Sensing and Monitoring for Small Water Treatment Systems
By Lawrence B. Kilham
Ozone is rapidly becoming the water treatment method of choice for bottled water, swimming pools, aquariums, municipal water treatment plants and many other applications. In comparison to chlorine, it creates less, if any, toxic by-products, is generated on site and the finished water is generally more attractive.
However, ozone itself is toxic. Safe workplace levels are specified by various government regulations.1 In most countries, levels at or below 0.05 ppm for continuous exposure of children, the sick and the elderly; 0.08 ppm/eight hours is an emerging threshold for general outdoor exposure; and 0.1 ppm is the generally recognized limit for continuous exposure in the workplace. Ozone’s principal health effect is that it damages lung and other delicate tissues.
On the other hand, ozone must be maintained above certain concentrations for given time periods in order to destroy pathogens in water, for example. There are limitless combinations of ozone concentrations and application times depending on the pathogens to be destroyed, minerals to be oxidized, color to be removed, etc. The applications formulae appropriate for equipment installation should already be programmed into the equipment or given by professionals for that application.
How is ozone detected at hazardous levels? Many who work with ozone say that the ‘sniff test’ is sufficient due to ozone’s distinct odor, even at low concentrations. That is not a responsible commercial safety practice, however. People who work around ozone often lose their sensitivity to its odor, at least at low levels. The sniff test is not an accurate means of locating leak sites in a maze of plumbing or of determining whether the leak is increasing or subsiding. If there has been major area ozone flooding, the sniff test doesn’t indicate when it is safe to return to the work area.
It is necessary to have an ozone sensor or monitor. These range from inexpensive single-use exposure badges, to costly portable electronic instruments, to very costly precision analyzers. A battery-operated hand-held sensor is indispensable during start-ups to detect and eliminate leaks and to make sure that the ozone is working its way throughout the part of the system where it should be. Once the system is running routinely, monitoring instruments ensure that everything is running according to design and that the local area is safe.
Dissolved ozone in water treatment systems
Ozone is generally dissolved in water to inactivate bacteria, viruses and other microorganisms, to oxidize and thus create removable solids of dissolved minerals, neutralize undesirable chemicals and remove color. It is also used for other purposes such as to allow lower temperature wash water and to minimize residuals in wash processes ranging from laundry to semiconductors. For any application, ozone should be efficiently dissolved into the water and the dissolved ozone should be measured at various points in the process.
Ozone is generated in air and is forced into water by a Venturi effect injector, a high porosity contact block or is generated directly in the water by electrolysis. The transfer efficiency, or mass transfer, refers to the amount of ozone immediately dissolved in the water compared to the amount of ozone injected into the water. The dissolved ozone concentration will vary according to the ozone dosage, transfer efficiency, the contact time of the ozone with the water and the ozone demand (less demand generally means higher water quality). A major variable affecting transfer efficiency is the water temperature (colder water will dissolve much higher concentrations of ozone than warmer water). Another variable that should not be overlooked is pH. Ozone dissolves most efficiently in the vicinity of a pH of seven, but to avoid corrosion of metal parts (such as piping and pumps in contact with the ozonated water) pH should be above seven.
The true dissolved ozone content of water will increase when the entrained ozone is forced into solution. This is often done in a vessel called a contact tank. Often there will be a higher concentration of ozone in the tank than in the piping before it. This is because the ozone in the pipe feeding the tank is not completely dissolved. The non-dissolved ozone is entrained as a mass of tiny bubbles. The contact tank, or contact chamber (or series of chambers), is (are) installed to more completely dissolve the ozone in the water (more ozone-water contact time) and to facilitate maximum contact with whatever needs to be destroyed or inactivated in the water by the ozone, such as bacteria or minerals.
In the flow channel after the contact tank or general contact area, the ozone concentration in the water declines due to consumption by such ozone-demand factors as microbes, minerals and organics. Ozone also declines exponentially over time due to its natural decomposition. Overall, the concentration decline can be quantified by the half-life (time for the concentration to fall by half). Half-life depends on ozone demand, temperature, pH and other factors.
Assuming a neutral pH (of about seven) and room temperature, typical ozone half-lives in water are shown in Figure 1.
Ozone measurement points
It is important to be sure what measuring point in the ozonated system someone is referring to, because these are often not specified. The ozone concentrations vary greatly from point to point. Ozone is often measured at one of the following three points in an optimized system:
- At the injection point (ozone dosage). Measurement can be made in the water just after the injection point. It is sometimes preferred to measure the ozone in air just before the injection point. In either case, care must be taken due to high pressures or vacuums, high ozone levels and high gas or liquid flow rates and volumes. Usually this is not an area for hand-held ozone instruments and in many cases ozone concentrations here should only be measured by specially designed probes.
- At the contact tank. In the area called the contact tank or contact chamber, where the ozone is designed to have prolonged contact with the water and its constituents, the dissolved ozone usually has its highest concentration and the flow rates and pressures are often lower than at the ozone injection point. On-line instruments or portable instruments using samples from taps can generally be used here. If the top of the contact has a gas pressure relief port, that is a good point to do a quick qualitative check that ozone is flowing through the system. A simple hand-held ozone sensor can be used.
- At the exit of the system (but before any dissolved ozone destruct unit). This is the point where the residual dissolved ozone is measured. This is where samples can be taken from bottled water bottles, swimming pools or spas, vegetable washing areas, cooling towers, etc. This is generally an ideal measurement point for hand-held sampling instruments, but on-line instruments can also be used here. Ozone concentrations are usually much lower than upstream and so are water pressures.
- Everything upstream is working correctly, including the ozone generator.
- There is a high ozone transfer efficiency.
- Any targets such as bacteria to be removed by the ozone would most likely have been removed because ozone was present throughout the system.
- There is not excessive ozone in the system outflow area where people or animals in the area would be harmed by the ozone.
ORP or redox
A two-electrode ORP (oxidation-reduction potential) or redox meter measures the oxidizing capacity in the water. It is not selective between different oxidizing agents such as chlorine or ozone, but is effective for destroying bacteria (and other contaminants) where distinction isn’t always important. ORP meters are inexpensive, but require on-site, individual, specific calibration based on local water chemistries and the probes should be maintained frequently. ORP meters don’t need temperature compensation.
The ozone migrates through a permeable membrane and then is converted to oxygen. The relative quantity of oxygen is measured by an electrochemical cell. Electrochemical instruments are usually configured for on-line use. The probes are expensive, easily damaged and require regular maintenance, but these instruments are much more ozone-selective than ORP meters and are widely used in larger systems.
Wet chemistries and test kits
Wet chemistries in use are mainly titrations of iodine solutions and indigo trisulfonate (ITS) test kits. With the test kits, the blue indigo is bleached with ozone and the color difference is usually measured by a digital colorimeter. The indigo reagent is purchased in one-time-use glass ampules. These test kits are very specific for ozone and are relatively inexpensive if not too many ampules are regularly consumed. However, they are fairly intricate to use and the reagents lose their calibration if they have been stored for a long time.
Ultraviolet (UV) absorption
The ozone is removed or ‘stripped’ from the water and the ozone is measured in air by a UV absorption analyzer. This method is very specific to ozone, is capable of high accuracy and many concentration ranges can be accommodated. High concentrations can also be measured by UV absorption directly in the water. The equipment cost tends to be out of range for small systems, however.
Headspace heated metal oxide sensors
A lower cost, simpler version of the UV system described above, is stripping the ozone from the water and measuring it in a ‘headspace’ using a low-cost, heated metal oxide semiconductor (HMOS) sensor. This works well in the 0.05-2.0 ppm range.
Ozone monitoring can be simple and inexpensive, but before purchasing equipment it is important to analyze your ozone system and needs in order to specify the appropriate kind of sensor or monitor. Plan ahead for the most effective operation.
- Ozone for Point-of-use, Point-of-Entry and Small Water System Water Treatment Applications, Water Quality Association, Lisle, Ill., USA, www.wqa.org (Hardcover book, 86 pp).
About the author
Since 1965, Larry Kilham has held positions in sales, engineering and management at high-tech companies. Currently, he is the owner of Eco Sensors, Inc. of Santa Fe, N.M. and Director of several start-up companies. Kilham earned a masters degree from the Sloan School of Management at MIT; holds three patents in instrumentation, received the IR 100 Award for Technical Innovation and has published numerous technical articles.
About the company
Eco Sensors, Inc. is a premier designer and worldwide supplier of low-cost ozone sensing instruments. These are used to monitor safety in the workplace, control ozone generators, monitor and control processes such as food production and storage, deodorization, sterilization and measuring ozone dissolved in water. Eco Sensor instruments are simple, user-friendly and inexpensive. Simple volatile organic compounds (VOC) detection instruments are also manufactured. The majority of Eco Sensor’s sales are through distributors and OEMs, totaling approximately 30,000 users across North America, Europe and the Pacific Rim. Products are sold by many specialty distributors and by ozone generator manufacturers and systems integrators; a full half of all sales are exported. Eco Sensors is a registered trademark of Eco Sensors, Inc. EcoZone and Ozone Switch are Eco Sensors Trademarks.