## 2 Materials and Methods  
**Olive mill wastewater:** The olive mill wastewater samples were all derived from four different three-phase or traditional mills in Sfax, Tunisia, in the season '22/'23. OMW 1 was collected from the same olive mill on two different dates within one month. Numerous olive kinds were mixed from many groves, but the most common is Chemlali [10]. The samples for analysis were taken and stored at 4 \({}^{\circ}\)C. The substrates for feeding the biogas reactor were stored outdoors under the sun and rain shelter for several weeks, depending on the feeding schedule of the biogas reactor. Depolyphenolized OMW samples were subjected to mechanical pretreatment, ultrafiltration, and a selective polyphenol extraction process by adsorption. The extracted polyphenols were further purified for valorization in industrial applications, such as feed additives or cosmetics [11]. The residue from the ultrafiltration process was reduced to roughly 40% of the feed volume. All OMW (derived) samples used for the experiments are characterized in Table 1.  
**Biomethane potential:** An adapted method based on [PERSON] et al. [12] and VDI 4630 (2016) was applied to determine the biomethane potential of the treated and untreated olive mill wastewater. Duplicates of five different samples were measured in 1 L Schott glass bottles, filled 2/3 (666 mL) with an inoculum-substrate ratio of 1:1. The digestate of an anaerobic biodigester treating market waste was used as an inoculum and mixed with three substrates derived from the same OMW sample, as characterized in Table 2. All samples are flushed with nitrogen prior to the experiment, sealed, and incubated at 37 \({}^{\circ}\)C. The produced biogas was measured using the liquid replacement method, utilizing NaOH to remove CO\({}_{2}\). **Anaerobic digester:** The digester used for the upscaled experiments [13] is a 1.95 m high, double-jacketed, thermostated stainless-steel column with an internal diameter of 0.45 m, resulting in an active fixed bed volume of around 270 L. It is filled with about 70 kg of carriers for biomass retention (Hiflow PVC rings--size 38-1), resulting in a specific surface of around 200 m\({}^{2}\)-m\({}^{-3}\) and a gap degree of over 92%. The temperature was kept at 37 \({}^{\circ}\)C with a thermostat. The feed was dosed with a BG 600 FJ-S peristaltic pump at a flow rate of 0.7 L-min\({}^{-1}\) (November-January) and 0.8 L-min\({}^{-1}\) (February). The inflow pH was adjusted with a 5 mol L\({}^{-1}\)-NaOH solution to pH 6. Biogas and digestate samples were taken irregularly to determine the methane content by gas chromatography. The feed substrates and durations are depicted in Table 1. The effects of operating parameters such as hydraulic retention times and organic loading rates on degradation efficiency and biomethane production were observed and determined over 150 days. **Germination Index:** The method is based on a modified Zucconi test and DIN EN 16086-2: Soil improvers and growing media--Determination of plant response--Part 2. Triplicates of 20 seeds are placed into Petri dishes with filter paper in the dish and in the lid. Lettuce Romaine seeds are used (Variety: NADER, Brand: Enza Zaden, Batch No. 6.121.702, Germination and Purity: 99%, Origin: South Africa). A total of 5 mL of sample is added per Petri dish, and 1 mL of distilled water is put on the filter paper in the lid of the Petri dish. For the control, 5 mL of distilled water was added to the seeds instead of the sample. The closed Petri dish is kept in a dark place at room temperature until day 5. Every day, the number of germinated seeds is counted. On the fifth day, the root length of all germinated seeds was measured.