Water Protection Program fact sheet


Division of Environmental Quality Director: Ed Galbraith

With the implementation of more protective discharge limits on domestic wastewater, several Missouri communities have proposed subsurface soil dispersal systems to eliminate direct discharge and the National Pollutant Discharge Elimination System (NPDES) permit. The purpose of this fact sheet is to provide a general discussion for community leaders and planners regarding subsurface soil dispersal systems in Missouri. This fact sheet is not intended to address specific design criteria nor is it meant to answer all situation-specific questions.

Subsurface soil dispersal systems can overcome a number of soil and site limitations and should be included among alternatives evaluated when planning a new wastewater system or upgrading an existing system. Several different approaches have proven successful in the design, operation and maintenance of large-scale subsurface soil dispersal systems. The references listed below represent some of the current practices. Regardless of the system chosen, the design engineer is responsible for properly assessing and documenting that the selected system will work in a specific location, under the specific soil and site conditions present, while ensuring the entire system is sustainable and protective of the environment and public health.

Although some consider a large-scale subsurface dispersal system nothing more than a “large septic system,” this is a misconception. For the purpose of this fact sheet, the term “septic” refers to a component of a system, such as a septic tank or anaerobic treatment.

The subsurface soil dispersal systems described below are for domestic wastewater (sewage) only, defined in RSMo 701.025(12) as “Human excreta and wastewater, including bath and toilet waste, residential laundry waste, residential kitchen waste and other similar waste from household or establishment appurtenances.”

What is Subsurface Soil Dispersal?
Subsurface soil dispersal is the method of distributing effluent uniformly into an unsaturated (vadose) zone within the soil allowing for the effective treatment of bacteria and nutrients along with the local reuse of the treated water.

Types of Subsurface Soil Dispersal Systems

Low Pressure Distribution Systems
A low-pressure distribution system is a shallow, low pressure-dosed subsurface soil dispersal system with a network of small-diameter perforated pipes placed 10 to 12 inches deep in narrow trenches commonly 12 to 18 inches wide, spaced 5 feet apart. The system also includes a dosing tank that holds the effluent and one or more pumps that deliver it to the lateral lines within the soil under relatively low pressure. These systems were developed to overcome shallow water tables and reduce anaerobic conditions (lack of oxygen) due to continuous soil saturation by undertreated effluent. This is usually done with timed dosing and alternating fields allowing effective use of the soil treatment area. The equal distribution of the effluent within a low-pressure distribution system allows for the sustainable use of the soil treatment area by enhancing the aerobic conditions and reducing the potential for the development of a restrictive biomat layer.

If there is sufficient soil treatment area available, a low-pressure distribution system can be scaled to accommodate flows in excess of 3,000 gallons per day (gpd). It can also accept wastewater that has only gone through anaerobic treatment with the separation of liquid and solids from a lagoon or septic tank. To enhance the life of the system, secondary (aerobic) treatment is strongly recommended for any system even if it is just serving a single-family home.

Limitations with low-pressure distribution systems include soils with shallow water tables or limiting layers, steep slopes, and limited available area. The potential for clogging the lateral line holes with solids or roots is also a shortcoming, along with limited storage capacity around the laterals. While a low-pressure distribution system can function in a variety of climate conditions, protection from freezing temperatures to protect certain components must be provided.

Drip Distribution Systems
Drip distribution is a method of dispersing effluent from a domestic wastewater treatment facility into the subsurface soil using polyethylene tubing with an approximate diameter of one-half inch with emitters manufactured into it every 24 inches. Depending upon the soils, landscape position and other site conditions, the tubing is usually installed between 6 and 12 inches below the surface on 24-inch centers. The drip system also includes a dosing tank where the effluent is held until one or more pumps deliver it under pressure to the drip tubing installed in the soil. This is usually done with timed dosing and alternating fields, allowing effective use of smaller soil treatment area.

Like a low-pressure distribution system, a drip system can receive anaerobic treated effluent, although that practice is discouraged for large-scale systems. All effluent, regardless of treatment, should pass through 100- to 120-micron filters prior to being dispersed in the drip tubing. Thus, many experts believe treating effluent with secondary (aerobic) treatment before it enters the drip system will not only increase the life of the system, but also allow a greater application rate potentially reducing the soil treatment area.

Drip distribution systems can be scaled to accommodate large flows in excess of 3,000 gpd in a variety of soil and site conditions, and with the tubing spaced on 2-foot centers, the overall size of the soil treatment area can be even further reduced. The flexibility in geometry, design, construction and the ability to distribute effluent uniformly allows the drip tubing to be installed in wooded areas; on steep slopes; in soils with high water tables or a limiting layer; and in areas where the depth to bedrock is limiting. Drip systems have the potential to be used in higher risk areas near sensitive water bodies that were previously unsuitable for wastewater treatment systems.

While drip systems can overcome many limitations, the components and soil treatment area must be protected from livestock and other heavy use. A sustainable management plan should be devised and followed to ensure that the system is sustainable. Also, while a drip system can function in a variety of climate conditions, protection from freezing temperatures for some of the components should be provided.

Certain residential developments require Department approval for the method of wastewater disposal to install onsite wastewater treatment. These developments must receive Department approval if the proposal has seven or more lots where each of those lots are less than five acres in size and use individual single-family on-site wastewater systems. The Department must also approve any expansion of an existing subdivision by three or more lots, where each lot is less than five acres in size. Refer to the On-Site Waste Disposal in a Subdivision Fact Sheet - PUB 2226 and 10 CSR 20-6.030 Disposal of Wastewater in Residential Housing Developments for more information.

Operation and Maintenance
The primary challenge with any wastewater treatment system is that it is not always managed by individuals trained for a specific technology. Long-term operation and maintenance by qualified persons is imperative to ensure the system functions optimally for its expected lifespan. Operation and maintenance must be addressed as part of the planning process and not an afterthought. Refer to Appendix 2: Operation and Maintenance Manual of the Missouri Wastewater Design Guides for additional guidance.

Notification to the Missouri Geological Survey
When applying for the State Operating Permit for a subsurface dispersal system, the consulting engineer must notify the Missouri Geological Survey. The Missouri Geological Survey manages the Underground Injection Control Program, an inventory of underground injection wells located within Missouri.

Subsurface soil dispersal systems are considered Class V underground injection control wells when they either receive domestic wastewater from two or more family residences or from a nonresidential establishment (schools, offices, shopping malls, etc.) when the system has a design capacity to serve 20 or more people per day. The purpose of the inventory is to maintain an accurate list of Class V underground injection control wells permitted in Missouri, there are no other requirements.

Subsurface soil dispersal systems can be a practical and cost effective wastewater solution in areas with soil and site limitations or in areas that are unevenly populated. To help answer more of your questions, the Department recommends reviewing the reference material below and reading Wastewater Basics for Small Community Leaders and Planners, also developed by WERF.

Reference Material
The following reference material is intended to provide a sample of what is available concerning the suitability, design, installation and the operation and maintenance of subsurface soil dispersal systems. Several of these resources were used in preparing this document.

Users should not rely solely on this document when making treatment technology decisions.It is important to consult closely with an experienced professional engineer in selecting a treatment technology. These costs are taken from the WERF Fact Sheets D2 and D3 for Decentralized Wastewater Systems, Performance and Cost of Decentralized Unit Processes, Dispersal Series. A copy can be found using the link listed within the Reference Material Section below. The actual costs can vary significantly depending upon local economic factors. WERF included 20 percent for overhead and profit for the contractor. Costs given within the WERF Fact Sheets as listed below reflect 2009 dollars. *Soil treatment area and the linear feet of lateral line was calculated as described in the previous section using standards set forth by the Missouri Clean Water Law and its regulations along with those set forth by RSMo 701.025 through 701.059 and the regulations promulgated under them.