Increasing the pH of wastewater to high levels with different gases--CO2 stripping

The potential of increasing the pH of wastewater to high levels by CO^sub 2^ stripping through air, N^sub 2^, O^sub 2^ and a gas mixture (95% N^sub 2^+ 5% CO^sub 2^) has been examined in this work. Wastewater collected after the biological treatment from a large-scale plant was monitored for pH in a continuous, computer-controlled laboratory setup. The use of air as the gaseous medium for CO^sub 2^ stripping increased the pH to a maximum value of approximately 8.53. Using pure N^sub 2^ instead of air increased the pH of wastewater to a final value of 10.3 after 24 h at a flow rate of 1.5 L min^sup -1^. An experimental system dismantling air from its CO^sub 2^ content by precipitation as CaCO^sub 3^ in a super-saturated lime solution (in a closed circulation operation), increased the pH of wastewater to a final value of 9.4 after 24 h at a flow rate of 0.7 L min^sup -1^. Applying the same closed circulation operation to an artificial carbonate solution instead of wastewater resulted in final pH values of 9.8 after 24 h using a flow rate of 0.7 L min^sup -1^. The results suggest that a closed circulation operation with CO^sub 2^-freed air has the potential to increase the pH of wastewater to high levels. The process may be applied for phosphorus precipitation from domestic wastewater not requiring chemicals for a pH increase.[PUBLICATION ABSTRACT]