Water Treatment Wisdom: Bioslime in a Cooling Tower is Bad



Cooling towers are notorious for growing things; especially microscopic things…  they are warm and moist. They have areas that get a lot of sunlight and other areas that kept fairly dark. Whether it’s algae or bacteria, it’s no wonder that cooling towers provide the perfect environment for all kinds of little organisms to proliferate. In fact, an improperly treated cooling tower can act like a giant Petri dish.

So what’s a couple hundred million microscope critters living in your tower mean anyway? It’s not such a big deal, right? A little bleach will keep them at bay, right?


The truth is that those little organisms can be much more of a liability than you think. Most facility operation managers are well versed in or have at least heard of the dangers of waterborne pathogens, like legionella, emanating from their cooling towers; it is why most facility managers find water treatment companies that provide water treatment services for their main water systems. While legionella presents a worst case scenario, it is not the only bug that can cause major operational issues in a cooling tower, and is why proper tower water treatment is so important to the overall operational health of a cooling tower system.

IMPORTANT NOTE:  ANSI/ASHRAE Standard 188-2015, Legionellosis: Risk Management for Building Water Systems has major potential impacts on how building owners need to think about legionella in their water systems.

Almost all bacteria have one thing in common… they really, really, really want to live! And they will do whatever they can to protect themselves. One of the clever things they do, especially in water, is to band together and produce a slimy matrix that covers them while they breed and grow. This material is actually quite resistant to most biocides. It is technically called biofilm, but many people also refer to it as bioslime (because hey, it’s slimey!) and if you have it in your tower, you probably have other issues.

One thing about bioslime that many people do not realize is that it acts as an incredibly good insulator. When you get bioslime on an active heat transfer surface, like on inside surfaces of the tubes of a chiller, it becomes harder and harder to produce chilling. In fact, a bioslime layer that is less than one fifth of one millimeter thick can impede heat transfer by more than 10%! That means that you will have to use more energy to produce the proper levels of cooling. In a larger chiller system that could mean tens of thousands of dollars in wasted energy… all because of a little slime in the towers.

Water Treatment Fun Fact: There are no issues with bioslime in boiler water treatment. The heat inside a boiler is typically way too hot to allow biological growth. The kind of issues that plague boilers are usually scale and/or corrosion related.

Check out this short video to see what bioslime is all about:

So what should I do about bioslime in my cooling tower?

The most important thing you can do is keep your cooling tower clean and make sure that your water treatment professional is aware of any issues that you see. Sometime it might not seem like you have a problem, but the best way to tell in not with your eyes. Even if the water looks completely clear, put on a rubber glove and run your hand over part of the cooling tower basin. If the basin feels “slick” or “slimy” you almost certainly have bioslime in your tower.

Options for removing bioslime vary from system to system, but it is almost always a good idea to get your water treatment professional involved and possible even employ the help of a reputable HVAC Cleaning company. Ongoing proper water treatment is the only way to prevent bioslime from ever forming. Your water treatment service provider should be able to describe their methods for keeping biological growth under control in your cooling tower.

Current industry standards dictate the use of a three chemistry system for cooling towers:

  1. An oxidizing biocide –  kills microorganisms by destroying their cell walls and oxidizing protein within the organism. Chlorine is an example of an oxidizing biocide.
  2. A non-oxidizing biocide –  kills by ways other than oxidation, including interference with cell metabolism and structure; more like a poison. DBNPA and isothiazoline are non-oxidizing biocides.
  3. A corrosion inhibitor –  provides protection to the metal surfaces against corrosion. Especially helps counteract the corrosive properties of the oxidizing biocide.

This standard water treatment protocol for cooling towers is often referred to as a “dual biocide program” because to the two biocides used, and it is still considered the best defense against microbiological growth in a cooling tower.

EcoSafe powered by Smart ReleaseThe latest advancements that have been made in the standard dual alternating biocide program doesn’t actually change the chemistry much at all, but instead changes the way in which those chemicals are fed into the cooling tower system. Most water treatment companies rely on liquid chemicals which are pumped into the open condenser system via chemical metering pumps. Up until now, the benefit to this type of system was precise control over the dosage rates of each chemical via highly calibrated, electronic metered pumps.

Today, there are highly improved solid feed delivery systems that use dry chemistry to achieve the same level of control without the headaches and safety issues of liquid chemistry. These newer types of solid feed systems, like EcoSafe Solid Feed System powered by Smart Release, have all but completely revolutionized the way that we treat cooling towers today.

If you would like more information about what to do if you have bioslime in your cooling tower, please do not hesitate to call us!

Thanks for reading!

About Clarity Water Technologies

Clarity Water Technologies is the leading industrial and commercial water treatment company, providing expert consulting and cutting-edge chemistry for your facility’s heating, cooling, potable and process applications.

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About the Clarity Blog

Clarity’s Water Treatment blog is written to provide tips and helpful information to anyone who is involved in managing a water treatment application at a facility. 

Our water treatment experts are here to help, no matter what your facility’s heating, cooling, potable or process application may be.  Check our blog for articles on topics like boiler and cooling water, cooling tower cleaning, legionella prevention and remediation, and environmentally-friendly water treatment options.

Clarity Water Technologies is the leading industrial and commercial water treatment company in the U.S.

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