Abstract: There is provided a hydrocyclone which operates on a relatively low pressure differential and on a linear flow principle. The hydrocyclone comprises an inlet chamber with a vortex creating a spiral insert therein, which inlet chamber opens in to a frustoconical portion of the hydrocyclone body. The frustoconical portion opens into a cylindrical portion which leads both to the accept and reject outlets at the end. The hydrocyclone is particularly useful for on sight separation of produced water in oil fields. The invention also provides hydrocyclone units of linked hydrocyclones according to the present invention, fluid separation systems incorporating hydrocyclones of the invention and methods of separating fluids using hydrocyclones in the invention.