Water pollutants and purification
- Nitrate fertilisers are very soluble in water and are easily washed off fields by the rain and then into rivers and reservoirs. Because nitrates are all soluble they cannot easily be removed from the water.
- Older houses and water supply systems still rely on lead pipes. Poisonous lead compounds formed on the inside of the pipes slowly dissolve into the water.
- Pesticides used by farmers to kill weeds or insects may be washed or blown into streams and rivers.
Raw water may contain dissolved salts and minerals, microbes, pollutants and insoluble materials like sand and stones. Water is treated to remove any harmful components before being fed into the public supply. This is achieved is several ways:
- Filtration - The water is sprayed onto specially prepared layers of sand and gravel. As it trickles through, different sized insoluble solids are removed. The filter beds are cleaned periodically by pumping clean water backwards through the filter.
- Sedimentation - A chemical is added which causes tiny solid particles (which would pass through a filter) to clump together into larger particles. These can then be allowed to settle out or may be filtered.
- Chlorination - Chlorine gas, injected into the water, kills microbes.