The aim of the invention is to provide a method for biotechnological production of carboxylic acids, in which the acid-forming micro-organisms are cultured in an unsterile manner in a submerged phase containing waste water containing all carbon and nutrient medium components necessary for the production of the carboxylic acid, which method avoids the disadvantages of known methods and enables high product concentrations and productivity while at the same time the resources of water and power are being conserved. This aim is achieved, according to the invention, in that micro-organisms are used that are cultured under unsterile conditions in a culture medium containing waste water with the addition of carbon-rich compounds.