12-12-1987 Planning Packet - Laserfiche WebLink

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V B-1 HOW SOIL TREATS SEWAGE TANK EFFLUENT Suitable soil is an effective treatment medium for sewage tank effluent. Soil contains a complex biological community. One tablespoon of soil contains over a million microscopic organisms, including bacteria, protozoa, fungi, molds, and other creatures. It is the bacteria and other microorganisms in the soil which treat the wastewater and purify it before it reaches the groundwater table. *^ut the wastewater must pass through the soil slowly enough to provide adequate contact time with soil particles and microorgeinisms. Soil bacteria or "bugs” as they are sometimes called, need the same things to live and grow as any other animal. They need a place to live, food and water to eat, air or oxygen to breathe, and time to grow. Soil bacteria attach themselves to soil particles using microbial slimes. The larger soil pores are filled with air, containing oxygen. To provide adequate time for treatment to take place, it is necessary to have at least 3 feet of aerated or unsaturated soil. WHAT IS SOIL? Soil is also a collection of particles of various sizes which have been ground up from the larger rocks by glaciers or by the action of wind and water. On page B-17 three schemes for classifying soil particles are presented. The U.S. Department of Agriculture classification system is used by the Soil Conservation Service and by soil scientists when classifying and mapping soils. The relative proportions of sand, silt, and clay particles determine the soil texture. The triangle graph on page B-l8 shows the various soil textures under the USDA classification system. A soil consisting of equal parts of sand, silt, and clay is called a clay loam. If there Is more sand, it becomes a sandy loam; if the clay content increases, it becor.es a clay. A field method for estimating soil texture is given in publication CD-FO-0817 at the end of Section B. Most people have a good idea of what a sand particle looks and feels like. But it is Impossible to see a single clay particle with a naked eye, and difficult to visualize the size of 0.002 mm. If a sand particle were magnified to a size 10 inches in diameter, in comparison, a silt particle would be about 1 inch in diameter and a clay particle about the size of a grain of sugar. Structure, which is the formation of larger soil particles by the cementing vtietner of individual sand, silt, and clay particles, affects the pore size and the rate at which water will move through soil. It is not the size of soil particles that determines how fast water moves through soil, but rather the size of the spaces or pores between the particles or aggregates. Loamy soils have a mixture of pore sizes, some large and some small, depending upon the arrangement of the different particles. A field soil with a high percentage of clay particles has more pores than a sandy soil but will drain much more slowly because the pores are very small. A soil which has a well-defined structure will transmit water much more rapidly than a soil where the structure has been destroyed by compaction or smearing. WHAT IS WASTEWATER? Wastewater which enters the soil in a sewage treatment system becomes the food and water for the biological community. Domestic wastewater is 99.water and 1