Untreated boiler feedwater frequently contains suspended matter such as mud, silt and bacteria. Left in the water, this material can cause problems, such as foaming or deposits in the boiler. The process of clarification or filtration removes most suspended matter. One common method involves both processes; the water is first passed through a clarifier which removes most of the suspended matter, then a filter, which re-moves the rest.
Filtration can be done several different ways. The most common filters are granular media filters, made from sand, anthracite (hard coal) and garnet. Other types of filters, such as cartridge filters, sock filters and strainers are used in some installations. Filter media choice, filter bed depth and other design parameters are determined by the quality of the water and boiler requirements.