The gradual decrease in the filtration flux rate with time, which occurred due to the capture of solid particles and/or organic substances onto the surfaces of pores in a thin filter medium, was measured in constant-pressure filtration of both pool water and pond water. A new theoretical background of the conventional characteristic filtration equation describing the variation of the filtration resistance of the clogged filter medium with time was afforded based on a consideration of the variation of both the porosity and the specific surface area of the filter medium caused by the capture of the solids in the Kozeny-Carman equation. The flux decline behavior depended solely on the initial filtration rate, influenced by the pore size of the filter medium and the filtration pressure. The variations of the filtration rate and the filtrate volume with the filtration time were very accurately described by the analytical solution based on the characteristic filtration equation. In the case of pond water with a relatively high turbidity compared to pool water, the transition point from depth to cake filtration was determined based on the pressure dependence of the specific filtration resistance of the filter cake, and then the flux attenuation behavior was evaluated by considering the transition from depth to cake filtration. (C) 2007 American Institute of Chemical Engineers.