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[Dark treatment effects on photoreactivation characteristics of Escherichia coli in a tertiary effluent disinfected by UV].

The effect of dark treatment on photoreactivation of Escherichia coli in a tertiary effluent disinfected by UV was studied. The effluent of sand filtration of a sewage treatment plant was disinfected by UV, kept in darkness for a period of time, and then put under photoreactivating light to investigate the photoreactivation. The experimental results indicated that the photoreactivation potential could recover immediately after water samples were kept in darkness for less than 6 h. The percentage photoreactivation of the water samples disinfected by UV with doses of 5 mJ/cm2 and 20 mJ/cm2 were 31.1% and 0.4%, respectively, and the photoreactivation rates were 1.67 x 10(4) CFU x (mL x h)(-1) and 125 CFU x (mL x h)(-1). When dark treatment time was prolonged to 24 h, the percentage photoreactivation and photoreactivation rates were lowered to 10.0% and 0.3%, 830 CFU x (mL x h)(-1) and 20 CFU x (mL x h)(-1), respectively. Those suggested that photoreactivation can only be weakened, but can not be controlled completely even after being kept in darkness for 24 h. So the safety concern on photoreactivation after UV disinfection of wastewater should be taken into account.