Open Cut » Environment
The forward osmosis (FO) process, also known as direct osmosis, is an emerging membrane technology for water reuse and desalination. In the FO process, water naturally traverses the semi permeable membrane from a lower solute concentration feed solution on one side of the membrane to a higher solute concentration solution, known as the draw solution, on the other side of the membrane. In this process, the two solutions of different concentration create an osmotic gradient, which is the driving force, which induces water permeation across the membrane. Ideally, as the higher osmotic pressure draw solution extracts the water across the membrane from the feed solution, the semipermeable FO membrane acts as a barrier that rejects salts and other contaminants and prevents its passage through the membrane. During the course of this process, feed water stream becomes more concentrated and the draw solution becomes diluted until an osmotic equilibrium is reached. Clean water is recovered from the diluted draw solution using a post treatment such as reverse osmosis. Over the last ten years, significant progress has been made in developing FO membranes and new draw solutions, resulting in the increase in its application to treat various water sources.
Environmental constraints prevent untreated mine water being discharged into water-ways. 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. Adopting efficient treatment technologies on-site would help to significantly minimise their inventory of saline mine impacted water storage and achieve consistent discharge quality water. It would also minimise the risk of wet season run-offs and freshwater contamination and allow segregation into different qualities of water to enable greater water recycling. Treated saline mine water can also offer new sources of supply of reusable quality water to coal mines, especially during periods of drought.
ACARP project C21043 previously introduced the osmotically driven forward osmosis (FO) process for coal mine water treatment and through a laboratory scale setup using flat sheet membranes, successfully demonstrated the proof-of-concept of integrating forward osmosis with reverse osmosis (RO) to obtain dischargeable or reusable quality water. The current project (ACARP C23031) further progressed this innovative FO-RO technology through the design and development of a modular pilot treatment unit of 1 m3/d treatment capacity. Two actual mine impacted waters, obtained from two different coal mines in Queensland were used in this study. Performance of the individual FO membranes and the integrated system with RO was evaluated and optimised for treating the mine impacted waters. Additional experiments were undertaken in the laboratory to test the performance of the FO membranes. These laboratory-scale tests used a solution of NaCl, which mimicked the properties of mine-impacted water. This study also investigated the feasibility of using actual RO brines as FO draw solution for providing the osmotic gradient across the membrane.
Optimum operating parameters for the overall system were determined through a series of stand-alone experiments using FO and RO processes and then as an integrated FO-RO system. The performances of the two FO membranes were evaluated with varying flowrates and concentrations of the feed and draw solutions. For a given starting concentration of mine water feed solution and with constant concentration of 5.5 wt% NaCl draw solution, the performance of FO-B membrane in terms of water flux was found to be about 1.5 times higher than the FO-A membrane. The experimental results obtained from the pilot FO-RO testing, have demonstrated that the combined FO-RO system was able to eliminate extensive pre-treatment steps that are generally required for conventional RO. Treated mine water was consistently of safe discharge quality that can also be reused for mining operations. This project also showed that the FO2 RO system can use coal mine water desalination brine wastes as the draw solution. This has implication for water treatment. Use of desalination brine wastes from coal mines in applications such as in FO draw solution help to minimise brine waste inventory at mine sites and potentially reduce the overall cost of desalination projects.
A preliminary economic evaluation was carried for a 10,000 m3 d-1 FO-RO mine water treatment system and was compared with a RO process using conventional pre-treatment with ultrafiltration (UF). Estimations for capital cost, operating cost and the total water cost (for treating one m3 of water), for the two treatment processes were compared. Compared to the conventional UF-RO process, FO-RO process is expected to provide about 15% reduction in CAPEX and about 10 % reduction in OPEX. Various basic assumptions for this estimation are detailed in this report. For an initial mine water feed concentration of 9.5 g L-1, the integrated FO-RO process would reduce the brine volume by 81 %. At least 8 % more reduction in brine waste volume for disposal is expected, compared to a conventional RO treatment. This would be an important cost consideration for this treatment option.