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NOW ..� Recirculating sand/gravel filters (RSFs or RGFs) are most popular for sites with medium to large wastewater flows. They are ideal systems for parks, restaurants, schools, office complexes, large developments, and even for communities which have septic tank effluent pump (STEP) or gravity effluent filter(STEG) collection systems. Typical recirculating sand/gravel filter design criteria are: Type of Design Recirc Filter Effective Uniformity Dose operation loading rate Ratio depth Size Coefficient limit ga . r •d R:R inches Div, mm C„ gal/orifice/dose Recirculating 5 5:1 f 24 f 1.5 to 5 2,-± 0.5 to 1.5 General Design Considerations Because the loading rate is dependent on the influent characteristics, designers need to carefully evaluate the contributing sources before sizing the filter. In dosing applications a screen pump vault located at the outlet of the septic tank draws liquid effluent from the clear zone of the tank. The screened effluent is dosed to the secondary treatment process. Primary treated effluent from a septic tank should not exceed the following parameters, shown in Table 2, when being further treated by onsite filtration and disposal application: Table 2 Typical Residential Wastewater Characteristics- Source Flow BODS TSS Grease Comments gpcd mg/1 mg/1 mg/l Sources Raw Domestic Sewage 47 371 338 73 EPA, M&E Building sewers Raw Domestic Sewage 50 450 503 164 Crites-Tchobanoglous, SDWM-1998 Septic Tank Effluent 48 156 84 17 EPA non-screened ST effluent Screened ST Effluent 60 133 30 Screened ST effluent (12 communities) Small-scale wastewater systems, typically serving individual homes or clusters of homes, need to meet the following requirements: • Efficient performance with highly variable wastewater strength and flow, including occasional hydraulic and biologic overloads • Low maintenance (e.g. at least many months between required servicing) • Adequate storage during typical power outages • No release of untreated sewage if a malfunction occurs • Low energy consumption Packed bed filters are inherently well suited for small-scale wastewater systems. They meet and typically exceed listed requirements utilizing specific design and operational features as described below: Watertight septic tanks,which precede most PBFs,provide several benefits: 1) allow primary settling of solids so that the packed bed filter treats largely liquid waste, 2) in conjunction with programmable timer controls, modulate and buffer large and uneven inflow, and 3)provide emergency storage during NTP-FLT-TBB-ESB-HL6-1 1100 Page 3