Biofilm has many features: conducive to cell separation; reactor can achieve a higher cell concentration; by optimizing the size of feature vectors can achieve maximum microbial activity; provided simultaneously in the same reactor, the possibility of a fixed different kinds of microorganisms ; processing, high efficiency, resistance to shock loading, small volume, easy operation and management, troubled by activated sludge bulking problem can be eliminated, can maintain a high sludge age, high microbial biomass, hydraulic retention time is short and so on. As the microorganisms are fixed on the carrier, the proliferation of nitrifying bacteria may also slow growth and reproduction of microorganisms. Biofilm biological phase is very rich, formed by bacteria, fungi and other microbial groups, composed of a series of relatively stable ecosystem.