United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/059 July 1985
&ERA Project Summary
The Rutgers Strategy for
Composting: Process
Design and Control
Melvin S. Finstein, Frederick C. Miller, Steven T. MacGregor, and
Kevin M. Psarianos
A strategy for sludge composting was
developed to counter the tendency of
other composting systems to operate at
high temperatures that inhibit and slow
decomposition. This method, known as
the Rutgers strategy, can be imple-
mented in a static pile configuration to
retain structural and operational sim-
plicity, or in a more elaborate enclosed
or reactor structure system. The method
maintains a temperature ceiling that
provides a high decomposition rate
through on-demand removal of heat by
ventilation (thermostatic control of a
blower).
Compared with the approach current-
ly in widespread use, the R utgers strate-
gy yields high-rate composting that
decomposes four times more waste in
half the time.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The main determinant of composting
process performance is decomposition
rate. This rate is negatively affected by
temperatures exceeding 60°C owing to
the inactivation of the responsible micro-
bial community. Nonetheless, composting
masses typically self-heat to 80°C, at
which point the rate of decomposition is
low. A method known as the Rutgers strat-
egy has been developed to counter this
tendency by removing heat through on-
demand ventilation (thermostatic control
of a blower). Compared with the Beltsville
method (the approach that is in current
widespread use), the Rutgers strategy
yields high-rate composting that decom-
poses approximately four times the waste
in half the time.
Though the method can be used in
enclosed (in-vessel) configurations, the
unenclosed static pile is very suitable for
its implementation. The static pile has the
advantage of being structurally and opera-
tionally simple and capital nonintensive.
Sludge Management Goals
Composting advances sewage sludge
management goals by decomposing pu-
trescible (odor-causing) material, decreas-
ing sludge volume, weight, and water
content, inactivating pathogenic orga-
nisms, and producing a stabilized process
residue. The residue is more easily stored
and transported than the sludge, is more
amenable to ultimate disposal, and might
be put to use. Traditionally, the residue is
used as a compost for application to soil.
Related uses include application to dis-
turbed land for reclamation purposes and
use as a partial landfill cover material. A
novel possibility resulting from the ability
of composting to remove water is the use
of the process residue as a waste-derived,
low-grade, solid fuel.
Importance of Decomposition
Rate
Composting advances sludge manage-
ment goals to varying degrees, depending
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primarily on whether it achieves a high
rate of organic matter decomposition. A
high rate is desirable because it leads to
odor control and cost effectiveness. Odor
control is promoted through the acceler-
ated decomposition of putrescible mate-
rial. Similarly, a high rate of decomposi-
tion promotes cost-effective construction
and operation by decreasing the facility
time and space needed to achieve a given
degree of stabilization, and by decreasing
the amount of material to be handled
during and after processing. However,
the composting system tends to produce
inhibitively high temperatures, which
lead to a low rate of decomposition and a
potential for malodors. The high tempera-
tures result from excessive accumulation
of biologically generated heat. Thus the
central issue in composting process de-
sign is temperature control.
Significance of the
Rutgers-Beltsville Comparison
The Beltsville static pile composting
process, which was developed specifically
for sewage sludge treatment and is widely
used, hasthe advantage of structural and
operational simplicity. However, this pro-
cess (like others) tends to produce high
temperatures that inhibit and slow de-
composition. A means of countering this
tendency is known as the Rutgers com-
posting process control strategy. The
Rutgers strategy can be implemented in
static pile configuration, thereby retaining
structural and operational simplicity while
benefiting from rapid decomposition. The
Rutgers and the Beltsville Processes are
compared in Table 1.
The different results produced by the
two strategies originate in the manage-
ment of ventilation. The Rutgers strategy
focuses on heat removal for temperature
control, whereas the Beltsville Process
maintains an oxygenated condition. These
respective operational objectives are met
through the different approaches taken to
blower control, sizing, and operation
mode. However, the physical, chemical,
and biological dynamics governing the
composting system dictate certain con-
sequences that may not be immediately
apparent: These are that a biologically
favorable temperature, so obtained, is
automatically accompanied by an oxygen-
ated condition, whereas focusing on
oxygen almost inevitably leads to inhib-
itively high temperature.
These principles pertain to composting
in general, regardless of the nature of the
waste or process configuration (i.e., static
pile or enclosed composting).
Materials and Methods
Process control stratgegy was the
experimental variable. Both strategies
were implemented in a common configu-
ration, the static pile, to permit a rigorous
analysis of how the strategy affected
system behavior and process perform-
ance.
Table 1. Fundamental Differences in the Rutgers and Beltsville Composting Strategies*
Item
Process Control Strategy
Rutgers Beltsville
Process control
operational objective
Blower control
Blower sizing
Blower operation mode
Consequences of strategy
Maintain 60°C
temperature ceiling
Fixed schedule
initially, followed
by temperature
feedback
Must meet peak
demand for heat
removal
Forced-pressure
System oxygenated;
a high rate of heat
generation^ and
vaporization; dryness
may come to inhibit
activity unless
prevented; good
pathogen kill.
Maintain O2 at 5%
to 15%
Fixed schedule
throughout
Prescribed as
1 /3 hp per
50-ton pile
Vacuum-induced
System oxygenated;
temperature peaks by
default at an
inhibitively high
level 1-80°C); a
low rate of heat
generation t and
vaporization; good
pathogen kill.
*Both strategies were implemented in an unenclosed static pile configuration.
^Heat generation is equivalent to decomposition.
The waste was a primary (raw) de-
watered (belt-filter press) sludge cake de-
rived from a municipal sewage. The
sludge cake had a nominal moisture
content of 76%, and a volatile solids
content of 74%. Sludge and bulking agent
(wooden ips or recycled compost) were
mixed in a pug mill and formed into piles
weighing 4 to 36 metric tons.
Results
Effect of Control Strategy on
System Behavior
Blower Operation
A direct side-by-side comparison of the
Rutgers and Beltsville strategies (Table 1)
was conducted with piles 9A and 9B (Fig.
1). In the Rutgers pile, a 10-hr period of
baseline blower operation scheduled by
timer (7% on time) was followed by a 460-
hr period of time variable, on-demand
operation mediated by temperature feed-
back control (Fig. 2). Demand for ventila-
tion peaked at 100% from hr 96 to hr 135.
After the onset of demand, the timer was
disconnected, hence baseline operation
did not resume when demand subsided.
The blower serving the Beltsville pile was
operated only as scheduled by timer, as
prescribed. The schedule was decreased
at hr 70 because the O2 level was higher
than prescribed.
O2and CO2
Similar interstitial Qz and CO2 levels
were observed in both piles (Fig. 2), but
they resulted from dissimilar circum-
stances. In the Rutgers strategy, vigorous
Oz uptake was matched by commensurate
ventilative resupply, resulting in a high Qz
level. In the Beltsville process, uptake and
resupply were both sluggish and also
resulted in a high O2 level. The former
reflects a high rate of decomposition,
whereas the latter reflects a low rate.
Thus maintenance of a high O2 level is a
necessary but not sufficient condition for
rapid decomposition.
Temperature
In the Rutgers pile, the temperatures
during the period of feedback control
(taken as hr 10 to hr 380) ranged from 24°
to 68°C, with a median of 53°C; they
were greater than 60°C in 13% of 1755
observations. For the Beltsville pile, the
period of summary is taken as hr 100 to
termination to exclude the initial temper-
ature ascent. Temperatures in this pile
ranged from 45°C to 82°C with a median
of 70°C; they were greater than 60°C in
91 % of 1519 observations.
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In both piles, a positive temperature
gradient was established in the direction
of airflow(Fig. 3). The gradient (upward in
the Rutgers pile, downward in the Belts-
ville pile) was steeper and more regular in
the Rutgers pile. This gradient represents
the transfer of heat from the solid phase
(composting matrix) to the gaseous phase
(flowing airstream). As such, a steep,
well-defined, gradient is indicative of
vigorous biological decomposition.
Moisture Content
The moisture content of the Rutgers
pile decreased from 67% to 29%, where-
as that of the Beltsville pile decreased
Controller
Timer
Blower
Pile
T
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Probe 18
Probe 15
Probe 11
Probe 6
Probe 1
Probe 14
Probe 11
Probe 7
Probe 2
200 300
(Hours)
100
Time
200 300
{Hours}
400 500
Figure 3.
Temperature of selected vertical series of probes. Left, Rutgers piles; right. Beltsville
pile.
hout = enthalpy of the outlet air
(energy/mass dry air)
hin = enthalpy of the inlet air
(energy/mass dry air)
The numerical constant (0.9) is based on
the estimate that 90% of the ventilative
heat removal is through vaporization (the
remainder is through sensible heating).
The validity of the expression is con-
strained by ambient air conditions, (hln),
and by the interaction between heat
generation and temperature. In this
summary of the project, only the latter
(overriding) factor is considered.
The goal in waste treatment is to
maximize Qv because it represents the
rate of decomposition. Mathematically, a
large Qv is obtainable by making m and/or
hout large. This procedure is unrealistic,
however, because the highest value of
hout attainable through biological action is
associated with inhibitively high temper-
ature (~ 80°C), which imposes a low rate
of heat generation. Consequently, such a
value of hout is sustainable only in com-
bination with a low m, yielding a low
value of Qv. This set of circumstances
corresponds to the Beltsville process.
The heat generation/temperature inter-
action dictates that a high value of Qv is
sustainable by manipulating m so that
hout corresponds to <60°C. Since the rate
of heat generation is time-variable, no
specific value of m (specific ventilation
rate) can be prescribed. Rather, ventila-
tion must be constantly adjusted to match
the instantaneous rate of heat generation.
This formalizes the rationale for on-
demand ventilative heat removal by
means of temperature feedback control of
a blower (i.e., the Rutgers strategy).
Bulking Agent: Replacement of
Woodchips with Recycled
Compost
Composting relies on gas exchange to
remove heat and water vapor and to
supply Oz. Although such exchange can
theoretically be provided through me-
chanical agitation, such a procedure is
operationally impractical and energy in-
tensive. Thus agitation is not a practical
means of temperature control, and its
main role is to mix and abrade the
materials.
The only practical means of removing
heat in reference to temperature (thereby
providing a high rate of decomposition) is
through ventilation, which requires por-
osity to permit the passage of air. Since
sludge cake by itself lacks porosity, it is
mixed with a bulking agent.
80
,, 70
.« 60
3
50
40
30
20
Rutgers Pile —
Beltsville Pile —
Figure 4.
100 200 300 400 500
Time (Hours)
Moisture content in central in-
terior region of the piles.
The usual bulking agent, woodchips,
has serious drawbacks. In routine Belts-
ville-type operations, the purchase of
woodchips and associated operations
(storage, translocation, mixing, screening)
represent about one-third of the overall
costs. Furthermore, woodchip stockpiles
are subject to colonization by Aspergillus
fumigatus, a fungus that produces spores
that can cause an allergic reaction and/or
infect the human lung. A desirable ap-
proach would be to replace the woodchips
with internally generated, recycled com-
post, provided that the advantages of the
static pile configuration (no mechanical
agitation) could be retained.
To do so, the recycled material must
consist of physically stable aggregates
and metabolically stable, dry material.
Furthermore, the composting process
itself should promote dryness to improve
porosity as the composting progresses.
These requirements are satisfied by the
Rutgers strategy.
Static pile composting of sewage sludge
using a compost bulking agent was
demonstrated in three piles (11 A, 11B,
and 11C) with recycle ratios (dry weight
recycle/dry weight recycle + dry weight
sludge) of 0.3,0.6, and 0.8. The behavior
of these piles was similar to that of the
sludge-woodchip mixtures controlled ac-
cording to the Rutgers strategy with
respect to blower operation, C*2 and CO2
levels, temperature, and moisture content
decrease. Without the rigidity imparted
by woodchips, the piles using recycled
compost decreased markedly in volume
(Fig. 6).
Effect of Control Strategy on
Materials Balance
Based on thermodynamic considera-
tions, a ratio was derived between the
decrements of organic matter and water
on a mass basis. This ratio permitted
calculation of materials balance (Table 2).
With respect to piles 9A and 9B (direct
comparison), the strategy designed to
maximize the rate of biological activity
resulted in the decomposition of 4.7 times
more volatile solids in three fourths of the
time. The overall result involving two
other comparable piles was 4 times more
decomposition in half the time. This result
would presumably translate into savings
in facility construction and routine oper-
ation, a more highly stabilized process
residue, and greater reliability.
Cost of Water Removal
Drying per se is an important sludge
treatment goal, and one of the benefits of
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0 WO 200 300 400 500 600 700 800 900 WOO 1100
Time (Hours)
Figure 5. Effect of water addition (arrows) on the behavior of a Rutgers pile. Upper graph,
blower operation; lower graph, temperature at the control thermistor site.
composting with the Rutgers strategy is
extensive water removal. In contrast,
nonbiological air drying is a process used
solely for water removal. Field observation
of air drying provided an estimate to
compare the cost of composting with that
of providing air for evaporative water
removal. Assuming electricity @ 0.06/
kWh, this estimate was as follows: Com-
posting (mean of six piles), $0.32/metric
ton of water removed ($0.31/ton); air
drying, $6.43/metric ton of water re-
moved ($6.33/ton). The 20-fold more
efficient use of air by composting results
from the biological generation of heat at
the expense of putrescible material in the
sludge.
Conclusions
• The composting system tends to ac-
cumulate metabolically generated heat
excessively, which leads to inhibitively
high temperatures. The threshold to
significant inhibition is approximately
60°C, and its severity increases sharp-
ly at higher temperatures. At 80°C
(common uncontrolled peak temper-
ature), the rate of decomposition is
extremely low.
• This tendency can be controlled
through ventilative heat removal in
reference to temperature. The main
mechanism of heat removal is evapo-
rative cooling, which establishes a
Table 2. Effects of Process Control Strategy on Materials Balance
Control
strategy
(pile no.)
Rutgers (9A)
Beltsville (9B)
Rutgers (1 IB)
Bulking
agent
Woodchips
Woodchips
Recycled
compost
Compost-
ing time
(days)
15.8"
21
7.8*
Total air
delivered
(ft3/initial
wet metric
tonx 1O3)*
307
74.5
323
Sludge
volatile
solids decom-
posed (%)
72.3\
15.4\
23.8
Water
removed
(%)
80.9
19.4
76.1
"In metric units (m3/initial wet metric ton) the values are: pile 9A, 9.580; pile 9B, 2,330; pile 11B,
10,100.
^Time-zero to cessation of blower demand.
'Calculation based on no woodchip decomposition.
drying tendency. Implementation is by
means of temperature feedback control
of one or more blowers using standard
(non-proprietary) equipment. The
forced-pressure mode of ventilation is
more efficient in removing heat and
vapor than the vacuum-induced mode.
In this manner, an operational ceiling
of 60°C is maintained.
• Blower capacity (head and volume)
must meet the peak demand for venti-
lation expressed through feedback
control. A strong waste (e.g., raw
sewage sludge) demands more venti-
lation than a weak one (e.g., digested
sludge).
• A temperature gradient is established
along the axis of airflow, whereas
drying is relatively uniform along this
axis. The temperature gradient impos-
es a height limitation above which a
high rate of decomposition is not
realizable.
• Managed in this way, decomposition
and drying are related in that the
decomposition generates heat, the
heat vaporizes water, and the vapori-
zation causes drying. Hence the strong-
er the observed drying tendency, the
faster the sludge decomposition.
• A consequence of temperature feed-
back control is that the composting
mass is well-oxygenated because more
air is needed to remove heat than to
supply Oz.
• Compared with a conventional ap-
proach, the Rutgers strategy resulted
in 4 times more sludge decomposition
in half the time.
• The cost of ventilation for water remov-
al through composting and nonbiolog-
ical air drying was as follows: Com-
posting, $0.32/metric ton of water
removed ($0.31/ton); air drying,
$6.43/metric ton of water removed
($6.33/ ton). This 20-fold more effi-
cient use of air in composting reflects
the biological generation of heat at the
expense of putrescible organic mate-
rial in the sludge.
Recommendations
• Achieving a maximum decomposition
rate should be the explicit goal of
composting process design and con-
trol.
• Achieving a maximum decomposition
rate should be approached through
temperature feedback control of one or
more blowers.
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• The rate of decomposition should be
assessed in terms of (1) the demand
for ventilation and (2) the course of
drying.
• This strategy (which employs on-
demand ventilation through temper-
ature feedback control) should be
implemented at the lowest possible
capital costs consistent with opera-
tional considerations.
• The unenclosed static pile configura-
tion is structurally simple and very
suitable for implementation; it should
be the preferred configuration.
The full report was submitted in fulfill-
ment of R806829010 by Rutgers Univer-
sity under the sponsorship of the U.S.
Environmental Protection Agency.
Figure 6. Before and after overview of piles using recycled compost as bulking agent. (Photos
by Dr. F. C. Miller.)
U. S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20611
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Melvin F. Finstein, Frederick C. Miller, Steven T. MacGregor, and Kevin M.
Psarianos are with Rutgers University, New Brunswick, NJ 08903.
Atal E. Eralp is the EPA Project Officer (see below}.
The complete report, entitled "The Rutgers Strategy for Composting: Process
Design and Control," (Order No. PB 85-207 538/AS; Cost: $23.50, 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:
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
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