A system (1; 101) and a method for purification of oil, said system comprising:—at least one feed tank (3) comprising oil to be purified;—a separation aid dosing device(13);—at least one basic sedimentation tank (21a, 21b) comprising at least one inlet (23a, 23b) connected to the feed tank (3) and to the separation aid dosing device (13) for receiving oil to be purified and separation aid, said at least one basic sedimentation tank (21a, 21b) further comprising at least one sludge phase outlet (41a, 41b) provided in a bottom part (37a, 37b) of the basic sedimentation tank and at least one oil phase outlet (39a, 39b);—at least one advanced sedimentation tank (121) comprising at least one sludge phase inlet (122) connected to the at least one sludge phase outlet (41a, 41b) of the at least one basic sedimentation tank (21a, 21b), said advanced sedimentation tank (121) further comprising at least one sludge phase outlet (141; 141a, 141b) connected to a sludge tank (143) and at least one oil phase outlet (139a, 139b; 39), wherein said advanced sedimentation tank (121) further comprises at least one sensor (55; 55a, 55b, 55c) for detecting the presence of an oil phase or a sludge phase.