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Design guide for the construction of mound type septic systems - septic mounds. This document uses the New York State wastewater treatment standard for individual household septic systems to provide an example of state regulated design and installation of mound type septic system designs.
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Model source for these septic mound system design specifications and regulations include Title: Appendix 75-A.9 - Alternative Septic Systems [Regulation and System Design Criteria for Raised Septic Systems, Septic Mound Systems, Intermittent Sand Filter Bed Systems, Evaporation-Transpiration Septic Systems, Evaporation-Transpiration Absorption Septic Systems, and Other Alternative Septic Systems] Effective Date: 12/01/1990
(1) General - Mound Septic Systems
A mound septic system (or septic mound system) is a soil absorption system that is elevated above the natural soil surface in a suitable fill material. It is a variation of the raised bed utilizing sandy fill material but not requiring a stabilization period prior to the construction of the absorption area.
On sites with permeable soils of insufficient depth to groundwater or creviced or porous bedrock, the fill material in the mound provides the necessary treatment of wastewater.
The overall size of the mound system will normally be substantially smaller than a raised bed.
[DF: Note: while they are similar in design "mound septic systems" discussed here are not identical to "raised septic systems" discussed at Raised Septics which have different site requirements.]
(2) Site Requirements for Mound Septic Systems
A mound system may be used where all the following conditions are found:
(i) The maximum high groundwater level must be at least one foot below the original ground surface.
(ii) Bedrock shall be at least two feet below the natural ground surface.
(iii) The percolation rate of the naturally occurring soil shall be faster than 120 minutes/inch.
(iv) The natural ground slopes shall not exceed 12%.
(v) All minimum horizontal separation distances can be maintained as described in Table 2.
(3) Design Criteria for Mound Septic Systems
(i) The designer shall consult with the health unit having jurisdiction regarding the method for detailing the hydraulic design.
(ii) The basal area of a mound system is defined differently than a raised bed. The basal area for a system on level ground includes all the area beneath the absorption trenches or bed and the area under the tapers.
On a sloping site, the basal area includes only the area under the absorption trenches/bed and the lower or downhill taper. The basal area is designed upon the percolation of the naturally occurring soil.
Where the percolation rate is 60 min/in or faster, refer to Table 4B. For soils of 61 to 120 min/in, a rate of 0.2 gpd/sq. ft. shall be used for determining the minimum basal area required.
(iii) Percolation tests for the fill material shall be conducted at the borrow pit. Only soils with a percolation rate between five and 30 minutes per inch shall be used for the fill material. Sands with greater than 10% by weight finer than 0.05 mm material must be avoided. At least 25% of the material by weight shall be in the range of 0.50 mm to 2.0 mm. Less than 15% of the material by weight shall be larger than a half-inch sieve. A sieve analysis may be necessary to verify this requirement. The required absorption area is based upon the percolation rate of the fill material as determined from Table 4B.
(iv) The system shall be designed to run parallel with the contours of the site. The width of the system (up and down the slope) shall be kept to a minimum, but in no case shall the absorption area be wider than 20 feet. In a distribution network using a center pressure manifold, distribution lines shall have a maximum total length of 200 feet.
In a network using an end manifold, distribution lines shall have a maximum length of 100 feet.
(v) Mound dimensions shall meet or exceed those required by the health unit having jurisdiction.
(vi) A pressure distribution network shall be required.
(vii) A dual chamber septic tank or two tanks in series in addition to the dosing tank shall be provided.
A gas baffle or other outlet modification that enhances solids retention is recommended.
(4) Construction Procedures for Mound Septic Systems
(i) Heavy construction equipment shall not be allowed within the basal area and area downslope of the system which will act as the dispersal area for the mound.
(ii) The vegetation shall not be scraped away, roto-tilled, or compacted. Excess vegetation shall be removed with trees cut at the ground surface but stumps left in place.
The area shall be plowed to a depth of seven or eight inches with a double bottomed blade/furrow plow and the furrow turned upslope.
(iii) The fill material is placed from the upslope side of the system to the full depth required in the design and shall extend to the edge of the basal area at a slope not to exceed one vertical to three horizontal.
(iv) The absorption area is then formed within the mound. A minimum of six inches of aggregate shall be placed beneath the distribution lines.
(v) The pressure distribution lines are placed parallel to the contours of the slope and a minimum of two inches of aggregate is placed above the lines.
(vi) A permeable geotextile is placed over the entire absorption area to prevent the infiltration of fines into the aggregate.
(vii) On sloping sites a diversion ditch or curtain drain shall be installed uphill to prevent surface water runoff from reaching the absorption area.
(viii) A minimum of six inches of finer materials such as clayey loam is placed over the top of the absorption area, and the entire mound including the tapers is then covered with six inches of top soil and seeded to grass.
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