We used whole microorganisms immobilized by the PVA-freezing method (in order to prevent microorganism wash-out) to investigate the efficiency of nitrification. In general, denitrified or oxidized effluents are recycled to the nitrification tank to provide alkalinity control in biological nitrogen removal systems. We found that allowable volumetric loading during nitrification was different for the recycling and once-through operations, using a biofilm nitrogen removal system in a fixed bed. Then, 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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