Assessment of the in-situ electrogenerated chloramines damaging polyamide NF membrane during secondary wastewater treatment

Abstract

Nanofiltration (NF) membrane is susceptible to chlorine damage during secondary wastewater treatment, rendering it critical to investigate the structure changes in polyamide membrane in chlorine-exposed environments. In this work, chlorinated secondary wastewater was pre-treated using an electro-oxidation (EO) process, and then the potential damage to polyamide membranes during the EO-NF process was assessed by the performance of NF90 nanofiltration membrane after exposure to the EO effluent. We observed a 90% reduction in the retention of Ca2+ and Mg2+ by the NF membrane at high concentration of chloramine (i.e., 20 mg/L), which was attributed to the formation of N-chloramides due to the binding of chloramine to the lone electron pair of the polyamide. Compared with the pristine membrane directly exposed to the chloramine-containing EO effluent, the NF membrane deposited with a biofouling layer did not exhibit any changes in chemical structure and compositions, probably because the biofouling layer blocked the chlorination of polyamide. Therefore, the biofouling layer formed by the deposition and development of microorganism on the membrane surface could prolong the lifetime of the polyamide membrane during the treatment of municipal wastewater in the EO-NF system.