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728 CHAPTER 11: Intermittent and Recirculating Packed-Bed f=ilters ' <br /> Inspection access <br /> riser and lid Conduit to Discharge assembly <br /> power source <br /> Effluent <br /> Influent <br /> Liquid level Level control <br /> J. float assembly <br /> Vault inlet ports <br /> High-head <br /> effluent pump Screened pump vault with <br /> removable filter cartridge <br /> (a) <br /> Effluent <br /> Influent <br /> Effluent filter External <br /> pump basin <br /> (b) <br /> FIGURE 11-11- <br /> Location of filter dosing pump: (a) in septic tank filter vault and (b) in external pump <br /> basin (courtesy Orenco Systems, Inc.). Pumps used for effluent dosing are typically <br /> the same as those used in STEP systems (see Chap. 6). <br /> Dosing frequency. As noted previously, increasing dosing frequency from 1' <br /> to 4 times/d to more than 12 times/d has improved the performance of sand filters, <br /> particularly those with larger.-diameter sands. It is recommended that a minimum of <br /> 18 doses/d be used for normal septic tank effluent. Should the BOD concentration in <br /> the septic tank effluent be elevated(e.g., greater than 200 mg/L),it is recommended <br /> that,the number of doses'be increased to 24 times/d. When-programmed dosing is <br /> used,the timer is typically set to dose the filter 18 times/d. To enhance treatment in <br /> 1SFs, the amount of flow discharged per orifice per dose should be limited to about <br /> 0.25 gaud (Bounds, 1996). <br /> The filter dosing pump can be located either in the septic effluent filter.vault or in <br /> a separate external pump basin (see Fig' 1 I-11).The dosing cycle is controlled from <br /> a control panel (see Fig. 11-12) using float switches and one or more programmable <br />