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

Slow sand and membrane filters can knock out nearly all of the tiny pollutants.
Water glasses being filled.
The health impacts of micro- and nano- particles is still relatively unknown. Unsplash

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. Small plastic particles between one micrometer and five millimeters in length are called microplastics; 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. In fact, studies have identified nanoplastics in tap water in China, lake water in Switzerland, and even ice samples in the Northern and Southern polar regions. However, the full extent of tiny plastic contamination of drinking water sources has yet to be known because it is challenging to detect them, which can make 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, a direct link has yet to be established.

Regardless of any risk considerations, releasing enormous amounts of non-biodegradable, synthetic material into the environment—which results in micro- and nano-plastic particles—is not wise, says Ralf Kägi, head of the Particle Laboratory at the Swiss Federal Institute of Aquatic Science and Technology.

“Nano-plastic particles may have unwanted effects on ecosystems and human health,” he adds. “The smaller the particles, the higher the likelihood that they can be taken up by any organism and distributed, for example, in the gastrointestinal tract.”

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 show that drinking water treatment plants can filter nanoplastics well enough. 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. In this method, water is treated using a thick, biologically active layer called schmutzdecke that lies on top of quartz sand. The untreated water passes through the biological layer first, and then the layers of sand 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 Kägi, who is one of the authors of the study. 

Pilot-scale filtration experiments were conducted at the Zurich Water Works to compare different water treatment processes and simulate the removal of nanoplastics in a full-scale drinking water treatment plant.

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 not as effective. For instance, ozonation or the infusion of ozone into water does not affect the retention of 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 is, slow sand filtration is actually a pretty 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. They are generally more expensive, but the efficiency is comparable to slow sand filters and they don’t take up as much space, says Kägi. 

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.

“Membrane filtration systems are expected to even outperform slow sand filtration systems regarding the retention of micro- and nano-plastic particles,” says Kägi. Although it’s very promising that some water treatment processes can be effective at removing plastic particles from contaminated water sources, the root of the problem must still be addressed. Minimizing plastic use as much as possible remains paramount in providing plastic-free, potable water.