Tennessee Wastewater Operator Practice Exam

What could cause the MLSS to gradually turn from light tan to black in several hours with normal DO concentration and no excess foaming present?

1. Organic overload

2. Powdered activated carbon

3. Increased bacterial growth

4. Algal blooms

The correct answer is: Powdered activated carbon

The transformation of mixed liquor suspended solids (MLSS) from light tan to black, particularly in the absence of excess foaming and while maintaining normal dissolved oxygen (DO) levels, suggests that powdered activated carbon is influencing the condition of the suspended solids. When powdered activated carbon is added to a wastewater treatment system, it can adsorb a variety of organic substances, which may lead to changes in the coloration of the MLSS. As powdered activated carbon interacts with the suspended organic matter and microorganisms, the solid phase may accumulate more carbon, resulting in a darker appearance due to the carbon’s coloration properties. This process does not involve an increase in foaming or dissolved oxygen levels, which could indicate other types of operational issues such as organic overload or excessive bacterial growth. Thus, the observed change to black MLSS correlates well with the introduction of powdered activated carbon, as it is known to darken the solids while also aiding in the treatment process by adsorbing additional contaminants. In contrast, organic overload would typically lead to an imbalance in the system, potentially causing excessive foaming and lower DO levels due to increased microbial respiration. Increased bacterial growth could also manifest differently, leading to turbidity or other color changes unrelated to carbon addition. Algal blooms,