Tennessee Wastewater Operator Practice Exam

How does the addition of primary sludge affect the oxygen demand in an aerobic digester?

1. It decreases oxygen demand

2. It has no effect on oxygen demand

3. It increases oxygen demand

4. It stabilizes oxygen demand

The correct answer is: It increases oxygen demand

The addition of primary sludge to an aerobic digester increases oxygen demand due to the presence of organic material that requires oxygen for microbial degradation. Primary sludge is rich in biodegradable organic compounds, and when it is introduced into the aerobic digester, microorganisms accelerate their metabolic processes to break down these compounds. This process relies on oxygen, thus raising the overall oxygen demand within the system. In aerobic digestion, the microorganisms consume dissolved oxygen as they decompose these organic materials. Consequently, adding more organic substrates, such as primary sludge, compels the microorganisms to utilize more oxygen, leading to a higher oxygen demand. Understanding this relationship is vital for appropriately managing aeration levels in a wastewater treatment process to ensure optimal performance and prevent issues such as under-aeration or excessive accumulation of organic matter.