## 3 Results and Discussion
### Impact of Treatment in a BER on the Concentration of Other Macro- and Micro Elements  
Previous studies that involved hydroponic effluent treatment have focused on nitrogen and phosphorus removal. However, not enough emphasis has been placed on the other macro- and micro elements [9]. The acceptable content of organic compounds, nitrogen and phosphorus in horticultural effluent (including hydroponic wastewater) is defined in the relevant legal regulations. Apart from the basic physicochemical parameters (TOC, TN, TP), hydroponic effluents should meet acceptable levels with respect to the other elements contained in them [5; 7; 8]. The macronutrients- and micro elements' concentration in the hydroponic wastewater, and their acceptable levels, according to Polish legislation are presented in Table 1.  
Figure 5: Organic compound load removed in the BER. Apart from the high nitrogen and phosphorus content, the concentrations of molybdenum, boron, sulphates and potassium prevent untreated hydroponic effluent from being discharged into the environment. According to the regulations on horticultural effluent discharge into the environment, the molybdenum and sulphate concentrations in effluent from hydroponic tomato cultivation also exceed the norms. Therefore, current and future studies should include extended physicochemical analysis. The copper and iron concentrations in the effluent, after treatment in BERs, grew with increasing electric current density (Figure 6). The mean total iron concentration in the effluent ranged from 0.133 \(\pm\) 0.062 mg Fe\({}_{\text{tot. }}\)/L (R1) to 0.244 \(\pm\) 0.076 mg Fe\({}_{\text{tot. }}\)/L (R4). It was higher in the R2-R4 reactors' effluent than in wastewater entering the reactors. The Cu\({}^{2+}\) concentration was 0.066 \(\pm\) 0.011 mg Cu\({}^{2+}\)/L at the lowest current density, and it was the highest in R4--0.162 \(\pm\) 0.048 mg Cu\({}^{2+}\)/L. The copper concentration in R3 and R4 was higher than in the untreated effluent. The manganese concentration ranged from 0.141 \(\pm\) 0.021 mg Mn\({}^{2+}\)/L (R1) to 1.160 \(\pm\) 0.217 mg Mn\({}^{2+}\)/L (R4). The Mn\({}^{2+}\) concentration in R4 was higher than in the hydroponic. The aluminium concentration in R1 was outside the detectability range (<0.2 mg Al\({}^{3+}\)/L), as was the effluent entering the reactor. The Al\({}^{3+}\) concentration grew with increasing current density, from 0.067 \(\pm\) 0.003 mg Al\({}^{3+}\)/L to 0.351 \(\pm\) 0.048 mg Al\({}^{3+}\)/L, in R1 \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Chemical Element**} & \multirow{2}{*}{**Mean Value with Standard Deviation**} & **Maximum Permissible Values of Pollutants for** & \multirow{2}{*}{**Permissible Values of Pollution**} \\  & & **Biodegradable Industrial Wastewater Generated** & \\  & & **during the Production and Processing of Fruit and** & \\  & & **V vegetables Discharged into Water or Ground** & \\ \hline Mo\({}^{+}\) & 2.87 \(\pm\) 0.17 mg Mo\({}^{+}\)/L & 1 & 1 \\ \hline Mn\({}^{2+}\) & 0.738 \(\pm\) 0.109 mg Mn\({}^{2+}\)/L & - & - \\ \hline Bi\({}^{2+}\) & 1.75 \(\pm\) 0.06 mg B\({}^{2+}\)/L & 1 & 10 \\ \hline Cu\({}^{2+}\) & 0.130 \(\pm\) 0.019 mg Cu\({}^{2+}\)/L & 0.5 & 1 \\ \hline Zn\({}^{2+}\) & 1.65 \(\pm\) 0.26 mg Zn\({}^{2+}\)/L & 2 & 5 \\ \hline K\({}^{*}\) & 1276.9 \(\pm\) 96.7 mg K\({}^{*}\)/L & 80 & - \\ \hline SO\({}_{2}\)\({}^{2-}\) & 1425.0 \(\pm\) 102.6 mg SO\({}_{2}\)\({}^{2-}\)/L & 500 & 500 \\ \hline Cl\({}^{-}\) & 91.8 \(\pm\) 6.1 mg Cl\({}^{-}\)/L & 1000 & 1000 \\ \hline Al\({}^{3+}\) & \textless{}0.02 mg Al\({}^{3+}\)/L & 3 & 3 \\ \hline Fe\({}_{\text{tot.}}\) & 0.127 \(\pm\) 0.006 mg Fe\({}_{\text{tot.}}\)/L & 10 & 10 \\ \hline Ca\({}^{2+}\) & 797.0 \(\pm\) 3.6 mg Ca\({}^{2+}\)/L & - & - \\ \hline Mg\({}^{2+}\) & 181.1 \(\pm\) 3.4 mg Mg\({}^{2+}\)/L & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The macronutrients- and micro elements’ concentration in the hydroponic wastewater, and their acceptable levels, according to Polish legislation [7; 8].