Choosing an On-Site Sewage Facility System

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Before an on-site sewage facility (OSSF), or septic system, can be constructed, installed, altered, extended, or repaired, the site must first be evaluated by licensed site evaluator or a licensed professional engineer. The table listed below can be used to determine which systems are suitable (S=Suitable, U=Unsuitable) for a property based on the results of the site evaluation.

On-Site Sewage Facility (OSSF)(9) Soil Texture or Fractured Rock(10)
(Most restrictive class along the media or 2 feet below excavation)		 Minimum depth to groundwater Minimum depth to restrictive horizon(1)
Disposal Method
Treatment		 Class Ia Class Ib, II(8), or III(8) Class IV Fractured Rock Measured from bottom of media(7) Measured from bottom of media(7)
Absorptive Drainfield(1)
Septic Tank		 U S U U 2 feet 2 feet(1)
Absorptive Drainfield(2)
Secondary Treatment		 S(5) S U S(5) 2 feet 2 feet(1)
Lined E-T(2)
Septic Tank		 S S S S N/A N/A
Lined E-T(2)
Secondary Treatment		 S S S S N/A N/A
Unlined E-T(2)
Septic Tank		 U S S U 2 feet 2 feet
Unlined E-T(2)
Secondary Treatment		 S(5) S U S(5) 2 feet 2 feet
Pumped Effluent Drainfield(3)
Septic Tank		 U S S U 2 feet 1 foot
Leaching Chamber(2)
Septic Tank		 U S U U 2 feet 2 feet(1)
Leaching Chamber(2)
Secondary Treatment		 S(5) S U S(5) 2 feet 2 feet(1)
Gravel-less Pipe(2)
Septic Tank		 U S U U 2 feet 2 feet(1)
Gravel-less Pipe(2)
Secondary Treatment		 S(5) S U S(5) 2 feet 2 feet(1)
Drip Irrigation
Septic Tank/Filter		 U S S U 2 feet 1 foot
Drip Irrigation
Secondary Treatment/Filter		 S(5) S S S(5) 1 foot 6 inches
Low Pressure Dosing
Septic Tank		 U S S U 2 feet 1 foot
Low Pressure Dosing
Secondary Treatment		 S(5) S S S(5) 2 feet 1 foot
Mound(4)
Septic Tank		 S S S S 2 feet 1.5 feet
Mound(4)
Secondary Treatment		 S S S S 2 feet 1.5 feet
Surface Application
Secondary Treatment		 S S S S N/A(6) N/A(6)
Surface Application
Non-standard Treatment		 S S S S N/A(6) N/A(6)
Soil Substitution(2)
Septic Tank		 S S U S 2 feet 2 feet
Soil Substitution(2)
Secondary Treatment		 S S U S 2 feet 2 feet
(1)An absorptive drainfield may be used, if a rock horizon is at least 6 inches above the bottom of the excavation, see §285.33(b)(1).
(2)If the slope in the drainfield area is greater than 30% or is complex, the area is unsuitable for the disposal method.
(3)Can only be installed in an area where the slope is less than or equal to 2.0%.
(4)Can only be installed in an area where the slope is less than 10%.
(5)Requires disinfection before disposal. A form of pressure distribution shall be used for effluent disposal into fractured or fissured rock.
(6)Requires vegetation cover and disinfection.
(7)When no porous media exists, measure from the bottom of the excavation or pipe, whichever is less.
(8)May require gravel analysis for further suitability analysis (see §285.30(b)(1)(B))
(9)If OSSF is located within a Flood Hazard, see §285.31(c)(2) for special planning requirements.
(10)Includes fissured rock.

All OSSFs require surface drainage controls if slope is less than 2%.

Treatment and Disposal System Descriptions

Treatment Systems

Primary treatment options

  • Septic tanks:
    • can be used as a primary treatment method for an OSSF.
    • may be installed by either an Installer Class I or Installer Class II.
    • are to be constructed in accordance with §285.32(b)(1).

Return to system selection table.

Secondary treatment options

For more information regarding the testing and approval of a secondary treatment unit, see §285.32(c).

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Non-standard treatment options

  • Non-standard treatment:
    • is usually all forms of:
      • activated sludge processes,
      • rotating biological contractors,
      • trickling type filters,
      • submerged rock biological filters,
      • recirculating sand filters, or
      • sand filters not described in §285.32(b)(2).
  • must be designed by a professional engineer or professional sanitarian.
  • can only be installed by an Installer Class II.
  • requires a continuous maintenance contract.

For more information regarding nonstandard treatment processes, see §285.32(d).

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Disposal Processes

Standard disposal processes

  • Absorptive Drainfields are:
    • constructed with perforated pipe and gravel.
    • installed by an Installer Class I or Installer Class II.
    • sized and constructed in accordance with §285.33(b)(1).
  • Unlined Evapotranspiration (E-T) systems:
    • are constructed with either pipe and gravel, leaching chambers, or gravel-less pipe in either a trench or bed configuration and backfilled with sand.
    • cannot be used if the annual rainfall exceeds the annual average evaporation.
    • can be installed by an Installer Class I or Installer Class II.
    • are sized and constructed in accordance with §285.33(b)(2).
  • Lined Evapotranspiration (E-T) systems:
    • are constructed similar to an unlined E-T bed with the exception of an impervious liner being placed between the excavated surface and the constructed disposal system.
    • may not be used if the annual rainfall exceeds the annual average evaporation.
    • can only be installed by an Installer Class II.
    • are sized and constructed in accordance with §285.33(b)(2).
  • Pumped Effluent Drainfields (PEDs):
    • dispose of effluent by use of a pump into small diameter perforated pipe in gravel.
    • can be installed by an Installer Class I or Installer Class II.
    • are sized and constructed in accordance with §285.33(b)(3).

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Proprietary disposal system options

  • Leaching Chambers:
    • are installed similar to an absorptive drainfield, however, the effluent is deposited into bottomless chambers that are linked together with solid walled pipe and are allowed a reduction in the absorptive drainfield sizing formula.
    • may be installed by an Installer Class I or Installer Class II.
    • are sized and constructed in accordance with §285.33(c)(2).
    • can be used in the following systems as a substitute for pipe and gravel without any reduction in drainfield sizing:
      • E-T systems
      • Low-pressure dosed drainfields, or
      • Soil substitution drainfields.
  • Gravel-less Pipe:
    • are installed similar to an absorptive drainfield, except the effluent in deposited into corrugated pipe with either an eight-inch or ten-inch diameter that is wrapped with geo-textile fabric.
    • may also be used in E-T systems as a substitute for pipe and gravel.
    • may be installed by an Installer Class I or Installer Class II.
    • are sized and constructed in accordance with §285.33(c)(1).
  • Drip Irrigation:
    • consists of small diameter pressurized lines containing pressure reducing emitters spaced at a maximum of 30 inches.
    • requires a design to be performed by either a professional engineer or professional sanitarian.
    • can only be installed by an Installer Class II.
    • requires an on-going http://www.tceq.state.tx.us/compliance/compliance_support/regulatory/ossf/ossfmaintenance.html maintenance contract.
    • are sized and constructed in accordance with §285.33(c)(3).

Return to system selection table.

Non-standard disposal options

  • Low-pressure dosed drainfields (LPDs):
    • dispose of effluent that is pumped, under low pressure, into a solid wall force main and then into perforated distribution pipe in gravel.
    • require a professional design by a professional engineer or professional sanitarian.
    • can only be installed by an Installer Class II.
    • are sized and constructed in accordance with §285.33(d)(1).
  • Surface application disposal:
    • is a method that disposes of secondary treated effluent onto the surface of the ground after http://www.tceq.state.tx.us/compliance/compliance_support/regulatory/ossf/ossfmaintenance.html disinfection.
    • cannot apply effluent to unseeded bare ground or areas used for growing food, gardens, orchards or crops which may be used for human consumption.
    • must be designed by a professional engineer or professional sanitarian.
    • can only be installed by an Installer Class II.
    • requires an on-going http://www.tceq.state.tx.us/compliance/compliance_support/regulatory/ossf/ossfmaintenance.html maintenance contract.
    • is sized and constructed in accordance with §285.33(d)(2).
  • Mounds:
    • are pressurized absorptive drainfields constructed above the native soil surface.
    • must be designed by a professional engineer or professional sanitarian.
    • can only be installed by an Installer Class II.
    • are sized and constructed in accordance with §285.33(d)(3).
  • Soil substitution drainfields:
    • are constructed similar to an absorptive drainfield with the exception that a 24 inch thick Class Ib, II, or III soil buffer is placed below and all sides of the drainfield excavation to an elevation equal to the top of the porous media.
    • must be designed by a professional engineer or professional sanitarian.
    • can be installed by an Installer Class I or Installer Class II.
    • are sized and constructed in accordance with §285.33(d)(4).

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RELATED LINKS:

On-Site Sewage Facility Permitting

Find an OSSF Approved Product
Last Modified 10/17/08
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