Sewage sludge digestion beyond biogas: Electrochemical pretreatment for biochemicals

Low economic gains from biogas drive research on shifting to volatile fatty acid (VFA) production during anaerobic sludge digestion. pH control and methanogenesis inhibition are widely used strategies for VFA production via anaerobic digestion of sludge. However, these strategies require perpetual dosing of chemicals, increasing cost and operation complexity. Here, we applied electrochemical pretreatment (EPT) (12 V/30 min) for VFA production during anaerobic sludge digestion. The underlying mechanisms of the VFA production induced by EPT were explored systematically through analyses of the changes in the EPT operation parameters, the sludge characteristics, and the microbial community structure and functional enzymes involving in the subsequent sludge digestion. EPT with carbon-based electrodes selectively inhibited methanogenesis by down-regulating heterodisulfide reductase without affecting enzymatic acidogenesis and hydrolysis, resulting in accumulation of VFAs (up to 389 ± 12 mg acetic acid equivalent/L). Propionate and acetate were, respectively enriched to 89 and 75% of the total VFAs after carbon- and graphite- EPT. Titanium-EPT produced lower levels of VFA; instead, biogas yield increased by ∼20%. We anticipate that EPT will advance VFA recovery from diverse organic wastes to meet the global challenge of resource supply and waste management.