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Breaking ground for Vienna's most significant ecological project: as of 2020, ebswien hauptkläranlage will cover its own energy demand entirely from renewable sources

E_OS groundbreaking ceremony © PID/Christian Houdek

Environment City Councillor Ulli Sima, Mayor Michael Häupl and District Governor Eva-Maria Hatzl (from left) attend the E_OS groundbreaking ceremony

Today, Monday, during the groundbreaking ceremony for the project "E_OS Energy Optimisation through Sludge Treatment", which was held on the premises of Vienna's Main Wastewater Treatment plant in the city district of Simmering, Mayor Michael Häupl said: “Vienna sets a new standard with this project, which will serve as a role model to many other countries around the world. The sustainable management of our existing resources is one of the biggest challenges the cities of the 21st century are facing." Smart answers are needed to cope with this challenge. “And this is what ebswien has successfully accomplished with its E_OS project!” Häupl added.

Wastewater treatment plants are among the major energy consumers of a municipality. Vienna's Main Wastewater Treatment Plant requires roughly one percent of Vienna's overall electricity consumption to clean all effluents produced in Vienna. By efficiently exploiting the energy contained in the sewage sludge, ebswien will be able to generate the entire amount of energy needed for wastewater treatment from its own sewer gas, a renewable fuel, as of 2020. Environment City Councillor Ulli Sima said: “Thanks to E_OS, Vienna's Main Wastewater Treatment Plant converts into an eco-power plant. This also significantly improves Vienna's climate footprint: as of 2020, CO2 equivalent emission will decrease by approximately 40,000 tonnes annually.” Accounting for the projected index increases, the overall cost of the E_OS project will be 250 million euros. Sima added: “Vienna remains prepared to invest in the environment - also in tough economic times!”

Innovation: Smart ideas for a higher energy output

“Sludge digesters were previously mainly used for stabilising and reducing sewage sludge, which is a ‘by-product’ of wastewater treatment," said Christian Gantner, Director General of ebswien. “The energy recovered during this process was only 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 is fed into the digester, it needs to be dewatered. The "thicker" the sludge, the more energy can be recovered. In order to be digested, the sludge as well as the water it contains need to be heated. A lower water content will therefore help to save energy. On the other hand, the sludge must not become too "thick" so it can still be pumped through the system. Gantner added: “Comprehensive tests have impressively confirmed our assumptions: we can operate the new digesters by using sludge with a double solids content than usual. This shows that smart ideas definitely help to produce more energy!"

This is how the new sludge treatment system works

The pollutants which are eliminated during the treatment process are bound in the sewage sludge; approximately two million cubic metres are produced in Vienna per year. The landmark of the new sludge treatment system are the six digesters, each 30 metres high, with a total capacity of 75,000 cubic metres. Here the "prethickened" sludge, which has been heated to a temperature of 38 °C, is processed. Bacteria break down the organic components of the sewage sludge under anaerobic conditions. During the 25-day decomposition process (also called "anaerobic stabilisation"), sewer gas is produced. Two thirds of this gas is energy-rich methane (CH4), with 20 million cubic metres produced every year. The digested sludge is extracted from the digesters and subsequently incinerated. The sewer gas leaves the gas tanks, passes through filter units and ends up in the co-generation plants, where it is combusted in gas engines. This results in mechanical energy, which is converted into electricity by means of generators, as well as heat, which may be used for heating and hot water. The co-generation plants achieve an overall efficiency of more than 80 percent.

Space for climate protection

The tanks which are used for preliminary sedimentation and for the first biological treatment stage have been continuously in operation since 1980 and have now reached the end of their service life. Necessary reinvestments in the “original” plant section also add energy efficiency to this part of the wastewater treatment process, minimise the risk of downtime and reduce maintenance costs. The volume of the tanks located on the ten-hectare building site will be enlarged in the framework of the E_OS project. Since the aeration and intermediate sedimentation tanks will be much higher in the future, their base area can be significantly reduced. This frees up space for the new sludge treatment system - and thereby helps to protect the climate.

Wastewater treatment secured during construction

After the successful completion of the EIA process and the Europe-wide tender procedures, construction activities are about to begin. The execution of the E_OS project is a huge logistic challenge as the upgrade takes place while the wastewater treatment plant remains fully 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.

E_OS project milestones

2010 ebswien hauptkläranlage receives mandate to prepare a feasibility study

2011 ebswien submits positive feasibility study

2012 E_OS project execution is unanimously approved by the Vienna City Council

2013 Environmental Impact Assessment (EIA)

2014 Positive EIA - Europe-wide tender procedure

2015 Groundbreaking ceremony and start of construction

2020 Commissioning

We'll clear it up – as of 2020 with eco-electricity from on-site production

The Main Wastewater Treatment Plant, which is located in the city district of Simmering, has been cleaning the entire wastewater of the Viennese population since 1980. More than 6,300 litres of wastewater per second are processed in this plant. Over a period of 20 hours, the wastewater passes through one mechanical and two biological treatment stages, where the 169 employees of ebswien hauptkläranlage use nature as a role model. More than 100,000 kilogrammes of pollutants are eliminated from the wastewater day after day. The purified wastewater flows through the Danube Canal into the Danube River, without impairing the quality of its water. So we really clear things up and take care that the Danube remains blue - as of 2020, exclusively with eco-electricity from on-site production.