Seawater pretreatment for thermal plant by pressure stimuli-responsive forward osmosis membrane

Scale formation and deposition on the heat exchanger of thermal desalination have serious consequences on the plant performance and energy consumption. This study presented a patented method for diluting the brine from the multi-stage flash (MSF) plant with seawater using nanofiltration (NF) membranes of specific characteristics in the forward osmosis (FO) process. Three flat-sheet NF membranes of < 200 µm structure parameter were tested as pressure stimuli-responsive (PSR) membranes in the FO process. The NF membranes are designed to reject divalent ions, such as sulfate, magnesium, and calcium, which are the main reason for scale formation and deposition in the MSF plant. In this study, the performance of cellulose triacetate (CTA) and thin film-composite (TFC) FO membrane was compared with the PSR NF membranes for seawater pretreatment using an MSF brine draw solution at 40 °C. The water flux in the PSR NF membrane was insignificant at 0 bar pressure but increased several times when the feed pressure increased to 2 and 4 bar pressure. For the FO tests at 4 bar pressure, the average water flux in the PSR membranes was up to 2.6 times more than that in the CTA and TFC FO membranes, all without increasing the energy requirements of the FO process. In the tight NF membrane, the MSF brine dilution occurred by the permeation flow, but in the loose NF membrane, it was by permeation flow and ions' reverse diffusion from the draw solution to the feed solution. Notably, the PSR NF membranes achieved a substantial cost reduction that is ten times cheaper than the FO membrane, which has excellent potential to reduce the cost of the FO process.