## 4 Discussion
Therefore, it could be hypothesized that the limited methylmercaptan abatement here obtained was not associated to mass transfer limitations but rather to a low microbial activity. Overall, Filtralite AIR AC outcompeted Filtralite Air 10-20 mm for the abatement of the three model odorants, while supporting the lowest pressure drop (despite its lower particle size) and the highest buffer capacity at 40 and 20 s. Previous studies on odour treatment using compost-based biofilters have achieved an efficient abatement of odorants (>95% for H\({}_{2}\)S, butanone and toluene) at GRTs ranging from 30 up to 94 s, although at the expense of an important pressure drop across the filter bed (from 6 up to 33 cm of water column) due to the compaction of the organic packing material [20]. Toluene degraders were mainly distributed among the Proteobacteria and Actinobacteria phyla in the BFs and BTFs, although Actinobacteria were less represented in the reactors packed with Filtrate Air 10-20 mm. Members of Pseudomonas, Burkholderiales, Xanthomonadales (within Betaproteobacteria) and Rhizales (within Alfaproteobacteria) are known to biodegradation toluene [24], and these microorganisms were present in all bioreactors. Among Actinobacteria, Mycobacterium, a well-known toluene degrader [25], was a key player in toluene degradation in BFs and BTFs. It is important to note that Filtrate Air AC packing material favored the development of Proteobacteria as the clear dominant phyla in BFs and BTFs, while the dominance of this group in BFs and BTFs packed with Filtrate Air 10-20 was less pronounced. In fact, aerobic oxidation of methyl mercaptan is performed by members of Proteobacteria such as _Thibacillus_, _Hyponomicobium_, _Pseudomonas_, _Methylplophag_ and _Klebsiella_[26]. Microorganisms of the genus _Thioobacillus_, _Halothiobacillus_ and _Acidithiobacillus_ within the Proteobacteria phylum were detected and were likely involved in methyl mercaptan degradation in the BF-FAC, BTF-F and BTF-FAC. _Thioobacillus thioparus_ is able to oxidize methyl mercaptan producing elemental sulfur and sulfate [27] and different _Thioobacillus_ species have been previously found in BFs and BTFs treating methyl mercaptan and other sulfur odorants [15; 28; 29]. Some species of _Thioacillus_ have been reclassified in the genera _Halothiobacillus_ and _Acidithiobacillus_[30] and their ability to degrade methyl mercaptan cannot be ruled out. On the contrary, BF-F did not show aerobic methyl mercaptan oxidizers. Instead, Archaea, that were exclusively present in this bioreactor, likely degraded this compound, since the ability of Archaea such as _Methanomethylovorans hollandica_, _Methanolobus_ and _Methanomethylovorans_ to anaerobically degrade methyl mercaptan has been demonstrated [31; 32; 33]. These results suggest that the higher performance in terms of methyl mercaptan removal in BF-FAC as compared to BF-F, could be mediated by the development of aerobic methyl mercaptan oxidizers, that were not found in the biofilter packed with Filtrate Air 10-20. Similarly, BF-FAC showed a higher population of _Thioacillus_, _Acidithiobacillus_ and _Halothiobacillus_ (56%) than BF-F (25%), which also could have influenced the better performance of BTF-FAC at GRT of 30 s; although none of the microbial communities were able to support methyl mercaptan removal at lower GRTs. Overall, these results demonstrate the relevance of the packing material in shaping the desired microbial communities.