A huge sewer network slumbers beneath Berlin, into which tons of warm waste water from bathtubs, showers, kitchens and washing machines flows – and then heats up the surrounding waters. In some places, however, this wastewater can also be used much more sensibly, namely as heating energy: “Three to five percent of the amount of heat in Berlin that is required for heating can be covered with it,” explains Hakan Kurc from Berliner Wasserbetriebe (BWB). in the “climate laboratory” of ntv – theoretically all households in Paderborn. However, the installation of the required heat exchangers in the sewage system is anything but cheap, and the service life of several decades is all the longer. “If these systems have at least 100 kW, you save 80 percent of your heating energy,” explains the project manager for waste water heat. Cities from abroad are now knocking on his door.
ntv.de: Berliner Wasserbetriebe generate heat from waste water. What do I have to do if I want to use this?
Hakan Kurc: It is of course important that the residents of this city use enough water when cooking, showering or wherever it is heated. Also with the washing machine, of course, if you select a warm program. After use, this heated water enters our network as waste water via the sewage system.
So, as a private person, is it of no use if I collect the warm water from the washing machine?
You can of course reuse your own water like you used to do in the past. Unfortunately, the quantity is not sufficient for our procedure. We once carried out a pilot project in a larger high-rise building with more than 70 residential units: A heat exchanger was installed at the point where the waste water flows out of the building and into the sewage system. We found that the waste water from 70 households is just enough to heat one household.
With heat for heating?
Yes, but that is a mixed temperature. Because if you shower at 40 degrees and or do the dishes at 38 or 39 degrees, the warm water then naturally mixes with colder water. The mixed temperature for consumers is usually between 20 and 30 degrees, so it is not so high that it makes the radiators bang. In the canal, the mixed temperature is around 16 degrees in winter and 21 degrees in summer. To increase the temperature, you need heat pumps. Then you can go into the radiator or surface heating with a temperature of 40, 50 or 60 degrees.
Which objects are ideal for the process? bigger? So far, it has mainly been used in Berlin by hardware stores, furniture stores, a swimming pool and residential buildings.
Yes and no. You need large amounts of waste water. But this must not only occur in the short term, but must be available in large quantities throughout the day. That doesn’t happen often in residential buildings, because the heated water comes from 70 parties at different times. If the wastewater from 10,000 to 15,000 households accumulates in our network, a larger wastewater flow is created throughout the day, which can be used efficiently with a heat exchanger.
The user of this heat can of course be a residential building, but also a commercial building, a hotel, a shopping center or an office building. At the end of the day, what matters is: Can he use that energy at a lower temperature level? Most houses in Berlin are unrenovated old buildings. They need temperatures of 80 or 90 degrees in their radiators. That doesn’t work there.
So you prefer a lot of medium-warm water to a little hot water?
Exactly.
And if the waste water from 10,000 to 15,000 households collects in your pipes – how much of it can be converted into heat?
This is different because the canals are different sizes. Depending on the location and size, different amounts of waste water collect in the individual sections. The channels are a network, like the veins in our body.
The sewers are the “pipes” of the sewer system?
Right. Sewers are pipes or masonry structures that sewage enters and flows freely down a slope to a low point. Since Berlin has a very flat topography and our sewage treatment plant is outside the city because of the odor nuisance, we have to transport the water a long way through the city. For this we need pumping stations that are distributed at the lowest points in the city. There are around 10,000 to 15,000 households per catchment area, sometimes even more. When the collection tank is full, the pump starts and conveys the water to the sewage treatment plant via sewage pressure pipes. Our ancestors cleverly planned and installed this. We maintain the network, expand and partially renew it.
And where in this network are the heat exchangers that generate heating energy from the wastewater?
We calculate how much wastewater is produced at which point in the network. A simulation of the theoretical flow. If the quantity is sufficient, we measure to be sure. If the amount has been confirmed or is even larger, you can install a heat exchanger at this point. The guide value is ten liters per second. The duct must also be large enough and have a cross-section of at least 600 millimeters.
From ten liters per second is it worth it?
Yes. Then the waste water has at least 100 kW. You can imagine the following: In a new building, you need 7 to 8 kW to heat a 70 to 90 square meter apartment. If they are well insulated, 6 kW is also sufficient. Then you could supply about 17 households.
The waste water from how many households is needed to get at least 100 kW out?
At ten liters per second, you need the waste water of 2,500 households. So you can already see a discrepancy: you need a lot of wastewater to ultimately supply a small number of households.
But doesn’t that mean that you should use as much water as possible at home?
No of course not. (laughs) It would actually be good for our purpose because we need a lot of warm water. On the other hand, of course, you first have to generate the heat at home, so you use up energy to do it. That wouldn’t be great.
We also currently have a situation where we need to be more careful with the water. That wasn’t the case for years. In the last 15 or almost 20 years we have never had a water shortage, on the contrary: we would have liked people to use more water to flush the sewers. That would have had a better cleaning effect.
But isn’t it a disadvantage if suddenly a lot of people buy an energy-saving shower head and reduce their water consumption in order to save energy?
This is right pocket, left pocket. We have to save energy overall wherever we can. We intervene with the waste water, which is normally not used at all, but ends up in the surrounding waters and heats them up. This is neither ecological nor desirable.
Overall, how great is the potential of this process? Is it also used in other cities?
As the largest city in Germany, Berlin also has the largest sewer network. There is no other city in Germany that could use wastewater heat to this extent. This also works in a few places in Frankfurt, Hamburg, Munich or Stuttgart, but there the necessary amount of waste water only collects in a few main pipes. This can be used to supply a manageable number of objects with energy.
Does this also apply to other large metropolises worldwide? There are still much larger cities in the world. Did Berlin copy other cities?
In fact, other cities are copying it from us. We regularly get inquiries from the USA, Canada and other European countries. Berlin is already relatively far ahead when it comes to the systematic use of waste water heat. How much energy can this ultimately save? The sheer amount of waste water in Berlin would be large enough to calculate 10 to 15 percent of the heating energy from the waste water. But that’s the theory. In practice, we have the limitations mentioned: you need sewers or pipes that continuously carry waste water in sufficient quantities. They also have to be big enough so that heat exchangers can be retrofitted.
How does it work?
The heat exchanger is either custom-made in the brine of the canal or pipe or it is wrapped around the pressure pipe of the pumping station. So you encase these pipes with another steel pipe. This creates a gap in which water as a medium absorbs energy from the wastewater. The water is then brought to the heat pumps in the building.
But it must be said: on a large scale, won’t this procedure help us to solve the energy crisis?
We analyzed and played through where the spots are where we can put this to good use. As a water company, we come to the conclusion that three to five percent of the amount of heat in Berlin that is required for heating can be covered with it. If you convert that to the amount of energy, that would be about 300 megawatts. That would correspond to a smaller combined heat and power plant. This could supply 60,000 to 70,000 households. Theoretically, this can cover Paderborn’s energy consumption.
Can industrial wastewater also be used for this? Tesla uses a lot of water in Grünheide.
Warm industrial wastewater also enters the network. When it comes to process water like Tesla, the question here is: what exactly is being done with it? how dirty is it Can this be reused? But waste heat is actually used in various branches of industry via on-site recovery. This is particularly useful in the food industry because the temperatures are sometimes very high. There, steams at 200 or 300 degrees are used. Then the amount of energy that can be recovered is relatively high. The same applies to cement or aluminum plants.
Is this already being used on a large scale? Can this be linked to your offer?
This is already being recuperated in many branches of industry and brought back into the system. However, this is called waste heat there and not waste water, because the heat occurs in various forms in industrial processes.
And how much does such a heat exchanger cost? The installation sounded like an expensive undertaking.
That’s correct. For apartment owners or house owners, it is relatively high sums if you want to use waste water heat. Let’s take the conventional systems: for an apartment, a gas boiler with all the trimmings costs 10,000 to 15,000 euros. For a homeowner, the value of a heating system is between 25,000 and 60,000 euros – depending on how big the house is.
Now comes your counter offer.
Installing the heat exchangers in a duct or integrating them into a pressure line costs at least 200,000 euros. Now you can ask: Who is interested in such expensive energy systems? But the quantity is crucial. If these systems have at least 100 kW, you save 80 percent of your heating energy. And they are designed for a very, very, very, very long time of use. Our contracts with heating customers have a term of at least 15 years and can be extended at any time. In theory, the systems have a service life of up to 50 or 60 years. We also have penstocks that are more than 100 years old.
Still very expensive.
For individuals yes. But actually I know a person who installed a geothermal system in their house for 300,000 euros. If you have the wherewithal…
At least it was put to good use.
But usually these are larger properties, housing associations or blocks of houses. If the system is used very efficiently, i.e. the amount of energy that is needed is very high, the costs can be amortized in seven to eight years. Then they get the money back. Like in a shopping center, for example, they use it all year round – also for cooling. With less heavy use, it takes 12 or 15 years to amortize, but usually still within the contractually agreed time.
In the end, however, the customer actually benefits from the fact that everyone in their immediate vicinity produces sufficient wastewater together. Consumers get none of it.
It’s a bit like the first-come-first-served principle: whoever looks for this technology first has the advantage and others lose out. But to make things fairer, we charge a fee for using the waste water heat. Those who do not benefit directly from this will at least be relieved with cheaper tariffs. That is the logic of the treaties.
Clara Pfeffer and Christian Herrmann spoke to Hakan Kurc. The conversation has been shortened and smoothed for better understanding.
