The right kind of filter can keep microplastics out of drinking water

The right kind of filter can keep microplastics out of drinking water thumbnail
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Out of all the plastic that has even been produced globally, less than 10 percent has been recycled. One of the biggest environmental dilemmas with this is that plastic does not decompose, it only breaks down into smaller pieces that can contaminate soil and water. Microplastics are small plastic particles between one and five millimeters in length. Those smaller than one micrometer are called nanoplastics.

So far, microplastics have been found in water sources like lake water, groundwater, and tap water, and they likely contain the even tinier nanoplastics too. Studies have shown that nanoplastics can be found in tap China, lake Switzerland, and even ice in Northern and Southern Polar regions. The full extent of tiny plastic contamination in drinking water sources is still to be determined . This is because it can be difficult to detect them, making it more difficult to address the problem.

The potential health impact of small plastic particles

Microplastics were recently found in human blood and living lung tissues for the first time, but their effects on human health are not yet fully understood. Ingested microplastic particles may cause an imbalance in the human gut microbiome, which can play a role in the development of gastrointestinal disorders like irritable bowel syndrome and inflammatory bowel disease. However, it is not clear if there is a direct connection.

Despite any risk considerations, releasing large amounts of synthetic material that is not biodegradable into the environment – which results in micro- and/or nano-plastic particles – is not wise, according to Ralf Kagi of the Swiss Federal Institute of Aquatic Science and Technology’s Particle Laboratory.

“Nanoplastic particles could have undesirable effects on ecosystems, and human health,” he says. “The smaller the particles are, the greater the chance that they will be taken up by any organism and distributed in any part of the body .”

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The number of nanoplastics in water sources is expected to increase in the future as plastics continue to degrade, therefore drinking water treatment processes must be equipped to remove them.

Various filtration processes may help provide drinkable water without plastics

Some studies have shown that drinking water treatment plants are capable of removing nanoplastics sufficiently. According to a study published in Science of The Total Environment, a conventional drinking water treatment plant that uses sand and granular activated carbon (GAC) filters–the kind of filter that many water pitcher filters use–can remove nanoplastics by about 88.1 percent. The removal efficiency can increase to 99.4 percent if a coagulation process is also used.

Meanwhile, a different study published in the Journal of Hazardous Materials found that a treatment process called slow sand filtration is just as effective at retaining nanoplastic particles from water sources, if not more. This method uses a thick, biologically active layer, called schmutzdecke, which is placed on top of quartz-sand. The untreated water is first passed through the biological layer, then the layers below it.

The biologically active layer–which consists of organisms like algae, bacteria, and protozoans–is especially effective at retaining the vast majority of particulate materials, including micro- and nano-plastic particles, says Kagi, who is one of the authors of the study.

Pilot scale filtration experiments were performed at Zurich Water Works in order to compare different water treatment processes, and simulate the removal nanoplastics in full-scale drinking water treatment plants.

In the pilot-scale slow sand filtration unit, about 70 percent of the nanoplastics were retained in the first 0.1 meters of the sand bed, and the retention reached 99.5 percent at 0.9 meters. Other processes were less effective. Ozonation, or the infusion ozone into water, does not affect the retention nanoplastics during water treatment. Meanwhile, activated carbon filtration retained only 10 percent in the first 0.9 meters of the filter.

As exciting as this news may be, slow sand filtering is actually an old technology. It was used in the United States for the first time back in 1875. Although it gradually fell out of favor in the late 1800s due to its slow flow rate and inadequacy to treat turbid source waters, it was still a promising filtration method for rural communities.

Slow sand filters are also being phased out in newly constructed water plants due to their extensive space requirements. These are then replaced by ultrafiltration, a kind of membrane filtration system, which uses synthetic polymer membranes to physically separate or strain substances from water, like sand or algae. Kagi says that although they are more expensive, their efficiency is comparable to slow-sand filters. They also don’t take up much space.

There is very limited research on the matter, but the removal of micro- and nano-plastic particles using membrane-based filtration technologies appears to be more effective compared to other techniques. A 2021 study published in Water Science & Technology found that the membrane filtration method displayed a 100 percent efficiency in removing microplastics from wastewaters, as demonstrated in both laboratory- and real-scale filtration results.

Kagi says

“Membrane filters are expected to outperform slow-sand filtration systems in terms of retention of micro-and nano-plastic particles.” While it is encouraging that some water treatment methods can remove plastic particles from contaminated waters sources, the root cause must still be addressed. It is important to reduce plastic use in order to provide potable water that is plastic-free.

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