Removal of microplastics from drinking water

Microplastics are one of the major environmental problems today. Whether they contaminate food, water or even the air, they have reached all parts of the globe and pose a threat to all living things that consume them. As plastic use continues to increase with seemingly little change on the horizon, groundwater has become an alarming victim of pollution. Fortunately, there are water filtration systems that can largely remove microplastics from drinking water.

How does microplastic get into the water?

Microplastics are pieces of plastic waste that are smaller than 5 millimeters and larger than 100 nanometers (0.0001 millimeters). They result from plastic products that degrade over time and products that intentionally contain microplastics. Microplastics can enter drinking water in one of the following ways:

  • Surface runoff
  • Wastewater
  • Degraded plastic waste
  • Atmospheric deposition
  • Plastic used to store water

Runoff and wastewater are the most common sources of microplastic pollution worldwide. Recent studies have found microplastics at a concentration of 15.2 particles per liter in two US aquifers.

How to remove microplastics from drinking water?

To remove microplastics from drinking water in domestic and industrial settings are reverse osmosis and ultrafiltration systems. Microplastics are between 100 nanometers and 5 millimeters in size, so a filter’s micron rating must be able to filter contaminants between 100 nanometers and 5 millimeters in order to effectively reduce levels of microplastics of all sizes. One micron is 1000 nanometers, so a filter must have a micron rating of 0.1 to remove contaminants 100 nanometers in size. Both reverse osmosis and ultrafiltration systems have a micron rating capable of filtering contaminants below 100 nanometers in size, while water stills separate the microplastics from the water during the distillation process.

Filtration of microplastics with reverse osmosis

Reverse osmosis (RO) systems are effective at reducing high levels of microplastics as well as most other contaminants in water. Standard reverse osmosis systems use three stages in the filtration process, while some systems contain four or five stages. In an RO system, the water begins by passing through a sediment filter. As the name suggests, this phase reduces the levels of large contaminants such as dirt, sand, rust and other microscopic particles. This not only filters the water, but also prevents these contaminants from damaging the reverse osmosis membrane in later stages. After the sediment filter, the water is subjected to an activated carbon filter. This step removes contaminants that cause the water to taste and smell bad, such as chlorine. The last step of the three-stage reverse osmosis system is the reverse osmosis membrane. Water enters under pressure as it passes through a semi-permeable membrane. At this stage, most pollutants are eliminated from the water. The RO membrane contains small pores (0.001 microns) that allow water to pass through while washing away small contaminants, such as microplastics, in the channel. Some reverse osmosis systems contain a carbon filter as a subsequent stage to ensure the purity of the final product. A remineralizing filter can be added to the system to reintroduce beneficial minerals lost in the filtration process.

The advantages of using reverse osmosis systems for the removal of microplastics are as follows:

  • Reduces levels of most pollutants;
  • Long system life;
  • Less maintenance costs than ultrafiltration.

The disadvantages of the systems on the other hand are as follows:

  • The reverse osmosis process generates wastewater;
  • Slower filter speed than ultrafiltration;
  • It requires a higher system inlet pressure, which necessitates the use of pumping systems.

Filtration of microplastics with ultrafiltration

Ultrafiltration systems use membranes with a permeability of 0.02 microns for filtration. This permeability is sufficient to effectively reduce levels of microplastics of any size. Unlike reverse osmosis, which forces water through the outside of the membrane, ultrafiltration systems filter water through the inside of the membrane. Also, unlike RO systems, ultrafiltration does not discharge wastewater during the filtration process. In areas where water is limited due to drought, ultrafiltration is a common choice over reverse osmosis. Ultrafiltration does not remove as many pollutants as reverse osmosis systems, but it does reduce the levels of bacteria, viruses, as well as heavy metals such as lead, copper. Some consumers may prefer the taste of water filtered by ultrafiltration, as minerals are not removed from the final product.

Advantages of ultrafiltration to remove microplastics:

  • Fast filtration rates
  • Maintains beneficial minerals in the final product
  • Easy installation
  • Works at low water pressure
  • It does not produce waste water

Disadvantages of ultrafiltration systems are as follows:

  • Short installation life
  • Higher maintenance costs than RO
  • Removes fewer contaminants than RO

Membrane-based technologies are effective in removing microplastics from polluted aquatic ecosystems. Microplastics are removed more efficiently at high inflow, higher concentration, using high quality membranes. Biological processes combined with porous membranes can improve removal efficiency by 99.9%. Despite the removal efficiency in research or small-scale application, there are still some unresolved issues in these technologies, such as relatively high energy consumption, varying removal efficiencies, and potential discharge of secondary pollutants, which would limit the promotion of large-scale application of these technologies .

Source: International Water Association


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