Microplastics in wastewater

Helena Ruffell is a PhD student at the University of Canterbury, investigating whether microplastics affect productive soil systems. This research follows on from her MSc which investigated if wastewater treatment plants are a significant source of microplastics to the environment. At the Water New Zealand Conference and Expo she presented the paper, ‘Microplastics in wastewater in New Zealand: current data and knowledge gaps’, written with Professor Sally Gaw, University of Canterbury; Dr Olga Pantos, Institute of Environmental Science and Research; Dr Grant Northcott, Northcott Research Consultants. Here is the abstract of their paper.

Microplastics – plastic particles smaller than 5mm 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) show microplastics were detected in wastewater influent and effluent at concentrations within the lower range of those detected overseas.

The concentration of microplastics at each WWTP decreased from influent to effluent at Christchurch, Kaiapoi, and Lyttelton WWTPs. This trend was not observed at Governors Bay WWTP.

The retention percentages of microplastics from influent to effluent in Christchurch, Kaiapoi, and Lyttelton WWTPs were lower than those observed in international studies. Few temporal trends were identified in terms of abundance, particle morphotype and polymer type, highlighting the complex nature of wastewater.

The conclusions and recommendations from this study are that WWTPs are a significant source of microplastics through the discharge of treated effluent to the Canterbury coastline, and further work is required to understand the environmental fate and impacts of discharged microplastics.

More research into the removal of microplastics from sludge and effluent during the WWTP process is required, however employing greater levels of treatment and filtration at WWTPs are costly to implement and will not effectively remove microplastics from all mediums.

Greater understanding of the relative contributions from both commercial and personal activities to influent microplastic load is needed in order to write more effective, targeted regulatory policy to mitigate sources of plastic waste to WWTPs, and the receiving environment.

To read the full paper, go to the Water New Zealand website