Method for utilizing microalgae in processing oilfield sewage and in co2 fixation

Provided in the present invention is a method for utilizing microalgae in treatment of oilfield sewage and in CO 2 fixation. The method is carried out according to the following steps: a. dissolved air floatation is utilized for biological degreasing pretreatment of oilfield sewage of an oilfield sewage combined station, the oilfield sewage that has been removed of most crude oil by floatation is introduced into an aerobic biological degreasing treatment system for further crude oil removal, N and P nutrient salts are added during the aerobic biological degreasing process, and the crude oil content in the aerobically-biologically treated oilfield sewage is less than 2 mg/L; b. the oilfield sewage treated per step a flows directly into a racetrack-type microalgae culture system, then a flue gas having CO 2 content of 3% to 15% is introduced, microalgae seeds are added into the racetrack-type microalgae culture system and left to grow for 5 to 15 days; and, c. the oilfield sewage treated by the racetrack-type microalgae culture system per step b is transferred to a microfiltration system, the sewage is filtered while the microalgae is collected by means of crossflow microfiltration by employing a ceramic microfiltration membrane, an algae sludge is filter-pressed with a filter press and made into an algae cake, and clean water produced by microfiltration is sterilized, deoxygenated, and then re-injected into a low-permeability or medium-low-permeability oil reservoir stratum for oil displacement.