Tenn. Comp. R. & Regs. 0400-48-01-.09 - DESIGN OF SEPTIC TANKS
(1) A septic tank shall be watertight, structurally sound, and not subject to excessive corrosion or decay. Septic tanks installed after January 1, 1991 shall be of two (2) compartment design. The inlet compartment of a two (2) compartment tank shall be between two-thirds (2/3) and three-fourths (3/4) of the total tank capacity.
(2) Minimum standards of design and construction of precast reinforced concrete septic tanks:
(a) The liquid depth may range from thirty (30) to sixty (60) inches for septic tanks of less than three thousand (3000) gallons capacity and may not exceed seventy-eight (78) inches for septic tanks with a capacity of three thousand (3000) gallons or greater.
(b) All tanks shall be manufactured with a partition so that the tank contains two (2) compartments. The partition shall be located at a point not less than two-thirds (2/3) nor more than three-fourths (3/4) the length of the tank from the inlet end. The tank wall thickness must remain not less than two and one-half (2 1/2) inches thick throughout the tank except for blockouts or the groove for a slide-in partition. The groove for the slide-in partition shall leave a concrete thickness of not less than two and one-fourth (2 1/4) inches in the tank walls. The partition shall be structurally sound and not subject to excessive corrosion or decay.
(c) There shall be three (3) blockouts in the inlet compartment, one (1) on the tank end and one (1) on each sidewall, and a blockout in the partition and the outlet end of the tank. The blockouts for these openings shall leave a concrete thickness of not less than one (1) inch in the tank wall. The blockouts shall be made for a minimum of four (4) inch pipe or a maximum of six (6) inch pipe. In lieu of the partition wall blockout, a four (4) to six (6) inch slot extending at least half way across the width of the tank may be used. The top of the slot shall be located no closer than twelve (12) inches to the liquid level and the bottom of the slot shall be no lower than four (4) inches below the midpoint of the liquid depth. A four (4) inch diameter, or equivalent, air passage opening in the partition shall be provided above the liquid level of the tank.
(d) The tees or baffles shall be a minimum diameter of either three (3) inch cast iron soil pipe tee branch, three (3) inch cast iron sanitary tee branch, three (3) inch cast-in-place baffle, or three (3) inch PVC tee branch or equivalent in durability and performance as determined by the Director of the Division of Water Resources.
(e) The inlet invert shall enter the tank at least one (1) inch, preferably three (3) inches, above the liquid level of the tank.
(f) An inlet tee or baffle shall be provided to divert the incoming sewage downward and extend at least twelve (12) inches below the liquid level.
(g) The partition tees or baffles and outlet tee or baffle shall extend eighteen (18) inches or one-third (1/3) the liquid depth, whichever is the lesser, below the liquid level of the tank. A tee or baffle shall be provided on the first compartment side of the partition at the same elevation as the outlet tee or baffle unless an intercompartmental connecting slot is utilized as described in subparagraph (c) of this paragraph.
(h) Air space equal to at least twenty (20) percent of the liquid depth shall be provided between the top of the tank and the liquid level.
(i) Adequate access openings above each tee or baffle must be provided in the tank top. Access shall be provided for cleaning or rodding out of the inlet pipe, and the interconnecting tees or baffles in the partition, for inserting the suction hose for tank pumping, and for entrance of a person if internal repairs are needed after pumping. If the knockouts on the inlet compartment sides of the tank are to be used access to these tees or baffles shall also be provided for cleaning and rodding of the inlet pipe. To accomplish this, it may be necessary to extend the tee so they will be located under an access port or a cleanout must be provided on the inlet line immediately outside the septic tank. A manhole opening shall be provided to each compartment with each having a minimum opening of eighteen (18) inches by eighteen (18) inches as the opening cuts the plane of the bottom side of the top of the tank. All circular shaped manholes shall have a minimum diameter of twenty (20) inches as the opening cuts the plane of the bottom side of the top of the tank. The manhole covers shall be beveled on all sides in such manner as to accommodate a uniform load of one hundred fifty (150) pounds per square foot without damage to the cover or the top of the tank. If the top of the tank is to be multislab construction, the slabs over the inlet of the tank, partition, and outlet of the tank must not weigh in excess of one hundred fifty (150) pounds each. Multislab construction must allow for a minimum access hole size of six (6) inches by six (6) inches to each compartment. Manhole covers, opening covers, or slabs shall have a handle of steel or other corrosion resistant material equivalent in strength to a No. 3 reinforcing rod (rebar).
(j) The tank shall be properly vibrated and rodded prior to curing to eliminate honeycomb.
(k) The top, bottom, ends, and sides of the tank must have a minimum thickness of two and one-half (2 1/2) inches except for blockouts or the slide in partition groove.
(l) After curing, tanks manufactured in two (2) sections shall be joined and sealed at the joint by the manufacturer by using a mastic sealant and/or pliable sealant that is both waterproof and corrosion resistant.
(m) Tank Labeling - Precast septic tanks shall be provided with a suitable legend, cast or etched in the wall at the outlet end and within six (6) inches of the top of the tank, identifying the manufacturer by name and address or registered trademark and indicating the liquid capacity of the tank in gallons.
(3) Plans for prefabricated tanks, other than those for precast reinforced concrete tanks, shall be approved by the Department on an individual basis as determined by the information furnished by the designer which indicates the tank will provide equivalent effectiveness as those designed in accordance with the provisions of paragraph (2) of this rule.
(4) Septic tanks other than approved prefabricated tanks shall be constructed consistent with the provisions of paragraph (2) of this rule except as follows:
(a) Cast-in-place concrete septic tanks shall have a minimum wall thickness of six (6) inches.
(b) Cast-in-place septic tanks of one thousand (1000) gallons or less shall have a minimum top and bottom thickness of four (4) inches.
(c) Cast-in-place concrete septic tanks with a capacity of greater than one thousand (1000) gallons shall have a minimum top and bottom thickness of six (6) inches.
Authority: T.C.A. §§ 68-221-401 et seq. and 4-5-201 et seq.
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