The invention relates to a process for drying sludge having the following steps: a) applying a sludge having a dry matter content of 15% to 30% to a floor of a drying hall (10), or sludge already present, b) predrying the sludge in the drying hall (10), wherein the sludge is mixed, c) introducing at least a part of the predried sludge into a heated thermal dryer (12), wherein the sludge introduced in each case into the thermal dryer (12) is replaced in the drying hall (10), by carrying out the step a), by a sludge having a dry matter content of 15% to 30% which then likewise passes through step b), d) drying the sludge in the thermal dryer (12) to a dry matter content of 80% to 95%, wherein air (14) taking up moisture from the sludge flows over or through the sludge, wherein the air (14) is brought to a temperature in a range from 70° C. to 160° C., or wherein, by heating the sludge, water vapor is liberated from the sludge, wherein the sludge, after achieving the dry matter content of 80% to 95%, is discharged from the thermal dryer (12), wherein the sludge discharged from the thermal dryer in each case is replaced by predried sludge by carrying out step c), which predried sludge then likewise passes through step d), e) passing the air (14) heated in step d), or the water vapor through a first region of a heat exchanger (17), through which ambient air (16) passes in a second region, wherein the air (14) heated in step d) or the water vapor releases heat to the ambient air (16), with condensation of the moisture present in the heated air or in the water vapor, wherein the heated air (14) or the water vapor is cooled to a temperature in the range from 10° C. to 60° C., and the ambient air (16) is heated to a temperature at most 40 K above the ambient temperature and the former temperature is at most 50° C., and f) promoting the predrying according to step b) by introducing the ambient air (18) heated in step e) into the drying hall (10), in such a manner that the ambient air (16) flows onto or over the surface of the sludge applied to the floor of the drying hall (10).