University of Canterbury researchers are pioneering the use of treated sewage to restore native plants on Te Pātaka-o-Rākaihautū Banks Peninsula.

Research​​ by Te Whare o Wānanga o Waitaha | University of Canterbury Environmental Science ​academics, Dr Maria Jesus Gutierrez-Gines and ​Professor Brett Robinson, shows treated sewage that would otherwise be discharged into waterways could be used for land irrigation. 

Drawing knowledge from UC Adjunct Fellow Dr Colin Meurk, an expert in native ecosystems, the team is working to identify the best match of native plant and nutrients for planting as well as benefits to the environment and wildlife. 

“What is irrigated onto the native trees has a low pathogen risk — it looks a lot like drinking water,” says Brett. 

Containing high concentrations of nitrogen and phosphorus, treated sewage discharged into waterways causes water degradation, algal growth and fouling on boats. 

“Continuing to discharge treated sewage into our harbours and waterways will cause ongoing damage to ecosystems, food chains and mahika kai,” Colin says. 

Brett says ​one​ solution is​ irrigation onto​ land, supporting native ecosystems and reducing contaminants ​entering​ water​ways​. 

“There have been lots of other side benefits to it,” Brett says. “Native plants like kānuka and mānuka have antimicrobial properties further reducing the risk of pathogens, while others can mitigate environmental contamination like reducing nitrate leaching.

“These could be used on paddock margins to intercept waste from animals and reduce the pathogen load into waterways.”

Brett says this is an example of turning what could have been an environmental problem into a solution that benefits both the land and the community with far-reaching possibilities. ‘It demonstrates the circular economy, where material is used to regenerate nature instead of becoming waste.”

Currently, New Zealand imports phosphate fertiliser from Morocco and West Sahara, which can be expensive, has supply constraints and contains a high level of cadmium, a toxic heavy metal which can cause ​kidney failure​.

“We don't want that in our soil, so high-quality treated sewage could be the solution.

“With cadmium levels increasing in our soils over the past 100 years, New Zealand has reached a point where we can no longer export ​some ​offal products and some of our vegetables exceed food safety standards for cadmium levels,” Brett says.

Maria and Brett ​have demonstrated​ the potential to generate valuable native products such as essential oils, honey, fibres and timber, in areas that have been irrigated with treated sewage​. However,​ there could be resistance from consumers.

“Sewage can contain components which you don't want in the food chain​, such as ​pathogens, hormones from contraceptive pills, or personal care products, so this needs to be investigated and addressed,” Brett says.

“Using traditional native plants like harakeke and totara could provide cultural and economic benefits while avoiding products related to the food chain,” Maria says.