Tennessee Wastewater Operator Practice Exam

What percentage of loss of MLSS constitutes a washout?

1. 10%

2. 20%

3. 30%

4. 40%

The correct answer is: 30%

A loss of 30% of Mixed Liquor Suspended Solids (MLSS) is considered a washout in wastewater treatment. This threshold is important because it indicates a significant reduction in the biomass that is essential for the treatment process. When the MLSS concentrations drop by this percentage, it typically means that the biological treatment process is severely compromised, potentially leading to decreased treatment efficiency. The presence of adequate MLSS is crucial for the biological decomposition of organic matter in wastewater. When there is a washout condition, it suggests that suspended solids, which house the microorganisms responsible for breaking down pollutants, are being removed from the system at an excessive rate. Understanding the implications of a 30% washout allows operators to respond promptly to rectify the situation, which might include adjusting operational parameters or investigating potential causes of the loss, such as hydraulic overloading or poor settling properties in the secondary clarifiers. Monitoring MLSS levels is vital for maintaining effective treatment processes and ensuring compliance with regulatory standards.