This report presents the results of a pilot project with the aim to gather information about the fate of some biocidal active substances in water treatment plants using different levels of treatment techniques. The aim was also to investigate whether the water temperature had any effect on the results of the treatment process. Four substances expected to be found in treatment plants (active substances notified in PT 1, 2, 3, 4, 6 or 9 of the Biocidal Products Directive) were chosen for the project. Samples for analysis were taken from four different treatment plants located in Sweden and representing the most common treatment techniques in the Nordic countries.