How much volume of septic should be below the liquid level for flows greater than 500 gallons?

For flows greater than 500 gallons, the correct determination of the required volume of septic tank capacity below the liquid level is based on a specific guideline used to ensure adequate treatment of wastewater. When the daily flow is multiplied by a factor of 1.5, it accounts for increased decanting and settling needs, leading to better treatment efficiency and preventing the tank from being overwhelmed by the volume or rate of incoming waste.

The factor of 1.5 is designed to help provide a buffer for extra capacity, ensuring that there is enough volume in the tank to allow solid materials to settle and sufficient retention time for the liquid to be treated effectively. This ultimately improves the performance of the septic system and helps in preventing issues like overflows or backups, which can lead to health hazards or environmental concerns.

In contrast, the other options, while specific numeric volumes like 1000, 1500, and 750 gallons might seem reasonable, they do not take into consideration the daily flow variability and needs of a system dealing with flows greater than 500 gallons. This reinforces that a formulaic approach based on daily flow is critical for the design and efficiency of septic systems.