Reverse osmosis removes more contaminants than any other water filter, while distillation, a form of water purification that doesn’t use filters removes even more. Picking the best water filter for your home is easier said than done – it is important to understand what filters remove which type of contaminants.
Distillation removes more contaminants than any other method of water filtration or purification. The distillation process removes heavy metals, organic compounds, microplastics, and mineral ions, whilst simultaneously killing microbes and viruses. Reverse osmosis removes many of the same contaminants, but does not kill microorganisms or viruses.
It’s helpful to know which water filters remove which contaminants, so this article covers which methods of water purification are best, and importantly, which brands remove more contaminants than everyone else.
Methods of Water Filtration
Here is a summary of the contaminants each method of water filtration and purification can remove. Keep reading to get the full breakdown of each method and which filters and purifiers are the best on the market.
| Method | Heavy Metals | Mineral Ions | Microplastics | Microbes |
|---|---|---|---|---|
| Distillation | ✓ | ✓ / ✗ | ✓ | ✓ |
| Reverse Osmosis | ✓ | ✓ | ✓ | ✓ / ✗ |
| Filtration | ✓ | ✗ | ✓ | ✓ / ✗ |
| Ultraviolet (UV) | ✗ | ✗ | ✗ | ✓ |
| Water Softeners | ✗ | ✓ / ✗ | ✗ | ✗ |
Distillation
Distillation is a purification process that involves the boiling and immediate cooling of water. Impure water is boiled, killing any microbes or pathogens that are present. The steam is then collected and re-condensed as clean, purified water.
Most microbes are killed or thermally inactivated. While heavy metals, organic compounds, and mineral salts are left behind as solids when the water evaporates. The distillation process can also naturally soften hard water by removing mineral ions.
Distillation can be used to remove arsenic, lead, cadmium, nitrates, sulfates, phosphorus, magnesium potassium, calcium, sodium, microbes, and copper. It also reduces the TDS of water. However, distilled water is so stripped of mineral salts that it may taste bland and flat.
Volatile chlorine compounds (with boiling points lower than or near that of water) may not be removed by distillation. Although, this can be rectified using a carbon filter after distillation.
Reverse Osmosis
Reverse Osmosis (RO) is one of the most effective purification mechanisms because it drastically reduces the amount of total dissolved solids (TDS) in water.
RO uses a semi-permeable membrane to filter suspended solids, heavy metals, and dissolved contaminants from water.
This membrane is placed between vats of water containing water with different concentrations of contaminants (as a solute). Pressure is applied to move the water from a region of higher contaminant concentration to a region of lower contaminant concentration. This is against the natural osmotic movement, which is why the process is called reverse osmosis.
The semi-permeable membrane, made of polymer matrix, allows only water molecules to pass through its pores. Molecules larger than water (including heavy metals, dissolved and suspended particles) are left behind.
Therefore, contaminants like fluoride, arsenic, nitrate, radium, uranium, sulphates, calcium, phosphorus, magnesium, potassium, lead, mercury, iron, copper, cadmium, chlorites, microplastics, and asbestos can be removed from water using RO.
RO also drastically reduces the TDS of water, making it an effective way to reduce the hardness of the water.
It’s important to remember that RO does not remove bacteria or viruses. Although, the Centers for Disease Control and Prevention (CDC) suggest RO systems can be effective at removing some microbes and waterborne viruses, such as Rotavirus.
Filtration (carbon filter)
Water is filtered by passing it through a physical filter with a pore size small enough to catch contaminants and particles – like a coffee filter. There are different types of filtration processes, depending on the size of the pores and the material of filtration.
- Microfiltration employs a filter with a pore size between 0.1 and 10 microns.
- Ultrafiltration uses filters with smaller pores, between 0.1 and 0.01 microns.
- Nanofiltration uses the finest filters, with pores of size 0.001 microns. At this level, most microplastics are removed.
These filters remove suspended solids, pesticides, organic compounds, chlorine, chlorine compounds, and microplastics, but cannot remove dissolved minerals, bacteria, or nitrates. Technically, nanofilters can remove viruses because of their small pore size.
Carbon filters are often used in addition to the semipermeable membrane in RO systems. These filters contain cartridges filled with activated carbon (from coconut fiber or other sources). Activated carbon removes contaminants through the process of adsorption.
Adsorption is the phenomenon by which particles stick to the surface of the adsorbing material. This is different from absorption because the particles are not sucked into the material itself – they are merely attracted to the surface of the adsorbing material like iron pieces stuck to a magnet.
When impure water is poured into a carbon filter, an initial filter removes contaminants according to its pore size. Then, as the water passes through the cartridge, materials like lead and microplastics stick to the activated carbon, and only filtered water passes through the filter.
Filtration removes arsenic, radium, lead, microplastics, mercury, iron, copper, chlorine compounds, cadmium, asbestos, and reduces turbidity of water. Filtration cannot remove dissolved mineral salts from water; therefore, it cannot alter the hardness of the water.
UV
Ultraviolet (UV) light is a form of non-ionizing radiation that has a wavelength between 100-400 nanometers. UV is of three bands – UVA, UVB, and UVC. While UVA and UVB are harmful to human skin, UVC can be used in the purification of water.
UVC (from both solar and artificial sources) has a wavelength between 100-280 nanometers. This has germicidal properties and can kill bacteria, viruses, and other microbes.
UV cannot reduce TDS or remove organic compounds and minerals, but it is very effective in killing bacteria, yeasts, spores, algae, and viruses. Pseudomonas, salmonella, clostridium, staphylococcus, saccharomyces, penicillium, mucor spores, influenza, hepatitis, poliovirus, rotavirus, and Chlorella algae are killed by UVC.
Under-Sink Filters
Under-the-counter and under-sink filters mostly use RO, which can remove heavy metals, sulfates, calcium, magnesium potassium, organic compounds, and hardness.
Some under-sink filters also use microfiltration, which removes pesticides, chlorine compounds, and other suspended solids. However, under-sink systems that use reverse osmosis or microfiltration cannot be used to effectively remove microbes (bacteria or viruses).
Gravity-fed Filter
Gravity based filters usually use a carbon filter mechanism. They are mainly used to remove chlorine and chlorine compounds from water (provided they have an NSF-42 certification). Their GAC (Granular Activated Carbon) filter will remove arsenic, heavy metals, and microplastics, but cannot remove dissolved minerals like potassium, magnesium, or calcium.
Softeners
Water softeners are used to soften naturally hard water.
Hard water contains high levels of calcium and magnesium salts, between 61-120 mg/L. Extremely hard water contains more than 180mg/L of dissolved salts.
Most groundwater is naturally hard because it passes through limestone underground. Hard water is not very tasty and may leave a scaly buildup in plumbing and household appliances.
Softeners use the principle of cation exchange to replace calcium and magnesium ions with sodium and potassium ions. Softeners do not remove any other contaminants – they are used solely to reduce the hardness of water and do not reduce the TDS. This is because TDS refers to the total number of all dissolved matter, and not just calcium and magnesium salts.
Types of Water Filters
Different types of water filters employ a combination of filtration methods to purify water. Let’s take a look at some brands available in the market, and what contaminants each type of filter will remove.
Water Filter Pitchers
Water filter pitchers are portable, compact, and inexpensive. They will contain some form of activated carbon, and some will use ion exchange. Together, water filter pitches typically remove the following contaminants:
- Asbestos
- Lead
- Cysts
- Chlorine
- Zinc
- Copper
- Cadmium
- Pesticides
- Sediments
- Mercury
- Organic compounds (e.g. TTHMs)
Note:
- Water filter pitchers certified to NSF/ANSI standards 42 can remove chlorine.
- Water filter pitchers can remove long and short chain PFAS if they contain granular activated carbon (GAC) or nanofiltration filters (>99% removal), or ion exchange resins (IER) (<90% removal).
- Water filter certified to NSF/ANSI standards P473 can remove PFOA and PFOS.
Microbial contaminant removal
In general, water filter pitchers with an ‘absolute’ pore size rating of less than or equal to 0.3 microns will be capable of removing cysts, and moderately effective at removing bacteria.
- Water filter pitchers certified to NSF/ANSI standards 53 can remove cysts.
- Water filter pitchers certified to NSF/ANSI standards 244 or P231 can remove certain bacteria and viruses.
Faucet-Mounted Filters
Faucet-Mounted filters use an activated carbon filtration mechanism to remove lead, mercury, chlorine, TTHMs, cysts, asbestos, benzene, herbicides, and pharmaceuticals.
Keep in mind that faucet filters are most effective at removing lead, but do not completely remove chlorine from the water.
Faucet filters cannot remove bacteria, dissolved minerals, cadmium, or copper.
On-Counter Filters
These filters use carbon filters to remove mercury, lead, asbestos, pesticides, and pharmaceuticals. Organic compounds can be removed effectively with these filters.
These filters can last longer than pitchers and faucet filters. Some variants also use RO for effective water purification. Most on-counter filters cannot remove bacteria – however, some newer models that incorporate UV sterilization can.
Under-Sink Filters
While these filters are sometimes more expensive than on-counter options, they employ more comprehensive purification mechanisms like RO and distillation. Under-sink filters are recommended if your water source is contaminated, or you get impure, untreated water from wells.
