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

Fontmorin, J.-M., Sillanpää, Mika
Environmental technology 2017 v.38 no.4 pp. 495-505
cadmium, chromium, copper, dewatering, environmental technology, heavy metals, hydrogen peroxide, iron, lead, oxidation, sludge, zinc
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 Fe ²⁺ and H ₂O ₂ in 1:10 and 1:1 ratios. Dewaterability of the raw sludge was highly improved: the addition of 36 mM Fe ²⁺ and 360 mM H ₂O ₂ 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 × 10 ¹¹ 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 Fe ²⁺ and H ₂O ₂ 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.