1. Introduction
Due to diminishing natural water resources and the increasing global population, the problem of searching for alternative water sources is becoming more intense. A trend in improving the management of freshwater resources is the utilization of innovative solutions [
1,
2].
One of the main consumers using significant amounts of drinking water is identified as the building sector [
3]. It should be emphasized that not all water draw points in buildings require the supply of potable water. This creates an opportunity for the reuse of water, which, in a typical plumbing system, would be directed into the sewerage [
4,
5]. By using recovered water for non-potable purposes, the demand for potable water decreases, and the volume of sewage in the sewage system is reduced. This results in both environmental and economic benefits [
6].
One of the solutions is hybrid systems that utilize both rainwater and greywater. This approach is due to the fact that rainwater availability is irregular and sometimes insufficient in some regions [
7,
8]. Leong et al. [
8] emphasize the complexities of utilizing hybrid rainwater–greywater systems to mitigate water scarcity. The variability in the quality and quantity of both rainwater and greywater leads to thorough treatment processes due to the presence of various pathogens. Chen et al. [
9] investigate the economic feasibility of hybrid systems by employing cooperative theory, focusing on a case study in Japan. These systems are found to be economically feasible with government subsidies. It has been noted by some researchers [
10,
11,
12] that the community exhibits a greater willingness to use greywater for toilet flushing and irrigation. On the other hand, a decrease in interest is observed as the potential contact with greywater increases. This was determined based on a community survey, where hygiene concerns were primarily raised. The study [
12] reveals that low applicability in residential buildings is observed due to the long return on investment without subsidization. Nonetheless, it could be cost-effective in households with high water consumption.
This work focuses on another group of solutions for wastewater reuse, which primarily aims at the recovery of greywater. It has been reported that 50–80% of household wastewater, excluding toilet waste, consists of greywater, marked by its low organic content and high volume, posing challenges for urban treatment facilities designed for more concentrated waste [
13]. Advantages of local greywater reuse are highlighted by its availability, and benefits tied to energy and sustainability. Conversely, obstacles are identified in the problematic quality of greywater posing health risks, the absence of legal frameworks, the need for safety monitoring, and negative public perceptions [
13,
14,
15]. In the study [
16], a greywater recovery system (GRS) with an existing centralized water system is examined. Greywater is treated using submerged membrane bioreactors for reuse in non-potable applications. For single-family zones, there is a 17–49% reduction in non-potable water demand. For multi-family zones, the reduction is 6–32%. Electricity use decreases by 17–49% in single-family zones. In multi-family zones, it drops by 32–41%. Furthermore, a life cycle assessment indicates a 20–41% improvement compared to the conventional system. In the research [
17], the recovery of greywater is identified as one of the components of alternative water supply systems to achieve the goal of net zero water (NZW) use. The authors conclude that, despite a significant reduction in water consumption, hybrid solutions are associated with higher costs and energy consumption. On this basis, the complexity of achieving NZW in urban areas is emphasized. Lam et al. [
18], compares four water management scenarios, highlighting the decentralized anaerobic fluidized bed membrane bioreactor system for greywater recycling as the most eco-efficient option for non-potable uses in domestic buildings.
From the perspective of legislation and subsidy programs, the first sector where the requirement for wastewater recovery can be introduced is single-family housing. In a typical single-family house, the use of greywater recovery can be considered an optimal solution in the context of standard living needs. This results from the fact that the production of greywater approximately balances with its demand. For irrigation, collecting rainwater can be seen as a more favorable solution. Taking this into account, the current study focuses on the application of greywater recovery in the single-family housing sector. European countries are considered, due to the adoption of water supply and sewage market data from report [
19] as input variables.
Based on the literature review conducted, it is observed that GRSs are primarily analyzed for their economic viability. Here, the main novelty can be identified as the analysis from multiple perspectives:
The primary objective of this research is to evaluate the integration of GRS in single-family buildings across European countries, focusing on these four areas. Traditional analyses of GRS primarily concentrate on their economic viability. However, this study aims to bridge a significant research gap by providing a comprehensive analysis from multiple perspectives. This holistic approach allows the coupling between water resources, investment profitability, and the impact of energy consumption on CO2 emissions to be captured.
The critical research questions addressed in this study are:
▪ How does greywater recovery contribute to sustainable water management in single-family houses?
▪ What are the economic, environmental, and energy consumption implications of adopting GRS compared to conventional water use strategies?
By answering these questions, the study aims to offer valuable insights into the water–energy–carbon nexus, thereby contributing to the development of sustainable water management practices [
20].
The structure of this paper is organized as follows. After the introduction, the integration of greywater recycling into building design is described. Additionally, information regarding the modeling of water consumption in single-family houses, along with a financial analysis, energy consumption, and assessment of carbon dioxide emission, is discussed. Subsequently, the results are obtained and the discussion thereof is presented. Finally, the conclusions are listed.
2. Materials and Methods
This part explains the materials and methods of this study. First, in
Section 2.1, integrating greywater recycling into building design is discussed. Then, in
Section 2.2 and
Section 2.3, the details about the daily water consumption profile and financial analysis are given. In
Section 2.4, the way to determine energy consumption and carbon dioxide emissions is explained. Finally, in
Section 2.5, the limitations of the used input data are disclosed.
2.1. Integrating Greywater Recycling into Building Design
The classic solution for the building is to use a sewerage system that discharges mixed domestic wastewater, which includes both blackwater and greywater. However, this approach restricts the potential for reusing greywater. To enable the use of alternative water sources in the building, a dual plumbing system must be designed, with one system for blackwater (fecal sewage) and another for greywater. According to the standard EN 16941 [
21], greywater is defined as wastewater from the kitchen (sink, dishwasher), the bathroom (shower, bath, sink), and the washing machine. Light greywater specifically refers to wastewater from showers, bathtubs, hand wash basins and washers, excluding kitchen wastewater (sink, dishwasher) due to its higher organic and grease content. The options for the collection and use of light greywater in the analyzed solution are shown in
Figure 1.
The treated greywater can be used for non-potable purposes such as flushing toilets, washing clothes and watering the garden (see
Figure 1). This requires separate and unconnected water supply systems, one for drinking water and the other for treated greywater. The implementation of a greywater recovery and reuse system in a building is only feasible if the wastewater has been treated to meet the accepted quality requirements for bacteriological and physicochemical properties [
21]. Greywater recycling systems can vary in terms of treatment method and resulting water disinfection. Various methods are available for treating greywater, including physical, chemical, biological, or a combination of these methods. The solution described involves collecting grey sewage through an internal sewer and storing it in a tank equipped with an emergency overflow to direct excess liquid to the external sewer. Furthermore, it is important to ensure that the tank capacity is suitable for the yield of greywater. After undergoing the treatment process, the water is directed to a storage tank. The water may be further disinfected using methods such as ultraviolet rays or chemicals before it is used in the domestic water supply for washing or flushing toilets.
2.2. Daily Water Consumption Profile
The development of a water usage simulation for a single-family house utilizes a detailed Matlab script. It includes input variables such as the number of occupants, and the average daily water usage per person. It accounts for the weekly variations in water use, differentiating between weekdays and weekends, with the observation that Saturday marks the peak of water consumption. To add realism to the simulation, a variance of ±30% on the daily water consumption has been introduced to reflect daily fluctuations.
While the average European household size is around two people, this figure includes multi-family buildings [
19]. It is assumed that single-family houses, which are the focus of this study, typically have a higher average household size. For the purpose of simplification and to better reflect the actual situation in single-family homes, a household size of four people is considered. Based on data from the same report [
19], it is determined that the average water consumption is 125 L per person per day, which is adopted for further analysis. This standardization helps establish consistent input conditions for the comparative analysis, allowing for a clearer identification of trends and their justification.
In this simulation, categorizing water usage is crucial for two primary reasons: it enables the estimation of potential greywater production and helps in assessing the household’s greywater needs. The consumption categories analyzed include showering/bathing, laundry, handwashing, toilet flushing, dishwashing, and cooking [
22,
23], as these activities significantly contribute to the total water usage and thus the potential for greywater recycling. This can be written as:
Based on the comprehensive review paper [
24], the daily probability of using water draw-off points was adopted. Finally, the developed simulation model generates an annual profile of water consumption in a household on a daily step, detailing the draw-off in the adopted categories of water consumption.
The model for simulating the performance of a GRS operates on a daily balance principle, asserting that the sum of greywater utilized each day cannot exceed the total daily greywater generated and stored within the system (Equations (2) and (3)). If the demand for greywater exceeds the current supply, the system is supplemented with water from the municipal supply. The inclusion of the washing machine as both a source and a use of greywater aligns with the standards outlined in reference [
21].
2.3. Financial Analysis
The economic feasibility of GRSs is evaluated using life cycle cost (LCC), net present value (NPV), and payback period (PBP) analyses, taking into account a set of basic assumptions listed in
Table 1. What needs to be emphasized is that Variant 0 (V0) represents the conventional plumbing system in a single-family house, while Variant 1 (V1) integrates a GRS into the conventional setup. Therefore, the economic analysis only considers the difference in additional costs resulting from the implementation of the GRS. It is assumed that the cost of conventional installation in both V0 and V1 are approximately the same.
The conducted LCC analysis assess the financial performance of various installation options, incorporating initial capital costs (ICCs), utility costs (UCs), but excluding disposal costs (DCs) [
10,
26]. As ICCs, only the costs of installing a GRS in a new single-family house are assumed. The formula for LCC is given by:
The NPV analysis complements the LCC by evaluating the profitability of the GRS. It calculates the present value of all cash flows (CFs) associated with the project, both inflows and outflows, over its entire life. The formula for NPV is:
PBP is ascertained by determining the time needed to recover the cost of an investment. The PBP value in years is taken as the first year in the analyzed period when the NPV turns positive.
2.4. Determining Energy Consumption and Carbon Dioxide Emissions
The calculation of annual energy consumption for a single household (
Euse) is derived from the cumulative energy consumption factor for the extraction, treatment, and distribution of drinking water, and for the collection and treatment of wastewater (
EFcumulative), as reported in [
19]. The report separately provides data for water supply and wastewater collection. For further analysis, the sum of these values is adopted as
EFcumulative. Additionally, the annual water consumption by the household (
Vtotal) must be taken into account. The formula for this calculation is expressed as follows:
To estimate carbon dioxide emissions (
CDEs) based on the given energy consumption, it is essential to understand that, according to literature data, the majority of the energy required for water and wastewater services is electrical. For simplification, carbon dioxide emission factors (
CDEFs) for electricity generation are applied. This approach is justified by findings that emissions from electricity constitute the largest share of water-related energy emissions in 98% of the investigated drinking water utilities, as reported in [
27]. It is important to note, however, that wastewater treatment utilities consume more fuel for heating purposes (e.g., natural gas), but electricity still accounts for approximately 88% of their energy use [
27]. The formula for calculating
CDE is given by:
The
CDEFs for gross electricity production in European countries, which account for combustion emissions, fuel upstream emissions, and construction emissions, were adopted according to [
28].
2.5. Limitations of Used Input Data
The exclusion of some European countries is due to challenges highlighted in [
19], which serves as the input data for our comparative analysis and the applicability of GRS. Figures 3, 4, and 6 show that not all countries have complete data sets needed for the conducted analyses.
Firstly, the diversity of national figures poses significant challenges, as data collection methods and standards vary widely across countries, making uniform reporting difficult. Additionally, specific data are not collected in some countries due to differences in national regulations, priorities, or resource availability, leading to gaps in the dataset beyond the authors’ control.
For coherence in our comparative analysis, data from countries with comprehensive information in four analyzed areas—water consumption, water tariffs, energy consumption, and consequently, CDEs for countries with provided energy consumption data—are utilized.
4. Conclusions
A holistic approach to the analysis of GRS application in single-family buildings is aimed at establishing frameworks for decision-making processes. Importantly, and requiring emphasis, economic and environmental aspects are considered to be mutually complementary and significant. A comprehensive comparative analysis is provided which considers potable water conservation, economic feasibility, energy consumption, and environmental impact. The following findings can be listed from this investigation:
GRSs significantly reduce potable water demand, up to 43.0% per user, compared to traditional systems;
Positive NPV is observed in countries such as Belgium, Germany, Denmark, Finland, and Norway, according to the base scenario;
The implementation of subsidies can reduce the PBP, enhancing economic incentives for adopting GRSs;
Significant energy savings of up to 42.60% are observed with the use of GRSs;
Notable reductions in CDEs, with reductions directly proportional to the decreases in energy use;
The highest CDE reduction was observed in Poland, from 227.65 to 130.68 kgCO2/year.
Moreover, further studies are recommended to explore the scalability and applicability of GRSs in different types of residential and public utility buildings. Future research on the social aspects of GRS adoption, such as public perceptions, is also recommended. Additionally, optimizing greywater treatment technologies to improve efficiency and safety in different applications is proposed for future research.