THE EUROPEAN DIRECTIVE The European Directive EN12255-15 defines the measurement and calculation of oxygen input in aeration tanks of municipal and industrial wastewater treatment plants in clear water. Standard Conditions The oxygen input is tested for the sake of comparability according to the following standard conditions: 1) Clear water in the tank: As the composition of different sewage has a significant influence on the oxygen input, all measurements have to be made in clear water. An adjustment of the oxygen input values to the indiviual situation in sewage is made via the alpha-factor (correction factor). 2) Water temperature 20°C: The efficiency of oxygen input in water is depending on the temperature of the medium. In general: the colder the medium, the more efficient, i.e. higher the oxygen input is. To allow fo the comparison in relation to the temperature, the temperature is defined as 20 degrees C according to the EN 12255-15. The onversion to the actual oxygen input into the sewage on site is done by mathematical calculations. 3) Atmospheric Pressure 1013hPa = 1 atm: Further, the atmospheric pressure has an influence on the efficiency of oxygen input. The higher the installation above sea level, the less efficient the oxygen input. The EN 12255-15 defines the atmospheric pressure at 1 atm (installation at sea level). The conversion to the situation on site is also done by mathematical calculations. The detailed proceedings during measurments according to EN 12255-15 are laid out in the European directive. If oxygen input values listed in catalogues were not measured according to the above mentioned standard conditions, then the oxygen input values are not conforming to EN 12255-15.