Effect of bioaugmentation combined with activated charcoal on the mitigation of volatile fatty acids inhibition during anaerobic digestion

Volatile fatty acids (VFAs) accumulation is a common problem in anaerobic digestion, which is constraining the process. In this study, bioaugmentation was initially adopted as a measure to tackle the acidification problem in a continuous anaerobic digester. However, after bioaugmentation stopped, the promotive effect of bioaugmentation was hard to sustain. In comparison, the combination of bioaugmentation and Activated charcoal (AC) enabled a better reactor performance. The positive impact of AC on recovery from acidification in sequential batch reactors was thus investigated by feeding either propionate or acetate. The results implied that the addition of AC significantly increased the degradation rate of propionate, thus accelerating the biogas production rate (104 ~ 371%). Moreover, the role of AC is mainly in the immobilization of microorganisms to form bio-AC owing to AC's physical characteristics (high-surface-area and large number of pores enables adsorption of microorganisms). The biofilms on the AC decreased the distance between the microbes and delayed the washout effect of microorganisms, enabling a longer-lasting effect of bioaugmentation. The 16S-rRNA analysis showed that the addition of AC led to the enrichment of syntrophic microbial guilds (syntrophic VFAs oxidizing bacteria and hydrogenotrophic methanogens). The potential function analysis (PICRUSt2) indicated the significant enhancement of formylmethanofuran dehydrogenase (EC 1.2.7.12) and coenzyme F₄₂₀ hydrogenase (EC 1.12.98.1) by AC. The results suggested that the addition of AC promoted interspecies electron transfer.