## 2 Materials and Methods  
### Experimental Model  
The experiment was performed in laboratory-scale models of biofilters operated at the Department of Environmental Engineering at the University of Warmia and Mazury in Olszqyn (Katedra Inzynierii Srodowiska UWM w Olsztynie). The biofilters were loaded with a granulate having the structure of expanded-clay aggregate (with a diameter d\({}_{60}\) = 8.2 mm, d\({}_{60}\)/d\({}_{10}\) = 2.27, bulk density of 0.88 g/cm\({}^{3}\), hydraulic conductivity of K\({}_{\text{h}}\) 0.52 cm/s and resistance to crushing of 12.6 N/mm\({}^{2}\)) prepared from fly ash from sewage sludge thermal treatment in the \"Debogorze\" Wastewater Treatment Plant in Gdynia (Poland). The granulate was prepared according to the method of mechanical plasticization and fragmentation of the raw material, followed by sintering of small balls in a rotary kiln at 1200 \({}^{\circ}\)C [15]. Biofilters consisted of a cylindrical polyethylene pipe and a cone-shaped bottom. In the bottom of the biofilter, which was outflow part, was a drain valve used to collect samples of wastewater. The technical parameters of the biofilter were: surface area 95 cm\({}^{2}\), volume 2500 cm\({}^{3}\), active volume 1552 cm\({}^{3}\), total height 0.24 m and biofilter filling height 0.19 m [6]. Four temperature variants were adopted: 0 \({}^{\circ}\)C, 4 \({}^{\circ}\)C, 8 \({}^{\circ}\)C, and 25 \({}^{\circ}\)C (control biofilter). The experiment was conducted at C/N levels of 0.5, 2.5, and 5.0. C/N ratio affects the efficiency of the biofilters (in relation to C and N removal). The C/N ratio applied in the study, was aimed to determine the kinetics under conditions favoring nitrification (C/N = 0.5), denitrification (C/N = 5.0) and intermediate conditions (C/N = 2.5). Stable operating temperatures were maintained by a thermostatic chamber that regulated the temperature with an accuracy of \(\pm\) 0.1 \({}^{\circ}\)C. Synthetic wastewater, prepared from a weighted sample including commonly used agents for de-icing airport pavements and tap water, was the substrate for the experiments. The composition and physicochemical parameters of the wastewater used in the study are given in Table 1.  
To promote nitrification, a low hydraulic load of 5 dm\({}^{3}\)/m\({}^{2}\cdot\)d and hydraulic retention time (HRT) of 4 d were adopted, equivalent to the cycle of de-icing agent application in airports. The study on the kinetics of pollutant removal started at the end of technological research lasting four months. This four month research was preceded by three months adaptive period [6]. Samples were collected after 0.5, 6, and 12 h of operation; then, at 12 h intervals for 84 h, with the final sample taken after 96 h of operation (Figure 1). The size of the final sample was 5 cm\({}^{3}\).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**C/N Ratio**} \\ \cline{2-4}  & **0.5** & **2.5** & **5.0** \\ \hline CH\({}_{4}\)N\({}_{2}\)O (urea) [mg/L] & 150.00 \(\pm\) 0.10 & 150.00 \(\pm\) 0.10 & 150.00 \(\pm\) 0.10 \\ \hline HCOONa (sodium formate) [mg/L] & 136.00 \(\pm\) 0.10 & 657.00 \(\pm\) 0.10 & 1326.00 \(\pm\) 0.10 \\ \hline CH\({}_{3}\)COOK (potassium acetate) [mg/L] & 49.00 \(\pm\) 0.10 & 237.00 \(\pm\) 0.10 & 478.00 \(\pm\) 0.10 \\ \hline  & \multicolumn{3}{c}{Parameter} \\ \hline Nog. [mgN/L] & 71.56 \(\pm\) 2.20 & 71.56 \(\pm\) 2.20 & 71.56 \(\pm\) 2.20 \\ \hline N Kjeldhal [mgN/L] & 70.80 \(\pm\) 3.02 & 70.80 \(\pm\) 3.02 & 70.80 \(\pm\) 3.02 \\ \hline COD [mg/L] & 100.66 \(\pm\) 1.34 & 386.80 \(\pm\) 1.94 & 765.50 \(\pm\) 2.90 \\ \hline pH & 7.47–8.03 & 7.47–8.03 & 7.47–8.03 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Composition and parameters of the wastewater tested.