NAC444.8354. Absorption trench system: Design criteria.

     1. The bottom of an absorption trench that is used in an individual sewage disposal system must be level and not less than 1 foot or more than 3 feet in width.

     2. Excavations for absorption trenches must be spaced apart at a distance that is equal to or greater than 4 feet plus 2 feet for each foot of depth which the trench is below the bottom of the distribution piping, as measured from the centerline of the trenches.

     3. An individual lateral may not be more than 110 feet long.

     4. An absorption trench must not be excavated if the soil is extremely wet. Surfaces in an absorption trench which are smeared or compacted must be scarified to the depth to which the soils are smeared or compacted, and all loose material must be removed.

     5. Distribution lines must be perforated drain pipe made of polyvinylchloride, unless otherwise approved by the administrative authority. The bottom of the distribution lines must be laid not less than 12 inches or more than 48 inches below the ground surface in continuous straight or curved lines with a slope of not less than 2 inches or more than 4 inches per 100 feet of pipe. Distribution lines must be equipped with end caps or vented to the surface at the end of the lines.

     6. At least 12 inches of clean, graded aggregate ranging in size from 3/4 to 2 1/2 inches must be placed in the trench below the distribution line, and the aggregate must extend at least 2 inches over the top of the distribution line.

     7. If an absorption trench is more than 6 feet in depth below the finished grade, the aggregate must extend to not less than 12 inches below the ground surface to avoid an anaerobic condition in the trench.

     8. The aggregate in an absorption trench must be covered with untreated building paper, straw, geotextile fabric, or a similar covering approved by the administrative authority, and the top of the trench must be overfilled with not less than 4 inches or more than 6 inches of soil.

     9. The percolation rate of the soil must be used to determine the design application rate in accordance with the following table:

Percolation Rate (minutes/inch)	Design Application Rate (gallons/square foot)
0-10	1.6
11-15	1.3
16-20	1.1
21-25	1.0
26-30	0.9
31-40	0.8
41-50	0.7
51-60	0.6

If the percolation rate of the soil is greater than 60 minutes per inch, the system must be designed by an engineer.

     10. The required capacity of the septic tank must be divided by the design application rate to calculate the minimum absorption area required.

     11. The area of the absorption trench must be determined by calculating the size of the effective area of the sidewall needed beneath the distribution line. Not more than 4 feet of aggregate below the distribution line may be used to calculate the effective area of the sidewall, except that aggregate which is in excess of 4 feet below the distribution line may be used to calculate the effective area of the sidewall with the approval of the administrative authority. The required length of distribution line must be determined as follows:

The minimum size required for the absorption area (in square feet) divided by [2 times the depth of the aggregate below the distribution line (in feet)] = required length of distribution line (in feet)

     12. The following is a diagram of an absorption trench:

     (Added to NAC by Bd. of Health by R129-98, eff. 3-25-99)