With automatic chlorination and filtration units, a metered pump continuously adds small amounts of chlorine to the water. Chlorine oxidizes the iron in the water, which causes the rust particles to become insoluble. The insoluble particles are removed by filtration. An activated carbon filter then removes any excess chlorine from the water.
Greensand is a material that contains manganese dioxide, which releases oxygen that in turn oxidizes the rust particles. The now insoluble rust particles are collected on the filter's bed. Periodic regeneration of these filters is required and involves adding new potassium permanganate to replace the oxygen needed for the process. The filter must also be backwashed periodically to remove accumulated iron particles to maintain the filter's efficiency.
Birm contains large quantities of manganese dioxide and works in a similar manner as greensand. The dissolved oxygen provided for by the manganese dioxide speeds up the oxidation of rust particles. As long as sufficient levels of dissolved oxygen are present, maintenance of these filters only involves occasional backwashing to remove accumulated iron particles from the filter. It's important to note that birm filters are not suited for water that contains hydrogen sulfide.