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

Current through July 13, 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 ½ inch and 2- ½
				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.