Tennessee Wastewater Operator Practice Exam

Why are ammonia concentrations higher in anaerobic digester supernatant compared to aerobic digester supernatant?

1. Aerobic digesters promote nitrogen gas release

2. Aerobic digesters support the growth of nitrifying bacteria

3. Anaerobic conditions encourage ammonia accumulation

4. Anaerobic digesters decrease ammonia levels

The correct answer is: Aerobic digesters support the growth of nitrifying bacteria

The observed higher ammonia concentrations in anaerobic digester supernatant compared to aerobic digester supernatant primarily results from the processes that occur under each condition. In anaerobic digesters, the absence of oxygen limits the ability of ammonia to be converted into nitrogen gas, leading to an accumulation of ammonia. Conversely, aerobic digesters foster the growth of nitrifying bacteria, which oxidize ammonia to nitrite and then further to nitrate. This biological process significantly reduces the levels of ammonia in the aerobic environment due to continuous conversion. Therefore, the presence of these nitrifying bacteria actively reduces ammonia concentrations, while in anaerobic conditions, the lack of such processes leads to higher levels of ammonia. Hence, the statement about the growth of nitrifying bacteria in aerobic digesters accurately explains why ammonia levels would be lower in that environment and higher in anaerobic digesters.