Substrate clogging caused by the accumulation of the particulate solids is the worst operational problem for vertical-flow constructed wetlands (VFCW). In this paper, the effects of particulate solids distribution and their accumulation in the substrate with different gravel sizes were investigated. The results demonstrated that the clogging layer can be considered as two parts: one is the blanket-like deposition layer, and the other is the upper substrate clogging layer. Furthermore, the clogging process shall be partitioned as three stages of puncture phase for the pollutants: the formation of the blanket-like deposition layer; and the formation and compaction phase to the whole clogging layer. With reference to the clogging mechanism, it is believed that the particulate solids (<100 mu m) were absorbed firstly by electrostatic forces and van der Waals' forces. This is followed by the "bridging" made by the accumulated solids which act as a "sieve", thus further restricting larger particulate solids to flow through. (C) 2010 Elsevier B.V. All rights reserved.