## 3 Results and Discussion
### Correlations between Electric Current Density and HRT and Pollutant Concentrations
The study results presented in Figures 4 and 5 demonstrate that the lowest pollutant concentrations in the effluent were obtained in both types of reactors (electrochemical and bioelectrochemical) at longer hydraulic retention times and higher current densities. Maintaining the appropriate HRT and J in the bioelectrochemical reactors ensures the conditions required for the development of hydrogenotrophic bacteria, such as an optimal concentration of electron donors, beneficial pH, and optimal duration of the reaction. However, an HRT that is too long leads to denitrification inhibition due to the accumulation of nitrites [29; 30], whereas current density that is too high contributes to a substantially exceeded saturation constant of hydrogen and to its excess remaining in the biofilm, which adversely affects denitrification [29; 31; 32]. In their study, [PERSON] et al. [33] proved that lower current intensities were more suitable for denitrification in bioelectrochemical reactors and that high intensities impaired this process. Their observation was confirmed in the present study conducted with electrochemical and bioelectrochemical Contactors (Table 3). At the highest electric current density tested, i.e., \(J\) = 10.0 A/m\({}^{2}\), the concentration of nitrites in the effluent from the RECDC exceeded 20.00 mg N/L regardless of HRT. Therefore, if wastewater is planned to be discharged into a municipal sewage system, the electrochemical contactor should operate at lower current densities to meet the requirements (Table 1). In turn, the concentration of ammonia nitrogen, determined at all HRTs and current densities tested, did not exceed 100 mg N/L, which made the effluent suitable to be discharged into a municipal sewage system. [PERSON] et al. [34] demonstrated a strong effect of HRT on nitrogen removal and nitrite nitrogen accumulation in a bioelectrochemical reactor. At short HRTs, for example, 5 h, they achieved 66.0% and 54.6% removal effectiveness of nitrates and total nitrogen, respectively. The concentrations of NO\({}_{3}\)-N and NO\({}_{2}\)-N in the effluent from the reactor were at 10.0 mg N/L and 3.5-4.0 mg N/L, respectively. The rate of denitrification was observed to increase along with extending hydraulic retention times and reached the maximal value at HRT = 10 h. Successive HRT extension (over 10 h) led to a decreased rate of denitrification. The longer HRTs (i.e., 10-12 h) contributed to the lesser accumulation of nitrite nitrogen in the effluent (below 1 mg N/L). When the environment is poor in organic carbon (i.e., at C:N = 1.5), the activities of both heterotrophic and autotrophic bacteria may prove to be important. The autotrophic bacteria need more time to remove nitrogen due to their relatively low growth rate. Therefore, a longer HRT can facilitate nitrogen removal by autotrophic denitfying bacteria and result in less accumulation of nitrite nitrogen. When organic carbon concentration is relatively low, the continuous mode of reactor operation with an HRT that is too long may lead to the extension of the endogenous phase of heterotrophic bacteria respiration. This, in turn, would reduce the number of heterotrophs and, by this means, decrease denitrification effectiveness. Extension of the HRT can improve nitrogen removal performance and reduce nitrite nitrogen accumulation. Nevertheless, an excessively long HRT is not recommended due to the interaction of heterotrophic and autotrophic mechanisms of denitrification [34]. This was also the case in the REBDC, where the concentration of nitrites exceeded 10.00 mg N/L only at the highest electric current density tested and three HRT values. At the other J and HRT values studied, the quality of the effluent met the requirements set for wastewater discharged into municipal sewage systems.