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Renewable wastewater energy reduces dependence on fossil gas

Buildings can be heated and cooled sustainably with innovative and environmental technology

The energy transition, especially the phase-out of natural gas, is experiencing a new urgency due to the war in Ukraine and the tense relationship between West and East. Quickly available, renewable and politically less explosive energy sources are in demand – the technology for this is already available in Austria, now it must be rolled out as quickly as possible. Great potential is seen, for example, in energy from waste water, a field in which the construction and environmental technology company Rabmer has specialised. According to the company, about 14% of the thermal energy required for buildings in Austria can be produced regionally and sustainably.

Until now, when people spoke of “sustainable energy”, they primarily meant renewable energy sources. With the war in Ukraine, the political component of the term and the security of supply are now also gaining in importance. Europe’s dependence on natural gas is suddenly no longer primarily a climate-related problem, but also a supply problem due to the high market share of Russian gas – in Austria it covers about 80 percent of the gas demand. The pressure of time to find reliable alternatives rapidly is increasing.

Fourteen per cent of the heat demand of all buildings in Austria can be covered with energy from waste water

Three quarters of the energy consumption in the building sector in this country is used for heating, cooling and hot water production, and at present almost exclusively CO2-critical energy sources such as oil or gas are used for this.

“Rising energy prices and the energy shortage caused by the Ukraine war show that we urgently need new energy sources that reduce dependence and guarantee security of supply. This also applies to delivery, spare parts and, last but not least, technical know-how – in the event of a crisis, we should no longer be dependent on global supply chains. The respective system must also be able to be implemented at relatively short notice, but promise long-term, reliable output. Moreover, it must also reduce CO2 emissions,” explains Ulrike Rabmer-Koller, managing partner of the Rabmer Group.

“The list of requirements is long, but there are systems that meet all these criteria. One of them is the use of wastewater, a renewable energy source for sustainable heating and cooling of buildings, which can be used to produce about 14% of the required heating energy as well as the increasing demand for cooling energy regionally and sustainably,” says Rabmer-Koller.

Especially in urban areas there are optimal conditions for energy from wastewater for renewable heating and cooling of buildings. In Vienna, for example, wastewater has an average temperature of 16 degrees Celsius throughout the year and does not drop below eleven degrees even in winter.  This high initial temperature, which is higher than that of geothermal energy, groundwater or outside air – the usual energy sources of heat pumps – makes the use of wastewater so interesting, as the heat pump works much more efficiently and therefore requires little additional energy itself.

Energy from wastewater has been recognised as renewable energy since 2018 and recently there have also been subsidies from the government. “In 2021, energy from wastewater was subsidised by the Climate and Energy Fund for the first time. This has now also given the technology a turbo boost in Austria. In the last 12 months alone, we have examined more than 20 major projects. Many of them could be implemented in the short term,” Rabmer-Koller explains.

One of the most recent systems in the country was installed by Rabmer in 2021 at the new Wien Kanal headquarters in Inzersdorf. It covers 100 per cent of the building’s heating and cooling needs and delivers up to 450 kilowatts of heating and 500 kilowatts of cooling power in full operation. Rabmer will soon implement another lighthouse project in Vienna, namely the supply of 1.2 MW of heating and 6 MW of cooling from the canal to the new real estate complex “VIO Plaza” at the U4 station Meidling Hauptstraße. Rabmer sources all the materials needed for the construction of the plants from within the EU.

How does energy extraction from wastewater work?

To make wastewater usable as a renewable energy source, heat exchangers are first installed in the public sewer or, in the case of larger plants, as a bypass outside the sewer or also in the wastewater treatment plant drain. The wastewater in the sewer flows around the heat exchangers and heats a separate water circuit, which in turn is connected to heat pumps in the building to be supplied. These heat pumps extract the heat from the water and bring it to the desired temperature level. In winter, a building can thus be heated economically or hot water can be prepared; in summer, the process is reversed to cool the building. In addition, wastewater energy can also be fed into district or local heating networks and district or local cooling networks.

The basic requirement for this type of energy generation is a sewer with a flow rate of at least ten litres per second and dimensions from DN 400. Furthermore, the wastewater temperature must be constantly above eight degrees Celsius, the distance to the consumer must not exceed 900 metres, and the demand for heating or cooling load must be at least 50 kW power.

© Rabmer

© Rabmer