During the past 15 years ozone has become a technology of choice in the point-of-entry (POE) water treatment industry. Sales of equipment incorporating ozone technology represent one of the fastest growing market segments within the industry.

The growth potential for this technology is enormous, as water treatment professionals develop a greater understanding of how to apply it.

POE water treatment systems incorporating ozone must include the following stages: air preparation and ozone generation; gas transfer (into solution) and retention; Degas destruction and venting and post-treatment.

The two stages discussed here are air preparation and ozone generation.

Dealers need to be aware that environmental factors can seriously affect the way ozone systems operate, especially when ozone is used to oxidize problematic inorganics common to private well water. To compensate for this, dealers should address several variables when designing ozone water treatment systems:

Air Supply

1. Ozone generators convert oxygen (O2) to ozone (O3). If you are designing a system to be installed in higher altitudes (i.e., Denver, CO) there is less oxygen per volume of air compared to what is found at sea level. Accordingly, if the oxygen level in the air supply is not enhanced with various air preparation technologies, the ozone generator's known output may need to be de-rated for high altitude applications.

2. Dewpoint. Increases in humidity cause decreases in ozone output. Also, ozone will combine with nitrogen and moisture within the air and produce nitric acid. This can significantly decrease the operating life of an ozone generator.

3. Temperature. Increases in temperature cause decreases in ozone output. Ozone generators produce heat during their operation, and a system's production rate is closely associated with how well heat dissipates from the chamber producing ozone. This problem is exacerbated when "hot" air is used as the feed gas. If this is the case, install the ozone system in the coolest location possible.

4. Variations. When an ozone generator is subjected to variations in temperature and feed gas dewpoint the ozone dosage changes accordingly and affects overall water treatment plant performance.

Gas preparation

To reduce variability of ozone performance based upon changes in climatic conditions and to enhance generator reliably, air preparation is mandatory. Two options exist:

1. Air dryers. Two types exist - expendable or regenerative. Each incorporates a desiccant medium such as silica gel or calcium sulfate.

The expendable type requires the homeowner to discard a dryer cartridge or have it recycled by a dealer once the media becomes saturated.

The more common type of regenerative air dryer in use today uses an automatic regenerative process where moisture is periodically driven out of a permanently installed desiccant cartridge with heat. In most cases, two cartridges are included to provide for continuous operation of the dryer while one canister is being regenerated.

2. Oxygen concentrators. This type of air prep system offers the benefit of producing elevated levels of oxygen while reducing humidity. If a generator is supplied with high concentrations of O2, it will produce more ozone. It is not unusual for ozone production to double if an oxygen concentrator is used for air prep. Oxygen generators either use a molecular sieve media capable of adsorbing nitrogen or a membrane capable of rejecting nitrogen. Each process requires air pressure to operate.

Ozone generation

Two technologies are commercially available for ozone generation in POE applications - ultraviolet (UV) and corona discharge (CD). UV ozone generation use a gas-filled quartz tube emitting light around the wavelength spectra of 183 nanometers (nm) when supplied with electrical energy. UV generators are simple in design and produce low concentrations of O3 gas.

CD ozone generation uses a high-energy electrical field. CD generators typically are complex in design, and are capable of producing high concentrations of O3 gas.

Consider the following when designing a generation system:

Production/concentration. Ozone generators are sold based upon performance capacity claims expressed as grams produced per hour (grams/hr). Unfortunately, this number alone is irrelevant as far as water treatment is concerned. Solubility of O3 gas in water depends on concentration of O3 delivered from the generator.

Although a generator is capable of producing 10 grams per hour, if the concentration of O3 gas delivered to the water supply is low, solubility of this gas in water is almost non-existent .

When reviewing performance specifications for ozone generators, it is critical to review production as measured in grams per hour (or equivalent) and concentration in percent by weight (or equivalent). This information best describes ozone generator performance .

Cooling system. As previously stated, increases in temperature cause decreases in ozone production. The generation of ozone requires energy, and this causes heat. If manufacturers do not incorporate an efficient method to dissipate heat from the ozone discharge chamber, it will cause significant decreases in ozone production.

Production rates can decrease by as much as 80 percent over the first 5 to 10 minutes of operation. Ask the manufacturer for test data measuring production rate against time of operation.

O3 production controller. The amount of ozone needed to satisfy each source water condition is unique. Changes in water chemistry or temperature affect the ozone dose required. While the net applied ozone dose can be calibrated to ozone demand by manipulating gas and/or water flow rates, an ozone generator with a variable output controller is preferred.

Water backflow. From the outlet of the generator, ozone gas is delivered to a water stream or contactor via a hose or pipe. Because the presence of water in a generator will cause innumerable service problems, a dealer must prevent water from entering this gas delivery line.

Most - if not all - check valves are not reliable over time in this application. Two check valves can be installed in series instead of one, which can be valuable if the check valves are inspected on a routine basis and it can be guaranteed that the valve closest to the water fails first.

By gaining a better understanding of the factors that affect ozone generation systems, dealers can design systems that more efficiently and effectively provide water treatment in POE applications.

Philip C. Olsen is president of Cartwright, Olsen and Associates, Cedar, MN.