Tennessee Wastewater Operator Practice Exam

Why do many wastewater treatment plants observe a decrease in settling velocity in the summer?

1. Decreased inflow

2. Water temperature affects settling velocity

3. Higher SRT

4. Increased MLSS

The correct answer is: Water temperature affects settling velocity

Water temperature plays a critical role in the settling velocity of particles in wastewater treatment. As water temperature increases during the summer months, the viscosity of the water decreases, which can alter how effectively solids settle out of the water column. Specifically, warmer water can cause microorganisms and suspended solids to exhibit different behaviors compared to cooler temperatures, impacting their ability to aggregate and settle. In warmer conditions, the kinetic energy of the particles is higher, which may lead to less efficient settling due to increased turbulence. Additionally, the biological activity in the treatment system can increase with temperature, leading to changes in the characteristics of the mixed liquor that affect settling. This interaction can result in a lower overall settling velocity. Understanding these temperature-related dynamics is vital for operators when adjusting processes and managing treatment performance throughout varying seasons.