Untreated water first pumped into the water system of rivers, lakes, aquifers and other sources. Some filtration plants may have more than one source. Usually it is pumped through a coarse screen, which stops logs and most fish from flowing in but allows substantially no less than about 2 inches in diameter through to be filtered out later. At this stage, the water is often unclear and cloudy, with a lot of particles in it.
coagulating and Floculating water
The water can not be effectively sterilized prior to most of the larger particles are removed. The raw, untreated water is first treated with coagulant chemicals which negate the electric charges of the dissolved particles, so that they clump together. To speed this process up, the water floculated or touched up. As lumps collide, they form larger lumps. The water can also be treated with other chemicals to remove taste or odor, or to target specific contaminants that are a problem in that water source. The water is then pumped into the settling tank.
Let it sit
The settling tank is a large, slow-moving pool of water. The water will flow either from one end to the other or from the middle to the outside, which takes several hours to make the trip. As it does, it makes up the herd out of the water, slowly drifting down to the bottom of the tank, where it forms a layer of sludge. The water that comes out of the tank is not clean, but it has much less dissolved material.
Filtering and decontaminating
The water flows from the tank through a filter. The simplest filter is a column of sand. Any dissolved particles that are still in the water will stick to the sand on the way down. Some filters are much more elaborate, employing certain species of plants that can trap heavy metals and other contaminants.
When the water leaves the filter there is a lot cleaner, but it can still harbor harmful bacteria or other microorganisms. It is then treated with chlorine or other chemicals to kill these organisms. Fluoride is often added as fluoridated water is good for people’s teeth. It is now ready to go into the municipal water supply.