The structure, chemistry, and the biological activity in some soils make them ideal to treat wastewater to protect ground and surface water. The suspended solids, organic matter (measured as BOD5), ammonia, bacteria, and viruses in wastewater can all be removed by an adequate layer of soil. Soil pores must be fine enough to trap suspended solids and disease-causing organisms. These same soils, however, must still have sufficient permeability to allow for the movement of air and water to accommodate the biological degradation of organic matter and ammonia by aerobic microorganisms that colonize the soil matrix. Finally, the soil must have the capability to adsorb viruses and other water pollutants, like phosphorus.
The suspended solids in wastewater give it a cloudy appearance. These small particles are easily filtered out through a layer of soil. After wastewater has flowed down through as little as 1 foot of unsaturated soil, it will be very clear and nearly free of suspended solids. While the wastewater at this point will look clean, it still contains dissolved pollutants and disease-causing organisms. This clear wastewater can still pollute ground or surface water. Therefore, a 1-foot-deep layer of soil is not enough to fully treat wastewater.
The organic matter dissolved in wastewater is an important source of food for microorganisms. That is why it is considered a water pollutant. The microorganisms in a stream or pond live and grow by consuming dissolved organic matter, utilizing the oxygen that fish and other aquatic organisms need, consequently stressing and even killing fish. Partially treated wastewater surfacing in a yard or discharged to a ditch may look clear, but when the little available oxygen is quickly consumed by microorganisms, the water soon becomes stagnant (or anaerobic) and will create strong odors and attract flies.
Fortunately, organic matter is a valuable soil constituent, not a soil pollutant. Soil structure, nutrient cycling, and biological activity benefit greatly from the addition of organic matter to soil. Naturally occurring microorganisms in the wastewater and the soil quickly colonize the surfaces of the soil particles when wastewater is applied through a septic system leach field. To remove nearly all of the organic matter, the wastewater must flow down through about 2 feet of unsaturated soil.
Ammonia is a very damaging water pollutant. Only small amounts of ammonia will kill fish and other aquatic organisms in a matter of minutes. Partially treated wastewater discharged to a ditch or field drainage tile that flows into a stream is very damaging to the environment.
Fortunately, in soil, ammonia can be rapidly converted to nitrate, a soil nutrient. Nitrogen is a necessary nutrient for the growth of plants. In fact, people frequently put ammonia fertilizer on their lawn or garden to increase plant growth. Ammonia is chemically attracted to soil particles and is converted to nitrate in the soil. Plants use the nitrate in the soil for growth. Naturally occurring microorganisms in the wastewater and soil that quickly colonize the surfaces of the soil particles also utilize ammonia as an important nutrient. Ammonia is mostly removed after the wastewater has flowed through about 2 feet of unsaturated soil.
Bacteria are present everywhere in the environment and most are helpful, not harmful. Unfortunately, human waste can contain disease-causing organisms that cause everything from diarrhea, chronic disease, and even death. That is why public health professionals are so careful to keep human waste from polluting drinking water, food, or even bodies of water where people of all ages may play, wade, swim, or fish. It is easy to forget that even partially treated sewage, while clear, discharged to a ditch in front of a home can threaten children that play, or touch pets that play, in the nearby yard.
Soil, however, is an excellent medium to remove bacteria from wastewater. Bacteria are both physically filtered out in the soil beneath a leach field or are chemically attracted to the surface of the soil particles (adsorbed). Once trapped in the soil, they either die in this now hostile environment or are preyed upon by other naturally occurring soil microorganisms.
Researchers have found that fecal coliform bacteria were removed through an 18-inch deep column of unsaturated fine loamy soil. Bacteria were removed after moving downward through 24 inches of fine sandy loam. Therefore, by the treatment depth that the organic matter and suspended solids are removed, the bacteria are also removed.
Viruses in wastewater are different than bacteria and are removed by soil in a different way. Viruses are smaller than bacteria and actually consist of nucleic acid and protein molecules. Because they are so small they are rarely physically filtered out in the soil. They must be removed mostly by being chemically attracted (adsorbed) onto the surface of the soil particles. Another complication of virus removal is that organic matter in the wastewater interferes with the adsorption of viruses. Virus removal does not effectively occur until the organic matter has been removed in the first 2 feet of depth in the soil column. Researchers have found that viruses from sewage effluents were removed in soil columns ranging from 0.5 to 2 feet deep after the organic matter is removed.
Up to four feet of unsaturated soil is required underneath a soil absorption system and above a limiting condition to protect public health and the environment (Figure 1). Limiting conditions are considered to be soil or geologic layers that are either insufficiently or excessively permeable. In Ohio, limiting conditions include ground or perched water tables, hard, unfractured bedrock, dense glacial till, compacted zones, dense clays, pans such as fragipans, sand and gravel layers, fractured rock, and soil layers with large amounts of coarse fragments (stones).
Soil saturated with water does not remove pollutants from wastewater. Pollutants move with the water through the saturated soil only to travel into wells, streams, and ditches. Researchers have found viruses in wells 200 to 400 feet from the point where the viruses were introduced into saturated soils. Ammonia is not effectively utilized by plants and microorganisms in saturated soil. Ammonia moves with the water through the saturated soil, polluting streams and ponds.