Ferrous activated calcium hypochlorite process combined with tannic acid conditioning for improving dewaterability of waste activated sludge

The high water content of sewage sludge presents a significant challenge for its subsequent treatment. With activation of hypochlorite treatment hydrophilic protein in sludge could not be effectively degraded which limits the performance of sludge dewatering, meanwhile, excessive radicals could be consumed. To solve this issue, in this study a novel Fe 2+ /Ca(ClO) 2 /tannic acid (TA) process was proposed for the first time, aiming at enhancing the dewaterability of waste activated sludge (WAS). Treatment with 1.2 mmol/g total solids (TS) of Ca(ClO) 2 , 1.0 mmol/g TS of Fe 2+ and 0.15 mmol/g TS of TA resulted in the reduction of 66.58 %, 88.31 % and 9.75 % in capillary suction time (CST), specific resistance to filtration (SRF) and water content (Wc) of sludge cake, respectively. Mechanistic investigations revealed that the ·OH and ·CH 2 CH 2 OH generated within the Fe 2+ /Ca(ClO) 2 /TA system facilitated the decomposition of extracellular polymeric substances (EPS) and sludge cells, leading to the release of bound water as free water. The value of α-helix/(β-sheet + random coil) with Fe 2+ /Ca(ClO) 2 /TA conditioning decreased from 50.43 % to 38.72 %. Furthermore, the formation of multivalent ions, such as Fe 3+ and Ca 2+ , along with TA, promoted re-coagulation, resulting in an increase in sludge particle size and enhanced flocculation. Additionally, TA effectively complexed hydrophilic proteins, facilitating the interaction between calcium and sulfate ions to form additional gypsum material as skeleton in sludge cake, which led to improved permeability and compressibility of the sludge cake. This study provides a new approach for efficient improvement in sludge dewaterability.