How Organic Deposits Affect Cooling Tower Operation: The Answer To Low Efficiency

Written by Sean Robbins

The efficiency of cooling towers is always a topic of discussion and worth a deeper dive. Even a well-controlled scale or corrosion inhibitor program can be significantly affected by the presence of flourishing microorganisms.  Because the interior of a cooling tower is usually moist and warm, it provides an ideal condition for microorganisms to flourish and they subsequently migrate throughout the entire cooling system.

Cooling Water Treatment Fact Sheet

There can be a variety of different microorganism species present within the tower and cooling system. Conditions in the tower support growth of both microorganisms that are present in the water supply and those that enter as airborne contaminants. The majority of the microorganisms are bacteria but may also include algae and fungi.

What microorganism can be found in cooling systems?

The type and the number of microorganisms that are present differ widely. Experts advise on checking the microorganism levels regularly and establishing a “trend” or baseline levels of microorganisms usually present in your cooling system. This can be accomplished with a variety of onsite test methods including: Dipslides (agar), BARTTM (culture tubes), and ATP testing. Another trend to watch is the approach temperature of the system since organic deposits will cause it to increase over time.

Lab technician holding up a a six-well plate containing bacterial or microbial colonies growing on an agar medium.

It is well established that cooling towers provide a good environment for growth and it is inevitable that microorganisms will grow throughout the system. The function of a cooling tower is its ability to efficiently reject heat from the water via evaporation. Cooling towers and associated equipment can be of several configurations, but all can have large microorganism populations living internally that are not visible. In severe cases, microbiological contamination results in visible or “touchable” growths in the tower basin. The presence of algae is visible by the accumulation of green patches on the structure and/or floating on the surface of the water. By simply wiping your hand along the basin walls you may detect the slimy presence of these growths. Oftentimes, there is a synergistic pattern of algae and bacteria growing together on these surfaces. 

Infographic explains biofilm formation in water systems: attachment, colonization, maturation, and detachment, with impacts.

How do microorganisms affect cooling tower efficiency?

The primary concern regarding the presence of bacteria and other microorganisms is that they can severely hamper the efficiency of the cooling tower’s heat rejection.

They can also cause restriction of water flow, form deposits, corrosion, or interfere with air circulation.

A few types of normally present microorganisms are, Sulfate Reducing Bacteria (SRB) and Iron Bacteria that create corrosive conditions Pseudomonas that produce biofilms, and disease causing Legionella.

Bacteria mostly grow in the heat exchanging equipment due to the warm temperature; however, they may be found throughout the entire cooling system. When they significantly increase in numbers, they create an insulating barrier and directly reduce the amount of heat exchanged. Some bacteria can also cause corrosion of metal due to the production of acidic waste products.

Table illustrates the relationship between heat transfer reduction and biofilm thickness increase across tubes types.

As displayed in the chart above*, even one micron of biofilm, 0.001 mm, can reduce the amount of heat being transferred by 2.8% on carbon steel and by 29.5% on copper tubes. This number only increases as the thickness of biofilm increases. A biofilm of only 5 microns thick can reduce the amount of heat being transferred to as high as 67% on carbon steel tubes. No matter what type of cooling system you use, the reality is you may be getting only half the efficiency due to the presence of biofilms. 

Cooling Water Treatment Fact Sheet

In addition, microorganisms reproduce at an exponential rate. Populations are typically measured in CFU/mL (Colony Forming Units/mL). Exponential reproduction means a single CFU/mL can become over 1014 CFU/mL over the course of 24 hours, if left uncontrolled. This is the primary reason for a well-regulated program of biocide additions to keep this under control. 

How to safely increase efficiency and prevent long-term damage

To prevent the loss of energy and time, controlling the microorganism level is a priority in cooling system maintenance. At Garratt-Callahan, our bundled services mean that our expertise comes with every chemical purchase. This allows us to serve our clients in every possible way and provide a solution for keeping your cooling systems operating at high efficiency as well. The “G-C Advantage” includes services from testing, to system evaluations, and field service reporting. 

A long used and proven way to prevent microorganism growth is the use of approved oxidizing and non-oxidizing biocides. While it is a common procedure, there can be many improvements in the end results with multiple and alternating additions rather than using a single biocide or chlorine alone. Garratt-Callahan also recommends using an organic deposit dispersant and penetrant with an oxidizing biocide such as bromine, chlorine, chlorine dioxide, other oxidants for best results. Garratt-Callahan Formula 9020 is an organic dispersant that penetrates and loosens the complex matrix of organic deposits allowing biocides to reach the organisms for more effective kill and control.  The product also reduces the need for hyperchlorination or other shock treatments by maintaining better system control.

The advantages of a clean system

A clean system with controlled scale, corrosion and biocide treatment is beneficial to you in both long- and short-term situations. Properly cleaned and maintained cooling towers will also reduce wasting of precious energy resources and downtime. 

Overall, the use of a properly administered biocide program with the help of Garratt-Callahan Formula 9020 organic dispersant and penetrant results in less energy consumption, less system downtime, a cleaner and more efficient cooling system that lasts for years without major damage to the heat exchange surfaces. Using Garratt-Callahan’s expert advice and choosing the right treatment can make a huge difference in operation and longevity of your cooling system. 

About Garratt-Callahan

Garratt-Callahan supplies more than water treatment chemicals; we deliver expertise for the care, management, and support industrial water systems. Garratt-Callahan’s experts assist our clients to treat their boilers, cooling towers, reverse osmosis systems, and wastewater. We protect their employees and equipment. 

Now in our second century of service, Garratt-Callahan is more than an experienced water treatment company. We’re a company of skillful professionals, from our field engineers to our technical support professionals. We build client relationships founded on fairness, honesty, and integrity by thoroughly understanding your needs and challenges. We can then roll up our sleeves to find solutions that work for YOU. 

Learn More about the G-C Advantage

*Chart provided by Bulletin B0015 Last Revision: 05. Oct.2012 Prepared by: Behrouz (Ben) Mosayebi, Aquarius Technologies.

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