Process for treating industrial wastewaters

A process for treating industrial wastewaters, preferably for wastewaters from a metal-processing system, comprising the steps of:

• passing wastewater through at least one chemical/physical flocculation/precipitation process, a filter system, a biological wastewater purification stage and also a nanofiltration unit to Form a wastewater mixture;
• neutralizing the wastewater mixture by adding an aqueous alkali solution to the wastewater mixture;
• passing the wastewater mixture through a separator to remove resulting flocks;
• removing free oil from the wastewater mixture by passing the mixture through an oil separator;
• passing the wastewater mixture through a biological filtration stage for decreasing the chemical oxygen demand of the wastewater mixture;
• feeding back the wastewater mixture for additional nanofiltration after the wastewater mixture has been passed through the nanofiltration a first time;
• and
• returning the wastewater mixture to the metal processing system for reuse therein.