Sewage water quality: CODcr: 250mg/L; BOD5: 120mg/L; SS: 150mg/L; NH3-N: 20mg/L; TP: 4mg/L.Through the comparison of various biological processes, including oxidation ditch process, SBR process, AB process and so on, combined with the water quality of the design of sewage, it is determined that the treatment process of A/O process is adopted in order to achieve the purpose of nitrogen and phosphorus removal and removal of organic stuff.

In the design, the main structures include coarse grille, fine grille, aerated grit chamber, A/O biological pool, horizontal sedimentation tank and sludge thickener. The sewage pump after the coarse grille rises the sewage first before flowing to other structures by gravity. First, the sewage through the fine grille and then the aeration grit tank, A/O biological pool and horizontal sedimentation tank.

This design has 8 sets of coarse grilles, combined with the intake pump room. All these grilles adopt the spiral drum type grille. The coarse grille is used to remove the coarse suspended solids that will block the valves and valves of the pump units so as to ensure the normal operation of the subsequent treatment facilities.

As to the 8 sets of fine grilles, the fine grille is combined with the aerated grit chamber and adopts the steel structure. The sewage first passes through the coarse grille and then passes through the fine grille to enter the aerated grit chamber. The working principle of the aerated grit chamber is as follows: the organic particles in the waste water are suspended through aeration. Then the sheer force of the aeration between the fraction of grains can remove the organic pollutants attached to the sand, and thus help to separate the relatively dense grain free particles from the sewage and benefit the subsequent treatment.

The aeration tank effluent flows into the A/O pool. The A/O process connects the anterior anaerobic stage with the subsequent aerobic stage. The A stage DO is no more than 0.2mg/L,and the O stage DO is 2 to 4mg/L. In anaerobic section, the heterotrophic bacteria hydrolyzed starch, fiber, carbohydrates, other suspended pollutants and soluble organic substances into organic acids in sewage, which decomposed macromolecule organic matter into small molecule organic matter, insoluble organic matter converted into soluble organic matter, and increased the bioavailability of sewage; in anaerobic stage, ammonia nitrogen is formed by ammonification of protein and other organic nitrogen pollutants. Under the condition of

adequate oxygen supply, the nitrification of autotrophic bacteria will oxidize NH3-N (NH4+) to

NO3- and return to A pool through the digestion solution. Under anaerobic conditions, the denitrification of the bacteria is reduced to molecular nitrogen (N2) by the denitrification of heterotrophic bacteria and the phosphorus is the most suitable for the anaerobic aerobic environment. After that, through the process of absorbing P through the release of P, the sludge was removed in the form of excess sludge and thus the transformation of C, N and P was completed to achieve the purpose of nitrogen and phosphorus removal and degradation of organic matter.

The sewage after biological treatment is separated by solid liquid in the two sedimentation tank. After precipitation, the supernatant enters the ultraviolet disinfection pool and sterilizes under the ultraviolet light within the specified wavelength range. Then it discharges after reaching the standard. The sterilization principle of ultraviolet disinfection is to destroy the DNA structure of viruses, bacteria and other pathogenic organisms in water by using the energy of ultraviolet photons. It is mainly to break the various structural keys in DNA or to produce photochemical polymerization so that all kinds of viruses, bacteria and other treatment bodies can lose their reproduction ability to achieve the effect of sterilization.The sludge from the secondary sedimentation tank flows into the radial flow gravity concentrator, which reduces the water content of the sludge and the volume of the sludge, thus reducing the subsequent treatment cost.The main structures in the design include coarse grids, fine grids, aerated grit chambers, A/O biological pools, advection sedimentation tanks and sludge thickening tanks. Confluent sewage is upgraded by the first stage of the sewage pump and flows to other structures by gravity. First, sewage flows through coarse grids to remove coarser suspended solids to ensure proper operation of subsequent structures, then passing through the fine grid, aerated grit chamber, A/O biological pool and advection sedimentation tank.The features of this design are as follows: (1) The sewage is pumped up once by the sewage pump and the whole process is completed by gravity. Without repeated lifting, it can save energy, reduce the cost of sewage treatment and facilitate the operation and management.(2) The design is technically feasible and economically reasonable. The investment cost is