Florida Wastewater Practice Test

What adjustment can create an increase in contact time in the aeration tank?

1. Increase the RAS rate

2. Decrease the RAS rate

3. Increase aeration

4. Reduce tank volume

The correct answer is: Decrease the RAS rate

Increasing contact time in the aeration tank is important for the effective treatment of wastewater, as it enhances the biological processes that remove contaminants. When the return activated sludge (RAS) rate is decreased, the amount of sludge being returned to the aeration tank is reduced. This means that less biomass is being removed from the system, allowing more time for the remaining microorganisms to interact with the wastewater. By lowering the RAS rate, the retention time of the mixed liquor in the aeration tank is effectively increased, allowing for more thorough degradation of organic material through biological processes. This extended contact time fosters better treatment efficiency as the microorganisms have a longer duration to assimilate nutrients and break down pollutants. Other options may not correctly enhance contact time. For example, increasing aeration mainly affects the oxygen transfer and may not directly impact the contact time of the mixed liquor with the influent. Similarly, reducing tank volume would decrease the overall contact time, not increase it.