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Clean wastewater. Clean energy.

More than 6,000 litres of wastewater - per second! - are processed at Vienna's Main Wastewater Treatment Plant. 20 hours later, the purified wastewater is discharged into the Danube, without impacting the river's water quality. To achieve such an excellent purification result, the Main Wastewater Treatment Plant devours roughly one percent of Vienna's overall electricity consumption. The lion's share of this energy - two thirds of the plant's overall consumption - is used by microorganisms to eliminate pollutants from the wastewater.

Energy has been a key focus of ebswien for many years. On the plant premises, solar energy, water and wind power are utilised and numerous measures to increase the plant's energy efficiency are implemented. Thanks to the subprojects carried out under SternE – "Strom aus erneuerbarer Energie" (electricity from renewable energy), electricity consumption has already been reduced by 11% or replaced by renewable energies. ebswien hauptkläranlage was one of the first Austrian companies that successfully passed the ISO 50001 certification audit and obtained a certificate for its energy management system in 2012. The project "E_OS Energy Optimisation through Sludge Treatment" takes us a giant leap forward: by using sewer gas as a renewable fuel, the entire energy needed for wastewater treatment will be produced on site as of 2020.

Space for climate protection

The tanks of the preliminary sedimentation and the first biological treatment stages have been continuously in operation since 1980 and have now reached the end of their service life. Necessary reinvestments in this section of the Main Wastewater Treatment Plant shall add energy efficiency, minimise the risk of downtime and reduce maintenance costs. The volume of the tanks located on the building site will be enlarged by 50 percent during the E_OS project. Since the aeration and intermediate sedimentation tanks will be much higher in the future, their base area will be significantly smaller. This frees up space for the new sludge treatment system - and thereby helps to protect the climate.

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E_OS Sewer gas helps to protect the climate


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Sludge digesters were previously mainly used for stabilising and reducing sewage sludge, a “by-product” of wastewater treatment. The energy recovered during this process was more or less a "pleasant side effect". The initial focus of the E_OS project therefore was to maximise energy recovery. In collaboration with the Institute for Water Quality, Resource and Water Management at the Vienna University of Technology, ebswien hauptkläranlage developed an innovative process. Before the sludge enters the digester, it needs to be dewatered. The "thicker" the sludge, the more energy can be recovered. This rationale is based on the understanding that in order to be digested, the sludge as well as the water it contains need to be heated. If the water content is low, this will save energy. On the other hand, the sludge must not become too "thick" so it can still be pumped through the system. Comprehensive test series in the E_OS pilot plant have impressively confirmed our assumptions that ebswien is able to operate the new digesters using sludge with the double solids content than usual. This shows that smart ideas definitely help to produce more energy!

Thanks to the E_OS project, ebswien progresses from a major energy consumer to an energy-positive wastewater treatment plant. Over a one-year period, the plant is capable of producing more energy (especially electricity) than is needed for wastewater treatment. This significantly improves Vienna's climate footprint: CO2 equivalent emission goes down by approximately 40,000 tonnes annually, an amount produced when circling the globe by car 6,700 times!

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E_OS News

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E_OS images

  • © Christian Houdek


    Umfangreiche Versuchsreihen lieferten wertvolle Erkenntnisse für die E_OS-Großanlage. © Christian Houdek

  • Fokus auf Energie:

    Die ebswien hauptkläranlage wird ab 2020 zum Öko-Kraftwerk. © LBS/Open Brand Design

  • © Christian Houdek


    Inbetriebnahme durch Umweltstadträtin Ulli Sima und ebswien-Generaldirektor Christian Gantner © Christian Houdek

  • © Eva Kelety

    Grundsteinlegung für E_OS

    durch BVin Eva-Maria Hatzl, Bürgermeister Michael Häupl, Umweltstadträtin Ulli Sima und ebswien-Chef Christian Gantner (von rechts). © Eva Kelety

  • Kajak-Erstbefahrung in der Vorklärung West. © Christian Houdek


    Zur Qualitätskontrolle sind die Becken mit Reinwasser gefüllt. © Christian Houdek

  • Belebungsbecken. © Christian Houdek


    Tellerbelüfter und Rührwerk in einem der neuen Belebungsbecken. © Christian Houdek

  • Zwischenklärbecken. © Christian Houdek


    Kettenbandräumer in den neuen Zwischenklärbecken. © Christian Houdek

  • Faulbehälter und Stiegenturm der Schlammbehandlungsanlage in der ebswien haupkläranlage vor morgenrotem Himmel. © Christian Houdek

    Happy Halloween 2018!

    Die Faulbehälter mit Stiegenturm im Morgenrot. © Christian Houdek

  • Idylle

    Nachklärbecken mit Faulbehältern. © Christian Houdek

  • Von oben

    Luftaufnahme vom August 2019. © MA 18/Christian Fürthner

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E_OS Construction works ahead

After the successful completion of the EIA process and the Europe-wide tender procedures, construction has been underway since 2015. The execution of the E_OS project is a huge logistic challenge  as the upgrade takes place while the wastewater treatment plant remains in operation. The quality of wastewater treatment in Vienna must be secured at all times. This results in a construction period of more than five years.

Overview of project milestones to date: 


2010ebswien hauptkläranlage receives mandate to prepare a feasibility study
2011ebswien submits positive feasibility study
2012E_OS project execution is unanimously approved by the Vienna City Council
2013Commissioning of pilot plant; Environmental Impact Assessment (EIA)
2014EIA process successfully completed; Europe-wide tender procedures
2015Groundbreaking ceremony and start of construction
2016Commissioning primary sedimentation west
2017Commissioning first biological cleaning stage (aeration tanks and intermediate sedimentation south)
2018Commissioning primary sedimentation east


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E_OS Videos

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E_OS Downloads

Download pictures of E_OS project300 dpi jpg

ebswien hauptkläranlage after completion of the E_OS project
© LBS/ebswien

E_OS project at ebswien hauptkläranlage - subject
© LBS/ebswien

Diagram of E_OS project at ebswien hauptkläranlage
© LBS/ebswien

E_OS pilot plant
© PID/Christian Houdek