By Roger Nathanson
Summary: Treatment of water comes in various forms. One such method gaining wider acceptance is ozone. In particular, those in the bottled water industry are finding it very useful. The argument for ozone in regards to this application follows.
There are a number of reasons why ozonating bottled water remains a good idea. Ozone extends shelf life, improves taste, kills bacteria and maintains a bacteria free environment. Ozone is required by public health agencies in many locations around the world. While not required in the United States, more utilities and bottled water operations are taking advantage of the added disinfection and oxidation capabilities it adds to their treatment regimen. As a result, more bottling operations around the country are being inspected to show they meet the highest standards. The International Bottled Water Standards are the criteria for inspection. The industry standards dictate the water quality and the disinfectant residual. The standards require ozone (O3) as the final treatment and there’s to be an ozone residual in the bottled water. Residual, in this instance, will be defined as a “remainder of ozone.” Still, ozone doesn’t produce a residual beyond a few minutes. Typical field applications last five minutes or less. Large commercial systems usually take approximately 20 minutes.
You’ll notice most if not all bottled water in the grocery store will say “ozonated” on the label. Ozone is a better choice as opposed to chlorine, because it virtually leaves nothing behind other than improved taste.
How much ozone is needed?
The amount of ozone in the water is referred to as the ozone residual. When ozone degrades, it reverts back to oxygen (O2). Many factors determine the residual but, generally, the higher the ozone production the longer/higher the residual.
The minimum ozone residual in the bottles is 0.2 parts per million (ppm) to a maximum of 0.5 ppm. The required residual is dictated by the amount and type of microbes to be killed. Lower residuals can be used to ensure water that’s relatively microbe-free. However, 50 colony forming units (cfu) of E. coli would require a 0.3-to-0.5 ppm ozone residual for a 3-to-5 minute contact time.
The main objective when using ozone for bottled water is to maintain ozone concentration in the water as it’s bottled.
Sizing an ozone system
You must be able to answer the following questions to size the ozone system. Then, your ozone supplier can design the ozone system for you.
- What’s the maximum flow rate—in gallons per minute (gpm)—of the bottling water?
- Is there bacteria present or is the ozone simply a preventative measure?
- Is there anything in the water that ozone can oxidize such as iron, sulfur, manganese, copper, organics, etc.?
- How many hours per day is the water used for bottling?
Ozonator Sizing: No. 1 and 2 above will determine the ozonator size. Ozone production levels are represented in grams per hour (gr/hr) for small-scale systems or pounds per day (lbs/d) for larger systems (see Figure 2 for an example).
Injector sizing: The ozone injector is used to get the ozone into the water. The two methods are: 1) venturi, and 2) ozone pump.
Venturis sometimes require a booster pump to increase the inlet pressure and flow to create adequate suction of the ozone. They are lower cost items and require little or no maintenance. It’s critical to size the venturi correctly according to the gpm and pounds per square inch (psi) of pressure. Incorrect sizing will result in insufficient ozone injection.
Ozone pumps do not restrict the flow or pressure as do venturis. Ozone pumps can be located anywhere in the plumbing without the need for booster pumps. They inject more ozone gas by volume than venturis. And they deliver high concentrations of micro-bubbles, providing the water pressure is 30 psi or more. The only drawback is that ozone pumps need to be serviced periodically. The benefit is the ease of installation and sizing. (The mass transfer of ozone is covered below in the discussion of Henry’s Law.)
Air dryer: These are critical pieces of equipment that ensure no degradation of ozone equipment. Air dryers remove all of the humidity and moisture from the air prior to the ozonator. Moisture/humidity, when combined with ozone gas can create nitric acid, which is a corrosive liquid. Also, moisture/humidity inhibits electrical conduction. Notice how the increase of static electricity occurs in winter and in dryer climates as opposed to more humid conditions.
Typically, the air is dried to a minimum of –40ºF dew point (very, very dry). Also, dry air greatly increases ozone production (2-to-3 times) compared to atmospheric conditions. Most ozone manufacturers rate their ozone production using dryer air, not atmospheric/moist air.
Contact/off gas tank: Pressure is what keeps the ozone in solution, thus extending the contact time. Once bottled, the ozone quickly dissipates due to lack of pressure. A contact tank eliminates this problem by allowing the ozone to be in contact with the water for an extended time prior to being bottled. The tank is simple, consisting of an in/out head, the tank and internal piping to direct flow. Some contact tanks have gas vents to release excessive gas/ozone build-up. These are called “off gas tanks.” Either will serve to extend contact time.
Actuation device: A flow switch will automatically turn on/off the ozonator when water flow is detected. Ozonators can also be connected to a pressure switch, automatic conveyor/filler switch or manual controls.
Ozone system design?
There are two basic designs:
Storage tank injection
Inject ozone into an atmospheric storage tank and rely on the ozone residual to build up enough to be of use in bottles at the end of the line. Every drop of water in the storage tank must be saturated with ozone in order to achieve a residual down-line in the bottles.
This requires far more ozone and equipment due to the loss of ozone in a non-pressurized tank, which equates to a higher priced system. In a non-pressurized tank, gas bubbles rise rapidly and break at the surface and there’s no mixing of the ozone and water. As we discussed earlier, ozone degrades rapidly, especially when there’s no pressure exerted on the tank.
Direct injection
Inject ozonate directly into the water under pressure as the water is being bottled. Ozone saturation and mixing is rapid due to “Henry’s Law” of physics. (Henry’s Law, in brief: The concentration of dissolved gas in a solution is directly proportional to the pressure exerted on the solution. For example, the higher the pressure, the more gas in solution. Likewise, the lower the pressure, the less gas in solution.) This is more efficient. Ozone will not escape or be wasted. Equipment costs are less than Option 1.
Ozone verification
Ozone residual must be verified in the bottles. Ozone gas, just like most gasses, is difficult to measure. The best test kit for this purpose is a hand-held electro-photometer. It tests the actual dissolved ozone.
Conclusion
The final question is—do you want to supply water that’s bacteria free?
Remember, the ozone advantage is simple:
- Bacteria-free water,
- No offensive chemical residual,
- Longer shelf life,
- Improved taste, and
- Complies with health department regulations.
About the author
Roger Nathanson is president of Ozone Pure Water Inc., of Sarasota, Fla. Ozone Pure Water has been a full service ozone/water treatment supplier since 1980. Nathanson heads the system design, system allocation and R&D departments. His background includes mechanical engineering, plumbing/pipe fitting, swimming pool remodeling/repair, sales and marketing. He holds a U.S. patent on a proprietary ozone unit/ozone generator design. Nathanson can be contacted at (941) 923-8528, (941) 923-8231 (fax) or email: [email protected]