Abstract: The method for treating wastewater includes a first flocculation-and-sedimentation process and a second flocculation-and-sedimentation process, wherein the first flocculation-and-sedimentation process include: passing wastewater through a first flocculation reaction tank; passing the first treated water through a third flocculation reaction tank; passing the third treated water through a fourth flocculation reaction tank; and passing the fourth treated water through a first sedimentation tank to generate fifth treated water and sludge, and the second flocculation-and-sedimentation process include: passing the fifth treated water through a fifth flocculation reaction tank to generate sixth treated water; passing the sixth treated water through a sixth flocculation reaction tank to generate sixth treated water; passing the sixth treated water through a seventh flocculation reaction tank to generate eighth treated water; and passing the eighth treated water through a second sedimentation tank to generate ninth t