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A sedimentation tank is a part of system for removing impurities from liquids, mainly water. In most cases, a sedimentation tank is nothing more than a large tank for holding water, often with a special solid material removal system at the bottom. Some tanks are tall and fat, while others are long and very shallow; this is based on the overall purpose of the tank. In some sedimentation systems, solid particles are simply allowed to settle at the bottom of the tank. Other methods require a special binding substance be added to the liquid before sedimentation occurs.
Sedimentation is a common method of removing particles from liquid. Most large systems apply to water, but other liquids can be cleaned using sedimentation. The most common non-water liquids that may employ a sedimentation tank are industrial solvents and cleaners, as well as some types of liquid polymer. Still, water is far more common for sedimentation than any other liquid.
The general idea behind a sedimentation tank is very simple. If water is kept still long enough, any solid material suspended in the water will simply fall to the bottom or float to the top of the tank. Since most natural solids are either heavier or lighter than water, all a system needs to do to remove solid impurities is scrape detritus off the top and bottom of the tank.
In order to speed up this process, tanks generally have a coagulant or flocking agent placed into the water. The introduced chemicals create a reaction that begins to draw impurities towards them. The agents bond with the impurities and create little balls of collected material. Since these balls are bigger than the solids were before, gravity pulls them to the bottom of the tank faster.
This process is common when treating waste water, but less so in sewage. In a typical waste water system, the particles are mostly benign impurities. These impurities are unwelcome, but not harmful, to processes using the water. In the case of sewage, the shear amount of bacteria and harmful agents in the water makes sedimentation more difficult and coagulation less effective.
There are two basic styles of sedimentation tank. Tall and fat tanks are generally used as an early stage part of water treatment and are as much for storage as treatment. These tanks have enough room for the water to sit for a long period, allowing large debris like rocks or sticks to settle out. Since they store the water as well as treat it, they need to be very large.
The other type of sedimentation tank is long and shallow. This tank design is typically inside a water purification system. Since the tank is so shallow, it takes less time for suspended particles to fall to the bottom. These tanks are typically used for the coagulation step in water treatment.