Design

Two men standing in hole in yard looking at cover of septic tank.

Well-designed and properly installed septic systems are critical to treating wastewater and protecting Minnesota's surface and groundwater from contamination. Designers must carefully evaluate soil conditions and proximity to groundwater, and create systems to function properly on specific sites. For example, designers may choose trench, bed, at-grade, or mound-style drainfields, depending on site conditions. SSTS designers and installers must use system components registered with the MPCA.

Septic systems in Minnesota are governed by a combination of state regulations, local government ordinances, and food/beverage/lodging establishment requirements. The technical criteria and inspection requirements for SSTS are governed by Minn. Rules Chapter 7080. Requirements for mid-size subsurface sewage treatment systems (MSTS) are in Minn. Rules Chapter 7081.

Mound systems

On sites with a high water table, shallow soil over bedrock, or slow or fast permeable soils, septic system designers may choose to install a septic system with a mound-style drainfield. Mounds are built above the natural soil surface with sand fill and a rock bed. Learn more:

Large SSTS

Counties, cities, and townships review designs and issue construction permits for most SSTS in Minnesota, but large sub-surface sewage treatment systems (LSTS) — designed to manage a flow of greater than 10,000 gallons a day — must be permitted by the MPCA (see the Wastewater permitting page).

Underground injection control: Class V systems

SSTS that are designed to receive sewage or nonsewage from a two-family or larger dwelling, or receive sewage or nonsewage from another establishment that serves more than 20 people a day, are regulated under Code of Federal Regulations, title 40, parts 144 and 146. Learn more on the U.S. EPA web site:

The EPA has banned two types of Class V wells: large-capacity cesspools and motor vehicle waste disposal wells. Existing Class V wells at vehicle maintenance shops that are not located within wellhead protection areas don't need to close, according to EPA rules. However, their owners must closely scrutinize what is going into them. All hazardous waste in the shops must be managed in accordance with Minnesota’s hazardous waste requirements.

Related resources

Advanced design guidance

All sizes of Type IV SSTS (systems with advanced treatment), and SSTS with a flow of 2,501 to 10,000 gallons per day must be designed by an MPCA-certified Advanced Designer. Some components need to be designed by AELSLAGID professionals.

Site evaluation and system size

Setbacks

Sizing non-registered gravelless pipe in SSTS expansions

Gravelless pipe is not currently a registered distribution media product; the manufacturer has not applied for registration in Minnesota. Gravelless pipe can't be used in new or replacement systems, but prior to 2008, it was allowed under Minnesota rules and is still used in some existing systems.

Gravelless pipe was typically sized at 3 square feet (ft2) per lineal foot of 12-inch-wide gravelless pipe when installed, but there's no data to support that sizing strategy. When expanding existing compliant systems, the MPCA recommends sizing gravelless pipe at 1 ft2 of infiltrative surface for each lineal foot of 12-inch-wide pipe. Existing 8-inch-wide gravelless pipe should be sized at 0.66 ft2 of infiltrative surface for each lineal foot of pipe.

Resources