12-12-1987 Planning Packet - Laserfiche WebLink

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B-3 HOW SOIL TREATS WASTEWATER In the soil, bacteria eat the food (literally break down the BOD and solids, and incorporate them). Pathogens become trapped in the soil, either by being adsorbed onto soil particles, or becoming stuck to the microbial slimes laid down by soil bacteria. Once trapped, some pathogens die because of differences in temperature, lack of moisture and food, and other causes. Others are inhibited or killed by antibiotics given off naturally by soil fungi and other organisms. Still others are actually preyed upon by soil bacteria and literally eaten. Nutrients are also removed or modified. Nitrogen from the septic tank is usually in the ammonia form (NH^). While some is used by the soil bacteria or adsorbed by the soil particles,'most of it is converted to nitrate (NO.) in the aerated soil. Nitrates are soluble and will move with soil water. Nitrate movement is one of the reasons for separation distances between sewage treatment systems and water supply wells. There is little evidence to show, however, that shallow sewage treatment systems such as drainfield trenches or seepage beds cause nitrate problems with water supply wells. Recent research results show that sewage treatment mounds utilizing pressure distribution do an excellent job of denitrification. Phosphates are removed from wastewater by being adsorbed onto soil particles, particularly particles with high concentrations of iron, manganese, and aluminum. Soils with a greater percentage of clay particles have more of these minerals than sands. When the adsorption sites are filled, phosphates will ir'^ve through the soil. Laboratory studies on coarse sands Indicate the maximum rate of phosphate movement is about 50 cm (20 inches) per year. Field studies have indicated that the rate is even slower under operating sewage treatment systems. Phosphate movement will be still less through loam or finer textured soils which have more adsorption sites. If the treatment system is functioning properly, problems from phosphate movement to surface or groundwaters should be minimal. UNSATURATED FLOW AND THE BIOMAT The soil bacteria must have air and sufficient time for treatment to be effective. These conditions will exist if the soil beneath the soil treatment system is unsaturated. In an unsaturated soil, water moves only through the smallest pores or in a thin film around soil particles surrounding the larger pores which are usually filled with air. The reason for this type of movement is that the driving force behind unsaturated flow is not gravity, but a soil tension force (sometimes called capillary attraction, wicking action, or "sucking power"). If all soil pores were filled with water (i.e., saturated conditions), most of the water would flow by gravity through the larger pores (much the same way one could put more water through a 12-inch culvert than a 1-inch garden hose). However, under unsaturated co»'ditions, the largest pores drain first, since they are able to exert the least tension (or "sucking power"). Water is pulled or "sucked" through the smaller pores. Because water is moving due to tension or "sucking" power, it does not have to go down but can move sideways or even up to wherever the soil is driest. The presence of lush, green grass over the drainfield is evidence of this "capillary" movement of unsaturated flow of water. 11 rn• r 11'rr- T i ri~~r .'T—7~r~