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742 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters <br /> TABLE 11-11 <br /> Typical performance of recirculating filters for BOD and TSS removal` <br /> Filter medium BOD, mg1L TSS, mg/L <br /> Locations C6,mm UC Influent Effluent influent Effluent <br /> Elkton, Oregon 3.5 1.8 141 6 32 6 <br /> Orcas Village,Washington 2.0 1.75 166 4 113 5 <br /> South Prairie,Washington 4.3 1.6 181 4 34 4 <br /> 'Courtesy Orenco Systems,Inc. <br /> Treatment Performance <br /> The removal of BOD in'recirculating filters depends on the loading rate and the <br /> size of the filter medium. Recirculating filters can produce an effluent with less than <br /> 10 mg/L of BOD and TSS when loading rates are less than 5 gal/ft2•d (200 mm/d) <br /> and the medium size is 3 mm or less. Typical removals for BOD are presented in <br /> Table 1141 for a range of filter medium sizes varying from sand to pea gravel. Per- <br /> formance data for the RGF used for the Stonehurst development described above are <br /> presented in Table 11-12. <br /> Recirculating filters can produce a high-quality, partially nitrified effluent. Ni- <br /> trogen removal with recirculating filters is typically 40 to 50 percent, as shown in <br /> TABU 11-12' <br /> Performance data for Stonehurst wastewater treatment system <br /> For the 28-month period from June 1994 through September 1996- Effluent samples taken after <br /> recirculating gravel filter and UV disinfection as shown in Fig. 11-22' <br /> Total Observed values <br /> Sample number of <br /> reporting samples Arithmetic Geometric <br /> Constituent' Unit period$ to date Range mean mean Median <br /> BODS mg1L Monthly 56 Oto c5 c5 <5 <br /> COD mg/L . Monthly 120 1.0-18.0 4.7 5.0 <br /> TSS mg/L Monthly 118 2.0-15.0 5.0 4.9 <br /> pH Unitless Monthly 120 6.96-8.65 t a 7.61 <br /> Total coliform MPN/100 mL Monthly 118 <2--12.5 ° <2 <br /> NH4 mg/L Quarterly 9 0-15 1 0.0 <br /> IV03 mg/L Quarterly 9 '-3.55--37.0 12.5 12.2 <br /> TKN ' mg/L Quarterly 9 0-3 ' 0.4 <br /> Oil and grease mg/L Quarterly 9 0--12 ' 0.0 <br /> TDS mg/L Quarterly 9 340-770 630 656 <br /> EC p.mhos/cm Quarterly 9 433-1200 894 1000 <br /> 'Crites et at.(1997). <br /> t TDS = total dissolved solids,EC=electrical conductivity. <br /> =Monthly values for BODS are based on an average of two samples per month.Monthly values for TSS,COD,pH,and total <br /> conform are based on an average of at least four samples per month. <br /> 4 Mean values cannot be reported for pH,which is a logarithmic function,and total coliform,which is based on a Poisson dis- <br /> tribution. <br /> 'Unable to define nature of distribution,because of the number of zero values reported. <br />