SR0033424_ENG DESIGNED PLAN - Laserfiche WebLink

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11-1 FUNCTIONAL FEATURES OF PACKED-BED FILTERS 707 mow' w } E � s 5 x � FIGURE 11-5 View of orifice shield used to protect orifice in distribution . � • w•~�:�=�-.__., _�;�`� lateral from clogging, I �I ness, as discussed subsequently, the total liquid volume to be.treated is applied equally anywhere from 12 to 72 doses per day. Because the liquid is applied period- ically, single-pass systems are known commonly as intermittent systems(e.g., inter- mittent sand filter). Increasing dosing frequency from 4 to 24 times/d was found to improve performance,particularly with larger-diameter sands(Furman et al., 1955). In more detailed research conducted at the University of California at Davis, it was found that dosing frequencies of 12/d were significantly better for ISF performance than 4 times/d(Nor, 1991;Darby et al., 1996).The impact of the number of doses can be understood more clearly after reading the material dealing with removal mecha- nisms presented in the following section. Multipass dosing. In multipass dosing, a portion of the liquid which has passed through the filter is diverted to reuse or disposal. The remaining liquid is returned to a recirculation tank where it is mixed with effluent from the septic tank and reapplied to the filter medium. Because a portion of the liquid is passed'through the filter a number of times,multipass systems are commonly known as recirculation systems (e.g.,"recirculating gravel filter). The typical recirculation ratios for multi- pass systems vary from about 3:1 to 5:1,based on forward flow from the septic tank. The principal effect of recirculation is to reduce the total organic loading applied to the filter with each dose, and to increase the dissolved oxygen in the filter. Typical dosing frequencies vary from 48 to 120 times per day (e.g., less than 2 to 3 minutes on every 10 to 25 minutes). , Filter Appurtenances The types of filter appurtenances required will depend on whether the packed bed is to function as a single- or multipass unit. The principal appurtenances for single- pass units include the necessary pump basins,pumps, and electrical control systems. Control systems for IPBFs. are considered in Sec. 11-3:In.addition to-the appurte- nances required for single-pass filters, multipass units require facilities for holding