Tennessee Wastewater Operator Practice Exam

What is the most likely occurrence when the VS loading rate to an anaerobic digester increases by 30% in a single day?

1. Accumulation of volatile acids

2. Decreased biogas production

3. Improved digestion efficiency

4. Increased sludge volume

The correct answer is: Accumulation of volatile acids

When the volatile solids (VS) loading rate to an anaerobic digester increases significantly, such as by 30% in a single day, it can lead to an accumulation of volatile acids. This is primarily because the microorganisms responsible for digestion may not be able to process the increased amount of organic matter quickly enough. As the organic material is broken down, it first produces volatile fatty acids as an intermediate product. If the load is too high, these acids can build up faster than they can be converted into biogas, leading to an acidic environment within the digester. This acidity can hinder the performance of the digestion process by inhibiting the activity of the methanogenic microorganisms, which are crucial for converting those volatile fatty acids into methane and carbon dioxide. Therefore, rather than seeing a balanced digestion process, the system might become overloaded, reflecting the importance of maintaining optimal loading rates for effective anaerobic digestion. The other potential outcomes are likely to either not occur or be less pronounced in this scenario. For instance, decreased biogas production may occur due to the aforementioned inhibition, but it is a consequence of acid accumulation rather than a direct or immediate result. Improved digestion efficiency would not happen under these conditions as the increase in VS loading is overwhelming the digester