Roofing rainwater cleaner production using pilot-scale electrocoagulation coupled with a gravity-driven membrane bioreactor (EC-GDMBR): Water treatment and energy efficiency

Abstract

A pilot-scale electrocoagulation method coupled with gravity-driven membrane bioreactor (EC-GDMBR) was conducted to roof rainwater cleaner production for long-term (120 days) operation. The utilization efficiency of roofing rainwater energy, water production efficiency, and water purification mechanism were systematically investigated. The permeate flux of the EC-GDMBR was stabilized at 17.0–20.0 L/(m2·h), 24.6 L/(m2·h) at given roofing rainwater heads of 0.4 and 0.6 m, respectively. Characterization of floc particle size showed that the large particle flocs (~145 μm) produced by the EC system enabled the stabilization flux improvement. A combined removal pathway (i.e., coagulation, adsorption, biodegradation, and coprecipitation) of the EC-GDMBR was found for particulate matter, ammonia nitrogen (NH3–N), total phosphorus (TP), organic substances, and heavy metal (i.e., Cr, Zn, and Cu) removal within synthetic or actual rainwater. In particular, EC could sweep the vast majority of bacteria deposited on the GDMBR, and the intensive biological activity of biofilms was demonstrated by analysis of bacterial abundance, extracellular polymeric substances (EPS), assimilable organic carbon (AOC), adenosine triphosphate (ATP), and confocal laser scanning microscopy (CLSM). Thus, a lower bacterial number (<100 cells/mL) within the effluent enabled us to ensure the biological safety of cleaner product water. Furthermore, a large amount of Nitrospira (10 %–12%) and Hyphomicrobium (2 %–3%) in the aggregates formed biofilms, clarifying the NH3–N and TP removal mechanism by microbial community analysis. In summary, the novel process is capable of roofing rainwater energy utilization considering building height and effective clean water purification.