Microplastics in wastewater in New Zealand: current data and knowledge gaps

Helena Ruffell (University of Canterbury)

Professor Sally Gaw (University of Canterbury) 

Dr Olga Pantos (Institute of Environmental Science and Research)

Dr Grant Northcott (Northcott Research Consultants, Ltd)

Microplastics, plastic particles <5 mm in diameter, are emerging contaminants of increasing concern. Microplastics have been detected in a range of remote locations, and are being shown to be ingested by a growing list of aquatic and terrestrial organisms. International literature has shown wastewater treatment plants (WWTPs) to be a major source of microplastics to the environment. Microplastics present in industrial and residential wastewater, particularly microfibres from the washing of textiles, are diverted to WWTPs which are not designed to remove microplastics during treatment. Microplastics from WWTPs are retained in the sewage sludge or are released directly to the environment through the discharge of effluent. There is currently a lack of data available on the concentration and types of microplastics entering and exiting WWTPs in New Zealand. This study is the first to have characterised microplastics in wastewater influent and effluent of four different WWTPs in New Zealand. Findings from this investigation will be presented along with a discussion of the wider impacts of microplastics exiting WWTPs.

Little is known worldwide about the fate, behaviour and potential impacts of the microplastics that are discharged from WWTPs to the environment. Microplastics have been shown to adsorb heavy metals and hydrophobic organic contaminants, and also act as a substrate for diverse microbial communities. These factors have been documented in a range of aquatic and terrestrial environments. Microplastics therefore act as a vector for these adsorbed contaminants and microorganisms, which are often sheltered from degradation during WWTP processes, and are subsequently released into sensitive aquatic and terrestrial ecosystems with the discharge of treated effluent. To address the knowledge gaps a study as part of the Aotearoa Impacts and Mitigation of Microplastics (AIM2) research programme is currently investigating the interactions of plastics, contaminants and microbes, and the associated risks to Aotearoa New Zealand’s environments. This is being done by deploying five different plastic types of known composition (polymer + additives), and age in the oxidation pond of a WWTP. Preliminary results of this year-long experiment (ending in June 2021) will be presented.