## 3 Results and Discussion
### Impact of Treatment in a BER on the Concentration of Other Macro- and Micro Elements
Figure 6: Concentration of Mn\({}^{2+}\), Cu\({}^{2+}\) and Fe\({}_{\text{tot. }}\) in BER effluent. and R4, respectively (Figure 4). This is probably because low-carbon steel, which was the material for the anode used in the experiment, can contain up to 0.3% aluminium [33]. The molybdenum concentration in treated wastewater from R1 and R2 was 74.9 \(\pm\) 14.0 mg Mo\({}^{6+}\)/L and 116.7 \(\pm\) 6.9 mg Mo\({}^{6+}\)/L, respectively. It was much higher than in the hydroponic wastewater. In R3, the molybdenum content was lower, and was 9.7 \(\pm\) 3.6 mg Mo\({}^{6+}\)/L. The molybdenum concentration in R4 was similar to that in the untreated hydroponic wastewater--2.8 \(\pm\) 0.5 mg Mo\({}^{6+}\)/L. A stainless steel cathode and a low-carbon steel anode were used in the reactor. It can be assumed, based on the steel's chemical composition, that part of the Mn\({}^{2+}\), Cu\({}^{2+}\), Fe\({}_{\text{tot}}\), and Mo\({}^{6+}\) ions were supplied to the effluent of the BERs as a result of electrode corrosion. Aluminium electrodes can undergo cathodic dissolution by hydroxyl ions formed during water hydrolysis [34; 35]. Electrode corrosion can also be caused by the action of salts, alkalis and acids [36]. A lower molybdenum concentration in BER's effluent at higher current densities is probably a consequence of the molybdenum consumption from the electrodes at the start-up, or coagulant removal on floccules, the amount of which is larger at higher current densities. The literature provides information on the possibility of removal of 99.9% of molybdenum from potable water (reduction from 9.95 mg/L to 0.006 mg/L) [37]. The findings of [PERSON] et al. [38] show that effective molybdenum removal from the solution takes place in an acidic environment at a suitable Fe/Mo ion ratio. The highest concentrations of Mg\({}^{2+}\) and Ca\({}^{2+}\) in the BER effluent were 132.9 \(\pm\) 41.4 mg Mg\({}^{2+}\)/L in R1 and 455.4 \(\pm\) 37.6 mg Ca\({}^{2+}\)/L in R4. The lowest calcium ion concentration was found in R1, and the lowest concentration of magnesium ions was found in R4 (196.2 \(\pm\) 16.5 mg Ca\({}^{2+}\)/L and 93.8 \(\pm\) 27 mg Mg\({}^{2+}\)/L, respectively). These ions are removed mainly by precipitation of calcium and magnesium phosphates in an alkaline environment [11; 26; 39; 40]. Calcium and magnesium in hydroponic effluent can form sludges with SO\({}_{4}\)\({}^{2-}\); they create an insulation layer on the electrodes, which can be dissolved by adding Cl\({}^{-}\)[41]. Earlier studies by [PERSON] et al. [11] and [PERSON] et al. [5] showed also that magnesium and calcium concentrations decreased following the application of bio-electrochemical and biological processes in an alkaline environment. No considerable concentration changes for boron ions were observed following the treatment in the BER. The lowest boron content was in R2 effluent (1.46 \(\pm\) 0.09 mg B\({}^{2+}\)/L), and the highest was in R4 (1.61 \(\pm\) 0.23 mg B\({}^{2+}\)/L). Similar levels were found in R1 and R3: 1.55 \(\pm\) 0.03 mg B\({}^{2+}\)/L and 1.53 \(\pm\) 0.02 mg B\({}^{2+}\)/L, respectively. The zinc content decreased during treatment with increasing current density, and it was similar in R1 and R2, as well as in R3 and R4. The Zn\({}^{2+}\) content in R1 and R2 was 0.078 \(\pm\) 0.090 mg Zn\({}^{2+}\)/L and 0.079 \(\pm\) 0.068 mg Zn\({}^{2+}\)/L, respectively. With these figures, the efficiency of removal was 95%. The Zn\({}^{2+}\) content in R3 and R4 were 0.313 \(\pm\) 0.295 mg Zn\({}^{2+}\)/L and 0.314 \(\pm\) 0.208 mg Zn\({}^{2+}\)/L, respectively. The mean removal efficiency in both reactors was approximately 81%. Potassium concentrations in treated hydroponic wastewater were 1077 \(\pm\) 86.5 mg K\({}^{+}\)/L and 1078.3 \(\pm\) 18.9 mg K\({}^{+}\)/L in R1 and R3, respectively. R2 and R4 effluent concentrations were higher--1140.0 \(\pm\) 32.0 mg K\({}^{+}\)/L and 1142.5 \(\pm\) 74.0 mg K\({}^{+}\)/L, respectively. The highest results for potassium removal efficiency were achieved in R1 and R3--15.6% and 15.5%, respectively.