Abstract: Process for the biological purification of effluent which makes it possible either to ensure complete treatment of the carbon pollution and the nitrogen pollution at the nominal output, or to remove the biological oxygen demand or BOD only during a substantial increase in the nominal output, especially if it rains, using bacteria in fixed cultures, said process comprising the step of using the modifications of the surrounding environment, that is to say the oxidation-reduction potential which depends on the presence or absence of dissolved oxygen, and the composition of the water in contact with the bacteria, either to modify the enzymatic system of the flora present in the bacterial film of the fixed culture in order to assign to the removal of the BOD a fixed culture which up until then carried out denitrification, or to promote, in the same bacterial film, the development of the most appropriate flora for the desired objective: the removal of the BOD or nitrification.