During a digester upset, pH is typically a lagging indicator.

• A. True
• B. False
• C. It is a leading indicator
• D. It is not used

Answer: (A)

In a digester upset, pH changes after the disturbance because the digester’s buffering capacity resists pH shifts until the upset has progressed and acid buildup occurs. When the process is upset, hydrolysis and acidogenesis can produce more volatile fatty acids, but alkalinity neutralizes much of these acids first. Only after the buffering capacity is overwhelmed does pH begin to fall, so pH reflects what has already happened rather than signaling trouble early. That buffering effect also means pH may recover slowly once conditions stabilize, even before the underlying issue is fully resolved.

If pH were a leading indicator, it would show changes before the disturbance becomes evident in other process parameters; that isn’t typically the case here. And since pH is routinely monitored as part of digester control, saying it isn’t used wouldn’t be accurate.

So stating that pH is typically a lagging indicator during a digester upset is correct.