In order to properly treat wastewater, unsaturated soil conditions must be present for beneficial bacteria and microbes to live. They feed on the harmful bacteria and viruses, effectively removing them from the wastewater. Saturated soil conditions do not allow the bacteria and microbes the proper conditions to live. There are a few ways to determine what depth saturated soils occur on each property. The most common methods involve using a hand tool called a bucket auger or by excavating a soil pit. The designer will examine
each layer and complete a soil log that documents changes in texture, color, and structure. Additionally, they will look for redoximorphic features, also referred to as 'redox,' and color deletions in the soil. Besides saturated soil, there are other limiting factors such as layers within the soil that contain 50% or more of rock fragments, bedrock, standing water, and disturbed or compacted soil. Once saturated soil or a limiting layer has been identified, the separation distance is measured from that depth to the surface.
In Minnesota, systems installed after March 31, 1996 require three feet of vertical separation to saturated soil. Systems installed before April 1, 1996, that are not in a Shoreland, area require two feet
of soil separation.
The difference in requirements occurred when Minnesota Rule 7080 changed. The new
code allows older systems to be grandfathered into the new rule.
Pressure beds and trenches are designed when you have a minimum of 48 inches of
unsaturated soil, and there aren't any other limiting layers present. Below grade
systems can use either gravity or pressure distribution.
At-grade and mound systems are designed when saturated soil or limiting layers are
located within 36 inches from the ground surface. These systems always use pressure
distribution.
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