Environ. Sci. Technol. 2001, 35, 2347-2351
Nitrous Oxide Emissions from Aerated Composting of Organic Waste Y A O W U H E , * ,† Y U H E I I N A M O R I , † MOTOYUKI MIZUOCHI,† HAINAN KONG,† NORIO IWAMI,† AND TIEHENG SUN‡ National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 3050053, Japan, and Institute of Applied Ecology, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110015, P.R. China
The composting of high organic content wastes has been shown to produce nitrous oxide (N2O). This study was initiated to investigate the mechanisms of N2O emissions from aerated composting and to determine the optimal operational conditions that minimize N2O emissions. The results of our experiment in laboratory-scale composters showed that more than 95% of N2O was produced during the later period of composting when readily available carbon sources had been depleted. Significant increases in N2O emission after nitrite (NO2-) addition, and good NO2- N2O correlation, indicates that N2O was transformed from NO2-. Extremely high N2O generation was observed after NO2- addition in the presence and absence of composted cattle manure. This suggests an identical mechanism for N2O production in both treatments. However, the addition of composted cattle manure resulted in an earlier initiation of the main N2O generation period. Intermittent feeding of fresh food waste postponed the main N2O generation period, and reduced the mass-based N2O emissions by 20%.
Introduction Interest in the sources of atmospheric N2O has recently been stimulated by the recognition that this trace gas exerts significant influence on the chemistry of the stratosphere and on the earth’s thermal balance. Reaction of N2O with singlet atomic oxygen represents the dominant source of nitric oxide in the stratosphere which plays a catalytic role in ozone destruction (1, 2). The high efficiency of N2O in absorbing infrared radiation (IR) also makes it an important greenhouse gas (3). Though atmospheric N2O accounts for only 6% of the greenhouse effect, the rapid increase of its mixing ratio in the atmosphere (currently at a rate of 0.250.31%/year) (4), has drawn attention to the need for research on all sources and sinks of this gas. In the global inventory of N2O, natural sources such as undisturbed soil, photolytic processes in the ocean, and atmospheric formation are estimated to account for about 60% of total N2O emissions (4, 5). Significant anthropogenic sources including agricultural and industrial activities release * To whom correspondence should be addressed. Current address: School of Biological Sciences, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, Australia. E-mail: li.li@ flinders.edu.au. † National Institute for Environmental Studies. ‡ Institute of Applied Ecology. 10.1021/es0011616 CCC: $20.00 Published on Web 04/19/2001
2001 American Chemical Society
5.5 Tg N2O-N/year, about 30-35% of the total emissions (5); however, this estimation does not include N2O production from waste management systems. Although the treatment of organic wastes has been frequently reported to generate N2O (6, 7), the total emission from these processes remains poorly quantified. Nitrous oxide is a byproduct of nitrification and denitrification, which are the main mechanisms for nitrogen removal in waste treatment processes; therefore, it is not surprising to detect high N2O emission from a variety of waste treatment facilities. Zheng et al. (8) reported that in anoxic treatment of wastewater 0-19% of nitrogen was removed in form of N2O. In activated sludge systems, Mizuochi et al. (9) and Hanaki (10) observed a production of 3 and 24.1 mg of N2O-N, respectively, from the treatment of 1 m3 wastewater. Apparent N2O emissions were also detected from treatment of solid waste. Tsujimoto et al. (7) reported emissions of 7.8 and 40.2 g‚day-1 from an active and a closed landfill site, respectively. Availability of oxygen is a determining factor in N2O production. Nitrification under strict aerobic condition and denitrification under strict anaerobic condition result in negligible N2O production. N2O is mainly formed at moderate O2 concentrations (6, 7). Up to now, most solid organic waste has been landfilled or incinerated; however, more and more concern has arisen in recent years because of leachate problems and air pollution associated with these systems. In particular, significant emissions of greenhouse gases were detected from these processes (7,11-13). In view of this, aerobic composting has been suggested as a more acceptable alternative to landfill disposal since the predominant aerobic environment involved can mitigate greenhouse gas emission by reducing methane generation (14). Nevertheless, the potential of N2O production from aerobic composting is unknown. In a practical scale sludge composting study, Czepiel et al. (6) found that N2O flux was a function of the compost age, pile depth, temperature, and water filled pore space. Aeration was reported to increase N2O production. The present paper describes the results of a comprehensive study conducted in laboratory-scale reactors to quantify N2O generation from the aerated composting of food waste. The effects of several factors were investigated to identify the main parameters controlling N2O emission. The resulting emission rate data were then used to optimize the operational conditions for aerated composting to minimize N2O emission.
Experimental Section Waste and Amendments (Table 1). To keep the uniformity of food waste composition, synthetic food waste was used instead of practical pre- and postcustomer garbage. It was fortified according to a Standard Composition for Food Waste used by Ministry of Construction, Japan, which contains chicken bone (8.16%, by weight), fish (10.20%), apple (10.20%), banana peel (10.20%), grape peel (10.20%), cabbage (18.38%), carrot (18.38%), rice (10.20%), and tea residue (4.08%). Chicken bone, fish, and rice were boiled. All materials were reduced in size by a disposer (Emerson Electric Co.). The food mixture was stored in the dark in a freezer prior to use. A commercial sawdust-based product of National Ltd. (Japan), Biochip, was used as the bulking agent. In some reactors composted cattle manure (CCM) purchased from Joriku Ranch (Ibaraki, Japan) was added, in simulation of the compost addition process in some small-scale composters in Japan, to investigate its effect on N2O generation. Composting Unit. Cylinder-shaped composters (18 L) were used in the study. As shown in Figure 1, waste and VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Amount of Food Waste and Amendments (fresh weight) in Different Experiments
experiments reproducibility test single-charged composters intermittent-charged composters effect of aeration rates effect of NO2- addition
food waste (kg)
composted cattle manure (kg)
sawdust (kg)
replicate
aeration rate (L‚min-1‚kg-1a)
2.0 2.0 2.0 2.0 2.0 + 4.0 + 6.0b 2.0 2.0 2.0 2.0
0.0 1.0 0.0 1.0 1.0 1.0 1.0 0.0 1.0
1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
2 2 2 2 2 2 2 2 2
0.70 0.70 0.70 0.70 0.70c 0.14 0.56 0.70 0.70
a Liters per minute per kilogram of initial dry weight of food waste. b On days 5 and 14, 4.0 and 6.0 kg of fresh food waste were added into the composters. c After readdition of food waste, aeration rate was increased correspondingly.
FIGURE 1. Schematic of laboratory composting apparatus. amendments were thoroughly mixed in the composter by slow stirring (0.8 rev‚min-1) of the motor. Four mixing arms were fixed to the central shaft and at the ends of two adjacent arms a connection plate was installed with only a narrow space left in the middle for the thermocouple. Such a design enabled the mixing paddles to move across the composter to guarantee a uniform mixing of waste and amendments. Aeration was achieved by continuously compressing air into the composter through a false floor. To mitigate effects of ambient temperature fluctuation on composting, the composter was kept in a thermostatic chamber throughout the testing period. Moreover, two electrical heaters were used, one to heat the aeration flow, the other to warm a 0.5 L‚min-1 airflow passed through the space between composter and the wall of chamber to establish a thermostatic environment. A trough was installed beneath the false floor to collect leachate which was then recycled into the composter. A thermocouple buried into the waste mixture automatically measured temperature. Gas Sampling and Analysis. Air samples were collected at 12-h intervals within the first week. After day 8, sampling frequency was gradually reduced to a rate of once in 3 days. A battery-operated pump (Sibata MP-2N, Japan) drew the headspace samples into aluminum bags. Samples were immediately transported to the lab, and allowed to equilibrate to room temperature and analyzed by a Shimadzu GC-14A gas chromatograph (Shimadzu Co., Japan) within 4 h. Samples and standards were cleaned-up across two glassmade columns packed with magnesium perchlorate (Nacalai Tesque, Inc., Japan) and ASCARITE II (Thomas Scientific), respectively, to remove moisture and CO2 as they were injected into the gas chromatograph injection loop. Analysis of N2O ((0.02 µL‚L-1) was accomplished using an electron capture detector after constituent separation by a Poropak Q column (2m × 3 mm i.d. stainless steel). The carrier gas was a 95% Ar-5% CH4 mixture at a flow rate of 40 mL‚min-1. Temperatures of detector and oven were 340 and 80 °C, respectively. O2, CO2, and NH3 concentrations in the exhaust air were measured by gas detector tubes (Gastec Co., Japan) packed with materials which change colors after reaction with O2, CO2, and NH3. The measurement was conducted by 2348
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eye at an accuracy of (0.1%, (0.1%, and (0.1 ppm for O2, CO2, and NH3, respectively. Waste Mixture Sampling and Analysis. Immediately after air sampling, the waste mixture was sampled from at least five different points in each composter and mixed thoroughly. Subsamples were then removed to determine pH and gravimetric water content. For analysis of NH4+-N, NO3--N, NO2--N, and TN (total N) concentration, 2.00 g of waste mixture was extracted by 20 mL of 2 M KCl solution for 30 min. After being centrifuged at 4000 rev‚min-1 for 10 min, the supernatant was filtered through glass fiber filter (0.45 µm, Whatman), analyzed by an automated colorimetric method on a TRAACS-800 instrument. To determine concentration of dissolved organic carbon (DOC) in waste, another 2.00 g subsample was extracted by 20 mL of water (Milli-Q SP TOC, Millipore) for 30 min, centrifuged at 4000 rev‚min-1 for 10 min, and filtered by glass fiber filter (0.45 µm, Whatman). The supernatant was stored at 4 °C and analyzed within 2 days on a TOC analyzer (Shimadzu TOC-5000, Japan).
Results Reproducibility of Composter Performance. The reproducibility of the performance of the composters was assessed by the analysis of oxygen and temperature profiles from two preliminary experiments, one with CCM the other without. In both experiments water content in the waste mixture was kept at around 65%. In the presence of CCM, both duplicates showed a 44-45 °C temperature peak at day 3 and a stabilizing temperature profile (30.6-32.3 °C) after day 10, whereas in the absence of CCM, temperature increased to 45-47 °C at day 5 and stabilized after day 11. The average difference between the replicate temperature profiles in two experiments was less than 2 °C throughout the composting period. Oxygen percentage in the offgases first decreased to 13 and 15% in treatments with and without CCM, respectively, and then returned to 19.1-19.5% after day 11. The average discrepancy between oxygen levels in offgases from composters with identical initial conditions was less than 0.5%. Analysis of the temperature and oxygen profiles showed that the composting system was able to provide reproducible test conditions for further experiments. Nitrous Oxide Emission. Significant N2O production was detected in all experimental runs. Figure 2 presents two typical N2O emission profiles versus composting time obtained in treatments with and without CCM, respectively. A N2O emission peak was observed immediately after the beginning of composting, and the peak value in treatments with and without CCM was 3.32 and 2.73 µL‚L-1, respectively. Charging of CCM increased the average peak value of N2O emission by 0.59 µL‚L-1. After 2 days, N2O concentration in the exhaust gas rapidly decreased to 0.53 µL‚L-1, slightly higher than the atmospheric background level of 0.45 µL‚L-1 (ranging from 0.41 to 0.47 µL‚L-1 during the composting
FIGURE 2. Profiles of N2O concentration in the exhaust gas versus composting time.
FIGURE 3. Profiles of N2O concentration in treatments with CCM at various aeration rates. period). From days 2 to 12, N2O production was low with an average emission rate of 3.38 µL‚h-1 from 1 kg food waste (dry weight). After day 12, N2O production from CCMamended treatments increased steadily and entered the second (main) production period. A peak of 202.67 µL‚L-1 was detected on day 36, then N2O generation decreased slowly to around 0.6 µL‚L-1 after day 100. In treatments without CCM, no significant change occurred in N2O production until day 55. Though a peak was detected at day 110, the average peak value was much lower compared with that in CCMapplied composters. Nitrous oxide generated during the main production period accounted for more than 95% of total emission throughout the experimental processes in trials with and without CCM. Effect of Aeration Rate. The effect of aeration rate on N2O production was studied by aerating composters with identical water contents and waste/amendment ratios at different aeration rate. As the aeration rate was reduced from 0.70 to 0.56 and 0.14 L‚min-1‚kg-1, N2O emission decreased significantly (Figure 3). The second N2O emission peak, which was observed at 0.70 L‚min-1‚kg-1 (Figure 2, treatments with CCM), was reduced by more than 90% at 0.56 L‚min-1‚kg-1 and did not appear at 0.14 L‚min-1‚kg-1. However, low aeration prolonged ammonification period and resulted in high ammonia volatilization (Figure 4), which would reduce the nutrient value of the composted product and cause more difficulties with odor control. Moreover, as the aeration rate was reduced methane production increased. The mass-based methane production was 1.72, 31.23, and 1328.94 mL‚kg-1 (dry weight), respectively, at 0.70, 0.56, and 0.14 L‚min-1‚kg-1. High methane production at 0.14 L‚min-1‚kg-1 offset its benefit in N2O emission control though IR absorbance of
FIGURE 4. Ammonia volatilization from composters aerated at different rates.
FIGURE 5. Changes of DOC in waste mixture and N2O concentration in the exhaust gas with composting time, the inset shows a plot of N2O concentration in offgas against available carbon content (n ) 111). CH4 is 10 times lower than that of N2O (9). Therefore, among the three aeration rates employed in the study, 0.56 L‚min-1‚kg-1 appeared to be the most suitable aeration rate if both N2O and CH4 emissions were considered. Effect of Intermittent Feeding of Fresh Waste. To assess the effect of carbon availability on N2O generation, DOC in the waste mixture was determined in parallel with gas analysis. In Figure 5, DOC in waste mixture increased sharply to 33.28 mg‚g-1 on day 3, showing a higher DOC production than removal, then decreased to 3.45 mg‚g-1 on day 10. During this period, N2O emission was relatively low, ranging from 0.45 to 3.32 µL‚L-1, most of N2O was produced after day 12 (Figures 2 and 5). A plot of N2O concentration against DOC content in the waste mixture (inset of Figure 5) showed no statistically significant correlation. However, high N2O production occurred while DOC content was low, indicating that N2O was mostly released after the depletion of available carbon source. The result implies that intermittent feeding of fresh waste in the presence of enough bulking agent and amendments, as it is widely used in commercially available composters which require sawdust addition once a month, may influence N2O production. To investigate the effect of intermittent feeding of waste on N2O generation, additional 4 and 6 kg fresh food waste were added at days 5 and 14, respectively, into two CCMamended treatments with an initial loading rate of 2 kg. Fresh waste was charged immediately after the pH in the waste mixture increased to 9.0. Similar DOC accumulationVOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Correlation between N2O concentration in offgas and NO2- content in waste mixture. FIGURE 6. Effect of intermittent feeding of food waste on N2O production consumption patterns were observed after addition of different amount of waste on days 0, 5, and 14 (DOC in waste mixture reached a maximum concentration of 31.88, 61.94, and 84.32 mg‚g-1 3 days after addition and returned to 6.18, 6.54, and 6.39 mg‚g-1 5 days after addition, respectively). The result suggested that sawdust used in the experiment was enough for removing organic carbon. A small N2O emission peak was detected immediately after each addition (Figure 6). Consequently, three emission peaks were obtained within 15 days in intermittent-charged composters, while in single-charged ones only one peak appeared during the same period. Nitrous oxide in the exhaust gas from single-charged composters began to increase after day 12 and reached a maximum concentration of 202.67 µL‚L-1 on day 36. In intermittent-charged composters, however, a significant increase in N2O production occurred on day 47 and reached a maximum concentration of 953.23 µL‚L-1 on day 70. The occurrence of the main production period was postponed in intermittent-feeding treatments. The mass-based N2O emission was 1.19 and 0.95L‚kg-1 (dry waste) for single-charged and intermittent-charged treatments, respectively, indicating a 20% reduction in N2O production in intermittent-feeding treatments. Nitrous Oxide-Nitrite Correlation. Ammonia, nitrite, nitrate, and TN concentration in waste mixture were monitored simultaneously with N2O, and the data was then utilized to formulate their relationships with N2O concentration in the exhaust gas. Significant increases in ammonia, nitrite, and nitrate concentrations in waste mixture occurred on days 8-10, 28-30, and 60-90, respectively, depending on the aeration rate and waste application pattern. The result suggested that nitrification was the predominant nitrogen transformation process in our system. A good linear correlation between nitrite and N2O concentration was found in all composting trials, independent of feeding patterns of waste and amendments (Figure 7). For ammonia, nitrate, and TN, however, no statistically significant correlation was obtained. To provide further evidence for the dependence of N2O production on nitrite in the aerated composting system, NaNO2 solution was added into composters to increase the theoretical nitrite concentration (calculated on the base of dry waste mixture) in waste mixture by 0.5 mg‚g-1. Six hours after addition of nitrite, N2O concentrations as high as 8838.83 and 1978.96 µL‚L-1 were detected in the offgases of the composters with and without CCM, respectively (Figure 8) and then sharply decreased to normal level (0.53-0.60 µL‚L-1) within 3 days. 2350
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FIGURE 8. Effect of nitrite addition on N2O production in composters in the presence (solid circles) and in the absence of CCM (open triangles).
Discussion Our results demonstrate the potential for significant N2O production from aerated composting of food waste. By microbial degradation, organic nitrogen was transformed into ammonia, leading to accumulation of ammonia in the waste mixture and high ammonia volatilization (Figure 4). Significant N2O emission started concurrently with ammonia oxidation, which was evidenced by the sequential occurrence of ammonia, nitrite, and nitrate accumulation. More than 95% of N2O was released during the later period of composting while available carbon had been used up (Figure 5). The time at which the N2O emission peak appeared depended on the waste application pattern and amendment. Intermittent feeding of fresh waste postponed its occurrence, whereas amendment of CCM resulted in a peak earlier in the composting cycle. The result implies that large amount of N2O can be released from old compost piles in which ammonia oxidation has started. Previous studies on aerobic wastewater treatment processes such as activated sludge and systems with high nitrification activity showed that high NO2- level was conducive to N2O production (8, 15-18). Similarly, linear NO2--N2O correlation was observed in our study, implying the existence of a nitrogen transformation pathway from nitrite to N2O in our system. Significant N2O emission after nitrite addition further certified that nitrite was one of main parent chemicals of N2O in the aerated composting process. The dependence of N2O production on nitrite was found both in the presence and absence of CCM, suggesting an identical mechanism for N2O production in both treatments.
In contrast to the results obtained previously in manure land application (19-21) and composting systems (22), no significant correlation was found between N2O production and ammonia, nitrate and total nitrogen. Aeration enabled the system to be predominantly an oxic environment, and reduction of nitrate was poor though anoxic/anaerobic microsites might exist in large waste particles. As a result of this, NO3- concentration as high as 3.53 mg‚g-1 detected in CCM-amended treatments at the end of composting did not cause apparent change in N2O generation. Poor NH4+-N2O correlation was possibly a consequence of the extensive period needed for ammonia oxidation. High N2O production occurred after the readily available carbon source was depleted. In intermittent-feeding composters, occurrence of the main N2O-production peaks was postponed from days 36 to 70. It’s unlikely the increased waste amount, which was expected to prolong DOC removal process, postponed N2O generation. With enough bulking agent, applying large amount of food waste did not prolong DOC removal process, as evidenced by the similar DOC accumulation-consumption patterns observed after readdition of different amount of waste in our study. Multiple feedings of fresh waste supported a continuous DOC production-consumption process and inhibited ammonia oxidation, and hence N2O production was postponed. In single-charged composters, a two-peak N2O emission curve was obtained after 5 months. The first peak observed immediately after feeding of fresh waste came from the release of N2O stored in food waste. Within 6 h, 200 g of food waste stored in refrigerator produced 0.75 µL of N2O according to our investigation. N2O was then sealed in the clump after being frozen and released after thawing. The first N2O emission peak in treatments with CCM was higher than that without CCM, as a result of N2O generation from the mature compost which was a known N2O generator(22-24). There is no direct evidence that CCM addition imported new N2O generation mechanisms based on the presently available data, but this study clearly showed that addition of CCM resulted in an earlier occurrence of N2O. In all experiments, a high N2O emission peak was found at the end of the composting process (Figures 2 and 3). Considering the good NO2--N2O correlation during this period and the immediate increase in N2O production after NO2- addition, N2O might be produced from NO2-. Usually this transformation step occurs under anoxic or anaerobic conditions (25, 26); however, aerobic denitrification has been frequently documented in recent years (27, 28). With the high aeration rates employed in the study, our composting system was kept predominantly aerobic throughout the experiment; however, anoxic or anaerobic microsites might still exist inside the waste particles similarly to the coexistence of aerobic and anaerobic microenvironment in aerobic soils (29, 30), denitrification conducted by denitrifiers in these microsites might contribute to NO2- reduction. If this is the case, N2O production should be proportional to the available carbon as shown previously (21, 31) because denitrifiers are mostly heterotrophs. However, most of the N2O was produced after the depletion of available organic carbon in our study, a result inconsistent with the active metabolism of denitrifiers. Considering the sequential appearance of NH4+, NO2-, and NO3- peaks, which proved the existence of nitrifiers, nitrifier denitrification appeared to be the predominant mechanism for N2O production in our system.
Acknowledgments The research was financially supported by Environmental Agency, Japan. We are grateful to Dr. Toshihiro Sankai and Ms. Keiko Kuto for their generous provision of equipment for food waste preparation, and Ms. Hideko Arai and Ms. Midori Toyama for their help on N, P analysis. Also, we wish to express our appreciation for the assistance of Dr. Nicholas McClure and Mr. Brett Roman in manuscript revisions.
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