Whole effluent toxicity reduction by ozone

An investigation of the effects of ozone and ozone‐induced hydroxyl radical on reducing whole effluent toxicity is discussed relative to the application of ozone for industrial water treatment. Results from operation of an ozone system treating industrial effluent from a lead/zinc mine in Colorado are presented. The mine discharges 1,000 gpm (227 m3/hr) of wastewater and has historically exceeded Whole Effluent Toxicity (WET) limits. On occasion, it has exceeded numeric limits for copper, ammonia, and cyanide. Based on test results, an applied ozone to COD ratio of 3:1 by weight and a contact time of 30 minutes was found to be effective for reducing whole effluent toxicity at pH 11 but not at pH 7, indicating oxidation by hydroxyl radical to be the dominant mechanism responsible for toxicity reduction. At an applied ozone to COD ratio of 3:1 and a pH of 11, toxicity was reduced with survival increasing improved from 0 percent survival to 100 percent survival for Ceriodaphnia dubia and fathead minnow (Pimephales promelas) based on 48‐hour and 96‐hour WET tests, respectively. This application rate of ozone with a 99 percent mass transfer efficiency was also effective in reducing total cyanide from an average of 0.45 mg/L to less than 0.05 mg/L and COD from 28 mg/L to 9 mg/L. The rate of ammonium nitrogen oxidation appeared to follow first‐order kinetics; however, the rate of oxidation was decreased significantly by the presence of COD.