The large nominal diameter Hydrocyclone process and the new process, Hydrocyclone equipped with Grit pot, were tested as a pre-treatment for run-off water. Many operational parameters such as inlet pressure, feed concentration and type of suspension solid were varied to study their performances. Both processes showed high efficiency in terms of solid separation and cutting diameter while the Hydrocyclone equipped with Grit pot has a very high concentration and low volume sludge compared to the classical Hydrocyclone process. Moreover, the advantage of the Hydrocyclone equipped with Grit pot was clearly demonstrated by comparing with the classical sedimentation tank. (C) 2002 Elsevier Science B.V. All rights reserved.