CaO2 activation by electrochemically generated Fe2+ for improving sludge dewatering at neutral pH: Influence of oxalic acid addition.

• Developed oxalic acid (OA) chelated electrochemical (EF) released Fe2+ activated CaO 2 pre-conditioned sludge process. • EF-CaO 2 /OA effectively enhanced sludge dewaterability at neutral pH. • EF-CaO 2 /OA provides durable and effective oxidizing power. • End-of-conditioned sludge was still pH-neutral. • Altered protein secondary structure transforms sludge from hydrophilic to hydrophobic. In this study, we investigated the efficiency of activation of CaO 2 by oxalic acid (OA) chelated with electrochemically (EF)-released Fe2+ (EF-CaO 2 /OA) in improving the performance of sludge dewatering under neutral conditions. We also explored the mechanism of OA-reinforcement of EF-CaO 2 in enhancing the performance of wastewater-activated sludge dewatering by investigating the water content (Wc), specific resistance to filtration (SRF), pH, bound water content, extracellular polymers in the sludge, the secondary structure of proteins, the soluble ferric ions, and the morphology of the sludge. The EF-CaO 2 /OA process was better than the EF-CaO 2 process in terms of sludge dewatering. OA introduction significantly inhibited the increase of solution pH, which in turn promoted CaO 2 dissolution and persistent H 2 O 2 generation. The persistent oxidative reaction contributed to the degradation of hydrophilic protein fractions in EPS and the reduction of the proportion of hydrophilic structures in the secondary structure of proteins, which in turn improved the hydrophobicity of the sludge. The changing profile of DNA content in EPS proved that EF-CaO 2 /OA led to the damage of more sludge cells. It suggested that OA enhanced the oxidizing efficacy of EF-CaO 2 , which in turn promoted the release of bound water from more sludge cells. Moreover, OA significantly increased the amount of dissolved iron in solution by chelating the electrochemically released iron, which contributed to the stabilization of the oxidation reaction. Under the optimal conditions, sludge Wc and SRF treated by EF-CaO 2 /OA were reduced by 30.1% and 96.9%, respectively, and the dewatered sludge cake remained neutral. In conclusion, EF-CaO 2 /OA may be an efficient and feasible technique for deep sludge dewatering. [ABSTRACT FROM AUTHOR]