The present invention is directed to a filtration system including filter media that use diffusion as a method of particulate reduction that balances the contact time required for a desirable level of interception and filter performance. The filter system of the present invention includes a filter medium that removes particulate contaminants by diffusion, means for providing sufficient contact time for an influent to contact the filter medium such that the filter medium can intercept sub-micron particulates at an average flow rate below that of an on-demand or instantaneous flow rate; and a storage buffer for providing a filtered effluent at a rate independent of the average flow rate required to achieve adequate particulate reduction through the filter medium.