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04/28/99 WED 10:36 FAI TTO 435 5997 FACILITY ENGRS 11004 r I� +4-6 oWStrE SYSTEMS roti rNtarorouar RESIDENCES 1037 r Recirculating Granular-Medium Filter. Functionally, a recirculating granular- medium filter is essentially the same;As an interinittent sand filter as described above. The major differences arse (1) effluent from n septic Lank or another treatment upit is xdrain It2In holea- recirculated through the filter as opposed to a single application,(2)the cffective filter 2a°racing down medium (coarse sand or fine gravel) si7.e is larger, and (3)the loading rate based on the effluent flowrare is greater than that for an intermittent sand filter. Recirculating granular-medium filters, used to provide an improved level of treatment for larger flows such as from an apartment building or small communities, are discussed in nil PVC aver greater detail in Section 1.4-8. i Shallow Sand-Filled Pressure-Dosed Disposal Field, in some locations where the groundwater is high.or the underlying strata may not be suitable for a conven- tionai disposal field, shallow sand-tilled pressure-dosed disposal fields have been used successfully (see Fig. 14-12). Operationally. the sand-filled pressure-dosed di%posal fields function like a combination incermirrent,,and filter and disposal field. The qual- an -Ironut th cap icy of the effluent after it asses through Raaroaduet�errier p eh the sand is very high. pressure distribution. LEx istinggrov+W which serves to distribure the effluent evenly over the sand in the trench iS a key sltnfactor contributing to the Success of this type of disposal field system. T Mound System. The mound system is essentially an intermittent sand filter char 211 is placed above the natural surface of the ground (see Pig. 14-13). Trenches or beds are constructed in sand placed above the natural soil. Septic tank- effluent is — Pve liner pumped or dosed through a pressure distribution .system placed in a gravel layer- A ;10k barrier material (geotextile) and a cap layer arc placed above the gravel layer- The C rtround system is covered with topsoil. Mound systems have been used in locations where: (1)The Soils are permeable and the water table is shallow, (2) the underlying $trata are.highly porous and convendolnal sy,',mms should not be used, (3) slopes are id(b)aectiort through filter. 00t6bution pica Bwiding reit Compacted sit ' B in o Qe-Uo00oOaL+_ ainmin Crevel (!Tin i• Sem - 1• �— 3 rt rrifn d under raised plBrTlers in trent ' IrreppNlDUS i ryaf,fracl�K.'(I rock FlGi1RE 14-12 ororoundwaiv level ryp�al shallow sand-filV@d Oldpressure-disposal frele trench.