## 3 Results and Discussion
### Removal Rate of Phosphorus and Nitrogen Loads per Time Unit  
The study results demonstrated that the removal rate of phosphorus and nitrogen loads per time unit in the RECDC was affected primarily by HRT. The highest daily phosphorus load removed was achieved at the shortest HRT tested (HRT = 4 h), whereas the lowest daily phosphorus load removed was achieved at HRT = 24 h (Figure 2a). The HRT extension contributed to a decrease in the daily load removed. In turn, an increase in electric current density led to an increase in pollutant load removed at all HRTs tested. In the case of HRT = 4 h, current density increases from 0.63 to 10.0 A/m\({}^{2}\) caused an over 20% increase in the phosphorus load removal rate, i.e., from 30.14 to 36.91 g P/m\({}^{2}\)-d. The HRT extension to 24 h caused a significant decrease in the daily phosphorus removal performance to values of 5.46 and 5.94 g P/m\({}^{2}\)-d, respectively. At HRTs of 8, 12, and 24 h, the changes in electric current densities had a negligible effect (below 5%) on the increase in the daily load removal rate (Figure 2a). As in the case of phosphorus, the HRT was the main driver of the daily nitrogen load removal (Figure 2b). The highest daily removed load was noted at HRT = 4 h, and the lowest daily removed load was at HRT = 24 h. In the case of HRT = 4 h, an electric current increase from 0.63 to 10.0 A/m\({}^{2}\) doubled the daily removed load from 0.95 to 2.03 g N/m\({}^{2}\)-d, while at HRT = 24 h, the respective values ranged from 0.36 to 0.87 g N/m\({}^{2}\)-d. At the remaining HRTs tested, the increases were more noticeable than at HRT = 4 h. At the same time, the removal rates noted were substantially lower thanthose determined for phosphorus (from 5.46 to 36.91 g P/m\({}^{2}\)-d). It should be remembered, however, that, in the electrochemical contact, phosphorus compounds were removed in the process of highly effective electrocoagulation, whereas nitrogen compounds were removed during less effective electrochemical reduction of nitrates [22; 23]. Figure 2: The influence of current density and HRT on the removed load (L\({}^{\text{A}}\)) per day in the electrochemical rotating disc contactor (RECDC): (**a**) Phosphorus (L\({}^{\text{AP}}\)\({}_{\text{Fe}}\)); (**b**) total nitrogen (L\({}^{\text{A}}\)N\({}_{\text{Fe}}\)). Figure 3: The influence of current density and HRT on the removed load per day (L\({}^{\rm A}\)) in the electrobiological rotating disc contactor (REBDC): (**a**) Phosphorus (L\({}^{\rm AP}\)**re**); (**b**) total nitrogen (L\({}^{\rm A}\)N\({}_{\rm ne}\)). Figure 4: The influence of current density and HRT on the concentration of pollutants in the treated wastewater in the electrochemical rotating disc contactor (RECDC): (**a**) Phosphorus (C\({}_{\rm Pe}\)); (**b**) total nitrogen (C\({}_{\rm Ne}\)); (**c**) nitrates (C\({}_{\rm NOSe}\)). Figure 5: The influence of current density and HRT on the concentration of pollutants in the treated wastewater in the electrobiological rotating disc contactor (REBDC): (**a**) Phosphorus (C\({}_{\rm Pe}\)); (**b**) total nitrogen (C\({}_{\rm Ne}\)); (**c**) nitrates (C\({}_{\rm NOSe}\)). In the bioelectrochemical contactor, the daily load of removed phosphorus (\(\mathrm{L^{AP}}_{\mathrm{pe}}\)) was higher than that in the electrochemical reactor (Figure 3a). At HRT = 4 h, it reached 33.56 and 37.08 g \(\mathrm{P/m^{2}}\)-d for electric current densities of 0.63 and 10.0 A/m\({}^{2}\), respectively. At HRT = 24 h, the respective values were 5.72 and 5.92 g \(\mathrm{P/m^{2}}\)-d, which confirmed the results obtained in the REDC, where HRT extension led to a decrease in the daily removal rate. It was also found that electric current density increases at a given HRT contributed to the neutralization of a difference noted between both reactors in the daily amount of removed phosphorus load. These observations were consistent with the results of our  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Parameter**} & \multirow{3}{*}{