Aeration with carbon dioxide-supplemented air as a method to control pH drift in toxicity tests with effluents from wastewater treatment plants

Environment Canada methods for acute toxicity tests with rainbow trout require continuous aeration of test solutions during exposure. Depending on the sample, this procedure can result in an increase in pH as dissolved carbon dioxide (CO2) is stripped from solution as a result of aeration. In samples that contain ammonia, the pH may increase to the point where the unionized fraction results in artifactual toxicity. Consequently, aeration with air supplemented with different CO2 concentrations was investigated as a method for maintaining pH at the level found in the original sample without adversely affecting other water quality parameters. Aeration with CO2 was an effective method for maintaining pH during exposure, depending both on the concentration of CO2 and the alkalinity of the sample. A multiple regression conducted on the data determined an equation that was effective at calculating the CO2 concentration necessary in an aeration mixture to maintain a target pH value as a function of sample alkalinity.