Integrated Sulphate Removal from Ion Exchange Brine Using Chemical Precipitation and Ceramic Nanofiltration

Abstract

In drinking water production, natural organic matter (NOM) is sometimes removed using ion exchange (IEX) resin. This treatment method has a limitation based on the exchanging capacity on the resin. Therefore, the resin needs to be regenerated when it is saturated with adsorbed NOM which leads to the production of brine. In general, NaCl is used to regenerate the resin, hence, the brine will contain NOM, high sodium and chloride concentrations. Moreover, some other anions are also found in the IEX brine, such as sulphate that is usually present in surface water and ground water. Because of its salinity, the disposal of IEX brine is not possible to be done conventionally due to its impact on the environment and high cost. Therefore, separating chloride from the brine is an interesting alternative that can be reused for the regeneration of the IEX in the later process.

Ceramic nanofiltration (NF) emerges to be an interesting alternative for water treatment. Compared to polymeric membranes, this type of membrane offers great mechanical robustness and can be operated under extreme conditions, and tolerates high-pressure backwash, chemical cleaning, and high-temperature sterilization, which leads to longer periods of reliable performance. Moreover, ceramic NF membranes are potentially capable to separate multivalent ions from monovalent ions. Hence, this method could be applicable to treat IEX brine. Alternatively, chemical precipitation using barium and calcium is widely used to remove sulphate from water which is more straight forward than membrane filtration. The precipitates can be mechanically separated from the supernatant for further treatment or use.

Combination of chemical precipitation and ceramic NF membrane (later called as integrated sulphate removal) was investigated to remove sulphate from IEX brine. Along with that, investigation using synthetic brines consisting of Na2SO4 and NaCl for a binary salt solution and only Na2SO4 for a single salt solution was also conducted to build the understanding in treating the IEX brine. Barium salt was proved to efficiently remove sulphate due to its very low solubility. However, calcium salt was not as effective as barium salt. The treatment was followed by NF using a ceramic membrane with MWCO of 900 Da. In the end, the integrated approach was able to remove 86% of the sulphate and 85% of NOM from IEX brine. Furthermore, the precipitation stage was also modelled in PhreeqC by using Pitzer database.

Barium salt (BaCl2.2H2O) was preferred in this research for precipitating the sulphate. However, due to its toxicity, alternative precipitation was desired. Ettringite (calcium sulfoaluminate) precipitation was considered since the involving salts were not toxic. The efficacy of this method was predicted through modelling in PhreeqC to give some insight to alternatively removing sulphate from IEX brine. Eventually, a comparison using cost estimation and Life Cycle Assessment (LCA) were performed to obtain some considerations to implement the treatment alternative in a full-scale application.