Influence of light conditions of a mixture of red and blue light sources on nitrogen and phosphorus removal in advanced wastewater treatment using Scenedesmus dimorphus

The effects of several light conditions (various light/dark cycles, light intensities, and light frequencies) of a mixture of intermittent red and blue light on nitrogen and phosphorus removal from wastewater using Scenedesmus dimorphus were evaluated. The nitrogen removal rates were almost the same (~ 10 mg/L/day) during light/dark cycles of 16:8, 20:4, and 24:0, and were two-fold higher than those observed during light/dark cycles of 12:12 and 8:16. The phosphorous removal rate also reached its highest value (~ 1.5 mg/L/day) during the light/dark cycles of 20:4 and 24:0. These results suggest that microalgae should be illuminated for at least 16 and 20 h for the efficient removal of nitrogen and phosphorous, respectively. Moreover, both the nitrogen and phosphorous removal rates were significantly enhanced when the light intensity was raised from 50 to 400 µmol/m2/sec. Increasing the light intensity above 400 µmol/m2/sec caused photo-inhibition, such that microalgae production decreased. Besides the light/dark cycle and light intensity, flashing frequency also controls the growth and nutrient removal rate of Scenedesmus dimorphus. It was found that the most appropriate flashing frequency was 2,500 Hz.