The particular characteristics of nitrifying bacteria are that they have a reduced tendency to flocculate, a lower specific growth rate and a higher specific nitrification rate. Accordingly, we used whole microorganisms immobilized by the PVA-freezing method (in order to prevent microorganism wash-out) to investigate the characteristics and efficiency of their nitrification. The following results were obtained.

  1. By comparsion with suspended microorganisms, the nitrification rate of immobilized microorganisms was not as affected by temperature and their storage stability was better.

  2. The nitrification efficiency of a recycling operation in a biological nitrogen removal system was compared with that of a once-through operation, using immobilized microorganisms. The results showed that the ratio of allowable volumetric loading in the recycling operation was 60% of that in the once-through operation.

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