Tennessee Wastewater Operator Practice Exam

If rising sludge occurs in a secondary clarifier due to denitrification, what adjustment should the operator make?

1. Increase the air to 1.0 mg/L

2. Reduce the air to 0.2 - 0.5 mg/L

3. Increase the wasting rate

4. Stop all aeration immediately

The correct answer is: Reduce the air to 0.2 - 0.5 mg/L

When rising sludge occurs in a secondary clarifier due to denitrification, it indicates that the sludge has become less dense and is floating instead of settling properly. This condition often arises because of the production of gas bubbles during the denitrification process, which can cause sludge to rise to the surface. Reducing the air to a lower concentration is effective because it can limit the amount of dissolved oxygen available in the system. A lower dissolved oxygen level, typically around 0.2 to 0.5 mg/L, encourages the growth of denitrifying bacteria while not supporting the growth of the organisms that produce the excess gas, helping to prevent rising sludge. This adjustment also allows the remaining bacteria to continue processing nitrogen without excessive foam or floating sludge. Increasing the air could exacerbate the gas production, worsening the rising sludge issue. Increasing the wasting rate could remove some sludge but may not address the root cause of the problem. Stopping aeration entirely can lead to other issues, including a lack of treatment efficiency and potential harm to the overall biological processes within the system.