Journal of Environmental Management 116 (2013) 163e171
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman
Gas emissions as influenced by home composting system
configuration
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, France
Université européenne de Bretagne, France
c
Department of Bioresource Engineering, Macdonald Campus of McGill University, 21 111 Lakeshore, Ste Anne de Bellevue, Québec, Canada H9X 3V9
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 January 2012
Received in revised form
27 November 2012
Accepted 7 December 2012
Available online 8 January 2013
Home composting systems (HC) are known to facilitate municipal solid waste management, but little is
known about their environmental impact including their greenhouse gas emissions (GGE). The present
research focused on selecting HC configuration producing the least CH4 and N2O. Thus, 4 HC types were
used to compost food and yard waste for 150 days and monitored for CO2, CH4 and N2O as of day 15: the
wood and plastic bins (WB and PB), the mixed and unmixed ground pile (GPM and GP). Using the same
waste recipe, all HC were filled at once (batch fed) to maximize gaseous emissions. Weekly as of day 15, CO2,
N2O and CH4 emissions were measured during 2-h sessions using a closed chamber inserted into the
compost surface. Monitored compost characteristics indicated little differences over time except for
moisture content. From day 15 to 150, CH4 emissions were not measurable. Generation of N2O occurred
between day 20 and 120 with PB producing the least because of top and bottom slots providing continuous
convective aeration, as compared to the WB with slats over its full height and the naturally aerated mixed
and unmixed ground piles. Total N2O emissions of 56 kg CO2-eq (tonne wet waste treated)1 for PB, 75 for
GP, 97 for WB and 99 for GPM represented average value for centralized composting facilities. Present and
past scientific works suggest the need for more research to establish the combined effect of management
and HC configuration on gaseous emissions, with close CH4 measurements from day 0 to 15.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Gas emissions
Home composting systems
Organic waste
1. Introduction
Greenhouse gas emissions (GGE), namely carbon dioxide (CO2),
methane (CH4) and nitrous oxide (N2O), have led to global warming
trends and their adverse climatic effects (IPCC, 2006; Recycled
Organics Unit, 2001; Friends of the Earth, 2000). Landfill is
considered 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 lowers
GGE to values of 0.03e8.0 kg CH4 (tonne wet waste treated)1 and
0.06e0.6 kg N2O (tonne wet waste treated)1, for a total averaging
200 kg CO2-eq (tonne wet waste treated)1 (Friedrich and Trois,
2011; Hermann et al., 2011; Rogger et al., 2011; Martínez-Blanco
et al., 2010; Lou and Nair, 2009; IPCC, 2006). Accordingly, Europe
and North America initiated policies for the diversion of the organic
* Corresponding author. Department of Bioresource Engineering, Macdonald
Campus of McGill University, 21 111 Lakeshore, Ste Anne de Bellevue, Québec,
Canada H9X 3V9. Tel.: þ1 514 398 7776; fax: þ1 514 398 8387.
E-mail address: [email protected] (S. Barrington).
0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jenvman.2012.12.008
waste fraction from the municipal solid waste stream (Pires et al.,
2011; Fenerty-McKibbon and Khare, 2005; Thompson and
Tanapat, 2005; Landfill Directive, 1999) and recycling through
either composting or anaerobic digestion.
Composting releases to the atmosphere, heat, CO2 and water
vapour 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 management
practice, composting can also generate volatile organic compounds,
ammonia (NH3), carbon monoxide (CO), nitric oxide (NO), N2O and
CH4 (Colón et al., 2010; Martínez-Blanco et al., 2010; IPCC, 2006;
Barton and Atwater, 2002), which have a negative impact on the
atmosphere. The CO2 emissions from organic waste decomposition
in natural environments have generally been overlooked to focus
on the short term production of CH4 and N2O (IPCC, 2006; ICF
Consulting, 2005). However, the high level of organic waste
production 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 conducted
through centralized facilities or decentralized systems such as
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B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
community centres and home composters (Bernstad and la Cour
Jansen, 2011; Schwalb et al., 2011). As compared to centralized
facilities, home composting eliminates the cost and energy
required for collection, transportation and processing (Boldrin
et al., 2011; Andersen et al., 2010). Therefore, home composting
systems (HC) can potentially reduce GGE for the waste management sector (Chan et al., 2011; Adhikari et al., 2010).
However, CO2, CH4 and N2O emissions can be affected by HC
configuration and management (Bogner et al., 2008; EPIC, 2002)
with a limited number of studies examining their GGE. Using
a weekly fed plastic bin, Colón et al., 2010 reported CH4 and N2O
emissions below their detection threshold. For bi-weekly fed home
composting bins, Andersen et al. (2010) measured CH4 and N2O
emissions in the range of 0.4e4.2 and 0.30e0.55 kg (ton wet waste
treated)1 respectively, totalling 100e239 kg CO2-eq (ton wet
waste treated)1. With weekly fed and mixed home composting
bins, Martínez-Blanco et al., 2010 measured GGE of
0.158 kg CH4 (tonne wet waste treated)1 and 0.676 kg N2O (tonne
wet waste treated)1. These reported GGE are within the range of
those produced for centralized composting facilities (IPCC, 2006),
but illustrates variability in GGE with HC configuration.
Accordingly, the objective of the project focused on testing HC
configuration for the least gaseous emissions. Thus, four common
types of home composting systems (HC) were used to compost
organic waste and monitored for CO2, CH4 and N2O from day 15 to
150: the wood and plastic bins (WB and PB), the mixed and
unmixed ground pile (GPM and GP). All HC were filled at once
(batch fed) with the same restaurant food waste and yard trimmings mixture, to produce conditions of maximize microbial
activity 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 composting
systems (HC) were monitored using four commonly used systems
in North America and Europe: the slatted wood bin (WB)
measuring 0.78 m 0.65 m by 0.75 m in height; the top and bottom
perforated plastic bin (PB) measuring 0.70 m 0.70 m by 0.80 m in
height, 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 waste
required, the experiment was not replicated.
The same food waste (FW) and yard trimmings (YT) were used
to prepare the common mixture batch fed to all four HC on the
same day. A typical FW:YT ratio of 1:1 on a wet volume basis, was
used and its average wet bulk density and water content of 235
(6) kg m3 and 76 (1.0)% respectively. The FW was source
separated and supplied by two restaurants in Rennes, France,
within 3 days of production. It consisted of vegetable and fruit
wastes. The YT were obtained from the green space surrounding
the IRSTEA research station of Rennes, France, and consisted of
a 90% grass clippings and 10% tree leaves on a wet weight basis.
While loading the four experimental HC, the initial compost
mixture was sampled in triplicate for physico-chemical characterization 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 at
the IRSTEA Research Centre, Rennes (France), to avoid rainfall and
direct sunshine interference. Representing mixtures typically used,
equal wet volumes of FW and YT were mixed by hand in a large tub
before being loaded without compaction into the HC. All four HC
were filled at once (batch fed) on the same day with the same
organic waste to 80% of their capacity, to create a high O2 demand
and enhance CH4 and N2O production. While filling the HC,
temperature sensors (model DS1921G-F5, Thermochron iButton,
Dallas Semiconductor, USA) were installed at their mass center
above 0.30 m from ground level.
While being monitored for 150 days, the HC composts were
naturally aerated and manually mixed weekly using a garden fork
except for that of GP. During the mixing operation, compost
samples were collected for analysis of dry matter (DM), total carbon
(TC), total nitrogen (TN), organic matter (OM) and pH every 15 days
during the first 60 days, and then every 30 days for the rest of the
experimental period. The leachate was not monitored in this study.
The GP treatment was sampled only on days 0 and 150. The
temperature sensors were retrieved during the 70 day mixing
operation, because the compost had reached ambient
temperatures.
During the composting process and before the mixing operation, all HC were monitored for GGE, namely methane (CH4),
nitrous oxide (N2O) and carbon dioxide (CO2) starting on day 15. For
all gas monitoring sessions, gases were trapped using a static
chamber placed for 2 h over the compost surface while monitoring
the greenhouse gas concentration (Parkin and Venterea, 2010;
Chadwick et al., 1998). The static chamber consisted of a metallic
chamber measuring 0.40 m in length by 0.15 m in width and 0.20 m
in height, opened at its bottom and pushed into the compost mass
to a depth of 5 mm (Fig. 1d). The upper chamber surface was
airtight and equipped with a silicone rubber septum to collect gas
samples using disposable plastic syringes.
Each monitoring session was repeated weekly for the first 30
days, 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 and
120 min. All air samples were analysed for CO2, CH4, N2O and O2
by gas chromatography (GC e HP6890N, Agilent, Santa Clara,
USA). The GC was equipped with an electron capture detector (ECD)
and a flame ionization detector (FID), and used nitrogen (N2) as
a vector gas.
After 150 days, the compost mass in each HC was weighed and
sampled in triplicate for physico-chemical characterization, to
compute 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 be
measured, although this value was required to compute with time,
losses in DM, TC and TN. Accordingly, an equation was developed
to predict the residual mass of wet compost in each home composting systems at a given time based on the hypothesis that the
mass of fixed solids (FS) is constant during the entire experimental period. Thus, the residual mass of wet compost at time t
was equated to the mass of FS plus that of the residual organic
matter and water:
Mt ¼ FSt þ MðtÞ OMt DMt 104 þ MðtÞ ð1 DMt =100Þ
(1)
where, Mt is total wet mass at sampling time t in kg; FSt is the fixed
solids mass at time t, in kg; OMt is the organic matter concentration
in % DMt, and; DMt is the dry matter concentration at time t, in % of
Mt. The values of OMt and DMt were measured only on days 0 and
150. Nevertheless, 14, 1, 8 and 0% of the original FS were lost as
leachate for WB, PB, GPM and GP, respectively. Therefore, Equation
(1) was corrected presuming a linear loss of FS over the entire
experimental composting period of 150 days.
B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
165
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).
Accordingly, Equation (1) can be rearranged to solve for Mt:
n
o
Mt ¼ FSt = 1 OMt DMt 104 ð1 DMt =100Þ
Q ¼
(2)
The mass evolution of TC and TN was thus computed using the
concentrations measured regularly and the compost mass Mt obtained from Equation (2).
For each gas emission monitoring session, individual gas
production rates were computed from the evolution of their
concentration in the air of the closed chamber, during 120 min,
neglecting the initial diffusion effect. Emissions in CO2, CH4, and
N2O were computed after each session as:
Ar
S 106 V r
OMi
(3)
where Q is the gas production rate in kg h1 (kg OMi)1, Ar is the
ratio of the composter to sampler cross sectional area in m2 m2
(8.7, 8.3 and 12.5 m2 m2 for WB, PB and the ground piles,
respectively), OMi is the initial mass of organic matter in the
composter in kg, S is the rate of gas production equal to the
slope of the linear gas concentration regression in ppmv h1, V is
the sampler volume in m3 and r is the gas density in kg m3.
The density used for the gases, namely CO2, CH4 and N2O were
1.842, 0.668 and 1.826 kg m3 respectively at the standard
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B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
temperature of 20
Toolbox, 2010).
C
and pressure of 101.3 kPa (Engineering
2.4. Analytical procedure
Before being analysed, all triplicate compost sample was dried
in an oven (SR 2000, Thermosi, France) at 80 C until a constant
weight 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 shaking
at 5 C, in just enough distilled water to use a pH electrode (pHElectode SenTix41, WTW, Weilhein, Germany). The OM was
quantified as volatile solids (VS) and determined by burning at
550 C for 3 h (Thermolyne 30400, Furnace, F30420 C-33, Essex,
UK), according to AFNOR (1985). The fixed solids concentration
was determined by the ash remaining following the OM procedure. The TC was determined by burning 10 mg samples at
900 C (Thermo Scientific, FLASH 2000 Series, Organic Elemental
Analyser, Courtaboeuf, France) according to AFNOR (2001).
According to AFNOR (1995), total Kjeldahl nitrogen (TKN) was
determined using an automatic distilling system (VAP 50c,
Gehardt automatic distilator, Gehardt, Germany), after digesting
0.5e1.0 g of sample with H2SO4 (automated Kjeldatherm TZ block
digester, Gehardt, Germany). The TKN value was assumed equal
to TN because of negligible amount of nitrite and nitrate in the
experimental material (Adhikari et al., 2009). To correct all
analytical results, residual moisture was determined by drying
grinded compost samples at 105 C for 24 h (SR 1000, Thermosi,
France).
2.5. Statistical procedure
The experimental HC were filled with the same organic waste
mixture (FW:YT wet mass and wet volume ratio of 1:1) on the same
day and mixed/sampled at the same time, to eliminate all effects
except for that of HC type. The only exception was the ground pile,
where one was not mixed while the other was mixed. Gaseous
emissions from the different composting systems could therefore
be compared with the repeated measures ANOVA procedure, using
PROC GLM procedure at 95% confidence level (SAS Institute Inc.,
2008). Regression equations relating compost CO2 and N2O emissions to TC and TKN losses were computed using Excel (Microsoft
2007).
3. Results and discussion
3.1. Initial characteristics of organic waste
Table 1 summarizes the characteristics of the organic waste
mixture fed into all four HC on day 0. The DM of the initial mixture
ranged 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 TN
varied 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. The
pH of all HC compost was at 6.1 initially. The slight variations in
initial characteristics of the OW amongst the tested HC occurred
due 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 supporting an active compost microbial activity (Adhikari et al., 2009;
Stabnikova et al., 2005; Haug, 1993). For GP, preparing the
Table 1
Initial characteristics of organic waste loaded into the 4 home composting systems (HC) and their evolutions during the 150 days of composting.
Composter
Day
a
Wet
mass (kg)
DM (%)
OM (%dm)
TC (%dm)
TN (g (kg dm)1)
C/N ratio
pH
WB
0
15
30
45
60
90
120
150
72.2
49.3
37.6
33.3
27.6
17.6
13.2
9.2
24.1
23.0
27.0
30.0
33.0
50.4
65.9
81.0
(1.2)
(0.8)
(2.3)
(0.7)
(1.3)
(0.6)
(0.7)
(0.8)
75.3
63.0
58.7
58.1
54.2
53.2
52.6
50.4
(0.2)
(0.1)
(0.2)
(0.1)
(0.1)
(0.3)
(0.2)
(0.3)
39.2
32.7
30.4
29.9
29.2
28.2
27.6
26.6
(0.7)
(0.2)
(0.6)
(0.2)
(0.1)
(0.2)
(0.2)
(0.3)
22.9
27.0
26.3
25.8
24.9
25.2
26.4
24.6
(0.2)
(0.2)
(0.2)
(0.2)
(0.4)
(0.2)
(0.2)
(0.1)
17.1
12.1
11.5
11.6
11.7
11.2
10.5
10.8
6.1
8.8
8.1
8.2
8.6
8.1
7.8
7.5
(0.2)
(0.1)
(0.2)
(0.0)
(0.3)
(0.4)
(0.1)
(0.1)
PB
0
15
30
45
60
90
120
150
75.5
50.9
44.1
38.7
34.6
24.2
16.3
12.5
24.1
22.0
24.0
26.0
27.0
34.5
50.9
66.0
(1.2)
(3.0)
(3.9)
(0.1)
(1.0)
(0.7)
(2.4)
(1.8)
75.2
60.5
58.4
56.2
52.9
47.0
47.4
45.8
(0.2)
(0.3)
(0.2)
(0.1)
(0.2)
(0.2)
(0.3)
(1.0)
39.2
31.2
30.4
29.5
28.7
25.5
25.3
24.6
(0.7)
(0.5)
(0.1)
(0.3)
(0.9)
(0.1)
(0.2)
(0.2)
22.9
26.6
26.2
25.0
23.3
21.3
21.4
20.6
(0.2)
(0.5)
(0.2)
(0.2)
(0.1)
(1.8)
(0.3)
(0.2)
17.1
11.7
11.6
11.8
12.3
12.0
11.8
11.9
6.1
8.4
8.4
8.2
8.8
8.7
8.4
7.7
(0.2)
(0.1)
(0.2)
(0.0)
(0.3)
(0.4)
(0.1)
(0.1)
GPM
0
15
30
45
60
90
120
150
75.0
42.9
36.4
31.4
24.3
15.7
11.2
9.4
23.7
25.0
27.0
30.0
36.0
54.1
75.6
85.0
(1.2)
(2.1)
(4.1)
(1.8)
(4.1)
(2.5)
(0.7)
(2.2)
75.2
59.6
56.0
54.1
50.8
49.6
49.5
49.0
(0.2)
(0.1)
(0.1)
(0.3)
(0.3)
(0.3)
(0.1)
(0.3)
39.4
32.1
30.1
28.7
27.6
26.2
26.1
25.1
(0.7)
(0.5)
(0.9)
(0.2)
(0.5)
(0.2)
(0.1)
(0.10
23.1
26.4
26.1
25.3
23.9
23.8
23.7
24.5
(0.2)
(0.2)
(0.1)
(0.1)
(0.4)
(0.3)
(0.4)
(0.1)
17.1
12.2
11.5
11.3
11.5
11.0
11.0
10.2
6.1
7.9
8.5
8.2
8.9
8.3
7.8
7.7
(0.2)
(0.4)
(0.3)
(0.1)
(0.0)
(0.2)
(0.0)
(0.0)
GP
0
150
63.3
11.8
22.3 (2.0)
60.0 (3.5)
23.3 (0.7)
23.2 (0.3)
17.1
11.2
6.1 (0.2)
7.5 (0.2)
77.0 (0.2)
50.4 (0.8)
39.8 (0.4)
25.9 (0.1)
a
The total mass was measured on days 0 and 150, otherwise estimated from Equation (2). WB e wood bin; PB e plastic bin; GPM-mixed ground pile; GP e unmixed ground
pile; dm e dry mass basis; FW e food waste; YT e yard trimmings; DM e dry matter; TC e total carbon; TN e total nitrogen; OM e organic matter; C/N e carbon to nitrogen
ratio. The numbers in parenthesis represent the standard deviation. All FW:YT dry mass ratio ranged between 0.93 and 0.96 except for the GP at 1.2, as a result of the
heterogeneity of the organic waste.
B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
75
70
65
Wood bin (WB)
Plastic bin (PB)
Mixed ground pile (GPM)
Unmixed ground pile (GP)
Ambient
60
55
Temperature (0 C)
50
45
40
35
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
60
Time (day)
Fig. 2. Temperature regime at the centre of compost mass for all four experimental
home composting systems against ambient temperature. All composters were filled at
once (batch fed) and mixed weekly except for the unmixed ground pile (GP).
heterogeneous organic waste mixture using a wet volume ratio of
1:1 produced a slightly higher FW:YT dry mass ratio (Table 1), with
the resulting DM being slightly lower at 22.3%.
167
The PB and GPM composts reached thermophilic conditions
(>45 C) after 2 days, as compared to 3 days for that of the GP and 7
days for that of the WB. The compost temperature reached almost
ambient levels by day 20 for all HC, with that of WB taking 3 more
days to stabilize.
During the active phase of composting, the temperature regime
reflects the rate of aeration and accordingly, the rate of microbial
activity. Thus, PB provided the best conductive aeration, because of
its 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. Comparing
the GP and GPM regimes, GP lagged in developing thermophilic
temperatures because of its lower initial DM at 22.3% as compared
with 23.7%. Nevertheless, GP sustained as high a temperature
regime as GPM, because of its higher FW content, producing a more
biodegradable organic waste mixture, and because of its large
exposed surface allowing for gas exchange. Comparing GP with WB,
the higher exposed surface of GP provided better aeration despite
the slightly lower DM at 22.3% compared to 24.1%. Thus, the slatted
opening configuration of WB provided an inefficient convective
aeration regime. Reflecting the rate of O2 supply and aerobic
microbial activity, temperature can also reflect the rate of CO2
emission and biodegradability of the organic waste (Epstein, 1997;
Diaz et al., 1993).
3.3. Emissions in CO2, N2O and CH4
3.2. Compost temperature regime
21.60
21.40
21.20
21.00
20.80
20.60
20.40
20.20
20.00
19.80
Within the closed gas monitoring chamber for one sampling
session, Fig. 3 illustrates typical concentration evolutions for CO2,
N2O and CH4, accumulating linearly, with little initial diffusion
14000
R2 = 0.98
12000
R2 = 0.97
CO2 (ppm)
O2 (%)
The temperature developed within the compost of all four HC
during the first 70 days of experimentation is presented in Fig. 2.
10000
8000
6000
4000
2000
0
0
0
25
50
75
25
100 125 150
50
75
100 125 150
Time (minute)
Time (minute)
(b)
(a)
R2 = 0.98
250
12
200
10
CH4 (ppm)
N2 O (ppm)
300
150
100
2
R = 0.96
8
6
4
2
50
0
0
0
25
50
75
100 125 150
Time (minute)
(c)
0
25
50
75
100
125
150
Time (minute)
(d)
Fig. 3. Typical trend of gas concentrations obtained in the closed chamber placed over the compost materials: (a) oxygen for the wood bin at 50 days; (b) carbon dioxide for the
wood bin after 50 days; (c) nitrous oxide for the wood bin after 50 days; and (d) methane for the unmixed ground pile on day 15.
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B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
effect. Measurable CH4 concentrations were obtained only for the
GP compost on day 15, likely because of its lower aeration rate, not
being mixed, its slightly higher moisture and FW levels, favouring
anaerobic pockets producing CH4.
Each greenhouse gas produced a specific emission curves during
the 150 day experimental period (Fig. 4). For CO2, the highest
emission rates were observed on day 15 in the range of 25e
28 g h1 (ton wet waste treated)1, with little variation among
HC. These values dropped to 1.5 g h1 (ton wet waste treated)1
on day 150, with some differences appearing especially for GP on
day 30 and 60 but producing an average drop in CO2 emission
rate 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 for
CO2. On day 15, all HC had low N2O emission of 7e105 mg h1 (ton
wet waste treated)1 with PB producing the highest value along
with the lowest temperature, followed by that of WB, GPM and
then GP. The N2O emission rate increased on day 20, at the end of
the thermophilic phase, likely because of the NH3 oxidation by
NH3 oxidizing bacteria (AOB) under nitrifying conditions (Kim
et al., 2010). Peak rate of 350e550 mg h1 (ton wet waste
treated)1 were reached on days 30 and 40 for all HC, except for
GP peaking on day 80 because of lower O2 availability.
Nevertheless, PB produced the least overall N2O, followed by GP
and then GPM and WB at similar levels. The only factor
corresponding to this N2O emission order is the final compost
DM with PB and GP at 66 and 60%, respectively as compared to
GPM and WB at 85 and 81%.
As for compost CH4 emissions, they were below detection
threshold between days 15 and 150, except for GP on day 15. Higher
CH4 emissions for GP likely resulted from: its lack of mixing,
resulting not only in less aeration, but in the collapse of aeration
pores, and; a higher FW content on a dry matter basis (Table 1),
resulting in a higher O2 demand because of its greater biodegradability but at the same time, a lower aeration rate because of
a higher moisture content.
3.4. Compost evolution and gas emissions
The evolution of all compost characteristics are presented in
Table 1 and the computed losses in mass are illustrated in Fig. 5. For
all HC, compost TC suffered the highest loss of 45e50% between days
0 and 15, followed from day 15 to 150 by an 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 also
reflected by higher N2O emissions. Using the initial and final HC
compost fixed solids mass, their losses through leachate was
found to be 14 and 8% for the WB and GPM composts, compared
to 0e1% for the PB and GP. Whereas PB was well aerated and
could lose water vapour through evaporation rather than leaching,
GP was not mixed, and suffered from the collapse of its aeration
and drainage pores preventing leachate drainage.
Wood bin (WB)
Plastic bin (PB)
Mixed ground pile (GPM)
Unmixed ground pile (GP)
80
Wood bin (WB)
Plastic bin (PB)
Mixed ground pile (GPM)
Unmixed ground pile (GP)
30
70
TC loss (%)
CO2 (g hr-1 (ton wet waste treated)-1 )
35
25
20
60
50
40
15
30
10
20
5
0
0
15
20
30
40
50
60
80
100
120
15
30
45
60
90
120
150
90
120
150
Time (day)
150
(a)
Time (day)
(a)
Wood bin (WB)
Plastic bin (PB)
Mixed ground pile (GPM)
Unmixed ground pile (GP)
600
Wood bin (WB)
Plastic bin (PB)
Mixed ground pile (GPM)
Unmixed ground pile (GP)
70
500
60
400
50
TN loss (%)
N2 O (mg hr-1 (ton wet waste treated)-1 )
700
300
200
100
40
30
20
10
0
0
15
20
30
40
50
60
Time (day)
80
100
120
150
(b)
Fig. 4. Measured emissions of CO2 and N2O for the four experimental home composting 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).
0
15
30
45
60
Time (day)
(b)
Fig. 5. Loss of total carbon (TC) (a) and total nitrogen (TN) (b) over time for the
compost of all four home composting systems estimated using Equation. Note: Composter GP was sampled only on days 0 and 150.
B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
Measured between day 15 and 150, CO2eC emissions corresponded to TC mass losses. For the WB compost for example, 27% of
the 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. For
the same period, measured CO2eC emissions amounted to
23 kg (ton wet waste treated)1. Accordingly and from day 15 to
150, all HC composts emitted CO2eC corresponding to TC losses
over time:
R2 ¼ 0:95
CO2 WBðtÞ ¼ 0:72 TCLðtÞ þ 1:78
CO2 PBðtÞ ¼ 1:55 TCLðtÞ þ 2:05
R2 ¼ 0:99
CO2 GPMðtÞ ¼ 1:08 TCLðtÞ þ 2:05
R2 ¼ 0:99
(4)
(5)
(6)
where CO2 and TCL are the loss of total carbon as measured by CO2
emissions 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 not
characterized regularly, no regression equation could be
formulated.
In terms of nitrogen, N2O emissions occurred at the end of the
active composting stage, between days 20 and 50, with the GP
compost showing a last peak on day 80. Nitrogen losses as N2OeN
amounted to 0.01e0.016 kg during 150 days of composting
represented only 4e7% of the final compost TN losses, suggesting
more important losses through leachate, and NH3 and N2
volatilization. Most of the emissions occurred between 15 and
40 C at a pH above 8, conditions favouring nitrification and
denitrification along with the production of N2O and NO (Kim
et al., 2010; de Bertoldi et al., 1983). During the thermophilic
phase, NH3 volatilization governs nitrogen losses (Pagans et al.,
2006; Barton and Atwater, 2002; de Bertoldi et al., 1983). Emissions in N2O were also correlated to TN losses from day 15 and 150:
N2 OWBðtÞ ¼ 0:08 TNLðtÞ þ 0:02
R2 ¼ 0:85
N2 OPBðtÞ ¼ 0:03 TNLðtÞ þ 0:0008
N2 OGPMðtÞ ¼ 0:11 TNLðtÞ þ 0:0003
R2 ¼ 0:94
R2 ¼ 0:96
(7)
(8)
(9)
where N2O is the compost N2OeN emission and TNL is the compost
loss of TN, both in kg N (ton wet waste treated)1, and; the
subscripts WB, PB and GPM correspond to individual HC. Because
the compost of the unmixed ground pile (GP) was not characterized
regularly, no regression equation could be formulated.
Considering the evolution in compost characteristics, CO2
emissions 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 wet
waste)1 for GPM and GP. Emissions of N2O were higher for the
drier compost of WB and GPM at 81 and 85% DM, as compared to
the wetter compost of PB and GP at 60 and 66% DM (Table 1). More
accurate compost CH4 monitoring is required for the first 15 days of
composting to properly characterize such emissions.
3.5. Total compost gas emissions
Table 2 summarizes CO2, CH4 and N2O emissions over time and
provides a CO2 equivalent value for N2O considering a contribution
of 298 times that of CO2 (IPCC, 2007). Because of limited CH4
measurements, this gas will not be discussed.
From day 15 to 150, HC produced level of CO2 ranging from 27 to
32 kg (ton of wet waste)1 except for the GP at 40 kg (ton of wet
169
Table 2
Greenhouse gas emission for the four home composting systems.
Gas/time interval
CO2
15e20 days
20e60 days
60e120 days
120e150 days
Total CO2 (kg (ton wet
waste treated)1)
CH4 (15e150 days)
Total CH4 (kg CO2-eq
(ton wet waste
treated)1)
N2O
15e20 days
20e60 days
60e120 days
120e150 days
Total N2O (kg (ton wet
waste treated)1)
Total N2O (kg CO2-eq
(ton wet waste
treated)1)
Composters (kg (ton wet waste treated)1)
WB
PB
GPM
GP
3
16
7
1
27
3
16
9
1
27
3
19
9
1
32
3
16
19
2
40
nd
e
nd
e
nd
e
0.010
0.289
0.032
0.002
0.333
99
0.014
0.150
0.022
0.001
0.187
56
0.001
0.274
0.051
0.001
0.327
97
0.002
0.05
0.006
0.017
0.228
0.002
0.253
75
WB e wood bin; PB e plastic bin; GPM e mixed ground pile; GP e unmixed ground
pile; CO2 e carbon dioxide; CH4 e methane; N2O e nitrous oxide; nd e not detected.
The greenhouse gas emission impact of CH4 and N2O are considered to be 25 and
298 times higher than CO2 (IPCC, 2007).
waste)1. For the same period, PB produced the lowest level of N2O
emissions at 0.187 kg (ton of wet waste)1 or 56 kg CO2-eq (ton wet
waste treated)1. These values were followed by GP at 75, and then
GPM and WB at 97 and 99 kg CO2-eq (ton wet waste treated)1,
respectively. Over time, the most N2O emission occurred between
days 20 and 60, except for GP loosing most of its N2O from day 60 to
120, because of its wetter compost.
Observed N2O emissions corresponded to the average value
reported for centralized composting facilities of 0.33 kg (tonne wet
waste treated)1 (IPCC, 2006) or 100 kg CO2-eq (ton of wet
waste)1. The N2O emissions from PB were slightly lower at
56 kg CO2-eq (ton of wet waste)1 demonstrating some improvement over the other HC. The values measured were low compared
to those observed by Andersen et al. (2010) and Martínez-Blanco
et 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 on
gaseous emissions as HC configuration. Furthermore, there may be
some interaction between HC configuration and management
practices, where for example, bins with top and bottom perforations may not need weekly mixing to produce low gaseous
emissions.
4. Conclusion and recommendations
To reduce earth warming trends, lower greenhouse gas emission
(GGE) technologies are required to manage the increasing
production of organic wastes. Besides landfill diversion, home
composting systems (HC) can help recycle residential organic waste
while reducing collection, transportation and treatment costs and
energies. Nevertheless, HC configurations must limit gaseous
emissions to levels comparable to other treatment alternatives. The
objective of the project therefore focused on testing HC configuration for the least gaseous emissions. Four common types of HC
were used to compost organic waste and monitored for CO2, CH4
170
B.K. Adhikari et al. / Journal of Environmental Management 116 (2013) 163e171
and N2O from day 15 to 150: the wood 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 carbon
and total nitrogen losses. Nevertheless, HC producing drier
composts such as the mixed ground pile and the wood bin, lost the
most N2O while the plastic bin with convective aeration created by
bottom and top perforations produced the least at half that of the
average emissions from centralized composting emissions.
Comparing the results of the present project to that of others, future
research should examine the interaction between management
practice and HC configuration to minimise CH4 and N2O emissions.
Emissions of CH4 need careful monitoring during the active phase
of the process.
Acknowledgements
This study takes part of a larger project entitled ECCOVAL funded by the regional council of Brittany in France; the authors also
acknowledge the financial and all necessary logistics supported by
the Rennes IRSTEA, France and the Natural Science and Engineering
Research Council of Canada.
Symbols and abbreviations
Ar
CH4
C/N
CO
CO2
DM
DMt
dm
FSt
FW
GGE
GP
GPM
HC
h
Mt
NH3
NH4
N2O
O2
OM
OMi
OMt
OW
PB
ppm
Q
S
TC
TN
V
YT
WB
r
ratio of the composter to sampler cross sectional area in
m2 m2 (8.7, 8.3 and 12.5 m2 m2 for WB, PB and ground
piles respectively)
methane
carbon to nitrogen ratio
carbon monoxide
carbon dioxide
dry matter
dry matter concentration at time t, in % of Mt
dry mass
fixed solids mass at time t, in kg
food waste
greenhouse gas emissions
unmixed ground pile
mixed ground pile
home composting system (composting bin or ground
pile)
hour
total wet mass at sampling time t in kg
ammonia
ammonium
nitrous oxide
oxygen
organic matter content
initial organic matter mass
organic matter concentration in % of DMt
organic waste
plastic bin
parts per million
gas production rate in kg h1 (kg OMi)1
rate of gas production (slope of the linear gas
concentration regression in ppmv h1)
total carbon
total nitrogen
closed gas collection chamber volume in m3
yard trimmings
wood bin
gas density in kg m3.
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