Application of robust MFI-type zeolite membrane for desalination of saline wastewater

Zeolites are potentially a more robust desalination alternative as they are chemically stable and possess the essential properties needed to reject ions. This work proposes to use zeolite membranes for desalination of saline recycled wastewater for the possibility of avoiding the costly pre-treatment needed for polymeric reverse osmosis membranes. The MFI-type zeolite membrane was developed on a tubular α-alumina substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N₂) permeation, and underwent desalination tests with saline wastewater under different conditions. When saline wastewater was fed at 7MPa to the zeolite membrane it showed a salt rejection of 80% based on electrical conductivity (EC) and a flux of 4Lm^-2h^-1. A₂₅₄ (organics absorption at the wavelength of 254nm measured by a HACH DR5000 spectrophotometer) removal exceeded 90%. Slightly lower salt removal and A₂₅₄ results were observed when operating at a lower pressure of 3MPa. During batch concentration runs on the saline wastewater, the EC of the feed water increased from the initial value of 1770μScm^-1 to 3100μScm^-1 over the 48h test time which indicates that a 43% water recovery was achieved. EC reduction remained >70% and flux was maintained at around 2Lm^-2h^-1 throughout the test period, indicating that the membrane resisted organic fouling. Chlorine stability studies showed that a long-term (7 days) strong hypochlorite clean did not significantly alter the flux or rejection confirming the chemical stability of zeolite membranes. Overall, the zeolite membrane showed excellent chemical resilience and produced a desalinated product suitable for reuse applications (e.g. irrigation, residential or industrial). Fluxes, however, need to be improved to be competitive with current polymeric membranes as do rejections for higher purity water applications.