Reciprocal enhancement of reverse osmosis and forward osmosis
Periodically operating a forward osmosis (FO) semi-permeable membrane element in reverse osmosis (RO), to yield product extraction on a feed side of the membrane, to flush out a concentration polarization (salt concentration of the feed side and dilution of draw solution on the draw side) across the membrane. Utilizing treated waste water to generate the flushing solution of low osmotic pressure, and gauge pressurizing the flushing solution to the gauge pressure of the draw solution to keep a constant gauge pressure of the solutions. FO process is interrupted every 5-20 minutes for a 10-60 seconds long flushing RO process, thereby increasing the FO throughput five fold and recovering power from the increased throughput, i.e. from the osmotic pressure of the brine.
1. A method of energy recovery in a reverse osmosis (RO) unit, comprising:
expanding a volume of gauge pressurized brine removed from the RO unit by forward osmosis over a semi permeable membrane from a low salinity solution, to enhance power recovery from the expanded pressurized brine via a pressure exchanger,
periodically injecting gauge pressurized low salinity flushing water instead of the gauge pressurized brine to remove concentration polarization from the semi permeable membrane, to enhance a forward osmosis efficiency thereof, and
gauge pressurizing seawater feed to the RO unit by the enhanced power recovery via the pressure exchanger.
2. The method of claim 1 wherein the periodical injection is carried out every 5 to 20 minutes for 10 to 60 seconds.
3. The method of claim 1 , further comprising measuring a time period from a beginning of operating the semi-permeable membrane in forward osmosis to a time in which the concentration polarization reaches a specified threshold, and wherein the periodical injection is carried out with a period equal to the measured time period.
4. The method of claim 1 , wherein the gauge pressurized low salinity flushing water has the same gauge pressure as the gauge pressurized brine.
5. The method of claim 1 , wherein the low salinity flushing water is pressurized by the gauge pressurized brine.
6. The method of claim 1 , further comprising selecting durations of a FO phase and of a flushing phase to maximize an overall pressure extracted by the pressure exchanger from the expanded pressurized brine and supplied by the pressure exchanger to the seawater feed.