Nitrous Oxide Emissions fromAerated Composting of OrganicWasteY A O W U H E , * , † Y U H E I I N A M O R I , †

M O T O Y U K I M I Z U O C H I , † H A I N A N K O N G , †

N O R I O I W A M I , † A N D T I E H E N G S U N ‡

National Institute for Environmental Studies, Onogawa 16-2,Tsukuba, Ibaraki 3050053, Japan, and Institute of AppliedEcology, Chinese Academy of Sciences, 72 Wenhua Road,Shenyang, 110015, P.R. China

The composting of high organic content wastes has beenshown to produce nitrous oxide (N2O). This study wasinitiated to investigate the mechanisms of N2O emissionsfrom aerated composting and to determine the optimaloperational conditions that minimize N2O emissions. Theresults of our experiment in laboratory-scale compostersshowed that more than 95% of N2O was produced during thelater period of composting when readily available carbonsources had been depleted. Significant increases inN2O emission after nitrite (NO2

-) addition, and good NO2- -

N2O correlation, indicates that N2O was transformedfrom NO2

-. Extremely high N2O generation was observedafter NO2

- addition in the presence and absence of compostedcattle manure. This suggests an identical mechanism forN2O production in both treatments. However, the addition ofcomposted cattle manure resulted in an earlier initiationof the main N2O generation period. Intermittent feeding offresh food waste postponed the main N2O generationperiod, and reduced the mass-based N2O emissions by20%.

IntroductionInterest in the sources of atmospheric N2O has recently beenstimulated by the recognition that this trace gas exertssignificant influence on the chemistry of the stratosphereand on the earth’s thermal balance. Reaction of N2O withsinglet atomic oxygen represents the dominant source ofnitric oxide in the stratosphere which plays a catalytic rolein ozone destruction (1, 2). The high efficiency of N2O inabsorbing infrared radiation (IR) also makes it an importantgreenhouse gas (3). Though atmospheric N2O accounts foronly 6% of the greenhouse effect, the rapid increase of itsmixing ratio in the atmosphere (currently at a rate of 0.25-0.31%/year) (4), has drawn attention to the need for researchon all sources and sinks of this gas.

In the global inventory of N2O, natural sources such asundisturbed soil, photolytic processes in the ocean, andatmospheric formation are estimated to account for about60% of total N2O emissions (4, 5). Significant anthropogenicsources including agricultural and industrial activities release

5.5 Tg N2O-N/year, about 30-35% of the total emissions(5); however, this estimation does not include N2O productionfrom waste management systems. Although the treatmentof organic wastes has been frequently reported to generateN2O (6, 7), the total emission from these processes remainspoorly quantified.

Nitrous oxide is a byproduct of nitrification and deni-trification, which are the main mechanisms for nitrogenremoval in waste treatment processes; therefore, it is notsurprising to detect high N2O emission from a variety of wastetreatment facilities. Zheng et al. (8) reported that in anoxictreatment of wastewater 0-19% of nitrogen was removed inform of N2O. In activated sludge systems, Mizuochi et al. (9)and Hanaki (10) observed a production of 3 and 24.1 mg ofN2O-N, respectively, from the treatment of 1 m3 wastewater.Apparent N2O emissions were also detected from treatmentof solid waste. Tsujimoto et al. (7) reported emissions of 7.8and 40.2 g‚day-1 from an active and a closed landfill site,respectively. Availability of oxygen is a determining factor inN2O production. Nitrification under strict aerobic conditionand denitrification under strict anaerobic condition resultin negligible N2O production. N2O is mainly formed atmoderate O2 concentrations (6, 7).

Up to now, most solid organic waste has been landfilledor incinerated; however, more and more concern has arisenin recent years because of leachate problems and air pollutionassociated with these systems. In particular, significantemissions of greenhouse gases were detected from theseprocesses (7,11-13). In view of this, aerobic composting hasbeen suggested as a more acceptable alternative to landfilldisposal since the predominant aerobic environment in-volved can mitigate greenhouse gas emission by reducingmethane generation (14). Nevertheless, the potential of N2Oproduction from aerobic composting is unknown. In apractical scale sludge composting study, Czepiel et al. (6)found that N2O flux was a function of the compost age, piledepth, temperature, and water filled pore space. Aerationwas reported to increase N2O production.

The present paper describes the results of a comprehen-sive study conducted in laboratory-scale reactors to quantifyN2O generation from the aerated composting of food waste.The effects of several factors were investigated to identifythe main parameters controlling N2O emission. The resultingemission rate data were then used to optimize the operationalconditions for aerated composting to minimize N2O emission.

Experimental SectionWaste and Amendments (Table 1). To keep the uniformityof food waste composition, synthetic food waste was usedinstead of practical pre- and postcustomer garbage. It wasfortified according to a Standard Composition for Food Wasteused by Ministry of Construction, Japan, which containschicken 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 materialswere reduced in size by a disposer (Emerson Electric Co.).The food mixture was stored in the dark in a freezer prior touse. A commercial sawdust-based product of National Ltd.(Japan), Biochip, was used as the bulking agent. In somereactors composted cattle manure (CCM) purchased fromJoriku Ranch (Ibaraki, Japan) was added, in simulation ofthe compost addition process in some small-scale compostersin 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

* To whom correspondence should be addressed. Current ad-dress: School of Biological Sciences, The Flinders University of SouthAustralia, GPO Box 2100, Adelaide 5001, Australia. E-mail: li.li@flinders.edu.au.

† National Institute for Environmental Studies.‡ Institute of Applied Ecology.

Environ. Sci. Technol. 2001, 35, 2347-2351

10.1021/es0011616 CCC: $20.00 2001 American Chemical Society VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2347Published on Web 04/19/2001

amendments were thoroughly mixed in the composter byslow stirring (0.8 rev‚min-1) of the motor. Four mixing armswere fixed to the central shaft and at the ends of two adjacentarms a connection plate was installed with only a narrowspace left in the middle for the thermocouple. Such a designenabled the mixing paddles to move across the composterto guarantee a uniform mixing of waste and amendments.Aeration was achieved by continuously compressing air intothe composter through a false floor. To mitigate effects ofambient temperature fluctuation on composting, the com-poster was kept in a thermostatic chamber throughout thetesting 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 andthe wall of chamber to establish a thermostatic environment.A trough was installed beneath the false floor to collectleachate which was then recycled into the composter. Athermocouple buried into the waste mixture automaticallymeasured temperature.

Gas Sampling and Analysis. Air samples were collectedat 12-h intervals within the first week. After day 8, samplingfrequency was gradually reduced to a rate of once in 3 days.A battery-operated pump (Sibata MP-2N, Japan) drew theheadspace samples into aluminum bags. Samples wereimmediately transported to the lab, and allowed to equilibrateto room temperature and analyzed by a Shimadzu GC-14Agas chromatograph (Shimadzu Co., Japan) within 4 h.Samples and standards were cleaned-up across two glass-made columns packed with magnesium perchlorate (NacalaiTesque, Inc., Japan) and ASCARITE II (Thomas Scientific),respectively, to remove moisture and CO2 as they wereinjected into the gas chromatograph injection loop. Analysisof N2O ((0.02 µL‚L-1) was accomplished using an electroncapture detector after constituent separation by a PoropakQ column (2m × 3 mm i.d. stainless steel). The carrier gaswas 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 exhaustair were measured by gas detector tubes (Gastec Co., Japan)packed with materials which change colors after reactionwith O2, CO2, and NH3. The measurement was conducted by

eye at an accuracy of (0.1%, (0.1%, and (0.1 ppm for O2,CO2, and NH3, respectively.

Waste Mixture Sampling and Analysis. Immediatelyafter air sampling, the waste mixture was sampled from atleast five different points in each composter and mixedthoroughly. Subsamples were then removed to determinepH 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 solutionfor 30 min. After being centrifuged at 4000 rev‚min-1 for 10min, the supernatant was filtered through glass fiber filter(0.45 µm, Whatman), analyzed by an automated colorimetricmethod on a TRAACS-800 instrument.

To determine concentration of dissolved organic carbon(DOC) in waste, another 2.00 g subsample was extracted by20 mL of water (Milli-Q SP TOC, Millipore) for 30 min,centrifuged at 4000 rev‚min-1 for 10 min, and filtered byglass fiber filter (0.45 µm, Whatman). The supernatant wasstored at 4 °C and analyzed within 2 days on a TOC analyzer(Shimadzu TOC-5000, Japan).

ResultsReproducibility of Composter Performance. The reproduc-ibility of the performance of the composters was assessed bythe analysis of oxygen and temperature profiles from twopreliminary experiments, one with CCM the other without.In both experiments water content in the waste mixture waskept at around 65%. In the presence of CCM, both duplicatesshowed a 44-45 °C temperature peak at day 3 and a stabilizingtemperature profile (30.6-32.3 °C) after day 10, whereas inthe absence of CCM, temperature increased to 45-47 °C atday 5 and stabilized after day 11. The average differencebetween the replicate temperature profiles in two experi-ments was less than 2 °C throughout the composting period.Oxygen percentage in the offgases first decreased to 13 and15% in treatments with and without CCM, respectively, andthen returned to 19.1-19.5% after day 11. The averagediscrepancy between oxygen levels in offgases from com-posters with identical initial conditions was less than 0.5%.Analysis of the temperature and oxygen profiles showed thatthe composting system was able to provide reproducible testconditions for further experiments.

Nitrous Oxide Emission. Significant N2O production wasdetected in all experimental runs. Figure 2 presents twotypical N2O emission profiles versus composting timeobtained in treatments with and without CCM, respectively.A N2O emission peak was observed immediately after thebeginning of composting, and the peak value in treatmentswith and without CCM was 3.32 and 2.73 µL‚L-1, respectively.Charging of CCM increased the average peak value of N2Oemission by 0.59 µL‚L-1. After 2 days, N2O concentration inthe exhaust gas rapidly decreased to 0.53 µL‚L-1, slightlyhigher than the atmospheric background level of 0.45 µL‚L-1

(ranging from 0.41 to 0.47 µL‚L-1 during the composting

TABLE 1. Amount of Food Waste and Amendments (fresh weight) in Different Experiments

experiments

foodwaste

(kg)

compostedcattle manure

(kg)sawdust

(kg) replicateaeration rate

(L‚min-1‚kg-1a)

reproducibility test 2.0 0.0 1.5 2 0.702.0 1.0 1.5 2 0.70

single-charged composters 2.0 0.0 1.5 2 0.702.0 1.0 1.5 2 0.70

intermittent-charged composters 2.0 + 4.0 + 6.0b 1.0 1.5 2 0.70c

effect of aeration rates 2.0 1.0 1.5 2 0.142.0 1.0 1.5 2 0.56

effect of NO2- addition 2.0 0.0 1.5 2 0.70

2.0 1.0 1.5 2 0.70a 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.

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period). From days 2 to 12, N2O production was low with anaverage emission rate of 3.38 µL‚h-1 from 1 kg food waste(dry weight). After day 12, N2O production from CCM-amended treatments increased steadily and entered thesecond (main) production period. A peak of 202.67 µL‚L-1

was detected on day 36, then N2O generation decreased slowlyto around 0.6 µL‚L-1 after day 100. In treatments withoutCCM, no significant change occurred in N2O production untilday 55. Though a peak was detected at day 110, the averagepeak value was much lower compared with that in CCM-applied composters. Nitrous oxide generated during the mainproduction period accounted for more than 95% of totalemission throughout the experimental processes in trials withand without CCM.

Effect of Aeration Rate. The effect of aeration rate onN2O production was studied by aerating composters withidentical water contents and waste/amendment ratios atdifferent aeration rate. As the aeration rate was reduced from0.70 to 0.56 and 0.14 L‚min-1‚kg-1, N2O emission decreasedsignificantly (Figure 3). The second N2O emission peak, whichwas observed at 0.70 L‚min-1‚kg-1 (Figure 2, treatments withCCM), 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, lowaeration prolonged ammonification period and resulted inhigh ammonia volatilization (Figure 4), which would reducethe nutrient value of the composted product and cause moredifficulties with odor control. Moreover, as the aeration ratewas reduced methane production increased. The mass-basedmethane 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 itsbenefit in N2O emission control though IR absorbance of

CH4 is 10 times lower than that of N2O (9). Therefore, amongthe three aeration rates employed in the study, 0.56L‚min-1‚kg-1 appeared to be the most suitable aeration rateif both N2O and CH4 emissions were considered.

Effect of Intermittent Feeding of Fresh Waste. To assessthe effect of carbon availability on N2O generation, DOC inthe waste mixture was determined in parallel with gasanalysis. In Figure 5, DOC in waste mixture increased sharplyto 33.28 mg‚g-1 on day 3, showing a higher DOC productionthan removal, then decreased to 3.45 mg‚g-1 on day 10. Duringthis period, N2O emission was relatively low, ranging from0.45 to 3.32 µL‚L-1, most of N2O was produced after day 12(Figures 2 and 5). A plot of N2O concentration against DOCcontent in the waste mixture (inset of Figure 5) showed nostatistically significant correlation. However, high N2O pro-duction occurred while DOC content was low, indicatingthat N2O was mostly released after the depletion of availablecarbon source. The result implies that intermittent feedingof fresh waste in the presence of enough bulking agent andamendments, as it is widely used in commercially availablecomposters which require sawdust addition once a month,may influence N2O production.

To investigate the effect of intermittent feeding of wasteon N2O generation, additional 4 and 6 kg fresh food wastewere added at days 5 and 14, respectively, into two CCM-amended treatments with an initial loading rate of 2 kg. Freshwaste was charged immediately after the pH in the wastemixture increased to 9.0. Similar DOC accumulation-

FIGURE 2. Profiles of N2O concentration in the exhaust gas versuscomposting time.

FIGURE 3. Profiles of N2O concentration in treatments with CCMat various aeration rates.

FIGURE 4. Ammonia volatilization from composters aerated atdifferent rates.

FIGURE 5. Changes of DOC in waste mixture and N2O concentrationin the exhaust gas with composting time, the inset shows a plotof N2O concentration in offgas against available carbon content (n) 111).

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consumption patterns were observed after addition ofdifferent amount of waste on days 0, 5, and 14 (DOC in wastemixture 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 experimentwas enough for removing organic carbon. A small N2Oemission peak was detected immediately after each addition(Figure 6). Consequently, three emission peaks were obtainedwithin 15 days in intermittent-charged composters, while insingle-charged ones only one peak appeared during the sameperiod. Nitrous oxide in the exhaust gas from single-chargedcomposters began to increase after day 12 and reached amaximum concentration of 202.67 µL‚L-1 on day 36. Inintermittent-charged composters, however, a significantincrease in N2O production occurred on day 47 and reacheda maximum concentration of 953.23 µL‚L-1 on day 70. Theoccurrence of the main production period was postponed inintermittent-feeding treatments. The mass-based N2O emis-sion was 1.19 and 0.95L‚kg-1 (dry waste) for single-chargedand intermittent-charged treatments, respectively, indicatinga 20% reduction in N2O production in intermittent-feedingtreatments.

Nitrous Oxide-Nitrite Correlation. Ammonia, nitrite,nitrate, and TN concentration in waste mixture weremonitored simultaneously with N2O, and the data was thenutilized to formulate their relationships with N2O concentra-tion in the exhaust gas. Significant increases in ammonia,nitrite, and nitrate concentrations in waste mixture occurredon days 8-10, 28-30, and 60-90, respectively, dependingon the aeration rate and waste application pattern. The resultsuggested that nitrification was the predominant nitrogentransformation process in our system. A good linear cor-relation between nitrite and N2O concentration was foundin all composting trials, independent of feeding patterns ofwaste and amendments (Figure 7). For ammonia, nitrate,and TN, however, no statistically significant correlation wasobtained. To provide further evidence for the dependenceof N2O production on nitrite in the aerated compostingsystem, NaNO2 solution was added into composters toincrease the theoretical nitrite concentration (calculated onthe base of dry waste mixture) in waste mixture by 0.5 mg‚g-1.Six hours after addition of nitrite, N2O concentrations as highas 8838.83 and 1978.96 µL‚L-1 were detected in the offgasesof the composters with and without CCM, respectively (Figure8) and then sharply decreased to normal level (0.53-0.60µL‚L-1) within 3 days.

DiscussionOur results demonstrate the potential for significant N2Oproduction from aerated composting of food waste. Bymicrobial degradation, organic nitrogen was transformedinto ammonia, leading to accumulation of ammonia in thewaste mixture and high ammonia volatilization (Figure 4).Significant N2O emission started concurrently with ammoniaoxidation, which was evidenced by the sequential occurrenceof ammonia, nitrite, and nitrate accumulation. More than95% of N2O was released during the later period of compostingwhile available carbon had been used up (Figure 5). Thetime at which the N2O emission peak appeared dependedon the waste application pattern and amendment. Intermit-tent feeding of fresh waste postponed its occurrence, whereasamendment of CCM resulted in a peak earlier in thecomposting cycle. The result implies that large amount ofN2O can be released from old compost piles in whichammonia oxidation has started.

Previous studies on aerobic wastewater treatment pro-cesses such as activated sludge and systems with highnitrification activity showed that high NO2

- level wasconducive to N2O production (8, 15-18). Similarly, linearNO2

--N2O correlation was observed in our study, implyingthe existence of a nitrogen transformation pathway fromnitrite to N2O in our system. Significant N2O emission afternitrite addition further certified that nitrite was one of mainparent chemicals of N2O in the aerated composting process.The dependence of N2O production on nitrite was foundboth in the presence and absence of CCM, suggesting anidentical mechanism for N2O production in both treatments.

FIGURE 6. Effect of intermittent feeding of food waste on N2Oproduction

FIGURE 7. Correlation between N2O concentration in offgas andNO2

- content in waste mixture.

FIGURE 8. Effect of nitrite addition on N2O production in compostersin the presence (solid circles) and in the absence of CCM (opentriangles).

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In contrast to the results obtained previously in manureland application (19-21) and composting systems (22), nosignificant correlation was found between N2O productionand ammonia, nitrate and total nitrogen. Aeration enabledthe system to be predominantly an oxic environment, andreduction of nitrate was poor though anoxic/anaerobicmicrosites might exist in large waste particles. As a result ofthis, NO3

- concentration as high as 3.53 mg‚g-1 detected inCCM-amended treatments at the end of composting did notcause apparent change in N2O generation. Poor NH4

+-N2Ocorrelation was possibly a consequence of the extensiveperiod needed for ammonia oxidation.

High N2O production occurred after the readily availablecarbon source was depleted. In intermittent-feeding com-posters, occurrence of the main N2O-production peaks waspostponed from days 36 to 70. It’s unlikely the increasedwaste amount, which was expected to prolong DOC removalprocess, postponed N2O generation. With enough bulkingagent, applying large amount of food waste did not prolongDOC removal process, as evidenced by the similar DOCaccumulation-consumption patterns observed after read-dition of different amount of waste in our study. Multiplefeedings of fresh waste supported a continuous DOCproduction-consumption process and inhibited ammoniaoxidation, and hence N2O production was postponed.

In single-charged composters, a two-peak N2O emissioncurve was obtained after 5 months. The first peak observedimmediately after feeding of fresh waste came from therelease of N2O stored in food waste. Within 6 h, 200 g of foodwaste stored in refrigerator produced 0.75 µL of N2O accordingto our investigation. N2O was then sealed in the clump afterbeing frozen and released after thawing.

The first N2O emission peak in treatments with CCM washigher than that without CCM, as a result of N2O generationfrom the mature compost which was a known N2O generator-(22-24). There is no direct evidence that CCM additionimported new N2O generation mechanisms based on thepresently available data, but this study clearly showed thataddition of CCM resulted in an earlier occurrence of N2O.

In all experiments, a high N2O emission peak was foundat the end of the composting process (Figures 2 and 3).Considering the good NO2

--N2O correlation during thisperiod and the immediate increase in N2O production afterNO2

- addition, N2O might be produced from NO2-. Usually

this transformation step occurs under anoxic or anaerobicconditions (25, 26); however, aerobic denitrification has beenfrequently documented in recent years (27, 28). With thehigh aeration rates employed in the study, our compostingsystem was kept predominantly aerobic throughout theexperiment; however, anoxic or anaerobic microsites mightstill exist inside the waste particles similarly to the coexistenceof aerobic and anaerobic microenvironment in aerobic soils(29, 30), denitrification conducted by denitrifiers in thesemicrosites might contribute to NO2

- reduction. If this is thecase, N2O production should be proportional to the availablecarbon as shown previously (21, 31) because denitrifiers aremostly heterotrophs. However, most of the N2O was producedafter 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 mechanismfor N2O production in our system.

AcknowledgmentsThe research was financially supported by EnvironmentalAgency, Japan. We are grateful to Dr. Toshihiro Sankai andMs. Keiko Kuto for their generous provision of equipmentfor food waste preparation, and Ms. Hideko Arai and Ms.Midori Toyama for their help on N, P analysis. Also, we wishto express our appreciation for the assistance of Dr. NicholasMcClure and Mr. Brett Roman in manuscript revisions.

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