Effect of settling time on aerobic granulation in sequencing batch reactor

Abstract

The essential role of hydraulic settling time in the formation of aerobic granules was studied in four column sequencing batch reactors (R1–R4). Results showed that aerobic granules were successfully cultivated and became dominant only in the R4 operated at the shortest settling time of 5 min, while a mixture of aerobic granules and suspended sludge was observed in R1–R3 run at settling times of 20, 15 and 10 min, respectively. After the steady state was achieved, the respective settling times of R1–R3 was shortened from 20 to 5 min, from 15 to 2 min and from 10 to 1 min; as the result, aerobic granules completely displaced suspended sludge in these three reactors after two-week operation. It was found that cell surface hydrophobicity was improved significantly at short settling times. Analyses of the polyvalent metals contents in aerobic granules showed that the accumulation of calcium in aerobic granules was related to the settling time, however, the total content of iron, magnesium and aluminium in aerobic granules was much pretty lower than the granule calcium content and almost remained constant. It is obvious that a short settling time favours aerobic granulation, and one may expect to manipulate the formation and characteristics of aerobic granules by controlling settling time.