Biogas is a renewable energy source that can be produced from different cheap recyclable organic waste streams combined with the reduction of greenhouse gas emission. Biogas production also contributes to economic benefits for farmers through the production of organic fertilizers, reduction of pathogenic microorganisms and removal of odours. Recent evaluations have indicated that the anaerobic digestion (AD) is an efficient alternative technology that combines biogas production with sustainable waste management (1, 2). The AD is a bioprocess during which complex organic materials are decomposed in the absence of oxygen through the following phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis (Fig. S1a (3)). The digestion of such complex organic biomass into a source of clean and renewable energy reduces the greenhouse effect and results in a production of two very valuable products: biogas and digestate. Biogas is composed of CH₄, CO₂, H₂S, NH₃, N₂ and traces of water vapour (4). Digestate is the residue (decomposed organic feedstock) of biogas production that contains different macro- and micronutrients. It can be used as a soil fertilizer due to its good C/N ratio and homogeneity, nutrient availability and significantly reduced odour.

The individual biomass degradation steps are carried out by different consortia of microorganisms. In the first step of the AD, the complex organic material, rich in carbohydrates, proteins and fats, is hydrolysed by extracellular enzymes of hydrolytic bacteria to simpler compounds: mono- and oligomers, amino acids and fatty acids. These compounds are converted to short-chain fatty acids (SCFAs), alcohols, H₂S, CO₂ and H₂ in an acidogenesis (fermentative) step. In the next step, acetogenesis, SCFAs and alcohols are oxidised into methanogenic substrates like acetic acid, H₂, CO₂ and water. The last step is methanogenesis in which products from acetogenesis are converted to CH₄ and CO₂. Methanogenesis can be conducted through the hydrogenotrophic, acetoclastic or methylotrophic pathways (Fig. S1b (3)). This is the rate-limiting step since members of Archaea domain are very sensitive to any changes in their environment. They can be easily influenced by many factors including the composition of the feedstock, feeding rate, volatile fatty acid (VFA) concentration or rapid changes in temperature and pH values, resulting in termination of methane production and process inhibition. The process conditions required for a stable one-stage AD system have to be as follows: pH=6.50-7.50, alkalinity as CaCO₃ 1.5-4.0 g/L, volatile solids/total solids >45%, total ammonia nitrogen <1500 mg/L, C:N ratio 20-30 and C:N:P ratio 100-120:5:1 (1, 5).

Low costs and the broad range of feedstock increase the biogas potential to be used for heat, steam, electricity or hydrogen production as well as a transportation fuel.

Biogas can be produced from various types of widely available organic feedstock such as animal manure and slurries, wastewater and sewage sludge, municipal solid waste, organic waste from dairy production and food industry, agricultural biomass, lignocellulosic residues (stalks, leaves, roots, seeds, seed shells), organic waste from households as well as energy crops. Industrial waste and agricultural biomass residues have been a promising feedstock without affecting natural resources, along with fewer risks and competition for food. As the expenses of lignocellulosic feedstock conversion to bioenergy exceed the price of fossil fuels, the development of economically viable production systems requires the use of low-value biomass and waste that are currently underutilized. Utilization of underused biomass feedstock is in alignment with the circular bio-economy concept by reducing the overall biogas production as well as waste treatment costs.

Depending on the used feedstock, the process of biogas production should be optimized in terms of composition of the substrate, total (TS) and volatile (VS) solids content, C/N ratio, organic loading rate (OLR), bioprocess parameters (temperature, pH, moisture content, loading rate, mixing intensity, retention time), presence of inhibitors (NH₃, heavy metals, oil, grease, phenols, antibiotics, VFAs) and biodegradability (6). In the following subsections, an overview of lignocellulosic and non-lignocellulosic feedstock, energy crops and co-digestion substrates is presented.

Lignocellulosic feedstock comprises a complex and recalcitrant matrix, constituted of cellulose, hemicellulose and lignin, resistant to microbial hydrolysis (7). High crystallinity and degree of polymerization of cellulose, lignin as a physical barrier that limits the access of the enzymes, and cellulose interweaving by hemicellulose are the major barriers for lignocellulose application as a feedstock for anaerobic digestion. In addition, slow hydrolysis rate, long retention time, formation of toxic compounds and low methane yields have been observed when using lignocellulosic feedstock (8). To overcome these issues, with the intention to enhance degradation and consequently biogas production, the lignocellulose should be treated prior to the AD. Several pretreatment methods have been developed, including physical, chemical, thermochemical, oxidative and biological methods (9). Pretreatment is responsible for opening the structure of such a complex matrix, thereby reducing the cellulose crystallinity and making it more accessible, separate the components of the lignocellulosic biomass and remove the lignin. During the pretreatment, inhibitors such as furfural and hydroxymethylfurfural can be generated. Novel methods with ionic liquids, supercritical CO₂ or oxidative pretreatment are promising techniques that avoid the use of strong acids/alkalis and the formation of inhibitory compounds (10).

The most utilized lignocellulosic feedstock for biogas production can be categorized into four main groups: (i) agricultural residues (e.g. bagasse, stalks of cotton plant and barley, empty fruit bunch from oil palm, different crop straws, corn cobs, maize and sorghum stover or peanut shell), (ii) fruit and vegetable waste, (iii) forestry residues (e.g. woody biomass like birch, eucalyptus, beech, cedar, pinewood or oak) and paper waste, and (iv) separated municipal solid waste (different organic materials, mainly from kitchen waste, from which lignocellulosic solid waste can be sorted out) (11). Table 1 (12-27) shows examples of lignocellulosic and non-lignocellulosic residues from agricultural and agro-industrial sectors, with the obtained biogas yields.

The main sources of biogas non-lignocellulosic raw materials are found in processing units of food, beverage, pharmaceutical and agro-industrial sectors (animal manure and slurries) together with a municipal solid waste. Animal manure is a highly favourable substrate for biogas production because it reduces CH₄ and N₂O emissions (28). However, due to low dry matter content (<10%), it is almost always mixed with lignocellulose residues to increase dry matter and the C/N ratio, making the bioprocess more efficient and economically profitable (29, 30). The chicken manure is problematic, not only because of wood chip fractions used as bedding material but also because of high nitrogen, uric acid and protein concentrations, which is in correlation with NH₃ concentration and process inhibition (31). The composition of different non-lignocellulosic residues generated by urban food processing and agro-industrial sectors is shown in Table 1.

Energy crops are cultivated feedstock (grains, silage and grasses) composed of cellulose, hemicellulose, lignin and monosaccharides like glucose, fructose and sucrose, fructans, extractives and pectins (32). They are primarily cultivated for biogas production and can be grown on soils that are not intended or appropriate for food production. They can be digested either alone or in co-digestion with other materials. Energy crops should comply with the following characteristics: high production yield (dry matter per ha), high methane yield ((V(CH₄)/m(VS))/(m³/kg)), low energy input for the cultivation of the plant, harvesting, and processing; low expenses; low content of contaminants; cultivation without using pesticides, herbicides and fertilizers; low nutrient demand; and lastly short growing season (33). Still, the production, harvesting time, nutrient composition, conservation and pretreatment technologies have to be improved for each. They can be divided into four categories: (i) sugar-based crops (sugar cane and beet, sweet sorghum), (ii) starch-based crops (wheat and corn), (iii) lignocellulose-based crops (fodder grass and switch grass), and (iv) woody (miscanthus, willow, poplar and oak). Grains, maize and grass are the most commonly cultivated energy crops with a high net energy yield per hectare (34). Recent studies on the production of biogas from energy crops point out dedicated energy crops specifically grown for energy and fuel production. These include corn and sugar cane, non-food crops as poplar trees and switchgrass (35). Biogas and crop yields with the calculated energy potential of diverse energy crops are listed in Table 2 (15, 34).

Simultaneous conversion of two or more substrates is necessary to improve the yield of anaerobic digestion, C/N ratio, bioreactor stability and obtain a well-balanced content of nutrients and buffer capacity. Therefore, animal manure is often mixed with carbon-rich substrates such as lignocellulose to provide a better nutritional balance in the AD system (36). Advantages of co-digestion include: (i) increased biogas and methane production compared to single substrates treated individually (37), (ii) better stability of digestion and homogenization (38), (iii) avoiding the use of additional fertilizers (39), (iv) greenhouse gas emission and odour reduction (38), (v) dilution of potentially toxic chemicals like ammonia and thus improving the stability of digestion (40), and (vi) avoiding the inhibition of the AD caused by the VFAs accumulation and a decrease in pH value (41).

The performance and efficiency of the AD, that is, the growth and activity of anaerobic microorganisms, especially methanogens, has been affected by some notable parameters monitored during the bioprocess. These parameters include temperature, pH, volatile fatty acids (VFAs), total (TS) and volatile solid (VS) concentration, organic loading rate (OLR), hydraulic retention time (HRT), solid retention time (SRT), toxic compounds, C/N ratio, CO₂ pressure, mixing and shear stress. It is essential to maintain these parameters in a range optimal for microbial growth and activity in order to decrease the risks of process failure (42).

The AD can be performed under three different temperature conditions: psychrophilic (<25 °C), mesophilic (25-55 °C) and thermophilic (55-70 °C). Under psychrophilic conditions, the conversion efficiency of VS to biogas is lower than under the other temperature regimes. Low temperatures correlate with VFA accumulation, a slightly higher fouling rate and significant methane loss (10). On the other hand, under thermophilic conditions the shortest HRT, higher microbial metabolic rates, degradation rates of organic matter, and the highest organic loading capacity have been established. Despite that, the high temperature raises NH₃ concentration, resulting in unstable AD and inhibition. In addition, some equipment (pumps, gasholder and mixers) can be destroyed by high temperatures (1). The mesophilic process does not require any specific care related to the selection of materials. It generally runs smoothly and requires little thermal energy to maintain the stable bioprocess temperature.

The direct relation between the process temperature and the HRT is noticeable. The minimum retention time needed for the psychrophilic stage is 70 to 80 days, for mesophilic 30 to 40 days and for thermophilic 15 to 20 days (43). Temperature variations have negative impact on the microbial consortium involved in the AD process. In the plant design process, it is crucial to prevent rapid temperature changes during plant operation. Temperature changes in bioreactors have to be slow so that the existing microbial populations have enough time to adapt to the new environmental conditions without affecting the efficiency and biogas production.

Optimal growth of different microbial groups involved in the AD happens under different pH values. This is the crucial reason for the separation of acidogenesis and methanogenesis. Methane production takes place within a relatively narrow pH interval, from about 6.5 to 8.5, while acidogenesis from 5.0 to 6.0 (44). As for the temperature, drastic changes in pH values inhibit the AD process. The solubility of CO₂ in water and the formation of carbonic acid is lower at higher temperature, which means that the pH value in thermophilic bioreactors is higher than in mesophilic ones. The pH value can be controlled by adding the NaOH and NH₃ or through the bicarbonate buffer system. The excess of Na⁺ ions, concentrations >3.5 g/L, leads to bioprocess inhibition (45). In addition, the pH value inside the bioreactor depends on the partial pressure of CO₂ and on the alkaline and acid concentrations in the liquid phase.

In microbial cells, CO₂ can participate in the metabolism as an intermediate, carbon donor, electron acceptor or final product of metabolic reactions. It can also contribute to the medium buffer system through the carbonate equilibrium. The response of microbial cells to the CO₂ partial pressure in the environment depends on its concentration. The high CO₂ partial pressures (4-30 MPa at 20-50 °C) used for sterilization are related to the cytoplasm acidification, inactivation of different enzymes and cell disruption. The lower CO₂ partial pressure (0.01-1 MPa) is usually related to the decreased intracellular pH and therefore reduced microbial activity of denitrifying bacteria when dissolved CO₂ concentrations were up to 30 g/L (46). Under these conditions, elevated CO₂ partial pressure can be related to the direct inhibition of carbon metabolism, enzyme activities and substrate consumption in order to increase buffer substance concentrations and to prevent pH drop (47, 48). The addition of CO₂ at atmospheric pressure in AD bioreactors coupled with stoichiometric H₂ supply is related to the increased CH₄ production due to enhanced hydrogenotrophic methanogenesis (49). CO₂ can be indirectly converted into CH₄ through homoacetogenesis affiliated with acetoclastic methanogenesis. This approach was used to explain the enlarged CH₄ production after direct CO₂ injection in pilot-scale anaerobic food waste digestion and two-stage anaerobic sewage digestion (50). Higher CO₂ partial pressure is also connected to the changes in microbial population diversity, activity, structure and interactions (51). Furthermore, it also has an impact on the propionate and butyrate accumulation that are correlated with AD disturbance through the pH changes, increased OLR and non-harmonized acidogenesis and methanogenesis (46).

VFAs are intermediate compounds (acetate, propionate, butyrate, lactate) produced during the hydrolysis and acidogenesis stages of the AD process. A reliable factor for the AD stability estimation is the VFA concentration, which is determined by their production and consumption rates, presence of inhibitors, temperature instability, loading rate and feedstock characteristics. Accumulation of these acids, more than 1.5-2 g/L, can notably reduce the pH and afterwards inhibit the methanogenesis (52). Co-digestion with other raw materials or using a two-stage digestion system are suggested to prevent the inhibition of biogas production by VFA accumulation (53). Different analytical methods have been performed to measure VFA concentration, like titration, photometric, colorimetric and chromatographic methods, steam distillation, alkalinity or buffering capacity measured in mg/L of CaCO₃, infrared spectrometry, high-pressure liquid chromatography (HPLC) or gas chromatography (GC).

The toxic compounds can be brought into the bioreactor together with the feedstock (oil, grease, phenols, antibiotics, heavy metals and volatile aromatics) or generated during the process (NH₃, H₂S, long chain fatty acids and VFAs) (54). The toxicity level of some inhibitors depends on their concentration and pH value in the bioreactor. The NH₃ is considered toxic at pH>7.0, while H₂S shows its inhibitory effects at pH<7.0 (55). The temperature influences the equilibrium between the NH₄⁺ ions and free NH₃. Therefore, NH₃ inhibition can be more pronounced under thermophilic than mesophilic conditions due to enlarged concentrations of free ammonia nitrogen (FAN) at higher temperatures (56).

The C/N ratio should be continuously controlled and monitored to achieve a high degradation rate of organic materials and consequently bioprocess potential. The C/N ratio is correlated with the nutrient composition of the substrates. An example of a balanced C/N ratio is co-digestion of animal manure that has a low C/N ratio and carbon-rich substrates such as silage (36). Above- or under-optimal C/N ratio is related to the unfavourable impact on the rate of methane production and activity of microbial culture (1). The optimal C/N ratio for the AD is in the range between 20 and 30 (57). Besides C/N ratio, phosphorus and sulphur are also essential to ensure maximal growth and activity of the microorganisms involved in the AD. The optimal ratio of these macronutrients is suggested to be C/N/P/S=600:15:5:1 (58).

Total solids (TS) represent the total content of organic and inorganic compounds in the substrate while volatile solids (VS) represent the content of the organic compounds:

The TS and VS mass fractions (in %) of the substrates give useful information about the biogas yield that is potentially produced. Moreover, the efficiency of mechanical parts such as pipes, cutters, pumps and mixers, selection of bioreactor as well as overall AD process relies on the TS mass fraction. In other words, the TS of the substrate organic matter and TS during the process are the two main factors for selecting the type of system and bioreactor design (59).

Organic loading rate (OLR) is the operational parameter that indicates how much organic dry matter (kg) can be introduced in the bioreactor per working volume V (m³) and time t (day) (60) as per the following equation:

The OLR depends on the type of the substrate and bioreactor and should be increased slowly by starting at the OLR as low as 0.5 kg/m³/day and by giving enough time to the microorganisms involved in the AD process to adjust (61). An appropriate OLR in the continuous stirred tank reactors (CSTR) with mesophilic conditions varies between 3 and 5 kg/m³/day depending on the type of the substrate (62). Excessively high OLRs boost hydrolysis and acidogenesis phase performance, leading to high VFA production, acidification and inhibition of methanogenesis (42). Higher OLR is preferred from the technical point of view since smaller bioreactor would be required. In this case, the above-mentioned limitations related to high OLRs can be overcome by using two-stage anaerobic processes (61).

Hydraulic retention time (HRT) is an average period of time (t/day) at which the substrate stays inside the bioreactor and solid retention time (SRT) is a retention time of solids in the bioreactor:

Since the HRT regulates the rate of bioconversion, it has to be long enough to avoid biomass washout. In other words, the number of microorganisms removed with the effluent should not be higher than the number of produced microorganisms per litre. The main parameters for adjusting HRT and SRT that have to be considered are the substrate composition and the AD temperature. The HRT and SRT are correlated with the economic aspects of biogas plants, meaning shorter retention times are associated with lower bioreactor volume as well as with lower capital and maintenance costs (63, 64). However, from a microbial point of view, shorter time can decrease substrate affinity, increase the risk of biomass washout, and lower CH₄ yields (44). Retention time values can range from a few hours up to 30 days, as reported by Jeong et al. (63). The optimal value of HRT and SRT should be determined from case to case by considering feed characteristics, mixing, sludge properties, bioreactor design and configuration (65).

Anaerobic digestion is carried out in a hermetic tank called digester or anaerobic bioreactor where organic waste is mixed with microbial culture and converted to biogas. There are numerous types of anaerobic bioreactors made of reinforced concrete, carbon and stainless steel, or specially coated steel, brick and plastic. They can be constructed in the shape of silos, troughs, basins or ponds. Their configuration and size depend on the scale of biogas plant (small household installations, large commercial plants), solid content of the feedstock (wet digestion – less than 15% and dry digestion - from 16 to 40% (m/m) on dry matter basis), temperature of digestion (psychrophilic, mesophilic, thermophilic), cultivation manner (batch, continuous), and number of stages (single-stage, two-stage, multistage).

The fundamental requirements of bioreactor construction design are to shorten the start-up period, reduce washout of active biomass, be easily adapted for variations in feedstock content, ensure process stability and produce a high volume of biogas. The choice for the type of bioreactor has to take into account some aspects including cost of construction, operation and energy consumption, abilities for effluent and digestate disposal, climate conditions, infrastructural support and waste type and composition. Biogas production recovers energy from organic waste as well as significantly alleviates the impact of the waste on the environment. Cost of installation, operation and maintenance are the main parameters that considerably affect the biogas economics. Many different bioreactor configurations can be used for biogas production and generally they are classified in three main groups: (i) conventional anaerobic bioreactors, (ii) sludge retention bioreactors, and (iii) anaerobic membrane bioreactors (Fig. 1).

The produced crude biogas typically needs to be purified and upgraded for further use. Besides CH₄ (φ=50-70%), biogas contains CO₂ (φ=25-50%), H₂S (0-5000 mg/L), NH₃ (0-500 mg/L), N₂ (φ=0-5%), H₂ (φ<3%), O₂ (φ<1%), volatile organic contaminants (<4500 mg/m³), siloxanes (<50 mg/m³), halocarbons (<200 mg/L) and water vapour (φ<5%), all considered as contaminants (3). These compounds have to be removed prior to biogas usage either in a heat/energy production or as fuel (biomethane). The presence of CO₂ and N₂ lowers the Wobbe index of biogas, which is by definition a quality of combustible gas that allows the air/fuel requirement to be determined (114). Unlike natural gas (35.8 MJ/m³), the calorific value of biogas (21.5 MJ/m³) is smaller due to the large volume of non-combustible CO₂ that increases the compression and transportation expenses, limiting the economic utility of biogas for generating power directly at the production site (115). The O₂ and H₂S are also considered as biogas impurities and the combination of these two forms H₂SO₄, which can corrode pipelines, gas storage tanks, compressors and engines. Only if these impurities were removed, the purified biogas would have high-quality CH₄ and could be used for heat and electricity (115). The use of biogas as fuel or as an alternative to natural gas requires a stricter purification. Thus, biogas must contain CH₄ volume fractions higher than 80-96%, φ(CO₂)<2-3%, φ(O₂)<0.2-1%, γ(H₂S)<5-15 mg/m³, γ(NH₃)<3-20 mg/m³, and γ(methylsiloxane)<5-10 mg/m³ (116). Purified biogas not only helps in reduction in greenhouse gas emissions but also emits fewer hydrocarbons, NO and CO than gasoline or diesel (117).

Methods for biogas purification are mainly based on the following mechanisms: absorption, adsorption and membrane-based separation. Absorption is one of the most extensively implemented technologies for CO₂ and H₂S separation. It is based on the transfer of CO₂/H₂S from the biogas to a liquid scrubbing solution, which can be water, an organic solvent or a chemical solution. Separation principle requires CO₂ to be more soluble in the scrubber than CH₄. It can be carried out in single-pass or multi-stage absorption columns. Adsorption is a process in which adsorbate travels from a gas or liquid phase and selectively binds to the surface of a microporous solid phase. The process can be reversed by decreasing or increasing pressure. Membrane separation works on a principle of the difference in chemical affinity and particle size of different molecules. The choice of appropriate technology is related to the specific biogas requirements, local circumstances, it is site-specific and case-sensitive. In the next section, the main physical, chemical, biological and current novel technologies applicable on industrial scale, with a special focus on the removal of CO₂, H₂S, O₂, N₂, siloxanes, and halocarbons, are described.

During the AD, together with biogas, a digested substrate called digestate is produced. The digestate is an abundant source of nutrients such as nitrogen, phosphorus, potassium, micronutrients and organic matter, which is applied to soils as agricultural biofertilizer (150). Digestate is a high-quality and preferable fertilizer to raw animal manure because of the better homogeneity and nutrient availability, lower C/N ratio, reduced odours and pathogens, and lower risk of soil contamination (151). Leaving the bioreactor, digestate is not fully stabilized as it contains pathogens and drains off heavy metals. Before its use on the soil, it needs proper handling and special treatment (150).

Digestate can be separated in semi-solid (10-30% dry matter) and liquid (5-15% dry matter) fraction. To reduce the costs (storage, transportation and application) a number of techniques are developed to upgrade the liquid digestate: dewatering, composting, drying, granulating, pelletizing, precipitation, filtration, steam evaporation, membrane separation and many others. The choice of upgrading technology depends on market demands and the location of the plant. The quality and stability of the solid fraction can be improved through composting (152). Dried or composed digestate is physically more stable with fewer emissions to air when kept in open storage. The digestate from the AD process can also be co-composted with fresh organic waste or applied as inoculant instead of commercial microbial cultures, which is economically more favourable (153). More applications of the solid and liquid fraction of various digestates have been shown in Table 5 (2, 153-163).

Since the AD is a complex microbial process, a broad range of studies has been recently performed in order to understand the relationship between the microbial community structure, operating conditions and bioprocess performance. Methanogenic archaeal community (or methanogens) has an important role in the last step of anaerobic organic matter degradation, that is, methanogenesis. The detailed understanding of how methanogens interact with other organisms in their environment is still a black box for microbiologists and engineers. The most abundant microbial populations present in the bioreactor can be identified by traditional molecular biology technologies. Recently, the determination of both the most abundant and minor populations can be done by the newly developed sequencing techniques (164). These sequencing techniques provide a valuable tool for the understanding of the microbiological systems and their function along with different aspects of anaerobic degradation and biogas process optimization. They are based on the detection and sequencing of DNA molecules extracted directly from microbial cells. Most of them use amplification of the 16S rRNA gene sequence, which has been specified for each microorganism and considered as a gold standard for identification and presence of bacteria and archaea in the environment. The standard choice of methanogen-specific genetic marker is the mcrA gene for methanogenic microbial populations. The most common, rapid and cost-effective techniques used for the precise detection of methanogenic populations are terminal restriction fragment length polymorphism (TRFLP) (165), denaturing gradient gel electrophoresis (DGGE) (164, 166), quantitative real-time polymerase chain reaction (qPCR) (167, 168) and ion torrent PGM (personal genome machine) technique (169). For a more complete characterization of the microbial community structure, current approach favours metagenomics, also called next-generation sequencing (NGS) technique. Nowadays, several NGS platforms are available and used to improve biogas optimization: 454 pyrosequencing (Qiagen), Illumina MiSeq and HiSeq (Illumina Inc.), SOLiD (Life Technologies), Ion Torrent (Thermo Fisher) and MinION (Oxford Nanopore Technologies) (76). Most studies have been focused on exploring the microbial community inside the bioreactors without taking into account the whole biogas production system (including storage and feeding together with the post-digestion systems). By using next-generation sequencing tools, it is possible to get useful information on functional diversity and gene expression at the community level as well as control the entire bioprocess in a more effective way. Future optimizations of biogas production systems have to be based on the combination of different NGS methods for the study of microbial community dynamics and functional activities.

The development of complex biorefinery systems based on underutilized and low-value biomass must be in focus of the biogas production. Generating energy and materials from these unconventional bio-resources is a promising solution for the environmental protection. Different biotechnological methods can be applied for high-strength organic waste treatment due to their low energy consumption, less residual sludge generation and efficient energy recuperation. The anaerobic digestion (AD) is a useful method of recovering energy from organic waste while diminishing the environmental impact of the waste. The produced biomethane can be used as a replacement of fossil fuels in the heat and electricity production, substitute for natural gas for domestic and industrial use, used in co-generation or as a vehicle fuel.

Many different bioreactor configurations and operational techniques have been developed for AD. The installation and operation costs, as well as maintenance, are factors that significantly influence the economics of biogas production. The choice of the bioreactor type has to consider nature and strength of the feedstock, construction and operation costs, infrastructural support, climatic conditions, availability and the level of skills of the local employees, and energy cost as well as prospects for disposal of effluent and digestate.

Regarding biogas purification and upgrading technologies, physical and chemical technologies are in general at high technology readiness levels, while biological methods are still new and commercially poorly applicable. Nevertheless, the development of biological methods is rapid and gives new perspectives for integrating different forms of renewable energy. Apart from upgrading, they can offer electricity storage advances and decoupling bioenergy production from biomass availability. These methods are able to support a simultaneous elimination of CO₂ and H₂S, with a CO₂ transformation into microalgal biomass for the production of biofertilizers or highly valuable products. The novel upgrading technologies are a prospective alternative for overcoming the challenges of current upgradation technologies. Since the AD is a complex microbial process that uses novel molecular biology and next-generation sequencing, it would be possible to control and regulate the process in a more effective way. It is necessary to pay attention to the whole biogas production system and to understand the relationship among the microbial community structure, operating conditions, bioprocess performance and the post-digestion step.