# Waste Water Heat Recovery Systems Types and Applications: Comprehensive Review, Critical Analysis, and Potential Recommendations

Zahra Wehbi, Rani Taher, Jalal Faraj, Thierry Lemenand, Mahmoud Khaled

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Zahra Wehbi, Rani Taher, Jalal Faraj, Thierry Lemenand, Mahmoud Khaled. Waste Water Heat Recovery Systems Types and Applications: Comprehensive Review, Critical Analysis, and Potential Recommendations. The International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability TMREES23, May 2023, Metz, France. pp.16-33, 10.1016/j.egyr.2023.05.243. hal-05437348

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# Waste Water Heat Recovery Systems types and applications: Comprehensive review, critical analysis, and potential recommendations

Zahra Wehbiª, Rani Taherᵇ, Jalal Farajª,ᶜ, Thierry Lemenandᵈ, Mehdi Mortazaviᵉ, Mahmoud Khaledª,ᶜ,\*

Received 17 May 2023; accepted 26 May 2023

Available online 6 June 2023

## Abstract

One of the most effective ways to deal with the effects of the current global energy crisis is to reduce energy consumption and rely energy management strategies. As a result, waste heat/energy recovery might be a helpful option for decreasing energy costs and environmental effect. It entails finding a practical way to apply any engineering system's wasted heat where the percentage of wasted heat is too high in global. In this context, waste hot water is a rich source of wasted energy that, if recovered, can significantly cut down on the amount of electricity used worldwide. Within this framework, the present research paper provides a thorough analysis of Waste Water Heat Recovery Systems (WWHRS) in terms of performance, design, tools and applications. In addition, it is highlighting the key elements associated with WWHRS, including the wastewater sources and approaches used in the literature for waste water heat recovery. Moreover, the present paper demonstrates the effect of the heat recovery from the drain waste water on economically and discuss the technical barriers to using WWHRS. It will be demonstrated that the feasibility of recovering heat from waste water could decrease significantly the cost of energy consumption for residential or industrial application. In addition, the main tools that are used for heat recovery systems is the heat exchanger where various types are used and the thermal performance are strongly affected by the choice of heat exchangers. Moreover, based on the literature review, it is shown that the methods of studying the thermal performance of the heat recovery system are experimental and/or numerical and in some cases the study is performed analytically. Finally, all discussed and presented studies demonstrate that the WWHRS has a significant benefits and could be considered for new residential construction.
© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Peer-review under responsibility of the scientific committee of the TMREES23-Fr, EURACA, 2023.

**Keywords:** Heat recovery; Wastewater; Comprehensive review; Critical analysis; Potential recommendations

<footnote>ª Energy and Thermo-Fluid Group, Lebanese International University (LIU), PO Box 146404 Bekaa, Lebanon</footnote>

<footnote>ᵇ College of Engineering and Technology, American University of the Middle East, Kuwait</footnote>

<footnote>ᶜ Energy and Thermo-Fluid Group, The International University of Beirut (BIU), PO Box 146404 Beirut, Lebanon</footnote>

<footnote>ᵈ LARIS EA 7315, University of Angers, Polytech Angers, Angers, France</footnote>

<footnote>ᵉ Department of Mechanical and Materials Engineering, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA</footnote>

<footnote>\* Corresponding author at: Energy and Thermo-Fluid Group, Lebanese International University (LIU), PO Box 146404 Bekaa, Lebanon. E-mail address: mahmoud.khaled@liu.edu.lb (M. Khaled).</footnote>
Nomenclature

<table><tr><td>A</td><td>Area of heat exchange, m²</td></tr><tr><td>Cp</td><td>Specific heat, J/kg °C</td></tr><tr><td>DWHRS</td><td>Drain water heat recovery system</td></tr><tr><td>ṁ</td><td>Mass flow rate, kg/s</td></tr><tr><td>Q̇</td><td>Heat rate, W</td></tr><tr><td>T</td><td>Temperature, °C</td></tr><tr><td>WWHRS</td><td>Wastewater heat recovery system</td></tr><tr><td>ΔTlm</td><td>Logarithmic mean temperature difference, °C</td></tr></table>

1. Introduction

The focus of current global attention is on environmental and climatic change. Urban residential areas and structures accounted for a sizable share of energy use, which is thought to be the primary source of greenhouse and CO₂ emissions that might be produced by burning fuels [1–4]. According to studies, the building industry accounts for two-thirds of energy use and one-third of gas emissions [5]. For instance, it is noticed that 30% of the energy is used to power buildings in China [5–8], which accounts for around 9.78% of global energy consumption [6]. Another study revealed that the building industry accounts for 36% of the European Union’s CO₂ emissions and consumes 40% of the total energy consumed in the region [7]. Energy consumption and intelligent building systems are now considered necessities for a comfortable lifestyle [9–12]. Several examples of energy use in buildings include air conditioning, lighting and the appliances used. Building energy consumption is being reduced as a result of agreements and initiatives aimed at reducing the cost of energy [13,14].

The globe is moving toward many options, such as renewable energy and energy management, to address energy consumption, cost, and environmental side effects. Renewable energy [15–19] comes from naturally replenishing sources including wind [20–22], solar [23–26], and geothermal [27–31] energy. By controlling energy equipment, energy management can also aid in lowering energy costs and consumption [32–35]. Several applications depend on energy to function on a daily basis, and it is currently the focus of millions of researchers [36–43]. The existing world pattern in energy investigations is toward energy consumption and carbon dioxide emission decrease [44–46]. Renewable energy sources and effective energy management can help with this [47,48]. Due to the significant heat emitted from engineering systems bound by the second law of thermodynamics, waste heat recovery is a procedure under energy management that is currently receiving a lot of attention [49–52].

Energy recovery is another technological advancement that enables decreasing the energy consumption by moving wasted energy from one system to another [53–56], in addition to renewable energy systems and energy management strategies. The most important component of energy recovery is heat loss. It is usually found in factories [57], internal combustion engines [58,59], and residential structures [60–63]. Many research, including those on HVAC recovery systems, have been done over the years to support heat recovery from building sectors. Yet, water heating accounts for the majority of energy consumption in buildings, and waste water that is dumped into the sewage system is seen as a significant source of heat loss from an energy recovery perspective.

In cities, a significant amount of heated wastewater, particularly from residential structures, with an average temperature about 27 °C. For example, a four family members residential building can consume about 1300 L/day of fresh water and generate about 800 L/day of wasted water [64]. Fijians et al. [65] made an overview on energy recovery options from water cycles (chemical and thermal energy recovery from wastewater) and presumed that the thermal energy can be saved through heat exchangers installed inside the sewerage systems where the domestic water cycle occurs.

Wastewater treatment facilities, drinking water sources, homes, and sewage systems inside of homes can all be used to generate energy [66,67]. The location, quantity used, and season all have an impact on the temperature of the wastewater which can vary between 28 and 29 °C in the summer and 9 to 14 °C in the winter respectively. In [68,69], the interaction between flow rate, wastewater temperature, and air temperature in sewage systems was examined. It was clear that flow rates—rather than the outside temperature—have a greater impact on effluent temperature variance. Another study was carried out within a home, where wastewater, water supply, and room
temperatures were monitored, along with water supply flow rates over the course of a month in the winter [70]. As much as 30% of the energy required to heat the water can be provided by wastewater energy recovery inside the home, according to research. In this context, the originality of the present paper is in the take into considerations of the recent published studies focusing on the types, the classifications and the configurations of Waste Water Heat Recovery System (WWHRS) systems. In all presented cases the used method (experimental, numerical or both) are analyzed in addition to the main outcomes. In most cases, the environmental and economic impacts are investigated and discussed. Accordingly, in all presented cases it was shown that the recovery of energy from WWHR is a promising benefits at all level.

The interest in the drainage wastewater heat recovery units has developed recently using various technologies, as is evident from research and observation. In this regard, the purpose of the current article is to provide information on the heat recovery systems for waste water that are currently in use. The paper is structured in this context as follows: The concept, potential and guiding principles of waste water heat recovery systems will be discussed in Section 2. Section 3 will provide clarification on the structural forms of waste water heat recovery systems already in use. The purpose of Section 3 is to present the classifications of the drain WWHRS and the studied configurations. The thermal modeling and the governing equations will be presented in Section 4. The Section 5 is the part of discussion where the main outcomes are presented in addition to the technical barriers. Finally, Section 6 presents the conclusion of the present paper.

## 2. Concept of waste water heat recovery system

From an economic perspective, recovering heat from wastewater in buildings is growing increasingly appealing. The main sources of drain water that interfere with WWHRS are showers, sinks, dishwashers, and washing machines. The fundamental idea behind this approach is to raise the temperature of the supply water after it has passed through the drainage system as a preheating step by heating the supply water before it enters the water heater using the heat carried from the drain water. The major part of a WWHRS, the heat exchanger, is shown in Fig. 1 as it functions. Systems can be equipped with a variety of heat exchanger types to perfectly adopt the ideal heat transfer state. The heat transfer rate that results from the temperature difference between the supply water and the drain water is influenced by the heat transfer area and the water thickness. Any residential building's potential to accommodate different types of WWHR units depends on a number of factors, including the building's construction and available space [71]. There are various kinds of heat exchangers, such as shell and tube heat exchangers, plate heat exchangers, etc., that can be employed in various commercial applications for wastewater heat recovery systems. Because to its high thermal conductivity, copper pipes are frequently utilized in WWHRS because they offer the highest possible heat transfer.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Main required component for any waste water heat recovery systems.</figcaption></figure>
### 3. WWHR classification, structure, and configuration

Vertical systems, horizontal systems, orientated systems, and storage tank based systems are the four categories into which heat exchanger types and structures can be subdivided based on their structure. According to their year of publication, application, type of heat exchanger, and performance estimation, the structure, capacity, performance, and financial aspects of waste water heat recovery systems have been carefully skimmed and reviewed. Papers are presented in stages based on their functions and years of publication, reviewing from the past to the present. Less of the papers rely on numerical simulations and more were tested empirically. Waste water heat recovery systems' energy and financial aspects are the focus of this investigation.

The development of wastewater heat recovery systems began many years ago. A British study from 1975 [72] gave an experimental setting that was tested in accordance with the way of life of the residents. In order to recover heat, a tiny storage tank was positioned within a tank used to collect wastewater. Measurements of this heat recovery system's energy use revealed that, in some situations, energy savings of up to 10% are possible. Another research [73] that studied the potential for heat recovery from dwellings in the UK during the following 30 years was carried out in 1981. [74] looked at heat recovery from wastewater during the dyeing process using an absorption heat pump.

The results of the operating analysis revealed significant energy savings, with an average efficiency that is twice as efficient as the principal energy input. A study that looked into the effectiveness of heat recovery from sewage effluent utilizing heat pumps can be found in [75]. The system may provide heat at a lower cost than a natural gas boiler, it has been demonstrated. Gonzalez et al. [76] explored the possibility of implementing therapeutic treatments in a spa by reclaiming energy from two sources of hot water in the year 2001. Calculations were made based on the demands of various spa services, losses, and thermal water use. A comparison was also done with the contribution of the spring and the evolution of the thermal water in tanks. The buffer tank was found to be adequate for meeting the demand. Up until 2001, all research was regarded as being in the early stages of the WWHR systems area. However, systems that were presented after 2003 can be divided into two categories: connected and non-coupled systems. Fig. 2 depicts the flow chart of the systems that were reviewed and the categories that each system falls within.

<figure><img src="image_3.png" /><figcaption>Fig. 2. WWHRS classification.</figcaption></figure>

#### 3.1. WWHR systems and types

##### 3.1.1. Vertical systems

The Fig. 3 is presenting a vertical waste water heat recovery system applied on a shower. A gravity film heat exchanger is what makes it unique. With a spiral coil wrapped around the main inner pipe, the system provides a larger heat transmission area than a straight pipe as shown in Fig. 3.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Schematic for vertical drain water heat recovery system applied on a shower drain.</figcaption></figure>

The design's shape has an impact on how much heat is transferred in this kind of heat exchanger. The Dean number and essential Reynolds number are utilized for the coiled pipe to characterize the flow profiles [77]. The hot water is supplied from the top of the main pipe, adhering to the inlet walls. The spiral coil receives cold water before the process of removing heat from hot water begins, and the system is finished with warmed water at the top of the unit. De Paepe et al. [78] suggest a heat recovery system for dishwashers and their results are obtained based on experimental study. Their results demonstrate that the drain waste heat recovery system is economically beneficial. Experiments demonstrate that a significant quantity of heat may be recovered. Torras et al. [79] suggest another study concerning the heat recovery storage-type unit for residential housing. According to the indicated volume flow rate and their numerical and experimental findings, the drain water heat recovery storage had the ability to recover between 34% and 60% of the energy present in the drain water. Ruiz el al. [80] suggest an enhancement of standard way of the DWHR storage unit using a phase change material. Their experimental and numerical analysis is suggested in order to present the performance of heat storage unit as a function of time. Accordingly, around 75% of the thermal energy from preceding process's wasted water heat can be stored by the PCM heat storage unit, and some of it can be recovered to provide about 50% of the thermal energy needed to heat the subsequent process. In this context, Table 1 summarizes the methodology, approach and the main findings for recent studies applied on vertical WWHR systems. It could be noticed that the methodology of their studies could be subdivided into numerical, experimental and experimental/numerical studies.

Another installation scenario that uses vertical systems and a storage tank is possible. In this scenario, the water is heated or preheated by the WWHR system before being stored, and then it is heated to the appropriate temperature to meet the needs of the building using a heat pump. Together with their encouraging performance, these studies have, in some cases, also demonstrated their positive effects on the economy and the environment. The problem with WWHR systems is that the drain water heat might sometimes be squandered inside or outside the system's enclosure due to the low temperature. To achieve better performance, the boiler/electric heater must be installed immediately following the recovery system.

### 3.1.2. *Horizontal systems*

Residential buildings might not always have adequate space to fit a vertical unit beneath shower trays. Horizontal WWHR systems may be sufficient where they can be used in various public sectors [71,82]. These systems, as shown
<table><caption>**Table 1.** Vertical DWWR studies.</caption><thead><tr><td>Ref.</td><td>Methodology</td><td>Main approach</td><td>Main findings</td></tr></thead><tbody><tr><td>[8]</td><td>Experimental</td><td>The solar water heater is compared to the drain heat recovery system (DWRS).</td><td>The DWRS is more expensive than the Shower waste heat recovery.</td></tr><tr><td>[81]</td><td>Experimental, Numerical</td><td>Based on preheating water the analysis is performed.</td><td>Water preheating is thought to have positive financial effects. With more time in the shower and more water used, it becomes more efficient.</td></tr><tr><td>[69]</td><td>Experimental</td><td>The flow rate impact on the performance study.</td><td>The performance of heat exchanger decreases when the flow rate increase.</td></tr><tr><td>[82]</td><td>Experimental</td><td>The impact of temperature at the inlet on the effectiveness.</td><td>No matter the shape, any rise in the temperatures on the main and drain sides will boost efficiency.</td></tr><tr><td>[58]</td><td>Experimental</td><td>The parametric study is based on experimental investigation.</td><td>The output temperature of boiler increases the performance.</td></tr><tr><td>[83]</td><td>Experimental, Numerical</td><td>A multi-drain heat recovery system's parametric analysis for preheating cold water.</td><td>According to a case study, with a drain temperature of 25 °C, the lowered energy, monthly savings, and CO2 emissions increase as the heat exchanger's length increases.</td></tr><tr><td>[84]</td><td>Experimental</td><td>Based on the efficiency of the heat exchanger and side wetting, the falling film DWHR system performs well.</td><td>The efficiency would decrease around 28% under partial wetness conditions.</td></tr><tr><td>[85]</td><td>Experimental</td><td>The effects of main and drain flow rates being different inside a falling film heat exchanger.</td><td>The DWHR system's performance can be significantly influenced by the operational conditions such as (flow rates) in inlet flow rates can have a negative impact on it regardless of other design factors.</td></tr><tr><td>[77]</td><td>Experimental, Numerical</td><td>The tank is charged with hot water.</td><td>Low flow rates were where the most heat transfer occurred. Ability to recover between 34% and 60% of the energy from the drained water.</td></tr><tr><td>[80]</td><td>Experimental, Numerical</td><td>A Phase Change Material (PC) based DWHR storage unit.</td><td>High heat recovery ratios from DWHR systems can be attained, and the next stage of a washing machine can get 50% of the thermal energy.</td></tr><tr><td>[67]</td><td>Experimental</td><td>The discharge of hot water into a cylindrical tank containing cold water through a spiral tube.</td><td>Less mixing of cold and hot flows allows for high temperature, which improves heat recovery.</td></tr></tbody></table>

in Fig. 4, directly connect a water heater to a simple single pass counter flow heat exchanger. It is obvious that in the vertical unit, water will flow completely around the pipe's perimeter, assuring excellent conductive efficiency. The heat transfer rates are reduced in the horizontal arrangement because water runs through the pipe's bottom [86]. This method was rarely employed since, in some situations, it was seen to be less effective than a vertical system. An experimental investigation based on shower time, water flow, and inlet temperature savings is provided in [71], and the findings reveal that the energy savings can reach up to 15%. Another experimental and numerical study showing an increase in efficacy of 45% with more system loops when the cold water pipe is thinner and the flow velocity of the drain water is low is reported in [86]. In addition, an experimental study is performed in [87]. The experimental study is based on Lab conditions and the main outcomes of the study is studying the effectiveness for two horizontal DWHR systems, (i) brand new baffled heat exchanger and (ii) normal heat exchanger. Their results show that the effectiveness of the baffled heat exchanger is higher than the standard one.

By comparing the aforementioned configurations, the efficiency of the heat exchanger used may be demonstrated to be up to 50% and 60% for horizontal and vertical installations, respectively. In contrast to horizontal installations, which only employ a piece of the pipe for heat transfer, vertically designed heat exchangers may guarantee heat
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Schematic for horizontal drain water heat recovery system applied on a shower drain system.</figcaption></figure>

transfer over the whole pipe system's border. The length of the shower, the cold water's temperature, the flow rates, the fouling effect, the pipe's flow regime, and the temperature difference between the shower head and the drain point, which vary between studies, are additional elements that have a significant impact on the system's overall effectiveness.

### 3.1.3. Inclined systems

The figure below presents the inclined system, a tilt angle between the vertical line and the heat exchanger of the drain system. This sort of slanted heat exchanger performs comparable duties to a vertical system. Due to construction of buildings several WWHR systems may not be fitted vertically [85]. In designing, a tilt angle (see Fig. 5) between the vertical line and the heat exchanger for the drain system should be taken into account. This kind of heat exchanger performs comparable duties to a vertical system. The efficacy of a system with a tilt angle in relation to the vertical line was discovered to be: (i) A non-uniform flow occurred with angles above 2° compared to a vertical one, where the effectiveness of the system decreased with the increase of the angle, 2° is the greatest acceptable angle where its effectiveness decreased about 4% relative to a vertical installation [85].

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Schematic of a tilted type heat exchanger [85].</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6. Storage tank heat exchanger.</figcaption></figure>

### 3.1.4. Storage tank based system

Storage tank [79] is another technique that can be used in WWHR systems. Currently, it is utilized to store domestic hot water. This type of systems is designed such that a spiral pipe carrying cold fresh water is inserted inside a hot water tank as shown in Fig. 6. This type of systems enables using a heat exchanger to extract heat at the bottom of the store, keeping the top of the tank hot and ready for future use.

The Table 2 is presenting the reference, the study methodology, research approach, assessments and the main outcomes of several research studies applied on tank heat exchangers.

<table><caption>**Table 2.** Case studies concerning the WWHR systems applied on storage tank.</caption><thead><tr><td>Ref</td><td>Study</td><td>Approach</td><td>Assessments</td><td>Outcomes</td></tr></thead><tbody><tr><td>[78]</td><td>Experimental</td><td>A spiral tube that transports the cold, fresh water is inserted into the cylindrical tank once the heated water has been released into it. Hot water will swirl because of its tangential flow rate to the wall.</td><td>Energy, economic</td><td>When the inlet flow rates are rather high, the inlet of swirl performs well. Because to the limited mixing of cooler and hotter water, the tank will be able to reach a high temperature, which will promote heat recovery.</td></tr><tr><td>[88]</td><td>Experimental and numerical</td><td>The behavior of a novel stratified, and two standard stores for domestic hot water. The stratified store is divided into lower and upper parts for pre-heating the domestic hot water using alternative systems and for heating respectively.</td><td>Energy</td><td>the inner arrangement of the heat exchangers inside the hot tank store significantly affects its performance and recovery process.</td></tr><tr><td>[80]</td><td>Experimental and numerical</td><td>A PCM plate-based drain water heat recovery storage unit. The device is made of a collection of PCM capsules that are arranged to create a network of water channels without branching out utilizing steel supports.</td><td>Energy</td><td>A portion of the stored energy can be recovered by the unit, which can then provide around 50% of the thermal energy needed for the washing machine's following stage. The highest heat recovery ratios for DWHR systems were achieved by this unit, according to field reports.</td></tr><tr><td>[79]</td><td>Experimental and numerical</td><td>Energy storage begins, and finishes when heat extraction starts, completing the process when the storage energy is extracted in the discharging process by cold water pumped through the accumulator coiled pipe.</td><td>Energy</td><td>When the flow rates are low, the most heat transfer took place. the ability to recover between 34% and 60% of the present energy in the drained water.</td></tr></tbody></table>
Based on the presented information in Table 2, it could be noticed that all applied methods (experimental/numerical) show that the recovered energy depends strongly on the operational parameters. The use of PCM has a positive effect on the recovery of energy for storage tank.

### 3.2. WWHR configurations

Without using a separate system where they are typically connected directly to the sewage pipe lines, non-coupled systems heat up incoming cold water using various heat exchanger arrangements.

A promising strategy is to combine energy applications and WWHR systems. For greater efficiency, very few studies [77,81] integrated the WWHR system with solar household water heaters. Nevertheless, integrating a heat pump with a WWHR system is the main focus [72,85]. It is common knowledge that a heat pump is a mechanical device that uses a fully reversible cycle to transfer energy from one medium to another using a refrigerant-based circulating fluid [73,74]. Air, ground, and water are its three usual thermal energy sources [75,89]. Due to their efficiency and performance, wastewater source heat pumps are now more frequently used [80,84]. Urban sewage-source heat pump installation has expanded significantly [90]. Several recent studies on the many uses for heat pumps are provided in Table 3. In New York city [64], a gray water heat recovery system was proposed in a two-story house with 4 family members. The system was adopted with a multiple-function heat pump that reclaims heat energy from drainage as a low-grade heat source for space and water heating. 14 other cities were selected to evaluate the performance of the system in various climates. The gray water sources considered in this study include sinks, laundry and showers. It was concluded that among all cities system is capable to save energy in a range of 17%–57.9% compared to a conventional system. Water saving have a range of 15%–34.1% and the higher savings can be observed in hot climates. The Table 3 presents some case studied of applying WWHR systems on heat pumps.

Only a small number of articles have explicitly examined the hybridization of heat pumps with WWHR systems. Heat recovery system and heat pump coupling can be used to raise the efficiency of the heat recovery system, provide heat for the evaporator, or both. By achieving these goals, residential structures’ heating and hot water needs may be more effectively met. In other words, the improved heat recovery ratio used here makes it easier to evaluate the heat recovered and is useful for a better supply-demand calculation.

### 4. Thermal modeling and simulation

After the literature review that is presented previously and the analysis that is presented, it was noticed that the heat exchanger is governing part in the drain WWHR systems. Accordingly, the thermal performance of heat exchangers is mainly covered by experimental, numerical and theoretical way. The conservation of mass and the conservation of energy are the main concept for this kind of investigation. In addition, the theory that is used to design and perform parametric study on heat exchangers is the method of Logarithmic Mean Temperature Difference (LMTD method) [83,100,101].

For this method the overall heat transfer coefficient (U) is one of the important variables that should be calculated which depend strongly on the operational conditions, geometrical design, thermophysical properties of fluid, heat exchangers types and materials. The estimation of U is demonstrated in [102–105]. This variable is the main parameter that permits to design the heat exchangers (length, diameter, number of tubes, area, etc.). Accordingly, the theories that are used for designing heat exchangers are based on the conservation of energy and conservation of mass. This permits to calculate the heat rate of cold water and hot water where both are assumed to be equal for an adiabatic system, as presented in the equations below.

$$ \dot{Q}_{\text{Cold-Water}} = \dot{m}_{\text{Cold-Water}} \cdot C_p \cdot \text{Cold-Water} \cdot (T_{\text{Cold-Water,out}} - T_{\text{Cold-Water,in}}) \quad (1) $$

$$ \dot{Q}_{\text{Hot-Water}} = \dot{m}_{\text{Hot-Water}} \cdot C_p \cdot \text{Hot-Water} \cdot (T_{\text{Hot-Water,in}} - T_{\text{Hot-Water,out}}) \quad (2) $$

where the $\dot{m}$ is the mass flow rate of fluids (cold and hot), $C_p$ is the specific heat at an average temperature of working fluids and $T$ is the temperature of fluids at inlet/outlet or outlet/inlet of heat exchanger. Accordingly, it is noticed that the geometrical parameters such as the length, diameters and area of heat exchange does not appear in these equations. For this reason, the LMTD method is used in the most presented cases in the literature review part. The Eq. (3) is the LMTD theory that is used when minimum three temperatures of cold and hot fluid are known.

$$ Q = U \cdot A \cdot \Delta T_{LM} \quad (3) $$
<table><caption>**Table 3.** Case studies concerning the WWHR systems applied on heat pumps.</caption><thead><tr><th>Ref</th><th>Config</th><th>Heat exchanger</th><th>Study</th><th>Approach</th><th>Assessments</th><th>Outcomes</th></tr></thead><tbody><tr><td>[91]</td><td>Heat pump</td><td>N/A</td><td>Experimental and numerical</td><td>A compression heat pump system using wastewater from sauna, and public baths, performing its analysis and capacity, with conditions like Korea.</td><td>Energy</td><td>The yearly mean Coefficient Of Performance (COP) of the system was 4.8 where it can supply 100% of hot water load except weekends in winter season.</td></tr><tr><td>[87]</td><td>Heat pump</td><td>Plate</td><td>Experimental</td><td>A modeled wastewater source heat pump was tested experimentally under different conditions. Refrigerant enters the compressor as saturated vapor after it is compressed, then leaves the condenser as a saturated liquid because of heat rejection to the circulating water.</td><td>Energy</td><td>The heating COP increased with the increase of the heat source temperature.</td></tr><tr><td>[92]</td><td>Heat pump</td><td>Shell and tube</td><td>Experimental</td><td>An urban sewage source heat pump composed of a filth block device, a wastewater heat exchanger, a sewage source heat pump and other devices like fans, pump and valves.</td><td>Energy</td><td>The heating and cooling COP was 4.3 and 3.5 respectively.</td></tr><tr><td>[93]</td><td>Heat pump and solar system</td><td>N/A</td><td>Experimental</td><td>A heat pump system for exhaust heat recovery for public shower facilities. The system composed of three subsystems: solar collection system, drainage collection system and a heat recovery system.</td><td>Energy, economic and environmental</td><td>The overall operating cost of the heat recovery system is 12% of the cost of oil boilers and 39% of coal boiler. The power consumption is lower than solar-only-system. Environmentally, the heat recovery systems were competitive with the gas boiler system.</td></tr><tr><td>[94]</td><td>Heat pump</td><td>N/A</td><td>Numerical</td><td>A system integrating a heat pump directly into the heat recovery from wastewater was demonstrated. The system is a heat recovery tank that accepts the outgoing warm wastewater which can be connected to any wastewater source in a house.</td><td>Energy</td><td>A COP of above 6 was demonstrated under conditions in the first scenario, while the second one a COP above 6.5 for all resident's datasets was reached. Energy saving analysis showed a low electrical energy inputs demands.</td></tr></tbody></table>

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<table><caption>**Table 3 (continued).**</caption><thead><tr><th>Ref</th><th>Config</th><th>Heat exchanger</th><th>Study</th><th>Approach</th><th>Assessments</th><th>Outcomes</th></tr></thead><tbody><tr><td>[95]</td><td>Heat pump</td><td>Counter flow</td><td>Experimental and numerical</td><td>A heat pump water heater for bathing assisted by shower wastewater. Thermodynamic models were settled besides a static and dynamic simulation based on MATLAB programming.</td><td>Energy</td><td>The temperatures of the condenser and evaporator were optimized at 51.5 and 11.68 °C respectively at an optimal COP of 4.97.</td></tr><tr><td>[82]</td><td>Heat pump</td><td>Shell and tube</td><td>Experimental and numerical</td><td>Untreated sewage source heat pump was considered in Dalian, China. The main equipment's of the system is an auto-avoiding-clogging, sewage heat exchanger, heat pump unit.</td><td>Energy, economic</td><td>The convective thermal resistance in sewage side accounts 60% of the total thermal resistance. The COP of the heat pump and the untreated source heat pump was 4.3 and 3.6 respectively. The unit operates in lower cost than the other conventional systems.</td></tr><tr><td>[96]</td><td>Heat pump</td><td>Gravity film</td><td>Experimental</td><td>Examination of the effectiveness of a heat pump-aided vertical drain water heat recovery system operating with various drain water flows.</td><td>Energy</td><td>It is crucial to size the recovery system to correspond with the drain water profiles.</td></tr><tr><td>[97]</td><td>Heat pump</td><td>N/A</td><td>Experimental</td><td>A direct fired absorption heat pump and an electrical heat pump are used in the heat recovery system. The tap water's temperature rises from 12 to 25 °C degrees when the wastewater is piped to the heat exchanger for heat exchange, then to the heat pump for additional heating.</td><td>Energy, economic and environmental</td><td>The natural gas consumption will be reduced every year and consequently less pollution will occur.</td></tr><tr><td>[77]</td><td>Heat pump</td><td>Gravity film</td><td>Experimental and numerical</td><td>The performance of a vertical inline drain water heat recovery system was studied boosted with a heat pump.</td><td>Energy</td><td>The contact heat resistance between the heat exchanger's inner and outside pipes has an impact on its capacity to recover heat. The effectiveness of the heat exchanger is affected by the flow rate while the heat recovery ratio is affected by the incoming temperatures. The proposed heat exchanger can recover about 25% of the available drain heat.</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 3 (continued).**</caption><thead><tr><th>Ref</th><th>Config</th><th>Heat exchanger</th><th>Study</th><th>Approach</th><th>Assessments</th><th>Outcomes</th></tr></thead><tbody><tr><td>[8]</td><td>Solar system</td><td>Gravity film</td><td>Experimental</td><td>Investigated the performance of a drain water heat recovery unit with two solar domestic water heaters. The two systems were installed in two different houses A (practical house with a flat plate solar thermal collector) and B (advanced technological houses with an evacuated plate solar thermal collector).</td><td>Energy</td><td>The unit can recover an annual heat recovery of 789 kWh and an overall efficiency of 50%. The flat and evacuated plates solar collectors produce an annual thermal energy output of 2038 kWh and 1383 kWh respectively. The system efficiency decreases with the increase of temperature difference.</td></tr><tr><td>[98]</td><td>Heat pump</td><td>N/A</td><td>Experimental</td><td>An innovative shower-use heat pump that uses the water from the Shower drain. A Waste heat Extraction Device (WHED) with a pre-heater for the water was fitted.</td><td>Energy, economic</td><td>Water pre-heated loops performed better, recovering more of the SDW's waste heat with an enhanced COP of 2.19 to 3.21. The system's energy usage grew as the water flow rates and shower temperatures were raised. When compared to an electrical water heater, the design can save 70% of energy.</td></tr><tr><td>[99]</td><td>Heat pump</td><td>Storage tank</td><td>Experimental</td><td>Performance of a heat pump heating system's thermal source, which is recovered heat from municipal sewage.</td><td>Energy, economic</td><td>A 12% increase in the COP over the air-to-water heat exchanger. The experiments conducted under varying external settings demonstrated that the performance of the heat pump may be impacted by ambient air temperatures. The energy recovery system's supplemental cost had a manageable payback period of 5 years.</td></tr></tbody></table>

$$ \Delta_{TLM} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1/\Delta T_2)} \qquad (4) $$

where $\Delta T_1 = T_{HW,i} - T_{CW,o}$ and $\Delta T_2 = T_{HW,o} - T_{CW,i}$, and A is the area of heat exchange that depends of the geometry of heat exchangers. The U is the overall heat transfer coefficient and it is calculated by $U = 1/R_{tot}$, and the $R_{total}$ is the total thermal resistance such as convection, conduction and fouling resistance. The conduction thermal resistance is calculated by knowing the thermal conductivity and the thickness of material, the convection coefficient is calculated by using specific Nusselt number correlations that depend on flow nature (laminar, turbulent) and geometries (pipe, ducts).

A multi drain heat recovery system was suggested recently [83]. In their study, an in-house code is developed based on the mentioned theory. They present a case study for a multi drain heat recovery system applied in several cases (washer machine, shower, washing machine). For varied input cold water temperatures and for various cold and hot water flow rates, the difference in heat rate between the numerical and experimental results is always less than 14.2%. The results of the parametric study, which examined the impact of hot and cold water inlet temperatures and
flow rates on system performance, allowed for the formulation of useful suggestions for the actual implementation of the proposed idea. It was shown that for a drain water temperature of 25 °C for the case of Lebanon, increasing the heat exchanger length from 0.2 m to 1 m results in an increase in monthly energy savings from 102.2 kWh to 410.9 kWh, monthly financial savings from 13.3 $ to 53.4 $, and monthly CO2 emissions reductions from 72.5 kg to 291.7 kg. In addition, several recent studies are published focusing on drain WWHR using numerical simulation using CFD software, such as Singh et al. [106]. The goal of their study is to determine whether it is possible to reduce the net energy needed for commercial hot water use in food-related activities by recovering heat from grease traps (GT) in commercial kitchens.

## 5. Discussions

Wastewater heat recovery systems' ability to reduce the use of energy and environmental damage was proved by the variety in their designs and applications. Governments are driven to implement this technology because vast volumes of water at a high temperatures and rapid flow rates pass through sewers each year, especially in showers in which it consumes the most energy. Several different closing observations were made throughout this review:

1. It is possible to categorize the setup designs from diverse sources into a number of configurations based on either single or hybrid systems. Hybrid systems include, for instance, heat pumps coupled with a heat recovery system or a solar heater. There may be further hybrid applications, such as a typical connection among wastewater and engine exhaust thermal management.

2. There is typically no difference between the two methods of analyses because the bulk of the studies employed an experimental setup and just a low amount assessed their study using a numerical simulation. Yet, the numerical technique should be expanded as it offers a greater understanding of the phenomenon of heat transfer that would occur in such systems.

3. The temperature difference between the hot and cold flows has a significant impact on the efficiency of heat transfer across the various types of heat exchangers that have been used in the studies. The most common type of heat exchanger used is a helical one. However, a heat exchanger combined with such a phase-change material might be employed since it has a higher efficiency and is a more sophisticated kind of heat exchanger that might be utilized to enhance heat transfer.

4. In most cases, the showers are a typical application of wastewater in residences. Nevertheless, no studies have been done on using other typical residential applications that generate hot waste water as a source of heat.

6. The efficacy of different trends is based on the fact that losses like conduction and convection through the system's walls have a minimal impact on the potential of the WWHRS, whereas flow rates and temperature changes among hot and cold flows have a significant impact on heat recovery efficiency.

7. Scarce are the experimental studies that present a complete and modular experimental setup that allows performing extensive parametric studies of the performance of the system.

8. Scarce are the numerical studies that present a wide and accurate thermal modeling that allows performing extensive parametric studies of the performance of the system for wide ranges of parameters with reduced calculation time.

9. Scarce are the studies that hybridize the use of the waste water heat recovery with other energy management or renewable energy sources, for example coupling the heat recovery exchanger with thermoelectric generators, coupling the heat recovery concept with solar and geothermal energy, etc.

Fig. 7 demonstrates the barriers that are subdivided into technological, social and economic barriers.

## 6. Conclusions and future work

The main conclusions of the present paper are subdivided into four main points:

* One indirect form of renewable energy is waste water heat recovery. It is a plentiful source of heat that is accessible in every home. Furthermore, it is simple to extract and reuse due to its thermal properties in various applications. The available WWHRS were thoroughly reviewed in the current research, with an emphasis on their potential for energy efficiency and cost-effective energy use. The studies are organized depending on their installation type and displayed in chronological order of publication.
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Schematic concerning the technological, social and economic barriers.</figcaption></figure>

* Practically speaking, the performance of a WWHRS is mainly influenced by the heat exchanger, which is the system's central component. Since a counter flow heat exchanger enables higher heat transfer, it is frequently utilized. WWHRS fall into one of three categories: vertical, which includes systems based on storage tanks; horizontal; or orientated. Moreover, there are two categories within which the recovery systems are divided: Single systems and Hybrid systems. The heat transfer process is primarily relied upon by single systems. While a hybrid system combines a WWHRS and a heat pump or solar system.

* Nevertheless, water viscosity, hot and cold temperatures, ambient temperature, and flow rates are the key factors that influence the heat performance of WWHRS. The effectiveness of WWHRS has been noted to increase when connected to a heat pump, where the COP may surpass 6 in some setups. Also, it was discovered that raising the hot flow rate and temperature would improve the system's effectiveness and efficiency. Economically speaking, hybrid systems have a five-year payback period compared to a seven-year average for single systems. In order to maintain the functionality of WWHRS, a number of challenges should also be taken into account, including fouling, pollution, and pressure decrease.

* As it is shown in the present paper, the operational parameters such as temperatures, flow rates have significant effects of the recovered energy. For these reasons, a multi-drain system could be a useful configuration that could enhance significantly the recovered energy.

As future work, the following are recommended:

1. Design and implementation of a complete and modular experimental setup that allows performing extensive parametric studies of the performance of the system.

2. Development and validation of a wide and accurate thermal modeling that allows performing extensive parametric studies of the performance of the system for wide ranges of parameters with reduced calculation time.

3. Invest more in hybridization of the use of the waste water heat recovery with other energy management or renewable energy sources, for example coupling the heat recovery exchanger with thermoelectric generators, coupling the heat recovery concept with solar and geothermal energy, etc.

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.
## Data availability

The authors are unable or have chosen not to specify which data has been used.

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