Domestic sewage is discharged into the presettling basin where it is mechanically cleaned first. Then the prepurified sewage is conveyed to the activating basin. In the first phase, the nitrification phase, oxygen is added in the form of compressed air to support the growth of oxygen consuming bacteria to form what is known as 'activated sludge'. In a second phase, the denitrification phase, no oxygen is added making the previously strongly multiplied bacteria suffer from oxygen deficiency. They now have to utilise other oxygen compounds.