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Biofilm – the Bad & the Ugly

If you think your pool water is clean and sanitary just because it looks clear, think again. Yes, you may have shocked the water and added algaecide and have even maintained good chlorine, bromine biguanide (Soft Swim or Baquacil) level, but you’ve only controlled about 1% of the bacteria in your swimming pool! That’s right, just 1%. The other 99% is on every pool surface that you can or can not see. And most of those places are virtually impossible to reach. What now?

biofilm1The more we deal with swimming pools and pool problems, the more we realize that we’re dealing with or treating symptoms rather than the root cause of the cloudy water, algae, scale build up, etc. As I’ve noted in other articles, there are a plethora of reasons for cloudy water from poor water chemistry to poor circulation to improper cleaning habits to environmental causes. And typically, these causes combine to create the problem.

As we look for the root cause, we see more and more that there are real “problems” that are often undetected. What do we mean? Have you noticed that there is a regular build up of film on the pool liner, up and down the walls or in the corners? If you are a regular brusher, the problem may not be as noticeable. How about when you take the filter apart for normal maintenance or cleaning and you see a whitish film on the inside of the tank or on the skimmer weir or skimmer body?
Biofilms in swimming pools can and often do lead to cloudy water, algae blooms, scale build-up on the heater (prevents efficient heating), and even corrosion (certain biofilms can have a pH of about 1.0 – very acidic) of any metal surface of the pool system including heaters, filter parts, ladders, rails, etc. Much of the biofilm found in swimming pools is hidden away in pool plumbing and the filter system. The average residential in ground pool can easily have 150 – 200 lineal feet of piping! Commercial or semi-public pools can have many times that.

All of those films or slimes are what we call biofilms. In biofilms lives the other roughly 99% of all pool bacteria. The 1% that is in the water is classified as “planktonic”. Like plankton or algae, planktonic bacteria free-floats in the water. That is the bacteria that your chlorine, bromine or other sanitizer can “easily” kill. The 99% of the bacteria in the biofilm can be quite another story and long-term headache. There is much information about biofilm from institutions around the world to back up our information to you. Montana State University’s Center for Biofilm Engineering is one of our key sources.


First, what is a biofilm? A biofilm is a film or large quantity of bacteria that is living in and as a vast colony in the microscopic world. In the “big” world, you could call a coral reef a “biofilm.” A biofilm is self-perpetuating and difficult to remove. Worst of all, biofilm love virtually any surface, especially wet or damp. But beware; even after drying out, the biofilm will not necessarily be dead but simply dormant. Did we mention that biofilms are relatively resistant to chlorine, bromine or other sanitizers?

Second, how do biofilms form? As just mentioned, biofilms form on any surface. In your pool that means the liner or the pool walls, bottom, ladder rails, skimmer baskets, ladder treads, filter tank bodies, pump bodies and impellers, directional returns (eyeballs), heater plumbing, and especially the piping. There is a 5 steps process as to the formation of biofilms: Attachment, Colonization, Protection, Growth, and finally what I call Distribution.