The clarified water at the top of the basin flows or is pumped to the next step of the treatment process. Clarified water is passed through a filter which consists of layers of granular material to remove any remaining particles of dirt, algae and microbes. Filtering material always include sand and gravel. It can also include crushed anthracite, a form of hard coal, or granular activated carbon. Chemicals are added for final disinfection, corrosion control and other purposes.
Chlorine is used for final disinfection.
Corrosion inhibitors may sometimes be added to ensure that the water is non-corrosive to the distribution system. A small amount of fluoride is added to help in the prevention of tooth decay. The treatment process used for Illinois-American's well sources in Pekin and Peoria include disinfection. Fluoride is added to all sources. Iron and manganese removal, plus corrosion control are sometimes necessary.