An experimental study of greywater irrigated green roof systems in an arid climate

Green roofs provide multi-functional benefits to the built environment. They minimize urban heat island effects, enhance biodiversity, reduce carbon footprints, provide hydraulic benefits to urban runoff, and improve overall environmental sustainability. However, their application is limited or rare in arid climates. On the other hand, greywater is becoming a popular alternative water resource in water-scarce regions. A greywater-fed green roof system was developed and studied in the city of Al Ain, United Arab Emirates (UAE). The effluent (treated greywater) from the green roofs can be used to irrigate amenity plantations. Two intensive and two extensive green roof prototypes were constructed, planted with reed canary grass (Phalaris arundinacea L.), and irrigated with greywater. The greywater influents and the green roof effluents were monitored for changes in greywater quality. The study showed that the intensive system performed well, which is attributable to the greater depth of soil media. Treated greywater effluent from the green roofs met the local standards for recycled wastewater-based irrigation for a number of parameters (pH, electrical conductivity, salinity, and total dissolved solids), but exceeded the maximum allowable limits for turbidity, COD, and sodium ions (Na+), which may be because of the short retention time of the experiment. Both the intensive and the extensive systems were inefficient in reducing the total bacterial count of the greywater