Dewatering and removal of metals from urban anaerobically digested sludge by Fenton’s oxidation

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Abstract
In this study, the relevance of Fenton’s reaction for the treatment of urban anaerobically digested sludge was investigated. In a first part, the impact of the oxidation process on the improvement of the sludge dewaterability was studied. In a second part, the removal of heavy metals from the sludge was examined. Fenton’s reaction was carried out with increasing concentrations of Fe2+ and H2O2 in 1:10 and 1:1 ratios. Dewaterability of the raw sludge was highly improved: the addition of 36 mM Fe2+ and 360 mM H2O2 led to specific cake resistance (SCR) and capillary suction time (CST) reductions of 99.8% and 98.8%, respectively. Indeed, under these conditions, SCR and CST of respectively 1.04 × 1011 m kg−1 and 18.5 ± 0.2 s were measured, and the treated sludge could be considered as having ‘good dewaterability’. A significant impact was also observed on the removal of heavy metals from the sludge. After 1-h oxidation, Cd, Cr, Cu, Pb and Zn could be removed by 81.1 ± 0.1%, 25.1 ± 0.1%, 87.2 ± 1.1%, 77.3 ± 4.8% and 99.6 ± 0.3%, respectively. These results were consistent with the heavy metals’ fractions in the sludge. It could be concluded that the addition of Fe2+ and H2O2 in a 1:10 ratio was more effective than in a 1:1 ratio. The results were consistent with the extracellular polymeric substance (EPS) contents in raw and treated sludge, since loosely bound EPS decreased significantly after the treatment. GRAPHICAL ABSTRACT

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