## 5 Conclusions  
OMW is a suitable co-substrate in agricultural biogas digesesters. As a single substrate, it is inapplicable due to the seasonal character of the biomass stream and the potential inhibition by polyphenols. Selective polyphenol recovery can reverse the inhibitory effects and add further value to the waste stream [31]. The biodegradability of OMW, indicated by the BOD\({}_{5}\):COD ratio, can be improved by selectively removing polyphenols. Mixing raw OMW with a co-substrate or mixing the two waste streams of the polyphenol recovery process, depolymerhenolized OMW and UF retenate, produces high biomethane potentials of around 350 NmL/g VSelimated at very long retention times of over 45 days. The methane content in the produced biogas varies between 61 and 72%. The anaerobic digester was able to continuously degrade an average of 71% with 19 days of HRT. Increasing the HRT from 9 to 29 days did not significantly increase the degradation. The organic loading rate was increased up to 5 kg VS m\({}^{-3}\) d\({}^{-1}\) without signs of over-acidification. The FOS/TAC indicated a stable biochemical equilibrium inside the reactor, even after terminating the pH adjustment of the feedstock after 70 days. The digestate needs to be applied as fertilizer to valorize the very high content of fertilizing minerals and humus buildup. Stabilizing the digestate further in an aerobic process (e.g., composting) is not necessary, as the VS can be reduced by up to 80%. A biogas yield of around 320 NL-(kg VS)\({}^{-1}\) is sufficient for an economic digester operation. The energetic and nutritional valorization of OMW in biodigesters is a feasible economic approach. Three parameters are decisive for the economic feasibility of biogas production from OMW:  
Biomethane yields are comparable to those of renewable raw materials  
Process stability  
A suitable long-year co-substrate plan with alternative biomass for anaerobic digestion between March and November  
Germination tests showed that undiluted digestate and OMW have phytotoxic properties for seeds. However, diluted samples increased germination compared to water. Full-grown plants with established root systems showed significantly increased growth compared to non-fertilized plants. One policy that helped increase the number of small and medium-sized biogas plants in Germany for local renewable energy production was part of the _Erneuerbaren Energie Gesetz_, which provided a degressive payment for small manure biogas plants up to 100 kW\({}_{\text{el}}\). In this way, a local valorization that helps reduce the negative impact of the waste stream on the climate and environment and generates renewable energy is economically feasible. In addition, regions where OMW management is an issue can implement regulations to incentivize or require biogas plant operators to accept a certain amount of OMW as a co-substrate that is diluted enough to have a minimal impact on biogas yields or process stability. Anaerobic digestion of OMW as a co-substrate in industrial-scale biogas digesesters is the most realistic solution for long-term sustainable OMW management. Further research on co-substrates, process stability, and kinetic optimization can reduce business risks for industrial implementation. Conceptualization, J.P., K.F., S.S., S.L., F.H. and S.-U.G. ; methodology, J.P., K.F., F.H. and S.L. ; software, J.P.; validation, J.P. and K.F. ; formal analysis, J.P. and K.F. ; investigation, J.P. and K.F. ; resources, J.P., S.S., S.-U.G., S.L. and M.C. ; data curation, J.P., K.F., S.L. and F.H. ; writing--original draft preparation, J.P. and K.F. ; writing--review and editing, J.P., K.F., S.-U.G. and S.S.; visualization, J.P. and K.F. ; supervision, M.C. and S.-U.G. ; project administration, J.P. and F.H. ; funding acquisition, S.S., S.-U.G. and J.P. All authors have read and agreed to the published version of the manuscript.