Tennessee Wastewater Operator Practice Exam

What happens to the performance of a grit basin when flow velocity is maintained at 1 ft/sec as flows increase throughout the day?

1. Increase

2. Remain constant

3. Decrease

4. Become erratic

The correct answer is: Decrease

When flow velocity in a grit basin is maintained at 1 ft/sec and flows increase, the performance actually decreases. This is related to the concept of effective settling and separation of grit from wastewater. As the volumetric flow increases while maintaining a constant velocity, the residence time of the wastewater in the grit basin decreases. This shortened residence time limits the ability of heavier particles, such as grit, to settle out effectively. The higher flow rates create turbulence, which can re-suspend settled solids, thereby reducing the overall efficiency of grit removal. Thus, even though the flow velocity is held constant, the dynamics of higher inflow diminish the grit basin’s performance, leading to a decrease in the system’s ability to remove grit and settleable solids. In summary, while the velocity remains unchanged, the increase in overall flow volume negatively impacts the grit basin's performance, confirming that it does indeed decrease.