The invention relates to methods and compositions for of treating water systems, particularly recirculating water systems contaminated by a biofilm containing sessile micro-organisms. The method includes the steps of forming a boron conditioned enzyme and contacting the biofilm with said boron conditioned enzyme, thereby planktonising the micro-organisms The boron conditioned enzyme retains a level of activity at least 40% of the initial activity of the unconditioned enzyme for at least two hours after contacting the biofilm, either in the presence or absence of a biocide or corrosion inhibitor which would normally deactivate the enzyme. In preferred embodiments a biocide is added with the conditioned enzyme. The method and compositions are also applicable to remediation of tepid water systems in which biofilm may harbour pathogens such as sessile Legionella.
