Environmental impact of biogas: A short review of current knowledge
, Francesco Petracchini
, Marco Segreto
, Laura Tomassetti
, Nour Naja
, and Angelo Cecinato
National Research Council of Italy, Institute of Atmospheric Pollution Research, Monterotondo, Italy;
Boston Northeastern University, Chemical
Engineering Department, Boston, Massachusetts, USA
Received 26 January 2018
Accepted 23 March 2018
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
according to several case studies conducted over the world. Direct emissions of gaseous pollutants are
then discussed, with a focus on nitrogen oxides (NO
); 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 NH
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
Air quality; anaerobic
digestion; biogas; digestate;
renewable energy; secondary
aerosol; waste management
The environmental beneﬁts of biogas technology are often
highlighted, as a valid and sustainable alternative to fossil
Together with the reduction of greenhouse gas (GHG)
emissions, biogas can enhance energy security, thanks to its
high energetic potential.
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-
Furthermore, while ashes from combustion ﬁnd
scarce agronomic applications,
the by-product of anaerobic
digestion, i.e. digestate, looks as a reliable material for agricul-
Another important advantage of biogas technol-
ogy is its easy scalability, allowing exploiting the energetic
potential of decentralized biomass sources.
can be upgraded to biomethane, suitably used as a vehicle fuel,
or injected into national natural gas grids,
The energy potential of biogas is reported in Figure 1, based
on data from the World Bioenergy Association.
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.
on which these opposition phenomena are observed depends
on different factors, including the inclusion strategies and the
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 scientiﬁc 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 ﬁnal 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,
the main measures to improve the global warming reduction
potential of biogas plants are: to use a ﬂare avoiding methane
discharge, to cover tanks, to enhance the efﬁciency 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
for the speciﬁc 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 email@example.com 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 ﬁgures 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, 899–906
The impact induced by biogas plants on global warming
needs to be studied case by case. Bachmaier and co-workers
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
, and the GHG saving was 2.31 –3.16 kWh
. 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,
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,
the GHG release reduction was esti-
mated equal to 177.0, 87.7 and 125.6 Mg of CO
dairy cow, sow and pig farms, respectively. Optimizing all pro-
cess parameters looks important with regard to ﬁnal environ-
mental impact: for instance, a speciﬁc case study on wastewater
treatment showed that the process optimization could result
into the emission abatement equal to 1,103 kg CO
O, 256 kg eq/d for CO
and 87 kg CO
eq/d for CH
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 efﬁcient
methane oxidation and conversion to CO
, with a rate of
83.6 kg per GJ (based on a biogas with 65% CH
). 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-
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
have investigated the CO
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
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 signiﬁcant contributions to total emis-
sion reduction in the case of speciﬁc 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 released by biogas processes is not considered relevant
for health issues: though exposure to hydrocarbon mixtures can
have some adverse effects on humans,
no evidence exists of
relevant interactions between methane and biologic systems.
However, methane is a greenhouse gas whose global warming
power is estimated to be 28–36 times higher than CO
years: as such, it is the second major component among anthro-
pogenic greenhouse chemicals.
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
methane emissions were also discussed; in
all investigated cases, the emission rates were below 5 g kg
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 ﬁeld without any pre-treatment.
, nitrous oxide (N
O) is another impor-
tant GHG: Due to its high greenhouse effect potential, N
emissions from biogas production processes can result into a
signiﬁcant contribution to global warming budget.
relative impact of nitrous oxide mostly depends on the chosen
climate metrics: indeed, N
O impact can even exceed those of
, when the considered metric is Global Tempera-
ture change Potential with a time horizon of 100 years (namely
Total GHG emission for energy production from biogas are
generally calculated in a range between 0.10 and 0.40 kg
, which is for instance 22–75% less than GHG
emissions caused by the present energy mix in Germany.
The wide uncertainty about the estimates of global warming
mitigation potential depends on N
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 beneﬁts, 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
for ﬂaring and
CO emissions related to energy produc-
tion are estimated in a range between 80 and 265 mg CO MJ
depending on the plant efﬁciency.
Sulphur dioxide (SO
) emissions from biogas plants manly
depend on the desulphurization degree of the introduced bio-
gas. The SO
emission rate of a CHP biogas plant is estimated
to lie in the range 19.2–25 mg MJ
The UK National Soci-
ety for Clean Air (NSCA) estimates an emission factor of 80
and 100 g
for ﬂaring and CHP, respectively.
The relatively high SO
concentrations in the proximity of bio-
gas plants can depend on different reasons, e.g.: direct emission
from biogas combustion, H
S oxidation from diffusive emis-
sions, and diesel truck exhausts.
Emissions of NO
are one of the most critical point with
regard to environmental impact of biogas plants.
to Kristensen and co-workers,
emission level of bio-
gas is, in general, higher than for natural gas engines: the aver-
aged aggregated emission factor is 540 g NO
, 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
can be assumed for
ﬂaring and CHP, respectively.
The importance of controlling
this pollutant is demonstrated by several case studies. For
instance, Battini and co-workers
in the above mentioned
case study of an intensive dairy farm situated in the Po valley
(Italy) reported a low enhancement in acidiﬁcation (5.5–6.1%),
particulate matter emissions (0.7–1.4%) and eutrophication
(C0.8%), while on the other hand a signiﬁcant enhancement in
photochemical ozone formation potential (41.6–42.3%) was
calculated. In another case study, Carreras-Sospedra and co-
estimated a potential enhancement of up to 10% of
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
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
, and in some cases
up to 1700 mg/Nm
Generally, only non-
methane volatile organic compounds (NMVOC) are consid-
ered in these studies. If combustion is assumed to reduce VOCs
concentration of 99%,
VOCs emission from biogas combus-
tion are in general lower, compared to liquid and solid biofuels.
However, a speciﬁc 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
, contributing for 0.3–0.9% 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
, resulting in a »2% contribution to the total.
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
Noticeably, fuel-cycle emissions can be strongly inﬂuenced
by the raw materials. For instance, CO
, CO, NO
bons and particles may differ by a factor of 3–4 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
due to the high variability of H
S and organic sulphur com-
pounds in the produced biogas.
Impact of feedstock and digestate storage
In the biogas combustion management, feedstock and digestate
storage and treatments can be the most important processes to
achieve the global warming beneﬁts of biogas production pro-
cesses. Indeed, the impact of a biogas plant on GHG emission
is heavily inﬂuenced by feedstock storage: most of N
O can be
abated when a closed storage is used for manure and co-diges-
Emissions from uncovered biomass storage have also been
identiﬁed as the main ammonia source along the whole biogas
and closed storage is strongly advised.
In a speciﬁc French case study of anaerobic digestion and
composting plant for municipal solid waste, Beylot and co-
have identiﬁed four conditions for process
operation, which highly inﬂuence the impact of the whole
plant; they are: (i) the features of degradation of the ferment-
able fraction; (ii) the collection efﬁciency of gas streams
released by biological operations; (iii) the abatement effective-
ness of collected pollutants; and (iv) NO
emission rate from
Table 1. Emission factors of biogas plants operating direct biogas combustion.
Emission factor (g
Carbon monoxide (CO) 310 Nielsen et al.,
Sulphur dioxide (SO
) 25 Nielsen et al.,
Nitrogen oxides (NO
) 202 Nielsen et al.,
Non-methane volatile organic
10 Nielsen et al.,
O) 8.7 Nielsen et al.,
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,
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.
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.
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.
mentioned above, avoiding leakages and using closed tanks are
among the most important ways to reduce the global warming
impact of biogas plants.
Impact of digestate ﬁnal 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 brieﬂy discussed.
A recent study on this topic
concluded that direct effects
of anaerobic digestion on long-term sustainability in terms of
soil fertility and environmental impact at the ﬁeld 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
signiﬁcantly 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.
In a speciﬁc 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
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”.
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-
investigated the emission of methane, nitrous oxide and
ammonia from untreated manure and digestate applied on sev-
eral soils: while methane emissions did not signiﬁcantly change,
O emissions were observed in the correspondence of
high carbon loadings. A signiﬁcative impact of soil moisture-
soil mineral-N interactions on N
O emissions was also
observed by Senbayram and co-workers.
O and CH
, digestate can give rise to signiﬁ-
cant emission rates into the atmosphere: however, these emis-
sions are generally lower than untreated biomass.
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
compared to fresh slurry application.
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
(Poeschl et al., 2012; Boulamanti
et al., 2013). As for methane emission, an exception is known
in the speciﬁc case of rice cultivation: indeed, adding digestate
to paddy results into the methane emission rate enhancement
from 16.9 to 29.9 g m
whilst no signiﬁcant effects are
observed for N
Based on the above-cited literature, N
O and CH
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.
It was also
reported that up to 30% of nitrogen can be lost by ammonia
volatilization, due to the enhancement of soil pH.
cally, Matsunaka and co-workers
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
(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
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.
With regards to pesticides, heavy metals and harmful micro-
organisms, the risk of food chain contamination is generally
but the soil burden of persistent organic pol-
lutants (POPs) caused by the use of digestate as biofertilizer still
needs to be fully assessed.
On the other hand, anaerobic
digestion can have relevant effects on phytotoxicity of speciﬁc
biomass: for instance, the phyto-toxic character of olive mill
efﬂuent is reduced after anaerobic digestion,
and the degra-
dation of aﬂatoxin B1 from corn grain can be reached.
Finally, an odour reduction up to 82–88% can be obtained.
In conclusion, the main critical issue in ﬁnal 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 difﬁcult 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
Impact on particulate matter
With regards to particulate matter (PM), biogas combustion is
not a signiﬁcant emission source when compared to other fuels:
emission factors of 0.238 and 0.232 g/Nm
have been esti-
mated for ﬂaring and CHP, respectively.
ary PM formation can occur, due to NO
emissions from CHP
volatilization from storage and digestate ﬁnal use.
Indeed, during secondary PM formation, the prominent roles
are ascertained. As reported by
Boulamanti and co-worker,
emissions are in general
the principal source of secondary PM from biogas. As discussed
above, closed storage can signiﬁcantly 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 efﬁcient approach to increase the
market share of biogas, resulting in a further reduction of fossil
fuels. The equivalent CO
saving raises considerably if methane
slip is limited to 0.05%,
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
and particulate matter. As a consequence,
biogas upgrading for vehicle fuelling purposes produces opti-
mum beneﬁts with respect to photochemical oxidant forma-
tion, marine eutrophication and ecotoxicity; on the other hand,
scarce beneﬁts are observed in terms of climate change com-
pared to biogas combustion in CHP.
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.
Starr and co-workers
reported that the most CO
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
; 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
Biomethane injection in the national grid
may also reduce residential solid fuels consumption in some
speciﬁc regions, with relevant beneﬁts on indoor air quality
and human health.
Global emission potential
The potential emission associated to biogas plants is reported in
Figure 2 (NO
and CO) and in Figure 3 (for formaldehyde,
NMVOC and SO
). Data are obtained combining emission fac-
tors reported in Table 1
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.
Biogas can signiﬁcantly contribute to abate greenhouse gas
emissions. However, attention must be payed towards unde-
sired emissions of methane and nitrous oxide (N
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, efﬁcient biomass and digestate
Figure 2. Emission potential of biogas plants for NO
Figure 3. Emission potential of biogas plants for formaldehyde, NMVOC and SO
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
) 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.
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.
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