Granular activated carbon as nucleating agent for aerobic sludge granulation: Effect of GAC size on velocity field differences (GAC versus flocs) and aggregation behavior

•Addition of GAC performed as the nucleating agent which facilitated granulation.•A novel quantitative evaluation method was used to investigate granulation process.•Increase the size of GAC enlarged velocity field differences between flocs and GAC.•Increase velocity field differences inhibited flocs–GAC coaggregates.•GAC with suitable size can perform as the nucleating agent to favor granulation.

Initial cell aggregation plays an important role in the formation of aerobic granules. In this study, three parallel aerobic granular sludge reactors treating low-strength wastewater were established using granular activated carbon (GAC) of different sizes as the nucleating agent. A novel visual quantitative evaluation method was used to discern how GAC size affects velocity field differences (GAC versus flocs) and aggregation behavior during sludge granulation. Results showed that sludge granulation was significantly enhanced by addition of 0.2 mm GAC. However, there was no obvious improvement in granulation in reactor amended with 0.6 mm GAC. Hydraulic analysis revealed that increase of GAC size enhanced the velocity field difference between flocs and GAC, which decreased the lifecycle and fraction of flocs–GAC aggregates. Overall, based on analysis of aggregation behavior, GAC of suitable sizes (0.2 mm) can serve as the nucleating agent to accelerate flocs–GAC coaggregation and formation of aerobic granules.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide