## 3 Results and Discussion
### Experimental Studies in Pilot Scale Biogas Plant  
Experimental studies of the TPAD of CSL in automatic and semi-automatic modes of the cascade system with simultaneous operation of both monitoring and control systems have been carried out. The supply of the feeding substrate from the tank at the inlet of BR\({}_{1}\) with peristalic pump P\({}_{1}\) and the monitoring and control of BR\({}_{1}\) is carried out hourly by the system built on LabVIEW. The withdrawal of culture medium from BR\({}_{1}\) and its supply to BR\({}_{2}\) is carried out with a peristalic pump P\({}_{2}\) and a connecting pipeline. This function, as well as the monitoring and control of BR\({}_{2}\), was performed twice a day by the system built on the Beckhoff industrial controller. Draining of the culture medium from BR\({}_{2}\) and leveling of the volumes is performed manually every 2-3 days. A mode of automatic regulation of biogas yields in BR\({}_{2}\) was also tested with the help of an intermediate tank with pulse operation of the pump.  
After long experiments separately with the two bioreactors (hydrogen BR\({}_{1}\) with a working volume of 8 L; methane BR\({}_{2}\) with a working volume of 80 L, i.e., the ratio is 1:10), it was switched to work as TPAD of CSL. Initially, the experimental work was conducted with an intermediate tank, i.e., a certain amount of culture fluid was drained from the hydrogen BR\({}_{1}\) into an intermediate tank with a volume of 50 L, from which a certain amount was fed into the pilot BR\({}_{2}\). In order to set up the two-phase system, this was carried out either manually or automatically. Then, a fully automatic mode of operation was initiated, where a certain amount of culture fluid from the hydrogen BR\({}_{1}\) was fed directly into the methane BR\({}_{2}\). Our literature studies show that this is the first automatic mode implemented in a two-phase AD system. Initially, 4 L of hydrogen BR\({}_{1}\) is removed every day and 4 L of water (D = 0.4 day\({}^{-1}\)) with 200 mL of CSL is introduced, i.e., the concentration of the input solution S\({}_{01}\) = 50 g/L, which is fed into the methane BR\({}_{2}\), from which 4 L of culture fluid is taken out once a day. From the 22 nd day of the experiment, the amount of culture medium to be removed is the same, but the concentration of CSL is twice lower (100 mL per 4 L of water, i.e., S\({}_{01}\) = 25 g/L). In the first case, the organic load is 20 g/L day\({}^{-1}\), and in the second is 10 g/L day\({}^{-1}\).  
As can be seen in Figures 7-9, the biogas yields from the two bioreactors, as well as the content of biohydrogen and biomethane, are quite high. The pH change in BR1 and BR2 is shown in Figure 10. Biomethane yields are approximately 50% higher compared to a single-stage process with the same substrate (performed early). The fluctuations in the daily yields of the biogas from BR1 and BR2 are mainly due to technical issues. Irregularities in the evolution of biohydrogen are also due to fluctuations in pH in BR\({}_{1}\), which sometimes go beyond the allowable range (Figure 10). The biodegradation process in BR\({}_{2}\) proceeds at a slower rate compared to BR\({}_{1}\), with smaller deviations in the content of methane and carbon dioxide in the resulting biogas.  
Figure 7: Biogas yields in BR\({}_{1}\) and in BR\({}_{2}\) in continuous mode of TPAD of CSL.  
The only larger deviation (around the 15 th day) is due to technical malfunctions of the system.