Rules & Regulations of the State of Tennessee
Title 0400 - Environment and Conservation
Subtitle 0400-48 - Water Resources (Groundwater Protection)
Chapter 0400-48-01 - Regulations To Govern Subsurface Sewage Disposal Systems (Transferred from 1200-01-06)
Section 0400-48-01-.07 - DESIGN OF THE CONVENTIONAL DISPOSAL FIELD
Universal Citation: TN Comp Rules and Regs 0400-48-01-.07
Current through September 24, 2024
(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:
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:
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:
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:
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.
Authority: T.C.A. §§ 68-221-401 et seq. and 4-5-201 et seq.
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