Influence of external circulation on sludge characteristics during start-up of internal circulation reactor

The start-up of external circulation — added internal circulation (IC) reactor was finished in 26 d, 32 d fewer than that of IC reactor. To evaluate the influence of the added external circulation on the development of granular sludge, the characteristics of the granular sludge taken from the two tested laboratory-scale reactors during start-up were studied. The results show that the added external circulation can enhance biomass granulation, accelerate granule development and improve sludge characteristics. At the end of start-up, the granular size of sludge in external circulation-added IC reactor greatly increases with a size distribution much better than that of sludge in IC reactor. The granular sludge originated from external circulation — added IC reactor contains more extracellular polymers and has a greater settling velocity than that from IC reactor. Methanogenic activity of the granular sludge from the external circulation — added IC reactor started 26 d ago reaches 358.23 mL · g−1 · d−1, 1.66 and 1.20 times as great as that of the sludge from the IC reactor started 26 d and 58 d ago respectively.