Performance and mechanism of biochar pretreatment on improving short-chain fatty acids production from anaerobic fermentation of waste activated sludge

As a technology to recover resources from sludge, anaerobic fermentation to produce short-chain fatty acids (SCFAs) faces the following problems: difficult dissolution of organic matter and slow electron transfer. This study proposes a novel technique for energy recovery from waste activated sludge (WAS), i.e., employing biochar pretreatment in anaerobic fermentation to enhance SCFAs production. The results showed that the maximum SCFAs production reached 348.61 mg COD/g VSS, with acetic acid making up 44.4 % at an optimal condition (i. e., 2.0 g biochar/g VSS). The value of SCFAs production represents an increase of 9.6-fold compared to the control group. Mechanism exploration reveals that biochar pretreatment disrupted the extracellular polymers (EPS) and cell wall structures through its strong alkalinity, thereby promoting the release of abundant biodegradable organic matter for SCFAs production. Meanwhile, biochar enhances electron transfer during anaerobic fermentation, which further promotes SCFAs production. Conversely, it suppressed the bioactivity of Coenzyme F420. Microbial community analysis confirms that biochar pretreatment significantly enriched several hydrolytic bacteria and acid-producing bacteria, such as Tissierella sp. and Proteiniborus sp. Importantly, the relative abundance of Proteiniclasticum sp. with electron transport ability increased from 0.04 % to 6.72 %. FTIR spectroscopy shows that biochar carried surface functional groups involved in electron transport (i.e., aromatic groups C-H and C-OH) and surface functional groups with strong redox activity (i.e., C--O, C--C). This work confirmed the feasibility of biochar pretreatment to enhance SCFAs production and revealed possible mechanisms, providing a new strategy for effective sludge treatment.