Vertical greening systems and green roofs for greywater treatment and reuse: experiment-based recommendations for research and practice

The increasing greening of urban areas to deliver multiple benefits is expected to face water-related challenges, as irrigation demand during drought periods can negatively affect drinking water supply. Consequently, the implementation of Nature-based Solutions (NbS), such as vertical greening systems and green roofs, should be combined with the use of greywater as alternative water source, enforcing a circular economy perspective. Both vertical greening systems and green roofs can either be irrigated with greywater or designed to act as decentralized and non-conventional water treatment plants, providing treated water for non-potable purposes. Although this topic has received increasing attention in recent years across different fields, clear design recommendations for urban NbS using greywater are still lacking. This research addresses this gap by analysing experimental studies published in scientific journals and grey literature, and by developing recommendations to improve both experimental design and practical implementation of vertical greening systems and green roofs for greywater use. The recommendations merge geometrical aspects (such as substrate depth and rooftop slope) with operational conditions (such as hydraulic/organic loading rate and hydraulic retention time) and are differentiated according to the main objective of the technology: direct greywater reuse (i.e., total evapotranspiration approach) or treatment and reuse for other non-potable-purposes. The findings of this analysis have been compared with the well-established field of treatment wetlands, setting the basis for a common approach and supporting the wider adoption of these sustainable technologies in urban environment.