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f f General Design Considerations In more than 12 years of research with textile media, several design variations have been tested in both intermittent and recirculating operations and in several different multi-pass recirculating"modes,"which optimize nutrient reduction. Like commercial sand and gravel filter installations, commercial textile filter installations are typically operated in multiple-pass recirculating modes. However,unlike residential sand filter installations,which are normally operated as single-pass systems,residential textile technology filters are also configured as multi-pass, recirculating systems, as shown in Figure 7. Figure 7:Perspective view of textile filter unit and - � - �a � ��-#cv �'!It3 eA� �� eu'3�§rE �°° �F as � j' � � g processing tank system E' C 'd 1 a� Because the loading rates are dependent on the influent characteristics, a careful and critical evaluation of the contributing source(s) is the first crucial step in adequately accounting for the filter's design size and capacity. Hydraulic, organic, and inorganic inputs (loads) are characteristics that will play critical short and long-term parts in all wastewater treatment designs. In single-pass applications, it is common to find a screened pump vault located at the outlet of the septic tank,where the septic tank's effluent is drawn from the clear zone of the tank in a decanting manner. The screened effluent is often dosed directly to the single-pass filter. In multiple-pass recirculation processes the clear-zone supernatant discharges into a secondary chamber or tank,which is typically called the recirculation, recirc/blend, or dilutioniblend chamber or tank. Typically primary treated effluent from a septic tank should not average higher than the fallowing parameters, shown in Table 2, when being further treated by onsite filtration and disposal: Table 2:Typical Residential Wastewater Characteristics Source Flow BOD TSS Grease Reference fgpcd N mg/L mg/L mg/L _._.._. Raw Domestic Sewage io� 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 n/a Screened ST effluent(12 Communities) Designing at 95% confidence levels, as shown in Figure 8a and 8b, tends to ensure reliability in meeting discharge limits consistently within the variability of the occasional excessive loading and operating conditions. NTP-FLT-TRB-1 Rev.1.1,11102 Page 10 0112