North Carolina Administrative Code › Title 15A - Environmental Quality › Chapter 18 - ENVIRONMENTAL HEALTH › Subchapter A - SANITATION › Section .1900 - SEWAGE TREATMENT AND DISPOSAL SYSTEMS › Section 18A .1954 - MINIMUM STANDARDS FOR PRECAST REINFORCED CONCRETE TANKS

Current through Register Vol. 39, No. 2, July 15, 2024
(a) The following are minimum standards of
				  design and construction of precast reinforced concrete septic tanks:
(1) The minimum requirement for the liquid
				  depth is 36 inches.
(2) A minimum
				  of nine inches freeboard is required, the freeboard being the air space between
				  the top of the liquid and the bottom side of the lid or cap of the
				  tank.
(3) The length of the septic
				  tank shall be at least twice as long as the width.
(4) There shall be three inlet openings in
				  the tank, one on the tank end and one on each sidewall of the inlet end of the
				  tank. The blockouts for these openings shall leave a concrete thickness of not
				  less than one inch in the tank wall. The blockouts shall be made for a minimum
				  of four-inch pipe or a maximum of six-inch pipe. The outlet pipe penetration of
				  the tank shall be through a resilient, watertight, sealed, non-corrosive and
				  flexible connective sleeve. The outlet pipe penetration shall be precast to be
				  compatible with the connective sleeve. No pipe penetration points or openings
				  shall be permitted below the tank liquid level.
(5) The inlet pipe in the tank shall be a
				  straight pipe.
(6) The outlet shall
				  be through an approved effluent filter secured in place in an effluent filter
				  support case. The effluent filter support case shall serve as a functioning
				  sanitary tee with the bottom inlet extending down between 25 and 40 percent of
				  the liquid depth. The approved effluent filter and support case shall be
				  furnished by the septic tank manufacturer. The invert of the outlet shall be at
				  least two inches lower in elevation than the invert of the inlet.
(7) Other equivalent methods of supporting
				  the effluent filter and for making the pipe penetrations shall be approved by
				  the On-Site Wastewater Section.
(8)
				  In order to obtain approval of an effluent filter, the filter manufacturer
				  shall submit to the State the following information with supporting
				  documentation:
(A) For each septic tank system
				  that is designed to treat 3,000 gallons per day or less of sewage, a written
				  certification that the effluent filter is designed, constructed, and performs
				  in compliance with
				  G.S.
				  130A-335.1(a)(1)(2)(3), and
				  (4);
(B) Sizing as to capacity and wastewater
				  strength for all models of proposed filters to be approved; and
(C) Specifications for application,
				  installation, operation, and maintenance.
(9) All tanks shall be manufactured with a
				  cast-in-place partition so that the tank contains two compartments. The
				  partition shall be located at a point not less than two-thirds nor more than
				  three-fourths the length of the tank from the inlet end. The top of the
				  partition shall terminate two inches below the bottom side of the tank top in
				  order to leave space for air or gas passage between compartments. The top and
				  bottom halves of the partition shall be cast in such manner as to leave a water
				  passage slot four inches high for the full width of the tank. The partition
				  (both halves) shall be reinforced by the placing of six-inch by six-inch No. 10
				  gage welded reinforcing wire. The reinforcing wire shall be bent to form an
				  angle of 90 degrees on the ends in order to form a leg not less than four
				  inches long. When the wire is placed in the mold the four-inch legs should lay
				  parallel with the sidewall wire and adjacent to it. It is recognized that there
				  are other methods of constructing a partition or two-compartment tank. Any
				  method other than the one described will be considered on an individual basis
				  for approval by the On-Site Wastewater Section. However, the tank wall
				  thickness must remain not less than two and one-half inches thick throughout
				  the tank except for the pipe penetrations.
(10) Adequate access openings must be
				  provided in the tank top. Access shall be provided for cleaning or rodding out
				  of the inlet pipe, for cleaning or clearing the air or gas passage space above
				  the partition, for pumping of each compartment, and for the maintenance of the
				  effluent filter. This shall be accomplished by properly locating two manholes
				  or access openings with each having a minimum opening of 15 inches by 15 inches
				  or 17 inches in diameter as the opening cuts the plane of the bottom side of
				  the top of the tank or other equidimensional opening with at least 225 square
				  inches. The manhole covers shall be beveled on all sides in such manner as to
				  accommodate a uniform load of 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 150 pounds each. Multislab construction
				  allows for the elimination of the manholes. Manhole covers, tank lids, access
				  opening covers, or slabs shall have a handle of steel or other rot-resistant
				  material equivalent in strength to a No. 3 reinforcing rod (rebar).
(11) The concrete tank and tank lid shall be
				  reinforced by using a minimum reinforcing of six-inch by six-inch No. 10 gage
				  welded steel reinforcing wire in the top, bottom ends, and sides of the tank.
				  The reinforcing wire shall be lapped at least six inches. Concrete cover shall
				  be required for all reinforcement. Reinforcement shall be placed to maximize
				  the structural integrity of the tank. The tank, tank lid, riser and riser cover
				  shall be able to withstand a uniform live loading of 150 pounds per square foot
				  in addition to all loads to which an underground tanks, riser, or riser cover
				  is normally subjected, such as the dead weight of the concrete and soil cover,
				  active soil pressure on tank walls, and the uplifting force of the ground
				  water. Additional reinforcement shall be required when the loads on a concrete
				  tank, riser, or riser cover are exceeded by subjecting it to vehicular traffic
				  or when the top of the tank is placed deeper than three feet below the finished
				  grade.
(12) The top, bottom, ends,
				  and sides of the tank must have a minimum thickness of two and one-half
				  inches.
(13) A minimum 28-day
				  concrete compressive strength of 3,500 pounds per square inch shall be used in
				  the construction of the septic tank, concrete access riser and riser cover. The
				  concrete shall achieve a minimum compressive strength of 3,000 pounds per
				  square inch prior to removal of the tank from the place of manufacture. It
				  shall be the responsibility of the manufacturer to certify that this condition
				  has been met prior to shipment. A septic tank shall be subject to testing to
				  ascertain the strength of the concrete prior to its being approved for
				  installation. Recognized devices for testing the strength of concrete include a
				  properly calibrated Schmidt Rebound Hammer or Windsor Probe Test. Accelerated
				  curing in the mold by use of propane gas or other fuels is prohibited, except
				  in accordance with accepted methods and upon prior approval of the
				  State.
(14) After curing, tanks
				  manufactured in two sections and as required, concrete risers shall be joined
				  and sealed at the joint by using a mastic, butyl rubber, or other pliable
				  sealant that is waterproof, corrosion-resistant, and approved for use in septic
				  tanks. The sealant shall have a minimum size of one inch nominal diameter or
				  equivalent. Before sealing, the joint shall be smooth, intact, and free of all
				  deleterious substances. Tank halves shall be properly aligned to ensure a tight
				  seal. The sealant shall be provided by the manufacturer.
(15) All tanks produced shall bear an imprint
				  identifying the manufacturer, the serial number assigned to the manufacturer's
				  plans and specifications approved by the State, and the liquid or working
				  capacity of the tanks. This imprint shall be located to the right of the
				  blockout made for the outlet pipe on the outlet end of the tank. All tanks
				  shall also be permanently marked with the date of manufacture adjacent to the
				  tank imprint or on the top of the tank directly above the imprint.
(16) Risers and access covers shall have a
				  clear opening sized to allow for maintenance and removal of internal devices of
				  the septic tank and shall not allow accidental entry. The access cover and tank
				  lid shall be designed, constructed, and maintained to prevent unauthorized
				  access. Risers shall be sealed watertight where they join the top of the septic
				  tank, and constructed to prevent water inflow through the lid or
				  cover.