Ozone applications Cooling tower water
|Cooling tower water requiress extensive treatment. During cooling tower water treatment, three main factors must be controlled [44,45]: |
- Corrosion of pipes and heat exchanger units
- Scaling in pipes and (mainly) in heat exchangers
- Microbial growth (bacteria, algae)
These three aspects cannot be viewed separately. For example, lower pH values can prevent scaling, but they increase metal corrosion. By means of ozone treatment, all these aspects can be controlled collectively, without further addition of chemicals.
Conventional treatment techniques are mainly applications of chemical biocides, corrosion inhibitors and scale inhibitors. Ozone is a reliable alternative that controls the above-mentioned factors sufficiently .
This page shows an overview of ozonation of scale, corrosion and microbial pollutants in cooling tower water.
Scale forms as a result of calcium and magnesium ion deposits on the units of a cooling system. This causes the formation of a core, which forms an isolation layer in heat exchangers. This negatively influences heat transport. Consequentially to water loss by evaporation, salt concentrations in the water increase. This process is called thickening. At a certain point, the saturation rate for these salts is reached, causing them to precipitate. This limits the number of times the cooling water can be recicled.
The thickening factor N is a measure for the increase in salt and ion concentrations in cooling water. As such, it indicates the number of times the water requires refreshment, in order to prevent salt precipitation.
A bio film can also be formed in the cooling water system, consequentially to clogging of microorganisms, such as algae. A bio film takes up ions that normally form micro crystals, which increases scaling. In time, inorganic and organic matter will thicken this core.
Ozone treatment limits scaling. Ozone is a disinfectant that decomposes bio film, causing ions to no longer be able to attach to it. This causes a decrease in scale formation. Even water with a high dissolved solids content can now be recicled, causing cooling water discharges to be decreased.
The only condition under which this system works, is that small rest concentrations of ozone must remain in the cooling water, to prevent further microbial growth that enhances scaling.
Each material has a limited life-span. The length of the life-span depends on the nature of the material and on environmental conditions. The first method of corrosion prevention is the choice for the most persistent material and a solid construction of the cooling system. Once the cooling water system is in use, corrosion can be prevented by alteration of the water quality. In practise, this is achieved by pH adjustment and alteration of the dissolved solids concentration. When these measures do not provide the desired result, corrosion-inhibitors may be added to the cooling water. However, corrosion-inhibitors are quite expensive, because of the supervision that must be carried out to quarantee the affectivity of the measure.
Another method to prevent corrosion is ozone application. Corrosion is mainly caused by microorganisms, which enhance corrosion forming conditions . Ozone limits microbial growth. Ozone also causes a certain electric current to occur in the water. This current causes metals to form a passive, corrosion-preventing film on the materials from its own oxides. Such a passivating oxidative film is found on stainless steel and aluminum. This film can only be formed at a certain water flow and on certain types of materials.
Various experiments have shown that corrosion always decreases when ozone is applied, usually by more than 50%.
Little ozone is required to form a corrosion-preventing film on metals. High dosages of zone can corrode some metals. In practise, about 0,1 g/m3 of ozone is dosed to recirculating water. The ozone that does not react with organic matter, decomposes to oxygen. There will be no toxic residues.
One cannot prevent microbial growth in a water system. This is because we always use water and air that contains bacteria. During processes, bacteria can enter the water, as well.
Ozone is a stronger disinfectant than any other chemical. Ozone is the most efficient disinfectant for the deactivation of Legionella bacteria. Usually, one cannot thicken the water in organic water treatment preparations more than three to five times. This means that of each 1000 litres of suppletion water, at least 200 litres is carried off (20%).
When ozone is used for water treatment, the water can be thickened more than 5 times. This means that for each 1000 litres of water, no more than 200 litres is carried off. In practise, the water can be thickened to about 20 times. This means that for each 1000 litres of water, no more than 50 litres is carried off to the sewer. Only evaporation and splatter losses are filled up with fresh water in this case.
Applications and measures
Ozone application in cooling water treatment is a good option for water quality control in a cooling water system. Ozone leads to savings of:
- Anti-scaling and anti-corrosion agents
- Acid dosages
- Water use
- Storage and transport costs of chemical biocides
- Pump capacity (more efficient heat transfer; lower energy use; higher yield heat exchanger)
Ozone application can only be done adequately when certain factors are taken into account:
- Water quality; suppletion water for cooling towers that is hard or has a high COD is less suitable for ozone treatment
- The time that ozone remains in the system. The half-life of ozone is usually less than 10 minutes in a cooling tower. To gain a significant residual concentration of ozone, the primary ozone concentration must be sufficient.
- Cooling water temperature. Ozone solubility and the half-life of ozone decrease at higher cooling water temperatures. This limits cooling water temperature for adequate application of ozonation. The limit is usually a cooling water temperature of 45 oC.
- Material choice. The material that a cooling tower consists of must be ozone-resistant.