This article explains the design and function of raised bed septic systems. In the sketch shown here the raised bed septic system is similar in design to a mound system but some use is made of original local soils for the raised bed. (Source: US EPA) Effluent is discharged to the effluent treatment and absorption system by a pumping chamber. Alternatively the raised bed component of a septic system, if suitably located, may be supplied effluent by gravity feed.
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RAISED BED SEPTIC SYSTEMS: a wastewater absorption trench system which has been constructed in soil-fill material which has been placed on top of the natural soil on a building lot.
Raised systems are often confused with "septic mound systems" but have different design requirements, and make at least partial use of existing soils for wastewater treatment. Raised bed septic systems are constructed in fill over soil which can accept septic effluent below the fill.
MOUND SEPTIC SYSTEMS are constructed in fill over soil which does not acceptably treat septic effluent below the fill - all of treatment occurs in the mound. Other requirements differ as well.
The following description of raised bed septic system design, installation, and maintenance is from: New York Appendix 75-A.9 Alternative Septic System Designs section 9.b Raised Bed Septic Systems
Section 75-A.9 Alternative Septic Systems: (b) Raised Septic Systems Design Criteria
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]
(1) General: Raised Septic Systems
A raised septic system [or raised bed septic system] is an absorption trench system constructed in fill material with acceptable permeability placed above the natural soil on a building lot.
[DF: Note: while they are similar in design "raised septic systems" discussed here are not identical to "mound septic systems" discussed at SEPTIC MOUND DESIGN SPECIFICATIONS which have different site requirements]
(2) Site Requirements for Raised Septic Systems
A raised system may be used where all the following conditions are found:
(i) There is at least one foot of original soil with a faster than 60 minutes percolation rate, above any impermeable soil layer or bedrock, but not more than two feet.
(ii) The maximum high groundwater level must be at least one foot below the original ground surface.
(iii) Slopes shall not exceed 15%.
(iv) All minimum vertical and horizontal separation distances can be maintained as described in Table 2.
(3) Design Criteria for Raised Bed Septic Systems
(i) Percolation tests shall be conducted in the fill material at the borrow pit and after placement and settling at the construction site. The slower percolation rate of these tests shall be used for design purposes.
(ii) The total area beneath the absorption trenches, extending 2.5 feet in all directions from the outer edge of all trenches, is defined as the basal area. The minimum size of the basal area of the raised bed shall be calculated based upon 0.2 gpd/sq.ft.
(iii) An absorption trench system as described in Section 75-A.8(b) is designed using the percolation rate of the fill material. The use of slowly permeable soils for the fill material will result in a trench system that will have a basal area larger than the minimum area calculated in (ii) above.
(iv) Sufficient soil with a percolation rate of between 5 - 30 min/in is required to maintain at least two feet separation between the proposed bottom of the trenches and any boundary condition such as groundwater, bedrock, clay or other relatively impermeable soil or formation.
(v) The edge of the fill material shall be tapered at a slope of no greater than one vertical to three horizontal with a minimum 20 foot taper.
(vi) Horizontal separation distances shall be measured from the outside edge of the taper.
(vii) The system shall incorporate siphon dosing or pressure distribution except where the following conditions are met:
(aa) The local health department has a program incorporating site evaluation, system design approval, and construction inspection/certification.
(bb) A minimum of two feet of fill material with a percolation rate of 5 - 30 min/in shall be placed between the bottom of the trenches and the existing ground.
(viii) Curtain drains may be used to intercept and carry underground water away where high groundwater levels exist. Curtain drains shall be upslope from the system and at least 20 feet from the toe of slope of the fill material.
(4) Construction details for raised bed septic systems
(i) Heavy construction equipment shall not be allowed within the area of the system. The underlying soil shall be undisturbed although the surface may be plowed with at least a double bottomed blade/furrow plow and the furrow turned upslope.
(ii) A system shall not be built in unstablized fill material. The fill material shall be allowed to settle naturally for a period of at least six months to include one freeze-thaw cycle.
(iii) The absorption trenches shall be constructed in the fill material.
(iv) The entire surface of the system including the tapers shall be covered with a minimum of six inches of topsoil, mounded to enhance the runoff of rainwater from the system and seeded to grass.
(v) On sloping sites a diversion ditch or curtain drain shall be installed uphill to prevent surface water runoff from reaching the bed area.
[See Curtain drains or intercept drains can protect septic drainfields in areas of wet soils or surface and subsurface groundwater]
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