Page - 70 - in Clean Water Using Solar and Wind - Outside the Power Grid

70 Clean Water Using Solar and Wind: Outside the Power Grid The salty water is given a high pressure (∆p) that exceeds the osmotic pressure (∆π). Then the water from the concentrated solution will pass through a semi-permeable membrane and leave the solid salt particles behind. The osmotic pressure for seawater with 3.5% salt is 2.6 MPa (or ≈26 bar). Brackish water needs less energy since the osmotic pressure is lower. This means that a RO system for salty water only begins to produce water when a pressure higher than the osmotic pressure is achieved. In fact, the flux of the water through the membrane is proportional to the difference in pressure between the applied pump pressure ∆p and the osmotic pressure ∆π: J C pw = ⋅ −( )∆ ∆π where Jw is the flux (ℓ/m2 membrane per hour) and C the so-called permeability constant. Consequently, the flow rate is Q J A C p Aw= ⋅ = ⋅ − ⋅( )∆ ∆π where A is the surface area of the membranes. The permeability depends on temperature. The higher the water temperature the higher the permeability will be. The change in permeability is about 3% per °C. As a result, the required pressure to achieve or keep a certain flux or capacity will be lower at higher temperatures. A small part of the dissolved substance also goes through the membrane with the water. Some 2% of common salt (NaCl) may go through the filter in RO. The actual required pressure and subsequent energetic cost of desalination is 2–3 times higher than the osmotic pressure due to inefficiency, material losses and membrane fouling. Typical operating pressure for seawater desalination is in the range 5.5–6.2 MPa (or 55–62 bar) but pressures in the range 6.9–8.3 MPa (69–83 bar) can be found. Brackish water desalination needs less energy since the osmotic pressure is lower. It decreases almost linearly with decreasing salt content. The osmotic pressure (in bar) can be estimated with a rule of thumb formula: ∆π ≈ ⋅ ⋅−07 10 3. C where C is the salt concentration (mg/ℓ). For 1% salinity the osmotic pressure is around 0.75 MPa (or 7.5 bar). Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user