INFLUENCE OF OZONE APPLICATION METHODS ON EFFICACY OF OZONE DECOLORIZATION OF PULP-MILL EFFLUENTS

This study statistically examined the impacts of ozone application methods on the two representative types of pulp mill effluents. To facilitate statistical evaluation, the experiments were designed as paired block experimental series, and the pooled experimental errors in each block were used for t-test and ANOVA analysis. Two different ozone application modes were investigated using two specially designed ozone reactor systems. System I consisted of a two-phased reactor which introduced the total amount of ozone to the wastewater in single instance with proper mixing. System II provided ozone to wastewater at a desired rate by controlling the flow and concentration of the ozone/oxygen gas mixture in a once-through flow mode. The investigations revealed that more than 10% of the used ozone dose was decomposed in the water-vapor-saturated gas phase in System I. A negligible amount of ozone was decomposed in the gas phase in System II. The observed effects were contributed to the decomposition of ozone initiated by water vapor in the gas phase. According to the ratio of decomposed ozone to used ozone, the consumed ozone dose was established and the performance of ozone treatment with each system was reevaluated. The new results demonstrated that over the wide range of ozone dose levels, the ozone application methods did not have statistically significant effects on ozone treatment of both wastewaters with regard to true color removal (at 1% significance level), COD and TOC removal (at 5% significance level), and the improvement of short-term biodegradability as BOD5 (at 5% significance level). The results also suggested that 1) the ozone application methods did not change the competitiveness of ozone reactions with various organics in the wastewater but affected the availability of ozone for reacting with the target organics; 2) the consumed ozone dose determined the level of ozone treatment but gas phase mass transfer could be the kinetically-controlling process in ozonation of pulp mill effluents; 3) biodegradable (non-colored) organics had the greatest potential to reduce the efficacy of ozone treatment on the pulp mill effluents.