Water Disinfection Techniques: Chlorination, Ozone Treatment, and UV Rays

Safe water is not just a luxury – it is a basic necessity for human health, livestock well-being, and food production. Raw water from rivers, wells, or reservoirs can harbour dangerous pathogens like bacteria, viruses, and protozoa that cause diseases ranging from diarrhoea to cholera. That is where water disinfection steps in. It is the process of destroying or inactivating these harmful microorganisms so that water becomes safe for drinking, dairy operations, food processing, and other uses. Three of the most widely used disinfection techniques today are chlorination, ozone treatment, and ultraviolet (UV) irradiation. Each works through a different mechanism, and each comes with its own set of strengths and limitations.

Why water disinfection matters 🔗

Waterborne diseases remain a serious global health concern. Contaminated water can carry pathogenic bacteria, protozoa, and viruses responsible for illnesses like gastroenteritis, myocarditis, and encephalitis. In agriculture and dairy settings, contaminated water directly impacts animal health, milk quality, and overall farm productivity. Disinfection is typically the final and one of the most critical steps in any water treatment process – it acts as the last barrier between harmful pathogens and the end user.

Several factors influence how well a disinfection method works. These include water pH, temperature, turbidity, the type of microorganisms present, disinfectant dose, and contact time. Understanding these variables helps in choosing the right technique for a given application.

Chlorination: the most widely used method 🔗

Chlorination is the oldest and most common water disinfection method globally. It involves adding chlorine-based compounds to water to kill harmful microorganisms. The three main forms of chlorine used in treatment are chlorine gas, sodium hypochlorite (liquid bleach), and calcium hypochlorite (granular form). When any of these dissolves in water, it produces hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), which are the active disinfecting agents.

How chlorination works 🔗

Hypochlorous acid is a strong oxidiser that penetrates the cell walls of bacteria and viruses, disrupting their enzyme systems and metabolic processes. This ultimately kills the microorganisms or renders them inactive. The effectiveness of chlorination depends largely on the concentration of free chlorine in the water and the contact time – the duration the chlorine remains in contact with pathogens before the water reaches the consumer.

One important concept in chlorination is breakpoint chlorination. When chlorine is first added to water, it reacts with metals like iron and manganese and then with ammonia and nitrogen compounds to form chloramines (combined chlorine). These chloramines are weaker disinfectants and can cause the familiar “chlorine smell” and eye irritation. As more chlorine is added, it begins to destroy these chloramines. The point at which all chloramine demand is satisfied and free chlorine begins to build up in the water is called the breakpoint. Only after the breakpoint is reached does true, effective disinfection begin.

Advantages of chlorination 🔗

Chlorination has several practical benefits that explain its widespread adoption. It is cost-effective and relatively simple to implement, even in resource-limited settings. One of its most valuable features is its ability to provide a residual disinfectant in the water. This means that even after initial treatment, chlorine continues to protect the water as it travels through the distribution system – pipes, tanks, and taps – preventing re-contamination along the way.

Chlorine is also effective against a wide range of bacteria and viruses at recommended doses. Additionally, it serves secondary purposes like removing iron, manganese, and controlling algal growth in water treatment plants.

Limitations of chlorination 🔗

Despite its benefits, chlorination has notable drawbacks. The biggest concern is the formation of disinfection by-products (DBPs). When chlorine reacts with naturally occurring organic matter in water, it can produce compounds like trihalomethanes (THMs) and haloacetic acids (HAAs), which have been linked to an increased risk of cancer with long-term exposure.

Chlorine is also less effective against certain pathogens. Protozoan parasites like Cryptosporidium and Giardia are highly resistant to chlorine at standard doses approved by regulatory agencies. Furthermore, chlorine can leave an unpleasant taste and odour in the water. Being a toxic, corrosive chemical, it also requires careful storage, handling, and transportation.

Ozone treatment: a powerful oxidiser 🔗

Ozone (O₃) is one of the strongest commercially available disinfectants for water treatment. It is a triatomic molecule made up of three oxygen atoms and is a far more powerful oxidiser than chlorine. Ozone treatment – also called ozonation – is used for both disinfection and broader water quality improvement, including removal of taste, odour, colour, and certain organic contaminants.

How ozone disinfection works 🔗

Ozone cannot be stored or transported like chlorine because it is unstable and decomposes quickly. It must be generated on-site using an ozone generator. The most common generation method is corona discharge, where a stream of dry air or pure oxygen is passed through a high-voltage electrical discharge, which splits oxygen molecules (O₂) and allows individual oxygen atoms to recombine into ozone (O₃). Ozone can also be produced using UV light, though this method yields lower concentrations.

Once dissolved in water, ozone attacks microorganisms by oxidising their cell walls, leading to cell lysis (rupture). This mechanism – called protoplasmic oxidation – is fundamentally different from how chlorine works. When ozone breaks down in water, it also produces highly reactive free radicals like hydroxyl radicals (OH) that contribute additional disinfection power.

Advantages of ozone treatment 🔗

Ozone offers several advantages over chlorination. It is effective against a broad spectrum of pathogens, including chlorine-resistant organisms like Cryptosporidium and Giardia. Its disinfection speed is also remarkable – ozone can achieve disinfection thousands of times faster than chlorine for equivalent pathogen inactivation.

Unlike chlorine, ozone does not produce harmful DBPs like THMs. After completing its oxidation reactions, ozone simply reverts back to oxygen (O₂), leaving no chemical residue in the treated water. This makes it an environmentally friendly option. Ozone also improves water quality by oxidising compounds responsible for unpleasant taste, odour, and colour, and it can reduce levels of iron, manganese, and sulphur.

Limitations of ozone treatment 🔗

The main drawback of ozonation is that it provides no residual disinfection. Since ozone decomposes rapidly back into oxygen, it cannot protect water during storage or distribution. For this reason, many treatment facilities use ozone as a primary disinfectant and then add a small dose of chlorine afterwards to maintain residual protection through the distribution network.

Ozone systems also involve higher capital and operational costs compared to chlorination, primarily due to the energy required for on-site ozone generation. The equipment demands careful maintenance and monitoring. Additionally, while ozone does not produce THMs, it can react with certain organic compounds to form other by-products like aldehydes and ketones, though these are generally considered less harmful. Ozone is also a respiratory irritant, so proper safety measures are needed at the generation site.

Ultraviolet (UV) irradiation: chemical-free disinfection 🔗

UV disinfection is a physical process that uses ultraviolet light to inactivate microorganisms without adding any chemicals to the water. It has gained significant popularity in recent decades as a safe, effective, and environmentally friendly alternative to chemical disinfection, particularly for applications where chemical residues are undesirable.

How UV disinfection works 🔗

UV disinfection systems work by passing water through a chamber that contains a UV lamp, typically a low-pressure mercury vapour lamp. This lamp emits UV-C radiation at a wavelength of approximately 254 nm, which falls within the germicidal range of the UV spectrum (200-280 nm). At this wavelength, the UV energy is strongly absorbed by the nucleic acids (DNA and RNA) of microorganisms.

When UV light is absorbed by a pathogen’s DNA, it causes the formation of pyrimidine dimers – abnormal chemical bonds between adjacent bases on the DNA strand. These dimers prevent the DNA from being read and copied correctly, which means the microorganism cannot replicate. An organism that cannot reproduce cannot cause infection. Importantly, UV does not kill the pathogen outright – it inactivates it by blocking its ability to multiply.

Components of a UV system 🔗

A typical UV disinfection system consists of four main parts: a reactor chamber (usually stainless steel) through which water flows, a UV lamp that generates germicidal UV-C light, a quartz sleeve that protects the lamp from water while allowing UV transmission, and a control unit that regulates electrical input and monitors UV intensity. Some advanced systems include automatic cleaning mechanisms to prevent fouling of the quartz sleeve, which can reduce UV output over time.

Advantages of UV irradiation 🔗

UV disinfection has several compelling benefits. It is entirely chemical-free, which means it produces no disinfection by-products whatsoever. It does not alter the taste, odour, or chemical composition of the water. The treatment is also extremely fast – effective disinfection can occur within seconds of exposure, compared to the 20-30 minutes of contact time that chlorination often requires.

UV is particularly valuable because it is effective against chlorine-resistant pathogens like Cryptosporidium and Giardia, which are among the most challenging waterborne threats. It is also effective across a broad spectrum of microorganisms including bacteria, viruses, and protozoa.

Limitations of UV irradiation 🔗

The biggest limitation of UV treatment is its dependence on water clarity. Turbid water – water containing suspended particles, sediment, or dissolved organic matter – can absorb or scatter UV light, shielding pathogens from the radiation and reducing effectiveness. Pre-filtration is almost always recommended before UV treatment to ensure clear water and optimal performance.

Like ozone, UV provides no residual disinfection. Once the water leaves the UV chamber, there is no ongoing protection against recontamination. Some microorganisms also possess repair mechanisms – known as photoreactivation (light repair) and dark repair – that can partially reverse UV damage under certain conditions, though modern systems deliver doses high enough to overwhelm these repair pathways. UV lamps also need periodic replacement (typically every 8,000-12,000 hours) and the quartz sleeves require regular cleaning.

Comparing the three methods 🔗

Each disinfection technique occupies a distinct niche, and understanding their differences helps in selecting the right approach for a specific application.

Chlorination is best suited for systems that need long-lasting residual protection, such as municipal water distribution networks where water travels long distances through pipes. It is affordable and easy to implement but carries the risk of DBP formation and cannot handle all types of pathogens.

Ozone treatment is ideal where high-level disinfection is needed quickly and where water quality improvement (taste, odour, colour removal) is also a priority. It handles resistant pathogens well and leaves no chemical residues, but its lack of residual effect and higher cost make it less suitable as a standalone solution for large distribution systems.

UV irradiation is the go-to option for chemical-free disinfection, especially in settings like dairy processing plants, pharmaceutical production, and point-of-use household systems. It works fast and adds nothing to the water, but it requires clear water and offers no residual protection.

Multi-barrier approach: combining methods for maximum safety 🔗

In practice, many modern water treatment facilities do not rely on just one disinfection method. Instead, they use a multi-barrier approach that combines two or more techniques to compensate for individual weaknesses. A common configuration is to use ozone or UV as the primary disinfectant to handle resistant pathogens, followed by a low dose of chlorine to provide residual protection through the distribution system.

For instance, a study on sequential ozone-UV-chlorine treatment demonstrated that combining these methods achieved significantly higher microbial inactivation than any single method alone. The ozone pre-treatment also improved water quality parameters like colour and turbidity, which in turn enhanced the effectiveness of subsequent UV and chlorine treatments.

This layered strategy is especially relevant in dairy and food processing environments, where water safety standards are stringent and any microbial contamination can have serious consequences for product quality and consumer health.

Choosing the right method for your needs 🔗

Selecting the best disinfection technique depends on several practical factors. Consider the source water quality – if it is highly turbid, UV may need pre-filtration, and chlorine demand may be very high. Think about the target pathogens – if Cryptosporidium is a concern, UV or ozone would be more appropriate than chlorine alone. Evaluate operational costs, available infrastructure, and whether residual disinfection is needed for the distribution system.

For small-scale dairy farms or rural water supplies, chlorination often remains the most practical choice due to its low cost and simplicity. For larger processing facilities with stricter quality requirements, UV or ozone – either alone or in combination with chlorine – can provide the higher level of safety needed.

What do you think? Given the specific water quality challenges in your region, which disinfection method – or combination of methods – would make the most sense for ensuring safe water on a dairy farm? And how might factors like energy availability and maintenance capacity influence that choice?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC9539188/
2. https://www.ncbi.nlm.nih.gov/books/NBK579455/
3. https://www.sciencedirect.com/topics/engineering/breakpoint-chlorination
4. https://absoluteozone.com/ozone-vs-chlorine-for-water-disinfection/
5. https://ozonesolutions.com/blog/ozone-vs-chlorine/
6. https://www.hydrotech-group.com/blog/chlorine-ozone-or-uv-light-how-is-wastewater-disinfected
7. https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation
8. https://matkuling.com/news/uv-water-treatment-systems-and-wavelength/
9. https://www.knowyourh2o.com/indoor-4/uv-disinfection
10. https://www.sciencedirect.com/science/article/abs/pii/S0048969720371722