United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-122 Sept. 1984
SEH\ Project Summary
Windrow and Static Pile
Composting of Municipal Sewage
Sludges
Mario lacoboni, Jack Livingston, and Thomas LeBrun
Several composting research projects
were conducted from 1972 to 1978 at
the Joint Water Pollution Control Plant
(JWPCP) in Carson. California, in
response to Federal mandates for
sludge treatment. The projects have
involved research in both windrow and
static pile composting.
The research on windrow composting
had three distinct phases because of
changes in sludge production and the
development of improved composting
methods. In the first windrow compos-
ting phase (1972 through 1976), the
sludge used prior to composting was
anaerobically digested and dewatered
by nine scroll centrifuges without the use
of any conditioning chemicals. Approx-
imately one-third of the solids were
removed from the sludge by the scroll
centrifuges, resulting in 90 dry metric
tons/day (100 short tons/day) of cake
containing about 35% total solids. In
the second phase, which began in
December 1976, sludge solids in the
centrate from the scroll centrifuges
were recovered by a centrifuge system
composed of 44 basket centrifuges.
The basket centrifuge system was
designed to remove 90% of the solids
from the centrate of the scroll centri-
fuges and produce a digested dewatered
sludge cake with 15% to 20% total
solids. The two types of centrifuged
sludge cakes were mixed in some of the
compost tests. The third phase began in
June 1978 and studied large windrows
for their ability to increase compost
productivity and produce more consis-
tent temperature elevations and micro-
organism kills. Alternative bulking
agents, odor and dust control techniques.
forced air aeration, and covered (shel-
tered) windrow experiments were also
investigated in the third phase of the
research program.
Static pile composting studies began
in November 1977 as a potential
replacement for the windrow process.
Static pile composting, however, was
only marginally successful. Windrow
composting was found to be the best
method for use at JWPCP. The research
projects conducted since 1972 have
resulted in the establishment of an
effective, full-scale windrow compos-
ting operation at this facility.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that are fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Composting is a biological process in
which organic matter is broken down
under aerobic and thermophilic condi-
tions into a humus-type end product with
byproducts of carbon dioxide, water, and
heat. Unlike many other treatment
process schemes for sewage sludge,
composting results in a stable product that
can be used as a soil conditioner. Many
municipalities are therefore considering
it as an alternative to other methods of
final sewage sludge treatment.
In an effort to comply with Federally
mandated sludge treatment at the Joint
Water Pollution Control Plant (JWPCP) in
Carson, California, the County Sanitation
Districts of Los Angeles County have
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conducted various composting research
projects since 1972. The sludge used in
these studies was anaerobically digested
and dewatered with centrifuges prior to
composting. In the early stages of this
project, only windrow composting was
studied When production of dewatered
sludge increased and sludge characteris-
tics changed, more research was con-
ducted not only to improve the windrow
composting operation but also to evaluate
other composting methods such as
aerated static piles. This report describes
three phases of Research on windrow
composting and a study of aerated static
piles
Monitoring the Composting
Process
Various parameters must be monitored
to evaluate the success or failure of the
composting process—temperature eleva-
tion, drying time, volatile solids reduc-
tion, and mactivation of selected microor-
ganisms Odor emissions were also
monitored because of special concerns
about the proximity of the community to
the compost fields
Temperature
The biological aerobic composting
process generates large amounts of heat
through biodegradation of organic matter.
This heat is important, because it
inactivates pathogens and aids in the
drying of compost material. Temperatures
of 55 to 65 °C (132 to 149 °F) should be
achieved and maintained in the compos-
ting material for extended periods. All
compost material should be exposed to
these elevated temperatures long enough
to inactivate pathogenic and parasitic
microorganisms. In the windrow process,
this exposure is accomplished by periodic
turning of the material. In the static pile
process, the elevated temperatures must
occur in all sections of the pile.
Microorganisms Inactivation
Most organisms present in raw sewage
are concentrated in the sludge produced
by standard wastewater treatment pro-
cesses Thus, many different types of
pathogens and parasites are found in the
initial composting mixture Because
compost is eventually used as a soil
conditioner, it is important that pathogenic
and parasitic microorganisms be inacti-
vated In fact, mactivation of these
microorganisms was the primary goal of
the composting research.
Samples were analyzed for indicator
microoganisms (total and fecal cohforms),
enteric pathogenic bacteria (Salmonella
sp.), parasites (e.g., Ascaris sp.), and
enteric viruses. Ascaris ova are the most
hardy in the natural environment and are
used as an indicator of the presence of
parasites. If Ascaris sp. are destroyed, it
was presumed all other parasites were
destroyed. Also Ascaris sp., as with other
parasites, do not regrow when inactivated.
Reduction of Ascaris ova to low levels
therefore indicates a successful compost
operation.
Total and Volatile Solids
Proper total and volatile solids contents
are important in the formation of win-
drows and static piles, and they help
determine when the composting process
is complete. For composting to proceed,
moisture content should not be excessive.
When using recycled finished compost as
the bulking agent, windrows should have
an initial total solids content of at least
40%—50% if static piles are used. Lower
total solids contents result in a mixture
with low porosity, which inhibits oxygen
transfer. In this study, the initial volatile
solids content (i.e. percent of total solids
that are volatile) was between 46% and
50%. The destruction of these volatile
solids fuels the composting process.
Composting activity decreases markedly
and the composting cycle is concluded
after the total solids content has in-
creased to between 60% and 65% and the
volatile solids content is reduced below
40%.
Windrow Composting
Process Description
Windrow composting was adopted in
1972 at the JWPCPto minimize problems
associated with the existing sludge
drying beds Spreading anaerobically
digested sewage sludge over drying beds
required more land area than was
available and was a source of offensive
odors and vectors to the surrounding
community Composting, on the other
hand, required less land area, yielded a
more uniform and stable product, and
produced elevated temperatures that
significantly reduced the densities of
pathogenic organisms. The full-scale
composting process at the JWPCP has
been modified intermittently since 1972
to incorporate research findings that
have been generated in response to
changes in sludge production and charac-
teristics
The current JWPCP windrow compos-
ting process typically uses previously
composted material as the bulking agent,
though rice hulls and wood shavings are
occasionally used Windrows are con-
structed using a tractor-trailer and front- •
end loader (Figure 1). Dewatered digested
sludge cake of a known weight is placed
in the trailer, and the loader then places a
known volume of bulking agent on top of
the sludge. This material is emptied from
the trailer and laid in piles up to 120 m
(400 ft) long on the compost field. The
windrows are turned and mixed by a
specially designed machine called a
composter. The composter straddles the
windrow and has a high-speed rotating
drum at ground level. The drum has flails
that lift the sludge up and over the drum,
depositing it behind the machine in
windrow form. Turning serves to mix the
sludge cake and finished compost,
increases the porosity in the windrow to
maintain aerobic conditions, promotes
drying of the sludge by exposure to air and
sun, and ensures that all of the sludge is
exposed to the higher temperatures
inside the windrow.
When the total solids level of the
compost material reaches 60%, it is
considered dry and stable enough to be
used by a fertilizer company. During the
summer, the windrow composting process
is completed in 4 to 5 weeks. Typical
winter cycles average 6 to 8 weeks,
depending on the Los Angeles weather
conditions.
The cost of the composting operation
can vary widely depending on the amount
of sludge cake that can be composted at
any given time. The unit cost per ton of
sludge composted is much less from
spring through autumn when approxi-
mately 450 wet metric tons of sludge
cake per day (500 short tons/day) at 23%
total solids are processed as compared
with the winter application rate of approx-
imately 135 wet metric tons/day (150
short tons/day). These productivities
coupled with fixed capital and operating
costs result in seasonal costs of approxi-
mately $5.50/wet metric ton ($5/short
ton) during most of the year and $ 11 /wet
metric ton ($10/short ton) during the
winter months. These costs are for 1983,
and they do not include land and
overhead costs.
Study Design
The windrow composting research was
conducted with the same basic procedure
used in the full-scale operation. The
testing involved three distinct phases
because of changes in sludge processing
and the development of improved com-
posting methods.
Phase I of the composting program at
JWPCP took place from the beginning of
windrow composting in 1972 through
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Jec
Trailer
Figure 1. JWPCP window composting operation
December 1976. During this phase,
anaerobically digested primary sludge
was dewatered by nine scroll centrifuges
without the use of any conditioning
chemicals. These centrifuges removed
about a third of the sol ids from the sludge,
resulting in 90 dry metric tons/day (100
short tons/day) of cake containing approx-
imately 35% total solids.
Phase II of the research program began
after a second centrifuge system with 44
basket centrifuges became operational in
December 1976 These centrifuges were
designed to remove 90% of the solids
from the centrate of the scroll centrifuges
and produced a cake containing from 15%
to 20% total solids. After the two-stage
centrifuge system began operating, the
total amount of dewatered sludge pro-
duced increased from 90 to approximately
250 dry metric tons/day (100 to 275
short tons/day) The combined total
solids content of the basket and scroll
centrifuge cakes was about 22%
Phase III began in June 1978 with the
discovery that high windrows 1 2 to 1.5m
(4 to 5 ft) instead of 0.7 to 1 1 m (2.5 to 3.5
ft) increased compost productivity with
more consistent temperature elevations
and microorganism kills Phase III studies
were expanded to include use of alterna-
tive bulking agents, odor and dust control
techniques, forced aeration, and covered
(sheltered) windrow experiments
Results
Phase I
Phase I studies focused on the effects
of climate, turning frequency, and volatile
solids content for achieving high temper-
atures in the Windrow compost piles.
With no rain and no composter break-
downs, temperatures as high as 65 °C
(149 °F) were measured inside the win-
drows when the initial volatile solids
contents were about 50% of the dry
weight. Low ambient temperatures,
rainfall, composter malfunctions, and/or
low initial volatile solids contents can
contribute to lower temperatures, 50 to
55 °C (122 to 131 °F), in the interior of the
compost windrows.
Parasitic ova were monitored, and
intact Ascaris sp., Trichuris trichiura, and
hookworm ova were isolated at the
beginning of the compost cycles. Viable
Ascaris ova were rarely found late in the
compost cycles, except when maximum
windrow temperatures were low. Final
cohform most probable number (MPN)
concentrations of less than 2 MPN/g
were achieved m interior samples of
warm weather compost cycles, and final
Salmonella sp concentrations of less
than 0.2 MPN/g were measured in both
interior and exterior windrow compost
pile samples collected during warm
months. Though some coliform regrowth
was observed in all cycles (predominantly
in samples from windrow exteriors), it
was considered insignificant during
periods with no rainfall.
Phase II
This phase involved applying the
techniques learned from earlier work to
the composting of sludge cake from the
two-stage centrifuge system. The new
dewatering system produced a sludge
cake with substantially different proper-
ties than a Phase I cake: higher water
content, a more dense and less porous
consistency, and a higher percentage of
fine particles. The new centrifuge cake (at
22% TS) required almost three times the
bulking material to raise the initial
windrow mixture to 40% total solids. This
extra bulking agent increased the amount
of land required to compost an equal
weight of sludge solids; it also proved
detrimental to the composting process
because of increased handling problems.
The method of building windrows did
not ensure a uniform mixture of wet cake
and finished compost at the beginning of
the compost cycles. The windrows varied
noticeably in moisture content and were
uneven in height and width. As a result,
all of the material did not experience the
same degree of composting, and internal
temperatures varied widely throughout
the length of the windrow. Average
temperatures were lower than in Phase I.
During Phase II, final coliform counts
averaged 10 to 100 times higher than in
Phase I, and Salmonella sp. concentra-
tions were also considerably higher.
Eventually, adequate bacterial and
pathogen inactivation was achieved, but
not consistently enough for overall
satisfactory composting performance.
Phase III
Phase III began m June 1978 and
centered on the composting properties of
windrows that were much larger than
those used in Phases I and II. These
results appear to reflect more accurately
the abilities of windrow composting to
achieve good temperature elevation and
consistent pathogen inactivation. Over a
2-year period of Phase III, 34 windrows
were monitored throughout their compost
cycle. In addition to the effects of
windrow composting and turning fre-
quency, other factors were studied such
as the use of alternate bulking agents,
forced aeration, enzymes and bacterial
additives, and the use of covers to protect
to the windrows. Near the end of the
study, additional monitoring was per-
formed to measure the odor levels
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emitted by the windrows so that odor
control strategies could be developed.
Temperature Elevation
A typical plot of the interior tempera-
ture of the windrow during an entire
compost cycle is shown in Figure 2.
Though turning frequency did have a
slight effect on internal temperature
elevation, three turns per week were
adequate and avoided the problems of
more frequent turning, which tended to
lower temperatures and shorten the
period that windrows maintained tem-
peratures above 55°C. Note that a good
percentage of the windrow material
experiences temperature in excess of
55°C (Figure 3). The periodic turning was
effective in moving all the material into
zones of elevated temperatures since it
resulted in good pathogen inactivation.
Microorganism Levels
The heat generated by the compost
process effectively reduces harmful and
indicator microorganisms to low levels
(Figure 2). By using such data for 20 test
windrows over the course of Phase III, it
was possible to construct probability plots
of expected concentrations of bacterial
organisms as a function of internal
windrow temperatures (Figure 4). The
curves in Figure 4 show the number of
consecutive days with internal windrow
temperature above 55 °C that were
required to reduce bacterial concentra-
tions to low levels. For example, after 7
consecutive days of temperatures above
55°C, 85% of the windrows indicated
Salmonella sp. levels below the minimum
detection limit of O.2 MPN/g of compost.
The rate of destruction of Ascaris ova
was not determined because of the cost
and complexity of the laboratory proce-
dure. To ensure that adequate destruc-
tion of these organisms had been
achieved, samples were routinely taken
at the beginning and end of select
compost cycles, and several of 20
windrows were extensively sampled at
locations least likely to have experienced
high temperatures (Figure 5). Excellent
Ascaris destruction was achieved, and a
summary of the laboratory results
appears in Table 1.
Drying Rates
Six windrows were monitored to
determine the effects of turning frequency
on drying rates. Evaluations were made
of two, three, and five turns per week.
Drying rates did increase slightly with
greater turning frequency, but not
enough to warrant the resulting increase
01
w
3
<5
w
0)
I
70
60
50
40
!
Figure 2.
5 70
75
20
25
30
35
40
• Total Coliform
• Fecal Coliform
Salmonella (sppj
10
15 20 25 30
Days into Compost Cycle
35
40
Bacterial inactivation resulting from elevated temperatures in a typical windrow
compost cycle.
15% of pile >60°C
35% of pile >55°C
50% of pile >50°C
4'W"
Note: - Windrow turning frequency of 3 times per week..
- Temperatures (°C) recorded on day 29 of cycle.
- Total so/ids = 58%, Volatile solids = 30%.
Figure 3. Temperature (°C) contour diagram for the recommended turning frequency at
the JWPCP.
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I
3
II
99
98
95
90
80
70
60
50
40
30.
20
•1.0 MPN/g
•10.0 MPN/g
0 4 8 12 16 20 24
Consecutive Days with Windrow
Temp. >55°r
A. Total col/form
§ "
I 98
95
90
8
11 $0
7°
60
50
30,
§,520
i
10
s.
1.0 MPN/g
F • 10.0 MPN/g -
'0 4 8 12 16 20 24
Consecutive Days with Windrow
Temp. >55°C
B. Fecal coliform
c 99
| 98
**
8 95
^ 90
lig SO
IS 70
eo
so,
^401
"oiQ. 30J
§i 20
* '
• 0.2 MPN/g
02 46 8 10 12
Consecutive Days with Windrow
Temp. >55°C
C. Salmonella spp.
Figure 4. Summary of bacterial inactivation at > 55°C for Phase III windrows.
Cross-Section Samples
A
Interior Longitudinal Sample
•J
•1C*
Sample
A -
B-D, £ -
C,
Description
top surface sample
intrior sample
ground level samples
Sample
F -
G -
H.J.K-
Figure 5. Final microbiological sampling locations lor windrows.
Table 1. Comparison of Total Coliform and Ascaris Inactivation
Total Coliforms
Description
material thrown from
windrow during turning
material at toe of
windrow
random interior samples
Ascaris sp.
Compost
System
Windrow
Aerated
static
pile
No. of Samples
from Finished
Compost
157
117
Samples with
less than
10 MPN/g
(%)
78
43
1 MPN/g
(%>
56
21
Samples with no
No. of samples Detectable
from Finished Viable Ova
Compost (%)
80
24
89
58
in operating time for daily turning.
Volatile solids levels were reduced to a
greater extent in windrows turned less
often.
An optimum turning frequency of three
turns per week was chosen based on
operational capabilities. This frequency
results in a yearly average composting
application rate of about 15 metric tons
(6 short tons of dry sludge solids/acre)
per day when using a dewatered sludge
cake of 22% total solids.
Odors
Because the JWPCP composting oper-
ation is located near a residential area,
odor and dust control on the compost
fields are of prime importance. Most
odors are emitted during the first 7 to 10
days of the compost cycle and are
primarily associated with the surface
odor emissions of the windrows in their
ambient or unturned states. Turning
odors account for approximately 15% of
the total emitted, but they are much more
concentrated than the ambient odors.
Several methods of odor and dust
control were evaluated, including chemi-
cal and biological additives. These did not
result in improved composting operations
or in reduced odor or dust emissions.
There are, however, several suggested
methods of odor control: (1) halt the
turning of windrows during times of
adverse meterological conditions or
increased odor complaints, (2) design
active composting areas so that they
provide a maximum buffer zone for the
surrounding residential area, and (3)
restrict the process of windrow turning to
hours of greatest vertical mixing in the
atmosphere.
Bulking Agents
Bulking agents are commonly used in
the construction of windrows to increase
initial percent total solids, improve
compost handling characteristics, and to
provide proper texture for better air
circulation. Recycled, finished compost is
frequently used as a bulking agent at the
JWPCP, but for comparative purposes a
study was conducted using alternate
bulking agents such as rice hulls and
wood shavings. Good composting per-
formance was achieved for all three
bulking agents tested. Rice hulls and
wood shavings produced shorter drying
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scrubber followed by activated carbon) fl
was the most effective method, removing
96% of the odor.
Further studies were conducted during
the second phase of the static pile
research to optimize the recycle ratio of
finished compost. Results of these studies
were not adequate, primarily because of
problems with mixing and developing a
uniform compost pile. Observations in
these and previous studies did show,
however, that a proper initial compost
mixture of dewatered sludge cake and
recycled, finished compost (i.e., mixture
ratios of 0.84 to 1.68 m finished compost
per metric ton of dewatered sludge cake
(1 and 2/yd3short ton) produced an initial
total solids content of about 50% and
resulted in the best overall temperature
elevations and microorganism inactiva-
tion. Too little initial moisture content (>
55% total solids) appeared to cause
premature drying of the compost pile and
inhibit biological activity. Too much
moisture (much less than 50% total
solids) tended to prevent oxygen transfer
through the pile.
The third phase of the static pile study
examined the performance of an extended
aerated pile consisting of five piles laid
side by side so that one continuous pile
resulted. Recycled, finished compost was
used as the bulking agent. Construction
of a continuous pile was difficult and
resulted in a nonuniform mixture of
dewatered sludge cake and finished
compost. Acceptable microorganism
inactivation was achieved, but internal
temperatures across the pile varied
considerably. This method is not recom-
mended for obtaining uniform results in
the final compost material.
Drying was a major problem and
consequently had to be investigated to
ensure that adequate total solids levels
were achieved throughout the pile. Part
of the extended pile was removed and
formed into individual windrows that
were turned regularly. The remainder of
the extended pile was allowed to dry by
continuous, forced aeration. The windrows
dried quickly, reaching 70% total solids
levels after 7 days and providing high
temperatures for preventing microorgan-
ism regrowth. The pile under continuous
aeration, on the other hand, dried un-
evenly (50% to 70% total solids level),
and internal temperatures dropped to low
levels (30 to 40°C).
Summary of Static Pile
Experiments
In summary, static pile composting was
only marginally successful, since good
times and reduced odor emissions, but
their performance was generally similar
to that of recycled, finished compost. Cost
is the most limiting factor when consider-
ing rice hulls or wood shavings as
alternative bulking agents.
Forced Aeration
A study was conducted of the compos-
ting characteristics of deep windrows
that used forced aeration in addition to
windrow turning. Forced aeration resulted
in more rapid temperature elevation; but
when the process was continued through-
out the cycle, internal temperatures were
lowered. Forced aeration did not appear
to be necessary for deep windrow com-
posting when using 1.2- to 1,5-m (4- to 5-
ft) high piles, but it may be useful fbr even
larger piles in which drying time would be
expected to increase.
Static Pile Composting
Static pile composting studies were
initiated in November 1977 based on the
success of the U.S. Department of
Agriculture in Beltsville, Maryland, who
developed this process for dewatered
municipal sewage sludge. The Sanitation
Districts were interested in the reported
advantages of odor control, productivity
per acre, and all-weather operations. At
the time, initial Phase II results for
windrow composting had not been es-
pecially encouraging. Thus, if the Belts-
ville process could have been modified to
use recycled, finished compost as the
only bulking agent, it would have been a
potential replacement for the windrow
process. The use of wood chips as a
bulking agent (as in the Beltsville process)
was not desirable because their recovery
and reuse added too much cost and
complexity to the overall composting
process
Process Description
In a typical aerated static pile, dewa-
tered sludge is combined with the bulking
agent and laid into windrows, the
material is then mixed by the composter
The aerated static piles in this study were
constructed using a front-end loader to
pile the mixed material on top of the
plastic air lines To maintain aerobic
conditions m the piles, air was usually
drawn through them intermittently Air
flow rates and the oxygen contents were
monitored The only wood chips used in
the process were the few needed to cover
the plastic air lines to prevent the lines
from being clogged with compost material
These chips were not recovered. After the
piles were constructed, about 0 3 rn (1 ft)
of finished compost was added to the
surface of each pile for heat and odor
insulation.
The piles remained intact for 3 to 4
weeks. Various data were collected to
gauge the performance of the composting
process. Little drying occurred during the
process, and thus some additional tech-
niques were required. These included air-
drying by increasing the air flow to
maximum and tearing down the pile and
forming a windrow. Both techniques
successfully raised the total solids
contents of the compost material to the
desired level of more than 60%.
Study Design
This study was to determine whether
the aerated static pile process could be
successfully run without the use of exter-
nal bulking agents (i.e., wood chips). Odor
control was also important. The hope was
that static piles would produce less odor
than windrows because turning was not
required. In addition, most odors would
be emanating from a point source (the
blower) and thus would be easier to
control. Finally, the performance consis-
tency of the static piles was examined
because the lack of routine turning or
mixing as used in windrow composting
might produce anaerobic zones and
consequently areas of poor pathogen
inactivation.
Results
During the first phase of the aerated
static pile research, the use of rice hulls
and wood shavings as bulking agents were
compared with the standard JWPCP
practice of using recycled, finished
compost. Wood chips were not used as a
bulking agent.
Good temperature elevations occurred
rapidly and were maintained throughout
the cycles for each of the experimental
piles (Figure 6). The static pile using fin-
ished compost bulking material produced
slightly lower temperatures (63 versus
70 °C), but all of the piles achieved excel-
lent microorganism inactivation.
As expected, odors were concentrated
in the blower exhaust, but they did not
differ significantly among the piles. In
fact, by day 10 of their cycles, virtually no
odor emissions were detected from the
surface of the static piles.
Several methods were tested for
treating exhaust gases from the blowers.
Finished compost material was tried as
an odor-adsorbing medium but was not
adequate. Wet scrubbers using water,
potassium permanganate, sulfuric acid,
or sodium hypochlorite removed 40% to
60% of the odor from the exhaust gases.
A two-stage scrubbing system (water
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80
70
I 60
I
50
40
T T
T 1 1 1
T—r
1 1 1
Pile Using Wood Shavings
Pile Using Rice Hulls
Pile Using Finished Compost
0 2 4 6 8 JO 12 14 16 18 20 22 24 26 28 30 32 34 '.36 }38
Days into Cycle
Figure 6. Average temperatures versus time for static piles.
performance was unreliable and unpre-
dictible. Alternative bulking agents such
as rice hulls and wood shavings imrpoved
compost handling and performance
slightly. The optimum total solids range
for good temperature elevation was 50%
to 55%.
Some specific disadvantages of the
static pile composting process were
nonuniform microoganism kills that
allowed regrowth, extended processing
time for drying of the piles, the need for
scrubbers a nd carbon f i Iters for treatment
of the blower exhaust, and the extreme
dependence of process performance on
the uniformity and consistency of the
initial compost mixture. This process was
also labor intensive.
Overall Summary of the
Composting Research Study
Sludge composting research conducted
since 1972 has helped produce an
effective full-scale windrow composting
operation at the JWPCP. Research
indicated that windrow composting of
JWPCP's anaerobically digested centri-
fuge dewatered primary sludge produced
results superior to those of the aerated
static pile composting process. This
conclusion is based on achieving consis-
tently the temperature elevations needed
for microorganism inactivation and
sludge drying with simpler operations.
The major operational parameters for
composting at the JWPCP were the type
and quantity of bulking agent used.
Windrow size and turn frequency were
also significant for windrow composting.
Odor control and microorganism regrowth
were the controlling output parameters.
Full-scale operation and research
studies of windrow composting at JWPCP
were very successful from June 1972
until December 1976, when sludge
production and characteristics were
significantly changed with the addition of
basket centrifuges for more complete
sludge dewatering. Windrow composting
methods used previously to compost the
existing scroll centrifuge cake were
inadequate for successful composting of
this new sludge, which had a much higher
moisture content. Research conducted
on the windrow and aerated static pile
composting processes from January
1977 through November 1977 proved
only marginally successful. But in June
1978, the use of larger windrows
produced significantly improved process
reliability and performance. Early com-
posting studies at JWPCP used shallow
windrows 0.7 to 1.1 m (2.5 to 3.5 ft) high.
Studies after June 1978 indicated that
the use of windrows 1.2 to 1.5 m (4 to 5 ft)
high would help assure good temperature
elevation and therefore acceptable micro-
organism destruction. Deep windrow
composting became possible because of
the development of a new composter
(Scarab)* that would adequately process
windrows up to 1.5 m (5 ft) high. Deep
windrows have the added advantage of
requiring less land to compost a given
amount of sludge.
Significant findings with respect to
other factors influencing windrow com-
posting include: (1) windrow turning
frequency of about three times per week
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
results in a well-mixed, aerated pile, (2)
mixture ratios of 0.84 to 1.68 m3 of
recycled finished compost per metric ton
of dewatered sludge cake (1 and 2
ydVshort ton) will provide the proper
total solids content (40%) and sludge
handling properties for successful win-
drow composting, and (3) the use of
alternative bulking agents such as rice
hulls and wood shavings improved
material handling characteristics for the
compost mixture as a result of lower bulk
density and indicated possibilities for
shorter drying times.
Because the JWPCP composting oper-
ation is located near a residential area,
odor and dust control are of prime impor-
tance. Some possible control methods
include stopping windrow turning during
times of adverse meteorological conditions
or increased odor complaints, using maxi-
mum buffer zones around active compos-
ting areas, and restricting windrow
turning to times of greatest vertical
atmospheric mixing.
The full report was submitted in partial
fulfillment of Contract No. 14-12-150 by
County Sanitation Districts of Los Angeles
County under the sponsorship of the U.S.
Environmental Protection Agency.
*USGPO: 1984-759-102-10682
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Mario lacoboni. Jack R. Livingston, and Thomas J. LeBrun are with County
Sanitation Districts of Los Angeles County. Los Angeles. CA 90607.
Gerald Stern is the EPA Project Officer (see below).
The complete report, entitled "Windrow and Static Pile Composting of Municipal
Sewage Sludges." (Order No. PB 84-215 748; Cost: $14.50. subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer 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
' .* -' 1]
Official Business
Penalty for Private Use $300
?*
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