Wastewater heat recovery systems for sustainable building energy efficiency and decarbonization: A comprehensive review with practical insights

Growing energy demand and carbon emissions pose a major challenge to the building environment, particularly in addressing heating and cooling needs sustainably. In this context, wastewater is recognized as an unconventional and largely untapped thermal energy resource due to its consistent supply and generally stable temperature. Wastewater heat recovery has the potential to significantly improve building energy efficiency and boost decarbonization efforts. This review provides a comprehensive and system-level assessment of wastewater heat recovery technologies for building- and district-scale applications, with particular emphasis on heat extraction locations, heat exchanger configurations, and wastewater source heat pump systems. Rather than treating individual components in isolation, the study integrates heat exchanger and heat pump technologies within a unified framework to support practical design and implementation decisions. State-of-the-art literature covering wastewater characteristics, recovery locations, heat exchanger technologies, and combined wastewater heat exchanger-heat pump systems is critically analyzed through comparative evaluation of performance indicators, benefits, and limitations. Key recovery locations, including building outlets, sewer networks, and wastewater treatment plants are examined in terms of thermal potential, operational constraints, and system compatibility. The review highlights that appropriately selected heat exchangers and wastewater source heat pump configurations can significantly improve energy recovery efficiency while reducing greenhouse gas emissions. Finally, practical recommendations are provided to guide the selection, installation, and integration of wastewater heat recovery systems, identifying fouling control, system optimization, and data-driven design as critical factors for advancing wastewater as a viable unconventional thermal energy resource in modern building energy systems.