Vienna's Main Wastewater Treatment Plant (MWTP) is one of the biggest (capacity: 4 million PE) and most technically advanced sewage treatment facilities in Europe. Its mechanical/biological processes are so efficient and render such excellent purification results that the water of the Danube River actually leaves Vienna with the same quality it has when it enters the city.
This, of course, requires an enormous input of energy, most of which goes into air diffusers, wastewater pumps, agitators, etc. Total energy consumption for the treatment of Viennese wastewater is about 175,000 kWh a day. In terms of specific energy consumption, the MWTP is already ranking at the top of the list. Large treatment plants usually have a per-capita energy consumption in the range of 20 to 25 kWh a year. The MWTP, with merely 17.8 kWh per capita and year, is thus performing really well.
Project for renewable energy recovery from purified wastewater
In addition to their prime objective to maximise plant efficiency and ensure a persistent operation while keeping energy consumption low, the plant operators are also challenged by the EMAS certification process (also read aqua press 3/2008, p. 6). MWTP operator Entsorgungsbetriebe Simmering GmbH (EbS) has therefore launched a project for renewable energy recovery from purified Viennese wastewater.
The aim is to find ways to further reduce specific energy consumption and thereby promote ecological sustainability. The concept makes use of the existing gradient between the plant outlet and the receiving water – the Danube Canal –, along which some 560,000 m3 (= approx. 6.5 m3/s) of purified effluents are discharged from the treatment plant per day.
“Based on the current amount of effluents and a normal level difference of five metres between headwater and tailwater, the use of a turbine typically designed for small hydropower plants with an appropriate efficiency presents itself as a viable and interesting option. This is why after testing several variants in 2008, the decision was made to launch such a project,” says EbS project manager Gerald Wandl.
Double-regulated Kaplan turbine by Kössler chosen
The turbine chosen for the project is a double-regulated Kaplan turbine with a vertical axis produced by Austrian turbine manufacturer Kössler (taken over by Voith Siemens Hydro Power Generation in 2007). Kössler also designed the pertaining hydraulic unit, gear unit and generator of the turbine installed in the plant's outflow pumping station.
The outflow pumping station also houses three pumps which are in operation only a few days per year, namely when the Danube Canal carries high flows and the purified effluents need to be pumped into the receiving water. With an effluent volume of 7.5 m3/s, the turbine achieves a maximum efficiency of 384 kW.
After the commissioning process in June this year, 1,500,000 kWh of electricity will be recovered and fed into the plant's own power grid. This corresponds to 2.6 percent of the overall demand, or to the amount of electricity consumed by 500 Viennese households.
Karl Wieder, project manager at Kössler, says proudly: “With the installation of a maintenance- free small hydropower plant in the outlet channel of a wastewater treatment plant, EbS and Kössler are diving into uncharted waters; so far such projects have only been tested in Greece.”
According to the technical expert, the main challenge for the MWTP lies in the daily and yearly flow fluctuations. The water flow goes down to 2.5 m3/s at night, while peak volumes of around 7.5 m3/s reach the plant around mid-day and remain largely constant until around midnight. During this period, the turbine operates at maximum efficiency.
The low head made it necessary to develop a relatively large runner with a diameter of 1.45 m. In the choice of material, Kössler had to take account of the chemical composition of the purified effluents, which differ from those of a natural watercourse. Any essential components are therefore made of stainless steel. The turbine boasts an excellent efficiency of 92 percent. According to the plant operators, investments into the SHP plant will pay off after 6.5 years.
Miklos Papp, who oversees plant operations, explains that the new SHP plant is only the first step towards renewable energy utilization from the MWTP. This trend will be consistently pursued, he says, and notes that a solar energy project is already in the pipeline. The project for renewable energy recovery from purified Viennese effluents has been nominated for the City of Vienna's 2009 Environmental Award.