Water innovation in industrial symbiosis - A global review

Motivated by the limited attention given to water management in industrial symbiosis research, this study presents the first global review of water innovation practices in the implemented industrial symbiosis cases reported in literature. We analyze the prevalence of global water innovation practices extending beyond the commonly used broad practices of water treatment and reuse to propose six categories, including utility sharing for alternative water supply, utility sharing for wastewater treatment, water recovery, energy recovery from water, material recovery from water, and material exchange to enhance water/wastewater treatment. Our findings highlight regional variations in adoption, with Asian and Europe showcasing diverse practices. Additionally, they indicate that most symbiosis cases center on the extensive role of public utilities and shared water facilities in pursuing water innovation, while ‘pure ’ interfirm water-related symbiosis is limited. Finally, this review highlights extensive knowledge gaps and research needs in advancing sustainable water management and innovation in industrial symbiosis. Overall, our study contributes to the development of a comprehensive framework for water innovation practices in industrial symbiosis and emphasizes the need for future research in this area.


Introduction
Freshwater is essential for any human activity.Yet the natural water resources have exceeded their renewal and assimilation capacity in many parts of the world, mainly because of human activities (Willet et al., 2019).As a result, more than 1 billion people have no access to clean water to meet their basic needs (UNESCO World Water Assessment Programme, 2019).Industrialization carries a significant burden in the current global water crises contributing to both water resource depletion and water pollution.
Industry currently accounts for 20% of the total freshwater consumption globally − 75% for energy production and 25% for manufacturing (Misstear et al., 2022).The stress on freshwater supply due to rapid industrialization is expected to increase, particularly in the emerging and developing countries striving for economic growth while facing fragmented and inefficient water systems.By 2050, the industrial water demand is estimated to increase by 800% in Africa and 250% in Asia, with possibly no significant increase in North America and Western Europe (Boretti and Rosa, 2019).However, there are major national and subnational variations in the level of water stress in the world, including developed regions such as Europe and North America (Boretti and Rosa, 2019;European Environment Agency, 2021).
To mitigate the impact of industrialization on the environment and to ensure a more sustainable growth, the field of industrial ecology has been introduced by taking an analogy from a natural and stable ecosystem (Ehrenfeld, 1997).In such a system, human activities are in harmony with the natural environment through an efficient cycle of materials and sharing of resources.Emphasizing the interaction among separate entities, the concept of "industrial symbiosis" seeks to reduce the flow of waste by establishing a local network of "physical" exchanges of water, material, and energy between companies (pure industrial symbiosis) but often also including exchanges between companies and utility facilities.The goal is to achieve environmental and competitive benefits in a collective approach (Chertow, 2000).To emphasize more on the sustainability aspect of industrial symbiosis, Lombardi and Laybourn, (2012) expanded the definition to a network of "diverse organizations" that foster "eco-innovation" and "long-term culture change".Innovation is an inevitable part of industrial symbiosis, whereas the concept of "eco-innovation" (i.e., combining economic and ecological-resource benefits) is very context-dependent (Levidow et al., 2016).
Academic research plays a significant role in pursuing ecoinnovation through industrial symbiosis by highlighting the context for current practices and promoting innovative approaches.There is an expanding body of literature on industrial symbiosis since its recognition by the scientific community in 1989 (Frosch and Gallopoulos, 1989), and uncovering the first comprehensive case of Kalundborg, Denmark (Christensen, 1992).Considering the growing number of successfully implemented industrial symbiosis cases worldwide, numerous research and review articles have evaluated successful practices to encourage new cases.Chertow (2000) studied 12 eco-industrial park cases in detail that were in late planning, implementation or operational stages and identified five different material exchange types, three of which were identified as industrial symbiosis in nature (i.e., inter-firm).Cerceau et al. (2014) created an international inventory of 23 port-based industrial symbiosis initiatives and analyzed their temporal and spatial characteristics.Côté and Cohen-Rosenthal (1998) studied some of the establishing eco-industrial parks in North America, Europe and Japan and proposed essential characteristics of eco-industrial parks derived from these experiences.With the purpose of identifying influential factors, Massard et al. (2014) performed a survey on more than 200 global cases of so-called "eco-innovation parks".More recently, Neves et al. (2020) provided a comprehensive list of literature references on global industrial symbiosis cases, including analyses on location, types of economic activities in each case and the methods employed in the research studies.Country-specific and sector-specific advancements in industrial symbiosis have also been studied, including the Netherlands (Eilering and Vermeulen, 2004), Portugal (Neves et al., 2019a), Italy (Susur et al., 2019), South Korea (Park et al., 2019), China (Fang et al., 2007;Tian et al., 2014;Zhang et al., 2010), Brazil (Colpo et al., 2022), North America (Heeres et al., 2004;Neves et al., 2019b), and Africa (Oni et al., 2022) as well as specific industries like chemicals (Yang et al., 2018), coal (Wang et al., 2017) and energy intensive manufacturing (Mendez-Alva et al., 2021).Many reviews of industrial symbiosis cases have led to the development of symbiosis knowledge databases, covering both implemented and potential/planned symbiosis activities; many described symbiosis plans/potentials are never implemented so the distinction is important.The most comprehensive database found in the literature is the outcome of a European research project, the MAESTRI project (Evans et al., 2017;Benedetti et al., 2017), which resulted in a database of 45 industrial symbiosis cases and includes 425 synergies.Other European projects such as SCALER 100 (Stéphane et al., 2019) and EPOS (Lessard et al., 2017) have produced symbiosis databases that focus solely on European countries.
While previous reviews have provided valuable insights into the principles, drivers, and barriers of industrial symbiosis, many have been broader in scope and have not specifically examined the role of water innovation in industrial symbiosis.Notably, we have observed that only about 10% of synergies identified in the MAESTRI database are water related.Water plays a critical role in industrial symbiosis, given its essentiality in industrial processes and its strong socio-environmental aspect (Moro et al., 2018).Industrial symbiosis offers a platform for water innovative approaches including establishing networks of water exchanges between companies and reducing the demand for freshwater.A significant body of literature has developed methodologies for integration of water networks including fit-for-purpose treatments (e.g., Aviso et al., 2010;Boix et al., 2015;Ramin et al., 2021).Despite major research advancements, the extent of water innovation practices, including resource recovery from water (e.g., energy and material) remains unknown.This gap in the literature highlights the need for a comprehensive overview of successful water innovation practices in industrial symbiosis, as well as identification of best practices and opportunities for future research and action.
This review article aims to address this gap through three fundamental research questions (RQs): RQ1.What are the main regional trends and influencing factors in adopting water innovation in industrial symbiosis, as evidenced by documented cases in the literature?RQ2.What are the key features and prevalence of water innovation practices in industrial symbiosis, and are there any dominant practices?RQ3.Which industries are primarily engaged in water innovation in industrial symbiosis, and to what extent does the public sector play a role in its adoption?
By addressing these questions, we have studied a large body of literature and extracted valuable information on water innovation practices on the existing global industrial symbiosis cases.The result is a comprehensive database on water-related practices, including type, distribution and the industrial sectors involved.Particularly, we assessed the involvement of municipal wastewater treatment plants in public-private partnerships, as they play an important role in the identified water-related symbiosis practices.
This review contributes to the documentation, characterization, and interpretation of successful water innovation practices in the context of industrial symbiosis and provides a deeper understanding of its prevalence.This knowledge could prove vital in enhancing potential developments on symbiotic water innovation worldwide, particularly in regions facing sever water stress.

Methodology
Fig. 1 summarizes the steps taken in performing the present literature review.The steps include: i) literature search in relevant databases; ii) screening to identify implemented cases; and finally, iii) processing to identify the cases with water-related synergies.Below, we describe the review process in detail: The literature search was done in a digital library made available by DTUfindit (https://findit.dtu.dk), which is mainly based on Scopus and Web of Science.The scope of the search was limited to three keyword strings ("industrial symbiosis" OR "eco industrial" AND "case").The aim was to identify all the literature studies on industrial symbiosis cases (with or without water innovation).The result was a list of 1510 literature items including journal articles, book chapters as well as master student and PhD theses.i.e., primarily scientific literature.In addition, we applied a snowball procedure (Greenhalgh and Peacock, 2005) to identify additional papers using the reference lists of the papers resulting from the initial search.
The screening process was done on the identified literature that was published in English, except for one Korean study (Park et al., 2015) and one Dutch paper (Van Waes and Huurdeman, 2009), both identified through the snowball procedure.The result of the screening process was identification of 213 existing/implemented industrial symbiosis cases reported in the reviewed literature (hypothetical and planned cases were excluded).The cases were considered as industrial symbiosis that involved at least three industrial entities with at least two physical synergies-a definition proposed by Chertow (2000).Publicly owned or public-private utility operations are counted as industrial entities (e.g., water supply, wastewater treatment plants, district heating).In the processing step, cases were examined and information on cases with water innovation was extracted from flow diagrams and text from relevant literature references.Water innovation was considered as any synergy related to water in its liquid form through by-product exchange or utility sharing.Fig. 2 shows the boundaries used for the identification of water-related synergies including industrial parks and agro-/urban-industrial symbiosis.
Finally, the identified global water innovation synergies were E. Ramin et al. analyzed through categorization and the type of industrial sectors involved.For sectoral analysis, we used the International Standard of Industrial Classification (ISIC) codes to group the industries by their primary economic activities (Phillips and Ormsby, 2016).

Overview of industrial symbiosis cases with water innovation
In this section, we address the regional developments in adopting water innovation based on the reviewed industrial symbiosis cases reported in literature.As a result of our comprehensive review, we identified 213 implemented industrial symbiosis cases, of which only 57 (27%), a little more than a quarter, were reported to have water-related synergies.The rest of the cases either did not have water innovation or no information on such synergies was found in the literature.
The details on the 57 industrial symbiosis cases with water innovation are provided in Table 1 including their name, the country, the main industrial sectors involved, the total number of synergies, the number of water-related synergies and the related references.

Geographical distribution of identified cases
Fig. 3 illustrates the geographical distribution of the identified cases, with a distinction made between total cases and cases with water innovation (Fig. 3a and b, respectively).Fig. 4 illustrates the extent of   water innovation practices in industrial symbiosis cases by country (blue color indicates the number of cases with water innovation and grey indicates the number of cases that either did not have any water innovation or for which no information was found).Fig. 3 reveals that the majority of the identified cases, particularly those with water innovation, are concentrated in Europe and Asia.The European countries (mainly UK, Sweden, Denmark, Finland, The Netherlands, and Italy), and Asian countries (mainly China, South Korea, Japan, and India) are also listed among the countries with the most successful implementation of industrial symbiosis cases (see Fig. 4).The distribution in the other regions appears too sparse.A similar observation is reported in the literature that analyzed the countries featured in the published academic articles on industrial symbiosis (see Chertow and Park, 2016;Neves et al., 2020).

Regional trends and factors in adopting water innovation
Research has indicated that the development of the major industrial symbiosis cases in the Asian countries is mainly policy driven (Chiu and Yong, 2004;Lowe, 2001).The focus is primarily on the planned eco-industrial parks, where industrial symbiosis can form part of the design (Behera et al., 2012), incentivized by governmental policies and investments (Lowe, 2001).Nevertheless, the focus of these policies in addressing water issues seems to vary considerable across countries.
China is the country with the highest number of successfully implemented industrial symbiosis cases (37 of the 213 cases identified in the literature).Notably, 21 of these cases (57%), practice water innovation.The implementation of water-related symbiosis projects in China seems to be strongly driven by policies, as is the prominence of industrial symbiosis in China in general (Chertow and Park, 2016).This reflects China's strong focus on water efficiency and recovery as part of its broader sustainable industrialization and circular economy efforts.The Chinese National Demonstration Eco-industrial Parks Program, established in 2006, includes 21 indicator standards, with four directly related to water conservation and reuse (Geng et al., 2009).The drive towards water efficiency and recovery is particularly critical, given the water stress in northern China (Moro et al., 2018).
Japan and South Korea also demonstrated significant advancements in industrial symbiosis, with 9 and 6 implemented cases identified, respectively (see Fig. 4).However, despite strong national policies supporting development of industrial symbiosis in both countries (Park et al., 2019;Van Berkel et al., 2009), there seems to be less focus on the implementation of water innovation practices.As reported by Park et al. (2019), in South Korea, a comprehensive national eco-industrial park program resulted in identification of 450 projects over a decade.However, only ten were related to water recovery (ca.2%).The success of the implemented water-related projects was also very limited.Out of 10 implemented water recovery projects, only five are still operational (Park et al., 2016(Park et al., , 2019)), indicating the difficulty in maintaining the water related symbiosis at the stage of development.
In Japan, the national eco-towns program, established in 1997, promotes "alternative industrial systems" with "zero emissions" with the   focus on waste management.As a result, 26 eco-towns were approved for implementation (Van Berkel et al., 2009).We could identify 9 successfully implemented cases documented in the literature, of which only one (Kawasaki) has water-related synergies (Van Berkel et al., 2009).
In India, where no national program directly promotes eco-industrial parks or industrial symbiosis, the government incentive programs for pollution control including tax exemptions and subsidies are the primary drivers for industrial symbiosis (Bain et al., 2010;Singhal and Kapur, 2002).However, lack of sufficient financial resources and infrastructure has challenged the implementation of industrial symbiosis in India (Neves et al., 2020).This is particularly relevant for water synergies that in many cases require major infrastructure.Only two out of the ten successful examples found in academic literature have water-related synergies (the cases of Naroda and Nanjangud).The case of Naroda was developed based on a partnership program between India and Germany in 1997 surveying more than 477 industries for potential symbiosis projects, of which only one water-related synergy (a common wastewater treatment plant) was successfully implemented (Lowe, 2001).
Europe is the leading region in the number of implemented industrial symbiosis cases reported in literature (110 out of the 213 -see Fig. 4).However only 18 cases pertain to water innovation practices, a figure that likely underestimates the actual number in Europe.This is due to commercial sensitivity, which prevents many of the industrial symbiosis cases in Europe, particularly in Germany, France, and Spain (reported in the international survey by Massard et al., 2014) to be documented in the literature in sufficient detail.However, considering the diversity of practice in the identified European cases (see section 4.3), the analysis of this limited number of cases could offer valuable insight into the common water innovation practices in Europe.Successful industrial symbiosis cases with water innovation are located in northern and central Europe with fewer instances in southern Europe (see Table 1).
In contrast to the Asian cases, the European industrial symbiosis cases vary in how they were developed (Domenech et al., 2019).Based on our findings, cases with water at their center or as a major part of the symbiosis were evolved mainly through local interfirm collaboration.Some were formed to secure water supply for the industry, such as Kalundborg in Denmark and Prato in Italy.The planned European cases (mainly located in Germanysee Massard et al., 2014) have typically water-related synergies in the form of shared utilities among single industries such as in chemical industrial parks.There are also European cases that have been created through facilitation (a combination of planned and spontaneous development- Costa and Ferrão, 2010), with the support of national policies while initiating the program locally; for instance the case of Humber in the United Kingdom was initiated based on inspiration from the case of Altamira-Tamaulipas (Mexico) and supported by the national industrial symbiosis program (Mirata, 2004).
Besides Asia and Europe, the other regions of the world have a very limited number of documented industrial symbiosis cases with water innovation: North America (three cases in the United States), Africa (three casesin Ethiopia, Namibia and Tanzania), Oceania (three cases two cases in Australia and one in Fiji), and South America (one case in Mexico).
In North America, the United States initiated the development of industrial symbiosis in the early 90s immediately after uncovering the case of Kalundborg in Denmark (Chertow, 2000).However, despite numerous research activities on the potential and planned cases (Neves et al., 2019a(Neves et al., , 2019b)), the implemented cases are low in number as compared to other developed regions, and even less when it comes to water innovation.The main reason, as Neves et al. (2020) pointed out, could be related to the tight legislations on waste management in the United States as compared to Europe.This could also apply to legislations for water-related waste streams, prohibiting the adaptation of water-related resource recovery solutions.The two well-known industrial symbiosis cases in the United States that have water innovation (Guayama and Barceloneta) are in Puerto Rico, a region with less strict environmental regulations (Chertow et al., 2008).
In Oceania, the implemented industrial symbiosis cases are limited to three in Australia, with two practicing water innovation (Kwinana and Gladstone, see Table 1).The case of Kwinana has the highest number of water-related synergies among the identified cases with water innovation (17 synergiessee Table 1).The tight regulations on groundwater abstraction is identified as the main driver for water innovation practices in Kwinana to diversify water supply (Harris, 2007).
In Africa, a region expecting significant industrial growth in the coming years and seeking to promote sustainable industrialization, the number of implemented industrial symbiosis cases is very limited so far.The reported cases, particularly those located in South Africa, are mainly focused on the recycling of waste streams (Oni et al., 2022), despite the fact that large parts of Africa are characterized as water scarce.In South Africa, a highly water stressed country with water demand expecting to exceed the available fresh water supply already by 2025 (Habiyaremye, 2020), no industrial symbiosis case with water innovation is identified in the literature.
Based on the above observations, the trends in adopting water innovation in industrial symbiosis vary highly by region.In Asia, the extent of water innovation practices seems to be mainly driven by policies, though not always in favor of water.The industrial symbiosis cases in China are mainly developed as a result of legislations strongly focusing on water, whereas in Japan and South Korea, the national programs directly promoting industrial symbiosis are mainly focused on waste management.In Europe, on the other hand, local (market-driven) initiatives seem to be the main drivers for adopting water innovation in industrial symbiosis facilitated by national/European policies.In other regions, the identified cases with water innovation were very sparse and therefore no general observation on the main drivers could be made.

Analysis of water innovation practices in industrial symbiosis
In this section, we present the results of analysis on the identified water related synergies, including categorization of water innovation practices (section 4.1), their specification in the reviewed cases and their global distribution (section 4.2).
As a result of our detailed review of the relevant literature on the 57 industrial symbiosis cases, we identified 179 water-related synergies, which are detailed in Table S1 of the supplementary information.We would like to highlight that about 130 of the water-related synergies, identified in this review, have not been reported in the available synergy databases in existing literature, including MAESTRI (Benedetti et al., 2017), EPOS (Lessard et al., 2017), and SCALER 100 (Stéphane et al., 2019).

Categorization of water innovation
While literature commonly defines industrial symbiosis as a network of "physical" exchange of resources (energy, water, and material), in principle many industrial symbiosis projects around the globe are in the form of utility sharing rather than demonstrating interfirm synergies.The primary motive for utility sharing is to increase performance reliability and reduce facility cost (Chertow et al., 2008).
The water-related synergies identified in this review can be divided into two main types: i) utility sharing; and, ii) by-product exchange, as suggested by (Van Beers et al., 2007).Additionally, Chertow et al. (2008) proposed a third category called "joint service provision", which is considered a form of utility sharing in this study.
Furthermore, we conducted a detailed analysis of the 179 identified water-related synergies and identified six categories related to water innovation practices (see the summary and main practices in Table 2): 1. Shared alternative water resources is a type of utility sharing synergy primarily formed to increase water supply security by sharing facilities that provide alternative water resources alongside where the effluent from the common treatment plant is first infiltrated into the groundwater downstream from an aluminum refinery for extraction and reuse.Another instance of water recovery for reuse includes on-site irrigation (e.g., Nanjangud, India).4. Energy recovery from water involves the direct recovery of energy from water, including exchange of a cold or warm wastewater stream for direct use (e.g.exchange of warm cooling water from power plant to fish farms in Kalundborg, Denmark; or exchange of warm municipal effluent to a cogeneration plant in Banwol and Shiwha, South Korea) or through heat exchangers (e.g.exchange of warm effluent for municipal district heating in Kalundborg, Denmark; exchange of cold chalk quarry's water from a cement plant for district cooling in a hospital in Aalborg, Denmark).Indirect energy recovery includes using sewage sludge (a by-product of wastewater treatment) as biofuel (e.g., in Yeosu, China; Forth Valley, UK; Kalle Albert, Germany) or for biogas production through anaerobic digestion (e.g., in Fiji project, Fiji; Tunweni, Namibia).5. Material recovery from water involves the exchange of an industrial or utility water stream containing a certain material (e.g., a nutrient, a salt) or the exchange of a by-product from the wastewater treatment process (primarily sewage sludge).Examples include the exchange of nutrient rich water (e.g., treated municipal effluent for fish farming in Dafeng, China; or brewery wastewater used for integrated farming in Tunweni, Namibia) and the exchange of salty water (e.g., effluent from a salt field plant to a potassium sulphate plant in Weifang, China).Another common form of material recovery from water is the exchange of sewage sludge as raw material for cement production (e.g. in S.Croce sull'Arno, Italy; Eco World Styria, Austria; Wu'an, China) or fertilizer production (e.g. in Camusca, Portugal).In some cases, with less legislative restrictions, sewage sludge is exchanged for direct land use (e.g., in Nanjangud, India).In a form of upcycling for material recovery, a firm in Kalundborg, Denmark, uses the biogas from an industrial wastewater treatment plant for single-cell protein production.6. Material exchange for enhancing (waste) water treatment is another water-related synergy, though less common than the practices mentioned above.However, it is worth mentioning it as a separate subcategory of by-product exchange as it has a strong focus on (waste) water treatment.This type of synergy mainly involves the exchange of organic sources and chemicals commonly used for

Table 2
The categorizations of water innovation and the main water-related practices identified in the reviewed global industrial symbiosis cases.water/wastewater treatment such as exchange of excess ethylene glycol from a cooling company to a municipal wastewater treatment plant in Aalborg, Denmark, or the exchange of acid residues from a chemical industry for water/wastewater treatment in both Humber, United Kingdom, and Ecosite, Switzerland; Other instances include the exchange of high strength ammonia containing wastewater from a metal recovery plant to enhance wastewater treatment in a pharmaceutical company in Ulsan, South Korea and directing a scrubber containing the wastewater stream from a print company to a municipal wastewater treatment plant as a carbon source.As an alternative practice, in Kymi, Finland, municipal wastewater sludge is exchanged to stabilize the biological treatment in a pulp and paper factory.

Prevalence of water innovation in the reviewed cases
Based on the above categorization, we studied the prevalence of water innovation practices in the reviewed industrial symbiosis cases.Fig. 5 provides an overview of category distribution for each case.The cases are sorted based on their total number of water-related synergies (also provided in Table 1).The cases with one water innovation are industrial symbiosis cases with one or more other forms of synergies that were not water related.As per definition of industrial symbiosis (see section 2), these cases have a total number of synergies exceeding two, involving at least three different entities-see Table 1.
Based on the analysis of water innovation types in the reviewed cases (see Table 3), we found that the majority of industrial symbiosis cases practice water innovation in both utility sharing (34 cases) and byproduct exchange (51 cases).Utility sharing for alternative water supply is practiced in fewer than one fourth of the cases (13 cases), while shared wastewater treatments are practiced by approximately half of the cases (26 cases).More than half of the cases practice water-related byproduct exchange for water recovery (35 cases), while around one third of the cases practice energy and material recovery from water.Material exchange to enhance wastewater treatment is observed in only seven cases.It is important to note that the numbers may not align with the sum of (sub)categories, as many cases practice multiple categories of water innovation.
Fig. 5 clearly illustrates the extent of water innovation in the reviewed industrial symbiosis cases, in terms of the number of waterrelated synergies and number of water innovation categories practiced.It is noteworthy that some of the successful examples of industrial symbiosis in the literature, such as Landskrona (Sweden), Styria (Austria), and Humber (United Kingdom) Gladstone (Australia), Guayama and Barceloneta (United States), Kawasaki (Japan), and Ulsan (South Korea) have relatively little focus on the implementation of water innovation, with only one or two water-related synergies that were identified.Therefore, as briefly mentioned in the introduction, the analysis of water innovation in this review relied also on many of the less-documented industrial symbiosis cases.
Nevertheless, the two industrial symbiosis cases with the highest number of water-related synergies are the well-known cases of Kwinana, Australia (18 water-related synergies) and the Kalundborg case in Denmark (12 water-related synergies).Both cases have a strong focus on water as the industrial symbiosis was initially developed due to stress on the available freshwater resources (groundwater in both cases).Therefore, there was a strong need to reduce water consumption mainly through water recovery and use of alternative water resources.The main difference between these two major cases is the extent of water innovation categories covered in the implemented water-related practices.In the case of Kwinana, the synergies are either in the form of by-product exchanges for water recovery (covering 80% of the water-related synergies) and utility sharing for alternative water supply.In Kalundborg, on the other hand, the water-related practices have been more diversified over time and cover five of the six identified water innovation categories.A similar diversity is observed in the case of Weifang industrial symbiosis (China), which has the 3rd highest number of water-related synergies (8 synergies).Other cases with a relatively high diversity despite a lower number of water-related synergies are Yongcheng (China), Yixing (China), and Kymi (Finland).We note that the diversity of practices within each water innovation category is not considered in the analysis.However, it is noteworthy that, in some of the categories, particularly in utility sharing resources for alternative water supply, the diversity can provide more "fit-for-purpose" opportunities and thereby increase the security of water supply within the network.The best example of the diversity of water supply is the abovementioned case of Kwinana, Australia, where utility sharing for alternative water supply (including reclaimed water, desalinated seawater, direct seawater use, and storm water) constitutes 44% of the total water supplythe rest is supplied by groundwater (Oughton et al., 2021).
As stated earlier, the analysis in this study of the number of waterrelated synergies and the categorization of water innovation is an attempt to compare the extent of water innovation practices (with a focus on resource recovery) in implemented industrial symbiosis cases worldwide.However, some cases we examined feature other forms of water innovation practices that are not highlighted in our analyses.These cases pertain to water-based landscape ecology and do not entail direct resource exchanges or shared facility usage but present alternative uses of water resources.For instance, the case of Jurong Islands in Singapore (Seetoh and Ong, 2008;Yang and Lay, 2004), while being low in number and diversity of water innovations, has strong focus on water.In addition to major infrastructure for alternative water supply, the development of "water corridors" has connected a network of industries and facilitated inter-firm material exchange.These water corridors represent a unique example of "landscape ecology", which sought to soften the industrial landscape of Jurong Islands and make it more aesthetically pleasing (Yang and Lay, 2004).Another case that incorporates landscape ecology is the industrial symbiosis case in Tianjin, China.In 2006 alone, 2.35 million tons of processed water was used for recharging an artificial wetland and landscaping (Shi et al., 2010).
As observed in Fig. 5, the reviewed industrial symbiosis cases exhibit significant variation in terms of the number of water-related synergies and the water innovation categories practiced.The level of implementation for each water innovation category on regional and global scale is provided in Fig. 6 (details are provided in supplementary information, Table S1).
As illustrated in Fig. 6, water recovery is the most reported byproduct exchange, accounting for 45% of the identified synergies.Over half of the recovery synergies are in Asia with China having the most (see Fig. 6 and Table S1 in SI).Water recovery is also predominant in cases in Oceania and North America.In Oceania, in particular, sixteen water recovery synergies are documented in only one industrial symbiosis case (Kwinana).In Europe, water recovery covers about one fourth of the water-related practices.Due to the limited number of cases in Africa and South America (see Table 3), only one water recovery synergy in each region was identified.Material and energy recovery from water are the other common by-product exchanges mainly practiced in Asia and Europe.Similarly, material exchange to enhance wastewater treatment is limited to Europe and Asia.Shared wastewater treatment is the most common form of utility sharing globally, while utility sharing for alternative water supply is practiced to a lesser extent (limited to Asia, Europe, and Oceania).
Besides clearly demonstrating the predominance of specific water innovation categories (i.e., water recovery), Fig. 6 also provides an overview of the diversity in water innovation practices in each region.Europe and Asia are found to be the only regions practicing all the six water innovation categories.In Europe, the distribution of the categories is fairly even, indicating high diversity in the adopted water innovation practices.*The numbers do not necessarily match the sum of (sub)categories as many of the cases practice multiple subcategories of water innovation.
E. Ramin et al.

Sectoral analysis
This section presents the result of sectoral analysis to identify the primary industrial sectors involved in water innovation synergies.From the literature review, we were able to identify and describe most of the companies involved in the examined synergies (in 110 out of 179 synergies).Some utility sharing synergies involve more than two companies, and synergies involving solely shared industrial wastewater plants were excluded from sectoral analysis.To simplify the analysis, we grouped some of the sectors.We performed sectoral analysis on both manufacturing (e.g., chemical, metal, paper) and non-manufacturing industries (sewerage and energy industry).
Table 4 summarizes the number of water-related synergies practiced by each industrial sector grouped based on their ISIC codes.In total, 222 individual companies (111 donators and 111 receivers) were identified and classified into 28 sectors, mostly at 3 digits except the cement sector, which uses a 4-digit code.Out of the 111 identified synergies, 100 are related to by-product exchanges and 11 involve utilities sharing.
Based on the information in Table 4, most water-related synergies involve three industrial activities: chemicals, power generation, and sewerage.Water intensive sectors with a high potential for water-related synergies such as food products, pulp and paper and textiles represent about 10% of the sample analyzed.Fig. 7 (left graph) highlights the type of water-related by-product exchange that each industrial group is participating in and their role as sender or receiver.It is noticeable that the exchanges involving the three main groups of sectors (chemicals, power generation and sewerage industry) consist mostly of water recovery.The synergies focused on energy recovery from water are related mostly to power generation and sewerage industry.As for material recovery from water, agriculture, and fishing, as well as chemicals companies are the main actors, followed by sewerage.Finally, the material exchange synergies aiming at enhancing wastewater treatment naturally involves sewerage companies.Overall, Fig. 7 shows that most synergies are focused on water recovery, especially on the three main groups of sectors.This indicates that there are a significant number of overlooked water synergy potentials involving other forms of exchange (material and energy from water), especially in those sectors that might be protagonists such as food producers.Fig. 7 (right hand side graph) highlights the role of each sector as sender or receiver of the water-related exchange.The sewerage industry is primarily identified as a sender, while the power generation industry is primarily identified as a receiver of the water-related synergies.The chemical industry is heavily involved both as sender and receiver.
Based on our analysis, we conclude that the sewerage industry has an extensive and diverse involvement in all the water innovation categories  (It is identified as the primary sender for water recovery as well as material and energy recovery exchanges and the main receiver of material exchange for enhancing treatment).In most instances, sewerage companies are public/municipal wastewater treatment plants (the common industrial wastewater treatment plants are not included in the sectoral analysis).Therefore, these types of synergies are also referred to as public-private partnerships.To elaborate more on the potentials for water innovation through industrial symbiosis with a sewerage company, in Table 5, we provide an overview of the water-related byproduct exchanges identified in the reviewed cases that involve the sewerage industry.
Based on sectoral analysis on the current global water innovation practices in industrial symbiosis, we conclude that industrial activities involving especially sewerage, power generation and chemicals can greatly increase the opportunity for water innovation when involved in industrial symbiosis activities.The sewerage industry, in particular, is shown to play an extensive and diverse role in pursuing water-related synergies.As we showed, the role of the sewerage industry in industrial symbiosis is beyond its primarily known role (sender of secondary effluent for water recovery) and can be expanded to sender of water for energy and material recovery and receiver of materials that can enhance wastewater treatment.This diverse role can involve other resource intensive and less "water-relevant" industries such as the cement industry receiving sewerage sludge as biofuel or raw material.On the other hand, we observe very limited involvement of the water intense manufacturing sectors (e.g., food production and textiles) in the documented water-related synergies in the literature.This reflects a likely major gap in water symbiosis innovation practices that needs to be addressed in future research.

Discussion
This comprehensive review addresses the potential of resourceintensive industrial sectors in pursuing a more efficient water management in practice through the analysis of the prevalence of water innovation in industrial symbiosis through successful case studies.We considered water not only as a resource and by-product, but also a carrier of material and energy, as diversifying the role of water in the context of industrial symbiosis would uncover more water-related resource recovery solutions.
As we illustrated in the analysis on water innovation, water recovery is by far the most common type of synergy among the water innovation practices identified in the review industrial symbiosis cases.As argued by Harris (2007), besides the necessity of water recovery to reduce water consumption, the recent advances in the water recovery technologies, particularly those relevant for industrial symbiosis (e.g., reverse osmosis) can help make water recovery a preferred option as compared to consumption of standard water sources.On the other hand, the economic challenge concerning the energy intensity of these technologies, particularly in the prevailing energy crisis, might hinder this advancement.In addition, industries are still facing technical challenges with regards to (reclaimed-) water reuse, such as (bio-)fouling and chemical or microbial-induced corrosion (Exall, 2004), as well as unforeseen pollution accumulations (authors' own knowledge of a failed industrial symbiosis case in Denmark).Therefore, considering that many of the potential synergies fail at the implementation level, by diversifying water innovation and taking advantage of other forms of recovery synergies (e.g., material and energy from water/wastewater), industries could increase their opportunities for more industrial symbiosis involvement and can further improve the efficiency in water management in the context of industrial symbiosis.
The limited literature on quantified industrial symbiosis presents a challenge in evaluating the effectiveness and potential of water-related synergies in environmental or economic terms (Van Berkel et al., 2009).While economic gains are a major driving factor for creating industrial synergies (Tudor et al., 2007), not all the potential water-related synergies may provide short-term economic benefits, particularly in the case of water treatment and recovery, as they cannot compete with other governmental incentives such as affordable water supply and low fees and taxes on discharge.
The cases that were reviewed in this study highlight a variety of factors that either facilitate or limit the progress in water innovation adaptation in industrial symbiosis.Table 6 summarized some of the significant global examples that were addressed throughout the paper.In addition to economic gains, other socio-economic factors such as societal approval and concern of local communities are equally important in pursuing industrial synergies, particularly when water is a shared  4 for more details on each industrial group.resource among industries and communities.For example, a power plant in Guayama, Puerto Rico, engaged in a symbiotic activity for using reclaimed wastewater, despite the high associated costs, to gain society's approval to operate in the region (Chertow and Lombardi, 2005).It remains to be evaluated how this synergy has improved the social image of that industry, given the potential damage it can cause to the local community and environment.Therefore, considering the concern of local communities can be decisive in pursuing the industrial symbiosis projects that directly impact their local environment, particularly with regards to depletion and pollution of fresh water resources.Based on the industrial sector analysis, we could illustrate how public-private partnerships between public utilities (sewerage industry) and nearby industries can foster water innovation.This clearly illustrates the role of the public sector with more environmental concern in balancing the economically driven motives of the industries involved in water-related synergies.On the other hand, inefficient public-private partnerships can hinder water innovation as well.In the example of Barceloneta, Puerto Rico, addressed by Ashton (2011), utility sharing for wastewater treatment failed due to poor operations management by the public sector in not meeting the regulatory requirements.Beyond participating in physical exchanges, public utilities are shown to act as facilitators for water-related synergies.The best examples are the cases of Kalundborg (Denmark) and Kwinana (Australia), where public water utilities are responsible for coordinating and providing alternative water supplies to various industries through shared facilities.
Research has shown that governmental legislation and policies can enable or hinder the development of industrial symbiosis (Costa et al., 2010).As Desrochers (2001) argues, the famous case of Kalundborg would have never happened if it was in the United States, considering the country's tight environmental legislations as compared to Denmark (or Europe in general).In section 3.1, we provided a short overview of the extent of water innovation addressed in some of the national policies promoting industrial symbiosis e.g., China, Japan, South Korea, and India.However, comprehensive research on the global water management policies is needed to understand how policies can influence the adoption of water innovation in industrial symbiosis.Moreover, as Lybaek et al. (2021) point out, this should not be limited to legislations directly promoting (or supporting) industrial symbiosis, and should include analysis of indirect policies at the local level.As water eco-innovation and water circular management are very context specific, local policies could play a vital role in promoting their adoption in industrial symbiosis cases.
The ultimate goal of water innovation in industrial symbiosis is to contribute to a more sustainable water management beyond the boundaries of individual industries.Industrial symbiosis is closely linked to circular economy, a framework for designing economic systems that aim to eliminate waste and keep resources in use for as long as possible.Water plays a critical role in both industrial ecology and circular economy.Through the implementation of water innovation in industrial symbiosis, industries can optimize their water use and minimize their water discharge, thereby reducing their environmental footprint.This approach also facilitates the recovery and reuse of valuable resources, such as materials and energy from water streams, contributing to a circular economy.To assure that, it is necessary to evaluate the sustainability aspects of the water innovation practices in each specific context.A recent review by Willet et al. (2019) has shown that, among the methodologies applied to assess the sustainability of water use in industries, holistic quantitative approaches such as life cycle analysis in combination with other methods that can address the local context allows for evaluation of environmental impact of water use at different geographical scales.Multi-scale sustainability assessment of water management strategies is particularly relevant in the context of industrial symbiosis, as synergetic resource recovery can have consequences on all the industries and communities involved through different pathways (e.g., avoiding discharge of (warm) water to environment, sewage sludge to landfills, etc.).However, as Nika et al. (2020) argued, symbiotic management of water resources requires valid flow and input information.In the particular case of industrial symbiosis, this requires active involvement of industries that want to pursue a more sustainable resource management, and here research can help facilitating the communication by providing best practices, frameworks and indicators.
Despite the limitations of current literature, the importance of symbiotic water innovation practices for the sustainable development of industries cannot be overstated.Further research and case studies in this area are necessary to better understand the potential of industrial symbiosis for achieving water sustainability goals.
Key Takeaways.Water plays a key role within the complex network of industrial symbiosis, serving as a cornerstone for not only fundamental industrial processes but also for its profound socio-environmental implications.Expanding beyond the traditional emphasis on water reclamation and reuse, diversifying water-related innovations can improve the efficiency of water management in industrial symbiosis.Achieving this transformation requires strong collaboration between public and private sectors, along with the adoption of policies that directly or indirectly encourage water innovation.Limitations of this study.The analysis performed in this article has certain limitations.The reviewed industrial symbiosis cases are limited to the successfully implemented cases that have been reported in the academic literature in English.Therefore, many of the cases that have not been reported in the literature or are not described in sufficient detail, are excluded from the analysis.The sectoral analysis performed does not distinguish between the public and private nature of companies.This is particularly relevant for sewerage and power generation industries that can be public service companies and subject to certain legislations in different countries, which can limit their involvement in industrial symbiosis activities.

Conclusions
This review provides valuable insights into global water innovation practices beyond traditional industry boundaries that support sustainable water management through a collective approach.The comprehensive literature review has enabled the identification of global water innovation practices in industrial symbiosis and the creation of a comprehensive database that includes the type of water innovation and the involved industrial sectors.Our analysis yielded the following findings, addressing the research questions posed in the introduction (RQ1, RQ2, and RQ3): • Regional differences in adopting water innovation in industrial symbiosis highlight policy-driven motives in Asia and spontaneous/ facilitated motives in Europe.Examples of influencing factors in adopting water innovation are derived from the reviewed cases.• The global water innovation practices were analyzed through six categories: utility sharing for alternative water supply, wastewater treatment, water recovery, energy recovery from water, material recovery from water, and material exchange for water/wastewater treatment.Water recovery was identified as the most common form of water-related synergy.Europe and Asia were identified as the regions with high diversity in adopted water innovation practices.• Involvement of chemicals, power generation, and sewerage industries increases the opportunity for water innovation.Publicprivate facilitated synergies involving sewerage industry have been identified as the main drivers in many of the cases.
Further research is needed to evaluate socio-economic factors and policies influencing the adoption of water innovation in industrial symbiosis and to develop a framework for sustainability assessment of water innovation for circular water management in industrial symbiosis.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.The flow diagram of the literature review for identifying water-related synergies in the implemented industrial symbiosis cases worldwide.

Fig. 2 .
Fig. 2. The boundaries used to identify water-related synergies (indicated by arrows) in the reviewed industrial symbiosis cases.

Fig. 3 .
Fig. 3. Geographic distribution of: a) all the 213 implemented industrial symbiosis cases identified in literature; and b) the identified 57 cases with water-related synergies (size of the bubbles indicates the number of water-related synergies).

Fig. 4 .
Fig. 4. Number of industrial symbiosis (IS) cases per country (blue: cases with water-related synergy, grey: cases with no water-related synergy or no specific information found in literature).(For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

E.
Ramin et al.   conventional water supply systems in the area.The main practices include shared facilities for extracting, distributing, and treating surface water (e.g., in Kalundborg, Denmark; Prato, Italy), using seawater through shared desalination plants (e.g., in Taranto, Italy) or directly in shared cooling systems (e.g., in Kwinana, Australia), and use of reclaimed water through shared reclamation plants (e.g., in Jurong Islands, Singapore; Pohang, South Korea).There are also synergies involving rainwater harvesting through shared facilities in a park (e.g., Ecopark Hartberg Steiermark, Austria) or facilitated (e. g., in Aalborg, Denmark, where harvested rainwater by farmers is directed to a power plant for reuse).Joint utilization of underground brine (e.g., Weifang, China) is another example.Although (nonpotable) aquifer recharge using treated (municipal) wastewater is a global practice to provide alternative water resources(Kanarek et al., 1993), no such example was found in the context of industrial symbiosis, i.e. shared facilities for water reclamation through aquifer recharge.2. Shared wastewater treatment is another form of utility synergy established primarily for pollution prevention.This subcategory includes both wastewater treatment plants and anaerobic digestion of sewage sludge (biogas plants).The synergy involves building shared facilities between similar firms (e.g.Chemical park in Shanghai, China; a cluster of tanneries in S.Croce sull'Arno, Italy; neighboring pharmaceutical/biotechnology plants in Barceloneta, Puerto Rico and Kalundborg, Denmark) or using the existing treatment facilities of a larger firm by the neighboring smaller firms (e.g.use of facilities at pulp and paper plants in both Kymi and Uimaharju, Finland).Major infrastructure for the whole industrial park is also established through public and/or private partnerships (e.g. in Tianjin, China; Naroda, India; Altamira-Tamaulipas, Mexico; Hawassa, Ethiopia).3. Water recovery is a common water-related synergy in the reviewed cases mainly involving the exchange (or cascading) of wastewater between two firms (e.g., Terneuzen, The Netherlands; Landskrona, Sweden), and the reuse of secondary effluent from municipal or shared industrial wastewater treatment facilities (e.g., in Suzho, Dalu and Jiangsu in China; Campbell, USA; Kawasaki, Japan).The exchange of tertiary treated effluent, typically between municipal wastewater treatment plants and industries, involves a shared (public and/or private) reclamation plant (e.g., Pohang, South Korea).Groundwater-based water recovery is identified in Kwinana,

Fig. 5 .
Fig. 5. Overview of the number of water-related synergies and the water innovation categories identified in each of the 57 reviewed industrial symbiosis cases.WWT: wastewater treatment.

Fig. 6 .
Fig. 6.Distribution of water innovation practices a) in each region, and b) globally.

Fig. 7 .
Fig. 7. Number of water-related by-product exchanges involving each of the industrial groups based on water innovation categories (left), and their roles as sender or receiver (right) -see Table4for more details on each industrial group.

Table 1
The reviewed industrial symbiosis cases with water innovation.
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Table 3
Number of reviewed global industrial symbiosis cases practicing each of the water innovation categories/sub-categories.

Table 4
Number of water-related synergies (utility sharing, by-product exchange, and total) for each of the industrial groups involved in water innovation in the reviewed global industrial symbiosis cases.
a Others include printing services, wholesale, landscape and maintenance services and hospital.E.Ramin et al.

Table 5
Primary water-related synergies in industrial symbiosis involving sewerage industry.
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Table 6
Primary examples of influencing factors on adoption of water innovation in industrial symbiosis derived from reviewed cases.