System water is dense and full of impurities which enter the water circulation through cooling towers, where water is in direct with ambient air. The water temperature is often between 25 and 50?C. Such conditions are ideal for formation of algae, fungi and bacteria. Bio films and accumulated algae reduce heat exchange and prevent resistance in installations.

To avoid such effects, we need to dose biocides. It needs to be emphasised that we do not achieve sterility of cooling water with the use of biocides, but we control biological growth and keep it within acceptable limits. There are several methods of the use of biocides available.

With the occasional so-called shock-dosing we take measures when biological growth exceeds the desired limit. Preventively, we can dose chemicals several times a week, or we opt for biocides based on regular biological tests.

Through constant dosing at a considerably lower concentration as the one using shock treatment we can prevent biological development in the system water. It depends on economic calculation and technological requirements which method will be selected.

With the use of system biocides resistance of microorganisms often occurs what demands replacement of the dosing recipe.

The second but not less important or smaller problem are particles, which enter the system water through cooling towers. Dust, pollen, leaves and other forms of impurities can be disposed on devices and installations and present a source of problems. Besides, this organic matter is food for microorganisms living in cooling water. This is the reason why it is advisable to place a filter into the circulation, which can remove such impurities from the water. The ratio between the circulation of the filtered water and the basic water flow depends on the quantity of impurities entering the system and on special technological requirements. With the use of circulation filtering cooling water can never be absolutely clean. However, its quality is stable. Filtration is performed on sand filters and self-rinsing filters with cartridges. Sand filters, compared to the ones using cartridges, have more advantages: they rinse well, they remove a wide range of sizes of particles from water, have the ability to bind large quantities of impurities. Their weak points are they take a lot of space in case of large flow devices or plants, and they are heavy and more expensive than filters with cartridges.

If absolute water quality is required, which is directed to the cooling process, the whole process must be filtered. In such a case filters with cartridges or replacement of cold cooling towers systems with the open ones are solution to the problem.