The size of
the conventional subsurface sewage disposal system shall be determined by the
(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.
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).
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
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
Systems - Those exceeding seven hundred fifty (750) gallons of estimated daily
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
3. Discharges from dosing
systems shall be designed to maximize the distribution of the effluent
throughout the system.
zones shall be required at a frequency and size as determined by a detailed
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
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
(d) A minimum of six (6)
feet of undisturbed earth between adjacent trench walls shall be
(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
(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.
shall not be excavated when the soil is wet enough to smear or compact
(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
(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.
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
(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.
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