If the underdrains at a treatment plant are not functioning, which treatment unit is most likely affected?

• A. Sedimentation basin
• B. Gravity Filter
• C. Disinfection contact chamber
• D. Rapid mix basin

Answer: (B)

Underdrains are essential components of the filtration stage, especially in a rapid gravity filter. Water passes through the filter media and then down through the underdrain system, which collects the filtered water and provides the path for backwash to flush the media. If the underdrains aren’t functioning, the filter can’t drain or be backwashed properly, effectively rendering the gravity filter inoperative. The sedimentation basin sits upstream and relies on settling, not underdrains. The disinfection contact chamber is downstream of filtration and depends on a steady flow but doesn’t rely on underdrains. The rapid mix basin is part of coagulation/flocculation and isn’t tied to the filter underdrains. So the gravity filter is the unit most likely affected.