Aeration has been used successfully in northern Canada and other cold regions for treating domestic and industrial waste waters by aerated lagoons, extended aeration package units and conventional activated sludge plants. Many of these installations used diffused aeration because this system has shown several advantages over mechanical surface aeration. The advantages of using diffused aeration in cold regions for oxygenation as well as for mixing are discussed in this paper.The parameters affecting the efficiency of oxygen transfer and the mixing of the tank contents are identified and the influence of varying these operating parameters on the performance of diffused aeration systems under cold climatic conditions is discussed. Equations showing the influence of temperature, airflow rates, submergence and tank geometry on the overall oxygen transfer coefficient are presented.A process of aeration, employing counter-current flow of air bubbles and waste water, is shown to result in increased contact time and therefore higher oxygenation efficiencies. A typical example for designing a diffused aeration system in cold regions has been worked out.