Method for controlling the supply of ambient air to an activation tank of a waste water purification plant, with the method steps characterized in that - lit a) by means of a collecting device arranged on the activation tank, the gas emerging from the activation tank is collected at a predefinable point on the surface of the activation tank, sealed off from the ambient air, and this emerging gas volume is subjected to an analysis, - lit b) as part of the analysis the oxygen and carbon dioxide content in the emerging gas collected on the surface of the activation tank is determined, - lit c) the difference between the sum of the oxygen content and the carbon dioxide content of the supplied ambient air and the sum of the oxygen content and the carbon dioxide content of the emerging gas collected on the surface of the activation tank is determined, - lit d) from the difference between the sums of the oxygen content and the carbon dioxide content according to lit c), produced by the biological pollutant load reduction and the gas components not actively involved in the biological pollutant load reduction in changing the concentration, the amount of the total oxygen consumption and the proportion of carbon dioxide produced in the activation tank from the supplied ambient air is determined, - lit e) the amount of ambient air supplied is reduced in pre-settable steps as long as the proportion between the difference mentioned in lit c) and the proportional carbon dioxide content remains unchanged and - lit f) if the proportion between the difference mentioned in lit c) and the proportional carbon dioxide content changes, the amount of ambient air supplied is increased by at least the reduction of the amount of air in the previous step. - lit a) by means of a collecting device arranged on the activation tank, the gas emerging from the activation tank is collected at a predefinable point on the surface of the activation tank, sealed off from the ambient air, and this emerging gas volume is subjected to an analysis, - lit b) as part of the analysis the oxygen and carbon dioxide content in the emerging gas collected on the surface of the activation tank is determined, - lit c) the difference between the sum of the oxygen content and the carbon dioxide content of the supplied ambient air and the sum of the oxygen content and the carbon dioxide content of the emerging gas collected on the surface of the activation tank is determined, - lit d) from the difference between the sums of the oxygen content and the carbon dioxide content according to lit c), produced by the biological pollutant load reduction and the gas components not actively involved in the biological pollutant load reduction in changing the concentration, the amount of the total oxygen consumption and the proportion of carbon dioxide produced in the activation tank from the supplied ambient air is determined, - lit e) the amount of ambient air supplied is reduced in pre-settable steps as long as the proportion between the difference mentioned in lit c) and the proportional carbon dioxide content remains unchanged and - lit f) if the proportion between the difference mentioned in lit c) and the proportional carbon dioxide content changes, the amount of ambient air supplied is increased by at least the reduction of the amount of air in the previous step.