Which of the following is a valid method to determine backwash timing for a rapid sand filter?

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Backwash timing is guided by the differential pressure (headloss) across the filter. As the rapid sand filter bed becomes loaded with trapped solids, resistance to flow increases, and the pressure difference between the inlet side and the outlet side grows. When this headloss reaches a preset limit chosen from design and operating criteria, backwashing is initiated to restore the bed’s porosity and filtration efficiency. This method directly tracks the condition of the filter medium and its ability to remove solids effectively.

Influent temperature isn’t a reliable indicator of when to backwash, since it doesn’t reflect bed fouling. Visual turbidity of the effluent can signal quality issues but isn’t a consistent trigger for backwashing and can lag behind headloss. A pressure reading on the outlet line alone doesn’t tell you the pressure drop across the entire filter bed, which is what matters for timing.