Untreated compressed air always contains contaminants because of the nature of the gas and how it is produced. The need for air treatment basically results from 3 characteristics of compressed air.
Liquid water - water aerosols - water vapor
As water is incompressible, the amount of moisture per m³ increases when air is compressed. The maximum amount of moisture per m³ air is however limited for a certain temperature. Condensation will thus be formed when air is compressed.