Gray water, an underused fuel

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Stéphanie Obadia

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4290 France - Last modified by the author on 04/01/2023 - 13:00
Gray water, an underused fuel

Recover calories from gray wastewater to produce domestic hot water. This is now a proven reality. Example with Quai d'Ivry, a student residence with 92 rooms delivered in 2020. A self-evident solution to decarbonize.

At the edge of the banks of the Seine, in the district in full construction of the 13th arrondissement of Paris, is the student residence Paris Quai d'Ivry (Linkcity, RRP and RIVP) designed by the architectural firm Ameller Dubois. Its originality: supplying all of the students' sanitary hot water needs via a waste heat recovery system from greywater. The principle is simple since it combines a specific network for collecting gray water from the building with high-efficiency “water/water” heat pumps for the production of domestic hot water at a temperature above 55°C. "A substantial saving knowing that the share of domestic hot water already represents 50% of the overall consumption of the building and will become preponderant in the near future with the improvements of the thermal envelopes of the building", argues Jérôme Gonçalves, sales manager at Solaronics ECS .


In practice, water from showers, baths, sinks, washing machines and dishwashers is collected and filtered before being stored (between 25°C and 35°C throughout the year) in a tank of 8,900 liters which supply the heat pumps.
“Grey water is the fuel for the heat pump, the primary energy,” continues Jérôme Gonçalves. Recovering waste heat from gray water by heat pumps raises the temperature of city water to 58°C and protects against any risk of legionellosis. The city water thus raised in temperature is stored in insulated hot water tanks (seven on this site with a capacity of 1,000 liters each) before supplying the student accommodation. The average consumption of this site is around 10 to 13 m3 per day.

The heat pump used has a theoretical COP of 6.4 and an average annualized real COP of 5 in operation – Ed. For 1 kWh of electricity consumed, a heat pump produces 6.4 thermal kWh. “The renewable energy rate of the domestic hot water produced is around 80%. The remaining 20% is used to power the system's pumps, compressors and auxiliaries. In the end, we divide the energy consumption of the DHW station by 5” . It is also equipped with a specific zeotropic mixture of two refrigerants in line with the regulatory threshold of 2030 (the global warming potential (WGP) is approximately 145 kg CO2 / kg fluid).

Other installations to follow

The system, called PAC Facteur 7, has been operating on this site for two years with a maintenance intervention every three months consisting essentially of cleaning. “All our sites are continuously monitored from the north of France. And benefit from predictive maintenance” . It is advisable to position the technical room at the low point of the building to collect gray water by gravity and thus avoid its lifting towards the PAC Factor 7 system. Solaronics ECS has about twenty references such as ABC La Montagne in Grenoble ,

The PAC Facteur 7 system is mainly intended for collective housing, hotels and public facilities, but it could just as well be considered for industrial uses such as collective laundries, canneries, etc. The investment is still significant: around 2,500 euros per dwelling for a building of 100 dwellings with a return on investment over ten years. However, the high renewable energy rate of the system opens up the possibility of its financing by the Ademe Heat Fund, thus reducing the initial investment to 1,800 euros per dwelling. "It becomes truly relevant from 100 housing units" , indicates Rémy Rochard, associate general manager.

In short, a self-evident solution to decarbonize with gray water!

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