The present invention relates to a process for the improved separation of clarified liquids from biomass or rather for the separation of clarified sludges in the (aerobic and/or anaerobic) biological treatment of sewage using magnetically separable materials as carrier masses. The carrier masses are organic materials containing magnetically separable inorganic materials incorporated in abrasion-resistant form. These magnetically separable carriers occupy a suspension volume of from 1 to 85% by volume, based on the clarified liquid.The carriers contain magnetically separable inorganic materials incorporated as fillers in organic polymeric materials, particularly in cellular polymeric carriers. Other organic or inorganic fillers may also be used in these organic carriers. The magnetically separable materials used are oxides or mixed oxides of heavy metals, preferably iron oxides, such as Fe.sub.3 O.sub.4 (magnetite) and .gamma.-Fe.sub.2 O.sub.3, which are used in average particle sizes of less than 50 .mu.m, preferably less than 10 .mu.m and more preferably less than 3 .mu.m, for example from 0.1 to 1 .mu.m.These magnetic carriers provide for magnetic separation of the biomasses containing these magnetic materials in the biological treatment of sewage.