What is the minimum volume of septic below the liquid level for flows less than 500 gallons?

The correct minimum volume of a septic tank below the liquid level for flows less than 500 gallons is 750 gallons. This volume is established to ensure that the tank has sufficient capacity to effectively treat sewage and prevent issues such as solids settling in the tank and causing premature system failure.

The 750-gallon requirement is specifically designed to accommodate not only the volume of wastewater generated by the household but also to allow for adequate retention time. This retention time is critical for the biological processes that break down waste. If the capacity is too low, the tank can quickly become overloaded, which may lead to surface discharge or backup.

Additionally, this regulation meets health and environmental standards, helping to avoid contamination of groundwater and ensuring that the treatment process is efficient. In the context of septic system design and regulations, adhering to this standard is essential for both the effectiveness of the system and public safety.