An apparatus for treating a flow of fluid with microwave radiation, the apparatus comprising: a vessel having a sidewall and opposed first and second end walls defining a substantially cylindrical chamber, the first end wall being disposed a predetermined distance d1 from the second end wall； a pipeline for flowing fluid through, the pipeline passing through the first end wall towards the second end wall of the vessel, the chamber and the pipeline being substantially co-axial and the pipeline being substantially transparent to microwave radiation； and a microwave radiation inlet in the side wall of the vessel for admitting microwave radiation of wavelength λ into the chamber, wherein the distance d1 is substantially equal to an integral multiple of λ/2 so that the chamber is a microwave resonator.
