A comparative Life Cycle Assessment (LCA) study of centralized and decentralized wastewater heat recovery in Stockholm, Sweden

Abstract: The cities bear a large role in the climate crisis. However, this also means that they have a big potential in the transition towards sustainable communities and a sustainable world. Up to 90 % of energy use within the urban water cycle is allocated to hot water heating for end users. A large portion of the heat that is provided to households in the form of hot water is lost through the drains. According to Schmid (2008), approximately 15 % of thermal energy supplied to conventional new buildings is lost through the sewers, while for new low-energy buildings, this number rises to 30 %. When buildings’ transmission, infiltration, and exfiltration losses decrease because of better building design, the share of losses to the drains can be expected to rise. In order to better utilize the heat still contained in the water that is flushed down the drains, wastewater heat recovery (WWHR) has been implemented and tested in various cities around the world. Wastewater (WW) is a reliable and renewable source of thermal energy with a relatively stable temperature throughout the year. Several techno-economi assessments, and a few lifecycle assessments have been conducted so far. However, no comparative lifecycle assessment of centralized and decentralized WWHR has been found. Bad or insufficient planning of WWHR can lead to competing technologies. Furthermore, uncoordinated decentralized WWHR can lead to the minimum influent temperature requirement of wastewater treatment plants (WWTPs) being jeopardized. Therefore, the environmental impact of centralized, and a future scenario with an increased amount of decentralized WWHR in Stockholm has been estimated and compared. This was achieved by systematically analyzing energy consumption, emissions, and natural resource extractions from manufacturing, transportation, operation, internal WWTP processes, biogas consumption, and disposal/recycling by developing a lifecycle assessment (LCA) model both in Excel and SimaPro. Centralized WWHR in Stockholm is compared with the case that 10, 20, …, 90, and 100 % of buildings installing shower wastewater heat exchanger (WWHEX). The decreased WW temperature and consequently the decreased centralized WWHR because of shower WWHR were estimated by calculating the resulting mixed WW temperature at the inlet of the sewer pipes, and adopting a simple model for the WW temperature decrease along the sewer pipes. The results of the lifecycle inventory (LCI), which was developed in Excel, were assigned to affected impact categories with the lifecycle impact assessment (LCIA). The midpoint and endpoint impact analyses of the ReCiPe2016 method showed that the centralized case has the lowest environmental impact per kWh WWHR. More specifically, the impacts of the centralized system were 0.131 kg CO2eq/kWh, 1.27×10-7 DALY/kWh, 3.73×10-10 terrestrial species years/kWh, 80.6 ktons CO2eqs, 1,780 DALYs, and 5.23 terrestrial species years. This can be-3-compared with the results of the 50 % decentralized case with 0.164 kg CO2eq/kWh, 1.59×10-7 DALY/kWh, 4.68×10-10 terrestrial species years/kWh, 82.8 ktons CO2eqs, 1,600 DALYs, and 4.72 terrestrial species years.The 100 % decentralized system had the biggest impact on all categories. The sensitivity of the model was inspected by varying major input parameters.