Using carbon dioxide gas to lower the pH of tunnelling waste water – laboratory test and assessment of toxic potential

Summary

Building of tunnels produces waste water that often is highly alkaline. Such water can be toxic to aquatic organisms, and pH should be lowered before discharge to recipient. A reactor tank was built to test the use carbon dioxide (CO2) gas for this purpose. Small scale experiments were conducted with three synthetic solutions prepared by diluting acids and bases, and one sample of tunnelling waste water from Gran, National road 4 (rv. 4). The pH decreased rapidly to pH 8.0 when CO2 gas was added to alkaline solutions. The rate of pH decrease was markedly slower below pH 7.0. The CO2 concentration was low (<40 mg/L) at pH values higher than 8.0, but increased rapidly as pH dropped below 8.0. Initially, the pH dropped faster and the CO2 concentrations increased faster at 4 °C than at 15 °C, but there were no clear differences when the experiments were terminated. The aqueous CO2 concentration in tunnelling waste water at pH 8.0 is probably not acutely toxic to fish, but can rapidly rise to toxic levels if pH declines further.