How to Filter Water: 2021 Ultimate Guide by Experts

How to Filter Water

The average drinking water source contains traces of more than 200 contaminants. It’s understandable if you’re concerned about the quality of the water you’re exposed to.

Luckily, there are plenty of ways to filter your water and make it safe to drink.

In this guide, I’ll be discussing the ins and outs of filtering your water, including the best methods, the benefits of filtration, and much more.

When Should I Filter My Water?

At Home

You might think that your home’s drinking water is relatively clean, but it could still be a whole lot cleaner. While water treatment centers must legally remove certain contaminants down to trace levels, that doesn’t mean your water is completely free from impurities.

You might want to filter your water at home if you have health concerns about some of the contaminants you’re drinking, or if water poses an aesthetic problem (such as limescale).

filtering tap water at home

In the Backcountry

Most backcountry water sources in developed countries can make us sick to drink. This water can contain contaminants like bacteria, which should be filtered out before you drink it.

Whether you’re hiking or backpacking in the backcountry or living in a developing country for the indefinite future, using a filter to make sure your water is clean and safe is essential.

filtering water in the backcountry

How to Filter vs Purify Water

There are two popular methods of water treatment currently available: filtration and purification.

Water filtration sifts out contaminants, producing filtered water.

Purification, however, uses a different method to eliminate certain contaminants, like microorganisms, in water.

Filtered water may still contain trace elements or other smaller particulates, while purified water is usually free from these impurities.

Related: Filtered Water vs Purified Water: The 2021 Definitive Comparison

Water Filter Methods

I’ve listed some of the most popular methods of water filtration below:

Activated Carbon

Activated carbon filters are some of the most widely available on the market. These filters are categorized into two types: carbon block and activated carbon, which both use adsorption to grab hold of contaminants as water passes through the media.

To this day, activated carbon filtration is considered one of the best water treatment options available for improving the aesthetic quality of water.

You can find activated carbon filter cartridges in a range of filtration systems, from countertop pitcher filters to faucet-mounted, under-sink and whole-home applications.

There are a number of raw materials that are used in an activated carbon filter, such as charcoal and coconut shell. Because these materials contain a naturally high carbon content, they’re ideal sustainable water filtration solutions.

The effectiveness of an activated carbon filter depends on its design. Typically, the larger the surface area of this filter media, the more thoroughly it will filter water, as there’s more opportunity for adsorption.

These filters are capable of removing chlorine, lead, and other aesthetic contaminants from water.

activated carbon filter cartridges
Activated carbon water filters

Ion Exchange

Ion exchange filter media is typically found in a water softening application. Water softeners are installed at your water’s point of entry in your home to provide whole-house softening benefits.

This type of water treatment system removes calcium and magnesium – known as hardness minerals – in water. It can also be used to reduce iron, manganese and sulfur.

The ion exchange method of filtration works by replacing hard water ions with another type of ion, usually sodium or potassium chloride.

When water flows through the negatively charged resin bed, the positively charged calcium and magnesium minerals are attracted to it, and stick to the bed. At the same time, positively charged sodium ions are released into the water to replace the missing mineral ions.

If you’re dealing with hard water issues in your home, such as limescale, stained laundry and itchy hair and skin, an ion exchange system is the most effective means of treatment. This method entirely removes hardness minerals from water, greatly improving water quality and eliminating your hardness issues going forward.

springwell salt-based water softener system
Ion exchanged water softening system

KDF Media

KDF, or kinetic degradation fluxion, is a filter media made from high-purity copper-zinc granules.

When water flows into this media, a redox reaction takes place, which can reduce or remove a select number of contaminants. This works a little like ion exchange: electrons are transferred between molecules and impurities are converted into a different form. They can then be removed from the system with backwashing.

A big advantage of KDF technology is that the process itself has been certified by NSF international to Standard 61. Having a certification from an accredited testing body in the United States means that KDF can be trusted as a safe, legitimate water treatment solution in your home.

KDF media can remove a broad range of contaminants, though the exact contaminants removed will depend on the type of media you opt for.

Some KDF technologies can remove contaminants like heavy metals, while others can filter out chlorine, iron and hydrogen sulfide. Some KDF systems can also remove magnesium and calcium, and even pathogens like algae, bacteria and fungi.

kdf media
KDF media

Reverse Osmosis

Reverse osmosis filters are so effective at what they do that they are often considered a purification solution – however, because they use filter media to remove contaminants, they’re still technically classed as filtration systems.

A reverse osmosis filter can be installed for under-sink, countertop and whole-house use. Under-sink RO filters tend to be the most common option.

In a reverse osmosis unit, unfiltered water flows through a number of filtration stages, namely a sediment pre-filter, a carbon filter, a semi-permeable membrane and a post-filter.

The most impressive feature of any RO system is the semi-permeable membrane. This membrane has tiny pores that prevent almost all minerals, salts, heavy metals, chemicals and even microorganisms from passing further into the system.

The result is that only tiny water particles make it out the other side, while a small amount of water containing impurities is washed down the drain.

RO water filtration can remove more than 99.9% of TDS (total dissolved solids).

reverse osmosis system for home
RO system


Deionization is a process that removes both positively and negatively charged ions from water, producing a solution that’s entirely free of charge. Ions in tap water can come from a number of sources, such as pipes and soils. Some examples of the impurities that may affect water’s charge are magnesium, calcium, iron, sodium, carbonates, nitrates and sulfates.

The deionization process takes place in a deionizer. Water runs through an electrically charged bed of resin, which holds onto the cations and anions responsible for water’s charge. When the process has finished, the system flushes out the resin bed to remove these impurities.

While deionization can effectively eliminate the majority of minerals and salts, it doesn’t tend to be an option for removing contaminants like chemicals and heavy metals. You would have to combine a deionizer with another water filtration solution if you wanted to tackle a broader range of contaminants – but deionized water isn’t typically recommended for drinking, anyway.

Mechanical Filtration

Mechanical filtration is a process in which contaminants are physically removed using a special blocking application. Normally, a mesh screen is used to filter out larger contaminants, like dirt, rust, sediment and sand, followed by a ceramic filter, which is designed to trap impurities like microorganisms.

The performance capabilities of a mechanical water filter depend on the micron size you opt for.

A mechanical filter with a 5-micron rating will be best suited to remove particles that are too small to be seen by the human eye, while a 1-micron mechanical filter will remove even smaller particulates that require a microscope for viewing. Finally, the lowest available micron rating for a mechanical filter is 0.5 microns; this can remove microorganisms like cysts, giardia and cryptosporidium.

Mechanical filtration is an effective solution for drinking water that contains dirt, but it won’t remove chemicals, so you may need to use it in conjunction with another filter for the best results.

mechanical water filtration
Mechanical water filter cartridges

Ozone Filtration

Ozone filters are a slightly lesser-known water treatment option that can kill bacteria and other microorganisms in drinking water. This FDA-approved filtration method uses ozone, or o3, to disinfect water and make it safe to drink.

A number of contaminants can be inactivated using ozone filtration, including fungi, pesticides, viruses, and other organic materials. Because ozone is a powerful oxidant, it can also oxidize metals like sulfur, manganese and iron, making it easier to filter them out.

Ozone is a much more appealing alternative to chlorine, a disinfectant that leaves chemical traces in water. With ozone, no chemicals are added, and the ozone actually disappears after it has been used for filtering purposes.

US Water Systems Matrixx Infusion Iron and Sulfur Removal System
Ozone injection system

DIY Water Filters

If you need to filter water unexpectedly, and you don’t have time to buy one of the water filters mentioned above, it’s possible to make your own filter using materials you may already have at home.

There are several different DIY water filters you could make, including a solar filter, a stovetop distiller, and a charcoal distiller. Some take more effort than others, and not all of them will provide you with instant clean water.

YouTube has plenty of great videos on how to make your own water filters if you’ve got lots of spare time on your hands and fancy a challenge.

Just keep in mind that, while these systems are cheaper to make and maintain, they’re a lot more effort, and they’re usually nowhere near as effective as water filters manufactured by an industry expert.

Water Purification Methods

Some of the most well-known water purification methods are as follows:


Distillation involves the method of boiling water until it evaporates, then allowing it to condense into a clean container.

When a distiller is used to boil water, the majority of water’s contaminants, which don’t have the same boiling points, are unable to evaporate. They’re left behind in the boiling chamber, and can be washed away when you clean out the system.

A huge advantage of using a distiller to purify water is that it does a really thorough job. We’re not just talking about filtering water to remove a certain amount of contaminants – this method massively improves water quality by eliminating nearly 100% of impurities.

There are a few setbacks of this type of purification, however.

Some impurities, like VOCs, may be able to evaporate and condense with tap water, which means they need to be filtered out by a carbon filter at a later stage. This filter will periodically need changing.

Additionally, distilling water is a very lengthy process, taking upwards of 4 hours to get the job done. The system also requires electricity to purify water, which can be pretty costly if you’re using your distiller constantly.

how water distillers work

Chemical Disinfection

Chemical disinfection is another common purification method. If you get your water from a city source, your local authority will most likely treat your water with chlorine or chloramine to kill dangerous or unhealthy contaminants, making it safe to drink.

Many people with a private well also use a chemical disinfectant to treat their water. Chlorine injection systems can add a measured amount of chlorine into water, which is then stored in a tank until the chemical has had time to disinfect the water.

Chlorine and chloramine are cheap and highly effective disinfectants, which is why they’re commonly used today. However, these chemicals are considered contaminants themselves, and while they’re not usually harmful in small amounts, you’d probably rather not consume them if you had the choice.


Boiling water is one of the simplest ways to kill germs in water. It’s fast, too – boiling your water for 3 minutes is usually enough to kill off any nasty germs.

You can boil water in any setting, as long as you have a heat source, a pan, and a storage container. It’s best to boil water in batches and drink it as soon as it’s cooled if you’re out in the wilderness, which will prevent recontamination.

Note that you can’t boil your water to remove contaminants – it can only kill them.

Impurities like heavy metals and chemicals aren’t technically “alive” in the first place, and they have very high boiling points, so heating them up will usually do nothing. It’s only live microorganisms like bacteria and viruses that can be killed during this type of purification.

Related: My guide to boil water notices & advisories

boiling water


Household pasteurization may be a lesser-known purification method, but if you can get your hands on the right tools for the job, it can be highly effective. Pasteurization doesn’t boil a water source – it simply brings it to a temperature that kills pathogens.

Many of us believe that our water needs to be boiled to kill germs, but actually, pasteurizing water at around 71 °C- 75 °C for up to 1 minute will have the same effect. Bacteria, a common waterborne pathogen, is killed at 60°C, while Hepatitis A is killed at 65 °C – so you don’t need as much heat as you may assume.

Pasteurization is a good option if you’re looking for an alternative to boiling your water that’ll save energy and money in the long run. Again, however, this type of purification won’t remove chemicals or metal contaminants from your water.

UV Treatment

UV treatment uses a UV lamp to kill pathogens like viruses, protozoa and bacteria in water. UV doesn’t remove microorganisms from water – instead, it alters their DNA, making them unable to reproduce or cause harm when consumed.

A big advantage of UV purification is that it doesn’t affect your water’s flow rate. It can be installed at the end of whole-house or under-sink filtration applications, and water will simply pass through the lamp without the sort of friction or pull-back you’d get from a water filter.

Note that UV can’t be used to remove odors, tastes, chemicals or heavy metals – you’d need to combine the system with a water filter like an RO unit if removing any of these impurities is important to you.

whole house uv water filter
UV disinfection at the end of a whole house system

Solar Disinfection (SODIS)

An alternative to UV treatment is solar disinfection, or SODIS.

Developed in the 1980s, SODIS is an inexpensive, sustainable method of removing harmful microorganisms from drinking water. This method was initially popular in backcountry regions where the only clean drinking water source was bottled water.

Though it is still most commonly used in developing countries, you can choose to use SODIS at home to remove bacteria, viruses and other pathogens from your water, no matter where in the world you live. If you use recycled plastic bottles for the job, the treatment costs nothing, and is very easy to set up.

The technique is simple: you just add water to a clear plastic bottle and expose this bottle to the sun.

This isn’t a quick fix – you could have to wait up to 48 hours for your water to be clean enough to drink, depending on the intensity of your local area’s UV rays and the concentration and types of pathogens in your water. However, it can be a great solution for emergencies, when clean drinking water isn’t available.

Important Considerations When Selecting a Treatment Option

There are so many ways to filter and purify your water that you’re probably wondering how on earth you can narrow your options down to one.

If you’re on the market for a water treatment solution today, I would recommend considering the following:

Water Source

The impurities you’re dealing with in your water source may determine which filtering or purifying solution you opt for. In some cases, a water filter application will be the best solution, while in others (such as if you’re dealing with microorganisms like bacteria), filtering water may not actually eliminate the problem.

If you’re considering buying a water filter or purifier, I’ll assume that you want to remove a particular contaminant.

Should you not know what’s in your water, however, I would recommend getting a lab test, which will give you the clearest indication of what you’re dealing with.

Method Effectiveness (Contaminants Removed)

Perhaps you’re just looking to remove a particular problem impurity, but equally, you might be looking for a water filter or purifier that can remove the largest number of impurities at once. In that case, consider filtering water with an RO system, or purifying it with a distiller.

It costs more money to buy filters that can do it all, and you may not need something so advanced. If you just need chlorine and lead removal, you can save a lot of money on carbon filters.

Additionally, if hard water is your biggest problem, avoid traditional filters and go for an ion exchange system.

Filter Capacity

Generally, the higher a filter’s capacity, the better.

Capacity is the amount of water which can pass through a filter before it needs to be replaced. This is often translated from number of gallons to a timeframe of months.

A system can filter water for longer if it has a good capacity, but capacity will vary from one filtering device to the next. Whole-house filtering systems naturally have the highest capacities, as they’re needed for whole-house use. Under-sink and countertop filters usually have slightly lower capacities, while pitcher filters generally have the lowest capacities.

If a filtering system lasts for over a year, it has a pretty decent capacity.

In most cases, however, especially with the smaller filters, you should expect a system to filter water for around three to six months before the filter needs replacing.

Flow Rate

The flow rate of a system will determine how quickly it’ll provide filtration.

The general consensus is that systems that are connected to your main waterline will naturally have a faster flow rate, as your water’s pressure will keep things moving quickly. Gravity filters, on the other hand, may take upwards of 10 minutes to filter a gallon batch, while distillation takes up to 4 hours.

Speed of filtration might not be massively important to you, but if you have a larger family, you may find that some systems are simply too slow to keep up with your demands.

Installation & Maintenace

Again, installation and maintenance varies widely depending on the filtration system you’re looking at.

While whole-home and under-sink systems are generally faster, they tend to be the most complex to install. This is especially the case when you’re dealing with a unit that has multiple components, like an RO filter.

Some filters, on the other hand, require no installation at all – like countertop filters and pitchers, which can be set up and ready to use in seconds.

In terms of maintaining a filter, if you’re looking at a filtering solution on this list, it’s a guarantee that you’ll have to replace the filters somewhere along the line.

Some purification methods require far less maintenance, however; distillation and UV purification can go years without major work (unless you choose to use your distiller’s filter), and boiling or pasteurizing water requires no work at all.

Costs Involved

Finally, your budget is something to consider carefully when deciding which water treatment solution to invest in.

The bigger the system, the more expensive you can expect it to cost.

If you’re new to the idea of filtration, you might want to start buy spending $20-$30 on a filter pitcher and testing it out.

You can then decide whether you’re keen to spend a bigger upfront cost on a larger system that requires less frequent filter changes (or in the case of a purification solution, no filter changes whatsoever).