1992-03-30 Septic System Design Report

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E-2 --� than 5 minutes pei inch. The allowable percolation rates also depend upon the slope of the original ground surface. A table of these relationships is presented on page E-8. It should be r,cted that mound construction be- gins with the 12-inch layer of clean sar.d upon which the rock is placed. The design material presented in section E of this Manual suggests a possible "cookbook pproach and is intended to deal primarily with mounds or "berms" for sinbie family residences, or daily sewage flow rates of no more than 1,200 gallons. A flow of 1,200 gallons per day can be treated with a rock bed 10 feet wide by 100 feet long in a properly constructed mound or "berm." However, the proper hydraulic operation of a mound depends upon lateral as well as vertical seepage. While there is little doubt that rock beds wider than 10 feet will operate satisfactorily on some soils as far as flow hydraulics is concerned, a careful analysis must be made of the ground slope and soil permeability underlying the clean sand layer of the mound. A vertical separation of at least 3 feet is required between the bottom of the rock bed and any restricting layer in order to maintain aerobic condi- tions in the clean sand under the rock layer. (When consolidated imaermeable bedrock is present the vertical separation distance is 7 feet.) When aerobic conditions exist in the clean sand, the long-term acceptance rate will be 1.2 gallons Der day per square foot. If the depth to the restricting layer is inadequ::-; or the rock bed is too wide, anaerobic conditions may exist and cause a much slower acceptance rate. To evaluate the possibility of anaerobic conditions and the subsequent hydraulic failure is the major design problem when sizing mounds larger than those required for single family residences. Thus, the design criteria of section E cannot be simply multiplied by a scale factor and expected to properly treat larger flows. The hydraulics of lateral and vertical movement in the clean sand layer and the soil under the elevated rock bed must be carefully analyzed to ascertain that anaerobic conditions will not exist. Thus, both lateral and horizontal permeabilities of the ,under- lying soil layers must be utilized to analyze the flow regime to estimate the height of the saturated zone. Where heavy clay soils with slow permeabilities and'high seasonal saturated conditions generally exist over an area, it is far better to utilize mounds for one or two single family residences than to collect the effluent from many residences than attempt to dispose and treat it at a single location. The flow hydraulics in clay soils will require either large depths of fill, or underdrainage, or both, in order to design a proper sewage treatment system to prevent anaerobic conditions under the rock layer. As an example, a mound designed to treat 450 gallons per day may function very well under certain clay soil conditions, while a single mound serving 5 or 10 residences will fail hydraulically if constructed according to the same vertical separation specifications. Prop,•, construction practices for mounds are extremely important but when carefully followed will produce a sewage treatment system that will function effectively on a long-term basis. There are an estimated 5,000 single family mounds successfully treating sewage in Minnesota. Many Minnesota counties have found that properly designed and constructed mounds or "berms" am an effective method of sewage treatment and accept them as a standard system.