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0 746 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters - Fig. 11-22a), a float in the recirculation tank is used to shut off an electrically actu- ated valve on the discharge line, so that all of the flow is returned to the recirculation tank. The recirculating orificelball float valve (see Fig. 11-22b) combines the advan- tages of the simple ball float valve and the flow splitter, without the need for any electrical controls. When the flow is low, all of the flow is returned to the recircu- lating tank. 1f the tank is full, only a small portion of the flow is diverted to the discharge line through the last orifice (Ball and Denn, 1997). Another mechanical method of flow splitting involves the uses of a baffle in the filter (see Fig. 11-22c). The amount of flow recirculated to the recirculation tank depends on the placement of the divider baffle. Although the system is straightforward,odor problems can de- velop if the occupants of the home leave for a few days, and the recirculating tank is pumped dry. Distributed dosing. When the design flows increase and more distribution laterals and orifices are required, a point will be reached where the flow from the required number of orifices will exceed the capacity of a single pump. Where the are- quired flow from the orifices exceeds the pump capacity,the filter area can be divided into sections and a mechanical distributing valve can be used to dose each section of the filter sequentially(see Fig. 11-23).Typically, a single pump can pressurize up to six separate sections, depending on the dosing frequency (Ball and Denn, 1997). 1n some designs, to provide redundancy, two pumps may be used, with each pump I pressurizing one or more of the sections. Where two pumps are used, the discharge piping from the two pumps is interconnected with appropriate valving. With inter- connected pumps, should one pump.fail,the remaining pump can be used to dose all of the sections until the faulty pump can be replaced. z W i r � J -Fn 3..' t•.��f i s�A 1r_k r..r-.3 +12. (a) (b) FIGURE 11-23 Mechanical distribution valve used to dose different sections of packed-bed filters: (a) CAD rendering (courtesy C. Jordan) and (b) photographic view.