When designing an ozone water treatment system, as with any engineering project, there are tradeoffs between cost and performance. One aspect of design for such systems involves the controls. Depending on the application, it is entirely possible to design a system with fully manual controls that will function well. Such systems require operator attention should one of the systems components fail. It is also to design and build a full automated ozone system that can run unattended and alert n operator should the need arise.
The cost of automation is usually the major reason why client opt for more manual controls, especially for smaller systems where the costs of controls can exceed the cost of the ozone generator. On the other hand, if the application where ozone is being applied is critical, the use of controls to maintain proper operation, regardless of operator attention is probably justified.
The basic components of an ozone water treatment system are gas preparation, ozone generator and ozone water mixing. Gas preparation involves the following options: compressing/drying air, concentrating oxygen from compressed air, or feeding purchased oxygen. Gas feed is critical for proper ozone generator operation in terms of both product quality and ozone generator protection. Moisture will not only reduce generator efficiency, but also damage the generating cell.
For new systems, pressure swing absorption (PSA) is used for both air drying and oxygen concentration. The process is highly reliable since it uses a solid state process with limited moving parts. Dew point monitors can directly measure the air drying process and oxygen monitors can follow the oxygen concentration process. These instruments are reliable and are routinely used in large installation on a regular basis. The instruments are moderately expensive and require periodic calibration.
An alternative is to use a pressure switch. A critical parameter in the operation of PSA systems is the feed pressure. If this drops below a certain level, the process will no longer work properly. This tends to be a factor is performance problems associated with such systems. So, a pressure switch is an inexpensive method to monitor the performance of PSA systems. While by no means fool proof, it can catch major faults allowing for eh shut down of the system before damage is done and alerting operators to a problem.
Another technique that is also relatively inexpensive is to use a silica gel that changes color with changes in moisture content of the gas. Both air dryers and oxygen concentrators should produce a gas with a dew point of less than -100 degrees F. The silica materials change color from a blue to red with the presence of moisture. While not an automated method of control, it can alert and operator to a problem.
If the quality of the feed gas is good, the operation of the ozone generator is usually good as well. Ozone generators with good feed gas will last for a long time between servicing. While near terms failures are not likely, they are possible. In larger installation, monitors follow the concentration of ozone coming out of the generator. These monitors use UV light to measure ozone. They are very accurate, but expensive and require periodic calibration by skilled technicians. In addition to measuring the concentration of ozone in the gas, these systems also follow the flow of the gas. They can then calculate the production of ozone in g/h or lbs/day and compare the values to the original specifications for the generators.
The objective of these controls is to insure that the dose of ozone in the water meets the application requirements. While the gas phase values can predict the amount of ozone in the water, the concentration of ozone in the water is also monitored. This can be done using a dissolved ozone monitor employing either UV or an electrochemical method. In both cases highly accurate measurements of ozone dissolved in water can be made. From a control point of view, one could do without the gas phase measurements since the critical parameter is the dissolved ozone concentration. While the gas phase measurements would be highly useful tools for assessing why the concentration of ozone in the water is low or detecting the problem before it occurs, it is not a requirement and most small systems do not use gas phase monitors.
The electrochemical dissolved ozone monitors are easily calibrated on site and are relatively inexpensive. They often come with built in control outputs that allow the monitor to control the power settings on the ozone generator thus controlling the dissolved ozone levels in the water without direct operator intervention.
Other considerations in the design of an ozone water treatment system are the handling of various systems failures. We already mentioned the potential for failure of gas preparation when compressed air is the feed source using pressure switches. Another potential failure is the back flow of water from the liquid side of the system to the ozone generator. Such an occurrence is catastrophic and would result in the destruction of the generator. As a result, protection must be put into place with back-up.
For small inexpensive systems, mechanical methods alone are popular and workable. These include liquid traps and check valves. Another level of protection involves liquid traps with sensors that detect the back flow and can provide a signal to a control panel to close valves, shut down generators and set off alarms. In systems that use venturi, back flow would also change the pressure in the line from negative to positive. This could be sensed by a pressure switch.
Leaks of ozone into closed spaces of a building can create potential hazards for people. Although ozone is easily detected at safe levels by our sense of smell, our olfactory nerves are easily fatigued by ozone. If the person exposed to ozone ignores the problem, they could unknowingly be expose themselves to dangerous levels. While there are no records of anyone dying from such exposure, it is not wise to rely on our sense of smell. The use of ambient monitors is recommended for these situations. Most monitors come with relays that can be used to shut down the ozone generator.
Ozone water treatment systems are designed to make ozone and mix it with water. So the primary thing to measure and control is the amount of ozone in the water. The next most important factor is to prevent water from backing up into the generator since this will result in the catastrophic failure of the generator. Finally, it is important to make sure the gas feed meets specifications in terms of oxygen quality or air dryness.
These objectives can be accomplished with simple manual methods or with sophisticated instruments and PLC. The decision of which to use has to do with overall system cost and how critical the application is. Spartan Environmental Technologies can supply ozone systems with a wide range of controls that can meet the specifications and budgets for a variety of applications and clients.