The Six Most Common Reasons An Ozone Water Treatment Systems Fail

The Six Most Common Reasons Why Ozone Water Treatment Systems FailIntroductionOzone water treatment systems are used for a variety of applications. Nearly 1.6 billion gallons of municipal drinking water is treated with ozone. Almost all bottled water has ozone added prior to the bottling step. A number of fruit and vegetable washing operations, especially for ready to eat foods, use ozone to keep the food safe from bacteria and other pathogens. It is important that the ozone systems in these applications work reliably.Knowing what types of things can go wrong can help municipalities and industry make smarter decisions regarding the type of ozone water treatment system to buy and why various features are important. In this article we are referring to municipal/industrial scale ozone generators. We will cover the six most common reasons why ozone systems fail, techniques for preventing these problems and proper instrumentation that can provide an early warning of potential problems.Ozone Water Treatment SystemsOzone water treatment systems take oxygen or dry air and convert the oxygen present into ozone. This ozone is then mixed with water for the specific water treatment application, e.g. disifection. There are a number of factors that can cause these systems to fail. This article tries to group them into six categories.The typical failure mechanisms include:Back Flow of Water into the Generator
Poor Feed Gas Quality
Under Sizing the System
Poor Ozone Transfer Efficiency
Ozone Generator Cooling
Incorrect Materials of ConstructionBack Flow of Water into the Ozone GeneratorCommercial scale ozone generator cannot tolerate water entering the generator without having severe damage. The potential for back flow exists since the gas must flow from the generator into the water, so there is a pathway for water to back flow into the generator. This is compounded because ozone can be injected, via a venturi, into water that is at a higher pressure than that inside of the ozone generator. If there is a problem with the operation of the venturi or some change in the downstream hydraulics, water can be forced into the generator.It is common to see check valves used to prevent the back flow of water, but check valves are not a reliable device for this purposes, especially given the severe damage that can result from back flow. Check valves in this application have a high probability of failure. Most high quality ozone water treatment systems use a multi barrier approach to back flow prevention employing several passive and active devices to detect and counter the back flow of water.An example might be check valves, liquid traps and differential pressure monitoring interlocked with a normally closed solenoid valve. Monitoring differential pressure is based on the fact that in normal operation the pressure in the generator must exceed the pressure at the point of injection, otherwise the gas would not flow into the water. Triggering a solenoid valve to close eliminates the pathway for the water to reach the generator. Using a normally closed valve means that even with a loss of power, the valve is closed and the generator protected.Another technique that is used is to place instruments in the gas line or liquid traps that can detect liquid water. These devices can be used to trigger the shutdown of the solenoid valve and the generator.Without such protection it is usually a matter of time until a set of circumstances arises that causes the back flow to occur and damage the ozone generator.Poor Feed Gas QualityOzone generators require a source of oxygen to make ozone. Ozone is simply three oxygen atoms connected together (O3). The air that we breathe contains about 21% oxygen, and is built from two oxygen atoms (O2). You can buy pure oxygen from industrial and medical gas suppliers. It is also possible to make oxygen using a so called oxygen concentrator. Small versions of these devices are used for home medical purposes. They also produce a gas that is extremely dry which is 90-95% oxygen.Different ozone generators have different optimal gas feeds. Some generators work best with pure oxygen, other require some nitrogen being present (1-4%). Other generators work from dry air. In all cases the gas feed must be extremely dry. This is normally measured as the dew point of the air, the temperature at which water in the air will condense. For ozone generators this value is typically around -100 degree F. This means that there are only a few parts per million of moisture in the gas stream.Moisture in the feed gas can result in the formation of nitric acid in the generator creating the conditions for severe corrosion. In addition, moisture also reduces the efficiency of the ozone formation reaction, reducing output.To insure that the feed gas is the proper quality oxygen monitors that report the concentration of oxygen in the gas can be used. Dew point monitors, hygrometers, are available that can measure the amount of moisture in a gas stream. These devices are often used in larger ozone generating systems.Finally, filtration is important to prevent particles, oil droplets and vapors of hydrocarbon from entering the ozone generator.Under Sizing the Ozone SystemEven a well design ozone system will not do any good if it is undersized. In some applications it is easy to predict the proper size of the ozone water treatment system and in other cases it is important that laboratory and pilot field studies be conducted. This is especially true for applications like treating surface water for municipal drinking water or industrial wastewater treatment.The complexities of these applications, including seasonal variations, require extensive testing prior to the final design of the system. Once a system has been built it is usually very difficult to increase its capacity due to the cost and space limitations that exist after the initial installation, if room for expansion was not planned for. For larger projects, competent engineering firms are unlikely to make this type of mistake, but for smaller projects it is possible that poor assumptions can lead to an undersized system.Poor Ozone Transfer EfficiencyIn water treatment applications the ozone water treatment system must make the proper amount of ozone and dissolve it into water. The ozone transfer efficiency is the percent of ozone that dissolves out of the total amount that was generated. Only ozone that dissolves into the water will be able to carry out reactions like disinfection, oxidation of organic molecules or enhancing filtration.Ozone has a limited solubility in water. It is more soluble than oxygen, but less soluble than chlorine. The solubility of ozone is affected by the following parameters:The ratio of gas volume to liquid volume (G/L ratio): lower ratio increases efficiency
Bubble size: smaller bubbles increase efficiency
Ozone demand of the water: higher demand increases efficiency
Ozone concentration: higher concentration increases efficiency
Pressure: higher pressure increases efficiency, specifically the venturi outlet pressure
Detention time: longer detention time increases efficiency
Temperature: lower temperature increases efficiencyA well designed ozone water treatment system will transfer >90% of the generated ozone into solution. While ozone can be generated from dry air, the concentration of ozone produced 1-3% is much lower than systems that use oxygen feed from either liquid oxygen or oxygen concnetrators. These systems can generate ozone concentrations of 6-10%. The most common feed gas systems for water treatment are oxygen based.Fine bubble diffusers or venturi are typically used to transfer the gas into the liquid. In either method of mixing the proper G/L ratio, temperature, pressure, and detention time to satisfy a given ozone demand need to be considered. If these considerations are not taken into account properly, even with the right amount of ozone being produced, the application may not be successful.Ozone Generator CoolingOzone generator output is directly proportional to the temperature of the cooling water, for water cooled systems, and air temperature, for air cooled systems. Normally ozone generator manufacturers provide production curves for their machines as a function of gas flow and power setting. These values are always based on a specific inlet water temperature or air temperature for their cooling systems.Typically, the output of the machine will decrease by 0.5-1.0% per degree higher than the value shown on the production curves or data table. So, if the cooling water entered at 20 degrees C instead of the specified 15 degrees C, the output of the generator might be decreased by as much as 5%.In cooling water systems that rely on chillers, the temperature of the water can normally be expected to be controlled, but if the water to be treated is used as the cooling medium, than the water temperature can vary. In drinking water systems, the treated water may be used to cool the generators. This water tends to be higher in the temperature in the summer than the winter, thus affecting output. Engineers will often oversize a system to account for the expected loss as water temperature increases. Where this is not done, the output may not meet the application demands.This problem is more acute with air cooled systems since ambient conditions can vary significantly. If the ozone system is not in an air conditioned space, it is likely that generator performance will drop off when temperatures rise. So, adding an air conditioning system to the ozone generator enclosure is often a good idea where significant temperature variations are expected.Materials of ConstructionOzone is a strong oxidizer and can attack many materials. This is especially true for rubber and Buna-N elastomers. Failure of materials in an ozone water treatment system can result in leaks of water or ozonated gas. It can also result in the failure of key components or instruments.In an ozone water treatment application, materials can be exposed to high concentration ozone 6-10 % (60,000 to 100,000 ppm) in the gas phase or low concentrations in the liquid phase, a few ppm. These applications are significantly different. Just because a material can work with high concentration gas phase ozone does not mean it will work in the liquid phase and vice a versa.Even materials that are supposed to be suitable for a specific ozone application can fail. Viton is generally considered acceptable for ozone use in both the liquid and gas phase. Viton, however, comes in different grades, can be compounded with different materials and in the case of diaphragms might have fabrics embedded in it. Some of these variants can fail in certain ozone environments.So, the question of materials of construction is a complicated one. Experience with specific materials for specific applications is required. Material compatibility charts should be considered a starting point. The manufacturers of the materials are the best place to consult about the use of the materials, grades available and certifications offered.

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