Abstract: The present invention relates to treating a medium and more particularly to treating a medium to reduce or inhibit biofouling.The invention provides a method for the reduction or inhibition of biofouling of a part of a system which is subject to contact with a medium capable of causing biofouling by providing biological material. The method comprises contacting the medium with a support material (which is preferably a high surface area material) so as to cause biological material to form on the support material in preference to the part of the system.The invention may be applied, for example, to inhibit or reduce biofouling of heat transfer surfaces in cooling water systems.