Hydrodynamic Cavitation as Pretreatment for Removal of Hardness from Reverse Osmosis Reject Water

In the present investigation, the treatment of reverse osmosis reject water hardness by hydrodynamic cavitation with the addition of sodium bicarbonate was studied. Hydrodynamic cavitation (HC), which is formed as a result of fluid pressure and velocity variation, has attracted great attention in industrial wastewater treatment due to its simple design and ease of operation. The influence of wastewater recirculation flow rate and the number of holes in the metal plate on the percentage removal of calcium hardness was studied. The study used a two-factor factorial design with three levels, plates with(3, 5 and 9 holes)and a recirculation flow rate of (0.6, 1 and 1.5 L/min) with a treatment time of 60 minutes. Calcium hardness, total dissolved solids, pH and liquid temperature were evaluated as a function of time. The results show that as the flow rate and the number of orifices increase, the percentage of hardness removal increases. A linear correlation of hardness removal with respect to flow rate and number of orifices in the metal plate is also observed. For the recirculation flow rate of 1.5 L/min and a plate with 9 holes, a maximum removal rate of 66.76 % was achieved. It was also observed that the temperature increases as a function of time, reaching up to 75°C, while pH and total dissolved solids decrease during the treatment time. Hydrodynamic cavitation represents an environmentally friendly mechanical treatment technology and, considering that the removal efficiency is higher than 60%, it is an alternative as a pretreatment for the removal of water hardness.