Start-up of granule-based denitrifying reactors with multiple magnesium supplementation strategies

Abstract In the present work, the effect of Mg 2+ supplementation on the start-up of a denitrification process and the granulation of denitrifying sludge was investigated in three upflow anaerobic sludge blanket (UASB) reactors. The reactors R 1 and R 2 were continuously and intermittently, respectively, supplied with 50 mg L −1 Mg 2+ , whereas R 0 was used as the control. The nitrogen loading rate (NLR) and organic loading rate (OLR) gradually increased, and extremely high values were obtained (36.0 kgN m −3  d −1 and 216.0 kgCOD m −3  d −1 , respectively). Granulation occurred in R 1 first, but the reactor capacities were comparable. Suffering from starvation, the R 0 –R 2 performances were comparable. At the end of the experiment, the average diameter of the granules in R 0 , R 1 , and R 2 were 1.67, 1.72 and 1.68 mm, respectively, and the settling velocities of the granules in R 1 and R 2 were 1.14-fold the speed of R 0 . The specific denitrifying activity (SDA) of the sludge from the reactors supplied with Mg 2+ was greater than the reactor without Mg 2+ . Intermittent Mg 2+ supplementation was identified as the best choice to be utilized to cultivate denitrifying granules, which was consistent with kinetic analysis.