Discharge of disinfected wastewater in recipient aquatic systems: fate of allochthonous bacterial and autochthonous protozoa populations

The discharge of disinfected effluents affects the bacterivorous ability of protozoa and the effect depends on the disinfectant applied. Chlorine provokes a decrease in the number of protozoa and a delay in the bacterivorous ability. The discharge of ozonated and peracetic acid-treated wastewater provokes only an initial slight decrease in bacterivorous ability. No correlation was found between toxicity values detected using the Microtox assay and the effect of disinfected effluents on freshwater protozoa population. After the disinfection processes, recipient systems (fresh and marine water) have different effects on the survival of Escherichia coli populations discharged to them. The effect of the freshwater recipient system is less negative than the effect provoked by sea-water, and the differences detected depend on the disinfection treatment applied. The wastewater bacterial population as a whole is able to grow after discharge of disinfected wastewater to receiving waters. However, in the absence of predation or competition, the recipient systems exert selection, with rod-shaped bacteria predominating.