Title: Appendix 75-A.9 - Alternative subsurface treatment systems

Section 75-A.9 Alternative subsurface treatment systems. (a) General. Alternative subsurface treatment systems described in this section can be installed when site conditions exist that do not allow the use of conventional subsurface treatment systems.

(b) Raised System. (1) A raised system is an absorption trench system constructed in fill material with acceptable permeability placed above the natural soil on a building lot.

(2) Site Requirements. 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. (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 system shall be calculated based upon 0.2 gpd/sq.ft. A conventional absorption trench system as described in subdivision 75-A.8 (b) is to be 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 using 0.2 gpd/sq.ft.

(iii) The minimum size of the basal area of a raised system designed to receive effluent from an ETU located in an area described in subclauses 75-A.6 (b)(6)(ii)(d)(1) or (2), shall be calculated based upon 0.3 gpd/sq ft. A conventional absorption trench system as described in subdivision 75-A.8 (b) is to be designed to distribute effluent evenly over the fill material basal area.

(iv) Sufficient fill material 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, pump dosing or pressure distribution. Gravity distribution may be allowed where both the following conditions are met:

(a) The local health department has a program incorporating site evaluation, system design approval, and construction inspection/certification, and

(b) 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. (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, or may be stabilized by mechanical compaction in shallow lifts if a fill material consisting of only a granular sand or sandy loam is used.

(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 raised system area.

(c) Mounds. (1) General. A 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.

(2) Site Requirements. 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. (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. (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.

(d) Intermittent Sand Filters. (1) General. In a sand filter, the septic tank or aerobic unit effluent is intermittently spread across the surface of a bed of sand through a network of distribution lines. Collector pipes beneath the filter collect treated effluent after it has passed through the sand.

(2) Site Requirements. (i) All horizontal separation distances shown in Table 2 must be met and the minimum required vertical separation to groundwater must be met from the bottom of the collector pipes. (ii) An environmental assessment determines that the development of the site with a sand filter is consistent with the overall development of the area and will cause no adverse environmental impacts.

(3) Design Criteria. (i) Septic tanks installed before a sand filter shall have dual compartments or two tanks in series. The use of a gas baffle on the outlet is strongly recommended.

(ii) The direct discharge of sand filter effluent to the ground surface or to a body of water shall not be approved by the Department of Health or a local health department acting as its agent.

(iii) Distributor lines shall be placed at three foot center lines as level as possible.

(iv) Collector pipes shall be centered between distribution lines at a slope of 1/16 to 1/8 inch per foot.

(v) Effluent shall be distributed to the sand filter by means of pressure distribution or dosing (siphon or pump). Gravity distribution may be used to apply effluent to smaller filters having less than 300 lineal feet of 4-inch diameter distributors or less than 900 square feet of filter area. Pressure distribution lines shall be a minimum of 1 inch and a maximum of three inches in diameter. Pressure distribution pumps shall be selected to maintain a minimum pressure of one pound per square inch (2.3 feet of head) at a downstream end of each distribution line during the distribution cycle. If siphon or pump dosing is allowed, the distributor pipe(s) shall have a diameter of three to four inches.

(vi) The distribution system shall be designed to dose the filter at least three times daily based upon the design flow rates with each dose.

(vii) The sand media shall have an effective grain size of 0.25 to 1.0 mm. If nitrification is not required by the local health department, the effective grain size shall be in the range of 0.5 to 1.00 mm. All sand shall pass a 1/4 inch sieve.

(viii) The uniformity coefficient of the sand shall not exceed 4.0.

(ix) The maximum allowed daily sand loading rate shall be 1.15 gal/day/sq. ft.

(x) Effluent from the collector pipes shall be discharged to an absorption bed located below the original ground level or a mound that is built up above the original ground surface. The size of the bed/mound shall be based upon the estimated quantity of effluent reaching the collector pipe and an application rate of 1.2 gal/day/sq. ft. regardless of the underlying soil percolation. The fill material for the bed/mound shall consist of medium sand with a percolation rate, tested at the borrow pit, not faster than five minutes per inch. All minimum vertical and horizontal separation distances shall be maintained as described in Section 75-A.4.

(4) Construction. (i) After excavation, the collector pipe shall be placed in 3/4 inches to 1 1/2 inches size aggregate.

(ii) There shall be a minimum of four inches of this aggregate beneath the entire system above the collectors.

(iii) A three inch layer of crushed stone or clean gravel with a size of 1/8 inches to 1/4 inches is carefully placed on top of the aggregate.

(iv) A minimum of 24 inches of the approved sand is placed above the crushed stone or gravel.

(v) The distributor pipes are placed in a layer of aggregate that provides a minimum of four inches across the entire surface of the filter and at least two inches above and below the distributor pipes.

(vi) A permeable geotextile, two inches of hay or straw, or untreated building paper is placed over the entire bed area to prevent the infiltration of fines into the filter.

(vii) The entire surface of the filter shall be covered with six to 12 inches of topsoil, mounded to enhance the runoff of rainwater from the system and seeded to grass.

(viii) The bed/mound following the filter shall be covered with 12 inches of topsoil and seeded to grass.