MBBR is a new type of biofilm reactor, which is an improved new type of composite biofilm reactor developed on the basis of fixed-bed reactor, fluidized bed reactor and biological filter. It overcomes the shortcomings of the fixed bed reactor that needs to be backwashed regularly, the fluidized bed reactor needs to fluidize the carrier, and the submerged biological filter plugs the filter material and replaces the aerator, and retains the characteristics of the traditional biofilm method of shock load, low sludge production and long sludge age. Compared with the activated sludge method, due to the longer age of the sludge, it can retain more nitrifying bacteria and has a better denitrification effect. The main principle is to use the sewage to continuously flow through the reactor packing carrier, form a biofilm on the carrier, and the microorganisms will multiply and grow on the biofilm while degrading the organic pollutants in the sewage, so as to play the role of purifying the sewage.