Wastewater New York State Practice Exam

What is the recommended design flow velocity to minimize odor generation in sewer lines?

1. Greater than 1.0 ft/sec

2. Less than 1.5 ft/sec

3. Exactly 2.0 ft/sec

4. Greater than 2.0 ft/sec

The correct answer is: Greater than 2.0 ft/sec

In the context of sewer line design, maintaining an appropriate flow velocity is crucial for minimizing odor generation, which is often caused by stagnant sewage and the development of anaerobic conditions. A design flow velocity greater than 2.0 ft/sec is recommended because it helps ensure that wastewater keeps moving through the system without allowing solids to settle. When velocities are too low, solids can accumulate, creating conditions conducive to anaerobic bacterial growth, which produces unpleasant odors. Higher velocities also promote better oxygen transfer and can help to limit the production of hydrogen sulfide, a common byproduct associated with odor issues in sewer systems. Additionally, by maintaining a velocity above this threshold, the system can better handle variations in flow without dropping below levels that can lead to negative consequences like odor emissions. Thus, designing sewer lines with a flow velocity greater than 2.0 ft/sec is an effective strategy for odor control, ensuring the sewage remains in continuous motion and reducing the potential for stagnant areas where odors can develop.