Surface active agents and their influence on oxygen transfer

Abstract

Oxygen transfer rates of fine bubble aeration systems in uniform arrangement are reduced down to 40% to 70% in wastewater compared to clean water conditions. Surfactants in wastewater are the main reason for the inferior and therefore uneconomic performance. The influence of different types of surfactants (anionic and nonionic) and of their concentration on oxygen transfer is investigated at various properties of pure water (content of electrolytes, hardness) by means of extensive experiments. The main results of the investigations are:in dependence of the type of surfactant, its concentration and the types of water:– the aeration coefficient kLa decreases (down to 55%)– the specific interfacial area (a) increases (up to 350%)– the oxygen transfer coefficient (kL) decreases (down to 20%)nonionic surfactants reduce the oxygen transfer more strongly than anionic surfactantsat the same surface tension, but different types of surfactant α-values can vary over a range of 0.12. Therefore α-values can not be calculated from surface tension measurementsα-values of approximately 0.55 should be taken for designing fine bubble aeration systemsIn new guidelines for the measurement of oxygen transfer rates, addition of 5 gm−3 of an arbitrary surfactant into clean water to simulate wastewater conditions must be abandoned.