Quantities of sludge are considerable: About 1,800,000 tons of thin sludge per year are produced in Vienna alone. By centrifuges these are processed to give about 180,000 tons of thick sludge per year, that are subsequently burnt in fluidisation furnaces.
Also other extracted refuse is burnt – gravel and metal particles contained in gravel, sand and fine particles included in sand and large quantities of textile, paper, fibre and residual organic substances as rests of fruit and vegetables from the raking (sieve) device. Added are residual substances from the cleaning process of Viennese sewers, that are also burnt because of hygienic reasons and the smell.
This does not only mean, that Vienna, as the only town in the whole world hygienelises all the content of waste water by burning it, but also involves an immense technical effort. While contents of waste water, gained by pure sedimentation – i.e. gravel, sand, raked material and refuse – may be burnt without any pre-processing, sludge from cleaned waste water requires exactly adapted processing. E.g. thin sludge gained by sedimentation/deposition in a thickening tank (about 4.5 % of contained substance) is itself a secondary product of the mixture (only about 0.7 % of contained substance) of the primary and secondary sludge in the principal sewage plant.
The thin sludge is pre-processed after careful preparation by the addition of a coagulant (synthetic substance in the form of a powder) and careful heating and then dehydrated out by centrifuging. This technology has been, in cooperation with suppliers of equipment and additives, improved in the plant Simmeringer Haide in 1986 to 1990 so, that the plant Simmering is able to operate with exemplary performance data till now. After dehydration solid contents in the thick sludge lie between 34 and 37 %. The values of the Simmeringer Haide were so instructive, that this process was adopted on a large scale by the US states New York and Connecticut.
After dehydration, sludge cannot burn as it is, but can be used in a very progressive burning technology for the production of energy. For this purpose three fluidisation furnaces were built in the plant Simmeringer Haide with an overall gas performance of 173,000 m3/hour.
Exhaust gases from the furnaces – these are working at a burning temperature of 850 to 880 °C – are used for the production of energy. Energy is extracted from exhaust gas by a steam boiler, so that steam at 53 bar is produced. Together with the steam produced by burning special refuse so much electricity is produced, that the plant Simmeringer Haide (without principal sewage treatment plant) is able to cover its own electric energy consumption.
The furnace lines are equipped with an energy-heating coupling serving to optimise this energy producing system. Remaining heat is used for heating and the production of warm water via the Viennese district heating. For comparison only: The heat produced in the plant Simmeringer Haide would be sufficient to heat the complete capital of Lower Austria, St. Poelten!
The steam boilers are followed by a cleaning section for combustible gas. Electric filters and four-stage gas washing devices provide very well cleaned exhaust gas. But the plant Simmeringer Haide has even more ambitious objectives. Therefore the gases are, with respect to energy neutrally, heated (from 60 to 110 ° C) and pass through active carbon filters. Only 0.003 nanograms of dioxine and furane per cubic meter, i.e. only three hundredth of the limiting value according to the Austrian air cleanness law, are finally released into the chimney.
Information & Contact:
Fernwaerme Wien GmbH
Plant Simmeringer Haide
Ing. Herbert Balz
Tel. +43 1 313 26-6320
Fax +43 1 313 26-6458
