Removal of colloidal impurities by thermal softening-coagulation-flocculation-sedimentation in steam assisted gravity drainage (SAGD) produced water: Performance, interaction effects and mechanism study

•TIC and SVI can be used as additional performance indicators.•Poly-DADMAC dose and mixing time with softeners only were the most important factors.•99.4 % particulate hardness removal and 38.1 mL/g SVI were obtained at optimal condition.•SFA results verified adsorption and bridging between polymers and particles.•Temperature could affect the removal mechanism by changing floc structure and component.