Temperature is one of the key parameters affecting the AD because it influences the microbial growth, enzymes activities, substrate characteristics, and consequently the methane yield. Although conventional AD is performed at mesophilic temperatures of 35–37°C (Forster-Carneiro et al., 2008), thermophilic AD has the advantages of higher specific growth rate, a faster metabolism, higher load-bearing capacity, and consequently results in a higher methane yield (Kim et al., 2011). Still, it should be noted that AD is affected negatively if the temperature is not constant. Compared to mesophilic AD, thermophilic AD is more sensitive to temperature change and it takes longer to adapt to a new temperature, as thermophilic methanogens have lower methanogenic diversity than mesophilic methanogens. Thermophilic AD has a higher risk of acidification, particularly when the biomass is rich in protein, which may consequently inhibit biogas production. This problem can be solved by the acclimatization of the inoculum. This problem can be solved by the acclimatization of the inoculum.

Due to the complex composition of biomass, it is difficult to reach a steady state during AD. Methanogenesis occurs in the pH range of between 6 and 8.5 with an optimum pH range of 7.0–8.0. It is dramatically inhibited when the pH decreases below 6.0 or increases above 8.5. The imbalances in pH are created especially during the bioconversion of proteins and lipids, ie, by the accumulation of free ammonia and VFA. This in turn affects the functions of the extracellular enzymes and the hydrolysis rate. Nutrient-rich biomasses can be easily acidified into VFA by fermentative microorganisms. VFA accumulation results in a decrease in pH, and so might inhibit the methanogenic system.

Anaerobic microorganisms can metabolize different organic compounds (carbohydrates, proteins, lipids, etc.). The methane content of the biogas mixture depends on the biomass, particularly the oxidative state of carbon found in the feedstocks. The more reduced the carbon is, the higher the content of biogas in methane (Gujer and Zehnder, 1983). The C/N ratio of the feedstock should be well-balanced. A C/N ratio of 20–30 has been reported to be ideal for AD (Gomez et al., 2005). Unfortunately, in most cases the feedstock's C/N ratio is out of the range. In such a situation, the codigestion of selected feedstocks is often applied in order to adjust the C/N ratio and stabilize biogas production.