Tennessee Wastewater Operator Practice Exam

What effect might returning supernatant from an anaerobic digester have on an activated sludge process?

1. Reduce effluent turbidity

2. Increase effluent ammonia concentrations

3. Improve sludge settleability

4. Enhance nitrification rates

The correct answer is: Increase effluent ammonia concentrations

Returning supernatant from an anaerobic digester to an activated sludge process can indeed lead to an increase in effluent ammonia concentrations. The supernatant typically contains higher levels of ammonia because during anaerobic digestion, organic matter is broken down, resulting in the release of ammonia and other nitrogen compounds. When this supernatant is reintroduced into the activated sludge system, it contributes additional ammonia to the influent. This can challenge the nitrifying bacteria in the system, potentially leading to elevated concentrations of ammonia in the effluent if the nitrification process cannot keep up with the increased load. To manage this situation effectively, operators must ensure that the system is appropriately balanced to handle the additional nitrogen load and that there are sufficient nitrifiers present to convert the excess ammonia to nitrate.