Greywater treatment by ornamental plants and media for an integrated green wall system

Greywater recycling through natural vegetated systems provides a low cost and sustainable approach to manage urban water scarcity but is space intensive. This study aims to treat greywater naturally using ornamental plant species that can be integrated vertically into green building structures and elucidate the main removal mechanisms occurring therein. To this end, various regional ornamental plants and growing media were tested in column experiments, with a bi-daily application-rest cycle of 500 mL. d−1 per plant. Removal percentages greater than 90% were achieved for all contaminants monitored (organics, solids, nitrogen and phosphorus) when using high surface area, small-diameter media such as coco coir, spent coffee grounds and sand. Media selection was found to be a much more dominant factor than plant selection for treatment performance. Using media replacement and plant removal studies, it was found that nitrification was linked with the presence of plant roots in the media. The removal rates of hydrophilic and hydrophobic organics were also analyzed and effluent specific UV absorbance measurements demonstrated hydrophobic compounds, which are typically more toxic in nature, were successfully removed from the system. This study demonstrates the complex interactions between the media, microbes, plant roots and greywater in natural treatment systems and their potential as an innovative approach for greywater treatment in the urban built environment.