Beyond filterability: Understanding the complexities of sludge dewatering through typical coagulation and advanced oxidation

Coagulation and advanced oxidation process (AOP) are the two most significant conditioning methods in sludge dewatering. However, previous studies mainly focus on their differences and overlook their similarities in dewatering mechanisms. In this study, PAC and Fe³⁺ were selected as typical coagulants, and Fenton and Fenton-like reactions as typical AOP. By regulating the reagent dosage to achieve basically the CST-based filterability (CST = 22.9 ± 1.5 s), AOP had superior dewatering efficiency (59.54–63.04 wt%) compared to coagulation (68.84–69.16 wt%). Obviously, the filterability of sludge could not accurately reflect its dewatering performance. The mechanism revealed that AOP had both “extracellular oxidation” and “intracellular oxidation”, whereas coagulation mainly affected the properties of extracellular organic matter. Both processes improved the sludge's relative hydrophobicity and reduced its interfacial free energy. AOP was superior to coagulation in improving sludge fluidity, hydrophobicity, permeability, cell disintegration and reducing bound water content. While coagulation had a better effect on electric neutralization, floc particle size growth and colloidal structure strength. Moreover, this study emphasized the significance of considering the vector-based relationship between sludge dewatering performance and various influencing factors for analyzing the dewatering mechanism. These findings are expected to deepen our understanding of the similarities and disparities in dewatering efficacy and the related mechanisms between the these two pretreatments.