## 3 Results  
### Phase I: Biotrickling Filters Operation  
#### 3.1.1 System Performance: Odorants  
Under abiotic conditions, Filtralite AIR 10-20 supported a complete removal of H2S, while Filtralite AIR AC removed more than 90% of this contaminant. The performance of SAINT GOBAIN materials without inoculation outcompet the removal efficiency of conventional plastic rings, which supported a 50% removal of H2S. Following inoculation, the BTF-FAC maintained a complete H2S removal, while the BTF-F and the BTF-K rapidly achieved similar H2S removals 2 and 4 days after inoculation, respectively. The three BTFs were capable of degrading inlet H2S concentrations of \(\sim\)25 ppm at GRTs of 2 and 1 min (Figure 2a). The GRT was reduced to 30 and 15 s by days 50 and 64, respectively, with no concomitant deterioration of the H2S removal performance observed in any of the BTFs. Nevertheless, a further reduction of the GRT to 7.5 s resulted in an increase in the outlet H2S concentration in BTF-F and BTF-K to 1.2 ppm (corresponding to a removal efficiency of 95%), while BTF-FAC was able to sustain a complete H2S abatement.  
Methylmercaptan was only analyzed from day 17 onwards of the experiment due to analytical issues at the initial stages of the project. No methylmercaptan was observed in the outlet gas stream of any BTF at GRTs of 2 and 1 min. However, when the GRT was reduced to 30 s, the outlet concentration of methylmercaptan increased to 2.5 mg m\({}^{-3}\) in BTF-F and 2 mg m\({}^{-3}\) in BTF-K and BTF-FAC, resulting in removal efficiencies of 28 and 42%, respectively. In the last operating stages, at GRTs of 15 and 7.5 s, no methylmercaptan removal was recorded in any of the BTFs (Figure 2b).  
In the particular case of toluene, no removal was observed for the BTF packed with Kaldnes plastic rings during the first 27 days of operation, despite this biotrickling filter  
Figure 2: Time course of the inlet (\(\times\)) and outlet concentration of H\({}_{2}\)S (**a**), methylmercaptan (**b**) and toluene (**c**) in BTF-F (\(\blacklozenge\)), BTF-FAC (\(\blacktriangle\)), BTF-K (\(\Circuit\)). Vertical dashed lines delimit different stages.  
was inoculated with activated sludge by day 9 (Figure 2c). On the contrary, both BTF-FAC and BTF-F reached toluene removal efficiencies of >99% within 11 and 15 days after inoculation at a GRT of 2 min. Surprisingly, a decrease in the residence time to 1 min promoted the abatement of toluene in BTF-K, achieving removals of >99% from day 37 onwards at this GRT. The high toluene removal was also maintained in the three BTFs at a GRT of 30 s. However, a further reduction in the GRT to 15 s resulted in a decrease in the toluene removal performance of BTF-K, increasing the outlet toluene concentration up to 0.65 mg m\({}^{-3}\) (which corresponded to a removal efficiency of 72%). On the contrary, both BTF-F and BTF-FAC were able to maintain toluene removal efficiencies >99%, which highlights the good performance of SAINT GOBAIN packing materials during biotrickling filtration of VOCs. Finally, the reduction in the GRT to 7.5 s resulted in a deterioration in the toluene removal performance regardless of the BTF, with average toluene outlet concentrations of 1.5 mg m\({}^{-3}\) (corresponding to removal efficiencies of 50%) for BTF-K and BTF-FAC and 1 mg m\({}^{-3}\) for BTF-F (corresponding to a removal efficiency of 67%).