United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-200 Mar. 1985
v>ERA Project Summary
Technology Assessment of
Carver-Greenfield Municipal
Sludge Drying Process
Henry C. Hyde
The innovative and alternative tech-
nology provisions of the Clean Water
Act of 1977 (PL 95-217) provide
financial incentives to communities
that use wastewater treatment alterna-
tives to reduce cost or energy con-
sumption. Some of these technologies
have only recently been developed and
are not in widespread use in the United
States. This document discusses the
technical and economic feasibility of
using one emerging technology, the
Carver-Greenfield* (C-G) municipal
sludge drying process, for municipal
wastewater treatment facilities.
The C-G process uses the principle of
multi-effect evaporation and is primarily
used in the food, pharmaceutical, and
industrial wastewater treatment in-
dustries. The C-G process can dry
aqeuous solutions or slurries with a
wide range of solids contents (4 to 45
percent).
The C-G drying process appears to be
a cost-effective, energy-efficient meth-
od applicable to the wastewater in-
dustry. Research and development for
application to municipal wastewater
solids drying has reached the point for
full-scale implementation.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the technology assess-
ment that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
* Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
Introduction
The objective of this technology as-
sessment was to evaluate the technical
and economic feasibility of using the
Carver-Greenfield (C-G) municipal sludge
drying process for municipal wastewater
treatment facilities. The C-G process
uses the principle of multi-effect evap-
oration and can dry aqueous solutions or
slurries with a wide range of solids
contents (4 to 45 percent).
The City of Los Angeles Hyperion
Energy Recovery System (HERS) project
will be the first full-scale municipal
wastewater solids facility in the United
States using the C-G process when it is
placed into operation in 1985. A Trenton,
New Jersey, plant is currently under
design, and Chicago, Illinois, is seriously
considering the process. Full-scale
facilities using the C-G process for
municipal sludge drying are operating in
Japan.
The C-G process is patented by De-
hydrotech Corporation (formerly Carver-
Greenfield Corporation) and is marketed
under exclusive license arrangements by
the Foster Wheeler Energy Corporation.
Use of patented process equipment and
appurtenant hardware can be negotiated
directly with Dehydrotech. These as-
sociated patent issues can increase costs
and may create complications with
federal funding that can cause delay in
project implementation. For the HERS
project, however, where the process is
being installed, the license fee was
approximately $1.4 million or only about
8 percent of the equipment capital cost.
Currently, no comparable sludge-
drying processes are available. Thermal
sludge drying or conditioning processes
(e.g., flash drying, wet-oxidation) are
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based on different thermodynamic prin-
ciples and are not analogous to the multi-
effect evaporation system. Indirect
contact steam dryers are the closest
conventional technology to the C-G
process.
The Carver-Greenfield Process
A flow diagram describing how the
process fits into a total sludge manage-
ment system is shown in Figure 1; the C-
G flow diagram is shown in Figure 2.
Sludge to be processed is first thickened
or dewatered to reduce the amount of
water to be evaporated. Thickened sludge
is then mixed with an oil (carrying
medium) such as No. 2 fuel oil or Isopar L
(an Exxon product) at a suggested ratio of
1 part dry solids to 5to 10 parts oil. By use
of an oil, fluidity is maintained in all
effects of the evaporation cycle: it permits
continuous pumping; facilitates heat
transfer in the later-stage evaporators
where the solids contents are higher as a
result of water evaporation; and minimizes
formation of scale or corrosion of the heat
exchangers. The sludge-oil slurry is then
pumped to the multi-effect evaporator
where water is vaporized. The remaining
solids-oil mixture is subsequently centri-
f uged to separate the oil and solids. The oil
is recycled and reused and the dry solids
(90 percent or greater) are discharged for
further processing or disposal
Multi-effect Evaporation
Multi-effect evaporation affords an
economy of scale over single-effect
operations through the reuse of heat. The
C-G process employs reverse flow, multi-
effect evaporation with steam being
added to the first effect. In a three-effect
system, vapor from the first effect is used
to heat the solution in the second effect,
and the vapor from the second furnishes
heat to the third. Vapor from the last
effect is removed, condensed, and
discharged. The oil-sludge mixture flows
in the opposite direction from effect to
effect, counter to the vapor flow between
effects. Through the reuse of heat in the
multi-effect process, the amount of water
removed per pound of steam supplied
increases with increasing number of
effects.
In its simplest theoretical form, a
single-effect evaporator can evaporate a
maximum of 1 kilogram of water per
kilogram of steam supplied, and a double-
effect evaporator will evaporate 2 kilo-
grams of water per kilogram of steam
supplied, etc., because of the reuse of
heat. The amount of kilojoules (Btu's)
required per kilogram (pound) of water
removed will depend on the number of
effects used. For a single-effect unit,
about 2300 kilojoules per kilogram (1000
Btu's per pound) of water removed is
required for a double-effect unit, 1150
kilojoules per kilogram (500 Btu's per
pound) of water removed, etc. A vacuum
Dewatering
Partial Water
Removal
Evaporation of Water
Sludge Drying
Combustion
Energy Recovery
End Products
for Reuse
or Sale
Sludge (Thickened/Unthickened)
Dewatering
Basic Carver-Greenfield
Multi-Effect Evaporator
with Hydroextractor
(Optimum Oil Recovery,
Use of Light Weight Oil)
(See Figure 2)
Pyrolyzer
Boiler and/or
Gas Turbine
"Use of heavier weight oil only; non-hydroextraction recovery system.
figure 1. Sludge management system utilizing the C-G process.
Pelleted
Dry Fuel
Fuel Oil
lor Sale
Oil for *
Reuse
Fertilizer
Steam for
Evaporation
Steam for
Electricity
Steam for
Sale
Electricity
for Sale
Ash
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Steam
Feed
(Water/
Solid)
Oil
Oil
Mixing
Multiple-Effect
Evaporation
J
Oil/Solid
Separation
Condensate/Oil
Separation
Condensate
Solid*
Product
Recycle Oil
•igure 2. Carver-Greenfield block flow diagram.
s applied to the various effects to reduce
he liquid vaporization temperature and
o maintain a positive temperature
difference within each effect so that heat
:an be transferred. Conventional heat
Irving processes normally require 3450
o 4600 kilojoules per kilogram (1500 to
2000 Btu's per pound) of water removed.
'herefore, in comparison, the C-G
jrocess is an energy efficient sludge
frying process.
It would appear that an infinite economy
)f scale would result from the use of an
nfinite number of effects. Several
actors, however, limit the number of
iffects in a system. Each affect operates
>nly on a fraction of the total temperature
Jrop across the system. The total drop is
leldom larger than that employed in
iingle-effect evaporation, and the capacity
ier unit area of heating surface is
•educed proportionately. Thus, a savings
n fuel requirements may be realized
hrough multiple-effect operation, but
iquipment costs will be greater. Cur-
rently, the system being constructed by
he City of Los Angeles will use four
sffects. In most cases no more than three
•31 four effects are economical, but the
ictual number is largely influenced by
sre vail ing fuel costs.
Most proposals for treating municipal
Judge with the C-G process include a
;ombustion reactor to recover the heat
i/alue of the dried product. Theoretically,
his is an attractive combination of
processes since water can be evaporated
with multi-effect efficiency before com-
bustion or gasification. Fuel gases
produced during pyrolysis or waste heat
from an incincerator can then be used to
supply the energy requirements of the C-
G process. The dried product may also be
marketed as a soil conditioner.
Technology Assessment
Procedure
There are over 70 operating C-G
installations throughout the world. For
the most part they are used in industry for
drying various industrial waste streams.
Two plants, the Fukuchiyama City and
Hiroshima plants in Japan, process
municipal sludge from conventional
activated sludge treatment plants.
Several full-scale systems have been in
continuous operation in other industries
(e.g., Adolf Coors Company, Golden,
Colorado).
This technology assessment was based
on an independent review of several
operating pilot-scale and full-scale
facilities. Much of the design and cost
information was derived from the City of
Los Angeles and the Los Angeles-Orange
County Metropolitan Area (LA/OMA)
regional sludge study.
Process Capabilities and
Limitations
The C-G process has several capabilities
and limitations based upon a review of
pilot-scale and full-scale facilities.
Important capabilities include:
• The C-G process can dry aqueous
solutions or slurries with a wide
range of solids contents (4 to 45
percent). The process can handle
any type of municipal sewage
sludge, can be designed to handle
any feed concentration, and can
evaporate water to any degree of
dryness.
• Multi-effect evaporation consumes
only a fraction of the energy required
by other heat drying processes.
• If the dried sludge is used as a fuel,
the process may be self-sufficient in
energy and in some cases provide
excess energy for export such as
with the City of Los Angeles HERS
project.
• Polychlorinated biphenyls (PCB's)
and other organic contaminants are
destroyed when sewage oil is burned
as a fuel in a boiler.
• The process produces a dry, easy-to-
handle product that is sterilized
during evaporation. The reduced
volume of fully dried sterile product
may be safely disposed of or may be
used as a fertilizer and soil condi-
tioner.
• Since it operates in a completely
closed system, odors are contained
within the system. The odoriferous
and other noncondensable gases
contained in the sludge feed, which
evolve during evaporation, can be
added to the air intake of the boiler
for combustion.
• The dried solid product can be stored
for an indefinite period.
Two significant limitations of multi-
effect evaporation are increasing viscosity
and resistance of the liquid to heat ex-
change as it is concentrated. If the in-
crease in viscosity is sufficient, the mate-
rial can clog or scale the evaporator tubes
of the heat exchanger and prevent
evaporation. To eliminate this problem,
using a fluidizing medium will keep the
material in a fluid state in each effect. In-
corporating a fluidizing oil with the multi-
effect evaporators is the basic principle of
the C-G process.
Design Considerations
General design criteria are listed in
Table 1. The minimum number of effects
and the required evaporation efficiency
depend on site-specific conditions. A
two-effect system may be most econom-
ical in some cases. Redundancy or
reliability requirements also help deter-
mine the number of effects required. The
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Los Angeles design incorporates three
trains of four effects each, with each train
capable of handling 50 percent of the
average design load. This level of
redundancy is dictated by the large
quantity of sludge being processed, the
lack of alternative disposal options in
emergency situations, the need to handle
peak sludge production rates, and esti-
mated downtime for routine main-
tenance. The City of Trenton, however,
will use a single process train, sized for
above-average production rates and
designed to operate 5 days a week. Isopar
will probably only be used as fluidizing oil
where a very high grade product is to be
produced, e.g., in food or pharmaceutical
applications. Other petroleum based oils
are more readily available and less
expensive for sludge processing.
Operational Considerations
The C-G process is quite flexible in
terms of variations during operation. The
heart of the process is the multi-effect
evaporator train, which consists of feed
and circulation pumps, heat exchanger,
vapor chamber, and connection piping.
As such, the system is mostly composed
of nonproprietary equipment that is
available from more than one manu-
facturer. These equipment sections (e.g.,
an evaporative effect unit) are amenable
to duplication or bypass arrangement to
ensure 100 percent reliability. Thus, an
extra evaporative effect (pumps, heat
exchanger, vapor chamber) may be added
in case one of the effects is shut down
temporarily for any reason. Reliability
without extra equipment can be ensured
by bypass arrangements: a normal four-
effect system can be operated as a three-
effect system using slightly more process
steam and higher temperature drop
across the system with attendant decrease
in efficiency. With these and certain other
essential spare equipment arrangements,
the C-G system can cope with upset
conditions without having 100 percent
redundancy.
The C-G process poses no special
maintenance problems. The maintenance
effort required in the dry materials phase
and distallate condensing operations will
be greater, however, than in other
process segments. All required main-
tenance procedures should be within the
capabilities of well-trained municipal
personnel. Equipment durability and
reliability are quite good. The employment
of proper preventive maintenance pro-
cedures for the C-G process can be
expected to result in smooth running
operation with long life.
Table 1. Carver-Greenfield Dehydration Design Criteria
Item Criteria
Number of effects
Evaporation rate
Steam characteristics
Boiler efficiency
Fuel value of extracted heavy oil
Fluidizing oil
Fluidizing oil make-up
Weight of Isopar-L
Outfeed
2, 3, or 4
2.3 kg water/kg steam
448.200 Pa (65 psia) saturated
75%
41.850 kj/kg (18.000 Btu/lb)
Isopar-L
1% by weight of dry solids fed (assumes
hydroextraction is employed for oil
recovery)
766 kg/cu m (6.388 Ib/gal)
95% solids
Table 2 lists the labor, power, and
chemical requirements for the C-G
process based on the City of Los Angeles
full-scale system.
Energy Considerations
The overall water evaporation energy
requirement of the C-G process is less
than, hence more energy efficient than,
that of comparable sludge drying processes
(Table 3).
Cost Comparison
The full technology assessment report
includes preliminary design criteria and
estimated costs for the HERS System at
Los Angeles. The estimated total (capital
and operating) cost for the C-G system is
$39/dry ton, which includes allowance
for a license fee. This compares very
favorably with that for a rotary dryer,
$100/dry ton.
Conclusions and
Recommendations
The C-G dewatering/drying process.
appears to be an energy-efficient, cost-
effective method applicable to the
wastewater industry.
Research and development for applica-
tion in the municipal wastewater industry
has reached the point for full-scale
implementation. Progress at Los Angeles,
Trenton, and Chicago should be carefully
followed.
Based on this assessment, the following
recommendations are made regarding
identified needs to fully develop this
technology for the municipal wastewater
industry:
• Municipal wastewater agencies
should consider the C-G process on
a site-specific basis because of the
variable process configurations,
energy and environmental consid-
erations, and cost.
• Pilot testing of the C-G process is
necessary to develop specific design
criteria! to guide full-scale projects.
• The construction cost and operating
characteristics of the full-scale C-G
facilities for the City of Los Angeles
and City of Trenton should be
tracked. Full-scale construction cost
and operating information is an
Table 2. Example Operation and Maintenance Requirements
Design
Labor
Power used
Chemical requirements
(carrier oil}
265 dry tons/day @ 20% solids
10 personnel® 1500 hr/yr each
1900 kWh/day
1.200 kg/day or 766 kg/cu m
(2.650 Ib/day or 415 gal/day @
6.388 Ib/gal)
Table 3. Comparative Energy Requirements
Unit
C-G (four-effect)
Spray dryer
Flash dryer
Rotary dryer
Indirect steam
Other devices that use
heat for drying and do
not employ multiple-
effect evaporation
Kj Input/ Kg of
Water Evaporated
810-1,050
4,650 minimum
5,210-6.280
5,580-6,510
1.9OO
2,330 (plus heat
lost due to in-
efficiencies of
system)
Btu Input/ Ib of
Water Evaporated
350-450
2,OOO minimum
2.2OO-2.700
2,400-2.800
1.250
1,000 (plus heat
lost due to in-
efficiencies of
system}
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important need at this time to
determine the widespread viability
of the process.
• There is a need to disseminate
technical and cost information on
specific C-G projects in the following
areas of concern:
— Municipal wastewater residual
solids dewatering and drying
performance.
— Construction and operating cost.
— Patent status of light oil tech-
nology.
The full report was submitted in
jlf illment of Contract No. 68-03-3016 by
/Wl Consulting Engineers under the
lonsorship of the U.S. Environmental
rotection Agency.
Henry C. Hyde is currently with Henry Hyde & Associates, Sausalito, CA 94965.
Robert P. G. Bowker was the EPA Project Officer (see below).
The complete report, entitled "Technology Assessment of Carver-Greenfield
Municipal Sludge Drying Process," (Order No. PB 85-138 634; Cost: $11.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information, contact Harry £• Bostian at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
<, US OOVERNUENT PRINTING OFFICE 1966 . 559-111/10793
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