Microalgal growth and ammonium removal in a P-free medium have been studied in two batch photobioreactors seeded with a mixed microalgal culture and operated for 46 days. A significant amount of ammonium (106 mg NH4-Nl(-1)) was removed in a P-free medium, showing that microalgal growth and phosphorus uptake are independent processes. The ammonium removal rate decreased during the experiment, partly due to a decrease in the cellular phosphorus content. After a single phosphate addition in the medium of one of the reactors, intracellular phosphorus content of the corresponding microalgal culture rapidly increased, and so did the ammonium removal rate. These results show how the amount of phosphorus internally stored affects the ammonium removal rate. A mathematical model was proposed to reproduce these observations. The kinetic expression for microalgae growth includes a Monad term and a Hill's function to represent the effect of ammonium and stored polyphosphate concentrations, respectively. The proposed model accurately reproduced the experimental data (r=0.952, P-value <0.01). (C) 2014 Elsevier Ltd. All rights reserved.