The invention discloses a method of applying a decolorizing agent to treat printing and
dyeing wastewater; the method comprises the steps of pretreating, to be specific, setting a thick grid to intercept fine suspended matters and floating matters in the printing and
dyeing wastewater and prevent them from entering a decolorizing tank, and performing regular aerating in the decolorizing tank; preparing an alkaline liquid, to be specific, mixing
lime and water in a
mass ratio of 1:100 to obtain the alkaline liquid; alkalizing the
wastewater, to be specific, injecting the alkaline liquid into the decolorizing tank of step I, and adjusting pH to 9-13; preparing a decolorizing agent, to be specific, using water 100 times the decolorizing agent to dilute the decolorizing agent that is a macromolecular
flocculation decolorizing agent, stirring well, adding
polyacrylamide into water 1000 times the
polyacrylamide, and stirring until full
dissolution; decolorizing; discharging wastewater, to be specific, adjusting pH of the wastewater to 6-7, and passing the wastewater through a filter outlet filled with spongy iron at certain speed so that the printing and
dyeing wastewater is finally decolorized. The method of applying the decolorizing agent to treat printing and
dyeing wastewater is more thorough and efficient in decolorizing the wastewater during treatment.