Integrated Forward and Reverse Osmosis for Coal Mine Wastewater Desalination

Water management is an integral part of coal mining operations. Due to the constraints on releasing saline water, coal mines require additional water storage facilities and therefore seek to minimise their inventory of saline water. Adopting efficient treatment technologies on-site would minimise the risk of wet season run-offs, freshwater contamination and allow segregation of different qualities of water to enable greater water recycling. This study aims to evaluate the application of an integrated forward osmosis (FO) and reverse osmosis (RO) system with three different actual coal mine waters, containing various concentrations of sulphates and silica that are generally associated with scaling and fouling of membrane systems. Three different FO draw solutions, di-sodium hydrogen phosphate (DHSP), sodium hexametaphosphate (SHMP) and sodium lignosulphonate (SLS) were evaluated. Two different modes of integrating the FO and RO systems were identified. The integrated system was able to concentrate the brackish mine waters, recovering more than 80% of the volume of mine water and obtaining dischargeable quality treated water. Simple physical cleaning with clean water circulation was found to be effective in restoring the FO water flux. The osmotic gradient between two mine waters was also utilised to adopt mine water as a draw solution. The effect of solution temperature on stand-alone and integrated FO and RO systems was also evaluated. The combination of FO with RO provided a better performance than individual FO or RO in treating coal mine wastewater. The FO unit served as an effective pre-treatment system prior to RO and the integrated FO–RO systems has a strong potential to successfully eliminate conventional pre-treatment processes for RO.