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Pilot plant delivers basic data to optimise energy and ecological efficiency of E_OS large-scale plant.

Umweltstadträtin Ulli Sima und ebswien-Generaldirektor Christian Gantner

Today, Wednesday, during the commissioning ceremony for the E_OS pilot plant, Environment City Councillor Ulli Sima said: "Thanks to the innovative project 'E_OS Energy Optimisation through Sludge Treatment', ebswien hauptkläranlage will become energy self-sufficient as of 2020. The wastewater treatment process is already very energy efficient, but to ensure this high level of purification still roughly one percent of Vienna's overall electricity consumption is needed. E_OS will allow us to produce all the energy required for wastewater treatment on site by using sewer gas as a renewable fuel." According to Sima, this project clearly demonstrates the City of Vienna’s commitment to invest in environmental and climate protection measures also in difficult economic times.

The E_OS pilot plant, which is located on the premises of ebswien hauptkläranlage, performs the same processes which are supposed to take place at the future large-scale plant, only at a scale of 1 : 600. While the digester of the pilot plant holds 130 cubic meters of sludge, the six digesters of the large-scale plant, each 35 metres high, will reach a total capacity of 75,000 cubic metres. "The pilot plant gives us a better insight into how the future sludge treatment system is supposed to work. Here the focus is on machines and technical equipment that cannot be simulated in the laboratory," says Christian Gantner, Director General of ebswien. "The results obtained during the pilot run will provide us with basic data for optimising energy and ecological efficiency at the E_OS large-scale plant.”

Innovative process engineering

ebswien can rely on the Institute for Water Quality, Resource and Waste Management of the Vienna University of Technology as a proven partner. The researchers have developed a new technology which seeks to preserve as much energy as possible in the sewage sludge already during wastewater treatment. Sludge digestion takes place at an elevated solids content. The sludge is thickened prior to the digestion process, which means water is extracted from the sludge. The "thicker" the resulting sludge, the less water must be heated up to 38 °C along with the sludge. This helps to save energy and improves the energy balance of sludge treatment enormously. In the E_OS pilot plant (and later on also in the large-scale plant), the sludge is “thickened” to a dry solids content (approx. 8%) which is considerably higher than in other wastewater treatment plants.

A high concentration of solids has yet another decisive advantage: sludge that contains less water also uses up less tank volume during sludge stabilisation. The large-scale plant will therefore only need six digesters. This helps to save construction costs as well as reduces the workload during operation and maintenance.

This is how the E_OS pilot plant works

First, the surplus sludge from the aeration tanks is mechanically thickened inside the E_OS pilot plant. In the storage tank, primary sludge from the primary sedimentation stage is added. The sludge mixture then passes through a heat exchanger where it is heated to 38 °C. Five cubic metres of heated sludge per day are fed into the digesters, where the sludge is kept in motion. Anaerobic bacteria break down the organic components of the sewage sludge under anaerobic conditions. During a minimum retention time of twenty days, which is known as the "anaerobic stabilisation" phase, energy-rich sewer gas is generated. This gas rises inside the digester and is intermediately stored in a 60 m³ gas tank, which is located directly above the digester.

Two thirds of the sewer gas is methane, which flows through a gas pipe to a co-generation plant. A gas engine transforms methane into mechanical energy, which his converted into electricity by a generator. The waste heat resulting from the process is used inside the heat exchanger to heat up the sludge mixture. The digested sludge is withdrawn from the digester and dewatered by using a worm extruder. The extracted water contains a lot of ammonia, which is largely decomposed by a biological process to assure that the reject water is safe to undergo wastewater treatment.

E_OS: Construction while treatment plant is in full operation

The project "E_OS Energy Optimisation through Sludge Treatment", which was unanimously approved by the Vienna City Council in February 2012, will be focused on the primary sedimentation and first biological treatment stages of the Main Wastewater Treatment Plant. To create space for the new project components, the existing tanks need to be removed. The new tanks will be partly twice as high as the previous ones, and their total capacity will be further increased. The landmark of the new sludge treatment system will be six digesters, each of them 35 metres in height. Also a sludge buffer tank and two gas tanks shall be added. 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 roughly six years.  

Technical parameters of the E_OS pilot plant

  • Sludge throughput
    • 5.2 m³ on a daily average
  • Sludge load
    • 0.4 t dry matter per day
  • Digester
    • Diameter and height: 6 metres each
    • Effective capacity: 130 m³ 
  • Gas tank
    • Effective capacity: 60 m³ 
    • Gas production: 200 m³ per day
  • Co-generation plant
    • Electricity output: 35 kWel
    • Thermal output: 58 kW 


EOS timetable

24 February 2012E_OS project is unanimously approved by the Vienna City Council
17 April 2013E_OS pilot plant starts operationAutumn
2013Environmental Impact Statement is submitted
2014Approval and tender procedures
Mid-2015Groundbreaking ceremony
2015 - 2020 Upgrade while plant remains in operation
End of 2020 Full Operation