Effects of surfactants on oxygen transfer in microbubble aeration

Surfactants were the important contaminants in wastewater and influenced oxygen transfer in aeration processes. The effects of four types of surfactants, including anionic, zwitterionic, cationic and nonionic surfactants, on oxygen transfer in microbubble aeration were investigated and compared with traditional bubble aeration. The results indicated that the oxygen transfer was enhanced with and without addition of surfactants in microbubble aeration. The K(L)a values were 7.44 similar to 11.56 h(-1) when the concentration of four types of surfactants varied from 5 to 20 mg/L, larger than that in traditional bubble aeration. Different types of surfactants showed different effects on oxygen transfer in microbubble aeration. When surfactant concentration increased, ionic surfactants showed strong positive effect on oxygen transfer and the a-factors were 0.77 similar to 1.20, even larger than 1.0 at high surfactant concentrations. The KLa values of ionic surfactants followed the sequence: cationic > zwitterionic > anionic under the same concentration condition. On the contrary, nonionic surfactant showed negative effect on oxygen transfer, similar with that in traditional bubble aeration. The gas holdup and bubble retention time were as high as 1.02 %similar to 2.67 % and 1.2 min similar to 3.2 min in microbubble aeration in the presence of surfactants, respectively, demonstrating their important role in oxygen transfer enhancement. The surfactant properties also influenced oxygen transfer greatly.