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Emission of greenhouse gases from home aerobic composting, anaerobic digestion and vermicomposting of household wastes in Brisbane (Australia)

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This study investigated greenhouse gas (GHG) emissions from three different home waste treatment methods in Brisbane, Australia. Gas samples were taken monthly from 34 backyard composting bins from January to April 2009. Averaged over the study period, the aerobic composting bins released lower amounts of CH(4) (2.2 mg m(- 2) h(-1)) than the anaerobic digestion bins (9.5 mg m(-2) h(-1)) and the vermicomposting bins (4.8 mg m(-2) h( -1)). The vermicomposting bins had lower N(2)O emission rates (1.2 mg m(-2) h(- 1)) than the others (1.5-1.6 mg m(-2) h( -1)). Total GHG emissions including both N(2)O and CH(4) were 463, 504 and 694 mg CO(2)-e m(- 2) h(-1) for vermicomposting, aerobic composting and anaerobic digestion, respectively, with N(2)O contributing >80% in the total budget. The GHG emissions varied substantially with time and were regulated by temperature, moisture content and the waste properties, indicating the potential to mitigate GHG emission through proper management of the composting systems. In comparison with other mainstream municipal waste management options including centralized composting and anaerobic digestion facilities, landfilling and incineration, home composting has the potential to reduce GHG emissions through both lower on-site emissions and the minimal need for transportation and processing. On account of the lower cost, the present results suggest that home composting provides an effective and feasible supplementary waste management method to a centralized facility in particular for cities with lower population density such as the Australian cities.
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Research Article
Emission of greenhouse gases from
home aerobic composting, anaerobic
digestion and vermicomposting of
household wastes in Brisbane (Australia)
Yiu C Chan
1
, Rajiv K Sinha
1
and Weijin Wang
2
Abstract
This study investigated greenhouse gas (GHG) emissions from three different home waste treatment methods in Brisbane,
Australia. Gas samples were taken monthly from 34 backyard composting bins from January to April 2009. Averaged over
the study period, the aerobic composting bins released lower amounts of CH
4
(2.2 mg m
2
h
1
) than the anaerobic digestion
bins (9.5 mg m
2
h
1
) and the vermicomposting bins (4.8 mg m
2
h
1
). The vermicomposting bins had lower N
2
O emission
rates (1.2 mg m
2
h
1
) than the others (1.5–1.6 mg m
2
h
1
). Total GHG emissions including both N
2
O and CH
4
were 463,
504 and 694 mg CO
2
-e m
2
h
1
for vermicomposting, aerobic composting and anaerobic digestion, respectively, with N
2
O
contributing >80% in the total budget. The GHG emissions varied substantially with time and were regulated by temper-
ature, moisture content and the waste properties, indicating the potential to mitigate GHG emission through proper man-
agement of the composting systems. In comparison with other mainstream municipal waste management options including
centralized composting and anaerobic digestion facilities, landfilling and incineration, home composting has the potential to
reduce GHG emissions through both lower on-site emissions and the minimal need for transportation and processing. On
account of the lower cost, the present results suggest that home composting provides an effective and feasible supplementary
waste management method to a centralized facility in particular for cities with lower population density such as the Australian
cities.
Keywords
Greenhouse gases, home composting, vermicomposting, anaerobic digestion, organic waste
Date received: 26 October 2009; accepted: 14 May 2010
Introduction
Disposal of municipal solid waste (MSW) has been mainly
through landfilling, incineration and centralized composting
and anaerobic digestion facilities in urban areas around the
world. These processes involve direct and indirect emissions
of greenhouse gases (GHGs) including carbon dioxide (CO
2
),
methane (CH
4
), nitrous oxide (N
2
O) and non-methane
hydrocarbons (NMHCs) and contribute to around 3–4%
of the anthropogenic GHG emissions in terms of CO
2
-equiv-
alent (CO
2
-e) (Pipatti and Savolainen, 1996; Australian
Greenhouse Office, 2007; Papageorgiou et al., 2009). More
than 70% of MSW is disposed of in landfills in Australian
and overseas cities (Ernst, 1990; Aumonier, 1996;
Queensland Environmental Protection Agency, 2002;
Mohareb et al., 2008). Anaerobic decomposition of these
wastes in the landfills results in the emission of CH
4
and as
such contributes significantly to the global greenhouse
budget (Hobson et al., 2005). Disposal of MSW contributed
17 million tonnes CO
2
-e of GHG emissions in Australia in
2005, equivalent to the emissions from 4 million cars or 2.6%
of the national emissions (Australian Greenhouse Office,
2007).
Due to the challenge of climate change and other
environmental concerns on landfills (Lisk, 1991), government
1
Griffith University, Nathan, QLD 4111, Australia.
2
Department of Environment and Resource Management, 80 Meiers
Rd, Indooroopilly QLD 4068, Australia.
Corresponding author:
Yiu C Chan, Griffith University, Nathan, QLD 4111, Australia
Email: a.chan@griffith.edu.au
Waste Management & Research
29(5) 540–548
!The Author(s) 2010
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DOI: 10.1177/0734242X10375587
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authorities around the world have introduced regulations
to phase out or reduce waste going to landfills
(Murphy and Power, 2006; Lee et al., 2007; Department of
Climate Change, 2007) and encouraged alternative waste
management options (Nolan ITU Pty Ltd, 2004). The reduc-
tion of emissions from waste disposal has mainly been
achieved by capturing the landfill gases and diversion of
MSW (mainly by paper and material recycling) (Australian
Greenhouse Office, 2007). Domestic food and garden wastes
contribute 15–70% of the MSW in urban areas. Composting
of domestic wastes in centralized facilities and/or at source
by residents has been perceived to have great potential in
reducing GHG emissions (Tchobanoglous et al., 1993; Wei
et al., 2000). For example, the Victorian, New South Wales
and South Australia Governments in Australia have set tar-
gets of increasing the recycling rate to up to 75% by 2010 by
promoting the composting of organic waste (Zero Waste
South Australia, 2005; Department of Climate Change,
2007; Victorian Government Department of Sustainability
and Environment, 2009).
Three types of methods are common for the recycling of
organic waste, namely aerobic composting, anaerobic diges-
tion and vermicomposting. In aerobic composting the waste
is aerated by air flowing constantly through the open system
and by intermittent turning of the waste by the operator.
Anaerobic digestion is carried out in the oxygen-deprived
environment inside the closed chamber. Vermicomposting
is similar to aerobic composting except that the composting
and aeration processes are aided by the use of detritivorous
worms. The type and emission rate of GHGs emitted from
these processes, in particular household practices, have not
been well studied (He et al., 2000). There have only been a
small number of studies on the emission of GHGs from cen-
tralized composting and anaerobic digestion facilities. The
findings from these studies indicate that GHG emissions
are dependent on the method used and other factors such
as the windrowing rate, age, depth, temperature and pore
space of the compost mix (He et al., 2000; Hobson et al.,
2005; Wihersaari, 2005; Lundie and Peters, 2005; Amon
et al., 2006; Majumdar et al., 2006). Under aerobic compost-
ing conditions, the gas emitted is mainly CO
2
rather than
CH
4
(Majumdar et al., 2006). Since this CO
2
is biogenic in
origin it is usually not counted in the GHG emission budget
(Intergovernmental Panel on Climate Change, 2007;
Department of Climate Change, 2007). Emission of
NMHCs has been found to be relatively small compared to
emissions of CH
4
and N
2
O. Anaerobic digestion has been
found to emit more CH
4
than aerated composting (Mata-
A
´lvarez et al., 2000). These findings show that the
use of different methods under different conditions could
result in very different type and amount of GHGs (Beck-
Friis et al., 2000). More information is required on the
actual effectiveness of different methods and conditions on
GHG reduction.
The aim of this study was to investigate GHG emissions
from different types of household organic waste treatment
methods (aerobic composting, anaerobic digestion and ver-
micomposting) in relation to environmental conditions and
the properties of the waste materials. The result will assist in
the identification of better MSW management systems and
practices with maximum GHG mitigation potential.
Materials and methods
Experimental design
This study was undertaken in 15 suburbs within 20 km from
the Brisbane Central Business District (27.5S 153E) in
south-east Queensland of Australia (Fig. 1). Brisbane has
a population of 1.95 million and a population density of
380 people km
2
in 2008 (Australian Bureau of Statistics,
2009). Twenty-two volunteer households with 11 aerobic
composting bins, 12 anaerobic digestion bins and 11 vermi-
composting bins in total were involved in this study. The bins
were located either in the backyard or front yard of the
households.
Results of a survey conducted at the beginning of the
study showed that the households in this study composted
on average 4–5 kg of food and green waste in each bin each
week. Similar to those reported in literature (Tchobanoglous
et al., 1993), the waste the households composted in this
study was about 50–70% food waste (mainly fruits and veg-
etables remnants and food left-over) and 30–50% green
waste (mainly grass clipping). The actual mass and compo-
sition of waste and the maintenance of the bins varied among
the volunteers. Most of the volunteers added the waste daily
and retrieved the compost products monthly.
The bins used were commercially available from hardware
stores with a volume of approximately 220 L (Fig. 2). The
aerobic bins have ventilation slots to facilitate airflow
through them. The anaerobic bins were only opened during
the addition of waste and retrieval of compost, and represent
an anaerobic digester when the lid is closed. The vermicom-
posting bins are similar in structure to the aerobic bins but
with a reservoir at the bottom to collect the leachates. The
vermicomposting bins normally contained approximately
1500–2000 worms. Tiger Worm (Eisenia fetida), Indian
Blue Worm (Perionyx excavatus), African Night Crawler
(Eudrilus euginae) and Red Worm (Lumbricus rebellus)
were the main worm species found in the vermicomposting
bins in this study (Sinha et al., 2002). General instructions
and guidance were also provided to the volunteers on the
proper operation of the three types of systems. The vermi-
composting bins were placed in shaded area. Meat products
were not used, but dried grass clippings (high in carbon) were
added to the food waste (high in nitrogen) to maintain a
carbon to nitrogen (C/N) ratio close to 25 to 1. Bulking
agents such as mulches were also added in aerobic compost-
ing and vermicomposting bins to increase air pockets.
Chan et al. 541
D’ AGUILAR
Mt D’ Aguilar
RANGE
BRISBANE
Moreton Bay
Moreton Island
North Strodbroke
Island
5km
10km
20km
15km
N0510
Kilometres
15 20
Figure 1. The 15 Brisbane suburbs of the volunteer households in this study (represented by the round dots in the figure).
Anaerobic digester bin
Aerobic composting bin Vermicompostin
g
bin
(a) (b)
Septum
Waste
surface
12.5cm
11.5cm
13.4cm 1cm
14.6cm
Figure 2. Examples of: (a) anaerobic digester, aerobic composting and vermicomposting bins; and (b) funnel static chamber
used to collect the greenhouse gas samples.
542 Waste Management & Research 29(5)
Waste in the aerobic composting bins was turned weekly.
Water was sprinkled in the bins to maintain a moisture con-
tent of 50–60% for the aerobic composting and anaerobic
digestion bins and 60–70% for the vermicomposting bins. A
two-bin rotating system was also recommended to allow 2–3
months for the compost to mature, although most of house-
holds in this study operated only one bin.
Sampling of CO
2
,CH
4
and N
2
O
Gas samples were collected during the last weekend of each
month during January–April 2009. This period covers the
humid summer and autumn seasons in Brisbane. In total
132 samplings were conducted (43 from aerobic bins,
45 from anaerobic bins and 44 from vermicomposting bins).
Gas samples were taken by using the static chamber method
(Zhang et al., 2008). A 0.7 L glass funnel with the stem cut off
and replaced with a removable septum was used as the collec-
tion chamber (Figure 2). Duplicate sampling from different
areas on the waste surface indicates good precision in results
(relative standard deviation <25% in general).
In each sampling, the inverted funnel with the septum
removed was first pushed approximately 1 cm deep into the
waste surface inside the bin. The rim of the funnel was
wrapped around with waste and water applied if needed to
provide a good air seal. The open funnel was left inside the
bin with the lid of the bin closed for approximately 5 min
before the septum was put back onto the funnel. Then the
first air sample was taken from inside the funnel by inserting
a 30 mL syringe through the septum. The air sample was then
transferred from the syringe into an evacuated container. The
bin was closed again for approximately 30 min then the
second air sample was taken. Results from preliminary sam-
plings show that the 30-min duration was sufficient to pro-
duce precise results comparable to the results of longer
duration.
Environmental and waste parameters including ambient
temperature, temperature near the waste surface (at 2 cm
below the surface) and inside the waste (at 8 cm below the
surface), and the pH and moisture content in the waste were
also determined. pH and moisture content were determined
with a soil pH and moisture tester (Takemura Electric Works
Model DM-15) by inserting the tester at three random posi-
tions to approximately 7 cm deep into the surface. DM-15
scales moisture content from 1 to 8. Most of the composting
bins in this study returned a high moisture scale of 8 or
beyond. Moisture-scale readings beyond 8 were treated as
9 in the statistical analysis. The measured values from the
replicate samplings were within 2.8C, 1.0 pH and 1.2
moisture scale. Measurements of a set of compost–water
mixtures with varying mass percentage of moisture showed
that moisture scale 1 was equivalent to a moisture content of
approximately 20%, whereas moisture scale 8 was equivalent
to approximately 33%.
Analysis of CO
2
,CH
4
and N
2
O and estimation
of GHG emission rates
The gas samples in the containers were analysed by a gas
chromatograph (Varian 3800) within one week after the sam-
pling. The gas chromatograph was equipped with a thermal
conductivity detector (GC-TCD), a flame ionization detector
(GC-FID) and an electron capture detector (GC-ECD) for
the analysis of CO
2
,CH
4
and N
2
O, respectively. Dual
Porapak columns using helium as the carrier gas for CO
2
and CH
4
, and nitrogen as the carrier gas for N
2
O were
used in the analysis. CO
2
standards of 513, 1000, 2009 and
4020 ppm were used for the calibration of results, while 1.8
and 5.1 ppm standards were used for CH
4
and 0.50, 5.02,
12.1 and 18.7 ppm standards were used for N
2
O.
The concentrations of CO
2
,CH
4
and N
2
O in the two
samples collected from each sampling were used to estimate
the emission rate of the GHGs from the bin. The emission
rate in mg m
2
h
1
was calculated by Equation (1), based on
the ideal gas equation:
Rateðmg m2h2Þ¼ C2C1
ðÞMW
V=0:08206 TA1DðÞ
ð1Þ
where C
1
and C
2
are the concentrations of the GHG in ppm
in the two samples; MW is the molecular weight of the GHG;
Vis the volume of air above the waste surface inside the
funnel ¼5.44 10
4
m
3
;A
1
is the area of the waste surface
inside the funnel ¼1.42 10
2
m
2
(Fig. 2); Tis the tempera-
ture in Kelvin; Dis the duration between the two samples in
hours. The emission rate in terms of kg GHG kg
1
waste was
estimated by Equation (2):
Rateðkg GHG kg1wasteÞ¼Rateðmg m2h1ÞA2
24 7106=Wð2Þ
where A
2
is the area of the waste surface inside the compost-
ing bin and Wis the amount of food and green waste treated
each week. The area of the waste surface was approximately
0.24 m
2
for the anaerobic bins and 0.25 m
2
for the aerobic
and vermicomposting bins. The volunteers in this study
added waste and retrieved compost products on a regular
basis and Wwas assumed to be 4.5 kg. The CO
2
-equivalents
(CO
2
-e) of CH
4
and N
2
O were estimated using the
global warming potential of 21 and 310, respectively
(Intergovernmental Panel on Climate Change, 2007).
Statistical analysis
Five of the 132 samplings conducted were found to
have extreme CH
4
and/or N
2
O emission rate values
(beyond average +4 s.d.) and were thus excluded from the
statistical analysis. All the measured parameters were tested
for normality by using the Kolmogorov–Smirnov test
Chan et al. 543
(Beck-Friis et al., 2000). At the significance level of 0.05, the
emission rates of CO
2
,CH
4
and N
2
O were found to be log-
normally distributed, and consequently the natural logarith-
mic values of these parameters were used in statistical
comparison.
Pearson correlation analysis was used to investigate
the correlation between the emission rates of the GHGs and
the other measured parameters (Beck-Friis et al., 2000).
ANOVA F-test was used to identify the influencing factors
(waste treatment method, month of sampling and time of
the day of sampling) of the GHG emission rates. Then a t-
test (least significant difference, LSD) was used to investigate
the influence of the identified factors on the emission rates.
The significance level of 0.05 was used in these statistical tests.
In this study, the samplings were conducted between 0800
and 1900 h, therefore the time of the day of sampling was
categorized as ‘am’ (0800–1000 h), ‘noon’ (1100–1300 h),
‘pm’ (1400–1600 h) or ‘late pm’ (1700–1900 h).
Results and discussion
Emission rates of GHGs
A summary of the GHG emission rates and the environmen-
tal and waste parameters measured in this study is presented
in Table 1. Anaerobic bins and vermicomposting bins emit-
ted significantly higher amounts of CO
2
and CH
4
than the
aerobic bins. This indicates that anaerobic bins and vermi-
composting bins were more efficient in decomposing the
carbon in waste into CO
2
and more favourable for CH
4
production than the aerobic bins.
On the other hand, aerobic bins and anaerobic bins emit-
ted significantly higher amounts of N
2
O than the vermicom-
posting bins. Presumably, the N
2
O emitted from the
composting bins were mainly from the denitrifying process
in the anaerobic zones in the compost, but might also be
created in the nitrifying process in aerobic zones (Beck-
Friis et al., 2000) and the activities of the denitrifying bacteria
within the earthworm gut (Hobson et al., 2005). The lower
emission of N
2
O from vermicomposting bins indicated that
the emission of N
2
O from worm gut was probably offset by
the reduction of anaerobic denitrification, due to the burrow-
ing action of the earthworms.
On the contrary, Hobson et al. (2005) found that for
larger scale systems windrow composting emitted more
CH
4
but less N
2
O than vermicomposting. This indicates
that the process and rate of decomposition of organic mate-
rials in larger scale systems could be very different to those
in home systems, due to the different level of ventilation
and different worm density in the two types of systems.
Table 1. Summary of GHG emission rates and regulating factors (min - max values in bracket)
Parameters Aerobic bins Anaerobic bins Vermicomposting bins
Number of samples 40 45 42
GHG emissions (mg m
2
h
1
)
CO
2a
882
B
(23–5764) 2950
A
(91–10069) 1675
A
(146–5669)
CH
4b
2.17
B
(0.00–38.05) 9.54
A
(0.00–52.90) 4.76
A
(0.00–40.89)
N
2
O
c
1.48
A
(0.01–16.25) 1.59
A
(0.00–16.37) 1.17
B
(0.00–24.78)
Total emissions (mg CO
2
-e m
2
h
1
)
Excluding CO
2
504 (4–5038) 694 (0.76–5073) 463 (4–8475)
Including CO
2d
1386
B
(28–7554) 3644
A
(259–14351) 2138
A
(189–14144)
Average % of GHG emissions
Excluding CO
2
CH
4
9.1 28.8 21.7
N
2
O 90.9 71.2 78.3
Including CO
2
CO
2
63.6 80.9 78.3
CH
4
3.3 5.5 4.7
N
2
O 33.1 13.6 17.0
Average GHG emissions (kg CO
2
-e kg
1
waste)
Excluding CO
2
0.0047 0.0062 0.0043
Ambient temperature (C) 28.7 (20.0–35.0) 28.1 (20.0–34.5) 28.5 (20.0–34.0)
Temperature below 2 cm (C) 30.2 (26.0–41.5) 31.6 (24.0–45.0) 26.9 (22.5–32.0)
Temperature below 8 cm (C) 31.2 (26.0–48.0) 32.4 (25.0–46.0) 27.1 (22.0–31.0)
pH 5.7 (3.1–6.7) 5.9 (4.1–6.7) 6.6 (5.8–7.0)
Moisture scale 6.4 (2.0–9.0) 6.8 (2.0–9.0) 8.2 (2.9–9.0)
Equivalent moisture content (%) 31 (25–>33) 31 (25–>33) 32 (26–>33)
a
LSD t-test shown that the waste treatment methods denoted by a superscript ‘A’ on their CO
2
emission values emitted significantly more CO
2
on average than those denoted by a superscript ‘B’ at the significance level of 0.05;
b,c,d
and similarly for CH
4
,N
2
O and total emissions.
544 Waste Management & Research 29(5)
Beck-Friis et al. (2000) found that the co-existence of both
anaerobic and aerobic conditions was not apparent in small,
intensively managed compost heaps, therefore resulting in less
CH
4
and N
2
O emissions. In their study, a very high level of
CH
4
emission was found from large compost heaps. They also
found that N
2
O emissions tended to be higher in compost
heaps after prolonged storage. In this sense home composting
systems reduce N
2
O emissions because the owners retrieve
compost products more frequently.
In terms of CO
2
-e, CO
2
emissions contributed approxi-
mately 64% on average of the total GHG emissions in the
aerobic bins, and about 80% of the total GHG emissions in
the anaerobic bins and vermicomposting bins. When CO
2
emission was excluded from the accounting as is common
practice, the three waste treatment methods emitted
463–694 mg CO
2
-e m
2
h
1
on average, largely attributable
to N
2
O emissions. LSD t-test showed that in terms of
CO
2
-e and with CO
2
emission included, anaerobic bins and
vermicomposting bins emitted significantly higher amounts
of GHGs than the aerobic bins. When CO
2
emission was
excluded, however, there was no significant difference in
the total GHG emissions between the three waste treatment
methods.
Variation in GHG emissions in relation
to environmental conditions and
waste properties
The environmental conditions and waste properties for the
three waste treatment methods are shown in Table 2.
The temperature inside the waste in this study (26–48C)
was lower than the optimum temperature (45–55C) sug-
gested by Ja
¨ckel et al. (2005) for the oxidation of CH
4
.
However, given the much smaller size of the waste heaps
for the home systems, attaining the optimum temperature
may not be possible under normal circumstances. The tem-
perature at 8 cm below the waste surface was higher than that
at 2 cm below the surface in the aerobic and anaerobic bins.
They were correlated to and higher than the ambient tem-
perature (r>0.32), reflecting the biological activities in the
waste. However, in the vermicomposting bins the tempera-
ture inside the waste was rather uniform and slightly lower
than the ambient temperature. This could be due to the
movement of the earthworms inside the waste making the
temperature in the waste more uniform. Furthermore, the
owners of the vermicomposting bins tend to moisturize
their bins more often, therefore effectively reducing the tem-
perature in the waste in the summer months. The moisture
content was also significantly higher in the vermicomposting
bins.
The pH value of the waste was acidic to neutral (5.8–7.0)
in all the waste treatment methods, which was within the
range of international control standards on pH of composts
(6.0–8.5; Wei et al., 2000; Tsai, 2008).
Large variations in emission rates have been reported by
other researchers for samples taken at different time of the
day (Christensen et al., 1996). The temporal trends of GHG
emissions in this study are shown in Table 3. The emission of
CO
2
was not significantly different in the different months,
and was higher at noon and lower in late afternoon. This is
Table 2. Comparison of the average environmental conditions and waste properties for different composting systems and at
different times
Ambient
temperature
(C)
a
Temperature at
2 cm below
the waste
surface (C)
b
Temperature at
8 cm below the
waste surface (C)
c
pH
d
Moisture
scale (Equivalent %
moisture)
e
Aerobic 28.7 30.2
A
31.2
A
5.7
C
6.4 (31)
B
Anaerobic 28.1 31.6
A
32.4
A
5.9
B
6.8 (31)
B
Vermicomposting 28.5 26.9
B
27.1
B
6.6
A
8.2 (>33)
A
January 25.9
B
30.1 31.5
A
5.8
B
8.0 (>33)
A
February 31.3
A
30.6
A
30.9 6.1
A
7.4 (31)
D
March 29.8
A
29.2 29.8 6.2
A
6.2 (31)
B,E
April 26.2
B
28.5
B
28.7
B
6.1
A
6.9 (31)
B
am 29.3
A
29.0 29.6 5.9
A
5.9 (30)
B,E
noon 29.3
A
30.0 30.7 6.3
A
7.4 (31)
D
pm 30.0
A
30.1 30.6 6.1
A
6.8 (31)
B
late pm 22.4
B
28.3 29.1 5.6
B
8.2 (>33)
A
a,b,c
The waste treatment methods (and similarly the months and the times of the day) denoted by a superscript ‘A’ on the temperature values
were significantly higher in the temperature on average than those denoted by a superscript ‘B’, and so forth;
d,e
and similarly for pH and
moisture scale. Also those denoted by a superscript ‘D’ were significantly higher in moisture scale on average than those denoted by a
superscript ‘E’.
Chan et al. 545
probably due to the higher temperature at noon speeding up
the decomposition of organic material in waste. On the other
hand the emissions of CH
4
and N
2
O did not show any con-
sistent temporal trends.
In the aerobic composting and vermicomposting bins,
CO
2
(r>0.45) and CH
4
(r>0.63) emissions were related to
the temperature inside the waste, whereas N
2
O emission was
not. These findings are different to those of Beck-Friis et al.
(2000) and Amlinger et al. (2008) in which N
2
O emission was
found to be related to waste temperature, perhaps because of
the different size of the compost heaps in their studies and
this study. Higher emission of CH
4
from wet waste stock has
been reported by Brown and Subler (2007) but this relation-
ship was not significant in this study. In the anaerobic bins,
CO
2
emission was correlated to both the temperature
(r>0.33) and moisture content (r¼0.32) inside the waste.
CH
4
emission correlated to moisture content only
(r¼0.32), whereas N
2
O emission correlated to temperature
in the waste only (r>0.48). There were smaller variations in
temperature, pH and moisture content in the vermicompost-
ing bins. No significant relationship between these factors
and CO
2
and N
2
O emissions was found, but CH
4
emission
increased with increasing temperature in the vermicompost
(r>0.40).
Implications on the environmental benefits of
home aerobic composting, vermicomposting
and anaerobic digestion
The equivalent emission rate of GHGs in kg CO
2
-e kg
1
waste for the three waste treatment methods, excluding
CO
2
emissions, are listed in Table 1. The average GHG emis-
sions from the home systems in this study (0.0043–
0.0062 kg CO
2
-e kg
1
waste) were generally lower than
those estimated in studies of the centralized waste
management options (Mata-A
´lvarez et al., 2000; Fukumoto
et al., 2003; Department of Climate Change, 2008; Amlinger
et al., 2008; Lou and Nair, 2009). These studies were gener-
ally based on the life-cycle analysis approach, using default
GHG emission factors for MSW (Intergovernmental Panel
on Climate Change, 2007; Department of Climate Change,
2007). For example, in the case of landfilling, the emission of
N
2
O is regarded as negligible (Pipatti and Savolainen, 1996;
Department of Climate Change, 2007). There are no GHG
emission rate values available for household systems in these
protocols. This raises the question of the applicability of
these default average GHG emission rates to different
waste treatment processes and under different climate condi-
tions (Beck-Friis et al., 2000).
Although life-cycle analysis is beyond the scope of this
study, the findings from this study have implications on the
environmental benefits of home aerobic composting, vermi-
composting and anaerobic digestion of MSW in countries
such as Australia. Results from life-cycle analysis studies
often indicate centralized composting and anaerobic diges-
tion facilities as preferable to landfilling and incineration, due
to lower GHG emissons in the processes and the higher
potential of recovery of methane gas for fuel use (Murphy
and Power, 2006). It has been found that for a centralized
composting facility, the GHG emissions due to kerbside sep-
aration and collection of waste and the use of machinery for
processing and turning the waste at the facility could con-
tribute a considerable amount of GHG emissions (Lundie
and Peters, 2005; Lou and Nair, 2009). Apart from green-
house impacts, the waste separation, transportation and pro-
cessing activities contribute substantially to the management
cost. For example, in Victoria (Australia) the cost of kerbside
collection is approximately 18% of the management costs
(Victorian Government Department of Sustainability and
Environment, 2009). This cost is particularly higher for
cities with low population density, such as Australian cities
(Lundie and Peters, 2005; Victorian Government
Department of Sustainability and Environment, 2009).
Home composting of organic waste in urban areas will
reduce the GHG emissions and cost not only from the
above-mentioned processes, but also from transportation of
compost products and chemical fertilizers to the households
and crop growers (Lou and Nair, 2009).
As discussed in the previous section, all three types of
systems investigated in this study have similar potential in
reducing GHG impacts in comparison with the other waste
treatment options. However, the findings from this study
show that properly maintained vermicomposting systems
have a greater potential of reducing N
2
O emissions while
producing more neutral compost products in comparison
with the other methods. Vermicomposting also results in
more efficient digestion of the carbon content in organic
waste. As such the compost products from these processes
should have lower C/N ratios and better quality. In addition,
Table 3. Temporal variations in GHG emissions (averages
in mg CO
2
-e m
2
h
1
)
Total GHG
excluding
CO
2
Total GHG
including
CO
2a
CO
2b
CH
4
N
2
O
January 321 2371 2050 5.51 0.66
February 805 2903 2099 6.41 2.16
March 751 2710 1959 6.32 1.99
April 317 1652 1335 4.13 0.74
am 639 2661
A
2023 6.05 1.65
noon 848 3143
A
2295
A
6.95 2.27
pm 313 1822
B
1509 4.18 0.73
late pm 338 1910
B
1572
B
5.33 0.73
a
The months (and similarly the times of the day) denoted by a
superscript ‘A’ on their GHG emission values emitted significantly
more GHG on average than those denoted by a superscript ‘B’;
b
and similarly for CO
2
emission.
546 Waste Management & Research 29(5)
Mitchell et al. (1980) found that earthworms also decrease
emission of volatile sulfur compounds which are readily emit-
ted from the conventional microbial composting process.
Lazcano et al. (2008) found that earthworms promoted the
retention of nitrogen and gradual release of phosphorus as
well as reduction in electrical conductivity, therefore produc-
ing improved organic fertilizers for agricultural uses in com-
parison with the aerobic thermophilic composting.
According to the Queensland Environmental Protection
Agency (2002), about half of the domestic solid waste in
Brisbane is food and green waste. Approximately 130 kg
green waste and 190 kg food waste were generated per
person per year in Queensland (Australia). This is equivalent
to about 3.5 kg green and food waste per day for a typical
family of two adults and two children. From the survey and
observation made in the study, the households composted
approximately 4–5 kg of food and green waste each week
on average. This amount is equivalent to about 20% of the
food/green waste generated and is also well within the typical
treatment capacity of 30–50 kg month
1
of the home systems.
A combination of home composting and other MSW man-
agement alternatives including recycling and centralized
composting and anaerobic digestion offers great potential
in reducing the GHG impact and management cost in the
waste sector.
Conclusion
This study investigated the rate of emission of GHGs from
different types of home composting bins. On average, aerobic
composting, anaerobic digestion and vermicomposting bins
released 504, 694 and 463 CO
2
-e m
2
h
1
as N
2
O and CH
4
,
with N
2
O accounting for >80% of total emissions. These
emission rates are equivalent to 0.0043–0.0062 kg
CO
2
-e kg
1
waste of GHGs assuming 4.5 kg green waste
were processed in each bin each week. Among the three
types of bins, vermicomposting bins had the lowest emission
of N
2
O.
The GHG emissions generally increased with increasing
temperature and/or moisture content. This indicates the
importance of proper maintenance of the bins to minimize
GHG emissions. Overall, vermicomposting provided more
stable and favourable composting conditions than the other
two systems.
The findings from this study indicate that smaller scale
home systems tend to emit less GHGs when compared with
larger scale systems. Other studies on the environmental
impacts of the mainstream MSW management options
have found centralized composting and anaerobic digestion
facilities preferable to landfilling and incineration. Home
composting and anaerobic digestion have the potential to
further reduce GHG emissions associated with separation,
transport and processing of the waste as well as transport
of the compost products and fertilizers to the users.
Among the three systems, vermicomposting showed greater
potential to provide better composting conditions and com-
post products with lower carbon/nitrogen ratio.
Acknowledgements
Special thanks to the Brisbane households who participated in
the sampling programme. Thanks are also due to Mr Kurnal
Chauhan and Mr Kulvaibhav Singh for conducting the sampling
programme and to Mr Steven Reeves for analysing the gas sam-
ples and proofreading the manuscript. This project was sup-
ported by the NRMA Insurance’s Community Help Grants
Program 2008.
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All over the world, organic wastes generated by modern human society are mounting significantly due to rising population and growing culture of consumerism. Millions of tons of organic wastes (food, farm and green wastes) are ending up in the landfills every day, creating great economic and environmental problems for the local governments to manage them safely. Only construction of secured waste landfills incurs $30–40 million US dollars before the first load of waste is disposed. Waste disposal in the landfills costs about $65 per ton. During 2002–03, the cost of waste disposal in Australia was $2458.2 million (Australian Bureau of Statistics, 2004). Organic wastes in landfills emit huge and powerful greenhouse gases (3640 mg CO2-e /m²/hour) inducing global warming and climate change. They also emit some toxic gases (Xylene and Toluene) if they are not properly dumped and covered. Waste landfills in Australia emitted some 17 million tons of greenhouse gases in 2005. Best solution is vermicomposting of organic wastes by waste-eater earthworms. Most voracious waste-eater species of earthworms are Eisenia fetida also called ‘Tiger Worms’, Eudrilus eugeniae also called ‘African Night Crawler’ and Perionyx excavates also called ‘Indian Blue Worms’. Vermicomposting involves about 100–1000 times higher value addition in the end-product (vermicompost – a highly nutritive organic fertilizers), converting waste into wealth for farmers. Given the optimum conditions of temperature and moisture, about 1000 earthworms can vermicompost 10 kg of organic wastes or 10,000 earthworms can vermicompost 1 ton of organic wastes in just 30 days. Vermicompost gives 5–7 times higher food productivity over the conventional composts and significantly higher over the chemical fertilizers and also protect crops from pests and diseases (Arancon and Edwards, 2004). Vermicomposting of organic wastes by earthworms will also significantly reduce emission of greenhouse gases as compared to the huge emissions from the landfills. It only emits 463 mg CO2-e/m²/hour. The body fluid of earthworms termed as vermiwash, produced during vermicomposting also works as a powerful bio-pesticides. The foods produced by vermicompost are chemical-free organic foods, highly nutritious (rich in vitamins and minerals) and also health protective (due to rich in antioxidants). Vermicompost also have high soil moisture holding capacity (nearly 30–40 percent) and hence also reduce the need of water for farm irrigation. Huge earthworms’ biomass also comes as a valuable by-product of vermicomposting of organic wastes as the earthworms multiplies very fast doubling their population every 60–70 days. Earthworms are finding many new uses in the modern world in the production of vermi-medicines to protect human beings from several diseases and also from heart diseases and cancers, protein rich vermi-meals for cattle, poultry and piggery farming increasing production of milk and meat. Earthworms are also being used for production of biodegradable detergents and lubricants. They can be used for vermicomposting of more organic wastes and promotion of technologies like vermifiltration of wastewater for re-use of clean and nutritive water in farm irrigation and vermiremediation of chemically contaminated lands and soils on earth to make them usable and even fertile for farming. Earthworms are blessed by nature with the capacity to detoxify and disinfect any product – solid or liquid. They also have innate defense mechanisms and strong immune system. Vermicomposting is self-promoted, self-regulated, self-improved and self-enhanced, low or no-energy requiring zero-waste technology, easy to construct, operate and maintain.
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Composting was originally developed in order to make use of used organic matter. However, there is a fine balance between the amount of greenhouse gases (GHG) emitted by composting as to be environmentally friendly, or not. Excessive emitting of GHG from the composting process can, after all, be harmful if not balanced. In order to add up the gas balance for composting, you have to divide the compost operation into three separate stages. The first stage involves what goes into the compost pile and where it would have gone if there were no pile to got to. The second stage revolves around the composting process itself, how much energy is used in composting and if gases other than CO2 are emitted from the compost pile. The final stage refers to what happens to the finished compost. How the compost is used also can have an impact on GHG accounting. There are two primary elements to consider. The first is to evaluate whether or not your are doing a good thing for the atmosphere by composting and how you can alter your operation to maximize the benefits associated with composting.
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Decomposition of sewage sludge in drying beds at Ley Creek (LC) and Meadowbrook-Limestone (MB) Wastewater Treatment facilities in metropolitan Syracuse, N.Y. was monitored. LC and MB beds were derived from an anaerobically digested, primary sludge and an aerobically digested, activated sludge, respectively. Fluxes of O2, CH4, and CO2, as determined by in situ incubation of cylinders and analysis by gas chromatography, showed that at LC, aerobic and anaerobic, decomposition and photosynthesis occurred concomitantly due to high moisture. Similar analyses for two separate trials at MB found that both total decomposition rate and percentage of anaerobic decomposition (1 to 90%), as indexed by CH4 evolution vs. O2 consumption, decreased with time and were inversely related to sludge moisture content. O2 consumption rates, when standardized to 15°C, ranged from 10 to 80 µliter/g dry wt. For both LC and MB sludge, aerobic and anaerobic bacteria were abundant (3 × 10⁷ to 6 × 10⁸ colony-forming units/g dry wt) and dominant bacteria were not enteric. Nematode densities ranged from 7 to 814 individuals/g dry wt. A computer simulation model on the role of macroinvertebrates in decomposition was used for analyzing the effects of the earthworm, Eisenia foetida. This oligochaete was introduced into one-half of the MB cylinders and was shown to accelerate decomposition and decrease the proportion of anaerobic decomposition if the sludge was below 375% moisture dry weight and 40% anaerobic decomposition. This stimulation was also reflected in increased nematode density and elevated Eh. Flooding of drying beds caused mortality to E. foetida. Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © . .
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Emissions of N2O and CH4 from an aerated composting system were investigated using small-scale simulated reactors. The results show relatively high emissions of N2O at the beginning of composting, in proportion to the application amount of food waste. After 2 days, the N2O emission decreased to 0.53 ppmv on average, near to the background level in the atmosphere (0.45 ppmv). The addition of composted cattle manure increased N2O emissions not only at the beginning of composting, but also during the later period and resulted in two peak emission curves. Good correlation was observed between the N2O concentration at the air outlet and NO2− concentration in waste, suggesting a generation pathway for N2O from NO2− to N2O. Methane was only detected in treatments containing composted cattle manure. The high emission of methane illustrates the involvement of anoxic/anaerobic microorganisms with the addition of composted manure. The result suggests the existence of anoxic or anaerobic microsite inside the waste particles even though ventilation was employed during the composting process.
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A number of options exist for reducing the greenhouse gas consequences of waste management. Carbon equivalent releases from four scenarios for domestic waste management are predicted and compared. Waste incineration is preferred for most parameter values. Only when ‘closed cycle’ releases of carbon are regarded as net contributions to greenhouse effects, and methane generation is very low, is this preference not robust. These assumptions are critical to the choice of option.
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Greenhouse impacts of alternative municipal waste management chains are assessed. The considered alternative chains consisted of landfilling, landfill gas recovery and flaring or energy production, biological treatment and incineration of wastes. The most favourable options to limit the greenhouse impact due to waste management are those that minimize the methane emissions to the atmosphere and those that reduce the greenhouse impact even further by replacing fossil fuels in energy production.
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The use of renewable energy sources instead of fossil fuels is one of the most important means of limiting greenhouse gas emissions in the near future. In Finland, wood energy is considered to be a very important potential energy source in this sense. There might, however, still be some elements of uncertainty when evaluating biofuel production chains. By combining data from a stack of composting biodegradable materials and forest residue storage research there was an indication that rather great amounts of greenhouse gases maybe released during storage of wood chip, especially if there is rapid decomposition. Unfortunately, there have not been many evaluations of greenhouse gas emissions of biomass handling and storage heaps. The greenhouse gas emissions are probably methane, when the temperature in the fuel stack is above the ambient temperature, and nitrous oxide, when the temperature is falling and the decaying process is slowing down. Nowadays it is still rather unusual to store logging residue as chips, because the production is small, but in Finland storage of bark and other by-products from the forest industry is a normal process. The evaluations made indicate that greenhouse gas emissions from storage can, in some cases, be much greater than emissions from the rest of the biofuel production and transportation chain.
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Composting can be a source of N2O andCH4 production. In this investigation, differentcompost heaps of organic household waste weremonitored with the focus on potential formation ofCH4 and N2O in the heaps and emission ofthese gases from the heaps. The studied compost heapshad different compost ages, turning intervals andcompost sizes. The analysed compost gases containedbetween 1–3421 L of N2O-N L-1 and 0–470 mL of CH4 L-1. The emission rates ofN2O and CH4 from the compost heaps werebetween 1–1464 mg N2O m-2 day-1 and0–119 000 mg CH4 m-2 day-1. These verylarge differences in compost gas composition andemission indicate the importance of compostmanagement. The results also give an understanding ofwhere in the composting process an increasing emissionof N2O and CH4 can occur.
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In the past, landfilling involved burying municipal refuse directly or after on-site burning. Typically, little attention was given to proper siting and engineering to obviate the hazards of the generation of CH4 and toxic leachates as wastes decomposed. Leachates were hopefully attenuated by natural processes (adsorption, precipitation, ion exchange, microbial decomposition or dilution in the unsaturated zone below landfills). Landfills slowly evolved by proper siting, design and management into efficiently operated bioreactors to produce purified CH4 for use as a fuel, and leachates, which were treated biologically and chemically to minimize groundwater pollution. Microbial reactions in landfills are outlined. The amounts and composition of landfill gas and leachate as determined by the interaction of factors such as refuse composition, degree of compaction, temperature, moisture content, refuse age and depth are discussed. Typical inorganic and organic composition of landfill gases and leachates are presented.The potential and real environmental effects on soils, plants, groundwater, aquatic organisms and humans of disposal of municipal refuse by landfilling are reviewed. Finally, the most recent trend in constructing refuse landfills to serve as final storage reservoirs which are deliberately kept dry to minimize gas and leachate production is discussed and illustrated. Present activities in waste recycling to conserve landfill space are outlined.
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The current situation of municipal solid waste (MSW) and sewage sludge production (in terms of volume as well as composition) in China is introduced. Composting and compost application in China are reviewed. In China, the production of municipal solid waste and sewage sludge is changing rapidly along with economic development. Composting is mainly applied for treating MSW, about 20% of the total amount of MSW being disposed. MSW composting is mainly co-composted with night soil or sewage sludge. Compost is used in agriculture, forestry and horticulture. Compost application is the key factor influencing the composting development in China. To promote composting and compost application in China, a state-wide survey on the production, composition and physical and chemical properties of MSW and sewage sludge should be carried out. More effort should be made to develop low cost and high efficient composting technologies according to China's conditions. The environmental impact of compost application should also be given more attention.
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Slurries are a significant source of CH4, NH3 and N2O emissions to the atmosphere. The research project aimed at quantifying CH4, NH3 and N2O emissions from liquid manure stores and after manure application under field conditions. The influence of the manure treatment options “no treatment”, “slurry separation”, “anaerobic digestion”, “slurry aeration” and “straw cover” on the emission level was investigated. Approximately 10 m3 of differently treated slurry were stored in pilot scale slurry tanks. Emissions were followed for c. 80 days. After the storage period, slurries were applied to permanent grassland. Greenhouse gas emissions from slurry were mainly caused by methane emissions during storage and by nitrous oxide emissions after field application of manures. Mitigation of GHG emissions can be achieved by a reduction in slurry dry matter and easily degradable organic matter content. Ammonia emissions mainly occurred after field application. Untreated slurry emitted 226.8 g NH3 m−3 and 92.4 kg CO2 eq. m−3 (storage and field application). Slurry separation (liquid fraction and composting of the solid fraction) resulted in NH3 losses of 402.9 g m−3 and GHG losses of 58.5 kg CO2 eq. m−3. Anaerobic digestion was a very effective means to reduce GHG emissions. 37.9 kg CO2 eq. m−3 were lost. NH3 emissions were similar to those from untreated slurry. Covering the slurry store with a layer of chopped straw instead of a wooden cover increased NH3 emissions to 320.4 g m−3 and GHG emissions to 119.7 kg CO2 eq. m−3. Slurry aeration nearly doubled NH3 emissions compared to untreated slurry. GHG emissions were reduced to 53.3 kg CO2 eq. m−3.