## 3 Results and Discussion
### First Experimental Cycle
It has been demonstrated that when switching is frequent enough (low switching period T), the PABR resembles the behavior of a UASBR (upflow anaerobic sludge blanket reactor) [7]. This was indeed verified also in this case when operating with a constant HRT/T ratio. Figure 4: Daily sCOD of the reactor’s feedstock and effluent. Figure 3: Daily methane concentration of the biogas generated in the PABR. Similar variation in the COD concentration was also noticed for other process parameters, such as the pH and the total alkalinity, as shown in Figures 6 and 7, respectively. The pH values have strong fluctuations during the higher HRT phases and after the third experimental phase are stabilized at values between 7 and 8. Due to the characteristics of the feedstock, the pH did not cause inhibition of the process as the pH of the feedstock is in the favorable range for anaerobic digestion. On the other hand, the alkalinity of the synthetic MWW proposed by the literature is significantly low (0.125 g/L NaHCO\({}_{3}\) = 78 mg CaCO\({}_{3}\)/L), and due to the water used in feedstock preparation, the total alkalinity of the feedstock was 913 mgCaCO\({}_{3}\)/L in the first experimental phase. As the HRT is lowered, the low alkalinity of the feedstock could introduce a limitation for the process. In order to avoid this problem, the proposed alkalinity of the synthetic mixture was changed and more NaHCO\({}_{3}\) was added as alkalinity buffer in order to stabilize the alkalinity of the process above the value of 2000 mgCaCO\({}_{3}\)/L. As the total alkalinity of the feedstock increased, the alkalinity of the reactor's compartments increased also. During the first experimental phase feeding the reactor with 912.50 mgCaCO\({}_{3}\)/L on average, the compartments average alkalinity was 1147 mg/L. Increasing the alkalinity of the feedstock to 2500 mgCaCO\({}_{3}\)/L led to an average alkalinity of 1821 mg CaCO\({}_{3}\)/L in the reactor. Further increase in the feedstock alkalinity led to higher increase inside the reactor compartments, with the highest amount noticed during the fourth experimental phase (3560 mg CaCO\({}_{3}\)/L). It is observed that as the alkalinity took higher values, the productivity of the reactor in biogas generation and biomethane concentration increased. This is an indication of the significant role of alkalinity for process efficiency. It can be concluded that if high-rate anaerobic digestion is to replace the activated sludge process for MWW treatment, the alkalinity of the feedstock can be a major limitation parameter for the process. In Table 4, the average daily pH and total alkalinity values are given for the reactor, the feedstock and the effluent. Figure 5: Average daily sCOD concentration and variation of in the four PABR compartments. Regarding the concentration of the biosolids, it seems that the solid content in the PABR compartments was reduced during the experiment. This phenomenon can lead to the assumption that as the HRT is reduced, the reactor reduces its biomass. However, the low concentrations of suspended solids and volatile suspended solids in the compartments, the effluent is caused due to biomass sedimentation in the reactor lower parts, and this is typical for a high-rate system such as this. As shown in Figure 8, in each PABR compartments a two-phase system is formed between the biomass (solid phase) and the treated effluent (liquid phase). The sampling and the effluent valve of each compartment are placed in the higher parts of the reactor. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Feedstock**} & \multicolumn{2}{c}{**Compartant a**} & \multicolumn{2}{c}{**Compartant b**} & \multicolumn{2}{c}{**Compartant c**} & \multicolumn{2}{c}{**Compartant d**} & \multicolumn{2}{c}{**Effluent**} \\ \hline
**Exp.