Chlorine dioxide or chlorine, what's the difference?
Chlorine and chlorine dioxide are both used for water disinfection, but they are two different chemical substances with different properties and modes of action.
Chlorine has been used for water treatment for a very long time and is still one of the world's most common disinfection methods. Chlorine dioxide is also used in water treatment but differs from traditional chlorination in several important respects.
Short answer
The main difference is that chlorine and chlorine dioxide act in different ways.
Chlorine is often used both as a disinfectant and to create a residual chlorine level in the water.
Chlorine dioxide is a selective oxidant used to inactivate microorganisms and can also be effective against biofilm in water systems. Its effect is also less affected by variations in pH than traditional chlorination.
The appropriate method therefore depends on the water system, water quality, and what the treatment aims to achieve.
What is chlorine?
When people talk about chlorine in water treatment, they usually refer to various chlorine-based substances used for disinfection.
Chlorine is used, among other things, to reduce bacteria and other microorganisms in drinking water, swimming pool water, and various technical water systems.
An important reason why chlorine is so widely used is that the method is well-established and it is possible to maintain a residual disinfection effect in the water.
At the same time, chlorine reacts with various substances in the water. The pH of the water also greatly influences the effectiveness of chlorination.
What is chlorine dioxide?
Chlorine dioxide, ClO₂, is an oxidant used for microbiological control and disinfection.
Despite its name, chlorine dioxide should not be considered the same as traditional chlorine.
The substance has different chemical properties and reacts differently in water.
Chlorine dioxide is used, among other things, in drinking water, process water, and technical water systems where bacteria and other microorganisms need to be controlled.
One property that makes chlorine dioxide particularly interesting in certain systems is its ability to affect biofilm.
What is biofilm?
Biofilm is a layer of microorganisms that adhere to a surface and embed themselves in a protective structure.
In a water system, biofilm can, for example, form on the inside of:
- Pipes
- Tanks
- Valves
- Filters
- Heat exchangers
- Other water-carrying components
Biofilm can be more difficult to manage than free-floating microorganisms in the water.
This means that it is not always enough to treat only the water volume itself. If microorganisms are established in biofilm, they can continue to affect the system.
Here, the choice of disinfection method can be of great importance.
Read more about biofilm
How do chlorine and chlorine dioxide differ against biofilm?
Traditional chlorination can be used for microbiological control, but organic matter and biofilm can consume chlorine and reduce its effect.
Chlorine dioxide has different oxidation properties and is therefore used in systems where biofilm is part of the problem.
This does not mean that every biofilm problem is automatically solved by adding chlorine dioxide. Dosing, contact time, water quality, system design, and the amount of existing biofilm always affect the result.
In a heavily loaded system, an initial sanitization may also be required before a lower continuous dosing is used to keep the system microbiologically stable.
How does pH affect it?
pH is an important difference between the methods.
The effectiveness of traditional chlorination is clearly dependent on the pH of the water.
When pH changes, the balance between the different forms of chlorine present in the water also changes, which affects the disinfection effect.
Chlorine dioxide works differently, and its disinfection effect is significantly less affected by normal variations in pH.
This can be an advantage in systems where water chemistry varies.
Taste and odor
Chlorine is often associated with the characteristic smell and taste that people recognize from, for example, swimming pools and chlorinated water.
How noticeable this is depends, among other things, on dosing, water quality, and the reactions that occur in the water.
Chlorine dioxide has other reaction properties and is used, for example, in contexts where one wants to avoid the typical taste and smell that can be associated with traditional chlorination.
For drinking water, this is an important practical factor, as effective water treatment also needs to provide water that people actually want to drink.
Byproducts and reactions in the water
All oxidative disinfection methods involve chemical reactions.
Traditional chlorination can react with organic matter and form various chlorinated byproducts.
Chlorine dioxide also forms reaction products, primarily chlorite and chlorate, which means that concentrations and dosing need to be controlled in applications where there are established limit values.
It is therefore misleading to describe any chemical disinfection method as entirely chemical-free.
The relevant question is instead what substances are used, in what concentration, what reactions occur, and what byproducts can arise.
When is chlorine used?
Chlorine is used on a very large scale worldwide.
Common uses include:
- Drinking water production
- Water distribution networks
- Swimming pools
- Process water
- Industrial water systems
The method is well-known, cost-effective, and there is extensive knowledge about control and measurement.
This makes chlorine a natural choice in many systems.
When is chlorine dioxide used?
Chlorine dioxide is used, among other things, when microbiological control needs to be combined with properties that differ from traditional chlorination.
This may be relevant, for example, in:
- Drinking water treatment
- Biofilm in piping systems
- Process water
- Technical water systems
- Industrial installations
- Buildings
- Water systems where pH varies
- Pool & SPA
The choice should always be based on the specific system and its requirements.
Is chlorine dioxide better than chlorine?
There is no universal answer.
To say that one disinfectant is always better than another is an oversimplification.
Chlorine has great advantages in many systems, not least because the technology is established, well-documented, and cost-effective.
Chlorine dioxide, however, can have clear advantages when, for example, biofilm, varying pH, odor, or specific microbiological problems are important factors.
The right comparison is therefore not just:
Which substance is strongest?
But rather:
Which treatment system provides the best control for the particular water and installation?
How does Xinix work with chlorine dioxide?
Xinix develops solutions for water and surface disinfection with FreeBact®.
The product line includes several different areas of application.
FreeBact® Water is developed for drinking water disinfection, from smaller volumes for personal use to larger water volumes and tanks.
FreeBact® Dosage consists of systems for continuous dosing in various types of water systems, such as properties and industrial installations.
FreeBact® Surface is developed for surface disinfection.
FreeBact® Fogging is a portable solution for comprehensive disinfection of enclosed spaces.
Which solution is relevant depends on the application, the water system, and the problem to be addressed.
Frequently asked questions about chlorine and chlorine dioxide
Is chlorine dioxide the same as chlorine?
No.
Chlorine dioxide, ClO₂, and the chlorine compounds normally used in traditional chlorination are chemically different substances and work in different ways.
Does chlorine dioxide work against biofilm?
Chlorine dioxide is used for microbiological control in systems where biofilm occurs and can be a relevant tool for reducing and controlling biofilm.
The result also depends on, among other things, concentration, contact time, water quality, and how established the biofilm is.
Is chlorine dioxide affected by pH?
Chlorine dioxide is less affected by normal variations in pH than traditional chlorination.
This is one of the reasons why the technology can be interesting in water systems with varying water chemistry.
Does chlorine dioxide taste like chlorine?
Chlorine dioxide has different chemical properties than traditional chlorination and therefore does not give the same typical chlorine character.
However, the actual experience is affected by concentration and water quality.
Can chlorine be replaced with chlorine dioxide?
In some water systems, chlorine dioxide can be an alternative to traditional chlorination, but it cannot be generally stated that one method can always replace the other.
This needs to be assessed based on the application, water quality, microbiological requirements, legislation, and how the system is structured.
Summary
Chlorine and chlorine dioxide are both used for microbiological control, but they are not the same.
Traditional chlorination is an established and very widespread method.
Chlorine dioxide has other chemical properties and can be particularly interesting in systems where, for example, biofilm, varying pH, odor, or specific microbiological problems play a major role.
The important thing is therefore not to designate a universal winner.
The important thing is to choose the right method for the right water system.
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Thank you for reading this far!
For questions & thoughts - please contact us.
Alexandra Németh, Marketing Manager, Xinix Global AB
alexandra@xinix.se
Mobile: 070-78 601 678
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