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Septic Systems Permissible in Oklahoma

Permissible Septic Systems in Oklahoma - Conventional System, Low-Pressure Dosing System, ET/A System, Aerobic Treatment Units, Lagoons.

I. Conventional System (Subsurface Sewage Treatment System)


  • The most-common and least expensive type of on-site wastewater treatment system
  • May be installed in approximately 60% of the soils in Oklahoma (DEQ).
  • Heavily relies on the ability of the soil to treat nutrient and microbial contaminants. Of utmost importance is the proper characterization of the soil profile. Specifics of the system design are highly dependent on the properties of the soil (primarily texture and structure).
  • Composed of two components:

1. The Septic Tank

  • Where primary treatment occurs - separation of solids (via sedimentation), separation of lighter grease and scum (floats), bacteria starts to degrade solid wastes
  • Solids must be periodically pumped-out to prevent clogging.

2. Subsurface Absorption Fields/Soil Treatment Areas (STAs)

  • The absorption field is composed of lateral lines (usually PVC pipes) that distribute the wastewater that was temporarily stored in the septic tank to a treatment field.
  • These lines buried in trenches.
  • Lines in the trenches may :
      • be surrounded by storage media that could either be gravel or tire chips and covered by a native soil as the back-fill material
      • be under a chamber and not surrounded by any storage media

  • Treatment of the wastewater is done by the soil underneath the media or the chamber. Success of the treatment process would depend on the soil's ability to sorb dissolved (nutrients and household chemicals) and microbial contaminants (bacteria and viruses) as well as the soil's ability to temporarily store water allowing for treatment before it recharges the underlying groundwater.

II. Low-Pressure Dosing System

  • Low-pressure Dosing Systems are essentially the same as Conventional Systems except that a pressure is used to distribute wastewater to the lateral lines in the soil treatment areas.
  • The use of pressure improves (compared to the gravity-driven conventional systems) the distribution of the wastewater throughout the soil treatment area. Because of this improvement in distribution, thinner vertical separation is required in Low-pressure Dosing Systems compared to those required in conventional systems.


III. Evapotranspiration/Absorption (ET/A) System

  • This system is designed for areas with high clay content (DEQ Soil Group 5 - Clay, silty clay, sandy clay with slickensides or weak structure).
  • There is a greater chance of being effective if installed in dry areas and in locations where evapotranspiration exceeds rainfall.
  • It still needs a septic tank and a site designated as a treatment area. The main difference (from conventional and from low-pressure system) is in the design of the trenches.
  • The system relies on having the wastewater removed from the area mainly via evapotranspiration --thus relying on the plant growing on the trench to facilitate the loss of water from the soil  into the atmosphere.
  • This system is only permissible in a site with an area of at least 1 acre.
  • Specifics of the design is also dependent on topography, location in Oklahoma (the net evaporation zone of an area), estimated water usage, water well locations (your and your neighbor’s), location of surface water bodies such as creeks, rivers, springs, ponds and lakes

  • The sand above the storage media in the trenches wicks-up the wastewater allowing it to be lost via evaporation or absorbed by plant roots and eventually lost to the atmosphere via transpiration.


IV. Aerobic Treatment Units

  • The Aerobic Treatment Units (ATU) has been the most popular household system in Oklahoma in the last few years. This is because, a large portion of new developments are taking place in very shallow clayey soils which can only be permitted for septic systems if ATUs are used.
  • Apart from having a septic tank (or trash tank) and a soil dispersal area, ATUs have "Aeration Tanks". Air is forced into the tank to change the aeration status of the  wastewater.
  • The following are the key processes in the aeration tank
    • The high oxygen content  is detrimental to the anaerobic microorganisms that thrived in the wastewater in the trash tank. This aerobic conditions could either kill the anaerobes or weaken them enough to be out-competed by the aerobic bacteria in the aeration tank.
    • The presence of oxygen facilitates aerobic decomposition of organic matter/compounds in the wastewater.
  • In most systems, there is a disinfection chamber after the aeration chamber. Chlorine is the widely used disinfectant.
  • Treated wastewater is disposed to a field by either Spray Irrigation or Drip Irrigation. Drip irrigation systems are more expensive by roughly $2,500 than spray irrigation systems.
  • In Oklahoma, there are no vertical separation requirement for Spray Irrigation ATU Systems.
  • Since drip irrigation is a soil-based system, the effluent is not required to be disinfected prior to dispersal.
  • Only disinfected effluent may be used for spray irrigation fields. The disinfected effluent is pumped out of the pump tank to sprinklers in the spray irrigation field. The sprinkler system is timed to automatically to spray the effluent in the early morning hours (1AM -6AM) over a large vegetated area (DEQ).

V. Lagoons

  • The Lagoon System uses an open pond as the storage area and relies on evaporation for the eventual disposal of the wastewater.
  • This system is a good alternative in poor soils located in areas where net evaporation exceeds rainfall amount.
  • It still has a septic tank for primary treatment.
  • Lagoon size increases from west to east across the state of Oklahoma. Lagoons are a much more effective treatment method in central and western Oklahoma. Lagoon sizes in the southeastern counties of Oklahoma can be quite large due to the high rainfall and low evaporation rates (DEQ).
  • In Oklahoma, lagoons are permitted to be constructed on any type of soil as long as the lot size is at least 2 ½ acres.
  • Specifics of the design is also influenced by the Oklahoma net evaporation zone restrictions.