Effect of light shielding on the photoinhibition of nitrifying bacteria immobilized on a light-shielding hydrogel

Microalgae-nitrifying bacterial consortia have attracted attention as energy-efficient nitrogen removal methods that do not require aeration. However, the photoinhibition of nitrifying bacteria is a hurdle to their practical application. We have developed a 'light-shielding hydrogel' that mitigates the photoinhibition of nitrifying bacteria, and it demonstrated the performance of high ammonia removal even under strong light irradiation. However, the effect of carbon black (CB) concentration in hydrogel and its optimization have not yet been examined. In this study, the light-shielding effect was evaluated by measuring the light transmitted through a light-shielding hydrogel layer and comparing the experimental and theoretical values. Increasing the CB concentration from 0.1 to 0.5% effectively reduced the light transmission from 13 to 2.5% at a thickness of 1 mm. The actual nitrification activity and the theoretical activity determined from the light transmission were close in the range of 900-1,600 μmol photons m^-2 s^-1 of irradiated light intensity. Based on these results, the theoretical predictions obtained in this study may contribute to the prediction of photoinhibition mitigation. Therefore, it is expected to provide suitable conditions for optimizing the preparation of light-shielding hydrogels and pave the way for their application in outdoor wastewater treatment processes.