Fermentation is a very delicate biological process that requires specific conditions in order to be successful. Regular monitoring of these conditions is essential for cost-effective operation.
In special fermentation reactors known as fermenters, bacteria convert substrates such as maize or food waste into biogas. Consisting mainly of methane, biogas can either be used directly for electricity generation or it can be fed into gas supply systems once it has been purified.
The system used for the four-stage fermentation process is highly sensitive. In order for the microbiological reactions to proceed smoothly and generate as much biogas as possible, parameters such as pH/redox potential, temperature, content of organic acids and DM content (dry matter) must be correct and need to be monitored on a regular basis.
HACH LANGE has an extensive portfolio of solutions designed specifically for fermenter monitoring:
- VOA/TIC (biogas titrator)
- pH (via pHD electrode)
- Redox potential (via pHD redox electrode)
- DM content (dry matter, via Solitax probe)
- Organic acids (photometric cuvette test LCK 365)
The most important parameter in the fermentation process is the concentration of organic acids that are formed during the process, primarily acetic acid. If its content is too high, and hence the pH too low, there is a risk that the entire biological process may be halted. This "biological meltdown" would paralyse biogas production in the long term, resulting in a massive loss of income for biogas producers.
The simplest method of determining the concentration of organic acids is VOA/TIC titration using the Nordmann method. The abbreviation VOA stands for "volatile organic acids", while TIC stands for "total inorganic carbon", referring to the carbonate buffer. The buffer potential of the carbonate buffer (also known as the lime reserve) prevents the fermenter from becoming hyperacidic. The lime reserve is also a vital parameter.
The biogas process is at its most efficient at a pH of around 7.3. Monitoring the pH on an ongoing basis allows critical problems in the process to be detected quickly so that countermeasures can be taken if necessary.
Microorganisms have an optimum "operating temperature", depending on the type of biomass. Mesophilic microorganisms, for example, work best at a temperature of 35 °C (mesophilic), whereas thermophilic microorganisms prefer a temperature in the region of 57 °C.