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I <br /> 11-4 MULTIPASS (RECIRCULATING) PACKED-BED FILTERS 745 <br /> '! Recirculation ratio and dosing frequency. Recirculation ratios <br /> are typi- <br /> tally 4 or 5 to 1,based on forward flow. The dosing interval can be set by a variable <br /> timer, and is typically less than l rein to 5 min every 12 to 25 min. For. higher- <br /> strength wastewaters, the dosing frequency has been increased to as high as 2 min <br /> every 6 min (Parker, 1996). x` <br /> Flow splitting. Because most of the flow from a recirculating filter is returned <br /> to the recirculation basin to be mixed with the flow from the septic tank,some method <br /> must be available to divide the flow. Several methods have been used to accomplish <br /> the required flow split including, as shown in Fig. 11-22, the use of: (1) a splitter <br /> basin, (2) a recirculating orifice/ball float valve, and (3) the use of a flow-splitting <br /> baffle in the filter (Ball and Denn, 1997). Although the simple ball float valve was <br /> used in early designs, its continued use is not recommended because, if the recircu- <br /> lation dilution tank is full,a portion of the flow from the septic tank may pass through <br /> the filter only once before being discharged. Where a splitter basin is used (see <br /> ` Discharge <br /> orifices <br /> Overflow <br /> i Inlet Outlet <br /> i <br /> I O <br /> Inlet y 0000 — <br /> �— Ball <br /> I 'o o d s <br /> Orifices `Q o y' <br /> Cage for <br /> ° ball i <br /> Outlet ° <br /> (a) (�) <br /> Divider placed in filter basin to Recirculating <br /> split the flow in the ratio of 4 to 1 + <br /> filter <br /> i F <br /> Recirculating <br /> Septic tank <br /> tanka <br /> I-I <br /> + u <br /> i <br /> i <br /> FIGURE 11-22 <br /> Methods available to split flow in recirculating packed-bed filters: (a) a splitter basin, <br /> (b) a recirculating orifice/ball float valve, and (c) a flow-splitting baffle,in the filter <br /> (a, b from Ball and Denn, 1997). ` <br />