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

This review relates the kinetics of anaerobic digestion (AD) to energy outcomes, including typical ranges of methane yields and volumetric methane productivities (down to hourly g L⁻¹ h⁻¹ scales relevant for industrial plants). It further translates these relationships into practical control principles that support stable, high methane productivity. Evidence spans substrate selection and co-digestion with emphasis on carbon/nitrogen (C/N) balance, pretreatment strategies, and reactor operation, linking process constraints with operating parameters to identify interventions that raise performance while limiting inhibition. Improving substrate accessibility is the primary step: pretreatment and co-digestion shift limitation beyond hydrolysis and allow safe increases in organic loading. Typical mesophilic operation involves hydraulic retention times of about 10–40 days for food waste and 20–60 days for different types of livestock manure and slowly degradable energy crops, with stable performance achieved when the solids retention time (SRT) is maintained longer than the hydraulic retention time (HRT). Stability is further governed by sustaining a low hydrogen partial pressure through hydrogenotrophic methanogenesis. Temperature and pH define practicable operating ranges; meanwhile, mixing should minimise diffusion resistance without damaging biomass structure. Early-warning indicators—volatile fatty acids (VFAs)/alkalinity, the propionate/acetate ratio, specific methanogenic activity, methane (CH₄)% and gas flow—enable timely adjustment of loading, retention, buffering, mixing intensity and micronutrient supply (Ni, Co, Fe, Mo). In practice, 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 operational choices to energy yield in combined heat and power (CHP) and biomethane pathways. 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. Priorities are set for adaptive control, micronutrient management, biomass-retention strategies, and standardised monitoring, providing a coherent route from kinetic understanding to dependable energy performance and explaining how substrate composition, pretreatment, operating parameters, and kinetic constraints jointly determine methane and energy yield, with particular emphasis on early-warning indicators.