Onsite sewage systems treat wastewater from about 25 percent of the homes in Missouri. Wastewater is treated and recycled usually on the property where it is produced by onsite systems rather than by a central sewer collection and treatment system. At the time of a property sale, a homebuyer or lender may want to know about the condition of an onsite sewer system. If a system has an improper discharge or the absorption system fails, untreated or under-treated wastewater might be exposed on the ground surface. Surfacing wastewater or a contaminated water supply could be a health hazard, for the home’s occupants or neighbors, and if repairs are needed for a failed or inadequate system, they can be costly.

Two types of onsite systems are commonly used to treat residential wastewater in Missouri. They are lagoons and sewage tanks with soil absorption systems.

Lagoons are above ground pond-like structures that are used to treat sewage from a single-family residence. Any overflow from a lagoon must soak into the soil, staying on the property where the lagoon is located. Air, sunlight and long retention time aid in this method of wastewater treatment.

Soil absorption systems follow primary wastewater treatment such as a septic tank or aerated treatment unit (ATU). Final treatment and dispersal is through lateral trenches in the soil. In septic tank systems, most of the wastewater treatment is provided by the soil. Soil or space limitations sometimes make it necessary to use an ATU or a more advanced pretreatment system. These systems include components such as filters, pumps, electric controls, distribution valves and bio-filters. If an onsite system is properly sized, located, installed, maintained, and operated, it can adequately treat wastewater indefinitely.

It is difficult for the homeowner, buyer, or lender to know the condition of an existing onsite system, since most components are underground. For the most part, local public health agencies no longer inspect existing systems related to real estate transfer. The help of a licensed inspector/evaluator can be obtained to check the condition of the system. Individuals are licensed by the Missouri Department of Health and Senior Services to perform private assessments of existing onsite systems for real estate transactions. There are two distinct types of assessments performed by these licensed individuals: inspections and evaluations.