Water Desalination Essay

Water desalination is the process of removing salts and minerals from water that has a high concentration of salt dissolved in it, and can therefore not be consumed as drinking water. Desalination of saline water can be achieved by a technique known as reverse osmosis. In osmosis the net movement of solvent molecules occurs from an area of low solute concentration to a more concentrated one through a semi-permeable membrane to balance the concentration of solute on either side of the membrane. However, in reverse osmosis the net flow of solvent is in the opposite direction this can be achieved by applying an external pressure that is great enough to stop osmosis from occurring, this is known as the osmotic pressure. If a pressure greater than

However, as carbon dioxide is released in the reaction it can dissolve into the water and lower the pH. So it is important that the pH is monitored so that it doesn’t make the water too acidic. The addition of coagulants reduces the charge on the colloid and means that there is less repulsion between adjacent particles. The next step in the process is known as flocculation where the particles begin to clump together. The particles will be more strongly attracted to each other and can form much larger clumps. These larger clumps can be removed much more easily than the smaller particles. As the colloids have a neutral charge the force dominating the attraction of the particles is van der Waals forces of attraction. As the particles come within a close proximity an induced dipole is generated on an adjacent particle and they begin to clump together. Once the clumps are large enough they can be separated by sedimentation as they settle at the bottom of the

Scaling is where salts such as: calcium carbonate, calcium sulphate and magnesium hydroxide precipitate out of the water due to super-saturation and deposit onto the partially-permeable membrane and block the pores. This results in a reduced water flow through the membrane and a larger pressure gradient which can cause increased strain on the partially-permeable membrane. The main cause of the build-up of scale is the saturation point of the salt, when this point is reached no more salt can be dissolved in the water and the salt begins to precipitate out causing scale to form. Magnesium and calcium possess a property known as inverse-solubility, as the temperature is increased the solubility decreases. This means that more of the substance precipitates out of the solution at higher temperatures causing increased scaling. It is important not to use high temperatures in the plant as this will cause the level of scaling to increase. Calcium carbonate is the largest cause of scale formation and is formed from bicarbonate and calcium ions in the following