The cooling tower has become a vital part of most cooling systems and is common in commercial facilities. It aids HVAC systems to transfer excess heat from various machinery and equipment to maintain optimal temperature. Also, like any other equipment, cooling towers maintenance is a major aspect of running these systems.
But the drawback with cooling towers is that they can cause some health hazards if not properly maintained. One of the major health hazards is the spread of Legionellosis or Legionnaires disease.
What is Legionellosis?
This disease is caused by the Legionella bacteria. It is a serious respiratory ailment that is spread via inhalation of water droplets infected with these bacteria. Such bacteria are tough survivors and can be found in central heating systems, humidifiers, and air-conditioning. They have been known to survive in pipes for over a year.
Legionella is found in natural water sources in benign quantities. They cause health risks only when they are allowed to grow and spread without treatment, especially in industrial cooling systems. They can proliferate in stagnant water and multiply dangerously when left untreated.
Such microbes tend to colonize in warm water environments such as cooling towers. In the latter, air intakes are located at the bottom. When they transfer heat to cool down warm water, there is a release of vapour into the air.
This moist air can transmit Legionella bacteria and eject them into the ventilation and air. These expelled bacteria can be inhaled by occupants of the building and result in a massive outbreak of Legionnaires disease.
By preventing the growth of Legionella bacteria in cooling towers, the spread of this disease can be halted. Cooling towers are a major source of concern as they feature pools of warm water that are open to the atmosphere and are ideal breeding grounds for these bacteria if they are not maintained, disinfected, and cleaned.
Hence the facility owner or manager has the responsibility of risk management of cooling tower Legionella. There are several risk factors for Legionellosis, and the following are the main ones:
Quality of source water
Makeup water typically comes from a well or municipal supply but can at times be sourced from holding tanks filled with sediment, rust, and sludge.
The solution is to clean and disinfect the tower system, focusing on the complete tower and cleaning tanks by removing such contaminants. When water is sourced from rivers or lakes, antimicrobial treatments must be used before the water enters the tower to prevent fouling.
Treatment of water
The risk is that flow velocities, environmental factors, and temperature of water in the cooling tower differ along with other parameters.
The solution is to design the system operation such that it coordinates with a total chemical treatment of water. Additionally, the system can be kept clean by removing all debris and dirt. When the cooling tower is required to be shut down for over 3 days, the complete system must be drained.
The risk is that since cooling towers are open to the environment, the system can quickly be re-colonized with microbes like Legionella, in spite of adding chemicals and other agents to accomplish sterilization.
The solution is to fluctuate the antimicrobial stresses used in the microbial control program of the cooling tower. This can be done by swapping two non-oxidizing biocides in a single dose or inter-changing oxidizing antimicrobials with non-oxidizing antimicrobials.
The risk is that of dust, dirt and other particles entering the water of the cooling tower. Based on the location of the tower, the quantum of the material can be substantial. The presence of such biofilms is vital for the survival of cooling tower Legionella and to encourage the growth of nutrients that aid the growth of such bacteria.
The solution is to use an environmentally acceptable and compatible detergent or dispersant for penetrating sediments and biofilms. One must ensure the best maintenance and cleaning of cooling towers to minimize the risk of Legionellosis.
Material and design
The risk is that pools of stagnant water will prevent the right chemical treatments of the towers, which helps in the flourishing of Legionella and their hosts.
The solution is to design the cooling tower for easy maintenance and cleaning to avoid the build-up of deposits and sludge. Though conditions of cooling towers depend mostly on chemicals used for water treatment, poor designs of piping can create an environment where Legionella pathogens can thrive.
The risk is that even under normal operations, droplets of water can leave the drift eliminators. The solution is to ensure that cooling towers are never installed near windows or air intakes that can be opened. Additionally, every six months, the condition of the drift eliminators must be inspected to ensure that there are no gaps to permit free in-flow of dirt.
In sum, many factors enhance the risk of Legionella infection: stagnant water, dead legs in the system of piping, temperatures in the range of 20 to 45 degree C, pH levels of water in the range of 5.5 to 8.1., presence of corrosion made of ferrous ions, presence of biofilm, etc. Treatment of such issues during cooling towers maintenance can help combat Legionnaires disease.