Tennessee Wastewater Operator Practice Exam

Why does the percentage of MLVSS decrease even as MLSS concentration increases with higher SRT?

1. More organic matter is introduced

2. Inorganic material accumulates in the process unchanged

3. Microbial activity is reduced

4. Oxygen availability decreases

The correct answer is: Inorganic material accumulates in the process unchanged

The percentage of MLVSS (Mixed Liquor Volatile Suspended Solids) can decrease in the face of increasing MLSS (Mixed Liquor Suspended Solids) concentration when the SRT (Solids Retention Time) is high due to the accumulation of inorganic materials in the biological treatment process. In wastewater treatment, the MLSS is comprised of both organic (volatile) and inorganic (non-volatile) solids. As the SRT increases, it allows for the settling and accumulation of solids in the treatment system, including inorganic solids that do not decompose or change over time. Since MLVSS primarily refers to the organic component of the suspended solids, an increase in inorganic material means that while the overall MLSS may rise, the proportion represented by MLVSS can decline. Consequently, MLVSS percentage drops even though MLSS increases because of the dilution effect from the additional inorganic matter, which does not contribute to the volatile solids measure that MLVSS represents. Understanding this relationship is crucial for operators, as it impacts treatment efficiency and the ability to remove organic pollutants effectively.