FLOCCULATION OF FINE-GRAINED SEDIMENTS DUE TO DIFFERENTIAL SETTLING

The flocculation of fine-grained particles depends on collisions due t o Brownian motion, fluid shear, and differential settling. Previous ex perimental work on the flocculation of fine-grained sediments has emph asized the effects of fluid shear. These effects are significant in hi gh-turbulence regions. However, as the turbulence and fluid shear decr ease, as, for example, in open waters away from shore, differential se ttling becomes the dominant mechanism for flocculation. In the present article, previous work on the effects of fluid shear is reviewed. How ever, the emphasis is on recent experimental work on the effects of di fferential settling on the flocculation of fine-grained, primarily ino rganic particles. The transition in effects between situations where f luid shear is dominant and the other extreme, where differential settl ing is dominant, was also investigated and is discussed. The sediments used in these studies were natural bottom sediments from the Detroit River inlet to Lake Erie. The tests were initiated with disaggregated sediments and were continued as the particles aggregated and formed fl ocs. These flocs then grew until a steady state size distribution was reached. In order to reach a steady state the differential settling te sts sometimes continued for as long as 30 days; they were done in both freshwater and seawater at sediment concentrations from 1 mg/L to 200 mg/L and with and without treatment to remove organic matter. Floc si ze distributions as a function of time were determined. From the exper iments it is shown that the times to steady state and the steady state median diameters are much larger when differential settling is the do minant mechanism for flocculation than when fluid shear is the dominan t mechanism for flocculation. It is also shown that the effects of sed iment concentration and salinity are qualitatively similar; i.e., as t hese quantities increase, both the time to steady state and the steady state floc size decrease. Settling speeds of the flocculated particle s were also measured; the settling speeds of flocs are much larger and increase more rapidly with floc diameter when produced by differentia l settling than when fluid shear is dominant.