An Oil-flooded rotary air compressor, compresses air while lots of oil is injected in the compression chamber !  The Oil is used to cool the air, to lubricate the rotors and to seal the compression chamber. Smart one would think. 

However, the irony is, that it is easy to inject the oil, but it is not easy to remove the oil ! 

A lot of filtration is needed to reduce the oilcontent into the compressed air ! An oil carry over of 2-5 ppm is achieved by the better designs. However, the remaining 2-5 ppm will over time contaminate your environment heavily ! 

The result is that a lot of waste is being produced, just to have a rotary running at a lower cost ! Why not having an Oil-free compressor ! Is it too expensive ?

Good alternatives exist to avoid the expensive waste !