pennsylvania (Proposed) (Flash Dryer)
k, Stamford, Connecticut (Flash Dryer)
5. Tenafly, New Jersey (Flash Dryer)
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6. Lansing, Michigan (Multiple Hearth)
7. Lederle Laboratories, Pearl River, New Jersey
At Frederick, the undigested primary sludge is dewatered by vacuum filter
to 30 percent solids, mixed with refuse and burned in a MMR incinerator. The
ratio of wet refuse to wet filter cake is 2.9:1. The Bradford and Bloomsburg
plants were not operating at the time of the report.
Little information is provided for the Stamford and Tenafly Installations,
but it is stated that the flash-dried sludge is incinerated in refuse
incinerators.
Lansing reportedly digests ground garbage with activated sludge, and the
vacuum-fi1ter-dewatered mixture is then burned in a multiple-hearth furnace.
Lederle Laboratories is reported to have a mechanically-stoked refuse
incinerator with an auxiliary sludge-drying hearth burning rubbish, sewage
sludge, and spent laboratory media.
Attempts at burning liquid sludge without dewatering in a multiple-hearth
incinerator at Piqua, Ohio, required 88 gal/ton (3031/metric ton) — oil/dry
sol ids.
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APPENDIX B
PLANT VISITS
VISIT TO THE ALTRINCHAM INCINERATOR, GREATER MANCHESTER METROPOLITAN
CITY COUNCIL, MANCHESTER (CHESHIRE COUNTY), ENGLAND, 19 SEPTEMBER
The principal contact was Mr. Davidson, the Waste Disposal Officer for
the Greater Manchester Council, which had recently taken over responsibility
for the plant (an administrative and financial consolidation of waste manage-
ment responsibilities within the Counties now taking place over all of
England). Mr. Davidson's offices were located at the County Engineering
Department, County Hall, Piccadilly Gardens, Portland Street, Manchester,
England.
The Altrincham plant consists of two furnaces, each having sections of
rocking grates, fed by a conventional pit and crane. As long as the plant has
been in operation (approximately a year), the refuse has, unaccountably, been
consistently damp and occasionally very wet. The low calorific value and the
operating problems discussed in the following paragraphs have made co-firing
of sludge almost impossible.
The sludge is fed to the furnace in a jet from the wall at the discharge
end of the furnace. In order to project the sludge stream over the residue
quench tank into the hotter parts of the furnace, a relatively small (perhaps
1 cm) pipe is appropriate. However, it has been recommended (and shown
prudent) that a 7.6-cm (3") diameter pipe be used in order to avoid plugging
problems with hair and other debris in the sludge. This rather obvious con-
flict has made sludge firing impractical. The small diameter pipe (about 2 cm)
used suffers repeated plugging. Compounded by other operating problems
(discussed later) and the high moisture content of the incoming refuse,
co-firing has been indefinitely discontinued.
The sludge is received at the plant in tank trucks from a nearby treat-
ment plant and stored in a large, rectangular concrete tank located under the
ramp leading up to the tipping area. The tank was not provided with an
agitator, and when high-water-content sludges were placed in the tank, they
settled out rapidly, making for difficult pumping and variable moisture con-
tent. A jerry-rigged agitator was installed in the tank, but did not
effectively maintain homogeneous sludge. Also, odor problems occurred when
the open sludge tank (which was not aerated to maintain aerobic conditions)
went septic. Sludge is pumped from the storage tank into the plant, where
a k to 8 cu m (1,000-2,000 gal) run tank is located. A second pump moves the
sludge from the run tank to the atomization point, with a return pipe to the
hold tank.
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In general, the plant operator was quite disenchanted with co-firing,
and felt that there was no good way to carry out the operation with the
Altrincham equipment. With only two grate sections, the burnout was already
bad, and adding wet sludge on top of the wet refuse tends to make the problem
worse.
VISIT TO ANSONIA INCINERATOR, INCINERATOR/WASTEWATER TREATMENT PLANT,
ANSONIA, CONNECTICUT
i
February 13, 1975 was the date of the visit to the Department of Public
Works, Ansonia, Connecticut, to inspect the incinerator and sludge dryer
facilities.
Ansonia is a small community of about 20,000 people. The residents are
served by an integrated wastewater treatment plant and municipal refuse Incin-
erator. Approximately half the sludge from the water treatment plant is dried
in a Nichols Spray Dryer, using off-gas from the incinerator.
Incinerator
Ansonia's Incinerator Plant consists of two rocking-grate, continuously
stoked incinerators with a combined capacity of 190 metric tons (200 tons)
per day (2k hours). The plant operates on an 8-hour day, 5-day per week
schedule, burning about 50 metric tons (55 tons) per day of refuse. This
represents about 70 percent of Ansonia's solid refuse load. The remaining
30 percent, consisting mostly of bulky refuse, is landfilled. Very little
landfill area remains in Ansonia, and a shredder has been installed (but is
not yet operational) at the plant to permit incineration of a greater.portion
of the town's refuse.
Refuse is continually charged to the incinerator. Combustion air is
supplied both under and over the grate; air is also used to cool the combustion
chamber walls. Ashes are quenched and landfilled. Combustion gases from the
two units are combined in a "secondary combustion chamber" before scrubbing in
a Detrick-Jens Scrubber, operating at a pressure drop of about 1" W.G. An
induced draft fan discharges the scrubbed gases to the stack.
Wastewater Treatment Plant
The Ansonia Treatment Plant handles all domestic and light industrial
wastewater, with both primary and secondary treatment. Incoming water is
degritted and settled, and the overflow is treated in an activated sludge
aeration pond. The treated water is again settled, and the overflow is
chlorinated and discharged to the Naugatuck River. Excess secondary sludge
is returned to the primary settling tank. Sludge from the primary settling
tank is about 50:50 primary/secondary.
The sludge js thickened to a consistency of about k percent. Thickener
overflow returns to the primary settling tank. About half the thickened
sludge is pumped to the incinerator for drying and ultimate use as a fer-
tilizer. The remainder goes to anaerobic digestion. Digester gases, about
510 cu m (18,000 cu ft) per day, are burned at the digester to maintain
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sludge temperature of 50° (120°F). Digested sludge is then sent to drying
beds. Drying of digested sludge is very Inefficient — there is virtually
no percolation. Sludge lagoons are being constructed to take excess sludge
from the water treatment plant. Figure B-l is a simplified flow diagram
of the wastewater treatment system.
Sludge Drying and Co-Incineration
The *f percent sludge is held in an air-agitated tank before drying in a
Nichols Spray Dryer. Feed to the dryer is from a variable-speed Moyno Pump.
A high-speed disc atomizes the sludge in the co-current dryer, inlet tempera-
ture is about 650°C (1,200°F) and outlet temperature is 150°C (300°F). Outlet
temperature controls the speed of the sludge feed pumps.
Heat input to the spray dryer is furnished by diverting a small portion
of the incinerator combustion gases (hot gases drawn from a "secondary
combustion chamber" just before the scrubber). Temperature at this point
generally was lower than expected, partially because of turbulence caused by
the draft control vent. The spray dryer was not able to operate at design
rate, requiring that half the water treatment plant sludge be diverted to the
digesters. The incinerator is being modified to permit take-off to the spray
dryer directly from the incinerator combustion chamber. Gas temperature at
this point is expected to be 980°C (1,800°F), thus significantly increasing
the capacity of the spray dryer.
Dried sludge solids are brought out in the cone of the dryer or in the
off-gas cyclone. Sludge moisture content at this point should hold 15 percent
less, to eliminate build-up on the dryer walls. Pneumatic conveying equipment
is available to return the dried sludge solids to the incinerator, where they
burn in suspension above the second grate; however, Ansonia generally does not
burn the sludge. Local residents or the State Highway Department use most of
the sludge solids as a fertilizer. Figure B-2 is a cross-section of the
incinerator.
VISIT TO B'ULACH UNIT, BULACH, SWITZERLAND
Upon arrival in Europe, it was learned that the Blilach Unit in Switzerland
was shut down. This was verified in Zurich. There was some talk about ex-
plosions and repair problems at this plant. Mr. Helle, of Lurgi in Frankfort,
advised contact with Mr. Van Der Kraan in Dordrecht, Holland, and to visit
the plant in that city.
VISIT TO DIEPPE INCINERATOR, WASTEWATER PLANT/INCINERATOR,
DIEPPE, FRANCE
The incinerator and wastewater treatment plant at Dieppe, France was visited
on May 16, 1975. The plant is about 3 1/2 years old. M. Jean Fossey is in
charge of the plant, as Chef d1 Exploitation.
Solid waste is delivered to the plant from Dieppe and the surrounding area
by truck, from a population of about 60,000 and from industrial sources.
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Raw Sewage
Degritter i i
Primary
I Settling Tank I-
-
Aeration Tank
t t
Air
Secondary
I f Settling Tank
Chlorinator
Thickener TO Stack
I
Detrick Scrubber
To Naugatuck
River
To Drying Beds
f Cyclone
Spray Dryer
Hot Gas
to Dryer
Dry Sludge
to Incinerator
Sludge Storage
Feed Pump
Refuse
Incinerator
To Landfill
Figure B-1. Water Treatment and Incinerator Flow Sheet,
Ansonia, Connecticut.
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GO
,L
Figure B-2. Longitudinal Section Through Incinerator,
Ansonia, Connecticut.
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The Incinerator plant consists of two von Roll Incinerators wfth
reciprocating grates and with an average capacity of 2.5 metric tons
(2.75 tons) per hour, or 60 metric tons (66 tons) per day, each. Operation
is 5 days per week, 24 hours per day. The maximum capacity of each furnace
is 70 metric tons (77 tons) per day.
The sludge that is co-incinerated in the plant comes from the sewage
treatment plant on the same grounds, sized to accommodate the flow of
40,000 inhabitants (equivalent with a theoretical daily flow of 81
cubic meters (21,000 gpd) and an average BODg loading of 250 mg/liter.
Municipal Sewage Treatment Plant
The biological wastewater treatment plant consists of a primary clarifier,
aeration basin, and secondary clarifier. Secondary sludge is recycled to the
primary clarifier, where both primary and secondary sludge, as well as skim-
mings, are removed. This sludge also contains sand, grit, etc. (Because of
the large amount of shellfish consumed in this fishing town, large amounts
of sand and shell parts are contained in the influent.) The primary and
secondary sludge, together with skimmings, are then pumped to the anaerobic
digesters, from which the digested sludge is transferred to the thickener by
way of a comminutor.
The thickened 4 percent digested sludge is then passed through a delumper
(macerateur) and is pumped to the day tank in the incinerator plant. Digester
gas is used to heat the digesters and the plant control building.
Incinerator Plant
A recirculating pump has been added to the sludge day tank to eliminate
plugging problems resulting from settling of solids. M. Fossey is looking
into the use of a rubber-vane pump, because standard centrifugal pumps are
subject to wear and clogging. The sludge piping is flushed with water each
shutdown, to prevent clogging.
The sludge is fed to the vertical thin-film evaporators by screw pumps.
These pumps are subject to considerable wear; the stator wears out in about
6 months, and the impeller lasts about a year. This wear is thought to be
caused by waste from shellfish. The pump capacity varies from the 750 1/hr
(200 gph) design flow to about 1,200 1/hr (320 gph) after excessive wear.
As the capacity reaches 1,000 1/hr (260 gph), because of wear, the internals
usually have to be replaced, because the evaporator discharge then becomes
too wet (more than 60 percent moisture content) for proper incineration.
The two thin-film evaporators are Luwa Double-Wall Dryers operated on
10 kg/sq cm (140 psi) steam at about 180°C (355°F). The evaporators are
vertical with top inlet and bottom outlet.
The feed to the dryers consists of 4 percent solids sludge, and the
discharge has an average consistency of 52 to 55 percent dry solids. The
solids discharge is sometimes very uniform and granular in appearance, but
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at other times it is very irregular, and big lumps, 10 to 30 kg (22-66 Ib),
drop out of the discharge.
The dryers discharge onto a Luwa discharge conveyer belt, which trans-
ports the dried sludge to chutes for discharge into the incinerator feed
hoppers. These batch-feed chutes are kept full with about 500 kg (1,100 Ib)
of solid waste from the pits. From 400 to 500 kg (880-1,000 Ib) of waste
are placed in a hopper, and the hopper is always kept full. The dried sludge
is continuously fed into the hoppers, but refuse is charged only as the
hopper is emptied. There is no danger from overflowing of the hoppers,
because they have large excess capacity and are continuously monitored by
remote T.V. from the control room. The feed hopper is equipped with hydrau-
lically operated flaps that batch-feed the furnace.
The evaporator vapors pass through a demister before flowing through
a heat exchanger (added after construction had been completed). The vapors
from the dryer were originally fed into the hottest zone of the furnace,
but now the vapors are fed into the undergrate hopper, in the burnout zone
(not the drying grate). These changes were made to the original installation
because problems were encountered with the waste-heat steam generator tubes.
Excessive fouling of the tubes was thought to be caused by combustion gas
condensation on the tubes. As already stated, a heat exchanger was moved
from the combustion chamber to the hopper under Zone 2 of the von Roll
reciprocating grates. Relocation of this discharge to the drop-section
between the combustion and ash burn-out grates is still being contemplated.
The solid waste consists of about 85 percent municipal refuse and about
15 percent commercial and industrial solid waste. Large items are removed
from the pits and landfilled. Automobile tires are burned at a maximum rate
of 5 per day; truck tires are not accepted. Grits and screenings are burned
as is secondary sludge; tertiary sludge is available, and chemical sludges
are handled. The light-off burner, with oil and gas connections, has never
been used.
Miscellaneous Data
For 197^ the following data are available:
17,062 metric tons 85 percent Municipal Refuse + 15 percent Industrial
2,123 metric tons Sludge (based on dry solids) processed.
19,185 metric tons Total incinerated
7,356 metric tons of residue, with 60 percent water content, remained.
Reinterpreted, these data indicate a capacity of approximately 2.277 metric
tons/hour (2.51 tph), which was low because of the problems encountered; the
rate for 1973 was 2.846 metric tons/hour (3.137 tph). The capacity of the
furnaces is 2.5 metric tons/hour (2.75 tph) design, with a maximum possible
rate of 3.0 metric tons/hour (3.3 tph). One furnace is usually on stream,
with the other down for maintenance or available for back-up capacity.
170
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The waste heat boiler produces 7,5 metric tons (8.3 tons) of steam
per hour at a pressure of 13 kg/sq cm (185 ps?) when the design waste
throughput rate is maintained. There is not enough demand to use all the
steam. One Luwa dryer uses 1,500 kg (3,300 Ib) per hour, and usually only
one is used; but even if two are running, which does occur occasionally,
the demand is only 3,000 kg (6,600 Ib) per hour, and there still is surplus
steam. The plant personnel, however, have added some other equipment to
utilize this steam, including preheating of combustion air and heating of
the vapors that come off the sludge dryers. The remaining excess steam,
generated by the waste heat boiler, is condensed in a heat exchanger that
is cooled by river water.
Sludge-to-refuse ratio is as follows:
Refuse: 2.5 metric tons/hour x 2k =60 metric tons per day (66 tpd)
Sludge: 20 cu m/hr x ^5 percent water = 23 metric tons per day (25 tpd)
Total Processed_ 83 metric tons per day (91 tpd)
55 percent Solids Sludge/Solid Refuse = 23/83 - 30 percent
Gases from the anaerobic digester have not been burned in the furnace. The
fuel oil nozzle is not used at all in running or starting the incineration
process.
This plant has never added water in the combustion chamber, either
through the nozzle that has been provided or in any other fashion, for fear
of waste heater problems and also, perhaps, to avoid refractory deterforatton.
An improved design has been suggested by M. Fossey to facilitate cleaning of
the heater tubes and removal of the dirt that is released in a rather inacces-
sible space.
The air pollution control method here consists of multi-clones, a number
of cyclones with a diameter of about 30 cm (12 in.). Some problems have been
encountered in the past when trying to clean these cyclones. Originally, a
solution was used to spray through these units, but this resulted in deteriora-
tion of many of the walls; now, all of these are mechanically cleaned with
better success.
The plant is located some distance from population in an industrial
park, and no odor problems have been encountered. Note that the pit is
ventilated by the incinerator forced draft fan, as shown in the flow sheet
and dryer diagram (Figures B-3 and B-*t).
VISIT TO DORDRECHT PLANT (LURGI SYSTEM), DORDRECHT, NETHERLANDS
This report contains information gathered during the plant visit to
"Gevudo" (Gemeenschappelijke VuiIverbranding Dordrecht En Omgeving) In
Dordrecht. Mr. J.M. Van Der Kraan is in charge of the sewage treatment plant
and the incinerator installation.
171
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Digested Sedimentation Sludge
ro
Figure B-3. Flow Sheet of Sewage Sludge and Refuse
Incinerating Plant, Dieppe, France.
-------
Figure B-4 Cross-Section of Von Roll Evaporator,
Dieppe, France.
173
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The Dordrecht Plant (2. J/2 years old) consists of: two parallel sewage
treatment plant systems, each with a capacity of 100,000 inhabitant equiva-
lents; three Martin-designed reciprocating grate-type incinerators with a
capacity of 6.4 metric tons/hr (7 tph) each; and one Lurgi multiple-hearth
sludge incinerator. The refuse incinerator serves 43 municipalities with a
total population of about 360,000. Energy reclamation was rejected for this
plant, because the nature of the operation would involve energy costs which
exceed those of conventionally generated electrical power.
Municipal Sewage Treatment Plant
The sewer system in Dordrecht is subject to ground-water infiltration
and also acts as storm sewer. The wastewater plant consists of two parallel
systems, each consisting of a degritter, primary clarifier, aeration basin,
and secondary clarifiers. Sludge from the secondary clarifiers is discharged
into the primary clarifiers; the primary and secondary sludge (2 percent
solids) are then pumped to the thickener. The thickened sludge (6 to 7 percent
solids) is pumped to the incinerator building for centrifuging. Grit and
skimmings are collected and burned in the refuse incinerator. No digesters
are used in this installation.
Incinerator Plant
Refuse is received by truck from municipal (91.5 percent) and from
industrial (8.5 percent) sources. The solid waste is dumped into a 300-cu m
(10,600 cu ft) pit; oversize material is dumped in a separate pit. The over-
size items are then reduced by means of the hydraulically operated Lindemann
Shear. Two bridge-cranes with 2.5"cu m (88 cu ft) grapples keep the hoppers
filled and can be used to feed the shear. Pieces that have been cut by the
Lindemann Shear are conveyed to the regular pit by a chute. The bottom of
the incinerator feed"chute is equipped with a ram feed, which discharges the
refuse onto a Martin grate. Undergrate (forced) air is exhausted from high
in the pit area to feed the five grate zones for each furnace.
The thickened sludge from the treatment plant is mixed with polyelectro-
lyte and is then centrifuged to increase the solids content from 6-7 percent
to 15-18 percent. The solids content is kept at this level to permit pumping
the sludge. There are 4 centrifuges, one of which is a standby unit. Each
centrifuge is operated for 60 hours/week at a rate of 1.2 metric tons/hr
(1.3 tph) dry basis. If normal (Design) sludge quantity is processed, it is
dumped on the first of twelve hearths; if the quantity is low, the sludge is
fed to the third or fifth hearth. (System design calls for burning of the
sludge on the ninth and tenth hearths.) Sludge-burning combustion gas temper-
ature is kept at 800°C (1,470°F); by law, the fumes must be held at 800°C
for 3 seconds. The unit is about 20 m (66 ft) high and 6 m (20 ft) in
diameter. The ashes from the bottom are about 40 percent of the original
solids volume and are landfilled.
All incinerator combustion fumes are moved to the scrubber by stainless
steel fans through stainless steel ducts; the scrubber and stack are also
made of 4,000 Series stainless steel. The scrubber treats the combustion
174
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gases from the Martin refuse incinerators as well as from the Lurgi
Multiple-Hearth sludge incinerator. This Lurgi Scrubber (which uses
clarifier overflow water) cools the combustion gases to 60°-70°C
(140-160°F). One third of the scrubber water evaporates, and the rest
is recycled by neutralizing it with lime effluent from the nearby drinking
water plant, which has a lime water surplus. The settled lime sludge is
disposed of with the other incinerator residue. The scrubber removes
90-95 percent of the HC1, 50-55 percent of the sulfur dioxide, and 90-95
percent of the HF. The scrubber discharge water has a pH of about 3-5
before neutralization. No odor problems have been encountered.
Miscellaneous Data
Processed in 1974:
97,000 metric tons Municipal Refuse (107,000 tons)
9,000 metric tons Industrial Refuse (9,900 tons)
106,000 metric tons Total Refuse (117,000 tons)
33,814 cu m (1.1932 x 106 cu ft) Sludge in 3,026 hours
Average number of furnaces in use: 2.4
Ferrous metals are separated from the residue, at a yield in 1974 of
1,998 metric tons (2,198 tons).
Capacity of Martin Furnaces with waste heat content of 1,400-2,000 kg-cal
(350-500 Btu) is 7.4 metric tons per hour (8.1 tph).
Oil is not processed in this plant. This pertains to used crankcase oil
as well as auxiliary fuel oil. Skimmings are a problem; they cannot be
centrifuged, and will be removed manually for incineration in the future.
There are plans to use the multiple-hearth cooling air as combustion air.
It is also planned in the near future to burn some industrial primary sludge
from neighboring industries (duPont) in the incinerators with the Martin
Grates. The duPont firm will have to dry the sludge to a consistency where
the material will not stick together, so that it can be mixed with the municipal
refuse. Note that the primary industrial sludge from duPont will not be burned
in the Lurgi Multiple-Hearth Unit, but in the regular municipal refuse inciner-
ators.
Figure B-5- presents a cross-section of the Dordrecht incinerator.
VISIT TO DOW INCINERATOR, DOW CHEMICAL COMPANY,
MIDLAND, MICHIGAN
In November 1974, the Dow Chemical Company in Midland, Michigan was
preparing a second rotary kiln incinerator for operation, with a target date
May of 1975.
175
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Figure B-5. Refuse Incinerator at Dordrecht, Netherlands.
176
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The new rotary kiln installation was to be similar to Dow's present
system, and would handle tars, refuse, and biological sludge. Dow is not
sure what pretreatment is scheduled for the biological sludge, but will
continue the present practice of firing the secondary combustion chamber.
Apparently, Dow plans to shut down the existing rotary kiln once the
new one is ready. Original plans called for the installation of two kilns
side by side. Present activities concern finishing the second kiln system,
with whatever modifications are considered appropriate.
VISIT TO EASTMAN KODAK, REFUSE/SLUDGE INCINERATOR,
KODAK PARK, ROCHESTER, NEW YORK
Eastman Kodak Company operates a combined refuse and sludge incinerator,
and power boiler at its Kodak Park Complex, in Rochester, New York. The
Eastman Kodak system is a Combustion Engineering VU-400 steam generating
boiler equipped for suspension burning and including a dump grate. The
biological sludge is dried in a Raymond Spray Dryer system, and suspension-
burned in the furnace. The hot gases for the Raymond system are withdrawn
from after the boiler passes and the vent gases from the Raymond system pass
into the furnace proper. The boiler was designed for 3,880 kg-cal/kg
(7,000 Btu/lb) refuse, and it is now running 4,880 kg-cal/kg (8,800 Btu/lb).
The facility is designed to burn 164 metric tons per day (180 tpd) of refuse
along with 104 metric tons per day (114 tpd) of 20 percent solids sludge.
(See Flow Diagram in Figure B-6.)
Wastewater Treatment Plant
The Wastewater Treatment Plant includes both primary and secondary
treatment, and produces about 13,600 kg (30,000 Ibs) per day of primary and
11,400 kg (25,000 Ibs) per day of secondary sludge (dry basis). The plant
receives industrial wastewater at an average rate of 106,000 cu m per day
(28 mgd), with very little sanitary sewage. About half of the BOD load has
been identified as water-soluble solvents. Primary and secondary sludge
are combined at the thickener, where they have thickened about to 4 percent
solids. The sludge is then vacuum-filtered to about 20 percent solids;
only synthetic polymer is used to aid filtration. Sludge solids are then
hauled to the remote refuse/sludge incinerator or burned in a rotary kiln
incinerator at the water treatment plant.
Sludge Drying
Sludge is stored at the incinerator until dried and burned. (This
creates a significant odor problem.) A series of screw and belt conveyors
moves the sludge to a mixer. Wet sludge is mixed with previously dried
sludge at a ratio of about one to one; moisture content at this point is
about 50 percent. The sludge is then dried in a Raymond Flash Dryer. Heat
is supplied to the cage mill from the incinerator/boiler. The inlet
temperature to the dryer is 540°C (1,000°F). Solids leaving the dryer
contain about 15 percent moisture. (Lower moisture content would increase
the potential for a dust explosion in other parts of the gas-handling
system.)
177
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Figure B-6. Flow Sheet of Combustible Waste Disposal System:
Eastman Kodak Company.
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The dried solids are separated from the air stream in a cyclone. When
only primary solids are burned, the cyclone has reasonable efficiency, but
secondary solids dry to a very fine powder which is not easily separated in
the cyclone. About 90 percent of the dried sludge is returned to the mixer
for combination with wet sludge. The remaining 10 percent is pneumatically
conveyed to the boiler. Off-gas temperature from the dryer is about J50°C
(300°F). This gas, containing some small particles of sludge solids which
escaped the cyclone, is returned to the combustion zone of the boiler.
Refuse Handling
Kodak's refuse consists almost entirely of waste paper and plastic.
Metals, glass, etc. are salvaged. Refuse is held in a pit prior to shredding.
An overhead crane charges refuse to the shredder hopper, where it is fed by
means of a ram and a variable-speed converging conveyor. An Eidal 600-kW
(800-hp) Shredder is used to reduce refuse size to about 5 cm x 5 cm (2" x 2").
A rotating hammer, atop the rotor, breaks any large pieces which will not fit
between the rotor and shell. This breaker bar also rejects oversized metal
and other objects which are not readily comminuted in the shredder. Shredded
refuse is air-classified and pneumatically conveyed to a storage silo.
The silo has four out-feed screws and 12 vertical mixing screws to
eliminate bridging. The feed screws charge a pneumatic conveying system,
which moves the shredded refuse to the boiler.
Incinerator/Boiler
The incinerator is a tangentially fired boiler fueled with No. 6 oil,
refuse, and dry sludge. (Oil is always fired, with or without the other fuels.)
Oil is fired from all four corners of the combustion chamber. When firing oil
only, virtually all combustion air is supplied at the burner port, and the
firing rate is about 3,200 kg/hr (7,000 Ibs/hr), which generates 68,200 kg
(150,000 Ibs) of steam per hour.
Refuse generally is burned in suspension in the combustion chamber, and
any refuse not burned in suspension drops to a tipping grate. The shredded
refuse is pneumatically conveyed to all four corners of the boiler. At
present, only a portion of the refuse available is burned in this unit.
Refuse is fired only 20 to 40 percent of the operating time. The remaining
refuse is used as a fuel in an adjacent waste chemical incinerator. Kodak
plans to modify the chemical incinerator to fire oil, and burn all refuse in
the boiler; refuse would then be fired about 80 percent of the time in the
boi ler.
Dried sludge is fired from two diagonal corners of the boiler. When
firing refuse, sludge, and oil, the boiler capacity is about 61,400 kg
(135,000 Ibs) of steam per hour, but oil consumption is reduced to 910 kg/hr
(2,000 Ibs/hr), or about 25 percent of the heat input. When firing refuse,
very little combustion air is fired at the oil burners. Combustion air is
then provided both over and under the grate.
179
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All combustion air is preheated to 260°C (500°F) using flue gas. The
boiler is a water-wall unit, providing preheated feed water. Steam pressure
in preheating the boiler is about 28.2 kg/sq cm (*»00 psi), with 56°C (100°F)
superheat. The steam is used to generate the power in a nearby turbine; or
the pressure is reduced, and the steam is incorporated into the plant system.
The returning dryer off-gas, at 150°C (300°F), enters the boiler at the
first tube bundle. The flue gas temperature after the boiler section is
maintained at 540°C (1,000°F) to provide heat for sludge drying. The combina-
tion of low-temperature before the boiler and high flue temperature after the
boiler significantly reduces fts efficiency. However, by providing all combus-
tion air under the burning refuse and by using the excess air for oil combustion,
Kodak is able to limit excess air in the flue to about 25 percent.
The flue gas passes through a cooler and then to an electrostatic pre-
cipitator (rated efficiency of 99 percent). The ID fan discharges flue gas
to a stack common to other combustion equipment in the area. Ash is discharged
from the tipping grate in the incinerator and from the electrostatic precipi-
tator to a pneumatic conveying system which moves the ash to a storage silo.
All ash is shipped to a smelter for recovery of trace quantities of silver.
Combustion System
Disposal of general plant waste and industrial waste treatment plant
sludge is accomplished by a combustion system where the waste fuels are
burned in a suspension-fired boiler. Four streams of general plant waste
are pneumatically conveyed into the four corners of the boiler and two
streams of flash-dried sludge are pneumatically conveyed into two diagonally
opposite corners of the boiler. The waste fuels are blown in tangential1y
to an imaginary circle in the center of the boiler. Much of the combination
occurs in suspension. Any material not burned in suspension is burned on
a dump grate.
The boiler is ignited by the use of No. 2 oil, and combustion is
stabilized by firing No. 6 oil. The boiler is of the balanced-draft type,
with a forced-draft fan and an induced-draft fan. The flue gases from the
boiler are cleaned by an electrostatic precipitator and pass to a stack and
then to atmosphere.
The boiler generates steam at 28.2 kg/sq cm (l»00 psi) and 290°C (550°F).
After leaving the boiler, the steam pressure is reduced to 18.3 kg/sq cm
(260 psi), and the steam is fed into an existing plant distribution system
for generation of power and process work. The boiler was designed to generate
35,000 kg/hr (77,000 Ibs/hr) of steam on waste fuels only, 61,400 kg/hr
(135,000 Ibs/hr) of steam on refuse, sludge and oil, or 68,200 kg/hr (150,000
Ibs/hr) of steam on oil only.
The bottom ash from the boiler and the fly ash from the precipitator
are pneumatically conveyed into an ash storage silo. The ash is unloaded
from the silo into gondola railroad cars and shipped to a smelter for recovery
of the silver contained in the wastes burned.
180
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This system was first started up In early 1970 and ran at partial
capacity because of material handling problems. After making system changes,
eht entire facility began operating in late 1973.
VISIT TO FRANKLIN CO-INCINERATOR, FRANKLIN, OHIO
This solid waste plant, with a design capacity of 136 metric tons/day
(150 tpd) of MMR and 6.4 metric ton/day (7 tpd) of MSS, has been partially
funded by EPA. The sludge from the Primary Municipal Clarifier goes into the
solid waste processing circuit just ahead of the press feeding the fluid-bed
reactor. Also, sludge from the Secondary Clarifier is fed back into the head
end of the plant, and effluent water from the sewage treatment plant is used
for the Ventur? Scrubber on the fluid-bed incinerator.
The process features a "Hydropulper" to shred the municipal refuse.
Ferrous and non-ferrous metals, glass, and paper fiber may be removed ahead
of the press feeding the fluid-bed unit. The partially dewatered material
from the press is pneumatically conveyed and introduced just above the sand
bed of the fluid-bed incinerator. Sufficient large material enters the
fluid-bed unit such that periodically excess solids must be drained from the
bed of the fluid-bed unit.
A simplified flow diagram is presented in Figure B-7.
VISIT TO HERSHEY SEWAGE TREATMENT PLANT, HERSHEY, PENNSYLVANIA
From 1963-1972 a Carver-Greenfield system was used to evaporate sewage
sludge to dryness in a triple-effect unit. Oil was separated in a centrifuge
and reused for a sludge suspension, and the oily sludge solids were burned in
a boiler. No additional fuel was required. The sludge has a high energy
content because 70 percent of the sewage is from the Hershey Chocolate Plant.
During an 8-hour period about 2,270 kg (5,000 Ibs) of dry sludge was burned;
the make-up oil to the Carver-Greenfield system during that period was 300-
380 L (80-100 gal). Sludge combustion in the boiler produced steam to
evaporate/dry the raw sludge. It is not known if it was necessary to burn
this much oil to make steam for evaporation, since there is no way of
completely removing the oil.
There were no corrosion problems in the boiler, although some corrosion
was noted on the shell side of the second effect because of the acidity of
the condensate. Injection of small amounts of ammonia into the shell
(controlled to pH 7.1) solved the problem. The BOD of the condensate was
about 5 ppm. The most significant problem was corrosion in the dryer section
and in the dry-sludge handling equipment.
The system has not been operated since 1972, and is in need of much
repair. Sewage treatment will soon be handled through a regional authority
(Envlrotech treatment system plant), with no biological treatment. Until
then, sludge at Hershey is to be digested and landfilled.
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Industrial Clarifier
oo
NJ
Soil Stabilization
Secondary Clarifier
Distribution Chamber
I ndustrial
Waste Water I
Municipal—*1
Waste Water
Aeration Basins
I |
No. 1 I No. 2 I No. 3
Junction Chamber
Municipal Clarifier
Scrubber Water Bleed
| Process Water Bleed
Hydropulper Liquid
Cyclone
I
Municipal Refuse
Recovered
Ferrous Metal
«
Non-Recoverable
Inorganics to Fill
Process Water
Fluid Bed
Reactor Scrubber
Waste Water to Treatment Plant
Recovered Fibre
I— _ _______ — _». Reclaimed Glass
Glass Recovery I
Plant I —— ——•*• Reclaimed Aluminum
Figure B-7 Flow Sheet of Franklin (Ohio)
Environmental Control Complex
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VISIT TO HOLYOKE CO-INCINERATOR, DEPARTMENT OF PUBLIC WORKS
HOLYOKE, MASSACHUSETTS
The Holyoke Department of Public Works operates a sludge drying and
co-incineration plant.
Raw primary sludge at 5 percent solids is vacuum filtered to a 25-30
percent consistency. The sludge cake is then dried in a 2.1 m (7 ft)
diameter by 12.2 m (40 ft) long rotary kiln. Heat is supplied from the
refuse incinerator and from auxiliary oil firing. Sludge leaves the drier
at 85 percent solids and is incinerated with municipal and industrial refuse
in one of two Pittsburgh fixed-grate incinerators. Spent gases from the
dryer are returned to the incinerator for deodorization.
The plant handles about 227 metric tons/week (250 tpw) of refuse and
14.5 metric tons/week (16 tpw) of dry (85 percent) sludge, on a 5-day basis.
However, sludge is dried and incinerated only three days per week. The ratio
of refuse to dry sludge is therefore about 9:1 on the three days co-incineration
is in operation.
Holyoke is a paper-converting town, and the incinerator plant handles both
industrial and domestic refuse; until recently, the mix was 55 percent indus-
trial and 45 percent domestic. With this ratio of high-energy industrial
waste, auxiliary fuel firing in the dryer has been required only when the
refuse was very wet. Some private landfill operations have opened, and the
industrial/domestic ration is now 1:1. Auxiliary fuel must be fired whenever
the dryer is in operation.
This plant handles about 2/3 of the sewage sludge from Holyoke, and burns
about 1/2 of the refuse load. Plans call for installation of another inciner-
ator at this site, but with no additional sludge handling capacity. The new
incinerator is expected to eliminate the need for auxiliary fuel in the sludge
dryer.
Incinerator
The incinerator section of the plant consists of two Pittsburgh, fixed-
grate, manually stoked, natural-draft incinerators with a combined capacity
of 205 metric tons/day (225 tpd)—24 hours. The plant actually operates at
a rate of 51 metric tons/day—12,700 metric tons/year (56 tons/day—14,000
tons/year) on a one-shift, five-day-per-week basis. Of the 51 metric tons
(56 tons), about 70 percent (9,100 metric tons/yr = 10,000 tons/yr) is typical
domestic refuse; the remaining 30 percent (3,600 metric tons/yr = 4,000
tons/yr) is industrial refuse consisting primarily of wastepaper from nearby
mills. The town of Holyoke generates about 10,900 metric tons (12,000 tons)
of refuse per year, of which 9,100 metric tons (10,000 tons)—85 percent is
incinerated. The remaining 1,800 metric tons/yr (2,000 tons/yr) are land-
filled.
Until recently, the incinerator burned about 9,100 metric tons/yr
(10,000 tons/yr) of industrial refuse, i.e., wastepaper; this was almost
183
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a 1:1 ratio with domestic refuse. However, participate emissions from the
pigmented paper coatings forced a significant reduction in incineration of
industrial waste. Much of the industrial wastepaper is now trucked to
landfill by private haulers.
Wastewater Treatment Plant
The wastewater treatment plant consists of primary treatment only.
After the clarifier, sludge is thickened to 5 percent solids; the thickeners
also serve as holding tanks during periods when the sludge dryer is not in
operation. Thickened sludge is treated with ferric chloride/lime and then
vacuum filtered. The vacuum-filtered sludge has solids content of 28 percent.
Filtrate and overflow from the thickener are returned to the clarifier head
box.
Overflow from the clarifier is discharged to the Connecticut River
without further treatment. Secondary treatment will be implemented in the
near future. The plant currently handles 65 percent of the wastewater from
Holyoke—all domestic wastewater. When secondary treatment is installed,
new interceptors will be included, and the plant will handle about 95 percent
of all wastewater from Holyoke, including significant amounts of industrial
wastewater,
Sludge Drying and Incineration
Filtered sludge cake is conveyed by belt to a pug mill, where previously
dried sludge solids are added and the cake is broken. The sludge enters a
2.1-m (7-ft) diameter by 12.2-m (^fO-ft) long direct-fired, steel-shell rotary
dryer. The dryer is equipped with lifting flights to minimize balling and
to provide better gas/solid contact. The solids leave the dryer with a
moisture content of about 15 percent.
The inlet temperature in the dryer is controlled to 650°C (l,200°F). The
heat load of the dryer is supplied by off-gas from the incinerator and from
auxiliary fuel firing. Hot gases are drawn from the incinerator at a temper-
ature of 480-650°C (900-1,200°F); if the temperature drops below if80°C (900°F),
auxiliary fuel is fired to maintain the specified inlet temperature. On the
average, 30 percent of the heat load is provided by auxiliary fuel oil burning.
The spent gases leave the dryer at 150°C (300°F), pass through a cyclone
and an induced draft fan, and into the incinerator stack breeching. The
temperature at this point is a minimum of 480°C (900°F).
Dried sludge is conveyed to the incinerator by bucket elevator and screw
conveyor. Half of the the dried sludge is diverted to the pug mill, to be
mixed with the wet, vacuum-filtered sludge cake. The remainder is injected
into the incinerator by a high-velocity air jet. Much of the sludge solids
burn in suspension above the burning refuse.
-------
IRAW SEWAGE
GO
vn
TO CONNECTICUT RIVER
Kti-use i
ID*
DRY SLUDGE TO INCINERATOR
TO LANDFILL
Figure B-8. Water Treatment and Incinerator Flow Sheet:
Holyoke, Massachusetts.
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VISIT TO HOUSTON MUNICIPAL INCINERATOR, HOUSTON, TEXAS
Attempts to burn raw sludge in this municipal incinerator were not
successful.
Local farmers pick up most of the sludge from South Houston, but
occasionally, the Houston plant has excess sludge. Attempts were made to
burn raw, unthickened sludge at about 95 percent moisture in an incinerator
at a refuse/sludge ratio of about 3:1. This immediately extinguished the
fire, and no further attempts at co-incineration were made.
Any excess sludge is landfilled.
Waste Control Systems, Inc., a division of Houston Natural Gas, which
makes rotary incinerators, supplied the unit for South Houston. It was
intended to burn "nearly dry" sludge, not raw sludge. The incinerator can
be equipped with a screw charger for heavier materials such as sludge, and
can use waste heat recovery to preheat incoming combustion air.
The company knows of no applications of co-incineration of refuse and
sludge in any of its equipment.
VISIT TO LANSING FACILITIES, LANSING, MICHIGAN
Lansing previously used a Raymond Flash Dryer to produce fertilizer from
sewage sludge. However, if the sludge was dried completely, it became
extremely dusty, and, resultingly, little was sold. Attempts were made to
reduce and dust by incomplete drying, but the remaining moisture caused the
bags of packaged fertilizer-sludge to break. Fertilizer production was dis-
continued, and the sludge was landfilled. However, the landfill was recently
condemned.
Lansing is now constructing a sludge-incineration facility, a Ztmpro
sludge heat treating process followed by vacuum filtration to produce a
40 percent solids sludge cake. The sludge cakes will be incinerated. No
additional fuel will be used to maintain sludge combustion. Waste-heat
recovery equipment will provide some heat for sludge pretreatment, but
additional gas-fired boilers will be necessary.
The sludge treating.faci1ity is to be in operation by late 1975.
VISIT TO THE NEWBURGH INCINERATOR, NEWBURGH, NEW YORK
Initial attempts at co-incineration of vacuum-filtered sludge failed.
They combined 25-30 percent solids sludge with trash, but the mixing was
poor. As the trash was deposited on the traveling grate and burned, the
sludge cake dropped through the grate.
In 1970, a Raymond flash drier was purchased. Attempts were made to
get the system into operation for about one year, but were never successful.
To start the operation, incinerator temperature was raised to 930°C (1,700°F)
186
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by burning cardboard. The hot gases were then directed to the flash dryer
(same 25 to 30 percent solids sludge as above). The sludge would dry for
2 to 3 hours, but during this time the incinerator temperature would drop
drastically; at times, temperatures went as low as 260°C (500°F). With
low-temperature flue gas, sludge drying was incomplete and clogged the
materials-handling equipment. The incinerator building also filled with
smoke.
The incinerator is very old, with many leaks and poor draft control.
The superintendent attributed this to poor operation, and believed that the
system would work with a modern incinerator. Several attempts to start up
again in 1972* netted the same results. Currently the sludge is landfilled,
but a new incinerator is in the planning stage. There are no plans to
include the dryer in the new facility.
VISIT TO RE1GATE INCINERATOR, RED HILL, SURREY COUNTY, ENGLAND
The Reigate Incinerator plant, a single furnace system, receives refuse
in a pit and, with an overhead crane, passes the material through a Tolemach
hammermill. With this mill, 910 metric tons (1,000 tons) can be processed
on a set of hammers before retipping with Armalloy-37 (a product of Australia).
The Tolemach mill has shown "good performance" with some pre-sorting (removal
of grossly oversized material).
The shredded material from the mill is passed over a magnetic separator
for the removal of ferrous metal. In moving out of the hammermill, the
material is leveled to a constant head on the pan conveyor passing the
magnetic separator and is thus fed at a relatively constant rate (with a
drag-chain conveyor) into a Lurgi multiple-hearth furnace. Feed blockages
have been a problem.
The Lurgi furnace, fired with oil at a rate of 285 L/hr (75 gal/hr)
during start-up, is fed with air corresponding to 100 percent excess for
refuse (averaging 6,980 J/g—3,000 Btu/lb). Thickened sewage sludge from a
nearby plant is fed to the top hearth of the furnace. The combined refuse
and sludge ash pass to the bottom of the furnace and into a dumpster-type
container. Few problems with rabble-arm fouling have been reported.
Flue gases are passed through a duct equipped with water sprays (in
case temperatures rise too high), through a Lurgi electrostatic precipitator,
and up a 38-m (125-ft) high, 1.37-m (4-fti 6-in) diameter stack. An
emergency vent and a stub stack are located directly above the furnace.
The Reigate plant was commissioned in February or March of 1973 but has
still not been accepted by the owners, because of a number of major and
minor design and operating problems. The plant had a total cost of 686,000
pounds sterling in 1969, and it is estimated that plant costs as of 197^
would be 1.1 million pounds. By and large, this cost increase represents
inflation, rather than the impact of design changes made during the start-up
experience.
187
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The Reigate plant was very clean and well-run. The sewage treatment
plant in the adjacent property had a landfill available for sludge disposal,
and thus was able to provide sludge on an as-needed basis. Fortunately, the
plant had the options available to dispose of the sludge if problems arose
in the incineration facility.
It should be noted that the tin cans separated by the magnetic separator
were stockpiled in 1.2 m by 1.8 m by 1.2 m (k ft by 6 ft by k ft) cages made
from reinforcing bar, and that the foodstuffs and adhering paper were burned
out in a separate refractory- 1 ined, oil-fired combustion chamber, for sub-
sequent sale as first-quality scrap.
At the time of the visit, the plant was down, to allow refractory to
be applied to the metal on the central drive shaft of the multiple-hearth
furnace. Problems due to local overheating (probably due to occasional
high heat release during firing of refuse with a high energy content) were
experienced,
Inquiries were made as to the problems concerning odor which, specu-
latively, could be associated with the plant (since it is similar to the
Biilach plant in Switzerland). Fortunately, few people live in the region
around the plant and the plant has a relatively high stack. As a consequence,
odor complaints to date have been minimal. During the visit, it was noted
that the sewage treatment plant next door, although not septic, had a dis-
tinctive odor.
VISIT TO SCRANTON INCINERATOR, SCRANTON, PENNSYLVANIA
Scranton uses a Raymond flash drier followed by incineration. No co-
incineration is employed.
Scranton starts with a 70 percent moisture, vacuum-filtered sludge,
which is then dried in the flash drier. There are facilities for bagging
dried solids as fertilizer, but there is no market. Sludge is incinerated
to produce heat for drying. Very little auxiliary fuel (natural gas) is
used. About 19 metric tons (21 tons) of dry sludge is incinerated in 16
hours. Most of the auxiliary fuel is used to reheat the system each morning.
with
sludge.
The only problem in the drier has been in drying sludge coagulated
polymerics. There has been no difficulty with ferric/lime treated
e.
VISIT TO STAMFORD CO- INCINERATOR, STAMFORD, CONNECTICUT
At the time of the visit, Stamford was building a co- incineration plant,
with start-up scheduled for January/February, 1975.
The sludge comes from the treatment plant at 5 percent solids (polymer-
floculated). It is centrifuged to 25 percent solids and pug milled, and
then enters a 2.7~m (9~ft) diameter x 18.3-m (60-ft)-long rotary kiln. The
sludge is dried to 75 percent solids using the hot gases from the incinerator.
The semi-dry sludge is screw fed to the top of a rocking grate incinerator.
-------
The incinerator will burn 300 metric tons/day (330 tpd) of refuse,
which typically has moisture content of 25 percent. Sludge burning capacity
is designed for 9.1 metric tons/day (10 tpd) of the 75 percent solids sludge
from the drier. The process design matches sludge solids and average refuse
solids, for the purpose of eliminating combustion problems.
Exhaust gases from the kiln are reintroduced into the incinerator for
deodorization, and an electrostatic precipitator controls particulate
emissions. The incinerator has auxiliary gas burners to operate the dryer.
when the incinerator is down.
Initially, the sludge will be primary, but Stamford is building a new
plant for secondary treatment. They see no problem when the new treatment
plant comes on line, because the ratio of sludge solids to refuse will be low.
This incinerator/sewage treatment plant handles all waste from Stamford
and Darien, Connecticut.
VISIT TO TWIN CITIES, MINNEAPOLIS/ST. PAUL, MINNESOTA
A pyrolizer-incinerator for Minneapolis/St. Paul is being designed and is
scheduled to be on line by 1980. Rust Engineering is the contractor for
the study. Pilot work was done by Vertiteck Labs., Louisville, Kentucky,
under Rust supervision.
The initial step will be heat treatment of raw sludge followed by
mechanical dewatering. Vacuum filters can produce a 30 percent sludge,
which can be co-incinerated at a 1:4 (dry weight) ratio with refuse.
Pressure filters can produce a k5 percent solids sludge cake, which can be
incinerated at a 1:2 ratio with refuse. The burning trash and sludge will
supply heat for raw sludge preconditioning.
A portion of the dewatered sludge will be pyrolyzed, and the recovered
fuel products will provide the heat for predrying and pyrolysis. A net
output is expected from this portion of the process. Char from the pyrolysis
unit will be reused in secondary wastewater treatment. All secondary sludge
will be incinerated after processing as described above. Only primary
sludge will be pyrolyzed.
The installation is expected to handle about 15 percent of the refuse
and 15 percent of the sludge (primary and secondary) of the Minneapolis/
St. Paul area. A portion of the fuel gas in incinerator afterburners may
have to be used. (Note: Recent information indicates that pyrolysis plans
have been abandoned.)
VISIT TO UNION CARBIDE, PUROX INCINERATOR, UNION CARBIDE CORPORATION,
SOUTH CHARLESTON, WEST VIRGINIA
The prototype Purox incinerator consists of one 182-metric ton/day
(200 tpd) shaft furnace using pure oxygen to burn and reduce municipal
refuse to a fritted residue while producing a combustible off-gas. The
189
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furnace system has been operating at approximately 91 metric tons/day
(100 tpd); Carbide personnel attribute this low capacity to the need for
prudent restriction of capacity at this stage of development.
On the day of the visit, a second run using shredded refuse was
performed. The first run on shredded refuse had been with a 10.2-cm (A-in.)
maximum size restriction on the shredded product, and Carbide had changed
the shredder for running on a 7.6-cm (3~in.) maximum size. Carbfde personnel
stated that it would be mid-1976 before any further attempt to incinerate
raw refuse.
Prior to visiting the prototype installation, Carbide sales personnel
made a lecture presentation on the Purox system. In this presentation, they
utilized data that had already been published; all the data were obtained
from their 4.5-metric ton/day (5~tpd) bench scale unit in Tarrytown, New York,
rather than on any runs at the prototype South Charleston unit.
We believe that the shift from raw to shredded refuse was caused by
problems in pyrolyzing raw refuse. Shifting from raw to shredded refuse
involved adding a 150-kW (200-hp) vertical-shaft Heil shredder, along with
a magnetic separator to reject the bulk of the ferrous metals. Another
change was an all-new furnace charging system that cannot be used to feed
raw refuse. The original configuration of the raw refuse feeder is unknown.
Mr. Jack Matthews, who is responsible for the operation of the furnace,
was our tour guide. In his opinion, once the constraints on capacity are
removed, Carbide will be able to easily exceed the rated capacity of the
furnace.
There have been no explosions in the shredder, but the term of operation
has been extremely short.
The explanation for the separation of ferrous metal from the feed
material includes the following:
1. The sii:e of the furnace hearth.
2. There is a single tap instead of dual taps, as is the practice
in the foundry industry.
3. Removal of the metal was an attempt to eliminate erosion of the
brick at the melt 1ine.
Carbide personnel conducted a walk-through type of tour, starting with
the floor dump arrangement for receiving refuse. A front-end loader is
190
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used to convey the raw refuse to the shredder feeder; two or three people
are positioned along the feeder to hand-pick material out of the raw refuse
ahead of the shredder. After shredding, a magnetic separator removes the
bulk of the ferrous metal and deposits it in a tote bin. A belt conveyor
conveys the shredded refuse to a feeder, which forces the material into the
pressurized furnace. The hot gases leaving the furnace are quenched with
a spray system positioned in the downcomer from the furnace and before any
gas-cleaning equipment. It was stated that the temperature drop in this
quench system was ?8°C (UO°F)--from 93°C to 15°C (20QOF to 60°F), and that
the liquid from this quench system normally went to the sewage treatment
plant. The quenched gases then pass through an electrostatic precipitator
of the tubular, single-field design, consisting of sixty 15.2-cm (6-ini)
tubes and incorporating a 320-V system.
The furnace is lined with 90 percent alumina brick, and it was stated
that the original brick was still in place, but Carbide personnel did not
know the total time that the brick has been in use since start-up. The
slag from the furnace is tapped out in a sealed enclosure, and drops into
a water bath, from which the fritted slag is withdrawn by a drag conveyor
which discharges into a tote bin.
The furnace charging system consisted of two hydraulically driven rams
that alternated in feeding refuse from the chute to the furnace. The ram
charging the refuse was covered by an oscillating piece while the other ram
feed point was being loaded by the continuously operating conveyor. Thus,
the ram being charged was always open while the other ram was feeding, and
the oscillating piece would shift to load the other ram feed point. The
ultimate discharge to the furnace was described as a restricted opening such
that the refuse being fed provided a plug to prevent the pressurized furnace
gases from exiting into the feeding compartment. However, the entire feed
point was then covered with a large box which was apparently a secondary
seal. We could not determine the exact configuration of this system.
VISIT TO THE UZWIL PLANT, NIEDER-UZWIL, SWITZERLAND
Municipal Sewage Treatment Plant
Sludge is generated in the treatment plant in the primary and secondary
clarifiers. Primary grit, sludge, and skimmings are hauled away. Secondary
sludge is digested in anaerobic digesters, which are heated by digester gas.
The digested sludge is then pumped to a thickener, where it is concentrated
to about 6 percent solids. The sludge production amounts to 20-25 cu m
(710-880 cu ft) per 24-hour operating day, at 6 percent solids content when
introduced into the incinerator. Secondary clarifier effluent is used in
the incinerator scrubber.
191
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Incinerator Installation
The plant operates 5 days per week, 2k hours per day. Refuse is
received and stored in the refuse pit, with oversize items diverted to a
separate location. The refuse is loaded on a conveyor that transports it
to a hammermill. Reduction size appeared to be about 7.6 to 10.2 cm
(3 to k in.).
The material then passes a magnetic drum, where all ferrous metal is
removed. The separated ferrous metal appeared clean, with very little
entrainment of other materials.
The remaining material is then conveyed onto a vibrating screen with
30-mm (1.2 in.) holes. The oversized material is transported to the top
of the multiple-hearth furnace by a vertical drag-link conveyor; the sludge
is also introduced at this location.
The undersized material is fed into another shredder, where it is
reduced to 1.3 to 0.64 cm (1/2 in. to 1/4 in.) size. This material is then
mixed with some sludge from the clarifier and stored for composting (or
loaded on trucks).
>
The multiple-hearth (Nichols) incinerator has three burners for auxiliary
heating. These nozzles introduce used oil (crankcase oil, etc.) with three
pumps, each with a capacity of 12 liters per hour (3.2 gph) to introduce
supplemental fuel when required to maintain proper hearth temperature. Not
all burners are used at the same time; one, two, or three can be used, as
requi red.
The first and second (from the top) stages have been removed from the
twelve-hearth incinerator.
The temperatures observed during the visit were:
Hearth 1 (top of furnace) 880°C (1,620°F)
Hearth 4 (now Hearth 2) 840°C (1,540°F)
Hearth 7 (now Hearth 5) 880°C (1,620°F)
The combustion gases pass first through a primary scrubber (in essence
a large vertical duct), where clarifier effluent is introduced into the
downward gas stream. This duct then connects into a 180° vertical bend,
where the spray water that has not evaporated is drained off the heel. The
combustion gases are ducted to a secondary scrubber, consisting of two
layers of Rasching Rings and spray nozzles. The sprays are fed with
secondary clarifier effluent water. An induced-draft fan conducts the
scrubbed gases to the chimney. There have been very few problems with
incinerator odors.
Municipal refuse is burned at a rate of roughly 78-80 metric tons per
day (86-88 tons per day)—24 hours a day, and about 20-25 cu m (710-880 cu ft)
of digested sludge at 6 percent solids fs incinerated dally.
192
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A special furnace (2.5 metric tons charge volume—88 cu ft) incinerates
oversize items, such as mattresses, tires, etc., that have been stored. The
combustion gases from this furnace are then fed to the Nichols Furnace,
which acts as a secondary combustion chamber to provide complete combustion
of any possible odors, smoke, etc.
REVIEW OF WASTE DISPOSAL OPERATIONS, WATERBURY, CONNECTICUT
Operational December 1951 and rehabilitated in 1968:
Two 136 metric ton/day (150-tpd) Batch-Fed Nichols monohearths to burn the
raw refuse, and a Combustion Engineering Raymond Flash drying System to dry
the dewatered primary sludge. The primary sludge is dewatered from 94
percent to 70 percent moisture on vacuum filters using ferric chloride and
lime for conditioning. The flash-dried sludge is screw-conveyed to one of
the two furnaces, but no details on feed method are included in available
data.
Dewatered sludge is available from the incinerator; the incinerator
also provides hot water for the plant. The hot water heater may be fired
with fossil fuel.
From the 1963 Annual report, we note that:
"Several major repairs at the incinerator cut heavily into
the normal burning time. This, together with abnormally high
moisture content of the refuse due to the very high rainfall,
reduced the average burning rate and forced us to bypass about
10.5 percent of all incoming material directly to landfill."
The report further states that on curved per capita basis the MMR
"breaks down to 1,530 pounds (695.5 kg) per year or k.2 pounds (U9 kg) per
person per day."
In 1973, the sludge to the filters averaged 4.58 percent dry solids and
was dewatered to 28.2 percent solids. After flash drying, the percent dry
solids was 49.9 percent.
i
All the sludge in 1973 was burned in Furnace No, 1 and it appears that
this is the only furnace equipped for sludge burning. Waterbury burned
25,035,000 kg (55,077,000 Ibs) of refuse along with 2,811,000 kg (6,184,000 Ibs)
of sludge. (We are assuming that the sludge is reported as fired.) Following
rehabilitation of the furnaces, which included the addition of wetted baffle
collectors and new high stack, the stack was tested for particulate emissions
in 1968. The following results were obtained:
Refuse Burning 0.528 kg/1,000 kg of flue gas
Sludge and Refuse 0.497 kg/1,000 kg of flue gas
193
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LIST OF CO-INCINERATION INSTALLATIONS
From catalogues and installation lists, the following list of sludge
flash drying co-incineration installations was compiled:
Location Status
Watervliet, New York (19^0) Shut down
Waterbury, Connecticut (1951) Standby
Bloomsburg, Pennsylvania (1953) Abandoned
Louisville, Kentucky (1959) Not co-incinerating
Neenah-Menasha, Wisconsin (1958) Shut down
New Albany, Indiana (1959) Shutdown
Trenton, Michigan (1964) Drying only
Newburgh, New York (1971) Abandoned
There is another installation at Eastman Kodak in Rochester, New York
(visited) and two rotary dryer installations at:
Holyoke, Massachusetts (1965) Visited
Stamford, Connecticut (1975) Recent start-up
The notations on status are based upon contact with Mr. George Simons,
Mr. R.D. Nickerson, and Mr. J.H. Fernandes of Combustion Engineering,
supplemented by contact with owner-operator and by plant visits. When the
status is "contacted" or "visited", a separate report is included in the
Appendix.
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APPENDIX C
GENERATION AND HANDLING
OF WASTEWATER TREATMENT SLUDGES
SOURCES AND EXTENT OF SLUDGE GENERATION
Suspended solids are usually present in the influent to municipal waste-
water treatment plants, at concentrations of 100 to 300 mg/1. In addition,
suspended solids are generated in biological and chemical precipitation
processes. These solids form the major by-product of a municipal wastewater
treatment plant: sludge.
The solids, removed by a variety of methods in wastewater treatment
plants, include grit, screenings, and scum, as well as sludge. (Grit,
screenings, and scum are not normally considered sludge.) Sludge is by far
the largest in volume, and its processing and disposal constitute perhaps
the most complex problem with which the engineer is faced in the field of
wastewater treatment.
"Sludge" is a broad term used to describe the various aqueous suspensions
of solids encountered during wastewater treatment. The nature and concentra-
tion of the solids control the processing characteristics of the sludge. The
unit processes used for capture, concentration, dewatering and disposal of
the solids encountered in municipal wastewater treatment plant operations are
frequently the most sensitive to changes and most difficult to design.
The full impact of the problems associated with disposal of the mixed
sludges resulting from secondary treatment has only recently begun to be
realized fully in the U.S. Coincidentally, the technical literature on sludge
reflects the growing concern for these problems. (All the entries in the
References Section for this document are from 1970 or later.) EPA has
published a technology transfer document^ on sludge treatment and disposal,
following up on previous EPA studies by Burd2 and Balakrishnan.3 The Burd
report is a comprehensive review of sludge handling and disposal practices,
while the Balakrishnan report deals specifically with sludge incineration
methodology. EPA has funded a number of other specific studies, and recently
many articles on sludge generation, production, handling, and disposal have
appeared in the literature.
The quantity of sludge produced in treating domestic sewage is a function
of wastewater characteristics and of the degree of treatment required to meet
effluent guidelines. The Federal Water Pollution Control Act Amendments of
195
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1972^* require application of secondary treatment as a minimum, with pro-
vision for applying (by 1983) the best practicable control technology.
WASTEWATER CHARACTERISTICS
The composition and concentration of wastewater constituents will vary
with time, sources, water quality, and the condition of the sewer system.
For any given area, the quantity and quality of the sewage should be deter-
mined and used in the design of the sewage treatment works. For the purposes
of feasibility studies, typical sewage characteristics and flows, as pre-
sented in Table C-1, are defined and can be used to calculate pollutant
loadings in municipal sewage. For example, with the medium concentrations of
Table C-1 and the widely used water usage rate of 380 liter/capita/day
(100 gal/capita/day), the suspended solids loading Is 0.077 kg/capita/day
(0.17 Ib/capita/day), and the BODc loading is also 0.077 kg/capita/day
(0.17 Ib/capita/day).
Zanoni and Rutkowski" have published a summary of literature findings
on per capita loading values. They reported that suspended solid values
range from 0.060 to 0.150 kg/capita/day (0.132-0.32k Ib/capita/day), and
BOD values from 0.0^5 to 0.12 kg/capita/day (0.099 to 0.26 Ibs/capita/day),
with five of eight sets of BOD data known to be on a total demand basis. The
authors also report studying an area that is entirely residential and report
the following data, representing strictly domestic wastewater, which includes
30 percent usage (reportedly the nationwide average) of garbage grinders:
Characteristics Value (per capita per day)
kg 1b
BOD (5-day, 20OC) 0.0^5 0.10
COD 0.090 0.20
BOD: COD ratio 1:2
SS 0.036 0.08
Wastewater flow
1/day/cap 220
gpd/cap 58
These loading values correspond to concentrations of 165 mg/1 of suspended
solids and 207 mg/1 of BODr, which approximate the medium concentrations
reported in Table C-1. Since it is impractical to isolate domestic sewage
totally from commercial, institutional, and light industrial sources or to
prevent sewer infiltration totally, the use of sewage at medium strength
and of a flow rate of 380 I/capita/day (100 gpcd) is a reasonable basis
for determining sewage sludge generation rates.
WASTEWATER TREATMENT PROCESSES AND
SLUDGES INVOLVED
The combinations of wastewater treatment unit processes are virtually
infinite, but can be categorized under the general headings of primary,
secondary and tertiary treatment. Figure C-1, from Eckenfelder and Ford7,
196
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TABLE C-1. TYPICAL COMPOSITION OF DOMESTIC SEWAGES
(All values except settleable solids are expressed in mg/liter)
Concentration
Constituent
Solids, total
Dissolved, total
Fixed
Volatile
Suspended, total
Fixed
Volatile
Settleable solids, (ml/liter)
Biochemical oxygen demand, 5-day, 20°C (8005-20°)
Total organic carbon (TOC)
Chemical oxygen demand (COD)
Nitrogen, (total as N)
Organic
Free ammonia
Nitrites
Nitrates
Phosphorus (total as P)
Organic
Inorganic
Chlorides*
Alkalinity (as CaC03)
Grease
Strong
1,200
850
525
325
350
75
275
20
300
300
1,000
85
35
50
0
0
20
5
15
100
200
150
Medium
700
500
300
200
200
50
150
10
200
200
500
ko
15
25
0
0
10
3
7
50
100
100
Weak
350
250
1*5
105
100
30
70
5
100
100
250
20
8
12
0
0
6
2
It
30
50
50
£
Values should be increased by amount in carriage water.
197
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Ultimote Disposal
PRIMARY
Concentrated Organic Stream I
ijjHiiNiiiiiiiiiiiiimiiiiiiiiniiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiHiiiiiiiiNiii
D
0
SECONDARY
TERTIARY
Spent Regenerate 8
Brine Disposal
t
Final
Effluent
CiMNHIIIIIIIIIIIIIIIIIIIIIIIIIIIIIinHIINIIIIIIIIIIIHUIIIIIIIIIIIIIUNIHIIIIIIUIinU'l^^j
oo
iiiiiiimiiiHiiiiiiiiiiniiii 0*' -lectro"
Supernatant Return 8
Secondary Treatment
Separate
Disposal
or
Oil Recovery
Figure C-1. Wastewater Treatment Processes:
Substitution and Sequence Diagram.
-------
indicates the multiplicity of available options, from which the designer
and operator select the one most appropriate, based on wastewater character-
istics, effluent quality requirements, and system cost. The sludges produced
in treating wastewaters must be disposed of, and Figure C-2 shows alternative
sludge processing steps that are feasible and widely used.
Conventional primary sedimentation plus aerobic secondary treatment
produces sludges which contain only a minimal amount of inert material and
are largely combustible, although the sludge solids are still tied up with
copious quantities of water. After incineration, only the inert ash remains
to be disposed of. Sludge incineration may, therefore, be considered a
sludge-processing step that burns the organics, evaporates the remaining
water, and reduces sludge solids to a much smaller volume and weight (of
organics) for ultimate disposal.
The addition of inorganic chemicals to the primary or secondary clari-
fiers of a conventional wastewater treatment plant to improve removal of
solids and BOD and/or phosphorus is a suspect procedure when the sludges
produced are to be incinerated before ultimate disposal of the residue.
The addition of inorganic chemicals (lime and the salts of iron or aluminum)
increases both the mass and the volume of sludge to be handled. The
non-combustible inorganics reduce the heating value of the dry sludge solids
and increase the quantity of incinerator residue. When the sludge solids
are incinerated with refuse, the sludge and refuse residues are mixed, and
any opportunity to recover chemical values is lost.
Addition of synthetic organic polyelectrolytes, either in the primary
or secondary treatment process, is an alternative to addition of inorganic
chemicals that can be used advantageously to improve the overall performance
of the treatment system. The combustible organics added are easily inciner-
ated and do not delrberately increase the quantity of incinerator residue.
In tertiary treatment, the use of inorganic chemicals to treat the
effluent from a primary-secondary treatment system allows segregation of
the resulting, largely inorganic, sludges produced. These sludges can then
be handled, processed, recycled, and disposed of independently of the
largely combustible solids produced in conventional secondary treatment. The
wastewater treatment model can, therefore, be defined as primary sedimenta-
tion and aerobic secondary treatment producing a reasonably combustible
sludge.
Sludge Quantities
The quantity of raw dry sludge solids produced by the model wastewater
treatment plant designed for an effluent suspended solids of 20 mg/1 and
BOD of Ik mg/1 is approximately 0.091 kg/capita day (0.20 Ib/capita/day).
The inert portion of the dry solids is approximately 20 percent. This
sludge quantity was calculated on the basis of medium-strength sewage
(Table C-1), 380 I/capita/day (100 gpcd), and Vesillnd'sS adaptation of
the method proposed by Kormanlk9.
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Wastewater
Treatment
Thickening
Primary
Secondary
Chemical
o
o
Conditioning
Stabilization
Gravity
Flotation
Centrifuge
Dewatering
Chemical
Elutriation
Thermal
Digestion
Freezing
Thermal
Processing
Drying Beds
and Lagoons
Screens
Vacuum Filters
Centrifuges
Press. Filters
Ultimate
Disposal
Drying
Incineration
Pyrolysis
Wet Oxidation
Landfill Burial
Ocean Disposal
Subsurface Burial
Surface
Wind Spreading
-^
Composting
-»
Figure C-2. Alternative Sludge Handling Process and Systems.
-------
Sludge Composition and Heating Value
The ultimate analysis of dry and ash-free sludge solids has been esti-
mated1" as follows:
Molecular
Element Weight Percent
Carbon, C 12 55
Hydrogen, H2 2 6
Oxygen, 02 32 35
Nitrogen, N2 28 3
Sulfur, S 32 1
The average high heat value for these dry volatile solids has been estimated'0
at 5,550 kg-cal/kg (10,000 Btu/lb).
SLUDGE HANDLING AND PROCESSING
SJudge Stabl lization
As shown in Figure C-1, the sludge to be wasted can be processed by
heat treatment or by digestion (aerobic or anaerobic). The net result of
either of these stabilization/reduction steps is to reduce the combustible
organic content of the resultant sludge. Anaerobic digestion of the raw
sludge produced by the model system reduces the heating value of the sludge
by 25 percent. Heat treatment reportedly" produces such a strong supernatant
that, when recycled to the treatment plant, it might increase plant design
capacity by as much as 15 percent.
Therefore, sludge stabilization/reduction process steps warrant no
further discussion or consideration in exploring the feasibility of the
co-incineration of sewage sludge and municipal refuse. We can now simplify
the sludge-handling options. The principal processing steps that are
potentially useful as preliminary to thermal destruction or incineration
are included in Figure C-2.
Sludge Thickening
The contaminants removed from the wastewater in the form of a dilute
aqueous sludge are typically concentrated by thickening, and the excess
water is returned to the head end of the treatment system. Gravity and
flotation thickeners are normally used, but, where space limitations or
sludge characteristics dictate, centrifuges may be more appropriate.
Thickening is used at most wastewater treatment plants, because it is an
efficient means of reducing the volume of sludge to be handled in subsequent
processing steps.
201
-------
Gravity Thickeners—
Both solids loading and hydraulic loading must be considered when
designing gravity thickeners. The following solids loadings have been used
for thickener design for different types of sludge:
Underflow
Type of Sludge Concentration Average Solids Loading (per day)
percent, TS kg/sq mIbs/sq ft
Primary 8-10 100 20
Primary and Trickling Filter 6-8 60 12
Primary and Waste Activated
(60:40 Weight Ratio) 3-6 40 8
Most thickeners are operated at a hydraulic loading of 2,400-3,300 liter/sq m
(600-800 gpd/sq ft). Thickeners with hydraulic loadings less than
1,600 liter/sq m (400 gpd/sq ft) have been found to produce odors.
Primary sludges are readily thickened in gravity thickeners, but waste
activated sludges are difficult to thicken; in both cases, the reduction in
sludge volume achieved by concentration is considerable. Experience with
biological sludge indicates that the sludge maintains its integrity in terms
of consolidation characteristics when mixed with other sludges. That is,
for a known quantity of primary and activated sludge, the floor loading and
underflow concentration can be calculated on a proportionate basis. It is
seldom desirable to achieve a solids concentration in excess of 10 percent,
because the thickened sludge would be viscous and very difficult to pump.
In summary, gravity thickening of combined primary and activated sludges
can produce a sludge containing 4 to 8 percent solids.
Flotation Thickeners—
Air flotation thickeners are better than gravity thickeners for waste
activated sludge. They can thicken this type of sludge to 6 percent, whereas
the maximum attainable by gravity thickeners Is 2-3 percent. The air
flotation process can also be applied to mixtures of primary and waste
activated sludge. The greater the ratio of primary sludge to waste activated
sludge, the higher the permissible solids loading to the flotation unit.
Because of its high operating cost, air flotation generally is considered
only for thickening waste activated sludge. Table C-2 summarizes typical
parameters used in the design of air flotation thickening units.
202
-------
TABLE C-2. TYPICAL AIR FLOTATION DESIGN PARAMETERS
— .
Parameter Values
Air Pressure,
kg/sq cm 3-5
Psig to-yo
" Effluent Recycle Ratio,
percent of Influent Flow 30-150
Air-to-Solids Ratio,
weight air/weight solids 0.02
Solids Loading, average
kg/sq m/day 50 (100-200)*
Ib/sq ft/day 10 (20-40)*
Polyelectrolyte Addition,
kg/metric ton dry solids 0 (2-4)
Ib/ton dry solids 0 (5-10)
Solids Capture, percent . 70-90 (90-96)
Total Solids, percent:
Unthickened 0.5-1.5
Thickened k-6
*
Figures in parentheses denote values with polyelectrolyte addition.
Typical operating data for various air flotation units are presented in
Table C-3. Combined primary and activated sludge produces a more concen-
trated float sludge than does waste activated sludge alone. Polymer and/or
chemical addition permits greater solids loading and improves solids recovery
without substantially increasing the float solids concentration.
Sludge Dewataring
A sludge dewatering process is one which removes sufficient water from
sludge so that its physical form is changed from essentially that of a fluid
to that of a wet solid cake. The principal sludge dewatertng methods are
vacuum filtration, centrifugation, and pressure filtration.
Vacuum FiItration—
Vacuum filtration is the most commonly used mechanical sludge dewatering
method in the United States. Conditioning of wet sludge is necessary to
achieve satisfactory yields from vacuum filters. Conditioning coagulates
the sludge particles and allows the water to drain freely. As a result, a
203
-------
TABLE C-3. AIR FLOTATION THICKENING PERFORMANCE
Type of
SI udge
Waste Activated
Waste Activated*
Waste Activated
Waste Activated
N> Waste Activated
o
.p-
Waste Activated
Waste Activated
Combined Primary and
Waste Activated
Combined Primary and
Waste Activated
Combined Primary and
Waste Activated
kg/sq m/day
60-90
120-230
68
35
97
128
141
120-230
100
227
Solids
Loading
]fas/sq ft/day
12-18
24-48
13.9
7.1
19.8
26.2
28.8
24-30
21
46.6
Feed
Sol ids
percent
0.5 to 1.5
0.5 to 1.5
0.81
0.77
0.45
0.80
0.46 '
1.5 to 3.0
0.64
^
2.30
Float
Solids
percent
4.0 to 6.0
4.0 to 5.0
4.9
3.7
4.6
6.5
4.0
6.0 to 8.0
8.6
7.1
Solids
Recovery
percent
85 to 95
95 to 99
85
99
83
93
88
85 to 95
91
94
-------
thicker filter cake is produced and the drum can be rotated at a higher
speed. The filtrate contains a high concentration of fine suspended solids
and is returned to the treatment plant.
The number and size of filters are based on the type of sludge to be
filtered and the number of hours of operation. At small plants, 30 hr/wk
constitutes a normal schedule; at large plants, 20 hr/day may be necessary.
The remaining hours in the day are used for conditioning, clean-up, and
possible delays. A plant may be designed for one-shift operation initially,
and for two- to three-shift operation of the same filters when the plant is
expanded to provide for increased sewage flows.
The performance of a vacuum filter is measured in terms of the yield of
solids on a dry weight basis, expressed as pounds per square foot per hour
(kg/sq m/hour). The quality of the filter cake is measured by its moisture
content expressed as a percent of the total (wet) weight.
Filters are operated to obtain the maximum production consistent with
the desired cake quality. Where the cake is to be heat-dried or incinerated,
the moisture content is a critical item, since all the water remaining in
the cake must be evaporated to steam. Typical performance data are shown in
Tables C-4 and C-5. Typical vacuum filtration chemical conditioning dosage
rates are shown on Table C-6.
TABLE C-6. ESTIMATED CHEMICAL CONDITIONING DOSAGE*
FOR VACUUM FILTRATION*
Type of Sludge CaO Dose FeCl^ Dose Polymer Dose
Ibs/ton Ibs/ton Ibs/ton
Primary Sludge
Limed Primary (212 Ib CaO/ton)
Digested Primary Sludge
Digested/Elutriated Primary
Raw (Primary + EAS)
Limed (Primary + EAS)
Digested (Primary + EAS)
Digested/Elutriated (Primary + EAS)
176
0
240
0
200
0
372
0
2iO
\t O
76
68
52
40
110
125
5
5
20
9
18
5
36
24
*Source: EPA Process Design Manual for Sludge Treatment and Disposal.
+1 Ib/ton = 0.412 kg/metric ton.
A vacuum filter can dewater combined (primary and secondary) thickened sludge
to approximately 20 percent solids (range 16-25 percent), inorganic chemicals
and/or polyelectrolytes are used to condition the sludge before filtration.
Typical inorganic chemical dosage rates are 2-6 percent ferric chloride
(FeCl^and 10 percent lime (CaO) used together.1 Alternatively, 0.9 percent
of a polymer can be used. All dosage percentages are expressed as percent
additions to dry sludge solids.
205
-------
TABLE C-4. TYPICAL ROTARY VACUUM FILTER RESULTS
FOR SLUDGE CONDITIONED WITH INORGANIC CHEMICALS*
Type Sludge
Chemical Dose,
kg/metric ton (ib/ton)
Ferric Chloride Lime
Yield,
kg/sq m/hr (Ib/sq ft/hr)
Cake
Solids
percent
Raw
Primary
0.4-0.8 (1-2) 2.5-3.3(6-8)
30-40 (6-8)
25-38
Anaerobically
Digested Primary
0.4-1.2 (1-3) 2.5-4.1 (6-10)
25-40 (5-8)
25-32
Primary +
Humus
0.4-0.8 (1-2) 2.5-3.3 (6-8)
20-30 (4-6)
20-30
Primary +
Air Activated
0.8-1.6 (2-4) 2.9-4.1 (7-10)
20-25 (4-5)
16-25
Primary + Oxygen
Activated
0.8-1.2 (2-3) 2.5-3.3 (6-8)
25-30 (5-6)
20-28
Digested Primary and
A? r Activated
1.6-3.6 (4-6) 2.5-7.8 (6-19)
20-25 (4-5)
14-22
*Source: EPA Process Design Manual for Sludge Treatment and Disposal.
-------
TABLE C-5 TYPICAL ROTARY VACUUM FILTER RESULTS
FOR POLYELECTROLYTE-CONDITIONED SLUDGES*
Type Sludge
Raw
P r i ma ry
Anaerobical ly
Digested Primary
Primary +
trickling filter Humus
Primary +
Air Activated
Primary + Oxygen
Activated
Anaerobical ly Digested Primary
and Air Activated
Chemical Cost,
$/metric/ton ($/ton)
1-2 (1-2)
2-6 (2-5)
3-7 (3-6)
6-13 (5-12)
6-11 (5-10)
7-16 (6-15)
Yield,
kg/sq m/hr (Ib/sq ft/hr)
1*0-50 (8-10)
35-40 (7-8)
20-30 (4-6)
20-25 (4-5)
20-25 (4-6)
17-30 (3.5-6)
Cake
Solids
percent
25-38
25-32
20-30
16-25
20-28
14-22
*Source: EPA Process Design Manual for Sludge Treatment and Disposal.
-------
Centrlfugation—
Centrifuges of various types have been employed for soUd-1 tqutd
separation processes in agriculture and Industry for at least 50 years. For
almost 25 years, the continuous solid-bowl conveyor-type centrifuge has been
used for dewatering municipal sludges. Objectives of centrifugal sludge
dewatering are the same as for rotary vacuum filtration. Sludge is fed into
the rotating bowl at a constant flow rate, and separates into a dense cake
containing the solids and a dilute stream called centrate. ^he centrate
contains fine, low-density solids, and is returned to the raVf-sludge
thickener or primary clarifier.
Centrifuges can produce dewatered cakes generally comparable to those
obtained by vacuum filtration. Polymers are used to improve the solids
recovery, but produce a slightly wetter cake. A summary of results achievable
with various sludges is shown in Table C-7.
TABLE C-7. TYPICAL SOLID-BOWL CENTRIFUGE PERFORMANCE*
Wastewater Sludge Type
Solids Chemical
Solids Recovery Addition
Raw or digested primary
Raw or digested primary, plus trickling
fi1ter humus
Raw or digested primary, plus activated
sludge
percent percent
28-35 70-90
(50-70F
20-30
15-30
80-95
60-75
80-95
50-65
no
yes
no
yes
no
^Source: EPA Process Design Manual for Sludge Treatment and Disposal.
+New data indicate performance is in this range.
Pressure Filtration—
Properly conditioned sludge can be dewatered in filter presses to very
high solids levels. Experience in the United States with pressure filtration
of municipal sludges has been limited. Table C-8 indicates typical sludge
concentrations that can be produced using pressure filtration, with the
addition of ash or a combination of ferric chloride and lime. These data
are based on European experience.
Filter presses can produce drier cakes than can vacuum filters or
centrifuges. Sludge conditioning is required, and pre-coating with inert
ash, diatomaceous earth, etc. often is combined with ferric chloride and
lime conditioning. Based on European practice, cake solids of 50 percent
208
-------
TABLE C-8. TYPICAL FILTER PRESS PRODUCTION DATA*
Sludge Type Suspended Solids
(percent)
*_,
Raw Primary *** 5-10
Raw Primary 3-6
with less than
50 percent EAS
Raw Primary 1-4
with more than
50 percent EAS
Digested with 6-10
less than
50 percent EAS
Digested with 2-6
more than
50 percent EAS
EAS Up to 5
Conditioning
Dry Solids
(percent)
Ash
Fed 3
Lime
Ash
FeCl3
Lime
Ash
Fed,
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
of
100
5
10
150
5
10
200
6
12
100
5
10
200
7,5
15
250
7.5
15
Cake Sol ids
(percent)
50
45
50
45
50
45
50
45
50
45
*
50
45
Time Cycle
(hrs)
1.5
2.0
2.0
2.5
2.0
2.5
;
1.5
2.0
1.5
2.5
2.0
2.5
*Source:EPA Process Design Manual for Sludge Treatment and Disposal.
209
-------
can be produced using 150 percent to 200 percent ash conditioning. Dewaterlng
to 45 percent solids can be accomplished by conditioning the sludge with 5-6
percent ferric chloride and 10 to 12 percent lime.
SUMMARY
The total quantity of sludge produced by the example primary-secondary
wastewater treatment plant is constant, but the quantities of water and of
conditioning chemicals are variables dependent on the dewatering method
selected. Table C~9 reports the sludge quantities produced.
REFERENCES
1. Anon. Process Design Manual for Sludge Treatment and Disposal.
EPA 625/1-74-006 (October 197*0.
2. Burd, R.S. A Study of Sludge Handling and Disposal. FWPCA (EPA)
Publication WP-20-4, NTIS No. PB 179514 (May 1968).
3. Balakrishnan, S. et al. State of the Art Review on Sludge Incineration
Practice. FWQA (EFA"T~No. 17070 DIV (April 1970).
4. Federal Water Pollution Control Act Amendments of 1972, Public Law
92-500 (October 18, 1972).
5. Metcalf and Eddy, Inc. Wastewater Engineering: Collection, Treatment,
Disposal. New York, McGraw-Hill Book Co. (1972).
6. Zanoni, A.E. and R.J. Rutkowski. Per Capita.Loadings of Domestic
Wastewater. Jour. Water Poll. Control Fed. 44,9:17-56 (September 1972).
7. Eckenfelder, W.W. and D.L. Ford. Water Pollution Control. New York,
Jenkins Book Publishing Co. (1970).
8. Vesilind, P.A. Treatment and Disposal of Wastewater Sludges. Ann Arbor,
Michigan, Ann Arbor Science Publishers Inc. (1974).
9. Kormanik, R.A. Estimating Solids Production for Sludge Handling.
Water & Sewage Works 9,12:72-74 (December 1972).
10. Unterberg, W. et_ aj_. Computerized Design and Cost Estimation for
Multiple-Hearth Sludge Incinerators. EPA Report 17070 EBP 07/71 (1971).
11. Krishnan, P. Process Design Manual for Upgrading Existing Wastewater
Treatment Plants. U.S. Environmental Protection Agency, Technology
Transfer (1971).
210
-------
TABLE C-9. SLUDGE QUANTITIES*
(Pounds per Day)+
Item
Volatile Solids
Inerts
Total Solids
Fed 3
CaO
Polymer
Ash
Subtotal
Water
Total Sludge
Thickened
1,600
400
2,000
•*«
—
—
--
—
48,000
50,000
(4% Solids)
Vacuum Fi
Chemical
Condi t ion ing
1,600
400
2,000
52
200
—
—
2,252
9,008
11,260
(20% Solids)
It rat ion
Polymer
Conditioning
1,600
400
2,000
—
—
18
—
2,018
8,072
10,090
(20% Solids)
Pressure Filtration
Ash
Conditioning
1,600
400
2,000
—
—
—
4,000
6,000
6,000
12,000
(50% Solids)
Chemical
Condi tioning
1,600
400
2,000
120
240
—
—
2,360
2,884
5,244
(45% Solids)
*Assumes 100% solids capture. Equivalent population is 10,000; sewage flow is 1 mgd (3,800 cu m/day).
+1 Ib/day = 0.454 kg/day.
-------
APPENDIX D
REFUSE
REFUSE GENERATION RATES
The following information has been taken from the "Baltimore Region Solid
Waste Management Plan", prepared by Roy F. Weston, Inc. for the Baltimore
Regional Planning Council. These data have been used because domestic refuse
was separated from manufacturing and commercially generated refuse and because
the available data included information on a variety of miscellaneous wastes.
Domestic waste quantities were correlated with population and with employment-
related waste generation in the manufacturing and commercial/institutional
sectors.
The study region consisted of the City of Baltimore and the five adjacent
counties—Baltimore, Anne Arundel, Carroll, Harford, and Howard. The bulk
of the population lives in an urban environment. However, only 16 percent
of the region's 1970 population lived in major incorporated urban areas; a
corresponding figure for 1960 was 20 percent.
Baltimore City, and its densely populated urban fringe, accounted for
the bulk of the urban population. Urban concentrations surround the core
city area (portions of Baltimore and Anne Arundel Counties), and there are
isolated concentrations around Bel Air and U.S. 40/1-95 in Harford County,
the Columbia area in Howard County, the Annapolis area in Anne Arundel County,
and the Westminster area in Carroll County.
The area is thus fairly representative of the kind of region where
incineration of refuse and sewage sludge needs to be considered, because the
availability of landfill disposal sites has been severely restricted by
urban sprawl.
In the Baltimore Region, the city and county governments are assuming
increasing responsibility, in one form or another, for the collection of
domestic solid waste, but basically have left the collection of manufacturing
and commercial solid waste to private parties. Anne Arundel County, Baltimore
City, and Baltimore and Howard Counties have assumed responsibility for
residential solid waste collection. Harford County licenses and regulates
private firms which collect solid waste from individual citizens. Only
Carroll County still avoids any involvement in residential solid waste col-
lection. While Baltimore City and Baltimore County do collect some
employment-re lated solid waste, it is primarily a private responsibility
throughout the Baltimore Region.
212
-------
In the Baltimore Regional Study wastes were categorized into three
groups, according to generating source:
• Domestic waste, which is generated in households.
•Employment-related waste, resulting from the production of goods
and services.
• Special waste, which requires special handling.
Both Baltimore City and Baltimore County had records of domestic waste
generation. The Department of Sanitation in Baltimore City has scales at
both incinerator facilities, and Baltimore County's Sanitation Bureau has
maintained weighing records of domestic waste generation since 1967. Per
capita domestic waste generation figures for the Region were based upon these
data recorded in Baltimore City and Baltimore County.
Similar records did not exist for employment-re lated waste generation
in the Baltimore Region. (Employment-re lated waste is defined as that
resulting from manufacturing, commercial, and institutional activities.)
Since the actual generation of employment-re lated waste is highly dependent
on the type and rate of manufacturing, commercial, and institutional activity
within a specific region, the refuse generation rates were determined by
survey.
The per capita domestic waste generation rates used to develop annual
residential waste load projections for Anne Arundel, Carroll, Harford, and
Howard Counties were assumed to be the same as those recorded by Baltimore
County.
The Baltimore County data disclosed a 1 July 1972 generation rate of
1 kg (2.2 Ibs) per capita per day. Data concerning the generation of domestic
waste in the City of Baltimore, disclosed a generation rate of 1.10 kg (2.^2 Ibs)
per capita per day. This generation rate is at the low end of the National
Average.
Domestic waste generation projections were based upon 2 percent, non-
compounded annual increase in per capita waste generation. Population estimates
(based upon data furnished by the Regional Planning Council) along with annual
domestic solid wastes quantities for the years 1970, 1980, and 1990 are pre-
sented in Table D-1. National averages included in Table D-1 are from EPA
publication "Comparative Estimates of Post Consumer Solid Wastes" by F.A. Smith,
EPA/53b SW-148, May 1975.
213
-------
TABLE D-1. PROJECTED POPULATION AND ANNUAL DOMESTIC REFUSE TONNAGE
BALTIMORE REGION—1970 TO 1990
Year
1970
1980
1990
Metric Tons ^Tons) Per Year
788
1,105
1,475
,909
,820
,900
(1
(1
(867
,216
,623
,800)
,400)
,500)
Population
2
2
2
,070
,435
,800
,670
,735
,800
Generation
1.05*
1.25
1.45
2.
2.
3.
Rate#
30*
74
18
Weighted average in kg per capita per day: assumes a 2% non-compounded,
annual increase.
Same--on a Ib per capita per day basis.
#U.S. National Average for 1971 is 1.06-1.55 kg (2.39-3.48 Ib) per capita
per day.
The composition of domestic solid waste in the Baltimore region (from
the Baltimore Region Plan) is presented in Table D-2. There have been three
studies analyzing the composition of domestic waste in Maryland, and these
analyses are reported and compared to the average U.S. Refuse Composition.
While domestic waste generation rates may be conveniently correlated
with population, any attempt to correlate employment-related waste with
population characteristics at the regional level would be very speculative.
Plant conditions, access to recycle markets, efficiency and productivity,
product mix, and choice of products are but a few of the hard-to-assess
factors which determine industrial refuse quantities and account for the
remarkable lack of uniformity in regard to waste-handling practices.
For these and other reasons, plus the unavailability of detailed local
data, the employment-related wastes were determined from a solid waste survey
of the Region. The Standard Industrial Classification (SIC), developed by
the Office of Statistical Standards in the Federal Bureau of the Budget, was
utilized for the classification of establishments in the Baltimore Region
according to their types of activity.
The actual generation rates per employee by SIC code and a detailed
description of the manner in which they were used are presented in a separate
document entitled "Solid Waste Generation Rates, Baltimore Region". The
rates determined by this survey were then applied to the employment in the
Baltimore Region to determine the total quantities of wastes generated by
census tract.
Solid waste generation rates are not likely to vary as greatly within
industrial groups as they do across industrial lines, especially when con-
sidering one region of the county. However, waste generation does vary
significantly between the manufacturing sector and the commercial/institutional
(C/l) sector, both by quantity and type. For the following reasons, the
214
-------
TABLE D-2. DOMESTIC SOLID WASTE COMPOSITION COMPARISON* -- BALTIMORE REGION
Classification
Garbage
Paper
Glass and Ceramics
Plastic and Rubber
Leather and Linoleum
Wood
Metals
Ashes
Paints and Oi Is
Forestry
Miscel laneous
Total
(Percent
Baltimore
Region
11
50
10
5
1
4
8
1
1
6
3
100
Total by Weight)
Northern
Baltimore Montgomery
County* County
11
47
13
5
1
4
8
1
1
6
3
100
11
53
8
4
1
k
8
1
1
5
k
100
Anne
Arundel
County
13
52
16
11
—
—
•MM
2
•»•»
—
6
100
U.S.
Average
8.5
60.0
8.0
3.9
0.4
2.5
8.0
0.9
0.3
6.5
1.0
100
*Roy F. Weston, Inc. analysis.
+Engineering Evaluation, Solid Waste Transfer-Reduction FaciUty at
Texas, Maryland, Green Associates, Inc., October 1972.
215
-------
two sectors were presented separately in the Baltimore reports on solid
waste management planning for the Baltimore Region:
•The average employee in the manufacturing sector
generates more than 7 times the waste generated
by the average employee in the C/l sector.
•The estimated current recycle rate is 25.6 percent
for the manufacturing sector, and 3-4 percent for
the C/l sector.
•A wide variety of wastes is generated in manufacturing
operations, while a high proportion of waste in C/l
sector is paper.
•The Regional Planning Council estimates that employment
will increase between 1970 and 1990 by 9 percent and
41 percent in the manufacturing and C/l sectors
respectively.
Table D-3 is a comprehensive survey of waste-generat ton tn the Region.
Table D-4 gives a detailed summary of special wastes.
The net employment-related column in Table D-3 includes both the
manufacturing and the C/l sectors. For the years in question, the percentage
of the net total reported attributable to the C/l sector is 48.6 percent (1970),
55 percent (1980), and 59-3 percent (1990).
The commercial and institutional (C/l) waste-generation category covers
a variety of sources. Commercial sources include: shopping centers,
restaurants, entertainment facilities, individual retail and wholesale
establishments, private-sector offices, motels and hotels, and other non-
manufacturing operations. The institutional group consists of hospitals,
school and public buildings, and public services.
The composition of wastes generated from activities in the commercial/
institutional sectors is fairly consistent. Although there are seasonal
variations in the business activities, these variations do not cause sig-
nificant corresponding variations in the types of solid wastes generated.
Survey data, which included waste composition breakdowns, disclosed
a high percentage of wastepaper generation. For 1970, paper accounted for
65 percent of the C/l waste composition, wood accounted for 13 percent,
mixed refuse 13 percent, garbage 8 percent, tires 1 percent, and metals
less than 1 percent.
Domestic refuse generation rates can conveniently be reported in terms
of per capita population. Manufacturing and C/l waste generation rates
should be generated for each region based upon the mix of SIC code enter-
prises; but we can, for the Baltimore Region, translate the available data
into a per capita generation rate.
216
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TABLE D-3. ANNUAL NET WASTE GENERATION
(Tons/year X 1.000)*
~ BALTIMORE REGION
Jurisdiction
Anne Arundel
County
Baltimore
City
Baltimore
County
Carroll
County
Harford
County
Howard
County
Region
Year
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
Net
Total
476.6
706.4
956.0
1,907.0
2,076.9
2,246.2
1,178.0
1,475.9
1,798.8
133.7
184.3
235.1
201.9
304.1
406.9
122.7
299-2
477.8
4,020.4
5,046.8
6,120.8
Gross,,
Total*
647.6
860.3
10,945.0
3,934.0
4,103.9
4,273.2
1,608.0
1,862.9
2,147.1
993.7
958.3
931.7
841.9
880.1
925.3
415.7
562.9
715.1
8,441.4
9,228.4
10,086.9
Domest ic
119-5
199-1
297.3
400.0
472.8
543.1
249.4
357.3
484.7
27.7
43.8
63.4
46.3
74.0
107-9
24.9
69.4
127.1
867.8
1,216.4
1,623.5
Net
Employment
Related
162.8
235.8
308.7
1,065.3
1,157.5
1,249.8
455.4**
544. 7**
634. 0**
50.4
67-3
80.8
80.9
131.6
176.2
43.2
104.5
152.6
1,858.0
2,241.4
2,602.1
* 1 metric ton = 0.907 ton.
+ Total quantity requiring disposal; excludes agricultural wastes,
dredgings, dunnage, and floatage debris.
# Does not include waste oil.
** Bethlehem Steel Corp. Sparrows Point Plant figures omitted.
217
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Jurisdiction
Anne Arundel
County
Baltimore
City
Baltimore
County
NJ
— • Carroll
OO county
Harford
County
Howard
County
Reg 1 on ^
Year
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
1970
1980
1990
Agricultural
Waste
171.0
153.9
138.5
--
~
~~
430.0
387.0
348.3
860.0
774.0
696.6
640.0
576.0
518.4
293.0
263.7
237.3
2.394.0
2,154.6
1,939.1
Net
Construction
and „
Demolition''
40.2
55.8
71.4
2,273.0
2,273.0
2,273.0
53.1
63.4
73.7
4,. 6
6.0
7.5
12.1
16.1
20.1
33.3
77.6
121.8
2,416.3
4,908,2
4,594.5
Dead
Animals
0.8
0.8
0.9
0.7
0.7
0.7
1.6
1.7
1.8
2.4
2.4
2.5
2.2
2,2
2.2
1.1
1.2
1.-3
8.8
9.0
9.4
Hospital
Waste
5.5
8.4
11.8
14.1
15.3
16,6
3.8
5.0
6.4
0.3
0.4
0.6
0.4
0.6
0.9
1.0
1.7
24.1
30.7
38.0
Junked and
Abandoned
Vehicles
11.3
16.9
23.2
35.0
40.1
45.9
29.4
43.5
62.7
2.4
3.2
4.0
3.5
4.7
5.8
2.3
3-7
8.3
83.9
112.1
149.9
Leaf
Waste
0.6
0.9
1.1
1.9
1.9
1.8
1.3
1.6
1.8
0.1
0.2
0.2
0.2
0.3
0.4
0.1
0.3
0.5
4.2
5-2
5.8
Recreational
Litter Waste
1.9
2.7
3.4
1.8
1.8
1.8
3.2
3.8
4.4
1.4
1.8
2.3
1.4
1.8
2.3
0.8
1.9
2.9
10.5
13.8
17.1
0.7
1.0
1.3
1.2
1.6
2.2
1.4
1.9
2.6
0.1
0.2
0.2
0.2
0.3
0.4
0.3
0.4
0.5
3.9
5.4
7.2
Residential
Bulky Waste
25.1
34.8
44.6
76.3
75.2
74.0
52.3
62.5
72.7
5.8
7.7
9.5
9.7
12.9
16.2
5.2
12.1
19.1
174.4
205.2
236.1
Scrap
Tires
2.8
3.9
5.0
8.6
8.3
8.3
5. -9
7.0
8.1
0.7
0.8
1.0
1.1
1.5
1.8
0.6
1.5
2:1
19.7
23.0
26.3
Sewage
Solids**
99.1
137.6
176.1
25-9
25.9
25.9
302.4
361.1
419.9
37.6
49.5
61.5
42.7
56.8
71.0
10.0
23.3
36.5
517.7
654.2
790.9
Street
Sweeping
6.3
8.7
11.2
28.1
27.3
27.3
18.8
22.4
26.0
0.7
1.0
1.6
1.2
1.3
1.7
0.9
2.3
3.4
56.0
63.0
71.2
Water
Treatment Waste
Solids Oil**
991.3
1,294.5
1,597.7
2.1 12,093.5
2.5 12,786.6
2.8 13,479.7
12,076.2
13,933.3
15,790.3
191.9
255.0
318.0
105.0
152.7
200.4
82.1
193.7
305.3
2.1 25,540.0
2.5 29,836.2
2.8 34,132.3
* 1 Metric tdn = 0.907 ton.
+ Projections assume a 10 percent decrease in waste quantity per decade.
# Includes 2,000,000 tons of dredgings and 27,000 tons of dunnage and flotage debris for Baltimore City.
** Excludes septic tank sludge.
•M- Gross total In thousand gallons per year (3.79 liters - 1 gal).
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The RFP has stipulated the sewage treatment plant (STP) sizes of 1,
10 and 100 mgd, which at 100 gpd per capita correspond to 10,000, 100,000,
and 1,000,000 people. Using 1980 as the base year and considering only
domestic refuse at 1.25 kg (2.74 lb) per capita per day, we can determine
the approximate size of a refuse incinerator (a further assumption is 2k
hours per day operation for 6 days per week operation and 50 weeks per year).
Thus, the incinerator size equivalent to a 3,785 cu m/day (1 mgd) STP is
15.5 metric ton/day (17 tpd), for a 37,850 cu m/day (10 mgd) STP the equiva-
lent incinerator is 155 metric ton/day (170 tpd), and for a 378,500 cu m/day
(100 mgd) STP the equivalent refuse incinerator is 1,550 metric ton/day
(1,700 tpd).
The foregoing information may be used as base 1ine data where the
sewage treatment plant handles largely domestic sewage and the refuse
incinerator handles largely domestic refuse.
While the net employment-re lated refuse quantities were developed on
the basis of specific industries and enterprises and their specific waste
generation rates, these data may be converted to an equivalent generation
rate per capita of 2.24 kg/C/D (4.92 lb/C/D) —1970, 2.29 kg/C/D (5.04
lb/C/D) —1980, and 2.31 kg/C/D (5.09 lb/C/D) —1990. The commercial and
institutional per capita waste rates are thus 1.09 (2.39) for 1970,
1.26 (2.77) for 1980, and 1.37 (3.02) for 1990, all data reported in kg
(pounds) per capita per day.
• For purposes of the co-incineration study, the domestic refuse genera-
tion rates derived from the Baltimore area report will be used as a base
line during the feasibility study of selected techniques. Since the refuse
generation rates attributable to manufacturing or to the commercial/institu-
tional sector are highly dependent upon the actual business structure of
the community, these data will be incorporated into the feasibility study
only when necessary to provide additional heat to allow burning the sewage
solids generated by an equivalent population.
Refuse Characteristics
The quantity of refuse generated in a region is important, but so is
the quality of the refuse. The quality (energy/mass as received) is highly
variable and dependent upon a number of factors:
•Geographic location
•Season of the year
•Climatic conditions
•Collection practice
•Industrial, commercial, residential sources.
Refuse characteristics are expected to change with time, thus reflecting
the changes in our consumption of disposable items. Municipal refuse
incineration is therefore a unique process in that the feed stock changes
from year to year, month to month, day to day, and probably from hour to
hour.
219
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To illustrate the variability, Table D-5 has been abstracted from a
review paper presented at an A.I.Ch.E. Meeting (E.M. Smith, "Munfcfpal
Incinerator Emissions—Current Knowledge," 72nd A.I.Ch.E. Meeting, St. Louis,
Missouri, May, 1972); Office of Solid Waste Management Programs data are also
included.
TABLE D-5. MUNICIPAL REFUSE COMPOSITION
ADL
OSWMP
Category
Food Wastes
Yard Wastes
Miscel laneous
Glass, Ceramics
Metal
Paper Products
Leather, Rubber, Plastics
Text! les
Wood
051, Paint, Chemicals, etc.
Ash, Rock, Dirt
Percent by Weight
0.8 to 34.6
1.6 to 33.3
0.2 to 23.6
2.0 to 17.9
4.6 to 14.5
17=5 to 61.8
1.0 to 5.8
0.4 to 4.8
0.3 to 22.4
0.8 to 12.0
Percent by Weight
2.2 to 30.0
0.0 to 26.0
0,9 to 24.6
4.2 to 15.9
21.6 to 76.6
1.0 to 6.6
0.2 to 13.4
0.0 to 11.5
0.0 to 32.2
The total incombustibles are highly variable, as can be seen from the
Table D-5, and another variable is the moisture content. The moisture
content reported in Table D-6 shows a range of 18.1 to 48.1 percent moisture.
Perhaps the best example of the extreme variability in refuse composi-
tion is to examine the "yard wastes" category in Table D-5, where, according
to OSWMP data, the percentage varies from 0.0 to 26.0 Both of these
percentages are reported for the same incinerator test series during July
and separated in time by only two days.
A recent review of solid waste heating values was published in 1974
(A.C.W. Eggen and R. Kraatz, "Relative Value of Fuels Derived from Solid
Wastes," Proceedings of 1974 National Incinerator Conference, ASME, N.Y.,
N.Y., pages 19~32, May, 1974). The authors review the work of previous
investigators who have reported upon the averages reported range from
19,400 to 20,900 J/g (8,340 to 9,000 Btu/lb) of combustibles. The authors
then suggest a heating value of 10,200 J/g (4,400 Btu/lb) of refuse as
received, with moisture content of 0.28 kg of water per kg of refuse and
inerts of 0.22 per kg of refuse. They also report the average kg of
combustible per kg of refuse as received ranging from 0.50 to 9.5 percent.
220
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TABLE D-6. REFUSE MOISTURE CONTENT
Incinerator
Location
Month
Moisture as
Sampled
Percent by Weight
Southern
July-August
D
E
Midwest
Southern
October
December
Southern
Southern
December
February
Middle Atlantic
January
20.7 Average
Average
21.1
20.2
21.8
21.0 Average
25.6
28.9
28.5
29.6
28.2 Average
(Reference: OSWMP open file test reports)
While average heating values are useful, it fs also important to under-
stand the effect of variations in refuse composition as it affects both the
heating value and the inerts content (water and residue). Figure D-1 relates
refuse heating value to the percent combustible, or (inversely) to the percent
inert content of the MMR. This figure was based on a series of test reports
prepared by the Office of Solid Waste Management Programs (OSWMP) of the
Federal EPA. The data from these reports indicate that the heating value
of fresh refuse moisture- and ash-free ranges from 14,000 to 18,000 kg-cal/kg
(7,920 to 9,800 Btu/lb, average 16,000 kg-cal/kg (8,960 Btu/lb). The
221
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center! ine, therefore, represents the average based on the OSWMP reports, and
the upper and lower lines represent the expected variation.
Available data indicate that the average combustible content of MMR is
approximately 50 percent; in Figure D-1, 50 percent combustible corresponds
to a refuse heating value of 8,100 kg-cal/kg (4,500 Btu/lb) as received. The
8,100 kg-cal/kg value compares favorably with the average proposed by Eggen
and Kraatz and is based on data available to all practf tioners-of the art. An
additional reason for basing the heating value on the OSWMP data is that the
information was derived using a methodology that was reasonably consistent
and which has been formalized in a testing manual for solid waste incinera-
tors (W.C. Achinger, and J.J. Giar, "Testing Manual for Solid Waste
Incinerators," open file report (SW-3ts) , U.S. Environmental Protection
Agency, 1973).
Baseline data will, therefore, be 8,100 kg-cal/kg (4,500 Btu/lb) on an
as-received basis of 50 percent combustible material and 50 percent inert
material; the inerts will be composed of 44 percent residue and 56 percent
moisture.
We also need to know the ultimate analysis of the combustible fraction
of the refuse. Using the OSWMP data, we have derived the following average
va 1 ues :
Element Percent
Carbon ............ 49.4
Sulfur
43.6
..... 0.9
i.... 0.3
..... 0.6
100.0
This ultimate analysis is on a dry ash-free basis.
222
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10,000
9,000
8,000
7,000
I 6,000
5,000
I 4,000
3,000
2,000
1,000
100 90 80 70 60 50 40 30 20 10
10 20 30 40 50 60 70
Percent Inert (HO + Residue)
80 90 100
Figure D-1. Refuse Heating Value as Received
(EPA-OSWMP Open File Reports).
223
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-288
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE ANP SUBTITLE
A REVIEW OF TECHNIQUES FOR INCINERATION OF SEWAGE
SLUDGE WITH SOLID WASTES
5. REPORT DATE
December 1976 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
W. Niessen, A. Daly, E. Smith and E. Gilardi
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Roy F. Weston, Inc.
Lewis Lane
Westchester, Pa. 19380
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/GRANT NO.
68-03-0475
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory - Cin., OH
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report discusses the state-of-the-art of co-incineration of municipal
refuse and sewage sludge. European and American practice is described. Four
co-incineration techniques are evaluated for thermodynamic and economic feasibility;
pyrolysis, multiple hearth, direct drying, and indirect drying. Each process is
compared with conventional separate incineration with respect to cost, practi-
cality, and project environmental impact. Recommendations for specific demon-
strations are made and EPA endorsement of co-incineration is proposed.
KEY WORDS AND DOCUMENT ANALYSIS
a.
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
sludge, sludge disposal, sludge drying,
refuse, refuse disposal, incinerators,
pyrolysis
combined incineration
codisposal
was tes-as-fuel
13B
8. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
236
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
224
U. S. GOVERNMENT PRINTING OFFICE: 1977-757-056/5561 Region No. 5-11
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