Tennessee Wastewater Operator Practice Exam

What may occur if sludge and supernatant are withdrawn from a fixed cover digester faster than feed sludge is added?

1. Stable biogas production

2. Reduction in volatile solids

3. Explosive mixture of air and biogas

4. Enhanced solid retention time

The correct answer is: Explosive mixture of air and biogas

When sludge and supernatant are withdrawn from a fixed cover digester at a rate faster than new feed sludge is added, it can lead to an imbalance within the system. Specifically, the reduction in liquid levels and the rapid removal of solids can expose the digester to an influx of air, particularly if a vacuum is created. This situation can result in an explosive mixture of air and biogas. In anaerobic digesters, biogas is produced through the breakdown of organic materials by microorganisms, which occurs in an oxygen-free environment. If air infiltrates the system, it can mix with the biogas and create a hazardous situation. The presence of air not only disrupts the anaerobic conditions vital for gas production but also poses a risk of explosion when ignited due to the presence of methane in biogas. Thus, maintaining the correct balance of inflow and outflow is critical to ensure a safe and efficient digestion process.