A process for working up alkaline process wastewater from the nitration of aromatic compounds to mono-, di- and trinitroaromatics, said alkaline process wastewater having a pH of 7.5 to 13, comprising the steps of a) acidifying the alkaline process wastewater by adding concentrated sulfuric acid which originates from the workup of the aqueous, sulfuric acid-containing phase obtained in the nitration to a pH below 5, which forms a mixture A consisting of organic phase which separates out and acidic aqueous phase, b) contacting the mixture A with fresh sewage sludge, and c) removing the sewage sludge.