Importance of Effective Microbiological Control in Open Cooling Tower Systems


Open cooling tower systems employ evaporative cooling to remove heat from manufacturing process and comfort cooling applications, such as building HVAC systems. Compared to air cooling, evaporative cooling can be a more cost-effective and energy-efficient process. For example, a 500-ton water-cooled chiller operating at 60 percent load uses 47 percent less energy than an air-cooled unit. This saves 565,000 kwh/yr in energy, reduces greenhouse emissions by 765,000 lb/yr, and lowers peak energy demand by 45 percent.


Unlike air-cooled systems, water-cooled systems require conscientious routine maintenance, including effective water treatment to prevent mineral deposits, corrosion, and microbial growth. Cooling towers provide an ideal environment for the growth of problem-causing microorganisms including bacteria, algae, fungi and protozoa. Cooling towers are excellent air scrubbers enriching the cooling water with oxygen, nutrients and ideal chemistries at temperatures suitable for the proliferation of microorganisms.

Most bacteria produce biofilm (complex communities of surface-attached bacteria) within which they grow, shelter, and reproduce. This polymeric substrate adheres to surfaces giving the bacteria a suitable environment to develop. A common example of biofilm is the clear, slick material that develops on the sides of a dog’s water bowl.

Biofilm can reduce heat transfer by over four times more than mineral deposits. Many microorganisms produce waste products that are corrosive to metal surfaces. Biofilm can also be utilized by pathogenic organisms causing illness and disease. Legionella bacteria is the most prominent pathogen associated with open cooling systems. Biofilm costs the U.S. economy billions of dollars annually in energy losses, equipment failures, product contamination, and health safety concerns. Biofilm interferes when developing an effective microbial control program.

Controlling the microbiological environment is essential in any cooling tower

In order to achieve good microbiological control, a few fundamental concepts need to be addressed. Preventing biofilm formation is easier and more practical than trying to regain control of a problematic situation. It is impossible to have a “sterile” system nor should that be the goal. Biocide selection, dosages and feed schedules may need to be periodically adjusted to remain effective. Successful corrosion and deposit control can also be affected by microbiological control.

The approach to controlling these problem-causing microorganisms is multifaceted but centers around four areas.

1. Develop a biocide program that includes proper product selection and the correct application.

2. Conduct biomonitoring to measure the effectiveness of the biocide program.

3. Provide regular cleaning and disinfection of the cooling system.

4. Eliminate stagnation and dead legs.

1: Biocide Program

Chemicals used for microbiological control are classified as microbiocides and are regulated by federal and state governments. These regulations include label identification, systems approved for application of the product, and dosage rates permitted for specific systems. It is a federal violation to use a biocide in a manner inconsistent with the label instructions.

It’s vital to keep chillers a help, not a hazard

Microbiocides fall into two classifications: non-oxidizing and oxidizing. Non-oxidizing biocides are designed to kill or inhibit growth by interfering with essential metabolic processes (essentially a poisoning effect). Oxidizing biocides are designed to kill or inhibit growth by physically destroying (oxidizing or “burning”) cell structures. A microbiological control program usually employs both types of biocides. Factors influencing the choice of biocide include types of microorganism, system water characteristics, environmental restrictions, and practical considerations (available feed points, feed equipment, and economics). The environmental effects of biocides need to be considered. Some biocides are greener than others breaking down to less toxic byproducts after use.

Effective microbiological control consists of selecting the correct biocides and proper application. Knowing the water volume of the system treated is essential to determining the correct dosages. If unknown, a volume study should be done. Feed methods to consider are intermittent (slug) or continuous feed. Frequency of biocide application, contact time at a toxic concentration, degradation (half-life) of a biocide, holding time index for a cooling system, and off line systems all must be considered for an effective program.

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Posted by on Jul 5th, 2017 and filed under Feature Story. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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