In 1985, twelve people were killed in Uster, Switzerland when the concrete roof of a swimming pool collapsed after only thirteen years of use. The roof was supported by stainless steel rods in tension, which failed. The Federal Materials Testing Institute, based in Duebendorf, Switzerland, and the Federal Materials Research and Testing Institute of Berlin concluded that the collapse was the result of chloride-induced stress corrosion cracking. In 2001, the suspended ceiling of a municipal swimming pool in Steenwijk, The Netherlands, collapsed due to a similar cause.
Case study - How UV can reduce corrosion in swimming pool structure
Swimming pool environments have changed significantly in recent years, most markedly in leisure pools. Higher water temperatures combined with an increase in the number of bathers, has led to higher levels of chemical disinfection. Chlorine-based disinfectants are the norm. Meanwhile, bathers in swimming pools introduce various nitrogenous substances to the pool water such as urine and sweat. These contaminants contain high levels of uric acid, urea, ammonia, histidine and creatinine. These compounds react with chlorine to form a number of by-products, notably chloramines (monochloramine, dichloramine and trichloramine). Chloramines are thought to be the most important factor in the corrosion of stainless steel in a swimming pool environment.
The temperature of the air in pool halls is generally held about 1 °C above water temperature. High air temperatures significantly accelerate corrosion. Atmospheric moisture in pool buildings comes from evaporation of pool water and as droplets from the turbulent water features that have become increasingly common in leisure pools. Higher levels of humidity can lead to condensation in cooler parts of the building and during the cool of the night. Recirculation of pool air (a common method of reducing energy costs) can increase humidity, as well as adding to the build-up of contaminants in the atmosphere.
Consequently the atmosphere of indoor swimming pools is one of the most aggressive to be found in a building environment. Under the specific temperature conditions near the ceiling, chlorine containing chemical species (chloramines) in water vapour from the pool can condense onto the stainless steel components, dry out and convert to chlorides. As this can be a repeated cycle, very aggressive concentrations of chlorides may build up. The situation is aggravated by the fact that components may not be easily accessible for regular cleaning.
UV treatment reduces the risk of stress corrosion cracking by removing the root cause – chloramines.
UV treatment is a very effective method of controlling chloramines. The polychromatic emission of medium pressure UV lamps breaks the chlorine-nitrogen bond in chloramine molecules by photolysis. Research has shown that the optimum wavelengths for chloramine photolysis are:
Monochloramine (NH2Cl): 245 nm
Dichloramine (NHCl2): 297 nm
Trichloramine (NCl3 ): 260 nm and 340 nm
The polychromatic spectrum of medium pressure lamps includes all these wavelengths. Medium pressure UV treatment is an effective method of keeping chloramine levels in check without the need for expensive dilution and back-washing, or shock dosing with chlorine.
In conclusion, UV technology has an important role to play in the provision of safe, healthy water for recreational use.