## 4 Conclusions  
The study showed that the rates of nitrification, denitrification, and organic compound removal were determined not only by the reactor operating temperature, but also by the organic carbon content (C/N ratios) in treated wastewater. Reaction rates were found to decrease at progressively lower temperatures in all of the experimental series. In terms of nitrification, the reaction rates fell as the organic compound levels (sodium formate and potassium acetate) rose. The nitrification rate was the highest (0.32 mg N/L-h) at 25 \({}^{\circ}\)C and C/N = 0.5, whereas the lowest rate (0.18 mg N/L-h) was produced at 0 \({}^{\circ}\)C for C/N = 2.5 and 5.0. The impact of the higher C/N ratios was more evident at lower biofilter operating temperatures. The study did not indicate that increased organic substrate levels led to improved denitrification rates, which were instead linked to the nitrification efficiency. The denitrification rate was found to be the highest (0.31 mg N/L-h) at 25 \({}^{\circ}\)C and C/N = 0.5, and the lowest (0.18 mg N/L-h) at 0 \({}^{\circ}\)C and C/N = 5.0. The lowest organic removal rates were noted for C/N = 0.5, ranging between 11.20 and 18.42 mg COD/L-h at 0 and 25 \({}^{\circ}\)C, respectively. The highest rates were recorded for C/N = 0.5, ranging between 27.83 and 59.43 mg COD/L-h at 0 and 25 \({}^{\circ}\)C, respectively. These results indicate that a treated wastewater with C/N = 0.5 does not inhibit the activity of nitrifying bacteria and actually reduces organic pollutant levels in the effluent, while also creating optimal conditions for simultaneous nitrification and denitrification, as long as suitable biofilter filling is provided.  
The following are available online at [[https://www.mdpi.com/2076-3417/11/4/1724/s1](https://www.mdpi.com/2076-3417/11/4/1724/s1)]([https://www.mdpi.com/2076-3417/11/4/1724/s1](https://www.mdpi.com/2076-3417/11/4/1724/s1)), Figure S1: Effect of temperature on the nitrification rate--series 1 (C/N = 0.5), Figure S2: Effect of temperature on the nitrification rate--series 2 (C/N = 2.5), Figure S3: Effect of temperature on the nitrification rate--series 3 (C/N = 5.0), Figure S4: Effect of temperature on the denitrification rate--series 1 (C/N = 0.5), Figure S5: Effect of temperature on the denitrification rate--series 2 (C/N = 2.5), Figure S6: Effect of temperature on the denitrification rate--series 3 (C/N = 5.0), Figure S7: Effect of temperature on the organic removal rate--series 1 (C/N = 0.5), Figure S8: Effect of temperature on the organic removal rate--series 2 (C/N = 2.5), Figure S9: Effect of temperature on the organic removal rate--series 3 (C/N = 5.0), Table S1: Goodness of fit and the rates of nitrification, denitrification, and organic compound removal depending on the adopted operating parameters of the biofilters.  
Conceptualization, W.J. and J.R.; Methodology, J.R.; Software, A.M.; Validation, K.O., J.R. and A.M.; Formal Analysis, J.R.; Investigation, K.O.; Resources, K.O. and W.J.; Data Curation, K.O. and A.M.; Writing--Original Draft Preparation, A.M. and W.J.; Writing--Review & Editing, J.R.; Visualization, A.M.; Supervision, W.J.; Project Administration, K.O.; Funding Acquisition, K.O. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Project No. 29.610.023 of the University of Warmia and Mazury in Olsztyn, Poland.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.