Unraveling the catalyzing behaviors of different iron species (Fe2+ vs. Fe0) in activating persulfate-based oxidation process with implications to waste activated sludge dewaterability

Dewatering of waste activated sludge (WAS) is of major interest in its volume reduction, transportation and ultimate disposal. Persulfate-based oxidation process is a newly developed option for enhancing WAS dewaterability through the generation of powerful sulfate radicals (SO4−·). However, the enhancement in WAS dewaterability by persulfate differs with the species of iron catalysts used. In this study, two types of iron catalysts (i.e. Fe2+ vs. Fe0) were employed to initiate the persulfate (S2O82−), and the catalyzing behaviors and the underlying principles in enhancing WAS dewaterability were investigated and compared. The Fe2+ exhibited the high effectiveness in catalyzing the decomposition of persulfate to sulfate radicals (SO4−·), inducing the greater improvement in WAS dewatering. The WAS dewaterability (indicated by dry solids content after filtration) increased with the added S2O82−/Fe2+ dosages, with the dry solids content reaching up to 5.1 ± 0.8 wt% at S2O82−/Fe2+ dosages of 1.2/1.5 mmol/g-VS after only 30 s’ filtration, roughly 1.8-fold increase than raw WAS (1.8 ± 0.1 wt%). In contrast, the influence of the persulfate oxidation when activated with Fe0 on WAS dewaterability was statistically insignificant. The WAS dewaterability remained nearly unchanged (i.e. dry solids content of 2.0 ± 0.0 wt%), irrespective of the employed S2O82−/Fe0 dosages. Further analysis demonstrated that the WAS dewaterability negatively corresponded to loosely bound extracellular polymeric substances (LB-EPS) and tightly bound EPS (TB-EPS). The abundant SO4−· from S2O82−/Fe2+ system could effectively disrupt the gel-like EPS matrix, break apart the cells and subsequently arouse the release of the water inside EPS and cells, facilitating water-solid separation. In the case of S2O82−/Fe0, the dissolution of Fe0 particles was the rate-limiting step, due to the formation of oxide iron layer near Fe0 metallic surface, which resulted in the slow SO4−· production and thus hardly promoted WAS dewaterability. The pH adjustment could accelerate Fe0 dissolution and enhance the dewatering performance of S2O82−/Fe0 process to a certain degree, but the effect was unsatisfactory. Additionally, the observations regarding the dissolved organic matters and ammonium collectively revealed that except for enhancing WAS dewatering, S2O82−/Fe2+ oxidation could concurrently degrade COD and ammonia from WAS filtrate, lighten the burden of the subsequent sewage treatment facilities and reduc