Florida Wastewater Practice Test

What condition might cause an anaerobic digester to become unstable, often referred to as "stuck" or "sour"?

1. Low pH levels

2. High acid production

3. Excessive temperature

4. High solids concentration

The correct answer is: High acid production

The condition that often leads to an anaerobic digester becoming unstable, known as "stuck" or "sour," is typically associated with high acid production. In anaerobic digestion, microorganisms break down organic material in the absence of oxygen, producing biogas. A key aspect of this process is maintaining a balanced microbial community that can efficiently convert substrates into biogas while minimizing the accumulation of volatile fatty acids (VFAs). When the production of these acids exceeds the system's ability to neutralize them, usually due to an overload of organic material or shifts in operational parameters, it can lead to a decrease in pH and the potential inhibition of methanogenic bacteria. These bacteria are critical for the conversion of the acids into methane, a primary goal of the digestion process. As the acid concentration rises and pH declines, the microbial community becomes unbalanced, resulting in a situation where biogas production stalls, and the digester is considered "stuck" or "sour." This understanding highlights the importance of controlling organic loading rates and monitoring acid levels to maintain the health and stability of anaerobic digesters, ensuring that they perform efficiently in converting waste into usable biogas.