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Monitoring/Process control of biogas plants

For the operator of a biogas plant it is necessary to keep the microbiological degradation process stable in order to enable a trouble-free and thus efficient operation. Particularly the start-up process of a new plant as well as the continuous operation shall be monitored.

In practice biogas plants often are started without scheduling the start-up phase. Sooner or later also that plants initiated by simple feeding will run. But in the case of a scheduled start-up of a plant including the monitoring of the most important analysis parameters the status of the microbiology can be estimated and thus occurring troubles can be identified and prevented at an early stage.

A rapid and controlled start of operation exhibits subsequent advantages:

  • Quick formation of a rich, stable and stress-resistant population of microorganisms
  • Inefficient operation at partial load of plant components only over a short period
  • By means of a quick start-up an increasing human-resource allocation can be reduced (Resch, Kirchmayr, Braun 2005: 6)
The following aims during continuous operation are:
  • Constant, high methane production
  • Constant, high utilization of the engine
  • Economically favourable input of substrates
At present automatic control of important parameters is not yet possible. Hence all analysis parameters only represent check values. At bottom the control of a biogas plant is based upon trial and error. Therefore analysis parameters are particularly important for the optimization of a biogas plant’s process control. (Resch, Kirchmayr, Braun 2005:9).

Analysis parameters for the assessment of fermenter condition:

  • pH value
The optimal pH for microbial strains in the fermenter ranges between 6.8 and 7.5. Metabolism and growth are possible within this range. If the pH value decreases too much methane bacteria are affected firstly in their metabolism.

  • Volatile Fatty Acids:
These are metabolic intermediates of the microorganisms. An increase of this value indicates a disequilibrium in the degradation chain and can be toxic to microorganisms at higher concentrations. Thus the fermentation process is inhibited.

  • UFA (undissociated Volatile Fatty Acids):
This value is dependent on the concentration of Volatile Fatty Acids, on the pH value and on the process temperature and acts toxic to microorganisms with increasing concentration. Consequence is the inhibition of the fermentation process.

  • COD (Chemical Oxygen Demand)
Too high COD values indicate either low degradation rates of digested sludge or a high fraction of crude fibre within the fermenter.

  • Dry matter
The dry matter content provides conclusions about the quality of mixing. Thoroughly mixing is the precondition for a homogenous fermentation process. With increasing dry matter content the electrical energy consumption of agitators is rising inevitably.

  • VS (volatile solids)
This value provides information about the already degraded organic fraction of digested sludge and is therefore directly related to the COD

  • TKN (Total Kjeldahl Nitrogen)
This parameter stands for the sum of nitrogen in organic compounds (especially in proteins) ammonia and ammonium. A high fraction of organically bound nitrogen is an indication for non-degraded proteins.

  • NH4-N (ammonia nitrogen)
A high fraction auf ammonia nitrogen indicates a high degradation rate of proteins.

  • UAN (unionized ammonia nitrogen)
This value is calculated from ammonia nitrogen, from pH value and from the process temperature. Too high concentration acts toxic to microorganisms and inhibits the fermentation process.

It is necessary for biogas plant operators to monitor these parameters in order to assess the status of the anaerobic digestion process. Exceeding a single threshold does not necessarily mean a malfunction of the whole biogas process. Microorganisms are partly very adaptive and they can adept themselves to changing conditions to a certain degree. If the threshold of several parameters is exceeded explicitly then one should contact experts trying to solve the problems and take measures to ensure a stable operation.

The coaction of technical dimensions of a fermenter on the one hand with chemical, physical and biological interactions of the anaerobic digestion on the other hand affect the stability of a microbiological process. The assessment of the process condition results from analytics considering technical factors.


LAABER, Michael: Aufbau eines Bewertungssystems für Biogasanlagen – „Infoblatt Fermentationsparameter“. Tulln: 2007

RESCH, Christoph/KIRCHMAYR, Roland/BRAUN, Rudolf: Start-Up und Prozesskontrolle. Unterlagen zum Anlagenbetreiberkurs. Tulln: 2005

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