# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 3 Results and Discussion
### Phosphorus Removal  
Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 on _Substances Particularly Harmful to the Aquatic Environment and Conditions to Be Met When Introducing Sewage into Waters or into the Ground, as Well as When Discharging_ defines the maximum permissible concentration of total phosphorus (TP)--3 mg/L (Dz.U. 2019, 1311) [31]. Mean TP concentrations in the influent (approx. 43.79 mg/L) and the treated effluent are provided in Figure 8. In each case, in the treated wastewater, the TP concentration far exceeds the value allowed by the regulation. Removal efficiencies across different organic carbon sources are given in Figure 9. The best-performing variants were PBS2 (42.48 \(\pm\) 10.3%) and Oxytree (31.14 \(\pm\) 6.17%). Beech chips and PBS1 provided similar removal efficiencies at 25.23 \(\pm\) 6.82% and 26.42 \(\pm\) 7.45%, respectively.  
Figure 7: Variation in DO concentration in different stages of treatment.  
[PERSON] et al. [44] noted high N and P removal in SBRs fed with food waste as the carbon source, with removal efficiencies of 92.70% and 92.38%, respectively. However, this study used influent with lower levels of phosphorus (approx. 5.36-6.19 mg PO\({}_{4}\)-P/L) and preceded the anoxic phase with an aerobic phase, which was responsible for the bulk of the P removal. A different study, by [PERSON] et al. [45], explored the effects of acetate and propionate on nitrogen and phosphorus removal during a A\({}^{2}\)O treatment. It was found that the choice of carbon source had a considerable impact on denitfying P removal in anoxic chambers, with 40.0% P removed in the acetate variant and 37.7% in the propionate variant. The experimental parameters were as follows: initial P = 8.75 mg/L, temperature = 21 \({}^{\circ}\)C, HRT = 8 h, and pH = 7.24-7.82. The lower TP removal efficiency in our study may be influenced by the much higher initial TP concentration in the influent wastewater.  
[PERSON] et al. [46] aimed to determine whether woodchips can release phosphorus through leaching in distilled water and aquaculture effluent. Phosphorus leaching was noted during the first 24 h (approx. 20-30 mg P per kg wood). Tests showed that woodchips can remove total phosphorus (15-54% removal efficiency; TP in the influent \(\approx\) 4 mg/L) after initial leaching and successive drying cycles. The present study (which employed denitfying P removal under anoxic conditions) did not detect any extra TP leaching from the woodchips.  
Figure 8: Concentration of total phosphorus in influent and treated wastewater.  
Figure 9: Total phosphorus removal efficiency according to the carbon source used.  
[PERSON] et al. [47] found that nitrates in the anoxic zone can inhibit phosphorus release until denitrification is finished. [PERSON] et al. [48] characterized P-removing denitrifying bacteria under anaerobic-anoxic conditions. This particular study used synthetic waste containing 20 mg P/L. Oxygen, nitrites, and nitrates served as electron acceptors. The study found that nitrites did not inhibit phosphorus removal, and instead acted as an alternative electron acceptor for oxygen and nitrates (maximal inhibitory concentration >115 mg NO\({}_{2}\)-N/L). This seems to suggest some groups of bacteria may be able to use both oxygen, nitrites, and nitrates as electron acceptors. Our study did not include a microbiological analysis, and thus we can only assume that the presence of NO\({}_{2}\)-N did not negatively impact P removal.