Optimizing biological pretreatment and bioleaching processes in sewage sludge: Heavy metal removal mechanisms and EPS-dependent dewaterability enhancement

Abstract

To overcome the inhibitory effect of organic matter on sludge bioleaching, this study systematically evaluated the impacts of two pretreatment methods (aeration and anaerobic digestion) on subsequent bioleaching performance. Results demonstrated that aeration pretreatment significantly accelerated the acidification process, enhanced overall heavy metals (HMs) removal efficiency, and effectively minimized phosphorus loss. Following the selection of aeration pretreatment, three bioleaching processes (direct, aerated, and anaerobic bioleaching) were compared. Aerated bioleaching achieved the fastest acidification (pH 2.08 in 6 days) and optimal removal of Cu²⁺, Zn²⁺, and Pb²⁺. Microbial community analysis revealed that the bioleaching process greatly changed community abundance and diversity, while particularly enriching key bioleaching genera such as Acinethiobacillus and Alicyclobacillus. This evolution was closely associated with the selectivity of HMs removal. All bioleaching processes achieved effective sludge reduction (MLSS decreased by 26.99% - 63.21%) and improved dewaterability. Aerated bioleaching yielded the optimal dewatering performance (specific resistance to filtration, SRF: 1.24 × 10¹³ m/kg), attributed to its induced reorganization of extracellular polymeric substances (EPS)—characterized by protein enrichment in tightly bound EPS and polysaccharide stripping from loosely bound EPS. This study provides critical theoretical insights for optimizing sludge bioleaching parameters and pretreatment-process combinations to maximize HM removal while minimizing nutrient loss.