12-12-1987 Planning Packet - Laserfiche WebLink

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L. B-4 As sewage tank effluent flows into a drainfield trench, It moves into the distribution pipe and down through the trench rock to the soil where treatment begins* A biological layer or biomat is formed by soil roicroorganisms which secrete a gluey or sticky substance and anchor themselves to the soil or rock particles. This biomat forms first along the trench bottom and as liquid begins to pond in the trench, forms along the soil surfaces on the sidewalls. When' fully developed, the gray to black slimy biomat layer is about 1/2 to 1 inch thick. Flow through the biomat is considerably slower than flow throug.' natural soil and is unsaturated flow. Thus, the biomat acta as a valve to slow down the flow of wastewater into the soil so that unsaturated conditions exist In the soil beneath the drainfield trench. A properly functioning system will have wastewater ponded in the trench while the soil a few inches outside of and below the trench will be unsaturated. The unsaturated soil has pores containing both air and water so that microorganisms living in the soil can effectively tr*i *• the wastewater through the soil systen,. A developed biomat is in equilibrium in that it remains at about the same thickness and the same permeability if proper effluent quality and quantity are maintained. The biomat and the liquid ponded within the trench are anaerobic and the organic materials in the wastewater are food for the anaerobic microorganisms which grow and multiply and increase the thickness and decrease the permeability of the biomat. On the soil side of the biomat, oxygen is present so that conditions are aerobic allowing soil bacteria to feed on and break down the biomat continuously. These two processes go on at about the same rate so that the thickness and permeability of the biomat remain about the same. If the wastewater contains a greater proportion of organic material because of failure to regularly pump the septic tank, the greater amount of food for the bacteria will increase the thickness of the biomat and decrease its permeability. If seasonally saturated conditions occur in the soil outside the trench, aerobic conditions will no longer exist, the biomat will thicken, and its permeability will be reduced. Soil of suitable texture is an excellent treatment medium for sewage tank effluent. The soil is a self-renewing treatment system and will maintain its treatment effectiveness and capacity as long as the sewage tank effluent remains of reasonable quality and as long as aerobic conditions exist in the soil outside of the trench. The design and construction procedures presented in this Manual are intended to show how to utilize the soil system in such a way as to assure that sewage tank effluent is properly treated. • • I