Greywater as a sustainable source for development of green roofs: Characteristics, treatment technologies, reuse, case studies and future developments

Greywater Reuse: Contaminant Profile, Health Implications, and Sustainable Solutions

Global water scarcity is becoming an increasingly critical issue; greywater reuse presents a promising solution to alleviate pressure on freshwater resources, particularly in arid and water-scarce regions. Greywater typically sourced from household activities such as laundry, bathing, and dishwashing, constitutes a significant portion of domestic wastewater. However, the reuse of greywater raises concerns about the potential risks posed by its complex composition. Despite the growing body of literature on greywater reuse, most studies only focus on specific contaminants, thus there is a limited understanding of the comprehensive profile of contaminants, health, and environmental effects associated with these pollutants. This review adds new knowledge through a holistic exploration of the composition and physico-chemical characteristics of greywater, with a focus on its organic and inorganic pollutants, heavy metals, EDCs, emerging microplastics, nanoparticles, and microbial agents such as bacteria, fungi, viruses, and protozoa. This review sheds light on the current state of knowledge regarding greywater pollutants and their associated risks while highlighting the importance of safe reuse. Additionally, this review highlights the removal of contaminants from greywater and the sustainable use of grey water for addressing water scarcity in affected regions.

Greywater reuse for irrigation: A critical review of suitability, treatment, and risks

Greywater accounts for approximately 75 % of domestic wastewater and generally contains fewer contaminants than domestic wastewater. Therefore, its treatment and reuse represent a promising approach to supplement irrigation demand. This study comprehensively evaluates the quality characteristics of greywater based on its source, applied treatment methods, and its potential health, environmental, soil, and agricultural impacts. Various physical, chemical, and biological treatment processes have been analysed, with the most commonly employed technologies including membrane bioreactors (MBRs), constructed wetlands, media filtration (sand, activated carbon), disinfection methods (UV, chlorine, ozone), and advanced oxidation processes. The effectiveness of these methods has been assessed concerning the intended reuse application, emphasizing the critical role of disinfection in ensuring safe irrigation use. The health and environmental implications of greywater reuse have been examined, focusing on the risks associated with pathogen contamination, detergent residues, and micropollutants, while also evaluating the efficiency of treatment processes in mitigating these risks. From an environmental perspective, the accumulation of essential nutrients such as nitrogen and phosphorus, the potential for salinity buildup, and alterations in soil microbial balance have been investigated. Regarding soil and agricultural impacts, this study analyzes how greywater reuse influences soil structure (e.g., permeability, infiltration), plant growth responses, and the accumulation of heavy metals. These findings contribute to the development of scientifically grounded recommendations for the safe and sustainable reuse of greywater within water management strategies, promoting its role as an alternative water source for irrigation.

Innovative approaches to greywater micropollutant removal: AI-driven solutions and future outlook

Greywater constitutes a significant portion of urban wastewater and is laden with numerous emerging contaminants that have the potential to adversely impact public health and the ecosystem. Understanding greywater's characteristics and measuring the contamination levels is crucial for designing an effective recycling system. However, wastewater treatment is an intricate process involving significant uncertainties, leading to variations in effluent quality, costs, and environmental risks. This review addresses the existing knowledge gap in utilising artificial intelligence (AI) to enhance the laundry greywater recycling process and elucidates the optimal treatment technologies for the most prevalent micropollutants, including microplastics, nutrients, surfactants, synthetic dyes, pharmaceuticals, and organic matter. The development of laundry greywater treatment technologies is also highlighted with a critical discussion of physicochemical, biological, and advanced oxidation processes (AOPs) based on their functions, methods, associated limitations, and future trends. Artificial neural networks (ANN) stand out as the most prevalent and extensively applied AI model in the domain of wastewater treatment. Utilising ANN models mitigates certain limitations inherent in traditional adsorption models, particularly by offering enhanced predictive accuracy under varied operating conditions and multicomponent adsorption systems. Moreover, tremendous success has been recorded with the random forest (RF) model, exhibiting 100% prediction accuracy for both sessile and effluent microbial communities within a bioreactor. The precise prediction or simulation of membrane fouling behaviours using AI techniques is also of paramount importance for understanding fouling mechanisms and formulating efficient strategies to mitigate membrane fouling.

Technical implications of light greywater production and quality for decentralized treatment and reuse: A case study in Bucaramanga, Colombia

Decentralized light greywater (LGW) treatment and reuse can help mitigate urban water scarcity, yet data on its characteristics at the household level in Latin America remain scarce, limiting system design and implementation. This study assessed LGW quantity and quality in a representative household in Bucaramanga, Colombia, and analyzed its implications for decentralized treatment and reuse. Potable water consumption and LGW production from showers and hand basins were monitored over 98 and 124 days, respectively, with 27 LGW samples collected for quality analysis. Results showed that LGW production was 33.83 ± 4.10 (L/person)/day, accounting for 21% of household potable water consumption, with no significant differences across days of the week. Showers contributed 94% of LGW, while hand basins accounted for 6%. The volume of LGW was sufficient to meet toilet flushing demands (10% of potable water consumption) and could also support other accepted uses, such as floor cleaning and garden irrigation. LGW quality exhibited high variability, low nutrient content (6.37 ± 1.84 mgN/L and 0.74 ± 0.33 mgP/L), high organic matter concentrations (COD: 879.68 ± 163.51 mg/L; BOD5: 387.92 ± 92.08 mg/L), and fluctuating fecal coliform levels (1.87 × 105 ± 4.03 × 105 CFU/100 mL), influenced by personal care product use and hygiene practices. Effective treatment systems must accommodate water quality fluctuations and incorporate processes to remove suspended solids, turbidity, dissolved organic matter, oil and grease, and pathogens. This study identified three key technical implications of LGW production and quality for household-level treatment and reuse systems in decentralized settings.

Weather dynamics affect the long-term thermal and hydrological performance of different green roof designs

Owing to climate change, numerous regions around the world are expected to experience heightened occurrences of extreme events, including heat waves and intense precipitation. This will disproportionately impact the well-being of urban populations. The implementation of green roofs is actively considered as a viable climate adaptation strategy enhancing the resilience of cities. Green roofs have the potential to mitigate elevated temperatures within urban environments and play a crucial role in retaining stormwater. While many cities have included green roofs as a proposed funding tool in their climate adaptation plans, there is still a knowledge gap regarding best practice design and the long-term performance of green roofs as they are sensitive to meteorological conditions. We analyzed lysimeter measurements for different roof designs (a traditional gravel roof, an extensive green roof, and a wetland green roof) in terms of their cooling potential (evapotranspiration rates) and retention capacity for stormwater events over 31 months in a time-series framework, to evaluate the impact of the local climate variability and conclude about the best maintenance practices. While the wetland green roof generally showed the best summertime cooling and runoff reduction (total retention of 82.4 % compared to 63.4 % for the extensive green roof, and 29.9 % for the gravel roof), respectively, under optimal conditions, the extensive green roof GR was able to perform comparably. However, when the evapotranspiration of the extensive green roof decreased during persistent dry periods (daily sum of from 1.27 mm to 0.73 mm), their cooling potential was low for these days. This is critical as hot days occurred more frequently for these periods (37 hot days of 106 days during dry periods; 7 hot days of 127 days during wet periods). This indicates the urgent need for summertime irrigation.

Treated greywater as a novel water resource: The perspective of greywater treatment for reuse from a bibliometric analysis

The current global water crisis has prompted research into technologies that can reuse different water resources to mitigate water scarcity. The use of treated greywater can be proposed to provide additional water resources. By reusing this water in different applications, this water crisis can be mitigated at the local scale. This study presents a bibliometric analysis to assess the state of the art of greywater treatment and its reuse technologies. This analysis is based on the scientific literature published until 2023 in Scopus regarding greywater treatment and 1,024 documents were found. The results showed a clear exponential increase in the accumulated number of publications in this topic, which was spurred during the mid-1990s. The most prolific country was the United States, while China, the other typical scientific superpower in most fields, occupied the sixth position in the ranking. Environmental Sciences was the knowledge subject with more documents, followed by Engineering and Chemical Engineering. The bibliometric study was complemented using SciMAT to create bibliometric networks that represent the dynamic evolution of the themes. The most important themes were identified, among which three key points stand out: greywater characterization, technologies for greywater treatment, and water management, including the reuse of treated greywater.

Long-term evaluation of soil-based bioelectrochemical green roof systems for greywater treatment

Water scarcity has generated the need to identify new sources. Due to its low organic contaminant load, greywater reuse has emerged as a potential alternative. Moreover, the search for decentralized treatment systems in urban areas has prompted research on using green roofs for greywater treatment. However, the performance of organic matter removal is limited by the type of substrate and height of the growing media. Bioelectrochemical systems (BESs) improve treatment performance by providing an additional electron acceptor (the electrode). In this study, nine reactors under three different conditions, i.e., open circuit (OC), microbial fuel cell (MFC), and microbial electrolysis cell (MEC), were built to evaluate the treatment of synthetic greywater in a substrate-growing medium composed of perlite and coconut fiber and operated in batch-cycle mode for 397 days. The results suggested that using BESs enables greywater treatment and the removal of pollutants to levels that allow their reuse for irrigation. Furthermore, electrical conductivity was reduced from 732.4 ± 41.2 μS/cm2 in OC to 637.32 ± 22.73 μS/cm2 and 543.15 ± 19.69 μS/cm2 in MEC and MFC, respectively. The soluble chemical oxygen demand in the latter treatments reached 76% removal, compared to levels above the OC, which only reached approximately 67%. Microbial community analysis revealed differences, mainly in the cathodes, showing a higher development of Flavobacterium, Azospirillum, and Zoogloea in MFCs, which could explain the higher levels of organic matter removal in the other conditions, suggesting that the BES could produce an enrichment of beneficial bacterial groups for treatment. Therefore, implementing BESs in green roofs enables sustainable long-term greywater treatment.

Nutrient treatment of greywater in green wall systems: A critical review of removal mechanisms, performance efficiencies and system design parameters

Greywater has lower pathogen and nutrient levels than other mixed wastewaters, making it easier to treat and to reuse in nature-based wastewater treatment systems. Green walls (GWs) are one type of nature-based solutions (NBS) that are evolving in design to support on-site and low-cost greywater treatment. Greywater treatment in GWs involves interacting and complex physical, chemical, and biological processes. Design and operational considerations of such green technologies must facilitate these pivotal processes to achieve effective greywater treatment. This critical review comprehensively analyses the scientific literature on nutrient removal from greywater in GWs. It discusses nutrient removal efficiency in different GW types. Total nitrogen removal ranges from 7 to 91% in indirect green facades (IGF), 48-93% for modular living walls (MLW), and 8-26% for continuous living walls (CLW). Total phosphorus removal ranges from 7 to 67% for IGF and 2-53% for MLW. The review also discusses the specific nutrient removal mechanisms orchestrated by vegetation, substrates, and biofilms to understand their role in nitrogen and phosphorus removal within GWs. The effects of key GW design parameters on nutrient removal, including substrate characteristics, vegetation species, biodegradation, temperature, and operating parameters such as irrigation cycle and hydraulic loading rate, are assessed. Results show that greater substrate depth enhances nutrient removal efficiency in GWs by facilitating efficient filtration, straining, adsorption, and various biological processes at varying depths. Particle size and pore size are critical substrate characteristics in GWs. They can significantly impact the effectiveness of physicochemical and biological removal processes by providing sufficient pollutant contact time, active surface area, and by influencing saturation and redox conditions. Hydraulic loading rate (HLR) also impacts the contact time and redox conditions. An HLR between 50 and 60 mm/d during the vegetation growing season provides optimal nutrient removal. Furthermore, nutrient removal was higher when watering cycles were customized to specific vegetation types and their drought tolerances.

Integrating of electrocoagulation process with submerged membrane bioreactor for wastewater treatment under low voltage gradients

Treating and reusing wastewater has become an essential aspect of water management worldwide. However, the increase in emerging pollutants such as polycyclic aromatic hydrocarbons (PAHs), which are presented in wastewater from various sources like industry, roads, and household waste, makes their removal difficult due to their low concentration, stability, and ability to combine with other organic substances. Therefore, treating a low load of wastewater is an attractive option. The study aimed to address membrane fouling in the submerged membrane bioreactor (SMBR) used for wastewater treatment. An aluminum electrocoagulation (EC) device was combined with SMBR as a pre-treatment to reduce fouling. The EC-SMBR process was compared with a conventional SMBR without EC, fed with real grey water. To prevent impeding biological growth, low voltage gradients were utilized in the EC deviceThe comparison was conducted over 60 days with constant transmembrane pressure and infinite solid retention time (SRT). In phase I, when the EC device was operated at a low voltage gradient (0.64 V/cm), no significant improvement in the pollutants removal was observed in terms of color, turbidity, and chemical oxygen demand (COD). Nevertheless, during phase II, a voltage gradient of 1.26 V/cm achieved up to 100%, 99.7%, 92%, 94.1%, and 96.5% removals in the EC-SMBR process in comparison with 95.1%, 95.4%, 85%, 91.7% and 74.2% removals in the SMBR process for turbidity, color, COD, ammonia nitrogen (NH3-N), total phosphorus (TP), respectively. SMBR showed better anionic surfactant (AS) removal than EC-SMBR. A voltage gradient of 0.64 V/cm in the EC unit significantly reduced fouling by 23.7%, while 1.26 V/cm showed inconsistent results. Accumulation of Al ions negatively affected membrane performance. Low voltage gradients in EC can control SMBR fouling if Al concentration is controlled. Future research should investigate EC-SMBR with constant membrane flux for large-scale applications, considering energy consumption and operating costs.

Performance study of an innovative concept of hybrid constructed wetland-extensive green roof with growing media amended with recycled materials

Implementation of green roofs requires a large amount of primary material, especially for constructing the growing media layer. In addition, irrigation of green roofs with potable water is uneconomical and unsustainable. The novel hybrid green roof system proposed in this paper is in line with the principles of circular economy as it incorporates recycled materials into green roof growing media and greywater for irrigation. Two experimental beds were built to evaluate the concept of treating greywater in a constructed wetland prior to using it to irrigate a dual-layer extensive green roof. The growing media in both two extensive green roof beds contained ca. 37.5% by volume of recycled crushed building rubble containing a large proportion of brick. One of the two beds additionally contained 9.5% by volume of sewage sludge-based biochar. The concept of the hybrid roof and novel growing media was evaluated based on laboratory analysis of the growing media and on onsite measurements of hydraulic and thermal performance. The growing media amended with recycled materials developed in this study had hydrophysical properties comparable to commercially available growing media without recycled materials. Observations made during one vegetation season from June to October and a ten day-intensive water quality monitoring campaign during September 2020 showed that the constructed wetland significantly reduced total nitrogen and orthophosphate concentrations in pre-treated greywater. Due to the irrigation method employed, in which water flowed predominantly through drainage mats below the growing media, nutrient-leaching by the irrigation water was avoided. Concentrations of nutrients in the effluent were observed to increase only in response to precipitation. The temperature peak of the bottom green roof layer was shifted by almost 9 h from the peak in air temperature, and temperature fluctuations were significantly reduced. Vegetation on the bed amended with biochar demonstrated more vigorous growth due to available nutrients in the biochar which increased the rate of temperature-reducing evapotranspiration. More water evapotranspirated more water, which provided more water retention capacity confirmed by a lower runoff coefficient. Simple storage routing hydraulic modeling of hybrid green roof runoff using a nonlinear reservoir was performed.

Sustainable green roofs: a comprehensive review of influential factors

Green roofs have gained much attention as a modern roofing surface due to their potential to deliver many environmental and social benefits. Studies have indicated that different GR designs deliver different ecosystem services, and there are important factors that affect GR performance. This article reviewed significant factors that influence GR performance and sustainability. Substrate and drainage layer material choice significantly affects stormwater retention potential, leachate quality, plant survival, and determines GR environmental footprints. Subsequently, type of plants, their form, and kinds used on GRs impact GR ecosystem function. Leaf area is the most studied trait due to its influence on the cooling potential and energy performance. In order to achieve a sustainable GR, it is essential to select the type of plants that have a high survival rate. Perennial herbs, particularly forbs and grass as dominant groups, are heat and drought tolerant, which make them suitable in GR experiment. Furthermore, selecting a suitable irrigation system is as important as two other factors for having a sustainable GR. Irrigation is essential for plant survival, and due to the current pressure on valuable water sources, it is important to select a sustainable irrigation system. This review presents three sustainable irrigation methods: (i) employing alternative water sources such as rainwater, greywater, and atmospheric water; (ii) smart irrigation and monitoring; and (iii) using adaptive materials and additives that improve GR water use. This review sheds new insights on the design of high-performance, sustainable GRs and provides guidance for the legislation of sustainable GR.

Social Acceptance of Greywater Reuse in Rural Areas

Like many countries, Palestine suffers from water scarcity. Here, treated greywater is considered an essential nonconventional water resource. We aim to identify some wastewater reuse and disposal practices in rural areas and assess the acceptance level of different reuses of greywater. We conducted a survey analysis in four villages with a strong agricultural activity of the western Bethlehem Governorate. The level of acceptance of greywater reuse was generally independent of demographic variables like family size, income, or water bill, with a few exceptions regarding gender, age, and level of education. Centralized treatment was more valued than treatment at home, which presented similar acceptance levels than no treatment and might indicate a lack of trust in this alternative. The only reuse alternative trusted across treatments was bush irrigation (3.53-3.86 on a five-point Likert scale), but other options without clear, direct human contact like crop irrigation (3.14-3.62), stone cutting (3.19-3.36), and construction (3.12-3.42) also received considerable support. Reused perceived as having direct contact with humans was rejected, as it was the flushing of public toilets (2.59-2.7), aquaculture (1.98-2.37), olive pressing (1.85-1.94), and drinking (1.62-1.72). Relatively new reuse, car washing (2.95-3.17), was somewhere in between, partially because of its novelty. To increase this and other reuses, we strongly encourage local authorities to inform the population about the potentialities of greywater reuse.

Granular media filtration for on-site treatment of greywater: A review

Rapid urbanization and industrialization have put pressure on water resources and centralized wastewater treatment facilities and the need for greywater treatment at decentralized levels is increasing. This paper reviews the studies that used granular filtration for the treatment of greywater. Filter media characteristics that helps in the selection of suitable sustainable and environmental friendly materials without compromising the quality of treated greywater is first reported. The effect of type of filter media, media size and media depth along with the effect of operating conditions are discussed in detail. The choice, role and effect of different pre-treatment alternatives to granular media filtration are also presented. The efficiency of the filters to remove different physicochemical and microbial parameters was compared with different reuse guidelines and standards. Reported studies indicate that not only filter media characteristics and operating conditions but also the quality of raw greywater significantly influence the filter performance. Based on the source of greywater and desired reuse option, different granular media filtration alternatives are suggested. Operation of filters with properly selected media at optimum conditions based on the source of greywater helps filter in achieve the different reuse standards.

Use of green roofs for greywater treatment: Role of substrate, depth, plants, and recirculation

This work focuses on the use of green roof as a modified shallow vertical flow constructed wetland for greywater treatment in buildings. Different design parameters such as substrate (perlite or vermiculite), substrate depth (15 cm or 25 cm), and plant species (Geranium zonale, Polygala myrtifolia or Atriplex halimus) were tested to determine optimum selection. In addition, the application of a 40% recirculation rate was applied during last month of the experiment to quantify the efficiency of pollutants removal. The experiment was conducted for a period of 12 months under typical Mediterranean climatic conditions in Lesvos island, Greece. Results showed that green roofs planted with Atriplex halimus and filled with 20 cm of vermiculite had the best COD (91%), BOD (91%), TSS (93%) and turbidity (93%) average removal efficiencies. In contrast, significant lower removals were observed when the substrate depth was decreased to 10 cm (60-75%). Green roof vegetation had significant impact on TN removal as the average TN concentration decreased from 6.5 ± 1.8 mg/L in the effluent of unplanted systems to 4.9 ± 2.7 mg/L in the effluent of green roofs planted with Atriplex halimus. The recirculation of a portion of the effluent in the influent had as a result a significant improvement of turbidity, organic matter and (especially) nitrogen removal. For example, BOD removal in green roofs planted with Atriplex halimus and filled with 20 cm of perlite increased from 76% to 92%, while TN removal in green roofs planted with the same plants and filled with 20 cm of vermiculite increased from 56% to 87%. Overall, the operation of green roofs as modified vertical unsaturated constructed wetlands seems a sustainable nature-based solution for greywater treatment and reuse in urban areas.

Modelling Treated Laundry Greywater Reuse for Irrigation Using an Affordable Treatment Method and Seed Germination Test

A potential solution in areas facing water shortages is greywater (GW) reuse. GW is produced in bathrooms, laundry rooms, and kitchens of households. With proper treatment, it can be an alternative source for the agriculture sector, which consumes approximately 70% of the world’s water. This paper represents the characterization of synthetic laundry GW fraction (LGW), its treatment and modelling of treated LGW reuse for irrigation using a seed germination test. LGW’s constant quality (pH = 8.0 ± 0.3, turbidity = 174 ± 73 NTU, BOD5 = 300 ± 60 mgL−1, TOC = 162 ± 40 mgL−1) is suitable for testing the treatment method’s efficiency. Coagulation–flocculation, applying iron(III) chloride and sand filtration as a simple treatment combination, generates good-quality irrigation water (pH = 7.27 ± 0.23, turbidity = 0.6 ± 0.4 NTU, BOD5 = 17 ± 8 mgL−1, TOC = 16 ± 6 mgL−1). Seed germination tests with different waters, and elemental analysis of water, roots, and stems of the plants were done to verify the plants’ quality. The sodium adsorption ratio (SAR) for the raw LGW (SAR = 4.06) was above the threshold (<3) for safe irrigation, thus it is not recommended for this purpose. Based on the elemental analysis results and SAR value of treated LGW (SAR = 2.84), it can potentially be used for irrigation purposes.