Evaluation of microalgae-nitrifying bacteria immobilized in light shielding hydrogel consortia under strong light irradiation

Author(s): Kento Nishi
Email (s): kento-nishi@soka.gr.jp
Institution or organization of origin: Soka University
Country: Japan


Wastewater containing nitrogen is normally treated through nitrification using aeration, and denitrification. However, aeration is an most energy-intensive and costly process and may account for 45–75% of energy consumption in wastewater treatment plants. Many studies using microalgae-bacterial consortia as a new wastewater treatment process have been investigated to reduce cost instead of using mechanical aeration. However, since it is known that nitrifying bacteria suffer from photoinhibition, it will limit nitrification performance. In my previous study, we developed light-shielding hydrogel to mitigate photoinhibition for nitrifying bacteria under strong light irradiation. In this study, the nitrification performance of the combined system of the nitrifying sludge-immobilized in the light-shielding hydrogel with microalgae were evaluated under strong light irradiation. The result shows that the nitrification rate of light-shielding hydrogel was 4.7 times higher than that of dispersion at 1600 μmol photons m-2 s-1. In addition, in the microalgae-nitrifying bacteria consortia without immobilization, it was clarified that high light intensity triggers photoinhibition of nitrifying bacteria, which increases the pH in the reactor, resulting in the production of free ammonia. The increased free ammonia inhibits not only the activity of nitrifying bacteria but also of microalgae, and finally leads to a process breakdown. On the other hand, the microalgae-nitrifying bacteria consortia using light-shielding hydrogel can mitigate photoinhibition and thus avoid this process breakdown. It is expected that the proposed technology will contribute to realizing the application of microalgae–nitrifying bacteria consortia in areas with high sunlight intensity.

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