The effectiveness of microalgae flocculation by pH increase was investigated using the jar test procedure. The results indicated that pH values between 11.8 and 12 induced extensive flocculation without need of adding magnesium. pH was increased by sodium hydroxide or calcium hydroxide. In both cases, the total solids abatement was more than 95% producing sludge of excellent settleability and good mechanical resistance quantified by a destructive test. The next step was the operation of a continuous flocculator which might optionally be packed with granular sand or inert resin. It was shown that a fluidized bed or any other mechanical energy transfer device was not required to obtain more than 95% efficiency with a superficial upflow velocity of 30 m/h corresponding to a residence time through the whole unit of 5 minutes only.