Sieve tray towers use a similar process to packed towers, but instead of packed materials being evenly distributed, the materials are separated into several trays with holes that allow water to drip through them. An electric air compressor is typically positioned at the bottom of the system where the air from the fans travels through the holes and becomes exposed to the water. A natural draft can also be used as an air source to separate contaminants from the water. A natural draft is used for removing more volatile substances such as hydrogen sulfide, radon, or vinyl chloride. On the other hand, mechanical air compressors are used to remove less volatile substances.