United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-104 Sept. 1984
c/ERA Project Summary
Thermal Treatment of Municipal
Sewage Sludges
Thomas J. LeBrun and Liberate D. Tortorici
A research program on the thermal
conditioning of sludge was conducted
as part of an overall, long-term sludge
management study for the Los Angeles
and Orange County metropolitan areas.
The major goal of this portion of the
study was to investigate the advantages
of the thermal conditioning of primary
and waste-activated sludges WAS
before anaerobic digestion on a contin-
uous-flow and pilot-scale basis. The
studies were designed to demonstrate
whether thermal conditioning would
increase gas production and volatile
solids destruction during subsequent
anaerobic stabilization of the sludge.
Anaerobic digestion and anaerobic
filtration were used for sludge stabiliza-
tion. The effects of thermal conditioning
on sludge dewaterability were studied
by means of dewatering with a filter
press, vacuum filter, scroll and basket
centrifuges, and belt filter press. Other
items studied were the fate of pathogens
and heavy metals and the production
and control of odors during the thermal
conditioning process.
The pilot-scale thermal conditioning
unit was tested under a variety of
operating conditions. Temperatures
and pressures were varied, and thermal
conditioning was investigated with and
without the use of oxygen. Two types of
wastewater sludges were used—WAS
and a blend of 65 percent raw primary
and 35 percent WAS. An energy
analysis was conducted to determine
the net energy demands of including
thermal conditioning in the sludge
process.
This Project Summary was developed
by EPA's Municipal Environmental
Research 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
In August 1976, a pilot-scale investiga-
tion of anaerobic digestion with thermal
pretreatment was initiated at the Joint
Water Pollution Control Plant (JWPCP) in
Carson, California. The specific objectives
were (1) to demonstrate on a continuous-
flow, pilot-scale basis the concept of
thermal treatment of primary and WAS
before anaerobic digestion, (2) to determine
the potential for increased methane
production as a result of thermal pretreat-
ment, (3) to determine the dewatering
properties of the digested sludge; (4) to
identify operational problems in the
system, (5) to determine the effectiveness
of a water-scrubber/carbon-adsorption
system for the control of odorous off-
gases, (6) to develop parameters for
design and operation of a full-scale
system, (7) to develop energy balances
for the entire system, and (8) to demon-
strate the treatment of thermally treated
decant liquors with an anaerobic filter.
Earlier laboratory studies at Stanford
University indicated that potential toxicity
problems existed in digesting sludges
that had been thermally conditioned at
temperatures above 177°C (350°F). This
question was also to be resolved in this
pilot-scale testing program.
A schematic representation of the
extensive experimental program conduc-
ted is presented in Figure 1. The program
was divided into three phases. The
specific flow scheme investigated under
Phase I incorporated thermal treatment
of flotation-thickened WAS followed by
(1) thermophilic anaerobic digestion and
mechanical dewatering or (2) decant
separation of the thermally conditioned
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Phases I and III
Oxygen (Air)
WAS
Phase III
Thermal Conditioning
HT
LPO
Dewatering
1. Centrifugation
2. Pressure Filtration
3. Vacuum Filtration
4. Belt Filter Press
I
Solids to
Digester
Filtrate (Centrate) to
Headworks
Figure 1. Schematic diagram of the sludge processing pilot facility.
sludge with anaerobic filtration of the
supernatant and mechanical dewatering
of the subnatant. Phase II was identical to
Phase I except that a blend of WAS and
primary sludge served as the feed to the
thermal-conditioning unit. A sludge
blend on a dry solids basis of 65 percent
raw primary sludge and 35 percent WAS
was used to approximate the projected
ratio at the JWPCP when full-scale
secondary treatment is implemented.
Phase III incorporated thermal treatment
of thickened WAS followed by decant
separation (thickening) of the sludge with
anaerobic digestion of the supernatant
and mechanical dewatering of the sub-
natant. Phase II! was conducted primarily
to study odor control techniques and to
test a two-stage odor control system for
decant tank off-gases.
Pilot System
A trailer-mounted, continuous-flow,
thermal-conditioning pilot plant was
leased from Zimpro Inc.* as the thermal
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
pretreatment system for these studies
(Figure 2). Feed sludge passes through a
grinder to reduce all particles to 0.8 cm
(0.3 in.) before pumping. In the low
pressure oxidation (LPO) mode, sludge
and air are combined and pumped into
the system. In the heat treatment (HT)
mode, air is not used. With or without air,
the sludge is then passed through heat
exchangers and brought to the initial
reaction temperature as it enters the
reactor. Oxidation takes place within the
reactor, and the oxidized products leaving
the reactor are cooled by countercurrent
heat exchange with the entering cold
sludge. -Steam is added directly to the
reactor for startup and whenever the
process is not thermally self-sustaining.
During these studies, the thermal
conditioning unit was operated under
either the HT mode or a modified LPO
mode. The term "modified" is used here
because under normal LPO conditions,
enough oxygen is contained in the air
added to the thermal reactor to oxidize 5
to 10 percent of the influent COD.
Because one of the main objectives of
these thermal pretreatment studies is to
maximize gas production upon subsequent
digestion, COD oxidation through the
pretreatment facility must be kept to
a minimum. To minimize the oxidation of
organics through the thermal reactor, the
quantity of air supplied was decreased
from 4.7 to 7.1 L/s (10 to 15 scfm) for
normal LPO conditions to 0.6 L/s (1.2
scfm) under modified operation. The
theoretical degree of COD oxidation under
these modified conditions approximates
1 percent and should not significantly
affect methane production when the
sludge is anaerobically digested. The small
amount of air still provides an oxidizing
environment in the reactor, and the
conditioned sludge should behave in a
similar manner to sludge that receives
normal LPO conditioning.
Pilot-scale anaerobic digestion studies
were carried out in a 45-m3(12,000-gal),
single-stage digester equipped with a gas
recirculation system for mixing and heat
exchangers to maintain thermophilic tem-
peratures. Anaerobic filtration studies
were conducted on a filter that was 1.2m
(4 ft) in diameter and 3 m (10 ft) high
(Figure 3). Feed was introduced at the top
through a 1.9-cm (0.75-in.), 120° spray
nozzle. The filter was loaded with three
0.6-m-deep sections (2 ft) of B.F. Goodrich
plastic medium with a surface area of 144
m2/m3(44ft2/ft3). Liquid level in the filter
was maintained to just cover the top layer
of the plastic medium. To reduce the
possibility of short circuiting along the
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Ground
Sludge
Holding
Tank
Positive
Displacement
Sludge Pump
Heat
Exchanger
Grinder
Air
Compressor
Oxidized
Sludge
Decant
Liquid Recycle to Tank
Plant or Separate -A
Sidestream Treatment
Anaerobic Digestion
Solids to Dewatering
Vapor Combustion
or Deodorization
by Water Scrubbing
and Activated Carbon
Vapor-Liquid
Separator
Pump
Figure 2. Schematic diagram of the Zimpro thermal conditioning process.
wall of the filter, a 1.9-cm-wide rubber
gasket (0.75 in.) was installed between
the medium sections and glued securely
against the inner surface of the filter wall.
The filter was kept at thermophilic
temperatures by means of a hot water
piping system attached to the outer wall
of the filter.
Results and Conclusions
Significantly improved energy efficiency
can result if thermal conditioning is
performed before anaerobic digestion
rather than after'digestion. The major
variable is the efficiency of the heat
exchangers in the thermal-conditioning
unit. If the efficiency is high enough, the
thermal-pretreatment/anaerobic-diges-
tion system could result in a net surplus
of energy. Advantages of thermal condi-
tioning before (rather than after) diges-
tion appear to be (1) stable digestion with
less recycling of degradable organics to
the wastewater treatment plants, (2)
greater methane gas recovery, (3) more
positive control of odors, and (4) a reduced
quantity of solids for ultimate disposal.
Advantages of thermal conditioning after
digestion with anaerobic treatment of
liquid side streams include (1) slightly
drier cake solids following dewatering
and (2) reduced polymer conditioning
requirements. Selection of the final mode
1 of operation is likely to depend on site-
specific factors and the ultimate disposi-
tion of the solids. Note that any type of
thermal treatment results in a significantly
smaller net energy surplus than when the
sludge receives anaerobic digestion
alone.
During the 17 months that the thermal
conditioning system operated, thermo-
philic anaerobic digestion of the thermally
conditioned WAS proved very reliable. No
toxicity problems or digester upsets
occurred, and digestion performance was
not significantly affected by the tempera-
ture of thermal pretreatment over the
range of temperatures from 170° to
220°C (340° to 425°F). The thermal-
pretreatment/anaerobic-digestion system
increased volatile solids destruction by
65 percent over standard mesophilic
digestion and 36 percent over standard
thermophilic digestion. When only those
volatile solids destroyed in the digester
were taken into account, the percent
increases dropped to 40 and 15 percent,
respectively, over standard mesophilic
and thermophilic digestion of WAS.
Thermal pretreatment and thermophilic
digestion of the 65 percent primary/
WAS blend did not produce results signi-
ficantly different from standard thermo-
philic digestion.
Thermal conditioning with temperatures
exceeding 150°C (300°F) for about 30 min
should assure the complete destruction
of all pathogen microorganisms. Data
indicated that viable parasitic helminth
ova were essentially eliminated and
bacterial organisms were significantly
reduced compared with levels in standard
digester effluent. Regrowth of coliform
organisms (and in some cases Salmonella
sp.) was observed occasionally, possibly
because of poor temperature distributions
in the thermal conditioning unit. But
salmonellae concentrations remained
lower than those in standard digester
effluent.
Thermally conditioned, digested sludges
dewatered well compared with sludges
receiving anaerobic digestion alone.
Filtration devices generally dewatered
these sludges more effectively than
centrifugal devices. But prethickening of
sludge was a necessary step before
vacuum or pressure filtration to attain ac-
ceptable cake consistency and discharge
characteristics. The mixture of primary
and WAS had slightly better dewatering
characteristics than WAS alone. Test
runs using undigested sludges indicated
that dewatering characteristics are
degraded slightly by subsequent digestion.
Cake solids concentrations are lowered,
and polymer doses are increased. Thermal
conditioning of WAS at temperatures
above 190°C (380°F) was needed to
produce successful dewatering.
Poly-Vinyl
Media
Sample
Valves
Effluent
1.9cm, 12O°
Spray Nozzle
Support
Flange
Note: 1"=2.54 cm
Figure 3. Schematic diagram of the pilot-
scale anaerobic filter.
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A two-stage water-scrubber/carbon-
adsorption unit successfully removed 97
percent of the odor concentration from
the thermal conditioning decant tank
exhaust gases. Alone, the water scrubber
portion removed about 68 percent of the
exhaust gas odor concentration.
Energy analysis data show that all
thermal treatment schemes combined
with the digestion step are net users of
energy. The thermal treatment processes
produce more biodegradable material but
the increase in the gas produced from the
anaerobic processes is not nearly suffi-
cient to match the energy needed to
operate the thermal conditioning unit.
The least energy-efficient system was
thermal treatment followed by decanting
supernatant from the sludge solids with
anaerobic digestion of the decant liquors
only at a 15-day hydraulic detention time.
Apparently, much degradable organic
material still remains associated with the
solids fraction of the thermally conditioned
sludge. No energy credit is given to this
material, since it is removed from the
system before anaerobic treatment.
Net energy efficiency calculations for
all systems studied are summarized in
Table 1 in which the treatment systems
are ranked in order of greatest to least
efficiency. The data also indicate the
amount of sludge solids remaining for
disposal after the thermal conditioning
anaerobic treatment process.
Important conclusions that can be
drawn from the tabulation are as follows:
1. For any given sludge, adding any
form of thermal treatment to anaero-
bic digestion reduces net available
energy, frequently to negative values.
2. For any given process combination,
there is more net available energy
with the blended sludge than with
WAS.
3. If thermal treatment is needed be-
cause of the demonstrated improve-
ments it produces in dewatering pro-
perties, more net energy will be pro-
duced if the heat treatment precedes
digestion rather than follows it (no
experiments with thermal treatment
after digestion were conducted so
advice on whether thermal treat-
ment should be conducted before or
after digestion for best dewatering
cannot be given).
4. Differences in net available energy
between LPO conditioning and heat
treatment are minor.
5. When thermal treatment is used, the
net available energy depends greatly
on the thermal approach attained in
the heat exchangers.
6. Anaerobic digestion of the entire
sludge after thermal treatment
produces much more net available
energy than treatment of only the
decant liquors by anaerobic digestion
or by the anaerobic filter.
Although net available energy is an
extremely important consideration, other
factors also influence process choices.
Process selection should also consider
capital cost and compatibility of the
sludge produced with subsequent process
steps and disposal choices.
The full report was submitted in
fulfillment of Contract No. 14-12-150 by
the Los Angeles County Sanitation
Districts under the sponsorship of the
U.S. Environmental Protection Agency.
Table 1. Ranking of Energy Balances for Various Thermal-Conditioning Systems
System
1. Digestion of blend
2. LPO conditioning— digestion of blend
3. Digestion of WAS
4. Digestion — heat treatment of blend
5. Digestion— LPO conditioning of blend
6. Heat treatment — digestion of WAS
7. LPO conditioning— digestion of WAS
8. Digestion— heat treatment of WAS
9. Digestion — LPO conditioning of WAS
JO. LPO conditioning— anaerobic
filtration of blend
1 1. LPO conditioning— anaerobic
filtration of WAS
12. LPO conditioning— anaerobic
digestion of WAS decant liquors
Net Energy W9 Joules
AT" = 22°C (40° F) A
427 ( 405)
243 ( 230)
206 ( 195)
127 ( 120)
106 ( 100)
95 ( 90)
95 ( 90)
-1791-170)
-2061-195)
-227 (-215)
-280 (-265)
-353 (-335)
(10* BTU)
T = 44°C (80°F)
427 ( 405)
95 ( 90)
206 ( 195)
-95 (-90)
-116 (-110)
-95 (-90)
-100 (-95)
-464 (-440)
-496 (-470)
-448 (-425)
-570 (-540)
-628 (-595)
Solids Remaining for Disposal
Metric Tons (Short Tons)
64 (70)
57 (63)
70(77)
59 (65)*
57 (63)*
55 (61)
55 (61)
64 (70)*
65 (72)*
79 (87)
83 (92)
85 (94)
Calculated assuming digested sludge behaves similar to raw sludge during thermal treatment; never confirmed by pilot testing.
*USGPO: 1984-759-102-10661
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Thomnas J. LeBrun and Liberate D. Tortorici are with the Los Angeles County
Sanitation Districts, Whittier, CA 90606.
Irwin J. Kugelman was the EPA Project Officer (see below).
The complete report, entitled "Thermal Treatment of Municipal Sewage Sludges,"
(Order No. PB 84-196 732; Cost: $20.50, 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, J. B. Farrell can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
0000529
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