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Journal of Environmental Management 116 (2013) 163e171

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Gas emissions as influenced by home composting systemconfiguration

Bijaya K. Adhikari a,b,c, Anne Trémier a,b, Suzelle Barrington b,c,*, José Martinez a,b, Mylène Daumoin a

a IRSTEA, UR GERE, 17 Avenue du Cucillé, CS 64427, F-35044 Rennes, FrancebUniversité européenne de Bretagne, FrancecDepartment of Bioresource Engineering, Macdonald Campus of McGill University, 21 111 Lakeshore, Ste Anne de Bellevue, Québec, Canada H9X 3V9

a r t i c l e i n f o

Article history:Received 23 January 2012Received in revised form27 November 2012Accepted 7 December 2012Available online 8 January 2013

Keywords:Gas emissionsHome composting systemsOrganic waste

* Corresponding author. Department of BioresouCampus of McGill University, 21 111 Lakeshore, SteCanada H9X 3V9. Tel.: þ1 514 398 7776; fax: þ1 514

E-mail address: suzellebarrington@sympatico.ca (S

0301-4797/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvman.2012.12.008

a b s t r a c t

Home composting systems (HC) are known to facilitate municipal solid waste management, but little isknown about their environmental impact including their greenhouse gas emissions (GGE). The presentresearch focused on selecting HC configuration producing the least CH4 and N2O. Thus, 4 HC types wereused to compost food and yard waste for 150 days and monitored for CO2, CH4 and N2O as of day 15: thewood and plastic bins (WB and PB), the mixed and unmixed ground pile (GPM and GP). Using the samewaste recipe, all HCwerefilled at once (batch fed) tomaximize gaseous emissions.Weeklyas of day 15, CO2,N2O and CH4 emissions were measured during 2-h sessions using a closed chamber inserted into thecompost surface. Monitored compost characteristics indicated little differences over time except formoisture content. From day 15 to 150, CH4 emissions were not measurable. Generation of N2O occurredbetween day 20 and 120with PB producing the least because of top and bottom slots providing continuousconvective aeration, as compared to the WB with slats over its full height and the naturally aerated mixedand unmixed ground piles. Total N2O emissions of 56 kg CO2-eq (tonne wet waste treated)�1 for PB, 75 forGP, 97 forWB and 99 for GPM represented average value for centralized composting facilities. Present andpast scientific works suggest the need for more research to establish the combined effect of managementand HC configuration on gaseous emissions, with close CH4 measurements from day 0 to 15.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Greenhouse gas emissions (GGE), namely carbon dioxide (CO2),methane (CH4) and nitrous oxide (N2O), have led to global warmingtrends and their adverse climatic effects (IPCC, 2006; RecycledOrganics Unit, 2001; Friends of the Earth, 2000). Landfill isconsidered one of the major contributors of CH4 emissions,accounting for over 12% of the total annual global CH4 emissions,equivalent to 734 kg CO2-eq (tonne wet waste treated)�1

(Matthews and Themelis, 2007; US EPA, 2006). Composting lowersGGE to values of 0.03e8.0 kg CH4 (tonne wet waste treated)�1 and0.06e0.6 kg N2O (tonne wet waste treated)�1, for a total averaging200 kg CO2-eq (tonne wet waste treated)�1 (Friedrich and Trois,2011; Hermann et al., 2011; Rogger et al., 2011; Martínez-Blancoet al., 2010; Lou and Nair, 2009; IPCC, 2006). Accordingly, Europeand North America initiated policies for the diversion of the organic

rce Engineering, MacdonaldAnne de Bellevue, Québec,398 8387.. Barrington).

All rights reserved.

waste fraction from the municipal solid waste stream (Pires et al.,2011; Fenerty-McKibbon and Khare, 2005; Thompson andTanapat, 2005; Landfill Directive, 1999) and recycling througheither composting or anaerobic digestion.

Composting releases to the atmosphere, heat, CO2 and watervapour while converting organic matter into a soil amendment(Epstein, 1997; Renkow and Rubin, 1996; Pace et al., 1995; Haug,1980). However, depending on the method and the managementpractice, composting can also generate volatile organic compounds,ammonia (NH3), carbon monoxide (CO), nitric oxide (NO), N2O andCH4 (Colón et al., 2010; Martínez-Blanco et al., 2010; IPCC, 2006;Barton and Atwater, 2002), which have a negative impact on theatmosphere. The CO2 emissions from organic waste decompositionin natural environments have generally been overlooked to focuson the short term production of CH4 and N2O (IPCC, 2006; ICFConsulting, 2005). However, the high level of organic wasteproduction in urban centres has changed the natural carbon cycle(Riebeek and Simmonlune, 2011), justifying GGE from composts,with and without accounting for CO2 emissions.

The composting of municipal organic waste can be conductedthrough centralized facilities or decentralized systems such as

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171164

community centres and home composters (Bernstad and la CourJansen, 2011; Schwalb et al., 2011). As compared to centralizedfacilities, home composting eliminates the cost and energyrequired for collection, transportation and processing (Boldrinet al., 2011; Andersen et al., 2010). Therefore, home compostingsystems (HC) can potentially reduce GGE for the waste manage-ment sector (Chan et al., 2011; Adhikari et al., 2010).

However, CO2, CH4 and N2O emissions can be affected by HCconfiguration and management (Bogner et al., 2008; EPIC, 2002)with a limited number of studies examining their GGE. Usinga weekly fed plastic bin, Colón et al., 2010 reported CH4 and N2Oemissions below their detection threshold. For bi-weekly fed homecomposting bins, Andersen et al. (2010) measured CH4 and N2Oemissions in the range of 0.4e4.2 and 0.30e0.55 kg (tonwet wastetreated)�1 respectively, totalling 100e239 kg CO2-eq (ton wetwaste treated)�1. With weekly fed and mixed home compostingbins, Martínez-Blanco et al., 2010 measured GGE of0.158 kg CH4 (tonne wet waste treated)�1 and 0.676 kg N2O (tonnewet waste treated)�1. These reported GGE are within the range ofthose produced for centralized composting facilities (IPCC, 2006),but illustrates variability in GGE with HC configuration.

Accordingly, the objective of the project focused on testing HCconfiguration for the least gaseous emissions. Thus, four commontypes of home composting systems (HC) were used to compostorganic waste and monitored for CO2, CH4 and N2O from day 15 to150: the wood and plastic bins (WB and PB), the mixed andunmixed ground pile (GPM and GP). All HC were filled at once(batch fed) with the same restaurant food waste and yard trim-mings mixture, to produce conditions of maximize microbialactivity and thus, gaseous emissions.

2. Material and methods

2.1. Composting systems and experimental organic waste

Emissions of CO2, CH4 and N2O from batch fed home compostingsystems (HC) were monitored using four commonly used systemsin North America and Europe: the slatted wood bin (WB)measuring 0.78m� 0.65m by 0.75m in height; the top and bottomperforated plastic bin (PB) measuring 0.70 m � 0.70 m by 0.80 m inheight, and; the mixed ground pile (GPM) and unmixed ground pile(GP), both measuring 0.65 m in height and 0.75 m in base diameter(Fig. 1aec). Considering the size of each HC and the organic wasterequired, the experiment was not replicated.

The same food waste (FW) and yard trimmings (YT) were usedto prepare the common mixture batch fed to all four HC on thesame day. A typical FW:YT ratio of 1:1 on a wet volume basis, wasused and its average wet bulk density and water content of 235(�6) kg m�3 and 76 (�1.0)% respectively. The FW was sourceseparated and supplied by two restaurants in Rennes, France,within 3 days of production. It consisted of vegetable and fruitwastes. The YT were obtained from the green space surroundingthe IRSTEA research station of Rennes, France, and consisted ofa 90% grass clippings and 10% tree leaves on a wet weight basis.While loading the four experimental HC, the initial compostmixture was sampled in triplicate for physico-chemical character-ization using standard methods as described below in Section 2.4.

2.2. Experimental procedure

The four experimental HC were set-up under an outside tent atthe IRSTEA Research Centre, Rennes (France), to avoid rainfall anddirect sunshine interference. Representing mixtures typically used,equal wet volumes of FWand YT were mixed by hand in a large tubbefore being loaded without compaction into the HC. All four HC

were filled at once (batch fed) on the same day with the sameorganic waste to 80% of their capacity, to create a high O2 demandand enhance CH4 and N2O production. While filling the HC,temperature sensors (model DS1921G-F5, Thermochron iButton,Dallas Semiconductor, USA) were installed at their mass centerabove 0.30 m from ground level.

While being monitored for 150 days, the HC composts werenaturally aerated and manually mixed weekly using a garden forkexcept for that of GP. During the mixing operation, compostsamples were collected for analysis of drymatter (DM), total carbon(TC), total nitrogen (TN), organic matter (OM) and pH every 15 daysduring the first 60 days, and then every 30 days for the rest of theexperimental period. The leachate was not monitored in this study.The GP treatment was sampled only on days 0 and 150. Thetemperature sensors were retrieved during the 70 day mixingoperation, because the compost had reached ambienttemperatures.

During the composting process and before the mixing opera-tion, all HC were monitored for GGE, namely methane (CH4),nitrous oxide (N2O) and carbon dioxide (CO2) starting on day 15. Forall gas monitoring sessions, gases were trapped using a staticchamber placed for 2 h over the compost surface while monitoringthe greenhouse gas concentration (Parkin and Venterea, 2010;Chadwick et al., 1998). The static chamber consisted of a metallicchambermeasuring 0.40m in length by 0.15m inwidth and 0.20min height, opened at its bottom and pushed into the compost massto a depth of 5 mm (Fig. 1d). The upper chamber surface wasairtight and equipped with a silicone rubber septum to collect gassamples using disposable plastic syringes.

Each monitoring session was repeated weekly for the first 30days, every 10 days for days 30e60, every 20 days for days 60e120,and then on day 150. During each 120 min monitoring session,duplicate air samples were drawn after 0, 10, 20, 40, 60 and120 min. All air samples were analysed for CO2, CH4, N2O and O2by gas chromatography (GC e HP6890N, Agilent, Santa Clara,USA). The GCwas equippedwith an electron capture detector (ECD)and a flame ionization detector (FID), and used nitrogen (N2) asa vector gas.

After 150 days, the compost mass in each HC was weighed andsampled in triplicate for physico-chemical characterization, tocompute the loss in DM, TC, TN and OM.

2.3. Computation of losses and gaseous emissions

Between day 0 and 150, the wet HC compost mass could not bemeasured, although this value was required to computewith time,losses in DM, TC and TN. Accordingly, an equation was developedto predict the residual mass of wet compost in each home com-posting systems at a given time based on the hypothesis that themass of fixed solids (FS) is constant during the entire experi-mental period. Thus, the residual mass of wet compost at time twas equated to the mass of FS plus that of the residual organicmatter and water:

Mt ¼ FSt þMðtÞ � OMt � DMt � 10�4 þMðtÞ � ð1� DMt=100Þ(1)

where,Mt is total wet mass at sampling time t in kg; FSt is the fixedsolids mass at time t, in kg; OMt is the organic matter concentrationin % DMt, and; DMt is the dry matter concentration at time t, in % ofMt. The values of OMt and DMt were measured only on days 0 and150. Nevertheless, 14, 1, 8 and 0% of the original FS were lost asleachate for WB, PB, GPM and GP, respectively. Therefore, Equation(1) was corrected presuming a linear loss of FS over the entireexperimental composting period of 150 days.

Fig. 1. Experimental home composting systems (a) wood bin, (b) plastic bin, (c) ground pile. The static gas collection chamber used to measure gaseous emissions (d).

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171 165

Accordingly, Equation (1) can be rearranged to solve for Mt:

Mt ¼ FSt=n1�

�OMt � DMt � 10�4

�� ð1� DMt=100Þ

o(2)

The mass evolution of TC and TN was thus computed using theconcentrations measured regularly and the compost mass Mt ob-tained from Equation (2).

For each gas emission monitoring session, individual gasproduction rates were computed from the evolution of theirconcentration in the air of the closed chamber, during 120 min,neglecting the initial diffusion effect. Emissions in CO2, CH4, andN2O were computed after each session as:

Q ¼ Ar

OMi� S� 10�6 � V � r (3)

where Q is the gas production rate in kg h�1 (kg OMi)�1, Ar is theratio of the composter to sampler cross sectional area in m2 m�2

(8.7, 8.3 and 12.5 m2 m�2 for WB, PB and the ground piles,respectively), OMi is the initial mass of organic matter in thecomposter in kg, S is the rate of gas production equal to theslope of the linear gas concentration regression in ppmv h�1, V isthe sampler volume in m3 and r is the gas density in kg m�3.The density used for the gases, namely CO2, CH4 and N2O were1.842, 0.668 and 1.826 kg m�3 respectively at the standard

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171166

temperature of 20 �C and pressure of 101.3 kPa (EngineeringToolbox, 2010).

2.4. Analytical procedure

Before being analysed, all triplicate compost sample was driedin an oven (SR 2000, Thermosi, France) at 80 �C until a constantweight was reached (Trémier et al., 2005; de Guardia et al., 2010)and then grinded to less than 0.5 mm (ZM model 1000 grinder,Retsch, Germany).

The compost pH was determined according to Adhikari et al.(2008) by soaking 10 g of wet sample for 24 h without shakingat 5 �C, in just enough distilled water to use a pH electrode (pH-Electode SenTix41, WTW, Weilhein, Germany). The OM wasquantified as volatile solids (VS) and determined by burning at550 �C for 3 h (Thermolyne 30400, Furnace, F30420 C-33, Essex,UK), according to AFNOR (1985). The fixed solids concentrationwas determined by the ash remaining following the OM proce-dure. The TC was determined by burning 10 mg samples at900 �C (Thermo Scientific, FLASH 2000 Series, Organic ElementalAnalyser, Courtaboeuf, France) according to AFNOR (2001).According to AFNOR (1995), total Kjeldahl nitrogen (TKN) wasdetermined using an automatic distilling system (VAP 50c,Gehardt automatic distilator, Gehardt, Germany), after digesting0.5e1.0 g of sample with H2SO4 (automated Kjeldatherm TZ blockdigester, Gehardt, Germany). The TKN value was assumed equalto TN because of negligible amount of nitrite and nitrate in theexperimental material (Adhikari et al., 2009). To correct allanalytical results, residual moisture was determined by dryinggrinded compost samples at 105 �C for 24 h (SR 1000, Thermosi,France).

Table 1Initial characteristics of organic waste loaded into the 4 home composting systems (HC)

Composter Day aWetmass (kg)

DM (%) OM (%dm)

WB 0 72.2 24.1 (1.2) 75.3 (0.2)15 49.3 23.0 (0.8) 63.0 (0.1)30 37.6 27.0 (2.3) 58.7 (0.2)45 33.3 30.0 (0.7) 58.1 (0.1)60 27.6 33.0 (1.3) 54.2 (0.1)90 17.6 50.4 (0.6) 53.2 (0.3)

120 13.2 65.9 (0.7) 52.6 (0.2)150 9.2 81.0 (0.8) 50.4 (0.3)

PB 0 75.5 24.1 (1.2) 75.2 (0.2)15 50.9 22.0 (3.0) 60.5 (0.3)30 44.1 24.0 (3.9) 58.4 (0.2)45 38.7 26.0 (0.1) 56.2 (0.1)60 34.6 27.0 (1.0) 52.9 (0.2)90 24.2 34.5 (0.7) 47.0 (0.2)

120 16.3 50.9 (2.4) 47.4 (0.3)150 12.5 66.0 (1.8) 45.8 (1.0)

GPM 0 75.0 23.7 (1.2) 75.2 (0.2)15 42.9 25.0 (2.1) 59.6 (0.1)30 36.4 27.0 (4.1) 56.0 (0.1)45 31.4 30.0 (1.8) 54.1 (0.3)60 24.3 36.0 (4.1) 50.8 (0.3)90 15.7 54.1 (2.5) 49.6 (0.3)

120 11.2 75.6 (0.7) 49.5 (0.1)150 9.4 85.0 (2.2) 49.0 (0.3)

GP 0 63.3 22.3 (2.0) 77.0 (0.2)150 11.8 60.0 (3.5) 50.4 (0.8)

a The total mass wasmeasured on days 0 and 150, otherwise estimated from Equation (pile; dm e dry mass basis; FW e food waste; YT e yard trimmings; DM e dry matter; TCratio. The numbers in parenthesis represent the standard deviation. All FW:YT dry maheterogeneity of the organic waste.

2.5. Statistical procedure

The experimental HC were filled with the same organic wastemixture (FW:YTwetmass andwet volume ratio of 1:1) on the sameday and mixed/sampled at the same time, to eliminate all effectsexcept for that of HC type. The only exception was the ground pile,where one was not mixed while the other was mixed. Gaseousemissions from the different composting systems could thereforebe compared with the repeated measures ANOVA procedure, usingPROC GLM procedure at 95% confidence level (SAS Institute Inc.,2008). Regression equations relating compost CO2 and N2O emis-sions to TC and TKN losses were computed using Excel (Microsoft2007).

3. Results and discussion

3.1. Initial characteristics of organic waste

Table 1 summarizes the characteristics of the organic wastemixture fed into all four HC on day 0. The DM of the initial mixtureranged from 22.3 (�2.0)% to 24.1 (�1.2)% amongst the tested HC.The OM ranged from 75.2 (�0.2) to 77.0 (�0.2)% and the TC and TNvaried from 39.2 (�0.7) to 39.8 (�0.4)% and 22.9 (�0.2) to 23.3(�0.7)% respectively, for an initial C:N ratio of 17, for all four HC. ThepH of all HC compost was at 6.1 initially. The slight variations ininitial characteristics of the OW amongst the tested HC occurreddue to the heterogeneous nature of the waste (de Bertoldi et al.,1983). Except for GP, the initial variations in properties for the PB,GPM and WB were considered negligible, and within a range sup-porting an active compost microbial activity (Adhikari et al., 2009;Stabnikova et al., 2005; Haug, 1993). For GP, preparing the

and their evolutions during the 150 days of composting.

TC (%dm) TN (g (kg dm)�1) C/N ratio pH

39.2 (0.7) 22.9 (0.2) 17.1 6.1 (0.2)32.7 (0.2) 27.0 (0.2) 12.1 8.8 (0.1)30.4 (0.6) 26.3 (0.2) 11.5 8.1 (0.2)29.9 (0.2) 25.8 (0.2) 11.6 8.2 (0.0)29.2 (0.1) 24.9 (0.4) 11.7 8.6 (0.3)28.2 (0.2) 25.2 (0.2) 11.2 8.1 (0.4)27.6 (0.2) 26.4 (0.2) 10.5 7.8 (0.1)26.6 (0.3) 24.6 (0.1) 10.8 7.5 (0.1)

39.2 (0.7) 22.9 (0.2) 17.1 6.1 (0.2)31.2 (0.5) 26.6 (0.5) 11.7 8.4 (0.1)30.4 (0.1) 26.2 (0.2) 11.6 8.4 (0.2)29.5 (0.3) 25.0 (0.2) 11.8 8.2 (0.0)28.7 (0.9) 23.3 (0.1) 12.3 8.8 (0.3)25.5 (0.1) 21.3 (1.8) 12.0 8.7 (0.4)25.3 (0.2) 21.4 (0.3) 11.8 8.4 (0.1)24.6 (0.2) 20.6 (0.2) 11.9 7.7 (0.1)

39.4 (0.7) 23.1 (0.2) 17.1 6.1 (0.2)32.1 (0.5) 26.4 (0.2) 12.2 7.9 (0.4)30.1 (0.9) 26.1 (0.1) 11.5 8.5 (0.3)28.7 (0.2) 25.3 (0.1) 11.3 8.2 (0.1)27.6 (0.5) 23.9 (0.4) 11.5 8.9 (0.0)26.2 (0.2) 23.8 (0.3) 11.0 8.3 (0.2)26.1 (0.1) 23.7 (0.4) 11.0 7.8 (0.0)25.1 (0.10 24.5 (0.1) 10.2 7.7 (0.0)

39.8 (0.4) 23.3 (0.7) 17.1 6.1 (0.2)25.9 (0.1) 23.2 (0.3) 11.2 7.5 (0.2)

2). WBewood bin; PBe plastic bin; GPM-mixed ground pile; GPe unmixed grounde total carbon; TN e total nitrogen; OM e organic matter; C/N e carbon to nitrogenss ratio ranged between 0.93 and 0.96 except for the GP at 1.2, as a result of the

0

5

10

15

20

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30

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40

45

50

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60

65

70

75

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (day)

Tem

pera

ture

(0 C

)

Wood bin (WB)Plastic bin (PB)Mixed ground pile (GPM)Unmixed ground pile (GP)Ambient

Fig. 2. Temperature regime at the centre of compost mass for all four experimentalhome composting systems against ambient temperature. All composters were filled atonce (batch fed) and mixed weekly except for the unmixed ground pile (GP).

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171 167

heterogeneous organic waste mixture using a wet volume ratio of1:1 produced a slightly higher FW:YT dry mass ratio (Table 1), withthe resulting DM being slightly lower at 22.3%.

3.2. Compost temperature regime

The temperature developed within the compost of all four HCduring the first 70 days of experimentation is presented in Fig. 2.

(a)

R2 = 0.97

19.8020.0020.2020.4020.6020.8021.0021.2021.4021.60

0 25 50 75 100 125 150

Time (minute)

O2 (

%)

(c)

R2 = 0.98

0

50

100

150

200

250

300

0 25 50 75 100 125 150

Time (minute)

N2O

(ppm

)

Fig. 3. Typical trend of gas concentrations obtained in the closed chamber placed over thewood bin after 50 days; (c) nitrous oxide for the wood bin after 50 days; and (d) methane

The PB and GPM composts reached thermophilic conditions(>45 �C) after 2 days, as compared to 3 days for that of the GP and 7days for that of the WB. The compost temperature reached almostambient levels by day 20 for all HC, with that of WB taking 3 moredays to stabilize.

During the active phase of composting, the temperature regimereflects the rate of aeration and accordingly, the rate of microbialactivity. Thus, PB provided the best conductive aeration, because ofits top and bottom perforations (Barrington et al., 2002;Karnchanawong and Suriyanon, 2011), followed by GPM and GP,and then WB with the least effective aeration regime. Comparingthe GP and GPM regimes, GP lagged in developing thermophilictemperatures because of its lower initial DM at 22.3% as comparedwith 23.7%. Nevertheless, GP sustained as high a temperatureregime as GPM, because of its higher FW content, producing a morebiodegradable organic waste mixture, and because of its largeexposed surface allowing for gas exchange. Comparing GPwithWB,the higher exposed surface of GP provided better aeration despitethe slightly lower DM at 22.3% compared to 24.1%. Thus, the slattedopening configuration of WB provided an inefficient convectiveaeration regime. Reflecting the rate of O2 supply and aerobicmicrobial activity, temperature can also reflect the rate of CO2emission and biodegradability of the organic waste (Epstein, 1997;Diaz et al., 1993).

3.3. Emissions in CO2, N2O and CH4

Within the closed gas monitoring chamber for one samplingsession, Fig. 3 illustrates typical concentration evolutions for CO2,N2O and CH4, accumulating linearly, with little initial diffusion

(b)

R2 = 0.98

0

2000

4000

6000

8000

10000

12000

14000

0 25 50 75 100 125 150

Time (minute)

CO

2 (p

pm)

(d)

R2 = 0.96

0

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Time (minute)

CH

4 (p

pm

)

compost materials: (a) oxygen for the wood bin at 50 days; (b) carbon dioxide for thefor the unmixed ground pile on day 15.

Wood bin (WB)

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171168

effect. Measurable CH4 concentrations were obtained only for theGP compost on day 15, likely because of its lower aeration rate, notbeing mixed, its slightly higher moisture and FW levels, favouringanaerobic pockets producing CH4.

Each greenhouse gas produced a specific emission curves duringthe 150 day experimental period (Fig. 4). For CO2, the highestemission rates were observed on day 15 in the range of 25e28 g h�1 (ton wet waste treated)�1, with little variation amongHC. These values dropped to 1.5 g h�1 (ton wet waste treated)�1

on day 150, with some differences appearing especially for GP onday 30 and 60 but producing an average drop in CO2 emissionrate corresponding to that of the other experimental HC.Accordingly, HC type had little if any effect on CO2 emissions.

Compost N2O emission rates were more variable than those forCO2. On day 15, all HC had low N2O emission of 7e105 mg h�1 (tonwet waste treated)�1 with PB producing the highest value alongwith the lowest temperature, followed by that of WB, GPM andthen GP. The N2O emission rate increased on day 20, at the end ofthe thermophilic phase, likely because of the NH3 oxidation byNH3 oxidizing bacteria (AOB) under nitrifying conditions (Kimet al., 2010). Peak rate of 350e550 mg h�1 (ton wet wastetreated)�1 were reached on days 30 and 40 for all HC, except forGP peaking on day 80 because of lower O2 availability.Nevertheless, PB produced the least overall N2O, followed by GPand then GPM and WB at similar levels. The only factorcorresponding to this N2O emission order is the final compostDM with PB and GP at 66 and 60%, respectively as compared toGPM and WB at 85 and 81%.

(a)

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15 20 30 40 50 60 80 100 120 150

Time (day)

CO

2 (g

hr-1

(ton

wet

was

te tr

eate

d)-1

)

Wood bin (WB)Plastic bin (PB)Mixed ground pile (GPM)Unmixed ground pile (GP)

(b)

0

100

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300

400

500

600

700

15 20 30 40 50 60 80 100 120 150Time (day)

N2O

(mg

hr-1

(ton

wet

was

te tr

eate

d)-1

)

Wood bin (WB)Plastic bin (PB)Mixed ground pile (GPM)Unmixed ground pile (GP)

Fig. 4. Measured emissions of CO2 and N2O for the four experimental home com-posting systems from day 15 to 150: (a) carbon dioxide, CO2; and (b) nitrous oxide,N2O. Emissions of CH4 between days 15 and 150 were below the detection threshold.Y-bars indicate the standard deviation (n ¼ 2).

As for compost CH4 emissions, they were below detectionthreshold between days 15 and 150, except for GP on day 15. HigherCH4 emissions for GP likely resulted from: its lack of mixing,resulting not only in less aeration, but in the collapse of aerationpores, and; a higher FW content on a dry matter basis (Table 1),resulting in a higher O2 demand because of its greater biodegrad-ability but at the same time, a lower aeration rate because ofa higher moisture content.

3.4. Compost evolution and gas emissions

The evolution of all compost characteristics are presented inTable 1 and the computed losses in mass are illustrated in Fig. 5. ForallHC, compost TC suffered thehighest loss of 45e50%betweendays0 and 15, followed fromday 15 to 150 byan additional 15 to 20% loss.As compared to TC, compost TN losses was slightly different,dropping by 30% from day 0 to 15, and then by an additional 25%between days 15 and 150, because of denitrification. This was alsoreflected by higher N2O emissions. Using the initial and final HCcompost fixed solids mass, their losses through leachate wasfound to be 14 and 8% for the WB and GPM composts, comparedto 0e1% for the PB and GP. Whereas PB was well aerated andcould lose water vapour through evaporation rather than leaching,GP was not mixed, and suffered from the collapse of its aerationand drainage pores preventing leachate drainage.

(a)

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80

0 15 30 45 60 90 120 150

Time (day)

TC

loss

(%

)

Plastic bin (PB)Mixed ground pile (GPM)Unmixed ground pile (GP)

(b)

0

10

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30

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70

0 15 30 45 60 90 120 150

Time (day)

TN

loss

(%

)

Wood bin (WB)Plastic bin (PB)Mixed ground pile (GPM)Unmixed ground pile (GP)

Fig. 5. Loss of total carbon (TC) (a) and total nitrogen (TN) (b) over time for thecompost of all four home composting systems estimated using Equation. Note: Com-poster GP was sampled only on days 0 and 150.

Table 2Greenhouse gas emission for the four home composting systems.

Gas/time interval Composters (kg (ton wet waste treated)�1)

WB PB GPM GP

CO2

15e20 days 3 3 3 320e60 days 16 16 19 1660e120 days 7 9 9 19120e150 days 1 1 1 2Total CO2 (kg (ton wet

waste treated)�1)27 27 32 40

CH4 (15e150 days) nd nd nd 0.002Total CH4 (kg CO2-eq

(ton wet wastetreated)�1)

e e e 0.05

N2O15e20 days 0.010 0.014 0.001 0.00620e60 days 0.289 0.150 0.274 0.01760e120 days 0.032 0.022 0.051 0.228120e150 days 0.002 0.001 0.001 0.002Total N2O (kg (ton wet

waste treated)�1)0.333 0.187 0.327 0.253

Total N2O (kg CO2-eq(ton wet wastetreated)�1)

99 56 97 75

WB ewood bin; PB e plastic bin; GPM emixed ground pile; GP e unmixed groundpile; CO2 e carbon dioxide; CH4emethane; N2Oe nitrous oxide; nde not detected.The greenhouse gas emission impact of CH4 and N2O are considered to be 25 and298 times higher than CO2 (IPCC, 2007).

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171 169

Measured between day 15 and 150, CO2eC emissions corre-sponded to TC mass losses. For theWB compost for example, 27% ofthe initial mass of 6.8 kg TC was lost between day 15 and 150,amounting to 1.84 kg TC or 25 kg TC (ton wet waste treated)�1. Forthe same period, measured CO2eC emissions amounted to23 kg (ton wet waste treated)�1. Accordingly and from day 15 to150, all HC composts emitted CO2eC corresponding to TC lossesover time:

CO2 WBðtÞ ¼ 0:72 TCLðtÞ þ 1:78 R2 ¼ 0:95 (4)

CO2 PBðtÞ ¼ 1:55 TCLðtÞ þ 2:05 R2 ¼ 0:99 (5)

CO2 GPMðtÞ ¼ 1:08 TCLðtÞ þ 2:05 R2 ¼ 0:99 (6)

where CO2 and TCL are the loss of total carbon as measured by CO2emissions and compost TC mass, in kg (ton wet waste treated)�1,and; subscripts WB, PB and GPM correspond to individual HC.Because the compost of the unmixed ground pile (GP) was notcharacterized regularly, no regression equation could beformulated.

In terms of nitrogen, N2O emissions occurred at the end of theactive composting stage, between days 20 and 50, with the GPcompost showing a last peak on day 80. Nitrogen losses as N2OeNamounted to 0.01e0.016 kg during 150 days of compostingrepresented only 4e7% of the final compost TN losses, suggestingmore important losses through leachate, and NH3 and N2volatilization. Most of the emissions occurred between 15 and40 �C at a pH above 8, conditions favouring nitrification anddenitrification along with the production of N2O and NO (Kimet al., 2010; de Bertoldi et al., 1983). During the thermophilicphase, NH3 volatilization governs nitrogen losses (Pagans et al.,2006; Barton and Atwater, 2002; de Bertoldi et al., 1983). Emis-sions in N2O were also correlated to TN losses from day 15 and 150:

N2OWBðtÞ ¼ 0:08 TNLðtÞ þ 0:02 R2 ¼ 0:85 (7)

N2OPBðtÞ ¼ 0:03 TNLðtÞ þ 0:0008 R2 ¼ 0:94 (8)

N2OGPMðtÞ ¼ 0:11 TNLðtÞ þ 0:0003 R2 ¼ 0:96 (9)

where N2O is the compost N2OeN emission and TNL is the compostloss of TN, both in kg N (ton wet waste treated)�1, and; thesubscripts WB, PB and GPM correspond to individual HC. Becausethe compost of the unmixed ground pile (GP) was not characterizedregularly, no regression equation could be formulated.

Considering the evolution in compost characteristics, CO2emissions were similar for PB and WB at 27 kg (ton wet waste)�1,and slightly higher for the ground piles at 32 and 40 kg (ton wetwaste)�1 for GPM and GP. Emissions of N2O were higher for thedrier compost of WB and GPM at 81 and 85% DM, as compared tothe wetter compost of PB and GP at 60 and 66% DM (Table 1). Moreaccurate compost CH4 monitoring is required for the first 15 days ofcomposting to properly characterize such emissions.

3.5. Total compost gas emissions

Table 2 summarizes CO2, CH4 and N2O emissions over time andprovides a CO2 equivalent value for N2O considering a contributionof 298 times that of CO2 (IPCC, 2007). Because of limited CH4measurements, this gas will not be discussed.

From day 15 to 150, HC produced level of CO2 ranging from 27 to32 kg (ton of wet waste)�1 except for the GP at 40 kg (ton of wet

waste)�1. For the same period, PB produced the lowest level of N2Oemissions at 0.187 kg (ton of wet waste)�1 or 56 kg CO2-eq (tonwetwaste treated)�1. These values were followed by GP at 75, and thenGPM and WB at 97 and 99 kg CO2-eq (ton wet waste treated)�1,respectively. Over time, the most N2O emission occurred betweendays 20 and 60, except for GP loosingmost of its N2O from day 60 to120, because of its wetter compost.

Observed N2O emissions corresponded to the average valuereported for centralized composting facilities of 0.33 kg (tonne wetwaste treated)�1 (IPCC, 2006) or 100 kg CO2-eq (ton of wetwaste)�1. The N2O emissions from PB were slightly lower at56 kg CO2-eq (ton of wet waste)�1 demonstrating some improve-ment over the other HC. The values measured were low comparedto those observed by Andersen et al. (2010) and Martínez-Blancoet al., 2010 at 0.3e0.55 and 0.676 kg (tonne wet waste treated)�1

where the HC were bi-weekly and weekly fed. Accordingly,management practices may have just as important an impact ongaseous emissions as HC configuration. Furthermore, there may besome interaction between HC configuration and managementpractices, where for example, bins with top and bottom perfora-tions may not need weekly mixing to produce low gaseousemissions.

4. Conclusion and recommendations

To reduce earthwarming trends, lower greenhouse gas emission(GGE) technologies are required to manage the increasingproduction of organic wastes. Besides landfill diversion, homecomposting systems (HC) can help recycle residential organic wastewhile reducing collection, transportation and treatment costs andenergies. Nevertheless, HC configurations must limit gaseousemissions to levels comparable to other treatment alternatives. Theobjective of the project therefore focused on testing HC configu-ration for the least gaseous emissions. Four common types of HCwere used to compost organic waste and monitored for CO2, CH4

B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171170

and N2O fromday 15 to 150: thewood and plastic bins (WB and PB),the mixed and unmixed ground pile (GPM and GP).

Emissions in CO2 and N2O were highly correlated to total carbonand total nitrogen losses. Nevertheless, HC producing driercomposts such as the mixed ground pile and the wood bin, lost themost N2O while the plastic bin with convective aeration created bybottom and top perforations produced the least at half that of theaverage emissions from centralized composting emissions.Comparing the results of the present project to that of others, futureresearch should examine the interaction between managementpractice and HC configuration to minimise CH4 and N2O emissions.Emissions of CH4 need careful monitoring during the active phaseof the process.

Acknowledgements

This study takes part of a larger project entitled ECCOVAL fun-ded by the regional council of Brittany in France; the authors alsoacknowledge the financial and all necessary logistics supported bythe Rennes IRSTEA, France and the Natural Science and EngineeringResearch Council of Canada.

Symbols and abbreviations

Ar ratio of the composter to sampler cross sectional area inm2 m�2 (8.7, 8.3 and 12.5 m2 m�2 for WB, PB and groundpiles respectively)

CH4 methaneC/N carbon to nitrogen ratioCO carbon monoxideCO2 carbon dioxideDM dry matterDMt dry matter concentration at time t, in % of Mt

dm dry massFSt fixed solids mass at time t, in kgFW food wasteGGE greenhouse gas emissionsGP unmixed ground pileGPM mixed ground pileHC home composting system (composting bin or ground

pile)h hourMt total wet mass at sampling time t in kgNH3 ammoniaNH4 ammoniumN2O nitrous oxideO2 oxygenOM organic matter contentOMi initial organic matter massOMt organic matter concentration in % of DMt

OW organic wastePB plastic binppm parts per millionQ gas production rate in kg h�1 (kg OMi)�1

S rate of gas production (slope of the linear gasconcentration regression in ppmv h�1)

TC total carbonTN total nitrogenV closed gas collection chamber volume in m3

YT yard trimmingsWB wood binr gas density in kg m�3.

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