Method for treating a waste water containing a nonionic surface active agent

A method for recovering a nonionic surface active agent from a nonionic surface active agent containing waste water to purify the waste water, which consists of separating by a foam separation step a low concentration waste water with less than 500 ppm of a nonionic surface active agent into a concentrated water containing more than 500 ppm of nonionic surface active agent and a treated water containing less than 3 ppm of nonionic surface active agent;

• adding an inorganic electrolyte selected from the group consisting of Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4, Al.sub.2 (SO.sub.4).sub.3, Fe.sub.2 (SO.sub.4).sub.3, NaCl, KCl, CaCl.sub.2, MgCl.sub.2, FeCl.sub.2, FeCl.sub.3 and AlCl.sub.3 to the concentrated water to insure a concentration of electrolyte in the concentrated water of more than 50 ppm, heating the resultant concentrated water containing electrolyte to 100.degree. to 200.degree. C under a pressure of 2 to 5 kg/cm.sup.2 to separate it into a high concentration nonionic surface active agent solution and a separated water, and recycling the separated water to the foam separation step, said nonionic surface active agent being selected from the group consisting of polyoxyethylenealkylether represented by a general formula:
• RO--(CH.sub.2 --CH.sub.2 --O).sub.n H,
• polyoxyethylenealkylphenylether represented by a general formula:
• ##STR4## and fatty ester of polyethyleneglycol represented by a general formula:
• R--COO--(CH.sub.2 --CH.sub.2 --O).sub.n H,
• where R denotes C.sub.6 to C.sub.24 alkyl radical or alkenyl radical and n denotes 2 to 40.