What is the correct procedure for backwashing a granular activated carbon (GAC) filter media bed?

Backwashing a Granular Activated Carbon (GAC) bed involves reversing the fluid flow upward through the vessel to fluidize the media bed, release trapped suspended solids, and prevent compaction or channeling.

• Step 1: Isolate and Drain: Take the vessel offline, relieve system pressure, and lower the water level to just above the top of the carbon bed.

• Step 2: Controlled Upflow Expansion: Introduce backwash water gradually, increasing flow until the bed expands by 10% to 30% (ensure expansion does not reach the vessel outlet collectors, which causes carbon loss).

• Step 3: Duration: Maintain the backwash cycle for 10 to 15 minutes, or until the discharge effluent runs visibly clear.

• Step 4: Settling and Fast Rinse: Shut off the backwash water, allow the carbon bed to settle undisturbed for a few minutes, and perform a downward fast rinse to repack the media before returning to service.

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