Considerations of energy consumption in desalination by reverse osmosis

The primary factors affecting the energy consumption of a reverse osmosis plant are considered. These are the osmotic pressure of the feedwater, the feedwater temperature, the water recovery, and the relationship between the water flux and salt flux characteristics of the membrane. In addition, the required permeate quality may have several indirect effects on the energy consumption. Permeate quality standards may impose minimum operating pressures, limit the recovery, and/or require treatment with a full or partial second stage. As a general rule, the energy required increases with increasing feed salinity and increasing permeate quality.For any given recovery, a single stage system will require less energy than a partial two stage system. However, for a specified permeate quality, a partial two stage system can operate at a higher overall recovery and a lower energy consumption than a single stage systemEnergy recovery systems can recover between 50 and 90 percent of the available energy in a reverse osmosis unit, thus significantly lowering the energy consumption. Studies have shown that with an energy recovery system, the minimum energy consumption occurs at a first stage recovery of 30 to 35 percent. Currently, very few energy recovery systems are in use due to their high capital cost, but as energy recovery systems become more available and reliable, they will greatly increase the energy efficiency of reverse osmosis plants.