Tennessee Wastewater Operator Practice Exam

Which form of nitrogen can become toxic to methanogens at pH values above 8.0?

1. Ammonium

2. Free ammonia

3. Nitric oxide

4. Nitrous oxide

The correct answer is: Free ammonia

Free ammonia is the correct answer because it exists in equilibrium with ammonium ion in solution and becomes more prevalent as pH levels rise. At pH values above 8.0, the concentration of free ammonia increases significantly. This free ammonia can be toxic to methanogens, which are anaerobic microorganisms responsible for methane production in wastewater treatment processes. The toxicity arises because free ammonia can inhibit the enzymatic activity of methanogens, impacting their ability to function effectively in the anaerobic digestion process. In contrast, ammonium ion is generally not toxic to methanogens at pH levels typically occurring in wastewater treatment. Nitric oxide and nitrous oxide are not forms of nitrogen that would primarily impact methanogen activity in the same pH range and context. Their roles are more closely associated with nitrogen cycling and denitrification processes rather than directly inhibiting methanogenesis.