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Sewage (Septic) System Basics

Well & Sewage SystemWhat is an onsite sewage system?

In areas where public sewer is not available, homeowners must install sewage systems (also known as septic systems) on their property that will treat wastewater coming from their home.

All facilities such as toilets, sinks, bathtubs, showers, washing machines, dishwashers, or anything else that generates sewage must be connected to a sewage system. Footing drains, roof drains, storm water pipes, and water softening waste should not be connected to sewage systems. These items should discharge away from the drainfield area, and in a manner that does not impact neighboring property. Also, water softener wastewater should not discharge near wells or surface water.

Onsite sewage systems must be designed according to Washtenaw County standards, and must be inspected and approved by a Sanitarian (health inspector). Sewage systems are different from municipal sewers because they have a limited life expectancy, which can be drastically reduced if the system is improperly used or not maintained. (See Maintaining Your Sewage System.)

What are typical sewage system components?

  • Pipe from House:  All of your household wastewater exits your home through a pipe that flows into the septic tank.Septic Tank - Courtesy of U.S. Environmental Protection Agency

  • Septic Tank:  The septic tank is a buried, watertight container that holds wastewater long enough for solids to settle out (sludge) and oil and grease to float to the surface (scum). Solid materials begin to decompose, and anaerobic breakdown of bacteria takes place. Compartments and a baffle or outlet tee in the septic tank prevent the sludge and scum from leaving the tank and entering the drainfield. The septic tank needs to be pumped every 3-5 years to remove the scum and sludge.

  • Drainfield:  Every time new wastewater enters the septic tank, the same amount of wastewater (or effluent) exits the tank and is pushed into the drainfield. If the drainfield is overloaded with too much liquid, it will flood. This prevents treatment of wastewater, and may cause sewage to flow to the surface of the ground or back up into the house. A reserve drainfield is an area on your property suitable for a new drainfield system if your current drainfield fails.

  • Soil:  Septic tank wastewater flows to the drainfield, where it percolates into the soil and is filtered. Natural processes remove most of the contaminants in the wastewater before it eventually reaches groundwater. Aerobic breakdown of bacteria also occurs. Soil that allows for “percolation”, or drainage, is necessary for successful wastewater treatment.


What are signs of sewage system problems?

A failed sewage system is a health hazard to you, your family and your neighbors. Call Washtenaw County Planning & Environment at (734) 222-3800 at the first signs of failure, and we will assist you in your efforts to remedy the situation.

If a sewage system fails, obvious sign appear:

  • Toilets back up; drains won't drain.
  • Excessive moisture or waste water surfaces over the drainfield.
  • Foul odors come from the drainfield or septic tank.


See Maintaining Your Sewage System for tips on keeping your sewage system functioning properly!


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