Page - 69 - in Clean Water Using Solar and Wind - Outside the Power Grid

Water treatment 69 5.3.3 Reverse osmosis RO units are available in a wide range of capacities due to their modular design. The RO technology is used in applications from family size to as large as 600,000 m3/day. This scalability together with similar scalability properties of solar PV and wind energy makes this combination extremely interesting. Large plants are built up with hundreds of units that are accommodated in racks. A large plant’s capacity far exceeds 100,000 m3/day or 1 m3/second, while very small units are made for flow rates as small as 0.1 m3/day or about 4 litres/hour. The small units are used for single households, marine purposes or hotels and are suited to and used in rural areas or islands where there is no other water supply available. Natural osmosis (or direct osmosis) governs how water transfers between solutions with different concentrations. It is the basis for the way in which human skin and organs function, and how flora and fauna maintain water balance. The osmosis process can be explained when there is a semi-permeable barrier such as a membrane located between two solutes with different salt content, as in Figure 5.1. In natural osmosis the water tends to flow from a solution with a lower concentration to a solution with a higher concentration if no external pressure is applied. Diluted solution Diluted solution Concentrated solution Concentrated solution Osmosis Osmotic equilibrium Reverse osmosis Applied pressure Semi-permeable membrane Figure 5.1 Illustration of osmosis and reverse osmosis. The arrows denote the water flow direction. In RO the water will flow in the opposite direction compared to natural osmosis, Figure 5.1. This will require that a pressure is created. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user