Absorption Fields

System Selection

Onsite Wastewater Treatment System (OWTS) types are determined based on a variety of factors:

  • Soil permeability
  • Karst topography
  • Soil conditions
  • Subsurface water levels
  • Restrictive layers
  • Bedrock

Typical Selection Criteria

Condition Likely System Choice
Deep/permeable soil Conventional Systems
Low-Pressure Pipe (LPP) Systems
Deteriorating soil conditions Aerobic Treatment Units (ATUs)
Shallow soils
Restrictive layers
Karst topography
Subsurface Drip Distribution (SDD)

Selection Based on Depth to Limiting Condition

System Type

Depth to Limiting Condition (in inches)*

>66

66-60

60-42

42-36

35-24

24-18

<18

Aerobic Treatment Unit (ATU)/Conventional Field

S

S

S

P

P

N

N

ATU/Conventional Field
w/ Shallow Placement

S

S

S

S

P

N

N

ATU/Low-Pressure Pipe (LPP)

S

S

S

S

S

P

N

ATU/Subsurface Drip Distribution (SDD)

S

S

S

S

S

S

P

ATU/Engineered Shallow Treatment System

S

S

S

S

S

S

S

S = Suitable for a system; P = Potentially suitable for a system; N = Not suitable

*Examples of limiting conditions include bedrock, seasonal water table, and restrictive layers.

Selection Based on Depth to Limiting Condition in Areas with Potential for Groundwater Contamination

System Type

Depth to Limiting Condition (in inches)*

>66

66-60

60-42

42-36

35-24

24-18

<18

Aerobic Treatment Unit (ATU)/Conventional Field

S

P

P

P

N

N

N

ATU/Conventional Field
w/ Shallow Placement

S

S

P

P

P

N

N

ATU/Low-Pressure Pipe (LPP)

S

S

S

S

P

P

N

ATU/Subsurface Drip Distribution (SDD)

S

S

S

S

S

S

P

ATU/Engineered Shallow Treatment System

S

S

S

S

S

S

S

S = Suitable for a system; P = Potentially suitable for a system; N = Not suitable

*Examples of limiting conditions include bedrock, water table, and restrictive layers.

Selection Based on Slope Limitations

System Type

Slope

>15%

15-30%

>30%

Conventional Field

S

*N

Low-Pressure Pipe (LPP)

S

*

N

Subsurface Drip Distribution (SDD)

S

S

*

S = Suitable for a system; N = Not suitable

*Deeper soils and more space between adjacent lines may be required to install the system to properly function.

Selection Based on Soil Texture

System Type

Soil Texture

Group I

Group II / III

Group IVA

Group IVB

Group V

Conventional Field

S

S

*

N

S

Low-Pressure Pipe (LPP)

S

S

S

N

S

Subsurface Drip Distribution (SDD)

S

S

S

S

**

S = Suitable for a system; P = Potentially suitable for a system; N = Not suitable

*In areas with high clay content, it is recommended another system be considered.
**Drip lines can be damaged in rocky soil.

Conventional Absorption Field Installation

Conventional absorption fields shall be designed based on the soil morphology and sewage flows.

The following conventional systems are often used and do not require a design professional.

Conventional System Design Criteria

Product Minimum Trench Width (in inches) Equivalent Treatment (in feet) Minimum Vertical Separation (in feet)
18-inch Pipe and Gravel 18 1.5 2
24-inch Pipe and Gravel 24 2 2
12-inch Polystyrene Wrapped Gravelless Pipe 18 2 2
15-inch Chamber 18 2 2
10-inch Geotextile Wrapped Gravelless Pipe 18 2.1 2
22-inch Chamber 24 3 2
36-inch Wide Pipe and Gravel 36* 3 2
(2) 12-inch Polystyrene Wrapped Gravelless Pipe 36* 3 2
34-inch Chamber 36* 3 2

*36-inch wide trenches are not allowed in locations with a loading rate less than 0.45.

Alternative Absorption Field Installation

Alternative absorption fields shall be designed based on the soil morphology and sewage flows by a professional engineer licensed in the State of Missouri. All construction plans shall include the seal, signature, and date of the licensed engineer.

Vertical Separation

System

Consideration

Minimum Vertical Separation (in feet)

Low-Pressure Pipe (LPP)

Typical

1

Groundwater Concern

4

Subsurface Drip Distribution (SDD)

Typical

1

Sand Mound

Typical

2

Threat of groundwater contamination

4

Subsurface Drip Distribution (SDD) Systems

Subsurface Drip Distribution (SDD) systems are alternative systems and require design by an engineer licensed in the State of Missouri.

System Design

All SDD systems must be designed by a professional engineer licensed in the State of Missouri and construction documents must be sealed, signed, and dated.

Line and Emitter Placement

Drip lines shall be placed two feet apart in a parallel layout, and emitters shall be located every two feet. Alternative separations and emitter placements are allowed, but calculations are based on a 2x2 configuration treating four square feet.

Group IVb Soils

Drips are permitted for Group IVb soil textures.

Application Rates for SDD Systems

Soil Group

Soil Texture

Classes

Application Rate 
(gpd/sq. ft.)

I

Sand, Loamy Sand

No Structure (Brown Colors)

0.50 – 0.40

II

Sandy Loam, Loam

Granular; Fine and Medium Subangular Blocky

0.40 – 0.35

Sandy Loam, Loam

Prismatic; Course Subangular and Angular Blocky

0.30 – 0.20

III

Silty Loam, Clay Loam, Sandy Clay Loam, Silty Clay Loam

Granular; Fine and Medium Subangular Blocky

0.30 – 0.20

Silty Loam, Clay Loam, Sandy Clay Loam, Silty Clay Loam

Prismatic; Course Subangular and Angular Blocky

0.20 – 0.15

IVa

Sandy Clay, Silty Clay, Clay (Low to Moderate Shrink/Swell)

Granular; Fine and Medium Subangular Blocky

0.20 – 0.10

Sandy Clay, Clay, Silty Clay

Prismatic; Course Subangular and Angular Blocky

0.10 – 0.05

IVb

Clay, Sandy Clay, Silty Clay Loam, Clay Loam, Silty Clay (High Shrink/Swell Potential)

Subangular, Angular Blocky, or Prismatic

0.05 – 0.10

V

Skeletal (Less than 50%), Silty Loam, Silty Clay Loam, Clay, Silty Clay

Anything but Platy or Massive

0.30 – 0.15


Maintenance and Inspection

SDD systems must be inspected by an OWTS Installer licensed with St. Charles County annually.

Low-Pressure Pipe (LPP) Systems

Low-Pressure Pipe (LPP) systems are alternative systems and require design by an engineer licensed in the State of Missouri. The systems are often used in the following conditions:

  • Shallow soil
  • Seasonally high water
  • Slow soil permeability

Site Considerations

Sites must be evaluated to ensure proper conditions for use with an LPP.

  • Slope – LPPs may be used on sites sloping less than 10 percent. For sites greater than 10 percent, additional design to distribute effluent is required.
  • Soil Separation – A minimum of 24 inches of soil is required between the soil surface and bedrock, seasonal high water, or evidence of Chroma-2 mottles.
  • Vertical Separation – A minimum of 12 inches of soil is required between the bottom of the trench and bedrock, seasonal high water, or evidence of Chroma-2 mottles. In areas where there is high chert, a minimum of 48 inches of soil is required.
  • Flooding – LPPs are not suitable for areas prone to flooding.
  • Depressions  LPPs are not suitable for placement in depressions.

System Design

LPP systems are alternative systems and must be designed by a professional engineer licensed in the State of Missouri. All construction documents must be sealed, signed, and dated. The following design conditions shall be utilized:

Trenches

  • Trench Depth – Typically not more than 12 inches in depth.
  • Trench Width – Minimum of 8 inches in width.
  • Trench Spacing – Minimum spacing not less than 5 feet on center.
  • Trench Bedding – A minimum of 5 inches of pea gravel (or minimum ¾-inch clean where pea gravel is not available) shall be placed below the required trench depth.
  • Trench Cover – A minimum of 2 inches of pea gravel (or minimum ¾-inch clean where pea gravel is not available) shall be placed above the distribution pipe and then covered with a minimum of 4 inches of soil.
  • Rocky Conditions – In areas where the soil is 50 percent or more rock, the trenches shall be lined with sand.

Distribution Lines

  • Line Material – One- to two-inch diameter perforated PVC pipe (160 lbs. per square inch or equivalent)
  • Hole Spacing – The spacing of holes in the perforated pipe must be spaced between 2 feet and 8 feet apart.
  • Hole Size – The minimum hole size shall be 5/32 inch.
  • Setbacks – Setbacks shall be measured from a point 2-1/2 feet from the distribution line (i.e. edge of the treatment area).
  • Line Spacing – Lines shall be spaced every 5 feet.
  • Discharge Equalization – Systems must be designed so that the discharge of each distribution does not vary by more than 10 percent.

Tanks

  • Treatment Tank – Tanks shall be a NSF Class 1 treatment tank.
  • Pump/Dosing Tank – Minimum tank size shall be the greater of 1,000 gallons or capacity capable to store a minimum of one day’s flow above the pump level.
  • Filter Required – Tanks shall include a filter or screen.

Pump

  • Pumping System – Pumping systems shall include on/off control for dosing.
  • Alarm – A high water alarm shall be provided.

Maintenance and Inspection

LPP systems must be inspected by an OWTS Installer licensed with St. Charles County annually.

Berms and Interceptors/Curtain Drains

Surface and subsurface water shall be intercepted or diverted around LPP fields.

Application Rates for LPP Systems

Soil Group

Soil Texture

Classes

Application Rate 
(gpd/sq. ft.)

I

Sand, Loamy Sand

No Structure (Brown Colors)

0.50 – 0.40

II

Sandy Loam, Loam

Granular; Fine and Medium Subangular Blocky

0.40 – 0.35

Sandy Loam, Loam

Prismatic; Course Subangular and Angular Blocky

0.30 – 0.20

III

Silty Loam, Clay Loam, Sandy Clay Loam, Silty Clay Loam

Granular; Fine and Medium Subangular Blocky

0.30 – 0.20

Silty Loam, Clay Loam, Sandy Clay Loam, Silty Clay Loam

Prismatic; Course Subangular and Angular Blocky

0.20 – 0.15

IVa

Sandy Clay, Silty Clay, Clay (Low to Moderate Shrink/Swell)

Granular; Fine and Medium Subangular Blocky

0.20 – 0.10

Sandy Clay, Clay, Silty Clay

Prismatic; Course Subangular and Angular Blocky

0.10 – 0.05

IVb

Clay, Sandy Clay, Silty Clay Loam, Clay Loam, Silty Clay (High Shrink/Swell Potential)

Subangular, Angular Blocky, or Prismatic

Not Suitable

V

Skeletal (Less than 50%), Silty Loam, Silty Clay Loam, Clay, Silty Clay

Anything but Platy or Massive

0.30 – 0.15

 

Curtain Drains

Curtain drains shall be installed in accordance with the following parameters to move subsurface water around the absorption field:

  • Location: Curtain drains shall be located a minimum of 10 feet uphill from the absorption field.
  • Width: Drain ditches shall be 12-24 inches in width.
  • Depth: The depth of the ditch shall extend a minimum of 6 inches into the restrictive layer.
  • Plastic Dam: A 6-mil plastic shall be installed on the downhill wall of the ditch from the ground to the ditch bottom.
  • Drain: A 4-inch diameter drainage pipe (drain tile) must be installed in the bottom of the ditch. The pipe shall extend around the field and daylight drain.
  • Gravel: The ditch is filled with 1-1/2 to 3-inch diameter clean gravel.

Berms

Berms shall be built above the absorption field when surface water needs to be redirected around the absorption field.