Nitrification of anaerobic digestate using a consortium of microalgae and nitrifiers in an open photobioreactor with moving bed carriers

Abstract

A consortium of microalgae and nitrifiers has attracted attention as an alternative to the expensive traditional nitrification process. A possible obstacle to achieving this is the inhibition of nitrifiers under strong light irradiation. This study evaluated the effect of moving bed carriers on anaerobic digestate nitrification in an open photobioreactor inoculated with microalgae and nitrifiers under an incident light intensity of 1000 μmol photons m−2 s−1. The results showed higher specific nitrification activity in the carrier-added photobioreactor (103.6 mg-N g-TSS day−1) than in one in which no carrier was added (11.7 mg-N g-TSS day−1). The empirical equations for determining the light intensity at different depths in the photobioreactor showed a significant contribution by carriers in attenuating the incident light intensity. This is due to the large light attenuation caused by the carrier (1.09 cm−1). The average light intensity inside of the photobioreactor decreased considerably in the carrier-added photobioreactor (342 μmol photons m−2 s−1), whereas it did not decrease in the one with no added carrier. It was found that specific nitrification activity was significantly negatively affected by average light intensity inside of the reactor, and not by incident light intensity, by combining the results from different studies including ours. This study demonstrated, for the first time, the effectiveness of adding moving bed carriers in photobioreactors to mitigate light inhibition of nitrifiers in a consortium of microalgae and nitrifiers.