PURPOSE: To reduce the amt. of sludge and to improve the removal rate for melamine polymers by adding inorg. acids to waste water contg. the melamine polymers to control the pH thereof to ≤5.0 and separating the melamine polymers by coagulation.

CONSTITUTION: Inorg. acids such as sulfuric acid, hydrochloric acid, nitric acid and others are added to waste water contg. melamine polymers discharged from paint, resin and textile plants, etc. to decrease the pH thereof to ≤5.0, and the melamine polymers are separated away by coagulation. There are methylated melamine resins, butylated melamine resins, etc. and mixtures of these and acrylic resins, etc. as the above-described melamine polymers. It is preferable to control the pH to about a 4.5W2.0 range by adding inorg. acids. In order to cause the coagulation of the melamine polymers effectively, it is preferable to agitate the inorg. acids mechanically and pneumatically and to agitate these for about ≥48hr at an ordinary temp.