# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing
### Treatments of the Solid Fraction
#### 5.2.1 Composting  
The organic material in the solid fraction of the DG is converted to CO\({}_{2}\), biomass, thermal energy (heat), and humus-like end-product [99] by microorganisms under aerobic conditions in the controlled self-heating composite process [98,100,101]. The ability of microorganisms to assimilate organic material depends on their ability to produce the enzymes needed to degrade the composted material. The more complex the composted material, the more comprehensive the enzyme system required. Typically, composting process goes through three phases: mesophilic (usually at 25-40 \({}^{\circ}\)C), thermophilic (lasts from a few days to several months, usually at 40-65 \({}^{\circ}\)C), and cooling and maturation (lasts for several months usually at 10-40 \({}^{\circ}\)C) [99,101]. The length of the composting phases depends on the nature of the OM and the efficiency of the process, which depends largely on the degree of aeration and agitation. At the beginning of composting, the biomass is at ambient temperature and is usually slightly acidic. In the early stages of composting, microorganisms utilize soluble and easily degradable C sources such as monosaccharides, starch, and lipids [99]. This is accompanied by the formation of organic acids formation which causes pH to be lowered (to 5 or less). In the next stage, microorganisms start to degrade proteins, which lead to the release of NH\({}_{3}\) and an increase in pH (to about 8 or 8.5) [101]. Then, more complex compounds such as cellulose, hemicellulose, and lignin are partly converted into humus which is the final product of the humification process. Humification is a process in which compounds of natural origin are partially converted into humic substances (humin, humic acids fulvic acids), which are considered the major reservoir of OC in soils and aquatic environments. In general, immature compost contains a high proportion of fulvic acids and a low proportion of humic acids, while as decomposition proceeds, fulvic acids either decrease or remain unchanged, while humic acids are formed. At the beginning of composting, mesophilic bacteria predominate, but after the temperature rises above 40 \({}^{\circ}\)C, thermophilic bacteria and fungi take over. When the temperature exceeds 60 \({}^{\circ}\)C, microbial activity decreases significantly [99]. Due to the high thermophilic temperatures maintained during the process, pathogens and antibiotics could be effectively removed [102]. After the compost has cooled, mesophilic bacteria and actinomycetes become dominant. In addition to aerobic microbes, anaerobic bacteria could also be found in compost piles. Since all anaerobic bacteria present in the compost are cellulolytic, they may play a significant role in the degradation of macromolecules [99]. The pH decreases slightly during the cooling phase to a range of 7 to 8 in mature compost. Composting can tolerate a wide pH range, from 5 to 8, with a pH between 7 and 8 generally considered optimal [103]. The process can be carried out in a closed (reactor) or open system (open-air or under a roof). Open system composting that is composting in the open air, is the oldest, the best known, and the cheapest composting method in which composted waste is piled with a triangular or trapezoidal cross-section. The piles are aerated by forcing air through the compost mixture by shifting or using blowers or fans. The main thermophilic phase of composting lasts 6 to 12 weeks, which depends on the moisture and oxygen content, C/N ratio, and air porosity of the composted material. Composting in reactors (containers, chambers, tunnels, and closed halls and towers) is quite similar to composting in piles, although it takes place in a closed reactor which allows better control of process parameters and facilitates the maintenance of low emissions to the environment through the ability to capture and purify polluted air [100].