Enhancing waste activated sludge dewatering and simultaneously inhibiting microbial viability by Fe(Ⅱ)-activated 1,3-Dichloro-5,5-dimethylhydantoin (Fe(Ⅱ)/DCDMH) process.

High moisture content critically constrains the disposal efficiency of waste activated sludge (WAS). Conventional flocculation conditioning exhibits limited efficacy as it fails to release water encapsulated in extracellular polymeric substances (EPS) and cells. This study proposes a novel process using Fe(Ⅱ)-activated 1,3-dichloro-5,5-dimethylhydantoin (Fe(Ⅱ)/DCDMH) to enhance sludge dewaterability. Under acidic conditions (pH 5), the synergistic oxidation and flocculation effects from in-situ generated reactive species (•OH, •Cl and Fe(Ⅳ)) and Fe(Ⅲ) greatly enhanced sludge dewatering performance. The capillary suction time (CST) and specific resistance to filtration (SRF) decreased by 57.34% and 85.79%, respectively. Mechanistic investigations revealed that reactive species altered the functional group structure of EPS and decreased the content of water-retaining components like proteins and polysaccharides. Such effects induced a loosened EPS structure, sludge flocs fragmentation and the increases in internal porosity and pore size, thereby facilitating water release. Concurrently, in-situ formed Fe(Ⅲ) also suppressed surface negative and hydrophilic surface area, promoting the re-agglomeration of fragmented fine flocs, which enlarged overall sludge particle size, reconstructed drainage channels, and enhanced flocculation performance. Moreover, the radicals preferentially degraded EPS to disrupt the water-retaining structure, while the molecular HClO penetrated cell membranes and induced intracellular oxidative damage. These processes collectively altered cell permeability, regulated cell death processes, and effectively suppressed microbial viability. It provides theoretical support for potential control of hazardous microorganisms in sludge during disposal.