This European Standard specifies the non-steady state measurement of the oxygen transfer rate and the oxygen transfer efficiency of aeration systems in activated sludge aeration tanks (see EN 12255-6) when filled with clean water. NOTE 1 This is known as the clean water test. NOTE 2 Since the method is based on completely mixed tanks or tanks with evenly distributed aerators or diffusers test results from certain aeration installations can be incorrect. NOTE 3 Under process conditions with mixed liquor the oxygen transfer rate and the oxygen transfer efficiency can be different from the clean water test results. This is expressed by the a-factor.