California Wastewater Treatment Operator Practice Exam

What is the typical concentration of mixed liquor suspended solids in a conventional activated sludge process?

1. Below 500 mg/l

2. 1000 to 4500 mg/l

3. Above 6000 mg/l

4. Dependent on influent quality

The correct answer is: 1000 to 4500 mg/l

In a conventional activated sludge process, the concentration of mixed liquor suspended solids (MLSS) typically falls within the range of 1000 to 4500 mg/l. This concentration is crucial because it reflects the amount of biomass present in the system, which is essential for the biological treatment of wastewater. The microorganisms in the MLSS are responsible for breaking down organic matter, and having a sufficient concentration helps ensure effective treatment. While concentrations above 4500 mg/l may occur in some systems, they can lead to operational challenges such as settling issues and the development of a thicker sludge blanket, which is not ideal for effective treatment. On the other hand, concentrations below 1000 mg/l would typically indicate insufficient biomass, potentially resulting in suboptimal treatment performance and lower removal efficiencies of contaminants from the wastewater. The influent quality can indeed impact the desired MLSS concentration; however, the typical operational range is most often seen between 1000 and 4500 mg/l.