The emission factors were transferred into CO2 equivalents according to IPCC (2007). The
overall CO2 emissions were in a range from 31 to 435 kg (Mg biowaste)-1. The CH4 emissions
from AD plants were more important than the emissions from N2O and NH3 (Figure 5A). The
emissions of CH4 accounted from 36 - 92 % while the emission of N2O and NH3 contributed
from 6.9 - 30 % and from 0.08 – 58 % respectively to the overall CO2 emissions. The median
CO2 equivalent emission was 105 kg CO2 (Mg biowaste)-1. The results were in line with a
previous study. [4] reported that an AD plant contributed up to 111 kg CO2 equivalent (Mg
waste)-1. The AD plants with open composting windrows (1, 2 and 9) showed higher CO2
equivalent emissions than the AD plant without open composting windrows.
Figure 5B shows the net total of CO2 equivalent from different emission sources at AD
plants. The open composting system resulted in high GHG emissions accounting from 73 to 96
% to the total emissions at plants 1, 2 and 9. CHP contributed from 5 to 50 % to the total
emissions at plants 3, 4 and 7. The liquid treatment system resulted in insignificant (3.7 %) to
the total CO2 equivalent emissions at plant 1.
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Journal of Science and Technology 54 (4B) (2016) 208-215
GREENHOUSE GAS EMISSIONS FROM ANAEROBIC
DIGESTION PLANTS
Nguyen Thanh Phong1, *, Joachim Clemens2, Carsten Cuhls3
1Faculty of Science and Technology, Hoa Sen University, 8 Nguyen Van Trang, District 1,
HCM city, Vietnam
2Faculty of Agriculture, University of Bonn, Karlrobert-Kreiten Str.13, 53115 Bonn
3Ingenieurgesellschaff für Wissenstransfer, Gewitra mbH, Im Moore 45, 30167 Hannover
*Email: phong.nguyenthanh@hoasen.edu.vn
Received: 15 August2016; Accepted for publication: 10 November 2016
ABSTRACT
This study investigated emissions of CH4, N2O and NH3 from nine anaerobic digestion
plants that treat biowaste. The treatment is in form of mechanical pre-treatment, anaerobic
digestion followed by a composting with or without intensive aeration. The exhaust gases from
the mechanical and anaerobic steps are treated by biofilters. The emission sources at the plants
consisted of biofilters, combined heat and power units (CHP), liquid digestate treatment systems
(LTS) and open composting windrows of the solid digestate. Overall, the emission factors were
0.4 - 16 kg (Mg biowaste)-1 for CH4, 7 - 170 g (Mg biowaste)-1 for N2O and 41 - 6,032 g (Mg
biowaste)-1 for NH3. Open composting windrows of solid digestate resulted in high emissions of
CH4 and N2O. Intensive aeration of the solid digestate could reduce greenhouse gas emissions.
Keywords: greenhouse gas, emissions, anaerobic digestion, windrows, organic waste, methane.
1. INTRODUCTION
Anaerobic digestion for treatment of biowaste is rapidly gaining interest in developed and
developing countries [1, 2]. The treatment is essentially based on the activities of
microorganisms that transform organic substances into biogas [3]. Biogas is used as renewable
energy source, and nutrients in the residue can be recovered in agriculture as fertilizer or soil
conditioner [4]. In addition, AD of biowaste is attracting attention as an effective method to
reduce Greenhouse gas (GHG) emissions according to Kyoto protocol [4]. According to the life
cycle analysis (LCA), AD results in negative GHG emissions. The total greenhouse gas (GHG)
emissions for AD can reduce up to one tonne CO2 equivalent/ Mg separated organic waste [5].
Actually, many studies have been conducted to show the benefits of AD treatment, for instance
the works of [2, 4, 6 and 7]. However, there is still missing an overall evaluation of GHG
emissions during treatment. For example, the GHG emissions associated to the pre-treatment
and post-treatment of AD were often excluded in previous studies. In fact, AD plants may have
fugitive emissions of CH4, N2O and NH3. The aim of the study was to investigate emission
Greenhouse Gas Emissions from Anaerobic Digestion Plants
209
factors of CH4, N2O and NH3 g (Mg biowaste)-1 and to compare emission sources in the plants.
Additionally, the efficiency of biofilters was taken into account. Nine operating AD plants, two
wet digestion plants, four dry digestion plants and three solid digestion plants, were evaluated.
2. MATERIALS AND METHODS
2.1. Measured locations and emission determinations
The emission sources at the plants consisted of biofilters, combined heat and power units (CHP),
liquid digestate treatment systems (LTS) and open composting windrows of the solid digestate.
The detailed inventories of the AD plants are listed in Table 1.
Table 1. Processing parameters of the anaerobic digestion plants.
Wet digestion Dry digestion Solid digestion
TS30 %
Plant AD 1 AD 2 AD 3 AD 4 AD 5 AD 6 AD 7 AD 8 AD 9
Pre-treatment yes yes yes yes yes yes yes yes yes
Feeding Cont. Cont. Cont. Cont. Cont. Cont. Batch Batch Batch
Temp. meso thermo thermo thermo thermo thermo meso meso meso
HRT (day) - 20 15-30 15-20 15-20 21 28 21 21
Digetate Separation yes yes yes yes yes yes no no no
Composting yes yes no yes yes yes yes yes yes
2.1.1. Biofilter
The gas inlet and outlet of biofilter was analysed at each plant for 1 week. At capsuled
biofilters the treated air left the biofilter in a chimney. Here the gases were measured (biofilters
at plants 1, 4, 6 and 9). At open biofilter (at plants 2, 3, 5, 7 and 8), 16 m2 of the biofilter (4x4 m)
was covered by a thin foil. Concentrations of the treated gases were measured under the foil.
Continuously monitored parameters included TOC, CH4 and N2O. TOC was measured by flame
ionisation detector (Bernath Atomic 3006) while CH4 and N2O were measured by an infrared gas
analyser. Gas concentrations in the treated and untreated exhaust air were recorded every
minute.
To control the accuracy of the infrared gas analyser, exhaust gases were sampled manually
by evacuated headspace vials and subsequently analysed on CH4 and N2O by GC (ECD/FID) in
the laboratory. A manual discontinuous analysis was applied for NH3 measurement: NH3 was
extracted from the waste gas stream by absorbing it in sulfuric acid and subsequently measured
Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls
210
colorimetrically in the laboratory. NH3 samples of treated and untreated gases were collected
twice. Air fluxes to the biofilter were measured by an anemometer (testo 435) or
micromanometer (Müller Instruments EPM-300-BA, Germany). It was assumed that the
volumes of treated and untreated air were the same.
2.1.2. Open composting windrows
To measure GHG emissions from composting windrows, a tunnel covers an area of around
50 m2 with a length of 10 m and a width of 5 m. The height of the tunnel may vary from 1.5 to 2
m. Two ventilators are used to ventilate the tunnel from one side. The ventilation rate is fixed at
1000 m3 h-1. Fresh air enters the tunnel from the front. In the tunnel, gas is emitted into the fresh
air and leaves the tunnel at the rear. At the outlet, a Teflon tube (4 mm in diameter) is installed
0.5 m above the windrows and used for gas sampling. The gas is pumped via a cooler to an
infrared gas N2O and CH4 analyser (Uras, ABB). The infrared detector has a sensitivity of 0.1
mg/m3 for N2O and 1 mg/m3 for CH4. When the tunnel was installed, it took ten to twenty
minutes for GHG concentrations to be constant. GHG concentrations were then recorded every
minute for one hour. Air fluxes were determined using an anemometer (testo 435) or a
micromanometer (Müller Instruments EPM-300-BA, Germany). In parallel, 60L of outgoing air
were flushed through two flasks containing 40 mL of a 0.05 M H2SO4 solution. NH3 was trapped
in the solution as NH4+ and subsequently analysed colorimetrically in the laboratory.
2.2. Other measuring points (e.g. CHP, receiving and pre-treatment hall, liquid digestate
treatment systems (LTS))
Other emission sources were point sources with preinstalled sampling points. For one hour
the TOC concentrations were recorded every minute by FID. In parallel, gas samples were taken
regularly using evacuated headspace vials for CH4 and N2O. For NH3, samples were taken by
absorbing it in sulfuric acid solution. Air fluxes were also determined by measuring velocity
(m/s) and cross section area (m2).
LTS: After anaerobic digestion, the digestate is dewatered by a second centrifuge. The solid
digestate is mixed with green waste and used for composting. The liquid is treated in
nitrification and denitrification tanks.
CHP: CHPs consist of a combustion engine and a generator. Biogas is used to generate
electricity and heat in these combustion engines.
2.3. Calculations of emissions factors for anaerobic digestion plants
The emission factors of CH4, N2O and NH3 g (Mg biowaste)-1 were calculated using the
aeration rates and concentrations of gases. The emission rates and emission factors for each gas
were calculated using the following formula:
1000
QEEMF
×
=
(g h-1) [8];
w
MF
f M
EE )724( ××= g (Mg waste)-1 [8]
with: E: concentration (mg x m-3), Q: air flow (m3 x h-1), EMF: emission mass flow (g × h-1),
Mw: total mass of incoming waste (Mg per week), Ef: emission factor g (Mg waste)-1
The emissions were calculated in form of CO2 equivalent according to Intergovernmental
Panel on Climate Change (IPCC) in 2007. N2O and CH4 are potential GHG with respective
Greenhouse Gas Emissions from Anaerobic Digestion Plants
211
global warming potentials 298 and 25 times higher than that of CO2 respectively. Additionally, it
was assumed that the CO2 equivalent of NH3 is 2.98 [8].
)98.229825(
3242
×+×+×= ∑ fNHOfNfCHequivalentfCO EEEE [8]
Overall GHG emissions from AD plants were calculated by the sum of emissions of CH4,
N2O and NH3 from open emission sources such as biofilter, CHP, open composting windrows
and liquid digestate treatment systems. Emissions from machinery and energy used in the plants
were not considered in the calculations.
)(1 LTSOWBFplant EEEE ++= ∑
)(2 OWBFplant EEE += ∑
)(3 CHPBFplant EEE += ∑
)(4 CHPBFplant EEE += ∑
)(5 ∑= BFplant EE
)(6 ∑= BFplant EE
)(7 CHPBFplant EEE += ∑ )(8 ∑= BFplant EE )(9 OWBFplant EEE += ∑
3. RESULTS AND DISCUSSION
3.1. Emission factors of CH4, N2O and NH3 from open emission sources in AD plants
The emission factors of CH4 varied from 16 to 819 g (Mg biowaste)-1 for liquid treatment
system (LTS), from 50 to 1500 g (Mg biowaste)-1 for CHP, and from 0.4 to 15.4 kg (Mg
biowaste)-1 for open windrows (Figure 1). Liquid digestate still contains potential to form CH4
[6]. Thus, CH4 emissions still occur in treatment systems of liquid digestate. Biogas produced at
the AD plants is burned in CHPs to produce electricity and heat. Since the combustion process is
not 100 %, some CH4 escapes unburned into the atmosphere. By this way, CHP contributes to
CH4 emissions.
The emission factors of N2O were in the range of 1.22 to 37.57 g (Mg biowaste)-1 for LTS,
0.1 to 2.7 g (Mg biowaste)-1 for CHP, and 56 to 201 g (Mg biowaste)-1 for open windrows. The
emissions of N2O at the CHP were insignificant, while the N2O emissions from LTS and open
windrows need to be considered. The results are in line with the findings of [4, 8].
The emission factors of NH3 were in the range of 0.1 to 0.16 g (Mg biowaste)-1 for LTS,
0.03 to 1.16 g (Mg biowaste)-1 for CHP and 65 to 3327 g (Mg biowaste)-1 for open windrows.
The emissions of NH3 from the LTS and CHP were low, while open windrows had high
emissions of NH3.
With
plantE : overall emission factor of an AD plant
BFE : emission factor of biofilter
OWE : emission factor of open windrows
LTSE : emission factor of liquid treatment system
CHPE : emission factor of CHP
Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls
212
LTS CHP BF OW
E
m
is
si
on
fa
ct
or
o
f C
H
4
g
(M
g
w
as
te
)-1
10
100
1000
10000
100000
LTS CHP BF OW
E
m
is
si
on
fa
ct
or
o
f N
2O
g
(M
g
w
as
te
)-1
0,1
1
10
100
LTS CHP BF OW
Em
is
si
on
fa
ct
or
o
f N
H
3 g
(M
g
w
as
te
)-1
0,01
0,1
1
10
100
1000
10000
Figure 1. Emission factors of CH4, N2O and NH3 from emission sources in AD plants. Error bars show
min and max values. Liquid treatment system (LTS) (n = 2), Combined heat and power units (CHP)
(n = 6), biofilter (n = 15), open windrow (n = 3).
3.2. Emissions factors of CH4, N2O and NH3 at the AD plants
The CH4 emission factors from AD plants were from 444 to 1,5713 g (Mg biowaste)-1
(Figure 2). The median CH4 emission factor was 3,397 g (Mg biowaste)-1. The plants 1, 2 and 9
with open composting windrows showed highest CH4 emissions. The CH4 emission factors from
composting windrows were 15,452, 5,763 and 10,254 g (Mg biowaste)-1 which contributed
relatively to 95 %, 73 % and 96 % and of the total CH4 emissions at the plants 1, 2 and 9
respectively.
Emission factors of CH4 from CHPs were measured only in the plants 3, 4 and 7. CH4
emission factors from CHPs varied from 52 to 2,040 g (Mg biowaste)-1. The results were higher
than a previous study: [4] reported that the emission factors of CH4 from CHP ranged from 16 to
819 g (Mg biowaste)-1.
The emission factors of CH4 from biofilters varied from 236 to 5,237 g (Mg biowaste)-1.
The results are in line with the findings of [4, 8] but comparatively higher than the results of [9],
who found that the emission factors of CH4 were about 100 g (Mg waste)-1.
plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9
Ei
ss
io
n
fa
ct
or
: g
C
H
4 (
M
g
w
as
te
)-1
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Biofilter
CHP
Open Composting Windrow
Liquid treatment
Figure 2. Median emission factors of CH4 g (Mg biowaste)-1 from different AD plants.
For plants 1, 2, 4, 5, 6, 8 and 9, CHP emission data are missing.
Greenhouse Gas Emissions from Anaerobic Digestion Plants
213
The emission factors of N2O were in a range of 7-170 g (Mg biowaste)-1 (Figure 3). Median
N2O emission factor was 67 g (Mg biowaste)-1. N2O emissions from open composting windrows
contributed significantly to the total N2O emissions of the AD plants. The contributions of open
composting windrows to the total N2O emissions were 76 %, 80 % and 94 % at the plants 1, 2
and 9 respectively. The N2O emissions from CHPs were from 0.5 to 5 g N2O (Mg biowaste)-1
and contributed only 2-7 % to the total N2O emissions. The N2O emission factors from biofilters
ranged from 6.7 to 78 g (Mg biowaste)-1. The N2O emissions from LTS contributed in the range
of 2-27 % of the total N2O emissions.
plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9
E
is
si
on
fa
ct
or
: g
N
2O
(M
g
w
as
te
)-1
0
50
100
150
200
Biofilter
CHP
Open Composting Windrow
Liquid treatment
plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9
Ei
ss
io
n
fa
ct
or
: g
N
H
3 (
M
g
w
as
te
)-1
1
10
100
1000
10000
Biofilter
CHP
Open Composting Windrow
Liquid treatment
Figure 3. Median emission factor of N2O g (Mg
biowaste)-1 from different AD plants.
Figure 4. Median emission factors of NH3 g (Mg
biowaste)-1 from different AD plants.
The emission factors of NH3 were in the range of 41-6,032 g (Mg biowaste)-1 (Figure 4).
Median NH3 emission factor was 101 g NH3 (Mg biowaste)-1. Open composting windrows
contributed 91 %, 86 % and 99 % to the total NH3 emissions at the plants 1, 2 and 9 respectively.
High NH3 emissions at the plant 3 were due to conversion of NH4+ in digestate to NH3 at a high
pH and a high temperature in the belt dryer.
3.3. The contribution of CH4, N2O and NH3 from AD plants to global warming potential
The emission factors were transferred into CO2 equivalents according to IPCC (2007). The
overall CO2 emissions were in a range from 31 to 435 kg (Mg biowaste)-1. The CH4 emissions
from AD plants were more important than the emissions from N2O and NH3 (Figure 5A). The
emissions of CH4 accounted from 36 - 92 % while the emission of N2O and NH3 contributed
from 6.9 - 30 % and from 0.08 – 58 % respectively to the overall CO2 emissions. The median
CO2 equivalent emission was 105 kg CO2 (Mg biowaste)-1. The results were in line with a
previous study. [4] reported that an AD plant contributed up to 111 kg CO2 equivalent (Mg
waste)-1. The AD plants with open composting windrows (1, 2 and 9) showed higher CO2
equivalent emissions than the AD plant without open composting windrows.
Figure 5B shows the net total of CO2 equivalent from different emission sources at AD
plants. The open composting system resulted in high GHG emissions accounting from 73 to 96
% to the total emissions at plants 1, 2 and 9. CHP contributed from 5 to 50 % to the total
emissions at plants 3, 4 and 7. The liquid treatment system resulted in insignificant (3.7 %) to
the total CO2 equivalent emissions at plant 1.
Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls
214
plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9
Em
is
si
on
fa
ct
or
: C
O
2 e
qu
i k
g
(M
g
w
as
te
)-1
0
100
200
300
400
500
CH4
N2O
NH3
Σ 435
Σ 31
Σ 85
Σ 105
Σ 203
327 197
3806
98
Σ 74
Σ 40
Σ 149
Σ 305
A
plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9
Em
is
si
on
fa
ct
or
: k
g
C
O
2 e
qu
iv
(M
g
w
as
te
)-1
0
100
200
300
400
500
Biofilter
CHP
Open Composting Windrow
Liquid treatment
Σ 435
Σ 31
Σ 85
Σ 105
Σ 203
Σ 74
Σ 40
Σ 149
Σ 305
B
Figure 5. A: The contribution of CH4, N2O and NH3 in form of CO2 equivalent emissions at AD plants. B:
The contribution of different emission sources at AD plants. For plants 1, 2, 4, 5, 6, 8 and 9, CHP
emission data are missing.
4. CONCLUSIONS
Anaerobic digestion plants are a source of GHG emissions. Emission sources are biofilter,
open windrows, CHP and liquid digestate treatment system. Especially, open windrows have
adverse impacts on environment. Inside the AD plants, the emissions at the receiving and pre-
treatment processes play less important roles, whereas the separation of digestate into a solid and
a liquid phase results in high GHG emissions.
Based on the results, the emissions factors were 3397 g (Mg waste)-1 for CH4 (85 kg CO2
equivalent) and 67 g (Mg waste)-1 for N2O (20 kg CO2 equivalent). In Germany, ca. 10.5 million
tonnes biowaste are produced per year. If all biowaste would be treated by AD, they would
result in contribution of 0.31 % for N2O and 1.83 % for CH4 to the overall national GHG
emissions (base: 2012).
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