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Article related to: People and perspectives
Tracking microplastics: from sewage sludge to the oceans
Microplastics contaminating freshwater and oceans is one of the world’s most pressing environmental concerns. It is estimated that 2 to 5 % of all plastics end up in the oceans. To learn more about the impact that microplastics have on our environment, we talked with Dr Rachel Hurley, a post-doctoral researcher based at the Norwegian Institute for Water Research (NIVA).
At wastewater treatment plants, wastewater is converted to water that can be reused. However, tiny pieces of microplastic debris (that are about the same size as a sesame seed or smaller) can pass through some of the physical, chemical and biological filtration systems in use at these plants. Although up to 99 % of pollutants can be trapped, there are some concerns over the efficiency of the treatments used to capture microplastics.
“Evidence seems to suggest that most microplastics are captured during primary and secondary treatment processes, but we need more research so that treatment plant processes can be tailored to capture more of the microplastics, and so the concentrations of microplastics discharged from the plants can be reduced,” Dr Hurley tells.
From sewage sludge to land
A proportion of the microplastic particles that evade the treatment plant’s processes are added to sewage sludge – a by-product of the wastewater treatment processes. NIVA’s study of microplastic content in sludge from eight treatment plants in Norway shows that there is variation in the concentrations of total microplastics and the composition of the contamination.
“We observed different particle types including beads, fragments, fibres and glitter composed of a wide range of polymers; however, polyethylene, polyethylene terephthalate, and polypropylene were the most common. It is likely that particle size, shape and density play an important role in determining the behaviour and potential risks of microplastic particles,” Dr Hurley informs.
NIVA’s researchers also revisited two treatment plants to check whether the microplastic content in the sludge remained stable over time or whether there were any variations. “In one treatment plant, the concentrations were steady. However, in the other, we noticed a significant difference in the amount of microplastic particles traced during each visit. This variability shows how important it is to gain a better understanding of the source of the microplastics entering the plants,” Dr Hurley says.
While there has been a lot of work done to optimise wastewater treatment processes, not much has been specifically focused on particles with similar sizes and densities to microplastics. It is also important to track how microplastic particles mobilise, to see whether sludge added to agricultural soils only contaminates the soil system or whether the microplastics transfer to the wider environment.
Through river systems towards the oceans
As part of Dr Hurley's doctoral research at the University of Manchester, patterns of microplastic contamination in channel sediments were also studied at 40 sites in river catchments near Manchester, in northwest England. Microplastics were found in all of the river channel beds.
However, after a period of severe flooding in winter 2015-2016, they resampled the areas and found that the concentrations of microplastics had drastically reduced and that microbeads had been completely eradicated at some sites. “High magnitude flood events like this are capable of flushing microplastic particles further downstream. In our study, the evidence shows the microplastic particles being exported from the catchment sites, moving downstream and eventually being discharged into the Irish Sea,” Dr Hurley says.
Therefore, it is important to control the source of microplastics entering the rivers to avoid them being further carried towards the oceans. Future studies on the same catchments by the Geography department at the University of Manchester may provide a deeper understanding of the processes underpinning the microplastic contamination of riverbeds.
What more can be done?
So far, the majority of research on environmental microplastics has focused on monitoring. “The next step would be to quantify processes related to microplastic contamination including identifying their sources and associated release, looking in more detail at the amount of time microplastics stay in different environmental compartments, and examining how they degrade in more detail,” Dr Hurley informs.
More research is also needed to understand the effects that ingesting microplastics could have on organisms. “We do not yet know whether ingestion of particles or accumulation of microplastics up the food chain leads to adverse effects,” she tells.
Several internationally-funded projects and working groups are currently tackling issues related to definitions and standardised methodologies for microplastics research. Much of this has been focused on further optimising methods and improving analytical capabilities related to analysing different sizes of microplastics and different environments.
“This process takes time, as there is not yet a ‘perfect’ method for analysing plastic particles, and research is necessary to identify methodological constraints and propose solutions. Over the coming months, we expect to see further clarifications on how to define and look for microplastic particles,” Dr Hurley tells.
It is also important to continue to improve methods to increase the quality and reduce the time and cost of microplastics research in the future. “At NIVA, we are working to contribute to this ongoing process by developing certified standard reference materials for different microplastic particle shapes and sizes. We hope this will help to validate methods and to ease the comparison of results from different laboratories,” she concludes.
Rachel Hurley is a post-doctoral researcher working on the Water JPI IMPASSE project (Impact of MicroPlastics on AgroSystems and Stream Environments). She is based at the Norwegian Institute for Water Research (NIVA) in Oslo, where she also works on other projects within the Microplastics research group.
The Norwegian Institute for Water Research (NIVA) is Norway’s leading competence centre for environmental and resource issues relating to the fields of water, biodiversity, sustainable development, contamination, and development work. The institute has an active, strategic research programme with several initiatives concerned with modelling and monitoring microplastics in different environmental compartments, as well as investigating potential effects of microplastic particles on biota.
What are microplastics?
Microplastics are small pieces of plastic debris that are typically less than 5 mm in size (about the same size as a sesame seed). They come in different types, shapes and sizes, including beads, fragments, fibres and glitter.
They come from a variety of sources, for instance, when larger plastics degrade into smaller pieces either by wear and tear caused by sea wave action, ultraviolet radiation or when marine animals shred them. In addition, they can also be intentionally added to household and personal care products, such as some cleaning products and toothpastes.
ECHA is currently working on a restriction on intentionally added microplastics. You can follow this work on ECHA’s website.
Interview by Paul Trouth
Published on: 13 September 2018
Top image: IStock/JonathanFilskov-Photography
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