Hybrid membrane - Thermal versus simple membrane systems

Conventional SWRO systems work under 6.5 MPa with 40% recovery. Developed in Japan, the BCS system can obtain up to 60% recovery under 9.0 MPa in the second stage RO modules. The recovery of desalted water by a high pressure RO (HPRO) can be increased up to 75-80% under 20 MPa in an integrated system for complete usage of components in seawater which is being developed also in Japan. In the process of salt production from seawater, raw seawater is concentrated to TDS of 195 g/1 by electrodialysis. Concentration to saturation of NaCl by ED and MD is also investigated. However, the production of such concentrated brine, by means of the previously mentioned membrane methods is questionable. Analysis of performance of the aforementioned systems shows that RO may compete with thermal methods if the inlet salt concentration does not exceed ∼70 g/1 and maximum profitable concentration of retentate is ∼90 g/l. While investigating the processing of coal-mine water (TDS 12 g/l) concentration to saturation in an ED-MSF system (as a first step of NaCI recovery), with simultaneous desalination to 1.1 g/l of Cl^-, an optimum value of TDS of ED concentrate was found. Taking into account co-generation of electricity production and coal-mine waters utilization, cost of electric energy as low as 0.03 $/k Wh was assumed and TDS 90 g/l obtained. ED-MSF system seems therefore to be more effective than ED alone in the investigated range of brine concentration. Thermal methods still seems to be more effective than membrane methods in terms of production of highly concentrated brines. The performance of desalination and concentration in hybrid membrane-thermal systems is then higher than the performance of simple membrane methods. Hybrid NF-RO-MSF-crystallization system offer a very promising performance: high water recovery (77.2%) and water cost as low as $0.37/m³.