The capacity of an aeration system to transfer oxygen to a given activated sludge oxidation ditch is characterised by the αkLa parameter. This parameter is difficult to measure under normal plant working conditions. Usually this measurement involves off-gas techniques or static mass balance. Therefore an on-line technique has been developed and tested in order to evaluate αkLa. This technique deduces αkLa from a data analysis of low cost sensor measurement: two flow meters and one oxygen probe. It involves a dynamic mass balance applied to aeration cycles selected according to given criteria. This technique has been applied to a wastewater treatment plant during four years. Significant variations of the αkLa values have been detected while the number of blowers changes. This technique has been applied to another plant during two months.

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