Modelling of reactivation after UV disinfection: Effect of UV-C dose on subsequent photoreactivation and dark repair

The increased use of UV radiation as a wastewater treatment technology has stimulated studies of the repair potential of microorganisms following treatment. In this study, samples of unfiltered secondary effluent were irradiated with seven levels of UV-C doses (50–200 mW s/cm2) from six low-pressure lamps in an open-channel UV disinfection system. Following irradiation, samples were incubated at 20 °C under photoreactivating light or in darkness. Samples were analysed for 240 min following incubation.The logistic model is proposed to explain the relation between photoreactivation and the UV-C dose received by the microorganisms. That model accurately fitted the data obtained in photoreactivation experiments, permitting interpretation of the estimated kinetic parameters: Sm and k2. In the experiments carried out in darkness, a slight reactivation is observed (<0.1%), followed by a decay period in which survival decreases. In order to model this last period, a modification was made to the logistic model by including a term of mortality that assumes a zero-order kinetic. The parameters Sm and k2, in both photoreactivation and darkness, show an exponential dependence on the UV-C inactivating dose. It is possible to predict their values, and hence the reactivation curve, from the equations proposed in this work.