Revealing the catalytic behavior of different types ZVI (powder Fe0, sponge Fe1, foam Fe2) in US/Fex/PDS and its effect on the dewaterability of waste activated sludge

The ultrasound (US) irradiation-coupled zero-valent iron (ZVI) activated persulfate system could improve dewatering performance, reduce WAS volume, decrease transport and final disposal costs. However, there are huge differences between various types of ZVI on activation efficiency, reaction mechanism, and application potential in WAS conditioning process. In this study, three ZVI (Fex) catalysts (Fe0 powder iron, Fe1 sponge iron and Fe2 foam iron) were used in the US/PDS system, whose catalytic performance and the fundamentals of the WAS conditioning were explored and compared. The results showed that the capillary suction time (CST) reduction rates were 89 %, 85 %, and 87 % in US/Fe0/PDS, US/Fe1/PDS, and US/Fe2/PDS, respectively, which indicated that the US/Fex/PDS system had an outstanding conditioning performance. The WAS reduction rate was US/Fe1/PDS > US/Fe2/PDS > US/Fe0/PDS attributed to Fe1 surface activate sites. Similarly, the organic matters characterization showed soluble protein content (PN), polysaccharide content (PS), and microbial by-products were significantly reduced in the US/Fe1/PDS system. Further examination of the thermodynamic parameters revealed that the WAS cohesive interfacial energy (Delta G(sls)(coh)) was minimally reduced to promote release of bound water in the system due to the effective regulation of intermolecular forces. In addition, examination of the trivalent iron/divalent iron ratio (Fe3+/Fe2+) to assess the efficacy of ferric ion recycling showed a significant advantage for US/Fe1/PDS system. On the basis of improving catalyst utilization, PDS produced SO4<middle dot>(-) that dominated free radicals to condition WAS, improve dewaterability and achieve WAS reduction. Finally, evaluating laboratory WAS conditioning results and costs, US/Fe1/PDS system has potential applications value.