Using Soil to Remove Pollutants From Wastewater

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

Almost one million homes in Ohio are beyond the reach of community sewage treatment systems. Communities that provide sewage services for homes must treat the wastewater to remove pollutants before discharging the effluent into Ohio’s environment. Unfortunately, for decades, households established in rural areas were not held to a similar standard and were often able to dispose of wastewater without providing treatment. As a result, Ohio has some of the most polluted swimming beaches in the nation, and rural neighbors have been sickened with waterborne diseases from contaminated wells.

Soil is the best medium to treat and disperse wastewater to protect the health of families, neighbors and visitors as well as the environment. Ohio has wonderfully diverse soil resources. Most of Ohio’s soils are best suited for growing food and supporting Ohio’s largest industry: agriculture. 

The soil’s ability to purify wastewater has been recognized for decades. The goals in any sewage treatment system are to 1) remove pollutants that cause disease before they contaminate drinking and recreational waters, and 2) remove pollutants that kill fish and aquatic life before they reach a lake or stream. Naturally occurring soils have varying capacity to accomplish pollutant removal. Three properties are recognized as most important in ensuring effective wastewater treatment:

  • Soil must be unsaturated to promote aerobic conditions.
  • Soil must be deep enough to remove all of the pollutants.
  • Soil must be permeable to air and water.

Removing Pollutants From Wastewater

As illustrated in Figure 1, three natural processes remove pollutants from wastewater as the wastewater moves through unsaturated soil:

  • Filtering is a physical treatment process. As water moves through the small soil pores, wastewater particles are removed, thus eliminating cloudiness. After passing through about 1 foot of soil, the wastewater is clear.
  • Naturally occurring microbes attach and grow on the surface of soil particles and consume the wastewater organic matter and nutrients as their food. Organic matter creates bad odors as it decomposes. Ammonia in wastewater is extremely toxic to fish. After filtering through about 2 feet of soil, ammonia is transformed to nitrate and the organic matter is consumed so the wastewater has no odor.
  • Adsorption of pollutants occurs due to the chemical nature of soils. Phosphorus is removed from wastewater as it attaches onto soil particles or forms insoluble compounds in the soil. Bacteria and viruses that cause disease are removed in the same way. They are attracted to and adhere onto soil particles in unsaturated soils. Bacteria are removed after filtering through 1.5 feet to 2 feet of soil. However, viruses are smaller and more difficult to remove, thus taking 3 feet to 4 feet of unsaturated soil depth for removal.
Figure 1. Illustration showing the wastewater treatment processes that occur in unsaturated soil with depth.

Limiting Layers

Many soils do not provide adequate depth to remove pollutants and treat wastewater. Like Goldilocks, some soil layers are too permeable for good contact between soil and effluent, and some are not permeable enough to allow for the movement of air and water. Only 16 percent of Ohio’s land area is just right and has soils ideally suited to traditional septic system leach fields. Zones in the soil where wastewater cannot be treated are called limiting layers. Limiting layers are:

  • hard or dense layers with low permeability.
  • coarse sand, gravel layers and fractured rock with excessive permeability.
  • water tables and saturated soil conditions.

Hard and Dense Layers

Much of Ohio was covered with glaciers thousands of years ago. The ground-up rock left behind as the glaciers melted is called “glacial till.” Glacial till consists of unsorted sand, silt, clay and rock fragments, and it has variable composition and physical properties. Many tills below Ohio soils are dense (almost like concrete), have little porosity, and have a low capacity to move water or air. Dense tills are hard and lack structure, so roots are unable to penetrate. Little effluent treatment occurs in dense till due to low permeability and limited biological activity. Dense tills may impede movement of water away from the site and cause the formation of perched saturated conditions in the soil profile, especially with the addition of wastewater.

In eastern and southeastern Ohio, similarly dense materials such as hard bedrock lay under the soils. Some bedrock, such as shale, is present in horizontal plates or beds making it difficult for water to move down and away.

Excessive Permeability

Fractured rock materials below soils not affected by glaciation are found mostly in eastern and southeastern Ohio. Found throughout Ohio—especially near Lake Erie and along rivers and streams—are dissolved limestone and layers of sand and gravel. Once wastewater moves through soil to these porous materials, it moves too quickly to provide treatment and can pollute nearby wells. Well pollution on South Bass Island, for example, is caused by rapid movement of septic effluent through thin layers of soil over porous rock.

Shallow Water Tables and Saturated Soil Conditions

Saturated conditions result when all soil pores are filled with water. These conditions are present below the water table or in saturated zones in the soil profile. Many Ohio soils have a water table present at shallow depths during some seasons, especially during winter and spring. Where saturation occurs, microorganisms rapidly deplete oxygen, and the resulting anaerobic conditions impede biological, chemical and physical treatment of contaminants in wastewater. Pollutants can move quickly through saturated soil and may move considerable distances with the risk of contaminating ground or surface water. Bacteria can move 100 feet to 300 feet through saturated soil, thus polluting nearby wells and making it into ditches, streams and ponds.

How Deep Is Ohio’s Soil?

Ohio, like much of the central United States, has shallow soils to a limiting condition. Soils with greater than a 3-feet depth to a limiting condition are present in only 16 percent of Ohio’s land area (Figure 2). Very shallow and seasonally wet (hydric) soils are found in 31 percent of Ohio’s land area.

Careful evaluation is needed to characterize the soil and site condition on a lot. Matching a suitable wastewater treatment system to the site is important to protect the public health and the environment. To learn more about site and soil evaluation, check Ohio State University Extension fact sheet AEX-724, Soil Evaluation for Home Septic Systems.

Figure 2. Percent of the land area of Ohio with various depths to a limiting condition.