Phosphorous simultaneous precipitation by coagulants reduces the volatile solids percentage which can be deleterious to the biological process. In this work a mathematical model was developed and biological indices were applied to control Fe(III)-dosed activated sludge systems. A molar ratio Fe:P = 1.9-2.3:1 on the aeration basin of a laboratory-scale activated sludge reactor caused a progressive enrichment of the sludge with inorganic solids deteriorating the system performance. Crawling and attached ciliates were the most sensitive organism groups to these changes. The proposed mathematical model estimated: (i) the threshold concentration of fixed suspended solids, above which the reactor performance deteriorates. and (ii) the decay of the most sensitive organism groups with time. The Shannon-Wiener and sludge biotic indices predicted the decrease of the reactor performance by coagulant overload. The simultaneous application of the mathematical simulation and the biological indices guarantees a successful control of systems operated with phosphorous simultaneous precipitation by Fe(III). (C) 2011 Elsevier B.V. All rights reserved.