Kinetics and Energy Yield in Anaerobic Digestion: Effects of Substrate Composition and Fundamental Operating Conditions

Temperature and pH define practicable operating ranges; meanwhile, mixing should minimise diffusion resistance without damaging biomass structure. Early-warning indi-cators—volatile fatty acids (VFAs)/alkalinity, the propionate/acetate ratio, specific meth-anogenic activity, methane (CH4)% and gas flow—enable timely adjustment of loading, retention, buffering, mixing intensity and micronutrient supply (Ni, Co, Fe, Mo). In prac-tice, robust operation is generally associated with VFA/alkalinity ratios below about 0.3 and CH4 contents typically in the range of 50–70% (v/v) in biogas. The review consolidates typical feedstock characteristics and biochemical methane potential (BMP) ranges, as well as outlines common reactor types with their advantages and limitations, linking opera-tional choices to energy yield in combined heat and power (CHP) and biomethane path-ways. Reported pretreatment effects span approximately 20–100% higher methane yields; for example, 18–37% increases after mechanical size reduction, around 20–30% gains at 120–121 °C for thermal treatments, and in some cases nearly a two-fold increase for more severe thermal or combined methods.