Process parameter development for ozonation of kraft pulp mill effluents

Wastewaters discharged from the aerated lagoon basin of a kraft pulp mill were ozonated using both bench scale and pilot scale fine bubble contactors. Effects of contactor scaling-up on ozonation effectiveness were examined by comparing the removal efficiencies of color, AOX (adsorbable organic halides) and other wastewater characteristics. The absorption efficiency of ozone was determined by monitoring the exit off-gas ozone over a wide range of operating conditions. Overall mass transfer coefficient (k_La) and its enhancement factor due to the occurrence of rapid ozone decay reactions (E) were then quantified by applying the two-film theory to an irreversible gas-liquid reaction. Results showed that ozone was very effective in oxidizing color and AOX-causing compounds. The effectiveness could be simply related to the amount of utilized ozone alone, regardless of the variations in gas flowrate, inlet ozone concentration, and contactor configurations. In contrast, the ozone absorption was affected by both operating conditions and wastewater characteristics. Its rate might be enhanced substantially due to the presence of ozone decay reactions, particularly in the cases of occurring rapid ozone decay reactions. Accordingly, a relationship was developed to describe the dynamic changes of Ek_La values versus the utilized ozone.