What perc rate shall be used for system design?

The slowest perc rate is used for system design because it ensures that wastewater is treated adequately before it enters the soil, reducing the risk of contamination and environmental impact. This approach provides a conservative estimate for the absorption capability of the soil, prompting the design to accommodate the least favorable conditions. By basing the design on the slowest rate, it minimizes the likelihood of system failure or overloading, which can occur if the system is designed based on faster perc rates that may not be indicative of the long-term performance of the soil.

Using the average or fastest perc rates could lead to designs that do not account for the limitations of the soil, while the mean of the highest 10% might also overlook lower rates that could critically affect the system's efficiency. Therefore, the slowest perc rate is a critical factor in ensuring the safety and reliability of the system over time.