The rational use of greywater in buildings reduces the consumption of many resources, such as water, heat, and electricity. In order to achieve full utilization of greywater, it is necessary to identify its resources, determine their availability, identify how to manage them, and find new reuse and recycling processes. Therefore, this section conducts a retrospective analysis of selected publications on the issue of greywater energy recovery, as well as articles addressing greywater recycling in buildings. A thorough understanding of the processes and technologies of the two main ways of reusing greywater will help identify weaknesses and possible ways of implementing them together.
3.1. Greywater Heat Recovery
The importance of domestic hot water preparation in the balance of energy consumption in buildings has been clearly increasing in recent years. Amanowicz [
58] indicates that this applies not only to the annual demand for energy but also to the selection of the peak power of the heat source. However, due to the complexity and multiplicity of factors determining the demand for domestic hot water, unambiguous prediction of its consumption is difficult [
59]. Therefore, it is necessary to search for ways of preparing domestic hot water that will ensure the comfortable and reliable operation of the installation for many years. In the face of the current energy crisis and the increase in energy prices, the diversification of energy production is also gaining importance [
60]. Østergaard et al. [
61], after analyzing the feasibility of low-temperature heating in buildings, identified heat recovery from wastewater as an effective way to reduce energy for domestic hot water preparation. On the other hand, Wehbi et al. [
62] noted that by using a wastewater heat recovery system, up to 50% of energy could be recovered in some cases.
The first articles on the use of energy carried by greywater appeared in 2001 [
63,
64]. These were technical notes in which the authors drew attention to the possibility of recovering energy contained in wastewater discharged from different therapy systems installed in the spa, including showers. For the next few years, the topic was not developed. It was not until 2009 that further publications appeared in which the issue of greywater energy recovery was addressed (
Figure 2). Therefore, it can be assumed that the increased interest in energy conservation and the use of waste heat was related to the global economic crisis. Initially, these were articles on the comparison of different ways of preparing domestic hot water, including systems based on energy recovery from greywater [
65,
66]. In the following years, research began to focus on drain water heat recovery (DWHR) systems [
67,
68]. This confirms that heat recovery from drain water is a relatively new technology [
69]. The authors analyzed both vertical [
70] and horizontal [
26] heat exchangers. Some publications that appeared at the time also highlighted the issues of using greywater as a lower energy source for heat pumps [
71,
72]. In the following years, research on energy recovery from greywater was continued, and the number of publications steadily increased. More and more emphasis was placed on the development and analysis of new DWHR units. For example, research by Morales-Ruiz et al. [
73] and Torras et al. [
74] concerned storage-type units. On the other hand, the subject of analysis for Gabor et al. [
75] was the heat pipe heat exchanger. Some authors centered their research on assessing the possibility of cooperation between the DWHR system and the heat pump [
76]. Others focused on evaluating the efficiency of the hybrid system [
77]. The papers published after 2016 also included numerical analyzes [
78], which was probably due to the development and increase in the popularity of computer tools. The topics of articles published in recent years do not differ significantly from those of older publications. Analyses concerning the modeling of DWHR systems [
79] and combined systems [
80], the use of greywater as a lower energy source for heat pumps [
81], and the assessment of the efficiency of new heat exchanger designs [
21,
82] are still being carried out. However, more and more publications address social, financial, and environmental issues. In order to show the scope of research undertaken in recent years,
Table 4 presents selected new research papers on greywater energy recovery.
Figure 3 presents the keywords co-occurrence maps of greywater energy recovery papers, which were generated using the VOSviewer software. Both Author Keywords and Keywords Plus from Web of Science were included in the analysis. Both of these groups of keywords are useful for studying the structure of knowledge in a scientific field [
98]. The size of nodes in the network is determined by the frequency of occurrence of the given keyword. The distance between the nodes indicates the frequency of co-occurrence of two keywords (the closer the nodes are to each other, the greater the correlation between them). And the thickness of the lines connecting nodes indicates the frequency of occurrence of two keywords in publications.
In addition,
Table 5 summarizes the most important information on the assignment of keywords to clusters, total link strength, occurrences, and the average number of citations. There were a total of 422 different keywords in the publications. Due to their large number, the analysis was limited to those keywords that appeared in at least five articles. This reduced the number of expressions to 30. The keywords and the nodes representing them were divided into four clusters. Each of them was assigned a different color on the map (
Figure 3a). Most keywords (10) were assigned to cluster 1, marked on the map in red. Subsequent clusters were assigned numbers in the order corresponding to the number of assigned elements. Therefore, the cluster number does not reflect the total number of connections and their strength. The individual keywords were connected by 296 lines with a total link strength of 601. It is worth noting that the values describing the number and strength of links are assigned to each keyword individually. Therefore, the total number of links resulting from the data presented in
Table 5 is twice as high as for the entire network.
The analysis showed that the keyword “performance” has the highest total link strength. It appeared in 27 publications, making 29 links with a total link strength of 111 with the other elements on the map. The word “waste-water” came in second place with a total link strength of 74, followed by such elements as “efficiency”, “system”, and “design”. This proves that research in the greywater energy recovery domain places great emphasis on improving the efficiency of dedicated systems and developing new heat exchanger designs. By contrast, analyzing the average number of citations, it can be seen that the phrase “residential buildings” is the most popular, appearing in only 7 publications. This reflects a significant interest in the use of sustainable energy technologies at the residential level and indicates the need for further research in this area.
The research was further extended by analyzing Author Keywords appearing in articles on greywater energy recovery. This is because they are more comprehensive in presenting the content of the article [
98]. The results of the analysis are shown in
Figure 4.
The phrase that appears most often is “energy efficiency.” It was used for the first time in 2013 and had been appearing regularly in subsequent publications since then. This is a positive trend, probably stemming from the need to meet environmental goals. Economic and political issues are also important motivators for improving the energy efficiency of heat recovery systems, as improving system efficiency, and reducing energy consumption make it possible to reduce dependence on energy imports, thus contributing to the implementation of SDG 7. However, it is worth noting that the keyword “energy efficiency” has replaced the term “energy saving” in publications. Therefore, energy conservation alone is not currently an indicator of sustainability. It should go hand in hand with process optimization. Only slightly fewer occurrences were recorded for the phrases “waste heat recovery”, “heat recovery”, and “heat exchanger”. The phrases “drain water heat recovery” and “energy recovery” were also among the most frequently used Author Keywords. All these keywords refer to the issue of recovering the energy carried by greywater. Among them, the phrase “heat exchanger” has seen the highest growth in recent years. This demonstrates the need to develop new systems for energy recovery from greywater. The new DWHR units should be characterized by high efficiency of heat recovery, and their design should allow for installation in all conditions, regardless of the purpose of the building and the configuration of internal installations. Among the most popular keywords was also the term “domestic hot water”, which was used for the first time in 2017. It can be concluded that the increased interest in technologies that allow the reduction of energy consumption for water heating is a consequence of the growing share of DHW preparation in the total energy consumption of buildings.
As part of the research, global geographic distributions of publications in the greywater energy recovery domain were also analyzed (
Figure 5). The analysis showed that the largest number of publications were developed in countries such as Canada, Poland, the United States, and China. Western European countries, including France, the UK, Switzerland, and Spain, also have a relatively large number of publications. In contrast, there is a noticeable lack of interest in this subject in South American countries and most African countries. The exception is Lebanon.
The next stage of the analysis was the identification of the most productive journals in the greywater energy recovery domain.
Table 6 lists journals with more than 5 articles published in this subject area. And
Figure 6 shows the number of publications in the top eight journals. The largest number of articles (16) was published in the journal
Energy and Buildings. In this case, the first article was published in 2013, and more are published every year. Second place was taken by the journal
Applied Energy, which also has a high total number of citations. However, it is worth noting that almost half of this value represents citations of a single article, which was published in 2010 and is one of the most cited publications in the set (
Table 7). The same number of papers were published in the journal
Energies. However, these are more recent publications. Most of them were published after 2020. Thus,
Energies is now becoming a leader in promoting the idea of energy recovery from greywater. The list in
Table 6 ends with the journal
Applied Thermal Engineering, where most of the articles are from before 2017. Other journals published up to three papers on greywater energy recovery.
In addition, the three-field plot, which is often used in bibliometric analyses, was developed using Bibliometrix software. The Sankey diagram presented in
Figure 7 shows the relations between countries, authors, and keywords of publications on greywater energy recovery research. The maximum number of elements in each field is 20. The individual elements of the diagram are illustrated by rectangles of different colors. Their height relates to the sum of the relationships between the elements.
Figure 7 also shows links made between the elements. For example, research by authors from Switzerland, a country with some of the most influential research topics, focuses mainly on energy efficiency, DHW preparation, energy recovery, and the use of heat pumps. Based on
Figure 7, it is also possible to assess in which parts of the world research is being carried out on the topic. For example, research related to falling film drain water heat recovery units is mainly carried out by authors from Canada.
The diagram shown in
Figure 7 further assesses the extent of international cooperation in the field of greywater energy recovery. It is clear that the authors representing the countries with the highest number of relationships (Poland, Switzerland, Canada) do not establish international cooperation in their research. The situation is different in the case of authors from France, Lebanon, the United States, or China.
3.2. Greywater Recycling
Global freshwater consumption for agricultural, industrial, and municipal purposes has increased nearly sixfold since 1900, with a marked slowdown since 2000 [
103]. For buildings, water consumption and its variability over time are determined by a number of factors, including the type of building, the type of plumbing fixtures and appliances installed, as well as weather conditions [
104,
105]. As a result of water consumption, significant amounts of wastewater are produced, of which about 40% [
106] to as much as 80% [
107] and more [
108] is greywater. Studies show that GW reuse can reduce unnecessary disposal of water resources and provide an alternative source of water in water-stressed areas [
109]. Reducing drinking water consumption can also contribute to minimizing the amount of wastewater produced in the city [
110]. In addition, Safarpour et al. [
111] indicate that technologies to reduce wastewater production are a sustainable option in terms of economic and social aspects.
Publications dealing with greywater recycling appeared as early as the beginning of this millennium [
112], as indicated by
Figure 8. The first papers addressed the issue of using greywater as an alternative water source in single-family houses or small public buildings [
113]. In the case of articles published at the beginning of the 21st century, there is a tendency to treat greywater recycling as a form of water conservation in underdeveloped countries with a significant share of people living under international poverty lines [
114].
The increased interest in the topic of greywater recycling resulted in increased awareness of the risks associated with the use of greywater and the resulting sanitation requirements [
115]. An often-discussed aspect of greywater recycling was the financial profitability of greywater collection and storage facilities [
116]. The financial viability of GWRSs was the factor determining the comparison of these systems to rainwater harvesting systems and the analysis of how the two systems could work together to maximize water savings [
117,
118]. It is worth noting that publications addressing these issues are among the most cited papers in the greywater recycling domain (
Table 8). In the following years, there was also a discussion of public attitudes toward greywater reuse. Such publications were mainly based on surveys of community groups using greywater recycling systems or considering this solution in their place of residence [
119]. Further interest in the topic resulted in publications addressing the implementation of greywater recycling techniques with consideration of minimizing investment costs [
120]. In recent years, greywater recycling has received a lot of attention from both researchers and potential users. Publications that have appeared in the last few years focus on both the social attitudes regarding the use of greywater [
10] and the economic aspect of GWRSs implementation [
121,
122]. Thus, these topics are still relevant. However, more and more authors are focusing on environmental and legal issues.
Table 9 shows selected publications from the last five years, along with the number of citations according to the Web of Science database.
Figure 9 presents the keywords co-occurrence maps of greywater recycling papers that were generated using the VOSviewer software. On the other hand,
Table 10 shows the keywords that make up the clusters and their assignment to each group. A total of 1009 keywords were analyzed. As was the case with the analysis of publications on energy recovery from greywater, the analysis was limited to those keywords that appeared most frequently in the papers. Due to having more than twice as many records, the minimum number of occurrences of a keyword (Author Keyword or Keyword Plus) was assumed to be 10. As a result, the number of keywords was reduced to the 32 most common phrases. As with the papers in the greywater energy recovery domain, the keywords were divided into 4 clusters, with each cluster represented by a different color on the map (
Figure 9a). The largest number of keywords (13) were assigned to cluster 1, marked in red. Cluster 2 (green) was assigned 11 keywords. There are 5 phrases in cluster 3 (blue), while 3 keywords were assigned to cluster 4 (yellow). The nodes representing the keywords were connected by 380 links with a total link strength of 1241.
Analysis using the VOSviewer software showed that the most frequently occurring phrase was the keyword “greywater” (cluster 4). It generates 31 links with a total link strength of 242. The second most frequent keyword was “reuse”, with 31 links with a total link strength of 221. The phrases “greywater reuse”, “grey water”, “water”, and “water reuse” were also relatively common. All of these keywords are directly related to greywater recycling and water resource management. Phrases such as “rainwater”, “rainwater harvesting”, and “rainwater harvesting systems” were also among the most frequently used keywords. This proves the considerable interest in the possibilities of using greywater and rainwater together and the significant potential of such combined systems. In addition to keywords directly related to alternative water sources, the phrases “management” and “performance” often appeared in the publications. This demonstrates the continuous effort to improve the efficiency of GWRSs and the concern for their reliable operation throughout their lifetime. Keywords related to the quality of greywater appeared in 38 papers. Thus, even for publications focused on technological and social issues, reference to the quality of the alternative water source is sometimes unavoidable. In addition, “sustainability” is a relatively frequent keyword, indicating that the implementation of the GWRSs can contribute to the achievement of the 17 SDGs.
When analyzing the average number of citations per keyword, it can be seen that the most frequently cited keyword (47.8 times on average) is the phrase “technologies”, despite the relatively low number of occurrences, equal to 11. A high average number of citations was also recorded for the phrase “potable water savings”. Therefore, the analysis indicates a significant interest in new tap water conservation techniques that can contribute to ensuring access to water for a wide range of consumers.
As in the case of greywater energy recovery papers, the review was extended with an analysis of Author Keywords (
Figure 10). The most common expression is “greywater”. The number of its occurrences is almost twice as high as the next highest-ranked keywords, and the largest increase in its use was recorded after 2017. Keywords such as “greywater reuse”, “reuse”, or “water reuse” are also used relatively often. All these expressions relate directly to the recycling of greywater. The phrases “rainwater harvesting” and “rainwater” were also among the Author Keywords with the highest number of occurrences. Single occurrences of these keywords appeared before 2010. This confirms that the potential of the combined use of various alternative water sources was noticed several years ago. However, it is in the last decade that the boom of papers in this field has become apparent. Some authors focus only on these two alternative water sources [
136,
137], while others analyze more [
138]. Phrases such as “sustainability” and “water conservation” were also among the most common Author Keywords. They are characterized by a similar number of occurrences and, starting in 2019, appear regularly in publications in this domain. Thus, the analysis confirmed that the implementation of GWRSs can contribute to SDG 6 by reducing potable water consumption and providing access to water.
The review also analyzed the global geographic distributions of publications in the greywater recycling domain (
Figure 11). This made it possible to identify the countries that are the most productive in greywater recycling publications. Most papers were published by authors from the United States and the UK. It is worth noting that in these countries a lot of publications on greywater energy recovery have also been developed. This creates opportunities for the authors to collaborate in research on the simultaneous recovery of energy from greywater and its recycling in internal installations of buildings. Chile, China, and Israel followed, with Australia, India, and Brazil close behind. Authors representing all these countries published a total of more than 20 items each. The number of papers prepared in other regions of the world is significantly lower. However, it should be noted that the topic of greywater recycling is taken up by representatives of different nationalities living on different continents. Therefore, it is far more widespread than the topic of greywater energy recovery, despite the fact that energy sustainability issues are currently one of the main research topics. This may be due to the fact that water is essential for life. As a consequence, the need to conserve water resources is familiar to representatives of many communities, regardless of the level of economic development. This need may be a result of water deficits, but it may also be due to an increase in the environmental awareness of the population.
As in the case of publications in the greywater energy recovery domain, the most productive journals were identified. The results of the analysis are presented in
Figure 12 and in
Table 11. Due to the significant number of publications,
Table 11 lists only those journals with more than 10 papers. The highest number of articles is from
Journal of Cleaner Production (19). The first of them was published in 2011, and after 2013 there was a rapid increase in the number of papers on the subject. This increase was particularly evident in 2018, but the journal also publishes articles on greywater recycling today. Other journals with the highest number of papers on greywater reuse are
Water and
Water Science & Technology. However, it should be noted that in the journal
Water Science & Technology, the first publications were already published at the beginning of the 21st century, while a clear slowdown in the growth of publications can now be observed. The situation is completely different for the journal published by the Multidisciplinary Digital Publishing Institute. The first publication was published in 2012, and in the following years, there was a clear boom in articles on this topic. The list is closed by
Resources, Conservation and Recycling. For this journal, there is a gradual increase in the number of publications in the greywater recycling domain. On the other hand, an analysis of the total number of citations shows that articles published by Elsevier are the most frequently cited. However, it should be noted that the number of citations of individual publications varies widely. Some of them have been cited more than 50 times, but there are also those that have been cited less than 5 times.
The next stage of the analysis was to develop a three-field plot (
Figure 13). As in the case of publications in the greywater energy recovery domain, this figure shows the relationships between countries, authors, and keywords within the analyzed database. It was generated assuming a maximum number of elements per field of 20. When analyzing
Figure 13, it can be seen that the issue of greywater recycling is being addressed by a larger group of researchers than greywater energy recovery. In addition, it is noticeable that many researchers are willing to cooperate with scientists representing other nationalities. Examples include authors from Chile or the UK. On the other hand, publications by researchers from Poland or Brazil are usually developed within a single country and sometimes even within a single university. This results in the need to expand the scope of international cooperation in order to make better use of the potential and facilities of the various research units and the communities they represent.
3.3. Greywater Recycling and Energy Recovery
The final stage of the review was to analyze a combined database of publications dealing with greywater energy recovery and recycling. The data, which were summarized in
Table 12, were determined using Bibliometrix software. They clearly show that only 5 publications out of the 308 analyzed address both the issues of energy recovery from greywater and its recycling. These are articles based mainly on financial analysis [
139]. The other records focus only on one way of using greywater. It is also notable that the relatively low number of Author Keywords appear in papers from both domains. Especially as both greywater energy recovery and greywater reuse are effective tools for achieving Sustainable Development Goals. Reducing the temperature of greywater not only does not prevent its reuse but may prove beneficial. Oteng-Peprah et al. [
38] indicate that the high temperature of greywater promotes the growth of microorganisms and can sometimes also cause the precipitation of inorganic salts. Therefore, the research carried out in the field of greywater recycling should be extended to include energy issues. This applies primarily to the possibility of recovering energy from greywater. However, the issues of energy consumption by GWRSs in various operating conditions should not be forgotten. Expanding the spectrum of research on greywater use seems to be a necessity. Only such an approach will be a step towards creating new value for potential users.
The lack of cooperation between units dealing with the use of greywater for various purposes is also confirmed by the low number of authors with publications in both fields (only 10 people). Meanwhile, authors representing different units can complement each other and share knowledge and skills in the course of their work. The formation of synergies between people representing different universities and even countries would allow the exchange, modification, and consideration of various concepts and their practical applications. As a result, the efficiency of their work would be raised, and the research potential would be strengthened. Carrying out joint research in the domains of greywater energy recovery and recycling could also contribute to an increase in the competitiveness of scientific units.
Harnessing the expertise of people involved in greywater recycling would also be beneficial for greywater energy recovery researchers. Papers dealing with greywater reuse issues are far more likely to be cited. The increased reach of such publications may be due to the greater internationalization of the projects. This approach to conducting scientific research affects its innovativeness and increases its usefulness. In addition, far more researchers devote their time and competence to the issues of greywater recycling, which also affects the interest in publications in this field.
In order to confirm the above thesis, keywords appearing in both groups of publications were analyzed. The results of this analysis are shown in
Figure 14. A total of 1329 expressions related to greywater energy recovery and recycling were analyzed. As in the previous case, the analysis was limited to keywords that appeared at least ten times in the publications. This reduced the number of analyzed expressions to 46. In this case, the keywords were assigned to four groups. Cluster 1 includes 15 phrases directly related to greywater energy recovery research. The expressions assigned to clusters 2 and 3 are related to greywater recycling. These groups include 15 and 11 terms, respectively. In contrast, keywords from both groups of publications were assigned to the smallest cluster 4. The nodes representing the keywords were connected by 632 links with a total link strength of 1825.
Analysis using the VOSviewer software showed that the most frequent keyword was “greywater”, found in cluster 2. It generates with other nodes 42 links with a total link strength of 264. However, the number of its occurrences is only slightly higher than in a case of the network shown in
Figure 9. This proves that only a small group of greywater energy recovery researchers use that phrase as a keyword. Such an approach can result in difficulties at the stage of searching for publications in databases. The problem is compounded by the different spellings of the term. In publications, not only the word “greywater” and its combinations with other words appear, but also “grey water”, “graywater” and “gray water”. Some authors use the general phrase “waste-water”, which can also make it difficult to find the paper in the database. The second most frequently appearing keyword, as in the case of publications in the greywater recycling domain, was “reuse”, with 38 links with a total link strength of 238. The podium is closed by “greywater reuse”.
When analyzing the average number of citations, it can be seen that publications with keywords such as “technologies” and “potable water savings” are the most frequently cited. Thus, it is confirmed that publications in the greywater recycling domain are of interest to a wider audience. It should also be noted that, as in previous cases, a high average number of citations is rarely associated with a high number of occurrences or high total link strength. This is due to the fact that frequently cited keywords tend to be related to a niche topic in a given research area. Because of the small number of publications on a given topic, existing articles are often used to create a list of references for new papers. In addition, when analyzing the timeline of occurrences network, it can be seen that the map is divided into two areas. Clusters 2 and 3 include keywords that were popular in earlier years. On the other hand, clusters 1 and 4 are assigned phrases that have been the most popular in the last few years. Based on this information, it can be concluded that the topic of greywater energy recovery is gaining popularity and meets the current need to reduce the consumption of natural resources. This should encourage authors to be interdisciplinary and broaden their horizons. Establishing cooperation between researchers representing both domains would provide an opportunity for new publications to reach a larger audience and adapt their topics to contemporary trends.