Resilience to flow variability of an open-air green wall for greywater treatment

Green walls have often been touted for their aesthetic and microclimatic benefits in urban environments, but recent research by Costamagna et al. (2025) explores their potential as effective, nature-based technologies for decentralised greywater treatment. The study, performed in collaboration between Politecnico di Torino and IRIDRA, examines a modular vertical green wall system installed in a real-world outdoor setting and assesses its resilience to variable hydraulic loads over a seven-month monitoring period. By treating actual household greywater, the system was exposed to fluctuating inflow rates, reflecting real-life usage patterns and operational challenges.

What makes this research particularly relevant is its focus on resilience, which is an increasingly decisive criterion for water treatment systems facing the uncertainties of climate change and urban growth. Despite irregular and sometimes intense flow peaks, the green wall consistently achieved high removal efficiencies for key pollutants such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), and nutrients. These results demonstrate that such low-tech, plant-based systems can operate reliably under variable and even sub-optimal conditions, without the need for complex infrastructure or high energy input.

In the broader context of the NICE project, this study reinforces the value of integrating nature-based solutions (NbS) into urban water cycles. It shows that decentralised, building-integrated NbS can provide robust water treatment performance while delivering co-benefits such as biodiversity support, space efficiency, and aesthetic enhancement. Importantly, they can be installed in existing buildings, making them attractive for retrofitting in dense urban areas where traditional water infrastructure upgrades may not be feasible.

For urban planners, water utilities, and municipal decision-makers, the implications are clear: NbS like vertical green walls offer scalable, resilient, and multifunctional alternatives for water reuse. They contribute to circular water management while enhancing urban liveability.

As cities worldwide look for innovative approaches to close the water loop and reduce reliance on centralised systems, the findings from this research highlight how combining ecology and engineering can lead to practical, sustainable, and adaptable solutions. Stakeholders involved in policy-making, urban regeneration, or climate adaptation strategies can draw from this evidence to promote the inclusion of NbS in future urban development plans, advancing both environmental goals and regulatory compliance.

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