Metabolic effects of Fe0 on simultaneously eliminating excessive acidification and upgrading biogas in mesophilic or thermophilic anaerobic reactor

Excessive acidification and low methane (CH4) percentage are technical challenges restricting anaerobic digestion (AD) of kitchen waste, especially under high organic loading rate (OLR) conditions. Although the contribution of Fe-0 powder to excessive acidification elimination and CH4 yield improvement in mesophilic AD (MAD) has been extensively reported, its impact on the thermophilic AD (TAD) process with more frequent excessive acidification problem is rarely studied. Moreover, the difference in microbial interactions and metabolic pathways between MAD and TAD reactors under Fe-0 regulation was still unclear. In this study, we compared the performance of Fe(0 )dosing on excessive acidification elimination and biogas upgrading quality in both MAD and TAD reactors at different OLRs. The results showed that compared with excessive acidification or low CH4 percentage (less than 70%) in control reactors without Fe0 addition, the CH(4 )yield of MAD and TAD reactors with addition of Fe0 at OLR of 35 gVS/L were 470.88 mLCH(4)/gVS and 270.40 mLCH4/gVS, with higher CH4 composition of 81.32%-82.23% and 83.68%-88.48% respectively. Microbial diversity analysis showed a sharp distinction between MAD and TAD reactors with addition of Fe-0. Analyses of co-occurrence networks and metabolic pathway presented hydrogenotrophic methanogenic pathway, in addition to acetoclastic methanogenesis, was dominant in MAD reactors as results of enhanced interspecies hydrogen transfer between Methanoculleus and some hydrogen producers, especially under high OLR condition. In contrast, syntrophilic acetate oxidization - hydrogenotrophic methanogenesis, which dominated by Methanothermobacter and Alkaliphilus or Pelotomaculum, was the sole methanogenic pathway in TAD reactors. The results will shed lights on the regulation mechanisms of Fe0 in AD of kitchen waste under mesophilic and thermophilic conditions, offering new insight into development of more efficient waste to energy conversion technologies.

Automated summary

Excessive acidification and low methane percentage are challenges in anaerobic digestion of kitchen waste. The addition of Fe-0 powder has been shown to improve these issues in mesophilic AD, but its impact on thermophilic AD and the microbial interactions are unclear. This study compared the performance of Fe0 dosing on acidification and biogas quality in both mesophilic and thermophilic reactors at different loading rates.