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

Abstract

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 (S₂O₈^2-), 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 S₂O₈^2-/Fe²⁺ dosages, with the dry solids content reaching up to 5.1 ± 0.8 wt% at S₂O₈^2-/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 S₂O₈^2-/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 S₂O₈^2-/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 S₂O₈^2-/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 S₂O₈^2-/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, S₂O₈^2-/Fe²⁺ oxidation could concurrently degrade COD and ammonia from WAS filtrate, lighten the burden of the subsequent sewage treatment facilities and reduce operational expense. Hence, from an environmental and economic perspective, the S₂O₈^2-/Fe²⁺ system possesses much greater promise for WAS dewatering.

Keywords: Dewatering; Extracellular polymeric substances; Iron catalysts; Persulfate-based oxidation process; Waste activated sludge.