Wastewater treatment facilities can produce sludge containing measurable amounts of radioactivity. The radionuclides that are concentrated may originate from radionuclides present in the original source of water or from water treatment. This treated water ends up most frequently in the aquatic environment, but it may also receive further application, for instance, as irrigation water in agriculture. Therefore, evaluation of its quality is essential to both protect the health of the environment and also of humans. The aim of the present work is to study the radioactivity in liquids and sludge generated at 11 wastewater treatment plants in Spain, working under a variety of conditions, as there is an increase in usage of wastewater treatment plants in Spain as a result of frequent drought periods. Gross alpha and gross beta activities were determined in influent and effluent liquids and sludge by ZnS solid scintillation detectors and low-background gas-flow counters. Natural and artificial gamma radionuclides in the sludge were analyzed with a germanium semi-conductor detector. The results indicate that the radiological characteristics of the effluents and sludge do not present a significant radiological risk and make them suitable for future applications. ; Postprint (published version)