What is the minimum size of a septic tank, in litres?

What is the minimum size of a septic tank, in litres?

• A. The greater of 3 600 L or twice the total daily design flow for residential; three times the total daily design sanitary sewage flow for non-residential
• B. Exactly 3 600 L
• C. Twice the total daily design flow for residential
• D. Three times the total daily design sanitary flow for residential

Answer: (A)

Septic tanks must have enough volume to hold the daily wastewater and give enough time for solids to settle and for the digestion process to begin, even when flows vary. For residential systems, the minimum size is the larger of 3,600 L or twice the total daily design flow, so you always have a safe lower bound while also accounting for the actual daily load. For non-residential systems, the minimum is three times the total daily design sanitary sewage flow, to handle larger or more variable waste streams. The option that includes both rules—minimum 3,600 L or 2× the residential daily design flow, and 3× the non-residential daily design sanitary flow—is therefore the correct one. For example, if a home’s daily design flow is 2,000 L, the minimum would be 4,000 L (since 2× 2,000 = 4,000, which is greater than 3,600 L). If a home’s daily design flow is 1,000 L, the minimum would be 3,600 L (the fixed minimum). This keeps the system sized appropriately for both typical and larger daily loads.

Septic tanks must have enough volume to hold the daily wastewater and give enough time for solids to settle and for the digestion process to begin, even when flows vary. For residential systems, the minimum size is the larger of 3,600 L or twice the total daily design flow, so you always have a safe lower bound while also accounting for the actual daily load. For non-residential systems, the minimum is three times the total daily design sanitary sewage flow, to handle larger or more variable waste streams. The option that includes both rules—minimum 3,600 L or 2× the residential daily design flow, and 3× the non-residential daily design sanitary flow—is therefore the correct one. For example, if a home’s daily design flow is 2,000 L, the minimum would be 4,000 L (since 2× 2,000 = 4,000, which is greater than 3,600 L). If a home’s daily design flow is 1,000 L, the minimum would be 3,600 L (the fixed minimum). This keeps the system sized appropriately for both typical and larger daily loads.