Community Assessment Report - Onsite Sewage Treatment ...

Below-Grade SystemsBelow-grade systems are constructed in original soil with distribution of effluent occurringbelow the soil surface. With below grade systems the soil treatment area is designed and installed suchthat the infiltrative surface is below the original ground elevation and a final cover of topsoil stabilizesthe completed installation, supports vegetative growth, and sheds runoff. It is the underlying soil thattreats the many harmful components in the effluent before it reaches surface or ground waters. The twotypes of below-grade soil treatment systems commonly used are trenches and seepage beds.Trenches have better oxygen transfer then beds and are recommended whenever the siteconditions allow although seepage beds are often more attractive due to reduced land arearequirements. In addition, the cost and time of construction, trenches are preferred because they havegreater infiltrative surface for the same bottom area, and less damage typically occurs to the infiltrativesurface during construction (Otis et al, 1977).The figure below shows minimum depths and separation requirements for trenches or seepagebeds. For systems without pretreatment, at least three feet of soil suitable for treatment should belocated below the bottom of the distribution media. The minimum depth of distribution media is sixinches, followed by a minimum soil cover of twelve inches, so that the total distance from theperiodically saturated or other limiting condition to the final grade is approximately 4.5 feet. Note thatthis total could be made up of 3.5 feet of original soil and one foot of soil (7080.2150, Subp. 3) over thedistribution media of the system.Figure 1 - Trench and Bed DepthFrom MN Rules 7080.2260 Subp. 3. If the distribution media in a trench or a bed isin contact with soil texture group 2 through 4 (medium sand, fine sand, coarse andmedium loamy sand) pressure distribution must be used.Below-Grade Systems: SpecificationsTrenches