This research studies the effect of the widely used coagulant ferric chloride on the activated sludge sedimentability through a vast array of hindered settling tests considering different application modes and a wide range of reagent doses. Direct application of ferric chloride improved the hindered settling velocity (up to twice the settling velocity of the activated sludge with no coagulant addition), but sharply decreased the pH to levels where the biological process was unfeasible (pH < 4). When the pH was adjusted during coagulation to avoid biological inhibition, the impact on the settling velocity depended on the adjusted pH value. When the added coagulant was previously precipitated and neutralized, no pH inhibition occurred and the hindered settling velocity increased linearly with the dose (up to 8 times). This velocity improvement was caused by the increase in flocs density due to the capture within the flocs of the formed precipitates. Based on these experimental results, the usefulness and reliability of the standard hindered settling velocity mathematical models used for the secondary settler design and optimization (Richardson & Zaki model and the Vesilind's exponential model), was expanded to situations in which precipitated ferric chloride is used in wastewater treatment plants. Two empirical equations were proposed and fitted to relate these mathematical models' parameters with the dose of coagulant. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.