020-25 Wyo. Code R. § 25-8

Current through April 27, 2019

Section 25-8 - Soil Absorption System Sizing

(a) The total infiltration surface area of a soil absorption system shall be calculated by dividing the design flow rates (gpd) from Table 1 or Table 2 by the loading rate (gpd/ft²) found in Table 5.

Table 5. Rates of Wastewater Application for Soil Absorption System Areas

Percolation Rate (mpi)	Loading Rate (gpd/ft²)	Percolation Rate (mpi)	Loading Rate (gpd/ft²)
5	0.80	21	0.45
6	0.75	22	0.44
7	0.71	23-24	0.43
8	0.68	25	0.42
9	0.65	26-27	0.41
10	0.62	28-29	0.40
11	0.60	30-31	0.39
12	0.58	32-33	0.38
13	0.56	34-35	0.37
14	0.54	36-37	0.36
15	0.52	38-40	0.35
16	0.50	41-43	0.34
17	0.49	44-46	0.33
18	0.48	47-50	0.32
19	0.47	51-55	0.31
20	0.46	56-60	0.30

(b) The total infiltration area shall be defined as follows:

(i) For standard trenches the total infiltration area shall be calculated based on the following formula:
A = L(W + 2S)

A = Total infiltration area

L = Total length of trench

W = Bottom width

S = Sidewall height of 12 inches or less
- (A) The sidewall height is the depth below the flowline of the pipe to the bottom of the trench.
- (B) The maximum credit for sidewall height shall not exceed twelve (12) inches even if the actual sidewall height exceeds twelve inches.

(ii) For chamber trenches, the total infiltration area shall be calculated based on the following formula:
A = L(E + 2S)

A =Total infiltration area

L =Total length of trench

E =Effective bottom width (Multiply width of the chamber by factor of 1.43 to get effective bottom width)

S =Sidewall height of 12 inches or less
- (A) The factor of 1.43 incorporates a thirty percent (30%) reduction of the bottom area.
- (B) The maximum credit for sidewall height shall not exceed twelve (12) inches even if the actual sidewall height exceeds twelve (12) inches.
- (C) The sidewall height is the height of the slotted sidewall of the chamber or depth below the flow line of the inlet pipe, whichever is less.
- (D) The total length of the trench is the number of chambers in a row multiplied by the length of one piece of chamber.

(iii) For standard bed systems, the total infiltration area shall be calculated based on the following formula:
A = LW

A=Total infiltration area

L= Total length of bed

W= Width of the bed
- (A) The sidewall credit shall not be used in calculating the total infiltration area for a bed system.

(iv) For chamber bed systems, the total infiltration area shall be calculated based on the following formula:
A = L(ExR)

A = Total infiltration area

L = Total length of bed

E = Effective bottom width of the chamber (Multiply width of the chamber by factor of 1.43 to get effective bottom width)

R = Number of chamber rows (Multiply effective bottom width of chamber by number of chamber rows to get effective bottom width of bed.)
- (A) The factor of 1.43 incorporates a thirty percent (30%) reduction of the bottom area.
- (B) The total length is the number of chambers in a row multiplied by the length of one piece of chamber.

(c) Coarse sand or soils having a percolation rate less than one (1) minute per inch (mpi) are unsuitable for subsurface effluent disposal. These soils may be used if a one (1) foot layer of fine sand or loamy sand is placed below the constructed soil absorption system. The soil absorption system shall be sized based on the percolation rate of the fill material.

020-25 Wyo. Code R. § 25-8

Amended, Eff. 7/27/2016.

Amended, Eff. 6/29/2018.