Application of biological indices and a mathematical model for the detection of metal coagulant overload in a laboratory scale activated sludge reactor with phosphate simultaneous precipitation

► Ferric chloride is widely used for the removal of phosphorous from wastewaters. ► Addition of Fe(III) on the secondary treatment stage is recommended. ► Biological indices predict the system performance. ► A mathematical model allows to define the adequate operating conditions. ► Simultaneous application of both methods guarantees a good control of the process. 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).