The invention provides a method of controlling biological fouling of water containing items. The method involves combining at least two reactants, a peroxygen source and an acyl group donor to form peracetic acid. Because the peracetic acid can be made in situ, the entire sum of its short lifespan can be used controlling biocidal fouling and storage and transport is simplified. The invention further facilitates the ease of controlling biocidal fouling by allowing the injection of the biocidal agent in liquid form, in a non-acidic environment, and without the need for any halogen. This allows the inventive biocide to be used with chemistries for which prior art biocides are inapplicable.