Vast amounts of hot water from household appliances, businesses and factories gurgle down the drain every day, wasting not only water, but also another precious resource: heat energy.
However, not in the Austrian town of Amstetten, where a pilot project by a local utility company is “recycling” this energy from a place where normally few dare to tread — the sewer.
It uses the energy to heat 4,000m2 of buildings, or cool them in summer, allowing it to dispense entirely with gas and reduce its carbon footprint.
Photo: AFP
“Almost every week we get a visit from a different delegation coming to look,” Robert Simmer, the enthusiastic boss of Stadtwerke Amstetten, told reporters as he showed off the project. “There is a French delegation coming next week, and then a Spanish one... We are starting to lack the resources. We need to beef up our personnel to handle all the tours we have to do.”
Along a 42m stretch of sewer where the water temperature can reach 27oC, the firm has put in place a high-tech installation.
Water running in pipes adjacent to — but separated from — the sewer is warmed by devices called heat exchangers that “suck” the warmth out.
This warmed water is then pumped to a nearby headquarters, where a highly efficient heat pump fires the central heating system.
“The water that is pumped over here is clean, there is no fecal matter. The equipment inside [the sewer] is also self-cleaning,” Simmer said.
Even though extra electricity for the heat pump — devices also used in refrigerators and freezers — costs the firm 6,500 euros (US$8,850) per year, the savings are substantial compared with what it used to spend on gas.
“We have invested 240,000 euros,” Simmer said. “This should be recouped within around 11 years. With any other renewable energy source like solar power, it wouldn’t be any sooner.”
Amstetten’s utility firm is lucky because a nearby paper factory pumps hot water into the sewer, making it warmer than normal.
In addition, the firm uses under-floor heating, which is more efficient than radiators on the walls.
Even in places without such advantages, the potential for the technology is high, said Florian Kretschmer, who is from Vienna’s University of Natural Resources and Life Sciences.
“The advantage from this technology is that you have a very regional resource, and wastewater is always present,” he said.
Other similar projects exist in Germany, and Austria’s Alpine neighbor Switzerland is particularly advanced, with more than 200, and the potential is considerable, Kretschmer said.
A study done by the university and others estimates that between 3 and 5 percent of buildings in Austria could be heated using this technology, with larger buildings such as schools or office blocks particularly well-suited.
This does not sound a lot, Kretschmer said, but combined with other “clean” technologies like solar and wind power, it can play a role in weaning Europe off fossil fuels.
“Of course this alone is not going to solve the world’s energy problems,” Kretschmer said. “But what we need in the future is a good mix [of ways to produce power], and energy from wastewater can play a part in this.”
One potential downside is the temperature of the sewer water might fall to such an extent that it would affect sewage treatment plants. However, this would only happen if the technology was used on a very large scale.
“Sewage treatment plants are highly dependent on temperature. If the water going into the treatment plants is cooled, then this has a negative effect on the performance of the treatment plant,” Kretschmer said.
As a result, the university is investigating the possibility of extracting heat from cleaned water downstream of treatment plans.
“In fact that would even be positive, because it will cool the water a bit before it flows into rivers,” Kretschmer said.
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