Control Parameters for Understanding and Preventing Process Imbalances in Biogas Plants. Emphas on VFA Dynamics

Anaerobic digestion is a widely used method for treatment of organic waste in bioreactors. Theanaerobic digestion process depends on a finely balanced action of several microbial groupswhere the product of one group serves as the substrate for others. The growth rates and thesensitivity towards environmental changes differ widely between the different groups.

As a consequence of this, an unrestrained reactor operation can lead to disturbances in the balance between the different microbial groups, which might lead to reactor failure. Therefore, reliable parameters and tools for efficient process control and understanding are necessary. The work of present study was directed towards this challenge.

Initially, the response of the anaerobic digestion process to various types of process imbalances was investigated with special focus on volatile fatty acid dynamics (VFA), methane production and pH. The experiments were carried out in lab-scale thermophilic continuously stirred tank reactors (CSTR) treating livestock waste.

The imbalances included inhibition by long chain fatty acids (LCFA), inhibition by ammonia, organic overloading with proteins and organic overloading with industrial waste, i.e. meat and bone meal and lipids. During the mainpart of the experiments one reactor was connected to an online VFA sensor giving a detailed profile of the VFA dynamics during the process imbalances.

Based on the results it was concluded, that propionate was the most reliable single parameter for indication of process imbalances in biogas plants. At Danish full-scale biogas plants the biogas production is normally the only continuously measured parameter. In order to examine the usability of propionate as control parameter a reactor experiment was constructed in which the reactor operation either was carried out on the basis of the methane production or on the basis of fluctuations in the concentration of propionate.

The experiment confirmed that propionate is a useful parameter for (1) indication of process imbalances and (2) for regulation and optimization of the anaerobic digestion process in CSTRs. In order to gain a better insight in the activity of the propionate degraders under different operational conditions the kinetic parameters of propionate degradation by biomass from 10CSTRs differing in temperature, hydraulic retention time (HRT) and substrates were estimated in batch substrate-depletion experiments.

In general, a good relationship was observed between the maximum degradation rate (Amax) of propionate and the overall reactor performances while the half-saturation constant (Km) was found to be in the same range (<1mM). However, when evaluating the accuracy of the substrate-depletion tests by use of radiotracer methodology a 14-15% underestimation of Amax in the substrate-depletion was observed.

This indicates a production of propionate during the experiments via degradation of higher organic compound. Therefore, when estimating the kinetic parameters of propionate degradation in substrate-depletion tests an important and difficult challenge, is to achieve a correct balance between the propionate concentration and biomass concentration.