The effects of immobilization conditions, such as freezing temperature, amount of seed sludge, gel size and mixing of other microorganisms with seed sludge, on the nitrification treatability of entrapped cell reactors using the PVA freezing method are discussed. Results indicated that the freezing temperature did not affect nitrification treatability within the range from −20 °C to −80 °C. The amount of seed sludge affected the start-up time and maximum nitrification rate, but the effects were not in proportion to the amount of seed sludge. Regarding the gel size, the maximum nitrification rate of the crushed gel (average gel size is 1mm3) reactor was 1.5 times that of the 3mm cubic gel reactor. Results also indicated that start-up time and maximum nitrification rate were enhanced when seed sludge was mixed with other microorganisms which did not have nitrification ability, before the immobilization procedure.

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