A sewage heat recovery via pump system is designed to withdraw heat from warm wastewater (12-15°C) through heat exchangers, in a closed loop to avoid mixing; preheating cold tap water, and reducing heating demands and fossil fuel consumption.
These systems can be implemented for individual buildings to pre-heat tap water for domestic hot water and space heating demands, or on a larger scale in sewer plants and channels having the potential to supply low-heat to a district heating network (DHN). The system can be implemented with heat pumps to raise its temperature for consumption.
The benefits are its simple and robust design with a long-life span, and that is an accessible low-heat source of energy, with great potential (5% of the heat demand of cities).
Other benefits are that it allows coupling with RES sources like solar thermal or other waste heat sources like industrial heat or exhaust air recovery and that it’s compatible with a low-heat DHN due to the fact that the points of source heat are the same as the points of consumption so the transmission losses are reduced.
The main drawbacks are economic for both cases, but especially for the sewer scale plant because of the expensive civil works required for a district heating network and greater installations; along with soiling by the wastewater and the big maintenance effort. Furthermore, at the building scale there might be limitations on space availability, it needs about 2 m of height; and the pumping consumption could be avoided using a passive configuration locating the tank in the lowest point of the sewage system.
With the sewage heat recovery, the hot water demand could be reduced up to 20% by raising its temperature by 10 degrees, that amount of savings can be translated to economic savings and reduction of GHG emissions, related to fossil fuel consumption.
The system promotes energy efficiency by heat recovery and circularity (Better energy efficiency)
This innovative solution can be coupled with low district heating, due to the fact that the source of heat is located really close to the points of consumption, reducing the heat transmission losses.
If combined with other solutions (like water purification technologies or water treatment, with nature-based or other), it could also lead to better water management (as the clean water post-treatment can be used for gardens or toilets in the same building).
Climate and geography
Sewage heat is an accessible low-heat source of energy that offers great thermal stability throughout the whole year within a range of (8-15˚C).
Urban form and layout:
To make heat recovery feasible it must be installed in places with high density of population, according to the IEE project ‘Stratego’ for big cities with more than 10,000 inhabitants and coupled with large heat pumps it could cover about a 5% of the heat demand.
This solution also requires specific infrastructure like existent District Heating and Cooling Network (DHCN), which has to be close to the points of consumption, within a radius of 300 metres, but it is preferable around 150m. (H.Erhorn, 2018)
Technical aspects/ infrastructure:
Wastewater can be used to produce both heating and cooling as base thermal load, needing back-up systems to cover peak loads.
This system requires two different and separated loops and great maintenance in order to separate solid substances on the sewage flow.
Existent wastewater heat pumps worldwide rate from 10 kW to 20 MW (LOWUP, 2020).
Policy and regulatory/legal framework:
Waste heat from sewage water has been defined as “ambient energy” in The new Renewable Energy Directive, and thus as part of the renewable sources.
In general, there are no regulatory restrictions for the supply of waste heat into DH network, but since sewage water channels and wastewater treatment plants are public infrastructures, there are specific regulations regarding its performance and safety in order to ensure that their operation will not be affected (Euroheat & Power, 2020).
Economic and social context:
There is little awareness regarding waste heat sources, especially from unconventional ones. It would be necessary to increase the public awareness about waste heat recovery benefits in order to increase its presence onto national and municipal renovation plans (Euroheat & Power, 2020).
Technical requirements:
It is a low-heat source, it requires high density of population and to be closer to the points of consumption, which reduces its application for large-scale plants, since wastewater treatment plants and industries are usually located far from the points of heat consumption.
It can produce both heating and cooling but needs to be coupled with backup systems to cover the loads like Heat Pumps.
Sewage heat recovery is a technical solution that already exists in the market but shows low-replication potential, mainly due to lack of experience and little awareness (consumer, manpower and municipalities). There are only around 120 systems all over Europe and 500 in the world, being more common in countries where district heating is well-developed, which makes sense since for large-scale applications the system requires existent DHCN infrastructure (Euroheat & Power, 2020).
Costs:
In general, this solution has high costs of operation due to maintenance and cleansing requirements that depends on the scale of the system and the wastewater quality.
For small-scale installations space requirements and pumping costs have to be considered.
Economic and social context:
There is little awareness of waste heat and can be difficult to sell as a sustainable product to end-users and customers, and even more if the waste heat is related to a fossil fuel driven activity.
The interest and know-how of a ‘waste heat owner’ for supplying waste heat to DH networks can be rather low since it is not part of their core business. The benefits from selling waste heat usually don’t match the investment and operation costs. Furthermore, waste heat owner could also need to change their processes due to the installation of heat exchangers, which could negatively affect their main activity. For example, in pharmaceuticals, industrial processes and waste heat recovery systems are decoupled, in order to not affect manufacturing (Euroheat & Power, 2020).
Regulation:
Waste heat potential is less straightforward to identify and plays a minor role in national decarbonisation strategies.
In addition, waste heat from sewage water channels and wastewater treatment plants have specific regulations that might result in an unbalanced treatment of the different waste heat sources when it comes to implementation.
Waste heat owners and DHC operators usually do not have contact with each other which limits their dialogue and there is a lack of standardised contracts which increases the project’s complexity (Euroheat & Power, 2020).
The main drawback is the maintenance, as if not done it can cause odours.
Sewage heat is a low-heat source that still requires of backup systems to increase or reduce its temperature level to cover thermal heating and/or cooling loads, which results in additional costs.
Waste heat recovery is often done on a case-by-case basis, there is no standard solution due to very individual and site-specific boundary conditions, as well as the limited number of real-life case studies, especially for unconventional waste heat sources. This results in high efforts to plan, design and operate systems. Consequently, the engineering and instrumentation efforts and connected costs are relatively high compared to other heat sources (Euroheat & Power, 2020).
For water fauna, the cooling of the downstream of wastewater treatments plant is even desirable. In the same aspect the use of this source for cooling purposes could be limited because the increase of the downstream temperature above levels defined in water protection regulation (Schmid F. , 2008).
LINKs to external catalogues
REFERENCES
- https://www.ecowec.com/
- http://eu-gugle.eu/de/aachen-flushes-heat-out-of-the-sewage-canal/
- https://www.kasag.com/en/renewable-energies-systems-plants-heat-exchanger-heat-from-wastewater/
- https://www.uhrig-bau.eu/en/division/heat-from-wastewater/technology/
- https://www.sciencedirect.com/science/article/pii/S1876610218305071/pdf
Emissions:
In Aachen district the new system with sewage heat recovery coupled with brine-to-water heat pumps saves about 264 tonnes of CO2 emissions each year.
The sewage heat recovery in ReUseHeat demo in Nice has expected savings of 4.000 tonnes of CO2 per year, for an installed capacity of 19 MW for heating and 15 MW for cooling.
Energy savings:
There is a demand reduction since less energy is required to cover thermal loads. To reduce even more the final consumption, it requires efficient backup systems such as booster HPs, which could use green electricity as PV on-site production and constitute a zero-emission system.
DNSH:
Climate change mitigation
This activity leads to a significant greenhouse gas emission reduction on a lifecycle basis
Sustainable use and protection of water and marine resources
This activity could be beneficial for bodies of water, including surface water and groundwater, or to the good environmental status of marine waters.
Circular economy
This activity promotes the circularity of the thermal energy use.
Pollution prevention and control
This activity leads to a reduction of electricity consumption, and consequently to a reduction of emissions.
The protection and restoration of biodiversity and ecosystems
This activity can help in maintaining optimal thermal conditions for water habitats and species.
- Governance EU Climate Neutrality Framework: https://netzerocities.app/resource-1728 for demand and GHG reduction, thanks to the use of waste heat sources.
- City water resilience assessment: https://netzerocities.app/resource-1738 referred to the treatment of waste water and the final quality and temperature level of the outlet flow previous to return to the natural environment.
- Integrated climate plans for cities: i.e.: SECAPs: https://netzerocities.app/resource-1698 large-scale systems uses public owned infrastructures and could require big civil works at city level.
- Loans for Energy Efficiency: EE: https://netzerocities.app/resource-1648
- Blended finance for Energy Efficiency: EE: https://netzerocities.app/resource-1658
- Platform for Enhancing Multi Stakeholder Dialogue to Implement NBS across EU: https://netzerocities.app/resource-1628 waste heat owners and DHC operators do not usually have contact with each other and could result into communication problems.
EU-GUGLE pilot buildings in the Aachen (DE) North district:
The system is composed of two 220kW brine-to-water heat pumps to boost the temperature of the sewage canal, whose values throughout the year are contained between 12-15°C, reaching sometimes 20°C.
In Aachen, the total cost of the investment amounts to €780,000 but will enable the city to save 264 tonnes of CO2 emissions each year.
Neckarpark Stuttgart (DE) urban district:
The DHCN grid has been installed to supply 450 residential NZEBs. The network includes heat recovery from a wastewater with an installed capacity of 2.1 MW, about 838 kW/K of heat extraction capacity from a flow which does not drop below 12 °C even during cold periods.
This area is composed of NZEBs so in this case the low-heat source proves to be more efficient than traditional district heating involving high supply temperatures.
ReUseHeat demosite of “Grand Arenas district” in Nice (FR):
This European project implemented heat recovery from the sewage system with temperatures between 25-30°C in summer and 13-8°C in winter; to a low temperature DHN. With an installed capacity of 19 MW for heating and 15 MW for cooling, the expected annual thermal energy production is 17 GWh/year and 22 GWh/year respectively.
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