A. Tank capacity. The minimum hydraulic detention time shall be 48 hours based on daily design flow. In no case shall the septic tank capacity be less than 750 gallons. Table 5.2 contains the minimum required septic tank capacities for dwelling units.
No. of Bedrooms
Approximate Tank Volume in Gallons
B. Tank dimensions. Septic tanks shall be rectangular in plan, cross-section and longitudinal view. The length to liquid depth to width ratio should be approximately equal or greater than 2 to 1 to 1 (2:1:1) and less than or equal to 3 to 1 to 1 (3:1:1). In no case shall the liquid depth be less than four feet or greater than eight feet. A minimum of one foot free board shall be provided. Inlet and outlet structures shall be placed on the longitudinal axis of the tank. Typical tank dimensions are found in Table 5.3.
C. Inlet-outlet structure.
1. General. The inlet and outlet structures shall function as a baffle. The invert of the inlet structure shall be greater than one inch but less than two inches higher than the invert of the outlet structure with the tank installed. The inlet structure shall extend six to eight inches below and eight to 10 inches above the normal liquid level. The outlet structure shall extend below the normal liquid surface to a distance of 35 to 40% of the liquid depth and eight to 10 inches above the normal liquid level. The inlet and outlet structures shall have an open space not less than four inches by four inches in cross-section or four inches in diameter.
2. Materials. All materials used for inlet and outlet structures shall have long term resistance to chemical and electrolytic corrosion. When pipe tees are used as inlet and outlet structures, the material shall be compatible with the material used in the sewer.
D. Top access and watertightness. All septic tanks shall be watertight and shall be provided with a watertight top. As a minimum, access manholes shall be provided over the inlet and outlet structures and shall have a minimum open space of 18 inches by 18 inches. When the septic tank has in excess of 30 inches of soil cover, an access manhole shall be brought to within 18 inches of the ground surface and shall be provided with a tight fitting cover. In wet areas the manhole covers shall be watertight.
E. Construction of septic tanks. The contractor and/or manufacturer shall design and construct the septic tank to withstand the lateral and bearing loads to which the septic tank is expected to be subjected.
F. Placement of septic tanks. The precast septic tank shall be bedded with at least six inches of sand or fine gravel where rock or other undesirable conditions are encountered. The tank shall be placed level. Where excavation is required, the hole shall be sufficiently large to permit placement of the tank. Backfilling the excavation for all septic tanks shall be done in layers with sufficient tamping to avoid settling. Backfill material shall be free of large stones and debris.