Inhibitory effects of a shock load of Fe(II)-mediated persulfate oxidation on waste activated sludge anaerobic digestion

The activated sludge process of wastewater leads to the production of a substantial amount of excess activated sludge. The increasing interest of Fe(II)-mediated persulfate (S2O82-) oxidation in improving sludge dewatering highlights a need to understand their environmental impacts on subsequent treatment, but the potential effects on sludge anaerobic digestion remains unknown. In this study, the response of waste activated sludge anaerobic digestion to a shock load of Fe(II)/S2O82- oxidation were investigated. It was observed that total suspended solids (TSS) and volatile suspended solids (VSS) removals were considerably lowered when Fe(II) and S2O82- dosages were greater than 0.8 and 1.0 mmol/g VSS, respectively, while 34.6-60.5% drop in hydrogen sulfide (H2S) took place. Degradation of extracellular polymeric substances (EPS) of sludge induced by Fe(II)/S2O82- oxidation resulted in their deteriorating protective role on the microorganisms, which may cause the inhibitory effects on the microbial activity, and subsequently reduce digestion efficiency and H2S generation. Then, sludge dewaterability during digestion process was investigated. It was found that dewaterability of sludge, firstly improved by Fe(II)S2O82- pretreatment, was gradually worsened during subsequent digestion because of slightly increased EPS. The visualization of EPS with three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy further revealed that the discernable rise in tyrosine and tryptophan protein-like substances together contributed to poorer dewaterability.