The Filtration of Microplastics in Drinking Water

Microplastics is a topic that regularly makes the headlines, especially with the increasing awareness around the impacts of plastic pollution. These tiny pieces of plastic have been found almost everywhere on our planet: in the ocean, in freshwater ecosystems such as lakes, ponds, and rivers, at the summit of Mount Everest, and even closer to home - in drinking water! A 2017 study of drinking water, which spanned five continents, found that over 80% of the samples analysed contained microplastics [1]. But why is this important, and what can be done about it?

What Are Microplastics?

To properly understand this issue, we first need to understand microplastics themselves. Microplastics are defined as small pieces of plastic less than 5mm in length; there is a further subclass of microplastics called nanoplastics, which are defined as being less than 1µm in size [2]. That’s 10,000 times smaller than a centimetre!

There are two categories of microplastics - primary and secondary, with the category determined based on how each microplastic enters the environment. Primary microplastics enter the environment directly, through situations such as product usage, spills from manufacturing or transport, or the weathering of synthetic fabrics during washing. Secondary microplastics are formed from the breakdown of larger plastics - this is usually due to erosion from sources such as waves, wind, or ultraviolet radiation from the sun [2].

Microplastics and Human Health

Now that we know what microplastics are, we can properly look at the possible health risks associated with them. The World Health Organization (WHO) has stated that “a number of research gaps need to be filled to better assess the risk of microplastics in drinking-water” [3]. However, they have identified three forms of potential hazards from microplastics in drinking water: the physical hazard caused by particles, chemicals from the plastics leaching into the water, and microorganisms that attach to and colonise microplastics in structures known as biofilms [3]. The physical particles may be absorbed into the human body, which could pose a threat due to the possible toxicity of chemicals that leach from the microplastics. Research has also shown that heavy metals, such as chromium, could be transported into the body through microplastics. Microplastics can adsorb heavy metals, and when consumed, may act as a vector for carrying these heavy metals into the human digestive system [4]. Risks from both the physical particles and the chemicals present in microplastics are poorly understood, just like the field in general. The concern around biofilms is mainly based on the potential for pathogens, such as Legionella, to spread through the biofilms on the surface of the microplastics. Legionella causes Legionnaires’ disease, a form of pneumonia which, while treatable, can be very dangerous. Typically, the disease is spread through the inhalation of droplets of water containing the bacteria, however, it can be spread through drinking water if droplets enter the lungs while drinking [5].

The risks from pathogens and bacteria like this are considered to most likely be low. However, we do not truly know the full effect these factors may have on human health. There has only been significant research into the topic in recent years, and therefore we are still working towards a better understanding of the risks, especially in the long term. Thus, it is important to look at the methods that we can use to filter out microplastics.

Microplastic Filtration: Methods and Its Importance

There are several ways we can filter out microplastics, and there are many companies that sell products to do so. However, their vested interest in product sales means that some of these companies over-exaggerate the need for filtering microplastics. Let’s consider the facts. Firstly, the levels of microplastics in water depends on where you live - in the United States, 94% of water samples tested had microplastics present. In comparison, Europe had lower levels of about 72% of water samples having microplastics present [1]. If you live in an area where there are generally lower amounts of microplastics in drinking water, having a personal water filter is probably not going to be a priority for you. Secondly, the WHO states that available data shows that water treatments are very effective - more than 90% of microplastics are removed by wastewater treatments [6]. This is a fact that many water filter companies fail to tell us while trying to convince us to buy their products. Having a filter for personal use might not have a huge impact on your life, but it is interesting to learn about some of the methods that are out there!

Granular Activated Carbon Filters

One method of filtering microplastics is with a granular activated carbon filter. Granular activated carbon filters can filter out contaminants as small as 5µm, which means that some microplastics will be filtered out [7]. They work by adsorption, where organic compounds (contaminants) are held to the surface of the carbon by Van der Waals forces [8].  Van der Waals forces are intermolecular forces of attraction that pull molecules together, and in this case, they pull the contaminants to the carbon. This results in the contaminants being removed from the water, as they are held to the surface of the carbon while the water flows past. Although this method of filtration is efficient for particles larger than 5µm, there is a clear limitation in that it does not remove the smallest microplastics, such as nanoparticles.

Carbon Block Filters

Carbon block filtration is another method of filtration, and this method may be able to remove contaminants as small as 0.5µm [9]. It uses mechanical filtration, which works in a similar way to a sieve; contaminants that are larger than the pores of the filter can't pass through and are separated out [10]. Some carbon blocks also use a specially designed outer wrap so that when the water passes through the block, the material in the wrap gains a positive molecular charge that will attract the negative ions of some pollutants. They also use the same form of adsorption as the granular activated carbon filters. The combination of these systems is what makes carbon block filters so efficient. Because carbon block filters can filter out contaminants that are as small as 0.5µm, they will filter out the majority of microplastics, and even some nanoplastics!

Reverse Osmosis Systems

The word osmosis might sound familiar, as it is a commonly taught concept in biology classes. Osmosis is the process of solvent molecules moving from a region of low solute concentration to an area of high solute concentration across a semipermeable membrane. Semipermeable membranes let some solutes through, but not all. Reverse osmosis systems use semipermeable membranes as well as pressure to remove contaminants from water [11]. Contaminated water is forced through the semipermeable membrane using pressure. The contaminant molecules do not fit through the membrane, and therefore the water on the other side of the membrane is free from contaminants, such as microplastics. This is the reverse of osmosis, as the contaminated water moves from an area of lower concentration of water to an area of higher concentration of water. Reverse osmosis systems are incredibly effective and can filter out particles that are as small as 0.001µm, so nearly all microplastics can be removed from the water, as well as a lot of nanoplastics.

Large Scale Prevention of Microplastics in Drinking Water

While personal water filters can help prevent the ingestion of microplastics on a small scale, we need to acknowledge the issue of microplastics on a larger scale, to reduce the impact of microplastics on our health. Primary microplastics are perhaps easier to reduce the numbers of as an individual. Some cosmetic products use microplastics called microbeads, so avoiding those products (or even joining campaigns that call for their ban) would be an easy way to make a difference. Another everyday source of primary microplastics is synthetic clothing, as when it gets washed, the erosion forms microplastics, so trying to reduce the frequency of machine washing synthetic clothes can also help [2]. Different washing methods and detergents also affect the number of fibres released from clothing when it is washed. One piece of research estimates that over 700,000 fibres could be released from a 6kg wash load of acrylic fabric [12].

To truly deal with the issue of microplastic, we need to address the wider and significantly larger issue of plastic pollution. Plastic production has been constantly rising since 2008, with 381 million tonnes of plastic being produced in 2015 [13]. As secondary microplastics are formed from the breakdown of larger plastics, decreasing the consumption of plastic would decrease the number of secondary microplastics. This is a huge issue to tackle, but by taking small steps in our personal lives, such as trying to avoid the use of plastics where possible, we can be part of a much bigger impact. Some countries, including the USA, UK, Italy, and Korea, have gone as far as to ban microbeads, a move that will certainly decrease the amount of microplastics in general [14]. As for industry, a large responsibility lies with them.  Moving towards circular economies, where processes are designed to keep out pollution and waste and keep materials in circulation, would make a substantive difference in reducing the amount of microplastics in our world and water systems, as well as reducing the overall amount of waste produced [15].

While we do not yet know how big of a risk the physical hazards from particles, chemicals from plastics, or biofilms pose to human health, it would be sensible to take caution, especially as we young people will be some of the first to feel the effects of them. However, we need to be aware of the facts and not be scared by companies trying to sell us products. Filters for microplastics may become more necessary in the future, as the scale of plastic pollution changes alongside our understanding of the health implications of consuming microplastics. However, for now, we should take caution, do our best to reduce the amount of plastic we use and call on our leaders to introduce policies that will minimise the microplastics that enter our environment.

References

[1] M. Kosuth, E.Wattenberg, S. Mason, C. Tyree, D. Morrison, “Synthetic Polymer Contamination in Global Drinking Water,” Orbmedia, May 16, 2017, [Online]. Available: https://orbmedia.org/stories/Invisibles_final_report. [Accessed 10 February 2021].

[2] K. Rogers, “Microplastics”, Encyclopedia Britannica, September 8, 2020, [Online]. Available: https://www.britannica.com/technology/microplastic [Accessed 10 February 2021].

[3] World Health Organization, "Microplastics in drinking-water,” WHO, 2019 [Online]. Available: https://www.who.int/water_sanitation_health/publications/information-sheet190822.pdf?ua=1. [Accessed 10 February 2021].

[4] Y. L Liao, J. Y. Yang. “Microplastic serves as a potential vector for Cr in an in-vitro human digestive model.” Science of The Total Environment, vol. 703, p.134805, 2020. Available: https://doi.org/10.1016/j.scitotenv.2019.134805.

[5] The Editors of Encyclopedia Britannica, “Legionnaire Disease”, Britannica, November 15, 2018 [Online]. Available: https://www.britannica.com/science/Legionnaire-disease [Accessed 27 February 2021].

[6] World Health Organization, “Information sheet: Microplastics in drinking-water,” WHO, 2019, [Online]. Available: https://www.who.int/water_sanitation_health/water-quality/guidelines/microplastics-in-dw-information-sheet/en/#:~:text=According%20to%20available%20data%2C%20wastewater,tertiary%20treatment%20such%20as%20filtration. [Accessed 20 February 2021].

[7] Water Quality Association, “Granular Activated Carbon (GAC) Fact Sheet,” Water Quality Association, 2013, [Online] Available: https://www.wqa.org/Portals/0/Technical/Technical%20Fact%20Sheets/2016_GAC.pdf. [Accessed 11 February 2021].

[8] The Editors of Encyclopedia Britannica, “Adsorption,” Encyclopedia Britannica, August 6, 2013, [Online]. Available: https://www.britannica.com/science/adsorption. [Accessed 20 February 2021].

[9] K. A. Reynolds, “Microplastics in Drinking Water”, Water Conditioning & Purification Magazine Online, July 15, 2019, [Online]. Available: http://wcponline.com/2019/07/15/microplastics-in-drinking-water/. [Accessed 20 February 2021].

[10] H. Nowicki, W. Schulinger, G. Nowicki, B. Sherman, “Activated Carbon Principles and Practices for Drinking Water Applications,” Water Conditioning & Purification Magazine Online, June 30, 2015, [Online]. Available: http://wcponline.com/2015/06/30/activated-carbon-principles-and-practices-for-drinking-water-applications/. [Accessed 20 February 2021].

[11] R. D. Braun, “Chemical Analysis”, Encyclopedia Britannica, April 1, 2016, [Online]. Available: https://www.britannica.com/science/chemical-analysis/pH-determinations#ref621100. [Accessed 20 February 2021].

[12] I. E. Napper, R. C. Thompson, “Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions.” Marine pollution bulletin, vol. 112, no.1-2, pp.39-45, Nov 2016. Available: https://doi.org/10.1016/j.marpolbul.2016.09.025.

[13] H. Ritchie, “Global plastics production, 1950 to 2015”, Our World in Data, [Online]. Available: https://ourworldindata.org/grapher/global-plastics-production?time=earliest..2015. [Accessed 20 February 2021].

[14] OECD, “Microbeads in cosmetics,” OECD, [Online]. Available: https://www.oecd.org/stories/ocean/microbeads-in-cosmetics-609ea0bf. [Accessed 27 February 2021].

[15] D. Mitrano, W. Wohlleben, “Microplastic regulation should be more precise to incentivize both innovation and environmental safety,” Nature Communication, vol.11, no.1, pp. 1-12, October 2020. Available: https://doi.org/10.1038/s41467-020-19069-1.