Effect of wastewater COD/N ratio on aerobic nitrifying sludge granulation and microbial population shift

The effect of COD/N ratio on the granulation process and microbial population succession was investigated. Four identical sequencing batch reactors, R1, R2, R3 and R4, were operated with various initial COD/N ratios ranging from 0/200 to 800/200 (m/m). Ethanol was fed as the source of COD. Aerobic granules were successfully cultivated in R2 and R3, operating with the COD/N ratio of 200/200 and 400/200, respectively. Scanning electron microscope observations indicated that short rod-shaped and spherical bacteria were dominant in R2, while granules produced in R3 were surrounded with a large amount of filamentous bacteria. The average specific nitritation rate in R2 and R3 were 0.019 and 0.008 mg N/(mg MLVSS.hr), respectively. Fluorescence in situ hybridization results demonstrated that nitrifying bacteria population was enriched remarkably in R2. It indicated that nitrification ability and nitrifying bacteria population were enriched remarkably at low COD/N ratio. However, no granules were formed in RI and R4 which might attribute to either limited or excessive extracellular polymeric substances production. This study contributed to a better understanding of the role of COD/N ratio in nitrifying sludge granulation.