Unveiling the Overlooked Roles of Divalent and Trivalent Metal Removal in Sludge Fermentation: Promising Outlooks in a Recyclable Cation Exchange Resin-Promoted Strategy

Anaerobic fermentation is a promising technology for sustainable sludge management; yet it nevertheless faces inherent limitations. Previous research has primarily focused on external factors while overlooking the endogenous regulatory approach, particularly the potential roles of divalent and trivalent metals. This Perspective highlights the distribution patterns and binding forms of various metals in sludge while elucidating their limiting effects: structural binding of sludge flocs (restricts hydrolysis), microbial inhibition by potential toxicity, and concealment of redox compounds (restricts metabolisms). These insights underscore the importance of identifying the specific impacts of the removal of divalent and trivalent metals on sludge hydrolysis as well as acidogenic and methanogenic pathways. In addition, several cation-exchange-based treatment approaches are critically reviewed to assess the feasibility and performance of metal removal in promoting sludge fermentation. The recyclable cation exchange resin-based strategy demonstrates strong potential in a staged process configuration. Overall, this Perspective clarifies the specific adverse roles of divalent and trivalent metals in sludge fermentation, revealing the previously overlooked contributions of metal removal to enhanced fermentation processes. Furthermore, the promising advances in recyclable cation exchange resin-promoted sludge fermentation are highlighted as a pathway toward sustainable sludge management in additive recycling and triple promotion patterns.