A cavitation apparatus, comprising a multi-stage cavitation subassembly comprising: a fluid inlet; a fluid outlet; a plurality of cavitation units; a housing surrounding said cavitation units and including blocking plates positioned between each cavitation unit to prevent fluid flow from bypassing each cavitation unit and to force fluid flow into each cavitation unit; and a rotating shaft; in which each cavitation unit comprises a front plate having a central aperture, a rotating impeller behind the front plate and a stationary stator and; in which each rotating impeller has vanes being part of or fitted onto the impeller, and has defined therein an aperture that couples to the rotating shaft so that the impeller and its vanes rotate with the rotating shaft, rotational motion of the vanes draws in fluid through the central aperture, such that fluid strikes the impeller which forces the fluid in a radial direction towards an inner surface of the housing; in which the stator of each cavitation unit further comprises at least one layer of Venturi-shaped slots arranged around a periphery of the vanes, each cavitation unit being arranged such that as the fluid leaves the vanes of the impeller, it enters the Venturi-shaped slots, the Venturi-shaped slots forming a cavitation-inducing feature arranged to introduce cavitation into the fluid; in which each cavitation unit is arranged such that as the fluid exits out of the Venturi-shaped slots, rotation of the impeller continues to force the fluid through a vacant region defined between the housing and each stator, the vacant region providing a flow path from the Venturi-shaped slots to a perforated spacer which has one or more apertures therein to allow fluid flow therethrough to a following cavitation unit, or for a final cavitation unit, to the fluid outlet. a fluid inlet; a fluid outlet; a plurality of cavitation units; a housing surrounding said cavitation units and including blocking plates positioned between each cavitation unit to prevent fluid flow from bypassing each cavitation unit and to force fluid flow into each cavitation unit; and a rotating shaft; in which each cavitation unit comprises a front plate having a central aperture, a rotating impeller behind the front plate and a stationary stator and; in which each rotating impeller has vanes being part of or fitted onto the impeller, and has defined therein an aperture that couples to the rotating shaft so that the impeller and its vanes rotate with the rotating shaft, rotational motion of the vanes draws in fluid through the central aperture, such that fluid strikes the impeller which forces the fluid in a radial direction towards an inner surface of the housing; in which the stator of each cavitation unit further comprises at least one layer of Venturi-shaped slots arranged around a periphery of the vanes, each cavitation unit being arranged such that as the fluid leaves the vanes of the impeller, it enters the Venturi-shaped slots, the Venturi-shaped slots forming a cavitation-inducing feature arranged to introduce cavitation into the fluid; in which each cavitation unit is arranged such that as the fluid exits out of the Venturi-shaped slots, rotation of the impeller continues to force the fluid through a vacant region defined between the housing and each stator, the vacant region providing a flow path from the Venturi-shaped slots to a perforated spacer which has one or more apertures therein to allow fluid flow therethrough to a following cavitation unit, or for a final cavitation unit, to the fluid outlet.