Effect of a partial substitution of energy crop with agricultural waste on the dynamics of bacterial communities in a two stage anaerobic digester

The two-stage (hydrolytic-methanogenic) anaerobic digestion has been largely used as a reliable alternative to single reactor, for being more efficient in preventing the biogas production failures, due to e.g. pH inhibition and metabolic unpairing of the microbial community. Concerning the various possible substrates, the substitution of dedicated crops with lower cost and environmental impacting food and agricultural waste is considered a priority for both economic and sustainability reasons. Despite their great potential, the change of feeding mix in AD might result in lower energy production, related for example to nutrient imbalance or the increase of the total ammonia nitrogen (TAN) content. The causes of AD system failures can be related to the unpredicted shift in the microbial communities and metabolic chain. Therefore, many studies have investigated and identified the microorganisms involved in the removal of nutrients like phosphorus and nitrogen, and other novel groups such as sulphate-reducing bacteria (SRB), methanotrophic bacteria (Reyes et al 2014), or syntrophic acetate oxidizing (SAO) bacteria, performing the oxidation of acetate to hydrogen and carbon dioxide whose dynamics in the AD process still needs to be fully elucidated. Recent findings supported the thesis that shifts from acetoclastic methanogenesis to syntrophic acetate oxidation (SAO) and hydrogenotrophic methanogenesis may be caused by high TAN concentration (Westerholm et al 2016). In a previous study, we investigated the influence of the slowly replacement of the energy crops with poultry manure litter (PML) in the feeding of a full-scale two-stage biogas plant on microbial community structure and the results disclosed that the change in the digester feeding and in the carbon:nitrogen (C:N) ratio had been key factors in determining the bacterial and archaeal communities (Bellucci et al 2019).