A mechanistic study on electrolytic free chlorine for fouling control in submerged membrane bioreactors

We interrogated the effects of in-situ electrochemical chlorination with Ti/IrO2 anodes on physicochemical properties of mixed liquor and fouling in membrane bioreactors (MBRs). Linear sweep voltammetry with variable concentrations of model organic foulant (alginic acid) indicated negligible direct oxidation. Batch electrolysis under current densities (j) up to 1.2 mA/cm(2) further indicated that free chlorine (FC) primarily mediates transformation of dissolved compounds such as NH4+-N and organic carbon (DOC) including protein and soluble microbial product (SMP, as noted in excitation-emission matrix, EEM). Owing to infinitesimal steady-state concentrations of FC under electrolytic generation, diffusion of FC determined the transformation kinetics in bulk, while floc/cell breakage and mineralization of extracellular polymeric substances (EPS) were marginal. Decay of negatively charged functionalities was found to compete with carbonyl generation on aromatic carbons (noted by red shift of EEM peak), which determined the magnitude of floc zeta-potential. A cathodic scale deposition was presumed to bring about PO43- removal and stimulate sludge flocculation. This study evinced that an increased floc size coupled with removal and/or transformation of hydrophobic DOC would contribute to the alleviated membrane fouling. However, significant decreases in microbial utilization rates of DOC under exposure to j > 0.8 mA/cm(2) accelerated fouling during continuous operations of MBRs. (C) 2019 Elsevier Ltd. All rights reserved.