Vienna turns sewage into electricity
In large digesters, the sludge is converted by anaerobic microorganisms partially in methane-containing fermentation gas, which then is burned in widely conventional power plants to generate electricity. In a process step called anaerobic stabilisation, energy-rich gas mixture is produced; two thirds of this mixture is methane. In a gas motor, the energy content of this gas is converted into mechanical energy used to run a generator, the last link in the conversion chain. Besides electric gas that eventually is turned into electric energy, the process also generates heat, which then is fed to a heat exchanger and is used to pre-heat the sludge.
The city of Vienna does not want to pass up this resource and has launched the project E_OS 2020, the hitherto largest environmental project of the Austrian Capital, worth €250 million euros. The sewage plant, currently one of the largest energy consumers of the community, will after the modernisation generate an output of 78 GWh of electric energy plus 82 GWh of thermal energy per year. Since the plant’s own use is about 60 GWh per year, the installation will produce excess energy which then can be sold. This method is estimated to save about 40.000 metric tons of CO2 per year.
Of course, such a project won’t work with a certain participation of high technology. Siemens will supply automation equipment in a value of some €24 million, including migration of the existing Simatic PCS7 process control system to the current version 8.1. The German industry group will also provide the process instrumentation and data network components.