The complex and interconnected global issues of climate change, dwindling resources, limited water availability, and declining biodiversity, call for comprehensive and enduring solutions. Floods and prolonged droughts pose severe challenges to cities worldwide. Floods are more immediate and destructive, causing immediate harm and infrastructure damage, while droughts, though slower, can have profound and long-lasting effects on urban environments and the well-being of populations.

Nature-based Solutions (NBS) have steadily emerged as a paradigm of an integrated approach for sustainable and resilient urban development. Nature-based Solutions (NBS) are approaches that take cues from natural systems to address various urban challenges. To harness the full potential of Nature-based Solutions (NBS) for providing ecosystem services, they integrate natural elements with engineering techniques. This approach enhances the ability of NBS to contribute to a sustainable and resilient urban water cycle.

Developing actions across sectors is crucial for successfully transitioning to a circular economy in cities. The introduction of circular economy frameworks in cities can face various challenges, many of which are rooted in the complexity of the urban context and the diverse urban forms present. Additionally, transitioning to circular cities make it difficult by socio-cultural, economic, and financial factors.

Collecting rainwater, treating and reusing stormwater, greywater, and wastewater, and utilizing seawater and brackish water through desalination and other means are essential processes that can help decrease the need for freshwater.NBS are designed to provide water purification services while also encouraging resource recovery and reuse within urban environments. The simplest circular function, which does not include the enhancement of reclaimed water, regards the release of filtered water back into water bodies through infiltration. In more sophisticated circular systems, the filtered water that has passed through biofiltration can be gathered using water harvesting systems or perforated pipes and subsequently reused for various purposes.

Non-Conventional Water Sources

Rainwater: The roof runoff collected directly in water tanks or systems such as green roofs.

Stormwater: The urban runoff collected from several impervious surfaces (e.g., roads, pavements).

Greywater: The amount of domestic wastewater that excludes toilet flushing and accounts for around 70–75% of the total household wastewater.

Municipal wastewater: It includes all wastewater of a city: wastewater from houses but also offices, restaurants, and industries.

Blackwater: Domestic wastewater that is collected from toilets and includes the highest amount of nutrients and organic matter.

Image: Non-Conventional Water Sources


The potential applications of reclaimed water sourced from circular water Nature-Based Solutions (NBS) can differ based on the degree of contamination. This discrepancy arises from considerations related to health risks and legal constraints that impact the suitability of its use.

NBS can provide some level of natural disinfection through the processes of filtration, sedimentation, predation, dehydration, and sun irradiation.

Reclaimed rainwater and stormwater, especially from systems like constructed wetlands and biofilters, can be a valuable resource for various non-potable purposes at the city level. It can be used for cleaning public spaces such as sidewalks, streets, parks, and recreational areas. It can supplement the water supply for fire hydrants. It can support the development of new artificial lakes or the expansion of existing ones.

However, there are standards globally regarding its quality for use when it comes to Municipal wastewater and black water treatment and valorization.

Circular Nature-Based Solutions (NBS) can be strategically positioned within urban areas, including climate-vulnerable zones, both in public and private spaces. Hence, it’s essential to take into account specific site-related attributes associated with socio-ecological factors and climate change susceptibility.

The write-up is based on the article: Nature-based solutions for circular urban water systems.

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