The potential of CO2 stripping for pretreating brackish and wastewater desalination feeds

Many scaling species such as CaCO3, Mg(OH)2 and Ca3(PO4)2 present in brackish and wastewater feeds can be removed by precipitation processes induced by raising the pH level of the raw water. These processes require dosage of alkaline chemicals such as NaOH, lime or magnesia. An interesting possibility for eliminating the need for chemical dosage exists in the case of feed waters having high carbonate contents. In this case, simple air bubbling enables stripping the acidic CO2 component of the carbonate system, thus acting to raise the pH level of the solution without chemical addition. The present work analyzes parameters affecting the pH level induced by CO2 removal from solution. Simulation results show that a high pH, exceeding 9, can be achieved with waters having a moderate carbonate level. Batch experiments were conducted to verify conclusions from the simulation study and to provide kinetic data relating the mass transfer coefficient with air sparging parameters. Continuous flow experiments showed that most of dissolved CO2 can be successfully stripped out by ambient air at retention times below 2 h. Results of this study indicate a marked potential for beneficial application of a CO2 stripping pretreatment process for brackish and wastewater feeds having a sufficient carbonate content.