This study focuses on a method for improving water quality via anoxification recovery using underwater LED treatment in an organically polluted reservoir. The main aim was to evaluate the effects of the optical spectrum and light intensity of LED irradiation on the maintenance of healthy aerobic conditions, as well as anoxification recovery, by promoting oxygen production via phytoplankton photosynthesis. Water quality was monitored via beaker and water tank experiments while using LED irradiation for 24 h (12 h on/12 h off) for 2 months in anoxic water, where the anaerobic decomposition of organic matter progressed under strong reductive conditions. As a result, red–green–blue (RGB) light was advantageous for promoting rapid oxygen production by phytoplankton photosynthesis compared with the mixed red and blue light. In particular, LED irradiation, including the green color, preserved the healthy dissolved oxygen environment without lowering the oxygen level in the dark. In addition, RGB irradiation (R:B:G = 1:1:1) did not only assist in preserving a healthy aerobic state in spite of low light intensities but also evidently decreased concentrations of total nitrogen and total phosphorous. In conclusion, the spatial effect of water quality improvement via LED irradiation was not limited to the vicinity of the light source but was applicable to a wide range, in which the light intensity contributed to one-fifth of the optimum photon intensity for phytoplankton photosynthesis.
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