Tennessee Wastewater Operator Practice Exam

Digesters typically do not achieve more than 50% volatile solids reduction (VSR) because?

1. Some VS in the feed sludge aren’t biodegradable

2. All the solids are converted to gas

3. The process operates at high temperature

4. Insufficient aeration occurs

The correct answer is: Some VS in the feed sludge aren’t biodegradable

The reason the statement is correct is that not all volatile solids (VS) present in the feed sludge are biodegradable. In any wastewater treatment process, especially in anaerobic digesters, there are fractions of organic material that are resistant to biodegradation. These materials may include certain fibers, fats, or other complex organic compounds that microorganisms cannot easily break down. As a result, even under optimal conditions in a digester, there will be a limit to how much of the volatile solids can be reduced. Understanding this concept is crucial, as it highlights the limitations of biochemical processes and underscores the importance of pre-treatment methods, which can enhance the digestibility of the sludge by making more of the organic material biodegradable. It also helps operators set realistic expectations about solids reduction in the digestion process. The other options each relate to aspects of the digestion process but do not directly address the inherent characteristics of the organic materials being treated. Therefore, focusing on the biodegradability of the volatile solids explains why reduction typically does not exceed 50%.