## 3 Results and Discussion
### Organic Removal Rate  
The temperature not only affects the rate of substrate diffusion into the cell, but also the activity of enzymes, and thus the rate of biochemical processes, which determine how quickly pollutants are biodegradable. What is more, microorganisms are able to grow and metabolize organic compounds as long as the liquid pollutant is provided, meaning that organic compounds can be efficiently removed from wastewater even at low temperatures [35]. It is believed that aerobic digestion of organic compounds proceeds at exponentially increasing rates as the temperature rises from 0 \({}^{\circ}\)C to 32 \({}^{\circ}\)C. Its rate is stable between 32 and 40 \({}^{\circ}\)C, then declines sharply, reaching zero at 45 \({}^{\circ}\)C. This correlation holds true in the systems where the process rate is not limited by substrate concentration [36]. In addition, biofilms are less susceptible to adverse variations in temperature than activated sludge systems [37]. The rates of organic compound removal obtained in the present study increased with the C/N ratios and biofiliter operating temperatures (Figure 2; Supplementary Figures S7-S9). The lowest rates of organic pollutant removal were recorded for series 1 reactors (C/N = 0.5), among which the control one (T = 25 \({}^{\circ}\)C) produced the highest reaction rate at 21.33 mgCOD/L'h. Lowering biofiliter operating temperatures to 8, 4, and 0 \({}^{\circ}\)C, with the other technical parameters being equal, reduced its performance by 18.4, 32.7, and 48.0%, respectively. Increasing organic carbon (C/N = 2.5) in the T = 0 \({}^{\circ}\)C biofiliter decreased organic compound removal by 65.5%, compared with the removal rates produced at 25 \({}^{\circ}\)C. The reaction rates were 51.8 and 57.3% lower in the other temperature variants (8 and 4 \({}^{\circ}\)C, respectively). The highest reaction rates were noted in the series with the greatest initial organic pollutant levels (C/N = 5.0). The peak value of organic compound removal, reaching 58.03 mgCOD/L'h, was reached in the control bioreactor (T = 25 \({}^{\circ}\)C). As in the C/N = 0.5 and 2.5 series, the organic removal performance in series 3 was the poorest at 0 \({}^{\circ}\)C (26.76 mgCOD/L)--being 53.9% lower than at T = 25 \({}^{\circ}\)C. The biofilters operating at 8 and 4 \({}^{\circ}\)C produced 41.5 and 45.7% lower reaction rates, respectively. Efficiencies and reaction rates were the highest in biofilters operating at 25 \({}^{\circ}\)C, and the lowest values in these operating at 0 \({}^{\circ}\)C (Figure 6).  
However, increased C/N ratios in the subsequent series also led to higher initial levels of organic compounds in the influent, necessitating the removal of higher pollutant loads. This caused the organic compound removal rates to decrease as the C/N increased, with the sole exception of the biofiliter operating at T = 25 \({}^{\circ}\)C and C/N = 5.0, which ensured an increase in organic pollutant removal rate from 55.4 \(\pm\) 2.8% (C/N = 2.5) to 61.2 \(\pm\) 3.1% (C/N = 5.0). Nevertheless, the pollutant levels (as expressed by COD) observed were the highest in series 3 (289.3-614.0 mgCOD/L), and the lowest in series 1 (17.2-35.1 mgCOD/L). These findings are in line with literature data, which indicate that as the organic pollutant load increases, so does the COD of the treated effluent. Thus, one way to improve the treatment efficiency at low temperatures is to reduce the organic load rate [37].  
Figure 6: Efficiency of organic compound removal from stormwater containing airport depicting agents.