## 3 Results and Discussion  
### Nitrogen and Phosphorus Removal  
The total nitrogen concentration in hydroponic effluent was 580.3 \(\pm\) 21.6 mg N/L. The concentrations of various nitrogen forms in treated wastewater are shown in Figure 2. The total nitrogen concentration in reactors R1 and R2 was found to be similar--338.7 \(\pm\) 30.2 mg N/L and 338.9 \(\pm\) 37.3 mg N/L, respectively. Subsequently, the TN concentration grew, with increasing current density, to 488.0 \(\pm\) 38.3 mg N/L (R3) and 528.0 \(\pm\) 37.9 mg N/L (R4). The nitrate's concentration grew with increasing current density. The lowest value was found in R1 (206.6 \(\pm\) 15.5 mg N/L) and the highest in R4 (542.8 \(\pm\) 41.0 mg N/L). The ammonium concentration was also the lowest in R1--0.5 \(\pm\) 0.5 mg N/L, and the highest in R4--7.5 \(\pm\) 5.1 mg N/L. The nitrite's concentration at  
Figure 1: Experimental design. the start of the treatment process was 0.2 \(\pm\) 0.1 mg N/L. The NO\({}_{2}\)\({}^{-}\) concentration in reactor R1 effluent increased to 131.5 \(\pm\) 14.2 mg N/L. Lower nitrite concentrations were found in reactors R3 and R4--1.1 \(\pm\) 0.9 mg N/L and 1.0 \(\pm\) 0.3 mg N/L, respectively. Despite a similar TN effluent concentration in the R1 and R2 reactors, the nitrite's concentration was lower in the R2 reactor. This implies that use of a higher electric current density lowers the nitrite concentration. [PERSON] et al. [22] used a synthetic effluent containing high levels of organic compounds and applied a current of 0-1600 mA/m\({}^{2}\). They also found that nitrite concentration could be reduced at an appropriate current density (400 mA/m\({}^{2}\)). Similar findings were observed by [PERSON] et al. [23] during treatment of synthetic groundwater at a current density exceeding 200 mA/m\({}^{2}\). The nitrogen-removal efficiency decreased with increasing current density--from 41.6% (R1) to 8.9% (R4) (Figure 3). This is probably a consequence of the fact that mainly heterotrophic denitrification processes dominated in R1 and R2 (because of low J), hence the high nitrate concentration in effluent from these reactors. However, in the R3 and R4 reactors, nitrogen was probably removed by cathode reduction and autotrophic denitrification processes. In an earlier study, authors treated synthetic hydroponic wastewater in a rotating electrobiological contactor (RBC), with an aluminium anode and sodium acetate as an external carbon source (C/N-0.5), and they found that TN removal efficiency increased, with increasing current density, from 23.4% (J = 0.63 A/m\({}^{2}\)) to 68.6% (J = 10 A/m\({}^{2}\)). The present study showed that the use of a current with a density exceeding 1.25 A/m\({}^{2}\) resulted in lower efficiency of total nitrogen removal. This may be due to different wastewater characteristics and different anode material. According to [PERSON] et al. [10], current with a density of 1.6 A/m\({}^{2}\) has an inhibitory effect on denitrification. [PERSON] et al. [24] removed 85.2% of nitrates in a bio-electrochemical reactor at an initial concentration of 50 mg N/L. Figure 3: Efficiency of total nitrogen and total phosphorus removal from hydroponic effluent in a BER. Figure 2: Concentration of various nitrogen forms in BER effluent. Phosphorus was removed owing to the formation of iron cations: Fe\({}^{2+}\) and Fe\({}^{3+}\) on the anode, which destabilises colloid particles in the aqueous solution and forms monomeric and polymeric hydroxy complexes. Hydroxy iron complexes show adsorptive properties and form large aggregates with phosphorus. Additionally, it should be taken into account that hydroponic effluent contains high concentrations of calcium and magnesium, which can form phosphates with phosphorus in the effluent, at a pH above 7.2 [14; 25]. The total phosphorus concentration decreased with increasing electric current density amperage. The lowest phosphorus concentration was in R4--1.28 \(\pm\) 0.97 mg P/L (J = 5 A/m\({}^{2}\)), and the highest in R1--7.11 \(\pm\) 1.8 mg P/L (J = 0.63 A/m\({}^{2}\)).