At what volatile acid to alkalinity ratio does an anaerobic digester become stressed?

• A. 0.2
• B. 0.3
• C. 0.4
• D. 0.5

Answer: (B)

In an anaerobic digester, keeping pH stable depends on buffering from alkalinity to neutralize the acids produced during digestion. When acids (volatile fatty acids) build up faster than the system can buffer them, the ratio of volatile acids to alkalinity rises and signals the digester is under stress.

About a ratio of 0.3, the buffering is stretched enough that the digester starts to show instability and becomes more susceptible to upset. If the ratio climbs higher, such as around 0.4 or 0.5, the risk of a significant pH drop and process upset increases even more because there isn’t enough alkalinity left to counter the acid production.

So, the 0.3 level is the point at which stress begins, making it the best indicator among the given options. Lower values suggest better buffering, while higher values indicate more severe stress.