Roskilde's wastewater supplies heat to approx. 3,550 households

For most of the year wastewater is warmer than both seawater and air. It is, therefore, an obvious heat source for a heat pump system that can produce district heating. Although there has been no technological revolution, the utilisation of wastewater in a heat pump system is a relatively new concept, and it is now starting to attract a lot of attention and gain popularity across Denmark.

Heat pump system at Bjergmarken

In January 2019, the first plans for a wastewater heat pump located at Fors' treatment plant in Bjergmarken in Roskilde were put on the drawing board. In March 2021, the plant was delivered, with NIRAS as consultant on its development, all the way from start to finish.

The utility company Fors previously received all its heat from the VEKS' transmission network - except for local peak and reserve load produced at boiler plants that provide supplementary heat production during cold periods or in the event of failure of other production units. After installation of the new heat pump system, approx. 64,000 MWh / year is now supplied from the wastewater heat pump. This corresponds to approx. 15% of Fors Varme's total district heating supply.

Fors Varme has long had challenges with high supply temperatures in several places in the district heating network. This is because the heat from VEKS' transmission network is supplied via heat exchangers in the eastern part of the district heating area, from where the heat must be distributed to the rest of the district heating distribution network. In order to maintain the required supply temperature for all their customers, in line with technical regulations, the temperature loss en route from the heat exchanger stations to customers must be compensated for. This is done by raising the supply temperature from the heat exchanger stations in the eastern part of the district heating area.

Benefit for both the environment and heating customers

There is great focus in the utilities sector on sector coupling, where different types of utility co-operate and extract gains from unused resources in their respective sectors. Sector coupling provides an overall benefit to the climate, the environment and the corporate and social economy. A heat pump system that utilises wastewater as a heat source for district heating production is a good example of this. Here, two types of utility are working together on resource utilisation, resulting in significant financial gain in the form of lower heating prices. There are also gains for the environment and climate due to lower emissions of organic substances (BOD) to Roskilde Fjord and a lower CO₂ footprint for heat production.

Sector coupling is not merely a fashionable notion. It is a real and important element in the green transition and better resource utilisation in the circular economy. A project with a high degree of sectoral coupling like this has required interdisciplinary consultancy in interplay with many professional disciplines in order to successfully achieve its goals.

NIRAS is among the leading consultancies in the field and is responsible for, and has been responsible for, consultancy on several large heat pump systems that use wastewater as a heat source.

How it works

• Two heat pump lines of 4 MW heat output, together totalling 8 MW heat output, have been installed at the treatment plant.
• Wastewater from the treatment plant's outlet line is first filtered before it ends up in a buffer tank, from where it is pumped to the plant's intermediate circuit with glycol. In the intermediate circuit, the wastewater is cooled down to a minimum of 2 ° C and discharged into Roskilde Fjord. The heat from the wastewater, which is transferred to the intermediate circuit, is used in the heat pump to raise the supply temperature of the district heating water in Fors' district heating network.
• The intermediate circuit gives a slightly lower COP *, but protects the heat pump's evaporator against dirt accumulation and provides extra security against ammonia leakage to the wastewater.
• Cleaning In Place (CIP) systems, to the extent they are needed, clean heat exchanger surfaces of dirt to ensure good heat transfer and thus maintain a high COP value.
• The post treatment of the wastewater has the side benefit that the concentration of organic matter (BOD), nitrogen (total-N) and phosphorus (total-P) in the wastewater is reduced to the benefit of the environment.

* Coefficient Of Performance (COP) indicates the ratio between the heat produced by the heat pump and the amount of power used in the heat pump.