Heat recovery from the sewer - alternative heat source for cities and neighbourhoods

Over the next few years, cities and local authorities in Germany will be obliged to draw up a municipal heating plan and develop climate-friendly, local heat sources in order to integrate them into the heating infrastructure.
In the search for productive sources, wastewater is increasingly becoming the focus of attention. Especially in densely populated cities and urban centres. Wastewater has a temperature of between 10 and 20 degrees Celsius in the sewer and is available in abundance throughout the day when heat (or cold) is needed. Around ten billion cubic metres of wastewater flow through the German sewer system every year.

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Definition of wastewater heat recovery

Wastewater is a good source of thermal energy that can be used for heating and cooling in modern heating networks - not just in the sewage treatment plant (as described in a previous blog post), but directly in the sewer.
Wastewater heat recovery refers to the process of extracting thermal energy from wastewater from households, industrial plants and other sources using heat exchangers to capture the thermal energy of the wastewater and channel it into heating or cooling systems. The recovered heat can be used to heat and cool buildings and to heat domestic hot water.

Man wearing safety waistcoat in the sewer, little water

How wastewater heat recovery works

Standardised stainless steel heat exchangers can be installed along the sewer floor both when constructing new sewers and when renovating existing ones. The warm wastewater flows over the heat exchanger, through which a cool fluid flows. This warm wastewater transfers energy to the cooler fluid and heats it up. It is then compressed and transported as heated water to the heat pump, where it is utilised. If the building is also to be cooled, a reversible heat pump is used: the cycle is then simply reversed in summer and the building is cooled via the wastewater flow. Here, the wastewater is used to remove excess heat that has been extracted from the building.
An alternative method to conventional sewer heat exchangers are concrete elements with built-in heat exchangers, which are mainly used in the construction of new sewers.
By extracting the heat, the wastewater in the sewer, which has an average temperature of 15 °C, is cooled down. However, the temperature of the wastewater rises again after a few metres because new inflows from the houses are constantly being added. The wastewater must not be cooled below 5 °C - most local authorities specify this value as the lower limit. At the inlet to the sewage treatment plant, the water must be at least 12°C.

Efficiency and costs compared with fossil heat

Most waste water heating systems are operated with electric heat pumps. Because wastewater is warmer than other heat sources such as air, groundwater or geothermal energy, these systems work very efficiently. The amount of useful energy generated (space heating, hot water) is significantly higher than the consumption of primary energy sources (coal, natural gas) to generate the required electricity. The efficiency of wastewater heat recovery varies depending on the type of system and the local conditions - but the annual coefficient of performance is usually 4 or higher.

In terms of costs, investments in wastewater heat recovery systems are generally higher than for conventional heating systems. Nevertheless, the long-term operating costs can be offset due to the stable and free availability of wastewater as an energy source. This makes wastewater heat recovery an economically attractive alternative in the long term. According to the European market leader Uhrig, "the heat generation costs for energy from wastewater at suitable locations are around 7 cents per kWh of heat output. This full cost calculation already includes the acquisition costs (heat exchanger, development, heat pump) and the heat pump electricity." (Source: www.uhrig-bau.eu/geschaeftsfeld/energie-aus-abwasser/wirtschaftlichkeit/).

Potential of waste heat from the sewer

According to the German Heat Pump Association, the energy contained in wastewater can cover 15 per cent of the heat demand in the building sector in Germany. The residual heat or residual energy from the wastewater is available free of charge. Utilising it reduces the consumption of other energy sources. The use of wastewater heat can contribute to decarbonisation and the energy transition in the heating market in the future.
The conditions are particularly favourable where there are buildings or neighbourhoods with a high heat demand in the vicinity of large sewers: Administrative buildings, housing estates, commercial and industrial buildings, schools, care homes and sports facilities. Such locations can be found in most municipalities with more than 10,000 inhabitants and in the catchment area of regional sewage treatment plants.
The prerequisite for economical energy utilisation from wastewater is a heating requirement of at least 100 kW and the proximity of the property to a large sewer or sewage treatment plant.

Requirements for efficient heat utilisation from wastewater

For the buildings:

Heating output with a power requirement of at least 100 kW (= 20 residential units+)
Proximity to the sewer: The closer a building is to the sewer, the more cost-effective it is to generate heat.
Building density: The higher the building density of an area, the more economical it is to operate a local heating network with wastewater heat.
Low temperature difference between source and sink - especially in new buildings (underfloor heating, component activation).
Air-conditioned cooling option: In summer, sewer wastewater can also be used for cooling. In this case, the heat pump is operated as a chiller. This allows the investment to be better utilised.

In the sewer:

Temperature of the wastewater above 10 °C even in winter
Size, location: Sewer cross-section over 1 metre and shallow installation depth with good access options (sewers from DN400 can be tapped)
Save costs through simple connection to the property

Examples of applications in Germany and Europe

Several successful wastewater heat recovery projects are already in operation in Germany and other European countries.

Haus der Statistik neighbourhood on Alexanderplatz in Berlin: renovation of a total area of 46,000 m2 and 65,000 m2 of new buildings, including flats, offices and spaces for educational and cultural activities. From 2029, the neighbourhood will be heated and air-conditioned using heat exchangers in the sewage system, several heat pump centres and a local heating network. The electrical energy required will be generated by photovoltaic systems. The conversion enables annual savings of around 722 tonnes of CO2 compared to an exclusive supply of natural gas (project by Berliner Standtwerke)
Hamburg, Germany: The city of Hamburg relies on wastewater heat recovery to generate heat for residential areas. Wastewater from households is used to heat homes and thus contribute to reducing CO2 emissions.
Vienna, Austria: Vienna has implemented innovative wastewater heat recovery systems that provide heat for public buildings and housing estates. These plants help to optimise the city's energy consumption and reduce its CO2 footprint. The large-scale heat pump currently under construction at the Ebswien wastewater treatment plant will supply up to 112,000 Viennese households with district heating from 2027.
Stockholm, Sweden: In Stockholm, modern wastewater heat recovery systems are used to supply heat to industrial areas. These applications help to reduce dependence on fossil fuels and support the city's sustainability goals.

The capacity of such plants can range from a few megawatts (MW) to several dozen gigawatts (GW), depending on the size of the plant and the amount of wastewater processed. Often the capacities are also tailored to the requirements of the buildings or industrial areas supplied.

Summary and assessment of the situation

Wastewater heat recovery offers a promising alternative to conventional heat generation in cities and urban centres. Not only does it enable the efficient use of wastewater as a renewable energy source, but it also helps to reduce the environmental impact by minimising the use of fossil fuels.
Despite the higher initial investment costs, successful implementations in various European cities show that the long-term benefits of wastewater heat recovery can outweigh the costs. With further technological developments and increased acceptance of this methodology, cities and urban centres could increasingly rely on this environmentally friendly heat recovery method in the future, paving their way to a more sustainable energy supply.

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