Tennessee Wastewater Operator Practice Exam

What does a higher SRT typically lead to in the context of MLSS concentration?

1. It decreases MLSS concentration

2. It increases MLSS concentration

3. It has no effect on MLSS concentration

4. It causes variability in MLSS concentration

The correct answer is: It increases MLSS concentration

A higher solids retention time (SRT) generally leads to an increase in mixed liquor suspended solids (MLSS) concentration. SRT is a measure of how long the microorganisms are retained in the system compared to the time it takes for treated water to be removed. When SRT is increased, microorganisms can thrive longer in the treatment process, leading to better biomass stabilization and growth. With a greater retention of biomass, you essentially allow more time for the bacteria to replicate and breakdown organic matter, contributing to a richer concentration of suspended solids in the mixed liquor. This process helps enhance the overall treatment efficiency as the population of microorganisms becomes better adapted for the conditions within the treatment system, thereby increasing MLSS levels. Higher MLSS concentrations can improve the facility's ability to treat wastewater effectively by increasing the amount of active biomass available for the degradation of organic materials. In summary, by retaining the microbes for a longer period, their concentration in the mixed liquor increases, which is why a higher SRT correlates with an increase in MLSS concentration.