Septic Tank Design

Septic Tank Materials  
  • A septic tank must be watertight.
  • The wastewater has not been fully treated, so it must be contained and prevented from escaping into the environment.
  • Septic tanks can be made of concrete, concrete blocks, fiber-reinforced plastic, high-density plastic or fiberglass.
septic tank in ground septic tanks
  • Metal tanks were allowed at one time, but are not allowed now because they were found to be prone to rusting. (At one time people used empty 55-gallon drums; these rusted out.)
  • Replace metal tanks when discovered if the system is in use or will be used.

 

Septic Tank Size  

 

Septic tank size is determined by the number of bedrooms, and if there is a whirlpool bath.

  • The minimum size for a septic tank is 1,000 gallons.
  • For example, the minimum required for a three-bedroom house with no whirlpool is 1,000 gallons. A four-bedroom house with no whirlpool is required to have a tank that holds at least 1,250 gallons.
  • Add 250 gallons additional tank capacity for a whirlpool.
  • The septic tank size is NOT based on the square footage of the house, or the number of people living in the house because occupancy can change.

 

Septic Tank Design  
  • A septic tank must be designed with acid-resistant inlet and outlet baffles or tees, inspection pipes at least 6 inches in diameter over both inlet and outlet, and an access manhole at least 12 inches in diameter.

Septic Tank with Tee-Baffles

 

Manhole 12" diameter minimum

  

6" minimum diameter
Inlet

septic operation drawing

 

6" minimum diameter

 

Outlet

 

 
  • A single tank is used most often, but multiple tanks can be hooked up in series.
  • Tanks may have one, two or three compartments.
Single-Compartment Tank
septic design drawing

Dual-Compartment Tank
septic design drawing

 

 
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