Tennessee Wastewater Operator Practice Exam

In a rotating biological contactor (RBC) process, where is nitrification most likely to occur?

1. First wheel

2. Second wheel

3. Third wheel

4. Fourth wheel

The correct answer is: Fourth wheel

In a rotating biological contactor (RBC) process, nitrification is most likely to occur in the fourth wheel due to the conditions present in that area of the system. Nitrification is a two-step microbial process that converts ammonia into nitrites and then nitrates, and it requires both oxygen and favorable environmental conditions. As the RBC rotates, the attached biofilm experiences cycles of immersion in wastewater and exposure to air. Early stages in the process, like in the first and second wheels, are primarily dedicated to the breakdown of organic materials, where heterotrophic bacteria dominate. By the time wastewater reaches the fourth wheel, the concentration of organic matter has typically decreased, allowing for a more conducive environment for nitrifying bacteria. Additionally, by this point in the system, there is often an adequate supply of oxygen since these wheels exhibit increased exposure to air, which supports the aerobic conditions required for nitrification. Therefore, the fourth wheel is optimized for nitrification due to the favorable balance of reduced organic loads, available oxygen, and the presence of established nitrifying microbial communities.