Absorption Fields

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)*
System Type	>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)*
System Type	>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.

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
Low-Pressure Pipe (LPP)	Groundwater Concern	4
Subsurface Drip Distribution (SDD)	Typical	1
Sand Mound	Typical	2
Sand Mound	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
II	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
III	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
IVa	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
II	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
III	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
IVa	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.