Industrial companies using cooling towers often require a cooling tower water treatment system to prevent corrosion, scaling, biofouling and biological growth. Ozone has found application in cooling towers as an effective biocide, but its share of the cooling tower biocide market has remained small. It appears that the reason for this low market share is primary due to economic factors. In conversations with industrial water treatment practitioners in the cooling tower industry, the vast majority consider ozone to be a good biocide capable of treating the type of micro-organisms that are found in cooling towers. However, due to the volatility and short half-life of ozone, they believe it is not economical relative to chlorine and bromine-based biocides.
Application of Biocides in Cooling Towers
Before discussing the particulars, it is worthwhile to briefly discuss the application of biocides in cooling towers. First, biocides need to kill free floating pathogens in the water. Second, biocides must reduce the presence of biofilms on heat exchangers and other towers components such as packing.
Cooling Tower Legionella Control
Removing free floating pathogens such as Legionella is critical since cooling towers aerosolize water which leaves the tower as the so-called drift. If the drift contains pathogens, they can expose people in the vicinity of the tower to these organisms. According to the CDC, “Legionella, the bacteria, was discovered after an outbreak in 1976 among people who went to a Philadelphia convention of the American Legion. Those who were affected suffered from a type of pneumonia (lung infection) that eventually became known as Legionnaires’ disease.”
It is believed that the Legionella came from a cooling tower. Since that time, the cases of Legionella have been increasing, although not all of them can be associated with cooling tower. Thus, a poorly maintained cooling tower can be a serious health risk.
Cooling Tower Biofilm
Biofilms, on the other hand, impact the functioning of the cooling tower. The most important function is heat transfer. If biofilms reduce the heat transfer in the chillers, then energy is wasted. This has direct economic and environmental consequences. Biocides are introduced into cooling water to control biological growth and biofilms. Currently, chlorine and bromine-based chemicals dominate the biocide market in cooling tower water treatment. Chlorine-based compounds (primarily hypochlorite) have the most market share due to their effectiveness and cost.
Cooling Tower pH Control
Typically, cooling towers operate in the range of pH 8-9, which is influenced by many factors including whether the tower employs pH control. Cooling tower water treatment specialists tend to use bromine chemicals in the upper end of the pH range since hypochlorite effectiveness as a biocide decreases with increasing pH. Bromine can be twice as expensive as chlorine compounds for treating a cooling tower, so bromine-based chemistries are typically used only where chlorine is not effective.
Ozone Use in Cooling Tower Water Treatment
Ozone is also used in cooling towers, although to a much lower extent as it has issues in the cooling tower application. First, ozone will be stripped out of the water as the water pours over the tower packing in the updraft of air. Second, high pH reduces the half-life of ozone. The higher the pH the shorter the half-life. Ozone half-life is also affected by temperature and cooling towers typically operate in the 80-90 F degrees range.
Cooling tower water treatment practitioners generally recognize that ozone is a very good biocide but believe that it is not economical and more complicated to apply. There are also some reputational issues associated with ozone in cooling towers. At one point it was aggressively promoted to do “chemical free treatment” of cooling towers. Essentially, it could not only prevent biofilms and kill free floating organisms, but also provide chemical scaling control and corrosion protection. Some towers treated with ozone as the only treatment method had poor performance and this may have soured some practitioners on using ozone.
There are cooling tower owners that want to use ozone or are not able to use chlorine/bromine-based biocides. One government agency that prefers ozone is NASA. NASA’s interest in ozone stems from limitations on blowdown discharges into the wetlands around the Kennedy Space Center (KSC). An article on this installation was covered in another Ozone News article (J. Baker et al, Volume 43, No.3). Other NASA installations followed, probably based on the success at KSC. Spartan Environmental Technologies has installed two ozone systems at the NASA Plum Brook Station in Sandusky, OH which have been in operation for over seven years.
Other cooling towers choose ozone because they want to minimize the use of chemicals perceived to be dangerous to store or handle. Spartan has sold an ozone system for cooling water treatment at a large primary/secondary school in Ohio. The Nashville Airport uses ozone cooling water treatment for their cooling tower for the same reason.
Applying ozone to cooling towers is not substantially different than other ozone water treatment applications. Because of the short half-life, ozone must be added continuously to replace ozone lost to air stripping at the tower and self-decomposition in the water. Normally, ozone is added to the cooling tower basin. Continuous dosing of ozone is different than chlorine, which is typically dosed in batches.
Ozone Application in Cooling Tower Water Treatment
The illustration below shows schematic of an ozone water treatment application in cooling towers.
Ozone generation is sensitive to ambient temperature and thus, generator cooling is important. Additionally, most systems use oxygen feed and use PSA oxygen concentrators.
The tower basin provides a convenient location to disengage the non-dissolved gases prior to the water flowing to the heat exchangers. In some case, ozone can be injected into the return line to the heat exchangers but in-line degassing may be required. An example of the latter approach is shown in a case study for an Arizona based cooling tower (J. Jackson, Industrial Water World, Jan/Feb, 2006, pages 48-49).
Transferred Ozone Dose
In terms of transferred ozone dose, the literature indicates 0.05-0.15 ppm of ozone immediately prior to the heat exchangers to protect them from biofouling (see references below). This ozone concentration is designed to keep the bacteria count in CFU/ml below 103. In terms of ORP readings, which is the typical way the cooling water treatment practitioners follow oxidant levels, values from +500-600 mV are targeted.
Transferred Ozone Dose Factors to Consider
Beyond transferred ozone dose, there are other factors to consider. Applications with high COD levels, (e.g., oil refineries), may make ozone impractical. Additionally, by minimizing chemical addition, customers/practitioners may believe that cycles of concentration for the towers can increase with the use of ozone. While this may be true, it is dependent on the cooling water chemistry. A study of ozone in cooling towers developed a Practical Ozone Scaling Index, which differs from the standard indices used by the cooling water industry such as LSI (A. Pryor et al, A New Practical Index For Predicting Safe Maximum Operating Cycles Of Concentration In Ozonated Cooling Towers, Ozone Science and Engineering, vol. 17, 1995, pgs. 71-96.)
The main take away is that ozone can be an effective cooling tower biocide, but the application of ozone in cooling towers involves different dosing and water chemistry considerations than the more common biocides. In addition, other treatment chemicals required to deal with chemical scaling and corrosion may be needed.
Additional References on Ozone Biocide us in Cooling Towers
- R. Strittmatter et al, Application of Ozone in Cooling Water Systems, Presented at the National Association of Corrosion Engineers Corrosion ‘92 Meeting, Nashville, Tennessee, April 27–May 1, 1992.
- P. Liechti et al, Case Study: One Year Full-Scale Study of Ozone Cooling Water Treatment at German Electric Power Station, Copyright © Ozonia Ltd, 2000
- H. Liou, An On-Site Cooling Tower Treated by Stand-Alone Low-Concentration Dissolved Ozone, Ozone: Science & Engineering,31:1,53 — 59, 2009