Reactor over-acidification

The cause of acidification is almost always an overload of the biogas reactor. The overload can be triggered by various factors:

• Overfeeding
• Excessively fluctuating feed quantities or feed qualities 
• Micro- and/or macronutrient deficiencies
• Temperature fluctuations
• Presence of toxic substances.

An overload usually has nothing to do with the organic load. Even when with the planned organic load the overload may occur. The overload takes place when the methane bacteria do not grow quickly enough to break down the feed into biogas.

In the day-to-day operation of biogas plants, the over-acidification can be quickly recognized by very simple means:

• methane content in biogas
• FOS/TAC value
• concentration of the organic acids.

A reduction in the methane content for 1-2% within a few days without changing the substrate is a sure indication that biological problems are emerging in the reactor biology. 

If methane has actually dropped, the FOS/TAC should be measured aditionally. If it has raised significantly, a sample should be tested in a laboratory to determine the concentration of the organic acids.

If the concentration of propionic acid is also very high (over 3,000 mg/l) or even higher than that of acetic acid, the feeding must be radically reduced until the acids are broken down. This can take up to two months.

The experienced biogas plant operator can derive the necessary measures from the acid pattern. When over-acidification begins, it is usually sufficient to reduce the feeding a little until the reactor biology has calmed down and then slowly feed it back up to the original level.