Carbon Dioxide in Pulp and Paper Mill Effluents from Oxygen-Activated Sludge Treatment Plants as a Potential Source of Distress and Toxicity to Fish

Rainbow trout exposed to effluent from a UNOX (oxygen activated sludge) treatment plant were found to exhibit signs of distress during the first hours of exposure. While the final effluent was not acutely lethal at full strength, it was still important to determine the cause of the distress to ensure that the mill's effluents remain in regulatory compliance. The use of a series of effluent manipulations involving pH adjustment and aeration as well as observations of symptoms exhibited by the fish were successful in identifying carbon dioxide as the cause of fish distress for the UNOX-treated effluent. For rainbow trout, the symptoms of exposure to elevated levels of carbon dioxide of 100 mg/L or greater included loss of equilibrium, erratic swimming, gasping at the surface, sinking to the bottom of the test container and paralysis. The fish were found to eventually recover as the carbon dioxide was stripped from the effluent due to aeration during the test. A carbon dioxide concentration of 250 mg/L, however, was found to be lethal to trout A survey of mills using oxygen activated sludge treatment systems indicated that the carbon dioxide levels in effluents from such operations can range from 48 to 251 mg/L. Solutions for eliminating the fish distress and possible mortality associated with such effluents would be to lower the carbon dioxide level to below 100 mg/L by aeration or pH adjustment.