Static and dynamic dissolved oxygen distributions in algal-bacterial granular sludge: mapping intragranular oxygen...

Algal-bacterial granular sludge (ABGS) exhibits pronounced intragranular dissolved oxygen (DO) heterogeneity. However, the internal DO microenvironments under different oxygenation strategies remain insufficiently understood. In this study, intragranular DO distributions in ABGS were characterized under darkness, illumination, and artificial aeration. Results show that intragranular DO distributions varied with granule size and were differently influenced by artificial aeration and photosynthetic oxygenation. After 60 min of artificial aeration at an air uplift velocity of 2.8 cm s^-1, DO at a depth of approximately 0.8 mm in granules with a diameter of around 3 mm remained nearly 0 mg L^-1. In contrast, oxygen generated in situ via photosynthesis rapidly elevated intragranular DO levels, exceeding 4 mg L^-1 at the same depth after 30-min illumination. This study shows that intragranular DO in ABGS can be dynamically restructured in response to distinct oxygen supply and consumption processes, which also provides an in-depth insight into better ABGS design and operation.