This technical report describes the literature review conducted on wastewater heat recovery (WWHR). As part of the urban water cycle, domestic hot water consumes the lion share – up to 90 % – of the total energy requirement for water management. Individual energy consumption of 780 to 1 150 kWh/cay/yr has been estimated in Sweden. Energy can be recovered from wastewater, in buildings close to the source or further downstream in the wastewater system. Depending on wastewater flow and temperature heat exchangers or heat pumps (or a combination of both) can be used for extracting heat the energy. Obstacles for utilizing this potential are for example: clogging and fouling of equipment, potentially negative system impacts and economic feasibility. Examples of various WWHR implementations have been found in Sweden, Switzerland and North America. Some installations have been running for a long time and technical function and financial viability has been evaluated and are reviewed in the report. Generally, heat pumps reach a coefficient of performance of 3 to 7, better the higher the wastewater temperature is, i.e. further up-stream.

WWHR application in a wastewater system can be modelled. The domestic hot water requirement and associated energy use has been modelled previously and concepts can be adapted for modelling the larger system. Equations for calculating performance and output variables from heat recovery equipment have been reviewed and is presented. For the purpose of assessing single WWHR installations in sewers, detailed models have been developed and presented. There are reviewed in the text. Concepts for estimating temperature variations in sewers are essential to assess the impact on wastewater treatment plants. Performance of wastewater treatment plants and their temperature dependence can be modelled with existing process models. Temperature variations along the course of the treatment plant might be important to consider.

In Sweden, there are currently some regulations related to WWHR. The temperature of hot water systems in buildings are regulated to prevent Legionella outbreaks. Furthermore, the practice of WWHR is limited in extent and requires a permit from the utility as by the contract between the consumer and the utility. Currently, this limits the implementation of WWHR in Sweden.