Homoacetogenesis is altering the metabolic pathway of acidogenic microbiome and combating volatile fatty acid accumulation in anaerobic reactors

Volatile fatty acids (VFAs) (especially propionate and butyrate) accumulation is known to be bottleneck limiting optimal use of anaerobic digestion (AD). Previous studies targeting optimization of the AD process, mainly focused on strategies for enhancing the degradation of propionate and butyrate. In this study, an alternative method to reduce risk of accumulation of higher VFA, was to promote homoacetogens, which are generating small molecules (such as formate, acetate) that can be easily utilized by methanogens by methanogens. Enrichment of homoacetogens would be advantageous for AD process. Two AD reactors operated at two different temperatures (35 & PLUSMN; 1 degrees C and 24 & PLUSMN; 1 degrees C) inoculated with heat-pretreated inoculum were used for this study. After 182 days operation, the quantity of homoacetogens, measured by the number of fhs gene, representing homo-acetogens, reached 1.24 x 1014 copies/g wet sludge in ambient temperature (24 degrees C) reactor, which was much higher than it in mesophilic (35 degrees C) reactor (1.89 x1012 copies/g wet sludge). The higher copies at ambient temperature compared to mesophilic, was because homoacetogens were surviving at this temperature. At ambient temperature, the acidification rate increased from 30.57 % to 34.25 %, with the composition of acetate and formate of total VFAs raised from 34.86 % to 55.70 %. It meant that enrichment of homoacetogens was resulting in re-routing carbon to methanogenic precursors, which had a significant positive effect on reducing the risk of VFAs accumulation. The genus Acetobacterium was the main homoacetogen enriched in 24 degrees C reactors. The abundance of Acetobacterium had a positive correlation with acetate concentration while it had a negative correlation with temperature. This study provides a new approach for mitigation the risk of VFAs accumulation.

Automated summary

The aim of this study was to reduce the risk of VFA accumulation and optimize anaerobic digestion by promoting the enrichment of homoacetogens. It was found that homoacetogens had a higher abundance at ambient temperature, which effectively reduced the risk of VFA accumulation.