Unravelling the UV/H2O2 process using bioelectrochemically synthesized H2O2 to reuse waste nutrient solution

Abstract

In this study, waste nutrient solution (WNS) was used as a catholyte in a bioelectrochemical cell to directly produce hydrogen peroxide (H2O2), after which the H2O2- containing WNS was integrated with the downstream UV oxidation process to meet quality standards for reuse. The generated current in the bioelectrochemical cell was successfully utilized at the cathode to produce H2O2 in WNS using a two-electron oxygen reduction reaction with different reaction times. The cathodic reaction time with the highest H2O2 production (504 ± 5.2 mg l−1) was 48 h, followed by that obtained from 24 h (368 ± 4.1 mg l−1), 12 h (158.8 ± 2.4 mg l−1), and 6 h (121.1 ± 4.1 mg l−1) reaction times. During H2O2 generation, calcium, magnesium, and phosphate in the WNS were recovered in the form of precipitates under alkaline conditions. The H2O2-containing WNS was further treated with different UV doses. After UV/H2O2 treatment, excitation-emission matrix and molecular weight distribution analyses demonstrated that aromatic compounds were reduced. Moreover, the gene expressions of sul1 (up to 95.65%), tetG (up to 93.88%), and aadA (up to 95.32%) were clearly downregulated compared with those of a control sample. Finally, a high disinfection efficiency was achieved with higher UV doses, resulting in successful seed germination. Thus, our results indicate that the developed method can be a promising process for reusing WNS in hydroponic systems.