Formation and characteristics of aerobic granular sludge for simultaneous phosphorus and nitrogen removal in a SBR

With inoculum sludge from a conventional activated sludge wastewater treatment plant, formation of aerobic granular sludge for simultaneous phosphorus and nitrogen removal was realized in a sequencing batch reactor by the selective pressure as a driving force. Selective pressure created by means of decreasing sedimentation time and increasing substrate loading enhanced the formation of aerobic granular sludge, which followed four consecutive stages: acclimation, granulation, growth and maturation. Under the condition that the substrate loading were increased to 500mg COD/(L.d) and 48mg NH4+-N/(L.d), the granules were the dominant sludge forms with most of diameter about 0.5-2.0 mm, a minimal settling velocity of 0.55 cm/s and a MLSS of 6800 mg/L after 120 days operation. The granules were composed of filamentous species with varying degrees of rod- and coccal-type bacteria. Microscopic examination revealed that granules microecosystem was more stable and should be less vulnerable to the changes of mixed liquor condition.