Ultrasound enhanced zero-valent iron-activated peroxymonosulfate oxidation for improving dewaterability of aerobically digested sludge

In this work, ultrasound (US) assisted zero-valent iron (ZVI)-activated peroxymonosulfate (PMS) oxidation was used to improve the dewaterability of aerobically digested sludge (AeDS). Capillary suction time (CST) reduction, centrifuged weight reduction (CWR), and water content of sludge cake (Wc) were determined to evaluate AeDS dewaterability. The results showed that the optimal reaction time, PMS dosage, ZVI dosage, and US power were 20 min, 50 mg/g total solids (TS), 100 mg/g TS, and 50 W, respectively. Under the optimum conditions, CST reduction, Wc, and CWR were 89.2%, 67.9%, and 40.4%, respectively. The quenching experiments showed that sulfate radical was the dominant radical species in the US-ZVI-PMS system while hydroxyl radical in the ZVI-PMS system. Compared to ZVI-PMS treatment, US-ZVI-PMS treatment was more conducive to improve sludge dewaterability, suggesting that US assistance destructed the passive film of ZVI and accelerated its corrosion, consequently resulting in more production of sulfate radical. The significant degradation of tightly bound extracellular polymeric substances (EPS) of AeDS after US-ZVI-PMS treatment led to the releases of EPS bound water and even the intracellular water, consequently improving AeDS dewaterability. Additionally, US-ZVI-PMS treatment presented the advantages of sludge disintegration and charge neutralization. The ZVI cycle tests demonstrated that the reusability and stability of the recovered ZVI in the US-ZVI-PMS system were desirable, leading to the cost savings.