In good digestion, volatile acids should be not more than which level?

• A. Not more than 200 mg/L
• B. Not more than 500 mg/L
• C. Not more than 250 mg/L
• D. Not more than 1000 mg/L

Answer: (C)

In anaerobic digestion, volatile acids are produced as organic matter is broken down, and the system relies on methanogenic microbes to convert those acids into methane. Keeping the level of volatile acids low is essential because high concentrations can push the pH downward, reducing buffering capacity and slowing down the methane-producing bacteria. A well-functioning digester typically aims to keep volatile acids to a level that the system can easily balance with its alkalinity, preventing instability.

The best target is not to exceed about 250 mg/L of volatile acids (expressed as acetic acid equivalents). Staying at or below this limit helps maintain a stable pH and smooth operation, ensuring the digestion process remains balanced and efficient.

Levels much higher—such as 500 mg/L or 1000 mg/L—signal that acids are accumulating faster than they can be consumed, increasing the risk of acid crash and process upset. A value around 200 mg/L is more stringent than necessary in many practice guidelines, but the commonly cited upper limit for good digestion is 250 mg/L, which is why that choice is considered correct.