A method and a system for purification of oil, wherein the method comprises the steps of:—adding to a contaminated oil provided in a preparation tank a chemical booster for boosting capturing of impurities and/or a separation of phases in the contaminated oil; —waiting for allowing different phases to be separated; —pumping content out from the preparation tank through a fluid connection; sensing at least one property of the content in the fluid connection; —controlling a valve provided in the fluid connection to transfer the content to a next step in the purification process or to another destination in dependence of the sensed at least one property.