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The problem with microplastics


29 Mar 2022

Beca associate environmental engineer – water, Bridget Rule, presented a paper on the problem on microplastics in the New Zealand water environment, asking should we be worried and what can we do? This is a summary of the paper written by Bridget and Garry Macdonald, business development director – water market segment at Beca.

One of the most concerning emerging contaminants in the water environment is microplastics. Plastics have created a global waste problem, prevalent in municipal and industrial waste streams, and the water environment.

A key feature and issue with plastics is that plastic products are durable and chemically resistant; however, plastics fragment under photo-oxidative and mechanical stress. This fragmentation results in a subset of plastic pollution: microplastics, characterised by plastic particles smaller than 5mm.

Microplastics represent a unique environmental threat due to exposure to hormonal disruptors and toxic additives.

Most plastics are mineral oil-based, though bio-based plastics do not solve this problem; they are not always biodegradable (for example, bio-based PET). Conversely, biodegradable plastics are not always fossil fuel-free (e.g. PBAT, PCL), and both can contain additives.

A significant proportion of macro and microplastics end up in the water environment, transported through sewerage and stormwater systems to the ocean.

Macroplastics are more visible, as well as highly buoyant. In contrast, microplastics are less visible but mobile, and therefore of more concern to global ecosystems as they are ingested by fish and other marine organisms, finding their way into the human food chain. A US study found that microplastics size decreased between freshwater and saltwater samples, with concentration of large particles higher in less saline water and concentration of smaller particles 

increasing with higher salinity – indicating that stormwater is also a source of microplastic pollution.

What happens in WWTPs?

Influent concentrations of microplastics have been found at 16 particles/L-effluent. In the US, where 160-trillion litres of wastewater are generated per day, an estimated 256-trillion plastic particles/day are passing through WWTPs.

WWTPs have been shown to be reasonably effective at removing microplastics, with removal efficiencies in the 75-100 percent range for conventional WWTPs utilising activated sludge processes and secondary clarification.

In a typical US facility, results from FTIR analysis showed 91 percent removal of total microplastic concentrations in the primary clarifier and an additional seven percent removal in secondary treatment.

In New Zealand, advanced plants (e.g. Christchurch) can achieve 97 percent removal. Less advanced plants do not result in reducing the level of microplastics (by number), as big pieces tend to break up as they pass through the system, especially if they go through the recycled activated sludge stream.

Though it appears that conventional treatment processes are successful at removing microplastics, given the large effluent volumes, even low concentrations of microplastics can still constitute significant discharge to the marine and freshwater environment.

So, where do they go?

While microplastic concentrations in effluent are lower than in influent, they are not digested. Where they are removed from the liquid stream, they end up in biosolids.

In New Zealand, nearly one quarter of WWTP biosolids are disposed of to open land application.

Given the high concentrations of microplastics found in biosolids – in one example, 99.7 percent of influent microplastics settled in digested sludge – disposing of biosolids through land application which are then exposed to the elements and able to re-enter the water cycle, implies that a significant part of the microplastics problem could be being temporarily diverted from the water environment rather than permanently removed. In New Zealand, we can expect, in future, higher standards for WWTP discharges – and probably stormwater discharges – and we must be in a position to determine the optimum liquid and solid stream processes to ensure that microplastics do not become more of a problem in our

aquatic and land environments.

What next?

Further work is needed on improving analytical methods to measure microplastics. This underpins the larger research goals of better understanding how sludge structure and properties affect microplastics' fate within WWTPs, the role of microplastics as microbial carriers in the environment, and how microplastics enter stormwater.

Given the prevalence of plastics in certain WWTP processes and in water and wastewater networks, and their tendency to break down over time into smaller pieces, more research is needed to understand how WWTPs may be inadvertently contributing to microplastic pollution via plastic media in secondary processes, especially those containing UV stabilisers and black colour.

Furthermore, considering that a primary vector for plastics into soils is through biosolids, more work is urgently needed on the extent of potential microplastic pollution entering topsoil via land application. Research into microplastics in the New Zealand environment is ongoing through ESR and others.

Key takeaways

  • • Conventional WWTPs are good at intercepting macroplastics and (to a certain size and extent) microplastics, as long as a clarification or filtration stage is present as a tertiary stage.
  • • Even at low concentrations, the discharge is continuous, and therefore the total quantum of released microplastics can be significant.
  • • Stormwater has no controls for macro or microplastics. Episodic discharges also give rise to significant discharges.
  • • Biosolids are a significant accumulator of microplastics (depending how effective inlet screens are at intercepting macroplastics) and are typically the primary means of environmental release of microplastics from WWTPs.

With thanks to Nicole Fahrenfeld, associate professor, civil and environmental engineering, Rutgers University, and Belinda Sturm, professor and associate vice chancellor for research, University of Kansas, for allowing us to share their research.

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