Microscopic observation of aerobic granulation in sequential aerobic sludge blanket reactor

Aims: This paper attempts to provide visual evidence of how aerobic granula tion evolves in sequential aerobic sludge blanket reactors. Methods and Results: A series of experiments were conducted in two column-t ype sequential aerobic sludge reactors fed with glucose and acetate as sole carbon source, respectively. The evolution of aerobic granulation was moni tored using image analysis and optical and scanning electron microscopy. Th e results indicated that the formation of aerobic granules was a gradual pr ocess from seed sludge to compact aggregates, further to granular sludge an d finally to mature granules with the sequential operation proceeding. Gluc ose- and acetate-fed granules have comparable characteristics in terms of s ettling velocity, size, shape, biomass density and microbial activity. Howe ver, the microbial diversity of the granules was associated with the carbon source supplied. In this work, an important aerobic starvation phase was i dentified during sequential operation cycles. It was found that periodical aerobic starvation was an effective trigger for microbial aggregation in th e reactor and further strengthened cell-cell interaction to form dense aggr egates, which was an essential step of granulation. The periodical starvati on-induced aggregates would finally be shaped to granules by hydrodynamic s heer and flow. Conclusions: Aerobic granules can be formed within 3 weeks in the systems. The periodical starvation and hydrodynamic conditions would pla a crucial r ole in the granulation process. Significance and Impact of the Study: Aerobic granules have excellent physi cal characteristics as compared with conventional activated sludge flocs. T his research could be helpful for the development of an aerobic granule-bas ed novel type of reactor for handling high strength organic wastewater.