## 3 Results
### Continuous Anaerobic Digestion of Raw and Derived OMW  
The fixed-bed biogas reactor was operated over 150 days, utilizing different OMW-derived substrates (Table 1). To investigate the effects on the control of process stability and degradation efficiency, the organic loading rates (OLR) and hydraulic retention times (HRT) were varied (Figure 3). The OLR varied between 1.8 and 5.8 kg COD\(\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\) and the HRT between 10 and 27 days. (Figure 3).  
Three phases can be distinguished in the substrate: The first phase lasted 40 days, and exclusively depolymerolized OMW was fed. In those 40 days, the organic loading rate doubled from 2 to 4 kg COD\(\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\) in two steps after 2 weeks of stable operation. From days 41 to 78, diluted OMW was used, with an average OLR of 3.1 kg COD m\({}^{-3}\) d\({}^{-1}\). Tap water was added to decrease the viscosity and concentration of potentially inhibiting monomeric polyphenols. The third and longest phase lasted from days 79 to 148 (phases V-VII, with substrates derived from OMW3 in the depolymerolization process). The mixing ratio of the different OMW substrates is the same as that produced during polyphenol extraction (25-45% retetate and 55-75% depolymerolized OMW).  
Figure 4 shows the daily biogas production and the measured methane content. The horizontal lines indicate the average biogas production for different OLRs, displaying a clear correlation. An increase in the produced gas volume is visible with increasing  
Figure 3: Chronological overview of OLR, HRT, and used substrates time periods of the substrates: I: OMW 1, II: OMW 2_1, III: OMW 2_2, IV: OMW 2_3, V + VI + VII: OMW 3.  
Figure 2: Biomethane potential with the inoculum as a baseline.  
With the TOC to COD ratio, the oxidation state of the inflow can be calculated, which allows an indication of its main components and an estimation of the methane content in the produced biogas [20,21]. The inflow derived from OMW 1 and OMW 4 has a COD to TOC ratio of 2.7, leading to an oxidation state of \(-0.12\) and \(-0.17\)[20]. The COD to TOC ratio for OMW 2 is 3.31 and 3.01 for OMW 3, which corresponds to an oxidation state of \(-0.52\) and \(-0.96\). A lower oxidation state leads to a higher methane content in the produced biogas [21], which is consistent with the measured increase in methane content (68-78%) during the feed period of OMW 3. Calculating the estimated methane content proposed by [PERSON] et al. [21] % CH\({}_{4}\) = COD/TOC-18.75, the estimated methane content is 52-62%, which is 10% lower than the measured methane values for the corresponding period.  
The feed COD concentration ranged from 35 to 58 g\(\cdot\)L\({}^{-1}\) in the first 105 days of operation of the continuous anaerobic digester. The COD degradation averaged 71%, with a residual COD of 11 g L\({}^{-1}\). Most of this COD in the digestate is not caused by organic carbon, as the TOC:COD ratio is low at 1:4. The biological oxygen demand was  
Figure 5: FOS, TAC, FOS/TAC, and pH feed and outflow. The grey bar indicates an ideal FOS/TAC value between 0.2 and 0.3.  
Figure 6: VS concentrations of the inflow and outflow with the resulting degradation efficiencies. The dotted line represents the average VS removal rate over the course of 100 days.  
not determined for this sample. However, it is likely that the digestate is stabilized and suitable for soil application.  
The inflow has a low nitrogen content, with an average ratio of COD:TN:P of 170:0.7:1, typical for OMW. Considering the ideal ratio of 300:5:1 for the anaerobic degradation of carbohydrates, the carbon-to-nitrogen ratio is not optimal. A mixture of the substrate or digestate with nitrogen- and phosphate-rich biomass, like manure, will provide sufficient nutrients.