Improving biomethane fermentation through trace elements-driven microbial changes: Different effects of Fe0 combined with Co/Ni

Anaerobic digestion at high organic loading rate (OLR) is prone to acid inhibition that reduces methane production. In this study, effects of Fe0 (+Co/Ni) on suppressing acid inhibition and improving methane production were studied using vegetable waste as substrate at OLR of 3.0 gVS/L/d for 160 days. Results indicated additional use of Co/Ni with Fe0 did not further improve the effects of Fe0 and use of Ni suppressed considerable the effects of Fe0. These differences were supported by microbial changes caused by addition of Fe0 (+Co/Ni). Analysis of 16S rRNA genes revealed microbial community without Ni addition was more stable, a factor largely influenced by pH and ammonia nitrogen levels. Relative abundances of Methanosarcina and Methanosaeta were approximately 60% in Fe0-Ni and Fe0-Co-Ni treatments, and 90% in control, Fe0 and Fe0-Co treatments. But linear discriminant analysis effect size and network analysis indicated Methanosaeta and Methanosphaera, not Methanosarcina, were key methanogens. They functioned with acid producing Acetobacterium, and ammonia nitrogen producing Aminobacterium to convert volatile fatty acids to methane in Fe0 treatments. Actinomyces were key microorganisms that hindered methanogenesis, especially in Ni addition treatments. This work suggests addition of Fe0 in anaerobic digestion at high OLR is theoretically feasible for engineering applications.