Effect of carbohydrate and protein in the EPS on sludge settling characteristics

EPSs have been believed to play a bonding role in microbial floc formation. However, the precise role is not well known. In this study, sludge settlin g characteristics and the carbohydrate to protein ratio in the EPS were tes ted at various airflow rates. Sludge was collected from three modified sequ encing batch reactors (MSBRs), which were operated with airflow rates of 0. 8 L/min, 2 L/min and 4 L/min, respectively. During the operation periods, t he reactor operated at an airflow rate of 0.8 L/min showed a sludge volume index (SVI) of 80 to 90 mL/g and a constant ratio of carbohydrate to protei n in the EPS, while a significant increase of this ratio and the SVI occurr ed in the other reactors. High airflow rate increased the amount of carbohy drate in the EPS, but the protein level was almost constant for reactors wi th airflow rates of 2 L/min and 4 L/min. The higher ratio of carbohydrate t o protein caused the bulking of the sludge; hence it was not favourable for sludge settling. The ratio of carbohydrate and protein in the EPS is infer red to be essential for solid floc formation.