How do water filters work?

How water filters work and why you need one?

How do water filters work and why should you care?

Despite the fact that fresh water is essential for human life and survival most people know very little about the topic. In-fact most people will spend a lot more time researching the purchase of a new TV than they have ever done with drinking water.

The default solution when concerns about the quality or taste of tap water is to buy bottled water. The problem is that in most cases there is no scientific evidence that bottled water is better.

600m households worldwide now choose bottled water instead of tap water every day based mostly on anecdotal evidence.

What if there was a much better and cheaper way for almost every household to get healthy fresh drinking water from the tap? There is and it’s called a water filter.

In this article we will address all the major concerns about tap water and how water filters such as EcoPro address these and how to choose the right water filter for you.

This is a long article about how water filters work so skip to the conclusion if you want a simple answer.

Why should we drink tap water?

Just a little bit over a hundred years ago most homes had to collect water from a local well and carry it home. It was heavy, time consuming and inconvenient. With the introduction of tap water in every home there was suddenly almost unlimited access to clean water at home from the tap. Tap water was generally affordable, convenient and safe.

Over the past 40 years as much as 40-50% of the population in some countries have replaced consumption of tap water with bottled water. Hundreds of millions of people are back to carrying home water. During this time the tap water quality in Europe and North America generally got better and not worse.

In addition to being inconvenient bottled water is also catastrophic for the environment and 100-1000x more expensive than tap water. Even worse it’s not healthier or safer than tap water. In fact in most places with hard water there’s more minerals in tap water than bottled water.

Where does tap water come from?

3% of the earth's water resources is fresh water but only 0.4% is usable for tap water. The most common source is fresh surface water such as lakes, rivers and springs. This makes up about 80% of the world’s water consumption. Second to this comes groundwater including aquifers. 98% of the earth’s fresh water is groundwater and it is about 60 times more plentiful than the surface water. Despite this ground water is becoming more and more of a scarce resource in densely populated areas due to overuse. Third but not last comes desalinated sea-water which is almost unlimited in supply but expensive due to the amount of energy needed.

The quality of the water source is essential for good tap water but not the only factor. Advanced water treatment can make dirty and contaminant water into clean pristine tap water. In Europe and North America over 99% of the public tap water is considered clean and safe to drink according to international standards.

So why do we need Water Filter if the tap water is good?

There’s a persistent myth that tap water is unsafe to drink. We know, at least in most of Europe, US and Canada, that this myth is not true. Your local water provider has already extensively filtered and treated the tap water before you use it.

Despite the safety of tap water, water filters aren’t just for hypochondriacs. The tap water may taste badly due to chlorine and it’s mineral content. Chemicals introduced to tap water during the cleaning process may be unhealthy to drink. It’s possible for bacteria and other germs to enter water after it undergoes the treatment process. If your pipes are old or corroded, water may shave away sediment or metal shavings, which will then get into your water supply.

Water filters can provide better tasting tap water and a “last line of defense” for those who want to make sure their water is safe. This is why it's important to understand how water filters work.

What are the most common issues with tap water?

Although most tap water in the developed world is safe, much of it is not. Inadequate management of urban, industrial, and agricultural wastewater means the drinking-water of hundreds of millions of people is dangerously contaminated or chemically polluted.

Contaminants that may be present in source water include:

  • Microbial contaminants, such as viruses and bacteria, which may come from sewage treatment plants, septic systems, agricultural livestock operations, and wildlife.
  • Inorganic contaminants, such as salts and metals, which can be naturally-occurring or result from urban stormwater run-off, industrial or domestic wastewater discharges, oil and gas production, mining, or farming.
  • Pesticides and herbicides, which may come from a variety of sources such as agriculture, urban stormwater runoff, and residential uses.
  • Organic chemical contaminants, including synthetic and volatile organic chemicals, which are by-products of industrial processes and petroleum production, and can also come from gas stations, urban stormwater runoff, and septic systems.
  • Radioactive contaminants, which can be naturally-occurring or be the result of oil and gas production and mining activities.

General tap water indicators

These indicators are parameters used to indicate the presence of harmful contaminants. Testing for indicators can eliminate costly tests for specific contaminants. Generally, if the indicator is outside of the acceptable limit there might be an issue.


Indicator Acceptable Limit Indication Notes
pH value 6.5 to 8.5 An important overall measure of water quality, pH can alter corrosivity and solubility of contaminants. Low pH will cause pitting of pipes and fixtures or a metallic taste. This may indicate that metals are being dissolved. At high pH, the water will have a slippery feel or a soda taste. Some health professionals suggest that pH below 7.4 is more difficult for the body to convert but there is no scientific research to back this up.
Turbidity Clarity of sample can indicate contamination.
Total Dissolved Solids (TDS) 500 mg/l or ppm Dissolved minerals like iron or manganese. High TDS also can indicate hardness (scaly deposits) or cause staining, or a salty, bitter taste. See our complete article on TDS and why it's a poor indicator of water quality.

Water hardness

Hard water is often pointed out a negative thing but there is no scientific evidence that water hardness has health effects. However, it can cause scale buildup in hot water heaters and other home equipment and reduce their effective lifetime.

Hardness in mg/l Hardness Level
0-60 soft
61-120 moderately hard
121-180 hard
Above 181* very hard

* level at which most people find hardness objectionable

The World Health Organization says that “there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans”.

Common nuisances of tap water

While these have no or little adverse health effects, they may make water unpalatable (undesired to drink) or reduce the effectiveness of soaps and detergents. Some nuisance contaminants also cause staining. Nuisance contaminants may include iron bacteria, hydrogen sulfide, and hardness.

Contaminant Acceptable Limit Effects/Taste
Chlorides 250 mg/l salty or brackish taste; corrosive; blackens and pits stainless steel
Copper (Cu) 1.3 mg/l blue-green stains on plumbing fixtures; bitter metallic taste
Iron (Fe) 0.3 mg/l metallic taste; discolored beverages; yellowish stains, stains laundry
Manganese (Mn) 0.05 mg/l or 5 ppb black stains on fixtures and laundry; bitter taste
Sulfates (SO4) 250 mg/l greasy feel, laxative effect
Iron Bacteria present orangeish to brownish slime in water

* Taste is subjective so what tastes good to one person might taste bad to someone else.

Common contaminants in tap water

These are some of the chemicals and contaminants that may cause issues:

Pathogens (Bacteria and Viruses)

If you drink raw tap water directly from a small mountain spring it will most likely contain bacteria such as coliform. This is completely normal and unlikely to make you sick in small amounts.

For public tap water supplies however, it’s critical that there is no coliform including e.coli.

Thankfully tap water in developed countries have been treated chlorine, chloramine or ozone to kill all pathogens. In many poor countries pathogens are still a major issue though. In addition to this old and poorly maintained water tanks on buildings and water filters can cause bacteria to grow and contaminate the local drinking water.

Chlorine and chlorine by-products

The most common disinfection method involves some form of chlorine or its compounds such as chloramine or chlorine dioxide. Chlorine is a strong oxidant that rapidly kills many harmful micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive solution used in household bleach that releases free chlorine when dissolved in water.

Chlorine levels up to 4 milligrams per liter (4 parts per million) are considered safe in drinking water. All forms of chlorine are widely used, despite their respective drawbacks. One drawback is that chlorine from any source reacts with natural organic compounds in the water to form potentially harmful chemical by-products. These by-products, trihalomethanes (THMs) and haloacetic acids (HAAs), are both carcinogenic in large quantities and are regulated by the United States Environmental Protection Agency (EPA), EU and WHO. The formation of THMs and haloacetic acids may be minimized by effective removal of as many organics from the water as possible prior to chlorine addition. Although chlorine is effective in killing bacteria, it also has limited effectiveness against pathogenic protozoa that form cysts in water such as Giardia lamblia and Cryptosporidium.

Read more about how to remove chlorine from tap water.


The use of chloramine is becoming more common as a disinfectant. Although chloramine is not as strong an oxidant, it provides a longer-lasting residual than free chlorine because of its lower redox potential compared to free chlorine. It also does not readily form THMs or haloacetic acids (disinfection byproducts).

It is possible to convert chlorine to chloramine by adding ammonia to the water after adding chlorine. The chlorine and ammonia react to form chloramine. Water distribution systems disinfected with chloramines may experience nitrification, as ammonia is a nutrient for bacterial growth, with nitrates being generated as a by-product.

Read more about how to remove chloramine from tap water.

Lead and other heavy metals

One of the most complicated contaminants is lead and other heavy metals that leach from pipes. The reason is that all house built prior to 1980 could in theory contain lead pipes. What makes the matter even worse is that there’s no reliable way of checking or measuring as leaching can happen on and off. Cities/regions with soft tap water are especially exposed.

Although WHO, EU and EPA have guidelines for lead and mercury there is no safe level for infants and kids. Therefore all lead exposure should be avoided.

Read more about how to remove lead from tap water.


Nitrate is used mainly in inorganic fertilizers. It is also used as an oxidizing agent and in the production of explosives, and purified potassium nitrate is used for glass making. Nitrates occur naturally in plants, for which it is a key nutrient but for humans mid to long term exposure

Nitrate can reach both surface water and groundwater as a consequence of agricultural activity (including excess application of inorganic nitrogenous fertilizers and manures), from wastewater treatment and from oxidation of nitrogenous waste products in human and animal excreta, including septic tanks.

Read more about how to remove nitrates from drinking water.


Arsenic makes up part of the Earth’s crust and is commonly found in groundwater. Because of this arsenic is a common issue for well water in many regions around the world including the United States. Arsenic is known to cause a variety of cancers as well as being linked to heart disease, strokes and diabetes. Recent research has found an association between arsenic below 10 parts per billion and IQ deficits in children.

Water treatment plants are very efficient in removing arsenic and therefore it’s generally not a risk in public tap water.


Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a large group of human-made chemicals that have been used in industry and consumer products worldwide since the 1950s. These chemicals are used to make household and commercial products that resist heat and chemical reactions and repel oil, stains, grease and water. PFAS chemicals include PFOA and PFOS. In recent years they’ve been found more and more frequently in tap water around the world. Since they are unregulated the normal water tests don’t detect PFAS.

Read more about how to remove PFAS from tap water.


Another unregulated contaminant is microplastics. Research carried out 2017-2018 found microplastics in 93% of all bottled water and 92% of tap water. We currently don’t know if there is any health impact on humans but it’s probably better to be safe than sorry.

Read more about microplastics in tap water.

Other contaminants that you probably don’t need to worry about

There’s been many alerts about pharmaceuticals including birth control pills and anti-depressives. However, even if frequently found the concentration is less than 1/1000 of what’s in a pill so very unlikely to impact humans.

Common tap water contaminants, potential health effect and accepted limits

These are some commons contaminants that have known health effects to watch out for in tap water. The accepted limit is based on WHO, EPA and EU standards.

Contaminant Acceptable Limit Sources/Uses Potential Health Effects at High Concentrations
Atrazine 3 ppb or .003 ppm used as a herbicide; surface or ground water contamination from agricultural runoff or leaching heart and liver damage
Benzene 5 ppb or .005 ppm gasoline additive; usually from accidental oil spills, industrial uses, or landfills blood disorders like aplasticaremia; immune system depression; acute exposure affects central nervous system causing dizziness, headaches; long term exposure increases cancer risks
Lead* 0.010 ppm or 10 ppb used in batteries; lead gasolines and pipe solder; may be leached from brass faucets, lead caulking, lead pipes, and lead soldered joints nervous disorders and mental impairment, especially in fetuses and infants; kidney damage; blood disorders and hypertension; low birth weights
Nitrates (NO3) 10 mg/l (nitrate-N)
45 mg/l (nitrate)
soil by-product of agricultural fertilization; human and animal waste leaching to groundwater methemoglobinemaia (blue baby disease) in infants (birth to 6 months); low health threat to children and adults
Total Coliform possible bacterial or viral contamination from human sewage or animal manure diarrheal diseases, constant high level exposure can lead to cholera and hepatitis
Radon 300 pCi/l** naturally occurring gas formed from uranium decay; can seep into well water from surrounding rocks and be released in the air as it leaves the faucet breathing gas increases chances of lung cancer; may increase risk of stomach, colon and bladder cancers

Continued... 3 mg / l

Chloramines (Mocnchloramine, Dichloramines and Trichloramine) are present both in chlorinated water and potentially as vapour above the surface of the water. No short or long term health effects have been associated with Chloramine. The only exception is dialysis patients that should avoid chloramine entirely.
Cyanogen chloride 0.3 mg / l Cyanogen chloride may be formed as a by-product of chloramination or chlorination of water. It is also formed by the
chlorination of cyanide ion present in raw water.
On inhalation, a concentration of 2.5 mg/m3 causes irritation. Cyanogen chloride was used as a war gas in the First World War. A concentration of 120 mg/m3 was lethal.
Trihalomethanes (THM) 0.08 mg / l or 80 ppb Trihalomethanes are formed as a by-product predominantly when chlorine is used to disinfect water for drinking. Th emost common is Chloroform. The trihalomethanes have demonstrated carcinogenic activity in laboratory animals. (Cancer Group B)
Haloacetic acids 60 ug/l Haloacetics acid are formed as a by-product predominantly when chlorine is used to disinfect water for drinking Excessive levels can cause nervous system and liver effects.
Chloroform 0.1 mg / l or 100 ppb*** Part of the trihalomethanes (see above) but may be listed on it’s own. The trihalomethanes have demonstrated carcinogenic activity in laboratory animals. (Cancer Group B)
Chlorite 1 mg / l Part of the trihalomethanes (see above) but may be listed on it’s own. Excessive levels can cause hemolytic anemia.
Bromate 10 ug / l Part of the trihalomethanes (see above) but may be listed on it’s own. Excessive levels causes gastrointestinal, kidney, and hearing effects.

* There is no safe limit of lead according to most scientists. If present, elevated levels of lead can cause serious health problems, especially for pregnant women and young children. Lead in drinking water is primarily from materials and components associated with service lines and home plumbing. Lead is unlikely to appear in hard to very hard water.

** Recommended level in water at which remedial action should be taken. No mandatory standards have been set

*** Based on EWG recommendations. WHO recommends 0.3 mg/l or 300 ppb

What about tap water in developing countries?

According to a 2007 World Health Organization (WHO) report, 1.1 billion people lack access to an improved drinking water supply; 88% of the 4 billion annual cases of diarrheal disease are attributed to unsafe water and inadequate sanitation and hygiene, while 1.8 million people die from diarrhea diseases each year. The WHO estimates that 94% of these cases are preventable through modifications to the environment, including access to safe water. Simple techniques for treating water at home, such as chlorination, filters, and solar disinfection, and for storing it in safe containers could save a huge number of lives each year. Reducing deaths from waterborne diseases is a major public health goal in developing countries.

TAPP Water sells a special version of TAPP 1 with Ultrafiltration developed and tested for countries/regions with poor water quality. Contact us for more information.

How do I know if my tap water is safe to drink?

The standards for drinking water quality are typically set by governments or by international standards. These standards usually include minimum and maximum concentrations of contaminants, depending on the intended use of the water.

If you have access to public tap water then it’s always best to first ask the water supplier for the most recent water quality report. Larger water utilities companies carry out thousands of tests every year and make these available on their websites. Others do it annually and require you to request the document. But as we’ve highlighted above public water companies don’t test for everything so there could still be microplastics or PFAS in your tap water.

To test yourself there are two alternatives:

  1. Self test Kits - These usually contain test strips that test for bacteria, lead, pesticides, nitrites/nitrates, chlorine, hardness, and pH. One example of this is The First Alert WT1 Drinking Water Test Kit or Complete Water Analysis Test Kit (about $30)
    2. Water labs - Send a water sample to a water lab and get a detailed analysis of the content. Once again this won’t tell you everything (microplastics and PFAS aer not included) but it will give a good indication of the quality.

What about bottled water?

There is no scientific evidence that bottled water is safer or healthier than tap water or filtered tap water. Read our extensive report on bottled water vs tap water and filtered water.

Do I need a water filter?

Like many others you are concerned about removing the contaminants in your tap water but what is water filtration or water purification and does it work?

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water fit for specific purposes such as human consumption (drinking water). The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

How do water filters work?

There are many different kinds of filter technologies. These are the most common with additional details about how they work available:

  • Activated Carbon (usually in the form of Granular Activated Carbon or Carbon Blocks): This is the most effective material available for water filters today removing over 70 common contaminants while leaving the healthy minerals. Read more about activated carbon.
  • Reverse Osmosis (RO): Very effective in removing most of the contaminants that activated carbon doesn’t but also removes all healthy minerals. RO is always combined with Activated Carbon and sometimes with other kinds of filters such as IX. Read more about reverse osmosis.
  • Ion Exchange (IX): Very effective in removing the hardness from tap water and thus reduce scale. IX targets specific substances for removal based on their ionic charges, while leaving desirable or innocuous minerals in solution.
  • Distillation: Distillation is probably the oldest method of water purification. Water is first heated to boiling. Then the water vapor rises to a condenser where cooling water lowers the temperature so the vapor is condensed, collected and stored. Most contaminants stay behind in the liquid phase vessel. Typically distilled water tastes poorly.
  • Ultraviolet light filters (UV): UV water filters effectively remove 99.9% of waterborne microorganisms. However, it does not remove other common contaminants so needs to be combined with e.g. activated carbon.

How do activated carbon block filters work?

There are different types of activated carbon such as Coal, Wood and Coconut shells. The most common carbon block used in water filters is made from Coconut Shells because it's the most efficient and eco-friendly.

The unfiltered water goes through the filter thanks to its pressure. At that moment, filtration takes place.

Microscopically, the carbon block is an assembly of many small “spherical” particles. All the contaminants are retained on the surface of these particles. In fact, they get “stuck” on the surface of the carbon block particles. This phenomena is called Adsorption.

But how about Bacteria and micro-plastics? Well, these contaminants are much bigger than the “chemical contaminants” such as Nitrate and chlorine. Thus, their filtration is mostly based on their size. Thanks to the particle size (2 microns) none of the microplastics and only a few of the bacteria pass through.,

What water filter should I choose?

This really depends on the source of your tap water. For public tap water in Europe and North America a high quality activated carbon water filter is generally more than enough. For well water or public tap water in other regions of the world a combination of RO, IX or UV filters might be needed to remove all contaminants.

Read about how EcoPro water filter works.

What’s the best water filter I can install myself?

We tested and compared the most common carafe/pitcher and faucet water filters to evaluate what filter will suit you best across 7 different attributes including total cost of ownership. Find out which filter scored best.

Check out the faucet water filter comparison. Also read about the best water filters 2024.

How much does a water filter cost?

The average bottled water consuming household spends about $280 or €240 per year on bottled water. Most people would guess considerably less as the cost per individual bottled is much lower.

The cost of a water filter is as low as €60 per year for a high quality filter such as EcoPro.

Read more about the cost of different kinds of filters.

Also, check out our bottled water cost calculator.

Why and how often do I need to change the filters?

You have to regularly change the filters, or don’t bother buying one. All filtering systems work the same way, with water passing through a removable cartridge filled with a filtering medium such as activated charcoal. Filters can grow bacteria, become clogged, and no longer effective. Follow the manufacturer’s recommended schedule for changing them.


We promised a summary of what you need to know about how water filters work at the end. Here you go:

  • Drink Water Quality varies a lot depending on where you live but generally public tap water in Europe and North America is safe to drink
  • Water Filters are great to improve taste and remove contaminants and thereby reducing risk of drinking unhealthy tap water
  • Bottled water is NOT healthier than tap water or filtered water and it’s terrible for the environment.
  • For public tap water in Europe and North America the best water filter is typically activated carbon faucet filter such as EcoPro or water pitchers. For well water or contaminated public tap water other filters may be needed.
  • Make sure you change your filters in accordance with the manufacturer's instructions. Otherwise you may end up with dirtier tap water than you started with.

External sources:

In addition to the links and sources provided throughout the text here's more info about tap water, where water comes from and how water filters work.

WHO Tap Water Guidelines:

Worldwater Reserve:

How Water Filters Work FAQ

These are the most common questions we get for this page:

How do Water Filters Work?

This article provides a simple but extensive overview about different types of water filters and what they remove as well as common tap water contaminants and recommendations on the best water filter for you. Read more

What are the most common contaminants in tap water?

Lead usually gets the most attention but the most common contaminants are actually microplastics, nitrates, chlorine by-products and pharmaceuticals. In this article we provide a full list of the contaminants and how to remove them. Read more

What water filter should I chose?

The water filter sales people will tell you that each individual household has a different need. But the truth is that a high quality carbon block filter such as EcoPro is sufficient for 95% or more of US and European households. 


Torna al blog