For any fuel, the content of water acts as an inhibitor in the incineration process. Thermal drying is one of the technologies available for removing some of the water content in municipal wastewater treatment plants to obtain an optimal energy utilisation before incineration. Municipal sludge is typically liquid containing 2 to 6 per cent solids prior to mechanical dewatering.
The thin sludge enters the drying line and is first mechanically dewatered in a decanter centrifuge or belt press. This is a cost-efficient process that brings the dry substance of the sludge up to 25-30 per cent and accomplishes both weight and volume reductions.
The dewatered sludge is then fed to the drier, where indirect heat is used to evaporate water from the wastewater solids in order to bring the dry substance up to approximately 40 to 45 per cent.
The heat value of the dried solids is thereby high enough to provide for an optimal energy balance in incineration plants.