The EU Directive 2018/2001 recognized wastewater as a renewable heat source. Wastewater from domestic, industrial and commercial developments maintains considerable amounts of thermal energy after discharging into the sewer system. It is possible to recover this heat by using technologies like heat exchangers and heat pumps; and to reuse it to satisfy heating demands. This paper presents a review of the literature on wastewater heat recovery (WWHR) and its potential at different scales within the sewer system, including the component level, building level, sewer pipe network level, and wastewater treatment plant (WWTP) level. A systematic review is provided of the benefits and challenges of WWHR across each of these levels taking into consideration technical, economic and environmental aspects. This study analyzes important attributes of WWHR such as temperature and flow dynamics of the sewer system, impacts of WWHR on the environment, and legal regulations involved. Existing gaps in the WWHR field are also identified. It is concluded that WWHR has a significant potential to supply clean energy at a scale ranging from buildings to large communities and districts. Further attention to WWHR is needed from the research community, policymakers and other stakeholders to realize the full potential of this valuable renewable heat source.
Although the food service sector is a major user of water, the potential for heat recovery from commercial kitchens' drain water remains largely unexplored. For the first time, we compare the life cycle environmental burdens of producing and installing a heat recovery system with the environmental credits arising from energy savings for a restaurant case study, and for the entire UK food service sector. Life Cycle Assessment was applied to determine the impacts of heat recovery systems made from different materials and comprising a heat exchanger in the shape of a concentric double-walled pipe, pipework and fittings. The design option with the smallest environmental footprint combined a heat exchanger made out of polypropylenegraphite (PP-GR) with polyethylene pipework, exhibiting 80-99% less environmental impact compared with components made out of (35% recycled) copper. Contrasting the environmental impacts of two heat recovery set-ups with energy savings shows that a PP-GR based system pays back all seven assessed environmental impacts within two years, while payback times for the copper-based system vary depending on the replaced energy source, and can exceed the 10 year operational lifetime of the system. When looking at typical flowrates in UK food outlets, net environmental savings can be realised across all analysed impact categories above a threshold water consumption of 555 L/day, using current technology. Extrapolation to the UK food service sector indicates annual greenhouse gas emission mitigation potential of about 500 Gg CO2 equivalent.
A large part of the thermal energy in buildings is lost through the drain and ends up as warm wastewater in the sewer system. The installation of heat exchangers in the sewer system enables a rise of the source temperature of heat pumps, increasing their coefficient of performance. To investigate the potential of such a technique in the Brussels Capital Region, a test facility named MYRTES has been installed in the sewer network, the starting point of this facility being to have one heat recovery system per residence. To estimate the heat recovery rate, potentially available in the Brussels Capital Region, the data from this test facility have been used as inputs and validation for a predictive model, considering both the heat recovery and its financial and environmental implications. Simulations show a minimum heating power of the heat pump of 6.3 kW, at a hot water temperature of 45 °C. A maximum of 35% of the buildings in the Brussels Capital Region are eligible for the use of such a system. At current tariffs, the levelized cost of energy for these systems, is lower than for traditional air heat pumps, but is higher than for gas boiler systems. The total equivalent warming impact, however, is estimated to be around 49% lower than for gas boiler systems and around 13% lower than for air heat pumps. In conclusion, heating through these types of systems is more expensive than gas boiler systems, but with increased consumption the competitiveness of these systems improves.
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