Abstract: A process for the disinfection and aerobic stabilization of concentrated sewage sludge in several steps, using one or more heat-insulated containers, preferably with stirring, with the addition of an oxygen-containing gas, described wherein(a) the sludge coming in, having been concentrated to at least 2% of dry matter, is caused to reach the thermophilic temperature range of >50.degree. C., with pre-heating if required, and then solely by an aerobic degradation without supplying external heat within from 2 to 4 days,(b) the sludge is disinfected by further aerobic degradation without supplying external heat at about 50.degree. C. to 55.degree. C. for at least 20 hours or at >55.degree. C. for at least 10 hours, and(c) the sludge is stabilized to become mesophilic by way of actively cooling it down to about 25.degree. C. to 45.degree. C. and a further aerobic degradation within about 2 to 8 days, and preferably within from 3 to 5 days.