Can Ozone Replace Chlorine? Comparing Water Disinfection Methods, Wastewater Treatment, and Water Safety

From water contaminants and E. coli testing to chlorination, ozonation, and advanced wastewater treatment, here's how scientists make drinking water safe—and why no single disinfection method is perfect.

Clean drinking water is fundamental to human health, yet ensuring its safety has become one of the greatest environmental and public health challenges of the 21st century. Rapid urbanisation, industrial expansion, agricultural runoff, and inadequate sewage treatment have significantly increased the contamination of freshwater resources across the globe, including India.

The seriousness of this challenge was recently highlighted by the National Green Tribunal (NGT), which observed that contamination of drinking water amounts to water pollution and directly violates the constitutional right to life guaranteed under Article 21 of the Constitution.

The Tribunal referred to national data showing that India recorded more than 20.98 crore cases of waterborne diseases between 2005 and 2022, underlining the enormous burden unsafe water continues to impose on public health.

These alarming figures raise several important questions.

• What exactly contaminates drinking water?

• How do scientists determine whether water is safe to drink?

• How is wastewater treated before being reused or discharged?

• Is chlorination still the best method of water disinfection?

• Can ozone or other advanced technologies replace chlorine?

Understanding these questions is increasingly important as climate change, population growth, freshwater scarcity, and pollution place unprecedented pressure on available water resources.

Why Safe Water Is Becoming One of the World's Biggest Challenges

Water covers nearly 71% of Earth's surface, yet surprisingly little of it is available for human consumption.

According to environmental engineering experts Gilbert M. Masters and Wendell P. Ela, Earth's total water distribution reveals just how scarce freshwater actually is.

Approximately:

• 96.5% of Earth's water exists in oceans and seas as saline water.

• Only 0.007% exists in freshwater lakes.

• Rivers and streams together account for merely 0.002% of global water resources.

The overwhelming majority of Earth's water is therefore unsuitable for drinking without expensive desalination.

At the same time, freshwater demand continues to increase because of:

• Rapid population growth

• Urban expansion

• Industrial development

• Agricultural irrigation

• Climate variability

• Rising standards of living

This growing imbalance between water demand and freshwater availability has made wastewater recycling one of the most practical and sustainable solutions for the future.

Why Wastewater Recycling Is No Longer Optional

For decades, wastewater was viewed simply as waste that needed to be discarded safely.

Today, environmental scientists increasingly regard wastewater as a valuable resource.

Properly treated wastewater can be safely reused for:

• Agriculture

• Industrial cooling

• Manufacturing

• Landscape irrigation

• Construction activities

• Groundwater recharge

• Environmental restoration

In many countries experiencing water scarcity, treated wastewater has become an essential component of water resource management.

Instead of extracting more freshwater from rivers and groundwater aquifers, governments are investing heavily in advanced wastewater treatment technologies that allow water to be recycled multiple times.

This approach provides several important benefits.

It helps:

• Conserve freshwater resources.

• Reduce environmental pollution.

• Improve water security.

• Support sustainable agriculture.

• Protect aquatic ecosystems.

• Lower pressure on groundwater extraction.

However, effective wastewater recycling depends entirely on one critical factor—proper treatment and disinfection.

Why Wastewater Treatment Is Essential Before Reuse

Wastewater leaving households, hospitals, industries, farms, and commercial establishments contains a complex mixture of contaminants.

If discharged without treatment, it can contaminate:

• Rivers

• Lakes

• Groundwater

• Reservoirs

• Coastal ecosystems

• Drinking water sources

Untreated wastewater also spreads infectious diseases and damages aquatic ecosystems by reducing oxygen levels and introducing toxic chemicals.

Consequently, wastewater treatment combines expertise from:

• Environmental engineering

• Microbiology

• Chemistry

• Public health

• Civil engineering

• Hydrology

The goal is not merely to remove visible dirt but also to eliminate microscopic organisms, toxic chemicals, and harmful pollutants that cannot be detected with the naked eye.

What's Really Present in Contaminated Water?

Many people assume dirty water simply contains mud or visible waste.

In reality, contaminated water contains a highly diverse mixture of biological and chemical pollutants.

Scientists generally classify these contaminants into two broad categories:

• Biological contaminants

• Chemical contaminants

Each category presents unique risks to human health and requires different treatment methods.

Biological Contaminants: Invisible Microorganisms That Cause Disease

Biological contamination remains the greatest immediate threat to drinking water safety.

These microorganisms are capable of causing widespread disease outbreaks when contaminated water is consumed.

The major categories include:

1. Viruses

Waterborne viruses are among the smallest infectious agents.

Examples include:

• Enteroviruses

• Rotaviruses

• Noroviruses

• Hepatitis A virus

These viruses can survive in contaminated water and spread rapidly through communities lacking adequate sanitation.

Symptoms may include:

• Severe diarrhoea

• Vomiting

• Fever

• Liver infection

• Dehydration

2. Bacteria

Several dangerous bacterial species contaminate drinking water through human and animal waste.

Common examples include:

• Salmonella

• Shigella

• Escherichia coli (certain pathogenic strains)

• Vibrio cholerae

These bacteria may cause illnesses ranging from mild gastroenteritis to life-threatening cholera and typhoid fever.

3. Protozoa

Protozoa are microscopic single-celled organisms that often survive conventional water treatment if disinfection is inadequate.

Examples include:

• Entamoeba

• Giardia

• Cryptosporidium

These organisms produce highly resistant cysts capable of surviving in water for extended periods.

Some protozoa are also relatively resistant to chlorine, making advanced treatment methods increasingly important.

4. Helminths

Helminths are parasitic worms transmitted through contaminated soil and water.

Common examples include:

• Hookworms

• Roundworms

• Tapeworms

Although less common in treated municipal water supplies, they remain an important concern in regions with poor sanitation.

Chemical Contaminants: A Growing Environmental Challenge

Unlike biological contaminants, chemical pollutants often accumulate gradually within the human body.

Long-term exposure may damage vital organs even when immediate symptoms are absent.

Scientists classify chemical contaminants into several major groups.

Oxygen-Demanding Wastes

Domestic sewage, industrial wastewater, food processing waste, and agricultural runoff introduce large quantities of biodegradable organic matter into water bodies.

As microorganisms decompose these materials, they consume dissolved oxygen.

This increases the Biological Oxygen Demand (BOD) of water.

High BOD levels reduce oxygen available for fish and other aquatic organisms, often resulting in:

• Fish kills

• Ecosystem degradation

• Foul odours

• Poor water quality

For environmental engineers, BOD remains one of the most important indicators of wastewater pollution.

Excess Nutrients and Eutrophication

Nitrogen and phosphorus compounds are essential nutrients for plant growth.

However, excessive quantities entering rivers and lakes create serious environmental problems.

Major sources include:

• Chemical fertilisers

• Animal waste

• Domestic sewage

• Industrial effluents

These nutrients stimulate explosive algal growth, a process known as eutrophication.

Large algal blooms block sunlight, reduce oxygen levels, and may release toxins harmful to humans and aquatic life.

Another major concern involves elevated nitrate concentrations in drinking water.

High nitrate levels have been linked to methemoglobinemia, commonly known as "Blue Baby Syndrome," a potentially life-threatening condition affecting infants by reducing the blood's ability to carry oxygen.

Heavy Metals: Silent but Dangerous Pollutants

Heavy metals are among the most persistent contaminants found in water.

Unlike many biological pollutants, metals do not degrade naturally.

Instead, they accumulate within living organisms over time.

Common water contaminants include:

• Lead

• Mercury

• Arsenic

• Cadmium

• Chromium

Long-term exposure has been associated with:

• Kidney damage

• Liver disease

• Neurological disorders

• Developmental problems in children

• Increased cancer risk

Industrial discharge, mining, and improper waste disposal remain major sources of heavy metal contamination.

Pesticides and Agricultural Chemicals

Modern agriculture relies extensively on pesticides, herbicides, and insecticides.

Rainfall and irrigation frequently wash these chemicals into nearby rivers and groundwater.

Besides harming aquatic organisms, some pesticides undergo biomagnification, becoming increasingly concentrated as they move up the food chain.

As a result, humans consuming contaminated fish or drinking polluted water may experience long-term health effects.

Volatile Organic Compounds (VOCs)

Volatile Organic Compounds, commonly called VOCs, include a broad class of industrial solvents and petroleum-based chemicals.

These compounds primarily contaminate groundwater through:

• Fuel leaks

• Industrial spills

• Chemical manufacturing

• Improper waste disposal

Many VOCs have been associated with:

• Liver damage

• Kidney damage

• Nervous system disorders

• Increased cancer risk

Because VOC contamination often occurs underground, it may remain undetected for years without regular monitoring.

Emerging Pollutants: The New Challenge for Water Scientists

In recent years, scientists have become increasingly concerned about emerging contaminants—pollutants that traditional wastewater treatment plants were not originally designed to remove.

These include:

• Pharmaceutical residues

• Personal care products

• Microplastics

• Antibiotic residues

• Hormones

• Endocrine-disrupting chemicals (EDCs)

One well-known endocrine-disrupting chemical is ethinylestradiol, a synthetic hormone commonly used in oral contraceptive pills.

Even at extremely low concentrations, studies have shown that these chemicals can disrupt reproduction and development in fish, amphibians, and other aquatic organisms.

Researchers are also investigating their potential long-term effects on human health.

Why Understanding Water Contaminants Matters

Every contaminant behaves differently inside water.

Some survive chlorination.

Others resist conventional filtration.

Certain chemicals accumulate gradually within the body, while some pathogens spread rapidly through communities.

Because no single treatment process removes every contaminant equally well, modern wastewater treatment relies on a carefully designed sequence of physical, chemical, and biological processes.

Before engineers decide how to treat contaminated water, however, they must first determine what is actually present in the water.

This raises another critical question:

How do scientists identify harmful microorganisms without testing for every single pathogen individually?

The answer lies in one of environmental microbiology's most important tools—indicator organisms such as Escherichia coli (E. coli) and other coliform bacteria.

How Scientists Test Water Quality: Why E. coli Is Used as an Indicator and How Wastewater Treatment Removes Dangerous Contaminants

Ensuring that drinking water is safe involves far more than simply making it appear clean. Clear water can still contain millions of disease-causing microorganisms, toxic chemicals, or invisible pollutants capable of causing serious illnesses.

This is why water quality assessment is one of the most important components of public health. Before water reaches homes, hospitals, industries, or farms, scientists perform a series of microbiological, chemical, and physical tests to determine whether it is safe for human use.

One of the biggest challenges in water testing is that contaminated water may contain thousands of different microorganisms. Testing individually for every virus, bacterium, protozoan, and parasite would be extremely expensive, time-consuming, and technically impractical.

Instead, environmental microbiologists rely on indicator organisms, particularly Escherichia coli (E. coli) and other coliform bacteria, to assess the microbial quality of water.

This approach has become the global standard for monitoring drinking water safety.

Why Scientists Use Indicator Organisms Instead of Testing Every Pathogen

A single litre of contaminated water may contain hundreds of microbial species.

These include:

• Disease-causing bacteria

• Viruses

• Protozoa

• Helminths

• Non-harmful microorganisms

Testing separately for every pathogen would require multiple laboratory techniques, specialised equipment, significant financial resources, and considerable time.

For routine monitoring, such an approach is simply not feasible.

Instead, scientists search for microorganisms whose presence strongly suggests faecal contamination.

These microorganisms are known as indicator organisms.

Among them, coliform bacteria, especially Escherichia coli (E. coli), are considered the most reliable indicators of recent faecal pollution.

What Is E. coli and Why Is It Used to Test Water Quality?

Escherichia coli (E. coli) is a bacterium that normally lives in the intestines of humans and warm-blooded animals.

Most strains are harmless and play an important role in digestion.

However, their presence in drinking water is a warning sign.

Because E. coli originates from faeces, finding it in water strongly indicates that sewage or animal waste has entered the water supply.

Once faecal contamination occurs, other disease-causing microorganisms may also be present.

Therefore, E. coli acts as a biological marker rather than the disease itself.

Why Coliform Bacteria Are the Global Standard for Water Testing

Environmental scientists use coliform bacteria as indicator organisms for several scientific reasons.

1. Faecal Contamination Is the Greatest Public Health Risk

Human and animal faeces contain numerous pathogens capable of causing serious diseases.

If faecal matter enters drinking water, multiple disease-causing organisms may accompany it.

Detecting faecal contamination therefore provides an early warning of broader microbial pollution.

2. Coliform Bacteria Exist Wherever Fresh Faecal Contamination Occurs

The human digestive system naturally contains enormous numbers of coliform bacteria.

Whenever untreated sewage contaminates water, coliform bacteria usually appear in very high concentrations.

Their presence therefore indicates recent contamination.

3. Pathogens Usually Survive for Shorter Periods Than Coliform Bacteria

Many disease-causing organisms die relatively quickly outside the human body.

Coliform bacteria generally survive longer under environmental conditions.

Consequently, if coliform bacteria are absent, it is less likely that recently introduced pathogens remain active.

4. Routine Monitoring Becomes Faster and More Reliable

Testing for one reliable indicator organism allows water treatment plants to monitor thousands of water samples efficiently.

This approach greatly reduces laboratory costs while maintaining high public health standards.

Why Indicator Organisms Are Not Always Enough

Although indicator organisms remain extremely useful, they do not guarantee complete water safety.

Scientists have discovered that some dangerous microorganisms survive treatment processes better than coliform bacteria.

Examples include:

• Giardia

• Cryptosporidium

• Enteric viruses

These organisms possess protective cysts or structures that make them more resistant to certain disinfectants.

As a result, eliminating E. coli from water does not automatically mean every pathogen has been destroyed.

Recognising this limitation, modern drinking water regulations now specify additional treatment performance targets rather than relying solely on coliform testing.

Understanding the 4-Log, 3-Log, and 2-Log Water Treatment Standards

To improve public health protection, modern drinking water regulations require treatment plants to achieve specific pathogen reduction targets.

These are commonly known as log reduction standards.

4-Log Reduction

Requires 99.99% inactivation of enteric viruses.

This means only one virus survives out of every 10,000 originally present.

3-Log Reduction

Requires 99.9% removal or inactivation of Giardia.

Only one organism remains from every 1,000 present before treatment.

2-Log Reduction

Requires 99% removal or inactivation of Cryptosporidium.

This standard recognises that Cryptosporidium is more resistant to conventional disinfection methods.

These performance targets ensure treatment plants achieve much higher levels of pathogen control than indicator testing alone can demonstrate.

How Scientists Distinguish Human and Animal Faecal Pollution

Sometimes identifying the source of contamination becomes equally important.

Was pollution caused by leaking sewage pipelines?

Or did livestock contaminate nearby rivers?

To answer this question, scientists often compare:

• Faecal coliform bacteria

• Faecal streptococci

The ratio between these two bacterial groups provides useful clues regarding whether contamination originated primarily from humans or animals.

This information helps authorities identify pollution sources and implement corrective measures.

How Is Water Tested for Coliform Bacteria?

The internationally accepted laboratory procedure follows Standard Methods for the Examination of Water and Wastewater, published by the American Public Health Association (APHA).

The testing process consists of three successive stages.

Step 1: Presumptive Test

The presumptive test provides the first indication that coliform bacteria may be present.

Scientists place water samples into lactose-containing culture media.

If coliform bacteria exist, they ferment lactose.

This produces:

• Acid

• Gas

Gas production serves as the initial evidence suggesting possible coliform contamination.

Because other bacteria may occasionally produce similar reactions, further confirmation becomes necessary.

Step 2: Confirmed Test

Samples showing positive presumptive results undergo additional laboratory testing.

Scientists culture them using specialised media such as:

• Brilliant Green Lactose Bile Broth

• Eosin Methylene Blue (EMB) Agar

Characteristic bacterial growth together with gas production confirms the presence of coliform organisms.

This step greatly improves test accuracy.

Step 3: Completed Test

The completed test provides final confirmation.

Scientists perform:

• Gram staining

• Additional culture verification

• Gas production assessment

Only after successful completion of all three stages is the presence of coliform bacteria officially confirmed.

This systematic approach minimises false-positive results while ensuring reliable monitoring of drinking water quality.

What Happens After Contamination Is Confirmed?

Detecting microbial contamination is only the beginning.

Once laboratory tests confirm unsafe water, treatment plants initiate a carefully designed sequence of purification processes.

These combine:

• Physical treatment

• Chemical treatment

• Biological treatment

Each stage removes different categories of contaminants.

No single treatment process can eliminate every pollutant present in wastewater.

Instead, engineers rely on multiple complementary treatment technologies.

Stage 1: Primary Wastewater Treatment

Primary treatment focuses mainly on removing large suspended materials.

The process usually begins with screening.

Large objects such as:

• Plastic

• Cloth

• Wood

• Leaves

• Stones

are mechanically removed before further treatment.

Rapid Mixing, Coagulation and Flocculation

After screening, chemicals called coagulants are rapidly mixed into the water.

These chemicals neutralise electrical charges on suspended particles.

Once destabilised, tiny particles begin sticking together.

The next process—flocculation—gently stirs the water.

Small suspended particles gradually combine into much larger clusters known as flocs.

This stage is one of the most important steps in water treatment because many impurities are too small to settle naturally.

Sedimentation Removes Heavy Flocs

The newly formed flocs are much heavier than individual particles.

They gradually settle to the bottom of sedimentation tanks under gravity.

This removes a substantial proportion of:

• Suspended solids

• Organic matter

• Attached microorganisms

The clarified water then moves to the next stage.

Filtration Produces Much Cleaner Water

Even after sedimentation, tiny suspended particles remain.

Water therefore passes through rapid sand filters.

These filters remove:

• Fine suspended solids

• Remaining flocs

• Many microorganisms

Filtration significantly improves water clarity and prepares it for final disinfection.

Stage 2: Biological Wastewater Treatment

While primary treatment removes physical particles, biological treatment removes dissolved organic pollution.

The most widely used biological treatment method worldwide is the Activated Sludge Process.

What Is the Activated Sludge Process?

The activated sludge process uses naturally occurring microorganisms to clean wastewater.

Large aeration tanks continuously pump oxygen into wastewater.

The added oxygen allows beneficial bacteria and other microorganisms to multiply rapidly.

These microorganisms consume:

• Organic waste

• Sewage

• Dissolved pollutants

As they feed, pollutants are converted into:

• Microbial biomass

• Carbon dioxide

• Water

The resulting biomass eventually settles during secondary sedimentation and is removed from the treated water.

This process dramatically lowers Biological Oxygen Demand (BOD) and improves overall water quality before final disinfection.

Why Wastewater Treatment Requires Multiple Stages

Each treatment stage targets different pollutants.

Treatment Stage	Primary Purpose
Screening	Removes large debris
Coagulation & Flocculation	Combines suspended particles
Sedimentation	Removes heavy solids
Filtration	Removes fine particles and microbes
Activated Sludge	Removes dissolved organic matter
Final Disinfection	Destroys disease-causing microorganisms

Together, these processes transform heavily contaminated wastewater into water that can be safely discharged—or further treated for reuse.

The Final and Most Critical Step: Water Disinfection

Even after filtration and biological treatment, some microorganisms may still survive.

This makes disinfection the final and perhaps most important stage of wastewater treatment.

Engineers use disinfectants to destroy the remaining disease-causing organisms before treated water enters rivers, reservoirs, irrigation systems, or drinking water supplies.

Among all available methods, chlorination remains the world's most widely used water disinfection technique.

However, it is no longer the only option.

Modern treatment plants increasingly use ozone, chlorine dioxide, ultraviolet (UV) radiation, and advanced oxidation technologies, each offering distinct advantages and limitations.

Chlorination vs Ozonation: Which Is the Most Effective Method of Water Disinfection?

After wastewater has undergone screening, sedimentation, filtration, and biological treatment, one final challenge remains—destroying the disease-causing microorganisms that survive these processes.

This final step, known as disinfection, is the last barrier between treated water and the public water supply.

Even crystal-clear water can still contain bacteria, viruses, protozoa, and other pathogens capable of causing serious outbreaks of diseases such as cholera, typhoid, dysentery, hepatitis A, and gastroenteritis.

For more than a century, chlorination has been the world's most widely used water disinfection technique. However, newer technologies such as ozonation, chlorine dioxide, and ultraviolet (UV) disinfection are increasingly being adopted by advanced water treatment plants.

Each method has unique advantages, limitations, costs, and environmental implications.

The question therefore is not simply whether chlorine or ozone is better, but which disinfection technology is most suitable under different conditions.

Why Water Disinfection Is the Most Critical Step in Treatment

The primary objective of water disinfection is to destroy harmful microorganisms before treated water reaches consumers.

Without effective disinfection, pathogens remaining after filtration can rapidly multiply or survive long enough to infect thousands of people.

History offers numerous examples of waterborne disease outbreaks caused by inadequate disinfection.

These include outbreaks of:

• Cholera

• Typhoid fever

• Dysentery

• Hepatitis A

• Giardia infection

• Cryptosporidiosis

For this reason, modern drinking water treatment always includes a final disinfection stage before distribution.

How Chlorination Works

Chlorination involves adding chlorine or chlorine-containing compounds to treated water.

The most commonly used chemicals include:

• Chlorine gas

• Sodium hypochlorite

• Calcium hypochlorite

When chlorine dissolves in water, it forms hypochlorous acid (HOCl).

Hypochlorous acid is the principal disinfecting agent responsible for destroying microorganisms.

It penetrates microbial cell walls, damages essential enzymes, disrupts metabolic functions, and destroys genetic material, ultimately killing bacteria, viruses, and many other pathogens.

Among the different chlorine species present in water, hypochlorous acid is by far the most effective disinfectant.

Why Chlorination Remains the Global Standard

Despite the availability of newer technologies, chlorination continues to dominate drinking water treatment worldwide.

Several factors explain its continued popularity.

1. Highly Effective Against Most Microorganisms

Chlorine effectively destroys:

• Most bacteria

• Many viruses

• Numerous disease-causing microorganisms

Its broad antimicrobial activity has made it one of the most reliable public health interventions ever introduced.

2. Affordable and Easy to Use

Compared with advanced technologies, chlorination is relatively inexpensive.

Treatment plants require:

• Simple dosing systems

• Lower operational costs

• Less energy

• Easier maintenance

This makes chlorine particularly suitable for developing countries and rural water supply systems.

3. Provides Residual Protection

One of chlorine's greatest advantages is something ozone cannot provide.

After treatment, a small amount of chlorine remains dissolved in water.

This residual chlorine continues protecting the water as it travels through pipelines, storage tanks, and household distribution systems.

If microorganisms enter the water after treatment—for example through leaking pipelines—the remaining chlorine helps destroy them before they reach consumers.

This secondary protection has saved countless lives worldwide.

Why Ammonia Is Sometimes Added After Chlorination

Some treatment plants deliberately add ammonia after chlorination.

This produces compounds known as chloramines.

Chloramines disinfect water more slowly than chlorine but remain active for much longer.

Their advantages include:

• Longer-lasting residual protection

• Reduced chlorine taste

• Lower odour

• Improved distribution system protection

However, chloramines are somewhat less powerful than free chlorine and therefore require careful operational control.

The Major Disadvantages of Chlorination

Although chlorination has transformed public health globally, it is not without drawbacks.

1. Formation of Harmful Disinfection By-products (DBPs)

Perhaps the biggest concern surrounding chlorination is the formation of disinfection by-products (DBPs).

When chlorine reacts with natural organic matter present in water, new chemicals may form.

Common DBPs include:

• Trihalomethanes (THMs)

• Chloroform

• Haloacetic acids (HAAs)

Long-term exposure to elevated concentrations of these compounds has been associated with an increased risk of certain cancers and other health concerns.

Modern treatment plants therefore carefully regulate chlorine dosage to minimise DBP formation while maintaining effective disinfection.

2. Reduced Effectiveness Against Certain Pathogens

Although chlorine performs exceptionally well against most bacteria, it is less effective against some resistant microorganisms.

Examples include:

• Giardia cysts

• Cryptosporidium oocysts

• Certain enteric viruses

Research has shown that chlorination below approximately 0.5 mg/L may be ineffective against some enteroviruses.

This limitation has encouraged many treatment plants to combine chlorine with other treatment technologies.

3. Taste and Odour Problems

Excess chlorine may produce:

• Unpleasant taste

• Swimming pool-like smell

• Consumer dissatisfaction

Although these effects rarely present serious health risks, they often influence public perception of drinking water quality.

Ozonation: A Powerful Alternative to Chlorination

Ozone (O₃) is another highly effective disinfectant increasingly used in modern water treatment plants.

Unlike chlorine, ozone is an unstable gas produced on-site using electricity.

It cannot be stored because it rapidly decomposes back into oxygen.

Ozone is one of the strongest oxidising agents available for water treatment.

It destroys microorganisms by rapidly attacking:

• Cell membranes

• Proteins

• Enzymes

• Genetic material

Its disinfection action is considerably faster than chlorine.

Advantages of Ozone Disinfection

Superior Performance Against Resistant Organisms

Ozone is particularly effective against:

• Giardia cysts

• Cryptosporidium

• Viruses

• Protozoa

These organisms are generally more resistant to chlorine.

Faster Disinfection

Ozone works much more rapidly than chlorine.

Treatment plants often require significantly shorter contact times to achieve equivalent microbial reduction.

Better Taste and Odour

Unlike chlorine, ozone leaves:

• No chemical taste

• No unpleasant smell

It can even improve the flavour of drinking water by oxidising compounds responsible for bad odours.

No Chlorinated By-products

Because ozone contains no chlorine, it does not produce:

• Trihalomethanes

• Chloroform

• Haloacetic acids

This eliminates one of chlorination's major disadvantages.

Why Ozone Cannot Completely Replace Chlorine

Despite its remarkable effectiveness, ozone also has important limitations.

No Residual Protection

The biggest disadvantage of ozone is its extremely short lifespan.

Once treatment is complete, ozone rapidly disappears.

Consequently:

• No disinfectant remains inside pipelines.

• Water receives no secondary protection.

• Microorganisms entering distribution systems later may survive.

This is precisely why many water treatment plants still add a small amount of chlorine after ozonation.

Bromate Formation

Another concern involves the formation of bromate when ozone reacts with naturally occurring bromide in water.

Bromate is regarded as a potentially harmful contaminant.

Its permissible concentration is extremely low—approximately 0.01 mg/L in drinking water.

Treatment plants must therefore carefully monitor bromate formation during ozonation.

High Installation and Operating Costs

Ozone systems require:

• Expensive generators

• High electricity consumption

• Skilled operators

• Continuous maintenance

These costs make ozonation less practical for many municipalities, particularly in developing countries.

Chlorine Dioxide: A Middle-Ground Solution

Another important disinfectant is chlorine dioxide (ClO₂).

It is highly effective against:

• Viruses

• Bacteria

• Biofilms

Unlike chlorine, chlorine dioxide produces far fewer trihalomethanes.

However, it generates chlorites, whose safe concentration is limited to approximately 1 mg/L.

Furthermore, chlorine dioxide systems are generally more expensive than conventional chlorination.

Ultraviolet (UV) Disinfection: An Increasingly Popular Technology

Many modern treatment plants now use ultraviolet (UV) radiation as an additional disinfection barrier.

UV light destroys microorganisms by damaging their DNA, preventing them from reproducing.

Advantages include:

• No chemical additives

• No taste or odour

• No harmful chlorinated by-products

• Highly effective against Cryptosporidium and Giardia

However, UV also has a major limitation:

Like ozone, it leaves no residual disinfectant in the distribution system.

Chlorination vs Ozonation: Which Is Better?

Feature	Chlorination	Ozonation
Cost	Low	High
Disinfection Speed	Moderate	Very Fast
Residual Protection	Yes	No
Virus Control	Good	Excellent
Giardia & Cryptosporidium	Moderate	Excellent
Taste & Odour	May affect water	Improves water quality
Harmful By-products	THMs, HAAs	Bromate
Operational Complexity	Low	High

Is Chlorination Still the Best Method?

There is no universal answer.

Each disinfection technology offers distinct advantages.

Chlorination is ideal when:

• Water must travel long distances.

• Distribution networks are extensive.

• Residual protection is essential.

• Budgets are limited.

Ozone is preferable when:

• Maximum pathogen removal is required.

• Taste and odour improvement is important.

• Advanced treatment facilities are available.

UV is valuable when:

• Chemical-free treatment is preferred.

• Highly resistant protozoa must be controlled.

• Combined treatment systems are used.

Consequently, many modern treatment plants now employ multiple disinfection barriers rather than relying on a single technology.

When Microorganisms Adapt to Disinfection

One of the newest challenges facing environmental microbiologists is microbial adaptation.

Repeated exposure to disinfectants creates selective pressure.

Some microorganisms gradually develop survival mechanisms that improve their ability to withstand stressful conditions.

Scientists are now studying these adaptations using advanced genomic technologies.

Recent research analyses:

• Metagenomes

• Metatranscriptomes

• Intracellular genetic material

• Extracellular genetic material

These techniques help researchers understand:

• Which microorganisms survive treatment.

• Which genes become activated under disinfectant stress.

• Whether resistance mechanisms are emerging.

Understanding these processes is becoming increasingly important as cities expand water reuse programmes.

The Future of Safe Drinking Water

Water treatment is rapidly evolving beyond traditional filtration and chlorination.

Emerging technologies include:

• Artificial intelligence-based monitoring

• Smart water sensors

• Advanced oxidation processes

• Nanotechnology-based filtration

• Membrane bioreactors

• Real-time microbial surveillance

• Genomic water quality monitoring

These innovations promise faster detection of contamination, improved treatment efficiency, and stronger protection against emerging pollutants.

Final Takeaway

Water disinfection remains one of the greatest public health achievements in modern history, preventing millions of deaths from waterborne diseases every year.

Although chlorination continues to be the world's most widely used disinfection method because it is affordable, effective, and provides lasting residual protection, it is no longer the only solution.

Ozone offers superior pathogen inactivation and better water quality but lacks residual protection and requires higher investment. Chlorine dioxide and ultraviolet disinfection provide additional alternatives, each with its own advantages and limitations.

Rather than competing technologies, these methods increasingly complement one another. Modern water treatment plants are moving towards multi-barrier treatment systems that combine physical, biological, and advanced chemical disinfection to ensure safe, reliable, and sustainable drinking water.

As freshwater resources become increasingly scarce and pollution continues to grow, investing in efficient wastewater treatment and scientifically robust disinfection systems will be essential for protecting both public health and long-term water security.