Official Website of Darjeeling Municipality
The water is being treated by the rapid sand filtration plant situated at Jorebunglow Filter house. Rapid sand filters use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals - typically salts of aluminum or iron. Water and flocs flows through the filter medium under gravity pressure and the flocculated material are trapped in the sand matrix.
Inside the filter box are layers of filter media (sand, anthracite, etc.) and gravel. Below the gravel, a network of pipes makes up the underdrain which collects the filtered water and evenly distributes the backwash water. Backwash troughs help distribute the influent water and are also used in backwashing. In addition to the parts mentioned above, most rapid sand filters contain a controller, or filter control system, which regulates flow rates of water through the filter. Other parts, such as valves, a loss of head gauge, surface washers, and a backwash pump, are used while cleaning the filter. The influent flows down through the sand and support gravel and is captured by the underdrain. However, the influent water in a rapid sand filter is already relatively clear due to coagulation/flocculation and sedimentation, so rapid sand filters operate much more quickly.