Greywater recycling in buildings using living walls and green roofs: A review of the applicability and challenges

Methodological framework for impact evaluation of Building-Integrated Greenery (BIG-impact)

Building-Integrated Greenery systems, i.e., green roofs, walls, and facades, are Nature-based Solutions that make possible the renaturing of cities when there is no room for traditional greenery solutions. These green systems provide several ecosystem services at both the building and city level, such as urban heat island effect mitigation and noise reduction, support for biodiversity, runoff control, thermal and acoustic insulation, etc. However, once implemented in real cases, their impact is almost never evaluated. This fact limits the possibility of carrying out cost-benefit analyses that contribute to justifying their long-term maintenance, thus putting at risk their long-term sustainability and consequently the provision of benefits. Unlike existing approaches, the method presented here offers a comprehensive and practical tool that addresses the gap in BIG systems' impact evaluation, facilitating informed decision-making and promoting the long-term sustainability of BIG systems.•In its design, the current references at European and global level for building-integrated systems impact assessment has been considered.•It is easily replicable in any real project and enables the collaboration of involved stakeholders.•The method is unprecedented and allows a holistic assessment of the impact of BIG in real cases, in terms of ecosystem services provided.

Recycled waste substrates: A systematic review

The growing interest in utilizing recycled waste substrates (RWS) in ecosystem services and environmental remediation aligns with the "waste to wealth" concept and the Sustainable Development Goals (SDGs). Despite the promising potential of RWS, research gaps remain due to a lack of comprehensive reviews on their production and applications. This systematic review attempts to synthesize and critically assess the scientific footprint of RWS through robust methodology and thorough investigation. Characterization of scientific literature, network analysis, and systematic review were conducted on articles indexed in the Web of Science and Scopus databases. Quantitative and qualitative analyses were performed on 140 articles selected by the rigorous article screening process executed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol. The findings map the scientific literature and research themes in RWS. Around 66 % of studies in RWS used a multiple research approach, primarily experiments with case studies. Key research topics identified include (A) Technical domains - types of wastes and recycling techniques in RWS production and parameters influencing the substrate quality; (B) Application domains: environmental remediation of soil and agriculture and horticulture. The use of RWS in urban green infrastructure, particularly for green roofs and vegetative walls, and the potential for LCA studies on RWS production and applications emerge as promising areas for future research. This systematic review also presents a conceptual framework model (CFM) on RWS research, encapsulating the state-of-the-art themes, risks, limitations and constraints, and future research avenues.

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.

A comparative analysis of the carbon footprint in green building materials: a case study of Norway

This paper critically examines the carbon cycle and environmental impacts associated with building materials, encompassing diverse impact categories for both midpoint and endpoint scenarios. The research encompasses a comparative analysis of five distinct scenarios, contrasting the environmental performance of green against conventional counterparts. Notably, previous research endeavors did not investigate the effects of varying percentages with and without phase change materials (PCM). The primary objective is to assess the impact of integrating phase change materials (PCM) with varying percentages of fly ash (20% and 35%) on energy consumption and carbon emissions, particularly in cold climates like Norway. The study employs the ReCiPe2016 Midpoint (E) method, which offers a robust life cycle assessment (LCA) framework aligned with European standards, making it particularly suitable for this context. Energy Plus, within the Design Builder software, was used to simulate and calculate the impact of PCM on energy efficiency. The findings underscore those environmental impacts attributed to green buildings amount to 9.79 × 104 kg of CO2 equivalent, while conventional buildings account for 1.04 × 105 kg of CO2 equivalent. Furthermore, among the cases studied, the optimal scenario pertains to a green building utilizing 35% wind ash cement and PCM, resulting in the equivalent of 9.68 × 104 kg of CO2 emissions. Remarkably, the best-case scenario involves a green building boasting a robust steel interior structure and aluminum windows, whereas the worst-case scenario entails a typical building devoid of PCM implementation. Furthermore, energy consumption analysis indicates that scenario 5, which utilizes PCM and 35% fly ash, achieves a 15% reduction in cooling energy and a 6.9% reduction in heating energy compared to scenario 3, resulting in an annual energy consumption of 97,453.09 kWh.

A systematic analysis on the efficiency and sustainability of green facades and roofs

In the modern urban space, green infrastructures have been gaining increasing relevance due to their positive impacts on sustainability issues, visual appeal, and the well-being of individuals. On the other hand, environmental sustainability has become mandatory in the agenda of governments and organizations. Thus, a systematic analysis on the efficiency and sustainability of green facades and roofs spanning key applications, benefits and implementation constraints is welcome. In this paper, we employed the PRISMA method to investigate how these matters were addressed in the recent literature, comprising articles published in scientific journals indexed to the SCOPUS database. Following the web search, selection, systematization, and analysis of that literature, it was revealed that the efficiency of green facades and roofs has been mostly associated with energy and thermal performance in buildings, which brings unequivocal multiple benefits (e.g., consumption savings, mitigation of urban heat island effects) despite of some barriers (e.g., installation and maintenance costs). Other discussions about green facades and roofs involved their valuable roles in stormwater management, considering their retention capacity, and in the treatment of wastewater for reuse in non-potable applications, considering their filtering capacity. It was also discovered the need to improve green infrastructures through the use of cleaner technologies and recycled materials, selection of plants that are appropriate for the local climate, and minimization of construction, transportation, disposal and maintenance costs. Efficiency and sustainability in these cases was prognosed to succeed if the costs were minimized throughout the entire life cycle, and complemented with incentive policies (e.g., tax reduction, agile administrative processes) and collaboration among multidisciplinary teams comprising designers, builders, municipality planners and the academic and market worlds.

Analysis of urban residential greening in tropical climates using quantitative methods

Urban green spaces play a crucial role in mitigating urban heat islands, providing shade, cooling, absorbing carbon dioxide, and releasing oxygen to enhance air quality. Understanding the user perceptions of residential greeneries is essential for effective planning and implementation of greening systems. This quantitative research explored user perceptions and preferences regarding residential greeneries through a structured questionnaire survey from 578 respondents. The responses from the densely populated Chennai city and the rest of Tamil Nadu, India, were analyzed. About 90% of residents are interested in having a garden, irrespective of location and residential characteristics. The most available space in Chennai's urban region is a balcony at 45%, followed by front and back gardens at 30% and vice versa for Chennai's suburban areas. The most preferred type is potted plants (30%) and climbers (20%) on balconies and near windows in Chennai. The most perceived challenges are installation and maintenance costs. The most influencing factors over the preference for greeneries and green walls are the house typology, house ownership, and site location. This study provides more insights to building designers and architects on planning and implementation of residential greeneries as per end users' preferences and perceptions.

Low-Cost Greywater Treatment Using Polyculture Microalgae-Microalgal Growth, Organics, and Nutrient Removal Subject to pH and Temperature Variations During the Treatment

Organics and nutrient removal studies are rarely done using polyculture microalgae, and that too in outdoor conditions, as they are often not deemed effective for wastewater treatment purposes. This study examined the organics and nutrient removal efficiency of polyculture microalgae cultivated in greywater. The reactor was operated in outdoor conditions. Hence, it was subjected to natural pH and temperature variations. A growth rate of 0.05 g L-1 day-1 was observed for temperatures up to 37 °C, beyond which the growth rate declined by 0.07 g L-1 day-1. During the treatment, the pH of the system was observed to be between 7.4 and 8.4. However, the growth rate would again pick up (0.05 g L-1 day-1) when the pH and temperature moved towards the optimum range, indicating that the polycultures adapt very quickly to their environment. The maximum biomass concentration reached 0.82 gL-1. The highest removal efficiency of organic carbon, ammonia, and phosphate was 80.7, 61.9, and 58.4%, respectively. Nitrate and nitrite concentrations remained ≤ 1.3 mgL-1 and ≤ 2 mgL-1, respectively, indicating the absence of nitrification/denitrification and ammonia volatilization. The mass balance of microalgae indicated that the primary removal mechanism of nitrogen and phosphorus removal was assimilation by the microalgae. The study proved polyculture microalgae to be as effective as some monoculture species in wastewater treatment, which require costlier controlled growth conditions. The high organics and nutrient removal by polycultures in outdoor conditions could pave the way to reducing wastewater treatment costs.

A review of interconnected challenges in the water-energy-food nexus: Urban pollution perspective towards sustainable development

The swift growth of cities worldwide poses significant challenges in ensuring a sufficient water, energy, and food supply. The Nexus has innovated valuable systems to address these challenges. However, a crucial issue is the potential for pollution resulting from these systems, which directly and indirectly impacts public health and the overall quality of urban living. This study comprehensively reviews the interconnected challenges of the water-energy-food (WEF) nexus and various forms of pollution in cities. The primary focus of this review article is to showcase the findings of WEF nexus studies regarding various pollutions across different geographical regions and spatial scales. It aims to examine the problems resulting from these pollutions, specifically their effects on human health and urban life. It also delves into the sources of pollution as identified in these studies. Furthermore, the article will highlight the proposed solutions from the research aimed at effectively mitigating pollution in each sector studied. This article is a systematic review which analyses research sources from the Scopus database. It extensively reviewed 2463 peer-reviewed published articles and focused explicitly on articles related to the WEF nexus that discussed pollution. Our study emphasizes, firstly, raising awareness about the crucial link between the WEF nexus, pollution, urban environments, and human health among policymakers and key stakeholders, including urban planners, industry partners and municipalities. This is to promote the development of policies that encourage sustainable practices and key stakeholders. Secondly, it evaluates WEF nexus and pollution research methods and findings, aiding in identifying research gaps technological innovation and potential, as well as enhancing decision-making. Lastly, it outlines future research challenges, providing a roadmap for researchers and policymakers to advance understanding in this domain and identify opportunities for resource efficiency and collaboration between different sectors.

Recent advances in green technology and Industrial Revolution 4.0 for a sustainable future

This review gives concise information on green technology (GT) and Industrial Revolution 4.0 (IR 4.0). Climate change has begun showing its impacts on the environment, and the change is real. The devastating COVID-19 pandemic has negatively affected lives and the world from the deadly consequences at a social, economic, and environmental level. In order to balance this crisis, there is a need to transition toward green, sustainable forms of living and practices. We need green innovative technologies (GTI) and Internet of Things (IoT) technologies to develop green, durable, biodegradable, and eco-friendly products for a sustainable future. GTI encompasses all innovations that contribute to developing significant products, services, or processes that lower environmental harm, impact, and worsening while augmenting natural resource utilization. Sensors are typically used in IoT environmental monitoring applications to aid ecological safety by nursing air or water quality, atmospheric or soil conditions, and even monitoring species' movements and habitats. The industries and the governments are working together, have come up with solutions-the Green New Deal, carbon pricing, use of bio-based products as biopesticides, in biopharmaceuticals, green building materials, bio-based membrane filters for removing pollutants, bioenergy, biofuels and are essential for the green recovery of world economies. Environmental biotechnology, Green Chemical Engineering, more bio-based materials to separate pollutants, and product engineering of advanced materials and environmental economies are discussed here to pave the way toward the Sustainable Development Goals (SDGs) set by the UN and achieve the much-needed IR 4.0 for a greener-balanced environment and a sustainable future.

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.

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.

Removal of pathogens from greywater using green roofs combined with chlorination

Greywater is an important alternative water resource which could be treated and reused in buildings, reducing the freshwater demand in drought affected areas. For the successful implementation of this solution, it is important to ensure the microbial safety of treated greywater. This study examined the microbiological quality of treated greywater produced by an emergent nature-based technology (green roofs) and a chlorination process. Specifically, the effect of substrate, substrate depth, and vegetation on the removal of total coliforms, Escherichia coli, and enterococci in experimental green roofs treating greywater was examined for a period of about 12 months. In addition, the ability of chlorination to inactivate the abovementioned pathogen indicators was evaluated and their potential regrowth was examined. Results shown that green roofs filled with 10 cm of perlite reduce total coliform concentration by about 0.4 log units while green roofs filled with 20 cm of vermiculite reduce total coliform concentration by about 1.2 log units. In addition, the use of vegetation in green roofs improves the removal of pathogenic bacteria by about 0.5 log units in comparison with unvegetated systems. In all cases, the effluents of green roofs failed to satisfy the criteria for indoor reuse of treated greywater for non-potable uses such as toilet flushing without a disinfection process. The addition of 3 mg/L of chlorine in the effluent provided safe greywater microbiological quality for storage periods of less than 24 h, while longer periods resulted in the significant regrowth of pathogens. In contrast, a chlorination dose of 7 mg/L completely secured inactivation of pathogen indicators for periods of up to 3 days.

Research and development of green roofs and green walls in Mexico: A review

The residential sector is one of the primary energy consumers and emitters of greenhouse gases. Given the environmental problem, one of the methods of mitigating electricity consumption and reducing the temperature in buildings is green infrastructure: green roofs and walls. This article presents a compilation of the studies carried out in México about green infrastructure; the energy, thermal and environmental benefits obtained were analyzed according to the vegetation, substrate, climate, and systems configuration. In addition, the development of policies, laws, regulations, and incentives in the field of green roofs in Mexico was also analyzed. The results indicate that using green infrastructure can help mitigate greenhouse gases since a green roof can reduce the indoor temperature up to 19.9 °C, save 28 % annually in electricity consumption and remove 80 % of rainwater pollutants. Finally, the results of this research can provide insight for researchers, legislators, and urban planners about the state in which Mexico is located, as well as help in decision-making.

Substrate modified with biochar improves the hydrothermal properties of green roofs

Green roof, as an important measure of sponge city construction, is considered as a win-win alternative for alleviating rainwater runoff and urban heat island. The ecological benefits of green roofs are highly dependent on the quality of substrates. Biochar (BC) prepared from agricultural waste biomass has the potential to be used as a substrate amendment for green roofs. However, the influences of BC properties on hydrothermal properties of green roofs remain unclear. We evaluated the effects of natural soils incorporated with two kinds of BCs (particle size and dosage) on runoff retention capacity and roof thermal performance. Results indicated that the runoff reduction benefit of green roofs declines with the increase of rainfall. When the rainfall is less than 10 mm, the green roofs with different substrates hardly generate runoff, otherwise runoff reduction rates of all green roofs reduce below 75%. BC particles have abundant micro-pores and higher specific surface area, significantly improving the water holding-capacity of roof substrate and playing a critical role in the runoff regulation and cooling effect of green roofs. Application of 20% finer BC particles is the optimal for stormwater retention in all BC addition substrates. Moreover, it could reduce the roof upper surface temperature by 3-5 °C and reduced the daily heat gain of the green roof by at least 0.06 MJ/m² compared with BC-free ones. Overall, adding BC into the substrates of green roofs can achieve better hydrothermal properties, which is beneficial to the design optimization of green roofs.

Green walls with recycled filling media to treat greywater

The continuous growth of urban areas in the last decades has resulted in an increase in water consumption, contributing to larger volumes of urban and domestic wastewater. Thus, stakeholders have been seeking for efficient alternatives for wastewater management, namely looking for new forms of treatment and reuse. The present work provides new insights on the application of a green wall for greywater treatment, aiming at water reuse and also at contributing to Circular Economy. Two types of recycled materials, crushed tiles and textile fibers, were tested as filling media combined with two plant species. Crushed tiles were mixed with coconut fibers in a 70 %-30 % ratio and textile fibers were used as single media. The tiles+coconut mix with plants performed the best, exhibiting on average removal efficiencies >70 % for Chemical Oxygen Demand (COD) and between 59 %-70 % for Total Suspended Solids (TSS). Fibers systems had on average removal efficiencies around 60 % for COD and 50 % for TSS and clogged at the end of the study. Overall the study demonstrated that green walls for greywater treatment can contribute to circular economy through the use of recycled material as filling media.

Application of leaf analysis in addition to growth assessment to evaluate the suitability of greywater for irrigation of Tilia cordata and Acer pseudoplatanus

Water is the key resource in fulfilling the cooling function of plants in urban environments and needs to be supplied reliably and adequately, especially during dry periods. To avoid an unsustainable use of high-quality drinking water for irrigation, the reuse of greywater should be implemented for Green Infrastructure irrigation in the sense of the circular economy. In this study, the influence of greywater irrigation on vitality of two trees species, Tilia cordata and Acer pseudoplatanus, was determined by investigating the effect of irrigation with raw or treated greywater in comparison to municipal tap water. Plant growth parameters were measured, including leaf area, number of leaves, average leaf area and annual growth. In addition, the relative chlorophyll content was determined and image analysis was used to identify vital and necrotic leaf parts. While treatment did not affect growth after one growing season A. pseudoplatanus had significantly higher leaf necrosis (34.8%) when irrigated with raw greywater compared to treated greywater (15.5%) and tap water (5.8%). Relative chlorophyll content of T. cordata irrigated with tap water decreased over time until it was significantly lower (28.5) then the greywater treatments (34.5 and 35). Image analysis of leaves to quantify necrosis proved to be a sensitive method to quantify plant health and showed negative effects earlier than an analysis of growth. Anionic surfactants and electrical conductivity had a significant influence on plant vitality. Therefore, plant selection should take these parameters into account, when planning green infrastructure irrigated with greywater.

Modelling the benefits of urine source separation scenarios on wastewater treatment plants within an urban water basin

Stringent discharge regulations are encouraging researchers to create innovative and sustainable wastewater treatment solutions. Urine source separation (USS) is among the potent approaches that may reduce nutrient peak loads in the influent wastewater and improve nutrient recovery. A phenomenological model was used to simulate dynamic influent properties and predict the advantages gained from implementing USS in an urban water basin. Several scenarios were investigated assuming different levels of deployment: at the entire city, or specifically in office buildings for men's urine only, or for both men and women employees. The results confirmed that all scenarios of urine source separation offered benefits at the treatment plant in terms of reducing nitrogen influent load. The economic benefits in terms of reducing energy consumption for nitrification and decreasing methanol addition for denitrification were quantified, and results confirmed environmental advantages gained from different USS scenarios. Despite larger advantages gained from a global USS rate in an entire city, implementation of a specific USS in office buildings would remain more feasible from a logistical perspective. A significant benefit in terms of reducing greenhouse gas emissions is demonstrated and this was especially due to the high level of N₂O emissions avoided in nitrifying biological aerated filter.

The regime of constructed wetlands in greywater treatment

There is an excellent need for supply-side threats due to the enhanced degradation and reclamation of existing water bodies in the present scenario. This led to the global water crisis. One of the easiest ways to fulfil the growing need for freshwater is the recycling of wastewater. Greywater is a form of wastewater from households, industries, etc., with some less toxic materials. The recycling of this greywater has provoked the development of new and sustainable technologies to meet the growing water demand. Engineered constructed wetlands are considered one of the most economically practical processes to treat greywater due to its minimal footprint. In this case study, we summarize several categories of constructed wetlands, operating conditions, and the effects of biological, physical, and chemical aspects of greywater on their treatment performance. On the other hand, the effluent quality from diverse wetlands is also summarized. Furthermore, it would be better to consider that constructed wetlands' integrated performance with disinfection may improve the effluent quality to desirable standards.

Decision-Making Framework for GI Layout Considering Site Suitability and Weighted Multi-Function Effectiveness: A Case Study in Beijing Sub-Center

The effectiveness of runoff control infrastructure depends on infrastructure arrangement and the severity of the problem in the study area. Green infrastructure (GI) has been widely demonstrated as a practical approach to runoff reduction and ecological improvement. However, decision-makers usually consider the cost-efficacy of the GI layout scheme as a primary factor, leading to less consideration of GI’s environmental and ecological functions. Thus, a multifunctional decision-making framework for evaluating the suitability of GI infrastructure was established. First, the study area was described by regional pollution load intensity, slope, available space, and constructible area. Then, to assess the multifunctional performance of GI, a hierarchical evaluation framework comprising three objectives, seven indices, and sixteen sub-indices was established. Weights were assigned to different indices according to stakeholders’ preferences, including government managers, researchers, and residents. The proposed framework can be extended to other cities to detect GI preference.

Economic impact assessment indicators of circular economy in a decentralised circular water system - Case of eco-touristic facility

The transition from a linear make-use-dispose model to a Circular Economy (CE) model has gained momentum in recent years. To date, substantive efforts have been put by researchers and practitioners on environmental assessment of circular water systems (CWS). Yet, the economic aspect of CWS has not received the same attention. This research is an attempt to bridge this gap by evaluating the economic viability of a decentralised hybrid rainwater- wastewater-greywater (HRWG) system. For this purpose, a framework of Shadow Pricing- Life Cycle Cost-Benefit (SLCCB) to analyse a CWS is proposed. Shadow pricing could compliment the established Life Cycle Costing (LCC) methods. The main parameters (costs and benefits) of the proposed SLCCB framework are divided into two types: Internal and External. The Internal pricing covers the capital expenditure (CAPEX) and operational expenditure (OPEX), while the External pricing covers the environmental and social costs-benefits of implementing CWS. The proposed SLCCB added to the classical Net Present Value (NPV) and Payback Period (PP) calculations could provide a more realistic evaluation of the economic performance of CWS. To demonstrate the efficacy of the new CE model, a new CWS in Greece was studied. A sensitivity analysis was conducted to assess the impact of the reclaimed water tariffs, internal costs, life span of the project, and the annual discount rate on the SLCCB. The results of the study reveal that the SLCCB of CWS is highly sensitive to these parameters. The economic feasibility of CWS boost with increasing discount rate and reclaimed water tariffs, as well as with decreasing project's life span and internal costs. The conclusion of this research demonstrates that investment in CWS is economically viable if External parameters are taken into consideration.

Performance of a green wall (Total Value Wall™) at high greywater loading rates and Life Cycle Impact Assessment

Nature-based greywater (GW) treatment and reuse in urban areas has become an up-and-coming option. A 14.4 m² green wall system called Total Value Wall (TVW) was installed at a terraced house in Gent (Belgium) for treating GW and reusing the effluent for toilet flushing. In a previous study, the TVW was loaded at 7 L.m^-2.d^-1 and efficiently removed TSS (67%), COD (43%), BOD₅ (83%) and total coliforms (log 2), but a number of issues were reported related to nutrient leaching from the substrate, and the excessive retention time in the storage tanks. In this study results are reported from a follow-up study during which an adapted TVW was subjected to both higher hydraulic and pollutant loading rates in order to investigate the treatment capability of TVW. The design of the system, i.e. substrate contained in geotextile bags, did not sustain the higher hydraulic loading rates as excessive leakage occurred. Despite this, the higher pollutant loading rates still resulted in an acceptable effluent quality with 15 mg.L^-1 TSS (90%), 85 mg.L^-1 COD (82%), and 15 mg.L^-1 BOD₅ (95%). Ammonium, E. coli and total coliforms were removed with removal rates of 98%, 63% (0.4 log units), and 36% (0.2 log units), respectively. Finally, a life cycle assessment (LCA) was performed for the TVW with and without treating GW to analyze the environmental burden. The LCA impacts showed that replacing tap water and chemical fertilizer by GW, and the reuse of effluent, have a positive impact. However, the energy use for pumping has a major impact and should be minimized by using an efficient pump and distribution system to reduce the overall footprint.

Development of an Intelligent Urban Water Network System

Water and wastewater services have been provided through centralised systems for more than a century. The operational and management approaches of the water systems face challenges induced by population growth, urbanisation, and ageing infrastructure. Recent advancements in water system engineering include the development of intelligent water networks. These intelligent networks address management and operational challenges associated with pressure and flow variations in the water network and it reduces the time for identification of pipe bursts and leakages. Research is required into the development of intelligent water networks to ensure consistent data collection and analysis that can filter and aggregate into actionable events to reduce water leakage, leakage cost, customer disruptions, and damages. Implementation of an intelligent algorithm with an integrated Supervisory Control and Data Acquisition (SCADA) system, high-efficiency smart sensors, and flow meters, including a tracking mechanism, will significantly reduce system management and operational issues and ensure improved service delivery for the community. This paper discusses the history of water systems, traditional water supply systems, need for intelligent water network, and design/development of the intelligent water networks. A framework for the intelligent water network has also been presented in this paper.

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.

Total value wall: Full scale demonstration of a green wall for grey water treatment and recycling

Greywater treatment and reuse for non-potable purposes in urban areas has become a widely researched topic to reduce the burden on fresh water resources. This study reports on the use of a green wall for treating grey water and reusing the effluent for toilet flushing, called Total Value Wall (TVW). Initially, the effectiveness of (mixtures of) different substrates, i.e. lava, lightweight expanded clay aggregates, organic soil and biochar was investigated by means of column tests. All substrates were first examined for hydraulic characteristics and later on the columns were fed with synthetic grey wastewater and followed up in terms of removal efficiency of COD and detergents. The mixture consisting of lava (50%), organic soil (25%) and biochar (25%) proved to be optimal both in terms of percolation rates and removal efficiencies, and was thus selected for the full-scale system. The full-scale TVW of 14.4 m² was installed at a terraced house in Ghent (Belgium), and was loaded with grey water at 100 L per day. Influent and effluent quality were routinely monitored by grab sampling, water savings were monitored by means of flow meters, and electricity consumption was also accounted for. The TVW was further equipped with sensors that measure temperature, Particulate Matter (PM₁₀) and CO₂ in the air. The full-scale system obtained effluent concentrations of 13 mg.L^-1 TSS, 91 mg.L^-1 COD and 5 mg.L^-1 BOD₅. Ammonium and total coliforms were removed with removal rates of 97% and 99% (2 log units) respectively. However, an increase in effluent concentration of nitrate and phosphate was observed due to leaching from the selected substrate. Available data from the temperature sensors have clearly demonstrated the additional benefit of the TVW as an insulating layer, keeping the heat outside on warmer days, and keeping the heat inside on colder days. Overall, this study demonstrated that the TVW is a sustainable system for greywater treatment and reuse.

Trends in global greywater reuse: a bibliometric analysis

The increase in water demand in recent years led to the expansion of research and public policies on the reuse of water, especially greywater (GW). Given the diversity of research in the area, this paper proposes an analysis of the evolution of the area through an objectivity metric. Metadata of 1,524 publications indexed in the Scopus database between 1974 and 2021 were analyzed using the VOSviewer tool, and showed exponential growth in publications from 2013. Six different spelling variations were found for GW in the database. Despite the highly geographical scattering of academic production, developed countries, who began researching greywater earlier, had more connections and published more papers; except for Israel, which had the highest average of citations per article. While developed countries lead the research area, developing countries are emerging in GW reuse research. These aspects reveal both the dispersion of the research structure development and a trend of intellectual production in GW from developed to developing countries. Also, we noted that countries suffering from water scarcity stood out with the highest activity in paper publishing. Thus, we expect that future research on GW reuse will take place in developing countries that face water scarcity.

Development and Assessment of a Web-Based National Spatial Data Infrastructure for Nature-Based Solutions and Their Social, Hydrological, Ecological, and Environmental Co-Benefits

Comprehensive datasets for nature-based solutions (NBS), and their diverse relationships have not yet been accumulated into a deployable format. This research describes the development of a novel National Spatial Data Infrastructure (NSDI) system for NBS co-benefits throughout the contiguous United States. Here, we gather and integrate robust geospatial datasets from the social, ecological, environmental, and hydrologic domains using seamless, cloud-based data services to facilitate the trans-disciplinary assessment of NBSs as a function of society and Earth. This research enhances practical decision making and research by assimilating web-based datasets and describing the missing links between national policy and robust adoption of NBSs as a sustainability solution. This NSDI serves to foster participatory planning capabilities and integrate local sustainability goals into decision–support frameworks. Such a platform strengthens the knowledge base necessary for addressing multiple, co-evolving issues of societal relevance, an essential component of fully espousing NBSs within the realm of socio-technological systems and improving policies and implementation regarding sustainable solutions. The efficacy of the proposed platform to serve as a holistic data information system is assessed by exploring important characteristics associated with geospatial NSDI tools, namely, openness, spatial functionality, scalability, and standardization. By placing GIS strengths and weaknesses in the context of transdisciplinary NBSs, we reveal strategic directions toward further co-production of such NSDIs. We conclude with recommendations for facilitating a shared vision of transdisciplinary technologies to strengthen the amalgamation of broad co-benefits and multi-disciplinary influences in sustainability planning.

Closing Water Cycles in the Built Environment through Nature-Based Solutions: The Contribution of Vertical Greening Systems and Green Roofs

Water in the city is typically exploited in a linear process, in which most of it is polluted, treated, and discharged; during this process, valuable nutrients are lost in the treatment process instead of being cycled back and used in urban agriculture or green space. The purpose of this paper is to advance a new paradigm to close water cycles in cities via the implementation of nature-based solutions units (NBS_u), with a particular focus on building greening elements, such as green roofs (GRs) and vertical greening systems (VGS). The hypothesis is that such “circular systems” can provide substantial ecosystem services and minimize environmental degradation. Our method is twofold: we first examine these systems from a life-cycle point of view, assessing not only the inputs of conventional and alternative materials, but the ongoing input of water that is required for irrigation. Secondly, the evapotranspiration performance of VGS in Copenhagen, Berlin, Lisbon, Rome, Istanbul, and Tel Aviv, cities with different climatic, architectural, and sociocultural contexts have been simulated using a verticalized ET0 approach, assessing rainwater runoff and greywater as irrigation resources. The water cycling performance of VGS in the mentioned cities would be sufficient at recycling 44% (Lisbon) to 100% (Berlin, Istanbul) of all accruing rainwater roof–runoff, if water shortages in dry months are bridged by greywater. Then, 27–53% of the greywater accruing in a building could be managed on its greened surface. In conclusion, we address the gaps in the current knowledge and policies identified in the different stages of analyses, such as the lack of comprehensive life cycle assessment studies that quantify the complete “water footprint” of building greening systems.

Rainwater Use for Vertical Greenery Systems: Development of a Conceptual Model for a Better Understanding of Processes and Influencing Factors

Vertical greenery systems (VGS) are promoted as a nature-based solution to mitigate the urban heat island effect. In order to ensure the long-term provision of this function, sufficiently available irrigation water is the key element. Currently, potable water is one of the main resources for irrigation of VGS. While rainwater is often mentioned as an alternative, only a few studies investigate the actual application of rainwater for irrigation. In this study a conceptual model is developed to present the processes and influencing factors for a holistic investigation of rainwater use for irrigation. In this model, five sub-modules are identified: the atmospheric, hydraulic, quality, rainwater harvesting and VGS sub-module. The conceptual model depicts which processes and influencing factors are involved in the water demand of VGS. Thus, the conceptual model supports a holistic understanding of the interrelations between the identified sub-modules and their relevance for VGS irrigation with harvested rainwater. The results of this study support the implementation of rainwater harvesting as a sustainable resource for VGS irrigation.

Nutrient leaching behavior of green roofs: Laboratory and field investigations

Despite the benefits of green roofs in managing stormwater quality, green roofs especially at their early age might leach nutrients. Research in this regard is still very limited. Therefore, this paper conducted both the laboratory and field observations to characterize and model the leaching of nutrients including nitrogen (N) and phosphorus (P) and to examine the discrepancy in knowledge produced from these two settings. The experiment revealed that the higher the initial nutrient contents of media were, the higher the degree of nutrient leaching was. The nutrient leaching from both the laboratory cells and the field green roof declined temporally, which was largely explained by the cumulative inflow. The semi-physically based nutrient leaching model generally captured the nutrient leaching from both the laboratory cells (R² in the range of 0.87-0.98) and the field green roof (R² in the range of 0.28-0.86). The mass balance analysis for the laboratory cells demonstrated that the masses of nutrients leached in outflow were 85-112% of the nutrients reduced in media in general (except P of two laboratory cells). The analysis and modeling results supported that media was the primary source for nutrients leached and the pattern of nutrient leaching was consistent with wash-off being the dominant process. The results also revealed the difference in the P leaching between the laboratory cells and the field green roof. Apart from the wash-off, other chemical and biological processes and/or nutrient sources might play non-negligible roles on the P leaching of the field green roof, implied by the relatively low performance of the models (R² of approximately 0.30 in both the regression analysis and the nutrient leaching model). The difference observed between the laboratory experiment and the field observation also calls into attention when translating knowledge derived from laboratory experiments into real practice.

Converting treatment wetlands into "treatment gardens": Use of ornamental plants for greywater treatment

Nowadays, the use of constructed wetlands for on-site greywater treatment is a very promising option. The successful application of this nature-based solution at full scale requires public acceptance, economic feasibility and the production of high-quality treated greywater. This work focuses on the use of ornamental plants as vertical flow constructed wetland (VFCW) vegetation for greywater treatment, aiming to improve aesthetic and acceptability of the system. The performance and economic feasibility of the proposed green technology were examined during a 2-years study. Results show that Pittosporum tobira and Hedera helix can grow in VFCW operating with greywater without any visible symptoms. These species tolerated both drought and flooding conditions, making them ideal for use not only in residential buildings but also in seasonal hotels and holiday homes. In contrast, partial defoliation of Polygala myrtifolia plants was observed during the winter period. High average removal efficiencies were observed for BOD (99%), COD (96%) and TSS (94%) in all examined VFCWs including unplanted beds. Phosphorus removal gradually decreased from 100% during first months of operation to 15% during second year of operation. In addition, total coliforms concentration reduced by 2.2 log units in the effluent of all planted systems, while lower removal efficiency was observed in the absence of plants. The mean concentration of BOD and TSS in the treated greywater met the standards for indoor reuse (<10 mg/L). Cost payback periods for the installation of the proposed technology in a multi-family building, a single house and a hotel in Greece were found 4.7, 16.6 and 2.5 years, respectively. Overall, the "treatment gardens" proposed in this study provide a technically and economically feasible solution for greywater treatment, with the additional benefit of improving the aesthetic of urban, semi-urban and touristic areas.

Greywater in Egypt: the sustainable future of non-conventional water resources

Egypt is a country with limited water resources. Egypt water needs are growing rapidly as a result of the population increase, climate change and development activities. The aim of the study is to analyse how Egypt can sustain its mega urban projects by utilizing greywater as a non-conventional water resource. A quantitative-based assessment has been conducted to investigate the overall evaluation of the greywater resources in Egypt. Greywater accounts for between 40 and 80% of the total water discharged from the house, and the total use of water for drinking and health use can be estimated at 10.4 billion cubic meters per year. The results show that the greywater resources may support a sustainable future of non-conventional water resources in a very positive way. Greywater can provide Egypt with about 4.15-8.30 billion cubic meters annually, which is a good support for water resources in Egypt. The paper concluded that greywater is an important resource to facilitate the success of the new mega urban projects. Egypt should maximize the share of greywater resources, especially in its new mega urban projects. Greywater resources can support Egypt and other arid and semiarid regions and countries in transition to a sustainable future.

Resolving the relative contributions of cistern and pour flushing to toilet water usage: Measurements from urban test sites in India

A challenge in water reuse for toilet flushing in India and other Asian countries derives from pour flushing practices. It is a common assumption that the amount of pour flushed water used for personal cleansing is small in comparison to the cistern flush volume, however there is a knowledge gap regarding the actual contribution of each water source to the blackwater amount. In this study, digital water meters were used to measure the fraction of water from personal wash tap relative to cistern water that is used for toilet flushing. High temporal resolution measurements were carried in three different urban sites in the city of Coimbatore in the southern Indian state of Tamil Nadu where onsite sanitation treatment prototypes that may provide reclaimed water for cistern flushing are being tested. Data collected over periods of up to 2 months show that the contribution of the cistern flush to the total blackwater volume is low (14-40%). These data highlight an important factor to inform interventions designed around water reuse for flushing in world geographies where personal toilet cleansing by water is the common practice.

Evaluation of Groundwater and Grey Water Contamination with Heavy Metals and Their Adsorptive Remediation Using Renewable Carbon from a Mixed-Waste Source

The contamination of water sources with heavy metals is a serious challenge that humanity is facing worldwide. The aim of this work was to evaluate and remediate the metal pollution in groundwater and greywater resources from Riyadh, Saudi Arabia. In addition, we investigated the application of ultrasonic power before adsorption to assess the dispersion of renewable carbon from mixed-waste sources (RC-MWS) as an adsorbent and enhance the water purification process. The renewable carbon adsorbent showed high ability to adsorb Pb(II), Zn(II), Cu(II), and Fe(II) from samples of the actual water under study. The conditions for the remediation of water polluted with heavy metals by adsorptive-separation were investigated, including the pH of the adsorption solution, the concentration of the heavy metal(s) under study, and the competition at the adsorption sites. The enhanced adsorption process exhibited the best performance at a pH of 6 and room temperature, and with a contact time of 60 min. Kinetic studies showed that the pseudo-second-order kinetic model was fitted with the adsorption of Pb(II), Zn(II), Cu(II), and Fe(II) onto the RC-MWS. The adsorption data were well fitted by Langmuir isotherms. The Freundlich isotherm was slightly fitted in the cases of Cu(II), Zn(II), and Fe(II), but not in the cases of Pb(II). The developed adsorption process was successfully applied to actual water samples, including water samples from Deria and Mozahemia and samples from clothes and car washing centers in Riyadh city.

A review of nature-based solutions for greywater treatment: Applications, hydraulic design, and environmental benefits

Recognizing greywater as a relevant secondary source of water and nutrients represents an important chance for the sustainable management of water resource. In the last two decades, many studies analysed the environmental, economic, and energetic benefits of the reuse of greywater treated by nature-based solutions (NBS). This work reviews existing case studies of traditional constructed wetlands and new integrated technologies (e.g., green roofs and green walls) for greywater treatment and reuse, with a specific focus on their treatment performance as a function of hydraulic operating parameters. The aim of this work is to understand if the application of NBS can represent a valid alternative to conventional treatment technologies, providing quantitative indications for their design. Specifically, indications concerning threshold values of hydraulic design parameters to guarantee high removal performance are suggested. Finally, the existing literature on life cycle analysis of NBS for greywater treatment has been examined, confirming the provided environmental benefits.

Performance evaluation of various individual and mixed media for greywater treatment in vertical nature-based systems

Nature-based systems (NBS) are a cost-effective, energy efficient and aesthetically pleasing approach for greywater treatment, but they are space intensive. Vertical NBS overcome this issue but must utilize lightweight media to reduce their construction costs. This study evaluates four common plant growing media: perlite, coco coir, LECA and sand, and compares them with two new media derived from local waste materials: date seeds and spent coffee grounds (SCG). The media are characterized and tested for their removal of various greywater pollutants. Further tests are conducted comparing mixtures of perlite-coco coir and date seeds-SCG. SCG was found to be an excellent media for greywater treatment, providing a similar degree of treatment as the best traditional media, coco coir and providing improved drainage. Drainage was further improved by mixing SCG with date seeds, which performed better than any mixture of perlite and coco coir. Most pollutants showed a slight deterioration in treatment performance with this mixture, although the removal of suspended solids and chemical oxygen demand was improved. An increased bed height improved the treatment performance with SCG, while increased hydraulic loading resulted in decreased treatment performance for all media. This study demonstrates the potential of date seeds and SCG as locally recycled waste materials to realize treatment of greywater in vertical NBS.

Development and application of relevance and reliability criteria for water treatment removal efficiencies of chemicals of emerging concern

With the growth in production and use of chemicals and the fact that many end up in the aquatic environment, there is an increasing need for advanced water treatment technologies that can remove chemicals of emerging concern (CECs) from water. The current lack of a homogenous approach for testing advanced water treatment technologies hampers the interpretation and evaluation of CEC removal efficiency data, and hinders informed decision making by stakeholders with regard to which treatment technology could satisfy their specific needs. Here a data evaluation framework is proposed to improve the use of current knowledge in the field of advanced water treatment technologies for drinking water and wastewater, consisting of a set of 9 relevance criteria and 51 reliability criteria. The two criteria sets underpin a thorough, unbiased and standardised method to select studies to evaluate and compare CEC removal efficiency of advanced water treatment technologies in a scientifically sound way. The relevance criteria set was applied to 244 papers on removal efficiency, of which only 20% fulfilled the criteria. The reliability criteria were applied to the remaining papers. In general these criteria were fulfilled with regards to information on the target compound, the water matrix and the treatment process conditions. However, there was a lack of information on data interpretation and statistics. In conclusion, a minority of the evaluated papers are suited for comparison across techniques, compounds and water matrixes. There is a clear need for more uniform reporting of water treatment studies for CEC removal. In the future this will benefit the selection of appropriate technologies.

Wetland Roofs as an Attractive Option for Decentralized Water Management and Air Conditioning Enhancement in Growing Cities—A Review

While constructed wetlands have become established for the decentralized treatment of wastewater and rainwater, wetland roofs have only been built in isolated cases up to now. The historical development of wetland roofs is described here on the basis of a survey of literature and patents, and the increasing interest in this ecotechnology around the world is presented. In particular, this article describes the potential for using wetland roofs and examines experience with applications in decentralized water management in urban environments and for climate regulation in buildings. Wetland roofs are suitable as a green-blue technology for the future—particularly in cities with an acute shortage of unoccupied ground-level sites—for the decentralized treatment of wastewater streams of various origins. Positive “side effects” such as nearly complete stormwater retention and the improvement of climates in buildings and their surroundings, coupled with an increase in biodiversity, make wetland roofs an ideal multi-functional technology for urban areas.

The governance mechanism of the building material industry (BMI) in transformation to green BMI: The perspective of green building

The transformation from the building material industry (BMI) to green BMI (GBMI) is not only of great significance to the upgrading of traditional BMI, but also has a pivotal role in promoting the sustainable development of cities. This paper studies the governance mechanism of the BMI in transformation to GBMI from the perspective of supply-side and demand-side of green building. First, evolutionary game theory is used to develop two three-party dynamic game models including building materials enterprises (BME), government, building developers (BD) and building consumers (BC). Second, numerical simulation experiment method is used to study the multi-stage governance mechanism based on the model deduction and the theoretical analysis of green transformation. The numerical simulation results show that the infrastructure construction projects on green building is an important governance mechanism for the rapid development of the GBMI, and the green innovation subsidy is a core governance mechanism for the high-quality development. The punishment of pollution and fraud compensation punishment and green innovation subsidy to green BME, tax incentives to green BD and purchase subsidy to green BC are conducive to promoting the change of the concept of green production and consumption. The aforementioned mechanisms and infrastructure construction contribute to promoting the production of green building materials in BME through developing and purchasing green buildings. The infrastructure construction and green innovation subsidy play a pivotal role in the high-quality development of the GBMI. This study not only reveals the theoretical mechanism of the BMI in transformation to GBMI under government-driven, but also provides a practical guidance for formulating high-quality development policies for GBMI. In addition, questionnaire method can be used to effectively solve the problem of bringing the four parties into a unified framework for future research.

Stormwater Retention and Reuse at the Residential Plot Level—Green Roof Experiment and Water Balance Computations for Long-Term Use in Cyprus

Green roofs can provide various benefits to urban areas, including stormwater retention. However, semi-arid regions are a challenging environment for green roofs as long dry weather periods are met with short but intense rainfall events. This requires green roofs to retain maximum volumes of stormwater, while being tolerant to minimal irrigation supplies. The objectives of this study are (i) to quantify the stormwater retention of two substrate mixtures with two plant species under natural rainfall; (ii) to assess the performance of two plant species under two levels of deficit irrigation; and (iii) to compute stormwater runoff reduction and reuse by green roofs and rooftop water harvesting systems for three standard residential plot types in urban Nicosia, Cyprus. A rooftop experiment was carried out between February 2016 and April 2017 and results were used to compute long-term performance. Average stormwater retention of the 16 test beds was 77% of the 371-mm rainfall. A survival rate of 88% was recorded for Euphorbia veneris and 20% for Frankenia laevis, for a 30% evapotranspiration irrigation treatment. A combination of a green roof, rainwater harvesting system and 20-m3 tank for irrigation and indoor greywater use reduced stormwater runoff by 47–53%, for the 30-year water balance computations.