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738 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters ' <br /> The headloss in distribution manifold with laterals: <br /> hfdp = 31.6 = 0.5 ft <br /> c. The total dynamic head is: <br /> Friction loss in pipe from septic tank = 6.3 ft <br /> Hose and valve assembly(estimated) = 5.0 ft <br /> Fittings (estimated) = 1.0 ft <br /> Friction loss in manifold = 0.5 ft <br /> Friction loss in lateral = 0.1 ft <br /> Residual head on orifices = 5.0 ft <br /> Elevation difference(assumed,from <br /> low water level in pump tank to manifold <br /> centerline) = 10.0 ft <br /> TDH(estimated) 27.9 ft <br /> Comment. In some cases, the presence of air in the piping system, leading from the <br /> septic tank to the filter, can cause high headloss,especially if there are high spots in the <br /> piping system. Use of small-diameter pipe and a high-head pump will force the air out, <br /> eliminating the problem.A pump should be selected to provide the required tlowrate at <br /> a minimum head of 5 to 10 ft on the orifices. Higher pressures (up to 30 ft) are usually <br /> not a problem. Some designers include a ball valve to adjust the residual head[typically <br /> 5 to 10 ft(1.5 to 3 m)]. The use of ball valves is not recommended because they do not <br /> 'hold their settings, and it is easy for someone to unknowingly change the setting. <br /> 11-4 MULTIPASS (RECIRCULATING) PACKED-BED FILTERS <br /> Multipass (recirculating) filters are similar to single-pass (intermittent) filters, with <br /> the exception that a portion of the treated effluent from the filter is returned to a' <br /> recirculation tank where it is used to dilute the effluent from the septic tank before <br /> being applied to the filter.By diluting the strength of the septic tank effluent, higher <br /> application rates can be used. <br /> Development and History of Use <br /> Recirculating sand filters evolved from ISFs in the 1970s when Hines introduced <br /> their use in Illinois (Hines and Farveau, 1974). The RSF used by Hines consisted <br /> of a septic tank, recirculation storage tank, and an open sand filter. The contents of <br /> the recirculation tank were pumped over the filter two to eight times each hour. A <br /> valve (or flow splitter) in the recirculation tank allowed filtered effluent to either <br /> enter the tank or be routed to reuse/disposal, depending on the liquid level in the <br /> tank. In the mid-1970s RSF systems were used to treat septic tank effluent prior <br /> to disposal in roadside ditches. In West Virginia, the sand medium was replaced <br /> with bottom ash, a hard, durable by-product from coal-fired power plants (Swan- <br /> son and Dix, 1988). In Oregon, sand was replaced with fine gravel (Ronayne et al., <br /> 1984). The significance of these alternative media types is that surface clogging is <br />