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eerier of trnl.a) motement resulting the fill raises thch sod e trenAbove the origoirl surtait .e sInldtra uenl w the sud particles. Water lie• lion of the sand filleted effluent m t un into the ttiparsd which latively stagnant as lateral or vertical u%uAll% has the highest i onducti%it-, of all moil horizons It ill. I and oral hydraulic gradients which are 2►. "Thus, surfAte seepage of potentially dangerous efflueut is pre rider these conditions is inadequate, vented. Once into the topsoil, the liquid in mote laterally until when this stagnant liquid surfaces in absorbed into the underlying clay subsoil whit to will purify the naideration is the principal justifica. liquid to a high quality water before disthargmg to the ground es within an arbitrary 90 cm (3 feet) water. age system or 1.5 in (5 feet) below Conditions in spring and tall, when natural perched water tables Alth codes. Obser%ations in unlined occur At shallow depth. the different design consicirratiom than 74 showed water levels near the soil il.. _e in summer and winter when the water table is lower. Dimen d Almena soils for short periods of s of the seepage t:encli should l,c properly designed its avoid in below the sod surface for w%rr)t .s Igor the perched water table into the fill when the ground water Ater level differed among augerholes s nigh and the Iota) basal area of the mound should lie sufficiently uiy affected by small differences in large to absorb and conduct the effluent downwards through slowly Is were not observe) within 1.50 inpermeable subsoil horizons when the ground water is low. urface in summer and early fall, and Medium sand is used As fill material within the mound. Luamy At that time was at least 15 m (50 sands or sandy IoAms have better filtration properties than sands cords. No water table measurements but their potential for clogging is higher (2). Column studies have Ilow levels are not anticipated since indicated that sands can be very effective as filters, however, and e precipitation as snow and effluent ., .: of medium-sized sands is therefore recommended (6). to the soil. The required downward 'The design of a mound for a particular site involves five steps. possible in periods without natural. They are (i) sizing of required basal area, (ii) sizing of the absorp- y if loading rates would not exceed lion trenches, (iii) design of the distribution system, (iv) final ng a perched "effluent" table. dimensioning of the mound, and (v) sizing of the dosing, hAfflbet. water table poses a problem that can The basal area of a mound should be large enough for the Linder- i? the perched water can be removed lying soil to absorb the maximum expected daily waste water now. the disposal system can be raised This calculation is based on the KsAt of the lent permeable soil rial, thereby creating an unsaturated horizon below the proposed site for the mound. Methods for tests ich and the seasonally high perched to measure this parameter are reported elsewhere (2). Daily now .lure is explored in this publication. is estimated by using 568 liters/day (150 gallons/dAy) per bedroom in the home served. ORES FOR MOUNDS The second step in the design is to size the absorption system within the mound. The system should consist of one or a series of mound is to provide treatment and small parallel trenches rather than one single absorption bed con - I in a safe and efficient manner in all taining all the distribution laterals. Trenches Are superior than beds , two criteria must be met: (i) con- because they will cause A smaller rise of perched liquid in the top - of the effluent discharged from the soil. A standard trench width (m) of 60 cm (2 feet) is ►ecom• of the effluent through a sufficient mended As follows from Applying the Dupuit-Forchheimer assump. ification before any possible human son for horizontal now applied to the topsoil 13). Figure 3 shows lewater. A schematic crun•sectiun of one-half of a mound on top of a •h these criteria. The fill below the permeable topsoil it h11 cm resting on an "impermeable" subsoil 8 mound acts as a sand filter to purify horizon. Infiltration of effluent from the seepage tied (I. cm/day) ication will increase as the rill thick• results in A rise of the ground water into the topsoil. Below the f 60 cm 112 feet) of sand is sufficient center of the bed. the ground -water level is equal to the depth of o very low levels (21. Much or the the topsoil Iho), below the edge of the beJ the level is h1. 1he OD) and suspended solids (SS) that calculation requires that ground water does not rise into the fill. .f the soil pores in the underlying Trench width (w) is calculated .is 66 cm with Eq. (1I, assuming is important since conductivities of / - 5 cm/dAy; K - 40 cmlday Isre Fig. I) ho ■ 30 cm, and h i marginal and even slight barriers to 25 cm. •nacceptably low levels. In addition. (hot - het) = (/ • tot112K. ill MOUND SURFACE ) �trouNo tstul-.� - ORIDIr1AL SOIL SURFACE HIGH a-wouNowarER 1 A - rtORItOM 0 - MOR1tMf n through a niotind system used to of the Qfsval II4141 in the mound L.rss.sr.w tit in • • 11.11 Mound I Sept :1 Mound 11 Sept ' 1 Hound III Aua 7. Mound 1% "us 71 • This analysis is approximate because (i) the subsoil 8 if nor im- permeable. (if) the wit usually has a slight slope, and (iii) the liquid is not applied as a steady flow but As a dose. However, calculations do indicate the need for small seepage trenches rather than large seepage beds. The total bottom area of the trenches is calculated by using a liquid application rate of 5 cm/day (1.2 gallons/fern per dav) (2). The height of the gravel-ndled trench should be at least ^I, tin (8 inchrsl to allow for sufficient liquid storage. If parallel trenches are uses, their spacing is determined by the hydraulic characte:istics of the underlving flibsoil. The area between the trenches should be sufficient to absorb all the liquid contributed toy the upilope trench, and Trenches should be lard parallel to the slope. Ihr design of the liquid distribution system is the next step. I he traditional %%stem using perforated 10 cm (4 inches► diameter tape does nit provide adequate distribution over the entire adsorp live area as is rrquired 12. 4). 'To solve this problem, pressure dis- tnbution %%stems utilizing smaller diameter pope have Bern etc signed. these s%%firms pnwnle uniform distribution ovri the entire trench IHmut.m Are.t .luring rath Apphtarici 1^dust'•) if effluent f41 [hut, .optimal use of the tandtill I% male by a%, Amg hHAI • m erloadig nit As%"i IAttit poor purer it anon And t loa,ftrtg t o ther delaih if three .%%iris Air torig puhlithed rl%rwhrrr • • Numbers in parrothe.,, ode. ale is r Numbers in parentheses incise ate f r Unknown quantJtrs of water from t Hoeoruwnrr damased d"wna tank The fourth step is the fine this can be done an appropria surfaec must be selected. Soi ly perched water tables at 3 several weeks, -ereas high Addition of 6V 2 feet) u would create is distatice of the absorption trenches level. This is tie.--tinimal di codes (8, 9). Mure important to obtain satisfactory purifica The depth of fill r the termined. This should be sufl northern Wisconsin have oper of cover. Side slopes should be stab off. Slopes of 5:1 are rerun cm to 15 cm (4 to 6 inches) c growth of grasses needed to filtration of precipitation. termined the total basal area area computed in the first st than or equal to the Area in it The last step in design is filtering efftceency of the fit: rate. Large mstantAneous app the sand resulting in lea p clogging. Smeller, more freq too small and too frequent I also lead to early clogging. mended and the sire of the t SECTION VIEW PLAN VIEW k no. 3, 1f7S