Drying of sludge using a disc dryer produces an excess of thermal energy which can be recovered with up to 80 % and reused for various purposes. The energy for sludge drying is transferred during the drying process from the heat source (live steam, thermal oil or pressurised hot water) to the vapours emitted from the dryer. This heat can to a large extent be recovered due to the high temperature level and the low amount of false air which characterises the disc drying system.
Preheating of wet sludge prior to mechanical dewatering results in a higher degree of dewatering and a reduced consumption of flocculation agent. The preheating of raw sludge also saves energy input to the digestor. The surplus of energy can alternatively be used for district heating if the wastewater treatment system is placed near consumers such as cities, large industrial areas, hospitals etc.