The Resuspension of Flocculent Solids in Sedimentation Basins

Abstract

The phenomenon of resuspension has been considered to be an important factor in the imperfect behavior of sedimentation basins receiving suspensions containing flocculent solids. Resuspension is the entrainment into the flow of particles that have once settled to the floor of the basin. This investigation has been undertaken to study the resuspension phenomenon and to find ways in which its harmful effects can be reduced. Resuspension has been found extremely difficult to isolate and define scientifically because many other factors produce the same overall effect upon the settling tank. Furthermore it is practically necessary to identify individual particles in order to know whether a particular one found in suspension in the downstream portion of a settling tank had earlier been settled out and resting on the floor. Laboratory studies on settling tank behavior were conducted on a glass-walled "scour flume" 1.27 ft wide by 14 ft long with depth adjustable from 0.5 to 2.0 ft (see Fig. 1.1). In order to simulate certain aspects of the behavior of full-scale settling tanks the laboratory flume was fitted with moving flight scrapers similar to those installed in primary sewage settling tanks. For some of the later tests the flume was fitted with 14 probes that made it possible to sample the tank contents at practically any point of five cross sections along the length of the flume. Most of the studies on the laboratory flume utilized a discrete suspension of gilsonite {s. g. 1.04) particles or a flocculent suspension of ferric chloride and bentonite clay particles. Tests for critical velocity required for entrainment of particles from a smooth bed showed that fine light particles are more easily lifted from the bed than was previously supposed. With scrapers moving upstream, the critical velocity for two sizes of gilsonite tested was found to vary between 10.5 and 14.5 times the particle settling velocity. Field and laboratory studies on one scheme proposed to improve the performance of settling tanks - a series of transverse sloping baffles installed throughout the main body of a rectangular settling tank - both indicate that baffling a tank is not the answer. Tests on full-scale settling tanks of a sewage treatment plant showed that conventional measures of settling tank performance are meaningless when the suspension entering the tank is flocculent (as is sewage). Newer measures of performance are proposed, which show promise in evaluating the behavior of settling tanks receiving flocculent suspensions.

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