SW686
THERMAL METHODS FOR THE CODISPOSAL
OF SLUDGES AND MUNICIPAL RESIDUES
This report (SW-686J was prepared by
t *-- n .m_,,..f. *""""'^ L t. \S
David B. Sussman and EarVey W. Gershman
and presented at the
Fifth National Conference
on
Acceptable Sludge Disposal Techniques
January SI - February 2., 1978
U.S. ENVIRONMENTAL PROTECTION AGENCY
1978
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2d Printing 1979
This report has been reviewed by the U.S. Environmental Protection
Agency and approved for publication. Its publication does not signify
that the contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of commercial products
constitute endorsement or recommendation for use by the U.S. Government.
An environmental protection publication (SW-686) in the solid waste
management series.
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Introduction
The intrinsic energy of municipal waste is now being
looked upon as a valuable resource that should not be thrown
away. Various thermal processes have been developed and
implemented in both Europe and the United States to recover
this energy and reduce the volume of the waste that must be
disposed of ultimately. As thermal sludge disposal requires
large amounts of energy, solid waste is now being considered
as a replacement for the fossil fuel heretofore used.
This paper describes some of the thermal co-disposal
techniques utilized in Europe and the United States with
emphasis on those techniques that are net producers of
energy. In addition, the use of refuse derived fuel in
existing sludge incinerators is assessed.
Background
At present there are approximately 22,000 publicly
owned wastewater treatment plants operating in the United
States. Over 5,000 of these are wastewater treatment
ponds/lagoons that generally have few or no sludge disposal
problems. Of the remaining 17,000 plants, only 350 have
greater than ten million gallon per day (mgd) capacities.
Together, all of these plants generate about five million
dry tons of sludge annually. This amount is likely to
double within the next eight to ten years as a result of
laws requiring higher quality wastewater treatment plant
effluents and construction of more plants.
Sludge Disposal Techniques
Sludge is disposed in a number of ways, but the most
common methods are landfilling, surface land application,
ocean disposl, and incineration. About 25 percent is
landfilled, which incurs the same problems as with the
landfilling of solid waste. Another 25 percent is disposed
by surface land application. This approach is also not
without problems. At present, about 15 percent of United
States sludge is dumped at sea, a practice which will be
eliminated by 1981. The remainder is incinerated in either
multiple hearth furnace (MHF) or fluidized bed furnace (FBF)
sludge incinerators. There are approximately 400 sludge
incinerators in the United States today combusting municipal
sludges.
Land application is the current preferred method of
sludge disposal. The Federal Water Pollution Control Act
(FWPCA) encourages that the end products of wastewater
treatment plants (clean water and sludge) be put to "bene-
ficial use" via resource recovery. The sludge, being organic,
can either be burned if dry enough and the energy recovered
or used as a soil conditioner.
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However, in areas where the sludge contains heavy
metals or where agricultural or reclaimable land is not
available, other disposal options such as incineration or
landfilling are indicated. Sludge incineration requires
energy both to dry the sludge mechanically to its autogenous
point and to reduce it to ash in the incinerator. Solid
waste is one such fuel that could be used as an energy
source for incineration.
Codisposal is the general term that best describes the
use of solid waste as a fuel to dispose of sludge in a
thermal process and at the same time dispose the solid
waste. There are two basic approaches in codisposal. Both
have the capability to recover the energy released in the
thermal process, and both are currently being demonstrated
or used. The first uses a solid waste incinerator, solid
waste fired steam generator, or waterwall combustion unit to
burn dewatered sludge. The second approach involves the use
of sludge incinerators which are already installed at the
wastewater treatment plant in many cases. The organic
portion of solid waste is used as a fuel to dry, burn and
reduce the volume of the sludge that must ultimately be
disposed.
The remainder of this paper is divided into two major
parts. First, the techniques used to codispose municipal
sewage sludge (MSS) and municipal solid waste (MSW) are
discussed. Two approaches, solid waste thermal technology
and sludge incinerator technology, are presented here, as
they are both being used on a day-to-day basis. Second, the
use of a prepared solid waste fuel refuse derived fuel
(RDF) as a replacement for fossil fuels in existing MHF and
FBF sludge incinerators is assessed.
Solid Waste Thermal Technology
Conventional solid waste thermal processing units
(incinerators) have been used to codispose solid waste and
sewage sludge for the last 50 years, although the results
have not always been satisfactory. Owing to the problems of
material handling, feeding, and firing which were never
successfully addressed, this concept has generally been
abandoned.
During the process, sludge, with a typical solids
content of 525 percent, was dumped into a refuse pit. It
was then mixed with solid waste in the charging chute by
feeding a bucket-load of sludge along with several bucket-
loads of solid waste. Since the furnace lacked the flexibility
to respond to this drastically different feed material, the
frequent result was a dousing of the fire, or failure of the
sludge to combust.
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As MSW incineration technology matured through the
introduction of more efficient and sophisticated devices,
codisposal was again considered and great strides were made.
At present, a number of incinerators and waterwall combustion
units have been tested as codisposal devices, various new
techniques have been demonstrated, and some plants are
operating on a day-to-day basis.
All of these techniques use the heat released from
solid waste combustion to dewater or dry the sludge to its
autogenous point. The released heat form is either hot flue
gas or steam from the waterwall combustion unit or waste
heat boiler. Once the sludge is dried, it can either be
burned in the furnace along with the solid waste on the
grate, or if in powdered form, burned in suspension above
the grates.
Two United States plants currently use flue gas to dry
the sludge solids in the furnace. The first, at Ansonia,
Conn., uses a 200 tpd (design) refractory incinerator.
About 55 tpd of refuse are disposed in an eight hour shift.
Sludge from the integrated wastewater treatment plant at
about four percent solids is dried in a high speed disc co-
current spray dryer. Hot flue gases from the secondary
combustion chamber at 1200F are then introduced into the
spray dryer. Vapors and dry solids are blown into the
furnace above the second grate where the solids burn in
suspension. However, at present the dried sludge from the
spray chamber is being used as fertilizer by local residents
rather than burned in the incinerator.
Another small refractory incinerator with 50 tpd average
throughput is located in Holyoke, Mass. It uses the same
general technique as Ansonia, but the sludge, is dried in a
rotary dryer after mechanical dewatering to 28 percent
solids. Hot flue gas from the incinerator is used to heat
the sludge directly in the dryer. The dried solids are then
burned in suspension above the refuse grates. However,
because of a reluctance on the part of the plant owners to
spend funds to bring the plant into compliance with the
present state air emission requirements, the facility is no
longer operating.
Approximately 200 solid waste fired steam gnerators or
waterwall combustion units are in operation in Europe.
These units range in size from throughputs of less than 100
to over 2500 tons of MSW per day, and generally produce some
form of exportable energy. Five of these plants are integrated
with wastewater treatment plants to codispose sewage sludge.
Three of the plants are located in France at Dieppe, Deauville
and Brive. These plants use steam to dry the MSS and then
burn the dewatered sludge along with the MSW on the grates
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in the furnace. The other two plants are located in West
Germany at Krefeld and Ingolstadt, and use hot flue gas to
dry the MSS which is then burned in suspension.
The three French plants are technically quite similar.
They are all of Von Roll design and use refractory lined
furnaces with vertical fire tube waste heat boilers. Dieppe
(population 60,000) was the first to be built and has been
in operation since 1971. Brive is the largest of the three,
serving 160,000 inhabitants, and came on-line in the codisposal
mode in 1975. Deauville is the newest and also smallest,
serving 50,000 persons. All the plants have two furnaces
each with design throughputs of 2.5 tonnes/hour at Dieppe
and Deauville and 3.5 tonnes/hour at Brive. The energy
recovered is in the form of saturated steam at the rate of
7.5 tonnes/hour (16 bar) at Dieppe, 9.7 tonnes/hour (16 bar)
at Brive, and six tonnes/ (12 bar) at Deauville.
MSW delivery, storage, charging, combustion and ash
handling are typical of other waste burning plants in Europe.
Emission controls are also coventional with multi-cyclones
at Dieppe and Brive and electrostatic precipatators at
Deauville (required after government regulations became more
stringent). The unique feature of these plants is their
ability to codispose MSS by using the energy recovered from
the solid waste combustion to dry the sludge to its autogenous
point. This process is carried out in thin film dryers,
units that have been used in both the food processing and
chemical industries.
The thickened sludge (digested, 7 to 8 percent solids
at Dieppe and Deauville and undigested, partially dewatered,
to 11.4 percent solids at Brive) from the adjacent waste-
water treatment plant is pumped to tanks located in the in-
cinerator building above the thin film dryers (single dryer
at Deauville). The sludge is then metered into each dryer
cylinder (two at Dieppe, three at Brive). As it flows onto the
rotating central shaft, it is thrown onto and spread over
the inner surface of the dryer which is surrounded by a
jacket heated by the steam from the boiler. The rotor which
revolves at 250 RPM has a number of selfadjusting vanes that
scrape off the dried sludge from the hot cylinder walls.
The sludge granules (now at 55 percent solids) fall onto a
belt conveyor and are metered into either one of the inciner-
ator feed chutes. At this moisture content, the sludge is
autogenous and burns along with the solid waste on the
grates in the furnace.
The vapors generated in the dryers travel upward
(concurrent to the sludge) and are removed from the dryer
head and blown by fans into the furnace where, as overfire
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air, they are deodorized at 800C. Each dryer has a capacity
of .75 cubic meters of wet sludge per hour at Dieppe, 2.5
cubic meters per hour at Brive, and .9 cubic meters per hour
at Deauville.
Designed by the VWK Company, the Krefeld plant is
located in the Rhur industrial area of West Germany near
Duesseldorf. The largest integrated solid waste disposal/
wastewater treatment facility operating in the world, it
serves a population area of 300,000 for solid waste disposal
and 600,000 for sludge disposal. Its waterwall combustion
unit consists of two watertube wall furnaces with design
throughputs of twelve tonnes of solid waste and seven tonnes
of dried sewage sludge per hour. The system generates
electricity for the incinerator and treatment plant, and
exports hot water for use in the community.
Undigested sludge, at five percent solids, is pumped
from the wastewater treatment section of the facility to the
waterwall combustion unit, where it is first dewatered with
the aid of a polymer in centrifuges to 25 percent solids.
The sludge is then flash dried in a vertical shaft flash
drying chamber using 800C flue gas taken from the top of the
radiation zone of the boiler.
Most of the drying take's place in the mill and fan
combination at the lower end of the drying chamber. The
dry, powdered solids (at 10 percent moisture), vapors, and
flue gas (now at 400C) are blown back into the furnace close
to the top of the flame in the radiation zone of the boiler.
The solids are then burned in suspension. As the flue gases
are recirculated, little energy is lost in the system and
the calorific content of the sludge itself more than offsets
the energy lost in the dewatering and drying steps.
The steam generated by combusting the solid waste and
sludge is used to generate electricity with a back pressure
steam turbine, and the back pressure steam is used to
produce exportable hot water. This integrated facility has
been on-line for almost two years, with apparently good
results.
The newest codisposal plant in Europe is located at
Ingolstadt, West Germany and serves a population of 170,000.
The plant is currently in start-up and should be operational
in the near future. The system is of the Widmer & Ernst
design and consists of a refractory solid waste incinerator
and a hot flue gas sludge drying unit. The plant has two
furnaces each with a design throughput of approximately 8.5
tonnes of solid waste and four tonnes of 25 percent solids
sludge per hour. No energy is exported from this facility
as the primary purpose of the plant is sludge and solid
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waste disposal. However, a waste heat boiler or a watertube
wall furnace could have been incorporated into the design
for energy recovery, if deemed desirable. The furnace is
straightforward with crane charging, ram feeding, recipro-
cating mass burning crates, and an extensive air pollution
control train consisting of an electrostatic precipatator
and scrubber.
Digested sewage sludge dewatered to 25 percent solids,
is conveyed to a storage bin in the incinerator plant.
Controllable screw-conveyors extract the sludge from the
storage bin and transport it to a twin mixing worm. This
twin mixing worm is also fed with already dryed sludge at a
moisture content of 10 to 15 percent. The two sludges are
blended together, resulting in a sludge of 35 to 40 percent
mositure. This material is flash dried in a manner similar
to that in Krefeld. The sludge enters the top of a vertical
flash drying chamber with hot flue gas (800C) and both are
sucked downwards together into a hammermill. In the mill
the sludge is disintegrated into very small particles and
drying continues. The fuel gas (now at about 400C) and the
sludge powder at 10 to 15 percent moisture enter a series of
cyclones and the sludge is de-entrained from the hot moisture
laden gas. The gas is then blown into the furnace above the
grates and deodorized at 800 to 1000C. Some of the dryed
sludge is conveyed to the mixing worms and the rest is
pneumatically conveyed and blown into the furnace and burned
in suspension. The conveying air assures complete combustion
of the sludge solids in the incinerator.
Sludge Incinerator Technology
In the past, fossil fuel fired sludge incinerators were
considered in many cases to be the most cost effective
option for ultimate sludge disposal. However, with the
drastic fuel cost increase of the early 1970s, the cost
effectiveness of this option was re-evaluated. As MSW
contains more than enough energy to maintain autothermic
conditions, it was considered as a possible fuel source in
sludge incinerators. Experiments were carried out in Europe
with mixed results. The use of raw, shredded MSW in sludge
incinerators was not completely successful and some demon-
strations were discontinued. However, the use of just the
organic portion of the MSW looked more promising. Thus, the
second approach to codisposal, using the organic portion of
the MSW as a fuel in conventional sewage sludge incinerators
was initiated.
Codisposal using sludge incinerator equipment was first
demonstrated in this country in an EPA-supported resource
recovery demonstration at Franklin, Ohio. The Franklin
plant utilizes a fluidized bed furnace (FBF). The fuel used
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in the FBF is composed of the rejected organic waste stream
coming from the solid waste fiber recovery operation. This
plant recovers a low grade paper fiber (as well as ferrous
and nonferrous metals and glass) from the solid waste stream
and uses the nonrecoverable organic portions of the waste to
fuel the sludge incinerator.
The resource recovery plant is co-located and integrated
with the wastewater treatment plant, but no energy is recovered.
Organic residue from the fiber recovery system, a 20 percent
solids slurry, is mixed with five percent solids sludge,
dewatered to 45 percent solids in a cone press, and combusted
in the fluidized bed incinerator. The incinerator requires
about 3000 Btu per pound of as-received material to sustain
combustion with no auxiliary fuel. As the combination of
solid waste and sludge contains about 3600 Btu per pound,
autogenous conditions are maintained. In this demonstration,
only 600 Btu per pound of input was available for energy
recovery. However, the furnace could burn much more solid
waste and improve the energy recovery capability of the
system.
The resource recovery technique demonstrated in Franklin
is being replicated elsewhere, but not for fiber recovery.
The recovered organic portion of the solid waste stream will
be used as a fuel in a boiler. A fluidized bed furnace
fired with the organic portion of MSW from a dry separation
process (refuse derived fuel RDF) is under construction in
Duluth, Minnesota.
Codisposal using a multiple hearth sludge incinerator
has also been demonstrated. The organic portion of the MSW
was converted into a fluff RDF through a dry separation
process, and that RDF was used to fire the multiple hearth
furnace. The demonstration was successful, but the longest
continuous run was only two days. In addition, operating
the incinerator in a pyrolysis mode (starved air) with an
afterburner greatly improved the system's performance.
This technique was demonstrated at a wastewater treatment
plant in Concord (central Contra Costa County), Calif. In
the demonstration, an existing multiple hearth sludge incin-
erator was modified to accept RDF as a fuel and to operate
in either an incineration or pyrolysis mode. An afterburner
was added to the device to burn the gas produced during the
pyrolytic operation. The RDF could be mixed with sludge
which had a solids content of 16 percent, and introduced
into the top hearth or fed directly into the third hearth.
The latter method proved more efficient. The system operated
best under starved oxygen conditions. Air flow was adjusted
to maintain 1400F off-gas temperature and the 130 Btu/DSCF
gases burned completely in the afterburner. The pyrolysis
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gas could be combusted at temperatures as high as 2500F in
the afterburner with no additional fuel. RDF could be fed
into the furnace at a much higher rate than necessary for
sludge disposal only, thereby producing exportable energy
that could be recovered in a waste heat boiler.
Systems are in the design phase for this approach,
which will be replicated in at least two known locations.
RDF in Sewage Sludge Incinerators
The use of RDF as a fuel in municipal sludge incinerators
is technically viable and could offer the following benefits:
0 Significant reduction in fuel consumption by
treatment sludge incinerators;
0 Provision of a hedge against large increases in
future solid waste disposal and sludge incineration
costs; and
0 Significant reduction in solid waste disposal
process steps such as hauling and landfilling.
Although RDF application in MSS incinerators is receiving
much attention, no information exists on the extent to which
this may be applicable in the many U.S. municipalities that
currently have municipal sludge incinerators. In order to
assess the applicability of RDF as a fuel in municipal sludge
incinerators, EPA's Office of Solid Waste contracted with
Gordian Associates, Inc.
The study consisted of two major tasks. The first was
to identify location of all municipal sewage sludge inciner-
ators in the United States and to obtain information relating
to the status, operating parameters, etc. of these installations
The second task was to analyze this data and determine the
RDF consumption potential of each unit surveyed. The methodolog
used to determine RDF potential in sludge incinerators and
the results are quite interesting. They are described
briefly below.
Assessment of RDF Potential
An inventory of sludge incinerators in the United
States was first compiled. The locations were provided by
the four manufacturers of sludge incinerators whose equip-
ment accounts for over 99 percent of the installed units in
this country. A statistically valid sample of the installed
units was conducted and the RDF capacity potential listed.
(See Tables 1, 2 and 3.)
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-------
The potential for using RDF multiple hearth and fluidized
bed furnaces is addressed here. The calculation of potential
RDF consump tion assumed full utilization of the available
incinerator capacity at each site; i.e., it was assumed that
all the incinerators that are currently operational and
those that could be reactivated would operate continuously,
24 hours per day, seven days per week.
Methodology for RDF Potential in Multiple Hearth Furnaces
The potential for RDF consumption in MHFs was calculated
under two operating modes: incineration and pyrolysis.
Prior to calculating the potential RDF consump tion in a
MHF, it was necessary to determine the following:
0 The application rates of RDF relative to sludge
for the incineration and pyrolysis modes,
expressed in wet Ibs per ft2 of hearth area
per hour;
0 The hearth area of the unit expressed in ft2;
0 The characteristics of sludge quantity expressed
in wet Ibs per hour, and solids content expressed,
as a percentage of the total.
The application rates for the incineration and pyrolysis
modes were obtained from information provided by MHF manu-
facturers and Brown and Caldwell. The range suggested was
36 Ibs/ft2/hr. for incineration, and 814 Ibs/ft2/hr. for
pyrolysis. Mid-range values of 4.5 and 11.0 Ibs/ft /hr.
respectively, were used for calculating the potential RDF
capacity.
The hearth area of each MHF was obtained from the
manufacturers. The total capacity of the unit, "A", in
pounds per hour, was calculated for each mode of operation
by multiplying the hearth area (ft2) by the application rate
(Ibs/ft2/hr.) for that mode. The quantity of sludge
produced by the treatment plant was known from the survey.
A continuous operating schedule (24 hours/day, 7 days/week)
was assumed in calculating the amount of sludge that would
be fed to the MHF each hour.
The capacity for RDF consumption can thus be calculated
by the following equation:
16
-------
R = AH
where R = RDF consumption (wet Ibs/hr)
A = MHF application rate (wet lbs/ft2 of
hearth area)
H = hearth area (ft2)
S1 = quantity of sludge (dry Ibs/hr)
s = percentage of solids in sludge
The above factors and results of the calculations are
shown for each MHF location surveyed. Table 1 presents
the Envirotech-BSP location results, while Table 2 presents
those for Nichols.
Methodology for RDF Potential in Fluidized Bed Furnaces
The methodology used in calculating the capacity for
RDF consumption in a FBF was developed by Dorr-Oliver, Inc.
This capacity is limited by either the maximum quantity of
heat (Btu/hr) that can be released in the FBF during
combustion (heat release capacity) or by the maximum
quantity of water that can be evaporated. The factors that
determine the capacity of a FBF depend on the heat value and
the water content of the combined sludge/RDF feed to the
FBF.
The calculations presented below assume an average
sludge with 70 percent volatile content of solids and a heat
value of 9500 Btu/lb. of volatile solids (6650 Btu/ Ib. dry
solids) . The RDF was considered to be 75 percent solids
with a heat value of 5000 Btu per wet pound. It was also
assumed that any retrofit would include the addition of heat
recovery (e.g., a hot windbox or recuperator) where the
combustion air is preheated. This would increase the
capacity of the unit to consume RDF.
The steps listed below were used to calculate the
capacity for RDF consumption in FBFs.
(1) The freeboard area (F) of the FBF was calculated
from the diameters supplied by the manufacturer:
F =
r = radius = 1/2 x diameter
17
-------
(2) The evaporation rate (E) for the FBF was
determined from a graph supplied by Dorr-Olever
(reproduced as Figure 1) for the solids content
of the sludge as indicated in the survey.
(3) The water evaporation capacity (W) was then
calculated:
W = E x F
(4) The capacity of the unit to incinerate sludge
was calculated:
C = W
w
w = % water in sludge
(5) Using the amount of sludge (S), in wet Ibs/hr.
that would be fed under continuous operating
conditions, the percent capacity required to
incinerate the treatment plant's sludge was
calculated:
U = S_ x 100
C
U = Incinerator capacity utilization.
(6) Knowing the sludge incineration capacity
utilization (U) and the % dry solids (S) in
the sludge, the ratio of wet RDF to wet sludge
(P) could be found from a series of plots
provided by Dorr-Oliver (reproduced as Figure 2)
(7) The capacity for RDF could then be calculated:
R = P x S
Since the graphs provided by Dorr-Oliver did not
consider situations where the incineration capacity
utilization (U) was below 25 percent; in these cases, the
calculation of RDF capacity was based on the heat release
capacity (Btu/hr). (When the solids content of the feed
material is high, the FBF capcity is determined by
its heat release capacity rather then its capacity to
evaporate water.) The heat release capacity of a FBF
with heat recovery was given as 188,797 Btu/hr/ft^ by
Dorr-Oliver. The RDF capacity in this was calculated as
follows:
18
-------
Figure 1
WATER EVAPORATION RATE IN
DORR-OLIVER FLUIDIZED BED FURNACE
Water Evaporation Rate
Pounds Water/Hour/Square Foot
41
40.8
40.6
40.4
40.2
40
39.8
39.6
39.4
39.2
39
15 20 25 30 25
% Dry Solids in Sewage Sludge
OURCE:
>rdian Associates Incorporated. Assessment of the use of
refuse-derived fuels in municipal wastewater sludge
incinerators; final report. [Washington], U.S. Environmental
Protection Agency, Dec. 30, 1977. [I57p.l
19
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Figure 2
RATIO OF WET SLUDGE TO WET RDF
Pounds of Wet RDF/
Pound of Wet Sewage Sludge
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
10
SOURCE:
Gordian Associates Incorporated. Assessment of the use of
refuse-derived fuels in municipal wastewater sludge
incinerators; final report. [Washington], U.S. Environmental
Protection Agency, Dec. 30, 1977. [I57p.]
15t dry solids In sewage sludge
205 dry solids In sewage sludge
255 dry sol Ids in sewage sludge
305 dry solids In sewage sludge
31.2% dry solIds In sewage sludge
20
30
50
60
70
80-
90
100
Sewage Sludge - % of Design Capacity - Hot Windbox Reactor
20
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