From Nearly Zero Water Buildings to Urban Water Communities: The Need to Define Parameters to Support the New Paradigms

Abstract

In the context of freshwater scarcity, effective water resource management is essential. This study explores techniques to optimise the use of local water sources and promote conservation, proposing a model to balance the supply and demand of non-potable water in urban environments. The model serves as an alternative or complement to public water supplies, particularly in drought-prone regions. Through a qualitative analysis of national and international regulations, specifications, and technical standards, this research identifies key trends in the use of alternative water sources and highlights knowledge gaps in urban water management, which are addressed through the proposed model. The study emphasises the transition from Nearly Zero Water Buildings (NZWBs) to Urban Water Communities (UWCs) as a sustainable and resilient solution, integrating decentralised water management approaches and resource recovery from wastewater. Additionally, a case study in a Portuguese urbanisation area illustrates the application of these principles and assesses their potential in real-world scenarios. The findings contribute to the development of comprehensive guidelines and public policies for sustainable urban water management, supporting the implementation of decentralised and integrated solutions that enhance resilience, water security, and resource efficiency.

1. Introduction

Water plays a key role in the achievement of the 17 Sustainable Development Goals (SDGs) approved by the United Nations in 2015. Cities, the epicentre of the SDG challenges, require innovation in water management to face the consequences of climate change, rapid urbanisation, and water security challenges. In addition, urban infrastructures, characterised by complexity and dependence on centralised systems, need to adapt to rapid urbanisation. Although fresh water is a constantly renewing natural resource, its availability is limited and increasingly pressured in several regions, like the Mediterranean basin. The decrease in rainfall and the occurrence of prolonged droughts, phenomena aggravated by climate change, exacerbate the related challenges of water resource management in these areas [1]. Contemporary strategies propose local hybrid systems that integrate existing infrastructures with decentralised and flexible solutions, promoting a progressive resilience to these emerging challenges. It is therefore imperative to adopt an integrated approach that considers both centralised and decentralised systems in urban water management to promote greater sustainability and resilience in the face of evolving challenges.

An analysis of the new paradigms of the urban water cycle and the transversal dynamics that govern the sector reveals essential areas for action, such as water efficiency in the built environment, water reuse, and rainwater harvesting. Innovation in technological tools and solutions is fundamental, following the principle of the “5Rs” [2]—reduce consumption and losses/waste, reuse and recycle water, and resort to alternative sources—which can lead to significant savings in drinking water in buildings, estimated at 30% [2]. Studies carried out in different urban contexts confirm that integrating water efficiency measures with decentralised solutions can lead to significant reductions in drinking water consumption. In commercial buildings, savings of up to 20% have been recorded [3], while educational institutions have implemented strategies that have led to reductions of over 33% [4,5]. In residential contexts, the adoption of hybrid strategies—combining rainwater harvesting, greywater use, and efficient appliances—has achieved savings of between 28% and 44%, depending on the level of integration of the systems [6,7,8,9,10]. The adoption of decentralised systems is effective in reducing potable water consumption, ensuring the continued use of alternative sources, and increasing water resilience during periods of drought [11]. This evidence, observed in various geographical and climatic contexts, reinforces the feasibility of achieving and even exceeding the estimated 30% reduction through the combined application of efficiency measures and alternative water sources.

Active leakage control is essential to improve the efficiency and performance of existing networks, while the digital transition, with technologies such as telemetry in meters and monitoring systems, allows for the rapid detection of problems and the effective implementation of corrective measures [12]. The use of treated wastewater offers an alternative source of water, with advanced technologies that transform it into regenerated water for various uses, with examples of application in Portugal, Singapore and Namibia [13,14], while also enabling the recovery and valorisation of by-products such as biogas, bioplastics and bionutrients [15]. The Lisbon Strategic Plan for Water Reuse aims to reduce the city council’s drinking water consumption by around 75% by 2025 through the implementation of a new recycled water network, including partnerships with large private consumers [13]. However, this centralised approach must be complemented by local solutions to strengthen water security at the community level. Integrated resource management (water, energy, and nutrients) in urban areas has benefits for sustainable development and food security [16,17]. Managing rainwater through nature-based solutions (NbSs) emphasises its multifunctionality compared to conventional structures [18,19]. These solutions help to naturally retain water, reduce pollution of watercourses, recharge aquifers, and improve urban aesthetics and biodiversity [18].

The integration of resource recovery practices throughout the water cycle is fundamental to sustainability and is emerging as a promising practice for UWCs and NZWBs, alongside the innovations already recognised in the energy sector. Inspired by the success of nearly zero energy buildings, NZWBs represent an innovative approach that promotes water efficiency not only in individual buildings but also as a basis for the development of UWCs. This transition from the concept of NZWBs to UWCs (with the management of surpluses and deficits involving buildings and public systems) not only amplifies the benefits but also establishes a crucial collective approach in urban contexts with high water demand. In an ideal scenario, NZWBs utilise alternative sources and treat all wastewater locally, forming a large closed loop [20]. In a more conventional model, buildings offset freshwater consumption with alternative sources and return the water to its original source through green infrastructure [20]. The development of specific regulations and technical-sanitary certification systems is essential to guarantee the quality and safety of NZWB and UWC practices, thus facilitating their widespread implementation.

The implementation of practices that promote the circularity of water in the urban environment requires the creation of robust technical regulations. Environmental certifications, such as BREEAM (Building Research Establishment Environmental Assessment Method) Communities [21], DGNB (German Sustainable Building Council) Districts [22], and LiderA (Lead for the Environment in Search of Sustainable Construction) [23], play an important role in validating these practices. Despite being multi-dimensional, these tools still carry little weight when it comes to assessing water efficiency, ranging from 3.2% to 9.5% of the total weighting. Strengthening these criteria could boost the transition to water-sensitive urban communities, especially if combined with financial and regulatory incentives.

However, the implementation of practices that promote the circularity of water in the urban environment reveals significant knowledge gaps. Most studies and standards focus on isolated solutions for buildings [5,24,25,26,27] or for centralised urban systems [15,28,29,30], without considering the interface needed for the transition to water-sensitive urban communities. The model developed in this study seeks to respond to this gap by integrating the requirements applicable to NZWBs and expanding them to a community approach. It is essential to carry out a detailed characterisation of consumption in the urban sector to determine the minimum quantities required for each use and to address the water quality requirements for each specific use, considering consumption habits. Rainwater harvesting and greywater use, for example, are promising practices for NZWBs and UWCs, but in many countries, such as Portugal, they lack defined regulations, guidelines, and quality standards [1]. This study aims to analyse national and international technical regulations on water management in buildings and communities (micro and meso scale, according to [19]), with special attention to local alternative water sources, in particular rainwater harvesting and greywater use. The aim is to develop a model that balances the supply and demand alternatives of water for non-potable domestic uses as an alternative or complement to the public water supply. Based on the knowledge acquired, a preliminary study of the application of these practices was carried out in an urbanisation area in central Portugal, specifically in Leiria. This research intends to contribute to the development of more comprehensive guidelines, fill the knowledge gaps, and provide guidance for public policies and private practices that promote water sustainability in urban contexts, especially in regions susceptible to prolonged droughts, such as the Mediterranean region.

The selection of the regulations analysed is based on the representativeness and pioneering spirit of the countries studied. Singapore is a benchmark in efficient and circular water management, having achieved near self-sufficiency through strategic planning and the use of innovative technologies [31,32]. The United Kingdom stands out for its investment in decentralised solutions and innovative strategies for urban water management [18,33]. Portugal is the main subject of the study and is a benchmark in the labelling and certification of water efficiency in buildings [2,34,35], as well as showing advances in public water services [15,36,37]. Brazil was included due to the similarities with Portugal and the shared challenges in implementing public policies for water reuse and water security [38,39]. To this end, an applied research study was carried out with a qualitative approach to textual elements of regulations, technical standards, and legal diplomas in force, identifying gaps and opportunities for the creation of guidelines that can serve as a reference for future public policies and specific regulations. This analysis was carried out using content analysis techniques [40]. By providing a structured analysis of existing regulations, this research seeks to contribute to the harmonisation of technical standards, ensuring that decentralised and integrated water solutions can be effectively implemented in different urban contexts.

This article is structured as follows: After the Introduction, Section 2 presents the concept and importance of integrated and decentralised urban water management, introducing sustainable solutions and strategies through a resilient model that balances the supply and demand of non-potable water. Section 3 explores the potential of wastewater resources, highlighting nutrient and energy recovery practices to promote circularity in water use. Section 4 presents a case study of an urbanisation area in Portugal, applying the principles analysed to demonstrate the potential of the approach in real-life scenarios. Finally, Section 5 presents a discussion of the results and conclusions of the study, reinforcing the importance of innovative and sustainable approaches to urban water management. The main contributions of the research are summarised, with an emphasis on the transition to UWCs and NZWBs, as well as indicating future directions for the implementation of the solutions analysed.

2. Integrated and Decentralised Urban Water Management: A Resilient Approach

It is essential to target the right type of water for each use, guaranteeing the quality required for its intended purpose (fit-for-purpose) [41]. This concept refers to the treatment of water to a quality level appropriate for its intended use, ensuring efficiency without unnecessary overtreatment. Among the functions that normally require water in the daily lives of different types of consumer activities, various demands do not require water with potability standards, competing with more noble uses, such as human consumption. For example, the average daily consumption of water per inhabitant in Portugal is approximately 195 L [42], while only 20 L are considered the minimum necessary to meet the World Health Organisation (WHO) guidelines for drinking water—essential for human health and hygiene [43]. This emphasises the importance of conserving and using water resources efficiently.

Integrated and decentralised urban water management aims to respond to these challenges by promoting a holistic vision that prioritises resilience. Based on the optimisation and reuse of water resources, this approach aims to develop a model that balances water supply and demand alternatives for non-potable uses, considering integrated water efficiency solutions, circular economy, and natural processes. Several sustainable solutions and technologies are available, such as NbSs, which support decentralised water treatment and reuse; compact greywater treatment or regeneration systems for residential use, which enable significant savings in drinking water; and, at the limit, water desalination, which is effective in regions near the sea with limited access to freshwater. The methodology proposed for this management is shown in Figure 1, which promotes circularity and efficiency at different scales, from individual buildings to urban communities, managing water sustainably from abstraction to its reintroduction into the hydrological cycle. Demand management includes strategies to promote water efficiency in the built environment, such as reducing consumption, controlling losses, using efficient devices, and raising user awareness. In addition, segmenting consumption according to its purpose contributes to more effective management of the resource, as highlighted in

Table 1. Waste segregation and possible utilisation options [44]. Different categories of waste segregation and their potential uses, highlighting opportunities for recovering resources from wastewater and rainwater in the urban water cycle.

Substances	Blackwater: Urine	Blackwater: Faeces	Greywater	Rainwater
Treatment	Hygienisation by storage or drying	Anaerobic digestion, drying, composting	Constructed wetlands, gardening, wastewater ponds, biological treatment, membrane technology	Filtration, biological treatment
Utilisation	Liquid or dry fertiliser	Biogas, soil improvement	Irrigation, groundwater recharge or direct reuse	Water supply, groundwater recharge

. Water supply management is also an important part of the process, involving the use of alternative sources, such as rainwater and groundwater, and the use of treated wastewater. These alternative sources help to reduce dependence on drinking water and promote the sustainability of water supplies. This approach aims to optimise the water cycle, maximise the use of resources, and minimise environmental impact while protecting public health. In short, the proposed resilient approach is not only a response to current challenges, but also a vision for the future, ensuring the adaptation and sustainability of urban water infrastructures.

Table 1 provides a succinct analysis of the possible options for utilising the different types of segregated water. The separation of flows such as urine, faeces, greywater and rainwater offers opportunities for efficient and economical resource recovery, avoiding unnecessary investment in treatment. This approach is particularly attractive for decentralised solutions, as they are close to the source of the effluent. This approach should be considered whenever possible, especially for stormwater and greywater, which provide less complex and costly treatment options, achieving higher levels of efficiency without compromising public health or causing an unacceptable environmental impact.

2.1. Water Balance of Buildings

The water balance is based on the building’s water characterisation, which covers water consumption and the generation of effluents from various consuming activities. It is drawn up specifically for each case, taking into account typology, population, occupancy, user behaviour, location, and water-consuming equipment. The aim is to identify supply and demand in the building, assisting in technical and economic feasibility studies aimed at suggesting efficient actions for water management [45,46].

Before assessing the balance between supply and demand, it is crucial to identify the sources of water available for utilisation and reuse, as well as the potential end uses for treated and/or regenerated water [46]. This makes it possible to diversify the building’s supply matrix and reduce potable water consumption in uses where potability is not essential [45].

In cases involving buildings or urban communities with representative populations, average estimates can be adopted for sizing [46]. If water efficiency measures are being implemented in conjunction with water reuse and utilisation, such as low-flow taps and low-flush or dual-flush toilets, the water balance should take into account the lower consumption rates of these water-efficient fixtures and fittings. This can be calculated using a water efficiency calculator [46], according to available manuals [2,47].

2.2. Greywater

Greywater is domestic wastewater that does not contain faecal matter, i.e., it is the water that constitutes the non-sanitary component of wastewater. It usually comes from bathtubs, showers, wash basins, laundry, and kitchens [48,49,50]. Its characteristics vary according to its source, and it requires different levels of treatment depending on its intended use. Greywater systems can therefore be quite diverse in terms of complexity and scale. Domestic wastewater with a low concentration of pollutants, such as that from baths, showers, and washbasins, can be considered for utilisation [48,49]. However, those coming from sinks and dishwashers are generally excluded due to the high levels of organic matter, fats, oils, and detergents, which require more complex and costly treatment.

The British standard BS EN 16941-2 [49] establishes a hierarchy for the supply and demand of greywater, prioritising the preferential use and collection of this water. For example, toilet flushing is the priority use, followed by external use without a sprayer, laundry, and external use with a sprayer. To facilitate treatment, the collection is prioritised according to source, starting with baths and showers, followed by wash basins, clothes washers, and, lastly, kitchen sinks and/or dishwashers [49].

As a calculation method, there are two main approaches to estimating greywater supply and demand: the simplified approach and the detailed approach. The simplified approach (

Table 2. Average daily supply and demand—greywater: simplified approach [49].

Occupancy	Supply (1)	Demand
		Toilet (WC)	Laundry (2)	Other Non-Potable Uses (3)
1 person	60	35	15	10

⁽¹⁾ Yield from showers, baths, and/or wash basins. ⁽²⁾ These figures are based on average daily demand. Note that a clothes washer normally uses 30 L to 60 L per cycle. ⁽³⁾ For example, garden watering.

) applies to residential buildings and is based on a balance of assumptions such as relatively constant daily demand per person for flushing toilets and washing clothes and constant daily greywater production per person [49].

In Singapore, greywater from bathrooms (bathtub, wash basin, and shower) accounts for around 40% of the total volume of usable greywater, while laundry water contributes around 20% [50]. In Portugal, according to ANQIP ETA 0905, 2023, in the absence of specific studies, in new or refurbished buildings that have water-efficient devices (e.g., ANQIP label “A” classification or European unified label green), average water consumption can be estimated at around 110 L per day per inhabitant, with greywater production of approximately 70 L per day per inhabitant (

Table 3. Water balance in residential buildings with efficient devices (average values in litres per inhabitant per day) [48].

Source of Water Used	Water Uses	Wastewater Produced	Water Destination
52 L of drinking quality water	40 L for showers, bathtubs, and wash basins	70 L of greywater	40 (to 58) L of regenerated greywater
	12 L for the kitchen
			12 (to 22) L of greywater discharged
40 (to 58) L of regenerated water	5 L for cleaning
	13 L for clothes washer
	35 L for flushing toilets	35 L of blackwater	35 L of blackwater discharged
	5 L for watering	-	Soil infiltration

).

When analysing the different standards, there is consistency and similarities in both the supply and demand of domestic greywater. For example, in the Portuguese context (Table 3), around 57% of the total volume of greywater produced comes from bathroom water, while 19% comes from clothes washers. These figures can be compared with Singaporean standards, showing a certain difference in bathroom consumption, which can be attributed to local consumption habits. It should be noted that the quantity and origin of greywater can vary considerably depending on the nature and activity of the buildings, as well as the consumption habits of the occupants [48,50]. These factors directly influence greywater production and use patterns in residential and commercial premises.

Another analysis to be made is concerning the 60 L of greywater supplied (Table 2), which are compared to the average values of 40 to 58 L of regenerated water specified in Table 3. For the specific uses, it is possible to compare that 35 L are for bathrooms, confirming the Portuguese specification of 35 L for flushing toilets, while 15 L are for laundry, very similar to the 13 L for clothes washers. Finally, the other consumptions mention 10 L for other non-potable uses, very similar to considering 5 L for cleaning and 5 L for watering. By comparing the values attributed to different greywater sources and their uses, it is possible to identify patterns and trends that can guide sustainable water management policies and practices in urban communities.

The detailed approach assumes that supply and demand are constant, excluding peaks in utilisation. This approach can be used for residential, commercial, industrial, or public installations [49]. The detailed approach considers specific variations in water consumption behaviour, offering a more accurate analysis of water management in different types of installations.

Greywater supply: to calculate production (yield), expression (1) is used [49]:

$$Y_G=n\times \left(Q_S\times t_S\times u_S+V_{BT}\times u_{BT}+Q_{HWB}\times t_{HWB}\times u_{HWB}+V_{WM}\times u_{WM}+Q_{KS}\times t_{KS}\times u_{KS}+V_{DW}\times u_{DW}\right)$$

(1)

where

YG	is the yield/supply of greywater in litres per day (L/d);
n	is the number of persons (p);
QS	is the volume flow of the shower in litres per minute (L/min);
tS	is the duration per usage of the shower in minutes (min);
uS	is the shower usage rate per person per day (1/(p⋅d));
VBT	is the volume of water per usage of the bathtub in litres (L)—not maximum filling volume;
uBT	is the usage rate of the bathtub per person per day (1/(p⋅d));
QHWB	is the volume flow of wash basin in litres per minute (L/min);
tHWB	is the duration per usage of wash basin in minutes (min);
uHWB	is the handwash usage rate per person per day (1/(p⋅d));
VWM	is the volume of water for clothes washer per operation cycle in litres (L);
uWM	is the number of operation cycles of clothes washer per person per day (1/(p⋅d));
QKS	is the volume flow of taps (warm and cold water) at the kitchen sink in litres per minute (L/min);
tKS	is the duration per usage of kitchen sink in minutes (min);
uKS	is the usage rate of tap at the kitchen sink per person per day (1/(p⋅d));
VDW	is the volume of water for dishwater per operation cycle in litres (L);
uDW	is the operation cycles of dishwasher per person per day (1/(p⋅d)).

Greywater demand: to calculate demand, expression (2) is used [49]:

$$D_G=n\times \left(V_T\times u_T+V_U\times u_U+V_{WM}\times u_{WM}\right)+V_{misc}$$

(2)

where

DG	is the demand of greywater in litres per day (L/d);
n	is the number of persons (p);
VT	is the volume of water for WC flushing per flush in litres (L);
uT	is the usage rate of WC per person and day (1/(p⋅d));
VU	is the volume of water for urinal flushing per flush in litres (L);
uU	is the usage rate of urinal per person and day (1/(p⋅d));
VWM	is the volume of water for clothes washer per operation cycle in litres (L);
uWM	is the operation cycles of clothes washer per person and day (1/(p⋅d));
Vmisc	is the volume of water for other purposes (e.g., garden watering, cleaning) in litres per day (L/d).

2.3. Rainwater

Alternative options to conventional drainage, such as rainwater harvesting and infiltration, with decentralised detention, not only reduce the demand for drinking water and water discharge but also maintain the natural water cycle [51]. The use of rainwater covers a variety of applications, from toilet flushing and washing in clothes washers to general cleaning and watering of green areas, as well as other uses that do not require drinking water (e.g., cooling towers, fire networks, air conditioning in buildings) [52], in urban communities and public spaces such as streets, parks, golf courses, and car parks [51].

However, it is crucial to consider several factors that affect both the quality and quantity of the water collected. These include the local rainfall pattern, collection area, surface drainage materials and characteristics, the sizing and material of pipework systems, pollution levels in the collection area, and the risk of contamination of the system [51].

To prevent the deterioration of water quality in the storage reservoir, it is recommended to install devices such as grids and screens to remove unwanted solids, which should be retained and/or diverted. It is also recommended that the first waters are not used, and it is preferable to install a device in the system to divert the initial runoff (first flush), preferably automatically [52,53]. If this is not sufficient to meet minimum quality standards, additional treatment using physical and chemical solutions may be necessary [53].

Rainwater supply: to determine the volume of rainwater to be used, expression (3) is used [51,52,53]:

$$Y_R=A\times P\times C\times \eta$$

(3)

where

YR	is the rainwater yield/supply per time interval, expressed in litres (L);
A	is the horizontal projection of the collection area/surface, expressed in square metres (m2); do not consider surfaces that are in contact with polluting sources;
P	is the total rainfall height for a given time interval, expressed in millimetres (mm); consider rainfall studies with historical rainfall series corresponding to periods of no less than 10 years;
C	is the surface runoff coefficient (ratio between the volume collected and the total volume of precipitation);
η	is the hydraulic efficiency coefficient of the filtration.

Rainwater demand:

Table 4. Unit and annual consumption by device or use [52].

Device or Use			Unit Consumption	Estimated Monthly or Annual Consumption
Flushing toilets (category “A”) (1) in residences			24 L/(person.day)	720 L/(person.month) 8800 L/(person.year)
Flushing toilets (category “A”) (1) in-service buildings (offices and others)			12 L/(person.day)	360 L/(person.month) 4400 L/(person.year)
Laundry (category “A” machine) (2)			10 L/(person.day)	300 L/(person.month) 3600 L/(person.year)
General Cleaning (3)		Floor washing	5 L/m2	100 L/(person.month) 1000 L/(person.year)
		Car washing (self-service)	50 L/car
Watering green areas (figures for average years) (4)	Total values: April to September	Lawns (5)	-	450 to 800 L/m2
		Gardens (6)	-	60 to 400 L/m2
	Maximum values (per day): Summer	Lawns (5)	5 to 7 L/m2	-
		Gardens (6)	1.5 to 5 L/m2	-

⁽¹⁾ Flushing toilet 6 L with dual flush. ⁽²⁾ Machine consuming 9 to 12 L per kg. ⁽³⁾ This is a rough estimate for homes, as the overall figure can vary significantly. ⁽⁴⁾ In mainland Portugal, the irrigation of green areas in rainwater harvesting systems is considered to be limited in time, as the highest irrigation demand occurs during prolonged dry periods. Therefore, the totals should not exceed half-yearly average estimates. In addition, there is a current trend towards gardens that do not require irrigation. In the Autonomous Regions, irrigation needs may be lower or non-existent. ⁽⁵⁾ Depending on the type of lawn, the type of soil, and the area of the country. ⁽⁶⁾ Depending on the type of crops, the type of soil, and the area of the country (considering a mix of lawns and shrub areas).

shows indicative consumption values by type of device or use, which can be taken into account when sizing systems. These values were estimated based on devices classified as efficient [52].

2.4. Quality and Treatment of Water for Non-Potable Reuse

Water reuse has evolved globally, covering a variety of uses, from agricultural, industrial, urban, and even potable. Treated wastewater is seen as an alternative source of water for various purposes. However, it is essential to ensure that potential adverse effects on health and the environment are minimised, which requires a strict definition of quality standards [1]. According to Decree-Law no. 119, 2019, which establishes the legal regime for the production and use of regenerated water from treated wastewater in an urban context, its reuse includes recreational applications, landscaping, street washing, firefighting, cooling, flushing toilets, and washing cars. On the other hand, ANQIP ETA 0905, 2023 defines technical criteria for building systems that use greywater, allowing it to be used for watering gardens, flushing toilets, washing clothes, and washing cars after appropriate treatment.

The quality of the water and the treatment required are determined by the end use, the likelihood of human contact, and the level of risk involved. Risk management should include treatment of reclaimed water, maintenance of systems, sampling for monitoring, control of system condition, and identification of pipes supplying water for reuse [46]. Greywater treatment systems vary in complexity and size, depending on the characteristics of the water collected and the quality requirements for the intended application. They can include one or more of the following stages [49]: (a) sedimentation/flotation: use of settling tanks to remove solid particles; (b) screening: filtering of large particles; (c) fine mechanical filtration: membrane filtration to remove fine particles; (d) biological treatment: aeration to promote the biological decomposition of organic matter; (e) chemical treatment: use of chemical agents, such as chlorine, for precipitation and disinfection; and (f) disinfection by ultraviolet radiation: elimination of pathogenic microorganisms. An effective greywater treatment system must include biological and filtration stages and disinfection processes to guarantee stable final quality. The installation of chlorine dosing in the storage tank is recommended to ensure that the treated water is free of pathogenic microorganisms and that the residual chlorine is maintained throughout the storage period [50], thus guaranteeing the quality and safety of the treated water. Treatment systems, which can be direct, short retention, basic physicochemical, biological, or biomechanical, are selected based on the specific needs of each application and considerations of sustainability and environmental impact [49].

In addition to greywater treatment, it is crucial to pay attention to rainwater utilisation systems, considering their purification and treatment. According to the technical specification ANQIP ETA 0701, 2022, in Portugal, these treatments include the following: (a) basic treatments by filtration (in the upstream filter) and by sedimentation and flotation (in the cistern), providing initial purification; (b) for activities such as watering green areas, washing pavements, and flushing toilets, rainwater may not require any additional treatment; (c) in cases of suspected or detected microbiological contamination, disinfection with appropriate treatment is recommended; and (d) when the catchment area includes polluted zones, such as vehicle traffic areas, appropriate supplementary treatments such as flocculation and/or disinfection should be considered.

The following tables show the regenerated water quality standards for different uses and appropriate treatment levels (

Table 5. Non-potable water quality standards. Comparison of different applications—part 1 (source: authors).

Use/Application Parameters		Firefighting	Cooling		Flushing Toilets			Landscaping/Watering Gardens
Regulations/Standards		DL 119/19 (1)	DL 119/19	PUB (2)	DL 119/19 (1)	ETA 0905	BS 16941-2	DL 119/19	ETA 0905 (3)	BS 16941-2 (4)
General and Quality	pH	6.0–9.0	6.5–8.5 (5)	6.0–9.0	6.0–9.0	6.0–9.0 (MAV)	5.0–9.5	6.0–9.0	6.0–9.0 (MRV)	5.0–9.5
	Turbidity (NTU)	≤5		<2	≤5	≤5 (MAV) ≤2 (MRV)	<10	≤5	≤5 (MAV) ≤2 (MRV)
	Colour (Hazen Units)			<15			(6)			(6)
	BOD5: Biochemical Oxygen Demand (mg/L O2)	≤25	≤25	<5	≤25	≤25 (MRV)		≤25	≤10 (MRV)
	Standard Plate Count/Heterotrophic Plate Count (CFU/mL)			<500
Organics and Nutrients	Ammoniacal Nitrogen (mg NH4+/L)		≤5 ≤1 (7)		≤10	≤10 (MRV)		≤5	≤10 (MRV)
	Total Nitrogen (mg N/L)								≤15 (MRV)
	Ptotal: Total Phosphorus (mg/L)							≤2 (8)	≤5 (MRV)
	Total Suspended Solids (mg/L)								≤10 (MAV)
Microbiological	Escherichia coli (CFU/100 mL)	≤10	≤200		≤10	≤250 (MAV) ≤10 (MRV)	≤250	≤10	≤200 (MAV) ≤10 (MRV)	≤250
	Total Coliforms (CFU/100 mL)			<10		≤103 (MRV)	≤103		≤104 (MRV)	≤103
	Enterococc (CFU/100 mL)					≤100 (MAV)	≤100		≤100 (MAV)	≤100
	Enteric Parasites (ova/10 L)								≤1 (MRV)
Disinfection	Residual Chlorine (mg/L)			0.5–2.0		≤2 (MAV;9)	<2		≤0.5 (MAV;9)	<0.5
	Residual Bromine (mg/L)						<5			0
Biological Risk	Legionella spp. (CFU/100 mL)			≤103					≤103 (MAV;10)

^(MAV) Maximum Acceptable Value; ^(MRV) Maximum Recommended Value; ^(DL) Decree-Law; ^(PUB) Public Utilities Board; ^(ETA) Technical Specification; ^(BS) British Standard. ⁽¹⁾ For high-pressure uses and any other routes of exposure by unintentional ingestion, water quality must be similar to that required for irrigation, as established by DL119/2019. ⁽²⁾ Cooling tower make-up only. ⁽³⁾ Applies to private garden irrigation, and it is recommended as an additional parameter that Salmonellae should be undetectable. ⁽⁴⁾ Non-spray applications. ⁽⁵⁾ Microbial growth can occur at values above or below this pH range. ⁽⁶⁾ Treated greywater should be visually clear, free of floating debris, and not objectionable in colour for all uses. ⁽⁷⁾ In the presence of copper. ⁽⁸⁾ When used in places subject to eutrophication (e.g., urban lakes, fountains). ⁽⁹⁾ When chlorine is used in the greywater regeneration process. ⁽¹⁰⁾ When there is a risk of aerosol formation (sprayers, sprinklers, and others).

,

Table 6. Non-potable water quality standards. Comparison of different applications—part 2 (source: authors).

Use/Application Parameters		General Washing					Non-Potable
Regulations/Standards		DL119/19 (street washing; 1)	DL119/19 (car washing; 1; 2)	ETA 0905 (3)	BS 16941-2 (laundry; 4)	BS16941-2 (pressure washing; 5)	PUB (6)	ABNT 16783 (7)
General and Quality	pH	6.0–9.0	6.0–9.0	6.0–9.0 (MAV)	5.0–9.5	5.0–9.5	6.0–9.0	6.0–9.0
	Turbidity (NTU)		≤5	≤5 (MAV) ≤2 (MRV)	<10	<10	<2	≤5
	Colour (Hazen Units)				(8)	(8)	<15
	BOD5: Biochemical Oxygen Demand (mg/L O2)	≤25					<5	≤20
Organics and Nutrients	Total Organic Carbon (mg/L)							<4 (9)
	Total Suspended Solids (mg/L)			≤10 (MRV)				≤2000
Microbiological	Escherichia coli (CFU/100 mL)		≤10	Not detected (MAV)	Not detected	Not detected		≤200
	Total Coliforms (CFU/100 mL)			≤10 (MRV)	≤10	≤10	<10
	Enterococc (CFU/100 mL)			Not detected (MAV)	Not detected	Not detected
	Pseudomonas aeruginosa (CFU/100 mL)			≤1 (MRV)
	Enteric Parasites (ova/10 L)			≤1 (MRV)
Disinfection	Residual Chlorine (mg/L)			≤2 (MAV; 10)	<2	<2	0.5–2.0	0.5–5.0 (MAV) 0.5–2.0 (MRV)
	Residual Bromine (mg/L)				<5	0
Biological Risk	Legionella spp. (CFU/100 mL)			≤103 (MAV; 11)		≤10

^(MAV) Maximum Acceptable Value; ^(MRV) Maximum Recommended Value; ^(DL) Decree-Law; ^(PUB) Public Utilities Board; ^(ETA) Technical Specification; ^(BS) British Standard; ^(ABNT) Brazilian National Standards Organization. ⁽¹⁾ For high-pressure uses and any other routes of exposure by unintentional ingestion, water quality must be similar to that required for irrigation, as established by DL119/2019. ⁽²⁾ Depending on the specific application of the water for reuse, some metals and ionic compounds such as iron, manganese, chlorides, sulphates, alkalinity, and silica may be controlled to minimise the occurrence of calcification or corrosion of water storage and distribution systems. ⁽³⁾ Applies to washing clothes (no lower than 55 °C) and car washing. ⁽⁴⁾ Application for non-spray applications. ⁽⁵⁾ Pressure washing, use of garden sprinklers, and car washing. ⁽⁶⁾ Applies to toilet flushing, general washing and irrigation, with the restriction that they are not allowed for high-pressure jet washing, irrigation sprinklers, and general washing in markets and food establishments to minimise risks and public health concerns. ⁽⁷⁾ Applies to flushing toilets and urinals, washing roads, patios, garages, outdoor areas and vehicles, irrigation for landscaping purposes, ornamental purposes, water cooling systems and roof cooling. ⁽⁸⁾ Treated greywater should be visually clear, free of floating debris, and not objectionable in colour for all uses. Colour is particularly relevant for washing machine use. ⁽⁹⁾ Only for water from lowering the water table. ⁽¹⁰⁾ When chlorine is used in the greywater regeneration process. ⁽¹¹⁾ This parameter applies only to car washing. When there is a risk of aerosol formation (sprayers, sprinklers, and others).

and

Table 7. Levels and types of treatment suitable for each use [1,41].

Uses Applications	Ecosystem Support	Firefighting	Cooling	Flushing Toilets	Recreation and Landscaping	Irrigation	Washing (Streets and Cars)
Treatment Level	Secondary or more advanced than secondary (1)	More advanced than secondary	More advanced than secondary	More advanced than secondary	More advanced than secondary	More advanced than secondary (disinfection)	More advanced than secondary
Type of Treatment	Secondary treatment (2) and possible disinfection (3) and/or removal of N and P	Secondary treatment (2), filtration (4) (e.g., membrane filtration) and disinfection (3) (advanced treatment systems)	Secondary treatment (2), filtration (4) (e.g., membrane filtration) and disinfection (3) (advanced treatment systems)		Secondary treatment (2), filtration (4) (e.g., membrane filtration) and disinfection (3) (advanced treatment systems)	Secondary treatment (2), filtration (4) (e.g., membrane filtration) and disinfection (3) (advanced treatment systems)

⁽¹⁾ Depends on the status of the body of water and its classification under Decree-Law no. 152/97, of 19 June, as amended by Decree-Laws no. 348/98, of 9 November, no. 149/2004, of 22 June, no. 198/2008, of 8 October, and no. 133/2015, of 13 July. ⁽²⁾ Conventional secondary treatment. ⁽³⁾ Disinfection includes UV radiation, ozonation, membrane processes, chlorination (only admissible to maintain residual disinfectant content), or other advanced oxidation processes. In the case of chlorination, the chlorine dosage should preferably be set based on chlorine deficiency to minimise the formation of by-products. ⁽⁴⁾ Filtration refers to microfiltration, ultrafiltration, cartridge filtration, high-performance sand filtration, membrane processes (including membrane reactors), the use of double filter media, and the use of textile and disc filters (with or without the addition of chemicals).

). By implementing appropriate treatment systems, it is possible to ensure the safe and efficient use of grey and rainwater for non-potable purposes, contributing to the sustainable management of water resources. The comparative analysis presented in Table 5 and Table 6 shows the differences in quality requirements for different contexts, including Portuguese (urban use and in buildings), British, Singaporean, and Brazilian regulations, making it easier to analyse national regulations. It is important to stress that infiltration into the ground or direct discharge into water bodies requires a licence from the relevant river basin district administration, under current legislation, for both reclaimed and non-reclaimed water [48]. Ecosystem support is assessed on a case-by-case basis, taking into account the ecological status and the respective support parameters, as established in Decree-Law no. 119, 2019.

The Portuguese Decree-Law no. 119, 2019 and ANQIP ETA 0905, 2023 are generally well harmonised with the Brazilian [45], the United Kingdom [49], and Singapore [50] legal specifications in many key aspects of non-potable water quality. However, differences in specific limits, such as turbidity and BOD₅, reflect different regulatory priorities and local contexts of water use. In addition, parameters such as ammoniacal nitrogen, total nitrogen, and total phosphorus are only mentioned in national regulations, whereas criteria such as Pseudomonas aeruginosa, and enteric parasites are specific requirements of ANQIP ETA 0905, 2023. Similarly, parameters such as standard plate count/heterotrophic plate count, total organic carbon, colour, and residual bromine are international requirements. This emphasises that when implementing policies or projects involving non-potable water, it is crucial to consider not only the numerical values of the parameters but also the specific guidelines and application contexts of each regulation to ensure compliance and adequate safety for end users and the environment.

When analysing the different uses of non-potable water, it can be seen that the Portuguese regulations [41,48] establish quality requirements for all the uses studied. This indicates that national regulations take a similar approach to the other countries surveyed, ensuring the safe and efficient use of water for different purposes. However, there are differences in the specific criteria of each regulation, which should be considered when implementing policies and projects involving the use of non-potable water in different contexts and specific applications.

Table 7 shows the levels and types of treatment suitable for each urban use, in accordance with the quality standards for water reuse established by Decree-Law no. 119, 2019 in Portugal.

This analysis shows that national and international regulations provide a solid basis for the development of guidelines that promote the sustainable management of non-potable water, in line with balancing supply and demand in urban contexts, intending to promote the UWCs and NZWBs.

3. Resource Potential—Wastewater

Recovering resources throughout the water cycle is crucial for sustainability, reducing costs, energy consumption, and emissions. Technological advances make it possible to recover nutrients, such as nitrogen and phosphorus, and to produce biogas from by-products of wastewater treatment and organic solid waste, with examples at the local level [54,55] and in centralised systems [15,55]. Figure 2 synthesises this transition, demonstrating how wastewater evolves from being a simple waste or cost to becoming a valuable resource, highlighting the increasing potential for recovery across the reuse value chain.

The advantages include energy recovery since wastewater contains chemical, thermal, and hydraulic energy. The chemical energy in domestic wastewater, which comes mainly from organic matter, is estimated at 1.5 kWh/m³, with anaerobic digestion being the most widely used technology for converting this organic matter into biogas. On the other hand, thermal energy, although less exploited, is significant, with the potential to extract approximately 5.8 kWh/m³ by reducing the temperature of wastewater by 5 °C [56].

Another advantage of utilising wastewater in the urban water cycle is the recovery of organic matter and nutrients such as nitrogen. The direct use of NH₄⁺ from wastewater in the production of protein for animal feed can have a significant impact on reducing carbon emissions since producing one kilogram of nitrogen as fertiliser requires two litres of fossil fuel [56]. Projects such as “Power to Protein” (https://www.powertoprotein.eu/about/ (accessed on 11 April 2024)) are committed to developing processes to produce protein from the ammonia that comes from anaerobic digesters, forming part of the circular economy and focusing on recovering and valorising ammonium from the wastewater cycle.

Phosphorus, although not a renewable resource, can be recovered from wastewater and sludge, making it a viable alternative for fertilisation. It is estimated that 22% of global phosphorus needs can be met by recycling domestic wastewater [56,57].

Urine, which accounts for approximately half of the phosphorus load in domestic wastewater, represents a highly concentrated nutrient stream that can be efficiently recovered through source separation technologies [55]. With the average adult excreting approximately 1 g of phosphorus per day through urine [57], these systems offer a sustainable and locally adaptable solution for nutrient recovery in urban environments. Recovered phosphorus can be used in urban agriculture, fertilising public gardens and green roofs. This practice not only contributes to food security by providing an essential fertiliser but also reinforces the fundamental link of the water-nutrient nexus, essential for a sustainable circular economy and the future of cities.

Ensuring adequate supplies of food, energy, and water faces significant challenges globally. Instead, local, adapted, and context-specific approaches are needed. Integrating these processes is key to promoting environmental and economic sustainability, as well as improving the quality of urban life. The relationship between water, energy, and nutrients plays an essential role in the sustainable development of the built environment. In urban environments, this integration should be seen as local resource management. Urban systems, including infrastructures and buildings, must adopt models that facilitate integrated management of energy and locally produced or recovered resources.

4. A Case Study

Located in the centre of Portugal, about 150 km north of Lisbon, Leiria is of great economic, environmental and social importance and plays a strategic role in the national context of sustainable development. The geographical location of the region is a pertinent factor in understanding its vulnerability to climate change [58,59,60]. The city partly extends over the floodplains of the Lis and Lena rivers, which exposes it to flooding and erosion risks [58,59], while its proximity to the coast exposes it to saltwater intrusion and the effects of rising sea levels [60]. Additionally, the region’s extensive forest cover, essential for biodiversity and ecological balance, also heightens the risk of wildfires, especially during severe droughts [60]. Since the 1990s, Leiria’s population has grown by around 25% [61], driving urbanisation and increasing pressure on water resources. This growth has created significant challenges—including increased pollution of watercourses, an increased risk of urban flooding, and water scarcity—all of which are exacerbated by extreme weather events [58,59,60]. Issues such as the artificial occupation of riverbanks, fragmentation of aquatic habitats, and point and diffuse pollution are highlighted in strategic reports like the Strategic Plan for the Rehabilitation of Water Lines and the Municipal Climate Change Adaptation Plan for Leiria. In this context, it is essential to develop integrated water management strategies that ensure efficient and resilient use of water resources, in line with local needs and sustainable urban planning.

Recognising Leiria’s environmental and social significance, this study examines the Detailed Urbanisation Plan for São Romão/Olhalvas—a strategic initiative under the Polis Programme aimed at revitalising the banks of the River Lis. The project aims to redefine the urban environment by fully integrating the River Lis and its margins into the urban fabric, thereby enhancing the river as a central territorial landmark [62]. To achieve this, the plan proposes creating an extensive green corridor with interconnected pedestrian and cycle routes, as well as multifunctional spaces for recreation, leisure, and community gatherings. It also proposes the creation of urban parks, public gardens, and agricultural zones for organic production and ecotourism, all contributing to a balanced urban ecosystem. Complementary infrastructure, such as sports facilities (e.g., tennis courts) and car parks, is also planned to support community activities and urban mobility. The redevelopment strategy extends to the riverfront itself, with plans to upgrade urban service networks and construct new buildings for residential, commercial, and service uses, all designed to meet high architectural standards and blend seamlessly into the local landscape. Key technical features include a managed floodplain with a naturalised water surface and a purpose-built weir, which serves as a retention basin for flood control and stormwater management. In addition, the strategic location of the São Romão Wastewater Treatment Plant in this area reinforces the project’s commitment to sustainable water management and environmental protection [63].

The data presented in

Table 8. Summary of the São Romão/Olhalvas Urbanisation Detailed Plan [63].

Description	Value
Intervention area	41.40 ha
Total construction area for housing, commerce, and services	46,828 m2
Proposed construction area for car parking	11,421 m2
Proposed construction area for equipment and support buildings	6475 m2
Number of dwellings (total)	242
Total area of public green areas and green spaces for collective use	86,409 m2

summarise the key elements of the São Romão/Olhalvas Detailed Plan, highlighting the spaces and infrastructures intended for the requalification and development of the area.

Based on the technical literature and applicable regulations [64,65], the daily water consumption for the area’s users was estimated. The residential area should accommodate around 1042 inhabitants, while the commercial and service spaces will have an average of 1534 occupants. This corresponds to an aggregate demand of approximately 205.9 m³/day of drinking water, which serves as the basis for developing a local water balance. This assessment integrated studies on diversifying the supply matrix and strategies to reduce potable water consumption in non-essential uses—as summarised in

Table 9. Summary of water supply/demand—Water Balance Urbanisation Detailed Plan.

Source of Water	Yield [m3/day]	Demand [m3/day]
Greywater	77.1 (A)	63 (B)
Rainwater
Months with the highest rainfall (C)	75.8 (D)
Months with less rainfall (C)	9.7 (D)
Without irrigation (applicable to green areas)		23.3 (E)
With irrigation (applicable to green areas)		455.3 (E)

Notes: ^(A) Water from washbasins, showers, and baths. ^(B) Water intended for flushing toilets. ^(C) Precipitation data were obtained from the National Water Resources Information System for the Lis/Ribeiras Costeiras Hydrographic Basin (Meteorological Station: 15E/01UG). ^(D) The catchment areas take into account 75% of the area available on the plots. ^(E) Water used for general cleaning (floors, washing clothes).

.

The analysis shows that the daily supply of greywater (77.1 m³/day) is sufficient to meet the demand of 63.3 m³/day for flushing toilets, demonstrating the potential to reduce drinking water consumption for non-potable uses. However, reusing this water for purposes such as cleaning and irrigation requires robust treatment and quality control solutions.

The availability of rainwater shows strong seasonal variability. In the rainiest months (generally from October to January), average catchment can reach approximately 75.8 m³/day, while in dry periods (from June to August), this supply drops to around 9.7 m³/day. This fluctuation highlights the need for properly selected and sized storage infrastructures, which can be integrated into the retention basin provided for in the plan, to optimise the use of this source during periods of low rainfall. Furthermore, the demand for irrigation of large green areas, which can reach 455.3 m³/day, contrasts with the demand for general cleaning (23.3 m³/day), highlighting the impact of green spaces on the water balance. In scenarios of high rainfall, disregarding irrigation, drinking water consumption can be reduced by up to 86.6 m³/day—representing a saving of approximately 42% of the 205.9 m³/day demanded. This result reinforces the viability of using greywater and rainwater for non-potable purposes, relieving pressure on resources intended for human consumption.

Among the solutions highlighted, the retention basin emerges as a highly relevant blue infrastructure, contributing not only to flood management but also to mitigating the effects of seasonality on rainwater availability [18]. The adoption of nature-based solutions (NbSs), such as green roofs integrated with solar panels—which can increase retention by up to 120 L/m² [66]—combined with the implementation of efficient irrigation systems and the use of native species adapted to aridity, reinforces the overall efficiency of the water system.

To optimise the sustainability of the urbanised area, an integrated and circular approach to water management is proposed, considering the synergy between centralised and decentralised systems. National and international studies show that converting wastewater into resources—through recycling and by-product recovery technologies—can significantly strengthen water security and promote urban sustainability. In this sense, the implementation of hybrid water management models, integrating the existing Wastewater Treatment Plant in the area, would enhance community resilience and align the region with global efficiency and sustainability trends.

Additionally, it is important to consider that water demand in an urban agglomeration is dynamic, influenced by factors such as climate, water quality and cost, population size, economic and cultural profiles, industrial and commercial presence, building characteristics, and the state of the supply system [64]. Although per capita water consumption has historically shown progressive growth, short- and medium-term forecasts—driven by environmental concerns—indicate the need to re-evaluate this trend [65]. In contexts of water scarcity, several countries have implemented legislation and ambitious targets to reduce consumption in residential buildings. For example, in the United Kingdom, the target for new housing is 110 L/(person.day) [47]; in Melbourne, the target is 100 L/(person.day) [67,68]; in Denmark, the limit is 105 L/(person.day) for new projects [69]; and the Netherlands aims to reach 100 L/(person.day) by 2035 [70].

Considering the context of the area under analysis, the implementation of the proposed measures—notably the use of greywater and rainwater—could reduce potable water consumption by up to 42%, aligning local indices with international targets and confirming the savings potential highlighted in the literature. Furthermore, the integrated analysis of urban natural resources indicates that by reducing irrigation water demand, up to 87% of non-potable needs could be met exclusively through rainwater during the rainiest months. This strategy not only supports existing infrastructures but also minimises the costs associated with transporting and treating wastewater from centralised public systems, reinforcing the interdependence between water, energy, food, and ecosystem security. Ultimately, the circular and efficient use of water represents a crucial step towards water sustainability, both locally and on a macro scale, by promoting accessible ecological spaces, strengthening social cohesion, and serving as a model for future urbanisations.

5. Discussion

As we face the emerging challenges related to water availability, the pressing need to rethink the urban water cycle and adopt innovative and integrated approaches to its management becomes evident. The transition towards more sustainable and water-sensitive urban communities requires a holistic approach that considers water efficiency, the circular economy, the resilience of urban infrastructures, and the active participation of stakeholders.

This study revealed critical gaps in the regulation and implementation of practices for water circularity in urban environments. The challenges identified include the following:

• Lack of integrated regulations for the community scale: Most regulations focus exclusively on the building scale or centralised urban systems, without clear guidelines for integrating decentralised solutions at the community level. In Portugal, for example, there is a distinction between DL 119/2019, which regulates the reuse of regenerated water for outdoor uses, and ETA 0905, which applies exclusively to buildings. This regulatory fragmentation makes it difficult to implement integrated approaches that consider both buildings and the urban environment in which they are located.
• Divergence between non-potable water quality criteria: National and international regulations have different requirements for treated wastewater and rainwater, reflecting specific regulatory priorities and local conditions. Countries such as the United Kingdom, Portugal, and Singapore have more restrictive standards for different uses, while Brazil, despite establishing quality requirements, does not differentiate specific applications, resulting in greater flexibility. However, this may require more robust treatments and additional costs, depending on the end use.
• Lack of financial and regulatory incentives: Financing and regulation still prioritise conventional infrastructures, hindering the adoption of hybrid models that combine centralised and decentralised systems. Although environmental certifications play an essential role in the transition to sustainable water management, the weighting given to water efficiency in these systems is low, suggesting an underutilised potential to drive more comprehensive change.

To assess the applicability of the proposed model, a case study was carried out in an urbanisation area in Portugal, demonstrating its practical feasibility. The results indicate that combined strategies, such as the use of greywater and rainwater, have great potential for reducing the demand for drinking water. In the context analysed, it was found that the use of greywater can fully meet the demand for flushing toilets, while the use of rainwater can represent around 40% of the reduction in drinking water consumption in months with a higher incidence of rainfall, making it an economically viable alternative due to lower treatment costs. In addition, the integration of blue-green infrastructure (NbSs), such as retention basins and green roofs, contributes to the water resilience of the system, offering multiple environmental and urban benefits. Harmonising these strategies aligns the model with national and international sustainable development goals, strengthening climate adaptation and water security.

Given these results, public policies and urban planning strategies must evolve to encourage more sustainable practices. Harmonising technical regulations between different scales and countries can facilitate the implementation of more efficient solutions, ensuring water security and regulatory flexibility. In addition, the creation of financial incentives, subsidies for decentralised technologies, and specific certifications for urban communities can accelerate the transition to hybrid water management models. Incorporating these systems into master plans and building regulations would contribute to the efficient use of water resources, reducing pressure on public networks and operating costs in the long term.

Advancing this model requires collaboration between governments, the private sector, the scientific community, and civil society, ensuring that the solutions implemented are scalable, adaptable to local realities, and economically viable. The valorisation of wastewater as a source, not only for water supply but also for nutrient and energy recovery, reinforces the importance of the water-energy-food nexus, promoting an integrated approach to urban sustainability.

The efforts of this research will continue with more advanced studies and the implementation of management models adapted to different realities to improve the understanding and effectiveness of the proposed solutions, ensuring a resilient, sustainable, and equitable future.