Characteristics of denitrifying granular sludge grown on nitrite medium in an upflow sludge blanket (USB) reactor

While inoculating pre-acclimatized floccular sludge, nitrite-denitrifying granular sludge was obtained after approximately 40 days of cultivation in a 10 L upflow sludge blanket (USB) reactor. The nitrite removal efficiency was approximately 95% when the nitrite concentration was 50 mg L⁻¹at an influent flow rate of 20 L h⁻¹. The nitrite granular sludge had several notable features including good settleability (110 m h⁻¹), high ash content (79%), and high density (1.248 g cm⁻³). The mixed liquor suspended solids (MLSS) of the sludge bed remained at 130.04 g L⁻¹, at a hydraulic upflow velocity of 2 m h⁻¹. These interesting characteristics were attributed to a high effluent pH (9.7) caused by the release of alkalinity during the nitrite denitrification process. The surfaces of the granules were dominated by cocci bacteria with a diameter of approximately 3 μm, which could be classified as Nitrosomonas-like species based on our analysis of 16 S rDNA sequences.