*4.2.2 Temperature control*

The temperature of MSW influences the success of the digestion process, as the activities of the anaerobes causing waste decomposition are temperature dependent. Optimum performance of an aerobic digestion system is affected largely by the operating temperatures of the reactor. There are three general ranges of temperature each favoring a specific type of microorganisms including; psychrophilic: about 10–20°C, or less than (<30°C), mesophilic: about (30–40°C), and thermophilic: about (50–55°C), or may reach 60°C [50, 106]. Previous studies have shown that anaerobic bacteria exhibit the highest activity within the mesophilic and thermophilic ranges [107]. Extreme cases of either very high or very low temperatures kill the anaerobes, hence inhibiting the whole AD process [96]. The rate of decomposition and gas production is sensitive to temperature, and, in general, the process becomes more rapid at high temperatures [3, 102]. The optimum temperature is 35°C [101, 108]. There are a couple of factors, which contribute to heat generation or transfer in a digester including process reaction, mixing (impellers), as well as heat exchangers (hot water or steam) [50].

## *4.2.3 Feedstock composition and nutrients*

A variety of digester kinds exists for the anaerobic treatment of organic wastes. Some diversity of biomass feedstocks could be used by anaerobic digestion techniques such as biowaste, agricultural crops, human waste, municipal sewerage and animal manure among others. The quality and quantity of the biogas yield is determined via nature of the feedstock used. Furthermore to the biogas yield, biomass generates vital nutrients and carbon that promote the sustainable growth of the microbes [96, 107, 109]. The selected kind rely on operational factors, involving the nature of the waste to be treated, e.g., its solid content. The Oregon Department of Energy [110], in its classification of kinds of digester, elucidates that "a plug-flow digesters are appropriate for ruminant animal dung having solid concentrations of 11–13%"; "a complete-mix digester is appropriate for manure that is 2–10% solids"; and "a covered lagoon digester" is used for liquid manure of less than 2% solids. The amount and kind of solid contents of the waste they considered were such that the

wastes are able to flow on their own or forming slurries with water and finally flowing, and thus can be used in a continuous operation [102].
