The present study is dedicated to a comparative analysis of operating modes of biogas plants (BGP), in which the biogas production process from anaerobic fermentation by bacteria obtained from plant and animal origin biomass (BM) and organic waste (OW). The stages of the anaerobic fermentation process and the mechanisms of their realization are described. An optimal mode has been shown for a specific case. Скачать в формате PDF
American Scientifi c Journal № ( 37 ) / 2020 41


UDK 662.76.05


Salamov Oktay Mustafa
Candidate of Physical and Mathematical Sciences, assistant professor
Institute of Radiation Problems of Azerbaijan National Academy of Sciences
Salmanova Firuza Aziz
Dr. of Philosophy in Technical Sciences, assistant professor
Institute of Radiation Probl ems of Azerbaijan National Academy of Sciences
Aliyev Farhad Fagan
Dr . of Philosophy in Technical Sciences
International Ecoenergy Academy city Baku


Саламов Октай Мустафа
Кандидат физ ико -мaтематических наук, доцент
Институт Радиационных Проблем Национальной Академии Наук Азербайджана
Салманова Фируза Азиз
Доктор философии по технике, доцент
Институт Радиационных Проблем Национальной Академии Наук Азербайджана
Алиев Фархад Фаган
Доктор философии по техническим наукам
Международная Академия Экоэнер гетики г. Баку
DOI: 10.31618/asj.2707 -9864.2020.1.37.8
Abstract . The present study is dedicated to a comparative analysis of operating modes of biogas plants
(BGP), in which the biogas production process from anaerobic fermentation b y bacteria obtained from plant and
animal origin biomass (BM) and organic waste (OW). Th e stages of the anaerobic fermentation process and the
mechanisms of their realization are described. An optimal mode has been shown for a specific case.
Аннотация . В настоящей работе проводится сравнительный анализ режимов работы биогазовых
установок (БГ У), предназначенных для реализации процессов получения биогаза из биомассы (БМ)
растительного и животного происхождения и органических отходов (ОО), путем анаэробного
сбраживания (ферментации) их, посредством бактерии. Разъясняются этапы процесса анаэробно го
сбраживания и механизмы реализации этих этапов. Указывается, что какой из режимов является
оптимальным, для конкретного случая.
Keywords: biomass, organic waste, sew age waste, biogas, biomethane, carbon dioxide, anaerobic
fermentation, psychophilic, mes ophilic, thermophilic, microorganisms, hydrolysis, acidification, methanation,
temperature regime .
Ключевые слова: биомасса, органические отходы, отходы сточных вод, би огаз, биометан, диоксид
углерода, анаэробное сбраживание, ферментация, психофильные, мез офильные и термофильные режимы,
микроорганизмы, гидролиз, подкисление, метанирование, температурный режим.

It’s known that the “20 -20-20” project has bee n
developed for environmental protection in the
European Union countries. The project aim is to reduce
toxic gases released to the atmosphere by 20% and
increase the share of renewable energy sources in total
energy sources up to 20%. In this regard, creat ing both
power plants and fuels used for various purposes have
great importance. Numerous works are being done in
this field in the world. In the 80s of the last century, in
Brazil began biomethanol and bioethe nol production by
biothermal decomposition (fe rmentation) of the special
productive specie of sugar beet and a few years later,
ethanol began to be used as a 20% additive to
automotive fuel. At present, the transport run entirely
on bioethanol, which is al so used for heating and
electricity generation . Moreover, Brazil exports
bioethanol to a number of Far Eastern countries.
However, later studies have shown that biogas
production by anaerobic fermentation is more efficient
than biochemical decomposition of methanol or ethanol
from various types of BM, including plant origin BM
and OW. So that, biogas production has the following
advantages: 1) unlike ethanol, biogas can be
transported via trunk pipelines, so it is both easy and
inexpensive to deliver biogas to enterprises and
residences in hard -to-reac h areas, and there is no need
for transportation costs; 2) Biogas can be produced
from numerous types of BM, including animal origin
OW, effluents from factories and plants, effluents from
human activities, etc . raw materials, which are
considered renewabl e raw materials; 3) it is possible to

42 American Scientific Journal № ( 37 ) / 2020
more ecologically efficient dispose of environmentally
harmful BM and OW, as well as industrial, construction
and household (municipal) origin solid combustible
waste durin g biogas production; 4) releasing of gases
whi ch generate heat effect such as CO 2, CH 4, N 2O etc
into the environment from the agro -industrial sector are
significantly prevent; 5) the quality of mineral residues
obtained as a result of fermentation in BGP i s better
than unprocessed BM and OW, in partic ular manure,
the intensity and alkalinity of unpleasant odors are
reduced when applied to the soil as fertilizer and
nutrients uptake by plants is significantly accelerated;
6) the possibility of the formations of pathogenic
microbes, pests and weeds durin g the fermentation
process goes down to a minimum; 7) in the use of toxic
drugs and pesticides are significantly saving, mineral
residues obtained at the end of the fermentation process
in the BGP are considere d to be better quality and more
environmentall y friendly substitute of conventional
organic fertilizers; 8) the amount of energy produced
from 1 Ha of arable land is 2 -4 times more than the
amount of energy from bioethanol and biodiesel
produced from the s ame area of arable land ühen using
energy -effi cient hybrid corn as a raw material; 9) 6,6 -
35 tons of wet manure can be obtained from one cattle
during per year, which give an opportunity to produce
257 -1785 m 3 of biogas. This indicators are equivalent
to 1 93-1339 m 3 of natural gas, 157 -1089 kg of gaso line,
185 -1285 kg of fuel oil and 380 -2642 kg of firewood in
terms of heat transfer capacity (HTC). This means 10
kW·h of energy production per day for a head of cow.
Therefore, a well -designed, energy -efficien t dwelling
with biogas produced from the manur e of 2 -3 cows can
be supplied with excess energy (heat and electricity). In
this case, moreover, a high -quality organic fertilizer is
obtained, which can be used both for the use of the
peasant in their own fie lds and for the purpose of
earning additional income by selling. In this case,
moreover, a high -quality organic fertilizer is obtained,
which can be used both for the use of the peasant in
their sown areas and for the purpose of earning
additional income b y selling [1, 2].
The main advantges of the u sing of BGP are: a
large amount of conventional fuels such as oil, gas,
coal, fuel oil, electricity, etc. and energy carriers are
being saved, the ecological condition of the areas near
livestock farms and poul try factories is improving, the
discharge of w astewater into the environment, as well
as sewage and other harmful wastes, is prevented due
to the use of BGP.
All these mentioned above confirms once again
the importance of obtaining biogas from various type s
of BM and OW. The existing methods to the pr oduction
of biogas by BM and OW fermentation, the required
temperature regimes in BGP and other requirements are
discussed below.
BC and OW fermentation methods and
comparative analysis of BGP operating modes
Biogas is a waste product of the metabolic
proc esses of specific bacteria (microorganisms) and is
obtained by fermentation of any type of substrate of
biological origin (BM or animal origin OW crushed and
mixed with hot water) under anaerobic conditions
(oxygen -free environment). In this regard, this p rocess
is called methanation fermentation and obtained BG is
contained from 50 -87% of CH 4, 13 -50% of CO 2 and a
small part of H 2 and H 2S. The biomethane which an
analogue of natural gas is obtained after purifi cation of
BG from CO 2. Therefore, the main gas component that
affects the energy performance of the obtained biogas
is methane.
Biogas obtaining process (biomethanogenesis) by
BM and OW fermentation methods usually consists of
three stages (when using cell ulose -containing OW) as
well as four stages in some cases:
1) Hydrolysis stage. In this stage, the stable
complex chemical compositions ( proteins, fats,
hydrocarbons, etc. ) are decomposed into simpler
substances ( amino acids, fatty acids, glucose, etc. ) as a
result of the hydrolysis with acetogenic aerobi c
bacterias (e.g. Syntrohobacter wolinii );
2) Acidification (Acidogenesis) (conversion of
hydrolyzed substrate to acids) stage . In this stage, the
mixed constituents obtained during the initial stage are
decomposed into other organic substances ( acetic and
propionic acids, alcohols and aldehydes ), as well as a
number of inorganic substances ( H2, CO 2, N 2, H 2S) by
acid -forming bacterias ( acidogenic type bacteria ). This
stage lasts until the growth of bacteri a is slowed down
under the influence of the formed acid s;
3) Acetogenesis stage . The acids formed in the
previous stage are converted into acetic acid by
acetogenic bacteria in this stage;
4) Methanation (Methanogenesis) stage . The
transformation process of acet ic acid, formed in stage 3
to methane, carbon dioxide a nd water occurs by
methanogenic anaerobic bacteria . Simultaneously,
carbon dioxide and hydrogen formed in the previous
(second) stage are transformed into methane and water.
Numerous microorganisms ty pes (up to
several hundred species of bacteria) are par ticipating to
occurs all these complex chemical transformation
processes. Most numerous of them are hydrolyzing,
fermenting, syntrophic and methanizing bacteria. The
quantitative and qualitative compos ition of bacteria
strongly depends on the content of th e fermented BM
or OW. On the other hand, the provided condition in
the bioreactor is also strongly influence to these
indicators. All above mentioned transformations
processes (reactions) occurs in sim ultaneously and in
this case, more stringent requiremen ts are imposed on
the ability of methanogenic bacteria to function.
However, according to the mechanism
provided by Barker, it is claimed that the methane
fermentation process of BM and OW occurs in t he two -
phase scheme. In this case, the above mentioned 3
stages together form the first phase and methanation
stage forms the second phase separately. According to
this scheme, in the first phase, the complex organic
substances ( proteins, hydrocarbons, fat s) are
decomposed to a number of acids ( vinegar, antaci d,
milk (lactate )), fatty ( butyric and propionic acids ),
alcohols ( ethyl, propyl, butyl etc. ), gases ( carbon
dioxide, hydrogen, hydrogen sulfide, ammonia ), amino
acids and glycerin with the presence of water through
acid bacteria. In the second phase, the intermediate

American Scientifi c Journal № ( 37 ) / 2020 43

products formed as a result of the vital activity of acid
bacteria in the first phase are decomposed by
methanogenic bacteria and as a result, methane, carbon
dioxide, nitrogen and hydroge n contain biogas is
Thus, the bacteria that are realizing each
subsequent stage during the fermentation of BM and
OW are fed by the vital activity products of the
previous bacteria. All these bacteria exist in the cattle
organism. Not only methanat ion bacteria, but all three
types of bacteria work toge ther in biogas production. It
should be note that, the cattle and other domesticated
animals including poultry manure, alcohol production
pulp from cereals and sugar beet, organic waste
generated durin g beer production, beet pulp, human
activity waste (fec es, etc), organic waste production
from fisheries and butcheries (blood, fat, intestine,
stomach etc), grass plants, organic household and
industrial waste, dairy plant waste (salted and raw milk
whey) , biodiesel fuel production wastes (technical
glycerin produced during the production of biodiesel
fuel from rapeseed), fruits, berries, juice production
pulp from vegetables and grapes, aquatic plants, starch
production wastes, unsweetened kernels and pur ee,
syrup, potato production waste, as well as grapes a nd
cotton swabs, sunflower, tobacco and corn stalks, corn
kernels and husks, crown part of sunflower, white and
red elderberry and crown parts of them, etc and OWs
refers to raw materials. Currently, a number of
artificially grown energy raw materials (cor n silage,
mollusk -eating insects, various types of aquatic plants
etc) also widely used in order to biogas production of
which it is possible to produce 300 m 3 biogas from each
ton. The maximum amount of biogas is 1300 m 3/t
produce from fat.
It is importan t that methanizing bacteria are active
under the following conditions: 1) adherence to the
temperature regime (average temperature, optimal
temperature and all owable temperature change
interval); 2) selection of fermentation mode; 3)
nutrients supply; 4) s election of fermentation time; 5)
ensuring uninterrupted operation of the bioreactor in
one session; 6) determination of daily loading of raw
materials; 7) det ermination of processing time of raw
materials; 8) alkaline -acid balance (pH indicator)
control; 9) carbon:nitrogen:phosphorus ratio control;
10) ensuring optimal moisture content of raw material
(substrate); 11) grinding solid and dry parts of raw
materia ls to the nominal level; 12) periodic mixing of
the substrate; 13) provision of the fermentation process
by inhibitors [4].
First of all, it should be noted that since the
anaerobic fermentation process occurs in the oxygen -
free environment, the hermeticit y of the bioreactor
must be ensured and always monitored. Most species of
methane bacteria die at temperatures above 700 °C with
the exception of several strains. Therefore, three
temperature regimes are used to optimize the
processing of BM and OW resulti ng in the production
of biogas and mineral fertilizers:
- psychrophilic mode - temperature varies in the
range of 20 -25 °C (optimal temperature is 25 °C )
- mesophilic regime - the temperature varies in the
range of 25 -40 °C (optimal temperature is 37 °C );
- thermophilic regime - the temperature is above
40 °C (optimal temperature is 55 °C ).
The rate o f methane production by bacteria
increase with increasing of temperature. However, the
amount of the ammonia also increases with increasing
temperature which c ause to slows down the
fermentation process. When using bioreactors
operating in psychrophilic mo de that are not heated by
an external source, sufficient biogas production is
possible only when the average annual temperature is
above 20 °C. The only advant age of this mode is that
the process occurs in natural conditions, i.e. an
additional heating sys tem is not used. However, this
regime does not allow to obtain mineral residues used
for fertilization. So that, the insect eggs, weed seeds and
other harmful substances for agriculture remain as in
the raw material, which significantly reduces the
quality of the mineral residue.
The selecting of the optimal temperature mode
strongly depends on the composition of the initial
substrate. So that, the optimal tempe rature regime for
the mesophilic regime is 34 -37 °C, and for the
thermophilic regime is 52 -54 °C when using mixed raw
materials (a mixture of cattle, pork and poultry
manure). Biogas production process occurs more
intense in the psychrophilic mode at 23 °C temperature.
There are special requirements for the temperature
change interval inside the bior eactor in the BGP. Thus,
for the psychophilic regime, the temperature is allowed
to change within ± 20 °C per hour, and for the
mesophilic and thermophilic reg imes, it is allowed to
change to ± 10 °C and ± 0.50 °C per hour, respectively.

44 American Scientific Journal № ( 37 ) / 2020

The main adv antages of the thermophilic regime
in comparison with the mesophilic regime are that,
firstly, the rate of decomposition of raw materials and
consequently, the production of biogas increases, and
secondly, the complete destruction of pathogenic
bacteria, e ggs of insects and weed seeds. The
dis advantage is the need to use an additional energy
source to maintain the required temperature mode and
in most cases, a large part of the biogas (up to 50%) is
used for this purpose. This, in general, leads to an
adequ ate reduction in the efficiency of the BGP.
Another disadvantage of the thermophilic regime
is the lower quality of the mineral residue (mineral
fertilizers) obtained during this method under the more
stringent conditions against temperature changes.
Altho ugh high amino acids are retained in t he
mineral residue during the mesophilic regime, it is not
possible to completely neutralize the toxins in the raw
material. Another disadvantage of the thermophilic
process is that the amount of methane in the obtaine d
biogas can be reduced by the percent age, which in turn
causes to reduces the heat capacity of the single volume
of biogas (1 m 3) compared to other methods. Although
there is not much difference between thermophilic and
mesophilic methods in the amount of biogas produced
per day, in the follo wing days there is a significant
increase in the amount of biogas produced per day in
BGP, operating with thermophilic mode. This
difference is obviously seen from the graphical
dependence presented in Figure 1. In man y countries
around the world, mesophil ic regimes have been more
preferred as a result of research and experiments on the
BGPs operating in different modes and currently, most
of the high -power and high -productivity BGPs used for
biogas production operate a t mesophilic temperature
The n utrient -rich nature of the raw materials used
in the production of biogas is also an important
condition for increasing the activity of methanizing
bacteria. It is important to have microelements such as
nitrogen, sulf ur, phosphorus, potassium, calcium and
magnesium, as well as small amounts of iron,
manganese, molybdenum, zinc, cobalt, selenium,
tungsten, nickel, etc. in the initial raw material in order
to be active of these bacteria. The cattle manure is very
rich wi th each of these chemical elements.
Th e optimal fermentation time of BM and OW
depends on the loading dose of the bioreactor and the
selected temperature regime, i.e. the fermentation
method. If the fermentation time is excessively reduced
due to the appli cation of a high temperature regime, t he
useful methanizing bacteria can be removed along with
the removal of the residues from the bioreactor. The
total surface area of raw material particles is one of the
decisive factors in biogas production. Thus, the smaller
the particle size of the raw m aterial (substrate), the
higher the decomposition rate of BC. Moreover, the
smaller the particles of the substrate, the easier it is to
mix, which allows the use of lower power electric
motors to automate the mixing pr ocess. Consequently,
the additional en ergy consumption is reduced, the total
coefficient of performance (c.o.p.) of BGP and the
efficiency of its work increases significantly.

0 2 4 6 8 10 12 14
Figure 1. Temperature dependence of biogas yield producing
from BGP for mesophilic and thermophilic modes:
curves 1 a nd 2, for thermophilic and mesophilic modes,
Fermentation time , day
Produced biogas amount ,

American Scientifi c Journal № ( 37 ) / 2020 45

Other physical parameters that affect the
fermentation process in BGP are: pressure in the BGP,
hydraulic regime, humidity of the environment, total
surface area of raw material particles (grinding degree),
transfer and mixing periods to the bioreactor of the
substrate, stimulating additives. Figure 2 shows
graphical dependencie s to the determination of the
amount water should be added to a given mass (100 kg)
of the substrate at values starting at 60% of the initial
moisture content to reach 85% and 92%, r espectively.
The curves 1 and 2 in Figure 2 are showing how much
water mus t be added to the substrate at different values,
starting from 60% of the initial moisture, to reach 85%
and 92% of moisture content of the substrate,
respectively. It is clearly see n that both curves change
linearly. Since in the first case the value of t he
maximum moisture content is 85%, and in the second
case it is 92%, the graphs intersect with the abscissa at
these values of moisture. The selected maximum
moisture values in Figu re 2 are justified by the fact that
the moisture content of the substrate loaded into the
bioreactor for the winter season should not be less than
85% and for the summer season not less than 92%. The
amount of hot water that must be added to the manure
to obtain the required moisture is determined using the
following formula [4] :
� = �⋅�2−�1
100 −�2
where M - is the amount of hot water to be added,
l; P - amount of manure loaded into the bioreactor, kg;
N1 – initial moisture of manure, %; N2 - required
moi sture of the substrate,%.
Experiments show that 36 kg of manu re with 65%
moisture can be obtained from a head of cattle during
per day. However, the daily amount of a mixture of
manure, urine and straw (excrement), which is more
productive in terms of the amount of produced biogas
is much higher, 55 kg with 85% mois ture, which does
not require additional water for the winter season and
only 7% extra water can be added for the summer
A two -stage technological process should be used
to ferment a number of mono -composite (pure) raw
materials. For instance, poult ry manure, alcohol
production pulp, etc. raw materials cannot be fermented
in conventional single -stage bioreactors. An additional
hydrolysis reactor must be used for this purpose. These
typ es of reactors allow to control the acidity level,
which eliminate s the possibility of the destruction of
irritating bacteria due to excess acidity or alkalinity. It
is also possible to process above mentioned and other
types of raw materials by the single -stage technology,
but in this case they include other types of ra w
materials, such as manure, straw, silage, etc. should be
added [5].
The use of rapidly decomposing raw materials,
such as sugar beet, food waste, etc leads to fast
oxidation of the bioreac tor, and therefore these raw
materials are considered unsuitable f or pure
fermentation. Therefore, these types of raw materials
are mixed with other types of raw materials for safe
fermentation. For this purpose, silage, various types of
grasses, corn, bit ter beans, grain wastes, etc are used in
most BGPs. For the Azerba ijan, the tobacco sprouts,
fern sprouts and leaves, white and red elderberry
sprouts, leaves and crown parts, achillea (yarrow), etc
can be successfully used as raw materials, which are
very rich in most mountainous and foothill regions of
the country. The se types of BM are usually used as an
additive to wet or dry manure that is not used cleanly.
The resulting gas yield shows that it is more efficient to
use these raw materials by mixing the m before
fermentation. It has been proved that While the biogas
yield is 0.380 m 3/kg per kg of raw material when using
cattle manure alone, the use of cattle manure mixed
with poultry manure increases this value up to 0.528
55 65 75 85 95
Moistur e, %
Amount of water to be added, l
Figure 2. The amo unt of water to be added depending
on the initial moisture of the substrate:
1- to reach 85% moisture of the substrate;
2- to reach 92% moisture of the substrate

46 American Scientific Journal № ( 37 ) / 2020
Following biological element s are included biogas
production process: 1) composition of fermen ted raw
materials (amount of proteins, fats, carbohydrates and
lignin); 2) composition of microflora (types and
quantity of microorganisms realizing decomposition
stages); 3) existing condit ions for the operating of
microorganisms (presence or absence of h armful
The presence of any inhibitors (antibiotics,
organic solvents, etc.) in the initial raw material has a
very negative impact on the vital activity of
microorganisms. A numb er of inorganic compounds
also have a negative effect on the activ ity of
microorganisms. For example, it is absolutely
unacceptable to use synthetic detergents rich hot water
removed from the washing machine for this purpose
when mixing grinded initial raw materials with hot
water [3 -5]. The decomposition degree of the B M or
OW depends directly on the composition of the raw
material and it is represented in the amount of produced
biogas. This indicator usually varies between 30 -70%.
The decomposition rate c an be up to 80% in the BGP
working with renewable raw materials. H owever,
currently it is possible to increase biogas yield from
60% to 95% in the simplest BGPs with using various
enzymes and enhancers (e.g. ultrasonic and liquid
shredders) to degrade the initial raw material.
The full completion time of the fermentation
process after loading the BGP reactor also depends on
the operating mode of the device. Thus, this period is
30 -40 days and more for psychrophilic regime, as well
as, 10 -20 days and 5 -10 da ys for mesophilic and
thermophilic regimes, respectively. However, if the
fermentation time is excessively short, then as
mentioned above, most of the bacteria are washed out
of the reactor without being able to multiply when the
fermented BM is discharged from the reactor.
Consequently, after a while, the fermentation p rocess
can stop completely. It is not efficient the gasification
process proceed excessively slow as in psychrophilic
mode. Thus, the amount of biogas prodced in this case
may not meet consu mer demand.
The daily loading dose of the bioreactor depends
on it s cycle time and increases in proportion to the
increase in temperature. The daily loading dose of the
reactor is assumed to be 1/10 of its turnover for
continuously operating BGP.
The selec ting of fermentation time also depends
on the type of tprocessed r aw material. So that, if the
BGP operates in the mesophilic mode, then the
fermentation time is 10 -15 days for separate wet
manure, and 40 -80 days for manure mixed with plant
origin BM speci es.
The alkaline -acid balance in the substrate should
be maintaine d so that the pH varies between 6,5 -8,5.
Better results are obtained when the C/N ratio varies
between 10 -20 [5].
In conclusion, it can be summarized that the
producing of biogas and mineral fertilizers by BM and
OW processing is of particular importance. It is more
expedient to carry out the fermentation process at
mesophilic temperature regime. Since Azerbaijan is
rich in both plant and animal origin raw materials, the
use of BGP is very ef ficient in our country.

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