Vertical Green Walls for Urban Water Resilience: Lessons from vertECO® and GRETA™ Pilots in Austria and Spain

Urban areas increasingly face compound hazards linked to climate change, including intensified pluvial flooding, heat stress and water scarcity. As a result, cities are turning towards green infrastructure (GI) and nature-based solutions (NbS) that can simultaneously reduce risk, enhance urban livability and enable circular resource management. In this contribution, we present alchemia-nova’s experiences from implementing and monitoring two building-integrated vertical NbS for decentralized water treatment and reuse: the vertECO® vertical constructed wetland system at the eco-community Cambium (Fehring, southeastern Austria) and the GRETA™ modular green wall at the St. Quirze social housing pilot (Barcelona metropolitan area, Spain).

At Cambium, vertECO® was installed in a wintergarden and represents, to our knowledge, the first full-scale vertical green wall receiving all fractions of mechanically pre-treated domestic wastewater (including blackwater, and not only greywater), with the aim of water and nutrient reuse in local agriculture . The system consists of four parallel (each 2-m long) modules with four stepwise aligned, aerated subsurface horizontal-flow basins, followed by treated water storage and ozonation recirculation . Monitoring results demonstrate that vertECO® alone already achieved average effluent quality compliant with the EU water reuse regulation thresholds for reclaimed water quality Class C (drip irrigation), while vertECO® combined with ozonation achieved Class B (broader irrigation methods), also meeting local Austrian permit requirements . The wintergarden setting maintained operational temperatures above freezing conditions during the monitoring period, supporting year-round performance in a temperate climate with cold winters.

In parallel, the GRETA™ pilot at St. Quirze demonstrates a vertical green wall for residential water management, combining bathroom greywater (three showers and two sinks) with rainwater harvested from a 120 m² roof area. The system was integrated into a renovated social housing building with dedicated greywater separation, highlighting the value of implementing source separation during new construction or refurbishment. GRETA™ treats ~125 L/day (peaks up to 180 L/day) using four parallel treatment lines across four stages of horizontal subsurface flow through modular planted units. Treated water is collected, disinfected via ozonation, and reused for toilet flushing in four apartments, with emergency tap water feeding options to improve reliability.

Monitoring from May 2023 to October 2024 (15-day intervals) indicates consistent performance, including strong reductions of turbidity, suspended solids, organic load, and ammonium. Hygiene indicators were already low in the influent and reached non-detectable levels after treatment and ozonation, supporting compliance with Spanish reuse requirements for urban non-potable applications. The pilot also yielded operational lessons: elevated installation reduced vandalism risk, and a heat period combined with automation failure caused major plant die-off. However, the system recovered quickly and maintained stable treatment efficiency, highlighting vertical GI resilience under disturbances.

Across sites, we show how vertical GI can contribute to integrated urban hazard management by reducing freshwater demand, strengthening resilience to drought and shortages, supporting rainwater buffering strategies, and acting as visible, community-facing infrastructure. We conclude with key research needs on scaling, cost–benefit assessment including co-benefits (e.g., greening and cooling), long-term robustness, and governance models for operation and maintenance.