Freezing-Activated Oxidation with Nitrite and H2O2 for Efficient Waste Activated Sludge Solubilization and Filterability

Nitrite-based pretreatment has been demonstrated to effectively improve waste activated sludge (WAS) solubilization. In this study, a freezing-activated oxidation method incorporating both nitrite and H₂O₂ was investigated, demonstrating significantly improved performance. At an optimal nitrite-to-H₂O₂ molar ratio of 1:1, the release of dissolved organic carbon (DOC) increased markedly from 38.4 ± 0.6 to 67.6 ± 0.6 mg C g⁻¹ VS, as the nitrite concentration rose from 100 mg L⁻¹ to 400 mg L⁻¹. In contrast, the freezing alone treatment yielded only 32.2 ± 1.0 mg DOC g⁻¹ VS. The freeze-concentration effect was hypothesized to facilitate the in-situ formation of peroxynitrous acid (HOONO) and its reactive intermediates, which substantially enhanced the disruption of extracellular polymeric substances and microbial cells. This led to a substantial release of low molecular weight (LMW) fractions, including hydrophobic organics, building blocks, LMW acids, and neutrals—particularly LMW acids, which increased by 4.5-fold. In addition, freezing-activated oxidation with nitrite and H₂O₂ significantly improved sludge filterability, with capillary suction time reduced by over 80% and median particle size increased by about 7-fold. Notably, secondary utilization of the freeze-thaw process further promoted WAS solubilization. These findings suggest that freezing-activated oxidation with nitrite and H₂O₂ represents a promising strategy for enhancing sludge treatment and resource recovery, particularly in cold climate regions.