At what volatile acid to alkalinity ratio does a digester begin to sour?

The volatile acid to alkalinity ratio is an important indicator of the health and stability of an anaerobic digester. A digester is considered to be in a stable condition when the ratio is managed correctly, as this balance affects the digestion process and microbial activity.

When the volatile acid to alkalinity ratio is at or below 0.35, the system is generally able to buffer against changes in acid concentration, allowing for a stable environment for the anaerobic bacteria responsible for the digestion process. Values higher than this threshold, specifically approaching or exceeding 0.40, can lead to conditions where volatile acids accumulate faster than they can be neutralized by the buffering capacity (alkalinity) of the digester contents. This imbalance can cause the digester to "sour," resulting in lower pH levels that can inhibit the activity of the digesting microorganisms, leading to reduced biogas production and potential operational failures.

Thus, the specified threshold of ≤0.35 indicates that maintaining volatile acids at this level supports a healthful digestion process, preventing souring and ensuring that the system operates effectively.