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A seepage bed system is a wide area (wider than three feet) prepared to accept septic tank effluent. It is created below the surface of the soil and built the same way as a trench system. Seepage beds treat the effluent effectively as long as they are located in appropriate soils. A seepage bed is defined as a soil treatment and dispersal system, the absorption width of which is greater than three feet but no greater than 25 feet. Figure 12.9 in the SSTS Manual for Septic System Professionals in Minnesota shows the design and installation specifications of a seepage bed. They can use either gravity or pressure distribution.
The construction of a seepage bed is essentially the same as that for a trench, except the bed is wider. In Minnesota, the bottom area of seepage beds with gravity distribution must be 50% greater than that of trenches to allow for the fact that there is very little sidewall with a seepage bed and low oxygen transfer. The bottom of the bed must be within 48 inches of the final grade and be covered with at least 12 inches of soil and a close-growing and vigorous vegetation.