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Water Treatment Processes: Beginner's Guide to Safe Clean Wholesome Drinking Water

Water Treatment works in UK

Imagine turning on your tap and having crystal-clear, refreshing water flow out, ready to drink. 

But have you ever stopped to wonder how that water travelled from its source to your glass? The answer lies in water treatment processes. These behind-the-scenes heroes play an important role in ensuring our drinking water is safe and clean.

Our water quality regulator in England and Wales, Marcus Rink, Chief Inspector at the Drinking Water Inspectorate (DWI) made a statement at the Drinking Water conference in 2023 that the UK is one of only six countries globally to be ranked as having no link between water and disease. He went on to point to almost 100% (99.7%) compliance with drinking water regulations and microbial compliance at 99.9% – citing this as “a fantastic achievement”. Adding: “We are in an incredibly fortunate position. We sometimes forget just what good quality drinking water we have.”

In this guide, we'll discover more about water treatment in UK, explaining each step in detail and highlighting the importance of this process for our health and well-being. 

From Source to Tap

Water is a precious resource, and its journey from its natural source to your tap can vary depending on your location. Common sources of drinking water are surface water and groundwater and to understand more about this, please read our article about where does UK tap water comes from.

Regardless of the source, all raw water can contain impurities and microorganisms that can be harmful if consumed.

Water Contaminants

Here are some of the potential contaminants found in raw water:

  • Micro-organisms: Some bacteria, viruses, protozoa and parasites can cause various waterborne diseases like E.coli, cholera, and cryptosporidiosis.

  • Chemicals: These can include industrial pollutants such as PFAS (colloquially known as ‘forever chemicals’), agricultural runoff (pesticides, herbicides such as metaldehyde and Diazinon), and naturally occurring minerals like arsenic and sulphur.

  • Sediment: Sand, silt, and other suspended particles can make the water turbid and aesthetically unpleasing.

These contaminants highlight the crucial role of water treatment processes in removing them and safeguarding our health. 

Catchment Management

It all starts at the source.

Managing land and activities surrounding our drinking water sources is essential to reduce pollution entering the body of water.

It involves every stakeholder understanding and working together for the benefit of the environment and raw water quality from water companies, local authorities, regulators, fisheries, amenity clubs and farmers in the catchment areas. 

It is possible to reduce pollution levels entering the water by making a concerted effort to understand appropriate non-polluting activities. 

For example when and where to apply pesticides -

  • ideally only on dry days - not when it's raining as this increases direct run-off to lakes and rivers, 

  • spray pesticides a safe distance away from the water body and 

  • leave a buffer zone of land to allow absorption into land not run-off to the water.

As a result, the water will require less treatment to make it safe and wholesome to drink. This in turn manages costs to customers in terms of water bills and reduces generating greenhouse gas emissions from treatment processes thus advancing water companies’ target to head towards net zero carbon. 

These techniques can also deliver environmental benefits.

For example, carrying out work to restore an area of upland moorland, could:

  • boost the environment’s natural capacity to store carbon and help mitigate climate change 

  • improve the variety of animal and plant life that the environment can support 

  • slow down the rate at which rainwater runs off land to reduce the risk of flooding and pollution of the water body

Key Water Treatment Processes

Water treatment works employ a multi-step process to remove impurities and contaminants from raw water, making it safe to drink. Here's a breakdown of the key processes involved:

A snapshot of a water treatment facility with turbid water on each side ponds and  clear water at the center passage.


This is the removal of large particles where water flows through a screen to capture any branches, leaves fish and other large objects. If left in the water they'd almost certainly clog up the treatment process.


a. Coagulation and Flocculation

Imagine a glass of murky water. Coagulation and flocculation work together to address the removal of this turbidity. Here's how:

Particles in water repel each other as they are naturally negatively charged, just like magnets repelling each other. Chemicals like ferric sulphate are added to the water. These positively charged chemicals bind to the negatively charged particles suspended in the water and allow the particles to join together to create a floc.

Slow mixing is introduced to gently stir the water, allowing the clumps (flocs) to grow larger and heavier. This process essentially makes the invisible (tiny particles) visible (larger flocs) for easier removal in the next step.

b. Sedimentation or Dissolved Air Flotation (DAF)

Thanks to coagulation and flocculation, the water now contains larger, heavier flocs. There are two types of common treatment to remove the floc at this point.

Sedimentation is the first common treatment where letting gravity do the work and the other is energy intensive and known as Dissolved Air Flotation (DAF).


Due to gravity, the heavier flocs settle at the bottom of the clarifying tanks, separating from the clearer water above.

This is a low-energy process that effectively removes a significant amount of impurities from the water.


Dissolved Air Flotation (DAF)

Alternatively, dissolved air flotation is where saturated air is added. Air bubbles are used as life jackets for the floc that float them to the surface. The floc forms a blanket which is scraped off and sent to the wastewater stream normally ending at a local wastewater treatment works. 

This clarifying process removes around 90% of the particulate matter from the water. It removes algae and bacteria as well.


This is the removal of the rest of the particulate matter.  Filtration acts like a sieve to remove even fine particles.

The water flows through layers of sand, gravel, and anthracite. This filtration media acts as a physical barrier, trapping any remaining flocs, sediment, and other particulates.

Filtration plays a crucial role in ensuring clear and sparkling water. Regular cleaning of the filters is important and known as backwashing, a process to remove the captured particulate matter.

Granular Activated Carbon (GAC) Adsorption

Tanks containing activated carbon remove any remaining chemicals such as pesticides from the water. This process also removes molecules that cause taste and odours in the water such as geosmin released from some algae.

The carbon does this through a process called adsorption in which the chemicals are attracted to a honeycomb surface by electrochemical processes.


The process of eliminating harmful microorganisms

Even after previous steps, some harmful microorganisms might still be present so disinfection is the final line of defence.

Different Ways of Disinfection

Now there are different ways of disinfection.

  1. Chlorination

This is the most common disinfection method at water treatment works where chlorine is added to the water, killing or inactivating bacteria, viruses, and other pathogens. 

Chlorine can sometimes react with naturally occurring organic matter in the water, producing disinfection byproducts (DBPs). While generally safe at regulated levels, some DBPs are a potential health concern.

2. Ultraviolet (UV) light

This method uses ultraviolet radiation to kill or inactivate microorganisms without using chemicals. An effective method to inactivate cryptosporidium oocysts.

3. Ozone

Ozone is a powerful oxidant that can effectively disinfect water. However, it decomposes quickly, so maintaining ozone residuals in long water distribution system can be challenging.

The choice of disinfection method depends on various factors, including cost, effectiveness against specific contaminants, type of source water and regulatory requirements.

Additional Processes

Depending on the specific water source and quality requirements, additional treatment processes might be employed such as aeration, ion exchange and reverse osmosis.

By understanding these core water treatment processes, you gain a deeper appreciation for the remarkable journey your tap water takes before reaching your glass.

Water Treatment at Home

Water companies provide a high level of treatment at their water treatment works and monitor the water quality from source to customer tap.  As mentioned our tap water in the UK is one of the top six in the world for the best drinking water quality. 

However, in some cases, you may have your own supply.  

Private Borehole Supply

If you rely on a private borehole for your water source, you'll be responsible for ensuring its safety. Where a borehole is permitted, up to 20,000 litres/day may be extracted without seeking additional permits for abstraction. Local Authorities act as the regulators for private water supplies. Each local authority must carry out a risk assessment of each private water supply system in its area at least every five years.

Testing your water supply regularly and implementing appropriate treatment based on the risk assessment and the test results is crucial. 

More information can be found either through your Local Authority or on the Drinking Water Inspectorate website.

Specific Contaminant Concerns

If you're concerned about specific contaminants in your tap water, check on the water company’s website for test results in your area. If there is a water quality emergency that is not listed on the events section of the water company website, contact your water company directly.

Lead Pipes

If your home was built before 1970, there's a chance your water may be supplied through lead pipes inside your home. Lead from these pipes can seep into the water which, over a very long period, can be harmful.  

To determine if there are lead pipes in a property,  unpainted lead pipes appear dull grey and when scraped, the pipe is gently with a coin. If it shows a shiny, silver-coloured metal underneath, it’s a lead pipe.  

It is the homeowner's responsibility to replace the lead pipe from the boundary of their property to the kitchen sink. Some water companies have schemes to support a replacement programme.


Taste and Odour Improvement

Individuals may find taste or odour not as desirable in certain areas of the UK sometimes arising from chlorine or mineral content. Filling a jug with water and storing it overnight in a fridge will release the chlorine and cool the water. This is highly likely to improve water taste and odour.

If using a home filtration system, it is essential to follow the manufacturer's guidelines to change the filter as any delay can create an increase in bacterial growth on the filter surface that could compromise water quality. Regular maintenance is crucial to ensure optimal performance.

Frequently Asked Questions (FAQ)

Here are some commonly asked questions about water treatment processes:

Can I drink untreated water?

It's generally not advisable to drink untreated water, especially from surface water sources like lakes or rivers. These sources can be contaminated with harmful bacteria, viruses, parasites, or chemicals from run off the land surrounding the water source. Groundwater can potentially contain contaminants depending on the location, depth of the boreholes and rock formation, and proximity to other water sources.

What are the signs of contaminated water?

Signs of contaminated water can include:

* Discolouration (cloudy, brown, or reddish water)

* Unpleasant odour or taste (chlorine or metallic)

* Visible particles

* Gastrointestinal issues after consumption (diarrhoea, nausea, vomiting)

If you experience any of these occurrences, it's best to seek professional advice from your doctor if you are feeling unwell, or your local water company if you observe that the water quality is impaired in any way.

Should I install a home water treatment system?

Drinking water quality in the UK is amongst the best in the world. There is no need to install additional home water treatment systems if your water is supplied by a water company.

If you are on a private water supply you must inform the Local Authority who will carry out a risk assessment.

If you do install a water treatment system in your home it will require regular maintenance to ensure optimal performance. This typically involves replacing filters or cartridges at recommended intervals. It's crucial to follow the manufacturer's instructions for your specific system. Some systems might also have indicator lights or gauges that signal when maintenance is needed.

What if I have concerns about my tap water quality?

If you're concerned about the quality of your tap water, you can take the following steps:

*Contact your water company:**  They can provide information about your specific water source and treatment processes used to maintain safe drinking water. If there is a water quality emergency, your water company website may provide up-to-date information or, contact your water company directly.

What if I have lead pipes supplying my drinking water?

It is recommended that homeowners replace lead pipe from the boundary of their property to the kitchen sink. Check with your water company as some water companies have schemes to support a replacement programme.


The journey of water from its source to your tap is a remarkable feat of engineering and science. 

Water treatment processes play a vital role in ensuring we have safe clean wholesome water for drinking, cooking, and countless other purposes. By understanding these processes, we gain a deeper appreciation of the efforts behind delivering this essential resource to our homes.

By valuing and understanding the importance of water treatment, we can ensure this precious resource continues to flow safely and abundantly for generations to come to our taps.

If you are new to UK Water Industry, you can learn more about water treatment process and more by joining our Introduction to the UK Water Industry course. Send us an email to for enquiry or check out our course page.

For further information, you can explore these resources:

  • The Drinking Water Inspectorate for England and Wales, the Drinking Water Quality Regulator for Scotland, Drinking Water Inspectorate as part of the Northern Ireland Environment Agency all provide a wealth of information on public water systems and drinking water regulations

  • The World Health Organisation (WHO) offers insights on safe drinking water, water quality guidelines, and health aspects: World Health Organisation's Water and Sanitation Page

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