Insights into the mitigative effects of microbial electrolysis cell on anaerobic digestion under high ammonia condition

During the process of nitrogen-rich substrates anaerobic digestion, methane production will be significantly inhibited due to the toxic effects of ammonia on anaerobic microorganisms. Previous study mainly focused on enhancing ammonia-tolerance abilities of anaerobic microorganisms, but were not able to fundamentally change the original characteristics of the excessive ammonia concentration in anaerobic digestion system. In this study, the authors applied 1.0 V voltage into a semi-continuous anaerobic digestor to establish a microbial electrolysis cell (MEC) system, which significantly improved the anaerobic digestion performance under high ammonia conditions. The results showed that the average methane yields in the anaerobic digestion system with voltage applied was about 40% and 110% higher than that in the system without voltage applied when the total ammonia nitrogen (TAN) concentration was 2000 and 3000 mg/L, respectively. Through bioinformation (16 s, qPCR, metagenomics) analysis, it was found that voltage application promoted the abundances of nitrogenmetabolism microorganisms and the expression of functional genes involved in ammonia oxidation (amoA) and denitrification (nirS, nirK) process, thus reducing the TAN concentration in digestion system. In addition, voltage application was also confirmed to maintain the microbial structure of digestate slurry and enhance the expression of key gene in methanogenesis (mcrA). This study provides a new insight into mitigating ammonia inhibition in anaerobic digestion through voltage application, which is expected to solve the problem of inefficient anaerobic digestion under high ammonia condition.