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708 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters the flow for recirculation and for splitting the flow that is to be sent to the disposal field, and that which is to be mixed with the incoming flow for reapplication to the filter. Flow splitting can be accomplished in a number of different ways, as detailed in Sec. 11-4. 11-2 REMOVAL MECHANISMS IN PACKED-BED FILTERS The constituent removal mechanisms operative in packed-bed filters, the failure mode, and the modeling of constituent removal in filters are considered in the fol- lowing discussion. Removal Mechanisms The removal mechanisms operative in single-and multipass systems for the removal of BOD, TSS, turbidity, nitrogen, total and fecal coliform bacteria, and viruses are considered in the following discussion. Although the mechanisms are for the most part similar for both filters, there are differences brought about by the added liquid that is applied in multipass filters. Single-pass filters. In single-pass filters, theprocess variables that affect the removal performance for BOD, TSS, oil and grease (definition includes fats; - see Chap. 2), turbidity, nitrogen, bacteria, and viruses include the size of the filter medium, the hydraulic application rate per dose, and the solids and organic applica- tion rate per dose.Shortly after a filter is put into service,a thin bacterial film'or slime layer begins to develop in the upper layers over the grains of the filter medium.The bacterial film is of fundamental importance in the operation of the filter, because it retains, by means of absorption, the soluble and small colloidal matter and mi- croorganisms found in settled wastewater.The retained material is decomposed and oxidized during the rest period between.doses. Soluble organic matter is taken up almost instantaneously, while the absorbed colloidal material is solubilized enzymatically.The solubilized material is then trans- . ferred across the cell membrane and converted to end products.'With each subse- quent dose, some of the end products are transported farther into the bed,eventually being removed from the bottom of the filter. Larger particles are retained within the filter by means of filtration, with mechanical straining and chance contact being the principal removal mechanisms. As with the soluble and colloidal matter, the coarse ` organic solids are also processed between doses,and in the early morning hours when the solids and organic loadings to the filter are reduced.As the filter matures,the film layer will develop throughout the filter.Because the larger solids are removed in the upper portion of the filter,the distribution of solids within the filter.is nonlinear,with the largest accumulation occurring in the upper 4 to 8 in (100 to 200 mm). The importance of filter medium size and the hydraulic application rate can be understood by referring to Fig. 11-6. As shown, when the volume of liquid applied is sufficient to fill the pore space, some organic material, colloidal particles, and