Biofouling layer maintains low hydraulic resistances and high ammonia removal in the UF process operated at low flux

In this study, the hydraulic resistance and pollutant removal efficiency of the biofouling layer (i.e., biofilm) were investigated in the ultrafiltration (UF) systems operating at various flux (i.e., 10, 15, 20, and 30 L/m2·h). The results showed that the biofouling layers successfully removed ammonia (70–100%) and assimilable organic carbon (AOC) (47–55%) although their thicknesses were approximately 50–200 μm. The ammonia removal efficiency exponentially increased as the average time of water passing through the biofouling layer increased. The fouling resistance sharply increased at the flux of 30 L/m2·h, while a negligible increase was observed at the flux of 10 L/m2·h. Moreover, membrane fouling was low even without backwashing or air sparging at the flux of 10 L/m2·h. According to the observation from optical coherence tomography (OCT) and the analysis of the specific resistance of biofouling layers, these phenomena of low fouling were related to the thin and loose biofouling layers at low flux, which formed because of the low accumulation of foulants, low compression of the biofouling layer at low trans-membrane pressures, and the activity of microorganisms. These findings suggested that we can utilize the benefits of biofouling to remove specific pollutants without a remarkable increase in fouling resistances.