For residential systems, which quantity determines the septic tank minimum size?

• A. The total daily design sanitary sewage flow
• B. Groundwater depth
• C. The size of the building
• D. The area of the lot

Answer: (A)

Sizing a residential septic tank is about hydraulic loading: the amount of wastewater generated in a day. The tank must hold that daily flow long enough for solids to settle and for partial digestion to begin before the liquid leaves for the drainfield. If the tank is too small for the daily design flow, solids will spill into the leach system, causing problems and possible failure. Groundwater depth affects how the drainfield is designed and installed, not how big the tank must be. The size of the building or the lot area doesn't set the tank volume directly, because what matters is how much wastewater the home is expected to produce daily, which is estimated from fixtures/bedrooms. Therefore, the quantity that determines the septic tank minimum size is the total daily design sanitary sewage flow.