TABLE OF CONTENTS Ross Gregory
WRc Swindon
Frankland Road, Blagrove
Swindon, Wiltshire
England
Thomas F.Zabel
WRc Medmenham
Medmenham, Oxfordshire
England
James K.Edzwald
Professor, Department of Civil and
Environmental Engineering
University of Massachusetts
Amherst, Massachusetts
U.S.A.
Sedimentation and flotation are solid-liquid separation processes used in water treatment mostly to lower the solids concentration, or load, on granular filters. As a result, filters can be operated more easily and cost effectively to produce acceptable-quality filtered water. Many sedimentation and flotation processes and variants of them exist, and each has advantages and disadvantages. The most appropriate process for a particular application will depend on the water to be treated as well as local circumstances and requirements.
Sedimentation for the improvement of water quality has been practiced, if unwittingly, since the day humans collected and stored water in jars and other containers. Water stored undisturbed and then poured or ladled out with little agitation will improve in quality, and this technique is used to this day.
As societies developed, reservoirs and storage tanks were constructed. Although constructed for strategic purposes, reservoirs and storage tanks did improve water quality. Various examples are known that predate the Christian era. Ancient surface water impounding tanks of Aden were possibly constructed as early as 600 B.C. and rainwater cisterns of ancient Carthage about 150 B.C. (Ellms, 1928). The castellae and piscinae of the Roman aqueduct system performed the function of settling tanks, even though they were not originally intended for that purpose.
