an investigation of the chemical and physical changes occurring

Compost Science & Utilization, (1998). Vol. 6, No. 2.44-66

An Investigation of the Chemical and Physical Changes Occurring During Commercial Composting

M. Day1, M. Krzymien', K. Shawl, L. Zarembal, W.R. Wilson2, C . Botden2 and 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.

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 in- fluence the composting process and the resulting product (Haug 1993; Naylor 1996; Fin- stein 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 to evaluate the composting process at a commercial composting facility, while at the same time determining the ability nf a !ab- oratory composting system to duplicate and/or monitor the behavior of the commer- cia1 facility. 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.

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Materials And Methods

The Commercial Composting Fucility

The commercial composting operation evaluated is operated by CORCAN and is located in Kingston, Ontario. The composting technology used by CORCAN in-

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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 au- tomatic 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.

I

Feedstock Material

The material processed by the CORCAN commercial composting facility con- sists 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 12.5 percent 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 1.5 percent 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. Mean- while, the starting C:N ratio of 24.6 and bulk density of 0.59 are typical for most com-

TABLE 1. Characteristics of the compost feed material

Material (%) (x,) (w,) (?A>)

Manure 25.0 71.3 7 2 16.9 162 42.5 5.2 2.1

Yiiid Waste 12.5 40.9 7.4 17.4 5: 0 27.4 27.: 1 C

Cardboard 1.0 32.5 0 7 1.6 8.4 48.2 6.1 0 2

Construction Waste 10.0 61.4 3.9 9.1 17.9 19.2 10.5 0.7

Restaurant Waste 25.0 794 5.1 121 120 23.5 10.3 2.7

Porridge 1.5 88.3 0.2 0.4 3.6 58.6 8.6 2.0

Recyclate 25.0 27.8 18.1 425 31.9 36.6 3.7 2.8

Compost Feed Calculated 100.0 57.5 42.5 100.0 23.3 33.1 9.5 2.1

Determined 100.0 64.0 20.4 44.7 5.4 1.8

Ratio (g/mL) (“AB)

19.9 0.48 52.0

27.7 C.33 6550

253.7 0.13 330

27.4 0.31 70.0

8.7 0.99 0.0

29.7 1.00 0.0

13.2 0.24 68.0

15.7 0.52 45.5

24.6 0.59 43.8

M . Day, M . Krzyniien, K. Shaw arid L. Zaremba, W.R. Wilson, C. Botden and B. Thomas

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 Instru- ments) 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,

MIXING BOX

EXHAUST

DETECTtON

INSULATION

CONTROLLER ASPIRATOR

Figure 1. Schematic of the laboratory composting system.

46 Compost Science a Utilization Spring 1998

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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 O ice 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 composting behavior 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.

Bulk Density

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 ab- sorbed by the composting material.

Material Homogenization

Many analytical procedures use small sample sizes. In order to obtain more representative samples of the test material for subsequent analysis the inherently het- erogenous 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.

PH 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 set- tled, 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 in- cluding moisture.

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

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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 desicca- tor 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 facility 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 over- come the natural variabilities of the panel members. Because of colds, illnesses and hol- idays it was not possible to maintain the same six panel members throughout the com- plete 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 out- liers (> 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 in- serted into a modified injection port of a HP5790 Gas Chromatograph (GC) (Krzymien 1987) where the sample was thermally desorbed and transferred by heli- um carrier gas on to a 30 m x 0.32 mm I.D., d, = 1 pm, 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

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 10th July 1996. The laboratory composters were operated on a 12 day cycle. Laboratory composting vessels number 3 and 4 were operat-

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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 Aerahon Time Pile In The 2 Hour Temperature Dstance

Time In Number Of Advanced Sampling Period During Turns Since Down Bay 10-12 am Sampling Sampling Compost Bay

Date (Days) Last Sample (W Win) Period (“C) ’ --- --- ----_I_-- --- --_ ___. __ - __ _.

May22,1996 0 May29,1996 7 June 5,1996 14 June 12,1996 21 Junel9,1996 28 June 26,1996 35 July 3,1996 JulylO, 1996 49

5 60 6 50 2 84 6 48 3 120 4 45 1 132 4 45 2 156 4 63

42 3 192 2 50 3 228 1.5 41

50 Compost Science 8 utilization spring 1998

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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 composting bay 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 t ~ ~ 51


TABLE 3. Changes in volatile matter in test samples during

the 12 day period in the laboratory composter

age volatile matter during the 12 day laboratory composting period. The data, summa-

Sample Time Initial Final Changein rized in Table 3, indicates that water aspiration of the in CORCAN Volatile Volatile

facility Matter Matter Matter (days) (Yo) (“Id (sa) air entering the laboratory

composting vessels prevents 0 64 1 67 8 3 7 the test samples from drying 14 63 9 66.4 2 5 out during the 12 day labora- 21 58 7 640 5 3 tory test period, especially 28 61 2 61 9 0 7 when high composting activi- 35 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 con-

tent 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 com- bine to cause an increase in moisture content and measured volatile matter. Subse- quent 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.

Volatiie

- -

7 63 6 66.5 2 9

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 feed- stocks 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,

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52 Compost Science & utilizatlm Spring 1998

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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,

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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 influ- ence the biodegradability of the organic fraction. Because the biological breakdown of the orPanic material involves

TABLE 6. Changes in inorganic content in test samples during

the 12 day period in the laboratory composter Samole Time " in C ~ R C A N Initial Final the consumption of oxygen

("m) (W (%) and the production of meta- facility Value Value Change

20.5 22.9 24.1 29.7 27.8 29.1 32.1 34.4

22.9 24.9 25.4 32.0 32.2 35.3 29.7 36.8

2.4 2.0 1.3 2.3 4.4 6.2 -2.4 2.4

bolic water and carbon dioxide, a net loss in organic matter occurs. Consequently, for a fixed weight of feed material, it can be anticipated that there will be a corresponding relative increase in the inorganic content of the material

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as the composting process progresses. This was indeed observed over the 49 days of composting in the CORCAN facility. This information which is presented graphi- cally 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 de- creasing 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 convert- ed 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 conse- quence 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 12 days of composting in the laboratory composters. This data is summarized

50

v

0 10 20 30 40 50

Time (days)

I Figure 6. Variation in the composition of carbon (v), hydrogen (0) and nitrogen (B) in the CORCAN test samples as a function of composting time.

Compost Science &Utilization Spring 1998 55

M. Day, M . Krzymien, K. Shaw and L. Zaremba, W.R. Wilson, C. Botden and B. Thomas

TABLE 7. Changes in C, H and N content in test samples during the 12 day perlod

in the laboratory composter Sample

N ___- Timein -- - - c CORCAN Initial Final Initial Final Initial Final facility Value Value Change Value Value Change Value Value Change

~ H - - _ _

(W (“4 (“w - (Yo) (“4 (“w (W I__________-

(74 -~ - (days) YW -

0 447 38 5 -6 2 544 456 -089 1 82 2 85 103 7 42 2 37 3 -4 9 528 442 -086 2 21 2 29 0 08 14 45 8 40 7 -5 1 589 512 -077 2 35 2 87 0 52 21 39 5 34 4 -5 1 4 48 427 -021 1 83 2 96 113 28 38 7 33 8 -4 9 468 380 -088 1 58 2 95 I 3 7 35 348 35 7 0 9 408 431 0 22 2 46 3 36 0 90 42 32 6 342 1 6 342 398 0 56 2 65 3 22 0 37 49 36 3 36 4 0 1 437 406 -032 2 65 2 74 0 08 _ _ _ _ _ _ _

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 commercial test 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.

p H Measuremenfs

The data on the pH measurements taken on the samples removed from the COR- CAN facility over the 49 day test period are presented in Figure 7. This data clearly indicates that the initial feed material used in the composting process 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

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Figure 7. The pH of the CORCAN test samples as a hrnction of composting time.

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An Investigation of the Chemical and Physical Changes Occurring During Commercial Composfing

weeks of composting a notice- able increase in the acidity of the test samples was noted as the pH fell to around 5.5. Af- ter the second week, however, the pH of the compost increased rapidly to a value of about 7.5, which was maintained for the remainder of the commercial composting period. Although only the initial and final pH's were measured in the laboratory composting systems the final pH of all the materials tested

TABLE 8. Changes in pH in test samples during the 12 day period in the laboratory composter

Sample Time in CORCAN facility Initial Final

Value Value Change

0 7 14 21 28 35 42 49

6.19 5.62 5.48 7.35 7.70 7.60 7.80 7.78

8.25 8.57 8.35 8.86 7.96 8.13 7.63 8.65

2.05 2.95 2.87 1.51 0.25 0.53

0.87 -0.17

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 exothermic nature of the composting process. 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, 2 a - 50 E a, P

t- E,

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

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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.

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58 Compost Science 8 utllizatii Spring 1998 COD1


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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 Fig- ure 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


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 composting at 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 composting had 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 pro- ductivity throughout the composting facility would be informative.

The measurement and quantifica- tion 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 All Selected Values Values Member

1 12 50 12 50 2 16 77 3 14.30 14.39 4 3.81 5 14.39 14 39 6 16.77 7 12.50 12 50 8 10.00 10.00 MIN 3.8 10.00 MAX 16.77 14.39 AVG 12.69 12 7 STD 3.81 1 6 AVG + STD 16.5 14.3 AVG - STD 8 79 11 1

- -

- -

0 0 10 20 30 40 50

Time (days)

Figure 12. Odor detection threshold values for gas samples taken from the CORCAN facility. Error bars represent 95% confidence limits.

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60 compost science a utilization sprine 1998


0 1 0 10 20 30 40 50

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 (v) plotted as a function of composting time in the CORCAN facility.

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 ob- servers can be time consuming and expensive. Consequently the following simplis- tic 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

Spr ingl~M 61 Compost Science 8 Utll lzat i

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 in- terpret 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 composting process 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.

Vofn t ile Orgo n ics

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

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with 2-pentanone were the three most abundant compounds. However, it is also in- teresting 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 ma-

TABLE 10. Major organic compounds identified in gas sample

(14 days in CORCAN facility) ___

R.T.

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

Name

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-Pen tanone 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 --

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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 83 84 78 83 93 81 78 92 a5 84

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spring1998 63


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- . 30 40

Retention time [min]

Figure 15. GC trace of volatile organics in thegas sample taken from the CORCAN fanllty on day 14.

Time (days)

Figure 16. Total abundance all the organic compounds (0) and sulphur compounds (m) detected in the ehaust gas from the laboratory composters after 2 days using CORCAN test samples taken over the 49 day test period.

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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.

Spring 1998 64 compost science utilization

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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 composting process 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 Hen- derson, 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 chemilumi- nescence detector. journal of American Society of Braving Chemists, 5053-58.

Finstein, M.S., F.C. Miller and P.F. Strom. 1986. Waste Treatment Composting as a Controlled System. In: W. Schonborn (ed.), Biotechnology, Biodegradations. VCH Verlagsgesellschaft, Weinheim, FRD, 8363-398.

Gies, G. 1995. Canadian Facilities: Composting Food Processing Residuals. BioCycle, 36(8):36- 39.

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Golueke, C.G. 1972. Composting: A Study of the Process and its Principles, Rodale Press Inc., Em- maus, PA.

Haug, R.T. 1993. The Practicnl Handbook of Compost Engineering, Lewis Publishers, Boca Ra- ton, FL.

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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. Aust. 1. Expt. Agriculture, 28:553-560.

Miller, F.C. 1989. Matric Water Potential as a Biological Determination in Compost, a Sub- strate Dense System. Microb. Ecol., 18:59-71.

Miller, F.C. 1993. Minimizing Odor Generation, In N.A.J. Hoitink und H.M. Kenner Ced,, Sci- ence and Engineering of Composting: Design, Environmental Microbiological and Utilization Aspects. Renaissance Publications: Wathyla Ohio. 219-241.

Nakassaki, K., A. Watanabe and H. Kubota. 1992. Effects of Oxygen Concentration on Com- posting Organics. BioCycle, 33(6):52-54.

Nakasaki , K., N. Aoki and H. Kubota. 1992. Accelerated Composting of grass clippings by controlled moisture level. Waste Management and Research, 12:13-30.

Naylor, L.M. 1996. Composting. In Composting Environmental Science and Pollution Control Se- ries, Marcel Dekker, New York. NY. 18193-269.

Reinhart, D.R., D.C. Cooper and B.L. Walker. 1992. Flux Chamber Design and Operation for the Measurement of Municipal Solid Waste Landfill Gas Emission Rates. J . Air, Waste Managemen t, 42(8): 1067-1 070.

Schulz, T.J. and Van Harreveld. 1996. International Moves Towards Standardization of Odor Measurements using Olfactometry. Waf. Sci. Tech., 34541-547.

Suler, D.J. and M.S. Finstein, 1977. Effect of Temperature Aeration, and Moisture on CO, For- mation in Bench-Scale, Continuously Thermophilic Composting of Solid Waste. Appl. Environ. Microbiol., 33345-350.

Tseng, D.Y., J.J. Chalmers, O.H. Tuovinen and H.A.J. Hoitink. 1995. Characterization of a Bench-Scale System for Studying the Biodegradation of Organic Solid Waste. Bioteclznol. Prog., ll(4): 443-451.

Van Durme, G.P., B.F. McNamara and McGinley. 1993. Bench scale removal of odor and volatile organic compounds at a composting facility. Water Environnrental Research, 64( 1):19-27.

66 Compost Science 8 utilization Spring 1998

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