By Harv Scholz, Sr. Project Engineer, Pureflow, Inc.
Even in a high purity water system, there is always a concern about having microbial growth within the system. Should this growth exceed system limits, product manufacturing can quickly come to a halt. One of the best ways to control bacteria in a high purity system is to use ozone as a disinfecting agent.
The most effective way of doing this is to have ozone dissolved in the high purity water while it is in the storage tank, and then remove it just prior to sending the water through the distribution loop. This is a very powerful and fast sanitizing agent, yet it’s easy to produce, and easy to remove at the beginning of a loop. Based on electrochemical oxidation potential (EOP), ozone ranks higher than hydrogen peroxide, sodium hypochlorite and gaseous chlorine. Some sources claim ozone is 2000 or 3000 times as fast as chlorine.
What is ozone?
Ozone is a gas. It is the tri-atomic form of oxygen (O3). It is the most powerful oxidizing agent readily available for water treatment, but along with its oxidizing ability is the fact that it is highly reactive and unstable. When dissolved in water, ozone has a half-life of only 20 minutes. Therefore, it must be regenerated constantly and can only be used on-site. As ozone breaks down, it simply reverts to harmless O2.
How does ozone work as a disinfectant?
It kills by cell lysing, causing the cell wall to rupture and release endotoxins, which are then further oxidized. It attacks all bacteria, virus, cysts and spores in varying degrees. So how much ozone is needed for bacterial control? Tests show that at 0.008 ppm ozone and higher, there will be no growth. However, in most applications, a residual in the tank of 0.1 to 0.2 ppm is recommended to keep the colony-forming units below 1 CFU/100ml.
Ozone Generation-Corona Discharge (Figure 1)
There are several methods for generating ozone. Low concentrations can be obtained from a UV operating at 185 nanometers. More practical for water systems is the production of ozone by corona discharge. In this method, either air or oxygen gas is passed through a small gap between parallel metal plates or between concentric stainless steel tubes. One of the plates or tubes, typically the inner tube, is covered with a dielectric (insulating) material which keeps the high voltage from arcing across the airspace. This results in a constant and controlled electrical discharge, also known as a corona discharge. As the oxygen passes through this electrical field, some of the oxygen molecules split into atoms and recombine to form ozone.
The ozone gas can then be dissolved in the storage tank water by means of a bubble diffuser or a venturi injector. For most pharmaceutical systems, the venturi injector would be preferred due to its high mixing capability. The venturi injector is typically located in a sidestream loop, where the injector can mix large quantities of ozone into the water. This highly ozonated water is returned to the tank, where it ozonates the remaining water. The ozone production can be controlled by varying the power supplied to the corona discharge cell. For optimum performance and to prevent unwanted gases in air from entering a pharmaceutical water system, the ozone generator should be supplied with oxygen from an oxygen generator or some form of oxygen storage. To prevent moisture from short circuiting the narrow corona discharge gap, a dew point of -60°C or lower must be maintained in the feed gas. In most cases where the corona discharge generator does not operate properly, the problem is due to moisture in the feed gas.
Oxygen generation for feed gas to the ozone generator is generally done with a swing adsorption unit, which has 2 tanks filled with a material that adsorbs nitrogen. Air contains 78% nitrogen, and the remaining gas is primarily oxygen (21%). The device is fed with clean, preferably dry and oil-free compressed air. Each tank goes through a cycle whereby it adsorbs nitrogen from air until the media is saturated. During this adsorption time, the oxygen from the air has continued on through the media tank into an oxygen storage tank. To regenerate the media, the tank pressure is released and nitrogen exhausts into the atmosphere. The media tank is now ready for another nitrogen adsorption cycle. In order to make this more of a continuous process, the tanks alternate cycles, so that while one tank is adsorbing nitrogen, the other is releasing it. Thus, the name pressure swing adsorption, or PSA. This equipment is very reliable and maintenance-free, except the valving that controls the cycles is constantly in use and therefore must be replaced or rebuilt every 2 years.
Ozone Generation-Electrolytic (Figure 2)
Another more sophisticated and more expensive technique for producing ozone is by means of an electrolytic cell. In this case, a small sidestream of water, approximately 0.8 gpm, is fed to a cell containing a membrane placed between porous, electrically charged anode and cathode plates. Some of the water is dissociated into hydrogen and oxygen atoms by the electrical attraction of the anode and cathode. The hydrogen, which in this case is a waste gas, is able to permeate through the membrane to vent to atmosphere. A part of the remaining oxygen atoms combine to form ozone molecules, and the balance become oxygen molecules. These molecules remain dissolved in the water, producing highly ozonated water that can then be returned to the storage tank to control bacteria.
The electrolytic ozone generator can be fed by a sidestream across the backpressure device at the end of the loop or by a separate sidestream from loop supply back to the tank. The electrolytic process also eliminates the concern some users have about corona discharge, where small amounts of other gases originally present in the compressed air feed gas can be carried through the ozone generator and into the high purity water.
Each of the ozone generation technologies has its advantages and disadvantages. Ozone production by the electrolytic process is very low, for example, leaving it more vulnerable should there be a malfunction in one of the cells. Another consideration is cost, both in initial installation and ongoing maintenance. The “TECH TIP” below this article shows a comparison between the corona discharge and electrolytic technologies.
As the ozonated water is drawn from the tank and pumped to the distribution loop, the ozone is removed post-pump by means of an ozone destruct UV (Figure 3). This UV must have 254 nm wavelength and be 3-4 times as powerful as a disinfect UV for the same flow rate. After the ozone is destroyed by the UV, the water passes by an ozone sensor that verifies no ozone remains. The high purity water is then distributed to the users via the distribution loop. A high ozone alarm is typically used with this sensor to alert the operator that the UV is not functioning properly.
To control microbial growth in the loop, the loop is periodically sanitized with ozone. This is accomplished by turning off the ozone destruct UV and allowing ozonated water from the tank to circulate through the loop for a pre-set time, usually 30 minutes to an hour. Tank ozone level may also be raised to a higher level (0.3- 0.5ppm) to insure microbial kill during loop sanitization. To verify the effectiveness of sanitization, another ozone sensor is used on loop return. When this sensor indicates a steady reading, meaning no more ozone is being consumed, the timing for sanitization can begin. For maximum effectiveness of sanitization, each user point should be opened briefly during sanitization, allowing ozonated flow to rinse and sanitize those areas of the valve downstream of the valve shutoff.
System Design Elements
A complete ozonated water storage and distribution system would consist of a storage tank, loop supply pump, ozone generator, ozone destruct UV, tank inlet vent filter, tank outlet vent destruct, and 3 dissolved ozone monitors. In addition to the ozone monitors post-UV and on loop return, there must be an ozone monitor sampling the water post-storage tank to determine if the tank has the proper ozone residual.
The tank must have a level sensor, and preferably a pressure relief device such as a rupture disk. A recommended option is a heat exchanger to cool the system to around 22°C. Without this exchanger, the constant pumping will warm the water in the tank, enhancing bacterial growth and making it more difficult to keep ozone dissolved. Ozonated storage tanks should also be fitted with dip tubes rather than spray balls on loop returns, since the spraying action will cause ozone to be removed from solution and off gas into the tank headspace.
Ozone Tank Venting
As with any other storage tank, the ozonated tank must be allowed to breathe in and out as water level changes. On the inlet vent, a 0.2 micron filter cartridge is typically used. On the outlet vent, a high temperature thermal ozone destruct or a catalyst ozone destruct can be used. If there are separate vent paths, a conservation vent or check valves must be used to direct the venting into the proper path. A possible side effect of the check valve design is that a level sensor based on head pressure will see false signals due to a slight pressure buildup prior to valve opening. This can be avoided by using a non-pressure based level sensor, such as radar or ultrasonic, or using a compensating pressure sensor on top of the tank to sense headspace pressure. An alternate vent design is to route both inlet and outlet venting through a filter and vent destruct piped in series. While simplifying piping, this design increases the ozone exposure of the vent filter, requiring that more expensive Teflon materials be used, or that the filter be changed more frequently.
Ozonated System Materials
Ozonated water storage tanks, pumps and piping must be an ozone- compatible material, typically 316L stainless steel and preferably electro-polished. Electro-polished (EP) stainless steel will actually resist rouging (a light rusting of the surface) longer than a standard mechanically-polished surface. Electro-polishing is a process where a combination of chemicals and an electrical current applied to the surface removes a minute amount of metal from the surface, creating a smoother finish than that achievable with mechanical polish alone. EP evens out the microscopic peaks and valleys left by mechanical polish, reducing the corrosive electrical potentials on the surface and eliminating some of the surface area available for corrosion. To avoid the rouging problem altogether, some parts of the system, such as valves and piping, can be made from ozone-resistant plastic materials, typically PVDF or Teflon. A tank lined with one of these materials would provide long term ozone resistance without the periodic passivation needed to maintain a rouge-free stainless steel system. Fiberglass cannot be used for ozonated tanks unless it is lined with an ozone resistant material.
Gasket and O-ring elastomers are especially susceptible to ozone degradation. Viton and silicone are frequently used, but should be inspected based on the degree of exposure. Gaseous ozone, as would be present in ozone injection lines or in the tank headspace, is more aggressive than dissolved ozone, and would prompt more frequent inspection. Teflon-covered O-rings and gaskets are preferred since they will resist ozone degradation indefinitely. Also Teflon- based rubber compounds such as Kalrez have good ozone resistance but are expensive.
Ozone System Safety
Another area that requires special attention in an ozone system is the detection of leaks in the ozone piping. These leaks are more likely to be a problem on a gaseous type of generator (corona discharge or UV), but can occur even on the electrolytic system, where there may be leaks from ozone in the tank headspace. Significant amounts of ozone gas in the room would be a hazard to employees working in the area.
Ozone has a characteristic pungent odor, and its presence at low levels can be smelled long before an ozone monitor would alert an operator of ozone in the air. Ozone will affect the upper respiratory tract, the lungs and the throat and cause eye irritation and headaches. The threshold for smell is 0.02 to 0.04 ppm.
To alert the employees of this danger, an ambient ozone monitor should be installed. The monitor senses ozone in the room air and alarms at high ozone levels. It can also be configured to turn the ozone generator off when ozone reaches an unacceptably high level. OSHA exposure limits for ozone are 0.1 ppm for a maximum exposure limit during an 8-hour period. The short term exposure limit is 0.3 ppm, 15 minutes at a time, no more than 4 exposures a day with a 1 hour break between exposures.
Ozone is a powerful and effective sanitizing agent for high purity water systems. Potential problems with ozone attack on materials or with personnel exposure to ozone gas can be controlled with proper system design. Performing regular preventive maintenance on the ozone generating and sensing equipment is key to having a trouble-free ozonated water system. The primary advantage of ozone is that it can be generated or destroyed at will, making loop sanitization easy and return to service timely.
Harv Scholz, PE, is Senior Engineer at Pureflowinc where he manages the Engineering Division. A graduate of United States Naval Academy, Harv has over 35 years experience in engineering, the last 8 of which have been with Pureflow. Harv’s expertise in high purity water and loop sanitization has played a significant role in the growth of the Company, founded in 1985.
Article is re-printed with permission of Harv Scholz. Unauthorized reproduction of this article and/or use in any form is strictly prohibited without the expressed written consent of Pureflowinc.