A new evaluation framework for the assessment of wastewater heat recovery potential coupled with wastewater reuse

Climate changes remain one of the most significant challenges faced by the global community. Sustainable energy plays a crucial role in combating climate changes and mitigates its impacts. Heat recovery from wastewater is a promising approach for sustainable and environmentally friendly energy utilization, offering advantages in terms of lower carbon emission compared to fossil fuel combustion. The thermal energy content of wastewater can be effectively recovered through a well-designed system that incorporates a heat exchanger and a heat pump installation. This system enables the extraction of thermal energy from wastewater, making it a valuable renewable heat source. In general, there are three different locations that are possible for heat recovery from wastewater: buildings, sewer systems, and the effluent of wastewater treatment plants (WWTPs) (Kretschmer et al. 2016a, 2016b; Huber et al. 2020). Recent studies have reported that heat recovery from WWTP effluent is advantageous in terms of stable flow rate and low variation temperature with a temperature drop of up to 8 °C (Spriet et al. 2020; Arnell et al. 2021; Nagpal et al. 2021), minimizing the fouling effect in the heat exchanger (Somogyi et al. 2018; Hao et al. 2019), while positively impacting the biological processes of the receiving water bodies (Neugebauer et al. 2015; Simperler 2015). Since the implementation of the heat recovery process within effluent WWTP, the decreased temperature due to thermal energy extraction does not have a negative impact on the biological nitrification process in WWTP (Wanner et al. 2005; Hao et al. 2019; Arnell et al. 2021). Thus, the high potential of heat recovery from WWTP effluent is evident when compared to heat recovery from other location. Additionally, the treated effluent resulting from the heat recovery process holds promise as a clean water resource suitable for wastewater reuse purposes.

The combination of heat recovery from WWTP effluent and its subsequent reuse has positively impact on sustainability and resource management. The recovery of thermal energy enhances the overall value of this renewable heat source, enabling its utilization in various applications like space heating, water heating, or industrial process. Concurrently, the utilization of treated WWTP effluent for non-potable purposes, including irrigation, industrial processes, and toilet flushing, significant water conservation is achieved, reducing the demand for freshwater resources (Lyu et al. 2016; Kehrein et al. 2020; Takeuchi & Tanaka 2020; Ofori et al. 2021). The concept of water–energy nexus (WEN) offers a scientifically approach to effectively integrate heat recovery and its subsequent reuse. This concept emphasizes the intricate interdependencies and interrelates between water and energy resources, highlighting the potential for integrated strategies to optimize resource utilization and enhance overall system efficiency (Hamiche et al. 2016; Magagna et al. 2019; Fayiah et al. 2020). Adopting the water–energy nexus perspective provide several key advantages including resource synergy and optimization, energy conservation and efficiency, as well as enhanced system efficiency (Baur et al. 2014; Lee et al. 2017; Dai et al. 2018).

The integration of heat recovery and treated wastewater reuse presents numerous advantages; however, the concept faced challenges in determining appropriate location for implementation. Key factors contributing to these challenges include geographical consideration and the variability in heat demand (Lichtenwoehrer et al. 2019). Additionally, the remote location of WWTPs often results in disadvantageous energy supply distances, leading to increased heat losses during distribution. Consequently, selecting an appropriate location is essential to ensure economic feasibility and process effectiveness. To address the challenges in determining appropriate locations, a comprehensive approach is essential, involving detailed feasibility studies, and site-specific evaluations. The approach for pre-assessment suitability locations was developed by applying relevance tree methods, which consider both energetic and wastewater perspective (Huber et al. 2020). The relevance tree in this method constitutes a scientifically rigorous tool for decision-making process with consider various criteria. However, the specific focus in this methodology lies in assessing the suitability location for sewer heat recovery, with a particular emphasis on predicting the effect of sewer heat recovery on the inflow temperature of WWTP. The study by Neugebauer et al. (2015) and Spriet et al. (2020) evaluated the performance of heat recovery location utilizing WWTP effluent, but the approach primary focus on spatial and temporal aspect, as well as energetic analysis.

Although these approaches have their strength, but existing literature lack a comprehensive evaluation framework that incorporates various criteria related to the combination of heat recovery and treated wastewater reuse for quick and efficient decision-making in selecting appropriate locations. To address this gap, this study develops a new evaluation framework to assess appropriate locations of WWHR coupled with treated wastewater reuse considering rigorous assessments of thermal energy potential, spatial distribution analysis, and infrastructure compatibility (potential consumer). A pre-assessment process was conducted to identify potential water–energy suppliers (WWTPs) and consumers, utilizing a geographical information system (GIS) to facilitate spatial analysis. The selection of assessment criteria and attributes was achieved through screening process, incorporating key criteria from the literature review and novel criteria generated from experts' group brainstorming. To evaluate the performance of criteria, the AHP was performed based on expert opinion. Through the application of the scientific evaluation framework, decision-makers gained valuable insights to determine the most suitable locations. The systematic integration of geospatial data, expert opinions, and assessment criteria ensured a thorough and objective analysis. To the best of authors' knowledge, this study presents the first conceptual evaluation framework specifically designed for identifying the suitable location of thermal energy recovery from WWTP effluent combined with wastewater reuse. The utilization of scientifically approach ensures the objectivity and reliability of the selection process suitable locations.

The methodological approach to assess appropriate locations of WWHR coupled with treated wastewater reuse involves a systematic three main steps process, comprising pre-assessment, develops evaluation framework by applying MCDA methods, and site prioritization.

The objectives of the pre-assessment phase encompass the systematic screening of wastewater treatment facilities and consumers. This phase is designed to establish a comprehensive foundation for the subsequent stages of the evaluation process. To facilitate this, a set of pre-assessment criteria was defined. These criteria serve as guiding principles in the selection of WWTPs and consumer clusters, which fulfill the roles of water–energy suppliers and consumers, respectively. Table 1 lists the three pre-assessment criteria used in this process:

The selection of the pre-assessment criteria involves a systematic consideration of key factors including the capacity of WWTP. This criterion serves as a defining parameter, quantifying both the water and heat energy potential (Kretschmer et al. 2016a, 2016b). According to the wastewater reuse policies in the enforcement decree of the act on promoting and supporting of water reuse (MOE 2022), Article 12 states that for a treatment facility that has a capacity of treating 5,000 m³ of wastewater per day, at least 10% of the volume of treated wastewater should be reused a day. The capacity of WWTP assumes paramount significance within the context of our study, as it serves as an essential parameter delineating the thermal energy potential inherent in treated wastewater and, in parallel, the potential for wastewater reuse. This particular parameter exhibits an interdependence with the broader water–energy supply potential. In accordance with these pivotal parameters and regulatory standards, a minimum WWTP capacity is set at 10,000 m³/day. This selection is predicated on the principle that such a WWTP can effectively operate as a potential water–energy supplier, which can fulfill the demands of a total floor area spanning approximately 60,000 m². Furthermore, the distance between WWTP and potential consumer emerges as a critical criterion, which reflects the economic feasibility of energy transmission. In consideration of the energy supply distance, the wastewater heat recovery manual for Japan have indicated an optimal spatial interval of approximately 500 m between a potential supplier and consumer (MLIT 2015). In addition, the total floor area of the buildings assumes significance as an indicator of potential consumers. This factor denotes the spatial capacity to accommodate the distribution and utilization of renewable thermal energy and treated wastewater reuse. Aligned with the enforcement decree of the act on promoting the new and renewable energy, specifically Article 15 (MOTIE 2021), and correspondingly in alignment with the enforcement decree of the act on promoting water reuse, particularly Article 11 (MOE 2022), we defined the minimum total floor area in the building cluster for the potential water–energy consumer of 60,000 m².

The fundamental goal underlying the spatial analysis through the utilization of geospatial information, pertains to the identification of potential consumers clusters that conform to the predefined criteria, specifically criterion 3 related to the demand area. The spatial analysis entails a comprehensive examination of potential consumers that fulfill the selected criteria (Scherson & Criddle 2014; Kollmann et al. 2017). Geospatial information data related to the total floor area of facilities were obtained from the national spatial data infrastructure (NSDI) portal. The geospatial information datasets were transferred into a GIS. Within this GIS layers, a features filtering process was applied to identified building facilities with a total floor area of 60,000 m² that align with the defined pre-assessment criteria, specifically criteria 3.

MCDA is a part of the decision-making methodologies that has garnered significant attention across various fields due to its effectiveness in facilitating complex decision process (Jaiswal et al. 2015; Martin-Gamboa et al. 2017). These methodologies provide substantial support for conducting a comprehensive evaluation of both criteria and sub-criteria for decision options, thereby contributing to investigating the best alternative to achieve the objective (Yalcin et al. 2022). In the context of this study, MCDA was utilized as an important tool to develop an evaluation framework by identifying the important criteria and their associated attributes. The AHP a component of MCDA was used to evaluate the performance of criteria. By utilizing the AHP, this research measures the relative importance of each criterion, thus facilitating an objective evaluation process.

In order to effectively mitigate the challenge of heat losses during distribution and ensure compliance with the regulations outlined in renewable energy act policies, this research introduces the concept of a water–energy station. Notably, the suggested facility is strategically situated within the confines of the consumer cluster. The attributes associated with each criterion are identified through a brainstorming process guided by expert insights. This process takes into account the operational dynamics of WWTP in South Korea, relevant details regarding the information of the potential consumer, and pertinent findings from previous studies.

The AHP procedure was applied to determine the relative importance or weight value of the criteria based on expert opinion through the AHP survey. The questionnaire survey was carried out with the active participation of 10 experts specializing in wastewater resource management and environmental engineering that consist of water and energy experts. This panel of experts encompassed individuals with diverse professional backgrounds, including five professors, three researchers, and two engineers, ensuring a well-rounded perspective on the subject matter. The survey was carefully organized through the administration of a structured questionnaire, and data collection was meticulously conducted within a defined time frame, spanning 1 week. Each of the 10 experts received a questionnaire electronically and distributed via e-mail. The questionnaire was structured into two distinct sections. In the first part, the 10 experts were asked to evaluate the relative importance of the main criteria, following the reference materials associated with each question. In the subsequent section, these same 10 experts were directed to assess the significance of the sub-criteria. All 10 questionnaires were completed and subsequently collected, thereby constituting the complete dataset that comprehensive analysis. AHP is one of the techniques in the MCDA used to evaluate relative importance through scoring and weighting criteria using pairwise comparisons and final ranking (Abel et al. 2018). The extant literature from Darko et al. (2018), emphasizes that there is no strict requirement for a minimum sample size of judges or experts in AHP analysis. The adequacy of the sample size depends on two key factors: the consistency of the judgments provided by the experts and the practical validity of these judgments (Saaty & Ozdemir 2014).

The determination of an evaluation framework to assess the potential location of WWHR coupled with wastewater reuse is made up of three subsequent steps: (i) the selected criteria, sub-criteria, and attributes that have been grouped into three aspects are integrated into a table; (ii) in addition, the derived weight value assigned to criteria, sub-criteria, and attributes, as determined through the AHP are transferred into a table; (iii) the global weight value of all criteria is calculated by multiplying the weight value of the criteria, sub-criteria, and attributes. It is important to highlight that the sum of these global weight values is 1, wherein this cumulative global weight value serves as the basis for evaluation ratings. To demonstrate its practicability and usefulness, the developed evaluation framework is applied in a case study application to select priority sites by assessing the selected potential WWTP sites with consumers from pre-assessment results.

The prioritization of the most suitable locations was conducted through an assessment of selected potential WWTP sites with the consumer cluster from preliminary assessment (Section 2.2), using an innovative evaluation framework. This procedure consists of a subsequent step: (i) initial grouping involves categorizing the selected WWTP sites and consumer cluster based on their geographical administrative district; (ii) subsequently, the novel evaluation framework is applied to evaluate the WWTP sites, encompassing a straightforward analysis of several factors: the accessibility of feasible water and heat energy suppliers including clean energy ratio in WWTP, capable of providing resources for WWHR and treated wastewater reuse. The identification of demand sources (consumers) to benefit from renewable heat energy and water reuse, and the availability of heat supply depends on the operation of HPS; (iii) the total global weight values ascribed to each WWTP sites facilitates the determination of priority rankings for both potential WWTP sites and the associated consumer clusters.

The identification of potential WWTP sites and consumers cluster was performed utilizing spatial analysis using GIS that was explained in the previous section (Section 2.2.2) that considers three pre-assessment criteria. The analysis of 650 WWTPs by applying pre-assessment criterion 1, resulting in the selection of 236 WWTPs that quantify as candidate water–energy suppliers with a capacity exceeding 10,000 m³/day. In a parallel, the analysis of potential consumer clusters led to the selection of 893 building facilities designated as candidates for water–energy consumers. This selection criterion was based on total floor area exceeding 60,000 m² based on spatial data infrastructure. To optimize of heat distribution and the process effectiveness, the analysis of these candidate water–energy supplier and consumer were expanded. Specifically, water–energy consumers were narrowed down based on their proximity to the WWTPs, limiting the distance less than 500 m. In alignments with the heat recovery manual from Japan that have mentioned an optimal spatial interval of approximately 500 m between a potential supplier and consumer (MLIT 2015). The detailed information regarding the selected potential WWTP sites with the consumers cluster as the water–energy supplier and consumers, is presented in Table 2.

A total of 13 WWTPs sites with a cluster of water–energy consumers were selected as potential water–energy suppliers and consumers. The information related water–energy consumer encompasses two key aspects: the total of buildings within the respective cluster and the specific categorization of each building type or facility. The total floor area of the building cluster for sites W₀₇ and W₀₉ was determined by summing the total floor areas of all buildings in the cluster of water–energy consumers. The larger the total floor area of water–energy consumers, the higher the use of renewable heat energy and wastewater reuse increases economic and environmental benefits (Rezaie & Rosen 2012). The selected of these 13 potential WWTP will become case study sites for applying a novel evaluation framework in determining priority sites for WWHR coupled with wastewater reuse.

• Treated wastewater flow rate. This criterion is a major factor reflecting the water and thermal energy source that determines the potential for WWHR and wastewater reuse. The higher the treated wastewater flow rate, the greater the water–thermal energy that can be recovered (Nagpal et al. 2021), strongly defining the potential locations for WWHR coupled with wastewater reuse. In alignment with the previously outlined concept and in compliance with the mandates of the water reuse act, a minimum threshold of 10% of the treated wastewater volume has been established as the requisite criterion for water–energy supply. Consequently, the attributes for the assessment were selected based on the potential of treated wastewater flow rate from the selected WWTP as the water–energy supplier as shown in Table 4.

• Treated wastewater temperature. As already mentioned before, this criterion is a major factor that determines the available heat energy potential for WWHR. Theoretically, the thermal energy potential can be calculated by following Equation (4) (modified from the original equation proposed by Chae & Kang, 2013):

  

  (4)

  where E is the thermal energy (kJ), Q_ef is the treated wastewater flow rate (m³), ρ_ef is the density of effluent (kg/m³), C_{p, ef} is the specific thermal capacity of effluent (kJ/kg. °C), and ΔT_ef is the decrease in temperature of effluent WWTP (°C) (extracted temperature from heat exchanger). The treated wastewater temperature in overall South Korea showed relatively small seasonal fluctuation, ranging between 12 and 28 °C as compared to atmospheric temperatures spanning −12 °C to 30 °C. The differences between the air and the effluent temperature varied with the seasons, with the maximum in winter (up to 26 °C) and the minimum in summer (0.4 °C) (Chae & Kang 2013). This observation suggest that the effluent water promise consistent heat source for cooling and heating during warmer and colder season. In this study, treated wastewater temperature was predicted to be directly related to the climate of the site's location, which was divided into upper, middle, and lower regions of the country. According to the data of treated wastewater temperature in overall the country, the treated wastewater temperature in the upper, middle, and lower regions in the fall season (November) is approximately 19–20 °C; 21–22 °C; and 23–24 °C, respectively.

• Clean energy ratio in WWTP. This criterion captures the use of renewable energy in WWTP. The use of renewable energy in the WWTP that gives an indirect impact on the WWHR and wastewater reuse, especially for pump systems of treated wastewater and heat energy flow in which the use of clean energy can reduce CO₂ emissions and operational costs in the facility. The attributes of this criterion were determined according to the data use of renewable energy in the selected WWTP from public authorities with the trend of clean energy ratio in the selected WWTP sites is approximately 1–23%.

• Water–energy supply distance. This criterion determines the effectiveness of the WWHR, efficiency of water distribution, and energy efficiency in the process of heat recovery and wastewater reuse, which is associated with the water and energy supply to the water–energy consumer. This criterion reflects the impact of water–energy supply distance on economic feasibility regarding heat losses during distribution. Regarding the distance between supplier and consumer, the appropriate distance was set in the pre-assessment section of less than 500 m and as a benchmark in determining attributes for these criteria.

• Type of water–energy consumer. This criterion determines the technical aspects and economic conditions of the water–energy supply related to the demand side as well as the operation of the HPS. The type of water–energy consumer is a major factor in the energy supply concept that will affect the WWHR process (Kollmann et al. 2017). The size of the building, its thermal insulation, and heating-cooling requirements are factors that can impact the energy-consuming aspects of heat recovery (Cecconet et al. 2020; Wehbi et al. 2022). The type of water–energy consumer was divided into the living facility, medical facility, public facility, business and commercial facility, and another facility (station, airport, etc.). The evaluation of water–energy consumer types is conducted by a thorough assessment of their advantages associated with the utilization of renewable energy and wastewater reuse. Additionally, closely interrelated with the operation of HPS. The use of renewable energy and wastewater reuse in living facilities such as apartment complexes is more advantageous in terms of economic value and reduces fossil fuel consumption.

• Total floor area of water–energy consumer. The technical aspects of the HPS in terms of design, capacity, and operation consider the type and total floor area of the water–energy consumer cluster. It is also an important factor that defines the water–energy supply demands that impact energy efficiency as well as decrease the CO₂ emissions and freshwater demands. The minimum total floor area of the water–energy consumer was determined from the pre-assessment section with a minimum of 60,000 m². This information was used in determining the attributes of the criterion.

• Usage of renewable heat energy by water–energy consumer. The purpose of using renewable heat energy by the water–energy consumer is used to define economic feasibility, CO₂ emissions, and energy efficiency. The usage purpose of renewable energy depends on the type of facility in the consumer cluster. For factories, renewable heat energy is used for pre-heating and pre-cooling processes, whereas in offices, warehouses, and commercial facilities, it is used for space heating and cooling in buildings.

• Efficiency of HPS. The coefficient of performance (COP) relates to the efficiency of HPS and directly depends on the treated wastewater temperature (Chae & Kang 2013; Culha et al. 2015). The COP is a major factor affecting the economic value, efficiency, and availability of thermal energy. The availability of thermal energy can be calculated following Equations (5) and (6):

  

  (5)

  

  (6)

  where E_c is the available energy for cooling purposes (kcal), and is the coefficient of performance for cooling. E_h is the available energy for heating (kcal), and is the coefficient of performance for heating purposes. The COP of the HPS was assumed based on the treated wastewater temperature, in which the average COP was approximately 5.35 for cooling purposes and 4.06 for heating purposes when the treated temperatures were 15–25 °C.

• Operation type of HPS. The energy supply-demand determines the operation type of HPS, where there is a correlation between the operation type of the HPS and the type of water–energy consumer that impacts on the efficiency and effectiveness of the HPS. The operation of HPS was divided into continuous and intermittent operations depending on the operation of the facility.

• Operation hours of HPS. The operating hours depend on the heat demands based on the type and total floor area of the water–energy consumers. The higher the operation hours of an HPS, the more efficiently it can be operated from an economic perspective. According to a study by Neugebauer et al. (2015), mixed facility structures such as residential buildings coupled with commercial uses such as offices and warehouses show operation hours of HPS of up to 4,500 full load hours, whereas simple residential buildings indicate1,500–2,200 full load hours. Based on these requirements, we considered operating hours of HPS for a medical facility up to 4,500 h per year or 12 h per day. For offices, factories, and warehouse facilities, the operating hours of a HPS were approximately 3,000–3,600 h per year or 8–10 h per day.

The selected assessment criteria and attributes have been integrated to construct an evaluation framework. To provide a better orientation for the user, the evaluation framework is grouped into three aspects: water–energy supplier, water–energy consumer, and water–energy station. Following the AHP steps to evaluate the performance criteria, the weight value was determined based on this obtained expert opinion. The pairwise comparison matrix was generated according to the expert answers from the AHP survey. The CI and CR were calculated with a CR of less than 0.1. Once the weight value of the criteria was obtained from the AHP process, a novel evaluation framework was structured consisting of criteria, sub-criteria, attributes, and detailed weight values which are listed in Table 5.

Simply multiplying the weight value of the criteria, sub-criteria, and attributes results in the global weight value. The sum of global weight is 1 and it is intended to be used as an evaluation grade for the assessment. According to the evaluation criteria by the AHP, the water–energy supplier is the most important criterion (0.560), followed by the water–energy consumer (0.319), and the water–energy station (0.121).

For the general assessment, the water–energy supplier that consists of treated wastewater flow rate, temperature, and clean energy ratio in WWTP becomes the first assessment criterion in the evaluation framework. Based on the results of local priorities from the AHP, treated wastewater flow rate and temperature are the most important factors as define the availability of water and thermal energy potential with the weight value of 0.297 and 0.186, respectively. Here, the attributes of each sub-criteria are determined according to the potentially treated wastewater flow rate indicated in Table 4, the overall treated wastewater temperature in South Korea, and the trend of clean energy ratio in the selected WWTP as discussed in Section 3.2.1. Water–energy supplier.

The next assessment criteria in the evaluation framework concern water–energy consumers as a demand side that consists of water–energy supply distance, type of water–energy consumer, total floor area of water–energy consumer, and usage of heat energy. According to the local priorities, water–energy supply distance and type of water–energy consumer are the third and fourth important factors with a weight value of 0.123 and 0.096, respectively. These results confirm that the potential of WWHR coupled with wastewater reuse depends on the water–energy supplier and water–energy consumer as supply and demand sources. The potential water–energy supplier depends on treated wastewater flow rate and temperature, as well as the potential consumers based on the water–energy supply distance and the type of water–energy consumer. The attributes of each criterion are defined following the potential water–energy consumer information indicated in Table 2 and discussed in Section 3.2.2. Water-energy consumer.

The last assessment criteria in the novel evaluation framework concern the water–energy station related to the availability of thermal energy based on the efficiency and operation of HPS. As discussed in Section 3.2.3. Water–energy station, the efficiency of HPS strongly depends on the treated wastewater temperature, and the operation of HPS depends on the type of water–energy consumers. Consequently, the attributes of each sub-criteria are determined in close relation to the two criteria as explained in Section 3.2.3. Water-energy station.

Through the assessment of 13 potential WWTP sites with the cluster consumer, site W₀₅ was selected as a priority site followed by sites W₀₆ and W₁₀ with the total global weight value of all criteria of 0.255, 0.240, and 0.229, respectively. As shown in Table 4, site W₀₅ has a potential treated wastewater flow rate as a source of water and heat energy of up to 26,939 m³/day. The treated wastewater temperature is approximately 20 °C and the clean energy ratio of the facility is 13.5%. Site W₀₅ has a target consumer consisting of a factory facility at a supply distance of 483 m and a total floor area of 101,443 m², with the purpose of the use of renewable energy for the pre-heating and pre-cooling process. The operation of the HPS at up to 3,600 h per year with an intermittent operation.

Additional studies involving other aspects, particularly micro-level studies, may help further validate the presented methodology, evaluation framework, and general results. The investigation or measurement of the treated wastewater temperature is strongly recommended for a more detailed site assessment.

A series of sensitivity analyses was performed to both validate the accuracy of the AHP method and to comprehensively examine the influence of modifications to the weight values of the main criteria on the prioritization of WWTP sites for the WWHR combined with treated wastewater reuse. When the priority ranking remains consistent, it indicates the stability and robustness of the proposed model (Milutinovic et al. 2017). The sensitivity analyses encompass three distinct scenarios to evaluate the impact of variations in the weighting factors assigned to the criteria. These scenarios are as follows: (i) The criterion WES has a weighting factor of 40%, while the remaining criteria, WEC and WET, each held a weighting factor of 30%; (ii) both ‘WES’ and ‘WEC’ were attributed a weighting factor of 35%, whereas ‘WET’ received a weighting factor of 30%; and (iii) all criterion has equal weighting factors of 33.33%.

This study introduces a new evaluation framework that considers various factors, including thermal energy potential, spatial distribution analysis, and the compatibility of HPS, to assess suitable locations for WWHR coupled with treated wastewater reuse. The novelty of this research lies in its integrated approach, focusing on heat recovery coupled with treated wastewater reuse, and its structured framework that incorporates multiple criteria and expert opinions. The methodology approach consists of three-step process: pre-assessment process, develops evaluation framework by applying MCDA methods, and site prioritization. The pre-assessment phase involving the screening of potential WWTP and consumers clusters through GIS based on pre-assessment criteria. Subsequently, an evaluation framework is formulated by using MCDA methods. This phase encompasses the identification and categorization of criteria into three aspects: water–energy supplier, water–energy consumer, and water–energy station. To establish the relative importance of these criteria, the AHP is utilized. Finally, the evaluation framework is applied to ascertain and designate the priority locations for integration WWHR and treated wastewater reuse.

Through the pre-assessment process with the support of pre-assessment criteria, 13 potential WWTP sites were selected from a total of 650 WWTP with the capacity more than 500 m³/day across South Korea. These selected sites are in proximity less than 500 m to consumer clusters. Analyzing of relative importance criteria indicates that the water–energy supplier is the most important criterion, followed by the water–energy consumer, and the water–energy station with weight values of 0.560, 0.319, and 0.121, respectively. The local priorities of criteria from AHP showed that, treated wastewater flow rate, treated wastewater temperature, water–energy supply distance, and type of water–energy consumer are the most important factor with weight values of 0.297, 0.186, 0.123, and 0.096, respectively. These results state that there is a strong correlation between the potential of WWHR coupled with wastewater reuse and the water–energy supplier as well as a water–energy consumer as supply and demand sources. A detailed comparison of the 13 potential WWTP sites confirms the selection of three priority locations: W₀₅, W₀₆, W₁₀. These sites exhibited higher total global weight values of 0.255, 0.240, and 0.229, respectively. Considering the findings and framework development from this study, future research could involve a comprehensive techno-economic analysis. Additionally, investigating the environmental impact and carbon footprint reduction resulting from the integration of these practices could provide valuable insights. Furthermore, the synergies between WWHR with other renewable energy sources, such as solar or wind, could offer optimizing energy utilization within the wastewater management.

E.R. contributed to conceptualization, methodology, validation, writing-original draft, formal analysis, data collection. J.O. contributed to conceptualization, methodology, validation, supervision, writing-review and editing, fund acquisition.

This work was supported by the Chung-Ang University Young Scientist Scholarship (CAYSS) in 2021.

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

The authors declare there is no conflict.