PURPOSE: To make DMF in waste water simply and surely harmless or useful at a low cost by mixing the waste water with alkali, forming dimethylamine and alkali formate and blowing air into the water.

CONSTITUTION: Waste water contg. DMF is mixed with alkali. In the case where sodium hydroxide is used as the alkali, liberated dimethylamine and sodium formate are formed by hydrolysis. Since this reaction is an equilibrium reaction, the formed dimethylamine is expelled into a vapor phase by blowing air into the water in the same way as ammonia in water is expelled. By this method, DMF in waste water is decomposed into dimethylamine and alkali formate and the dimethylamine is transferred to a vapor phase and made useful by absorption in an acidic substance.