Analysing the difference between Chlorine and Chlorine Dioxide as water sanitation treatments

When reviewing the best water sanitation treatment for your farm there are many options available, and it can often be difficult to identify the best solution for you.

With a market saturated with treatment options, including various chemical and infrastructure applications, and with an increasing demand to keep the bottom line low, having a clear understanding of the pro’s and con’s that come with each option is ever more important.

At FarmWater, the majority of our work is based on the use of Chlorine Dioxide (ClO₂). Whilst on paper ClO₂ may look to be very similar to Chlorine (Cl), the difference in these chemical compounds is vast, and so in this article our aim is to address the differences between the two, and to demonstrate the benefits of using a ClO₂ based system.

Why is water sanitation so important?

Many drinking water sources contain levels of disease-causing micro-organisms known as pathogens. One of the primary objectives of the treatment of drinking water is the removal or reduction of these pathogens which helps to maintain and support the health and wellbeing of livestock. Additionally, in a time where the consumer market is becoming more aware of what food producing animals consume during their lifespan, it is even more important that potable water is at the top of the agricultural and farming agendas. 

Would you drink from these water pipes?

The answer is that many of us already do. The pipes above were extracted from every day drinking water supplies around the UK. With this being the case, what is the problem in exposing this form of contamination to our livestock?

The answer is quite simple. As we go about our daily lives we are not under the same pressure to grow and perform to the same level that we have come to expect from our livestock. It is with this ever increasing pressure on our stock that the impact of a contaminated water supply is polarised, and the effects can be much more prevalent. 

Amongst the human population, we will all be aware of the effects of changes in water quality and composition, for example, when you travel abroad and treatment systems are variable to those we are used to at home. These variations in water supply can affect us all in different ways as the mineral content and bacterial composition of water changes to one which we are not accustomed to. Similarly, it is the mineral and composition changes, as well as bacterial contamination within a water supply that can add complications and negatively impact the performance of our stock. It is therefore important to ensure a stable and consistent water supply with an effective water sanitation treatment protocol in order to limit the risk of poor sanitation compromising the health, wellbeing and performance of our livestock.

A short introduction to Biofilm

Contrary to common belief, most bacteria are not “free living” – instead, they live in a self-sustaining community where they cooperate with other bacteria and are largely protected from outside influences. This is properly known as a “Biofilm”.

Biofilm removal is the single most important part of any drinking water treatment system. If the sanitation product used does not have the ability to remove biofilm it is impossible to have a safe, pathogen free water supply. Many products – even down to the level of citrus juices – have the ability to kill free living bacteria, but only the best have the ability to penetrate and destroy biofilm, meaning that identifying and using an effective treatment in the correct way is of primary importance.

For more information on biofilm, click here to read our article.

Water Treatment Approaches

There are several solutions that are available that can effectively remove biofilm from water supplies and which you should use depends on many different factors including; your infrastructure, the water source and identifying the best product to deliver the solution that is right for your farm.

Here we outline the difference between Cl and ClO₂ treatment for comparison.

Chlorine Disinfection of Water

Chlorine (Cl) disinfection can be achieved by using either chlorine gas or sodium or calcium hypochlorite. Chlorine gas can be generated by combining salt and acid or it can be purchased as a bottled gas. When free chlorine gas is dissolved in water, a mixture of hydrochloric and hypochlorous acids is formed which will react with water to varying degrees of success to an extent determined by pH. At pH 7 approximately 80% remains effective as hypochlorous acid, but as the pH increases this figure falls dramatically meaning that at pH 8 only around 20% remains active. Therefore when using Cl it is important to closely monitor and control the pH to ensure optimum effectivity of the chemical.

When sodium or calcium hypochlorite are used, the chemical reactions involved also produce sodium and calcium hydroxide. This reaction causes an increase in the pH, which results in a reduction in the efficacy of the active acids. Chlorine (Cl) as hypochlorous acid readily reacts with both organic and non-organic material such as sulphides and ammonia. Initially the chlorine will be used up in these reactions and testing may show that none remains as a residual in the water. As further hypochlorous acid is added, ammonia and some organics will react to form chloramines and chloro-organic compounds which can result in taste and odour problems. Additional hypochlorous acid will be used in oxidising the compounds just formed, and only after that has happened will there be an available residual in the water to ensure thorough sanitation.

As described above, in water with a high organic load Cl is used up in complex multiple reactions. These reactions are temperature, and pH related, and Cl will start to dissociate or “gas off” from water at about 40oC.

Although Cl will oxidise as a tertiary reaction, its primary reaction is chlorination and many of the chloro-organics formed will not be re-oxidised. These residuals include the group trihalomethanes, which are known carcinogens and difficult and costly to remove, especially once they have contaminated the ground water.

Free chlorine gas is toxic and corrosive when in solution. Biocidal effectiveness is limited to simpler organisms and viruses in operating conditions of pH6.5 – 8 and below 40oC. Cl has limited effectiveness against biofouling and will not kill all of the more complex organisms such as cysts and protozoa. Research has shown amoebic cysts infected with hundreds of legionella bacteria surviving in well chlorinated systems. When the host dies the legionella bacteria are released and infect the bases of calorifiers, silt traps and re-seed themselves in the biofouling. Because Cl readily combines with both organic and inorganic material to form both toxic and carcinogenic by-products, its use as a potable water disinfectant is rapidly declining and will continue to do so.

Chlorine Dioxide Disinfection of Water

Chlorine dioxide (ClO₂) is a powerful oxidising biocide and has been successfully used as a water treatment disinfectant for several decades in many countries. Rapid progress has been made in the technology for generation of the product and knowledge of its reactivity has increased with improved analytical techniques.

ClO₂ is a relatively stable radical molecule, it is highly soluble in water, has a boiling point of 110C, absorbs light and breaks down into ClO3- and Cl. Because of its oxidising properties, ClO₂ acts on Fe2, Mn2+ and NO2- but does not act on Cl, NH4+ and Brwhen not exposed to light. These ions are generally part of the chemical composition of natural water.

Because of its radical structure, ClO₂ has a particular reactivity – totally different from that of chlorine. Chlorine behaves as an electron acceptor or is electrophilic, while chlorine dioxide has a free electron for a homopolar bond based on one of its oxygens. The electrophilic nature of Cl or hypochloric acid can lead, through reaction of addition or substitution, to the formation of organic species while the radical reactivity of ClO₂ mainly results in oxycarbonyls.

The oxidising properties and the radical nature of ClO₂ make it an excellent virucidal and bactericidal agent in a large pH range. The most probable explanation is that in the alkaline media the permeability of living cell walls to gaseous ClO₂ radicals seems to be increased, which allows easier access to vital molecules. It is the reaction of ClO₂ with vital amino acids that is one of the dominant processes in its action on bacteria and viruses.

Chlorine dioxide is efficient against viruses, bacteria and protoza clumps usually found in raw water. A rise in pH level further increases its action against f2 bacteriophages, amoebic clumps, polioviruses and anterovirus. It is efficient against Giardia and has an excellent biocidal effect against Cryptosporidium which are resistant to chlorine and chloromines. It has been demonstrated that ClO2 has greater persistence than Cl. In a recent report, for dosages 3 times lower than those of Cl at the station outlet, the residual of ClO2 used alone was always higher than that of Cl2 which also required 3 extra injections of chlorine in the distribution system.

There is also a reduction of bad tastes and odour with the use of ClO2. This reduction is the result of the elimination of algae and on the negligible formation of organo-chlorinated derivatives. The latter formed under chlorination give rise to very unpleasant odours. By its action on dissolved organic materials, without the formation of organic halogen compounds, ClO2 limits problems of taste and colour. In addition the low dosages used post-disinfection are an advantage.

When comparing ClO₂ with Cl, the oxidation of ClO₂ is more selective in that it is highly reactive with certain amino acids that make up proteins. Chlorine dioxide also provides a better downstream residual, making it the ideal biocide for use in water systems.

ClO₂’s behaviour as an oxidising agent is quite dissimilar to non-oxidising biocides. Instead of combining with the aromatic rings, chlorine dioxide breaks these rings apart. In addition, as the use of ClO₂ increases, the generation of chlorinated organics fall dramatically. ClO₂’s chemistry also explains why it is such an effective oxidant, or bleaching agent. It’s 2.5 times more powerful than chlorine gas, and also much more selective.

In water treatment applications, ClO₂ has broad spectrum activity over a wide range of micro-organisms, and has the ability to penetrate biofilms, and other heavily contaminated areas, where Cl treatment is simply not effective. Because of its increased efficiency, far less of the ClO₂ product is needed within a treatment programme thus eliminating waste, and reducing handling costs and risk.


Because of their fundamentally different chemistries, ClO₂ and Cl react in distinct ways with organic compounds, and as a result generate very different by-products. It’s this difference that explains the superior environmental performance of chlorine dioxide in a number of industrial applications.

Chlorine and chlorine dioxide are oxidising agents – electron receivers. Chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five. This property, along with the complex but well-known ways in which chlorine combines with certain organic materials, to form chlorinated organics that cause numerous environmental problems, explains the superiority of chlorine dioxide based products.

For more information on ClO₂, or on the FarmWater Chlorine Dioxide Generator Systems, please visit the links below: