## 2 Materials and Methods  
The effluent from hydroponic tomato cultivation in an open system, on a mineral wool substrate, was used. It was collected in summer and subsequently used in the tests during a 3-month period. This allowed for maintenance of its constant physical and chemical parameters. The properties of the effluent were: pH 5.35 \(\pm\) 0.42; electrolytic conductivity (EC) 7.572 \(\pm\) 0.479 mS/cm; temperature 19.5 \(\pm\) 1.0 \({}^{\circ}\)C; total organic carbon (TOC) 15.8 \(\pm\) 1.2 mg C/L; total nitrogen (TN) 592.3 \(\pm\) 9.7 mg N/L; ammonium nitrogen 0.67 \(\pm\) 0.19 mg N/L; nitrites 0.18 \(\pm\) 0.07 mg N/L; nitrates 591.3 \(\pm\) 10.1 mg N/L and total phosphorus (TP) 145.9 \(\pm\) 4.4 mg P/L.  
A bio-electrochemical reactor was used in the experiment. The active reactor volume was 2 L. Five stainless steel disks were used as the cathode, and they also acted as the reactor packing (chemical composition C \(\leq\) 0.030; Si \(\leq\) 0.75; Mn \(\leq\) 2.25; P \(\leq\) 0.025; S \(\leq\) 0.01; N \(\leq\) 0.1; Cr 17.00-19.00; Ni 13.00-15.00; Mo \(\leq\) 2.25; Cu \(\leq\) 0.5). An anode made of carbon steel was mounted inside the reactor (chemical composition: C \(\leq\) 0.12; Mn \(\leq\) 0.60; P \(\leq\) 0.045; S \(\leq\) 0.045).  
Each reactor was connected to a direct current (DC) with the following density (J): 0.63 A/m\({}^{2}\) (R1), 1.25 A/m\({}^{2}\) (R2), 2.5 A/m\({}^{2}\) (R3), 5 A/m\({}^{2}\) (R4) (Figure 1). The current density was selected based on earlier research by [PERSON] et al. [19]. The power was supplied with a Rohde & Schwarz HMP 4040 (Munich, Germany).  
Sodium acetate, at a carbon to nitrogen (C:N) ratio of 1.0 (TOC 592 \(\pm\) 4.2 mg C/L), was added to facilitate the biofilm development in the BER. The reactor operation supervision began after the reactor start-up period of 4 weeks. Subsequently, the experiment was conducted for two months. Hydraulic retention time (HRT) in the reactor was 24 h, and the reactor operated at 20-22 \({}^{\circ}\)C. Reactors worked in anaerobic conditions.  
Physicochemical analyses of raw and treated wastewater were carried out on filtered samples.  
The analyses used a TOC-L CPH/CPN total organic carbon analyser (Shimadzu, Japan) and included: the content of total organic carbon (TOC) (the oxidizing incineration--infrared) and the content of total nitrogen (the incinerating-chemiluminescence method).  
Physicochemical analyses made on a UV-VIS 5000 DR spectrophotometer included: the content of total phosphorus (TP, with the colorimetric method, HACH Lange LCK 348-350); the content of nitrates (with the colorimetric method, [PERSON] LCK 339 and LCK 340); the content of ammonia, nitrogen and nitrites (with the colorimetric method HACH Lange LCK 303 and LCK 342); the content of aluminium, (with the colorimetric method, HACH Lange LCK 301); the content of iron (with the colorimetric method, HACH Lange LCK 301); the content of molybdenum (with the colorimetric method, HACH Lange LCK 330); the content of copper (with the colorimetric method, HACH Lange LCK 329); the content of chloride (with the colorimetric method, HACH [PERSON] LCK 311); the content of sulphate (with the colorimetric method, HACH [PERSON] LCK 153); the content of manganese (with the colorimetric method, HACH Lange LCW 032); the content of boron (with the colorimetric method, HACH [PERSON] LCK 307); the content of zinc (with the colorimetric method, HACH Lange LCK 360); the content of potassium (with the colorimetric method, HACH Lange LCK 328); the content of calcium and magnesium (with the colorimetric method, HACH Lange LCK 327).  
Electrolytic conductivity, pH, and temperature were measured with a CX-461 meter (Elmetron, Zabrze, Poland).