United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/026 May 1987
v>EPA Project Summary
Shredded Rubber Tires as a
Bulking Agent for Composting
Sewage Sludge
Andrew J. Higgins, Jeffrey L Suhr, M. Siddiqur Rahman, Mark E. Singley,
and Vijay S. Rajput
Shredded rubber tires were evalu-
ated as a bulking agent for composting
wastewater sludges to determine the
optimum particle size and mix ratio for
efficient composting. Three sludges
(raw primary, anaerobically digested,
and secondary biological), two amend-
ments (sawdust and recycled com-
post), three sizes of shredded rubber
(1.27 to 2.54 cm, 2.54 to 5.08 cm, and
greater than 5.08 cm), and three
shredded-rubber-chip-to-sludge mix
ratios (1:1, 2:1, and 3:1) were evalu-
ated. The smallest size rubber chip, 2:1
mix ratio, and sawdust amendment
were found to be optimums.
Test results with raw primary sludge,
shredded rubber, and no amendments
produced undesirable odors and han-
dling difficulties. A high initial moisture
content and low carbon-to-nitrogen
(C/N) ratio led to conglomeration of the
sludge particles, anaerobic conditions,
and the conversion of excess nitrogen
into ammonia gas. Tests with all three
sludges and recycled compost pro-
duced similar results.
When amended with sawdust, all of
the sludges were effectively composted
using shredded rubber. Because of high
moisture content and low C/N ratio, all
sludges required the moisture ab-
sorbency and supplemental carbon that
the sawdust provided.
Heavy metal levels increased during
composting with raw primary sludge
and rubber chips as a result of the con-
centrating effect of organic matter de-
composition. In addition, the shredded
rubber chips contributed Zn and Fe to
the finished compost. Recycling the
rubber chips reduced the Zn and Fe con-
centrations, but they were still high
after five cycles. However, the levels
were not high enough to limit the use
of shredded rubber in the composting
of the sludge.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search 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
The static pile composting of sewage
sludge has historically used wood chips
as a bulking agent. Because wood chips
are biodegradable and easily lost in the
composting process, there is a need for
recoverable, low-cost bulking agents.
Previous studies have shown that
shredded rubber tires are a potential
substitute for wood chips as a bulking
agent. Experiments using raw sludge
showed that temperatures of 55°C could
be developed and maintained for patho-
gen destruction, but that the shredded
rubber contributed Fe and Zn to the fin-
ished compost. Based on past experi
ence, this study had four goals: (1) to
determine the optimum rubber chip size
and mix ratio for efficient composting;
(2) to evaluate which types of sewage
sludges could be composted with
shredded rubber; (3) to determine if
supplemental sources of carbon would
be required to enhance the composting
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process, particularly for biological and
digested sludges with low energy val-
ues; and (4) to determine the heavy
metal contribution of shredded rubber
and the number of reuse cycles required
before a reduction in metal levels oc-
curred.
Procedure
Three compost vessels were con-
structed of wood with dimensions of
1.83 by 1.83 by 1.37 m. The front wall of
the vessel was hinged for access, and
the vessels were insulated with styro-
foam board.
The vessels had an airflow plenum at
a height of 15.2 cm from the bottom for
air distribution. One blower, controlled
by a time clock and thermostat, pulled
air through each vessel. The thermostat
was located in the center of the com-
posting mass and overrode the time
clock during peak activity periods to
maintain the temperature at 55°C. At the
beginning of each experiment, the mini-
mum airflow rate was set at 31 cubic
meters per hr per dry metric ton
(m3/h-dt). During peak activity periods,
the maximum airflow rate was 500
m3/h-dt.
Three dewatered sewage sludges
(raw primary, anaerobically digested,
and secondary biological) were ob-
tained from three local wastewater
treatment plants. Shredded rubber tires
that were relatively free of protruding
steel belts were obtained from a local
tire company. The shredded tires were
classified by screening into three sizes,
1.27 to 2.54, 2.54 to 5.08, and greater
than 5.08 cm. Supplemental amend-
ments (sawdust and recycled compost)
were used for moisture control and car-
bon sources. The amendments were
used to adjust the initial moisture con-
tent of the rubber chip and sludge mix-
ture to between 50% and 60%.
Temperature and airflow were
recorded for each composting vessel.
Four thermocouples at both the 0.3- and
0.6-m levels were connected in parallel
to produce an average temperature
reading for each level. Because the tem-
perature of the compost vessel was con-
trolled by a thermostat, the temperature
was not used to evaluate performance.
The heavy metal (Cd, Cu, Fe, Ni, Pb,
and Zn) contents of the sludge and the
final compost were measured to deter-
mine any possible contribution of
metals to the compost by the shredded
rubber tires.
Results and Discussion
The composting experiments were
grouped into five sets, and within each
set were several trials. Each trial used
the same sludge in each of the three
composting vessels.
Trials with Raw Primary
Sludge
Determination of the Optimum
Rubber Chip Size
The first experimental set consisted
of five trials to determine the optimum
shredded rubber chip size for compost-
ing. These trials used raw primary
sludge with a C/N ratio of 11 to 19,
which was lower than the optimum
range for composting of 25 to 30. The
first three trials compared small,
medium, and large rubber chips, and
the last two trials compared small,
medium, and mixed-size (1:1 small and
medium) chips. The results were meas-
ured by reductions in moisture, volatile
solids, and total carbon content, and
were statistically analyzed using a Latin
square design. The statistical analysis
of the first three trials showed that the
large rubber chips performed poorly;
therefore, no further trials were con-
ducted with the large chips. Analysis of
the last two trials showed that the small
rubber chip outperformed the medium
and mixed-size chips. Based on these
results, the small size chip was deter-
mined to be the optimum and was used
for all subsequent trials.
Determination of the Optimum
Rubber-Chip-to-Sludge Mix
Ratio
The second experimental set con-
sisted of three trials conducted with raw
primary sludge (C/N ratio of 7 to 9) to
determine the best rubber-chip-to-
sludge mix ratio. Three mix ratios were
compared (1:1, 2:1, and 3:1; volume
basis) based on experience with the
static pile composting method used in
the United States. The performance of
each mix ratio was statistically evalu-
ated using moisture, volatile solids, and
total carbon content data. In addition,
qualitative evaluations were made from
observations during loading, compost-
ing, and screening.
The statistical analysis, confirmed by
field observations, identified the 1:1
mix ratio as being unsatisfactory. No
statistically significant differences were
found between the 2:1 and 3:1 mix ra-
tios. These two mix ratios were approx-
imately equal in ease of handling and
composting performance, but the 2:1
mix ratio was considered optimum be-
cause fewer rubber chips were re-
quired. During loading, the 1:1 mix ratio
was observed to be the stickiest and to
form the most sludge balls. The 2:1 and
3:1 mixed more uniformly and formed
fewer sludge balls. A test for one trial
showed that the porosity of the mixture
increased from 43% to 52% as the mix
ratio increased from 1:1 to 3:1. Com-
posting odors were most noticeable
from the 1:1 mix ratio and least notice-
able from the 3:1. Screening carryover
was greatest for the 1:1 mix and pockets
of wet, partially composted sludge were
observed during unloading.
Contribution of Heavy Metals
by Shredded Rubber Tires
Using shredded rubber tires signifi-
cantly increased the concentrations of
Zn and Fe. Recycling the rubber chips
reduced these levels somewhat, but
they were still high after five cycles. Zinc
levels ranged from 900 to 1,200 mg/kg
in the raw sludge and 1,400 to 2,800 mg/
kg in the finished compost. Iron levels
ranged from 6,900 to 14,000 mg/kg in
the raw sludge, and 11,000 to 27,000
mg/kg in the finished compost. Zinc
oxide, used in the manufacturing of
tires, was the source of Zn in the com-
post. The increase in Fe was because of
steel belts and rim beads found in rub-
ber chips that had eluded magnetic sep-
aration. Although steel was infrequently
observed, it caused no handling prob-
lems and appeared to be randomly dis-
tributed among the chip sizes.
In four of the first five trials, the
highest levels of Zn were found in the
compost that used the small chips, and
the lowest levels when the large chips
were used. This was probably because
of the abrasion of the rubber chips dur-
ing loading, unloading, and screening
that resulted in small particles being in-
corporated into the sludge or compost
matrix. The small chips had the greatest
surface area per unit volume and, there-
fore, contributed more abraded parti-
cles to the compost samples. The
greater the percentage of chips in the
mix, the higher the concentration of Zn
and Fe in the compost.
The percentage change in concentra-
tion of Cd, Pb, Ni, and Cu showed that
increases in the level of these metals
were not significant and were due to the
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concentrating effect of composting as
organic matter was destroyed. No cor-
relation was found between chip size
and metal concentration for these
metals.
Temperature
Temperature readings for the middle
of the compost pile (0.6 m level) were
averaged for each day. The set point
temperature of 55°C was maintained for
at least 3 consecutive days only about
60% of the time. There was no pattern
between the temperature and the chip
size or mix ratio.
A temperature profile was observed
in the composting pile, highest at the
bottom and lowest at the top. This was
because of cool ventilation air entering
the composting vessels at the top and
warmed exhaust air exiting from the
bottom.
Screening Efficiency
A rotary trommel screen with a 1.27-
cm mesh opening was used to separate
the shredded rubber from the compost.
The screen removed all of the chips
from the compost, but some compost
was carried over into the shredded rub-
ber. The amount of carryover was
highly moisture dependent: the least
amount occurred when the moisture
content was 40% or less; the highest
amount occurred when the moisture
content exceeded 50%. Because the
rubber chips did not absorb water from
the sludge to lower the moisture con-
tent, sludge balls formed while mixing
and were difficult to break during
screening. Thus the screening efficiency
was poor when the compost was wet
and became more severe when the
sludge balls dried.
Trials with Raw Primary
Sludge Plus Amendments
The third experimental set consisted
of two trials with raw primary sludge
mixed with amendments (one with saw-
dust, the other with recycled compost)
and small rubber chips.
Adding both amendments to the raw
sludge lowered the moisture content of
the initial mixture to the 50% to 60%
range. Adding sawdust also increased
the C/N ratio from 14 to 27. Reducing
the moisture and increasing the C/N
ratio produced a dry, screenable com-
post with no objectionable odors during
the compost process, goals which were
not achieved by adding recycled com-
post.
The average daily temperature at the
center of each composting vessel was
maintained close to the 55°C set point
for both trials, but showed greater fluc-
tuation for the recycled compost trial.
Trials with Digested Sludge
Plus Amendments
The fourth experimental set consisted
of two trials in which either sawdust or
recycled compost was mixed with aner-
obically digested sludge and the small
rubber chips. The anaerobically di-
gested sludge had a moisture content
of about 82%. The amount of amend-
ment required to lower the initial mix-
ture to the 50% to 60% range was calcu-
lated to be about equal to the volume of
sludge. The rubber-chip-to-sludge-to-
amendment mix ratio was 2:1:1 for
both trials.
As a result of digestion, the anaerobi-
cally digested sludge had a high total
nitrogen content and a low carbon con-
tent that yielded a low C/N ratio of 3 to
4, which was not favorable for compost-
ing. Therefore, a carbon source was es-
sential.
As expected, adding sawdust to the
anaerobically digested sludge in-
creased the volatile solids content, total
carbon content, and C/N ratio of the ini-
tial mixture. But, the recycled compost
did not supply enough carbon for effec-
tive composting. Adding recycled com-
post to the anaerobically digested
sludge decreased the volatile solids
content and thus produced odors, but
not as strong as those produced during
the trials with raw primary sludge.
When sawdust was used as an amend-
ment, odors were not a problem. Also,
the recycled compost trial did not com-
post well or uniformly. During unload-
ing, ammonia gas was present and
numerous pockets of wet, uncom-
posted sludge were found.
There were no problems screening
the compost from the sawdust trial. The
compost from the recycled compost
trial was wet and formed balls during
unloading and screening, resulting in a
high percentage of compost staying
with the rubber chips.
The average temperature at the cen-
ter of each composting vessel was
maintained near the 55°C set point for
both trials. The temperature in the saw-
dust trial had less variation.
Trials with Secondary
Biological Sludge Plus
Amendments
The last experimental set consisted of
two trials in which secondary biological
sludge was mixed with either sawdust
or recycled compost and the small rub-
ber chips. With a moisture content over
85%, the amount of amendment
required to lower the initial moisture
content was greater than for previous
trials. The rubber-chip-to-sludge-to-
amendment mix ratio was 2:1:1.5.
As expected, the secondary biological
sludge had a high total nitrogen content
and low carbon content. The resultant
C/N ratio was low (4) and a supplemen-
tal carbon source was necessary. The
sawdust increased the C/N ratio from 4
to 17, but adding recycled compost only
increased the C/N ratio from 4 to 9.
The 2:1:1.5 mix ratio yielded initial
mixtures that had about 56% moisture
content for both trials, but had different
physical characteristics depending on
which amendment was used. The saw-
dust aided the mixing of the sludge and
rubber chips, the mixture had a uniform
consistency that was easy to handle,
and the finished compost screened
well. The mixture with recycled com-
post appeared wet, was very sticky, and
formed balls and clumps during the
loading phase of the experiment. This
occurred in the mixer and in the con-
veyor used to load the compost vessels.
The formation of sludge balls and
clumps also contributed to composting
odors and a high amount of screening
carryover.
During the composting with sawdust,
no odors were detected because the
moisture content was lowered into the
optimal range, carbon was provided to
increase the C/N ratio, and the mixing
was easier. Odors were detected during
composting with recycled compost, but
these odors were not as strong and ob-
jectionable as those produced when
composting raw primary sludge. The
odors were present, even though the
moisture content was in the optimal
range, because the recycled compost
did not prevent the formation of sludge
balls and clumps or provide sufficient
carbon to adequately increase the C/N
ratio.
The average daily temperature meas-
ured in the center of the pile did not
reach 55°C for two of the three vessels
in the sawdust trial. Two of the three
vessels in the recycled compost trial
reached 55°C for at least 3 consecutive
days.
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Conclusions
Of the three rubber tire chip sizes
(1.27 to 2.54, 2.54 to 5.08, and greater
than 5.08 cm) and three rubber-chip-to-
sludge mix ratios (1:1, 2:1, and 3:1)
evaluated, the smallest size chip and the
2:1 mix ratio were found to be opti-
mums for effective composting.
The porosity of the rubber chip and
raw primary sludge mixtures were all
acceptable and did not limit the com-
posting process. Average porosities
were 46%, 47%, and 50% for the 2:1
chip-to-sludge mixtures of the small,
medium, and large chips, respectively.
The combination of raw primary
sludge and shredded rubber produced
thermophilic composting conditions.
However, it also produced anaerobic
conditions, undesirable odors, and han-
dling difficulties, even at the optimum
chip size and mix ratio. These problems
were caused by the high initial moisture
content and the balling of sludge parti-
cles during mixing. In addition, a low
C/N ratio produced noticeable concen-
trations of ammonia gas.
Adding sawdust to the raw primary
sludge and shredded rubber mixture
lowered the moisture content and in-
creased the C/N ratio and thus elimi-
nated odors and screening problems. A
minimum of one part of shredded rub-
ber is recommended to provide suffi-
cient porosity and structure. The quan-
tity of sawdust required depends on the
moisture content and C/N ratio of the
sludge mixture; a moisture content of
55% to 60%, combined with a C/N ratio
of about 25:1, successfully eliminated
odors and produced a dry, stabilized
compost.
Using recycled compost to lower the
moisture content did not effectively
eliminate odors and screening prob-
lems, probably because of the lack of
carbon for increasing the C/N ratio. In
addition, recycled compost was not to-
tally effective in preventing the forma-
tion of sludge balls. Hence, recycled
compost is not recommended as an
amendment when using shredded rub-
ber unless it is combined with another
amendment such as sawdust.
Because of the low C/N ratio and the
high moisture content of the sludges
used, amendments containing available
carbon, such as sawdust, were essen-
tial. When mixed with these sludges,
sawdust effectively eliminated odors,
screening problems, and the formation
of sludge balls, and ensured compost-
ing temperatures of 55°C.
The shredded rubber contributed Zn
and Fe to the compost, but repeated re-
cycling of the rubber lowered the metal
concentrations somewhat. Elevated Zn
levels are expected to be found in the
finished compost for the life of the
shredded rubber because zinc oxide is
part of the rubber matrix and will con-
tinue to be abraded from the surface
during handling. On the other hand, Fe
comes from the steel belts and rim
beads and should eventually be com-
pletely oxidized and no longer con-
tribute to the finished compost. The in-
creased Zn and Fe levels do not appear
high enough to limit the use of shred-
ded rubber in the composting of
sewage sludge.
When only sludge and shredded rub-
ber were composted, all heavy metals
increased in concentration because of
the concentrating effect as organic mat-
ter was decomposed. However, amend-
ments that supplied organic matter,
such as sawdust, lowered the metals
concentrations in the finished compost.
The use of shredded rubber tires as a
bulking agent in the composting of all
types of sewage sludge is recom-
mended in combination with amend-
ments such as sawdust, which reduce
moisture content and supply carbon. In
spite of the need for additional amend-
ments, using shredded rubber tires may
represent a cost advantage over the
sole use of other materials. Although
other materials may supply carbon and
moisture absorbency as well as bulk,
they are biodegradable and must be re-
placed more frequently than non-
biodegradable shredded rubber. The
cost-effectiveness of using shredded
rubber as opposed to other amend-
ments will depend on the local cost and
availability of each material.
The full report was submitted in fulfill-
ment of Cooperative Agreement No.
CR-810255-01 by Rutgers University
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
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Andrew J. Higgins, Jeffrey L Suhr, M. Siddiqur Rahman, Mark E. Singley,
and V/jay S. Rajput were with Rutgers University, New Brunswick, NJ 08903.
Donald S. Brown is the EPA Project Officer (see be low).
The complete report, entitled "Shredded Rubber Tires as a Bulking Agent for
Composting Sewage Sludge," (Order No. PB 87-175 535/AS; Cost: $13.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-87/026
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