Abstract: In the aerobic digestion of sludges, particularly but not exclusively primary sedimentation sewage sludges, a layer of sludge foam is deliberately maintained over the sludge surface to improve the efficiency of the process. The layer of foam may have a volume of at least 50% of that of the sludge. The digestion process is effected in a digester, in which the amount of foam may be reduced (as necessary) by injecting sludge directly into the foam to cause local collapse thereof. A digester vessel especially suited for use in the process is described.