© Christian Houdek - MA 53

 The Vienna main wastewater treatment plant is on its way towards energy self-sufficiency. This annually saves 40,000 tons of CO2.

The ebswien main wastewater treatment plant is on the way to becoming energy self-sufficient rather than a large user of energy: with the project SternE, the use of energy by the subsidiary of the city of Vienna has already been reduced by 11%. Through the energy optimizing sludge treatment (EOS) project, ebswien will be able to generate over 100% of the energy required for wastewater treatment beginning in 2020 by using sewage gas.

One of the most energy efficient wastewater treatment facilities at this time is the main wastewater treatment plant in Vienna. In the future, it will set a new standard for sustainable environmental protection within its size classification.

Process of the energy savings

The EOS project has the goal of optimizing the use of the energy found in sewage sludge. The sludge, which is a residual product of wastewater treatment, is condensed and heated to 38°C, after which it is pumped into six 35 m high digesters with a total volume of 75,000 m³. The digesters are hermetically sealed, and sewage gas is produced during the anaerobic stabilization phase. Two thirds of this gas consists of energy-rich methane and is recognized as a renewable energy source. Thermal power stations then convert the sewage gas into electricity and heat.


The EOS project considers the following steps to be the most advisable solution technically, economically and ecologically:

  • New construction of preliminary sedimentation and biological stage
  • construction of sludge digestion with sewage gas extraction in six digesters as well as usage of gas in thermal power stations

During the planning of the project, in order to ensure the highest possible energy gain, innovative aspects were taken under consideration. This includes maximizing the electrical energy production for water treatment, feeding surplus energy into the public network, using the total heat produced in the form of district heating, as well as utilizing residual heat for sludge digestion:

  • The technology for the new first stage is specifically selected so that the energy use for ventilation is minimized, while the energy content of the sludge is maximized. This increases the gas yield and the resulting energy production considerably in contrast to conventional water treatment plants.
  • The surface ventilation in shallow tanks will be converted to a more energy-efficient pressure ventilation system in deep tanks.
  • A separate treatment center for wastewater containing a high level of nitrogen created by sludge dewatering will be, in the best-case scenario, tied into the total process. This will ensure that the nitrogen removal will continue at the highest possible quality level.
  • Sludge introduced during the digestion process contains a high level of dry matter. A minimum level of 8% is ensured with machine pre-thickening. This means that the energy required to heat the sludge in the digester to a temperature of circa 38°C, as well as the required size of the digesters is reduced.

Test facility confirms projections

At the EOS test facility, which was put into operation at the beginning of 2013 on a 1:600 scale, the projections for energy yield made during the feasibility study were verified. In addition, important knowledge for system implementation was gained. It has become clear that as of 2020 the entire energy requirement for the main water treatment plant in Vienna – while a small percentage of the total use of electricity in Vienna – can be created out of the renewable energy source of sludge. The result is an energy optimized water treatment plant, which will set a new standard for sustainable environmental protection within its size classification. It will do this while ensuring the water purification levels required by law. The output of CO2-equivalents is reduced by 40,000 tons per year through the EOS project– an important contribution to reaching Vienna’s climate protection goals.


  • 2010: presentation of feasibility study in December
  • 2012: EOS is unanimously approved by the municipal council of Vienna in February
  • 2013: EOS trial facility is put into operation in March
  • 2013: submittal of environmental impact statement in August
  • 2014: tendering process for the main works
  • 2015: start of construction in the summer
  • 2017 bis 2019: partial initial operation
  • 2020: construction completion and fully operational at year end


Mag. Karl Wögerer

ebswien hauptkläranlage Ges.m.b.H.

E-Mail: k.woegerer@ebswien.at

Website: http://www.ebswien.at/hauptklaeranlage/

This post is also available in: German