Some designs incorporate coagulation, flocculation, and sedimentation a single unit (either up flow solids contact units or sludge blanket units). Most up flow solids contact units use recirculation of previously formed floes to enhance floc formation and maximize usage of treatment chemicals. Sludge blanket units force newly forming flocs to pass upward through a suspended bed of floc. In both styles of units, the cross-sectional surface of the basin increases from bottom to top, causing water flow to slow as it rises, and allowing floc to settle out. Combination units generally use higher rise rates and shorter detention time than conventional treatment. Numerous manufacturers market proprietary units based on these design concepts. These units are more compact and require less land for plant site location. On-site pilot plant evaluation, by a qualified engineer familiar with the water quality, is recommended prior to design.