The Definitive Guide to Water Purification

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Water purification is the process of removing all the impurities in a water supply, making it pure.

In this ultimate guide to water purification, we’ve shared everything you need to know about purifying water, including the goals of purification, the numerous purification processes, and more.

πŸ“Œ Key Takeaways:

  • Water purification involves removing all the particles suspended in a water supply, aside from water (H20) molecules.
  • However, the term “water purification” is often used to describe any process that filters water by removing contaminants or killing microorganisms.
  • The purpose of purifying water is to make it suitable and safe for a specific application, such as drinking, cleaning, or use in the medical space or food manufacturing industry.
  • The 5 main stages of community water treatment are screening, coagulation/flocculation, sedimentation, filtration, and disinfection.

🚰 What Is Water Purification?

Water purification is the process of purifying water by removing undesirable chemicals, dissolved minerals, heavy metals, microorganisms, and other inorganic and organic particles.

The true definition of water purification is the process of removing all impurities from water, leaving only water (H20 particles).

However, the term “water purification” is also commonly used to refer to various chemical and physical processes that treat contaminated water – even if they’re not capable of removing every single contaminant and purifying water.

Water purification system

πŸ€” What Is The Purpose Of Water Purification?

The purpose of water purification is to make water safe and suitable for its intended use.

This could be for drinking, using in a medical or industrial setting, or for agricultural use.

Water purification may be a legal requirement. For example, in the US, municipal water suppliers are legally obliged to remove certain contaminants down to certain levels to make drinking water safe for consumption according to guidelines set out by the Environmental Protection Agency (EPA).

The purpose of a water purification process is linked to the treatments used. Some water supplies (such as drinking water supplies) will need to be more thoroughly treated than others.

The treatment method is also determined by the initial water quality. Generally, the poorer the quality of the water, the more extensive treatment it will require.

πŸ”Ž What Is Involved In Water Purification?

The exact processes involved in water purification depend on a couple of factors:

  • The quality of the source water
  • The water’s intended use

For instance, water treated for drinking will require different processes than water treated for the cosmetics industry. Drinking water still contains impurities like minerals and trace contaminants, while water used in cosmetic products needs to be as pure as possible – just H20.

Generally, water purification will usually involve the removal of sediments, have some form of disinfection, and some form of filtration.

Lake livingston water dam

πŸ“Š Stages Of Community Water Purification

When treating water for drinking, most water treatment plants use the following stages of water purification, in this order:

Stage 1: Pretreatment

Pretreatment provides initial treatment processes, depending on the quality of the water source.

Generally, pretreatment will involve one or several of the following:

  1. Pumping and containment – Pumping water from the source into clean holding tanks or pipes.
  2. Screening – Removing large, physical particles and debris like leaves, sticks, and rubbish, preventing them from clogging later filtration stages. Screening is most commonly used as pre-treatment for surface water, not groundwater, supplies, since groundwater will usually be naturally screened already.
  3. Storage – River water may be stored in reservoirs for months, which allows water supplies to be maintained during droughts and pollution incidents, and enables natural biological purification to occur.
  4. Pre-chlorination – Many water treatment plants used to chlorinate their incoming water supplies to prevent contamination and fouling inside the storage and treatment infrastructure. This practice is now less common due to the potential adverse effects of early chlorination on water quality.
  5. pH adjustment – Purified water is neither acidic nor alkaline and has a neutral pH value of around 7. Acidic water can have its pH boosted to neutral with the use of soda ash, lime, or sodium hydroxide. This is an important pre-treatment as it makes the next stage of treatment – coagulation and flocculation – more effective and reduces lead leaching from water transportation pipes. Alkaline water can be treated with acid, such as sulfuric acid or carbonic acid, to bring its pH back down to neutral.
Municipal city water treatment plant

Stage 2: Coagulation and Flocculation

Following pre-treatment, the next stage of community water treatment is coagulation and flocculation.

In this stage, chemicals are added to the water to support the removal of suspended particles that affect water’s color and turbidity, such as inorganic matter (silt and clay) and organic matter (viruses, protozoa, algae, and bacteria).

There are several inorganic coagulants that might be added to water to stimulate coagulation and flocculation, including iron chloride and aluminum sulfate. These coagulants cause chemical reactions amongst the particles, causing them to bond together and form larger particles called “flocs”.

These coagulated particles can be removed in the subsequent sediment and filtration stages.

Stage 3: Sedimentation

Once water exits the flocculation tank, it flows into a sedimentation basin, or settling basin. This basin may be circular, with a flow from the center outward, or rectangular, with water flowing from end to end.

There are a few stages of sedimentation that follow.

Sedimentation basin

Sludge Collection

In the sedimentation tank, the heavy coagulated particles produced by the earlier coagulation and flocculation process sink to the bottom of the basin. Only the very top layer of water is able to exit the basin.

The bigger the surface area in the basin, the more particles can be removed. It’s common for newer sedimentation basins to have inclined plates and tubes to increase their surface area while taking up less space than conventional sedimentation basins.

Sludge storage & removal

Over time, the buildup of floc at the bottom of the sedimentation basin forms a thick layer of sludge. To prevent eventual clogging of the basin, this sludge must be removed.

The exact process of clearing the sludge depends on a specific water treatment plant’s setup. For instance, some water treatment plants have mechanical cleaning devices that provide continuous cleaning of the sedimentation basin, while others require periodic manual cleaning while the sedimentation basin in question is out of service.

Stage 4 (Optional): Dissolved Air Flotation

Some particles that need to be removed from a water supply don’t easily settle out of the solution. In this case, water treatment plants may use dissolved air flotation (DAF) after coagulation and flocculation to target these particles.

A DAF tank contains air diffusers at the bottom, which produce tiny bubbles in the water that grab onto the floc and cause it to float to the surface. This blanket of floating particles can then be removed from the surface of the water. DAF is most commonly used to treat water supplies that are vulnerable to algal blooms.

Dissolved air flotation system
Stefan.p21 (Maciej PreΕ›), CC BY-SA 3.0, via Wikimedia Commons

Stage 5: Filtration

The next stage of drinking water treatment is to filter any remaining particles that didn’t coagulate to form floc in the earlier treatment stages.

There are numerous types of filters that may be used, including membrane filters, slow sand filters, and rapid sand filters. Natural filtration processes like bank filtration might also be used.

Membrane Filters

Membrane filters are commonly used in drinking water and sewage water purification.

Drinking water membrane filters are capable of removing particles as small as 0.1 micrometers, including pathogens like cryptosporidium and giardia.

Drinking water treatment plants aren’t the only facilities to use this form of filtration. Membrane filters are also used widely for industrial and commercial applications, including preparing beverage products like bottled water.

However, membrane filtration has its limitations. It can’t remove contaminants that are already dissolved in the water, like heavy metals, nitrates, and phosphates.

Slow sand filters

A slow sand filter plant uses layers of fine sand, medium sand, and gravel to gradually filter out contaminants of various sizes.

In regions where there’s plenty of space and land, slow sand filters can be used to very slowly remove impurities with biological treatment processes. The bottom of these filters has the coarsest sand and gravel, while the top of the filters has the finest sand. Treated water is carried away for disinfection in drains at the base.

The advantage of slow sand filters is that they effectively lower water’s available nutrient level – something that is rarely achieved by physical filtration processes. By reducing water’s nutrient levels, the volume of disinfectants used in the water can also be minimized, reducing the impact of water chlorination.

Biological growth on the top layer of sand eventually obstructs a slow sand filter’s water flow, and the top layer of sand must be scraped off to maintain the filter’s performance.

Slow sand filtration plant

Rapid sand filters

Rapid sand filters are the most commonly used filters in the filtration stage of the municipal water purification process. In these filters, water travels vertically through several filter layers – usually including sand topped with anthracite coal or activated charcoal.

Organic compounds (which affect water’s taste and odor) are removed by the top layer, and numerous contaminants that are able to pass through this surface layer end up trapped in pore spaces between the layers. By trapping contaminants gradually as water flows deeper and deeper into the layers, the filter is able to last a long time without quickly clogging.

A rapid sand filter can be cleaned by backwashing: sending water quickly through the filter layers in the opposite direction. This lifts the embedded particles. The backwashing water is usually disposed of rather than recycled due to its potential bacteria content.

Bank filtration

Finally, bank filtration is a natural form of filtration that may be used as a filter stage before other stages.

Bank filtration involves sending surface water through the banks of a lake or a river, allowing the natural sediments and soil microorganisms of the bank to perform physical filtration, removing or digesting chemical nutrients and dissolved or suspended organic material.

This filtering process alone doesn’t typically yield water that’s clean enough for human consumption, but it’s a better option than taking raw, untreated water directly from the source.

Stage 6: Dissolved Substance & Ion Removal

The majority of dissolved mineral ions and other dissolved substances are unable to be removed by physical filtration. These substances can simply slip through a filter’s pores along with water particles, and a special kind of water purification process is required to remove them.

An ultrafiltration membrane is commonly used for this purpose. UF membranes have microscopic pores that can filter out contaminants down to 0.02-0.05 microns in size.

One of the most effective water purification methods for removing unwanted ions is with an ion exchange resin or a zeolite-packed column. Ion exchange resins are primarily employed for removing calcium and magnesium hardness ions and exchanging them with sodium ions, and these resins usually also remove lead, mercury, nitrate, and other toxic ions.

The deionization process (which sends water between a positive electrode and a negative electrode) may also be used to purify water by completely removing all ions from water for various industrial and commercial uses. However, the source water needs to have the right conditions for the DI process to be effective.

mixed bed deionization
Ultrafiltration Process

Stage 7: Disinfection

Disinfection is one of the key stages of drinking water purification. None of the above water purification methods can completely eliminate biological contaminants like bacteria and viruses because they’re small enough to remain in the water alongside hydrogen and oxygen molecules. The easiest way to make sure these biological contaminants aren’t dangerous in a water supply is to kill them with disinfection.

Water treatment plants use pumps to add measured amounts of disinfectant chemicals – enough to kill potential waterborne pathogens and to leave a residual dose of disinfectant in the water as it travels through storage and distribution systems.

There are several methods of disinfection, but most water treatment plants use some form of chlorine/chloramine disinfection or ozone disinfection.

  • Chlorine disinfection involves adding some form of chlorine to a water supply to rapidly kill microorganisms. The most common type of chlorine used is sodium hypochlorite, a solution that releases free chlorine when dissolved in water. Up to 4 PPM (parts per million) of chlorine is considered safe for human consumption. The biggest drawback of chlorine is that it’s very reactive with organic compounds in water, which results in the formation of dangerous chemical byproducts like trihalomethanes (THMs) and haloacetic acids.
  • Chloramine disinfection uses chloramine (a combination of chlorine and ammonia) rather than chlorine as a disinfectant. Chloramine use is becoming more popular because it has a lower redox potential than chlorine and doesn’t readily form disinfection byproducts.
  • Ozone disinfection uses ozone, an unstable molecule that provides an oxidizing agent that kills most waterborne pathogens – particularly cyst-causing protozoans. Ozone is more difficult to use for water disinfection than chlorine and chloramine because it must be made on-site, but fewer byproducts are created, ozone residual isn’t left over, and water doesn’t have issues with taste and odor as it does with the common disinfectants.

Stage 8 (Optional): Post-Treatment Options

In its clean, purified form, water is now ready to be distributed to customers. However, many water treatment plants provide a further post-treatment to target specific impurities or further improve water quality.

Some of these post-treatment options include:

  • Water fluoridation: The process of adding fluoride to drinking water after disinfection. The aim is to prevent tooth decay.
  • Fluoride removal: Fluoride is a mineral that occurs naturally in most water supplies, and some regions already have too much fluoride in their source water. Some fluoride may need to be removed, usually with bone char or activated alumina filter media.
  • Water conditioning: Some water systems may condition water to reduce the effects of hardness minerals. Water is treated to precipitate calcium carbonate, which can then be sold to toothpaste manufacturers.
  • Radium removal: Radium is common in groundwater sources. If radium is found in excess in a water supply, it’ll need to be removed with a dedicated treatment process, such as ion exchange or water conditioning.
  • Reducing plumbosolvency: Plumbosolvency is water’s ability to adsorb lead and other impurities from underground service lines. Water that’s naturally acidic is more capable of dissolving these impurities, and may need to be treated with low levels of phosphate to increase its pH and reduce its plumbosolvency.

πŸ“‹ Other Forms Of Water Purification

The above methods are all commonly used for municipal drinking water purification at water treatment plants. But these aren’t the only purification methods that exist. Keep in mind that many water treatment plants have cost limitations, too, which restrict the types of methods that can be used for large-scale, long-term water treatment.

There are other methods that are better suited for other purposes, such as at-home residential water treatment, and water treatment for various industries, including food and beverage, cosmetics, and mechanics.

We’ve shared an overview of the other forms of water purification below.

Ultraviolet disinfection

UV light is a highly effective chemical-free method of water purification. UV purification systems emit ultraviolet waves, which scramble the DNA of viruses and other microorganisms, killing them in water.

A UV purifier only works with clear (non-turbid) water supplies because the ultraviolet light needs to be able to travel clearly through the water to target all the microorganisms. UV purification is best used in residential applications where there’s a very low risk of recontamination of the purified water because no residual disinfectant remains after treatment.

Read More: The Best UV Water Purifiers: Our Expert Reviews and Recommendations

Springwell blackcomb uv next to a water softener


Activated carbon filters remove contaminants with a process called adsorption. These filters have a large surface area that enables them to grab hundreds of suspended solids as water passes through the filter media.

The most common use of adsorptive filters is to improve water quality by removing contaminants affecting taste and odor, like chlorine. Adsorptive filters are most commonly found in household drinking water filters and fish tank filters. The filters need to be replaced regularly to prevent bacteria buildup in the filter media.

Related: Get the Best Filter for Your Family’s Health and Safety


Distillation is a process that boils water until it vaporizes. The distilled water vapor condenses back into a pure liquid when it comes into contact with a cool surface. Most suspended solids in water can’t vaporize with water when they’re heated, so they remain behind in the boiling chamber.

The exception is a few solutes, like some VOCs, which can vaporize and condense with water. However, distillation still obtains up to 99.9% pure water, depending on its initial quality.

Again, large-scale distillation is usually too costly for water treatment plants, but it’s used in some industries, like cosmetic manufacturers, laboratories, and automotive businesses, and at-home water distillation units are increasing in popularity.

Continue Reading: Get the Best Water Distiller for Your Family with These Top Picks

Distillation Process

Reverse Osmosis

Reverse osmosis is a process of membrane separation, which removes all total dissolved solids from contaminated water by sending it quickly through a semipermeable membrane with tiny pores (usually 0.0001 microns).

Reverse osmosis is a highly effective purification method, removing up to 99% of all dissolved contaminants. However, it isn’t feasible for a water treatment plant to use RO for large-scale purification because the process wastes water and is expensive to operate.

Reverse osmosis systems are most commonly used in residential applications, but they’re also used by various industries, including food & beverage, automotive, aerospace, pharmaceutical, and industrial.

Related: Find Your Ideal Reverse Osmosis System for Clean and Tasty Water

Tank-based RO system


One of the simplest methods of purifying drinking water is to boil it.

Boiling water (bringing it to a rolling boil for 10 minutes at about 100 Β°C or (212 F) kills microorganisms like bacteria and viruses, and encourages chlorine to dissipate at a faster rate. However, boiling doesn’t offer complete purification because it can’t remove any suspended solids in water, like heavy metals, chemicals, and minerals.

Boiling is a temporary disinfection solution, but it doesn’t leave a residual disinfectant, so water should be stored safely to prevent it from picking up new pathogens.

Hydrogen Peroxide

Some water treatment systems use hydrogen peroxide (H2O2) as an alternative to chlorine or chloramine to disinfect water. When synthesized on-site, this chemical is highly effective at killing E. Coli (more than 100 times more effective than chlorine), and can also bind to and degrade other compounds.


Bioremediation uses microbes to get rid of unwanted waste products in contaminated groundwater. The process has been used since the early 90’s to remove perchlorates (which are highly soluble and otherwise very difficult to remove), metals, alkanes, and other impurities. Bioremediation encourages the growth of microorganisms that use certain impurities as a source of energy and nutrition.

Illustration of the bioremediation process
Source: USGS

❔ Water Purification: FAQ

What are the 5 stages of water purification?

In a water treatment plant, the 5 most common stages of water purification are screening, coagulation/flocculation, sedimentation, filtration, and disinfection. Additional stages may be included depending on the quality of the source water.

What is the main purpose of water purification?

The main purpose of water purification is to make it safe for its intended use by removing the contaminants found in the natural water source. For instance, drinking water is purified to remove heavy metals, VOCs, and other impurities that pose a human health risk.

What is the healthiest way to purify water?

The healthiest way to purify your water is with a series of natural filters and purifiers that remove impurities without adding anything unhealthy or dangerous to the water. You could use ultraviolet light, activated carbon filters, and RO semi-permeable membranes to treat your water supply.

What is the difference between a water filter and a water purifier?

The technical difference between a water filter and a water purifier is that water filters improve water quality by filtering out contaminants (but they don’t remove everything) while water purifiers purify water by eliminating all impurities, including microorganisms. An example of a water filter would be an activated carbon filter, while a water distiller is a water purifier. However, many manufacturers use the terms “water filter” and “water purifier” interchangeably nowadays.


  • Jennifer Byrd
    Water Treatment Specialist

    For 20+ years, Jennifer has championed clean water. From navigating operations to leading sales, she's tackled diverse industry challenges. Now, at Redbird Water, she crafts personalized solutions for homes, businesses, and factories. A past Chamber President and industry advocate, Jennifer leverages her expertise in cutting-edge filtration and custom design to transform water concerns into crystal-clear solutions.

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