NextGenBiogas – Flexible production of bioenergy can contribute stability for power grids

Applied concept

The anaerobic digestion process is a so-called two-stage anaerobic system: in a first, hydrolytic stage, acid formation takes place, and in a subsequent, methanogenic stage, biogas is formed. For running the two-stage system efficiently, the following criteria should be considered: the construction of reactors involved in both stages, the operating conditions, the composition of the inoculum, and the composition of the feeding substrate.

In order to be able to quickly increase methane production in the second stage if required, easily convertible organic acids and alcohols are produced as storage medium in the first hydrolytic stage. The identification of microorganisms with good adaptation properties to changes in system operation for fast and efficient methane production is an important topic within the NextGenBiogas project.

The hydrolytic stage and the methanogenic stage have been tested separately under laboratory conditions.

Efficient acid production in the hydrolytic phase

In the hydrolytic stage, the optimal conditions for fast and efficient acid production have been identified corresponding to the highest potential methane production, which can be obtained when applying this hydrolysate in the methanogenic reactor. The optimal conditions were found under t = 51°C, pH = 5.5, the addition of organic dry residues in the amount of 30 g. The highest acid formation was achieved at the ratio of shredded corn silage to cattle slurry of 50:50 in the substrate input.

Flexible and demand-oriented biogas production in the methanogenic phase

In the methanogenic stage, different scenarios of flexible feeding have been tested, aiming at the fastest microorganisms’ recovery and the highest biogas production efficiency after selected starvation phases. The synthetic medium corresponding to the hydrolysate from the first stage is used as a feed substrate.

Process monitoring and regulation through microbial approach

Alongside monitoring and evaluating the experimental trials, metagenome and metatranscriptome analyses of the inoculum and the samples from the two-stage system are carried out [1–3]. Based on the acquired microbial data, we were able to both classify the species involved in each case and determine their relative abundance. This information enables us to map the microorganisms involved in the fermentation process. By conducting the metagenome and metatranscriptome analysis, the microorganisms’ behavior in response to the experimental changes will be identified and assessed. This enables targeted inoculation of the process with these microorganisms, and in addition, the search for the optimal grow conditions could be streamlined.

Outlook

The ongoing lab-scale experiments are aimed at optimizing biogas production in the methanogenic phase, which is carried out in a fixed-bed circulating reactor. The obtained results are of the most interest for designing and operating a two-stage anaerobic system for biogas production in full-scale.