Most cooling towers are likely to become contaminated with Legionella at some point in their serviceable life. It is unrealistic to try to prevent entry of the organism into a cooling tower or to create an environment that entirely precludes its growth and multiplication. However, an effective water treatment program that reduces the risk of legionellosis and thus ensures safer operation of the system also leads to more efficient operation and longer system life. Good maintenance of cooling towers is also necessary to keep legionella at bay, but no water treatment and maintenance system is guaranteed to fully and permanently eliminate the organism.
Cooling towers are a concern because they contain pools of warm water that are open to the atmosphere and present perfect breeding grounds for Legionella bacteria if they are not properly cleaned, disinfected and maintained. Therefore, the responsibility for managing risk belongs to the facility owners or manager. There are many components that can lead to hazards and the presence of Legionella bacteria and identifying the control measures for prevention.
1) Source Water Quality
Risk – Make-up water usually comes from a municipal or well supply but can sometimes come from a holding tank, which can contain sludge, rust, and sediment. Less commonly used, surface water that comes from lakes, rivers, or reservoirs could be full of microorganisms and nutrients from the environment.
Prevention – Twice a year the cooling tower should be cleaned and disinfected focusing on the entire tower and any holding tank by removing any rust, sludge and sediment. If the water is coming from a lake, river or reservoir, antimicrobial treatments should be used before the water enters the cooling system to prevent fouling.
2) Water Treatment
Risk – The temperature, environmental factors, and flow velocities of cooling tower water vary as well as other parameters affecting water treatment.
Prevention – The system should be designed so water can circulate throughout the system whenever it is operating. System operation should be coordinated with a full chemical treatment of the water. Also, keep the system clean by removing all dirt and debris. If the cooling tower needs to be shut down for more than three days, the entire system should be drained. If your cooling tower is metal, extra caution and measures are necessary, ensure free flow of water to prevent the proliferation of microorganisms responsible for corrosion and degradation. Another reason to consider our HPDE engineered plastic cooling towers.
Risk – Because cooling towers are open to the environment, the system can rapidly become recolonized with microorganism even after chemicals or other agents were added to achieve sterilization.
Prevention – Vary the antimicrobial stresses applied in the cooling tower microbial control program by alternating between two non-oxidizing biocides as a single dose or alternate an oxidizing antimicrobial with a non-oxidizing antimicrobial.
Risk – Dirt, dust, and other particles enter the cooling tower water during the cooling tower process. Depending on where the cooling tower is located, the amount of material can be extensive. The presence of biofilms is important for Legionella survival and growth providing nutrients in water systems.
Prevention – It is strongly advised to use a compatible and environmentally acceptable dispersant and/or detergent to penetrate biofilm and sediments. Make sure to follow proper cleaning and maintenance for your cooling system to ensure top performance and to minimize the risk of disease.
5) Design & Material Used
Risk – Areas of stagnant water prevent proper chemical treatment of the towers and allow Legionella and their hosts to flourish.
Prevention – Your cooling tower should be designed to easily be cleaned, provide easy access for maintenance of internal services, and avoid the buildup of sludge and deposits. While cooling tower conditions are often managed by water treatment chemicals, such treatment is sometimes inadequate and poor piping designs, lead to ‘dead legs,’ creating an environment in which pathogens – including Legionella – can thrive.
6) Spray Drifts
Risk – Even under normal operation, water droplets can leave the drift eliminator.
Prevention – It is for this reason that cooling towers should never be installed next to air intakes or windows that may be opened. Also, every six months or so, inspect the conditions of the drift eliminators making sure there are no gaps to allow free flow of dirt. For best results, install the best drift eliminators available with drift rates as low as 0.0005% of the circulating water flow.
There are many innovative technologies on the market today that can help prevent the growth of Legionella in cooling towers. One of the most effective include an anti-microbial cooling tower from Delta Cooling Towers.
Delta’s cooling tower has the only anti-microbial efficacy against Legionella bacteria out of all the common cooling tower materials. The anti-microbial resin is compounded into the plastic material so it will not fade or wash away.