Raised bed septic system designs:

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)

Wastewater 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.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author.

Raised Septic Systems Designs

Raised bed septic designs are a wastewater absorption trench system that 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 mound septic systems, but they are not identical: raised bed septics have different design requirements, and make at least partial use of existing soils for wastewater treatment while in a septic mound system disposal of septic effluent depends entirely on and occurs within soils brought to the site and shaped into a long elevated mound.

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 DESIGN CODES 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]
Effective Date: 12/01/1990

(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.

Example Raised Bed & Mound Septic Design Specifications

• Ohio - SAND MOUND SEPTIC DESIGN SPECIFICATIONS [PDF] retrieved 2017/10/20, original source: http://www.odh.ohio.gov/
• Ohio - SEPTIC SEPARATION from SEASONAL HIGH WATER LEVEL [PDF] Drainage to manage the level of seasonal water in soils
  
  Watch out: also see Bicki, T. J., and R. B. Brown. "ON-SITE SEWAGE DISPOSAL: THE IMPORTANCE OF THE WET SEASON WATER TABLE." [PDF] Journal of Environmental Health 52, no. 5 (1990): 277-279.

Also see CURTAIN DRAINS or intercept drains can protect septic drainfields in areas of wet soils or surface and subsurface groundwater

Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed atReferences or Citations .

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Freeze worries with raised bed septic?

Is there any concern for freezing. The discharge pipe from the tank is always full out to the bed.I am confused;( On 2011-10-10 by Richie

Answer by (mod) -

Richie:

If the discharge pipe from a septic tank is always full there are two different conditions:

If the pipe is designed to drain into a drainfield by gravity then it should not be full and there is a clog or blockage that needs repair. Indeed freezing and damage are possible depending on pipe depth.

If the septic tank discharge line is designed to be pumped by a septic grinder pump up to a raised septic drainfield system or bed or soakaway bed, or any effluent disposal system that is higher on the site than the elevation of the septic tank itself, then the pumping line or "forced main" needs to be protected by frost, either by burying it below the local frost line or by a combination of insulation and burial depth.

We're told if we install a raised bed septic we can't use the area for a garden or anything else: why not?

We've been told we have to get this raised bed pure flow system for our new Scout Den which is basically some old sheds under renovation.

They have said that if we use it though we will be unable to use the garden it's going into for anything else.

We had hoped to be able to camp back there and generally hold meetings and use the green area and it seems an awful shame to lose the area. In what I've read however I don't see any reason why we couldn't use the garden once it's installed.

Can you tell me if it's correct that we won't be able to use the garden and why? Thanks! On 2011-07-18 by Emma

Answer by (mod) -

Emma there are these concerns about using a raised bed septic area for anything but growing grass:

- traffic and worse vehicles that risk compacting the soil can ruin the system

- there could be bacterial hazards in foods grown on such an area

- less obvious, if the system should fail and effluent reach the ground surface, a scout could become infected or sickend by exposure to septic effluent.

In the ARTICLE INDEX left take a look at

DRIVING or PARKING OVER SEPTIC COMPONENTS?
and at

PLANTS & TREES OVER SEPTIC SYSTEMS

for details.

Is it OK to drive over our raised septic tile bed system?

Can you drive or disturb soil on top we have a raised tile bed that is all bumpy cannot cut or ride tracot on too steep any suggestions? On 2011-06-23 by carol and darryl

by (mod) - not with anything heavy

Driving over a raised bed septic system:

I would not recommend driving any heavy equipment over the raised bed septic mound as you risk compressing the soil or breaking a pipe. If you want to level or smooth the top surface you can do so using lightweight equipment such as a modest-sized lawn mower or walk-behind grader. Or better, a hand rake.

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• MOUND SEPTIC SYSTEMS
• New York Appendix 75-A.9 ALTERNATIVE SEPTIC DESIGN CODES
• SEPTIC MOUND DESIGN SPECIFICATIONS
• SEPTIC SYSTEM DESIGN ALTERNATIVES - home

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