PURPOSE: To efficiently remove a nonionic surfactant contained in a waste soln. in low cost by adjusting a waste soln. containing a nonionic surfactant to predetermined temp. and applying flocculation treatment thereto to form flock containing the nonionic surfactant.

CONSTITUTION: When a waste soln. containing a nonionic surfactant is treated, at first, an oil component is removed from the waste soln. and the pH of the waste soln. is adjusted to 3 or less by sulfuric acid if necessary and, subsequently, an alkali aq. soln. such as potassium hydroxide is added to neutralize the waste soln. to adjust the pH thereof to 8-11. Next, the waste soln. is adjusted to required temp. and a flocculant (org. polymeric flocculant) is added to the waste soln. to form floc which is, in turn, sedimented to be separated. The temp. of the waste soln. after pH adjustment is adjusted to temp. equal to or higher than the cloud point of the nonionic surfactant contained in the waste soln., pref., to a temp. range of the cloud point -80°C to efficiently perform flocculation treatment.