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Combined overflow sewage（CSO） is a kind water mixed of sewage and rainwater. Itcontains a large number of polluting substances, which easy to causing serious impact onthe water. Therefore,the study of CSO has become one of the most important research toimprove the water environment. In this paper, the CSO in Wuhan Yu Jia Lake area is takenas the research object, during the several rainstorm in Wuhan, the CSO’s concentration ofSS, COD, TN, TP are measured. With the storm water management model as a tool，referring to simulating coefficient of domestic and international model and generalizabilityof the researched area，build a calculation model for the area．And then analyse the waterquality of the CSO. According to the test results, the water quality of CSO changes greatly.Firstly the concentration of SS, COD shows a general trend of increase,then withing theweakening of the rainfall intensity decrease, eventually it restores to the concentration ofthe sewage.The concentration of TN,TP decrease with the enhancement of rainfallintensity,then it will restore to the concentration of the sewage.With the numerical simulation of SWMM and measurements,using the simulationoverflow sewage as the test object, the best parameters of magnetic flocculation process isresearched. First studying the flocculation effect under different feeding order offlocculant polyaluminum chloride （PAC）, the coagulant aid polyacrylamide （PAM） andmagnetic particle.The best order is magnetic particle→PAC→PAM. Secondly, byresearching the impact of treatment effect on CSO which uses different G value （velocitygradient）, T value （stirring time），the best rapid stirring speed is300r/min (G value62.1s-1),rapidly stirring time is60s,slow stirring rate is70r/min (G value8.8s-1), and slowstirring time is10min.By the research of sewage of average concentration and highestconcentration,using Box-Behnken model, according to laboratory analysis at theexperimengtal conditions, the best reaction value for the sewage is shown as follows:Average concentration: PAC dosage is150mg/L；PAM dosage is2.95mg/L，magneticdosage is300mg/L. The removal of TP is96.95%, and the COD removal efficiency is93.78%. Highest concentration: PAC dosage is250mg/L；PAM dosage is2.5mg/L， magnetic dosage is188mg/L. The removal of TP is96.79%, and the COD removalefficiency is96.31%. Pilot study provides an effective basis for applying of the magneticflocculation to the CSO.
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