Onsite wastewater treatment
Residents of areas without access to sanitary
sewers and municipal treatment plants own, operate, and
maintain their own mini-treatment plants usually called
septic systems, on-site sewage treatment system (on-sites),
or individual sewage treatment systems (ISTSs).
Residential onsite wastewater treatment in Minnesota is a large (about 500,000
residences representing about 30% of the population use onsite treatment),
troublesome (nearly half may not comply with State rules or be failing), and
Failing systems (called hydraulic failure) are those where the
wastewater ponds on the surface instead of disappearing into the
subsurface soil. Non-compliant systems may not adequately treat
wastewater and pose a health risk due to pathogenic (disease-causing)
microorganisms. Conventional systems, such as standard trenches
or mounds, may work well by filtering out microbes to reduce disease
risks, but are not designed for nutrient removal and may contribute
to lake eutrophication and groundwater nitrate contamination.
Contrary to general public perceptions, properly designed and
maintained onsite systems may provide at least as effective a level
of treatment as from a large municipal plant at an equal or lower
cost. Since federal subsidies to municipalities have largely disappeared
in recent years, many small communities are considering the onsite
option. Because of reduced size, construction and maintenance costs,
they may also be an important component of a comprehensive land
use plan and provide a means to minimize the potentially rapid
growth that may be associated with new or expanded treatment plants.
The complete septic system has three primary components:
- Plumbing: collects wastewater
- Septic tank: primary treatment - settles out the larger solids + some breakdown
of organic matter + storage capacity (usually from 3-10 days for a family of
- Soil treatment: secondary and tertiary treatment - filters pathogens + final
breakdown of organic matter + some phosphorus removal depending on soil + disperses
Additional treatment capacity may be added between the septic
tank and the final soil- dispersal.
The restrictive site and soil conditions in
the Northland present special challenges to standard soil-based
treatment systems in unsewered areas. Particularly along lakeshores,
lot sizes may be too small for effective treatment and/or the
soils are too thin or too "tight" to allow the septic
tank effluent to disperse without ponding on the surface. Compliance
with State rules requires the drain field to have at least 3
feet of seasonally unsaturated soil but in some areas, particularly
during spring snowmelt, the water table may be too high.
A variety of so-called alternative systems now exist which
may alleviate some of the waste disposal problems at these
difficult sites where conventional
drainfields are inadequate. They are also called "experimental", "pre-engineered",
or "performance" systems, the latter two because they may be engineered
to perform to a specified set of effluent standards as opposed to the one size
fits all prescriptive approach of conventional systems.
Dealing with Problem Areas and Evaluating Onsite System Performance
A University / multi-industry / local, state, and federal agency project was established in 1995 to design, construct,
and monitor the performance of alternative treatment systems in Minnesota. Although the research program was largely terminated
after 2001, results are available via the University of Minnesota Extension
Onsite Sewage Treatment Program
(OSTP) website (which contains lots of useful information).
Northland results may be found under the
"NRRI-UMD Technical Reports" heading near the bottom of the
Research page in the OSTP website.
The research site is located at the Northeast Regional Correctional Center (NERCC). This project and the Grand Lake
cluster-constructed wetland project were the product of a collaboration between the Natural Resources Research Institute,
St. Louis County Health Department and WLSSD.
Download a November 2004 report
(PDF file -- 1.2 MB) that summarizes all of the performance data for the NERCC peat and sand filters and constructed wetlands.
Grand Lake Cluster Collection System
& Constructed Wetland Treatment System information may also be found at this website. This project linked 9 lakeshore homes
having chronic wastewater problems to form their own Sanitary Sewer District with a common (cluster) collection system that
delivered wastewater to a constructed wetland and seepage cell for effective treatment.