Photoreactivation and subsequent solar disinfection of Escherichia coli in UV-disinfected municipal wastewater under natural conditions.

Photoreactivation of ultraviolet (UV)-disinfected wastewater of different qualities was experimentally assessed. Photoreactivation ability of secondary effluent and microstrained inflow was analyzed in different samples of 50 mL (Petri dish) and 7,000 mL volume to describe open channel effluent situations of wastewater treatment plants in a more realistic approach. The small sample of secondary effluent revealed a total log10 inactivation of 1.8 units and the small sample of microstrained inflow a total log10 inactivation of 3.2, with an applied UV-254 fluence of 84 and 253 J/m², respectively. Maximum net photoreactivation for secondary effluent and microstrained inflow was in the order of 1.2 log10 and 0.37 log10 units, respectively, for both sample sizes. However, significantly faster photoreactivation performance was generally determined for small sample volumes. The photoreactivation processes were completely compensated for by solar disinfection within a 120 min exposure time. Solar disinfection processes were negligible in the larger sample volumes of microstrained inflow. For municipal wastewater treatment systems with open channel effluents, it is essential to take into consideration the dependence of solar UV-365 fluence rate on water depth and wastewater characteristics.