Wastewater Treatment Operator Certification Practice Test

What could occur if the volatile acid to alkalinity ratio increases without corrective action?

1. Decrease in carbon dioxide content in digester gas

2. Fluctuation in carbon monoxide content in digester gas

3. Increase in pH of the digester sludge

4. Increase in carbon dioxide content in digester gas

The correct answer is: Increase in carbon dioxide content in digester gas

When the volatile acid to alkalinity ratio increases in a digester system, it indicates that there is an excess of volatile fatty acids relative to the buffering capacity provided by alkalinity. This imbalance can lead to an increase in carbon dioxide content in the digester gas as a result of enhanced microbial activity, particularly from acidogenic bacteria that thrive under such conditions. As these bacteria break down organic material, they produce more volatile fatty acids, resulting in a surge of carbon dioxide alongside other gases. This process often occurs when the system is overloaded or when there's insufficient alkalinity to neutralize the excess acids, leading to more acid production than can be buffered adequately. Ultimately, if this trend continues without corrective measures—such as adding alkaline substances to restore balance—the system risks moving towards instability. Such shifts in gas composition not only impact digester efficiency but could also lead to operational challenges in the wastewater treatment process. Thus, an increase in carbon dioxide content in digester gas is a direct consequence of a rising volatile acid to alkalinity ratio.