Florida Wastewater Practice Test

When the VA/ALK ratio in an anaerobic digester increases, what effect is noticeable?

1. An increase in pH and a decrease in CO2

2. An increase in CO2 and a decrease in pH

3. A decrease in both pH and CO2

4. A constant pH and fluctuating CO2

The correct answer is: An increase in CO2 and a decrease in pH

The increase in the VA/ALK ratio in an anaerobic digester indicates that there is a rise in volatile fatty acids (VA) relative to alkalinity (ALK). This imbalance occurs when there is an excess of organic material being decomposed by anaerobic bacteria, leading to the production of more acids than the system can neutralize with its buffering capability (alkalinity). As volatile fatty acids accumulate, the system becomes more acidic, resulting in a decrease in pH. This reduction in pH typically occurs alongside an increase in carbon dioxide (CO2) concentration in the digester. The microbial metabolism happening within the digester often leads to increased CO2 production, especially when organic acids are broken down. Therefore, a higher VA/ALK ratio points to a state of ongoing fermentation with an abundance of fatty acids, further driving the production of CO2, contributing to the reduction in pH. This concept illustrates the delicate balance required in anaerobic digesters and highlights the importance of monitoring these parameters to maintain optimal operating conditions.