Long-term continuous treatments of synthetic groundwater containing 15.06 ± 0.55 mg-N/L nitrate were conducted using an in situ denitrification and oxidation process under different operating conditions. In the experiments, electrolytic hydrogen and oxygen were injected upstream and downstream in a laboratory-scale aquifer, respectively, and measurements were made for nitrate, nitrite, pH, dissolved oxygen, dissolved hydrogen, total organic carbon (TOC), turbidity, and chromaticity. Experimental results demonstrated that steady-state denitrification and oxygenation of groundwater were achieved over 820 days. During the initial phase of the experiments, nitrite was accumulated at a level of several mg-N/L in a hydrogen-injected zone, but it was almost completely oxidized to nitrate in a downstream oxygen-injected zone. In subsequent experiments, nitrite accumulation was not observed in effluent, and satisfactory treatments were achieved. Effluent concentrations of total nitrate and nitrite, TOC, turbidity, and chromaticity were generally below World Health Organization guidelines for drinking water (11.3 mg-N/L). It should be mentioned that water quality parameters such as TOC, turbidity, and chromaticity were lower in effluent than influent, and no clogging problem was observed. From these results, we concluded that the present in situ process has superior properties in terms of long-term performance and stability.

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