## 3 Results and Discussion
### EBRT Study under Feedback Control  
With the selected controller (PI-MO), the influence of EBRT on the control of the H\({}_{2}\)S outlet concentration at a set point of 15 ppmv was studied. Figure 4 shows the performance of the outlet H\({}_{2}\)S concentration (Figure 4a) and of the nitrite flow rate (Figure 4b) for the three EBRT studies (117, 92 and 67 s). Figure 3: Staircase perturbation for PID (**a**) and PI (**b**) adjusted by MO. Blue lines: nitrite flowrate, red lines: H\({}_{2}\)S\({}_{\text{out}}\), dashed black lines H\({}_{2}\)S IL. The PI-MO controller was able to adjust the control variable at 117 s and 92 s in 14.98 \(\pm\) 0.37 ppm\({}_{\rm V}\) and 15.00 \(\pm\) 0.56, respectively, while at the lowest EBRT tested, the adjustment of the H\({}_{2}\)S\({}_{\rm out}\) concentration was shifted up to 14.21 \(\pm\) 0.98 ppm\({}_{\rm V}\). As a consequence of the control action, the variation in the nitrite flow rate was significant among the three EBRT analyzed (Figure 4b), specially at high values of the studied profile (500 - 1000 ppm\({}_{\rm V}\)). Thus, the nitrite flow rate of 117 s was incremented by 50.88% (0.13 - 0.32 g N-NO\({}_{2}\) h\({}^{-1}\)) and 96.13% (0.14 - 0.48 g N-NO\({}_{2}\)- h\({}^{-1}\)) for EBRT of 92 s and 67 s, respectively. For the three EBRT studied, the low output concentration of H\({}_{2}\)S allowed other uses of the treated biogas, as it could be fuel cells [23; 24]. The H\({}_{2}\)S IL were between: 7-39, 9-50 and 12-69 gS-H\({}_{2}\)S m\({}^{-3}\) h\({}^{-1}\), for 117 s, 92 s and 67 s, respectively. Previous studies have usually been carried out at H\({}_{2}\)S IL higher than 100 gS-H\({}_{2}\)S m\({}^{-3}\) h\({}^{-1}\)[12; 16; 31] because it has been used to treat biogas with high H\({}_{2}\)S concentrations, such as the biogas produced in a waste water treatment plant (WWTP) [32]. However, it is possible to find biogas with lower H\({}_{2}\)S concentrations, such as landfill biogas, which has H\({}_{2}\)S concentration in the range studied in this section (between 180-1000 ppm\({}_{\rm V}\)) [24]. Thus, with landfill biogas, a lower EBRT could be used, allowing to obtain an outlet H\({}_{2}\)S concentration of above 15 ppm\({}_{\rm V}\), which would offer more profitable biogas applications than burning in a cogeneration engine. Moreover, a lowest EBRT reduces the size of the package and therefore the installation cost [33]. In addition, the presence of leachate in landfills and ammonium-rich effluents could be used to biologically produce nitrites [34]. As far as we know, this study has tested the lowest EBRT on anoxic BTFs; EBRTs are usually larger (342 s [14], 321 s [35], 176 s [36] or 117 s [16]). Effects of Feedforward Control Strategy on the Outlet H\({}_{2}\)S Concentration and Sulfate Selectivity  
Figure 5 shows the outlet H\({}_{2}\)S concentration (Figure 5a) and inlet molar ratio N:S (Figure 5b) for experiments with and without feedforward control for the stair's sinusoidal perturbation on H\({}_{2}\)S IL. Figure 4: Profiles obtained in EBRT studies at 67 s, 92 s and 117 s. (**a**) H\({}_{2}\)S outlet concentration, (**b**) nitrite flowrate. Green lines: 67 s; blue lines: 92 s; red lines: 117 s; dashed black lines: H\({}_{2}\)S\({}_{\rm in}\). An improvement on the BTF operation was observed using the feedforward control, with respect to the experiment without control (Figure 5a). The maximum output H2S concentration was reduced by 26.18%, since in the experiment without control, the maximum outlet H2S concentration reached was 233 ppmV and 172 ppmV for the feedforward control with an average of 117 \(\pm\) 21 ppmV, corresponding to an elimination percentage of 93 \(\pm\) 2 %. Moreover, the emitted mass of the system (gS-H2S) was reduced by 2.6% (from 0.18 gS-H2S to 0.15 gS-H2S). Therefore, the results were similar to a study previously done using nitrate as an electron acceptor [12].