Opportunities and Challenges for Research on Heat Recovery from Wastewater: Bibliometric and Strategic Analyses

Abstract

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.

1. Introduction

The density of sewage networks for the disposal of domestic and industrial wastewater is increasing year by year. This phenomenon is observed both in cities and in areas with a lesser degree of urbanization, and its scale can be considered an indicator of economic development and quality of life. Along with the increase in the length and density of sewage networks, the amount of wastewater transported by sewers to wastewater treatment plants also increases. This wastewater is usually perceived as waste and the main source of surface water pollution [1,2]. Bugajski et al. [3] also drew attention to the high costs of their purification, especially in the case of combined sewage systems. However, more and more often, the positive aspects of wastewater in the form of resource recovery possibilities are recognized [4]. For example, greywater, which constitutes the majority of wastewater produced in households [5], can be used as an alternative water source [6]. Water reclamation is also gaining popularity in industrial plants and wastewater treatment plants [7,8]. Actions to recover nutrients and chemical energy are increasingly being taken [9]. However, less attention is paid to the heat carried by wastewater, although Hao et al. [10] showed that the potential of thermal energy contained in wastewater is significantly higher than that of chemical energy. In addition, the process of heat recovery from wastewater can be carried out not only within wastewater treatment plants (WWTPs) or industrial plants, but virtually anywhere where there is wastewater. Hidalgo et al. [11] and Nagpal et al. [12] noted that this can be any stage of the cycle, starting from the production of wastewater to its disposal.

The use of the energy potential of wastewater is gaining particular importance in the face of global challenges related to the increase in energy demand and the resulting deterioration of the natural environment. Although climate change is not felt the same everywhere [13], the destructive impact of the energy sector on the environment is visible throughout the planet. Urbanized areas with a high population density are particularly vulnerable [14]. At the same time, these areas generate the largest amounts of wastewater, which is a potential source of heat. As the number of residents increases, the amount of wastewater generated increases. Wilson and Worrall [15] also noted, using data from England, that the temperature difference between wastewater flowing from the WWTP and river water is increasing annually, so wastewater heat is a growing resource. Considering that waste energy recovery not only has a positive impact on the environment but can also contribute to reducing energy costs [16], more attention should be paid to methods, technologies, and tools enabling the recovery of heat carried by wastewater. However, this process cannot be carried out uncritically without considering the potential consequences of the actions taken. Although lowering the temperature of raw wastewater may, under certain conditions, have a positive effect on some processes occurring in the sewers [17], its excessive cooling may have a destructive effect on the treatment process [18,19]. Golzar and Silveira [20] pointed out that this problem may also apply to local systems in buildings. Therefore, integrating and analyzing the existing knowledge in the field of heat recovery from wastewater is necessary in the context of making the most of its energy potential.

Depending on the location of the wastewater heat recovery system (WHRS), characteristics, type, and amount of wastewater, as well as the potential ways of using the recovered thermal energy, both heat exchangers (HEs) [21] and heat pumps (HPs) [22] can be used. In the former case, the purpose of installing the unit is usually to preheat domestic hot water (DHW) [23]. In residential or commercial buildings, horizontal [24], vertical [25], oriented [26] and plate [27] heat exchangers, as well as storage units [28] and those intended for installation in the shower tray [29] are used. Units based on phase change materials are also gaining popularity [30]. All these solutions are often referred to as drain water heat recovery (DWHR) units. There are also solutions in which special wastewater heat exchanger designs are used to preheat ventilation air [31] or are used to recover heat from industrial wastewater [32]. The possibility of using HPs is considered when there is a larger amount of available wastewater. Although the use of these devices requires the supply of additional drive energy, the range of potential applications of the recovered heat is significantly wider. These may be, for example, central heating and DHW installations in buildings [33,34] or WWTPs facilities [35], as well as industrial installations [36,37]. More and more often, wastewater is also considered as a source of energy in low-temperature district heating networks of the 4th [38] and 5th [39] generation.

Despite the fact that there have been several literature reviews in the field of heat recovery from wastewater in recent years, most of them focus on selected stages of the cycle. For example, Piotrowska and Słyś [40] and Wehbi et al. [16] focused on the technical feasibility and profitability of WHRSs in buildings. On the other hand, Culha et al. [41] analyzed and classified heat exchangers used in wastewater heat pumps. Some authors addressed the issue of heat recovery from wastewater in review papers, in which this topic is not leading. An example of this is a paper by Pomianowski et al. [42], in which the authors discussed sustainable DHW heating systems, and an article by El Hage et al. [43] on waste heat recovery in buildings. Only Nagpal et al. [12] focused on the possibilities of using the energy carried by wastewater at all points of the cycle, considering both the use of HEs and HPs. They analyzed WHRSs in light of environmental, technical, and economic criteria. However, this paper was published several years ago, and as a result, it does not take into account the latest achievements in the field of wastewater heat recovery. Considering that new innovative solutions of devices for wastewater heat recovery, as well as methods of monitoring and controlling their operation, are constantly being developed, it is necessary to update the knowledge in this field. It should also be noted that the technology of wastewater heat recovery is becoming increasingly popular in recent years, and more and more scientists have decided to undertake research related to it. Identification of research gaps is necessary in order to set research priorities and identify areas that have the greatest potential. A bibliometric analysis was used as a tool. Its application allows us to recognize trends in research, assess their interdisciplinarity, and measure the strength of influence in the world of science [44,45]. This analysis has not yet been used to assess the possibilities of heat recovery from wastewater at all points of the system. It was only used as a tool to assess greywater as a sustainable source of water and energy in buildings [46]. However, this paper did not include systems that allow for the recovery of thermal energy carried by raw and treated wastewater, systems located within WWTPs, or those operated in industrial plants.

The literature review provided a basis for developing SWOT (strengths, weaknesses, opportunities, threats) and SOAR (strengths, opportunities, aspirations, results) analysis matrices. A holistic look at the issue of heat recovery from wastewater will allow for better use of the strengths of these systems and the opportunities associated with their implementation.

In summary, this review paper addresses the following issues:

• Presentation of the results of the bibliometric analysis of publications in wastewater heat recovery domain (Section 3.1).
• Presentation of the latest research on the use of wastewater heat exchangers (Section 3.2.1) and wastewater heat pumps (Section 3.3.1), as well as WHRSs in industrial facilities (Section 3.4).
• Indication of strengths and weaknesses of wastewater heat exchangers (Section 3.2.2) and wastewater heat pumps (Section 3.3.2).
• Identification of potential opportunities and threats for the development of wastewater heat recovery technology based on the use of heat exchangers (Section 3.2.3) and heat pumps (Section 3.3.3).
• Discussion of the aspirations of wastewater heat recovery systems based on the use of heat exchangers (Section 3.2.4) and heat pumps (Section 3.3.4), as well as the results that are possible to achieve.
• Indication of examples of the use of WHRSs in practice (Section 3.5).

2. Materials and Methods

2.1. Bibliometric Analysis

The first stage of the analysis involved formulating a search query (Figure 1), which was subsequently implemented in the Web of Science (WoS) database. This database was chosen due to its credibility, interdisciplinary nature, and high quality data. It is widely used by researchers worldwide, and journal indexing in this database guarantees increased recognition and prestige. The publications presented in the Web of Science database often form the basis for literature reviews conducted in various scientific disciplines [47]. It is also used for bibliometric analyses [48,49], including those addressing energy conservation and natural resources protection [50,51]. While formulating the search query, it was assumed that the title (TI), abstract (AB), or keywords (AK) should contain words or phrases related to wastewater. Simultaneously, one of the mentioned search areas should include a phrase describing the purpose of wastewater utilization, namely, heat recovery. No time restrictions were defined to identify the first papers on this topic and to show how the number of publications has increased over the years. As a result, 1127 records potentially related to the analysis topic were identified. The database subjected to further analysis was downloaded on 30 April 2023.

Next, from the database of 1127 publications, non-English records were excluded (Figure 2). These were primarily articles in German (6 records), Chinese (4 records), Russian (4 records), and Japanese (2 records) languages. The database also contained publications in Czech, Croatian, French, Polish, Korean, Spanish, and Turkish languages. As a result, 24 records were discarded, and 1092 papers were subjected to further analysis. In the next step, the types of publications were analyzed. As many as 2 letters, 2 editorial materials, and 1 correction were removed from the list.

Further selection was carried out on 1087 publications. The authors of the manuscript read their abstracts and, if necessary, the full texts to identify records discussing heat recovery from wastewater. The bibliometric analysis included papers focusing on heat recovery from greywater, industrial wastewater, raw wastewater in sewer systems, as well as wastewater during and after the treatment process. Publications presenting technical solutions for wastewater heat recovery installations and devices, along with their reliability and associated risks, were taken into account. Articles that estimated the potential of waste heat from wastewater and explored possible utilization methods at various levels, such as buildings, low-temperature district heating networks, and wastewater treatment plants, were also included. Both publications solely dedicated to wastewater heat recovery and those where heat recovery was one of the considered options were analyzed. However, papers discussing wastewater treatment technologies, energy optimization of these processes, and the recovery of various substances (e.g., nutrients) from wastewater were excluded. Furthermore, papers related to biogas production and utilization, sewage sludge disposal, and wastewater treatment during the cleaning of heat exchangers were not considered in the analysis. As a result, the database of publications was reduced to 504 records, which were then subjected to performance and science mapping analyses.

For this purpose, VOSviewer version 1.6 [52] and Bibliometrix ver. 4.1 with the biblioshiny application [53] were used. VOSviewer is one of the most popular software tools for bibliometric analysis. This program enables the visualization of data related to extensive collections of scientific publications [54]. The tool allows for the analysis of co-authorship networks, research topics, and exploration of relationships among keywords [55,56]. VOSviewer is commonly used for generating partially editable networks depicting associations between co-authors, keywords, citations, or author affiliations. The input data for the software can be RIS, EndNote or BibTeX files typical for scientific databases and related reference managers. However, the software also allows us to work with text files exported directly from databases like Scopus or Web of Science. The imported database allows the generation of co-authorship networks, keyword heatmaps, co-occurrence maps of scientific terms, as well as textual files presenting compilations of constituent elements of the generated graphics.

Bibliometrix is an Rstudio package developed by Massimo Aria and Corrado Cuccurullo. It serves as a comprehensive tool for generating graphics and charts based on statistical data related to scientific publications [57]. The software combines the functionalities of an Rstudio package with a web-based application called “biblioshiny app”. The user-friendly interface of the application allows for convenient usage of the software without the need for knowledge of the R language.

Based on the R language, Bibliometrix also provides the opportunity for integration with other packages and extends the analysis with additional functions available in R. This program enables the analysis of dependencies using a wide range of information describing scientific publications [58]. In addition to dependency charts, Bibliometrix offers various modules that allow for a more in-depth understanding of the evolution of topics within the analyzed database. It also enables the comparison of generated charts with model curves such as Lotka’s Law, as well as the comparison of local dependencies within the analyzed database with global statistics across the entire WoS or Scopus database. Similarly to VOSviewer, Bibliometrix allows for importing data from various popular formats and exporting analysis results to XLSX and JPG files.

2.2. SWOT and SOAR Analyses

Bibliometric mapping has been additionally expanded to include SWOT and SOAR strategic analyses. Both SWOT analysis and SOAR analysis allow for identifying strengths and opportunities in the context of a given project, or solution [59]. Moreover, SWOT enables the recognition of weaknesses and threats, while SOAR focuses on defining aspirations and results [60,61].

SWOT analysis focuses on evaluating internal and external factors and is highly regarded for its methodological clarity. However, in practice, it does not always guarantee reliable results and can sometimes lead to incorrect decisions, mainly because the traditional approach to this type of analysis is based on qualitative assessment, which may contain subjective opinions from planners [62]. Therefore, this analysis is often combined with other methods [63], allowing for a more comprehensive view of the situation.

In practice, one of the methods that address these aspects is the SOAR analysis. SOAR is a strategic approach that focuses on identifying and developing strengths, which includes unique resources, skills, and characteristics that provide a competitive advantage. The analysis also emphasizes the identification of opportunities, which are external factors offering perspectives for the development [64]. Aspirations, representing the ambitions and goals of the project, play a crucial role in this type of analysis, serving as motivation for achieving success and excellence. Additionally, SOAR analysis incorporates an assessment of solution results by evaluating achievements and outcomes in relation to the established objectives [65]. While SWOT analysis is more centered on the current state of the project and its environment, SOAR analysis is perceived as a more positive approach to identifying aspects related to the functioning of the project, which can affect its development and the involvement of people managing it. However, it is important to highlight that both methods have their unique characteristics and applications, and they can be successfully used in the process of organizing and examining available data to understand their structure, significance, and relationships [66]. The conducted literature analysis was the basis for identifying the factors characterizing the analyzed waste heat recovery technologies, i.e., the technology based on the use of heat exchange units in buildings and the technology used on a larger scale, based mainly on the use of heat pumps.

The definition of strengths involved identifying internal factors that contribute to the competitive advantage of the analyzed technological solution for recovering waste energy from wastewater [59,67]. These factors were determined with the assumption that they represent unique resources or distinctive characteristics that contribute to the success of the solution in the market. On the other hand, the group of weaknesses included internal factors limiting the possibilities of a given solution or project, which may relate to limitations or shortcomings that hinder the achievement of set objectives.

The analysis of opportunities required describing external factors that can bring benefits to the considered solution. These factors may encompass market trends, socio-economic adaptations, and technological changes that the project can exploit to develop the solution or meet the needs of decision-makers [65,67]. On the other hand, threats referred to external factors that can negatively impact the analyzed WHRSs. These factors may involve risks, uncertainties, competition, and changes in the market or regulatory environment, which can affect, among others, the efficiency of waste energy recovery, financial performance, reputation, or competitiveness [67].

When analyzing the results of implementing heat recovery systems, the focus was placed on specifically defined effects that can be achieved. These factors determine the success of the given solution and may include achieving strategic objectives or developing new solutions [66]. Positive internal factors providing motivational support for the considered solution, which can influence the pursuit of intended goals, such as the optimization and popularization of wastewater heat recovery technologies, were assigned to the category of aspirations [67].

Based on the results of the SWOT and SOAR analyses concerning wastewater heat recovery technologies, it was shown that the analyzed systems use their strengths and take advantage of opportunities (SWOT and SOAR analyses), minimize weaknesses, and manage threats (SWOT analysis). Additionally, aspirations and potential results were identified that can be achieved by using the strengths and opportunities offered by wastewater heat recovery technologies at various stages of wastewater generation and transport.

3. Results and Discussion

This section presents the most important findings of the holistic literature review, which was based on the elements of bibliometric analysis and strategic analysis methods. We believe that this has allowed us to present a comprehensive and up-to-date state of this field, contributing to its further development.

3.1. Bibliometric Analysis

The first stage of the analysis involved examining publication trends and the number of citations of articles in the analyzed database. Figure 3 presents a comparison of the number of published articles and the average number of citations over the years 1995–2023. The period from 1977 to 1994 was not included in Figure 3 due to a very low number of publications. Over the course of 18 years, only 11 articles were published on the topic. On the left axis, the average citations per article (ACPA) published in a given year were marked, and these values are represented by red columns. On the right axis of the chart, there are values corresponding to the number of articles in each year, represented on the graph by purple points connected by lines.

From 1997 to 2006, there was a gradual but slow increase in the number of published papers. The high average number of citations is directly influenced by the low number of publications during that period. It is important to note that these papers were often used as a basis for new articles in later years. For example, in 2001, only one article on wastewater heat recovery was published and indexed in WoS, but it received over 60 citations in total. Articles from the initial period of the analysis, which received a negligible number of citations, are usually proceedings papers, which often have less impact than publications from scientific journals. From 2007, there has been a significant increase in the number of publications on the discussed topic, while the number of citations per article decreased. This is a common phenomenon as the interest in a particular research area grows, leading to an overall increase in the number of publications, including those of lesser importance for the general development of the research field. As a result, the average number of citations per article decreases. In the years 2021–2022, there was a slight decline in the number of publications. This fact may be attributed to the SARS-CoV-2 pandemic or the lack of indexing of publications from the end of 2022.

Figure 4 presents a visualization of the network of co-occurrences between the most frequently occurring keywords in the analyzed database. The author keywords that appeared in at least 5 articles in the database were taken into consideration. The keywords were divided into 8 clusters, indicated by different colors, where keywords within each cluster share similar themes or frequently co-occur together (Figure 4a). Individual nodes are connected by lines, while the thickness of the lines represents the number of co-occurrences within the analyzed database of scientific publications. The most frequently occurring keywords were “heat pump” (68 occurrences, Cluster 6), “heat recovery” (64 occurrences, Cluster 2), “waste heat recovery” (31 occurrences, Cluster 4), “sewage source heat pump” (31 occurrences, Cluster 7), and “energy efficiency” (29 occurrences, Cluster 4). Figure 4b illustrates the network of keywords in relation to the average number of citations received by documents in which each keyword appears. The keywords with the highest average number of citations were “wastewater heat exchanger,” “buildings”, “wastewater treatment”, and “wastewater”. All of these keywords were cited on average over 30 times. On the other hand, the keywords with the lowest average number of citations were “falling film”, “energy saving”, “COP” (Coefficient of Performance), and “urban sewage”. These terms received an average of fewer than 3 citations. These findings suggest a considerable interest in the application of wastewater heat exchangers in buildings and wastewater treatment plants, as indicated by the high average citations for related keywords. On the other hand, keywords describing the operation of heat exchangers and their coefficients of efficiency seem to be less popular, implying that they may pertain to more specialized or newer research areas that are not yet widely recognized in the literature. Among the author keywords with the lowest number of citations are also general terms related to energy saving and urban sewage. This could be due to the fact that such expressions are very commonly used in scientific publications, including those from other fields, leading to lower citation averages for these keywords. Figure 4c shows the average publication year of the keywords included in the generated network of co-occurrences. The most recent publications frequently used keywords such as “wastewater heat recovery” and “climate change”. This suggests an interest in renewable energy sources, including heat recovery from wastewater, as a means of mitigating excessive greenhouse gas emissions and related climate change. Similarly, keywords like “COP” and “falling film” were also commonly used in recent publications. This justifies the lower number of citations for these terms and reflects the niche nature of research areas concerning the efficiency of devices and the impact of film on their ability to recover heat from wastewater.

The next part of the review was the analysis of the geographic distribution of publications in the wastewater heat recovery domain, as presented in Figure 5. The analysis included 55 countries. Therefore, the total number of records was 923, despite only 504 unique documents were taken into consideration in the database. This is because articles co-authored by scientists from different countries are counted separately for each country involved. The countries with the highest number of publications were China (318), the United States (47), Turkey (44), the United Kingdom (44), and Sweden (43). In Europe, apart from the previously mentioned countries, other countries with a significant number of publications were Austria (39), Poland (32), France (31), and Switzerland (27). Asian countries, excluding previously mentioned China, showed lower interest in the topic of heat recovery from wastewater. Among the most frequently publishing countries were Japan (22) and South Korea (20). Generally, the median number of publications was 6, while the standard deviation was over 43. These values suggest a wide range in the number of articles among countries and the presence of a few leading countries in the discussed field. Despite having significantly fewer publications, there is visible interest in wastewater heat recovery in Europe, especially among countries in Northern and Western Europe. Conversely, the aspect of wastewater heat recovery is less popular in Central American and African countries. This may be due to the lower demand for energy in these regions and the significant potential of solar energy.

Figure 6 presents a compilation of the top ten sources with the highest number of publications in the analyzed database. Along with the number of publications, the average number of local citations per article is also provided. Local citations refer only to citations occurring between articles within the analyzed database. These local citations do not represent the total number of citations for individual articles in the Web of Science database but better reflect the significance of the publications’ impact on the development of research in the discussed topic. The highest number of publications appeared in the journals Energy and Buildings, Applied Thermal Engineering and Energies. When examining the average number of citations per article, notable journals include Energy Conversion and Management, Energy, and Water Research. This indicates the significant influence of articles published in these journals on research in the area of wastewater heat recovery. However, it should be noted that a high average number of citations can be influenced by one or a few frequently cited publications, which can significantly inflate the average for a particular journal. The total number of citations also does not capture the dynamics of a journal’s development. For example, the low average number of citations for the journal Energies might be due to the fact that the articles published there are relatively new. This trend is well illustrated in Figure 7, where the number of publications over time for the top ten sources with the highest number of articles is shown.

Figure 8 illustrates the thematic evolution in the field of waste heat recovery from wastewater using heat exchangers and heat pumps. The diagram was developed based on the most frequently occurring keywords in successive time intervals. The time frames and their division into periods are derived from the segmentation of the database into stages with a similar number of publications. Each period contains 90–120 articles. The first analyzed period covers the years 1977–2011. During this time, 105 publications were recorded in the analyzed database. The most popular keyword combination was “heat pump”. Keywords such as “energy saving” and “fouling” were also frequently used, indicating an interest in heat recovery both in elaborate systems [68] and in multi-family residential buildings [69]. In the period 2012–2014, interest in heat pumps did not diminish, but the broader concept of energy recovery from wastewater gained popularity [70]. Greater attention was also given to the potential of heat recovery from wastewater using heat exchangers installed directly in the sewer system channels, as indicated by the appearance of the phrase “urban sewage” [71]. The years 2015–2017 witnessed an increase in popularity of the wastewater source heat pump [41]. This trend resulted in a series of publication topics related to heat exchangers and heat pumps based on wastewater in later years. There was also noticeable interest in potential energy savings associated with the installation of heat recovery systems [72]. The potential of computer simulations and optimization of wastewater heat exchangers and heat pumps for maximizing financial gains and minimizing investment costs was also recognized [73], as indicated by the increasing appearance of phrases such as “simulation” and “energy efficiency”. Subsequent years (2018–2020) were characterized by the development of previously outlined research directions. Topics related to energy recovery from wastewater, wastewater heat pumps, and niche subjects concerning wastewater heat recovery, such as district cooling, remained popular. Recent research indicated a significant increase in interest in optimizing the operation of heat recovery systems [74], the environmental impact of energy systems, and the possibility of emission reduction through the use of alternative energy sources [75]. Issues related to the operation of wastewater heat exchangers and the impact of characteristic wastewater properties on the performance of devices within the heat recovery system were also noted, as indicated by the appearance of the phrase “falling film” [76].

One of the indicators of the real impact of a publication on the development of research in the discussed field is the number of citations. However, the overall number of citations is not a reliable indicator, especially in bibliometric analyses. For this reason, an alternative to the overall number of citations is the number of local citations. This means that citations are counted only within the analyzed database of publications. Figure 9 presents the top locally most cited publications in the database in relation to the total number of citations for these articles. The lack of a visible correlation between the total and local number of citations for a certain publication may indicate its broad and multidisciplinary research topic. A good example of such a situation is represented by two publications ranked fifth and sixth in terms of the number of local citations. Both articles were published in 2013. In the first article [24], the authors described and analyzed heat recovery from shower water in a horizontal heat exchanger. This publication focused on a narrow specialization of heat recovery, with 48 local citations, while the number of global citations is only 9 more, representing a 19% increase. On the other hand, the sixth position in Figure 9 is occupied by a publication with the same number of local citations [70] but with over three times more global citations for the article. The authors of this publication not only discussed heat recovery from wastewater but also explored the possibilities of recovering chemical energy through the combustion of biogas from wastewater sludge and the impact of heat recovery on wastewater treatment processes. The concentration of various research areas in one publication is evident, which has resulted in the potential for increasing the number of citations for the article and its rapid popularization.

3.2. Heat Recovery in Residential and Commercial Buildings Using Wastewater Heat Exchangers

3.2.1. The Latest Research on the Use of Wastewater Heat Exchangers

One of the basic and simplest ways to recover thermal energy from wastewater is the use of heat exchangers. These devices can be used in both residential and commercial buildings, contributing to reducing their energy consumption. Depending on the design and hydraulic layout, they can be installed directly next to the sanitary facility or recover heat from more facilities. However, in each case, the source of heat is greywater. The idea of using this energy carrier appeared in some regions of the world in the last century. For instance, Parker and Tucker [82] already described in 1991 the studies of a DHW installation cooperating with solar panels and a wastewater heat exchanger. Their experiments showed that energy savings from the application of the HE can exceed 30%. However, under the climatic conditions of New Zealand, the use of solar energy proved to be a more favorable option. Perhaps for this reason, the design and optimization of wastewater HEs did not constitute a significant research topic in the following years, and as a consequence, they were not widely commented on in the literature. The beginning of the 21st century saw a slight increase in interest in the possibilities of greywater heat recovery in buildings, resulting in attempts to design systems for dishwashers [83]. Although in some regions of the world wastewater HEs were already available on the market, the increase in interest in this field only occurred around 2010, probably as a result of the global economic crisis. It was then that papers on the assessment of the efficiency of various models of shower heat exchangers appeared [69,84]. In the following years, research was carried out to assess the efficiency of existing HE solutions, as well as to present innovative alternatives to commonly known types of heat exchangers. The conducted research focused not only on experimental analyses but also on the assessment of the financial and environmental efficiency of different types and configurations of systems under specific operating conditions. Recently, there has been a noticeable increase in interest in using modern research methods, such as numerical modeling or social research techniques, as well as assessing the validity of the application of such a solution on a global scale. To show the scope of current research and the wide range of journals and publishers interested in the discussed issue,

Table 1. Examples of the latest papers on the use of HEs in residential and commercial buildings (HE—heat exchanger).

Authors	Aim of the Paper	Source Title
Ovadia and Sharqawy [85]	Experimental and analytical analyses aimed at determining the transient characteristics of the vertical drain water heat recovery unit	Case Studies in Thermal Engineering (Elsevier)
Singh et al. [86]	Analysis of heat recovery from commercial kitchen grease traps using two different HEs designs	Journal of Cleaner Production (Elsevier)
Jadwiszczak and Niemierka [87]	Experimental studies of a horizontal plate heat exchanger with determination of its thermal efficiency and the number of heat transfer units under balanced and unbalanced flow conditions	International Communications in Heat and Mass Transfer (Elsevier)
Kordana-Obuch and Starzec [23]	Analysis of the possibility of increasing the efficiency of the horizontal shower heat exchanger by installing baffles	Energies (MDPI)
Piotrowska and Słyś [21]	Analysis of the efficiency of the HE in the form of linear drainage with a wavy middle wall	Journal of Building Engineering (Elsevier)
Jaleel and Jaffal [88]	Experimental assessment of the influence of the shape of the fins on the efficiency of the phase change material-based HE	Frontiers in Heat and Mass Transfer (Global Digital Central)
Schestak et al. [25]	Presentation of a calculator supporting decision-making in the field of heat recovery from drain water in commercial kitchens, considering financial and environmental criteria	Water (MDPI)
Ravichandran et al. [75]	Evaluation of the impact of local conditions on the legitimacy of using drain water heat recovery systems and a comparison of these systems with a medium-scale district heating network	Journal of Cleaner Production (Elsevier)
Manouchehri and Collins [89]	Presentation of a model for predicting the efficiency of vertical HEs, generated based on experiments on eight units of various lengths and diameters	Resources (MDPI)
Selimli et al. [90]	Experimental research on heat recovery from dishwasher greywater	Sustainable Energy Technologies and Assessment (Elsevier)

provides examples of the latest papers devoted to the application of wastewater heat exchangers in residential and commercial buildings. These and other publications addressing the issue of wastewater heat recovery using HEs served as the basis for developing SWOT/SOAR analysis matrices.

3.2.2. Strengths and Weaknesses

As mentioned in Section 2, both SWOT and SOAR analyses are considered strategic analysis methods, whose aim is to indicate how to use the opportunities facing the system under analysis. SOAR analysis is more action-oriented but does not allow for a complete analysis of the existing state. On the other hand, the assumption of SWOT analysis is to indicate both positive and negative aspects accompanying a given project, providing a more comprehensive view of the situation. Therefore, these methods complement each other, forming the basis for the further development of wastewater heat exchangers and increasing their significance in the context of sustainable energy policy.

The primary advantage of the application of wastewater heat exchangers (HEs) in buildings (Figure 10) is the ability to reduce the consumption of fossil fuels used to heat tap water. Considering that fossil fuel resources are rapidly shrinking [91], finding and implementing alternative heat sources is no longer a vision of the future, but becomes a necessity. Virtually every building where DHW is used represents a potential site for the application of wastewater HEs, regardless of its size and purpose. It is estimated that in 2021 there were about 2.3 billion houses worldwide [92]. This should be augmented by commercial buildings, hotels, hospitals, sports halls, and many others. Numerous studies on the efficiency of wastewater HEs have shown that the application of high performance vertical units at the outflow of warm greywater from showers can ensure heat recovery at a level ranging from about 30% to even 75% [40]. However, Manouchehri et al. [26] pointed out that a deviation from vertical, especially above 2°, significantly reduces its efficiency due to delamination and unequal film thickness. In the case of other types of HEs, the projected energy savings are correspondingly lower, which may result from a smaller heat exchange surface compared to vertical units. The thickness of the wastewater layer adhering to the heat exchange surface is also important. The higher it is, the less efficiently the heat deposited in wastewater is transferred. For example, Vavřička et al. [27] showed that at a flow rate of 2 L/min, a plate shower HE can achieve an efficiency of even 62%. At higher flows, corresponding to the flow rates of mixed water from a showerhead, its efficiency was definitely lower. On the other hand, studies concerning horizontal DWHR units and HEs intended for installation in linear shower drains showed that such units are usually characterized by efficiency not exceeding 35% [23,29], although some authors [24] pointed out that achieving efficiency of even above 50% is realistic. However, it should be noted that regardless of the model of the heat exchanger, its efficiency is dependent on the conditions in which it is operated, mainly flow rates of water and greywater through the HE and the temperatures of these media. The efficiency of heat recovery generally increases with decreasing water flow rate and increasing temperature difference between the media. For this reason, particularly favorable conditions for the recovery of thermal energy from wastewater occur in winter, when the temperature of cold water supplied to the building is the lowest. Regardless of the weather conditions, the third-generation renewable energy source, which is warm wastewater, can be used efficiently, contributing to reducing the costs of heating DHW and limiting greenhouse gas emissions [23,93]. In the case of single shower installations, this reduction will not be impressive. However, considering entire settlements, cities or countries, this potential becomes significant.

It is estimated that between 15% and 30% of the energy supplied to a building is lost with wastewater [94]. Considering the increasing requirements regarding energy consumption and emission levels of buildings, it can be assumed that we will increasingly be dealing with these higher values. The application of wastewater HEs can reduce this loss. It is true that these units can only recover a portion of the lost energy, as a result of which it is impossible to completely eliminate the need to burn fossil fuels. However, it should be noted that wastewater HEs can successfully be used in conjunction with other devices based on renewable energy sources (RESs), for example, solar panels [93,95]. In such a situation, support for the conventional water heating system will be provided throughout the year. This is a particularly beneficial solution because the operation of the described HEs does not require external energy supply. Wastewater flows through the unit by gravity, and the water flows under pressure from the water supply network. However, it should be kept in mind that the flow of water through the HE is associated with a decrease in its pressure [96]. In the case of low available pressure, it is worth considering the installation of the HE models, the design of which allows to minimize pressure losses. If there is a significant consumption of mixed water in the building, the installation of batteries of parallel connected heat exchangers may be beneficial.

There are currently many models of wastewater heat exchangers available on the market, adapted to various conditions of use of internal installations in buildings. New designs are also being developed, which creates the possibility of improving the efficiency of heat recovery. Particularly noteworthy are instantaneous HEs in which the energy recovered from greywater is used on an ongoing basis, without the need to store it. However, the realization of a circular economy using wastewater HEs has weaknesses that may hinder the development and implementation of these units. The most important of these is the relatively high investment outlay. It is related to both the cost of purchasing the HE and the costs of its installation. The price of a heat exchanger can vary widely depending on its type and size. For example, vertical DWHR units for 2” drains can be purchased for about $500, while the prices of units for 4” drains can exceed $1000 [97]. The prices of plate heat exchangers can exceed $2500 [98]. Installation costs are particularly noticeable in existing buildings, where the installation of the unit may require significant modifications to the plumbing system. The scope of the required works results from both the location of individual installation elements and the selected type of the HE. It is usually much higher in the case of vertical HEs, which is tantamount to an increase in the assembly costs of these units [29]. Meanwhile, as emphasized by Jahanbin et al. [99], the modernization of the DHW system requires minimization of nuisance for users, as well as construction works and costs incurred. When choosing a wastewater HE model, in addition to efficiency and cost, its purpose should also be analyzed. The fact is that the application of wastewater HEs gives the possibility of using the recovered thermal energy only for preheating water. However, it should be considered whether the unit is to be installed directly under the shower, or the source of heat will be greywater discharged from more sanitary facilities. Equally important is the availability of space for the installation of the HE. In many cases, the application of high efficiency vertical units may prove impossible, especially in existing buildings.

Before purchasing a wastewater HE, a decision should also be made about how to install it. Numerous studies in the field of assessing the energy and economic efficiency of units installed in the shower drain [40] emphasize that the optimal system configuration is one in which preheated water is supplied to both the DHW heater and the shower mixing valve. However, it should be noted that these studies assume that the heat exchanger cooperates with an instantaneous DHW heater. If DHW is prepared centrally or the building is connected to the district heating network, the use of such a method of installing the HE may prove unjustified, or even impossible.

The weaknesses of local greywater heat recovery systems, installed directly on the drain water outlet from a sanitary facility, also include the slight delay in heat recovery in relation to the start of water intake. The financial analysis described in the literature [100] usually overlook this issue. Meanwhile, Ovadia and Sharqawy [85] demonstrated a 37% overall decrease in annual financial savings after considering the transient performance of the unit, with a 13% reduction for the shower. Frequent but short-term use of DHW was found to be the most unfavorable in terms of the amount of energy saved [80]. Another problem may be the adverse effect of pollutants contained in wastewater on the efficiency of the heat recovery system. To overcome this problem, wastewater HEs are made of appropriate materials, mainly copper, which reduces the need for periodic cleaning of the unit [101]. However, in the case of some heat exchanger models, it may turn out that during long-term operation, this procedure will be necessary.

In addition to the aforementioned weaknesses of wastewater HEs, one should also consider those that are not directly experienced by users of these units, but their importance can be considered on a global scale. Although Hadengue et al. [102] demonstrated, using the example of a catchment area located in Switzerland, that the implementation of shower heat exchangers in 50% of buildings would result in a wastewater temperature reduction in the system by only 0.3 K, Golzar and Silveira [20] reached different conclusions. Based on data from Stockholm, they showed that implementing local installations on a large scale could, over time, have a negative impact on centralized systems, such as WWTPs [20].

3.2.3. Opportunities and Threats

The research also analyzed the opportunities for development and growth in the implementation of wastewater heat exchangers as well as potential threats that could delay, or even completely prevent, this development. Energy price fluctuations are of critical importance. As with any investment aiming to reduce the consumption of traditional energy carriers, an increase in their prices will result in increased profitability of using wastewater heat exchangers. On the other hand, lowering their prices will result in decreased net profits. However, current forecasts [103] indicate that we should expect a clear increase in energy prices in the coming years, which should encourage the implementation of such solutions. The introduction of government and non-government subsidy programs and other financial benefits, for example tax reliefs, may also be helpful. Studies on other pro-ecological investments [104] indicate that subsidy programs significantly increase the number of implemented systems. On the other hand, lack of funding sources can be a critical element causing problems in the involvement of stakeholders [105]. In financial matters, the availability of funds for scientific research can also be significant. Increasing the financing of studies on heat recovery from wastewater can contribute to improving the efficiency of existing solutions, as well as developing new highly efficient models of wastewater heat exchangers.

Another significant aspect that, depending on the development of the situation, can both positively and negatively affect the development of greywater heat recovery systems, is the ecological awareness of society. Wang et al. [106] noticed that increased environmental awareness can boost the demand for clean energy production. Wu et al. [107] showed that a lack of it can be the main barrier to the development of green building. The implementation of sustainable energy-saving technologies, including wastewater HEs, can also be hindered by the enactment of unfavorable or unclear legal regulations. For example, in Poland [108], unclear legal regulations are one of the main obstacles to the implementation of RESs-based installations. On the other hand, the introduction of strict regulations requiring a reduction in emissions and energy consumption in buildings could provide a significant opportunity for the development of heat recovery technology, especially if the introduction of these regulations would go hand in hand with financial support. An extreme example of such regulations could be the mandate to use DWHR units in new buildings. In some parts of the world, such as Canada [89], such regulations already exist. In most cases, however, the use of wastewater HEs is not regulated by any legal acts. Therefore, depending on the knowledge and level of engagement of relevant government bodies, the regulations that will be introduced in the future may either contribute to an increase in the use of these units or prevent their further development. The scope of requirements regarding the principles of sustainable development and the approach to their implementation will also be significant in this area. The application of these principles can serve to assess energy efficiency while considering all human needs [109].

An undeniable opportunity for the considered WHRSs is also the potential to improve the state of the natural environment. Installing one HE will certainly not solve the global problem of atmospheric pollution. However, implementing such a solution on the scale of entire settlements, districts or cities could have tangible effects. Considering that residential buildings alone can be responsible for as much as a dozen or so percent of emissions [110], reducing emissions from point sources poses a particular challenge that deserves more attention. The incentive for increased interest in using wastewater HEs may also be the increase in the number of buildings, as each such facility is a potential place for the installation of these units. Particularly favorable conditions will occur when this growth is associated with an increase in interest in passive and energy-saving construction. In the case of passive buildings, preparing DHW becomes the main fraction of the building energy demand [78]. It should also be noted that improving the ecological aspects of buildings increases their market value and attractiveness to potential buyers [109,111]. In addition, taking into account that consumers’ requirements for the comfort of using DHW installations are constantly increasing [112,113], improving this comfort by using wastewater HEs seems to be a beneficial option. This applies especially to installations equipped with instantaneous DHW heaters, where reducing the temperature difference at the input and output of the device can significantly increase its efficiency.

However, the use of the potential of the renewable energy source, such as greywater, will in many cases be difficult due to the unresolved issue of building ownership. Gilbert [114] pointed out that approximately 1.2 billion people live in rented apartments, and although the trend has been declining in recent years, there are countries where the proportion of tenants exceeds 50%. Given that many tenants are not willing to pay more for a sustainable built environment [115], it can be expected that in the absence of appropriate incentives, building owners will also be reluctant to invest in sustainable solutions. That is why it is so important to implement strategies aimed at motivating tenants to look for buildings with a lower environmental impact [116].

When considering the potential for development of greywater heat recovery technology and the research conducted in this field, one must also take into account the possible increase in the potential of competitive technologies. Research is being conducted worldwide to optimize the use of RESs. Improving the efficiency and lowering the costs of obtaining other sources of energy [117] will certainly act to the disadvantage of wastewater HEs.

3.2.4. Aspirations and Results

The last stage of the analysis concerning wastewater HEs was to indicate aspirations and results. Aspirations are the goals of researchers dealing with the topic of heat recovery from greywater, manufacturers and installers of these units, as well as people promoting sustainable energy policy and green construction. These goals include optimizing the efficiency of existing heat exchanger solutions, as well as developing new designs for these units. As highlighted in Section 3.2.2, vertical HEs are more efficient, but in many cases they cannot be used due to the lack of available space to install the unit. Therefore, special attention should be paid to heat exchangers that can be installed as part of the waste pipe or below the shower tray. A tangible effect of the works in this area should be to increase the efficiency of horizontal wastewater HEs to a level corresponding to the efficiency of vertical units. Nevertheless, improving vertical DWHR units should also be the subject of future research. Only the use of highly efficient DWHR units will allow following the changing legal regulations in the field of building energy efficiency. It should also be borne in mind that the increase in unit efficiency goes hand in hand with the reduction of combustion product emissions to the atmosphere. The use of low-efficiency HEs will not allow greywater heat recovery technology to compete with technologies based on other RESs.

Another important goal of activities carried out in the field of thermal energy recovery from greywater should be to optimize the production costs of heat exchangers and the process of their installation. It is advisable to search for and use durable, and at the same time resistant to corrosion and deposition of pollutants, biodegradable materials that will not be a burden on the environment after the end of the heat exchanger service life. The most commonly used material for the production of wastewater HEs is copper. It is a durable material with high thermal conductivity, but relatively expensive, which results in relatively high purchase costs of DWHR units. In addition to reducing the production costs of these units, efforts should also be made to reduce the scope of the required installation work, which will certainly result in reducing the costs of installing a wastewater HE. It is expected that the effect of such an approach will be an increase in financial savings resulting from the reduction of energy consumption for DHW heating and a faster return on investment. In an ideal scenario, the profitability of using DWHR units should be higher than that of other technologies based on RESs. However, one should keep in mind the weaknesses of wastewater HEs associated with the inability to completely eliminate fossil energy resources and a limited range of applications of recovered thermal energy, which may prevent wastewater HEs from achieving such a level of profitability.

Future actions should also be aimed at increasing the availability of wastewater heat exchangers in local markets. In some countries, such as Canada, the United States, Norway or Denmark, these systems are known and used. In most cases, however, purchasing a wastewater HE is quite a challenge, mainly due to the limited range of available HE models. In addition, in many countries, for example in Poland, it is not possible to purchase DWHR units in brick-and-mortar stores. Meanwhile, introducing these units to regular sales could be one of the ways to increase social acceptance for such solutions. In combination with systematic education of society and promotion of the solution at various levels, this could lead to increased interest in their use. Considering the perspective of the next few years, it can be expected that the result of such actions will be the dissemination of wastewater HEs installations in new buildings and an increase in the value of properties equipped with these units. Additionally, the widespread recovery of thermal energy from wastewater will contribute to the diversification of energy sources used in buildings and ensure their independence from external energy supplies.

3.3. Heat Recovery from Wastewater Using Heat Pumps

3.3.1. The Latest Research on the Use of Wastewater Heat Pumps

Recovery of waste heat from wastewater using heat pumps is an efficient and environmentally friendly solution that can use wastewater from technological processes and those related to human existence as a lower source of energy for heat pumps. Currently, wastewater HPs technologies are used both on a large scale in sewage systems and facilities, such as sewer pipes, wastewater treatment plants, or wastewater pumping station [35,118], as well as on a smaller scale, i.e., in buildings [119,120]. Regardless of the system’s scope, the energy recovered from wastewater can be used for domestic hot water preparation, space heating in winter, and cooling in summer. Moreover, the application of wastewater HPs technology is justified by the fact that the average annual temperature of wastewater constituting the lower heat source is higher than in the case of other low-temperature sources, such as air or water. Additionally, as shown by Schmid [94], in sewer systems, the temperature of wastewater remains relatively constant throughout the year, ensuring system stability.

Recovery of heat from wastewater using heat pumps is not a pioneering solution. In Switzerland, pilot studies on heat recovery from sewage network were already conducted in the 1980s [94]. Currently, wastewater heat pumps are one of the fastest developing energy-saving technologies, as evidenced by the dynamically conducted research in recent years on the subject of maximizing energy efficiency and reducing investment outlays for this type of investment. Research conducted on the subject of recovery of energy deposited in wastewater with the use of heat pumps mainly covers the technical and economic assessment of the legitimacy of their implementation and concerns environmental aspects [121]. There are also known analyses in which the authors took into account the integration of wastewater heat pumps and solar energy systems [122]. On the other hand, Gou et al. [123] drew attention to the possibility of using a dual-source heat pump to use energy from wastewater and air heat. Table 2 includes examples of papers published in recent years, the subject of which is devoted to the use of wastewater heat pumps, and the further part of the paper presents a SWOT/SOAR analysis of wastewater energy recovery technology using heat pumps (Figure 11).

In recent years, an increase in interest in wastewater heat pumps among decision-makers has been observed, which was influenced, among others, by an increase in the ecological awareness of the society, an increase in electricity prices, but also the need to implement the objectives of the climate and energy policy promoting the implementation of low-emission technologies. From the perspective of potential users of wastewater HP technology, the implementation of demonstration projects is also crucial, such as the Borders College project in Galashiels, Scotland. Todorović et al. [124] assessed that this project will contribute to popularizing the heat recovery technology from wastewater using heat pumps in the United Kingdom.

Wastewater heat recovery technology using heat pumps is successfully applied in Scandinavian countries as well [125]. Even in less developed countries, the first investments are currently being made to recover deposited heat energy from wastewater. For example, in Poland, there is an investment related to the installation of a heat pump that will be powered by energy obtained from wastewater and rainwater. The investment will be located at a wastewater pumping station near Wroclaw and is expected to supply up to 5% of the city’s annual district heating demand [126].

Table 2. Examples of the latest papers dedicated to the utilization of wastewater HPs (HP—heat pump).

Authors	Aim of the Paper	Source Title
Zhang et al. [120]	Analysis of the operation of a domestic hot water installation based on the use of solar collectors and a wastewater heat pump using the TRNSYS	Energy (Elsevier)
Todorović et al. [124]	Demonstration of a domestic hot water preparation system in a hotel based on a heat pump, where the lower energy source is greywater and rainwater	Journal of Thermal Analysis and Calorimetry (Springer)
Pokhrel et al. [119]	Economic, technical, and environmental analysis of utilizing two methods to replace conventional heat sources for space heating and water heating, including wastewater heat pump, in two multi-family buildings	Energy and Buildings (Elsevier)
Qin and Tian [127]	Demonstration of a wastewater heat pump system with a plate heat exchanger and wastewater filter	Heat and Mass Transfer (Springer)
Kim et al. [118]	Experimental research of two types of HPs applied in block heating and cooling networks	Energies (MDPI)
Zhu et al. [128]	Investigation of the relationship between various parameters related to the application of a wastewater HP, such as flow rate and wastewater temperature, thermal power, and system location	Urban Water Journal (Taylor & Francis)
Ali and Gillich [121]	Demonstration of the potential of raw and treated wastewater as a source of heat for HPs in London, along with the political implications for the UK’s district heating strategy	Building Services Engineering Research & Technology (Sage Publications)
Fadnes et al. [129]	Presentation of a modern thermal energy plant consisting of wastewater heat pumps, a biogas boiler, thermal solar collectors, and greywater recycling, the concept of which was created as a result of cooperation between three partners: the industrial plant designer, the municipal plant owner, and the local academic institution	Frontiers in Energy Research (Frontiers Media)
Łokietek et al. [35]	Determining the potential of wastewater heat recovery in various sections of a wastewater treatment plant on the example of a facility located in Poland	Energies (MDPI)
Wang et al. [130]	Analysis of the feasibility of utilizing reclaimed water from a wastewater treatment plant as a lower energy source for a heat pump to provide heating buildings in a district heating network	Energy Technology (Wiley)

Table 2. Examples of the latest papers dedicated to the utilization of wastewater HPs (HP—heat pump).

Authors	Aim of the Paper	Source Title
Zhang et al. [120]	Analysis of the operation of a domestic hot water installation based on the use of solar collectors and a wastewater heat pump using the TRNSYS	Energy (Elsevier)
Todorović et al. [124]	Demonstration of a domestic hot water preparation system in a hotel based on a heat pump, where the lower energy source is greywater and rainwater	Journal of Thermal Analysis and Calorimetry (Springer)
Pokhrel et al. [119]	Economic, technical, and environmental analysis of utilizing two methods to replace conventional heat sources for space heating and water heating, including wastewater heat pump, in two multi-family buildings	Energy and Buildings (Elsevier)
Qin and Tian [127]	Demonstration of a wastewater heat pump system with a plate heat exchanger and wastewater filter	Heat and Mass Transfer (Springer)
Kim et al. [118]	Experimental research of two types of HPs applied in block heating and cooling networks	Energies (MDPI)
Zhu et al. [128]	Investigation of the relationship between various parameters related to the application of a wastewater HP, such as flow rate and wastewater temperature, thermal power, and system location	Urban Water Journal (Taylor & Francis)
Ali and Gillich [121]	Demonstration of the potential of raw and treated wastewater as a source of heat for HPs in London, along with the political implications for the UK’s district heating strategy	Building Services Engineering Research & Technology (Sage Publications)
Fadnes et al. [129]	Presentation of a modern thermal energy plant consisting of wastewater heat pumps, a biogas boiler, thermal solar collectors, and greywater recycling, the concept of which was created as a result of cooperation between three partners: the industrial plant designer, the municipal plant owner, and the local academic institution	Frontiers in Energy Research (Frontiers Media)
Łokietek et al. [35]	Determining the potential of wastewater heat recovery in various sections of a wastewater treatment plant on the example of a facility located in Poland	Energies (MDPI)
Wang et al. [130]	Analysis of the feasibility of utilizing reclaimed water from a wastewater treatment plant as a lower energy source for a heat pump to provide heating buildings in a district heating network	Energy Technology (Wiley)

3.3.2. Strengths and Weaknesses

The use of wastewater heat pumps can efficiently reduce the energy consumption needed for domestic hot water preparation, space heating and cooling. In addition, properly adapted wastewater heat pumps enable direct cooperation with the district heating, reducing the consumption of energy from conventional sources [130].

The described wastewater heat recovery technology is versatile and flexible, which creates favorable conditions for its application on various scales. The lower source of energy in heat pumps can be both wastewater from individual systems [77] and those from extensive communal systems [131,132]. In the latter case, wastewater is discharged collectively from larger agglomerations, so there is a much greater potential for heat recovery due to the widespread availability of the raw material [133]. However, this requires incurring high investment outlays and implementation of the project on a large scale. On the other hand, the application of a wastewater heat pump in a building allows the process of heat recovery from wastewater to be carried out even by individual users [12]. The condition is having a sufficient amount of wastewater as a lower source of energy, which is why it is more commonly employed in larger facilities. Furthermore, the strength of systems based on the use of wastewater heat pumps is the widespread and stable availability of the lower energy source, which is wastewater, and the possibility of efficient operation of the system in an annual cycle [134]. Wastewater is considered a stable source of heat with a relatively high temperature and volume flux, and thus also a significant energy potential [41,135]. Moreover, the heat deposited in the wastewater, unlike some other renewable energy sources, such as solar or wind energy, is a solution independent of the prevailing weather conditions. It is available on a large scale, wherever people live or industrial facilities operate [42]. However, the difference between the availability and stability of the lower source of energy in the form of raw wastewater and treated wastewater should be noted. Raw wastewater is available over a larger area, which allows for the application of wastewater HPs practically in any location, including in close proximity to the recipients of recovered heat [80]. During the design phase of such a system, however, the risk of significant fluctuations in wastewater temperature and flow rate, both in daily and annual cycles, must be taken into account. This issue is particularly noticeable in the case of combined sewer systems. Additionally, contaminants present in wastewater may deposit on the heat exchanger surfaces, leading to reduced efficiency. Therefore, energy recovery from raw wastewater is often achieved using an additional heat exchanger [81], which increases investment costs and reduces system reliability. These drawbacks do not occur when utilizing treated wastewater. During the winter season when energy demand is at its peak, the temperature of treated wastewater may be slightly lower, but it can be further decreased by a few degrees. However, in many cases, the recovered thermal energy must be utilized within the wastewater treatment plant due to the considerable distance between the system and potential recipients of recovered heat [79].

The results of research [34,130] on the use of HPs in wastewater heat recovery systems confirm that this technology allows for significant energy savings, and thus financial savings. This applies to both heating and cooling systems. For this reason, the implementation of heat recovery from wastewater with the use of technology based on the use of heat pumps can have a real impact on reducing the effects associated with excessive greenhouse gas emissions and is part of the scope of activities consistent with the principles of sustainable development [136]. Considering that the use of wastewater HPs guarantees the recovery of much larger amounts of energy compared to wastewater heat exchangers, their potential for saving fossil fuel resources and reducing greenhouse gas emissions is significantly higher. Zhang et al. [33] demonstrated that they can even compete with heat pump systems based on more conventional energy sources, such as geothermal energy or air.

Another strength of heat recovery systems based on heat pumps is the ability to combine waste energy utilization with other renewable energy sources, such as solar energy [129,137]. For example, Chae et al. [138] conducted research on the energy activity of a wastewater treatment plant using photovoltaic panels, small hydropower plants and an installation for collecting energy from wastewater based on a heat pump. As a result, they created an efficient hybrid energy system based on zero-emission technologies.

Systems based on wastewater HPs are characterized by a wide range of applications, making them an attractive solution that can be implemented in various types of installations and buildings. At the same time, the heat recovery technology using HPs is constantly modernized and optimized [134]. As a result, there are many proven and technologically advanced solutions for wastewater heat pumps available in the market [12]. A wide range of options is also available for heat exchangers that work in conjunction with wastewater HPs. Depending on the size and condition of the sewer pipes, as well as the prevailing hydraulic conditions, heat exchangers can be used within the pipes or outside of them. In the first case, there are heat exchanger models integrated into the sewer pipe structures, models designed for installation within existing pipes, as well as solutions where installation can be combined with the renovation of sewer pipes in poor technical condition.

As in the case of heat recovery technology based on the use of DWHR units, the investment costs associated with the application of wastewater HPs depend on several variables, including the scope of required installation work, system location, and the specific heat pump solution chosen [12,139]. Typically, such investments require increased financial outlays, especially if waste heat recovery concerns large-scale systems and heavily polluted wastewater, e.g., industrial wastewater, which may negatively affect the efficiency and reliability of heat pumps [140]. Therefore, regular maintenance and monitoring of the entire system are crucial to ensure its optimal and trouble-free operation. The increased investment costs for such projects also result from the need to use more advanced materials with enhanced corrosion resistance and resistance to the chemicals present in wastewater. This issue is particularly relevant when dealing with industrial wastewater. Hence, from the perspective of eliminating potential failures and ensuring proper operation of the heat recovery system, it is essential to adapt the technological setup of the HPs system to the type and quality of wastewater [32].

The category of weaknesses related to the application of technology based on wastewater HPs also includes the existence of limitations in the collection of waste heat, caused by the adverse effect of low wastewater temperatures on the processes of their treatment. This issue pertains to raw wastewater. Due to the recommended optimal values of their temperature in individual treatment processes and the need to protect receivers to which treated wastewater is discharged [138,141], it is not recommended to lower the temperature of wastewater below the legally defined limit value. For example, in Poland it is 12 °C. In many countries, the temperature limits of wastewater fed to WWTPs are determined based on the characteristics of the geographical area. In the United States, the optimal temperature of wastewater at the inflow to the treatment plant is 15 °C [142]. However, the European Investment Bank (EIB) report [143] states that depending on the region of the European Union (assuming that the minimum wastewater temperature at the inflow of treatment plants is not specified in national law), this temperature should range from 10 °C to 25 °C. Therefore, the concept of waste heat recovery in a given wastewater system must be coordinated with local authorities, and the project should be included in municipal planning [143]. Wanner et al. [19] demonstrated that reducing the temperature of raw wastewater by just 1 °C can lead to a 10% increase in the required capacity of the nitrification chamber. Hence, each investment in a raw wastewater heat recovery installation requires an individual approach and detailed analysis, for example, using dedicated computer simulation software [125]. In some cases, the optimal solution may involve only slight cooling of the wastewater, while in other circumstances, the construction of a bypass may be necessary.

3.3.3. Opportunities and Threats

Analyzing the opportunities for the development of waste heat recovery technology with the use of wastewater heat pumps, it was pointed out that these systems may positively affect the improvement of the region’s energy balance through local use of the widespread availability of waste heat resources [121]. Such action favors the diversification of energy sources in the region and may reduce dependence on conventional energy sources, while increasing the energy security of the residents [144].

It should be noted that the development and selection of appropriate operating parameters of HPs technology requires specialist knowledge, which generates a demand for specialists and engineers, especially in the field of engineering sciences, including power engineering. The implementation of wastewater heat pumps affects the development of the industry related to renewable energy sources in terms of production, assembly, and service of these devices, which in turn may positively affect the support of local enterprises, specialists, and suppliers, contributing to the economic growth of the region [79]. Moreover, the development of infrastructure based on renewable energy sources, including waste energy from wastewater, may have an impact on the development of external investments, and even the implementation of research grants in the field of energy and environmental engineering in the context of the development of low-emission technologies and the climate change mitigation.

The prospect of changes, including tightening of legal regulations requiring the reduction of CO₂ emissions and the increase in the use of renewable energy sources, may be an incentive for potential recipients of technology based on HPs [40]. The introduction of new regulations may encourage greater cooperation between the public and private sectors to promote wastewater heat pump applications. Such cooperation can significantly accelerate the development of waste heat recovery technology, while enabling the achievement of ambitious climate and energy policy goals [145]. However, it should be noted that in some countries, the prospect of wastewater heat recovery is still not considered in guidelines and regulations. Even in the European Union (EU), where wastewater has been officially recognized as a renewable energy source, there are countries where the thermal energy potential of wastewater has not been recognized by those developing energy strategies. Therefore, potential changes in legal regulations can be considered both as opportunities and threats. The vision of introducing unclear or simply unfavorable legal regulations that may limit investments and development of this technology is still apparent. As a result, companies may be less willing to invest in research and development of wastewater heat pumps, as unclear legal regulations do not provide certainty as to market stability and long-term development prospects [146]. Additionally, complicated and costly procedures related to obtaining appropriate permits and certifications necessary for the admission of this technology to general sale may be problematic.

Significant with regard to potential threats to HP-based systems are fluctuations in energy prices. Their increase will noticeably increase the profitability of the implementation of wastewater heat pumps. In addition, as in the case of the technology based on wastewater HEs, the increase in energy prices predicted by analysts and the possible introduction of support programs for financing energy recovery technology from wastewater may significantly popularize this technology both among local government units and individual decision-makers. On the other hand, lower energy prices will result in reduced financial benefits, which may translate into a decrease in interest in this technology.

A significant barrier to the implementation of wastewater heat pumps may be the lack of societal acceptance for such technologies. Such a phenomenon may result from the lack of knowledge of waste heat recovery technology and may raise concerns among decision-makers about the existence of potential threats related to the operation of HPs [42], e.g., contamination of water intended for human use at the stage of its introduction to the heat recovery system. A significant part of society may be skeptical about new technologies only due to the lack of basic knowledge related to its functioning.

Issues with integrating wastewater heat pumps to existing energy systems can also pose a significant threat to the efficient implementation of this technology. The lack of appropriate infrastructure may make it difficult or even impossible to efficiently connect wastewater heat pumps to district heating or cooling systems [147]. In some cases, problems may arise in obtaining permits to connect wastewater heat pumps to existing municipal networks, and operators of existing energy systems may not be interested in collaborating to integrate wastewater HPs.

3.3.4. Aspirations and Results

Defining the aspirations resulting from the development and implementation of wastewater heat pumps, it was indicated the need to conduct research in the field of optimization of this technology in order to increase the energy efficiency of HPs. This includes the introduction of new, more advanced technological solutions and materials, which may contribute to minimizing heat losses during energy transfer between the wastewater and the intermediate. Simultaneously, it is crucial to conduct research aimed at reducing production costs and enhancing the reliability of system components. It is also important that HPs scientists and system designers aspire to develop wastewater heat pump solutions in a way that allows them to be easily integrated into existing heating and cooling systems. Another important aspect is also the need to conduct educational campaigns that will increase public awareness of the benefits and principles of operation of wastewater heat pumps, as well as the involvement of local communities in the process of planning and implementing investments in the field of renewable energy sources. It also seems necessary to cooperate with local and government authorities to introduce favorable regulations and financial support that can stimulate the development and popularization of wastewater heat pumps.

In the coming years, it can be expected that the result of activities related to the development of HPs technology will be the possibility of achieving a significant reduction in energy consumption, especially that from conventional sources, and thus reducing CO₂ emission. Such a scenario would make it possible to meet the energy goals set by the EU and other communities. The development of HPs technology can also have a significant impact on the transformation of the model of energy systems in a more sustainable and ecological direction. Moreover, the recovery and use of energy from wastewater will be an important tool for diversifying energy sources used in municipal networks, in residential and service buildings and industry. The group of results also includes the development of qualifications of employees who in the future will deal with the design, assembly and operation of technology based on wastewater HPs.

3.4. Heat Recovery in Industrial Facilities

In industrial facilities, significant amounts of waste heat are often generated as a by-product of various production processes. These frequently unrecovered heat sources are lost without being utilized, which is unjustified and financially disadvantageous for the company and harmful to the environment. Considering the increasing energy prices, it is crucial to optimize its utilization by implementing energy-efficient technologies and waste heat recovery systems, including recovering heat from industrial wastewater. The potential for utilizing waste heat is closely related to the type of industrial activity and the scale of production in specific company. Large factories and industrial plants, such as laundries, chemical, petrochemical, or pulp and paper mills, produce substantial amounts of high temperature industrial wastewater (>40 °C) [148,149], which supports the use of technologies enabling waste heat recovery. This heat can be reused in the plant’s technological processes and can also be utilized for industrial facilities’ utility purposes, such as space heating or preparing DHW [150]. Implementing waste heat recovery systems not only reduces the company’s energy costs but also contributes to environmental protection by minimizing the wasteful discharge of excess heat into the environment.

Despite the potential requirement for expensive technology and infrastructure, current research [140,151] showed that the long-term financial benefits associated with energy savings in industrial facilities confirm the legitimacy of using heat recovery systems from industrial wastewater [32]. According to Etemoglu [152], reusing waste heat will become one of the most crucial actions for energy generation in the industry in the near future.

Depending on the nature of the industrial facility and the wastewater produced within it, both heat pumps and heat exchangers can find application in recovering waste heat. Examples of manuscripts published in recent years, related to waste heat recovery from industrial wastewater, are presented in

Table 3. Examples of the latest papers on the use of wastewater HPs in industrial plants.

Authors	Aim of the Paper	Source Title
Seo et al. [32]	The efficiency research of three design variants of a high temperature wastewater heat recovery system in the textile industry, consisting of a heat exchanger and a water tank; the three analyzed configurations of the integrated heat recovery system were developed based on the lower temperature limit of the wastewater	Energies (MDPI)
Dimoglo et al. [140]	Research conducted for a pilot installation concerning the reuse of wastewater from industrial laundries, including the analysis of the efficiency of waste heat recovery	International Journal of Environmental Science and Technology (Springer)
Lu et al. [153]	Thermodynamic and economic analysis of a high temperature cascade heat pump system using low-temperature wastewater heat to generate steam for industrial processes	Processes (MDPI)
Sun et al. [154]	Research on the prototype of a sinusoidal-corrugated plate heat exchanger and thermoelectric modules for heat recovery from industrial wastewater, including comparative analyses of heat exchange efficiency with a plate heat exchanger	Energy Conversion and Management (Elsevier)
Kim et al. [151]	Research on the energy efficiency of the heat exchanger network in the heat recovery system from wastewater from the textile industry	Energies (MDPI)
Oliveira et al. [155]	Variant analysis of water and energy consumption reduction in a process industry using three different optimization methodologies	Frontiers in Chemical Engineering (Frontiers Media SA)
Hu et al. [156]	Research on a new centrifugal heat pump for recovering waste heat from a steel plant for district heating purposes, including environmental and economic analysis of the waste heat recovery system	Energy (Elsevier)
Szkarowski et al. [150]	Research on two heat exchangers used for thermal utilization of industrial wastewater generated in the plant for the production of refined fibreboards	Rocznik Ochrona Środowiska (Middle Pomeranian Scientific Society of the Environment Protection)
Maddah et al. [157]	Comparative analysis of energy, exergy, economic, and environmental aspects of an industrial wastewater heat pump with an air-source heat pump and a geothermal heat pump	Journal of Cleaner Production (Elsevier)
Yang et al. [158]	Research on the Organic Rankine Cycle system using an evaporative condenser to recover heat from wastewater in the petrochemical industry	Energy Conversion and Management (Elsevier)
Etemoglu [152]	Analysis of the efficiency of industrial combined heat and power generation systems based on waste heat including the analysis of the selection of the working fluid	International Communications in Heat and Mass Transfer (Elsevier)

. Considering the diversity of wastewater generated in different industrial sectors and even within different branches of the same industry, it is impossible to determine a single strategy for developing industrial wastewater heat recovery systems. However, undoubtedly, such systems have the potential that should be individually assessed for each facility.

3.5. Heat Recovery from Wastewater in Real Conditions

3.5.1. Examples of the Use of Wastewater Heat Recovery Technology

Wastewater heat exchangers are solutions that can be successfully used in residential buildings and public utility buildings, both in new and existing buildings. This technology is known and used primarily in the USA and Canada. In Europe, HEs are implemented mainly in the Scandinavian countries, as well as in Switzerland [40], and although the bibliometric analysis indicated a significant interest in wastewater heat exchangers, in fact the implementation of this type of solutions in residential buildings is not common [159]. To encourage the public to use DWHR units, the Canadian Green Building Council enabled it to receive a point in the Efficient Domestic Hot Water Equipment category in the LEED Canada for Homes 2009 rating system [89]. In Poland, this technology has been commercially implemented in residential buildings in Kędzierówka, where the world’s first housing estate is located, where all the energy needed to heat water comes from renewable sources [160].

HPs technology is much more common. The world’s first wastewater heat pump was developed and commissioned in 1981 in Sweden. This technology was to be used for heating in residential buildings [34]. Currently, wastewater heat pumps are commonly used, for example, to heat swimming pool water, enabling the maintenance of a comfortable temperature in the pool while reducing energy consumption for domestic hot water preparation. In addition, HPs are known in facilities in Sweden, Denmark, Finland and Switzerland and other European countries. There are many examples of the implementation of large-scale heat recovery systems based on the use of HPs in urban networks, including:

• Lund, Sweden (treated wastewater) [161],
• Västerås, Sweden (treated wastewater) [161],
• Turku, Finland (treated wastewater) [162],
• Kalundborg, Denmark (treated wastewater) [163],
• Oslo, Norway (raw wastewater) [164],
• Budapest, Hungary (raw wastewater) [165].

HPs are also used in the post office building in Muelligen or, for example, in the wastewater treatment plant in London [121]. The potential of wastewater heat recovery technology using heat pumps was also described by Łokietek et al. [35] on the example of a wastewater treatment plant in Poland. In recent years, a wastewater heat recovery system has been implemented in Hamburg with the use of a 100-metre heat exchanger placed in the sewer pipe and four heat pumps, which annually provide about 2000 MWh of energy for the needs of over 200 apartments [166]. HPs are also successfully used in hotels. Zhang et al. [33] studied the energy, environmental and economic performance of a hotel wastewater heat pump in China, while Todorovic et al. [124] analyzed the legitimacy of using a wastewater heat pump to prepare domestic hot water in a hotel in Serbia comparing this solution to a conventional gas boiler. The rooms of the office building in Singen have been heated in winter since 2004 with heat coming from the recovery of wastewater energy, which is received by means of an 80-m heat exchanger placed in the sewer pipe and a heat pump. In summer, this system is used for cooling [166].

3.5.2. Evaluation of the Economic Aspects

Economic analyses of wastewater heat exchangers showed that this technology can pay off in noticeable financial benefits resulting from the reduction of energy consumption, but its profitability depends on a number of factors, such as the efficiency of the DWHR unit, the price of energy, as well as the purchase costs of the heat exchanger [24]. Some authors [167] have noted that the payback period may be as little as two years. In some cases, however, it may be extended to several or more years, and in extreme cases even exceed the period of technical life of the unit [100]. There are known studies [21] in which the authors showed that due to the cost of the DWHR unit, the use of a greywater heat recovery system cooperating with a gas water heater may turn out to be economically unjustified because the investment payback period exceeds the probable technical durability period of the tested horizontal heat exchanger. In any case, however, the profitability of the investment will increase with the extension of the life of the HE and the increase in water consumption per shower. In general, the application of high efficiency vertical HEs is the most cost-effective, but in many cases it may not be possible due to the lack of space for the installation of the unit. In such a situation, it is possible to use horizontal units, which are usually characterized by lower efficiency. Therefore, worth mentioning are the results of research on the use of baffles in the construction of DWHR units, which can improve the efficiency of the heat exchanger by up to 40% [23]. These baffles incur a negligible additional capital expenditure and can be used in both new and existing heat exchangers. As a result, the implementation of this solution allows for a significant reduction in the costs incurred for the preparation of domestic hot water [23]. On the basis of the published analyses it was noticed that in Polish conditions, the use of a DWHR unit in buildings is more financially advantageous when the heat source is an electric water heater than a gas water heater, which results from the fact that gas prices in Poland are lower than electricity prices [23,100]. In addition, studies [93,95,120] confirmed that the integration of wastewater heat recovery systems and renewable energy sources has great potential to be a financially viable technical solution in the residential sector, especially for areas with a cool climate and significant insolation. Analyses conducted for a WHRS using the multi drain heat recovery system in a residential building in Lebanon have shown that it is possible to achieve savings due to the reduction of energy demand for hot water preparation from about $13 to $53 per month [168].

Analyses published in recent years on the economic viability of using HPs [129,139] indicated that it is possible to develop an efficient energy system based on wastewater energy and solar energy. Li and Wang [169] showed that the use of wastewater HPs in buildings deserves popularization due to the relatively low investment outlays and dynamic annual computation cost of the HP compared to other heating and cooling technologies. Hirvonen et al. [170] have also shown that the use of wastewater as an additional source of energy for a ground source heat pump can improve system sustainability and reduce its life cycle costs by up to 20%. The use of wastewater HPs, like other renewable energy sources, can also be profitable for larger installations. For example, Sandvall et al. [171] noted that the use of urban excess heat, including heat from wastewater, in district heating systems can significantly improve their competitiveness compared to individual heating systems. However, Nielsen et al. [172] pointed out that urban excess heat sources may affect local district heating systems, while their impact on national energy systems will be negligible.

4. Conclusions

The analysis of the results of research on the recovery of thermal energy from wastewater has demonstrated that this process can be an efficient tool for achieving the Sustainable Development Goals (SDGs). This is particularly applicable to ensuring universal access to energy (SDG 7) and mitigating climate change (SDG 13). However, that is not all. An environmentally sustainable approach to managing energy resources, such as recovering waste energy, can indirectly contribute to the development of sustainable industries, the establishment of resilient cities, and the preservation of ecosystems. These issues relate to the implementation of sustainable development goals such as SDG 9, SDG 11, and SDG 15.

The bibliometric analysis of 504 papers indexed in the Web of Science database led to the following conclusions:

• The greatest interest in research on wastewater heat recovery is observed in such countries as China, USA, Turkey, United Kingdom, and Sweden.
• The most influential journals in terms of number of publications are Energy and Buildings, Applied Thermal Engineering and Energies, while in terms of the average number of citations it is Energy Conversion and Management.
• The number of publications on wastewater heat recovery has been growing steadily since 2007. In recent years, it ranged from 30 to 50 items per year.
• The keywords referring to the use of wastewater heat recovery technology in buildings and wastewater treatment plants are most often cited.
• In recent years, scientists have focused on analyses related to the optimization of heat recovery systems and the impact of this technology on the environment, as well as on the integration of wastewater heat recovery systems with other alternative energy sources.

The bibliometric analysis was extended with a SWOT/SOAR strategic analysis in order to identify internal and external factors determining the current state and directions of development of wastewater heat recovery technologies using HEs and HPs. The results of the analysis indicated a significant potential of HEs and HPs for the recovery of energy deposited in wastewater, which results from the existence of a number of often similar positive aspects accompanying the implementation and development of wastewater heat recovery technologies. The SWOT/SOAR analysis led to the following conclusions:

• The use of wastewater HPs is particularly recommended in places where significant amount of wastewater is available as an energy source. These can be WWTPs, sewage networks or large residential and commercial buildings. In smaller building installations wastewater HEs can be successfully used.
• Both methods of recovering the heat deposited in wastewater are an effective tool for implementing the circular economy and sustainable development goals.
• Wastewater is a relatively stable and widely available source of energy that can be used together with other renewable energy sources, both in local systems and large-capacity installations.
• Recovering heat from raw wastewater, even for local systems, carries the risk of potentially adversely affecting wastewater treatment processes. For this reason, the scale of heat recovery should be adapted to individual conditions each time.
• The selection of the WHRS and the materials used should be preceded by a thorough analysis of the quality of wastewater, which may have a potentially aggressive impact on the components of the installation.
• Properly conducted pro-environmental and legislative policy, educational initiatives for society and an effective system of financial incentives, which will result in increasing the profitability of wastewater HEs and HPs and improving social acceptance, may contribute to the application of WHRSs.
• Optimization of wastewater heat recovery technology, production costs of individual devices, as well as their installation are also of great importance.
• The increase in the degree of implementation of WHRSs, especially HPs, will increase energy security through diversification of the energy sources used, which will also improve the condition of the economy and the environment.

Continuing research in the integration of wastewater heat recovery systems with other alternative energy sources, both at the building level and on a broader scale, appears to be justified due to the requirements imposed by the EU on member states. In the coming years, studies should focus on the issues of threats and weaknesses related to the use of the heat recovery technologies. Such analysis can play a crucial role in the implementation of heat exchangers and heat pumps, as well as in the optimization and commercialization of new devices and heat recovery systems. These studies are immensely important for achieving sustainable development goals, promoting energy efficiency, and reducing CO₂ emission, which are priorities in the pursuit of climate and energy policy objectives.