Formation mechanisms of emerging organic contaminants during on-line membrane cleaning with NaOCl in MBR

On-line chemical cleaning with sodium hypochlorite (NaOCl) is widely employed for sustaining MBR permeability, during which the inevitable contact between activated sludge and NaOCl had been shown to trigger substantial release of dissolved organic matter (DOM). Therefore, this work further explored the formation mechanisms of such DOM by looking into the respective reactions of intracellular organic matter (IOM) and cell debris in activated sludge with NaOCl. The results showed that DOM was primarily produced from the dissolution of cell wall, while IOM release was insignificant at the NaOCl concentration of 25 mg/L as Cl₂. On the basis of experimental observations, a three-step mechanism was proposed for elucidating the DOM formation from activated sludge upon NaOCl exposure: (i) NaOCl first damaged cells by perforating cellular wall, producing a considerable amount of humic-like substances and low-molecular-weight halogenated byproducts; (ii) IOM was released but rapidly degraded and humified by NaOCl, accompanied with the formation of relatively high-molecular-weight halogenated byproducts; (iii) the residual NaOCl and combined chlorine continued to react with cell wall or likely diffused into cells leading to the deactivation of DNA/enzymes. Consequently, this study offers mechanistic insights into the origination of emerging contaminants during on-line membrane cleaning of practical MBR.