Code Corner: 2024 IPSDC Chapter 8: Tanks

The International Codes® (I-Codes®), developed by the International Code Council (ICC), are a family of fifteen coordinated, modern building safety codes that help ensure the design and construction of safe, sustainable and affordable structures.    

The I-Codes are the most widely adopted set of model codes globally, implemented in all 50 U.S. states and many countries around the world.     

The Building Safety Journal’s series, Code Corner, explores sections of the I-Codes each month, focusing on key elements of these essential codes. This month, we’re spotlighting the scope of Chapter 8 of the 2024 International Private Sewage Disposal Code® (IPSDC®): Tanks. 

Tanks are an integral part of any private sewage disposal system whether they serve as treatment (septic) tanks, pump tanks or holding tanks for leveling the peaks in flow to the system.  

Where tanks are used for treatment, the dimensions, volume and location of internal features are very important to ensure that the solid wastes are kept within the tank so as to not clog the effluent distribution system. Where tanks are used for holding purposes, they must be sized large enough to accommodate the total of peak flows coming from a building and adequate retention between servicing. Chapter 8 provides the necessary requirements for tanks. 

802.2 Design of septic tanks 

Septic tanks shall not have less than two compartments. The inlet compartment shall not be less than two-thirds of the total capacity of the tank, not less than a 500-gallon (1893 L) liquid capacity and not less than 3 feet (914 mm) wide and 5 feet (1524 mm) long. The secondary compartment of a septic tank shall have not less than a capacity of 250 gallons (946 L) and not more than one-third of the total capacity. The secondary compartment of septic tanks having a capacity of more than 1,500 gallons (5678 L) shall be not less than 5 feet (1524 mm) long. The liquid depth shall be not less than 30 inches (762 mm) and a maximum average of 6 feet (1829 mm). The total depth shall be not less than 8 inches (203 mm) greater than the liquid depth. Rectangular tanks shall be constructed with the longest dimensions parallel to the direction of the flow. Cylindrical tanks shall be not less than 48 inches (1219 mm) in diameter. 

Effluent treatment tanks must have two compartments with a baffle to provide for settling or retention of solids in the first compartment. The first compartment must have a volume capacity of at least two-thirds of the total capacity of the tank with a minimum 500-gallon capacity and not less than 3 feet wide by 5 feet long. The second compartment must be not more than one-third of the total capacity of the tank with a minimum of 250-gallon capacity. If the second compartment of the tank is more than 1,500 gallons, then the compartment must be at least 5 feet in length. The liquid depth of the tank (depth from the bottom of the inlet pipe to the bottom of the tank) must be a minimum of 30 inches and a maximum average of 6 feet. The total depth of the tanks must be at least 8 inches greater than the liquid depth. For rectangular tank design, tanks must be constructed with the longest dimension parallel to the direction of flow. Cylindrical tanks must be at least 48 inches in diameter (see Commentary Figure 802.2). 

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