Designing green walls for greywater treatment: The role of plants and operational factors on nutrient removal

Abstract

Green walls, installed on the side of the buildings, mainly for their aesthetics and micro climate benefits, could become effective on-site greywater treatment solutions. However, far more research is needed to convert these nature-based systems into low-cost water purification technologies. This process study presents the first step in development of greywater-treating green walls by examining how variation in plant spaces and operational conditions (hydraulic loading rate, inflow concentrations and intermittent drying) influence nutrient removal from light greywater. The experiment was conducted over 12 months on a large-scale pilot green wall located in a laboratory in Melbourne, Australia. The results showed that ornamental plant species can successfully adapt to greywater only irrigation regime and play an important role in nitrogen and phosphorus uptake from greywater. Higher performing plants, C. appressa, N. obliterate, M. parvifolium and L. muscari, had on average 7–10% higher TN removal than unvegetated configuration (over 88% removal overall), which is attributed to consistently high NOx uptake. On the other hand, poorer performing plants O. japonicus, P. occidentalis and N. officinale tended to leach NOx, likely due to their shallow or slow developing root systems. While low retention times hindered high TP removal from all tested configurations, high dependency on plant species and operational conditions was still observed, with C. appressa and N. obliterata being the two highest performing plants (34–53% removal during standard operation). The overall phosphorus uptake was found to improve over time, suggesting that plant growth is a significant TP removal mechanism in green walls. While drying caused a performance drop for all designs, higher performing plant species were less affected by changes in operational conditions. The tested configurations showed good resilience to sudden inflow concentration increases, suggesting green walls could be used as a robust and aesthetically attractive on-site greywater treatment system.