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r r 7 704 cWAFTEFI 11: Intermittent and Recirculating Packed-Bed Filters PVC lateral with orifice shields yyr,. olL. Surface soil • Filter fabric - Rode -.:.R. Filter sand Pea gravel Rock 30 mil PVC liner - 4 in slotted PVC underdrain pipe To drainfield or pump vault FIGURE 11-1 Cutaway view of the basic elements of a packed-bed filter including. geomembrane liner (shown without container), underdrain pipe, support gravel, filter sand, lateral distribution pipe with orifice shield in gravel layer, and cover layer. Container for Medium Lined earthen pits, lined wooden structures, and concrete structures (see Fig. 11-2) have been used to contain the faltering material. The choice depends on the type of packed-bed filter (PBF) process to be used, the site conditions with respect to the placement of the unit, and economics. The most common liner material is 30- mil (0.76-mm) PVC or polypropylene. Bottomless filters have been used where the underlying soils have rapid permeability and adequate separation from the ground- water. Bottomless filters are significantly less expensive than contained filters be- cause the liner can be omitted. Underdrain System The underdrain system is used to collect the treated liquid and transport it to a pump basin or to the watertight line leading to the disposal field.The most commonly used underdrain systems include the use of slotted or perforated pipe, with the slotted pipe being favored.The slots,cut halfway through the pipe, are 0.25 in(6 mm)wide to prevent biomat bridging. Where slotted pipe is used, the slots are placed facing upward, as shown in Fig. 11-3. The slots are placed upward so that they are not blocked by settlement into the liner. In intermittent PBFs and recirculating PBFs, the underdrain is surrounded with a granular medium of sufficient size that will not clog the slot openings.