## 2 Materials and Methods
### Experimental Setup and Operational Procedure  
The experiment was divided into two operational phases as described below. Phase I. The experimental set-up for malodorous air treatment consisted of three identical PVC biotrickling filters (internal diameter = 0.1 m, height = 1 m, total working volume = 2.5 L) packed with three different materials: Filtralite AIR 10-20 mm (BTF-F), Filtralite AIR AC (BTF-FAC) and Kaldnes K2 plastic rings (BTF-K) (Figure 1) (Table 1). The air stream entering the system was first pumped through an activated carbon filter, where any residual pollutant from the atmospheric air was eliminated. The air was then humidified in a water column, entering afterwards a mixing chamber. In the mixing chamber, the clean and humidified air stream was mixed with a synthetic mixture of H\({}_{2}\)S, toluene and methyl mercaptan in N\({}_{2}\) (purchased to Abello Linde, Barcelona, Spain). The resulting polluted stream was divided into three identical streams of 1.25 L min\({}^{-1}\) by means of rotameters. The individual streams were then fed at the bottom of each biotrickling filter. The clean air exiting the biotrickling filters was discharged outside the room using an extraction fan. Three 0.5-L holding tanks magnetically stirred were used as liquid mineral salt medium (MSM) reservoirs, containing the required nutrients for the microbial community. The liquid was continuously recycled through the packed beds at a flowrate of 0.15 L min\({}^{-1}\) countercurrently with the polluted air stream (liquid trickling velocity of 2 m h\({}^{-1}\)). A 9-days abiotic test was performed prior inoculation of the BTFs in order to evaluate the possibility of physical-chemical removal of the target pollutants by the packing materials. After that, three aliquots of 200 mL of fresh activated sludge were centrifuged for 10 min at 10,000 rpm. The supernatant was removed, and the biomass was resuspended in 100 mL of fresh MSM. The inoculum was then added to the holding tanks and recycled through the bed. The BTFs were initially operated at a gas retention time (GRT) of 2 min, which was decreased to 1 min, 30 s, 15 s and 7.5 s in subsequent operating stages. Following the inoculation period, 150 mL of the cultivation medium were daily withdrawn for analysis and replaced with fresh MSM. This MSM renewal also maintained the pH above inhibitory levels. The mineral salt medium exchange rate was increased when decreasing the gas residence time, following an exchange ratio of 2.5 L MSM (g H\({}_{2}\)S fed)\({}^{-1}\). Inlet and outlet gas concentrations of H\({}_{2}\)S, methylmercaptan and toluene were daily analysed. Liquid samples from the recycling medium were periodically withdrawn in order to determine pH and sulfate concentration. The pressure drop across the packed beds was also periodically measured. Phase II. The experimental setup consisted of two identical PVC biofilters (internal diameter = 0.1 m, height = 1 m, total working volume = 2.5 L) packed with two different materials: Filtralite AIR 10-20 mm (BF-F) and Filtralite AIR AC (BF-FAC), and a two-phase biotrickling filter packed with Filtralite AIR 10-20 mm (2 PBTF-F) (Figure 1). The \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Acronym** &
\begin{tabular}{c} **Density** \\ **(kg m\({}^{-3}\))** \\ \end{tabular} & **Porosity** & **Particle Size** & **Material** \\ \hline Kaldnes K2 & K & 950 & 85\% & 10 mm & Polyethylene (PEHD) \\ Filtralite AIR AC & FAC & Bulk: 700 & 63\% & 0.5–2 mm & Solid grains of thermally treated clay \\ Filtralite AIR 10-20 & F & Bulk: 275 & 43\% & 8–20 mm & Expanded clay \\ \hline \end{tabular}
\end{table} 
Table 1: Main characterization parameters of the packing materials tested. Figure 1: Schematic representation of the experimental setup (red: Phase I, blue: Phase II). Irrigation to the bioreactors (dotted lines) was either intermittent (BFs) or continuous (BTFs). SP: sampling port; MSM: mineral salt medium.