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

-------
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 processtemperature 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

-------
                                                                        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

-------
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 55C.  Note that  a good
percentage of the windrow material
experiences  temperature  in excess of
55C (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
55C,  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 >60C
35% of pile >55C
50% of pile >50C
                                                                         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.

-------
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. >55r
           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. >55C
                                               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. >55C
                                                      C.  Salmonella spp.
 Figure 4.    Summary of bacterial inactivation at > 55C 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

-------
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 40C).

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

-------
   80
   70
I  60

I
   50
   40
T	T
                       T	1	1	1
                                Tr
                                                       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

-------
      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
                            ?*

-------