Tennessee Wastewater Operator Practice Exam

What may cause low DO concentrations and odors in an aerobic digester?

1. High solid retention time

2. Low mixed liquor levels

3. High VS loading rate

4. Inadequate stirring

The correct answer is: High VS loading rate

The correct answer highlights an important aspect of the operation of an aerobic digester. A high volatile solids (VS) loading rate can lead to low dissolved oxygen (DO) concentrations and undesirable odors due to the excessive amount of organic material being introduced into the system. When the loading rate of volatile solids is too high, the microorganisms responsible for breaking down the organic material may become overwhelmed, leading to an insufficient balance between the biological demand for oxygen and the aeration capacity of the system. This can cause oxygen depletion, which is essential for aerobic processes to efficiently convert waste into stable end products. Additionally, the breakdown of organic matter at high loading rates can result in the production of gases such as methane or hydrogen sulfide, which contribute to foul odors. Proper management of the VS loading rate is crucial for maintaining the aerobic conditions necessary for effective digestion and odor control in wastewater treatment processes.