Do We Need Design Guidance for Mound Systems?

We have the answer to this and other burning mound system questions. Plus, learn the key factors for designing sewage treatment mounds.
Do We Need Design Guidance for Mound Systems?
Mounds provide excellent treatment when they are designed and installed properly. Shown here, onsite technicians from Florian & Sons Excavating rake rock over the pipes in a mound system. (Photo by Eric Hylden)

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A reader submitted a question recently asking why there was not design guidance for mound systems loaded by gravity. The easy answer is that mound systems are designed to address specific soil and site problems, and a key aspect of their performance depends on pressure distribution of the effluent over both time and space (area). So, there should not be any guidance for gravity distribution because the effluent will not be distributed in a manner to allow adequate treatment.

Mounds are designed to address shallow soils over bedrock or some limiting soil layer such as a fragipan or hardpan; slowly permeable soil, shallow soil over permanent or seasonal water tables; and soils that are rapidly permeable.

In the case of slowly permeable soils and the presence of hardpans, the main consideration is to provide enough area for the effluent to infiltrate; so it is primarily a hydraulic acceptance problem. It’s a constant balance between systems accepting the amount of effluent generated by the residence with the need for the necessary amount of treatment. In the case of shallow soils and rapidly permeable soils, the main concern is to provide adequate treatment. 

Mounds provide excellent treatment when they are designed and installed properly, including a nitrate reduction of 40 to 50 percent. I think they have been underutilized in many parts of the country, mostly due to confusion and misinformation about the proper design considerations. For instance, in Minnesota and Wisconsin, we have thousands of sewage treatment mounds that are properly treating septic tank effluent from single-family residences. When properly maintained, these mounds will last for many years beyond the 20-year design life. 

There are some important factors to remember when designing sewage treatment mounds: 

  • Landscape location
  • Size and shape
  • Soil surface preparation
  • Construction procedures
  • Distribution of effluent
  • Dosing quantity and timing
  • Quality of clean sand fill 

If you’re designing a mound for a site other than a single-family residence and it needs to handle more, the likelihood of hydraulic failure increases, so it is not enough to simply “scale up” single residence designs to handle the larger flows. I have seen this quite often, which is why many people believe mounds do not work.

About the Author

Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians.

Send him questions about septic system maintenance and operation by email to


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