Development of a novel green wall system with integrated greywater treatment for on-site water reuse

Urbanization is rapidly increasing, with projections indicating that 68% of the global population will reside in urban areas by 2050. This growth exerts substantial pressure on water resources, necessitating sustainable water management and the exploration of alternative water sources such as greywater. Greywater treatment technologies suitable for household-level implementation are critical for sustainable urban water management and on-site water reuse. Green walls, which integrate greywater treatment capabilities, represent a promising nature-based solution. Additionally, green walls have the potential to address other pressing issues such as the heat island effect within cities and increasing biodiversity locally. These systems typically function like constructed wetlands, using substrates to support both plant growth and pollutant removal. This study, however, introduces a novel green wall system that separates the plant layer from the greywater treatment, incorporating a trickling filter-like setup. The research focused on various aspects to realise this novel green wall system: (1) identifying optimal carrier materials serving as trickling filter for greywater treatment in the back layer of the system, (2) suitable plant selection and substrates for the given conditions in such a system for the front layer, (3) prototyping the functional setup of the green wall system through various iterations and (4) testing the system at various scales and in various locations internationally. Different commercially available substrates were tested as carrier materials and results showed that a expanded shale performed best compared to a plastic carrier (Hel-x), lecca and foam carriers with removal efficiencies up to 90 % based on Chemical Oxygen Demand (COD). Despite the superior performance of expanded shale, plastic Hel-x was selected due to its lower weight, which is advantageous for the structural requirements of green wall systems. Greywater is continuously trickling through the back layer, the water is then drawn by a special fleece to the front layer, which waters the plants. This, however, creates constant moist conditions similar to a hydroponic system. A plant selection was finally established that copes these moist conditions. Four iterations of prototyping were needed to create a functional green wall pilot system incorporating the previous results. The developed pilot systems was first tested at the university, then at a building with greywater separation in Zurich, Switzerland. International tests were carried out in South Africa at a community centre in Khayelitsha, Cape Town together with the organization VPUU, and at a building in Stellenbosch together with Stellenbosch University. All installed pilot systems operated successfully and show high COD removal efficiencies up to 90%. Other measured parameters are total nitrogen, ammonia, total phosphorous, turbidity and pH. Detailed results for the various aspects of the development of this novel green wall system will be presented. The findings underscore the potential of green walls to address urban water challenges by treating greywater and simultaneously providing environmental benefits such as reducing the urban heat island effect and increasing biodiversity. The study advances the development of green wall systems that can effectively integrate greywater treatment, contributing to more sustainable urban water management practices.