The project aimed to provide a site‑specific, plant‑specific solution for the nitrogen problem that arises when nitrogen‑rich substrates are used in biogas plants. By adapting a mass‑ and nitrogen‑balance matrix that had been developed at the German Federal Research Institute for Biogas and Bioenergy (DBFZ) to a user‑friendly format, the team was able to evaluate the feasibility of different nitrogen‑reduction routes for three representative biogas plants. The core of the technical work involved laboratory‑scale experiments with nitrogen‑rich substrate combinations that mimic the operational conditions of the pilot plants, followed by a pilot‑scale up‑scaling of the most promising process for one of the plants.

In the laboratory phase, the team investigated anaerobic digestion of mixed substrates, percolation of liquid digestate through straw or green‑cutting, and subsequent aerobic treatment of the resulting digestate. Two biological nitrogen‑reduction strategies were tested: conventional nitrification/denitrification and the Anammox process. The experiments demonstrated that the digestate could be treated to achieve nitrogen reductions of 20 % to 80 %, depending on the chosen route and operating conditions. The Anammox reactor, in particular, showed a high nitrogen removal efficiency while consuming less oxygen than the nitrification/denitrification pathway. The pilot‑scale implementation combined a biogas reactor with an aerobic treatment train that included a percolation step over straw, thereby converting a liquid digestate into a solid, transportable organic fertilizer. Mass‑balance calculations confirmed that the nitrogen content of the final product was reduced by up to 80 % compared with untreated digestate, and that the energy yield of the biogas plant increased because a larger share of nitrogen‑rich agricultural residues could be fed into the digester.

The project also produced a comprehensive mass‑balance model for the entire process chain, which was validated against data from the pilot plant. This model was used to assess the economic viability of the two nitrogen‑reduction options. The cost analysis revealed that the Anammox route, while slightly more capital intensive, offered lower operating costs due to reduced aeration requirements. A comparative assessment of greenhouse‑gas emissions and eutrophication potential showed that both routes lowered the nitrogen load on the soil and reduced the risk of nitrate leaching and nitrous‑oxide emissions, thereby improving the environmental performance of the biogas plants.

Collaboration was central to the project’s success. The German Federal Research Institute for Biogas and Bioenergy (DBFZ) led the research and coordinated the work with industry partners and academic collaborators. The project was funded under the German research funding scheme (grant numbers FKZ 22042118, 2219NR140, 2219NR138) and ran over a two‑year period from 2020 to 2022. The partners contributed expertise in process engineering, microbiology, environmental assessment, and economic analysis, ensuring that the developed solutions were technically robust, economically feasible, and environmentally sound. The outcome is a set of scalable, site‑adapted nitrogen‑reduction strategies that can be integrated into existing biogas plants, thereby enhancing the utilisation of nitrogen‑rich agricultural residues, reducing the need for nitrogen fertiliser application, and mitigating environmental impacts associated with excess nitrogen.