Chapter 11 – Rapid Filtration

Chapter 11 introduces rapid filtration theory and practice. Key construction elements of a rapid sand filter are a sand bed in which filtration occurs, a support for the sand bed, an underdrain system to remove filtered water and to admit backwash water, an inlet for water, an outlet for used wash water, and means for controlling the flow through the filter. The importance of the sand bed and underdrain system as well as basic hydraulic rules of filtration are outlined. Finally, a comparison with slow sand filters is presented. Rapid sand filters operate at rates some 10 times those of slow sand filters. It follows that impurities are drawn deep into the bed. Hence, cleaning is automated and hydraulic. The rapid sand filter takes up a relatively small land area, the water requires pretreatment, and the operator skill level required is high. After studying this chapter, readers should be aware of the design and operation principles of rapid (gravity sand) filtration.