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The use of ozone for the treatment of logyard run-off Zenaitis, Michael Guy

Abstract

This work explored the use of ozone in the treatment of run-off from logyards in an effort to remove its toxicity to aquatic wildlife. In this survey, an EC50 range of 1.9%-26.8%, as measured by Microtox, was obtained for samples from a pair of sawmills on the British Columbia coast. Canadian law prohibits toxic discharges into fish-bearing waters; toxicity is defined as a 96-hour rainbow trout LC50 less than 100%. Ozone has been found to be an effective way of reducing the toxicity of logyard run-off. For treatment of run-off at pH 7, the reduction in the levels of COD and BOD was moderate (-35% and 25%, respectively) but reduction in the levels of toxicity and parameters associated with toxicity to aquatic organisms was significant. Acute toxicity as measured by Microtox was reduced by over 85% while DHA (a resin acid toxic to fish at low concentrations) and tannins and lignins were reduced by 100% and 90%, respectively. A decrease in the pH of the treatment from 7 to 5 was found to have a negative effect on the effectiveness of ozone in reducing the levels of the toxicity-related parameters. Although run-off samples had quite different initial COD levels (2380 mg/L-8760 mg/L), the fractional reduction of COD and toxicity-related parameters displayed good consistency when expressed in terms of ozone consumed / initial COD (0.57 mg/mg). Batch biological treatment of run-off resulted in reductions of BOD, COD, tannins and lignins, and Microtox toxicity of 98%), 80%, 90%, and 96%, respectively. The kinetics of biodegradation are similar to those for a bleached kraft mill effluent. Ozonation of logyard run-off in conjunction with biological treatment was examined. Ozone treatment of biologically-treated run-off resulted in further reductions of COD (22%) and tannins and lignins (68%); however, these were from quite reduced starting levels of 1130 mg/L and 105 mg/L, respectively. Microtox toxicity was not improved and BOD increased slightly from a low initial concentration of 94 mg/L. The ozonation of run-off affected subsequent biological treatment. The BOD of preozonized samples decreased faster than that of non-ozonized samples during biological treatment but the final residual COD at the end of biological treatment was higher for ozonized samples. Although starting from quite different levels (200 mg/L -677 mg/L), the tannin and lignin levels of ozonated and non-ozonated run-off attained similar levels (80 mg/L-105 mg/L) by the end of biological treatment. Toxicity levels (6.8%-23.9% EC50) displayed the same relationship as tannins and lignins (final EC50 55%-60%). The oxidation of DHA by ozone was examined using Matlab to empirically fit the data. The reaction rate constant between the two compounds was determined to be l . l x l O 2 L/mols at 23°C. The reaction was found to consume 3 moles of ozone per mole of DHA consumed, and to also generate 3 moles of hydrogen peroxide per mole of DHA consumed. Radical scavengers were found to have a deleterious effect on the rate of oxidation of DHA by ozone, especially as pH becomes basic.

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