Beware of Wide Beds in Mound Systems

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I have noticed in a number of recent articles that the onsite sewage treatment industry is moving toward very wide beds in mounds and other wide systems with a series of beds. I am concerned that this is a very dangerous trend and I predict hydraulic failure of these wide mounds when flows are near or at design values.

Mounds designed for single-family residences have been used successfully for years when the rock layer width does not exceed 10 feet. For a mound to work properly it is important that effluent is distributed through the clean sand layer to the underlying soil. As the effluent moves downward through the rock layer and into the clean sand layer, it flows sideways into the lesser permeable soil underneath the mound.

If the rock layer width is increased, a greater width of the lesser permeable soil must be available under the mound for the mound to operate successfully, because flow per linear foot of bed is more concentrated. This creates a situation where anaerobic conditions occur at the sand-soil interface, restricting flow and resulting in further restricting flow through the less-permeable material. These conditions can then result in effluent mounding in the sand layer underneath the rock bed, ultimately causing hydraulic failure. The wide mound will fail to accept the design effluent flow and hydraulic failure will take place.

The University of Wisconsin Small-Scale Waste Management Project discovered the flow problems into the underlying soils of wide mounds in an experimental mound installed at Westboro, Wisconsin, many years ago. The failure of that wide mound and the results of that study were published as a technical paper by the American Society of Agricultural Engineers.

The designers of every very-wide mound should be accurately evaluating the fluid flow pattern in the soil layers under the mound. That evaluation must use the permeability of the various underlying soil layers to determine the liquid flow pattern.

A mass failure of the wide mounds or beds now being installed will be extremely harmful to the future of the onsite sewage treatment industry. The advocates of the “big pipe” approach will certainly welcome the news of any such failures and use that information when new onsite collector systems are proposed.

Roger E. Machmeier, Ph.D.
Professor Emeritus, University of Minnesota
Former Septic System Answer Man author


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