It’s in all pools and spas--that slimy, sticky, gooey stuff that appears along the tile line, in skimmer baskets, around air jets, on ladders, in the filters and in the pipes.

While facility operators assumed it was just suntan and body oils, scientists concerned with problematic environments have another name for it, “biofilm.”

Over the past 20 years, an abundance of studies have attempted to understand biofilm’s effect on swimming pool and spa water. What have we learned? Trapped within the biofilm are microorganisms like algae, bacteria, fungi, protozoa or a mixture of these populations.

The U.S Centers for Disease Control and Prevention (CDC) estimates that 65 percent of human bacterial infections in aquatic environments involve biofilm (NSPF Pool & Spa Handbook, 2011 edition).

Many of these microorganisms are resistant to standard disinfection processes because the biofilm actually protects the microorganisms from being destroyed.

Even in well-maintained spas, microbes -- especially Pseudomonas aeruginosa -- can rapidly recolonize within spa environments. The spa’s hot water dilates the skin pores, which invites the bacteria and toxins into the skin, causing a rash, or an allergic reaction.

Rash outbreaks are an indicator of poor overall sanitation in an aquatic facility (Meyer and Klueger, Arch Chemical). This “hot tub rash,” indicated by itchy, red bumps, grows in warm water environments and survives in biofilm.

In addition to a rash, pathogens may be released from the biofilm into the air, causing respiratory diseases like Legionella (Miller, WAHC 2009). Legionnaires’ disease -- named after the 1976 outbreak at a convention of the American Legion in Philadelphia -- is a severe form of pneumonia. The Legionella bacteria are found naturally in warm water environments; therefore, they can be established (trapped) in the biofilm. If this pathogen is inhaled in or near the surface of a pool or spa -- or through misters -- the effects can be quite serious.

Most bacteria grow in biofilm attached to a surface. We all actually experience this -- the slimy feeling in the mouth, plaque buildup on teeth, contact-lens solution kits, on toilets and in bathroom basins -- all are examples of contamination lurking in biofilm.

Combating biofilm requires an understanding of what the substance actually is and then finding how to treat it.

What Is Biofilm?

Biofilm is a collection of microbes living together on a wet surface. Extracellular polysaccharide substance (EPS) is the slimy goo which encases the organisms and anchors them to a surface.

Pools and spas are excellent media for microbes; the wet, warm and nutrient-enriched waters encourage growth. Biofilm harbors algae and water mold as well as recreational water illnesses, such as Cryptosporidium, Giardia and E. coli.

If there is a wet surface, there is biofilm. Black, yellow and green algae are microscopic forms of plant life and can be free-floating or attached to the surface (biofilm). Algae on pool surfaces are much harder to kill. Operators who have examined black algae confirm there is a crusty film or gelatinous sheath over the black algae spore. This is actually a biofilm covering.

Black algae, the slowest growing of the three algae, are very resistant to chemicals and can become embedded within the plaster surfaces of the pool or spa. The green algae are free-floating and grow quickly; they also are easily controlled with sanitizers and algaecides. Yellow, or mustard algae, appear on walls, mainly in the shade.

Water mold has a different structure. It may be called white or pink algae and may appear to be white or gray and may even be of a tissue-paper-like substance.

All of these algae may attach to the walls through the biofilm process; these can require excessive labor to kill if a preventative maintenance program is not adopted.

Combating Biofilm

Conventional methods of microbial control have proven inadequate, so the best way to combat biofilm is to perform regular, preventative maintenance. Don’t let the biofilm get started because it is much harder to kill the organisms once they are established. First, maintain a sanitizer residual at all times in the pool or spa.

The ideal range for chlorine disinfection is 3 to 5 parts per million (ppm). Pool operators should pay close attention to maintaining this residual, either through manual testing and/or automatic chemical-feed control systems with constant monitoring. The sanitizer residual must be maintained in all parts of the pool, paying close attention to coves and dead zones. Second, daily brushing must be performed on the tile and pool/spa walls and floor.

We know that the pool/spa wall surface is one place where the biofilm attaches; brushing the walls detaches the biofilm-containing algae spores from the walls, and forces them into the water where the disinfectants and algaecides can kill them.

A thorough cleaning of all filter systems with an efficient backwashing of sand filters is mandatory. The proper cleaning of cartridges and Diatomaceous Earth (D.E.) grids requires degreasing the media prior to any acid-washing. If grids are acid-washed prior to degreasing, the biofilm will not be eradicated.

The use of tri-sodium phosphate has been recommended as a degreaser for D.E. grids. Muriatic acid can be applied as a supplemental cleaner to remove scale after the grids have been degreased.

Other recommended tactics include the addition of surfactants and chelants, weekly shocking with 10 ppm of Free Available Chlorine and use of enzymes (Klueger and Meyer).

Above all, a chemical cleansing of the filter systems is highly recommended. Roy D. Vore, with DuPont, a member of the Disinfection and Water Quality task force for the CDC Model Aquatic Health Code, suggests “the use of alkaline cleaners containing either bleach or quats will reduce biofilm; however, they do not result in total kill.”

Aquatic operators should take the time to learn from all the recent studies being performed by chemical manufacturers. Our job is to ensure safe and healthy swimming venues. Staying ahead of biofilm by adding these effective, regular maintenance recommendations is the right thing to do, and will help protect bathers from unhealthy environments, curtail recreational water illnesses, create an enjoyable experience, and keep the patrons coming back!

Thanks to the following for their expertise and input into this article’s research:

Roy Vore, Ph.D, and Ed Lightcap, Dupont.

Todd Klueger and Ellen Meyer, Ph.D., Arch Chemicals

James J. Miller, M.S.

World Aquatic Health Conference (

National Swimming Pool Foundation (

Montana State University, Center for Biofilm Engineering

Connie Sue Centrella is a professor and Program Director for the online Aquatic Engineering Program at Keiser University eCampus. She is a five-time recipient of the Evelyn C. Keiser Teaching Excellence Award “Instructor of Distinction.” Centrella is an industry veteran with over 40 years experience in the pool and spa industry. She is a former pool builder with extensive knowledge in pool construction and equipment installation as well as manufacturing.

Bryan BuchkoComment