Co-pretreatment (calcium peroxide + freeze) to improve anaerobic fermentation of waste activated sludge: Method optimization and mechanism insights

Abstract

The reduction, harmlessness, and recycling of waste-activated sludge (WAS) can be accomplished by anaerobic fermentation, where sludge disintegration is the key speed-limiting process. Co-pretreatment, as a derivative of pretreatment, has attracted more and more attention. However, obtaining optimal co-pretreatment conditions is difficult. Response surface methodology (RSM) can combine statistical analysis with mathematics to optimize the parameters. Therefore, in the current work, RSM optimization for CaO2-freezing co-pretreatment conditions to improve the production of short-chain fatty acids (SCFAs) from WAS was carried out using the Box-Behnken Design based on Design Expert software. We obtained the optimal values of three pretreatment parameters: −18 °C freezing temperature, 48.6 h freezing time, and 0.19 g CaO2/g VSS addition. After 5 days of fermentation, the maximum cumulative yield of SCFAs was 162.34 mg COD/g VSS. CaO2-freezing co-pretreatment promoted the solubilization and release of organic matter in WAS during anaerobic fermentation, which not only provided more organic matter for the subsequent production of SCFAs but also improved the methane production capacity in the anaerobic digestion stage. 16S rRNA genes sequencing revealed that the abundance of SCFAs-producing bacteria (i.e., Macellibacteroides and Proteiniclasticum) was significantly enriched in the fermentation tank after CaO2-freezing co-pretreatment. All in all, this study successfully optimized the co-pretreatment conditions based on RSM, providing new insights and effective solutions for the environmentally friendly and efficient realization of solid waste recycling.