Dani Mulas defends his thesis on the presence of radioactivity in water treatment plants

The Barcelona metropolitan area (BMA; 3.2 M inhabitants) has an integrated urban water cycle management. Different type of treatment plants are located along the drinking, sewerage and reuse networks where specific treatments are applied to guarantee a good enough standard of the water quality. The presence of radionuclides in treatment plants and in the aquatic environment is well known. Primordial and daughters, cosmogenic, global fall-out and nuclear-legacy radionuclides can be found in the urban water cycle. Moreover in nuclear medicine (NM), short-lived radionuclides are administered to patients, excreting part directly into the sewage network and entering into the urban water cycle. Thus, the levels of radionuclides in waters and materials from water treatment plants were experimentally checked in the BMA in order to understand the behaviour during the water treatment and to perform a risk assessment. An integrated study focused on the study of the radionuclides levels at different three types of water treatment plants from the same network was carried out. A total of 233 samples were taken at 1 drinking water treatment plant (DWTP), 7 wastewater treatment plants (WWTPs) and 1 reclaimed water treatment plant (RWTP). The concentrations were determined by gamma-spectroscopy techniques after acquisitions performed by high-purity germanium detectors. Primordial and daughters radionuclides were found in materials from the DWTP studied and the highest specific activities found for sludge, sand and reverse osmosis brine corresponded to K-40. Nevertheless, the maximum concentration in the case of granular activated carbon was found for U-238. Moreover cosmogenic Be-7 and fall-out Cs-137 were found in sludges. A total of 5 different NM radionuclides were found in the analysis carried out in the samples from the 7 WWTPs. In the case of water and sewage sludge the highest maximum values and detection frequencies corresponded to Tc-99m and I-131. Moreover Ga-67, In-111 and I-123 were found but showing significantly lower levels. The detection frequencies and the mean levels found at the WWTPs of Ga-67, Tc-99m, In-111 and I-131 agreed with the NM radionuclides total activity administered in the region studied. Furthermore the concentrations and detection frequencies were significantly higher in the sewage sludge samples taken at the very large-sized WWTP-1 (325,000 m3/d), partially explained by their low sludge age. Medically-derived I-131 was also found in reclaimed water for reuse from the RWTP and materials from DWTP, which represent novel contributions to the current knowledge in this field.

Taking into account the present findings some considerations from the radiological protection point of view can be done. Despite the presence of radionuclides in the DWTP materials, they do not pose a radiological risk. In the case of the WWTPs and the RWTP studied, the levels found in waters and materials do not represent a significant risk, however, I-131 concentrations were pointed out as the most significant. With the aim to achieve a better understanding of I-131 behaviour in WWTPs and predict the I-131 levels novel methods of I-131 partitioning analysis as well as prognosis models were adapted successfully to a WWTP. I-131 partitioning results pointed out that the settling fraction predominates in the reactor while in the rest of the WWTP samples dissolved iodide fraction was the most significant. Furthermore the activated sludge reactors from WWTPs were revealed as the key step for I-131 removal from wastewater. Specifically, reactors with the highest total nitrogen kjeldahl removal were also the most effective for I-131 reduction. Regarding the I-131 modelling a total of 82 % of simulated data fit with the experimental results in the sewage effluent within uncertainties.