Onsite Wastewater Treatment Systems

In Missouri, where central sewer service is not available, homeowners and small business owners use various types of onsite wastewater treatment systems (OWTS). An OWTS is a system that uses natural processes and mechanical components to collect, treat and disperse wastewater from single dwellings or other buildings or structures. Onsite systems can effectively treat domestic wastewater and disperse it into the environment and protect public health. However, it is important that an appropriate type of system is selected, it is properly installed, and the owner insures it is operated and maintained, as it should be. Several factors should be considered when choosing the type of onsite system for a site including: soil/site limitations, available space, operation and maintenance (O & M) requirements, initial costs as well as O & M costs, landscape disturbance, and the owners' preferences and ability to manage the system. Of these considerations, often the most limiting is the soil resource or site and space limitations. Below are two lists of OWTS, basic and advanced. When the soil and site are suited to a lagoon or to a septic tank and conventional soil absorption system, any registered OWTS installer can assist with the permitting and can install a basic onsite system. When site limitations or other factors lead to an advanced OWTS, the installer must be registered as an advanced OWTS installer. Basic and advanced registered installers are listed on this website.

Basic Onsite Wastewater Treatment Systems and components.

Building sewers and other sewer lines: watertight pipes, which carry waste by gravity from a building to the onsite system or carry effluent by gravity from sewage tanks to other system components.

Septic tanks: a watertight, covered container designed and constructed to receive the discharge of sewage from a building sewer. Its function is to separate solids from liquid, digest organic matter, store liquids through a period of detention and allow the clarified liquids to discharge to other components of an onsite system. Solids are stored and periodically need to be pumped out and hauled to a point for further treatment.

Aerobic Treatment Units (ATUs): a mechanical onsite treatment unit that provides secondary wastewater treatment by mixing air (oxygen) and aerobic and facultative microbes with the wastewater in a sewage tank. In Missouri, the minimum construction standards require that ATUs comply with NSF Standard 40.

Gravity effluent distribution devices: divide and/or transport the liquid effluent from a septic tank or ATU to absorption trenches for dispersal into the soil. These devices include distribution boxes, drop boxes, and step-downs.

Gravity laterals: a system of trenches excavated along ground contours used to distribute effluent by gravity flow from a septic tank or ATU and apply the effluent to the soil infiltrative surface. Generally, 18-inch deep trenches are used; however, with approval trenches can be up to 30 inches deep. Gravity lateral systems include:

• 4-inch perforated distribution pipe in trenches filled with gravel or tire chips;
• chamber systems (an open bottom structure, which forms an underground effluent storage cavity over the soil's infiltrative surface);
• large diameter gravelless pipe (a filter wrapped corrugated plastic pipe); and
• 12-inch expanded polystyrene (EPS) bundles (a 4-inch corrugated plastic distribution pipe enclosed in a bundle of EPS)

Shallow placed gravity laterals: Lateral trenches with the trench bottom 12 to 18 inches deep in natural soil with suitable soil fill material properly installed to provide adequate cover over the system.

Dosed gravity systems: use siphons or pumps to dose into a gravity distribution device or through a pressure manifold into the ends of gravity lateral trenches. Pressure manifolds can be used to more equally divide effluent between gravity lateral trenches or to proportion effluent to unequal length trenches; however, effluent is still moved along the length of a trench by gravity.

Lagoons (wastewater stabilization ponds): sealed earthen basins, which use natural unaided biological processes to treat wastewater.

Advanced Onsite Wastewater Treatment Systems and components include:

Sand filters: a packed-bed filter of sand or other granular materials used to provide advanced secondary treatment of septic tank effluent. Sand/media filters consist of a lined (e.g., impervious PVC liner on sand bedding) excavation or structure filled with uniform washed sand that is placed over an under-drain system. The wastewater is dosed onto the surface of the sand through a distribution network and allowed to percolate through the sand to the under-drain system, which collects the filter effluent for further processing or discharge.

Other media bio-filters: packed-bed filters using other more porous materials, (e.g., peat, textile, or foam) to provide advanced secondary treatment of septic tank effluent.

Constructed wetlands: an OWTS that incorporates an aquatic treatment system consisting of one or more lined basins which may be filled with a medium and where wastewater undergoes some combination of physical, chemical, and/or biological treatment and evapotranspiration.

Sand mounds: an above ground treatment system that incorporates at least 12 inches of clean sand above the original soil surface and disperses the treated wastewater into the original soil.

Low-pressure distribution systems: an OWTS in which pressurized small diameter distribution lines are used for equal distribution of effluent within the final treatment and dispersal component. These systems include low-pressure pipe (LPP) distribution systems, as described in the Missouri Minimum Construction Standards, and other systems such as an otherwise conventional system with a pressurized distribution network.

Drip irrigation systems: a subsurface soil dispersal system that distributes treated wastewater through drip irrigations lines.

Modified shallow placed gravity lateral trenches: (six to 12 inches deep in natural soil) and other engineered distribution systems using fill soil material.

Other advanced engineered treatment or distribution systems not specifically mentioned.