Gas holdup and oxygen transfer in an aerobic granule-based sequencing batch reactor

Aerobic granules with mean radius of 0.25 mm were cultivated in a sequencing batch reactor (SBR) fed with soybean-processing wastewater, and the gas holdup and oxygen transfer were investigated for this granule-based reactor. At the discontinuous point in the variation of gas holdup with superficial gas velocity at 1.73 cm/s, bubble flow regime was changed into churn-turbulent flow one. Gas holdup of the granule-based SBR was greater than that of a floc-based SBR, indicating that higher oxygen transfer efficiency could be achieved in the former. The volumetric gas-liquid oxygen transfer coefficient increased with increasing superficial gas velocity, but declined with increasing solid fraction. Observed Thiele modulus and effectiveness factor were effectively used to describe the efficiency of oxygen diffusion in the granules and the oxygen utilization rate of the reactor. With the porous pellet model, the oxygen concentration profiles and effectiveness factor for aerobic granules with different sizes were predicted. Effectiveness factor of 0.98 for the granules with radius of 0.25 mm suggests that the oxygen diffusion limitation could be neglected. For the granules with radius of 1.00 mm, a severe oxygen limitation occurred, as indicated by an effectiveness factor of 0.55.