Gravity-driven membrane bioreactor coupled with electrochemical oxidation disinfection (GDMBR-EO) to treat roofing rainwater

Abstract

Clean water scarcity is a pressing global water resource issue that affects a large population. Rainwater utilization carries an enormous potential as an effective option to alleviate water crisis. In our work, a gravity-driven membrane bioreactor (GDMBR) coupled with electro-oxidation (EO) was investigated for long-term roofing rainwater treatment and disinfection over a duration of 265 days. It was found that the disinfection performance of EO was significantly increased with an increased operational current and hydraulic retention time (HRT), and the optimal applied currency density was 2 mA/cm2 at an HRT of 60 min. The GDMBR-EO process exhibited desired rainwater treatment efficiency at 8 ~ 12 L/(m2·h) followed by the intermittent EO. Aided by GDMBR rejection, EO could realize complete microbial disinfection (i.e., bacterial and E.coli) during treatment of both synthetic rainwater and actual roofing rainwater (from a reinforced concrete roof, metal roof, and oil-polluted roof), due to the production of reactive oxygen substances (ROS) (i.e.,·OH, H2O2, and O3, etc.) and reactive chlorine species (RCS) (i.e., Cl2, HClO, ClO-, ·Cl, etc.). Removal of organic matter (i.e., ATP, protein, and polysaccharide) was achieved via membrane retention, biodegration and oxidation in the presence of ROS and RCS. GDMBR obtained a stable ammonia removal, attributed to the enrichment of ammonia-oxidizing bacteria (AOB) (i.e., Nitrospira), as confirmed by bacterial community analysis. RCS was responsible for the transformation of nitrogenous substances (i.e., NH3-N, NO– 3-N,NO– 2-N and N2) during the EO process. TP removal and slight increase of turbidity in the EO unit was caused by FePO4 precipitation, and CaCO3 and Mg(OH)2 precipitation, respectively, as demonstrated by an X-ray diffraction (XRD) and Roman spectra analysis.