Which of the following is NOT typically part of the design flow considerations?

The correct choice indicates that daily rainfall is not typically considered when designing a septic system's flow. In the context of septic system design, the focus is generally on factors that directly affect the treatment and disposal of sewage.

Operating capacity refers to the amount of wastewater the system can handle on a regular basis without being overwhelmed, which is crucial for ensuring effective treatment. Surge capacity is also essential, as it accounts for the potential influx of higher-than-normal wastewater volumes that can occur, such as after heavy usage or during certain times of day. Lastly, the volume of sewage generated is a critical consideration because it directly impacts the system's sizing and overall functionality.

In contrast, while rainfall can affect soil saturation and perhaps indirectly influence a system's performance, it is not a primary factor when assessing the design flow of a septic system. Therefore, daily rainfall is less relevant to the direct operational parameters that the septic system must effectively manage.