Abstract: Colloidal particle and microorganism fragment in secondary effluent are removed well by dissolved air flotation.The first, the flotation pretreatment conditions were tested and the results indicate that the flocculation conditions are three stirring levels, stirring strengths are 50/30/20 s^-1 and reaction time is 20 minutes.The second, the removal efficiency of COD and turbidity were improved as the dosages of polyaluminium chloride (PAC) were increased, and the flotation tank has better ability of resistance to different influent load.The effluents reach Water Quality Standard for Urban miscellaneous Water Consumption (GB-T18920—2002) and Urban Wastewater discharge standards (GB-T18918—2002) .In the end, the test shows that the removal efficiency of turbidity, COD and TP were improved to 43%, 42% and 27% respectively when the surface load of flotation tank reached 12.7 m³/ (m²·h) , and the increasing height of separation zone is of advantage to effluent quality, but the invest of tank may be high.