If rapid sand filters are not washed adequately, dirt on sand grains can cement the grains together, forming what?

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In a rapid sand filter, keeping the filter media clean and properly backwashed is essential for normal filtration. If backwashing is not done adequately, fine particles and dirt build up on and between the sand grains. These fines can bind with minerals and organic matter, acting like cement that glues the grains together. The result is a cemented bed—a hardened, less permeable layer at the top of the filter. This reduces pore space, raises headloss, and suppresses filtration efficiency, often leading to poor removal of turbidity and a need for more frequent backwashing or media replacement.