COOLING TOWERS AND LEGIONNAIRE’S DISEASE, PART. 4
In this fourth and last part, we will conclude our analysis on Legionnaires’ disease, indicating further maintenance measures for the towers to counteract its formation, and establishing the conditions that might facilitate the bacterias multiplication.
In addition to the impurities contained in the feed water of the evaporating circuit, those contained in the air or other biological substances can also be brought into the tower and introduced into the circulating water. In addition to the necessary and continuous purging of a small amount of water, a water treatment program specifically designed to contain scale, corrosion and biological formations should be activated as soon as the system is installed, and then constantly maintained.
Excessive scale formation on the exchange surfaces of an evaporative cooler or evaporative condenser significantly reduces its efficiency. This can cause the cooling temperatures to increase compared to the design temperatures and possibly plant shutdowns. The formation of scale also increases energy consumption, and this happens throughout the year regardless of the workload of the system. While fouling itself is not considered a nutrient for bacteriological growth, a large formation of fouling is a preferred site for the reproduction of microorganisms and this can thus increase the risk of bacterial contamination.
There is a wide variety of chemical biocides, both oxidising and non-oxidising, that are effective in controlling bacterial levels, including legionella. In addition, there are other non-chemical methods for biocidal treatment such as ozonization, UV irradiation and the use of copper and silver ion release treatments. A water treatment specialist should recommend the most suitable biocide treatment depending on the type of installation.
Conditions that favour the multiplication of legionella
Normally, a charge concentration level below 104 cfu/ml (cfu = colony forming units) indicates that the system is under control. Conversely, concentration levels above 105 cfu/ml require immediate corrective action to be taken to reduce them. Within the total bacterial count, the value of the Legionnaires’ disease virus can be measured separately, and this should be below 103 cfu/l.
By Francesco Viola, Thermodynamic Engineer
Sources:
Guidelines for the prevention and control of Legionellosis, edited by the Italian Ministry of Health
Eurovent, Guidelines on how to maintain efficient and safe cooling systems
