Greenhouse gas emissions from piggery and biogas digesters in the North of Vietnam
ISSN 2588-1299
VJAS 2020; 3(4): 843-853
Vietnam Journal
of Agricultural
Sciences
Greenhouse Gas Emissions from Piggery
and Biogas Digesters in the North of
Vietnam
Pham Van Dung1, Duong Cong Hoan2, Jacobo Arango3, Tran
Dai Nghia4, Nguyen Tri Kien1, Ashly Arevalo3 & Sabine
Douxchamps1
1Alliance of Bioversity International and International Center for Tropical Agriculture
(CIAT), Hanoi 143330, Vietnam
2Department of Livestock Production, Ministry of Agriculture and Rural Development,
Hanoi 124332, Vietnam
3Alliance of Bioversity International and International Center for Tropical Agriculture (CIAT),
Apartado Aéreo 6713, Cali, Colombia
4Institute of Policy and Strategy for Agriculture and Rural Development (IPSARD), Hanoi
124332, Vietnam
Abstract
Increases in pig farm densities have caused great pressures on waste
management systems and produce massive manure and urine
quantities in Vietnam. This study aimed to identify the role and
contributions of biogas digesters to better manage the sources of
greenhouse gas (GHG) emissions from pig wastes for different types
of pig farms in the north of Vietnam. Four provinces, namely Thanh
Hoa, Phu Tho, Thai Binh, and Vinh Phuc, were identified. A total of
24 farms were purposively selected including 16 small-size farms
and 8 larger-size farms. The findings showed that GHG emissions
from small-size farms (154.8 t CO2-eq.yr-1) did not significantly
differ from the amounts measured in larger-size farms (139.1 t CO2-
eq.yr-1) in the four surveyed provinces. The sampling position did
not significantly affect the GHG emission rates, with 173.9 t CO2-
eq.yr-1 inside piggeries and 120.8 t CO2-eq.yr-1 outside the outlet of
the biogas digesters (p-value = 0.09). N2O emissions require further
measurements at different farm sizes and sites. These results
confirmed that the pig waste management of biogas digesters for
both small-size and larger-size pig farms is not completely efficient
and that efforts need to be invested in to mitigate GHG emissions in
pig production. Reducing pig density per piggery is highly
recommended. The application of other alternative aerobic or
anaerobic digestion technologies like vermicompost, effective
microorganisms, and composting should also be encouraged and
promoted.
Received: August 3, 2020
Accepted: January 20, 2021
Keywords
Biogas digester, emission rate, greenhouse gas, pig production,
Northern Vietnam
Correspondence to
Vietnam Journal of Agricultural Sciences
843
Greenhouse gas emissions from piggery and biogas digesters in the North of Vietnam
al., 1996; USAID, 2012). It is predicted that the
amount of GHG emissions will continue to rise in
the coming years.
Introduction
Livestock is one of the fastest-growing sub-
sectors of agriculture in Vietnam. In the past,
livestock raising activities based on feeding
agricultural by-products were popular in
smallholder farms in all agro-ecological zones.
However, these have been sharply shifting from
small-size to larger-size or industrial levels
during the last decade. Under the orientations of
the livestock production development strategies
of the Ministry of Agriculture and Rural
Development of Vietnam (MARD) from 2008 to
2020, the herd size and growth rate of livestock
in general has quickly advanced towards
industrial productions in areas where appropriate
conditions for livestock raising, such as types of
animals, housing systems, location, farm size,
land for waste disposal, and policy support, are
met. Consequently, animal populations have
remarkably increased, especially pig herds,
which reached 27.4 million heads in 2017 at an
annual growth rate of 2.3% between 2013 and
2017. More than 14,858 intensive pig farms at
different production levels are nationally listed
(General Statistical Office of Vietnam, 2019).
Two-thirds of the intensive farms are in the Red
River Delta and Northern provinces and the rest
are in the South.
However, livestock population intensification
is linked to an increase in waste production,
reaching 26.5 million tons and 33.7 million m3 for
solid and liquid wastes, respectively (Nguyen Van
Bo, 2017). Waste disposal is not yet organized,
with an estimation of about 60% of wastes treated
and used effectively through technologies as such
biogas digesters and composting (Ginting et al.,
2003; (Nguyen Van Bo, 2017; Yaman, 2020;
Yaman et al., 2020). The rest remains untreated
and is directly released into the environment. The
dumping and inappropriate management of
wastes before discharging them into the
surrounding environment have caused varying
degrees of water, soil, and air pollution, and
epidemic diseases to human and animal habitats.
These not only cause losses from recycling wastes
for use as fertilizers and biogas, but also increase
GHG emissions such as carbon dioxide (CO2),
methane (CH4), and nitrous oxide (N2O).
There is potential to switch from waste to
energy and contribute to economic, social, and
environmental benefits (Aracil et al., 2018;
Yılmaz & Abdulvahitoğlu, 2019; Yaman et al.,
2019; Yaman et al., 2020). Biogas digesters have
been known for their multi-purpose nature,
treating waste and producing energy at the same
time. Biogas and digestates produced through the
anaerobic digestion of organic matter inside the
digester are important products to feed trees and
improve environmental issues. Biogas could also
replace other energy sources such as fossil fuels,
firewood, and agriculture residues that are
commonly used for households in rural areas
(Müller, 2007; Amigun et al., 2008; Adu-Gyamfi
et al., 2012; Molino et al., 2013; Hinh, 2017;
Chen, 2018). Recently, about 500,000 biogas
digesters were built, mainly in the north of
Vietnam where the density of pig production is
the largest. Of these, 83.3 thousand digesters are
now used in the northern midlands and
mountainous areas, 123.8 thousand digesters in
the northern central and coastal areas, and 154.0
thousand digesters in the Red River Delta ( Tong
Xuan Chinh, 2017; Hinh, 2017). However, recent
Manure management is one of the mitigation
components of agriculture under the Nationally
Determined Contribution (NDC)'s framework that
the Vietnam government is undertaking to
implement during the period of 2020-2030.
To achieve its mitigation goals, the
Vietnamese government has planned specific
actions to develop an additional 300,000 biogas
digesters, which are expected to mitigate 1.92
million tons of CO2-equivalent (CO2-eq), and
improve animal feeds, which are anticipated to
mitigate 0.13 million tons of CO2-eq. These plans
are essential contributions to the Vietnamese
government's policy implications and international
commitments on climate change prevention, global
warming, and GHG mitigations. Previous reports
have shown that manure management practices
contributed to 15.1% of the total agricultural
emissions between 1992 and 2012 (Misselbrook et
Vietnam Journal of Agricultural Sciences
844
Pham Van Dung et al. (2020)
findings showed that the quality of manure
decomposition of these systems is limited, with
animal density causing high pressure on the
biogas digesters’ volume, as their size is not big
enough for treatments anymore. In addition,
GHG emissions from the biogas digesters were
not quantified, neither for small-size nor for
larger-size pig production systems. Therefore,
this study aimed to quantify the GHG emissions
from biogas digesters in different types of pig
production systems and at different locations of
piggery.
antibiotics was found in the selected farms. The
study was done on three types of biogas
digesters, specifically the KT1, KT2, and
composite plastic structures. These biogas
digesters were usually built underground beneath
the piggery. The digesters were filled through the
inlet tank and the inlet pipe. The produced biogas
then accumulated at the upper part of the
digesters.
Description of the static chamber operation
The static chamber system has been applied
extensively to measure rates of trace gas
emission sources (Hutchinson & Mosier, 1981;
Hutchinson & Livingston, 1993; Kusa et al.,
2008), and allows the detection of gases emitted
from a surface of a volatile solid within a known
volume during a known period of time. In this
study, a static chamber system was designed
following the GHG emission measurement
protocol that was developed by Ashly et al.
(2018). This system was connected to a Gasmet
Methodology
Description of study sites
This study was carried out in four districts in
four provinces: Thanh Hoa (TH), Phu Tho (PT),
Thai Binh (TB), and Vinh Phuc (VP). These
provinces represent the areas of the North of
Vietnam where the largest pig populations and
densities are observed per farm. A non-
probability convenience sampling method based
on suggestions from the Department of Livestock
Production, Ministry of Agriculture and Rural
Development Vietnam was applied to select the
pig farms. Due to constraints of financial and
various resources, the study size was calculated
to account for one percent out of the 2,674
livestock farms in the four provinces in 2017
(General Statistical Office of Vietnam, 2018). A
total of 24 farms were selected, among which, 16
were small-size farms and 8 were larger-size
farms corresponding to the different levels of pig
herd size in the selected provinces. Six pig farms
were randomly sampled in each district of the
four provinces, and were named as the TH farms,
TB farms, VP farms, and PT farms according to
the initials of the provinces. The farm category
was defined according to the number of fattened
pig heads per farm. Farms fell in the larger-size
category when the total pig heads were more than
10 pigs per farm. The farms that had less than 10
pigs per farm were as classified as small-size
farms. Pig farms of each type were characterized
by the same raising practices. Concentrated feed
was the main input for both farm sizes, while a
small quantity of agricultural by-products was
also used in the small-size pig farms. No use of
DX-4040
Fourier
Transform
Infrared
Multicomponent Trace Gas Analyzer. The
chamber was programmed to be closed for fifteen
minutes (one observation), with three
observations performed in one hour. The total
number of observations was 72. The FTIR gas
analyzer measured the main greenhouse gases at
low concentrations in parts per million units per
second (ppm.s-1) including CO2, CH4, N2O, and
water vapor. The response time of the analyzer
was 20 seconds for one reading and the flow
speed of the sample pump was 1.5 liters min-1.
The gas analyzer was calibrated with pure
nitrogen N2 (2 liters min-1) prior to each
measurement. The chamber was inserted into the
base and sealed with a black rubber ring while
the base was inserted into the sample and sealed
with water.
Gas sampling procedure
Gases were sampled from October 1 to
November 11, 2018 from pig manure collected at
two locations per farm, inside the piggery and
outside the biogas digester. Inside the piggery, a
composite sample was obtained from fresh solid
manure or slurry taken at two random positions,
while at the outlet of the biogas digester, one
845
Greenhouse gas emissions from piggery and biogas digesters in the North of Vietnam
sample of digested wastes was taken directly. Pig
manure was then kept on white plastic plates
(radius = 9.25cm). The plates with pig manure
were then weighed to note the initial mass using
an electronic scale (Model-HY K17, 5kg). Fresh
solid pig manure was sampled in the same
locations, and weighted and dried in the
microwave until the mass was stable. The dry
matter content in the samples was recorded to
calculate the equivalent rate of water and pig
waste in each type of farm size. Parameters were
recorded at the time of measurement for each
sampling duration.
The GHG fluxes data were first tested for
normality. The GHG emissions rates were
determined from linear regressions, using the
goodness of fit and the significance level for
model selection. The significance of the
differences between emission fluxes in the
different piggeries was tested by a one-way
ANOVA analysis. These statistical analyses
were performed using the stats package in the R
software, version 3.5.1. As N2O gas fluxes were
nonlinearly distributed, the concentrations C0,
C1, C2, corresponding to the time intervals of 0,
5, and 10m, were used for calculations.
Calculation of GHG emission fluxes
Results
Emission fluxes were computed from the
change in gas concentration with time. There are
two main approaches of GHGs emission rate
calculations based on the static chamber method,
namely the linear and non-linear models
(Anthony et al., 1995; Phillips et al., 2000). For
the linear model, the gas concentration within the
container headspace increase linearly with time.
As such, fluxes are calculated from the slope of
the linear regression between gas concentrations
versus time (Whalen & Reeburgh, 2001). The
equation is described as follows:
Descriptive characteristics of the pig
production systems in the surveyed locations
The characteristics of the piggery structures
and pig populations are shown in Table 1. The
TB farms had the largest number of fattened pigs
and sows while the number of piglets was the
greatest in the TH farms. The average area of
piggeries in the TB and TH farms was two times
larger than the one of the farms from the other
two provinces. The largest average volume (m3)
of the biogas digesters was found in the TH
farms. Feces and urine were gathered in the same
inlets of the biogas digesters without separation.
The period of manure storage inside a biogas
digester was usually one year. However, the
biogas digester sizes, commonly ranging from
10.8 to 13.5m3, and the treatment duration were
not large or long enough to digest and decompose
the amount of produced manure. The digested
wastes, after being removed from the biogas
digesters and discharged into the surrounding
environment of the piggery and pig raiser's
residences, still provided odour emissions and
polluted the water and soil.
∆C P 273.15 v M
F =
∆t P TKelvin A 푉
0
푠
where, F is the flux rate (mass unit. m-2.h-1),
P is the measured ambient pressure (mbar), P0 is
the standard pressure (1013.25 mbar), v is the
total system volume (L), V is the volume
occupied by 1 mol of the gas at standard
temperature and pressure (STP) (0.024 m3, or
22.4 L) calculated by the equation:
∑ 푉 = 푉headspace + 푉tubing + 푉
cell of gas analyzer
where, A is surface area of the chamber over
the emission source (m2), T is the ambient
temperature in degrees Celsius (oC), TKelvin is the
temperature T in Kelvin (K) = (273.15 + Tc),
∆C/∆t is the change in concentration in time
interval t or the slope of the gas concentration
curve (ppm.s-1), and M is the molecular weight
(g mol-1).
Greenhouse gas emissions from piggery and
manure management
Table 2 presents the GHG (CO2, CH4, N2O)
concentrations (ppm) from the pig farms in the
surveyed provinces. Notably, the concentration
of CO2 was the main contributor to the increased
emissions, followed by the CH4 concentration.
The concentrations of these gases were measured
Vietnam Journal of Agricultural Sciences
846
Pham Van Dung et al. (2020)
Table 1. Average pig production characteristics by province and by farm size
PT farms
TB farms
TH farms
VP farms
Indicators (units)
Small
Large
2
Small
Large
2
Small
Large
2
Small
4
Large
2
Sample size (farm)
Number of sows (head)
4
4
4
2.2
7.8
10.3
0.0
3.7
8.6
14.3
5.0
1.3
6.7
18.1
2.8
2.2
6.5
8.8
2.6
Number of fattened pigs (head)
Number of piglets (head)
25.5
20.4
52.5
24.0
41.4
72.5
23.0
30.0
Piggery’s area (m2)
35.6
8.4
84.5
14.5
51
136.3
13.5
57.8
9.0
640.4
16.0
68
137.7
14.3
Volume of biogas digesters (m3)
9.0
8.5
Note: PT = Phu Tho; TB = Thai Binh; TH = Thanh Hoa; VP = Vinh Phuc
Table 2. Greenhouse gas concentrations from the 24 pig farms in the four study provinces
PT farms TB farms TH farms
VP farms
Concentration (ppm)
R-
R-
R-
R-
squared
Mean (SD)
Mean (SD)
Mean (SD)
Mean (SD)
squared
0.994***
0.995***
0.658**
squared
0.861***
0.844***
0.703***
squared
0.928***
0.862***
0.459
CO2
CH4
N2O
1162 (428)
33.3 (25.1)
0.5 (0.3)
1078 (403)
28.7 (22.8)
0.5 (0.1)
1259 (311)
47.6 (25.5)
0.3 (0.07)
1257 (679)
45.6 (31.3)
0.4 (0.03)
0.848***
0.845***
0.471**
Note: SD means standard deviation; ***, ** means significant difference at 1% and 5% level.
at higher levels in Thanh Hoa and Vinh Phuc
provinces than in the remaining provinces. The
ANOVA indicated that the GHG concentrations
had a normal distribution and did not differ
significantly among the selected provinces for
both CO2 (F = 1.79; P = 0.61) and CH4 (F = 1.47;
P = 0.68). The concentrations of CO2 and CH4
increased linearly over time within the chamber
headspaces. The regression R-squared values of
these two gases were significant in all the
surveyed sites.
GHG emissions in terms of mass with a wide
variation between farms. The final value was
converted to the CO2-eq, and the contribution
from CH4 emissions played an important role.
The emission rates did not vary significantly
between the sampling locations, whether inside
the piggery or outside the outlet of the biogas
digester at the sampling points (Figure 1),
although the emissions of CO2 and CH4 were
insignificantly higher inside compared to outside
the outlet of the digester tank.
The regression R-squared values were lower
for N2O because the curve of this gas
concentration increase was non-linear (quadratic,
exponential). Therefore, the equation for F as
described above was applied for CO2 and CH4
while the calculation of N2O emissions was the
concentration at specific time intervals (0, 5, and
10 minutes). The N2O concentrations among the
samples increased in the PT and TB farms and
decreased in the TH and VP farms.
In order to detect the effect of feed
components on emissions, the relationship
between GHG emissions rates and the dry matter
content of the pig manure is demonstrated in
Figure 2. The water-dry matter ratio for solid
manure was 4:1. The average temperature and
humidity conditions at the sampling sites were
24.75oC and 80.15%, respectively. Emission
rates tended to increase as the dry matter content
in the manure increased. However, the variation
needs to be reinforced from further studies. The
emission rates of CO2 and N2O showed higher
variation in the small-size farms than in the
larger-size farms (Figure 3). The CH4 emission
The relationship between the intensification
of GHG emissions (CO2 and CH4; tons.yr-1) from
pig manure and the number of pig heads and feed
intake is shown in Table 3. Remarkably, CO2
emissions were by far the largest contributor to
847
Greenhouse gas emissions from piggery and biogas digesters in the North of Vietnam
Table 3. Greenhouse gas emissions from pig manure by pig head and feed inputs
Total number of pigs (head)
GHG emission
Feed input (kg day-1)
(tons yr-1)
Mean
8.04
SD
10.9
Mean
5.61
SD
7.71
0.13
CO2
CH4
0.13
0.19
0.09
0.0002
11.4
0.0012
0.0001
7.9
0.0008
N2O
CO2-eq (tons yr-1)
Note: SD means standard deviation
Figure 1. Emission rate of pig manure inside the piggery and outside storage
rates were significantly greater in the larger-size
farms compared to the small-size farms.
could be explained because pig raising
conditions and infrastructures were similar in all
these provinces. This means that the capacity of
the biogas digesters in the piggeries to
decompose organic matter and digest pig manure
was not completely efficient. Otherwise, the
biases of sampling selection, the standards of pig
farm classification based on Vietnamese
regulations, and the heterogeneity among the
selected sites were factors that could explain the
differences in GHG emissions between these
types of pig farms.
Discussion
Pig farm location and stall structure
characteristics affecting the GHG emissions
Although the structure of the pig house does
not directly affect GHG emissions, it determines
how manure is handled, stored, processed, and
used. For the pig house location, this study found
that most of the small-size farmers built their pig
houses near or inside their residential area at a
very close distance (around 17 to 67m). Piggeries
were placed within farmers’ residences. This
Unlike the larger-size pig farms, which were
required to arrange outside far away from the
community's residences, the location of small-
Vietnam Journal of Agricultural Sciences
848
Pham Van Dung et al. (2020)
size pig farms in residential areas was commonly
seen. This made it more convenient to clean and
take care of the pig herds. However, there were
risks of water and air contamination, as well as
epidemic diseases. The exchange between GHG
and odor emissions of excreta indoor and the
atmosphere outdoor was restrained by the
surrounding infrastructures. In conditions of high
temperature, the emissions were stronger,
especially for CO2 and CH4. These results agreed
Figure 2. Relationship between the GHG emission rate and dry matter in the pig manure
Figure 3. Emission rate of pig manure by farm size
849
Greenhouse gas emissions from piggery and biogas digesters in the North of Vietnam
with the previous findings of Misselbrook &
Powell (2005), Ngwabie et al. (2011), and
Korkut et al. (2018). For this reason, the manure
management method using biogas digesters
should be appropriate to deploy under the floor
or outside near the pig house. In addition, the pig
house type and structure played an important role
in CO2, CH4, and N2O emission intensification.
The most frequent problems with structural
components were related to the floor
characteristics. The natural ventilation pig house
structure and slatted floor were popular for both
larger-size and small-size farms in the study
sites. Housing systems with slatted floors were
realized to be more likely to accumulate manure
in liquid or slurry form. Depending on the floor
type, how to design the pig house could increase
or decrease GHG emissions. Different types of
floors can have varying effects on GHG
emissions. The effect of slatted floor areas on
GHG emissions has also been shown to have
conflicting results, especially CH4 and N2O
emissions (Fitamant et al., 1999; Philippe et al.,
2007; Guingand et al., 2010; Philippe et al.,
2012; Philippe et al., 2014). Pig houses with fully
slatted floor systems were observed to reduce
CO2 production by 7-13% (Sun et al., 2008;
Guingand et al., 2010; Sun et al., 2010). In
comparison, bedded systems combine a wide
range of raising techniques that impact the level
of emissions. Bedded floor systems are usually
associated with reduced CH4 emissions, but
increased CO2 and N2O emissions. Therefore,
the selection of suitable materials is significant
not only for the raising conditions of the pigs and
benefits of production costs, but also practical
issues such as manure storage and drainage.
Together with the pig house site arrangement, the
floor type and structure system also determined
the feasibility of using anaerobic digestion or
composting to treat the manure with its
associated effects on GHG emissions. However,
more evidence related to different floor types is
needed to confirm their effects on gas emissions
in pig houses in the North.
The biogas digester volume calculation
depends on the volume of manure produced. In
the case of Northern Vietnam, small-size biogas
digesters are still predominant, with the most
common sizes being from 6 m3 to 12 m3 (Roubík
et al., 2016). However, results from this study
showed that an average quantity of manure from
5 sows, 58 market pigs, and 59 piglets per farm
produced 2.7 kg per day, which was a lot for the
average size of the biogas digesters (12.6 m3) to
handle pig wastes. Another key finding was that
there was no significant difference between the
amounts of GHG emission produced before and
after the use of the biogas digesters, showing the
inefficiency of the process. This leads to a lack
of organic matter in the digestate, usually caused
by the use of excessively high water per manure
ratios (Tran et al., 2011; Thu et al., 2012; Roubík
et al., 2016). The imbalance between the manure
and water ratios promoted the digester filling
faster and reduced the retention time of the feces.
Thu et al. (2012) showed that 55-60% of the
digesters had a retention time between 1 and 20
days. In these biogas digesters the organic matter
was inefficiently transformed to biogas. This
means that the waste decomposition process
continued to happen after being discharged into
the environment (Yaman, 2020).
Gap of knowledge for future studies
This case study has shown that waste
management and treatment processes were only
moderately effective in mitigating GHG
emissions and improving fertilizer utilization, as
the level of GHG emissions before and after the
use of biogas digesters remained high. Several
gaps in knowledge could be addressed by future
studies. The study initially evaluated the
efficiency of GHG emission reduction in biogas
digester systems. Comparisons with other waste
management technologies are needed, such as
composting technology, solid and liquid manure
separation technology, EM technology, etc. This
knowledge will be helpful to target interventions
for different manure management technologies
and prioritize them for government programs.
Care should be taken when extrapolating
research results to other regions, and more
Biogas digester volume and design affecting
GHG emissions
Vietnam Journal of Agricultural Sciences
850
Pham Van Dung et al. (2020)
during a pig production cycle are highly
recommended. Modifications to the biogas
digester structure are also necessary to separate
solid pig manure and urine. Otherwise, the
application of other alternative aerobic or
assessments are needed in larger-size and small-
size pig farms in different geographical zones to
consolidate findings of emission levels at
different production scales. Feed sources play an
important role in total GHG emitted. Most
households currently use concentrated feed, and
the comparative emissions following feeding
with alternative products should be investigated.
anaerobic
digestion
technologies
like
vermicompost, effective microorganisms, and
compost should also be encouraged and
promoted when economic, income, and
environmental benefits are met. Biogas digesters
in pig production have a significant role to play
in the Vietnam government’s mitigation
strategies, as well as from the perspective of
biosafety and animal husbandry policies.
This study analyzed emissions at the time of
sampling, but did not evaluate emissions over a
longer time period, especially changes in
seasonal emission rates. Future studies could
focus on the impact of seasonal environmental
changes, such as the fluctuation of temperature
and humidity, on emission rates over time. These
results were restricted to specify which type of
pig (market pig, sow, and piglet) had high GHG
emission rates per pig head. Similarly, the results
did not identify GHG emission rates for the
specific type of feed input. Further research is
needed to provide additional data on GHG
emission and feed intakes to confirm these
results.
Acknowledgments
The authors would like to gratefully
acknowledge anonymous local supporters, the
People’s Committees, and the Department of
Agriculture and Rural Development of the
surveyed provinces for approving the interviews
and arranging the site visits. Special appreciation
and thanks also to the farmers in the visited sites
for their invaluable contributions and openness in
sharing with us their experiences and insights.
This study was funded by the Livestock and
Environment flagship of the CGIAR Research
Program on Livestock, which is carried out with
support from CGIAR Fund Donors and bilateral
funding agreements.
Conclusions
Overall, the consequences of this inefficient
process were poor GHG emissions mitigation,
the lack of organic matter in the digestates, poor
quality of biogas, spreading smells of biogas, and
loss of nutrients to the environments. The study
confirmed that the biogas digesters were
overloaded and the quality of manure
decomposition was not optimal. The GHG
emission amounts (tCO2-eq.yr-1) from pig wastes
inside and outside of the biogas digesters had
great potential to cause greenhouse effects and
global warming. However, the GHGs emission
rate did not significantly differ between the
small-size and larger-size farms in the four
surveyed provinces. Sampling position (between
inside piggeries and outside the outlet of the
biogas digesters) did not significantly affect the
GHG emission rate. These results confirmed that
the pig waste management of biogas digester
systems for both small-size and larger-size pig
farms was not efficient and that efforts need to be
invested in to mitigate GHG emissions in pig
production. This case study suggested that
adjustments in pig population numbers and the
density of pig heads per piggery floor area unit
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