Which acid/alkalinity ratio indicates the digester is stressed?

• A. 0.30-0.40
• B. 0.50
• C. 0.10
• D. 0.80

Answer: (A)

The idea being tested is how the balance between produced acids and the system’s buffering shows digester stability. In anaerobic digestion, alkalinity acts as a buffer against pH drops caused by accumulating acids. When acids build up faster than buffering can handle, the acid/alkalinity ratio rises, signaling stress on the digester.

A ratio in the range of about 0.30 to 0.40 indicates the digester is under stress but not yet collapsing; buffering is being challenged and closer monitoring or adjustments are warranted. If the ratio is much lower, such as around 0.10, the system has ample buffering and is stable. If the ratio climbs to around 0.50 or higher, stress increases and the risk of process upset grows. A ratio near 0.80 points to severe acidification and a high likelihood of failure without corrective action.

So, the correct interpretation of a stressed digester is the moderate range around 0.30–0.40.