In case study 3, biomass waste is treated in a three-step process. First, a mixture of C-rich industrial wastes and grey wastewater will be used to produce organic acid, e.g., citric acid and itaconic acid, via yeast-based biotechnological process (see figures 1 and 2). In the second step, after extracting citric acid from the mixture, residues will be used for biogas production through co-digestion with industrial C-rich substrate. As an output of this process, biogas will be produced for the use in power generation or heating. In the final step, the anaerobic sludge will be treated in a hydrothermal carbonization (HTC) process transforming waste biomass to valuable ready-to-use soil fertilizers. The combination of these processes will enable a material and energetic re-cyclization of wastes, thus contributing to closing life cycles in the bio-economy. Preliminary results are very promising and indicate that the INCOVER yeast-based process is a very suitable option for organic acid and biogas production. This case study will be implemented in Germany.