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.