Backwash bed expansion to prevent media loss is designed to reach what percentage?

• A. 25%
• B. 30%
• C. 40%
• D. 50%

Answer: (D)

Backwash bed expansion is about protecting the filter media while you clean it. When you backwash, water flows upward through the bed, making it rise and loosen. Reaching about fifty percent expansion gives enough space for fines and smaller particles to be carried out and flushed away, and for the media to move freely without being pushed out the underdrain. This level of expansion is a practical target that reliably prevents media loss across typical rapid filter beds, while still allowing an efficient cleaning cycle. If expansion is much less, fines can escape or clog the underdrain; if it’s significantly more, you’re wasting backwash time and energy, though a bit more expansion won’t typically hurt.