Review on Water and Energy Integration in Process Industry: Water-Heat Nexus

Abstract

The improvement of water and energy use is an important concern in the scope of improving the overall performance of industrial process plants. The investment in energy efficiency comprehended by the most recent sustainability policies may prove to be an effective response to the fall of energy intensity rates associated with the economic crisis brought by the COVID-19 pandemic. The improvement in water efficiency may also prove to be a potential approach due to its interdependencies to energy use, whose exploitation comprises part of the study of the water-energy nexus. Waste heat recovery and water reclamation practices have been exploited to improve water and energy efficiency. A specific method designated “Combined Water and Energy Integration” has been applied to water recycling as both an additional water source and a heat recovery source in a set of water-using processes. In scientific and industrial domains, there is still a need for integrated approaches of water-using and combustion-based processes for overall water and energy efficiency improvements in industrial plants. In this work, an innovative approach for a simultaneous improvement of water and energy use is proposed based on process integration and system retrofitting principles. This proposal is based on the delineation of two innovative concepts: Water and Energy Integration Systems (WEIS) and Water-Heat Nexus (WHN). A review on existing technologies for waste heat recovery, thermal energy storage and heat-driven wastewater treatment is performed, following a conceptualisation design.

1. Introduction

Water and energy efficiency are prominent concerns for industry and the improvement of the use of both of these resources is prominent in sustainability policies established by each country worldwide [1]. The adoption and planning of water and energy management strategies is essential to securing an efficient and sustainable operation of plants [2]. Considering the inevitable use of such resources in industries, water and energy use have obligatory interdependencies. Such interdependencies are encompassed in the concept of the water-energy nexus [3,4,5,6]. The improvement of energy efficiency in industries has been a subject of study by several authors (namely regarding the identification and analysis of improvement measures [7,8,9,10], their application into industrial processes factories [11] and the optimization of single equipment [12]). Within the scope of energy efficiency improvement, the authors of the present study have assessed potential water efficiency improvements, namely measures which simultaneously improve water and energy use [13], through simulation models [14].

Water efficiency and energy efficiency are concepts that have been gradually introduced in the sustainability policies adopted by each country in the world. In the European Union, targets for an efficient water and energy use were respectively established in the 2009 Water Framework Directive [15] and the 2012 Energy Efficiency Directive [16]. The most relevant objectives of the EU regarding sustainability have been established in the most recent 2030 Climate and Energy Framework (up to a 40% reduction in GHG emissions from 1990 levels, 32% share for renewable energy and 32.5% energy efficiency improvement) [17]. Furthermore, the European Green Deal was introduced as a roadmap for the promotion of circular economy, biodiversity restoration and pollution mitigation [18].

At the level of engineering studies, process integration (PI) research has emerged on the scope of the study of water and energy efficiency, with a methodology based on the analysis and assessment of several processes existing in plants and all potential interdependencies [19]. The implementation of Water and Energy Integration mandatorily involves a new planning of the overall operation of a plant, namely in respect to the recirculation of plant streams containing reusable water and energy [20]. Overall, such implementations require the installation of several technologies to promote water and waste heat recirculation in a perspective of system retrofitting [21].

This work presents two innovative concepts: Water and Energy Integration Systems (WEIS) and Water-Heat Nexus (WHN). These concepts are established through a review on the implementation of Water and Energy Integration in the process industry, namely through waste heat recovery, thermal energy storage and heat-driven wastewater treatment technologies. This work is part of ongoing research involving all aspects associated with the implementation of heat recovery, water treatment and recirculation and eco-efficiency promotion practices in the process industry [22,23,24,25,26].

2. Strategic Framework

The principle of progress towards industrial sustainability is based on the reduction in resource consumption and the environmental impacts associated with the waste produced in plants—mostly water, electricity, fuels and process raw materials [27]. In practice, the promotion of industrial sustainability is achieved by improvement measures which optimise water and energy use [11,28], the application of renewable energy resources [29] and the application of waste-to-energy technologies [30]. The concept of a circular economy has been emerging on the scope to transform waste into potential by-products, promoting reuse, recovery and recycling, in which the life cycles of the production chains are optimised [31]. Within the limit, the application of several measures converges on the reuse of resources (either material or energy) within the same industrial site.

2.1. EU Energy System Integration Strategy

The scope of this work is driven by EU Energy System Integration Strategy. This strategy originated from a combination of the European Green Deal [18] and the 2050 long-term strategy [32]. The definitions of the guiding principles of each pillar of this strategy and their correlations to this work are presented in

Table 1. Framework of water and energy contextualization based on EEIS.

Pillar	Definition of the Guiding Principles	Contextualization of the Work
1st Pillar Energy Efficiency and Circular Economy Nexus	Energy efficiency first principle (giving priority to energy demand-side solutions in relation to energy supply-side ones, if more cost-effective) Waste heat recovery (WHR) from industrial sites at the centre of intra-plant energy efficiency improvement and the functioning of district heating and cooling networks (promotion of WHR practices in a wider perspective and the surpassing of existing barriers) Wastewater-to-energy (production of biofuels)	Focus on heat recovery and water valorisation (either as feed water, waste heat streams or through further valorisation in waste-to-energy) Recirculation of these streams allows the conceptualization of systems encompassing industrial processes which are circular in nature (resources inevitably produce wastes as by-products which are in turn recirculated as additional resources)
2nd Pillar Renewable-Based Electrification	Compensation of growing electricity demand through use of renewable energy resources as primary energy forms Electrification of industrial processes Implementation of energy storage technologies	Electricity-producing heat recovery-based thermodynamic cycles, which are also included within the scope of this work
3rd Pillar Alternative Low-Carbon Fuels	Promotion of the use of green hydrogen in sectors with more difficult decarbonisation Promotion of carbon capture and storage (CCS) and carbon capture and use (CCU)	The application of waste-to-energy to the sludge from wastewater treatment units may result in the production of renewable biofuels (through anaerobic digestion) and hydrogen (through electrolysis)

.

2.2. The Water-Heat Nexus (WHN) General Concept

In this work, a new concept of the Water-Heat Nexus (WHN) is introduced, which is based on a specific application of the water-energy nexus. It primarily consists of the analysis of the thermal energy supply and demand requirements of water systems which may be fulfilled by waste heat stream recirculation. Secondly, it consists of the use of considered wastes from water systems as valorised energy inputs in combustion-based processes. Much of the understanding of this concept is related to the analysis of the waste streams generated in a plant. Firstly, it is of note that this waste may not only be considered as material waste (such as the one present in wastewater [33]), but also as thermal energy that is wasted in processes, such as combustion (waste heat) [34]. Heat recovery systems consist of the application of energy management principles to plan the installation of several technologies. It is based on the recirculation of streams with an associated waste heat potential so as to obtain savings in fuel or electricity consumption [35,36]. The recirculation of water streams has a similar principle to the one of heat recovery, in which the water stream at the outlet of a certain process is recirculated in order to produce savings on freshwater consumption [37]. In the case of the planning of a system containing several water-using processes, it is often necessary (although not obligatory in all industrial cases) to treat wastewater streams for further recirculation [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65]. Two types of streams result from wastewater treatment units. One of these types is treated water streams, which may be recirculated to produce savings in freshwater use [66] (this work uses water recirculation as a term to include both water recycling to the process of origin and water reuse from one process to another [67]). Another type is sludge streams, which in turn may be valorised through the implementation of wastewater-to-energy technologies [68]. The general interpretation of the WHN paradigm is pictorially represented in Figure 1.

3. Water and Energy Integration Technologies and System Retrofitting

The understanding of the application of Water and Energy Integration in Process Industry involves the analysis of the existing literature in Process Integration (PI) concepts and methodologies for the purpose of water and energy resource valorisation. In this work, process integration principles are set to be implemented for the purpose of system retrofitting of the existing industrial sites, which involve the installation of several technologies, for instance:

• Waste heat recovery [21,36,69] (including thermal energy storage [70]);
• Heat-driven wastewater treatment [71].

Table 2. Progress on Water and Energy Integration at the industry level.

Aspect	Progress	Ref.
Overview of Process Integration	Framework of process integration within the improvement of the use of utilities in industry and the involvement of numerical methods, including: Use of several mathematical programming methods (including multi-objective programming); Directions of the implementation of process integration; Attainment of general results on water savings, energy savings and reduction in environmental impacts.	[72,73,74,75,76,77,78,79,80,81,82,83,84,85]
Pinch Technology for Energy Efficiency Improvement	Pinch analysis-based heat integration in industry, including: Assembling of heat exchanger networks (HEN) in a plant; Application of pinch technology to improve the operation of industrial processes and systems (for instance, ORC and Kalina cycle); Simultaneous application of pinch and exergy analysis methods; Implementation of algorithms for the assembling of energy management scenarios.	[86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105]
Water Minimisation and Recirculation	Strategies for the minimisation of freshwater consumption and reduction in water footprint, through: The application of water pinch analysis (WPA) and water cascade analysis (WCA) methods; The study of water management strategies; The study of the reduction in wastewater discharge.	[67,81,106,107,108,109,110,111,112,113]
Combined Water and Energy Integration	Research on the simultaneous application of Water and Energy Integration within a water network, considering: Simultaneous application of water and energy pinch; Application of the concept of water-energy nexus from a process-based perspective.	[54,114,115,116,117]

presents a summary of the progress on Water and Energy Integration in the industrial sector.

From the analysis of Table 2, it is observed that process integration in respect to heat recovery and water recirculation has been successfully developed through numerical models for further industrial implementations. Research in this field has also accounted for the requirements of related areas, such as energy management and water management. The existing methodology of Combined Water and Energy Integration has been shown to be effective for the minimization of the use of freshwater and hot/cold utilities in a water system with a variable number of water-using processes. This corresponds to the approach of the Water-Heat Nexus concept. However, it considers in its analysis water streams as the only streams to be recirculated and as a single type of process (in this case, a water network). Therefore, the integration of the aforementioned technologies is relevant in the scope to expand the applicability of this approach.

3.1. Heat Recovery Technologies (HR)

Waste heat recovery (WHR) technologies encompassing improved combustion systems, heat exchangers and electricity generation systems have been studied by several authors [118,119]. Within these, thermal energy storage (TES) technologies emerged to address the dynamic heat supply and demand levels in a plant. Such storage presents advantages, such as better capacity factors, avoidance of heat losses and reduced investment cost (in combination with cost-expensive components) [71,120,121], making up for the issues of standard WHR technologies, such as the significant distance between heat source and sink, the lack of identification of existing heat sinks, operation disturbances and overall economic viability. Table 3 presents a characterization of several WHR technologies applied from a multisectoral perspective.

Table 3. Characterization of HR Technologies and Strategies.

Technology	Characterization	Ref.
Heat Exchanging Units
Air–Gas Heat Exchangers(Air Preheaters)	Commonly applied for waste heat from exhaust gases of combustion processes to an air stream, having different configurations (recuperator, regenerator, rotary regenerator and run-around coil units) [122] and designs (plate heat exchangers [123,124] and heat pipe heat exchangers, HPHEs [125,126,127]). The HPHEs are specific air preheaters applied with a higher heat transfer capability and a need for less heat transfer surface area for the same quantity of heat. They have over fuel savings of 10–30% and less than a two-year payback time.	[122,123,124,125,126,127,128,129,130,131,132,133,134]
Liquid–Gas Heat Exchangers(Economisers)	Applied for the heating of the liquid stream, such as a water stream at the inlet of a boiler or a steam boiler. They have different configurations (non-condensing, condensing, finned tube and coiled tubes). In some economisers, the liquid stream may be recycled for thermal energy efficiency improvement. They have a typical fuel savings of 5–10% at present and a payback time of less than two years.	[135,136,137,138,139]
Heat Recovery Steam Generators (HRSG)	Complex technologies commonly applied to generate steam for heating processes within a plant and the operation of thermodynamic cycles for electric energy generation. They are normally consist of an economiser (in which the liquid stream is preheated in order to attain the boiling point), an evaporator (in which the saturated liquid is converted into gas) and a superheater (in which the vapour is overheated beyond its saturation point). They have an overall plant efficiency of 85–90% and an electricity generation system efficiency of 75–85%.	[140,141]
Thermodynamic Cycles
Organic Rankine Cycle (ORC)	Similar to the Clausius–Rankine cycle (CRC), the working principle of this thermodynamic cycle consists of the capture of thermal energy from a heat source to evaporate an organic fluid. It is commonly implemented for low-grade WHR. The cycle generally consists of a turbine, an HRSG unit, a condenser and a pump (in many installations a regenerator is also installed to even further increase the system efficiency). With appropriate working fluid, it may lead to about a 6% increase in overall plant efficiency;.The organic fluids R-12, R-123, R134a and R-717 have been demonstrated as suitable to produce high-efficiency systems and a considerable amount of electric energy. It has an associated typical payback time four to five years.	[142,143,144,145,146,147,148,149]
Kalina Cycle	A system similar to the CRC and ORC, using water-ammonia mixture as the working fluid and suitable for medium and high temperature applications. It is structurally similar to the Regenerative ORC, with an additional separator due to the high ammonia concentration of the turbine outlet gas stream (so as to assist the full condensation of the water-ammonia mixture). It presents an overall better WHR performance compared to the ORC, although it requires more maintenance. It has been assessed as having a minimum payback time of 5.8 years.	[150,151,152,153,154,155]
Supercritical CO2 Brayton Cycle (SCBC)	A system with a similar arrangement to a common Brayton cycle, using CO2 at a supercritical state as the working fluid. It has several advantages: a higher thermal efficiency, operation at a lower pressure across the system and a reduction in number of stages in the turbine. The heat extraction capability may be limited due to the heat transfer in the heater being processed at a low temperature range close to the maximum cycle temperature. It has been assessed as having a payback time range between eight and twelve years.	[156,157,158,159]
Thermal Energy Storage
Liquid Thermal Tank	A sensible TES technology based on the heating of a liquid continuum within a tank. It may be a part of systems with several configurations, with different material streams being used as heat source streams (in addition to solar thermal collectors and boiler units), with a generic configuration being presented in Figure 2a. Water (working temperature of 0–100 °C), thermal oils (working temperature of 0–400 °C), molten salts (working temperature of 150–565 °C) or sodium (working temperature of 100–882 °C) may be used as the liquid media. It has been assessed to have an average payback time of 14 years.	[70,160,161,162,163,164]
Phase Change Material (PCM) Heat Exchanger	A latent TES technology consisting of a heat exchanger operating in a transient mode. A specific application of this technology includes the preheating of combustion air at the inlet of a combustion-based process, using an exhaust gas stream as the heat source, as presented in Figure 2b. The transportation of the exhaust gas stream to the PCM–TES unit functions as the charging phase (in which the PCM phase in turn is almost completely melted) and the transportation of the air stream functions as the discharging phase (in which the PCM phase in turn is again solidified by releasing the latent heat). Common latent materials include: molten salts [165,166], metal alloys [167,168], eutectic inorganic [169] and organics [170,171,172]. It has been assessed to have a potential of 28.6–66.2% total energy savings and a payback time of 7.5–14.5 years.	[122,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200]

Table 3. Characterization of HR Technologies and Strategies.

Technology	Characterization	Ref.
Heat Exchanging Units
Air–Gas Heat Exchangers(Air Preheaters)	Commonly applied for waste heat from exhaust gases of combustion processes to an air stream, having different configurations (recuperator, regenerator, rotary regenerator and run-around coil units) [122] and designs (plate heat exchangers [123,124] and heat pipe heat exchangers, HPHEs [125,126,127]). The HPHEs are specific air preheaters applied with a higher heat transfer capability and a need for less heat transfer surface area for the same quantity of heat. They have over fuel savings of 10–30% and less than a two-year payback time.	[122,123,124,125,126,127,128,129,130,131,132,133,134]
Liquid–Gas Heat Exchangers(Economisers)	Applied for the heating of the liquid stream, such as a water stream at the inlet of a boiler or a steam boiler. They have different configurations (non-condensing, condensing, finned tube and coiled tubes). In some economisers, the liquid stream may be recycled for thermal energy efficiency improvement. They have a typical fuel savings of 5–10% at present and a payback time of less than two years.	[135,136,137,138,139]
Heat Recovery Steam Generators (HRSG)	Complex technologies commonly applied to generate steam for heating processes within a plant and the operation of thermodynamic cycles for electric energy generation. They are normally consist of an economiser (in which the liquid stream is preheated in order to attain the boiling point), an evaporator (in which the saturated liquid is converted into gas) and a superheater (in which the vapour is overheated beyond its saturation point). They have an overall plant efficiency of 85–90% and an electricity generation system efficiency of 75–85%.	[140,141]
Thermodynamic Cycles
Organic Rankine Cycle (ORC)	Similar to the Clausius–Rankine cycle (CRC), the working principle of this thermodynamic cycle consists of the capture of thermal energy from a heat source to evaporate an organic fluid. It is commonly implemented for low-grade WHR. The cycle generally consists of a turbine, an HRSG unit, a condenser and a pump (in many installations a regenerator is also installed to even further increase the system efficiency). With appropriate working fluid, it may lead to about a 6% increase in overall plant efficiency;.The organic fluids R-12, R-123, R134a and R-717 have been demonstrated as suitable to produce high-efficiency systems and a considerable amount of electric energy. It has an associated typical payback time four to five years.	[142,143,144,145,146,147,148,149]
Kalina Cycle	A system similar to the CRC and ORC, using water-ammonia mixture as the working fluid and suitable for medium and high temperature applications. It is structurally similar to the Regenerative ORC, with an additional separator due to the high ammonia concentration of the turbine outlet gas stream (so as to assist the full condensation of the water-ammonia mixture). It presents an overall better WHR performance compared to the ORC, although it requires more maintenance. It has been assessed as having a minimum payback time of 5.8 years.	[150,151,152,153,154,155]
Supercritical CO2 Brayton Cycle (SCBC)	A system with a similar arrangement to a common Brayton cycle, using CO2 at a supercritical state as the working fluid. It has several advantages: a higher thermal efficiency, operation at a lower pressure across the system and a reduction in number of stages in the turbine. The heat extraction capability may be limited due to the heat transfer in the heater being processed at a low temperature range close to the maximum cycle temperature. It has been assessed as having a payback time range between eight and twelve years.	[156,157,158,159]
Thermal Energy Storage
Liquid Thermal Tank	A sensible TES technology based on the heating of a liquid continuum within a tank. It may be a part of systems with several configurations, with different material streams being used as heat source streams (in addition to solar thermal collectors and boiler units), with a generic configuration being presented in Figure 2a. Water (working temperature of 0–100 °C), thermal oils (working temperature of 0–400 °C), molten salts (working temperature of 150–565 °C) or sodium (working temperature of 100–882 °C) may be used as the liquid media. It has been assessed to have an average payback time of 14 years.	[70,160,161,162,163,164]
Phase Change Material (PCM) Heat Exchanger	A latent TES technology consisting of a heat exchanger operating in a transient mode. A specific application of this technology includes the preheating of combustion air at the inlet of a combustion-based process, using an exhaust gas stream as the heat source, as presented in Figure 2b. The transportation of the exhaust gas stream to the PCM–TES unit functions as the charging phase (in which the PCM phase in turn is almost completely melted) and the transportation of the air stream functions as the discharging phase (in which the PCM phase in turn is again solidified by releasing the latent heat). Common latent materials include: molten salts [165,166], metal alloys [167,168], eutectic inorganic [169] and organics [170,171,172]. It has been assessed to have a potential of 28.6–66.2% total energy savings and a payback time of 7.5–14.5 years.	[122,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200]

3.2. Heat-Driven Wastewater Treatment Technologies (HDWTT)

A specific set of wastewater treatment technologies (WTT) which use the supply of thermal energy as the driving force are designated as heat-driven wastewater treatment technologies [71,201,202]. These may be framed within the overall conceptualization of Water and Energy Integration by analysing the potential for a determinate waste heat stream (e.g., exhaust gases [203]) to serve as the heat source for the operation of these units. In Table 4, several heat-driven wastewater treatment technologies are characterized.

Table 4. Characterization of WWT Technologies.

Technology	Description	Ref.
Multi-Effect Distillation (MED)	An HDWWT technology with several operational advantages including: the use of low-temperature operational levels, the production of high-quality treated water, high thermal performance, the use of low pumping power, the requirement of minimum water pre-treatment and the requirement for minimum labour. Its limitations include: level of investment (highly expensive technology), considerable susceptibility to corrosion and existence of relatively low recovery ratio. A conventional MED unit may be divided into three sections: (i) first effect section (in which the heat source is a hot liquid); (ii) the second-to-last effects section (in which the heat source is the vapour stream produced in the immediately previous step for each particular effect); (iii) the condenser section (which condensates the last effect outlet vapour stream through heat transfer with the cold inlet saline water stream). It has been assessed to have a payback time of four to sixteen years. The overall process is represented in Figure 3a.	[71,201,204,205,206]
Multi-Stage Flash Distillation (MSFD)	An HDWWT technology with an operation principal similar to MED, with the following advantages: higher resistance against scaling compared to MED, large capacity for freshwater production, independence from the salinity of feed water, operational and maintenance simplicity, low performance degradation, production of high quality freshwater and potential for combination with other processes. It has disadvantages, including: the level of investment (high investment technology), requirement for high-level technical knowledge, highly thermal energy-intensive process (high operational temperatures) and low recovery ratio. It has been assessed to have an average of 3.3 years payback time. The overall process is represented in Figure 3b.	[21,207,208,209,210,211,212]
Membrane Distillation (MD)	An HDWWT technology that presents advantages in relation to conventional distillation technologies: low energy requirements, non-dependability from concentration polarization and lack of limit in feed water concentration. Its disadvantages include: investment cost (high investment cost associated to MD module) and possibility of membrane wetting in the case of the presence of surfactant and amphiphilic contaminants. It has been assessed to have a payback time between six and fourteen years. The temperature gradient within the MD module may be generated by a heat recovery system: single-loop as represented in Figure 3c (in which the heat source is directly connected to the membrane) or two-loop as represented in Figure 3d (in which a heat exchanger is implemented and also commonly a TES unit for transient mode systems). The MD module may have the following configurations: direct contact membrane distillation (DCMD); vacuum membrane distillation (VMD); air gap membrane distillation (AGMD); and sweeping gas membrane distillation (SGMD).	[213,214,215,216,217]

Table 4. Characterization of WWT Technologies.

Technology	Description	Ref.
Multi-Effect Distillation (MED)	An HDWWT technology with several operational advantages including: the use of low-temperature operational levels, the production of high-quality treated water, high thermal performance, the use of low pumping power, the requirement of minimum water pre-treatment and the requirement for minimum labour. Its limitations include: level of investment (highly expensive technology), considerable susceptibility to corrosion and existence of relatively low recovery ratio. A conventional MED unit may be divided into three sections: (i) first effect section (in which the heat source is a hot liquid); (ii) the second-to-last effects section (in which the heat source is the vapour stream produced in the immediately previous step for each particular effect); (iii) the condenser section (which condensates the last effect outlet vapour stream through heat transfer with the cold inlet saline water stream). It has been assessed to have a payback time of four to sixteen years. The overall process is represented in Figure 3a.	[71,201,204,205,206]
Multi-Stage Flash Distillation (MSFD)	An HDWWT technology with an operation principal similar to MED, with the following advantages: higher resistance against scaling compared to MED, large capacity for freshwater production, independence from the salinity of feed water, operational and maintenance simplicity, low performance degradation, production of high quality freshwater and potential for combination with other processes. It has disadvantages, including: the level of investment (high investment technology), requirement for high-level technical knowledge, highly thermal energy-intensive process (high operational temperatures) and low recovery ratio. It has been assessed to have an average of 3.3 years payback time. The overall process is represented in Figure 3b.	[21,207,208,209,210,211,212]
Membrane Distillation (MD)	An HDWWT technology that presents advantages in relation to conventional distillation technologies: low energy requirements, non-dependability from concentration polarization and lack of limit in feed water concentration. Its disadvantages include: investment cost (high investment cost associated to MD module) and possibility of membrane wetting in the case of the presence of surfactant and amphiphilic contaminants. It has been assessed to have a payback time between six and fourteen years. The temperature gradient within the MD module may be generated by a heat recovery system: single-loop as represented in Figure 3c (in which the heat source is directly connected to the membrane) or two-loop as represented in Figure 3d (in which a heat exchanger is implemented and also commonly a TES unit for transient mode systems). The MD module may have the following configurations: direct contact membrane distillation (DCMD); vacuum membrane distillation (VMD); air gap membrane distillation (AGMD); and sweeping gas membrane distillation (SGMD).	[213,214,215,216,217]

4. Water and Energy Integration Systems (WEIS) for Process Industry

Heat recovery and water recirculation principles result from the conceptualization of systems encompassing a set of recirculated streams from one process to several other processes. This study proposes a concept that goes beyond conventional ways of thinking, such as open-loop and linear economy-modelled systems (in which water and energy recirculation is mere or non-existing). In contrast, the soon-to-be conceptualized systems are closed-loop and circular economy-modelled. The most generalist conceptualization of such systems and its comparison with linear economy-based systems are represented in Figure 4.

Such industrial systems are associated with several designations in the literature:

• Heat exchanger networks (HEN) (for heat exchanger installation-based heat recovery) [105];
• Near-zero liquid discharge (NZLD) (for water systems with significantly decreased wastewater discharge owing to water treatment and recirculation) [218,219,220];
• Water–energy networks (WEN) and water-allocation and heat-exchanger networks (WAHEN) (for water systems in which previously discharged water streams are recirculated to simultaneously decrease the use of freshwater and hot and cold utilities) [45].

This work introduces and conceptualizes a new type of systems, designated as Water and Energy Integration Systems (WEIS). WEIS include all conceptualized superstructures encompassing a set of water-using processes and thermal processes and also the application of several technologies and strategies. WEIS consist of thermal processes (making up part of the constituent thermal processes system) and a set of water-using processes (making up part of the constituent water system). Within the conceptualization of WEIS, several technologies may be implemented that overall promote:

• Water recirculation simultaneously as a heat recovery source (hot utility and cold utility savings) and as an additional water source (freshwater savings) [50,53,64];
• Waste heat stream recirculation to thermal processes (fuel savings);
• Waste heat stream recirculation to the heaters of the water system (hot utility savings);
• Waste heat stream recirculation to the heat-driven wastewater treatment units [201];
• Use of the sludge resulting from wastewater treatment as an additional energy source (fuel savings).

Figure 5 represents the general rationale behind the concept of WEIS, with schematics of a water treatment and recirculation network (a set of water-using processes) and a heat recovery system (a set of thermal processes).

The two systems represented in Figure 5 are conceptualized to consider all the potential recirculation within water-using processes and combustion-based thermal processes, respectively. The overall WEIS concept, however, requires further adaptation for the interdependencies between the water recirculation network (Figure 5a) and the heat recovery network (Figure 5b). Such analysis is relevant to further conceptualize these systems and also for the newly coined concept of the Water-Heat Nexus (WHN). This analysis involves identifying the unit operations for the water recirculation network, which requires thermal energy inputs—corresponding to the red spots in Figure 5a, namely:

• Heating spots (total waste heat from thermal processes that may be allocated for savings in hot utilities);
• Wastewater treatments units (particularly in the heat-driven ones, wastewater treatment requiring minimum energy input).

The analysis of thermal energy supply and demand requirements associated to each of the water networks points corresponds to the concept of the Water-Heat Nexus (WHN). This is a specific implementation that investigates the use of waste heat streams to be supplied as thermal energy inputs in another system for the promotion of water recirculation. In the general scheme represented in Figure 5, this paradigm corresponds to an amplification of the Industrial Processes part, in which the interdependencies of specific processes (categorised as water-using processes or thermal processes) are exploited.

4.1. Mass and Enthalpy Balances of a WEIS

The implementation of WEIS models relies on the definitions of mass and enthalpy balance equations. It serves to ultimately assess the difference between the water and energy inputs in the baseline case (with no implemented measures) and in the improved case (with implemented measures).

Table 5. General delineation of balance equations describing Water and Energy Integration Systems.

Process/ System		Balance	Equation
Thermal Process		Mass Balance	$M_{Fuel}+M_{C.Air}=M_{ExGas}$	(1)
		Enthalpy Balance	$M_{Fuel}•\left(h_{Fuel}+LHV\right)+M_{C.Air}•h_{C.Air}=q_{useful}+M_{ExGas}•h_{ExGas}$	(2)
		Heat recovery equations	$q_{supply}=M_{{Fuel}_{initial}}•LHV$	(3)
			$q_{supply}=M_{Fuel}•LHV+q_{additional}$	(4)
Water System	Water-Using Process	Mass Balance	$M_{WP,in}+M_{cont.}=M_{WP,out}$	(5)
		Enthalpy Balance	$M_{WP,in}•h_{WP,in}=M_{WP,out}•h_{WP,out}$	(6)
	Wastewater Treatment Unit	Mass Balance	$M_{WWT,in}=M_{WWT,out}+M_{sludge}$	(7)
		Enthalpy Balance	$M_{WWT,in}•h_{WWT,in}+q_{WWT}=M_{WWT,out}•h_{WWT,out}$	(8)
	Heater	Mass Balance	$M_{Heater,in}=M_{Heater,out}$	(9)
		Enthalpy Balance	$M_{Heater,in}•h_{Heater,in}+q_{HU}=M_{Heater,out}•h_{Heater,out}$	(10)
	Cooler	Mass Balance	$M_{Cooler,in}=M_{Cooler,out}$	(11)
		Enthalpy Balance	$M_{Cooler,in}•h_{Cooler,in}=M_{Cooler,out}•h_{Cooler,out}+q_{CU}$	(12)
	Recirculation Point (Splitting)	Mass Balance	$M_{water,in}=M_{water,out}+{\displaystyle {\displaystyle \sum }_i}{M}_{recir. water,i}$	(13)
		Enthalpy Balance	$\begin{array}{cc}\hfill M_{water,in}•h_{water,in}& \\ & =M_{water,out}•h_{water,out}+{\displaystyle {\displaystyle \sum }_i}{M}_{recir. water,i}\hfill \\ & •h_{recir. water,i}\hfill \end{array}$	(14)
	Recirculation Point (Joint)	Mass Balance	$M_{water,in}+{\displaystyle {\displaystyle \sum }_i}{M}_{recir. water,i}=M_{water,out}$	(15)
		Enthalpy Balance	$\begin{array}{c}{M}_{water,in}•h_{water,in}+{\displaystyle {\displaystyle \sum }_i}{M}_{recir. water,i}•h_{recir. water,i}\\ =M_{water,out}•h_{water,out}\end{array}$	(16)

presents a general delineation of the mass and enthalpy balances that describe the phenomena occurring in a WEIS.

The sets of equations presented in Table 5 must be furtherly adapted to each reality specifically for the purpose of model development. For instance, the categories of thermal processes include a variety of equipment which includes other streams in addition to gas and material products (such as the material cooling fluid streams), and as such, the useful heat parcel ($q_{useful}$) varies for different processes.

4.2. Computational Tools for WEIS

The assessment of the implementation of WEIS in real-life plants may strongly benefit from the use of computational tools. As highly complex systems, computational tools are comprehensive, from a developmental level to further optimisation modelling.

Table 6. Description of existing computational tools.

Type	Description	Ref.
Simulation Tools	Applicable for scenario analysis, sensitivity analysis and further API-based integration with optimisation models. The unit operations simulation models included in WEIS may be developed overall using existing software packages and modelling languages, such as Modelica [221,222,223,224,225,226,227], TRNSYS [228], ASPEN HYSYS [229,230,231], Apros [232], AMESim [233] and MATLAB/Simulink [234,235]. The technologies/measures being simulated include hot air recycling as combustion air, the implementation of heat exchangers for water/air preheating, organic Rankine cycles, multi-effect distillation units and PCM heat exchangers.	[23,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235]
Optimisation Tools	Optimisation models for heat recovery/water recirculation systems overall are within the categories of mathematical programming (MP) and pinch analysis (PA) method applications. The PA method is a holistic perspective for the application of a systematic approach for the planning of industrial systems, although it suffers from the adoption of a steady-state perspective only and a high number of assumptions about fluid properties and temperature profiles [48,52,54,57,236,237,238,239]. MP methods may be formulated to require less assumptions, but are less systematic in nature, and include linear and non-linear programming (LP, NLP, MILP and MINLP) [39,40,42,43,46,47,51,55,56,59,62,65,240,241,242,243,244,245], multi-objective programming (MOP) [45,246,247,248,249,250] and dynamic programming (DP) [251,252,253,254,255,256]. These optimisation models have been prominently developed using the Modelica, Python and GAMS languages.	[22,36,37,39,40,41,42,43,44,45,48,49,51,52,53,54,55,56,57,59,61,62,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257]

presents a synthesis of models and methods of existing computational tools commonly used for the analysis of processes and phenomena involved in WEIS.

As may be verified from Table 6, several simulation and optimisation models conjointly achieve the aims of analysis and assessment of the technologies and phenomena involved in WEIS. In terms of component-level models, the existing models overall fulfil each type of required technology/phenomenon comprised the general concept of WEIS. The existing optimisation methodologies are also sufficient for the development of optimisation models for WEIS, considering both steady-state and dynamic perspectives. With the aim to assemble system-level models, the existing models and methodologies require adaption for the analysis of several levels of stream recirculation within WEIS and all the optimisation of involved costs (overall minimization of water costs, energy costs and investment costs).

5. Conclusions and Future Work

This work aimed to establish the concepts of Water and Energy Integration Systems (WEIS) and Water-Heat Nexus (WHN) by identifying and characterizing several technologies for waste heat recovery (WHR), thermal energy storage (TES) and heat-driven wastewater treatment (HDWWT). Such identification and characterization of technologies are the foundation of the understanding of all the potential interdependencies between water-using processes and thermal processes in an industrial site, which the aforementioned two concepts depend on.

In respect to the exploitation of existing heat recovery technologies (encompassing in this case both standard WHR and TES technologies):

• The existing WHR technologies fulfil the technical specifications for the conceptualized WEIS;
• The existing TES technologies are adequate within the conceptualization of the proposed WEIS, although it is still necessary to perform a further study with respect to the applications of these in specific cases (e.g., within the scope of the recirculation of heat streams from thermal process systems to water systems encompassing an energy storage component).

In respect to the exploitation of existing heat-driven wastewater treatment technologies:

• The existing technologies are adequate in the scope of the developed concept, although it is still necessary to research the specifications of these in terms of the characterizations of contaminant removal (namely specific types of contaminants and whether single- or multi-contaminant removal).

In respect to the implementation of WEIS and the concept of Water-Heat Nexus in general:

• The conceptualized WEIS concept is overall in accordance with the aims of the EU Energy System Integration, namely in terms of the practical implementation of circular economy principles;
• The conceptualized general WEIS scenario (as represented in Figure 5) may still have to be adapted in order to consider all potential recirculation of waste heat streams (namely as the heat source in the heaters and heat-driven wastewater treatment units of the water allocation and heat exchanger network), and furthermore, according to the requirements of different case studies;
• The existing computational tools (comprising simulation and optimisation models) are overall sufficient for the development of WEIS models, although further adaptations at the level of stream recirculation (connections between unit operations through material streams) must be performed.

A specific set of technologies that apply sorption and reaction phenomena designated as thermochemical technologies may be proposed for implementation to enhance the maturity of WEIS as viable market options. These comprise both energy storage and wastewater-to-energy technologies; their adequacy for WEIS may be explored in a future works.