Appendix 75-A.8 - Conventional subsurface treatment systems

Section 75-A.8 Conventional Subsurface Treatment Systems. (a) General for all treatment systems.

(1) All wastewater effluent from septic tanks or ETUs shall be discharged to a subsurface treatment system.

(2) The minimum distances that all treatment system components shall be separated from other site features are listed in Table 2.

(3) Absorption systems shall not be located under driveways, parts of buildings or under above-ground swimming pools or other areas subject to heavy loading. Surface waters shall be diverted from the vicinity of the system.

(b) Absorption Trench Systems. (1) Site requirements. A minimum of four feet of useable soil shall exist above bedrock and groundwater with a minimum separation of two feet to the lowest part of any absorption trench system.

(2) Design criteria. The required length of absorption trench is determined from Table 4A based upon the percolation test results and confirmed by the soil evaluation. The maximum trench depth shall be 30 inches below ground surface. The maximum trench width for design purposes shall be 24 inches. Where trenches exceed 24 inches in width, calculations of absorptive area shall be based on a width of 24 inches.

(ii) Adjacent trenches shall be separated by at least four feet of undisturbed soil. Individual trenches shall be constructed parallel to the ground contours with trench bottoms as near level as possible. They need not be perfectly straight but abrupt changes in direction shall be avoided.

 

 

TABLE 4B

APPLICATION RATES FOR

NON-STANDARD DESIGN FLOWS

Percolation Rate

(minutes/inch)

Application Rate

(gal/day/sq. ft.)

1 - 5

1.20

6 - 7

1.0

8 - 10

0.90

11 - 15

0.80

16 - 20

0.70

21 - 30

0.60

31 - 45

0.50

46 -  60

0.45

Soil with a percolation rate of less than 1 minute/inch is unsuitable for a conventional system

Required Area (sq. ft.) =

Flow Rate (GPD)/Application Rate (GPD/sq. ft.)

Required Absorption Trench Length =

Required Area (GPD)/2 feet (trench width)

 

(3) Materials. (i) Perforated distributor pipe shall be used in the trenches. Solid (non-perforated) pipe shall be used between the distribution box and the trenches. Perforated pipe shall be made of rigid or corrugated plastic and be labelled as fully meeting ASTM standards for use in septic systems. Corrugated plastic pipe delivered in coils is not to be used unless provision is made to prevent the recoiling or movement of the pipe after installation.

(ii) Aggregate shall mean washed gravel or crushed stone 3/4 - 1 1/2 inches in diameter. Larger diameter material or finer substances and run-of-bank gravel are unacceptable.

(iii) The aggregate shall be covered with a material that prevents soil from entering the aggregate after backfilling, yet must permit air and moisture to pass through. The preferred material for covering the aggregate is a permeable geotextile. Untreated building paper or a four inch layer of hay or straw is acceptable. Polyethylene and treated building paper are relatively impervious and shall not be used.

(iv) Alternate aggregate. Materials may be used as a substitute for conventional gravel or stone aggregate when it can be demonstrated that the material provides at least the equivalent soil infiltration area and storage volume as conventional gravel or stone aggregate. Materials shall also maintain structural integrity and be non-degradable by wastewater effluent.

(v) Tire Derived Aggregate (TDA). Properly manufactured tire chips have physical characteristics similar to conventional gravel or stone aggregate. TDA may be used as a substitute for gravel or stone aggregate on a one-to-one basis, volumetrically, when;

(a) The TDA manufacturer shall have a written case-specific beneficial use determination from the New York State Department of Environmental Conservation (NYSDEC) for use in onsite wastewater treatment systems, and

(b) TDA shall meet the following size and gradation requirements.

(1) Two-inch nominal size, and

(2) Maximum dimension in any direction shall not exceed four inches; minimum dimension in any direction shall not be less than ½ inch, and

(3) Exposed wire shall not protrude more than ½" from the chip, and

(4) Fine particles and foreign materials are prohibited, and

(5) At least 95% of the TDA shall comply with the above specifications.

(4) Construction. (i) Trench locations and depths should be marked by stakes before the trenches are excavated. The natural surface shall not be significantly disturbed. If the site is regraded or similarly disturbed, the soil shall be allowed to stabilize and new percolation tests conducted.

(ii) The trench depth shall be as shallow as possible, but not less than 18 inches. At least six inches of aggregate is placed below the distribution line and two inches above the line. The earth cover over the aggregate should not exceed 12 inches in order to enhance natural aeration and nitrogen uptake by plant life. Trenches shall be excavated to design depth with bottoms practically level. Heavy equipment shall be kept away from the field because the weight may permanently alter soil characteristics due to compaction, cause trench cave-ins, and/or misalign and break pipe.

(iii) Trench bottoms are to be raked and immediately covered with at least six inches of aggregate.

(iv) Any smeared surfaces on the trench walls are to be raked. Distributor lines are carefully placed on the aggregate and covered with aggregate to a depth of at least two inches over the top of the pipe. Additional aggregate may be required to bring the top of the aggregate to within six to 12 inches of the surface.

(v) In gravity distribution systems, the pipe shall be carefully sloped at between 1/16 inch and 1/32 inch per foot. Grades shall be determined by an engineer's level, transit or carpenter's level.

(vi) After the upper aggregate is placed, the geotextile, untreated building paper, hay or straw is to be immediately installed and the trench backfilled with native soil. If the trenches cannot be immediately backfilled, they should be temporarily covered with an impervious material such as treated building paper to prevent sidewall collapse and siltation into the aggregate.

(vii) The earth backfill is to be mounded slightly above the original ground level to allow for settling and after settlement the entire area should be graded without the use of heavy equipment and seeded with grass.

(c) Gravelless Absorption Systems. (1) General. Gravelless trench products must be designed to distribute effluent and provide at least the equivalent soil surface area for wastewater treatment as a conventional absorption trench without the use of gravel or stone aggregate. All gravelless systems must also be capable of withstanding typical construction equipment and residential use loads without deformation.

(2) Site requirements. These products may be used as an alternative to conventional gravel or stone absorption trenches in wastewater treatment systems. All other treatment system design requirements shall apply.

(3) Design criteria. Unless otherwise specified, all absorption trench system designs incorporating gravelless products shall have the same trench length as a conventional (24-inch wide) absorption trench as listed in Table 4A or as calculated from Table 4B.

(i) Open-bottom gravelless chambers. Absorption area designs may use a 25% reduction in the total absorption trench length listed in Table 4A or as calculated from Table 4B when the product can demonstrate the following features:

(a) minimum soil infiltration bottom area of 1.6-square feet per linear foot, and

(b) a minimum volumetric capacity of 7.5-gallons per linear foot , and

(c) open sidewall area for aeration and infiltration.

(ii) Gravelless media-wrapped corrugated pipe sand-lined systems. Absorption area designs may use a 25% reduction in the total absorption trench length as listed in Table 4A or as calculated from Table 4B, when the product can demonstrate the following features and installation criteria:

(a) corrugated pipe with a minimum outside diameter of 12-inches, and

(b) wrapped in a media that allows wastewater distribution and prohibits sand infiltration, and

(c) installed with a minimum of 6-inches of washed concrete sand surrounding the pipe.

(iii) Gravelless geotextile sand filter. Absorption area designs may use a trench bottom sizing criteria of 6-square feet per linear foot of trench when the product demonstrates the following features and installation criteria:

(a) a minimum unit width of 3-feet, and

(b) a minimum storage capacity of 12-gallons/linear foot of unit, and

(c) a minimum of 6-square feet per linear foot of geotextile surface area per linear foot of unit, and

(d) installed with 6-inches of washed concrete sand below and on the sides of each unit.

(4) Special Conditions.

(i) The gravelless product trench length reductions specified above in paragraph 75A.8(c)(3) are not applicable at properties located within the New York City Watershed.

(ii) The trench length reduction, may only be used for conventional absorption trench systems and shallow absorption trench systems.

(iii) The gravelless trench length reductions may not be further reduced by the trench length reduction allowed for Enhanced Treatment Units (ETUs) as specified in paragraph 75-A.6(b)(6).

(5) Construction.

(i) Gravelless absorption system products shall be installed in conformance with the manufacturer's instructions because of the proprietary design of some products.

(ii) The gravelless trench sidewalls shall be separated by a minimum of 4-feet of undisturbed soil.

(iii) All gravelless trenches shall be equal in length. The total trench length shall be increased if necessary.

(d) Deep Absorption Trenches. (1) Site Requirements. These are used on sites where an useable layer of soil is overlaid by three to five feet of impermeable soil.

(2) Design Criteria. (i) There shall be at least four feet of useable solid beneath the impermeable layer.

(ii) The required length of absorption trench is determined from Table 4A based upon percolation tests conducted in the underlying soil.

(3) Construction. (i) Trenches are excavated at least two feet into the useable layer and backfilled with aggregate or coarse sandy material containing a low percentage of fines more permeable than the underlying material to a level 30 inches below the original ground surface.

(ii) An absorption trench system as described in Section 75-A.8(b) is constructed in the upper 30 inches of the excavation.

(e) Shallow Absorption Trenches. (1) Site Requirements. These systems are used where there is at least two feet but less than four feet of useable soil and/or separation to boundary conditions.

(2) Design criteria. (i) A minimum two foot separation must be maintained between the bottom of each trench and all boundary conditions.

(ii) The bottom of each trench must not be above the original ground surface.

(iii) Material of the same permeability as the underlying original soil shall be used as fill material. The depth of the fill shall not be greater than 30 inches above the original ground elevation.

(iv) An absorption trench system as described in Section 75-A.8(b) is designed using the percolation of the underlying original soil.

(3) Construction. (i) Heavy equipment shall be kept out of the absorption area.

(ii) Fill material is carefully placed within the absorption area.

(iii) The edge of the fill material shall be tapered at a slope of no greater than one vertical to three horizontal. On sloped sites a diversion ditch shall be placed on the uphill side to prevent runoff from entering the fill.

(iv) The absorption trench system is constructed in the fill material, extending into the existing natural soil.

(f) Cut and Fill Systems. (1) A cut and fill system is an absorption trench system installed on sites where impermeable soil overlays a permeable soil.

(2) Site Requirements. Cut and fill systems may be used where all the following conditions are found:

(i) A soil with a percolation rate slower than 60 minutes per inch, such as clay or clay loam, overlays a useable soil with a percolation rate faster than 60 minutes per inch;

(ii) At least three feet of useable soil is available beneath the tight soil;

(iii) All minimum vertical and horizontal separation distances can be maintained as described in Table 2.

(3) Design criteria. (i) It shall provide for the removal of the overlaying unusable soil and replacement by soil having a percolation rate comparable with the underlying soil;

(ii) An absorption trench system is designed as described in Section 75-A.8(b).

(iii) The required length of absorption trench is based upon the percolation of the underlying soil or the fill material, whichever has the slower percolation (lower permeability).

(4) Construction. (i) The area excavated and filled must provide at least a five foot buffer in each direction beyond the trenches.

(ii) The material placed above the trenches shall have a percolation rate faster than 60 minutes per inch.

(iii) Original surface material shall not be used as backfill above the trenches.

(iv) The surface area of the fill system must be mounded and graded to enhance the runoff of rainwater from the system and seeded to grass.

(g) Absorption Bed Systems. (1) General. An absorption bed system operates on a principal similar to the absorption trench except that several laterals, rather than just one, are installed in a single excavation. This reduces the effective sidewall infiltration area per linear foot of lateral or leachline.

(2) Site Requirements. (i) A bed system may be built in soils with a percolation rate between one and 30 minutes per inch. A bed shall not be built where the soil evaluation indicates silty loam, clay loam, or clay.

(ii) Slope of the site shall not exceed eight percent.

(iii) Bed systems are more practical on sites that are long and narrow with a minimal slope.

(iv) All vertical and horizontal separation distance requirements shall be met.

(3) Design Criteria. (i) Pressure distribution is required for the installation of an absorption bed system. The local health department having jurisdiction may allow the use of siphon dosing on specific sites.

(ii) The maximum width of the bed shall be 20 feet. The maximum length of each lateral from a pressure manifold shall be 100 feet. Utilizing a center manifold system, a bed may then have a maximum length of 200 feet. Laterals for siphon dosing systems in beds are limited to 75 feet.

(iii) The depth of the bed shall be between 18 and 30 inches below original ground level.

(iv) Laterals shall be spaced five (5) feet apart. Two and one-half feet (2 1/2') must be provided between the laterals and the sidewalls. In the maximum width of 20 feet, only four laterals may be installed.

(v) Using pressure distribution with a center manifold, a bed system shall have maximum dimensions of 205 feet by 20 feet.

(vi) The required bed bottom area shall be calculated from the application rates shown in Table 5.

(4) Construction. (i) Heavy construction equipment shall be kept outside the proposed bottom area of the bed.

(ii) The required bed bottom area is excavated as level as practical. The bottom and sides of the excavation are hand raked to reduce soil smearing.

(iii) After excavation, a six inch layer of aggregate is placed across the bottom of the bed.

(iv) The laterals are laid level on the aggregate and covered with aggregate to a level two inches above the top of the pipe.

(v) The entire bed area is covered with a permeable geotextile. Untreated building paper or a four inch layer of loose hay or straw may be substituted if a permeable geotextile is unavailable.

(h) Seepage Pits. (1) General. A seepage pit, sometimes called a leaching pit, leaching pool, or incorrectly a cesspool, is a covered pit with an open-jointed or perforated lining through which septic tank effluent seeps into the surrounding soil.

(2) Site Requirements. (i) If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.

(ii) A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.

(3) Design Criteria. (i) The required "effective seepage pit area" is obtained from Tables 6 and 7.

(ii) No allowance for infiltration area is made for the bottom area of a pit or the surface area of impervious soil layers (percolation rate slower than 60 minutes/inch).

(iii) The effective diameter of a pit includes the diameter of the lining plus the added diameter provided by the annular ring of aggregate. Any area surrounding the liner with rock smaller than 2 1/2 inches in size shall not be included as part of the effective diameter.

(iv) Effective depth is measured from the invert of the seepage pit inlet to the floor of the pit, with the thickness of impervious layers deducted.

TABLE 7

SEEPAGE PITS (CYLINDRICAL) - DIMENSIONS FOR

REQUIRED ABSORPTIVE AREA (IN SQUARE FEET)

Note: See Official Compilation for Table 7

(v) Linings may be precast concrete, cast-in-place concrete, or built in place with unmortared hollow cinder or concrete blocks. Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended. Material with comparable structural strength, determined in accordance with commonly accepted sewage construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced.

(vi) The separation between the outside edges of seepage pits shall be three times the effective diameter of the largest pit. This separation is measured as the undisturbed soil between pit excavations.

(vii) Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.

(4) Construction. (i) Laterals leading to each seepage pit must be at least four inches in diameter with a minimum slope of 1/8 inch per foot.

(ii) Seepage pits shall not be connected in series. A distribution box shall be required where more than one seepage pit is installed.

(iii) The pit excavation is to be raked to minimize sidewall smearing that may occur and reduce infiltration capacity. If groundwater is encountered, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum groundwater and adjustments made in the pit dimensions.

(iv) The linings are placed upon a concrete block, poured concrete, or precast footing and surrounded by a six inch minimum annular ring of large aggregate (2 1/12 - 4 inches in size).

(v) The rock is covered to prevent soil from filling the void spaces. Building paper, a four inch thick layer of hay or straw may be used.

(vi) The seepage pit cover shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be precast concrete or cast-in-place and shall be reinforced. A manhole with an opening of at least 20 inches in the shortest dimension shall be provided.

Effective Date: 
Wednesday, February 3, 2010
Doc Status: 
Complete