[Growth model of aerobic granule under different salinities]

The stability of aerobic granules that developed under salinities of 1%, 2.5% and 5%, respectively, was evaluated using microscopic observations and size distribution. It was found that the size of granules increased and the diversity of size distribution became wider during the granulation period, and the size of mature granules distributed mainly in the range of 0.3-0.5 mm. Granules under the salinity of 1% showed high porosity and narrow size distribution and extracellular polymeric substances (EPS) linked the particles in the granules. In contrast, those under the salinity of 5% showed low porosity and wide size distribution and entanglement of filaments formed the frame work of granules. Disintegration of aerobic granules was found under the salinity of 2.5%, and the size distribution was found became wider, which may due to the unbalance growth between floc-forming and filamentous microorganisms. Therefore, it is reasonable to conclude that size distribution could effectively describe the stability of aerobic granules, namely, narrow size distribution indicates stability of aerobic granules. Furthermore, granules under low salinity were predominated by the floc-forming bacteria and thus EPS-linking was the main mechanisms of granulation, on the other hand, those under high salinity were predominated by filamentous microorganisms and thus the entanglements of filaments was the main mechanisms of granulation. Two models have been proposed for the growth of aerobic granules. One is heterogeneous growth of which the granule size distributes in a narrow spectrum and the granules growing with this model are stable because the conditions of growth and substrate utility are similar for all granules. Another is homogenous growth of which the granule size distributes in a wide spectrum and granules with this model are unstable.