Abstract：Activated sludge bulking, mainly caused by filamentous bacteria-Microthrix Parvicella, was one of the most important reasons in activated sludge process under low temperature. This study was aimed at resolving the problem of sludge bulking and low nitrification ability caused by low temperature, low DO and low influent loading in a variable-velocity oxidation ditch pilot with fine bubble. Experiments results showed that taking the whole DO tank as aerobic zone was benefit for improving nitrifying bacteria content and meeting effluent NH4+-N concentration below 5 mg/L. And then A: O ratios of 0, 0.1, 0.5, 1.1, 1.8 were gradually increased the quantity of nitrifying bacteria quickly in accordance with above the standard of effluent NH4+-N. Also identifying filamentous bacteria showed that dominant microorganism was M. Parvicella with hydrophobicity. Flocs with mass M. Parvicella was floated due to denitrifying nitrogen and formed foaming on the surface of selector and OD. After the foaming was ripped off in order to decrease the quantity of M. Parvicella, nitrifying as increasing the quantity of nitrifying bacteria was improved. The further analysis of nitrification and denitrification performance under different activated sludge contents of M. parvicella showed that M. parvicella was benificial to denitrifying and adverse to nitrifying.It also showed that M. parvicella the maijor reason of impacting nitrifying, denitrifying and sludge settleability except for DO concentration, water temperature and influent load.
