A method and a system for purification of oil, said method comprising the steps of: -providing separation aid and oil to be purified to at least one sedimentation tank; -waiting for allowing a sludge phase to sediment to a bottom part of the sedimentation tank, said sludge phase comprising the separation aid together with impurities from the oil; -removing an oil phase not comprising said sludge phase from the at least one sedimentation tank; and -filtering said oil phase through a depth filter for removing any possible remaining separation aid and impurities.