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Water treatment 71 Brackish water reverse osmosis (BWRO) systems typically require pressures in the range 1.5–2.5 MPa (or 15–25 bar). Consequently, less power is needed for brackish water desalination (BWRO) compared to seawater desalination (SWRO). Also, the lower pressures found in BWRO systems permit the use of low-cost plastic components. Therefore, the total cost of water from brackish water is considerably less than that from seawater, and systems are beginning to be offered commercially. The required pressure-pump power is proportional to the flow rate of the feed and to the pressure difference between the exit and inlet pressure of the primary pump. The RO plants are sensitive to the feedwater quality, such as salinity, turbidity and temperature. Energy is consumed not only to achieve a sufficiently high pressure for the RO process; it is also needed for the pumping that pulls the water into the desalination facility as well as for the filters for the pre-treatment. Up to 72% of the total energy required for seawater desalination with RO membranes is consumed by the high-pressure pumps (Voutchkov, 2012). Therefore, it is essential to make sure that the pressure pump has a high efficiency. 5.3.4 Reverse osmosis membranes In an RO membrane the water moves by flowing through the polymer structure, which is always hydrophilic and “swelled” by the water. The RO membranes are arranged in spiral-wound modules so that the membrane unit looks like a tube. It is very important to pre-treat the feedwater. Suspended particles need to be removed via pre-filters and chemicals may be added ahead of the RO filters. A well-functioning pre-treatment is the key to reliable fresh water production. This will minimise membrane washes and will give the RO modules a longer life. Still membranes can be affected by fouling that can be caused by high-turbidity feedwater where membranes are clogged with suspended solids, marine organisms and their metabolic products. Fouling is also a result of the sedimentation Energy for desalination decreases almost linearly with decreasing salt content. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user