Current Research Trends and Challenges Related to the Use of Greywater in Buildings

Abstract

This paper presents a review of current trends and challenges in greywater use in buildings, with particular emphasis on toilet = flushing applications. It discusses the quantitative and qualitative characteristics of greywater, including its generation sources, share in total domestic wastewater volume (50–89%), and flow variability depending on residents, building type, and user habits. Implementation of greywater recycling technologies faces several challenges, such as parameter variability, stringent sanitary and epidemiological standards, and the presence of micropollutants, including pharmaceuticals. Technological barriers include the integration of multi-stage treatment systems (physical, biological, and chemical) and ensuring effective disinfection for indoor use. The paper also highlights the lack of uniform international regulations and the significant variation in recovered water quality requirements. Key physicochemical and microbiological indicators that determine treatment system requirements are presented, with particular emphasis on the removal of organic pollutants and indicator bacteria. Various physical, chemical, and biological treatment technologies are described, with hybrid systems offering high efficiency and user safety. The implementation of greywater recycling systems encounters technical, regulatory, and social barriers. Social acceptance and transparent monitoring are identified as key challenges for widespread adoption. This critical literature review summarises current knowledge on effective greywater management in buildings, representing an increasingly important issue for sustainable water resource management.

1. Introduction

The reuse of greywater represents one of the major challenges in contemporary water management. In the context of the global water crisis and growing pressure to manage natural resources efficiently, it must be recognised that this issue has evolved beyond a purely scientific concern and has become a pressing necessity for implementing practical solutions. Although the secondary use of wastewater has long been known and practiced, particularly in regions facing drinking water shortages, the recovery and reuse of wastewater have, in recent years, been implemented on an increasingly wide scale.

Despite the absence of comprehensive legal regulations in this area, technical initiatives for greywater reuse have been undertaken for some time. Countries such as Canada, the USA, Japan, Australia, Jordan, Israel, and Oman, as well as several member states of the European Union, have been implementing innovative programmes and investments in greywater recovery systems for years [1]. Today, many nations regard greywater reuse technologies in new residential or public buildings as standard practice. Western European countries, in particular, have observed significant technological advancements in dual installation systems (sewage installations discharging greywater and black water through separate systems). The pioneers in this field include the Scandinavian countries and the nations of the Mediterranean basin, such as Spain [2,3].

The estimated costs of implementing greywater reuse systems in buildings vary considerably depending on the selected technology, system size, and local water and wastewater tariffs [4,5]. The economic efficiency of such systems increases with the number of users and the proportion of variable water costs, particularly in regions with high water prices or regulatory incentives for recycling. In multi-family residential and public utility buildings, where water and wastewater charges are typically higher, the average payback period ranges from 7 to 15 years [6].

With the growing interest in this topic, users’ concerns regarding the sanitary safety of treated greywater have prompted extensive research on its composition—including the presence of pollutants such as dyes, heavy metals and micropollutants—and on various treatment technologies [7,8]. In parallel with ongoing research and the implementation of technical solutions, efforts are also being made to develop proposals for legal regulations in this field.

In recent years, several review articles have been published on the reuse and treatment technologies of greywater in buildings [9,10,11,12], discussing in detail aspects such as biodegradability and biological treatment processes. These studies primarily focus on technological issues and the performance of specific treatment methods under laboratory conditions. The present paper extends this body of knowledge by providing a comprehensive analysis of current implementation challenges, as well as the legal and social aspects related to greywater recovery in buildings, with particular emphasis on its use for toilet flushing. It also addresses the issue of micropollutant presence and summarises the existing regulatory frameworks and social acceptance barriers that hinder large-scale implementation of these technologies, thereby broadening the perspective of previous studies. Furthermore, the paper outlines recommendations for future research directions in the field of greywater treatment, highlighting the need for clear regulatory provisions and harmonised guidelines that would enable comparison and selection of optimal technical solutions. Greywater reuse remains a pressing and evolving issue, whose importance will continue to grow systematically—particularly in the context of the challenges posed by the circular economy within both the construction sector and the water and wastewater management sector.

2. Quantitative and Qualitative Characteristics of Greywater

A detailed quantitative and qualitative analysis of greywater provides the basis for assessing its suitability for various applications. The values of physicochemical and microbiological indicators determine the selection of appropriate treatment processes and, consequently, the potential for greywater reuse in applications such as toilet flushing, irrigation of green areas, vehicle washing, and industrial processes. The largest volumes of greywater are typically generated in buildings with high occupancy rates—such as public facilities, office buildings, and hotels—where efficient recovery and reuse systems form an important component of water and wastewater management. However, the potential for implementing such solutions in residential buildings should not be overlooked. Even though smaller volumes of wastewater are produced in these settings, the use of local treatment systems can still significantly reduce potable water consumption. The following section presents a review of the literature on the composition, variability, and quality assessment of greywater from different sources.

2.1. Quantitative Characteristics of Greywater

Domestic wastewater can be categorised into black water (wastewater from toilets) and greywater (wastewater from all other household sources). Depending on its origin, greywater can be further classified as “light greywater”—generated from showers, baths, and washbasins—or “dark greywater”—produced from kitchen sinks and washing machines (Figure 1).

Greywater constitutes a significant proportion of the total volume of wastewater generated in households. Its share typically ranges between 50 and 89% of total domestic wastewater [10,14]. These values vary depending on the country, level of economic development, infrastructure, residents’ lifestyle, and water availability. The daily amount of greywater produced also shows substantial variability, primarily influenced by the country’s economic development. In highly developed countries (e.g., the USA, Australia, Israel), greywater generation ranges from 100 to over 200 litres per person per day. The highest reported values are observed in Australia (up to 201 L/p/d, 89%), the USA (up to 208 L/p/d, 80%), and Israel (100–150 L/p/d, 80%). High values have also been reported in some developing countries, such as Brazil and Kuwait. In Brazil, this is attributed to the specific type of building studied—an academic facility with more intensive use of sanitary installations compared to residential buildings. Similarly, in Kuwait, unique climatic conditions contribute to the elevated greywater generation rates. In general, however, developing countries (e.g., India, China, Ghana) are characterised by lower greywater production, typically ranging between 70 and 120 L/p/d. The lowest values are observed in China and Ghana (approximately 73–100 L/p/d, 42.9–70% of total wastewater), largely due to limited access to running water, differences in infrastructure, and hygiene practices. In some cases (e.g., India, Ghana), significant variation in greywater generation is directly related to water availability. For example, in Ghana, households with access to running water generate approximately 73 L/p/d, while those without such access produce only 32 L/p/d [15]. The amount of greywater produced is also influenced by factors such as the number of residents, type of sanitary equipment, and hygiene habits. Daily variation is notable, with the highest flows occurring during morning and evening hours, when household members most frequently use showers, washbasins, and washing machines [16]. The literature typically analyses wastewater from bathrooms (showers, baths, washbasins), laundries, and kitchens. In some studies, however, kitchen wastewater is excluded due to its high content of fats and organic substances [10,17,18].

Table 1. Data on the quantitative characteristics of greywater produced in selected countries.

Country (Reference)	Building Type	Greywater (GW)	Quantity by Source	Share in Domestic Wastewater	Domestic Wastewater (WW)
		[L/p/d]	[L/p/d]	[%]	[L/p/d]
Brazil [19]	university building	248.2	SH: 63.6 WB: 39.2 WM: 145.3	nd	nd
Netherlands [20]	households	82	SH: 38 BT: 9 WM: 25 WB: 4 KS: 6	70–75	nd
China–Pekin [21]	urban household	86	nd	nd	nd
India [17]	urban household	110	LGW + WM: 80 KS: 30	LGW: 48	165
Greece [18]	domestic households	82.6	SH/BT: 33.9 WM: 21.3 WB: 8.6 KS: 12.2 DW: 6.6	58	142
Jordan [22]	domestic households	50	nd	nd	nd
Vietnam [23]	domestic household	80–110	SH/BT: 30–60 KS: 15–20 WM: 15–30	nd	nd
Nepal [24]	domestic household	72	nd	nd	nd
Israel [16]	domestic household	100.4	KS: 26.6 WM: 16.6 WB: 18 BT: 22.4 SH: 16.8	73	138.1
Israel [25]	domestic household	90–120	nd	60–70	nd
Kuwait [26]	domestic households	200	WB: 79 SH: 43 KS: 76 WM: 2	84	238
Ghana [15]	domestic household (in-house water supply)	73.4	nd	74.2	82.51
Ghana [15]	domestic household (outside sources of water)	32.4	nd	88.6	36.64
Sweden [27]	domestic household area	66	nd	nd	nd
Switzerland [28]	domestic household	110	SH/BT: 52 WM: 30 KS: 28	nd	nd
Netherlands [11]	domestic household	60–100	nd	80	nd
UK [29]	university residential	102.9	BT/SH: 45.5 WB: 43.8 WM: 13.6	nd	nd
UK [29]	domestic household	84	BT/SH: 33.6 WB: 24 WM: 26.4	nd	nd

Notes: nd—no data; KS—kitchen sink, DW—dishwasher, WM—washing machine, BT—bath, SH—shower, WB—washbasin.

summarises data from various countries on greywater generation, taking into account the type of building and the sources of its origin (where specified).

The quantitative data presented in Table 1 are derived from both direct measurements (metre readings and flow measurements) and indirect estimates based on water consumption surveys. For example, Antonopoulou et al. [18] estimated greywater production in Greek households using a questionnaire, whereas Abusam et al. [26] monitored water consumption in a building over 9-month period, allowing for an accurate determination of wastewater generation—distinguished by individual sources. It should be emphasised that results obtained from direct measurements in buildings generally provide more reliable data than those derived from surveys and may provide a better basis for planning greywater recycling systems. Across all studies, greywater constitutes a significant proportion of total domestic wastewater. In most cases, its share exceeds 50%, confirming its considerable potential for reuse. In developed countries, greywater from bathrooms and laundry rooms typically accounts for 65–75% of the total, while in countries with lower economic status, a larger contribution originates from kitchens and washbasins. The data presented in Table 1 also indicate that the type of building (single-family house, multi-family dwelling, dormitory) affects the quantity of greywater generated. Nevertheless, when lifestyle and sanitary equipment are taken into account (l/person/day) are comparable across different regions.

It is important to note that one of the major challenges in the quantitative characterisation of greywater for reuse system design is in the discontinuity of greywater generation. This issue is particularly relevant for small-scale systems (e.g., single-family dwellings), where substantial temporal variability occurs on both daily and weekly scales, closely linked to user behaviour patterns. In many cases, it is not feasible to generate a sufficient volume of greywater at a given moment to meet short-term demand—such as during periods of increased toilet flushing activity. To address this issue, storage tanks are required to accumulate greywater during periods of surplus production, ensuring its availability when generation is reduced or halted. The sizing of the storage tank must account for the maximum and minimum production and consumption cycles specific to the building. An undersized tank may fail to supply the required volume during peak demand, whereas an oversized tank can lead to stagnation and subsequent deterioration of water quality, including microbial growth and unpleasant odours. Moreover, it is essential to incorporate a mechanism for switching the supply source to potable water during greywater shortages. This requires continuous monitoring of greywater levels and automated flow control systems to maintain operational reliability and ensure uninterrupted water supply.

The issue of optimising small-scale greywater systems was addressed by Rodriguez et al. [30], who identified key challenges in the effective design of greywater storage tanks, as well as opportunities to optimise both the costs efficiency and environmental performance. According to the authors, modelling results are highly sensitive to assumptions regarding flow characteristics and variability, which considerably limits the reproducibility of findings and the practical applicability of the proposed solutions.

2.2. Qualitative Characteristics of Greywater

In the literature, the quality of greywater is characterised using the same indicators commonly applied to domestic wastewater. The primary physicochemical parameters include pH, turbidity, suspended solids content, BOD, COD, total nitrogen, and total phosphorus. In some studies, additional parameters such as electrical conductivity, total organic carbon, oil and fat content, chlorides, potassium, calcium, magnesium, nitrates and sulphates have also been analysed [15]. Eriksson et al. [31] identified over 900 organic compounds potentially present in greywater, based on data concerning the composition of household products. Many of these substances are considered priority pollutants due to their potential environmental and health risks. These include surfactants, solvents, and fragrance compounds. Examples of compounds with the highest potential environmental threat include benzalkonium chloride, bis-(2-ethylhexyl) phthalate/DEHP, bronopol. Recent studies have further demonstrated that greywater often contains a variety of micropollutants, such as nonylphenol, triclosan, bisphenol A, and caffeine [32].

These compounds enter greywater primarily through the use of personal care products, cleaning agents, and cosmetics that are rinsed off during washing and bathing activities. Nonylphenol is released mainly through the use of detergents and cleaning products during laundry and dishwashing. In the European Union, this compound is classified as a priority hazardous substance, with a maximum permissible concentration of 0.3 μg/L in freshwaters. Triclosan, as an antibacterial and antifungal agent, is a major ingredient in personal hygiene products such as antibacterial soaps, shower gels, shampoos, toothpastes, and mouthwash solutions. Bisphenol A (BPA) may enter greywater through washing plastic containers and utensils containing this compound [33]. Caffeine is also a common contaminant present in greywater. The kitchen wastewater stream constitutes its dominant source, primarily due to the rinsing of espresso machines, cups, and dishes contaminated with caffeine-containing beverages. Secondary sources include cosmetic products containing caffeine (anti-cellulite creams, shampoos) washed off during bathing, as well as coffee-stained clothing during laundry. Turull et al. [34] reported the following caffeine concentrations in individual greywater sources: kitchen > 40,000 ng/L, laundry 2360 ± 241 ng/L, and the lowest concentrations in shower water (670 ± 12 ng/L).

The presence of the aforementioned compounds in greywater reused for toilet flushing poses potential risks due to skin contact and inhalation of aerosols, particularly in children. Bisphenol A can readily penetrate the skin and mucous membranes, making its presence in such applications especially concerning. Triclosan, when it comes in contact with chlorine (commonly used as a final disinfection agent), may lead to the formation of chloroform, a carcinogenic compound. In the context of greywater reuse for crop irrigation, the release of these contaminants into the environment—including soil, surface waters, and groundwaters—can result in their persistent accumulation. This persistence not only poses threats to ecosystems but also contributed to the spread of antibiotic resistance among microbial communities [33,35,36].

An increasingly detected contaminant in greywater that requires systematic monitoring and in-depth analysis of both its sources and potential environmental impacts is also microplastic. Recent research demonstrates that the dominant source of microplastics in greywater is fibres from synthetic textiles such as polyester, polyamide, and acrylic, which are released during laundry washing. De Falco et al. [37] reported that a single washing cycle can release between 640,000 and 1.5 million microfibers (equivalent to 124–308 mg/kg of fabric), depending on the type of garment. A second significant source of microplastics arises from cosmetic and personal care products containing microbeads of polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC), which enter greywater during washing and bathing. Additionally, wastewater from washing machines and dishwashers may contain polyvinyl alcohol (PVA) derived from laundry and dish detergent pods. Kitchen sink wastewater can also introduce microplastics through the washing of plastic packaging materials.

The presence of microplastics in greywater highlights the complexity of its composition, which also includes microbiological contaminants that must be carefully monitored to ensure both public health and environmental safety.

Microbiological indicators used to characterise greywater typically include the total coliform bacteria count, which serves as a marker of potential faecal contamination, and E. coli, which acts as a more specific indicator. Gonçalves et al. [38] also included Rotavirus, Cryptosporidium spp., and Campylobacter spp. in their microbiological risk assessment. The concentrations of these pathogens were estimated based on E. coli data and established pathogen/E. coli ratio indicators (10⁻⁶–10⁻⁵ for Rotavirus and Campylobacter spp.; 10⁻⁷–10⁻⁶ for Cryptosporidium spp.). The authors highlighted important limitations of using E. coli as a sole indicator for risk assessment, especially in small greywater recycling systems. This concern arises because the methodology assumes fixed ratios between E. coli and pathogens, which may not accurately reflect real conditions. For instance, when all users are healthy, E. coli concentrations may be high while pathogens are absent; however, infection in even a single user can dramatically change the pathogen/E. coli ratio. Jefferson et al. [39] attempted to identify other pathogenic microorganisms, such as Campylobacter spp., Cryptosporidium spp., Salmonella spp., Shigella, or Entamoeba, but none were detected in any sample. Detected microorganisms such as Candida albicans, Pseudomonas aeruginosa, and Staphylococcus aureus are normal human microbiota, and their presence in greywater is therefore considered natural. Other studies also investigated microorganisms commonly occurring in humans but similarly failed to detect pathogens [40,41]. Although greywater is not directly contaminated by faecal matter from toilets, microbiological contamination occurs through routine hygiene-related activities such as bathing, washing, and laundry.

Table 2. Characteristic of greywater by source and country (physicochemical indicators).

Country [Reference]	Source	pH	Turbidity	TSS	COD	BOD	TN	NH4+-N	TP
		[-]	[NTU]	[mg/L]	[mgO2/L]	[mgO2/L]	[mgN/L]	[mg/L]	[mgP/L]
Germany [1]	SH	nd	nd	nd	113–633	70–300	nd	nd	nd
	BT + SH	nd	nd	nd	100–200	50–100	nd	nd	nd
Denmark [31]	BT+ SH+ WB	6.4–8.1	60–240	40–200	113–633	76–200	5–17	<0.1–15	0.11–2
	WM	8.1–10	nd	120–280	725–1815	48–290	1–21	<0.1–1.9	0.06–57
	KS	6.3–7.4	nd	235–720	26–1380	5–1460	0.31–74	0.2–23	0.06–74
England [39]	SH	nd	84.8	89	420	nd	nd	nd	nd
	BT	nd	59.8	58	367	nd	nd	nd	nd
	mixed	nd	164	100	451	146	10.4	nd	0.35
Germany, Denmark, Sweden, Spain, Japan China, USA, Canada, Australia, Oman, Israel [12]	SH/BT	6.4–8.1	44–375	7–505	100–633	50–300	3.6–19.4	nd	0.11–>48.8
	WM	7.1–10	50–444	68–465	231–2950	48–472	1.1–40.3	nd	0–>171
	KS	5.9–7.4	298	134–1300	26–2050	536–1460	11.4–74	nd	2.9–>74
	mixed	6.3–8.1	29–375	25–183	100–700	47–466	1.7–34.3	nd	0.11–22.8
Brazil [19]	SH	7.0–8.2	100	155.8	272.8	123.1	nd	50.3	5.3
	WB		48.7	18.5	208.1	101.1	nd	5.1	3.3
	WM		33.5	32.7	274.1	77	nd	4.3	2.3
Greece [18]	SH/BT	7.22	nd	63	399	nd	nd	8.4	0.4 *
	WB	7.07	nd	61	335	nd	nd	2.6	0.7 *
	KS	6.72	nd	299	775	nd	nd	4.0	0.4 *
Ghana [15]	mixed	7.0	nd	297–538	644–758	204–253	nd	14.2–14.8	2.3
Poland [42]	SH + WB	6.9–8.19	10–362	13–110	11.2–329	nd	10.1–10.6	nd	<0.5–0.814
Australia, China, India, Israel, France, Netherlands, Greece, Spain, Brazil, Egypt, Malaysia, South Africa [14]	BT/SH + WB	6.0–7.7	68–81	57.5–88	324–445	130–349	5.3–23	0.022–0.24	3–18
	KS	5.94–8.74	54.4–245	308–3589	384–2074.5	200–604.5	28.05	4.7–6.89	5.3–15.7
	WM	7.1–9.95	45–162	60–95	466–1353	290	33	<0.28	1.3
	mixed	6.5–8.63	4–173	16–122.9	74–668	117	10.4	1.9–29.7	0.89–9.4

Notes: nd—no data, KS—kitchen sink, WM—washing machine, BT—bath, SH—shower, WB—washbasin. * mg PO₄³⁻/L.

and

Table 3. Characteristics of greywater by source and country (microbiological indicators).

Country [Reference]	Source	Total Coliforms	E. coli	Other Indicators
		[CFU/100 mL]	[CFU/100 mL]	[CFU/100 mL]
Germany [1]	SH	10−1–103	10−1–101 CFU/mL	Total counts: 107–108
	BT + SH	102–103	10−1–101 CFU/mL	Total counts: 107–108
Denmark [31]	BT + SH + WB	70–2.4 × 107	1–3.3 × 103	Total counts: 107–3 × 108 Enterococcus: 1–7 × 104
	WM	85–8.9 × 105	9–16 × 103	Total counts: 107–3 × 108 Enterococcus: 1–1.3 × 106
	KS	nd	0.13–250 × 106	Enterococcus: not detected
England [39]	SH	6800	2050	nd
	BT	6350	40.1	nd
	mixed	7387	1740	nd
Germany, Denmark, Sweden, Spain, Japan China, USA, Canada, Australia, Oman, Israel [12]	SH/BT	10–2.4 × 107	0–3.4 × 105	nd
	WM	200.5–7 × 105	50–1.4 × 103	nd
	KS	>2.4 × 108	nd	nd
	mixed	56–8.03 × 107	0.1–1.5 × 108	nd
Brazil [19]	SH	4.0 × 105	5.06 × 104	nd
	WB	7.6 × 106	2.08 × 104	nd
	WM	50	0	nd
Ghana [15]	mixed	3.7–3.8 × 106	1.8–2.4 × 104	Salmonella spp.: 3.1–2.4 × 103
Poland [42]	SH + WB	0.1–3.2 × 101 CFU/mL	nd	Total counts: 76–>3 × 105 Enterococcus: 20–3.2 × 103
Australia, China, India, Israel, France, Netherlands, Greece, Spain, Brazil, Egypt, Malaysia, South Africa [14]	BT/SH + WB	1.1 × 108	4.5 × 105–6.1 × 105	nd
	KS	nd	nd	nd
	WM	4.8 × 105	3.6 × 105	nd
	mixed	2.2 × 105–1 × 106	1 × 105–6 × 105	nd

Notes: nd—no data; KS—kitchen sink, WM—washing machine, BT—bath, SH—shower, WB—washbasin.

summarise data from various countries on the qualitative characteristics of greywater, taking into account its different sources.

Greywater is characterised by high variability in quality, depending on both its source and geographical location. Different household sources—such as showers, washbasins, kitchen sinks, and washing machines—contribute unequally to the overall pollutant load [18]. The concentration of pollutants in greywater generated from kitchens and washing machines is significantly higher than that in that bathroom-generated greywater (SH, BT, WB), particularly in terms of COD, TSS, and phosphates. Given these differences, it may be advisable to separate greywater streams, for example, by excluding kitchen wastewater from recycling systems. The typical pH of greywater ranges between 6.7 and 8.2, a level that is generally favourable for biological treatment processes. The data presented in Table 2 also indicate that an important consideration in selecting appropriate greywater treatment systems is the removal of turbidity and TSS, especially when their values exceed 100 NTU and 300 mg/L TSS, respectively. The organic matter content in greywater is also notable, with COD and BOD values fluctuating across very wide ranges—26–2950 mgO₂/L COD and 5–1460 mgO₂/L BOD, respectively. Such substantial variability in wastewater characteristics is a key factor influencing the design and operation of wastewater treatment systems.

The very high pollutant concentrations reported by Oteng-Peprah et al. [15]—with values reaching up to 758 mg/L COD, 253 mg/L BOD, and 538 mg/L TSS—are considerably higher than those typical of greywater from developed countries and those recommended by local and international environmental standards. These elevated concentrations primarily reflect differences in residents’ lifestyles and the state of water supply infrastructure. In particular, high pollutant concentrations are directly related to limited access to running water.

The high concentrations of total phosphorus (TP, up to 74 mg P/L) are primarily attributed to the widespread use of phosphate-based detergents in Danish households during the study period. The authors emphasise that in countries where formal restrictions on the phosphate content of detergents have not been implemented, TP values in greywater—particularly from laundry and kitchen sources—can range from 6 to 23 mg TP/L, and may locally reach even higher values [31]. These levels therefore reflect local practices, detergent composition, and individual user habits.

A review of the available literature shows that investigations of microbiological indicators are considerably less common than those addressing physicochemical characteristics of greywater (Table 3).

Available research results indicate the presence of coliform bacteria, including faecal types, in greywater regardless of its source. High concentrations of both coliform bacteria and E. coli bacteria have been observed particularly in greywater from showers and washbasins [19].

Although greywater is primarily used for purposes such as toilet flushing and/or watering plants, it must undergo treatment not only for sanitary safety reasons but also due to its potential, yet unexplored impact on sanitary facilities and living organisms, as well as for aesthetic reasons.

3. Greywater Treatment Technologies

Recent research on greywater treatment has focused on the selection, evaluation, and optimisation of various treatment technologies, with the goal of enhancing efficiency and ensuring sanitary safety. These studies aim to identify and minimise risks associated with the potential presence of pathogens and hazardous substances. A wide range of physical, chemical, and biological methods [15] are employed in integrated systems, ensuring high contaminant removal efficiency and sanitary safety [43,44,45]. The classification of applied technologies is presented in Figure 2. The first stage of treatment typically involves physical processes such as sedimentation and filtration, which enable the removal of suspended solids from greywater. More advanced physical technologies employ membrane processes (micro-, ultrafiltration, nanofiltration, or reverse osmosis) to effectively separate fine particles, microorganisms, and dissolved contaminants. Additionally, granular activated carbon (GAC) is often used to adsorb complex organic compounds and improve the organoleptic parameters of the treated water. The second group of technologies comprises chemical processes, including both traditional coagulation for the removal of colloids, as well as innovative methods such as electrocoagulation (EC) and advanced oxidation processes (AOPs) based on ozonation or photocatalysis (TiO₂/UV), along with disinfection using UV or chlorination. Disinfection usually constitutes the final stage of treatment and is essential for pathogen removal and ensuring user safety. The most commonly applied methods are chlorination with sodium hypochlorite (NaOCl) and ultraviolet (UV) irradiation. Sodium hypochlorite is widely used due to its high effectiveness against bacteria and ease of application, although residual chlorine levels and disinfection by-products must be carefully monitored when selecting this method. UV disinfection, on the other hand, does not generate by-products and is also extensively applied in practice. Numerous studies have reported that greywater exhibits high biodegradability [12,14,43]. Biological technologies primarily utilise aerobic reactors, such as sequencing batch reactors (SBR), membrane bioreactors (MBR), and moving bed biofilm reactors (MBBR), as well as anaerobic reactors (UASB).

The choice of greywater treatment technology and the required treatment level should be adapted to local conditions and the intended reuse application (“fit for purpose”). Greywater intended for irrigation or watering green areas can be treated using simple systems based on filtration, sedimentation, or flotation processes. In contrast, indoor applications, such as toilet flushing, require more advanced treatment systems that combine multiple processes. In commercial buildings, treated greywater may also be utilised in cooling systems or fire protection installations, where the primary consideration in system design and operation must always be sanitary and epidemiological safety [46].

There is growing interest in Nature-Based Solutions (NBS) for greywater treatment, including constructed wetlands (CWs), green roofs (GRs) and walls, as well as solar systems that utilise solar energy to power AOP, distillation, or photodisinfection processes, thereby supporting the principles of the circular economy. Boano et al. [44] documented the effectiveness and applicability of such treatment approaches, but emphasised the need to supplement these systems with an additional disinfection stage (most often UV), especially for the most restrictive applications (e.g., watering food crops). Integrated wastewater treatment systems, combining of the mentioned technologies, offer the potential for maximising greywater reuse in both high-consumption facilities and in residential buildings.

Another aspect increasingly highlighted in recent scientific literature is the need to implement standards for real-time monitoring of treatment efficiency (e.g., through sensors for key quality parameters) in order to guarantee sanitary safety [46].

Table 4. Efficiency of wastewater treatment depending on the technology applied.

Treatment Type	Scale	Building Type	Greywater Source	Removal Efficiency (%)										Final Effluent Quality	Country [References]
				TSS	TOC	COD	BOD5	Turbidity	TN	TP	Total Coliforms	E. coli	Enterococcus
SBR	lab	single-family house	SH + WB + WM + DW	-	91	92	-	-	-	-	-	-	-	7 mg/L TOC; 44 mg/L COD 4 NTU >100 CFU/100 mL E. coli 3 CFU/100 mL Enterokoki	Poland [45]
ultrafiltration				-	>95	>95	99	84	-	-	-	100	100	0.47–2.19 mg/L TOC; 5.8–18.1 mg/L COD <2 mg/L BOD5 <1 NTU 0 CFU E. coli/100 mL 0 CFU Enterokoki/100 mL
MBBR	pilot	university building	SH + WB + WM	87	-	70	59	66	-	-	-	100	-	11.3 mg/L TSS 73.9 mg/L COD 13.7 NTU 18.2 mg/L BOD	Brazil [19]
SBR	lab	housing estate	SH + WB + KS	-	-	90	-	-	-	-	-	-	-	82 mg/L COD	Netherlands [47]
sand + GAC filters	lab	university building	SH + WB + KS + WM	-	-	>96	>95	97–100	-	49.1	-	-	-	0–2.7 NTU 1–2.3 mg/L BOD 2.3–5.7 mg/L COD 42–163 CFU/100 mL Faecal coliforms 3300–78,000 CFU/100 mL Total coliforms	Ethiopia [48]
RBC	on-site	hotel	SH + BT + WB + WM	-	-	-	-	-	-	-	-	>99	-		Germany [1]
MBBR + filtration + UV	on-site	university building	SH + BT + WB + WM	94	-	-	95.6	95.9	-	-	100	100	-	2.0 mg/L TSS 3.0 mg/L BOD 2.0 NTU	UK [29]
SMBR	on-site	single house	SH + WB + WM	92	-	87	-	97	40	69	100	100	-	8 mg/L TSS 59 mg/L COD 5.0 NTU 20 mg/L TN 69 mg/L TP	Greece [8]
filtration + sedimentation + NaOCl	on-site	hotel	BT + WB	58	31	54	-	18	38	-	-	100	-	18.6 mg/L TSS 39.9 mg/L TOC 16.5 NTU 78 mg/L COD 7.1 mg/L TN	Mallorca (Spain) [3]
coagulation + filtration + GAC	lab	single houses	SH + WB + KS	97	-	96	-	-	-	-	-	-	-	11 mg/L TSS 28 mg/L COD	Greece [18]

presents sample data on the efficiency of greywater treatment depending on the technology applied.

Greywater treatment technologies differ considerably in terms of both their complexity and treatment efficiency. However, the use of different performance indicators to evaluate system effectiveness often makes it difficult to compare results and complicates the critical assessment and selection of the optimal treatment solution. The most commonly analysed physicochemical parameters include TSS, COD, BOD, and turbidity. In terms of microbiological indicators, researchers most frequently assess the removal efficiency of coliform bacteria and E. coli bacteria from wastewater.

The highest treatment efficiency and sanitary safety are achieved in hybrid systems, which allow for consistent compliance with restrictive international microbiological and physicochemical standards, even under variable operating conditions and with minimal user intervention [43]. A groundbreaking study by Eriksson et al. [31] was the first to document the presence of hundreds of xenobiotic organic compounds (XOCs) potentially occurring in grey wastewater. Subsequent research has consistently confirmed the presence of a variety of micropollutants, ranging from surfactants and personal care products (PPCPs) to UV filters and preservatives, as well as biocides and fragrances [49,50,51]. The most recent study by García-Gomez et al. [52], employing advanced analytical techniques, identified a total of 97 compounds (e.g., caffeine up to 2678 µg/L, piperine 268–426 µg/L, cotinine 45–91 µg/L, tapentadol up to 55 µg/L, telmisartan around 15 µg/L, tributyl phosphate 37 µg/L, lauryldiethanolamide 1.6 µg/L, PFOA 3 ng/L) in greywater discharged from vessels (ships). These findings demonstrate that the presence of micropollutants in greywater is a global issue, extending across different building types.

Research on the removal of these compounds from greywater has been conducted by several authors, including Hernández-Leal et al. [49], who confirmed the high effectiveness of ozonation and adsorption on activated carbon in removing micropollutants from biologically pre-treated greywater. In aerobically treated greywater, 79–94% of micropollutants were removed. Solar photocatalysis demonstrated the highest efficiency in removing benzophenone-3 (BP-3), achieving 98.5% removal using NP-TiO₂ on gravel. This technology also allowed for 75% removal of surfactants and 93.7% of total organic carbon (TOC) [8]. Najmi et al. [53] employed an SMBR system for removal of PCPs from greywater, achieving high efficiency, including: triclosan (98.2%), methylparaben (99.96%) and propylparaben (99.97%). The membrane bioreactor (MBR) proved to be very effective in removing various micropollutants, achieving 98.20% removal of triclosan, 99.96% of methylparaben, and 99.97% of propylparaben [14]. MBR is also a promising technology for microplastic removal, with reported efficiencies of up to 90% [54].

Despite the availability of technological solutions capable of achieving high-efficiency greywater treatment, the large-scale implementation remains a challenge. In addition to social and regulatory constraints, high implementation and operation costs should also be mentioned [55].

4. Legal Regulations—Rules and Guidelines

In recent years, increasingly precise regulations and standards governing greywater recovery systems have been developed and implemented across Europe, the US, and Australia. Examples of such regulations are presented in Table 5.

At the EU level, there are currently no pan-European regulations directly governing the reuse of greywater in buildings. The existing legal framework relies primarily on general safety principles, minimum quality requirements (mainly related to agricultural reuse), and recommendations encouraging Member States to establish more specific standards for indoor applications [56].

It is worth mentioning Directive (EU) 2024/3019 [57], which establishes requirements for wastewater treatment and disposal, mainly aimed at the protection of surface waters. However, this directive does not specify standards for greywater reuse within buildings. The directive focuses on the removal of biogenic compounds and micropollutants from municipal wastewater, and introduces monitoring obligations for emerging contaminant groups (e.g., pharmaceuticals, microplastics).

Also relevant is Regulation (EU) 2020/741 [58] of the European Parliament and of the Council on minimum requirements for water reuse. This document mainly addresses the reuse of municipal wastewater for agricultural irrigation, but it also defines general principles regarding minimum water quality standards, risk management, and public health protection, referring to the principles of “fit for purpose” (treatment adequate for the intended use).

In regulatory documents, industry guidelines, and technical manuals, the microbiological parameters (e.g., coliform bacteria or E. coli) for treated greywater often refer to the values specified in Directive 2006/7/EC [59]. However, this directive does not directly regulate greywater recycling, rather it establishes reference values for health safety in relation to recreational waters. The document nonetheless recommends UV disinfection as the final treatment step to ensure hygienic safety and maintain a high standard of water quality for human consumption.

It is also worth noting that greywater recycling systems are evaluated within global building certification frameworks such as LEED and BREEAM, which require compliance with specific technical and sanitary standards.

In practice, several countries in Western Europe, Australia and the USA, as well as the United Kingdom and France, have developed their own regulations or guidelines governing the reuse of greywater in buildings.

Within the European Union, the most widely applied reference document addressing quality and bacteriological requirements for greywater reuse is the series of British standards, which also provides guidelines for the design, installation, and operation of the system and its labelling, depending on the intended use of the treated water [60,61]. These standards specify recommended quality parameters for treated greywater intended for various applications, including toilet flushing, supply of automatic washing machines, and watering gardens.

Beginning in 2025, the recycling of greywater has become mandatory in new residential buildings, hotels, office buildings, and major renovations across Barcelona and several other cities in Catalonia [62], Spain. The relevant regulation requires the installation of systems that recover water from showers and bathtubs for use in toilet flushing and the irrigation of shared green areas. Investors are obliged to provide separate plumbing installations and ensure an appropriate level of water treatment. The implementation of greywater recycling in buildings is driven not only by local regulations but also by environmental certification systems (BREEAM, LEED) [63,64].

In individual European countries, standards and regulations concerning water quality and wastewater management are typically regulated by national environmental and public health authorities. In addition, local, regional, or municipal institutions may develop their own guidelines and programmes related to water reuse and wastewater management. Countries such as Germany (TrinkwV, 2023)—through the Federal Water Act (WHG, 2021), or France (2024 Decree on the reuse of greywater and rainwater), enforce administrative notification prior to the commissioning of greywater systems [65,66,67]. These regulations also mandate a system for monitoring, separation of installations, and technical labelling. Another example is the guidelines of the Association for Rainwater Harvesting and Water Use [68].

International recommendations (WHO, EPHC, Italy, Germany, Slovenia) emphasise the importance of achieving complete elimination of indicator bacteria, maintaining low turbidity levels (<2–10 NTU), and ensuring very low concentrations of total suspended solids [43].

Table 5. Greywater reuse guidelines for toilet flushing in different countries.

Indicator	USA (California) 1	USA 2	Australia 3	Canada 4	UK 5	China 6	Japan 7	Spain 8
pH	6.0–9.0	6.0–9.0	6.0–9.0	6.0–9.0	5.0–9.5	6.5–9.5	5.8–8.6
Turbidity [NTU]	<5	≤2	-	≤2	<10	<2	≤2	≤2
TSS [mg/L]	≤10	≤10	≤10	≤10	-	≤10	≤10	≤10
BOD5 [mg/L]	≤10	≤10	≤10	≤10	-	≤10	≤10	≤10
Cl2 residual [mg/L]	-	≥0.5	-	≥0.5	<2.0	0.3–1.0	trace amount	-
E. coli [CFU/100 mL]	≤14	≤2.2–14	≤1	ND	250	≤3	ND	≤10
Total coliforms [CFU/100 mL]	-	-	-	ND	1000	≤100	≤3	-
Enterococcus [CFU/100 mL]	-	-	-	-	100	-	-	-
Other	-	-	-	-	visually clear, not objectionable contamination	odour not unpleasant	odour not unpleasant, not objectionable contamination	odour not unpleasant, not objectionable contamination

Notes: ND—not detected. ¹ NSF/ANSI Standard 350 [69]. ² Water Reuse Guidelines 2012 (US EPA 2012) [70]. ³ AS 1546.4:2016 [71]. ⁴ CSA B128.1-06:2006 [72]. ⁵ British Standards BS 8525-2:2011 [61]. ⁶ GB/T 50336-2008 [73]. ⁷ Guidelines for the Reuse of Treated Wastewater (MLIT, 2007) [74]. ⁸ UNE-EN 16941-2:2021 [63].

Table 5. Greywater reuse guidelines for toilet flushing in different countries.

Indicator	USA (California) 1	USA 2	Australia 3	Canada 4	UK 5	China 6	Japan 7	Spain 8
pH	6.0–9.0	6.0–9.0	6.0–9.0	6.0–9.0	5.0–9.5	6.5–9.5	5.8–8.6
Turbidity [NTU]	<5	≤2	-	≤2	<10	<2	≤2	≤2
TSS [mg/L]	≤10	≤10	≤10	≤10	-	≤10	≤10	≤10
BOD5 [mg/L]	≤10	≤10	≤10	≤10	-	≤10	≤10	≤10
Cl2 residual [mg/L]	-	≥0.5	-	≥0.5	<2.0	0.3–1.0	trace amount	-
E. coli [CFU/100 mL]	≤14	≤2.2–14	≤1	ND	250	≤3	ND	≤10
Total coliforms [CFU/100 mL]	-	-	-	ND	1000	≤100	≤3	-
Enterococcus [CFU/100 mL]	-	-	-	-	100	-	-	-
Other	-	-	-	-	visually clear, not objectionable contamination	odour not unpleasant	odour not unpleasant, not objectionable contamination	odour not unpleasant, not objectionable contamination

Notes: ND—not detected. ¹ NSF/ANSI Standard 350 [69]. ² Water Reuse Guidelines 2012 (US EPA 2012) [70]. ³ AS 1546.4:2016 [71]. ⁴ CSA B128.1-06:2006 [72]. ⁵ British Standards BS 8525-2:2011 [61]. ⁶ GB/T 50336-2008 [73]. ⁷ Guidelines for the Reuse of Treated Wastewater (MLIT, 2007) [74]. ⁸ UNE-EN 16941-2:2021 [63].

It should be noted that the solutions proposed in various studies are often specific to individual countries and may differ due to national requirements for installation materials or the habits of sanitary installation users, such as the type of chemicals used, as well as the quantity, quality, and temperature of generated wastewater. Inadequately treated greywater may be rich in sodium, potassium, and detergents, which can contribute to corrosion and accelerated deterioration of plumbing fittings. Among non-European countries, the USA was the first to introduce legal regulations on water reuse, with the earliest enacted in California in 1918. The current regulations include an extensive system of codes and guidelines at the state and local levels, focusing on public health protection and installation safety [75]. The minimum quality requirements (e.g., E. coli limits) that national systems must meet are set out in NSF/ANSI Standard 350 [69]. In Australia, a national certification system governs water and sewage products and appliances used for greywater recycling [76]. Additionally, national codes establish detailed design, quality, and operational requirements for greywater treatment and reuse systems in residential buildings, allowing the reuse of greywater for toilet flushing, provided that strict criteria are met, e.g., 10 mgO₂/L BOD, no detectable faecal bacteria [71,77]. At the international level, the World Health Organisation has published a comprehensive set of guidelines and procedures for the safe reuse of greywater [78]. Similarly, in 2010, Health Canada issued guidelines for household-recovered water for toilet and urinal flushing, complementing the country’s applicable standard [72]. As interest in sustainable water management continues to grow, it is expected that greywater reuse regulations will evolve and expand. The development of an appropriate legal framework is not only justified by increasing sanitary requirements and the need to reduce drinking water consumption, but also essential for the effective implementation of water reuse systems.

5. Social Aspects of Water Reuse in Buildings

Another important aspect is the growing public awareness of water management, sustainable development, and environmental protection, which increasingly promotes acceptance of greywater reuse systems. Recent research has focused on assessing the level of public acceptance and awareness among residents regarding greywater reuse practices in buildings.

In the literature, several key social and psychological barriers have been identified that hinder the development and public acceptance of greywater reuse systems [55,79,80]. The most frequently cites concerns relate to sanitary safety, including fears of pathogen, faecal bacteria, and chemical contamination. These apprehensions are often linked to a lack of knowledge about the effectiveness of available treatment and disinfection technologies,, as well as a low level of trust in system reliability and monitoring practices, especially in the case of individual solutions. Other significant barriers include limited knowledge and awareness of the environmental and economic benefits of greywater reuse, and the growing need to adopt alternative water sources in response to increasing water scarcity. Aesthetic and functional concerns—such as fears of unwanted odours, discolouration, or residues of detergents and foam—also reduce user acceptance. Moreover, unclear or absent regulations defining minimum quality standards for treated greywater further increase mistrust and reluctance to invest in or use new solutions. Despite these challenges, acceptance levels area gradually growing in less developed countries, and in regions experiencing water shortages or temporary restrictions on water use. March et al. [3] analysed user perceptions and acceptance of a greywater recycling system used for toilet flushing, finding that clear communication about system functionality, technical performance, and environmental benefits significantly improved public acceptance. Reported concerns were limited to organoleptic parameters (odour, increased turbidity). The authors recommend that information programmes be supported by local authorities or public institutions to foster confidence and acceptance. Portman et al. [81] found that public greywater applications were more widely accepted than domestic uses, And that toilet flushing was the most accepted reuse option, while laundry applications elicited the greatest resistance. Akpan et al. [82] reached similar conclusions, noting that applications with “high” user contact, such as laundry and dishwashing, were significantly less accepted by residents (in the academic environment). Marks and Zadoroznyj [83] reported very high (over 94% of respondents) general acceptance and willingness of residents to use greywater treatment systems, yet with a clear preference for applications that do not require direct user contact. Conversely, Portman et al. [81] also observed that higher economic status and level of education of users translated into lower willingness to use treated greywater domestically, possibly due to higher expectations of wealthier social groups regarding water quality, less pressure to implement additional systems, or water costs that are low enough that the financial savings resulting from the use of recovery systems are not a motivation. However, Alhumoud and Madzikanda [84] reported the opposite trend, showing that higher social status and level of education of users correlated with greater openness to wastewater reuse.

Based on available literature, it is evident that the key aspect of public concern is the need to ensure sanitary safety.

Nonetheless, economic conditions remain a fundamental factor influencing the willingness to implement such systems. The investment costs related to installation and subsequent operation can represent a key determinant for both building managers and individual investors. The anticipated economic benefits, such as genuine reductions in potable water bills, particularly in the context of increasing water utility prices, are therefore of significant importance.

The challenges outlines require not only the development and implementation of advanced and efficient systems, but also educational activities aimed at raising public awareness, ensuring transparent monitoring, and building public confidence in both the quality of recovered water and the benefits of greywater reuse. Achieving this goal also depends on the active participation of the public in planning solutions.

6. Conclusions

Reliable data on the quantity and quality of wastewater produced, including knowledge of the range of basic indicators, such as total suspended solids concentration, turbidity, and organic compound content, are essential for determining the minimum required treatment level of, selecting appropriate treatment technologies, and assessing the potential for greywater reuse. Such data also enable the optimisation of system performance, leading to increased efficiency, reduced demand for tap water in the building and, consequently, economic benefits. Looking ahead, one of the key challenges lies in the development of standardised Life Cycle Assessment (LCA) methodologies, which offer a promising framework for evaluating the true environmental benefits of greywater reuse by enabling comprehensive comparisons of different variants of greywater treatment and recovery systems, taking into account their environmental impact, external social costs, and resource use efficiency. Future research should particularly focus on improving removal of micropollutants, especially pharmaceuticals and personal care products, which are increasingly detected in greywater and may pose a threat to public health and the environment. The implementation of advanced hybrid treatment systems, combining conventional biological processes with modern technologies, requires in-depth analysis of investment, operational, and environmental costs to determine the optimal system configuration suited to local conditions and intended applications. Research shows that effective greywater treatment for non-potable purposes requires the integration of biological and physical processes to meet established quality standards. Nevertheless, despite the availability of proven technological solutions, large-scale implementation remains constrained by regulatory barriers, high investment costs, low user confidence, and a lack of precise quality standards and sanitary requirements. Current approaches tend to focus primarily on the technical and economic aspects of greywater reuse.

The successful implementation of greywater reuse systems in buildings depends primarily on overcoming social and legal barriers, followed by addressing technical and environmental challenges. Increasing public acceptance requires the establishment of effective quality monitoring systems, transparency of information, and educational initiatives targeting both users and system designers. Future research should focus on the optimisation of scalable, maintenance-free, and adaptive hybrid systems, whose implementation would be economically feasible in both public utility and residential buildings.

Given the pivotal role of economic factors in investment decisions, future research initiatives and implementation programmes should incorporate comprehensive cost-effectiveness analyses of these systems. It is equally important to clearly communicate to potential users that greywater reuse can be cost-competitive with conventional water supply infrastructure, especially in the context of rising water prices. Under these conditions, the widespread adoption and broad public acceptance of such technologies can be realistically achieved—an outcome that is essential for advancing the water sector’s transition toward the principles of a circular economy.

It is also important to consider the implications of large-scale implementation of greywater reuse systems in buildings for the operation of wastewater treatment plants. The reduction in wastewater inflow to treatment plants, accompanied by an increase in pollutant concentrations in blackwater, may present significant challenges, necessitating the adaptation of treatment technologies and the modernization of existing sewage infrastructure. Future research should therefore prioritise the development of guidelines for the technological adaptation of wastewater treatment plants and the modernization of sewage infrastructure.

Greywater reuse, especially for toilet flushing, represents promising strategy for promoting sustainable water management and enhancing environmental safety in buildings. In light of the increasing challenges associated with water resource scarcity, the continued development, standardisation, and popularisation of greywater recovery systems in buildings with consideration of life cycle costs and investment efficiency management models, can be essential for achieving more effective water management within the construction sector.