Page - 72 - in Clean Water Using Solar and Wind - Outside the Power Grid

72 Clean Water Using Solar and Wind: Outside the Power Grid of natural organic material (NOM) and mineral particles on the membrane surface. Fouling restricts water flow and ultimately affects the water recovery of the membrane system. It can also damage membranes. Fouling can be prevented by adding special chemical agents or anti- scalants. The presence of fouling increases energy use. Frequent cleanings and chemical injection reduce the RO membrane life. Even with good pre-treatment design it is vital to provide for periodic washing of the modules. If washing is inadequate, it generally must be repeated quickly. The desalination process produces two streams of water, one with the fresh water (having low salinity) and one with more salt. From the feedwater flow about half the volume is converted to drinkable water and the other half will be about twice as salty as the incoming water. The second stream is called brine, and its disposal is a key issue. The brine is denser than the seawater. As noted before, chemicals like anti-scaling agents and coagulants are added to the seawater in the desalination process. The chemicals do not pass through the membrane and are left in the brine. As soon as the brine is released, biological activities will begin, and microorganisms will feed on the chemicals. This consumes dissolved oxygen in the receiving water. Phosphate and nitrate are also released when chemicals in the reject water break down, causing eutrophication of the receiving water. So, if the brine is discharged into the sea, there is a risk for marine life. Too much salt can be just as deadly for sea life as seawater is for land animals and crops. Furthermore, the brine is often quite hot, so its disposal can have a negative impact both on land and in water. One way to minimise the potential negative effects of the salts and the chemicals in the brine is to mix the reject water rapidly with the surrounding water. A desalination plant located at the ocean has practically unlimited access to seawater. Inland desalination plants, on the other hand, can be used for water reuse and drinking water production from used water where traditional sources are inadequate. For inland plants the disposal options must be carefully considered. The primary environmental concern with the disposal of concentrate to surface water, to sewers or by land application, is salt-loading the receiving waters, whether they be surface water or groundwater. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user