The Optimization of Hydrodynamic Cavitation as an Advanced Oxidation Option for the Removal of Persistent Contaminants in Wastewater

Wastewater is increasingly becoming the primary source of potable water in many cities, thanks to the development of recycling facilities. Persistent contaminants such as dyes and perfluorinated compounds from textile industries as well as other contaminants necessitate the design of removal technologies to treat wastewater to reduce these chemicals before discharge or being used as feed to a potable water plant. Several chemical treatment techniques have been reported but the most utilized advanced chemical treatments lead to high costs and further environmental concerns. This study investigated an alternative approach to wastewater treatment using a hydrodynamic cavitation pilot plant combined with a venturi as a way to remove recalcitrant compounds. The optimization of the removal process was explored by testing the effect of orifices with size 2, 3, 4, 5, or 6 mm on the decoloration of orange II dye. The impact of the catalyst: iron(II); oxidizing agent: hydrogen peroxide; and contact time was evaluated to find the ideal conditions under which the removal of perfluorooctanoic acid (PFOA) could be achieved. The decoloration of 20 ppm of orange II dye in simulated industrial textile wastewater was achieved at 90% efficiency when the pressure at the inlet was maintained at 300 kPa, the temperature at 34 °C, the pH at 2, and the orifice size at 2 mm of diameter. The kinetic study proved the decoloration reaction was pseudo first order and the rate of decolourisation of orange II was 0.23/min.Ten parts per million of PFOA could not be degraded by free radical attack using advanced oxidation processes when the inlet pressure was maintained at 300 kPa, the temperature at 34 °C, the pH of 2, and the orifice diameter of 2 mm. This resistance to removal is due to the structure of PFOA which is made up of a fluorine ion which stabilizes the compounds by inductive effects while dye is made up of nitrogen ion and is compatible with the above removal methods. The study demonstrated that the combination of venturi and orifice requires the throat size of the venturi to be similar or equal to that of the orifice for better efficiency.