In a rapid sand filtration system, which component receives the filtrate after the water passes through the filter media?

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In rapid sand filtration, the clean water must be collected directly beneath the filter media. The underdrain system sits at the bottom of the filter and has perforated outlets that collect the filtrate from the filter bed, guiding it into the filtrate trough and onward to the downstream treatment steps. This bottom collection is what separates the filtrate from the dirty water that passes through the media. The backwash line is used to return water (and air) during cleaning of the bed, not to collect the filtrate; the filter basin is the entire tank that houses the media and the underdrain; the influent channel brings water into the filter. So the component that receives the filtrate after filtration is the underdrain.