(A) Final treatment and disposal of all sewage tank effluent shall be by discharge into the soil treatment system. The required soil treatment area shall be determined by the daily sewage flow and measured percolation rate of the soil.
(Prior Code, Ch. 506 § 14.1)
(Prior Code, Ch. 506 § 14.2)
(C) Distribution shall be made in accordance with § 157.36.
(Prior Code, Ch. 506 § 14.3)
(D) Location of trenches and seepage beds:
(1) On slopes greater than 12%, the soil profile shall be carefully evaluated in the location of the proposed soil treatment system and down slope to identify the presence of layers with different permabilities that may cause side hill seepage. In no case shall a trench be located within 15 feet of where such a layer surfaces on the down slope.
(2) Seepage bed construction shall be limited to areas having natural slopes of less than 6%. Beds shall not be placed in soils with percolation rates slower than 60 minutes per inch or in floodplain areas.
(4) Soil treatment areas shall not be placed in areas subject to flooding or in floodplains delineated by local ordinances adopted in compliance with the Statewide Standards and Criteria for Management of Floodplain Areas of Minnesota, Chapter 6120, or in areas for which regional flood information is available from the DNR, except that in areas where 10-year flood information is available from and/or approved by the DNR, soil treatment systems may be installed in accordance with the provisions of § 157.39(C).
(Prior Code, Ch. 506 § 14.4)
(E) Distribution medium for trenches and seepage beds.
(1) Distribution medium shall consist of drainfield rock, gravel-less drainfield pipe or chambered systems.
(a) Drainfield rock shall meet the requirements of its definition in § 157.05. There shall be a layer of at least 12 inches but no more than 24 inches of drainfield rock below the distribution pipe. The drainfield rock shall completely encase the top and sides of the distribution pipes to a depth of at least 2 inches. The total thickness of rock-filled trenches shall not exceed 30 inches.
(b) Gravel-less drainfield pipe including appurtenances shall be:
1. Of commercially fabricated corrugated pipe completely encased by the manufacturer in a geotextile wrap specific to this purpose;
2. An 8-inch or 10-inch nominal ID pipe that conforms to the requirements of this section and meets the requirements of American Society of Testing Materials (ASTM) F667, which is incorporated by reference. Requirements under this section also include the following:
a. The pipes must be marked with an alignment stripe visible through the geotextile wrap and installed with this stripe at top center; and
b. The pipes shall contain a row or rows of cleanly cut 3/8-inch to 1-inch diameter holes located in such a manner to provide storage of solids. Each row shall contain a hole in every other corrugation valley, staggered such that every corrugation valley contains 1 hole.
3. Geotextile wraps specifically designed and tested for use with gravel-less pipe and for installation and use in individual sewage treatment system and designed to transmit sewage at a long-term acceptance rate which corresponds to the sizing factor as prescribed in division (F)(2) below; and
4. Protected from heat and ultraviolet rays prior to installation.
(c) Chamber media including all piping appurtenances shall be constructed:
1. Of commercially fabricated materials specific to this purpose;
2. Of materials resistant to sewage tank effluent;
3. With an open bottom;
4. To support the load of overburden and sidewall soil;
5. With slotted or perforated sides to allow sewage to move laterally into the soil and prevent soil penetration into the chamber;
6. No greater than 3 feet in width; and
7. With vertical outside dimensions less than 30 inches.
(Prior Code, Ch. 506 § 14.5)
(F) Sizing of trenches and seepage beds.
(1) Drainfield rock media. Table V gives the required trench bottom assuming 12 inches below the distribution pipe. The required bottom area may be reduced, for trenches only, by the following percentages: 20% for 18 inches of drainfield rock below the distribution pipe; 34% for 24 inches. Unless pressure distribution is used, all seepage bed bottom areas must be 1.7 times the soil treatment area required in Table V. With pressure distribution, the bottom area must be 1.2 times the soil treatment area required in Table V.
(2) Gravel-less drainfield pipe media. Sizing shall be based on a factor of 1.2 times the soil treatment area required in Table V, except that no reduction shall be given as specified in division (E)(1)(a) above). An 8-inch ID pipe shall be equivalent to a 2-foot wide rock-filled trench with 6 inches of drainfield rock below the distribution pipe and a 10-inch ID pipe shall be equivalent to a 3-foot wide rock filled trench with 6 inches of drainfield rock below the distribution pipe.
(3) Chambered media. Sizing shall be based on Table V, with the depth of slated sidewalls being equivalent to the corresponding depth of rock below the distribution pipe.
Percolation Rate (Minutes per Inch) | 0.1-5 | 6-15 | 16-30 | 31-45 | 46-60 | ||
Number of Bedrooms | Tank Size | Gallons Per Day | Square Feet | ||||
Percolation Rate (Minutes per Inch) | 0.1-5 | 6-15 | 16-30 | 31-45 | 46-60 | ||
Number of Bedrooms | Tank Size | Gallons Per Day | Square Feet | ||||
2 | 1,000 + 500 | 300 | 250 | 380 | 500 | 600 | 660 |
3 | 1,000 + 1,000 | 450 | 380 | 570 | 750 | 900 | 990 |
4 | 1,000 + 1,000 | 600 | 500 | 760 | 1,000 | 1,200 | 1,320 |
5 | 1,500 + 1,000 | 750 | 630 | 950 | 1,250 | 1,500 | 1,650 |
6 | 1,500 + 1,000 | 900 | 750 | 1,140 | 1,500 | 1,800 | 1,980 |
7 | 2,000 + 1,000 | 1,050 | 870 | 1,330 | 1,750 | 2,100 | 2,310 |
8 | 2,000 + 1,000 | 1,200 | 990 | 1,520 | 2,000 | 2,400 | 2,640 |
Square Feet Per Gallon | .83 | 1.27 | 1.67 | 2 | 2.20 | ||
Percolation Rate in Minutes per Inch (MPI) | Soil Texture | Square Feet Per Gallon Per Day | Gallons Per Day Per Square Foot |
Percolation Rate in Minutes per Inch (MPI) | Soil Texture | Square Feet Per Gallon Per Day | Gallons Per Day Per Square Foot |
Faster than 0.1* | Coarse Sand | ----- | ----- |
0.1 to 5** | Sand | 0.83 | 1.20 |
0.1 to 5 | Fine Sand*** | 1.67 | 0.60 |
6 to 15 | Sandy Loam | 1.27 | 0.79 |
16 to 30 | Loam | 1.67 | 0.60 |
31 to 45 | Silt Loam | 2.00 | 0.50 |
46 to 60 | Clay Loam | 2.20 | 0.45 |
Slower than 60**** | Clay | ----- | ----- |
Notes: | |||
* Soil too coarse for sewage treatment | |||
** Distribution of sewage effluent shall be by pressure flow over the treatment area or by dividing treatment area into a minimum of 4 equal parts connected serially, by means of drop boxes | |||
*** For soils having more than 50% of very fine sand by weight, plus fine sand having a particle size range of 0.05 millimeters (sieve size 270) to 0.25 millimeters (sieve size 60), the required soil treatment area is 1.67 square feet per gallon of sewage flow per day | |||
**** Soil with too high a percentage of clay for installation of an in-ground standard system | |||
(Prior Code, Ch. 506 § 14.6)
(G) Design and construction of trenches and seepage beds.
(1) The bottom and sides of trenches and beds shall be in original soils at least 3 feet above the saturated soil or bedrock. In no case shall the bottom of the distribution medium be deeper than 42 inches from the final grade.
(2) The trenches shall not be less than 18 inches nor more than 36 inches wide. Any excavation wider than 36 inches shall be considered a bed. No bed may be wider than 25 feet and parallel beds must be at least 10 feet apart. The width of the excavation for gravel-less drainfield pipe and chambered systems shall be installed per manufacturer's recommendation.
(3) Drainfield rock must be used as the distribution medium in seepage beds.
(4) There shall be a layer of at least 12 inches but no more than 24 inches of drainfield rock in the bottom of the trenches. The drainfield rock shall completely encase the top and sides of the distribution pipes to a depth of at least 2 inches. The top of the drainfield rock in trenches, beds, and mounds must be level in all directions.
(5) The bottom and side of the soil treatment system to the top of the distribution medium shall be excavated in such a manner as to expose the original soil structure in an unsmeared and uncompacted condition. Excavation into the soil treatment area shall be made only when the soil moisture content is at or less than the plastic limit.
(6) Excavation equipment or other vehicles shall not be driven on the soil treatment area. Once the trench or seepage bed is excavated, it shall not be exposed to rainfall prior to placement of the final backfill.
(7) A vertical inspection pipe at least 1-1/2 inches in diameter shall 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 3/8-inch or larger perforations spaced vertically no more than 6 inches apart. At least 2 perforations must be located in the distribution medium. No perforations shall be located above the geotextile cover or wrap. The inspection pipe must extend to the bottom of the distribution medium and must be capped flush with or above finished grade.
(8) The top and bottom of the distribution medium shall be level in all directions.
(9) Drainfield rock must be covered by a durable nonwoven geotextile cover specific to this purpose. The cover must be of sufficient strength to undergo installation without rupture. In addition, the cover must permit passage of water without allowing the passage of overlying soil material into drainfield rock.
(10) The minimum depth of cover over the distribution medium shall be at least 6 inches. The maximum depth of cover over the distribution pipes shall be no more than 24 inches.
(11) The trenches or beds shall be backfilled and crowned above finished grade to allow for settling. The top 6 inches of soil shall have the same texture as the adjacent soil.
(12) A vegetative cover shall be established over the soil treatment system. The soil treatment system shall be protected until a vegetative cover is established. The vegetative cover shall not interfere with the hydraulic performance of the system and shall provide adequate frost and erosion protection.
(13) All joints for gravel-less drainfield pipes or chambered systems must be secured as recommended by the manufacturer.
(14) Backfilling for gravel-less drainfield pipe and chambered systems shall not crush or damage the medium.
(Prior Code, Ch. 506 § 14.7)
(H) Dual field systems.
(1) Dual field systems shall be used only where the percolation rate is slower than 5 minutes per inch unless a liner or pressure distribution system is employed as specified in § 157.37(C).
(2) Dual field systems shall be sized, designed, and constructed as set forth above for standard systems except as follows:
(a) The soil treatment area shall be divided into 2 or more parts.
(b) Alternating soil treatment areas shall each be connected to a valve box outlet.
(3) A part of the soil treatment area shall be used no more than 1 year unless the effluent level indicates that a longer duration can be used.
(Prior Code, Ch. 506 § 14.8)
(I) Rapidly permeable soils. Soil treatment systems placed in soils with a soil sizing factor of 0.83 gallons per day per square foot must provide at least 1 of the following treatment techniques:
(1) Distribute the sewage tank effluent by pressure flow over the treatment area as specified in § 157.37(C); and/or
(2) Divide the total soil treatment area into at least 4 parts with no part larger than 25% of the area required by division (F) of this section and the parts constructed for serial application.
(Prior Code, Ch. 506 § 14.9)
(J) Mounds.
(1) Location of mounds.
(a) Mounds must be constructed on original soils so that there is at least 36 inches of separation between the drainfield rock layer and limiting soil characteristics as defined in § 157.05.
(b) There must be at least 18 inches of original soil with a percolation rate faster than 60 minutes per inch above the limiting soil characteristics as defined in § 157.05.
(c) Exceptions are provided to divisions (J)(1)(a) and (b) for previously developed sites: a depth of 12 inches of original soil may be used and a 61 to 120 minutes per inch percolation rate may be used.
(d) If original soil conditions do not exist on a site proposed for a mound, as defined in § 157.05, the site is deemed unsuitable for a mound.
(e) Setbacks for mounds shall be as established in Table III and shall be measured from the absorption area.
(f) Absorption areas shall not be placed in areas subject to flooding as described in § 157.39(C)(4).
(g) On slopes of 1% or greater, and where the percolation rate in the top foot of original soil is in the 61 to 120 minutes-per-inch range, mounds shall not be located where the ground surface contour lines directly below the long axis of the rock bed represent a swale or draw, unless the contour lines have a radius of curvature greater than 100 feet. Mounds must never be located in swales or draws where the radius of curvature of the contour lines is less than 50 feet. In no case shall mounds be placed on slopes greater than 12%.
(2) Design of mounds. Drainfield rock must be used as the distribution of medium in mounds.
(a) The bottom area of the rock bed shall be calculated by multiplying the average design flow by 1.0 square feet per gallon per day.
(b) The width of a single rock bed must not exceed 10 feet.
(c) A minimum of 12 inches of clean sand must be placed where the rock bed is located.
(d) The required absorption width is calculated by multiplying the rock bed width by the absorption ratio. The absorption ratio shall be determined according to Table VI, using percolation rate of the upper 12 inches of soil in the proposed absorption area.
(e) The required absorption width for mounds constructed on slopes from 0 to 1% shall be centered under the rock bed width. The required absorption width for mounds constructed on slopes greater than 1% shall be measured down slope from the down slope edge of the rock bed width and measured in the direction of the original land slope and perpendicular to the original contours.
(f) The side slopes on the mound must not be steeper than 3 horizontal united to 1 vertical unit and shall extend beyond the required absorption area, if necessary.
(g) On slopes of 1% or greater, the upslope edge of the level drainfield rock must be placed on the contour.
(h) Whenever mounds are located on slopes greater than 1%, a diversion must be constructed immediately upslope from the mound to intercept and direct runoff.
(i) A maximum of 2 ten-foot wide beds may be installed side by side in a single mound if the original soil percolation rate is between 5 and 60 minutes per inch to a depth of at least 24 inches below the sand layer. The beds must be separated by at least 4 feet of clean sand.
(j) Distribution of effluent over the rock bed must be by level perforated pipe under pressure. A pump must be used as specified in § 157.37(C).
(k) The rock bed shall completely encase the top and side of the distribution pipes to a depth of at least 2 inches above the pipe. The rock shall extend 9 inches below the pipe.
(l) A vertical inspection pipe at least 1-1/2 inches in diameter shall be installed and secured at each rock bed/sand interface of every mound. The inspection pipe must have 3/8-inch or larger perforations spaced vertically no more than 6 inches apart. At least 2 perforations must be located in the rock bed. No perforations shall be located above the permeable synthetic fabric. The inspection pipe must extend to the bottom of the rock bed and must be capped flush with or above finished grade.
(m) The rock bed must be covered with a durable nonwoven geotextile cover specific to this purpose. The cover must be of sufficient strength to undergo installation without rupture. In addition, the cover must permit passage of water without passage of overlying soil material into the drainfield rock.
(n) Sandy to loamy soil material must be placed on the rock bed to a depth of 1 foot in the center of the mound and to a depth of 6 inches at the sides. When 2 rock beds are installed side by side, the soil material must be 18 inches deep at the center of the mound and 6 inches deep at the sides.
(o) Six inches of topsoil must be placed over the entire mound. Topsoil does not include peat soil texture.
(3) Surface preparation for mounds.
(a) The supply pipe from the pump to the mound area must be installed before mound soil surface preparation. The trench excavated for the supply pipe must be carefully backfilled and compacted to prevent seepage of effluent.
(b) All vegetation in excess of 2 inches in length and dead or organic debris must be removed from the absorption area. Trees must be cut nearly flush with the ground and stumps should not be removed.
(c) All surface preparation must take place when the upper 12 inches of soil has a moisture content of less than plastic limit and soil conditions allow field testing of soil properties, and these properties are maintained throughout installation.
(d) The absorption area must be roughened by backhoe teeth or moldboard, or chisel plowed to a depth of 8 inches. Discing is allowed if the upper 8 inches of soil has a texture of sand loam or coarser. If plowed, furrows must be thrown uphill and there must not be a dead furrow in the absorption area. A rubber tired tractor may be used for plowing or discing. Rototilling or pulverizing the soil is not allowed. The original soil must not be excavated or moved more than 1 foot from its original location during soil surface preparation.
(e) Prior to placement of 6 inches of clean sand, no vehicle shall be driven on the absorption area after the surface preparation is completed. If rainfall occurs on the prepared surface, the site must be allowed to dry below the plastic limit and roughened as specified in division (J)(3)(d) above.
(4) Mound construction.
(a) The clean sand must be placed by using a construction technique that minimizes compaction. If the clean sand is driven on for construction, a crawler or tract-type tractor must be used for mound construction. At least 6 inches of sand must be kept beneath equipment to minimize compaction of the prepared surface.
(b) The sand layer upon which the rock bed is placed must be level in all directions.
(c) The top of the rock bed must be level in all directions.
(d) Construction vehicles must not be allowed on the rock bed until backfill is placed.
(e) A vegetative cover must be established over the entire area of the mound. The soil treatment system mound shall be protected until a vegetative cover is established. The established vegetative cover shall not interfere with the hydraulic performance of the system and shall provide adequate frost and erosion protection.
(f) Shrubs must not be planted on the top of the mound. Shrubs may be placed at the foot and side slopes of the mound.
(Prior Code, Ch. 506 § 14.10)
(K) At-grade systems.
(1) Location of at-grade systems.
(a) At-grade systems must be constructed on original soils so that there is at least 36 inches of separation between the bottom of the rock bed and saturated soil or bedrock.
(b) Percolation tests shall be conducted in the upper 12 inches of original soil. At-grade systems are only allowed if constructed on soils with percolation rates faster than 61 minutes per inch.
(c) At-grade systems shall not be installed in areas with slopes greater than 12%.
(d) Setbacks must be in accordance with Table III. Setbacks shall be measured from the edge of the rock bed.
(2) Design of at-grade systems.
(a) Rock bed absorption width shall be calculated by multiplying the linear loading rate by the soil sizing factor as identified in division (F) above, Table V, using the percolation rate of the upper 12 inches of soil in the proposed absorption area. The linear loading rate shall be between 2 and 8 gallons per day/square feet as determined by the relationship between vertical and horizontal water movement in the soil. Total rock bed width for sloping ground shall consist of the rock bed absorption width plus enough rock on the upslope side to provide stability.
(b) Rock bed length shall be calculated by multiplying the soil sizing factor by the average design flow and dividing by the rock bed width.
(3) Construction of at-grade systems.
(a) Surface preparation for at-grade systems shall be the same as for mound construction, division (J)(3)(c), (d), and (e).
(b) Drainfield rock must be used as the distribution medium in at-grade systems.
(c) The upslope edge of an at-grade system shall be installed along the natural contour.
(d) The rock bed shall completely encase the top and side of the distribution pipe to a depth of at least 2 inches above the pipe. There shall be at least 9 inches of rock below the distribution pipe.
(e) The entire rock bed shall be covered with durable nonwoven geotextile cover specific to this purpose. The cover must be of sufficient strength to undergo installation without rupture. In addition, the cover must permit passage of water without allowing the passage of overlying soil material into the drainfield rock.
(f) One foot of loamy or sandy cover material shall be installed over the rock bed. Cover shall extend at least 5 feet from the ends of the rock bed and be sloped to divert surface water. Side slopes shall not be steeper than 4 horizontal united to 1 vertical unit. The upper 6 inches of the loamy soil cover must be topsoil. Topsoil must be of a quality that provides a good vegetative cover on the at-grade system and must exclude peaty material.
(g) Three vertical inspection pipes of at least 1-1/2 inches in diameter shall be installed and secured along the down slope portion of the rock bed. These pipes shall be located within 3 feet of the down slope edge of the rock bed at the middle and 1/6 of the total rock bed length and placed as measured from the ends of the rock bed. The inspection pipes shall have 3/8-inch or larger perforations spaced vertically no more than 6 inches apart. No perforations shall exist above the permeable synthetic fabric. The inspection pipes must extend to the rock bed/soil interface and must be stabilized and capped flush with or above finish grade.
(h) A vegetative cover must be established over the entire area of the at-grade system. The soil treatment at-grade system shall be protected until a vegetative cover is established. The established vegetative cover shall not interfere with the hydraulic performance of the system and shall provide adequate frost and erosion protection.
(Prior Code, Ch. 506 § 14.11)