United States
Environmental Protection
Agency
Research and Development
Risk Reduction Engineering
Laboratory
Cincinnati, OH 45268
EPA/600/S2-90/038 Sept. 1990
w EPA Project Summary
Fuel-Efficient Sewage Sludge
Incineration
Michael J. Walsh, Albert B. Pincince, and Walter R. Niessen
A study was performed to evaluate
the status of incineration with low fuel
use as a sludge disposal technology.
The energy requirements, life-cycle
costs, operation and maintenance re-
quirements, and process capabilities of
four sludge incineration facilities were
evaluated. These facilities used a range
of sludge thickening, conditioning, de-
watering, and incineration technologies.
The results provided realistic cost
and energy requirements for a fuel-effi-
cient sludge incineration facility and
highlighted operational, managerial, and
design features that contributed to the
fuel efficiency of the incineration pro-
cess. This information provides a basis
for evaluating both the applicability of
sludge incineration in future facilities
and the cost and energy efficiency of
existing incineration facilities.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
To be considered an alternative sludge
technology under the U.S. EPA's Con-
struction Grants program, an incineration
system is required to be "self-sustaining" or
a net energy producer. In determining if a
system is a net energy producer, the energy
used for sludge dewatering, combustion,
and pollution control equipment is included.
The purpose of this study was to determine
if, in fact, a fuel-efficient, well-operated
sludge incineration system could be "self-
sustaining."
The energy requirements, life-cycle
costs, operation and maintenance (O&M)
requirements, and process capabilities of
four sludge incineration facilities were
evaluated to determine the status of incin-
eration with lowfuel use. The four facilities
used a variety of sludge thickening, condi-
tioning, dewatering, and incineration tech-
nologies.
Plant Information
Detailed information for the following
four facilities is included as appendices in
the Project Report.
Upper Blackstone WPCF
The Upper Blackstone Water Pollution .
Control Facility (WPCF) in Millbury, MA is a
secondary wastewatertreatment facility with
an average capacity of 2,400 L/s (56 mgd).
Currently, the facility treats an average of
1,400 L/s (33 mgd) and processes ap-
proximately 27 dry metric tonnes (30 dry
tons) of dewatered sludge cake per day.
The solids handling processes include
flotation thickening of the waste activated
sludge (WAS), storage and mixing of the
WAS and primary sludge, polymer condi-
tioning, beltfilterpress dewatering, multiple-
hearth (MH) incineration, and ash disposal
by landfilling. The MH does not include an
afterburnerchamberorany means of waste
heat recovery.
Metropolitan WPCF
The Metro facility in St. Paul, MN is a
secondary wastewatertreatmentfacility with
-------
an average capacity of 11,000 Us (250
mgd). The plant currently receives an av-
erageof 9,600 Us (220 mgd). Approximately
163 dry metric tonnes (180 dry tons) of
dewatered sludge cake are processed at
the plant daily.
Sludge handling processes include
gravity thickening of the primary sludge,
flotation thickening of the WAS, sludge
storage, heat conditioning (Zimpro*) and
decanting of the WAS, blending of the pri-
mary sludge and WAS, polymer condition-
ing, roll press dewatering, and MH incin-
eration. The MH incineration system in-
cludes a waste heat boiler system for heat
recovery and a zero hearth afterburner
chamber for air emission control. Auxiliary
fuelisnotaddedtotheafterburnerchamber.
Duffln Creek WPCP
The Duffin Creek Water Pollution
Control Plant (WPCP) in Pickering, Ontario
is a secondary wastewater treatment plant
with an average capacity of 2,100 L/s (48
mgd). Currently, the plant treats an average
of2,100 L/s (47 mgd) and processes about
24dry metrictonnes (27drytons) perday of
dewatered sludge cake.
WAS is returned to the primary clarifi-
ars and co-settled with the primary sludge.
The solids handling processes include two-
stage anaerobic digestion, sludge storage,
polymer conditioning, diaphragm filter press
dewatering, fluidized bed (FB) incineration,
and ash disposal by landfilling. The incin-
eration system has a hot windbox design
and includes a waste heat boiler system for
energy recovery.
Cranston WPCF
The Cranston, Rhode Island WPCF is
a secondary wastewater treatment facility
with an average capacity of 1000 Us (23
mgd). The plant currently receives an av-
erage of 400 L/s (10 mgd) and processes
about 8 dry metric tonnes (9 dry tons) of
dewatered sludge cake daily.
The solids handling processes include
dissolved air flotation thickening of WAS,
gravity thickening of primary sludge, sludge
storage and blending, chemical condition-
ing, dewatering with fixed-volume filter
presses, and MH incineration. The MH
system includes an external afterburner
chamber and a separate scrubbing and
heat recovery system. The furnaces were
designed for operation in the pyrolysis
(starved air) mode but have only operated in
' this manner during start-up. No auxiliary
fuel is added to the afterburner chamber.
Mention of trade names or commercial
products does not constitute endorse-
ment or recommendation for use.
Cost and Energy Consumption
The estimates of cost and energy con-
sumption presented here reflect plant op-
eration under current emission control
regulations. Changes in these regulations
could significantly affect the cost and en-
ergy efficiency of the incineration process.
Exhaust gas temperatures for the three MH
furnaces ranged from 480°C (900°F) to
590°C (1100°F). None of the MH systems'
employs afterburning of the furnaces ex-
haust gas stream, although two of the instal-
lations have an afterburner chamber incor-
porated into the incineration system. Each
of these facilities employs a venturi wet
scrubbing system. If new air emission
regulations require operation of an after-
burner for all MH systems, cost and energy
requirements for MH incinerationwill jn- .
crease significantly. Requirements for more
sophisticated emissions control technolo-
gies will also increase the capital, labor, and
power costs for sludge incineration.
Basis of Analysis
For the purpose of this evaluation, the
solids handling train included all processes
following the clarifiers through the incin-
eration system, including airemission control
and waste heat recovery systems. The cost
of energy consumption required to treat the
sidestream flow that is returned to the head
of the plant was considered essentially equal
at each facility and therefore was not included
in this evaluation. However, at the Metro
facility, the sidestream flow is of much higher
strength due to the heat conditioning process
and must be treated to reduce its strength
before it can be returned to the head of the
plant. Because this additional level of
sidestream treatment represents an addi-
tional cost to the solids handling system,
costs and energy consumption associated
with the rotating biological contactors used
forsidestream treatment at the Metrofacility
were included in this evaluation.
Plants with waste heat recovery sys-
tems must have an auxiliary boiler system
to supplement steam generation from the
waste heat boilers during periods of low
sludge production or high steam demand.
The auxiliary boiler system is considered a
part of the incineration/heat recovery system;
therefore, energy, fuel, and costs associated
with auxiliary boilers were accounted for
under the incineration system.
Raw data for each facility were ob-
tained through a review of available plant
records and discussions with plant staff.
For the purposes of the economic evalua-
tion, both capital cost and O&M cost data
were obtained for each unit process, and
O&M cost data were further broken down
into six components: (1) labor; (2) electric-
ity; (3) fuel (eitherfuel oil or natural gas); (4)
chemicals; (5) materials and supplies; and
(6) contracted services.
Because of the lack of complete data
regarding actual metered electrical con-
sumption, electrical consumption at each
facility was estimated by using a combina-
tion of available plant data and typical
consumption figures presented in industry
publications.
Economic Evaluation
Variations in the unit costs for labor,
electricity, fuel, and other consumables
amongthefourfacilities were accounted for
by converting O&M costs to a common set
of unit costs. Original capital costs were
updated to" 1988 dollars using the Engi-
neering News Record cost index. Updated
capital costs were amortized assuming a40
year useful life for all structures, a 20 year
useful life for all equipment, and a discount
rate of 8%.
The actual load on a plant (versus
desig n capacity) can sig nif icantly affect O& M
costs. Forthis reason, a facility operating at
design capacity should not be compared
directly with a facility that is in the early
stages of its design life. Because of the
influence of the difference between load
and capacity, costs were evaluated for op-
eration at current loads and for operation at
capacity. To estimate the costs at capacity,
the capacity of each solids handling system
was estimated, and costs for operation at
capacity were developed.
The capacity of each solids train was
taken to be the maximum amount of sludge
that the solids train could process while
maintaining a reasonable amount of stand-
by capacity.
Each facility's expenditures for labor,
electricity, chemicals, and other
consumables were scaled up to reflect op-
eration at capacity. Costs were scaled up
based upon the percent increase in sludge
production, the increase in the number of
units on-line under average conditions, or
some other appropriate parameter.
All costs presented below are on the
basis of dewatered sludge cake, and are
expressed as dollars per dry metric tonne
(per dry ton). Based on the methodology
presented above, the following ranges
represent a reasonable estimate of capital
and O&M costs for a well-operated sludge
incineration system operating at capacity,
including furnaces, heat recovery system,
air pollution control system, and ash dis-
posal system.
-------
Annual
O&M $ 77 to $ 99 ($ 70 to $ 90)
Amortized
Capital $110 to $138 ($100 to $125)
Total Annual
Cost .$187to$237 ($170to$215)
The following ranges represent a rea-
sonable estimate of capital and O&M costs
for a complete, well-operated incineration
solids train operating at capacity, when the
thickening and dewatering processes are
also included.
Annual
O&M $198 to $220 ($180 to $200)
Amortized
Capital $220 to $253 ($200 to $230
Total Annual
Cost $418 to $473 ($380 to $430)
Limitations on Cost Estimates
It is important to recognize the limits of
these cost estimates. Capital costs are
presented on the basis of dollars per ton of
instal led capacity, allowing for a reasonable
amount of reserve capacity. It must be
recognized that capital costs can vary sig-
nificantly due to site-specific factors such as
subsurface conditions, local materials costs,
size constraints, constructions market
conditions, and the amount of redundancy
built into the system. These factors make it
difficult to generalize about capital costs;
therefore the capital costs figures presented
here-should be applied carefully.
Estimates of O&M costs were based
upon operation at system capacity. O&M
costs can vary significantly over the life of a
facility depending on thedifference between.
initial and design year sludge quantities, the
use of multiple units rather than one large
unit in equipment selection, and otherdesign
factors. A system that operates at a fairly
constant sludge feed throughout its design
life will see little change in per ton O&M
costs.
Actual per ton O&M costs for sludge
incineration at the four subject facilities
ranged from 6% to 186% greater than the
per ton costs estimated for operation at
capacity. The fluctuation in per ton costs
over the design life of an incineration sys-
tem should be considered when using the
figures presented in this study.
Energy Evaluation Summary
Energy efficiency was examined on
several levels, which were defined by how
energy inputs and outputs for the system
were defined. The following generalizations
were made regarding the energy consump-
tion of a well-operated facility.
Level A -
Based on the auxiliary fuel consumed
within the furnaces only.
- Total annual auxiliary fuel consump-
tion within the furnace itself should
range from 29 to 38 L fuel/dry metric
tonne of sludge cake processed (7 to
9 gal/dry ton).
- Frequent downtime (whether sched-
uled or unscheduled) increased the
auxiliary fuel consumption by a factor
of 10.
Level B -
In addition to Level A, auxiliary fuel use
by the heat/auxiliary boiler equipment, the
emission control equipment, and the ash
disposal system was included. When steam
was produced and used outside of the incin-
eration system, it was included as an energy
output.
- When -a waste heat recovery system
was included in the incineration sys-
tem, the incineration system could be
a net energy producer when only aux-
iliary fuel (no electricity) was consid-
ered an energy input to the system.
Level C -
. In addition to Level B, electricity for
equipment was also included as an energy
input.
- An incineration system with a waste
heat recovery system could still be
energy producer when operated at a
maximum efficiency. Two facilities
approached the goal of being a net
producerattheircurrent loading rates.
It is possible that, if these facilities
were operating at capacity, the in-
crease in energy efficiency that re-
sults from complete equipment utili-
zation might make those facilities net
energy producers.
Level D -
In addition to Level C, energy inputs to
the sludgeconditioning/dewatering system
were included.
- When the definition of the incineration
system was expanded to include the
sludge conditioning/dewatering sys-
tem, the goal of net energy production
by the incineration system did not
appear achievable.
Level E -
In addition to Level D, energy to the
entire solids handling train was included.
Auxiliary fuel, electricity for equipment, and
electricity for general building requirements
were considered energy inputs.
- Total energy consumption for a solids
train using sludge incineration varied
widely depending on the thickening
and dewatering technologies em-
ployed. Total energy consumption for
a well-operated solids train ranged
from 5.8 to 10.4 million kJ/dry metric
tonne (5 to 9 million Btu/dry ton) of
sludge cake processed.
Overall, a variety of technologies could
achieve energy-efficient sludge incinera-
tion. The most complex systems proved to
be very energy efficient under each set of
conditions evaluated. The simplest system
also proved to be very energy efficient,
especially in terms of overall energy con-
sumption by the entire solids train.
Keys to Fuel-Efficient Operation
Although each of the facilities had
several features that contributed to the
success of its own sludge incineration pro-
cess, there were some common operational
and management features that operators at
each facility agreed were essential to af uel-
efficient sludge incineration system.
Sludge Equalization
A uniform flow of sludge to the incinera-
tion system was essential, both in terms of
quantity and quality. Each time the quality
or quantity of the furnace feed changed, the
operator had to adjust the excess air level,
rabble arm rotation speed, auxiliary fuel
use, or some other operational variable to
maintain complete combustion conditions
instead of concentrate on fine tuning the
incineration system. .
To create a uniform furnace feed, some
sludge storage and mixing should be pro-
vided within the solids train. Sludge storage
also allows the incineration system to be
taken off-line for regular maintenance and
equipment calibration.
Staff Motivation/Training
The real key to a successful sludge
incineration facility was in the plant O&M
staff. An incineration system is a relatively
complex system to operate and maintain.
The operations staff had to understand the
effect that changes in operational variables
had on the combustion process, and the
effect that the performance of the preceding
solids handling processes had on the
furnace's operation. The maintenance staff
had to have the manpower and skill to
provide regular maintenance on a variety of
equipment. Management had to create a
positive workingenvironmentthat motivated
the plant staff.
U. S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20099
-------
Communication among the operators
of all of the solids handling processes re-
garding changing sludge conditions was
essential. Management encouraged this by
implementing programs such as a rotating
employee program or by forming problem
solving committees comprised of engineers,
operators and maintenance personnel from
throughout the solids handling facility.
Management should encourage the
plant staff to pursue advanced levels of
training through in-house programs, op-
eratorcert'rfication programs, graduate level
engineering programs, and activity in pro-
fessional organizations and societies.
Maintenance Program
A strong maintenance program in-
creased the cost-effectiveness and energy
efficiency of the incineration process; it:
- reduced auxiliary fuel use by minimiz-
ing unscheduled maintenance shut-
downs;
- minimized costs by extending the
useful life of furnace components; and
- provided the operators with accurate
information by keeping the instru-
mentation and monitoring equipment
operable and up-to-date.
Over the life of a facility, it appeared to
be more economical to pay the annual cost
of proper preventive maintenance than to
pay for major repairs on a periodic basis.
Conclusions
1. If all energy inputs were considered
(Level D), none of the incineration
systems studied could be defined as
"self-sustaining." These systems did,
however, provide several keys to fuel-
efficient operation:
A uniform sludge flow to the incin-
eration system was essential, both in
terms of quantity and quality.
the real key to a successful sludge
incineration facility was in the plant
operations and maintenance staff.
."'- a strong maintenance program in-
creased the cost-effectiveness and
energy efficiency of the incineration
process.
2. For a well operated system, total an-
nual auxiliary fuel consumption within
the furnace itself should range from
29 to 38 L fuel/dry metric tonne of
sludge cake processed (7 to 9 gal/dry
ton). Frequent downtime (whether
scheduled orunscheduled), however,
increased the auxiliary fuel consump-
tion by a factor of 10.
3. A reasonable estimate of total annual
cost (including amortized capital and
O&M) for a well-operated sludge in-
cineration system operating at capac-
ity, including furnaces, heat recovery
system, air pollution control system,
and ash disposal system ranged from
$187 to $237 per dry metric tonne
($170 to $215 per dry ton) (see note
on limitations above).
4. A reasonable estimate of total annual
cost (including amortized capital and
O&M) for a complete well-operated
solids train, operating at capacity, in-
cluding thickening, dewatering, and
incineration, ranged from $418to $473
perdrymetrictonne($380to$430per
dry ton) (see rioteon limitations above).
The full report was submitted in partial
fulfillment of Contract No. 68-03-3346 by
Camp, Dresser & McKee, Inc., under the
sponsorship of the U.S. Environmental
Protection Agency.
MichaelJ. Walsh, Albert B. Pincince, and WalterR. Niessen are with Camp, Dresser
& McKee, Inc., Boston, MA 02108.
Donald S. Brown is the EPA Project Officer (see below).
The complete report, entitled "Fuel-Efficient Sewage Sludge Incineration" (Order No.
PB90-261 827/AS ; Cost: $39.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-90/038
------- |