Influence of osmotic mediation on permeation of water in reverse osmosis: Experimental and numerical analysis

Osmotically mediated reverse osmosis (RO), whose concept is to lower the osmotic pressure difference (Δπ) across the membrane by supplying a “mediate” solution of low or equal concentration (relative to feed solution concentration) in the permeate side, has been recently suggested as a viable way to treat hypersaline brines. However, most of the studies on the osmotically mediated RO process have focused on numerical analyses or simulation. Accordingly, we experimentally delved deeper into the permeation of water through the membrane and numerically analyzed this permeation for the process. When Δπ across the membrane was mediated in the bulk phase for the feed solution of 0.2 to 1.2 M, the water flux significantly decreased with the feed solution concentration. Specifically, for the feed solutions of 0.6 and 1.2 M, the water fluxes of 22.1 and 5.76 L m-2 h-1 were obtained at ΔP of 40 bar and Δπ of 0 bar in the bulk phase, respectively. Numerical results showed that even when Δπ was mediated in the bulk phase, effective osmotic pressure difference (Δπeff) for each concentration increased with ΔP increase and increased further with the feed solution concentration. Nevertheless, the osmotically mediated RO process can be regarded as hydraulic dilution process. By virtue of this process, the high-pressure single stage RO process may be replaced by a moderate-pressure two-stage RO process for low-energy process.