The air to be cleaned flows horizontally through the packing bed as the scrubber is positioned horizontally. This way, the waste air gets in contact with the washing liquid sprayed from above in a “crossflow” manner. Thereby, the contaminants from the waste air are being transferred to the washing liquid. Depending on the application either a chemical-oxidative or biological operation mode is possible. The waste air is flowing through various milieus. These can be modified because each stage has its own self-sufficient washing liquid system. An optimal and target-oriented removal of contaminants within one plant is ensured.