Sodium hypochlorite-assisted osmotic backwashing for mitigating forward osmosis membrane fouling during pre-concentrating wastewater

Abstract

Forward osmosis (FO) membrane has drawn substantial attention to pre-concentrating organic matter in wastewater for biogas production. However, membrane fouling control is an ongoing challenge for the feasibility of the FO system. Although chemical cleaning using a caustic solution (e.g., solution pH = 11) can remove organic foulants on the polyamide-based membranes, it can damage cellulose triacetate (CTA)-based FO membrane due to hydrolysis. This study assessed the efficacy of sodium hypochlorite (NaClO)-assisted osmotic backwashing to mitigate CTA-based FO membrane fouling during pre-concentrating primary wastewater effluent. The cleaning strategy employing a low concentration of NaClO showed a restoration of water flux higher than conventional hydraulic flushing and sole osmotic backwashing. The water flux after NaClO-assisted osmotic backwashing (NaClO concentrations = 10–80 mg/L) reached 97–102%. Irreversible foulants on the membrane surface were fully removed by NaClO concentration from 20 to 80 mg/L. During four filtration cycles using a 40 mg-NaClO/L cleaning solution, the reverse salt flux increased from 4.3 to 6.3 g/m2h. This indicates that for a long-term operation, applying lower concentrations of NaClO (e.g., 20 mg/L) may be needed to avoid the changes in reverse salt flux. This study suggests that FO membranes during pre-concentrating primary wastewater effluent can be stably operated with NaClO-assisted osmotic backwashing.