Recreational Water Disinfection

June 1, 2017, Department, by Ellen Meyer

2017 June Operations RecreationalWaterDisinfection 410

This summer while kids are happily enjoying a cool splash in the pool, aquatics staff will be busy ensuring that aquatics facilities are clean and safe. With obesity and diabetes reaching epidemic proportions, it is important that kids and adults are provided with fun ways to stay active. A recreational water illness (RWI) outbreak in park facilities could be disastrous for the people who become ill, the aquatic staff responsible for keeping the water safe and for people who need to stay active but may not swim because of concerns about the safety of the water.

Every two years, the Centers for Disease Control and Prevention (CDC) provides a survey of disease outbreaks in pools and spas. In the latest survey (2011–2012), there were 69 outbreaks in treated water (i.e., not a lake or river). A summary of disease-causing organisms (i.e., pathogens) and outbreak statistics are shown in Table 1.

Table 1: Disease-Causing Organisms and Outbreak Statistics
Group Species Typical Health Effect Outbreaks*


Bacteria Pseudomonas aeruginosa Rash 2 (4)   16 (56) 0 (0)
E. coli  Diarrhea 2 21 5
Shigella sonnei Diarrhea 1 5 1
Legionella spp. Respiratory illness 9 (2)  33 (52)   18 (1)
Parasites Cryptosporidium spp. Diarrhea 36 874 44
Giardia Diarrhea 1 21 0
Viruses Norovirus Diarrhea 2 (2) 122 (21) 0 (4)

*Values in parentheses are not confirmed to be the specific pathogen.

There are three basic steps to prevent RWIs:

  • Minimize contamination
  • Maintain a sanitizer residual in all parts of the pool at all times
  • Respond to contamination according to CDC protocols

Since most of the pathogens are introduced by the swimmers themselves, patron education is a vital component of contaminant minimization. Signage should be used to inform patrons about the following:

      i. They should shower before entering the pool.
     ii. They should not enter the pool if they are ill with diarrhea.
    iii. They should not urinate in the pool.

Free posters and brochures are available online from the CDC to help educate patrons about the role they play in keeping the water clean.

Pathogens are constantly being introduced into the pool, so maintaining a sanitizer residual in all parts of the pool at all times is essential to prevent RWIs. The sanitizer needs to be out in the water where the swimmers are to protect them from bather-to-bather disease transmission. A sanitizer residual also needs to be present in areas of the pool where swimmers rarely go (e.g., the deepest part of the pool) and during times when swimmers are not present (e.g., overnight or during periods of closure). The reason for this is because bacteria, such as Pseudomonas and Legionella, can grow and reproduce in unsanitized water. These organisms can form a slimy layer, called biofilm, which is very hard for sanitizers to get into to kill the organisms harboring there. So, even if a sanitizer residual is reestablished, it may not be able to penetrate the biofilm.

There are numerous products that claim to kill bacteria, but unless the product is a sanitizer registered by the Environmental Protection Agency (EPA), as indicated on the label, it is not a sanitizer. The EPA regulates chemicals used to kill pathogens in pools. The only chemicals and systems that are EPA-registered for use in sanitizing pools are chlorine, bromine, PHMB (polyhexamethylene biguanide) and some metal systems. There are some treatments, such as UV and ozone, which are very good at inactivating hard-to-kill pathogens, like Cryptosporidium (crypto). However, UV and ozone cannot be used as primary sanitizers in pools because a residual cannot be maintained in the pool to prevent bather-to-bather disease transmission. UV and ozone are often used as secondary disinfectants in high-risk facilities, but a primary sanitizer residual must always be present. Chlorine is the most common sanitizer used in pools, and its concentration should be maintained between 1 and 4 parts per million (ppm).

We know that pathogens are constantly being introduced into the pool, but there are also certain events where gross contamination occurs: vomit, formed stool and diarrheal accidents. The CDC has studied the pathogens that may be present in these events and has provided guidelines for treatment on its Healthy Swimming website and in its Model Aquatic Health Code. The steps in these remediation procedures should be carefully followed to prevent an outbreak. For example, below are the basic steps for treatment of diarrhea in the water:

Hyperchlorination Tables
No Stabilizer Present With Cyanuric Acid Stabilizer Present
Free Chlorine, PPM    Time, Hours Free Chlorine, PPM     Time, Hours
20                                    12.75 20                                      28
10                                      25.5 30                                       18
  40                                        8.5


     Step 1: Close the pool to swimmers
     Step 2: Remove as much fecal material as possible (vacuuming is NOT recommended)
     Step 3: Using unstabilized chlorine, hyperchlorinate the pool (see tables above for dosages and times) with the filtration system operating and maintaining the pH below 7.5
     Step 4: Backwash the filter
     Step 5: Reduce chlorine concentration to 1 to 4 ppm.
     Step 6: Allow swimmers back into the water

If stabilizer is greater than 15 ppm, partially drain pool to reduce stabilizer to less than or equal to 15 ppm.

The reason there are different hyperchlorination instructions with the stabilizer is because the stabilizer slows the disinfection rate of the chlorine. The CDC performed a study with crypto and found that with greater than 15 ppm of cyanuric acid, it was not able to obtain the required reductions. The values in the stabilizer table were obtained from its study.

It is essential that all members of the pool staff are trained in RWI prevention and their local health code(s). They should also be familiar with how and why (i) patrons can contribute contamination, (ii) an EPA-registered sanitizer residual is maintained in all parts of the pool at all times and (iii) contamination response protocols are followed. The pool operators, lifeguards and swimmers all play a part in preventing disease transmission and ensuring that a safe, healthy pool can be enjoyed by everyone.

Ellen Meyer is the Product Safety and Government Affairs Manager at Lonza.


  1. The prevalence of obesity among U.S. youth aged 2-19 years in 2011-2014 was 17%. From Ogden, C.L., Carroll, M.D., Fryar, C.D., Flegal, K.M., Prevalence of obesity among adults and youth: United States, 2011-2014, National Centers for Health Statistics Data Brief No. 219, November 2015.
  2. 29.1 million people or 9.3% of the U.S. population have diabetes. From Centers for Disease Control and Prevention. National Diabetes Statistics Report: Estimates of Diabetes and Its Burden in the United States, 2014. Atlanta, GA: US Department of Health and Human Services; 2014.
  3. Hlavsa MC, Roberts VA, Kahler AM, Hilborn ED, Mecher TR, Beach MJ, Wade TJ, Yoder, JS. Outbreaks of illness associated with recreational water — United States, 2011–2012 . MMWR Morb Mortal Wkly Rep. 2015;64(24):668-672.
  4. Centers for Disease Control and Prevention - Healthy Swimming Posters 
  5. Centers for Disease Control and Prevention - Healthy Swimming Brochures 
  6. Moritz, M.M., Flemming, H.C., Wingender, J., Integration of Pseudomonas aeruginosa and Legionella pneumophila in drinking water biofilms grown on domestic plumbing materials, Int J Hyg Environ Health. 2010 Jun;213(3):190-7, DOI: 10.1016/j.ijheh.2010.05.003.
  7. Abdel-Nour, M., Duncan, C., Low, D.E., Guyard, C., Biofilms: The stronghold of Legionella pneumophila, Int. J. Mol. Sci. 2013, 14, 21660-21675; doi:10.3390/ijms141121660.
  8. Fecal Incident Response Recommendations for Aquatic Staff 
  9. The Model Aquatic Health Code (MAHC): An All-inclusive Model Public Swimming Pool and Spa Code 
  10. Murphy JL, Haas CN, Arrowood MJ, Hlavsa MC, Beach MJ, Hill VR. Efficacy of chlorine dioxide tablets on inactivation of Cryptosporidium oocysts. Environ Sci Technol. 2014;48(10):5849–56.