Ultraviolet (UV) light disinfection has increasingly been used as an alternative method to replace conventional chlorine disinfection as it has been found to be a more efficient disinfection method. As UV disinfection only damages the nucleic acids of the microorganisms to prevent replication, there is a possibility of microorganisms repairing the damage sites. As few studies have investigated the reactivation of microorganisms after exposure to medium-pressure UV disinfection, it is essential for reactivation related to medium-pressure UV disinfection to be studied as medium-pressure lamps are gaining in popularity. Besides, disinfection by-products (DBPs) produced by UV disinfection have been discovered recently and may serve as a carbon source in the finished water, resulting in regrowth of the bacteria. It is therefore important to know the regrowth potential of bacteria with the existence of DBPs. In this study, the repair and regrowth of Escherichia coli after UV disinfection were investigated. Results showed that E. coli underwent photo repair (up to 5 log under fluorescent light conditions) more significantly than dark repair (up to 0.8 log in terms of bacterial count increase). The repair was generally found to be higher at low doses. At the same UV dose, it seems medium-pressure UV irradiation is able to control the repair to a lesser extent. In addition, the bacterial regrowth potential was studied with the addition of DBPs typically found in UV processes, such as acetic acid and formaldehyde. The maximum increase in bacterial count was found to be 0.3 log. Generally, the level of regrowth was insignificant compared with the increase of bacterial count due to bacterial repair.

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