PURPOSE: To efficiently and economically treat nitrophenolic by-products by mixing a nitration waste water with a solvent and an acid to extract the nitrophenolic by-products, distilling the extract liquid to recover the solvent and ashing the residue.

CONSTITUTION: A nitration waste water is mixed with a solvent (e.g. toluene) and an acid (e.g. sulfuric acid), and the mixture is extracted at a high temp. (e.g. about 60 to 65.6°C) in an acid pH range (e.g. 1.0 to 1.2) to produce a solvent soln. containing nitrophenolic by-products. The nitrophenolic solvent soln. is distilled to recover the solvent from the solvent soln. and to produce a residue containing the nitrophenolic by-products. Then, the residue is ashed. As a result, the nitrophenolic by-products are converted into a form suitable for environmental and safe treatment and can be efficiently and economically treated.