Formation of Chlorination By-Products in Sanitary Hot Water Networks Treated by Continuous Chlorination

The French Public Health code has defined the drinkability criteria of tap water, and especially the absence of pathogenic microorganisms. Sanitary hot water should respect the same criteria and in this context, Veolia has deployed specific measures to limit the proliferation of Legionella pneumophila. These bacteria can be found in any hydric environment but sanitary hot water networks had good conditions for their development: temperature, nutriments, and solid surfaces. Legionella pneumophila is pathogenic for human and is the bacterium responsible of Legionnaires 'disease. To limit the proliferation of Legionella pneumophila in domestic hot water network, Veolia use sometimes a continuous disinfection process called continuous chlorination. It is based on the injection of chlorine continuously in the hot water loop. The injection is controlled to obtain a residual concentration of chlorine about 1ppm in the recycling loop. This process is effective to control the development of Legionella pneumophila in water, but is susceptible to improve the formation of chlorination by-products. The impact of continuous chlorination on the formation of by-products has not bee described in the literature.In this study, the concentrations of AOX (Absorbable Organic Compounds), THM (Trihalomethan) and halophenols have been measured in cold water and in hot water after chlorine injection for 6 sanitary hot water networks in France. In the French regulation, the authorized levels in tap water are 100 μg.L-1 for THM and 0.1 mg.L-1 for halophenols. The results showed that the concentration of AOX and THM was higher in hot water than in cold water. When the continuous chlorination was efficient (free chlorine > 0,5ppm), the concentrations of AOX and THM increase. But for all analysis achieved in this study the concentration limit for THM was not reached. The concentration of halophenols was not modified by the continuous chlorination and was lower than the limit. This study showed that it is important to control the concentration of free chlorine in the network to combine the efficiency against Legionella development and the respect of the regulation in terms of byproducts concentrations.