Reverse Osmosis

Part 1: Limits of Energy Savings in Seawater RO

By Wayne Bates, Craig R. Bartels, Ph.D., and Rich Franks


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In recent years, there has been continuing development of seawater reverse osmosis (SWRO) processes to reduce the energy consumption, one of the main contributors to SWRO operational costs. As reported in one paper (1), the specific energy for the RO process has been reduced from 27.6 kilowatt hours per thousand gallons (kWhr/kgal) (7.3 kilowatt hours per cubic meters [kWhr/m3]) to a current value of 9.1 kWhr/kgal (2.4 kWhr/m3). Another study (2) reported pilot testing low energy seawater membranes, with the achievement of 6 kWhr/kgal (1.58 kWhr/m3), but operated at unique conditions. Also, there are other papers which report the actual low energy consumption of new state-of-the-art SWRO plants. At the Gold Coast Desalination plant in Australia, it has been reported that this plant is operating at around 11 kWhr/kgal (3 kWhr/m3) of energy consumption at 19oC and 35,500 milligrams per liter (mg/L) feed salinity (3). Another report (4) indicated that the SWRO unit at Kindasa, Saudi Arabia, is operating at 11.6 kWhr/kgal (3.07 kWhr/m3) on a feed of 42,500 mg/L with total dissolved solids (TDS) at about 30oC and 48% recovery. These plants have demonstrated that very low energy can be obtained with current SWRO technology. Much of the recent improvement in energy savings has resulted from the use of new membranes, high efficiency energy recovery devices, and process design improvements. This significant energy savings then leads engineers in the industry to the question of how much more energy savings is possible. This is especially important since energy consumption by the SWRO process generally accounts for 30% to 50% of the total water cost to produce desalinated water.

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