Assessment of the fate of Escherichia Coli in different stages of wastewater treatment plants

Seven full scale wastewater treatment plants were investigated to highlight the effectiveness of each treatment stage on removing E.coli. The primary sedimentation achieved an average E.coli removal efficiency of 30.5% which was much lower than the suspended solids (58%), thus revealing the absence of a linear relationship between the two parameters. Biological processes proved to be very important in the removal of E.coli through adsorption inside the sludge flocs and complex decay (mortality). In biological processes with a long retention time, such as activated sludge denitrification-nitrification, the decay was very important, whereas in the more traditional activated sludge process, without nitrification, the contribution of adsorption and mortality was quite balanced. Overall, the mechanical-biological treatment achieved a removal efficiency of 91.8-96.5% depending on the process. Additional removal can be achieved by disinfection. The effectiveness of E.coli removal with sodium hypochlorite was strictly depended on the product of residual chlorine (CR) with the contact time (t). The experimental curve fitted the Collins model well, with a standard deviation of less than 7%.