Characteristics of Substrates Used for Biogas Production in Terms of Water Content

Abstract

New technologies based on the anaerobic digestion process make it possible to manage problematic waste. Methane efficiency depends largely on the level of the hydration of the substrates used for biogas production and their ability to decompose easily. The aim of this study was to present the current state of knowledge and practices in substrate hydration characteristics, focusing on pretreatment methods as the preferred method for improving efficiency. The paper discusses issues related to the degree of hydration of substrates in the context of their use in biogas plants. Reference was also made to topics related to the transportation and logistics of raw material supply regarding environmental impact. Biogas plant projects should be expanded to include an element related to assessing the impact of raw material deliveries on the immediate environment. Previous papers have not sufficiently analyzed the aspect related to the hydration of substrates used in anaerobic digestion processes. The presented and discussed research results can be implemented to optimize biogas plant water management processes. By replacing standard feedstock transportation methods with a pipeline, the environmental impact can be reduced by nearly ten times.

Full text

Citation: Czekała, W.; Nowak, M.;
Bojarski, W. Characteristics of
Substrates Used for Biogas
Production in Terms of Water
Content. Fermentation 2023,9, 449.
https://doi.org/10.3390/
fermentation9050449
Academic Editor: Alessia Tropea
Received: 11 April 2023
Revised: 25 April 2023
Accepted: 4 May 2023
Published: 9 May 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
fermentation
Review
Characteristics of Substrates Used for Biogas Production in
Terms of Water Content
Wojciech Czekała * , Mateusz Nowak and Wiktor Bojarski
Department of Biosystems Engineering, Pozna´n University of Life Sciences, Wojska Polskiego 50,
60-637 Pozna´n, Poland; mateusz.nowak@up.poznan.pl (M.N.); wiktor.bojarski@up.poznan.pl (W.B.)
*Correspondence: wojciech.czekala@up.poznan.pl; Tel.: +48-749-265-938
Abstract:
New technologies based on the anaerobic digestion process make it possible to manage
problematic waste. Methane efficiency depends largely on the level of the hydration of the substrates
used for biogas production and their ability to decompose easily. The aim of this study was to present
the current state of knowledge and practices in substrate hydration characteristics, focusing on
pretreatment methods as the preferred method for improving efficiency. The paper discusses issues
related to the degree of hydration of substrates in the context of their use in biogas plants. Reference
was also made to topics related to the transportation and logistics of raw material supply regarding
environmental impact. Biogas plant projects should be expanded to include an element related to
assessing the impact of raw material deliveries on the immediate environment. Previous papers have
not sufficiently analyzed the aspect related to the hydration of substrates used in anaerobic digestion
processes. The presented and discussed research results can be implemented to optimize biogas
plant water management processes. By replacing standard feedstock transportation methods with a
pipeline, the environmental impact can be reduced by nearly ten times.
Keywords: anaerobic digestion; fermentation; water content; pretreatment; biomass; biogas
1. Introduction
1.1. Climate Problems
Climate change resulting from relentless economic development poses a significant
threat to future generations. It is necessary to make reasonable and practical changes,
primarily in economic sectors related to food production and energy [
1
]. One of the
most noticeable climate changes around the world is the phenomenon of global warming.
Emissions of greenhouse gases such as methane, carbon dioxide, and nitrous oxide, for
example, contribute to this situation [
2
]. Replacing fossil fuels with biomass-derived fuels
is one way to reduce greenhouse gas emissions into the environment. Biogas produced
by anaerobic digestion is a solution for the environmental and energy crisis [
3
]. The use
and production of this type of biofuel are possible both in developing countries whose
economy is based on agriculture and in developed ones that produce significant amounts of
waste with high energy potential. Despite the possibilities, the development level of biogas
technologies in many countries currently varies [
4
]. This sector ’s problems on the road to
expansion are often due to inadequate legal conditions in this area and a lack of capital.
However, it should be noted that countries in Europe and Asia building biogas plants are
solving an important aspect: the possibility of processing various types of waste [5].
1.2. Biomass Characteristics and Utilization
The quantity and quality of research on processing and ways to use hard-to-find
biomass have recently increased significantly [
6
]. Biomass is a valuable energy material.
Biomass includes products and wastes of plant and animal origins. Sawdust is also an
example of biomass, the waste generated after felling trees [
7
]. Some other agricultural
Fermentation 2023,9, 449. https://doi.org/10.3390/fermentation9050449 https://www.mdpi.com/journal/fermentation

Fermentation 2023,9, 449 2 of 15
wastes, e.g., olive pruning and tea leaf residues and wetland vegetations, are also biomass-
rich lignocellulosic materials that can be subsequently reused for biogas production [
8
,
9
].
Ongoing research indicates that it is possible to use available biomass sources, such as
composting or anaerobic digestion [
10
,
11
]. By processing and removing excess moisture
from such renewable substrates, the possibilities for their use can be expanded. Biomass,
briquettes or pellets are an alternative that reduces fossil fuel extraction [
12
]. In addition,
biomass is also used to produce biochemical compounds or biocatalysts that enable chemi-
cal transformations [
13
]. In a breakdown of the world’s total energy demand, biomass, and
waste provide 55.7 EJ, ranking fourth. For years, the top three have consistently been coal,
oil, and natural gas [
14
]. Biomass is the primary energy resource in developing countries
with limited access to fossil fuels [15].
The use of waste biomass is currently becoming a popular solution worldwide [
16
].
One can distinguish between sources of traditional biomass that have been used since
the beginning of time on earth, such as wood, and biomass, which has been divided
into three distinct generations [
17
]. The first generation of biofuels are products derived
from crops that compete with human food production. This includes liquid biofuels
produced from feedstocks with high starch and sucrose content. There are concerns about
the competition between food and energy production from such sources, hence, second-
generation biofuels have been introduced [
18
,
19
]. A good example is biofuels from inedible
plants or agricultural and forestry residues. The second generation is thus a solution to
produce green energy while processing waste [
20
]. The last type is third generation biofuels,
which use algae for energy. These are microorganisms characterized by their easy culture
and simple cellular structure. Major advantages of algae are: no competition with food
crops for arable land, high growth rates, and low fractions of lignin, which reduces the need
for energy-intensive pretreatment. However, some disadvantages, such as the presence
of high water content, seasonal chemical composition, and the occurrence of inhibitory
phenomena during anaerobic digestion, make algal biofuels not yet economically feasible,
even though they are more environmentally friendly than fossil fuels. They also thrive in
polluted water, which they filter during growth [21].
There are various ways and technologies to convert biomass to energy. The current
climate policy of EU countries seeks to reduce greenhouse gas emissions by 40% by 2030
from 1990 levels [
22
]. Biomethane facilities play an essential role in the energy transition,
enabling the production of biomethane to replace the need for natural gas. As is well
known, using and replacing hard coal with natural gas or biomethane reduces smog
and high particulate matter emissions in many countries, including Poland [
23
,
24
]. The
increase in demand for gaseous fuel contributes to developing and constructing new
biogas plants. This potential is significant in countries producing significant amounts of
waste with appropriate energy parameters [
25
]. The versatility of modern biogas plants
makes it possible to use a variety of feedstock substrates. The waste processing through
anaerobic digestion and the generation of environmentally friendly renewable energy fits
perfectly with the goal of climate neutrality [
26
]. The resulting by-product of the process,
the digestate, is a valuable organic fertilizer used in the agricultural sector [
27
,
28
]. Research
conducted by Kumar et al. [
27
] suggest the sustainable up-cycling of spent mushroom
substrates inspired by a circular economy approach through a synergistic production of
bioenergy and secondary fruit crops, which could potentially contribute to minimize the
carbon footprints of the mushroom production sector.
1.3. Water Requirements for Biomass Production
In addition to the fertilizer aspect, energy crops require certain amounts of water
for growth and development. Despite the availability of good fertilizer in the form
of digestate, the current problem for agriculture is drought. Research conducted by
Mathioudakis et al.
[
29
] presents results on substrate conversion technology in terms of
water use. According to the paper’s authors, biogas is the energy source with the smallest
water footprint, among other biomass conversion technologies, such as pyrolysis and

Fermentation 2023,9, 449 3 of 15
gasification [
28
,
29
]. A closed-loop operation of the plant has a beneficial effect on water
management. It is possible to use digestate pulp for fertilizer or, after careful separation, also
for crop irrigation [
30
,
31
]. Comparing total freshwater demand globally, the agricultural
sector uses about 92% of the resource [
32
]. Authors Mekonnen and Gerbens-Leenes [
33
]
point to a high water footprint for certain biofuels, particularly the first hydropower gen-
eration. For biomass from crops directed for use as biogas feedstock, the total water
footprint for this type of agricultural production always remains the same. However, some
directed crops have a relatively high water footprint due to limited opportunities to harvest
residues [16,33].
Research on the carbon footprint is increasingly being analyzed, but recent years have
also directed another essential aspect—water footprint analysis. The study aim was to dis-
cuss the topic of water content in substrates used for biogas production. The main chapters
analyze our own and the literature research considering the importance of the proper water
management of modern biogas plants. The paper also presents the latest methods used
during substrate pretreatment that increase the efficiency of anaerobic digestion processes.
Activities and topics related to the work are necessary to increase the efficiency of water
use and conservation, which can contribute to achieving climate neutrality.
2. Characteristics of Substrates Used for Anaerobic Digestion
2.1. Types and Potential of Raw Materials Used for Biogas Production
By analyzing the literature on the use of raw materials and waste for biogas production,
it is predicted that it is possible to reach 108 EJ by 2030. Using the potential of biomass, it is
possible to replace 20% of primary energy in this way [
34
]. Regardless of the projections,
reaching this level is challenging for the biofuel sector due to the limited availability of
raw materials and the difficulty of their conversion. The residues used present significant
differences in chemical composition and dry matter content. In addition, some chemical
compounds in the feedstock can severely limit the biogas production potential, negatively
affecting the anaerobic digestion [
35
]. Using biogas facilities is, therefore, a rational alter-
native to reduce the consumption of fossil fuels. In the anaerobic digestion process, it is
possible to use a variety of substrates, including highly hydrated waste from the agro-food
industry. After anaerobic digestion, the waste is transformed into an environmentally
safe substance as digestate [
36
,
37
]. In addition, biogas facilities are part of conducting a
closed-loop economy. In the case of biogas combustion in cogeneration engines, the emitted
carbon dioxide is reabsorbed in the photosynthesis process of plants that are substrates in
the anaerobic digestion process, such as corn. The seasonality of the production of some of
the substrates is also an important aspect. In most installations, it is necessary to build a
storage area to ensure continuous production [38,39].
Dividing the substrates used in anaerobic digestion into appropriate categories is
possible. According to Yuan and Gerbens-Leenes [
32
], the first category is biomass from
agriculture. Substrates can come from crops directly directed to produce high-energy
crops, e.g., corn, and from crop residues and residues such as manure. Forestry-related
residues are included in the following category. This sector is characterized by logging
waste or waste from the paper industry. All other municipal and industrial organic wastes
were included in the third category of anaerobic digestion substrates [
32
]. The primary
criterion for selecting a suitable feedstock mix is biogas efficiency, feedstock availability,
and economic and environmental aspects. The introduction of an additional substrate can
have a beneficial effect on increasing the energy efficiency of the process. Carbohydrates
and proteins are characterized by a faster rate of decomposition compared to a substance
made up of fats. However, according to Table 1, it is fats that have the highest methane
efficiency [
40
]. An essential parameter for the substrates used in the anaerobic digestion
process is also the C/N ratio, which should be at an equilibrated level to ensure the stability
of biogas production [41].
The potential for using selected feedstocks for biogas production is determined by
methane efficiency. In addition to the differences in methane efficiency, an important aspect

Fermentation 2023,9, 449 4 of 15
is the availability of substrates near the biogas plant [
42
]. Table 1shows the theoretical
potential for the methane efficiency of selected compounds contained in substrates used by
biogas plants.
Table 1. Maximum theoretical biogas efficiency (own elaboration based on [43–45]).
Component Methane Efficiency
(m3kg−1VS) Reference
Carbohydrates 0.42 [43]
Proteins 0.50 [43]
Fats 1.01 [4]
Table 1shows that the most significant potential in the amount of methane produced is
in the substrates containing significant amounts of fat. Types of fat-rich substrates include
oil leachates. A side effect of using fat-rich substrates is the formation of long-chain fatty
acids in the decomposition process, which become toxic to bacteria at excessive concen-
trations [
46
]. Another group of substrates exhibiting high methane efficiency are products
containing significant amounts of proteins. During the decomposition of proteins, NH
4+
compounds are formed, which are converted into ammonia compounds at a later stage of
the process. Too high a concentration of ammonia can lead to the inhibition of anaerobic
digestion. However, it is possible for microorganisms to adapt to an environment with
higher concentrations of ammonia [
47
,
48
]. In addition to protein, lipid, and carbohydrate
compounds, it is also possible to use biomass with high levels of lignocellulose. Lignocellu-
losic biomass makes it more difficult to decompose the substance, which translates into
lower methane yields compared to the other groups [49].
The key findings and the preferred direction of biogas plants is to process and use
problematic substrates to manage and treat waste, which is also costly to dispose of [
50
]. An
essential aspect of the efficient operation of biogas plants is to ensure the correct proportions
of the feedstock, which allows for the optimal anaerobic digestion process and the growth
of new structures of methane bacteria. Meeting the difficulty of achieving a high level
of biogas efficiency is not possible if the anaerobic digestion process is carried out using
only one substrate. New trends indicate using two or more substrates, most often in the
co-digestion process, thus increasing plant profitability [51,52].
2.2. Water Content of Substrates vs. the Environmental Effect of Their Transport
Using various feedstocks for biogas production often necessitates transporting and
delivering them to facilitie several or tens of kilometers away. Ongoing research indicates
that the logistics of supplying feedstock substrates and the export and management of
digestate are components of biogas plant operations that adversely affect the environment.
The distance involved in transportation has the most significant impact on environmental
pollution and affects global warming in particular [
53
,
54
]. Another critical issue is how raw
materials are stored on the biogas plant site before they are used for anaerobic digestion.
Adequate storage has a positive effect on preserving the freshness of the products and
keeping their methane efficiency constant. Water in the form of moisture bound in many
substrates creates problems during improper storage, as it can lead to the appearance of
bacteria and mold [55].
Based on the problems associated with the operation of a biogas plant, Tucki et al. [
56
]
analyzed factors affecting operational efficiency. The main aspect impacting the efficiency
of biogas plants is the transportation distance of substrates. In addition, it is also necessary
to prepare appropriate technological infrastructure to facilitate the process of feeding
digesters. The authors also point out an essential and often overlooked aspect: the need to
manage the digestate. The use of the by-product in question on one’s fields or the provision
of guarantees in receiving the digestate determines the efficiency of the plant’s operation.
However, the analyzed article does not specify a vital aspect: the minimum distances that

Fermentation 2023,9, 449 5 of 15
guarantee the economic justification of the transportation of pulp and substrates. It is
proposed to research transportation’s economic and environmental aspects [56].
The carbon footprint associated with substrate supply is also significantly affected
by the parameter of dry matter and dry organic matter. Running the anaerobic digestion
process with a highly hydrated substrate decreases methane efficiency per ton of fresh
matter. Therefore, it is a good practice to use and collect the raw material from a fixed
location to ensure commensurate parameters for a specific type of substrate. In addition, if
the owner of a biogas plant contracts a significant amount of substrate, it is necessary to
establish acceptable deviations from the assumed parameters indicative of its quality. An
important aspect is also the possibility of transporting acceptable and, at the same time,
maximum quantities of transported substrates by vehicles with a large capacity [57,58].
Research conducted by Muradin and Foltynowicz [
59
] describes the environmental im-
pact aspect of feedstock transports for an example of biogas plants. The authors conducted
their experiments with four different biogas plants. This impact was expressed in [Pt]
points based on ISO 14040-44 and LCIA Impact 2002+ [
58
,
59
]. The results on the logistics
of transporting raw materials to biogas plants are presented in the analyzed article. The
standard transportation of slurry using a farm tractor with a barrel to a modern pipeline
was compared. The results also show that transportation has a negative impact on the
environmental effect. Using a pipeline, it is possible to reduce the negative environmental
impact by up to 10 times compared to the standard transportation method [59].
As mentioned earlier, the key findings is the dry matter level for liquid and solid
substrates. The feedstock dry matter parameter mainly determines the methane efficiency.
In general, the higher the feedstock dry matter value, the higher the methane efficiency per
ton of fresh substrate mass [60].
3. Feedstock Hydration and Anaerobic Digestion Technology
Processing biomass to produce biogas can be carried out in many different ways. Clas-
sification of the anaerobic digestion process requires the determination of an appropriate
criterion. The most popular division of anaerobic digestion technologies is based on the
temperature range at which the process occurs. Determining the optimal temperature level
is related to the type of bacteria involved in the anaerobic decomposition process. The
optimum temperature value allows for the proper decomposition of organic matter and
provides a suitable environment for a particular group of bacteria. Using this variable, the
different types of anaerobic digestion technology have been characterized as psychrophilic,
mesophilic, and thermophilic [
61
]. Another possibility is the division resulting from the
number of process steps, the method of substrate dosing, or the degrees of separation
of the different phases of anaerobic digestion. Depending on the type of substrate pro-
cessed, the critical factor determining the anaerobic digestion technology is the degree of
hydration. The dry matter content of the chamber determines the type of technology used,
distinguishing between wet and dry anaerobic digestions [62].
In biogas plants, the average daily feedstock requirement ranges from a few to tens
of Mg of feedstock. A biogas plant with a capacity of 0.5 MW
e
requires a daily supply
of 30 Mg of feedstock to the digester [
63
]. A biogas facility’s size and electrical capacity
also depend on factors related to the amount and type of substrates used, which are found
in the immediate vicinity. Regardless of the biogas plant size, each consists of the same
essential components, including those related to feedstock storage. In the case of solid
feedstock such as corn silage, for example, it is essential to ensure proper conditions during
preparation and storage [
55
]. A portion of waste is characterized by odor nuisance and
requires storage in storage halls with biological or activated carbon filters. In the case
of liquid substrates, problems are encountered with the need to install mixers or heating
systems at the storage site. Therefore, the choice of the appropriate anaerobic digestion
technology can be determined, in addition to the availability of raw material and the
convenience of its storage [
55
,
64
]. In addition, it should be noted that the selected anaerobic

Fermentation 2023,9, 449 6 of 15
digestion method is not determined by the dry weight compactness of the individual
feedstock but by the hydration of the feedstock mixture entering the digester [64].
3.1. Wet Anaerobic Digestion
For years, the wet anaerobic digestion process has continued to be the most common
and dominant way of treating various types of substrates processed in biogas plants [
62
].
This type of anaerobic digestion is popular due to the nature of the process, in which
the bacteria for each of the different phases require an aqueous environment in their
surroundings. If the reactor mixture is not hydrated enough, the bacteria will have difficult
access to the substrate, which can cause only partial decomposition and a decreased
biogas production. Moisture is also a factor that provides a substrate for bacteria to grow
and multiply, ensuring the high efficiency of the ongoing process. In addition, for wet
technology, natural process inhibitors are distributed throughout the mixture, ensuring
their low concentration and not interfering with the process. Low hydraulic retention
time (HRT) for highly hydrated substrates such as slurry can result in the leaching of
microorganisms. When selecting substrates for biogas plants, it is necessary to take into
account the dry matter input and dry organic matter input load of the digester [65,66].
Dry matter values in the reactor chamber are conventional and are defined differently
depending on the available literature sources. The most commonly accepted values for the
wet process are <10–15% dry matter [
67
–
69
]. The low solids content and liquid consistency
create the possibility of pumping the mixture between chambers and the subsequent
management of digestion residues [
30
]. In wet technology, operating biogas plants are
characterized by a full flow of substances through the entire volume of the reactor. Before
refilling the reactor with a new portion of the substrate, a conditioning process is used,
i.e., adding process water or the liquid fraction of the digestate to maintain the assumed
level of the dry mass of the feedstock. Ensuring a suitable mixing ratio is possible through
vertical and horizontal mechanical mixers or a hydraulic mixing process (liquid recycling).
The advantages of building this type of plant are the ease of construction of the tank and
the possibility of carrying out maintenance work without emptying the reactor [
69
]. Based
on research conducted by Luning et al. [
70
], a mass flow model was developed for the wet
anaerobic digestion process for a full-scale plant, as shown in Figure 1.
Fermentation 2023, 9, x FOR PEER REVIEW 6 of 15
liquid substrates, problems are encountered with the need to install mixers or heating
systems at the storage site. Therefore, the choice of the appropriate anaerobic digestion
technology can be determined, in addition to the availability of raw material and the con-
venience of its storage [55,64]. In addition, it should be noted that the selected anaerobic
digestion method is not determined by the dry weight compactness of the individual feed-
stock but by the hydration of the feedstock mixture entering the digester [64].
3.1. Wet Anaerobic Digestion
For years, the wet anaerobic digestion process has continued to be the most common
and dominant way of treating various types of substrates processed in biogas plants [62].
This type of anaerobic digestion is popular due to the nature of the process, in which the
bacteria for each of the dierent phases require an aqueous environment in their sur-
roundings. If the reactor mixture is not hydrated enough, the bacteria will have dicult
access to the substrate, which can cause only partial decomposition and a decreased bio-
gas production. Moisture is also a factor that provides a substrate for bacteria to grow and
multiply, ensuring the high eciency of the ongoing process. In addition, for wet technol-
ogy, natural process inhibitors are distributed throughout the mixture, ensuring their low
concentration and not interfering with the process. Low hydraulic retention time (HRT)
for highly hydrated substrates such as slurry can result in the leaching of microorganisms.
When selecting substrates for biogas plants, it is necessary to take into account the dry
maer input and dry organic maer input load of the digester [65,66].
Dry maer values in the reactor chamber are conventional and are dened dierently
depending on the available literature sources. The most commonly accepted values for the
wet process are <10–15% dry maer [67–69]. The low solids content and liquid consistency
create the possibility of pumping the mixture between chambers and the subsequent man-
agement of digestion residues [30]. In wet technology, operating biogas plants are charac-
terized by a full ow of substances through the entire volume of the reactor. Before rell-
ing the reactor with a new portion of the substrate, a conditioning process is used, i.e.,
adding process water or the liquid fraction of the digestate to maintain the assumed level
of the dry mass of the feedstock. Ensuring a suitable mixing ratio is possible through ver-
tical and horizontal mechanical mixers or a hydraulic mixing process (liquid recycling).
The advantages of building this type of plant are the ease of construction of the tank and
the possibility of carrying out maintenance work without emptying the reactor [69]. Based
on research conducted by Luning et al. [70], a mass ow model was developed for the wet
anaerobic digestion process for a full-scale plant, as shown in Figure 1.
Figure 1. Mass balance in the wet anaerobic digestion process (own elaborated based on [70]).
One biogas plant analyzed by Luning et al. [70] used a wet substrate separation pro-
cess to eliminate sand and other materials that adversely aect the process. The process
uses the phenomenon of sedimentation. The substrate is then mixed and directed to the
digester. The nal stage is dewatering the digestion residue using belt presses. It should
be noted that dewatering the digestate from the wet anaerobic digestion process results in
Figure 1. Mass balance in the wet anaerobic digestion process (own elaborated based on [70]).
One biogas plant analyzed by Luning et al. [
70
] used a wet substrate separation process
to eliminate sand and other materials that adversely affect the process. The process uses
the phenomenon of sedimentation. The substrate is then mixed and directed to the digester.
The final stage is dewatering the digestion residue using belt presses. It should be noted
that dewatering the digestate from the wet anaerobic digestion process results in about
40% wastewater and 42% solid fraction. This waste should be managed appropriately. The
costs associated with the disposal of solid fraction waste are significantly higher than the
disposal of process wastewater. This aspect has a significant impact on the profitability of
the biogas plant and the resulting water footprint. In the case of dry anaerobic digestion,
the final mass balance is in the ratio of 10% wastewater and 77% solid fraction digestate.

Fermentation 2023,9, 449 7 of 15
This ratio facilitates the introduction of technology for drying the solid fraction and using
it as an energy fuel [69].
3.2. Dry Anaerobic Digestion
The dry anaerobic digestion method allows for the processing of various types of
the feedstock of varying compositions, such as manure or food waste [
11
]. Using such
substrates without prior dilution and preparation is impossible in wet technology, as they
exceed standard equipment’s ‘pumpability’ limit. The finished batch substrate mixture
in the reactor contains a high amount of dry matter at levels ranging between 20 and
40% [
69
]. Large-scale biogas production using dry anaerobic digestion technology is
currently considered a fledgling method. This may be due to the lack of prevalence among
most industrial biogas plants [
11
,
70
]. However, differences in process parameters and the
feedstock used and the bacteria for dry and wet conditions are similar [71].
A system based on dry anaerobic digestion is devoid of the drawbacks of a wet
digestion process. It is possible to conduct anaerobic digestion in a discontinuous process,
and no additional energy inputs are required for mixing and grinding the feedstock [
72
].
The dry process prevents the formation of foam and crusts on the surface of the digester
and also counteracts the phenomenon of sedimentation [
73
]. The benefit of dry technology
is the processing of waste in its original form. In addition, a process of this type does not
require additional diluent liquid and, according to Jha et al. [
74
], is capable of producing
a higher volume of methane per m
3
of bioreactor volume [
74
]. A 540–750 NL kg VS
biogas efficiency was obtained for the substrate studied using the wet anaerobic digestion
process [
75
]. Comparing methane efficiency for the dry process with the wet process in the
literature has been analyzed at the laboratory scale [
74
] and at the large scale of industrial
biogas plants [
70
]. The authors of the paper [
75
] established, based on their research, the
higher methane efficiency of the dry-fermentation process of grass compared to the dry-wet
method. A 420–540 NL kg VS biogas efficiency was obtained for the substrate studied
using the dry anaerobic digestion process [
75
]. A study by Angelonidi and Smith [
66
]
compared nine plants using dry and wet technologies for anaerobic digestion processes.
For the dry method, a greater flexibility related to the type of feedstock used, reduced water
consumption, shorter retention times, and an easier management of the end products of
the process were evaluated. In contrast, the wet technology was characterized by a more
favorable final energy balance [69].
In recent years, Europe has seen progress related to dry anaerobic digestion efficiency
by up to 50%, although methane anaerobic digestion is responsible for managing only 35%
of all waste [
76
]. Additionally, on other continents, e.g., in China, interest in dry anaerobic
digestion has increased, which is an ideal solution for managing the
0.9 billion tons
of
straw generated annually in China. Straw is a problematic feedstock for anaerobic di-
gestion, as there are difficulties with its breakdown by bacteria, so pretreatment may be
necessary [
77
]. The use of dry anaerobic digestion technology allows for treating wastes
containing high values of organic matter in their composition, such as municipal solid
waste and agricultural waste [
76
]. By analyzing the water footprint and substrate hydration
for dry digestion technology, it was estimated that it requires four to ten times less water
than wet processes [
78
]. This fact also has the benefit of reducing the volume of the digester
and the savings associated with substrate pretreatment processes, as it only requires the re-
moval of very large particles—larger than 5 cm [
79
]. Challenges posed by the dry anaerobic
digestion process relate to generating a homogeneous mixture and transportation. The use
of equipment for transporting solid substrates is more expensive than for liquid substrates.
It is possible to design and build pumps, belts, or screw conveyors transporting substances
with a high dry matter value [
80
]. The authors of the paper [
81
] show the problem associ-
ated with the inhibition of the dry fermentation process when the concentration of total
solids is above 40%. At such a high level of total solids, methane bacteria have difficulty
decomposing substrates. An excessively high amount of total solids can also cause plant
heating problems [78,81].

Fermentation 2023,9, 449 8 of 15
4. Substrate Pretreatment as Key to Increasing Anaerobic Digestion Process Efficiency
Biogas production technology is constantly optimized for biological processes and
chemical reactions to increase productivity and reduce operating costs [
82
]. It is theoretically
possible to carry out the methane anaerobic digestion process for the organic fraction of
any substrate. Depending on availability, it is possible to process wood, crop residue,
lignocellulosic waste, food waste, or cotton stalks [
19
]. Some feedstocks are not naturally
adapted to anaerobic digestion, as they are characterized by complicated or very long
decomposition by bacteria. In addition, some of the raw materials may also contain natural
inhibitors of the process. Therefore, it is becoming necessary to develop technology related
to the pretreatment of substrates allowing the management of difficult-to-degrade wastes,
which can contribute to increasing the potential of biogas facilities worldwide [83,84].
Most raw materials inherently characterized by susceptibility to fermentation, such as
slurry and sewage sludge, are used. This type of waste, too, often requires pretreatment
processes due to excessively high levels of hydration, which reduces methane efficiency.
Using appropriate pretreatment technology, it is possible to increase the availability of valu-
able biogas substrates while helping to reduce environmental pollution [
83
,
85
]. Substrates
containing large amounts of cellulose (40–60%) and hemicellulose (20–40%) after pretreat-
ment have high biogas potential [
86
]. Waste containing lignocellulosic compounds is one
of the most considerable organic resources in the world, providing an average of about
200 billion tons
per year [
87
]. Organic wastes, such as bird feathers, contain more than 90%
creatine in their content [
88
]. The use of pretreatment allows creatine to be broken down
into oligomers, which are easily submitted to anaerobic fermentation [
89
]. Waste from fruit
and food processing are also substrates with high methane potential after pretreatment [
83
].
In recent years, research has been conducted to manage waste polyethylene terephthalate
(PET). Yoshida et al. [
89
] found that a new type of bacteria Ideonella sakaiensis 201-F6 can
degrade PET compounds, but further research is needed to expand the scale and possibili-
ties of this type of technology [
89
]. An innovative option for converting problematic waste
is gasification technology before anaerobic digestion. Gasification converts the substrate
into synthesis gas, which is directed into the environment with anaerobic bacteria, thus
producing biogas [90]. Combining the two types of technology brings some benefits [83].
Substrate pretreatment processes are designed to improve availability and facilitate
anaerobic microorganisms’ utilization of organic matter. Several pretreatment types are
classified as chemical, biological, mechanical, and thermal, respectively [
91
]. In addition,
substrates are often prepared by removing solid contaminants such as sand and metals,
which can be found in municipal waste. In addition, pretreatment allows for grinding
the feedstock or even removing process inhibitors such as oils. An important aspect also
concerns the compaction of substrates before they are delivered to the biogas plant site.
This procedure is an opportunity that allows for reductions related to greenhouse gas
emissions. The reason for the unfavorable water and carbon footprint of biogas plants is
the necessity to transport substrate characterized by low biogas efficiency and then manage
the highly hydrated digestate pulp [
83
,
92
,
93
]. Similarly, in the case of chemical methods,
various types of chemical compounds, such as strong acids and alkalis, are used. The acidic
chemical treatment process allows lignocellulosic compounds.
Analyzing the aspect of the application of storage solution leads to significant cost
reduction at the storage and handling stage of the biomass supply chain, which results in
considerable cost savings for the whole biomass logistics function. This reduction exceeds
by far the extra cost imposed by biomass material losses. However, side-effects of applying
cheaper storage solutions without biomass drying, such as a heating value reduction in the
biomass, health and fire risks, etc., should be further investigated [94,95].
4.1. Chemical and Biological Pretreatment
The use of the pretreatment of substrates by biological and chemical methods is rela-
tively the same. In the case of chemical methods, various types of chemical compounds,
such as strong acids and alkalis, are used. The acidic chemical treatment process allows lig-

Fermentation 2023,9, 449 9 of 15
nocellulosic compounds to be broken down into simpler monosaccharides [
96
]. In addition,
the acidic reaction of this type of treatment can be controlled by the presence of hydrolytic
bacteria [
97
]. High costs resulting from the pretreatment of substrates by acidic methods
limit the development of this technology on the industrial scale of anaerobic digestion
processes [
98
]. Another type of chemical method is alkaline compounds such as hydroxides,
which allow them to be used at ambient temperatures [
99
]. The anaerobic digestion process
may require the addition of alkaline reagents to balance the pH, and alkaline pretreatment
is a preferred method over acidic pretreatment [
100
]. Research by Liew et al. [
101
] showed
that the pretreatment of leaves using a 3.5% NaOH solution allows for a 20% increase in
methane efficiency within three days. This type of treatment contributes to an increased
anaerobic digestion efficiency for substrates containing lignocellulose compounds. Con-
tinuous anaerobic digestion with substrates after alkaline pretreatment can lead to salt
accumulation and pH increase, thus contributing to the inhibition of the methanation
process [101,102]. Chemical treatment processes are also not widely used on an industrial
scale due to the lack of economic viability currently resulting from the high cost of alkali.
However, this type of treatment may be the only option for highly acidic or lignin-rich
substrates to ensure a stable anaerobic digestion process [103].
Biological pretreatment of substrates can be carried out at a low temperature and
without additional chemicals. It includes processes under aerobic and anaerobic conditions,
but this type of substrate preparation is not used for municipal waste [
96
]. The treatment
conditions of anaerobic technology are often used as a pre-acidification or dark anaerobic di-
gestion. This technology makes it possible to separate the methane generation process from
hydrolysis and acid production. The pH value for the pre-acidification stage is between
4 and 6, which inhibits the methane generation process and causes volatile fatty acids to
accumulate. In a study by Liu et al. [
104
], a 21% higher methane efficiency was achieved by
introducing a pre-acidification stage for household waste. In addition, it resulted in the ben-
efit of a higher methane concentration in biogas [
103
,
104
]. Compared to other pretreatment
methods, biological methods require a more extended period of substrate preparation while
having low efficiency [
105
]. Research conducted by
Mshandete et al.
[
106
] also presents
pretreatment methods involving aeration. Their tests showed an increased methane pro-
duction efficiency of 26% for a 9 h treatment stage. Prolongation of the assumed aeration
stage leads to the aerobic breakdown of the substrate and reduces the amount of methane
produced [106].
4.2. Mechanical and Thermal Pretreatment
Pretreatment of substrates using mechanical and thermal methods is categorized as a
physical process. The rationale for using mechanical pretreatment is to reduce the particle
size of substrates while increasing the surface area of bacterial activity [
18
]. In addition,
grinding the substrates facilitates the mixing process in the digesters and reduces the
problem associated with dross formation. Most industrial biogas plants use mechanical
processing of substrates, allowing for uniformity of the feedstock mixture [
101
]. Nah
et al. [
107
] conducted research related to substrate pretreatment using high-pressure particle
breaking. This process reduced the hydraulic retention time of sludge from 13 days to
6 days
while maintaining the same process efficiency. This research was conducted only on
a laboratory scale, and the final stage did not test the methane content of the biogas [
107
].
Fine grinding of waste containing lignocellulosic compounds increases the anaerobic
digestion process’s efficiency, but too fine particles can cause acidification because they
dissolve quickly [
96
]. In addition, the disadvantages of mechanical treatment processes
include the possibility of damage from inert materials such as stones [
96
]. A number of
researchers have studied the effect of knife milling on biogas production. Menind and
Normak [
108
] found that about a 10% higher gas yield was obtained when hay was ground
to 0.5 mm compared to 20–30 mm. A different investigation found that grinding sisal fibers
from 100 mm to 2 mm produced about 20–25% higher gas yields [109].

Fermentation 2023,9, 449 10 of 15
The thermal pretreatment process is based on processing substrates under high tem-
peratures (typically 125 to 190
◦
C) and pressure while maintaining the set conditions for
about an hour [
96
]. Biogas plants using substrates after thermal pretreatment can use an
increased reactor load factor and conduct a more stable anaerobic digestion process [
108
].
A crucial factor during thermal treatment is selecting an appropriate temperature value.
If the value is too high, feedstock substrates can be degraded. In addition, conducting
the thermal process allows the destruction of pathogenic pathogens contained in some
wastes [
96
,
110
]. An example of the thermal pretreatment of feedstock on an industrial
scale is the thermo-druck-hydrolyse (TDH) process. The technology has been used for
diluting to about 10–15% dry weight of kitchen waste and plastics. The TDH reactor is
pressurized for about 20 min at 20 to 30 bar and 170–200
◦
C. It was found that this type of
technology increased biogas production by 20 to 30% for the substrates analyzed. Thermal
pretreatment plays a unique role in locations with a free supply of waste heat from plants
such as factories or power plants [
96
]. Table 2shows the advantages effect of pretreatment
on the fermentation process from selected methods.
Table 2. Effect of pretreatment on the fermentation process (own elaboration based on [94–107]).
Pretreatment Process Advantages Reference
Chemical decomposition of lignocellulose
only solution for highly acidic substrate [98,101,102]
Biological
Mechanical
used as pre-acidification
low temperature
increasing the surface area for bacterial activity
[96,104]
[94,97,105]
Thermal more stable anaerobic digestion process [94,95,107]
5. Conclusions and Directions for Further Research
In addition to managing waste, biogas plants also process significant amounts of ballast
water in anaerobic digestion. The agricultural sector and related agricultural production
are the largest consumers of water resources. The development and exploitation of biogas
plants that use agricultural residues optimize aspects of the water footprint. In addition,
waste biomass is generally characterized by high hydration, so pretreatment methods are
required. Removing excess moisture even before transporting the substrates to the biogas
plant site is good practice.
Recently, the situation related to protecting the world’s water resources is also not
insignificant. Creating assumptions in the context of water limits in the future can help
avoid the problem of unsustainable water consumption. Numerous studies are being
conducted to determine the water footprint for products and industrial plants, including
biogas plants and their substrates. It is becoming necessary to look for solutions related
to the low efficiency of the anaerobic digestion process when over-hydrated substrates
are used. Due to the multitude of available technologies related to the pretreatment of
substrates and for the direct execution of the anaerobic digestion process, further research
needs to optimize the discussed solutions. It should also be emphasized that pretreatment
processes are often an expensive and energy-intensive technology. The most common
method used to pretreat substrates is a mechanical one. Reducing the particle size of
substrates results in a greater surface accessibility for microorganisms.
Transportation distance is essential in the production and delivery of substrate for a
biogas plant. Depending on the conditions of the selected location for a newly built biogas
plant, the impact of harmful greenhouse gas emissions associated with transportation
should be considered. It is proposed to initiate new research on transportation elements and
biogas substrate management. The use of modern technologies related to the continuous
analysis of selected substrate parameters and control of delivery processes can allow for a
breakthrough optimization of the biogas sector in Poland and the world.

Fermentation 2023,9, 449 11 of 15
Author Contributions:
Conceptualization, M.N. and W.C.; methodology, M.N. and W.C.; software,
M.N.; validation, M.N. and W.C.; formal analysis, M.N. and W.C.; investigation, M.N. and W.C.;
resources, M.N. and W.C.; data curation, M.N. and W.C.; writing—original draft preparation, M.N.,
W.B. and W.C.; writing—review and editing, M.N. and W.C.; visualization, M.N.; supervision, W.C.;
funding acquisition, M.N. and W.C. All authors have read and agreed to the published version of
the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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