Facilities that specialize in filtration of water, which is a method of purification that typically passes water through various filtering agents. Water filtration was developed for large scale implementation in 19th-century Germany. In slow-filtration methods, the water is allowed to pass through a deep layer of fine sand. In rapid-filtration plants, the water is treated with a coagulant, such as aluminum sulfate, ferric chloride, or ferric sulfate, which flocculates particles; suspended matter to the bottom in sedimentation tanks and the water is passed at a relatively rapid rate through small beds of coarse sand. Heavily polluted waters may be chlorinated both before and after filtration. Aeration, involving the mixing of air with the water, is carried out if undesirable amounts of iron and manganese are present, since they are held in solution in water only in the absence of oxygen.