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 or to sludge already present there, b) predrying the sludge in the drying hall, wherein the sludge is mixed, c) introducing at least a part of the predried sludge into a heated thermal dryer, wherein the sludge introduced in each case into the thermal dryer is re-placed in the drying hall by a sludge having a dry matter content of 15% to 30% by carrying out step a), which sludge then likewise passes through step b), d) drying the sludge in the thermal dryer to a dry matter content of 80% to 95%, wherein air taking up moisture from the sludge flows over or through the sludge, wherein the air is brought to a temperature in a range from 70°C to 160°C, or wherein, by heating the sludge, water vapor at a temperature of 100°C to 160°C is liberated from the sludge, wherein the sludge, after achieving the dry matter content of 80% to 95%, is discharged from the thermal dryer, 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 heated in step d), or the water vapor, through a first region of a heat exchanger, through which, in a second region, ambient air flows, wherein the air heated in step d) or the water vapor, with condensation of the moisture present therein, releases heat to the ambient air, wherein the air or the water vapor is cooled to a temperature in the range from 10°C to 60°C, and the ambient air is heated to a temperature of at most 50°C which is at most 40 K above the ambient temperature and f) promoting the predrying according to step b) by introducing the ambient air that is heated in step e) into the drying hall, in such a manner that the ambient air flows on or above the surface of the sludge applied to the floor of the drying hall, wherein an increase or decrease in the pressure of the air, the water vapor or the ambient air in the overall process serves solely for transporting the air, the water vapor or the ambient air. a) applying a sludge having a dry matter content of 15% to 30% to a floor of a drying hall or to sludge already present there, b) predrying the sludge in the drying hall, wherein the sludge is mixed, c) introducing at least a part of the predried sludge into a heated thermal dryer, wherein the sludge introduced in each case into the thermal dryer is re-placed in the drying hall by a sludge having a dry matter content of 15% to 30% by carrying out step a), which sludge then likewise passes through step b), d) drying the sludge in the thermal dryer to a dry matter content of 80% to 95%, wherein air taking up moisture from the sludge flows over or through the sludge, wherein the air is brought to a temperature in a range from 70°C to 160°C, or wherein, by heating the sludge, water vapor at a temperature of 100°C to 160°C is liberated from the sludge, wherein the sludge, after achieving the dry matter content of 80% to 95%, is discharged from the thermal dryer, 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 heated in step d), or the water vapor, through a first region of a heat exchanger, through which, in a second region, ambient air flows, wherein the air heated in step d) or the water vapor, with condensation of the moisture present therein, releases heat to the ambient air, wherein the air or the water vapor is cooled to a temperature in the range from 10°C to 60°C, and the ambient air is heated to a temperature of at most 50°C which is at most 40 K above the ambient temperature and f) promoting the predrying according to step b) by introducing the ambient air that is heated in step e) into the drying hall, in such a manner that the ambient air flows on or above the surface of the sludge applied to the floor of the drying hall,