Underground » Environment - Subsidence and Mine Water
The forward osmosis (FO) process, also known as direct osmosis, is an emerging membrane technology for water reuse and desalination. The driving force for the FO process is the osmotic gradient between the two solutions on either side of the membrane, which induces the water permeation across the membrane from a lower solute concentration feed solution to a higher solute concentration solution, known as the draw solution. The semipermeable FO membrane acts as a barrier that rejects salts and other contaminants and prevents its passage through the membrane. During this process, the feed water stream becomes more concentrated and the draw solution becomes dilute due to the permeated water. The reduced volume feed is the brine reject and the permeated water is recovered as product reusable water using the treatment process such as the reverse osmosis and at the same time regenerating the draw solution and returned to FO, thereby maintaining the osmotic gradient for the FO membrane. Over the last decade, progress has been made in FO membrane development and the use of draw solutions, resulting in the increase in its application to treat various impaired water sources.
Effective water management initiatives on-site help to maximise the amount of water being reused and to minimise the volume discharged off-site without harming the environment. Adopting efficient treatment technologies on-site would help to significantly minimise the inventory of saline mine impacted water storage and achieve consistent discharge quality water. Treated saline mine water can also offer new sources of supply of reusable quality water to mines and surrounding areas, especially during periods of drought. Reverse osmosis (RO) is one of the most commonly used and mature desalination technologies. However, a conventional RO would require extensive pre-treatment to reduce membrane fouling and to ensure acceptable performance.
In this project a site trial demonstration of an integrated FO-RO process was undertaken in treating the coal mine waters under actual mine site conditions. A prototype FO-RO test unit of capacity 10m3/day was designed, developed and trialled at a mine site. The performance of the FO-RO unit was tested with different feed mine water characteristics: filtered mine water, clarified mine water and mine water without pre-treatment. The performance of individual FO and RO membranes were studied, and the integrated FO-RO system was then evaluated and optimised for treating the mine waters.
The integrated FO-RO unit was able to successfully treat the mine waters and for every 100L of mine water treated, about 90-95L of reusable quality water was able to be recovered (water recovery 90-95%) with total dissolved solids of less than 300 mg/L, with a specific energy consumption for treatment between 2.85-3.3 kWh/m3 of water produced. With suitable post treatment steps such as disinfection and remineralization, the recovered water would meet the potable water quality guidelines. The FO and RO membranes acted as a double barrier for the rejection of contaminants. The site trial testing demonstrated that a combined FO-RO system was able to eliminate extensive pre-treatment steps that are generally required for conventional RO. The FO unit was also able to handle very high suspended solids, with waters having turbidity of about 1000 NTU.