EFFECTS OF OZONE ON KRAFT PROCESS PULP-MILL EFFLUENT

Effluent from a kraft process pulp mill was studied in a batch reactor for ozone doses between 50 and 200 mg O3/L to identify the relative suitability of ozone application locations in the treatment process and see the improvements in biotreatability of wastewaters from a kraft process pulp mill. Laboratory acclimatized seed were used for BOD tests for ozonated and unozonated samples. The inhibitory effects were minimized by using optimum dilutions. The studies were divided into three major sections: characterization of mill effluent; ozone system calibration, and reactor design; and ozonation of mill effluent. Seed for Biochemical Oxygen Demand tests were acclimatized in batch units for primary, bleach and secondary effluents separately. The inhibitory effects which were noted with unacclimatized seed, were reduced by using laboratory acclimatized seed and optimum dilution which were determined during the characterization phase. The batch reactor designed for the studies consisted of a cylindrical section for holding effluent, and a top spherical section for ozone/oxygen mixture. The reactor proved to be effective for controlling ozone dose. The variation in the applied ozone dose was less than 5 mg/L. Bleach and primary effluents were treated with 50 and 100 mg/L ozone doses. Duplicate experiments were conducted for these effluents. Secondary effluent was studied for 50, 100, 150 and 200 mg/L ozone doses. Six replicate experiments were conducted for 50 and 100 mg/L, ozone doses, whereas two experiments were carried out for 150 mg/L and one experiment for 200 mg/L ozone dose. The results were analyzed using 't' test for paired experiments an ANOVA table for statistical confirmation. Residuals were plotted to check the assumptions of constant variance and normal distribution. The results indicated that 50 and 100 mg O3/L effectively removed color from bleach effluent and primary effluent, but did not significantly change the BOD. Ozone was found to be effective for secondary effluent, as BOD5 was increased by 65% for 50 and 100% for 100 mg O3/L doses. The corresponding reduction in color was 62% and 82%, respectively. K(e) and L(o) values for the BOD equation were calculated by using the non-linear least square method for the BOD equation, giving joint confidence regions for the calculated parameters. It was concluded that ozone is most effective for the removal of color and the increase of BOD in secondary effluent.