The ProPhaSep project, funded by the German Federal Ministry of Education and Research (grant 03EI5409B) and carried out from 1 July 2020 to 31 December 2022, aimed to increase the operational flexibility of biogas plants. The consortium, consisting of the Technical University of Berlin (TU Berlin), the engineering firm FWE GmbH, and the Kurt‑Schwabe Institute for Measurement and Sensor Technology (KSI Meinsberg), focused on separating the different stages of anaerobic digestion and on recirculating thin sludge between two fermenters. This approach was intended to allow independent optimisation of the hydrolytic/acidogenic phase in the first reactor and the acetogenic/methanogenic phase in the second, thereby improving robustness against difficult‑to‑hydrolyse substrates and enabling flexible substrate feeding.
At TU Berlin a laboratory‑scale, two‑stage biogas plant was designed and built. The system incorporated 3‑D‑printed components and a semi‑automatic screw press supplied by FWE GmbH to separate thin sludge from thick sludge before recirculation. Thin sludge was returned to the reactors, allowing microorganisms in the liquid phase to be flexibly re‑introduced. Phase separation was maintained by a pH‑controlled regime: the first fermenter operated at a lower pH to favour hydrolysis and acidogenesis, while the second reactor was kept at a higher pH to promote acetogenesis and methanogenesis. Dynamic operation tests varied substrate load, recirculation volume, and stirrer power, and monitored metabolite concentrations, gas composition, and substrate utilisation. The dissolved‑hydrogen sensor developed by KSI Meinsberg was installed in both fermenters for the first time. It enabled real‑time detection of hydrogen concentrations, allowing the team to optimise hydrogen injection from both external sources and from the first fermenter into the methanogenic phase. Iterative testing and optimisation of the sensor and the injection strategy were carried out during a long‑term experiment, demonstrating that hydrogen addition could be precisely controlled and that methane yields could be increased without compromising process stability.
The project also produced an ecological and economic assessment based on laboratory data and scaled‑up scenarios. The assessment evaluated the potential of using residual feedstocks such as straw and corn silage within the phase‑separation concept, highlighting the benefits of integrating waste streams into biogas production. The results showed that the two‑stage configuration with thin‑sludge recirculation could maintain stable methane production while allowing flexible substrate feeding, thereby improving the overall efficiency and resilience of biogas plants.
Collaboration within the consortium was structured around clear roles: TU Berlin led the experimental design and data analysis, FWE GmbH provided engineering support for the reactor build and sludge handling, and KSI Meinsberg supplied the dissolved‑hydrogen sensor and performed sensor optimisation. The project was managed by Dr. Jens Zosel of KSI Meinsberg, who coordinated the integration of the technical components and the evaluation of the results. The consortium’s joint effort produced a set of technical recommendations for the design and operation of flexible biogas plants, and the findings are being prepared for publication in peer‑reviewed journals.