## 3 Results and Discussion
### Nitrogen Removal  
The activity of heterotrophic denitrifying bacteria is less dependent on the temperature than that of nitrifying bacteria [27]. [PERSON] and [PERSON] [28] reported that 20-35 \({}^{\circ}\)C was the optimal temperature range for denitrification, which slows down considerably at temperatures lower than 10 \({}^{\circ}\)C and greater than 30 \({}^{\circ}\)C. [PERSON] et al. [29] noted that providing a source of readily biodegradable organic carbon helped maintain high nitrification efficiencies even at temperatures under 10 \({}^{\circ}\)C. In the present study, the denitrification rate increased at higher temperatures and decreased at higher C/N ratios (Figure 2; Supplementary Figures S4-S6), even though the presence of a readily available source of organic carbon is one of the main drivers of effective denitrification [30; 31]. In series 1 (C/N = 0.5), oxidized nitrogen was reduced in the control reactor (T = 25 \({}^{\circ}\)C) at a rate of 0.31 mgN/L-h. Nitrogen removal was impeded by lowered biofilter operating temperatures, with the other technical parameters being equal. The lowest oxidized nitrogen reduction rates, at 0.22 mgN/L-h, were recorded for the biofilter operating at 0 \({}^{\circ}\)C. The denitrification rate was 0.30 mgN/L-h at 25 \({}^{\circ}\)C in series 2 and 3 (C/N = 2.5 and 5.0). Lowering the biofilter operating temperature to 0 \({}^{\circ}\)C reduced the performance by 36.7% and 40.0% against the control, respectively. These findings indicate that the nitrogen removal rates recorded in the study across the different experimental series did not correlate directly with the processing temperature or the C/N ratio--instead, the nitrification rate and efficiency served as the main factors. This pattern was the most strikingly evident in series 3, where the biofilters were fed with wastewater having the highest C/N ratio and were operating at optimal temperature conditions (T = 25 \({}^{\circ}\)C). A decreased ammonia nitrogen concentration in the effluent is indicative of intensive nitrification in the reactors. The difference between oxidized ammonia nitrogen and the nitrate/nitrite concentration in the treated wastewater shows how much nitrogen was removed. High rates of ammonia nitrogen oxidation and a low nitrite/nitrate content in the biofilter effluent, which did not exceed 12% TN in series 1 (no available organic substrate after approx. 24 h; Supplementary Figure S7) and 8% TN in the other series, suggest that nitrification and denitrification occurred simultaneously (Figures 4 and 5). The main factor enabling simultaneous nitrification and denitrification is the gradient of dissolved oxygen levels, determined by inhibited diffusion in the film [32]. [PERSON] et al. [33] found that a dissolved oxygen concentration of 0.5-3.0 mgO\({}_{2}\)/dm\({}^{3}\) reduced oxygen penetration and enabled denitrification in the internal layers. The authors obtained a denitrification efficiency of 71%, with the simultaneous nitrification reaching 96-98%. Another study, by [PERSON] and [PERSON] [34] obtained 46-50% nitrogen removal through simultaneous nitrification and denitrification at dissolved oxygen levels between 2.5 and 3.0 mgO\({}_{2}\)/dm\({}^{3}\), depending on the filling used in the MBBR. The nitrogen removal rates recorded in the presented study ranged from 30.1 \(\pm\) 1.5% (T = 0 \({}^{\circ}\)C) to 43.0 \(\pm\) 2.1% (T = 25 \({}^{\circ}\)C) in series 1 (C/N = 0.5). In the series with the higher organic compound contents (higher C/N ratios), the biofilters operating between 0 and 8 \({}^{\circ}\)C exhibited lower performance. The nitrogen removal rate ranged from 26.5 \(\pm\) 1.3% (T = 0 \({}^{\circ}\)C) to 35.9 \(\pm\) 1.8% (T = 8 \({}^{\circ}\)C) for C/N = 2.5, and from 25.0 \(\pm\) 1.3% (T = 0 \({}^{\circ}\)C) to 31.2 \(\pm\) 1.6%  
Figure 3: Nitrification efficiency in the experiment depending on temperature and C/N ratio in storm-water containing airport de-icing agents.