Abstract: Processes and apparati for inactivating microorganisms in flowing or pulsed liquids are provided. The processes consist of pressurizing the liquid and then depressurizing the liquid by directing it through one or more constrictions into an area of reduced pressure. The apparati comprise two fluid containing vessels, one of which is capable of being pressurized, the two vessels being in fluid communication through one or more constrictions, one or more of which may be adjustable, and a means for increasing pressure in one vessel. Specific embodiments include the use of a pressure intensifer to pressurize the liquid and an adjustable orifice, such as a micrometering valve, to form the constriction(s).