Zero valent iron catalyzing sulfite improves the quality and quantity of short-chain fatty acids from anaerobic fermentation of sewage sludge

The quality and quantity of short chain fatty acids (SCFAs) production from anaerobic fermentation of sewage sludge is generally constrained by low organics solubilization and poor substrate-availability. This study provided an environmentally friendly strategy with zero valent iron (ZVI) activating sulfite pretreatment to overcome this limitation and advance the sludge anaerobic fermentation. Experimental results showed that the maximal SCFAs production and acetate proportion promoted by 5.7 and 2.75 times respectively at 500 mg S/L and the ZVI/sulfite ratio of 1.25. Mechanism analysis elucidated that the free radicals of S O 4 - · ${\mathrm{S}\mathrm{O}}_4^{-}·$ and ·OH produced from ZVI-sulfite pretreatment accelerated the disintegration of sludge-flocs and decreased the value of α-helix and ratio of α-helix/ (β-sheet + random coil) from 21.64 % and 44.23 % to 17.80 % and 38.67 %, respectively, which provided more easily- and bio- available substance for functional anaerobes. The ZVI-sulfite pretreatment enhanced the ability of functional anaerobes for SCFAs production processes as hydrolysis, acidogenesis and acetogenesis and reduced the SCFAs consumption as methanogenesis, which ultimately promoted SCFAs accumulation. Microbial community illustrated that ZVI-sulfite pretreatment enriched the relative abundance of functional anaerobes related to hydrolysis, acidogenesis and acetogenesis bioprocesses. PICRUSt analysis demonstrated that the pretreatment boosted the functional genes abundance involved in pathway of butyrate and acetate formation. Moreover, the enrichment genes of acetate formation were consistent with the increase of acetate proportion.