Powdered biochar doubled microbial growth in anaerobic digestion of oil

Oil is a desirable substrate for anaerobic digestion (AD) for its high energy recovery potential. However, low solubility of oil and long-chain fatty acids (LCFAs) inhibition hinder microbial growth and further reduce the biogas production in most cases. Biochar is a cheap but reliable additive in AD. Its functions to enrich microbial enrichment and enhance biogas production were confirmed previously, In present study, effects of biochars with different particle sizes on oil AD were investigated under thermophilic and mesophilic conditions. Results from the quantification and space distribution of microbes indicate that powdered biochar (< 5 mu m) aggregated more microbes than granular biochar (0.5-1 mm). Especially in the mesophilic digesters, powdered biochar doubled the number of microbes in the whole digester, which was a satisfactory result of acclimation. In addition, powdered biochar was able to adjust the microbial communities and further increased CH4 production by 13.3% in thermophilic digesters. Granular biochar enhanced the maximum CH4 potential by 32.5% under mesophilic condition, which was most promising from the perspective of energy recovery. Biochar of both particle sizes relieved propionic acid accumulation in thermophilic digesters. Hence, biochar addition might be an ideal approach to elevate energy recovery efficiency by overcoming the barriers of anaerobic microbes to utilize LCFAs.