REDUCTION IN THE TOXICITY OF BOILER BLOWDOWN EFFLUENT BY ADDITION OF CALCIUM OR HUMIC-ACID

Boiler blowdown effluent from both nuclear and thermal electric genera ting stations was frequently toxic to both rainbow trout and Daphnia m agna in monitoring tests. The toxicity could be caused either by the l ow ionic content of the effluent, or through interactions of the low i onic content and elevated metal concentrations. Followup studies were conducted to characterize the toxicity of the effluent and develop met hods to reduce its toxicity. Addition of calcium, either by adding cal cium chloride (CaCl2) or recirculating the effluent over a limestone f ilter for approximately 12 h, largely eliminated the toxicity of boile r blowdown effluent to trout; however, the treated effluent remained t oxic to D. magna. Adding humic acid (approximately 20 mg L-1 as dissol ved organic carbon) significantly reduced the toxicity of boiler blowd own effluent to both D. magna and rainbow trout. The toxicity of boile r blowdown effluent appears to result from the interactions of its low ionic content and elevated metal concentrations, but addition of humi c acid effectively mitigated this toxicity.