Tenn. Comp. R. & Regs. 0400-48-01-.07 - DESIGN OF THE CONVENTIONAL DISPOSAL FIELD

(1) The size of the conventional subsurface sewage disposal system shall be determined by the following:

(a) The suitability of the site shall be determined by a high or extra-high intensity soil map completed by an approved soil consultant and other criteria established by these regulations. The soil absorption rates that range from ten (10) through seventy-five (75) minutes per inch are acceptable. Soil absorption rates for soil series and phases of soil series that are established in Appendix I shall apply. The absorption rates for soil series and phases that have been recognized by the Natural Resource Conservation Service in Tennessee, but not listed in Appendix I shall be established by the Department. Rates for soil variants and miscellaneous land types may be established by an approved soil consultant, but may require approval by the Department.

(b) Where percolation tests are conducted the size of the subsurface sewage disposal system shall be determined by the rate found in Appendix II. The minimum square footage of trench bottom installed per bedroom shall be three hundred seventy (370).

(c) On individual lots where the Commissioner determines site suitability, an estimated soil absorption rate up to and including seventy-five (75) minutes per inch may be established. The size of the conventional subsurface sewage disposal system shall be determined by the rate found in Appendix II.

(d) Soils with absorption rates greater than seventy-five (75) minutes per inch as determined by a soil evaluation shall be considered unsuitable for conventional subsurface sewage disposal. Soils with percolation rates less than one hundred six (106) minutes per inch may be used for conventional systems under authority of T.C.A. § 68-221-403(c)(1).

(2) Where conventional subsurface sewage disposal systems are installed, sufficient additional area must be available for the expansion of the disposal field in an amount large enough to install a secondary subsurface sewage disposal field as defined by these regulations.

(3) Design of the disposal field shall be of the recirculating (level lot) or serial distribution type or a modification of either, depending on the characteristics of the site.

(a) Recirculating Design - A recirculating design provides equal distribution of the effluent throughout the entire system by connecting successive trenches on both ends and by maintaining the grade in the bottom of these trenches from level to no more than four (4) inches. In this manner, the entire absorption area within the sewage system is utilized concurrently.

(b) Serial Distribution - In serial distribution, each adjacent trench is connected to the next by a closed pipe laid on an undisturbed section of ground. The arrangement is such that all effluent is discharged to the first trench until it is filled. Excess liquid is then carried by means of a closed line (relief line) to the next trench. In this manner, each portion of the subsurface system is used in succession.

(c) Large Conventional Systems - Those exceeding seven hundred fifty (750) gallons of estimated daily flow volume.

1. When the estimated daily flow volume exceeds seven hundred fifty (750) gallons for a single system, a properly designed dosing system shall be used for discharging septic tank effluent into the disposal field. The Commissioner shall require design plans by a licensed engineer. The Commissioner may also require:

(i) Prior to design approval, a licensed engineer must agree, in writing, to monitor the installation and construction of the system and upon completion, provide a final set of construction as-built plans encompassing all components of the system and certification that the installation is in accordance with the design specifications.

(ii) Prior to design approval, it shall be the responsibility of the Department to review the aforementioned design and notify the engineer, in writing, of approval of the plans, denial of the plans or needed modifications to the plans.

2. When the design daily flow from a single source exceeds three thousand (3,000) gallons per day, siphons or pumps shall be used which shall discharge to separate disposal fields. Each system shall not exceed a design capacity of three thousand (3,000) gallons per day.

3. Discharges from dosing systems shall be designed to maximize the distribution of the effluent throughout the system.

4. Buffer zones shall be required at a frequency and size as determined by a detailed soil/site evaluation.

(4) Construction Procedure for Disposal Field

(a) The pipe size from the septic tank to the disposal field or to the dosing chamber shall not be less than three (3) inches in diameter (inside measurement) and shall be functionally equivalent to Schedule 40 PVC except when Rule 0400-48-01-.15 applies. The pipe from the septic tank to the disposal field shall be of sufficient length to rest on undisturbed earth.

(b) Materials and equipment used in subsurface sewage disposal systems shall be those which have specifications outlined in these regulations.

(c) The disposal field trenches shall follow the ground surface contours so that variations in trench depth will be minimized.

(d) A minimum of six (6) feet of undisturbed earth between adjacent trench walls shall be required.

(e) Adjacent trenches in a serial distribution system shall be connected with a relief line in such a manner that each trench is completely filled with septic tank effluent to the full depth of the media before effluent flows to a succeeding trench.

(f) In constructing relief lines, care must be exercised to insure that an undisturbed block of earth remains between trenches. The trench for the relief pipe, where it connects with the preceding absorption trench, shall be dug no deeper than the top of the media. The relief line shall rest on undisturbed earth and backfill must be carefully tamped. This section pertains primarily to a serial distribution system. Pipe for relief lines shall have no perforations and shall have a minimum inside diameter of three (3) inches and shall be Schedule 40 PVC or functionally equivalent. All couplings and/or connections must be accomplished with materials/fittings manufactured specifically for Schedule 40 PVC or functionally equivalent to the point of connection with the pipe in the disposal field and shall provide a secure connection. The lengths of pipe used for relief lines shall not be considered part of the required absorption area.

(g) The influent and effluent relief lines in each individual trench shall be as far from each other as practical in order to prevent short circuiting.

(h) Invert of the overflow pipe in the first relief line shall be at least four (4) inches lower than the invert of the septic tank outlet.

(i) Trenches shall not be excavated when the soil is wet enough to smear or compact easily.

(j) Media for the disposal fields shall consist of crushed rock, gravel or other suitable material as approved by the Department, and shall be size number 2, 3, 4 or 24 coarse aggregate, as defined by ASTM D-448-86. The material shall be free from dust, sand, clay or excessive fines. At least ninety (90) percent of the material must pass a two and one-half (2 1/2) inch screen and not more than five (5) percent may pass a one-half (1/2) inch screen.

(k) Media for the disposal fields shall extend from at least two (2) inches above the top of the perforated field line pipe to at least six (6) inches below the bottom of the perforated field line pipe to achieve a minimum of twelve (12) inches total, except for systems designed pursuant to T.C.A. § 68-221-403(i).

(l) The top of the disposal field media shall be below the invert of the septic tank outlet.

(m) The media for the disposal fields shall be covered with untreated building paper, a layer of straw at least two (2) inches thick or other material determined to be equivalent by the Commissioner.

(n) Soil material excavated from trenches should be used in backfilling and should be left mounded over the trenches until initial settling has taken place.

(o) There shall be a minimum of twelve (12) inches of ground cover over the disposal field media and the maximum shall be thirty-six (36) inches of fill, except for systems designed pursuant to T.C.A. § 68-221-403(i).

(p) The bottom of the trenches and the distribution lines shall have a grade from level to no greater than four (4) inches.

(q) All pipes (tight lines) under paved areas or under driveways shall be Schedule 40 PVC or functionally equivalent and have a minimum inside diameter of three (3) inches. The lengths of pipe used for relief lines and tight lines shall not be considered part of the required absorption area.

(r) The pipe used in the disposal field trenches shall have a minimum diameter of four (4) inches, be perforated with one-half (1/2) inch holes and conform to or exceed the standards of the most current version of ASTM F-405.

(s) The area of the disposal field shall not be used for vehicular traffic or vehicular parking. Dozers, trucks and other heavy vehicles shall not be allowed to run over the septic tank, field lines or other parts of the system.

(t) Systems with electrical components must obtain an electrical inspector's approval associated with the subsurface sewage disposal system electrical component(s) prior to being considered for construction inspection.

(u) The maximum depth of a trench shall be forty-eight (48) inches. The minimum depth shall be twenty-four (24) inches, except for systems designed pursuant to T.C.A. § 68- 221-403(i).

(v) The area of the disposal field shall not be used for any underground utilities.

(w) A septic tank must not be bypassed by direct line (laundry, grease, etc.) to field line.

(x) Down-spouts shall not be connected to the subsurface sewage disposal system. Down-spouts or other surface water shall be diverted away from the subsurface sewage disposal system.

(y) Water lines shall not cross, pass through, or go under the subsurface sewage disposal field. Water lines may cross, but not be located in the same trench with, a tight line leading from a septic tank or dosing tank to a disposal field provided the water line is sleeved in a continuous twenty (20) feet section of Schedule 40 PVC pipe or equivalent (a minimum of ten (10) feet on either side of the tight line) and the water line is a minimum of one (1) foot vertically above the tight line.

(5) No system shall be covered without the inspection and authorization of the Commissioner.

(6) Conventional Substitute Products

(a) A Large Diameter Gravelless Pipe (LDGP) System is a subsurface sewage disposal system that has one (1) basic design principle different from conventional subsurface sewage disposal systems, which is that an eight (8) or ten (10) inch inside diameter corrugated polyethylene perforated pipe is used for the storage and distribution of effluent in a trench in lieu of a four (4) inch diameter pipe and gravel. A filter, fabric wrap around the pipe prevents soil infiltration into the pipe; and prevents small, suspended solids from moving out of the pipe.

LDGP systems exceeding seven hundred fifty (750) gallons of estimated daily flow volume shall meet the minimum requirements established in subparagraph (3)(c) of this rule.

1. Site and Soil Requirements

(i) The site and soil requirements are the same as for a conventional subsurface sewage disposal system. Where the soil absorption rate exceeds sixty (60) minutes per inch, gravel backfill, leveled with the top of the LDGP, shall be required in accordance with subparagraphs (4)(j) and (m) of this rule and a trench width of twenty-four (24) inches.

(ii) An area of suitable soil must be available equivalent in size to that necessary to install and duplicate a conventional subsurface sewage disposal system.

(iii) The size of the LDGP system shall be determined by the following:

(I) The suitability of the site shall be determined by a high or extra-high intensity soil map completed by an approved soil consultant and other criteria established by these regulations. The soil absorption rates that range from ten (10) through seventy-five (75) minutes per inch are acceptable. Soil absorption rates for soil series and phases of soil series that are established in Appendix I shall apply. The absorption rates for soil series and phases that have been recognized by the Natural Resource Conservation Service in Tennessee, but not listed in Appendix I shall be established by the Department. Rates for soil variants and miscellaneous land types may be established by an approved soil consultant, but may require approval by the Department. The rate found in Table I shall determine the size of the LDGP system.

(II) On individual lots where the Commissioner determines site suitability, an established soil absorption rate up to and including seventy-five (75) minutes per inch may be established. The size of the LDGP system shall be determined by the rate found in Table I.

(III) Where percolation tests are required to determine the absorption rate, the size of the LDGP system shall be determined by the rate found in Table I (370 linear foot minimum).

Table I

Soil Absorption Rates and Corresponding Trench Length Requirements for LDGP Systems

Absorption Rate (mpi)	Trench Length (ft/gal) 8 in Pipe 10 in Pipe		Trench Length (ft/bedroom) 8 in Pipe 10 in Pipe
10	0.600	0.400	83	55
15	0.700	0.467	95	64
30	1.000	0.667	125	84
45	1.250	0.834	150	100
60	1.450	0.967	165	110
75	1.600	1.067	185	124
80	1.650	1.100	190	126
85	1.700	1.134	195	130
90	1.750	1.167	200	134
95	1.800	1.200	208	139
100	1.850	1.234	215	144
105	1.900	1.267	223	149

Examples:

The soil absorption rate is thirty (30) minutes per inch. A three (3) bedroom home is to be located on this site. If an eight (8) inch inside diameter pipe is used:

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The soil absorption rate is ninety-five (95) minutes per inch. A four (4) bedroom house is to be located on this site. If a ten (10) inch inside diameter pipe is used:

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The soil absorption rate is forty-five (45) minutes per inch. A commercial building generating five hundred (500) gallons of wastewater per day is to be located on this site. If a ten (10) inch inside diameter pipe is used:

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The soil absorption rate is eighty-five (85) minutes per inch. An industrial plant generating three hundred seventy-five (375) gallons of wastewater per day is to be located on this site. If an eight (8) inch inside diameter pipe is used:

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2. Layout of the LDGP System

(i) The linear footage required is determined from Table I.

(ii) The location of the septic tank and the disposal field shall be in accordance with Rule 0400-48-01-.11.

(iii) The lateral lines shall be placed on contour. The maximum length of a single line should not exceed one hundred (100) feet unless conditions require a longer line.

(iv) The trench bottom of each lateral shall have a grade from level to no greater than two (2) inches per one hundred (100) feet.

(v) A minimum of six (6) feet of undisturbed earth between adjacent trench walls shall be required.

(vi) Trench width shall be a maximum of eighteen (18) inches when the soil absorption rate is sixty (60) minutes per inch or less. Trench width shall be a minimum of twenty-four (24) inches with gravel backfill when the soil absorption rate exceeds sixty (60) minutes per inch.

(vii) Trench depth shall range between twenty-two (22) and forty-eight (48) inches for eight (8) inch pipe and twenty-four (24) and forty-eight (48) inches for ten (10) inch pipe.

(viii) The large diameter pipe shall be positioned in the trench so that the top location stripe is on top. Sections of pipe shall be securely joined and the filter wrap must cover all joints and the ends of each line. All couplings and/or connections must be accomplished with material/fittings manufactured specifically for large diameter gravelless pipe. Where a supply or tight line ties into the side of a large diameter gravelless pipe, the supply line or tight line shall not penetrate the pipe more than two (2) inches.

(ix) Soil material excavated from trenches should be used in backfilling and should be left mounded over the trenches until initial settling has taken place.

(x) The invert of the overflow pipe in the first relief line shall be at least four (4) inches lower than the invert of the septic tank outlet. The trench for the relief line, where it connects with the preceding LDGP, shall be dug no deeper than the top of the LDGP. In a recirculating design, the top of the LDGP must be a minimum of one (1) inch below the invert of the septic tank outlet.

3. Equipment and Material Specifications

(i) Septic Tank - Rules 0400-48-01-.08 and 0400-48-01-.09 shall apply.

(ii) Pipe -

(I) The eight (8) and ten (10) inch inside diameter tubing shall be corrugated polyethylene, meeting the requirements of ASTM F667, Standard Specification for Large Diameter Corrugated Polyethylene Tubing.

(II) Perforations shall be cleanly cut and uniformly spaced along the length of the tubing as follows: two (2) rows of three-eighths (3/8) to one-half (1/2) inch diameter holes located 115° - 125° apart along the bottom half of the tubing with each row of holes 57.5° - 62.5° up from the bottom centerline. These perforations should be staggered so that there is only one (1) hole in each corrugation. Perforations must be located in the minimum diameter portions of the pipe.

(III) The tubing shall be marked with an easily visible top location stripe.

(iii) Filter Wrap - All large diameter pipe shall be encased with a spun bonded nylon, or other material of similar strength and durability. If the filter wrap is installed at the point of manufacture, then the corrugated pipe and filter wrap shall be shipped in a protective covering that will prevent damage to the filter wrap. This wrap shall meet or exceed the following general qualities:

Physical Properties	Minimum Values
Weight	0.75 oz/yd2
Thickness	4.4 mm
Grab Strength Machine Direction	19 lbs
Transverse Direction	11 lbs
Burst strength	26 psi
Air Permeability	500 cfm/ft2
Water Flow Rate (3 ft head pressure)	200 gpm/ft2

(b) A Chamber System is a media replacement system consisting of a plastic arch-shaped open bottomed chamber. Chamber systems that exceed seven hundred fifty (750) gallons of estimated daily flow volume shall meet the minimum requirements established in subparagraph (3)(c) of this rule.

1. Site and Soil Requirements

(i) The site and soil requirements are the same as for a conventional subsurface sewage disposal system.

(ii) An area of suitable soil must be available equivalent in size to that necessary to install and duplicate a conventional subsurface sewage disposal system.

2. Layout of the Chamber System

(i) The size of a chamber system shall be equivalent to the total linear footage required for a three (3) feet wide conventional subsurface sewage disposal system. However, where the soil absorption rate is from ten (10) to sixty (60) minutes per inch, the total linear footage may be reduced by thirty (30) percent. If a portion of a unit is left over after determining the total linear footage required, round up to the nearest whole unit.

(ii) The location of the septic tank and the disposal field shall be in accordance with Rule 0400-48-01-.11.

(iii) The lateral lines shall be placed on contour. The maximum length of a single line should not exceed one hundred (100) feet unless conditions require a longer line.

(iv) The trench bottom of each lateral shall have a grade from level to no greater than two (2) inches per lateral line.

(v) A minimum of six (6) feet of undisturbed earth between adjacent trench walls shall be required.

(vi) Trench width shall be no more than six (6) inches wider than the product width, with the maximum trench width being thirty-six (36) inches.

(vii) Trench depth shall range from twenty-four (24) to forty-eight (48) inches.

(viii) Soil material excavated from trenches should be used in backfilling and should be left mounded over the trenches until initial settling has taken place.

(ix) The top of the chambers shall be below the invert of the septic tank outlet.

(x) The trench for the relief pipe, where it connects with the preceding absorption trench, shall be dug no deeper than the invert of the opening in the end plate.

3. Equipment and Material Specifications

(i) Septic Tank - Rules 0400-48-01-.08 and 0400-48-01-.09 shall apply.

(ii) Chamber -

(I) Chambers of varying heights and widths may be approved on an individual basis by the Director of the Division of Water Resources.

(II) Each chamber must be designed to interlock with adjacent chambers, inlet plate or end plate forming a complete disposal trench that consists of an inlet plate and a solid end plate to be located at the distal end of each terminal trench.

(III) The chamber sidewall must be designed to allow effluent to pass laterally into the soil.

(IV) Chambers shall be constructed of materials meeting the material property requirements of the International Plumbing and Mechanical Officials (IAPMO) Property Standard 63-2005.

(c) Expanded Polystyrene

Expanded polystyrene (EPS) synthetic aggregate systems are cylinders comprised of EPS aggregate contained in high strength polyethylene netting. Cylinders may vary in diameter and are produced both with and without a distribution pipe.

1. Site and Soil Requirements

(i) The site and soil requirements are the same as for a conventional subsurface sewage disposal system.

(ii) An area of suitable soil must be available equivalent in size to that necessary to install and duplicate a conventional subsurface sewage disposal system.

2. Layout of the EPS System

(i) The size of an EPS system shall be equivalent to the total linear footage required for a three (3) feet wide conventional subsurface sewage disposal system. However, where the soil absorption rate is from ten (10) to sixty (60) minutes per inch, the total linear footage may be reduced by thirty (30) percent for approved EPS product configuration totaling 36 inches in width. If a portion of a unit is left over after determining the total linear footage required, round up to the nearest whole unit.

(ii) The location of the septic tank and the disposal field shall be in accordance with Rule 0400-48-01-.11.

(iii) The lateral lines shall be placed on contour. The maximum length of a single line should not exceed one hundred (100) feet unless conditions require a longer line.

(iv) The trench bottom of each lateral shall have a grade from level to no greater than two (2) inches per lateral line.

(v) A minimum of six (6) feet of undisturbed earth between adjacent trench walls shall be required.

(vi) Trench width shall be no more than six (6) inches wider than the product width, with the maximum trench width being thirty-six (36) inches.

(vii) Trench depth shall range from twenty-four (24) to forty-eight (48) inches.

(viii) Soil material excavated from trenches should be used in backfilling and should be left mounded over the trenches until initial settling has taken place.

(ix) The top of the EPS bundles shall be below the invert of the septic tank outlet.

(x) The trench for the relief pipe, where it connects with the preceding absorption trench, shall be dug no deeper than the top of the EPS bundles.

(xi) All couplings and/or connections must be accomplished with material/fittings manufactured specifically for EPS products.

(xii) The EPS bundles shall be covered with untreated building paper or other material determined to be equivalent by the Commissioner.

3. Equipment and Material Specifications

(i) Septic Tank - Rules 0400-48-01-.08 and 0400-48-01-.09 shall apply.

(ii) EPS Product -

(I) Each EPS product configuration shall be approved independently.

(II) Each EPS bundle responsible for assuring effluent distribution must be designed to connect to adjacent bundles.

Notes

Tenn. Comp. R. & Regs. 0400-48-01-.07

Original rule filed June 20, 2013; effective September 18, 2013. Rule renumbered from1200-01-06.

Authority: T.C.A. §§ 68-221-401 et seq. and 4-5-201 et seq.