Soil Management: On-site wastewater treatment systems

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When central sewers are not available, households, businesses, and institutions with indoor plumbing must use an onsite wastewater treatment and dispersal system. For new home builders in rural areas in particular,  selecting the location of the onsite wastewater system is not often the first thing they think about. It is important to give that some consideration early in the planning process. The location, size of the home, and type of system can vary greatly in cost. The simplest onsite systems are septic tanks — laterals (Figure 1-left) and lagoon ponds (Figure 1-right). With proper design, good construction, and regular maintenance, onsite wastewater systems can effectively eliminate most health and environmental threats caused by pollutants in wastewater.

Figure 1. Rural home with a septic system: septic tank and soil dispersal field (left image) and a rural home with a lagoon pond system. Graphics from K-State Research and Extension.


Site Requirements

Underground septic systems with a drainfield for wastewater dispersal are common and preferred onsite sewage systems. It is essential to match the type of treatment and dispersal system to the soil properties in the drainfield. Fine-textured soils with high clay are often not well-suited for traditional soil dispersal. Instead, a lagoon may be viable when the code permits it, and there is adequate area. Because of soil and/or site limitations, a feasible onsite system may not be possible on all lots. For this reason, it is preferable to have an onsite soil evaluation done by a qualified person before obtaining a permit to install a wastewater system.

Each onsite system must be designed specifically for the flow and site conditions. What may be suitable for one location might not fit the needs at a different site. The area required by a typical system for a three-bedroom home could range from 2,000 square feet for a tank and lateral field to over 6,000 square feet for a lagoon. No part of an onsite system (septic tank, dispersal field, lagoon, or other component) should be placed on an easement or in an area subject to flooding. In order to avoid possible backup or failure, all underground septic system components must be correctly designed for the maximum flow of wastewater expected during a defined time period, usually three days. A surface system (lagoon) should not be subject to backup by high wastewater flow, so it is usually designed for average flow.

Septic Tank

A septic tank is a buried, watertight container made of durable material — such as concrete, plastic, or fiberglass — that is strong enough to withstand soil forces and resistant to corrosion or decay. The tank slows wastewater flow, allowing solids to settle or float to the top. The solids are stored in the tank where they can decompose. The clarified liquid protects the soil dispersal field from potential clogging and early failure. The septic tank wastewater, though mostly clear, is sewage and contains dissolved organics, microbes, and pathogens. The wastewater flows to the dispersal field where it is absorbed and treated as it percolates through the soil.

Inspection ports, access manholes, and risers from the tank to the surface serve three important purposes:

  1. Provide easy access to check the condition of the tees and sludge depth in the tank.
  2. Allow access to the tank for inspection, maintenance, and removal of solids.
  3. Allow quick identification of the tank’s location. Regular inspection and periodic pumping of the septic tank will ensure long-term, efficient functioning of the septic system.

Soil Dispersal Field

The dispersal field absorbs, filters, and treats the wastewater, making it suitable for re-entry into the groundwater. Dispersal laterals are typically 1.5 to 3 feet wide and the top is approximately a foot below the surface. In Kansas, at least 4 feet of suitable, aerated soil is required beneath the bottom of the lateral. This depth helps ensure adequate treatment (or purification) before wastewater reaches a limiting condition such as bedrock, impervious soil, groundwater, or seasonally saturated soil.

Septic permits usually require a depth of 6 feet of suitable soil above the limiting condition to allow the construction of dispersal laterals and ensure adequate treatment. Minimum standards require the dispersal field to be at least 50 feet from any surface water or well and at least 25 feet from the house and property lines. Greater separation distances from wells and surface water are recommended, and some county codes require these.

Septic system construction is limited by steep slopes, greater than 25 percent. Slopes less than 15 percent are preferred for easier construction of the dispersal field. The size of the dispersal field depends on the amount of wastewater flow, the site conditions, and the local code. The number and length of laterals are often determined by a county health or environmental department official.


In areas of Kansas with slowly permeable, high clay soils, a lagoon may be best suited for an onsite wastewater system. A lagoon treats the waste, and the wastewater evaporates or slowly seeps into the soil. Lagoons are simple in design, easy to construct, and, when correctly managed, have little or no odor. However, protective fencing is essential to avoid accidental drowning and disease transmission, especially for children, the elderly, and pets.

Not every site is suitable for a lagoon, however. Lagoons require a soil with a depth of at least 7 feet to bedrock and some homeowner associations may not allow them, despite the fact that they are a very effective solution, particularly for very clayey soils.

To determine the best option for the home site, contact the county or city health department. All health departments have environmental specialists who work with homeowners in evaluating the soil and make recommendations on system locations. When soil, site, or code requirements are not the most suitable for a basic system, an alternative system may be a better option. Alternative systems include components that give higher levels of pretreatment and usually involve different methods of soil dispersal.

More information about on-site wastewater systems is available in publications from K-State Research and Extension, including:

Onsite Wastewater Systems — Overview

Site and Soil Evaluation for Onsite Wastewater Systems

Selecting an Onsite Wastewater or Septic System

Septic Tank Maintenance – A key to longer system life

Why Do Onsite Wastewater (Septic) Systems Fail?


Stay tuned for next week’s eUpdate which will feature an article on tips to implement to avoid wastewater system failure.


DeAnn Presley, Soil Management Specialist

Tags:  wastewater septic systems