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Published online 20 February 2008
Published in J Environ Qual 37:592-598 (2008)
DOI: 10.2134/jeq2006.0429
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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Carbon Monoxide from Composting due to Thermal Oxidation of Biomass

H.J. Hellebranda,* and G.W. Schadeb

a Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB), Max-Eyth-Allee 100, D-14469 Potsdam, Germany
b Dep. of Atmospheric Sciences, Texas A&M Univ., 3150 TAMU, College Station, TX 77843-3150, USA

* Corresponding author (jhellebrand{at}atb-potsdam.de).

Received for publication October 5, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Emissions of carbon monoxide (CO) were observed from decomposing organic wastes and litter under laboratory, pilot composting plant, and natural conditions. Field studies included air from inside a compost heap of about 200 m3, emissions from composting of livestock wastes at a biologically operating farm, and leaf litter pile air samples. The concentration of CO was up to 120 µmol mol–1 in the compost piles of green waste, and up to 10 µmol mol–1 in flux chambers above livestock waste windrow composts. The mean CO flux rates were approximately 20 mg CO m–2 h–1 for compost heaps of green waste, and varied from 30 to 100 mg CO m–2 h–1 for fresh dung windrows. Laboratory studies using a temperature and ventilation-controlled substrate container were performed to elucidate the origin of CO, and included hay samples of fixed moisture content at temperatures between 5 and 65°C, including nonsterilized as well as sterilized samples. The concentration of CO was up to 160 µmol mol–1 in these experiments, and Arrhenius-type plot analyses resulted in activation energies of 65 kJ mol–1 for thermochemically produced CO from the nonsterilized compost substrate. Sterilized samples showed dramatically reduced CO2 but virtually unchanged CO emissions, albeit at a slightly lower activation energy, likely a result of the high-temperature sterilization. Though globally and regionally these CO emissions are only a minor source, thermochemically produced CO emissions might affect local air quality in and near composting facilities.

Abbreviations: ATB, Leibniz Institute for Agricultural Engineering Potsdam-Bornim • BMU, Bundesministerium für Umwelt, Naturschutz, und Reaktorsicherheit (Federal Environment Ministry of Germany) • FT-IR, Fourier-transform infrared spectroscopy • MPI, Max-Planck-Institut • PLS, partial least squares • UBA, Umweltbundesamt (Federal Environment Office of Germany)


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
IN 1993, the German federal government passed new legislation called the "Technical Instruction on Municipal Waste" (BMU, 1993) that would eventually mandate organic waste composting. As a result, composting has become an important way for municipal garbage processing in Germany. Composting has been accepted by the public as an environmentally friendly way of organic waste processing. However, in 1993, few figures were available on the emissions of ammonia (NH3) and greenhouses gases, such as nitrous oxide (N2O), and methane (CH4), from composting. Numerous studies have since been performed on greenhouse gas emissions from several different composting processes and technologies, and the literature review by Zeman et al. (2002) summarized previous findings. During the actual composting procedure, nitrous oxide and methane emissions are strongly dependent on compost scale and process management, as recently documented by several authors (Beck-Friis et al., 2000; He et al., 2000; Sommer and Möller, 2000; Hao et al., 2001; Monteny et al., 2001; Clemens and Cuhls, 2003; Fukumoto et al., 2003; Thompson et al., 2004; Hobson et al., 2005; Paillat et al., 2005). Methane is a main product of anaerobic methanogenesis, while nitrous oxide is a by-product of denitrification, which follows nitrification-produced nitrate in the substrate. Hence, substrate quality and quantity, its aeration rate, and the duration of composting influence these emissions. A sufficient aeration rate during aerobic composting generally minimizes CH4 emissions but may not reduce N2O emissions.

At the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB), research on emissions during composting was first launched in 1994 on a laboratory scale and at pilot plants to investigate leaching and emissions from compost heaps in the frame of landscape management and conservation. To analyze the concentrations of NH3, N2O, CH4, and other gases in compost air, a high resolution Fourier-transform infrared spectroscopy (FT-IR) measuring system was chosen for laboratory studies (Hellebrand and Kleinke, 1996, 1997). Additionally, this system was used for gas analysis from field measurements using gas sampling bags. Results reported earlier (Hellebrand, 1998; Hellebrand and Kalk, 2001a) are in line with the literature cited above.

The FT-IR measurement technique was chosen due to its ability to distinguish between the greenhouse gases N2O, CH4, and CO2 on the one hand and on the other hand, NH3 as a typical agricultural emission. High resolution FT-IR can analyze complex gas samples, unambiguously identifying infrared active trace species in the µmol mol–1 range on the basis of their absorption spectra. As an alternative and at concentrations below 1 µmol mol–1, gas chromatography can be used for the greenhouse gases. However, for NH3 measurement, another method would have been necessary. In addition, high concentrations of NH3 may interfere with the operability of or contaminate GC systems. In comparison, FT-IR also enables the evaluation of measured and stored spectra for other trace gases later on, which initially were not of interest.

A second check of the recorded FT-IR spectra from compost air in the course of our compost studies revealed previously unidentified absorption lines near the intensive CO2–absorption band (2060–2100 cm–1, Fig. 1 and 2 ). Detailed analysis showed that these lines reflected CO in the mid to high µmol mol–1 range (10–100 µmol mol–1), which was not expected for green waste compost air. The findings of our first evaluation were included in a publication on composting (Hellebrand, 1998), and a more detailed search on the sources of CO during composting started. At that time, no other study on CO emissions from composting had been published. The aim of the study presented here is therefore to characterize the type and magnitude of the source of CO during composting.


Figure 1
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  		Fig. 1. Fourier-transform infrared spectroscopy (FT-IR) spectrum of ambient air between 600 and 4000 cm–1 (reverse scale increases with wavelength; measured with 20 m long path gas cell). The IR lines of the strong absorbers CO2 and H2O stand out. Sections with absorption lines of environmentally relevant trace gases are marked by circled numbers: 1: CO; 2: NH3; 3: N2O; 4: CH4.

 

Figure 2
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  		Fig. 2. Example section of Fourier-transform infrared spectroscopy (FT-IR) absorption spectrum of an air sample during laboratory scale composting of hay (mixed herbage from landscape conservation with a moisture content (w.b.) of 0.7 x 10–3 m3 kg–1) amended with calcium ammonium nitrate (resolution 0.2 cm–1, temperature 35°C, aeration rate 17 cm3 min–1 kg–1 substrate). Circled numbers indicate sections analyzed in detail: 1: Superposition of CO2, N2O, and CO; 2: Mainly CO lines; 3: Mainly CO2 lines; 4: Minimally disturbed CO lines.

 
Carbon Monoxide on a Global Scale
To put the potential sources of CO during composting into context, we briefly review the existing knowledge on biogenic CO sources. CO is an air pollutant and toxic gas. Its dominantly anthropogenic sources are incomplete combustion of fossil fuels and biomass (1350 Tg; IPCC, 2001), and photochemical oxidation of atmospheric hydrocarbons (1230 Tg). Direct emissions from vegetation (ca. 150 Tg; Sanderson, 2002) and the ocean (ca. 50 Tg) contribute on a smaller scale. Though CO is not a direct greenhouse gas, emissions of CO have indirect effects on climate through enhanced levels of tropospheric O3 and CH4 as a result of its reaction with hydroxyl radicals (Crutzen, 1974; Logan et al., 1981; Fuglestvedt et al., 1996; Daniel and Solomon, 1998). Therefore, Working Group I of the Intergovernmental Panel on Climate Change included CO in its latest report (IPCC, 2001).

The mean atmospheric CO abundance is approximately 80 nmol mol–1 with an atmospheric lifetime of 1 to 3 mo (Daniel and Solomon, 1998). The Northern Hemisphere contains almost twice as much CO as the Southern Hemisphere, mostly due to anthropogenic emissions. The most important sink for CO is the reaction with hydroxyl radicals (Logan et al., 1981), with a smaller sink via soil uptake (Conrad, 1996). The latter results from soil microbial processes (Conrad and Seiler, 1980; Conrad et al., 1981; Conrad and Seiler, 1985a; Scharffe et al., 1990; Conrad, 1996), whereas simultaneously occurring CO production in soils has been assigned to thermochemical oxidation of organic matter (Conrad and Seiler, 1985a, 1985b; Seiler and Conrad, 1987; Zepp et al., 1997). Although the global budget of CO is fairly well understood, there are significant uncertainties associated with the CO fluxes from natural sources.

Direct and Indirect Biogenic Sources of Carbon Monoxide
Atmospheric CO from biogenic sources has different sources—direct CO release from plants (Bauer et al. (1979), and reference therein), production or consumption by specialized microorganisms (Radler et al., 1974; Conrad and Seiler, 1980, 1985a; Conrad, 1988, 1996; Scharffe et al., 1990; King, 1999, 2003), generation by photochemical and thermochemical oxidation of biomass (McConnell et al., 1971; Fischer and Lüttge, 1978; Lüttge and Fischer, 1980; Conrad and Seiler, 1985b; Tarr et al., 1995; Kanakidou and Crutzen, 1999; Schade et al., 1999; Kisselle et al., 2002; Varella et al., 2004), and by oxidation of CH4 and of non-methane hydrocarbons (NMHC) of biogenic origin, such as isoprene (McConnell et al., 1971; Crutzen, 1974; Novelli et al., 1992; Miyoshi et al., 1994).

The production of CO from plant litter (Sanhueza et al., 1994, 1998; Tarr et al., 1995; Zepp et al., 1996a, 1996b, 1997; Hellebrand, 1998; Kisselle et al., 2002; Varella et al., 2004;) in the dark is most likely caused by thermochemical oxidation processes, and has previously been studied in more detail (Schade, 1997; Schade et al., 1999). Thermochemically (or thermally induced) CO production from grass and leaf litter obeys the Arrhenius law with activation energies mostly between 60 and 90 kJ mol–1, with strong evidence that CO is not produced biologically (Schade, 1997; Schade et al., 1999). The authors estimated global CO emissions due to thermochemical oxidation of organic carbon in plant litter of 40 Tg CO (Schade and Crutzen, 1999). Uncertainties are related to the fact that thermal CO production does not only depend on temperature, but also on type of plant litter, moisture content, and oxygen availability. Increasing moisture content and oxygen availability generally promote thermochemically caused CO production (Schade, 1997).


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Fourier-Transform Infrared Spectroscopy Measuring System
Trace gas analysis was performed with a high resolution FT-IR spectrometer (Spectrum 2000, PERKINELMER). The system was equipped with two heated, long path multi-pass gas cells (FOXBORO-Invensys Process Systems). The temperature of the cells was kept at 80°C to avoid condensation and adsorption of gaseous organic and nonorganic trace components from compost air, especially on the gold-coated mirrors of the cells. The primary gas cell had a variable optical path length of up to 20 m, a measuring volume of 5.4 x 10–3 m3, and was used for quantification when samples with at least this volume were available. The second gas cell had a fixed optical path length of 7.2 m (48 passes with a base of 0.15 m), a volume of only 0.5 x 10–3 m3, and was used in experiments with limited sample size or to achieve sufficient time resolution for continuous flow measurements under changing gas composition.

The FT-IR spectrometer used here operates as a so-called one-beam system. To get a pure sample spectrum, at first the background spectrum (evacuated gas cell) must be measured. Then a sample spectrum is obtained by a second measurement with sample air in the gas cell. To minimize stochastic and systematic errors, the FT-IR spectrometer housing was purged with dry, CO2–free air (BALSTON FT-IR Purge Gas Generator; –73°C dew point). Separate mercury-cadmium-tellurium alloy (MCT) detectors for each gas cell were used to optimize sensitivity. Generally, full spectra were recorded with 64 scans.

A constant range (600 to 4000 cm–1) and fixed-point intervals (0.05 cm–1) were applied for calibration and for all quantitative measurements. The detection limit was better than 10 nmol mol–1 for CO in nitrogen inside the 20 m gas cell (0.2 µmol mol–1 for 7.2 m gas cell). Because of absorption line overlapping in the case of compost air containing concentrations of CO2 above 50 mmol mol–1, N2O above 300 µmol mol–1, and water vapor saturation, the detection limit for CO deteriorated by a factor of 5 to 10 dependent on compost air composition (Fig. 2). The systematic error in the determination of CO concentrations due to these interferences was estimated to be less than 20% for CO concentrations below 1 µmol mol–1 (20 m cell) and approximately 10% for CO above 10 µmol mol–1. The error depends on the type of gas and the concentration span to be evaluated (line positions and changes in line intensities), on calibration and spectrum evaluation, and on measurement procedure. Errors of the FT-IR measurements of gas samples, caused by electronic noise, gas handling, and other stochastic sources had a coefficient of variation of 5%.

Fourier-Transform Infrared Spectroscopy Calibration and Concentration Measurements
Spectrum Quant+ software (PERKINELMER) was applied for concentration evaluation of the FT-IR spectra. The software operates with chemometric procedures (PLS, partial least squares) based on inputs from calibration spectra, a method developed by Wold (1966). Cross validation was used. For baseline corrections, first derivatives were chosen. The spectra had to contain the range of gas concentrations of the gas (or gases) to be evaluated and, if possible, different concentration levels of interfering gases such as water vapor. In addition, during the development of the calibration method the most suitable wave number ranges for the calibration procedure had to be input, or in other words, nonrelevant and interfering spectral regions were to be "blanked." In the calibration methods developed here, 15 to 30 different calibration spectra over the range of observed CO mixing ratios were measured.

For multi-gas analysis, the concentration distribution with the maximum mutual linear independency was calculated. Then, either specific instrument calibration mixtures (Linde HiQ Specialty Gas) were applied or, in most cases, calibration gas mixtures produced using plastic bags (Linde Plastigas) with a volume of 10 x 10–3 m3 were used. The latter were filled with nitrogen or ambient air of known composition and defined quantities of pure gases added by micro-liter syringes through the septum of the bags. It was found that the best calibrations were achieved using the PLS1 algorithm of the software and blanking all spectral parts except for regions with no or only weakly overlapping spectral lines. These lines were determined by qualitative measurements with variable concentrations and by spectrum simulation using USF-HITRAN-PC (HITRAN, 1992). The highest accuracy was achieved when the standard lines and sample lines did not differ in magnitude of absorbance. Although it is possible to develop a calibration method, which works for example, for CO2 from 200 µmol mol–1 to 200 mmol mol–1, usually strong deviations between predictions and actual concentrations will be measured on the low or high end of the concentration range. Therefore, intensity adopted calibration software tools had to be developed and applied for this compost air study.

Carbon Monoxide Flux Measurements
For laboratory experiments, the plant material (1 kg sun-dried mixed herbage from landscape management and conservation) was wetted up to a moisture content (wet basis, WB) of approximately 0.7 x 10–3 m3 kg–1 by incubating with 2 L of water for 24 h. Composting studies were performed with nitrogen amendments, and specific CO studies at a later date without any amendment except water. The wetted plant material was then placed into a temperature-controlled, ventilated chamber with a volume of 36 x 10–3 m3 applying a constant air exchange rate (Hellebrand, 1998). Substrate temperature was monitored with thermocouples in the center, top, and bottom of the chamber. Experiments were performed to study emission rates of different gases as influenced by substrate temperature, moisture content, nitrogen content, and chamber aeration rate. The latter was manually regulated via a peristaltic pump and viscosity flow meters and the exact rate was measured by a drum gas meter (Ritter). The composition of the input air and of the air leaving the chamber was analyzed by FT-IR. Gas production rates were calculated on the base of volume flow rates and concentration differences.

Green Waste Compost Heaps
For the composting of green waste, a typical trapezoidal compost heap was set up using mixed green litter from landscape management and conservation. The initial dimensions of the compost heap were 11 m x 7 m x 3 m high. The initial fresh mass of the compost heap was 14,800 kg with a carbon content of 4300 kg. Its mass was measured by means of a trailer and two axle balances (HAENNI, Model WL 103) with a resolution of 20 kg and an accuracy of 50 kg at maximum load of 25,000 kg. Mass and carbon content were determined at the start of the experiment, repeated following heap turnover after 32 and 70 d, and at the end of composting after 194 d. The carbon content of the material was analyzed by means of three mixed samples of the compost material using a CN analyzer (Elementar Analysensysteme GmbH, Model VARIO EL III). The initial carbon mass decreased to 730 kg after rotting. Moisture content and temperature profiles of the heap were measured weekly at each compost air measuring day (Kleinke et al., 1996). Compost air samples from inside the heap were sampled through inserted polyethylene pipes by a small diaphragm pump into gas bags of 10 x 10–3 m3 volume each (Fig. 3 ). Total emissions were estimated by calculating the mean concentration from measurements at nine inserted tubes in connection with weighing and CN analysis of the rotting compost material (Hellebrand, 1998).


Figure 3
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  		Fig. 3. Schematic of the compost heap with inserted polyethylene tubes, and the variation of CO concentrations measured in samples extracted through them. Arrows indicate dates of turning the compost.

 
Dung Windrows
The CO flux from dung windrows was calculated from concentration increases inside static gas flux measuring chambers, which had had a volume (V) of 0.189 m3 and a bottom area (A) of 0.292 m2. Preliminary studies using CO2 sensors with data loggers (Testo AG, Model 950) and a photo-acoustic multi-gas monitor (Brüel & Kjær, Model 1302) indicated that a sampling time of 10 min was sufficient for a flux measurement under most circumstances (Hellebrand and Kalk, 2000, 2001a, 2001b). Samples were acquired into 10 x 10–3 m3 gas bags as described above.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Carbon Monoxide from Green Waste Compost Heap, Dung Windrows, and Leaf Litter
Originally, the different studies on composting of green waste and dung windrows were not performed to measure CO emissions, but served for the evaluation of environmental aspects of composting (Kleinke et al., 1996; Hellebrand and Kalk, 2001a). However, in both cases FT-IR spectroscopy was applied for trace gas analysis, and thus, after CO emissions during composting had been identified, the spectra were re-evaluated for CO absorption lines and CO flux determination. As shown in Fig. 3, already shortly after establishing the green waste compost heap, highly elevated CO concentrations were observed at all measuring tubes. During the initial phase, little to no methane was produced and denitrification rates were low as indicated by low N2O concentrations. As shown in Fig. 3, this mostly aerobic composting period showed the highest CO concentrations. Just after turnover and partial collapse of the compost heap, highest CO concentrations were observed at the top measuring tubes 1, 4, and 7, and lowest CO levels were measured at the bottom part of the heap (pipes 3, 6, and 9 in Fig. 3), consistent with a typical, passive aeration, i.e., O2 availability gradient inside a compost heap. The previously published results for CH4 and N2O concentrations at this heap (Hellebrand, 1998) showed that anaerobic conditions developed toward the middle and bottom of the heap after it was first turned over and had partially collapsed. Higher CO concentrations near the top of the heap after its turnover possibly occurred as a result of re-aeration of previously slightly anaerobic parts as indicated by the measured N2O levels. The experimental results from the compost heap support earlier results that CO production depends on the availability of oxygen. Over the 6 mo of composting, 3570 kg carbon was released as CO2–C and an estimated 1.7 kg as CO-C (approximately 0.5 g kg–1 with reference to the initial carbon mass). The mean CO flux of 20 mg CO m–2 h–1 was estimated based on the overall mean of CO, CO2, and CH4 concentrations and the total decrease of carbon content of the heap, and is given in relation to the estimated mean surface area of the heap, calculated from dimensional changes between onset and heap turnover (Hellebrand, 1998).

Carbon monoxide emissions were also measured during composting of animal farm waste. At fresh windrows of animal waste (dung), the measured flux rates ranged from 30 to 100 mg CO m–2 h–1. After a period of 1 to 2 wk, emissions dropped to very low levels, and during early spring and late fall uptake was occasionally recorded when the ambient CO mixing ratio was higher. The accumulated CO emissions during composting of farm waste were calculated by interval integration. Per composting period, total fluxes were between 1.6 and 3.7 g m–2. Six periods between 7 wk and up to 3 mo of windrow composting of farm waste were evaluated. The mean total flux of CO was 3 g m–2 and a mean value of 0.4 mmol mol–1 was calculated for the ratio between total CO-C and CO2–C fluxes.

As a comparison to the above studies, we also sampled air from the center of heaps of fallen leaves and heaps of mowed grass in different parks of Potsdam, Germany, similarly as described above. The measured CO concentrations ranged from 3 to 22 µmol mol–1, similar to the in-depth compost heap study (Fig. 3), and reaffirmed our findings about significant CO emissions during biomass decomposition.

Carbon Monoxide from Sterilized and Nonsterilized Hay
After evidence of CO emissions from composting had been established, the influence of temperature and aeration rate on emissions was determined by laboratory measurements using sun-dried, mixed grass clippings (hay). The results were puzzling at first, as they showed an initial maximum of CO a few hours into the experiment (Fig. 4 ), dropping then to much lower levels. In the case of substrate temperatures of 35 and 50°C, a slight increase and subsequent drop of the CO concentration was observed after about 100 to 200 h measuring time.


Figure 4
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  		Fig. 4. Temperature influence on CO concentration trends in the substrate container containing degrading herbage mixture from landscape conservation with a moisture content (w.b.) of 0.7 x 10–3 m3 kg–1. The ventilation rate was 25 cm3 min–1 kg–1 substrate. 35°C was measured twice.

 
The initial maximum likely resulted from a build-up of CO generated in the closed container during a time with limited aeration between container preparation and the start of measurements. In several experiments, aeration rates were too low to keep the substrate aerobic during periods of high microbial activity (note high CO2 concentrations for about 100 to 200 h and substrate temperatures above 20°C, Fig. 5 ). The increasing O2 consumption rate, caused by the growing microorganism population, then probably resulted in an oxygen gradient inside the substrate with partially anaerobic zones. If CO emissions are indeed of thermochemical instead of biochemical origin, the previously established emission dependence on O2 availability and wetness (Schade, 1997) predicts a higher initial emission when the material is wet and has not consumed the chamber's O2 significantly yet. As there is only a slight change in wetness of the substrate in the closed container within the first days of container measurements (calculated water loss rates did not exceed 35 g H2O d–1 at 65°C assuming saturation vapor pressure), CO emissions likely dropped with high compost decay rates because the compost substrate became partially anaerobic. The CO emissions rose again with dropping composting rates, possibly due to aeration again becoming sufficient (second CO maximum at both 35 and 50°C, Fig. 4). Toward the end of the container composting experiment CO emissions decreased very slowly, possibly a result of material drying and a reduction of the molecular sources of CO emissions in the degrading plant matter (Schade, 1997; Schade et al. (1999), and references therein).


Figure 5
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  		Fig. 5. Temperature influence on CO2 concentration trends in the substrate container containing degrading herbage mixture from landscape conservation with a moisture content (w.b.) of 0.7 x 10–3 m3 kg–1. The ventilation rate was 25 cm3 min–1 kg–1 substrate. 35°C was measured twice.

 
To test the thermochemical CO emission theory, the filled substrate container, separated by bacteria-tight 0.2-µm filters from ambient air, was sterilized in an autoclave at 136°C for 3 h. The subsequent measurements of CO2 inside the chamber (previously 200 vs. now 2 mmol mol–1 [Fig. 6 and 7 ]) demonstrated the effectiveness of the sterilization, while, at the same time, nearly unchanged CO levels showed evidence of thermochemical oxidation as the dominant CO emission process.


Figure 6
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  		Fig. 6. Carbon monoxide and CO2 concentration variation in air exiting the substrate chamber containing degrading mixed herbage from landscape conservation (without sterilization, moisture content (w.b.) of 0.7x10–3 m3 kg–1, temperature 50°C, ventilation rate 25 cm3 min–1 kg–1 substrate).

 

Figure 7
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  		Fig. 7. Carbon monoxide and CO2 concentration variation in air exiting the substrate chamber containing degrading mixed herbage from landscape conservation (sample sterilized for 3 h at 136°C, moisture content (w.b.) of 0.7x10–3 m3 kg–1, temperature 50°C, ventilation rate 25 cm3 min–1 kg–1 substrate).

 
The levels of CO and CO2 inside the chamber were dependent on temperature. An Arrhenius plot of calculated formation rates after the initial minimum (mean values of the periods from 96 to 144 h) resulted in an activation energy of 65 kJ mol–1 (CO) In experiments with sterilization, the Arrhenius plot of production rates (mean values of the periods from 44 to 144 h) gave activation energies for the thermochemical oxidation of 53 kJ mol–1 for CO and 31 kJ mol–1 for CO2 (Fig. 8 ), in line with previous findings (Schade, 1997; Schade et al., 1999).


Figure 8
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  		Fig. 8. Arrhenius plot of CO and CO2 production rates (sterilized wet substrate, ventilation rate 25 cm3 min–1 kg–1 substrate) between 278 and 338 K. The exponential approximation functions and determination coefficients are given in the figure. The activation energy results from multiplication of the exponential factor with the gas constant.

 
A recent report by Harstad et al. (2006) also showed CO production during the composting process. While the authors concluded from their data that CO production is higher under low oxygen conditions, it appears likely that CO was formed instead from anaerobic bacteria in their system as evident from very high methane concentrations. Nevertheless, the results of Harstad et al. (2006) are largely in agreement with our findings.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Very high concentrations of CO have been observed inside dedicated composting and other heaps of degrading organic material. Flux chamber measurements confirmed that CO is emitted to the atmosphere from the composting process and its molar ratio to simultaneously emitted CO2 lies between 0.1 and 1 mmol mol–1. Furthermore, our laboratory and field measurements confirmed that the production process of CO is probably independent of microbial activity in the substrate, but is promoted by increased temperatures and needs oxygen.

Our results are also consistent with the conclusions of Conrad and Seiler (1985b) and Schade (1997; Schade et al., 1999) on organic matter CO production, and reconfirm that the slow decomposition is a source of atmospheric CO. Furthermore, the results confirm that, in principle, CO emissions can be calculated from the activation energy and the pre-exponential factor calculated from Arrhenius plots based on laboratory measurements (Schade and Crutzen, 1999). Previously estimated global CO emissions from this process (~ 40 Tg CO yr–1) were low compared to the dominant anthropogenic sources of CO, such as fossil fuel combustion. However, they may often occur in regions that receive comparatively low inputs of anthropogenic CO emissions, particularly in the tropics outside the burning season (Sanderson, 2002).

The specific composting processes evaluated here are commonly performed in Germany for a multitude of organic wastes, including yard waste and municipally collected household biowaste. Composting facilities in Germany that use the more common aerobic composting process handled an annual throughput of ~ 7.2 x 109 kg of fresh waste in 2003 (Statistisches Bundesamt, 2005). Based on a water content of 0.5 x 10–3 m3 kg–1 and an assumed average carbon mass loss during the composting process of approximately 0.75 kg kg–1, we estimate that between 3 and 30 x106 kg CO-C may be emitted into the atmosphere from biowaste composting in Germany. That is, compared to dominant road traffic emissions of 1.7 x 109 kg CO per year (UBA, 2005), a relative amount of less than 0.01, and therefore likely not contributing significantly to regional tropospheric ozone formation. Although negligible compared to dominant road traffic emissions of CO, emissions from the composting facilities may contribute significantly to local air pollution under stagnant meteorological conditions, and to elevated CO levels inside and near composting facilities.


   ACKNOWLEDGMENTS
 
We are grateful to Dr. Christine Idler (ATB) for providing and operating the autoclave used for sterilizations in this study. We also thank Professor Ralf Conrad (MPI Marburg) for helpful discussions about how to interpret the laboratory measurements.


   NOTES
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