Which filtration method uses a granular media filter with periodic backwashing and is widely used in municipal water treatment?

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Rapid sand filtration is a high-rate filtration method that uses a bed of granular media (sand, often with gravel or coal) and requires periodic backwashing to restore capacity. Water moves through the media at a relatively fast rate, capturing suspended particles, and as head loss builds, the flow is reversed to scour and flush the accumulated solids out to waste. This approach is widely used in municipal treatment because it can handle large flow rates and works well with upstream coagulation and sedimentation steps, making it a robust, scalable option for community water supplies. Other options don’t fit as neatly: cartridge filtration relies on replaceable cartridges rather than a fixed granular bed; membrane filtration uses membranes rather than a granular media bed; and slow sand filtration operates at a much lower rate and relies on a biological layer without routine backwashing.