Integrating membrane, ozonation, and biological processes for the treatment of alkaline bleach plant effluent

The removal of organic compounds from alkaline bleach plant pulp mill effluent was investigated using integration of ozonation, biological treatment, and ultrafiltration processes. The synergies of combining these processes were studied. O3-Bio, Bio-O3-Bio, and UF-(O3) r-(Bio)rf combined treatments, that used 0.26-0.35 mg O3/mL wastewater in the bubble column, provided about 57-65% COD removal from the alkaline effluent. This amount of removal was up to three times more than the COD removal obtained by stand-alone ozonation or biotreatment. The significantly greater COD removal indicated the presence of synergies between the treatment methods. Significant changes in BOD5, COD, TC (or TOC), pH, and colour were obtained for the ozonation stage of Bio-O 3-Bio and UF-(O3)r -(Bio)rf treatments. Ozonation alone that was conducted on the alkaline effluent increased the biodegradability (measured as BOD5/COD) of the whole effluent by 30-40% using 0.7-0.8 mg O3/mL wastewater. The improvement in the biodegradability is related to the cleavage of high molecular weight (HMW) compounds, which were found non-biodegradable, and production of low molecular weight (LMW) organics, which were very biodegradable. When ozone was applied to each molecular size fraction, it did not change the biodegradability of LMWs and BOD5/COD stayed constant at about 50%. Ozonation, on the other hand, increased the biodegradability of HMWs by 50%. Hence, it was found important to remove the LMW organics before ozonating the wastewater to reduce the size of the bubble column and improve the overall performance of ozonation through reducing scavengers of oxidizing radicals. Statistical analysis of variance (ANOVA) showed that the initial pH (range: 9 to 11) and temperature (range: 20 and 60°C) of the effluent did not influence the biodegradability improvement during the ozonation at 95% confidence level. However, the effect of pH became significant when a wider range of pl1 (4.5 vs. 11) was examined. The rate of COD removal during the ozonation followed a first order kinetics with respect to COD. The percentage COD removal during the actual biological treatment was found more than the value estimated using BOD5/COD and a linear function was obtained to correlate them.