Current through Register Vol. 48, No. 39, March 25, 2024
Subpart 1.
Characteristics.
To qualify as a trench or seepage bed system, the system must
meet the requirements of items A to E:
A. employ flow values in parts
7080.1850 to
7080.1885;
B. meet applicable technical requirements of
parts
7080.1900 to
7080.2030,
7080.2050, and
7080.2100;
C. provide flow measurement if a pump is to
be employed;
D. meet the
requirements of part
7080.2150, subparts
2 and
3, except subpart
3, item M; and
E. meet the requirements of subparts
2 to
4.
Subp. 2.
General.
Seepage bed placement must be limited to areas having natural
slopes of less than six percent. Absorption areas for seepage beds and trenches
must not be placed in soils with a loading rate of less than 0.45 gallons per
day per square foot or as shown in Table IX or IXa in part
7080.2150, subpart
3, item E. Seepage beds must
not be located in floodplains.
Subp.
3.
Sizing of trenches and seepage beds.
A. The trench bottom absorption area is
calculated by dividing the design flow by the appropriate soil loading rate in
Table IX or IXa in part
7080.2150, subpart
3, item E. If gravity
distribution is used in seepage beds, the seepage bed absorption area is
calculated by dividing the design flow by the soil loading rate in Table IX or
IXa in part
7080.2150, subpart
3, item E, multiplied by 1.5.
If pressure distribution is used in seepage beds, the seepage bed absorption
area is determined by dividing the design flow by the soil loading rate in
Table IX or IXa in part
7080.2150, subpart
3, item E.
B. The minimum sidewall absorption is six
inches. The bottom absorption area is allowed to be reduced, for trenches only,
by the following:
| Sidewall absorption - inches
|
Bottom area reduction
|
| 12 to 17 |
20% |
| 18 to 23 |
34% |
| 24 |
40% |
A 40 percent reduction is not allowed with a loading rate of
1.2 gallons per day per square foot.
Subp. 4.
Design and construction of
trenches and seepage beds.
A. Trenches
must be no more than 36 inches wide. Any excavation wider than 36 inches is a
seepage bed. A seepage bed must not be wider than 12 feet if gravity
distribution is used and 25 feet if pressure distribution is used. Natural,
undisturbed soil must exist between multiple trenches and seepage beds.
Multiple seepage beds must be spaced at one-half the bed width.
B. A vertical inspection pipe at least four
inches in diameter must be installed and secured in the distribution medium of
every trench or seepage bed. The inspection pipe must be located at an end
opposite from where the sewage tank effluent enters the medium. The inspection
pipe must have three-eighths inch or larger perforations spaced vertically no
more than six inches apart. At least two perforations must be located in the
distribution medium. Perforations must not be located above the geotextile
cover or wrap. The inspection pipe must extend to the bottom of the
distribution medium, be secured, and be capped flush with or above finished
grade.
C. The top and bottom of the
distribution medium must be level along the contour. Sidewalls must be as
vertical as practical and not intentionally sloped.
D. The minimum depth of soil cover, including
topsoil borrow, over the distribution medium is 12 inches.
E. Trenches or seepage beds must be
backfilled and crowned above finished grade to allow for settling. The top six
inches of the backfill must have the same texture as the adjacent
soil.
F. Trenches and seepage beds
in which the distribution media is in contact with any of the United States
Department of Agriculture soil textures classified as sand or loamy sand or
soils with a percolation rate of 0.1 to 5 minutes per inch must employ one or
more of the following measures:
(1) employ
pressure distribution according to part
7080.2050, subpart
4;
(2) divide the total dispersal area into
multiple units that employ serial distribution, with each dispersal unit having
no greater than 15 percent of the required bottom absorption area; or
(3) have a vertical separation distance of at
least five feet.
Statutory Authority: MS s
115.03;
115.55
