Anaerobically-digested sludge conditioning by activated peroxymonosulfate: Significance of EDTA chelated-Fe2+

Abstract

This study investigates the efficiency of anaerobically-digested sludge (ADS) conditioning using peroxymonosulfate (PMS) activated by EDTA chelated-Fe2+ process (PMS-EDTA-Fe2+), and the roles and mechanism of EDTA are explored by investigating the capillary suction time (CST), specific resistance to filtration (SRF), bound water content (BWC), extracellular polymeric substances (EPS), iron species transformation, sludge morphology, and active radicals formation. PMS-EDTA-Fe2+ process exhibits better performance in sludge dewatering than PMS-Fe2+ process. Upon the introduction of EDTA at optimum molar ratio of EDTA to Fe2+(REDTA:Fe2+) of 1 : 3, CST, SRF, and BWC are decreased by 60.0%, 70.6%, and 1.5%, and the cell viability is decreased from 75.4% to as low as 24.7%. EDTA introduction significantly enhances the yield of active radicals (i.e., SO4−· and O⋅H) by nearly 50% as compared to PMS-Fe2+ process. This effect contributes to stronger cells lysis as indicated by SEM and CLSM, and the contents of polysaccharide and protein in total EPS fractions are increased by 19.2% and 37.4% accordingly. Additionally, EDTA increases the iron mobility by forming FeII/III-EDTA complexes and potentially inhibits ADS coagulation at high doses. The total dissolved iron (FeTot_dis) concentration increases from 203.4 mg/L to 601.4 mg/L due to the introduction of EDTA at 0.4 mmol/g VSS, and the dominant iron species in supernatant transformed from Fe2+ to Fe3+ accordingly. The optimization of EDTA doses is significant to balance oxidation and coagulation effects and to achieve better dewatering performance.