By Ben Couch
Ozone was discovered in 1783 by Dutch scientist Martinus Van Marum from the odor produced by an electrostatic machine. It was initially thought to be the ‘odor of electricity.’
Its unique chemistry and reactivity made it a relatively elusive molecule to identify. A chemical curiosity in the eyes of the science world, ozone’s reactive nature revealed valuable uses from the outset.
Christian Friedrich Schönbein brought scientific knowledge further with his experiments; by the time of his death in 1868, ozone was an established factor in chemistry. It was utilized for drinking water treatment in Nice, France beginning in 1906.
Today, ozone is used around the world in many industries—everything from agriculture to commercial laundries to semiconductor manufacturing. It is the most rapidly growing and safest oxidation and disinfection treatment available. Each industry uses ozone in a unique way and the specific factors of each application affect the nature of how they design and control their ozone application.
Using ozone can be like operating a hybrid car. Processes need to be controlled very specifically to achieve the delicate balance necessary for the vehicle to function properly. That all happens smoothly thanks to the car’s control computer and interfaces. In fact, these automobiles are surprisingly simple to operate because they have been engineered so well. Much the same is true for today’s ozone application.
In drinking water treatment, ozone was found to be an effective alternative to chlorine, but the two chemicals are quite different. One encyclopedia states: “Ozone is a relatively unstable molecule of oxygen which readily gives up one atom of oxygen providing a powerful oxidizing agent which is toxic to most waterborne organisms. It is a very strong, broad-spectrum disinfectant that is widely used in Europe. It is an effective method to inactivate harmful protozoans that form cysts. It also works well against almost all other pathogens. Ozone is made by passing oxygen through ultraviolet light or an electrical discharge. The U.S. Food and Drug Administration has accepted ozone as being safe; and it is applied as an anti-microbiological agent for the treatment, storage and processing of foods.”
Because of ozone’s aggressive and complete oxidation and disinfection power, employing even some simple controls will allow you to manage your company’s or your customer’s application confidently. For everything from a rural municipality to a water treatment solutions dealer to a large-scale industrial wastewater treatment facility, controlling your ozone use will give you more bang for your buck.
Ozone controls—the tools of the trade
In order to understand what controls can do for ozone use, it’s useful to be familiar with the equipment beyond ambient and dissolved ozone monitors and the standard pressure, temperature and flow meters.
A Proportional Integral Derivative (PID) is a relatively simple processor that adheres to user-set parameters for an algorithm. It responds to an input signal from a dissolved ozone monitor. The PID feeds the ozone generator a proportional signal based on current power and ozone outputs to either ramp up or down until the predetermined set point is reached.
A Programmable Logic Controller (PLC) is a software-based tool that both runs multiple pre-programmed algorithms and employs programmable variables towards a desired output.
A Supervisory Control And Data Acquisition (SCADA) system reads and coordinates the signals from the monitors, analyzers and sub-controllers. It makes interfacing with the system and points of control more user-friendly.
One challenge of ozone control equipment is the capital investment. When setting up a control system for accurate reading and feedback, the sensors and instruments required can be prohibitively expensive. But as many companies find out, not having accurate ozone controls will cost you even more.
Without proper controls, dosing is guesswork, nothing more than costly trial and error. For years in the fish hatchery industry, testing and design of new recirculation systems was severely limited by the availability of accurate dosing and efficiency data. Too little ozone and pathogens survive; too much ozone and the fish suffer. When residual ozone analyzers and programmed control system functions were set in place, ozone as a sterilant for nursery tank systems became feasible from both economic and design standpoints.1
There are several areas of ozone generation and application that need to be considered when making a decision of what and how to monitor and control in one’s system.
One area of ozone controls, especially as ozone use expands in more labor-intensive industries like agriculture, food processing and beverage bottling, is the safe-to-start control. In areas where ozone is being applied for surface disinfection, or indoors, as in a food processing plant or wine cellar, the chance of off-gas buildup increases. For example, ozone is used to disinfect the insides of wine barrels and the surfaces and tanks in wineries because it eliminates pathogens and then decomposes rapidly, leaving no chemical trace to affect the bouquet, color or taste of the wine.
The scent of ozone can be detected in the air at very low, innocuous concentrations. However, safety steps need to be taken, because at higher concentrations and exposure time, ozone can be irritating to breathe.
The OSHA standard for environmental ozone is 0.1 ppm (0.2 mg/m3)—time weighted for an eight-hour period. If an ambient ozone monitor detects a level close to that point before the generator is to start, it would relay that signal to the control computer for an alarm or prevent the system from engaging, even opening a vent to an ozone decomposer or ventilation system.
Another case where an ambient ozone monitor can save you a world of trouble is in the case of an ozone off-gas leak due to oversizing—trying to apply more ozone than is needed or can be transferred into the target water.
At the predetermined set point (OSHA’s 15-minute exposure limit for ozone is 0.3 ppm), the ambient ozone monitor would send a signal to the controller, which can in turn shut off the generator’s power supply.
Ambient ozone monitors will alert personnel to excessive off-gas. This condition may indicate the need to adjust the level of ozone application or risk compromising your ozone destruct. It also indicates wasted ozone and the energy used to produce it.
When running any ozone application system, but especially for large or multi-stage applications like municipal water treatment or food processing, power efficiency is a key focus. Ozone’s reactivity provides opportunities for creative process engineering. Several different operating conditions and applied ozone doses can be employed with a given effluent quality to achieve the same absorbed ozone concentration.2
Flexibility of operating conditions means that a system can be optimized for proper ozone dosing, minimum power consumption or any setup that works best for the specific individual application.
For example, in bottled water operations, ozone is applied at multiple locations at varying concentrations: source water, bottle rinsing and final bottle filling for residual. Economic, effective dosing via dissolved ozone monitors and PID control will allow for sufficient disinfection without wasted energy.
A well-programmed and flexible control system is valuable when dealing with variable ozone demand. Many municipal water systems draw either directly or indirectly from lakes and surface water, which can be heavily impacted by summer algae blooms and other seasonal factors.
The ozone output, ozone concentration and carrier gas flow rate can be changed to maintain the proper conditions for both optimized ozone production and ozone transfer. This requires mapping or monitoring both the ozone generator power requirements and ozone contactor transfer capabilities to combine this information into the most economical operating condition.2
It’s also important to understand how controls apply to the key processes of the ozone generation sequence: feed gas, generation and contacting.
Whether the feed gas is liquid oxygen vapor, compressed or dried air or concentrated oxygen, the condition and flow of the fuel greatly impact the nature of the ozone production.
For corona discharge ozone generators, the dry air or concentrated oxygen must be within the specific standards as stated by the ozone generator manufacturer and fed into the reactor cell at a specific flow rate and pressure for consistent and economic ozone production. The reactor cell pressure must also be carefully maintained. About three to nine psig is an optimal operating pressure range for most corona discharge generators. This pressure affects not only the performance of the gas in the generator, but also the stress on the reactor cell and the efficiency of ozone gas mass transfer.
Just as important as the monitoring of the feed gas and generation of ozone is its delivery to and contact with its disinfection target. Whether it is applied directly as gas or transferred into suspension in a stream of water that is the disinfection vehicle, a tool as powerful as ozone needs efficient application.
One of the primary methods for monitoring real-time residual ozone in a process is a dissolved ozone monitor. By measuring dissolved ozone after contacting and then again after application and oxidation, the amount of ozone consumed (and therefore the amount of particulate or organic elements) can be determined.
Future of controls
Control informs and maximizes every component of the ozone generation process. Increasingly powerful, integrated and cost-effective controls will lead to more capital investment to the extent that manufacturers and end users can track and evaluate the processes and results of their ozonation efforts.
This convergence propels development of improved control systems, as more industries appreciate the value and opportunity of ozone. As more firms implement fully integrated systems and make use of powerful logic software and control technologies, ozone becomes more efficient and more manageable, leading to more value for manufacturers, dealers and end users of water treatment systems.
- Monroe, “The Feasibility of Ozone For Purification of Hatchery Waters,” Ozone: Science and Engineering, Vol. 2 pp. 203-224, 1980.
- Stover, “Optimizing Operational Control of Ozone Disinfection,” Ozone: Science and Engineering, Vol. 4 pp. 131-145, 1982. http://en.wikipedia.org/wiki/Water_purification
About the author and company
Ben Couch is the Marketing Administrator at Pacific Ozone Technology, Inc., a world leader in air-cooled corona discharge ozone generators. Pacific Ozone Technology offers a complete line of ozone generators, integrated ozone contact systems, ozone destructs and controls. For questions or feedback, contact Couch by telephone at (707) 747-9600 ext. 29 or email him at firstname.lastname@example.org. Visit the company’s website, www.pacificozone.com.