A green solution for decentralised water treatment: a pilot-scale study on the performance of green walls irrigated with greywater, under a Mediterranean climate

Rapid urbanisation, climate change and scarcity of freshwater leads to conservative water consumption practices, including wastewater recycling for non-potable ‘low exposure risk’ end-use, such as sub-surface landscape and selective garden irrigation. Small-scale decentralised and cost-effective water treatment technologies like green walls require low energy, and are ideal for implementation in both residential and commercial areas. Green walls have been shown to attenuate nutrients, with the treatment efficiency mostly dependent on soil characteristics and plant types. While green wall systems have long been used for thermal comfort under temperate climates, there has been less research on its optimised performance under Mediterranean climates, where long, dry periods in summer and sometimes water-logged conditions in winter, create challenges for both plant and soil health. Our pilot-scale research project used planters (2.5 m x 0.7 m x 0.75 m) to establish detached green façades irrigated by greywater, and to test the impact on façade viability and treatment performance of planter orientation, plant species,  deciduous and non-deciduous plants and the projected total leaf area. Influent and effluent volumes from the planters were carefully monitored, and water balances were established for the planters. The water requirements of green walls in east, west and north facing orientations, and using different plant species, were quantified under different seasons. We determined that annual water requirements for the deciduous plants were almost half that of the non-deciduous plants; as expected the leaves appeared on deciduous plants as air temperatures increased and then both type of plants showed similar water requirements. The evapotranspiration as estimated by the water balances, was validated by quantifying the plant water loss (transpiration) using a portable photosynthetic unit (LI-6400XT, Licor Inc., Lincoln, NE, USA). The transpiration measured on a single leaf (in triplicate) was scaled up to the projected total leaf area of the façade, to estimate the total transpiration from the planter. The influents and effluents were also monitored for water quality, to determine how their treatment performance changed with vegetation maturity and season. The green walls showed up to 90% total nitrogen and 80% total phosphorus removal efficiencies throughout the two years study period. However, the pathogen count was greatly impacted by the irrigation water temperature and the effluents had higher pathogen counts than the influents, irrespective of facade orientation or plant species. The results of the leaf area analysis and water balance measurements, as well as their effect on water quality, will be presented to identify suitable orientation and plant species for improving the urban micro-climate that could thrive under greywater irrigation, and in particular under Mediterranean climates.