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Published April 2010

Septic systems: how do they work?

By Ivan Raconteur

There is nothing glamorous about septic systems, but when properly designed, installed, and maintained, they can provide effective and economical sewage treatment.

Sean Riley of the environmental health division of the Wright County Planning and Zoning Department provided information that helps explain how septic systems work.

“A septic system has to collect, treat, and dispose of wastewater,” Riley said.

To do this, septic systems have three basic components; plumbing for wastewater collection, a septic tank for primary treatment, and a soil treatment system for final treatment.

After entering the septic tank, materials separate into three layers. On top is a floating “scum” layer, composed of soap, grease, and toilet paper. Below the scum layer is a liquid layer. At the bottom is a sludge layer that includes heavy organic and inorganic materials.

The primary treatment occurs when the bacteria in the sewage begins to break down the organic materials.

Wastewater from the house enters the tank through an inlet baffle, settles into the three layers, and the liquid portion eventually leaves the tank through an outlet baffle and runs into a soil treatment system (drain field).

While the plumbing and septic tank are the same for all systems, the soil on the site determines what type of soil treatment system is required.

Riley explained that three feet of separation are required between saturated soils and the treatment system.

Traditional systems include perforated pipes running through trenches. The pipes are placed on a bed of 3/4 to 2-inch diameter rock covered by natural or synthetic fibers.

In a properly-working system, the soil will destroy disease-causing pathogens in the effluent.

In cases where the saturated soil is closer to the surface, mound systems are required. These are seepage systems constructed with washed sand to provide the necessary separation from the saturated soil.

Traditional systems may cost $7,000 to $9,000, while mound systems typically cost $4,000 to $5,000 more due to the additional materials required, Riley said.

In Wright County, about half of the systems are mound systems, he added.

McLeod County Environmentalist Roger Berggren said that due to the high-clay soil in the county, about 75 percent of the systems installed in McLeod County are mound systems.

The majority of the systems in Carver County are mound systems, according to Kim Jopp of the Carver County Office of Environmental Services.

This is due to the heavier clay soils in the area.

“Areas that are good for farming are usually bad for septics,” Jopp commented.

Riley said some people have misconceptions about mound systems that date back to when this type of system was new. He said much has been learned about the systems, and when properly installed and maintained, they work well.

One example of an improper use is that some people drive snowmobiles over their mound systems, which drives down the frost and causes them to freeze, Riley said.

In addition to drain fields and mound systems, there are also performance-based systems that are designed for small lakeshore lots. These systems include a pre-treatment component, almost like a miniature city treatment plant, Riley said.

Many counties require certification of the septic system when a property changes hands. Some counties also check the records when a resident applies for a building permit for remodeling.

Septic systems are sized based on the number of bedrooms in a house, which dictates the number of potential residents. Systems are not sized based on the number of bathrooms.

Berggren said McLeod County emphasizes the importance of maintenance. He said it costs about $110 to $150 to have a system pumped every three years, which is much less expensive than if a system is not maintained and fails. He compared regular tank pumping to changing the oil in a car.

Riley provided the following tips for septic systems:


• Homeowners should have their septic systems inspected every three years, and pumped to remove scum and sludge when necessary (usually needed every three years).

• Repair any leaky faucets, toilets, and water-using appliances which can cause unnecessary pressure on the system.


To get the best performance from your septic system:

• try not to have one big water-using day, such as laundry Saturday. Spread out water use when possible.

• remember that septic systems are not garbage cans; don’t flush cigarette butts, hygiene products, or medications.

• limit or avoid using a garbage disposal. Compost organic waste or throw it away.

• use white toilet paper. Some dyes can hurt the bacteria in the septic system.

• try to minimize the use of cleaners in the house.

• consider purchasing a front-loading, high-efficiency washing machine to use less water. Install a filter on the washer to remove lint. Use phosphate-free liquid detergents.

• install low-flow toilets and shower heads to reduce water use.

• minimize recharge settings on water softeners.

• don’t locate dog kennels, pet areas, parking areas, or other improvements on top of septic systems.

• don’t put additives that are advertised to improve performance into the septic system. They are unnecessary.

• maintain good grass cover over the system, and do not plant trees or plants with deep, invasive roots within five feet of the system.

Jopp said the University of Minnesota Extension web site includes information about plants to avoid, and plants that are OK to plant on or near septic systems.

• in late fall, let grass grow thick over the system, and let snow cover build up. Do not walk or drive over the system in winter.


• system must be designed, installed, inspected, and maintained/repaired by a contractor licensed by the Minnesota Pollution Control Agency.

• Wright County requires inspections every three years. In McLeod and Carver counties, newly-installed systems do not need to be inspected for five years, and then inspections are required every three years thereafter (as they are with existing systems).

• Wright, McLeod, and Carver counties require septic systems to be certified as compliant for property transfer. The state requires system disclosure.

• Carver County evaluates septic systems when there is a triggering event, which could include a change in use of the property, such as starting a day care business or contractor’s yard.

• Wright County evaluates conformance issues on septic systems when homeowners apply for building permits.

• In all three counties, inspections are required when homeowners apply for building permits in shoreland areas (within 300 feet of a river or stream, or within 1,000 of Department of Natural Resources protected water), or when adding a bedroom.

• New state rules implemented Feb. 4 require that when a system is installed, the county and the contractor have to verify and agree on where the saturated soil begins.

• Berggren said if an inspection determines that a system is out of compliance, the county is notified. The county determines the date a system must be brought into compliance. In cases where there is an imminent health threat, a system must be brought into compliance within 10 months. If the non-compliance is due only to lack of required separation, the system must be brought into compliance within three years (two years if it is within a shoreland district). If a system needs to be replaced, a limited amount of funding is available for low-interest loans on a first-come-first-served basis.

• Jopp said Carver County is in the process of updating its code regarding septic systems, and there may be changes as a result of this process.

For more information:

To learn more, visit:


• or contact your local planning and zoning office, environmental health office, soil and water conservation district, or the University of Minnesota Extension Service, www.mes.umn.edu.

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