Deep‐Bed Filtration

Hundreds of water-treatment plants around the world incorporate rapid sand filters with a media depth in excess of 4 ft. This type of plant will be discussed here, with particular reference to downflow filtration as differentiated from the upflow "contact clarifier" with media depth of 6 ft found in the Soviet Union. By definition the deep-bed filters are of the rapid sand type, the applied water flowing downward through at least 4 ft of active media. Gravity units are more common than pressure units. Numerous transport and capture mechanisms have been identified, measured, and mathematically modeled for precise descriptions of how the filtration process works and what a rational approach to design would be.1-2 Surface screening by definition is not a predominant mechanism of removal in