Walnut shell biochar-assistant anaerobic digestion towards carbon-energy recovery from thermal-hydrolyzed excess sludge: Specific illustration in carbon source fate

Biochar enhanced methane production from excess sludge through anaerobic digestion, a renewable carbon-energy alternative to fossil fuels. This study produced a novel walnut shell (WS)-based biochar to enhance anaerobic digestion of excess sludge, which greatly improved the kinetics of endogenous carbon release and biotransformation by 1.26–2.06 times in a short-term digestion (16 d). Considerable particulate carbon sources of 95.17 mg C/g VSS were solubilized from sludge solid, mainly composing of proteins, carbohydrates and various metabolic intermediates. Simultaneously, 76.26 % of the originally released carbon sources were bio-transformed into the carbon-energy in methane form. Correspondingly, the WS biochar contributed to the energy-conversion bacteria screening and microbial community modification, the well-known electroactive microorganisms (e.g. Clostridia, Methanosarcina, etc.) were selectively enriched. The direct interspecies electron transfer (DIET) was established via WS biochar. Owing to the dual driving mechanisms of bacterial metabolism improvement and DIET acceleration, the WS biochar strategy enhanced the carbon migration rate by 2.06 times and reduced the lag phase for methane production by 11.95–53.89 %. An impressive carbon-energy recovery rate of 23.54 % was achievable. [Display omitted] •WS biochar increased carbon release by 16.56 %, converting 62.84 % to carbon- energy.•Remarkable carbon recovery of 23.54 % was achieved in 16-d digestion.•The combined effects of enhanced bacterial metabolism and increased DIET speed•“wastewater → sludge → energy recovery” pathway clarified the carbon sources fate.