Tennessee Wastewater Operator Practice Exam

What does white foam in a wastewater treatment process typically indicate?

1. SRT higher than 2 days

2. SRT lower than 2 days

3. High MLSS concentration

4. Excessive nutrients

The correct answer is: SRT lower than 2 days

White foam in a wastewater treatment process typically indicates a lower solids retention time (SRT), generally less than 2 days. This phenomenon occurs because a shorter SRT can lead to increased activity of microorganisms that produce excess foam or scum. These microorganisms, known as foam-forming bacteria, thrive when conditions favor rapid growth and high nutrient availability, which can be prevalent in systems with a short SRT. In contrast, a longer SRT often allows for better settling of solids and decreases the likelihood of foam formation, as there are more established and diverse populations of microorganisms effectively breaking down organic matter. Therefore, when operators observe white foam, it signals the need to evaluate the SRT and possibly make adjustments to the treatment process to alleviate this condition, ensuring optimal operation of the wastewater treatment system. While high mixed liquor suspended solids (MLSS) concentration and excessive nutrients can influence foam production, they are not direct indicators of the foam's presence like an inappropriate SRT is.