Green walls for greywater treatment: A comprehensive review of operational parameters and climate influence on...

Green walls for greywater treatment have emerged as a solution to increase green spaces in densely urbanized areas while providing treated greywater for reuse. Over the past decade, numerous studies have focused on optimizing these systems, though most address specific operational conditions and evaluate a limited set of performance parameters. This review synthesizes the existing literature using a meta-analysis to identify key operational factors and treatment performance metrics. A systematic search was conducted across Google Scholar, Scopus, and Web of Science, resulting in the selection of 33 studies. These studies were classified using the Köppen-Geiger climate classification, and a comprehensive database with over 8500 entries was built to analyse performance of COD, BOD, TOC, TSS, NH₄⁺, TN, TP, and bacteriological parameters across different climate zones. Results revealed performance variations across climate zones, with temperate climates outperforming dry regions. Regression equations between areal mass load and removal efficiency are proposed as design guidelines, and recommendations are made regarding optimal filling media. Additionally, for specific reuse applications, the inclusion of a disinfection unit is advised to meet microbiological quality standards.