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r <br />( <br />A. SKV/AGE TREAT^EIJT BY SOIL <br />The effluent discharging from a septic tank contains appreciable <br />amounts of nitrogen, phosphorus, bacteria, and viruses, <br />-Nitrate nitrogen (NO^) in anoxmts gre-ater than 45 fflg/l in drinking <br />water can cause neth^oglobinemia in infants (also called "blue baby <br />syndrome"), <br />-Phosphates entering lakes and rivers cause eutrophication, but are <br />not considered a drinking water problem, <br />-Bacteria and viruses cause many enteric (digestive tract) diseases. <br />The soil in most cases is capable of treating these contaminants <br />through a series of complex chemical, physical, and biological processes. <br />Bacteria and viruses <br />1. Adsorption to soil particles <br />Z, "Feed" on sewage and form a slime layer. <br />3, i*i;ne layer acts as a filter and also acts as a valve to slow the <br />.. nward percolation of effluent, <br />by attrition due to lack of nutrients, presence of <br />'u-.eria-toxic chemicals, natural soil antibiotics. <br />Phoaphc:nib <br />1, r<\,ad.ly adsorbed by mineral particles in the soil, <br />2, Movement through soil occurs as adsorption sites are used up, <br />3, Greatest movement in sands (up to 20"/year); less in clays <br />(up tc 4'/year). <br />liitrogen <br />1, Nearly all N in effluent is in ammonia form (NH^) <br />2, Under anaerobic conditions such as hea*^ clay soils and sAturAtdd <br />zones, it stays in this form as it moves through the soil and is <br />relatively hainless, <br />3, Under aerobic conditions, nitrification occurs, changing into <br />nitrate (NO3). In deep sandy or gravelly soils this may move <br />directly into the water table, where it ray ot nay not be diluted <br />depending on the load and the housing density. <br />The above processes are just abbrief overview of the complex proc*sc^s <br />which actually occur. <br />B, SITE AND SOIL CONDITION'S WHICH NECESSITATE ALTERKATI’/E TREATENT <br />AND DISPOSAL METHODS <br />In the standard trench drainfield system, the bacterial slime forms <br />a "biomat" at the rock/soil interface, which slows the downward movement <br />of effluent. The rate of water movement through uhis bionvut depends <br />to a great extent on the permeability of the soil, <br />1, Coarse sands and gravels may allow such rapid percolation that the <br />biomat won't form. This allov/s effluent to rove directly into <br />the water table. <br />It r tk 4