Long term performance of gravity driven membrane (GDM) pilot filtration system for household drinking water treatment with differing MF and UF membranes

Abstract

Gravity-driven membrane (GDM) filtration technology has been shown to be a promising option for household water treatment (HWT) in low-income countries as it can produce safe drinking water without backwashing or chemical membrane cleaning. Stable water fluxes in the range of 2 – 10 L/(m2h) can be achieved under ultra-low hydrostatic pressure (e.g., 50 mbar) thank to the formation of membrane fouling layers with soft and heterogeneous structure. Biological activities such as predation or grazing in the fouling layers also help to maintain the stabilized flux. In this study, the performance of GDM pilot filtration system designed for a single house (production capacity of 20 L/day) were tested to treat lake waters over 400 days using differing MF and UF membranes such as PES-UF (100 kDa), PVDF-UF (100 kDa), and PTFE-MF (0.3 m). Previous studies have demonstrated good performance of GDM filtration systems mainly with PES-UF membranes while other membranes with differing properties have not been intensively tested. It has been of interest whether the membrane characteristics (e.g., polarity, roughness etc) affect the structure of fouling layers and the stabilized flux levels in GDM filtration. Our results showed that even though stabilized fluxes were observed for all three membranes, however, the level of the stabilized flux was variable and the highest for PVDF-UF (6.34 5.6 L/(m2h)), followed by PTFE-MF (5.62 3.5 L/(m2h)), and the lowest for PES-UF (1.67 1.4 L/(m2h)). The three membranes also exhibited differing fouling layer structures based on SEM, AFM, and ATR-FTIR analyses. The PVDF-UF and PTFE-MF showed more heterogeneous and open fouling layers with higher biomass concentrations compared to the PES-UF. Various species of nematode were found in the fouling layers of PVDF-UF and PTFE-MF membranes and their numbers per unit area (#/cm2) showed a positive correlation with the stabilized fluxes. Nematode was not observed in the PES-UF membrane. Considering the differences of the three tested membrane characteristic, surface roughness appears to play an important role for the flux performance of GDM filtration. Possible mechanisms for the effect of surface roughness on the fouling layer structure and the stabilized flux are discussed. These results can be useful in designing and optimizing the GDM filtration process.