Annually around 1,000,000 m3 of wastewater sludge is generated during the wastewater treatment with a dry matter content of about 10,000 tons. This wastewater sludge is dewatered to such an extent that it can be thermally recycled in one of the two fluidized-bed furnaces. During the incineration and subsequent flue-gas purification, problematic compounds are destroyed or separated. The ash generated during the incineration contains valuable nutrients, such as phosphorus compounds, and is supplied to the fertilizer industry as a resource. The incineration of the sewage sludge is CO2-neutral, similar to wood or other renewable raw materials. Two steam turbines use the emitted heat to generate electrical energy.
01 Sludge dewatering
The sludge from the primary and secondary clarifiers is pumped into the thickening tank. Via the receiver tank the sludge is transported into the centrifuge, where it is mechanically dewatered. Before centrifugation, the sludge is conditioned with the flocculation agents to improve its dewatering characteristics.
The dewatering liquids are introduced back into the biological treatment stage. From the centrifuges, the dewatered sludge is moved into the sludge drying plant and pre-dried to the extent that it can be burned without additional fuel.
02 Thermal recycling
The incineration of the dried sludge takes place in fluidized bed incinerator at a temperature of approx. 850° C. The flue- gas is cooled down to 200° C in a heat recovery boiler. The steam created in the boiler is used as a heat source in the dryer and to generate electricity via steam turbines.
03 Flue-gas purification
A deNOx system, an electrostatic precipitator, a two-stage flue-gas wet scrubber and a activated carbon reactor with fabric filter are used to purify the flue-gas. Sophisticated processes and measuring technology ensure that the flue gas emissions remain far below the limit values of the 17th German Federal Emissions Protection Regulation.
04 Residue recycling
Thermal treatment destroys harmful substances in the sludge. 99% of the phosphorus remains in the ash. The phosphate ash is used as fertilizer in agriculture. The residues from the fabric filter are used as fill materials in mining operations.