Ozonation dynamics and its implication for off-gas ozone control in treating pulp mill wastewaters

Ozonation of biologically pretreated pulp mill wastewaters was studied usin g both bench and pilot scale fine bubble contactors to determine the oxidat ion efficiencies, mass transfer coefficients (k(L)a) and enhancement factor s (E) due to the occurrence of chemical reactions. A sensitivity analysis b ased on the measured process parameters was then used to reveal the interre lated effects of key factors on off-gas ozone concentrations. It was shown that the removal efficiencies of color and AOX were simply related to the a mount of utilized ozone, regardless of variation of other operating conditi ons. Furthermore, the rate of absorption fell within the fast or instantane ous kinetics regimes due to the occurrence of rapid chemical reactions. The Ek(L)a values were found to vary substantially during the course of ozonat ion, indicating that the enhancement factors were not only affected not onl y by operating conditions but also by wastewater characteristics. To effect ively control the off-gas ozone emission, measures should be taken to minim ize the backmixing, use a counter-current flow arrangement and provide adeq uate contact time.