The growing interest in the use of unconventional energy sources is a stimulus for the development of dedicated devices and technologies. Drain water heat recovery (DWHR) units can be an example of such devices. They allow the recovery of part of the heat energy deposited in grey water. This paper describes the results of research on the assessment of the financial profitability of the use of two horizontal heat exchanger solutions, taking into account the actual distribution of cold water temperature during the operating year in the plumbing and two operating regimes of the premises as the residential and service facilities. The analysis showed that the use of a horizontal heat exchanger with increased efficiency in a dwelling in a 15-year life cycle allowed for achieving more than twice as much savings (reaching up to EUR 1427) compared to a classic horizontal heat exchanger. At the same time, it was shown that the installation of this type of equipment was more profitable the greater the water consumption of the premises. The article also notes the impact of cold water temperature in the installation on the results of the analysis. It was featured that taking temperature on the basis of installation design recommendations led to significant distortions in the financial analysis. On the other hand, comparing the method of averaging the cold water temperature (daily, monthly and yearly), it was determined that averaging the temperature over the annual cycle was an acceptable simplification of the model. The research results presented in the paper have a practical aspect and may constitute guidelines for designers and potential investors. In addition, they can be an incentive to continue research on heat exchangers by other scientific centers, which on a global scale will increase the universality of their use.

The need for efficient use of energy and sustainable energy management and the fact that large quantities of heat are deposited in the discharged sewage have contributed to the development of research on waste heat recovery. Gray water began to be seen not just as waste, but also as an alternative source of energy. Research related to the development, improvement, and finally, the popularization of waste energy recovery devices and systems has evolved rapidly over the last two decades. Initially, technologies for gray water reuse were not widely used, which was due to the low efficiency of the current heat exchangers and the significant investment outlays that would have to be covered by potential users. Research conducted by scientists from around the world has allowed us to eliminate construction flaws, improve efficiency, and also provide information on the selection of optimal waste heat recovery technology, depending on the installation conditions and operating parameters. The ability to correctly select the device allows for effective energy collection from gray water, which improves the investment profitability. This paper reviews the research regarding issues related to waste heat recovery from gray water in sewage installations and systems. A critical analysis of the current state of knowledge was carried out with a special consideration to the technologies intended for the residential buildings.

The use of energy from waste can be a key means of reducing the consumption of fossil fuels and thus reduction of greenhouse gas emissions. Waste energy can be a worthy alternative to conventional energy sources in construction. This paper presents data on energy consumption for the preparation of domestic hot water in residential buildings. A review of the literature sources and inventions in the area of waste energy recovery from grey water was carried out. It also presents the results of research on prototypes of devices used to receive energy deposited in wastewater, published in recent years. The benefits of using drain water heat recovery systems for preheating utility water in residential buildings are presented. An analysis of technical solutions for grey water energy collection units was made, revealing their advantages and disadvantages. Great importance was attached to the review of patent sources as well as devices available on the market. According to the authors, the results of the technical review may be useful for contractors and designers of heat recovery equipment and installations, researchers and potential users of these technologies.

The potential for recovering heat from wastewater exists at various stages, including generation, transport, and treatment. As a result, various technologies for thermal energy recovery from wastewater are now successfully employed in many countries. In order to synthetically present the current state of knowledge on heat recovery from wastewater, a bibliometric analysis of previously published studies indexed in the Web of Science database was performed. The review was further extended with strategic SWOT and SOAR analyses to identify internal and external factors determining the competitive advantage and weaknesses related to the use of wastewater heat exchangers and heat pumps. These analyses indicated the need for further research on the possibilities of heat recovery from wastewater as the use of this technology, both at the building level and on a larger scale, contributes to the implementation of sustainable development goals, especially in terms of improving energy efficiency and reducing CO₂ emissions. Particular emphasis should be placed on research into the use of warm wastewater together with other, better known and accepted, renewable energy sources. It is also important to continuously educate the public and promote heat recovery technologies at various levels, as well as to increase the involvement of legislators and other stakeholders.

The use of greywater heat exchangers (GHEs) is an effective way to reduce energy consumption for heating domestic water. However, the available characteristics of this type of device are often insufficient and consider only a few selected parameters of water and greywater, which results in the need to look for tools enabling the determination of the effectiveness of GHEs in various operating conditions with incomplete input data. The aim of this paper was to determine the usefulness of artificial neural networks (ANNs). For this purpose, comprehensive experimental tests were carried out on the effectiveness of the horizontal heat exchanger, taking into account a wide range of water and greywater flow rates and temperatures of these media, as well as the linear bottom slope of the unit, which allowed for the creation of a database of 32,175 results. Then, the feasibility of implementing the full research plan was assessed using ANNs. The analysis showed that the impact of the media temperatures on the heat exchanger effectiveness values obtained using ANNs is limited, which makes it possible to significantly reduce the number of necessary experiments. Adopting only three temperature values of at least one medium allowed the generation of ANN models with coefficient values R² = 0.748–0.999 and RMSE = 0.077–1.872. In the case of the tested GHE, the slope and the flow rate of the mixed water are of key importance. However, even in the case of parameters of significant importance, it is possible to reduce the research plan without compromising the final results. Assuming five different values for each of the four input parameters (a total of 625 combinations) made it possible to generate an ANN model (R² = 0.993 and RMSE = 0.311) with high generalization ability on the full research plan covering 32,175 cases. Therefore, the conducted analysis confirmed the usefulness of ANNs in assessing the effectiveness of GHEs in various operating conditions. The approach described in this paper is important for both environmental and economic reasons, as it allows for reducing the consumption of water and energy, which are necessary to carry out such scientific research.