Cooling towers are a relatively new sector for ozone treatment. As such, the benefits of the technology are still being discovered by the users in this sector. The main advantages of ozone water treatment over the traditional chemical water treatment are in the water and energy savings that can be made. The reduction, and possible elimination, of chemical use also creates cost benefits for the user.
Cooling water is an ideal medium for breeding bacteria due to the warm water temperature and sunlight, which helps in proliferating growth of algae. If not checked, the algae can choke the heal transfer tubes. Biocides are required to control the growth of these micro-organisms. Due to the water loss in water evaporation in the cooling process, and subsequent addition of make-up water, the dissolved solids content of the water increases as it cannot escape with the evaporation. This increases the concentration of dissolved solids in water over a period of time necessitating periodic rejection of some portion of water; called blow down; to dilute the solids concentration in this water. The increased concentration leads to the increased scaling on the heat exchanger surfaces which leads to the loss of heat transfer resulting in the drop in efficiency of the heat transfer system incurring heavy losses. Increased scaling also contributes to the higher rate of corrosion. The formation of scaling is also attributed to the formation of a slimy bacterial layer on the metal surfaces called Biofilm. Scale inhibitors and corrosion inhibitors are used to control these factors.
Ozone as a cooling tower biocide is not only environmentally benign, it also brings a significant number of other benefits. The circulating water of cooling tower systems has always been an ideal place for growth of microorganisms. The warm temperature of the water and the addition of nutrients from the scrubbed air flowing through the tower provide a virtual nirvana for all sorts of microbes and algae. In turn, these organisms multiply at fantastic rates. The result is degraded system efficiency due to blockage of flow, shortened equipment life due to corrosion, increased operating cost due to higher pumping power requirements, and the spread of disease or even death.
Ozone as a cooling tower biocide offers a means to this end. It is not only environmentally benign, but ozone use also brings a significant number of other benefits to the cooling tower system operator.
Ozone is a practical oxidizing biocide because it is the fastest acting and the most efficient oxidizing agent against all microbiological organisms - when properly applied, ozone will kill all viruses, bacterium, cysts, and algal species. And, any ozone not consumed as a biocide naturally decays to oxygen, making ozonation the preferred choice for biological control of many cooling tower systems.
Field tests have demonstrated that the use of ozone in place of chemical treatment can reduce the need for blowdown, and, in some cases where make-up water and ambient air are relatively clean, can eliminate it. As a result, cost savings accrue from decreased chemical and water use requirements and from a reduction of wastewater volume.
There are also environmental benefits as fewer chlorine or chlorinated compounds and other chemicals are discharged. There is also a belief within the industry (and some evidence) that under certain conditions ozone acts as a descaling agent. The premise is that ozone oxidizes the biofilm that serves as a binding agent adhering scale to heat exchange surfaces. When scale buildup on condenser tubes is reduced, higher heat transfer rates are achieved. Increasing the condenser heat transfer rate will reduce the chiller head pressure, which then allows the chiller to operate more efficiently and consume less energy.
Biological growth, scaling, and corrosion are the main maintenance concerns with these cooling towers.
Benefits of Ozone over conventional method
Will not load any substance in water unlike chlorine which builds up the TDS till the cooling power is lost.
Virtually eliminate the necessity for the blow down.
Saves you from the 'who-pollutes-pay tax' which is becoming standard world wide (people are made to pay according to the load they dump even in the permitted drains)
Makes the water crystal clear, eliminating all turbidity.
Removes all bacteriae, including the dreaded 'Legionella'.
Costs almost nothing, once installed. Just a little powered!
Dissolves the scale and inhibits further scaling, hence improving heat transfer and further saving on chemicals and pollution.
Decreases the corrosion by 50% (acts by passivation like chromates which are used as corrosion inhibitors, but dangerous)
Removes froth and does not generate further froth.
Brings the bacteriae count down to below 1000CFU/ml compared to 105 to 107 for chlorine.
Makes you free from the suppliers of chemicals and maintenance personal as well.
Reduced Chemical Usage i.e. Chlorine can substitute for several chemicals in a cooling tower treatment package, reducing and possibly eliminating the use of such chemicals.
Increases dramatically Heat Transfer Efficiency.
Repays for its capital cost in 6 to 18 months.
After this, it adds good money to the bottom line of your clients, balance sheet. . . and yours by their repeated orders.
The first problem facing water cooling towers is the build-up of biological growth and minerals, otherwise known as scale. These problems inhibit the cooling towers' heat transfer efficiency. The way this problem has been solved in the past has been through the use of chemical agents such as chlorine and chelating agents. While this serves as an adequate solution to the original problem, the chemicals lead to other problems. Because of the evaporation of water in the tower, the remaining water reaches a high level of chemical and contaminant concentration. To regulate this, water is bled out of the system, and replaced by fresh "make up" water. It is the bleed off water that can be problematic to dispose of, with extra sewage cost being incurred.
Ozone treatment solves the original problem with a vastly reduced number of secondary costs and considerations. As well as being a powerful biocide, killing virus and infectious bacteria, ozone has been proven to have a positive de-scaling effect. It also greatly reduces the level of bleed off water, as well as the per unit cost of disposing it due to the environmentally friendly nature of ozone. Added to this are the savings due to reduced storage costs and handling of chemicals as ozone is produced on site. This fact significantly simplifies regulatory compliance.
Ozone is an extremely powerful oxidizing and disinfecting agent. It destroys all the micro-organisms and is over 3000 times more powerful when compared with chlorine. The use of ozone in cooling water completely replaces the biocide for controlling the growth of micro-organisms including algae. It also destroys the biofilm responsible for the scaling, thus helps in the control of scaling, which also helps in reducing the corrosion rate. The use of ozone thus brings down the requirement of scale and rust inhibitor chemicals improving the quality of the blow down water which, otherwise may require downstream effluent treatment due to the presence of these chemicals.
The result of using ozone in cooling water treatment is better control of bio-growth and scaling and corrosion of the metal parts. The ozonator require no maintenance, no operators and have no consumables. The raw water is air and comparatively low electric power. The capital cost of ozone plant and cost of electricity used for running the ozone plant, when compared with the cost of all the cooling water chemicals, cost of inventory, storage space cost, cost of personnel involved in handling the chemicals and sometimes the cost of downstream water treatment, the ozone plant pays for itself in 2 to 4 years depending upon the cooling water capacity and the water circuit.
In the design of a proper ozone disinfected cooling tower, one needs to take care of the following points :-
Suppletion water with high hardness or high COD levels is less suitable for ozone based cooling towers
Retention time in the system : - The half time of ozone is normally less than 10 minutes in cooling systems. To assure an adequate residual ozone concentration it is necessary to start with an adequate begin concentration.
Dead spots : - Because ozone depletes to oxygen, spots with little circulation have to be prevented.
Temperature : - The solubility of ozone and the half life time of ozone decreases at higher temperatures. This limits the temperature of the cooling water in which ozone can be used. This limit is in most cases around the 45°C.
Material : - The materials used in the cooling tower need to be ozone resistant.
Low corrosive effects
Corrosion effects are a usual common concern when using ozone. However, because a very low concentration is required and the short half-life, corrosive effects of ozone are low (or even half that resulting from chlorination treatment). Moreover, the effectiveness as a biocide minimizes significant corrosion effects induced by microbiological activity. Also, ozone treatment has been shown to increase the corrosion protection by forming a passive film covering and protecting the exposed surface.