Influences of different iron forms activated peroxydisulfate on volatile fatty acids production during waste...

The treatments of peroxydisulfate (PDS) activated with different iron forms (zero-valent iron (ZVI), ferrous iron (Fe²⁺) and nano zero-valent iron (NZVI)) all contributed to the generation of volatile fatty acids (VFAs) during waste activated sludge (WAS) anaerobic fermentation. The maximal VFAs generation was 3036, 5537 and 3533 mg COD/L in the PDS/ZVI, PDS/Fe²⁺ and PDS/NZVI reactors, respectively, while it was only 702 mg COD/L in the control. The enhancing effects followed the order of PDS/Fe²⁺ > PDS/NZVI > PDS/ZVI. ZVI and NZVI showed no dual promoting effects with PDS on the VFAs production. Mechanisms exploration indicated that the simultaneous improvement of WAS solubilization and hydrolysis (high concentrations of soluble proteins and carbohydrates) and enrichment of fermentative bacteria (i.e. Bacteroides, Clostridium, Fonticella, and etc.) involved in VFAs generation were the main causes of VFAs promotion in the PDS treated systems. However, the reductive ZVI and NZVI partially consumed the generated free radicals (i.e. SO₄^- and/or OH), which possess strong oxidative potentials and are the main contributors to extracellular polymeric substances disintegration in WAS. This consumption of free radicals accounted for the lower efficiency of solubilization and hydrolysis and consequently reduced VFAs production in the PDS/NZVI and PDS/ZVI reactors compared with that in PDS/Fe²⁺ reactor. Moreover, the treatment of PDS activated by different forms of iron improved the VSS reduction extent and dewaterability of fermented sludge compared with that of the control, which is advantageous to the ultimate disposal of WAS.