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HIGH-RATE GRANULAR MEDIA FILTRATION 8.41

mixed media, low washwater collection troughs, and leakage through the support gravel
or underdrains. Filter performance and the effectiveness of surface wash decline when
media loss exceeds about 20% of the original depth (Kawamura, 1991).
Lost media should be replaced with appropriately specified material following the
guidelines presented earlier to ensure that the entire bed of media approximates original
specifications. Replacement media for dual- and mixed-media filters require special at-
tention. If a layer of fines has accumulated on the surface of the filter, it may be wise to
scrape off a thin layer of top media before adding new media. After replacing the new
media on top of the filter, the operator should initiate two complete filter wash cycles to
fluidize the media, wash off fines, and ensure restratification.

Filter Evaluation. Filter evaluation procedures can be found in Nix and Taylor (2003).

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