Compost Science & Utilization, (1998). Vol. 6, No. 2.44-66
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
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M. Day1, M. Krzymien', K. Shawl, L. Zarembal,
W.R. Wilson2, C . Botden2and B. Thomas2
1. Institute for Chemical Process and Environmental Technology,
National Research Council of Canada, Ottawa, Ontario, Canada
2. CORCAN, Pittsburgh Institution, Kingston, Ontario, Canada
In this study, the chemical and physical changes in the composting material, along
with the emissions of volatile compounds, have been monitored during a 49 day composting period in a commercial composting operation. In addition, samples of composting material, taken from the commercial operation, have been monitored in automated laboratory scale composters. The measurements conducted on the solid
samples included: pH, volatile matter, bulk density, air voids, carbon, hydrogen and
nitrogen. In addition, the gaseous volatiles were monitored for odor, as well as gas
composition as determined by gas chromatography /mass spectrometry. The results
clearly indicated that while the behavior of the composting material was different in
the laboratory scale unit, in comparison to what was observed in a commercial composting operation, the laboratory method gave valuable information on the compostability of the material, unobtainable in the larger unit. Based upon an evaluation
of the physical and chemical parameters measured, a great deal of information was
obtained regarding the progression of the composting process and the identification
of possible problem areas where biological activity may have been compromised.
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Introduction
A wide variety of composting methods are employed in Canada from the basic
windrows to sophisticated in-vessel systems. It is well recognized that many factors influence the composting process and the resulting product (Haug 1993; Naylor 1996; Finstein et al. 1986).These include temperature, degree of aeration and moisture (Golueke
1972;Suler and Finstein 1977; Miller 1989;Nakasaki et a/. 1992; Nakasaki et al. 1994; Tseng
et a1.1995), which may be controlled in the composting process. Other factors such as carbon: nitrogen (C:N) ratio, particle size, moisture content, bulk density and air voids can
be controlled during the preparation of the feed material (Haug 1993, Naylor 1996, Jeris
and Regan 1973).Consequently, the onus is on the operator of a composting facility to
prepare the feedstock in such a manner that ensures a quality compost while using only
aeration, mixing and temperature (and maybe moisture) as process control parameters.
The objective of this current study was t o evaluate the composting process at a
commercial composting facility,while at the same time determining the ability nf a !aboratory composting system to duplicate and/or monitor the behavior of the commercia1facility. The study focuses on the physical and chemical changes occurring during
the comgosting of an organic waste fraction, as well as monitoring the odors and chemical composition of the volatile gases produced.
Pan
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Materials And Methods
R~CY
The Commercial Composting Fucility
Comi
The commercial composting operation evaluated is operated by CORCAN and
is located in Kingston, Ontario. The composting technology used by CORCAN in44
compost Science 8 Utilization
Spring 1998
Calm
ate?
--
Cow
A n Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
corporates the IPS system developed by Wheelabrator Clean Waste Systems Canada
Inc. In this process the mixed feed material is loaded into one of six concrete composting bays. Each bay is 80 meters long with a cross section of approximately two
meters square. A special agitator machine then mixes and moves the composting material through the bays. The desired composting temperature is maintained by an automatic temperature feedback system which controls the air flow to the bays and consequently the degree of aeration. The unit processes on average about 25 tonnes of
waste per day. Agitation of the material in the bays occurs, on average, two to three
times per week. It usually takes approximately 28 days to compost the material in
the bays, prior to outside curing.
Feedstock Material
The material processed by the CORCAN commercial composting facility consists primarily of food residues, yard trimmings, agricultural wastes and wood
wastes from various institutions and organizations. The feed material used in this
study was made up of 12.5percent yard waste, 10 percent construction waste, 25 percent food residues, 25 percent barnyard bedding material, and 25 percent oversized
recyclate. In addition the material also contained about one percent soiled cardboard
and paper products and about 1.5percent porridge whey. Because the feed material contained over 25 percent food residues from the Institutional, Commercial and
Industrial (ICI) sector, it had a high moisture content. This high moisture content
combined with the putrescible nature of the materials in the feed made it particularly
prone to odor formation. The results of the analysis of the individual feed materials
and the resulting feed mixture are presented in Table I. The measured and calculated physical and chemical properties of the feed mixture are also presented for comparison. The starting moisture content in the feed (64 percent) was intentionally high,
to compensate for lack of moisture additions during the composting process. Meanwhile, the starting C:N ratio of 24.6 and bulk density of 0.59 are typical for most com-
TABLE 1.
Characteristicsof the compost feed material
Material
(%)
(x,)
Ratio
(g/mL)
(w,)
(?A>)
(“AB)
5.2
2.1
19.9
0.48
52.0
1C
27.7
C.33
6550
Manure
25.0
71.3
72
16.9
162
42.5
Yiiid Waste
12.5
40.9
7.4
17.4
5: 0
27.4
Cardboard
Construction
Waste
Restaurant
Waste
1.0
32.5
07
1.6
8.4
48.2
6.1
02
253.7
0.13
330
10.0
61.4
3.9
9.1
17.9
19.2
10.5
0.7
27.4
0.31
70.0
0.0
27.:
25.0
794
5.1
121
120
23.5
10.3
2.7
8.7
0.99
Porridge
1.5
88.3
0.2
0.4
3.6
58.6
8.6
2.0
29.7
1.00
0.0
Recyclate
25.0
27.8
18.1
425
31.9
36.6
3.7
2.8
13.2
0.24
68.0
Compost Feed
Calculated
100.0
57.5
42.5
100.0
23.3
33.1
9.5
2.1
15.7
0.52
45.5
Determined
100.0
64.0
20.4
44.7
5.4
1.8
24.6
0.59
43.8
I
M . Day, M . Krzyniien, K. Shaw arid L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
Val.
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ed
the!
Laboratory Composting
The vessels used in the laboratory scale studies were constructed from Pyrex glass
piping, 10 cm in diameter. With a height of 60 cm, each vessel has a capacity of about
6 liters. An outline of one of these vessels along with the associated controls systems
is shown schematically in Figure 1.A total of four such composting vessels can be monitored and controlled from one computer operating with LabVIEW (National Instruments) software.
Air supply for each laboratory composter was controlled at a flow rate of 150
mL/min by a MKS mass flow controller. This air then either directly enters the base of
the laboratory composters or passes through a water aspirator to saturate the air with
water vapour and prevent the drying out of the compost. By switching between the two
air input circuits it is possible to control the moisture content in the compost. In all the
tests, a 20 minute aspiration period was followed by a 40 minute direct dry aeration period. The air supply entering the base of the compost vessel was then distributed by
means of a mushroom shaped air diffuser. A wire mesh screen placed directly above
the air diffuser acts as a support for the test material and ensures linear air flow through
the material and prevents blockage of the inlet air ports. Air, after passing through the
composting vessel exited at the top through one of several ports. One port lead to a gas .
bleeding device where the gases were diluted and presented to a panel of experts for
odor evaluation. This port was also used to collect gas samples for subsequent analysis
by gas chromatography/mass spectrometry (GC/MS). Another port lead to a solenoid
valve. This valve allowed the exhaust gas to be either vented or routed to a gas analyzer. Gases routed for gas analysis were first dried and then analyzed for carbon dioxide
(CO,), oxygen (0,)and methane (CH,) by a portable, Triple Landfill Gas Analyzer
(ADC LFG20). The control of the solenoid valve and data collection of CO,, 0, and CH,
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CONTROLLER
ASPIRATOR
Figure 1. Schematic of the laboratory composting system.
46
Compost Science a Utilization
Spring 1998
Cow
b
A n Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
values were all controlled from the computer operating under LabVIEW software.The
temperature of the composting material was monitored by three thermocouples located at heights of 8 cm, 24 cm and 44 cm inside the composting vessels, and recorded by
the computer.
All experiments with the laboratory composters were performed in a room maintained at 35 & 1°C. In addition each vessel was insulated with 5 cm thick polyurethane
foam to minimize heat loss.
Sampling and Examination of Materials
Sampling of the Compost Material from the CORCAN Facility
The object of the study was to follow a particular batch of compost feed material
as it progressed through the commercial composting process.
The first sample taken was the freshly mixed feed material as it was loaded into
the front of the composting bay. A typical sample essentially consisted of 12 kg (20 L)
of material which was taken to be representative of the batch being processed. It should
be noted that at the commencement of the study individual samples of the various feed
ingredients were also collected for analysis.
Each subsequent week, a sample of the composting material was taken and analyzed. The sampling location of the specific compost material required for the study
was determined based upon the number of agitations since the last sampling. This
gave an indication of the advancement of the specific test material as it moved down
the compost bay. The location of the desired test material was also ensured by the
incorporation of markers within the initial feed material. The detection of these
markers on subsequent samplings verified the progression of the material down the
composting bay.
Prior to the removal the compost test sample gas samples were taken. These gas
samples were collected using a flux chamber similar to that described by Reinhardt et
al. (1992).This unit is comprised of an open bottomed chamber, 60 cm in diameter and
30 cm deep. This chamber was placed upon the compost surface at the desired location. Air from an air tank was swept into the chamber at a flow rate of 10 L/min. The
chamber air exited through a five port manifold and converged at the sampling port
to obtain a discrete whole gas sample for subsequent analysis. Samples were taken using both TEDLAR bags for use in odor assessment, and trapped on to Carbotrap tubes
for subsequent analysis of gas composition by GC/MS.
Once the gas samples had been taken, a representative 12 kg sample of the composting material was taken from a 1 m depth in the compost. The sample was sealed
in a plastic bag which was then placed in a cardboard box along with ~ W Oice packs for
immediate transportation to the NRC laboratories located in Ottawa.
Material Analysis
Once received in the Ottawa laboratories, the test sample was loaded into two laboratory composting vessels. This loading of the vessels was carried out in a
standard manner in order to ensure uniform compaction of the material within each
vessel, while ensuring the correct placement of the thermocouples. Once loaded,
the composting vessels were closed and the systems placed under computer control.
The laboratory compostingbehavior of the test material was then followed for the next
12 days. During this period, the computer monitored the three thermocouples located
in the test mixture as well as the concentration of CO, 0, and CH, in the exit gases.
Compost Science & Utilization
sprh-tg1998
47
M . Day, M. Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
In addition to monitoring the composting behavior of the test samples, a series of
material characterization tests were also conducted. These characterization tests were
performed on the material as received from the CORCAN facility, prior to loading into
the laboratory composting vessels, as well as on material after the 12 day laboratory
test period.
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Bulk Density
011
The measurement of bulk density of composting material was found to be a very
subjective determination. In order to improve the reproducibility of the determination
a standardized test was developed. This involved the determination of the weight of
the material contained in a 450 ml wide mouthed (7.5 cm in diameter) jar filled using
a standard compaction procedure. The compaction involved dropping a plastic bottle
(7.0 cm diameter base) containing water and weighing 300 g from a height of 7.5 cm,
three times. Following compaction, more composting material was added to the rim
of the jar and the process repeated.
Percentage Air Voids
Following the determination of the bulk density, distilled water was added to the
jar containing the compacted material. When almost full with water the lid was placed
on the jar and it was inverted slowly with some tapping to displace the trapped air
bubbles with water. The jar was then completely filled with water and the total weight
of added water was measured and used to calculate a value referred to as the air voids
within the sample. Unfortunately this method fails to compensate for the water absorbed by the composting material.
Material Homogenization
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Many analytical procedures use small sample sizes. In order to obtain more representative samples of the test material for subsequent analysis the inherently heterogenous material was homogenized in a commercial blender. This homogenization
process produced a material with a particle size of about 3 mm which was used in subsequent tests.
thl
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PH
let(
A 10 g sample of homogenized material was added to 500 mL of distilled water
and stirred rapidly with a magnetic stirrer for 5 minutes. Once the sediment had settled, the pH of the !ic;uid was measured.
(v) Volatile Matter (moisture)
Three 15 g samples of the homogenized material were weighed accurately into
porcelain crucibles. These crucibles and contents were dried in an oven at 105°C for 24
hours. The loss in weight recorded was taken as a measure of the volatile matter including moisture.
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Inorganics
Analysis of the inorganic content and elemental composition of C, H and N was
performed on the dried samples from (v) using material ground into a powder with a
Wic
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PIT
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An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
mortar and pestle. For the determination of inorganic content (ash) the dried ground
material was weighed into porcelain crucibles with the lids. The crucibles were placed
in a furnace at 600°C for two hours. After a two hour period the crucibles were removed and the lids opened. The lids and crucibles were then returned to the furnace
for a further two hours at 600°C. The crucibles and lids were then cooled in a desiccator prior to reweighing. The weight of ash remaining was taken as a measure of the inorganic content.
C, Hand N Analysis
These analyses were performed on 0.1 g samples of the dried ground material, using a LECO CHN-1000 (carbon, hydrogen and nitrogen) Analyzer. This instrument is
a non-dispersive, infrared, microcomputer based instrument which provides a direct
reading of the composition of elemental C, H and N in the test specimen.
Odor Testing
Odor levels were measured on both gas samples taken from the CORCAN facili-
ty using TEDLAR bags as well as on gas samples taken directly from the exit from the
laboratory composting vessels. The approach taken in this assessment was to determine the dilution detection threshold. This testing involved an assessment of the odor
detection level by at least six individuals. The methodology involves taking the test gas
and diluting it with a known quantity of clean air. The dilution ratio of gas to air was
then gradually decreased (the concentration of gas increased) until the test individual
could just detect an odor. The odor dilution thresholds were then averaged to overcome the natural variabilities of the panel members. Because of colds, illnesses and holidays it was not possible to maintain the same six panel members throughout the complete test program. Consequently, substitutes had to be found, which in part may have
contributed to the wide variability in the results of some of the tests data. Because of
this variability it was found necessary to use a statistical approach in which data outliers (> 2u) were removed from the analysis.
Chemical Composition
Of the Compost Gases
In addition to collecting gases for odor evaluation, compost gases were also collected for the identification and semi-quantitative analysis of the major organic compounds by GUMS. The analytical approach taken was as follows. The compost gas
was collected with 76 mm x 6 mm ID Pyrex glass tubes filed with a 40 mm column
of Carbotrap 20/40 mesh (Supelco, Mississauga, Ont.). The sampling rate was 50
ml/min and sampling time was 5 minutes. For analysis the absorber tube was inserted into a modified injection port of a HP5790 Gas Chromatograph (GC)
(Krzymien 1987) where the sample was thermally desorbed and transferred by helium carrier gas on to a 30 m x 0.32 mm I.D., d, = 1pm, DB-1701 capillary column U&W
Scientific, Folsom, California). The column temperature was programmed from 20°C to 180°C at 5"C/min after a 3.5 min hold at -20°C. The separated compounds
were detected by a HP 5970A Mass Selective Detector (MSD) operated in a scan mode
at mass range of 20-200. The total ion chromatogram peaks were identified using a
Probability Based Matching (PBM) search and retrieval algorithm and NBS REVF
spectral library.
Compost Science (L Utilization
Spring1998
49
M . Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
Data Analysis
etc
The total composting period for the material evaluated in this study was 49 days.
This time period started on the day the material was mixed and loaded into the front
end of the composting bay (22nd May 1996). Composting material and gas samples
were taken at the CORCAN facility every seven days for the next seven weeks with
the last samples being taken on the 10thJuly 1996. The laboratory composters were operated on a 12 day cycle. Laboratory composting vessels number 3 and 4 were operat-
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Time
(Days)
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Commerciel
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Sample
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Characterization Analysis
34
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1
2
3
4
5
6
7
8
9
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11
12
13
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15
16
17
18
19
20
21
22
23
24
25
26
27
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28
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29
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Figure 2. Schematic outline of the testing schedule adopted in this study. * denotes tests on CORCAN test samples, 0,
and 0.4
denote the commencement and stopping of the laboratory composting systems, while 0 and denote analysis of the laboratory compost gas samples for odor and composition Open symbols denote tests conducted in duplicate
with composter 3 and 4 while closed symbols represent tests conducted with composters 1 and 2.
+
,i
TABLE 2.
CORCAN testing schedule
Total
Sampling
Date
--May22,1996
May29,1996
June 5,1996
June 12,1996
Junel9,1996
June26,1996
July 3,1996
JulylO, 1996
50
Time In
Compost Bay
--- (Days)
Number Of
Turns Since
Last Sample
_
I
_
-
0
7
14
21
28
35
42
49
3
Compost Science 8 utilization
Dstance
Advanced
Down Bay
---
(W
--_
5
60
2
84
3
1
2
3
Aerahon Time
In The 2 Hour
Sampling Period
10-12 am
Win)
_
_ ___
.
-
Pile
Temperature
During
Sampling
__Period (“C)
_.
120
132
156
6
6
4
4
4
50
48
45
45
63
192
228
2
1.5
41
’
50
spring 1998
coo1
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
ed with test samples removed from the CORCAN facility after 0,14,28 and 42 days of
composting, while vessels number 1 and 2 were used for test samples removed after
7,21,35 and 49 days. Gas samples from the laboratory composting vessels were sampled on days 1,2,5,8,9 and 12. An overall schematic of the sampling and testing procedure is outlined in Figure 2. Meanwhile Table 2 provides information on the agitation record of the material as it progressed down the compostingbay at the CORCAN
facility. Also provided in this table is the aeration time period (i.e. blowers on) during
the two hour sampling period, along with temperatures recorded in the compost pile
at the sampling position.
Results and Discussion
The Physical Characteristics of the Composting Material
Volatile Matter (Moisture)
As previously mentioned the CORCAN composting facility does not incorporate
any moisture addition during the composting process. Consequently, the moisture
content in the starting feed material is usually adjusted to be on the high side to compensate for the tendency of the composting material to dry out due to the forced air
aeration and the agitation of the material. Although moisture loss was minimized during the composting process by controlling the blower application time, aeration was
required to ensure proper temperature control and optimize the biological activity.
The variation in the percentage volatile matter in the test material as a function of composting time is presented in Figure 3. This figure clearly indicates that over the first
three to four weeks there is a gradual decline in the volatile matter from a value of
about 65 percent at the start of the composting process to about 58 percent after four
weeks. Over the last three weeks, however the material starts to rapidly dry out, such
that when the material reached the end of the composting period (seven weeks) the
volatile matter was only 30 percent.
Meanwhile, for test samples placed in the laboratory composters, where moisture
addition was controlled by water aspiration, little changes were noted in the percent-
70
&
0
10
20
30
40
50
Time (days)
Figure 3. Volatile matter in CORCAN test samples as a function of composting time.
Compost Science & Utilization
s p r ~ n g t51
~~
M . Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
TABLE 3.
age volatile matter during the
12 day laboratory composting
period. The data, summaSample Time
Initial
Final
Changein
rized in Table 3, indicates
in CORCAN
Volatile
Volatile
Volatiie
that water aspiration of the
facility
Matter
Matter
Matter
(Yo)
(“Id
(sa)
air entering the laboratory
(days)
composting vessels prevents
0
64 1
67 8
37
the test samples from drying
7
63 6
66.5
29
14
63 9
66.4
25
out during the 12 day labora21
58 7
640
53
tory test period, especially
28
61 2
61 9
07
when high composting activi35
52 8
52 0
4 8
ty was noted. In effect it
42
40 1
37.3
-2.8
appears that water aspiration
49
30 1
24.1
-6 0
may increase the water content of the test samples.
However, when low activity test samples were being processed in the laboratory composting systems (i.e. samples obtained after 35-49 days) the aspiration does not appear
to have a major effect, since some moisture losses were noted. These observations regarding percentage volatile matter or moisture content variations can be attributed to
the actual composting process. During the laboratory composting of the initial highly
active test samples (0-21 day samples), in addition to water augmentation from aspiration, water was also being produced by biological activity. Both these factors combine to cause an increase in moisture content and measured volatile matter. Subsequent test samples obtained from the CORCAN facility after day 28, appear to have
lower biological activity (i.e. most of the readily bio available material has been consumed at the CORCAN facility). Consequently the production of water by biological
activity was substantially reduced and the addition of aspirated water to the air stream
was not sufficient to balance that loss due to the aeration process.
Changes in volatile matter in test samples during
the 12 day period in the laboratory composter
Fii
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fai
(dl
0
7
14
211
Bulk Densify and Air Voids
Air porosity through the composting material is a key factor in maintaining an aerobic composting environment. Porosity also influences heat exchange for the exothermic process of the biological degradation of the organic material. Since both the measured bulk density and the percentage air voids are linked to air porosity they are
important parameters used by composting facility operators in the blending of feedstocks to achieve optimum efficiency of the biological process.
The starting test samples used in this study had an initial bulk density of 0.59
g/mL with 43.8 percent air voids. However, as the composting process proceeded the
bulk density of the test samples decreaszd, while the percentage air voids increased.
These changes during the CORCAN composting process are clearly demonstrated in
Figure 4.The changes noted in both these parameters are significant during the full
seven week composting period. For example the bulk density fell from 0.59 g/mL at
the start to 0.35 g/mL by the end of the composting period. Meanwhile, the percentage air voids in the material increased from 43.8 percent at the start to 62.5 percent at
the completion of the composting period. While both these parameters give an indication of the progress of the composting process, a large percentage of the noted
changes can be attributed to the drying out of the material as was noted by the loss in
volatile matter.
In the case of test samples composted in the laboratory composting vessels,
52
Compost Science & utilizatlm
289
35
422
459
-
Sa
ink
fait
(di
.-
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0
7
141
211
283
35:
42
493
C(i
Spring 1998
i
M . Day, M . Kuzymien, K. Shaw nnd L. Znremba, W.R. Wilson, C. Botden and B. Thomas
as t::
corn
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sam
the
con
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is si1
mercially composted and laboratory composted samples, it appears that a significant
contribution to these changes in bulk density and percentage air voids can be attributed to changes in the moisture content due to the dehydration of the material at the
CORCAN composting facility.
lnorgnnics
In addition to the organic compostable material, the feed material also contains
a certain amount of inorganic matter which remains as an ash when the material is
combusted. This inorganic material, sometimes referred to as ash content or fixed
solids, is principally composed of a variety of inorganic minerals such as calcium,
1
36
34
-
32
c
30
5
28
5
C
c
pos
that
creii
born
ed
meii
net:
a vi:
of c
to c
0
.E 26
m
24
- 22
C
20
18
4
0
IO
20
30
40
50
que
Time (days)
Sam
Figure 5. Inorganic composition ot CORCAN test samples as a function of composting time.
cilitt
and
magnesium, sodium, iron and manganese along with other trace metals. These
cations are usually associated with carbonates, bicarbonates, sulphates, phosphates,
nitrites etc. Because these materials are generally unaffected by biological action,
they should pass through the composting process unaltered. However, it should be
noted that they may influence the biodegradability of
TABLE 6.
Changes in inorganic content in test samples during
the organic fraction. Because
the 12 day period in the laboratory composter
the biological breakdown of
the orPanic
material involves
Samole Time
"
in C ~ R C A N
Initial
Final
the
consumption
of oxygen
facility
Value
Value
Change
("m)
(W
(%)
and the production of metabolic water and carbon diox2.4
22.9
20.5
ide, a net loss in organic mat2.0
24.9
22.9
ter occurs. Consequently, for
1.3
24.1
25.4
a fixed weight of feed mater2.3
29.7
32.0
4.4
32.2
27.8
ial, it can be anticipated that
6.2
35.3
29.1
there will be a corresponding
-2.4
29.7
32.1
relative increase in the inor2.4
36.8
34.4
ganic content of the material
spring 19aa
f
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
as the composting process progresses. This was indeed observed over the 49 days of
composting in the CORCAN facility. This information which is presented graphically in Figure 5, shows the inorganic content of the composting material increasing
from 20.5 percent at the start of the process and reaching 34.4 percent by the end of
the 49 day composting period. A similar increase in the inorganic content of the test
samples was noted with material in the laboratory composters (Table 6), although
the magnitude of the changes were not nearly as large. For example, the inorganic
content of the initial feed materials increased only 2.4 percent during the 12 day period in the laboratory composter, while the inorganic content of the CORCAN material increased 3.6 percent over the initial 14 day period. The rationalization of these
apparent discrepancies are still under investigation although sample heterogeneity
is suspected as a possible contributor to some of the variabilities noted.
The results of the elemental analysis of the test samples during the 49 day composting period are presented in Figure 6. From this data, there is a clear indication
that the concentration of carbon and hydrogen present in the dry test samples is decreasing as a function of composting time. These results are to be expected as the carbon and hydrogen present in the biologically degradable organic matter is converted into volatile CO, and H,O and removed from the compost. Meanwhile the
measured amount of nitrogen in the test samples remained relatively constant. The
net effect of these changes is that the C:N ratio of the composting material fell from
a value of 24.5 at the start of the composting process to a value of 13.6 after 49 days
of commercial composting. Although’ the concentration of nitrogen did not appear
to change during the composting process, some ammonia was released as a consequence of the composting process, and its characteristic smell was detected during
sampling operations.
In addition to measuring the C , H and N in samples taken from the CORCAN facility, the concentration of these elements was also measured on test samples before
and after 12days of composting in the laboratory composters. This data is summarized
50
v
0
10
20
30
40
50
Time (days)
Figure 6. Variation in the composition of carbon
function of composting time.
Compost Science &Utilization
(v),hydrogen (0)and nitrogen (B)in the CORCAN test samples as a
Spring 1998
55
I
M. Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
n
TABLE 7.
-
Changes in C, H and N content in test samples during the 12 day perlod
in the laboratory composter
Sample
Timein
-- - - c
CORCAN Initial
Final
facility
Value
Value
(days)
YW - (W
0
7
14
21
28
35
42
49
___
447
42 2
45 8
39 5
38 7
348
32 6
36 3
_ _
38 5
37 3
_ _
40 7
34 4
33 8
35 7
342
36 4
~
Change
(“4
-6 2
-4 9
-5 1
-5 1
-4 9
09
16
01
Initial
Value
(“w
H
- - _ _
Final
Value
Change
544
528
589
4 48
468
408
342
437
456
442
512
427
380
431
398
406
N ___Final
Value
Change
Initial
Value
(“4
(74-~
- (Yo)
a
(“w
(W
2 85
2 29
2 87
2 96
2 95
3 36
3 22
2 74
103
0 08
0 52
113
I37
0 90
0 37
0 08
I__________-
-089
-086
-077
-021
-088
0 22
0 56
-032
1 82
2 21
2 35
1 83
1 58
2 46
2 65
2 65
in Table 7. Examination of this data and comparison of the results with those obtained
from the CORCAN operation, a certain amount of similarity was noted. For example
the C:N ratio fell from about 24.6 to about 13.5 in both cases, as a result of the reduction in carbon and the relative increase in nitrogen contents. It was also evident from
these results that commercialtest samples taken over the first 28 days had the greatest
bioavailable carbon. After the first 28 days the available carbon for conversion to CO,
was reduced, and consequently little or no changes were noted in the carbon content
from this point in time onwards.
tt
tt
ti
tt
C
a?
tt
t:
F
t:
si
F
a!
w
H
t
w
rtt
m
UI
p H Measuremenfs
The data on the pH measurements taken on the samples removed from the CORCAN facility over the 49 day test period are presented in Figure 7.This data clearly indicates that the initial feed material used in the compostingprocess was slightly acidic
with an initial pH of 6.2. This acidic nature of the feed may be due to the presence of
the restaurant waste component which had a measured pH of 5.5. During the first two
8.0
,
5.0
-/
0
sn
w+
tu
all
att
I
J
10
20
30
40
50
Time (days)
Fig;
Figure 7. The pH of the CORCAN test samples as a hrnction of composting time.
CO)
Corr
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composfing
weeks of compostinga noticeTABLE 8.
Changes in pH in test samples during the
able increase in the acidity of
12 day period in the laboratory composter
the test samples was noted as
the pH fell to around 5.5. AfSample Time
in CORCAN
ter the second week, however,
facility
Initial
Final
the pH of the compost inValue
Value
Change
creased rapidly to a value of
2.05
0
8.25
6.19
about 7.5, which was main2.95
7
8.57
5.62
2.87
14
tained for the remainder of
5.48
8.35
1.51
21
7.35
8.86
the commercial composting
28
0.25
7.70
7.96
period. Although only the ini0.53
35
7.60
8.13
tial and final pH's were mea42
-0.17
7.80
7.63
sured in the laboratory com49
0.87
7.78
8.65
posting systems the final pH
of all the materials tested
were generally higher than those of the initial pH. In most cases the measured pH at
the end of the 12 day period was about 8.3 which appears consistently higher than values reported from the commercial composting facility (Table 8).
Compostability
Because the laboratory composting systems are fully automated, a continual
record of temperature, CO, and 0,, is available. This data can provide valuable information on the biological activity of the material in each vessel. In this study we have
used the temperature output from the middle thermocouple in each vessel as a measure of the exothermicnature of the compostingprocess. Values evaluated in this study
were the maximum composting temperatures, as well as a time integrated temperature, which gave a measure of the heat output.
In addition to using heat output as a measure of composting activity, we have
also used respiration measurements as indicators of composting action. Because the
aerobic decomposition of organic matter consumes oxygen and liberates CO,, the
70
60
G
0,
-E
2
a
50
a,
P
E,
t-
40
30
0
50
100
150
200
250
300
Time (hours)
Figure 8. Temperature recorded by the middle thermocouple (A)
in the laboratory composter as a function of time for a
CORCAN test sample collected on day 0.Room temperature in the composting laboratory shown (0).
M. Day, M. Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
0
50
100
150
200
250
0
Time (hours)
Figure 9. Variation in O2(A)
and CO, (a)concentrattons in the exhaust gas from the laboratory composters as a function
of time for a CORCAN test sample collected on day 0
degree of 0,depletion and CO, evolution provides another useful measure of composting activity.
A typical temperature-time profile for a test sample, in the laboratory composter,
is shown in Figure 8. This curve was obtained using the original feed material as the
test sample, taken from the CORCAN facility on day zero. It will be noted that the material showed an initial rapid self heating to 50°C within the first 12 hours, followed by
a cool down period to 46°C. Thematerial then once again self heated to a high of 68°C
over the next four days, remaining above 60°C for several days. Eventually the temperature of the composting material gradually declined until the process was stopped
at the end of the 12 day test period.
The CO, and 0, profiles of the exit gas from the same experiment are shown in
Figure 9. It will be noted that there was a sharp decrease in 0, and a corresponding
4000
a0
e
Fig;
CO)
incc
UT
sla
CG
CO!
tha
sa::
rail
0
n
3500
70
.-C 3000
c
C
a
2500
4m
I
:2000
a
60
CI
50
E
En
n
1500
c
m
I"
e
3
.-*C
8.
altr
incl
cill
la11
ha
ac:
to1
PC
5
0
1000
40
500
0
30
0
20
30
40
Ea
.-xE
f
50
Time (days)
'y'
Figure 10.Variation in the integrated heat output (a)and maximum temperatures (m) recorded in the laboratory composters for CORCAN test samples taken over the 49 day test period.
58
Compost Science 8 utllizatii
Spring 1998
PC
COD1
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
1600
.--c
.II
2
1400
1200
,
t
Oxygen Depletion
J; 1000
m
e
a
800
u)
U
600
c
E
P
a
-
1
Carbon Dioxide Evolution
\
400
200
04
0
\
10
20
30
40
50
Time (days)
Figure I I . Variation in the integrated 0, depletion (0)and CO, evolittion (V)
recorded in the laboratory composters for
CORCAN test samples over the 49 day test period.
increase in CO, that coincides with the initial temperature rise as indicated in Figure 8. The initial steep decrease of 0, concentration in the exit gas is followed by a
slow decrease to a minimum value of about 12 percent while the concentration of
CO, rose to a maximum of about seven percent. These trends then reversed as the
composting process proceeded, with the observed values reflecting the changes in
the temperature.
These measurements of temperature, 0, and CO, were recorded for all the test
samples taken from the CORCAN facility. From each of these plots the following parameters were measured:
The maximum temperature recorded by the middle thermocouple
Integrated heat outputs for a typical composting test. This involved integration of the area between the compost temperature/time curve from t =
0 hours to t = 270 hours and the laboratory temperature curve (35 percent).
Integrated oxygen depletion. This once again involved integration of the
area between the measured oxygen concentration and the baseline oxygen
concentration of 21% from t = 0 hours to t = 270 hours.
Integrated carbon dioxide formation. This was calculated by integrating the
area under the measured CO, curve from t = 0 hours to t = 270 hours.
The results of the heat evolution characteristics of the CORCAN test samples
along with maximum compost temperatures are summarized in Figure 10. This data
indicates that all test samples taken during the first 28 days from the CORCAN facility have high composting activities in terms of high maximum temperatures and
large integrated heat output values. Meanwhile test samples taken after five weeks
had significantly reduced activity. For example, the 35 day CORCAN sample only
achieved a maximum temperature of 55"C, while after 42 days the material struggled
to reach a temperature of 50°C. Finally the test sample removed after 49 days of composting showed only a marginal temperature increase when placed in the laboratory composter, suggesting that most of the biologically available carbon in the composting material had been consumed.
Confirmation of these changes in composting activity for test samples from the
Compost Science & Utilization
Spring1998
59
M . Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
CORCAN facility are evident from the CO, evolution and 0, depletion cullres. The integrated areas for CO, evolution and 0, depletion are presented in Figure 11.These results reflect very closely the data obtained with the integrated heat output data shown
in Figure 10. Both 0, depletion and CO, evolution are high for test samples taken during the first 28 days of compostingat the CORCAN facility. After day 28, however, the
integrated evolution of CO, and depletion of 0, show marked declines as the readily
compostable material available in the test samples decreases. Eventually, after 49 days
the test sample from the CORCAN facility hardly had any measurable CO, evolution
or 0, depletion. This indicated that the majority of the compostinghad ceased and the
material was ready for removal to the outside maturing area.
Odor Measurements
The generation of odors at commercial composting facilities is a subject of
major concern for the operators of these
facilities. Numerous publications have
presented information concerning the
sources and compounds responsible for
the offensive smells (Miller and
Macauley 1988; Hentz et al. 1992; Miller
(1993); Van Durme et al. 1993).While the
CORCAN facility uses a biofilter to control the release of these offensive odors it
was felt that an investigation of their productivity throughout the composting facility would be informative.
The measurement and quantification of odor is a difficult and exacting
challenge which involves a sound understanding of other scientific disci-
TABLE 9.
Threshold Dilution Level (x 103)
Panel
Member
-
1
2
3
4
5
6
7
8
MIN
MAX
AVG
STD
AVG + STD
AVG - STD
All
Values
Selected
Values
12 50
16 77
14.30
3.81
14.39
16.77
12.50
10.00
3.8
16.77
12.69
3.81
16.5
8 79
12 50
-
14.39
14 39
12 50
10.00
-
-
10.00
14.39
12 7
16
14.3
11 1
II
tt
0
0
10
20
30
Time (days)
40
50
II
ii
Figure 12. Odor detection threshold values for gas samplestaken from the CORCAN facility. Error bars represent 95%
confidence limits.
60
compost science a utilization
sprine 1998
tt
ii
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
0 1
20
10
0
50
40
30
Time (days)
Figure 13. Odor detection threshold values for gas samples taken from the exhaust of the laboratory composter after 1
day ( 0 )and 2 days
plotted as a function of composting time in the CORCAN facility.
(v)
16
2-1
0
2
4
6
8
10
12
Time (days)
Figure 14. Odor detection threshold values for gas samples taken from the exhaust gas of laboratory composter during
test runs with CORCAN test samples taken on day 0 (0)and day 7
(v).
plines such as physiology, medicai science and psychoiogy (Biiss et ai. i996, Schuiz
and Van Harreveld 1996). In addition sensory odor measurements using human observers can be time consuming and expensive. Consequently the following simplistic approach was taken in this study. A diluted compost gas sample was presented
to members of a test panel. The value sought was the dilution that gave an odor that
was just perceptible to the panel member. The testing commenced at the highest dilution attainable based upon the limitations of available flow rates for the compost
gas and diluting air supply. The compost gas concentration in the mixture was then
increased until the odor was just detectable. This technique known as "olfactometry" is used routinely by the cosmetic industry with trained panel members. The approach taken in our study was very rudimentary and found to be greatly dependent
Compost Science 8 Utlllzati
Springl~M
61
M. Day, M. Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
upon the test individuals. To illustrate this point consider the data obtained in the
evaluation of the odor from the CORCAN facility after seven days of composting
(Table 9). Clearly a wide discrepancy in the readings was noted from a high of 16.7
to a low of 3.8. Because of these large differences in the panel responses (coefficient
of variation = 30 percent) a data reduction step was found necessary to correctly interpret the panels responses. This involved the determination of the standard deviation for all responses. Values that deviated from the average by more than one standard deviation were eliminated. Applying this process to the results presented in
Table 9 caused only a slight shift in the average, but resulted in a more acceptable
coefficient of variation of 12.6 percent.
This approach was used on all the gas samples taken from the CORCAN facility
over the 49 days of the study and the results are presented in Figure 12. This data clearly indicates that the odors were strongest during the first few weeks when most of the
composting activity was occurring. After 28 days the measured odors were substantially reduced. A similar trend was noted in the odors of the gases sampled from the
laboratory composter. The data presented in Figure 13 represents the estimated odor
levels for compost samples taken from the CORCAN facility after one and two days in
the laboratory composter. The odor values determined from these samples are very
similar to those noted for the gas samples taken directly from the CORCAN composting facility. While little odor was generated from the initial feed samples during day
one and two in the laboratory composter, test samples obtained from the CORCAN facility on days seven to 28, gave the highest odor levels. These values then decreased in
intensity as the compostingprocess proceeded. Generally speaking it will be noted that
the odor levels in gas samples measured after two days in the laboratory composter
were higher than those measured after one day. This observation was in agreement
with the known build up in composting activity which usually takes two days in the
laboratory composters.
The variation in odor intensity with time for the laboratory composters is plotted
in Figure 14. The odor associated with the initial feed material was low. It then gradually increased in magnitude reaching a maximum after about nine days. The test sample taken from the CORCAN facility on day seven, meanwhile, commenced with a
high odor level which was maintained for most of the 12 days in the laboratory composting system.
n
ti
11
x
a
m
1('
1..
111
I!!
1::
1::
1::
1:
1U
155
2u
2u
2K
2c
222
22
22
221
22
231
23%
231
231
24:.
25;.
25:.
26.,.
Vofnt ile Orgonics
Malodorous gases are the subject of most of the complaints against commercial
composting operations. However, it should be noted that odors measured at the detection threshold need not be malodorous. In fact many of the odors being detected at
the threshold levels were nct regarded by the panel members as offensive. This observation caused some psychological problems with some panel members who expected a malodorous smell. Consequently, in order to get a better understanding of the
odor problem it was decided to investigate the nature of the chemical species present
in the gas emissions from the composting test samples.
Figure 15 is the GC trace of the chemical compounds separated and detected
from the gas sample taken from the CORCAN facility after 14 days. Clearly many organic compounds were detected and the majority of these chemicals have been identified. The results of this analysis are summarized in Table 10. From this table it appears that aldehydes and ketones appear to be the dominant products, especially the
C, species. For example the two aldehydes, 3-methyl-butanal and pentanal along
62
compost science a Utltization
Spring 1998
26..
26..
27..
29
^^
LYJ
30.::
30..:
30.11
31."
33.::
33.'.
33.55
34.::
36.;
39.55
-
Com
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
with 2-pentanone were the three most abundant compounds. However, it is also interesting to note the presence of several esters of the fatty acids as well as pinene,
limonene and other terpenes among the detected gases. Many of these compounds
are characteristic of food residues and wood products used in the compost feed maTABLE 10.
Major organic compounds identified in gas sample
(14 days in CORCAN facility)
___
R.T.
Name
10.331
13.720
15.266
15.939
17.008
17.153
17.761
17.995
18.527
19.070
20.620
20.785
20.89~
20.985
22.275
22.328
22.414
22.543
22.654
23.067
23.601
23.830
23.950
24.375
25.707
25.978
26.460
26.623
26.734
27.181
29.380
29 629
30.228
30.438
30.668
31.453
33.379
33.480
33.976
34.500
36.783
39.988
1,3-Pentadiene
I-Hexene
2-Methyl-propanal
2-Methyl-furan
2-Methyl-1,3-pentadiene
1-Hexyn-3-ol
Acetic acid, ethyl ester
2-Butanone
Butyl-cyclopropane
2-Butanol
3-Methyl-butanal
Pentanal
2,5-Dimethyl-furan
3-Ethyl-pentane
2-Pentanone
Pentanat
3-Pentanone
Butanoic sad, methyl ester
2,3--l’mLtnedione
Oftane
Dimethyl disulfide
Toluene
Acetic acid, 1-methylpropyl ester
2-Methyl-3-pentanone
Butanoic acid, ethyl ester
3-Hexanone
5-Methyl-2-hexanone
Hexanal
Pentanoic acid, methyl ester
Nonane
alpha-Pinene
Pentanok add, ethyl ester
Camphene
2-Heptanone
Hexanoic acid, methyl ester
beta-Pinene
Hexanoic acid, ethyl ester
Limonene
Methyl(1-methylethyl) benzene
Bezaldehyde
Heptanoic acid, ethyl ester
Octanoic acid, ethyl ester
Total peak area
-Co”
Science & UtUmtion
PBM
Match ‘%
Peak
Area
89
94
83
96
83
70
62
83
67
86
81
60
95
67
66
73
89
94
70
92
94
87
78
79
97
59
60
79
117
3,057
448
234
40
127
1,158
5,294
1,325
4,533
36,845
10,856
253
223
12,074
2,408
450
1,393
191
637
1,163
116
822
150
2,436
78
1,079
1,571
354
119
1,533
573
120
83
84
78
83
93
81
78
92
a5
84
89
67
71
89
~~~
Detected
In Feed
X
X
X
X
X
X
80
465
505
2,388
1,882
106
332
814
129
107,455
X
_~_________.____
spring1998
63
M . Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas
3-Methyl butanol
44
I1
U
10
30
ll
.
11.
40
E
Retention time [min]
\I
tt
tt
t:
t:
?
!.
Figure 15. GC trace of volatile organics in thegas sample taken from the CORCAN fanllty on day 14.
a
iu
ltt
dl
hi
a
P’
r
Time (days)
B11
(m)
Figure 16. Total abundance all the organic compounds (0)and sulphur compounds
detected in the ehaust gas from
the laboratory composters after2 days using CORCAN test samples taken over the 49 day test period.
terial. Head-space anaIysis of the individual feed materials was also conducted to
determine what compounds were associated with the feed materials. The compounds identified by this analysis have been identified in Table 10. This helped to
distinguish the compounds being produced by the composting process and those
volatilizing from the feed material.
64
compost science utilization
Spring 1998
Bit
Fii
G
co,
An Investigation of the Chemical and Physical Changes
Occurring During Commercial Composting
It should also be noted, that the concentrations of the compounds detected during
the composting were very much dependent upon the sampling temperature, because
vapour pressure is temperature dependent. Consequently, both laboratory gases, and
the CORCAN gas samples demonstrated a clear a relationship between degree of composting activity, compost temperature and measured gas concentrations.
In terms of malodorous gases detected, dimethyl disulphide appeared to be the
most offensive compound identified (Burmeister 1992). Figure 16 gives the analytical
results for the sulphur compounds detected in the gas samples taken after two days in
the laboratory composters as a function of days in the CORCAN facility. From this data
it can be seen that the liberation of sulphurous compounds are initially high during the
first 21 days of composting, but rapidly decrease as the compostingprocess progresses. Consequently, adequate odor control is essential during this initial period if odor
complaints are to be avoided.
Conclusion
Numerous physical and chemical changes take place within a commercial
composting process. In this study we have shown that many of these changes can
be monitored and used to give an excellent indication of the composting process. It
has also been shown that while the laboratory scale composters described in this
study may not duplicate ”real” world conditions, they were capable of providing
valuable insight into the composting process. In terms of odor generation, it appears
that the first two weeks of composting activity may be the most critical. During
this period, the highest biological activity occurs, which results in higher temperatures and greater gas evolution rates (higher vapour pressures). Consequently, either temperatures have to be moderated, or remedial action taken to maintain the
gas emissions at an acceptable level. As the composting process progresses, the biological activity begins to slow down due to a depletion of readily available organic matter. At this point in time gaseous emissions are reduced and odors become
less of a problem.
Acknowledgements
The authors would like to thank the following high school students: Mary Henderson, Ariel Grostern, Ryan Poulin, Christine O’Malley and Lindsay Petherick who
helped with many of the experiments described in this study and who have no doubt
a better appreciation of ”composting science” as a result of their co-op educational
placement at the NRC.
References
Bliss, P.J.,T.J. Schulz, T. Senget and R.B. Kaye. 1996. Odor Measurement - Factors affecting
olfactometry panel performance. Waf.Sci. Tech., 34:549-556.
Burmeister, M.S., C.J. Drummond, E.A. Pfisterer and D.W. Hysert. 1992. Measurement of
volatile sulfur compounds in beer using gas chromatography with a sulfur chemiluminescence detector. journal of American Society of Braving Chemists, 5053-58.
Finstein, M.S., F.C. Miller and P.F. Strom. 1986.Waste Treatment Compostingas a Controlled
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Weinheim, FRD, 8363-398.
Gies, G. 1995. Canadian Facilities: Composting Food Processing Residuals. BioCycle,
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Compost Science & UtilizaUoo
Spring1998
65
M . Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson,C. Botden and B. Thomas
Golueke, C.G. 1972. Composting: A Study of the Process and its Principles, Rodale Press Inc., Emmaus, PA.
Haug, R.T. 1993. The Practicnl Handbook of Compost Engineering, Lewis Publishers, Boca Raton, FL.
Hentz, L.H., C.M. Murray, J.L. Thompson, L.L. Gasner and J.B. Dunson. 1992. Odor control
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Jeris, J.S. and R.W. Regan. 197313. Controlling Environmental Parameters for Optimum Composting: 11. Moisture, Free Air Space and Recycle. Compost Science, March-April 8-15.
Jeris, J.S. and R.W. Regan. 1973c. Controlling Environmental Parameters for Optimum Composting: 111. Compost Science, May-June 16-22.
Krzymien, M.E. 1987. Analysis of the Effluent from Polyurethane Foam Heated at 80°C. Am.
Ind. Hyg. Assoc. J., 48(1):67-72.
Miller, F.C. and B.J. Macauley. 1988. Odors arising from mushroom composting: a review.
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