Wyoming Administrative Code
Agency 020 - Environmental Quality, Dept. of
Sub-Agency 0011 - Water Quality
Chapter 25 - SEPTIC TANKS, SOIL ABSORPTION SYSTEMS, AND OTHER SMALL WASTEWATER SYSTEMS
Section 25-12 - Standard Soil Absorption Systems
Universal Citation: WY Code of Rules 25-12
Current through September 21, 2024
(a) General Design Requirements:
(i) All soil absorption systems
shall be designed in such a manner that the effluent is effectively filtered
and retained below the ground surface. The absorption surface accepts, treats,
and disperses wastewater as it percolates through the soil.
(ii) Soil absorption systems shall not be
excavated when the soil is wet enough to smear or compact easily. Open soil
absorption system excavations shall be protected from surface runoff to prevent
the entrance of silt and debris. All smeared or compacted surfaces shall be
raked to a depth of one (1) inch, and loose material removed before filter or
filler material is placed in the soil absorption system excavation.
(iii) Soil absorption systems shall be
designed to approximately follow the ground surface contours so that variation
in excavation depths will be minimized. The trenches may be installed at
different elevations, but the bottom of each individual trench shall be level
throughout its length.
(iv) Shallow
soil absorption system depths are encouraged to promote treatment and
evapotranspiration. The minimum soil cover depth over the soil absorption
system is one (1) foot. The maximum depth to the bottom absorption surface of a
soil absorption system is five (5) feet. Finished grading shall prevent ponding
and promote surface water runoff.
(v) Pipes, chambers or other products shall
be bedded on firm, stable material. Heavy equipment shall not be driven in or
over soil absorption systems during construction or backfilling.
(vi) Standard trenches refer to perforated
pipe embedded in aggregate-filled trenches that shall conform to the following:
(A) The perforated pipe shall have a minimum
diameter of 4 inches. Suitable pipe materials include: ASTM D-2729-11 PVC, ASTM
D-3034-08 PVC, Schedule 40 PVC ASTM d1784-11, and ASTM F810-07 PE.
(B) The aggregate shall be crushed rock,
gravel or other acceptable, durable and inert material that is free of fines,
and has an effective diameter between 1/2 inch and 2- 1/2 inches.
(C) Prior to backfilling, the aggregate shall
be covered throughout with a woven/non-woven geotextile material or a three (3)
inch layer of straw.
(D) Aggregate
shall extend the full width and length of the soil absorption system to a depth
of at least twelve (12) inches with at least six (6) inches of drain gravel
under the distribution pipe and at least two (2) inches over the distribution
pipe.
(E) Maximum width of trench
excavation is three (3) feet.
(F)
Minimum spacing of trenches (wall to wall) is three (3) feet. Trench spacing
shall be increased to nine (9) feet when the area between each trench is
considered as reserve area. For clay loam soils that have percolation rates
greater than 60 min/in., the nine (9) foot spacing shall also be required but
it is not considered as reserve area.
(vii) Standard beds shall conform to the same
pipe and aggregate requirements for trenches as found in subparagraphs (vi)(A
through D) of this section. Standard beds shall also conform to the following:
(A) The soils shall have percolation rates
less than 60 minutes per inch (5-60 mpi). The bottom of the bed must be level,
therefore the site shall be relatively flat, sloping no more than one (1) foot
from the highest to the lowest point in the installation area.
(B) Distribution laterals within a bed must
be spaced on not greater than six (6) feet centers. Sidewalls shall not be more
than three (3) feet from a distribution lateral.
(C) Beds must not be wider than twenty-five
(25) feet if gravity distribution is used. Multiple beds must be spaced at
one-half the bed width.
(D) Rubber
tired vehicles must not be driven on the bottom surface of any bed
excavation.
(viii)
Chambered trenches, when used in lieu of perforated pipe and aggregate, shall
be installed in conformance with the manufacturer recommendations. No cracked,
weakened, modified, or otherwise damaged chamber units shall be used in any
installation.
(A) All chambers shall be an
open, arch-shaped structure of durable, non-degradable design, suitable for
distribution of effluent without filter material.
(B) All chamber endplates shall be designed
so that the bottom elevation of the inlet pipe is at least six (6) inches from
the bottom of the chamber.
(C)
Inlet and outlet effluent sewer pipes shall enter and exit the chamber
endplates. Inspection ports shall be installed at all outlet effluent sewer
pipes.
(D) All chambers shall have
a splash plate under the inlet pipe or another design feature to avoid
unnecessary channeling into the trench bottom.
(E) The maximum width of the bottom
absorption surface for a chambered trench is three (3) feet. The excavation to
install a chambered trench may exceed three (3) feet.
(F) Minimum spacing of trenches (wall to
wall) is three (3) feet. Trench spacing shall be increased to nine (9) feet
when the area between each trench is considered as reserve area. For clay loam
soils that have percolation rates greater than 60 min/in., the nine (9) foot
spacing shall also be required but it is not considered as reserve
area.
(ix) Chambered
beds shall conform to the same requirements for chambered trenches as found in
subparagraphs (viii)(A through D) of this section. Aggregate, as specified in
subparagraph (vi)(B) of this section, or native soil shall be used to fill the
space between the chambers.
(x)
Serial Sidehill Trench:
(A) A minimum of six
(6) feet of undisturbed soil shall be maintained between adjacent trench or bed
side walls.
(B) The bottom of each
serial trench or bed system shall be level.
(C) The overflow pipe between serial soil
absorption systems shall be set no higher than the mid-point of the upstream
distribution pipe. The overflow pipe shall not be perforated.
(b) A design package for standard soil absorption systems is provided online at the Division's website to assist the applicant in submitting a completed application for coverage under the general permit for small wastewater systems. The worksheet and calculations were prepared by a registered professional engineer employed by the Wyoming Department of Environmental Quality, Water Quality Division. The general design requirements stated in this section are incorporated into the worksheets such that by properly completing the forms and installing the components, the system will comply with these requirements.
Disclaimer: These regulations may not be the most recent version. Wyoming may have more current or accurate information. We make no warranties or guarantees about the accuracy, completeness, or adequacy of the information contained on this site or the information linked to on the state site. Please check official sources.
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