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• fS 746 CHAPTER 11: intermittent and Recirculating Packed-Bed Filters (RPBF) are coarse sand and fine gravel. Gravel and expanded shale are also used commonly for RPBFs.In the plastic foam filter, developed by Jowett and McMaster (1995), open-cell foam is used as the filtering medium. Distribution and Dosing System A distribution system is required to apply the liquid to be filtered uniformly over the filtering medium. Both gravity- and pressure-dosed systems have been used. Meth- ods of application that have been used include spray nozzles,tilting buckets,special molded plastic sheets, and pressure-dosed manifold distribution systems. The most common method for dosing IPBFs is with a distribution manifold with evenly spaced '-in (3-mm) orifices. The size of the manifold distribution pipe is determined such that the difference in discharge between orifices is not greater than 1.4 percent (see Fig. 11-4). The dosing manifold, with orifices facing upward, is usually placed on a gravel layer. The-orifices are usually covered with small orifice shields (see Fig. I1-5). The orifice shields prevent the gravel medium around the distribution Sys- tem from blocking the orifices and also prevent discharge from breaking through the surface.In very cold climates,the dosing system must be designed to allow for drain- back of any wastewater remaining in the distribution system after the completion of the dosing cycle, or the orientation of every fourth orifice must be downward. Single-pass dosing. Dosing of the liquid to be treated onto the packed bed can be intermittent single-pass or multipass. In single-pass dosing, the liquid to be treated is applied to the packed bed only once. To optimize the treatment effective- �r° T FIGURE 11-4 Typical distribution system for packed-bed filters.The distribution system is being tested hydraulically to check the uniformity of the discharge between orifices.