Ambient pressure and gas-liquid oxygen transfer at high altitude: A critical review of an uncorrected design assumption

Aeration is the most energy-intensive step in biological wastewater treatment, and is most constrained at high altitude, where low pressure and low temperature coincide. Standard design corrects the driving force for pressure by scaling the saturation dissolved oxygen C s , and corrects the volumetric oxygen transfer coefficient K La for water quality and temperature, but not for pressure. Across the design frameworks examined here, K La is the only core term corrected for some environmental variables and not for pressure. Using the decomposition K La = k L · a , this review traces the paths by which lower pressure may alter K La across the bubble life cycle. The Sauter mean diameter should grow through three concordant channels: a larger formation size, stronger rise expansion, and weaker gas-density-driven breakup. The gas holdup responds with a sign set by the gas-supply control basis, mass flow or volumetric flow, while the liquid-film coefficient k L is insensitive to pressure directly. These paths oppose one another, so the net effect cannot be predicted from theory alone, and no controlled sub-atmospheric measurement of K La exists to resolve it. A worked design example shows what is at stake: the driving-force correction alone raises the required air mass flow by 40% and the blower power by 92% at 3000 m, and an uncorrected error in the oxygen transfer efficiency E A would move both by a further −20% to +22%. The review calls for controlled-decompression experiments, sets out three falsifiable hypotheses and the measurements that would refute them, and proposes a pressure correction for E A .