Chemical–Biological Process Based on H2O2/O3 Lysis and Cryptic Growth for the In Situ Reduction of Waste Activated Sludge

In addressing the expensive transportation and disposal cost of the waste activated sludge (WAS), an in situ sludge reduction strategy was developed in this study based on H₂O₂/O₃ lysis and a cryptic growth process. The critical roles of sludge solubilization and biodegradability of sludge lysate were analyzed comprehensively. Under a dosage of 0.36 g O₃/g mixed liquor volatile suspended solids (MLVSS), H₂O₂ addition (H₂O₂/O₃ molar ratio was 0.25) led to a significant improvement of sludge cell lysis performance in terms of the MLVSS reduction ratio (35.2% vs 26.3%). Compared with O₃ alone, the sludge lysate treated with H₂O₂/O₃ exhibited a higher biodegradability, as the value of BOD₅/COD increased from 0.72 to 0.79, accompanied by mean specific rates of 6.71 and 7.42 h^–1, respectively. Reintroduction of the H₂O₂/O₃-treated sludge into the mainstream biological process facilitated further hydrolysis of oxidized sludge fragments, achieving an overall sludge reduction ratio of 29.1% without adversely affecting the effluent quality adversely. A pilot-scale experiment at 2.4 m³/d treatment capacity also demonstrated a similar sludge reduction performance of 28.1% in term of MLVSS. Economic analysis revealed that the net benefit of the in situ sludge reduction reached 132/t dry sludge (DS), highlighting the significant potential of the combined H₂O₂/O₃ method for effective sludge reduction.