# Digestate Management and Processing Practices: A Review
## 3 Anaerobic Digestion  
AD is a biochemical process in an anaerobic digester in which a consortium of microorganisms converts complex OM into biogas and DG under controlled conditions in the absence of oxygen [34]. It is a widely used technology by which the volume of different types of biomass, such as the organic fraction of municipal and industrial waste, energy crops, agricultural wastes, and forestry residues, is effectively reduced while renewable energy is recovered [35; 36; 37; 38; 39; 40; 41; 42; 43]. During digestion, three groups of microorganisms have been identified in anaerobic digesters where they grow optimally and survive and function in a wide range of temperature regimes [44]. These include psychrophilic, mesophilic, and thermophilic microorganisms. The psychrophilic microorganisms grow and function in a range of 10 to 25 \({}^{\circ}\)C and require a longer retention time of up to 70 to 80 days to convert biomass into biogas. The mesophilic microorganisms grow and operate in a range of 30 to 40 \({}^{\circ}\)C and require 30 to 40 days to convert biomass to biogas. The thermophilic organisms grow and function in a range of 45 to 60 \({}^{\circ}\)C under which they require a shorter retention time to generate biogas of 15 to 20 days [45]. Although the mesophilic temperature regime is most commonly applied in biogas plants, the thermophilic regime is often considered more useful as it offers several advantages, such as higher microbial diversity, faster and higher methanogenic activity, higher organic loading rate (OLR), accelerated hydrolysis and acidogenesis of the recalcitrant substrate, higher biogas production, higher rates of pathogen inactivation compared to the mesophilic regime. However, the thermophilic regime is a more energy-consuming and less stable process, producing biogas with lower methane (CH4) content [44; 45; 46]. The process of biogas production is carried out by the combined action of three physiological groups of microorganisms: hydrolytic-acidogenic bacteria and most likely fungi, syntrophic acetogenic bacteria, and methanogenic archaea. The process is divided into four main steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 1). The first step (hydrolysis) is performed by the hydrolytic bacteria and possible fungi and in this step, the conversion of polymers into soluble monomers occurs which are further converted to short-chain fatty acids, hydrogen, carbon dioxide (CO2), and alcohols by various fermentation reactions. During the steps of hydrolysis and acidogenesis polymers are converted into soluble monomers while acidogenesis polymers are converted to soluble monomers which are further converted to short-chain fatty acids, hydrogen, and carbon dioxide (CO2), and alcohols by various fermentation reactions. After the hydrolysis and acidogenesis comes acetogenesis in which the acids and alcohols are degraded by acetogens to form CO2, hydrogen, and acetate which are used by methanogens during the last step (methanogenesis) to produce biogas. Methanogenesis is the last step in the anaerobic food chain which is catalyzed by methanogens. In methanogenesis, products that are formed in previous steps are converted into CH4. Acetogens and methanogens are in syntropy, meaning that they cannot operate without one another and when they do operate together they exhibit certain metabolic activities which they could not achieve on their own. A large number of microorganisms that participate in the process of biogas production have been found but not all of them have been identified (Table 1). Because of requirements for low oxygen concentration and because of a high degree of commensalism and mutualism in microorganism communities the isolation, cultivation, and identification of those microorganisms are rather difficult.