## 3 Results and Discussion
### Proposals of Technological Systems for the Treatment of Wastewater from Soilless Tomato Cultivation  
The regulations pertaining to the quality of treated wastewater discharged into the natural environment and a sewage network also included requirements related to COD levels (Table 1). If the effluent is to be discharged into natural waters or the ground, its COD should not exceed 125 mg O\({}_{2}\)/L. In the present study, the COD did not exceed 60 mg O\({}_{2}\)/L in any of the effluents. Its value was higher in the wastewater treated in the REBDC, which was due to the external carbon source fed to the reactor. In this reactor type, the COD decreased along with extended HRT, which was consistent with the literature data [35]. Because the reported concentrations of organic compounds were very low, the COD value of the effluent should not hamper its discharge to a municipal sewage system [7]. The requirements pertaining to treated wastewater also include concentrations of iron and aluminum ions. Electric dissolution of the aluminum anode and the dissolution of contact case and disks made of steel result in the formation of aluminum and iron ions, respectively, in both the electrochemical and bioelectrochemical contacts [36]. The Fe and Al concentrations in the effluents from both contacts were lower than 0.5 mg/L (Table 3), which means that the presence of these ions would not hamper effluent discharge to both natural receivers and municipal sewage system (Table 1). The pH values of the effluents from both reactors met the requirement of pH values falling within the range of 6.6-9.5 at all \(J\) and HRT values tested. In the case of temperature, it was higher than the permissible value of 35 \({}^{\circ}\)C in both contactors upon the flow of electric current with a density of \(J\) = 10.0 A/m\({}^{2}\). The study results presented in this manuscript demonstrate that soilless tomato cultivation wastewater can be treated in both electrochemical and electrobiological disk contactors, and afterwards can be discharged to municipal sewage facilities. Regardless of the intended effluent receiver (natural waters, the ground, or a sewage network), the technological system of the proposed installation should also include a reservoir, which also functions as an equalizing tank (with an agitator), a pump system (delivering wastewater evenly to the contactors 24 h a day), a one-stage electrobiological disk contactor, and a secondary sedimentation tank (Figure 6). Due to the characteristics of the generated wastewater, there is no need to use devices for mechanical wastewater treatment (screens, grit chambers, and sedimentation tank). The system should be supplemented by devices for sludge collection, thickening, and dehydration. Given the low organic matter concentration in the sludge, there is no need to provide sludge stabilization devices in the system. This should be important in relation to vertical farming located in cities. The study results presented in Figures 2 and 3 establish the operating conditions of the electrobiological disk contactor (REBDC), i.e., adjusting the \(J\) and HRT to enable achieving the earlier specified performance of the removal of the required pollutant load. In turn, correlations shown in Figures 4 and 5 establish the technological parameters of the REBDC depending on the required quality of the effluent, including concentrations of phosphorus, nitrogen, and nitrates.