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11-3 SINGLE-PASS (INTERMITTENT) PACKED-BED FILTERS 729 <br /> ft :-.._ . <br /> F <br /> Lf <br /> } � <br /> 2J, FIGURE 11-12 <br /> Typical control panel used to <br /> ` µ house the electrical elements <br /> F t that control the operation of <br /> 1 an intermittent sand filter. <br /> See Fig. 11-13 for typical <br /> 'Y <br /> schematic wiring diagram. <br /> I timers (see Fig. 11-13). By controlling the on and off times with the programmable <br /> G timer, the filter is dosed more or less uniformly throughout the day. Further, short- <br /> terra hydraulic surges (e.g.,the discharge from a washing machine) are stored in the <br /> septic tank or external pump basin. By limiting the applied dose and maintaining a <br /> fixed off time, the performance of the filter is enhanced (Bounds, 1996). Although <br /> closing controlled only by float switches is often used, it is not recommended, be- <br /> cause the resulting wastewater doses and resting times are unequal, which can lead <br /> to premature clogging of the filter. <br /> When the dosing frequency is set at 18 times/d, the corresponding total flow is <br /> equal to 1.5 times the average flow. Thus, at average flow the filter will be dosed <br /> 12 times/d. When an extreme event occurs (e.g-, peak flow greater than 1.5 times <br /> the average flow) the normal cycle is completed and, depending on the adjustment . <br /> of the upper-alarm float,40 to 60 gal will be dosed. The timer can also.be set to dose <br /> i a fixed volume on an hourly basis over the remaining 6 h until the next timed cycle <br /> is initiated.A programmable control unit is currently under development which will <br /> make it possible to adjust the volume per dose on a 24-h basis (Bounds, 1996). An { <br /> effective alternative is to increase the size of the septic tank, or the external pump <br /> basin, so that they will function as equalization basins. The timer is then set to dose . <br /> the filter 24 times per day,based on average flow. In many states the storage volume <br /> required in the external pump basin is 2 times the daily average flowrate. Extreme <br /> flow events are handled by pumping a fixed flow in response to the high-water alarm j <br /> float. <br /> l <br />