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

This study statistically examined the impacts of ozone application met hods on the two representative types of pulp mill effluents. To facili tate statistical evaluation, the experiments were designed as paired b lock experimental series, and the pooled experimental errors in each b lock were used for t-test and ANOVA analysis. Two different ozone appl ication modes were investigated using two specially designed ozone rea ctor systems. System I consisted of a two-phased reactor which introdu ced the total amount of ozone to the wastewater in single instance wit h proper mixing. System II provided ozone to wastewater at a desired r ate 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 dec omposed in the gas phase in System II. The observed effects were contr ibuted to the decomposition of ozone initiated by water vapor in the g as phase. According to the ratio of decomposed ozone to used ozone, th e consumed ozone dose was established and the performance of ozone tre atment with each system was reevaluated. The new results demonstrated that over the wide range of ozone dose levels, the ozone application m ethods did not have statistically significant effects on ozone treatme nt of both wastewaters with regard to true color removal (at 1% signif icance level), COD and TOC removal (at 5% significance level), and the improvement of short-term biodegradability as BOD5 (at 5% significanc e level). The results also suggested that 1) the ozone application met hods did not change the competitiveness of ozone reactions with variou s organics in the wastewater but affected the availability of ozone fo r reacting with the target organics; 2) the consumed ozone dose determ ined the level of ozone treatment but gas phase mass transfer could be the kinetically-controlling process in ozonation of pulp mill effluen ts; 3) biodegradable (non-colored) organics had the greatest potential to reduce the efficacy of ozone treatment on the pulp mill effluents.