Simultaneous Algorithm-based Monitoring of Volatile Fatty Acids (VFA), Bicarbonate & Alkalinity in anaerobic wastewater of bio-digesters - Kristof De Neve

The anaerobic process in Bio-digesters is based on a complex ecosystem of bacterial species that degrade organic matter in waste streams. In recent years this process has gained increasing importance at industrial scale. Especially in the agro and food industry big quantities of wastewater are treated using this process. The success of the anaerobic waste treatment is due to the low production of sludge (disposal of sludge is the highest cost for an aerobic treatment), recovering energy by combustion of the produced methane, and the possibility of treating wastewater with a high organic load. Figure 1 shows the schematic degradation process and the several involved bacterial species.

The anaerobic process however is susceptible to variations and therefore eventually less stable than the classical aerobic wastewater treatment. Methanogenic bacteria that are strictly anaerobic, grow slowly and are very sensitive to pH changes. The formation of methane from Volatile Fatty Acids (VFA) by these bacteria is also the slowest step in the process. Acidogenic bacteria however have a more rugged nature. Therefore a good balance between the several microbial populations is necessary.

After periods of non-activity of the Bio-digester, like in the agro-industry where production is season related, the process is extra vulnerable and needs to be monitored more frequently. Instability will lead to a less efficient process, instable gas production and a possible total failure of the Bio-digester.

The first step in destabilisation of the anaerobic process begins with an increase of the VFA concentration in the Bio-digester. Due to a high organic loading, failure of the heating or inhibition of the methanogenic bacteria (lack of nutrients or the presence of toxic compounds), VFA are more rapidly formed than degraded. While the concentration of VFA is increasing, the bicarbonate concentration will decrease and the CO2 concentration in the biogas will increase. If the biogas is used for energy recovery, an unstable gas production will result in a less efficient energy production.