Energy performance enhancements of a 950 m3/d seawater reverse osmosis unit in Grand Cayman

This paper describes the upgrade of a seawater reverse osmosis (SWRO) unit by replacing the hydraulic turbocharger energyrecovery system with a Dual Work-Exchanger Energy-Recovery (DWEER) system. Before the upgrade, the unit operated at a capacity of 1,071m3/d, and a specific electricity of 3.00kWh/m3. Tests conducted on the unit after the upgrade, when operated at essentially the same membrane conditions, showed a 59% capacity increase to 1,699m3/d, and a 26% reduction in specific electricity to 2.22kWh/m3. A further test was then conducted on the unit, by temporarily modifying it such that it operated at essentially the same conditions as before the upgrade. The test resulted in a specific electricity of 2.32kWh/m3, a reduction in specific electricity of 23% compared to the original performance. This confirmed that the reduction in specific electricity was due to the DWEER system and not economies of scale from the increased capacity. A detailed efficiency analysis showed that the upgraded unit had an overall desalination energy efficiency of 45%, which was broken down into 85% for pumping, 92% for energy recovery; and 57% for the membrane array. This indicates that further improvements in efficiency are most likely to be achieved in the membrane array. The operating experience of the unit for the first 6 months is presented.