Rapid Formation of Aerobic Granular Sludge and Its Mechanism in a Continuous-Flow Bioreactor

Based on the principle of self-coagulation of microorganisms, the flocculant-producing denitrifying bacterial TN-14 sludge was added to the continuous-flow reactor for treating domestic sewage. The bacterial TN-14 sludge acted as the main seed sludge to promote the rapid formation of aerobic granular sludge. The sludge morphology, sludge volume index (SVI) values, amounts of extracellular polymeric substances (EPS), and the role of calcium in the granulation process of the sludge were investigated. Results showed that brown aerobic granules with the particle size of 0.5 ~ 2.0 mm was successfully cultivated at 40 days, and its SVI 30 decreased from 122.62 mL g −1 initially to 46.61 mL g −1 and remained at 44.28 ~ 60.51 mL g −1 afterwards. The protein (PN) content in sludge EPS increased from 76.4 mg g −1 initially to 512.3 mg g −1 . Compared with PN, the polysaccharide (PS) content did not change much throughout the operation process of the bioreactor. Energy-dispersive spectrum (EDS) showed that Ca elements were deposited inside the granular sludge, and X-ray diffraction (XRD) showed that Ca elements existed in the granular sludge in the forms of CaCO 3 , K 2 CaP 2 O 7 , Ca 2 P 2 O 7 , and Ca 4 O(PO 4 ) 2 . The formation mechanism of continuous-flow aerobic granular sludge was that bacterial TN-14 sludge could promote the EPS content of sludge, and PN content of EPS increases the hydrophobicity and settling performance of the sludge. Calcium mainly exists in the granular sludge in the form of inorganic calcium phosphate, and therefore plays the role of nucleation in sludge granulation.