## 4 Discussion  
The BMP value for untreated OMW of 276 NL CH\({}_{4}\). (kg VS)\({}^{-1}\) is comparable to the result of [PERSON] et al. [15], who obtained in a mesophilic BMP-test of raw OMW a methane yield of 243 NL CH\({}_{4}\). (kg VS)\({}^{-1}\) at a total polyphenol concentration of 1 g:L\({}^{-1}\). The removal of polyphenols did not show a positive effect on the methane yield, as the methane yield of the OMW after adsorption is 62% lower in comparison to the untreated OMW. Besides the polyphenol content, the main difference is the amount of solids. [PERSON] et al. [15] studied the effect of pretreatment steps on the anaerobic digestion of OMW in a mesophilic 3-L CSTR. The authors emphasized the presence of solids as a crucial factor for process stability. After a thermal pretreatment step and sedimentation to  
Figure 7: The picture depicts the digestate sample (day 37) in different dilutions (100%, 50%, 25%, and 13%) and distilled water on the 5 th day of the GI experiment. The bar graph depicts the percentage of germinated seeds compared to distilled water, clearly indicating positive effects for highly diluted digestate. The fertilizing effects cannot be quantified by this experimental setup. remove solids, stability and methane production rates decreased compared to the anaerobic digestion of raw OMW. [PERSON] and [PERSON] [22] investigated the biomethane potential of 3-phase olive mill solid waste under mesophilic and thermophilic conditions. The result for the cumulative methane yield obtained after 60 days of digestion at mesophilic conditions was 139 NL CH\({}_{4}\)-(kg VS)\({}^{-1}\). For thermophilic conditions at 55 \({}^{\circ}\)C, a higher methane yield of 239 NL CH\({}_{4}\)-(kg VS)\({}^{-1}\) was achieved. [PERSON] et al. [13] used the same fixed bed reactor with diluted OMW mixed with 10% liquid poultry manure and produced a maximum biogas production of 0.507 L-(g CODintroduced)\({}^{-1}\). Numerous approaches to improving process stability and biogas yields have been investigated. [PERSON] et al. [16] have found a positive effect of biochar addition to the digestion mixture regarding methanogenesis inhibition and digestate phytotoxicity. [PERSON] et al. [23] have investigated an anaerobic hybrid reactor to treat a mixture of piggery effluent and OMW. The reactor proved stable, with biogas production rates of 3.16 m\({}^{3}\) m\({}^{-3}\) d\({}^{-1}\) at an organic loading rate of 7.1 kg m\({}^{-3}\) d\({}^{-1}\). Partially substituting the feedstock of an anaerobic digester for OMW in a relatively short period of time is a key challenge to the treatment of the seasonally arising OMW. [PERSON] et al. investigated cheese whey and laying hen litter as co-substrates for the anaerobic digestion of OMW, resulting in a strong increase in BMP [24]. Aerobic pretreatment, partial chemical oxidation, flocculation, ultrasound, and others have been investigated to increase process stability [7; 25; 26; 27]. Bioaugmentation and co-digestion optimization in the mainstream are two options to increase the biomethane yield. Bioaugmentation can be an economical option when dealing with recalcitrant feedstocks or disturbances to inhibitory compounds [28]. The sulfate concentrations between 0.4 and 2.2 g\(\cdot\)L\({}^{-1}\) in the OMW are reduced to H\({}_{2}\)S in the biogas, demanding proper desulfurization for biogas valorization. The high potassium concentration and, to a lesser extent, phosphate content in the digestate make them highly suitable for fertilizing. The concentrations varied between 0.1 and 0.5 g phosphate and 2.7 and 4.8 g potassium per liter of digestate. To produce a full fertilizer, nitrogen-rich substrates like manure should be added. The increase in the GI with higher dilution is consistent with the observations of [PERSON] et al. [13]. The germination index was determined using cress seeds and a digestate obtained after anaerobic treatment of diluted OMW mixed with 10% liquid poultry manure.