Vienna’s main wastewaster treatment plant gears up for energy self-sufficiency! The energy-optimising sludge treatment (EOS) project is due for completion in late 2020. This means that in future ebswien will be able to self-generate over 100% of the energy it requires for wastewater treatment from biogas, thus saving 40,000 tonnes of CO2 emissions per year. In the new facility at ebswien the sludge, a residual product of wastewater treatment, is condensed and heated to 38°C before being pumped into six 35-metre-high anaerobic digestion tanks. Inside these hermetically sealed digesters, biogas is produced (during the anaerobic stabilisation phase). Biogas consists of two-thirds energy-rich methane, a recognised renewable energy source, which is subsequently converted into electricity and thermal power in cogeneration plants.
Innovative process engineering
To implement EOS, ebswien has completely reconstructed the preliminary sedimentation and first biological stages of the plant. The new process technology for the first stage minimises the energy required for ventilation while maximising the energy content of the sludge, so the gas yield and resulting energy production are higher than in conventional sewage treatment plants. The project also included the installation of new facilities for sludge digestion and biogas extraction and utilisation. Mechanical pre-thickening of the sludge ensures a high dry matter content of around 8%, which means that less energy is needed to heat the sludge in the digester to the required temperature of approx. 38°C. It also allows the volumetric capacity of the digestion tanks to be reduced to around 75,000m³ each. Reject water from sludge dewatering containing high levels of nitrogen is treated separately, thus ensuring that most of the nitrogen continues to be removed. ebswien thus complies with all the water purification standards required by law.
Pilot plant confirms projections
ebswien made careful preparations for EOS, building a 1:600-scale pilot plant which confirmed the energy-yield projections of the 2010 feasibility study. Vienna City Council approved the project in 2012, and construction work began in 2015 following the positive outcome of the environmental impact assessment. Partial operation has been implemented step by step, and ebswien will become fully operational at year-end 2020. Vienna’s main wastewaster treatment plant will then generate 15 gigawatt-hours more electricity and 42 gigawatt-hours more thermal energy per annum than it consumes itself, meaning that its entire energy requirement – almost one per cent of the city’s total energy consumption – will be covered by renewables. Any surplus will be fed into the public grid. In addition, the heat generated during the process will be used on site for sludge digestion or fed into Vienna’s district heating network. ebswien is thus setting new standards for sustainable wastewater treatment within its size category and making a substantial contribution to achieving the objectives of the Smart City Wien Framework Strategy.
Mag. Karl Wögerer
ebswien hauptkläranlage Ges.m.b.H.
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