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) tre atment plant were found to exhibit signs of distress during the first hours of exposure. While the final effluent was not acutely lethal at full stren gth, it was still important to determine the cause of the distress to ensur e that the mill's effluents remain in regulatory compliance. The use of a s eries of effluent manipulations involving pH adjustment and aeration as wel l as observations of symptoms exhibited by the fish were successful in iden tifying carbon dioxide as the cause of fish distress for the UNOX-treated e ffluent. 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 contai ner 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 c arbon dioxide concentration of 250 mg/L, however, was found to be lethal to trout. A survey of mills using oxygen activated sludge treatment systems i ndicated that the carbon dioxide levels in effluents from such operations c an range from 48 to 251 mg/L. Solutions for eliminating the fish distress a nd possible mortality associated with such effluents would be to lower the carbon dioxide level to below 100 mg/L. by aeration or pH adjustment.