High rate filtration system includes an upflow filter vessel containing a bed of buoyant media followed by a downflow filter vessel containing a bed of nonbuoyant media. The upflow filter vessel is unobstructed by cross-sectional, media-confining screens. A horizontal trough structure collects filter...http://www.google.com.au/patents/US4547286?utm_source=gb-gplus-sharePatent US4547286 - Water filtration process and apparatus having upflow filter with buoyant filter media and downflow filter with nonbuoyant filter media
