Bien qu'il soit très courant de comprendre ce qu'est l'ozonation, il est moins courant de comprendre comment l'ozone assure la désinfection ou comment l'ozone est créé dans les générateurs. Vous trouverez ici ces réponses ainsi que des estimations CAPEX/OPEX !
Les techniques de désinfection de l’eau sont de plus en plus variées, que l’on pense à l’ajout de produit chimique comme le métabisulfite de sodium ou le chlore et ses dérivés, ou que l'on pense à des techniques physiques comme l'irradiation UV chacune d’entre elles présente des avantages et des faiblesses. Toutefois, il existe une technique de désinfection qui se démarque des autres par ses forces et sa polyvalence: l’ozonation.
In this article, we discuss the subject of ozone from its manufacture, its operation, its uses and much more. At the end of the article, you will find graphs comparing the costs (CAPEX & OPEX) associated with different ozonation situations.
What Is Ozonation?
In the water treatment industry, ozonation is a chemical disinfection technique. Therefore, ozonation attacks pathogenic organisms in order to deactivate or destroy them, and thus prevent the spread of waterborne diseases. In a very simple way, it is enough to add ozone to the water in order to allow it to attack the contaminants and sanitize the water.
How does ozone work?
To begin with, ozone is an unstable gas consisting of 3 oxygen atoms. Having a very powerful oxidizing power and having the quality of virucide, ozone cleans water thanks to 4 mechanisms of disinfection:
- Direct oxidation: Ozone attacks directly the cell walls. By destroying the cell walls, leakage of the cell component occurs and "kills" the contaminating organism.
- Reaction with the radicals of the by-products due to the decomposition of ozone. When the ozone molecule decomposes, hydroxyl radicals are created. This oxygen species is considered one of the most reactive and attacks organic molecules to oxidize them.
- The damage of nucleic acids and their compounds such as purines and pyrimidines. Ozone will therefore attack the DNA and RNA in the water to be treated.
- Lastly, ozone can break the carbon-nitrogen bonds between the polymer molecule to make them monomeric. This is called depolymerization. By the way, depolymerization is a process widely used in recycling.
Since the ozonation process operates under these four mechanisms, ozone has a much wider range of disinfection capabilities than other types of treatment. Most importantly, ozone is considered to be the strongest oxidizer on earth and is said to be up to 3000 times more powerful than chlorine.
What are the uses of ozone in water treatment?
The ozonation process is found in many industrial sectors when it comes to water treatment. Whether used as a secondary treatment or as an additional treatment to ensure optimal disinfection of industrial wastewater, ozonation ensures optimal water treatment.
The sectors where the use of chemicals is prohibited or not recommended are those where ozone is most often used as a disinfectant. One can think of the food industry, some pharmaceuticals or the production lines of conductors/electronic products. This is due to the fact that ozone decomposes quickly and does not leave any residue.
Some of the Benefits of Using Ozone
As mentioned above, ozone has an oxidizing power up to 3000 times more powerful than chlorine. Thanks to this quality, the treatment efficiency against viruses and bacteria is far superior to that of chlorine. In addition to being very powerful, the time required for ozone treatment is relatively short. Depending on the type of contaminants to be oxidized, the contact time required for ozone to treat the water is approximately 10 to 30 minutes. Furthermore, unlike many other disinfection techniques, no hazardous residues are produced by the ozonation process since ozone decomposes rapidly.
In reference to its rapid decomposition, ozone cannot be transported or stored, which is why generators are needed at the place of use. Although this may seem like a hassle, it has the advantage of reducing the risks associated with handling and storing chemicals.
Secondly, unlike other types of disinfection, after ozonation there is no bacterial recontamination due to the way ozone destroys contaminants in the water.
Last but not least, ozonation increases the amount of dissolved oxygen present in the water. By doing so, ozone can allow you to avoid the addition of an aeration step before discharging the water.
Some of the Disadvantages of Ozonation
To begin with, the oxidizing power of ozone is so powerful that special equipment with increased resistance to oxidation is required to minimize the risk of corrosion or premature wear.
In addition, the complexity of the ozonation process requires state-of-the-art equipment to ensure proper dosing, optimal contact time and effective treatment. Without the use of the right equipment or the addition of too low a concentration of ozone, the treatment can be ineffective against certain contaminants. Moreover, ozone is a very irritating gas and can be toxic, which means that in case of leaks, minor or not, complications can occur.
Finally, ozonation is not economically efficient for the treatment of suspended solids, BOD/COD and total organic carbons. Although ozone would theoretically have the ability to treat these contaminants, the option would be far too costly to be financially viable. Ultimately, these systems are expensive to purchase, which is why it is important to choose a solution that is optimized for the company's needs, otherwise, unnecessary expenses may occur.
How Ozone Is Produced
As mentioned above, ozone must be produced at the location where it will be used. Therefore, several techniques have been developed to ensure the continuous and constant production of this gas.
What Are the Types of Ozone Generator
There are three types of ozone generators operating under three different principles: UV, corona discharge and electrolytic. Each one of these technologies offers advantages and disadvantages, depending on the situation.
Ozone Generator by Ultraviolet Radiation
UV ozone is produced from air, so the production capacity of these generators is generally very low, since air molecules (O2) cannot produce more than 1% of their weight in ozone. The use of ozone in water treatment systems is generally more demanding than what UV generators can provide, so they are rarely used. However, they can be seen installed in high flow treatment systems that require a very low ozone concentration. On the other hand, this type of generator can become a financial burden if production exceeds 10 grams of ozone per hour.
Regardless of the type of ozone generator, the principle of ozone production remains the same. This principle consists in the separation of oxygen molecules (O2) into individual oxygen atoms (O). When they are alone, the oxygen atoms are very unstable and suddenly get agitated until they collide with other oxygen atoms and agglomerate together to form O3.
Although long wave UV (315-400 nm) has the ability to split oxygen atoms to create ozone, it is the short-wave UV (160 nm) that is most effective and allows for reasonable ozone production. Under the same principle as UV irradiation, the ultraviolet waves will attack the covalent bonds of oxygen in order to break them. Once separated, the ions are left alone and now want to "attack" each other and bond again in the form of ozone.
Ozone Generator by Corona Discharge
To begin with, a corona discharge is a gas discharge whose geometry restricts the ionization process of the gases around the active electrodes. When the corona discharge is activated, the oxygen between the dielectric materials is ionized. This ionization breaks the covalent bonds of the oxygen atoms and accelerates their movement. Collisions occur between these atoms and ozone is produced as a result of this reaction.
To operate, corona discharge ozone generators need electrodes, a dielectric material, a space where the discharges are produced and an oxygen supply. This supply can come from a nearby compressor, oxygen generator or oxygen cylinder. Corona discharge ozone generators are the most common in the water treatment industry as they allow the production of ozone at very high levels. For example, the Ozonia XF series can produce over 250 kg/h of ozone.
Although these systems are very efficient, they produce a lot of heat. It is therefore important to ensure that the surrounding temperatures do not affect the efficiency of the ozonation process since ozone is greatly affected by high temperatures.
Electrolytic Ozone Generator
When choosing an electrolytic ozone generator, ozone is produced directly from water, which means that no O2 is added. To do this, an electrical discharge is used in the water to separate the H2O molecules to form H2 + O2 molecules. Then, with the help of catalysts, anodes and cathodes, the O2 is separated again to form oxygen atoms alone. Once in its individual form, the oxygen atoms activate and together form O3. However, in order to function effectively, the water must be free of contamination. In other words, ultra-pure water is required to allow the transition from H2O -> H2 + O3.
Even though the oxygen addition step is subtracted, electrolytic ozone generators are very energy consuming because they proceed to several reactions to achieve ozone production. Moreover, even if their design is compact, the life span of the anodes/cathodes is relatively short, which drastically increases the maintenance and operation costs.
Costs Associated With Ozone
As we have discussed a few times in this article, ozonation is a very effective water treatment technology, but it is far from being the most affordable. In short, in order to help you understand the costs associated with this technology, we have prepared a few graphs to help you estimate these costs.
How much Does It Cost to Buy and Operate an Ozone Generator
Before we begin, it is important to note that the price of an ozone system and its use is highly dependent on the situation it will face. The quality of the water, the purpose of its use, the quantities required and the price of the related expenses will have a significant impact on the final price.
How to Calculate the Amount of Ozone Needed
Even though we mentioned it above, we would like to take the time to come back to the subject of the ozone concentrations needed for water treatment. It is important to note that ozone is not preferential, so the addition of this gas will oxidize all contaminants present in the water. Therefore, the more contaminated the water is, the higher the ozone concentration must be.
For example, to be oxidized, iron requires about 0.2 to 0.5 mg/l of ozone. For manganese, it is 0.88 mg/l. And finally, hydrogen sulfite requires 3 mg/l. What is also important to understand is that since ozone decomposes quickly, it is ideal to add 25% to the production. In addition, changes in water temperature will greatly affect the effectiveness of the treatment. So if your water line is subject to drastic changes, this must be taken into account.
In other words, to know exactly how much ozone you need for your treatment, you need to do a water analysis.
Once you know your ozone requirements, the following formula allows you to calculate your relative ozone generation needs.
- (mg/L x L/h) + 25%
Example situation: need for 8 mg/L of ozone with a flow rate of 50 gpm
- 50 gpm = (50 x 3.78) x 60 = 11 340 litres per hour
- 8 mg/L x 11 340 = 90 720mg/h + 25%
- 113 400 / 1000 = 11,34 g/h of ozone
In short, in the graph below, you will find the costs for 3 corona discharge ozone generators. Each of them has a variable production capacity, but all work on the same principle.
In terms of operating costs, ozone generators are relatively energy consuming. This means that the price of your electricity will have an impact on the annual OPEX of your system. In addition to that, some parts may need to be changed and this usually requires trained technicians. Anyway, here is an estimate of the operational costs depending on the type of generator chosen. In this estimate, the systems are operational 24/7:
A Little Reminder
Thus, as we have seen, ozonation is a particularly effective disinfection process: concentrations of 2 mg/L are sufficient to disinfect pretreated water. Although relatively expensive, these systems have various advantages and are widely used in many industries.
We consider it important to remind you that ozone is a corrosive gas and that ozone generation system, regardless of the model, are very specialized systems and have characteristics that can be dangerous for health and safety. This is why we advise not to work on this equipment without having had specific training.
In the meantime, we remind you that if you have any questions about water treatment technologies or any other related subject, we will be happy to answer them. Do not hesitate to write to us privately or in comments.