Performance and mechanism of calcium peroxide reducing H2S production from sludge anaerobic fermentation

Calcium peroxide (CaO2) addition is widely regarded as a high-efficient sludge treatment method to degrade pollutants and enhance resource recovery. In this study, it was further found that CaO2 addition changed sulfur distribution during sludge anaerobic treatment, which promoted sulfur flow from sludge solid to liquid phase but significantly inhibited toxic hydrogen sulfide (H2S) gas production. With CaO2 addition increased from 0 to 0.25 g/g VSS, the accumulated H2S yield decreased from 314.6 × 10−4 to 61.3 × 10−4 mg/g VSS while the accumulated SCFAs and hydrogen yields increased from 46.2 to 266.5 mg COD/g VSS, and 0.9 to 10.2 mL/g VSS. Mechanism analysis showed that the released OH– and oxidizing substances (e.g., ·OH and ·O2–) firstly destroyed functional groups and hydrogen bonding networks contained in sludge EPS, with the contents of C=O group, β-sheet and α-Helix being reduced by 24%, 19.7%, and 23.2%, respectively. This caused the cracking of sludge EPS and releases of soluble metal ions (e.g., Al, Mg, and Fe). The released metal ions in alkaline environment promoted the transformation of H2S gas and dissolved sulfide to metal sulfide, which was one important reason to reduce H2S gas release. Then, the free radicals further attacked sludge cells without EPS protection and caused cell lysis. Flow cytometry analysis showed that 0.25 g/g VSS CaO2 reduced the rate of living cells from 90.7% to 81.4%. Microbial analysis unveiled that CaO2 reduced the content of S2− producers (e.g., Desulfobulbus and Desulfovibrio) and the expression of related genes (e.g., PepD and Sat). Moreover, the activities of S2− producers, e.g., methionine lyase and adenosine-5′-phosphosulfate reductase also reduced by 41.3% and 64.8% with 0.25 g/g VSS CaO2 addition. These findings provide new insights into CaO2-based sludge treatment and might have significant implications for sludge treatment.