Oxidation ditch is absolutely impossible. I have visited many manufacturers' oxidation ditch processes, and the best one for nitrogen and phosphorus removal is Handan East Sewage Treatment Plant. They control the aeration rate in different ditch zones, which has reached the standard that the amount of nitrogen and phosphorus removal can never reach Grade A. Although the effluent COD is very low, it is stuck in the control of P in general, and phosphorus removal is really difficult, especially in the end, the sedimentation tank leads to the anaerobic release of P, and the effluent P generally exceeds the standard, but at least there is absolutely no drug injected into it. This is true in theory, but it is difficult to do it in the actual operation of sewage plants. The supernatant of the secondary sedimentation tank can not be directly added into the secondary sedimentation tank, and the sludge needs to be refluxed and precipitated, so the civil construction cost will go up. Generally speaking, A2/O can remove nitrogen and phosphorus, but the residence time of the secondary sedimentation tank must be controlled. MBR and membrane technology are used around Beijing, reaching A standard!