As global water resources become increasingly strained, the development and utilization of non-conventional water sources have become a key direction for urban sustainable development. Domestic wastewater can be classified into blackwater and greywater based on pollution levels. Blackwater refers to wastewater contaminated by feces, while greywater originates from showers, washbasins, washing machines, and bathtubs [1]. Greywater typically contains only 30-50% of the total organics and 9-20% of household nutrient loads from domestic wastewater, with biochemical oxygen demand (BOD₅) around 40–70 mg/L and suspended solids (SS) about 30-50 mg/L. Although greywater contains some organics, oils, detergents, and microorganisms, its pollution load is significantly lower than that of blackwater (toilet sewage). With appropriate treatment, it can be reused for non-potable purposes such as toilet flushing and landscape irrigation [2]. Studies show that in the European Union (EU) and high-income countries, greywater accounts for up to 75% of domestic wastewater discharge; in low-income countries, average daily greywater discharge per household is 75-90 L [3]. With its stable volume and strong recyclability, household greywater is considered a highly promising reclaimed water resource.
I. Global Practical Applications of Greywater Reuse
Reusing greywater can transform large amounts of wastewater from waste into valuable water resources. While reuse is relatively common in developed countries, only a few developing countries have begun adopting greywater recovery technologies to alleviate freshwater supply pressures. Countries supporting greywater research and reuse include the United Kingdom, United States, Canada, Japan, Germany, Israel, Sweden, and Australia. The United States has issued relevant guidelines, while Spain, Australia, and others have legislated to permit greywater reuse under specific conditions.
California was the first U.S. state to approve greywater reuse [22], and Arizona offers residents up to $1,200 in subsidies for greywater use [26]. In Germany, residential systems tend to favor rainwater harvesting over greywater recovery [27]. In Tokyo, buildings with floor areas exceeding 30,000 m² or potential reuse volumes reaching 100 m³/day are required to implement greywater recovery. Singapore and Namibia supplement drinking water supplies with treated greywater to address freshwater shortages [28]. Acceptance of residential greywater reuse is high in countries like the UK [29]. In Sant Cugat del Vallès, Spain, over 5,000 households have installed greywater reuse systems, with other municipalities in Catalonia increasingly joining local and regional rainwater and greywater recovery projects [30]. Sydney, Australia, has established various guidelines and implemented greywater storage policies; Brisbane City Council has successfully achieved household greywater recovery and approved standard greywater treatment systems or diversion devices for uses such as toilet flushing and garden irrigation based on greywater characteristics and treatment levels [31]. Israel collects greywater in various ways and uses ultraviolet or chlorination disinfection, with treated greywater applied to horticultural irrigation and restroom cleaning [32]. In China, Beijing has achieved a 60% wastewater reuse rate, making it the most successful region for wastewater reuse in the country. However, in 65% of China's regions, wastewater reuse rates remain below 8.8% [33].
II. Conventional Greywater Treatment Systems and Application Schemes
Greywater treatment systems primarily fall into three categories: physical, chemical, and biological [4].
1. Physical Treatment Systems
Common physical methods include coarse sand filtration, sedimentation, and membrane separation. Conventional physical processes purify water mainly through: (1) physical particle filtration; (2) chemical adsorption of pollutants on soil surfaces; (3) adsorption by aerobic microorganisms absorbing nutrients from wastewater. Oron et al. [5] used a decentralized greywater treatment device combining sand filtration and electrolysis in private lawns in Israel, finding that organic content in the treated water slightly increased. Al-Mughalles et al. [6] combined a granular activated carbon (GAC) reactor with a sand filter to treat greywater from a mosque in Sana'a, achieving a 65% COD removal rate. Sand filters generally perform well in reducing turbidity and biochemical oxygen demand but are inadequate for removing organics, nutrients, pathogens, and surfactants that pass through porous media pores, resulting in limited overall water quality improvement [7-10].
2. Chemical Treatment Systems
Reported chemical methods include precipitation, electromagnetic resin ionization, catalytic degradation, granular activated carbon activation, electrode-enhanced ultraviolet, and electrolysis. Researchers studied precipitation and electromagnetic resin ionization on greywater from student dormitories at Cranfield University, UK, achieving high organic removal rates but failing to meet certain national greywater reuse standards [11]. A hotel in Spain combined precipitation, filtration, and hypochlorite disinfection for an indoor greywater recovery system for toilet flushing [12]; satisfactory disinfection was achieved at 75 mg/L chlorine dosage. However, due to limited treatment depth, some water quality parameters did not reach ideal standards.
3. Biological Treatment Systems
Since the late 20th century, biological treatment systems have been used directly or combined with various physical treatments for greywater. Membrane bioreactors (MBR), rotating biological contactors (RBC), and upflow anaerobic sludge blanket (UASB) systems have been studied and reported in multiple studies [13-15]. Pretreatment such as sedimentation, septic tanks, and filtration is typically applied before biological treatment. Researchers used sequencing batch reactors to treat greywater from student dormitory restrooms in Tunisia [16], achieving up to 90% COD removal, effective organic biodegradation, and good sludge settling performance. In a residential area on Crete, Greece, a submerged biofilm reactor was used for decentralized greywater remediation, achieving average removal rates of 80% for COD and anionic surfactants; the treated greywater was safe and reliable for toilet flushing [17]. Although membrane bioreactors have made significant progress in research and practical applications, membrane fouling remains a challenge, leading to increased energy consumption and maintenance costs [18].
III. Ecological Advantages of Traditional Reuse Combined with UVC-LED Disinfection
UVC-LEDs emit deep ultraviolet light at 265–280 nm, effectively damaging microbial DNA/RNA structures and preventing replication, making them one of the most effective disinfectants in water treatment [19]. Studies show that at 265 nm, E. coli inactivation efficiency reaches 99.99% with doses as low as 6–10 mJ/cm², depending on water quality and reactor configuration [20].
Compared to traditional mercury lamp UVC systems, UVC-LEDs offer significant advantages: 1) No chemical residues: pure physical disinfection, avoiding carcinogenic byproducts like trihalomethanes from chlorine disinfection; 2) Instant on/off: no preheating required, supporting smart sensor control suited to intermittent greywater flows; 3) Modular integration: small size and low power consumption, easily integrated into household or building-scale greywater treatment systems; 4) Environmentally safe: mercury-free, compliant with RoHS and other environmental directives [21]. UVC-LED disinfection effectiveness highly depends on water ultraviolet transmittance (UVT); suspended solids, oils, and organics in greywater significantly absorb or scatter UV light, reducing effective dose. Therefore, UVC-LED is typically used as a final disinfection unit, requiring upstream pretreatment.
Friedler et al. [22] applied UV devices at the effluent end of biological rotating contactors (RBC) and membrane bioreactors (MBR) for lightly polluted greywater. Results showed excellent disinfection efficiency against fecal coliforms and Staphylococcus aureus, completely removing (100%) virus indicators (F-RNA phages, host: E. coli) injected into the system. When greywater UVT ≥ 70%, UVC-LED achieves >4-log (99.99%) E. coli inactivation, meeting microbial safety requirements for greywater reuse [23].
Barzegar et al. [24] combined electrocoagulation with ozonation and UV (EC+O₃+UV), reducing total organic carbon (TOC) and COD in greywater by about 70%-85%. After UV irradiation, TOC and COD removal reached 87%-95%, achieving 4-log total colony count reduction and 96% E. coli removal, significantly enhancing electrocoagulation/ozonation performance. Sanchez et al. [25] placed UV lamps in quartz sleeves for hotel greywater treatment, finding low dissolved organic carbon (DOC) content made TiO₂ photocatalytic treatment particularly suitable.
IV. Conclusion and Outlook
Greywater reuse, as an efficient water-saving approach, not only significantly reduces freshwater consumption in agricultural irrigation, municipal landscaping, and building toilet flushing but also lowers sewage discharge loads at the source. It provides core support for building closed-loop water resource cycles and sustainable ecosystems, serving as a key driver for global ecological and economic sustainable development. Given its value, governments worldwide should adopt dual strategies in policy and awareness: on one hand, introduce targeted greywater reuse standards, subsidy policies, and regulatory details, clearly defining reclaimed water quality requirements and application scenarios; on the other, conduct regular public education through science popularization, case demonstrations, etc., to raise awareness of greywater characteristics, treatment principles, and reuse value, eliminating public concerns about reclaimed water.
Technologically, traditional greywater recovery methods—physical (sedimentation, filtration), biological (activated sludge, biofilms), and chemical (coagulation, oxidation)—are mature and widely applied but have unavoidable drawbacks: physical methods offer limited precision for dissolved pollutants; biological methods are sensitive to water quality fluctuations and require large land areas with long maintenance cycles; chemical methods risk secondary pollution and high reagent costs.
To overcome these bottlenecks, the “efficient pretreatment + UVC-LED disinfection” combined process is emerging as the preferred solution for on-site greywater reuse. This process removes suspended solids and large-molecule organics via grilles, membrane filtration, etc., then relies on UVC-LED disinfection for efficient pathogen inactivation, achieving unity in safety, economy, and sustainability for greywater reuse. Compared to traditional mercury lamp UV disinfection, UVC-LED offers green environmental benefits (no mercury pollution), low energy consumption, fast response, and modular assembly, flexibly adapting to different scales of greywater treatment needs and promoting a shift from centralized, large-scale to distributed, intelligent, small-scale greywater treatment models.
Looking ahead, with continuous improvements in UVC-LED photoelectric conversion efficiency and declining production costs, this technology holds greater application potential in building water-saving systems, distributed community water treatment stations, sponge city rainwater-greywater combined facilities, etc. It will provide technical support for water-saving practices in water-scarce regions and contribute to achieving global sustainable water resource utilization goals.
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