In a batch test of sedimentation with flocculated suspensions, from the evolution 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 visible. On the other hand, the design corresponds to sediment heights much smaller than the initial height of the batch tests, and consequently to very small sediment heights. In the method presented in this paper, the semi-batch test is carried out with periodical withdrawals of supernatant and additions of fresh suspension at the top of the liquid, obtaining a zig-zag variation. 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 initial height of the suspension. In this way, it is possible to obtain the exact relationship between the settling flux density of solids in the hindered settling or non-compression zone, and consequently the design parameters for a continuous thickener can be obtained for an underflow solids concentration range and sediment height range wider than those obtained from a batch test.