Effectiveness of conventional surface water treatment processes in reducing natural radionuclides in Nile River drinking water.

Understanding the stage-specific performance of conventional water treatment processes in removing natural radionuclides is crucial for optimizing public health protection, particularly in regions dependent on major river systems like the Nile. This study comprehensively evaluates the effectiveness of each treatment stage in conventional water treatment plants across Upper Egypt in reducing natural radionuclides, radon-222 (Rn-222), radium-226 (Ra-226), radium-228 (Ra-228), and potassium-40 (K-40), in Nile River-derived drinking water. We collected 40 water samples from 10 representative Nile-fed treatment plants in Upper Egypt, at 4 key stages: raw water intake, post-coagulation/sedimentation, post-filtration, and final treated water. Rn-222 concentrations were measured using the RAD7 detection system with RAD H2O accessory, while gamma-emitting radionuclides (Ra-226, Ra-228, and K-40) were analyzed via NaI(Tl) gamma spectrometry after achieving secular equilibrium between each parent and its short-lived progeny. The multi-stage conventional treatment process demonstrated differential effectiveness across radionuclides through distinct removal mechanisms. The treatment sequence achieved cumulative removal efficiencies (Reff) of 74.19% for Rn-222 through volatilization during various stages, 28.86% for Ra-226 through coagulation and particulate capture, 46.84% for Ra-228, and 20.17% for K-40, the lowest among the studied radionuclides, due to its predominantly dissolved ionic nature. Treatment stages contributed sequentially: coagulation removed 29.17% of Rn-222, 17.46% of Ra-226, 32.16% of Ra-228, and 11.85% of K-40; filtration further reduced Ra-226, Ra-228, and K-40, resulting in cumulative Reff values of 26.06%, 42.04%, and 12.84%, respectively, by the end of this stage, while for Rn-222, filtration significantly enhanced its removal to a cumulative Reff of 59.42%; final treatment (disinfection) achieved the aforementioned cumulative efficiencies for all radionuclides. The sequential multi-barrier approach resulted in calculated annual effective doses (Dan) of 18.8 µSv/year (adults), 28.3 µSv/year (children), and 15.4 µSv/year (infants), all well below the international screening level of 100 µSv/year for a single source of radiation in drinking water, applicable to all age groups. These processes effectively mitigate radiological risks, with filtration being particularly crucial for volatile radionuclides and coagulation-filtration being essential for radionuclides that are associated with suspended particles, such as Ra-228. These findings provide critical insights for water treatment optimization and regulatory compliance in river-dependent communities.