(1) Septic tanks shall be constructed of sound and durable watertight materials not subject to excessive corrosion, decay, or frost damage, or cracking or buckling due to settlement or backfilling. Septic tanks may be constructed of the following materials:

1. (a) poured-in-place concrete;
2. (b) precast reinforced concrete;
3. (c) fiberglass reinforced plastic;
4. (d) polyethylene; or
5. (e) other materials as approved in writing by the Department.
6. (f) metal septic tanks are prohibited.

(2) Tank construction materials shall meet the following minimum specifications or an ASTM equivalent standard:

1. (a) Concrete

   1. 1. Concrete Strength f'c 4,000 PSI @ 28 days. Density 140 PCF
   2. 2. Cement, Portland Type I or III per ASTM C150-96
   3. 3. Admixtures per ASTM C233-95
   4. 4. Reinforcing per ASTM A615 for wire fabric. Grade 40/60 R'd or equivalent.
   5. 5. Design loading H-10
   6. 6. Minimum wall thickness: four inches; three inch thickness is allowable if reinforced.
   7. 7. The tank shall be watertight.

2. (b) Fiberglass Reinforced Plastic.

   1. 1. Unless otherwise indicated, plastics terminology is in accordance with the definitions given in the most recent edition of ASTM D 883 or equivalent method.
   2. 2. Fiberglass reinforced plastic (FRP) tanks shall be made of a chemical resistant grade of polyester resin reinforced with a suitable grade of glass fiber (E glass) treated with a coupling agent that will provide a compatible bond between the resin and the glass. Glass fiber surfacing materials, if used, shall be of a chemical resistant glass (C glass) bonded with a suitable binder.
   3. 3. The resins shall not contain fillers except as required for viscosity control. Up to 2% by weight of the total resin content of thixotropic agent that will not interfere with visual inspection may be added to the resin for viscosity control.

   4. 4. The tank shall be constructed as a laminate consisting of the following:

      1. a. Primary chemical resistant surface between 0.13 and 0.30 mm of a reinforced resin rich surface.
      2. b. Internal wicking barrier shall be not less than 2.5 mm of chemical resistant laminate next to the inner surface and reinforced by not less than 20% and not more than 30% by weight of mat and chopped strand.
      3. c. Structural layer of chemically resistant construction of minimum ¼ inch thickness meeting a tensile strength of a minimum of 12,000 psi when tested in accordance with the most recent edition of ASTM D-638. It shall meet a flexural strength minimum of 19,000 psi and flexural modulus of elasticity of 800,000 psi both of which tested in accordance with the most recent edition of D-790.
      4. d. External wicking barrier as in 310 CMR 15.226(2)(b)4.b.
      5. e. Exterior surface layer between 0.13 and 0.30 inch thick consisting of resin. No glass fibers are allowed in this layer.
   5. 5. All cut edges shall be treated and shall be coated with resin so that no glass fibers are exposed and all voids are filled. Structural elements having edges exposed to the chemical environment shall be made with chopped strand glass reinforcement only.
   6. 6. The tank laminate shall have a Barcol Hardness of at least the manufacturers minimum specified hardness for cured resins when tested in accordance with the most recent edition of ASTM D-2583.
   7. 7. The tank shall be free from visual defects. The most recent edition of ASTM D-2563 visual acceptance level 3 shall be the minimum standard for acceptance.
   8. 8. The tank shall be watertight.

3. (c) Polyethylene.

   1. 1. The polyethylene used shall be Type II or III and category three per the most recent edition of ASTM D-1248, Class B (requiring an ultraviolet stabilizer) or Class C (requiring a minimum of 1% carbon black), and shall have a stress crack resistance of 150 hours, Condition C (20% failure rate) when measured in accordance with the most recent edition of ASTM D-1693 and ASTM-638 with a value of equal to or greater than 2400 psi; ASTM-790, with a flexural modulus of elasticity equal to or greater than 85,000 psi.
   2. 2. Wall thickness of the sides, top and bottom shall be a minimum of ¼ inch. The thickness of the inlet and outlet ends shall be at least ¼ inch and the thickness of internal walls shall be at least 3/16 inches.
   3. 3. The tank shall be watertight.