For aerobic sludge stabilisation, the sludge from the wastewater treatment plants is aerobically treated until all organic components are oxidised and deposition is possible without any odour issues. This procedure proves itself to be economical for sludge digestion, which works anaerobically, i.e. using air exclusion, as the treatment time can be considerably shortened by the self-heating of the sludge. An aeration system is needed here, which intensively aerates the sludge and does not additionally cool down through turbulence. Our immersible TA or TAK aerators are particularly well suited for this. After trials and operating results, a process temperature of 45°C and more is achieved without difficulties. This is known as aerobic thermophilic sludge stabilisation. The treatment time of the sludge is reduced to less than 5 days as a result. On the other hand, according to Imhoff, the anaerobic sludge digestion, which tends to work at 30°C, requires a treatment time of 27 days. The immersible aerators used to ventilate the sludge are also well suited because they have a high circulation rate and no blockages are expected with the generally very high sludge dry weights.