Civil Engineering Reference
In-Depth Information
Fig. 12-9.
Typical vertical pressure filter with concrete grout fill in the bottom head, pipe headers,
lateral underdrains, gravel supporting bed, and filter sand (Courtesy of Infilco Degremont, Inc.)
Filter Underdrains
Filter underdrains have a twofold purpose. The more important is to allow uniform
distribution of backwash water without disturbing or upsetting the filter media above.
The other is to collect the filtered water uniformly over the area of the bed. There are
a wide variety of filter underdrain types available for use in both gravity and pressure
filters. All of the systems accomplish the uniform distribution of wash water by intro-
ducing a controlling loss of head, usually about 3 to 15 feet (1 to 5 m), in the orifices
of the underdrain system. The orifice loss must exceed the sum of the minor (manifold
and lateral) headlosses in the underdrain to provide good backwash flow distribution.
The underdrain systems differ basically with respect to the type of filter washing
system and whether a gravel layer between the media and underdrain is employed.
The filter underdrains are needed to perform a variety of functions, including:
Supporting and retaining the filter media
•
Distribution of backwash supply water and air scour
•
Collection of flow through the filter bed
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