New insights into the factors influencing methanogenic pathways in anaerobic digesters.

Introduction: Anaerobic digestion integrates waste treatment, energy generation, and nutrient recycling, producing methane mainly through acetoclastic (AM) and hydrogenotrophic methanogenesis (HM). Methanogenic pathway management can improve biogas productivity and quality. The balance between pathways is influenced by environmental and physicochemical conditions, with conflicting results on the effect of different factors often reported. This systematic review aims to clarify the influence of various parameters on methanogenic pathways in anaerobic digesters.

Methods: Literature search was conducted in the Web of Science and Scopus databases. The effects of different parameters on the predominant methanogenic pathway were evaluated using Kruskal-Wallis tests and Spearman's rank correlation.

Results: Thermophilic temperatures and high free ammonia nitrogen concentrations (>300 mg L^-1) increase HM, with a strong combined effect of these variables. Conversely, under moderate temperature and ammonia concentrations, the primary feedstock influences the methanogenic pathway, with algae biomass, pig manure, and food industry wastewater showing the lowest contribution of hydrogenotrophic methanogens. pH effect varied with temperature, with acidic and alkaline pH favoring HM in mesophilic and thermophilic digesters, respectively. Furthermore, higher levels of volatile fatty acids (>2000 mg L^-1), carbohydrates (>10 g/L) and lipids (>10 g/L) also appeared to favor HM over AM, while most metals - especially Cr, Se and W - promoted AM.

Conclusion: This study emphasizes the role of various factors in methanogenic pathway selection, highlighting the impact of previously overlooked parameters, such as inorganic elements and organic matter composition. These insights are essential for understanding the methanogenic pathway balance and optimizing biogas processes.