Ozone – Go Zone or No Zone?
This is an often talked about a topic that has a rather polarized following. In terms of system filtration components that can sterilize water, the most common methods in our industry are UV and ozone. We have already covered UV and its complexities so this time we will cover ozone.
First, what is the difference between ozone and UV? Well, ozone is a nonselective oxidizer that breaks down anything that is organic by rupturing the cell wall. UV changes the DNA structure of the organism through irradiation. For UV to be effective, the UV light must penetrate the organism and cannot be shaded by particulates in the water flow. Ozone, on the other hand, can be effective even in less clear water.
For most, if not all of our applications, ozone is injected into system water and when managed properly a residual level can be maintained throughout the entire system. This allows for a higher efficiency in maintaining lower overall bacterial loading. It can remove color and odor from the water. There are also several studies on how ozone removes off flavor using ozone during purge cycles.
Unlike oxygen, ozone cannot be stored because it rapidly converts back to oxygen, therefore O3 must be generated on site. The ozone molecule does not differentiate in that it will attack anything that is organic. It is something that does require forethought when adding it to a filtration system as well as safety programs. However, that should not deter the use of it in systems with heavy loading or critical situations that require sterile water.
Ozone is created when an oxygen molecule is broken into two single atoms and one atom attaches to an existing oxygen molecule. This happens in nature with ultraviolet (UV) radiation from the sun and even lightning. When it is produced by a machine, there are several ways it is produced. This article will review the three most common ways that ozone is produced commercially. It can be produced using a UV light and corona discharge using both traditional dielectrics and plasma systems.
The UV light is possibly the simplest of all three ways we will discuss. These systems are based on the same principals that are found in nature. While ozone is produced in wavelengths less than 240 nanometers (nm), there is a specific UV wavelength for producing ozone which is 185 nanometers (nm). This is not to be confused with the effective germicidal UV wavelength range of 260-270 nm for use in water. Ozone is generally destroyed when exposed to wavelengths between 240 and 315nm.
Keep in mind that there are several products in the market that are purported to be able to be used for both germicidal use in a water filtration system by passing the water through the vessel and harvesting the ozone produced in the air space between the UV lamp and the quartz sleeve. While there is a small percentage of ozone produced in those systems, it is not generally enough to be effective as an ozone source. Logically speaking, if it works to kill organisms in water, it will also have a wavelength high enough to break down the ozone molecule.
This style of machine generally has the oxygen or air source pass by UV lamp where it creates the ozone. Machines are available in very small sizes up as well as commercial size. Although the majority of this style system available in our industry tend to be on the smaller side. The highest concentration of ozone through this style of machine is about 0.2 percent by weight which is the lowest concentration of the three styles that will be featured.
Corona Discharge Systems
These systems tend to be the most prevalent and have been the ones that we have used most until recently. Corona discharge systems operate by passing the air or oxygen through an electrical field. The electrical field is created by using a dielectric that has air space between points of electricity which creates a “corona.” As the gas passes through, the bonds on the O2 molecule are broken and ozone is created.
Corona discharge systems require that the air be very dry prior to entering the generator. Typically, an air prep system is an additional piece that needs to be included in the design. For smaller systems, onboard oxygen generators can be included which allows the generator to be turnkey. If the generator does not include an on-board dryer or oxygen generator then a separate compressor or oxygen generator should be put in-line before the ozone generator.
These systems can run on both pure oxygen or ambient air. Of course, running pure oxygen more than doubles the amount of ozone that can be produced thereby taking up a smaller footprint. Corona discharge systems can produce up to 6 percent by weight.
Plasma systems are another type of corona discharge system. The main difference is that they use thin layers in a block style dielectric rather than a tubular design that is used in the more traditional generators. They are much more efficient and compact than the traditional corona discharge tube style. They also have a smaller footprint which allows the end user to save space. This style can produce ozone over 10 percent by weight.
“So, ozone is the best thing going,” right? It certainly can be but there are some things to consider before incorporating it into the filtration systems.
First and foremost, safety! Ozone gas in the air can very quickly cause a very serious medical emergency. It is really strong and will attack our lungs which can in extreme cases cause death. Now that I have said that, I think it’s also important to say that it is VERY easy to manage the safety concerns. There are well written OSHA safety rules when using ozone in the United States and most likely even stronger rules in the EU. If the application is not located in the United States or European Union, it might be worth researching those rules to incorporate them into your operation. ALWAYS follow your local safety guidelines.
In terms of safety, most systems have the ability to incorporate ambient ozone monitors which can alarm (via visual and audio) and turn off the generator if the concentration in the room exceeds the safety set point. These monitors are critical and require annual calibration.
A consideration for using ozone is where the generator will be located. Ideally, it would be in a dedicated climate-controlled room. This is important for a few reasons. First safety, if the generator starts releasing ozone into the ambient air, a smaller space will be affected. The climate-controlled room reduces humidity and keeps the temperature stable.
It is important to provide clean, cool, dry air to the machine. Ozone is destroyed with heat, so a hot room will reduce the output of the generator and rooms above about 90F will render the generator useless. If the air has high humidity, the machine will begin to create nitric acid in the dielectrics. This reduces the output of the generator and corrodes the internal working pieces.
Ozone can be diffused in several different ways but most commonly it is put into a contact tower. Its dosage is determined based on a concentration over time so please make sure that the method chosen is able to produce the desired effects. This is something that most engineers, with experience, can size for the system.
Ozone can and is a very effective tool for cleaning water. It is heavily used in the food and beverage industry and has a place in aquaculture when applied correctly.