At the operation of biogas plants acidification of the digester is one of the most expensive problems. When the digester biology is acidotic, it can possibly last months, till the plant has its full performance again and the planned revenues flow again. Hence it is especially important to control the digester biology continually and to react at upcoming problems quickly and correctly.
Nearly always an overload of the digester biology is the cause for an acidification. The overload can be initiated by different factors:
Overfeeding of the digester or too strong fluctuating substrate quantities respectively substrate qualities overload the digester biology.
Fluctuations of the temperature inhibit the biological activity of the bacteria.
A lack of micro- and/or macronutrients limits the biological activity of the bacteria.
An addition of toxic substances inhibits the biological activity of the bacteria.
Normally an overload of the digester biology has nothing to do with the loading rate of the digester. The overload is caused by the fact, that the methane bacteria do not grow fast enough to degrade the substrate to biogas. The cause for the too slow growth of the methane bacteria lies in a lack (for example of micronutrients) or in an inhibition, for example by too high concentrations of ammonia or a too fast increase of temperature.
In practical operation of biogas plants a beginning acidification of the digester biology can be detected with rather simple methods. The well accessible measuring parameters are the methane content of the biogas directly out of the digester and the FOS/TAC value. Both parameter should be recorded and documented at least two times a week.
A reduction of the methane content from for example 52 % to 51 % within a few days without a change of the substrate, is a sure indication for upcoming biological problems in the digester biology. An experienced plant operator can detect the reduction of the methane content in the biogas of a usually stable running plant already with the gas consumption of the CHP or with the position of the gas mixer.
Every biogas plant should be equipped with a mobile gas analyzer or at least a quick access to such a device. If there is any reason to suspect that the biogas gets worse, the methane content must be measured immediately. If the methane content actually decreased, the FOS/TAC should be measured immediately. If this value increased considerably, a sample should be sent to the lab promptly in order to determine the concentration of the organic acids.
If you take samples and send it to the lab, take care that the sample is cooled directly after the removal and is best sent freezed. Even if the sample stands around at outside summer temperatures only a few hours or is brought to the lab by car uncooled, the analysis result will be wrong, because the acids in the sample are possibly already degraded.
From the acid values the experienced biogas plant operator can derive the necessary measures. At a beginning acidification it is mostly sufficient to reduce the feeding a bit till the digester biology has calmed down again and then to feed up again to the original level slowly.
If the concentration of the propionic acid is also very high (over 3,000 mg/l) or even higher than that of the acetic acid, the feeding must be drastically reduced till the acids are degraded. This can last up to two months.
In every case the causes that lead to the acidification, must be eliminated.
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