In most of Ohio, the soil is too shallow to construct a septic system leach field. Many people try to construct a septic system leach field anyway. These systems fail within months, creating a public health and environmental hazard. Untreated sewage either comes to the surface in the yard or moves through saturated soil to pollute nearby wells and groundwater.
To keep sewage from surfacing in lawns, some systems are installed with shallow curtain drains placed about 26 inches beneath the soil surface about 8 feet from the leach field. These curtain drains carry pollutants away from the home, but discharge pollutants directly into streams or into ditches or agricultural drainage systems that drain into streams. Shallow curtain drains that intersect untreated wastewater are an important source of nonpoint source pollution in Ohio and should never be used.
Sand has been shown by researchers at Ohio State University to be a medium to aid in wastewater treatment. Organic matter (BOD5), total suspended solids, and ammonia is removed from septic tank effluent through a 2-foot layer of specially sized sand. The sand does not, however, do the entire job. Sand does a poor job of removing all of the disease-causing bacteria and viruses from wastewater. Sand does not have the same chemical or physical properties as natural soil and has limited capacity to attract bacteria and viruses to adhere (adsorb) onto particle surfaces. If sand is used to remove organic matter, suspended solids, and ammonia, a second treatment step is needed to remove the disease-causing organisms.
In 1978, Dr. Converse of the University of Wisconsin presented an onsite wastewater treatment system design that could be used in areas with shallow soil depths to a limiting condition. Known as a mound system, a layer of sand is placed on top of the natural soil to augment its treatment capacity as shown in Figure 5. The sand layer of up to 2 feet acts to reduce suspended solids, organic matter, and ammonia with continued removal, along with bacteria and virus removal in the underlying soil. Dr. Converse found that with sand augmentation, onsite wastewater treatment systems could be used in areas with more slowly permeable soils, with permeabilities as low as 0.5 inches per hour. Ohio State University Extension Bulletin 813 adapted the concept of a mound presented by Drs. Converse and Tyler for Ohio's soil conditions and regulatory requirements.