The first step in the oil extraction process is cleaning the olives and removing the stems, leaves, twigs, and othersleft with the olives. The olives should be washed with water to remove pesticides, dirt, etc. Rocks and sand will damage a hammermill and quickly wear out a centrifugal decanter or oil separator, reducing life span from 25 to as little as 5 years. It is amazing, and sometimes entertaining, to see what can be found in the bins with the olives. Light contaminants are removed by a heavy air flow (blower) and heavy objects sink in the water bath.