The performance of gravity-driven membrane (GDM) filtration for roofing rainwater reuse: implications of roofing rainwater energy and rainwater purification

Rainwater harvesting (RWH) coupled with gravity-driven membrane (GDM) filtration was used to simultaneously treat rainwater and recover energy. A pilot GDM could obtain a relatively stable level of permeate flux (~4.0 L/(m²·h)) under a set water head (ΔH = 0.4 m) over 140 days of operation. An increase water head (ΔH = 0.6 m) did not achieve a sharp increase in stabilized flux (~2.4 L/(m²·h)) over 20 days of operation until the end. It was found that GDM filtration could produce a permeate that was almost free of particles. However, only a small amount of organic matter and trace metals (i.e., Cr, Al, Fe, Cu, Al, Mn and Ca) were removed, as demonstrated by excitation-emission matrix (EEM) and energy dispersive spectrometry (EDS) analysis. Additionally, the bacterial abundance within the permeate ((8.45 ± 0.11) × 10² cells/mL) decreased compared to that within the GDM tank ((1.85 ± 0.14) × 10⁵ cells/mL), revealing that the rejected bacteria might enhance biofilm formation. The presence of extracellular polymeric substances (EPS), adenosine triphosphate (ATP) and assimilable organic carbon (AOC) indicated a high level of microbial activity within the biofilm, which was also demonstrated by the porous cake layer morphology observed by scanning electron microscopy (SEM) and results from confocal laser scanning microscopy (CLSM) imaging of the biofilm. NH₃-N was removed by Nitrospira within the biofilm, which was identified by microbial community analysis. Overall, this novel approach has the potential to improve municipal water availability and stormwater management practices.