Sludge samples from eleven potable water treatment plants (PWTP), three waste water treatment plants (WWTP) and an industrial water treatment plant (IWTP), located in different areas of Spain, mainly in Catalonia, were analyzed for their radiological content in order to determine whether they could be considered as industries affected by naturally occurring radioactive material (NORM). In general, samples from the PWTPs showed higher activity values for the alpha and gamma emitting isotopes than the WWTPs and the IWTP. For example, samples from the area located in the north of Catalonia show values of U-234, U-235 and U-238 in the range of 84.4-792.1 Bq/kg, 3.3-26.8 Bq/kg and 63.8-585.9 Bq/kg, respectively. In general, for PWTP, the values obtained for the gamma emitter and alpha emitter isotopes showed that both the geology and the industrial activities correlate with the values measured. The magnitude of these results demonstrates the need to measure the radionuclide content of these samples before reaching a decision about their final disposal. (C) 2010 Elsevier B.V. All rights reserved.