## 4 Discussion  
The results here obtained confirmed clay-based packing materials from SAINT GOBAIN company as a promising platform for the optimization of the performance of biofilters and biotrickling filters devoted to the removal of odorants from different characteristics (VOC and VIC). In this context, Filtralite materials showed better adsorption capacities when the bioreactors were operated under abiotic conditions, also supporting a more rapid start-up after inoculation. The presence of water in the materials under abiotic conditions enhanced their ability to remove H2S, as confirmed by the superior performance of the BTFs compared to BFs. A similar H2S and MM abatement was supported by the three BTFs, achieving almost complete removals at 7.5 s and 1 min, respectively. On the contrary, Filtralite materials showed a superior toluene removal efficiency, exhibiting values >99% at 15 s of GRT, while toluene removal in the BTF packed Kaldnes plastic rings remained at ~70%. At this point it should be highlighted that the Henry's law constants of H2S, toluene and methyl mercaptan are 1\(\times\)10\({}^{-3}\), 1.5 \(\times\)10\({}^{-3}\) and 3.5 \(\times\)10\({}^{-3}\) mol (m\({}^{3}\)Pa)\({}^{-1}\). This rules out a potential mass transfer limitation causing the deterioration of methylmercaptan removal based only on the mass transfer gradient. Gas-liquid mass transfer in biotrickling filters depends, not only on the gas-liquid concentration gradient, but also on the mass transfer coefficient (K\({}_{\text{L}}\)a), which itself is a function of the pollutant. Indeed, K\({}_{\text{L}}\)a is proportional to the (molar volume of the pollutant) ~ 0.4, where the molar volume for H2S, toluene and methylmercaptan are 33, 106 and 55 mL mol\({}^{-1}\), respectively. The results suggest that Filtralite Air AC was able to support better methylmercaptan removal efficiencies due to the higher metabolic activity of the microbial community as a result of the higher pH values provided by the packing material compared to plastic rings. In this context, the sharp initial decrease in the pH of the trickling liquid recorded in the BTF-K was likely detrimental for the microbial activity and might explain the lower MM and toluene removal performance observed in the biotrickling filter packed with plastic rings. In addition to the superior abatement performances obtained at 7-15 s (typical gas residence times for the operation of BTFs at a commercial scale), SAINT GOBAIN materials provided pressure drops as low as those of conventional plastic rings, which foresees a good performance of these materials in terms of energy demand during odour abatement. In this context, 10 cm of water column is often considered as the maximum tolerable pressure drop in biofiltation to avoid severe operating costs [20,21]. In our particular study, this value was not exceeded at any of the gas flowrates tested, even at 7.5 s of gas residence time (<3.5 cmH2O), which demonstrates the technical viability of the tested materials. In biofilters, lower air velocities, and thus higher residence times are typically provided to accomplish an effective air pollutant treatment. For instance, for H2S and VOCs removal, typical values ranging from 40 to 60 s are usually suggested [22]. However, negligible H2S concentrations were recorded at the outlet of the biofilters and the two-phase biotrickling filter packed with SAINT GOBAIN materials even at the lowest GRT of 20 s. On the contrary, the two-phase BTF did not show any improvement regarding methylmercaptan and toluene removal. Previous studies have demonstrated a superior removal performance of hydrophobic VOCs such as hexane in two-phase partitioning bioreactors, due to an enhanced mass transfer of the pollutant from the gas phase to the microorganisms [4]. Similarly, [PERSON] et al. (2010) reported gas-silicone oil partition coefficients for sulfur compounds such as DMS and DMDS up to 30 times lower than those values for gas-water systems [23].