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Ohio State University Extension


Septic Tank: Soil Treatment System

Agriculture and Natural Resources
Karen Mancl, Professor, Food, Agricultural and Biological Engineering
Brian Slater, Associate Professor, Environment and Natural Resources

Homes not served by a public sewer must treat and disperse wastewater on the lot. Septic tanks with soil treatment systems (also known as leach fields) are a common onsite system. The septic tank removes most settable and floatable solids from the wastewater to protect the soil from clogging. The soil under the pipes buried in the yard filter out pollutants and pathogens to treat the wastewater before moving into groundwater or surface water.  

 Pie chart showing the percent of land area in feet from less than 1 foot to greater than 3 feet

Septic tank soil treatment systems require a suitable soil depth of at least 36 inches before hitting a limiting layer. Limiting layers are bedrock, sand, and gravel, dense and compacted layers, and water tables. About 16 percent of soils in Ohio are deeper than 36 inches to a limiting layer, as shown in Figure 1. Therefore, not every rural lot can use a soil treatment system. Mound, drip, or spray irrigation systems may need to be installed on lots with shallow soils.

Figure 2 shows a typical soil treatment system. Effluent from a septic tank discharges through perforated pipes in trenches buried in the yard. The wastewater soaks in along the bottom of the trench where the soil and soil microbes remove the pollutants and disease-causing organisms.  

Soil treatment systems are long and narrow, and must be constructed along the lot contour. For a three-bedroom home, a system can be as long as 138 feet, depending on soil and site conditions. For larger homes, up to 23 feet in length per bedroom is added to a system. Systems are also divided into two equal sections with a diversion device to alternate the wastewater flow. Depending on the number of trenches needed, the system may be 50 to 100 feet wide. A replacement area is also required to allow for construction of a new system if the system is damaged or the soil severely compacted after construction.

A soil treatment system is constructed in trenches about 18 inches deep (Figure 3). A layer of gravel or a chamber is placed on top of the natural soil at the trench bottom to create a space for wastewater to flow out of a perforated pipe. After a final covering with construction fabric, a layer of soil fill covers the trench to protect the pipes from freezing. The depth of the soil underneath the trench is called the vertical separation distance. To protect Ohio’s groundwater, most sites will require a vertical separation distance of at least 18 inches.

Diagram illustrating the leach field replacement including septic tank, distribution box, and leach field trenches.

Diagram of a soil treatment system with soil fill on top of landscape fabric and gravel surrounding a distribution pipe, vertical distance between chamber and limiting layer.

When building a soil treatment system the installer will be careful not to compact the soil. After construction, the lot will be carefully graded to divert any runoff water away from the soil treatment system.

For specific information on how to design and construct a system, see Ohio State University Extension Bulletin 939 Septic Tank: Soil Treatment Systems available for purchase online at Learn more at

As with all household sewage systems, the homeowner must maintain the system to ensure trouble-free operation. The homeowner should:

  • pump the septic tank every one to five years.
  • use water wisely and install water-saving devices in the home.
  • never compact the soil by paving, constructing a building, or parking cars.
  • avoid clogging pipes with roots by not planting trees or shrubs on the treatment system.
Originally posted Aug 13, 2019.