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The social acceptance of biogas is often hampered by environmental and health concerns. In this study, the current knowledge about the impact of biogas technology is presented and discussed. The survey reports the emission rate estimates of the main greenhouse gases (GHG), namely CO2, CH4 and N2O, according to several case studies conducted over the world. Direct emissions of gaseous pollutants are then discussed, with a focus on nitrogen oxides (NOx); evidences of the importance of suitable biomass and digestate storages are also reported. The current knowledge on the environmental impact induced by final use of digestate is critically discussed, considering both soil fertility and nitrogen release into atmosphere and groundwater; several case studies are reported, showing the importance of NH3 emissions with regards to secondary aerosol formation. The biogas upgrading to biomethane is also included in the study: with this regard, the methane slip in the off-gas can significantly reduce the environmental benefits. Free eprint at: https://www.tandfonline.com/eprint/zzE2IdqnpJu5QnbCZarc/full
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Environmental impact of biogas: A short review of current knowledge
Valerio Paolini
a
, Francesco Petracchini
a
, Marco Segreto
a
, Laura Tomassetti
a
, Nour Naja
b
, and Angelo Cecinato
a
a
National Research Council of Italy, Institute of Atmospheric Pollution Research, Monterotondo, Italy;
b
Boston Northeastern University, Chemical
Engineering Department, Boston, Massachusetts, USA
ARTICLE HISTORY
Received 26 January 2018
Accepted 23 March 2018
ABSTRACT
The social acceptance of biogas is often hampered by environmental and health concerns. In this study,
the current knowledge about the impact of biogas technology is presented and discussed. The survey
reports the emission rate estimates of the main greenhouse gases (GHG), namely CO
2
,CH
4
and N
2
O,
according to several case studies conducted over the world. Direct emissions of gaseous pollutants are
then discussed, with a focus on nitrogen oxides (NO
x
); evidences of the importance of suitable biomass
and digestate storages are also reported. The current knowledge on the environmental impact induced by
nal use of digestate is critically discussed, considering both soil fertility and nitrogen release into
atmosphere and groundwater; several case studies are reported, showing the importance of NH
3
emissions with regards to secondary aerosol formation. The biogas upgrading to biomethane is also
included in the study: with this regard, the methane slip in the off-gas can signicantly reduce the
environmental benets.
KEYWORDS
Air quality; anaerobic
digestion; biogas; digestate;
renewable energy; secondary
aerosol; waste management
Introduction
The environmental benets of biogas technology are often
highlighted, as a valid and sustainable alternative to fossil
fuels.
[1]
Together with the reduction of greenhouse gas (GHG)
emissions, biogas can enhance energy security, thanks to its
high energetic potential.
[24]
As a renewable energy source, it
allows exploiting agricultural and zootechnical byproducts and
municipal wastes, with a lower impact on air quality when
compared to combustion-based strategies for these bio-
masses.
[57]
Furthermore, while ashes from combustion nd
scarce agronomic applications,
[8,9]
the by-product of anaerobic
digestion, i.e. digestate, looks as a reliable material for agricul-
tural uses.
[10]
Another important advantage of biogas technol-
ogy is its easy scalability, allowing exploiting the energetic
potential of decentralized biomass sources.
[11,12]
Finally, biogas
can be upgraded to biomethane, suitably used as a vehicle fuel,
or injected into national natural gas grids,
[13,14]
The energy potential of biogas is reported in Figure 1, based
on data from the World Bioenergy Association.
[15]
For Europe,
China and USA, data are detailed in terms of the following
sources: manure, agriculture residues, energy crops, organic
fraction of municipal solid waste (MSW), agro-industry waste
and sewage sludge. For the total world biogas potential, data
are only divided into waste (i.e. organic fraction of MSW, agro-
industry waste and sewage sludge) and agricultural byproducts
(i.e. manure, agriculture residues and energy crops).
In spite of the above cited advantages, social opposition is
often observed towards biogas plants, generally based on con-
cerns about environmental and health issues.
[16]
The frequency
on which these opposition phenomena are observed depends
on different factors, including the inclusion strategies and the
considered country.
[17,18]
In order to overcome social and
cultural barriers hampering a wider diffusion of biogas, the
accurate and complete evaluation of the environmental impact
of these processes remains an issue of high scientic and tech-
nical relevance. The aim of this work is to report an updated
state of the art of current knowledge about the environmental
impact of biogas and biomethane.
Greenhouse gas emissions
A main objective of biogas industry is the reduction of fossil fuel
consumption, with the nal goal of mitigating global warming.
However, anaerobic digestion is associated to the production of
several greenhouse gases, namely carbon dioxide, methane and
nitrous oxide. As a consequence, dedicated measures should be
taken in order to reduce these emissions. According to Hijazi,
[19]
the main measures to improve the global warming reduction
potential of biogas plants are: to use a are avoiding methane
discharge, to cover tanks, to enhance the efciency of combined
heat and power (CHP) units, to improve the electric power uti-
lisation strategy, to exploit as much thermal energy as possible,
to avoid leakages. Similar conclusions were obtained by Buratti
and co-workers
[20]
for the specic case study of cereal crops in
Umbria, Italy. Biomethane chain exceeds the minimum value of
GHG saving (35%) mainly due to the open storage of digestate;
usual practices to improve GHG reduction (up to 68.9%) include
using heat and electricity produced by the biogas CHP plant, and
covering digestate storage tanks.
CONTACT Dr. Valerio Paolini v.paolini@iia.cnr.it National Research Council of Italy, Institute of Atmospheric Pollution Research, via Salaria km 29,300; 00015,
Monterotondo (RM), Italy.
Color versions of one or more of the gures in the article can be found online at www.tandfonline.com/lesa.
© 2018 Taylor & Francis Group, LLC
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
2018, VOL. 53, NO. 10, 899906
https://doi.org/10.1080/10934529.2018.1459076
The impact induced by biogas plants on global warming
needs to be studied case by case. Bachmaier and co-workers
[21]
calculated the GHG impact of ten agricultural biogas plants.
GHG emissions coming from electricity production in the
investigated biogas plants ranged from ¡85 to 251 g
CO
2
-eq/kWh
el
, and the GHG saving was 2.31 3.16 kWh
fossil
/
kWh
el
. The results obtained also highlighted that reliable esti-
mates of GHG emissions in the case of electricity production
from biogas can be only made on the basis of individual moni-
toring data, for instance: reduction of direct methane emission
and leakage, exploiting of heat obtained from cogeneration,
amount and nature of input material, nitrous oxide emission
(e.g. from energy crop cultivation) and digestate management.
Battini and co-workers,
[22]
in a case study of an intensive dairy
farm situated in the Po valley (Italy), calculated a GHG emis-
sion reduction due to anaerobic digestion ranging between
¡23.7% and ¡36.5%, depending on digestate management. In
a Finnish case study,
[23]
the GHG release reduction was esti-
mated equal to 177.0, 87.7 and 125.6 Mg of CO
2
eq. yr
¡1
for
dairy cow, sow and pig farms, respectively. Optimizing all pro-
cess parameters looks important with regard to nal environ-
mental impact: for instance, a specic case study on wastewater
treatment showed that the process optimization could result
into the emission abatement equal to 1,103 kg CO
2
eq/d for
N
2
O, 256 kg eq/d for CO
2
and 87 kg CO
2
eq/d for CH
4.[24]
Carbon dioxide emissions
Harmful compounds and air contaminants are introduced into
the environment during biogas production and use through
both combustion processes and diffusive emissions. Consider-
ing carbon dioxide, combustion of biogas leads to efcient
methane oxidation and conversion to CO
2
, with a rate of
83.6 kg per GJ (based on a biogas with 65% CH
4
and 35%
CO
2[25]
). Other releases of this contaminant are related to
transport and storage of biomass, as well as digestate use. In the
case of both biogas combustion and biomass/digestate emis-
sion, CO
2
is considered as biogenic and calculated neutral with
regards to the impact on climate. Taking into account the
reduction of fossil fuel, it can be demonstrated that biogas pro-
duction leads globally to mitigation of anthropogenic green-
house impact of the environment. Poeschl and co-workers
[26]
have investigated the CO
2
emissions associated to biogas pro-
duction from several feedstocks, and the relative contribution
of feedstock supply, biogas plant operation and infrastructure,
biogas utilization and digestate management. According to this
study, biogas use gives rise to a negative CO
2
balance because
CO
2
caption results every time higher, in absolute values, than
positive emissions from feedstock supply and biogas plant
operation. As expected, biogas production from byproducts
(e.g. from food residues, pomace, slaughter waste, cattle
manure, etc.) is a more sustainable approach than energy crops
utilization such as whole-wheat plant silage. Besides, digestate
management provides signicant contributions to total emis-
sion reduction in the case of specic feedstock such as munici-
pal solid waste. A dedicated section of this study will below
discuss the impact of digestate in full details, in paragraph 5.
Methane emissions
Methane released by biogas processes is not considered relevant
for health issues: though exposure to hydrocarbon mixtures can
have some adverse effects on humans,
[27]
no evidence exists of
relevant interactions between methane and biologic systems.
[28]
However, methane is a greenhouse gas whose global warming
power is estimated to be 2836 times higher than CO
2
over 100
years: as such, it is the second major component among anthro-
pogenic greenhouse chemicals.
[29]
Hence, in evaluating the
impact of biogas industry on climate change, methane emis-
sions are a point of primary importance. Methane can be
released during biogas incomplete combustion; however a
strong contribution to this contaminant comes out from diffu-
sive emission related to biomass storage and digestate
management. On the other hand, other biomass management
strategies must be taken into account to abate emissions related
to biogenic methane. In the above mentioned study of Poeschl
and co-workers,
[26]
methane emissions were also discussed; in
all investigated cases, the emission rates were below 5 g kg
¡1
.
Considering cattle manure, important reductions in methane
emission are related to digestate processing and handling, since
this kind of biomass is characterized by high methane emission
rate when spread in the eld without any pre-treatment.
Nitrous oxide
Besides CO
2
and CH
4
, nitrous oxide (N
2
O) is another impor-
tant GHG: Due to its high greenhouse effect potential, N
2
O
emissions from biogas production processes can result into a
signicant contribution to global warming budget.
[30,31]
The
relative impact of nitrous oxide mostly depends on the chosen
climate metrics: indeed, N
2
O impact can even exceed those of
CO
2
and CH
4
, when the considered metric is Global Tempera-
ture change Potential with a time horizon of 100 years (namely
GTP-100).
[32]
Total GHG emission for energy production from biogas are
generally calculated in a range between 0.10 and 0.40 kg
CO2-eq/kWh
el
, which is for instance 2275% less than GHG
emissions caused by the present energy mix in Germany.
[33]
The wide uncertainty about the estimates of global warming
mitigation potential depends on N
2
O emission rate assessment
Figure 1. Energy potential of biogas.
900 V. PAOLINI ET AL.
as well as on storage and use as a fertilizer of digestate, as dis-
cussed in paragraphs below.
Gaseous pollutants from biogas combustion
Along GHG reduction benets, it must be considered that bio-
gas combustion is associated to release of pollutants in the
atmosphere; therefore, the correct assessment of these emis-
sions is a key point in social acceptance of this technology. A
summary of emission factors for the main gaseous pollutants
are reported in Table 1.
Carbon monoxide (CO) is produced in all oxidation pro-
cesses of carbon containing materials, and is an important by-
product of incomplete combustion of biogas. Methane emission
rates are 0.74 and 8.46 and g CO per Nm
¡3
CH
4
for aring and
CHP, respectively.
[34]
CO emissions related to energy produc-
tion are estimated in a range between 80 and 265 mg CO MJ
¡1
,
depending on the plant efciency.
[35]
Sulphur dioxide (SO
2
) emissions from biogas plants manly
depend on the desulphurization degree of the introduced bio-
gas. The SO
2
emission rate of a CHP biogas plant is estimated
to lie in the range 19.225 mg MJ
¡1
.
[25]
The UK National Soci-
ety for Clean Air (NSCA) estimates an emission factor of 80
and 100 g
SO2
/tonn
waste
for aring and CHP, respectively.
[36]
The relatively high SO
2
concentrations in the proximity of bio-
gas plants can depend on different reasons, e.g.: direct emission
from biogas combustion, H
2
S oxidation from diffusive emis-
sions, and diesel truck exhausts.
[37]
Emissions of NO
x
are one of the most critical point with
regard to environmental impact of biogas plants.
[38]
According
to Kristensen and co-workers,
[35]
the NO
x
emission level of bio-
gas is, in general, higher than for natural gas engines: the aver-
aged aggregated emission factor is 540 g NO
x
GJ
¡1
, which is
more than three times the rate from natural gas engines. When
emission factor is reported to methane consumption, an emis-
sion factor of 0.63 and 11.6 g
NOx
/Nm
3CH4
can be assumed for
aring and CHP, respectively.
[34]
The importance of controlling
this pollutant is demonstrated by several case studies. For
instance, Battini and co-workers
[22]
in the above mentioned
case study of an intensive dairy farm situated in the Po valley
(Italy) reported a low enhancement in acidication (5.56.1%),
particulate matter emissions (0.71.4%) and eutrophication
(C0.8%), while on the other hand a signicant enhancement in
photochemical ozone formation potential (41.642.3%) was
calculated. In another case study, Carreras-Sospedra and co-
workers
[39]
estimated a potential enhancement of up to 10% of
NO
x
emission in 2020 in California (US); nevertheless, their
study included both biogas and biomass burning. Indeed, the
lower emissions of methane from storage and the credits from
substituted electricity are not enough to compensate the
increase in NO
x
emissions from the biogas combustion.
Biogas is a gaseous fuel rich in volatile organic compounds
(VOCs), compared to natural gas: indeed, VOCs concentration
normally ranges between 5 and 500 mg/Nm
3
, and in some cases
up to 1700 mg/Nm
3
were observed.
[40,41]
Generally, only non-
methane volatile organic compounds (NMVOC) are consid-
ered in these studies. If combustion is assumed to reduce VOCs
concentration of 99%,
[42]
VOCs emission from biogas combus-
tion are in general lower, compared to liquid and solid biofuels.
However, a specic critical issue can be highlighted for formal-
dehyde. In a case study conducted on anaerobic waste treat-
ment plants in Barcelona (Spain), VOC emission factors was in
the range 0.9 §0.3 g s
¡1
, contributing for 0.30.9% of total
VOCs in the area. On the other hand, formaldehyde emission
factors from biogas engines were found between 0.2 and
3.0 mg s
¡1
, resulting in a »2% contribution to the total.
[43]
It is
important to remark that a similar emission pattern is observed
for natural gas: indeed, formaldehyde is a by-product of meth-
ane oxidation. Compared to natural gas, emissions of VOCs
are 40% lower in biogas engines, while formaldehyde emissions
are slightly lower and higher aldehydes (present in natural gas
due to the presence of higher hydrocarbons) are almost
absent.
[35]
Noticeably, fuel-cycle emissions can be strongly inuenced
by the raw materials. For instance, CO
2
, CO, NO
x
, hydrocar-
bons and particles may differ by a factor of 34 between ley
crops, straw, sugar beet byproducts, liquid manure, food indus-
try waste and municipal solid waste. On the other hand, differ-
ences by a factor of up to 11 can be observed in SO
2
emissions,
due to the high variability of H
2
S and organic sulphur com-
pounds in the produced biogas.
[44]
Impact of feedstock and digestate storage
and treatment
In the biogas combustion management, feedstock and digestate
storage and treatments can be the most important processes to
achieve the global warming benets of biogas production pro-
cesses. Indeed, the impact of a biogas plant on GHG emission
is heavily inuenced by feedstock storage: most of N
2
O can be
abated when a closed storage is used for manure and co-diges-
tion feeding.
[45]
Emissions from uncovered biomass storage have also been
identied as the main ammonia source along the whole biogas
production chain,
[46]
and closed storage is strongly advised.
In a specic French case study of anaerobic digestion and
composting plant for municipal solid waste, Beylot and co-
workers
[38]
have identied four conditions for process
operation, which highly inuence the impact of the whole
plant; they are: (i) the features of degradation of the ferment-
able fraction; (ii) the collection efciency of gas streams
released by biological operations; (iii) the abatement effective-
ness of collected pollutants; and (iv) NO
x
emission rate from
Table 1. Emission factors of biogas plants operating direct biogas combustion.
Pollutant
Emission factor (g
GJ
¡1
) Source
Carbon monoxide (CO) 310 Nielsen et al.,
[25]
256 Kristensen
et al.,
[35]
Sulphur dioxide (SO
2
) 25 Nielsen et al.,
[25]
Nitrogen oxides (NO
x
) 202 Nielsen et al.,
[25]
540 Kristensen
et al.,
[35]
Non-methane volatile organic
compounds (NMVOC)
10 Nielsen et al.,
[25]
21.15 Kristensen
et al.,
[35]
Formaldehyde (CH
2
O) 8.7 Nielsen et al.,
[25]
14 Kristensen
et al.,
[35]
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A 901
biogas combustion. The importance of digestate storage step
has been highlighted by Battini and co-workers,
[22]
in the above
mentioned case study of intensive dairy farm situated in the Po
valley (Italy): GHG emission reduction due to AD, calculated
as equal to ¡23.7%, can reach ¡36.5% when a gas-tight tank is
used for digestate storage.
A proper design and management of feedstock and digestate
storage units looks also important in order to mitigate the
odour impact of the plant. Indeed, the two major sources of the
olfactory annoyance are biomass storage production of biogas
and digestate composting units.
[47]
Closed-operated hydrother-
mal hydrolysis has positive effects on overall fugitive odour
control in plants; on the other hand, eventual fugitive emissions
during high-temperature and seemingly open pre-treatments
can be the principal source of odours.
[48]
In conclusion, gas tight storage should always be advised,
since the corresponding GHG and ammonia fugitive emissions
are even more important those coming from fertilizers.
[49]
As
mentioned above, avoiding leakages and using closed tanks are
among the most important ways to reduce the global warming
impact of biogas plants.
[19]
Impact of digestate nal use
The use of agricultural and zootechnical byproducts and MSW
as soil improver and fertilizer is a sustainable approach, allow-
ing to reduce the production, transport and use of synthetic
chemicals: however, spreading untreated biomass on soils
sometimes implies the release into the atmosphere of huge
amounts of chemicals such as methane, nitrous oxide, ammo-
nia, volatile hydrocarbons, etc. Anaerobic digestion of biomass
followed by the use of digestate as biofertilizer is a common
practice related to biogas production. In this paragraph, the
current knowledge concerning the environmental impact of
this practice is briey discussed.
A recent study on this topic
[50]
concluded that direct effects
of anaerobic digestion on long-term sustainability in terms of
soil fertility and environmental impact at the eld level are of
minor relevance; indeed, the most relevant issue (with regard
to both emissions to atmosphere and in soil fertility) is related
to possible changes in cropping systems. According to this
study, the main direct aftermaths of anaerobic digestion are
short-term effects on soil microbial activity and changes in the
soil microbial community. Considering soil quality, digestate is
signicantly more inert vs. atmospheric and biological agents
than the biomass itself: this property results into a lower degra-
dation rate of the organic matter. In fact, labile fractions of
original biomass such as carbohydrates are rapidly degraded,
causing the enrichment of more persistent molecules such as
lignin and non-hydrolysable lipids.
[51]
In a specic case study
on pig slurry anaerobic digestion, a high biological stability of
biomasses was achieved, with a Potential Dynamic Respiration
Index (PDRI) close to 1,000 mg O
2
kg VS
¡1
h
¡1
.
[10]
With regard to nitrate leaching and release into the atmo-
sphere of ammonia and nitrous oxide, the current state of
knowledges needs to be improved: however, the impact is con-
sidered negligible or at least ambiguous.
[50]
The ambiguity
of previous studies, as highlighted by this Author, is probably
due to the different impact of digestate depending on the type
of considered soil. For instance, Eickenscheidt and co-work-
ers
[52]
investigated the emission of methane, nitrous oxide and
ammonia from untreated manure and digestate applied on sev-
eral soils: while methane emissions did not signicantly change,
high N
2
O emissions were observed in the correspondence of
high carbon loadings. A signicative impact of soil moisture-
soil mineral-N interactions on N
2
O emissions was also
observed by Senbayram and co-workers.
[31]
Considering N
2
O and CH
4
, digestate can give rise to signi-
cant emission rates into the atmosphere: however, these emis-
sions are generally lower than untreated biomass.
[53]
As for
nitrous oxide, digested products are more recalcitrant than
fresh slurry; thus, microbial degradation is slower, in which
leads to relatively few anoxic microsites and poor N
2
O emission
compared to fresh slurry application.
[5456]
Conversely, meth-
ane emissions from digestate are generally lower than those of
original biomass, since the methanogenic potential is reduced:
this is particularly relevant in the presence of reduced methane
coming from manure
[26,45]
(Poeschl et al., 2012; Boulamanti
et al., 2013). As for methane emission, an exception is known
in the specic case of rice cultivation: indeed, adding digestate
to paddy results into the methane emission rate enhancement
from 16.9 to 29.9 g m
¡2
,
[57]
whilst no signicant effects are
observed for N
2
O.
[57,58]
Based on the above-cited literature, N
2
O and CH
4
emissions
from digestate are not critical, while ammonia release and
nitrate leaching are still a critical point. For instance, ammonia
emissions from digestate higher than from original manure
have been observed in several studies.
[56,59,60]
It was also
reported that up to 30% of nitrogen can be lost by ammonia
volatilization, due to the enhancement of soil pH.
[59,60]
Speci-
cally, Matsunaka and co-workers
[61]
reported a 13% nitrogen
volatilization as ammonia, when anaerobically digested cattle
slurry was used as soil fertilizer for grassland. The practice of
fertilizing soil with anaerobically digested materials increases
soil concentration of NO
3
¡
(C30/40% compared to raw cattle
slurry): this is associated to the four times more readily degrad-
able organic C increased microbial biomass, depleting nitrogen
and oxygen concentration in soil and resulting in the 10 times
increase of CO
2
and N
2
O emissions.
[62]
A proper management
of digestate can mitigate its environmental impact: ammonia
emission rates ranging from 1.6 to 30.4 were reported, depend-
ing on the adopted practice.
[63]
With regards to pesticides, heavy metals and harmful micro-
organisms, the risk of food chain contamination is generally
considered low,
[64]
but the soil burden of persistent organic pol-
lutants (POPs) caused by the use of digestate as biofertilizer still
needs to be fully assessed.
[65]
On the other hand, anaerobic
digestion can have relevant effects on phytotoxicity of specic
biomass: for instance, the phyto-toxic character of olive mill
efuent is reduced after anaerobic digestion,
[66]
and the degra-
dation of aatoxin B1 from corn grain can be reached.
[67]
Finally, an odour reduction up to 8288% can be obtained.
[63]
In conclusion, the main critical issue in nal use of digestate
is nitrogen release into the environment, which can be reduced
by applying the best practices for preserving soil quality. The
management of nitrogen dosage is sometimes difcult because
of the feedstock variability. It is also important to remark that
fugitive emissions from digestate storage are generally more
902 V. PAOLINI ET AL.
important than those released by its use into soil, as indicated
above.
[20,49]
Impact on particulate matter
With regards to particulate matter (PM), biogas combustion is
not a signicant emission source when compared to other fuels:
emission factors of 0.238 and 0.232 g/Nm
3CH4
have been esti-
mated for aring and CHP, respectively.
[34]
However, second-
ary PM formation can occur, due to NO
x
emissions from CHP
and NH
3
volatilization from storage and digestate nal use.
Indeed, during secondary PM formation, the prominent roles
of ammonia
[68]
and NO
x[69]
are ascertained. As reported by
Boulamanti and co-worker,
[45]
NO
x
emissions are in general
the principal source of secondary PM from biogas. As discussed
above, closed storage can signicantly abate ammonia emis-
sions, resulting also into the global reduction of PM formation
from this contaminant.
Impact of biogas upgrading to biomethane
Biomethane production is an efcient approach to increase the
market share of biogas, resulting in a further reduction of fossil
fuels. The equivalent CO
2
saving raises considerably if methane
slip is limited to 0.05%,
[70]
while the process results no longer
sustainable when methane losses reach 4%. Biomethane use as
an alternative to gasoil is expected to improve local air quality,
with regards to NO
x
and particulate matter. As a consequence,
biogas upgrading for vehicle fuelling purposes produces opti-
mum benets with respect to photochemical oxidant forma-
tion, marine eutrophication and ecotoxicity; on the other hand,
scarce benets are observed in terms of climate change com-
pared to biogas combustion in CHP.
[71]
Depending on several factors such as energy consumption,
production and transport of materials used, produced waste
and methane slip, the environmental impact of biomethane
production depends on the upgrading technology adopted. In
PSA, the eventual recovery of the off-gas plays a key role.
[72,73]
Starr and co-workers
[74]
reported that the most CO
2
-efcient
upgrading technology for MSW biogas is the BABIU (bottom
ash upgrading) based on ash produced by municipal waste
incinerators. The condition required is that the incinerator lies
within 125 km from the biogas upgrading plant. Considering
water scrubbing in basic solutions, a lower impact can be
achieved by replacing KOH with NaOH. Water from biogas
upgrading plants can be recycled in the process or treated as
wastewater, depending on chemical composition: the most
common VOC in the wastewater of biogas upgrading plants
are p-cymene, d-limonene and 2-butanone
[75]
; the maximum
VOC content is observed in MSW treatment plants, reaching
up to 238 mg/L, but no inhibition is observed when waste-
waters are recycled in the plant.
Along its impact on climate, biomethane use as gasoil substi-
tute of is expected to improve urban air quality, because emis-
sion factors of methane are up to 10 times lower than those of
liquid fuels, considering PM, VOCs and polycyclic aromatic
hydrocarbons.
[76]
Biomethane injection in the national grid
may also reduce residential solid fuels consumption in some
specic regions, with relevant benets on indoor air quality
and human health.
[77]
Global emission potential
The potential emission associated to biogas plants is reported in
Figure 2 (NO
x
and CO) and in Figure 3 (for formaldehyde,
NMVOC and SO
2
). Data are obtained combining emission fac-
tors reported in Table 1
[25]
and energy potential reported in
Figure 1. For Europe and China, the contribution of energy
crops is reported separately, since their use is often disregarded
due to its negative impact on land availability for food. In the
case of the global potential, the relative contribution of energy
crops is not available.
Conclusions
Biogas can signicantly contribute to abate greenhouse gas
emissions. However, attention must be payed towards unde-
sired emissions of methane and nitrous oxide (N
2
O). The emis-
sion budgets of the two compounds are scarcely related to
direct release from biogas/biomethane combustion, whilst bio-
mass storage and digestate management are the critical steps.
Similar considerations apply to ammonia: to reduce its impact
on secondary aerosol formation, efcient biomass and digestate
Figure 2. Emission potential of biogas plants for NO
x
and CO.
Figure 3. Emission potential of biogas plants for formaldehyde, NMVOC and SO
2
.
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A 903
storage should always be recommended. Among all the gaseous
pollutants considered in direct emission from biogas combus-
tion, nitrogen oxides (NO
x
) level were worth of some concern
in several case studies. On the other hand, volatile organic com-
pounds do not seem to constitute a critical issue. Considering
the aftermaths of digestate spreading on soil quality, further
studies are needed in order to fully assess the long-term impact.
In the medium-short term, digestate seems to be preferable
compared to untreated biomass. The upgrading to biomethane
can generally improve air quality and reduce GHG emissions;
however methane losses in the off-gas can affect the sustainabil-
ity of the whole process.
Acknowledgments
The study is part of the ISAAC project (Increasing Social Awareness and
Acceptance of biogas and biomethane), funded by the European Commis-
sion within the Horizon 2020 programme under grant agreement no.
691875.
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906 V. PAOLINI ET AL.
... According to Paolini et al. [36], to remove the social and cultural barriers that hinders the diffusion of biogas, a complete evaluation of the environmental impact of the related processes is an issue of high scientific and technical relevance. The Life Cycle Assessment (LCA) methodology is widely adopted in this field for the evaluation of the environmental sustainability [37]. ...
... Most of the studies belong to hard sciences, followed by the economic ones, the social-psychological and others (Fig. 3). In some cases, non-academic organisations were involved in the studies: governmental research centres [33,36,[49][50][51][52][53][54][55][56][57], private companies [37,53,58,59] and non-profit organisations [23,53,60,61]. ...
... Most papers were empirical studies. The review articles were 4 [34,36,52,62]. Finally, in 9 works, both reviews and case studies were presented [33,44,51,53,57,[63][64][65][66]. ...
Article
Social acceptance is considered the main non-technical barrier to the development of bioenergy projects. This paper presents the results of a systematic literature review aimed to cover a lack in state-of-the-art literature about socio-cultural factors affecting the acceptance of biogas projects at a global scale. Moreover, this study is aimed at identifying which methods are used for studying this phenomenon, with a focus on the Life Cycle Thinking-oriented ones. Journal articles and conference proceedings were considered. At the end of the screening phases, 54 documents were selected and reviewed. The results showed that acceptance concerns two main issues: biogas plants and its presence in a given location and digestate application on fields. This review showed different results between high-income and low-middle-income countries. As regards the former, trust was the most mentioned socio-cultural factor. Education, as well as women's living conditions were considered important in the latter. However, a contextualisation of every outcome based on local peculiarities is needed in order to understand in a better way the accepting/refuting phenomena of the projects. As regards the second objective of this study, Life Cycle Analysis resulted the most widespread Life Cycle Thinking methodology. In conclusion, the outcomes of this work may be useful to identify the non-technical factors and the most suitable approach that should be considered for a successful implementation of site-specific biogas projects.
... In general, silage maize cultivation has increased significantly in organic farming due to high energy yields, efficient cultivation practices, high digestibility and methane formation potentials of maize [35,36]. The use of renewable raw materials for the production of sustainable energy should primarily conserve fossil raw materials and contribute to the reduction of man-made greenhouse gases, thus helping to combat climate change [37]. This only works if the emissions from the production of energy crops and fertilization with biogas digestates, the nutrient-rich residue of biogas production, are lower than those produced by fossil fuels. ...
... This continues until July. At this time, the maize is in the BBCH stage of shooting (30)(31)(32)(33)(34)(35)(36)(37)(38)(39). In the white clover variant, the nitrogen application of the fertilizer could be partially removed by the clover and thus reduced the emissions, but also the yields. ...
... The advances in AD procedures, strategies and technologies, such as the simultaneous application of different pretreatments over the years promoted a reduction of toxic compounds, GHG emission and an increase in organic matter degradation (Paolini et al. 2018). Pretreatment of the substrate promotes further solubilization of the organic matter in simpler, bioavailable molecules for microorganisms' metabolism, leading to an increase in biogas production and methane yield (Agbor et al. 2011). ...
... Pretreatment of the substrate promotes further solubilization of the organic matter in simpler, bioavailable molecules for microorganisms' metabolism, leading to an increase in biogas production and methane yield (Agbor et al. 2011). Pretreatments are usually an investment to maximize the biogas yield and economic balance (Paolini et al. 2018). In this sense, the application of pretreatments in all kinds of organic substrates has the potential to, indirectly, benefit aquatic environments (Prabakar et al. 2018). ...
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The large global generation and improper management of waste lead to the pollution of the environment and efforts toward reducing the impacts of anthropogenic activities on aquatic environments should be prioritized. The United Nations declared 2018-2028 as the international decade for action on “Water for Sustainable Development” and integrated management of water resources. Several international initiatives, such as the UN 2030 Agenda, the Sendai Framework for Disaster Risk Reduction and the Paris Agreement, have highlighted and strongly recommended the development of new technologies to reverse the current environmental scenario of global water bodies. The use of anaerobic digestion for treating organic wastes can minimize and avoid several adverse effects on aquatic environments while promoting nutrient cycling and the production of biogas, a renewable energy source that can replace fossil fuels and therefore decrease the emission of greenhouse gases. We performed a systematic review to evaluate the contribution of anaerobic digestion in preventing and reducing human impacts on aquatic ecosystems. China (15.1%), Spain (7.3%) and Italy (7.3%) are countries with a pronounced research focus on this topic, indicating their awareness on the importance of managing and preserving their water resources. The integration of co-digestion and pretreatment methods into anaerobic digestion improved the production of byproducts (especially energy and biofertilizer). Thus, this review highlights the success of AD technology as a waste treatment strategy, while reducing the damage inflicted to aquatic systems and its consequences to human health and aquatic biodiversity.
... Methane (CH4), nitrous oxide (N2O), and fluorinated gases are all significant GHGs in addition to carbon dioxide (CO2). As a GHG, methane is roughly 25 times more potent than CO2 (Paolini et al., 2018;Nevzorova and Kutcherov, 2019;Hasan et al., 2020). Methane is produced in nature as a decomposition of plant and animal matter, but there are also natural sinks that remove excess methane (Nevzorova and Kutcherov, 2019;Yasmin and Grundmann, 2019;Hasan et al., 2020). ...
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Biogas is a gas that is produced when organic matter decomposes. Biogas produced by anaerobic digestion may be a viable energy source for rural Ethiopia. It is suitable for cooking as well as generating electricity. There are over 18,000 biogas digesters dispersed throughout Ethiopia, and despite the benefits of addressing energy-related issues like environmental and energy scarcity, the country's use of biogas is not rising dramatically. This article provides an overview of biogas technology in Ethiopia and discusses the obstacles and opportunities associated with its expansion. High initial investment costs for digesters, a lack of biogas substrates, a lack of biogas research, a failure of the biogas pilot phase, a lack of public awareness campaigns, insufficient construction and maintenance expertise, low biogas technology efficiency, minimal biogas application, and a lack of appropriate bio-slurry management were identified as barriers to biogas technology expansion in the country. It was stressed that biogas plants installed throughout the country, particularly in rural areas, should be sized appropriately for the substrate available. Furthermore, the calorific value of biogas should be increased in order for it to be used to power generator sets and internal combustion engines.
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The international sustainable development agenda identifies two fundamental problems to be solved before 2030: the optimal management of Urban Solid Waste (USW) and the generation of clean and affordable energy. The energy use of residual biomass provides scientific and technological solutions, in an interdisciplinary way, to solve these problems together. The objective of this investigation is to examine the alternatives for bioenergetic use of USW, with emphasis on the possibilities of implementation in Mexico. With a documentary review, the thermochemical and biological methods for the bioenergetic use for the biomass of the USW, the expected products (industrial heat, biogas, bio-hydrogen, bioethanol, charcoal, and pellets), their applications, as well as the advantages and limitations of its implementation in Mexico are analyzed. It is concluded that the bioenergetic use of the USW allows: a) to reduce the environmental impacts from management, transport and disposal of the USW, as well as the emission of Greenhouse Gases by them, and by the fuels used in transportation, heating, and electricity generation; b) to value waste and change the financial balance by reducing the costs of managing USW, and generating income from the commercialization of bioenergetics, and c) to generate new sources of employment, contributing to sustainable development and without putting the country's food security and sovereignty at risk.
Chapter
Waste biomass is an emerging renewable feedstock for sustainable biogas production. Rapid progress in the scientific world has driven higher demand for renewable energy resources. Fossil fuel reserves are limited, so environmental imbalances require a renewable solution, including biogas production through waste biomass recycling. Waste biomass contains diverse and complex polymeric constituents, such as carbohydrates, proteins, cellulose, lignin, and fat. However, these various waste biomasses are not easily accessible for biogas generation. Several pretreatment approaches have been implemented to break down stubborn waste biomass into monomeric residues that can be further transformed for biogas production. A range of microbial regimes are key players directly or indirectly involved in accelerating waste biomass degradation and biogas production using a broad spectrum of metabolic traits. However, classical approaches are not efficient, cost-effective, or sustainable enough to improve biogas production using waste biomass to meet increasing demand. Hence, further research and development need to be conducted to solve specific concerns for improving biogas generation and waste biomass accessibility in the near future. Based on this current scenario, the present work provides a thorough discussion on advanced avenues for improving biogas generation and waste biomass degradation with a special emphasis on metabolic engineering and synthetic biology for a sustainable future.
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Worldwide population growth and corresponding energy demands are pushing the limits of natural resource exploitation. Fossil fuel reserves formed over millions of years are being swiftly consumed. This consumption is causing increased concentrations of greenhouse gases (GHGs) in the atmosphere, with global temperatures rising in response. Hence, there is a need to switch to renewable, clean, and sustainable energy sources. Carbon capture and storage and carbon capture and utilization are the two primary domains of focus for controlling global warming. Several techniques, such as GHG utilization, storage of CO2 as gas hydrates under the seabed, CO2 utilization in enhanced oil recovery and other processes, and tree planting to expand the primary GHG sink, are remedies for global warming. The sustainable development vision can be achieved via a mixed approach of maintaining the carbon balance while developing clean energy alternatives to fossil fuels.
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Life cycle analysis allows for the assessment of the qualitative and quantitative relationship between selected areas of human activity and the consequences for the environment. One of the important areas is the production of electricity and heat, for which the main raw material in Poland is hard coal. An alternative may be to use biogas as a fuel for energy purposes. This article presents the assessment of environmental hazards caused by the production of energy from biogas. The analysis took into account the change of the substrate from maize silage, commonly used in Polish biogas plants, to waste from the domestic agri-food industry. The evaluation covered the acquisition of substrates, their transport to a biogas plant, generation of electricity from biogas, and management of the generated by-products. The analysis was done in terms of both the impact and sensitivity categories. It was found that the emission of pollutants related to the acquisition of the substrate plays a key role and the use of waste for the production of biogas used for energy production brings environmental benefits. The analysis has shown that replacing coal with biogas, regardless of the raw materials used in its production, results in a positive environmental effect, especially in the areas of human health and resources categories. The positive environmental effect of the production of electricity from biogas can be enhanced by switching raw materials from purpose-grown crops to waste from the agri-food industry and agriculture. An important factor influencing the environmental impact is the degree of heat utilization (the greater the percentage of heat utilization, the greater the environmental benefits) and management of all by-products.
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Biogas is competitive, viable, and generally a sustainable energy resource due to abundant supply of cheap feedstocks and availability of a wide range of biogas applications in heating, power generation, fuel, and raw materials for further processing and production of sustainable chemicals including hydrogen, and carbon dioxide and biofuels. The capacity of biogas based power has been growing rapidly for the past decade with global biogas based electricity generation capacity increasing from 65 GW in 2010 to 120 GW in 2019 representing a 90% growth. This study presents the pathways for use of biogas in the energy transition by application in power generation and production of fuels. Diesel engines, petrol or gasoline engines, turbines, microturbines, and Stirling engines offer feasible options for biogas to electricity production as prme movers. Biogas fuel can be used in both spark ignition (petrol) and compression ignition engines (diesel) with varying degrees of modifications on conventional internal combustion engines. In internal combustion engines, the dual-fuel mode can be used with little or no modification compared to full engine conversion to gas engines which may require major modifications. Biogas can also be used in fuel cells for direct conversion to electricity and raw material for hydrogen and transport fuel production which is a significant pathway to sustainable energy development. Enriched biogas or biomethane can be containerized or injected to gas supply mains for use as renewable natural gas. Biogas can be used directly for cooking and lighting as well as for power generation and for production of Fischer-Tropsch (FT) fuels. Upgraded biogas/biomethane which can also be used to process methanol fuel. Compressed biogas (CBG) and liquid biogas (LBG) can be reversibly made from biomethane for various direct and indirect applications as fuels for transport and power generation. Biogas can be used in processes like combined heat and power generation from biogas (CHP), trigeneration, and compression to Bio-CNG and bio-LPG for cleaned biogas/biomethane. Fuels are manufactured from biogas by cleaning, and purification before reforming to syngas, and partial oxidation to produce methanol which can be used to make gasoline. Syngas is used in production of alcohols, jet fuels, diesel, and gasoline through the Fischer-Tropsch process.
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Utilizing agricultural residues for energy and heating purposes is gaining momentum in the EU and even more in areas where biomass is a fundamental pillar of the economy like, e.g. South Tyrol in Northern Italy. In previous studies, the authors studied the combustion, in a commercial boiler of four different types of agricultural residues that were previously pelletized. This study has the scope to assess the suitability of the deriving biomass ashes as soil fertilizers. In this analysis potassium is identified as a key indicator for the nutrient content of biomass residue ashes. Therefore, this study introduces a new factor for assessing the utilization potential of biomass residues ashes, which is defined as the “potassium utilization potential factor” or KUP factor (with K representing potassium). The analysis is based on combining the initial concentration of potassium in the feedstock and the efficiency of statistical entropy. Statistical entropy analysis showed that combustion had a diverse substance concentrating efficiency of potassium for the four different types of biomass, ranging from 25 to 40%. In total, apple logs and apple pruning had the best performance in respect to their suitability as soil fertilizer since these feedstocks had the highest KUP factors.
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A vacuum swing adsorption (VSA) prototype plant wasdeveloped and tested, based on natural zeolites from tuffscontaining Chabazite. The regeneration procedure of naturalzeolites was optimized, and the operative pressure resulted tobe the most important parameter. The VSA prototype was fedfor 3 months with a real biogas, produced by agrozootechni-cal byproducts in an anaerobic baffled reactor (ABR). Biogascomposition from ABR was very stable and allowed to deeplyinvestigate the performances of natural zeolites in real bio-gas conditions. Biogas was desulfurized and dehydratedbefore the VSA unit. In our experimental conditions, a bio-methane average purity > 98% was achieved, with methanerecovery > 95%. CO2concentration was always lower than0.3%, while oxygen ranged from 0.2 to 1.5%. A light desul-furization effect was also observed. CH4concentrationwas < 0.1% in the VSA off-gas.
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The environmental influence of biomass burning for civil uses was investigated through the determination of several air toxicants in the town of Leonessa and its surroundings, in the mountain region of central Italy. Attention was focussed on PM10, polycyclic aromatic hydrocarbons (PAHs) and regulated gaseous pollutants (nitrogen dioxide, ozone and benzene). Two in-field campaigns were carried out during the summer 2012 and the winter 2013. Contemporarily, air quality was monitored in Rome and other localities of Lazio region. In the summer, all pollutants, with the exception of ozone, were more abundant in Rome. On the other hand, in the winter, PAH concentration was higher in Leonessa (15.8 vs. 7.0 ng/m3), while PM10 was less concentrated (22 vs. 34 μg/m3). Due to lack of other important sources and to limited impact of vehicle traffic, biomass burning was identified as the major PAH source in Leonessa during the winter. This hypothesis was confirmed by PAH molecular signature of PM10 (i.e. concentration diagnostic ratios and 206 ion mass trace in the chromatograms). A similar phenomenon (i.e. airborne particulate levels similar to those of the capital city but higher PAH loads) was observed in other locations of the province, suggesting that uncontrolled biomass burning contributed to pollution across the Rome metropolitan area.
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Biogas will constitute a significant fraction of future power supply, since it is expected to contribute a large share of the EU renewable energy targets. Biogas, once produced, can be combusted in traditional boilers to provide heat, or to generate electricity. It can be used for the production of chemical compounds, or fed into a pipeline. This review paper will briefly analyze the current most promising emerging biogas technologies in the perspective of their potential uses, environmental benefits, and public acceptance; draw a picture of current conditions on the adoption of a biogas road map in the several EU Member States; analyze incentive and support policy implementation status and gaps; discuss non-technological barriers; and summarize proposed solutions to widen this energy's use.
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Solutions for resource scarcity should be sought from urban waste management and sanitation, which are characterised by central plants and long networks. The socio-technical transition to more sustainable infrastructure is expected to include partial decentralisation based on local conditions. This paper focuses on drivers, barriers and enablers in implementing a decentralised circular system in a new residential area (Tampere, Finland). In the alternative system, biowaste and feces are treated in a local biogas plant, and nutrient and energy output are utilised within the area. This research aims to understand what kind of urban planning enables alternative infrastructure, as well as the characteristics of an innovation capable of making a breakthrough. Seventeen infrastructure planning experts were interviewed, then assembled to re-develop ideas arising from the interviews. Based on these qualitatively analysed data, 11 factors which help the adoption of the alternative system were formulated. The results indicate that sustainability transition can be facilitated through impartial urban planning that allows the early participation of actors and improved communications. Additionally, studying the impact of alternative solutions and city guidance according to environmental policy aims may enhance transition. Innovation success factors include suitable locations, competent partners, mature technology and visible local benefits.
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This study assesses the environmental sustainability of electricity production through anaerobic co-digestion of sewage sludge and organic wastes. The analysis relies on primary data from a biogas plant, supplemented with data from the literature. The climate impact assessment includes emissions of near-term climate forcers (NTCFs) like ozone precursors and aerosols, which are frequently overlooked in Life Cycle Assessment (LCA), and the application of a suite of different emission metrics, based on either the Global Warming Potential (GWP) or the Global Temperature change Potential (GTP) with a time horizon (TH) of 20 or 100 years. The environmental performances of the biogas system are benchmarked against a conventional fossil fuel system. We also investigate the sensitivity of the system to critical parameters and provide five different scenarios in a sensitivity analysis. Hotspots are the management of the digestate (mainly due to the open storage) and methane (CH4) losses during the anaerobic co-digestion. Results are sensitive to the type of climate metric used. The impacts range from 52 up to 116 g CO2-eq./MJ electricity when using GTP100 and GWP20, respectively. This difference is mostly due to the varying contribution from CH4 emissions. The influence of NTCFs is about 6% for GWP100 (worst case), and grows up to 31% for GWP20 (best case). The biogas system has a lower performance than the fossil reference system for the acidification and particulate matter formation potentials. We argue for an active consideration of NTCFs in LCA and a critical reflection over the climate metrics to be used, as these aspects can significantly affect the final outcomes.
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ABSTRACT Municipal wastewater treatment plants with multi-stage activated sludge technology generate significant amounts of greenhouse gases (GHG) in the form of nitrous oxide N2O, methane (CH4) and carbon dioxide (CO2). Although the exact magnitudes of the specific emissions are difficult to estimate, they strongly affect the energy balance of a plant. This article presents a simulation study carried out on a model municipal wastewater treatment plant. The research aimed to analyse the potential for the reduction in GHG emissions through the operational optimization of some core operational parameters and its effects on the plant’s energy balance. The results showed that the combined effect of optimization of the dissolved oxygen concentration in the aerobic zone, the solids retention time and the ratio of chemical oxygen demand to total nitrogen (COD:TN) in the influent may lead to a reduction in the N2O emissions by 1,103 kgCO2eq/d and also a slight reduction in the CO2 and CH4 emissions, by 256 and 87 kgCO2eq/d, respectively. This was coupled with an improvement in the plant’s net energy balance by 34 kW through the reduction in energy consumption for aeration of the activated sludge by 18 kW and the increased energy production from biogas by 16 kW. An author copy of the accepted manuscript is available at: http://www.tandfonline.com/eprint/W3faq8ShqGSQYD393GX8/full
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Emission factors of formaldehyde and VOCs were determined for two waste treatment plants (WTP) located in the metropolitan area of Barcelona city. Formaldehyde emission factors were determined from the biogas engines exhausts and the process chimneys (after the biofilter process), and VOC emission factors were determined in the process chimneys. Formaldehyde and VOC were dynamically sampled using DNPH-coated adsorbent tubes with ozone scrubber and multi-sorbent bed tubes (Carbotrap, Carbopack X and Carboxen 569), respectively, using portable pump equipment. Formaldehyde emission factors from biogas engines were found between 0.001–0.04 g s− 1. Additionally, formaldehyde and VOC emission factors from process chimneys were found to be between 0.0002–0.003 g s− 1 and 0.9 ± 0.3 g s− 1, respectively. Employing real emission factors, the expected concentrations derived from the WTPs in their nearby urban areas were calculated using The Atmospheric Pollution Model (TAPM, CSIRO), and impact maps were generated. On the other hand, ambient air formaldehyde and VOC concentrations were determined in selected locations close to the evaluated waste treatment facilities using both active and passive samplers, and were between 2.5 ± 0.4–5.9 ± 1.0 μg m-3 and 91 ± 48–242 ± 121 μg m-3, respectively. The concentrations of formaldehyde and VOC derived exclusively from the waste treatment plants were around 2% and 0.3 ± 0.9% of the total formaldehyde and VOC concentrations found in ambient air, respectively.
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Characterisation of biogases is normally dedicated to the online monitoring of the major components methane and carbon dioxide and, to a lesser extent, to the determination of ammonia and hydrogen sulphide. For the case of Volatile Organic Compounds (VOCs), much less attention is usually paid, since such compounds are normally removed during gas conditioning and with exception of sulphur compounds and siloxanes represent a rather low risk to conventional downstream devices but could be a hindrance for fuel cells. However, there is very little information in the literature about the type of substances found in biogases generated from biowaste or co-fermentation plants and their concentration fluctuations. The main aim of this study was to provide information about the time dependencies of the VOCs in three biogas plants spread out through Germany from autumn until summer, which have different process control, in order to assess their potential as biofuels. Additionally, this study was an attempt to establish a correlation between the nature of the substrates used in the biogas plants and the composition of the VOCs present in the gas phase. Significant time-dependent variations in concentration were observed for most VOCs but only small changes in composition were observed. In general, terpenes and ketones appeared as the predominant VOCs in biogas. Although for substances such as esters, sulphur-organic compounds and siloxanes the average concentrations observed were rather low, they exhibited significant concentration peaks. The second biogas plant which operates with dry fermentation was found to contain the highest levels of VOCs. The amount of total volatile organic compounds (TVOCs) for the first, second and third biogas plants ranged from 35 to 259 mg Nm(-3), 291-1731 mg Nm(-3) and 84-528 mg Nm(-3), respectively.