Sawdust-Derived Biochar Much Mitigates VFAs Accumulation and Improves Microbial Activities To Enhance Methane Production in Thermophilic Anaerobic Digestion

Abstract

Sawdust-derived biochar (SDBC) was added to an anaerobic digester at 15g/L for thermophilic codigestion under semicontinuous operation for more than 130 days. With a stepwise increase of organic loading rate (OLR) from 1.6 to 5.4 gVS/L·d–1, steady CH4 yields achieved between 462.3 and 500.1 mL/gVS. In contrast, for the control digester (without biochar addition) and another digester added with sewage sludge-derived biochar (SSBC), a steady working condition could only be maintained for about 80 days with OLR and CH4 yields up to 2.7 gVS/L·d–1 and 322.3–337.9 mL/gVS, respectively. The total volatile fatty acids (VFAs) accumulation up to about 30gCOD/L tended to be the threshold to restrain steady CH4 production, but this occurred much earlier for the control and SSBC digesters at much lower OLR. By batch experiments of digestion in a methanogenesis inhibition condition and with propionate or butyrate as the sole substrate, it was found that with SDBC addition, the substrate was oxidized steadily in the 40-day experimental duration, while without SDBC addition, substrate oxidation almost did not occur. SDBC apparently showed excellent electron-accepting capacity for syntrophic oxidation of VFAs. High-throughput sequencing analysis identified that SDBC significantly altered the microbial community structure and brought about enrichment of Tepidimicrobium and Methanothermobacter – two microorganisms with potential capacity of extracellular electron transfer. It could thus be concluded that the main effects of SDBC would be the improvement of VFAs syntrophic oxidation and microbial activities which jointly enhanced CH4 production under thermophilic anaerobic condition. The high specific surface area might have provided favorable condition for microbial attachment growth and certain electro-active organic functional groups on its lignocellulosic structure might have stimulated direct interspecies electron transfer, which needs further study.