# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 3 Results and Discussion  
### Nitrogen Removal  
N-NO\({}_{3}\) levels by treatment step and nitrate removal by carbon source are shown in Figures 2 and 3. Nitrate concentration in the influent wastewater was 710.0 mg/L. The highest removal efficiencies (43.11 \(\pm\) 14.31% (\(\pm\)CI)) were noted for PBS2 (FD92 PB), whereas Oxytree chips performed the worst (8.80 \(\pm\) 4.88%). Beech chips and PBS1 (FZ91 PB) had similar denitrification rates at 14.10 \(\pm\) 9.09% and 15.55 \(\pm\) 9.09%, respectively. PBS2 has a higher degree of biodegradability than PBS1. Thus, access to organic carbon from this medium was easier for microorganisms performing denitrification, resulting in higher treatment efficiency. However, these values are relatively low compared to carbon sources tested by other researchers. Polish legislation contains information on the maximum permissible concentrations of pollutants entering the aquatic environment from the production and processing of fruits and vegetables. For nitrate nitrogen, the value is 30 mg/L, while for total phosphorus, it is 3 mg/L [31]. The treatment efficiencies achieved did not guarantee a reduction in nitrate concentrations to acceptable values. [PERSON] et al. [32] managed to achieve high nitrate removal of 96.42-99.52% for a PHBV/PLA polymer at the same HRT (24 h). However, it should be noted that this study used synthetic waste containing much lower levels of nitrates (50 mg/L NO\({}_{3}\)-N) similar to [PERSON] et al. [33] who utilized low-nitrate wastewater too--50-150 mg/L NO\({}_{3}\)-N using polybutylene succinate (PBS)--71.77% and 96.73% removal. The denitrification process is sensitive to pH change, and the enzymatic activity of denitrifiers is highly dependent on temperature [23]. Wastewater pH and temperature were controlled during each step of treatment. Table 2 shows mean parameter values by carbon source. Both factors were within the optimal range for denitrification: temperature was approx. 20-24 \({}^{\circ}\)C for all media, with average pH not exceeding 8. In each case, there is a slight increase in pH (the highest with the use of PBS2, the lowest--PBS1). The denitrification process produces alkalinity (OH\({}^{-}\) ions are formed), which can increase the pH of the denitrification system [9; 23]. [PERSON] and [PERSON] [34] used PCL-filled beds and achieved 92% nitrate removal at 7.2 pH, and 20 \({}^{\circ}\)C. [PERSON] et al. [14] tested maize cobs, green waste, wheat straw, and soft/hard woodchips as potential sources of organic carbon. For almost all these feedstocks, nitrogen removal rates were higher at 23.5 \({}^{\circ}\)C than at 14 \({}^{\circ}\)C. [PERSON] and [PERSON] [35] investigated how temperature affects nitrate reduction, using acetic acid, propionate acid, and a mixture of both as exogenous carbon sources. Increasing the temperature from 10 \({}^{\circ}\)C to 20 \({}^{\circ}\)C had  
Figure 3: Nitrate removal efficiency according to the carbon source used. Figure 2: Nitrate concentration in influuent and treated wastewater. more of an impact on denitrification and carbon removal parameters than the increase from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C. The authors found that denitrification is possible at low temperatures, and that the exact rate of denitrification and carbon consumption at a given temperature are affected by the initial nitrogen and carbon concentrations, respectively. Denitrification was found to be feasible under all tested conditions, as long as the initial pH was 6.5. Other authors have pointed to 20 \({}^{\circ}\)C as the optimal temperature for nitrate reduction [36]. According to [PERSON] et al. [37] denitifying microbes are the most active at neutral pH, with highest performance noted within the pH range of 6.0-8.0. It requires 0.646 g of organic matter expressed as COD to reduce 1 g of N-NO\({}_{3}\). For a value less than 1.13 g COD/g N-NO\({}_{3}\), nitrogen removal efficiency is low [38]. Figure 4 shows the COD/N-NO\({}_{3}\) ratio for the tested carbon sources at different stages of the experiment.