The use of CO₂ for the treatment of alkaline tunnel construction wastewater: Efficiency and influencing factors

Alkaline wastewater from tunnel construction can cause environmental problems when draining into water streams like rivers and lakes. Research interest in the neutralization of alkaline wastewater using CO2 based on the carbon utilization concept has been increasing. In this study, the effects of key parameters such as the flow rate (FR) of CO2, initial water temperature (T), pH, and turbidity (TURB) on CO2 neutralization of alkaline wastewater were investigated through laboratory experiments and field tests using alkaline wastewater collected from the Yunnan Dianzhong Water Diversion Engineering, China. Results revealed that 1.5 L min(-1) was the optimal flow rate for CO2 gas. The water pH (p < 0.001) and TURB (p < 0.001) significantly affected the neutralization performance of CO2. The pH, TURB, electrical conductivity and total dissolved solids of alkaline wastewater can be effectively reduced by CO2. The least time required for the treatment was noted at a T, pH, and TURB of 20 degrees C, 10.5, and 80 NTU, respectively. Laboratory and field stability experiments have shown CO2 to be effective in reducing the pH of alkaline wastewater and consistently maintaining its pH within the discharge standard compliance range (6-9). A cost saving of $0.26 per 1 m(3) of water was achieved comparing with neutralization of alkaline wastewater using citric acid. In summary, this study for the first time explored the efficiency and potential influencing factors for the treatment of alkaline tunnel construction wastewater using CO2. It proves the effectiveness and economic benefits of the proposed technology.