Ceramic nanofiltration membranes

Abstract

Natural Organic Matter (NOM) is always present in the drinking water sources such as rivers, lakes and reservoirs. It causes several problems, including the unfavourable colour and odour of water, formation of disinfectant by-products and harmful microbial growth. In drinking water treatment, anion exchange (anion-IEX) is used for NOM removal. However, the regeneration of anion-IEX produces a brine, which is a high saline waste stream containing the desorbed NOM and anions (i.e. sulphate) as well as the residual sodium chloride added during the regeneration process.
One possible approach to manage the waste brine is to recover the valuable compounds from the brine, and consequently, reduce the brine volume that has to be disposed. The separation could be done by ceramic nanofiltration (NF) membranes. However, the separation performance of ceramic NF membranes at high salinity conditions is still not fully understood. Therefore, the use of ceramic NF membranes to treat the brine-like wastes that contain NOM and high salt concentrations were investigated.
Commercial 500 Dalton(Da) ceramic NF membranes were used in the salt experiments. Several experimental conditions such as pH and ionic strength were investigated to understand their influence on the rejection of sulphate (SO42-) and chloride (Cl-). The results show that the salt rejection was governed by charge effect, and it changed depending on pH and ionic strength. When ionic strength was 0.1M, the mitigated charge effect led to a low rejection of SO42- (<20%) and Cl- (<5%) by the 500Da membranes. On the other hand, the 560Da membrane used in salt&NOM experiments exhibited more than 95% rejection of NOM but less than 25% rejection of both SO42- and Cl- when ionic strength was 1M.
Since the 500Da membranes were unable to reject divalent ions at high ionic strength, it is suggested to decrease the membrane pore size to achieve the separation of SO42- and Cl-. The size of pores in ceramic membranes can be narrowed down by an approach called Atomic Layer Deposition (ALD) (Shang et al., 2017). In this study, the vacuum TiO2 ALD was applied to coat thin films on the commercial ceramic NF membranes that have Molecular weight cut-off (MWCO) ranging from 600Da to 900Da. After the first coating, the water permeability of the membranes decreased dramatically while the MWCO of the membranes decreased slightly. After the second coating, an increased MWCO was observed, which might be attributed to the plugging of small pores. In addition, the flux distribution and pore size distribution were theoretically analysed to investigate the change of the membrane pores before and after ALD.