What is reverse osmosis, and how does it filter microplastics?

What are microplastics, and how can we filter them from drinking water?

Microplastics are plastic particles less than 5 millimeters in length and shed from larger pieces of plastic such as plastic waste, road tires, synthetic clothing, and any other plastics exposed to high heat, UV rays, mechanical abrasion, or chemical solvents.

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These tiny particles can be ingested, inhaled, and even absorbed dermally by humans - and their health effects are already showing in just two generations since the rise of plastic industrialization..

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By far, the largest medium for our exposure to microplastics by quantity is through contaminated drinking water.

While avoiding plastic bottles and replacing them with non-toxic reusable water bottles is a great first step to prevent adding microplastics into your water, the tap water you’re drinking from has an incredibly high chance of containing microplastics.

Logically, it makes sense to tackle the largest methods of microplastic exposure first. So how can we do that? The answer is through water filtration, but not just any kind of filtration.

Popular name brand filters (think Brita and Zerowater pitcher filters, faucet filters, refrigerator filters, etc.) are usually activated carbon filters, which rely on ‘adsorption’ (yes with a ‘d’) methods to attract and trap contaminants in the carbon’s surface. You’ll notice these brands advertise excellent filtration capacity for things like chlorine, mercury, and some bacteria improving taste and odor, but not microplastics.

However, microplastics can be nearly completely removed from water with a quality and well-maintained reverse osmosis water filtration system. 

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The key difference is that reverse osmosis filters have much smaller ‘pores’ in their filter membrane, allowing only water molecules to pass through. Let’s look into exactly how this more powerful type of filter works and why it’s the better option if you’re trying to remove a wider variety of extremely small contaminants, like microplastics.

How does reverse osmosis filtration work, and how does it remove microplastics?

Quick chemistry/biology class refresher. Osmosis is the natural tendency for a less concentrated solution to migrate to a solution with a higher concentration.

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The opposite of this process, reverse osmosis, occurs when energy is applied to the concentrated solution, forcing water molecules through a semipermeable membrane to produce uncontaminated water.

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Reverse osmosis point-of-use (source) filters use this core principle along with a variety of other filtration stages (usually 4-5) to aid a nearly complete removal of nearly all contaminants.

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A sediment pre-filter is the first filter stage in most RO models, removing large particles like dirt, sand, and rust. This is the same technology you might find in a saltwater filter for a pool.

While we mentioned that carbon filters alone cannot remove most microplastics, they are still found in RO systems as the second stage (another pre-filter) to increase the lifespan of the RO membrane, removing chemicals that affect taste and odor. Sometimes, the sediment and first carbon filters might be combined.

The third step in the majority of consumer-grade RO filters is the reverse osmosis membrane itself, made from a thin film composite or cellulose triacetate. This is the step that benefits specifically from high pressure water extruded through the semipermeable membrane with incredibly small pores the size of 1 nanometer.

Another technology frequently paired in RO systems is a UV filter. UV filters use exactly that - ultraviolet light - to kill harmful microorganisms in water. Not all RO systems include it, but it can be worth the feature, as it can extend the life of the system and cartridges by preventing fouling, or the buildup of bacteria inside of the system.

A second post-carbon filter might be included to further remove any lingering taste or odor from the purified water.

Lastly, some reverse osmosis filtration systems might have a remineralization stage built-in. It’s essential to remineralize your filtered water by re-adding these minerals and other trace elements in order to make it safe and drinkable for long-term consumption by humans and animals. Drinking RO-filtered water without remineralization can lead to deficiencies, negatively impacting normal bodily functions.

Depending on the specific model of a reverse osmosis filtration system, the order and quality of these specific stages could vary, but ultimately the technology is largely the same.

Reverse osmosis filters can produce up to 3 to 5 gallons of effluent water (wastewater) for every gallon of pure, contaminant-free water, so this is something to consider when deciding between models. Some brands use technologies that allow for an impressive 1:1 ratio or higher of pure-to-waste water.

These stages make RO systems incredibly effective at removing nearly all contaminants, such as heavy metals (lead, mercury, arsenic, etc), volatile organic compounds (VOCs), microorganisms (bacteria, viruses, cysts), PFAS/PFOS (“forever chemicals”, including fluoride), BPA (bisphenol-A, a toxic plasticizer), and 99.9% of microplastics.

The 3 types of reverse osmosis water filters, and how to choose the right one for you

Reverse osmosis water filters come in three variants for your home - countertop, under-sink, and whole-house systems. All three allow you to filter your own tap water for consumption, and using them is simple. 

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Countertop reverse osmosis systems are the most common and the easiest to install. Typically the size of a large computer tower, they require zero installation meaning no plumbing work is required. The main drawback of this variant is that they do need to be refilled by hand, so that’s something to keep in mind if you have a large, thirsty household.

This type of filter is best for people who aren’t able to change their plumbing such as apartment renters, or those who simply want a plug-and-play way to immediately get microplastic-filtered water. They are effective and work just as well at removing microplastics as larger RO systems, but might need their filter cartridges replaced the most often at around 6 months of regular use.

For around $250-400, a countertop reverse osmosis filter is one of the best upfront investments you could make to reduce most of your microplastic ingestion.

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As the name implies, under sink reverse osmosis filters are installed directly under your kitchen sink. Instead of being manually refilled, these filters are supplied by a direct connection to the incoming water line and connect to your kitchen faucet (no special faucet needed), dispensing filtered water.

Some under-sink filters optionally come with external tanks for storing pre-filtering water at an added cost. Even with added features, under-sink filters are actually less expensive than most countertop filters at around $200-$300, as the overall construction is less complex and might not require digital buttons at all.

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Whole house reverse osmosis filters are the largest, highest flow-rate filters you can invest in for reducing microplastics in your water. Whole house filters do last significantly longer between replacements and can provide clean filtered water for applications beyond the kitchen sink, such as laundry machines, bathroom sinks, and showers.

These large-scale filters can be much more expensive than the previous two options, starting at about $2,000, but they provide the highest gpd (300 or more gallons per day) of any other type of RO system.

It’s worth mentioning that consumers who live in parts of the US with notoriously undesirable ‘hard’ water (meaning containing calcium and magnesium ions) can benefit dually from a whole house filter. Reverse osmosis, in its filtration process, completely softens water and reduces hardness, making it a great two-for-one solution. 

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Microplastic reduction beyond filtering your drinking water

There are plenty of other ways to reduce your daily plastic consumption, such as filtering your home’s air, avoiding synthetic clothing, swapping out toxic kitchenware, and more. 

Check out our full guide on minimizing exposure below.

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