## 4 Discussion  
The AD of the PM+CWM, PM+CWM+SWW, and PM+CWM+SBT substrate mixtures in a continuous operation mode using the three different EGSBs reactors yielded three main conclusions regarding the mixtures (see below). 1. The synergistic effects described by the batch model in [24] were also found in the continuous operation. 2. The maximum methane yields in the continuous operation of any mixture of these four substrates were predicted using the batch model and multiplying the BMP\({}_{\infty}\) by a coefficient between 0.7 and 0.8. 3. The employment of the Stover-Kincannon model showed that all three mixtures had a different kinetical behavior, which could even be noticed among the two triple mixtures. The synergistic effects due to the addition of a third substrate were most likely related to the C/N values. The high C/N values in the three-substrate mixtures explained the good performances observed; however, a higher ratio was no indication of the better performance of mixture 3 versus 2. Hence, the performance of the co-digestion of these mixtures should not be oversimplified by the C/N values without having taken into account other influential factors. Nevertheless, the most recommended C/N values in the literature were from 0 to 30 [4,13,63]. We note that all our mixtures had a C/N value < 20 (Table 1), which strongly suggested the increased proportion of the carbon-rich-substrates within the mixtures. The concept of integrating an EGSB reactor in a typical agricultural biogas plant is also of relevance. Compared to a typical agricultural biogas plant, where the representative HRT values are between 50 and 150 d [12,64], high-rate reactors provide an alternative system for the treatment of liquid substrates or their liquid fractions at much smaller HRTs. Substrates mixtures that have influenced HRTs should be applied as suggested by [PERSON] and [PERSON] [63]. They stated that a degradation rate follows an inverse function with the HRT depending on substrate complexity. Consequently, higher HRTs need to be applied and lower degradation rates were expected for lignin-rich substrates than for protein- or sugar-rich substrates. Agricultural biogas plants in Germany typically co-digest animal manure with either maize or grass silage [11]; therefore, higher HRTs are expected for the three-substrate mixtures digested in this paper due to their complexity. However, the differences in the HRTs were always noticeably large. [PERSON] et al. [12] studied 21 full-scale plants in the region of Baden-Wurttemberg (southern Germany), which performed either  
Figure 8: Graphical optimizations of the optimum MPR and MY of R2 and R3 using the OLR. single digestion or co-digestion of cattle manure, maize silage, and grass silage at different solid contents. They found that high values of degradability were reached at a HRT of \(\geq\)100 d. Thus, a more sophisticated scheme of treatment that involves multistage processes has been suggested for more efficient energy production [4; 63]. Thus, the integration of a high-rate reactor in a typical treatment plant could lead to an increment in energy production, as found by [PERSON] et al. [65] in their co-digestion of fruit/vegetable and food wastes in two stages (UASB+CSTR). This approach allowed them to work at higher OLRs and increase MPR values up to 15% over a single-stage digestion (UASB). We were able to operate all three reactors for up to 1 d, where the tanks were refilled daily. Consequently, the daily preparation of the mixtures was a logistical and practical challenge. Also, the MVs obtained at a HRT of 1 d were the lowest among the three reactors. It was probably not ideal to run the reactors at such a low HRT; however, this was possible and can be especially useful when the demand for biogas is peaking or excess amounts of substrates need to be processed. The results of the continuous operation were significantly influenced by the lack of proper mixing in the storage tanks and the seasonal behavior of the substrates.