| 摘要 |
A filter having a gravel-less underdrain and a filter layer installed directly on top of the gravel-less underdrain. The filter layer is designed such that when fluid is directed through the filter layer at controlled rates a protective barrier is formed by the larger particles of the filter layer directly above the gravel-less underdrain to prevent clogging and/or structural failure of the gravel-less underdrain thereby obviating the need for one or more layers of gravel or torpedo sand.
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