Lab-Scale Nano-Filtration Membrane Performance With High Silica Concentration Feed Water

Abstract

Water scarcity emphasizes the necessity of water recovery from saline sources, which are plentifully available. Nanofiltration (NF) is widely implemented in water reclamation treatment particularly in industrial field. However, the presence of silica hinders the possibility of obtaining a high recovery in a NF membrane system. In this work, the effect of silica concentration on the scaling of a NF membrane was investigated. Three types of feed water (demineralized water, water with 100 mg L-1 and 300 mg L-1 of SiO2) were fed to a single stage nanofiltration element. All cases were carried out at room temperature, pH values ranged 7 - 8 and a constant recovery of 11±1% for 24 hours continuously. The mass transfer coefficient (MTC) was used as method to determine the scaling of the membrane module. Results showed that no silica scaling occurred during the system runtime. Nonetheless, for all cases, an initial decrease in MTC could be detected. On the one hand, based on some literature reports, it is likely that a part of silica (mainly colloid) had precipitated on the membrane surface forming a more porous layer than the precipitation of polymerized monomeric silica. But to verify this argument, a further autopsy method is demanded. On the other hand, concentration polarization may also contribute to the initial decline in mass transfer coefficient. But the reason for the decrease for demi-water case remains unknown. In brief, Nanofiltration membrane can operate at a silica concentration up to 300 mg L-1 for 24 hours in lab-scale. The guideline (120 mg SiO2 L-1) suggested in the industrial standard (ASTM, D4933-08) was judged to be relatively conservative. For more practical purpose, further study is needed.