Anaerobically-digested sludge disintegration by transition metal ions-activated peroxymonosulfate (PMS): Comparison between Co2+, Cu2+, Fe2+ and Mn2+

This study investigates anaerobically-digested sludge (ADS) disintegration by activated peroxymonosulfate (PMS) with transition metal ions of Co2+, Cu2+, Fe2+, and Mn2+ (PMS-Me2+). The activating performances of Me2+ are quantitatively compared by capillary suction time (CST), extracellular polymeric substances (EPS), bound water content (BWC), particle size distribution, and metal speciation. At Me2+ dose of 1.2 mmol/g VSS, PMS-Fe2+ and PMS-Cu2+ achieve the lowest normalized CST, i.e., CST/CST0, of 0.47, and the higher normalized CST values of 0.71 and 0.74 are observed for PMS-Mn2+ and PMS-Co2+, respectively. BWC shows little extent of decrease upon PMS-Me2+ oxidation, and the most significant decrease from 89.5% to 88.3% is observed for PMS-Mn2+. PMS-Co2+ contributes to the decrease of DOC in total EPS fractions (DOCtot) from 698.0 mg/L to 496.6 mg/L, whereas the increased DOCtot to 713.6, 734.4, and 755.0 mg/L is observed with the introduction of Cu2+, Fe2+, and Mn2+, respectively. Fe2+ tends to transform to Fe3+ and the coagulation effect increases the median particle diameter (D50) from 15.8 μm to 91.1 μm. Comparatively, much lower D50 values of below 20.0 μm are observed for other divalent ions. The European Community Bureau of Reference (BCR) sequential extraction method is used to analyze the metal speciation in ADS sediment after PMS-Me2+ disintegration. The dominant species of Co and Mn are acid extractable fractions with the ratios of 91.0% and 87.3%, whereas the main Fe and Cu species are observed to be residual and oxidizable fractions. The pre-captured Me2+ ions with 50-days aging interestingly exhibit activating efficacy towards PMS, and CST values were observed to decrease by 11.5%, 30.5%, 11.8%, and 27.3% with the presence of pre-captured Co2+, Cu2+, Fe2+, and Mn2+ at 1.2 mmol/g VSS. This study proposes the potentially valuable strategy for the disintegration and dewatering of sludge with high contents of transition metal ions.