Sulfite serving as a pretreatment method for alkaline fermentation to enhance short-chain fatty acid production from waste activated sludge

Alkaline anaerobic fermentation has been reported to be effective for short-chain fatty acids (SCFAs) production from waste activated sludge, but is still limited by the high alkalis consumptions and long fermentation time. This study offers a novel pretreatment strategy for alkaline fermentation, i.e., using sulfite to treat sludge for 1 d. Experimental results showed the optimal SCFAs production of 324.8 +/- 9.5 mg COD/g VSS (volatile suspended solids) was obtained at 500 mg Sulfite-S/L pretreatment integration with 4 d of pH 9.5 fermentation, which was 16.2-, 2.0- and 2.9-fold of that in the blank, sole pH 9.5 and sole sulfite pretreatment systems, respectively. Acetic acid comprised 56.2% of SCFAs in this integration system compared with 32.4% in the blank and 45.7% in the sole pH 9.5 systems. And the total time of this integration system was only 5 d, whereas the corresponding time was 9 d in the blank and 7 d in the sole pH 9.5 systems. Mechanism investigations revealed that sulfite pretreatment significantly facilitated the solubilization of both extracellular polymeric substances and cell envelope of sludge flocs, as well as the biodegradability of the released substrates. The reactive derivatives from acidified-sulfite (e.g., H2SO3, HSO3-) contributed the most to the enhanced SCFAs production, while the residual SO32- facilitated the conversion of propionate and butyrate acid to acetate during alkaline fermentation. Illumina MiSeq sequencing analyses showed that the abundance of bacteria related to hydrolysis and acidification, especially the acetogenesis, including Proteiniclasticum sp., Alkaliphilus sp., Romboutsia sp., Tissierella sp., etc, were improved in integration system.