Plug-flow based dark fermentation for short-chain carboxylic acid accumulation from biogenic residues

The process of anaerobic digestion is an important and widely applied bioprocess to make use of biogenic resources. If methanogenesis is prevented, the first process steps can be optimized towards high hydrolytic activity and the production of hydrogen and short-chain carboxylic acids (SCCAs) in a so called dark fermentation (DF). Converting many different types of feedstock to SCCAs allows material use in various biorefinery processes in which SCCA serve as substrates for microbial polyunsaturated fatty acid production or bioplastic production, among others.
Plug-flow reactors with an exclusively radial mixing of the substrate can evolve concentration gradients along the direction of flow, and thus a selective enrichment of microbial populations and the segregation of the process steps like hydrolysis and acidogenesis. Although the influence of a plug-flow on anaerobic digestion is well-investigated, the potential of gradient monitoring in the liquid phase was not investigated so far. Therefore, sensors for the measurement of the temperature, pH, redox potential and conductivity are are used to obtain a spatial resolution of these parameters. Different loads, retention times, bioaugmentation with hydrolytic microbes, enzyme addition and process disturbances have been applied to investigate, whether i) the gradient-based monitoring is suitable to detect process disturbances and thus can be applied for process control and ii) yield optimization is feasible. In future, also a two-phase operation is envisaged, in which gaseous products of the first part of the process are utilized in a second part to boost SCCA accumulation, while reducing any CO2 emission of the microbial process.

The measurements in different plug-flow reactors over the duration of more than one year made it possible to evaluate the benefits of gradient-based measurements. Correlation analysis showed that the formation of gradients, as measured with conductivity and the redox potential, is correlated with the hydrolysis efficiency and the SCCA concentration under certain process conditions. In particular the gradient of conductivity along the reactor allows the location of the region of maximum metabolic activity along the reactor’s liquid phase and is correlated and thus replaces off line acid concentration measurement. If process conditions are unfavorable due to insufficient forward flow or hydrolytic activity, the zone of highest activity is detected at the inlet or outlet, which is leading to lower yields while either acids are reassimilated or, in the latter case, hydrolysis remains incomplete. Monitoring the gradients in PFR allows a real-time process evaluation compared to gas phase, which is a ideally mixed, and thus an average value of the release from the whole liquid phase without any spatial information. In case of process instability, thin slurry recirculation, bioaugmentation or enzyme addition can be applied. Then, gradient based monitoring support the evaluation of such options for process intensification and stabilization as shown in this study at several examples.
The novel combination of gradient-based monitoring in combination with plug-flow assisted anaerobic digestion / DF led to a high feedstock flexibility and enables automated and decentralized application, e.g. close to the source of biogenic residues.