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The Water Resources Center is affiliated with the College of Food, Agricultural and Natural Resource Sciences and University of Minnesota Extension.

Solving Sewage Problems on Small Lots and Poor Soils


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Septic systems are a safe and effective soil based system to treat household wastewater provided there's enough soil area and soil conditions are conducive to treatment. Septic systems treat sewage as well as or better than municipal treatment facilities when they are properly designed, installed and maintained.

There are many situations in wet soils or on small lots where traditional septic system designs must be modified or they won't work. Space problems are common in small rural towns, around lakes, and in existing or new small-lot suburban developments. Poor or wet soil conditions may occur in any of these same areas, as well as in wide-open spaces.

The soil treatment portion of a septic system, often referred to as a "drainfield", is the most important part of the treatment. It kills the disease-causing pathogens and filters out most of the nutrients in the sewage. All of this depends on having two to three feet of unsaturated soil and separation from bedrock. Treatment will not occur if untreated wastewater is allowed to leak into bedrock or enter soil that is filled with water at any time during the year. This separation determination is made at the time the system is designed by doing a soil boring.

If you don't have the two to three feet of separation necessary for a traditional trench drainfield, the treatment area must be raised (mound), relocated on the property, or the raw sewage stored and hauled off of the property (holding tank). If the individual property is not large enough or does not have the right soil conditions, additional property may be needed. Two or more property owners may join together to locate an appropriate treatment site nearby.

Several treatment alternatives are available. Individual or multiple-household septic systems may utilize an in-ground trench, an at-grade trench, a mound, a constructed (lined) wetland, or drip or spray irrigation system to disperse and treat septic tank effluent. Special enhancement devises may be added to improve the performance or allow for the modification of the system. These include peat or sand filters, aerobic septic tanks, or the separation of solid wastes into a composting system. Current research and trial systems are evaluating the effectiveness of these units. Some of them may result in reduced size requirements and/or smaller soil separation distances.

The importance of proper operation and maintenance is increased as multiple-household systems and special enhancements are introduced. Private interests such as homeowner associations, private joint ventures, or water quality cooperatives can do this management. Public management options include municipal utilities, sanitary sewer districts or Environmental Subordinate Service Districts.

In solving sewage treatment problems on small lots or in difficult soil conditions, it is very important to remember that the ultimate goal is to "achieve proper sewage treatment for the protection of human health and the environment". The most cost- effective solution to the problem may not be "one system for everyone" but rather a combination of treatment and management options.

More details about treatment and management options are available from your local planning and zoning or environmental services office. The Residential Cluster Development: Fact Sheet Series available from all University of Minnesota Extension County Offices and the Extension Distribution Center 1(800)876-8636 also has more details of these management options.