Tennessee Wastewater Operator Practice Exam

What is the approximate composition of anaerobic digest gas?

1. 50% methane and 50% carbon dioxide

2. 65% methane and 35% carbon dioxide

3. 80% methane and 20% carbon dioxide

4. 100% methane

The correct answer is: 65% methane and 35% carbon dioxide

The composition of anaerobic digest gas typically falls within a range where methane is the predominant component. Given the general understanding of anaerobic digestion processes, it is most commonly reported that the digestate gas consists of approximately 60-70% methane and around 30-40% carbon dioxide. The choice indicating 65% methane and 35% carbon dioxide aligns well with average reported values from various sources regarding anaerobic digest gas composition. This is significant because methane is the main product of the anaerobic digestion process and is the gas of interest for energy recovery, while carbon dioxide is a natural byproduct. While options showing purely methane or a higher proportion of carbon dioxide do not reflect the typical outputs of anaerobic digestion, the choice that states 65% methane and 35% carbon dioxide accurately matches the expected outcomes, making it the most reliable option among the others provided.