The activated sludge process, which is used in the purification of an oil synthesis effiuent, produces an effiuent which is high in turbidity. The water also contains a lot of dispersed bacterial growth which is not removed during sedimentation. Flocculation is essential to remove these materials. The average diameter of the colloids was found to be 0,92/'m. The zeta potential of the colloids was determined in an attempt to correlate it to the amount of flocculant required. The value was relatively constant at -13,88 mV, which indicates that a cationic·coagulant was required. The required dosage to achieve clarification by means of sedimentation only was very high. Both the recirculation of floes and the addition of bentonite reduced the required dosage drastically. The usage offlocculant aids led to small reductions in flocculant dose and cost. Ferric chloride as primary coagulant, followed by the addition of polyelectrolyte, led to large savings in certain cases. Large reductions in flocculant dosage were obtained by the additional use of sand filtration for the removal of smaller floes.