Pilot-plant studies on the treatment of three wastewaters (biological aerated flooded filter (BAFF) backwash water, post-primary settling tank effluent and BAFF effluent) have been conducted using a ballasted sedimentation process (ACTIFLO(R)) coupled with ferric chloride and polyaluminium chloride (PACl) coagulants. The response of the process to shock loads of high solids concentrations was also investigated. Results showed over 80% turbidity, 70% suspended solids, and 50% COD removals to be achievable at rise rates in excess of 80 m h(-1) and dose rates of 10-20 mg dm(-3) Fe. Slightly improved performance was attained using PACL at half the weight concentration (but about the same molar concentration) as that of Fe. No pH adjustment was necessary and process performance was not significantly influenced by changes in hydraulic loading rate. Effluent quality was largely unaffected by shock organic loads provided coagulant dosing was commensurately increased. Non-ideal flow through the plant was apparent from the measured residence time, which was similar to25% less than that calculated for plug flow. Clarification data were comparable to those reported for other high-rate clarification (HRC) processes, with somewhat improved performance in terms of hydraulic retention time and coagulant use. (C) 2004 Society of Chemical Industry.