The determination and monitoring of 131I activity in sewage treatment plants based on A2/O processes

131I, a radionuclide used in nuclear medicine facilities, is habitually dumped into domestic sewer systems, which causes its control in municipal sewage treatment plants (STP) to become necessary. In this paper, we describe a procedure for the determination of the 131I activity based on its precipitation as silver (I) iodide, its dissolution with cyanide in ammonia medium and its measurement by the liquid scintillation counting technique (LSC). After optimization, this LSC procedure allows the determination of 131I activities with a limit of detection (CCβ) of 11 mBq kg−1. The procedure was successfully validated by comparison with the ISO 10703:2007 normative and requires both less sample volume and measurement time than the standard procedure. The method has been applied to the monitoring of 131I in the STP of Valladolid (Spain), which found activities below the maximum allowed by legislation and showed a near constant background 131I activity in the influents with punctual increases corresponding to authorized spills. The mean 131I removing efficiency of the STP was 52%.