Green Walls: An Opportunity to Treat and Reuse Greywater in New Zealand

Moeen Gholami1, Aisling D. O’Sullivan1,2, Hamish R. Mackey1

1Department of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand; 

2Centre for Ecological Technical Solutions (CELTS.org.nz), University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand.

The national average household water use is 252 L/ p.e./d. The majority (60%) of this used water ends up as greywater and, combined with blackwater, is treated in wastewater treatment plants. Greywater is generated from household activities such as laundry, cleaning, handwashing, dishwashing, showering, whereas blackwater is from toilet waste. Greywater has a low pathogenic pollutant load compared to blackwater and so separating black and grey wastewater streams could offer numerous opportunities for greywater reuse such as irrigation, toilet flushing, and laundry, widely implemented overseas but minimally in New Zealand. Demands on freshwater consumption for outdoor irrigation (17%) and toilet flushing (18%) could be lessened by substituting freshwater with greywater for these activities. It would also reduce wastewater volumes, and associated collection and pumping costs, currently being treated in wastewater treatment plants.

Green walls provide multiple benefits including energy conservation, air purification, noise and thermal insulation and enhanced biodiversity. As living systems, they sequester carbon through photosynthesis. Additionally, green walls could be adapted to treat greywater at source, removing pollutants from the low pathogenic wastewater stream. Internationally, lab-scale green walls were demonstrated to remove organic matter (>90%), suspended solids (>99%), nitrogen (>85%) and phosphorus (>60%) from greywater. These early data are promising to support the principle of on-site wastewater treatment using green walls.

This paper reviews international practice on green walls capabilities and design considerations for treating greywater, with a focus on applying this knowledge to the national context. The effects of climate variables and native vegetation; potential for repurposing local waste substrates; as well as key legislative and cultural values in the New Zealand context; are discussed to suggest fit-for-purpose solutions to greywater treatment and reuse.