## 3 Results
### Phase II: Biofilters and Two-Phase Biotrickling Filter Operation  
#### 3.2.1 System Performance: Odorants Abatement  
Under abiotic conditions, 2 PBTF-F and BF-FAC supported a complete removal of H\({}_{2}\)S by day 0, while the H\({}_{2}\)S removal efficiency recorded in the BF-F gradually dropped from 90% by day 1 to \(\sim\)55% by day 15 of abiotic operation (Figure 4a). The three bioreactors were capable of biologically degrading inlet H\({}_{2}\)S concentrations of 19.9 \(\pm\) 2.2 ppm, completely removing the H\({}_{2}\)S immediately after inoculation of the systems. The decrease in GRT from 2 min to 1 min and 40 s did not affect the H\({}_{2}\)S removal performance, no H\({}_{2}\)S being detected in the treated air exiting the bioreactors throughout these operational stages (regardless of the packing material or presence of trickling aqueous-silicone oil solution). Similarly, BF-FAC and 2 PBTF-F supported a complete H\({}_{2}\)S removal at 20 s of GRT, whereas the steady H\({}_{2}\)S removal efficiency of BF-F slightly decreased during this last experimental period to 97.5 \(\pm\) 0.3%.  
Figure 3: Time course of (**a**) pH and (**b**) pressure drop in the BTF-F (\(\blacklozenge\)), the BTF-FAC (\(\blacktriangle\)) and the BTF-K (\(\Circuit\)). Vertical dashed lines delimit different stages.  
Figure 4: Time course of the inlet (\(\times\)) and outlet concentration of H\({}_{2}\)S (**a**), methylmercaptan (**b**) and toluene (**c**) in BF-F (\(\blacklozenge\)), BF-FAC (\(\blacktriangle\)) and 2 PBTF-F (\(\Circle\)). Vertical dashed lines delimit different stages.  
Methylmercaptan concentration was reduced from 3 to 1.5 ppmv regardless of the packing material and bioreactor configuration from day 8 of the abiotic period (Figure 4b). After inoculation, MM removal remained similar in both biofilters, while the two-phase BTF packed with Filtralite AIR 10-20 mm achieved a complete removal of MM at a GRT of 2 min. Surprisingly, a decrease in the GRT from 2 to 1 min resulted in a higher MM abatement in BF-FAC (up to \(\sim\)80%), which supported a removal efficiency \(>\)78% at 40 and 20 s of GRT. On the contrary, when the GRT was reduced to 1 min, a concomitant decrease in the MM abatement performance to \(\sim\)50% was observed in the BF-F and the 2 PBTF-F. A further decrease in the GRT to 40 s resulted in average removal efficiencies of 63 \(\pm\) 7% and 42 \(\pm\) 9% in BF-F and 2 PBTF-F, respectively. Finally, both bioreactors supported average MM removals of \(\sim\)45% at 20 s of GRT.  
In the particular case of toluene, a negligible removal was observed in the biofilters packed with Filtralite AIR 10-20 mm or Filtralite AIR AC during the 15 days of abiotic operation (Figure 4c). On the contrary, the 2 PBTF-F reached toluene removal efficiencies \(>\)99% by day 7 under abiotic conditions. After inoculation, toluene was completely removed regardless of the bioreactor, although 6 days were necessary for BF-F to achieve a complete removal. A decrease in the GRT from 2 min to 1 min and 40 s slightly reduced toluene removal in the 2 PBTF-F to \(\sim\)98 and 96%, respectively, while no effect was observed in the biofilters performance. Finally, the biofilter packed with Filtralite AIR AC (BF-FAC) was able to completely remove the toluene from the polluted air stream, while the toluene removal efficiency of BF-F and 2 PBTF-F dropped to 96 \(\pm\) 3% and 89 \(\pm\) 4%, respectively, at the lowest GRT of 20 s.