## 3 Results and Discussion
### Nitrification  
The nitrifier's activity is very temperature-sensitive [3]. However, exposure to low temperatures did not impact it to the point of halting ammonia nitrogen oxidation, which followed the zero-order reaction, meaning that changes in activity levels over time were linear. Similar findings have been reported in the literature, indicating that longer HRTs may be used to improve removal performance at low temperatures [18]. The rate and efficiency of nitrification of airport de-icing wastewater increased at higher biofiliter operating temperatures. In contrast, the higher organic matter content in series 2 and 3 caused a slight reduction in the ammonia nitrogen oxidation rate at lower biofiliter operating temperatures (Figures 2 and 3; Supplementary Figures S1-S3). At C/N = 0.5 group, the highest nitrification rate in the entire series, i.e., 0.32 mgN/L-h, was determined in the biofiliter operating at 25 \({}^{\circ}\)C. Lowering biofiliter operating temperatures, with the other technical parameters being equal, reduced the process rate by 31.3% (to 0.22 mgN/L-h) at 0 \({}^{\circ}\)C. In the two remaining reactors (T = 4 and 8 \({}^{\circ}\)C), the nitrification rates were 25.0 and 15.6% lower, respectively. It is noteworthy that the effect of temperature on reaction kinetics is much harder to determine in biofilm reactors than in suspended-growth biomass, and not as drastic as predicted by the van't Hoff-Arrhenius equation [19]. Other factors, such as the reactor operating parameters, HRT or wastewater composition, may lessen the impact of temperature [20]. While lower temperatures do reduce nitrification rates, they also increase dissolved oxygen concentration in the water, meaning that the effect of temperature is mitigated by the higher oxygen availability. Thus, the resultant reduction in the biofilm nitrification rate was ultimately negligible (1.108% per 1 \({}^{\circ}\)C) [19]. The decreases in nitrification rates produced in the present experiment were similar. Autotrophic nitrification rates are, in large part, driven by high levels of readily digestible organic compounds, which promote heterotrophic bacterial growth [21]. According to [PERSON] et al. [22], fast-growing heterotrophic microorganisms compete for oxygen and ammonia nitrogen with nitrifying bacteria, and their metabolic intermediates may inhibit the activity of the latter. To achieve optimal nitrification conditions, the influent organic carbon must be kept under 2 kg COD/m\({}^{3}\cdot\)d [22]. In an experiment by [PERSON] et al. [23], the highest nitrification rates were produced at C/N ratios under 6. Another study, by [PERSON] et al. [24], showed that C/N values as low as 0.3 led to the competition between heterotrophs and autotrophs, impairing nitrification. In turn, [PERSON] et al. [25] reported that the inhibiting effect of high C/N ratios on ammoniacal nitrogen removal from treated wastewater was more pronounced at lower temperatures. This finding is corroborated by the present study, which showed that the reductions in nitrification rate and efficiency were higher at the lowest of the test temperatures (T = 0 \({}^{\circ}\)C), but minor at 25 \({}^{\circ}\)C and increased organic compound load. In series 2 (C/N = 2.5), the nitrification rate was similar to that of series 1 (0.31 mgN/L-h at 25 \({}^{\circ}\)C). As in series 1, the lowest nitrification rate, reaching 0.18 mgN/L-h (41.94% lower), was observed at the lowest treatment temperature (0 \({}^{\circ}\)C). The nitrification rates were similar in the 8 \({}^{\circ}\)C and 4 \({}^{\circ}\)C variants, reaching 0.22 and 0.21 mgN/L-h, respectively. With the higher organic carbon levels (C/N = 5) in series 3, the nitrification rate remained stable at 0.31 mgN/L-h in the control reactor (T = 25 \({}^{\circ}\)C). In the other reactors analyzed in this series, it was approx.