Rule 15-18-77-5.1.4 - Design All septic tanks (prefabricated concrete, steel, fiberglass or polyethylene) must be designed according to minimum standards as follows:
1. General a. The septic tank shall be watertight, structurally sound and not subject to excessive corrosion or decay. The outlet of the septic tank should be placed so as not to be located below the Seasonal High Water Table.b. The minimum hydraulic detention time of the septic tank must be two (2) days (48 hours) based on daily sewage flows. In no case shall the septic tank have a minimum effective liquid capacity of less than seven hundred fifty (750) gallons. c. All tanks manufactured in two (2) sections must have an interlocking type joint. Tanks manufactured in two sections must be sealed and joined with an approved sealant such as butyl rubber or other approved pliable sealant that is waterproof, corrosion-resistant and is warranted by the manufacturer for sealing concrete septic tanks.d. All septic tanks with a capacity of greater than fifteen hundred (1,500) gallons shall be deemed structurally sound by a licensed Professional Engineer via stamped letter. 2. Tank Dimensions a. The inside length of a rectangular septic tank shall be a minimum of 1.5 times the width. The minimum inside width of a septic tank shall not be less than 3.5 feet.b. The minimum liquid depth of all septic tanks shall be thirty (30) inches.c. A minimum air space of seventeen (17) percent of the liquid depth must be provided.3. Tank Inlet and Outlet a. The inlet and outlet of the septic tank must be large enough to accommodate a four (4) inch schedule forty (40) pipe and be equipped with a sanitary tee or baffle.b. The inlet and outlet pipes must extend a minimum of three (3) feet onto undisturbed soil before entering and after exiting the septic tank.c. The inlet invert shall enter the septic tank a minimum of two (2) inches above the liquid level of the tank. The inlet tee or baffle shall be provided to divert the incoming sewage downward and extend a minimum of six (6) inches below the liquid level of the tank.d. The outlet tee or baffle shall extend eighteen (18) inches below the liquid depth of the tank.4. Baffle Walls and Two Compartment Tanks a. The first compartment shall be between sixty (60) and sixty-seven (67) percent of the total capacity of the tank.b. The baffle forming the two (2) compartments shall have an opening four (4) to six (6) inches wide, located in the center of the baffle and at fifty (50) percent of the liquid depth of the tank.c. If the tank is to be made of concrete, the baffle wall shall be constructed of concrete and be structurally sound. This shall be interpreted as a minimum of three thousand (3000) pound concrete containing six (6) inch by six (6) inch number ten (10) concrete wire and having a minimum thickness of two and one-half inches.d. Baffle walls shall be securely and permanently fastened to the septic tank. All fasteners shall be of sound and durable material not subject to corrosion or decay.5. Access Openings a. A resealable opening above each tee and baffle must be provided in each tank top. These openings provide for cleaning or rodding out of the inlet or outlet pipe and access for pumping.b. Openings covering the inlet and outlet shall be accessible and visible at finished grade once the septic tank is covered.c. Rectangular openings shall be a minimum of fifteen (15) inches by fifteen (15) inches as measured from the bottom side of the lid of the septic tank.d. Circular openings shall be a minimum diameter of seventeen (17) inches as measured from the bottom side of the lid of the septic tank.e. Multi-slab tank lids and one piece lids that can be removed manually to include but not limited to steel and fiberglass require the slab or lid over the inlet and outlet tee or baffle to have a minimum access opening of 6 inches by 6 inches if rectangular or 8 inches in diameter if round.f. All concrete covers, access openings and slabs must have a handle of 3/8 inch steel rebar or other corrosive resistant material of the size necessary to facilitate the removal of the cover, opening or slab.15 Miss. Code. R. 18-77-5.1.4