Semi-batch test of sedimentation. Application to design

In a batch test of sedimentation with flocculated suspensions, from the evo lution of the supernatant-suspension discontinuity height with time, it is possible to obtain the design parameters for continuous gravity thickeners. Nevertheless, the design is only approximate because normally the surface height of the sediment, growing on the bottom of the cylinder, is not visib le. On the other hand, the design corresponds to sediment heights much smal ler than the initial height of the batch tests, and consequently to very sm all sediment heights. In the method presented in this paper, the semi-batch test is carried out with periodical withdrawals of supernatant and additio ns of fresh suspension at the top of the liquid, obtaining a zig-zag variat ion. Considering the Kynch theorems, that are applicable to the suspension above the sediment, it is possible to obtain the variation of the sediment surface height versus time, and this sediment height can be close to the in itial height of the suspension. In this way, it is possible to obtain the e xact relationship between the settling flux density of solids in the hinder ed settling or non-compression zone, and consequently the design parameters for a continuous thickener can be obtained for an underflow solids concent ration range and sediment height range wider than those obtained from a bat ch test. (C) 2000 Elsevier Science B.V. All rights reserved.