Light wavelength-dependent E. coli survival changes after simulated solar disinfection of secondary effluent

In this study, the photoreactivation and the modification of dark repair of E. coli in a simulated secondary effluent were investigated after initial irradiation under different conditions. The simulated solar exposure of the secondary wastewater was followed by exposure to six different low-intensity fluorescent lamps (blacklight blue, actinic blacklight, blue, green, yellow and indoor light) for up to 8 h. When photoreactivation was monitored, blue and green colored fluorescent light led to increased bacterial regrowth. Blacklight lamps further inactivated the remaining bacteria, while yellow and indoor light led to accelerated growth of healthy cells. Exposure to fluorescent lamps was followed by long term storage in darkness, to monitor the bacterial repair in the dark. The response was correlated with the pre-exposure dose of applied solar irradiation, and to a lesser extent with the fluorescent light dose. Bacteria which had undergone extensive exposure showed no response under fluorescent light or during storage in the dark. Finally, the statistical treatment of the data allowed us to suggest a linear model, which is non-selective in terms of the fluorescent light applied. The estimation of the final bacterial population was predicted well (R-sq similar to 75%) and the photoreactivation risk was found to be more important for cultivable cells.