Treated greywater as a novel water resource: The perspective of greywater treatment for reuse from a bibliometric analysis

One of the main consequences of climate change is the modification of the precipitation regime and the prolonged drought periods resulting from these new climatic conditions (Padulano et al. 2019; Wu et al. 2022; Booker & Snelder 2023). Water stress is already one of the main problems affecting several regions worldwide and refers to the scarcity of water in a region or for a particular population where the available resources cannot cover the required demand. Water demand has increased due to population growth, urbanization, industrialization, and agricultural intensification (Chen et al. 2020). As a result, many countries face water scarcity, and water stress is becoming an increasingly severe and common problem. According to the World Resources Institute, 17 countries have extremely high levels of water stress (most of them in the Middle East and North Africa) and 27 countries have high levels of water stress (World Resources Institute 2023).

Water stress affects the availability of water for human consumption, as well as other socioeconomic activities and natural systems. Water stress could severely affect human health and quality of life and negatively affect the environment. Lack of safe drinking water can lead to waterborne diseases and lack of irrigation water for agriculture can affect food production, leading to food insecurity (Meng 2023). In addition, water stress can lead to land desertification and the loss of natural habitats for flora and fauna. To minimize these problems, measures must be taken to manage water better and reduce water stress at global and local levels. Therefore, improving water use efficiency, promoting water conservation, and developing innovative water management models such as wastewater treatment and reuse are required (Al-Najjar et al. 2021; Crutchik & Campos 2021).

In this sense, in Chile (the country suffering the most severe hydric stress in South America), an initiative known as ‘Escenarios Hídricos 2030’ (translated as Water Scenarios 2030 in English) emerged in 2016, which aims to move towards a transition in the complete water management system (Escenarios Hídricos 2030, 2019). It plans a sustainable future that allows economic, environmental, and social development based on sufficient resources in quality and quantity, incorporating various water sources along with the creation of a national water policy with the participation of all sectors. This initiative has four main axes. One of the axes of the water transition includes the incorporation of new water sources, promoting the management of water resources through the reuse of water for different uses: from non-potable purposes (agricultural irrigation, irrigation for recreational activities in gardens, golf courses, or parks and industrial uses such as refrigeration, boilers, or process water) to potable purposes.

One type of wastewater that has great potential to be reused after treatment is domestic greywater, which includes the water used in sinks, showers, baths, washing machines, or dishwashers (Antonopoulou et al. 2013). Greywater is safer to handle and easier to treat and reuse onsite than blackwater (directly related to the discharges from toilets) because of its lower pollutant load. Basic treatment of greywater allows its reuse for toilet flushing, floor cleaning, and ornamental irrigation, while more advanced greywater treatment could achieve higher-quality water for reuse in other applications (Ghawi 2017; Craig & Richman 2018). Moreover, the onsite treatment and reuse of domestic wastewater can significantly contribute to sustainable water management since it reduces the demand for potable water and helps preserve aquatic systems by decreasing the volume of wastewater discharged into them (Godfrey et al. 2009).

The treatment of greywater has become a very relevant topic and a huge number of scientific documents focused on it have been published. This fact is corroborated by the presence of several reviews related to greywater. Some of them present the quantification of greywater production and the main physicochemical characteristics of greywater (Boyjoo et al. 2013; Ghaitidak & Yadav 2013; Shaikh & Ahammed 2020). Specific technologies for greywater treatment have also been deeply reviewed (Gassie & Englehardt 2017; Arden & Ma 2018; Cecconet et al. 2019; Wu 2019; Boano et al. 2020; Khajvand et al. 2022) while more complete reviews revising all the available treatment alternatives are not so frequent (He et al. 2022; Awasthi et al. 2024). Other aspects investigated and reviewed consider the possible uses of recovered greywater (Glover et al. 2021; Shahmohammad et al. 2022; Gholami et al. 2023; Kurniawan et al. 2023; Chen et al. 2024a), the consequences over traditional wastewater management systems (Filali et al. 2022; Van de Walle et al. 2023), and all the environmental, social, legal, and other relevant sustainability considerations of greywater treatment and reuse (Cobacho et al. 2012; Benami et al. 2016a; Benami et al. 2016b; Rodríguez et al. 2022; Madzaramba & Zanamwe 2023). The management of the extensive scientific literature regarding greywater requires the application of systematic tools.

Bibliometric tools have shown to be extremely important in the field of library and information sciences since they allow for a quantitative description of document distribution based on a variety of specified categories, including year, author, country, institution, source, or knowledge category. Indeed, according to the definition proposed by Pritchard, bibliometrics involves analysing books and other knowledge communication mediums using statistical and mathematical methods (Pritchard 1969). The enormous amounts of scientific data that are currently available can be methodically examined, arranged, and analysed with great advantage by applying this methodology. As a result, researchers can obtain pertinent data and understand the state of the art in a specific field, which makes it easier to recognize novel research patterns.

Two examples of bibliometric studies related to greywater have been found. On the one hand, a bibliometric analysis focused on greywater recycling and recovery of energy from greywater was carried out (Kordana-Obuch et al. 2023). On the other hand, the potential reuse of greywater was investigated by means of a bibliometric analysis (Pinto et al. 2021). In both cases, the treatment of the greywater before its reuse, recycling, or recovery is not the focus of the papers. This work presents a bibliometric analysis that points out the available technologies for greywater treatment in order to obtain a valuable water resource, paying also attention to the qualitative and quantitative characteristics of greywater that influence its treatment and the water management systems to consider treated greywater for reuse. The identified documents were systematically analysed to determine the most relevant quantitative characteristics of the research on this topic. This way, further information about global research trends can be provided, which may be a useful tool to identify the challenges in this field.

The Scopus database was used to complete the bibliometric analysis, which offers a comprehensive catalogue of bibliographic documents, with the feasibility of ordering and classifying them at the user's disposal. Scopus is recognized as the most complete abstract and citation database of peer-reviewed literature. It contains more than 25,100 titles, of which more than 23,450 have been peer-reviewed, and more than 5,000 international publishers are included (Elsevier 2022).

The data search was performed in March 2023, where the search parameters were defined, in this case, the interval of years and all types of documents. In the search, the following words were entered: gr*ywater AND treatment. For the case of the first word, the symbol * was used. With this, the database omits the specification of the letter and focuses on the others to search for matches. This was done due to the inflection of the language and its words because, in American English, the word would be ‘graywater’ while in British English, the word would be ‘greywater’. In addition, within the search parameters, it was limited to 2022 since it was the last full year available for document publication when the search was carried out. Thus, the search was recorded as ‘TITLE-ABS-KEY (gr*ywater AND treatment) AND PUBYEAR < 2023’. The search yielded 1,204 documents. An Excel spreadsheet was created to save all the available information on the identified documents and analyse the most important quantitative parameters.

A network analysis based on bibliometric scientific mapping was performed. SciMAT software, developed by researchers at the University of Granada in Spain, was used (Cobo et al. 2012). SciMAT is an open-source software licensed (GPLv3) tool developed to perform science mapping analysis in a longitudinal framework. It provides different modules that help the analyst perform the science mapping workflow steps.

The grouping of keywords in normalized sets with the same spelling was the initial task in order to encompass the same concepts in the face of the differences in typing that these may contain. For words that had the same meaning, it was decided to group them according to the word with the highest number of quotes, which was generally the option that belonged to American English. When the difference was due to the presence or not of a hyphen (-), it was decided to consider the option that did not have a hyphen. In the cases where plural words appeared, the singular form was selected to name the set.

The following parameters were established for forming the clusters and strategic diagrams. First, the periods over which the strategic diagrams will be made were established. Three periods were determined: the first contemplates the documents published between 1977 and 2013; the second considers those from 2014 to 2019, and the third one, between 2020 and 2022. This selection was made to seek equality of the number of documents published in the three periods. The parameter on which the analysis will be carried out was selected and the option ‘Words’ was selected. Because of the large volume of documents analysed, reducing the frequency of the keywords used is necessary. A minimum number of times the keyword should appear to be considered was imposed. Consequently, keywords falling below this frequency threshold (which was defined as 12) were excluded from consideration. The network formation was based on ‘Co-occurrence’ matrixes. The frequency of co-occurrence was determined according to the criterion ‘Edge value reduction’ and the value four was selected as the threshold. The ‘Equivalence index’ was selected for the normalization of the networks and the option ‘Simple centers algorithm’ was selected as a clustering algorithm, where minimum and maximum sizes of the network were requested. The options ‘Core mapper’ and ‘Secondary mapper’ were selected to obtain the graphs and the bibliometric measures ‘h-index’ and ‘sum citations’ were considered to evaluate the quality of the documents. Finally, the ‘Jaccards index’ and ‘Inclusion index’ were chosen to define the longitudinal analysis. A summary of the most relevant parameters defined for the bibliometric network analysis is presented in Table 1.

This exponential increase could be more easily observed when accumulated production was graphed. Figure 1(b) presents the information obtained regarding the number of documents in an accumulated manner (each year presents the documents published that year and all the previously published). In fact, as can be seen in Figure 1(b), there is a clear exponential growth for the topic under study as indicated by the included trendline. The equation of the curve with an exponential fitting was y = 2.089·e^0.140x, where y represents the number of accumulated documents and x the year (starting with the value 1 for the year 1977). It can be noted that according to the coefficient of linear correlation (R² = 0.9963), there is a clear affinity to the model. Therefore, it is possible to infer that this topic is still hot and there will be an increase in publications related to the topic in the coming years.

The different document types which were categorized and the identified results are presented in Table 2. The highest percentage of contribution corresponds to Articles, with more than 74% of the total number of publications, which is a very typical result since scientific journals have become the most popular way for research dissemination. In second place, with a 15% contribution, Conference Papers can be found, which indicate the popularity and importance of research on greywater treatment, as it is fully considered in scientific meetings and congresses. Finally, Reviews contributed with 6%, indicating a considerable amount of documentation related to the topic to be reviewed and commented on by the scientific community, while the contribution of the rest of the types was insignificant (the joint contribution was around 4%). Reviews are often expected to outnumber Conference Papers in terms of contribution, especially when a huge number of documents have been published about a specific topic (Mao et al. 2015; Abejón et al. 2018). However, there are bibliometric studies that show that the contribution of Conference Papers can surpass the contribution of Reviews (Abejón et al. 2017; Chang et al. 2020) and even that of Articles, positioning them as the type of document with the highest contribution in some specific fields (Abejón & Moya 2022).

Table 3 provides information on the language used in the documents, clearly showing the superiority of documents written in English. According to the information in the table, more than 97.5% of the publications found in the database correspond to documents written in English. Although it is not the most widely spoken language in the world, English is considered the most globalized one nowadays. For instance, more than 80% of the electronic information stored worldwide is written in English, and two out of every three researchers worldwide consider English as the most important language for knowledge diffusion (NIÑO-PUELLO 2013). Bibliometric studies have demonstrated that more than 75% of the documents published in social sciences and humanities and more than 90% published in engineering and environmental sciences are written in English (Abejón 2022). The second position in the ranking was occupied by German, with 11 documents (0.9% contribution), while the rest of the languages were only used in five or fewer documents, which implies a contribution below 0.5%. The case of China is quite surprising since China is a superpower in research production but only three documents written in Chinese were found.

This section presents the most prolific journals publishing documents on the studied subject. The data collected in Table 4 show the journals with the corresponding number of documents, and some indexes related to the impact and reputation of the journal are included. On the one hand, the Scimago Journal Rank Indicator (SJR) is a size-independent prestige indicator that ranks journals by their average prestige per article. It represents the average number of weighted citations received during a selected year per document published in that journal during the previous three years (Khalid & Khalid 2016). On the other hand, SJR is considered an alternative to the journal impact factor (JIF), provided by Web of Science. It is most often used as a proxy measure for journal quality and is calculated as the ratio between the number of citations received in that year for publications in that journal that were published in the two preceding years and the total number of citable items published in that journal during the two preceding years (Severin et al. 2023). Therefore, higher SJR and JIF values indicate greater journal prestige, but both indexes are absolute and it is difficult to compare journals from different disciplines. To solve this problem, the journal citation indicator (JCI) index was defined, which is a bibliometric measure of impact normalized by the field that represents the average citation impact by category of the articles published in the previous three years. This metric makes it possible to evaluate in relative terms the impact of scientific journals in the corresponding discipline (Torres-Salinas et al. 2022). A JCI value above 1 indicates that the journal performs above average, while a value below 1 indicates that its performance is below average. For instance, a journal with a JCI value of 1.35 has 35% more citations than the average in that discipline.

The most productive journal was Water Science and Technology, with 91 papers and contributions above 7.5%. This journal publishes papers covering various aspects of the sciences and technologies linked to water treatment, paying special attention to water transport processes, treatment of stormwater, greywater, domestic and industrial effluents, or sources of pollution and its consequences on water bodies. The corresponding JCI value (below 0.4) clearly indicated that this journal is not among the most relevant in the discipline, but the journal that occupied the second position of the ranking presented a higher JCI value: Science of the Total Environment presented a JCI value equal to 1.77. It is a journal with a multidisciplinary approach in the field of environmental sciences, considering the atmosphere, lithosphere, hydrosphere, biosphere, and anthroposphere, which contributed to 48 documents. The third position of the ranking is shared between two journals that published 35 papers, which once again demonstrated the disparity of the relevance of the journals. While Desalination is among the most important journals related to water treatment with a JCI value of 2.02, Desalination and Water Treatment presented a low JCI value (0.24). The highest JCI value among the journals included in Table 4 corresponded to Water Research in the fifth position, with a JCI value of 2.13, which means that the two journals among the most productive in research related to greywater treatment received double the number of citations as the average, as indicated by the JCI values above 2. This way, we know that high-quality journals have covered the topic, which can give an idea of the relevance of the topic.

Regarding scientific disciplines, since certain documents can be labelled within two different disciplines, the sum of the contribution percentage can exceed 100%. Table 5, which is a compilation of the most important disciplines, shows that the highest contribution corresponded to Environmental Sciences, containing 935 documents (more than 77% contribution). This leading discipline is followed by Engineering in the second position and Chemical Engineering in the third position, with 305 and 171 documents, respectively. These results indicate that the topic is mainly approached from an environmental perspective, although a keyword such as treatment was considered in the search string. This fact can be justified by the great concern about the environmental repercussions of treated and untreated greywater or the research efforts applied to the characterization of greywater before the treatment application. Regarding the engineering point of view, the contribution of Engineering was more than 10% higher than that of Chemical Engineering (25 vs. 14%), so the technical aspects of the solutions and treatments for the reuse of greywater have been focused more significantly from branches of engineering other than chemical engineering. Therefore, relevant contributions by civil, environmental, hydraulic, or sanitary engineers must be highlighted. Finally, the last two positions in Table 5 corresponded to two basic sciences: Agricultural and Biological Sciences and Chemistry, with both contributions around 10%. The implementation of biological and chemical technologies is the most frequent option for greywater treatments and the complete understanding of the involved mechanisms is a priority.

The United States emerged as the leading research superpower in the investigated topic (Table 6 presents the most productive countries), with 196 documents and above 16% contribution. Once again, as in the case of the knowledge disciplines, this is not an exclusive category since a document can be assigned to several countries due to international collaborations. Consequently, the sum of the contribution percentage can exceed 100%. The United States was followed by India, with a contribution of close to 10%. In most scientific fields, the United States and China nowadays share the leading positions in scientific literature production, but this is not the case for the topic under study in this work. Indeed, China occupied the sixth position of the ranking in Table 6, with lower production than Australia, the United Kingdom, and Germany. This is not a frequent situation, so the reduced interest of China in greywater treatment must be emphasized.

Regarding the second position of India, this country is living through a great promotion of its scientific resources, and it appeared among the most productive countries in several fields, including those related to water (Zapata-Sierra et al. 2023). In fact, India is home to 1.391 billion people, constituting 17.7% of the world's population. However, it only possesses 4% of the planet's potable water, and an estimated 70% of the country's surface water is considered inadequate for human consumption (Instituto de Comercio Exterior de España 2023). These circumstances prioritize the scientific research focused on the optimal management of the water resources, including greywater. Similar situations occur in other highly and densely populated countries in Asia that appear in Table 6, such as Malaysia (58 documents) and Indonesia (29 documents), where the limited access to safe water justified the identified research efforts in topics related to water treatment and reuse.

Australia, which occupied the third position of the ranking, is a country with vast arid regions, where the agricultural sector is the main national water consumer, which is equivalent to approximately 65% of the total consumption, so water is an essential resource for the national development (Dolnicar & Hurlimann 2010). In addition, due to the severe drought in many parts of the countries, the construction of wastewater treatment plants to treat and recover water has been promoted. Moreover, the country has promoted new policies and practices that encourage citizens to be more aware of water expenditure and moderate their consumption. Israel, which occupied the eighth position with 57 documents and a close to 5% contribution, is a country with high similarity to Australia, as water efficiency has become a critical national priority. Israel is one of the countries that best manages the use and exploitation of its water resources. They have developed efficient irrigation methods consisting of drip irrigation, which has allowed them to save up to 30% of water for the agricultural sector. In addition, Israel is one of the countries that reuses almost all its greywater. Currently, 95% of greywater is treated, and 86% is used to cover the agricultural sector and to irrigate parks (Mainardis et al. 2021). Other countries with arid regions can be found among the most productive ones, such as South Africa (31 documents) and Jordan (23 documents).

The leading institutions involved in publishing documents related to greywater treatment are presented in Table 7. Two Israeli institutions occupied the first and second positions of the ranking. The first place belonged to the Technion – Israel Institute of Technology, with 35 documents, which imply a 3% contribution. It is the oldest technological institute in Israel and the campus, located in Haifa, has more than 13,000 students. The second place belonged to the Ben-Gurion University of the Negev, with 25 documents and a 2% contribution. This university was founded in 1969 and accounts for more than 19,500 students. Both institutions have carried out relevant projects related to greywater (much of them as collaborations), especially considering the safe use of treated greywater for irrigation (Busgang et al. 2018; Hadad et al. 2022; Maimon et al. 2023).

After the two leaders from Israel, three institutions contributed 24 documents: Colorado State University from the United States, S. V. National Institute of Technology (SVNIT) from India, and Universiti Tun Hussein Onn from Malaysia. These three countries have been previously identified as among the most productive ones. In the case of the United States, the other two American institutions can be found in Table 7: the United States Environmental Protection Agency and Purdue University, while SVNIT was the only Indian institution in the ranking, despite the second position of India in the ranking of countries. This fact demonstrated that Indian production is highly shared among different institutions. In fact, only two other countries apart from the United States inserted more than one institution in the ranking: Australia with Monash University and UNSW Sydney, and Germany with Hamburg University of Technology and Technische Universität Berlin.

The top 10 documents, according to the number of citations received in Scopus, are compiled in Table 8. The number of citations is generally a synonym of the prestige and relevance of a scientific document. The most cited article in the list was ‘Challenges and Opportunities for Electrochemical Processes as Next-Generation Technologies for the Treatment of Contaminated Water’ (Radjenovic & Sedlak 2015). This review had received 624 citations when the database was consulted, and it covers the use of electrochemical processes for treating different types of wastewater, including greywater. In addition, other reviews were found in the ranking. The second most cited document was ‘Greywater reuse: Towards sustainable water management’ (Al-Jayyousi 2003). With 282 citations, this review deals specifically with greywater reuse and its importance for sustainable water management. The fifth position corresponded to the review ‘Gravity-driven membrane filtration for water and wastewater treatment: A review’ (Pronk et al. 2019), which reviewed the research about this low-energy membrane technology for wastewater treatment, including greywater. The eighth and ninth positions were occupied by two reviews focused on the characteristics of greywater and their influence on the treatment (Friedler 2004; Ghaitidak & Yadav 2013).

The documents in the third and sixth positions corresponded to studies focused on greywater reuse in multi-storey buildings. It is possible to highlight the document ‘Greywater reuse systems for toilet flushing in multi-storey buildings – over 10 years’ experience in Berlin’ (Nolde 2000). It had 270 citations and deals with implementing greywater reuse systems for toilet flushing in buildings in Berlin, Germany. Continuing with the analysis of Table 8, the documents in the fourth and tenth positions are focused on the treatment of the microbiologic pollution of greywater and the corresponding associated microbial risks, as specified in the paper ‘Faecal contamination of greywater and associated microbial risks’ (Ottoson & Axel Stenstr om 2003) (cited 230 times). The remaining document in the seventh position of the ranking, titled ‘Recycled vertical flow constructed wetland (RVFCW) – a novel method of recycling greywater for irrigation in small communities and households', was another example of the studies performed by Israeli researchers on irrigation with treated greywater (Gross et al. 2007).

Other families of keywords highly represented in the ranking were those related to the greywater characterization and the presence of contaminants, such as Chemical Oxygen Demand, Water Pollution, Nitrogen, Escherichia coli, Water Pollutants (Chemical), Turbidity, Phosphorus, Ammonia, or Bacteria, which are typically present in most greywater samples. In addition, the keywords Bioreactors, Wetlands, Filtration, Bioreactor, Constructed Wetland, Disinfection, Biological Water Treatment, or Adsorption correspond to a series of technologies and related keywords for the treatment and reuse of greywater.

The SciMAT tool was used to obtain the corresponding strategic and dynamic diagrams, which were generated utilizing the word selection and grouping algorithms of the software. The strategic diagrams are based on the values of centrality, which indicates the internal cohesion of the keywords that compose the cluster, and density, which reflects the interaction of a cluster with other clusters and its relevance in the scientific field studied (Herrera-Viedma et al. 2020). The relative centrality and density values are represented in the horizontal and vertical axes of the strategic diagram, respectively. This way, the themes represented by the keyword clusters are assigned to one of the four quadrants of the strategic diagram, which are categorized as follows:

• - Motor themes (upper right quadrant): These themes have the most remarkable development and set the trends within the topic research.

• - Basic transversal themes (lower right quadrant): These themes are fundamental to the topic due to high centrality but have low density values. They are not as developed as the motor themes but can evolve into motor themes in the near future.

• - Emerging or declining themes (lower left quadrant): These themes require further analysis, and their evolution throughout time must be taken into account in order to identify if they are declining themes that will disappear because of further development or if they are new emerging themes with chances to be developed and upgraded.

• - Developed isolated themes (upper left quadrant): These themes are well developed, but they have low centrality values due to their high specificity or the appearance of new alternatives, so they are considerably distant from the other themes and are no longer trendy.

Cluster size in the strategic diagrams is represented by the number of documents related to each cluster (the exact values appear in each cluster). In addition, the h-index, absolute centrality, and density values are presented in Table 9 for each cluster, as well as the number of citations received by the documents linked to the clusters. The absolute centrality measures the degree of interaction of a cluster with other clusters, and it indicates the external cohesion of the network. The absolute density measures the internal strength of the cluster, and it indicates the internal cohesion of the network.

Based on the results of the bibliometric analysis (most cited documents and frequently used keywords) and the bibliometric network analysis, a quick review of the most relevant topics related to greywater treatment and reuse was carried out, paying special attention to three main axes identified from the compiled bibliography: greywater characterization, greywater treatment, and treated greywater management.

Because of the scarcity of traditional water resources, the option of covering specific areas of consumption with nontraditional resources, such as treated wastewater (including greywater) has gained relevance (Jaeel & Abdulkathum 2018). It is possible to differentiate two types of greywater as a function of the pollutant load: light greywater (corresponding to showers, bathtubs, and bathroom sinks) and dark greywater (corresponding to kitchen sinks, dishwashers, and washing machines). The typical lower pollution content of light greywater facilitates the treatment with less complex systems (Leiva et al. 2021).

The physicochemical characteristics of greywater depend entirely on the household from which it comes. Factors such as the number of people living, their age, lifestyle, or social status strongly influence the pollutant load of the produced greywater (Ghaitidak & Yadav 2013). All the housework habits and the employed cleaning and personal hygiene products can significantly impact the chemical composition of greywater since they determine the quantity and nature of the chemicals that will appear in the greywater. Among the typical components present in greywater, surfactants, soaps, foaming agents, wetting agents, dispersants, dyes, fats, and oils can be mentioned, while other organic chemicals are present in lower concentrations, such as plasticizers, antibacterial agents, pharmaceutical drugs, radiation absorbers, insect repellents, or recreational drugs (Glover et al. 2021; Ren et al. 2021; Van de Walle et al. 2023). Higher organic matter and solid material can be expected in dark greywater from kitchen sinks and dishwashers, and, depending on the original source of freshwater, heavy metals can be found, too. The microbiological content of greywater is a critical aspect. Although it is expected to present clearly lower pathogen load than blackwater from toilets, concerning pathogens such as E. coli, Salmonella, and Shigella, together with protozoan parasites such as Giardia and Cryptosporidium, may be found in greywater (Birks & Hills 2007; Dalahmeh et al. 2016; Dwumfour-Asare et al. 2017; Busgang et al. 2018; Schoen et al. 2018; Craddock et al. 2020). These microorganisms could cause severe diseases if they are not eliminated or at least reduced in their pathogen load from the treated greywater. In addition, nutrients such as nitrogen and phosphorus are present in greywater, which can contribute to eutrophication when released into the environment (Huiliñir et al. 2020). Consequently, the inadequate management of untreated or unproperly treated greywater can cause considerable environmental problems, contaminating water bodies and posing a severe health risk to humans (Shaikh & Ahammed 2020; Mahmoudi et al. 2021).

Similar to the qualitative characteristics, the quantitative generation rates of domestic greywater differ from case to case, depending on age, gender, habits, lifestyle, living standards, geographic water availability, and sources (Chowdhury et al. 2015). Greywater generally constitutes about 50–80% of the total household wastewater and typical flow rates range from 40 to 130 L/day/capita (Eriksson et al. 2003). Indeed, economic income is a critical variable for the greywater production. High-income households even in arid regions like Oman or the United Arab Emirates can produce more than 150 L/day/capita of greywater, while the generation rates in developing countries are frequently below 50 L/day/capita (Jamrah et al. 2008; Chowdhury et al. 2015; Ntibrey et al. 2021). In these developing countries, there are significant differences in the quantity and quality of greywater between in-house sources and outside sources, since the lack of domestic running water favours the reduction of water consumption and the subsequent greywater production (Oteng-Peprah et al. 2018). The amount of greywater collected from public establishments and institutions is clearly lower than from households; for instance, the greywater produced in schools falls in the interval from 3 to 16 L/day/capita (Alsulaili & Hamoda 2015). The contribution of light greywater to total greywater is around 70%, with showers and bathtubs being the main contributors, followed by kitchen sinks, which are the main sources of dark greywater (Jamrah et al. 2008; Mandal et al. 2011).

The design of a greywater reuse system primarily depends on the quantity to be treated, the characteristics of the greywater, and the planned reuse applications. Greywater characteristics must be determined before deciding on its treatment and reuse onsite since the type and extent of the treatment depend on such characteristics. The full characterization of greywater has to consider the determination of the complete physicochemical profile, which includes the assessment of the concentrations of the main pollutants present in the sample as well as other significant indicators, such as colour, turbidity, density, and odour (Noutsopoulos et al. 2018; Oteng-Peprah et al. 2018; Shaikh & Ahammed 2020; Joseph Perumpully et al. 2023; Van de Walle et al. 2023). Several authors have reviewed and compiled the main characteristics of greywater, including the identification of the most frequent pollutants and their concentrations (Eriksson et al. 2002; Friedler 2004; Boyjoo et al. 2013; Maimon & Gross 2018; Noman et al. 2019; Vuppaladadiyam et al. 2019; Wu 2019).

One of the most common technological options for greywater treatment is mechanical filtration. This process involves the removal of solid particles and grease by filtration through meshes, sieves, or sand filters, and is commonly used as a preliminary stage to other types of treatment and as a post-treatment process in cases where particulate matter is formed. Screen filters, textile filters, metal filters, and granular media such as sand, gravel, and ceramic waste have been used to treat greywater on a pilot scale for the treatment of real bathroom greywater (Mohamed et al. 2018). In addition, biofilters composed of plant-based products such as ground coffee or hemp fibre are also available (Sami et al. 2023). In many cases, filtration can be completed with a previous sedimentation stage, which retains the larger and denser solid particles, while filtration is effective for removing suspended solids that cannot be sedimented. A laboratory scale study for greywater treatment at Addis Ababa University obtained a 60.8–100% removal for the parameters turbidity, biological oxygen demand (BOD), chemical oxygen demand (COD), total carbon (TC), and faecal coliforms (FCs) using a filter filled with gravel, sand, and granular activated carbon (GAC) (Tusiime et al. 2022). For this reason, other alternatives have been developed, such as activated carbon filters or zeolite filters. However, they are not very effective in removing detergents or dyes and are materials that generate waste since they are not reusable. Physical treatments are inexpensive and easy to implement, but they are ineffective in removing most contaminants or microorganisms.

Direct filtration with or without mechanical pretreatment by pressure-driven membranes has been the first approach to greywater treatment by membrane technologies. For example, it is possible to mention microfiltration (Babaei et al. 2019; Rezaei et al. 2023), ultrafiltration (Nghiem et al. 2006; Kim & Park 2021), and nanofiltration (NF) (Guilbaud et al. 2012; Rutten et al. 2023) as well as reverse osmosis (Gual et al. 2008; Lanjewar et al. 2021). In general, microfiltration and ultrafiltration are preferred for greywater treatment because of their lower pressure and reduced tendency to fouling compared to NF and reverse osmosis membranes; however, previous research shows that pretreatment of greywater is still necessary when using ultrafiltration membranes (Gual et al. 2008). Some of the pollutants present in greywater are oils and fats, whose removal is quite complex due to the formation of emulsions, in this case, oil-in-water (O/W). In this sense, Shi et al. (2019) successfully removed emulsions (O/W) using ultrafiltration ceramic membranes at lab scale (Shi et al. 2019). Other constituents of these waters, such as dyes, can be removed by membrane systems (Wei et al. 2022; Li et al. 2023).

A disadvantage of these low-pressure technologies is that they are not capable of removing dissolved organic matter, nutrients, and emerging organic pollutants such as caffeine, paracetamol, bisphenol A, and octocrylene, among others, which are present in greywater (Schäfer et al. 2006; Glover et al. 2021). High-pressure technologies, such as NF, can attain the removal of dissolved organic contaminants but present problems of membrane fouling (Nghiem et al. 2006). Another disadvantage is that applied pressures are required to remove contaminants from water since ultrafiltration membranes operate between 50 and 120 psi (3.4–8.3 bar), raising operational costs in a possible domestic use. Thus, the membrane distillation system has advantages over conventional filtration technologies as it requires lower pumping pressures (Li et al. 2023). Therefore, the long-term operation of these membrane technologies is limited by: (i) fouling of the membranes and (ii) the use of pumps to achieve the required high pressures, which makes necessary the application of pretreatments and, probably, a programme of periodic chemical washes to the membranes. It implies increased operating costs and the appearance of residual washing streams to be treated.

In summary, membrane technologies would allow the effective treatment of greywaters with short residence times, such as, for example, those containing surfactants (Fernández et al. 2005; Kowalska 2008). Finally, the most critical advantage of membrane distillation systems is that they can produce pure water, removing even some dissolved micro-pollutants present in greywater, such as plasticizers, personal care products (PCPs), and antimicrobials, among others (More & Tyagi 2020; Costa et al. 2023). Therefore, there is a considerable opportunity for membrane technologies to be implemented in domestic systems for greywater treatment with short residence times, low space utilization, easy installation, and maintenance.

The basis of the operation of biological technologies involves several reactions associated with the use of living organisms, such as microorganisms (aerobic or anaerobic) and plants that use the pollutants and molecules present in the medium for their growth (Arunbabu et al. 2015). Aerobic and anaerobic biological treatments have proven useful, but aerobic conditions are usually preferred since anaerobic processes tend to achieve lower levels of organic contaminant and surfactant removal, implying higher risk (Gonçalves et al. 2021).

Initially, biological treatments aimed to remove organic matter in greywater, but scientific research and technological progress have made it possible to extrapolate their use for different pollutants, such as those derived from nitrogen or phosphorus (Van de Walle et al. 2023). It is typical for these biological processes to consider a filtration stage to retain the biosolids involved in the treatment. Membrane bioreactors (MBRs) combine the biological process and membrane filtration, where the latter acts as an effective barrier preventing the outflow of biosolids with the treated water stream (Atanasova et al. 2017).

Biological systems can effectively reduce organic loads and nutrients over the entire range of concentrations in greywater. However, contaminants of emerging concern (CEC) are mostly highly resistant to biological degradation, making it difficult to eliminate their presence by these methods. In addition, a final disinfection step is required to eliminate pathogens (Van de Walle et al. 2023), and the residence times of these processes are usually very long, reaching up to 199 days for MBRs (Ren et al. 2021; Ongena et al. 2023) that achieved PCPs removal rates >80% using the MBR coupled with ultraviolet (UV) or electrochemical disinfection at lab scale for synthetic greywater and constructed wetland (CW) methods. The main pathway for PCP removal in the MBR was sludge adsorption and biodegradation, while the contribution of the membrane module was weak (Ren et al. 2021). Bathing water and laundry wastewater were used as the inlet greywater of the MBR and CW devices on a pilot scale. An advantage of these processes is their greater effectiveness in removing dissolved contaminants compared to mechanical filtration, while the main disadvantages include high residence times and higher maintenance.

Finally, according to the literature, the biological processes proposed range from simple solutions that mimic natural systems, such as artificial wetlands (Abd-ur-Rehman et al. 2022), through biological contactors and sequencing batch reactors (SBR) sequential reactors, to more advanced solutions, such as Upflow Anaerobic Sludge Blanket reactors or MBRs (Shafiquzzaman et al. 2021; Nurmahomed et al. 2022). Regarding CWs, five PCPs, including DEET (diethyltoluamide), diethyl phthalate, HHCB (Jiale musk), AHTN (Tuina musk), and ethylhexylmethoxycinnamate, were selected in one study and removal efficiencies above 80% were obtained, where the main mechanism of PCP removal in CW was the combined action of plant uptake, microbial biodegradation, and substrate adsorption, depending on the type of PCP (Ren et al. 2021).

Another study investigated the use of climbing and ornamental plants at a pilot scale for synthetic greywater treatment in vertical flow CWs. Different design parameters were evaluated such as substrate (sand or vermiculite), use of saturation zone, and plant species (Trachelospermum jasminoides, Lonicera japonica, Callistemon laevis) for optimal removal of the studied pollutants. The results revealed that, in relation to the substrates tested, sand with or without saturation zone presented significantly higher removal rates for turbidity (94 ± 5%) and chemical oxygen demand: (96 ± 7%) than vermiculite (turbidity: 54 ± 23%; chemical oxygen demand: 73 ± 29%). Slightly higher removal rates were recorded in vegetated systems compared to unvegetated systems (Stefanatou et al. 2023).

Electrochemical and other physicochemical processes have been studied for a wide range of pollutants, such as organic matter, pathogenic microorganisms, and some metals since their multiple advantages include environmental compatibility, versatility, and high efficiency in removing contaminants. Among the most widely used physicochemical methods for treating greywater are electrocoagulation (EC), electro-oxidation (EO), and adsorption.

EC, also known as electrochemically assisted coagulation, involves the production of coagulant agents by applying a specific current density to destabilize contaminants present in an aqueous matrix, removing them from the liquid phase to a solid phase (Shahedi et al. 2020; Tegladza et al. 2021). It is based on using an electric current to dissolve the anodes, thus generating metal ions in different oxidation states (Boinpally et al. 2023).

This technique depends on various variables, as follows:

• - Electrode material: This parameter is one of the most important in EC treatment, as it will determine the electrochemical reactions involved in the system. Among the most common materials for this process, zinc (Fajardo et al. 2015), magnesium (Kruk et al. 2014; Devlin et al. 2019), and copper (Hong et al. 2013) can be mentioned, although iron and aluminium are undoubtedly the two most common electrodes used in EC (Bote 2021; Patel et al. 2022; Sandoval et al. 2022; Bani-Melhem & Rasool Al-Kilani 2023; Bassyouni et al. 2023; Simon et al. 2023).

• - pH: This variable influences the efficiency of the current process since pH variation affects solubility, concentration, and charge of flocs. The literature has observed that pH varies during the process (Hussein & Jasim 2021), affecting the nature of the Fe and Al monomeric species. In general terms, the best removal of different contaminants has been achieved at acidic pH values and values close to neutrality (Khorram & Fallah 2017; Zhang et al. 2023).

• - Applied current density: The current density regulates the kinetics of a process in EC, as it influences the production of Fe or Al ions (depending on the anode used). The quantity of these ions determines the rate of coagulum production and bubble formation. Therefore, current density studies are recommended as initial studies of the EC process. In general, an increase in current density leads to an increase in the pH of the solution due to the formation of hydroxide ions at the cathode, and it determines the concentration of monomeric species (Zampeta et al. 2022).

• - Reactor configuration: The reactors for EC can be classified according to the flow direction, which can be horizontal or vertical, multichannel or single-channel, and the type of electrode connection, which can be parallel, monopolar, or bipolar. The monopolar arrangement of electrodes is typically the most used in EC (Barısçi & Turkay 2016; Zampeta et al. 2022).

In general, when comparing Fe and Al electrodes, both widely studied in the EC for greywater treatment, the optimal results in terms of operating costs are obtained with an Fe anode in conditions of pH close to neutrality and applying a low current density (Ucevli & Kaya 2021). Another study that compares the combination of these electrode materials as anode and cathode (Al–Al, Fe–Fe, Al–Fe, and Fe–Al) determined that at initial pH values of 3 and 6, the most effective combination was Fe–Al, while at pH 9 it was Fe–Fe, confirming that iron as an anode material is better to obtain greater COD removal (Bote 2021). On the one hand, when comparing electrodes of this same chemical nature with mild steel, it was shown that the latter allowed a reduction of over 80% of the COD when applying a low current density and that although this increases, the removal does not increase. On the other hand, the Fe electrode reached a similar percentage of COD decay by doubling the current density applied with the mild steel electrode. These differences have to do with physical surface differences because they are chemically similar. A smoother surface such as mild steel would account for the formation and production of finer bubbles, while larger bubbles, such as those formed with the Fe electrode, could destroy the flocs formed in EC and also reduce the probability of contact between coagulant ions and colloidal species present in greywater (Bani-Melhem & Rasool Al-Kilani 2023). In relation to the EC parameters for greywater, it has been reported that applying low current densities for a treatment time not exceeding 60 min allows achieving greater removals of COD, phosphates, nitrates, and BOD at low operating costs, just 0.114 US$m⁻³ (Moosavirad 2017; Patel et al. 2022).

The EC presents some disadvantages, such as (i) electrode consumption can lead to a decrease in process efficiency because they should be replenished periodically, (ii) formation of sludge, and (iii) formation of oxides on the anode. However, it has significant advantages that make it an attractive technique, such as (i) a simple implementation system, (ii) coagulants are produced in situ, (iii) gas bubbles formed during the process promote the flotation of pollutants, (iv) efficiency in a broader range of pH values, and (v) although the anodes must be replaced periodically, they are not expensive.

EO is one of the most popular electrochemical advanced oxidation processes for the elimination of organic compounds present in water (Moreira et al. 2017; Ganiyu et al. 2020; Seibert et al. 2020; Titchou et al. 2021). It is a process with a relatively easy configuration that does not require a prior pH adjustment but is limited by the presence of suspended solids. In this process, the oxidation of contaminants can occur directly through an electronic transfer with the anode and indirectly because of electro-generated oxidants (Nair et al. 2023). The power of these oxidants will depend on the chemical nature of the anodic material used, and this is why anode materials have been classified as active and non-active electrodes. As examples of active anodes, it is possible to mention the Pt electrode, glassy carbon, and mixed metal oxide (Cotillas et al. 2016; Singla et al. 2020). Conversely, as examples of non-active anodes, it is possible to mention the PbO₂, SnO_2, and boron-doped diamond (Cotillas et al. 2018; Feng et al. 2018).

The EO has also been applied for greywater treatment (Drennan et al. 2019), but to a lesser extent than the EC. Although EO, in most cases, degrades 100% of the contaminants and allows high mineralization percentages, there is a risk in the formation of intermediates and byproducts that, in some cases, could be more toxic than the initial contaminants. For example, the formation of active chlorine, perchlorate, haloacetic acids (HAAs), and trihalomethanes (THMs) was assessed in a divided and undivided cell at 12.5 mA/cm². Perchlorate formation was observed in undivided experiments (>50 μg/L), but not detected in divided experiments because they were removed on the cathode surface in the divided cell, while HAAs and THMs were generated anodically (dos Santos et al. 2023).

Greywater composition depends on the demographic zone as well as the culture of a country. There is limited literature on treating this type of water by EC and EO compared to other wastewaters containing other specific pollutants, e.g., dyes. In this sense, since EC and EO present some significant limitations but also some advantages, it becomes essential to consider a sequential combination of both processes to improve the effectiveness of each one, in which EC could be used as a pretreatment to EO for greywater treatment.

Several adsorbents meet the requirements for their application to the treatment of pollutant removal in greywater. One of the best-known and widely used is activated carbon, an efficient, simple, and low-cost material. The synthesis of activated carbon from different sources such as coal, coke, peat, wood charcoal, sugar cane bagasse, and rice husk is possible. In addition, due to its origin, it could improve the biodegradability of greywater, reducing the concentrations of pollutants that biological treatment cannot remove efficiently (Alateeqi et al. 2023). The active sites on it determine the type of chemical reactions that occur with other molecules, and it is crucial to consider this since it will determine the type of contaminant that can be removed using this technology. For example, the treatment of wastewater from a wastewater treatment plant with activated carbon showed high effectiveness in the removal of copper, chromium, and iron in addition to a noticeable decrease in the total organic load of the sample (Hu et al. 2015; Torrellas et al. 2015).

Other studies have reported the adsorption of five model emerging contaminants (CECs) such as caffeine, hydrochlorothiazide, saccharin, sulfamethoxazole, and sucralose with activated carbon in fixed bed column experiments with different aqueous solutions and obtained an effective removal of the CECs, although when the flow rate is increased, the lifetime of the column decreases (Diniz & Rath 2023).

Another widely studied material is zeolite. This has great potential for removing contaminants present in greywater due to their adsorption capacity and low operating cost. The chemical composition of zeolites comprises a series of crystalline and hydrated aluminium and silicate compounds containing alkaline and alkaline earth cations. These compounds have a three-dimensional structure characterized by their open structure. This characteristic gives them a remarkable ability to adsorb and release water and cations without significantly modifying their arrangement (Abd-ur-Rehman et al. 2022). Although only a reduction of 20% in the total organic carbon was demonstrated in the treatment of liquid industrial waste, the addition of this material to the treatment has prevented the leaching of metals present in the sludge generated in the physicochemical treatment (Li et al. 2017). Although natural zeolites are a promising compound for greywater treatment, the high specificity of the matrix of contaminants that may be present in a greywater sample makes it difficult to corroborate this treatment's effectiveness in removing contaminants more accurately. However, the removal of 75.95% ammonium in a filter composed of zeolite and cocoa shell activated carbon in greywater treatment has been reported. The composition of the filler mixture showed the best results (75:25% zeolite and activated carbon, respectively) when synthesized at a temperature of 700°C (Susilawati et al. 2023).

The reuse of greywater has led to a significant number of scientific documents covering the social, legal, environmental, and economic aspects (Cobacho et al. 2012). Various topics, such as the legislative frameworks designed to consider greywater reuse, the life cycle assessment of greywater treatment and reuse, the corresponding economic costs and benefits, and the social perception of treated greywater reuse, have been deeply investigated.

Sustainable access to safe water resources is a priority in the agenda of the most important international organizations, and the Sustainable Development Goal 6 about clean water and sanitation must be highlighted as the most illustrating example (Basu & Dasgupta 2021). This context has allowed the definition of several general objectives to improve conditions related to water, such as access, sanitation, and reuse. Nevertheless, water management players require the specification from these general targets to a detailed, valid institutional framework that regulates the reuse of treated greywater (Vera-Puerto et al. 2022). This legal and regulatory context requires the establishment of tools such as water laws, public health standards, technology standards, or building codes, which clearly pave the way to define sources, technologies, and uses for greywater treatment and reuse. The lack of this legal skeleton can be the main institutional barrier to the implementation of greywater reuse systems (Hacker & Binz 2021).

Some countries have worked to develop this legal framework, providing tools to manage greywater resources under safe conditions for human health. However, there is a huge diversity of approaches and strictness in the proposed greywater regulations, ranging from guidelines with few restrictions to laws that prohibit greywater reuse in all circumstances (Stevens 2021). While in some regions there is a clear explicit government encouragement for the reuse of greywater, others have not developed specific policies or direct regulations. In these latter cases, alternative norms, such as building or plumbing codes or health standards not specifically focused on greywater reuse, define the scenario. The publication of the Guidelines for the Safe Use of Wastewater, Excreta, and Greywater (where a specific volume was dedicated to excreta and greywater reuse in agriculture) by the World Health Organization in 2006 opened the door to the appearance of the different national documents for greywater reuse (World Health Organization 2013). Israel was a pioneer in the definition of the requirements for treated wastewater to be reused in irrigation and considered the strong public health and environmental implications that make mandatory the implementation of adequate treatments in accordance with soil sensitivity and without risk to human health, flora, soil, and water sources (Inbar 2007). The United Kingdom prepared an information guideline for greywater reuse, taking into account the lack of specific legislation about this issue (Enviromental Protection Agency 2012). Therefore, as in many other European countries, the standards included in the Bathing Water Directive were proposed (EUR – Lex 2014). The Environmental Protection Agency published the Guidelines for Water Reuse in 2012 (Enviromental Protection Agency 2012), but the final decisions about greywater treatment and reuse in the United States lay on the different states. For instance, California has promoted greywater reuse and the California Plumbing Code possesses a complete chapter focused on greywater systems (International Association of Plumbing & Mechanical Officials 2016). Australia has vast experience in greywater reuse, and the different territories have prepared codes for the promotion of acceptable long-term greywater reuse and conservation of the quality of ground and surface water supplies without compromising public health (Australian Government 2010). Asian territories like Hong Kong and Singapore have developed technical guidelines for safe greywater reuse, but further detailed information about guides and technical standards for greywater treatment and reuse in different countries can be consulted (Chaillou et al. 2011; Boyjoo et al. 2013; Rodríguez et al. 2022). Some limits imposed on greywater characteristics by water reuse guidelines and standards for different countries are compiled in Table 10.

Traditional greywater treatment is carried out in centralized wastewater treatment plants. These centralized systems are built at a large scale, and although they require significant initial investment, they benefit from economies of scale over time. Greywater reuse is planned based on decentralized systems that treat greywater in situ on a smaller scale. The economic analysis of the costs and benefits of decentralized greywater reuse must consider two different players: the final users and the social stakeholders (Friedler & Hadari 2006). On the one hand, the final users can significantly reduce the consumption of freshwater, as recovered treated greywater can replace freshwater for several uses, so reduced water and sewage bills should be expected. Nevertheless, the investment and operation costs for the installation and processing of the greywater treatment and reuse system, including the design of separated water distribution networks for greywater, blackwater, and treated greywater, might be covered by the final users. Moreover, additional energy consumption could be anticipated because of the greywater treatment system. On the other hand, social stakeholders, such as public water utilities, private water companies, and governmental institutions are involved in the economic considerations of greywater management. Basically, these actors obtain several economic benefits for the implementation of greywater reuse: the reduced freshwater consumption of final users implies lower uptake from water resources and subsequent decreased conditioning and distribution costs, while the reduced wastewater production results in lower wastewater collection costs and other decreased costs in wastewater treatment plants (less energy and chemical consumption). The only additional costs attributable to the implementation of greywater reuse may be linked to additional blockages in wastewater collection systems due to reduced flows or the adaptation of the wastewater treatment plants to new conditions characterized by higher concentrations of pollutants due to the increased blackwater-to-greywater ratio. Therefore, the consideration of different cost-sharing scenarios between final users and social stakeholders must be more deeply investigated to identify optimal conditions for all the parts involved. Different examples of economic analysis of greywater systems have been published (Yerri & Piratla 2019; Arden et al. 2020; Dewalkar & Shastri 2020; Abdelhay & Abunaser 2021; Ghafourian et al. 2022; Chen et al. 2023).

The motivation to promote onsite greywater treatment systems is the reduction of freshwater demand and the expected overall environmental emissions (Yoonus & Al-Ghamdi 2020). The use of comparative life cycle assessment demonstrated that centralized drinking water supply coupled with onsite greywater treatment can be more energy- and carbon-efficient than conventional (drinking water and sewerage) centralized systems (Xue et al. 2016). Indeed, other environmental impacts can be reduced when greywater treatment and reuse are compared to traditional centralized systems. For instance, the consideration of in situ treatment and reuse of greywater reduced around 30% of the freshwater eutrophication, freshwater ecotoxicity, and human toxicity impacts of the water management system (Gilboa et al. 2023). Nevertheless, the environmental performance of a greywater treatment and reuse installation depends on the definition of the specific system, including the characteristics of the coupled freshwater treatment facilities and wastewater treatment plants, the possible uses of the treated greywater, or the existence of other alternative water resources, such as rainwater or treated wastewater. Examples of cases where the implementation of greywater in situ treatment and reuse were not environmentally favoured alternatives have been identified. When a threshold production of greywater per capita was not achieved, as in rural schools where the amount of produced water is scarce, the reuse of greywater was not effective according to the impact analysis since the savings in drinking water are not enough to compensate the impacts associated with the construction, operation, and maintenance of the greywater system if the recovered water was only used for irrigation (Rodríguez et al. 2021). The scale of the greywater system is another variable that has great influence on the environmental performance. While totally centralized systems have been opposed to totally decentralized in situ systems, intermediate situations can be possible, too. A study compared the performance of three different scales for greywater treatment implementation (a community with 3,500 persons equivalent, a neighbourhood with 350 persons equivalent, and a single household with up to five persons equivalent) to conventional centralized wastewater treatment (Kobayashi et al. 2020). Moreover, two different greywater treatments were considered: CWs or membrane bioreactors. This way, MBRs allowed additional non-potable applications for treated greywater due to their higher quality when compared to CWs, where only irrigation was considered. For scenarios with the same treatment technology, larger scales reduced global warming potential, eutrophication, and human toxicity, although more complex distribution networks were required. The CWs were preferred at household and neighbourhood scales, while the community-scale system based on MBRs may be the optimal option when a large amount of greywater can be reused. Other researchers have found that semi-distributed greywater treatment and reuse at cluster scale showed better environmental performance than fully distributed at building scale reclamation systems (Opher et al. 2019). Renewable energy sources and possible heat recovery can affect the environmental evaluation (Bonoli et al. 2019), as using solar energy is a highly effective solution with reduced environmental loads (Dominguez et al. 2018; Ramezanianpour & Sivakumar 2019).

The successful implementation of greywater treatment and reuse systems is subject to public acceptance. If final users are not convinced to reuse treated greywater, the entire systems are doomed to fail. The critical role played by several factors in the social acceptability of treated greywater reuse has been investigated. Gender, age, educational level, water expenditure level, and associated costs are relevant determinants. For instance, when urban farmers in the Colombo District were consulted about their willingness to reuse greywater for irrigation, elderly farmers were more reluctant to reuse greywater, while better-educated farmers and female farmers were more open to irrigation with greywater (Dona et al. 2023). Nevertheless, previous knowledge about greywater reuse must be highlighted among the most decisive factors affecting greywater acceptability (Amaris et al. 2020). Other internal factors, such as religious customs, personal values, and environmental awareness, must be considered to understand the behaviour of individuals when facing the reuse of treated greywater, as well as external factors, such as geographic situation, sociocultural context, and water availability, which can affect the decision-making processes (Nkhoma et al. 2021; Singha & Eljamal 2022). In order to check if the geographical location and the corresponding availability of water resources affect the public's attitude towards alternative water systems (including greywater reuse), a survey was carried out among respondents from 12 different countries (Poland, The Czech Republic, Slovakia, Hungary, Spain, Portugal, Italy, Sweden, Turkey, Iraq, Egypt, and Brazil). People in the countries with low water resources view alternative water sources more favourably (Stec 2023).

Not all the uses to be given to the treated greywater present the same acceptability. The closer the contact between the treated greywater and the user, the higher the opposition to this use, so it is evident that non-potable reuse approaches might be easier to implement when compared to potable reuses (Oteng-Peprah et al. 2019). Consequently, drinking water appeared as the most rejected reuse option, followed by cooking or bathing (Craddock et al. 2021). Public spaces, including parks, gardens, sport facilities, or industrial installations, were preferred over particular households for treated greywater reuse, and in the latter case, outside uses (irrigation or car washing) were more easily accepted than inside uses (toilet flushing or laundry) (Po et al. 2003). Although professional agricultural technicians from Minas Gerais State in Brazil showed a positive attitude towards reusing greywater in agriculture, the willingness to consume food grown with greywater was the least supported issue, and irrigation of non-food crops was preferred (Silva et al. 2023). Other uses for treated greywater perceived as having direct contact with humans, such as aquaculture or olive pressing, had lower acceptance rates in rural areas of Palestine than uses without clear direct human contact, such as crop irrigation, stone cutting, or construction (Al-Khatib et al. 2022).

The role of the governmental institutions is critical in the definition of the legislative scenario that must regulate the reuse of greywater. These institutions can promote the implementation of greywater treatment and reuse systems. The town of Sant Cugat del Vallès, which is located about 20 km from Barcelona in Spain, is a great example (Domenech & Saurí 2010). The local water-saving regulation developed in this town states that all residential buildings with more than eight apartments, as well as other kinds of buildings with showers and baths consuming more than 400 m³ of water per year, must install a greywater reuse system. This regulation demands the separation of greywater from showers and baths, and after treatment, its reuse for toilet flushing. Apart from the definition of the normative and legal frameworks, public institutions must take part in the proposal of measures to increase the acceptability of greywater reuse. Economic benefits, in the form of subsidies or rebates for the installation costs, can promote the adhesion to greywater reuse systems, and users would even trade off some comfort level, such as changes in water smell or colour, if they could observe the positive impacts of reuse on the environment (Hurlimann & McKay 2007; Juan et al. 2016). Nevertheless, good greywater aesthetics is one of the principal factors affecting willingness to use and the acceptability of greywater, so colourless and odourless greywater encourages its reuse (Hyde et al. 2017). Without the economic support of institutions, the implementation of alternative water systems, such as greywater reuse, can be financially unprofitable, and examples of cities where this situation occurs have been reported (Stec & Słyś 2022). As the importance of the information that the user has about the greywater reuse system is critical, campaigns to communicate the fundamentals of the treatment, the associated environmental benefits, and the successful implementation of equivalent systems must be carried out (Furlong et al. 2019).

The understanding of the influence of all the factors affecting the willingness to adopt greywater reuse systems is not trivial and the prediction of the users' behaviour requires advanced mathematical models that combine economic theory and behavioural foundations, including advanced discrete choice models and social cognitive theories (Oteng-Peprah et al. 2018; Amaris et al. 2020). The influence of emotion, risk, and threat perception on individual decisions to accept and adopt decentralized water systems, such as greywater systems, has been deeply investigated (Mankad 2012). The original theory of planned behaviour and its extended versions have been successfully applied to explore the willingness of consumers to adopt greywater treatment and reuse systems (Oteng-Peprah et al. 2020). Although attitude, personal norms, subjective norms, and perceived behavioural control seemed to all contribute to intentions, attitude and personal norms must be highlighted as the strongest determinants. More advanced models have also been proposed, for example, a formal mathematical model for understanding public acceptance of water reuse (Hartley et al. 2019). This model conceptualized how governments, water utilities, and consumers interact to facilitate or hinder acceptance of alternative water supply sources, including potable reuse. The model provided a theoretical basis to systematize empirical studies and allowed the design of policy initiatives that cultivate support for water reuse, particularly around benefit- or risk-focused narratives as constitutive of broader communications and information campaigns.

Implementing large-scale systems for water treatment and obtaining potable quality water can be costly and complex due to the logistics of system and technology implementation. By contrast, the reuse of greywater enhances simpler water reutilization, even from the perspective of the circular economy. Therefore, it is extremely important to analyse the capital costs and operation and maintenance costs of greywater reuse applications. The reuse of treated greywater, for example, as ornamental irrigation water or in agriculture, has a significant impact in areas or regions where water resources are scarce and its management is expensive. Additionally, treated greywater contains essential nutrients such as phosphorus and nitrogen, which reduces the costs due to added fertilizers (Tripathy et al. 2024).

The use of a submerged membrane bioreactor (SMBR) was compared with an NF module for the treatment of greywater to be reused, considering two greywater reuse capacities of 3 and 30 m³/day, equivalent to the greywater from the bathrooms of 50 and 500 inhabitants of a building in France. For a production capacity of 3 m³/day, the associated costs were similar, with NF costing $8.49/m³ while an SMBR costs $8.05/m³. These costs changed significantly when the production was projected to be 30 m³/day, with the costs for the NF and SMBR systems being $5.25/m³ and $4.79/m³, respectively. The costs were calculated as the sum of fixed expenses associated with equipment, depreciation, and maintenance, as well as variable costs related to chemicals and electricity, along with labour costs and water treatment costs (Humeau et al. 2011).

Another study related to the reuse of greywater from 20 toilets in different schools in Kuwait applied various treatment processes such as screening, sand filtration, chlorination, and UV disinfection. A pilot plant was developed to treat 5 m³/day for schools with 500 students, applying a process based on filtration and UV treatment. A cost–benefit analysis was conducted in which the costs were related to the capital cost for constructing a small greywater treatment unit, the operation and maintenance cost, and the energy cost. The benefits of the cost savings on potable water flow to be used instead of greywater and wastewater reductions due to using recycled greywater were also taken into account. A net savings value of $1,600/year was obtained, with a payback period of six years and 11 months (Alsulaili et al. 2017).

The combination of processes aimed at optimizing the benefits each one provides is not limited to physical treatments but also includes combining them with advanced oxidation processes. This is the case with the combination of hydrodynamic cavitation (HC) with the use of hydrogen peroxide and ozone (HC + H₂O₂ + O₃) to treat greywater from a kitchen. The total cost of this integrated process was $1.40/m³, which was lower than the values reported for each of the processes separately (Mukherjee et al. 2020).

A simple filtration system consisting of a gravel sand filter followed by GAC was also designed to evaluate the performance of a treatment plant with a capacity of 1 m³/day and an organic load of 0.32 kg COD/m³. The cost to produce treated greywater was determined based on the capital cost to build the treatment system, the operation and maintenance costs, and the energy cost.

The capital cost for constructing the treatment plant, which included the filter, pump, tanks, and piping, was approximately $2,480 (this value does not include the land purchase price). The operational cost depends on the energy consumed by the pump, which was around $3/year with a rate of $0.073/kWh in Nepal. To maintain the quality of the effluent, it was necessary to conduct laboratory tests of the water's physicochemical parameters every 3–4 months, costing around $100/year. Finally, the maintenance cost of the filter, including cleaning, repair, and replacement of the filter media, was approximately $30/year, considering a system lifespan of two years (Samal et al. 2020).

Nature-based technologies for treating greywater, such as artificial wetlands, have gained attention due to their low operating and maintenance costs. However, the investment costs for constructing wetlands can vary significantly. Additionally, they require a large area for construction. A horizontal subsurface flow wetland requires between 5 and 10 m² per equivalent population, while a vertical flow wetland requires about 2 m² per equivalent population. Another important point is the lack of control over the operational parameters in these systems (Gkika et al. 2015).

In this way, it is possible to see that the costs associated with these processes are variable, just like the variability of the physicochemical properties of greywater, as they are influenced by various factors. Among these, what stand out are the geographical area, which determines aspects such as climate and topography; the cultural habits of the population, which affect the type and amount of waste generated; and the size of the population, which influences the total volume of greywater produced. Therefore, the costs are not clearly limited and can vary widely depending on these contextual variables.

The reuse of treated greywater is a promising option to increase the availability of hydric resources in the domestic framework. The bibliometric analysis carried out in this work demonstrated that this is a hot topic. Since the pioneering works in the late 1970s and early 1980s, the interest in the reuse of greywater due to the concern generated by water stress and drought has exponentially increased the research and publication of scientific papers that contribute to advancement in this field. The most frequently published type of document was an Article, which is typical in most scientific areas, followed by a Conference Paper, giving additional indications about the importance and relevance of the topic nowadays, as it is still the object of interest of the research community that meets in scientific conferences and congresses. English was the most employed language (clearly the lingua franca of scientific research distribution). The United States was the leading country in publishing documents related to the topic, while China was in sixth position, which demonstrates the little interest by this research superpower in greywater treatment and reuse. Regarding the most prolific institutions, two universities from Israel, the Technion – Israel Institute of Technology and the Ben-Gurion University of the Negev, occupied the first positions of the ranking, revealing the great role of this country in the management of greywater. Although Environmental Sciences was the discipline with most publications, it was followed by Engineering in the second position and Chemical Engineering in the third position. Moreover, two basic sciences, such as Agricultural and Biological Sciences and Chemistry, occupied the fourth and fifth positions, respectively. Therefore, the multidisciplinary approach to greywater treatment and reuse was clear, paying attention to the environmental consequences of greywater reuse, the search of technical solutions for treatment and reuse of this type of wastewater, and the basic fundaments of biological and chemical technologies developed for this purpose.

The information derived from the bibliometric analysis of the most frequent keywords and the corresponding scientific mapping allowed the identification of the most important themes regarding greywater treatment and reuse. First, the characterization of greywater in both quantitative and qualitative terms revealed the great diversity of greywater production, which depends on different social, economic, and geographical factors. Moreover, the increasing consideration of emerging contaminants has promoted the search for effective treatment processes for their removal. Regarding the treatment technologies and their effectiveness, the combined use of the available alternatives must be emphasized. Generally, pretreatment processes suggest the application of filtration to eliminate particles, but in order to remove dissolved chemicals and microorganisms, more specific technologies are necessary, based on physicochemical and biological treatments. Nevertheless, the successful implementation of systems for greywater treatment does not depend only on technical factors, as it is highly influenced by economic, social, and legislative aspects. The reuse of greywater has a series of environmental benefits, especially with the mitigation of water stress due to the decrease in demand for drinking water and reduced volume of wastewater to be treated.

On the one hand, the environmental benefits related to the reduction of drinking water consumption thanks to the reuse of treated greywater must be considered the more relevant motivation to consider the implementation of the corresponding treatment processes. In addition to this direct water saving, indirect energy savings can be achieved due to greywater reuse since the energy consumption related to transporting water in centralized water management systems can exceed the energy consumption required for the actual water treatment. Therefore, small-scale decentralized greywater reuse systems have the potential to reduce energy consumption for water provision significantly.

On the other hand, some knowledge gaps have been identified, which must be addressed by future research in order to solve the related challenges. From a technical perspective, the great variability of greywater production, both in qualitative and quantitative terms, is a great concern. The characteristics and flow rates of greywater highly depend on several factors and vary from household to household and even from source to source in the same household. Moreover, different alternatives for greywater reuse require different characteristics, and the imposed pollutant limits must take into account these different requirements. Therefore, optimized systems that fit adequately to each particular case must be identified. In addition, the investigation of the long-term performance of greywater treatment systems is still scarce, and further research efforts must be applied to this topic in order to minimize the risks associated with greywater reuse. Adequate testing, monitoring, and control must be warranted to assure the quality of the recovered water to be reused to avoid environmental or health hazards. From a legal perspective, public institutions must provide an adequate legislative scenario to promote greywater treatment and reuse. The lack of an adequate legal framework can obstruct the development and implementation of this type of system, so the development of guidelines, laws, and other legislative documents must be a priority. The consequences of the different legal scenarios defined must be deeply investigated, and further work on the promotion of greywater treatment and reuse by public and private institutions must be completed. Finally, from a social perspective, the acceptance of greywater reuse by final users is critical. The perception of consumers defines the success of these treatment systems, but they present varying degrees of acceptance of greywater reuse systems, depending on a wide range of factors. Further studies to identify all these factors are required, as well as the development of models and tools that contribute to enhancing the social acceptability of treated greywater for reuse.

The authors acknowledge the funding provided by the Chilean National Agency for Research and Development (ANID) under the Projects ANID ANILLO ATE 220024 and ANID/Millennium Science Initiative Program/ICN2021_023.

Funding acquisition and project administration were performed by E. Q.-M. and M. J. A. The conceptualization was performed by R. A. and J. R. The first draft of the paper was written by D. Y. under the supervision of R. A., based on a previous document by I. V. (who carried out the data collection and software tasks), and with contributions by L. C. Espinoza under the supervision of R. A. All authors have commented on previous versions of the paper and contributed to the writing and review of the last version of the paper.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.