Water Purification
Water is rarely pure. Depending on the ground materials wherein water passes through, different substances and micro-organisms can be dissolved or incorporated in the water. That is why water, prior to its distribution, often needs to be purified.
The type of purification process depends on the actual final use of the water. Water destined for human consumption, must be free from disease-causing microbes and toxic substances. Besides, people prefer water not to have turbidity, and free of taste and odours.
Water from boreholes usually is fit for drinking purposes but may require disinfection. The addition of a disinfectant like chlorine is needed to kill the bacteria and other micro-organisms that could be present in the water and to prevent the pollution of the water with living microbes when passing through the reservoirs and the pipes of the distribution network until the tap of the consumers.
On the contrary, surface water from rivers, lakes and dams usually need a more complete treatment before distribution to the population. The process of a typical drinking water treatment plant includes a stage of sedimentation and another stage of sand filtration besides disinfection.
A major part of particles in water are in the form of stabilised colloids that stay in suspension. In order to make them settle, chemical products called coagulants and flocculants are added to the water. Those chemicals, which are added in very small proportions, promote the formation of flocs that settle down in the sedimentation stage. As a result of this settling of flocs, sludge is formed at the bottom of the settlers. This sludge is periodically withdrawn out of the settlers.
After water has been clarified in the sedimentation stage, it is passed through sand filters for the further removal of impurities from the water. Sand filters are periodically backwashed with air and water to clear the retained impurities out of the sand.
Finally, a disinfectant (usually chlorine) is added to the water to kill bacteria and other diseases-causing microbes.
Powdered activated carbon (PAC) is sometimes also added to the water on order to adsorb the taste-and-odour causing compounds.
In the next figure the process described is represented.
