Tennessee Wastewater Operator Practice Exam

What indicates the end of denitrification when monitoring ORP in an SBR?

1. Nitrite peak

2. Nitrate knee

3. Biological oxygen demand drop

4. Phosphate balance

The correct answer is: Nitrate knee

The "nitrate knee" is a critical indicator in the denitrification process when monitoring the oxidation-reduction potential (ORP) in a Sequencing Batch Reactor (SBR). Denitrification is the biological reduction of nitrate (NO3-) to nitrogen gas (N2), which can occur under anoxic conditions. During denitrification, as nitrate is being consumed, one may observe a particular point known as the nitrate knee on the ORP curve. This knee represents a sharp decrease in the concentration of nitrate as it is being converted to nitrogen gas. As the nitrate is reduced, it influences the ORP values, which typically shift to more reduced potentials. The presence of the nitrate knee in the monitoring data signifies that denitrification is in full swing and nearing completion, as the nitrate concentration drops significantly. This allows operators to accurately gauge the effectiveness of the biological treatment process and make necessary adjustments in real time to maintain process efficiency. The other indicators, while potentially useful in various contexts, do not specifically signal the conclusion of denitrification. For instance, a nitrite peak indicates a transient increase in nitrite during the denitrification process rather than its completion. A drop in biological oxygen demand may suggest decreased organic matter,