Emission of Carbon Monoxide During Composting of Municipal Solid Waste

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Emission of Carbon Monoxide During Composting ofMunicipal Solid WasteE.A. Phillipa, O.G. Clarka, K. Londryb, S. Yud & J. Leonardbc

a Department of Bioresource Engineering, McGill University, Ste. Anne de Bellevue, QC,Canadab Edmonton Waste Management Centre of Excellence, Edmonton, AB, Canadac Department of Agricultural, Food, and Nutritional Science, Agriculture/Forestry Centre,University of Alberta, Edmonton, AB, Canadad Department of Civil and Environmental Engineering, Natural Resources Engineering Facility,University of Alberta, Edmonton, AB, CanadaPublished online: 23 Jul 2013.

To cite this article: E.A. Phillip, O.G. Clark, K. Londry, S. Yu & J. Leonard (2011) Emission of Carbon Monoxide DuringComposting of Municipal Solid Waste, Compost Science & Utilization, 19:3, 170-177, DOI: 10.1080/1065657X.2011.10736996

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170 Compost Science & Utilization Summer 2011

Introduction

The Edmonton Composting Facility (ECF) is locat-ed at the Edmonton Waste Management Centre(EWMC) in Alberta, Canada, one of the largest multi-functional municipal waste treatment and research cen-ters in North America. The ECF is an enclosed cocom-poster in which the organic fraction of the municipalsolid waste is combined with thickened biosolids fromthe city’s wastewater treatment plant and convertedinto a marketable soil amendment. The ECF processes200,000 t of residential waste per year in three aeratedcomposting bays. Elevated concentrations of carbonmonoxide (CO) have occasionally been observed nearthe compost bays in the ECF. High CO concentrationsin this enclosed facility pose a potential health threat tothe workers, because atmospheric CO binds readilywith hemoglobin and can impair the O

2supply to the

body’s tissues. Identifying the source of CO and the fac-tors influencing its production could allow operationsmanagers at the facility to reduce the CO emissions andconsequently reduce related workplace health threats.

The objectives of this study were to: (1) Assess thetemporal and spatial variability of CO emissions fromthe composting bays in the ECF, by direct measure-ment using Fourier Transform Infrared (FTIR) spec-troscopy; and (2) Identify any correlations betweenthe CO emission rate and operating parameters orphysicochemical properties of the compost.

Literature Review

Carbon Monoxide Production

Holloway et al. (2000) estimate the total global COinput to the atmosphere to be approximately 2.5 Pgy-1. The major sources of CO include incomplete com-bustion of fossil fuels and oxidation of biogenic hy-drocarbons. There may also be a possible chemicalorigin of CO due to an atmospheric equilibrium reac-tion involving CO and CO

2(Conrad and Smith 1995).

Common biological sources of CO emission to theatmosphere include dissolved organic matter in aquat-ic systems, and the degradation of organic matter in

Compost Science & Utilization, (2011), Vol. 19, No. 3, 170-177

Emission of Carbon Monoxide During Composting of Municipal Solid Waste

E.A. Phillip1, O.G. Clark1*, K. Londry2, S. Yu4 and J. Leonard2,3

1. Department of Bioresource Engineering, McGill University, Ste. Anne de Bellevue, QC, Canada2. Edmonton Waste Management Centre of Excellence, Edmonton, AB, Canada

3. Department of Agricultural, Food, and Nutritional Science, Agriculture/Forestry Centre, University of Alberta, Edmonton, AB, Canada

4. Department of Civil and Environmental Engineering, Natural Resources Engineering Facility, University of Alberta, Edmonton, AB, Canada

*E-mail contact: [email protected]

Elevated concentrations of carbon monoxide (CO) have been observed at the enclosed municipal wastecomposting facility (ECF) in Edmonton, Canada. Elevated concentrations of CO in an enclosed facilitypose a potential health risk to workers. The objectives in this study were to: (1) assess temporal and spa-tial variability of CO emissions from the composting bays in the ECF using Fourier Transform Infrared(FTIR) spectroscopy; and (2) identify any correlations between the CO emission rate and the physico-chemical properties of the compost through bench-scale incubation experiments. Repeated gas measure-ments were taken above and within the compost bed in the ECF using a probe connected to an FTIR gasanalyzer, which continuously collected concentration data. These preliminary field measurementsshowed maximum CO concentrations of 112 µL L-1 within the compost. Autoclaved and non-sterilizedcompost samples from the ECF were incubated under aerobic and hypoxic conditions, and gas emissionswere quantified using gas chromatography (GC). These trials showed a positive correlation between COemission rate and incubation temperature for all samples, indicating a physico-chemical source of CO gen-eration. Lower concentrations of CO were observed in the non-sterilized compost under both aerobic andanaerobic conditions, presumably due to the microbial metabolism of CO.

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Compost Science & Utilization Summer 2011 171

soils and marine sediments (Hellebrand and Schade2008; King and Weber 2007). For instance, studies inthe fields of soil science and forestry indicate that sub-stantial quantities of CO may be emitted from forestedsoils. Kuhlbusch et al. (1998) found significant COemission from upper soil horizons (50 mm) in uplandCanadian boreal forests. The CO emission from thetopsoil was attributed to temperature, moisture vari-ability, and exposure of the topsoil to fire and humanactivities. Carbon monoxide emission was also record-ed from the upper layers of Scottish soils (Moxley andSmith 1998). King (1999) found that CO emission fromsoils is highly dependent on organic carbon availabili-ty and temperature. The study showed that CO emis-sion increases with high organic carbon availabilityand high temperature (30 to 40oC) during the daywhereas soils become a net sink of CO during the nightor when temperature falls below 30oC.

Few studies regarding the emission of CO fromcompost can be found in the literature. Haarstad et al.(2006) studied the emission of CO from the decompo-sition of organic waste under aerobic and anaerobicconditions. Hellebrand and Kalk (2001) studied theemissions of CO from the composting of dung andgreen waste, and found that the release of CO is di-rectly related to the availability of O

2and the temper-

ature during composting. Hellebrand and Schade(2008) then measured the emission of CO from steril-ized and non-sterilized samples of green waste com-post at different temperatures and found that theemission of CO was similar among both sets of sam-ples, indicating that the production of CO did not de-pend on biological activity.

According to the national ambient air quality stan-dards (NAAQS), the maximum acceptable 8 h rolledaverage exposure limit for CO is 13 µL L-1 (15 mg/m3)(Health Canada 1994). The NAAQS guideline statesthat the 1 h maximum desirable and maximum accept-able CO exposure level is 13 and 30 µL L-1, respectively.

Carbon Monoxide Removal

Carbon monoxide is toxic to many microorganisms,due to its capacity to inhibit certain electron transportchains (Techtmann et al. 2009). Some bacterial species,however, use CO as an energy and carbon source (Kingand Weber 2007). Due to the widespread and varied nat-ural and anthropogenic sources of CO, a wide physio-logical diversity can be found among bacteria that canexploit CO as an energy source (King and Weber 2007).In these organisms, CO is first oxidized to produce CO

2and a pair of reducing equivalents (2H+ and 2e-).

CO + H2O —>CO

2+ 2H+ + 2e-

Both aerobic and anaerobic bacteria are capable ofoxidizing CO, and the fate of the two reducing equiv-alents depends on the type of metabolism. In obligateanaerobes, the reducing equivalents can be coupled tosulphate reduction to form sulphide (S2-), or to CO

2re-

duction to form acetate (OAc–) or methane (CH4). In

aerobic metabolism, reducing equivalents are coupledto O

2reduction (King and Weber 2007). The total CO

consumption through microbial activity in soils, sedi-ments and freshwater is unknown, however, King andWeber (2007) estimate that CO consumption by CO-oxidizing bacteria amounts to approximately 20% ofthe total global input of CO into the atmosphere.

Another mechanism for the removal of CO fromthe atmosphere is through tropospheric oxidation. Re-actions involving the hydroxyl radical (OH) providethe dominant sink for tropospheric carbon monoxide(Logan et al. 1981).

CO + OH —>CO2

+ H

The atmospheric concentration of CO can affectthe climate by buffering the reaction of the OH radicalwith methane and other greenhouse gases, thus ex-tending their residence times in the atmosphere (Zell-weger et al. 2009). Badr and Probert (1995) estimatethat ~85% of the atmospheric CO that is destroyed an-nually is removed by reactions with the OH radical,and only ~10% is consumed in soils.

Material and Methods

Edmonton Composting Facility Field Study

A field study was carried out at the ECF facility inEdmonton. The co-composter contains three com-posting bays, each of which is 175 m long, 22 m wideand 2.5 m deep. Each bay is divided into four zones.The compost in each zone is aerated by a downdraftventilation system. The compost is turned and shiftedsideways across the bay, in sequence from Zone 1 toZone 4, by twin augers mounted on an overheadbridge crane. Water is added to the compost throughsprayers mounted with the mixing augers (Figure 1).Aeration rate, moisture content, and turning frequen-cy can be controlled independently in each zone, withthe potential to optimize composting conditions ineach. During this study, the substrate residence timesin the four zones were 2, 3, 5, and 6 days, respective-ly. Due to this arrangement, the temporal evolution ofthe composting process corresponds to the spatial po-sition of the compost relative to the width of each bay,with fresh compost feedstock being introduced intothe outer edge of Zone 1 and the most mature com-post being removed from the opposite edge of Zone 4.

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A Fourier Transform Infrared (FTIR) (Model CX-4015, Gasmet Technologies, Helsinki) gas analyzerwas used to measure the concentrations of CO, CO

2,

and CH4. The accuracy of the FTIR was verified using

reference gases before the beginning of the study. Thebackground absorption spectrum of the gas analyzerwas recorded by filling the sample chamber with ni-trogen gas (N

2) before each sampling episode. Gas

concentrations were measured in two of the threecomposting bays at the ECF, as the third bay was shutdown for maintenance during the field study. Gasconcentrations were measured at each of seven sam-pling locations in each of all four zones, at 1 m abovethe compost bed, 1 m inside the compost bed, and inthe aeration pipes below the compost. An oxygenprobe (Model OT-21, Demista Instruments, ArlingtonHeights, IL) was used to measure the concentration ofO

2. A 1 m-long dial gauge thermometer was used to

measure temperature within the compost bed at eachsampling location. A hotwire anemometer (Velocicalcmodel 8345/8346, STI, St. Paul, MN) was used to mea-sure air velocity and temperature in the aeration pipesbelow each sampling location.

Compost was also sampled at each location andanalyzed for chemical and physical properties, includ-ing moisture content, ash content, pH, C:N ratio, andtotal phosphorous. Additional compost samples werecollected for microbial community analysis.

The FTIR measured the infrared transmissionspectrum of the gases pumped through its sample cellat 0.5 Hz, and the associated software calculated theapproximate concentration of the component gasesbased on a best-fit composite spectrum built from a li-brary of transmission spectra. These raw concentra-tion data were then processed to obtain an estimate ofthe mean gas concentrations during the samplingevent. The time series of concentration data for CO,CO

2and CH

4was imported into MATLAB™ (The

Mathworks, Natick, MA). A peak in the data signalwas defined as a rapid rise in gas concentration fol-lowed by a plateau and then a rapid fall, as illustratedin Figure 2 and, in these data, corresponds to mea-surements taken within the compost bed. A valley inthe data signal is defined as the steady state period be-tween two peaks, and corresponds here to measure-ments taken above the compost bed.

Figure 3 illustrates the identification of a signalpeak for CO

2concentration, delineated by the outer

dotted vertical lines, and plateau delineated by thesolid inner vertical lines, for one of the sampling loca-tions in the composting bed. The gas concentrationswere calculated by taking the mean value of the datapoints that constituted the steady state regions(plateaus or valleys) in the data signal. The algorithmthat was used to calculate the gas concentration valuesat each sampling location in the composting bay is de-scribed in detail elsewhere (Phillip and Clark 2010).



FIGURE 1. Configuration of composting bays and sampling loca-tions in plan and elevation views (not to scale).

FIGURE 2. Carbon dioxide concentration measured using theFTIR gas analyzer. Labeled regions are: (1) signal peak, (2) signalvalley, (3) signal plateau. The sample number is the sequentialrecord number of a raw datum generated by a single FTIR mea-surement at 0.5 Hz.

FIGURE 3. Estimation of gas concentration from the instrumentsignal: (1) signal peak; (2) signal plateau.

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Laboratory Microcosm Experiments

Two sets of experiments were conducted in orderto measure the effects of temperature on CO genera-tion from compost, to determine whether: (1) the gen-eration of CO from the ECF was dominantly a physic-ochemical or biological process; and (2) to observe theimpacts of O

2availability on CO generation.

Compost was collected from the ECF from regionswith a temperature of 65–70ºC. The samples werestored in large ZiplocTM bags and transported to thelaboratory at room temperature. Compost (250 ± 15 g)was transferred, inside a fume hood, into 250 ml con-tainers and the containers were sealed with caps withsepta. The containers were approximately 3/4 full ofcompost, leaving a headspace of approximately 60 ml.For experiments in which some samples were auto-claved, the non-sterilized samples were stored at 4ºCfor up to two days prior the start of the trials.

Sterilization was accomplished by placing com-post samples in an autoclave at 121°C for 1 h for threesuccessive cycles, 24 h apart. The effectiveness of thissterilization procedure was confirmed by taking sam-ples of an extra sterilized container at the start of incu-bation, and plating on two different kinds of media.The first medium was BD BactoTM Soybean-Casein Di-gest Medium/Tryptic Soy Broth (SCB) (Becton, Dick-inson and Co. (BD), Franklin Lakes, NJ) with an incu-bation time of 7 days at 35ºC. The second medium wasBD BactoTM Fluid Thioglycollate Medium (FTM) (BD,Franklin Lakes, NJ), with an incubation time of 14 daysat 25ºC. Aseptic technique was used for all sterile con-trols, including filtering of gasses through syringe-tipfilters, and the use of alcohol wipes on all surfaces.

During the incubation experiments, containerswere placed into hot water baths at the appropriatetemperatures and incubated for pre-determinedamounts of time. Containers on the bench were kept at20ºC. Gas samples (7 ml) were collected from thesealed containers by syringe. For the sterile controls,the samples were collected aseptically. The sampleswere transferred to 5-ml Vacutainer™ evacuated con-tainers (BD, Franklin Lakes, NJ), and stored at roomtemperature. An equal volume of gas (air, filter-steril-ized air, N

2, or filter-sterilized N

2) was added to the

containers to replace the sample volume. Note that thesample volume represented only about 10% of theheadspace volume of the containers, but the overall ef-fect of the replacement would be dilution of the gassesin the headspace by N

2or air, which would lead to

slight underestimates of CO production.The gas samples were transported in sealed vials

for analysis at the University of Alberta, Edmonton.Samples were analyzed by gas chromatography (GC)

using an HP 5890 GC with a Thermal ConductivityDetector (Agilent Technologies, Inc., Santa Clara, CA).Carbon monoxide and O

2were measured using an HP

Molsieve column, and CO2

was measured using anHP PlotQ. The CO concentration of ambient air wasnot analyzed during these experiments. Concentra-tions of gasses in the samples were determined bycomparison of peak areas to external standard curvesof 60–300 µL L-1 CO, 0.2–1.0% CO

2, and 4–20% O

2.

Experiment #1

A first series of experiments was conducted to de-termine the effects of temperature on the emission ofCO, CO

2and O

2. Triplicate sterilized and non-sterilized

samples were submerged in water baths at 35, 55 and75ºC. Gas samples were taken from the headspaces at 2,4, 6, and 25 h from the start of incubation, as describedabove. Carbon monoxide and O

2gas analysis were per-

formed together, within a few days of the collection ofthe gas samples. Thereafter, the gas samples were re-tained, and CO

2analysis was performed only after the

samples had been stored at 4ºC for several weeks.

Experiment #2

In order to determine whether the generation ofCO from compost varied with the availability of O

2,

sterilized and non-sterilized compost samples wereprepared under aerobic and hypoxic conditions. Halfof the containers were flushed with inert N

2gas prior

to being sealed, to create hypoxic conditions. A cylin-der of N

2supplied the gas source, which was filter

sterilized with Fisherbrand™ 0.2 µm disposable sy-ringe sterile filters. The containers were flushed for 20min, allowing the gas to escape through sterile nee-dles. During incubation, gas samples were taken fromthe containers first and an equal volume of N

2was

then injected with a syringe to compensate for the lostgas volume. For aerobic incubations, air from a com-pressor was humidified by bubbling through waterand then added to the aerobic containers after sam-pling. All containers were incubated at 65ºC. Gas sam-ples were taken after 0, 2, 4, and 6 h. In this experi-ment, CO, O

2, and CO

2concentrations were all

measured by GC at the same time, as described above,within a few days after collection of the gas samples.

Results and Discussion

Field Study

The first objective in the field study was to assessthe temporal and spatial variability of CO emissions in

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the ECF composting bays. The gas emissions data col-lected during the field study from two of the com-posting bays were neither consistent among the sam-pling points within each zone, nor among the zones ofeach bay. This extreme variability is typical of gasemissions from compost. The field data set, moreover,did not include data from all of the planned samplinglocations, due to logistical problems during the study.Consequently, beyond the confirmation that the com-post itself was the source of the CO emissions, a verynarrow set of observations can be drawn from thefield study. The first observation was that the highestmeasured concentration of CO was 112 µL L-1. Thesecond observation was that the concentration of COwas higher in Zone 2 (56.7 ± 39.6 µL L-1) than in Zone1 (4.3 ± 3.1 µL L-1).

In Zone 1 of Bay 1 there was a correlation betweenthe amount of CO and the amount of CO

2in the sam-

ples (r2 = 0.981, p = 0.0001), which suggested a rela-tionship between CO

2and CO emissions during the

composting process. Zone 1 of Bay 1was the only zonefrom which the data collected were adequate to calcu-late the correlation between the concentrations of thetwo gases.

In Zone 1 of Bay 1, there were also differences ob-served in the amount of CO at the seven sampling lo-cations and at different elevations at each location.Concentrations of CO were highest within the com-post and in the aeration pipe, and were low at theprocess floor and above the compost. This confirmsthat CO was generated within the compost bed andwas drawn mostly downward into the aeration sys-tem, although some appears to have diffused upwardinto the airspace of the building. The 8 h rolled aver-age CO concentration from the compost beds for themonitoring period was below the national maximumacceptable level (13 µL L-1 (15 mg/m3) and may notpose a threat to the workers (Health Canada 1994).

The second objective in the field study was to ex-amine the influence of physicochemical propertiesand facility operating parameters on CO emissionrates. The low quality of the field data, however, pre-cluded any observations of this nature. A series of con-trolled laboratory experiments was therefore conduct-ed in order to examine any correlation between COemission rates and process conditions.

Laboratory Microcosm Experiments

Experiment #1

The first set of microcosm experiments was astudy of the influence of temperature on the emissionrate of CO from sterilized and non-sterilized compost.

Figure 4 illustrates that the production of CO inboth sterilized and non-sterilized containers increasedwith temperature. At 35°C, the rate of CO productionin the 2–6 h time frame was 7.4 ± 4.1 µL L-1 h-1 and 7.4± 3.7 µL L-1 h-1 for the sterilized and non-sterilizedtreatments, respectively. These rates increased to 64.2± 21.9 and 43.4 ± 20.9 µL L-1 h-1 respectively at 55°C,and to 181 ± 6 and 148 ± 22 µL L-1 h-1 at 75°C.

These results support the previous finding ofHellebrand and Schade (2008) that the origin of COappears to be of a thermochemical nature and is pro-portional to temperature. This is indicative of an Ar-rhenius relationship:

k reaction rate coefficient (s-1)A constant (s-1)E

aactivation energy (kJ mol-1)

R gas constant (kJ mol-1 K-1)T temperature (K)

The activation energies for the production of COfrom sterilized and non-sterilized samples, respective-ly, were estimated to be ~69 kJ/mol and ~64 kJ/mol(Figure 5). These results are similar to those found byHellebrand and Schade (2008).

It was assumed for the sake of this analysis thatthe free air space within the compost in each vial wasnegligible compared to the headspace (60 mL), so thelatter was taken to represent the total available air-space and used in the conversion of measured concen-trations to total mass of volatile compound.

In addition to CO, CO2

and O2

were also ana-lyzed in the samples of gas taken from the headspaceof the containers of compost incubated at the three



FIGURE 4. Carbon monoxide production in vials of sterilized ornon-sterilized compost, incubated at 35°, 55°, or 75°C. Error barsindicate the standard deviation among triplicates.

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temperatures (Figures 6 and 7). Oxygen concentra-tions decreased faster at the higher temperatures

and, as with CO2, the rate of change was similar for

the sterilized and non-sterilized containers. In spiteof the decrease, aerobic conditions (>5% O

2) were

maintained throughout the 25 h of incubation. Theconcentration of CO

2was significantly lower in Ex-

periment #1 compared to Experiment #2, under sim-ilar conditions (Figure 10). This difference is likelydue to the loss of CO

2during the storage of compost

samples in Experiment #1.

Experiment #2

A series of experiments was designed to examine thedifferences in the generation of CO under hypoxic andaerobic conditions, from both sterilized and non-ster-ilized samples. Carbon monoxide and CO

2produc-

tion, along with the concentration of O2, were moni-

tored over a 6 h period from samples incubated at65ºC (Figures 8–10).

The measured O2

concentrations indicate thatboth the hypoxic and aerobic conditions were main-tained for non-sterilized samples, since the O

2concen-

tration did not drop below 15% for the aerobic sam-ples and the O

2concentration did not exceed 6.5% in

the hypoxic samples (Figure 9). For the non-sterilizedsamples, however, the O

2concentrations in the aero-

bic sample containers dropped below 5% after ap-proximately 2 h of incubation and remained there forthe duration of the experiment (Figure 9). This de-crease in O

2concentration was likely to do the con-

sumption of O2

by aerobic microorganisms in thesealed sample containers.

Under both hypoxic and aerobic conditions, netCO accumulation was greater in sterilized samplescompared to non-sterilized samples. In these experi-ments, biological activity limited the net accumulationof CO, likely due to the oxidation of CO by aerobic and



FIGURE 5. Production rates of CO between 308 K (35°C) and 348K (75°C) plotted against inverse temperature for sterilized andnon-sterilized substrate. Error bars indicate the standard devia-tion of calculated production rates, among triplicates.

FIGURE 6. Oxygen concentration in vials of sterilized or non-ster-ilized compost. Error bars indicate the standard deviation amongtriplicates.

FIGURE 7. Carbon dioxide concentration in vials of sterilized ornon-sterilized compost. Error bars indicate the standard deviationamong triplicates.

FIGURE 8. Carbon monoxide concentrations in vials with steril-ized or non-sterilized compost, under aerobic or hypoxic condi-tions. Error bars indicate the standard deviation among triplicates.

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anaerobic bacteria. These results clearly demonstrateda dominant physicochemical source of CO, mitigatedby biological activity, which corroborates the resultsfound in Experiment #1. Note that the initial values (0h) in the figure were taken from samples of air or N

2gas, rather than from the containers themselves. It istherefore possible that the initial CO concentrationswere greater than zero, and that accumulation of CO inthe non-sterilized samples was negligible.

The results in Figure 8 also indicate that the pres-ence of O

2increased CO generation during the incuba-

tion in the sterilized containers. Although the aerobicconditions were not maintained in the non-sterilizedsamples throughout the entire experiment, the rate ofCO accumulation in the non-sterilized sample for thefirst 2 hours of the experiment suggests that the pres-ence of O

2also promotes CO generation in the pres-

ence of biological activity. Greater production of CO inthe presence of O

2for both sterilized and non-sterilized

samples was consistent with earlier findings from

Hellebrand and Schade (2008).Net accumulation of CO

2was limited in the steril-

ized containers, yet rapid in the non-sterilized contain-ers (Figure 10). Sterilized containers produced CO

2at

low rates under both aerobic and hypoxic conditions,whereas the non-sterilized containers produced CO

2at

higher rates under both aerobic and hypoxic condi-tions, over the entire 6 h period. Carbon dioxide pro-duction was the fastest in the aerobic, non-sterilizedcontainers during the first 2 h, but then diminishedduring the latter 4 h. Overall, the CO

2concentrations

confirmed high rates of microbial activity at 65ºC un-der hypoxic and especially aerobic conditions, andnegligible microbial activity in the sterile samples.

Overall Observations of Changes in CO Concentrations

The emission of CO from the compost in the labo-ratory microcosms was dependent on temperature, O

2concentration, and microbial activity; increased tem-perature and O

2concentration were correlated with

enhanced CO emission, whereas biological activitywas correlated with less net accumulation of CO, like-ly due to the microbial oxidation of CO.

The net rate of CO emission from compost de-pends on a number of competing processes, each ofwhich is impacted by temperature and O

2concentra-

tion, as well as additional factors not quantified in thisstudy, such as moisture content and substrate compo-sition. The bench-scale laboratory studies were asound approach for the purpose of this study becausethey enabled the selective control of some of theseprocesses, such as the suppression of biological activ-ity by autoclaving and the minimization of aerobic ac-tivity by creating hypoxic conditions. We were unable,however, to maintain aerobic conditions in the non-sterilized samples throughout the experiment, due tomicrobial consumption of O

2. In future experiments it

might be useful to use a ventilated headspace to main-tain constant O

2concentrations.

The impacts of other process variables, such assubstrate composition and moisture content, are moredifficult to determine. Future research could involvethe experimental manipulation of additional factorssuch as these to gather further insight into the process-es involved in the emission of CO, and develop recom-mendations for the net reduction of CO emission to theatmosphere. For instance, the rate of CO production inthe compost was difficult to determine in the non-ster-ilized samples because some of the CO presumablywas oxidized biologically to CO

2. The net result of the

microbial activity under both aerobic and hypoxic con-ditions, therefore, was the mitigation of CO generationfrom the system. Microbial activity thus appears to



FIGURE 9. Oxygen concentrations in vials with sterilized or non-sterilized compost, under aerobic or hypoxic conditions. Errorbars indicate the standard deviation among triplicates.

FIGURE 10. Carbon dioxide concentration in vials with sterilizedor non-sterilized compost, under aerobic or hypoxic conditions.Error bars indicate the standard deviation among triplicates.

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play an important role in the mitigation of CO emis-sions in the ECF. A detailed analysis of the microbialcommunity could provide further insight into theseprocesses.

Conclusions

The preliminary field measurements taken at theECF confirmed that the compost bays were the likelysource of the CO emissions. Although logistical chal-lenges during the initial field study resulted in the col-lection of only a limited number of reliable data, thosedata did reveal elevated CO concentrations within thecompost beds and aeration pipes. The maximum ob-served concentration was 112 µL L-1. The CO concen-trations were highly variable, even among sampling lo-cations within zones. Carbon monoxide concentrationswere strongly correlated with CO

2concentrations.

The results of the laboratory microcosm studiescorroborated the hypothesis that CO generation in thecompost was dominantly thermochemical, and waspositively correlated with temperature and O

2concen-

tration. The microbial oxidation of the CO producedwithin the compost reduced the net emission of CO tothe atmosphere. These findings are in agreement withliterature published about small-scale composting ofgreen waste and the release of CO from soils.

Acknowledgements

This work was supported by the City of Edmontonand the Edmonton Waste Management Centre of Ex-cellence (EWMCE). The authors of this report alsowish to acknowledge the contributions of Dr. ChristianFelske and Jennifer Chiang of the City of Edmonton,and the staff of the Edmonton Composting Facility.

References

Badr, O., and S.D. Probert. 1995. Sinks and environmentalimpacts for atmospheric carbon monoxide. Applied En-ergy, 50 (4): 339-372.

Conrad, R., and K.A. Smith. 1995. Soil microbial processesand the cycling of atmospheric trace gases. Philosophi-cal Transactions: Physical Sciences and Engineering, 351:219-230.

Haarstad, K., O. Bergersen and R. Sorheim. 2006. Occur-rence of carbon monoxide during organic waste degra-dation. J. Air & Waste Management Assoc., 56: 575-580.

Health Canada. 1994. National Ambient Air Quality Objec-tives for Carbon Monoxide - Executive Summary. On-line at http://www.hc-sc.gc.ca/ewh-semt/pubs/air/naaqo-onqaa/carbon-monoxyde-carbone/index_e.html. Accessed: Nov.6, 2006.

Hellebrand, H.J. and W.-D. Kalk. 2001. Emission of carbonmonoxide during composting of dung and green waste.Nutrient Cycling in Agroecosystems, 60: 79-82.

Hellebrand, H.J. and G.W. Schade. 2008. Carbon monoxidefrom composting due to thermal oxidation of biomass.Journal of Environmental Quality, 37(2): 592-598.

Holloway, T., H. Levy, II and P. Kasibhatla. 2000. Global dis-tribution of carbon monoxide. Journal of Geophysical Re-search-Atmospheres, 105 (D10): 12,127-12,147.

King, G.M. and C.F. Weber. 2007. Distribution, diversity andecology of aerobic CO-oxidizing bacteria. Nature Re-views Microbiology, 5(2):107-118.

King, G.M. 1999. Attributes of atmospheric carbon monox-ide oxidation by Maine forest soils. Applied and Environ-mental Microbiology, 65: 5257-5264.

Kuhlbusch, T.A.J., R.G. Zepp, W.L. Miller and R.A. Burke.1998. Carbon monoxide fluxes of different soil layers inupland Canadian boreal forests. Tellus, 50: 353-365.

Logan, J. A., M. J. Prather, S. C. Wofsy and M. B. McElroy.1981. Tropospheric chemistry: A global perspective.Journal of Geophysical Research, 86 (C8): 7210-7254.

Moxley, J.M. and K.A. Smith. 1998. Carbon monoxide pro-duction and emission by some Scottish soils. Tellus, 50B:151-162.

Phillip, E.A. and O.G. Clark. 2010. An algorithm for thedetection of steady-state measurements of gas emis-sions from compost. In-press. Applied Engineering inAgriculture.

Techtmann, S.M., A.S. Colman and F.T. Robb. 2009. ‘Thatwhich does not kill us only makes us stronger’: the roleof carbon monoxide in thermophilic microbial consor-tia. Environmental Microbiology, 11(5): 1027-1037.

Zellweger, C., C. H¸glin, J. Klausen, M. Steinbacher, M.Vollmer and B. Buchmann. 2009. Inter-comparison offour different carbon monoxide measurement tech-niques and evaluation of the long-term carbon monox-ide time series of Jungfraujoch. Atmos. Chem. Phys., 9:3491-3503.



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Emission of Carbon Monoxide During Composting of Municipal Solid Waste