## 3 Results and Discussion
### Correlations between Electric Current Density and HRT and Pollutant Concentrations  
Table 1 presents the highest permissible concentrations of pollutants in wastewater from soilless plant cultivation discharged into natural waters or the ground and to sewage systems. Figure 4 shows correlations between operating parameters (electric current density and HRT) and pollutant concentrations in the effluent treated in the electrochemical contactor. At all HRTs tested, an increase in electric current density resulted in decreased effluent phosphorus concentration. A similar tendency was observed while changing HRT. Its extension was found to improve the effluent's quality. The lowest phosphorus concentrations were at HRT = 24 h and ranged from 6.2 to 0.14 mg P/L at current densities increasing from 0.63 to 10.0 A/m\({}^{2}\). At HRT = 24 h, the pollutant concentrations in the effluent failed to meet the requirements set for the industrial wastewater discharged into natural waters or the ground (3.0 mg P/L, Table 1) only at the lowest current density tested (0.63 A/m\({}^{2}\)). At HRT = 12 h, the concentration of phosphorus was lower than 3.0 mg P/L only at current densities of 5.0 and 10.0 A/m\({}^{2}\). At all HRTs and \(J\) = 10.0 A/m\({}^{2}\), the concentration of phosphorus in the effluent was below 1.0 mg P/L, which means it could be discharged into natural waters and the ground. Figure 4b,c presents concentrations of total nitrogen achieved at various current densities and HRTs, and reveals similar tendencies as those observed in Figure 4a, i.e., increased current density and extended HRT decreased nitrogen concentrations in the effluent. At HRT = 24 h and \(J\) = 10.0 A/m\({}^{2}\), the concentration of total nitrogen was at 217.7 mg N/L, which means that the effluent treated in the electrochemical reactor could not be discharged into natural waters or the ground as it failed to meet the requirements set for industrial wastewater (Table 1). The concentration of phosphorus in the effluent from the bioelectrochemical reactor (REBDC) was lower than in that from the electrochemical reactor (Figure 5a). At HRT = 24 h, the lowest phosphorus concentrations in the effluent from the RECDC reached 2.80 and 0.10 mg P/L at electric current densities of 0.63 and 10.0 A/m\({}^{2}\), respectively. At HRT = 24 h and all current densities tested, the effluent concentration met the requirements set for industrial wastewater discharged into natural waters and the ground (3.0 mg P/L). At HRT = 12 h, the phosphorus concentration exceeded 3.0 mg P/L only at \(J\) = 0.63 A/m\({}^{2}\). At current densities of 2.5, 5.0, and 10.0 A/m\({}^{2}\) and all HRTs tested, the effluent met the requirements set for industrial wastewater discharged into natural waters and the ground. At all HRTs and \(J\) = 10.0 A/m\({}^{2}\), the phosphorus concentration in the effluent was lower than 1.0 mg P/L, which made the effluent suitable for discharge into natural waters and the ground [8]. The concentrations of nitrogen and nitrates in the effluent discharged from the REBCD (Figure 5) were lower than the respective values achieved in the experiment with the RECDC. The lowest values were determined at HRT = 24 h and \(J\) = 10.0 A/m\({}^{2}\), when the concentration of total nitrogen reached 147.7 mg N/L, which means that the wastewater treated in the bioelectrochemical reactor could be discharged into natural waters and the ground as it met the requirements set for industrial wastewater (Table 1). Nevertheless, it needs to be remembered that the external carbon source (sodium acetate) used in the present study was a very low dose, to reach a C:N ratio of 0.5 and to ensure the growth of biofilm biomass, and simultaneously allow for autotrophic denitrification [10]. Increasing the carbon dose has a direct impact on the effectiveness of denitrification and nitrogen removal [27; 28].