Effects of increasing phenyl acid concentrations on the AD process of a multiple-biogas-reactor system

Anaerobic digestion (AD) of organic waste materials is an ecological way to produce biogas. However, organic wastes can comprise high amounts of aromatic compounds like phenyl acids (PA) which can cause process impairments. Here, three of six thermophilic, semi-continuously fed biogas reactors were exposed to increasing PA concentrations at two different input speeds (consecutive experiments). Biochemical (gas, volatile fatty acid (VFA) and PA analyses) as well as molecular biological analyses (qPCR and amplicon sequencing) were done to monitor the AD process. Biogas production was completely inhibited at the end of both approaches. An increase in acetate and propionate concentrations were the earliest signs of process impairments. Methanosarcina spp. was the dominant methanogen during biogas production; however, their absolute abundances were low in impaired reactors. Syntrophic VFA oxidation and hydrogenotrophic methanogenesis became relevant in impaired but still biogas producing reactors. The combination of biochemical and metagenomic data and the consideration of input speed using six parallel, lab-scale bioreactors showed that a decrease in biogas production is a time-delayed sign of process impairments and VFA monitoring on-site would be useful to early counteract impeding restrictions. Moreover, this study confirmed that syntrophic VFA oxidation is relevant in stressed AD systems and that further knowledge on these cooperations is pending.