Nitrite accumulation will drop down when the nitrite oxidation bacteria have acclimated to the nitrifying conditions after stable performance for some times. Accumulation is a gradating course, nitrite disappear will take only a few days. A long MCRT can not maintain a long term stable partial nitrifying process. The refreshment of system sludge and suitable MCRT is key for nitrite accumulation, and bacillar sludge is its typical feature.