Treatment and reuse of domestic greywater through green walls

The principles of circular economy and sustainability involve also water management. Since both scarcity and demand of water are increasing, wastewater reuse represents a necessary element to preserve the environment while guaranteeing human development. Greywater is the amount of wastewater that is more suitable for reuse purposes: it comes from sinks, showers, bath tubes and laundry. Greywater has low pollutant concentration and developed countries generate high volumes of it everyday.

Nature-based solutions are well suited for greywater treatment purposes thanks to their environmental and energetic advantages. In fact, these green systems have low energy consumption (that means also low CO₂ emissions), improve the quality of the air (e.g. capturing CO₂), reduce heat island and promote biodiversity. However, their efficiency in treating greywater needs to be deeply investigated in order to couple their efficacy with the lack of space in urban areas.

In this study we have realized a pilot system to treat greywater through green walls, in order to exploit the unused surfaces of buildings and improve urban areas, increasing their sustainability and resilience as recommended from the Sustainable Development Goal 11 (Make cities and human settlements inclusive, safe, resilient and sustainable) of the UN 2030 Agenda. Our innovative system produces treated greywater that can be reused for non-potable purposes (e.g. gardening and toilet flushing), driving a reduction of potable water consumption in our houses.

In order to guarantee aesthetic requirements, we selected three types of evergreen plants that are able to survive a great amount of water per day. We prepared different porous media mixes in order to evaluate the effects of additives on the common media used in usual green walls. We built six modular panels with three replicates per media mix, in order to assess the statistical variability of the results. Each panel has four independent columns of three pots. Each column contains a different porous media mix and is planted with the same sequence of three plant species. We daily fed each panel with around 100 L of synthetic greywater and monitored different parameters (e.g. BOD, COD, DO, Nitrogen, Phosphorus, E. coli).

In a first phase we evaluated differences in treatment performance among different mixes. Removal efficiency exhibits some variability depending on the considered parameter but in general our results show statistically significant differences between configurations. In a second phase we consider the treatment performances along each column. Preliminary results of this phase show a significant decrease in pollution after the second line of pots already. In summary, concentrations at the system outlet respect the most common reuse guidelines for many parameters without any other treatment.