How are clogged rapid filters cleaned by backwashing?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

In the context of public health, rapid filters are essential for removing suspended impurities from water. When these filters become clogged with trapped particles, they are cleaned through a process known as backwashing.1

During backwashing, the standard downward flow of water through the filter is reversed. Clean water is forced upward through the filter media, which causes the bed to expand slightly. This expansion releases the trapped impurities, which are then carried away into wash troughs. To ensure the process is effective, an underdrain system—typically consisting of perforated pipes or porous tile blocks—distributes the backwash water uniformly across the bottom of the filter.1

This maintenance process is critical for ensuring the continued efficiency of water treatment plants, as it allows the filter media to be reused after removing the accumulated floc and other contaminants.1