Digestibility, microbiome dynamics, and biogas generation in anaerobic digestion with integrated additives and artificial intelligence.

Addition of abiotic and biotic factors as single or combined in anaerobic digestion (AD) improves the substrate hydrolysis, microbial nexus, and enzymatic activity. The effect of a single abiotic (salinity, micronutrients, and conductive material) or biotic factor (bacteria, fungi, and archaea) on AD has been reviewed previously. However, the recent research trend on combined factors strategies (CFSs) (abiotic with biotic or two abiotic or biotic) in AD has not been reviewed. Thus, this review aims to collect, summarize, and compare the recent studies on CFSs in AD. The impact of CFSs on substrate digestion, biomethanation, microbial abundance, enzyme production, and metabolic pathways is elaborated. The time (initial, middle, and end) of CFSs addition, artificial intelligence (AI) approaches, and technoeconomic analysis are also discussed. Biochar + micronutrients were the most studied coupled abiotic factors, followed by magnetite + activated carbon, which are reported to improve the hydrolysis efficiency, biomethane production, and microbial abundance. Combined biotic factors (bacteria + archaea) improved key methanogens (Methanothrix and Methanosarcina). The majorly reported integrated abiotic and biotic factors were biochar + archaea, which promoted microbial growth and direct inter-species electron transport, resulting in increased biomethane production (14.4-79 %). Modelling, prediction, and optimization of CFSs in AD through AI has high potential. CFSs also offer a short payback period and return on investment. However, further research is required to understand the substrate-specific CFSs, addition time, and mechanisms in AD.