# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 3 Results and Discussion
### Nitrogen Removal
The ratio, which was 0.05 g COD/g N-NO\({}_{3}\) in the influent, surged after the first stage (I\({}^{\circ}\)) of treatment to 0.49 g COD/g N-NO\({}_{3}\), 0.52 g COD/g N-NO\({}_{3}\), 0.70 g COD/g N-NO\({}_{3}\), and 1.20 g COD/g N-NO\({}_{3}\) for been chips, Oxytree chips, PBS1, and PBS2, respectively. After the second stage (II\({}^{\circ}\)) of treatment, the PBS2 variant boasted the highest ratio (5.13 g COD/g N-NO\({}_{3}\)), though still insufficient for good denitrification performance. A solid carbon source is first hydrolyzed by extracellular enzymes (such as lipase) secreted by the microorganisms, then broken down into soluble and small-molecule substrates. Therefore, [PERSON] and [PERSON] [18] posit that the volume of carbon released is mainly regulated by bacteria as they react to nitrate levels in the water. However, carbon leaching can occur, as evidenced by [PERSON] et al. [39]--42.3-94.1% of the carbon was released during the initial stage (<24 h). [PERSON] et al. [40] also noted that plant-based organic carbon is released much faster in the initial phase. Thus, the increase in total organic carbon (TOC) and COD in the effluent is indicative of leaching and biodegradation of the solid carbon source. Figures 5 and 6 show trends in TOC and COD during the first and second treatment step. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Temperature [\({}^{\circ}\)C]**} & \multicolumn{4}{c}{**pH**} \\ \cline{2-10}  & **Arithmetic** & **Minimum** & **Maximum** & **Standard** & **Arithmetic** & **Minimum** & **Maximum** & **Standard** \\  & **Mean** & **Value** & **Value** & **Deviation** & **Mean** & **Value** & **Value** & **Deviation** \\ \hline Influent & 22.20 & 21.00 & 23.50 & 0.77 & 6.37 & 6.20 & 6.63 & 0.13 \\ Beech wood & 22.51 & 20.80 & 24.00 & 0.95 & 7.28 & 6.56 & 7.98 & 0.37 \\ Oxytree chips & 22.46 & 20.80 & 23.80 & 0.88 & 7.22 & 6.56 & 8.17 & 0.45 \\ PBS 1 & 22.49 & 20.90 & 23.90 & 0.90 & 7.06 & 6.03 & 7.98 & 0.65 \\ PBS 2 & 22.65 & 20.90 & 24.10 & 0.93 & 7.49 & 7.06 & 7.91 & 0.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average temperature and pH values depending on the carbon source used. Figure 4: COD/N-NO\({}_{3}\) ratio at each treatment stage. By far the highest TOC was noted for PBS2 at approx. 170.9 \(\pm\) 96.6 mg/L and approx. 236.6 \(\pm\) 125.5 mg/L after steps 1 and 2 of treatment, respectively. The respective values for other sources were: PBS1--61.5 \(\pm\) 28.7 mg/L and 95.0 \(\pm\) 45.0 mg/L; been chips--33.2 \(\pm\) 21.5 mg/L and 33.9 \(\pm\) 8.5 mg/L; Oxytree chips--30.3 \(\pm\) 10.5 mg/L and 44.0 \(\pm\) 17.7 mg/L. By comparison, [PERSON] et al. [32] recorded dissolved organic carbon (DOC) levels of approx. 136.11-202.51 mg/L. Influent COD was 34.96 mg/L, but rose significantly after treatment with PBS1 and PBS2 (to 763.39 \(\pm\) 219.60 mg/L and 1177.50 \(\pm\) 479.32 mg/L, respectively). The COD increase for woodchips was less pronounced, but still sufficient to qualify as recontamination (approx. 300 \(\pm\) 65 mg/L). The high DOC and COD may be attributable to carbon leaching or may indicate that the carbon load released by the metabolizing microorganisms exceeded the amount required by the denitrifiers to grow and remove nitrogen. Further research is needed on how to control the biodegradability of solid carbon sources to ensure optimal DOC in the wastewater (without it accumulating as a recontamination). According to [PERSON] et al. [41] heterotrophic denitrification performance can be affected by the presence of dissolved oxygen (DO). This explains the observed low efficiency of the denitrification process in all biofilters (Figure 3). In our results, denitrification performance peaked at DO = 3.97 mg/L for PBS2 after the second-step treatment. The reactor filled with PBS2 had more favorable oxygen conditions for the denitrification process to occur.