## 3 Results and Discussion
### Proposals of Technological Systems for the Treatment of Wastewater from Soilless Tomato Cultivation
The remaining data required for signing a contract with a network operator regarding effluent discharge into a sewage system can be determined from the data presented in Table 3.  
In compliance with the EU and Polish legal regulations [37], the concentrations of nitrogen and nitrates in treated wastewater from soilless tomato cultivation discharged to the natural environment should not exceed 30 mg N/ L. The present study demonstrated that, in the case of the REBDC, it was feasible to obtain 150 mg N/L nitrate nitrogen concentration in the effluent at the highest current density, i.e., \(J\) = 10.0 A/m\({}^{2}\), and the longest HRT = 24 h tested. The respective value recorded in the electrochemical reactor reached 210 mg N/L, which meant that the RECDC was not a viable solution to obtain effluent suitable to be discharged into the natural environment. In turn, the discharge of soilless tomato cultivation wastewater treated in the REBDC directly into natural waters or the ground would be feasible upon the use of a solution enabling nitrate nitrogen concentration reduction below 30 mg N/L. While searching for a solution involving the use of the electrobiological disk contactor, an assumption should be made that, at HRT = 4 h and all electric current densities tested, the concentration of nitrite nitrogen in the effluent will exceed the permissible level (1.0 mg N/L). Due to the possibility of nitrite nitrogen presence in the effluent, wastewater cannot be treated at electric current densities of \(J\) = 5.0 A/m\({}^{2}\) and \(J\) = 10.0 A/m\({}^{2}\). At \(J\) = 2.5 A/m\({}^{2}\), the HRT cannot be shorter than 12 h, whereas at \(J\) = 0.63 A/m\({}^{2}\) and \(J\) = 1.25 A/m\({}^{2}\), the HRT cannot be shorter than 8 h. To ensure the permissible nitrate nitrogen concentration in the effluent, the selected treatment technology should envisage the supply of an external carbon source to the disk contactor, providing carbon concentrations exceeding C:N = 0.5 (used in the present study). These carbon sources may include commercial chemical agents, for example, acetic, propionic, or citric acids. Our previous investigations have shown the feasibility of affecting nitrate concentration in the effluent from the biofilm reactors by adjusting the type and dose of the organic substrate [16; 27; 38]. In that instance, the technological system of the wastewater treatment plant should additionally provide tanks for storing acids, solution tanks, and a feeding system (Figure 7). A more viable solution considering the environmental protection perspective and the idea of a circular economy would be to support the technological system with an installation for the production of volatile fatty acids (VFAs) from tomato stem and leaf biomass (Figure 8). It would be necessary to include devices for grinding and homogenizing plant residues and a reactor for prepared biomass fermentation. The system should include a tank for storing the digestate containing VFAs and a feeding installation. In both cases, higher suspended solid loads in the effluent and a much greater proportion of organic matter in the sludge must be taken into consideration. Therefore, apart from sludge dehydating devices, additional devices for its stabilization should be designed. An alternative to sludge management may be its disposal to a municipal wastewater treatment plant. This would, however, require constructing a sludge retention reservoir or a thickener. The  
Figure 6: Scheme of a technological system with electrochemical or electrobiological rotating disc contacts for treating wastewater discharged to sewage systems. The discharge of treated soilless tomato cultivation wastewater from a REBDC into natural waters or the ground would be feasible only upon the application of technological treatments ensuring nitrate nitrogen concentration reduction below 30 mg N/L. In order to ensure nitrate concentrations in the effluent at an acceptable level, an external carbon source should be supplied to the REBDC in a technological system for a wastewater treatment plant.