In a system and process for ultrasonically treating a liquid having a thermal conductivity, an elongate treatment chamber housing has an inlet and an outlet such that liquid flows longitudinally through an interior space of the chamber from the inlet to the outlet. At least part of the interior space of the chamber housing is filled with a bed of particles having a thermal conductivity substantially greater than that of the liquid whereby a ratio of the thermal conductivity of the particles to the thermal conductivity of the liquid is in the range of about 2:1 to about 400:1. An elongate ultrasonic waveguide assembly extends longitudinally within the interior space of the housing and is operable at a predetermined ultrasonic frequency to generate mechanical ultrasonic vibration within the housing in direct contact with the liquid flowing therein as the liquid flows through the bed of particles.