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How To Choose a Water Filter: What Kind of Filter Do I Need?

Wondering what type of water filter is best for your home?

You’ve come to the right place.

In this 15-minute post, I’ll explain the different types of residential water filtration technologies available, how they work, what contaminants they remove and their pros and cons.

This post is for anyone who is concerned about the quality of their tap water but doesn't know what kind of water filter they should buy:

This guide to the different kinds of water filter technologies & types of water filters will help you answer this important question - in 5 minutes or less.

What's In This Guide

Water Filtration Technologies Explained

Soft Water vs Hard Water

Even thousands of years ago, people knew that it was a good idea to purify water before drinking it.

Boiling water was a common method of purification. They also used gravel and sand to filter out sediment from water.

Then, people were mostly concerned about water turbidity – cloudiness caused by suspended particles like dust and silt.

They did not yet know about microscopic bugs like bacteria that were much worse than a bit of sand in the water.

We obviously know better today and we’ve invented a dozen new filtration methods to make sure water is actually clean and not just clear.

Modern filtration technologies work through different methods.

Some filter out impurities from water, only allowing water molecules to pass through the media.

Some use chemical process such as chlorination while others make use of electromagnetic radiation – specifically UV light – to purify water.

Here are the most common water filtration technologies used in home water filters.

Activated Carbon


Activated carbon in block and powder form. Source: Wikimedia 

This is the most common type of filtration media used in home water filters.

Activated carbon is charcoal that has been processed, using either hot gasses or chemicals, to increase its porosity.

It has a huge number of micro-pores that increase its surface area and which adsorb impurities in water.

When water goes through activated carbon, molecules of chemicals, small sediment particles and other impurities become trapped in the tiny pores within the internal surface area.

The carbon also reacts with chlorine in water to form chloride ions. This eliminates the unpleasant taste of chlorine and chloramines used in municipal water treatment.

What can Activated Carbon Remove?

  • Chlorine and chloramines
  • A wide range of potentially harmful chemicals including dioxane, arsenic, benzene, chromium and mercury. (see the full EPA list here)
  • Certain microbes including Calicivirus, Cryptosporidium and Saxitoxin.
  • A couple of radiological compounds –cobalt and radium.
  • Small particles of sediment (although most filter systems use a sediment pre-filter to avoid clogging up the carbon block).

What Can’t Activated Carbon Remove? 

  • Inorganic contaminants such as asbestos, nitrates, nickel, fluoride and selenium. However, some modern carbon filters can reduce some of these contaminants.
  • Heavy metals like lead and cadmium. As with inorganic contaminants, some carbon filters do reduce the levels of some heavy metals like mercury.
  • Dissolved minerals that cause water hardness
  • .Most bacteria and viruses.

Reverse Osmosis water filter


If you remember half of your high school biology, you may know a bit about osmosis.

It’s a process where water moves from a less concentrated solution through a semi-permeable membrane into a more concentrated solution.

Plant roots use it to absorb water from the ground and our kidneys use it to absorb water from the blood stream.

Reverse osmosis is the opposite. Water moves from a more concentrated solution to a less concentrated one.

But unlike normal osmosis, reverse osmosis cannot happen on its own. Pressure is needed to push water back through the semi-permeable membrane.

The membrane only allows water through. 99% of impurities are left behind.

Here’s a gif showing how it works.

That’s how a reverse osmosis (RO) filter works.

It is highly effective at ridding water of a wide range of chemicals, dissolved solids and harmful microbes.

What can a Reverse Osmosis Filter Remove? 

  • Dissolved solids such as calcium, sodium, potassium, iron and zinc.
  • Heavy metals including mercury, lead and Nickel.
  • Chemicals including those used in insecticides and herbicides.
  • Chlorine and chloramines.
  • Most microorganisms including cysts, bacteria and viruses.

Note: Most RO systems consist of multiple filters.So in reality, the RO membrane in your filter may never come into contact with the chlorine in your water (which can actually damage it) since it has already been removed by a carbon pre-filter.

What can’t a Reverse Osmosis Filter Remove? 

  • Some pesticides and herbicides that have molecules smaller than water.
  • Certain ions and metals.
  • Some organic chemicals like Toluene and Benzene

Ion Exchange


Ion exchange is a physical-chemical reaction where an exchange of ions between contaminants in water and a resin bed is used to purify water.

An ion exchange filter works by passing water through bead-like materials called resins. As the water passes through, contaminants exchange their ions with the ions on the beads.

Ion exchange removes many dissolved solids in water including hardness minerals, toxic heavy metals and other dissolved inorganics.

Like most other filters, an ion exchange resin bed has a finite lifespan after which it needs to be replaced.

What can an Ion Exchange Filter Remove? 

  • Heavy metals like lead and chromium.
  • Hardness minerals like potassium.

What can’t an Ion Exchange Filter Remove? 

  • Harmful microbes.
  • Sediment and large particles.
  • Dissolved organic chemicals.
  • Chlorine (it can actually degrade the resin bed).

Because of these limitations, ion exchange systems usually consists of multiple-stages with several filters to remove different kinds of contaminants.

 So while an ion exchange resin bed might not remove chlorine, the carbon pre-filter can.

UV Water Filters


Ultra violet radiation is an extremely effective way of disinfecting water. UV filters specifically target microbes like bacteria, viruses and cysts, neutralizing over 99% of them.

UV disinfection works by radiating the cells of these microbes. This damages their DNA, preventing reproduction.

UV filters are easy to install, last a long time before you need to replace the bulb and consume very little energy.

But because they can only target microbes, you should only use it to complement other filters that can remove the other contaminants.

What can a UV Filter Remove?

  • Harmful water-borne microbes that can cause a variety of diseases in humans and pets.

What can’t a UV Filter Remove? 

  • Dissolved solids including hardness minerals.
  • Heavy metals.
  • Chlorine and chloramines.
  •  Organic chemicals.

Other Filtration Technologies


The above are the most common. Here are a few others you may not have heard about, including some advanced modern technologies.

Ceramic filters: Like activated carbon and RO membranes, ceramic filters also use a physical process to filter out contaminants. As water passes through the tiny pores in the ceramic, contaminants that can’t fit through are filtered out.

Ceramic filters can remove some microbes and sediment. On their own, they are not very effective at removing viruses (too small), chemicals and dissolved solids.

That’s why they are often combined with other types of filters, usually an activated carbon filter.

Activated alumina: This is a filtration media made from aluminum hydroxide. It is processed to create a highly porous material that works the same way as activated carbon.

Activated alumina is often used to filter out fluoride, selenium and arsenic from water.

Like activated carbon, activated alumina cannot remove microbes from the water. So it is necessary to combine it with a UV or RO filter to get pure drinking water.

Hollow fiber filters: Another filter that utilizes the size exclusion technique to remove contaminants from water. In this case, tiny pores in the fibers only allow water to pass through, leaving microbes trapped within the fibres.

This type of filter is only ideal for removing bacteria, cysts and sediment. It does not remove chemicals and dissolved solids like iron and potassium.

Types of Water Filters Explained

types o filters

Most of the filtration technologies/media we have discussed above are not used on their own. They are combined with other filters to create a multi-stage system.

This has a couple of benefits:

  • It enhances filtration performance.What cannot be removed in the first stage is removed in the second one. By the time the water gets out on the other end, almost all contaminants are gone.
  • It prolongs the life of individual filters. If an activated carbon filter had to filter out sediment, it wouldn’t last long before it became too clogged. If chlorine reaches an RO membrane, it can degrade it. Having multiple filters ensures each filter lasts longer and functions at its best.
Here are the main filter systems used in homes. They go down in size from whole-house water filters to carafe water filters.

Whole House Water Filters 

Aquasana EQ-1000

A whole-house water filter treats all the water coming into your house. You install it on the main line into your house just after the water meter and before the heater.

It’s a good choice when you want all faucets and showers to produce treated water.

A whole-house filter typically has at least three stages though some systems can have as many as five or six.

The stages include a sediment pre-filter, an activated carbon filter and a post-filter.

Some systems also include a UV filter to neutralize microbes. Systems designed for use with well water have stages to handle dissolved solids like iron and manganese.


  • Filters all the water coming into the house. This eliminates the need to install additional filters around the house (though you should still consider an under-sink or countertop RO filter for drinking water if your whole-house system doesn’t include a UV filter).
  • Prevents unsightly deposits of iron on surfaces, clothes and utensils.
  • Improves the quality of your shower water, which is great for your skin.


  • Expensive
  • Takes more time and tools to install –you may have to hire a plumber.
  • Most whole-house systems do not neutralize harmful microbes.

Under Sink Water Filters

osmosis system

If you only need filtered water for drinking or cooking, an under-sink water filter is a much more convenient and cheaper alternative to a whole-house system.

It goes under the sink – obviously – and connects either to the existing faucet or to a dedicated one.

Under-sink filters have at least 2-3 stages but some can have as many as six or more.

The stages includes sediment pre-filter and an activated carbon block.

RO under-sink filters also have an RO membrane plus an alkalization stage to reduce the acidity of filtered water. Most also pass the water through a mineralization stage to return stripped minerals.


  • Cheaper than a whole-house system.
  • Better quality drinking water (especially if you use an RO system).
  • Easy to install and maintain.
  • They are installed under the sink out of sight – great for aesthetics.


  • Filters water only at one access point. All other faucets will still produce unfiltered water. This can cause problems if you receive hard water or well water laden with dissolved solids.
  • Most under-sink systems reduce water pressure, making them frustrating to use at times. This is especially the case with RO systems.

Countertop Water Filters 

choose a water filter

Instead of going under the sink, these filters sit on your kitchen countertop.

You connect a countertop filter directly to the end of your faucet and the filtered water comes out of a different faucet integrated into the filter housing.

They work essentially the same way as under-sink filters except that they are smaller, easier to install and connect to the faucet instead of the water line.


  • Small and compact.Ideal for small kitchens, RVs and boats.
  • Easy to install. No tools needed.


  • Take up space on the countertop.
  • Filtered water comes out with low pressure.
  • They only filter water at a single access point.

Water Filter Pitchers & Carafe Filters 

choose a water filter

Under-sink and countertop filters are good and all but they are not very helpful when you are away from home.

If you want a portable solution, carafe filters are great.

This setup consists of a pitcher with an integrated filter. It can be a single type of filter, usually activated carbon, or a combination of 2 or 3 filters.

When you pour water into the pitcher, it passes through the filtration media. When you pour yourself a glass of water from the pitcher, it is free of chlorine, lead, sediment and a variety of other impurities.

Carafe filters are great for RVs, camping or for those who don’t want to go through the hassle of installing an under-sink or whole-house system.


  • The cheapest water filtration solution.
  • Portable.
  • Perfect for small households.


  • Limited amount of filtered water.Even a large carafe cannot compare to a steady flow from an under-sink filter.
  • Not ideal for dirty water containing a lot of impurities. Unlike an under-sink filter that can have as many as six stages, a carafe filter contains at most two types of filters that cannot remove everything.

What Kind of Water Filter Should You Buy? 


The answer is - it totally depends on your budget and needs:

If you are looking for a low-cost filter for a mid-size or large family, then buy an under-sink or countertop filter.

If you live on hard water or well water, then a whole-house water filter with a water softener option will protect your plumbing, clothes and appliances from the effects of dissolved minerals.

If you are looking for a filter for your RV or boat, then buy a countertop or carafe filter.

If your family travels a lot, buy a carafe or portable water filter and carry it with you.

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Ed Carmichael