Johnson Lake

CAVITATION

Although “cavitation” is a significant cause of wear in some engineering contexts, the phenomenon of cavitation creates physical-chemical effects which can be used to isolate and oxidize contaminants in water.  A rapid change of pressure in a liquid leads to the formation of small vapor-filled cavities with relatively low pressure, when these low-pressure cavities are exposed to higher pressure, these cavities collapse and will generate a shock wave adjacent to the bubble, the shock wave rapidly weakens as it propagates away from the bubble.  “These microbubbles generated by pressure pulses inside a liquid act as small reactors, reaching extreme P-T (pressure/temperature) conditions during short periods of time and generating highly oxidant radicals such as OH·. This chemical behavior is analogous to that of Advanced Oxidation Processes (AOPs).”[1]

Wastewater treatment through hydrodynamic cavitation or acoustic cavitation generate and grow when the external pressure over the liquid decreases to the original value, which is usually similar to that of the vapour pressure of the liquid.  As pressure recovery begins to take place, and upon reaching a maximum size, the non-equilibrium state gives rise to bubble collapse, which under certain conditions, can be considered implosive.  When the bubbles implode, the compression effects upon its internal gases may cause temperature increases, due to the extreme P – T conditions inside the cavities (bubbles), water dissociates into H· and OH· radicals.  The hydroxyl radical represent a strong oxidant and many substances will oxidize in it’s presence.  Hydroxyl Radicals have a noticeably short half life and then reduces to Ozone (O₃), which also has beneficial characteristics in the treatment process.

[1] Hydrodynamic Cavitation as a low-cost AOP for wastewater treatment: preliminary results and a new design approach Y. Benito1 , S. Arrojo1 , G. Hauke2 & P. Vidal2 1 Effluent Treatment Program, Ciemat, Spain 2 Centro Politécnico Superior. Universidad de Zaragoza, Spain