Conventional Drinking Water Treatment Plants (DWTPs) have a fairly standard sequence of processes which essentially consist of solid separation using either physical processes such as settling and filtration, or chemical processes such as coagulation and disinfection. Consequently, large quantities of solid wastes (sludge, filtration media, exhausted ion exchange resin and astewaters) are generated every year by DWTPs. These solid wastes may contain all kinds of pollutants, some with significant levels of radioactivity and may cause a radiological impact either on operating personnel, or on the public if the waste is recycled, e.g. as fertilizer or for cement manufacturing. The occurrence of radionuclides in sludge samples from drinking water treatment plants has been studied by a few authors with the main aim of evaluating their impact on human exposure. For instance, Wisser, 2003 reported relatively high activity concentrations for natural radionuclides in sludge samples from a water treatment plant in Canada. Kleinschmidt and Akber, 2008 also examined radioactivity concentrations in solid wastes from Australian’s DWTPs and elevated residual concentrations were identified in these waste products. Palomo et al., 2010 have recently reported results of liquid and sludge samples from a Spanish DWTP. We propose to study the temporal evolution of the radioactivity content in waste products at a Catalan DWTP, in particular, sludge from the decanter cleaning process and granular activated carbons. Rejects from the reverse osmosis process applied to a fraction of the water were also monitored. Radiological hazard effects were evaluated by the gamma index because one of the end uses of this sludge was the manufacture of cement.