Cultivation of aerobic granules in a bubble column and an airlift reactor with divided draft tubes at low aeration rate

A bubble column and a novel airlift reactor with divided draft tubes were used for the cultivation of aerobic granules at the superficial air velocity of 1.2 cm s−1. It was found that profiles of biomass concentration, average particle size and SVI of biomass showed the same changing trends during the first 40-day operation in the bubble column and the airlift reactor. After 60-day operation, the biomass concentration, average particle size, and SVI of biomass reached 4.5 g l−1, 1300 μm and 30 ml g−1, respectively, and then maintained at these values during the next 3-month operation in airlift reactor. However, morphology of granular sludge in bubble column changed at day of 50 and then disintegrated into bioflocs at day of 67 when biomass concentration reached to 8.0 g l−1, which led to the great increase of SVI of biomass and the wash-out of lots of biomass from reactor. Clear and regular granules were seen again at day of 75 and granules gradually grew to bigger sizes. But the system deterioration happened again when biomass in bubble column increased to about 8.0 g l−1. It was found that DO concentration in feast period was about 35–40% with biomass concentration of 8.0 g l−1 in bubble column while DO concentration was above 60% with biomass concentration of 4.5 g l−1 in airlift reactor. Thus, it was thought that lower oxygen concentration due to higher biomass concentration was mainly the reason of unstable operation of aerobic granules in bubble column at superficial air velocity of 1.2 cm s−1.