Endotoxin removal efficiency in conventional drinking water treatment plants, a case study in Egypt

The present study determines the endotoxin removal efficiency of drinking water treatment plants (DWTPs) in Egypt, as examples of conventional treatment methods used in developing countries. The total endotoxin in source water (Nile River) of these DWTPs ranged from 57 to 187 EU∙mL−1, depending on the location of treatment plants.  Coagulation/ flocculation/sedimentation (C/F/S) after chlorine pre-oxidation removed bound endotoxins by 76.1–85.5%, but caused cell lysis and increased free endotoxins by 28.2–33.3% of those detected in raw waters. Rapid sand filtration had not significant effect on free endotoxins, but reduced bound endotoxins by 23–33.3%. Final chlorine disinfection also reduced bound endotoxins to levels around 1 EU/mL, accompanied by an increase in free endotoxins (37–112 EU∙mL−1) in finished waters. Simultaneously, final chlorine disinfection removed all heterotrophic bacteria, with low cyanobacterial cell numbers (348–2 450 cells∙mL−1) detected in finished waters. Overall, conventional treatment processes at these DWTPs could removal substantial amounts of bound endotoxins and bacterial cells, but increase free endotoxins through cell lysis induced by pre-oxidation and final chlorine disinfection. The study suggests that conventional processes at DWTPs should be optimized and upgraded to improve their performance in endotoxin removal and ensure safe distribution of treated water to consumers.