The High-Rate Algal Pond is an efficient treatment for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient. The removal of phosphate is of great importance considering eutrophication. Deterministic modelling of the HRAP ecosystem could provide a rational basis for pond management.

The aim of this paper is to underline the difficulties in modelling orthophosphate evolution in an HRAP. A model simulating the evolution of orthophosphate concentrations in an experimental HRAP was developed taking into account the usual processes: inflow, outflow, mineralization of dead algae and uptake by phytoplankton. These processes allow us to simulate quite well the evolution of the orthophosphate concentration.

According to the literature, the precipitation of calcium phosphate (at high pH) should be taken into account in calcium-rich waters but the models allowing calculation of PO4 concentration according to pH variations neglect PO4 uptake by algae.

The problem is that both direct (uptake) and indirect (precipitation) phenomena act on phosphate removal when algae grow.

It is possible, by striking chlorophyll a and phosphorus balance, to assess the precipitation of phosphates; the results are consistent with the fact that uptake by phytoplankton is the main process acting on PO4 evolution.

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