Open Cut » Environment
Mines across Australia face enormous water challenges and desalination options are limited and expensive. Managing the saline water generated during coal mining is becoming an important and costly exercise for the coal industry. Due to the constraints in releasing the saline water, coal mines require additional water storage facilities and therefore seek to minimise their inventory of saline water. Also, during periods of drought, the treated saline mine water can offer new supply sources of reusable quality water to coal mines. Adopting efficient treatment technologies on-site would minimise the risk of wet season run-offs and freshwater contamination and allow segregation into different qualities of water to enable greater water recycling. Conventional reverse osmosis (RO) desalination would require extensive and expensive pre-treatment to reduce membrane fouling and to ensure reliable performance. This project aimed to establish proof-of-concept for this technology by adopting a laboratory scale osmotically driven forward osmosis (FO) process as the pre-treatment to a RO unit and to evaluate the performance of the integrated FO-RO system to treat mine impacted water to a level safe for discharge or reuse.
In the RO process, the water is separated from the salt by applying a hydraulic pressure over the osmotic pressure of the feed to force water through a semi-permeable membrane. In the FO process, the separation is achieved by the concentration gradient (difference in osmotic pressures) between a lower solute concentration feed solution (in this case, mine impacted water) on one side of the membrane and a higher solute concentration solution, known as draw solution, on the other side of the membrane. Water, from the brackish mine impacted water, naturally permeates through the membrane to the draw solution due to the osmotic gradient. Clean water is then recovered from the diluted draw solution via RO. In this integrated FO-RO system, the FO unit prior to RO unit serves as an effective barrier to the RO membrane for the removal of suspended solids and micro-organisms present in the mine impacted water and also a range of dissolved solids that represent a fouling and scaling threat to the RO system.
The experimental results obtained from the laboratory scale flat sheet membrane FO-RO test unit, developed through this project, have demonstrated the proof-of-concept of the integrated FO-RO system for the treatment of mine impacted water. The combination of FO with RO provided better performance than stand-alone FO or RO systems. The FO unit served as an effective pre-treatment system for RO and the integrated FO-RO system has a strong potential to successfully eliminate conventional RO pre-treatment processes. The elimination of the extensive and expensive pre-treatment currently used for RO at mine sites, offers a significant reduction in energy utilisation, chemical usage, piping infrastructure and operating costs. The improved sustainability of the integrated FO and RO desalination process is an additional benefit to the savings in the overall desalination cost.
However, before developing a full scale FO-RO unit (nominal capacity of 100 m3/day) for mine site implementation, it is necessary to develop a pilot scale integrated FO-RO unit with spiral wound modules and then trial it at a mine site to obtain reliable scale up information including optimum operational parameters and also to gain practical site operation experience. The optimum operating conditions of the laboratory size small flat sheet membrane cell will be radically different from those of an FO-RO system using a spiral-wound membrane module.
An e-newsletter has also been published for this project, highlighting its significance for the industry.