A pilot plant had been built to test the behaviour of ultra fi ltration (UF), reverse osmosis (RO), and electrodialysis reversal (EDR) in order to improve the quality of the water supplied to Barcelona met- ropolitan area from the Llobregat River. This paper presents results from two studies to reduce natural radioactivity. The results from the pilot plant with four different scenarios were used to design the full- scale treatment plant built (SJD WTP). The samples taken at different steps of the treatment were analysed to determine gross alpha, gross beta and uranium activity. The results obtained revealed a signi fi cant improvement in the radiological water quality provided by both membrane techniques (RO and EDR showed removal rates higher than 60%). However, UF did not show any signi fi cant removal capacity for gross alpha, gross beta or uranium activities. RO was better at reducing the radiological parameters studied and this treatment was selected and applied at the full scale treatment plant. The RO treatment used at the SJD WTP reduced the concentration of both gross alpha and gross beta activities and also produced water of high quality with an average removal of 95% for gross alpha activity and almost 93% for gross beta activity at the treatment plant.