The Effects of Ozone in Cooling Water Systems

Ozone, like chlorine or hypochlorite is an oxidizing biocide. Ozone produces oxidation byproducts, and these several secondary compounds must be accounted for in the set up of cooling tower system ozonation. Both iron and manganese will be oxidized by the ozone to form insoluble particulate matter, which will collect in basins, on screens, or in any scale that is formed. Excessive amounts of either of these two elements in the make-up water will require pretreatment, such as softening, to facilitate removal. Organic compounds that may either be in the make-up water or introduced through the atmosphere will react with ozone to form various substances. These substances, particularly peroxides, aldehydes, ketones, and alcohols, are efficient biocides themselves, which will further disinfect the water. If bromide is present, ozone can convert it to hypobromous acid and hypobromite ion. These two are also effective biocides, and would be considered helpful in controlling biological fouling, but because they are such effective biocides, they are generally detrimental in the blow down discharge to municipal waste treatment systems. Excessive ozone concentrations in the water will further oxidize the hypobromite ion to bromate, reducing the biocidal effectiveness of this component.

The effects of ozone on biofilms: On heat exchange surfaces, biofilms degrade heat transfer efficiency, increase pressure drop (and pumping power required) through exchanger tubes and plates by substantially reducing flow rates through these machines, and can lead to corrosion problems under the film. Ozone kills organisms by rupturing their cell walls, a process to which the microorganisms cannot develop immunity. When oxidized, the mucilaginous material secreted by these microorganisms is loosened from the heat exchange surfaces, and the biofilm, along with inorganic precipitates which were adhered to the secretions, are flushed away by the motion of the flowing water. Once biofilms have been removed, ozone concentrations of less than 0.1 mg / L have proven effective at preventing new growth of biofouling material.

The effects of ozone on algae: Most algal species are readily oxidized upon application of ozone; different species will require different exposure times for removal. The oxidation process can proceed to total decomposition of the algae to carbon dioxide and water with sufficient concentrations of ozone and contact time. A combination of controlled sunlight exposure and a low level of residual ozone will minimize algae growth in the cooling tower fill and basin. Destruction of many algal species using ozonation will liberate nucleic acids, proteins, polysaccarides, and other biopolymers. The production of polysaccarides by exposure of biofilms and algae to ozone liberates surface active materials with ability to complex iron, manganese, and calcium, effectively removing them from solution in the tower water, and thereby reducing scaling potential. These surface active substances, through micro-flocculation, may contribute to the observation that ozone treated cooling tower water becomes crystal clear.

The effects of ozone on bacteria: High levels of bacteria ("bug counts") can lead to an increase in microbially influenced corrosion (MIC). Certain sulfate reducing and iron metabolizing bacteria can destroy a system's steel or iron piping in less than one year. Ozone kills bacteria by rupturing their cell walls, a process to which microorganisms cannot develop immunity. Residual ozone concentrations of 0.4 mg / L or higher result in a 100% kill in 2 to 3 minutes for Pseudomonas fluorescens (a biofilm producer) in an established biofilm, while residual concentrations as low as 0.1 mg / L will remove 70 to 80% of the biofilm in a three-hour exposure. Studies have also proven that maintained ozone concentrations of less than 0.1 mg / L will reduce the populations of Legionella pneumophila, the bacteria responsible for Legionnaire's Disease, in cooling tower system water by 80%.

The effects of ozone on scale: Another common trouble in a cooling tower system which requires prevention is mineral buildup, commonly referred to as scale. Minerals such as calcium and magnesium, which are normal dissolved solids in make up water, are deposited by two different mechanisms, thermal and biological. As the water in a tower evaporates, dissolved solids become concentrated in the circulating water. When the concentration of these reaches the solubility limit of the water, they begin to precipitate out of solution. When biofilms are present on the walls and other components of the tower, the biofilm acts like a binder, cementing the mineral micro-crystals to the surfaces of the cooling tower system and piping and components. Over time, additional deposition of organic and inorganic matter increases scale thickness, and the effects of scale thickness versus heat transfer efficiency degradation are well documented. Ozone will oxidize the biological matter, and by removing the "glue", the mineral scale is allowed to fall away from the affected surfaces. However, if the scaling is not due to the presence of biofilm, ozone will probably be ineffective in removing the scale. Biofilm is rarely the dominant fraction of scale formation where the temperature of the heat exchanger is in excess of 135 degrees F. Scale-forming minerals are less soluble at these higher temperatures and will deposit from solution directly onto hot heat exchange surfaces.

The effects of ozone on corrosion: There have been several studies on corrosion rates in ozonated systems conducted and reported in trade journals and other literature. The initial premise for cooling tower systems using ozone for water treatment was that because ozone is such a powerful oxidizer, those metals capable of developing a passive oxide film would be protected from the ozone residual. But, Meier and Lammering tested and observed the corrosion rates of 1010 carbon steel, copper, brass, 90 / 10 cupro-nickel, and 304 stainless steel using ozone and chlorine in physically separate pilot cooling tower systems. In testing periods of 14 and 35 days, the observed corrosion rates were comparable with control tests without treatment. Corrosion rates on mild steel were reported to be lower when using ozone (4.6 mpy) as opposed to chlorine alone (28 mpy). Wellauer and Oldani reported test results for an open cooling tower system in which a 0.05 mg / L ozone residual was maintained. Copper alloy samples had lower corrosion rates than samples in water without ozone. There was no detectable corrosion on Cr - Ni steel alloys or on titanium, each of which developed a protective oxide layer.

Ozone is not a corrosion inhibitor; however, the higher concentration ratios resulting from the reduced blow down volumes raise the pH of the circulating water, which helps protect the system from corrosion. The high pH condition will also promote the precipitation of silicates and calcium carbonate if pretreatment of make-up water is not provided. Lower pH will remove scale, but will also increase the corrosion rate from the ozone.

It is clear that maintaining a small ozone residual (0.1 gm / L or less) will maintain algae and biofilm free surfaces, and thereby avoid or substantially reduce the under-film corrosive attack. Any corrosion that occurs in a clean system will be uniform throughout the system, as opposed to localized, and the corrosion that does occur will be very unlikely to cause component failures. The heat transfer efficiency, and overall system efficiency, will be restored and maintained at original installation levels with clean heat transfer surfaces after elimination of biological growth in the cooling tower system.

It is important to note that substantially higher levels of ozone residual above 0.1 mg/l may create ozone related corrosion.

References:

A Comparative Use of Ozone versus Other Chemical Treatments of Cooling Water Systems, D. A. Meier and J. D. Lammering, ASHRAE Transactions, Part 2, 1987.
Cooling Tower Water Treatment with Ozone, R. Wellauer and M. Oldani, Ozone: Science and Engineering 12(3):243 - 253, 1990.
Federal Technology Alert - Ozone Treatment for Cooling Towers, Pacific Northwest National Laboratory, Richland, WA, December 1995.
Ozonation in Cooling Water Systems, Thomas Ruisinger, Plant Engineering Magazine, August 1996.
Ozone and Cooling Tower Treatment, Dennis Kelly, Water Conditioning and Purification, August 1993.
Ozone for Cooling Towers - Facts, Update, and Predictions, J. Fred Wilkes, PE, Titusville, FL.
Ozone in Water Treatment: Application and Engineering, a cooperative research report edited by Bruno Langlais, David Reckhow, and Deborah Brink, American Water Works Association Research Foundation, 1991.