Presence of powdered activated carbon/zeolite layer on the performances of gravity-driven membrane (GDM) system for drinking water treatment: Ammonia removal and flux stabilization

Gravity-driven membrane (GDM) filtration is a promising alternative for decentralized water supply, while its widespread application was hindered by the poor removals of organics and ammonia during long-term operation. In this study, powered activated carbon (PAC) and granular zeolite were selected as typical adsorbents to investigate the impacts of pre-deposited adsorbent layers on contaminant removal and membrane fouling. Results showed that the pre-deposited PAC layers exhibited higher removal of organics than the control, while the zeolites deposited layers exhibited low removal of organics. The presence of PAC only enhanced the NH4+ removal at subsequent stable stage, while zeolites were effective in deal with sudden high NH4+ concentration due to ion exchange. The presence of mixed adsorbents layers had similar organic removal with PAC and NH4+ removal with zeolite. The pre-deposited PAC layers could effectively alleviate membrane fouling in short-term UF tests, while the stable fluxes (5.88–6.54 L/(m2·h)) in long-term GDM operation were slightly lower than the control (6.63 L/(m2·h)). The zeolites deposited layer aggravated membrane fouling in both short-term ultrafiltration and long-term GDM (5.03–3.84 L/(m2·h)), but a higher stable flux (6.10 L/(m2·h)) was observed for GDM using the mixed adsorbents. The pre-deposited adsorbent layers resulted in increased concentrations of biomass, tri-phosphate (ATP) and extracellular polymeric substances (EPS), forming cake layers with a denser structure than the control. Finally, the fouling mechanism for GDM using different adsorbent layers was proposed based on fouling analysis and characteristics of biological fouling layer. The results and conclusion in this study could provide helpful information for the application of GDM with pre-deposited adsorbent layer in treating raw water with organics and/or sudden high ammonia concentration to produce potable water. [Display omitted] •PAC enhanced the removals of organics in short- and long-term UF processes.•Zeolites could deal with sudden rather than long-term high NH4+ concentration.•Mixed adsorbents had similar organic removal with PAC and NH4+ removal with zeolite.•Pre-deposited adsorbent layers increased fouling in GDM but reduced pore blocking.•Low stable flux was due to increase in ATP and EPS of biological fouling layer.