Ozone for Cooling Towers Water Treatment
In this article you’ll learn more about the use of ozone in cooling tower water treatment. Ozone in industry means a chemical-free solution and also a cost reduction in procedures. When it comes to cooling towers disinfection methods, it doesn’t have a difference.
If you want to learn more about how ozone means cost reduction, just scroll down!
Cooling tower water must be treated to limit the growth of mineral and microbial deposits that can reduce the heat transfer efficiency of the cooling tower.
Ozone is a strong disinfectant and an interesting alternative for the chemical biocides in cooling tower water treatment.
In the prevention and control of legionnaires disease (legionella) causing microbes, ozone has taken an eminent roll.
The main advantages of ozone water treatment disinfection procedure in cooling towers over the traditional chemical water treatment disinfection method are in the water and energy savings that can be made, caused by the reduction/elimination of chemicals used.
The use of an ozonation system in free cooling towers entails, in addition to the benefits comprising, represent significant savings to the levels of:
Reduction of water consumption;
Reduction of consumables;
Reduction of anti-scaling and anti-corrosion agents;
Reduced maintenance costs;
Remove the costs of chemical biocides storage and transport;
Reduction of energy consumption due to the increased efficiency of the cooling operation.
More than the reduced savings in chemical disinfection for cooling towers, the use of ozone as a maintenance treatment for cooling towers has good potential for operation and maintenance savings because, a small amount of ozone acts as a powerful biocide that decreases or nearly eliminates the need to remove quantities of water from the cooling tower in order to decrease the concentration of organic and mineral solids in the system.
The ozone applied to cooling towers associated with air-conditioning coolers is an excellent ally, as it eliminates bacteria, fungi and viruses, including legionella, in a very effective way and without leaving any residues, however, ozone can be a corrosion stimulant rather than an inhibitor which may imply optimization of the system in order to combat corrosion thereby ensuring a clean system rather than a biologically and mineralogically contaminated system.
According to a study carried out by the U.S. Department of Energy Federal Technology Alert at the Lockheed Martin Facility’s premises, in Florida, , where they compared the operating costs of cooling towers with the traditional (chemical) system and with ozone treatment, we can conclude that annual costs are reduced by one-quarter of the initial value. This represents a reduction from $ 198,168 to $ 57,415 per year, two values that stand out are relative to the operational costs (labor) and the costs with the power consumption.
Ozone Action on Cooling Towers
Effect on algae
Most algae species oxidize immediately when exposed to ozone (different species require different exposure times for complete removal). When algae are exposed to sufficient ozone concentration and exposure time, they completely decompose into carbon dioxide and water.
Destruction of algae by ozonation releases nucleic acids, proteins, polysaccharides, and other biopolymers. The production of polysaccharides by the action of ozone on biofilms and algae releases surfactants that can complex iron, manganese, and calcium, which removes these substances from the cooling tower water, reducing the risk of encrustation. These surface-active substances help make ozone-treated cooling tower water crystal clear through microflocculation, leaving no taste or chemical residue. Ozone is a very powerful disinfectant and oxidizer and can remove disinfection by-products from the water.
Effect on bacteria
High bacterial concentrations can lead to an increase in microbial influenced corrosion (MIC). Certain sulphate-reducing and iron-metabolising bacteria can destroy steel or iron pipes in a system in less than a year. Ozone kills bacteria by breaking down their cell walls, a process to which microorganisms cannot develop immunity.
Residual ozone concentrations of 0.4 mg/litre or more result in complete reduction of Pseudomonas fluorescens (a biofilm producer) in an established biofilm within 2 to 3 minutes, while residual concentrations as low as 0.1 mg/l remove 70% to 80% of the biofilm in a 3- to 4-hour exposure.
Studies have also shown that an ozone concentration of less than 0.35 mg/l for 30 seconds reduces the population of Legionella pneumophila, the bacterium responsible for Legionnaires’ disease, in cooling tower water by 99%.
Effect on biofilms
On heat exchanger surfaces, biofilms degrade heat transfer efficiency, increase pressure drop, significantly reduce flow rates, and can lead to corrosion problems. Ozone kills organisms by breaking down their cell walls, a process to which microorganisms cannot develop immunity.
Oxidation dissolves the viscous material secreted by the microorganisms from the heat exchanger surfaces, and the biofilms are washed away by the movement of flowing water. Once the biofilm is removed, it is possible to prevent the re-growth of biofouling at ozone concentrations below 0.1 mg/l.
Effect on corrosion
Several studies of corrosion rates in ozonated systems have been conducted and published in journals and other literature, as it was believed that ozone is a strong oxidizing agent and could oxidize metals.
However, tests using ozone and chlorine separately found that the corrosion rates of 1010 carbon steel, copper, brass, 90/10 cupro-nickel, and 304 stainless steel increased.
Corrosion rates comparable to untreated control tests were observed for 14- and 35-day tests. For mild steel, lower corrosion rates were observed when ozone (4.6 mpy) was used than when chlorine (28 mpy – Mils per Year) was used. Test results were reported for an open cooling tower system in which an ozone residual of 0.05 mg/l was maintained. Copper alloy samples exhibited lower corrosion rates than samples in water without ozone. No corrosion was detectable in Cr-Ni steel alloys and titanium, each of which formed a protective oxide layer.
Ozone is not a corrosion inhibitor, but the higher concentration rates resulting from reduced discharge rates increase the pH of the circulating water, which helps protect the system from corrosion. Maintaining a low ozone residual (0.1 g/L or less) keeps surfaces free of algae and biofilm and significantly prevents or reduces microbial corrosion (MIC) or corrosive attack under the film.
Effect on scaling
Another common problem in a cooling tower system that should be avoided is mineral deposits commonly referred to as scale.
Minerals such as calcium and magnesium, which are solids normally dissolved in fresh water, settle by two different mechanisms: thermal and biological. When water evaporates from a tower, the dissolved solids concentrate in the circulating water. When the concentration of these solids reaches the limit of water solubility, they begin to precipitate.
When biofilms are present on the walls and other components of the tower, the biofilms act as a binder that binds mineral microcrystals to deposit organic and inorganic materials.
Ozone oxidizes the biological material, and removal of the binder allows the mineral encrustations to fall off the affected surfaces.
Ozone is produced on-site, this means that it does not requires storage of dangerous chemicals;
Environmentally friendly treatment, facilitating regulatory compliance;
Disinfectant with a high efficiency level (a residual ozone concentration of 0.1 to 0.2 ppm is, in most cases, very effective to keep the cooling tower and the cooling circuit clean);
Ozone requires no additional disinfectants, micro-organisms cannot get resistant to ozone after prolonged use of ozone;
Ozone is 3,125 times more germicidal than chlorine;
Insignificant buildup of disinfectant or disinfectant byproducts. Ozone does not leave any chemical residues or disinfectants, at the end of its cycle, ozone decomposes again into oxygen;
Eliminates the use of chemicals (except for pH balancing). Ozone does not change the pH;
Ozone is effective in a wide pH range;
Destroys all types of micro-organisms instantly;
Very effective in removing biofilms;
Very effective against Legionella, due to good biofilm removal capacities;
Low corrosion rates in the system. Reduces the corrosion rate of metals, including copper heat exchangers, because there are no chlorinated compounds;
Ozone decomposes organic waste through oxidation;
Reduced permit costs for discharge of treated water to environment;
Low maintenance costs;
Lower operational costs and in many cases a lower overall cost;
Lower energy costs, because the increasement of the heat transfer efficiency of the chiller, increasing the cooling operation efficiency;
Safe and easy in use;
Effective for mussel growth;
Minimizes condenser fouling;
And much more.
EMMANUEL I. EPELLE, ANDREW MACFARLANE, MICHAEL CUSACK, ANTHONY BURNS, JUDE A. OKOLIE, WILLIAM MACKAY, MOSTAFA RATEB, MOHAMMED YASEEN | February 15th | Ozone application in different industries: A review of recent developments
PIERRE A. LIECHTI, DR. RÜDIGER KAULBACH | October 17th | A Full Year- Scale Study of Ozone Cooling Water Treatment at a German Electric Power Station
ALAN E. PRYOR, MARK FISHER | October 17th | Practical Guidelines For Safe Operation Of Cooling Tower Water Ozonation Systems
M. F. HUMPHREY | July 23rd | Cooling Tower Water Conditioning Study
WILLIAM K. McGRANE | July 23rd | Ozone Cooling Tower Treatment With and Without Mineral Removal
R. J. STRITTMATTER, B. YANG, D. A. JOHNSON | February | A Comprehensive Investigation On The Application Of Ozone In Cooling Water Systems –– Correlation of Bench–Top, Pilot Scale and Field Application Data