Plant Feedstocks and their Biogas Production Potentials

2.1. Energy Crops

Energy crops can be herbaceous as grass, maize (Zea mays L.), raps (Brássica nápus L.) or woody like willow (Salix), poplar (Populus), oak (Quercus), although the woody crops need special delignification pretreatment before AD [5]. The high content of lignin is not degradable in AD and makes the woody substrates more suitable for gasification or incineration or for the composting process as a bulking agent instead of biogas production [22]. Herbaceous energy crops have several essential characteristics that make them suitable for AD: efficient solar energy conversion resulting in high yields, low agrochemical inputs, low nutrient and water requirement due to their extensive rooting system, which holds onto fertilizers and water, and low moisture levels at harvest [15]. Plants with perennial growth habits also have the benefits of low establishment costs and fewer annual operations [15]. Using crops producing large biomass yields and/or growing fast crops that can provide high biogas output as a feedstock source makes biogas production cost-effective and profitable [23].

Key determining factors for a maximum biogas yield are the following: species and variety of energy crops, time of harvesting, mode of conservation and pretreatment of the biomass prior to the digestion process and nutrient composition [24]. Clearly, DEC which are more suitable for cultivation in one country might be less suitable for the other country [25]. To enhance the biogas production process from agricultural residues, manure and organic waste, energy crops such as maize, triticale (× Triticosecale) and sweet sorghum (Sorghum bicolor L. Moench) are often used [26].

To date, maize is the dominant substrate among energy crops for biogas production [25, 27-32] because it is assumed to possess the combination of high biomass (9-30 tons dry matter (DM) per hectare) and methane yields [24, 28, 29, 33-35]. Meanwhile, methane yield depends not only on the kind of crop and the genotype but also on biomass treatment [32, 36]. Some research investigations, presented in Table 1, found no differences in methane content between maize and barley (Hordeum vulgare L.), rye (Secale cereale L.), triticale [32] and several other energy crops [29, 31, 32, 34].

Maize is considered to be one of the three major cereal-based calorie sources for food supply, along with wheat and rice [37], which brings controversy over the concurrent use of the crop for food or fuel. It is important to note that maize monocultures may facilitate soil erosion [38], nitrate leaching [39], cause negative long-term effects on soil organic matter and reduction of biodiversity [28]. Maize is also sensitive to water deficit and other environmental stresses abound flowering [40]. In Germany, maize-specific pests Diabrotica virgifera subsp. virgifera has become increasingly important [41] and encourages investigations of other energy crops for biogas production [28].

Concerning DEC, recent EU projects, such as presented in Table 2, and extensive scientific evidence suggest that ‘second-generation’ lignocellulosic feedstock production systems (e.g., switchgrass (Panicum virgatum L.), miscanthus (Miscanthus × giganteus J.M.Greef & Deuter), cardoon (Cynara cardunculus L. var. altilis), giant reed (Arundo donax L.), and removal of crop residues) are amongst the most promising candidates to be grown on less favourable agricultural lands [23, 42-45]. For example, it was shown [23] that the use of alternative energy crop A. donax comparing to maize silage decreased climate change impact because of high biomass productivity per specific area and inputs, low agronomic inputs necessary (operation, fuel, chemicals) to produce the biomass and reduced need of N and P allowing better nutrient management.

In the framework of the MAGIC project Table 2, the concept of marginal agricultural land low-input systems for industrial crop cultivation was introduced [46] as sets of agricultural low-input practices to form viable cropping systems on marginal agricultural lands under specific climatic conditions. Additionally, Horizon2020 project BIOPLAT-EU aims at promoting and supporting the uptake of sustainable bioenergy projects on marginal, underutilized and contaminated lands, and for this purpose, the project is producing a tool and a database of maps through a web-based platform [47].

Ukraine is a large country in Central and Eastern Europe with a territory of 60.4 mln ha 70% of which is agricultural lands [48]. As estimated in the international project FORBIO Table 2, 4 mln ha in Ukraine have been set-aside and can be used for DEC cultivation [49]. As long as the war in the Eastern part of Ukraine is making a badly detrimental impact on land and water resources, the study supported by NATO Science for Peace and Security Program [50] shows a good potential of Miscanthus x giganteus phytomanagement on metal (loid)-polluted military soils in Ukraine and Slovakia. Furthermore, the data [51] indicates that Miscanthus × giganteus could improve the composition and structure of marginal lands as it leads to an increase in soil organic content, reduces the bulk density, improves the microbial activity and soil porosity. Several EU projects such as MISCOMAR and MisChar also focus on Miscanthus biomass options for contaminated and marginal land.

In many countries in sub-Saharan Africa, connecting the national electricity grid is highly expensive because their populated regions are spread over vast areas. In this context, decentralized renewable energy production from DEC cultivated on marginal lands offer an attractive solution [52]. The Crassulacean acid metabolism is a photosynthetic adaptation that facilitates carbon dioxide uptake at night and allows prickly pear (Opuntia ficus-indica (L.) Mill.) from Cactaceae family and African milkbush (Euphorbia tirucalli L.) to grow under arid and semi-arid climates and produce biomass for methane yield of 1860 m³ and 1791 m³ per 1 ha of marginal land respectively [52].

Another approach in sustainable energy crop cultivation is the development of integrated crop rotation that offers the supply with food and feed, the production of raw materials and energy sources such as biogas and the maintenance and further promotion of a multifaceted arable landscape [24]. For the development of sustainable crop rotation, experiments [24] elucidated optimum time of harvesting and methane yield in the cultivation of maize, winter wheat, triticale, winter rye, sunflower and grasses varieties. The highest methane yields of 7500-10200 m³_N ha^-1 were achieved from maize varieties, and the other crops resulted in 2-4 lower methane yields.

In an integrated crop rotation systems catch crops are plants that have short growth periods and can be cultivated between the vegetation periods of the main crops, thus increasing the efficiency of cultivation with regard to limited water and land resources. In this way, summer catch crops in Europe grow from June to October, and winter ones - from October to April [31]. Sorghum is a fast-growing crop with a shorter growth period compared to maize. It is characterized by wide adaptability to different environmental conditions and can be used as a summer crop [31, 40]. Sorghum is drought resistant due to the ability to delay reproductive development and a dense and prolific root system that is capable of extracting soil water deep in the soil profile, the ability to maintain stomatal opening at low levels of leaf water potential through osmotic adjustment [40, 53, 54].

The extensive study [26] was dedicated to energy crop combinations to produce biomass for biogas in reduced territories using double-cropping systems comparing to single crop: varieties of triticale, rye, and grass species as autumn-winter crops as well as varieties of sorghum and maize hybrids as spring-summer crops. In the single crop test, the fresh matter production per hectare was the highest in the case of sorghum (122.5 ± 10 Mg ha^-1) owing to double harvest, but in DM yield context, the most efficient were maize and sorghum (17.6-21.5 Mg DM ha^-1). Crop successions increased fresh and DM production from the territory as well as the biogas yield. Maize by itself resulted in up to 12 969 ± 812 Nm³ ha^-1 biogas, and the most productive combination was triticale + maize with 18 737 ± 2357 Nm³ ha^-1 of biogas. The biomass and biogas outputs in the research performed in Italy [26] were 15-20% higher in comparison to other reports from Austria [24], Germany [27], and Finland [25] due to the soil fertility and climate conditions.

Thereby, the cultivation of DEC brought new farming practices and new crop rotation systems, where intercropping and combined crop cultivation are the subjects of intensive research [5]. Further research is however required to ensure that the land-use change implications are well understood and the cultivation of energy crops is not detrimental to other land use or associated water resources, particularly for food production [17].

2.2. Agricultural Residues

Agricultural crop residues are one of the main concerns of the farming industry and are known to be significant contributors to GHG generation [55]. According to the definitions [48], primary agricultural residues are materials which remain in fields as by-products after the primary product of crops has been harvested (include cereal grain straws, wheat (Triticum), barley, rice (Oryza), corn stovers, stalks, leaves, etc.); and secondary agricultural residues arise during the processing of agricultural products for food or feed production (such as bagasse, sunflower husks, nutshells and so on). When using agricultural residues for biogas production, two sustainability issues should be considered: the potential competition with feeding in animal husbandry and possible depletion of organic matter in the soil and nutrients in agricultural lands because of the removal of straw from fields [48].

Some lignocellulose biomass has high lignin content which results in a slow and incomplete anaerobic fermentation [30, 55]. Plant materials may also contain a high percentage of carbon so the C/N ratio is too high [6]. In such cases, co-digestion with other substrates is more preferable than mono-digestion [6, 55]. On the other hand, the high content of water and the high fraction of fibers in manure cause low biogas yield typically ranging from 10 to 20 m³/t whereas the operation is considered profitable when biogas yields higher than 30 m³/t of treated material are achieved [56, 57]. To increase the biogas yield, a co-digestion with crop residues can be introduced. Thus, the addition of 4.6 kg of wheat straw (240 m³ CH₄ per 1 t straw) to 1 t of swine manure resulted in a 10% methane production increase [57]. Biogas and methane potentials of some plant residues are comparable to energy crops on a dry weight basis (Table 3).

2.3. Wastes

A rapid increase in the global population is leading to an increased demand for living resources such as food, feed, and fuel and, at the same time, to the generation of a large amount of wastes [58]. Improper management of organic waste can act as a major contributor to multifold environmental problems, such as climate change, ecosystem damage and resource depletion [59].

3.1. Parameters of Anaerobic Fermentation

AD and biogas production depends on many important parameters (Table 5) including temperature, pH, substrate DM rate, Organic Dry Matter (ODM) rate, C/N ratio, Hydraulic Retention Time (HRT), and Organic Loading Rate (OLR) [12, 61].

Different species of methanogenic bacteria function optimally in peculiar temperature ranges: psychrophilic, mesophilic, and thermophilic [6]. Higher temperature significantly improves the microbial growth kinetics, enzymes secretion, substrate diffusion, and mixing but affects the gas solubility, influencing the sulfide and ammonia toxicity, especially to methanogens [12]. Although the thermophilic process is generally the most efficient, large-scale AD is frequently carried out under the mesophilic conditions which are much easier to control and require less energy for heating [6, 12]. Moreover, the thermophilic process needs to be well buffered as it has a higher VFA/alkalinity ratio [12].

The pH is one of the most important operational parameters due to a wide range of optimal pH for particular microbial groups in the AD process: 5.5-6.5 for hydrolysis and acidogenesis as well as 7.8-8.2 for methanogenesis, therefore, neutral pH 6.8-7.4 is fitted for mixed culture if the whole process is performed in one digester [12].

Retention Time (RT) is the duration for the substrate and microorganisms to be kept together in a digester to achieve the desired extent of degradation [6]. Accordingly, HRT (or substrate retention time) is the time that an organic material stays in a digester from the moment it is added into the digester to the time of its evacuation [6]. In practice, HRT may last from a few hours in case of easily degradable soluble wastes up to several months in case of energy crop digestion [34]. For the efficient anaerobic fermentation, solid (microorganisms) retention time to HRT ratio should be more than 1 [12]. The shorter SRT the smaller would be the size of the reactor which accounts for 70-80% of the process total cost, and microorganism retention time, on the contrary, should be maximized [6]. In reactor configurations with biomass retention/recycling, the time microbes are retained in the bioreactors is much longer than the time a soluble substrate was kept resulting in a low substrate retention time without biomass washout [12].

RT and HRT are tightly connected with the OLR, the amount of ODM to be injected in the reactor. Substrate properties such as concentration and degradability, milieu parameters (e.g., temperature, pH), and the reactor performance (configuration, mass transfer capability, scum and bottom layer formation, etc.) determine the maximum allowable loading rate [34]. Depending on the influential factors listed, volumetric loadings for the same substrate can vary in the range between 5-40 gI^-1d^-1 VS [34]. It is important to avoid overloading or inadequate mixing of substrates which can cause a significant rise in VFA concentration, a sharp drop in pH and system failure [6, 12]. This is the reason for the concentration of total VFA under control (Table 5).

A properly balanced nutrient composition must be provided in the substrate for stable fermentation [12, 34]. The performance of digesters fed with nutrient-deficient substrates such as lignocellulosic feedstock can be greatly improved by co-digestion with nutrient-rich substrates such as food waste or animal manure [12]. The carbon to nitrogen ratio is one of the crucial substrate properties in AD. The high C/N ratio results in VFA accumulation [65], besides, the nitrogen is consumed rapidly by the methanogens to meet their protein requirements and is no longer available to react on the left-over carbon content in the material, and both circumstances may inhibit biogas production [6]. If the C/N ratio is too low, nitrogen is liberated and accumulates in the form of ammonia/ammonium which consequently increases the pH of the material and inhibits AD as well [6, 104]. Hence, a higher C/N ratio leads to a higher methane production but it lowers buffer capacity, while a low C/N ratio decreases methane production, increases the buffer capacity. To balance the C/N a correlation co-digestion strategy can be applied [104]. In addition, supplementing trace elements, particularly iron, nickel, cobalt, molybdenum, selenium, magnesium, zinc, manganese has been shown to have a positive effect on the stability of the AD process and methane production [12].

Inhibiting or toxic components of the substrate can cause retarded methane formation, a decrease of the methane content in biogas or even complete failure of methanogenesis [34]. Inhibition has been reported to occur from long-chain fatty acids such as oleate and stearate, antibiotics, phenols, organic solvents, e.g., chloroform, and higher levels of heavy metals [6, 34]. However, gradual adaptation to increased levels of inhibitory substances or complete degradation/detoxification can be observed for some toxic components such as pentachlorphenol, nitroaromatic compounds, chlorinated aliphatic or aromatic hydrocarbons, and azo dyes [34].

Anaerobic microorganisms can also be inhibited by metabolic by-products generated during the fermentation, e.g., hydrogen, ammonia, sulphide, and VFA [34, 105]. The extensive review indicates [34] that maintenance of a low hydrogen partial pressure (1-10 Pa) regulates the degradation of propionate and butyrate. Besides, it also allows the acetate-utilizing methanogens to sustain the pH by conversion of acetic acid to methane and CO₂. The increased partial pressure of hydrogen results in the accumulation of intermediate organic metabolites with subsequent pH drop below 6, ceasing methane generation [34]. Meanwhile, a competition for the available hydrogen can occur during methanogenesis, and sulphate-reducing bacteria obtain hydrogen and acetate more easily than methane-forming bacteria under low-acetate concentrations which increase sulphide levels and inhibit all reaction stages [34].

The destruction of organic matter, primarily the proteins [5, 34, 105] but also nitrates [34], releases ammonia nitrogen. As do sulphate reducers, nitrate-reducing bacteria compete successfully for the hydrogen normally used for CO₂ reduction to methane, subsequently, substrates with high protein or nitrate contents may suppress methane formation and cause considerable ammonia toxicity during AD [34]. Increasing pH and temperature will lead to an increased fraction of free ammonia [5] that can penetrate the cell membrane [105]. Methanogenic bacteria are especially sensitive to the unionized molecules of ammonia [5, 105], sulphide, and VFA [105]. Some process parameters, such as temperature, retention time inside the digester, pH, etc., have a direct or indirect impact on the sanitation efficiency of the AD process [5]. However, there are feedstock types which require separate pre-sanitation process (pasteurization or pressure sterilization), e.g. wastewaters from slaughterhouses, food and catering wastes and flotation sludge [5].

The main target product of biogas production is methane; any other gasses contained in biogas are unwanted and are considered as biogas pollutants [103, 106]. Such components reduce the density, calorific value, and Wobbe index of biogas [106]. H₂S and NH₃ components in biogas are toxic and extremely corrosive, damaging the combined heat and power (CHP) unit and metal parts via emission of SO₂ from combustion [106]. The relative content of CH₄ and CO₂ in biogas mainly depends on the nature of the substrate and pH of the reactor, N₂ appears from the dissolved air in the substrate, water vapours come from substrates at thermophilic temperatures, hydrogen sulfide recovers from substrate sulfates, and ammonia originates from the degradation of nitrogen-containing compounds such as protein, urea, and uric acid [103, 106]. To remove unwanted compounds, various biogas upgrading methods are commercialized; the major ones are water scrubbing, chemical adsorption, pressure swing absorption, membrane and cryogenic separation [107].

3.2. Substrate Utilization

The composition and digestibility of substrates are the most important parameters in methane yield [61]. It is determining for the structure and the composition of biogas substrates that an increasing amount of lignin is deposited into the cell walls in the process of plant crop growth to ensure their stability [30]. Lignin is a strongly cross-linked hydrophobic three-dimensional polymer consisting of three primarily monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) with amorphous structure resistant to microbial degradation [30, 108, 109]. The lignin content and composition varies among species, phylogenetic groups, cell types, and developmental stages of plants [109]. Most studies on specific methane yield prediction provide linear or non-linear models across crops with lignin content as major regressor variable [36].

Cellulose is a fibrous water-insoluble linear polysaccharide, a homopolymer of glucose joined by β-1,4 linkages [110]. Hemicelluloses are branched polysaccharides composed of pentoses (arabinose, xylose), hexoses (galactose, glucose, mannose) and acetylated sugars that serve as cement compounds between lignin and cellulose fibers in the united cell structure to form lignocellulosic complex [30, 108]. Hemicelluloses in agricultural biomass like straw and grasses are composed mainly of xylan that can be extracted easily in an acidic or alkaline milieu, whilst softwood hemicelluloses contain mainly glucomannan which requires a stronger alkaline milieu for extraction [108]. Since hemicellulose, unlike cellulose, has a lower molecular weight, is not crystalline but highly branched and amorphous, it has hydrophilic properties and is more easily accessible to hydrolytic enzymes for microbiological degradation than cellulose and lignin [30, 108].

Due to the strong integration of lignin, an increase of lignin content makes the lignocellulosic complex more resistant to biochemical degradation, and hydrolysis is the limiting step in methane fermentation of lignocellulose [30]. Since plant biomass contains in general 40-50% cellulose, 20-40% hemicellulose, and 20-30% lignin in DM, consequently, its characteristics such as lignin content, cellulose accessibility to cellulase, and cellulose crystallinity to a large extent determine the overall digestibility of the plant feedstock [108]. Although hemicelluloses, being lignocellulosic components, are highly thermo-chemically sensitive, severity parameters must be carefully optimized to avoid the formation of hemicellulose decay products such as furfurals and hydroxymethyl furfurals, which have been reported to inhibit the microbiological fermentation process. Such inhibition is more pronounced in aerobic conditions than in anaerobic [108, 111, 112]. In the fermentation of lignocellulosic hydrolysates, phenolic compounds have also been suggested to exert a considerable inhibitory effect with low molecular weight compounds being the most toxic [112]. At the same time, the inhibition of the third phase of anaerobic fermentation, acetogenesis, in lignocellulosic substrates is rather unlikely to happen due to the slow-running previous phases of hydrolysis and acidogenesis which prevent the accumulation of organic acids [30].

Pectins, proteins and the low-structure carbohydrates such as sugar and starch can be easily attacked by microorganisms and decomposed relatively quickly [30]. The overall scheme of organic constituents’ anaerobic degradability is illustrated in Fig. (2).

If the elemental composition of a substrate is known, the theoretical biogas yield can be calculated on the basis of the stoichiometric reaction. In methane fermentation of glucose the formula will be as follows [34]:

C₆H₁₂O₆ = 3 CH₄ + 3 CO₂

As a result, under standard conditions, 1 mole of the carbohydrate glucose gives 3 moles of methane and 3 moles of carbon dioxide, or 134.4 l biogas out of 180 g glucose (0.747 l/g) [34]. Similarly, the theoretical biogas yield from lipids and proteins will be 1.25 l/g (32% CO₂ + 68% CH₄) and 0.7 l/g (29% CO₂ and 71% CH₄) correspondingly [34].

Theoretical expectations that oil plants, due to the high content of lipids, should attain particularly high specific methane yields, are not supported by empirical studies [113]. For instance, oil crop sunflower (Helianthus annuus L.) produced substantially lower specific methane yields than maize [113]. It is known [34, 114] that long-chain fatty acids and higher oil concentrations can inhibit the anaerobic fermentation, though as shown in the study [115], in some cases adding a small amount of oil may significantly increase biogas yield.

A plant biomass-based renewable energy research has recently turned towards the investigation of quick and reliable valuation methods to determine the substrate quality of plant biomass for biogas processing which enables biogas plant owners to optimize the substrate production and purchase [36]. A laboratory procedure has been invented at the University of Hohenheim to determine the quantity and quality of biogas, e.g., methane gained as a result of an organic break-down of substrate samples less than 5 g of dry biomass in a batch process [116].

3.3. Biomass Treatment

The substrate quality of plant biomass for biogas processing widely varies not only depending on the plant species but also concerning management practices [32, 36]. Biomass treatment of plant crops for biogas production consists of cultivation, harvesting, transportation, storage, pretreatment, configuration/control of the anaerobic process, treatment, storage and use of digestate [29, 117].

The time of crop harvest can influence the composition of the ingredients regarding the methane yield [29, 30]. In the case of grasses, late harvest leads to higher lignin content causing slowing down and decreasing methane yield [29]. Although Miscanthus × giganteus cultivated for combustion purposes is harvested after winter when lignin and DM contents are high, for biogas plants and improved digestibility it is necessary to harvest miscanthus green, before winter [118]. The silage quality is better when miscanthus is harvested in mid-October due to the highest lactic acid content (average: 3.0% of DM) and the lowest pH (average: 4.39) compared to the harvests in mid-September and beginning of October [119]. On the contrary, late hybrids of maize, compared to medium-early hybrids, attained the highest specific methane yield and maximum methane hectare yields at the final harvest date taking full advantage of the growing season and providing higher whole-plant DM concentration [113]. Consequently, maize for biogas is recommended to harvest later than the forage maize.

Biomass storage serves to compensate for the seasonal fluctuations of feedstock supply and to facilitate mixing different co-substrates for non-interrupted uploading to the digester [5]. Solid feedstock can be stored as shown in Fig. (3) in large heaps on the ground or in bunker silos and require pressing by tractor or covering to minimize aerobic reactions and undesired gas emissions [5, 120].

According to the general principle of organization of protective measures in biogas technology, which states that the formation of hazardous gases outside the feeding system must be minimized, mixing of substrates outside closed tanks should be avoided to prevent chemical reactions and formation of hazardous gases, such as hydrogen sulfide, carbon dioxide or ammonia [103].

3.4. Anaerobic Digesters Design

There are multiple types of biogas digesters operating in the world: made of concrete, steel, brick or plastic, shaped like silos, troughs, basins or ponds, placed underground or on the surface [5]. The DM content of the digested substrate determines the design of a biogas plant and the type of digestion [5].

Historically, research on high-solids anaerobic fermentation was focused on the single charge (batch), non-mixed reactor concept, with recirculation of the effluent [6]. In non-mixed systems, the rate of gas production is much slower than in mixed systems, but this technology allows treating solid wastes in larger quantities and saving water [6]. An example of batch digesters is a “garage type” made of concrete, for the treatment of source separated biowaste from households, grass cuttings, and energy crops with 15-40% (TS) [5]. Instead of mechanical mixing, the leachate collected at the bottom of the reactor is either recirculated back to the top of the reactor or mixed with a fresh substrate prior to feeding into the reactor [12]. The temperature of the process and of percolation liquid is regulated by a built-in floor heating system and by a heat exchanger, a reservoir for the liquid [5].

Suspended growth anaerobic reactor includes CSTR, up-flow of anaerobic sludge blanket digesters, anaerobic contact reactors, anaerobic sequencing batch reactors, and anaerobic baffled reactors [12]. Due to simple design and operation as well as the ability to be fed with diverse feedstocks at various total solids content, CSTR is the most common reactor design for currently working biogas plants [12, 131].

Mixing is required for the most anaerobic reactor designs to combine the incoming material with the bacteria, to maintain pH of the fluid, temperature, and nutrients homogeneity, to facilitate the up-flow of gas bubbles as well as to prevent the formation of swimming layers and sediments [5, 6]. If the rate-limiting step throughout the startup duration is methanogenesis vigorous mixing is viewed [121] to be counterproductive, whereas high mixing is recommended if the rate-limiting step is hydrolysis. Stirring is applied using mechanical, hydraulic or pneumatic equipment [5].

To eliminate biomass stratification and inhomogeneity, the rotary digester concept was introduced [132-134]. In the rotating digester, substrate mixing is performed by raising the mineral component of biomass and methane-forming bacteria accumulating in the lower part of the digester and immersing the organic component accumulating in the upper part [133]. The digester is designed as a horizontal cylinder rotating around a horizontal axis, partially immersed in the liquid of the outer casing [132, 134].

Attached growth anaerobic reactor allows microorganisms to attach on to diversified inert media with high specific surface area and design for treating low total solids (< 5%) containing wastewaters [12]. The variants of this configuration include an anaerobic filter, expanded bed reactor, and fluidized bed reactor [12].

Large farm-scale co-digestion plants usually consist of several digester tanks, including one or several main digesters and post digesters [5]. The storage tanks for digestate serve also as post-digesters and should be covered with a gas-tight membrane [5].