At the onset of a digester upset, the pH will drop before the volatile acids concentration begins to increase.

• A. True
• B. False
• C. They occur at the same time
• D. pH rises first

Answer: (B)

In anaerobic digestion, pH changes track the balance between acids produced and the system’s buffering. When an upset starts, acid-forming bacteria generate volatile fatty acids, so their concentration rises and, as these acids accumulate, the pH drops. The pH decline is a result of, not a precursor to, higher VFA levels, because the acids must build up before the pH can fall. If buffering is strong, the pH can stay relatively stable until VFAs overwhelm the system; once that happens, the pH drops. So the statement is false: VFAs increase first, and the pH falls as a consequence.